Distinct caspase cascades are initiated in receptor-mediated and chemical-induced apoptosis

Release of cytochrome c is important in many forms of apoptosis. Recent stu dies of CD95 (Fas/APO-1)-induced apoptosis have implicated caspase-8 cleava ge of Bid, a BH3 domain-containing proapoptotic member of the Bcl-2 family, in this release. We now demonstrate that both receptor-induced (CD95 and t umor necrosis factor) and chemical-induced apoptosis result in a similar ti me-dependent activation of caspases-3, -7, -8, and -9 in Jurkat T cells and human leukemic U937 cells. In receptor-mediated apoptosis, the caspase inh ibitor, benzyloxycarbonyl-Val-Ala-Asp fluoromethyl ketone (Z-VAD.FMK), inhi bits apoptosis prior to commitment to cell, death by inhibiting the upstrea m activator caspase-8, cleavage of Bid, release of mitochondrial cytochrome c, processing of effector caspases, loss of mitochondrial membrane potenti al, and externalization of phosphatidylserine, However, Z-VAD.FMK inhibits chemical-induced apoptosis at a stage after commitment to cell death by inh ibiting the initiator caspase-9 and the resultant postmitochondrial activat ion of effector caspases. Cleavage of Bid but not release of cytochrome c i s blocked by Z-VAD.FMK demonstrating that in chemical-induced apoptosis cyt ochrome c release is caspase-independent and is not mediated by activation of Bid. We propose that caspases form an integral part of the cell death-in ducing mechanism in receptor-mediated apoptosis, whereas in chemical-induce d apoptosis they act solely as executioners of apoptosis.