Functional Regulation of Histone Methyltransferase Activity by Hepatitis C Virus Core Protein

Posttranslational modification of histone proteins constitutes the histone code which plays an important role in eukaryotic gene expression as well as viral pathogenesis. Previous studies have shown that viral proteins functionally interact with histone acetyltransferases/deacetylases. Whether cellular histone methyltransferases (HMTs) are also targeted by viral oncoproteins remains unclear. Our previous study indicated that the core protein of the hepatitis C virus (HCV) which causes chronic hepatitis, cirrhosis, fibrosis and hepatocellular carcinoma in infected individuals represses core histone methylation by coactivator-associated arginine methyltransferase 1 (CARM1), protein arginine methyltransferase 1 (PRMT1), and SET domain-containing lysine methyltransferase 9 (SET9) in in vitro HMT assays. It prompted us to investigate the relationship between HCV core protein and the three HMTs in vivo. The three HMTs (CARM1, PRMT1 and SET9) are the coactivators of p53, and p21 is the downstream of p53. We used human hepatoma cell line Huh 7 as our study model. We demonstrated that in reporter assays HCV core protein repressed CARM1- and PRMT1-enhanced p21 promoter activities as well as p21 synthesis. In addition, we showed the in vivo interactions between core protein and CARM1 and PRMT1. Using the chromatin-immunoprecipitation (ChIP) analysis, we further demonstrated that (1) core protein was recruited to the p21 promoter, (2) core protein did not affect p53 binding to the p21 promoter and (3) inhibited histone H3 acetylation and H3K4 mono-methylation, the active histone modification pattern. Furthermore, we unraveled that core protein 191 repressed promoter activities of four kinds of NF-κB target genes in reporter assay, and R17 site of core protein c173 was methylated. Taken together, our results indicate that HCV core protein inhibits HMT activities on core histones in vivo, which in turn resulted in the inhibition of the function of p53 and NF-κB.