Inhibition of c-Myc expression sensitizes hepatocytes to tumor necrosis factor-induced apoptosis and necrosis

The typical proliferative response of hepatocytes to tumor necrosis factor (TNF) can be converted to a cytotoxic one by transcriptional arrest. Althou gh NF-kappaB activation is critical for hepatocyte resistance to TNF toxici ty, the contribution of other TNF-inducible transcription factors remains u nknown. To determine the function of c-Myc in hepatocyte sensitivity to TNF , stable transfectants of the rat hepatocyte cell. line RALA255-10G contain ing sense and antisense c-myc expression vectors were isolated with increas ed (S-Myc cells) and decreased (AN-Myc cells) c-Myc transcriptional activit y. While S-Myc cells proliferated in response to TNF treatment, AN-Myc cell s underwent 32% cell death within 6 h. Fluorescent microscopic studies indi cated that TNF induced apoptosis and necrosis in AN-Myc cells. Cell death w as associated with DNA hypoploidy and poly(ADP-ribose) polymerase cleavage but occurred in the absence of detectable caspase-3, -7, or -8 activation. TNF-induced, AN-Myc cell death was dependent on Fas-associated protein with death domain and partially blocked by caspase inhibitors. AN-Myc cells had decreased levels of NF-kappaB transcriptional activity, but S-Myc cells ma intained resistance to TNF despite NF-kappaB inactivation, suggesting that c-Myc and NF-kappaB independently mediate TNF resistance. Thus, in the abse nce of sufficient c-Myc expression, hepatocytes are sensitized to TNF-induc ed apoptosis and necrosis, These findings demonstrate that hepatocyte resis tance to TNF is regulated by multiple transcriptional activators.