The X-gene product of hepatitis B virus (HBX) modulates a variety of viral and cellular genes relevant to hepatocarcinogenesis, where α-fetoprotein (AFP) is produced by hepatoma cells. In the present study, the possible mechanism by which HBX regulates AFP expression was investigated using three human hepatoma cells, HepG2, HuH-7 and Hep3B, which are known to contain the wild-type, the mutant-type and the deletion of p53, respectively. Transfection with the HBX expression vector stimulated the co-transfected AFP reporter gene expression in HepG2 cells and HuH-7 cells, but not in Hep3B cells. Transfection with the p53 expression vector repressed the AFP reporter gene expression in all three hepatoma cells, while overexpression of HBX counteracted the p53-induced repression. In addition, a G↷A substitution at nucleotide -119 in the AFP promoter sequence abrogated the stimulatory effect of HBX on the AFP promoter in HepG2 cells. These results suggest that HBX interacts with p53 to up-regulate AFP gene transcription probably by restoration of the p53-mediated repression of the AFP promotor activity.

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