Hepatitis B virus-X protein upregulates the expression of p21(waf1/cip1) and prolongs G1 -> S transition via a p53-independent pathway in human hepatoma cells

Progression through the cell cycle is controlled by the induction of cyclin s and activation of cognate cyclin-dependent kinases, The human hepatitis B virus-X (HBV-X) protein functions in gene expression alterations, in the s ensitization of cells to apoptotic killing and deregulates cell growth arre st in certain cancer cell types, We have pursued the mechanism of growth ar rest in Hep3B cells, a p53-mutant human hepatocellular carcinoma (HCC) cell line, In stable or transient HBV-X transformed Hep3B cells, HBV-X increase d protein and mRNA levels of the cyclin-dependent kinase inhibitor (CDKI) p 21(wafl/cipl), increased binding of p21(wafl/cipl) with cyclin-dependent ki nase 2 (CDK2), markedly inhibited cyclin E and CDK2 associated phosphorylat ion of histone HI and induced the activation of a p21 promoter reporter con struct, By using p21 promoter deletion constructs, the HBV-X responsive ele ment was mapped to a region between -1185 and -1482, relative to the transc ription start site, Promoter mutation analysis indicated that the HBV-X res ponsive site coincides with the ets factor binding sites. These data indica te that in human hepatocellular carcinoma cells, HBV-X can circumvent the l oss of p53 functions and induces critical downstream regulatory events lead ing to transcriptional activation of p21(wafl/cipl). AS a consequence, ther e is an increased chance of acquisition of mutations which can enhance the genesis of hepatomas, Our results also emphasize the chemotherapeutic poten tial of p21(wafl/cipl) inhibitors, particularly in the HBV-X infected hepat oma which lacks functional p53.