Hepatitis B virus X protein acts as a tumor promoter in development of diethylnitrosamine-induced preneoplastic lesions

Chronic infection with hepatitis B virus (HBV) is one of the major etiologi cal factors in the development of human hepatocellular carcinoma. Transgeni c mice that express the HBV X protein (HBx) have previously been shown to b e more sensitive to the effects of hepatocarcinogens. Although the mechanis m for this cofactor role remains unknown, the ability of HBx to inhibit DNA repair and to influence cell cycle progression suggests two possible pathw ays. To investigate these possibilities in vivo, we treated double-transgen ic mice that both express HBx (ATX mice) and possess a bacteriophage lambda transgene with the hepatocarcinogen diethylnitrosamine (DEN). Histological examination of liver tissue confirmed that DEN-treated ATX mice developed approximately twice as many focal lesions of basophilic hepatocytes as trea ted wild-type littermates. Treatment of mice with DEN resulted in a six- to eightfold increase in the mutation frequency (MF), as measured by a functi onal analysis of the lambda transgene. HBx expression was confirmed by immu noprecipitation and Western blotting and was associated with a modest 23% i ncrease in the MF. Importantly, the extent of hepatocellular proliferation in 14-day-old mice, as measured by the detection of proliferating cell nucl ear antigen and by the incorporation of 5-bromo-2'-deoxyuridine, was determ ined to be approximately twofold higher in ATX livers than in wild-type liv ers. These results are consistent,vith a model in which HBx expression cont ributes to the development of DEN-mediated carcinogenesis by promoting the proliferation of altered hepatocytes rather than by directly interfering wi th the repair of DNA lesions.