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

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Abstract
To determine the role of phosphatidylinositol 3-kinase (PI3K)/Akt and nuclear factor-κB (NF-κB) in protecting hepatocytes from tumor necrosis factor-α (TNF-α)- and Fas-mediated apoptosis, we pretreated primary cultures of mouse hepatocytes with pharmacological and adenovirus-mediated inhibitors of the PI3K/Akt and NF-κB pathways followed by treatment with TNF-α or Jo2, an anti-Fas antibody. Jo2 and, to a lesser extent, TNF-α phosphorylate Akt. The PI3K inhibitor LY-294002 blocks TNF-α- and Fas-mediated Akt phosphorylation. LY-294002 pretreatment reduces NF-κB binding activity and transcriptional activity and NF-κB-responsive gene expression by TNF-α or Jo2. LY-294002 promotes apoptosis after TNF-α or Jo2. The expression of dominant-negative Akt blocks NF-κB activation and sensitizes hepatocytes to TNF-α- and Fas-mediated apoptosis. The expression of constitutively active Akt rescues LY-294002-pretreated cells from TNF-α- and Fas-mediated apoptosis. Active Akt induces NF-κB transcriptional activity but not NF-κB binding activity or IκB degradation. Furthermore, LY-294002 pretreatment blocks TNF-α- 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-α-induced apoptosis after sensitization by actinomycin D or the IκB superrepressor. Together, the PI3K/Akt pathway has a protective role in Fas-mediated apoptosis, which requires NF-κB activation, partially through the subsequent induction of Bcl-xL.