Letter to the Editor: Angiotensin-converting enzyme 2: an ally or a Trojan horse? Implications to SARS-CoV-2-related cardiovascular complications

to the editor: Emerging clinical reports from China, Italy, and the United States reveal that SARS-CoV-2 is associated with a prominent incidence of cardiovascular morbidities and complications (1, 36), including myocarditis, acute myocardial infarction, and worsening heart failure. These cardiovascular manifestations have been encountered in preceding epidemics of corona viruses, namely Severe Acute Respiratory Syndrome (SARS) and Middle-East Respiratory Syndrome (MERS), as well as during H1N1 influenza outbreaks (1, 36). Moreover, several preliminary reports indicate that a vast majority of patients with comorbidities are prone to SARS-CoV-2 complications (50–86%). Congestive heart failure (CHF), chronic kidney disease (CKD), diabetes, and pulmonary diseases are the principal-identified clinical conditions predisposing to SARS-CoV-2-induced morbidities and mortality (7). Although pneumonia, the principal alarming feature of SARS-CoV-2 infection, could by itself evoke cardiovascular complications as a result of hypoxemia, systemic inflammation and enhanced myocardial oxygen demand, a direct cardiovascular injury, likely develops, initiated by binding of SARS-CoV-2 to angiotensin-converting enzyme 2 (ACE2), widely expressed in myocardial and vascular endothelial cells, leading to adverse cardiovascular consequences. The following short commentary outlines potential mechanisms by which elimination of ACE2 by this virus may lead to deleterious cardiovascular outcome.

ACE2 is a transcellular protein predominantly expressed in the heart, vasculature, kidney, lung, brain, intestine, and testis and is usually located at the apical side of cells attached to basal membrane (Fig. 1) (30). In the heart, ACE2 is widely expressed in all cardiac cell types, including endothelial cells, smooth muscle cells in the myocardial vasculature and in cardiac myocytes (2, 9). ACE2 has 400-fold affinity to angiotensin II (ANG II), as compared with the classic ACE, and it converts ANG II to angiotensin-(1–7) [Ang-(1–7)] (31). The latter short peptide exerts vasodilatory, natriuretic/diuretic, anti-inflammatory, and antifibrotic effects via Mas receptor (MasR). Noteworthy, both clinical (2, 3, 10, 13, 37) and experimental (2) heart failure is characterized by upregulation of cardiac ACE2 and enhanced Ang-(1–7) generation, which may represent a cardioprotective compensatory response aimed at reducing or preventing cardiac remodeling (28) (Fig. 1). In agreement with this notion, targeted overexpression of cardiac ACE2 by applying local injection of lenti-viral vector in Sprague-Dawley normotensive rats significantly attenuated cardiac hypertrophy and myocardial fibrosis induced by prolonged ANG II administration (15). Similarly, overexpression of ACE2 in cardiac tissues of spontaneously hypertensive rats decreased cardiac remodeling in as was evident by reduced left ventricular wall thickness and perivascular fibrosis (6), probably via reduction of collagen production (11). Collectively, these animal studies highlight a cardioprotective role for the ACE2–Ang-(1–7)–MasR axis.

Fig. 1.

The initial step after the invasion of Severe Acute Respiratory Syndrome (SARS)-CoV-2 is binding to membranal angiotensin-converting enzyme 2 (ACE2) widely expressed in cardiac cells including endothelial cells, smooth muscle cells in the myocardial vasculature and in cardiac myocytes. ACE2 is responsible for the conversion of ANG II to Ang-(1–7) that exerts beneficial effects on the cardiac tissue such as vasodilation, antifibrosis, and anti-inflammation via Mas receptor (MasR). The binding of SARS-CoV-2 to ACE2 is preceded by furin-mediated exposure of the viral receptor binding protein (RBP) localized to S-glycoprotein (S1 domain of the viral spike). Furin is abundant in the heart both intracellulary and in the circulation as a free enzyme, making it a key factor in the uncovering of RBP and eventually in SARS-CoV-2 transmission. In addition, furin enhances the affinity of the virus to ACE2 by not only exposing the viral binding site on S1 domain but also revealing the effusion site on the S2 domain in the viral spike. Consequently, the virus undergoes endocytosis and massive replication accompanied by profound activation by cathespsin L (CatL) and the abundant intracellular furin. The activated intracellular SARS-CoV-2 undergoes exocytosis where it binds again to ACE2 elsewhere, thus creating a vicious feed-forward devastating cycle. Importantly, heart failure is characterized by enhanced expression of myocardial ACE2, which is further upregulated by ACE-I, angiotensin receptor blockers (ARBs), and mineralocorticoid-receptor (MR) antagonists, thus sensitizing ACE2 expressing target organs to SARS-CoV-2. ADAM metallopeptidase domain 17 (ADAM 17) is responsible for shading of ACE2, a process stimulated by ANG II type 1 receptors (AT₁-R) and may explain why renin-angiotensin-aldosterone system inhibitors augment ACE2 expression. ER, endoplasmic reticulum.

Binding of the SARS viral spike glycoprotein to ACE2 triggers its internalization along with the virus (13). This might be of supreme importance for cardiomyocytes of patients with heart failure, characterized by intense upregulation of ACE2 (9, 10, 37) (Fig. 1). Possibly, intracellular translocation of SARS-CoV-2 coupled with ACE2 leads to its depletion in cell membranes. It is tempting to assume that consequent ACE2 elimination might participate in many features of acute corona virus infection. Among such clinical characteristics are decompensation of preexisting CHF, respiratory distress irrespective to left-ventricular backward failure (due to impaired pulmonary capillary endothelium and endothelial barrier function), acute kidney failure (reflecting altered renal microcirculation and hypoxic injury), and diarrhea (caused by injured gut microcirculation with hypoxic damage and injured mucosal barrier.

Indeed, corona virus has already been shown to induce myocardial inflammation and dysfunction accompanied with adverse cardiac outcomes in patients with SARS, assumedly due to downregulation/elimination of the myocardial ACE2 system (25). Support for this concept emerges from previous experimental reports demonstrating cardiac contractility defects in rats with reduced X chromosomal-derived ACE2 expression and heart failure with pulmonary congestion in ACE2-knockout mice (ACE2-KO) (4, 34). These undesired changes were prominent in males and progressed with age, coincidentally overlapping the observations that elderly and men are more susceptible to SARS-CoV-2-induced serious infection. Interestingly, the hearts of animals depleted of ACE2 exhibited similar changes that occur after coronary artery disease or bypass surgery in humans (5). Subsequent studies demonstrated extended infarct size, reduced contractility, altered ventricular remodeling, and increased mortality following myocardial infarction (MI) induced by ligation of the proximal LAD in mice with ACE2 deletion, as compared with their wild-type controls (20). Moreover, these mice showed enhanced oxidative stress and concomitant upregulation of proinflammatory cytokines, plausibly parallel to the observed hypersensitive immunological response reported in patients with SARS-CoV-2 infection.

Furthermore, preliminary alarming data from SARS-CoV-2-infected patients suggested that those treated with renin-angiotensin-aldosterone (RAAS) inhibitors such as angiotensin-converting enzyme inhibitors (ACE-I) or angiotensin-receptor blockers (ARBs) experienced severe symptoms with a higher mortality rate as compared with nonuser counterparts (7, 26). Noteworthy, cardiac ACE2 expression is markedly enhanced in response to RAAS blockade by ACEi (24), ARB (8, 18, 19), and even by mineralocorticoid receptor (MR) antagonist (19, 21) (Fig. 1). Conceivably, this is translated into increased vulnerability of patients with RAAS blockade during SARS-CoV-2 infection. Furthermore, several studies have demonstrated that binding of ANG II to its AT₁ receptors in target organs, including the heart, activates ADAM 17, a sheddase affecting ACE2 (22) (Fig. 1). Conclusively, blocking ANG II synthesis or its binding to AT₁ receptors by RAAS inhibitors likely leads to the upregulation of ACE2 and eventually hypersensitizing the heart to SARS-CoV-2 infection. Enhancement of myocardial invasion by the virus due to enhanced ACE2 likely plays an important role.

An additional player that may contribute to the vulnerability of patients with heart failure to SARS-CoV-2 is furin, also termed paired basic amino acid-cleaving enzyme (PACE). Furin is essential for permeating viral functionality as it cleaves viral envelope trimeric transmembrane glycoprotein (S) (12, 14, 29). This S-glycoprotein, vital for the entry of the virus into the cell, contains two functional domains: an ACE2-binding domain (also called receptor-binding domain (RBD) and a second domain essential for fusion of the viral and cell membranes (23, 33, 35). Furin activity exposes the binding and fusion domains essential for the entry of the virus into the cell (32). Furin presents mainly intracellularly and to a lesser extent in the circulation (16) (Fig. 1), where it converts ventricular proBNP to active BNP, an important physiological process in heart failure subjects. Patients with heart failure are specifically characterized by upregulation of cardiac furin, providing an additional potential explanation for their vulnerability Covid-19 infection (17) (Fig. 1). Moreover, furin is detected in circulating T cells that are activated during infections (27). This may form a feed-forward loop of furin-facilitated coronavirus replication that may be responsible for hypersensitive immunological response (cytokine storm) in some patients, leading to fulminant myocarditis, devastating lung injury, and lethal multiorgan failure.

Collectively, evidently ACE2 exerts beneficial effects on cardiac function under normal conditions and particularly in the presence of heart failure. Moreover, some of the cardioprotective effects of ACE inhibitors, ARBs, and MR blockers are mediated by their positive impact on ACE2 abundance in cardiac tissues. Nevertheless, in patients infected with SARS-CoV-2, ACE2 may transform to a Trojan horse. Its binding with ACE2 neutralizes the advantageous cardiac effects of this enzyme, especially in patients with heart failure. The susceptibility of these subjects to life-threatening SARS-CoV-2 infection could be attributed to the simultaneous upregulation of both ACE2 and furin in the diseased myocardium and to the wide use of RAAS inhibitors in this population (Fig. 1). Therefore, temporary blockade of the viral binding site on ACE2 or furin by immunological or pharmacological means in patients infected with SARS-CoV-2 may compose new therapeutic strategies in combating this unprecedented formidable viral threat.

GRANTS

Z. Abassi acknowledges research support from the Israel Science Foundation Grant 544/18.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

Z.A.A. and E.E.K. prepared figure; Z.A.A., S.A., E.E.K., and S.N.H. drafted manuscript; Z.A.A., S.A., E.E.K., and S.N.H. edited and revised manuscript; Z.A.A., S.A., E.E.K., and S.N.H. approved final version of manuscript.