X-GENE PRODUCT OF HEPATITIS-B VIRUS INDUCES APOPTOSIS IN LIVER-CELLS

Hepatitis B virus is a causative agent of hepatocellular carcinoma, an d in the course of tumorigenesis, the X-gene product (HBx) is known to play important roles. Here, we investigated the transforming potentia l of HBx by conventional focus formation assay in NIH3T3 cells. Cells were cotransfected with the HBx expression plasmid along with other on cogenes including Ha-ras, v-src, v-myc, v-fos, and Ela. Unexpectedly, the introduction of HBx completely abrogated the focus-forming ability of all five tested oncogenes, In addition, the cotransfection of Bcl- 2, an apoptosis inhibitor, reversed the HBx-mediated inhibition of foc us formation, suggesting that the observed repression of focus formati on by HBx is through the induction of apoptosis. Next, to test unequiv ocally whether HBx induces apoptosis in liver cells, we established st able Chang liver cell lines expressing HBx under the control of a tetr acycline inducible promoter. Induction of HBx in these cells in the pr esence of 1% calf serum resulted in typical apoptosis phenomena such a s DNA fragmentation, nuclear condensation, and fragmentation, Based on these results, we propose that HBx sensitizes liver cells to apoptosi s upon hepatitis B virus infection, contributing to the development of hepatitis and the subsequent generation of hepatocellular carcinoma.