Release of methyl CpG binding proteins and histone deacetylase 1 from the Estrogen receptor alpha (ER) promoter upon reactivation in ER-negative human breast cancer cells.

Estrogen receptor alpha (ER) is an epigenetically regulated gene. Inhibitors of DNA methyltransferases (DNMTs) and histone deacetylases (HDACs) synergistically activate the methylated ER gene promoter in ER-negative MDA-MB-231 human breast cancer cells. Chromatin immunoprecipitation was used to examine the chromatin status and repressor complex associated with silenced ER and changes in the key regulatory factors during reactivation by inhibitors of DNMT (5-aza-2'-deoxycytidine) and HDAC (trichostatin A). The silencing of ER due to CpG hypermethylation correlates with binding of specific methyl-binding proteins, DNMTs, and HDAC proteins. Inhibition of HDAC activity by trichostatin A results in the accumulation of hyperacetylated core histones. The activation of ER gene expression by 5-aza-2'-deoxycytidine also involves the release of the repressor complex involving various methyl-binding proteins, DNMTs, and HDAC1. HDAC and DNMT inhibitors modulate histone methylation at H3-K9 and H3-K4 to form a more open chromatin structure necessary for reactivation of silenced ER transcription. Together these results impart a better understanding of molecular mechanisms of chromatin remodeling during ER reactivation by DNMT and HDAC inhibitors. These findings will aid in the application of agents targeting epigenetic changes in the treatment of breast cancer.