The glucocorticoid antagonist 17 alpha-methyltestosterone binds to the 10 S glucocorticoid receptor and blocks agonist-mediated dissociation of the 10 S oligomer to the 4 S deoxyribonucleic acid-binding subunit.

The glucocorticoid antagonist 17 alpha-methyltestosterone inhibits binding of the agonist [3H]triamcinolone acetonide ot the glocucorticoid receptor in cytosol prepared from rat pituitary tumor GH1 cells. Competitive binding studies indicate that the dissociation constant for 17 alpha-methyltestosterone is about 1 microM. After incubation of intact GH1 cells with 10 nM [3H]triamcinolone acetonide at 37 C and subsequent cell fractionation at 4 C, three glucocorticoid receptor forms are observed: cytosolic 10 S receptor, cytosolic 4 S receptor, and nuclear receptor. Concurrent incubation with 17 alpha-methyltestosterone reduces the amount of [3H]triamcinolone acetonide bound to each of these receptor forms. Ligand-exchange assays performed at 0 C in intact cells using [3H]triamcinolone acetonide show that the exchangeable antagonist is associated predominantly with cytosolic 10 S receptor. Immunochemical analysis using monoclonal antibody BuGR2 indicates that 17 alpha-methyltestosterone does not cause substantial accumulation of glucocorticoid receptors in GH1 cell nuclei and, when present together with agonist, reduces nuclear accumulation of receptor seen with agonist alone. Results from dense amino acid labeling studies show that unlike [3H]triamcinolone acetonide, 17 alpha-methyltestosterone does not reduce the total amount of cellular glucocorticoid receptor and does not reduce receptor half-life. These results are consistent with a model for glucocorticoid receptor transformation in which binding of agonist promotes the dissociation of an oligomeric 10 S cytosolic receptor protein to its DNA-binding 4 S subunit. The antagonist 17 alpha-methyltestosterone competes with agonist for binding to the 10 S cytosolic receptor but does not appear to promote dissociation of the oligomer, thus inhibiting agonist-mediated nuclear actions of the glucocorticoid receptor.