Hepatitis B virus (HBV) infection is a major cause of hepatocellular carcinoma (HCC). Previous studies including our own have identified recurrent nonsense mutations of the HBV S gene from cancerous parts of the liver tissues from HBV core antigen-positive HCC patients. These nonsense mutants have been shown to be oncogenic in mouse xenograft models using a mouse embryonic fibroblast cell line.
Here, we expressed in a liver cell line Huh-7 the carboxy terminally truncated HBV large S (LHBs) protein from one of the missense mutants of the HBV S gene, sW182* (nonsense mutation at the codon for tryptophane-182). Although the sW182* mutant protein appeared not to be very stable in the cultured liver cells, we confirmed that the protein can be sustainably expressed in the mouse liver tissue by hydrodynamic injection of the liver specific expression construct for the mutant. In Huh-7 cells, the sW182* mutant downregulated tumor suppressor proteins p53 and Smad4. This downregulation was reversed by a proteasome inhibitor MG132, implying the involvement of proteasome-based protein degradation in the observed regulation of the tumor suppressors. On the other hand, we found that c-Jun activation domain-binding protein-1 (Jab-1) physically interacted with the sW182*, but not wild type LHBs protein. RNA interference (RNAi) of Jab-1 restored the levels of the downregulated p53 and Smad4. The sW182* mutant inhibited the promoter activity of genes that are known to be regulated by the tumor suppressors. Consistently, Jab-1 RNAi alleviated the inhibition.
These results suggest that the HBV LHBs nonsense mutant antagonizes the tumor suppressor pathways through Jab-1, which may contribute to HCC development.