Modular structure of glucocorticoid receptor domains is not equivalent to functional independence. Stability and activity of the steroid binding domain are controlled by sequences in separate domains

Effects of parabens on adipocyte differentiation

Parabens are a group of alkyl esters of p-hydroxybenzoic acid that include methylparaben, ethylparaben, propylparaben, butylparaben, and benzylparaben. Paraben esters and their salts are widely used as preservatives in cosmetics, toiletries, food, and pharmaceuticals. Humans are exposed to parabens through the use of such products from dermal contact, ingestion, and inhalation. However, research on the effects of parabens on health is limited, and the effects of parabens on adipogenesis have not been systematically studied. Here, we report that (1) parabens promote adipogenesis (or adipocyte differentiation) in murine 3T3-L1 cells, as revealed by adipocyte morphology, lipid accumulation, and mRNA expression of adipocyte-specific markers; (2) the adipogenic potency of parabens is increased with increasing length of the linear alkyl chain in the following potency ranking order: methyl- < ethyl- < propyl- < butylparaben. The extension of the linear alkyl chain with an aromatic ring in benzylparaben further augments the adipogenic ability, whereas 4-hydroxybenzoic acid, the common metabolite of all parabens, and the structurally related benzoic acid (without the OH group) are inactive in promoting 3T3-L1 adipocyte differentiation; (3) parabens activate glucocorticoid receptor and/or peroxisome proliferator-activated receptor γ in 3T3-L1 preadipocytes; however, no direct binding to, or modulation of, the ligand binding domain of the glucocorticoid receptor by parabens was detected by glucocorticoid receptor competitor assays; and lastly, (4) parabens, butyl- and benzylparaben in particular, also promote adipose conversion of human adipose-derived multipotent stromal cells. Our results suggest that parabens may contribute to obesity epidemic, and the role of parabens in adipogenesis in vivo needs to be examined further.

Glucocorticoid resistance in two key models of acute lymphoblastic leukemia occurs at the level of the glucocorticoid receptor

Glucocorticoids (GCs) specifically induce apoptosis in malignant lymphoblasts and are thus pivotal in the treatment of acute lymphoblastic leukemia (ALL). However, GC-resistance is a therapeutic problem with an unclear molecular mechanism. We generated approximately 70 GC-resistant sublines from a GC-sensitive B- and a T-ALL cell line and investigated their mechanisms of resistance. In response to GCs, all GC-resistant subclones analyzed by real-time polymerase chain reaction (PCR) showed a deficient up-regulation of the GC-receptor (GR) and its downstream target, GC-induced leucine zipper. This deficiency in GR up-regulation was confirmed by Western blotting and on retroviral overexpression of GR in resistant subclones GC-sensitivity was restored. All GC-resistant subclones were screened for GR mutations using denaturing high-pressure liquid chromatography (DHPLC), DNA-fingerprinting, and fluorescence in situ hybridization (FISH). Among the identified mutations were some previously not associated with GC resistance: A484D, P515H, L756N, Y663H, L680P, and R714W. This approach revealed three genotypes, complete loss of functional GR in the mismatch repair deficient T-ALL model, apparently normal GR genes in B-ALLs, and heterozygosity in both. In the first genotype, deficiency in GR up-regulation was fully explained by mutational events, in the second by a putative regulatory defect, and in the third by a combination thereof. In all instances, GC-resistance occurred at the level of the GR in both models.

Chaperoning of glucocorticoid receptors

A multiprotein hsp90/hsp70-based chaperone machinery functions as a 'cradle-to-grave' system for regulating the steroid binding, trafficking and turnover of the glucocorticoid receptor (GR). In an ATP-dependent process where hsp70 and hsp90 act as essential chaperones and Hop, hsp40, and p23 act as nonessential co-chaperones, the machinery assembles complexes between the ligand binding domain of the GR and hsp90. During GR-hsp90 heterocomplex assembly, the hydrophobic ligand-binding cleft is opened to access by steroid, and subsequent binding of steroid within the cleft triggers a transformation of the receptor such that it engages in more dynamic cycles of assembly/disassembly with hsp90 that are required for rapid dynein-dependent translocation to the nucleus. Within the nucleus, the hsp90 chaperone machinery plays a critical role both in GR movement to transcription regulatory sites and in the disassembly of regulatory complexes as the hormone level declines. The chaperone machinery also plays a critical role in stabilization of the GR to ubiquitylation and proteasomal degradation. The initial GR interaction with hsp70 appears to be critical for the triage between hsp90 heterocomplex assembly and preservation of receptor function vs CHIP-dependent ubiquitylation and proteasomal degradation. The hsp90 chaperone machinery is ubiquitous and functionally conserved among eukaryotes, and it is possible that all physiologically significant actions of hsp90 require the hsp70-dependent assembly of client protein-hsp90 heterocomplexes.

Ligand binding is without effect on complex formation of the ligand binding domain of the ecdysone receptor (EcR)

The ligand-binding domain (LBD) encompassing the C-terminal parts of the D- and the complete E-domains of the ecdysteroid receptor (EcR) fused to Gal4(AD) is present in two high molecular weight complexes (600 and 150 kDa) in yeast extracts according to size exclusion chromatography (Superdex 200 HR 10/30). Hormone binding is mainly associated with 150-kDa complexes. Complex formation is not influenced by hormone, but the ligand stabilizes the complexes at elevated salt concentrations. Mutational analysis of Gal4(AD)-EcR(LBD) revealed that formation of 600-kDa, but not 150-kDa, complexes depends on dimerization mediated by the EcR(LBD). Deletion of helix 12 is without effect. Mutation of K497 in helix 4, known to be essential for comodulator binding, abolishes 600-KDa complexes, but does not interfere with the formation of 150-kDa complexes. In contrast, the DE-domains of USP fused to Gal4(DBD) elute as monomer after elimination of the dimerization capacity of the ligand-binding domains by mutation of P463 in helix 10. The data presented here reveal that the complex formation of ligand-binding domains EcR and USP ligand is different.

Glucocorticoid Receptor Ligand Binding Domain Is Sufficient for the Modulation of Glucocorticoid Induction Properties by Homologous Receptors, Coactivator Transcription Intermediary Factor 2, and Ubc9

Several factors modulate the position of the dose-response curve of steroid receptor-agonist complexes and the partial agonist activity of antagonist complexes, thereby causing differential gene activation by circulating hormones and unequal gene repression during endocrine therapies with antisteroids. We now ask whether the modulatory activity of three factors (homologous receptor, coactivator transcription intermediary factor 2, and Ubc9) requires the same or different domains of glucocorticoid receptors (GRs). In all cases, we find that neither the amino terminal half of the receptor, which contains the activation function-1 activation domain, nor the DNA binding domain is required. This contrasts with the major role of activation function-1 in determining the amount of gene expression and partial agonist activity of antisteroids with most steroid receptors. However, the situation is more complicated with Ubc9, where GR N-terminal sequences prevent the actions of Ubc9, but not added GR or transcription intermediary factor 2, at low GR concentrations. Inhibition is relieved by deletion of these sequences or by replacement with the comparable region of progesterone receptors but not by overexpression of the repressive sequences. These results plus the binding of C-terminal GR sequences to the suppressive N-terminal domain implicate an intramolecular mechanism for the inhibition of Ubc9 actions at low GR concentrations. A shift from noncooperative to cooperative steroid binding at high GR concentrations suggests that conformational changes reposition the inhibitory N-terminal sequence to allow Ubc9 interaction with elements of the ligand binding domain. Collectively, these results indicate a dominant role of GR C-terminal sequences in the modulation of the dose-response curve and partial agonist activity of GR complexes. They also reveal mechanistic differences both among individual modulators and between the ability of the same factors to regulate the total amount of gene expression.

The distinct agonistic properties of the phenylpyrazolosteroid cortivazol reveal interdomain communication within the glucocorticoid receptor

Recent structural analyses of the nuclear receptors establish a paradigm of receptor activation, in which agonist binding induces the ligand binding domain (LBD)/activation function-2 helix to form a charge clamp for coactivator recruitment. However, these analyses have not sufficiently addressed the mechanisms for differential actions of various synthetic steroids in terms of fine tuning of multiple functions of whole receptor molecules. In the present study, we used the glucocorticoid receptor (GR)-specific agonist cortivazol (CVZ) to probe the plasticity and functional modularity of the GR. Structural docking analysis revealed that although CVZ is more bulky than other agonists, it can be accommodated in the ligand binding pocket of the GR by reorientation of several amino acid side chains but without major alterations in the active conformation of the LBD. In this induced fit model, the phenylpyrazole A-ring of CVZ establishes additional contacts with helices 3 and 5 of the LBD that may contribute to a more stable LBD configuration. Structural and functional analysis revealed that CVZ is able to compensate for the deleterious effects of a C-terminal deletion of the LBD in a manner that mimics the stabilizing influence of the F602S point mutation. CVZ-mediated productive recruitment of transcriptional intermediary factor 2 to the C-terminally deleted LBD requires the receptor's own DNA binding domain and is positively influenced by the N-terminal regions of GR or progesterone receptor. These results support a model where ligand-dependent conformational changes in the LBD play a role in GR-mediated gene regulation via modular interaction with the DBD and activation function-1.

Identification of glucocorticoid receptor domains involved in transrepression of transforming growth factor-beta action

The transforming growth factor-beta (TGF-beta) and glucocorticoid signaling pathways interact both positively and negatively in regulating a variety of physiological and pathologic processes. We previously reported that liganded glucocorticoid receptor (GR) repressed TGF-beta induction of human plasminogen activator inhibitor-1 gene transcription by directly targeting the transcriptional activation function of Smad3. To identify the domain(s) in the glucocorticoid receptor involved in this repression, we have examined the ability of various GR truncation, deletion, and substitution mutants to repress TGF-beta transactivation in Hep3B human hepatoma cells that lack functional endogenous GR. Partial deletions in the ligand-binding domain (LBD), including the tau2 and tauc regions, greatly reduced or eliminated GR repression, whereas deletion of the N-terminal AF1 (tau1) domain and substitution mutations in the DNA-binding domain had little or no effect. Liganded androgen receptor repressed TGF-beta transactivation, whereas mineralocorticoid receptor did not, and studies with rat GR-mineralocorticoid receptor chimeras confirmed that the GR C-terminal domains were required for repression. RU486, a strong antagonist of transactivation by GR, partially reversed repression by wild type GR. Co-immunoprecipitation experiments in Hep3B cells indicated that physical interaction between GR and Smad3 is necessary but not sufficient for repression. Physical interaction required activation of Smad3 by TGF-beta but not dexamethasone binding to GR. Glutathione S-transferase pull-down assays demonstrated that several regions of the LBD could mediate GR-Smad3 physical interaction. We conclude that the LBD of GR, but not the DNA-binding domain or the N-terminal activation domain, is required for GR-mediated transrepression of TGF-beta transactivation.

Mutations at positions 547-553 of rat glucocorticoid receptors reveal that hsp90 binding requires the presence, but not defined composition, of a seven-amino acid sequence at the amino terminus of the ligand binding domain

Glucocorticoid receptors (GRs) must heterocomplex with hsp90 to have an open steroid binding cleft that can be accessed by steroid. We reported that a seven-amino acid sequence (547-553 of rat GR) overlapping the amino-terminal end of the ligand binding domain is required for hsp90 binding to GR. We have now conducted saturation mutagenesis of this sequence, which appears to be part of the surface where the ligand binding cleft merges with the surface of the ligand binding domain. No single point mutation causes significant changes in any of a variety of biochemical and biological properties in addition to hsp90 binding. A triple mutation (P548A/T549A/V551A) increases by >100-fold the steroid concentration required for half-maximal induction without affecting the level of maximal induction or coactivator response. Interestingly, this triple mutant displays reduced binding of steroid and hsp90 in whole cells, but it possesses wild type affinity for steroid and normal hsp90 binding capacity under cell-free conditions. This phenotype of a dramatic shift in the dose response for transactivation would be expected from an increase in the rate of disassembly of the triple mutant GR.hsp90 heterocomplex in the cell. Mutation of the entire seven-amino acid region to CAAAAAC maintains the presence of a critical alpha-helical structure and heterocomplex formation with hsp90 but eliminates steroid binding and transcriptional activation, thus disconnecting hsp90 binding from opening of the ligand binding cleft and steroid binding.

Mutations flanking the polyglutamine repeat in the modulatory domain of rat glucocorticoid receptor lead to an increase in affinity for hormone

A polyglutamine repeat in the N-terminus of the rat glucocorticoid receptor shows polymorphism, with variants of Q2RQ5, Q2RQ15-21. We investigated whether these natural polymorphisms affect receptor function, and whether alleles with polyglutamine repeats shorter than Q2RQ5, between Q2RQ6-14, or longer than Q2RQ21 are not found naturally because they encode a dysfunctional receptor. Ligand binding and transactivation properties of sets of natural (Q2RQ5-Q2RQ21) and artificial (Q4-Q80) alleles were compared following expression in CV-1 cells. The sequence of artificial alleles at sites flanking the repeat region was altered slightly to facilitate cloning. Western blotting showed that all constructs expressed GR protein in CV-1 cells. When co-expressed with an MMTV-lacZ reporter plasmid, all GR proteins were shown to be transcriptionally active in the presence of hormone. Scatchard analysis of ligand binding curves showed that affinities for dexamethasone and corticosterone were not affected by variation in the polyglutamine repeat either the natural or artificial sets of alleles. However, affinities were greater for the artificial compared with the natural alleles (2-3-fold for dexametasone, p < 0.001; and 4-fold for corticosterone,p < 0.001). These differences provide evidence of a direct or indirect interaction within GR between the ligand binding domain and residues flanking the polyglutamine repeat of the N-terminal domain.

The proper folding of a long C-terminal segment of the yeast Lys14p regulator is required for activation of LYS genes in response to the metabolic effector

Transcription of lysine genes in Saccharomyces cerevisiae is dependent on Lys14p and on alpha-aminoadipate semialdehyde (alphaAASA), an intermediate of the pathway. The two-thirds C-terminal end of Lys14p is sufficient to ensure the activation function of the protein and its modulation by alphaAASA. Here, we show that no single discrete domain of Lys14p is able to activate transcription and that most of the deleted LexA-Lys14p proteins are inactive even in the presence of a high alphaAASA concentration. The point mutations abolishing the activation capacity of Lys14p are distributed all over the entire C-terminal segment. Although the deletion of 20 residues rich in leucine and located downstream of the DNA-binding domain converts Lys14p to a constitutive transcriptional activator, our analysis provides evidence that the modulation process of Lys14p activity does not involve an effector-dependent masking/unmasking mechanism. Furthermore, we show that the protein chaperone Hsp82p is required for full activation of LYS genes by the alphaAASA-activated Lys14p as well as by the constitutive Lys14p. Our results suggest that the proper folding of the two-thirds C-terminal portion of Lys14p is essential not only to activate transcription but also to modulate it according to alphaAASA concentration.

Transactivation specificity of glucocorticoid versus progesterone receptors. Role of functionally different interactions of transcription factors with amino- and carboxyl-terminal receptor domains

A major unanswered question of glucocorticoid and progesterone action is how different whole cell responses arise when both of the cognate receptors can bind to, and activate, the same hormone response elements. We have documented previously that the EC(50) of agonist complexes, and the partial agonist activity of antagonist complexes, of both glucocorticoid receptors (GRs) and progesterone receptors (PRs) are modulated by increased amounts of homologous receptor and of coregulators. We now ask whether these components can differentially alter GR and PR transcriptional properties. To remove possible cell-specific differences, we have examined both receptors in the common environment of a line of mouse mammary adenocarcinoma (1470.2) cells. In order to segregate the responses that might be due to unequal nucleosome reorganization by the two receptors from those reflecting interactions with other components, we chose a transiently transfected reporter containing a simple glucocorticoid response element (i.e. GREtkLUC). No significant differences are found with elevated levels of either receptor. However, major, qualitative differences are seen with the corepressors SMRT and NCoR, which afford opposite responses with GR and PR. Studies with chimeric GR/PR receptors indicate that no one segment of PR or GR is responsible for these properties and that the composite response likely involves interactions with both the amino and carboxyl termini of receptors. Collectively, the data suggest that GR and PR induction of responsive genes in a given cell can be differentially controlled, in part, by unequal interactions of multiple receptor domains with assorted nuclear cofactors.

Asymmetric catalytic ene-cyclization approach to 2-fluoro-19-nor-1,25-dihydroxyvitamin D(3) A-ring analog with significant transactivation activity

1alpha,25-Dihydroxyvitamin D(3) (1alpha,25(OH)2D3) has been shown to modulate not only proliferation and differentiation, but also apoptosis in malignant cells, indicating that it could be useful for the treatment of cancer and psoriasis. However, little information has been available on the binding conformation of the 1alpha,25(OH)2D3 molecule and its analogs with the vitamin D receptor (VDR). Therefore, we synthesized 2alpha-fluorinated A-ring analogs of 19-nor-1alpha,25(OH)2D3 in order to investigate the VDR-binding conformation of the A-rings on the basis of the (19)F NMR analysis. The 2alpha-fluoro-19-nor-1alpha,25-dihydroxyvitamin D3 A-ring analog thus synthesized via a asymmetric catalytic carbonyl-ene cyclization, shows significant activity in transactivation.

Drug discovery and the intracellular receptor family

Drug discovery using intracellular receptors (IRs) as targets presents its own set of unique complications and advantages. The natural ligands for these receptors are, in many cases, already used as drugs. To effectively exploit these targets, newer molecules must have either increased receptor selectivity or increased tissue or gene selectivity to reduce side effects. The search for these molecules will yield new therapeutics as well as new insights into the mechanism of action of these receptors and their ligands.

Differential estrogen receptor binding of estrogenic substances: a species comparison

The study investigated the ability of 34 natural and synthetic chemicals to compete with [3H]17beta-estradiol (E2) for binding to bacterially expressed glutathione-S-transferase (GST)-estrogen receptors (ER) fusion proteins from five different species. Fusion proteins consisted of the ER D, E and F domains of human alpha (GST-hERalphadef), mouse alpha (GST-mERalphadef), chicken (GST-cERdef), green anole (GST-aERdef) and rainbow trout ERs (GST-rtERdef). All five fusion proteins displayed high affinity for E2 with dissociation constants (K(d)) ranging from 0.3 to 0.9 nM. Although, the fusion proteins exhibited similar binding preferences and binding affinities for many of the chemicals, several differences were observed. For example, alpha-zearalenol bound with greater affinity to GST-rtERdef than E2, which was in contrast to other GST-ERdef fusion proteins examined. Coumestrol, genistein and naringenin bound with higher affinity to the GST-aERdef, than to the other GST-ERdef fusion proteins. Many of the industrial chemicals examined preferentially bound to GST-rtERdef. Bisphenol A, 4-t-octylphenol and o,p' DDT bound with approximately a ten-fold greater affinity to GST-rtERdef than to other GST-ERdefs. Methoxychlor, p,p'-DDT, o,p'-DDE, p,p'-DDE, alpha-endosulfan and dieldrin weakly bound to the ERs from the human, mouse, chicken and green anole. In contrast, these compounds completely displaced [3H]E2 from GST-rtERdef. These results demonstrate that ERs from different species exhibit differential ligand preferences and relative binding affinities for estrogenic compounds and that these differences may be due to the variability in the amino acid sequence within their respective ER ligand binding domains.

Catalytic asymmetric syntheses and biological activities of singly dehydroxylated 19-nor-1alpha,25-dihydroxyvitamin D(3) A-ring analogs in cancer cell differentiation and apoptosis

BACKGROUND

1alpha,25-Dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)) has been shown to modulate not only proliferation and differentiation, but also apoptosis in malignant cells, indicating that it could be useful for treating cancer. Little information is available concerning the structural motifs of the 1alpha, 25(OH)(2)D(3) molecule responsible for modulation of differentiation and apoptosis, however. We set out to synthesize singly dehydroxylated A-ring analogs of 19-nor-1alpha,25(OH)(2)D(3) in a catalytic asymmetric fashion, and to investigate their biological activities in leukemia HL-60 cells.

RESULTS

A series of singly dehydroxylated 19-nor-1alpha,25-dihydroxyvitamin D(3) A-ring analogs were synthesized using a combinatiorial sequence of regioselective propiolate-ene reaction and catalytic asymmetric carbonyl-ene cyclization. Surprisingly, the analogs could be clearly divided into two categories; one group, bearing 1alpha-hydroxy or 3beta-hydroxy groups in the A-ring, were potent differentiators and the second group, bearing 1beta-hydroxy or 3alpha-hydroxy groups, were potent stimulators of apoptosis.

CONCLUSIONS

We have clearly identified the structural motifs of 19-nor-1alpha,25(OH)(2)D(3) analogs responsible for differentiation and apoptosis in HL-60 cells. These findings will provide useful information not only for development of therapeutic agents for treatment of leukemia and other cancers, but also for structure-function studies of 1alpha,25(OH)(2)D(3).

Cloning and expression of rat glucocorticoid-receptor DNA-binding domain

The inability to obtain sufficient quantities of purified glucocorticoid receptor (GR) causes difficulties in studying in vitro the interactions of GR with other proteins which contribute to the transcriptional activation of chromatin. Thus, we overexpressed the DNA-binding domains (DBD) of GR in E. coli using glutathione S-transferase (GST)-fusion protein expression vector. The DBD-GST protein (about 57 kDa), recovered in the soluble fraction, was purified by glutathione-agarose column. However, during the chromatography, more than 90% of the DBD-GST protein was cleaved into three species with molecular sizes of about 40, 35 and 30 kDa. The 40 kDa and 35 kDa proteins were immunostained with the anti-GR antibody, and the 30 kDa protein was stained with anti-GST antibody. Among these proteins, only the 57 kDa DBD-GST protein bound to the GRE-agarose. These data indicate that the properly folded 57 kDa protein has a DNA binding function and is resistant to intrinsic cleavage of the protein.

Functional analysis of R651 mutations in the putative helix 6 of rat glucocorticoid receptors

Trypsin digestion of steroid-free, but not steroid-bound, rat glucocorticoid receptor (GR) has recently been reported to occur at arginine-651 (R651). This residue is close to the affinity labeled Cys-656 and thus could be a sensitive probe of steroid binding. This hypothesis is supported by the current model of the GR ligand binding domain (LBD), which is based on the X-ray structures of several related receptor LBDs and places R651 in the middle of the putative alpha-helix 6 (649-EQRMS-653 of rat GR), close to the bound steroid. To test this model, R651, which could be involved in hydrophilic and/or hydrogen bonding, was mutated to alanine (A), which favors alpha-helices, the helix breakers proline (P) and glycine (G), or tryptophan (W). All receptors were expressed at about the same level, as determined by Western blots, but the cell-free binding activity of R651P was reduced twofold. The cell-free binding affinities were all within a factor of 10 of wild type receptors. Whole cell biological activity with transiently transfected receptors was determined with a variety of GR agonists (dexamethasone and deacylcortivazol) or antagonists (dexamethasone mesylate, RU486, and progesterone). Reporters containing both simple (GRE) and complex (MMTV) enhancers were used to test for alterations in GR interactions with enhancer/promoter complexes. Surprisingly, no correlation was observed between biological activity and ability to preserve alpha-helical structures for each point mutation. Finally, similar trypsin digestion patterns indicated no major differences in the tertiary structure of the mutant receptors. Collectively, these results argue that the polar/ionizable residue R651 is not required for GR activity and is not part of an alpha-helix in the steroid-free or bound GR. The effect of these mutations on GR structure and activity may result from a cascade of initially smaller perturbations. These LBD alterations were the most varied for interactions with deacylcortivazol and RU 486, which have recently been predicted to be sub-optimal binders due to their large size. However, further analyses of ligand size versus affinity suggest that there is no narrowly defined optimal size for ligand binding, although larger ligands may be more sensitive to modifications of LBD structure. Finally, the changes in GR activity with the various mutations seem to result from altered LBD interactions with common, as opposed to enhancer specific, transcription factors.

The seven amino acids (547-553) of rat glucocorticoid receptor required for steroid and hsp90 binding contain a functionally independent LXXLL motif that is critical for steroid binding

Hsp90 association with glucocorticoid receptors (GRs) is required for steroid binding. We recently reported that seven amino acids (547-553) overlapping the amino-terminal end of the rat GR ligand-binding domain are necessary for hsp90 binding, and consequently steroid binding. The role of a LXXLL motif at the COOH terminus of this sequence has now been analyzed by determining the properties of Leu to Ser mutations in full-length GR and glutathione S-transferase chimeras. Surprisingly, these mutations decreased steroid binding capacity without altering receptor levels, steroid binding affinity, or hsp90 binding. Single mutations in the context of the full-length receptor did not affect the transcriptional activity but the double mutant (L550S/L553S) was virtually inactive. This biological inactivity was found to be due to an increased rate of steroid dissociation from the activated mutant complex. These results, coupled with those from trypsin digestion studies, suggest a model in which the GR ligand-binding domain is viewed as having a "hinged pocket," with the hinge being in the region of the trypsin digestion site at Arg(651). The pocket would normally be kept shut via the intramolecular interactions of the LXXLL motif at amino acids 550-554 acting as a hydrophobic clasp.

Steroid-induced conformational changes of rat glucocorticoid receptor cause altered trypsin cleavage of the putative helix 6 in the ligand binding domain

Steroid-induced changes in receptor protein conformation constitute a logical means of translating the variations in steroid structures into the observed array of whole cell biological activities. One conformational change in the rat glucocorticoid receptor (GR) can be readily discerned by following the ability of trypsin digestion to afford a 16-kDa fragment. This fragment is seen after proteolysis of steroid-free receptors but disappears in digests of either glucocorticoid- or antiglucocorticoid-bound receptors. The location of this cleavage site has now been located unambiguously as R651, in helix 6 of the ligand binding domain, by a combination of point mutagenesis, arginine specific protease digestion, and radiochemical sequencing. This 16-kDa species, corresponding to amino acids 652-795, was non-covalently associated with another, approximately 17-kDa species that was determined to be amino acids 518-651 after a comparison of co-immunoprecipitated fragments from wild type and two chimeric receptors. These assignments revise our earlier report of amino acids 537-673 being the 16-kDa fragment and suggest that sequences of the entire ligand binding domain are required for high affinity and specificity binding. This was supported by the observation that trypsin digestion of the steroid-free R651A mutant GR gave rise to the 30-kDa meroreceptor (amino acids 518-795), which displayed wild type affinity. This 30-kDa species is thus the smallest non-associated fragment of GR possessing wild type steroid binding affinity. This suggests that other GR regions do not influence steroid binding affinity. The above results are reminiscent of those observed for the estrogen receptor. However, unlike the estrogen receptor or the more closely related progesterone receptor, the precise proteolytic cleavage points of both the steroid-free and -bound GR fall within regions that are predicted, on the basis of X-ray crystal structures of related receptors, to be alpha-helical and resistant to proteolysis. Thus, the tertiary structure of the GR ligand binding domain may be distinctly different from that of estrogen and progesterone receptors.

Discrimination between NL1- and NL2-mediated nuclear localization of the glucocorticoid receptor

Glucocorticoid receptor (GR) cycles between a free liganded form that is localized to the nucleus and a heat shock protein (hsp)-immunophilin-complexed, unliganded form that is usually localized to the cytoplasm but that can also be nuclear. In addition, rapid nucleocytoplasmic exchange or shuttling of the receptor underlies its localization. Nuclear import of liganded GR is mediated through a well-characterized sequence, NL1, adjacent to the receptor DNA binding domain and a second, uncharacterized motif, NL2, that overlaps with the ligand binding domain. In this study we report that rapid nuclear import (half-life [t1/2] of 4 to 6 min) of agonist- and antagonist-treated GR and the localization of unliganded, hsp-associated GRs to the nucleus in G0 are mediated through NL1 and correlate with the binding of GR to pendulin/importin alpha. By contrast, NL2-mediated nuclear transfer of GR occurred more slowly (t1/2 = 45 min to 1 h), was agonist specific, and appeared to be independent of binding to importin alpha. Together, these results suggest that NL2 mediates the nuclear import of GR through an alternative nuclear import pathway. Nuclear export of GR was inhibited by leptomycin B, suggesting that the transfer of GR to the cytoplasm is mediated through the CRM1-dependent pathway. Inhibition of GR nuclear export by leptomycin B enhanced the nuclear localization of both unliganded, wild-type GR and hormone-treated NL1(-) GR. These results highlight that the subcellular localization of both liganded and unliganded GRs is determined, at least in part, by a flexible equilibrium between the rates of nuclear import and export.

Quantity of partial agonist activity for antiglucocorticoids complexed with mutant glucocorticoid receptors is constant in two different transactivation assays but not predictable from steroid structure

An unsolved question in steroid hormone action is why the amount of agonist activity displayed by antisteroids is not constant but varies with the assay conditions. Receptor mutations have provided insight into hormone action, presumably due to changes in the tertiary structure of the receptor that alter its interaction surfaces with the transcriptional machinery or/and co-factors. We have now employed two mechanistically different induction assays to determine whether disparate transactivation processes are similarly altered by receptor mutations. The two activation assays studied were (i) the standard induction of GREtkLUC in transiently transfected CV-1 cells and (ii) a novel modulation of endogenous receptor activity by transiently transfected receptors in HeLa cells. Five different mutations in the ligand binding and DNA binding domains of the rat glucocorticoid receptor (CS1, CS1/CD, 451/9, C656G, and R732Q) and seven steroids of varied structures (five antagonists and two agonists) were selected for use. The results in both induction assays were the same. However, no generalizations regarding steroid structure and activity emerged. Neither of two potent glucocorticoids were active with GR-CS1, or GR-CS1/CD, while RU 486 was the only antisteroid with appreciable agonist activity. With the GR-451/9 mutant, three antagonists afforded partial agonist activity. We confirmed that the C656G mutant is both "super-sensitive" and "super-selective" for transactivation. In contrast, the R732Q mutation caused significant decreases in activity with both antagonists and subsaturating concentrations of agonists. This inability to generalize about the behavior of any class of steroids with mutant receptors may reflect an induced fit for each receptor steroid complex. Nevertheless, the activity of a given steroid appeared to be constant in two different transactivation assays for a given mutant receptor. Thus, disparate transactivation processes may utilize identical receptor surfaces, even in the expression of partial agonist activity for specific antiglucocorticoids.

Binding of hsp90 to the glucocorticoid receptor requires a specific 7-amino acid sequence at the amino terminus of the hormone-binding domain

The glucocorticoid receptor (GR) HBD must be bound to the protein chaperone hsp90 in order to acquire the high affinity steroid binding conformation. Despite this crucial role of hsp90, its binding site in GR remains poorly defined. Large portions of the GR HBD have been implicated and no similarity has been established between steroid receptor HBDs and the catalytic domains of the protein kinases (e.g. pp60(src), Raf) that also form stable heterocomplexes with hsp90. Thus, it has been thought that some general property of the proteins, such as exposure of hydrophobic residues in partially denatured regions, determines the assembly of stable hsp90 heterocomplexes. In this work, we have studied fusion proteins containing glutathione S-transferase (GST) and very short amino-terminal truncations just before and at the beginning of the rat GR HBD that are otherwise intact to the carboxyl terminus. Overexpression in COS cells of the chimeras GST537C and GST547C was found to yield receptors that were bound to hsp90 and had wild-type steroid binding affinity. However, removal of 7 more amino acids to form GST554C resulted in a fusion protein that did not bind either hsp90 or steroid. Additional mutations revealed that the role of these 7 amino acids was neither to provide a spacer between protein domains nor to expose a protein surface by introducing a bend in the conserved alpha-helix. Instead, these observations support a model in which the sequence of the 7 amino acids directly or indirectly affects hsp90 binding to the GR HBD. Thus, a region of GR that has not been thought to be relevant for hsp90 binding is now seen to be of critical importance, and these data argue strongly against the commonly accepted model of receptor-hsp90 heterocomplex assembly in which the chaperone initially interacts nonspecifically with hydrophobic regions of the partially denatured HBD and subsequently assists its folding to the steroid binding confirmation.

The antibiotic rifampicin is a nonsteroidal ligand and activator of the human glucocorticoid receptor

The glucocorticoid receptor (GR) belongs to a superfamily of ligand-regulated nuclear steroid hormone receptors. The steps in the signal transduction pathway leading to the biological effects of glucocorticoids (GCs) include sequentially binding of the steroid to the GR ligand binding domain (LBD), receptor transformation, nuclear translocation and either positive or negative gene transactivation. Rifampicin (RIF) is a macrocyclic antibiotic used as an antituberculosis agents. As the incidence of tuberculosis has been increasing, in part because of the AIDS epidemic, a growing number of patients are being exposed to the adverse effects of this antibiotic. Indeed, this compound, as are the GCs, is often implicated in noxious drug interactions, because of its strong ability to induce drug-metabolizing enzymes. Moreover, in humans, RIF, as are the GCs, has been described as a potential immunodepressor, associated notably with the reduction of mitogenic responsiveness of human peripheral blood lymphocytes. Here, we report that RIF activates the human glucocorticoid receptor (hGR). Transient expression of wild-type, deleted or mutated GRs; sucrose density gradient sedimentation; and the BIAcore technique strongly suggest that RIF binds to the receptor with the physiological consequence that this antibiotic acts as an immunodepressor. Given the wide use of RIF in the treatment of coinfection of tuberculosis and HIV, this report is highly relevant to current medical practice.

Identification of amino acids in the tau 2-region of the mouse glucocorticoid receptor that contribute to hormone binding and transcriptional activation

The tau 2-region of steroid hormone receptors is a highly conserved region located at the extreme N-terminal end of the hormone-binding domain. A protein fragment encoding tau 2 has been shown to function as an independent transcriptional activation domain; however, because this region is essential for hormone binding, it has been difficult to determine whether the tau 2-region also contributes to the transactivation function of intact steroid receptors. In this study a series of amino acid substitutions were engineered at conserved positions in the tau 2-region of the mouse glucocorticoid receptor (mGR, amino acids 533-562) to map specific amino acid residues that contribute to the hormone-binding function, transcriptional activation, or both. Substitution of alanine or glycine for some amino acids (mutations E546G, P547A, and D555A) reduced or eliminated hormone binding, but the transactivation function of the intact GR and/or the minimum tau 2-fragment was unaffected for each of these mutants. Substitution of alanine for amino acid S561 reduced transactivation activity in the intact GR and the minimum tau 2-fragment but had no effect on hormone binding. The single mutation L550A and the double amino acid substitution L541G+L542G affected both hormone binding and transactivation. The fact that the S561A and L550A substitutions each caused a loss of transactivation activity in the minimum tau 2-fragment and the full-length GR indicated that the tau 2-region does contribute to the overall transactivation function of the full-length GR. Overall, the N-terminal portion of the tau 2-region (mGR 541-547) was primarily involved in hormone binding, whereas the C-terminal portion of the tau 2-region (mGR 548-561) was primarily involved in transactivation.

Steroid-induced conformational changes at ends of the hormone-binding domain in the rat glucocorticoid receptor are independent of agonist versus antagonist activity

The underlying molecular mechanism for the expression of agonist versus antagonist activity for a given receptor-steroid complex is still not known. One attractive hypothesis, based on data from progesterone receptors, is that agonist versus antagonist binding induces unique conformations at the C terminus of receptors, which can be detected by the different fragments produced by partial proteolysis. We now report that the determinants of glucocorticoid receptor (GR)-antagonist complex activity are more complex. Steroid binding did cause a conformational change in the GR that was detected by partial trypsin digestion, as described previously (Simons, S. S., Jr., Sistare, F. D., and Chakraborti, P. K. (1989) J. Biol. Chem. 264, 14493-14497). However, there was no uniformity in the digestion patterns of unactivated or activated receptors bound by a series of six structurally different antagonists including the affinity labeling antiglucocorticoid dexamethasone 21-mesylate. A total of four resistant bands were observed on SDS-polyacrylamide gels in the range of 30-27 kDa. Using a series of point mutations and epitope-specific antibodies, it was determined that the 30-kDa species represented the entire C-terminal sequence of amino acids 518-795, whereas the other bands arose from additional N-terminal and/or C-terminal cleavages. Bioassays with GRs containing various point and deletion mutations failed to reveal any C-terminal alterations that could convert antagonists into biologically active agonists. Thus, the presence or absence of C-terminal amino acids of the GR did not uniquely determine either the appearance of smaller trypsin-resistant fragments or the nature of the biological response of receptor-bound antisteroids. When compared with the current model of the ligand-binding domain, which is based on the x-ray structures of the comparable region of thyroid and retinoic acid receptors, the present results suggest that sequences outside of the model structure are relevant for the binding and biological activity of GRs.

Induction properties of a transiently transfected glucocorticoid-responsive gene vary with glucocorticoid receptor concentration

Transient transfections of steroid receptors have yielded much of the data used to construct the current models of steroid hormone action. These experiments invariably examine the ability of receptors to regulate transcription when occupied by saturating concentrations of steroid. We now report that other induction properties of a transiently transfected gene are not constant but vary with the concentration of transiently transfected glucocorticoid receptors. Thus, the percentage of maximal induction seen with subsaturating concentrations of glucocorticoid could be dramatically increased, and an antiglucocorticoid could be converted into a partial glucocorticoid, simply by increasing the concentration of glucocorticoid receptors. This behavior was observed in HeLa cells, containing endogenous receptors, or in CV-1 cells, containing almost no endogenous receptor, with either homologous or heterologous receptors. These increases were relatively insensitive to the concentration of reporter gene, suggesting the titration of some transcription factor(s) involved in regulating the position of the glucocorticoid dose-response curve and the agonist activity of an antiglucocorticoid. This property of transfected glucocorticoid receptors required a full-length, functionally active receptor but was retained, albeit reduced in magnitude, in the absence of binding to a glucocorticoid response element. Furthermore, this phenomenon was specific in that the A form of the human progesterone receptor had no effect under the same conditions. These variations in induction properties of antiglucocorticoids and of subsaturating concentrations of glucocorticoid, in a manner that was proportional to the amount of transfected receptor, reveal processes that are not operative with saturating concentrations of glucocorticoid. These variations also demonstrate that caution should be exercised in making mechanistic conclusions based solely on experiments conducted with saturating concentrations of glucocorticoid.