The relationship between glucocorticoid biopotency and receptor binding in the AtT-20 cell

The AtT-20 cell is a commonly used model for investigations of glucocorticoid agonist action. As yet, however, no reports quantify the relationship between the magnitude of agonist binding to the intact cell's glucocorticoid receptor and the magnitude of response to agonists. Because this correlation may illuminate the molecular mechanism of hormone action, the abilities of triamcinolone acetonide, dexamethasone, and corticosterone to inhibit the AtT-20 cell's production of ACTH were determined and compared with their binding affinities for the intact cell's glucocorticoid receptor. Both variables were assayed under similar conditions. Each of the three steroids caused a dose-dependent suppression of ACTH production. To compare the potencies of the three, the concentration of agonist that caused a 50% reduction in ACTH secretion was determined and was found to be 0.12 nM for triamcinolone acetonide, 0.58 nM for dexamethasone, and 4.8 nM for corticosterone. The affinities of these steroids for the intact cell's glucocorticoid receptor were determined by Scatchard analysis. The Kd values were 3.4 nM for triamcinolone acetonide, 8.5 nM for dexamethasone, and 51 nM for corticosterone. Comparison of the biopotency and binding data suggests that the agonists are more potent than one would predict from the binding studies and that only a fraction of the cell's receptors need be filled for the agonist to be fully active. Control studies showed that this quantitative discrepancy between steroid binding and biopotency was not caused by misdilution of steroids, steroid metabolism, or steroid-binding components in serum. Thus, this discrepancy leads to the conclusion that the AtT-20 cell has a large number of "spare" glucocorticoid receptors.