c-FLIPL enhances anti-apoptotic Akt functions by modulation of Gsk3β activity.

Akt is a serine-threonine kinase that has an important role in transducing survival signals. Akt also regulates a number of proteins involved in the apoptotic process. To find new Akt interactors, we performed a two-hybrid screening in yeast using full-length Akt cDNA as bait and a human cDNA heart library as prey. Among 200 clones obtained, two of them were identified as coding for the c-FLIP(L) protein. c-FLIP(L) is an endogenous inhibitor of death receptor-induced apoptosis through the caspase-8 pathway. Using co-immunoprecipitation experiments of either transfected or endogenous proteins, we confirmed the interaction between Akt and c-FLIP(L). Furthermore, we observed that c-FLIP(L) overexpression interferes with Gsk3-β phosphorylation levels. Moreover, through its effects on Gsk3β, c-FLIP(L) overexpression in cancer cells induced resistance to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). This effect was mediated by the regulation of p27(Kip1) and caspase-3 expression. These results indicate the existence of a new mechanism of resistance to TRAIL in cancer cells, and unexpected functions of c-FLIP(L).