Genome-wide activity of unliganded estrogen receptor-α in breast cancer cells

Estrogen receptor-alpha (ER alpha) has central role in hormone-dependent breast cancer and its ligand-induced functions have been extensively characterized. However, evidence exists that ER alpha has functions that are independent of ligands. In the present work, we investigated the binding of ER alpha to chromatin in the absence of ligands and its functions on gene regulation. We demonstrated that in MCF7 breast cancer cells unliganded ER alpha binds to more than 4,000 chromatin sites. Unexpectedly, although almost entirely comprised in the larger group of estrogen-induced binding sites, we found that unliganded-ER alpha binding is specifically linked to genes with developmental functions, compared with estrogen-induced binding. Moreover, we found that siRNA-mediated down-regulation of ER alpha in absence of estrogen is accompanied by changes in the expression levels of hundreds of coding and noncoding RNAs. Down-regulated mRNAs showed enrichment in genes related to epithelial cell growth and development. Stable ER alpha down-regulation using shRNA, which caused cell growth arrest, was accompanied by increased H3K27me3 at ER alpha binding sites. Finally, we found that FOXA1 and AP2 gamma binding to several sites is decreased upon ER alpha silencing, suggesting that unliganded ER alpha participates, together with other factors, in the maintenance of the luminal-specific cistrome in breast cancer cells.