What is known about the mechanism(s) by which the EC50 and PAA 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 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 Amax, EC50, and PAA 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, EC50, and PAA of GR-regulated repression. A complication in any mechanistic description of gene induction and repression is the that 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 limiting 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 our continuing collaboration with Carson Chow (NIDDK, NIH), we are developing a similar reference for the changes in the Amax and EC50 of GR-repressed genes. The preliminary results with two factors indicate that the mechanism and site of action of both factors is the same in gene repression as in gene induction. These studies are investigating whether our earlier conclusions regarding the modulation by cofactors of all three transcriptional parameters of GR-mediated gene induction (Amax, EC50, and PAA) can be extended to GR-regulated gene repression. The answers to this question are critical for our understanding of how GRs alter gene expression. If the basic mechanism of action of each factor is preserved, regardless of whether the gene expression goes up or down, that will greatly simplify the task of defining the action of steroid hormones at a molecular level. Such a preservation of factor functioning 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.
