The most frequent co-morbidities reported with CoVID-19 patients on treatment with ACE inhibitors [22], suggests that ACE inhibition might worsen the consequences of infection. As the host response plays a determinant role in the pathogenesis of infectious diseases, strategies for treatment need to consider both the nature of host response as well as the microbial factors [23, 24]. We therefore questioned whether the effect of ACE inhibition on outcome of SARS-CoV infection is a consequence of its ability to modulate host response to infection rather than target the infection itself. As we reviewed the signalling pathways associated with aberrant inflammation in SARS-CoV infection under the lenses of lung cancer, the existence of a common ACE2- BDKRB1-inflammation cross talk in both became evident.
A common ACE2-BDKRB1 cross talk in lung cancer and SARS-CoV2 infection regulate release of nitric oxide
Given the implication of ACE2-BDKRB1 axis in the pathogenesis of acute lung inflammation [12], BDKRB1 has emerged as an important pharmacological target for treatment of inflammatory lung disorders. Phase I trial with the dual BDKRB1/2 antagonist B9870 (breceptin) has been conducted in lung cancer patients [25]. The predominant function of NOSTRIN is to reduce nitric oxide (NO) levels by sequestering eNOS within intracellular vesicular structures and rendering it inactive [26]. As positive co-relation was noted between ACE2 and NOSTRIN, it is likely that diminished NOSTRIN levels under ACE inhibition could elevate NO levels. Activation of BDKRB1 leads to very high and prolonged release of NO [27], and ACE inhibitors increase availability of NO by preventing the degradation of Bradykinin [28]. Antagonizing bradykinin receptors or nitric oxide synthase abolishes the cardioprotective effect of ACE inhibitor [29]. BDKRB1 is a potent activator of inducible form of NOS (iNOS), and the BDKRB1-iNOS auto-induction and amplification loop enhances inflammation in diabetic retinopathy [30]. This could explain severe COVID-19 infection with co-morbidities in patient with diabetes mellitus who are often treated with ACE inhibitors [31]. mARC-1/2 which showed a positive correlation with ACE2 is known to catalyze NADH-dependent nitrite reduction to NO [32]. It is possible that co-expression of mARC and NOSTRIN under conditions of ACE inhibition play a role in regulating NO levels. Also, monoamine oxidases (MAOA) which is positively correlated with ACE2 negatively regulate IFNg inducible nitric oxide synthase gene expression [33]. Also, the association between PKD1 and ACE2 is interesting as loss of PKD1 expression increases the malignant potential of lung cancer [34], and acute renal failure is an important negative prognostic indicator for survival with SARS [35].
ACE2-BDKRB1 nexus regulates inflammatory millieu
Bradykinin sensitizes EGF induced signalling via Src-dependent enhanced phosphorylation of EGFR [36]. NO also induces activation of EGFR and its downstream signalling pathway [37]. It is tempting to speculate that elevated Bradykinin and NO levels upon ACE inhibition might heighten EGFR activation. Overactive EGFR signaling leads to increased pulmonary fibrosis after SARS-CoV infection [38], indicating that inhibiting EGFR signaling may prevent an excessive fibrotic response to corona viruses [39]. EGFR enables sustained IL-6 dependent STAT3 activation to promote inflammation [40], and IL-6R antagonist Sarilumab (Kevzara) is set to enter for clinical trial in COVID-19. Besides, several EGFR inhibitors are among FDA approved drugs for lung cancer (Supplementary List). Bradykinin induces IL-6 secretion in human airway smooth muscles [41]. While IFN-γ is capable of partly inhibiting SARS-CoV replication through down-regulation of ACE2 [42], it can also amplify pro- inflammatory cytokine mediated increase in BDKRB1 expression and abundance in endothelial cells [43].
Folate and viral load
Considering its positive co-relation with ACE2, inhibiting FOLR1 carries the risk of activating BDKRB1. However, as folic acid and its active metabolite 5-methyl tetrahydrofolate improves NO bioavailability [44], FOLR1 inhibition can reduce NO mediated inflammation. In patient with primary lung cancer, elevated exhaled NO production from alveolar macrophages and increase in iNOS activity has been noted [45]. Given its role in the pathogenesis of inflammation, NO inhibitors are considered in the management of inflammatory diseases [10]. NG-monomethyl-L-arginine (L-NMMA) a nitric oxide synthase inhibitor has been suggested [46], and is in clinical trial for lung cancer (ClinicalTrials.gov Identifier: NCT03236935). Importantly, FOLR1 is over-expressed in lung cancer and among the several FDA approved drugs for lung cancer, folate inhibitors such as Methotrexate (Trexall) and Pemetrexed disodium finds a place (Supplementary List). An ongoing phase II clinical trial is evaluating farletuzumab (a monoclonal antibody targeting FOLR1) in patients with lung adenocarcinoma [47].
Most interestingly, folic acid has a critical role in determining the outcome of lymphocytic choriomeningitis (LCM) infection, as a folic acid antagonist amethopterin enhanced survival of the host despite prolonged viraemia [48]. FOLR1 is a significant cofactor for cellular entry for Marburg and Ebola viruses [49]. Human cytomegalovirus (HCMV) infection stimulates activity of cellular dihydrofolate reductase (DHFR) and inhibition of DHFR activity by methotrexate prevented HCMV replication [50]. Pemetrexed disodium- a novel multitargeted antifolate is used in the treatment of lung cancer (Supplementary list). Recent review has highlighted the importance of folate in natural killer cell dysfunction and viral etiology in type 1 diabetes [51].
Evaluation of anti-lung cancer agents for COVID-19
Our findings suggest that ACE2-BDKRB1 associated molecular pathways regulate inflammatory milieu in the lungs under pathological conditions (Fig. 3b). Also, existing literature points to the requirement of folate for efficient viral replication. Pro-inflammatory cytokines are highly up-regulated in serum from SARS-CoV patients [52, 53]. As the signalling pathways associated with inflammation show striking similarities between SARS- COV2 infection and lung cancers, repurposing of anti-lung cancer drugs for COVID-19 has considerable promise (Fig. 3c). The Damage–Response Framework (DRF) tool that integrates clinical findings with microbiology and immunology [24], highlights the indispensable role of host response in determining the outcome of microbial pathogenesis in response to anti- inflammatory agents. Most COVID-19 deaths involved older adults having had underlying chronic illnesses, with several on ACE inhibitors. Therefore, leveraging host response while targeting (i) ACE2-BDKRB1- inflammatory network and (ii) folate-nitric oxide for COVID- 19 treatment will determine therapeutic success.
Folate, malaria and CoV: Where does the connection lie?
Folate metabolism impairment is associated with malarial parasite infection [54]. The anti- malarial agent hydroxyl-chloroquine (HCQ) with immunomodulatory properties widely used for the treatment of rheumatoid arthritis and systemic lupus erythematosus, has been effective in inhibiting SARS-CoV-2 infection in vitro [55]. It has been suggested that HCQ attenuates COVID-19 progression by inhibiting the cytokine storm [56]. As Bradykinin induced release of prostaglandins is inhibited by chloroquine [57], it is possible that HCQ controls the inflammatory milieu in COVID-19 patients by attenuating Bradykinin induced inflammatory mediators. Also, HCQ inhibits NO production [58]. Though no direct association between HCQ and folate metabolism has been reported, it is also possible that effect of HCQ on COVID-19 is a consequence of its action on FOLR1 dependent nitric oxide generation.