Activation-induced Degradation of FLIPL 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 (FLIPL) and FLIP short (FLIPS) mRNA was induced by tumor necrosis factor (TNF) alpha, mediated through NF-kappa B. However, we observed TNF alpha reduced the protein level of FLIPL, but not FLIPS, at 1 and 2 h. Similar results were observed with lipopolysaccharide. The reduction of FLIPL by TNF alpha 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 FLIPL resulted in the short term induction of caspase 8-like activity, which augmented NF-kappa B activation. A co-immunoprecipitation assay demonstrated that Akt-1 physically interacts with FLIPL. Moreover, TNF alpha enhanced FLIPL serine phosphorylation, which was increased by activated Akt-1. Serine 273, a putative Akt-1 phosphorylation site in FLIPL, was critical for the activation-induced reduction of FLIPL. Thus, these observations document a novel mechanism where by TNF alpha facilitates the reduction of FLIPL protein, which is dependent on the phosphatidylinositol 3-kinase/Akt signaling.