Rapid induction of cell death by selenium-compromised thioredoxin reductase 1 but not by the fully active enzyme containing selenocysteine.

Mammalian thioredoxin reductases are selenoproteins. For native catalytic activity, these enzymes utilize a C-terminal -Gly-Cys-Sec-Gly-COOH sequence (where Sec is selenocysteine) forming a redox active selenenylsulfide/selenolthiol motif. A range of cellular systems depend upon or are regulated by thioredoxin reductase and its major protein substrate thioredoxin, including apoptosis signal-regulating kinase 1, peroxiredoxins, methionine sulfoxide reductase, and several transcription factors. Cytosolic thioredoxin reductase 1 (TrxR1) is moreover inhibited by various electrophilic anticancer compounds. TrxR1 is hence generally considered to promote cell viability. However, several recent studies have suggested that TrxR1 may promote apoptosis, and the enzyme was identified as GRIM-12 (gene associated with retinoid interferon-induced mortality 12). Transient transfection with GRIM-12/TrxR1 was also shown to directly induce cell death. To further analyze such effects, we have here employed lipid-mediated delivery of recombinant TrxR1 preparations into human A549 cells, thereby bypassing selenoprotein translation to facilitate assessment of the protein-related effects on cell viability. We found that selenium-deficient TrxR1, having a two-amino acid-truncated C-terminal -Gly-Cys-COOH motif, rapidly induced cell death (38 +/- 29% apoptotic cells after 4 h; p < 0.005 compared with controls). Cell death induction was also promoted by selenium-compromised TrxR1 derivatized with either cis-diamminedichloroplatinum (II) (cisplatin) or dinitrophenyl moieties but not by the structurally related non-selenoprotein glutathione reductase. In contrast, TrxR1 with intact selenocysteine could not promote cell death. The direct cellular effects of selenium-compromised forms of TrxR1 may be important for the pathophysiology of selenium deficiency as well as for the efficacy of antiproliferative drugs targeting the selenocysteine moiety of this enzyme.