A novel protein overexpressed in hepatoma accelerates export of NF-κB from the nucleus and inhibits p53-dependent apoptosis

NF-kappaB is a transcription factor that can protect from or contribute to apoptosis. Here we report identification of HSCO that binds to NF-kappaB and inhibits apoptosis. HSCO mRNA was overexpressed in 20 of 30 hepatocellular carcinomas analyzed. Overexpression of HSCO inhibited caspase 9 activation and apoptosis induced by DNA damaging agents, while it augmented apoptosis induced by TNFalpha. Like IkappaBalpha, HSCO inhibited NF-kappaB activity and abrogated p53-induced apoptosis. However, the underlying mechanism was different. HSCO is a nuclear-cytoplasmic shuttling protein, bound to ReIA NF-kappaB, and HSCO sequestered it in the cytoplasm by accelerating its export from the nucleus. These results suggest that overexpression of HSCO suppresses p53-induced apoptosis by preventing nuclear localization of NF-kappaB during signaling and thus contributes to hepatocarcinogenesis.