Transcriptional regulation of TAX1BP2 in Hepatocellular carcinoma

Abstract

Hepatocellular carcinoma (HCC) is one of the most common and deadly cancer in South East Asia, mainly due to the high prevalence rate of chronic hepatitis B virus (HBV) infection. TAX1 binding protein 2 (TAX1BP2) is a tumor suppressive protein in HCC. It has two isoforms, and both are often under-expressed in HCC cell lines and patient samples. Aiming to uncover the molecular mechanism of TAX1BP2 under-expression in HCC, we have studied the potential DNA methylation of TAX1BP2 promoter and the alteration of transcription factor (TF) binding to its promoter. Our data showed that TAX1BP2 isoform 1 was hyper-methylated in HCC cell lines and immortalized hepatocyte cell line, MIHA, whereas isoform 2 is more methylated in HCC cell lines than MIHA. However, there is no significant difference in promoter methylation of isoform 1 or 2 between tumorous and non-tumorous tissues. As predicted by LASAGNA software, the two isoforms of TAX1BP2 were found to have distinct transcription factor binding sites. After deleting the predicted TF binding sites of HNF4α and AP-4, the isoform 1 promoter activity decreased significantly, indicating a potential activating role for HNF4 and AP-4 on isoform 1 promoter. Furthermore, by Western blotting, HNF4 and TAX1BP2 protein expression showed a positive correlation in the HCC cell lines, including SMMC-7721, HepG2, Huh-7, Hep3B, PLC/PRF/5, LO2, MIHA, H2P, H2M and MHCC97L. Consistently, ectopic expression of HNF4α upregulated the promoter activity, mRNA and protein expression levels of TAX1BP2 isoform 1. Taken together, these results suggested that HNF4 is an activating transcription factor of TAX1BP2 isoform 1. On the other hand, the isoform 2 promoter activity was significantly changed with the deletion of the TFs, SP-1 and YY1, binding sites, suggesting that they are potential regulating TFs for isoform 2. In summary, our data suggested that the transcription of TAX1BP2 isoforms is regulated by different TFs.