X-gene Product of Hepatitis B Virus Induces Apoptosis in Liver Cells

Abstract

Hepatitis B virus is a causative agent of hepatocellular carcinoma, and in the course of tumorigenesis, the X-gene product (HBx) is known to play important roles. Here, we investigated the transforming potential of HBx by conventional focus formation assay in NIH3T3 cells. Cells were cotransfected with the HBx expression plasmid along with other oncogenes including Ha-ras, v-src, v-myc, v-fos, and E1a. 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 focus formation, suggesting that the observed repression of focus formation by HBx is through the induction of apoptosis. Next, to test unequivocally whether HBx induces apoptosis in liver cells, we established stable Chang liver cell lines expressing HBx under the control of a tetracycline-inducible promoter. Induction of HBx in these cells in the presence of 1% calf serum resulted in typical apoptosis phenomena such as DNA fragmentation, nuclear condensation, and fragmentation. Based on these results, we propose that HBx sensitizes liver cells to apoptosis upon hepatitis B virus infection, contributing to the development of hepatitis and the subsequent generation of hepatocellular carcinoma.

Hepatitis B Virus X Protein Induced Expression of the Nur77 Gene

“…Apoptotic cell death of host cells is considered as one of the primary anti-viral defense mechanisms, and many animal viruses are known to encode one or more proteins that interact with apoptotic regulatory pathways. In fact, several lines of evidence suggest that the HBx protein triggers apoptotic cell death in HBV infected hepatocytes (41)(42)(43). However, the experimental models we employed did not show significant apoptotic changes upon expression of either HBx or Nur77 (data not shown), supports the former possibility.…”

p53-independent apoptotic effects of the hepatitis B virus HBx protein in vivo and in vitro

“…Evidence that HBx expression is detrimental to cell viability was provided in earlier studies by the di culty in generating stable cell clones that express the X protein (Kim et al, 1998;Oguey et al, 1996). We succeeded in establishing permanent cell lines which stably expressed HBx by isolating hepatocytes from HBx/p53-null and HBx/AT-cyto-MET bitransgenic animals.…”

UVDDB p127-binding modulates activities and intracellular distribution of Hepatitis B virus X protein

“…Results obtained using the lacZ reporter gene (right) are expressed as in (B) (Bergametti et al, 1999;Chirillo et al, 1997;Kim et al, 1998;Su and Schneider, 1997) although both the underlying mechanism and the biological signi®cance of these e ects are still elusive. Moreover, in several studies, HBx overexpression has been shown to directly trigger apoptosis, resulting in strong suppression of colony formation (Chirillo et al, 1997;Kim et al, 1998). In agreement with the latter observations, transfection into CCL-13 cells of an episomic vector (pDR2) carrying the wt WHx gene, reduced e ciency of hygromycin-resistant colony formation, in a dosedependent way ( Figure 4A).…”

“…In others, X appears to act far more upstream, by modulating the signal transduction cascade in the cytoplasm (Benn et al, 1996;Chirillo et al, 1996;Henkler et al, 1998;Kekule et al, 1993;Klein and Schneider, 1997;Lee and Yun, 1998;Su and Schneider, 1996). In addition to its widely documented transactivation activity, X has been reported to interfere with DNA repair (Becker et al, 1998;Wang et al, 1994), cell cycle control (Benn and Schneider, 1995) and apoptosis (Bergametti et al, 1999;Chirillo et al, 1997;Kim et al, 1998;Su et al, 1998). However, the functional connections between these activities and the cellular Xbinding proteins are still missing.…”