Differential role of caspase-8 and BID activation during radiation- and CD95-induced apoptosis

Activation of the CD95 death receptor as well as ionizing radiation induces apoptotic cell death in human lymphoma cells. The activation of caspases i s a hallmark of apoptosis induction irrespective of the apoptotic trigger. In contrast to death receptor signaling, the exact mechanisms of radiation- induced caspase activation are not well understood. We provide evidence tha t both, radiation and CD95 stimulation, induce the rapid activation of casp ase-8 and BID followed by apoptosis in Jurkat T-cells, To analyse the relat ive position of caspase-8 within the apoptotic cascade we studied caspase a ctivation and apoptosis in Jurkat cells overexpressing Bcl-2 or Bcl-x(L). C aspase-8 activation, proapoptotic BID cleavage and apoptosis in response to radiation were abrogated in these cells, while the responses to CD95 stimu lation were only partially attenuated by overexpression of Bcl-2 family mem bers. In parallel, the breakdown of the mitochondrial transmembrane potenti al (Delta Psi(m),) in response to radiation was inhibited by overexpression of Bcl-2/Bcl-x(L) Jurkat cells genetically deficient for caspase-8 were fo und to be completely resistant towards CD95, However, radiation-induced apo ptotic responses in caspase-8-negative cells displayed only a modest reduct ion. We conclude that ionizing radiation activates caspase-8 and BID downst ream of mitochondrial damage suggesting that, in contrast to CD95, both eve nts function as executioners rather than initiators of the apoptotic proces s.