Roles of cellular FLICE-inhibitory protein (c-FLIP) and PI3K/Akt in Fas (CD95)-induced NF-kappaB activation and apoptosis through death effector domains

The cell survival protein kinase Akt/protein kinase B (PKB) is a known regulator of apoptosis but its role in Fas-mediated cell death and its regulatory mechanisms are unclear. Here, we show that stimulation of the Fas receptor by its ligand (FasL) induces rapid phosphorylation of Akt/PKB and a parallel increase in cell apoptosis in epidermal Cl41 cells. Inhibition of PI3K/Akt by overexpression of dominant-negative mutants of PI3K (Deltap85) and Akt (Akt-T308A/S473A) protects the cells from apoptosis induced by FasL, indicating an unexpected pro-apoptotic role of PI3K/Akt in Fas signaling process. Electron spin resonance (ESR) studies also show that FasL treatment induces rapid generation of reactive oxygen species (ROS) and inhibition of ROS by antioxidants effectively inhibits Akt/PKB signaling, suggesting that FasL activation of Akt/PKB is redox-sensitive. In cells transfected with dominant-negative PI3K/Akt, Fas receptor expression is downregulated but FLICE-inhibitory protein (FLIP) expression is unaffected.; We also investigate the role of TNF-alpha and NF-kappaB in FasL-mediated cell death in macrophages. Gene transfection studies using NF-kappaB-dependent reporter plasmid showed that FasL did indeed activate NF-kappaB promoter activity. Gel shift studies revealed that FasL mobilized the p50/p65 heterodimeric form of NF-kappaB. Inhibition of NF-kappaB by specific NF-kappaB inhibitor, caffeic acid phenylethyl ester, or by dominant expression of the NF-kappaB inhibitory subunit IkappaB caused an increase in FasL-induced apoptosis and a reduction in TNF-alpha expression. However, neutralization of TNF-alpha by specific anti-TNF-alpha antibody had no protective effective effect on FasL-induced apoptosis. These results indicate that FasL-mediated cell death in macrophages is regulated through NF-kappaB and is independent of TNF-alpha activation.; The wild-type FLIPL and various N-terminal deletion plasmids were generated and used to study the role of FLIP in Fas-mediated NF-kappaB activation and apoptosis. NF-kappaB is activated by FLIPL and its deletion mutants except Delta7. Also, FLIPL and its deletion mutants Delta2-Delta6 (but not Delta7) inhibit FasL-induced apoptosis. We also tested whether our truncated mutants are capable of binding to TRAF2. We performed immunoprecipitation experiments and found that both DED1 and DED2 domains of FLIP are required for TRAF2 recruitment. These results are in good agreement with our apoptosis and NF-kappaB activation data.