X Protein of Hepatitis B Virus Inhibits Fas-mediated Apoptosis and Is Associated with Up-regulation of the SAPK/JNK Pathway

Abstract

The X protein from a chronic strain of hepatitis B virus (HBx) was determined to inhibit Fas-mediated apoptosis and promote cell survival. Fas-mediated apoptosis is the major cause of hepatocyte damage during liver disease. Experiments demonstrated that cell death caused by anti-Fas antibodies was blocked by the expression of HBx in human primary hepatocytes and mouse embryo fibroblasts. This effect was also observed in mouse erythroleukemia cells that lacked p53, indicating that protection against Fas-mediated apoptosis was independent of p53. Components of the signal transduction pathways involved in this protection were studied. The SAPK/JNK pathway has previously been suggested to be a survival pathway for some cells undergoing Fasmediated apoptosis, and kinase assays showed that SAPK activity was highly up-regulated in cells expressing the HBx protein. Normal mouse fibroblasts expressing HBx were protected from death, whereas identical fibroblasts lacking the SEK1 component from the SAPK pathway succumbed to Fas-mediated apoptosis, whether HBx was present or not. Assays showed that caspase 3 and 8 activities and the release of cytochrome c from mitochondria were inhibited, in the presence of HBx, following stimulation with anti-Fas antibodies. Coprecipitation and confocal immunofluorescence microscopy experiments demonstrated that HBx localizes with a cytoplasmic complex containing MEKK1, SEK1, SAPK, and 14-3-3 proteins. Finally, mutational analysis of HBx demonstrated that a potential binding region for 14-3-3 proteins was essential for induction of SAPK/JNK activity and protection from Fas-mediated apoptosis.

Genetics of hepatocellular tumors

“…Insertional mutagenesis seems to be implicated in 20-40% of the HCC cases related to HBV infection. The third mechanism of carcinogenesis linked to HBV infection is based on the expression of viral protein, in particular HBX, to modulate cell proliferation and viability (Andrisani and Barnabas, 1999;Diao et al, 2001). Moreover, HBX binds to p53 and inactivates p53-dependent activities, including p53-mediated apoptosis (Feitelson et al, 1993).…”

Activation of calcium signaling by hepatitis B virus-X protein in liver cells

“…The x gene product of HBV (HBx) is known to be a transactivator of a variety of viral and cellular transcriptional elements, including AP-1, CREB, NFκB and C/EBP, but the mechanisms underpinning these transactivations by HBx remain controversial (Rossner 1992;Feitlson, 1997;Murakami, 1999;Diao et al, 2001). The association between HBx with the transactivation of transcription factors has been reported to be mediated through the up-regulation of the MAPK signal transduction pathway (Benn and Schneider, 1994;Doria et al, 1995;Chirillo et al, 1996;Klein and Schneider, 1997).…”

Hepatitis B virus X protein promotes hepatoma cell proliferation via upregulation of MEKK2

“…The expression of HBx induced the transformation of human liver cells and caused the development of hepatocellular carcinoma in certain strains of transgenic mice [6,7] . In addition, HBx is involved in many cell signaling transduction pathways, such as the mitogen-activated protein kinase (MAPK), c-jun N-terminal kinase (JNK), phosphatidylinositol 3-kinase (PI 3-kinase), and JAK/STAT pathways [8][9][10][11] . Our laboratory has focused on the investigation of HBx-mediated hepatocarcinogenesis.…”