A dynamic model of glucocorticoid receptor phosphorylation and cycling in intact cells

Multifaceted Control of GR Signaling and Its Impact on Hepatic Transcriptional Networks and Metabolism

Glucocorticoids (GCs) and the glucocorticoid receptor (GR) are important regulators of development, inflammation, stress response and metabolism, demonstrated in various diseases including Addison's disease, Cushing's syndrome and by the many side effects of prolonged clinical administration of GCs. These conditions include severe metabolic challenges in key metabolic organs like the liver. In the liver, GR is known to regulate the transcription of key enzymes in glucose and lipid metabolism and contribute to the regulation of circadian-expressed genes. Insights to the modes of GR regulation and the underlying functional mechanisms are key for understanding diseases and for the development of improved clinical uses of GCs. The activity and function of GR is regulated at numerous levels including ligand availability, interaction with heat shock protein (HSP) complexes, expression of GR isoforms and posttranslational modifications. Moreover, recent genomics studies show functional interaction with multiple transcription factors (TF) and coregulators in complex transcriptional networks controlling cell type-specific gene expression by GCs. In this review we describe the different regulatory steps important for GR activity and discuss how different TF interaction partners of GR selectively control hepatic gene transcription and metabolism.

Retinoic acid increases glucocorticoid receptor phosphorylation via cyclin-dependent kinase 5

Glucocorticoid receptor (GR) function is modulated by phosphorylation. As retinoic acid (RA) can activate some cytoplasmic kinases able to phosphorylate GR, we investigated whether RA could modulate GR phosphorylation in neuronal cells in a context of long-term glucocorticoid exposure. A 4-day treatment of dexamethasone (Dex) plus RA, showed that RA potentiated the (Dex)-induced phosphorylation on GR Serine 220 (_pSer220GR) in the nucleus of a hippocampal HT22 cell line. This treatment increased the cytoplasmic ratio of p35/p25 proteins, which are major CDK5 cofactors. Roscovitine, a pharmacological CDK5 inhibitor, or a siRNA against CDK5 prevented RA potentiation of GR phosphorylation. Furthermore, roscovitine counter-acted the effect of RA on GR sensitive target proteins such as BDNF or tissue-transglutaminase. These data help understanding the interaction between RA- and glucocorticoid-signalling pathways, both of which have strong influences on the adult brain.

Differential regulation of the transcriptional activity of the glucocorticoid receptor through site-specific phosphorylation

Post-translational modifications such as phosphorylation are known to play an important role in the gene regulation by the transcription factors including the nuclear hormone receptor superfamily of which the glucocorticoid receptor (GR) is a member. Protein phosphorylation often switches cellular activity from one state to another. Like many other transcription factors, the GR is a phosphoprotein, and phosphorylation plays an important role in the regulation of GR activity. Cell signaling pathways that regulate phosphorylation of the GR and its associated proteins are important determinants of GR function under various physiological conditions. While the role of many phosphorylation sites in the GR is still not fully understood, the role of others is clearer. Several aspects of transcription factor function, including DNA binding affinity, interaction of transactivation domains with the transcription initiation complex, and shuttling between the cytoplasmic compartments, have all been linked to site-specific phosphorylation. All major phosphorylation sites in the human GR are located in the N-terminal domain including the major transactivation domain, AF1. Available literature clearly indicates that many of these potential phosphorylation sites are substrates for multiple kinases, suggesting the potential for a very complex regulatory network. Phosphorylated GR interacts favorably with critical coregulatory proteins and subsequently enhances transcriptional activity. In addition, the activities and specificities of coregulators may be subject to similar regulation by phosphorylation. Regulation of the GR activity due to phosphorylation appears to be site-specific and dependent upon specific cell signaling cascade. Taken together, site-specific phosphorylation and related kinase pathways play an important role in the action of the GR, and more precise mechanistic information will lead to fuller understanding of the complex nature of gene regulation by the GR- and related transcription factors. This review provides currently available information regarding the role of GR phosphorylation in its action, and highlights the possible underlying mechanisms of action.

Modulation of glucocorticoid receptor function via phosphorylation

The glucocorticoid receptor (GR) is phosphorylated at multiple serine residues in a hormone-dependent manner. It has been suggested that GR phosphorylation affects turnover, subcellular trafficking, or the transcriptional regulatory functions of the receptor, yet the contribution of individual GR phosphorylation sites to the modulation of GR activity remains enigmatic. This review critically evaluates the literature on GR phosphorylation and presents more recent work on the mechanism of GR phosphorylation from studies using antibodies that recognize GR only when it is phosphorylated. In addition, we present support for the notion that GR phosphorylation modifies protein-protein interactions, which can stabilize the hypophosphorylated form of the receptor in the absence of ligand, as well as facilitate transcriptional activation by the hyperphosphorylation of GR via cofactor recruitment upon ligand binding. Finally, we propose that GR phosphorylation also participates in the nongenomic activation of cytoplasmic signaling pathways evoked by GR. Thus, GR phosphorylation is a versatile mechanism for modulating and integrating multiple receptor functions.

Modulation of glucocorticoid receptor transcriptional activation, phosphorylation, and growth inhibition by p27Kip1

The cyclin-dependent kinase inhibitor p27Kip1 is frequently inactivated in human cancers. Glucocorticoids, acting through the glucocorticoid receptor (GR), are frequently used to treat certain malignancies and are growth inhibitive, but the relationship between GR activity and p27 status has not been explored. We have therefore examined GR-dependent transcriptional activation, receptor phosphorylation, and glucocorticoid-dependent growth inhibition in p27-deficient (p27-/-) murine embryonic fibroblasts (MEFs). We find that GR transcriptional enhancement as well as receptor phosphorylation at two putative cyclin-dependent kinase sites are elevated in p27-/- MEFs, relative to control cells. This increased GR transcriptional activation appears to be mediated through the GR N terminus, and coexpression of the GR N-terminal coactivator, DRIP150, further enhanced GR-dependent transcriptional activation. Furthermore, p27-/- MEFs are partially resistant to the growth inhibitory effects of glucocorticoids. Thus, p27 appears to be an important element in the GR transcription and growth inhibitory responses.

Steroid receptor and molecular chaperone encounters in the nucleus

Steroid hormone receptors interact with several different molecular chaperones. DeFranco and Csermely discuss how the molecular chaperones p23 and Hsp90 may serve to regulate the activity of the ligand-bound steroid receptors within the nucleus. The authors hypothesize that these chaperone proteins may have a proactive role in promoting recycling of receptors once they have interacted with chromatin and in allowing rebinding of ligand once the receptors have been recycled.

Role of molecular chaperones in subnuclear trafficking of glucocorticoid receptors

The delivery of activated steroid receptors to high-affinity genomic sites must be efficient enough to account for the rapidity and selectivity of many transcriptional responses to steroid hormones. Thus, the signal transduction capacity of steroid hormone receptors will be influenced by the efficiency of receptor trafficking both between different subcellular compartments (that is, the cytoplasm and nucleus) and within a specific compartment (that is, the nucleus). Molecular chaperones, such as heat shock proteins, have long been recognized to play important roles in the management of protein folding in both stressed and nonstressed cells. In recent years, the participation of these proteins in various signal transduction pathways (for example, steroid hormone responses) has also been recognized. In this review, recent results that implicate a role for distinct heat shock proteins in subnuclear trafficking of glucocorticoid receptors are discussed. These studies also highlight the importance of mobilizing the cellular chaperone machinery for managing steroid receptor folding within the nucleus.

Chromatin recycling of glucocorticoid receptors: implications for multiple roles of heat shock protein 90

Unliganded glucocorticoid receptors (GRs) released from chromatin after hormone withdrawal remain associated with the nucleus within a novel subnuclear compartment that serves as a nuclear export staging area. We set out to examine whether unliganded nuclear receptors cycle between distinct subnuclear compartments or require cytoplasmic transit to regain hormone and chromatin-binding capacity. Hormone-withdrawn rat GrH2 hepatoma cells were permeabilized with digitonin to deplete cytoplasmic factors, and then hormone-binding and chromatin-binding properties of the recycled nuclear GRs were measured. We found that recycled nuclear GRs do not require cytosolic factors or ATP to rebind hormone. Nuclear GRs that rebind hormone in permeabilized cells target to high-affinity chromatin-binding sites at 30 C, but not 0 C, in the presence of ATP. Since geldanamycin, a heat shock protein-90 (hsp90)-binding drug, inhibits hormone binding to recycled nuclear GRs, hsp90 may be required to reassemble the receptor into a form capable of productive interactions with hormone. Geldanamycin also inhibits GR release from chromatin during hormone withdrawal, suggesting that hsp90 chaperone function may play multiple roles to facilitate chromatin recycling of GR.

Enhancement of phosphorylation and transcriptional activity of the glucocorticoid receptor in human synovial fibroblasts by nimesulide, a preferential cyclooxygenase 2 inhibitor

OBJECTIVE

To examine the effect of 2 nonsteroidal antiinflammatory drugs (NSAIDs), nimesulide (NIM), a preferential cyclooxygenase 2 (COX-2) inhibitor, and naproxen (NAP), on the functional parameters and transcriptional activity of the glucocorticoid receptor (GR) system in cultured human synovial fibroblasts (HSF).

METHODS

HSF were incubated with NIM (0.3, 3, and 30 microg/ml), NAP (15, 30, and 90 microg/ml), and dexamethasone (DEX; 0.01, 0.1, and 1 microM) on a time- and dose-dependent basis. The numbers of GR binding sites per cell were determined by radioligand receptor assay. Total cellular, cytoplasmic, or nuclear GR protein was measured by Western analysis using a specific anti-human GR antibody. Phosphorylation of GR was determined by specific immunoprecipitation of protein extracts from 32P-orthophosphate-labeled HSF. Mitogen-activated protein kinase p44/42 (MAPK) phosphorylation was followed by Western analysis using a specific anti-phosphoMAPK antibody. Levels of activated nuclear GR capable of binding specifically to a 32P-labeled oligonucleotide harboring the glucocorticoid/hormone response element (GRE) were evaluated by gel electrophoretic mobility shift analysis. The effects of NIM and DEX on transcriptional activation of the mouse mammary tumor virus (MMTV) promoter was determined by transfecting HSF with MMTV-luciferase (reporter gene) constructs.

RESULTS

NIM had no effect on the number of GR binding sites, in contrast to NAP and DEX. NIM and NAP did not influence cellular GR protein levels or nucleocytoplasmic shuttling, although DEX lowered GR messenger RNA and protein levels after 48 hours. NIM, but not NAP, markedly increased MAPK phosphorylation (suggesting an increase in MAPK cascade activity), GR phosphorylation, GR binding to GRE, and transcriptional activation of MMTV promoter through the GRE site in the promoter.

CONCLUSION

This study is the first to report that the antiinflammatory effects of NIM, an NSAID, may be partly related to its activation of the GR system.

Correlation between the effects of retinoic acid and dexamethasone on liver tyrosine aminotransferase

A single dose of 50 microg of trans-retinoic acid administered to rats significantly raised the level of hepatic tyrosine after a few hours. This effect was compared with that of dexamethasone and a possible correlation between these effectors was also investigated. An equal increase in enzyme activity level caused by retinoic acid was observed in adrenalectomized rats, leading to the suggestion that the effect of retinoic acid on liver tyrosine aminotransferase is independent of glucocorticoids. However, the study of the binding activity of the liver nuclear glucocorticoid receptors vs dexamethasone showed that this activity is favoured by retinoic acid, whereas no variation was evidenced for retinoic acid receptors caused by dexamethasone. In the adrenalectomized rat, the synergistic effect produced by the association of retinoic acid and dexamethasone at the lowest doses used led us to conclude that retinoic acid is an efficient effector of liver tyrosine aminotransferase. It probably affects tyrosine aminotransferase activity in a dependent and an independent way, regulated respectively by the glucorticoid status and by the provision of retinoic acid.

Subnuclear trafficking of glucocorticoid receptors in vitro: chromatin recycling and nuclear export

We have used digitonin-permeabilized cells to examine in vitro nuclear export of glucocorticoid receptors (GRs). In situ biochemical extractions in this system revealed a distinct subnuclear compartment, which collects GRs that have been released from chromatin and serves as a nuclear export staging area. Unliganded nuclear GRs within this compartment are not restricted in their subnuclear trafficking as they have the capacity to recycle to chromatin upon rebinding hormone. Thus, GRs that release from chromatin do not require transit through the cytoplasm to regain functionality. In addition, chromatin-released receptors export from nuclei of permeabilized cells in an ATP- and cytosol-independent process that is stimulated by sodium molybdate, other group VI-A transition metal oxyanions, and some tyrosine phosphatase inhibitors. The stimulation of in vitro nuclear export by these compounds is not unique to GR, but is restricted to other proteins such as the 70- and 90-kD heat shock proteins, hsp70 and hsp90, respectively, and heterogeneous nuclear RNP (hnRNP) A1. Under analogous conditions, the 56-kD heat shock protein, hsp56, and hnRNP C do not export from nuclei of permeabilized cells. If tyrosine kinase inhibitors genistein and tyrphostin AG126 are included to prevent increased tyrosine phosphorylation, in vitro nuclear export of GR is inhibited. Thus, our results are consistent with the involvement of a phosphotyrosine system in the general regulation of nuclear protein export, even for proteins such as GR and hnRNP A1 that use distinct nuclear export pathways.

Differential effects of agonist and antagonists on autoregulation of glucocorticoid receptors in a rat colonic adenocarcinoma cell line

The relative abilities of a potent glucocorticoid receptor (GR) agonist (RU 28362), a weak GR agonist (aldosterone) and a potent GR antagonist (RU 38486) to promote in vivo activation/transformation and subsequent down-regulation of GR mRNA and protein levels were compared using the PROb rat colonic adenocarcinoma cell line. In vivo activation, which is followed immediately by nuclear translocation, by these ligands (1 microM) was evaluated in terms of their abilities to deplete cytoplasmic GR protein levels after a 30 min incubation period. Western blotting experiments with the anti-GR monoclonal antibody BuGR2 demonstrated that a brief incubation with RU 28362 resulted in nearly complete depletion of cytoplasmic GR, whereas incubation with aldosterone resulted in a 50% depletion of the cytoplasmic GR protein. Incubation with RU 38486 was even more effective than aldosterone in promoting this key step in the GR pathway. Prolonged treatment (18 h) with RU 28362 resulted in significant down-regulation of GR mRNA and total cellular GR protein levels. Similar incubation with aldosterone resulted in a transient decrease in the GR mRNA level and also down-regulated the total GR protein level. Although prolonged incubation with RU 38486 did not result in detectable down-regulation of the GR mRNA level, this antagonist very effectively down-regulated total cellular GR protein levels. Taken collectively, these data demonstrate that agonists capable of promoting in vivo activation (and subsequent nuclear translocation) of GR are also effective at down-regulating GR at both the mRNA and protein levels. Although the antagonist RU 38486 is also capable of down-regulating GR protein levels by shortening the half-life of the receptor, it appears to be incapable of altering the rate of transcription of the GR gene. Glucocorticoid target tissue sensitivity may thus be decreased via two independent mechanisms: agonist-induced repression of GR gene transcription; and/or ligand-induced degradation of total cellular GR protein levels.

The influence of dietary vitamin A on triiodothyronine, retinoic acid, and glucocorticoid receptors in liver of hypothyroid rats

The properties of nuclear receptors belonging to the superfamily of receptors acting as transcription factors are modulated by nutritional and hormonal conditions. We showed recently that retinoic acid (RA) restored to normal the expression of receptors attenuated by hypothyroidism. The present study was designed to find out whether dietary vitamin A (as retinol) had the same effect. Propylthiouracil in drinking water induced both hypothyroidism and a vitamin A-deficient status in rats. The maximum binding capacity (Cmax) of triiodothyronine nuclear receptors (TR) was unchanged, while that of nuclear RA receptors (RAR) and nuclear glucocorticoid hormone receptors (GRn) was reduced in the liver of these hypothyroid rats. The reduced Cmax of RAR stemmed from a lower level of RAR mRNA, while the reduced Cmax of GRn was assumed to be due to reduced translocation of the receptor from the cytosol to the nucleus. Feeding the hypothyroid rats with a vitamin A-rich diet did not restore the Cmax of either RAR or GRn to normal. The lack of effect of dietary retinol on RAR expression may be attributed to either genomic (unoccupied TR block the expression of RAR genes) and/or extragenomic (hypothyroidism decreases the availability of retinol and/or its metabolism to RA) mechanisms. Triiodothyronine is thought to favour the translocation of glucocorticoid hormone receptors from cytosol to nucleus. These findings provide more information on the relationship between vitamin A and hormonal status, showing that a vitamin A-rich diet is without apparent effect on the expression of nuclear receptors in hypothyroid rats.

ATP-dependent release of glucocorticoid receptors from the nuclear matrix

Glucocorticoid receptors (GRs) have the capacity to shuttle between the nuclear and cytoplasmic compartments, sharing that trait with other steroid receptors and unrelated nuclear proteins of diverse function. Although nuclear import of steroid receptors, like that of nearly all other karyophilic proteins examined to date, requires ATP, there appear to be different energetic requirements for export of proteins, including steroid receptors, from nuclei. In an attempt to reveal which steps, if any, in the nuclear export pathway utilized by steroid receptors require ATP, we have used indirect immunofluorescence to visualize GRs within cells subjected to a reversible ATP depletion. Under conditions which lead to >95% depletion of cellular ATP levels within 90 min, GRs remain localized within nuclei and do not efflux into the cytoplasm. Under analogous conditions of ATP depletion, transfected progesterone receptors are also retained within nuclei. Importantly, GRs which accumulate within nuclei of ATP-depleted cells are distinguished from nuclear receptors in metabolically active cells by their resistance to in situ extraction with a hypotonic, detergent-containing buffer. GRs in ATP-depleted cells are not permanently trapped in this nuclear compartment, as nuclear receptors rapidly regain their capacity to be extracted upon restoration of cellular ATP, even in the absence of de novo protein synthesis. More extensive extraction of cells with high salt and detergent, coupled with DNase I digestion, established that a significant fraction of GRs in ATP-depleted cells are associated with an RNA-containing nuclear matrix. Quantitative Western blot (immunoblot) analysis confirmed the dramatic increase in GR binding to the nuclear matrix of ATP-depleted cells, while confocal microscopy revealed that GRs are bound to the matrix throughout all planes of the nucleus. ATP depletion does not lead to wholesale collapse of nuclear proteins onto the matrix, as the interaction of a subpopulation of simian virus 40 large tumor antigen with the nuclear matrix is not quantitatively altered in ATP-depleted Cos-1 cells. Nuclear GRs which are not bound to the nuclear matrix of metabolically active cells (i.e., a DNA-binding domain deletion mutant and a beta-galactosidase chimera possessing the GR nuclear localization signal sequence) are not recruited to the matrix upon depletion of cellular ATP. Thus, it appears that ATP depletion does not expose the GR to nuclear matrix interactions which are not normally encountered in cells but merely alters the dynamics of such interactions. The dynamic association of steroid receptors with the nuclear matrix may provide a mechanism which is utilized by these regulable transcription factors to facilitate their efficient scanning of the genome.

Effects of adrenalectomy and corticosterone replacement on glucocorticoid receptor levels in rat brain tissue: a comparison between western blotting and receptor binding assays

A sensitive Western blotting technique, using a commercially available antibody, was developed herein to study glucocorticoid receptor (GR) autoregulation in brain tissue. A prominent immunoreactive band at approximately 94 kDa, representing the GR, was observed in soluble fractions prepared from rat hippocampus whereas two bands (approximately 97 and 94 kDa) were detected in frontal cortex preparations. Four-day adrenalectomy significantly increased immunoreactive GR levels in both brain regions. In contrast, adrenalectomized animals implanted with corticosterone pellets of varying concentrations displayed dose-dependent decreases in immunodetectable GR levels. Radioligand binding assays ([3H]dexamethasone +/- RU 28362), performed on these same tissue preparations, revealed a similar pattern of GR response to that measured by Western blotting. However, changes in GR binding capacity were generally greater in magnitude than corresponding changes in immunoreactive GR levels. This discrepancy was most pronounced in adrenalectomized animals administered a bolus of corticosterone 1 h prior to sacrifice where a 60-70% reduction in receptor binding sites occurred, in sharp contrast to the 25-30% decrease in immunoreactive GR levels. Taken together, our findings suggest that Western blotting can be used to study GR regulation in brain tissue and that changes in steroid-binding capacity may not necessarily reflect changes in receptor protein levels.

Role of acidic and phosphorylated residues in gene activation by the glucocorticoid receptor

To investigate the role of acidic and phosphorylated amino acids in the function of the major transactivation domain (tau 1) of the glucocorticoid receptor, we have performed a mutagenesis study. Aspartic and glutamic acid residues were neutralized in clusters of 2 to 4 amino acids throughout the tau 1 domain. The activity of the mutant proteins was determined using transactivation assays in yeast and mammalian cells. Some acidic residues in the core region of tau 1 appear to play a minor role in tau 1 activity, but, generally, individual acidic residues are not critical for activity. Mutagenesis of five serine residues that are phosphorylated in the mouse glucocorticoid receptor and which are conserved in the human receptor did not affect the transactivation activity of the tau 1 domain in yeast. As in mouse cells, these serine residues are the predominant sites of phosphorylation for ectopically expressed receptor in yeast, since the mutant protein lacking all five sites had a severely reduced phosphorylation level. Mutant proteins in which larger numbers of acidic residues are neutralized show a progressive decrease in activity indicating that acidity in general is important for tau 1 function. However, our results are not consistent with the "acid blob" theory of transactivator function that has been suggested for some other activator proteins. Other putative roles for the acidity of tau 1 are discussed.

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.

Retinoids modulate the binding capacity of the glucocorticoid receptor and its translocation from cytosol to nucleus in liver cells

The binding capacity (Cmax) of the glucocorticoid hormone receptor (GR) was affected by vitamin A status in rat liver. In rats fed on a vitamin A-overloaded diet as well as in rats administered with retinoic acid (RA) there was an increased ratio Cmax of nuclear GR (expressed as fmol/mg liver): Cmax of cytosolic GR (expressed as fmol/mg liver) while in rats fed on a vitamin A-deficient diet this ratio was decreased. These results suggested that an increased amount of RA, resulting from either metabolization of an increased amount of dietary retinol or RA administration, enhanced the translocation of GR from the cytosolic compartment to the nuclear compartment. Moreover such an increased amount of RA could also induce the observed decreased Cmax of the total GR that we observed. These observations were similar to the well known effects of dexamethasone administration on the properties of GR. It is probable that RA, similarly to dexamethasone treatment, induces a dissociation of the tetrameric form of the cytosolic GR and thus enhances translocation of the monomeric form from cytosol to nucleus and also resulting in an increased proteolytic degradation of the receptor.

Third-generation model for corticosteroid pharmacodynamics: roles of glucocorticoid receptor mRNA and tyrosine aminotransferase mRNA in rat liver

A third-generation pharmacokinetic/pharmacodynamic model was proposed for receptor/gene-mediated corticosteroid effects. The roles of the messenger RNA (mRNA) for the glucocorticoid receptor (GR) in hepatic GR down-regulation and the mRNA for hepatic tyrosine aminotransferase (TAT) induction by methylprednisolone (MPL) were examined. Male adrenalectomized Wistar rats received 50 mg/kg MPL iv. Blood and liver samples were collected at various time points for a period of 18 hr. Plasma concentrations of MPL, free hepatic cytosolic GR densities, GR mRNA, TAT mRNA, and TAT activities in liver were determined. Plasma MPL profile was biexponential with a terminal t1/2 of 0.57 hr. Free hepatic GR density rapidly disappeared from cytoplasm after the MPL dose and then slowly returned to about 60% of starting level after 16 hr. Meanwhile, GR mRNA level fell to 45% of baseline within 2 hr postdosing, and remained at that level for at least 18 hr. The GR down-regulation of GR mRNA and protein turnover rate were modeled. The TAT mRNA began to increase at about 2 hr, reached a maximum at about 5 hr, and declined to baseline by 14 hr. TAT induction followed a similar pattern, except the induction was delayed about 0.5 hr. Pharmacodynamic parameters were obtained by fitting seven differential equations in a piecewise fashion. The cascade of corticosteroid steps were modeled by a series of inductions for steroid-receptor-DNA complex, two intermediate transit compartments, TAT mRNA, and TAT activity. Results indicate that GR mRNA and TAT mRNA are major controlling factors for the receptor/gene-mediated effects of corticosteroids.

Characterization of mechanisms involved in transrepression of NF-kappa B by activated glucocorticoid receptors

Glucocorticoids are potent immunosuppressants which work in part by inhibiting cytokine gene transcription. We show here that NF-kappa B, an important regulator of numerous cytokine genes, is functionally inhibited by the synthetic glucocorticoid dexamethasone (DEX). In transfection experiments, DEX treatment in the presence of cotransfected glucocorticoid receptor (GR) inhibits NF-kappa B p65-mediated gene expression and p65 inhibits GR activation of a glucocorticoid response element. Evidence is presented for a direct interaction between GR and the NF-kappa B subunits p65 and p50. In addition, we demonstrate that the ability of p65, p50, and c-rel subunits to bind DNA is inhibited by DEX and GR. In HeLa cells, DEX activation of endogenous GR is sufficient to block tumor necrosis factor alpha or interleukin 1 activation of NF-kappa B at the levels of both DNA binding and transcriptional activation. DEX treatment of HeLa cells also results in a significant loss of nuclear p65 and a slight increase in cytoplasmic p65. These data reveal a second mechanism by which NF-kappa B activity may be regulated by DEX. We also report that RU486 treatment of wild-type GR and DEX treatment of a transactivation mutant of GR each can significantly inhibit p65 activity. In addition, we found that the zinc finger domain of GR is necessary for the inhibition of p65. This domain is also required for GR repression of AP-1. Surprisingly, while both AP-1 and NF-kappa B can be inhibited by activated GR, synergistic NF-kappa B/AP-1 activity is largely unaffected. These data suggest that NF-kappa B, AP-1, and GR interact in a complex regulatory network to modulate gene expression and that cross-coupling of NF-kappa B and GR plays an important role in glucocorticoid-mediated repression of cytokine transcription.

Charge heterogeneity in wildtype and variant glucocorticoid receptors

Two-dimensional electrophoresis was used to examine charge heterogeneity in glucocorticoid receptors (GCRs) from sublines of the thymic-derived, mouse P1798 lymphosarcoma which were sensitive (S) or resistant (R) to glucocorticoid-mediated apoptosis. Previous work had identified the 97 kDa wildtype GCR (WT-GCR) in S cells and two variant GCRs in R cells: a 45 kDa, steroid-binding truncated GCR (TR-GCR), and a 97 kDa non steroid-binding GCR (NSB-GCR). Using denaturing isoelectric focusing, we now show that S cells as well as adult mouse thymus gland also express the NSB-GCR at pI 5.6 in addition to the WT-GCR which resolves between pH 5.9-7.1. Thus, the NSB-GCR is detected in steroid-sensitive cells and is not unique to R cells. Separation of receptors by native isoelectric focusing suggested that the TR-GCR in R cells resolved at a single, high pI (8.1) relative to the WT-GCR which resolved in a broad range (pI 5.8-8.0). The high pI of the TR-GCR may alter its functional activity thereby contributing to the resistance phenotype.

Glucocorticosteroid resistance in humans. Elucidation of the molecular mechanisms and implications for pathophysiology

Familial glucocorticoid resistance (FGR) is a rare hereditary disorder characterized by hypercortisolism and the absence of stigmata of Cushing's syndrome. The inability of glucocorticoids to exert their effects on target tissues is compensated for by increases in circulating corticotropin (ACTH) and cortisol, the former causing excess secretion of both adrenal androgens and adrenal steroid-biosynthesis intermediates with salt-retaining activity. There is considerable variability in the clinical presentations of FGR ranging from asymptomatic, to isolated chronic fatigue and to hypertension with or without hypokalemic alkalosis or to hyperandrogenism, or both. In women, hyperandrogenism can result in acne, hirsutism, menstrual irregularities, oligoanovulation, and infertility; in men it may lead to infertility and in children to precocious puberty. The reported molecular defects in FGR, such as point mutations and a microdeletion of the glucocorticoid receptor (GR) gene, cause partial resistance by, respectively, compromising the function of the GR or decreasing its intracellular concentration in glucocorticoid target tissues. Complete glucocorticoid resistance is believed to be incompatible with life in humans. Hence, the glucocorticoid resistance cases reported have been partial and of variable degree. The extreme variability in the clinical manifestations of the disorder can, additionally, be explained by differing sensitivity of target tissues to mineralocorticoids or androgens or both, and perhaps by different biochemical defects of the glucocorticoid receptor, causing selective resistance of certain glucocorticoid responses in specific tissues. Isolated tissue-resistance from a somatic mutation of the GR in a corticotropinoma from a patient with Nelson's syndrome was also found, suggesting that this may be a mechanism of tumorigenesis. There is additional evidence that defects of GR function can appear surreptitiously in a variety of clinical conditions, suggesting that glucocorticoid resistance in humans may be involved in the pathogenesis and/or clinical picture of a plethora of disease states, of which FGR is the archetype.

Phosphorylation sites in ligand-induced and ligand-independent activation of the progesterone receptor

Steroid hormone receptors are phosphoproteins that undergo hyperphosphorylation upon binding of hormone. The mechanism and the role of this reaction remain poorly understood. Two-dimensional analysis of ligand-free progesterone receptor (PR) tryptic digests showed the existence of seven main phosphopeptides. Incubation of the cells with the progestin R5020 led to a global increase in the levels of PR phosphorylation. However, the same phosphopeptides were seen, and their levels of labeling relative to each other were unchanged. A similar result was observed after incubation of cells with the antiprogestin RU486. The antiprogestin ZK98299 demonstrated only half of the activity of RU486 in terms of receptor hyperphosphorylation, but the same phosphopeptides, proportionally labeled to the same extent, were observed by chromatography electrophoresis. Ligand-induced DNA binding did not play a role in receptor hyperphosphorylation since the mutant delta 547-592, which is devoid of the first zinc finger region, exhibited the same phosphopeptides, labeled to the same extent, as did wild-type receptor after incubation of cells with hormone. These results suggest that the same kinase(s) act in vivo on ligand-free and on agonist or antagonist-bound progesterone receptor. Binding of different ligands produces different conformational changes in the ligand binding domain of the receptor which enhance, to varying extents, affinity of the receptor for the kinase(s). The DNA binding region also plays a role in the interaction with the kinase(s), although binding to DNA per se is not necessary for the hyperphosphorylation of the receptor to take place.(ABSTRACT TRUNCATED AT 250 WORDS)

Hormone-nuclear receptor interactions in health and disease. Glucocorticoid resistance

Familial glucocorticoid resistance results from the partial inability of glucocorticoids to exert their effects on their target tissues throughout the organism. The condition is associated with compensatory elevations of circulating ACTH and cortisol, with the former causing excess abnormal secretion of steroids with mineralocorticoid and androgen activity. The manifestations of glucocorticoid resistance vary from asymptomatic to chronic fatigue, to varying degrees of hypertension and/or hypokalaemic alkalosis and hyperandrogenism. The latter can be manifest in women as acne, hirsutism, menstrual irregularity, oligoanovulation and infertility, in men as infertility, and in children as precocious puberty. Different molecular defects of the highly conserved glucocorticoid receptor gene, altering its concentration and functional characteristics, appear to cause the syndrome of familial glucocorticoid resistance. Depending on the molecular defect, this syndrome is transmitted by an autosomal dominant or recessive trait. There are recent suggestions that non-generalized forms of glucocorticoid resistance may exist, resulting in autoimmune-inflammatory phenomena or psychiatric manifestations.

Glucocorticoid receptors: ATP-dependent cycling and hormone-dependent hyperphosphorylation

The dependence of hormone binding to glucocorticoid receptors (GRs) on cellular ATP levels led us to propose that GRs normally traverse an ATP-dependent cycle, possibly involving receptor phosphorylation, and that without ATP they accumulate in a form that cannot bind hormone. We identified such a form, the null receptor, in ATP-depleted cells. GRs are basally phosphorylated, and become hyperphosphorylated after treatment with hormone (but not RU486). In mouse receptors we have identified 7 phosphorylated sites, all in the N-terminal domain. Most are on serines and lie within a transactivation region. The time-course of hormone-induced hyperphosphorylation indicates that the primary substrates for hyperphosphorylation are the activated receptors; unliganded and hormone-liganded nonactivated receptors become hyperphosphorylated more slowly. After dissociation of substrates for hyperphosphorylation are the activated receptors; unliganded and hormone-liganded nonactivated receptors become hyperphosphorylated more slowly. After dissociation of hormone, most receptors appear to be recycled and reutilized in hyperphosphorylated form. From these and related observations, we have concluded that the postulated ATP-dependent cycle can be accounted for by hormone-induced or spontaneous dissociation of receptor-Hsp90 complexes, followed by reassociation of unliganded receptors with Hsp90 via an ATP-dependent reaction like that demonstrated in cell-free systems. Other steroid hormone receptors might traverse a similar cycle. Four of the 7 phosphorylated sites in the N-terminal domain are in consensus sequences for p34cdc2 kinases important in cell cycle regulation. This observation, along with the known cell cycle-dependence of sensitivity to glucocorticoids and other evidence, point to a role for receptor phosphorylation in controlling responses to glucocorticoids through the cell cycle.

Enhancement of tissue delivery and receptor occupancy of methylprednisolone in rats by a liposomal formulation

A liposomal formulation of methylprednisolone (L-MPL) was developed to improve localization of this immunosuppressant in lymphatic tissues. Liposomes containing MPL were formulated from a mixture of phosphatydylcholine and phosphatydylglycerol (molar ratio, 9:1) and sized by extrusion through a 0.1-micron membrane. Male Sprague-Dawley rats received a bolus dose of 2 mg/kg of L-MPL or free MPL in solution (control). Samples of blood, spleen, liver, thymus, and bone marrow were collected at intervals over a 66-hr period. Concentrations of MPL in plasma and organs and free cytosolic glucocorticoid receptors (GCR) in spleen and liver were determined. The plasma MPL profiles for free and L-MPL were bi- and triexponential. Although the alpha phase kinetics of both dosage forms were similar, L-MPL showed a substantially slower elimination phase than did free drug. Incorporation of MPL into liposomes caused the following increases: terminal half-life, from 0.48 (MPL) to 30.13 hr (L-MPL); MRT, from 0.42 to 11.95 hr, Vss, from 2.10 to 21.87 L/kg; and AUC, from 339 to 1093 ng x hr/mL. Uptake of liposomes enhanced significantly the delivery of drug to lymphatic tissues and liver; AUC tissue:plasma ratios for spleen increased 77-fold; for liver, 9-fold; and for thymus, 27-fold. The duration of GCR occupancy was extended 10-fold in spleen and 13-fold in liver by the liposomal formulation. Lymphatic tissue selectivity and extended receptor binding of liposome-delivered MPL offer promise for enhanced immunosuppression.

An analysis of autologous glucocorticoid receptor protein regulation in AtT-20 cells also reveals differential specificity of the BuGR2 monoclonal antibody

When the anti-glucocorticoid receptor monoclonal antibody (BuGR2) was initially incorporated either into a new immunoassay strategy or into a traditional sedimentation analysis technique, both methods failed to reveal any change in the cellular content or distribution of BuGR2-reactive antigen following glucocorticoid treatment of AtT-20 cells. Furthermore, the immunoassay also generated strong positive signals with cytosol and nuclear extracts from a receptor-negative cell line (E8.2) derived from L929 cells. However, when the BuGR2 antibody was incorporated into a combined immunoprecipitation/Western blot analysis of AtT-20 cell extracts, only the glucocorticoid receptor protein produced a signal on the Western blot, even though other proteins had been specifically immunoprecipitated by BuGR2 antibody and were clearly present on the Western blot membrane. Applying the latter approach to AtT-20 cells chronically treated with glucocorticoid, we observed not only that the receptor protein rapidly and persistently (1-96 h) accumulated in the nucleus, but also that its total cellular content was first depleted (24 h) and then was progressively replenished (48-96 h). From these studies in AtT-20 cells we conclude: (i), the BuGR2 antibody can exhibit differential immunospecificity dependent upon whether antigen mixtures are denatured or not; (ii), glucocorticoid receptor protein resided almost exclusively in the nucleus during four days of glucocorticoid treatment and (iii), the same treatment regimen resulted in total receptor protein levels being regulated in a biphasic pattern. Together, these results suggest that receptor regulation in AtT-20 cells is a complex event, and that, since steroid was constantly present during our experiments, other factors are involved in regulation of receptor levels.

Glucocorticoid stimulation of amnion cell prostaglandin synthesis: suppression by protein kinase C inhibitors and independence of phorbol ester-sensitive protein kinase C

Glucocorticoids stimulate the prostaglandin E2 production of confluent amnion cell cultures, but have no stimulatory effect on the PGE2 output of freshly isolated human amnion cells. Since protein phosphorylation may modify the responsiveness of target cells to steroids, and activators of protein kinase C (PKC), as well as corticosteroids, promote amnion cell PGE2 output by stimulating the synthesis of prostaglandin endoperoxide H synthase (PGHS), we investigated the possibility that PKC is involved in the glucocorticoid-induction of PGE2 synthesis in cultured amnion cells. The dexamethasone-induced PGE2 output of arachidonate-stimulated cells was blocked by the protein kinase inhibitors staurosporine, K-252a, H7, HA1004, and sphinganine, in a manner consistent with their effect on PKC. However, dexamethasone increased the PGE2 production of cultures treated with maximally effective concentrations of the PKC-activator compound TPA. Moreover, dexamethasone stimulated PGE2 synthesis in cultures which were desensitized to TPA-stimulation by prolonged phorbol ester treatment. Concentration-dependence studies showed that staurosporine completely (greater than 95%) blocked glucocorticoid-provoked PGE2 synthesis at concentrations which did not inhibit TPA-stimulated prostaglandin output, and that K-252a inhibited the effect of TPA by more than 95% at concentrations which decreased the effect of dexamethasone only moderately (approximately 40%). Dibutyryl cyclic AMP had no influence on the basal- or dexamethasone-stimulated PGE2 production, and on the staurosporine inhibition of the steroid effect. These results show that glucocorticoids and phorbol esters control amnion PGE2 production by separate regulatory mechanisms. It is suggested that the response of human amnion cells to glucocorticoids is modulated by protein kinase(s) other than phorbol ester-sensitive PKC and cyclic AMP-dependent protein kinase.

Receptor-mediated methylprednisolone pharmacodynamics in rats: Steroid-induced receptor down-regulation

Several approaches to receptor down-regulation were examined to extend previous receptor/gene-mediated pharmacokinetic/dynamic models of corticosteroids. Down-regulation of the glucocorticoid receptor was considered as an instantaneous event or as a gradual steroid-receptor-mediated process. Concentrations of plasma methylprednisolone, free hepatic cytosolic receptors, and the activity of hepatic tyrosine aminotransferase (TAT) enzyme were measured for 16 hr following administration of 0, 10, and 50 mg/kg methylprednisolone sodium succinate to 93 adrenalectomized rats. Receptor down-regulation was best described by a fractional decrement in the rate of return of free cytosolic glucocorticoid receptor. Predicted values for free receptor, bound receptor, nuclear bound receptor, and transfer compartments were in accord with the expected rank order values based on the high and low steroid doses. Model parameter estimates were independent of dose and described the rapid depletion of free cytosolic receptor, late-phase return of cytosolic receptor to a new baseline level that was 20-40% lower than control, and the TAT induction/dissipation pattern following steroid dosing. The microscopic association and dissociation constants describing the steroid-receptor interaction were 0.23 L/nmole per hr (k(on)) and 4.74 hr-1 (k(off)) for methylprednisolone compared to previously obtained values of 0.20 L/nmole per hr and 15.7 hr-1 for the related steroid prednisolone. The time course of TAT induction was similar to that observed previously for prednisolone. Efficiency of TAT induction was more closely related to steroid receptor occupancy than plasma methylprednisolone concentrations due to receptor saturability and receptor recycling.

Comparison of the nuclear and cytosolic forms of the Ah receptor from Hepa 1c1c7 cells: charge heterogeneity and ATP binding properties

2-[125I]iodo-7,8-dibromo-p-dioxin ([125I]Br2DpD) and 2-[125I]iodo-3-azido-7,8-dibromo-p-dioxin ([125I]N3Br2-DpD) are both capable of binding to the Ah receptor (AhR) with a high degree of specificity in cultured Hepa 1c1c7 cells. After incubation with either [125I]N3Br2DpD or [125I]Br2DpD Hepa 1c1c7 cytosolic and high salt nuclear extracts were analyzed by sucrose density gradient analysis with the following results: (i) With both radioligands an approximately 9 S form of the AhR was observed in cytosolic extracts. (ii) Nuclear extracts labeled with [125I]N3Br2DpD revealed both approximately 6 S and approximately 9 S forms of the AhR. (iii) In contrast, analysis of nuclear extracts labeled with [125I]Br2DpD revealed only an approximately 6 S form of the AhR. The approximately 9 S [125I]N3Br2DpD-labeled AhR was preferentially extracted with 100 mM KCl from a nuclear fraction and mixed with monoclonal antibody 8D3, an anti-90-kDa heat shock protein antibody. Monoclonal antibody 8D3 was able to bind to the approximately 9 S nuclear form of the AhR and caused the receptor to sediment as a heavier complex on sucrose density gradients. This would indicate that the AhR can reside in the nucleus bound to 90-kDa heat shock protein. The [125I]N3Br2DpD-labeled approximately 6 S peak fractions were collected and subjected to denaturing two-dimensional gel electrophoresis. A comparison of [125I]N3Br2DpD-labeled cytosolic (9 S) AhR preparations with the nuclear (6 S) AhR by 2-D gel electrophoresis was performed. The cytosolic form of the AhR was present in the apparent pI range of 5.2-5.7; the nuclear form focused between 5.5 and 6.2. The [125I]N3Br2DpD-labeled nuclear extracts were incubated with ATP-agarose and 43% of the photoaffinity-labeled AhR bound to the affinity gel. In contrast, approximately threefold lower binding of [125I]N3Br2DpD-labeled receptor was obtained when GTP-, AMP-, or ADP-agarose was used. Only 2% of the [125I]N3Br2DpD-labeled cytosolic AhR was able to bind to ATP-agarose. These results suggest that after the AhR translocates into the nucleus the following biochemical changes occur: (i) The sedimentation value for the AhR changes from an approximately 9 S to an approximately 6 S species. (ii) The AhR attains the ability to bind with specificity to ATP. (iii) The AhR undergoes a shift to a more basic pI.

Glucocorticoid receptors in human malignancies: a review

The present knowledge of the human glucocorticoid receptor (hGCR) in primary malignancies is reviewed. It is concluded that hGCR is present in a large number of these tissues; in all tissue specimens of lymphoid malignancies and in varying fractions of the different solid tumors. The hGCR functions as a hormone dependent, specific enhancer interacting protein in mediating the considerable effects of glucocorticoids on growth regulation, both through stimulation and inhibition of expression of the target genes, including other transcription regulation systems. The processes of receptor activation and regulation, as well as the effects of glucocorticoids, are tissue-specific. Subjects for future research are proposed: Establishment of more cell lines and animal models to extend investigation beyond the present concentration on only a few cell lines, especially CEM-C7, application of 'dynamic' assays to cells obtained from patients, in an attempt to predict development of glucocorticoid resistance, and further investigation of the relationships among GCR and growth factors and oncogenes.

Autoregulation of glucocorticoid receptor gene expression

Glucocorticoid receptors are members of a highly conserved family of steroid receptor proteins, which are ligand-dependent transcription factors. Previous studies have shown that the presence of functional glucocorticoid receptors is a prerequisite for manifestation of cellular responses to hormone. Glucocorticoid receptors undergo down-regulation following treatment with glucocorticoids. To define the molecular mechanisms that are involved in this process we have analyzed the down-regulation of glucocorticoid receptors both in HeLa cells, which contain endogenous receptors, and in cells containing receptors that have been introduced by DNA transfection. Our results show that cells that contain glucocorticoid receptors--either endogenous or transfected--undergo down-regulation of steroid-binding capabilities, as well as reductions in receptor protein and mRNA levels, in a remarkably similar fashion. DNA sequences in the coding region of the human glucocorticoid receptor cDNA appear to be sufficient to account for down-regulation of receptor. This novel finding suggests that unique mechanisms are involved in controlling glucocorticoid receptor homeostasis.