Renin Angiotensin System and Cardiovascular Disease (Contemporary Cardiology)

Contemporary Cardiology C HRISTOPHER P. C ANNON , SERIES EDITOR

For other titles published in this series, go to http://www.springer.com/7677

MD

Walmor C. DeMello · Edward D. Frohlich Editors

Renin Angiotensin System and Cardiovascular Disease

Editors Walmor C. DeMello Department of Pharmacology University of Puerto Rico P.O. Box 5067 San Juan PR 00936 Medical Sciences Campus USA [email protected]

Edward D. Frohlich Ochsner Clinic Foundation 1514 Jefferson Highway New Orleans LA 70121 USA [email protected]

ISBN 978-1-60761-185-1 e-ISBN 978-1-60761-186-8 DOI 10.1007/978-1-60761-186-8 Library of Congress Control Number: 2009933642 © Humana Press, a part of Springer Science+Business Media, LLC 2009 All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Humana Press, c/o Springer Science+Business Media, LLC, 233 Spring Street, New York, NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights. While the advice and information in this book are believed to be true and accurate at the date of going to press, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein. Printed on acid-free paper springer.com

Preface

Experimental and clinical evidence supports the view that the activation of the renin angiotensin aldosterone system is involved in cardiovascular pathology including hypertension, heart failure, myocardial ischemia, and atherosclerosis. The present volume describes the intricacies involved in these processes, including the influence of prorenin/renin, angiotensin II, angiotensin (1-7), and aldosterone on cardiac and vascular functions as well as their involvement in the generation of cardiovascular diseases. Fundamental aspects like intracellular signaling, regulation of cell volume in the failing heart, and the presence of an intracrine renin angiotensin system are discussed. Moreover, the role of the mineralocorticoid receptor as an important component of the intracrine renin angiotensin system and as a regulator of extracellular action of angiotensin II is described, reinforcing the view that aldosterone inhibitors are helpful in the treatment of heart failure and hypertension. Let us hope the important topics included here motivate basic and clinical investigators and contribute to the development of new therapeutic approaches for cardiovascular diseases. We want to thank the distinguished authors and Humana Press for the opportunity to publish this important book. Walmor C. DeMello Edward Frohlich

v

Contents

1 Systemic Versus Local Renin Angiotensin Systems. An Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Walmor C. DeMello and Richard N. Re

1

2 Clinical Import of the Local Renin Angiotensin Aldosterone Systems . . . . . . . . . . . . . . . . . . . . . . . . . Edward D. Frohlich

7

3 Renin, Prorenin, and the (Pro)Renin Receptor . . . . . . . . . . . Genevieve Nguyen and Aurelie Contrepas

15

4 Local Renin Angiotensin Systems in the Cardiovascular System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Richard N. Re

25

5 Renin-Angiotensin-Aldosterone System and Pathobiology of Hypertension . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pierre Paradis and Ernesto L. Schiffrin

35

6 AT1 Receptors, Angiotensin Receptor Blockade, and Clinical Hypertensive Disease . . . . . . . . . . . . . . . . . . Robert M. Carey

59

7 Structural and Electrophysiological Remodeling of the Failing Heart . . . . . . . . . . . . . . . . . . . . . . . . . . Walmor C. DeMello

81

8 Inhibiting the Renin Angiotensin Aldosterone System in Patients with Heart Failure and Myocardial Infarction . . . . . . Marc A. Pfeffer

93

9 Left Ventricular Hypertrophy and Treatment with Renin Angiotensin System Inhibition . . . . . . . . . . . . . . . . . . . . Edward D. Frohlich and Javier Díez

103

vii

viii

Contents

10 Angiotensin-(1-7), Angiotensin-Converting Enzyme 2, and New Components of the Renin Angiotensin System . . . . . . Aaron J. Trask, Jasmina Varagic, Sarfaraz Ahmad, and Carlos M. Ferrario

121

11 Kinin Receptors and ACE Inhibitors: An Interrelationship . . . . Ervin G. Erdös, Fulong Tan, and Randal A. Skidgel

135

12 Kinins and Cardiovascular Disease . . . . . . . . . . . . . . . . . Oscar A. Carretero, Xiao-Ping Yang, and Nour-Eddine Rhaleb

151

13 CMS and Type 2 Diabetes Mellitus: Bound Together by the Renin Angiotensin Aldosterone System . . . . . . . . . . . . . Deepashree Gupta, Guido Lastra, Camila Manrique, and James R. Sowers 14 Renin Angiotensin Aldosterone System and Cardiovascular Disease . . . . . . . . . . . . . . . . . . . . . Swynghedauw Bernard, Milliez Paul, Messaoudi Smail, Benard Ludovic, Samuel Jane-Lise, and Delcayre Claude

187

207

15 Renin Angiotensin System and Atherosclerosis . . . . . . . . . . . Changping Hu and Jawahar L. Mehta

215

16 Renin Angiotensin System and Aging . . . . . . . . . . . . . . . . León F. Ferder

231

Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

245

Contributors

Sarfaraz Ahmad, MD, PhD Hypertension and Vascular Research Center, Department of Physiology and Pharmacology, Wake Forest University School of Medicine, Winston-Salem, NC Robert M. Carey, MD, MACP Division of Endocrinology and Metabolism, Department of Medicine, University of Virginia Health System, Charlottesville, VA Oscar A. Carretero, MD Hypertension and Vascular Research Division, Department of Medicine and Heart and Vascular Institute, Henry Ford Hospital, Detroit, MI Aurelie Contrepas, BS Institut de la Santé et de la Recherche Médicale and Collège de France, Experimental Medecine Unit, Paris, France Claude Delcayre, PhD Centre de Recherches Cardiovasculaires INSERM Lariboisière, PARIS, France Walmor C. DeMello, MD, PhD Department of Pharmacology, School of Medicine, Medical Sciences Campus, University of Puerto Rico, San Juan, PR Javier Díez, MD, PhD Área de Ciencias Cardiovasculares, Edificio CIMA, Pamplona, Spain Ervin G. Erdös, MD Department of Pharmacology, University of Illinois College of Medicine, Chicago, IL León F. Ferder, MD Departments of Physiology, Pharmacology and Medicine, Ponce School of Medicine, Ponce, PR Carlos M. Ferrario, MD Hypertension and Vascular Research Center, Department of Physiology and Pharmacology, Wake Forest University School of Medicine, Winston-Salem, NC Edward D. Frohlich, MD Ochsner Clinic Foundation, Louisiana State University School of Medicine, New Orleans, LA Deepashree Gupta, MD Diabetes and Cardiovascular Center, University of Missouri School of Medicine, and VA Medical Center, Columbia, MO

ix

x

Contributors

Changping Hu, MD, PhD Division of Cardiovascular Medicine, University of Arkansas for Medical Sciences and the Central Arkansas Veterans Healthcare System, Little Rock, AR Guido Lastra, MD Diabetes and Cardiovascular Center, University of Missouri School of Medicine, and VA Medical Center, Columbia, MO Benard Ludovic, BS Centre de Recherches Cardiovasculaires INSERM Lariboisière, Paris, France Camila Manrique, MD Diabetes and Cardiovascular Center, University of Missouri School of Medicine, and VA Medical Center, Columbia, MO Jawahar L. Mehta, MD, PhD Division of Cardiovascular Medicine, University of Arkansas for Medical Sciences and the Central Arkansas Veterans Healthcare System, Little Rock, AR Genevieve Nguyen, MD, PhD Institut de la Santé et de la Recherche Médicale, (INSERM) and Collège de France, Experimental Medecine Unit Marcelin Berthelot Paris, France Pierre Paradis, MD Hypertension and Vascular Research Unit, Lady Davis Institute for Medical Research, McGill University, Montreal. Canada Milliez Paul, MD Centre de Recherches Cardiovasculaires INSERM Lariboisière, Paris, France Marc A. Pfeffer, MD, PhD Department of Medicine, Division of Cardiology, Brigham and Women’s Hospital, Dzau Professor of Medicine, Harvard Medical School, Brigham and Women’s Hospital, Boston, MA Richard N. Re, MD Ochsner Clinic Foundation, New Orleans, LA Nour-Eddine Rhaleb, PhD, FAHA Hypertension and Vascular Research Division, Department of Medicine and Heart and Vascular Institute, Henry Ford Hospital, Detroit, MI Jane-Lise Samuel, MD, PhD Centre de Recherches Cardiovasculaires INSERM Lariboisière, Paris, France Ernesto L. Schiffrin, MD Department of Medicine, Sir Mortimer B. Davis-Jewish General Hospital, Hypertension and Vascular Research Unit, Lady Davis Institute for Medical Research, McGill University, Montreal, Canada Randhal Skidgel, PhD Department of Pharmacology, University of Illinois College of Medicine, Chicago, IL Messamoudi Smail, PhD Centre de Recherches Cardiovasculaires INSERM Lariboisière, Paris, France James R. Sowers, MD Diabetes and Cardiovascular Center, University of Missouri School of Medicine, and VA Medical Center, Columbia, MO

Contributors

xi

Bernard Swyngedauw, MD, PhD Centre de Recherches Cardiovasculaires INSERM Lariboisière, Paris, France Fulong Tan, PhD Department of Pharmacology, University of Illinois, College of Medicine, Chicago, IL Aaron J. Trask, BS Hypertension and Vascular Research Center, Department of Physiology and Pharmacology, Wake Forest University School of Medicine, Winston-Salem, North Carolina Winston-Salem, NC Jasmina Varagic, MD, PhD Hypertension and Vascular Research Center, Department of Physiology and Pharmacology, Wake Forest University School of Medicine, Winston-Salem, NC Xiao-Ping Yang, MD Hypertension and Vascular Research Division, Department of Medicine and Heart and Vascular Institute, Henry Ford Hospital, Detroit, MI

Chapter 1

Systemic Versus Local Renin Angiotensin Systems. An Overview Walmor C. DeMello and Richard N. Re

Abstract The concept of local renin angiotensin systems in the cardiovascular system is discussed, and evidence is presented that these systems work independently of the systemic one. Particular attention was given to the presence of an intracrine renin angiotensin aldosterone system in the heart and the novel role of the mineralocorticoid receptor. Furthermore, the influence of the renin angiotensin system on cell volume regulation is briefly discussed. This chapter includes an overview of these important biological concepts and provides an introduction to the topics that are discussed in detail by different authors throughout the book. The renin angiotensin system (RAS) is an enzymatic cascade in which renin derived from the juxtaglomerular cells (JG) of the kidney acts on an hepatically synthesized substrate, angiotensinogen, to generate the decapeptide angiotensin I. This peptide is cleaved by angiotensin-converting enzyme (ACE), primarily in the pulmonary circulation, to the vasoconstrictor and aldosterone secretagogue, angiotensin II. The blood pressure-elevating action of angiotensin II, together with its direct suppressive action on JG cells and the volume expansion produced by enhanced aldosteronedriven sodium retention, leads to the suppression of JG renin secretion, thereby forming a negative feedback loop. Volume depletion or lowered blood pressure stimulates renin release, leading to pressure elevation and volume retention. Elevated blood pressure or hypervolemia suppresses renin release and tends to lower blood pressure and intravascular volume. However, as powerful as this construct is, accumulating evidence indicates that it is incomplete in that it focuses solely on angiotensin synthesis in the circulation. For example, the blood pressure response to ACE inhibitors, which block ACE-driven angiotensin I generation, is not predicted by circulating renin activity, suggesting that RAS activity in tissues may be relevant [1]. Indeed, early on it was shown that most angiotensin II generation takes place in the arterial wall where angiotensin II is generated from RAS components taken W.C. DeMello (B) School of Medicine, Medical Sciences Campus, UPR, San Juan, PR USA e-mail: [email protected] W.C. DeMello, E.D. Frohlich (eds.), Renin Angiotensin System and Cardiovascular Disease, Contemporary Cardiology, DOI 10.1007/978-1-60761-186-8_1,  C Humana Press, a part of Springer Science+Business Media, LLC 2009

1

2

W.C. DeMello and R.N. Re

up from the circulation [2]. In the follow-up to these observations, it was then noted that components of RAS were taken up by many tissues [3], leading to the possibility that angiotensin II synthesis could be locally influenced by the relative uptake of components at those sites [4] and the synthesis of the various components can be detected in various tissues under various conditions [3]. Together, these observations made the concept of local RASs in tissues compelling. It must be recalled that renin in the circulation is not strictly rate-limiting for angiotensin I production. That is, angiotensinogen circulates at a concentration close to the K m for the generation of AI by renin. Therefore, an increase in angiotensinogen will lead to an increased production of angiotensin I [3, 5]. Thus, if a tissue were to augment tissue concentration of angiotensinogen by local production, more angiotensin I and likely angiotensin II would be generated in that tissue as compared to a tissue that did not augment the concentration of angiotensinogen with local synthesis. Also, it must be noted that the alteration of JG renin secretion cannot possibly equalize the angiotensin concentrations in the two tissues, which clearly indicates local regulation of local angiotensin II [5]. This, in turn, is particularly important because, of all the components of the RAS, the synthesis of renin in tissues (with a few exceptions) is the most contentious. Indeed, in many tissues the reported renin secretion is very low, suggesting that this renin could influence angiotensin production in only a small area [4]. Nonetheless, renin upregulation has been reported in the adrenal gland following nephrectomy where it helps maintain aldosterone secretion, as well as in the left ventricle and other tissues such as the heart in specific circumstances [6]. But it is clear from the arguments presented above that even in the absence of local renin synthesis, local regulation of angiotensin production can occur through local synthesis of other RAS cascade components [3]. Differential uptake of renin into tissues provides another mechanism for local RAS regulation. Although in normal heart cardiac renin seems to be related to its uptake from plasma [5], evidence is available that renin expression is increased after myocardial infarction [7] and after stretch of the cardiomyocytes [8]. On the other hand, a renin transcript that does not encode a secretory signal [9] and remains inside the cell is overexpressed during myocardial infarction – suggesting that intracellular renin has functional properties. Indeed, previous studies showed that intracellular renin and Ang II administration impairs cell coupling in the heart [10, 11] and intracellular Ang II reduces the inward calcium current in the failing heart [12], supporting the view that there is a functional intracrine renin angiotensin system [13–17]. This intracrine angiotensin II must properly also be considered an aspect of the tissue RASs, and it may well play an important role in such pathological processes as left ventricular hypertrophy, cardiac arrhythmias and cardiac myocyte apoptosis [13, 17] (see also Chapters 4 and 7). Other studies have demonstrated upregulation of angiotensinogen and angiotensin-converting enzyme (ACE) in tissues under normal or pathological conditions. The enzyme chymase, which can substitute for ACE in the conversion of angiotensin I to angiotesnin II, is also expressed in multiple tissues and upregulated in some circumstances [17]. Even more telling is the recent demonstration of a (pro)renin receptor in mesangial and other cells, which signals using classical

1

Systemic Versus Local Renin Angiotensin Systems

3

second messengers following the binding of prorenin or renin [18, 19]. This reveals the hormonal nature of (pro)renin. At the same time, binding of prorenin to the receptor activates its binding site so that the prohormone becomes enzymatically active, generating angiotensin I in the vicinity of cell surface receptors [19]. Similarly, renin bound to the receptor becomes more enzymatically active [4, 20]. These observations make clear that the biological activity of the RAS in a tissue can be powerfully influenced by the level of expression of the (pro)renin receptor in the tissue – a variable totally hidden from any analysis of the concentrations of circulating RAS components. The potential importance of this finding is suggested by the fact that prorenin levels are elevated in diabetic patients, and high concentrations of circulating prorenin are a predictor of retinopathy – a finding made all the more compelling by the observation that prorenin can be synthesized locally in the Mueller cells of the retina [19]. In addition, it now appears that there exist countervailing systems which while not influencing angiotensin II action at the receptor nonetheless offset some of its effect. For example, an ACE homologue, ACE2, has recently been described and studied [21]. ACE2, unlike ACE, does not convert angiotensin I to angiotensin II, but rather its principal action seems to be the conversion of angiotensin II to the hepatapeptide angiotenin (1–7), which operating through its own receptor offsets many of the vasoconstrictive and growth-promoting actions of angiotensin II [22, 23], improving impulse propagation during ischemia reperfusion through activation of the sodium pump, reducing the incidence of slow conduction and the generation of cardiac arrhythmias [24] (see also Chapter 10). Recently, it was found that chronic administration of eplerenone, a mineralocorticoid receptor blocker, reduces the expression of AT1 receptors at surface cell membrane as well as intracellularly inhibiting the intracrine and extracellular actions of Ang II on the inward calcium current in the failing heart [25]. These findings indicate that the mineralocorticoid receptor is involved in the regulation of intracellular and extracellular actions of Ang II and lead to the concept that there is an intracrine renin angiotensin aldosterone system (see also Chapter 7). It is possible to conclude that the beneficial effects of eplerenone in patients with heart failure are in part explained by the suppression of fibrosis, hypertrophy and electrophysiological abnormalities elicited by Ang II [26]. It is well known that regulation of cell volume is essential for normal cellular function. Recent evidence is available that the renin angiotensin system is involved in the regulation of heart cell volume [27] because extracellular Ang II increases cell volume through inhibition of the sodium pump and activation of the Na-K-2Cl cotransporter, while intracellular Ang II reduces the cell volume by activating the Na-K pump [27]. These findings are relevant particularly to myocardial ischemia which by itself causes cell swelling. According to these observations, the activation of the circulating renin angiotensin system is particularly harmful during myocardial ischemia while the activation of the intracrine renin angiotensin system might be beneficial by decreasing the cell volume (see Chapter 7). In conclusion, evidence is available that there are local renin angiotensin systems in the cardiovascular system, and that a functional intracrine renin angiotensin aldosterone system contributes to cardiovascular pathology [13, 25, 28, 29].

4

W.C. DeMello and R.N. Re

References 1. Mazzolai, L., Nussberger, J., and Aubert, J.F. et al. (1998) Blood-pressure independent cardiac hypertrophy induced by local activated renin-angiotensin system. Hypertension 31, 1324–30 2. Muller, D.N., and Luft, F.C. (1998) The renin angiotensin system in vessel wall. Basic Res Cardiol 93(Supl 2), 7–14 3. Kurdi, M., DeMello, W.C., and Booz, G.W. (2005) Working outside the system: an update on unconventional behavior of the renin angiotensin system components. Intern J Biochem Cell Biol 37, 1357–67 4. Nguyen, G., Delarue, F., and Bu, C. et al. (2002) Pivotal role of the renin/prorenin receptor in angiotensin II production and cellular responses to renin. J Clin Invest 109, 1417–27. 5. Danser, A.H.J., van Katz, J.P., and Admiraal, P.J.J. et al. (1994) Cardiac renin and angiotensins; uptake from plasma versus in situ synthesis. Hypertension 24, 37–48 6. Peters, J., Obermuller, N., Woyth, A., Peters, B., Maser-Gluth, C., Kranzlin, B., and Gretz N (1999) Losatan and angiotensin II inhibit aldosterone production in anephric rats via different actions on the intraadrenal renin-angiotensin system. Endocrinology 140, 675–82. 7. Passier, R.C.J.J., Smits, J.F.M., Verluyten, M.J.A., and Daemen, M.J.A.P (1996) Expression and localization of renin and angiotensinogen in rat heart after myocardial infarction. Am J Physiol 271, H1040–8 8. Malhotra, R., Sadoshima, J., Broscius, F.C., and Izumo, S. (1999) Mechanical stretch and angiotensin II differentially upregulated the renin angiotensin system in cardiac myocytes in vitro. Circ Res 85, 137–46 9. Clausmeyer, S., Reinecke, A., and Farrenkopf, R. et al. (2000) Tissue-specific expression of a rat renin transcript lacking the coding sequence for the prefragment and its stimulation by myocardial infarction. Endocrinology 141, 2963–70 10. DeMello, W.C. (1994) Is an intracellular renin angiotensin system involved in the control of cell communication in the heart? J Cardiovasc Pharmacol 23, 640–6 11. DeMello, W.C. (1995) Influence of intracellular renin on heart cell communication. Hypertension 25, 1172–7 12. DeMello, W.C. (1998) Intracellular angiotensin II regulates the inward calcium current in cardiac myocytes. Hypertension 32, 976–82 13. DeMello, W.C., and Danser, A.J.H. (2000) Angiotensin II and the heart: on the intracrine renin angiotrensin system. Hypertension 35, 1183–8 14. Re, R.N (2000) On the biological actions of intracellular angiotensin. Hypertension 35, 1189–90 15. Cook, J.L., Zhang, Z., and Re, R.N. (2001) In vitro evidence for an intracellular site of angiotensin action. Circ Res 89, 1138–46 16. Singh, V.P., Le, B., Bhat, V.B., Baker, K.M., and Kumar, R. (2007) High-glucose-induced regulation of intracellular ANG II synthesis and nuclear redistribution in cardiac myocytes. Am J Physiol Heart Circ Physiol 293(2), H939–48. 17. Paul, M., Poyan, M.A., and Kreutz, R. (2006) Physiology of local renin-angiotensin systems. Physiol Rev 86(3), 747–803. 18. Nguyen, G., Burckle, C.A., and Sraer, J.D. (2004) Renin/prorenin receptor biochemistry and functional significance. Curr Hypertens Rep 6, 129–32 19. Nguyen, G., Delarue, F., Berrou, J., Rondeau, E., and Sraer, J.D. (1996) Specific receptor binding of renin on human mesangial cells in culture increases plasminogen activator inhibitor-1 antigen. Kidney Int 50, 1897–903. 20. Nguyen, G., and Danser, A.H. (2008) Prorenin and (pro)renin receptor: a review of available data from in vitro studies and experimental models in rodents. Exp Physiol 93(5), 557–63 21. Donoghue, M., Hsieh, F., and Baronas, E. et al. (2000) A novel angiotensin converting enzyme-related carboxypeptidase(ACE2) converts angiotensin I to angiotensin (1–9). Circ Res 87, E1–E9.

1

Systemic Versus Local Renin Angiotensin Systems

5

22. Ferrario, C., Chappell, M., and Tallant, E.K. et al. (1997) Counterregulatory actions of angiotensin (1-7). Hypertension 30, 535–41 23. Crackower, M.A., Sarao, R., and Oudit, G.Y. et al. (2002) Angiotensin-converting enzyme 2 is an essential regulator of heart function. Nature 417, 799–802. 24. DeMello, W.C. (2004) Angiotensin (1-7) re-establishes impulse conduction in cardiac muscle during ischaemia-reperfusion. The role of the sodium pump. J Renin Angiotensin Aldosterone Syst Dec 5(4), 203–8. 25. DeMello, W.C., and Gerena, Y. (2008) Eplerenone inhibits the intracrine and extracellular actions of angiotensin II on the inward calcium current in the failing heart. On the presence of an intracrine renin angiotensin aldosterone system. Regul Pept 151, 54–60. 26. De Mello, W.C. (2006) Beneficial effect of eplerenone on cardiac remodelling and electrical properties of the failing heart. J Renin Angiotensin Aldosterone Syst 7(1), 40–6. 27. DeMello, W.C. (2008) Intracellular and extracellular renin have opposite effects on the regulation of heart cell volume. Implications for myocardial ischaemia. J Renin Angiotensin Aldosterone Syst. 9(2), 112–8. 28. Re, R.N., and Cook, J.L. (2008) The basis of intracrine physiology. J Clin Pharmacol 48, 344–50. 29. Re, R.N., and Cook, J.L.M. (2007) Mechanisms of disease: intracrine physiology in the cardiovascular system. Nat Clin Pract Cardiovasc Med Oct 4(10), 549–57.

Chapter 2

Clinical Import of the Local Renin Angiotensin Aldosterone Systems Edward D. Frohlich

Abstract The concept of local renin angiotensin (and possibly aldosterone) systems has been a relatively recent interjection to the investigative milieu. Much interest and important studies have resulted, and reference to applicability to disease and disease mechanisms is still of innovative and imaginative clinical and experimental studies. To this end, there are several areas of pertinence which have evolved including the underlying causations, mechanisms, and treatment of a number of diseases. Among those fascinating and provocative study areas is the need for additional motivated investigation related to ventricular and vascular hypertrophy, remodeling, and cardiac and renal failure and new thinking related to lifestyle modifications (including those related to salt excess, obesity, and responses to various drugs, clinically useful or otherwise). We have much confidence that these and other areas for study will be productive and useful and will lead to important clinical approaches and contributions on the issue of existing local RAAS. Much of the present-day clinical and investigative considerations of the renin angiotensin aldosterone system (RAAS) as well as this monograph concern the classically accepted endocrine RAAS system. The overall concepts involved have been extremely important in understanding the biology, physiology, and clinical relevance of this system as it pertains to cardiovascular and renal diseases, and they have led to the synthesis of new classes of therapeutic agents which have changed dramatically approaches to disease. Consequently, these changes have resulted in remarkable reductions in the morbidity and mortality of cardiovascular, renal, brain, and other diseases.

2.1 The Classical System The framework of this classically understood system embodies the synthesis of the enzyme renin in the kidney, the variety of mechanisms that promote and stimulate its release by the renal juxtaglomerular apparatus, and its action on the complex E.D. Frohlich (B) Ochsner Clinic Foundation, New Orleans, LA e-mail: [email protected] W.C. DeMello, E.D. Frohlich (eds.), Renin Angiotensin System and Cardiovascular Disease, Contemporary Cardiology, DOI 10.1007/978-1-60761-186-8_2,  C Humana Press, a part of Springer Science+Business Media, LLC 2009

7

8

E.D. Frohlich

protein angiotensinogen which is synthesized in the liver. The consequence of this action is the production of the decapeptide angiotensin I which, as it passes through the pulmonary circulation, loses its terminal two peptides by virtue of the proteolytic action of the angiotensin-converting enzyme. The resultant octapeptide angiotensin II is the potent vasopressor agent which is responsible for vasoconstriction; release of aldosterone by the adrenal cortex and consequent retention of sodium and water by the kidney; stimulation of specific brain centers responsible for increased cardiovascular adrenergic outflow and thirst; and local endothelial actions that promote mitogenesis, hypertrophy, collagen synthesis and tissue fibrosis, apoptosis, inflammation, and, no doubt, other intracellular signaling and other biological and pathophysiological effects [1]. Already, many of these latter actions have been incorporated in our consideration of the clinical diagnosis used clinically with respect to “endothelial dysfunction” [2, 3]. Although relatively recently described, there have been several additional components to the RAAS which have intriguing biological actions that have the potential for developing new physiological and pathological implications [4].

2.2 The Local Systems Although several of the foregoing actions of angiotensin II are relatively new, they have already been inculcated into a new dimension of the classical RAAS. This then relates to the overall concept of this monograph. It therefore concerns the concept of local RAAS (hereafter to be considered in plurality) that affect the structure and function of specific target organs of disease, including heart, blood vessels, kidney, and, no doubt, other organs [5]. To this end, although to some extent considered by some to be controversial, each of the components of these local RAAS has been identified within these foregoing organs although certain specific components (e.g., the putative synthesis of the enzyme renin within the heart) of the system. Indeed, these local systems already have important clinical and even therapeutic considerations and implications in health and disease [6]. In this respect, we also have deliberately included the hormone aldosterone in this local system since this hormone has already been identified to be present in some of these systems as for vital consideration of the existence of local RAAS [7]. Thus, although perhaps still in the realm of speculation, consideration of these local systems and newer components and metabolites of the system is neither premature, irrelevant, nor speculative for present-day consideration in this monograph. This monograph has been conceived and organized to stimulate further fundamental and clinical investigations dealing with the impact of the RAAS in disease. Thus, the participants of this workshop are of the unanimous opinion that these local RAAS are no longer a subject of debate; indeed, this is an important area of fundamental and clinical study, which is the intellectual commitment of this entire volume. To this end, the existence of these local systems in certain organs and the information derived from recent and current investigations provide the substance of

2

Clinical Import of the Local Renin Angiotensin Aldosterone Systems

9

tentative (but appealing) and exciting information which relates to specific clinical problems. Thus, this rather selective and speculative discussion of local RAAS in disease is included to tantalize the interested reader, student, and investigator in certain specific clinical situations including the pathogenesis and pathophysiology of ventricular hypertrophy in hypertension; ventricular and vascular remodeling in hypertensive and ischemic cardiovascular diseases; secondary (e.g., renal) therapeutic responses to disease; structural and functional responses of organs and in toxemias of pregnancy, to certain lifestyle and other interventions (e.g., salt excess) in hypertension. Ever since the Framingham Heart Study demonstrated that left ventricular hypertrophy (LVH) was a major risk factor predisposing the hypertensive patient to increased morbidity and mortality associated with coronary heart disease [8], we have been intrigued about the fundamental pathophysiological mechanisms of LVH that account for this risk. Thus, soon after this landmark epidemiological study, we initiated our earliest clinical and pathophysiological studies of this problem in which we elucidated the clinical correlates associated with the development of LVH [9]. We perceived the well-recognized concept that arterial pressure increased as an adaptive response of the left ventricle to the progressive increase in afterload in response to the increasing total peripheral resistance imposed by arteriolar constriction imposed. Our subsequent studies introduced the feasibility of the new noninvasive technology of M-mode echocardiography in order to identify the pathophysiological sequence in the clinical development of LVH [10]. We confirmed that coincident with the developing increased left ventricular (LV) mass and wall thicknesses, the earlier events associated with electrocardiographic evidence of left atrial abnormality were also identified with increased left ventricular mass and LVH. Moreover, these structural changes were associated with functional changes of LV functional impairment early in LVH [10]. These early findings suggested to us our ongoing concern that the development of LVH in hypertension were not solely the consequence of “adaptive hypertrophy”. We soon focused our attention on the functional events associated with antihypertensive therapy and whether it reversed the increased LV mass [11–14]. These studies indicated that certain agents decreased LV mass and impaired the ventricular functional responses. However, other agents decreased LV mass and were associated with normal ventricular function following reversal. We also showed some of those therapeutic agents that reduced LV mass also maintained normal function when the ventricular afterload was abruptly increased to pretreatment levels; other agents did not maintain that normal function [15–23]. These findings suggested to us that associated with treatment were intrinsic biological and physiological alterations which were related to the “reversal of hypertrophy” and were also responsible for these disparate functional changes. Our ensuing hypothesis was supported by our subsequent reports that the reduction of LA mass was achieved within only 3 weeks of therapy at a time when arterial pressure had not been reduced. In some studies, this was achieved with doses of some of these agents that had not even reduced arterial pressure [18, 20]. We therefore restated our concept to the development and reversal of the increased LV mass in hypertension, which were associated with nonhemodyanamic as well

10

E.D. Frohlich

as hemodynamic factors [24, 25]. These provocative findings permitted a further assessment of the issue concerning whether there were additional comorbid pathophysiological alterations associated with LVH. This concept was soon supported by our studies in untreated naturally developing spontaneously hypertensive rats (SHR) and their normotensive (control) Wistar-Kyoto (WKY) rats, matched for gender and age. In these studies we learned that they developed progressive ventricular ischemia not only in the hypertrophied LV but also in the nonhypertrophied right ventricles and in the LV of the WKY rats. Furthermore, this progressive ischemia with aging was closely related to increased hydroxyproline deposition and histological evidence of fibrosis in the extracellular matrix of the ventricle as well as surrounding the intramural arterioles in the chamber [26]. These findings were supported further by additional reports demonstrating pathological changes of apoptosis [27] and inflammatory changes [25]. Hence, we concluded that the underlying mechanisms of risk associated with LVH in hypertension related to ischemia, fibrosis, apoptosis, and inflammatory changes [23, 25]. More recently, we added yet another factor that complicates risk associated with LV – certain environment factors including excessive dietary salt-loading (vide infra) (28–30]. In this chapter, I shall not discuss the important experimental and clinical evidence that provides abundant clinical and experimental data demonstrating that angiotensin II contributes importantly to the development of LVH as well as the remodeling of the LV and the arterioles in clinical and experimental hypertension. This is the subject of separate chapters in this monograph [31, 32]. The evidence is abundant with reference to the numerous well-designed placebo-controlled multicenter pharmacological clinical trials involving administration of either angiotensinconverting enzyme agents or angiotensin II type 1 receptor blocking agents to patients following myocardial infarction. These trials demonstrated the efficacy of these drugs in reducing not only arterial pressure but cardiovascular morbidity and mortality, cardiac failure, and even a second myocardial infarction [6].

2.3 Structural and Function Response of Organs to Salt-Loading Abundant clinical and experimental evidence has accumulated in recent years to the response of various organs (i.e., heart, vessels, kidney) to excessive salt-loading [28–30, 33). Until relatively recently, much evidence of risk with salt-loading has been ascribed to increase in arterial pressure; however, more recent reports have demonstrated clearly that salt-loading (experimentally as well as clinically) was associated with increased cardiovascular morbidity and mortality as well as structural and functional alterations of heart, aorta, and kidney [34–36). Recent data have shown that co-treatment with angiotensin II receptor antagonists or angiotensinconverting enzyme inhibitors along with the salt-loading will prevent the structural and functional end-organ damage [30, 33, 35]. The reader is referred to those specific references that provide abundant data and references to support the foregoing statements. Moreover, sodium-restricted diets in prehypertension patients will significantly reduce cardiovascular morbidity and mortality as compared with

2

Clinical Import of the Local Renin Angiotensin Aldosterone Systems

11

control group patients whose daily sodium diet was not reduced [36]. These findings provide important data that relates the data derived from chronic salt-loading diets in the earlier epidemiological studies that demonstrated a close relationship between salt-loading and the prevalence of hypertension in large population groups [37–39].

2.4 Secondary Organ Responses of Therapy to Certain Treatment Over the past five or more decades of antihypertensive therapy and the welldocumented evidence of associated reduction in cardiovascular morbidity and mortality, a disturbing conundrum has complicated this therapeutic effort [33]. Thus, each national and international report has attested to the remarkable reduction in morbidity and mortality of such disease endpoints in hypertensive emergencies, stroke, and coronary heart disease [40, 41]. However, over the years, the successive publications of these very same reports have continued to provide an ever-increasing prevalence, morbidity and mortality resulting from end-stage renal disease and of cardiac failure [40, 41]. How can we reconcile these startling data? In response to this shocking and as yet unresolved conundrum, we have suggested that this may be the result of long-term stimulation or ineffective inhibition of the local cardiac and renal renin angiotensin systems. Indeed, there are abundant experimental data which have demonstrated that prolonged diuretic treatment promotes structural and functional renal abnormalities which can be prevented by co-existent treatment with an angiotensin-converting enzyme agent or an angiotensin II type 1 angiotensin receptor blocker [42, 43]. This led to our suggestion resulting from long-term diuretic therapy, there is a secondary increase of renin generation in the kidney that promotes the local synthesis of angiotensin II and its attendant pathophysiological alterations from secondary renal renin generation [34]. These latter studies have demonstrated that in addition to promoting renin release from the juxtaglomerular apparatus of the kidney, a second source of renin production occurs in renal tubular cells [44, 45]. In addition, salt-loading without adequate treatment with either an ACE inhibitor or an angiotensin II type 1 receptor blocker may not protect or prevent stimulation of the local cardiac RAAS. These salt/pharmacological stimuli or inhibition of local renal and cardiac RAAS may be analogous to the multiplicity of Yin/Yang biological systems in the body. Therefore, unless the consequent events stimulating the increased renin synthesis and angiotensin II generation are prevented, the adverse structural and functional cardiac and renal biological events may result.

2.5 Toxemias of Pregnancy Finally, a word or two may be in order concerning yet another clinical expression of pathological stimulation of a local RAAS in the uterus or other female genital organs. Several recent reports have suggested that the utero-placental unit may be the source of stimulated synthesis of components of the RAAS [46, 47]. In part, this

12

E.D. Frohlich

may be related to inadequate perfusion of the utero-placental unit and/or a consequent relative hypoxemia stimulation, endothelial dysfunction of that unit, upregulation of specific genes, generation of autoantibodies, and generation of certain humoral or hormonal factors, inflammatory changes and production of an increased arterial pressure and proteinuria that are characteristic of pre-eclampsia or eclampsia [48, 49]. Each of these possible pathophysiological changes may be responsible for the establishment of toxemia alone or in association with preexisting or otherwise predisposed underlying mechanisms of hypertensive disease. Although these provocative findings are of great significance, what is most important is that this much neglected area for study has now captured much needed interest and work.

References 1. Fyhrquist, F., and Saijonmaa, O. (2008) Renin-angiotensin system revisited. Intern Med 264, 224–236. 2. Harrison, D.G., and Cai, H. (2003) Endothelial control of vasomotion and nitric oxide production. Cardio Clin 21, 289–302. 3. Besler, C., Doerries,C., Giannotti,G., Luscher,T.F., and Landmesse, U. (2008) Pharmacological approaches to improve endothelial repair mechanisms. Expert Rev Cardiovasc Ther 6, 1071–1082. 4. Varagic, J., Trask, A.J., Jessup, J.A., Chappell, M.C., and Ferrario, C.M. (2008) New angiotensins. J Mol Med 86, 663–671. 5. Paul, M., Mehr, A.P., and Kreutz R (2006) Physiology of local renin angiotensin systems. Physiol Rev 86, 747–803. 6. Pfeffer, M.A., and Frohlich, E.D. (2006) Improvements in clinical outcomes with the use of angiotensin converting enzyme inhibitors: cross-fertilization between clinical and basic investigation. Am J Physiol Heart Circ 291, H2021–H2025. 7. Frohlich, E.D., and Re, R.N. (eds) (2006) The Local Cardiac Renin Angiotensin-Aldosterone System. Springer, New York. 8. Kannel, W.B., Dawber, T.R., Kagan, A., Revorskie, N., and Sacks, J. (1961) Factors of risk in the development of coronary heart disease: six year follow up experience: the Framingham Study. Ann Intern Med 55, 33–56. 9. Frohlich, E.D., Tarazi, R.C., and Dustan, H.P. (1971) Clinical-physiological correlations in the development of hypertensive heart disease. Circulation 44, 446–455. 10. Dunn, F.G., Chandraratna, P., de Carvalho, J.G.R., Basta, L.L., and Frohlich. E.D. (1977) Pathophysiologic assessment of hypertensive heart disease with echocardiography. Am J Cardiol 39, 789–795. 11. Frohlich, E.D., and Tarazi, R.C. (1979) Is arterial pressure the sole factor responsible for hypertensive cardiac hypertrophy? Am J Cardiol 44, 959–963. 12. Frohlich, E.D. (1983) Hemodynamics and other determinants in development of left ventricular hypertrophy: conflicting factors in its regression. Fed Proceed 42, 2709–2715. 13. Tarazi, R.C., and Frohlich, E.D. (1987) Is reversal of cardiac hypertrophy a desirable goal of antihypertensive therapy? Circulation 75, 113–117. 14. Frohlich, E.D. (1988) State of the Art. The heart in hypertension: unresolved conceptual challenges. Hypertension 11, 19–24. 15. Sasaki, O., Kardon, M.B., Pegram, B.L., and Frohlich, E.D. (1989) Aortic distensibility and left ventricular pumping ability after methyldopa in Wistar-Kyoto and spontaneously hypertensive rats. J Vascular Med Biol 1, 59–66.

2

Clinical Import of the Local Renin Angiotensin Aldosterone Systems

13

16. Natsume, T., Kardon, M.B., Pegram, B.L., and Frohlich, E.D. (1989) Ventricular performance in spontaneously hypertensive rats with reduced cardiac mass. Cardiovasc Drug Ther 3, 433–439. 17. Frohlich, E.D. (1989) Overview of hemodynamic and non-hemodynamic factors associated with LVH. J Mol Cell Cardio 21(Suppl V), 3–10. 18. Frohlich, E.D., and Sasaki, O. (1990) Dissociation of changes in cardiovascular mass and performance with angiotensin converting enzyme inhibitors in Wistar-Kyoto and spontaneously hypertensive rats. J Am Coll Cardiol 16, 1492–1499. 19. Frohlich, E.D., and Horinaka, S.(1991) Cardiac and aortic effects of angiotensin converting enzyme inhibitors. Hypertension 18, 2–7. 20. Ando, K., Frohlich, E.D., Chien, Y., and Pegram, B.L. (1991) Effects of quinapril on systemic and regional hemodynamics and cardiac mass in spontaneously hypertensive and WistarKyoto rats. J Vascular Med Biol 3, 117–123. 21. Frohlich, E.D., Sasaki, O., Chien, Y., and Arita, M. (1992) Changes in cardiovascular mass, left ventricular pumping ability, and aortic distensibility after calcium antagonist in Wistar-Kyoto and spontaneously hypertensive rats. J Hypertens 10, 1369–1378. 22. Soria, F., Frohlich, E.D., Aristizabal, D., Kaneko, K., Kardon, M.B., Hunter, J., and Pegram, B.L. (1994) Preserved cardiac performance with reduced left ventricular mass in conscious exercising spontaneously hypertensive rats. J Hypertens 12, 585–589. 23. Frohlich, E.D (1994) Okamoto International Award Lecture: The spontaneously hypertensive rat. Jpn Heart J 35, 487–491. 24. Susic, D., Nunez, E., Hosoya, H., and Frohlich, E.D. (1998) Coronary hemodynamics in aging spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats. J Hypertens 16, 231–237 25. Frohlich, E.D. (1999) Risk mechanisms in hypertensive heart disease. Hypertension 34, 782–789. 26. Frohlich, E.D. (2001) Fibrosis and Ischemia: The real risks in hypertensive heart disease. Am J Hypertension 14, 194S–199S. 27. Fortuño, M.A., González, A., Ravassa, S., López, B., and Díez, J. (2003) Clinical implications of apoptosis in hypertensive heart disease. Am J Physiol Heart Circ Physio 284, H1495– H1506. 28. Ahn, J., Varagic, J., Slama, M., Susic, D., and Frohlich, E.D. (2004) Cardiac structural and functional responses to salt loading in SHR. Am J Physiol (Heart Circ Physiol) 287, H767–H772. 29. Varagic, J., Frohlich, E.D., Diez, J., Susic, D., Ahn, J., Gonzalez, A., and Lopez, B. (2006) Myocardial fibrosis, impaired coronary hemodynamics, and biventricular dysfunction in saltloaded SHR. Am J Physiol (Heart Circ Physiol) 290, H1503–H1509. 30. Matavelli, L.C., Zhou, X., Varagic, J., Susic, D., and Frohlich, E.D. (2007) Saltloading produces severe renal hemodynamic dysfunction independent of arterial pressure in spontaneously hypertensive rats. Am J Physiol (Heart Circ Physiol) 292, H814–H819. 31. Pfeffer, M.A. (2009) Inhibiting the renin angiotensin aldosterone system in patients with heart failure and myocardial infarction. In: DeMello, W.C., Frohlich, E.D., (eds.) Renin Angiotensin Aldosterone System and Cardiovascular Disease. Humana Press, Totowa, NJ, Chapter 8 of this book. 32. Diez, J., and Frohlich, E.D. (2009) Left ventricular hypertrophy and treatment with renin angiotensin system inhibition. In: DeMello, W.C., Frohlich, E.D., (eds.) Renin Angiotensin Aldosterone System and Cardiovascular Disease. Humana Press, Totowa, NJ, Chapter 9 of this book. 33. Frohlich, E.D. (2007) The salt conundrum: a hypothesis. Hypertension 50, 161–166. 34. Frohlich, E.D. (2008) The role of salt in hypertension: the complexity seems to become clearer. Nat Clin Pract Cardiovasc Med 5, 2–3.

14

E.D. Frohlich

35. Varagic, J., Frohlich, E.D., Susic, D., Ahn, J., Matavelli, L., Lopez, B., and Diez, J. (2008) AT1 receptor antagonism attenuates target organ effects of salt excess in SHRs without affecting pressure. Am J Physiol Heart Circ 294, H853–H353. 36. Cook, N.R., Cutler, J.A., Obarzanek, E., Buring, J.E., Rexrode, K.M., Kumanyika, S, K., Appel, L.J., and Whelton, P. K. (2007) Long term effects of dietary sodium reduction on cardiovascular disease outcomes: observational follow-up of the trials of hypertension prevention (TOHP), BMJ 334, 885–894. 37. Kurlansky, M. (2003) Salt: A World History. Penguin Books, New York 38. Dahl, L.K., and Love, R.A. (1954) Evidence for a relationship between sodium (chloride) intake and human essential hypertension. Arch Intern Med 94, 525–531. 39. Stamler, J. (1997) The INTERSALT study: background, methods, findings, and implications. Am J Clin Natr 65, 626–642. 40. The Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure (JNC-7) (2003). JAMA, 289, 2560–2572. 41. International Society of Hypertension Writing Group. International Society of Hypertension (ISH): Statement on blood pressure lowering and stroke prevention (2003). J Hypertens 21, 651–663. 42. Ono, Y., Ono, H., and Frohlich, E.D. (1996) Hydrochlorothiazide exacerbates nitric oxideblockade nephrosclerosis with glomerular hypertension in spontaneously hypertensive rats. J Hypertens 14, 823–828. 43. Zhou, X., Matavelli, L.C., Ono, H., and Frohlich, E.D. (2005) Superiority of combination of thiazide with angiotensin-converting enzyme inhibitor or AT1 – receptor blocker over thiazide alone on renoprotection in L-NAME/SHR. Am J Physiol Renal 289, F871–F879. 44. Schunkert, H., Ingelfinger, J.R., Jacob, H., Jackson, B., Bouyounes, B., and Dzau, V.J. (1992) Reciprocal feedback regulation of kidney angiotensinogen and renin RNA expressions by angiotensin II. Am J Physiol E863–E869. 45. Navar, L.G., Prieto-Carrasquero, M.C., and Kobori, H. (2005) Regulation of renin in JGA and tubules in hypertension. In: Frohlich, E.D., Re, R.N., (eds.) The Local Cardiac Renin Angiotensin-Aldosterone System. Springer Science –Business Media, Inc, New York, 22–29. 46. Herse, F., Dechend, R., Harsem, N.K., Wallukat, G., Jurgen, J., Fatimunnisa, Q., Hering, L., Muller, D.N., Lucct, F.C., and Staff, A.C. (2007) Dysregulation of the circulating and tissuebased renin-angiotensin system in preeclampsia. Hypertension 49(2), 604–611. 47. LaMarca, B.D., Gilbert, J., and Granger, J.P. (2008) Recent progress toward the understanding of the pathophysiolsogy of hypertensions during preeclampsia. Hypertension 51, 982–988. 48. Robert, J.M., Pearson, G., Cutler, J., and Lindheimer, M. (2003) Summary of the NHLBI working group on research on hypertension during pregnancy. Hypertension 41, 437–445. 49. Robert, J.M., and Von Versen-Hoeynck, F. (2007) Maternal fetal/placental interactions and abnormal pregnancy outcomes. Hypertension 49, 15–16.

Chapter 3

Renin, Prorenin, and the (Pro)Renin Receptor Genevieve Nguyen and Aurelie Contrepas

Abstract The discovery of a receptor for renin and for its inactive precursor prorenin, and the introduction of renin inhibitors in therapeutic, has renewed the interest for the physiology of the renin angiotensin system (RAS) and has brought prorenin back in the spotlight. The receptor known as renin for (Pro)Renin Receptor binds both renin and prorenin, and binding triggers intracellular signaling involving the MAP kinases ERK1/2 and p38. The MAP kinases activation in turn upregulates the expression of profibrotic genes, potentially leading to fibrosis, growth, and remodeling. Simultaneously, binding of renin to (P)RR increases its angiotensin I-generating activity, whereas binding of prorenin induces the inactive prorenin to become enzymatically active. These biochemical characteristics of (pro)renin binding to (P)RR allow to distinguish two aspects for the new (pro)renin/(P)RR system, an angiotensin-independent function related to the intracellular signaling and its downstream effects and an angiotensin-dependent aspect related to the increased generation of angiotensin I on the cell surface. Ongoing experimental studies should now determine which of the two aspects is the most important in pathological situations.

List of Abbreviations (pro)renin: AOG: Ang I and Ang II: ACE: HRP: (P)RRB:

designate renin and prorenin angiotensinogen angiotensin I and angiotensin II angiotensin-converting enzyme handle region peptide (pro)renin receptor blocker

G. Nguyen (B) Institut de la Santé et de la Recherche Médicale, París, France e-mail: [email protected]

W.C. DeMello, E.D. Frohlich (eds.), Renin Angiotensin System and Cardiovascular Disease, Contemporary Cardiology, DOI 10.1007/978-1-60761-186-8_3,  C Humana Press, a part of Springer Science+Business Media, LLC 2009

15

16

G. Nguyen and A. Contrepas

3.1 Introduction The discovery of a specific receptor for renin and for its precursor, prorenin, has modified our conception of renin being just an enzyme responsible for the cleavage of angiotensinogen and of prorenin being just an “inactive” proenzyme. The receptor named (P)RR binds with similar affinity to renin and prorenin. Binding to the receptor allows these enzymes to display increased enzymatic activity on the cell surface and trigger intracellular signaling that in turn modifies gene expression. This implies that renin may also be considered as a hormone and that a function was finally found for prorenin. Information on the role of the (P)RR in organ damage was obtained only recently, and experimental models suggest that (P)RR may play a role in the development of high blood pressure and of glomerulosclerosis, in cardiac fibrosis, in diabetic nephropathy and retinopathy by “non-proteolytically” activating prorenin. Importantly, blocking prorenin/(P)RR interaction with a putative (P)RR blocker called “handle region peptide” (HRP) was claimed to not only prevent diabetic nephropathy but also reverse the glomerulosclerosis of diabetic nephropathy. If this is true, then it would make (P)RR a major therapeutic goal.

3.2 Renin and Prorenin The term “renin” is used to cover two entities: – renin, the mature enzyme which is catalytically active in solution and – prorenin, the proenzyme form of renin which is virtually inactive in solution. Prorenin is synthesized in many organs, the kidney of course, and also the eye, the brain, the adrenal gland, the submandibular gland, the glands of the reproductive system and the adipose tissue. All these tissues are able to secrete inactive prorenin in the surrounding milieu and in plasma, but the only tissue able to mature and secrete active renin is the kidney. Indeed, prorenin, but not renin, is still detectable in blood after bilateral nephrectomy, although prorenin levels are lower than in normal subjects indicating that, although kidney is the main if not the only source of renin in the body, other tissues are able to release prorenin in the circulation [1, 2].

3.2.1 Renin Renin is an aspartyl protease with a typical structure made of two lobes. The cleft in between the lobes contains the active site characterized by two catalytic aspartic residues. Renin is a highly specific enzyme and has only one known substrate, angiotensinogen (AOG). Renin cleaves AOG to generate angiotensin (Ang) I that is converted into Ang II by the angiotensin-converting enzyme. In addition to its substrate specificity, renin catalytic activity is species-specific and renin can only

3

Renin, Prorenin, and the (Pro)Renin Receptor

17

cleave AOG of the same species. Renin is synthesized by the renin-producing cells of the juxtaglomerular apparatus (JGA) and is stored as active enzyme in secretory granules from which it is released upon acute stimulation of the JGA. Renin has also been called “active” renin in opposition to the enzymatically “inactive” form of renin, prorenin [3].

3.2.2 Prorenin Being the precursor of renin, prorenin was assumed to have no function of its own, and yet it represents 70–90% of total renin in human plasma. The absence of enzymatic activity of prorenin is due to the fact that a 43-amino acid N-terminal prosegment covers the cleft of the active site. Unlike for the proenzymes of trypsin and of cathepsin D, prorenin does not undergo auto-activation in the plasma ant its activation takes place under two circumstances: a proteolytic activation by a proconvertase which identity is still not established and that removes the prosegment, an irreversible process that occurs in physiology in the renin- producing cells of the juxtaglomerular apparatus exclusively; and non-proteolytic activation in a test tube by exposure to low pH (pH < 3.0) or cold (4◦ C) and which can be imagined as a reversible unfolding of the prosegment. In plasma and in physiological conditions, however, approximately 2% of prorenin is in the open, active form and can display enzymatic activity, whereas 98% is in closed and inactive form [3]. In contrast to renin, prorenin is released constitutively and renin and prorenin levels are usually well correlated. However, under some physiopathological circumstances such as pregnancy and diabetes, prorenin levels exceed by far those of renin. In diabetes mellitus complicated by retinopathy and nephropathy, prorenin is increased out of proportion to renin and this increase starts before the occurrence of microalbuminuria, so that prorenin level was suggested to be a marker of microvascular complications in diabetic patients [4, 5]. Pregnant women also have high plasma prorenin levels, likely derived from the ovaries. The reason for the elevated prorenin levels in diabetes and pregnancy is unknown.

3.3 The (Pro)Renin Receptor Interestingly, the renal vasodilator response to captopril in diabetic subjects correlated better with plasma prorenin than plasma renin [6]. Possibly therefore, prorenin rather than renin is responsible for tissue angiotensin generation despite the absence of prorenin–renin conversion that cannot occur elsewhere than in the JGA cells [7]. In support of this concept, transgenic rodents with inducible prorenin expression in the liver display increased cardiac Ang I levels, cardiac hypertrophy, hypertension, and/or vascular damage without evidence for increased circulating renin or angiotensin [8–10]. Even more surprising, increased tissue Ang I formation occurred even when expressing a non-activatable prorenin variant mutated in the

18

G. Nguyen and A. Contrepas

site of cleavage of the prosegment [11]. Therefore, it seems logical to assume that prorenin accumulates in tissues, e.g., via a receptor-dependent mechanism, where it can be activated in a non-proteolytic manner. Several proteins able to bind renin and prorenin have been described, an intracellular renin-binding protein (RnBP) [12] and the mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF2R) [13–15]. The intracellular RnBP was found to be an inhibitor of renin activity and its deletion affected neither blood pressure nor plasma renin [16], and it is now believed that the M6P/IGF2R is a clearance receptor for renin/prorenin [17]. This leaves the (Pro)Renin Receptor [(P)RR] as the most promising candidate for the tissue uptake of circulating renin/prorenin.

3.3.1 Biochemistry of the (P)RR The (pro)renin receptor is a 350-amino acid receptor with a single transmembrane domain, like receptors for growth factors [18]. There is no homology with any known protein based on the nucleotide and the amino acid sequence of (P)RR. Homologies in the tertiary structure have not yet been determined due to the lack of knowledge on the crystal structure of (P)RR. The receptor binds both renin and prorenin, with affinities in the nanomolar range, and the encoding gene, called ATP6AP2 (see below), is located on the X chromosome in locus p11.4. The initial characteristics of the (P)RR were: 1. Renin and prorenin bound to the receptor are not internalized or degraded but remain on the cell surface. 2. Renin bound to the receptor displays increased catalytic activity as compared to renin in solution. 3. Receptor-bound prorenin displays Ang I-generating activity in the absence of cleavage of the prosegment, most likely due a conformational change induced by binding and non-proteolytic activation of prorenin. 4. (Pro)renin binding triggers intracellular signalization involving the mitogenactivated protein (MAP) kinase ERK1/2 and p38. Further studies confirmed ERK1/2 phosphorylation and showed that it was due to MEK phosphorylation and provoked Elk phosphorylation [19–22]. Moreover, ERK 1/2 activation resulted in the upregulation of transforming growth factor ß1 gene expression, the subsequent upregulation of genes coding for profibrotic molecules such as plasminogen-activator inhibitor-1, fibronectin, and collagens, and the induction of mesangial cell proliferation [19, 20, 23]. The ERK1/2 pathway is not the only signaling pathway linked to the (P)RR as the receptor also appears to activate the MAP kinase p38-heat shock protein 27 cascade [24] and the PI3K-p85 pathway [25]. Importantly, the latter results in the nuclear translocation of the promyelocytic zinc finger transcription factor, which downregulates the expression of the (P)RR

3

Renin, Prorenin, and the (Pro)Renin Receptor

19

itself [25, 26]. In other words, high (pro)renin levels will suppress (P)RR expression, thereby preventing excessive receptor activation. Prorenin binding (Fig. 3.1) and its subsequent non-proteolytic activation was confirmed both in primary cells [27] and in cells with transient overexpression of (P)RR [28]. Data in rat aortic vascular smooth muscle cells overexpressing the human (P)RR suggested that prorenin binds with higher affinity to the receptor than renin, so that in vivo prorenin might be the endogenous agonist of the receptor [27]. The fact that both prorenin and renin are capable of binding to the (P)RR implies that the domains involved in the interaction between (P)RR and the (pro)renin molecule are different from the active site and are not restricted to the prosegment of prorenin. Unfortunately, due to the difficulties in generating purified recombinant (P)RR, no structure–function studies are currently available, which would allow the identification of the domains of the (P)RR and (pro)renin involved in binding. In the absence of such structure–function studies or of an X-ray crystallographic structure of the (P)RR, it is difficult to design antagonists for the (P)RR. Nevertheless, Suzuki et al. [29] made the interesting observation that, when bound to prorenin, an antibody against the sequence I11P FLKR15P of the prosegment was able to open the pro-fragment, yielding a “non-proteolytically” activated prorenin in a manner similar to the putative mechanism of (P)RR binding-induced prorenin activation. They named this region of the prosegment the “handle” region. Based on this observation, Ichihara et al. [30] tested a 10 amino acid peptide encompassing the handle region and called HRP for handle region peptide as a blocker of prorenin-(P)RR binding. In diabetic rodents, they reasoned that diabetes would increase prorenin synthesis, thus

Fig. 3.1 Schematic representation of the angiotensin II-dependent and independent consequences of (pro)renin binding to (P)RR and of (P)RR activation. Adapted from [47]

20

G. Nguyen and A. Contrepas

creating optimal conditions to test the efficacy of HRP in vivo. Indeed, HRP could totally prevent or even reverse diabetic nephropathy [30, 31] and blocked ischemiainduced retinal neovascularization and ocular inflammation in endotoxin-induced uveitis [32]. Moreover, it diminished cardiac fibrosis in stroke-prone spontaneously hypertensive rats [31]. Taken together, these data strongly suggest that the prorenin(P)RR axis plays an essential role in end-organ damage in diabetic and inflammatory pathologies. HRP was subsequently renamed a (P)RR “blocker.” However, in vitro and in vivo studies by others did not reproduce the protective effect of HRP on organ damages well as they did not support the inhibition of prorenin binding to its receptor by HRP [22, 27, 33]. Even more surprising, an FITC-labeled HRP also bound to cells devoid of the (P)RR on the plasma membrane [22]. If there is no demonstration that HRP can really block (pro)renin binding to the (P)RR, thus one may wonder why it is so successful if not blocking renin– (P)RR interaction. At this moment, it cannot be ruled out that HRP also exerts other non-(P)RR related effects, particularly in diabetic animals. Clearly, more work is needed to unravel its mechanism of action before HRP can truly be called a (P)RR blocker.

3.3.2 (P)RR in Experimental Models of Cardiovascular and Renal Diseases The high blood pressure occurring in a transgenic rat model targeting human (P)RR expression to vascular smooth muscle cells suggests a pathological role of the (P)RR in raising blood pressure [34]. Ubiquitous over-expression of the human (P)RR resulted in proteinuria and glomerulosclerosis [35] and in cyclooxygenase-2 upregulation [36]. Both targeted and ubiquitous (P)RR expression left the plasma levels of renin and angiotensin unaltered, but did cause a rise in plasma aldosterone. Finally, in a Goldblatt model of hypertension, a parallel increases in (P)RR and renin was suggested to be profibrotic in the clipped kidney [37] and an increase of (P)RR expression was described in diabetic rats [38]. Although the claimed beneficial effects of HRP in diabetic rodents and stroke-prone spontaneously hypertensive rats are suggestive for a role of the (P)RR in fibrosis and glomerulosclerosis, no increased (P)RR expression was described in these models [30, 31, 39]. In addition, it should be noted that glomerulosclerosis did not occur in transgenic ren-2 rats with inducible prorenin expression [10], despite the fact that such rats, following induction, displayed 200-fold higher prorenin levels, with no change in renin. This argues against the concept that prorenin, through a direct interaction with its receptor, induces glomerulosclerosis. Of the two means classically used to establish the role of a receptor in pathology, the antagonist, HRP, is still speculative and the total knock-out of the (P)RR is, surprisingly for a component of the rennin angiotensin system (RAS), not possible [40]. Therefore, the generation of (P)RR conditional knock-out mice is becoming mandatory and such animals will allow to further establish the role of (P)RR in disease.

3

Renin, Prorenin, and the (Pro)Renin Receptor

21

3.3.3 Unexpected Properties and Ontogeny of the (P)RR There is only one gene called ATP6ap2 coding for the full-length protein known as (P)RR. All other truncated forms of (P)RR derive from intracellular processing of the full-length form. The reason why the (P)RR gene is called ATP6ap2 was because a truncated form of the (P)RR, composed of the transmembrane and cytoplasmic domains of (P)RR, had been co-purified with a vacuolar H+ -ATPase (V-ATPase) [41]. This V-ATPase is a complex, 13-subunit protein, essential to maintain an acidic pH in intracellular vesicles and to regulate cellular pH homeostasis [42], but (P)RR is not a subunit of this V-ATPase. The necessity of an intact (P)RR/ATP6ap2 gene in early development is stressed by the observations that in zebra fish, the mutation of (P)RR/ATP6ap2 gene provoked the death of the fish before the end of the embryogenesis [43] and that in rodents (P)RR/ATP6ap2 gene expression is ubiquitous and early in development [44]. Whereas renin expression can be detected in large intrarenal arteries only at 15.5 days of gestation, (P)RR mRNA is already present on day 12 in the ureteric bud and at later stages in vesicles and S-shaped bodies (Fig. 3.2). In newborn mice (P)RR expression is high in epithelial cells of distal, proximal, and collecting tubules and low in glomeruli and arteries [44]. These observations in zebra fish and in the developing kidney suggest that the (P)RR has functions essential for cell survival and proliferation that are unrelated to the RAS.

Fig. 3.2 In situ hybridization with riboprobes

35 S-labelled

mouse (P)RR (left) and mouse renin (right)

Analysis of the sequence of (P)RR-coding cDNA shows that sequence coding for the transmembrane and the intracellular domain putatively associated with the V-ATPase is remarkably conserved between invertebrates and vertebrates, whereas the cDNA sequence coding for the extracellular domain responsible for renin and prorenin binding is conserved in vertebrates only [45]. This leads to the postulate that the (P)RR/ATP6ap2 gene may result from the fusion of two genes, an ancient gene (corresponding with the C-terminus) coding for a protein essential for cell survival and a more recent gene in vertebrates (corresponding with the N-terminus)

22

G. Nguyen and A. Contrepas

which binds renin and prorenin [40]. However, to date, we have no arguments to confirm or to infirm that the (P)RR role in cell survival is related to V-ATPase activity.

3.4 Conclusion The discovery of the (P)RR has confirmed the hypothesis of Tigerstedt and Bergman more than a century ago that renin is a hormone [46]. Now, the (P)RR also endows prorenin with a function that was suspected over 25 years ago by Luetscher and Wilson in diabetic patients (1985). Experimental studies suggest that the (P)RR might be a major target in cardiovascular disease and in diabetes-induced organ damage, and tissue-specific knock-out of (P)RR should soon establish whether the (P)RR plays a role in cardiovascular pathologies and in diabetes and to what degree HRP exerts (P)RR-dependent effects.

References 1. Danser, A.H.J., Derkx, F.H.M., Schalekamp, M.A.D.H., et al. (1998). Determinants of interindividual variation of renin and prorenin concentrations: evidence for a sexual dimorphism of (pro)renin levels in humans. J Hypertens 16, 853–862. 2. Krop, M., and Danser, A.H.J. (2008). Circulating versus tissue renin-angiotensin system: on the origin of (pro)renin. Curr Hyp Rep 10, 112–118. 3. Danser, A.H.J., and Deinum, J. (2005). Renin, prorenin and the putative (pro)renin receptor. Hypertension 46, 1069–1076. 4. Luetscher, J.A., Kraemer, F.B., Wilson, D.M., et al. (1985). Increased plasma inactive renin in diabetes mellitus. A marker of microvascular complications. N Engl J Med 312, 1412–1417. 5. Wilson, D.M., and Luetscher, J.A. (1990) Plasma prorenin activity and complications in children with insulin-dependent diabetes mellitus. N Engl J Med 323, 1101–1106. 6. Stankovic, A.R., Fisher, N.D.L., and Hollenberg, N.K. (2006). Prorenin and angiotensindependent renal vasoconstriction in type 1 and type 2 diabetes. J Am Soc Nephrol 17, 3293–3299. 7. Lenz, T., Sealey, J.E., Maack, T., et al. (1991). Half-life, hemodynamic, renal, and hormonal effects of prorenin in cynomolgus monkeys. Am J Physiol 260, R804–R810. 8. Véniant, M., Ménard, J., Bruneval, P., et al. (1996). Vascular damage without hypertension in transgenic rats expressing prorenin exclusively in the liver. J Clin Invest 98, 1966–1970. 9. Prescott, G., Silversides, D.W., and Reudelhuber, T.L. (2002). Tissue activity of circulating prorenin. Am J Hypertens 15, 280–285. 10. Peters, B., Grisk, O., Becher, B., et al. (2008). Dose-dependent titration of prorenin and blood pressure in Cyp1a1ren-2 transgenic rats: absence of prorenin-induced glomerulosclerosis. J Hypertens 26, 102–109. 11. Methot, D., Silversides, D.W., and Reudelhuber, T.L. (1999). In vivo enzymatic assay reveals catalytic activity of the human renin precursor in tissues. Circ Res 84, 1067–1072. 12. Maru, I., Ohta, Y., Murata, K., et al. (1996). Molecular cloning and identification of N-acylD-glucosamine 2-epimerase from porcine kidney as a renin-binding protein. J Biol Chem 271, 16294–16299. 13. van Kesteren, C.A.M., Danser, A.H.J., Derkx, F.H.M., et al. (1997). Mannose 6-phosphate receptor-mediated internalization and activation of prorenin by cardiac cells. Hypertension 30, 1389–1396.

3

Renin, Prorenin, and the (Pro)Renin Receptor

23

14. Saris, J.J., Derkx, F.H.M., de Bruin, R.J.A., et al. (2001a). High-affinity prorenin binding to cardiac man-6-P/IGF-II receptors precedes proteolytic activation to renin. Am J Physiol 280, H1706–H1715. 15. van den Eijnden, M.M.E.D., Saris, J.J., et al. (2001). Prorenin accumulation and activation in human endothelial cells. Importance of mannose 6-phosphate receptors. Arterioscler Thromb Vasc Biol 21, 911–916. 16. Schmitz, C., Gotthardt, M., Hinderlich, S., et al. (2000). Normal blood pressure and plasma renin activity in mice lacking the renin-binding protein, a cellular renin inhibitor. J Biol Chem 275, 15357–15362. 17. Saris, J.J., van den Eijnden, M.M.E.D., Lamers, J.M.J., et al. (2002). Prorenin-induced myocyte proliferation: no role for intracellular angiotensin II. Hypertension 39, 573–577. 18. Nguyen, G., Delarue, F., et al. (2002). Pivotal role of the renin/prorenin receptor in angiotensin II production and cellular responses to renin. J Clin Invest 109, 1417–1427. 19. Huang, Y., Wongamorntham, S., Kasting, J., et al. (2006). Renin increases mesangial cell transforming growth factor-beta1 and matrix proteins through receptor-mediated, angiotensin II-independent mechanisms. Kidney Int 69, 105–113. 20. Huang, Y., Noble, N.A., Zhang, J., Xu, C., et al. (2007b). Renin-stimulated TGF-beta1 expression is regulated by a mitogen-activated protein kinase in mesangial cells. Kidney Int 72, 45–52. 21. Sakoda, M., Ichihara, A., Kaneshiro, Y., et al. (2007). (Pro)renin receptor-mediated activation of mitogen-activated protein kinases in human vascular smooth muscle cells. Hypertens Res 30, 1139–1146. 22. Feldt, S., Batenburg, W.W., Mazak, I., et al. (2008a). Prorenin and renin-induced extracellular signal-regulated kinase 1/2 activation in monocytes is not blocked by aliskiren or the handleregion peptide. Hypertension 51, 682–688. 23. Huang, Y., Border, W.A., and Noble, N.A. (2007a). Functional renin receptors in renal mesangial cells. Curr Hypertens Rep 9, 133–139. 24. Saris, J.J., ’t Hoen, P.A.C., Garrelds, I.M., et al. (2006). Prorenin induces intracellular signalling in cardiomyocytes independently of angiotensin II. Hypertension 48, 564–571. 25. Schefe, J.H., Menk, M., Reinemund, J., et al. (2006). A novel signal transduction cascade involving direct physical interaction of the renin/prorenin receptor with the transcription factor promyelocytic zinc finger protein. Circ Res 99, 1355–1366. 26. Schefe, J.H., Neumann, C., Goebel, M., et al. (2008) Prorenin engages the (pro)renin receptor like renin and both ligand activities are unopposed by aliskiren. J Hypertens 26, 1787–1794. 27. Batenburg, W.W., Krop, M., and Garrelds, I.M., et al. (2007). Prorenin is the endogenous agonist of the (pro)renin receptor. Binding kinetics of renin and prorenin in rat vascular smooth muscle cells overexpressing the human (pro)renin receptor. J Hypertens 25, 2441–2453. 28. Nabi, A.H., Kageshina, A., Uddin, M.N., Nakagawa, T. ,et al. (2006) Binding properties of rat prorein and renin to recombinant rat renin prorein receptor prepared by a baculovir expression system. Int. I Mol Med 18, 483–488 29. Suzuki, F., Hayakawa, M., Nakagawa, T., et al. (2003). Human prorenin has “gate and handle” regions for its non-proteolytic activation. J Biol Chem 278, 22217–22222. 30. Ichihara, A., Hayashi, M., Kaneshiro, Y., et al. (2004). Inhibition of diabetic nephropathy by a decoy peptide corresponding to the “handle” region for nonproteolytic activation of prorenin. J Clin Invest 114, 1128–1135. 31. Ichihara, A., Kaneshiro, Y., Takemitsu, T., et al. (2006a). Nonproteolytic activation of prorenin contributes to development of cardiac fibrosis in genetic hypertension. Hypertension 47, 894–900. 32. Satofuka S, Ichihara A, Nagai N, et al. (2006). Suppression of ocular inflammation in endotoxin-induced uveitis by inhibiting nonproteolytic activation of prorenin. Invest Ophthalmol Vis Sci 47, 2686–2692.

24

G. Nguyen and A. Contrepas

33. Müller, D.N., Klanke, B., Feldt, S., et al. (2008). (Pro)renin receptor peptide inhibitor “handleregion” peptide does not affect hypertensive nephrosclerosis in Goldblatt rats. Hypertension 51, 676–681. 34. Burcklé, C.A., Danser, A.H.J., Müller, D.N., et al. (2006). Elevated blood pressure and heart rate in human renin receptor transgenic rats. Hypertension 47, 552–556. 35. Kaneshiro, Y., Ichihara, A., Sakoda, M., et al. (2007). Slowly progressive, angiotensin II-independent glomerulosclerosis in human (pro)renin receptor-transgenic rats. J Am Soc Nephrol 18, 1789–1795. 36. Kaneshiro, Y., Ichihara, A., Takemitsu, T., et al. (2006). Increased expression of cyclooxygenase-2 in the renal cortex of human prorenin receptor gene-transgenic rats. Kidney Int 70, 641–646. 37. Krebs, C., Hamming, I., and Sadaghiani, S., et al. (2007). Antihypertensive therapy upregulates renin and (pro)renin receptor in the clipped kidney of Goldblatt hypertensive rats. Kidney Int 72, 725–730 38. Siragy, H.M., and Huang, J. (2008) Renal (pro)renin receptor upregulation in diabetic rats through enhanced angiotensin AT1 receptor and NADPH oxidase activity. Exp Physiol 93(5), 709–714 39. Ichihara, A., Suzuki, F., and Nakagawa, T., et al. (2006b). Prorenin receptor blockade inhibits development of glomerulosclerosis in diabetic angiotensin II type 1a receptor-deficient mice. J Am Soc Nephrol 17, 1950–1961. 40. Burcklé, C., and Bader, M. (2006). Prorenin and its ancient receptor. Hypertension 48, 549–551. 41. Ludwig, J., Kerscher, S., Brandt, U., et al. (1998). Identification and characterization of a novel 9.2-kDa membrane sector-associated protein of vacuolar proton-ATPase from chromaffin granules. J Biol Chem 273, 10939–10947. 42. Nishi, T., and Forgac, M. (2002). The vacuolar (H+)-ATPases–nature’s most versatile proton pumps. Nat Rev Mol Cell Biol 3, 94–103. 43. Amsterdam, A., Nissen, R.M., Sun, Z., et al. (2004). Identification of 315 genes essential for early zebrafish development. Proc Natl Acad Sci USA 101, 12792–12797. 44. Contrepas, A., Praizovic, N., Duong Van Huyen, J.P., et al. (2007). Expression of (pro)renin receptor in mouse embryonic and newborn kidney and proliferative effect of soluble (P)RR on mesangial cells. Hypertension 50, e145 (Abstract). 45. L’Huillier, N., Sharp, M.G.F., Dunbar, D.R., et al. (2006). On the relationship between the renin receptor and the vacuolar proton ATPase membrane sector associated protein (M8-9). In: E.D. Frolich and R.N. Re. (eds.) The Local Cardiac Renin Angiotensin-Aldosterone System. Chapter 3. Springer, 233 spring street, new york NY10013, USA, pp. 17–34. 46. Tigerstedt, R., and Bergman, P.G. (1898). Niere und Kreislauf. Scand Arch Physiol 8, 223–271. 47. Nguyen, G., and Danser, A.H. (2008) Prorenin and (pro)renin receptor: a review of available data from in vitro studies and experimental models in rodents. Exp Physiol 93, 557–563.

Chapter 4

Local Renin Angiotensin Systems in the Cardiovascular System Richard N. Re

Abstract The renin angiotensin system (RAS) is an established regulator of intravascular volume and arterial pressure. It is now clear that complete and partial RASs exist in multiple tissues, including the cardiovascular system, with the result that local regulation of angiotensin can occur. In addition, newly identified factors such as ACE 2 and the (pro)renin receptor expand the potential physiological actions of these tissue RASs. Here evidence for the existence and functional relevance of local RASs in cardiovascular tissues is reviewed.

4.1 Introduction There is abundant evidence to indicate that components of the renin angiotensin system (RAS) are found in, and act in, cardiovascular tissues. In some cases evidence strongly points to local synthesis of these moieties, while in others uptake from the circulation predominates [1–5]. However, in either case, there exists the opportunity for local regulation of angiotensin production in various tissues. Moreover, tissue-level regulation also occurs via the actions of newly discovered angiotensinconverting enzyme homologue (ACE-2), which cleaves angiotensin II to produce angiotensin [1–7], a peptide with actions that in many instances counteract those of angiotensin II itself [6, 7]. Receptor interactions and modulation can affect the net action of RAS peptides in tissues. For example, heterodimerization of the AT1 angiotensin II receptor with the bradykinin receptor leads to enhanced activity of the AT-1 receptor, and this may play a role in the angiotensin II sensitivity seen in patients with pre-eclampsia [8, 9]. Also the AT-1 receptor can transactivate the epidermal growth factor (EGF) receptor at the cell surface and thereby enhance cell growth [10]. A reciprocal interaction between the AT-1 receptor and the lectin-like oxidized low-density lipoprotein (LDL) receptor results in upregulated R.N. Re (B) Ochsner Clinic Foundation, New Orleans, LA e-mail: [email protected]

W.C. DeMello, E.D. Frohlich (eds.), Renin Angiotensin System and Cardiovascular Disease, Contemporary Cardiology, DOI 10.1007/978-1-60761-186-8_4,  C Humana Press, a part of Springer Science+Business Media, LLC 2009

25

26

R.N. Re

angiotensin signalling in the environment of high-circulating oxidized LDL [11]. In addition, two receptors for (pro)renin have recently been described: one an apparent clearance receptor and the other having the capacity to enhance angiotensinogen cleavage at the cell membrane, leading to enhanced generation of angiotensin II in the near vicinity of the AT-1 receptor [12–14]. Moreover, the binding of prorenin to this receptor results in the activation of prorenin such that angiotensinogen can be cleaved and angiotensin II subsequently produced at the target cells. This latter receptor is widely expressed and in particular is found on coronary artery smooth muscle cells and on mesangial cells [12–14]. The physiological relevance of these findings is suggested by the recognition that elevated circulating prorenin levels are found in diabetics, and although this circulating prorenin was heretofore thought to be inert, circulating prorenin concentrations correlate with the extent of microvascular complications in these patients [15]. The marked expression of prorenin in the diabetic eye also suggests a role for prorenin activation at its receptor in human disease [16]. Thus, the actions of local RAS systems in cardiovascular tissues are complex and in some cases remain controversial. Early on, there was considerable debate over whether or not renin was synthesized in cardiovascular tissues [17]. Although the great majority of renin in these tissues is taken up from the circulation, there are persistent reports that renin can be synthesized in cardiovascular tissues of experimental animals and in other tissues [2, 18]. But irrespective of the degree of local synthesis in the cardiovascular tissues of man or experimental models, it must be recognized that the availability of angiotensinogen and ACE in tissues can nonetheless lead to dramatic differences in tissue angiotensin II production in different tissues – that is, tissue angiotensin II production can be locally regulated [3]. Differential uptake of these components could produce this kind of local regulation. In addition, there is evidence for the synthesis of ACE and angiotensinogen in various tissues (for example, the left ventricle in the case of angiotensinogen and the vessel wall in the case of ACE) under specific physiological conditions. These findings are not as controversial as is the issue of tissue renin synthesis; local synthesis of ACE and angiotensinogen therefore provides yet another means of local tissue RAS regulation in the cardiovascular system [3, 18, 19]. Moreover, there is good evidence for the synthesis of chymase, a second enzyme capable of converting angiotensin I to angiotensin II in heart and other tissues depending on species [20]. It must also be noted that renin gene expression resulting in prorenin synthesis occurs in multiple sites [1–5, 21–27]. Thus, even in the absence of conversion to active renin, tissuederived prorenin could generate angiotensin II independent of circulating renal renin after prorenin binding to its receptor in tissue or it could function by direct receptor stimulation [26, 28]. In order to approach this complex subject, the evidence for local cardiovascular RAS regulation in man will first be discussed. Thereafter, the more extensive literature on local cardiovascular systems in selected experimental models will be reviewed both to point the way towards new lines of experimentation and to suggest novel physiologic roles of locals RASs in human physiology.

4

Local Renin Angiotensin Systems in the Cardiovascular System

27

4.2 Studies in Man The most direct studies of the cardiovascular RAS in man have been conducted by Sernieri and colleagues. Initially, they studied normal volunteers on both lowand high-salt diets and determined the angiotensin II gradient across the heart [29]. Depending on conditions, the hearts of these volunteers either extracted or released angiotensin while transiting the heart. Surprisingly, cardiac synthesis of angiotensin I and angiotensin II was lowest when the patients were on a low-sodium diet (that is, when circulating renin and angiotensin II were high) and greatest on the highsodium diet (when circulating renin and angiotensin were low). Thus, this study provided clear evidence of locally regulated angiotensin II production. Later, this group took advantage of the fact that some patients suffering from severe heart failure undergo cardiac transplantation, meaning that both diseased and normal myocardia can easily be biopsied for study [30]. Hearts removed from patients suffering from cardiomyopathy reveal increased tissue expression of angiotensinogen, ACE and chymase by PCR and in situ hybridization as compared to normal donor hearts. No renin signal was detected in the tissues, however, and presumably the local upregulation of angiotensinogen and ACE allowed renin to be taken up from the circulation to drive local synthesis of angiotensin II. Also, the gradients of angiotensin I and II across the heart were elevated in patients with congestive heart failure and were correlated with wall stress to a greater degree than to circulating renin activity. Once again, this study points to a role for locally generated angiotensin in the heart and, taken with studies in diabetic patients, suggests that increased tissue concentrations of angiotensin II are associated with apoptosis, a role in cardiac pathobiology [2]. An area of some controversy is the possibility that cardiac tissue can synthesize aldosterone from circulating steroids or even directly from cholesterol. Catheterization studies revealed a gradient of aldosterone, suggesting cardiac production in patients with congestive heart failure or hypertension [31–33]. These studies, however, do not necessarily imply aldosterone synthesis in the heart but could alternatively reflect differential uptake and release. This, coupled with ongoing controversy regarding the possibility that cardiac tissue can synthesize aldosterone, has led to uncertainty regarding the physiologic significance of these results. But the issue of local synthesis aside – and therefore the related issue of whether the local cardiac RAS regulates aldosterone levels so as to form a local renin angiotensin aldosterone system – the variability of aldosterone release from the heart and its association with disease suggests that local regulation of cardiac aldosterone concentration by one mechanism or another does in fact occur. Additional support for local regulation of angiotensin in cardiovascular tissues comes from studies of diabetic patients. In this group plasma renin activity is low; it had been conjectured that the diabetic kidney was not capable of activating prorenin adequately. However, when renovascular resistance was determined in these patients before and after ACE inhibition, AT-1 blockade, or renin inhibition, it was found that renovascular resistance responded briskly to all three agents [34–36]. Also, treatment with ACE inhibitors resulted in marked increases in circulating plasma rennin activity [34]. Collectively, these results suggest the likelihood that renal tissue

28

R.N. Re

production of angiotensin II results in increased renovascular resistance and renin suppression. The fact that all three agents reverse the effect on resistance indicates that any tissue source of angiotensin II is renin driven. In fact, the response to the renin inhibitor aliskiren is greater than that to the other agents, raising the possibility that prorenin activation at the (pro)renin receptor – a site in close proximity to the AT-1 receptor – could account for this differential sensitivity to aliskiren, which could block the action of prorenin directly and therefore effectively blunt the RAS cascade [36]. These findings must be seen in the context of animal studies demonstrating synthesis of renin in the renal tubules with subsequent generation of angiotensin II from locally produced and filtered angiotensinogen. It is noteworthy that angiotensin II infusion actually upregulates the local production of renin and angiotensinogen, indicating a local renal tubular RAS [37, 38]. To the extent that angiotensin II so produced can migrate to the juxtaglomerular cells to suppress renin release and can traffic to the renal vasculature to cause vasoconstriction, this system could explain the findings described above in diabetic subjects.

4.3 Studies in Animal Models Compared to the relative paucity of direct data related to the cardiovascular RAS in man, there is a large body of work studying this system in animals. Interpretation of this work is made complex by virtue of known and unknown species differences in the RAS. For example, the mouse has a second renin gene (REN 2) expressed primarily in salivary glands and producing a nonglycosylated renin moiety [39]. The rat expresses two related but distinct AT-1 receptors [40]. This divergence in the RAS among species carries over to the local systems and their functioning; this demands caution in extrapolating animal studies to man. An additional caveat derives from the fact that many cell culture studies of the local RAS involve neonatal cells, which are more easily cultured than adult cardiomyocytes. Here too, caution must be exercised in extrapolating results to adult animals. That said, there is good evidence for the regulated synthesis of various RAS components in the cardiovascular and other tissues of experimental animals. Here local RAS activity in the heart, adrenal gland and kidney will be discussed.

4.3.1 Cardiac RAS More than 25 years ago, renin gene expression was first reported in the rodent heart, and this observation has been repeated in various species over the ensuing decades [2, 18, 41]. To be sure, expression is low and in some systems is only seen in pathological states. For example, rapid cardiac pacing leads to the upregulation of renin gene expression in the dog heart [42]. In rats myocardial infarction is associated with the upregulation of renin expression in monocytes and myofibroblasts in the infarction zone [18]. At the same time, cardiac tissue angiotensin I and II levels fall to

4

Local Renin Angiotensin Systems in the Cardiovascular System

29

very low levels following nephrectomy, indicating the primacy of circulating renin in the generation of angiotensin in the cardiac interstitium. This has led some to the dogmatic view that all cardiac renin is derived from the circulation [17]. This view assumes that the entire physiological relevance of tissue renin lies in the generation of interstitial angiotensin. This need not be the case. Recently, a second renin transcript, encoding a renin lacking the signal sequence for secretion, has been described by three groups. In the rodent adrenal gland both genes are expressed, while in the juxtaglomerular cells only the secreted form is found. However, in the heart only the nonsecreted form is expressed, and this expression is markedly upregulated by myocardial infarction [21–23]. This intracellular renin may be pathogenic (see Intracellular RASs). Irrespective of the role of locally produced renin in the heart, there is clear evidence for renin uptake from the circulation along with local synthesis of angiotensinogen, leading to locally regulated angiotensin I production followed by its conversion to angiotensin II by ACE or chymase. The upregulation of angiotensin production in the ventricles of transgenic animals has been shown to lead to left ventricular hypertrophy, and so this local system is likely relevant to myocardial stress responses and pathology [43]. The synthesis of aldosterone in the hearts of experimental animals has been reported, but this remains controversial.

4.3.2 Adrenal RAS There is a consistent body of evidence to indicate the synthesis of renin by the rodent adrenal gland. Indeed, following nephrectomy adrenal renin synthesis increases in these animals to the extent that circulating renin levels remain detectable as the result of spillover into the circulation [44]. Moreover, this adrenal renin appears to be biologically important in so far as it maintains adrenal aldosterone synthesis in the absence of renal renin [22, 24, 25, 45, 46]. It is also of note that following nephrectomy renin accumulates in crystalline structures in adrenal mitochondria, the sites of aldosterone synthesis. Thus, it can be conjectured that a local intracellular RAS exists in these cells. Indeed, several groups have reported the existence of a renin variant termed renin exon1A, which lacks the signal sequence for secretion and is predicted to encode a nonsecreted but active renin (as opposed to prorenin), which could act within the cell without the necessity of proteolytic activation [21– 23]. Studies indicate that the renin that accumulates in the mitochondria of anephric animals is this nonsecreted form of renin and is active in aldosterone regulation [22, 24, 25].

4.3.3 Renal RAS As noted earlier, there is good evidence for an autonomous tubular renin angiotensin system that is upregulated in high angiotensin II states, including renal artery

30

R.N. Re

stenosis. This local system appears to play a role in renal autoregulation and may spill over to influence juxtaglomerular renin secretion and renovascular resistance [37, 38]. At the same time there is evidence to suggest that direct (pro)renin effects at mesangial and possibly other kidney cells are physiologically important. It has been reported that a peptide designed to mimic the handle region of prorenin, and therefore to block the binding of prorenin to the prorenin receptor, can blunt the glomerular pathology in a rodent model of diabetes. It did this in an angiotensin-independent manner as evidenced by a beneficial effect in animals lacking the AT-1 receptor [47]. These results suggest a novel approach to the prevention of glomerulosclerosis. However, there exists controversy about these findings because the observations have not yet been successfully repeated. Also, it is unclear why a peptide directed against prorenin binding would be so effective given the fact it is expected to have no effect on renin binding, and signalling, at the receptor. Therefore, while these findings are exciting, much work remains before the role of the prorenin receptor in glomerulosclerosis will be understood. An additional point, however, is that recent in vitro studies indicate that while the clinically available renin inhibitor aliskiren will likely block angiotensin I formation by receptor-bound renin or prorenin, it does not appear to be capable of blocking direct signalling at the receptor by these factors [48].

4.3.4 Intracellular RASs In 1978, it was reported that tritiated angiotensin II injected into the circulation of a rat localized to myocardial cell nuclei and mitochondria [49]. Subsequent studies revealed that hepatic cell nuclei possessed angiotensin II receptors, some associated with the nuclear membrane and some with euchromatin and nucleosomal protein/DNA particles; binding of angiotensin II to these receptors was associated augmented RNA synthesis. Subsequently, these binding sites were found to be AT1-like in character, and binding of angiotensin II to hepatic nuclear sites was associated with the upregulation of renin and angiotensinogen transcription [50, 51]. Over the ensuing years, considerable evidence accumulated to support the existence of an intracellular site of angiotensin action: (i) Electron-microscopy immunohistology revealed angiotensin II associated with nuclear euchromatin, consistent with the previously reported chromatin receptors, in unmanipulated animals [52]. (ii) The introduction of angiotensin I or II into cardiac myocytes produced definite changes in calcium currents; the angiotensin II effects were blocked by an angiotensin II receptor blocker, the angiotensinogen effects by an ACE inhibitor. These results also suggest the presence of active renin in the cells [53–56]. (iii) Cells engineered to synthesize intracellular angiotensin II in the absence of secreted angiotensin (either by introducing an angiotensinogen construct lacking the signal sequence for secretion or by introducing a construct encoding octapeptide angiotensin II as an EGF fusion protein) showed marked proliferation in the absence of extracellular angiotensin II. This effect was not inhibited by the angiotensin II receptor blocker candesartan but was blocked by

4

Local Renin Angiotensin Systems in the Cardiovascular System

31

renin antisense, again indicating the existence of an intracellular RAS [57–59]. (iv) A second renin transcript encoding a nonsecreted but active renin was identified by several groups. This renin is found in the adrenal gland and supports aldosterone synthesis after nephrectomy. It is also upregulated in the rodent left ventricle following myocardial infarction [21–25]. (v) It was shown that nonglycosylated renin, which does in fact circulate to some extent in man, can be internalized by cardiac myocytes via a heretofore uncharacterized (pro)renin receptor and generate intracellular angiotensin II, thereby producing cardiac pathology [28]. (vi) Angiotensinogen was shown to be synthesized in some circumstances by glial cells and to reside in the nucleus. This retention of angiotensinogen by the cells appeared to result from altered posttranslational phosphorylation [60]. (vii) Immunohistochemical studies revealed renin, angiotensin [1–7] and N-terminal ACE immunoreactivity in mesangial cells [61]. (viii) The transfection of cardiac myocytes with a construct encoding a nonsecreted angiotensin II construct led to marked hypertrophy within 96 h; the same construct injected into mice produced overt myocardial hypertrophy in the same time period [62, 63]. (ix) Elevated glucose concentrations were demonstrated to upregulate renin and angiotensinogen synthesis in cultured neonatal cardiac myocytes, an effect blocked by the renin inhibitor aliskiren [62, 63]. Collectively, these and other results strongly point to the existence of intracellular RASs in the cardiovascular system. As these results were being developed, a theory of intracellular peptide action (i.e. intracrine action) was developed to encompass intracrine factors in the RAS as well as the large and growing number of other intracrine peptides/proteins. This, in turn, led to the development of an intracrine physiology and pharmacology, which remain under active investigation [50, 63–65].

4.4 Conclusion Considerable evidence has accumulated over recent decades to indicate that locally regulated angiotensin II synthesis occurs in many tissues and that this synthesis has potentially important physiological and therapeutic implications. In addition, physiologically relevant actions of tissue renin, prorenin, angiotensin [1–7] and other RAS components have been described, as has the operation of intracellular systems. Collectively, these findings demonstrate that the renin angiotensin system is more than a circulating enzymatic cascade regulating blood pressure and volume: It is an important regulator of tissue biology and structure in the cardiovascular system and elsewhere.

References 1. Re, R. N. (1987) The renin angiotensin systems. Med Clin North Am 71, 877–895. 2. Re, R. N. (2004) Tissue renin angiotensin systems. Med Clin North Am 88, 19–38. 3. Dzau, V. J., and Re, R. N. (1994) Tissue angiotensin system in cardiovascular medicine. A paradigm shift? Circulation 89, 493–498.

32

R.N. Re

4. Re, R. N. (2001) The clinical implication of tissue renin angiotensin systems. Curr Opin Cardiol 16, 317–327. 5. Re, R. N. (1989) The cellular biology of angiotensin: paracrine, autocrine and intracrine actions in cardiovascular tissues. J Mol Cell Cardiol 5, 63–69. 6. Brosnihan, K. B., Li, P., Tallant, E. A., and Ferrario, C. M. (1998) Angiotensin-(1–7): a novel vasodilator of the coronary circulation. Biol Res 31, 227–234. 7. Crackower, M. A., Sarao, R., Oudit, G. Y., et al. (2002) Angiotensin-converting enzyme 2 is an essential regulator of heart function. Nature 417, 822–828. 8. AbdAlla, S., Lother, H., el Massiery, A., and Quitterer, U. (2001) Increased AT(1) receptor heterodimers in preeclampsia mediate enhanced angiotensin II responsiveness. Nat Med 7, 1003–1009. 9. AbdAlla, S., Lother, H., Abdel-tawab, A. M., and Quitterer, U. (2001) The angiotensin II AT2 receptor is an AT1 receptor antagonist. J Biol Chem 276, 39721–39726. 10. Tang, H., Nishishita, T., Fitzgerald, T., Landon, E. J., and Inagami, T. (2000) Inhibition of AT1 receptor internalization by concanavalin A blocks angiotensin II-induced ERK activation in vascular smooth muscle cells. Involvement of epidermal growth factor receptor proteolysis but not AT1 receptor internalization. J Biol Chem 275, 13420–13426. 11. Singh, B. M., and Mehta, J. L. (2003) Interactions between the renin angiotensin system and dyslipidemia: relevance in the therapy of hypertension and coronary heart disease. Arch Intern Med 163, 1296–1304. 12. Saris, J. J., Derkx, F. H., Lamers, J. M., Saxena, P. R., Schalekamp, M. A., and Danser, A. H. (2001) Cardiomyocytes bind and activate native human prorenin: role of soluble mannose 6-phosphate receptors. Hypertension 37, 710–715. 13. Nguyen, G., Delarue, F., Burcklé, C., Bouzhir, L., Giller, T., and Sraer, J. D. (2002) Pivotal role of the renin/prorenin receptor in angiotensin II production and cellular responses to renin. J Clin Invest 109, 1417–1427. 14. Nguyen, G., Delarue, F., Berrou, J., Rondeau, E., and Sraer, J. D. (1996) Specific receptor binding of renin on human mesangial cells in culture increases plasminogen activator inhibitor-1 antigen. Kidney Int 50, 1897–1903. 15. Wilson, D. M., and Luetscher, J. A. (1990) Plasma prorenin activity and complications in children with insulin-dependent diabetes mellitus. N Engl J Med 323, 1101–1106. 16. Kida, T., Ikeda, T., Nishimura, M., et al. (2003) Renin-angiotensin system in proliferative diabetic retinopathy and its gene expression in cultured human müller cells. Jpn J Ophthalmol 47, 36–41. 17. von Lutterotti, N., Catanzaro, D. F., Sealey, J. E., and Laragh, J. H. (1994) Renin is not synthesized by cardiac and extrarenal vascular tissues. A review of experimental evidence. Circulation 89, 458–470. 18. Dostal, D. E., and Baker, K. M. (1999) The cardiac renin-angiotensin system: conceptual, or a regulator of cardiac function? Circ Res 85, 643–650. 19. Diet, F., Pratt, R. E., Berry, G. J., Momose, N., Gibbons, G. H., and Dzau, V. J. (1996) Increased accumulation of tissue ACE in human atherosclerotic coronary artery disease. Circulation 94, 2756–2767. 20. Reid, A. C., Silver, R. B., and Levi, R. (2007) Renin: at the heart of the mast cell. Immunol Rev 217, 123–140. 21. Lee-Kirsch, M. A., Gaudet, F., Cardoso, M. C., and Lindpaintner, K. (1999) Distinct renin isoforms generated by tissue-specific transcription initiation and alternative splicing. Circ Res 84, 240–246. 22. Clausmeyer, S., Reinecke, A., Farrenkopf, R., Unger, T., and Peters, J. (2000) Tissue-specific expression of a rat renin transcript lacking the coding sequence for the prefragment and its stimulation by myocardial infarction. Endocrinology 141, 2963–2970. 23. Sinn, P. L., and Sigmund, C. D. (2000) Identification of three human renin mRNA isoforms from alternative tissue-specific transcriptional initiation. Physiol Genomics 3, 25–31.

4

Local Renin Angiotensin Systems in the Cardiovascular System

33

24. Clausmeyer, S., Stürzebecher, R., and Peters, J. (1999) An alternative transcript of the rat renin gene can result in a truncated prorenin that is transported into adrenal mitochondria. Circ Res 84, 337–344. 25. Peters, J., Obermüller, N., Woyth, A., et al. (1999) Losartan and angiotensin II inhibit aldosterone production in anephric rats via different actions on the intraadrenal renin-angiotensin system. Endocrinology 140, 675–682. 26. Morgan, T., Craven, C., and Ward, K. (1998) Human spiral artery renin-angiotensin system. Hypertension 32, 683–687. 27. Li, C., Ansari, R., Yu, Z., and Shah, D. (2000) Definitive molecular evidence of reninangiotensin system in human uterine decidual cells. Hypertension 36, 159–164. 28. Peters, J., Farrenkopf, R., Clausmeyer, S., et al. (2002) Functional significance of prorenin internalization in the rat heart. Circ Res 90, 1135–1141. 29. Neri Serneri, G. G., Boddi, M., Coppo, M., et al. (1996) Evidence for the existence of a functional cardiac renin-angiotensin system in humans. Circulation 94, 1886–1893. 30. Serneri, G. G., Boddi, M., Cecioni, I., et al. (2001) Cardiac angiotensin II formation in the clinical course of heart failure and its relationship with left ventricular function. Circ Res 88, 961–968. 31. Delcayre, C., Silvestre, J. S., Garnier, A., et al. (2000) Cardiac aldosterone production and ventricular remodeling. Kidney Int 57, 1346–1351. 32. Mizuno, Y., Yoshimura, M., Yasue, H., et al. (2001) Aldosterone production is activated in failing ventricle in humans. Circulation 103, 72–77. 33. Yamamoto, N., Yasue, H., Mizuno, Y., et al. (2002) Aldosterone is produced from ventricles in patients with essential hypertension. Hypertension 39, 958–962. 34. Price, D. A., Porter, L. E., Gordon, M., et al. (1999) The paradox of the low-renin state in diabetic nephropathy. J Am Soc Nephrol 10, 2382–2391. 35. Lansang, M. C., Osei, S. Y., Price, D. A., Fisher, N. D., and Hollenberg, N. K. (2000) Renal hemodynamic and hormonal responses to the angiotensin II antagonist candesartan. Hypertension 36, 834–838. 36. Fisher, N. D., Jan Danser, A. H., Nussberger, J., Dole, W. P., and Hollenberg, N. K. (2008) Renal and hormonal responses to direct renin inhibition with aliskiren in healthy humans. Circulation 117, 3199–3205. 37. Prieto-Carrasquero, M. C., Botros, F. T., Pagan, J., et al. (2008) Collecting duct renin is upregulated in both kidneys of 2-kidney, 1-clip goldblatt hypertensive rats. Hypertension 51, 1590–1596. 38. Gonzalez-Villalobos, R. A., Seth, D. M., Satou, R., et al. (2008) Intrarenal angiotensin II and angiotensinogen augmentation in chronic angiotensin II-infused mice. Am J Physiol Renal Physiol 295, F772–F779. 39. Field, L. J., McGowan, R. A., Dickinson, D. P., and Gross, K. W. (1984) Tissue and gene specificity of mouse renin expression. Hypertension 6, 597–603. 40. Gembardt, F., Heringer-Walther, S., van Esch, J. H., et al. (2008) Cardiovascular phenotype of mice lacking all three subtypes of angiotensin II receptors. FASEB J 22, 3068–3077. 41. Dzau, V. J., and Re, R. N. (1987) Evidence for the existence of renin in the heart. Circulation 75, I134–I136. 42. Barlucchi, L., Leri, A., Dostal, D. E., et al. (2001) Canine ventricular myocytes possess a renin-angiotensin system that is upregulated with heart failure. Circ Res 88, 298–304. 43. Mazzolai, L., Nussberger, J., Aubert, J. F., et al. (1998) Blood pressure-independent cardiac hypertrophy induced by locally activated renin-angiotensin system. Hypertension 31, 1324–1330. 44. Volpe, M., Gigante, B., Enea, I., et al. (1997) Role of tissue renin in the regulation of aldosterone biosynthesis in the adrenal cortex of nephrectomized rats. Circ Res 81, 857–864. 45. Peters J. (2008) Secretory and cytosolic (pro)renin in kidney, heart, and adrenal gland. J Mol Med 86, 711–714.

34

R.N. Re

46. Wanka, H., Keßler, N., Ellmer, J., et al. (2008) Cytosolic renin is targeted to mitochondria and induces apoptosis in H9c2 rat cardiomyoblasts. J Cell Mol Med, Jul 30. [Epub ahead of print] 47. Inagami, T., Nakagawa, T., Ichihara, A., Suzuki, F., and Itoh, H. (2008) Renin/prorenin receptor, (P)RR, in end-organ damage: current issues in 2007. J Am Soc Hypertens 2, 205–209. 48. Feldt, S., Batenburg, W. W., Mazak, I., et al. (2008) Prorenin and renin-induced extracellular signal-regulated kinase 1/2 activation in monocytes is not blocked by aliskiren or the handleregion peptide. Hypertension 51, 682–688. 49. Robertson, A. L. Jr., and Khairallah, P. A. (1971) Angiotensin II: rapid localization in nuclei of smooth and cardiac muscle. Science 172, 1138–1139. 50. Re, R. N. (2003) The intracrine hypothesis and intracellular peptide hormone action. Bioessays 25, 401–409. 51. Erdmann, B., Fuxe, K., and Ganten, D. (1996) Subcellular localization of angiotensin II immunoreactivity in the rat cerebellar cortex. Hypertension 28, 818–824. 52. De Mello, W. C. (1995) Influence of intracellular renin on heart cell communication. Hypertension 25, 1172–1177. 53. De Mello, W. C. (2001) Cardiac arrhythmias: the possible role of the renin-angiotensin system. J Mol Med 79, 103–108. 54. Eto, K., Ohya, Y., Nakamura, Y., Abe, I., and Iida, M. (2002) Intracellular angiotensin II stimulates voltage-operated Ca(2+) channels in arterial myocytes. Hypertension 39, 474–478. 55. Haller, H., Lindschau, C., Quass, P., and Luft, F. C. (1999) Intracellular actions of angiotensin II in vascular smooth muscle cells. J Am Soc Nephrol Suppl 11, S75–S83. 56. Cook, J. L., Zhang, Z., and Re, R. N. (2001) In vitro evidence for an intracellular site of angiotensin action. Circ Res 89, 1138–1146. 57. Cook, J. L., Mills, S. J., Naquin, R., Alam, J., and Re, R. N. (2006) Nuclear accumulation of the AT1 receptor in a rat vascular smooth muscle cell line: effects upon signal transduction and cellular proliferation. J Mol Cell Cardiol 40, 696–707. 58. Cook, J. L., Giardina, J. F., Zhang, Z., and Re, R. N. (2002) Intracellular angiotensin II increases the long isoform of PDGF mRNA in rat hepatoma cells. J Mol Cell Cardiol 34, 1525–1537. 59. Sherrod, M., Liu, X., Zhang, X., and Sigmund, C. D. (2005) Nuclear localization of angiotensinogen in astrocytes. Am J Physiol Regul Integr Comp Physiol 288, R539–R546. 60. Camargo de Andrade, M. C., Di Marco, G. S., de Paulo Castro Teixeira, V., et al. (2006). Expression and localization of N-domain ANG I-converting enzymes in mesangial cells in culture from spontaneously hypertensive rats. Am J Physiol Renal Physiol 290, F364–F375. Erratum in: Am J Physiol Renal Physiol 291, F921. 61. Kumar, R., Singh, V. P., and Baker, K. M. (2008) The intracellular renin-angiotensin system: implications in cardiovascular remodeling. Curr Opin Nephrol Hypertens 17, 168–173. 62. Singh, V. P., Le, B., Bhat, V. B., Baker, K. M., and Kumar, R. (2007) High-glucose-induced regulation of intracellular ANG II synthesis and nuclear redistribution in cardiac myocytes. Am J Physiol Heart Circ Physiol 293, H939–H948. 63. Re R. N. (2003) The implications of intracrine hormone action for physiology and medicine. Am J Physiol Heart Circ Physiol 284, H751–H757. 64. Re R. N, and Cook J. L. (2006) The intracrine hypothesis: an update. Regul Pept 133, 1–9. 65. Re R. N, and Cook J. L. (2008) The basis of an intracrine pharmacology. J Clin Pharm 48, 344–350.

Chapter 5

Renin-Angiotensin-Aldosterone System and Pathobiology of Hypertension Pierre Paradis and Ernesto L. Schiffrin

Abstract The renin-angiotensin-aldosterone system (RAAS) plays a critical role in the pathophysiology of elevated blood pressure, both in experimental models and in humans, in essential hypertension and some forms of secondary hypertension, such as renovascular hypertension and primary hyperaldosteronism. This is supported by studies measuring its components, such as renin activity or concentration, angiotensin II or aldosterone in plasma and tissues, the receptors for these mediators and their signaling in cells, as well as by inhibition of the different steps in the RAAS cascade with renin inhibitors, angiotensin I–converting enzyme (ACE) inhibitors, or angiotensin receptor blockers (ARBs). The effects of the RAAS on blood pressure are exerted on blood vessels to induce vasoconstriction, inflammation, growth, and remodeling and accelerate the progression of both atherosclerosis and arteriosclerosis in large vessels and remodeling of resistance arteries, on the kidney to retain salt and water, a critical effect to induce long-term blood pressure elevation, on the heart to induce left ventricular hypertrophy and coronary artery disease, and on the brain to stimulate vasopressin secretion and sympathetic nervous system activity. These different aspects of the role of the RAAS in hypertension will be reviewed in this chapter. Classically, the renin-angiotensin-aldosterone system (RAAS) was described as a peripheral system regulating blood pressure and water and salt balance (Chapter 1). Renin secreted from juxtaglomerular (JG) cells in the afferent arterioles of the kidney cleaves angiotensinogen, an α-2-globulin produced and released into the circulation by the liver, to generate the decapeptide angiotensin (Ang) I. Ang I is then cleaved by the dipeptidyl carboxypeptidase AngI-converting enzyme (ACE) in the lungs to generate the vasoactive octapeptide Ang II. Ang II, classically considered the final mediator of the RAAS, binds to Ang type 1 (AT1 ) receptors. Ang II increases blood pressure by causing vasoconstriction of blood vessels or, indirectly, by increasing blood volume through enhanced renal sodium and water reabsorption. E.L. Schiffrin (B) Hypertension and Vascular Research Unit, Lady Davis Institute for Medical Research/Sir Mortimer B. Davis-Jewish General Hospital, McGill University, Montreal, QC, Canada e-mail: [email protected] W.C. DeMello, E.D. Frohlich (eds.), Renin Angiotensin System and Cardiovascular Disease, Contemporary Cardiology, DOI 10.1007/978-1-60761-186-8_5,  C Humana Press, a part of Springer Science+Business Media, LLC 2009

35

36

P. Paradis and E.L. Schiffrin

In addition, by causing release of aldosterone from the adrenal glands which also causes increased renal reabsorption of salt and water, Ang II contributes to increased blood pressure. Ang II mediates most of its effects through AT1 receptors, which are expressed ubiquitously. In rodents there are two AT1 receptor subtypes: AT1a which is the predominant AT1 receptor subtype in most organs and AT1b which is highly expressed in the adrenal cortex and the pituitary gland [1]. Studies with AT1a receptor null (agtr1a) mice have revealed that blood pressure and vascular tone are regulated by AT1a receptors [2, 3]. Agtr1a mice had a reduced resting blood pressure and no pressor response to Ang II infusion. Ang II also binds to AT2 receptors, which are highly expressed in fetal tissues, but whose expression decreases dramatically after birth. The density of AT2 receptors is low in adult tissues. AT2 receptors are expressed in tissues involved in blood pressure regulation, such as the heart, kidneys, adrenal glands, brain, and vascular smooth muscle (VSMC) and endothelial cells (EC) in the adult [4]. Although there is some evidence that AT2 receptors may counteract the action of AT1 receptors, their role in blood pressure regulation remains unclear. In addition to the peripheral RAAS of renal origin, there are local tissue RAASs (Chapter 2). More recently, our understanding of the complexity of the RAAS has been significantly enhanced with the finding that pro-renin is more than a precursor protein but also an active molecule, and the discovery of renin receptors (Chapter 3), ACE2 (Chapter 11), and new vasoactive peptides such as Ang IV, Ang 1-7, and Ang 1-12 (Chapter 11), and new receptors like Ang 4R/IRAP and Ang 1-7R/Mas, as well as the rediscovery that Ang III may play important roles in the brain and in the kidney that had not been detected in the past. From the initial finding of the different components of the RAAS, it has been assumed that this system is involved in the development and maintenance of hypertension. This was first recognized in acquired disorders, particularly in renovascular hypertension for renin and Ang II, and primary aldosteronism in the case of aldosterone, and also in essential hypertension (reviewed in reference 2). Both Ang II, the main peptide effector of RAAS, and aldosterone exert effects that participate in the mechanisms leading to the development of hypertension by acting on the vasculature, the kidneys, and the central and peripheral nervous system. Ang II and aldosterone also act on the heart beyond their effects on the coronary vessels. These effects on the myocardium are part of the target organ damage associated with hypertension.

5.1 Blood Vessels Essential hypertension is characterized by increased peripheral vascular resistance to blood flow [5], occurring mostly through energy dissipation in small resistance arteries with a reduced lumen diameter. It is important to note that according to the law of Poiseuille, flow resistance is inversely related to the fourth power of the vessel radius, and therefore, small decreases in the diameter of the lumen significantly increase resistance. Small resistance arteries with a lumen diameter of 100–300 μm play an important role in the development of hypertension [6], and contribute to

5

Renin-Angiotensin-Aldosterone System and Pathobiology of Hypertension

37

its complications [7], to myocardial ischemia [8–10], stroke [11], and renal failure [12]. In essential hypertension, resistance arteries undergo eutrophic remodeling characterized by reduced outer diameter and lumen and increased media-to-lumen ratio, but with unaltered media cross-sectional area [13–20]. Eutrophic remodeling has been most often observed in animal models of hypertension with an activated RAAS [4, 21, 22]. This remodeling is characterized in humans by the absence of VSMC hypertrophy or hyperplasia [23] but with rearrangement of VSMCs around the smaller lumen [24–26]. The VSMC rearrangement may result from increased constriction [27, 28] due to activation of the RAAS and/or the sympathetic nervous system or secretion of growth factors such as endothelin-1 (ET-1), which becomes embedded in an increased extracellular matrix, also resulting from the action of RAAS components, such as Ang II or aldosterone, or other agents. Hypertrophic remodeling, which is characterized by increased media-to-lumen ratio and media cross-sectional area, has been observed in small arteries of patients with renovascular hypertension [29], hypertensive diabetic subjects [30, 31], and acromegalic patients [32]. This remodeling is associated with VSMC hypertrophy without evidence of hyperplasia. It is noteworthy that VSMC hyperplasia and hypertrophy have been shown to contribute to vascular remodeling in animal models of hypertension, such as spontaneous hypertensive rats (SHRs), stroke-prone SHR (SHR-SP), and Ang II-induced hypertension in rodents [33–37]. We demonstrated that Ang II induced proliferation (hyperplasia) and growth (hypertrophy) of cultured vascular VSMCs isolated from resistance arteries from subcutaneous gluteal biopsies from human healthy subjects [38] through AT1 receptors via the ERK-dependent signaling pathway and increased generation of reactive oxygen species (ROS) [39]. Furthermore, Ang II induced growth and proliferation of VSMCs through a crosstalk between AT1 receptors and epidermal growth factor (EGF), platelet-derived growth factor (PDGF), and insulin-like growth factor (IGF) receptors [28]. Aortic media thickening in animal models of hypertension is accompanied by re-expression of fetal genes in VSMCs associated with a shift of VSMCs from a contractile to a synthetic phenotype and/or expansion of preexisting immature VSMC population [40–42], and accordingly, in TGRen2 transgenic rats overexpressing the mouse Ren2 gene, Ang II induced re-expression of fetal muscle genes (SM-MyHC and MyHC-α2) and EIIIA-fibronectin (FN) in aortic VSMCs, which may play a role in the changes observed in VSMC phenotype [40] and vascular stiffness (see below). Apoptosis in the periphery of small arteries triggered by Ang II, combined with growth of VSMCs toward the lumen, may contribute to the VSMC rearrangement in eutrophic remodeling [28, 43]. However, Rizzoni et al. observed apoptosis in small mesenteric arteries of older SHRs while no apoptosis was present in younger rats in which eutrophic remodeling was already present [44]. Interestingly, whereas Ang II infusion induced apoptosis in aortic VSMCs of Wistar rats, AT1 or AT2 receptor blockers did not prevent but actually enhanced apoptosis [45], suggesting a role for both receptors in this process. Although the role of AT2 receptors is still unclear, they may counteract and fine-tune AT1 pro-proliferative actions [46]. Fibrosis of the media is observed in hypertension in blood vessels in both eutrophic and hypertrophic remodeling [31], and may contribute to increased resis-

38

P. Paradis and E.L. Schiffrin

tance by augmenting vasculature stiffness [47]. Vascular fibrosis is characterized by the accumulation of extracellular matrix components such as collagen, FN, fibrillin, and proteoglycans in the media and perivascularly [48]. Ang II regulates the synthesis and degradation of collagen in the vascular media. Results from in vivo studies have indicated that Ang II through AT1 receptors induced collagen deposition in the media [40, 49]. In vitro studies further demonstrated that Ang II increased expression of collagen in VSMCs and fibroblasts through AT1 receptors [50, 51]. In addition, Ang II decreased the expression of enzymes that degrade collagen, the matrix metalloproteinases (MMPs), and stimulated production of the tissue inhibitors of MMPs (TIMPs) [52, 53]. Ang II–AT1 receptor-dependent signaling pathways upregulated the expression of a fetal extracellular matrix gene, EIIIA-FN [48]. Increased levels of Ang II in TGRen2 transgenic rats increased EIIIA-FN in aortic SMCs, thus contributing further to vascular stiffness [40]. The increase in FN preceded the rise in blood pressure in Ang II-dependent hypertensive models [48]. The profibrotic effects of Ang II via AT1 receptors were mediated by transforming growth factor (TGF) β [54] or connective tissue growth factor (CTGF) [55, 56]. Increased vasoconstriction may contribute to development of hypertension. Indeed, we have observed exaggerated vasoconstriction response to Ang II in resistance arteries of patients with essential hypertension [16]. Increased contractile sensitivity has also been found in rodent models of hypertension [57]. This enhanced contractile response may be mediated by aldosterone-induced increase in AT1 receptor expression [58, 59] and/or through enhanced signaling responses by the RhoA/Rho kinase-dependent pathways in VSMCs [57]. Endothelial dysfunction may also play a role in the increases in peripheral resistance found in both experimental and human hypertension. We and others have observed endothelial dysfunction in humans [60, 61] and in SHRs [60, 62]. This altered endothelial function is mainly dependent on reduced bioavailability of nitric oxide (NO), as reported in human subjects with essential hypertension [60, 63, 64] and in animal models such as SHR-SP and Ang II-induced hypertension in rabbits [65, 66]. Decrease in NO bioavailability could result from a reduction in synthesis or an increase in degradation of NO. Inhibition of NO synthesis could result from increases in asymmetric dimethylarginine (ADMA), which is a potent endogenous inhibitor of NO synthase (NOS) [67]. ADMA is produced by methylation of arginine by protein arginine N-methylytansferase (PRMT) and release by proteolysis. Normally, most of ADMA is degraded by dimethylamine dimethylaminohydrolase (DDAH) and the remaining is excreted in the urine. Increased levels of ADMA have been shown in hypertensive subjects [64, 68, 69] and in SHR [63]. Interestingly, the levels of ADMA and NO were, respectively, decreased and increased by blockade of the RAAS independently of blood pressure [63, 69]. Ang II-induced oxidative stress may contribute to decrease in biovailability of NO [70, 71]. Ang II induces the production of superoxide (• O2 – ) mainly through activation of reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, which is expressed in all vascular layers including ECs, VSMCs, and adventitial fibroblasts [70], although other enzymes such as xanthine oxidase, the respiratory chain, etc., probably also

5

Renin-Angiotensin-Aldosterone System and Pathobiology of Hypertension

39

contribute to different degrees. Increase in ROS was documented in humans with essential, renovascular, and malignant hypertension and in women with preeclampsia [63, 72–74]. In experimental models of hypertension, increases in oxidative stress in different tissues including the vasculature have been demonstrated in deoxycorticosterone acetate (DOCA)-salt mice, SHRs, and SHR-SPs [75–78]. We observed that basal and Ang II-induced NADPH oxidase-driven • O2 – production increased in VSMCs from SHRs with the level of blood pressure elevation [75]. Furthermore, we and others found that blockade of the RAAS, for example, with the AT1 receptor blocker (ARB) valsartan, decreased the production of ROS in SHRSP and in Dahl salt-sensitive hypertensive rats [77–79]. Decreases in NO bioavailability could also be caused by oxidation of NO to peroxynitrate (• ONOO– ) by • O – . In addition, Ang II-induced generation of • O – and • ONOO– may oxidize 2 2 tetrahydrobiopterin (BH4 ), an essential co-factor of endothelial NOS (eNOS), to 7,8-dihydrobiopterin (BH2 ). This will cause uncoupling of eNOS and change its state from one in which it produces NO to one in which it produces • O2 – . BH4 levels and the rate-limiting enzyme for the de novo synthesis of BH4 , guanyltriphosphate cyclohydrolase (GTPCH) I, as well as endothelium-dependent acetylcholineinduced vascular relaxation, were decreased in DOCA-salt hypertensive mice [76]. Interestingly, endothelium-specific overexpression of GTPCH I restored endothelial function in DOCA-salt mice. Several other studies have demonstrated that treatment with BH4 decreased blood pressure and improved endothelial function in Dahl saltsensitive hypertensive rats and in Ang II-induced hypertension in rats [79–81]. Peripheral resistance is modulated by up to slightly under 20% by the density of capillaries. Factors affecting angiogenesis such as hypoxia and inflammation modify capillary density and therefore blood pressure. Most forms of human and experimental hypertension are associated with decreased density of microvessels (rarefaction), which can increase blood pressure further and exacerbate hypertension-induced end-organ damage [28, 82]. Apoptosis of ECs may also be involved in microvascular rarefaction, contributing thus to hypertension [83, 84]. This could be mediated in part by Ang II, since inhibition of the RAAS with an ACE inhibitor has been shown to increase microvascular density [82]. Vascular remodeling and endothelial dysfunction are accompanied by local inflammation, which may contribute to the development of hypertension and its complications [28]. Inflammation is actually a normal process involved in recovery of tissue integrity, but if repair is not well regulated, this may lead to persistent changes and tissue damage. ROS have been implicated in all the stages of inflammation. Growing evidence indicates that low-grade inflammation plays a significant role in the pathophysiology of RAAS-induced high blood pressure and its complications. Ang II-induced • O2 – by activation of NADPH oxidase in the vasculature may be an early step in the initiation of local inflammation. Furthermore, Ang II has been implicated in all the subsequent steps of inflammation characterized by an increase in vascular permeability, leukocyte recruitment, and activation of tissue repair [28]. Ang II increased vascular permeability by inducing the synthesis of prostaglandins (PGs) and vascular endothelial growth factor (VEGF) in VSMCs and ECs through

40

P. Paradis and E.L. Schiffrin

AT1 receptors [85]. Ang II-induced leukocyte recruitment and activation via AT1 receptors, by inducing the expression of selectins (P and E) [86, 87], integrins (β2 and α4) [86], intercellular adhesion molecule (ICAM)-1 and vascular cell adhesion molecule (VCAM)-1 [87], cytokines such as monocyte chemotactic protein (MCP)1, interleukin (IL)-6, IL-8, IL-18, osteopontin (OPN), tumor necrosis factor (TNF)α [88–92], and chemokines such as cytokine-inducible neutrophil chemoattractant (CINC), keratinocyte-derived chemokine (KC), macrophage inflammatory protein (MIP)-2, and CC chemokine ligand 5 (CCL5) [89, 90, 93]. Furthermore, Ang II induces low-grade inflammatory effects independently of its blood pressureraising actions. Inhibition of VEGF by soluble VEGF receptor 1 (sFlt-1) gene transfer attenuated Ang II infusion-induced inflammation and vascular remodeling in mice without normalization of blood pressure [94]. A sub-pressor dose of Ang II induced leukocyte adhesion in mesenteric arteries via pressor-independent mechanisms [89]. Ang II may also induce inflammation by acting directly on lymphocytes. Nataraj et al. observed that T and B cells and macrophages isolated from the spleen express AT1a receptors and that in vitro Ang II regulated the proliferation of wildtype but not agt1ar–/– splenic lymphocytes [95]. More recently, it was demonstrated that T lymphocytes are required for Ang II to induce vascular remodeling [96]. Ang II-induced vascular remodeling was impaired in rag1–/– mice, which are deficient in both T and B lymphocytes, although expression levels of aortic AT1 and AT2 receptors were unaltered. The vascular effects of Ang II were restored by transfer of T but not B cells. Finally, the last step of inflammation, tissue repair, which is a normal process that restores tissue integrity, may be impaired by the action of Ang II. Indeed, Ang II causes VSMC growth, proliferation and apoptosis, and vascular fibrosis, leading to vascular remodeling and hypertension (see above). Furthermore, Ang II-induced inflammation is a major contributor to the progression of atherosclerosis. The ACE inhibitor (ACEi), quinapril, blunted the increased expression of the CC chemokine MCP-1 and macrophage infiltration in the neointima of injured femoral artery in a rabbit model of accelerated atherosclerosis [97]. The ARB ibesartan reduced both the progression of the lesions and the expression of CC chemokines such as MCP-1 and macrophage inflammatory protein (MIP)-1α and CXC chemokines (MIP-2 and KC) in artherosclerotic lesions of apoE–/– mice [98]. The ARB losartan decreased both intima proliferation and the expression of P-selectin and macrophage infiltration in aorta of hypercholesterolemic rabbits [99]. The ARB irbesartan reduced the serum level of • O2 – ,VCAM-1, and TNF-α in normotensive patients with coronary artery disease (CAD) who had undergone coronary artery bypass graft, percutaneous transluminal coronary angioplasty, or both prior to the study. The ARB candesartan reduced the serum levels of two proinflammatory molecules in hypertensive patients, the acute-phase reactant C-reactive protein (CRP) produced by the liver and adipocytes and CD40 ligand expressed by activated T lymphocytes, independently of blood pressure reduction [100]. In addition, Ang II-induced inflammation may further exacerbate artherosclerotic coronary artery disease (CAD) or stroke by causing atherothrombosis, which is characterized by an unpredictable, sudden rupture or erosion/fissure of an atherosclerotic plaque,

5

Renin-Angiotensin-Aldosterone System and Pathobiology of Hypertension

41

which leads to platelet activation and thrombus formation. Plasminogen activator inhibitor (PAI)-1, the major inhibitor of fibrinolysis, is increased in atherosclerotic plaques [101, 102], and Ang II has been demonstrated in vitro to induce expression of PAI-1 in endothelial cells [103, 104] and in vivo to cause a rapid increase in circulating PAI-1 in normotensive and hypertensive patients [105]. Inhibition of the RAAS with the ACE inhibitors ramipril or fosinopril decreased, respectively, the plasma levels of PAI-1 in patients with acute CAD [106] and in type 2 diabetic hypertensive patients [107]. Administration of the ARB valsartan to diabetic hypertensive subjects corrected structural remodeling of small arteries [108] and also reduced proinflammatory cytokines [109]. Interestingly, in the diabetic hypertensive subjects, the ARB treatment was associated with upregulation of AT2 receptors, which could contribute to the beneficial effects of valsartan on remodeling and inflammation [110]. Renin receptors have been described in different tissues including blood vessels [111], but their significance with respect to blood pressure elevation remains unclear. This subject is treated extensively in Chapter 3. So far, this chapter has concentrated on effects attributable to actions of Ang II. However, aldosterone, whose secretion by the adrenal glomerulosa is stimulated by Ang II, has important vascular effects (for more complete review see reference 112). These affect not only the endothelial layer [113] and the media but also the adventitia. Aldosterone binds to the mineralocorticoid receptor (MR) and has proliferative, proinflammatory, and profibrotic actions that are mediated in part via its genomic effects and in part via its nongenomic actions. These effects in part appear to mediate vascular actions usually attributed to Ang II [114–116]. Aldosterone appears to contribute to vascular actions of angiotensin II in part via upregulation of Ang receptors [58, 117] and other components of the RAS, not only in the vasculature but also in the brain [118]. It may also act in concert with Ang II to contribute to enhance its actions, area which is under active research. Aldosterone exerts its nongenomic effects by stimulating oxidative stress mainly through activation of NADPH oxidase and stimulation of MAPK (ERK 1/2) and other kinases [119]. Through genomic effects, aldosterone and other mineralocorticoids stimulate the expression of different proteins and peptides, including ET-1, which contributes to the oxidative stress, proliferation, inflammation, and fibrosis [120]. Increased oxidative stress participates in the endothelial dysfunction associated with effects of aldosterone [121], as NO is scavenged by ROS. The contribution of aldosterone to the hypertensive process is highlighted by the fact that it participates not only when aldosterone is produced in excess by an adrenal adenoma, which will not be discussed here [122], but also in essential hypertension, where there may be inappropriate secretion of aldosterone, particularly in resistant hypertension [123] and among obese hypertensive subjects [124]. Treatment of hypertensive subjects with blockers of MR, such as eplerenone, resulted in lowering of blood pressure (demonstrating the participation of aldosterone in blood pressure elevation) but as well decreased stiffness of large and small arteries of hypertensive patients [125]. Typically, collagen and fibronectin deposition in the media was reduced by eplerenone, whereas elastin was enhanced and vessels became more distensible.

42

P. Paradis and E.L. Schiffrin

5.2 The Kidney The effects of Ang II and aldosterone that impact on the kidney to contribute to the pathophysiology of hypertension will be dealt with only briefly. True renovascular hypertension in humans, in which increased renin secretion occurs in response to renal artery stenosis, or renin-secreting tumors, or those forms of experimental hypertension in which renin secretion by the juxtaglomerular cells is increased, such as Goldblatt hypertension in rodents or dogs, will not be addressed. Rather, it is the effects that the RAAS has on the kidney, which may lead to hypertension, and more specifically that of Ang II, which will be described. The specific effects of aldosterone on salt and water balance are dealt with in Chapter 15. The role of the kidney in long-term control of blood pressure and in the development and maintenance of hypertension was underlined by the computer modeling carried out by Arthur Guyton [126], and its relation to Ang II clarified in large measure by studies of John Hall [127] and others. Infusion of Ang II impairs pressure natriuresis, and blood pressure becomes highly sensitive to sodium intake, resulting in blood pressure elevation in the presence of small increments in sodium in the diet, and this is corrected by inhibition of Ang II generation by an ACE inhibitor (Fig. 5.1) [128]. This is particularly important in view of the fact that the kidney has large amounts of Ang II generated locally [129], which may be increased in hypertension as suggested by studies with RAAS blockade selectively administered

Fig. 5.1 Chronic relationships between blood pressure and sodium intake and excretion in dogs with a normal RAS, after blockade of Ang II formation with an ACE inhibitor and after continuous infusion of a low dose of Ang II (5 ng/kg per minute) to prevent suppression of circulating Ang II upon increased sodium intake. Inability to modulate Ang II levels decreases the slope of pressure/natriuresis relationship, causing marked salt sensitivity of blood pressure. Reproduced from [127] with permission

5

Renin-Angiotensin-Aldosterone System and Pathobiology of Hypertension

43

to the kidney, which improve natriuresis [127]. Ang II, through its hemodynamic and tubular effects on the kidney (for review see reference 130), and aldosterone, through its sodium-retaining action, will contribute thus to blood pressure elevation. The similarity of what happens with Ang II infusion, which can be reversed by ACE inhibitors as shown in Fig. 5.1 [128], and the displacement to the right of the blood pressure/urinary volume output relationships in both salt-sensitive and nonsalt-sensitive essential hypertension compared to normal as shown in Fig. 5.2 [131] suggests that indeed Ang II plays this role in renal hemodynamics and natriuresis, leading to blood pressure elevation. Interestingly, studies by T. Coffman and his group have demonstrated the critical importance of the AT1a receptor in the kidney in blood pressure elevation in response to Ang II infusion in mice using gene deletion and cross-transplantation studies (Fig. 5.3) [132]. From these studies, it would appear that in mice, kidney AT1a receptors that seem to be tubular rather than vascular play a more important role in blood pressure elevation induced by Ang II than systemic AT1a receptors. Whether this also applies to other species remains to be determined.

Fig. 5.2 Blood pressure/volume output relationships in normal conditions to the left or in saltsensitive and non-salt-sensitive essential hypertension to the right. Reproduced from [131] with permission

A word should be said about renin receptors and the kidney [111]. These have been identified in the kidney, but little is known about their importance in relation to blood pressure control by the RAAS. However, it has been suggested that renin may

44

P. Paradis and E.L. Schiffrin

Fig. 5.3 (a) Kidney cross-transplantation groups used in the study by Coffman’s group. Wild-type (+/+) or AT1a receptor-deficient (–/–) mice were transplanted with kidneys from either AT1a (+/+) or AT1a (–/–) mice. Systemic knockout (KO) mice express AT1a receptors only in the kidney since they have received a kidney transplant from AT1a (+/+) mice. Kidney KO animals express AT1a receptors in all tissues except the kidney, since they have been transplanted with kidneys from AT1a (–/–) mice. Total KO animals lack AT1a receptors completely. (b) Daily blood pressures in cross-transplanted mice during 21-day Ang II infusion. By day 12 of Ang II infusion, the severity of blood pressure elevation in systemic KO reaches that of the wild-type mice. Absence of renal AT1a receptors in kidney KO mice blunts the development of Ang II-induced blood pressure elevation. Total KO blood pressure shows minimal response to Ang II infusion (∗ P≤0.03 vs wild type; §PTIMP-1

Alterations of the extracellular matrix

Hypertrophy Hyperplasia

>ECM

Alterations of the intramyocardial vasculature

Myocardial hypertrophy and remodeling

Fig. 9.2 Schematic representation of the actions of extracellular (ANG IIE ) and intracellular (ANG III ) angiotensin II in cardiac cells that result in structural alterations of the myocardium responsible for myocardial hypertrophy and remodeling that characterize hypertensive left ventricular hypertrophy. (Cm, means cardiomyocyte; Fb, fibroblast; Ec, endothelial cell; Vsmc, vascular smooth muscle cell; AT1 , angiotensin II type 1 receptor; MMP-1, matrix metalloproteinase-1; TIMP-1 tissue inhibitor of matrix metalloproteinases-1; ECM, extracellular matrix)

blood pressure in young hypertensive individuals [25]. Therefore, locally acting myocardial angiotensin II can be considered as a major determinant of LVH in hypertension. In fact, interacting with its specific G protein-coupled AT1 receptor, angiotensin II elicits Gaq pathways in the cardiomyocyte that result in activation of several Ca2+ -dependent signaling molecules (i.e., Ca2+ -dependent protein kinases and mitogen-activated protein kinases, as well as the Ca2+ -calmodulin-dependent phosphatase calcineurin) which, in turn, participate in the transduction of hypertrophic stimuli to the nuclei [26, 27]. This will activate gene expression and promote protein synthesis, protein stability, or both, with consequent increases in protein content and in the size and organization of force-generating units (sarcomeres) that, in turn, will lead to increased size of individual cardiomyocytes [28]. Of interest, the AT1 receptor itself is directly activated by mechanical stress and invokes the aforementioned signaling routes that lead to cardiomyocyte hypertrophy, which can be blocked by an inverse agonist of the receptor [29]. In addition, angiotensin II binding of AT1 receptors triggers apoptosis by a mechanism involving stimulation of p38 MAP kinase activity, activation of p53 protein and subsequent decrease of the Bcl-2-to-Bax protein ratio, activation of caspase-3, stimulation of calcium-dependent DNase I, and internucleosomal DNA fragmentation [30]. Although angiotensin II has been shown to induce apoptosis in other cardiovascular cells through stimulation of the AT2 receptor, several findings suggest that it is unlikely that this receptor is a strong signal to induce cardiomyocyte apoptosis in vivo. In fact, apoptosis is not increased in the heart of

108

E.D. Frohlich and J. Díez

transgenic mice overexpressing AT2 receptors in the myocardium [31]. In addition, Ikeda and colleagues [32] have reported that blockade of AT1 receptors with losartan is accompanied by normalization of cardiac apoptosis in rats with angiotensin II-induced hypertension that exhibit increased expression of AT2 receptors in the heart. On the other hand, increasing evidence supports the notion that angiotensin II influences both fibrillar collagen synthesis and degradation [33]. In vitro studies of rat and human cardiac fibroblasts and myofibroblasts have shown that angiotensin II stimulates cell proliferation and fibrillar collagen synthesis via the AT1 receptor. The proliferative response of fibroblasts to angiotensin II might well be mediated by stimulation of the synthesis of growth or inflammatory substances like plateletderived growth factor (PDGF) and cytokines, by integrin activation due to secreted extracellular matrix proteins (e.g., osteopontin), or by a combination of these mechanisms [34]. A number of studies provide strong evidence that angiotensin II stimulates collagen synthesis by cardiac fibroblasts via specific growth factors [35]. For instance, angiotensin II has been shown to induce collagen I gene expression via activation of both MAP/ER kinase pathway and TGF-β1 signaling pathways (e.g., connective tissue growth factor - CTGF - and Smad proteins). Finally, angiotensin II stimulation of the AT1 receptor has been shown to inhibit collagen degradation by attenuating interstitial matrix metalloproteinase-1 (MMP-1) or collagenase synthesis and secretion in cardiac fibroblasts [36] and by enhancing TIMP-1 production in rat heart endothelial cells [37]. Studies of human vascular smooth muscle cells and of vessels from experimental animals have demonstrated that angiotensin II binding to the AT1 receptor leads to activation of receptor tyrosine kinases, such as epidermal growthfactor receptor (EGFR), platelet-derived growth factor receptor (PDGFR) and insulinlike growth factor-1 receptor (IGF-1R), and nonreceptor tyrosine kinases, such as c-Src [38]. In addition, AT1 receptor binding by angiotensin II induces activation of NAD(P)H oxidase resulting in intracellular generation of reactive oxygen species, which influence redox-sensitive signaling molecules, such as mitogen-activated protein (MAP) kinase p38MAP kinase, JNK, ERK1/2, and ERK5), and transcription factors (NF-κB, AP-1, and hypoxia-inducible factor-1) [39]. These signaling events stimulate vascular smooth muscle cell growth and extracellular matrix production. Some of the cardiac effects of angiotensin II seem to be independent of the AT1 receptor but the result of the ability of the peptide to operate in the intracellular space. As early as 1971, it was reported that tritiated angiotensin II is internalized and rapidly localized to the nuclei and mitochondria of cardiomyocytes [40]. Subsequently, Re and colleagues [41] demonstrated the presence of specific, high-affinity nuclear receptors for angiotensin and showed that nuclear binding of angiotensin II enhanced gene transcription. In 2004, Baker and colleagues [42] demonstrated that the transfection of cardiac myocytes with a construct encoding a nonsecreted type of angiotensin II led to the rapid induction of cell hypertrophy. In fact, when a plasmid encoding the nonsecreted angiotensin II, the expression of which was driven by the α-myosin heavy chain promoter, was injected into mice, marked LVH developed within 96 h [42].

9

Left Ventricular Hypertrophy and Treatment with RAS Inhibition

109

9.3 Effects of Pharmacological Inhibition of the RAS on Hypertensive LVH 9.3.1 General Aspects Antihypertensive drugs are effective in reducing LVM. In fact, Mosterd and colleagues [43] analyzed recently the data from 10333 participants in the Framingham Heart Study, and reported that the increasing use of effective antihypertensive therapy has caused a decrease in the prevalence of both hypertension and LVH in the general population. A large number of trials and meta-analyses have attempted to compare the effects of different antihypertensive agents on LVM, but flawed study designs and methodological problems have limited the utility of these studies. Nevertheless, a recent meta-analysis by Klingbeil and colleagues [44] including 80 double-blind, randomized controlled trials with 146 active treatment arms (n = 3767 patients) and 17 placebo arms (n = 346 patients) showed that after adjustment for treatment duration and change in diastolic blood pressure, there was a significant difference among medication classes in regressing LVH. In fact, the decrease in LVM (indexed by body surface area or LVM index) induced by the different classes was as follows: AT1 receptor antagonists>calcium channel blockers>angiotensin-converting enzyme (ACE) inhibitors>diuretics>beta-blockers (Fig. 9.3). In pair-wise comparisons, AT1 receptor antagonists, ACE inhibitors, and calcium channel blockers were more effective at reducing LVMI than were diuretics and beta-blockers. Why might AT1 receptor antagonists and ACE inhibitors decrease LVMI more effectively than would the other antihypertensive agents? The findings from the meta-analysis by Klingbeil and colleagues [44] indicate that blockade of the RAS reduces LVM beyond the effects of blood pressure reduction. This has been clearly demonstrated in two studies. A substudy of the HOPE trial showed that compared with patients given placebo, ramipril-treated patients without LVH at baseline had lower rates of subsequent LVH [45]. In addition, ramipril-treated patients with LVH at baseline had higher rates of regression of LVH, which was associated with

Reduction of LVMI (%)

20 16 12

ARAs CCBs

ACEIs

8

Ds 4

BBs

0

Fig. 9.3 Reduction (expressed as 95% confidence interval) of the left ventricular mass index (LVMI) by different classes of antihypertensive drugs (ARAs, angiotensin receptor antagonists; CCBs, calcium channel blockers; ACEIs, angiotensin converting enzyme inhibitors; Ds, diuretics; BBs, beta-blockers) (Adapted from [44])

110

E.D. Frohlich and J. Díez

improved prognosis and was independent of blood pressure reduction. Devereux and colleagues [46] conducted an echocardiographic substudy of the LIFE trial, in which LVM was measured yearly up to 5 years. Patients randomly assigned to receive losartan-based therapy had a significantly greater reduction in LVMI compared with patients receiving atenolol-based therapy. The larger relative reduction in LVMI in the losartan group was detected after 1 year of treatment and persisted to year 5. Consistent with the clinical findings of the LIFE trial, regression of LVH was greater in losartan-treated patients than in atenolol-treated patients even though blood pressure reductions were similar for the two groups. Therefore, the high effectiveness of pharmacological inhibition of the RAS to reduce LVM and regress LVH seems to be related to the reduction of the direct pro-hypertrophic and pro-remodeling actions of angiotensin II on the myocardium.

9.3.2 Emerging Clinical Aspects The time has come to revisit the current management of hypertensive LVH simply focused on controlling blood pressure and reducing LVM. In fact, it is necessary to pay attention also to the correction of alterations in LV function and coronary microcirculation that associate with LVH. Although several trials have been performed to analyze these aspects, methodological problems of design and the confounding influence of factors such as the antihypertensive and antihypertrophic effects of treatment make difficult to evaluate the available information. Nevertheless, from the available data it has been proposed that the use of either ACE inhibitors or AT1 receptor antagonists provides a higher benefit than the use of other agents [47, 48]. Similarly, it is now the time to develop new approaches in the treatment of hypertensive LVH aimed to repair myocardial structure (i.e., cardiomyocyte apoptosis and myocardial fibrosis). The in vivo effects of antihypertensive drugs on cardiac apoptosis in SHRs have been reviewed elsewhere [49]. Collectively, the available findings suggest that the ability of antihypertensive drugs to inhibit cardiomyocyte apoptosis is independent of their antihypertensive efficacy, but can be related to their capacity to interfere with the pro-apoptotic actions of humoral factors such as angiotensin II. This is further supported by clinical findings showing that despite an identical antihypertensive efficacy, the AT1 receptor antagonist losartan, but not the calcium channel blocker amlodipine, reduced cardiomyocyte apoptosis in hypertensive patients with LVH after 1 year of treatment [50] (Table 9.1). Brilla and colleagues [51] showed that treatment with the ACE inhibitor lisinopril, but not with the diuretic hydrochlorotiazide, reduced myocardial fibrosis in hypertensive patients, independently from blood pressure control and LVH regression, and that this was associated with improved left ventricular diastolic function. We have shown recently that treatment with the AT1 receptor antagonist losartan was associated with inhibition of collagen type I synthesis and regression of myocardial fibrosis in hypertensive patients with LVH [52] (Table 9.1). In contrast, hypertensive patients treated with the calcium-channel blocker amlodipine did not show significant changes in collagen type I metabolism or myocardial fibrosis [52] (Table 9.1). Interestingly,

9

Left Ventricular Hypertrophy and Treatment with RAS Inhibition

111

Table 9.1 Effect of antihypertensive treatment on hemodynamic, left ventricular mass and myocardial histological parameters in hypertensive patients Losartan group

Study 1 N Systolic BP (mm Hg) Diastolic BP (mm Hg) LVMI (g/m2 ) CVF (%) Study 2 N Systolic BP (mm Hg) Diastolic BP (mm Hg) LVMI (g/m2 ) AI (TUNEL positive nuclei/106 nuclei)

Amlodipine group

At baseline

After treatment

At baseline

After treatment

21 173±6 95±2 131±6 5.65±0.44

137±2∗∗ 81±2∗∗ 105±4∗∗ 3.96±0.32∗

16 162±11 95±3 134±12 4.93±0.27

137±4∗∗ 79±2∗∗ 119±11 4.31±0.38

14 173±2 94±3 134±9 2843±730

136±2∗∗ 78±3∗∗ 105±6∗∗ 1118±176

14 176±7 96±3 127±13 1658±244

139±3∗∗ 82±2∗∗ 124±11 3211±639

Study 1 adapted from [52]. Study 2 adapted from [50]. N means number of subjects in each group. BP means blood pressure. LVMI means left ventricular mass index. CVF means myocardial collagen volume fraction. AI means cardiomyocyte apoptotic index. Data are expressed as mean±SEM. ∗ P102 cm (>40 in) >88 cm (>35 in) ≥150 mg/dl