The task of comparing the mechanistic details of GR- vs. PR-regulated gene transcription gained additional criteria with our development of a theoretical model of steroid receptor action (Ong et al., 2010, Proc Natl Acad Sci U S A, 107, 7107-7112). Three novel features of this model, and its associated graphical analysis, permit an unprecedented level of mechanistic information regarding steroid receptor-regulated gene transactivation. First, it is now possible to determine the kinetically-defined type of action being displayed by the factor (competitive inhibitor, uncompetitive inhibitor, coactivator, etc.). Second, it is usually possible to define where the factor acts relative to a reference point called the concentration limiting step (CLS), which is the steady state analog of the rate limiting step of enzyme kinetics. Third, the model and its graphical analysis have recently been extended to the analysis of two competing factors in the same assay (Dougherty et al., 2012, PLoS ONE, 7, e30225). As opposed to making the situation more obscure, this competition assay actually yields greater mechanistic information. Not only can such competition assays determine how and where each factor acts, relative to the CLS, but the site of action of the two factors relative to each other is usually revealed. Thus, one can now assemble an ordered sequence of reactions based on the biological function of cofactors, much as in epistasis analysis, even when the biochemical properties of the cofactors are not known. Initial experiments with a several cofactors have not exposed any major differences between GR and PR action. Recent studies in our collaboration with Dr. David Levens (NCI, NIH) have uncovered one cofactor that differentially affects GR in a cell-specific manner. As with the other factors examined, no significant differences between GR and PR are seen with this new factor. However, the hybrid transcription factor GAL/VP16 also responds similarly to GR so we do not appear to have yet found a factor that discriminates between different transcription factors. Nonetheless, these studies greatly contribute to our long-term goal of defining the action of GRs vs. PRs at a molecular level and of understanding their role in human physiology.
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