Transcriptional regulation of TAX1BP2, a putative tumor suppressor in hepatocellular carcinoma

Abstract

Hepatocellular carcinoma (HCC) is one of the most widespread and deadly cancers in South East Asia, mainly because of the high incidence rate of chronic hepatitis B virus (HBV) infection. The clinical outcome of HCC patients is often poor due to the highly metastatic nature and commonly late diagnosis of HCC. Together with a limited choice of chemotherapy, this makes HCC a serious health threat in Hong Kong. Thus, better treatment is desperately required for HCC. Centrosomal Tax1- binding protein 2 （TAX1BP2）protein is a putative tumor suppressor in HCC and functions to suppress centrosome overduplication. TAX1BP2 is first recognized as a novel interacting protein of Tax, an oncoprotein encoded in HTLV-I virus, and is frequently underexpressed in HCC. TAX1BP2 down-regulation is associated with poorer overall survival of HCC patients. To understand how TAX1BP2 is downregulated in HCC, we examined the transcriptional regulation of TAX1BP2. With the epigenetic analysis, including methylation profile, demethylation and histone acetylation analysis, we ruled out the possibility of differential epigenetic modification as the major cause for TAX1BP2 underexpression. By LASAGNA online software, we predicted the potential transcriptional factors (TFs) binding sites on TAX1BP2 promoter and showed that deletion of the binding sites for Hepatocyte Nuclear Factor 4 alpha (HNF4α or HNF4A) caused a drastic decrease in TAX1BP2 promoter activity. Consistently, ectopic expression of HNF4α up-regulated the promoter activity and expression levels of TAX1BP2 both transcriptionally and translationally. Inhibition of HNF4α expression by its antagonist and siRNA also suppressed TAX1BP2 expression. By Chromatin Immunoprecipitation, we found that HNF4α binds to TAX1BP2 promoter. Moreover, we demonstrated that down-regulation of HNF4α was related to a poorer survival outcome of HCC patients. Using the quantitative real-time PCR (qRTPCR), a positive correlation of TAX1BP2 and HNF4α expression at clinical level was observed. Interestingly, ectopic expression of hepatitis B virus X protein (HBx), an oncoprotein encoded by HBV, was able to suppress HNF4α and TAX1BP2 protein expression. Furthermore, the HBx-mediated suppression of TAX1BP2 promoter activity could be rescued by ectopic expression of HNF4α, which suggested that HBx and HNF4α worked within the same signaling pathway. This study focuses on the transcriptional regulation of TAX1BP2. Here we provide the evidence on positive regulation of TAX1BP2 by HNF4α transcription factor. Furthermore, we showed that HBx suppressed HNF4α, leading to a downregulation of TAX1BP2. Further studying the HBx/HNF4α/TAX1BP2 pathway will shed light on the molecular regulatory mechanism of HBV-associated hepatocarcinogenesis.