Localization of the glucocorticoid receptor in rat liver cells: evidence for plasma membrane bound receptor

The Interface of Nuclear and Membrane Steroid Signaling

Steroid hormones bind receptors in the cell nucleus and in the cell membrane. The most widely studied class of steroid hormone receptors are the nuclear receptors, named for their function as ligand-dependent transcription factors in the cell nucleus. Nuclear receptors, such as estrogen receptor alpha, can also be anchored to the plasma membrane, where they respond to steroids by activating signaling pathways independent of their function as transcription factors. Steroids can also bind integral membrane proteins, such as the G protein-coupled estrogen receptor. Membrane estrogen and progestin receptors have been cloned and characterized in vitro and influence the development and function of many organ systems. Membrane androgen receptors were cloned and characterized in vitro, but their function as androgen receptors in vivo is unresolved. We review the identity and function of membrane proteins that bind estrogens, progestins, and androgens. We discuss evidence that membrane glucocorticoid and mineralocorticoid receptors exist, and whether glucocorticoid and mineralocorticoid nuclear receptors act at the cell membrane. In many cases, integral membrane steroid receptors act independently of nuclear steroid receptors, even though they may share a ligand.

Nongenomic glucocorticoid effects and their mechanisms of action in vertebrates

Glucocorticoids (GC) act on multiple organ systems to regulate a variety of physiological processes in vertebrates. Due to their immunosuppressive and anti-inflammatory actions, glucocorticoids are an attractive target for pharmaceutical development. Accordingly, they are one of the most widely prescribed classes of therapeutics. Through the classical mechanism of steroid action, glucocorticoids are thought to mainly affect gene transcription, both in a stimulatory and suppressive fashion, regulating de novo protein synthesis that subsequently leads to the physiological response. However, over the past three decades multiple lines of evidence demonstrate that glucocorticoids may work through rapid, nonclassical mechanisms that do not require alterations in gene transcription or translation. This review assimilates evidence across the vertebrate taxa on the diversity of nongenomic actions of glucocorticoids and the membrane-associated cellular mechanisms that may underlie rapid glucocorticoid responses to include potential binding sites characterized to date.

Further evidence for a membrane receptor that binds glucocorticoids in the rodent hypothalamus

In parallel with their well-characterized delayed genomic effects, steroid hormones exhibit rapid, non-genomic effects at molecular, cellular and behavioral levels. We have proposed a model of rapid, non-genomic glucocorticoid inhibition of hypothalamic neuroendocrine cells through a putative membrane-associated glucocorticoid receptor (GR). Here we tested for plasma membrane GR immunoreactivity and binding in the hypothalamic supraoptic and paraventricular nuclei. Selective cross-linking of membrane proteins with membrane-impermeant BS³ and subsequent Western blot analysis with a monoclonal GR antibody revealed a reduction in the intensities of a ∼98kDa immunoreactive band and a ∼64kDa band in the rat paraventricular and supraoptic nuclei, and of a 64kDa band in hippocampal tissue, which suggested that these proteins are associated with the membrane. Saturation binding of [³H]-corticosterone and [³H]-dexamethasone in rat and mouse hypothalamic tissue revealed a K_d 4-24-fold lower and a B_max 4-7-fold lower for the membrane-associated GR compared to the intracellular GR, suggesting a lower affinity and abundance of the glucocorticoid binding sites in the membrane than in the cytosol. Together, these findings suggest the presence of a low-affinity, low-abundance membrane-associated GR in the hypothalamus that shares homology with the intracellular GR, and are consistent with physiological evidence of rapid, non-genomic glucocorticoid actions in hypothalamic neuroendocrine cells that are GR dependent.

Membrane glucocorticoid receptor activation induces proteomic changes aligning with classical glucocorticoid effects

Glucocorticoids exert rapid nongenomic effects by several mechanisms including the activation of a membrane-bound glucocorticoid receptor (mGR). Here, we report the first proteomic study on the effects of mGR activation by BSA-conjugated cortisol (Cort-BSA). A subset of target proteins in the proteomic data set was validated by Western blot and we found them responding to mGR activation by BSA-conjugated cortisol in three additional cell lines, indicating a conserved effect in cells originating from different tissues. Changes in the proteome of BSA-conjugated cortisol treated CCRF-CEM leukemia cells were associated with early and rapid pro-apoptotic, immune-modulatory and metabolic effects aligning with and possibly "priming" classical activities of the cytosolic glucocorticoid receptor (cGR). PCR arrays investigating target genes of the major signaling pathways indicated that the mGR does not exert its effects through the transcriptional activity of any of the most common kinases in these leukemic cells, but RhoA signaling emerged from our pathway analysis. All cell lines tested displayed very low levels of mGR on their surface. Highly sensitive and specific in situ proximity ligation assay visualized low numbers of mGR even in cells previously thought to be mGR negative. We obtained similar results when using three distinct anti-GR monoclonal antibodies directed against the N-terminal half of the cGR. This strongly suggests that the mGR and the cGR have a high sequence homology and most probably originate from the same gene. Furthermore, the mGR appears to reside in caveolae and its association with caveolin-1 (Cav-1) was clearly detected in two of the four cell lines investigated using double recognition proximity ligation assay. Our results indicate however that Cav-1 is not necessary for membrane localization of the GR since CCRF-CEM and Jurkat cells have a functional mGR, but did not express this caveolar protein. However, if expressed, this membrane protein dimerizes with the mGR modulating its function.

Mechanisms regulating the susceptibility of hematopoietic malignancies to glucocorticoid-induced apoptosis

Glucocorticoids (GCs) are commonly used in the treatment of hematopoietic malignancies owing to their ability to induce apoptosis of these cancerous cells. Whereas some types of lymphoma and leukemia respond well to this drug, others are resistant. Also, GC-resistance gradually develops upon repeated treatments ultimately leading to refractory relapsed disease. Understanding the mechanisms regulating GC-induced apoptosis is therefore uttermost important for designing novel treatment strategies that overcome GC-resistance. This review discusses updated data describing the complex regulation of the cell's susceptibility to apoptosis triggered by GCs. We address both the genomic and nongenomic effects involved in promoting the apoptotic signals as well as the resistance mechanisms opposing these signals. Eventually we address potential strategies of clinical relevance that sensitize GC-resistant lymphoma and leukemia cells to this drug. The major target is the nongenomic signal transduction machinery where the interplay between protein kinases determines the cell fate. Shifting the balance of the kinome towards a state where Glycogen synthase kinase 3alpha (GSK3alpha) is kept active, favors an apoptotic response. Accumulating data show that it is possible to therapeutically modulate GC-resistance in patients, thereby improving the response to GC therapy.

Glucocorticoid receptor (DlGR1) is expressed in pre-larval and larval stages of the teleost fish Dicentrarchus labrax

Glucocorticoid hormone receptors (GR), members of the nuclear hormone receptor superfamily, are ligand-dependent transcription factors expressed in various tissues by binding to specific DNA sequences. Since glucocorticoids have a role in maintaining the homeostatic status in fish, we previously cloned and sequenced a GR (DlGR1) of adult Dicentrarchus labrax; we also showed mRNA expression (in situ hybridization) and tissue immunohistochemical localization of DlGR1 in several organs. This work has now been extended to the examination of the expression, tissue distribution, and cytolocalization of DlGR1 in larval developmental stages by similar methods to those used for the adult organs. The riboprobe included the DlGR1 cDNA transcriptional activation domain (1.0–1,300 nucleotide sequence) showing no significant similarity with a known second GR cDNA sequence of sea bass. The antibody was specific for an opportunely selected peptide sequence of the DlGR1 transcriptional domain. In histological sections of brain, head kidney, gills, liver, anterior intestine, and spleen cells, the riboprobe was mainly located in the cell nucleus. The antibody identified DlGR1 in the head kidney, gills, liver, and anterior intestine, mainly located in the cytosol. These results are in agreement with the receptor location in adult tissues. The greater presence of both the transcript and protein of DlGR1 in the late developmental stages suggests an increasing expression of this receptor. The cytolocalization (nuclear-cytosolic) and presumptive roles of DlGR1-containing tissues are discussed.

The effects of non-genomic glucocorticoid mechanisms on bodily functions and the central neural system. A critical evaluation of findings

Mounting evidence suggests that--beyond the well-known genomic effects--glucocorticoids affect cell function via non-genomic mechanisms. Such mechanisms operate in many major systems and organs including the cardiovascular, immune, endocrine and nervous systems, smooth and skeletal muscles, liver, and fat cells. Non-genomic effects are exerted by direct actions on membrane lipids (affecting membrane fluidity), membrane proteins (e.g. ion channels and neurotransmitter receptors), and cytoplasmic proteins (e.g. MAPKs, phospholipases, protein kinases, etc.). These actions are mediated by the glucocorticoids per se or by the proteins dissociated from the liganded glucocorticoid receptor complex. The MR and GR also activate non-genomic mechanisms in certain cases. Some effects of glucocorticoids are shared by a variety of steroids, whereas others are more selective. Moreover, "ultra-selective" effects-mediated by certain glucocorticoids only-were also shown. Disparate findings suggest that non-genomic mechanisms also show "demand-specificity", i.e. require the coincidence of two or more processes. Some of the non-genomic mechanisms activated by glucocorticoids are therapeutically relevant; moreover, the "non-genomic specificity" of certain glucocorticoids raises the possibility of therapeutic applications. Despite the large body of evidence, however, the non-genomic mechanisms of glucocorticoids are still poorly understood. Criteria for differentiating genomic and non-genomic mechanisms are often loosely applied; interactions between various mechanisms are unknown, and non-genomic mechanism-specific pharmacological (potentially therapeutic) agents are lacking. Nevertheless, the discovery of non-genomic mechanisms is a major breakthrough in stress research, and further insights into these mechanisms may open novel approaches for the therapy of various diseases.

Glucocorticoid signaling: a nongenomic mechanism for T-cell immunosuppression

Glucocorticoids were long believed to exert their effects through transcriptional regulation of glucocorticoid-receptor target genes. However, there is accumulating evidence for nongenomic glucocorticoid-receptor-dependent modulation of signal transduction pathways. Here, we review rapid glucocorticoid activities and focus on a novel mechanism that underlies nongenomic glucocorticoid-induced immunosuppression in T cells. The findings demonstrate a physical and functional interaction between the glucocorticoid receptor and the T-cell receptor (TCR) complex. In its unligated state, the glucocorticoid receptor has an important role in TCR signaling but, after glucocorticoid-receptor-ligand binding (caused by short-term treatment with the synthetic glucocorticoid dexamethasone), the TCR complex is disrupted, leading to impaired TCR signaling. These data reveal a dichotomal functional role for glucocorticoid receptors: one in the cytosol as part of the TCR complex and the other as a nuclear regulator of gene transcription. Drugs that selectively target membrane-bound glucocorticoid receptors might represent a novel immunosuppressive approach.

Glucocorticoid receptors are involved in the regulation of pulsatile urea excretion in toadfish

The objectives of this study were to characterize the pattern of pulsatile urea excretion in the gulf toadfish in the wake of exogenous cortisol loading and to determine the receptors involved in the regulation of this mechanism. Toadfish were fitted with indwelling arterial catheters and were infused with isosmotic NaCl for 48 h after which fish were treated with cortisol alone, cortisol + peanut oil, cortisol + RU486 (a glucocorticoid receptor antagonist) or cortisol + spironolactone (a mineralocorticoid receptor antagonist). Upon cortisol loading, fish treated with cortisol alone, cortisol + oil or cortisol + spironolactone experienced a two- to threefold reduction in pulsatile urea excretion. This reduction was due to a decrease in urea pulse size with no effect on pulse frequency compared to values measured during the control NaCl infusion period. In addition, these fish showed an increase in plasma urea concentrations upon treatment. These apparent effects of cortisol treatment were abolished in fish treated with cortisol + RU486. In contrast, these fish showed an increase in pulsatile urea excretion mediated by a twofold increase in pulse size with no change in frequency. Likewise, fish treated with cortisol + RU486 showed a significant decrease in plasma urea concentrations over the course of the experiment. The findings of this study indicate that high levels of cortisol reduce pulsatile urea excretion by decreasing pulse size. In addition, it appears that glucocorticoid receptors and not mineralocorticoid receptors are involved in the regulation of the toadfish pulsatile urea excretion mechanism.

Corticosteroid receptors, 11 beta-hydroxysteroid dehydrogenase, and the heart

Mineralocorticoid and glucocorticoid hormones are known as corticosteroid hormones and are synthesized mainly in the adrenal cortex; however, more recently the enzymes involved in their synthesis have been found in a variety of cells and tissues, including the heart. The effects of these hormones are mediated via both cytoplasmic mineralocorticoid receptors (MRs) and glucocorticoid receptors (GRs), which act as ligand-inducible transcription factors. In addition, rapid, nongenomically mediated effects of these steroids can occur that may be via novel corticosteroid receptors. The lipophilic nature of these hormones allows them to pass freely through the cell membrane, although the intracellular concentration of mineralocorticoids and glucocorticoids is dependent on several cellular factors. The main regulators of intracellular glucocorticoid levels are 11 beta-hydroxysteroid dehydrogenase (11 beta HSD) isoforms. 11 beta HSD1 acts predominantly as a reductase in vivo, facilitating glucocorticoid action by converting circulating receptor-inactive 11-ketoglucocorticoids to active glucocorticoids. In contrast, 11 beta HSD 2 acts exclusively as an 11 beta-dehydrogenase and decreases intracellular glucocorticoids by converting them to their receptor-inactive 11-ketometabolites. Furthermore, P-glycoproteins, by actively pumping steroids out of cells, can selectively decrease steroids and local steroid synthesis can increase steroid concentrations. Receptor concentration, receptor modification, and receptor-protein interactions can also significantly impact on the corticosteroid response. This review details the receptors and possible mechanisms involved in both mediating and modulating corticosteroid responses. In addition, direct effects of corticosteroids on the heart are described including a discussion of the corticosteroid receptors and the mechanisms involved in mediating their effects.

Nongenomic steroid action: controversies, questions, and answers

Steroids may exert their action in living cells by several ways: 1). the well-known genomic pathway, involving hormone binding to cytosolic (classic) receptors and subsequent modulation of gene expression followed by protein synthesis. 2). Alternatively, pathways are operating that do not act on the genome, therefore indicating nongenomic action. Although it is comparatively easy to confirm the nongenomic nature of a particular phenomenon observed, e.g., by using inhibitors of transcription or translation, considerable controversy exists about the identity of receptors that mediate these responses. Many different approaches have been employed to answer this question, including pharmacology, knock-out animals, and numerous biochemical studies. Evidence is presented for and against both the participation of classic receptors, or proteins closely related to them, as well as for the involvement of yet poorly understood, novel membrane steroid receptors. In addition, clinical implications for a wide array of nongenomic steroid actions are outlined.

Effects of dexamethasone on K(+)-evoked glutamate release from rat hippocampal slices

Dexamethasone (DEX) at physiologically elevated (stress) concentration (1 microM) decreased K(+)-evoked glutamate release from rat hippocampal slices under superfusion in the presence of Ca2+. On the contrary 10 microM DEX increased this K(+)-evoked glutamate release while 0.1 microM DEX had no effect. The glucocorticoid antagonist for the "classic" receptor, RU 486, completely reversed the effect of 1 microM DEX. Actinomycin D had no effect. Dexamethasone at 1 microM had no effect on the Ca2(+)-independent (10 mM Mg2+ replacing 1 mM Ca2+) K(+)-evoked glutamate release. Dexamethasone at 1 microM or 10 microM had no effect on the phosphate-activated glutaminase--the key enzyme for the biosynthesis of neurotransmitter glutamate. These results suggest that the effect of DEX on K(+)-evoked glutamate release: (i) depends on its concentration; (ii) is exerted on the Ca2(+)-dependent (neurotransmitter release), at least at physiological stress concentrations; and (iii) is exerted via the classical receptor but is nongenomic.

Localization of the Glucocorticoid Receptor in Rat Brain Mitochondria

The distribution of glucocorticoid receptor in subcellular fractions of brain cortex and hippocampus, two regions rich in glucocorticoid receptor, has revealed its presence in nuclei, cytosol, mitochondria, synaptosomes, and synaptosomal mitochondria. The identification of glucocorticoid receptor has been accomplished both by Western blotting using antibodies recognizing the carboxy and the amino terminus of the glucocorticoid receptor and by immunogold electron microscopy using the same anti-glucocorticoid receptor antibodies. Antibody-glucocorticoid receptor interaction is abolished by preincubation of each antibody with its competing peptide. In addition to the intact 95-kDa glucocorticoid receptor in all fractions, lower molecular weight glucocorticoid receptor fragments have been also detected by Western blotting. The presence of glucocorticoid receptor in brain mitochondria supports the concept of a direct action of glucocorticoids on mitochondrial gene transcription, parallel to the established primary actions of the hormones on nuclear gene transcription, as a mechanism of coordinate regulation of respiratory enzyme biosynthesis by steroid hormones.

Nongenomic membrane actions of glucocorticoids in vertebrates

For decades, it was widely assumed that glucocorticoids (GCs) work solely through changes in gene expression to exert their physiological actions, a process that normally takes several hours to occur. However, recent evidence indicates that GCs might also act at the membrane through specific receptors to exert multiple rapid effects on various tissues and cells. GCs modulate hormone secretion, neuronal excitability, behavior, cell morphology, carbohydrate metabolism and other processes within seconds or minutes. These early actions occur independent of the genome and are transduced by the same biochemical effector pathways responsible for mediating rapid responses to neurotransmitters. The biological significance of most rapid GC effects are not well understood, but many might be related to the important functions that this hormone plays in modulating stress responses.

Glucocorticoid-recognizing and -effector sites in rat liver plasma membrane. Kinetics of corticosterone uptake by isolated membrane vesicles. III. Specificity and stereospecificity

In previous papers we provided evidence for a glucocorticoid (GC) responsive site in a highly purified rat liver plasma membrane (PM) fraction, which has proved to be osmotically active, 'right side-out' vesicles, free of CBG, glucocorticoid receptors (GR) and ATP (J. Steroid Biochem. Molec. Biol. 42 (1992) 737-756 and 757-771). This site, now called 'GC importer', mediates active transmembrane transport of corticosterone (B). Pronounced specificity, including stereo- and enantiomeric specificity, of ligand-GC importer interaction was demonstrated by competition assays using 54 different steroidal hormones and molecules. Important structural prerequisites for ligands with high specificity for the GC importer are plane C21-steroid hormones with 1-ene and/or 4-ene or 5alpha-reduced configuration, and/or OH-group(s) at C11beta>C17alpha>C21. Unexpectedly, other preferred ligands are C17alpha-ethynyl steroids like estrogens with an OH- or OCH3-group at C3 (EE2, mestranol) as well as progestins with C3-OH and 4-ene configuration (ethynodiol). C21-steroids with 11alpha-OH, 11-keto, 16alpha-CH3, 16beta-CH3, 16alpha-OH or 5beta-reduced configuration are low specificity ligands. The importer even displays different specificity for enantiomers (levonorgestrel>L-norgestrel). Altogether, the GC importer preferentially recognizes active GC and natural progestins which act as GC-antagonist (e.g. prednisolone>11beta-cortisol = B > or = progestins). Synthetic GC-agonists (e.g. dexamethasone, betamethasone, triamcinolone), most synthetic progestins, biologically inactive GC (e.g. 11alpha-cortisol, prednisone, cortisone, 11-dehydro-B), mineralocorticoids (aldosterone), natural estrogens (e.g. E1, E2, E3), DES and vitamin D3 derivatives do not interact with the GC importer. Osmotic shrinkage experiments revealed that interaction of high as well as low specificity ligands with the GC importer comprises reversible binding and transport through the PM. The ligand specificity profile of the GC importer and the GR exhibit pronounced differences, suggesting that both GC recognizing sites are different proteins. Performing immunoblotting, using specific mono- and polyclonal antibodies directed against the intracellular rat GR, of the PM pretreated with the membrane protein solubilizing detergent CHAPSO, we found that specific steroid binding to the PM site is not due to contamination with GR. Colchicine, daunorubicine, quinine, reserpine, verapamil and vinblastine, representatives of lipophilic xenobiotics which are known to be transported out of cells by the glycoprotein P170, did not compete with B for uptake into PM-vesicles, indicating that the GC importer is not a member of the ABC/mdr superfamily. The GC importer seems to be an additional link in the chain of steroid signal transduction and may be functionally involved in the action of natural GC-agonists and GC-antagonists.

The effect of adrenalectomy on stress-induced c-fos mRNA expression in the rat brain

Previously, we determined the pattern of stress-induced c-fos mRNA expression throughout the brain in order to gain further insight into the identification of the neural circuits mediating stress-induced regulation of the hypothalamic-pituitary-adrenal axis. In the present study, we determined if rapid effects of increased glucocorticoid levels after stress contribute to changes in c-fos mRNA expression. To this end, stress-induced c-fos expression was characterized in adrenalectomized (ADX) or adrenalectomized and corticosterone replaced (ADX/B) male rats. Animals were sacrificed 30 min post-onset of a 10 min swim stress, and in situ hybridization histochemistry was used to detect c-fos mRNA throughout the brain. The pattern of c-fos induction in the ADX and ADX/B animals was similar to that observed in the sham operated animals. Additionally, densitometric measurements were made to quantify the c-fos response in the paraventricular nucleus of the hypothalamus and the CA1/2 region of the hippocampus. We found that ADX did not alter the magnitude of the c-fos response to stress in these areas, but there was a slight dampening of the response in ADX/B animals. In sum, these results suggest that the pattern of c-fos expression observed 30 min post-stress is independent of stress-induced increases in circulating glucocorticoid concentrations.

Identification and localization of steroid-binding and nonsteroid-binding forms of the glucocorticoid receptor in the mouse P1798 lymphosarcoma

Glucocorticoid receptors (GCRs) were characterized in sublines of the mouse P1798 lymphosarcoma that are sensitive (S) or resistant (R) to glucocorticoid-mediated apoptosis. Previous work had identified two steroid-binding GCRs in S and R cells: a 97 kDa wild-type GCR in S cells (WT-GCR), and a 45 kDa truncated GCR in R cells (TR-GCR). A third GCR, a 97 kDa nonsteroid-binding GCR (NSB-GCR), was also identified in R cells. Using subcellular fractionation and Western blotting, we now show that in contrast to the WT-GCR which is localized in both the cytoplasm and nucleus of S cells, the NSB-GCR is localized predominantly in R cell nuclei. Moreover, gel filtration chromatography revealed that treatment with 400 mM NaCl and heat did not significantly alter the Stokes radius of the NSB-GCR suggesting that this receptor is not present in a heterooligomeric complex with other proteins. The TR-GCR was localized predominantly in the soluble cytoplasmic fraction but also in the crude membrane fractions of R cell nuclei, suggesting that this receptor is tightly associated with nuclear structures. It was not detected in the soluble nuclear fraction. Unexpectedly, a 45 kDa nonsteroid-binding immunoreactive protein was detected in crude membrane fractions of S cells. These studies describe a complex GCR system in the P1798 lymphosarcoma that necessitates a further consideration of glucocorticoid signaling in S and R cells.