Agonist versus antagonist induce distinct thermodynamic modes of co-factor binding to the glucocorticoid receptor

Single-molecule force spectroscopy reveals folding steps associated with hormone binding and activation of the glucocorticoid receptor

The glucocorticoid receptor (GR) is a prominent nuclear receptor linked to a variety of diseases and an important drug target. Binding of hormone to its ligand binding domain (GR-LBD) is the key activation step to induce signaling. This process is tightly regulated by the molecular chaperones Hsp70 and Hsp90 in vivo. Despite its importance, little is known about GR-LBD folding, the ligand binding pathway, or the requirement for chaperone regulation. In this study, we have used single-molecule force spectroscopy by optical tweezers to unravel the dynamics of the complete pathway of folding and hormone binding of GR-LBD. We identified a "lid" structure whose opening and closing is tightly coupled to hormone binding. This lid is located at the N terminus without direct contacts to the hormone. Under mechanical load, apo-GR-LBD folds stably and readily without the need of chaperones with a folding free energy of [Formula: see text] The folding pathway is largely independent of the presence of hormone. Hormone binds only in the last step and lid closure adds an additional [Formula: see text] of free energy, drastically increasing the affinity. However, mechanical double-jump experiments reveal that, at zero force, GR-LBD folding is severely hampered by misfolding, slowing it to less than 1·s^-1 From the force dependence of the folding rates, we conclude that the misfolding occurs late in the folding pathway. These features are important cornerstones for understanding GR activation and its tight regulation by chaperones.

In vitro and in silico assessment of the structure-dependent binding of bisphenol analogues to glucocorticoid receptor

Widespread use of bisphenol A (BPA) and other bisphenol analogues has attracted increasing attention for their potential adverse effects. As environmental endocrine-disrupting compounds (EDCs), bisphenols (BPs) may activate a variety of nuclear receptors, including glucocorticoid receptor (GR). In this work, the binding of 11 BPs to GR was investigated by fluorescence polarization (FP) assay in combination with molecular dynamics simulations. The human glucocorticoid receptor was prepared as a soluble recombinant protein. A fluorescein-labeled dexamethasone derivative (Dex-fl) was employed as tracer. Competitive displacement of Dex-fl from GR by BPs showed that the binding affinities of bisphenol analogues were largely dependent on their characteristic functional groups. In order to further understand the relationship between BPs structures and their GR-mediated activities, molecular docking was utilized to explore the binding modes at the atomic level. The results confirmed that structural variations of bisphenol analogues contributed to different interactions of BPs with GR, potentially causing distinct toxic effects. Comparison of the calculated binding energies vs. experimental binding affinities yielded a good correlation (R ² = 0.8266), which might be helpful for the design of environmentally benign materials with reduced toxicities. In addition, the established FP assay based on GR exhibited the potential to offer an alternative to traditional methods for the detection of bisphenols.

Org 214007-0: a novel non-steroidal selective glucocorticoid receptor modulator with full anti-inflammatory properties and improved therapeutic index

Glucocorticoids (GCs) such as prednisolone are potent immunosuppressive drugs but suffer from severe adverse effects, including the induction of insulin resistance. Therefore, development of so-called Selective Glucocorticoid Receptor Modulators (SGRM) is highly desirable. Here we describe a non-steroidal Glucocorticoid Receptor (GR)-selective compound (Org 214007-0) with a binding affinity to GR similar to that of prednisolone. Structural modelling of the GR-Org 214007-0 binding site shows disturbance of the loop between helix 11 and helix 12 of GR, confirmed by partial recruitment of the TIF2-3 peptide. Using various cell lines and primary human cells, we show here that Org 214007-0 acts as a partial GC agonist, since it repressed inflammatory genes and was less effective in induction of metabolic genes. More importantly, in vivo studies in mice indicated that Org 214007-0 retained full efficacy in acute inflammation models as well as in a chronic collagen-induced arthritis (CIA) model. Gene expression profiling of muscle tissue derived from arthritic mice showed a partial activity of Org 214007-0 at an equi-efficacious dosage of prednisolone, with an increased ratio in repression versus induction of genes. Finally, in mice Org 214007-0 did not induce elevated fasting glucose nor the shift in glucose/glycogen balance in the liver seen with an equi-efficacious dose of prednisolone. All together, our data demonstrate that Org 214007-0 is a novel SGRMs with an improved therapeutic index compared to prednisolone. This class of SGRMs can contribute to effective anti-inflammatory therapy with a lower risk for metabolic side effects.

Differential nuclear localisation and promoter occupancy play a role in glucocorticoid receptor ligand-specific transcriptional responses

The glucocorticoid receptor (GR) is a ligand-activated transcription factor for which a number of endogenous and synthetic ligands exist. A key question in steroid receptor biology is how different ligands elicit different maximal transcriptional responses via the same receptor and on the same promoter. This question was addressed quantitatively for the GR, using a panel of agonists, partial agonists and antagonists, on the endogenous GILZ gene in two different human cell lines. It was found that the extent of GR nuclear localization correlated with the efficacy for GILZ transactivation by the GR in U2OS cells. However, in A549 cells there was no significant correlation, with all ligands resulting in similar levels of GR nuclear localization, despite different levels of transcriptional activation of the GILZ gene. Chromatin immunoprecipitation analysis on the other hand, revealed ligand-specific differences in GILZ promoter occupancy in the A549 cells, which correlated with the transcriptional efficacy of the subset of ligands investigated. This suggests that ligand-specific differences in promoter occupancy by activated GR play a major role in discrimination between agonist, partial agonist and antagonist responses on the endogenous GILZ gene in A549 cells, while differences in nuclear localisation of liganded GR play a role in determining the transcriptional outcome in U2OS cells. These cell line-specific differences were not dependent on the amount of GR present, since transient overexpression of GR in U2OS did not alter the relative ligand-selective nuclear localisation. Our results show that there is a relationship between ligand-specific transactivation efficacy, extent of nuclear translocation and recruitment of GR to the promoter. However, the relative contribution of nuclear translocation and GR promoter recruitment to ligand-specific transactivation efficacy is cell-specific.

Hormone binding and co-regulator binding to the glucocorticoid receptor are allosterically coupled

The glucocorticoid receptor initiates the cellular response to glucocorticoid steroid hormones in vertebrates. Co-regulator proteins dock to the receptor in response to hormone binding and potentiate the transcriptional activity of the receptor by modifying DNA and recruiting essential transcription factors like RNA polymerase II. Hormones and co-regulators bind at distinct sites in the ligand binding domain yet function cooperatively to mediate transcriptional control. This study reveals and quantifies energetic coupling between two binding sites using purified components. Using a library of peptides taken from co-regulator proteins, we determine the pattern of co-regulator binding to the glucocorticoid receptor ligand binding domain. We show that peptides from co-regulators differ in their effects on hormone binding and kinetics. Peptides from DAX1 and SRC1 bind with similar affinity, but DAX1 binding is coupled to hormone binding, and SRC1 is not. Mechanistic details of co-regulator binding and coupling to the hormone binding pocket are uncovered by analysis of properties endowed by mutation of a key residue in the allosteric network connecting the sites.

Identification of the functional domain of glucocorticoid receptor involved in RU486 antagonism

Mifepristone, also known as RU486, is a potent glucocorticoid receptor (GR) antagonist that inhibits GR-mediated transactivation. As an alternative to existing antidepressants, RU486 has been shown to rapidly reverse psychotic depression, most likely by blocking GR. Although a number of studies have demonstrated RU486-induced GR antagonism, the precise mechanism of action still remains unclear. To identify the GR domain involved in RU486-induced suppression, GR transactivation and nuclear translocation were examined using cells transfected with human GR (hGR), Guyanese squirrel monkey GR (gsmGR), and GR chimeras into COS-1 cells. RU486 showed a much more potent suppressive effect in gsmGR-expressing cells versus hGR-expressing cells, without significant cortisol- or RU486-induced changes in nuclear translocation. A GR chimera containing the gsmGR AF1 domain (amino acids 132-428) showed a marked decrease in luciferase activity, suggesting that this domain plays an important role in RU486-induced GR antagonism. Furthermore, fluorescence recovery after photobleaching (FRAP) analysis indicated that, in the presence of RU486, gsmGR AF1 domain contributes to GR mobility in living COS-1 cells. Taken together, these results demonstrate, for the first time, that the antagonistic effects of RU486 on GR transactivation involve a specific GR domain.

Ligand-selective transactivation and transrepression via the glucocorticoid receptor: role of cofactor interaction

The mechanisms that determine ligand-selective transcriptional responses by the glucocorticoid receptor (GR) are not fully understood. Using a wide panel of GR ligands, we investigated the relationships between the potency and maximal response for transactivation via a glucocorticoid response element (GRE) and transrepression via both nuclear factor small ka, CyrillicB (NFsmall ka, CyrillicB) and activator protein-1 (AP-1) sites, relative binding affinity for the GR, as well as interaction with both coactivators and corepressors. The results showed ligand-selective differences in potency and efficacy for each promoter, as well as for a particular ligand between the three promoters. Ligand potency correlated with relative affinity for the GR for agonists and partial agonists in transactivation but not for transrepression. Maximal response was unrelated to relative affinity of ligand for GR for both transactivation and transrepression. A good and significant correlation between full length coactivator binding in two-hybrid assays and efficacy as well as potency of different receptor-steroid complexes for both transactivation and transrepression supports a major role for coactivator recruitment in determination of ligand-selective transcriptional activity. Furthermore, ligand-selective GR binding to GRIP-1, as determined by both two-hybrid and DNA pull down assays, correlated positively with ligand-selective efficacy for transactivation of both a synthetic GRE reporter with expressed GR as well as of an endogenous gene via endogenous GR. The receptor interacting domain of the corepressor SMRT exhibited strong interaction with both agonists and partial agonists, similar to the results for coactivators, suggesting a possible role for SMRT in activation of transcription. However, there was no correlation between ligand affinity for the GR and cofactor interaction. These results provide strong quantitative biochemical support for a model in which GR-mediated ligand-selective differential interaction with GRIP-1, SRC-1A, NCoR and SMRT is a major determinant of ligand-selective and promoter-specific differences in potency and efficacy, for both transactivation and transrepression.

A survey of the year 2007 literature on applications of isothermal titration calorimetry

Elucidation of the energetic principles of binding affinity and specificity is a central task in many branches of current sciences: biology, medicine, pharmacology, chemistry, material sciences, etc. In biomedical research, integral approaches combining structural information with in-solution biophysical data have proved to be a powerful way toward understanding the physical basis of vital cellular phenomena. Isothermal titration calorimetry (ITC) is a valuable experimental tool facilitating quantification of the thermodynamic parameters that characterize recognition processes involving biomacromolecules. The method provides access to all relevant thermodynamic information by performing a few experiments. In particular, ITC experiments allow to by-pass tedious and (rarely precise) procedures aimed at determining the changes in enthalpy and entropy upon binding by van't Hoff analysis. Notwithstanding limitations, ITC has now the reputation of being the "gold standard" and ITC data are widely used to validate theoretical predictions of thermodynamic parameters, as well as to benchmark the results of novel binding assays. In this paper, we discuss several publications from 2007 reporting ITC results. The focus is on applications in biologically oriented fields. We do not intend a comprehensive coverage of all newly accumulated information. Rather, we emphasize work which has captured our attention with originality and far-reaching analysis, or else has provided ideas for expanding the potential of the method.

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.