“Increased ACE 2 expression is a risk factor for SARS-CoV-2 infection”

ACE 2 expression has been detected for example in the heart, kidneys, gastrointestinal tract (GIT), lungs, brain, testes, bladder, adipose tissue, vascular system.5,8,14,15 Recently, ACE 2 expression was detected in cholangiocytes,16 in the epithelial cells of the oral17 and nasal mucosa.18 Various modes of SARS-CoV-2 transmission are possible due to the ubiquitous expression of ACE 2. The possibility of infection through the GIT is discussed in the initial transmission from an animal to humans.19 Intrapopulation faecal-oral transmission is reported, especially in children.20 Nonetheless, the droplet transmission appears to be the most significant way for the intrapopulation transmission.21

Respiratory tract

Why respiratory tract and ARDS?

Zhao et al.22 analysed lung tissue of 8 human lung donors and detected concentrated expression of ACE 2 RNA in a small group of pulmonary cells. Most of them (83%) were type II alveolar cells (AT 2). Surprisingly, only 1.4 % of the AT 2 expressed ACE 2 RNA, and, compared to the AT 2 not expressing RNA ACE 2, this group expressed other genes facilitating viral reproduction and transmission. The SARS-CoV-2 spike protein contains a sequence that can be cleaved into the S1 and S2 domains. Subsequent cleavage facilitates fusion of the virion into the host cell.23,24 This cleavage may be performed via a protease called furin 23 which is also used by other respiratory viruses to penetrate into a host cell (but it is not utilised e.g. by the original SARS-CoV),23,25 or by transmembrane serine protease 2 (TMPRSS2).24 Both proteases are significantly expressed in the respiratory tract.25 So, it seems to be crucial that in addition to the ACE 2 expression itself, SARS-CoV-2 requires the expression of further genes and activation of metabolic pathways for its replication. This explains why lungs are the most vulnerable organ.

Smoking

Several studies documenting increased ACE 2 expression in smokers have been published in relation to the current COVID-19 pandemics. Immunohistochemistry analyses revealed higher expression of ACE 2 proteins in smokers as compared to non-smokers and increased ACE 2 expression in smokers with chronic obstruction pulmonary disease (COPD) as compared to healthy smokers.26,27 Similarly to Zhao et al.,22 ACE 2 was expressed in AT 2,26 in alveolar macrophages26 and the small airway epithelium.26,27
Also studies28–30 based on transcriptome analyses confirm the increased expression of RNA ACE 2 in lungs of smokers when compared to non-smokers. Increased expression of ACE 2 RNA correlated with the expression of genes included in the process of replication of SARS-CoV-2.28,29Furthermore, there is a positive correlation between expression of ACE 2 RNA and the length and intensity of exposition to cigarette smoke in mice.28 Higher ACE 2 expression in smokers seems to be part of a complex change of metabolic processes facilitating the replication of SARS-CoV-2. Considering the fact that smoking is a general risk factor for many respiratory diseases31, including the Middle East Respiratory Syndrome (MERS),32 smoking seems to be a clear risk factor for incidence of COVID-19 infection.
It is therefore surprising that we do not have evidence significantly showing epidemiological association between smoking and increased incidence of SARS33 or COVID-19. Contrary to that, Miyara et al.,34 for instance, compared the ratio of smokers among 482 French patients with symptomatic COVID-19 with the ratio of smokers in the general population and found out that smoking seems to be a protective factor for development of symptomatic COVID-19. Pharmacologically, they explained their results by nicotine-induced reduction of ACE 2 expression.35 This hypothesis is therefore contrary to studies where smoking increased ACE 2 protein expression in pulmonary tissue in humans.26,27However, the level of ACE 2 expression in pulmonary tissue of patients using nicotine patches compared to smokers would be of interest.
We have evidence of undetected patients infected with SARS-CoV-2.36–38It is therefore possible that current data does not correspond to the prevalence of SARS-CoV-2 infection among smokers compared to non-smokers in the general population, but only to the ratio of smokers and non-smokers in severe cases. But, studies analysing smoking as a risk factor worsening the course of COVID-19 provide contradictory findings. There are studies showing smoking as a risk factor for COVID-19 severity,39,40while other studies disprove the association between smoking and COVID-19 severity.41,42 So, it will be extremely interesting to wait for the results of larger epidemiological studies on smoking, e.g. from Iceland colleagues who are testing a large part of the population for the presence of SARS-CoV-2.43,44

Other factors increasing the expression of ACE 2

The relationship between other factors and possible increase in ACE 2 expression in the respiratory tract is more ambiguous. Several transcriptome analyses with non-coherent conclusions are available. Cai30 did not detect a difference in ACE 2 RNA expression between women and men, individuals younger and older than 60 years of age, or various ethnic groups, which contradicts the study of Chen et al..45 Chen et al. found increased expression of ACE 2 RNA in women, East Asian population and suggests negative correlation between ACE 2 expression and age. Furthermore, he found decreased expression of ACE 2 RNA in diabetic patients, contradicting Pinto et al.,29 who found increased expression of ACE 2 RNA in patients with diabetes mellitus, hypertension and COPD. In addition to the ambiguity, the possible discrepancy between RNA expression and protein levels of ACE 246 is yet another limiting factor of these analyses.

Benefit of reduced pulmonary ACE 2 expression?

With increased expression of ACE 2 being a risk factor for SARS-CoV-2 infection, a theoretical possibility to reduce the risk of SARS-CoV-2 infection appears to be ACE 2 blockage in the pulmonary tissue. Inhibitor of ACE 2 was discovered in silico and it is supposed to prevent the interaction between ACE 2 and SARS-CoV,47but the consequences of the inhibition of the generally protective ACE 2 have not been evaluated in vivo . Chloroquine and hydroxychloroquine are used in the clinical practice wherein the inhibition of the synthesis of sialic acids and the subsequent decrease in ACE 2 glycosylation have been described as one of their mechanisms of action in the treatment of COVID-19. Reduced ACE 2 glycosylation decreases the affinity between SARS-CoV-2 and ACE 2.48

Tissues outside the respiratory tract

Despite high ACE 2 expression, tissues outside the respiratory tissue seem less significant for the pathogenesis of COVID-19. This may be caused by the absence of further enzymes amplifying the replication cycle of the SARS-CoV-2. However, extrapulmonary tissue cannot be completely excluded from the pathogenesis of COVID-19. It is clearly important for the initial contact between SARS-CoV-2 and the host with the subsequent transfer of the virus into the pulmonary tissue. Furthermore, there is evidence of the role of SARS-CoV-2 in the pathogenesis outside the respiratory tract. The presence of SARS-CoV-2 and the subsequent tissue damage are suggested in GIT,20,49 central nervous system (CNS),50 kidneys51 and liver.16 Decreased expression of ACE 2 on the surface of platelets may contribute to the procoagulant state.52 Similarly, SARS affected extrapulmonary tissue, e.g. in a study analysing cardiac tissue of patients who succumbed to SARS, virus was detected in 35% of patients, with correlating downregulation of ACE 2 expression in cardiac tissue.53

Pharmacological increase of ACE 2 expression

We hold evidence that some medication can increase the expression of ACE 2. Regrettably, there are no studies focusing on the pharmacological modulation of ACE 2 expression specifically in the pulmonary tissue. The influence of ACE-I and AT1 R blockers is a matter of intense discussion.54–59 But, mineralcorticoid receptor antagonists, statins, thiazolidinediones,59 ibuprofen, as representatives of non-steroidal anti-inflammatory drugs,60 are also associated with increased ACE 2 expression.
Recently published studies analysing the available evidence linking the effect of ACE-I and AT1 R blockers to increased ACE 2 expression did not demonstrate their clear and general effect on the increase of ACE 2 expression in humans.55,59 Furthermore, the different effect of ACE-I and AT1 R blockers on ANG II concentration is also discussed. While ACE-I, beta blockers or direct renin inhibitors decrease the plasma concentration of ANG II, AT1 R increases the plasma concentration of ANG II.58 Some authors57 state that the increased concentration of ANG II as a substrate for ACE 2 may lead to an increase in ACE 2 expression. On the other hand, ANG II has been shown to downregulate ACE 2 mRNA and protein expression levels in hypertensive conditions.61 Therefore, some authors58 warn of a possible upregulation in ACE 2 expression induced by beta blockers or direct renin inhibitors.
The extent of the possible increase in ACE 2 expression is also unknown. There are data pointing to a fold increase in ACE 2 expression compared to control groups,59 as well as studies where pharmacotherapy significantly increased the expression of ACE 2, but by far did not reach the level of the control groups.60,62 It is clear that any suggestions to changes in pharmacotherapy56 must be better supported by experimental data. All the more so, as the data published so far have not shown any negative association between drugs affecting the RAS system and COVID-19.63–68