Bcl-2 and caspase inhibition cooperate to inhibit tumor necrosis factor-α-induced cell death in a Bcl-2 cleavage-independent fashion

The ability of proteins of the Bcl-2 family to either induce or inhibit apoptosis is dependent on both cell type and the apoptotic stimulus. We have shown in the murine pro-B cell line FL5.12 that Bcl-2 is incapable of inhibiting tumor necrosis factor alpha (TNF alpha)-induced cell death and is cleaved during this process. One potential explanation for this observation is that caspase activation directly or indirectly inhibits Bcl-2 function. It has been suggested that caspase cleavage of Bcl-2 is responsible for its inability to block certain cell deaths. Consistent with Bcl-2 cleavage being a caspase-mediated event, this cleavage is inhibitable by 50 mu M CBZ-Val-Ala-Asp-fluoromethylketone (zVAD-fmk). Furthermore, Bcl-2 can cooperate with the caspase inhibitor zVAD-fmk in a dose-dependent manner to block TNF alpha-induced cell death. Overexpression of Bcl-2 results in a 10-fold decrease in the amount of zVAD-fmk required to inhibit TNF alpha-induced apoptosis. However, cleavage-defective mutants (D31A and D34A) show no enhanced viability relative to wild-type Bcl-2 in response to TNF alpha-induced cell death and also show the same cooperativity with zVAD-fmk. These results suggest that Bcl-2 cleavage is not important for the inhibition of TNF alpha-induced cell death but do not preclude an involvement in a post-commitment phase of apoptosis.