Glucocorticoid hormone plays a major role in metabolism and disease. The hormone-bound glucocorticoid receptor (GR) binds to a specific set of enhancers in different cell types, resulting in unique patterns of gene expression. We addressed the role of chromatin structure in GR binding by mapping nucleosome positions in mouse adenocarcinoma cells. Before hormone treatment, GR-enhancers exist in one of three chromatin states: (1) Nucleosome-depleted enhancers that are DNase I-hypersensitive, associated with the Brg1 chromatin remodeler and flanked by nucleosomes incorporating histone H2A.Z. (2) Nucleosomal enhancers that are DNase I-hypersensitive, marked by H2A.Z and associated with Brg1. (3) Nucleosomal enhancers that are inaccessible to DNase I, incorporate little or no H2A.Z and lack Brg1. Hormone-induced GR binding results in nucleosome shifts at all types of GR-enhancer, coinciding with increased recruitment of Brg1. We are developing a model wherein nucleosome-depleted GR-enhancers are formed and maintained by other transcription factors which recruit Brg1, whereas, at nucleosomal enhancers, GR behaves like a pioneer factor, interacting with nucleosomal sites and recruiting Brg1 to remodel the chromatin.
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