[Show abstract][Hide abstract] ABSTRACT: Formation of transcriptional repression complexes such as DNA methyltransferase (DNMT) 1/histone deacetylase (HDAC) or methyl-CpG binding protein/HDAC is emerging as an important mechanism in silencing a variety of methylated tissue-specific and imprinted genes. Our previous studies showed that treatment of estrogen receptor (ER)-alpha-negative human breast cancer cells with the DNMT inhibitor 5-aza-2'-deoxycytidine (5-aza-dC) led to ER mRNA and protein re-expression. Also, the HDAC inhibitor trichostatin A (TSA) could induce ER transcript about 5-fold. Here we show that 5-aza-dC alone induced ER transcript about 30-40-fold, and the addition of TSA elevated ER mRNA expression about 10-fold more in the human ER-negative breast cancer cell lines MDA-MB-231 and MDA-MB-435. Overall, the combination of 5-aza-dC and TSA induced a 300-400-fold increase in ER transcript. Restoration of estrogen responsiveness was demonstrated by the ability of the induced ER protein to elicit estrogen response element-regulated reporter activity from an exogenous plasmid as well as induce expression of the ER target gene, progesterone receptor. The synergistic activation of ER occurs concomitantly with markedly reduced soluble DNMT1 expression and activity, partial demethylation of the ER CpG island, and increased acetylation of histones H(3) and H(4). These data suggest that the activities of both DNMT1 and HDAC are key regulators of methylation-mediated ER gene silencing.
Cancer Research 11/2001; 61(19):7025-9. · 8.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recent findings have established a connection between DNA methylation and transcriptionally inactive chromatin characterized by deacetylated histones. Because the absence of estrogen receptor alpha (ERalpha) gene expression has been associated with aberrant methylation of its CpG island in a significant fraction of breast cancers, we tested whether histone deacetylase activity contributes to the transcriptional inactivation of the methylated ER gene in a panel of ER-negative human breast cancer cells. Treatment of these cells with trichostatin A, a specific histone deacetylase inhibitor, led to dose- and time-dependent re-expression of ER mRNA as detected by reverse transcription-PCR without alteration in ERalpha CpG island methylation. Trichostatin A-induced ER re-expression was associated with increased sensitivity to DNase I at the ER locus in MDA-MB-231 cells. These data implicate inactive chromatin mediated by histone deacetylation as a critical component of ER gene silencing in human breast cancer cells. Therefore, histone deacetylation may be a potential target for therapeutic intervention in the treatment of a subset of ER-negative breast cancers.
Cancer Research 01/2001; 60(24):6890-4. · 8.65 Impact Factor