Understanding Selenium and Glutathione as Antiviral Factors in COVID-19: Does the Viral Mpro Protease Target Host Selenoproteins and Glutathione Synthesis?

Abstract

Glutathione peroxidases (GPX), a family of antioxidant selenoenzymes, functionally link selenium and glutathione, which both show correlations with clinical outcomes in COVID-19. Thus, it is highly significant that cytosolic GPX1 has been shown to interact with an inactive C145A mutant of M pro , the main cysteine protease of SARS-CoV-2, but not with catalytically active wild-type M pro . This seemingly anomalous result is what might be expected if GPX1 is a substrate for the active protease, leading to its fragmentation. We show that the GPX1 active site sequence is substantially similar to a known M pro cleavage site, and is identified as a potential cysteine protease site by the Procleave algorithm. Proteolytic knockdown of GPX1 is highly consistent with previously documented effects of recombinant SARS-CoV M pro in transfected cells, including increased reactive oxygen species and NF-κB activation. Because NF-κB in turn activates many pro-inflammatory cytokines, this mechanism could contribute to increased inflammation and cytokine storms observed in COVID-19. Using web-based protease cleavage site prediction tools, we show that M pro may be targeting not only GPX1, but several other selenoproteins including SELENOF and thioredoxin reductase 1, as well as glutamate-cysteine ligase, the rate-limiting enzyme for glutathione synthesis. This hypothesized proteolytic knockdown of components of both the thioredoxin and glutaredoxin systems is consistent with a viral strategy to inhibit DNA synthesis, to increase the pool of ribonucleotides for RNA synthesis, thereby enhancing virion production. The resulting “collateral damage” of increased oxidative stress and inflammation would be exacerbated by dietary deficiencies of selenium and glutathione precursors.

Identification of Host Cellular Protein Substrates of SARS-COV-2 Main Protease

“…The relatively higher accessibilities of cleavage sites are in agreement with the susceptibilities of CTBP1 and IRAK1 for proteolysis in vitro ( Figure 5 ) and indicate that probabilities of cleavage sites predicted by the NetCorona webserver need to be interpreted by considering surface accessibilities of putative sites, as well. Our result highlights that determination of apparent accessibilities of cleavage sites in the protein structures is not sufficient enough ( Figure 2 ); in agreement with the results of Taylor and Radding [ 29 ], we also suggest the detailed determination of structural characteristics, especially the calculation of numerical SASA values ( Figure 7 ) for more reliable cleavage site prediction.…”

“…Additionally, we assumed that structural contexts of the putative cleavage sites need to be considered, therefore, accessibilities of target regions were also determined. A similar in silico approach has already been applied for the identification of potential cleavage sites in host selenoproteins and enzymes of glutathione synthesis [ 29 ], but neither proteomic [ 21 ] nor specifically targeted analyses proved cleavages of these targets in vitro to date. Interestingly, PAI2 was identified as a substrate of SARS-CoV and hCoV-NL63 3CLpro, as well, while cleavage of PLMN was not detected by a proteomic analysis [ 21 ].…”

“…This algorithm was applied previously to predict cleavage sites in the nucleocapsid protein of porcine epidemic diarrhea virus (PEDV) 3CLpro [ 27 ], in the equine coronavirus polyprotein [ 28 ], or in human protein targets of SARS-CoV 3CLpro while developing the method [ 26 ]. In the case of SARS-CoV-2 3CLpro, glutathione peroxidase 1, selenoprotein F, and thioredoxin reductase 1 were proposed to be host substrates by using in silico algorithms [ 29 ], but the results were not validated in vitro. These proteins were not identified in the recently reported proteomic analysis as substrates of SARS-CoV-2 3CLpro [ 21 ], although, in vitro identification of host targets in additional cell types remain to be performed.…”

SARS-CoV-2 suppresses mRNA expression of selenoproteins associated with ferroptosis, endoplasmic reticulum stress and DNA synthesis

“…This is interesting in light of a recent report that SELENOF may be targeted for proteolysis by the SARS-CoV-2 main protease M pro , because the SELENOF protein contains a sequence (TVLQ/AVSA) that is almost identical to a known viral M pro cleavage site (TVLQ/AVGA). 16 Taken together, these observations suggest that disruption of SELENOF function may be particularly important for SARS-CoV-2 replication.…”

Severe acute respiratory syndrome coronavirus 2 ORF3a induces the expression of ACE2 in oral and pulmonary epithelial cells and the food supplement Vita Deyun® diminishes this effect

“…Regarding selenium, Taylor and Radding ( 55 ) mentioned that the resulting collateral damage due to increased oxidative stress and inflammation would be exacerbated by dietary deficiency of selenium and GSH precursors. In the review by Hiffler and Rakotoambinina ( 39 ), it was observed that low-selenium status is common in individuals considered at risk of developing severe COVID-19, particularly in the elderly.…”