Heat and chemical shock potentiation of glucocorticoid receptor transactivation requires heat shock factor (HSF) activity - Modulation of HSF by vanadate and wortmannin

Heat shock and other forms of stress increase glucocorticoid receptor (GR) activity in cells, suggesting cross-talk between the heat shock and GR sign al pathways. An unresolved question concerning this cross-talk is whether h eat shock factor (HSF1) activity is required for this response, We addresse d 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 OR-responsive CAT reporters, we demonstrate that SV inhibits both HSF1 acti vity and the stress potentiation of GR, while having no effect on the hormo ne-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 f or DNA-dependent protein kinase, wortmannin was found to increase the GR re sponse. However, in cells expressing a minimal promoter lacking negative re gulatory 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 th at the mechanism by which CR activity is increased in stressed cells requir es intrinsic HSF1 activity.