Tumor necrosis factor signaling in hepatocyte apoptosis

Death receptor-mediated hepatocyte apoptosis is implicated in a wide range of liver diseases including viral hepatitis, alcoholic hepatitis, ischemia/reperfusion injury, fulminant hepatic failure, cholestatic liver injury, and cancer. Our aim was to clarify the protective pathway in death receptor-mediated hepatocyte apoptosis and the significance of apoptosis in liver injury. In vitro: AdI kappa Bsr plus tumor necrosis factor (TNF)-alpha/Jo2 rapidly induced apoptosis in mouse hepatocyte, whereas TNF-alpha/Jo2 alone produced little cytotoxicity. The combination of the mitochondrial permeability transition (MPT) inhibitors, cyclosporine A and trifluoperazine, protected AdI kappa Bsr-infected hepatocytes from TNF-alpha- but not Fas-mediated apoptosis. The TNF-alpha and Jo2 induced iNOS through NF-kappa B. Nitric oxide donor (S-nitroso-N-acetylpenicillamine) inhibited Bid cleavage, the MPT, and caspase activation and reduced TNF-alpha- and Fas-mediated cell killing. Inhibition of PI3K by LY294002 and a dominant-negative Akt, which attenuated NF-kappa B activation by TNF-alpha or Jo2, sensitized hepatocytes to TNF-alpha or Jo2. In vivo: apoptosis as well as necrosis may play an important role in hepatic ischemia/reperfusion injury. Adenoviral gene transfer of myrAkt could inhibit apoptotic cell death and subsequent hepatic ischemia/reperfusion injury in the rat, through Bad not NF-kappa B. Bile acids cause liver injury during cholestasis by inducing hepatocyte apoptosis. Hepatocyte apoptosis has a major role in hepatic injury by bile duct ligation. At least, early hepatic injury by bile duct ligation involved Fas-mediated and Bcl-xL insensitive apoptotic pathway. In conclusion, the role of apoptosis in various liver diseases may suggest possible treatments.