X protein of hepatitis B virus inhibits Fas-mediated apoptosis and is associated with up-regulation of the SAPK/JNK pathway.
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 Fas-mediated 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.
Diao
J
J
Department of Medical Biophysics, University of Toronto, Toronto, Ontario M5G 2M9, Canada.
Khine
A A
AA
Sarangi
F
F
Hsu
E
E
Iorio
C
C
Tibbles
L A
LA
Woodgett
J R
JR
Penninger
J
J
Richardson
C D
CD
D017209:Apoptosis; D053148:Caspase 3; D053181:Caspase 8; D053453:Caspase 9; D061945:Caspase Inhibitors; D002460:Cell Line; D022781:Hepatocytes / Q000502:physiology; D006801:Humans; D048031:JNK Mitogen-Activated Protein Kinases; D048670:MAP Kinase Kinase 4; D048728:MAP Kinase Kinase Kinase 1; D048055:Mitogen-Activated Protein Kinase 8; D020929:Mitogen-Activated Protein Kinase Kinases / Q000502:physiology; D020928:Mitogen-Activated Protein Kinases / Q000502:physiology*; D017346:Protein Serine-Threonine Kinases / Q000502:physiology; D015534:Trans-Activators / Q000502:physiology*; D016159:Tumor Suppressor Protein p53 / Q000502:physiology; D015854:Up-Regulation; D054334:Viral Regulatory and Accessory Proteins; D019014:fas Receptor / Q000502:physiology*
D061945:Caspase Inhibitors; D015534:Trans-Activators; D016159:Tumor Suppressor Protein p53; D054334:Viral Regulatory and Accessory Proteins; D019014:fas Receptor; C057628:hepatitis B virus X protein; D017346:Protein Serine-Threonine Kinases; D048031:JNK Mitogen-Activated Protein Kinases; D048055:Mitogen-Activated Protein Kinase 8; D020928:Mitogen-Activated Protein Kinases; D048728:MAP Kinase Kinase Kinase 1; C482576:MAP3K1 protein, human; C482578:Map3k1 protein, mouse; D048670:MAP Kinase Kinase 4; C482559:MAP2K4 protein, human; C482562:Map2k4 protein, mouse; D020929:Mitogen-Activated Protein Kinase Kinases; C505407:CASP3 protein, human; C505530:CASP8 protein, human; C506433:CASP9 protein, human; C505409:Casp3 protein, mouse; C505526:Casp8 protein, mouse; C506434:Casp9 protein, mouse; D053148:Caspase 3; D053181:Caspase 8; D053453:Caspase 9