The hepatitis B x antigen effector, URG7, blocks tumor necrosis factor α-mediated apoptosis by activation of phosphoinositol 3-kinase and ß-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 (TNF alpha) is also an important mediator of chronic hepatitis, and partially shares signalling with Fas, experiments were designed to test whether URG7 blocks TNF alpha killing of HepG2X cells. HepG2X cells expressing URG7 and HepG2 cells overexpressing URG7 in the absence of HBxAg were resistant to TNFa killing compared with HepG2CAT cells. URG7 small interfering RNA restored the sensitivity of HepG2X cells to TNFa 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 P-catenin, experiments were designed to determine whether URG7 blocked apoptosis via activation of ss-catenin. Both HBxAg and URG7 activated fragments of the ss-catenin promoter, and also promoted expression of ss-catenin target genes. Hence, URG7 inhibits TNF alpha-mediated killing by blocking one or more caspases in the apoptotic pathway and by activating phosphoinositol 3-kinase and ss-catenin, thereby overriding the apoptotic signalling of TNF alpha. This suggests that URG7 helps to protect virus-infected hepatocytes during chronic hepatitis B virus infection.