Bcl-2 inhibits apoptosis induced by mitochondrial uncoupling but does not prevent mitochondrial transmembrane depolarization

Bcl-2 overexpression protects cells hom apoptosis induced by many cytotoxic agents. In this study, we investigated the effects of uncoupling mitochond rial electron transport in both HL60 wild-type and Bcl-2-overexpressing cel ls using the protonophore carbonyl cyanide m-chlorophenylhydrazone. We foun d that uncoupling mitochondrial electron transport induced apoptosis in wil d-type, but not in Bcl-2-overexpressing cells. To investigate the mechanism of action of Bcl-2-mediated inhibition of cyanide m-chlorophenylhydrazone- induced apoptosis, we measured the mitochondrial transmembrane potential (D elta Psi (m)) after uncoupling mitochondrial electron transport and found t hat both HL-60 wild-type and Bcl-2-overexpressing cells similarly depolariz e following cyanide m-chlorophenylhydrazone exposure. Western blot analysis demonstrated that Bcl-2 overexpression did not completely block cytochrome c release from mitochondria after uncoupling mitochondrial electron transp ort. Since Bcl-2 may act as an antioxidant, me studied the effect of alteri ng the cellular redox state prior to uncoupling mitochondrial electron tran sport in Bcl-2-overexpressing cells. Depletion of mitochondrial (but not cy tosolic) glutathione induced apoptosis in Bcl-2-overexpressing cells and ne gated the protective effect of Bcl-2. Furthermore, following glutathione de pletion, Bcl-2-overexpressing cells were sensitized to undergo cyanide m-ch lorophenylhydrazone-induced apoptosis. These data suggest that the action o f Bcl-2 is dependent, in part, on the cellular and mitochondrial redox stat e. (C) 2001 Academic Press.