The molecular chaperone Hsp90 can negatively regulate the activity of a glucocorticosteroid-dependent promoter

Abstract

Hsp90, a molecular chaperone required for the functioning of glucocorticosteroid receptor (GR), ensures, by direct interaction, the conformational competence of the steroid-binding pocket. In addition to having this positive function, Hsp90 maintains steroid receptors in an inactive form in the absence of hormone. However, neither the participation of Hsp90 once the pathway has been activated by the ligand nor the importance of increased Hsp90 levels in determining the amplitude of the response has ever been assessed directly. Here, by increasing the Hsp90͞GR ratio in the nuclear compartment, we found an attenuation of the response to glucocorticosteroids that was not due to a nonspecific or toxic effect of the Hsp90 modified by nuclear targeting. Since this negative effect was more pronounced at high levels of hormone, when receptor and Hsp90 are maximally dissociated, the possibility of an interaction between Hsp90 and GR, already activated to a DNA-binding form, was directly investigated. Indeed GR, after in vivo activation by ligand, was still able to reassociate with Hsp90, suggesting that this interaction plays a role in vivo, possibly in receptor recycling. Moreover, the GR binding to its DNA response element was inhibited by an excess of Hsp90, pointing to a function of Hsp90 in the nuclear compartment. It is thus proposed that an increased Hsp90͞GR ratio inf luences the responsiveness to ligand at a step that is after receptor activation. This increased ratio may be of pathophysiological relevance in the different circumstances that lead to an elevated level of nuclear Hsp90.

Somatostatin Increases Glucocorticoid Binding and Signaling in Macrophages by Blocking the Calpain-specific Cleavage of Hsp 90

“…There is recent evidence that the latter inhibitory function of Hsp 90 prevails when Hsp 90/GR ratio is highly increased, especially in the nucleus (47). Thus, whether Hsp 90 accumulation following calpain inhibition increases glucocorticoid responsiveness in addition to glucocorticoid binding was questionable.…”

Unliganded and hormone-bound glucocorticoid receptors interact with distinct hydrophobic sites in the Hsp90 C-terminal domain

“…HM9-binding site to compete with the coactivator LxxLL motifs for binding to the hydrophobic groove provides a potential explanation for the observed ability of Hsp90 to inhibit the transcriptional activity of GR (11,13). These interactions of Hsp90 with apo-and holo-GR appear to optimize the transition between inactive and active states of GR, resulting in a sharpening of the cellular response to hormone.…”

“…Hormone binding by GR induces the replacement of the GR-Hsp90 heterocomplex component FKBP51 by FKBP52, whose interaction with the motor protein dynein has been implicated in the nuclear import of holo-GR (9-10). Hsp90 and p23 are present at glucocorticoid response elements (GREs) and regulate the interaction of GR with GREs and transcriptional coregulators (11)(12)(13).…”

The Corticosteroid Receptor Hypothesis of Depression

“…There is increasing evidence that (co)chaperones are not only important for GR activity in the cytosol, but also in the nucleus. For example, hsp90 may participate in the nuclear-cytoplasmic shuttling of GR (Kang et al 1999) and it may also facilitate chromatin recycling of GR (Liu and DeFranco 1999). More recently, the cochaperone RAP 46 (also known as HAP 46 or BAG-IL) which can associate with hsp90 and GR has been identified as a non-specific DNA-binding protein which may act as a general transcriptional activator (Zeiner et al 1999), on the other hand, Schneikert and coworkers (1999) reported that RAP 46 downregulates GR-mediated transactivation, but not transrepression.…”