Stoichiometric analysis of the specific interaction of the glucocorticoid receptor with DNA

Klf9 plays a critical role in GR -dependent metabolic adaptations in cardiomyocytes

Glucocorticoids through activation of the Glucocorticoid receptor (GR) play an essential role in cellular homeostasis during physiological variations and in response to stress. Our genomic GR binding and transcriptome data from Dexamethasone (Dex) treated cardiomyocytes showed an early differential regulation of mostly transcription factors, followed by sequential change in genes involved in downstream functional pathways. We examined the role of Krüppel-like factor 9 (Klf9), an early direct target of GR in cardiomyocytes. Klf9-ChIPseq identified 2150 genes that showed an increase in Klf9 binding in response to Dex. Transcriptome analysis of Dex treated cardiomyocytes with or without knockdown of Klf9 revealed differential regulation of 1777 genes, of which a reversal in expression is seen in 1640 genes with knockdown of Klf9 compared to Dex. Conversely, only 137 (∼8%) genes show further dysregulation in expression with siKLf9, as seen with Dex treated cardiomyocytes. Functional annotation identified genes of metabolic pathways on the top of differentially expressed genes, including those involved in glycolysis and oxidative phosphorylation. Knockdown of Klf9 in cardiomyocytes inhibited Dex induced increase in glycolytic function and mitochondrial spare respiratory capacity, as measured by glycolysis and mito stress tests, respectively. Thus, we conclude that cyclic, diurnal GR activation, through Klf9 -dependent feedforward signaling plays a central role in maintaining cellular homeostasis through metabolic adaptations in cardiomyocytes.

Fighting the Fire: Mechanisms of Inflammatory Gene Regulation by the Glucocorticoid Receptor

For many decades, glucocorticoids have been widely used as the gold standard treatment for inflammatory conditions. Unfortunately, their clinical use is limited by severe adverse effects such as insulin resistance, cardiometabolic diseases, muscle and skin atrophies, osteoporosis, and depression. Glucocorticoids exert their effects by binding to the Glucocorticoid Receptor (GR), a ligand-activated transcription factor which both positively, and negatively regulates gene expression. Extensive research during the past several years has uncovered novel mechanisms by which the GR activates and represses its target genes. Genome-wide studies and mouse models have provided valuable insight into the molecular mechanisms of inflammatory gene regulation by GR. This review focusses on newly identified target genes and GR co-regulators that are important for its anti-inflammatory effects in innate immune cells, as well as mutations within the GR itself that shed light on its transcriptional activity. This research progress will hopefully serve as the basis for the development of safer immune suppressants with reduced side effect profiles.

More than meets the dimer: What is the quaternary structure of the glucocorticoid receptor?

It is widely accepted that the glucocorticoid receptor (GR), a ligand-regulated transcription factor that triggers anti-inflammatory responses, binds specific response elements as a homodimer. Here, we will discuss the original primary data that established this model and contrast it with a recent report characterizing the GR-DNA complex as a tetramer.

DNA binding triggers tetramerization of the glucocorticoid receptor in live cells

Transcription factors dynamically bind to chromatin and are essential for the regulation of genes. Although a large percentage of these proteins appear to self-associate to form dimers or higher order oligomers, the stoichiometry of DNA-bound transcription factors has been poorly characterized in vivo. The glucocorticoid receptor (GR) is a ligand-regulated transcription factor widely believed to act as a dimer or a monomer. Using a unique set of imaging techniques coupled with a cell line containing an array of DNA binding elements, we show that GR is predominantly a tetramer when bound to its target DNA. We find that DNA binding triggers an interdomain allosteric regulation within the GR, leading to tetramerization. We therefore propose that dynamic changes in GR stoichiometry represent a previously unidentified level of regulation in steroid receptor activation. Quaternary structure analysis of other members of the steroid receptor family (estrogen, androgen, and progesterone receptors) reveals variation in oligomerization states among this family of transcription factors. Because GR's oligomerization state has been implicated in therapy outcome, our findings open new doors to the rational design of novel GR ligands and redefine the quaternary structure of steroid receptors.

Self-association energetics of an intact, full-length nuclear receptor: the B-isoform of human progesterone receptor dimerizes in the micromolar range

We are focused on understanding the mechanisms underlying eukaryotic gene regulation, using the human progesterone receptor (PR) and its interactions with its DNA response elements as a model system. An understanding of PR function is complicated by the presence of two transcriptionally distinct isoforms, an 83 kDa A-receptor (PR-A) and a 99 kDa B-receptor (PR-B). The two isoforms are identical except the B-receptor contains an additional 164 residues at its N-terminus. As a first step toward understanding the principles by which the two isoforms assemble at complex promoters, we examined the energetics of PR-B self-association using sedimentation velocity and sedimentation equilibrium methods. Full-length human PR-B was purified to 95% homogeneity from baculovirus-infected insect cells. Using a fluorescence hormone binding assay, we determined the purified protein to be highly active in its ability to bind ligand. Sedimentation velocity studies of hormone-bound PR-B at pH 8.0, 4 degrees C, and 50 mM NaCl demonstrate that it undergoes a concentration-dependent change in its sedimentation coefficient, existing as a 4.0S species at submicromolar concentrations but forming a 5.7S species at higher concentrations. These results strongly suggest that PR-B undergoes self-association in the micromolar range. This hypothesis was examined rigorously using sedimentation equilibrium. Global analysis of the sedimentation equilibrium data demonstrated that PR-B self-association was well described by a monomer-dimer model with a dimerization free energy of -7.2 +/- 0.7 kcal/mol. The role of NaCl in regulating PR-B dimerization was examined by carrying out sedimentation velocity and equilibrium studies under high salt conditions. At 300 mM NaCl, PR-B is exclusively monomeric in the micromolar range, thus revealing a significant ionic contribution to the assembly energetics. Further, the monomer sediments as a structurally homogeneous, but highly asymmetric, 4.0S species. Limited proteolysis of PR-B demonstrated that the hydrodynamic asymmetry is due in part to an extended, nonglobular conformation localized to the N-terminal region of PR-B. In contrast, the DNA binding domain (DBD) and hormone binding domain (HBD) exist as independent structural units, and the activation function N-terminal to the DBD (AF-1) shows moderate structure. These results represent the first rigorous analysis of the self-assembly energetics of an intact nuclear receptor and suggest that PR function is more complex than envisioned by traditional models.

Recruitment of octamer transcription factors to DNA by glucocorticoid receptor

Glucocorticoid receptor (GR) and octamer transcription factors 1 and 2 (Oct-1/2) interact synergistically to activate the transcription of mouse mammary tumor virus and many cellular genes. Synergism correlates with cooperative DNA binding of the two factors in vitro. To examine the molecular basis for these cooperative interactions, we have studied the consequences of protein-protein binding between GR and Oct-1/2. We have determined that GR binds in solution to the octamer factor POU domain. Binding is mediated through an interface in the GR DNA binding domain that includes amino acids C500 and L501. In transfected mammalian cells, a transcriptionally inert wild-type but not an L501P GR peptide potentiated transcriptional activation by Oct-2 100-fold above the level that could be attained in the cell by expressing Oct-2 alone. Transcriptional activation correlated closely with a striking increase in the occupancy of octamer motifs adjacent to glucocorticoid response elements (GREs) on transiently transfected DNAs. Intriguingly, GR-Oct-1/2 binding was interrupted by the binding of GR to a GRE. We propose a model for transcriptional cooperativity in which GR-Oct-1/2 binding promotes an increase in the local concentration of octamer factors over glucocorticoid-responsive regulatory regions. These results reveal transcriptional cooperativity through a direct protein interaction between two sequence-specific transcription factors that is mediated in a way that is expected to restrict transcriptional effects to regulatory regions with DNA binding sites for both factors.

Very-low-density lipoprotein response element in the promoter region of the human plasminogen activator inhibitor-1 gene implicated in the impaired fibrinolysis of hypertriglyceridemia

Hypertriglyceridemia and impaired fibrinolytic function are linked to coronary heart disease and other atherothrombotic disorders. Triglyceride-rich lipoproteins may attenuate fibrinolysis by increasing the plasma levels of plasminogen activator inhibitor-1 (PAI-1). Furthermore, a common 4/5 guanosine (4G/5G) polymorphism in the promoter region of the PAI-1 gene has been indicated to influence plasma PAI-1 activity and to be involved in an allele-specific response to triglycerides. Herein we show by transfection assays that VLDLs induce transcription of the human PAI-1 promoter in endothelial cells. A VLDL response element (VLDLRE) is located to residues -672 to -657 in the promoter region by electromobility shift assay, methylation interference, and DNase I footprinting, and its activity is shown to be influenced by the common 4G/5G polymorphism located adjacent to and upstream of the binding site of a VLDL-inducible transcription factor. These findings may provide a molecular explanation to the link between VLDL and PAI-1 activity elevation in plasma and to the interaction between the 4G/5G polymorphism and plasma triglycerides.

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)

Phosphorylation of immunopurified rat liver glucocorticoid receptor by the catalytic subunit of cAMP-dependent protein kinase

We have examined phosphorylation of the rat liver glucocorticoid receptor (GR) and GR-associated protein kinase (PK) activity in the immunopurified receptor preparations. Affinity labeling of hepatic cytosol with [3H]dexamethasone 21-mesylate showed a covalent association of the steroid with a 94 kDa protein. GR was immunopurified with antireceptor monoclonal antibody BuGR2 (Gametchu & Harrison, Endocrinology 114: 274-279, 1984) to near homogeneity. A 23 degrees C incubation of the immunoprecipitated protein A-Sepharose adsorbed GR with [gamma-32P]ATP,Mg2+ and the catalytic subunit of cAMP-dependent PK (cAMP-PK) from bovine heart, led to an incorporation of radioactivity in the 94 kDa protein. Phosphorylation of GR was not evident in the absence of the added kinase. Of the radioinert nucleotides (ATP, GTP, UTP or CTP) tested, only ATP successfully competed with [gamma-32P]ATP demonstrating a nucleotide specific requirement for the phosphorylation of GR. Other divalent cations, such as Mn2+ or Ca2+, could not be substituted for Mg2+ during the phosphorylation reaction. Phosphorylation of GR was sensitive to the presence of the protein kinase inhibitor, H-8, an isoquinoline sulfonamide derivative. In addition, the incorporation of radioactivity into GR was both time- and temperature-dependent. The phosphorylation of GR by cAMP-PK was independent of the presence of hsp-90 and transformation state of the receptor. The results of this study demonstrate that GR is an effective substrate for action of cAMP-PK and that the immunopurified protein A-Sepharose adsorbed GR lacks intrinsic kinase activity but can be conveniently used for the characterization of the phosphorylation reaction in the presence of an exogenous kinase.

Glucocorticoid receptor binding sites in the promoter region of milk protein genes

The action of glucocorticoids on the induction of the two milk protein genes beta-casein and whey acidic protein (WAP) is delayed and appears to be indirect. The response requires a co-stimulation of cells with prolactin, is restricted to mammary epithelial cells and is dependent on cis-acting sequences localized in the promoter region of the two genes. We have searched for glucocorticoid receptor (GR) binding sites in these hormone response regions. In vitro DNaseI footprinting experiments were performed with a purified preparation of the GR from rat liver. The sequences between -231 and -7 and between -250 and -79 of mouse WAP and rat beta-casein gene promoter, respectively, were found to contain multiple sites which were protected from the attack of DNaseI by the GR preparation. The sites comprise sequence motifs related to the delayed secondary glucocorticoid response elements described (Chan et al., J. Biol. Chem. 266, 22,634-22,644, 1991), which represent half sites of classical GR binding sites. In the WAP gene promoter, the motifs are arranged head to tail with irregular spacing. The GR binding sites are in close proximity or even overlap with the bindings sites for candidate factors involved in mammary cell specific gene expression. The results suggest a direct co-operation between GR and mammary cell specific transcription factors in mediating the expression of milk protein genes.

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.

H-2 gene complex and corticosteroid responsiveness: evidence that the corticosteroid hormone signal transduction pathway in the adult mouse lung is not associated with haplotype-specific responses to corticosteroids

Differential responsiveness to corticosteroids (CORT) has been shown to be related to HLA haplotype. A strong association between the mouse homolog to the human HLA complex, the H-2 complex, and intrauterine responses to CORT have also been demonstrated; haplotype differences alter CORT-induced susceptibility to cleft palate and temporal differences in lung maturation. Since variation in the glucocorticoid receptor (GR) is associated with tissue specific responses to CORT, we hypothesize that haplotype-specific CORT responsiveness may be regulated by H-2 associated modification of GR expression and/or function. Given that H-2 congenic mice are genetically identical except at the H-2 complex on mouse chromosome 17 and the GR structural gene is encoded on chromosome 18, the GR gene is identical in these mice. However, any step in the GR signal transduction pathway may be regulated by gene(s) at or near the H-2 complex and result in haplotype-specific differences in CORT responsiveness. We have investigated differences in qualitative and quantitative characteristics of the adult B10 (H-2b) and B10.A (H-2a) pulmonary GR by Scatchard analysis, immunochemical and biochemical assays. No differences in the GR binding parameters (BMAX and Kd), receptor form and level, or ligand-GR complex binding to glucocorticoid response element (GR-GRE) were detected, leading us to conclude that H-2 associated factors do not regulate the relative intrauterine responses to CORT by modulating the adult GR.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.

One-step 10(4)-fold purification of transformed glucocorticoid receptor. Method for purifying receptors associated with Mr ca. 90,000 heat-shock protein

Chromatography of rabbit glucocorticoid-receptor complexes in the absence of sodium molybdate on a Mono Q anion-exchange column induces the transformation of the receptor and allows the resolution of the transformed and non-transformed molecular species. These abilities were used to design a new purification scheme for the glucocorticoid receptor from rabbit liver in its transformed state. Microgram amounts of receptor were obtained using this single-step procedure in less than 2 h. The purification yield was 50-60%. Immunoblot experiments showed that the glucocorticoid receptor was present as an Mr approximately 94,000 polypeptide in these preparations and represented 20-30% of the eluted proteins as determined by densitometric scanning analysis of silver-stained sodium dodecyl sulphate polyacrylamide gels. Finally, the purified receptor was able to interact quantitatively with bulk DNA.

DNA binding of glucocorticoid receptor protein A fusion proteins expressed in E. coli

In an effort to obtain large quantities of glucocorticoid receptor (GR) protein for functional and structural studies, several truncated versions of the human glucocorticoid receptor (hGR) have been expressed in E. coli as C-terminal fusion proteins with protein A. The amount of expressed protein was between 5 and 25 mg/l in the culture. South-Western blotting was initially used to demonstrate the DNA binding capacity of fusion proteins containing the DNA binding domain of GR. The hybrid proteins were highly enriched in the insoluble fraction after cell lysis. For further purification and characterization the fusion proteins were solubilized in 8 M urea. The concentration of denaturing agent was reduced by dilution and the fusion proteins were allowed to refold. The renatured GR protein A fusion proteins bound to DNA in a nitrocellulose filter binding assay. We also show that it is possible to purify the renatured fusion protein to apparent homogeneity using a single chromatographic step on DNA-cellulose.

The non-activated glucocorticoid receptor: structure and activation

Glucocorticoid hormone receptors are present in the soluble fraction of target cell homogenates as large entities (Mr approximately 300,000) that are unable to interact with DNA. These large complexes contain an Mr approximately 94,000 steroid- and DNA-binding polypeptide, in association with an Mr approximately 90,000 non-ligand-binding entity, which has been identified as a heat shock protein, hsp90. This protein has been purified to near homogeneity as a component of the non-activated receptor complex. Characterization of the purified protein revealed its presence as a dimer in the large receptor form. Dissociation of the receptor-hsp90 complex can be induced by heat treatment only when ligand is bound to the receptor, as demonstrated by specific DNA-binding assay and sucrose gradient ultracentrifugation, hsp90 represents ca 1% of total proteins in rat liver cytosol, and milligram amounts were purified using a combination of high performance ion exchange and gel permeation chromatography. Monospecific antibodies were raised in rabbits. They were found to precipitate the intact non-activated glucocorticoid receptor, as well as the Mr approximately 27,000 steroid-binding fragment of the receptor generated by trypsin treatment, indicating that hsp90 interacts with the steroid-binding domain of the glucocorticoid receptor. Finally, translation of glucocorticoid receptor mRNA in reticulocyte lysate yields a protein which also interacts with hsp90 and binds to DNA only after ligand-binding and heat treatment. Thus, the glucocorticoid receptor is synthesized in a non-activated form also in vitro.

Physicochemical characterization of the nuclear form of Ah receptor from mouse hepatoma cells exposed in culture to 2,3,7,8-tetrachlorodibenzo-p-dioxin

Molecular properties of nuclear aromatic hydrocarbon (Ah) receptor from Hepa-1c1c9 (Hepa-1) cells were assessed by velocity sedimentation on sucrose gradients and by gel permeation chromatography on Sephacryl S-300. Nuclear Ah receptor was obtained by exposing intact cells to [3H]-2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) for 1 h at 37 degrees C in culture followed by extraction of receptor from nuclei with buffers containing 0.5 M KCl. The nuclear Ah receptor was compared to the cytosolic Ah receptor from the same cells. Under conditions of low ionic strength, the Ah receptor from Hepa-1 cytosol sedimented as a single 9.4 +/- 0.63 S binding peak that had a Stokes radius of 7.1 +/- 0.12 nm and an apparent relative molecular mass of 271,000 +/- 16,000. After prolonged (24 h) exposure to high ionic strength (0.5 M KCl), cytosol labeled with [3H]TCDD exhibited two specific binding peaks. The large form of cytosolic Ah receptor seen under high ionic strength conditions sedimented at 9.4 +/- 0.46 S, had a Stokes radius of 6.9 +/- 0.19 nm, and an apparent Mr 267,000 +/- 15,000. The smaller ligand-binding subunit generated by exposing cytosol to 0.5 M KCl sedimented at 4.9 +/- 0.62 S, had a Stokes radius of 5.0 +/- 0.14 nm, and an apparent Mr 104,000 +/- 12,000. Nuclear Ah receptor, analyzed under high ionic strength conditions, sedimented at 6.2 +/- 0.20 S, had a Stokes radius of 6.8 +/- 0.19 nm, and an apparent Mr 176,000 +/- 7000. Nuclear Ah receptor from rat H4IIE hepatoma cells was analyzed and found to have physicochemical characteristics identical to those of nuclear Ah receptor from the mouse Hepa-1 cells. The molecular mass of Hepa-1 nuclear Ah receptor was found to be statistically different from both the Mr approximately 267,000 cytosolic Ah receptor and the Mr approximately 104,000 subunit which were present in cytosol under high ionic strength conditions. Hepa-1 nuclear Ah receptor could not be converted to a smaller ligand-binding subunit by treatment with alkaline phosphatase, ribonuclease, or sulfhydryl-modifying reagents or prolonged exposure to 1.0 M KCl. Cytosolic Ah receptor from Hepa-1 cells was "transformed" by heating at 25 degrees C in vitro into a form with high affinity for DNA-cellulose. The transformed cytosolic Ah receptor, when analyzed under conditions of high ionic strength, sedimented at approximately 6 S, had a Stokes radius of approximately 6.7 nm, and an apparent Mr approximately 167,000.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular interactions of steroid hormone receptor with its enhancer element: evidence for receptor dimer formation

A steroid hormone responsive element (GRE/PRE), sufficient to confer glucocorticoid and progesterone inducibility when linked to a reporter gene, was used in band-shift assays to examine its molecular interactions with steroid hormone receptors. Both progesterone and glucocorticoid receptors bound directly and specifically to the GRE/PRE. The purine contact sites for both form A and form B chicken progesterone receptor, as well as those for rat glucocorticoid receptor, are identical. A peptide fragment produced in bacteria that primarily contain the DNA binding domain of the glucocorticoid receptor binds first to the TGTTCT half-site of the GRE/PRE, and a second molecule binds subsequently to the TGTACA (half-site) of the GRE/PRE in a cooperative manner. Utilizing the peptide fragment and the protein A-linked fragment, we demonstrated that the receptor interacts with its cognate enhancer as a dimer.

Functional probing of glucocorticoid receptor structure

The glucocorticoid receptor (GR) consists of three functional domains. The steroid-binding domain is situated at the C-terminal end and N-terminal sequence analysis has identified the border of this domain at residue 518. The DNA-binding domain is central and lies between residues 414 and 517. The remaining N-terminal half of the protein constitutes the third domain. Probing of the steroid-receptor interaction by affinity-labelling and radiosequence analysis has identified three steroid-binding amino acid residues, Met-622, Cys-656 and Cys-754.

Hormone-mediated repression: a negative glucocorticoid response element from the bovine prolactin gene

We have defined and characterized a region upstream of the bovine prolactin gene that confers repression by glucocorticoids. This 'negative glucocorticoid response element' (nGRE) contains multiple footprinting sites for purified glucocorticoid receptor protein between -51 and -562 bp. A strong consensus sequence for receptor binding within the nGRE has not yet been defined, but it is apparent that nGRE sequences differ from the GRE consensus elements that confer positive glucocorticoid regulation. Unlike 'positive' GREs, the nGRE enhances promoter activity in the absence of glucocorticoids or receptor, presumably through the action of a protein that binds in the same region and activates transcription. The hormone-receptor complex appears to negate this enhancement by competing or inactivating the second factor. As with positive GREs, nGRE sequences confer hormonal regulation upon linked heterologous promoters within various cell types; a 34-bp subfragment containing a single receptor binding site is sufficient for nGRE activity. We speculate that nGRE sequences might alter the structure of bound receptor, thereby preventing it from functioning as a positive regulator when bound at those sites.

Activation of the human glucocorticoid receptor: evidence for a two-step model

The relationship between glucocorticoid receptor subunit dissociation and activation was investigated by DEAE-cellulose and DNA-cellulose chromatography of monomeric and multimeric [3H]triamcinolone acetonide ([3H]TA)-labeled IM-9 cell glucocorticoid receptors. Multimeric (7-8 nm) and monomeric (5-6 nm) complexes were isolated by Sephacryl S-300 chromatography. Multimeric complexes did not bind to DNA-cellulose and eluted from DEAE-cellulose at a salt concentration (0.2 M KCl) characteristic of unactivated steroid-receptor complexes. Monomeric [3H]TA-receptor complexes eluted from DEAE-cellulose at a salt concentration (20 mM KCl) characteristic of activated steroid-receptor complexes. However, only half of these complexes bound to DNA-cellulose. This proportion could not be increased by heat treatment, addition of bovine serum albumin, or incubation with RNase A. Incubation of monomeric complexes with heat inactivated cytosol resulted in a 2-fold increase in DNA-cellulose binding. Unlike receptor dissociation, this increase was not inhibited by the presence of sodium molybdate. Fractionation of heat inactivated cytosol by Sephadex G-25 chromatography demonstrated that the activity responsible for the increased DNA binding of monomeric [3H]TA-receptor complexes was macromolecular. These results are consistent with a two-step model for glucocorticoid receptor activation, in which subunit dissociation is a necessary but insufficient condition for complete activation. They also indicate that conversion of the steroid-receptor complex to the low-salt eluting form is a reflection of receptor dissociation but not necessarily acquisition of DNA-binding activity.

Requirement of hormone for thermal conversion of the glucocorticoid receptor to a DNA-binding state

A central question arising from the model of eukaryotic gene regulation by steroid hormone receptors is whether or not proteins represent pre-existing gene regulatory proteins that are activated on exposure to the extracellular signal. It has been generally believed that the ligand-binding of steroid hormone receptors triggers an allosteric change in receptor structure, manifested by an increased affinity of the receptor for DNA in vitro and nuclear target elements in vivo, as monitored by nuclear translocation. But this model has been challenged by recent reports indicating that glucocorticoid and progesterone receptors bind specifically in vitro to target DNA sequences even in the absence of hormone. On the other hand, it appears that the hormone induces protection in vivo of the glucocorticoid response element of the tyrosine amino transferase gene. Here we show that under conditions permitting minimal in vitro manipulation, the steroid-free glucocorticoid receptor in crude cytosol associates with the hsp90 heat shock protein (relative molecular mass Mr approximately equal to 90,000) to form a large 300K complex, rather than the 94K liganded receptor monomer. More importantly, we have developed an assay to demonstrate the requirement of hormone to dissociate the 300K complex by heat treatment. Specific DNA-binding activity of the receptor becomes apparent in this process, showing that DNA binding occurs but is inhibited in the large heteromeric complex. We propose a model in which receptor function is repressed by association of the receptor with hsp90. Dissociation of this complex is induced by the binding of steroid and is apparently an irreversible process.

Structure and function of the glucocorticoid receptor

The glucocorticoid receptor protein is a transcriptional regulatory protein that interacts with specific enhancer sequences. A stoichiometric analysis of the interaction indicates an equimolar relationship between the receptor protein and the specific enhancer sequences. The activity of the receptor protein is itself regulated by the binding of glucocorticoids. The two functional domains (DNA-binding and steroid-binding) are adjacent and lie within the C-terminal half of the receptor protein. The N-terminal half of the protein appears to modulate the interaction with DNA but does not have any role in the binding of the steroid. The functional domains have also been defined at the sequence level.