Hepatitis B virus-X protein upregulates the expression of p21wafl/cipl and prolongs G1→S transition via a p53-independent pathway in human hepatoma cells

Abstract

Progression through the cell cycle is controlled by the induction of cyclins and activation of cognate cyclindependent kinases. The human hepatitis B virus-X (HBV-X) protein functions in gene expression alterations, in the sensitization of cells to apoptotic killing and deregulates cell growth arrest in certain cancer cell types. We have pursued the mechanism of growth arrest in Hep3B cells, a p53-mutant human hepatocellular carcinoma (HCC) cell line. In stable or transient HBV-X transformed Hep3B cells, HBV-X increased protein and mRNA levels of the cyclin-dependent kinase inhibitor (CDKI) p21 wa¯/cipl , increased binding of p21 wa¯/cipl with cyclin-dependent kinase 2 (CDK2), markedly inhibited cyclin E and CDK2 associated phosphorylation of histone H1 and induced the activation of a p21 promoter reporter construct. By using p21 promoter deletion constructs, the HBV-X responsive element was mapped to a region between 71185 and 71482, relative to the transcription start site. Promoter mutation analysis indicated that the HBV-X responsive site coincides with the ets factor binding sites. These data indicate that in human hepatocellular carcinoma cells, HBV-X can circumvent the loss of p53 functions and induces critical downstream regulatory events leading to transcriptional activation of p21 wa¯/cipl . As a consequence, there is an increased chance of acquisition of mutations which can enhance the genesis of hepatomas. Our results also emphasize the chemotherapeutic potential of p21 wa¯/cipl inhibitors, particularly in the HBV-X infected hepatoma which lacks functional p53. Oncogene (2000) 19, 3384 ± 3394.

Oncogenic role of p21 in hepatocarcinogenesis suggests a new treatment strategy

“…Inversely, HBx enhanced p21 in some reports [34,35] . Park et al [34] reported that when the cell cycle was prolonged by enhancement of p21 by HBx, cells had survival advantages and chances for gene mutations, eventually leading to preneoplastic hepatocytes. In addition, Yano et al [36] reported that HBx enhanced cytoplasmic p21 in protein kinase C (PKC)-dependent manner to induce cell proliferation.…”

Genetic mechanisms of hepatocarcinogenesis

“…MMP-7 expression is also up-regulated in early, di erentiated tumor cells, promoting angiogenesis and the intrahepatic spread of tumor. The ®nding that bcatenin and the Ets oncoprotein stimulate of MMP-7 expression (Ozaki et al, 2000), and that HBxAg transactivation maps to Ets binding sites on cellular genes (Park et al, 2000), implies that HBxAg may modulate the outgrowth of early HCC by de®ning the integrity of the extracellular matrix during chronic infection.…”

The Enigmatic X Gene of Hepatitis B Virus

“…In some established cell lines and in mouse primary hepatocyte cultures, HBx alters expression of p21 and p27 cell cycle regulation genes, influencing cell cycle progression profiles in these cells (1,42,61,89,93). However, there are no clear answers as to whether HBx induces or inhibits expression of p21 and p27, as conflicting reports have been published; these differences may reflect influences of the experimental systems or levels of HBx expression (61,93).…”