E1A Sensitizes Cells to Tumor Necrosis Factor Alpha by Downregulating c-FLIP(S)

Tumor necrosis factor alpha (TNF-α) activates both apoptosis and NF-κB-dependent survival pathways, the former of which requires inhibition of gene expression to be manifested. c-FLIP is a TNF-α-induced gene that inhibits caspase-8 activation during TNF-α signaling. Adenovirus infection and E1A expression sensitize cells to TNF-α by allowing apoptosis in the absence of inhibitors of gene expression, suggesting that it may be disabling a survival signaling pathway. E1A promoted TNF-α-mediated activation of caspase-8, suggesting that sensitivity was occurring at the level of the death-inducing signaling complex. Furthermore, E1A expression downregulated c-FLIP(S) expression and prevented its induction by TNF-α. c-FLIP(S) and viral FLIP expression rescued E1A-mediated sensitization to TNF-α by restoring the resistance of caspase-8 to activation, thereby preventing cell death. E1A inhibited TNF-α-dependent induction of c-FLIP(S) mRNA and stimulated ubiquitination- and proteasome-dependent degradation of c-FLIP(S) protein. Since elevated c-FLIP levels confer resistance to apoptosis and promote tumorigenicity, interference with its induction by NF-κB and stimulation of its destruction in the proteasome may provide novel therapeutic approaches for facilitating the elimination of apoptosis-refractory tumor cells.