Association of TRAF2 with the short form of cellular FLICE-like inhibitory protein prevents TNFR1-mediated apoptosis

Background: We have previously shown that c-FLIPL is a more potent inhibitor than c-FLIPS of Fas ligand-induced apoptosis and that c-FLIPL physically binds to Daxx, an alternative Fas-signaling adaptor. Here we examined whether c-FLIPS effectively inhibits TNFR1-mediated apoptosis and triggers JNK activation through its interaction with TRAF2. Results: Some cancer cell lines, such as DU145, AGS, and PC3, have higher levels of c-FLIPS than other cell lines, such as SNU-719 and T24. The expression of c-FLIPS correlated with the susceptibility to TNFR1-mediated apoptosis. In contrast to DU145 and PC3, which are resistant to TNFR1-mediated apoptosis, T24 and SNU719 were sensitive to TNF-α treatment. To address the role of c-FLIP_s in TNFR1-mediated apoptosis, we examined the molecular interaction between c-FLIP_s and TRAF2. As expected, western blot analysis revealed that TRAF2 antibody immunoprecipitated a greater amount of c-FLIP_s than c-FLIP_L. Also, we measured the involvement of c-FLIP_s in TNF-α-induced JNK activation and apoptosis by comparing these in TNF-α-resistant and TNF-α-sensitive cell lines. Treatment with TNF-α increased the phosphorylated JNK level in SNU719 and T24 cells, whereas DU145 and AGS cells were resistant to TNF-α-mediated apoptosis. Conclusion: We now report that the short form of c-FLIP_s is a more efficient inhibitor of TNF-receptor 1-mediated apoptosis signaling than the long form of the protein.