Bcl-x(L) inhibits cytochrome c release but not mitochondrial depolarization during the activation of multiple death pathways by tumor necrosis factor-alpha

Cells can respond differently to anti-CD95 antibody treatment. Type I cells show strong activation of caspase-8 and directly activate caspase-3, Type II cells weakly activate caspase-8 and must amplify their death signal thro ugh the mitochondria, These cells can be rescued by Bcl-x(L). Here we show that tumor necrosis factor-alpha induces both Type I and II pathways, which can be inhibited by benzyloxycarbonyl-val-Ala-Asp-fluoromethyl ketone (Z-V AD-fmk) and Bcl-x(L) in a cooperative fashion. Death induced in the presenc e of Z-VAD-fmk was associated with a partial inhibition of caspase-8, where as no effects on cytochrome c release, DEVDase activity, and intranucleosom al DNA cleavage were ob served. Thus, Z-VAD-fmk is likely weakening the dea th-inducing signaling complex-mediated activation of caspase-8 and divertin g cells to a Type TI pathway, Bcl-x(L) cooperates with Z-VAD-fmk by blockin g the Type II pathway at the level of cytochrome c release. Surprisingly, a lthough Bcl-xL was able to block cytochrome c release, it was unable to blo ck mitochondrial depolarization, suggesting that these are separate events, This suggests that mitochondria occupy two places in apoptotic signaling, as initiators of apoptosis through the release of cytochrome c as well as a target for effector caspases.