Eliminating epigenetic barriers induces transient hormone-regulated gene expression in estrogen receptor negative breast cancer cells

In breast cancer, approximately one-third of tumors express neither the estrogen receptor (ERα) nor estrogen-regulated genes such as the progesterone receptor gene (PR). Our study provides new insights into the mechanism allowing hormone-activated expression of ERα target genes silenced in ERα-negative mammary tumor cells. In cell lines derived from ERα-negative MDA-MB231 cells, stable expression of different levels of ERα from a transgene did not result in transcription of PR. A quantitative comparative analysis demonstrates that inhibiting DNA methyltransferases using 5-aza-2′-deoxycytidine or specific disruption of DNMT1 by small interfering RNAs and treatment with the histone-deacetylase inhibitor trichostatin A enabled ERα-mediated hormone-dependent expression of endogenous PR. We show that demethylation of a CpG island located in the first exon of PR was a prerequisite for ERα binding to these regulatory sequences. Although not a general requirement, DNA demethylation is also necessary for derepression of a subset of ERα target genes involved in tumorigenesis. PR transcription did not subsist 4 days after removal of the DNA methyltransferase blocking agents, suggesting that hormone-induced expression of ERα target genes in ERα-negative tumor cells is transient. Our observations support a model where an epigenetic mark confers stable silencing by precluding ERα access to promoters.