Selenite Negatively Regulates Caspase-3 through a Redox Mechanism*

Selenium, an essential biological trace element, exerts its modulatory effects in a variety of cellular events including cell survival and death. In our study we observed that selenite protects HEK293 cells from cell death induced by ultraviolet B radiation (UVB). Exposure of HEK293 cells to UVB radiation resulted in the activation of caspase-3-like protease activity, and pretreatment of the cells with z-DEVD-fmk (N-benzyloxycarbonyl-Asp-Glu-Val-Asp-fluoromethylketone), a caspase-3 inhibitor, prevented UVB-induced cell death. Interestingly, enzymatic activity of caspase-3-like protease in cell lysates of UVB-exposed cells was repressed in vitro by the presence of selenite. Selenite also inhibited the in vitro activity of purified recombinant caspase-3 in cleaving Ac-DEVD-pNA (N-acetyl-Asp-Glu-Asp-p-nitroanilide) or ICAD(L) (inhibitor of a caspase-activated deoxyribonuclease) and in the induction of DNA fragmentation. The inhibitory action of selenite on a recombinant active caspase-3 could be reversed by sulfhydryl reducing agents, such as dithiothreitol and beta-mercaptoethanol. Furthermore, pretreatment of cells with selenite suppressed the stimulation of the caspase-3-like protease activity in UVB-exposed cells, whereas dithiothreitol and beta-mercaptoethanol reversed this suppression of the enzymatic activity. Taken together, our data suggest that selenite inhibits caspase-3-like protease activity through a redox mechanism and that inhibition of caspase-3-like protease activity may be the mechanism by which selenite exerts its protective effect against UVB-induced cell death.