Over the two decades since its discovery in 2000 [1,2], angiotensin-converting enzyme 2 (ACE2) has been shown to protect against certain actions of the classical renin-angiotensin system (RAS) (Fig. 1). The RAS is physiologically essential, but contributes to the pathogenesis of many diseases. The RAS is composed of an ACE, angiotensin II (Ang II) and the angiotensin type 1 receptor (AT1R). Suppression of the ACE-Ang II-AT1R axis by ACE inhibitors/AT1R blockers (ACEi/ARBs) has become the “dogma” of management for patients with kidney and cardiovascular diseases [3-6]. In contrast, the ACE2-angiotensin-(1-7) (Ang-(1-7))-Mas receptor (MasR) axis largely mitigates the consequences of AT1R activation, thus counterbalancing activation of the classical RAS [7]. Thus, blockade of the classical RAS via concurrent activation of the ACE2-Ang-(1-7)-MasR axis has emerged as an attractive therapeutic strategy [8-11]. The evidence indicates that ACEi/ARBs enhance ACE2-Ang-(1-7)-MasR axis activity [12-15], although the agents do not directly target ACE2, Ang-(1-7), or MasR, further suggesting that ACEi/ARBs are valuable for patients with kidney or cardiovascular diseases.

This dogma, however, is being threatened by the viral pandemic coronavirus disease 2019 (COVID-19). Structural studies [16-18] have shown that the causative virus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), utilizes airway epithelial ACE2 as the receptor for entry into the human host. It has been suggested that ACEi/ARBs may increase vulnerability to SARS-CoV-2 infection by upregulating the viral ACE2 receptor [19-21], triggering an intense debate as to whether ACEi/ARBs should be continued or stopped in patients with COVID-19. Indeed, clinicians have been forced to continue or discontinue ACEi/ARBs in patients with COVID-19 and underlying kidney and/or cardiovascular diseases, without reliable evidence for such decisions.

In this review, the molecular structure, function (as an enzyme of the RAS), and distribution of ACE2 will be discussed. We will explore the role played by ACE2 in kidney diseases, highlighting studies that have demonstrated the benefits afforded when ACEi/ARBs activate the local ACE2-Ang-(1-7)-MasR axis. The roles played by ACE2 in cardiovascular and pulmonary diseases will also be summarized. Finally, the differing opinions on the use or disuse of ACEi/ARBs in patients with COVID-19 will be reviewed by drawing on the evidence that has accumulated to date; decision-making must be reasonable.

More than a century after the discovery of renin, ACE2 was near-simultaneously discovered by two research groups in 2000 [1,2]. Its enzymic properties were soon established [22] and the ACE2-Ang-(1-7)-MasR axis defined [23]. The human ACE2 gene is located in chromosome Xp22 and is 40 kb in size. The gene features 18 exons, most of which resemble those of ACE [2]. As is true of ACE, the N-terminus of ACE2 is a zinc metalloprotease domain exposed to the extracellular surface. Structurally, ACE exhibits two enzymatically active sites, whereas ACE2 has only one (Fig. 2). A major difference between ACE and ACE2 lies in the distinct substrate specificities of the N-terminal domains. ACE cleaves C-terminal dipeptide residues from susceptible substrates (and is thus a peptidyl dipeptidase), converting Ang I to Ang II [22,24] and bradykinin to inactive metabolites [25]. Ang-(1-7) is also a substrate of ACE, and is converted to an inactive metabolite, Ang-(1-5), by the enzyme [26,27]. In contrast, ACE2 is a simple carboxypeptidase that hydrolyzes Ang I and Ang II to Ang 1-9 and Ang-(1-7), respectively, although the affinity for Ang II is 400-fold greater than that for Ang I [24]. ACE2 does not cleave bradykinin. Conventional inhibitors of ACE, including ramipril, block the enzymatic activity, but ACE2 is insensitive to this class of agents [22]. The C-terminus of ACE2 is a transmembrane domain with a cytosolic tail lacking any similarity to ACE. This is termed the collectrin-like domain, being a homolog of collectrin, a protein expressed in the kidney, and regulates the trafficking of amino acid transporters to the cell surface, conferring a unique function on ACE2. Unexpectedly, ACE2 serves as the host receptor for coronavirus. Interaction between the receptor-binding domain of the viral spike protein and the protease domain of the host ACE2 was shown, in the early 2000s, to be essential for entry of the SARS-CoV virus and, more recently, SARS-CoV-2 [16,17,28,29]. Compared to SARS-CoV, several mutations in amino acid residues in the interface between SARS-CoV-2 and ACE2 are evident; some may strengthen the interactions between SARS-CoV-2 and ACE2 but others may reduce the affinity by negatively affecting hydrophobic interactions and salt bridge formation [17], although the overall binding affinity of SARS-CoV-2 to ACE2 is 10- to 20-fold higher than that of SARS-CoV [29], which may partly explain the persistence of COVID-19.

Renin converts angiotensinogen to Ang I, which is subsequently cleaved by ACE to form Ang II (Fig. 1). Ang II is a major effector molecule of the classic RAS, and binds to its cognate G-protein-coupled receptor (GPCR), termed AT1R. As AT1R activation mediates vasoconstrictive, proliferative, proinflammatory, and profibrotic processes, receptor antagonists such as ACEi/ARBs have become the cornerstone of kidney and cardiovascular disease therapeutics [4-6,30]. ACE2 cleaves Ang II to Ang-(1-7), which binds to another GPCR (MasR) [7,23,31]. As MasR activation abrogates the pathogenic processes mediated by AT1R, the ACE2-Ang-(1-7)-MasR axis essentially counterbalances the actions of the classical RAS, preventing the organ damage that will be discussed later. Conversely, RAS blockade by ACEi/ARBs upregulates ACE2 expression, although the precise mechanism was long elusive despite robust evidence from several animal studies [15,32-34]. However, the role played by the TNF-α converting enzyme (TACE) in cleavage of the ACE2 ectodomain was then discovered [35]. Using p47^phox-/- mice, in which the p47^phox subunit of nicotinamide adenine dinucleotide phosphate plays a crucial role in the superoxide generation induced by Ang II, it was found that oxidative stress followed by RAS activation enhanced TACE expression/activity via phosphorylation of p38 mitogen-activated protein kinase, to cleave ACE2 from cardiomyocytes (Fig. 3). Specific deletion of Tace from the myocardium prevented ACE2 shedding despite Ang II infusion [35].

The role of the angiotensin type 2 receptor (AT2R) is related to that of the ACE2-Ang-(1-7)-MasR axis, but is poorly understood (Fig. 1). AT2R is a receptor for Ang II, but the consequences of AT2R activation are opposite to those of AT1R activation. The anti-inflammatory vasoprotective effect of AT1R blockade by valsartan was significantly attenuated in AT2R knockout (KO) mice, implying that AT2R stimulation after AT1 blockade is important in terms of vascular protection [36]. Furthermore, Ang-(1-7) seems to act as an AT2R agonist, as best illustrated by the report that the anti-atherosclerotic effect of Ang-(1-7) in ApoE^-/- mice was abolished by chemical inhibition of AT2R. AT2R blockade also revealed that the anti-hypertensive effect of Ang-(1-7) was largely mediated by AT2R rather than MasR [37,38]. More recently, an organ-specific protective role for ATR2 has been implied by a series of studies using a non-peptide AT2R agonist [39-42]. AT2R activation in the proximal tubules prevented sodium retention via internalization/inactivation of the major sodium transporters, and reduced the blood pressure of hypertensive rats [42]. As AT2R also exerts anti-inflammatory effects in the kidney [41], AT2R stimulation significantly ameliorated renal pathology in a rodent model of type 1 diabetes mellitus (T1DM). Thus, together with MasR, AT2R seems to be the principal receptor of an alternative, counterbalancing arm of the RAS. Organ-specific functions by the site of AT2R expression, and AT2R functions that differ from those of MasR, require further study.

Northern blotting initially implied that ACE2 expression was restricted to the heart, kidney, and testis [2]. However, later studies expanded the anatomical distribution. Lung type 2 pneumocytes and the endothelial cells (ECs) and vascular smooth muscle cells (VSMCs) of various organs express ACE2 [43]. The epithelial cells of intestinal villi also express ACE2, which regulates amino acid transport [44]. ACE2 is normally expressed by the hepatocytes of healthy humans, and also in bile duct epithelial cells and sinusoidal ECs of patients with cirrhotic liver disease [45]. In contrast, direct evidence of ACE2 expression in the human brain remains lacking, although ACE2 expression was reported in a subset of mouse paraventricular neurons [46]. Most importantly, with the emergence of COVID-19, data based on single-cell transcriptome analysis of ACE2 expression are growing explosively [47-49], yielding many high-quality insights that will revolutionize our knowledge of ACE2 expression.

The SARS-CoV-2 was first identified in late 2019, but lies on a continuum shared by two other highly pathogenic human coronaviruses (CoVs) described during the past two decades, thus SARS-CoV and Middle East respiratory syndrome (MERS)-CoV [114]. During the first SARS-CoV epidemic, human ACE2 was identified as the receptor for the surface spike protein (S protein) of SARS-CoV [108,109]. It remains unclear whether SARS-CoV-2 infection changes ACE2 expression; recent studies found that ACE2 expression increased after infection [115], and suggested that this was triggered by induction of interferons when the cell detected viral entry [49]. Any concern that ACEi/ARBs use might increase vulnerability to SARS-CoV by upregulating the viral ACE2 receptor was not marked prior to the emergence of COVID-19, probably because most studies on the effects of ACEi/ARBs on ACE2 expression were reported thereafter.

Initial reports indicate that comorbidities are very common in patients infected with COVID-19; these include hypertension, diabetes, coronary artery disease, and CKD [116-118]. Moreover, the frequency of pre-existing underlying conditions is considerably higher in patients exhibiting more severe clinical courses of COVID-19 infection, compared to patients with mild clinical courses [119]. Soon after identification of ACE2 as the receptor for SARS-CoV-2 [16-18], it was suggested that continued use of ACEi/ARBs in patients with underlying diseases would facilitate SARS-CoV-2 infection and increase the risk of severe disease and a fatal outcome [19]. Others later raised similar concerns [20,21,120], triggering an intense debate on continued use or cessation of ACEi/ARBs in patients with COVID-19 and underlying cardiovascular or kidney diseases. In fact, a recent letter reported that many patients from South America, Central America, and Spain, have already stopped or intend to interrupt their treatments with such drugs [121].

It should be emphasized; however, that, although these concerns have been (mostly) expressed by experts, they are grounded on very weak scientific evidence (Table 1) [122-129]. To date, no clinical evidence strongly recommends either the cessation or continued use of ACEi/ARBs in patients infected with COVID-19 [28,130-132]. Rather, the evidence that is available favors continued use of ACEi/ARBs in patients with COVID-19 (Table 2) [133-135], although one study suggested that ACEi/ARBs increased the risk of acute kidney injury in patients with severe COVID-19 infections [136]. For example, inhibition of AT1R by losartan attenuated SARS-CoV-induced acute lung injury in mice [108], and SARS-CoV infection downregulated ACE2 expression in mouse lungs and cultured cells, implying that uncontrolled RAS activation plays a crucial role in the pathogenesis of SARS-CoV-induced acute lung injury. One study enrolling a small number of COVID-19 patients reported that Ang II plasma levels were linearly associated with the viral load and extent of lung injury, and the authors even suggested that ARBs should be repurposed to treat COVID-19 [137]. More recently, a large retrospective multicenter study including 1,128 COVID-19 patients with hypertension on ACEi/ARBs showed that inpatient use by COVID-19 patients lowered all-cause mortality [133]. Despite potential biases attributable to extrapolation of data from mice infected with SARS-CoV, those of studies with limited numbers of patients, and those of retrospective analyses, the evidence that ACEi/ARB therapies are dangerous seems to be balanced by evidence for the opposite view. The fears are groundless. Appropriately, learned societies have recommended continuation of RAS antagonist therapies in COVID-19 patients with underlying conditions such as HF, hypertension, ischemic heart disease, or kidney disease. Unless concrete evidence supporting ACEi/ARB cessation is available; treatment should be individualized by reference to patient hemodynamic status and clinical presentation [131,132].

In the time since ACE2 was discovered, our understanding of how the RAS affects kidney and cardiovascular disease progression has expanded. Over the last two decades, the protective role played by ACE2 (in terms of antagonizing activation of the classical RAS axis) has been demonstrated in many clinical and experimental settings. The positive effects of ACEi/ARBs on ACE2-Ang-(1-7)-MasR axis activation encouraged their prescription for patients with kidney and cardiovascular diseases, until COVID-19 appeared. The human virus receptor is ACE2; the previously unchallenged utilities of ACEi/ARBs have thus been challenged. It has been suggested that ACEi/ARB use may increase the risk of SARS-CoV-2 infection by upregulation of the ACE2 receptor; however, the scientific evidence is minimal. Much accumulated evidence to date indicates that SARS-CoV-2 infection does not imply that ACEi/ARB therapy should cease in patients conventionally indicated for such drugs. Planned directed trials [122,138] will soon guide clinical decision-making in the ACEi/ARB context in COVID-19-infected patients.