Structure of mammalian steroid receptors: evolving concepts and methodological developments

Hormonal control of inflammatory responses

Almost any stage of inflammatory and immunological responses is affected by hormone actions. This provides the basis for the suggestion that hormones act as modulators of the host reaction against trauma and infection. Specific hormone receptors are detected in the reactive structures in inflamed areas and binding of hormone molecules to such receptors results in the generation of signals that influence cell functions relevant for the development of inflammatory responses. Diversity of hormonal functions accounts for recognized pro- and anti-inflammatory effects exerted by these substances. Most hormone systems are capable of influencing inflammatory events. Insulin and glucocorticoids, however, exert direct regulatory effects at concentrations usually found in plasma. Insulin is endowed with facilitatory actions on vascular reactivity to inflammatory mediators and inflammatory cell functions. Increased concentrations of circulating glucocorticoids at the early stages of inflammation results in downregulation of inflammatory responses. Oestrogens markedly reduce the response to injury in a variety of experimental models. Glucagon and thyroid hormones exert indirect anti-inflammatory effects mediated by the activity of the adrenal cortex. Accordingly, inflammation is not only merely a local response, but a hormone-controlled process.

Energy-dependent conversion of transformed cytosolic glucocorticoid receptors from soluble to particulate-bound form

We have recently published that soluble cytosolic glucocorticoid receptors are converted to a particulate form when they are incubated at 37 degrees C in a tubulin-polymerizing buffer [Pratt, W. B., Sanchez, E. R., Bresnick, E. H., Meshinchi, S., Scherrer, L. C., Dalman, F. C., & Welsh, M. J. (1989) Cancer Res. (Suppl.) 49, 2222s-2229s]. In this work, we further define this phenomenon and demonstrate that the L-cell glucocorticoid receptors are binding to a protein particulate composed largely of cytoskeletal proteins. Incubation of L-cell cytosol with glutamate at 37 degrees C converts the glucorticoid receptor to a form that pellets when cytosol is centrifuged at 150000g. The particulate material formed in a temperature-dependent and glutamate-dependent manner contains a large amount of tubulin, actin, and vimentin, but it is not the product of a cold-labile, colchicine-sensitive polymerization process. Very few cytosolic proteins are present in this complex, but the glucocorticoid receptor is tightly bound to it. Binding of the receptor to the cytoskeletal complex occurs after receptor transformation and is at least partially energy-dependent. Examination of the behavior of beta-galactosidase receptor fusion proteins and the nti glucocorticoid receptor demonstrates that residues 445 to the COOH-terminus of the receptor (DNA-binding and hormone-binding domains) contain the features required for binding to the cytoskeletal complex. Although it is the transformed receptor that associates tightly with the complex, DNA-binding activity is not required for association with the cytoskeletal particulate.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of nonylphenol, 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (p,p'-DDE), and pentachlorophenol on the adult female guinea pig reproductive tract

The guinea pig exhibits cyclic and luteal similarities to the human, a feature not present in other small experimental animals such as rats, mice, or rabbits. Studies were undertaken to investigate the in vivo effects of three persistent environmental xenobiotics (nonylphenol, 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene [p,p'-DDE], and pentachlorophenol) on the microanatomy of the adult female guinea pig reproductive system. The effects brought about by these compounds (40 mg/kg/day) were compared to those caused by the synthetic estrogen diethylstilbestrol (DES; 50 microg/kg/day). Adult female guinea pigs, intact and castrated, were treated with 14 daily subcutaneous (s.c.) doses of one of these agents. The 50% decline in the weight of the tract that occurred following castration, was prevented by administration of nonylphenol, p,p'-DDE, and DES, but not of pentachlorophenol. Nonylphenol produced weak estrogenic stimulation of the tract of intact animals and maintained a relatively normal histologic appearance in castrated animals. Focal mucinous metaplasia of the endometrium, however, was observed in both groups. Treatment of intact and castrated animals with p,p'-DDE resulted in cystic hyperplasia and mucinous metaplasia of the endometrium, hyperplasia of the cervical epithelium, estrogenic stimulation of the vagina, and dilation of the rete ovarii. Treatment of intact or castrated animals with DES resulted in effects that were qualitatively similar to those caused by p,p'-DDE. The appearance of the vaginal epithelium, however, was abnormal and the rete ovarii were less dilated. Pentachlorophenol had minimal effect on the histology of the tract of castrated or intact animals. These data support our hypothesis that some environmental toxicants can substitute for estradiol in regulating the microanatomy of the female reproductive tract. They indicate the potential of these compounds to act as endocrine disrupting agents.

Local anti-inflammatory activity and systemic side effects of NM-135, a new prodrug glucocorticoid, in an experimental inflammatory rat model

The local anti-inflammatory activity and systemic side effects of NM-135 (6alpha,9-difluoro-11beta-hydroxy-16alpha-methyl-21[[2 ,3,4,6-tetrakis-O-(4-methylbenzoyl)-beta-D-glucopyranosyl]oxy]-pregna-1, 4-diene-3,20-dione) in croton oil-induced granuloma pouches and ear edema in rats were studied. The local anti-inflammatory activity of NM-135 was stronger than that of betamethasone 17-valerate (BV). As to systemic side effects, BV and diflucortolon valerate (DFV) caused thymolysis at the doses required for the anti-inflammatory activity. In contrast, no clear systemic side effect was observed in rats administered NM-135 at the dose producing the anti-inflammatory activity. These results suggest that NM-135 is a drug exhibiting a high degree of dissociation between the local anti-inflammatory activity and systemic side effects.

The presence of cortisol receptors in the human amnion

Cytosol extracts of human amnion tissue contained high affinity binding of cortisol (Ka=2.48+/-1.06 x 10(9) M(-1); n = 30) and low capacity binding of cortisol (Nmax=279+/-15.5 fmol mg(-1) protein). Kinetic studies of cortisol binding resulted in a similar value of Ka to that obtained by Scatchard analysis. Nuclear extracts of amnion tissue contained high affinity binding of cortisol (Ka=5.8+/-1.91 x 10(7) M[-1]) and low binding capacity (Nmax=91.4+/-21.4 fmol mg(-1) protein). Ka values were an order of magnitude higher in cytosol than in blood serum when amnion and blood were obtained from the same individuals. Differences in competitive ligand binding, especially dexamethasone, were observed between the amnion receptor and transcortin in serum. Gel permeation chromatography gave only one peak at 320 kDa for amnion receptor and only one peak at 48 kDa for transcortin from serum. When amnion tissue was incubated with or without cortisol, cytosol receptor activity was significantly lower in cortisol treated tissue than in control. The nuclear extracted receptor activity was significantly higher in cortisol treated tissue than control. The Ka values from cortisol treated tissue were significantly lower from control. Together the data support the presence of a specific cortisol receptor in the human amnion that is different from transcortin.

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)

Dose-response assessment strategies for endocrine-active compounds

Hazard identification provides evidence for the potential of compounds to cause effects in exposed people. Dose-response assessments define the range of exposure conditions associated with minimal risks of adverse effects. With endocrine-active compounds (EACs), the vast majority of resources are presently being applied to hazard identification. In the past, dose-response assessments have been based on empirical analysis of these relationships. The empirical underpinnings of these models do not permit conclusions about the low-dose and interspecies extrapolation of the animal study results. Biologically based dose-response assessments relying on knowledge of mode-of-action (pharmacodynamics) and dosimetry (pharmacokinetics) offer promise to develop broadly applicable strategies for quantitative dose-response assessments with these EACs. These approaches would focus on normal physiological endocrine signaling processes in the body, their associated control mechanisms, and the interaction among different internal signaling pathways. A critical element of signaling is regulation of the concentration of the signaling compound, e.g., steroid sex hormone. Exogenous compounds that act as signals but evade the normal homeostatic control of signaling compound concentrations represent one class of EACs. Other molecular components of these signaling systems include receptors, second messengers, and DNA-accessory/transcriptional protein complexes; EACs may interfere with the functions of any of these components. The challenge facing the toxicology and risk assessment professions is to base regulatory strategies on the interaction of these EACs with the fundamental control mechanisms which regulate responses throughout the body and to determine the extent to which these interactions create specific dose-response behaviors in the living animals.

Retrieval of estradiol receptor in paraffin sections of resting porcine uteri by microwave treatment. Immunostaining patterns obtained with different primary antibodies

The unmasking of estradiol receptor in paraffin sections of Bouin's-fixed uterine tissue from ovariectomized gilts was attained with microwave treatment. Immunocytochemistry of the receptor was performed using a polyclonal or five monoclonal antibodies, two of which are commercially available, reacting with different domains of the protein and an amplified-peroxidase system for detection. With five of the antibodies, a predominance of nuclear staining was observed in cells of endometrial glands, while one monoclonal antibody (13H2), reacting with the receptor's domain E, showed a preference for the cytoplasmic receptor. In stroma, all antibodies detected more receptor in nuclei than in cytoplasm. In epithelium, the commercially available antibody H222, our monoclonals 13H2 and HT65, and the polyclonal antibody 402 demonstrated more receptor in cytoplasmic than in nuclear areas. In myometrium, the nuclei from longitudinal and ring muscles were definitely stained with the antibodies. We conclude that the accessibilities of the antibody epitopes of the receptor differ according to the functional uterine cell type.

Surface mapping of the ligand-filled C-terminal half of the porcine estradiol receptor by restricted proteolysis

The ligand-filled 32-kDa fragment of the porcine estradiol receptor extending from His267 to the C-terminal Ile595 was purified to homogeneity by adsorption to mAb 13H2. The native protein was exposed at 4 degrees C to a panel of proteases: thermolysin, subtilisin, pronase, elastase, ficin, bromelain, endopeptidase Lys-C, both in the dimer and the monomer state, and chymotrypsin at pH 8.2 only. The digests were analysed by SDS/PAGE/Western blotting for Coomassie staining and immunostaining. Peptides were sequenced from blots. The majority of cleavage sites in upper domain E (8 out of 11) amassed in the Leu296-Leu310 stretch. Cleavage at Leu319 was seen with subtilisin and at Tyr328 with chymotrypsin. Susceptability to enzymic proteolysis was also pronounced in Thr465-Glu470 at the center of domain E. Three peptides, 13 kDa with thermolysin, beginning at Leu337, 6 kDa and, in low yield, 5 kDa with endopeptidase Lys-C beginning at Asp473 resp. Cys417 were only obtained from the monomer substrate. The various digests featured either 27-23-kDa peptides or mixtures of 17-13-kDa and 12-7-kDa peptides separable by SDS/PAGE. All peptides with N-termini between Leu297 and Ser329 reacted with mAb 13H2. The digests showed high peaks of bound estradiol in the dimer position of 32-kDa fragment controls on density gradient centrifugation at pH 7.4. However, the property of proton-driven dissociation was only preserved in the pronase, elastase and chymotrypsin digests with peptides extending beyond the His547-ArgLeuHis550 motif. The preservation of the estradiol-binding niche in the tightly complexed peptides of domain E was also demonstrated by refilling after steroid removal. The sites exposed to proteolytic enzymes and the epitope for 13H2 attachment are in good agreement with surface probability plots.

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.

Structural characterization of the trypsinized estrogen receptor

Structural differences between the unoccupied and ligand-occupied rat uterine estrogen receptors (ERs) were investigated using partial proteolysis followed by immunoblotting, affinity labeling, and gel filtration chromatography. Trypsin digestion of the unoccupied ER at 4 degrees C resulted in retention of 70-80% of high-affinity [3H]estradiol binding. Only two fragments of the rat ER were detected after prolonged trypsin treatment of the unoccupied ER followed by affinity labeling with [3H]tamoxifen aziridine. One fragment represents the intact steroid binding domain (28 kDa), and the other fragment is about 10 kDa. The small 10-kDa fragment of the ER detected by denaturing gel electrophoresis is shown to be held in a large oligomeric complex in solution using gel filtration chromatography. This oligomeric complex probably represents the steroid binding domain, which has its tertiary structure maintained predominantly by noncovalent interactions between the trypsin-generated fragments. The estrogen, anti-estrogen, and unoccupied trypsinized ERs all result in similar patterns of fragments after separation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and detection by immunoblotting. Although no new trypsin cleavage sites were exposed, the sensitivity of the available trypsin sites was altered by heating the ER and, to a lesser extent, by hormone treatment. Gel filtration chromatography of the trypsinized estradiol- and 4-hydroxytamoxifen-occupied ERs demonstrates similar, diffuse peaks centered at about the correct size for the intact steroid binding domain (28 kDa), whereas the trypsinized unoccupied ER results in a sharp, discrete peak centered at about 80 kDa.(ABSTRACT TRUNCATED AT 250 WORDS)

Use of automated smoothing and deconvolution procedures for the determination of human breast cancer estrogen and progesterone receptor isoforms, after high-performance size-exclusion chromatography

Automated smoothing and deconvolution procedures were used to analyse complex chromatographic patterns of human breast cancer estrogen and progesterone receptors, obtained by size-exclusion chromatography. By injecting, first, different known amounts of a radio-iodinated protein in a TSK G-3000 SW column, constructed complex chromatograms (twelve chromatograms) with known peak positions were obtained, and were further treated by mathematical functions to determine a smooth-deconvolution strategy, which could be used with unknown chromatographic patterns. The determination of peak areas by a "curve-fit" program showed a good correlation with the amounts of protein injected (r = 0.91). Human breast cancer estrogen receptors (56 cases) and progesterone receptors (45 cases) were chromatographed in a TSK G-3000 SW column, and further analysed with the smoothing and deconvolution procedures: eight different estrogen receptor isoforms were detected with molecular masses ranging from 530,000 [Stoke's radius (Rs) = 7.7 nm] to 23,000 (Rs = 2.6 nm), and eight progesterone receptor isoforms were observed with masses ranging from 680,000 (Rs = 8.6 nm) to 50,000 (Rs = 3.1 nm). The dissociation effect of KCl (0 to 1 M) on receptor structure yielded different proportions of receptor isoforms, but did not modify their different peak positions.

Characterization of the 4 S polycyclic aromatic hydrocarbon-binding protein in human liver and cells

The 4 S polycyclic aromatic hydrocarbon (PAH)-binding protein (PBP) is a soluble protein that binds PAHs with high affinity in mouse, rat, and rabbit. Until now, this protein had not been detected in human placenta or human cells in culture by cytosol labeling and gradient centrifugation assay. Thanks to a preliminary fractionation of cytosol by sedimentation on sucrose gradient or/and gel permeation chromatography, we found that PBP was present in liver, MCF-7 cell line, and hepatocytes of human. To accurately quantitate PBP binding and determine specific binding parameters, a reduction in the amount of charcoal used to adsorb nonspecifically bound benzo[a]pyrene was required. By saturation analysis, the concentration of specific binding sites for [3H]BP in PBP fraction from human liver was 4.6 pmol/mg of protein compared with 14.7 +/- 1.4 pmol/mg in the same fraction from DBA/2J mouse liver. Kinetic studies analyzed by Scatchard and Woolf plots indicate that human liver and MCF-7 cells contain a low-affinity PBP form: the Kd derived from Woolf plot analysis were 14.2 +/- 1.4 and 26.2 +/- 1.8 nM, respectively. DBA/2J mouse possesses a higher-affinity PBP form, the same analysis indicating a Kd of 6.1 +/- 0.3 nM. These data demonstrate that, by comparison to the mouse liver, a lower-affinity form of PBP is present in reduced concentration in human liver, explaining the impossibility of detecting this protein by sedimentation of human cytosol in sucrose gradient.

Immunogold labelling of the cytoplasmic estradiol receptor in resting porcine endometrium

Serial sections of resting porcine endometrium were analyzed with the monoclonal antibody 13H2 using goat antimouse IgG/5 nm gold as secondary reagent or with either polyclonal antibodies from goat #402 or the rat monoclonal antibody H222, both in combination with protein G/12 nm gold. A modestly higher labelling of nuclei than of cytoplasm was seen only with the monoclonal antibody H222. Polyclonal #402 and monoclonal 13H2 showed fewer attachments over nuclear than over cytoplasmic areas. The highest densities of attachment and of predominantly cytoplasmic labelling were obtained with the monoclonal antibody 13H2. The results confirm the earlier assumption of a restricted accessibility of estradiol receptor in the cytoplasm of resting cells for immunoreagents.

Enzyme induction and receptor-binding affinity of steroidal 20-carboxamides in rat hepatoma tissue culture cells

The steroidal 20-carboxamides [(20R)- and (20S)-21-(N-substituted amino)-11 beta,17,20-trihydroxy-3,21-dioxo-1,4-pregnadiene] recently have been shown to possess anti-inflammatory activity in animal models of inflammation. These N-substituted methyl, ethyl, n-propyl, and benzyl derivatives also exhibited suppressive effects on plasma corticosterone and thymus function. Generally, the (20R)-hydroxy-20-carboxamides were more potent than the corresponding (20S)-epimers. In continuing investigations on the glucocorticoid effects of these compounds, we have studied their ability to induce tyrosine aminotransferase (TAT), inhibit uptake of [3H]thymidine into DNA, and complete with [3H] dexamethasone for binding to the hepatoma tissue culture glucocorticoid receptor. Results indicated that the N-substituted methyl, ethyl, and n-propyl derivatives were full glucocorticoid agonists in the three measurements. Receptor binding affinities of the N-substituted carboxamides correlated well with their ability to induce TAT activity and to inhibit thymocyte proliferation. Structure-activity relationships indicated that the larger the N-substituent, the weaker the agonist activity in this system, and 20R isomers exhibited higher glucocorticoid agonist activity than the corresponding 20S isomers. This investigation is part of our effort to elucidate structure-activity relationships of steroidal carboxamides synthesized on the basis of the antedrug concept.

A ligand-induced conformational change in the estrogen receptor is localized in the steroid binding domain

Upon binding estrogen, the estrogen receptor (ER) is proposed to undergo some form of conformational transition leading to increased transcription from estrogen-responsive genes. In vitro methods used to study the transition often do not separate heat-induced effects on the ER from estrogen-induced effects. The technique of affinity partitioning with PEG-palmitate was used to study the change in the hydrophobic surface properties of the ER upon binding ligand with and without in vitro heating. Upon binding estradiol (E2), the full-length rat uterine cytosolic ER undergoes a dramatic decrease in surface hydrophobicity. The binding of the anti-estrogen 4-hydroxytamoxifen (4-OHT) results in a similar decrease in surface hydrophobicity. These effects are independent of any conformational changes induced by heating the ER to 30 degrees C for 45 min. The use of the human ER steroid binding domain overproduced in Escherichia coli (ER-C) and the trypsin-generated steroid binding domain from rat uterine cytosolic ER demonstrates that the decrease in surface hydrophobicity upon binding E2 or 4-OHT is localized to the steroid binding domain. Gel filtration analysis indicates that the change in surface hydrophobicity upon binding ligand is an inherent property of the steroid binding domain and not due to a ligand-induced change in the oligomeric state of the receptor. The decrease in surface hydrophobicity of the steroid binding domain of the ER upon binding E2 or 4-OHT represents an early and possibly a necessary event in estrogen action and may be important for "tight" binding of the ER in the nucleus.

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.

Protein interactions with surface-immobilized metal ions: structure-dependent variations in affinity and binding capacity with temperature and urea concentration

We have used equilibrium binding analyses to evaluate the influence of temperature and urea on the affinity of hen egg white lysozyme and bovine pancreatic ribonuclease A for surface-immobilized Cu(II) ions. Linear Scatchard plots suggested that these model proteins were interacting with immobilized metal ions via a single class of intermediate-affinity (Kd = 10-40 microM) binding sites. Alterations in temperature had little or no effect on the immobilized Cu(II) binding capacity of either protein. Temperature effects on the interaction affinity, however, were protein-dependent and varied considerably. The affinity of lysozyme for immobilized Cu(II) ions was significantly decreased with increased temperature (0 degree C-37 degrees C), yet the affinity of ribonuclease did not vary measurably over the same temperature range. The van 't Hoff plot (1n K vs 1/T) for lysozyme suggests a straight line relationship (single mechanism) with a delta H of approximately -5.5 kcal/mol. Urea effects also varied in a protein-dependent manner. A 10-fold reduction in the affinity of lysozyme for the immobilized Cu(II) was observed with the urea concentrations up to 3 M; yet urea had no effect on the affinity of ribonuclease for the immobilized metal ions. Although the interaction capacity of lysozyme with the immobilized Cu(II) ions was decreased by 50% in 3 M urea, ribonuclease interaction capacity was not diminished in urea. Thus, temperature- and urea-dependent alterations in protein-metal ion interactions were observed for lysozyme but not ribonuclease A. The complete, yet reversible, inhibition of lysozyme- and ribonuclease-metal ion interactions by carboxyethylation with low concentrations of diethylpyrocarbonate provided direct evidence of histidyl involvement. The differential response of these proteins to the effects of temperature and urea was, therefore, interpreted based on calculated solvent-accessibilities and surface distributions of His residues, individual His residue pKa values, and specific features of the protein surface structure in the immediate environment of the surface-exposed histidyl residues. Possible interaction mechanisms involved in protein recognition of macromolecular surface-immobilized metal ions are presented.

Subunit structure of the glucocorticoid receptor and activation to the DNA-binding state

Glucocorticoid receptors of S49.1 mouse lymphoma cells were analyzed under a variety of conditions. The complexes with an agonist or a steroidal antagonist can be formed in cytosolic extracts, they are of high molecular weight, Mr approximately 330,000 and have a Stokes radius of 82 A. Cross-linking by several agents stabilized this structure against subunit dissociation which produces the activated receptor form of 60 A and DNA-binding ability. Careful analysis of intermediate cross-linked forms lead to the conclusion that the large receptor structure is a hetero-tetramer consisting of one hormone-bearing polypeptide of Mr approximately 94,000, two 90 kDa subunits and a protein component of Mr approximately 50,000. The 90 kDa subunits are the heat shock protein hsp90. The high molecular weight receptor form also exists in intact cells as revealed again by cross-linking. The cytosolic complex with the antagonist can become activated to the DNA-binding form upon warming but simultaneously looses the ligand. Ligand rebinding does not occur subsequent to receptor dissociation. Upon incubation of intact cells at 37 degrees C with agonist or antagonist the respective receptor-ligand complexes are formed. The agonist complex is immediately activated, however, the antagonist complex remains stable in the undissociated state. This explains the biological effect of the antagonist.

Estradiol increases phosphorylation of the 90 kDa heat shock protein not associated with estradiol receptor in MCF-7 cells in culture

MCF-7 cells in monolayer culture were incubated with [32P]orthophosphate for 18 h followed by covalent whole cell labelling of the estradiol receptor with tritiated tamoxifen aziridine [( 3H]TA). The heat shock protein (hsp-90) bound to receptor was precipitated with monoclonal antibodies H222 or JS 34/32, coupled to protein A-Sepharose and purified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under reducing conditions. Hsp-90 not associated with receptor was similarly purified after isolation with the monoclonal antibody AC88. It was found that estradiol treatment of the cells markedly increased phosphate incorporation in the free hsp-90, without affecting heat shock protein bound to receptor. A 6-fold increase in phosphate content was observed after 10 min incubation of the cells with estradiol. A similar effect was seen after treatment of the cells with the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA). The calcium ionophore A23187 had no influence on hsp-90 phosphorylation, and treatment of the cells with forskolin to increase the cellular content of cAMP had a reverse effect. A 50% reduction of the phosphate content in the free hsp-90 was observed after 15 min treatment. The observation that estradiol, TPA and forskolin had effect only on hsp-90 not bound to receptor is an indication that the receptor-hsp-90 complex exists in vivo. Time course studies show that the effect of estradiol is non-genomic. Two possible explanations of the results seem to exist. Either estradiol induces an increase in the degree of phosphorylation of hsp-90, or hsp-90 is translocated to the cytosol from a different cellular compartment.

Ligand-binding properties of estrogen receptor proteins after interaction with surface-immobilized Zn(II) ions: evidence for localized surface interactions and minimal conformational changes

The site- or domain-specific immobilization of steroid receptor proteins with preserved structure and function would facilitate the identification and purification of receptor-associated regulatory components and nucleic acids. We have demonstrated previously that restricted surface regions of the estrogen receptor protein contain high affinity binding sites for immobilized Zn(II) ions. Possible conformational changes in receptor at the stationary phase immobilized metal ion interface were evaluated by monitoring alterations in the equilibrium dissociation constant (Kd) for [3H]estradiol. Soluble estrogen receptor proteins (unliganded) present in immature calf uterine cytosol were immobilized via surface-exposed Zn(II)-binding sites to beads of agarose derivatized with iminodiacetate (IDA)-Zn(II) ions. The IDA-Zn(II) bound receptor was incubated with increasing concentrations of [3H]estradiol (0.01-20 nM) in the presence and absence of unlabeled competitor (diethylstilbestrol) to determine the level of specific hormone binding. Steroid-binding experiments were performed in parallel with identical aliquots of soluble receptor. Analyses of the equilibrium binding data revealed the presence of a single class of high-affinity (Kd = 2.44 +/- 1.5 nM, n = 10) steroid-binding sites which were only marginally affected by receptor immobilization via surface-exposed Zn(II) bindings sites (Kd = 2.58 +/- 0.56 nM, n = 4). These data are consistent with the location of surface accessible Zn(II) binding site(s) on the receptor at or near the DNA binding domain which, upon occupancy, do not influence the steroid binding domain.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the Ah receptor mediating aryl hydrocarbon hydroxylase induction in the human liver cell line Hep G2

The Ah receptor, a soluble cytoplasmic receptor that regulates induction of cytochrome P450IA1 and mediates toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), was detected and characterized in the continuous human liver cell line Hep G2. The mean concentration of specific binding sites for TCDD was 112 +/- 26 (SEM) fmol/mg cytosol protein as determined in eight separate cytosol preparations in the presence of sodium molybdate. This is equivalent to 14,000 binding sites per cell, approximately 40% of the sites per cell found in the mouse hepatoma line Hepa-1. The cytosolic Ah receptor from Hep G2 cells sedimented at 9 S and was specific for those halogenated and nonhalogenated aromatic compounds known to be agonists for the Ah receptor in rodent tissues and cells. Specific binding in the 9 S region was detected with both [3H]TCDD and 3-[3H]methylcholanthrene. 3-[3H]Methylcholanthrene did not bind to any component besides that at approximately 9 S. Phenobarbital, dexamethasone, and estradiol did not compete with [3H]TCDD for binding to the Hep G2 Ah receptor. Specific binding of [3H]triamcinolone acetonide to glucocorticoid receptor could also be demonstrated in Hep G2 cytosol. The apparent equilibrium dissociation constant (Kd) for binding of [3H]TCDD to Hep G2 Ah receptor was 9 nM by Woolf plot analysis, about an order of magnitude weaker than the affinity of [3H]TCDD for the mouse Hepa-1 Ah receptor or for the C57BL/6 murine hepatic Ah receptor. [3H]TCDD.Ah receptor complex, which was extracted from nuclei of Hep G2 cells incubated with [3H]TCDD at 37 degrees C in culture, sedimented at approximately 6 S under conditions of high ionic strength. Aryl hydrocarbon hydroxylase (AHH) activity was significantly induced after 24 h of incubation with polycyclic aromatic hydrocarbons: the EC50 for AHH induction was 5.3 microM for benz(a)anthracene and 1.3 microM for 3-methylcholanthrene. Modification of the preparative technique for cell cytosol, especially inclusion of 20 mM sodium molybdate in homogenizing and other buffers, was necessary to detect cytosolic Hep G2 Ah receptor. Hep G2 cells appear to conserve drug-metabolizing activity associated with cytochrome P450IA1 as well as the receptor mechanism which regulates its induction.

Immunological and physiological analysis of human breast cancer progesterone receptor heterogeneity, following KCl dissociation and size exclusion chromatography

Progesterone receptor (PR) levels were determined in 69 human breast cancer specimens by both radioligand assay (RLA) and enzyme immunoassay (EIA). These methods did not detect the same number of sites, and for each tumor there was a constant ratio between epitopes and PR-binding sites corresponding to 1/4, 2/4, ... 8/4. High performance size exclusion chromatography was performed to separate the various PR isoforms, and the ability of these isoforms to interact with the monoclonal antibodies was assessed. Determination of PR in the chromatographic fractions by EIA and RLA showed that the various isoforms isolated by chromatography presented variable quantities of steroid-binding sites and epitopes, thus confirming the differences observed in the cytosol assays. The dissociation of molybdate-stabilized PR by KCl and measurement by RLA and EIA of the isoforms obtained showed two different types of chromatographic patterns, particularly in the 8S polymeric form where the monoclonal antibodies appeared to detect mainly the heavier 8S fraction, which may correspond to the 8S-B form of the progesterone receptor. The monoclonal antibodies also detected an intermediate PR polymeric form (236 kDa) which was not always detected by the tritiated ligand. Our results suggest that breast cancer PR exhibit a marked molecular heterogeneity which may partially explain the differences in response to hormonal therapy, particularly for tumors with high receptor levels which nonetheless fail to respond to treatment.

Evidence that 5 S intermediate state in glucocorticoid receptor transformation contains hsp90 in addition to the steroid-binding protein

Previous studies have shown that the exposure of molybdate-stabilized nontransformed glucocorticoid receptor (GR) of the chick embryonic neural retina to 0.4 M KCl dissociated the 9.5 S complex to a 5 S GR complex, which is an intermediate state in GR transformation. The present study was designed to characterize the 5 S GR complex. It shows that molybdate-stabilized nontransformed 9.5 S GR complex and 5 S GR interact with monoclonal antibodies (MAb) directed against 90 kDa heat shock protein (hsp90), as evidenced by the increase in the sedimentation velocity of these GR-complexes. Electrofocusing of the partially purified molybdate-stabilized nontransformed GR, prepared from [32P]-labeled neural retinas, and of the 5 S GR (derived from molybdate-stabilized preparation) showed that nontransformed GR complex, which has an apparent pI (pI') value of 5.0 +/- 0.2, and 5 S GR, which was resolved in a major peak with a pI' value of 5.8, are phosphorylated. Partially purified 5 S GR, cleared of molybdate and exposed to 25 degrees C, was resolved by electrofocusing into two phosphorylated fractions, one with a pI' value of 6.5, representing the monomeric GR form and the other with a pI' value of 5.1, apparently representing the acidic hsp90. The dissociation of hsp90 from the molybdate-cleared 5 S heterodimer seems to account for the decrease in the negative net charge of 5 S GR from pI' 6.5. Monomeric GR, derived from a molybdate-cleared, partially purified GR preparation, by the exposure to 25 degrees C, did not retain glucocorticoid-binding activity. Molybdate-stabilized 5 S GR was apparently re-assembled into the oligomeric nontransformed state when the salt concentration was reduced. This phenomenon was evident under the low-salt conditions of electrofocusing, by the shift in pI' value of GR from 5.8 to 5.0; and in glycerol density gradients containing 0.15 M KCl, by the shift in the sedimentation of the GR complex from 5 S to 9.5 S.

Ah receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin in rat liver: lack of sensitivity to alkaline phosphatase when compared with that of glucocorticoid receptor

Rat hepatic cytosol was treated with alkaline phosphatase in order to determine if dephosphorylation altered the ability of Ah receptor to bind 2,3,7,8-[3H]tetrachlorodibenzo-p-dioxin (TCDD). Glucocorticoid receptor was studied for comparison. As previously had been shown in other laboratories, treatment of cytosol with purified alkaline phosphatase dramatically reduced the subsequent ability of glucocorticoid receptor to bind hormone. However, alkaline phosphatase had no effect on the ability of Ah receptor to bind [3H]TCDD. If either glucocorticoid receptor or Ah receptor was occupied by its ligand prior to exposure to alkaline phosphatase there was no loss in ligand binding capacity. Crude alkaline phosphatase (containing some protease activity) substantially reduced the ability of glucocorticoid receptor to bind hormone and shifted the sedimentation position of the glucocorticoid receptor from approximately 8 S to approximately 2 S. Crude alkaline phosphatase did not reduce the ability of Ah receptor to bind [3H]TCDD and did not alter sedimentation of the 9 S [3H]TCDD. Ah receptor complex. Although the Ah receptor appears to be a member of the steroid receptor superfamily, the lack of effect of alkaline phosphatase on Ah receptor (compared to the sensitivity of glucocorticoid receptor) highlights another significant difference in molecular characteristics between the Ah receptor and the receptors for steroid hormones.

Identification of dexamethasone-binding sites on male-rat liver plasma membranes by affinity labelling

Binding studies with [3H]dexamethasone identified two binding sites on plasma membranes prepared from the male rat liver, a low-capacity site with a KD of 7.0 nM and a higher-capacity site with a KD of 90.1 nM. Both sites exhibited glucocorticoid responsiveness and specificity for glucocorticoids and progestins. Triamcinolone acetonide, which competes well for the binding of dexamethasone to the cytosolic glucocorticoid receptor, did not compete well for the binding of [3H]dexamethasone to the plasma-membrane binding sites. The binding sites were sensitive to protease and neuraminidase treatment, and resistant to extraction with NaCl, but were extracted with the detergent Triton X-100. As these experiments indicated the presence of plasma-membrane protein components which bind glucocorticoids at physiological concentrations, affinity-labelling experiments with dexamethasone mesylate were conducted. Two peptides were specifically labelled, one at approx. Mr 66,000 and one at Mr 45,000. The Mr-66,000 peptide was not sensitive to glucocorticoids, and was extracted by NaCl, and so did not correspond to either of the sites identified in the dexamethasone-binding studies. The Mr-45,000 entity, on the other hand, resembled the dexamethasone-binding sites in its response to glucocorticoid manipulation of the animal and in its resistance to salt extraction. This peptide was not present in rat serum. Thus we have identified a plasma-membrane peptide which binds dexamethasone. Whether this peptide is involved in transport of the glucocorticoid across the plasma membrane remains to be determined.

Glucocorticoid-receptor complexes are associated with small RNA in vitro

Identification of RNA associated with soluble glucocorticoid-receptor complexes of HeLa cells was performed by immunoprecipitation of receptor complexes with a monoclonal antibody raised against rat liver glucocorticoid receptor. Polyacrylamide gel electrophoresis of RNA extracted from immunoprecipitates of cytosolic complexes revealed the presence of eight RNA bands, consisting of 28S, 18S, and small RNAs, including 5.8S, 5S and tRNA. A comparison of RNA species immunoprecipitated by monoclonal anti-glucocorticoid receptor antibody and IgG purified from normal mouse serum showed that four small RNAs were preferentially recovered after immunoprecipitation with anti-glucocorticoid receptor antibody. When these species were analyzed on sequencing gels, their nucleotide lengths coincided with those of 7-3, 7S, U2, and U1 RNA. Immunoprecipitation of nuclear extracts containing glucocorticoid-receptor-RNA complexes showed that the same set of small RNAs was preferentially immunoprecipitated by anti-glucocorticoid receptor antibody. The four small RNAs we detected represented minor species in whole extracts, and their preferential immunoprecipitation by anti-glucocorticoid receptor antibody was prevented by removal of glucocorticoid-receptor complexes from HeLa cell extracts. We conclude that 7-3, 7S, U2, and U1 RNA are associated with glucocorticoid-receptor complexes in vitro, and hypothesize that post-transcriptional effects of glucocorticoids may in part be mediated through interaction of receptor complexes with these small RNAs.

Biochemical characterization and subunit structure of quail oviduct progesterone receptor

The cytosolic quail oviduct progesterone receptor (PR) was studied under conditions that lead either to its stabilization or activation. Sedimentation coefficients and Stokes radii were respectively 7.8 +/- 0.2 S and 7.6 +/- 0.8 nm for the non transformed receptor (8S PR) and 3.9 +/- 0.4S and 4.8 +/- 0.6 nm for the transformed receptor (4S PR). The calculated molecular weight was 261 +/- 29 KDa for the 8S PR and 83 +/- 10 KDa for the 4S PR. Density gradient centrifugation analyses showed that the monoclonal antibody BF4, directed against the 90 KDa hsp of the chick oviduct, cross-reacted with the quail 8S PR but not with the 4S PR. In contrast, polyclonal IgG-G6 antibodies, raised against the purified non transformed chick PR, cross-reacted with the non transformed as well as the transformed quail PR. The quail 8S PR was partially purified using NADAc-Sepharose affinity chromatography and DEAE-Sephacel chromatography from cytosol prepared with protease inhibitors. The subunit structure of the purified quail and chick 8S PR were compared using SDS-PAGE, photoaffinity labeling and western immunoblotting. The quail PR was composed of two different proteins: a non-hormone binding protein (Mr approximately 90 KDa) which exhibited the same properties as the 90 KDa hsp protein of the chick oviduct and a single hormone binding subunit (Mr approximately 101 KDa). Based on its binding and immunological properties, this protein corresponded to the "B" form of the chick PR but was significantly smaller. In the quail cytosol or in purified PR preparations the "A" form of the PR was virtually absent; this observation is discussed.

Study of the heteromeric structure of the untransformed glucocorticoid receptor using chemical cross-linking and monoclonal antibodies against the 90K heat-shock protein

The untransformed rat glucocorticoid receptor is assumed to be a hetero-oligomeric complex, containing a non-steroid binding component, the 90K heat-shock protein (HSP 90). Direct measurement of its molecular weight by chemical cross-linking provides new evidence for a trimeric structure with a Mr of ca. 270,000. Resorting to an anti HSP 90 probe (AC 88), we show that the native dimeric HSP 90 possess two accessible epitopes for this monoclonal antibody, while when bound to the steroid-binding subunit, only one epitope remains accessible. These data clearly suggest that the untransformed rat glucocorticoid receptor is an asymmetrical hetero-oligomeric complex.

Ligand-dependent interaction of the dioxin receptor with target DNA

Wild type and nuclear transfer deficient mouse hepatoma cell lines were used to study the specific DNA binding of a dioxin inducible factor. This factor interacts with XRE only after dioxin treatment and is absent in receptor mutant containing cells even after treatment. Thus, evidence is provided to substantiate the claim that the dioxin receptor is involved in the specific DNA interaction with dioxin response enhancer elements. It is also shown that the molybdate stabilised dioxin-receptor interacts with hsp90 suggesting that, in similarity to the glucocorticoid receptor, the dioxin receptor is kept in a non-transformed state in the absence of ligand.

Interaction of the chicken progesterone receptor with heat shock protein (HSP) 90

The chicken progesterone receptor A (PRA) was expressed from cDNA by in vitro transcription and translation and also by transient transfection of receptor-negative COS M6 cells. These receptors synthesized from cDNA exhibited functional properties similar to those of oviduct PRA. The ability of PRA to form an 8S complex and to bind to DNA was studied. PRA, synthesized by either expression system, formed an 8S complex which was dissociated by incubation in vitro with 0.4 M NaCl or 20 nM progesterone to generate a 4S species able to bind to DNA-cellulose. The presence of HSP 90 in the PRA 8S complex was confirmed by use of an HSP 90-specific antibody, AC-7. Expression constructs coding for various receptor deletions were studied in order to identify the site of interaction of PRA with HSP 90. Deletions of 290 amino acids from the C-terminus resulted in the loss of ability to form an 8S complex. Truncated receptor proteins lacking 153 amino acids from the C-terminus or 369 amino acids from the N-terminus were able to interact with HSP 90. These data suggest that the site of interaction between PRA and HSP 90 responsible for 8S complex formation may be in this region (amino acid 369-506). However, small internal amino acid deletions in this region of PRA did not result in the loss of interaction of mutant receptor proteins with HSP 90. Thus, it appears that there may be more than one site of interaction between PRA and HSP 90 in this region.

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.

Chemical crosslinking: a useful tool for evaluations of steroid receptor structures and their functional states in intact cells

A procedure of chemical crosslinking of intact cells with glutaraldehyde was employed to contribute to the understanding of glucocorticoid receptor structures and their functional states in vivo. Under optimal experimental conditions, glucocorticoid binding sites were found almost equally distributed between cytosolic and nuclear fractions of crosslinked cells. Sedimentation properties of crosslinked receptor complexes in cytosolic and nuclear extracts revealed that these entities were oligomers, which heterogeneously sedimented between 11 and 4S in the presence of 0.3 M NaCl. By anion exchange chromatography, we could establish that these receptor complex oligomers behaved as untransformed forms.