Chromatin recycling of glucocorticoid receptors: Implications for multipleroles of heat shock protein 90

Unliganded glucocorticoid receptors (GRs) released from chromatin after hor mone withdrawal remain associated with the nucleus within a novel subnuclea r compartment that serves as a nuclear export staging area. We set out to e xamine whether unliganded nuclear receptors cycle between distinct subnucle ar compartments or require cytoplasmic transit to regain hormone and chroma tin-binding capacity. Hormone-withdrawn rat GrH2 hepatoma cells were permea bilized with digitonin to deplete cytoplasmic factors, and then hormone-bin ding and chromatin-binding properties of the recycled nuclear GRs were meas ured. We found that recycled nuclear GRs do not require cytosolic factors o r ATP to rebind hormone. Nuclear GRs that rebind hormone in permeabilized c ells target to high-affinity chromatin-binding sites at 30 C, but not 0 C, in the presence of ATP. Since geldanamycin, a heat shock protein-90 (hsp90) -binding drug, inhibits hormone binding to recycled nuclear GRs, hsp90 may be required to reassemble the receptor into a form capable of productive in teractions with hormone. Geldanamycin also inhibits GR release from chromat in during hormone withdrawal, suggesting that hsp90 chaperone function may play multiple roles to facilitate chromatin recycling of GR.