Mechanism of glucocorticoid receptor down-regulation by glucocorticoids

The effect of glucocorticoids on the regulation of glucocorticoid receptor mRNA was studied in two different cell lines, human IM-9 lymphocytes and rat pancreatic acinar AR42J cells. Using a glucocorticoid receptor cDNA probe, glucocorticoid receptor mRNA was examined by Northern blot hybridization and quantitated by slot-blot hybridization. In IM-9 and AR42J cells, dexamethasone decreased steady-state glucocorticoid receptor mRNA levels to approximately 50% of control. This decrease occurred with a one-half time of 3 h for IM-9 cells and 6 h for AR42J cells. Dexamethasone was the most potent steroid tested with a one-half maximal effect occurring at 10 nM and a maximal effect occurring at 100 nM. Glucocorticoid receptor mRNA half-life and gene transcription were then studied to determine the mechanism of decreased mRNA levels. The glucocorticoid mRNA half-life was approximately 120 min in IM-9 cells and 240 min in AR42J cells; these rates were not affected by dexamethasone treatment. In contrast, the rate of glucocorticoid gene transcription as measured by run-on assays in IM-9 cells was decreased to 50 ± 6% of control by dexamethasone. These results indicate therefore that glucocorticoids regulate glucocorticoid receptor mRNA levels by influencing gene transcription.