The hepatitis B x antigen effector, URG7, blocks tumour necrosis factor alpha-mediated apoptosis by activation of phosphoinositol 3-kinase and beta-catenin.

Hepatitis B x antigen (HBxAg) contributes significantly to the pathogenesis of chronic infection and development of hepatocellular carcinoma. To discern some of its operative pathways, HepG2 cells were stably transduced with HBx or the bacterial chloramphenicol acetyltransferase (CAT) gene. Differential gene expression has previously revealed an upregulated gene, clone 7 (URG7), that conferred resistance to anti-Fas killing on HepG2X cells. Given that tumour necrosis factor alpha (TNFalpha) is also an important mediator of chronic hepatitis, and partially shares signalling with Fas, experiments were designed to test whether URG7 blocks TNFalpha killing of HepG2X cells. HepG2X cells expressing URG7 and HepG2 cells overexpressing URG7 in the absence of HBxAg were resistant to TNFalpha killing compared with HepG2CAT cells. URG7 small interfering RNA restored the sensitivity of HepG2X cells to TNFalpha killing. Killing was associated with the activation of caspases 3 and 8, suggesting that URG7 blocked these caspases. This resistance was also associated with activation of phosphoinositol 3-kinase/Akt. Given that Akt and HBxAg also activate beta-catenin, experiments were designed to determine whether URG7 blocked apoptosis via activation of beta-catenin. Both HBxAg and URG7 activated fragments of the beta-catenin promoter, and also promoted expression of beta-catenin target genes. Hence, URG7 inhibits TNFalpha-mediated killing by blocking one or more caspases in the apoptotic pathway and by activating phosphoinositol 3-kinase and beta-catenin, thereby overriding the apoptotic signalling of TNFalpha. This suggests that URG7 helps to protect virus-infected hepatocytes during chronic hepatitis B virus infection.