Trichostatin A (TSA), an inhibitor of histone deacetylase. is a well-known antitumor agent that effectively and selectively induces tumor growth at-rest and apoptosis Recently, it was reported that hTERT is one of the primary targets for TSA-induced apoptosis in cancer cells but the mechanism of which has not yet been elucidated. In the present Study, to better understand the epigenetic regulation mechanism responsible for the repression of hTERT by TSA. we examined expression of hTERT in the HCT116 colon cancer cell line after treatment with TSA and performed site-specific CpG methylation analysis of the hTERT promoter. We found that TSA-induced the demethylation of site-specific CpGs on the promoter of hTERT, which was Caused by down-regulation of DNA methyltransferase I (DNMTI) Among the demethylated region. the 31st-33rd CpGs contained a binding site for CTCF, an inhibitor of hTERT transcription ChIP analysis revealed that TSA-induced demethylation of the 31st-33rd CpGs promoted CTCF binding on hTERT promoter, leading to repression of hTERT taken together, down-regulation of DNMTI by TSA caused demethylation of a CTCF binding site oil the hTERT promoter, the result of which was repression of hTERT via recruitment of CTCF to the promoter (C) 2009 Elsevier Inc All rights reserved