Agonist-Activated Glucocorticoid Receptor Inhibits Binding of Heat Shock Factor 1 to the Heat Shock Protein 70 Promoter in Vivo

We have previously shown that activation of glucocorticoid receptor (GR) signaling in stressed cells will cause inhibition of the heat shock response as mediated by heat shock transcription factor 1 (HSF1). In that work, a full-length human heat shock protein 70 (Hsp70) promoter was used to measure HSF1 transactivity, and the data suggested inhibition of HSF1 through the transactivation or transrepressive properties of GR. Here, we show that the inhibitory effect of glucocorticoid agonist (dexamethasone) upon Hsp70 promoter activity is rapid, occurring within 1 h of hormone addition. Moreover, addition of hormone during the first hour of recovery from stress was sufficient to inhibit HSF1. Thus, dexamethasone is able to rapidly reverse HSF1 transactivity, suggesting a transrepressive mode of action for GR. Yet, GR transrepression of HSF1 by analysis of putative negative glucocorticoid response elements in the Hsp70 promoter was not found. To further investigate the in vivo nature of this fast-acting mechanism, we used the chromatin immunoprecipitation assay with primers specific to the human Hsp70 promoter. Dexamethasone inhibited HSF1 binding at the Hsp70 promoter in response to heat or chemical shock (sodium arsenite). Moreover, dexamethasone also blocked promoter binding by a constitutively active mutant of HSF1 (hHSF1-E189) expressed under nonstress conditions. In all cases, inhibition of HSF1 recruitment to the promoter by dexamethasone was blocked by the GR antagonist RU486, a result that was consistent with promoter activity based on chloramphenicol acetyl transferase gene expression. The ability of dexamethasone to prevent HSF1 recruitment to the promoter was fast acting (occurring in as little as 15 min), and the hormone also caused release of HSF1 already bound to the promoter. Although these results suggest GR can effectively prevent HSF1 binding to Hsp promoters, fractionation and Western blot experiments showed that stress-activated HSF1 was not released from the nucleus in response to hormone. Thus, this effect of dexamethasone is either specific to the Hsp70 promoter or causes shunting of HSF1 to other high-affinity nuclear sites. These observations provide evidence of a novel mechanism for attenuation of the heat shock response by glucocorticoids: prevention or reversal of HSF1 recruitment to Hsp promoters through the rapid actions of GR.