Hepatitis B virus X gene expression is activated by X protein but repressed by p53 tumor suppressor gene product in the transient expression system.

Hepatitis B virus (HBV) X gene is known to exhibit a transcriptional activation function and is considered to play a major role in hepatocarcinogenesis. We determined a 20-bp promoter element for the HBV X gene transcription and found a binding protein to this promoter element, designated as an X-PBP. We then examined the effects of HBV X protein and p53 tumor suppressor gene product on X gene transcription from the 20-bp promoter element using the transient expression technique. Activity of the X gene promoter was stimulated by X protein expression, but, in contrast, was repressed by transfected normal p53 gene. On the other hand, mutant p53 gene product exhibited no repression. Moreover, the p53 repression of X gene transcription was canceled by X protein coexpression. Thus, the effects of X protein and normal p53 product appear to be mutually antagonistic in the regulation of X gene expression. However, mutated promoter elements which failed to bind to X-PBP still responded to X protein or p53, indicating that the process of X transactivation or p53 repression may be independent of X-PBP binding to the promoter element. Our data suggest that X protein could disrupt function of normal p53 protein in X gene-transfected cells.