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The expression of the glucocorticoid receptor (GR) gene and the phosphorylation of the GR protein has been studied as a function of time after hormone addition to NIH 3T3 cells. We detected a ligand-induced decrease of GR gene expression at both the level of RNA and protein. GR mRNA declined to 25% of the control within 3 h of dexamethasone treatment and remained at this level for at least 24 h. GR protein was analyzed with a monospecific antiserum directed against the DNA- and hormone-binding domains of the rat GR synthesized in Escherichia coli. Pulse-chase experiments revealed a decrease in GR half-life from 8 h in the absence of hormone to 3 h following hormone treatment. Both effects resulted in a reduction of total GR protein to 20% as determined by quantitative immunoblotting. The level of the GR protein returned to that of untreated cells within 24 h after withdrawal of hormone. Furthermore, dexamethasone treatment led to a 3-4-fold increase in GR phosphorylation within 60 min. The glucocorticoid antagonist 17 beta-hydroxy-11 beta-(4-dimethylamino-phenyl)-17 alpha-(1-propynyl)-oestra-4,9-dien-3-one (RU 486) did not change the phosphorylation state of the receptor, but a down-regulation of the GR was still observed. These results suggest that ligand-dependent receptor phosphorylation is not involved in down-regulation of the GR but may be important in the transcriptional activation of hormone responsive genes.
Hoeck et al. (Tue,) studied this question.