Is COVID-19 an endothelial disease? Clinical and basic evidence

1. Department of Advanced Medical and Surgical Sciences, University of Campania “Luigi Vanvitelli”, Naples, Italy; 2. Department of Medical Sciences, International University of Health and Medical Sciences “Saint Camillus”, Rome, Italy; 3. Department of Advanced Biomedical Sciences, International Translational Research and Medical Education Academic Research Unit (ITME), "Federico II" University, Naples, Italy; 4. Department of Medicine, Albert Einstein College of Medicine, Wilf Family Cardiovascular Research Institute, Fleischer Institute for Diabetes and Metabolism (FIDAM), Montefiore University Hospital, New York, NY, USA.


Introduction
Coronavirus disease 2019 (COVID-19) represents a public health crisis of global proportions.
Caused by SARS-CoV-2, which stands for severe acute respiratory syndrome coronavirus 2, COVID-19 was first announced in December 2019 in Wuhan, the capital of China's Hubei province, and has since spread globally1. The symptoms most commonly reported include cough, fever, and shortness of breath. The pathophysiology of the disease explains why respiratory symptoms are so common: indeed, the virus accesses host cells via the protein angiotensin-converting enzyme 2 (ACE2)2, 3, which is very abundant in the lungs4. Nevertheless, ACE2 is also expressed by endothelial cells (ECs)5, 6, and other major clinical events usually observed in COVID-19 patients (e.g. high blood pressure7, thrombosis8, pulmonary embolism9) seem to suggest that the virus is targeting the endothelium, one of the largest organs in the human body10.

Pathogenesis of COVID-19
SARS-CoV-2 uses a surface glycoprotein (peplomer) called spike to access host cells and ACE2 has been shown to be a co-receptor for coronavirus entry11, 12. Therefore, the density of ACE2 in each tissue may correlate with the severity of the disease in that tissue13-16. Other receptors on the surface of human cells have been suggested to mediate the entry of SARS-CoV-23, including transmembrane serine protease 2 (TMPRSS2)17, sialic acid receptors18, 19, and extracellular matrix metalloproteinase inducer (CD147, also known as basigin)20.
Intriguingly, all of these 4 receptors are known to be expressed by ECs21-24 (Figure 1). ACE2 remains the most studied of these receptors16, 25-29: for instance, its genetic inactivation has been shown to cause severe lung injury in H5N1-challenged mice30, whereas administration of recombinant human ACE2 ameliorates H5N1 virus-induced lung injury in mice30. Importantly, ACE2 is currently at the center of a heated debate among cardiologists31-34, and there are concerns that medical management of hypertension, including the use of inhibitors of the reninangiotensin-aldosterone system (RAAS), may contribute to the adverse health outcomes observed16, 35, 36; TMPRSS2 has been shown to bind the viral spike glycoprotein17; recent structural 4 assays have suggested that coronaviruses can bind sialic acid receptors18; CD147 has been shown to be essential for the entry of cytomegalovirus into ECs24.
Endothelial dysfunction refers to a systemic condition in which the endothelium loses its physiological properties, including the tendency to promote vasodilation, fibrinolysis, and antiaggregation; moreover, endothelial dysfunction appears to be a consistent finding in patients with diabetes37. Here we will discuss clinical and preclinical findings supporting our hypothesis that COVID-19 impairs endothelial function (Figure 2).

Hypertension and COVID-19
Several investigators have called attention to the potential over-representation of hypertension among patients with COVID-1938, 39. Moreover, hypertension appears to track closely with advancing age, which is emerging as one of the strongest predictors of COVID-19-related death8, 40. Specifically, observational trials and retrospectives studies conducted near Wuhan area have actually shown that hypertension is the most common co-morbidity observed in patients affected by COVID-19, ranging from 15 % to 31.2%8, 41-44. The largest study has been conducted by Guan and colleagues between December 11, 2019, and January 29, 2020, providing data on 1099 hospitalized patients and outpatients with laboratory-confirmed COVID-19 infection41; in this cohort, 165 of them (~15%) had high blood pressure41. The authors also evaluated the severity of disease, infection43. Notably, in the latter report, the rate of hypertension was 31.2%, and 58.3% of hypertensive patients with COVID-19 infection were admitted to ICU compared to 21.6% of individuals with normal blood pressure43; evidencing the hypertensive state as a common co- These studies also raise numerous questions regarding the association between hypertension and COVID-19. Indeed, it is well known that hypertension is one of most common disease and comorbidity worldwide, considered a silent killer for worldwide population45. We speculate that the higher rate of hypertension and the worse prognosis in patients with COVID-19 infection could be seen as the spy of a cause-effect mechanism more than of a casual pre-existing association between these two different diseases.
Recent reports evidenced higher morbidity and mortality rates of COVID-19 in African-Americans compared to Caucasian subjects in United States46. Of note, several studies have shown a higher prevalence of hypertension in blacks than in whites47, and ACE inhibitors (ACEi) and angiotensin II receptor blockers (ARS) have not been shown to be as effective in black populations compared with white populations48.

ACE2 and anti-hypertensive drugs: what do we know?
ACEi and ARS represent very effective strategies for the treatment of hypertension45. These drugs reduce the effects of renin-angiotensin axis by inhibiting ACE (ACEi) or by blocking the angiotensin receptors (ARS). A rising question for the scientific community and physicians is to understand whether ACEi/ARS could affect the prognosis of hypertensive COVID-19 patients16, 49, 50. 6 Unfortunately, there are no data regarding specific anti-hypertensive medications and hypertensive COVID-19 patients with infection.
The role of ACEi/ARS in the control of ACE2 molecular pathways is controversial: indeed, preclinical studies evidenced that the selective blockade of either angiotensin II synthesis or activity in rats induces increases in ACE2 gene expression and activity51-54; similarly, treating infarcted rats with ARBs increased plasma concentration of angiotensin 1-7 and ACE255. In mice, ARB treatment augmented ACE2 mRNA and protein levels56, 57 and prevented the decrease in ACE2 protein levels induced by Angiotensin II58. Equally important, mineralocorticoid receptor blockers prevented aldosterone induced reduction in cardiac ACE2 mRNA expression in rat cardiomyocytes59 and increased ACE2 expression and activity in murine hearts and in monocytederived macrophages obtained from 10 patients with heart failure60.
Nevertheless, there is no clinical evidence that ACEi directly affect molecular pathways linked to ACE2 activity. For instance, urinary ACE2 levels were reported to be higher in patients treated with olmesartan vs untreated controls, but this finding was not observed in patients treated with other ARS or enalapril61; instead, another study reported no difference in ACE2 activity in patients who were taking ACEi or ARS vs untreated patients49. Of note, clinically prescribed ACEIs have been shown to not inhibit ACE2, which function as a carboxypeptidase62. In particular, ACE2 acts to counterbalance the effect of ACE63: indeed, whereas ACE generates angiotensin II from angiotensin I, ACE2 converts angiotensin II into an active heptapeptide (angiotensin 1-7) with vasodilative, anti-oxidant, and anti-inflammatory properties64-66. A major role in the pathogenesis of (as well as in the clinical response to) COVID-19 could be also played by ACE2 polymorphisms, which are relatively under-investigated if compared to ACE71, 72. Finally, we have to consider the higher rate of cardiac injury and adverse outcomes in hypertensive patients during COVID-19 pandemic48, 73, 74. Therefore, ACEi/ARS chronic therapy should not be discontinued in hypertensive patients with COVID-19. Indeed, the loss of their pneumo-and cardioprotective effects could be detrimental45. In addition, in absence of adequate follow-up visits, switching from ACEi/ARS to another anti-hypertensive therapy could cause a suboptimal control of blood pressure. Thus, as suggested by several medical associations67, in absence of definitive clinical studies and without clear evidence, hypertensive patients should avoid discontinuation and/or therapeutic switching during COVID-19 infection.

Diabetes and COVID-19
Diabetes mellitus is a frequent co-morbidity and a cause of worse prognosis in patients with COVID-19 infection75-77. Indeed, evaluating pneumonia cases of unknown causes reported in Wuhan and in patients with history of exposure to Huanan seafood market before Jan 1, 2020,  41. In summary, diabetes is a frequent co-morbidity, a risk factor, and an independent prognostic factor in COVID-19 patients. A strong evidence of the negative effects of diabetes in COVID-19 patients is confirmed by two meta-analyses78, 79. The worse prognosis in patients with diabetes and COVID-19 could be attributable to the fact that the pneumonia evolves towards clinical stages more refractory to medical therapies, oxygen administration and mechanical ventilation, with necessity of ICU care. These data have been In COVID-19 patients, the incidence of diabetes is two-folds higher in ICU/severe vs non-ICU/severe cases79. Indeed, the diagnosis of diabetes in a cohort of patients with COVID-19 Infection evidenced a sub-group of patients with a 2.26-fold higher risk to experience adverse disease outcome analyses78. Unfortunately, no data are available on anti-diabetic medications and glucose homeostasis in COVID-19 patients. This aspect is really limiting, because the diagnosis of diabetes diagnosis and the altered glucose homeostasis during a condition of severe pneumonia with SARS are reported as main factors of worse prognosis and deaths80. Therefore, the investigation of anti-diabetic medications and glucose homeostasis could be harnessed to evaluate patients with higher risk to experience worse prognosis and death by COVID-19. We speculate that the amelioration of glucose homeostasis in diabetic COVID-19 patients by specific hypoglycemic drugs could result in the amelioration of clinical outcomes with death reduction. However, these data are not reported in trials on COVID-19, and they need to be investigated in further studies.

Thrombosis and COVID-19
Patients with COVID19 often show clotting disorders, with organ dysfunction and coagulopathy, resulting in higher mortality81. Important data came from the analysis of coagulation tests including

Other therapeutic approaches
Recently, chloroquine and hydroxychloroquine have been suggested as a potential therapy for COVID-19102, 103, although the exact molecular mechanisms remain unknown; if our hypothesis on the key role of ECs in COVID-19 disease is confirmed, these drugs may exert their beneficial effects via an amelioration of endothelial dysfunction. Indeed, consistent with our view, both these antimalaric agents have been shown to improve endothelial function104, 105

Endothelial dysfunction is a major determinant of COVID-19.
Endothelial dysfunction is a common feature of hypertension, diabetes, and thrombosis, critical clinical findings in COVID-19 patients.