Initial Intracellular Events Of Steroid Hormone Action

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The focus of our research is the determinants of two transcriptional properties of steroid receptors: the dose-response curve of agonists and the partial agonist activity of antisteroids. The dose-response curve defines the EC50, or steroid concentration at which half-maximal response is seen, and is a crucial but poorly understood component of steroid hormone endocrinology. Differences in the EC50s of regulated genes provide a mechanism for differential expression by the common concentration of circulating steroid hormone in an organism. The partial agonist activity of antisteroids is an important consideration for limiting unwanted side effects during endocrine therapies by potentially allowing partial expression of genes other than the one targeted for suppression. While these two properties were long considered to be invariant, mounting evidence indicates that they can be varied. The first modulator to be described for these two transactivation properties of any steroid receptor was the glucocorticoid modulatory element (GME), which we identified as an upstream DNA sequence in the glucocorticoid-inducible tyrosine aminotransferase gene and appears to require the binding of two new proteins that we have cloned (GMEB-1 and -2) and are present as a heterooligomer. We have also found that coactivators and corepressors modulate the properties of glucocorticoid receptors (GRs) in a manner consistent with an equilibrium competition of coactivator vs. corepressor binding to GRs. We previously reported the structure-activity relationships for GMEB-1. We have now completed a similar study for GMEB-2. As for GMEB-1, most of the activities of GMEB-2, such as homo- and hetero-oligomerization, binding to GR and to the transcription factor CBP, DNA binding, and modulation of the above GR transcriptional properties, require large regions of the protein, presumably due to the multiple functions required for each activity. Furthermore, the location of these domains in each protein is very similar. Only the short domain for intrinsic transactivation activity is differently positioned in each protein. These studies shed light on the mechanism of action of both GMEBs and further support our previous conclusion that the ability of factors to modulate the position of the dose-response curve, and the partial agonist activity, of GR complexes is unrelated to effects on the total levels of GR-induced gene expression. The ability of coactivators and corepressors to also modulate the EC50 and partial agonist activity of GR complexes is an exciting recent addition to the known effects of these factors on the total level of gene activation by receptor-steroid complexes. We now report that truncated forms of TIF2 (such as TIF2.4) and SRC-1 lacking the two activation domains (AD1 and AD2) have significantly less ability to increase transactivation but retain most of the activity for modulating the dose-response curve and partial agonist activity. The ability of a TIF2.4 fragment (i.e., TIF2.37), which is not known to interact with proteins, to block the actions of TIF2.4 suggests that an unidentified binder mediates the modulatory activity of TIF2. These studies support our hypothesis that the capacity of coactivators such as TIF2 to modulate the partial agonist activity of antisteroids is mediated by the binding of coactivators to GR-antagonist complexes. Furthermore, the modulatory activity of coactivators with GR-agonist and -antagonist complexes is shown to be mechanistically distinct from the ability of coactivators to augment the total levels of transactivation and appears to involve the binding to both GR-steroid complexes and an unidentified TIF2-associated factor(s). We next inquired if the whole cell modulatory activity of corepressors entails binding to both GR-agonist and -antagonist complexes and whether the association of corepressors and coactivators with GR complexes involves competitive equilibrium reactions. In mammalian two-hybrid assays with two different cell lines, and cell-free pulldown assays, corepressors NCoR and SMRT associate with agonist and antagonist complexes of GRs. Importantly, whole cell GR interactions with corepressors are competitively inhibited by excess coactivator and visa versa. However, the regions of the coactivator TIF2 that compete for GR binding to corepressor and coactivator are not the same, implying a molecular difference in GR association with coactivators and corepressors. Additional studies with thyroid receptor beta allow us to conclude that mutually antagonistic equilibrium interactions of corepressors and coactivators modulate the dose-response curve and partial agonist activity of GR complexes in a manner that is responsive to the intracellular ratio of these two classes of cofactors. The above studies were all conducted with GRs. We previously reported that coactivators and corepressors modulate the EC50 and partial agonist activity of progesterone receptors. To determine the generality of these observations, we asked whether a similar modulation can be seen for the transcriptional properties of mineralocorticoid receptors (MRs) and estrogen receptors (ERs). The studies of MR were greatly facilitated by our discovery that the antiglucocorticoid dexamethasone 21-mesylate (Dex-Mes) is a new antimineralocorticoid with partial agonist activity. Elevated levels of MR, the coactivators TIF2 and SRC-1, and the corepressor SMRT do modulate the dose-response curve and partial agonist activity of MR complexes. The precise responses are indistinguishable from those seen with GRs in the same cells. Thus, the unequal transactivation of common genes by MRs vs. GRs cannot be explained by differential responses to changing cellular concentrations of homologous receptor, coactivators, or corepressors. At the same time, we find that the dose-response curve of ER-estradiol complexes is left-shifted to lower steroid concentrations by higher amounts of exogenous ER. As a result of the above studies, we have gained detailed information about two disparate processes for modulating GR transcriptional properties. The second process involving coactivators and corepressors affords data supporting our hypothesis that the modulation of steroid-bound receptor transactivation properties is of general and widespread importance for the differential control of gene expression during development, differentiation, and homeostasis. The findings that coactivators and corepressors competitively inhibit the interactions of each other with both agonist- and antagonist-bound GRs strengthen our proposal that the properties of GR complexes, and probably the other receptor-steroid complexes, are affected by the equilibrium interactions of coactivators and corepressors. 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.