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 hepat ocellular 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 carcinog ens such as aflatoxin B1 (AFB1) prevail. In this study, we report the effec t 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 CCL1 3. 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 simila r conditions, expression of HBx promotes apoptosis in a subset of cell popu lation. Exposure to AFB1-8,9-epoxide alone induces a low frequency of AGG t o AGT mutation in codon 249 of the p53 gene, as determined by an allele-spe cific 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 H Bx enhances liver cell susceptibility to carcinogen-induced mutagenesis, po ssibly through alteration of the balance between DNA repair and apoptosis, two cellular defense mechanisms against genotoxic stress. (C) 2000 Publishe d by Elsevier Science B.V.