Fig. 6.

Model of intranuclear GR responsiveness to pulses of GC. The first pulse of endogenous GC, corticosterone or cortisol, resulted in nuclear translocation and enhanced association at specific GRE sequences at the chromatin template. p300 and CBP are recruited to this regulatory site, increasing acetylation at histone H4 and the association of activated RNA Pol2. At this phase (pulse peak), there is an increased rate of transcription of the PER1 gene. As hormone is cleared (the half-life of corticosterone is <10 min in vivo), the cells can sense the declining levels of ligand, because the endogenous ligand off-rate from GR is relatively rapid. The mechanism underlying this acute response to declining ligand is the rapid GR cycling on chromatin (timescale sec). At each exchange event, there is a probability that ligand will be lost from GR. Unliganded GR will then reenter the chaperone cycle, with the reassociation of the molecular chaperones HSP90 and p23. Unliganded GR is then retained within the nuclear compartment in complex with these molecular chaperones. At this phase (pulse trough), GR and cofactors are not chromatin associated, and the acetylation state returns rapidly to basal levels. RNA Pol2 association at the promoter is also decreased to basal levels, and the transcription rate is low. The next pulse of ligand can activate the intranuclear pool of GR, enabling another cycle of transcriptional activity, because the chaperones hold the ligand binding cleft in the correct conformation. Inhibition of the HSP90 chaperone assembly affects the intranuclear chaperone cycle at multiple points. Inhibition of a productive HSP90-GR interaction prevents pulsatile GR reactivation by directly acting at the ligand binding step. Therefore, all cyclical transcriptional activities initiated by GR as a pioneer factor at the chromatin template ceases. NET, The amount of GR (measurable by ChIP) at the chromatin template at equilibrium position.