Heat shock and other forms of stress increase glucocorticoid receptor (GR) activity in cells, suggesting cross-talk between the heat shock and GR signal pathways. An unresolved question concerning this cross-talk is whether heat shock factor (HSF1) activity is required for this response. We addressed this issue by modulating HSF1 activity with compounds acting by distinct mechanisms: sodium vanadate (SV), an inhibitor of protein phosphatases; and wortmannin, an inhibitor of DNA-dependent protein kinase. Using HSF1-and GR-responsive CAT reporters, we demonstrate that SV inhibits both HSF1 activity and the stress potentiation of GR, while having no effect on the hormone-free GR or HSF1. Paradoxically, SV increased hormone-induced GR activity in the absence of stress. In contrast, wortmannin increased HSF1 activity in stressed cells and had no effect on HSF1 in the absence of stress. Using the pMMTV-CAT reporter containing the negative regulatory element 1 site for DNA-dependent protein kinase, wortmannin was found to increase the GR response. However, in cells expressing a minimal promoter lacking negative regulatory element 1 sites, wortmannin had no effect on the GR in the absence of stress but increased the stress potentiation of GR. Our results show that the mechanism by which GR activity is increased in stressed cells requires intrinsic HSF1 activity. The glucocorticoid receptor (GR)1 is a member of the nuclear receptor family of proteins that act as hormone-activated transcription factors (1, 2). Since the GR is known to be involved in the organismal response to stress (3), and since the hormonefree GR is known to reside in the cytoplasm as a complex containing heat shock proteins (HSPs) (4), we and others have investigated the role of the heat shock response in controlling GR function. Early evidence to suggest a relationship between these responses includes the ability of heat shock or chemical stress (arsenite) to cause nuclear translocation of hormone-free GR in mouse L929 and Chinese hamster ovary cells (5, 6) and a partial increase in GR-mediated gene expression in Chinese hamster ovary cells subjected to heat or chemical shock in the absence of hormone (6). Heat shock-induced nuclear translocation of hormone-free GR has also been documented in the liver of rats subjected to whole-body hyperthermia (7, 8) as well as in COS cells expressing human GR (9). Evidence to suggest that heat shock-induced nuclear translocation of hormone-free GR can result in altered expression of endogenous genes has also been accumulating. For example, heat shock treatment of COS and Hela cells in the absence of hormone results in GR-mediated transrepression of the collagenase promoter (9), while heat shock treatment of murine macrophages results in a pattern of Fc receptor expression or repression that is identical to that observed in response to hormone (10). More recently, heat shock-induced translocation of hormone-free GR has been implicated in the process of stress-induced apoptosis in leukemic cells (11).In the presenc...
Using mouse L929 cells stably transfected with a glucocorticoid receptor (GR)-responsive murine mammary tumor virus-chloramphenicol acetyltransferase (MMTV-CAT) reporter gene (LMCAT2 cells), we have shown that cellular stress (heat or chemical shock) can cause a dramatic increase in the levels of dexamethasone (Dex)-induced CAT gene expression. We refer to this response as the heat shock potentiation effect, or HSPE. As the cellular heat shock response also involves the activation of heat shock transcription factor (HSF), we have, in the present study, examined the role of HSF in the stress potentiation of GR by use of a flavonoid compound, quercetin, recently shown to selectively inhibit the stress response in a variety of human and murine cell lines. Analysis of the HSPE, as well as heat shock protein synthesis and activation of HSF during time-courses of recovery following heat shock, revealed a similar pattern for each response, with peak activities occurring about 16 h after stress. These data suggest a correlation between the activation of both GR and HSF in stressed cells. In L929 cells stably transfected with a CAT reporter plasmid under the control of the HSF-responsive hsp70 promoter (LHSECAT cells), pretreatment with quercetin was found to cause a dose- and time-dependent inactivation of HSF activity following heat shock, but only when added before the stress event. In LMCAT2 cells, quercetin similarly inhibited both heat and chemical shock potentiation of Dex-induced GR activity. This activity of quercetin was not the result of post-transcriptional or general cytotoxic properties, as quercetin (1) did not significantly affect GR or HSF activities when added after the stress event, (2) did not reduce CAT gene expression as controlled by the constitutive SV40 early promoter, and (3) did not alter normal (non-stress), Dex-induced MMTV-CAT expression. Thus, quercetin appears to be an effective and selective inhibitor of HSF stress-induced activation and its ability to prevent the stress potentiation of GR suggests either a direct or indirect involvement by stress-activated HSF in this process, or the existence of a regulatory step common to both the heat shock and HSPE responses.
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