Comparison of glucocorticoid receptor depletion and the suppression of ACTH secretion in the AtT-20 cell.

Glucocorticoids exert two effects on the cloned AtT-20 mouse pituitary tumor cell: they inhibit its production of ACTH and decrease its content of glucocorticoid receptors. In this paper the time courses of these two processes are compared to ascertain whether there is a link between them. The initial part of the time course of glucocorticoid receptor depletion was measured during an incubating of intact cells with 10 nM tritiated glucocorticoid. This showed that receptor depletion commenced at 4-6 h, but the half time of depletion was 30 h. Cell fractionation studies demonstrated that the decrease in receptor number occurred proportionately in both the nuclear and cytosolic fractions. The time course of dexamethasone suppression of ACTH secretion was measured under similar conditions. These studies showed that suppression was complete within 24 h. Comparison of these results led to the conclusion that although dexamethasone's effect on both receptor depletion and ACTH secretion is rapid, its effect on ACTH secretion is complete clearly before its effect on receptor depletion. Long term (3 week) dexamethasone incubations showed that even after dexamethasone had depleted the cell's receptor content to only a fraction of its initial value, its inhibitory effect on ACTH was undiminished. These results suggest that agonist action is not dampened with time. Taken together, these studies suggest that depletion is not a step in the mechanism of steroid-hormone action in the AtT-20 cell nor is it involved in diminishing the magnitude of agonist action. Instead, more likely, depletion is the process which removes redundant, spare glucocorticoid hormone receptor complexes.