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.