Government College of Pharmacy, Bengaluru, India

Government College of Pharmacy, Bengaluru, India Golden Jubilee of Dept of Pharmacology Lecture series 10 August 2017, 11:00 am The never ending story of Renin-Angiotensin, an inclusive, an invigorative, an interactive and an interminable system G. Jagadeesh, Ph.D. Division of Cardiovascular and Renal Products Center for Drug Evaluation and Research US Food and Drug Administration Silver Spring, Maryland, USA The story of angiotensin (Ang) began with the discovery of pressor effect of renin by Tigerstedt and Bergman in 1898. This led to the discovery of several components of renin-angiotensin system (RAS) starting with Ang I, Ang II and ACE in 1956. This was followed by identification of two major subtypes of Ang II receptors, AT1 and AT2 in 1970 (1). The AT2 receptor activation might oppose the detrimental effects of AT1 receptor overactivation as AT1R mediates vasoconstriction, hypertrophy, proliferation and inflammation. In contrast, AT2R causes vasodilation and natriuresis. The AT-1 receptor has strong competition with the AT-2 receptor in cardiovascular control of Ang II. The RAS is incessantly expanding with the discovery of ACE2 in 2000 and a host of angiotensin peptides such as Ang (1-7), alamandine and angioprotectin that have functions opposite to that of Ang II. Their effects are mediated through Mas (coupled to Gs/Gi/Gq/G12/13) and MrgD (coupled to Gi/Gs) receptors. They have a wide range of influence modulating or opposing the predominant canonical pathway, ACE-Ang II-Gq-AT1R. This is physiologically counterbalanced by more than one protective arm of the RAAS. Reduce endogenous Ang II levels by enzymatic cleavage to Ang IV and to Ang (5-8) by dipeptidyl peptidase (DPP III) or increase the availability of endogenous Ang (1–9)/Ang (1-7) and alamandine by ACE2. Thus, the vasodepressor and cardio-renal defensive arms of the RAAS are (a) Ang II/APA/AngIII/AT2 receptor/NO/cGMP, and (b) Ang I/Ang II/ACE2/Ang (1–7)/Mas receptor (1,2). The cross-regulated signaling networks have been beneficial in identifying potential drug targets in the treatment of cardiovascular, renal and hypertension-related cerebrovascular diseases. The system is growing with endless possibilities. Renin-angiotensin is a dual hormone system, serving as a circulating and local tissue hormone system as well as a neuromodulator function in the CNS. Control of blood pressure by the RAS is exerted through multiple actions of Ang II, the central product of the RAS. Ang II is a potent and direct vasoconstrictor, and a stimulator of aldosterone biosynthesis in the adrenal cortex. It releases noradrenaline, endothelin and vasopressin. Ang II plays a pivotal role in the pathophysiology of hypertension, cardiac hypertrophy and remodeling, heart failure, vascular thickening, atherosclerosis and electrolyte balance. The importance of the RAS is reflected in the observation that angiotensin II levels are elevated in renovascular and malignant hypertension. The agonist activity of octapeptide Ang II at the AT1 receptor (359 amino acids) is defined by binding of specific amino acids on Ang II (Phe8 and Tyr4, are the first and 2nd agonist switches, respectively) with the key amino acids in the extracellular loops and transmembrane domains (TMDs) of the receptor. Subsequent to this action, G protein interaction occurs on the TMD at the amino terminus and the cytoplasmic domains at the 2nd and 3rd intracellular loops (3). Serine and threonine residues in the carboxy tail of the receptor are sites for phosphorylation by G protein receptor kinases. This

interaction prevents further stimulation and simultaneously recruits β-arrestins initiating receptor internalization. The ensuing ‘secondary signaling’ is G protein-independent that has led to the development of ‘biased agonists’. Such agonists (e.g., TRV120027) block detrimental actions of G protein activation, demonstrating their role in cardiovascular diseases. Other G protein-independent signal is activation of JAK-STAT, Wnt signaling pathways contributing to CV and renal abnormalities. AT1 receptors coupling to classical Gq/11 protein activate multiple downstream signals (1) that play a critical role in the development of hypertension, cardiac hypertrophy and heart failure. Therefore, preventing the formation of Ang II or its precursor renin from prorenin (4) at various stages in the RAAS cascade dominates in the treatment of hypertension and cardiac insufficiency. Interrupting the functioning of the RAAS at different points has different effects on its components. The first approach, not necessarily a preferred choice, is to block the action of enzyme renin (e.g., aliskiren), a rate limiting step in the synthesis of Ang I from angiotensinogen (5). This prevents the production of Ang peptides by the renin-ACE and non-ACE/chymase pathways. On the other hand, angiotensin converting enzyme (ACE) inhibitors (e.g., captopril, ramipril), a second and frequently employed approach in targeting the RAAS, act by blocking the formation of Ang II from Ang I. ACE inhibitors are also involved in the activation of bradykinin, enkephalins, and other biologically vasoactive peptides. The resultant increase in circulating bradykinin promotes the release of potent vasodilator, nitric oxide (endothelium-derived relaxing factor) via activation of the endothelial B-2 receptor. Thus, part of the reduction in blood pressure that occurs with ACE inhibitor therapy is bradykinin-mediated. These actions adversely result in induction of dry cough, increased bronchial reactivity, and angioedema. A third line of RAAS intervention for the treatment of hypertension is to block the actions of angiotensin II at its target site, AT1 receptors (e.g., losartan, telmisartan). Aldosterone, an important component of RAAS, is known to cause increased sodium retention and increased volume, both contributing toward hypertension. Thus, the fourth class, are effective in lowering blood pressure in patients afflicted with essential hypertension by blocking the actions of aldosterone on mineralocorticoid receptors and beyond (e.g., spironolactone, eplerenone). A new and fifth strategy opened up recently is simultaneous inhibition of the AT-1 receptor and neprilysin (e.g., valsartan-sacubitril combination) (6). Currently, our knowledge on the various components of RAAS including the recent discovery of several Ang peptides and their receptors in understanding the physiopathology of RAS is strident to a new height. Every substrate matters as each one is likely to invigorate a new concept and is vital to the development of novel targets for the treatment of cardiovascular and renal diseases. Disclaimer: The opinions expressed herein are those of the speaker and do not necessarily reflect those of his employer. Reference 1. Balakumar P, Jagadeesh G. A century old renin–angiotensin system still grows with endless possibilities: AT1 receptor signaling cascades in cardiovascular physiopathology. Cellular Signalling 2014; 26: 2147–2160 2. Balakumar P, Anand-Srivastava MB, Jagadeesh G. Editorial. Renin-angiotensin-aldosterone: An inclusive, an invigorative, an interactive and an interminable system. Pharmacol Res 2017; in press 3. Balakumar, P, Jagadeesh, G. Structural determinants for binding, activation, and functional selectivity of the angiotensin AT1 receptor. Mol. Cell. Endocrinol. 2014; 53: R71-R92. 4. Balakumar P, Jagadeesh G. Cardiovascular and renal pathologic implications of prorenin, renin, and the (pro)renin receptor: promising young players from the old renin-angiotensin-aldosterone system, J. Cardiovasc. Pharmacol. 2010; 56: 570–579. 5. Jagadeesh G, Balakumar P, Stockbridge N. How well do aliskiren's purported mechanisms track its effects on cardiovascular and renal disorders? Cellular Signalling 2012; 24: 1583–1591 6. Balakumar P, Jagadeesh G. Drugs targeting RAAS in the treatment of hypertension and other cardiovascular diseases, in: G. Jagadeesh, P. Balakumar, U.K. Maung (Eds.), Pathophysiology and Pharmacotherapy of Cardiovascular Disease, Springer, Germany, 2015 (pp. 751–806).