Molecular Mechanisms Of The Induction Of Cyclooxygenase-2Gene Expression By Hepatitis B Virus

Human hepatitis B virus (HBV) is a non-cytopathic partially double-stranded DNA virus that causes acute and chronic liver disease. HBV infection affects approximately400million people in the world, and is strongly associated with the development of liver cirrhosis and hepatocelluar carcinoma (HCC). Even though multiple factors have been implicated in HBV-associated pathogenesis and carcinogenesis, the exact molecular mechanism underlying chronic HBV-induced liver cancer remains largely unknown.Cyclooxygenase-2(COX-2) is an inducible COX isoform required for the biosynthesis of prostaglandins, such as PGE2, and is primarily involved in pathological processes such as inflammation, fibrogenesis, and carcinogenesis. The increased expression of COX-2had been reported in chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma. Our work focuses on this phenomenon and explores the mechanism in it.The comparison between the circulating PGE2level in chronic hepatitis B patients and that in healthy people indicates that the serum levels of PGE2were significantly higher in patients with chronic hepatitis B infection in comparison with healthy individuals. Three aspects of the promoter, mRNA and protein in the HepG2cell lines can prove HBV induces the COX-2expression. It is found in the seven HBV-coded proteins that only HBx can obviously upregulates the expression of COX-2, which indicates HBV may induce the expression of COX-2mainly through HBx.To further expound the mechanism of HBV and HBx-induced COX-2, we begin with the regulation of transcription on one hand. Truncated and site-specific mutation of promoter shows the sites of C/EBP and CRE play an important role in the HBx-induced COX-2expression. HBx interacts with the transcription factor C/EBPβ and promotes the binding of C/EBPβ to the corresponding site of COX-2promoter, thereby activating the transcription. In addition, the expression of HBx can also increase the recruitment of coactivator CBP/p300to COX-2promoter. The existence of CBP/p300promotes the binding of transcription factors and the assembling of the transcription machine, thus further activating the transcription of COX-2.On the other hand, we study the role of epigenetic modification. In view of the close relationship between methylation status and activity of COX-2promoter, bisulfite sequencing analysis reveals the hypomethylation and demethylation of certain motifs of COX-2promoter in HBV-expressing cells. The demethylation of NF-AT site increases the recruitment of NF-AT factor, leading to the activation of COX-2promoter. The research indicates among three different methyltransferases that DNMT3B plays a pivotal role in the methylation of COX-2promoter. The expression of HBV can lower the binding of DNMT3B to promoter as well as the total enzymatic activity of DNMT within the nucleus, thus leading to the hypomethylation and demethylation of COM-2promoter.The combination of epigenetic modification and transcription regulation in our research shows that upregulation of COX-2by HBV and HBx is mediated by both demethylation events and recruitment of multiple transcription factors to the promoter, which provides a new idea and approach for deepper understanding the molecular mechanism of virus-regulated gene expression.