Molybdate-stabilized nonactivated glucocorticoid-receptor complexes contain a 90-kDa non-steroid-binding phosphoprotein that is lost on activation

The Nuclear Receptor Field: A Historical Overview and Future Challenges

In this article we summarize the birth of the field of nuclear receptors, the discovery of untransformed and transformed isoforms of ligand-binding macromolecules, the discovery of the three-domain structure of the receptors, and the properties of the Hsp90-based heterocomplex responsible for the overall structure of the oligomeric receptor and many aspects of the biological effects. The discovery and properties of the subfamily of receptors called orphan receptors is also outlined. Novel molecular aspects of the mechanism of action of nuclear receptors and challenges to resolve in the near future are discussed.

Enhancing nuclear translocation: perspectives in inhaled corticosteroid therapy

Corticosteroids are widely used in the treatment of asthma and chronic obstructive pulmonary disease (COPD). In contrast to their use in mild-to-moderate asthma, they are less efficacious in improving lung function and controlling the underlying inflammation in COPD. In most clinical trials, corticosteroids have shown little benefit in COPD, but have shown a greater clinical effect in combination with long-acting bronchodilators. Impaired corticosteroid activation of the glucocorticoid receptor (GR) has been reported in corticosteroid-insensitive individuals. Reversal of corticosteroid-insensitivity by enhancing GR nuclear translocation is a potential therapeutic target. Preclinical studies suggest members of the nuclear receptor superfamily may facilitate glucocorticoid receptor nuclear translocation. Unravelling the mechanisms that govern GR nuclear translocation may identify novel therapeutic targets for reversing corticosteroid-insensitivity.

Molecular Actions of Glucocorticoids in Cartilage and Bone During Health, Disease, and Steroid Therapy

Cartilage and bone are severely affected by glucocorticoids (GCs), steroid hormones that are frequently used to treat inflammatory diseases. Major complications associated with long-term steroid therapy include impairment of cartilaginous bone growth and GC-induced osteoporosis. Particularly in arthritis, GC application can increase joint and bone damage. Contrarily, endogenous GC release supports cartilage and bone integrity. In the last decade, substantial progress in the understanding of the molecular mechanisms of GC action has been gained through genome-wide binding studies of the GC receptor. These genomic approaches have revolutionized our understanding of gene regulation by ligand-induced transcription factors in general. Furthermore, specific inactivation of GC signaling and the GC receptor in bone and cartilage cells of rodent models has enabled the cell-specific effects of GCs in normal tissue homeostasis, inflammatory bone diseases, and GC-induced osteoporosis to be dissected. In this review, we summarize the current view of GC action in cartilage and bone. We further discuss future research directions in the context of new concepts for optimized steroid therapies with less detrimental effects on bone.

The 'active life' of Hsp90 complexes

Hsp90 forms a variety of complexes differing both in clientele and co-chaperones. Central to the role of co-chaperones in the formation of Hsp90 complexes is the delivery of client proteins and the regulation of the ATPase activity of Hsp90. Determining the mechanisms by which co-chaperones regulate Hsp90 is essential in understanding the assembly of these complexes and the activation and maturation of Hsp90's clientele. Mechanistically, co-chaperones alter the kinetics of the ATP-coupled conformational changes of Hsp90. The structural changes leading to the formation of a catalytically active unit involve all regions of the Hsp90 dimer. Their complexity has allowed different orthologues of Hsp90 to evolve kinetically in slightly different ways. The interaction of the cytosolic Hsp90 with a variety of co-chaperones lends itself to a complex set of different regulatory mechanisms that modulate Hsp90's conformation and ATPase activity. It also appears that the conformational switches of Hsp90 are not necessarily coupled under all circumstances. Here, I described different co-chaperone complexes and then discuss in detail the mechanisms and role that specific co-chaperones play in this. I will also discuss emerging evidence that post-translational modifications also affect the ATPase activity of Hsp90, and thus complex formation. Finally, I will present evidence showing how Hsp90's active site, although being highly conserved, can be altered to show resistance to drug binding, but still maintain ATP binding and ATPase activity. Such changes are therefore unlikely to significantly alter Hsp90's interactions with client proteins and co-chaperones. This article is part of a Special Issue entitled: Heat Shock Protein 90 (HSP90).

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.

Evidence Denies the Presence of O-GlcNAcylation on Mouse Glucocorticoid Receptor and Its Potential Involvement in Receptor Transcriptional Activity

O-linked N-acetylglucosamine (GlcNAc) glycosylation (O-GlcNAcylation) is commonly found on many cytoplasmic and nuclear proteins. It can play a role in protein trafficking, signal transduction, and in some nuclear proteins it is involved in the control of gene expression. The steroid receptor family consists of proteins that have similar domain architecture including individual DNA and hormone-binding domains that have closely related three-dimensional structures. The discovery of O-linked GlcNAc on both androgen and estrogen receptors and the realization that the GlcNAc plays a role in the transcriptional activity of these receptors raise the possibility that this glycosylation is a common mechanism involved in transcriptional modulation in all members of the steroid receptor family. To test this hypothesis, we affinity purified the mouse glucocorticoid receptor from cell lines engineered to overexpress the receptor and used GlcNAc-specific lectin chromatography, lectin-blotting analysis, and galactosylation assay to assess the presence of GlcNAc modification. All three techniques were found to be highly sensitive when used with proteins known to harbor GlcNAc yet they failed to show the presence of GlcNAcylation on the mouse GR. We also determined the effect of mutation at seven major potential glycosylation sites of the receptor on its transcriptional activity. We conclude that either the mouse GR is not modified by GlcNAc or that the amount of the modification is so low that it cannot be detected. Therefore, the O-GlcNAcylation appears not to be a common mechanism used to modify the activity of all steroid receptors.

Insulin modulates rat liver glucocorticoid receptor

This investigation used cytosol fraction of rat liver to examine the effects of insulin (INS) on functional properties of glucocorticoid receptor (GR). Male Wistar rats (220-250 g b.wt.) were injected with INS (50 microg/200 g b.wt, i.p.) and 18 h after INS administration used for experiments. INS-stimulated dissociation of G-R complexes was significantly increased by 133% compared to control level. However, INS treatment significantly stimulated stability of GR protein by 138% above control value. Furthermore, results show that INS stimulated activation of formed cytosol [3H] TA-R complexes by 143% in respect to control. [3H]TA-R complexes from INS treated animals could be activated and accumulated at higher rate in cell nuclei of control animals. The physiological relevance of the data was confirmed by INS-related stimulation of Tryptophan oxigenase (TO) activity. It was observed that INS stimulated TO activity while INS injected to adrenalectomized rats, exhibited less effects compared to control. The results indicate that a glucocorticoid hormone (CORT) enhances INS induced stimulation of TO activity, as evidenced by enhanced enzyme activity. Presented data suggest: that INS treatment leads to modifications of the GR protein and the nuclear components and that INS activates the rat liver CORT signaling pathway which mediates, in part, the activity of TO.

Use of [125I]4'-iodoflavone as a tool to characterize ligand-dependent differences in Ah receptor behavior

We have synthesized [(125)I]4'-iodoflavone to study Ah receptor (AhR)-ligand interactions by a class of AhR ligands distinct from the prototypic ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). This radioligand allows the comparison of AhR-ligand interactions using a ligand that differs in AhR affinity, and yet has the same radiospecific activity as [(125)I]2-iodo-7,8-dibromodibenzo-p-dioxin. Specific binding of [(125)I]4'-iodoflavone with the AhR was detected as a single radioactive peak ( approximately 9.7 S) following density sucrose gradient analysis. Cytosolic extracts from both Hepa 1 and HeLa cells were used as the source of mouse and human AhR, respectively. A approximately 6.7 S form of radioligand-bound Ah receptor was detected in the high salt nuclear extracts of both cell lines. In HeLa cells approximately twofold more [(125)I]4'-iodoflavone-AhR 6 S complex, compared with [(125)I]2-iodo-7,8-dibromodibenzo-p-dioxin, was recovered in nuclear extracts. A comparison of the ability of 4'-iodoflavone and TCDD to cause time-dependent translocation of AhR-yellow fluorescent protein revealed that 4'-iodoflavone was more efficient at enhancing nuclear accumulation of the receptor. These results suggest that [(125)I]4'-iodoflavone is a particularly useful and easily synthesized ligand for studying the AhR.

A new first step in activation of steroid receptors: hormone-induced switching of FKBP51 and FKBP52 immunophilins

We have identified a new first step in the hormonal activation of the glucocorticoid receptor (GR). Rather than causing immediate dissociation of the cytoplasmic GR heterocomplex, binding of hormone-induced substitution of one immunophilin (FKBP51) for another (FKBP52), and concomitant recruitment of the transport protein dynein while leaving Hsp90 unchanged. Immunofluorescence and fractionation revealed hormone-induced translocation of the hormone-generated GR-Hsp90-FKBP52-dynein complex from cytoplasm to nucleus, a step that precedes dissociation of the complex within the nucleus and conversion of GR to the DNA-binding form. Taken as a whole, these studies identify immunophilin interchange as the earliest known event in steroid receptor signaling and provide the first evidence of differential roles for FKBP51 and FKBP52 immunophilins in the control of steroid receptor subcellular localization and transport.

Vanadate increases glucocorticoid receptor-mediated gene expression: a novel mechanism for potentiation of a steroid receptor

Transition metal oxyanions, such as molybdate, tungstate and vandadate, have been shown to prevent in vitro hormone-induced activation of the glucocorticoid receptor (GR) by blocking dissociation of the GR/heat shock protein heterocomplex. In this work, we report a novel effect of vanadate: in vivo potentiation of GR-mediated gene expression. In cells stably-transfected with complex (mouse mammary tumor virus (MMTV)) or minimal GR-regulated CAT reporters, treatment with 500muM vanadate caused CAT gene expression to dramatically increase, even at saturating concentrations of dexamethasone; while no such effect was seen in response to RU486 antagonist. Similar treatment with molybdate had no effect on GR activity, suggesting that the response to vanadate was not a general property of transition metal oxyanions. Treatment with vanadate after hormone-induced nuclear translocation of the GR also caused potentiation, demonstrating that vanadate was acting on a post-transformation event, perhaps by affecting the transactivation function of DNA-bound GR. Paradoxically, vanadate caused an apparent but temporary "loss" of GR protein immediately after treatment (as measured by loss of reactivity to BuGR2 antibody and of hormone-binding capacity) that returned to normal at approximately 8h post-treatment, suggesting that potentiation of GR transactivation function (as measured by our CAT assays) was probably occurring during the later stages (8-24h) of this assay. However, gel shift analyses revealed that vanadate could induce binding of the hormone-free GR to glucocorticoid response element (GRE)-containing oligonucleotides immediately after treatment. Thus, the rapid vanadate-induced "loss" of GR was not due to degradation of GR protein. Yet, vanadate in the absence of hormone had no effect on CAT reporter expression, demonstrating that this form of the GR still requires agonist for its enhanced transcriptional activity. As an indication of the potential mechanism of vanadate action, vanadate was found to dramatically stimulate the mitogen-activated protein kinases, ERK-1 and ERK-2. In addition, vanadate potentiation of GR reporter gene expression was completely blocked by the tyrosine kinase inhibitor herbimycin A. Taken as a whole, our results suggest that vanadate can have dramatic and complex effects on GR structure and function, resulting in hormone-free activation of GR DNA-binding function, as well as alterations to the BuGR2 epitope and hormone-binding domains--while at the same time stimulating tyrosine phosphorylation pathways controlling GR-mediated gene transcription.

Progesterone receptors as neuroendocrine integrators

Intracellular progesterone receptors (PRs) are ligand-inducible transcription factors that mediate the majority of the effects of progesterone (P) on neuroendocrine functions. During the past decade, evidence has accumulated which suggest that PRs can also be activated independently of P, by signals propagated through membrane-bound receptors to the interior of cells. The activation of PRs by this type of "cross-talk" mechanism has been implicated in the physiological regulation of several important neuroendocrine processes, including estrous behavior and periovulatory hormone secretions. We review evidence that both ligand-dependent and ligand-independent activation of PRs occurs in central neurons and in anterior pituitary cells and that the convergence and summation of these signals at the PR serves to integrate neural and endocrine signals which direct several critically important neuroendocrine processes. An integrative function for PRs is reviewed in several physiological contexts, including the display of lordosis behavior in female rodents, the neurosecretion of gonadotropin-releasing hormone surges, secretion of preovulatory gonadotropin surges, and release of periovulatory follicle stimulating hormone surges. The weight of evidence indicates that cross talk at the intracellular PR is an essential component of the integrative mechanisms that direct each of these neuroendocrine events. The recurrence of PR's integrative actions in several different physiological contexts suggests that other intracellular steroid receptors similarly function as integrators of neural and endocrine signals in other neuroendocrine processes.

Glucocorticoid receptor interaction with Hsp90 remains unaltered after whole body hyperthermia

The influence of 41 degrees C whole body hyperthermic stress on glucocorticoid receptor (GR) association with 90-kDa heat shock protein (Hsp90) in the rat liver cytosol was examined. Total cytosolic GR and Hsp90 concentrations, as well as the amount of Hsp90 co-immunoprecipitated with the GR were determined by quantitative Western blotting using BuGR2 as anti-GR and AC88 as anti-Hsp90 monoclonal antibody. After exposure of the animals to the heat stress, the level of cytosolic Hsp90 increased, while its ratio to apo-receptor within non-activated GR heterooligomeric complexes remained unaltered. Therefore, the Hsp90 recruitment by the GR was not dependent on Hsp90 total cytosolic concentration.

Ligand-independent activation of the glucocorticoid receptor by beta2-adrenergic receptor agonists in primary human lung fibroblasts and vascular smooth muscle cells

The glucocorticoid receptor (GR) is a ubiquitously expressed transcription factor present in most cell types. Upon ligand binding, the GR is activated and translocates into the nucleus where it transmits the anti-inflammatory actions of glucocorticoids. Here, we describe the ligand-independent activation of GR by the beta2-adrenergic receptor (beta2-AR) agonists, salbutamol and salmeterol, in primary human lung fibroblasts and vascular smooth muscle cells. Immunohistochemistry demonstrated expression of GR and the beta2-AR by fibroblasts and vascular smooth muscle cells. Treatment of the cells with the beta2-AR agonists, salbutamol or salmeterol, resulted in translocation of GR into the nucleus beginning at 30 min, as shown by immunohistochemistry and Western blotting of cytosolic and nuclear cell extracts. In comparison, activation of GR induced by the corticosteroids dexamethasone and fluticasone occurred at the same time after treatment (30 min) but resulted in a more complete depletion of GR from the cytosolic compartment. Electrophoretic mobility shift assays confirmed that nuclear GR, activated by both beta2-AR agonists and glucocorticoids, actively bound to the GR consensus sequence (GR element). Functional activation of the GR was confirmed by a Luciferase reporter gene assay, using a GR driven promoter fragment from the p21((WAF1/CIP1)) gene. The effects of the beta2-AR agonists, salbutamol and salmeterol, were dependent upon binding to the beta2-AR, because blocking of beta2-AR with propranolol abrogated GR activation. GR activation appeared to involve cAMP. In summary, these data show that beta2-AR agonists are potent activators of GR. Ligand-independent activation of GR by beta2-AR agonists may substantially mediate the anti-inflammatory actions of these drugs observed in vitro and in vivo.

Steroid receptor interactions with heat shock protein and immunophilin chaperones

We have provided a historical perspective on a body of steroid receptor research dealing with the structure and physiological significance of the untransformed 9S receptor that has often confused both novice and expert investigators. The frequent controversies and equivocations of earlier studies were due to the fact that the native, hormone-free state of these receptors is a large multiprotein complex that resisted description for many years because of its unstable and dynamic nature. The untransformed 9S state of the steroid and dioxin receptors has provided a unique system for studying the function of the ubiquitous, abundant, and conserved heat shock protein, hsp90. The hormonal control of receptor association with hsp90 provided a method of manipulating the receptor heterocomplex in a manner that was physiologically meaningful. For several steroid receptors, binding to hsp90 was required for the receptor to be in a native hormone-binding state, and for all of the receptors, hormone binding promoted dissociation of the receptor from hsp90 and conversion of the receptor to the DNA-binding state. Although the complexes between tyrosine kinases and hsp90 were discovered earlier, the hormonal regulation or steroid receptor association with hsp90 permitted much more rapid and facile study of hsp90 function. The observations that hsp90 binds to the receptors through their HBDs and that these domains can be fused to structurally different proteins bringing their function under hormonal control provided a powerful linkage between the hormonal regulation of receptor binding to hsp90 and the initial step in steroid hormone action. Because the 9S receptor hsp90 heterocomplexes could be physically stabilized by molybdate, their protein composition could be readily studied, and it became clear that these complexes are multiprotein structures containing a number of unique proteins, such as FKBP51, FKBP52, CyP-40, and p23, that were discovered because of their presence in these structures. Further analysis showed that hsp90 itself exists in a variety of native multiprotein heterocomplexes independent of steroid receptors and other 'substrate' proteins. Cell-free systems can now be used to study the formation of receptor heterocomplexes. As we outlined in the scheme of Fig. 1, the multicomponent receptor-hsp90 heterocomplex assembly system is being reconstituted, and the importance of individual proteins, such as hsp70, p60, and p23, in the assembly process is becoming recognized. It should be noted that our understanding of the mechanism and purpose of steroid receptor heterocomplex assembly is still at an early stage. We can now speculate on the roles of receptor-associated proteins in receptor action, both as individuals and as a group, but their actual functions are still vague or unknown. We can make realistic models about the chaperoning and trafficking of steroid receptors, but we don't yet know how these processes occur, we don't know where chaperoning occurs in the cell (e.g. Is it limited to the cytoplasm? Is it a diffuse process or does chaperoning occur in association with structural elements?), and, with the exception of the requirement for hormone binding, we don't know the extent to which the hsp90-based chaperone system impacts on steroid hormone action. It is not yet clear how far the discovery of this hsp90 heterocomplex assembly system will be extended to the development of a general understanding of protein processing in the cell. Because this assembly system is apparently present in all eukaryotic cells, it probably performs an essential function for many proteins. The bacterial homolog of hsp90 is not an essential protein, but hsp90 is essential in eukaryotes, and recent studies indicate that the development of the cell nucleus from prokaryotic progenitors was accompanied by the duplication of genes for hsp90 and hsp70 (698). (ABSTRACT TRUNCATED)

The role of the hsp90-based chaperone system in signal transduction by nuclear receptors and receptors signaling via MAP kinase

The multicomponent heat-shock protein (hsp) 90-based chaperone system is an ubiquitous protein-folding system in the cytoplasm of eukaryotes. Several signal transduction systems utilize an interaction with hsp90 as an essential component of the signaling pathway. The steroid and dioxin receptors are bound to hsp90 through their hormone-binding domains, and several of them must be bound to hsp90 in order to have a ligand-binding site. The binding of ligands to these receptors promotes their dissociation from hsp90, an event that is the first step in their signaling pathways. Several protein kinases, including the Src and Raf components of the MAP kinase system, are also bound to hsp90. Genetic studies in yeast have demonstrated that hsp90 is required for normal signaling via steroid and dioxin receptors and for the activity of Src in vivo. The hsp90-based chaperone system has been reconstituted from purified components, permitting detailed analysis of the molecular basis of the chaperone's role in signal transduction.

Attenuation of glucocorticoid receptor levels by the H-ras oncogene

Certain oncogene products are known to affect the cellular response to glucocorticoids. In particular, glucocorticoid-induced transcription is impaired in H-ras-transformed cells. In this study, we examine the mechanism for this effect in NIH3T3 cells containing stably integrated H-ras genomic sequences. NIH3T3ras cells transfected with the MMTV-CAT reporter exhibit a pronounced reduction in the level of glucocorticoid-induced CAT activity, compared to normal NIH3T3 cells. As the response to glucocorticoids depends on the amount of glucocorticoid receptor protein, we have examined the cellular receptor content in both cell lines. The cytosolic and total cellular GR protein are both markedly lower in NIH3T3ras cells, suggesting that the reduced response is directly due to an attenuation of receptor levels. The steady-state level of glucocorticoid receptor mRNA is appreciably reduced in NIH3T3ras cells, which accounts for the attenuated level of glucocorticoid receptor protein. The rate of glucocorticoid receptor gene transcription is concomitantly decreased in NIH3T3ras cells. Theras effect maps to the proximal promoter of the glucocorticoid receptor gene. These results suggest that a target for activated H-Ras protein may be a transcription factor which partially represses transcription of the glucocorticoid receptor gene.

Stabilization in vitro of the untransformed glucocorticoid receptor complex of S49 lymphocytes by the immunophilin ligand FK506

Untransformed steroid receptors are large heteromeric complexes which have been shown to contain the mammalian heat shock proteins hsp56, hsp70 and hsp90. Based on functional and sequence homology studies, it was recently discovered that hsp56 also belongs to the FKBP class of immunophilin proteins, which are thought to mediate the actions of the immunosuppressive drugs FK506 and rapamycin. This discovery has led to the speculation that FK506 and related drugs could influence the actions of steroid receptors. In this work, we have examined the effects of FK506 on the transformation and hormone-binding properties of glucocorticoid receptors (GR) present in the cytosolic fraction of mouse S49 lymphocyte cells. Based on immunoprecipitation studies, it was found that hsp56 was indeed a component of untransformed GR complexes in S49 cytosols. It was also found that the untransformed but not the transformed GR was retained following affinity chromatography with FK506-affigel resin, reinforcing the possibility that hsp56 within the untransformed GR complex could be a target for the actions of FK506. Using a DNA-cellulose-binding assay, FK506 exhibited a 60% inhibition of dexamethasone (Dex)-induced transformation of the GR to the DNA-binding state, while sodium molybdate, a transition metal oxyanion known to stabilize GR complexes, was 100% effective. This inhibition of GR transformation by FK506 was shown to correlate with an inhibition of Dex-induced GR/hsp90 dissociation, with 10 microM FK506 preventing 48% of the GR/hsp90 complexes from dissociating. Scatchard analysis of GR hormone-binding function was performed, with FK506 treatment of cytosols causing Kd values to decrease (3.36 nM) as compared to vehicle (8.42 nM) and no-addition (9.82 nM) controls. Taken together, our results suggest that FK506 can stabilize the untransformed GR complex of S49 cells and that this stabilization in turn results in an increase in GR ligand-binding affinity. Although we speculate that these actions of FK506 on the GR complex are mediated by the associated hsp56 component, other possible mechanisms are also discussed.

Oligomeric structures of cytosoluble estrogen-receptor complexes as studied by anti-estrogen receptor antibodies and chemical crosslinking of intact cells

The structure of estrogen-receptor complexes recovered in cytosolic extracts of MCF-7 cells treated with hormone at 2 degrees was probed by chemical crosslinking of intact cells and sample analysis with four monoclonal anti-estrogen receptor antibodies. When MCF-7 cells were treated with either glutaraldehyde or dithiobis(succinimidyl propionate), cytosoluble estrogen-receptor complexes consisted of two major forms sedimenting as 4 S monomers and 8-9 S salt-resistant oligomers. By high salt sucrose density gradient centrifugation, we could observe that the four monoclonal anti-estrogen receptor antibodies bound different forms of receptor complexes from crosslinked cells. While H222 and H226 antibodies could interact with any form we detected, the D75 and D547 monoclonals could only recognize those showing sedimentation coefficients lower than 7 S. When cytosolic extracts from [35S]-methionine-labeled cells were subjected to immunoprecipitation with H222 and D75 anti-estrogen receptor antibodies, electrophoretic analysis of material extracted from immunoprecipitates revealed the presence of 65 kDa estrogen receptors. If extracts were prepared from crosslinked cells, instead, two more components with estimated molecular masses of 220 and 100 kDa were specifically immunoprecipitated by the H222 antibody, whereas only the 100 kDa component and the estrogen receptor were found in immunoprecipitates obtained with the D75 monoclonal. When estrogen-receptor complexes were immunopurified from extracts prepared after cells had been crosslinked with dithiobis(succinimidyl propionate), and the oligomers were dissociated by treatment with beta-mercaptoethanol, electrophoretic analysis of our samples showed that only the 65 kDa estrogen receptor and a 50 kDa protein were selectively immunoprecipitated by anti-estrogen receptor antibodies. We concluded that the structures of cytosoluble estrogen-receptor complexes in MCF-7 cells treated with hormone at 2 degrees C, include oligomeric forms which contain a 50 kDa non-steroid binding protein.

Heat shock protein 70 is associated in substoichiometric amounts with the rat hepatic glucocorticoid receptor

The 70-kDa heat shock protein (hsp70) has been shown to be an important participant in several intracellular events, including protein folding and trafficking. Hsp70 binds to many, if not all, proteins during their translation and maintains its association with some protein complexes as a subunit. We have examined the possibility that hsp70 may be associated with one or more forms of the rat hepatic glucocorticoid receptor (GR). Unliganded GR was immunoprecipitated from cytosol with the anti-GR monoclonal antibody BUGR2 and then subjected to western blotting. Both hsp70 and the 90-kDa heat shock protein (hsp90) were found to be specifically associated with the GR. Hsp70 was also bound to the liganded unactivated and activated (transformed) forms of the GR complex, while as expected, hsp90 was absent from the activated GR. Immunoprecipitation of cytosolic hsp70 with the anti-hsp70 monoclonal antibody N27 resulted in coprecipitation of GR. The components of the immunopurified GR were also analyzed by laser densitometry after polyacrylamide gel electrophoresis and Coomassie blue staining. These experiments revealed that hsp70 is bound to the GR in an approximate 1:5 ratio. Unactivated GR complexes isolated via a ligand affinity purification scheme contained hsp90 and trace amounts of hsp70. Collectively, these experiments demonstrate that hsp70 is specifically associated with several forms of the native rat hepatic GR, although its binding is substoichiometric. This is in direct contrast to hsp90, which binds as a dimeric subunit to the heteromeric unactivated GR complex.

Structure and function of the glucocorticoid receptor

Glucocorticoids cause changes in the expression of target genes via interaction with an intracellular receptor protein, the glucocorticoid receptor. This signal transduction process can be divided into a number of steps, each of which represents a functional facet of the receptor protein. These steps include (i) receptor transformation to an active form resulting from specific interaction with glucocorticoid steroid hormones, (ii) homo-dimerization, (iii) DNA-binding to specific hormone response elements in the genome and (iv) modulation of the expression levels of linked genes. These aspects of glucocorticoid receptor function have been studied using a combination of tertiary structure determination, biochemical assays and a genetic approach using a yeast system to screen for mutant receptors that are altered in function. The results show that contacts involving both the DNA and steroid binding domains are involved in dimerization and high affinity DNA binding. Genetic experiments have illuminated the role of amino acids within the recognition helix of the DNA-binding domain in discriminating between cognate DNA response elements for the glucocorticoid receptor and closely related binding sites for other nuclear receptors. Squelching experiments suggest that the N-terminal transactivation domain of the receptor contacts components of the general transcriptional machinery that appear to be distinct from the TATA binding protein, TFIID, during transactivation of gene expression by the DNA-bound receptor.

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.

Purification and characterization of a macromolecular-translocation inhibitor III of activated glucocorticoid-receptor-complex binding to nuclei from rat liver

Macromolecular-translocation inhibitors (MTI) of binding of the activated glucocorticoid-receptor complex (GRC) to nuclei from rat liver are separated into at least three components (MTI-I-III) by DEAE-cellulose column chromatography [Okamoto, K., Isohashi, F., Horiuchi, M. & Sakamoto, Y. (1982) Biochem. Biophys. Res. Commun. 108, 1655-1660]. In this study, we have purified MTI-III from the livers of adrenalectomized rats to apparent homogeneity, as determined by SDS/PAGE. The purification procedure consisted of DEAE-cellulose chromatography, acid treatment and sequential chromatographies using gel filtration, S-Sepharose and Mono S columns. The purified protein had a molecular mass of approximately 69 kDa, as estimated by SDS/PAGE, and the molecular mass of the inhibitor was approximately 68 kDa, as estimated by gel filtration. Thus, MTI-III exerts its inhibitory activity as a monomer. The sedimentation coefficient of MTI-III was approximately 3.7 S. Purified MTI-III was fairly stable at 4 degrees C but at higher temperatures, especially above 25 degrees C, it was rapidly inactivated. Under low-salt conditions, MTI-III was associated with activated GRC (4.2 S) and the resulting complex was detected on sucrose density gradients as a larger species (6.8 S). Initial treatment of nuclei or DNA-cellulose with MTI-III did not alter their abilities to bind activated GRC. These results indicate that MTI-III acts through an interaction with GRC.

Localization of the approximately 12 kDa M(r) discrepancy in gel migration of the mouse glucocorticoid receptor to the major phosphorylated cyanogen bromide fragment in the transactivating domain

The intact wild-type mouse glucocorticoid receptor has a theoretical molecular weight of approximately 96 kDa based on amino acid sequence, but on SDS-polyacrylamide gel electrophoresis it migrates as a protein of approximately 98 kDa. It is not known where the unusual primary structure or covalent modification responsible for this anomalous migration is located within the amino acid chain. In the course of examining the pattern of fragmentation of 32P-labeled glucocorticoid receptors from Chinese hamster ovary (CHO) cells containing amplified mouse receptor cDNA, we have found a localized region in the amino-terminal half of the receptor that accounts for this anomalous behavior. Cyanogen bromide treatment of the intact receptor produces a 23.4 kDa (theoretical) fragment consisting of residues 108-324 and containing all of the identified phosphorylated serines within the receptor. We find that the only large resolvable 32P-labeled receptor fragment produced after complete cyanogen bromide cleavage of intact receptors migrates with an apparent molecular weight of approximately 35 kDa. Because the apparent difference between the theoretical and the experimentally observed molecular weights of this cyanogen bromide fragment is essentially the same as the difference between the theoretical and experimental molecular weights of the intact mouse glucocorticoid receptor, we propose that some feature lying within this fragment accounts for slower migration. Although the existence of an additional phosphorylation site lying within the 15 kDa tryptic receptor fragment containing the DNA-binding domain has been contested, we also demonstrate that this fragment of the mouse glucocorticoid receptor is phosphorylated in vivo upon incubation of CHO cells in growth medium containing [32P]orthophosphate.

Biochemical and immunological evidence that an acidic domain of hsp 90 is involved in the stabilization of untransformed glucocorticoid receptor complexes

Polyclonal antibodies (AS 232-266) have been raised against the 232-266 amino acid sequence of the mouse hsp 84. This sequence possesses 54% acidic residues. AS 232-266 react with both the denatured and the free native murine hsp 84, but not with the bound hsp 84 present in the untransformed glucocorticoid receptor complexes (GR). Both AS 232-266 and peptide 232-266 were shown to decrease [3H]dexamethasone binding by GR. Moreover synthetic peptide 232-266, when added to 7 nm untransformed GR, convert them into 5 nm hsp 84-free GR. Taken together these data suggest that the acidic 232-266 sequence of hsp 84 is involved in the stabilization of the hsp 84-GR interaction, which is known to result in 7 nm complex formation and in GR ligand binding activity improvement. Both peptide 232-266 and AS 232-266 destabilize this interaction.

Glucocorticoid receptors in leukemias, lymphomas and myelomas of young and old

In this paper we have briefly reviewed the nature of leukemias and lymphomas in the old and the young. We surveyed in general the ways in which lymphoid cells and other hematologic elements respond to glucocorticoids, mentioning that there may be direct or indirect effects on their growth by these ligands. We have reviewed the current general model for the action of glucocorticoids in all cells, namely the fact that the actions of these steroids are mediated to a large extent through binding with ligand-activated transcription factors, their receptors. The growing wealth of detail about the nature of the interaction of these receptors with regulatory sites in the genome is discussed. Finally, we have described our results with lines of tissue culture cells representing clones from a typical leukemia of the young, and of myeloma, a typical hematologic malignancy of the elderly. Several features of the effects of glucocorticoids on these cells point up areas that would be pertinent to explore in aging and in the relationship of hematologic diseases to survival and response to therapy in the older versus the younger patient.

Control of steroid receptor function and cytoplasmic-nuclear transport by heat shock proteins

As targeted proteins that move within the cell, the steroid receptors have become very useful probes for understanding the linked phenomena of protein folding and transport. From the study of steroid receptor-associated proteins it has become clear over the past two years that these receptors are bound to a multiprotein complex containing at least two heat shock proteins, hsp90 and hsp56. Attachment of receptors to this complex in a cell-free system appears to require the protein unfolding/folding activity of a third heat shock protein, hsp70. Like the oncogenic tyrosine kinase pp60src, steroid receptors bind to this complex of chaperone proteins at the time of their translation. Binding of the receptor to the hsp90 component of the system occurs through the hormone binding domain and is under strict hormonal control. The hormone binding domain of the receptor acts as a transferable regulatory unit that confers both tight hormonal control and hsp90 binding onto chimaeric proteins. The model of folding and transport being developed for steroid receptors leads to some general suggestions regarding the folding and transport of targeted proteins in the cell.

DNA-binding by the glucocorticoid receptor: a structural and functional analysis

The glucocorticoid receptor belongs to a family of ligand activated nuclear receptors. This family includes, in addition to the receptors for steroid hormones, receptors for thyroid hormone, retinoic acid and 1,25-dihydroxy vitamin D3 as well as some receptors with as yet unknown ligands. The glucocorticoid receptor DNA-binding domain has been expressed in E. coli. The purified protein binds to the same DNA sequences as the native receptor and is therefore suitable for biochemical and structural studies of the DNA-binding function of the receptor protein. This protein has been shown to bind as a dimer to its DNA-binding site. Protein-protein interactions facilitate DNA-binding and a segment responsible for these interactions has been identified close to the C-terminal zinc-binding site. The family of nuclear receptors, with their related DNA-binding sites, provides an opportunity to study determinants for DNA sequence recognition. A segment close to the N-terminal zinc ion has been shown to be responsible for the target specificity of glucocorticoid and estrogen receptors. DNA-binding domains of nuclear receptors include nine conserved cysteine residues which have been shown to coordinate two zinc ions and zinc has been shown to be required for the structural integrity and DNA-binding ability of the glucocorticoid receptor DNA-binding domain. A motif for DNA recognition, based around zinc ions, was first described for transcription factor IIIA and nuclear receptors were believed to recognize DNA via a similar motif. However, the three-dimensional structure determination of the glucocorticoid receptor DNA-binding domain shows that its structure is clearly different from that of the TFIIIA type zinc-binding domains.

A model of glucocorticoid receptor unfolding and stabilization by a heat shock protein complex

It has recently been reported that incubation of avian progesterone receptors, mouse glucocorticoid receptors, or the viral tyrosine kinase pp60src with rabbit reticulocyte lysate reconstitutes their association with the 90 kDa heat shock protein, hsp90. The reassociation is thought to require unfolding of the steroid receptor or pp60src before hsp90 can bind. The unfoldase activity may be provided by hsp70, which is also present in the reconstituted receptor heterocomplex. In this paper we review evidence that hsp70 and hsp90 are associated in cytosolic heterocomplexes that contain a limited number of other proteins. From an analysis of known receptor-hsp interactions and a predicted direct interaction between hsp90 and hsp70 we have developed an admittedly very speculative model of glucocorticoid receptor unfolding and stabilization. One important feature of the model is that the receptor becomes attached to a heat shock protein heterocomplex rather than undergoing independent unfolding and stabilization events. The model requires that hsp70 and hsp90 bind directly to the receptor at independent sites. Importantly, the model accommodates the stoichiometry of 2 hsp90 per 1 molecule of receptor that has been assayed in the untransformed GR heterocomplex in cytosols prepared from hormone-free cells.

Subunit composition of the untransformed glucocorticoid receptor in the cytosol and in the cell

We have used bifunctional reagents to examine the subunit composition of the non-DNA-binding form of the rat and human glucocorticoid receptor. Treatment of intact cells and cell extracts with a reversible cross-linker, followed by electrophoretic analysis of immunoadsorbed receptor revealed that three proteins of apparent approximate molecular masses, 90, 53 and 14 kDa are associated with the receptor. The first of these was identified immunochemically as a 90-kDa heat-shock protein (hsp90). The complex isolated from HeLa cells contained 2.2 mol hsp90/mol steroid-binding subunit. Cross-linking of the receptor complex in the cytosol completely prevented salt-induced dissociation of the subunits. The cross-linked receptor was electrophoretically resolved into two oligomeric complexes of apparent molecular mass 288 kDa and 347 kDa, reflecting the association of the 53-kDa protein with a fraction of the receptor. Since no higher oligomeric complexes could be generated by cross-linking cell extracts under different conditions, we conclude that most of the untransformed cytosolic receptor is devoid of additional components.

Methotrexate-induced overexpression of functional glucocorticoid receptors in Chinese hamster ovary cells

We have used a modified cotransfection and selection strategy to create a series of mammalian cell lines that stably express high levels of intact glucocorticoid receptors. These cell lines were produced by subjecting Chinese hamster ovary (CHO) cells, which had been previously cotransfected with a glucocorticoid-responsive dihydrofolate reductase (DHFR) gene and the human glucocorticoid receptor gene, to growth in increasing concentrations of methotrexate (MTX). By linking the MTX selection process to glucocorticoid receptor function via the DHFR gene, stable cell lines resistant to a range of MTX concentrations (50 nM to 3 microM) were isolated that were strictly dependent upon glucocorticoids for growth. Quantitation of steroid binding capacity in MTX-resistant cells revealed a progressive increase in the number of glucocorticoid receptors as a function of increasing MTX concentration. This increase in receptor content was maximal at the highest level of MTX resistance examined (3 microM MTX) and represented a 25-fold elevation in glucocorticoid receptor number relative to CHO cells expressing only endogenous hamster receptor. The increases in steroid binding obtained after MTX selection were reflected by similar increases in the level of glucocorticoid receptor protein as determined by immunoblot analysis. Examination of glucocorticoid receptor structure by sucrose density gradient centrifugation revealed that oligomeric (9 S) steroid receptor complexes were formed at all levels of receptor expression. Subcellular localization of the glucocorticoid receptor protein by immunocytochemical staining revealed effective nuclear translocation of the overexpressed receptors in MTX-resistant cells. Functional transfection studies using a glucocorticoid-responsive reporter gene indicated that the additional glucocorticoid receptors in CHO cells were competent to activate transcription. To determine the molecular basis for the MTX-induced increases in functional glucocorticoid receptors, steady-state levels of glucocorticoid receptor mRNA were examined. MTX selection produced a 5- to 7-fold increase in transfected glucocorticoid receptor gene expression relative to untreated cells. MTX-resistant cells also expressed increased levels of a putative hamster glucocorticoid receptor mRNA species. Interestingly, the observed increases in receptor gene expression in these cells could not be accounted for by amplification of either the human or the hamster glucocorticoid receptor genes.

Regulation of glucocorticoid receptor expression in cultured fibroblasts from a patient with familial glucocorticoid resistance

The thermolabile glucocorticoid receptor (GR) in fibroblasts from a patient with familial glucocorticoid resistance (FGR) was characterized by solution hybridization, Northern blot analysis and Western immunoblotting using an hGR and cRNA probe and a GR specific monoclonal antibody. Specific DNA binding was measured by binding of cytosolic GR to mouse mammary tumour virus (MMTV) DNA. Northern blot analysis of total cellular RNA isolated from the fibroblasts showed hybridization of the hGR probe to 7.0 and 6.1 kb RNA species. Basal expression of hGR mRNA was 1.8 times higher in fibroblasts derived from the patient compared to control fibroblasts as assayed by solution hybridization. Even though nonsignificant, dexamethasone treatment maximally caused at 60% down-regulation of GR mRNA in normal fibroblasts after 12 h but only a 40% down-regulation in fibroblasts from the patient. In both cases, the initial mRNA values were restored after 72 h. No difference in GR mRNA stability was observed between fibroblasts from the patient and from controls. The induction of the glucocorticoid-regulated gene metallothionein IIA (MTIIA) by dexamethasone and cadmium sulphate was studied at different temperatures using a cRNA probe for human MTIIA. At elevated temperatures, cadmium sulphate but not dexamethasone increased MTIIA mRNA levels approximately three-fold in fibroblasts from the patient, whereas in normal fibroblasts regardless of temperature both cadmium sulphate and dexamethasone increased MTIIA mRNA levels approximately three- and two-fold, respectively. Cytosolic GR from FGR-fibroblasts showed an increased specific binding to MMTV DNA at 4 degrees C. These data support our previous findings of a thermolabile GR, probably due to a defect intrinsic to the GR protein, in this patient with primary cortisol resistance and indicate a compensatory mechanism at the transcriptional level of GR expression. The data also indicate a receptor defect affecting specific DNA binding in vitro.

Association of heat shock protein 90 with the 16 kDa steroid binding core fragment of rat glucocorticoid receptors

We have recently described a 16 kDa steroid binding core (Thr537-Arg673) of the rat glucocorticoid receptor [Simons et al. (1989) J. Biol. Chem. 264, 14493-14497]. Sedimentation analysis and size exclusion and anion exchange chromatography now suggest that other proteins are associated with the 16 kDa receptor, just as has been seen for the intact 98 kDa receptor. The 16 kDa fragment was also immunoprecipitable with anti-heat shock protein 90 (hsp90) antibody. These results argue that hsp90 binds to the 16 kDa core fragment and directly position the site of hsp90 association between Thr537 and Arg673 of the rat glucocorticoid receptor.

Mechanisms controlling steroid receptor binding to specific DNA sequences

Mammalian progesterone receptors activated by hormone binding in nuclei of intact cells exhibit substantially higher binding activity for specific DNA sequences than receptors bound with hormone and activated in cell-free cytosol. Differences in DNA-binding activity occur despite the fact that both activated receptor forms sediment at 4S on sucrose gradients and are apparently dissociated from the heat shock protein 90. This suggests that hormone-induced release of heat shock protein 90 from receptors is necessary, but not sufficient for maximal activation of DNA binding. This report is a review of studies from our laboratories that have examined the role of receptor interaction with other nuclear protein factor(s), and receptor dimerization in solution, as additional regulatory steps involved in the process of receptor activation and binding to specific gene sequences.

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.