The BH3 domain is required for caspase-independent cell death induced by Bax and oligomycin

Bax causes apoptosis by associating with mitochondria and triggering cytochrome c release, which activates the caspase cascade. Bax can also kill some cells independently of caspases, but the requirements for such killing are poorly understood. Here we describe an inducible fibroblast line that expresses Bax when tetracycline is withdrawn; the resulting apoptosis can be blocked by the caspase inhibitor zVAD-fmk. Even when caspases are inhibited, however, treating the Bax-expressing cells with the mitochondrial toxin oligomycin efficiently triggers death with features resembling apoptosis. Bax mutants lacking the BH3 domain remain able to cause cytochrome c release and caspase-mediated death, but cannot support this caspase-independent killing. Mutating specific BH3 residues needed for binding Bcl2 does not prevent synergy with oligomycin, implying that no such binding is required. These findings illuminate a caspase-independent pathway of death that depends on the Bax BH3 domain and on effectors emanating from mitochondria.