The HSP90 molecular chaperone cycle regulates cyclical transcriptional dynamics of the glucocorticoid receptor and its co-regulatory molecules CBP/P300 during ultradian ligand treatment

Glucocorticoid hormones are secreted from the adrenal gland in a characteristic pulsatile pattern. This ultradian secretory activity exhibits remarkable plasticity, with distinct changes in response to both physiological and stressful stimuli in humans and experimental animals. It is therefore important to understand how the pattern of glucocorticoid exposure regulates intracellular signalling through the glucocorticoid receptor (GR). We have previously shown that each pulse of ligand initiates rapid, transient GR activation in several physiologically relevant and functionally diverse target cell types. Using chromatin immunoprecipitation (ChIP) assays we detect cyclical shifts in the net equilibrium position of GR association with regulatory elements of glucocorticoid-target genes and have investigated in detail the mechanism of pulsatile transcriptional regulation of the glucocorticoid-induced Period 1 gene. Transient recruitment of the histone acetyl transferase (HAT) complex CBP/P300 is found to precisely track the ultradian hormone rhythm, resulting in transient localized net changes in lysine acetylation at glucocorticoid-regulatory regions following each pulse. Pulsatile changes in histone H4 acetylation and concomitant recruitment of RNA Pol2 precede ultradian bursts of Period 1 gene transcription. Finally, we report the crucial underlying role of the intranuclear HSP90 molecular chaperone complex in pulsatile GR regulation. Pharmacological interference of HSP90 with geldanamycin during the intranuclear chaperone cycle completely ablated GR’s cyclical activity, cyclical CBP/P300 recruitment and the associated cyclical acetylation at the promoter region. These data imply a key role for an intact nuclear chaperone cycle in cyclical transcriptional responses, regulated in time by the pattern of pulsatile hormone.