Angiotensin-(1-7): Translational Avenues in Cardiovascular Control

Abstract

Despite decades of research and numerous treatment approaches, hypertension and cardiovascular disease remain leading global public health problems. A major contributor to regulation of blood pressure, and the development of hypertension, is the renin-angiotensin system. Of particular concern, uncontrolled activation of angiotensin II contributes to hypertension and associated cardiovascular risk, with antihypertensive therapies currently available to block the formation and deleterious actions of this hormone. More recently, angiotensin-(1–7) has emerged as a biologically active intermediate of the vasodilatory arm of the renin-angiotensin system. This hormone antagonizes angiotensin II actions as well as offers antihypertensive, antihypertrophic, antiatherogenic, antiarrhythmogenic, antifibrotic and antithrombotic properties. Angiotensin-(1–7) elicits beneficial cardiovascular actions through mas G protein-coupled receptors, which are found in numerous tissues pivotal to control of blood pressure including the brain, heart, kidneys, and vasculature. Despite accumulating evidence for favorable effects of angiotensin-(1–7) in animal models, there is a paucity of clinical studies and pharmacokinetic limitations, thus limiting the development of therapeutic agents to better understand cardiovascular actions of this vasodilatory peptide hormone in humans. This review highlights current knowledge on the role of angiotensin-(1–7) in cardiovascular control, with an emphasis on significant animal, human, and therapeutic research efforts.

Introduction

Hypertension is a major public health concern contributing to increased risk of cardiovascular disease, cerebrovascular incidents, and development of end-organ damage including chronic kidney disease and heart failure.¹ While the prevalence of hypertension is approximately 29% among adults in the United States, only half of these individuals have controlled blood pressure.² This finding illustrates the need to better understand mechanisms involved in blood pressure regulation, to develop novel therapeutic targets for hypertension and related cardiovascular disease. A pivotal player contributing to both physiological and pathophysiological control of blood pressure is the renin-angiotensin system (RAS).³ To date, the majority of research on the role of the RAS in hypertension has focused on overactivation of canonical pathways involving angiotensin (Ang) II. The actions of Ang II at AT₁ receptors (AT₁R) elevates blood pressure through multiple mechanisms including vasoconstriction, sympathetic nervous system activation, arterial baroreceptor reflex dysfunction, and increases in aldosterone, inflammation, and oxidative stress.^3,4 Uncontrolled RAS activation has been identified as a leading cause of hypertension and a major contributor to progressive cardiac, renal, and vascular dysfunction.³

More recently, a noncanonical arm of the RAS has emerged that is characterized by Ang-(1–7), a biologically active heptapeptide serving as an endogenous antagonist to Ang II.⁵ In animal models, Ang-(1–7) elicits antihypertensive, antihypertrophic, antiarrhythmogenic, antiatherogenic, antifibrotic, and antithrombotic effects through activation of mas G protein-coupled receptors (MasR) distributed to numerous cardiovascular-related tissues.⁵ There are limited clinical studies, however, and thus the importance of Ang-(1–7) to cardiovascular regulation in humans remains unclear. This review highlights current translational evidence for a beneficial role of Ang-(1–7) in cardiovascular control by summarizing significant animal, human, and therapeutic research efforts.

RAS pathways contributing to cardiovascular control

The RAS is a series of enzyme–substrate interactions generating bioactive peptides involved in cardiovascular control (Figure 1). Renin, an aspartyl protease, is released from renal juxtaglomerular cells in response to decreased perfusion in the afferent renal arterioles, increased sympathetic activity, locally acting prostanoids and nitric oxide (NO), and decreased sodium chloride concentration in the macula densa.⁶ In canonical RAS pathways, renin cleaves angiotensinogen to produce Ang I, which is hydrolyzed by Ang-converting enzyme (ACE) to form Ang II, the main effector of the RAS.³ Ang II is an octapeptide that binds AT₁R to induce vasoconstriction, sympathetic activation, baroreflex dysfunction, and increases in aldosterone, oxidative stress, and inflammation.⁴ Long-term Ang II exposure is associated with several cardiovascular-related pathologies including hypertension, cardiac hypertrophy, heart failure, stroke, stent restenosis, renal fibrosis, and chronic kidney disease.⁷ Conversely, pharmacotherapies inhibiting Ang II activity, such as ACE inhibitors and AT₁R antagonists, lower blood pressure and are cardiovascular and renal protective in clinical populations. Ang II also binds type 2 receptors (AT₂R) to counteract the deleterious actions of AT₁R stimulation by promoting vasodilation, natriuresis, baroreflex sensitivity, and NO production.⁸ While precise mechanisms underlying AT₂R actions are still under debate, activity of this receptor is limited due to low affinity and tissue expression in adults.⁸

Figure 1.

Simplified schematic of the renin-angiotensin system, with a focus on angiotensin-(1–7) forming pathways. ACE, angiotensin converting enzyme; ACE2, angiotensin converting enzyme 2; AT₁R, angiotensin II type 1 receptor; AT₂R, angiotensin II type 2 receptor; MasR, angiotensin-(1–7) mas receptor; NEP, neprilysin (neutral endopeptidase); POP, prolyl oligopeptidase; TOP, thimet oligopeptidase.

A noncanonical arm of the RAS has more recently emerged that is characterized by Ang-(1–7), a biologically active heptapeptide that counteracts the deleterious cardiovascular actions of Ang II.⁵ As shown in Figure 1, Ang-(1–7) is formed from degradation of Ang I by endopeptidases (e.g., neprilysin, prolyl oligopeptidase, thimet oligopeptidase) or degradation of Ang II by carboxypeptidases such as ACE2. While having lower catalytic efficiency compared with other formation pathways, Ang I can also be converted to Ang-(1–9) by ACE2, which is subsequently cleaved by ACE or neprilysin to form Ang-(1–7).⁵ While the majority of in vivo evidence suggests that Ang-(1–7) binds MasR to promote beneficial cardiovascular effects, recent studies provide evidence for potential heterodimerization and functional interactions between MasR and AT₁R or AT₂R.^9,10 Finally, it is important to note that RAS pathways involved in cardiovascular control have become increasingly complex with discovery of additional biologically active components including Ang III, Ang IV, Ang-(1–12), prorenin, the prorenin receptor, alamandine, and tissue and intracellular RAS components.⁴

Cardiovascular effects of Ang-(1–7) in animal models

Ang-(1–7) has been suggested as a novel hormonal target for hypertension based on findings from animal models. As described below, numerous studies have shown that Ang-(1–7) elicits vasodilatory and cardioprotective effects in animal models via actions at MasR found in the blood vessels, heart, kidneys, and brain.⁵ While not a focus of this review, Ang-(1–7) has also been implicated in numerous other functions including glucose and lipid metabolism, angiogenesis, reproduction, cellular growth, inflammation, and cognition.¹¹

Vascular

Ang-(1–7) is formed and metabolized in endothelial cells, with several studies providing evidence for vasodilatory properties of this peptide.^12–14 Ang-(1–7) promotes endothelium-dependent vasodilation in numerous regional vascular beds in animal models, with effects mediated by several signaling mechanisms including bradykinin potentiation, release of vasodilatory prostaglandins, and stimulation of the phosphoinositide-3 kinase-Akt-endothelial NO synthase-NO-cyclic guanosine monophosphate (PI3K-Akt-eNOS-NO-cGMP) intracellular signaling pathway.^5,13,15 In normotensive animals, Ang-(1–7)-mediated vasodilation unloads the arterial baroreceptors to reflexively increase cardiac output and decrease systemic vascular resistance to prevent blood pressure changes.¹⁶ In hypertensive rats with impaired baroreflexes, however, Ang-(1–7) dilates mesenteric, coronary, and pulmonary arteries to lower blood pressure, with effects abolished by MasR blockade or NO synthase, guanylate cyclase, or protein kinase G inhibition.¹⁷ Ang-(1–7) also counteracts obesity- and Ang II-induced vascular oxidative stress and endothelial dysfunction.^5,18 Conversely, MasR knockout mice have increased vascular resistance in the kidneys, lung, and adrenal glands.¹⁹

In addition to effects on the endothelium, Ang-(1–7) protects vascular smooth muscle cells by reducing neointimal thickness and area in a rat stent model, counteracting Ang II-induced proliferative effects, and reducing osteogenic transition in rats with vascular calcification.⁵ Ang-(1–7) also promotes antithrombotic effects in mice by increasing NO and prostacyclin release from platelets, with global MasR knockout mice conversely having increased venous thrombus size and short bleeding times.²⁰ In addition, Ang-(1–7) protects against aneurysm development and rupture.²¹ Global MasR deficiency conversely enhances Ang II-mediated atherosclerosis and abdominal aortic aneurysm rupture in hypercholesterolemic mice by augmenting oxidative stress, apoptosis, and inflammatory pathways.²² The collective effects of Ang-(1–7) on the vasculature are summarized in Figure 2.

Figure 2.

Proposed mechanisms involved in the beneficial cardiovascular actions of angiotensin-(1–7) in animal models at the level of the vasculature, heart, kidneys, and brain. MI, myocardial infarction.

Cardiac

The presence of Ang-(1–7), ACE2, and MasR is established in cardiomyocytes, coronary vessels, and sinoatrial cells in animal models.⁵ Ang-(1–7) is generally cardioprotective by promoting antifibrotic, antihypertrophic, and antiarrhythmogenic effects (Figure 2).⁵ In terms of cardiac structure, Ang-(1–7) mediates beneficial effects of AT₁R blockade on cardiac remodeling and fibrosis in a rat model of myocardial infarction.²³ Ang-(1–7) also prevents cardiomyocyte proliferation by downregulating extracellular signal-regulated kinase 1/2 (ERK1/2) pathways and promoting NO synthesis.²⁴ In addition, Ang-(1–7) protects against Ang II- and isoproterenol-induced cardiac hypertrophy and fibrosis in rodents, in part, by reducing oxidative stress and collagen deposition and increasing atrial natriuretic peptide secretion.²⁴ Moreover, stimulation of MasR prevents Ang-II induced cardiac hypertrophy in a transforming growth factor beta-1 (TGFβ1)/Smad2 dependent manner, implicating downregulation of profibrotic cytokine pathways.²⁵ In contrast, global MasR deletion or pharmacological MasR antagonism with A779 adversely impacts cardiac function during ischemia/reperfusion in isolated perfused mouse hearts.²⁶ Global genetic deletion of MasR in mice also increases fibronectin and collagen deposition in cardiac tissue leading to a profibrotic phenotype to decrease cardiac function.²⁴ These profibrotic changes are mediated by upregulation of growth-promoting signaling cascades such as ERK1/2 and p38.²⁴

In addition to cardiac structure, Ang-(1–7) influences coronary artery relaxation and cardiac rhythm, two factors affecting systemic blood pressure. While high doses produce no effect or vasoconstriction, lower Ang-(1–7) concentrations elicits vasodilation in coronary arteries of dogs, pigs, and rats.^5,27 A recent study showed Ang-(1–7) promotes coronary vasodilation in rats through MasR coupling and interactions with ACE and ACE2.²⁷ Vasodilatory responses are also observed in isolated hearts from aorta-coarcted hypertensive rats when using picomolar Ang-(1–7) concentrations.²⁸ In addition to coronary vessels and cardiomyocytes, all components of the vasodilatory arm of the RAS are expressed in sinoatrial node cells of rats.²⁹ Low dose Ang-(1–7) has generally been shown to promote antiarrhythmogenic effects. For example, Ang-(1–7) reduces tachyarrhythmias in isolated perfused rat atria in a MasR-dependent manner.²⁹ Moreover, Ang-(1–7) decreases cardiac arrhythmias without changing fractional shortening, a measure of contractility, in mice via MasR- and NO-mediated mechanisms.³⁰ The improvements in myocardial function and arrhythmic events with Ang-(1–7) may involve alterations in atrial natriuretic peptide, bradykinin, prostaglandins, the sodium pump, and calcium handling proteins.^5,24 High doses of Ang-(1–7), however, can conversely elicit pro-arrhythmogenic effects.³¹

Renal

As recently described, Ang-(1–7) biosynthesis has been described in several nephron segments within the kidney, with this peptide contributing to local homeostatic control of blood volume and hydroelectrolyte balance.⁵ An initial clinical study showed higher urinary versus systemic Ang-(1–7) levels in healthy volunteers suggesting local renal production.³² Furthermore, urinary Ang-(1–7) excretion was reduced in hypertensive patients, and inversely correlated with blood pressure suggesting an association with hypertension.³² In terms of renal vascular function, Ang-(1–7) induces MasR-dependent afferent arteriolar relaxation and NO release in isolated kidneys from rabbits.³³ Furthermore, Ang-(1–7) increases renal blood flow in anesthetized rodents, and prevents abnormal renal vascular responsiveness to vasoactive agents (e.g., Ang II, norepinephrine, endothelin) in diabetic rats.^16,34 Similarly, intra-arterial Ang-(1–7) infusion induces renal vasodilation in clinical populations, as described later in this review.^35–37 In contrast, global MasR knockout mice have increased vascular resistance and reduced renal blood flow.¹⁹

Ang-(1–7) has also been shown to have non-vascular renal actions including promotion of diuresis and natriuresis in vitro and in vivo in normotensive, hypertensive, and diabetic rodent models under baseline conditions.⁵ Ang-(1–7) promotes natriuresis via modulation of sodium transport in the distal and proximal tubules as well as in the thick ascending loop of Henle, in part via MasR-mediated increases in NO production.³⁸ Ang-(1–7) also decreases proteinuria, lipid accumulation, and renal inflammatory cytokines and oxidative stress markers in rat models of renovascular hypertension, diabetic nephropathy, and chronic kidney disease.^5,39,40 Conversely, global MasR knockout mice have reduced urinary volume and fractional sodium excretion and increased microalbuminuria.⁴¹

It is important to note that while most studies support vasodilatory, natriuretic and diuretic effects of Ang-(1–7) as summarized in Figure 2, disparate findings exist in the literature. For example, preclinical studies have not consistently shown effects of Ang-(1–7) on renal vascular function in vivo.⁵ In addition, in water-loaded mice and rats, Ang-(1–7) induces a potent antidiuretic effect mediated by AT₁R.⁵ As recently reviewed, these complex findings may reflect differences in sex, species, nephron segment or cell type studied, hydroelectrolyte status, receptor interactions (MasR, AT₁R, AT₂R, vasopressin), and experimental conditions.⁵

Neural

Ang-(1–7) and ACE2 immunoreactivity have been reported throughout the brain including in cardiovascular regulatory brain regions such as the solitary tract nucleus (NTS), caudal ventrolateral medulla (CVLM) and rostral ventrolateral medulla (RVLM) of the brainstem and in the paraventricular nucleus (PVN) and arcuate nucleus (ARC) of the hypothalamus. MasR are found in these brain regions as well as in autonomic preganglionic neurons, ganglia, and nerve terminals and in circumventricular organs allowing access to circulating hormones.^42,43 While these components are thought to have primary neuronal localization, astroglia have been suggested as a possible cellular substrate for Ang-(1–7) effects in cardiovascular brain regions such as the RVLM.⁴⁴ The source of Ang peptides in brain, however, remains controversial with some reports supporting derivation from the circulation as opposed to local production.⁴⁵

In contrast to Ang II, chronic Ang-(1–7) administration within the central nervous system lowers blood pressure, reduces cardiac and renal sympathetic tone, and improves measures of parasympathetic tone such as heart rate variability and baroreflex sensitivity for control of heart rate in hypertensive animal models (Figure 2).^4,46,47 These neuromodulator actions of Ang-(1–7) influencing arterial pressure are mediated by MasR.⁴⁸ Furthermore, ACE2 overexpression in brain regions critical to control of sympathetic cardiovascular drive such as RVLM and PVN decreases blood pressure in hypertensive rats.⁴⁹ In terms of sympathetic actions, Ang-(1–7) reduces presynaptic norepinephrine release from isolated hypothalamus via a MasR- or AT2R-mediated mechanism involving bradykinin and NO release in normotensive and hypertensive rats.⁵⁰ Furthermore, as recently reviewed, Ang-(1–7) stimulates norepinephrine reuptake and decreases tyrosine hydroxylase activity and expression in hypothalamic neurons from normotensive and hypertensive rats.⁵

In addition to reducing sympathetic tone, intracerebroventricular or NTS administration of Ang-(1–7) facilitates the parasympathetic component of the arterial baroreceptor reflex under normal conditions, in antenatal betamethasone-exposed sheep, and in rat models of hypertension, metabolic syndrome, and aging.^4,5,51 Within the NTS, the primary site for termination of vagal afferents fibers originating from the arterial baroreceptors, a non-glial source for endogenous Ang-(1–7) appears to contribute to enhancement of baroreflex sensitivity in older rats.⁵² Ang-(1–7) pathways also contribute to the improvement in baroreflex sensitivity produced by ACE inhibition in spontaneously hypertensive rats.⁵³ In contrast, MasR antagonism with A779 attenuates baroreflex sensitivity in normotensive rats, with no effect on the impaired baroreflex function in hypertensive rats suggesting loss of protective endogenous Ang-(1–7) tone in hypertension.⁵ Furthermore, global MasR knockout mice have elevated resting blood pressure that is associated with impaired arterial baroreflex sensitivity.⁵⁴

As recently reviewed,^4,5 the central actions of Ang-(1–7) are complex and duration- and region-specific. For example, similar to Ang II, Ang-(1–7) produces depressor and bradycardic effects when administered acutely into the NTS, CVLM, or anterior hypothalamus. Several studies have shown, however, that Ang II and Ang-(1–7) engage different pathways to elicit depressor responses within these brain regions. When administered acutely into the PVN or RVLM, Ang-(1–7) conversely increases blood pressure and renal and splanchnic sympathetic nerve activity as well as enhances the cardiac sympathetic afferent reflex suggesting tonic sympathoexcitatory actions. These sympathoexcitatory effects appear to involve increased glutamate release and decreases in GABA, glycine, and taurine release.

Cardiovascular effects of Ang-(1–7) in humans

While Ang-(1–7) elicits antihypertensive and cardioprotective effects in animal models,⁵ there are limited studies in humans. This reflects, in part, conflicting findings for Ang-(1–7) vasodilatory effects in early clinical studies, which halted progression of further research for over a decade. As summarized in Table 1, clinical studies to date have largely focused on the ability of Ang-(1–7) to increase blood flow in the forearm or renal arteries in healthy subjects and patients with essential hypertension, heart failure, and obesity. While these studies provide insight into cardiovascular effects of Ang-(1–7) in humans, additional research is needed, particularly related to systemic administration.

Table 1.

Published clinical studies on angiotensin-(1–7) and cardiovascular regulation

Authors	Year	Population	Route	Endpoint	Finding
Kono et al. 65	1986	Healthy men	Intravenous	Blood pressure	Pressor response
Davie et al. 59	1999	Heart Failure patients treated with an ACE inhibitor	Intrabrachial	Forearm blood flow, blood pressure	No effect
Ueda et al. 63	2000	Healthy men	Intrabrachial	Forearm blood flow	Antagonized Ang II vasoconstriction
Ueda et al. 62	2001	Healthy men	Intrabrachial	Forearm blood flow	Potentiated bradykinin- mediated vasodilation
Sasaki et al. 61	2001	Healthy and hypertensive subjects	Intrabrachial	Forearm blood flow	Increased
Wilsdorf et al. 60	2001	Healthy subjects	Intrabrachial	Forearm blood flow	No Effect
Plovsing et al. 66	2003	Healthy men	Intravenous	Blood pressure	No Effect
Van Twist et al. 37	2013	Hypertensive subjects	Intrarenal	Renal blood flow	Increased
Van Twist et al. 36	2014	Hypertensive subjects with and without renal artery stenosis	Intrarenal	Renal blood flow	Increased; Attenuated in hypertensive subjects with stenotic kidneys
Van Twist et al. 35	2016	Hypertensive subjects with multifocal renal artery FMD	Intrarenal	Renal blood flow	Increased
Schinzari et al. 64	2018	Obesity	Intrabrachial	Forearm blood flow	Increased

Abbreviations: ACE, angiotensin converting enzyme; Ang, angiotensin; FMD, fibromuscular dysplasia.

Vascular and systemic effects

Ang-(1–7) dilates several vascular beds in animal models such as coronary, pulmonary, and renal arteries.⁵ The vasodilatory properties of Ang-(1–7) have also been investigated in human blood vessels. Ang-(1–7) significantly reduces Ang II-induced vasoconstriction in mammary arteries from healthy subjects and splanchnic vessels from cirrhotic subjects.^55–57 Ang-(1–7) also vasodilates adipose and atrial arterioles from patients without coronary artery disease, with effects attenuated in patients with coronary artery disease.⁵⁸ This suggests protective effects of Ang-(1–7) in human coronary and peripheral microvasculature, similar to preclinical observations.

To date, the majority of studies in humans have utilized an intra-arterial approach, by measuring forearm blood flow in response to escalating doses of Ang-(1–7). In heart failure, intrabrachial Ang-(1–7) infusion produced no vasodilating effect; however, these patients were chronically treated with ACE inhibitors, which increase levels of circulating Ang-(1–7).⁵⁹ Similarly, intrabrachial Ang-(1–7) produced no effect on forearm blood flow in healthy subjects.⁶⁰ In contrast, another study showed that intrabrachial Ang-(1–7) increases forearm blood flow in healthy and hypertensive subjects.⁶¹ Furthermore, intrabrachial Ang-(1–7) potentiated bradykinin-mediated vasodilation,⁶² and antagonized Ang II-induced vasoconstriction, in healthy male subjects.⁶³ A recent study showed that intrabrachial Ang-(1–7) infusion also improves insulin-dependent vasodilation in obese subjects.⁶⁴

The role of Ang-(1–7) in renal vascular function has also been investigated in a handful of clinical studies. In an initial study, intra-arterial Ang-(1–7) infusion in the kidneys increased renal blood flow in hypertensive subjects.³⁷ This vasodilatory effect appears dependent on RAS activation as it was attenuated by low sodium diet and/or coinfusion of Ang II.³⁷ Another study showed renal vasodilating effects of Ang-(1–7) are reduced in hypertensive subjects with renal artery stenosis compared to hypertensive subjects without renal-related pathology.³⁶ It was postulated this attenuated vasodilation could reflect, in part, low NO levels in stenotic vascular beds, such as observed in renal stenosis. Finally, intrarenal Ang-(1–7) increased renal blood flow in hypertensive patients with multifocal renal artery fibromuscular dysplasia, to a similar extent as hypertensive patients without renal abnormalities.³⁵

In terms of systemic infusion, intravenous Ang-(1–7) produced a weak pressor response and renin-suppressing actions in healthy men when given at supraphysiologic doses.⁶⁵ At physiologic doses, acute intravenous Ang-(1–7) did not alter blood pressure or heart rate in healthy men.⁶⁶ Overall, clinical trials with Ang-(1–7) have produced conflicting results, thus bringing into question the role of this hormone in blood pressure control in humans. These disparate clinical findings could reflect differences in methodologies, populations studied, dose selection, and the region-specific vascular beds analyzed. In addition, circulating levels of Ang-(1–7) in healthy individuals versus patients with cardiovascular diseases remain unclear. It is possible, for example, that Ang-(1–7) infusion only impacts cardiovascular outcomes under conditions of hormone deficiency.

Ongoing clinical trials

Research to better understand how Ang-(1–7) impacts local and systemic hemodynamics in humans is ongoing (Table 2). The effects of acute intravenous Ang-(1–7) infusion on blood pressure and heart rate, and their variability, are being examined in healthy and hypertensive subjects. Clinical trials are also examining blood pressure responses to acute intravenous Ang-(1–7) infusion in primary autonomic failure patients and in subjects with essential hypertension following ganglionic blockade, to eliminate potential baroreflex interferences in vascular homeostasis. Finally, studies are examining effects of intravenous Ang-(1–7) on brachial artery diameter, blood pressure, and sympathetic activation in obesity hypertension; resting metabolic rate and adipose thermogenesis in obesity; and leg blood flow and systemic inflammatory markers in peripheral arterial disease. These clinical trials will hopefully offer new insight into the connection between Ang-(1–7) and blood pressure control, and inform on the potential for targeting this hormone in cardiovascular-related diseases.

Table 2.

Ongoing clinical trials examining cardiovascular effects of angiotensin-(1–7) pathway activation

Institution	Study Title	Population	Route, Drug	Endpoints	NCT number
Penn State	Cardiovascular Effects of Angiotensin-(1–7) in Obesity	Obesity Hypertension	Intravenous, Ang-(1–7)	Brachial and coronary blood flow, BP, HR, MSNA	NCT03604289
Penn State	Protective effects of Angiotensin-(1–7) in PAD	Peripheral Arterial Disease	Intravenous, Ang-(1–7)	Leg blood flow, inflammatory markers, BP, HR	NCT03240068
Vanderbilt	BP lowering effects of Angiotensin-(1–7) in primary autonomic failure	Primary Autonomic Failure	Intravenous, Ang-(1–7)	BP, HR	NCT02591173
Vanderbilt	Cardiovascular effects of Angiotensin-(1–7) in essential hypertension	Essential hypertension	Intravenous, Ang-(1–7)	BP	NCT02245230
Penn State	Angiotensin-(1–7) and Energy Expenditure in Human Obesity	Obesity	Intravenous, Ang-(1–7)	Energy expenditure, BP, HR	NCT03777215
Vanderbilt	Metabolic Effects of Angiotensin-(1–7)	Obesity	Intravenous, Ang-(1–7)	Insulin sensitivity, BP, HR, cardiac output, stroke volume, vascular resistance	NCT02646475
Instituto de Cardiologia do Rio Grande do Sul	Acute Effect of Angiotensin-(1–7) in Healthy and Hypertensive Subjects	Healthy and Essential Hypertension	Intravenous, Ang-(1–7)	Adverse effects, BP, BP variability, HR, HR variability	NCT3001271
University Hospital Basel	Safety and Tolerability Study of APN01 (Recombinant Human Angiotensin Converting Enzyme 2)	Healthy	Intravenous, rhACE2	Adverse events, vital signs (BP, RR, HR), blood chemistry, ECG parameters	NCT00886353
GlaxoSmithKline	A Dose-escalation Study in Subjects with Pulmonary Arterial Hypertension	Pulmonary Arterial Hypertension	Intravenous, rhACE2	Pulmonary vascular resistance, cardiac output, mean pulmonary artery pressure, adverse events	NCT03177603
Vanderbilt	Hormonal, Metabolic, and Signaling Interaction in Pulmonary Arterial Hypertension	Pulmonary Arterial Hypertension	Intravenous, rhACE2	Sex hormone metabolites, insulin resistance, oxidative stress, triglycerides	NCT01884051
GlaxoSmithKline	The Safety, Tolerability, PK and PD of GSK2586881 in Patients with Acute Lung Injury	Acute Lung Injury and ARDS	Intravenous, rhACE2	BP, HR, ECG parameters, hematology parameters, blood chemistry	NCT01597635

Abbreviations: Ang, angiotensin; ARDS, acute respiratory distress syndrome; BP, blood pressure; ECG, electrocardiogram; GSK2586881, recombinant human ACE2; HR, heart rate; MSNA, muscle sympathetic nerve activity; NCT, https://clinicaltrials.gov registry number; rhACE2, recombinant human ACE2.

Novel therapeutic formulations

Despite limited clinical evidence, Ang-(1–7) remains an attractive therapeutic target. It has been difficult to translate findings from animal models to the clinical setting, however, due to unfavorable pharmacokinetic properties of Ang-(1–7) including a very short half-life due to rapid cleavage by peptidases.⁶⁷ Based on this, novel tools to increase Ang-(1–7) levels or activity have been developed and tested in animal models including MasR agonists, oral Ang-(1–7) formulations, ACE2 activators, and recombinant human ACE2.⁵ Several studies have shown these novel therapeutic formulations reduce blood pressure and attenuate cardiovascular damage in animal models, illustrating their attractiveness for hypertension and cardiovascular diseases.

Mas receptor agonists

Ang-(1–7) elicits vasodilatory and cardioprotective effects via activation of MasR in various tissue beds.⁵ The first MasR agonist synthesized, AVE0991, was initially described in 2002.⁶⁸ This nonpeptide compound mimics effects of Ang-(1–7) on the endothelium, including ~5-fold higher NO release.⁶⁹ Similar to Ang-(1–7), AVE-0991 restores vascular responsiveness in mesenteric, renal, and carotid arteries from diabetic animals.³⁴ AVE0991 also attenuates cardiac remodeling and improves baroreflex sensitivity in renovascular hypertensive rats, attenuates myocardial infarction-induced heart failure in rats, and decreases portal hypertension in animal models of cirrhosis.^25,34,70,71 In addition, AVE0991 exhibits antiatherosclerotic properties by stabilizing plaques and decreasing perivascular and plaque inflammation in early stage atherosclerosis in mice.⁷² More recently, another nonpeptide MasR agonist, CGEN-856S, was shown to lower blood pressure in spontaneously hypertensive rats, and to induce vasodilation in aortic rings and antiarrhythmogenic effects in isolated rat heart.⁷³ MasR agonism, with either AVE0991 or CGEN-856, also prevents myocardial infarction-related injury remodeling, and attenuates myocardial hypertrophy in rats.^25,74

Oral Ang-(1–7) formulations

Recent efforts to improve the pharmacokinetic profile and therapeutic potential have led to testing of Ang-(1–7) encapsulation by hydroxyl-propyl-b-cyclodextrin (HPβCD) to prevent rapid degradation and improve oral bioavailability. HPβCD-Ang-(1–7) reduces blood pressure and heart rate, improves cardiac function and fibrosis, and promotes antithrombotic effects in rat models of hypertension and myocardial infarction.^75–77 Beneficial effects of Ang-(1–7) on cardiac function in postmyocardial infarction rats may involve downregulation of C-X-C chemokine receptor type 4 (CXCR4) pathways.⁷⁸ Cyclic Ang-(1–7), a thioether stabilized analogue, also improves cardiac morphology and function in rats postmyocardial infarction and enhances endothelium-dependent vasodilation in rat aortic rings.^79,80

ACE2 activators

Numerous studies have examined targeting ACE2 levels or activity, as a means to increase Ang II to Ang-(1–7) conversion.⁸¹ In general, ACE2 is protective for cardiovascular function and fluid balance. A decrease or absence of ACE2 in animal models increases circulating Ang II levels and subsequent AT₁R overstimulation to promote cardiovascular and renal pathologies including impaired cardiac contractility and cardiomyopathy.⁸² Conversely, ACE2 overexpression increases Ang-(1–7) and decreases Ang II levels, as well as modifies expression of their respective receptors, leading to antihypertensive effects in mice.⁸¹ ACE2 overexpression also attenuates myocardial damage due to ischemia-reperfusion injury, and improves vasoconstriction and endothelial dysfunction to reduce blood pressure in hypertensive rats.⁸³ Moreover, ACE2 gene therapy prevents cardiac hypertrophy and improves right ventricle function in animal models of pulmonary arterial hypertension (PAH).⁵

Likewise, activation of endogenous ACE2 activity with the small synthetic molecule xanthenone (XNT) elicits a dose-dependent hypotensive response when acutely administered in spontaneous hypertensive rats.⁸⁴ Chronic XNT treatment also improves cardiac function and elicits cardiac and renal antifibrotic effects in animal models.⁸⁴ Moreover, XNT prevents cardiac remodeling in animal models of pulmonary hypertension and diabetes, and blunts thrombus formation in spontaneous hypertensive rats.^85–87 Another ACE2 activator, diminazene aceturate (DIZE), improves renal injuries in renovascular hypertensive rodents.³⁹ DIZE also decreases heart mass and enhances cardiac function and hemodynamics in mice following acute myocardial infarction, to a greater extent than the ACE inhibitor enalapril.^88,89 Furthermore, DIZE inhibits Ang-II-mediated pressor responses and myocardial collagen deposition in mice, with antifibrotic effects mediated in part by downregulation of intermediate-conductance Ca2+-activated K+ channel (KCa3.1).⁹⁰ While ACE2 activators are promising, human translation may be limited due to interspecies biochemical differences in this enzyme.⁹¹ It also remains unclear if beneficial effects of ACE2 activation reflects reduced Ang II versus increased Ang-(1–7) levels, cleavage of other substrates, or a combination of these mechanisms.⁵

Recombinant human ACE2

The use of recombinant human ACE2 (rhACE2) to improve cardiovascular function is currently under investigation in preclinical models and clinical populations. Daily rhACE2 administration attenuates Ang II-induced vascular dysfunction, cardiac hypertrophy, and superoxide production in animal models of hypertension.⁹² In addition, rhACE2 attenuates vascular remodeling and improves right ventricular function in mouse models of PAH. Of particular importance, rhACE2 (GSK2586881) has been used clinically, with intravenous administration shown to be well tolerated and to improve pulmonary vascular resistance and cardiac output in PAH.⁹³ Intravenous rhACE2 also decreases circulating Ang II and inflammatory cytokine levels, and increases Ang-(1–7), in subjects with acute respiratory distress syndrome.⁹⁴ Clinical trials are ongoing to examine the therapeutic potential of rhACE2 in PAH, hypertension, and cardiovascular and renal diseases (Table 2).

Conclusions

Ang-(1–7) is a biologically active RAS component, which antagonizes Ang II actions as well as promotes direct blood pressure lowering effects in animal models via vascular, cardiac, renal, and neural mechanisms. In addition to antihypertensive effects, Ang-(1–7) has antihypertrophic, antiarrhythmogenic, antiatherogenic, antifibrotic, and antithrombotic properties in animal models. A critical gap in knowledge is whether Ang-(1–7) is capable of providing similar cardioprotection clinically for hypertension and cardiovascular diseases. There is limited clinical data for Ang-(1–7) and cardiovascular outcomes, with most studies focused on intra-arterial administration and often showing conflicting results. Novel formulations to enhance stability and activity of Ang-(1–7) have been developed, but there are limited data supporting safety and efficacy of these emerging therapies in clinical populations, with most studies focused on rhACE2. Overall, targeting the vasodilatory arm of the RAS in the setting of hypertension could lead to powerful additional treatment options, but further studies are needed to clarify the role of Ang-(1–7) in cardiovascular control in clinical populations.

Disclosure

The authors declared no conflict of interest.