Archives of Medicine and Health Sciences

REVIEW ARTICLE
Year
: 2018  |  Volume : 6  |  Issue : 2  |  Page : 238--246

Angiotensin-converting enzyme 2 and its potential protective effect upon heart


Leta Melaku 
 Department of Biomedical Sciences, College of Health Sciences, Arsi University, Asella, Oromia, Ethiopia

Correspondence Address:
Mr. Leta Melaku
Department of Biomedical Sciences, College of Health Sciences, Arsi University, Asella, Oromia
Ethiopia

Abstract

To circumvent the major threats of low blood volume and low blood pressure, animals need powerful mechanisms for salt and water conservation, which is renin–angiotensin system (RAS). Activation of the RAS is, therefore, a useful response in many demanding situations. However, increased activity of the RAS, especially in combination with other cardiovascular risk factors, may lead to a cascade of deleterious effects such as hypertension, atherosclerosis, myocardial remodeling, heart failure, ischemic stroke, and diabetes mellitus. Although many pathophysiological actions of angiotensin (Ang) II may still be viewed as being homeostatic in principle, its over-activation can be detrimental.. Numerous experimental studies have indicated that angiotensin-converting enzyme (ACE) 2 efficiently hydrolyzes the potent vasoconstrictor Ang II to Ang 1–7. Thus, the axis formed by ACE 2/Ang 1–7/Mas appears to represent an endogenous counterregulatory pathway within the RAS, the actions of which are in opposition to the vasoconstrictor/proliferative arm of the RAS consisting of ACE, Ang II, and AT1 receptor. Although most of the well-known cardiovascular and renal effects of RAS are attributed to ACE, an important enzyme in the generation of Ang II, much less is known about the function of ACE 2. This review summarizes the recently published data on basic properties of ACE 2 and Ang 1–7 and the evidence from experimental and clinical studies of various pathological conditions related to the biological roles of ACE 2/Ang 1–7/Mas in the heart.



How to cite this article:
Melaku L. Angiotensin-converting enzyme 2 and its potential protective effect upon heart.Arch Med Health Sci 2018;6:238-246


How to cite this URL:
Melaku L. Angiotensin-converting enzyme 2 and its potential protective effect upon heart. Arch Med Health Sci [serial online] 2018 [cited 2020 Jan 20 ];6:238-246
Available from: http://www.amhsjournal.org/text.asp?2018/6/2/238/248666


Full Text



 Introduction



To circumvent the major threats of low blood volume and low blood pressure, animals and our ancestors, with a diet relatively poor in sodium, needed powerful mechanisms for salt and water conservation, and these organisms relied heavily on one of the oldest hormone systems called the renin–angiotensin system (RAS).[1] ACE 2, as human new homolog of angiotensin-converting enzyme (ACE), was discovered by two independent research groups in 2000.[2],[3] ACE 2 (ACE-related carboxypeptidase or ACE homolog) is a monocarboxypeptidase type 1 transmembrane protein that contains 805 amino acids and it has an extracellular (ecto) domain (amino acids 18–739), a transmembrane region (amino acids 740–768), and an intracellular tail. The extracellular part of ACE 2 [Figure 1] contains the catalytic domain (amino acids 147–555), which has a substrate-binding region (amino acids 273–345) and a typical HEMGH metalloproteinase zinc-binding site (amino acids 374–378).[4],[5],[6],[7] The catalytic domain of ACE 2 is 42% identical to that of ACE.[3] The peptidase activity of ACE 2 is dependent on the C-terminus sequence of the substrate (sequence specificity). The C-terminal part of ACE 2 (614–805) is homologous (48% identity) to a transporter protein known as collectrin.[5],[8] ACE 2 substrates generally have a hydrophobic or basic residue at the C-terminal end, preceded by a Pro-X-Pro motif, where either one of the two proline residues is sufficient to allow ACE 2-dependent hydrolysis.[9] In this circumstance, ACE 2 displays potent peptidase activity to angiotensin (Ang) II (Pro-Phe), Ang I (Pro-Phe-His-Leu), and des-Arg9-bradykinin (BK) (Ser-Pro-Phe) but shows no activity toward Ang 1–9, Ang 1–7, or BK[10] [Figure 2]. ACE 2 can also hydrolyze other bioactive peptides, such as apelin-13, β-casomorphin, dynorphin A 1–13, and ghrelin.[10] Within the RAS, ACE 2 competes with ACE because it is capable of hydrolyzing the inactive decapeptide Ang I into the nonapeptide Ang 1–9, thus decreasing the amount of Ang I available for pressor Ang II generation by ACE. To the same extent, ACE 2 degrades the vasoconstrictor Ang II into Ang 1–7, which is the most important active product.[11],[12] The Ang 1–7 can be primarily generated via two routes. First, both ACE 2 and prolylcarboxypeptidase can directly hydrolyze Ang II to yield Ang 1–7; second, this monocarboxypeptidase can also remove the amino acid leucine from the C-terminus of Ang I to form the biologically active peptide Ang 1–9,[3],[13] which is then cleaved by either neprilysin (NEP) or ACE to yield Ang 1–7[6],[14],[15] [Figure 2]. The heart, brain, and kidney are major sources of Ang 1–7 production.[14]{Figure 1}{Figure 2}

In the human coronary circulation, NEP seems to have a more prominent role in Ang 1–7 production than ACE 2.[16] Pharmacokinetic experiments have determined that, in humans, Ang 1–7 has a short half-life of ~0.5 h.[17] Following subcutaneous injection, the peptide is quickly available in the blood and reaches its peak plasma concentration at ~1 h.[17] In rats, the plasma half-life of Ang 1–7 is only 9 s.[18] It has been established that Ang 1–7 binds to a non-Ang II type 1/Ang II type 2 receptor originally identified as the Mas oncogene receptor[19] and mediates vasodilatation; myocardial protection; antiarrhythmic at low concentration, antihypertensive, and positive inotropic effects; and inhibition of pathological cardiac remodeling within the cardiovascular system.[15],[20],[21] In addition, it is thought to have favorable effects on metabolism by lessening insulin resistance.[22],[23],[24] Although most effects are protective, some seem to be variable. Many of the diverse actions of Ang II, the major end product of the RAS, can be viewed in a single conceptual framework, as serving to prevent life-threatening shrinkage of intravascular volume (rapid actions of Ang, in combination with the sympathetic nervous system), to help maintain volume homeostasis by minimizing the changes in arterial pressure and fluid volumes required to achieve sodium balance (prevention of salt sensitivity), and to increase the efficiency of cardiovascular dynamics by promoting the growth of the heart and vessels and sensitizing blood vessels to vasoconstrictor agents (slowest actions of Ang) through its coordinated effects on the heart, blood vessels, kidneys, and nervous system.[25],[26] Activation of the RAS is therefore a useful response in many demanding situations. However, an increased activity of the RAS, especially in combination with other cardiovascular risks factors, may lead to a cascade of deleterious effects such as hypertension, atherosclerosis, myocardial remodeling, heart failure, ischemic stroke, and diabetes mellitus.[27],[28] Many of these pathophysiological actions of Ang II may still be viewed as being homeostatic in principle, but harmful if carried to excess. Thus, ACE 2 may have a role to counterbalance the action of ACE in producing the vasoconstrictor Ang II, leading to have protective effects in various tissues and to prevent overactive RAS-associated diseases, including hypertension.[29],[30],[31],[32],[33] The affinity of ACE 2 to Ang II (Km= 2.0 μmol/l, which represents the concentration of substrate required for the enzyme to achieve half maximum catalytic velocity; that is, the higher the Km value, the lower the affinity) is higher than to Ang I (Km= 6.9 μmol/l). The ACE 2 catalytic efficiency for Ang II is >300 times that for Ang I.[10]

A newly described RAS component, ACE 2, has been characterized recently in humans[2],[3] and in mice.[34] In humans, ACE 2 was found at various levels in 72 tissues that also express ACE mRNA,[35] and it is highly expressed in kidney, blood vessels, heart, lung, brain, and testis.[2],[3],[36],[37] Interestingly, it has also been reported to be localized in glucose-regulating tissues such as pancreas, including β-cells,[38],[39],[40] adipose tissue,[36] and liver.[41]

 The Cardiac Action of the ACE 2



The Components of Local Cardiac

For a number of years, ACE and its main biologically active peptide Ang II have assumed a central position in the cardiac RAS.[43] With the discovery of ACE 2, a new regulator entered within the established metabolic RAS pathways.[44] The presence and synthesis of RAS components in the heart suggest that locally produced bioactive Ang peptides modulate cardiac structure and function.[43],[44] Components of the local cardiac RAS are heterologously distributed on different cell types within the heart.[45] For instance, angiotensinogen (AGT) is primarily distributed in the atrial muscle and the neuronal fibers of the conduction system, with small amounts in the subendocardial region of the ventricle.[46] In contrast, ACE is primarily expressed by coronary endothelial cells and cardiac fibroblasts.[46] In addition, ACE expression can be detected in all four heart valves, coronary blood vessels, aorta pulmonary arteries, endocardium, as well as epicardium.[47],[48] However, ACE 2 is localized to the endothelium and smooth muscle cells of most intramyocardial vessels, including capillaries, venules, and medium-sized coronary arteries and arterioles.[49] Furthermore, ACE 2 protein expression was detected in cardiac myocytes from failing human hearts.[49] Ang I is extensively metabolized during a single pass through the coronary bed, leading to the generation of Ang II, Ang III, Ang IV, and Ang 1–7 in isolated hearts from normal[50],[51] and diabetic rats.[51] As a result of its hydrolyses from Ang II into Ang 1–7 than Ang I into Ang 1–9,[11],[12] recent studies report that ACE 2 is an important regulator of cardiac pathophysiology.[52],[53] However, it should be stressed that the role of ACE 2 in heart function and structure might depend on the species.[54] Interestingly, ACE 2 expression has been reported to be increased in failing human heart ventricle.[49],[55],[56] Nevertheless, there are contrasting findings in rat hearts. While an increase of both ACE and ACE 2 was found by Burrell et al.[49] in hearts from Sprague–Dawley rats after myocardial infarction, Ishiyama et al.[57] observed an increase in ACE 2 expression only after AT1 blockade in Lewis normotensive rats. These divergent results further suggest that ACE 2 effects are strain dependent. ACE 2 gene transfection using lentiviral vectors significantly attenuated cardiac damage in spontaneously hypertensive rats (SHRs)[58] and in Ang II-infused Sprague–Dawley rats.[59] Further, the stage of the disease apparently influences the expression of ACE 2.

At the early phase of myocardial infarction, ACE 2 activity in the plasma and left ventricles is increased in rats while the plasma and left ventricular ACE 2 activities and mRNA levels are lower than in controls at 8 weeks postinfarction.[13] Similar findings were observed regarding the cardiac expression of Mas, i.e., its changes depend on the nature and duration of the physiological and pathological stimuli.[60] Ang 1–7, which is also one of the components of RAS, is present within the hearts. The localization and local generation of Ang 1–7 have been demonstrated within aortic root, coronary sinus, and right atrium of dogs at basal conditions, and its levels were markedly reduced following treatment with the ACE inhibitor CGS-14831.[61] In addition, immunohistochemical staining revealed that Ang 1–7 is expressed in rat cardiac myocytes[62] and sinoatrial node cells.[63] Of note, the Ang 1–7, Ang 1–7 receptor Mas, mRNA, and protein of Ang 1–7 are localized in human cardiac tissues.[35],[63],[64] It is important to note that although all the components of RAS are present in the heart, not all of them are believed to be synthesized in the heart. For example, the question whether renin is synthesized in heart or is derived primarily from circulation remains still unresolved.[65]

The role of ACE 2 on conductivity of the heart

In several published studies, Ang II has been implicated in conduction abnormalities although some results appear contradictory. Slowed conduction was associated with increased myocardial and plasma ACE activity. Moreover, administration of an ACE inhibitor improved conduction velocities in cardiomyopathy using a Syrian hamster model.[66],[67],[68] These observations suggest that Ang II slows cardiac conduction. This conclusion is further supported by the findings of slowed ventricular conduction in mice overexpressing the AT1 receptor.[69] However, in contrast, in cardiac myocyte cultures, Ang II stimulated an increase in connexion 43, a protein implicated in the upregulation of cardiac conduction,[70] implying that Ang II may accelerate cardiac conductance. Interestingly, in ACE 2-null mice, elevated levels of Ang II did not affect normal conductivity, and the mice appear to have a normal life span, at least under nonstress laboratory conditions.[71]

The question whether cardiac conduction is, in fact, influenced by the RAS under physiological condition was examined, and it has been demonstrated that Ang 1–7, a main product of ACE 2 enzymatic activity in the heart, decreases the incidence and duration of ischemia–reperfusion arrhythmias in isolated perfused rat hearts,[63] apparently involving activation of the sodium pump.[72] These effects were abolished by ouabain.[72] In addition, Ang 1–7 decreased total (Na+, K+, Mg2+)-ATPase activity in sheep atrium.[73] Further, the antiarrhythmogenic effect of Ang 1–7 was blocked by the Ang 1–7 antagonist A-779 and by the cyclooxygenase inhibitor indomethacin.[74] This peptide also improved postischemic contractile function in isolated heart preparations by a mechanism involving Mas and the release of BK and prostaglandins.[75] However, at concentrations nearly 10,000-fold higher, Neves et al.[76] found that Ang 1–7 facilitated reperfusion arrhythmias in isolated perfused rat hearts. In keeping with these latter data, transgenic mice overexpressing ACE 2 in the heart presented sudden death due to cardiac arrhythmias.[71] These observations suggest that only very high local concentrations of Ang 1–7 exert deleterious effects in the heart possibly through activation of NADPH oxidase[77] or release of norepinephrine.[78] In fact, transgenic rats presenting a local increase of Ang 1–7 of up to 20-fold in the heart did not show any sign of arrhythmias.[79]

The role of ACE 2 on contractility of the heart

Although hearts from young ACE 2-mutant mice are functionally normal, hearts of old ACE 2-deficient mice in this particular mouse background display a reduction in cardiac contractility as demonstrated by 40% reduction in fractional shortening and velocity of circumferential shortening (heart rate corrected) with slight ventricular dilation.[52] The significance of ACE 2 in regulating cardiac function is further highlighted by the thinning of the left ventricular wall in aged ACE 2-mutant mice. This progressive cardiac dysfunction occurred without myocardial fibrosis or hypertrophy and in the absence of the myosin heavy chain isoform switches typically found in other animal models of heart failure. Thus, one may speculate that the observed phenotype closely resembles the defective heart found in patients with cardiac stunning/hibernation.[80] Cardiac stunning and hibernation reflect adaptive responses to prolonged tissue hypoxia that occurs in coronary artery disease or after bypass surgery.[81]

In these human diseases and related animal models, chronic hypoxic conditions lead to compensatory changes in myocyte metabolism,[82] upregulation of hypoxia-induced genes,[83] and reduced heart function.[84] Accordingly, the hearts of ACE 2-null mice show upregulation of mRNA expression of hypoxia-inducible genes, such as BNIP362 and PAI-1.[84] The magnitude of increased expression of these hypoxia-inducible genes resembles previously observed levels in other hypoxic models such as the myocyte-specific vascular endothelial growth factor mutant mice.[85] However, the link between cardiac stunning/hibernation and the heart defect observed in ACE 2-knockout mice has to be investigated further. Whether ACE 2 expression levels indeed change under conditions of hypoxia remains to be demonstrated. ACE 2-knockout mice show also increased local heart Ang II levels.[52] Interestingly, both the cardiac phenotype and increased Ang II levels were completely reversed by additional deletion of ACE gene (i.e., ablation of ACE expression on an ACE 2-mutant background abolished the cardiac dysfunction phenotype of ACE 2 single-knockout mice).[52] The heart function of ACE/ACE 2 double-mutant mice was similar to that in ACE single-mutant and wild-type littermates. The normal cardiac functions of ACE/ACE 2 double-mutant mice suggest that the catalytic products of ACE account for the observed contractile impairment of old ACE 2 single-mutant mice. These observations for the first time demonstrated at the genetic level that ACE 2 counterbalances the enzymatic actions of ACE. It seems that increased local cardiac Ang II might have been the cause for the cardiac abnormalities in ACE 2-deficient mice. However, it remains unclear why despite the elevated plasma and heart Ang II levels, the heart of the ACE 2-deficient mice did not show any evidence for cardiac hypertrophy. In fact, it is well established that cardiac myocytes express Ang II receptors and undergo hypertrophy in response to Ang II. However, in vivo, elevated cardiac Ang II levels alone do not directly induce cardiac hypertrophy but do increase interstitial fibrosis.[86] Thus, it is important to note that Ang II-independent pathways could also play an important role in ACE/ACE 2-regulated heart function. Apparently, the generation of Ang 1–7 directly from Ang II through the cleavage of the C-terminal amino acid phenylalanine by ACE 2 is physiologically and biochemically more relevant.[10] According to Loot et al.,[87] chronic infusion (8 weeks) of Ang 1–7 improved coronary perfusion and preserved cardiac function in an experimental rat model of heart failure induced by ligation of the left coronary artery. The vascular endothelial dysfunction observed in the aortic rings from rats with myocardial infarction was also reversed by chronic infusion of Ang 1–7.[87]

In addition, Ang 1–7 immunoreactivity was significantly increased in the tissue surrounding the infarct area of rat hearts with myocardial infarction[62],[88] published the first study demonstrating that the compound AVE 0991 is a nonpeptide and orally active Ang 1–7 receptor agonist that mimics the Ang 1–7 effects in bovine endothelial cells. Pinheiro et al.[89] and Lemos et al.[90] reported that this compound acts as a Mas agonist in the kidney and isolated aortic rings, respectively. Another study also revealed that AVE 0991 preserves cardiac function and attenuates the development of hypertrophy and fibrosis in hearts from rats chronically treated with isoproterenol.[91] This nonpeptide Ang 1–7 analog also significantly improved the cardiac function in hearts subjected to myocardial infarction and preserved the myocardium after ischemia.[92] Furthermore, long-term treatment with AVE 0991 prevented the end-organ damage in hearts from SHRs treated with NG-nitro-L-arginine methylester (L-NAME).[93] Recently, it has been shown that the inclusion of Ang 1–7 into the cavity formed by the oligosaccharide hydroxypropyl β-cyclodextrin (HPβCD) could protect the peptide during the passage through the gastrointestinal tract. Taking advantage of this formulation, Marques et al.[94],[95] found that chronic oral administration of HPβCD/Ang 1–7 significantly attenuated the impairment of heart function and cardiac remodeling induced by isoproterenol treatment and myocardial infarction in rats. The actions of Ang 1–7 in coronary vessels include biochemical and functional alterations, leading to vasodilatation either directly in artery rings or indirectly through BK potentiation or by opposing Ang II actions.[96] In isolated canine coronary artery rings precontracted with the thromboxane A2 analog, U46619, Ang 1–7 elicited a dose-dependent vasorelaxation, which was completely blocked by the nonselective Ang II antagonist (Sar1, Thr8)-Ang II, but not by the selective AT1 or AT2 antagonists, CV11974 and PD 123319, respectively.[97] This heptapeptide induced a concentration-dependent dilator response in porcine coronary artery rings, which were markedly attenuated by nitric oxide (NO) inhibition.[98] However, Gorelik et al.[99] observed a vasodilator effect of Ang 1–7 only in BK-stimulated pig coronary artery rings. Furthermore, Ang 1–7 elicited an increase in the vasodilator effect of BK in isolated perfused rat hearts. This effect was dependent on Mas and NO and prostaglandin release.[100] Ang 1–7 also evoked vasodilation in isolated perfused mouse hearts involving interaction of Mas with AT1- and AT2-related mechanisms.[101] Together, these data suggest that Ang 1–7 is a vasorelaxant peptide in the coronary bed and that this effect involves coupling to Mas and release of NO and prostaglandins.

Nevertheless, because Neves et al.[76] found that, at high concentrations (>25 nM), Ang 1–7 induces a concentration-dependent decrease in the coronary flow in isolated perfused rat hearts, it remains to be demonstrated whether Ang 1–7 directly causes vasodilation in the coronary bed. This effect was not accompanied by consistent changes in contraction force and heart rate. A similar finding was observed in isolated hamster hearts.[102] The other most important beneficial effect of Ang 1–7 is its ability to regulate the expression of extracellular matrix proteins and cardiac remodeling. Iwata et al.[103] reported that Ang 1–7 binds to isolated adult rat cardiac fibroblasts, which play a critical role in cardiac remodeling. Treatment of these cells with Ang 1–7 inhibited Ang II-induced increases in collagen synthesis [Figure 3]. Importantly, deletion of Mas produced impairment of cardiac function associated with a significant increase in collagen type 1, 3 and fibronectin content in the heart.[104],[105] On the other hand, Ang 1–7 also attenuated either fetal bovine serum-stimulated or endothelin 1-stimulated 3H-leucine incorporation into isolated neonatal rat cardiac myocytes through a mechanism involving inhibition of serum-stimulated ERK 1/2 MAP kinase activity and activation of Mas.[106] Chronic administration of this peptide significantly attenuated left ventricular hypertrophy and fibrosis in pressure-overloaded rats[107] and fibrosis in Ang II-infused and deoxycorticosterone acetate–salt rats.[108],[109] In addition, these animals showed a slight, but significant, increase in daily and nocturnal dP/dt, more resistance to isoproterenol-induced cardiac hypertrophy, reduced duration of reperfusion arrhythmias, and improved postischemic function in isolated perfused hearts,[110] further supporting a beneficial role for Ang 1–7 in cardiac function at physiological concentrations. Altogether, these findings indicate that the ACE 2/Ang 1–7/Mas axis is a functional cardioprotective arm of the RAS [Figure 4]. The signal transduction pathways following activation of Mas in the heart are not fully characterized but probably involve release of prostacyclin and/or NO release[74],[100],[101] since Ang 1–7 stimulated NO production and activated endothelial NO synthase and Akt in cardiomyocytes.[60] Of note, the antihypertrophic effects of Ang 1–7 on Ang II-treated cardiomyocytes were prevented by the blockade of the NO/ cyclic guanosine monophosphate (cGMP) pathway.[111] Moreover, amplification of the actions of BK[99],[100] and decrease of Ang II levels in the heart[112],[113] may also be possible mechanisms involved in the beneficial cardiac effects of Ang 1–7.{Figure 3}{Figure 4}

 Conclusion



ACE 2 is a new component of the RAS. This transmembrane protease has emerged as a negative regulator of the RAS that counterbalances the multiple functions of ACE. Because ACE 2 efficiently hydrolyzes the potent vasoconstrictor Ang II to Ang 1–7, this has changed overall perspective about the classical view of the RAS, because it represents the first example of a feed-forward mechanism directed toward mitigation of the actions of Ang II. Ang 1–7 appear to play a central role in the RAS because it exerts a vast array of actions, but many of them are opposite to those attributed to the main effector peptide of the RAS, Ang II. It is now generally accepted that the RAS is dual and that, besides the well-known mainly deleterious arm if stimulated excessively (ACE/Ang II/AT1), there is a second beneficial axis consisting of ACE 2, Ang 1–7, and Mas to re-correct the deleterious arm. A summary of the evidence from both experimental and clinical studies shows that the overall biological role of ACE 2/Ang 1–7/Mas axis has a protective role within normal function of heart [Figure 5]. Therefore, the development of drugs that could activate ACE 2 function would allow extending treatment options in hypertension, heart failure, and other cardiovascular diseases.{Figure 5}

Financial support and sponsorship

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Conflicts of interest

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