HBx-induced hepatic steatosis and apoptosis are regulated by TNFR1- and NF-kappaB-dependent pathways

Hepatitis B virus X (HBx) protein is an important regulator of hepatic steatosis observed in patients with hepatitis B virus; however, its underlying molecular mechanism remains unclear. TNF receptor 1 (TNFR1) is an essential pathway for the HBx-mediated nuclear factor kappaB (NF-kappaB) activation involved in hepatic liver injury. Here, we show that HBx-mediated steatosis and apoptosis are regulated by TNFR1- and NF-kappaB-dependent pathways. HBx-mediated tumor necrosis factor alpha (TNF-alpha) production and NF-kappaB activation were completely diminished in anti-TNF-alpha-treated cells and TNF-alpha(-)(/-) or TNFR1(-/-) mice. HBx and TNFR1, which are potentiated by TNF-alpha, are physically associated and colocalize in the plasma membrane. Similarly, TNFR1 depletion inhibits lipid droplets, and lipogenic genes such as sterol regulatory element binding protein (SREBP) 1 and peroxisome proliferator-activated receptor (PPAR) gamma increased in HBx-Tg mice and HepG2-GFPHBx stable cells. Furthermore, lipid accumulation and expression of SREBP1c and PPAR gamma are significantly increased in AdHBx-GFP-injected (intravenous) wild-type mice, but not in TNFR1(-/-) mice. HBx-enhanced transcriptional activities of SREBP1 and PPAR gamma are significantly attenuated by the NF-kappaB inhibitor Bay 11-7082, as well as by TNFR1 depletion. Also, AdHBx-GFP potentiates TNF-alpha-induced apoptosis, which is completely inhibited in TNFR1-depleted cells. Our results suggest that HBx-induced NF-kappaB activation was mediated by direct interaction with TNFR1 and thereby induced TNF-alpha production. HBx-induced NF-kappaB activation is also associated with the induction of hepatic steatosis and apoptosis, which is determined by a TNFR1-dependent pathway.