Ultradian glucocorticoid (GC) secretion is highly conserved, having been detected in all species studied. The GC corticosterone (CORT) is a ligand for the glucocorticoid receptor (GR), inducing GR recruitment to glucocorticoid responsive elements (GREs) to modulate transcription of GC-target genes. We have previously demonstrated that pulsatile GR recruitment to GREs upstream of the Period1 gene is associated with its pulsatile transcription in rat liver. Similarly the gene pulsing phenomenon was demonstrated in cell lines, where constant GC treatment was found to induce prolonged GR activity and overexpression of GC-target genes. However the effects of GC pattern dysregulation on GR transcriptional dynamics at more physiologically relevant metabolic targets in vivo are less well understood.
Adrenalectomized male Sprague Dawley rats were administered with physiological CORT replacement over 3 hr, as hourly 20 min pulses or matched constant infusion. Vehicle-infused rats served as controls. Liver samples were collected at 2 hr 20 min and 3 hr (times corresponding to pulse peak and trough respectively). Samples were ChIPd with GR and RNA Polymerase (Pol2) antibodies and sequenced. GR binding at a large range of genomic sites was found to faithfully track the pulsatile peak and trough, whereas constant CORT generally induced sustained GR recruitment.
Pol2 activity was found to be highly dynamic and differentially regulated in a gene-specific manner. Notably Pol2 activity at metabolic targets involved in gluconeogenesis and lipolysis, such as Tyrosine aminotransferase, Lpin1 and Angptl4, was markedly increased with constant infusion relative to both pulsatile CORT and vehicle-infused controls. Interestingly pulsatile infusion resulted in pulsatile Pol2 activity along entire GC-regulated gene bodies in many cases, and actually reduced Pol2 activity relative to both constant and vehicle-infusion in some cases such as for Angptl4.
Therefore, we have demonstrated that disrupting the ultradian GC rhythm causes complex genome-wide dysregulation of metabolic targets potentially resulting in development of adverse metabolic phenotypes.
07 Nov 2016 - 09 Nov 2016