Functional Switching of TGF-β1 Signaling in Liver Cancer via Epigenetic Modulation of a Single CpG Site in TTP Promoter

BACKGROUND & AIMS: Acquisition of resistance to the antiproliferative effect of transforming growth factor (TGF)beta 1 is crucial for the malignant progression of cancers. In this study, we sought to determine whether deregulated expression of tristetrapolin (TTP), a negative posttranscriptional regulator of c-Myc, confers resistance to the antiproliferative effects of TGF-beta 1 on liver cancer cells. METHODS: The epigenetics of TIP promoter regulation and its effects on TGF-beta 1 signaling were examined in hepatocellular carcinoma (HCC) cell lines and patient tissues. RESULTS: TIP was down-regulated in HCC cell lines (10/11), compared with normal liver, as well as in tumor tissues (19/24) from paired HCC specimens. Methylation of a specific single CpG site located within the TGF-beta 1-responsive region (TRR) of the TIP promoter was significantly associated with TIP downregulation in both HCC cell lines and tumor tissues (r = -0.606383, P < .001). The singly methylated CpG site was specifically bound by a transcriptional repressor complex consisting of MECP2/c-Ski/DNMT3A and abolished the TGF-beta 1-induced as well as basal-level expression of TTP. The epigenetic inactivation of TTP led to an increased half-life of c-Myc mRNA and blocked the cytostatic effect of TGF-beta 1. Statistically significant correlations were observed between the single CpG site methylation and expression levels of TTP or c-Myc in clinical samples of HCC. CONCLUSIONS: Abrogation of the post-transcriptional regulation of c-Myc via methylation of a specific single CpG site in the TTP promoter presents a novel mechanism for the gain of selective resistance to the antiproliferative signaling of TGF-beta 1 in HCC.