The hepatitis B virus X gene induces p53-mediated programmed cell death

Abstract

The human hepatitis B virus (HBV) protein pX is a multifunctional regulatory protein that is known to affect both transcription and cell growth. Here we describe induction of apoptosis in NIH 3T3 polyclonal cell lines upon stimulation of pX expression from a dexamethasone inducible mouse mammary tumor virus (MMTV)-X expression vector. The effect of long-term pX expression on the cell survival of mouse fibroblasts was confirmed in colony generation assays. This effect is not shared either by the other HBV products and it is c-myc mediated, as shown by the use of a dominant negative deletion mutant of c-myc. pX also sensitize cells to programmed cell death after exposure to DNA damaging agents. Taking advantage of stable transfectants carrying the p53val135 temperature-sensitive allele, we directly demonstrate that induction of apoptosis by pX requires p53. In p53 null mouse embryo fibroblasts pX activates transcription and confers an evident growth advantage without loss of cell viability. Although pX protein was not detectable in the experimental conditions we used, our results indicate that its expression affects both cell growth and cell death control.

Hepatitis B virus-related hepatocellular carcinoma: paradigms for viral-related human carcinogenesis

“…HBx can also exert antiapoptotic functions independently of p53 via modulation of activities of the serine protease hepsin (Zhang et al, 2005a) and upregulation of survivin (Li et al, 2003). HBx protein may also promote apoptosis dependent on p53 (Chirillo et al, 1997) or by regulating the expressions of Fas/FasL (Terradillos et al, 1998;Shin et al, 1999;Lee et al, 2002;, inactive procaspase-8, cFLICE (Kim and Seong, 2003), Bax/Bcl-2 (Miao et al, 2005), HSP60 (Tanaka et al, 2004), UV-DDB1 (Sitterlin et al, 2000;Bergametti et al, 2002;Leupin et al, 2003) and c-myc gene (Kalra and Kumar, 2004). HBx negatively regulates proteasome function and, thus, controls degradation of cellular and viral proteins (Sirma et al, 1998;Hu et al, 1999;Zhang et al, 2004).…”

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.…”

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

“…HBx was shown to sequester the p53 protein in the cytoplasm and to inactivate its transactivating and apoptotic properties (Elmore et al, 1997;Takada et al, 1997;Truant et al, 1995;Wang et al, 1994Wang et al, , 1995. In other reports however, inhibition of known cellular functions of p53 could not be evidenced in cells with replicative HBV (Puisieux et al, 1995), and the apoptotic activity of the X gene product was found to require wild-type p53 (Chirillo et al, 1997).…”

“…In human HBV carriers, HBx expression is preferentially maintained through the multistage process from preneoplastic nodules to HCC and peritumoral tissues (Su et al, 1998) and X-speci®c mRNAs selectively accumulate in liver tumors from HBsAg-negative patients (Paterlini et al, 1995). Relevant to how HBx might participate in cell transformation, the X gene product stimulates cell cycle progression, DNA synthesis and apoptosis (Benn and Schneider, 1995;Chirillo et al, 1997;Su and Schneider, 1997). There is also evidence to indicate that HBx may bind a variety of cellular target genes (Fischer et al, 1995;Lee et al, 1995;Sirma et al, 1998), and notably the tumor suppressor protein p53 (Feitelson et al, 1993;Lin et al, 1997;Ueda et al, 1995).…”