Genome-wide dynamics of chromatin binding of estrogen receptors alpha and beta: mutual restriction and competitive site selection.

Estrogen receptors ERalpha and ERbeta, members of the nuclear receptor superfamily, exert profound effects on the gene expression and biological response programs of their target cells. Herein, we explore the dynamic interplay between these two receptors in their selection of chromatin binding sites when present separately or together in MCF-7 breast cancer cells. Treatment of cells (containing ERalpha only, ERbeta only, or ERalpha and ERbeta) with estradiol or ER subtype-selective ligands was followed by chromatin immunoprecipitation analysis with a custom-designed tiling array for ER binding sites across the genome to examine the effects of ligand-occupied and unoccupied ERalpha and ERbeta on chromatin binding. There was substantial overlap in binding sites for these estradiol-liganded nuclear receptors when present alone, but many fewer sites were shared when both ERs were present. Each ER restricted the binding site occupancy of the other, with ERalpha generally being dominant. Binding sites of both receptors were highly enriched in estrogen response element motifs, but when both ERs were present, ERalpha displaced ERbeta, shifting it into new sites less enriched in estrogen response elements. Binding regions of the two ERs also showed differences in their enrichments for other transcription factor binding motifs. Studies with ER subtype-specific ligands revealed that it was the liganded subtype that principally determined the spectrum of chromatin binding. These findings highlight the dynamic interplay between the two ERs in their selection of chromatin binding sites, with competition, restriction, and site shifting having important implications for the regulation of gene expression by these two nuclear receptors.