Commentary for "Endocrine significance of SARS-CoV-2's Reliance on ACE2".

Twenty years ago, a homolog of angiotensin-converting enzyme (ACE) was identified as ACE2; however, this peptidase does not share similar catalytic properties of ACE that converts angiotensin I (Ang I) to the active peptide hormone Ang II and rapidly metabolizes bradykinin, as ACE2 preferentially degrades Ang II to Ang-(1-7) (1). Crackower et al (2) subsequently found that ACE2–/– null mice exhibited altered cardiac function associated with increased Ang II expression and that the ACE2–/– phenotype was rescued by the knockdown of ACE consistent with the lack of Ang II metabolism in the ACE2–/– murine heart (3) Recognition of ACE2 as a major pathway in the metabolism of Ang II was key to establishing the concept that the renin-angiotensin system (RAS) comprises 2 major peptide pathways that have opposing actions (1, 4). The classical endocrine RAS characterized as the ACE-Ang II-AT1 receptor (AT1R) axis mediates an increase in blood pressure, and is likely a contributing factor in hypertension, heart failure, diabetes, and kidney disease that reflects inflammatory, fibrotic, and oxidative stress pathways (1). In contrast, the ACE2-Ang-(1-7)-MasR axis antagonizes the actions of Ang II through both the metabolism of Ang II and the generation of Ang-(1-7) that binds the MasR to stimulate nitric oxide, but attenuates inflammation, fibrosis, and oxidative stress. Indeed, the main therapeutic approaches to block the RAS include AT1R antagonists and ACE inhibitors that may augment the ACE2-Ang-(1-7)-MasR axis, contributing to the efficacy of RAS blockade (1). New therapeutic approaches are proposed to use soluble but catalytically active ACE2 to reduce Ang II and augment the levels of Ang-(1-7) (1).

Our view of ACE2 as a beneficial component within the RAS was radically altered by the coronavirus disease 2019 (COVID-19) pandemic currently responsible for more than 293 000 deaths in the United States and 1.59 million deaths worldwide (as of December 11, 2020). SARS-CoV-2 enters the cell by binding to ACE2 with high affinity that subsequently stimulates internalization of the ACE2-viral complex following cleavage of the SPIKE protein subunits by furin and TMPRSS2, which are ubiquitous host cell proteases (4). The viral-induced internalization of ACE2 may also comprise a biological inflection point within the RAS by altering the local balance of endogenous Ang II and Ang-(1-7) to exacerbate inflammatory events possibly leading to the “cytokine storm” associated with SARS-CoV-2 (4). In the present issue of Endocrinology, Lazartigues et al (5) review the potential role of SARS-CoV-2 and ACE2 on various endocrine systems. Although numerous reports focus on SARS-CoV-2 in pulmonary and cardiovascular function, the present review addresses the potential effects of SARS-CoV-2 on endocrine disorders. The authors emphasize that ACE2 is widely expressed in multiple tissues that include both endocrine and exocrine elements of the pancreas, adrenal gland, thyroid, adipose tissue, hypothalamus, testes, ovary, and kidney that may be the cellular basis for the multiple organ dysfunction associated with SARS-CoV-2 (5). The authors initially discuss preclinical data regarding ACE2 null mice that exhibit impaired glucose tolerance and reduced β-cell mass and proliferation, as well as improved pancreatic function in diabetic mice following ACE2 gene therapy (5). The authors cite clinical evidence for the acute onset of diabetes in patients with the original SARS virus, which also internalizes ACE2, and in COVID-19 patients who exhibit indices of pancreatic injury (5). Diabetes is one of the major risk factors among hypertension and obesity for the increased incidence of mortality and severity of symptoms in patients with SARS-CoV-2 (4, 6). The authors also discuss the increased incidence of thyroid dysfunction or injury (follicular cell apoptosis) in SARS patients that has been initially reported with SARS-CoV-2, although sufficient clinical data are lacking in COVID-19 patients. Finally, the authors review the data on SARS-CoV-2 and testicular function given the high expression of both ACE2 and TMPRS22 in this tissue, particularly as TMPRS22 is positively regulated by androgens. Interestingly, the authors discuss one clinical study in which patients treated for prostate cancer by androgen depletion exhibited a lower incidence of SARS-CoV-2 infection (5). TMPRSS2 inhibitors are currently undergoing clinical trials for their ability to block SARS-CoV-2 infection (NCT04321096).

The present study has reviewed the extensive preclinical data on the role of ACE2 in cardiovascular and endocrine function, but also emphasizes the need for additional clinical studies on the functional consequences of SARS-CoV-2 infection and the potential downregulation of ACE2. Apart from the critical need to identify effective treatment regimens for COVID-19 patients, the long-term consequences of SARS-CoV-2 infection need to be addressed as well.