What little is known about the mechanism(s) by which the EC50 and percent partial agonist activity are determined derives from our studies of GR-regulated gene induction (reviewed in Simons Jr., 2003, TIPS, 24, 253-259;Simons Jr., 2006, Current Topics in Medicinal Chemistry, 6, 271-285;Simons Jr., 2008, Bioessays, 30, 744-756). However, the most commonly prescribed clinical use of glucocorticoids is for their capacity to repress gene induction, such as in the treatment of lymphomas by causing cell death and in the suppression of inflammatory responses. Furthermore, the mechanism of GR-regulated induction and repression is often different, with induction proceeding via GRs bound directly to DNA sequences called hormone response elements (HREs) while repression often involves GRs indirectly bound to DNA through some other DNA-bound factor, such as AP-1 or NF-κB. Finally, the EC50 of GR repression of gene expression is usually 10-fold lower than that for gene induction. Thus, at least some of the mechanistic details for GR-regulated induction and repression are different. Our studies of GR-regulated gene induction at physiological levels of steroid have documented that the EC50 and percent partial agonist activity for gene induction can be significantly altered simply by varying the concentration of a variety of transcription factors, such as the receptor itself, p160 coactivators, corepressors, Sur2, GMEB-1 and GMEB-2, and STAMP. As gene repression accounts for about half of all of the GR-mediated responses, it is clearly important to determine whether these various factors can similarly modulate the EC50 and percent partial agonist activity of GR-regulated repression. The approach of this study was two-fold: (1) to determine whether several factors known to influence GR-regulated gene induction would similarly alter gene repression by GRs and (2) to see if treatments causing modulation of the parameters of gene repression in tissue culture cells are similarly effective with endogenous genes in primary human PBMCs. In the first approach, we examined the effects of five modulators (coactivators TIF2 GRIP1, SRC-2 and SRC-1, corepressor SMRT, and comodulators STAMP and Ubc9), a glucocorticoid steroid (deacylcortivazol DAC) of very different structure, and an inhibitor of histone deacetylation (trichostatin A TSA). These factors interact with different domains of GR and thus are sensitive topological probes of GR action. These agents altered the Amax, EC50, and percent partial agonist activity of endogenous and exogenous repressed genes similarly to that previously observed for GR-regulated gene induction. Collectively, these results suggest that GR-mediated induction and repression share many of the same molecular interactions and that the causes for different levels of gene transcription arise from more distal downstream steps. In the second approach, we asked whether lowering the endogenous concentration of TIF2 with added siRNA would have the same effect on three endogenous GR-regulated genes (GILZ, CD163, and THBS1) in human PBMCs as seen with an exogenous reporter in tissue culture cells. As expected, lower levels of TIF2 did reduce the percent partial agonist activity and increase the EC50 but the effects were gene-selective, with no one gene exhibiting both responses. The heterogeneity of outcomes is consistent with our frequent observation that different pathways and/or factors exist for the selective control of Amax, EC50, and percent partial agonist activity. We therefore conclude that the repression of some GR induction properties after reducing the level of a specific endogenous transcription factor is a physiologically relevant response. These studies demonstrate that two important transcriptional properties (EC50 and percent partial agonist activity) are similarly modified by several factors for both of the major actions of glucocorticoid steroids: gene induction and gene repression. These manipulations are physiologically relevant and thus permit a continuum of responses and constitute new therapeutic targets for differential control of gene expression by steroid hormones during development, differentiation, homeostasis, and endocrine therapies. These combined findings contribute to our long-term goal of defining the action of steroid hormones at a molecular level and of understanding their role in human physiology.
