Epigenetic repression of E-cadherin expression by Hepatitis B virus x Antigen (HBx) in liver cancer

摘要

Loss of E-cadherin is associated with acquisition of metastatic capacity. Numerous studies suggest that histone deacetylation and/or hypermethylation of CpG islands in E-cadherin gene (CDH1) are major mechanisms responsible for E-cadherin silencing in different tumors and cancer cell lines. The hepatitis B virus (HBV)-encoded X antigen, HBx, contributes importantly to the development of hepatocellular carcinoma using multiple mechanisms. Experiments were designed to test if in addition to CDH1 hypermethylation HBx promotes epigenetic modulation of E-cadherin transcriptional activity through histone deacetylation and miR-373. The relationships between HBx, E-cadherin, mSin3A, Snail-1 and miR-373 were evaluated in HBx expressing (HepG2X) and control (HepG2CAT) cells by western blotting, immunoprecipitation (IP), chromatin IP as well as by immunohistochemical staining of liver and tumor tissue sections from HBV-infected patients. In HepG2X cells, decreased levels of E-cadherin and elevated levels of mSin3A and Snail-1 were detected. Reciprocal IP with anti-HBx and anti-mSin3A demonstrated mutual binding. Furthermore, HBx-mSin3A colocalization was detected by immunofluorescent staining. HBx downregulated E-cadherin expression by the recruitment of the mSin3A/histone deacetylase complex to the Snail-binding sites in human CDH1. Histone deacetylation inhibition by Trichostatin-A treatment restored E-cadherin expression. Mir-373, a positive regulator of E-cadherin expression, was downregulated by HBx in HepG2X cells and tissue sections from HBV-infected patients. Thus, histone deacetylation of CDH1 and downregulation of miR-373, together with the previously demonstrated hypermethylation of CDH1 by HBx, may be important for the understanding of HBV-related carcinogenesis.