Hepatitis B Virus X Protein Induces Hepatic Steatosis by Enhancing the Expression of Liver Fatty Acid Binding Protein.

Hepatitis B virus (HBV) has been implicated as a potential trigger of hepatic steatosis although molecular mechanisms involved in the pathogenesis of HBV-associated hepatic steatosis still remain elusive. Our prior work has revealed that the expression level of liver fatty acid binding protein 1 (FABP1), a key regulator of hepatic lipid metabolism, was elevated in HBV-producing hepatoma cells. In this study, the effects of HBV X protein (HBx) mediated FABP1 regulation on hepatic steatosis and the underlying mechanism were determined. mRNA and protein levels of FABP1 were measured by quantitative RT-PCR (qPCR) and Western blotting. HBx-mediated FABP1 regulation was evaluated by luciferase assay, coimmunoprecipitation, and chromatin immunoprecipitation. Hepatic lipid accumulation was measured by using Oil-Red-O staining and the triglyceride level. It was found that expression of FABP1 was increased in HBV-producing hepatoma cells, the sera of HBV-infected patients, and the sera and liver tissues of HBV-transgenic mice. Ectopic overexpression of HBx resulted in upregulation of FABP1 in HBx-expressing hepatoma cells, whereas HBx abolishment reduced FABP1 expression. Mechanistically, HBx activated the FABP1 promoter in an HNF3β-, C/EBPα-, and PPARα-dependent manner, in which HBx increased the gene expression of HNF3β and physically interacted with C/EBPα and PPARα. On the other hand, knockdown of FABP1 remarkably blocked lipid accumulation both in long-chain free fatty acids treated HBx-expressing HepG2 cells and in a high-fat diet-fed HBx-transgenic mice. Therefore, FABP1 is a key driver gene in HBx-induced hepatic lipid accumulation via regulation of HNF3β, C/EBPα, and PPARα. FABP1 may represent a novel target for treatment of HBV-associated hepatic steatosis. Accumulating evidence from epidemiological and experimental studies has indicated that chronic HBV infection is associated with hepatic steatosis. However, the molecular mechanism underlying HBV-induced pathogenesis of hepatic steatosis still remains to be elucidated. In this study, we found that expression of liver fatty acid binding protein (FABP1) was dramatically increased in the sera of HBV-infected patients and in both sera and liver tissues of HBV-transgenic mice. Forced expression of HBx led to FABP1 upregulation, whereas knockdown of FABP1 remarkably diminished lipid accumulation in both in vitro and in vivo models. It is possible that HBx promotes hepatic lipid accumulation through upregulating FABP1 in the development of HBV-induced nonalcoholic fatty liver disease. Therefore, inhibition of FABP1 might have therapeutic value in steatosis-associated chronic HBV infection.