Signaling events triggered by tumor necrosis factor-related apoptosis-inducing ligand (TRAIL): Caspase-8 is required for TRAIL-induced apoptosis

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a TNF family member and potent apoptosis inducer. In contrast to TNF-α or Fas ligand, relatively little is known about the signaling events activated by TRAIL. In particular, the initial caspase(s) required for TRAIL-induced apoptosis remains to be determined. Caspase-3-like protease but not caspase-1-like protease (YVADase) activity rapidly increased in HeLa cells in response to TRAIL treatment. The increase in protease activity correlated with the profile of apoptotic cell death that was inhibited by the pan-caspase inhibitor Z-VAD-fmk. In response to TRAIL, caspase-8, an initiator caspase in death receptor-mediated apoptosis, was activated within 1 h in association with Bid cleavage, cytochrome c release, caspase-3 activation, and DNA fragmentation factor 45 cleavage. Z-IETD-fmk, a caspase-8 inhibitor, completely blocked caspase-8 activation and resulted in inhibition of caspase-3 (a caspase-3-like protease) activation and apoptotic cell death. Overexpression of a caspase-8 dominant negative mutant inhibited apoptosis induced by TRAIL. Caspase-8-deficient Jurkat cells were resistant to both TRAIL- and Fas-induced apoptosis, whereas wild-type Jurkat cells were susceptible to both TRAIL- and Fas-induced apoptosis. The caspase-8-reintroduced caspase-8-deficient Jurkat cells acquired normal susceptibility to both TRAIL and agonistic Fas antibody. Reverse transcription-PCR and sequence analyses have revealed that these caspase-8-deficient Jurkat cells express wild-type caspase-10. Therefore, our data indicate that caspase-8 is required for TRAIL-induced apoptosis and suggest that caspase-10 may play a minor role, if any, in TRAIL-induced apoptosis.