Phosphatidylinositol 3-kinase/Akt activity regulates c-FLIP expression in tumor cells

The caspase-8 homologue FLICE-inhibitory protein (FLIP) functions as a casp ase-8 dominant negative, blocking apoptosis induced by the oligomerization of the adapter protein FADD/MORT-1. FLIP expression correlates with resista nce to apoptosis induced by various members of the tumor necrosis factor fa mily such as TRAIL. Furthermore, forced expression of FLIP renders cells re sistant to Fas-mediated apoptosis. Although FLIP expression is regulated pr imarily by MEK1 activity in activated T cells, the oncogenic signaling path ways that regulate FLIP expression in tumor cells are largely unknown. In t his report, we examined the roles of the MAP kinase and phosphatidylinosito l (PI) 3-kinase signaling pathways in the regulation of FLIP expression in tumor cells. We observed that the MEK1 inhibitor PD98059 reduced FLIP level s in only 2 of 11 tumor cell lines tested. In contrast, disruption of the P I 3-kinase pathway with the specific inhibitor LY294002 reduced Akt (protei n kinase B) phosphorylation and the levels of FLIP protein and mRNA in all cell lines evaluated. The introduction of a dominant negative Akt adenovira l construct also consistently reduced FLIP expression as well as the phosph orylation of the Akt target glycogen synthase kinase-3. In addition, infect ion of the same cell lines with a constitutively active Akt adenovirus incr eased FLIP expression and the phosphorylation of GSK-5, These data add FLIP to the growing list of apoptosis inhibitors in which expression or functio n is regulated by the PI 3-kinase-Akt pathway.