Regulation of nuclear retention of glucocorticoid receptor by nuclear Hsp90.

Heat shock protein 90 (Hsp90) has been demonstrated in both cytoplasm and nucleus, and regulates cytoplasmic retention of glucocorticoid receptor (GR). However, the role of nuclear Hsp90 in GR trafficking is less characterized. The present study examined the role of Hsp90 in nuclear retention of GR after ligand withdrawal. Hsp90 inhibitors; geldanamycin (GA) and radicicol (Rad), significantly accelerated nuclear export of GR after withdrawal of ligands including dexamethasone, corticosterone and RU486. GA accelerated relocalization of GR in the cytoplasm even when reimport of GR into the nucleus was inhibited by okadaic acid or when novel GR synthesis was inhibited by cycloheximide. Overexpression of wild type or nuclear-targeted Hsp90 attenuated Hsp90 inhibitor-induced acceleration of GR nuclear export, although nuclear Hsp90 showed higher activity than the wild type. Only nuclear-targeted Hsp90 prolonged basal nuclear retention of GR after withdrawal of dexamethasone and corticosterone. These results suggest that nuclear Hsp90 regulates the nuclear retention of GR.

Hsp90 inhibitors attenuate effect of dexamethasone on activated NF-kappaB and AP-1.

Heat shock protein 90 (Hsp90) regulates the functions of glucocorticoid receptor (GR). Hsp90 inhibitors geldanamycin (GA) and radicicol (Rad) have been studied as anti-inflammatory agents; however, their effects on glucocorticoid-mediated anti-inflammatory mechanism are not known. In the present study, we examined the effects of dexamethasone (Dex) and Hsp90 inhibitors, alone and in combination, on the activation of GR and proinflammatory transcription factors such as nuclear factor-kappaB (NF-kappaB) and activator protein-1 (AP-1). In cell-based reporter assay, Hsp90 inhibitors inhibited Dex-induced nuclear import and transcriptional activity of GR. Both tumor necrosis factor-alpha-activated NF-kappaB and phorbol ester-activated AP-1 were inhibited by Dex and Hsp90 inhibitors alone. When the cells were treated with a combination of these drugs, the inhibitory effect of Dex was significantly attenuated by Hsp90 inhibitors. We further examined the effects of Dex and Rad on lipopolysaccharide-induced gene expressions of the proinflammatory cytokine interleukin (IL)-1beta in macrophages. Dex, but not Rad, inhibited IL-1beta expression. Rad concentration-dependently attenuated the inhibitory effect of Dex. These results suggest that Hsp90 inhibitor itself inhibits the activation of NF-kappaB and AP-1, however, impedes Dex-induced inhibition of IL-1beta induction by attenuating Dex-mediated activation of GR and inhibition of the proinflammatory transcription factors.