Thiophosphate (SPO3) was recently shown to promote cysteine insertion at Sec (selenocysteine)-encoding UGA codons during selenoprotein synthesis. We reported previously that irreversible targeting by cDDP [cis-diamminedichloroplatinum(II) or cisplatin] of the Sec residue in TrxR1 (thioredoxin reductase 1) contributes to cDDP cytotoxicity. This effect could possibly be attenuated in cells expressing less reactive Sec-to-cysteine-substituted TrxR1 variants, or pronounced in cells with higher levels of Sec-containing TrxR1. To test this, we supplemented cells with either SPO3 or selenium and subsequently determined total as well as specific activities of cellular TrxR1, together with extent of drug-induced cell death. We found that cDDP became less cytotoxic after incubation of A549 or HCT116 cells with lower SPO3 concentrations (100–300 μM), whereas higher SPO3 (>300 μM) had pronounced direct cytotoxicity. NIH 3T3 cells showed low basal TrxR1 activity and high susceptibility to SPO3 cytotoxicity, or to glutathione depletion. Supplementing NIH 3T3 cells with selenite, however, gave increased cellular TrxR1 activity with concomitantly decreased dependence on glutathione, whereas the susceptibility to cDDP increased. The results suggest molecular mechanisms by which the selenium status of cells can affect their glutathione dependence while modulating the cytotoxicity of drugs that target TrxR1.

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