Novel arylpyrazole compounds selectively modulate glucocorticoid receptor regulatory activity

Biased regulation of glucocorticoid receptors signaling

Glucocorticoids (GCs), steroid hormones that depend on glucocorticoid receptor (GR) binding for their action, are essential for regulating numerous homeostatic functions in the body.GR signals are biased, that is, GR signals are various in different tissue cells, disease states and ligands. This biased regulation of GR signaling appears to depend on ligand-induced metameric regulation, protein post-translational modifications, assembly at response elements, context-specific assembly (recruitment of co-regulators) and intercellular differences. Based on the bias regulation of GR, selective GR agonists and modulators (SEGRAMs) were developed to bias therapeutic outcomes toward expected outcomes (e.g., anti-inflammation and immunoregulation) by influencing GR-mediated gene expression. This paper provides a review of the bias regulation and mechanism of GR and the research progress of drugs.

Hidden modes of DNA binding by human nuclear receptors

Human nuclear receptors (NRs) are a superfamily of ligand-responsive transcription factors that have central roles in cellular function. Their malfunction is linked to numerous diseases, and the ability to modulate their activity with synthetic ligands has yielded 16% of all FDA-approved drugs. NRs regulate distinct gene networks, however they often function from genomic sites that lack known binding motifs. Here, to annotate genomic binding sites of known and unexamined NRs more accurately, we use high-throughput SELEX to comprehensively map DNA binding site preferences of all full-length human NRs, in complex with their ligands. Furthermore, to identify non-obvious binding sites buried in DNA-protein interactomes, we develop MinSeq Find, a search algorithm based on the MinTerm concept from electrical engineering and digital systems design. The resulting MinTerm sequence set (MinSeqs) reveal a constellation of binding sites that more effectively annotate NR-binding profiles in cells. MinSeqs also unmask binding sites created or disrupted by 52,106 single-nucleotide polymorphisms associated with human diseases. By implicating druggable NRs as hidden drivers of multiple human diseases, our results not only reveal new biological roles of NRs, but they also provide a resource for drug-repurposing and precision medicine.

Variation in glucocorticoid sensitivity and the relation with obesity

Increasing evidence points to a relation between increased glucocorticoid (GC) exposure and weight gain. In support, long-term cortisol measurements using hair analysis revealed that many individuals with obesity appear to have cortisol values in the high physiological range. The mechanisms behind this relationship need to be determined in order to develop targeted therapy to reach sustainable weight loss in these subgroups. The effect of GCs is not only determined by the plasma concentration of GCs but also by individual differences in GC sensitivity and the target tissue, which can be analyzed by functional GC assays. GC sensitivity is influenced by multiple genetic and acquired (e.g., disease-related) factors, including intracellular GC availability, hormone binding affinity, and expression levels of the GC receptors and their isoforms, as well as factors involved in the modulation of gene transcription. Interindividual differences in GC sensitivity also play a role in the response to exogenous GCs, with respect to both therapeutic and adverse effects. Accordingly, in this review, we summarize current knowledge on mechanisms that influence GC sensitivity and their relationships with obesity and discuss personalized treatment options targeting the GC receptor.

Expression of glucocorticoid receptor (GR) and clinical significance in adrenocortical carcinoma

Adrenocortical carcinoma (ACC) is a rare endocrine tumor, and most cases present with hormone excess with poor prognosis. Our research aims to determine the clinical and biological significance of glucocorticoid receptor (GR) expression using large cohorts of ACC patients. Immunohistochemistry was used to assess the expression of GR in 78 ACC cases from the West China Hospital (WCH) cohort. RNA-seq data were retrieved from The Cancer Genome Atlas database (TCGA, n=79). Clinicopathological and follow-up data were obtained from two cohorts. The correlation between the GR gene and tumor immune status was estimated using TIMER and GEPIA2. Kaplan-Meier analysis was performed to identify the prognostic value of GR in ACC. In the WCH cohort, positive nuclear GR staining was identified in 90% of the primary ACC cases. Cortisol-secreting ACCs demonstrated significantly lower GR protein expression than did nonfunctioning tumors (P<0.001). This finding was validated by the mRNA data analysis of the TCGA cohort (P = 0.030). GR expression was found to be positively correlated with the immune cell infiltration level and immune-checkpoint-related gene expression in ACC. Survival comparison and multivariate analysis showed that GR expression is an independent prognostic predictor of disease-free survival and overall survival in ACC patients in both cohorts. Our findings suggest that low GR expression is significantly correlated with excess cortisol, immune signatures and poor survival in ACC patients. We propose that GR signaling may play an important role in ACC behavior and thus may be a therapeutic target, which deserves further research.

Potent Anti-Inflammatory, Arylpyrazole-Based Glucocorticoid Receptor Agonists That Do Not Impair Insulin Secretion

Glucocorticoids (GCs) are widely used in medicine for their role in the treatment of autoimmune-mediated conditions, certain cancers, and organ transplantation. The transcriptional activities GCs elicit include transrepression, postulated to be responsible for the anti-inflammatory activity, and transactivation, proposed to underlie the undesirable side effects associated with long-term use. A GC analogue that could elicit only transrepression and beneficial transactivation properties would be of great medicinal value and is highly sought after. In this study, a series of 1-(4-substituted phenyl)pyrazole-based GC analogues were synthesized, biologically screened, and evaluated for SARs leading to the desired activity. Activity observed in compounds bearing an electron deficient arylpyrazole moiety showed promise toward a dissociated steroid, displaying transrepression while having limited transactivation activity. In addition, compounds 11aa and 11ab were found to have anti-inflammatory efficacy comparable to that of dexamethasone at 10 nM, with minimal transactivation activity and no reduction of insulin secretion in cultured rat 832/13 beta cells.

GR-mediated anti-inflammation of α-boswellic acid: Insights from in vitro and in silico studies

Although multiple bioactivities of α-boswellic acid have been reported, the molecular mechanism of its anti-inflammatory action is not yet clear. Hence, glucocorticoid receptor (GR)-mediated anti-inflammation of α-boswellic acid was investigated in this work. Fluorescence polarization assay suggested that α-boswellic acid bound to GR with IC₅₀ value of 658.00 ± 0.21 μM. Upon binding to α-boswellic acid, GR translocated from cytoplasm into nucleus of HeLa cells, facilitating sequential transcriptional regulation of GR-related genes. Luciferase reporter assay suggested that α-boswellic acid lacked GR transcriptional activity, indicating its potential as a dissociative GR ligand. Interestingly, α-boswellic acid selectively modulated the anti-inflammatory gene CBG (marker for GR transrepression), while leaving the "side-effect" gene TAT (marker for GR transactivation) unaffected in HepG2 cells. Furthermore, α-boswellic acid inhibited lipopolysaccharide-stimulated cytokines production in U937 macrophages, confirming its anti-inflammation property in vitro. Molecular docking showed that both hydrogen-bonding and hydrophobic interactions helped to stabilize α-boswellic acid-GR binding. Their binding stability was further confirmed in a 70-ns dynamics simulation. In summary, α-boswellic acid could bind to and translocate GR but did not induce glucocorticoid response element-mediated transcription. Since α-boswellic acid showed the dissociated characteristic that separated transrepression from transactivation, it might be a selective GR modulator against inflammatory disorders.

Deacylcortivazol-like pyrazole regioisomers reveal a more accommodating expanded binding pocket for the glucocorticoid receptor

Glucocorticoids (GCs) are widely used, potent anti-inflammatory and chemotherapeutic drugs. They work by binding to the glucocorticoid receptor (GR), a ligand-activated transcription factor, inducing translocation to the nucleus and regulation of genes that influence a variety of cellular activities. Despite being effective for a broad number of conditions, GC use is limited by severe side effects. To identify ligands that are more selective, we synthesized pairs of regioisomers in the pyrazole ring that probe the expanded binding pocket of GR opened by deacylcortivazol (DAC). Using an Ullmann-type reaction, a deacylcortivazol-like (DAC-like) backbone was modified with five pendant groups at the 1'- and 2'-positions of the pyrazole ring, yielding 9 ligands. Most of the compounds were cytotoxic to leukemia cells, and all required GR expression. Both aliphatic and other aromatic groups substituted at the 2'-position produced ligands with GC activity, with phenyl and 4-fluorophenyl substitutions exhibiting high cellular affinity for the receptor and >5× greater potency than dexamethasone, a commonly used strong GC. Surprisingly, phenyl substitution at the 1'-position produced a high-affinity ligand with ∼10× greater potency than dexamethasone, despite little apparent room in the expanded binding pocket to accommodate 1'-modifications. Other 1'-modifications, however, were markedly less potent. The potency of the 2'-substituted and 1'-substituted DAC-like compounds tracked linearly with cellular affinity but had different slopes, suggesting a different mode of interaction with GR. These data provide evidence that the expanded binding pocket opened by deacylcortivazol is more accommodating that expected, allowing development of new, and possibly selective, GCs by substitution within the pyrazole ring.

A General Introduction to Glucocorticoid Biology

Glucocorticoids (GCs) are steroid hormones widely used for the treatment of inflammation, autoimmune diseases, and cancer. To exert their broad physiological and therapeutic effects, GCs bind to the GC receptor (GR) which belongs to the nuclear receptor superfamily of transcription factors. Despite their success, GCs are hindered by the occurrence of side effects and glucocorticoid resistance (GCR). Increased knowledge on GC and GR biology together with a better understanding of the molecular mechanisms underlying the GC side effects and GCR are necessary for improved GC therapy development. We here provide a general overview on the current insights in GC biology with a focus on GC synthesis, regulation and physiology, role in inflammation inhibition, and on GR function and plasticity. Furthermore, novel and selective therapeutic strategies are proposed based on recently recognized distinct molecular mechanisms of the GR. We will explain the SEDIGRAM concept, which was launched based on our research results.

Mechanisms and Approaches for Overcoming Enzalutamide Resistance in Prostate Cancer

Enzalutamide, a second-generation small-molecule inhibitor of the androgen receptor (AR), has been approved for patients who failed with androgen deprivation therapy and have developed castration-resistant prostate cancer. More than 80% of these patients develop bone metastases. The binding of enzalutamide to the AR prevents the nuclear translocation of the receptor, thus inactivating androgen signaling. However, prostate cancer cells eventually develop resistance to enzalutamide treatment. Studies have found resistance both in patients and in laboratory models. The mechanisms of and approaches to overcoming such resistance are significant issues that need to be addressed. In this review, we focus on the major mechanisms of acquired enzalutamide resistance, including genetic mutations and splice variants of the AR, signaling pathways that bypass androgen signaling, intratumoral androgen biosynthesis by prostate tumor cells, lineage plasticity, and contributions from the tumor microenvironment. Approaches for overcoming these mechanisms to enzalutamide resistance along with the associated problems and solutions are discussed. Emerging questions, concerns, and new opportunities in studying enzalutamide resistance will be addressed as well.

Allosteric pathways in nuclear receptors - Potential targets for drug design

The nuclear receptor family of transcription factor proteins mediates endocrine function and plays critical roles in the development, physiology and pharmacology. Malfunctioning nuclear receptors are associated with several disease states. The functional activity of nuclear receptors is regulated by small molecular hormonal and synthetic molecules. Multiple sources of evidence have identified and distinguished between the different allosteric pathways initiated by ligands, DNA and cofactors such as co-activators and co-repressors. Also, these biophysical studies are attempting to determine how these pathways that regulate co-activator and DNA recognition can control gene transcription. Thus, there is a growing interest in determining the genome-scale impact of allostery in nuclear receptors. Today, it is accepted that a detailed understanding of the allosteric regulatory pathways within the nuclear receptor molecular complex will enable the development of efficient drug therapies in the long term.

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 receptor control of transcription: precision and plasticity via allostery

The glucocorticoid receptor (GR) is a constitutively expressed transcriptional regulatory factor (TRF) that controls many distinct gene networks, each uniquely determined by particular cellular and physiological contexts. The precision of GR-mediated responses seems to depend on combinatorial, context-specific assembly of GR-nucleated transcription regulatory complexes at genomic response elements. In turn, evidence suggests that context-driven plasticity is conferred by the integration of multiple signals, each serving as an allosteric effector of GR conformation, a key determinant of regulatory complex composition and activity. This structural and mechanistic perspective on GR regulatory specificity is likely to extend to other eukaryotic TRFs.

Mechanisms of Glucocorticoid-Regulated Gene Transcription

One fascinating aspect of glucocorticoid signaling is their broad range of physiological and pharmacological effects. These effects are at least in part a consequence of transcriptional regulation by the glucocorticoid receptor (GR). Activation of GR by glucocorticoids results in tissue-specific changes in gene expression levels with some genes being activated whereas others are repressed. This raises two questions: First, how does GR regulate different subsets of target genes in different tissues? And second, how can GR both activate and repress the expression of genes?To answer these questions, this chapter will describe the function of the various "components" and how they cooperate to mediate the transcriptional responses to glucocorticoids. The first "component" is GR itself. The second "component" is the chromatin and its role in specifying where in the genome GR binds. Binding to the genome however is just the first step in regulating the expression of genes and transcriptional regulation by GR depends on the recruitment of coregulator proteins that either directly or indirectly influence the recruitment and or activity of RNA polymerase II. Ultimately, the integration of inputs including GR isoform, DNA sequence, chromatin and cooperation with coregulators determines which genes are regulated and the direction of their regulation.

Identification of gene signatures for prednisolone-induced metabolic dysfunction in collagen-induced arthritic mice

BACKGROUND

Prednisolone is a potent anti-inflammatory glucocorticoid (GC) but chronic use is hampered by metabolic side effects. Little is known about the long-term effects of GCs on gene-expression in vivo during inflammation.

AIM

Identify gene signatures underlying prednisolone-induced metabolic side effects in a complex in vivo inflammatory setting after long-term treatment.

MATERIALS & METHODS

We performed whole-genome expression profiling in liver and muscle from arthritic and nonarthritic mice treated with several doses of prednisolone for 3 weeks and used text-mining to link gene signatures to metabolic pathways.

RESULTS

Prednisolone-induced gene signatures were highly tissue specific. We identified a short-list of genes significantly affected by both prednisolone and inflammation in liver and involved in glucose and fatty acid metabolism. For several of these genes the association with GCs is novel.

CONCLUSION

The identified gene signatures may provide useful starting points for the development of GCs with a better safety profile.

Glucocorticoid receptor binds half sites as a monomer and regulates specific target genes

Background

Glucocorticoid receptor (GR) is a hormone-activated, DNA-binding transcriptional regulatory factor that controls inflammation, metabolism, stress responses, and other physiological processes. In vitro, GR binds as an inverted dimer to a motif consisting of two imperfectly palindromic 6 bp half sites separated by 3 bp spacers. In vivo, GR employs different patterns of functional surfaces of GR to regulate different target genes. The relationships between GR genomic binding and functional surface utilization have not been defined.

Results

We find that A477T, a GR mutant that disrupts the dimerization interface, differs from wild-type GRα in binding and regulation of target genes. Genomic regions strongly occupied by A477T are enriched for a novel half site motif. In vitro, GRα binds half sites as a monomer. Through the overlap between GRα- and A477T-bound regions, we identify GRα-bound regions containing only half sites. We further identify GR target genes linked with half sites and not with the full motif.

Conclusions

Genomic regions bound by GR differ in underlying DNA sequence motifs and in the GR functional surfaces employed for regulation. Identification of GR binding regions that selectively utilize particular GR surfaces may discriminate sub-motifs, including the half site motif, that favor those surfaces. This approach may contribute to predictive models for GR activity and therapy.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-014-0418-y) contains supplementary material, which is available to authorized users.

Glucocorticoid receptor confers resistance to antiandrogens by bypassing androgen receptor blockade

The treatment of advanced prostate cancer has been transformed by novel antiandrogen therapies such as enzalutamide. Here, we identify induction of glucocorticoid receptor (GR) expression as a common feature of drug-resistant tumors in a credentialed preclinical model, a finding also confirmed in patient samples. GR substituted for the androgen receptor (AR) to activate a similar but distinguishable set of target genes and was necessary for maintenance of the resistant phenotype. The GR agonist dexamethasone was sufficient to confer enzalutamide resistance, whereas a GR antagonist restored sensitivity. Acute AR inhibition resulted in GR upregulation in a subset of prostate cancer cells due to relief of AR-mediated feedback repression of GR expression. These findings establish a mechanism of escape from AR blockade through expansion of cells primed to drive AR target genes via an alternative nuclear receptor upon drug exposure.

Bifunctional ligands allow deliberate extrinsic reprogramming of the glucocorticoid receptor

Therapies based on conventional nuclear receptor ligands are extremely powerful, yet their broad and long-term use is often hindered by undesired side effects that are often part of the receptor's biological function. Selective control of nuclear receptors such as the glucocorticoid receptor (GR) using conventional ligands has proven particularly challenging. Because they act solely in an allosteric manner, conventional ligands are constrained to act via cofactors that can intrinsically partner with the receptor. Furthermore, effective means to rationally encode a bias for specific coregulators are generally lacking. Using the (GR) as a framework, we demonstrate here a versatile approach, based on bifunctional ligands, that extends the regulatory repertoire of GR in a deliberate and controlled manner. By linking the macrolide FK506 to a conventional agonist (dexamethasone) or antagonist (RU-486), we demonstrate that it is possible to bridge the intact receptor to either positively or negatively acting coregulatory proteins bearing an FK506 binding protein domain. Using this strategy, we show that extrinsic recruitment of a strong activation function can enhance the efficacy of the full agonist dexamethasone and reverse the antagonist character of RU-486 at an endogenous locus. Notably, the extrinsic recruitment of histone deacetylase-1 reduces the ability of GR to activate transcription from a canonical GR response element while preserving ligand-mediated repression of nuclear factor-κB. By providing novel ways for the receptor to engage specific coregulators, this unique ligand design approach has the potential to yield both novel tools for GR study and more selective therapeutics.

The preclinical efficacy, selectivity and pharmacologic profile of MK-5932, an insulin-sparing selective glucocorticoid receptor modulator

Glucocorticoids are used widely in the treatment of inflammatory diseases, but use is accompanied by a significant burden of adverse effects. It has been hypothesized that gene- and cell-specific regulation of the glucocorticoid receptor by small molecule ligands could be translated into a therapeutic with an improved risk-benefit profile. MK-5932 is a highly selective glucocorticoid receptor modulator that is anti-inflammatory in vivo with an improved profile on glucose metabolism: Bungard et al. (2011). Bioorg. Med. Chem. 19, 7374-7386. Here we describe the full biological profile of MK-5932. Cytokine production following lipopolysaccharide (LPS) challenge was blocked by MK-5932 in both rat and human whole blood. Oral administration reduced inflammatory cytokine levels in the serum of rats challenged with LPS. MK-5932 was anti-inflammatory in a rat contact dermatitis model, but was differentiated from 6-methylprednisolone by a lack of elevation of fasting insulin or glucose levels after 7 days of dosing, even at high exposure levels. In fact, animals in the vehicle group were consistently hyperglycemic at the end of the study, and MK-5932 normalized glucose levels in a dose-dependent manner. MK-5932 was also anti-inflammatory in the rat collagen-induced arthritis and adjuvant-induced arthritis models. In healthy dogs, oral administration of MK-5932 exerted acute pharmacodynamic effects with potency comparable to prednisone, but with important differences on neutrophil counts, again suggestive of a dissociated profile. Important gaps in our understanding of mechanism of action remain, but MK-5932 will be a useful tool in dissecting the mechanisms of glucose dysregulation by therapeutic glucocortiocids.

The glucocorticoid receptor dimer interface allosterically transmits sequence-specific DNA signals

Glucocorticoid receptor binds to genomic response elements and regulates gene transcription with cell- and gene-specificity. Within a response element, the precise sequence to which the receptor binds has been implicated in directing its structure and activity. We use NMR chemical shift difference mapping to show that non-specific interactions with particular base positions within the binding sequence, such as those of the “spacer”, affect the conformation of distinct regions of the rat glucocorticoid receptor DNA binding domain. These regions include the DNA-binding surface, the “lever arm” and the dimerization interface, suggesting an allosteric pathway that signals between the DNA binding sequence and the associated dimer partner. Disrupting this path by mutating the dimer interface alters sequence-specific conformations, DNA-binding kinetics and transcriptional activity. Our study demonstrates that glucocorticoid receptor dimer partners collaborate to read DNA shape and to direct sequence specific gene activity.

A ligand-specific kinetic switch regulates glucocorticoid receptor trafficking and function

The ubiquitously expressed glucocorticoid receptor (GR) is a major drug target for inflammatory disease, but issues of specificity and target tissue sensitivity remain. We now identify high potency, non-steroidal GR ligands, GSK47867A and GSK47869A, which induce a novel conformation of the GR ligand-binding domain (LBD) and augment the efficacy of cellular action. Despite their high potency, GSK47867A and GSK47869A both induce surprisingly slow GR nuclear translocation, followed by prolonged nuclear GR retention, and transcriptional activity following washout. We reveal that GSK47867A and GSK47869A specifically alter the GR LBD structure at the HSP90-binding site. The alteration in the HSP90-binding site was accompanied by resistance to HSP90 antagonism, with persisting transactivation seen after geldanamycin treatment. Taken together, our studies reveal a new mechanism governing GR intracellular trafficking regulated by ligand binding that relies on a specific surface charge patch within the LBD. This conformational change permits extended GR action, probably because of altered GR-HSP90 interaction. This chemical series may offer anti-inflammatory drugs with prolonged duration of action due to altered pharmacodynamics rather than altered pharmacokinetics.

Understanding stress-effects in the brain via transcriptional signal transduction pathways

Glucocorticoid hormones exert crucial effects on the brain in relation to physiology, endocrine regulation, mood and cognition. Their two receptor types, glucocorticoid and mineralocorticoid receptors (GR and MR), are members of the nuclear receptor superfamily and act in large measure as transcription factors. The outcome of MR/GR action on the genome depends on interaction with members from different protein families, which are of crucial importance for cross-talk with other neuronal and hormonal signals that impinge on the glucocorticoid sensitive circuitry. Relevant interacting proteins include other transcription factors that may either tether the receptor to the DNA, or that bind in the vicinity of GR and MR to tune the transcriptional response. In addition, transcriptional coregulator proteins constitute the actual signal transduction pathway to the transcription machinery. We review the current evidence for involvement of individual coregulators in GR-dependent effects on stress responses, and learning and memory. We discuss the use of in vitro and in silico tools to predict those coregulators that are of importance for particular brain processes. Finally, we discuss the potential of selective receptor modulators that may only allow a subset of all interactions, thus allowing more selective targeting of glucocorticoid-dependent processes in the brain.

Ligand-binding dynamics rewire cellular signaling via estrogen receptor-α

Ligand-binding dynamics control allosteric signaling through the estrogen receptor-α (ERα), but the biological consequences of such dynamic binding orientations are unknown. Here, we compare a set of ER ligands having dynamic binding orientation (dynamic ligands) with a control set of isomers that are constrained to bind in a single orientation (constrained ligands). Proliferation of breast cancer cells directed by constrained ligands is associated with DNA binding, coactivator recruitment and activation of the estrogen-induced gene GREB1, reflecting a highly interconnected signaling network. In contrast, proliferation driven by dynamic ligands is associated with induction of ERα-mediated transcription in a DNA-binding domain (DBD)-dependent manner. Further, dynamic ligands displayed enhanced anti-inflammatory activity. The DBD-dependent profile was predictive of these signaling patterns in a larger diverse set of natural and synthetic ligands. Thus, ligand dynamics directs unique signaling pathways, and reveals a novel role of the DBD in allosteric control of ERα-mediated signaling.

Noncanonical mechanisms to regulate nuclear receptor signaling

Nuclear receptor (NR)-targeted therapies comprise a large class of clinically employed drugs. A number of drugs currently being used against this protein class were designed as structural analogs of the endogenous ligand of these receptors. In recent years, there has been significant interest in developing newer strategies to target NRs, especially those that rely on mechanistic pathways of NR function. Prominent among these are noncanonical means of targeting NRs, which include selective NR modulation, NR coactivator interaction inhibition, inhibition of NR DNA binding, modulation of NR cellular localization, modulation of NR ligand biosynthesis and downregulation of NR levels in target tissues. This article reviews each of these promising emerging strategies for NR drug development and highlights some of most significant successes achieved in using them.

Alternate glucocorticoid receptor ligand binding structures influence outcomes in an in vivo tissue regeneration model

Since their characterization, glucocorticoids (GCs), the most commonly prescribed immunomodulatory drugs, have undergone numerous structural modifications designed to enhance their activity. In vivo assessment of these corticosteroid analogs is essential to understand the difference in molecular signaling of the ligands that share the corticosteroid backbone. Our research identified a novel function of GCs as modulators of tissue regeneration and demonstrated that GCs activate the glucocorticoid receptor (GR) to inhibit early stages of tissue regeneration in zebrafish (Danio rerio). We utilized this phenomenon to assess the effect of different GC analogs on tissue regeneration and identified that some GCs such as beclomethasone dipropionate (BDP) possess inhibitory properties, while others, such as dexamethasone and hydrocortisone have no effect on regeneration. We performed in silico molecular docking and dynamic studies and demonstrated that type and size of substitution at the C17 position of the cortisol backbone confer a unique stable conformation to GR on ligand binding that is critical for inhibitory activity. In the field of tissue regeneration, our study is one of the first Structure Activity Relationship (SAR) investigations performed in vertebrates demonstrating that the in vivo tissue regeneration model is a powerful tool to probe structure function relationships, to understand regenerative biology, and to assist in rational drug design.

A chemical screening system for glucocorticoid stress hormone signaling in an intact vertebrate

Glucocorticoids, steroid hormones of the adrenal gland, are an integral part of the stress response and regulate glucose metabolism. Natural and synthetic glucocorticoids are widely used in anti-inflammatory therapy but can have severe side effects. In vivo tests are needed to identify novel glucocorticoids and to screen compounds for unwanted effects on glucocorticoid signaling. We created the Glucocorticoid Responsive In vivoZebrafish Luciferase activitY assay to monitor glucocorticoid signaling in vivo. The GRIZLY assay detects stress-induced glucocorticoid production in single zebrafish larvae, measures disruption of glucocorticoid signaling by an organotin pollutant metabolite, and specifically identifies a compound stimulating endogenous glucocorticoid production in a chemical screen. Our assay has broad applications in stress research, environmental monitoring, and drug discovery.

Non-steroidal Dissociated Glucocorticoid Receptor Agonists

Synthetic glucocorticoids, such as dexamethasone and prednisolone, are amongst the most commonly used drugs due to their potent and efficacious anti-inflammatory and immunosuppressive properties. However, their long-term and/or high-dose administration is limited by a number of deleterious side-effects, including glucocorticoid-induced diabetes and osteoporosis. Glucocorticoids exert their effects through binding to the glucocorticoid receptor. Since the discovery of multiple differentiated down-stream functions of the glucocorticoid-bound receptor, such as gene transrepression and transactivation, researchers in academia and industry have been on a quest to discover novel glucocorticoids that achieve functional selectivity, hence dissociating the desired anti-inflammatory from the undesired side-effects. This review describes the current state of discovery and development of non-steroidal glucocorticoid receptor agonists. Several small-molecule drug candidates have advanced into clinical trials, and have shown promising early biomarker data, as well as beneficial effects in topical applications. However, a clinically efficacious and systemically available glucocorticoid with significantly reduced side-effects as compared to current steroidal drugs, the “Holy Grail” in immunology, is still elusive.

Ligand structural motifs can decouple glucocorticoid receptor transcriptional activation from target promoter occupancy

Glucocorticoid (GC) induction of the tyrosine aminotransferase (TAT) gene by the glucocorticoid receptor (GR) is a classic model used to investigate steroid-regulated gene expression. Classic studies analyzing GC-induction of the TAT gene demonstrated that despite having very high affinity for GR, some steroids cannot induce maximal TAT enzyme activity, but the molecular basis for this phenomenon is unknown. Here, we used RT-PCR and chromatin immunoprecipitation to determine TAT mRNA accumulation and GR recruitment to the TAT promoter (TAT-GRE) in rat hepatoma cells induced by seven GR ligands: dexamethasone (DEX), cortisol (CRT), corticosterone (CCS), 11-deoxycorticosterone (DOC), aldosterone (ALD), progesterone (PRG) and 17-hydroxyprogesterone (17P). As expected, DEX, CRT, CCS and ALD all induced both TAT mRNA and GR recruitment to the TAT-GRE, while PRG and 17P did not. However, while DOC could not induce significant TAT mRNA, it did induce robust GR occupancy of the TAT-GRE. DOC also induced recruitment of the histone acetyltransferase p300 to the TAT-GRE as efficiently as DEX. These DOC-induced effects recapitulated at another GR target gene (sulfonyltransferase 1A1), and DOC also failed to promote the multiple changes in gene expression required for glucocorticoid-dependent 3T3-L1 adipocyte differentiation. Structural simulations and protease sensitivity assays suggest that DOC and DEX induce different conformations in GR. Thus, although steroids that bind GR with high affinity can induce GR and p300 occupancy of target promoters, they may not induce a conformation of GR capable of activating transcription.

Identical gene regulation patterns of T3 and selective thyroid hormone receptor modulator GC-1

Synthetic selective thyroid hormone (TH) receptor (TR) modulators (STRM) exhibit beneficial effects on dyslipidemias in animals and humans and reduce obesity, fatty liver, and insulin resistance in preclinical animal models. STRM differ from native TH in preferential binding to the TRβ subtype vs. TRα, increased uptake into liver, and reduced uptake into other tissues. However, selective modulators of other nuclear receptors exhibit important gene-selective actions, which are attributed to differential effects on receptor conformation and dynamics and can have profound influences in animals and humans. Although there are suggestions that STRM may exhibit such gene-specific actions, the extent to which they are actually observed in vivo has not been explored. Here, we show that saturating concentrations of the main active form of TH, T(3), and the prototype STRM GC-1 induce identical gene sets in livers of euthyroid and hypothyroid mice and a human cultured hepatoma cell line that only expresses TRβ, HepG2. We find one case in which GC-1 exhibits a modest gene-specific reduction in potency vs. T(3), at angiopoietin-like factor 4 in HepG2. Investigation of the latter effect confirms that GC-1 acts through TRβ to directly induce this gene but this gene-selective activity is not related to unusual T(3)-response element sequence, unlike previously documented promoter-selective STRM actions. Our data suggest that T(3) and GC-1 exhibit almost identical gene regulation properties and that gene-selective actions of GC-1 and similar STRM will be subtle and rare.

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.

The antagonists but not partial agonists of glucocorticoid receptor ligands show substantial side effect dissociation

A synthetic glucocorticoid receptor (GR) ligand with the efficacy of a glucocorticoid, but without the accompanying side effects, would meet an unmet medical need for the treatment of inflammatory diseases. It was hypothesized that a GR ligand that shifted helix 12 in a manner distinct from an agonist and an antagonist would confer a distinct GR conformation, resulting in differential gene expression and, ultimately, dissociation of antiinflammatory activity from side effects. A structural feature expected to interfere with helix 12 was incorporated into a nonsteroidal, tricyclic scaffold to create novel, high-affinity, and selective GR ligands that manifested a dual function in cellular assays, partial but robust agonist activity for inflammatory cytokine inhibition, and full antagonist activity for reporter gene activation. In contrast, analogs not likely to hinder helix 12 exhibited partial agonist activity for reporter gene activation. The requirement of full antagonist activity for substantial side effect dissociation was demonstrated in primary human preadipocytes, hepatocytes, and osteoblasts in which effects on adipogenesis, key genes involved in gluconeogenesis, and genes important for bone formation were examined, respectively. The dissociated GR ligands, despite lacking significant reporter gene activation, weakly recruit a limited number of coactivators such as peroxisomal proliferator-activated receptor-γ coactivator 1α. Transcriptional activation was sensitive to both peroxisomal proliferator-activated receptor-γ coactivator 1α and GR levels, providing a basis for cell-selective modulation of gene expression. The antiinflammatory activity of the dissociated ligands was further demonstrated in mouse models of inflammation. Together these results suggest that these ligands are promising candidates with robust antiinflammatory activity and likely dissociation against glucocorticoid-induced side effects.

The glucocorticoid receptor and the coregulator Brm selectively modulate each other's occupancy and activity in a gene-specific manner

The diverse transcriptional patterns that distinguish metazoan cells are specified by multifactor regulatory complexes containing distinct combinations of factors that assemble at genomic response elements. To investigate combinatorial control, we examined a set of glucocorticoid receptor (GR)-regulated genes bearing nearby regulatory complexes that include both GR and the coregulator Brm, an ATPase subunit of the Swi/Snf chromatin remodeler. We analyzed how GR and Brm affect each other's occupancy and activity by utilizing glucocorticoid treatment and Brm knockdown to modulate GR-mediated transcriptional regulation and Brm-mediated chromatin remodeling, respectively. GR occupancy and activity were altered differentially by Brm knockdown at specific activated and repressed primary GR target genes. Brm knockdown decreased GR occupancy at activated Brm-dependent genes, whereas we identified two classes of repressed genes, at which Brm knockdown either increased or decreased GR occupancy. Glucocorticoid treatment increased both Brm occupancy and chromatin accessibility at Brm-dependent and Brm-independent GR-regulated genes. However, chromatin remodeling activity decreased after Brm knockdown only at genes with Brm-dependent transcription. Our study revealed multiple distinct patterns of GR and Brm interdependence. Thus, monitoring as few as two factors within regulatory complexes is sufficient to reveal functionally distinct assemblies, providing an analytical method for gaining insights into combinatorial regulation.

Identification of 9-cis-retinoic acid as a pancreas-specific autacoid that attenuates glucose-stimulated insulin secretion

The all-trans-retinoic acid (atRA) isomer, 9-cis-retinoic acid (9cRA), activates retinoic acid receptors (RARs) and retinoid X receptors (RXRs) in vitro. RARs control multiple genes, whereas RXRs serve as partners for RARs and other nuclear receptors that regulate metabolism. Physiological function has not been determined for 9cRA, because it has not been detected in serum or multiple tissues with analytically validated assays. Here, we identify 9cRA in mouse pancreas by liquid chromatography/tandem mass spectrometry (LC/MS/MS), and show that 9cRA decreases with feeding and after glucose dosing and varies inversely with serum insulin. 9cRA reduces glucose-stimulated insulin secretion (GSIS) in mouse islets and in the rat β-cell line 832/13 within 15 min by reducing glucose transporter type 2 (Glut2) and glucokinase (GK) activities. 9cRA also reduces Pdx-1 and HNF4α mRNA expression, ∼8- and 80-fold, respectively: defects in Pdx-1 or HNF4α cause maturity onset diabetes of the young (MODY4 and 1, respectively), as does a defective GK gene (MODY2). Pancreas β-cells generate 9cRA, and mouse models of reduced β-cell number, heterozygous Akita mice, and streptozotocin-treated mice have reduced 9cRA. 9cRA is abnormally high in glucose-intolerant mice, which have β-cell hypertropy, including mice with diet-induced obesity (DIO) and ob/ob and db/db mice. These data establish 9cRA as a pancreas-specific autacoid with multiple mechanisms of action and provide unique insight into GSIS.

Transcriptional regulation of human dual specificity protein phosphatase 1 (DUSP1) gene by glucocorticoids

BACKGROUND

Glucocorticoids are potent anti-inflammatory agents commonly used to treat inflammatory diseases. They convey signals through the intracellular glucocorticoid receptor (GR), which upon binding to ligands, associates with genomic glucocorticoid response elements (GREs) to regulate transcription of associated genes. One mechanism by which glucocorticoids inhibit inflammation is through induction of the dual specificity phosphatase-1 (DUSP1, a.k.a. mitogen-activated protein kinase phosphatase-1, MKP-1) gene.

METHODOLOGY/PRINCIPAL FINDINGS

We found that glucocorticoids rapidly increased transcription of DUSP1 within 10 minutes in A549 human lung adenocarcinoma cells. Using chromatin immunoprecipitation (ChIP) scanning, we located a GR binding region between -1421 and -1118 upstream of the DUSP1 transcription start site. This region is active in a reporter system, and mutagenesis analyses identified a functional GRE located between -1337 and -1323. We found that glucocorticoids increased DNase I hypersensitivity, reduced nucleosome density, and increased histone H3 and H4 acetylation within genomic regions surrounding the GRE. ChIP experiments showed that p300 was recruited to the DUSP1 GRE, and RNA interference experiments demonstrated that reduction of p300 decreased glucocorticoid-stimulated DUSP1 gene expression and histone H3 hyperacetylation. Furthermore, overexpression of p300 potentiated glucocorticoid-stimulated activity of a reporter gene containing the DUSP1 GRE, and this coactivation effect was compromised when the histone acetyltransferase domain was mutated. ChIP-reChIP experiments using GR followed by p300 antibodies showed significant enrichment of the DUSP1 GRE upon glucocorticoid treatment, suggesting that GR and p300 are in the same protein complex recruited to the DUSP1 GRE.

CONCLUSIONS/SIGNIFICANCE

Our studies identified a functional GRE for the DUSP1 gene. Moreover, the transcriptional activation of DUSP1 by glucocorticoids requires p300 and a rapid modification of the chromatin structure surrounding the GRE. Overall, understanding the mechanism of glucocorticoid-induced DUSP1 gene transcription could provide insights into therapeutic approaches against inflammatory diseases.

Classic glucocorticoids versus non-steroidal glucocorticoid receptor modulators: survival of the fittest regulator of the immune system?

The search for novel glucocorticoid receptor (GR) modulators with similar anti-inflammatory properties as conventional steroids, but with a reduction in the number or severity of the side effects has been a long-standing goal, and still remains a challenge today. The quest for these so-called 'dissociated GR ligands' is mainly based on the hypothesis that the occurrence of undesirable side effects is mostly associated with GR-mediated transactivation, whereas transrepression of many pro-inflammatory genes (e.g. cytokines and enzymes involved in inflammatory processes) is more involved in GR-mediated anti-inflammatory effects. As glucocorticoids (GCs) can also enhance the transcription of anti-inflammatory genes, the GR-mediated activation-repression dissociation hypothesis has to be nuanced. However, an enhanced selectivity of GR-affected genes, while upholding the desired anti-inflammatory potential, is still believed to contribute to a more beneficial therapeutic profile with fewer side effects. The initial pharmacological focus on steroidal scaffolds as a basis to dissociate the functionalities of GR has, due to a lack of success, recently been shifted to a focus on non-steroidal ligands. The current work reviews recent advances on the characterization of a generation of novel non-steroidal GR ligands.

Dissociated non-steroidal glucocorticoids: tuning out untoward effects

The endogenous glucocorticoid (GC), cortisol, is involved in maintaining homeostatic balance in glucose regulation and immune response while allowing stress adaptation. The glucocorticoid receptor (GR) is required to maintain life and is the target of numerous FDA-approved drugs. Synthetic steroidal GCs are useful in a plethora of conditions characterized by excessive inflammatory or immune responses. Unfortunately, the GCs used at present have potentially dose-limiting and debilitating side effects, some of which derive from the glucose regulatory role of GCs. Consequently, there is a great need to find agents which preserve the potent immune effects without the side effects. This manuscript reviews the existing patent literature on these intensely sought non-steroidal agents that dissociate the therapeutic from metabolic effects, or specifically retain certain GR target effects with attenuated untoward effects. The chemical classes and underlying mechanisms (when known) for these non-steroidal GCs are discussed.

Selective Glucocorticoid Receptor modulators

The ancient two-faced Roman god Janus is often used as a metaphor to describe the characteristics of the Glucocorticoid Receptor (NR3C1), which exhibits both a beneficial side, that serves to halt inflammation, and a detrimental side responsible for undesirable effects. However, recent developments suggest that the Glucocorticoid Receptor has many more faces with the potential to express a range of different functionalities, depending on factors that include the tissue type, ligand type, receptor variants, cofactor surroundings and target gene promoters. This behavior of the receptor has made the development of safer ligands, that trigger the expression program of only a desirable subset of genes, a real challenge. Thus more knowledge-based fundamental research is needed to ensure the design and development of selective Glucocorticoid Receptor modulators capable of reaching the clinic. Recent advances in the characterization of novel selective Glucocorticoid Receptor modulators, specifically in the context of anti-inflammatory strategies, will be described in this review.

Crosstalk in inflammation: the interplay of glucocorticoid receptor-based mechanisms and kinases and phosphatases

Glucocorticoids (GCs) are steroidal ligands for the GC receptor (GR), which can function as a ligand-activated transcription factor. These steroidal ligands and derivatives thereof are the first line of treatment in a vast array of inflammatory diseases. However, due to the general surge of side effects associated with long-term use of GCs and the potential problem of GC resistance in some patients, the scientific world continues to search for a better understanding of the GC-mediated antiinflammatory mechanisms. The reversible phosphomodification of various mediators in the inflammatory process plays a key role in modulating and fine-tuning the sensitivity, longevity, and intensity of the inflammatory response. As such, the antiinflammatory GCs can modulate the activity and/or expression of various kinases and phosphatases, thus affecting the signaling efficacy toward the propagation of proinflammatory gene expression and proinflammatory gene mRNA stability. Conversely, phosphorylation of GR can affect GR ligand- and DNA-binding affinity, mobility, and cofactor recruitment, culminating in altered transactivation and transrepression capabilities of GR, and consequently leading to a modified antiinflammatory potential. Recently, new roles for kinases and phosphatases have been described in GR-based antiinflammatory mechanisms. Moreover, kinase inhibitors have become increasingly important as antiinflammatory tools, not only for research but also for therapeutic purposes. In light of these developments, we aim to illuminate the integrated interplay between GR signaling and its correlating kinases and phosphatases in the context of the clinically important combat of inflammation, giving attention to implications on GC-mediated side effects and therapy resistance.

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.

Repression of DNA-binding dependent glucocorticoid receptor-mediated gene expression

The glucocorticoid receptor (GR) affects the transcription of genes involved in diverse processes, including energy metabolism and the immune response, through DNA-binding dependent and independent mechanisms. The DNA-binding dependent mechanism occurs by direct binding of GR to glucocorticoid response elements (GREs) at regulatory regions of target genes. The DNA-binding independent mechanism involves binding of GR to transcription factors and coactivators that, in turn, contact DNA. A small molecule that competes with GR for binding to GREs could be expected to affect the DNA-dependent pathway selectively by interfering with the protein-DNA interface. We show that a DNA-binding polyamide that targets the consensus GRE sequence binds the glucocorticoid-induced zipper (GILZ) GRE, inhibits expression of GILZ and several other known GR target genes, and reduces GR occupancy at the GILZ promoter. Genome-wide expression analysis of the effects of this polyamide on a set of glucocorticoid-induced and -repressed genes could help to elucidate the mechanism of GR regulation for these genes.

Angiopoietin-like 4 (ANGPTL4, fasting-induced adipose factor) is a direct glucocorticoid receptor target and participates in glucocorticoid-regulated triglyceride metabolism

Glucocorticoids are important regulators of lipid homeostasis, and chronically elevated glucocorticoid levels induce hypertriglyceridemia, hepatic steatosis, and visceral obesity. The occupied glucocorticoid receptor (GR) is a transcription factor. However, those genes regulating lipid metabolism under GR control are not fully known. Angiopoietin-like 4 (ANGPTL4, fasting-induced adipose factor), a protein inhibitor of lipoprotein lipase, is synthesized and secreted during fasting, when circulating glucocorticoid levels are physiologically increased. We therefore tested whether the ANGPTL4 gene (Angptl4) is transcriptionally controlled by GR. We show that treatment with the synthetic glucocorticoid dexamethasone increased Angptl4 mRNA levels in primary hepatocytes and adipocytes (2-3-fold) and in the livers and white adipose tissue of mice (approximately 4-fold). We tested the mechanism of this increase in H4IIE hepatoma cells and found that dexamethasone treatment increased the transcriptional rate of Angptl4. Using bioinformatics and chromatin immunoprecipitation, we identified a GR binding site within the rat Angptl4 sequence. A reporter plasmid containing this site was markedly activated by dexamethasone, indicative of a functional glucocorticoid response element. Dexamethasone treatment also increased histone H4 acetylation and DNase I accessibility in genomic regions near this site, further supporting that it is a glucocorticoid response element. Glucocorticoids promote the flux of triglycerides from white adipose tissue to liver. We found that mice lacking ANGPTL4 (Angptl4(-/-)) had reductions in dexamethasone-induced hypertriglyceridemia and hepatic steatosis, suggesting that ANGPTL4 is required for this flux. Overall, we establish that ANGPTL4 is a direct GR target that participates in glucocorticoid-regulated triglyceride metabolism.

Ligand-based gene expression profiling reveals novel roles of glucocorticoid receptor in cardiac metabolism

Recent studies have documented various roles of adrenal corticosteroid signaling in cardiac physiology and pathophysiology. It is known that glucocorticoids and aldosterone are able to bind glucocorticoid receptor (GR) and mineralocorticoid receptor, and these ligand-receptor interactions are redundant. It, therefore, has been impossible to delineate how these nuclear receptors couple with corticosteroid ligands and differentially regulate gene expression for operation of their distinct functions in the heart. Here, to particularly define the role of GR in cardiac muscle cells, we applied a ligand-based approach involving the GR-specific agonist cortivazol (CVZ) and the GR antagonist RU-486 and performed microarray analysis using rat neonatal cardiomyocytes. We indicated that glucocorticoids appear to be a major determinant of GR-mediated gene expression when compared with aldosterone. Moreover, expression profiles of these genes highlighted numerous roles of glucocorticoids in various aspects of cardiac physiology. At first, we identified that glucocorticoids, via GR, induce mRNA and protein expression of a transcription factor Kruppel-like factor 15 and its downstream target genes, including branched-chain aminotransferase 2, a key enzyme for amino acid catabolism in the muscle. CVZ treatment or overexpression of KLF15 decreased cellular branched-chain amino acid concentrations and introduction of small-interfering RNA against KLF15 cancelled these CVZ actions in cardiomyocytes. Second, glucocorticoid-GR signaling promoted gene expression of the enzymes involved in the prostaglandin biosynthesis, including cyclooxygenase-2 and phospholipase A2 in cardiomyocytes. Together, we may conclude that GR signaling should have distinct roles for maintenance of cardiac function, for example, in amino acid catabolism and prostaglandin biosynthesis in the heart.

Signaling networks that control the lineage commitment and differentiation of bone cells

Osteoblasts and osteoclasts are the two major bone cells involved in the bone remodeling process. Osteoblasts are responsible for bone formation while osteoclasts are the bone-resorbing cells. The major event that triggers osteogenesis and bone remodeling is the transition of mesenchymal stem cells into differentiating osteoblast cells and monocyte/macrophage precursors into differentiating osteoclasts. Imbalance in differentiation and function of these two cell types will result in skeletal diseases such as osteoporosis, Paget's disease, rheumatoid arthritis, osteopetrosis, periodontal disease, and bone cancer metastases. Osteoblast and osteoclast commitment and differentiation are controlled by complex activities involving signal transduction and transcriptional regulation of gene expression. Recent advances in molecular and genetic studies using gene targeting in mice enable a better understanding of the multiple factors and signaling networks that control the differentiation process at a molecular level. This review summarizes recent advances in studies of signaling transduction pathways and transcriptional regulation of osteoblast and osteoclast cell lineage commitment and differentiation. Understanding the signaling networks that control the commitment and differentiation of bone cells will not only expand our basic understanding of the molecular mechanisms of skeletal development but will also aid our ability to develop therapeutic means of intervention in skeletal diseases.

Discovery of selective glucocorticoid receptor modulators by multiplexed reporter screening

Glucocorticoids are widely used to suppress inflammation and treat various immune-mediated diseases. Some glucocorticoid receptor (GR)-regulated genes mediate the therapeutic response, whereas others cause debilitating side effects. To discover selective modulators of the GR response, we developed a high-throughput, multiplexed system to monitor regulation of 4 promoters simultaneously. An initial screen of 1,040 natural products and Food and Drug Administration-approved drugs identified modulators that caused GR to regulate only a subset of its target promoters. Some compounds selectively inhibited GR-mediated gene activation without altering the repression of cytokine expression by GR. This approach will facilitate identification of genes and small molecules that augment beneficial effects of GR and diminish deleterious ones. Our results have important implications for the development of GR modulators and the identification of cross-talk pathways that control selective GR gene regulation.

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.

Thiazolidinediones are partial agonists for the glucocorticoid receptor

Although thiazolidinediones were designed as specific peroxisome proliferator-activated receptor (PPAR)-gamma-ligands, there is evidence for some off-target effects mediated by a non-PPARgamma mechanism. Previously we have shown that rosiglitazone has antiinflammatory actions not explicable by activation of PPARgamma,but possibly by the glucocorticoid receptor (GR). Rosiglitazone induces nuclear translocation both of GR-green fluorescent protein, and endogenous GR in HeLa and U20S cells but with slower kinetics than dexamethasone. Rosiglitazone also induces GR phosphorylation (Ser211), a GR ligand-binding-specific effect. Rosiglitazone drives luciferase expression from a simple glucocorticoid-response element containing reporter gene in a GR-dependent manner (EC50 4 microm), with a similar amplitude response to the partial GR agonist RU486. Rosiglitazone also inhibits dexamethasone-driven reporter gene activity (IC50 2.9 microm) in a similar fashion to RU486, suggesting partial agonist activity. Importantly we demonstrate a similar effect in PPARgamma-null cells, suggesting both GR dependence and PPARgamma independence. Rosiglitazone also activates a GAL4-GR chimera, driving a upstream activating sequence promoter, demonstrating DNA template sequence independence and furthermore enhanced steroid receptor coactivator-1-GR interaction, measured by a mammalian two-hybrid assay. Both ciglitazone and pioglitazone, structurally related to rosiglitazone, show similar effects on the GR. The antiproliferative effect of rosiglitazone is increased in U20S cells that overexpress GR, suggesting a biologically important GR-dependent component of rosiglitazone action. Rosiglitazone is a partial GR agonist, affecting GR activation and trafficking to influence engagement of target genes and affect cell function. This novel mode of action may explain some off-target effects observed in vivo. Additionally, antagonism of glucocorticoid action may contribute to the antidiabetic actions of rosiglitazone.