Molecular chaperones and cochaperones as regulators of the transcriptional activity of the glucocorticoid receptor

Abstract

Glucocorticoids play important roles in anti-inflammation, immnunosuppression and induction of apoptosis. Glucocorticoids function via binding to the glucocorticoid receptor (GR) which is localized in the cytoplasm of glucocorticoid target cells. The action of the GR is modulated by molecular chaperones and cochaperones that interact with the aporeceptor to provide the correct conformation for hormone binding. Recent studies suggest that the molecular chaperones and cochaperones may also act in the nucleus to modulate the transcriptional activity of the GR. Bag-1M, a member of the Bag-1 family of cochaperones, is transported by the GR from the cytoplasm into the nucleus where it inhibits the transcriptional activity by the receptor, but does not affect the action of the structurally related mineralocorticoid receptor. However the mechanism by which it inhibits the action of the receptor is not known. In this study, the modes of action of Bag-1M in inhibiting GR-mediated transactivation have been analyzed. The hinge region of the GR was found to be essential for Bag-1M-mediated repression of the transactivation by the receptor. Bag-1M binds to lysine 496 and isoleucine 497 in the hinge region of the GR to inhibit DNA binding by the receptor in a process that requires ATP hydrolysis. Using a combination of siRNA and chromatin immunoprecipitation techniques, it has also been demonstrated that inhibition of the DNA binding by the receptor is one of two ways by which Bag-1 negatively regulates GR action. The second mechanism occurs through the recruitment of histone deacetylases 1 and 2 to glucocorticoid regulated genes to repress their action once the GR had bound Bag-1M. This action of Bag-1M is cell type specific, since it was observed in kidney cells but not cervical or hepatoma cells. The negative regulation of GR activity may therefore be relevant in kidney cells that possess both glucocorticoid and mineralocorticoid action where the action of the glucocorticoid needs to be downregulated for the mineralocorticoid receptor to function.