Since the first months of the coronavirus disease 2019 (COVID-19) pandemic, several specific physiologic traits, such as male sex and older age, or health conditions, such as overweight/obesity, arterial hypertension, metabolic syndrome, and type 2 diabetes mellitus, have been found to be highly prevalent and associated with increased risk of adverse outcomes in hospitalized patients. All these cardiovascular morbidities are widespread in the population and often coexist, thus identifying a common patient phenotype, characterized by a hyper-activation of the “classic” renin-angiotensin system (RAS) and mediated by the binding of angiotensin II (Ang II) to the type 1-receptor. At the same time, the RAS imbalance was proved to be crucial in the genesis of lung injury after severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, where angiotensin-converting-enzyme-2 (ACE2) is not only the receptor for SARS-CoV-2, but its down-regulation through internalization and shedding, caused by the virus binding, leads to a further dysregulation of RAS by reducing angiotensin 1-7 (Ang 1-7) production.
New-onset hyperglycemia or acute decompensated diabetes mellitus have been frequently observed in COVID-19 patients [47,48][31][32]. Moreover, susceptibility to other overlapped secondary infections, together with the use of glucocorticoid therapy, can further precipitate acute hyperglycemia [49][33] with increased plasma osmolality, osmotic polyuria and dehydration, endothelial dysfunction, thrombophilia, and amplified pro-inflammatory cytokine secretion, all key factors in SARS-CoV-2-related multi-organ dysfunction. In addition, binding of SARS-CoV-2 to ACE2 in pancreatic cells can damage islets and reduce insulin release, leading to acute hyperglycemia and transient diabetes [50][34]. The evidence that diabetes mellitus causes a pro-inflammatory environment has been corroborated by serum levels of inflammation biomarkers, such as interleukin-6 (IL-6), C-reactive protein (CRP), and ferritin, and D-dimer that are markedly higher in COVID-19 patients with diabetes mellitus compared to controls without diabetes [51][35]. Generally, diabetes mellitus is associated with weakened immune response and enhanced susceptibility to infections, due to inherent neutrophil dysfunction, reduced T-cell responses, and disordered humoral immunity [52][36]. As other cardio-metabolic comorbidities, type 2 diabetes mellitus bears the fingerprint of RAS imbalance: indeed, chronic activation of “classic RAS” is typical in diabetes and insulin-resistance, despite high sodium intake and high blood pressure, and contributes to microvascular and macrovascular complications and is clearly involved in diabetic kidney disease [53][37]. Furthermore, chronic hyperglycemia also reduces the ACE2 expression, with a loss of its anti-inflammatory effects and protection of endothelial function, because of a decreased counter-regulation of Ang II [54,55][38][39].
In the context of ACE/Ang II/AT1R axis hyper-activation, the insulin receptor also uses mitogen-activated protein (MAP) kinase as a downstream mediator of its action [35][23], mediating growth-factor-like effects, such as vascular smooth muscle growth and cardiac hypertrophy [56][40]. Even AT1R can activate MAP kinase in its post-receptor cascade, so it can be postulated that these two receptors synergize to exacerbate and perpetuate inflammation, fibrosis, and tissue injury. These findings are in line with the hypothesis that RAS dysregulation could be the backbone in the pathogenesis of severe COVID-19 in patients with diabetes mellitus. This vulnerability may play a synergistic role with the underpinning inflammatory milieu and immune defects associated with diabetes, providing SARS-CoV-2 a pathway for causing prolonged lung injury.
Worldwide epidemiologic data provided evidence that hypertension is a pivotal comorbidity related to COVID-19 disease severity. In addition to the hypertensive condition, anti-hypertensive therapy is also likely to affect the course of COVID-19. A large meta-analysis enrolling 101,949 COVID-19 patients found a significant association between treatment with RAS inhibitors and mortality reduction among patients with hypertension [62][41].
The mechanisms by which hypertension leads to increased risk of worse outcome in COVID-19 are likely to be many. Hypertension is a major cardiovascular risk factor that promotes arteriosclerosis of large and small arteries and accelerates atherosclerosis, leading to cardiovascular disease and death. Left ventricular hypertrophy and myocardial fibrosis, with increased filling pressures and impaired coronary reserve, are key features that contribute to atrial fibrillation, myocardial ischemia, and heart failure with preserved ejection fraction [76][42]. Certainly, RAS hyper-activation or, at least, “inappropriately normal” renin activity and aldosterone levels are typical of overweight/obese hypertensive patients [33][21]. In these patients, normal or increased Ang II production results in a stimulation of the ACE/Ang II/AT1R pathway, leading to small arteries constriction, hypertrophy, fibrosis, and tissue injury [77][43]. It also leads to the activation of NADPH oxidases, with subsequent generation of reactive oxygen species, protein oxidation, and dysregulated cell signaling [78][44]. Moreover, animal models support a hypothetical link between hypertension and reduced ACE2 expression, corroborated by data showing lower expression of ACE2 mRNA and ACE2 protein expression in the kidneys of hypertensive rats [79][45].