Retroviral expression of the hepatitis B virus x gene promotes liver cell susceptibility to carcinogen-induced site specific mutagenesis.

Mutational inactivation of the tumor suppressor gene p53 is common in hepatocellular carcinomas (HCC). AGG to AGT transversion in codon 249 of exon 7 of the p53 gene occurs in over 50% of HCC from endemic regions, where both chronic infection with the hepatitis B virus (HBV) and exposure to carcinogens such as aflatoxin B1 (AFB1) prevail. In this study, we report the effect of the HBV x protein (HBx) on carcinogen-induced cytotoxicity and AGG to AGT mutation in codon 249 of the p53 gene in the human liver cell line CCL13. Expression of HBx, as revealed by its transactivation function, results in enhanced cell susceptibility to cytotoxicity induced by the AFB1 active metabolite, AFB1-8,9-epoxide, and benzo(a)pyrene diol-epoxide. Under similar conditions, expression of HBx promotes apoptosis in a subset of cell population. Exposure to AFB1-8, 9-epoxide alone induces a low frequency of AGG to AGT mutation in codon 249 of the p53 gene, as determined by an allele-specific polymerase chain reaction (AS-PCR) assay. However, expression of HBx enhances the frequency of AFB1-epoxide-induced AGG to AGT mutation compared to control cells. In summary, this study demonstrates that expression of HBx enhances liver cell susceptibility to carcinogen-induced mutagenesis, possibly through alteration of the balance between DNA repair and apoptosis, two cellular defense mechanisms against genotoxic stress.

Modulation of glutathione S-transferase alpha by hepatitis B virus and the chemopreventive drug oltipraz.

Persistent infection by hepatitis B virus (HBV) and exposure to chemical carcinogens correlates with the prevalence of hepatocellular carcinoma in endemic areas. The precise nature of the interaction between these factors is not known. Glutathione S-transferases (GST) are responsible for the cellular metabolism and detoxification of a variety of cytotoxic and carcinogenic compounds by catalysis of their conjugation with glutathione. Diminished GST activity could enhance cellular sensitivity to chemical carcinogens. We have investigated GST isozyme expression in hepatocellular HepG2 cells and in an HBV-transfected subline. Total GST activity and selenium-independent glutathione peroxidase activity are significantly decreased in HBV transfected cells. On immunoblotting, HBV transfected cells demonstrate a significant decrease in the level of GST Alpha class. Cytotoxicity assays reveal that the HBV transfected cells are more sensitive to a wide range of compounds known to be detoxified by GST Alpha conjugation. Although no significant difference in protein half-life between the two cell lines was found, semi-quantitative reverse transcription-polymerase chain reaction shows a reduced amount of GST Alpha mRNA in the transfected cells. Because the HBV x protein (HBx) seems to play a role in HBV transfection, we also demonstrated that expression of the HBx gene into HepG2 cells decreased the amount of GST Alpha protein. Transient transfection experiments using both rat and human GST Alpha (rGSTA5 and hGSTA1) promoters in HepG2 cells show a decreased CAT activity upon HBx expression, supporting a transcriptional regulation of both genes by HBx. This effect is independent of HBx interaction with Sp1. Treatment with oltipraz, an inducer of GST Alpha, partially overcomes the effect of HBx on both promoters. Promoter deletion studies indicate that oltipraz works through responsive elements distinct from AP1 or NF-kappaB transcription factors. Thus, HBV infection alters phase II metabolizing enzymes via different mechanisms than those modulated by treatment with oltipraz.