The proposal of a new pharmacotherapy: the targeting of ACE2 and ADAM17 zinc-metalloproteases
In previous works, we have highlighted that the binding of the SARS-CoV spike proteins to the zinc-metalloprotease ACE2 has been shown to induce ACE2 shedding by activating the zinc-metalloprotease ADAM17, which ultimately leads to systemic upregulation of ACE2 activity (27,28, 31 ). Indeed, the interaction of ACE2 with spike protein of SARS-CoV is able to induce a cellular “protective” ACE2 shedding feedback response that initially limits viral entry (31 ). However, an excessive systemic ACE2 activity produced by ACE2 shedding would be at the origin of COVID-19, which would be induced by positive feedback-loops initially triggered by SARS-CoV-2 and subsequently sustained independently on viral trigger (see 27,28 ). Indeed, a few experimental models have shown that an upregulation of pathways downstream ACE2 activity (i.e. Mas receptor and angiotensin receptor 2) can leads to “clinical” manifestations resembling COVID-19 (see28 ). Moreover, strong upregulation of circulating ACE2 activity was recently reported in COVID-19 patients (32 ) and different comorbidities associated with patients’ critical status are characterised by pre-existing increased ACE2 activity (see27,28 ), thus supporting the hypothesis that COVID-19 may derive from excessive upregulation of ACE2 activity. Notably, in contrast to SARS-CoVs, HNL63-CoV, which similarly binds to ACE2 through its spike protein, infects ACE2-bearing cells that leads to common cold without inducing both ACE2 shedding and SARS (31 ). That means that the recruitment of ADAM17 in close proximity to ACE2 is a crucial event in order to induce both ACE2 shedding and consequent deleterious effects of SARS-CoVs. It is well known that spike protein has a receptor binding domain (RBD) that is decisively involved in the ACE2-mediated viral binding/entry. Within the RBD, there is an immunodominant SARS-CoV-2 receptor binding motif (RBM, spanning residues 438 to 506), which is the primary target of the neutralizing antibody response induced by SARS-CoV-2 vaccination or infection. The selective pressure of the immune system in vaccinated or convalescent people is known to induce an adaptation process of SARS-CoV-2 that constantly generates a heterogeneous pool of SARS-CoV-2 variants. As expected, the RBM is one the highest variable regions of the S protein (17 ) and SARS-CoV-2 spike variants that increase the binding affinity to human ACE2 and/or confer resistance to neutralising antibodies (such as N439K, Y453F, E484K and N501Y mutations, 17 ) are often located in the RBM. Notably, the highest variable region of the entire S protein is surprisingly the N-terminal domain (17 ) and recurrent deletions are preferentially detected in the NTD of the spike glycoprotein (33 ), suggesting that this region may have yet unknown functions involved in SARS-CoV-2 infection. Interestingly, a two-amino-acid deletion at position 69-70 in the NTD often co-occurs with one of the mutations in RBM (N501Y, N439K, or Y453F) (9, 17, 24 ), suggesting that RBM and NTD could participate together in the binding process and in the infectious development of SARS-CoV-2 variants. If this would be the case, these mutations could possibly affect both the binding affinity of SARS-CoV-2 virions to ACE2-ADAM17 expressing cells and the shedding of ACE2 (mediated by ADAM17 recruitment) that leads downstream to induction of pathological events.
Based on this, a reasonable hypothesis of using inhibitors that curb the upregulation of both ACE2 and ADAM17 zinc-metalloprotease activities can be proposed as therapy for COVID-19. In particular, zinc-chelating agents such as citrate and ethylenediaminetetraacetic acid (EDTA) alone or in combination are expected to act in protecting from COVID-19 at different levels thanks to their both anticoagulant properties and inhibitory activity on zinc-metalloproteases (see 27,28 ). Several zinc-chelating agents as well as specific RAS pathway inhibitors (e.g., phytates, nicotianamine, zeolites, MLN-4760, Dx600, A779, aliskiren) and specific ADAM17 inhibitors (34 ) have been already proposed as anti-SARS-CoV-2 agents whose administration routes and safety concerns have been widely discussed in previous works (see27,28 ). Among them, chelating agent supplementation during the SARS-CoV-2 pandemic as an adjunct treatment to reduce the risk of infection and severe disease progression is a feasible treatment because oral supplementation of chelating agents is well-tolerated, safe, promptly available, easily deliverable and storable, inexpensive and practically usable also in developing countries. Unfortunately, clinical trials employing chelating agents in COVID-19 are currently absent and urgently needed in order to shed more light on the efficacy of zinc chelators against SARS-CoV-2 infection in vivo.