Induction of apoptosis by the transactivating domains of the hepatitis B virus X gene leads to suppression of oncogenic transformation of primary rat embryo fibroblasts

Abstract

Epidemiology shows a clear correlation between chronic infection with the hepatitis B virus (HBV) and development of hepatocellular carcinoma (HCC). The potential role of the transactivating hepatitis B virus X protein (HBx) in transformation by HBV is controversial. Here we report that HBx suppresses transformation of primary rat embryo ®broblasts (REFs). Cooperating oncogenes like c-Ha-ras and c-myc transform REF very eciently but cotransfection with HBx suppressed transformation of REFs down to 5%. Similarly, transfection of HBx together with the cooperating oncogenes Ha-ras and SV40 LTAg or c-Ha-ras and mutant p53 reduced the number of foci to 13%. Comparable results were obtained with HBx in the context of the whole HBV. Suppression of focus formation in REF could be partly relieved by cotransfection of apoptosis inhibitors Bcl-2 or E1B. However, cotransfection of apoptosis inhibitors crmA and p35 did not in¯uence the proapoptotic functions of HBx. Thus, HBx may speci®cally activate the Bcl-2 sensitive pathway leading to apoptosis. Experiments with 13 HBx linker scanning mutants revealed that the domains necessary for HBx dependent transactivation overlap with the domains needed for the apoptotic/growth arrest functions of HBx.

Activated ras oncogene collaborates with HBx gene of hepatitis B virus to transform cells by suppressing HBx-mediated apoptosis

“…In contrast to a previous report (Kim et al, 1998;Schuster et al, 2000), signi®cant collaboration between HBx and activated H-ras oncogene was observed. Our observation that HBx mediated apoptosis was suppressed by activated H-ras underlies the molecular basis of the collaboration between HBx and H-ras.…”

“…When HBx was expressed from the strong CMV promoter, collaboration of HBx with H-ras was not detected in colony formation assay (Kim et al, 1998;Schuster et al, 2000). Consistently, it has been reported that a low level expression of HBx sensitizes cells to apoptotic stimuli while overexpression of HBx triggers apoptotic death in transfected cells (Terradillos et al, 1998).…”

Molecular functions and biological roles of hepatitis B virus x protein

“…(59,60) But later the expression of HBx alone was reported to show apparent antitransforming ability in rat embryo fibroblasts due to the induction of cell apoptosis. (61,62) Cellular senescence and crisis are the host defense mechanisms to suppress tumorigenesis. Knowledge of cellular senescence has accumulated during the last decade demonstrating the profound difference between mouse and human systems in several aspects, (63)(64)(65)(66) especially the critical contribution of telomere checkpoint control in cellular senescence of human cells.…”

The hepatitis B virus X protein abrogates Bcl-2-mediated protection against Fas apoptosis in the liver

“…Bcl-2 represses cell death by preventing insertion of Bax in the mitochondrial membrane and the following release of cytochrome c (Eskes et al, 2000;Kluck et al, 1997;Murphy et al, 1999;Yang et al, 1997). It has been recently reported that Bcl-2 can block HBx-induced inhibition of focus formation (Kim et al, 1998;Schuster et al, 2000).…”