The compensatory renin–angiotensin system in the central regulation of arterial pressure: new avenues and new challenges

Introduction

Hypertension is classically defined in adults as systolic blood pressure (BP) >140 mmHg and/or diastolic BP >90 mmHg. Prehypertension is defined as systolic BP >120 mmHg and diastolic BP >80 mmHg, but not reaching hypertension levels according to the US Eighth Joint National Committee (JNC 8) for hypertension guidelines [James et al. 2014]. Thresholds for pediatric patients vary according to age.

Hypertension is a multifactorial disease, the etiology of which is not completely understood [Yemane et al. 2010]. It has reached epidemic proportions worldwide and contributes significantly to the burden of heart disease, stroke, kidney failure, disability and premature death [Neupane et al. 2014]. It is estimated that about 17 million deaths occur worldwide because of cardiovascular diseases every year, of which complications of hypertension alone account for 9.4 million [Centers for Disease Control and Prevention, 2011]. A third of US adults are reported to be prehypertensive [Yoon et al. 2015]. Prevention is an important step in the prehypertensive group to reduce the number of people affected by high BP and consequently the number of deaths due to cardiovascular complications arising from hypertension. Emphasis is placed on those with prehypertension in order to prevent the development of high BP by adopting a healthier lifestyle such as increasing exercise activity, reducing salt consumption and reducing smoking, among others. However, despite the effort and self-care, there is still a large population of patients who will eventually require pharmacological treatment.

Current strategies to manage hypertension

According to the recent JNC 8 meeting, the current recommendations for treatment of hypertension are mainly based on the use, alone or in combination, of four classes of medications which include diuretics, calcium channel blockers, angiotensin receptor blockers (ARBs) and angiotensin converting enzyme inhibitors (ACEIs). All of these medications have proved to be adequate in reducing BP enough to maintain it below the threshold levels for the age population [James et al. 2014]. However, there are over 30% of patients whose hypertension cannot be controlled [Yoon et al. 2015] with the current recommendations and who require novel interventions or additional drugs to their treatment regimen with variable rates of success [Persell, 2011]. Therefore, the question of finding new treatment strategies is still a major concern in pharmacological research.

The renin–angiotensin system

The classic renin–angiotensin system (RAS) is a complex system composed of numerous peptides, enzymes and receptors that are involved in BP regulation and fluid homeostasis. It starts with angiotensinogen (AGT) generated in the liver. AGT is converted to angiotensin I (Ang-I) by renin released from juxtaglomerular cells of the kidney under salt deprivation and fluid imbalance. Ang-I acts as a substrate for angiotensin converting enzyme (ACE) which cleaves it to form Ang-II (Figure 1). Ang-II signaling via G protein-coupled Ang-II receptor type 1 (AT1R) promotes vasoconstriction and increases sympathetic tone, vasopressin release and aldosterone secretion. All of these effects ultimately increase the effective fluid volume and elevate BP. To a lesser extent, Ang-II also signals through a second G protein-coupled receptor, namely Ang-II receptor type 2 (AT2R), promoting vasodilatory effects opposite to AT1R through the release of nitric oxide (NO).

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Figure 1.

Schematic of the circulating or classical renin angiotensin system. Angiotensinogen is cleaved by the renin, secreted from the kidneys, to form the inactive peptide Angiotensin-I. This decapeptide is then hydrolyzed by ACE, essentially in the lung circulation, to form Angiotensin-II. Angiotensin-II by acting on type 1 receptors (AT1R) lead to increased sympathetic activity, tubular reabsorption of NaCl and excretion of potassium within the kidney, secretion of aldosterone from the adrenal reinforcing the renal effects, secretion of antidiuretic hormone from the pituitary and direct vasoconstriction on arteries. All these effects contribute to water and NaCl retention which feedbacks negatively on renin secretion. Overactivity of the system leads to a rise in blood pressure an hypertension. Renin–angiotensin system.

Although RAS is classically thought to act as an endocrine system with Ang-II circulating throughout the bloodstream, components have been shown to exist locally in several tissues including the heart, lung, adrenal gland, kidney, blood vessels and brain, to name only a few [Lavoie and Sigmund, 2003; Paul et al. 2006].

The brain RAS produces Ang peptides locally in several regions involved in the central regulation of BP, such as the paraventricular nucleus of the hypothalamus (PVN), subfornical organ (SFO), rostral ventrolateral medulla (RVLM), area postrema, and nucleus tractus solitarius (NTS) [Davisson, 2003; Gironacci et al. 2014]. Increased circulating Ang-II acts by controlling sodium and water intake by acting on structures of the anteroventral region of the third ventricle (AV3V), stimulating the secretion of vasopressin from the PVN and supraoptic nuclei of the hypothalamus, and controlling autonomic function by increasing sympathetic nerve activity in several nuclei including the PVN and RVLM [Ferguson and Bains, 1997].

Overactivity of the brain RAS has been implicated in neurogenic hypertension and is associated with excessive levels of Ang-II [Grobe et al. 2010]. In addition to the circumventricular organs (CVO), which are brain regions lacking a blood–brain barrier and allowing small, circulating peptides like Ang-II to cross [Johnson and Gross, 1993], evidence suggests that the blood–brain barrier becomes ‘leaky’ during the development of hypertension [Ueno et al. 2004], thus allowing more Ang-II to enter the central nervous system (CNS).

Therefore, not only locally generated Ang-II interacting with RAS components within the blood–brain barrier (PVN and RVLM), but also circulating Ang-II can enter the brain and interact with Ang receptors, notably AT1R, to produce sympathetic activation in addition to peripheral effects and further exacerbate the overactivity of the brain RAS in neurogenic hypertension [Lazartigues et al. 2007; Xia et al. 2009].

Targeting the RAS

Strategies to treat hypertension and heart failure have focused on blockade (Figure 2) of Ang-II formation (i.e. ACEI and more recently renin inhibitors) and blockade of ATIR (i.e. AR blockers). Blockade of the RAS with these drugs is beneficial but not absolute and results in arterial and venous vasodilation, natriuresis and diuresis, decrease in sympathetic nervous activity, and inhibition of cardiac and vascular hypertrophy. These effects lead to a reduction of BP and less vascular remodeling in the pathology of hypertension [Brunner et al. 1979]. Accordingly, RAS inhibitors are also effective antihypertensive treatments in patients with low plasma renin levels, a phenomenon seen in neurogenic hypertension [Sigmund, 2010] and which correlates with 25% of those diagnosed with low renin hypertension [Centers for Disease Control and Prevention, 2011]. Renin inhibitors, such as aliskiren, target the conversion of AGT to Ang-I and thus reduce the substrate available for metabolism into Ang-II. Another peptide in the RAS, Ang-(1-12), can be cleaved by ACE and converted into Ang-I, followed by a second cleavage by ACE into Ang-II, thus bypassing the effects of renin [Westwood and Chappell, 2012] and suggesting that renin inhibitors may not completely block the formation of Ang-II. Nagata and colleagues reported that administration of Ang-(1-12) in the periphery increases BP. This pressor response was attenuated by the ATIR blocker candesartan and the ACEI captopril [Nagata et al. 2006], suggesting that Ang-II stimulation of AT1R remains the main pressor mechanism.

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Figure 2.

Site of action of drugs targeting the ACE/Ang-II/AT1R axis. Historically, the ACE inhibitors were the first developed and prevent the conversion of Ang-I into Ang-II. The sartans (Angiotensin rece) were then developed to inhibit the Angiotensin-II signaling via the AT1R. Another antagonist exists to experimentally target AT2R but it does not have any clinical relevance. Finally, in the last decade, renin inhibitors have made their entry into the clinic and are efficient at blocking the system upstream. ACE, angiotensin converting enzyme; AT1R, angiotensin II receptor type 1; AT2R, angiotensin II receptor type 2.

ACEIs like captopril, lisinopril and enalapril block the cleavage of Ang-I to Ang-II, and lead to an overall decrease in circulating Ang-II levels. However, chymase has been shown to catalyze the formation of Ang-II from Ang-I [Park et al. 2013] independently of ACE. This was particularly evident in the face of diabetic and hypertensive nephropathy, which allows Ang-II to bypass the effects of ACEI to some extent. ACEI also inhibit the catabolism of bradykinin [Tom et al. 2001], a molecule that produces vasodilatory effects via its B2 receptor and mediates inflammation through its B1 receptor. This dual action of ACEI provides great therapeutic benefits in treating cardiovascular diseases.

A final set of inhibitors targeting the RAS blocks the AT1R, through which Ang-II promotes the majority of its signaling effects [Allen et al. 2000]. These drugs are commonly referred to as sartans or ARBs. While only minor effects of Ang-II are attributed to the AT2R in normal conditions, the role of this receptor appears to be more prominent in pathological conditions (e.g. hypertension), as evidenced by recent data [Xu et al. 2011; Bruce et al. 2014; Sampson et al. 2015]. Blocking AT1R increases circulating Ang-II levels, leading to increased signaling through AT2R [and angiotensin converting enzyme type 2 (ACE2)]. Like ACEI, ARBs also provide beneficial effects in the treatment of hypertension by decreasing Ang-II signaling via the ATIR.

The ACE2/ANG-(1-7)/Mas axis

In the past decade, a new axis of the RAS, ACE2/Ang-(1-7)/Mas receptor, has been identified (Figure 3) and shown to exhibit compensatory mechanisms in the face of an overactive RAS [Xu et al. 2011]. The components of this new axis have been identified in regions of the brain involved in the central regulation of BP, as well as the periphery, and are thought to play a role in neurogenic hypertension [Lazartigues et al. 2007]. ACE2 is a homologue of ACE that cleaves Ang-II into the vasodilator peptide Ang-(1-7), which acts on the Mas receptor to produce effects opposite to Ang-II, such as vasodilatation, antiproliferation and antihypertrophy [Ferrario et al. 2005]. ACE2 (and AT2R) may also be providing some of the beneficial effects seen with ARBs, which increase Ang-II levels, by allowing for ACE2 to cleave more Ang-II substrate into Ang-(1-7). ACE2 can also cleave Ang-I into Ang-(1-9), which could then be converted into Ang-(1-7) by ACE or neprilysin [Vickers et al. 2002]. However, the affinity for this reaction is much less than for Ang-II and it appears that this path would only be activated when Ang-I levels are elevated, for example, in the presence of ACEI due to blocking the conversion of Ang-I into Ang-II, leaving Ang-I available for cleavage by ACE2 (Figure 3).

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Figure 3.

Proposed hypothesis for the potential role of brain ACE2 shedding in the central regulation of blood pressure. Activation of AT1R by Angiotensin-II leads to up-regulation of ADAM17 on the neuronal plasma membrane. ADAM17-mediated shedding results in the release of ACE2 within the cerebrospinal fluid potentially limiting the conversion of Angiotensin-II into Angiotensin-(1-7) and thus the compensatory response resulting from Mas receptors activation. Under this scheme, ADAM17 up-regulation would contribute to the deleterious effects of ACE/Ang-II/AT1R axis overactivation and the resulting hypertension. ACE, angiotensin converting enzyme; ADAM17, A Disintegrin and Metalloproteinase 17; AT1, angiotensin II type 1; sACE2, soluble ACE2.

Several studies have highlighted ACE2 as a pivotal player in counterbalancing the vasoconstrictive actions of Ang-II in the development of cardiovascular diseases. The development of Ang-II-induced hypertension was shown to be blunted by overexpression of ACE2 in the CNS [Feng et al. 2010]. In similar studies, rodents were also protected from impaired baroreflex and autonomic dysfunction induced by Ang-II when ACE2 was overexpressed in the brain [Yamazato et al. 2007; Xia et al. 2009]. Other studies have reported that ACE2 prevents cardiac hypertrophy in Ang-II infused animals [Crackower et al. 2002; Diez-Freire et al. 2006; Feng et al. 2012]. Finally, high BP is decreased by ACE2 overexpression from lentivirus introduced to the RVLM of spontaneously hypertensive rats [Yamazato et al. 2007]. In these studies, ACE2 not only reduces the levels of Ang-II but also increases the levels of Ang-(1-7), promoting beneficial effects in various cardiovascular diseases. Therefore, therapies to enhance ACE2 could become a new approach for the treatment of cardiovascular diseases.

Potential new targets and new challenges

Although not part of the compensatory RAS per se, the previously identified (pro)-renin receptor (PRR) [Nguyen and Contrepas, 2008] has recently been shown to play a critical role in the central regulation of BP. Not only was the PRR shown to be expressed in key brain nuclei involved in the regulation of autonomic function, but it is also upregulated with hypertension [Li et al. 2012, 2014; Zubcevic et al. 2013]. Interestingly, one of these groups recently reported the generation of a new PRR antagonist, termed PRO20, capable of dose-dependently inhibiting PRR-induced hypertension in mice [Li et al. 2015]. Prevention of increased Ang-II levels within the CNS was shown to contribute to the antihypertensive effects of PRO20, which could potentially be the first of a new class of antihypertensive agents.

At the other end of the RAS cascade, Ala¹-Ang-(1-7), aka alamandine, was identified as a new metabolite of Ang-(1-7) through an unidentified enzyme [Lautner et al. 2013]. Alamandine can also be formed, via ACE2, from Ang-A, an analogue of Ang-II identified in human plasma [Jankowski et al. 2007; Etelvino et al. 2014]. So far, reports show that alamandine has similar properties to Ang-(1-7), capable of promoting NO signaling, reducing Ang-A-mediated vasoconstriction, and with antihypertensive properties. Alamandine was reported to bind the MrgD receptor which can also be activated, in vitro, by β-alanine, γ-aminobutyric acid (GABA), β-aminobutyric acid (BABA) and Ang-(1-7) [Solinski et al. 2014]. However, recent studies have suggested that heterodimerization of receptors, including the AT2R, could mediate the effects of alamandine and Ang-(1-7) [Villela et al. 2015]. More work is needed to determine the specificity of alamandine for these receptors in vivo. Of particular interest, a new endopeptidase was recently identified in the brain and kidney, capable of hydrolyzing Ang-(1-7) and alamandine [Wilson et al. 2015]. Once identified, this new endopeptidase might be of importance as its inhibition would theoretically prolong the halflives of Ang-(1-7) and alamandine and therefore their antihypertensive properties.

Besides the promising targets described above, researchers are also facing new challenges with the previously identified target, ACE2. Our laboratory recently identified two new post-translational mechanisms that may impair the ongoing efforts to elevate ACE2 activity and reinforce the enzyme’s compensatory activity.

ADAM17 (A Disintegrin and Metalloproteinase 17) is a type I transmembrane protein that belongs to a superfamily of Zn-dependent metalloproteases. ADAM17 plays a key role in the regulation of the proteolytic release from cellular membranes of some cytokines, chemokines, growth factors and their receptors [Dreymueller et al. 2012]. Increased ADAM17-mediated shedding has been described in a variety of diseases such as ischemia, heart failure, arthritis, atherosclerosis, diabetes, cancer, neurological and immune diseases. Recently, we demonstrated enhanced ADAM17 expression through a RAS overactivity-mediated mechanism leading to ACE2 downregulation from the plasma membrane through shedding of its catalytic site, in the brain of deoxycorticosterone acetate (DOCA) salt hypertensive mice [Xia et al. 2013]. At the same time, treatment of neurons with Ang-II was shown to upregulate the expression of ADAM17. Furthermore, knockdown of ADAM17 in the CNS using chronic infusion of small interference RNA blunted the development of hypertension in these mice. This may suggest a deleterious effect of ADAM17 activity on ACE2 compensatory effects and potentially a new target for the treatment of neurogenic hypertension.

Another mechanism of ACE2 downregulation has been proposed by our group, by suggesting that the hypertensive effects of ANG-II are in part mediated by internalization of membrane-bound ACE2 and subsequent lysosomal degradation of this carboxypeptidase through an AT1R-dependent mechanism [Deshotels et al. 2014]. Interestingly, a similar mechanism of AT1R-mediated internalization was recently highlighted for another enzyme, Cox2 [Sood et al. 2014]. This feedforward mechanism, supporting the reduction of ACE2, would limit the formation and therefore availability of Ang-(1-7), as well as alamandine, and enhance the hypertensive actions of Ang-II.

These studies emphasize the need for a better understanding of the mechanisms limiting ACE2 compensatory effects before the introduction of new therapies to enhance ACE2 activity.

Conclusion

A great advance has been made in understanding the role of RAS and ACE2 in the brain. The potential therapeutic implications of ACE2 and downstream peptides like Ang-(1-7) and alamandine make for ideal target candidates for further research in hypertension, heart failure and other cardiovascular diseases. Increasing brain ACE2 by stimulating endogenous ACE2 activity and/or expression, or administration of exogenous ACE2, may provide beneficial effects in some pathologic conditions, mostly by reducing sympathetic activity, oxidative stress and inflammation which are the main mechanisms activated by the ACE/Ang-II/AT1R axis. Although the discoveries of ACE2 and alamandine have been recent advancements in our understanding of the compensatory RAS, further clarification of its role and therapeutic potential in the CNS in health and disease is needed.