Endocrine Abstracts

Ligand binding of the glucocorticoid receptor (GR) initiates recruitment to GR Response Elements (GRREs). GR transcriptional activation of genes manifests many of the pleiotropic actions attributed to glucocorticoids. However, excessive exposure to GCs produces detrimental impacts, shifting GR behaviour and driving dysregulated metabolic states. Moreover, GR, its interacting partners, and influencing proteins remain ambiguous. Poly-(ADP-ribosyl) polymerase 1 (PARP1) is a chromatin-interacting protein, shown to influence a range of biological pathways. PARP1 is also described as co-operating with pioneer factors to direct transcription factor binding, through genome accessibility. Our existing data show GCs reduce PARP1 activity, and we hypothesise that PARP1 plays a role in the GR binding events triggered by GCs. To evaluate this, we gave C57BL/6J mice drinking water containing corticosterone (CORT) or vehicle control for 3 weeks. Liver tissue was subject to ChIPseq for GR and PARP1 binding. CORT significantly increased GR binding events within 2kb of transcriptional start sites (TSS) from 410 to 602 (+46.83%) (peaks called using MACS2 Pvalue 0.05). GR recruitment to the whole genome was also elevated (+9.29%). PARP1 binding events were reduced from 4975 to 1004 (-79.06%). CORT reduced overlap of PARP1 and GR binding sites from 248 to 115 (-53.63%). Together, findings indicate PARP1 is influenced by intracellular GC availability as its genomic interaction is greatest before steroid receptor-ligand binding. Pathway analysis indicates necroptosis (adjP 5.7x10-38) and RNA transport (adjP 3.6x10-22) are differentially regulated as a result. RNAseq will reveal the impacts of PARP1 genome. It will also show the influence of PARP1 over transcriptionally productive GR binding. These data explore PARP1 cooperation with pioneer factor proteins to improve accessibility of the chromatin prior to steroid hormone receptor binding. Together it reveals mechanisms of transcription factor-directed gene expression and provides new knowledge into how GC excess may impact fundamental biological programmes.