Activation-induced degradation of FLIP_L is mediated via the phosphatidylinositol 3-kinase/Akt signaling pathway in macrophages

Cellular FLIP (Flice-like inhibitory protein) is critical for the protection against death receptor-mediated cell apoptosis. In macrophages, FLIP long (FLIP_L) and FLIP short (FLIP_S) mRNA was induced by tumor necrosis factor (TNF) α, mediated through NF-κB. However, we observed TNFα reduced the protein level of FLIP_L, but not FLIP_S, at 1 and 2 h. Similar results were observed with lipopolysaccharide. The reduction of FLIP_L by TNFα was not mediated by caspase 8, or through JNK or Itch, but was suppressed by inhibition of the phosphatidylinositol 3-kinase/Akt pathway employing chemical inhibitors, a dominant negative Akt-1, or Akt-1 small interfering RNA. The reduction of FLIP_L resulted in the short term induction of caspase 8-like activity, which augmented NF-κB activation. A co-immunoprecipitation assay demonstrated that Akt-1 physically interacts with FLIP_L. Moreover, TNFα enhanced FLIP_L serine phosphorylation, which was increased by activated Akt-1. Serine 273, a putative Akt-1 phosphorylation site in FLIP_L, was critical for the activation-induced reduction of FLIP_L. Thus, these observations document a novel mechanism where by TNFα facilitates the reduction of FLIP_L protein, which is dependent on the phosphatidylinositol 3-kinase/Akt signaling.