Role of activation function domain-1, DNA binding, and coactivator GRIP1 in the expression of partial agonist activity of glucocorticoid receptor-antagonist complexes

The determinants of the partial agonist activity of most antisteroids complexed with steroid receptors are not well understood. We now examine the role of the N-terminal half of the glucocorticoid receptor (GR) including the activation domain (AF-1), the DNA binding site sequence, receptor contact with DNA, and coactivator binding on the expression of partial agonist activity in two cell lines for GRs bound by five antiglucocorticoids: dexamethasone mesylate (Dex-Mes), dexamethasone oxetanone (Dex-Ox), progesterone (Prog), deoxycorticosterone (DOC), and RU486. Using truncated GRs, we find that the N-terminal half of GR and the AF-1 domain are dispensable for the partial agonist activity of antiglucocorticoids. This contrasts with the AF-1 domain being required for the partial agonist activity of antisteroids with most steroid receptors. DNA sequence (MMTV vs a simple GRE enhancer) and cell-specific factors (CV-1 vs Cos-7) exert minor effects on the level of partial agonist activity. Small activity differences for some complexes of GAL4/GR chimeras with GR- vs GAL-responsive reporters suggest a contribution of DNA-induced conformational changes. A role for steroid-regulated coactivator binding to GRs is compatible with the progressively smaller increase in partial agonist activity of Dex-Mes > Prog > RU486 with added GRIP1 in CV-1 cells. This hypothesis is consistent with titration experiments, where low concentrations of GRIP1 more effectively increase the partial agonist activity of Dex-Mes than Prog complexes. Furthermore, ligand-dependent GRIP1 binding to DNA-bound GR complexes decreases in the order of Dex > Dex-Mes > Prog > RU486. Thus, the partial agonist activity of a given GR-steroid complex in CV-1 cells correlates with its cell-free binding of GRIP1. The ability to modify the levels of partial agonist activity through changes in steroid structure, DNA sequence, specific DNA-induced conformational changes, and coactivator binding suggests that useful variations in endocrine therapies may be possible by the judicious selection of these parameters to afford gene and tissue selective results.