GLUCOCORTICOID RECEPTOR CONVERSION TO HIGH-AFFINITY NUCLEAR-BINDING AND TRANSCRIPTION ENHANCEMENT ACTIVITY IN CHINESE-HAMSTER OVARY CELLS SUBJECTED TO HEAT AND CHEMICAL STRESS

The untransformed glucocorticoid receptor (GR) is a heteromeric comple x containing two molecules of the 90-kDa heat shock protein (hsp90) an d one molecule of the 56-kDa heat shock protein (hsp56). In the absenc e of hormone, this complex is found in the cytosolic fraction of cells , and upon hormone-binding the complex dissociates and the GR is recov ered in the nuclear pellet fraction. Given the association of heat sho ck proteins with the cytosolic form of the GR, we have examined the ef fects of heat shock on GR subcellular localization and transcription e nhancement activity in a series of Chinese hamster ovary (CHO) cells w hich express either low levels of endogenous GR (CHOd cells), high lev els of the mouse GR (WCL2 cells), or high levels of mutated mouse GR u nable to bind DNA (NB cells). It was found that heat shock treatment o f WCL2 cells results in wild-type mouse GR that is recovered almost en tirely within the nuclear pellet fraction, a response similar to that seen in hormone-treated cells. In contrast, heat shock treatment of NB cells results in complete loss of GR from the cytosolic fraction, but almost no shift of CR to the nuclear pellet. These results indicate t hat heat shock-mediated conversion to high-affinity nuclear binding by the wild-type GR requires a functional DNA-binding domain, and that h eat shock will result in loss of GR to proteolysis in the absence of n uclear sequestration. Analysis of MMTV-CAT reporter gene expression in these cells revealed that heat or chemical shock, in comparison to ho rmone-treatment, results in a small induction of MMTV-CAT expression i n the WCL2 cells, but not in the CHOd or NB cells. These results indic ate that cellular stress can cause at least a partial induction of hor mone-independent GR-mediated gene expression.