DNA binding activity of the glucocorticoid receptor is sensitive to redox changes in intact cells

The effect of changes of redox conditions on glucocorticoid receptor (GR) activity in intact cells has been studied using two approaches. One was to evaluate the GR-DNA binding in extracts of COS2 cells transiently overexpressing GR and in which reactive oxygen intermediates (ROI) accumulate as a consequence of glutathione (GSH) depletion. GR-DNA binding was significantly decreased in COS2 cells treated with diethylmaleate (DEM), which causes GSH depletion by forming GSH-DEM complexes. A similar effect was observed for Sp1, another Zn-finger transcription factor, whereas no difference was observed for the C/EBP transcription factor, which is known to be unaffected by redox changes in vitro. N-Acetylcysteine (NAC), which counteracts the effects of DEM by increasing GSH biosynthesis, prevents the decrease of GR-DNA binding in cells treated with DEM. The GR-DNA binding efficiency was similarly decreased using extracts from H2O2-treated COS2 cells and from COS2 cells treated with buthionine sulphoximine, which causes GSH depletion via a mechanism different from that of DEM. The other approach was to evaluate the efficiency of a GR-regulated promoter under different redox conditions. In HeLa cells, transfected with a plasmid containing the CAT gene under the control of the glucocorticoid responsive element (GRE) within the mouse mammary tumor virus promoter, and treated with dexamethasone to activate GR, exposure to DEM significantly impaired the activation of CAT gene expression induced by dexamethasone. Also in this case NAC treatment inhibited the effects of DEM.