An active nuclear retention signal in the glucocorticoid receptor functions as a strong inducer of transcriptional activation

The Biologist's Guide to the Glucocorticoid Receptor's Structure

The glucocorticoid receptor α (GRα) is a member of the nuclear receptor superfamily and functions as a glucocorticoid (GC)-responsive transcription factor. GR can halt inflammation and kill off cancer cells, thus explaining the widespread use of glucocorticoids in the clinic. However, side effects and therapy resistance limit GR's therapeutic potential, emphasizing the importance of resolving all of GR's context-specific action mechanisms. Fortunately, the understanding of GR structure, conformation, and stoichiometry in the different GR-controlled biological pathways is now gradually increasing. This information will be crucial to close knowledge gaps on GR function. In this review, we focus on the various domains and mechanisms of action of GR, all from a structural perspective.

RNA binding by the glucocorticoid receptor attenuates dexamethasone-induced gene activation

The glucocorticoid receptor (GR) is a ligand-activated transcription factor that regulates a suite of genes through direct binding of GR to specific DNA promoter elements. GR also interacts with RNA, but the function of this RNA-binding activity remains elusive. Current models speculate that RNA could repress the transcriptional activity of GR. To investigate the function of the GR-RNA interaction on GR's transcriptional activity, we generated cells that stably express a mutant of GR with reduced RNA binding affinity and treated the cells with the GR agonist dexamethasone. Changes in the dexamethasone-driven transcriptome were quantified using 4-thiouridine labeling of RNAs followed by high-throughput sequencing. We find that while many genes are unaffected, GR-RNA binding is repressive for specific subsets of genes in both dexamethasone-dependent and independent contexts. Genes that are dexamethasone-dependent are activated directly by chromatin-bound GR, suggesting a competition-based repression mechanism in which increasing local concentrations of RNA may compete with DNA for binding to GR at sites of transcription. Unexpectedly, genes that are dexamethasone-independent instead display a localization to specific chromosomal regions, which points to changes in chromatin accessibility or architecture. These results show that RNA binding plays a fundamental role in regulating GR function and highlights potential functions for transcription factor-RNA interactions.

FX5 as a non-steroidal GR antagonist improved glucose homeostasis in type 2 diabetic mice via GR/HNF4α/miR-122-5p pathway

Type 2 diabetes mellitus (T2DM) is a chronic metabolic disease characterized by glucose metabolic disorders, and gluconeogenesis inhibiting is a promisingly therapeutic strategy for T2DM. Glucocorticoid receptor (GR) is tightly implicated in the regulation of gluconeogenesis, although the underlying mechanism remains obscure. Here, we discovered that small molecule, 5-chloro-N-[4-chloro-3-(trifluoromethyl)phenyl]thiophene-2-sulfonamide (FX5) as a new non-steroidal GR antagonist efficiently ameliorated glucose homeostasis in db/db and HFD/STZ-induced T2DM mice. The mechanism underlying the suppression of FX5 against gluconeogenesis was highly investigated. FX5 suppressed gluconeogenetic genes G6Pase and PEPCK in mouse primary hepatocytes and liver tissues of T2DM mice. Results of mammalian one-hybrid and transactivation as well as nuclear translocation assays totally evaluated the antagonistic features of FX5 against GR. Moreover, siRNA and overexpression related assays verified that FX5 alleviated gluconeogenesis either directly by antagonizing GR or indirectly through GR/HNF4α/miR122-5p signaling pathway. Our work has presented a new mode for GR antagonist in the regulation of gluconeogenesis, which is expected to highlight the potential of FX5 in the treatment of T2DM.

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.

New Insights in Glucocorticoid Receptor Signaling-More Than Just a Ligand-Binding Receptor

The clinical use of classical glucocorticoids (GC) is narrowed by the many side effects it causes and the resistance to GC observed in some diseases. Since the great majority of GC effects depend on the activation of a glucocorticoid receptor (GR), many research groups had focused to better understand the signaling pathways involving those receptors. Transgenic animal models and genetic modifications of the receptor brought a huge insight into GR mechanisms of action. This in turn opened a new window for the search of selective GR modulators that ideally may have agonistic and antagonistic combined effects and activate one specific signaling pathway, inducing mostly transrepression or transactivation mechanisms. Another important research field concerns to posttranslational modifications that affect the GR and consequently also affect its signaling and function. In this mini review, we discuss many of those aspects of GR signaling, as well as findings like the ligand-independent activation of GR, which add another layer of complexity in GR signaling pathways. Although several recent data have been added to the GR field, much work has yet to be done, especially to find out the biological relevance of those alternative GR signaling pathways. Improving the knowledge about alternative GR signaling pathways and understanding how these pathways intercommunicate and in which situations they are relevant might help to develop new strategies to take benefit of it and to improve GC or other compounds efficacy causing minimal side effects.

Glucocorticoid therapy and ocular hypertension

The projected number of people who will develop age-related macular degeneration in estimated at 2020 is 196 million and is expected to reach 288 million in 2040. Also, the number of people with Diabetic retinopathy will grow from 126.6 million in 2010 to 191.0 million by 2030. In addition, it is estimated that there are 2.3 million people suffering from uveitis worldwide. Because of the anti-inflammatory properties of glucocorticoids (GCs), they are often used topically and/or intravitreally to treat ocular inflammation conditions or edema associated with macular degeneration and diabetic retinopathy. Unfortunately, ocular GC therapy can lead to severe side effects. Serious and sometimes irreversible eye damage can occur as a result of the development of GC-induced ocular hypertension causing secondary open-angle glaucoma. According to the world health organization, glaucoma is the second leading cause of blindness in the world and it is estimated that 80 million will suffer from glaucoma by 2020. In the current review, mechanisms of GC-induced damage in ocular tissue, GC-resistance, and enhancing GC therapy will be discussed.

Comprehensive overview of the structure and regulation of the glucocorticoid receptor

Glucocorticoids are among the most prescribed drugs worldwide for the treatment of numerous immune and inflammatory disorders. They exert their actions by binding to the glucocorticoid receptor (GR), a member of the nuclear receptor superfamily. There are several GR isoforms resulting from alternative RNA splicing and translation initiation of the GR transcript. Additionally, these isoforms are all subject to several transcriptional, post-transcriptional, and post-translational modifications, all of which affect the protein's stability and/or function. In this review, we summarize recent knowledge on the distinct GR isoforms and the processes that generate them. We also review the importance of all known transcriptional, post-transcriptional, and post-translational modifications, including the regulation of GR by microRNAs. Moreover, we discuss the crucial role of the putative GR-bound DNA sequence as an allosteric ligand influencing GR structure and activity. Finally, we describe how the differential composition and distinct regulation at multiple levels of different GR species could account for the wide and diverse effects of glucocorticoids.

Bisphenol A increases aP2 expression in 3T3L1 by enhancing the transcriptional activity of nuclear receptors at the promoter

Environmental pollutants, such as bisphenol A (BPA), have the potential to affect the differentiation processes and the biology of the adipose tissue. The 3T3-L1 model is one of the murine cell models used extensively for the investigation of the molecular events that govern the differentiation of adipocytes from a committed preadipocyte to a mature, lipid laden adipocyte. Most of the studies investigating the effects of BPA on preadipocyte differentiation have investigated the effects of this chemical in the presence of an optimal differentiation cocktail containing high concentrations of the synthetic glucocorticoid dexamethasone, conditions that result in 90% to 100% of differentiated adipocytes. Our studies employed the 3T3-L1 cell model in the absence of exogenous glucocorticoids. We show that BPA is able to increase the differentiation of the 3T3-L1 cells under these conditions. Furthermore, the effect of BPA was observed in the absence of the synthetic glucocorticoid (dexamethasone), a hormone known to be required for the differentiation of the 3T3-L1 cells. In addition, BPA upregulated the mRNA expression and protein levels of the terminal marker of adipogenesis the fatty acid binding protein (aP2) in these cells. Interestingly, the known modulators of adipogenesis such as the peroxisome proliferator-activated receptor (PPAR) γ or CCAAT enhancer binding protein (C/EBP) α were not elevated at the mRNA or protein level in response to BPA. Furthermore, BPA upregulated the expression levels of the marker of adipogenesis aP2, through an effect on the transcriptional activity of C/EBPδ and the glucocorticoid receptor (GR) at its promoter.

Prenatal alcohol exposure modifies glucocorticoid receptor subcellular distribution in the medial prefrontal cortex and impairs frontal cortex-dependent learning

Prenatal alcohol exposure (PAE) has been shown to impair learning, memory and executive functioning in children. Perseveration, or the failure to respond adaptively to changing contingencies, is a hallmark on neurobehavioral assessment tasks for human fetal alcohol spectrum disorder (FASD). Adaptive responding is predominantly a product of the medial prefrontal cortex (mPFC) and is regulated by corticosteroids. In our mouse model of PAE we recently reported deficits in hippocampal formation-dependent learning and memory and a dysregulation of hippocampal formation glucocorticoid receptor (GR) subcellular distribution. Here, we examined the effect of PAE on frontal cortical-dependent behavior, as well as mPFC GR subcellular distribution and the levels of regulators of intracellular GR transport. PAE mice displayed significantly reduced response flexibility in a Y-maze reversal learning task. While the levels of total nuclear GR were reduced in PAE mPFC, levels of GR phosphorylated at serines 203, 211 and 226 were not significantly changed. Cytosolic, but not nuclear, MR levels were elevated in the PAE mPFC. The levels of critical GR trafficking proteins, FKBP51, Hsp90, cyclophilin 40, dynamitin and dynein intermediate chain, were altered in PAE mice, in favor of the exclusion of GR from the nucleus, indicating dysregulation of GR trafficking. Our findings suggest that there may be a link between a deficit in GR nuclear localization and frontal cortical learning deficits in prenatal alcohol-exposed mice.

The effect of dexamethasone on lentiviral vector infection is associated with importin α

Importin α (Imα) plays an important role during the shuttling of the HIV-1 preintegration complex (PIC) from the cytoplasm to the nucleus. Imα may bind to the glucocorticoid receptor (GR), which is localized to nucleus following hormone binding. However, it remains unclear whether the binding of dexamethasone (Dex) to GR affects the Imα redistribution and, thus, alters PIC import. In our study, 293T cells were transfected with the lentiviral vector (LV) carrying the luciferase (Luci) gene following Dex or RU486 pretreatment. The Luci activity (LucA) in the Dex or RU486 group was significantly higher compared to that in the control group (P≤0.01). The effects of Dex and RU486 were inhibited by the Imα inhibitor Bimax1 (P≤0.01), although the inhibitory effect of Bimax1 was alleviated by increasing the Dex dose. Furthermore, it was observed that the LucA in the 30-min Dex treatment group was lower compared to that in the 30-min Dex pretreatment group (P≤0.01). These results suggested that Dex may improve PIC import via increasing the cytoplasmic Imα levels. Kunming mice were transfected in vivo with the LV, either 30 min or 15 h following an intraperitoneal injection of Dex. The LucA in the liver of the 30-min group mice was significantly lower compared to that of the 15-h group mice (P≤0.01), suggesting that the effect of Dex on LV infection depends mainly on the suppression of immune and inflammatory responses in vivo. Taken together, our data indicated that the effect of Dex on LV infection may be associated with Imα, constituting a novel signaling pathway mediating the effects of Dex on HIV-1 infection.

Prenatal alcohol exposure is associated with altered subcellular distribution of glucocorticoid and mineralocorticoid receptors in the adolescent mouse hippocampal formation

Background

Accumulating evidence indicates that several of the long-term consequences of prenatal alcohol exposure (PAE) are the result of changes in the development and function of cortico-limbic structures, including the hippocampal formation. The glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) are key regulators of hippocampal formation development, structure, and functioning and, thus, are potential mediators of PAE’s effects on this brain region. In the present studies, we assessed the impact of PAE on components of corticosteroid signaling pathways in the mouse hippocampal formation.

Methods

Throughout pregnancy, mouse dams were offered either 10% (w/v) ethanol sweetened with 0.066% (w/v) saccharin (SAC) or 0.066% (w/v) SAC alone using a limited (4-hour) access, drinking-in-the-dark paradigm. The hippocampal formation was isolated from naïve postnatal day 40 to 50 offspring, and subcellular fractions were prepared. Using immunoblotting techniques, we measured the levels of GR, MR, 11-β-hydroxysteroid dehydrogenase 1 (11β-HSD1), and the FK506-binding proteins 51 (FKBP51, FKBP5) and 52 (FKBP52, FKBP4). Finally, we determined the effect of PAE on context discrimination, a hippocampal-dependent learning/memory task.

Results

PAE was associated with reduced MR and elevated GR nuclear localization in the hippocampal formation, whereas cytosolic levels of both receptors were not significantly altered. FKBP51 levels were reduced, while FKBP52 levels were unaltered, and 11β-HSD1 levels were increased in postnuclear fractions isolated from PAE mouse hippocampal formation. These neurochemical alterations were associated with reduced context discrimination.

Conclusions

The data support a model in which PAE leads to increased nuclear localization of GRs secondary to reductions in FKBP51 and increases in 11β-HSD1 levels in the adolescent mouse hippocampal formation. Persistent dysregulation of GR subcellular distribution is predicted to damage the hippocampal formation and may underlie many of the effects of PAE on hippocampal-dependent functioning.

Glucocorticoid receptor concentration and the ability to dimerize influence nuclear translocation and distribution

Glucocorticoid receptor (GR) concentrations and the ability of the GR to dimerize are factors which influence sensitivity to glucocorticoids. Upon glucocorticoid binding, the GR is actively transported into the nucleus, a crucial step in determining GR function. We examined the effects of GR concentration and the ability to dimerize on GR nuclear import, export and nuclear distribution using both live cell microscopy of GFP-tagged GR and immunofluorescence of untagged GR, with both wild type GR (GRwt) and dimerization deficient GR (GRdim). We found that the observed rate of GR nuclear import increases significantly at higher GR concentrations, at saturating concentrations of dexamethasone (10(-6) M) using GFP-tagged GR, while with untagged GR it is only discernable at sub-saturating ligand concentrations (10(-10)-10(-9) M). Loss of dimerization results in a slower observed rate of nuclear import (2.5- to 3.3-fold decrease for GFP-GRdim) as well as a decreased extent of GR nuclear localization (18-27% decrease for untagged GRdim). These results were linked to an increased rate of GR export at low GR concentrations (1.4- to 1.6-fold increase for untagged GR) and where GR dimerization is abrogated (1.5- to 1.7-fold increase for GFP-GRdim). Furthermore, GR dimerization was shown to be required for the appearance of discrete GC-dependent GR nuclear foci, the loss of which may explain the increased rate of GR export for the GRdim. The reduction in the observed rate of nuclear import and increased rate of nuclear export displayed at low GR concentrations and by the GRdim could explain the lowered glucocorticoid response under these conditions.

It takes two to tango: dimerisation of glucocorticoid receptor and its anti-inflammatory functions

For a number of years, there has been a widespread view that the adverse side-effects of prolonged glucocorticoid (GC) treatment are a result of glucocorticoid receptor (GR)-mediated gene activation, whilst the beneficial anti-inflammatory effects result from GR-mediated 'transrepression'. Since the introduction of the dimerisation-deficient GR mutant, GR(dim), was apparently unable to activate gene transcription, yet still able to repress pro-inflammatory gene transcription, the search for novel GR modulators has centred on the separation of gene activation from repression by prevention of GR dimerisation. However, recent work has questioned the conclusions drawn from these early GR(dim) studies, with evidence that GR(dim) mutants not only activate gene transcription, but that, in direct contradiction to the initial GR(dim) work, are also capable of forming dimers. This review of the current literature highlights the versatility of the GR in forming homodimer interactions, as well as the ability to bind to alternate nuclear receptors, and investigates the potential implications such varying GR dimer conformations may have for the design of GR ligands with a safer side effect profile.

Differential immunoreactivity of glucocorticoid receptor and GABA in GABAergic afferents to parvocellular neurons in the paraventricular nucleus

We assessed the distribution of glucocorticoid receptors (GR), GABA (γ-aminobutyric acid) neurons and co-localization of GR in GABA-positive neurons for four hypothalamic sources of GABAergic projections to the parvocellular neurons in the paraventricular nucleus (PVH) from normal, sham-surgery and adrenalectomized male rats subjected to intraperitoneal injections of saline or dexamethasone. Blood samples were collected to measure corticosterone by radioimmunoassay. The distribution of GR, GABA-positive neurons and co-localization of GR in GABA-positive neurons were analyzed by immunofluorescence in sections from the paraventricular nucleus (PVH). In intact and sham rats, dexamethasone induced expression of GABAergic neurons in the regions of the anterior periventricular nucleus (PVa) coincident with anterior (PVHap) and medial (PVHmp) parvocellular subdivisions of thePVH. However, the co-expression of GR in GABAergic neurons was found only in the region of the PVa coincident with PVHmp. These findings confirm that glucocorticoids may directly act on GABAergic neurons through GR. PVHap and PVHmp present differentiated patterns of GABA and GR expression between then. The co-localization of GR in GABA-positive neurons in the region of the PVa coincident with PVHmp demonstrates a critic importance of this region to control the hypothalamus-pituitary-adrenal axis through GABAergic mediation.

Glycogen synthase kinase-3β is involved in ligand-dependent activation of transcription and cellular localization of the glucocorticoid receptor

Glucocorticoids (GC) induce cell cycle arrest and apoptosis in different cell types and therefore are widely used to treat a variety of diseases including autoimmune disorders and cancer. This effect is mediated by the GC receptor (GR), a ligand-activated transcription factor that translocates into the nucleus where it modulates transcription of target genes in a promoter-specific manner. Glycogen synthase kinase-3 (GSK3) regulates GR response by genomic and nongenomic mechanisms, although the specific role of each isoform is not well defined. We used GSK3 pharmacological inhibitors and isoform-specific small interfering RNA to evaluate the role of GSK3 in the genomic regulation induced by GC. GSK3 inhibition resulted in the reduction of GC-induced mRNA expression of GC-induced genes such as BIM, HIAP1, and GILZ. Knockdown of GSK3β but not GSK3α reduced endogenous GILZ induction in response to dexamethasone and GR-dependent reporter gene activity. Chromatin immunoprecipitation experiments revealed that GSK3 inhibition impaired the dexamethasone-mediated binding of GR and RNA polymerase II to endogenous GILZ promoter. These results indicate that GSK3β is important for GR transactivation activity and that GSK3β inhibition suppresses GC-stimulated gene expression. Furthermore, we show that genomic regulation by the GR is independent of known GSK3β phosphorylation sites. We propose that GC-dependent transcriptional activation requires functional GSK3β signaling and that altered GSK3β activity influences cell response to GC.

Molecular dynamics of ultradian glucocorticoid receptor action

In recent years it has become evident that glucocorticoid receptor (GR) action in the nucleus is highly dynamic, characterized by a rapid exchange at the chromatin template. This stochastic mode of GR action couples perfectly with a deterministic pulsatile availability of endogenous ligand in vivo. The endogenous glucocorticoid hormone (cortisol in man and corticosterone in rodent) is secreted from the adrenal gland with an ultradian rhythm made up of pulses at approximately hourly intervals. These two components - the rapidly fluctuating ligand and the rapidly exchanging receptor - appear to have evolved to establish and maintain a system that is exquisitely responsive to the physiological demands of the organism. In this review, we discuss recent and innovative work that questions the idea of steady state, static hormone receptor responses, and replaces them with new concepts of stochastic mechanisms and oscillatory activity essential for optimal function in molecular and cellular systems.

Glucocorticoid receptor mediated suppression of natural killer cell activity: identification of associated deacetylase and corepressor molecules

Physical and psychological stressors reduce natural killer cell function. This reduction in cellular function results from stress-induced release of glucocorticoids. Glucocorticoids act upon natural killer cells to deacetylate and transrepress immune response genes through epigenetic processes. However, other than the glucocorticoid receptor, the proteins that participate in this process are not well described in natural killer cells. The purpose of this study was to identify the proteins associated with the glucocorticoid receptor that are likely epigenetic participants in this process. Treatment of natural killer cells with the synthetic glucocorticoid, dexamethasone, produced a significant time dependent reduction in natural killer cell activity as early as 8h post treatment. This reduction in natural killer cell activity was preceded by nuclear localization of the glucocorticoid receptor with histone deacetylase 1 and the corepressor, SMRT. Other class I histone deacetylases were not associated with the glucocorticoid receptor nor was the corepressor NCoR. These results demonstrate histone deacetylase 1 and SMRT to associate with the ligand activated glucocorticoid receptor within the nuclei of natural killer cells and to be the likely participants in the histone deacetylation and transrepression that accompanies glucocorticoid mediated reductions in natural killer cell function.

On the trail of the glucocorticoid receptor: into the nucleus and back

The glucocorticoid receptor (GR) belongs to the superfamily of steroid receptors and is an important regulator of physiological and metabolic processes. In its inactive state, GR is unbound by ligand and resides in the cytoplasm in a chaperone complex. When it binds glucocorticoids, it is activated and translocates to the nucleus, where it functions as a transcription factor. However, the subcellular localization of GR is determined by the balance between its rates of nuclear import and export. The mechanism of GR nuclear transport has been extensively studied. Originally, it was believed that nuclear import of GR is initiated by dissociation of the chaperone complex in the cytoplasm. However, several studies show that the chaperone machinery is required for nuclear transport of GR. In this review, we summarize the contribution of various chaperone components involved in the nuclear transport of GR and propose an updated model of its nuclear import and export. Moreover, we review the importance of ligand-independent nuclear transport and compare the nuclear transport of GR with that of other steroid receptors.

Glucocorticoid receptor mutants: man-made tools for functional research

The glucocorticoid receptor (GR) is a ligand-dependent transcription factor that can bind to glucocorticoids (GCs). Upon ligand binding, GR sheds its cytoplasmic chaperoning complex and translocates to the nucleus, where it can act as a ligand-dependent transcription factor, transactivating or transrepressing specific gene promoters. Often, GR interacts with specific cofactors to implement a variety of gene promoter effects. GR activity and function is further modulated by post-translational modifications. To assess the diverse aspects of GR mechanisms of activation and gene regulation, researchers continue to use a range of artificial GR mutants. In this review we analyze the characteristics of GR mutants with the aim of assisting the design and interpretation of GR mutant-based experiments.

Cell cycle phase regulates glucocorticoid receptor function

The glucocorticoid receptor (GR) is a member of the nuclear hormone receptor superfamily of ligand-activated transcription factors. In contrast to many other nuclear receptors, GR is thought to be exclusively cytoplasmic in quiescent cells, and only translocate to the nucleus on ligand binding. We now demonstrate significant nuclear GR in the absence of ligand, which requires nuclear localisation signal 1 (NLS1). Live cell imaging reveals dramatic GR import into the nucleus through interphase and rapid exclusion of the GR from the nucleus at the onset of mitosis, which persists into early G(1). This suggests that the heterogeneity in GR distribution is reflective of cell cycle phase. The impact of cell cycle-driven GR trafficking on a panel of glucocorticoid actions was profiled. In G2/M-enriched cells there was marked prolongation of glucocorticoid-induced ERK activation. This was accompanied by DNA template-specific, ligand-independent GR transactivation. Using chimeric and domain-deleted receptors we demonstrate that this transactivation effect is mediated by the AF1 transactivation domain. AF-1 harbours multiple phosphorylation sites, which are consensus sequences for kinases including CDKs, whose activity changes during the cell cycle. In G2/M there was clear ligand independent induction of GR phosphorylation on residues 203 and 211, both of which are phosphorylated after ligand activation. Ligand-independent transactivation required induction of phospho-S211GR but not S203GR, thereby directly linking cell cycle driven GR modification with altered GR function. Cell cycle phase therefore regulates GR localisation and post-translational modification which selectively impacts GR activity. This suggests that cell cycle phase is an important determinant in the cellular response to Gc, and that mitotic index contributes to tissue Gc sensitivity.

Glucocorticoid receptor alpha isoform-selective regulation of antiapoptotic genes in osteosarcoma cells: a new mechanism for glucocorticoid resistance

Glucocorticoids regulate a variety of physiological processes and are commonly used to treat disorders of inflammation, autoimmune diseases, and cancer. Glucocorticoid action is predominantly mediated through the classic glucocorticoid receptor (GR)α isoform. Recent data suggest that the mature GRα mRNA is translated into multiple N-terminal isoforms that have distinct biochemical properties and gene regulatory profiles. Interestingly, osteosarcoma cells stably expressing the GRα-D translational isoform are unique in that they are resistant to glucocorticoid-induced apoptosis. In this study, we investigate whether GRα isoform-specific differences in the regulation of antiapoptotic genes contribute to this resistant phenotype. We now show that GRα-D, unlike the other receptor isoforms, does not inhibit the activity of a nuclear factor κB (NF-κB)-responsive reporter gene and does not efficiently repress either the transcription or protein production of the antiapoptotic genes Bcl-xL, cellular inhibitor of apoptosis protein 1, and survivin. The inability of GRα-D to down-regulate the expression of these genes appears to be associated with a diminished interaction between GRα-D and NF-κB that is observed in cells, but not in vitro, and likely reflects the sequestration of GRα-D in the nucleus. Deletion of the GRα N-terminal amino acids 98-335 also results in a nuclear resident GR, which fails to interact with NF-κB in cells and promote apoptosis in response to glucocorticoids. These data suggest that the N-terminal translational isoforms of GRα selectively regulate antiapoptotic genes and that the GRα-D isoform may contribute to the resistance of certain cancer cells to glucocorticoid-induced apoptosis.

Selective mutations in estrogen receptor alpha D-domain alters nuclear translocation and non-estrogen response element gene regulatory mechanisms

The three main mechanisms of ERα action are: 1) nuclear, genomic, direct DNA binding, 2) nuclear, genomic, "tethered"-mediated, protein-protein interactions, and 3) non-nuclear, non-genomic, rapid action responses. Reports suggest the D-domain or hinge region of ERα plays an important role in mechanisms 1 and 2 above. Studies demonstrating the functionality of the ERα hinge region have resected the full D-domain; therefore, site directed mutations were made to attribute precise sequence functionality to this domain. This study focuses on the characterization and properties of three novel site directed ERα- D-domain mutants. The Hinge 1 (H1) ERα mutant has disrupted nuclear localization, can no longer perform tethered mediated responses and has lost interaction with c-Jun, but retains estrogen response element (ERE)-mediated functions as demonstrated by confocal microscopy, reporter assays, endogenous gene expression and co-immunoprecipitation. The H2 ERα mutant is non-nuclear, but translocates to the nucleus with estradiol (E2) treatment and maintains ERE-mediated functionality. The H2+NES ERα mutant does not maintain nuclear translocation with hormone binding, no longer activates ERE-target genes, functions in ERE- or tethered-mediated luciferase assays, but does retain the non-genomic, non-nuclear, rapid action response. These studies reveal the sequence(s) in the ERα hinge region that are involved in tethered-mediated actions as well as nuclear localization and attribute important functionality to this region of the receptor. In addition, the properties of these ERα mutants will allow future studies to further dissect and characterize the three main ERα mechanisms of action and determine the mechanistic role each action has in estrogen hormone regulation.

Orphan nuclear receptors in breast cancer pathogenesis and therapeutic response

Nuclear receptors comprise a large family of highly conserved transcription factors that regulate many key processes in normal and neoplastic tissues. Most nuclear receptors share a common, highly conserved domain structure that includes a carboxy-terminal ligand-binding domain. However, a subgroup of this gene family is known as the orphan nuclear receptors because to date there are no known natural ligands that regulate their activity. Many of the 25 nuclear receptors classified as orphan play critical roles in embryonic development, metabolism, and the regulation of circadian rhythm. Here, we review the emerging role(s) of orphan nuclear receptors in breast cancer, with a particular focus on two of the estrogen-related receptors (ERRalpha and ERRgamma) and several others implicated in clinical outcome and response or resistance to cytotoxic or endocrine therapies, including the chicken ovalbumin upstream promoter transcription factors, nerve growth factor-induced B, DAX-1, liver receptor homolog-1, and retinoic acid-related orphan receptor alpha. We also propose that a clearer understanding of the function of orphan nuclear receptors in mammary gland development and normal mammary tissues could significantly improve our ability to diagnose, treat, and prevent breast cancer.

Abrogation of glucocorticoid receptor dimerization correlates with dissociated glucocorticoid behavior of compound a

Compound A (CpdA), a dissociated glucocorticoid receptor modulator, decreases corticosteroid-binding globulin (CBG), adrenocorticotropic hormone (ACTH), and luteneinizing hormone levels in rats. Whether this is due to transcriptional regulation by CpdA is not known. Using promoter reporter assays we show that CpdA, like dexamethasone (Dex), directly transrepresses these genes. Results using a rat Cbg proximal-promoter reporter construct in BWTG3 and HepG2 cell lines support a glucocorticoid receptor (GR)-dependent transrepression mechanism for CpdA. However, CpdA, unlike Dex, does not result in transactivation via glucocorticoid-responsive elements within a promoter reporter construct even when GR is co-transfected. The inability of CpdA to result in transactivation via glucocorticoid-responsive elements is confirmed on the endogenous tyrosine aminotransferase gene, whereas transrepression ability is confirmed on the endogenous CBG gene. Consistent with a role for CpdA in modulating GR activity, whole cell binding assays revealed that CpdA binds reversibly to the GR, but with lower affinity than Dex, and influences association of [(3)H]Dex, but has no effect on dissociation. In addition, like Dex, CpdA causes nuclear translocation of the GR, albeit to a lesser degree. Several lines of evidence, including fluorescence resonance energy transfer, co-immunoprecipitation, and nuclear immunofluorescence studies of nuclear localization-deficient GR show that CpdA, unlike Dex, does not elicit ligand-induced GR dimerization. Comparison of the behavior of CpdA in the presence of wild type GR to that of Dex with a dimerization-deficient GR mutant (GR(dim)) strongly supports the conclusion that loss of dimerization is responsible for the dissociated behavior of CpdA.

The human glucocorticoid receptor: molecular basis of biologic function

The characterization of the subfamily of steroid hormone receptors has enhanced our understanding of how a set of hormonally derived lipophilic ligands controls cellular and molecular functions to influence development and help achieve homeostasis. The glucocorticoid receptor (GR), the first member of this subfamily, is a ubiquitously expressed intracellular protein, which functions as a ligand-dependent transcription factor that regulates the expression of glucocorticoid-responsive genes. The effector domains of the GR mediate transcriptional activation by recruiting coregulatory multi-subunit complexes that remodel chromatin, target initiation sites, and stabilize the RNA-polymerase II machinery for repeated rounds of transcription of target genes. This review summarizes the basic aspects of the structure and actions of the human (h) GR, and the molecular basis of its biologic functions.

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.

Insulin sensitization of human preadipocytes through glucocorticoid hormone induction of forkhead transcription factors

Glucocorticoids are synthesized locally in adipose tissue and contribute to metabolic disease through the facilitation of adipose tissue expansion. Here we report that exposure of human primary preadipocytes to glucocorticoids increases their sensitivity to insulin and enhances their subsequent response to stimuli that promote differentiation. This effect was observed in primary human preadipocytes but not in immortalized 3T3-L1 murine preadipocytes or in fully differentiated primary human adipocytes. Stimulation of insulin signaling was mediated through induction of insulin receptor (IR), IR substrate protein 1 (IRS1), IRS2, and the p85 regulatory subunit of phosphoinositide-3-3-kinase, which led to enhanced insulin-mediated activation of Akt. Although induction of IRS2 was direct, induction of IR and IRS1 by glucocorticoids occurred subsequent to primary induction of the forkhead family transcription factors FoxO1A and FoxO3A. These results reveal a new role for glucocorticoids in preparing preadipocytes for differentiation.

Suppression of GATA-3 nuclear import and phosphorylation: a novel mechanism of corticosteroid action in allergic disease

BACKGROUND

GATA-3 plays a critical role in regulating the expression of the cytokines interleukin (IL)-4, IL-5, and IL-13 from T helper-2 (Th2) cells and therefore is a key mediator of allergic diseases. Corticosteroids are highly effective in suppressing allergic inflammation, but their effects on GATA-3 are unknown. We investigated the effect of the corticosteroid fluticasone propionate on GATA-3 regulation in human T-lymphocytes in vitro and in vivo.

METHODS AND FINDINGS

In a T lymphocyte cell line (HuT-78) and peripheral blood mononuclear cells stimulated by anti-CD3 and anti-CD28 in vitro we demonstrated that fluticasone inhibits nuclear translocation of GATA-3 and expression of Th2 cytokines via a mechanism independent of nuclear factor-kappaB and is due, in part, to competition between GATA-3 and the ligand-activated glucocorticoid receptor for nuclear transport through the nuclear importer importin-alpha. In addition, fluticasone induces the expression of mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1), the endogenous inhibitor of p38 MAPK, which is necessary for GATA-3 nuclear translocation. These inhibitory effects of fluticasone are rapid, potent, and prolonged. We also demonstrated that inhaled fluticasone inhibits GATA-3 nuclear translocation in peripheral blood lymphocytes of patients with asthma in vivo.

CONCLUSIONS

Corticosteroids have a potent inhibitory effect on GATA-3 via two interacting mechanisms that potently suppress Th2 cytokine expression. This novel mechanism of action of corticosteroids may account for the striking clinical efficacy of corticosteroids in the treatment of allergic diseases.

Glucocorticoid receptor plays an indispensable role in mineralocorticoid receptor-dependent transcription in GR-deficient BE(2)C and T84 cells in vitro

The mineralocorticoid receptor (MR) plays an important functional role in the central nervous system; however, the molecular mechanism of MR-dependent gene expression is not entirely clear. In this study, we examined the MR-dependent transcriptional regulation using a human neuronal cell line BE(2)C and an MR/GR-dependent reporter gene (HRE-luciferase) in vitro. Western blot analysis revealed that the cell line expresses MR but not glucocorticoid receptor (GR). In this experimental condition, unexpectedly, the MR-specific ligand aldosterone did not induce HRE-dependent transcription in a native or MR-overexpressed condition, whereas significant transcriptional induction by aldosterone was observed when the GR was co-expressed. The effect of aldosterone was completely inhibited by the MR antagonist spironolactone, indicating an MR-dependent effect. We found similar results in T84 colonic cells expressing neither MR nor GR, such that the aldosterone effect was obtained only when both receptors were co-expressed. The co-operative effect of GR was not obvious with the dimer-deficient mutant GR. Finally, the above findings were reproducible with different promoters containing HRE such as ENaC and MMTV. These results suggest that GR plays an indispensable role in MR-dependent transcription, possibly by forming a MR/GR heterodimer or by acting as a co-activator of MR/MR homodimer.

Minireview: latest perspectives on antiinflammatory actions of glucocorticoids

Taking into consideration that glucocorticoid (GC) hormones have been used clinically for over half a century and that more than 20 yr have passed since the cloning of the GC receptor (GR), it is hard to imagine that novel aspects in the molecular mechanism by which GCs mediate their antiinflammatory actions are still being unveiled today. Partly, this is because almost on a daily basis, novel insights arise from parallel fields, e.g. nuclear receptor cofactor and chromatin regulation and their concomitant impact on gene transcription events, eventually leading to a revisitation or refinement of old hypotheses. On the other hand, it does remain striking and puzzling why GCs use different mechanisms in so many different cell types and on many different target genes to elicit an antiinflammatory effect. Meanwhile, the obvious question for the clinic remains: is the separation of GR functionalities through differential ligand design the strategy of choice to avoid most GC-mediated side effects? This minireview aims to highlight some of the latest findings on aspects of the antiinflammatory working mechanisms of GCs.

The A/B domain of the teleost glucocorticoid receptors influences partial nuclear localization in the absence of hormone

The glucocorticoid (GR) and mineralocorticoid receptor (MR) of extant jawed vertebrates emerged after duplication of an ancestral corticosteroid receptor. The ancestral corticosteroid receptor resembled extant MRs in hormone selectivity, and the different ligand specificity of extant GRs is a secondary derived characteristic. An additional characteristic that distinguishes the mammalian GR from the MR is the cellular distribution pattern in the absence of hormone: the naïve GR resides in the cytoplasm, whereas the naïve MR is found in both the nucleus and cytoplasm. Our results show, by the use of green fluorescent protein-tagged fusion proteins, that the GRs [rainbow trout (rt) GR1 and rtGR2] from a lower vertebrate, the teleost fish, rainbow trout (Oncorhynchus mykiss) resemble mammalian MR rather than GR in their subcellular localization pattern. The addition of cortisol caused the remaining cytoplasmic rtGR1 and rtGR2 to migrate to the nucleus. The speed of nuclear localization was cortisol concentration dependent, with rtGR2 being more sensitive than rtGR1, mimicking the transactivational properties of the receptors in which the cortisol EC50 value is an order of magnitude lower for rtGR2. By the use of chimera constructs between the trout GRs and the rat GR C656G, we show that the E domain of the trout receptors are not involved in the nucleocytoplasmic localization of naïve trout GRs, but the A/B domain, especially if linked to the corresponding trout CD region, plays a pivotal role in the cellular distribution pattern. This is unrelated to the difference in the trout GRs transactivation sensitivity, which is determined by the receptor's E-domains.

Altered subcellular distribution of MSK1 induced by glucocorticoids contributes to NF-kappaB inhibition

Glucocorticoids are widely used anti-inflammatory and immunomodulatory agents, of which the action mechanism is mainly based on interference of hormone-activated glucocorticoid receptor (GR) with the activity of transcription factors, such as nuclear factor-kappaB (NF-kappaB). In addition to the well described interaction-based mutual repression mechanism between the GR and NF-kappaB, additional mechanisms are at play, which help to explain the efficacy of glucocorticoid-mediated gene repression. In this respect, we found that glucocorticoids counteract the recruitment of activated Mitogen- and Stress-activated protein Kinase-1 (MSK1) at inflammatory gene promoters resulting in the inhibition of NF-kappaB p65 transactivation and of concurrent histone H3 phosphorylation. Additionally, we observed that activated GR can trigger redistribution of nuclear MSK1 to the cytoplasm through a CRM1-dependent export mechanism, as a result of an interaction between liganded GR and activated MSK1. These findings unveil a novel aspect within the GR-mediated NF-kappaB-targeting anti-inflammatory mechanism.

Site-specific androgen receptor serine phosphorylation linked to epidermal growth factor-dependent growth of castration-recurrent prostate cancer

The androgen receptor (AR) is required for prostate cancer development and contributes to tumor progression after remission in response to androgen deprivation therapy. Epidermal growth factor (EGF) increases AR transcriptional activity at low levels of androgen in the CWR-R1 prostate cancer cell line derived from the castration-recurrent CWR22 prostate cancer xenograft. Here we report that knockdown of AR decreases EGF stimulation of prostate cancer cell growth and demonstrate a mechanistic link between EGF and AR signaling. The EGF-induced increase in AR transcriptional activity is dependent on phosphorylation at mitogen-activated protein kinase consensus site Ser-515 in the AR NH(2)-terminal region and at protein kinase C consensus site Ser-578 in the AR DNA binding domain. Phosphorylation at these sites alters the nuclear-cytoplasmic shuttling of AR and AR interaction with the Ku-70/80 regulatory subunits of DNA-dependent protein kinase. Abolishing AR Ser-578 phosphorylation by introducing an S578A mutation eliminates the AR transcriptional response to EGF and increases both AR binding of Ku-70/80 and nuclear retention of AR in association with hyperphosphorylation of AR Ser-515. The results support a model in which AR transcriptional activity increases castration-recurrent prostate cancer cell growth in response to EGF by site-specific serine phosphorylation that regulates nuclear-cytoplasmic shuttling through interactions with the Ku-70/80 regulatory complex.

Role of the hinge region of glucocorticoid receptor for HEXIM1-mediated transcriptional repression

We previously reported that HEXIM1 (hexamethylene bisacetamide-inducible protein 1), which suppresses transcription elongation via sequestration of positive transcription elongation factor b (P-TEFb) using 7SK RNA as a scaffold, directly associates with glucocorticoid receptor (GR) to suppress glucocorticoid-inducible gene activation. Here, we revealed that the hinge region of GR is essential for its interaction with HEXIM1, and that oxosteroid receptors including GR show sequence homology in their hinge region and interact with HEXIM1, whereas the other members of nuclear receptors do not. We also showed that HEXIM1 suppresses GR-mediated transcription in two ways: sequestration of P-TEFb by HEXIM1 and direct interaction between GR and HEXIM1. In contrast, peroxisome proliferator-activated receptor gamma-dependent gene expression is negatively modulated by HEXIM1 solely via sequestration of P-TEFb. We, therefore, conclude that HEXIM1 may act as a gene-selective transcriptional regulator via direct interaction with certain transcriptional regulators including GR and contribute to fine-tuning of, for example, glucocorticoid-mediated biological responses.

Structural basis for the nuclear import of the human androgen receptor

Ligand-dependent nuclear import is crucial for the function of the androgen receptor (AR) in both health and disease. The unliganded AR is retained in the cytoplasm but, on binding 5alpha-dihydrotestosterone, it translocates into the nucleus and alters transcription of its target genes. Nuclear import of AR is mediated by the nuclear import factor importin-alpha, which functions as a receptor that recognises and binds to specific nuclear localisation signal (NLS) motifs on cargo proteins. We show here that the AR binds to importin-alpha directly, albeit more weakly than the NLS of SV40 or nucleoplasmin. We describe the 2.6-angstroms-resolution crystal structure of the importin-alpha-AR-NLS complex, and show that the AR binds to the major NLS-binding site on importin-alpha in a manner different from most other NLSs. Finally, we have shown that pathological mutations within the NLS of AR that are associated with prostate cancer and androgen-insensitivity syndrome reduce the binding affinity to importin-alpha and, subsequently, retard nuclear import; surprisingly, however, the transcriptional activity of these mutants varies widely. Thus, in addition to its function in the nuclear import of AR, the NLS in the hinge region of AR has a separate, quite distinct role on transactivation, which becomes apparent once nuclear import has been achieved.