Endocrine Abstracts

Localized transitions in chromatin structure accompany nuclear receptor binding events in mammalian cells. These remodeling processes are critical to determine the binding landscape for steroid receptors (SRs) in cancer cells. Multiple reports indicate that steroid receptors (ER, GR, AR, PR) can regulate the binding patterns for each other, particularly during cancer progression. Elucidation of the mechanisms by which these ‘chromatin opening’ processes occur is central to our understanding of steroid receptor cross-talk at the genome level. A widely accepted model suggests that SRs function in concert with pioneer factors to open enhancer chromatin, creating DNase ‘hypersensitive’ sites (DHS), relieving the inhibitory activity of closed chromatin. These localized sites are often assumed to represent nucleosome free regions (NFRs). We have mapped nucleosome positions at high resolution in mouse mammary adenocarcinoma cells, and characterized GR dependent factor recruitment and changes in nucleosome structure. We find little correlation between the extent of hypersensitivity and nucleosome presence. GR-enhancers exhibit a complex range of states; in some cases, the receptor attacks pre-existing nucleosomes and recruits the Brg1 remodeler, behaving at these sites as a classic pioneering activity. There is also controversy regarding the multimeric status of SRs in their enhancer bound state. GR is typically represented as acting as a monomer or dimer. We described a tetrameric state for GR bound to the MMTV response element in live cells, and tetrameric states for other transcription factors have been reported. Using a mutation that mimics the DNA bound state, we have examined the chromatin binding landscape for a putative tetrameric form of GR. This receptor dramatically increases the binding profile for GR, penetrating chromatin that is closed in mammary cells, but available in other cell types. These findings will be discussed in terms of a model wherein receptors and many transcription factors act to achieve chromatin remodeling and enhancer activation through a highly dynamic mechanism termed ‘dynamic assisted loading.’

DOI: 10.1530/endoabs.54.IS14