What is known about the mechanism(s) by which the EC50 and Amax are determined derives mostly 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; Simons Jr., 2010, Current Opin. Pharmacology, 10, 613-619). 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. Induction proceeds via GRs bound directly to DNA sequences called hormone response elements 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 often 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 Amax and EC50 for gene induction can be significantly altered simply by varying the concentration of a variety of transcription factors. As gene repression accounts for about half of all of the GR-mediated responses, it is clearly important to determine whether the same factors can similarly modulate the Amax and EC50 of GR-regulated repression. A complication in any mechanistic description of gene induction and repression is that the currently employed terms of coactivators and corepressors are phenomenological descriptions without mechanistic information. Further confounding the issue is that there is no consensus on whether, for example, a coactivator (which increases the Amax of steroid receptors in gene induction) should increase or decrease the Amax in gene repression. An unbiased solution to this question is possible with the application of our recently developed theoretical framework of steroid hormone action (Ong et al., 2010, Proc Natl Acad Sci U S A, 107, 7107-7112). This theory, and derived competition assay (Dougherty et al., 2012, PLoS ONE, 7, e30225), is able to determine the kinetically-defined mechanism of action, and the site of action, of a cofactor relative to a reaction step called the concentration limited step (CLS), which is similar to the rate limiting step in enzyme kinetics. Thus is now possible to classify factor action on the basis of first principals during both gene induction and gene repression by steroid receptors. We have been able to define types of graphs of Amax and EC50 that are associated with different kinetically-defined mechanisms and site of action in gene induction. In order to disambiguate the local action from the global action, we use the term activator or accelerator for a factor that increases the output of the local reaction independent of the observed final response (e.g. amount of gene product). Similarly, we use inhibitor or decelerator or a factor that decreases the output of a local reaction. In our continuing collaboration with Carson Chow (NIDDK, NIH), we have now developed a theory for gene repression in which the mechanisms of factor action are defined kinetically and are consistent for both gene repression and induction. The theory is applicable if the dose-response curve for gene repression is non-cooperative with a unit Hill coefficient, which is observed for GR-regulated repression of AP1LUC reporter induction by phorbol myristate acetate. The theory predicts the mechanism of GR and cofactors, and where they act with respect to each other, based on how the dose-response curve changes under their influence. We show that the kinetically-defined mechanism of action of each of four factors (reporter gene, p160 coactivator TIF2, and two pharmaceuticals NU6027 and phenanthroline) is the same in GR-regulated repression and induction. What differs is the site of GR action. This insight should simplify clinical efforts to differentially modulate factor actions in gene induction vs. gene repression. These studies demonstrate that our earlier conclusions regarding the modulation by cofactors of all three transcriptional parameters of GR-mediated gene induction (Amax and EC50) can be extended to GR-regulated gene repression. This insight is critical for our understanding of how GRs alter gene expression. The current results indicate that the basic mechanism of action of accelerators is preserved, regardless of whether the gene expression goes up or down. Such a preservation of factor functioning will greatly simplify the task of defining the action of steroid hormones at a molecular level. It will also greatly facilitate the task of achieving our two long-range objectives of (1) understanding the role of each factor in human physiology and (2) developing pharmaceutical agents to alter factor actions in the clinical setting.
