Akt protects mouse hepatocytes from TNF-α- and Fas-mediated apoptosis through NK-κB activation

To determine the role of phosphatidylinositol 3-kinase (PI3K)/Akt and nuclear factor- kappaB (NF-kappaB) in protecting hepatocytes from tumor necrosis factor-alpha (TNF-alpha)- and Fas-mediated apoptosis, we pretreated primary cultures of mouse hepatocytes with pharmacological and adenovirus-mediated inhibitors of the PI3K/Akt and NF-kappaB pathways followed by treatment with TNF-alpha or Jo2, an anti-Fas antibody. Jo2 and, to a lesser extent, TNF-alpha phosphorylate Akt. The PI3K inhibitor LY-294002 blocks TNF-alpha- and Fas-mediated Akt phosphorylation. LY-294002 pretreatment reduces NF-kappaB binding activity and transcriptional activity and NF-kappaB-responsive gene expression by TNF-alpha or Jo2. LY-294002 promotes apoptosis after TNF-alpha or Jo2. The expression of dominant-negative Akt blocks NF-kappaB activation and sensitizes hepatocytes to TNF-alpha- and Fas-mediated apoptosis. The expression of constitutively active Akt rescues LY-294002-pretreated cells from TNF-alpha- and Fas-mediated apoptosis. Active Akt induces NF-kappaB transcriptional activity but not NF-kappaB binding activity or I kappaB degradation. Furthermore, LY-294002 pretreatment blocks TNF-alpha- and Jo2-induced Bcl-xL levels in hepatocytes, with no effect on the phosphorylation levels of Bad. Bcl-xL overexpression protects hepatocytes from Fas-but not TNF-alpha -induced apoptosis after sensitization by actinomycin D or the I kappaB superrepressor. Together, the PI3K/Akt pathway has a protective role in Fas-mediated apoptosis, which requires NF-kappaB activation, partially through the subsequent induction of Bcl-xL.