Genome-wide activity of unliganded Estrogen Receptor alpha in breast cancer cells [RNA-Seq]

ERa is essential for the anti-proliferative response of breast cancer cells not only to estrogen antagonists, but also to estrogen withdrawal by means of aromatase inhibitors. We explored here one of the simplest explanation for this, consisting in the possibility that ERa may have a wide genomic function in absence of ligands. The genomic binding of ERa in the complete absence of estrogen was then studied using hormone-dependent MCF7 cells, by chromatin immunoprecipitation sequencing. From these data, 4.2K highly significant binding events were identified, which were further confirmed by comparing binding events in cells expressing ERa to cells silenced for ERa. Apo-ERa binding sites were distributed close to genes with functions associated to cell growth and epithelial maintenance and show significant overlap with binding of other transcription factors important for luminal epithelial breast cancer. Interestingly, we found that upon ERa silencing cognate gene transcription in absence of estrogen is downregulated and this is accompanied by increased H27Kme3 at ERa binding sites. RNA-Seq experiments showed that unliganded ERa controls basal transcription widely, including both coding and noncoding transcripts. Genes affected by ERa silencing can be easily functionally related to mammary epithelium differentiation and maintenance, especially when considering downregulated genes. Additional functions related to inflammatory and immune response was observed. Our data unravel unexpected actions of ERa in breast cancer cells and provide a novel framework to understand success and failure of hormone therapy in breast cancer. Overall design: Examination of unligandend estrogen receptor alpha (aERa) DNA interactions in control and aERa siRNA treated MCF7 cells.