DNA binding cofactors contribute to plasticity of progesterone receptor cistromes (#200)
The ovarian hormone progesterone regulates a diverse range of normal female reproductive functions and is essential for normal mammary gland development. However, it is now well established that progesterone analogues in hormone replacement therapy increase breast cancer risk, and there is evidence that endogenous progesterone exposure also influences breast cancer development. The mechanisms driving that increased risk and the factors confining that influence to mammary epithelial cells are incompletely understood. Progesterone effects are mediated via the nuclear progesterone receptor (PR), a DNA binding transcriptional regulator. Where examined, the transcriptional responses to progesterone in different normal and malignant cell types are quite distinct, suggesting that differing patterns of PR genomic interaction may underlie this cell type specific response. We used genome wide PR ChIP sequencing to compare PR cistromes in two cell lines: T-47D breast cancer cells and MCF-10A immortalized normal breast cells stably expressing PR (AB32). Comparing the PR cistromes in the two cell lines revealed a remarkably low overlap in PR binding, which was reflected in a similarly low overlap in transcriptional response. Analysis of PR binding regions for enriched motifs revealed the pioneer factor, FOXA1, as a cofactor in a subset of binding events in T-47D cells. In AB32 cells, which lack FOXA1, AP-1 and NF1 transcription factors were identified as likely PR cofactors in these cells. Introduction of FOXA1 into AB32 cells profoundly altered progesterone response in these cells, demonstrating the potential for cell type specific factors to influence PR action. Our data suggest that cell-specificity of PR binding is determined by the coordinated effects of key binding cofactors and that altered availability of these cofactors may result in aberrant progesterone regulation in breast cancer. Our current work focuses on the ligand and PR isoform-specificity of this phenomenon.