NF-KAPPA-B INACTIVATION CONVERTS A HEPATOCYTE CELL-LINE TNF-ALPHA RESPONSE FROM PROLIFERATION TO APOPTOSIS

Toxins convert the hepatocellular response to tumor necrosis factor-al pha (TNF-alpha) stimulation from proliferation to cell death, suggesti ng that hepatotoxins somehow sensitize hepatocytes to TNF-alpha toxici ty. Because nuclear factor-kappa B (NF-kappa B) activation confers res istance to TNF-alpha cytotoxicity in nonhepatic cells, the possibility that toxin-induced sensitization to TNF-alpha killing results from in hibition of NF-kappa B-dependent gene expression was examined in the R ALA rat hepatocyte cell line sensitized to TNF-alpha cytotoxicity by a ctinomycin D (ActD). ActD did not affect TNF-alpha-induced hepatocyte NF-kappa B activation but decreased NF-kappa B-dependent gene expressi on. Expression of an I kappa B superrepressor rendered RALA hepatocyte s sensitive to TNF-alpha-induced apoptosis in the absence of ActD. Apo ptosis was blocked by caspase inhibitors, and TNF-alpha treatment led to activation of caspase-2, caspase-3, and caspase-8 only when NF-kapp a B activation was blocked. Although apoptosis was blocked by the NF-k appa B-dependent factor nitric oxide (NO), inhibition of endogenous NO production did not sensitize cells to TNF-alpha-induced cytotoxicity. Thus NF-kappa B activation is the critical intracellular signal that determines whether TNF-alpha stimulates hepatocyte proliferation or ap optosis. Although exogenous NO blocks RALA hepatocyte TNF-alpha cytoto xicity, endogenous production of NO is not the mechanism by which NF-k appa B activation inhibits this death pathway.