ACE2 Regulation and Cardiovascular Disease
Because of the importance of the RAS in cardiovascular disease, its regulation via ACE inhibitors, ARBs, and MRAs has played an essential role in the management of cardiovascular diseases (Central Illustration ).
Central Illustration The Renin-Angiotensin System Interaction With COVID-19
Normally, angiotensin I (Ang I) is converted to Ang II via angiotensin-converting enzyme (ACE), which could be inhibited by ACE inhibitors. The pro-inflammatory effects of Ang II are mediated through AT1R in several ways: 1) in the zona glomerulosa of the adrenal medulla, it stimulates aldosterone secretion and binding to mineralocorticoid receptors to promote water reabsorption and to increase salt retention; it is inhibited by mineralocorticoid receptor antagonists (MRAs); 2) in the posterior pituitary, Ang II stimulates antidiuretic hormone secretion to promote water retention; and 3) in other tissues, it stimulates pathways responsible for hypertrophy, fibrosis, oxidative stress, and apoptosis. These effects are attenuated by angiotensin receptor blockers (ARBs), which block Ang II binding to AT1R. Ang II can also be converted to angiotensin 1-7 (Ang 1-7) via ACE2, which stimulates the Mas receptor promoting anti-inflammatory benefits. The ACE2/Ang1-7/Mas axis acts as a counter regulatory pathway to the traditional renin-angiotensin system (RAS). AT1R and ACE2 are coupled. Ang II binding to AT1R allows dissociation of ACE2 and subsequent degradation. ARB prevents dissociation of ACE2 and renders it availability for unused Ang II conversion to Ang 1-7. ACE2 has been identified as the targeted receptor for both the severe acute respiratory syndrome coronavirus (SARS-CoV) 2 and SARS-CoV. ACE2 mediates S protein binding that stimulates viral entry into the host cytosol that results in infection and viral replication. Diversion of Ang II towards ACE2 could competitively inhibit viral binding and also counter regulate the adverse effects caused by AT1R and improve outcomes by Mas R−based favorable effects.
Several studies have elucidated the role of these drug classes on the modulation of the ACE2/Ang1-7/Mas axis. Mouse peritoneal macrophages treated in vitro with aldosterone, demonstrated significantly increased ACE activity as well as ACE mRNA and significantly reduced ACE2. However, in mouse peritoneal macrophages treated with nicotinamide adenine dinucleotide phosphate oxidase inhibitor, aldosterone could not increase ACE or decrease ACE2, which suggested these effects were mediated in part by nicotinamide adenine dinucleotide phosphate oxidase (42). These effects were also attenuated with treatment with an MRA (eplerenone) (42). Human monocyte-derived macrophages obtained from patients with heart failure before and after 1 month of treatment with another MRA (spironolactone; 25 mg/day) showed 47% reduction in ACE activity and 53% reduction in ACE mRNA expression. At the same time, ACE2 activity increased by 300% and ACE2 mRNA expression increased by 654% (42). In mice treated for 2 weeks with eplerenone, cardiac ACE2 activity increased 2-fold and was paralleled by increased ACE2 activity in macrophages (42). This study demonstrated that the MRA reduced oxidative stress, decreased ACE activity, and increased ACE2 activity and/or expression, which suggested the protective role played by increased generation of Ang 1-7 and decreased formation of Ang II. Overall, aldosterone decreased ACE2 transcription through a nicotinamide adenine dinucleotide phosphate oxidase−mediated pathway (42), and in vascular smooth muscle cells, potentiated Ang II signaling with increased phosphorylation of ERK1/2 and c-Jun kinase, which are also dependent on reactive oxygen species generation (43). Thus, the beneficial effects of MRAs are likely associated with reduction of oxidative stress and differential control of these angiotensinases. MRAs appeared to promote membrane ACE2 expression and suppress the peripheral effects of Ang II; however, the effect of MRAs on soluble ACE2 remains unclear.
Similar upregulation of ACE2 was observed in studies focused on the effects of ARB treatment. Spontaneously hypertensive rats treated with olmesartan demonstrated a 5-fold greater expression of ACE2 mRNA and increased Ang1-7 in their thoracic aortas, whereas those treated with atenolol and hydralazine exhibited no change in ACE2 expression or Ang1-7 (44). Comparison of vessel wall dimensions showed that olmesartan selectively reduced the thoracic aorta media-to-lumen ratio, whereas vascular hypertrophy was unchanged in spontaneously hypertensive rats given atenolol or hydralazine (44). There was no change in ACE2/Ang1-7 expression and/or activity in the carotid arteries of the treated animals. The possibility that the effects of olmesartan on vascular ACE2 gene and protein expression were the result of reduced arterial blood pressure was ruled out because of the comparative effect observed in mice treated with atenolol or hydralazine (44).
Sprague-Dawley rats treated with a 4-week course of Ang II infusion showed Ang II upregulated AT1 receptor, downregulated AT2 receptor, ACE2 activity, endothelial nitric oxide synthase expression, as well as increased CD44 expression and hyaluronidase (45). However, rats treated with telmisartan exhibited significantly increased ACE2 activity and endothelial nitric oxide synthase expression in intracardiac vessels and intermyocardium, as well as downregulated local expression of the AT1 receptor. Treatment with telmisartan also inhibited membrane CD44 expression and reduced transforming growth factor−β and Smad expression (45). Studies in normotensive rats with post-coronary artery ligation left ventricular remodeling and dysfunction exhibited partial resolution following losartan and olmesartan treatment while augmenting plasma concentrations of the angiotensins (46). This was associated with recovery of cardiac AT1 receptor mRNA and increased ACE2 mRNA post-myocardial infarction, which implied the beneficial effects of ARBs on cardiac remodeling were accompanied by direct blockade of AT1 receptors and increased ACE2 expression and/or activity (46). The literature offers conflicting results pertaining to ARB use and the level of ACE2 expression on the myocardium; most of the controversy arises from the difference in ACE2 cell surface expression and plasma ACE2 levels. In the Sprague-Dawley rats with left coronary artery ligation and myocardial infarction, plasma Ang II and Ang1-7 were not elevated, but plasma ACE2 was elevated, along with enhanced cardiac ACE2 and AT1 receptor mRNA at the infarct border (47). Receptor upregulation was not observed in the remote myocardium (47). Treatment with ramipril and valsartan resulted in increased plasma Ang I and Ang II and suppression in plasma ACE and ACE2 activity; however, neither monotherapy nor combination therapy affected ACE2 or AT1 receptor expression, both of which remained at levels comparable to non-myocardial infarction control (47). However, a previous study in the same murine model showed ACE and ACE2 upregulation in the border, infarct zones, and in viable myocardium after myocardial infarction. Treatment with ramipril reduced ACE expression, whereas ACE2 remained elevated compared with the noninfarcted control subject (48). A recent study in the same murine model demonstrated that treatment with olmesartan or telmisartan increased both cardiac ACE2 mRNA and protein expression while augmenting plasma Ang1-7/Ang II ratios, which resulted in improved cardiac function and alleviated collagen disposition (49). These experiments suggested that both ACE inhibitors and ARBs variably upregulated ACE2 expression (49). ARBs inhibited binding of Ang II to the AT1 receptor, which permitted circulating Ang II to be shunted to ACE2 for conversion to Ang1-7. These studies suggest that the ACE2/Ang1-7 axis collaborates with or is regulated by the AT1 receptor and may be important in mediating the vascular and cardiac remodeling effects of Ang II.
The mechanisms by which ACE inhibitors act are complex. Although ACE2 is not inhibited by ACE inhibitors (19), an increase in Ang1-7 suggests their clinical effects are partly mediated by the angiotensinases. ACE inhibitors inhibit the conversion of Ang I to Ang II and inhibit the hydrolysis of bradykinin. ACE inhibition promotes the vasodilatory effects of bradykinin, improved endothelium-dependent vasodilation through increased prostaglandin and nitric oxide production, and down regulation of the AT1 receptor (50, 51, 52). Studies that elucidated the effect of ACE inhibition on the ACE2 gene showed that inhibition of Ang II synthesis regulated ACE2 mRNA but not ACE2 activity (53). However, ACE inhibition alone or in combination with losartan was demonstrated to increase plasma Ang1-7 while reducing plasma Ang II (53). Compared with the degree of ACE2 mRNA upregulation seen with post-losartan monotherapy, combination of losartan and lisinopril resulted in suppressed upregulation of ACE2 mRNA, which suggested ACE inhibitors might override a signal that regulates ACE2 transcription (53). Although Ang II is the predominant substrate, ACE2 can also convert Ang I into Ang 1-9, which, in turn, could be converted to Ang 1-7 via ACE; Ang I can be directly converted into Ang 1-7 via zinc metallopeptidase neprilysin (17), although with less favorable kinetics at baseline. Thus, it can be assumed ACE inhibitors disrupt the balance between catalytically active ACE and ACE2, resulting in favored activation of the ACE2/Ang1-7/Mas axis.
Overall, because of the demonstrated anti-inflammatory, antifibrotic, and antithrombotic effects associated with the ACE2/Ang1-7/Mas axis, upregulation could serve as a valuable therapeutic target.