E1A-induced Processing of Procaspase-8 Can Occur Independently of FADD and Is Inhibited by Bcl-2

Expression of the 243-residue form of the adenovirus E1A protein in the absence of other viral proteins triggers apoptosis by a pathway that requires p53. This pathway includes processing and activation of initiator procaspase-8, redistribution of cytochrome c, and activation of procaspase-3. Bcl-2 functions at or upstream of procaspase-8 processing to inhibit all of these events and prevent cell death. This contrasts with the anti-apoptotic influence of Bcl-2 family proteins in the cell death pathway induced by Fas ligand or tumor necrosis factor (TNF), in which Bcl-2 typically acts downstream of Fas/TNFR1-mediated activation of caspase-8. Moreover, E1A induces procaspase-8 processing and cell death in cells deleted of FADD, an adaptor protein critical for Fas/TNFR1 activation of caspase-8. The results indicate that E1A is capable of activating caspase-8 by a Bcl-2-inhibitable pathway that does not involve autocrine stimulation of FADD-dependent death receptor pathways.