Induction properties of a transiently transfected glucocorticoid-responsive gene vary with glucocorticoid receptor concentration

An Approach to Greater Specificity for Glucocorticoids

Glucocorticoid steroids are among the most prescribed drugs each year. Nonetheless, the many undesirable side effects, and lack of selectivity, restrict their greater usage. Research to increase glucocorticoid specificity has spanned many years. These efforts have been hampered by the ability of glucocorticoids to both induce and repress gene transcription and also by the lack of success in defining any predictable properties that control glucocorticoid specificity. Correlations of transcriptional specificity have been observed with changes in steroid structure, receptor and chromatin conformation, DNA sequence for receptor binding, and associated cofactors. However, none of these studies have progressed to the point of being able to offer guidance for increased specificity. We summarize here a mathematical theory that allows a novel and quantifiable approach to increase selectivity. The theory applies to all three major actions of glucocorticoid receptors: induction by agonists, induction by antagonists, and repression by agonists. Simple graphical analysis of competition assays involving any two factors (steroid, chemical, peptide, protein, DNA, etc.) yields information (1) about the kinetically described mechanism of action for each factor at that step where the factor acts in the overall reaction sequence and (2) about the relative position of that step where each factor acts. These two pieces of information uniquely provide direction for increasing the specificity of glucocorticoid action. Consideration of all three modes of action indicate that the most promising approach for increased specificity is to vary the concentrations of those cofactors/pharmaceuticals that act closest to the observed end point. The potential for selectivity is even greater when varying cofactors/pharmaceuticals in conjunction with a select class of antagonists.

Glucocorticoid-independent modulation of GR activity: Implications for immunotherapy

Pharmacological doses of glucocorticoids (GCs), acting via the glucocorticoid receptor (GR) to repress inflammation and immune function, remain the most effective therapy in the treatment of inflammatory and immune diseases. Since many patients on GC therapy exhibit GC resistance and severe side-effects, much research is focused on developing more selective GCs and combination therapies, with greater anti-inflammatory potency. GCs mediate their classical genomic transcriptional effects by binding to the cytoplasmic GR, followed by nuclear translocation and modulation of transcription of target genes by direct DNA binding of the GR or its tethering to other transcription factors. Recent evidence suggests, however, that the responses mediated by the GR are much more complex and involve multiple parallel mechanisms integrating simultaneous signals from other receptors, both in the absence and presence of GCs, to shift the sensitivity of a target cell to GCs. The level of cellular stress, immune activation status, or the cell cycle phase may be crucial for determining GC sensitivity and GC responsiveness as well as subcellular localization of the GR and GR levels. Central to the development of new drugs that target GR signaling alone or as add-on therapies, is an in-depth understanding of the molecular mechanisms of GC-independent GR desensitization, priming and activation of the unliganded GR, as well as synergy and cross-talk with other signaling pathways. This review will discuss the information currently available on these topics and their relevance to immunotherapy, as well as identify unanswered questions and future areas of research.

Theory of partial agonist activity of steroid hormones

The different amounts of residual partial agonist activity (PAA) of antisteroids under assorted conditions have long been useful in clinical applications but remain largely unexplained. Not only does a given antagonist often afford unequal induction for multiple genes in the same cell but also the activity of the same antisteroid with the same gene changes with variations in concentration of numerous cofactors. Using glucocorticoid receptors as a model system, we have recently succeeded in constructing from first principles a theory that accurately describes how cofactors can modulate the ability of agonist steroids to regulate both gene induction and gene repression. We now extend this framework to the actions of antisteroids in gene induction. The theory shows why changes in PAA cannot be explained simply by differences in ligand affinity for receptor and requires action at a second step or site in the overall sequence of reactions. The theory also provides a method for locating the position of this second site, relative to a concentration limited step (CLS), which is a previously identified step in glucocorticoid-regulated transactivation that always occurs at the same position in the overall sequence of events of gene induction. Finally, the theory predicts that classes of antagonist ligands may be grouped on the basis of their maximal PAA with excess added cofactor and that the members of each class differ by how they act at the same step in the overall gene induction process. Thus, this theory now makes it possible to predict how different cofactors modulate antisteroid PAA, which should be invaluable in developing more selective antagonists.

Research resource: modulators of glucocorticoid receptor activity identified by a new high-throughput screening assay

Glucocorticoid steroids affect almost every type of tissue and thus are widely used to treat a variety of human pathological conditions. However, the severity of numerous side effects limits the frequency and duration of glucocorticoid treatments. Of the numerous approaches to control off-target responses to glucocorticoids, small molecules and pharmaceuticals offer several advantages. Here we describe a new, extended high-throughput screen in intact cells to identify small molecule modulators of dexamethasone-induced glucocorticoid receptor (GR) transcriptional activity. The novelty of this assay is that it monitors changes in both GR maximal activity (A(max)) and EC(50) (the position of the dexamethasone dose-response curve). Upon screening 1280 chemicals, 10 with the greatest changes in the absolute value of A(max) or EC(50) were selected for further examination. Qualitatively identical behaviors for 60% to 90% of the chemicals were observed in a completely different system, suggesting that other systems will be similarly affected by these chemicals. Additional analysis of the 10 chemicals in a recently described competition assay determined their kinetically defined mechanism and site of action. Some chemicals had similar mechanisms of action despite divergent effects on the level of the GR-induced product. These combined assays offer a straightforward method of identifying numerous new pharmaceuticals that can alter GR transactivation in ways that could be clinically useful.

Modulation of glucocorticoid receptor induction properties by core circadian clock proteins

Glucocorticoid (GC) plays important roles in diverse physiological processes including metabolism and immune functions. While circadian control of GC synthesis and secretion is relatively well appreciated, circadian control of GC action within target tissues remains poorly understood. Here, we demonstrate that CLOCK/BMAL1, the core circadian clock components, reduces maximal GR transactivation (A(max)) as well as efficacy (EC₅₀) by a novel mechanism that requires binding to DNA and transactivation of target genes. Accordingly, we observe that PER1 and CRY1, the primary targets of CLOCK/BMAL1 action, reduce maximal GR transactivation while not affecting the efficacy. Moreover, we observe hyper-activations of GRE-dependent transcription in BMAL1- or PERs-deficient MEFs. In addition, endogenous GC target genes expression negatively correlates with the CLOCK/BMAL1 activity. Considering that GC sensitivity is widely implicated in human health and diseases, these results provide valuable insights into plethora of GC-related physiology and pathology.

A structural explanation of the effects of dissociated glucocorticoids on glucocorticoid receptor transactivation

There is a therapeutic need for glucocorticoid receptor (GR) ligands that distinguish between the transrepression and transactivation activity of the GR, the later thought to be responsible for side effects. These ligands are known as "dissociated glucocorticoids" (dGCs). The first published dGCs, RU24782 (9α-fluoro-11β-hydroxy-16α-methylpregna-21-thiomethyl-1,4-diene-3,20-dione) and RU24858 (9α-fluoro-11β-hydroxy-16α-methylpregna-21-cyanide-1,4-diene-3,20-dione), do not have the 17α-hydroxyl group that characterizes dexamethasone (Dex; 9α-fluoro-11β,17α,21-trihydroxy-16α-methylpregna-1,4-diene-3,20-dione), and they differ from one another by having C21-thiomethyl and C21-cyanide moieties, respectively. Our aim was therefore to establish the structural basis of their activity. Both RU24782 and RU24858 induced a transactivation activity highly dependent on the GR expression level but always lower than dexamethasone. They also display less ability than dexamethasone to trigger steroid receptor coactivator 1 (SRC-1) recruitment and histone H3 acetylation. Docking studies, validated by mutagenesis experiments, revealed that dGCs are not anchored by Gln642, in contrast to Dex, which is hydrogen bonded to this residue via its 17α-hydroxyl group. This contact is essential for SRC-1 recruitment and subsequent dexamethasone-induced GR transactivation, but not transrepression. The ability of dGCs to make contacts with Ile747, for both RU24858 and RU24782 and with Asn564 for RU24858 are not strong enough to maintain GR in a conformation able to efficiently recruit SRC-1, unless SRC-1 is overexpressed. Overall, our findings provide some structural guidelines for the synthesis of potential new dissociated glucocorticoids with a better therapeutic ratio.

Impact of glucocorticoid receptor density on ligand-independent dimerization, cooperative ligand-binding and basal priming of transactivation: a cell culture model

Glucocorticoid receptor (GR) levels vary between tissues and individuals and are altered by physiological and pharmacological effectors. However, the effects and implications of differences in GR concentration have not been fully elucidated. Using three statistically different GR concentrations in transiently transfected COS-1 cells, we demonstrate, using co-immunoprecipitation (CoIP) and fluorescent resonance energy transfer (FRET), that high levels of wild type GR (wtGR), but not of dimerization deficient GR (GRdim), display ligand-independent dimerization. Whole-cell saturation ligand-binding experiments furthermore establish that positive cooperative ligand-binding, with a concomitant increased ligand-binding affinity, is facilitated by ligand-independent dimerization at high concentrations of wtGR, but not GRdim. The down-stream consequences of ligand-independent dimerization at high concentrations of wtGR, but not GRdim, are shown to include basal priming of the system as witnessed by ligand-independent transactivation of both a GRE-containing promoter-reporter and the endogenous glucocorticoid (GC)-responsive gene, GILZ, as well as ligand-independent loading of GR onto the GILZ promoter. Pursuant to the basal priming of the system, addition of ligand results in a significantly greater modulation of transactivation potency than would be expected solely from the increase in ligand-binding affinity. Thus ligand-independent dimerization of the GR at high concentrations primes the system, through ligand-independent DNA loading and transactivation, which together with positive cooperative ligand-binding increases the potency of GR agonists and shifts the bio-character of partial GR agonists. Clearly GR-levels are a major factor in determining the sensitivity to GCs and a critical factor regulating transcriptional programs.

Identification of location and kinetically defined mechanism of cofactors and reporter genes in the cascade of steroid-regulated transactivation

A currently obscure area of steroid hormone action is where the component factors, including receptor and reporter gene, act. The DNA binding of factors can be precisely defined, but the location and timing of factor binding and action are usually not equivalent. These questions are addressed for several factors (e.g. glucocorticoid receptor (GR), reporter, TIF2, NCoR, NELF-A, sSMRT, and STAMP) using our recently developed competition assay. This assay reveals both the kinetically defined mechanism of factor action and where the above factors act relative to both each other and the equilibrium equivalent to the rate-limiting step, which we call the concentration limiting step (CLS). The utility of this competition assay would be greatly increased if the position of the CLS is invariant and if the factor acting at the CLS is known. Here we report that the exogenous GREtkLUC reporter acts at the CLS as an accelerator for gene induction by GRs in U2OS cells. This mechanism of reporter function at the CLS persists with different reporters, factors, receptors, and cell types. We, therefore, propose that the reporter gene always acts at the CLS during gene induction and constitutes a landmark around which one can order the actions of all other factors. Current data suggest that how and where GR and the short form of SMRT act is also constant. These results validate a novel and rational methodology for identifying distally acting factors that would be attractive targets for pharmaceutical intervention in the treatment of diseases involving GR-regulated genes.

The hypersensitive glucocorticoid response specifically regulates period 1 and expression of circadian genes

Glucocorticoids regulate gene expression by binding and activating the glucocorticoid receptor (GR). While ligand affinity determines the global sensitivity of the response, additional proteins act on the genome to tune sensitivity of some genes. However, the genomic extent and specificity of dose-specific glucocorticoid responses are unknown. We show that dose-specific glucocorticoid responses are extraordinarily specific at the genomic scale, able to distinctly express a single gene, the circadian rhythm gene for Period 1 (PER1), at concentrations consistent with the nighttime nadir of human cortisol. We mapped the PER1 response to a single GR binding site. The specific GR binding sequence did not impact sensitivity, and we instead attributed the response to a combination of additional transcription factors and chromatin accessibility acting in the same locus. The PER1 hypersensitive response element is conserved in the mouse, where we found similar upregulation of Per1 in pituitary cells. Targeted and transient overexpression of PER1 led to regulation of additional circadian rhythm genes hours later, suggesting that hypersensitive expression of PER1 impacts circadian gene expression. These findings show that hypersensitive GR binding occurs throughout the genome, drives targeted gene expression, and may be important to endocrine mediation of peripheral circadian rhythms.

Deducing the temporal order of cofactor function in ligand-regulated gene transcription: theory and experimental verification

Cofactors are intimately involved in steroid-regulated gene expression. Two critical questions are (1) the steps at which cofactors exert their biological activities and (2) the nature of that activity. Here we show that a new mathematical theory of steroid hormone action can be used to deduce the kinetic properties and reaction sequence position for the functioning of any two cofactors relative to a concentration limiting step (CLS) and to each other. The predictions of the theory, which can be applied using graphical methods similar to those of enzyme kinetics, are validated by obtaining internally consistent data for pair-wise analyses of three cofactors (TIF2, sSMRT, and NCoR) in U2OS cells. The analysis of TIF2 and sSMRT actions on GR-induction of an endogenous gene gave results identical to those with an exogenous reporter. Thus new tools to determine previously unobtainable information about the nature and position of cofactor action in any process displaying first-order Hill plot kinetics are now available.

Inferring mechanisms from dose-response curves

The steady state dose-response curve of ligand-mediated gene induction usually appears to precisely follow a first-order Hill equation (Hill coefficient equal to 1). Additionally, various cofactors/reagents can affect both the potency and the maximum activity of gene induction in a gene-specific manner. Recently, we have developed a general theory for which an unspecified sequence of steps or reactions yields a first-order Hill dose-response curve (FHDC) for plots of the final product versus initial agonist concentration. The theory requires only that individual reactions "dissociate" from the downstream reactions leading to the final product, which implies that intermediate complexes are weakly bound or exist only transiently. We show how the theory can be utilized to make predictions of previously unidentified mechanisms and the site of action of cofactors/reagents. The theory is general and can be applied to any biochemical reaction that has a FHDC.

Ligand binding domain mutations of the glucocorticoid receptor selectively modify the effects with, but not binding of, cofactors

We previously reported that several point mutations in the ligand binding domain (LBD) of glucocorticoid receptors (GRs) marginally affect the binding affinity of the synthetic glucocorticoids dexamethasone (Dex) and deacylcortivazol (DAC). However, these mutations dramatically alter the efficacy (A(max)) and potency (EC(50)) of agonists, along with the partial agonist activity (PAA) of the antisteroid Dex-mesylate (DM), for gene induction and repression in a steroid-dependent manner. This was proposed to result, in part, from altered protein-protein interactions in the complex of GR with the coactivator TIF2 despite normal TIF2 binding. To explore the generality of this phenomenon, we now ask whether these mutations also affect the transactivation properties, but not binding, of other GR-bound factors. We find that an elevated concentration of GR, to probe unidentified cofactors, or of the comodulator Ubc9 does not reverse the effects of GR LBD mutations that increase the EC(50) and lower the PAA with the GREtkLUC reporter in both CV-1 and U2OS cells. This behavior is more dramatic with Ubc9 and the isolated GR LBD fused to the GAL4 DNA binding domain, despite normal binding of Ubc9 to the mutant GRs. Similar effects, albeit gene, steroid, and transcriptional property-specific, are seen with full-length GRs and three endogenous genes in U2OS cells. Thus, changes in simple steady-state binding capacities of mutant receptors for factors cannot account for the modified transcriptional properties. In all cases, the nuclear translocation of Dex- and DAC-bound wild-type and mutant receptors is the same. These results are consistent with the earlier results with TIF2 and support the hypothesis that small changes in the GR LBD can alter the activities of the bound cofactor without modifying cofactor binding. We propose that this separation of binding and the modulation of transactivation parameters occurs for a wide variety of GR-associated cofactors.

A theoretical framework for gene induction and experimental comparisons

Ligand-mediated gene induction by steroid receptors is a multistep process characterized by a dose-response curve for gene product that follows a first-order Hill equation. This behavior has classically been explained by steroid binding to receptor being the rate-limiting step. However, this predicts a constant potency of gene induction (EC(50)) for a given receptor-steroid complex, which is challenged by the findings that various cofactors/reagents can alter this parameter in a gene-specific manner. These properties put strong constraints on the mechanisms of gene induction and raise two questions: How can a first-order Hill dose-response curve (FHDC) arise from a multistep reaction sequence, and how do cofactors modify potency? Here we introduce a theoretical framework in which a sequence of steps yields an FHDC for the final product as a function of the initial agonist concentration. An exact determination of all constants is not required to describe the final FHDC. The theory predicts mechanisms for cofactor/reagent effects on gene-induction potency and maximal activity and it assigns a relative order to cofactors in the sequence of steps. The theory is supported by several observations from glucocorticoid receptor-mediated gene induction. It identifies the mechanism and matches the measured dose-response curves for different concentrations of the combination of cofactor Ubc9 and receptor. It also predicts that an FHDC cannot involve the DNA binding of preformed receptor dimers, which is validated experimentally. The theory is general and can be applied to any biochemical reaction that shows an FHDC.

Pharmacological strategies for improving the efficacy and therapeutic ratio of glucocorticoids in inflammatory lung diseases

Glucocorticoids are widely used to treat various inflammatory lung diseases. Acting via the glucocorticoid receptor (GR), they exert clinical effects predominantly by modulating gene transcription. This may be to either induce (transactivate) or repress (transrepress) gene transcription. However, certain individuals, including those who smoke, have certain asthma phenotypes, chronic obstructive pulmonary disease (COPD) or some interstitial diseases may respond poorly to the beneficial effects of glucocorticoids. In these cases, high dose, often oral or parental, glucocorticoids are typically prescribed. This generally leads to adverse effects that compromise clinical utility. There is, therefore, a need to enhance the clinical efficacy of glucocorticoids while minimizing adverse effects. In this context, a long-acting beta(2)-adrenoceptor agonist (LABA) can enhance the clinical efficacy of an inhaled corticosteroid (ICS) in asthma and COPD. Furthermore, LABAs can augment glucocorticoid-dependent gene expression and this action may account for some of the benefits of LABA/ICS combination therapies when compared to ICS given as a monotherapy. In addition to metabolic genes and other adverse effects that are induced by glucocorticoids, there are many other glucocorticoid-inducible genes that have significant anti-inflammatory potential. We therefore advocate a move away from the search for ligands of GR that dissociate transactivation from transrepression. Instead, we submit that ligands should be functionally screened by virtue of their ability to induce or repress biologically-relevant genes in target tissues. In this review, we discuss pharmacological methods by which selective GR modulators and "add-on" therapies may be exploited to improve the clinical efficacy of glucocorticoids while reducing potential adverse effects.

Modulation of glucocorticoid receptor induction properties by cofactors in peripheral blood mononuclear cells

Glucocorticoids are widely used for their anti-inflammatory and immunosuppressive properties. Changing concentrations of transcriptional cofactors or chemicals in transiently transfected tissue culture cells modify several properties of glucocorticoid receptor-regulated gene expression including total activity (A(max)), agonist steroid potency (EC(50)), and the percentage of full agonist activity for antisteroids (percent partial agonist activity). However, no reports exist for endogenous genes in primary human cells. Here we document that reduced concentrations of TIF2, a p160 coactivator, in peripheral blood mononuclear cells modulate these parameters for endogenous genes in a gene-selective manner, thus establishing the physiological relevance of this behavior.

Modulation of transcription parameters in glucocorticoid receptor-mediated repression

Glucocorticoid receptors (GRs) affect both gene induction and gene repression. The disparities of receptor binding to DNA and increased vs. decreased gene expression have suggested significant mechanistic differences between GR-mediated induction and repression. Numerous transcription factors are known to modulate three parameters of gene induction: the total activity (Vmax) and position of the dose-response curve with glucocorticoids (EC50) and the percent partial agonist activity with antiglucocorticoids. We have examined the effects on GR-mediated repression 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 Vmax, 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.

What goes on behind closed doors: physiological versus pharmacological steroid hormone actions

Steroid-hormone-activated receptor proteins are among the best-understood class of factors for altering gene transcription in cells. Steroid receptors are of major importance in maintaining normal human physiology by responding to circulating concentrations of steroid in the nM range. Nonetheless, most studies of steroid receptor action have been conducted using the supra-physiological conditions of saturating concentrations (> or =100 nM) of potent synthetic steroid agonists. Here we summarize the recent developments arising from experiments using two clinically relevant conditions: subsaturating concentrations of agonist (to mimic the circulating concentrations in mammals) and saturating concentrations of antagonists (which are employed in endocrine therapies to block the actions of endogenous steroids). These studies have revealed new facets of steroid hormone action that could not be uncovered by conventional experiments with saturating concentrations of agonist steroids, such as a plethora of factors/conditions for the differential control of gene expression by physiological levels of steroid, a rational approach for examining the gene-specific variations in partial agonist activity of antisteroids, and a dissociation of steroid potency and efficacy that implies the existence of separate, and possibly novel, mechanistic steps and cofactors.

Differential modulation of glucocorticoid and progesterone receptor transactivation

The determinants of the different biological activities of progesterone receptors (PRs) vs. glucocorticoid receptors (GRs), which bind to the same DNA sequences, remain poorly understood. The mechanisms by which differential expression of a common target gene can be achieved by PR and GR include unequal agonist steroid concentrations for half maximal induction (EC50) and dissimilar amounts of residual partial agonist activity for antisteroids in addition to the more common changes in total gene induction, or Vmax. Several factors are known to alter some or all of these three parameters for GR-regulated gene induction and some (i.e., the corepressors NCoR and SMRT) modulate the EC50 and partial agonist activity for GR and PR induction of the same gene in opposite directions. The current study demonstrates that other factors known to modulate GR properties (GME, GMEB-2, Ubc9, and STAMP) can also differentially interact with PRs or alter several of the above induction parameters under otherwise identical conditions. These results support the hypothesis that the modulation of EC50, partial agonist activity, and Vmax by a given factor is not limited to one receptor in a specific cell line. Furthermore, the number of factors that unequally modulate PR and GR induction parameters is now greatly expanded, thereby increasing the possible mechanisms for differential gene regulation by PRs vs. GRs.

A Holy Grail of asthma management: toward understanding how long-acting beta(2)-adrenoceptor agonists enhance the clinical efficacy of inhaled corticosteroids

There is unequivocal evidence that the combination of an inhaled corticosteroid (ICS) -- i.e. glucocorticoid -- and an inhaled long-acting beta(2)-adrenoceptor agonist (LABA) is superior to each component administered as a monotherapy alone in the clinical management of asthma. Moreover, Calverley and colleagues (Lancet 2003, 361: 449-456; N Engl J Med 2007, 356: 775-789) reporting for the 'TRial of Inhaled STeroids ANd long-acting beta(2)-agonists (TRISTAN)' and 'TOwards a Revolution in COPD Health (TORCH)' international study groups also demonstrated the superior efficacy of LABA/ICS combination therapies over ICS alone in the clinical management of chronic obstructive pulmonary disease. This finding has been independently confirmed indicating that the therapeutic benefit of LABA/ICS combination therapies is not restricted to asthma and may be extended to other chronic inflammatory diseases of the airways. Despite the unquestionable benefit of LABA/ICS combination therapies, there is a vast gap in our understanding of how these two drugs given together deliver superior clinical efficacy. In this article, we review the history of LABA/ICS combination therapies and critically evaluate how these two classes of drugs might interact at the biochemical level to suppress pro-inflammatory responses. Understanding the molecular basis of this fundamental clinical observation is a Holy Grail of current respiratory diseases research as it could permit the rational exploitation of this effect with the development of new 'optimized' LABA/ICS combination therapies.

Regulation of the rat UGT1A6 by glucocorticoids involves a cryptic glucocorticoid response element

Glucocorticoids precociously induce fetal rat UGT1A6 and potentiate polycyclic aromatic hydrocarbon (PAH)-dependent induction of this enzyme in vivo and in isolated rat hepatocytes. To establish whether induction was due to glucocorticoid receptor (GR), luciferase reporter vectors were tested in transfection assays with HepG2 cells. Using a reporter construct containing approximately 2.26 kilobases of the 5'-flanking region of the UGT1A6-noncoding leader exon (A1*), dexamethasone increased basal activity 3- to 7-fold in cells cotransfected with an expression plasmid for GR. PAH increased gene expression 23-fold, but the presence of dexamethasone only induced PAH-dependent expression by 1.5-fold, suggesting interaction between GR and the aryl hydrocarbon (Ah) receptor. Furthermore, the GR antagonist RU 38486 [17beta-hydroxy-11beta-(4-dimethylamino-phenyl)-17alpha-(prop-1-ynyl)-estra-4,9-dien-3-one] was a partial agonist that increased, rather than inhibited, basal activity 3-fold. 5'-deletion analysis defined the 5'-boundary for a functional glucocorticoid-responsive unit between base pairs -141 and -118 relative to the transcription start site. This region contains the Ah receptor response element (AhRE), and both PAH and glucocorticoid-dependent gene activation were lost when this area was deleted. Mutation of a single base pair located in the AhRE region simultaneously reduced induction by PAH and increased glucocorticoid induction. Thus, the sequences of both the AhRE and glucocorticoid response elements seem to overlap, suggesting that Ah receptor binding may decrease glucocorticoid-dependent induction due to interactions of these two cis-acting elements. Mutation of a putative GRE located between base pair -81 and -95 reduced, but did not completely eliminate, glucocorticoid-dependent induction of the reporter, suggesting that a nonclassic mechanism of induction is involved in this response.

Amino-terminal domain of TIF2 is involved in competing for corepressor binding to glucocorticoid and progesterone receptors

Both agonist- and antagonist-bound glucocorticoid receptors (GRs) and progesterone receptors (PRs) regulate gene transcription with the assistance of corepressors (NCoR and SMRT) and coactivators (TIF2/GRIP1, SRC1, and AIB1). Receptor binding of these cofactors is competitive and is considered to involve interactions between the C-terminal ligand binding domain of receptors and receptor interaction domains (RIDs) in the middle and C-terminus of coactivators and corepressors, respectively. Therefore, our recent finding that an amino terminal fragment of TIF2 (TIF2.0 = amino acids 1-627) competed for GR and PR interactions with corepressors in mammalian two-hybrid assays was unexpected. Here, we use biochemical approaches (mammalian two-hybrid, pull-down, and coimmunoprecipitation assays) to locate an N-terminal GR region that is sufficient to bind TIF2.0. In contrast, an N-terminal sequence of PR-B that is largely missing in the shorter PR-A is necessary but not sufficient for TIF2.0 binding. Mutagenesis studies of NCoR establish that the more amino-terminal RID#1, but not RID#2, is necessary for binding to both GR and PR agonist and antagonist complexes. ChIP assays indicate that PR and NCoR each selectively localize to the enhancer element (PRE) of a transiently transfected PREtkLUC reporter in the presence of antagonist steroid, whereas exogenous TIF2.0 reduces the amount of PRE-associated NCoR. Importantly, exogenous TIF2.0 also inhibits the biological responses to added NCoR under the same conditions as those used in the ChIP assays. These results suggest that both N-terminal and middle sequences of TIF2 participate in competing with corepressor for regulating the gene transcriptional responses of GRs and PRs.

The glucocorticoid agonist activities of mifepristone (RU486) and progesterone are dependent on glucocorticoid receptor levels but not on EC50 values

Mifepristone is an antagonist of the glucocorticoid receptor (GR) that also has significant agonist activity in some cell types. We examined the partial agonist activity of mifepristone in COS-7 cells transfected with increasing amounts of a glucocorticoid receptor expression vector pmGR. As pmGR levels increased, the response of the reporter, pMTVCAT to dexamethasone increased, consistent with increasing levels of receptor expression; the response to mifepristone also increased but at a higher rate, resulting in increasing mifepristone agonist and decreasing antagonist activity. In contrast, increasing pMTVCAT levels increased CAT activity induced by both dexamethasone and mifepristone, but did not change the relative agonist activity of mifepristone. We also examined the relationship between agonist activity and receptor level in a series of clones of the E8.2.A3 cell line expressing various levels of GR. Again, the relative agonist activity of mifepristone increased as GR increased. This increase was not due to changes in the dose response curves to these two ligands since their EC50 values were independent of receptor levels. These results indicate that the degree of glucocorticoid agonist activity exhibited by mifepristone is dependent on the concentration of GR in the cell. Similar results were obtained with another partial agonist of the GR, progesterone, whereas the complete antagonist ZK98.299 had no agonist activity under any condition. Taken together, these results suggest that the phenomenon of receptor concentration-dependence is a property of partial GR agonists in general.

The ligand binding domain controls glucocorticoid receptor dynamics independent of ligand release

Ligand binding to the glucocorticoid receptor (GR) results in receptor binding to glucocorticoid response elements (GREs) and the formation of transcriptional regulatory complexes. Equally important, these complexes are continuously disassembled, with active processes driving GR off GREs. We found that co-chaperone p23-dependent disruption of GR-driven transcription depended on the ligand binding domain (LBD). Next, we examined the importance of the LBD and of ligand dissociation in GR-GRE dissociation in living cells. We showed in fluorescence recovery after photobleaching studies that dissociation of GR from GREs is faster in the absence of the LBD. Furthermore, GR interaction with a target promoter revealed ligand-specific exchange rates. However, using covalently binding ligands, we demonstrated that ligand dissociation is not required for receptor dissociation from GREs. Overall, these studies showed that activities impinging on the LBD regulate GR exchange with GREs but that the dissociation of GR from GREs is independent from ligand dissociation.

STAMP, a novel predicted factor assisting TIF2 actions in glucocorticoid receptor-mediated induction and repression

The coactivator TIF2 was predicted to interact with an unknown factor to modify both the relative inhibition in glucocorticoid receptor (GR)-mediated gene repression and several parameters of agonists and antisteroids in GR-regulated induction. Here, we describe the isolation and characterization of the predicted factor as a new 1,277-amino-acid endogenous protein (STAMP). STAMP associates with coactivators (TIF2 and SRC-1) and is selective for a subset of the steroid/nuclear receptors including GRs. Transfected STAMP increases the effects of TIF2 in GR-mediated repression and induction. Conversely, the levels of both induction and repression of endogenous genes are reduced when STAMP small interfering RNAs are used to lower the level of endogenous STAMP. Endogenous STAMP colocalizes with GR in intact cells and is recruited to the promoters of endogenous GR-induced and -repressed genes. We suggest that STAMP is an important new, downstream component of GR action in both gene activation and gene repression.

Single-cell analysis of glucocorticoid receptor action reveals that stochastic post-chromatin association mechanisms regulate ligand-specific transcription

The glucocorticoid receptor (GR) dynamically interacts with response elements in the mouse mammary tumor virus (MMTV) promoter to regulate steroid-dependent transcription. In a clonal mammary carcinoma cell line containing a tandem array of MMTV promoter-reporter gene cassettes integrated at a single genomic locus, direct binding of a green fluorescent protein (GFP)-GR fusion protein to the MMTV regulatory elements can be observed in living cells. After ligand treatment, MMTV-dependent transcription in individual cells was detected by RNA fluorescence in situ hybridization (FISH). High-resolution fluorescence images were acquired from large numbers of randomly selected cells. Images were analyzed with a novel automated computer algorithm, measuring the RNA FISH signal and the relative GFP-GR fluorescence intensity at the MMTV array for each cell. Although dexamethasone increased the mean RNA FISH signal approximately 10-fold, RU486 produced only about a 2-fold induction, as expected for this mixed antagonist. For all treatment conditions, the relative GFP-GR fluorescence at the array for the averaged cells paralleled the RNA FISH measurements, suggesting that image analysis accurately detected an increase in steady-state GR association with the MMTV array that was responsible for the increase in transcriptional activity. The antagonist-dependent decreases in GR association with the MMTV promoter were confirmed by chromatin immunoprecipitation experiments, supporting the image analysis results. A pronounced cell-to-cell variability was observed in RNA FISH signal and GR-MMTV association within treatment groups. We observed a nonlinear relationship between GR-MMTV association and RNA FISH in individual cells, indicating that differences in GR-MMTV interaction account for some, but not all, of the transcriptional heterogeneity between individual cells. In selected cell subpopulations with equal levels of GR-MMTV association, there was a decrease in RNA FISH signal with RU486 treatment compared with dexamethasone treatment. These results indicate that stochastic events occurring after GR-promoter association, such as the actions of chromatin remodeling complexes or other cofactors, change in a ligand-dependent manner and regulate heterogeneous transcription in individual cells.

Effects of acetylation, polymerase phosphorylation, and DNA unwinding in glucocorticoid receptor transactivation

Varying the concentration of selected factors alters the induction properties of steroid receptors by changing the position of the dose-response curve (or the value for half-maximal induction=EC(50)) and the amount of partial agonist activity of antisteroids. We now describe a rudimentary mathematical model that predicts a simple Michaelis-Menten curve for the multi-step process of steroid-regulated gene induction. This model suggests that steps far downstream from receptor binding to steroid can influence the EC(50) of agonist-complexes and partial agonist activity of antagonist-complexes. We therefore asked whether inhibitors of three possible downstream steps can reverse the effects of increased concentrations of two factors: glucocorticoid receptors (GRs) and Ubc9. The downstream steps (with inhibitors in parentheses) are protein deacetylation (TSA and VPA), DNA unwinding (CPT), and CTD phosphorylation of RNA polymerase II (DRB and H8). None of the inhibitors mimic or prevent the effects of added GRs. However, inhibitors of DNA unwinding and CTD phosphorylation do reverse the effects of Ubc9 with high GR concentrations. These results support our earlier conclusion that different rate-limiting steps operate at low and high GR concentrations versus high GR with Ubc9. The present data also suggest that downstream steps can modulate the EC(50) of GR-mediated induction, thus both supporting the utility of our mathematical model and widening the field of biochemical processes that can modify the EC(50).

Medroxyprogesterone acetate binds the glucocorticoid receptor to stimulate alpha-ENaC and sgk1 expression in renal collecting duct epithelia

Medroxyprogesterone acetate (MPA), a widely used synthetic progestational contraceptive, occasionally leads to Cushingoid side effects such as hypertension, fluid retention, and centripetal obesity. We investigated the effect of MPA on classic mineralocorticoid target genes, alpha-epithelial Na channel (ENaC) and sgk1, in the collecting duct. In adrenalectomized mice, aldosterone, dexamethasone, and MPA increased alpha-ENaC mRNA levels in kidney cortex. MPA and dexamethasone, but not progesterone, dose dependently increased alpha-ENaC and sgk1 mRNA in M-1 and in Madin-Darby canine kidney-C7 cells, both collecting duct cell lines. The stimulatory effect of MPA and dexamethasone on alpha-ENaC expression was inhibited by RU-38486, a combined glucocorticoid receptor (GR) and progesterone receptor (PR) antagonist, but not by Org31710, a pure PR antagonist. MPA and dexamethasone dose dependently increased alpha-ENaC promoter-driven luciferase activity in M-1 cells, which was not inhibited by Org31710, indicating that MPA regulates alpha-ENaC in a PR-independent manner. When tested in HT29 cells, MPA could only stimulate alpha-ENaC-driven reporter activity when GR was coexpressed, confirming the requirement for functional GR in the transcriptional effect of MPA. The activation of steroid receptors such as GR can explain the apparent glucocorticoid effects of MPA, independent of PR activation.

Role of activation function domain-1, DNA binding, and coactivator GRIP1 in the expression of partial agonist activity of glucocorticoid receptor-antagonist complexes

The determinants of the partial agonist activity of most antisteroids complexed with steroid receptors are not well understood. We now examine the role of the N-terminal half of the glucocorticoid receptor (GR) including the activation domain (AF-1), the DNA binding site sequence, receptor contact with DNA, and coactivator binding on the expression of partial agonist activity in two cell lines for GRs bound by five antiglucocorticoids: dexamethasone mesylate (Dex-Mes), dexamethasone oxetanone (Dex-Ox), progesterone (Prog), deoxycorticosterone (DOC), and RU486. Using truncated GRs, we find that the N-terminal half of GR and the AF-1 domain are dispensable for the partial agonist activity of antiglucocorticoids. This contrasts with the AF-1 domain being required for the partial agonist activity of antisteroids with most steroid receptors. DNA sequence (MMTV vs a simple GRE enhancer) and cell-specific factors (CV-1 vs Cos-7) exert minor effects on the level of partial agonist activity. Small activity differences for some complexes of GAL4/GR chimeras with GR- vs GAL-responsive reporters suggest a contribution of DNA-induced conformational changes. A role for steroid-regulated coactivator binding to GRs is compatible with the progressively smaller increase in partial agonist activity of Dex-Mes > Prog > RU486 with added GRIP1 in CV-1 cells. This hypothesis is consistent with titration experiments, where low concentrations of GRIP1 more effectively increase the partial agonist activity of Dex-Mes than Prog complexes. Furthermore, ligand-dependent GRIP1 binding to DNA-bound GR complexes decreases in the order of Dex > Dex-Mes > Prog > RU486. Thus, the partial agonist activity of a given GR-steroid complex in CV-1 cells correlates with its cell-free binding of GRIP1. The ability to modify the levels of partial agonist activity through changes in steroid structure, DNA sequence, specific DNA-induced conformational changes, and coactivator binding suggests that useful variations in endocrine therapies may be possible by the judicious selection of these parameters to afford gene and tissue selective results.

Corepressor binding to progesterone and glucocorticoid receptors involves the activation function-1 domain and is inhibited by molybdate

Corepressors are known to interact via their receptor interaction domains (RIDs) with the ligand binding domain in the carboxyl terminal half of steroid/nuclear receptors. We now report that a portion of the activation function-1 domain of glucocorticoid receptors (GRs) and progesterone receptors (PRs), which is the major transactivation sequence, is necessary but not sufficient for corepressor [nuclear receptor corepressor (NCoR) and silencing mediator of retinoid and thyroid hormone receptor (SMRT)] RID binding to GRs and PRs in both mammalian two-hybrid and coimmunoprecipitation assays. Importantly, these two receptor sequences are functionally interchangeable in the context of GR for transactivation, corepressor binding, and corepressor modulatory activity assays. This suggests that corepressors may act in part by physically blocking portions of receptor activation function-1 domains. However, differences exist in corepressor binding to GRs and PRs. The C-terminal domain of PRs has a higher affinity for corepressor than that of GRs. The ability of some segments of the coactivator TIF2 to competitively inhibit corepressor binding to receptors is different for GRs and PRs. With each receptor, the cell-free binding of corepressors to ligand-free receptor is prevented by sodium molybdate, which is a well-known inhibitor of receptor activation to the DNA-binding state. This suggests that receptor activation precedes binding to corepressors. Collectively, these results indicate that corepressor binding to GRs and PRs involve both N- and C-terminal sequences of activated receptors but differ in ways that may contribute to the unique biological responses of each receptor in intact cells.

Glucocorticoid Receptor Ligand Binding Domain Is Sufficient for the Modulation of Glucocorticoid Induction Properties by Homologous Receptors, Coactivator Transcription Intermediary Factor 2, and Ubc9

Several factors modulate the position of the dose-response curve of steroid receptor-agonist complexes and the partial agonist activity of antagonist complexes, thereby causing differential gene activation by circulating hormones and unequal gene repression during endocrine therapies with antisteroids. We now ask whether the modulatory activity of three factors (homologous receptor, coactivator transcription intermediary factor 2, and Ubc9) requires the same or different domains of glucocorticoid receptors (GRs). In all cases, we find that neither the amino terminal half of the receptor, which contains the activation function-1 activation domain, nor the DNA binding domain is required. This contrasts with the major role of activation function-1 in determining the amount of gene expression and partial agonist activity of antisteroids with most steroid receptors. However, the situation is more complicated with Ubc9, where GR N-terminal sequences prevent the actions of Ubc9, but not added GR or transcription intermediary factor 2, at low GR concentrations. Inhibition is relieved by deletion of these sequences or by replacement with the comparable region of progesterone receptors but not by overexpression of the repressive sequences. These results plus the binding of C-terminal GR sequences to the suppressive N-terminal domain implicate an intramolecular mechanism for the inhibition of Ubc9 actions at low GR concentrations. A shift from noncooperative to cooperative steroid binding at high GR concentrations suggests that conformational changes reposition the inhibitory N-terminal sequence to allow Ubc9 interaction with elements of the ligand binding domain. Collectively, these results indicate a dominant role of GR C-terminal sequences in the modulation of the dose-response curve and partial agonist activity of GR complexes. They also reveal mechanistic differences both among individual modulators and between the ability of the same factors to regulate the total amount of gene expression.

Receptor density dictates the behavior of a subset of steroid ligands in glucocorticoid receptor-mediated transrepression

By co-expressing glucocorticoid receptor (GR) and transcriptional reporter systems in GR-deficient Cos-7 cells, we profiled potency and efficacy of a panel of GR ligands as a function of GR expression levels (density). Our results show that potency and efficacy for GR full agonists, such as dexamethasone, in these transrepression assays are affected by receptor density. Intriguingly, receptor density dramatically influenced the behavior of the GR antagonist RU486 or the GR agonist medroxyprogesterone acetate (MPA). At high receptor density, both MPA and RU486 behaved as full agonists in transrepression: reducing GR density, however, resulted in conversion of these ligands from full agonist to full antagonists. In contrast, varying GR density could not convert cortisol and budesonide from GR agonists to antagonists. These results have clearly demonstrated, for the first time, an effect of receptor density on the agonist and antagonist properties of RU486 and MPA in GR-mediated transrepression.

Modulation of transcriptional sensitivity of mineralocorticoid and estrogen receptors

Recent reports describe the ability of factors to modulate the position of the dose-response curve of receptor-agonist complexes, and the amount of partial agonist activity of receptor-antagonist complexes, of androgen, glucocorticoid (GRs), and progesterone receptors (PRs). We now ask whether this modulation extends to the two remaining steroid receptors: mineralocorticoid (MRs) and estrogen receptors (ERs). These studies of MR were facilitated by our discovery that the antiglucocorticoid dexamethasone 21-mesylate (Dex-Mes) is a new antimineralocorticoid with significant amounts of partial agonist activity. Elevated levels of MR, the co-activators TIF2 and SRC-1, and the co-repressor SMRT do modulate the dose-response curve and partial agonist activity of MR complexes. Interestingly, the precise responses are indistinguishable from those seen with GRs in the same cells. Thus, the unequal transactivation of common genes by MRs versus GRs probably cannot be explained by differential responses to changing cellular concentrations of homologous receptor, co-activators, or co-repressors. We also find that the dose-response curve of ER-estradiol complexes is left-shifted to lower steroid concentrations by higher amounts of exogenous ER. Therefore, the modulation of either the dose-response curve of agonists or the partial agonist activity of antisteroid, and in many cases the modulation of both properties, is a common phenomenon for all of the classical steroid receptors.

Aldosterone resistance in kidney transplantation is in part induced by a down-regulation of mineralocorticoid receptor expression1

BACKGROUND

After renal transplantation immunosuppressive drugs-like cyclosporin A (CsA) and FK506 induce either hypoaldosteronism or pseudo-hypoaldosteronism presenting with hyperkalemia and metabolic acidosis. We investigated the relationship between renal allograft function under CsA therapy and plasma aldosterone concentration, potassium- and water homeostasis and mineralocorticoid receptor (MR) expression level in peripheral leukocytes.

METHODS

We studied 21 renal transplant patients under CsA therapy and 12 healthy controls. Transplant recipients were studied before and under fludrocortisone treatment. Using quantitative reverse-phase polymerase chain reaction (RT-PCR) specific for the MR, we analyzed the level of expression of MR in peripheral leukocytes.

RESULTS

In acidotic transplant recipients (HCO(3) 18.5 +/- 1.2 mM) renal function was only slightly impaired with 2.0 +/- 0.2 mg creatinine/dL when compared with 1.8 +/- 0.3 mg/dL (ns) in non-acidotic patients (HCO(3) 23.0 +/- 2.8 mM). Mean plasma aldosterone levels in renal transplant recipients did not differ from control levels (150 +/- 33 pg/mL vs. 148 +/- 33 pg/mL, ns). In contrast, the expression level of MR in peripheral leukocytes of renal transplant recipients treated with CsA was significantly decreased when compared with healthy controls without renal disease (120 +/- 78 vs. 423 +/- 73 RNA molecules/0.5 microg total RNA, p < 0.01). The level of expression of MR in renal transplant recipients did not differ between acidotic patients and non-acidotic patients (ns). The application of fludrocortisone reversed hyperkalemia and metabolic acidosis without significant effect on MR expression.

CONCLUSIONS

The present data demonstrate that hyperkalemia and metabolic acidosis following CsA treatment in kidney transplantation might be associated with a down-regulation of MR expression on peripheral leukocytes. Electrolyte imbalance is reversible on application of fludrocortisone. This observation supports fludrocortisone treatment in transplant patients with severe electrolyte disturbances.

Structure/Activity Relationships for GMEB-2: The Second Member of the Glucocorticoid Modulatory Element-Binding Complex

The position of the dose-response curve of agonist complexes of glucocorticoid receptors (GRs), and the partial agonist activity of GR-antagonist complexes, can be modulated by two proteins (GMEB-1 and -2), which bind as oligomers to a DNA element that is called a glucocorticoid modulatory element, or GME. This element is active when located upstream of the glucocorticoid response element that controls the expression of a reporter gene. Here, we report the structure/activity relationships of GMEB-2 and compare them to our previous findings for GMEB-1. Most of the activities of GMEB-2, such as homo- and heterooligomerization, binding to GR and to CBP, DNA binding, and modulation of the above GR transcriptional properties, require large regions of the protein. Only the intrinsic transactivation activity could be localized to a small region of the protein. These studies shed light on the mechanism of action of GMEB-2 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. These studies also identify regions of GMEB-2 possessing yet unidentified properties that are critical for several activities. Finally, as the domain organization of GMEB-2 and -1 is extremely similar, we conclude that the quantitative differences in activities derive from variations in amino acid sequence rather than more global features of protein structure.

Phenotypic alterations in breast cancer cells overexpressing the nuclear receptor co-activator AIB1

BACKGROUND

Estrogen signaling plays a critical role in a number of normal physiological processes and has important implications in the treatment of breast cancer. The p160 nuclear receptor coactivator, AIB1 (amplified in breast cancer 1), is frequently amplified and overexpressed in human breast cancer and has been shown to enhance estrogen-dependent transactivation.

METHODS

To better understand the molecular and physiological consequences of AIB1 overexpression in breast cancer cells, an AIB1 cDNA was transfected into the low AIB1 expressing, estrogen-receptor (ER) negative breast cancer cell line, MDA-MB-436. The features of a derivative cell line, designated 436.1, which expresses high levels of AIB1, are described and compared with the parental cell line.

RESULTS

A significant increase in the levels of CREB binding protein (CBP) was observed in 436.1 cells and immunofluorescent staining revealed altered AIB1 and CBP staining patterns compared to the parental cells. Further, transient transfection assays demonstrated that the overall estrogen-dependent transactivation in 436.1 cells is approximately 20-fold higher than the parental cells and the estrogen dose-response curve is repositioned to the right. Finally, cDNA microarray analysis of approximately 7,100 cDNAs identified a number of differentially expressed genes in the 436.1 cells.

CONCLUSION

These observations lend insight into downstream signaling pathways that are influenced by AIB1.

The importance of being varied in steroid receptor transactivation

A major response of steroid receptors to steroid hormones is the induction of gene transcription. Two relevant, albeit less studied, properties of these receptors are the EC(50) values of the receptor-agonist complexes and the partial agonist activity of the receptor-antagonist complexes. Contrary to earlier expectations, neither the EC(50) value nor the partial agonist activity is constant for a given receptor-steroid complex. This variation is, however, beneficial to cells and organisms because it provides a mechanism both for differential control of gene expression by a single concentration of circulating hormone and for limiting side-effects during endocrine therapies. In this article, the factors and proposed mechanisms for the modulation of the EC(50) value and partial agonist activity of receptor-steroid complexes are discussed.

A second pathway for modulating glucocorticoid receptor transactivation properties

We recently reported that three factors (a cis-acting element and changing concentrations of receptor or coactivator TIF2) act at a common rate-limiting step to modulate the position of the dose-response curve and the partial agonist activity of glucocorticoid receptors (GRs). The ability of saturating levels of GR, and added inhibitors, to prevent the actions of the three modulators (cis-acting element, GR, and TIF2) but not the currently investigated C-terminal fragment of E1A-13S (E1A-133C) indicates that E1A-133C alters GR properties via a second pathway that is downstream of the common step for the original three modulators. hSur2 binds to E1A-133C. We find that hSur2 modulates GR transactivation properties, thus suggesting that the effects of E1A-133C are due to the recruitment of hSur2. hSur2 also modifies GR activities in the presence of saturating GR concentrations, which is consistent with hSur2 acting downstream of the common step for the original three modulators. The H160Y mutation, which eliminates hSur2 binding to E1A, blocks most of the activity of E1A-133C. This suggests that the modulatory activity of E1A-133C is largely due to the binding of hSur2, which is a component of the Mediator complex. Collectively, these data support the existence of a new pathway for modulating GR transactivation processes, thereby increasing the number of cellular mechanisms that permit differential control of gene expression by endogenous levels of glucocorticoid hormones.

Modulation of induction properties of glucocorticoid receptor-agonist and -antagonist complexes by coactivators involves binding to receptors but is independent of ability of coactivators to augment transactivation

Coactivators such as TIF2 and SRC-1 modulate the positioning of the dose-response curve for agonist-bound glucocorticoid receptors (GRs) and the partial agonist activity of antiglucocorticoid complexes. These properties of coactivators differ from their initially defined activities of binding to, and increasing the total levels of transactivation by, agonist-bound steroid receptors. We now report that constructs of TIF2 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. Mammalian two-hybrid experiments show that the minimum TIF2 segment with modulatory activity (TIF2.4) does not interact with p300, CREB-binding protein, or PCAF, which also modulates GR activities. DRIP150 and DRIP205 have been implicated in coactivator actions but are unable to modulate GR activities. The absence of synergism by PCAF or DRIP150 with SRC-1 or TIF2, respectively, further suggests that these other factors are not involved. 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. Pull-down experiments with GST/TIF2.4 demonstrate a direct interaction of TIF2 with GR in a hormone-dependent fashion that requires the receptor interaction domains of TIF2 and is equally robust with agonists and most antiglucocorticoids. These observations, which are confirmed in mammalian two-hybrid assays, suggest 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. In conclusion, the modulatory activity of coactivators with GR-agonist and -antagonist complexes is 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).

Glucocorticoid receptors in hippocampal neurons that do not engage proteasomes escape from hormone-dependent down-regulation but maintain transactivation activity

The glucocorticoid receptor (GR) protein is subjected to hormone-dependent down-regulation in most cells and tissues. This reduction in receptor levels that accompanies chronic hormone exposure serves to limit hormone responsiveness and operates at transcriptional, posttranscriptional, and posttranslational levels. The ability of glucocorticoid hormones to trigger GR down-regulation may be not universal, particularly in mature and developing neurons in which conflicting results regarding hormone control of GR protein have been reported. We find that endogenous GR is not down-regulated in the HT22 mouse hippocampal cell line and in primary hippocampal neurons derived from embryonic rats. Because GR has the capacity to be ubiquitylated in HT22 cells, receptor down-regulation must be limited by defects in either targeting of polyubiquitylated receptor to the proteasome or processing of the targeted receptor by the proteasome. Despite the lack of GR down-regulation in the HT22 cells, glucocorticoid-induced transcription from transiently transfected templates is attenuated upon prolonged hormone treatment. This termination of GR transactivation is not due to inefficient nuclear import or nuclear retention of the receptor. Furthermore, GR efficiently exports from HT22 cell nuclei in hormone-withdrawn cells, indicating that the receptor has access to both nuclear and cytoplasmic degradation pathways. Our results suggest that appropriate maturation of proteasomal degradative or targeting activities may be required, particularly in hippocampal neurons, for hormone-dependent down-regulation of GR.

In vivo analysis of glucocorticoid-induced reporter gene expression using gene gun DNA delivery

Glucocorticoid regulates various physiological processes via the activation and repression of gene expression. The anti-inflammatory effects and the adverse effects are believed to be dependent on the repression and the activation of genes, respectively. Reporter gene assay is a useful technique to separately evaluate these two functions and has been used for in vitro screening of novel ligands for the glucocorticoid receptor (GR). We report here the application of a reporter gene assay for the in vivo determination of the GR-mediated gene activation. A reporter plasmid containing glucocorticoid response elements was introduced to abdominal mouse skin using a gene gun. Administration of prednisolone induced the expression of the reporter gene, only when the GR expression plasmid was co-transfected with the reporter plasmid. Endogenous levels of corticosterone appeared to be negligible in this protocol. The dose response for this induction was comparable to those for the decreases in thymus weight and serum corticosterone. These results suggest that gene gun-mediated skin transfection enables the in vivo reporter gene assay and that this technique can be used to predict the potency of ligands for the GR-mediated gene activation.

Structure/activity elements of the multifunctional protein, GMEB-1. Characterization of domains relevant for the modulation of glucocorticoid receptor transactivation properties

GMEB-1 was initially described as a component of a 550-kDa heteromeric DNA binding complex that is involved in the modulation of two properties of glucocorticoid receptor (GR) transactivation, the dose-response curve of agonists and the partial agonist activity of antagonists. Subsequently, GMEB-1 was also found to bind to hsp27, to associate with the coactivator TIF2 in yeast cells, and to participate in Parvovirus replication. To understand these multiple activities of GMEB-1 at a molecular level, we have now determined which regions are associated with the various activities associated with the modulation of GR transactivation properties. These activities include, homooligomerization, heterooligomerization, DNA binding, binding to GR and the transcriptional cofactor CBP, and GR modulation. Complex activities such as DNA binding and GR modulation, are found to require the physical combination of those domains that would be predicted from the involved biochemical processes. We have previously documented that GMEB-1 possesses both GR modulatory and intrinsic transactivation activity. However, the domains for these two activities of GMEB-1 are found not to overlap. This separation of activities provides a structural basis for our prior biological observations that the modulation of the dose-response curve and partial agonist activity of GR complexes is independent of the total levels of gene activation by the same GR complexes.

Ubc9 is a novel modulator of the induction properties of glucocorticoid receptors

The EC(50) of agonists and the partial agonist activity of antagonists are crucial parameters for steroid hormone control of gene expression and endocrine therapies. These parameters have been shown to be modulated by a naturally occurring cis-acting element, called the glucocorticoid modulatory element (GME) that binds two proteins, GMEB-1 and -2. We now present evidence that the GMEBs contact Ubc9, which is the mammalian homolog of a yeast E2 ubiquitin-conjugating enzyme. Ubc9 also binds to glucocorticoid receptors (GRs). Ubc9 displays no intrinsic transactivation activity but modifies both the absolute amount of induced gene product and the fold induction by GR. With high concentrations of GR, added Ubc9 also reduces the EC(50) of agonists and increases the partial agonist activity of antagonists in a manner that is independent of the ability of Ubc9 to transfer SUMO-1 (small ubiquitin-like modifier-1) to proteins. This new activity of Ubc9 requires only the ligand binding domain of GR and part of the hinge region. Interestingly, Ubc9 modulation of full-length GR transcriptional properties can be seen in the absence of a GME. This, though, is consistent with the GME acting by increasing the local concentration of Ubc9, which then activates a previously unobserved target in the transcriptional machinery. With high concentrations of Ubc9 and GR, Ubc9 binding to GR appears to be sufficient to permit Ubc9 to act independently of the GME.

New androgen response elements in the murine pem promoter mediate selective transactivation

The Pem homeobox transcription factor is expressed under androgen control in the testis and epididymis. It is also transcribed in the ovary, muscle, and placenta. The mouse Pem gene promoter was cloned and sequenced. It was analyzed in transactivation tests using CV-1 and PC-3 cells expressing the AR and found to be strongly stimulated by androgens. EMSAs and mutational analysis of the Pem promoter allowed the identification of two functional androgen response elements named ARE-1 and ARE-2. They both differed from the consensus semipalindromic steroid response element and exhibited characteristics of direct repeats of the TGTTCT half-site. Unlike the steroid response element, both Pem androgen response elements were selectively responsive to androgen stimulation. Specific mutations in the left half-site of Pem ARE-1 and ARE-2, but not of the steroid response element, were still compatible with AR binding in the EMSA. In addition, Pem ARE-1, but not ARE-2 or the steroid response element, showed some flexibility with regard to spacing between half-sites. These results strongly suggest that the AR interacts differently with direct repeats than with inverted repeats, potentially leading to cis element-driven selective properties. Thus, the existence of several classes of DNA response elements might be an essential feature of differential androgen regulation.

Transactivation specificity of glucocorticoid versus progesterone receptors. Role of functionally different interactions of transcription factors with amino- and carboxyl-terminal receptor domains

A major unanswered question of glucocorticoid and progesterone action is how different whole cell responses arise when both of the cognate receptors can bind to, and activate, the same hormone response elements. We have documented previously that the EC(50) of agonist complexes, and the partial agonist activity of antagonist complexes, of both glucocorticoid receptors (GRs) and progesterone receptors (PRs) are modulated by increased amounts of homologous receptor and of coregulators. We now ask whether these components can differentially alter GR and PR transcriptional properties. To remove possible cell-specific differences, we have examined both receptors in the common environment of a line of mouse mammary adenocarcinoma (1470.2) cells. In order to segregate the responses that might be due to unequal nucleosome reorganization by the two receptors from those reflecting interactions with other components, we chose a transiently transfected reporter containing a simple glucocorticoid response element (i.e. GREtkLUC). No significant differences are found with elevated levels of either receptor. However, major, qualitative differences are seen with the corepressors SMRT and NCoR, which afford opposite responses with GR and PR. Studies with chimeric GR/PR receptors indicate that no one segment of PR or GR is responsible for these properties and that the composite response likely involves interactions with both the amino and carboxyl termini of receptors. Collectively, the data suggest that GR and PR induction of responsive genes in a given cell can be differentially controlled, in part, by unequal interactions of multiple receptor domains with assorted nuclear cofactors.

Correlation between different gene expression assays designed to measure trans-activation potencies of systemic glucocorticoids

The glucocorticoids (GC) betamethasone, dexamethasone, hydrocortisone, methylprednisolone, prednisolone and triamcinolone acetonide are currently used in the treatment of inflammatory diseases. Through a process called trans-activation, GC activate gene expression and produce various physiological and pharmacological effects. In particular, by inducing gluconeogenic enzymes, long-term GC treatment may cause diabetes. Using three different assays, we have extensively compared the capacity of the above GC to activate gene expression. trans-Activation of a GC inducible luciferase gene was assessed in HeLa and A549 cells after stable and transient transfection, respectively. In hepatoma tissue culture cells, we measured trans-activation of the endogenous gene encoding tyrosine aminotransferase, a gluconeogenic enzyme. Half-maximal effective concentrations of GC were determined by dose-response analyses. Results obtained with these assays were highly correlated and GC were ranked in three groups according to their trans-activation potency: betamethasone, dexamethasone, and triamcinolone acetonide > methylprednisolone and prednisolone > hydrocortisone. Potencies were not strictly related to receptor binding affinities and not significantly affected by the amount of endogenous GC receptor.

New antiprogestins with partial agonist activity: potential selective progesterone receptor modulators (SPRMs) and probes for receptor- and coregulator-induced changes in progesterone receptor induction properties

A pharmacologically relevant property of steroid hormone-regulated gene induction is the partial agonist activity of antisteroid complexes. We now report that dexamethasone-mesylate (Dex-Mes) and dexamethasone-oxetanone (Dex-Ox), each a derivative of the glucocorticoid-selective steroid dexamethasone (Dex), are two new antiprogestins with significant amounts of agonist activity with both the A and B isoforms of progesterone receptor (PR), for different progesterone-responsive elements, and in several cell lines. These compounds continue to display activity under conditions where another partial antiprogestin (RTI-020) is inactive. These new antiprogestins were used to determine whether the partial agonist activity of PR complexes can be modified by changing concentrations of receptor or coregulator, as we have recently demonstrated for glucocorticoid receptors (GRs). Because GR and coregulator concentrations simultaneously altered the position of the physiologically relevant dose-response curve, and associated EC(50), of GR-agonist complexes, we also examined this phenomenon with PR. We find that elevated PR or transcriptional intermediary factor 2 (TIF2) concentrations increase the partial agonist activity of Dex-Mes and Dex-Ox, and the EC(50) of agonists, independently of changes in total gene transactivation. Furthermore, the corepressors SMRT (silencing mediator for retinoid and thyroid receptors) and NCoR (nuclear receptor corepressor) each suppresses gene induction but NCoR acts opposite to SMRT and, like the coactivator TIF2, reduces the EC(50) and increases the partial agonist activity of antiprogestins. These comparable responses of GR and PR suggest that variations in receptor and coregulator concentrations may be a general mechanism for altering the induction properties of other steroid receptors. Finally, the magnitude of coregulator effects on PR induction properties are often not identical for agonists and the new antagonists, suggesting subtle mechanistic differences. These properties of Dex-Mes and Dex-Ox, plus the sensitivity of their activity to cellular differences in PR and coregulator concentrations, make these steroids potential new SPRMs (selective progesterone receptor modulators) that should prove useful as probes of PR induction properties.

Paradoxical interactions among estrogen receptors, estrogens and SERMS: mutual annihilation and synergy

The phenomenon of mutual annihilation of action between 17beta estradiol (E(2)) and a selective estrogen receptor modulator (SERM), previously described in prepubertal rat diaphysis, epiphysis and uterus, has been investigated in ROS 17/2.8 rat osteoblastic cells and in transiently co-transfected cells in culture. In ROS 17/2.8 cells, the estrogen-induced marker enzyme creatine kinase B (CKB) was stimulated by raloxifene, tamoxifen and tamoxifen methiodide to a specific activity equal to or greater than that induced by 10 nM E(2). However, when a fully inhibitory dose of any of these SERMS was given simultaneously with E(2), no stimulation of CK activity resulted. Therefore, SERMS can be full agonists when acting alone, but complete antagonists to a super-physiological dose of estrogen. It is expected that excess tamoxifen would prevent the action of a SERM, but that the agonist activity of a SERM is abolished by 1000-fold less estrogen is a phenomenon without obvious explanation by classical pharmacology of competitive inhibition. To probe the mechanism of this interaction further, a ckb-CAT reporter plasmid, plus the human receptor expression plasmid, HEO, was transfected transiently into several cell types. In MCF-7 cells, a 1:10 ratio of E(2) to tamoxifen produced mutual annihilation, but the same ratio in ROS 17/2.8 or HeLa cells led to synergistic stimulation. In HeLa cells, co-transfected with the more efficient wild-type estrogen receptor plasmid, HEGO, synergy was demonstrated only at sub-saturation levels of HEGO. We speculate that, in the presence of estradiol and a SERM, not only active homodimers would be formed, but also hetero-dimers of estrogen-liganded and tamoxifen-liganded receptor monomers, depending on the molar ratio of their ligands and their relative affinities. The resulting hetero-dimer conformation would change the specific receptor surface for interactions with the growing number of co-activators and co-repressors, structural changes which could help to explain the mutual annihilation and synergy phenomena and their cell selectivity.