p38α, but not p38β, inhibits the phosphorylation and presence of c-FLIPS in DISC to potentiate Fas-mediated caspase-8 activation and type I apoptotic signaling

Pharmacological inhibitors of JNK (SP600125) and p38 (PD169316) sensitize tumor cells to Fas-mediated apoptosis. PD169316 is less potent than SP600125 and diminishes its effect when present together. Because the p38 isoforms that promote (p38α) or inhibit (p38β) apoptosis are both suppressed by PD169316, we investigated their regulatory involvement in Fas-signaling. We report here, that p38α, but not p38β, exerts its proapoptotic effect by inhibiting the phosphorylation and presence of c-FLIPS, but not c-FLIPL, in the DISC to promote caspase-8 activation and type I signaling in Fas-activated Jurkat cells. Its effect was enhanced by enforced expression of Flag-tagged p38α and was attenuated by its inactive mutant (p38α-AGF) or by translational silencing. By contrast, type II signaling was facilitated by p38α-dependent mitochondrial presence of tBid and inhibition of Bcl-2 (Ser70) phosphorylation as well as by p38α/β-dependent mitochondrial localization of Bax and inhibition of phosphorylation of Bad (Ser112/Ser155). Potentiation of Fas-mediated apoptosis by the inhibition of JNK1/2 correlated with the loss of Bad (Ser136) phosphorylation and was dependent on the stimulatory effect of p38α on DISC and the downstream effects of both p38α and p38β. These data underscore the need to reassess the findings obtained with pan-p38 inhibitors and suggest that activation of p38α coupled with targeted inhibition of p38β and JNK1/2 should optimally sensitize tumor cells to Fas-mediated apoptosis.