Glucocorticoid Receptors: Evidence for a Second, Non-Glucocorticoid Binding Site

Abstract

Previous studies on cytoplasmic glucocorticoid receptors and enzyme induction led to the classification of steroids as inducers (optimal or sub-optimal), antagonists, or inactive steroids, with respect to their activity as glucocorticoids. The receptor was postulated to exist in allosteric equilibrium between 2 conformational states, 1 active and the other inactive. Steroids behaved as inducers (optimal or sub-optimal), antagonists or inactive steroids depending on their relative affinity for the active and inactive conformational state of the receptor. Another possible model invokes multiple binding sites on a single receptor with interactions between the binding sites depending upon the particular steroid bound. To test this latter possibility, an experimental technique was developed to measure the rate of dissociation of 3H-dexamethasone ([3H]DM) or 3H-aldosterone ([3H]A) from the glucocorticoid receptor of rat liver or kidney cytosol. The dissociation of the [3H]DM-receptor at 25.degree. C was not due to irreversible denaturation, and minimal recombination of the receptor with [3H]DM occurred. Progesterone and a number of other steroids consistently increased the dissociation rate of [3H]DM-receptor complexes in both liver and kidney cytosol. An identical effect was seen with hepatic glucocorticoid receptors labeled with [3H]A, like DM an optimal inducer. All steroids which enhanced glucocorticoid-receptor dissociation were either antagonists or sub-optimal inducers. Glucocorticoid receptors may have at least 2 classes of binding sites, and occupation of the 2nd site may increase the dissociation rate of agonists from glucocorticoid receptors.