Quantity of partial agonist activity for antiglucocorticoids complexed with mutant glucocorticoid receptors is constant in two different transactivation assays but not predictable from steroid structure

Abstract

An unsolved question in steroid hormone action is why the amount of agonist activity displayed by antisteroids is not constant but varies with the assay conditions. Receptor mutations have provided insight into hormone action, presumably due to changes in the tertiary structure of the receptor that alter its interaction surfaces with the transcriptional machinery or/and co-factors. We have now employed two mechanistically different induction assays to determine whether disparate transactivation processes are similarly altered by receptor mutations. The two activation assays studied were (i) the standard induction of GREtkLUC in transiently transfected CV-1 cells and (ii) a novel modulation of endogenous receptor activity by transiently transfected receptors in HeLa cells. Five different mutations in the ligand binding and DNA binding domains of the rat glucocorticoid receptor (CS1, CS1/CD, 451/9, C656G, and R732Q) and seven steroids of varied structures (five antagonists and two agonists) were selected for use. The results in both induction assays were the same. However, no generalizations regarding steroid structure and activity emerged. Neither of two potent glucocorticoids were active with GR-CS1, or GR-CS1/CD, while RU 486 was the only antisteroid with appreciable agonist activity. With the GR-451/9 mutant, three antagonists afforded partial agonist activity. We confirmed that the C656G mutant is both “super-sensitive” and “super-selective” for transactivation. In contrast, the R732Q mutation caused significant decreases in activity with both antagonists and subsaturating concentrations of agonists. This inability to generalize about the behavior of any class of steroids with mutant receptors may reflect an induced fit for each receptor–steroid complex. Nevertheless, the activity of a given steroid appeared to be constant in two different transactivation assays for a given mutant receptor. Thus, disparate transactivation processes may utilize identical receptor surfaces, even in the expression of partial agonist activity for specific antiglucocorticoids.