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1. Introduction

Cardiovascular disease (CVD) and hypertension have emerged as critical comorbid risk factors affecting the survival of patients with COVID-19(1). Inflammation is a major player in the progression of CVD, and the renin-angiotensin-aldosterone system (RAAS) plays an important role in producing and maintaining vascular inflammation (2). While RAAS serves a key role in regulating blood pressure and hypertension, it also mediates pro-inflammatory functions. Most importantly, blocking RAAS has beneficial and protective outcomes in CVD treatment. Indeed, the use of major RAAS inhibitors, such as angiotensin receptor blockers (ARBs) and angiotensin-converting enzyme inhibitors (ACEIs) improves CVD by effectively treating hypertension-induced injury (3). Angiotensin-converting enzyme 2 (ACE2) is a major component of RAAS, and the receptor to which SARS-CoV2 binds to enter the cells. Endothelial cell dysfunction (ED) driven-ACE2 depletion is associated with an increase in inflammation and blood coagulation; both are considered critical factors in the progression of COVID-19. To date, this association remains unclear and thus, there should be an increased effort to better understand the relationship between ACE2, blood hemostasis and inflammation in the pathogenesis of COVID-19 disease.

2. CVD, ED and RAAS

In CVD, chronic inflammation leads to ED and the initiation and progression of atherosclerosis by enhancing the migration of inflammatory cells into the vessel wall, foam cell formation and the stimulation of smooth muscle cell hyperplasia, which ultimately leads to tissue injury (4). Strong associations between Angiotensin II (AngII), a major actor in RAAS, and inflammation have been demonstrated, implicating AngII in enhancing pro-inflammatory responses through the upregulation of pro-inflammatory cytokines and chemokines, including IL-6, MCP-1, VCAM-1 and TNF-α (5-8). In addition, AngII is a strong pro-oxidant and it mediates its effects through the activation of NADH/NADPH signaling, production of superoxide anions and reduction in nitric oxide (NO) bioavailability, which is a key marker for a healthy endothelium (9-12).

Inhibiting RAAS signaling pathways reduces CVD mortality (13). Blocking RAAS involves either ACEIs, which inhibit AngII formation or ARBs, which block angiotensin receptors (Fig. 1). Targeting RAAS decreases inflammation, vascular remodeling and oxidative stress, and improves endothelial cell function by increasing NO production (14).

Figure 1 RAAS is essential for regulating blood pressure and inflammation in the body. While the ACE/AngII/AT1R pathway is responsible for the increase in blood pressure and inflammation, the other ACE2/Ang(1-7)/MasR arm of RAAS is involved in the opposing effects, and in lowering blood pressure and inflammation. ACE2 mediates SARS-CoV-2 entry to the cell and is a key player in abrogating the detrimental effects of the ACE/AngII/AT1R arm of RAAS and mitigating virus-driven lung injury. ACEIs and ARBs block ACE (and therefore the conversion of AngI to AngII) and AT1R respectively, which affect the progression of COVID-19 symptoms. RAAS, renin-angiotensin-aldosterone system; ACE, angiotensin converting enzyme; AngII, angiotensin II; AT1R, Angiotensin Type 1 Receptor; MasR, MAS1 oncogene; ACEI, ACE inhibitor; ARB, angiotensin receptor blocker.

3. ACE2 and COVID-19

ACE2 is the target receptor to which SARS-CoV-2 binds with to gain entry into cells (1). Since ACE2 is an essential component of RAAS, concerns arise regarding the plausible relationships between hypertension, the use of ACEIs and ARBs, and the role of cardiovascular disease in aggravating COVID-19 symptoms, restoring the balance in the RAAS system may be a critical factor in attenuating organ injuries. Indeed, this was addressed early during the COVID-19 pandemic; drugs that block RAAS could affect the severity of the disease (15). Results from the initial outbreak in China showed that a majority of patients with COVID-19 with severe symptoms had hypertension; this led to speculations that ACEIs and ARBs may increase the risk of viral infection since their administration enhances ACE2 expression (16,17). However, studies in humans and animal models did not provide any convincing proof of this association, and thus remains unclear and contested (18-20). Adding to this controversy is the fact that the virus-binding target, ACE2, converts AngII to Ang(1-7), which decreases inflammation and lowers blood pressure. ACE2 therefore plays an important role in balancing the two RAAS arms, the pro-inflammatory and hypertensive arm mediated by ACE, AngII and Angiotensin Type 1 Receptor (AT1R), and the cardioprotective, anti-inflammatory arm mediated by MAS1 oncogene (MasR) and AT2R. Disruption of this balance is a crucial player in the pathophysiology of CVD and COVID-19(21). Indeed, while the ACE/AngII pathway is important in vasoconstriction, hypertension and oxidative stress, which leads to inflammation, the ACE2/Ang(1-7) pathway counteracts the above effects, and both pathways coexist in various tissues including in the lungs, heart, blood vessels and kidneys where they regulate blood pressure and contribute to CVD pathophysiology (22,23).

The SARS-CoV-2 spike protein recognizes, with high affinity, ACE2 present on the surface of host cells mediating the entry of the virus. Endocytosis of the virus-ACE2 complex can potentially lead to ACE2 downregulation and shedding from the surface of the cell (24). This loss of ACE2 function in infected cells could be a critical factor in the progression and course of the disease (25). Even though there is no compelling evidence that links ACEI and ARB treatment with an increase in SARS-CoV-2 infection, it is becoming evident that these drugs may attenuate AngII-driven lung injury (26). Since AngII promotes inflammation and acute lung injury (27), any increase in ACE2 activity could reduce AngII concentration in the lungs and mitigate virus-driven lung injury. Indeed, a recent study revealed correlations between biochemical and clinical markers of lung injury, viral load and AngII concentrations in patients with COVID-19(28). Similarly, results link SARS-CoV-2 with a decrease in ACE2 expression and acute heart injury (29). However, it was reported that the use of ACEIs or ARBs in hospitalized patients with COVID-19 had no effect on their survival rate; actually, there was no significant difference in the mean number of days alive for patients who were hospitalized with mild to moderate symptoms of COVID-19 and who were assigned to continue vs. discontinue these medications (30). Conversely, another cohort study that assessed ACEIs and ARBs and included more than 8 million individuals, has shown that these drugs are associated with significantly reduced severe risks of the disease, such as requiring intensive care. This study also hinted to the role of ethnicity in modulating ACEIs/ARBs effects in relation to the severity of the disease; it was shown that the risk of the disease in association with the use of these drugs was higher in Black African and Caribbean groups when compared with the Caucasian group (31). Overall, the use of ACEIs/ARBs is still a paradoxical issue that requires extended investigation to resolve; it is also an area of research where the benefit/risk analysis and potential efficacy of those drugs should be addressed in connection with other comorbidities that are related to COVID-19(22).

4. COVID-19, the exposure of a masquerading illness

There is growing evidence for COVID-19 being a disorder of blood clotting where the virus uses the respiratory route to gain entry to blood circulation (32). It has been initially reported that COVID-19 is strongly associated with ischemic strokes in patients that required vacuum and clot retrieval devices as well as blood thinning medications (33,34). It was shown that when the virus enters the blood stream, it triggers a cascade of events resulting in blood clotting and strokes. This all starts with the attachment of the virus to the ACE2 receptor on endothelial cells, making use of transmembrane protease, serine-2 (TMPRSS-2) which initiates the process of ED (35). Thus, SARS-CoV-2 mediated ACE2 downregulation on the surface of the cell results in AngII accumulation and NADPH activation fueling the generation of reactive oxygen species (ROS) and thus increasing oxidative stress (36,37). ROS assists in the conversion of β2-glycoprotein 1 into its oxidized form, which can no longer bind competitively to the von Willebrand factor (vWF) that is secreted by dysfunctional endothelial cells; subsequently, this will promote the coagulation cascade, as vWF binds to the sub-endothelial layer, crosslinking collagen and platelets together and accentuating coagulation mechanisms that lead to strokes of the large vessels (38,39).

During viral infection, the dysfunctional endothelium plays a detrimental role by worsening inflammation which is associated with a poor prognosis in patients with COVID-19 (Fig. 2) (35). As the coagulation mechanism is a highly organized process that involves endothelial cells, endotheliitis plays a critical role in the pathogenesis of SARS-CoV-2 by increasing the risk of excessive and disseminated intravascular coagulation and the rates of fatality (40). During infection, pro-inflammatory cytokines, such as IL-1β, IL-6 and TNF-α are amplified and lead to a simultaneous increase in the vWF and tissue factor release from endothelial cells, which will promote blood clotting through the increase in platelet aggregation and the initiation of the clotting cascade (41). Similarly, those cytokines enhance blood clotting by downregulating pro-fibrinolytic and anticoagulant factors, including endothelial protein C receptor and thrombomodulin and by upregulating anti-fibrinolytic factors, namely plasminogen activator inhibitor-1 (PAI-1) (42,43). There is cumulative proof that ACE2 downregulation may contribute to an increase in the thrombotic risk in patients with COVID-19(44). It has been speculated that the decrease in ACE2 activity seen in patients with COVID-19 may lead to a series of mechanisms that are promoted by the dysfunctional endothelium and that affect blood hemostasis. This comprises an increase in vascular permeability, as well as an upregulation of tissue factor and PAI-1 culminating in the activation of the extrinsic coagulation pathway and the reduction in fibrinolysis (45). On this same note, it has been reported that, in animal models of thrombosis, there is a clear association between coagulation and ACE2 pathways. In rats with an induced thrombosis, ACE2 inhibition is significantly correlated with the increase in blood clot weight; conversely, ACE2 administration induced a decrease in thrombus size as well as a reduction in platelet adhesion to the endothelium (46). Similarly, it has been shown that a decrease in ACE2 activity is associated with an increase in blood coagulation in spontaneous hypertensive rats, and that the activation of ACE2 attenuates thrombosis by reducing the attachment of platelets to the vessel wall (47).

Figure 2 SARS-Cov-2 binds to ACE2 in order to enter the cell. Subsequent activation of the RAAS pathway induces endothelial dysfunction, which will lead to the generation of ROS and the secretion of pro-inflammatory cytokines (IL-1, IL-6, TNF-α), resulting in a cytokine storm as well as hypercoagulation and a rise in thrombotic events culminating in an increase in the severity of the disease. ACE, angiotensin converting enzyme; RAAS, renin-angiotensin-aldosterone system; ROS, reactive oxygen species; IL, interleukin; TNF-α, tumor necrosis factor-α; Ang, angiotensin.

Interestingly, it was reported that SARS-CoV-2 can directly bind to platelets through its spike protein, which will enhance platelet activation. It was shown that platelets are hyperactive in patients with COVID-19 and that they express ACE2 and TMPRSS2. ACE2-mediated viral binding to platelets stimulated them to release inflammatory and coagulation factors, which lead to an enhancement in leukocyte-platelet aggregation (48).

5. Ongoing therapeutic strategies and disease management

At present, the treatment of COVID-19 is limited to alleviating the symptoms of the disease, with no specific antiviral drugs that are effective in targeting the virus (49). Accordingly, there exist numerous ongoing clinical trials and treatments that aim to target COVID-19-associated ED in order to mitigate disease progression and the high mortality rate associated with it. Such treatments include the use of RAAS inhibitors, serine protease inhibitors, recombinant human ACE2, monoclonal anti-spike antibodies, heparin, corticosteroids as well as other agents directed towards specific cytokines and inflammatory signaling pathways (Fig. 3) (50-54). Serine protease inhibitors may affect SARS-CoV-2 entry to the cell by inhibiting TMPRSS-2, which plays an instrumental role in mediating S protein fusion to the endothelial cell membrane (49). One study showed that targeting TMPRSS2 using a clinically proven protease inhibitor can effectively prevent SARS-CoV-2 infection in vitro (25). Additionally, numerous studies and ongoing clinical trials point to the vital role that the RAAS inhibitors may contribute to improving ED and the pathogenesis of COVID-19 (55-59). On this same note, statins were also reported to improve endothelial cell function in a manner distinct from their major lipid-lowering activities. These drugs can increase the expression of NO synthase, whilst inhibiting NADPH oxidase, which leads to the suppression of pro-inflammatory pathways in endothelial cells (60). Meanwhile, there is a growing evidence showing that statins can improve the prognosis of COVID-19 through the decrease in the production of inflammatory biomarkers (61). In addition, heparin is known to have anti-inflammatory and protective effects in endothelial cells, and recent studies confirmed its role in improving the prognosis of severely infected patients and reducing mortality rates through its well-known anticoagulation properties (62). Furthermore, given the importance of inflammation in the pathophysiology of COVID-19, clinical evaluation of the anti-inflammatory effects of corticosteroids has gained high priority recently. One study has shown the efficacy of methylprednisone in treating severely ill patients with acute respiratory distress syndrome (63). Another study also confirmed the significant role of dexamethasone in decreasing mortality rates in patients who are severely affected with COVID-19(64). Lastly, other promising therapeutic approaches include targeting cytokines, such as interferon-γ, IL-1 and IL-6 and, as well as the VEGFA/VEGFR2 signaling pathways in order to alleviate virus-driven injury and inflammation (54).

Figure 3 Therapeutic processes that address inflammation, oxidative stress and blood coagulation, which are considered hallmarks of ED during SARS-CoV-2 infection. The proposed mechanisms include targeting the interaction between the virus and endothelial cells through the use of rhACE2, serine protease inhibitors and SARS-CoV-2 anti-spike monoclonal antibodies in order to decrease ED, viral infection and tissue injury. The therapeutic procedures described here also target ED-induced hyperinflammation and hypercoagulability syndromes by using ACEIs, ARBs and antioxidants, as well as anticoagulant and anti-inflammatory drugs and antibodies, such as dexamethasone, methylprednisone, heparin, anti-IL1 and anti-IL-6. TMPRSS-2, transmembrane protease, serine-2; ED, endothelial dysfunction; ACE, angiotensin converting enzyme; rhACE2, recombinant human ACE2; ARB, angiotensin receptor blocker; IL, interleukin; NO, nitric oxide.

6. Conclusion and future perspectives

Overall, ACE2 is a key player in SARS-CoV-2 infection and in abrogating the detrimental effects of the ACE/AngII/AT1R arm of RAAS; namely, the ACE2/Ang(1-7) pathway instigates a shift away from ACE/AngII/AT1R, which affects the progression of COVID-19 symptoms. Moreover, there is a clear association linking ACE2 and blood coagulation pathways, which could play an important role in COVID-19. This relationship suggests that ACE2 may be a novel target for the treatment of thrombogenic diseases, including COVID-19. Future investigations into the role of ACEIs and ARBs in this disease shall expose their potential value for managing COVID-19 symptoms. In addition, there is an urgent need to better understand the effect of recombinant human ACE2 as a potential treatment for SARS-CoV-2 driven injury. Equally important is the need to define the promising role of anti-Spike monoclonal antibodies in alleviating COVID-19 symptoms by reversing SARS-CoV-2 spike protein-induced platelet activation, excessive coagulation and the rates of strokes and fatalities. Future research shall hopefully address these issues as there remain significant gaps in our knowledge pertaining to these related subjects. Accordingly, it is extremely essential that future clinical practice deals with the precise therapeutic processes pertaining to the action of anti-spike monoclonal antibodies and recombinant human ACE2 as fully integrated subjects of high priority. This should help scientists in confirming and verifying the efficacy of those recommended therapeutic strategies in well-designed clinical trials since, as to date, the pathogenesis of COVID-19 is still a vaguely understood subject. Shedding more light onto ED in clinical practice may be more significant than we expect; in this context, a collaborative effort of biomedical and clinical science is urgently required, as this will assist in completing our understanding of the paradigm of the pathogenesis of COVID-19, and in translating our current understanding of the disease to successful treatment strategies.

Acknowledgements

The author would sincerely like to thank the University of Balamand for their unbounded support.

Funding

Funding

No funding was received.

Availability of data and materials

Not applicable.

Authors' contributions

JD wrote and revised the manuscript, and has read and approved the final manuscript. Data authentication is not applicable.

Ethics approval and consent to participate

Not applicable.

Patient consent for publication

Not applicable.

Competing interests

The author declares that he has no competing interests.

References