Reversible denaturation of the estrogen receptor and estimation of polypeptide chain molecular weight

Interaction between estrogen receptor and Pit-1 protein is influenced by estrogen in pituitary cells

The estrogen responsiveness of the rat prolactin gene expression requires the presence of both the estrogen receptor (ER) and the tissue-specific transcription factor, Pit-1 protein. We performed protein interaction assays using anti-rat Pit-1 antiserum (a-rPit-1) to investigate the physical interactions which occur between ER and Pit-1 proteins following estrogen treatment. After fusing maltose binding protein (MBP) and Pit-1 protein, we used the resulting MBP Pit-1 fusion protein to prepare a-rPit-1. Our results show that the estrogen receptor readily co-precipitated with the Pit-1 protein drawn from the lysates of two prolactin-expressing pituitary cell lines GH3 and PR1. The rate of precipitation appears to be both estrogen- and time-dependent. Cellular levels of estrogen receptors and Pit-1 proteins did not show significant changes during the time of estrogen treatment. We therefore suggest that an estrogen-dependent physical interaction between ER and Pit-1 protein exists in vivo, and that this interaction may play an important role in the regulation of prolactin gene expression.

Effect of ligand binding and DNA binding on the structure of the mouse oestrogen receptor

We have investigated the effects of ligand and DNA binding on the structure of the oestrogen receptor by performing limited proteolysis and analysing DNA binding activity by gel shift analysis. The effects of oestradiol, 4-hydroxytamoxifen and ICI 164,384 have been examined and we have found that despite differences in the DNA binding activity or relative mobility of the receptor-DNA complex we were unable to detect differences in the cleavage pattern produced by trypsin, chymotrypsin, Staphylococcus aureus V8, papain or elastase. Inhibition of DNA binding by ICI 164,384 was lost in receptor fragments that lacked the hormone binding domain. In contrast to the full-length receptor, proteolytic fragments produced by chymotrypsin differed in their ability to bind to an oestrogen response element (ERE) vs a thyroid response element (TRE). Evidence is presented that this difference can be accounted for by the inability of fragments lacking the hormone binding domain to dimerise on a TRE.

Proteolytic activity of the purified hormone-binding subunit in the estrogen receptor

The hormone-binding subunit of the calf uterus estradiol receptor was purified as a hormone-free molecule. Immunoaffinity chromatography with a specific monoclonal antibody was used as the final step. The purified subunit was specifically labeled by radioactive diisopropyl fluorophosphate. The diisopropyl fluorophosphate-labeled amino acid was serine. The purified receptor was able to release the fluorogenic or chromogenic group from synthetic peptides containing phenylalanine at the carboxyl terminus. This occurred only in the presence of estradiol and was hampered by aprotinin and diisopropyl fluorophosphate. Estradiol-dependent hydrolytic activity was also found in the eluate from gel slices after SDS/PAGE of purified receptor. This activity comigrated with the renaturable estradiol-binding activity. The estradiol antagonists 4-hydroxytamoxifen and ICI 164,384 as well as other steroid hormones were unable to activate this hydrolytic activity.

Characteristics of the calf uterine androgen receptor

The highest molecular weight form of the calf uterine androgen receptor separates as an 11S form in glycerol gradients. This "cytosolic" receptor, prepared in the presence of molybdate, polyethyleneimide and low ionic strength, dissociates into 9S and 7.2S forms with increasing KCl concentration. A 4.5S androgen binding component appears as the predominant form of the receptor in the absence of polyethyleneimide and this unit quantitatively converts to a stable 3.5S form in the absence of molybdate. Renaturation of partially purified protein, separated by SDS-PAGE electrophoresis, demonstrates the presence of an androgen binding component in the 110 kDa region of the gel. This renatured protein separates as a 4.5S component in glycerol gradients and has a Stokes radius of 6 nm. Photoaffinity labelling of partially purified receptor preparations, followed by SDS-PAGE electrophoresis, reveals the presence of an androgen binding component having a molecular weight of 115 kDa. The binding characteristics and specificity of the receptor binding to R1881 have been studied and a DHT-affinity chromatography resin used to purify the receptor.

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)

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.

Size heterogeneity of affinity labeled estrogen receptors in the MtTF4 tumor whose growth is inhibited by estradiol, in pituitary gland and uterus

Estrogen receptors (ER) of the MtTF4 tumor whose growth is inhibited by estradiol (E2) were analyzed and compared to those of tissues whose growth is stimulated by E2 (uterus and pituitary gland). Cytosol prepared in buffer containing protease inhibitors was incubated with [3H]tamoxifen aziridine ([3H]TAZ) in the presence or absence of non-radioactive competitor. The labeled proteins were precipitated, separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis in denaturing conditions and detected by fluorography. Two classes of ER were identified. The first class is of high molecular weight (Mr = 65,000-64,000). In normal tissues, it is indeed frequently made up of two subtypes as revealed by the presence of a doublet on autoradiograms. In the MtTF4 tumor these subtypes were only rarely suspected and never they were as marked and distinct as in normal tissues. The second class, of low molecular weight (Mr ! 54,000-52,000), is also frequently made up of two subtypes in the uterus and the proportion of this class is higher in the uterus of mature than of immature rats. The MtTF4 tumor contains this class of ER but, due to the presence of non-specifically labeled proteins in this region, its relative amount cannot be estimated and the doublet was exceptionally revealed. In the pituitary gland, this small receptor has not been found.

CONCLUSIONS

(i) On the basis of molecular weight analyses, estrogen receptors are heterogeneous, (ii) the ER pattern depends on the type of tissue and the sexual maturity of rats but all the tissues examined contained at least one type of the "classic" high molecular weight receptor, and (iii) no evident correlation was found between the ER pattern and the positive or negative response to estradiol.

Phenylmethylsulfonyl fluoride (PMSF) inhibits 17 beta-estradiol binding to estrogen receptor from human prostate

Estrogen receptor binding studies were performed on cytosol obtained from human benign prostatic hyperplasia (BPH) tissue. Binding assays were done in the absence or presence of various concentrations of phenylmethylsulfonyl fluoride (PMSF). Saturation analysis and Scatchard plots showed that the binding of 17 beta -estradiol to the estrogen receptor (ER) was inhibited by PMSF. The nature of the inhibition appears to be uncompetitive, as determined from double-reciprocal plots. Glycerol density gradient centrifugation analysis also confirmed the results obtained with Scatchard plots. The inhibition observed in the presence of dithiothreitol (DTT) was greater than the inhibition observed in the absence of DTT. The maximum number of binding sites (Bmax) observed in our present study was 59.1 +/- 34.1 fmol/mg protein with an equilibrium dissociation constant (KD) of 2.2 +/- 2.2 nM. Our study indicates that PMSF significantly affects 17 beta -estradiol binding to ER and consequently alters the estimation of ER in Human BPH.

Proteins associated with untransformed estrogen receptor in vitro. Perturbation of hydrophobic interactions induces alterations in quaternary structure and exposure of the DNA-binding site

Estrogen receptors from calf uteri have been analyzed by high-performance size-exclusion chromatography, chromatofocusing, and DNA affinity chromatography using conditions designed to evaluate the relative contribution of hydrophobic interactions between the steroid-binding subunit and other receptor-associated proteins. The single large (untransformed) species of soluble estrogen-receptor consistently (n = 9) found in calf uteri displayed a rapid change in Stokes radius from 8.0 to 3.5 nm upon exposure to elevated ionic strengths (0.4 M KCl). However, equilibration of the estrogen-receptor complex into urea (up to 6 M) did not dissociate the untransformed receptor into the 3.5-nm receptor form (subunit) observed in hypertonic (0.4 M KCl) buffers. Exposure to 6 M urea did result in conversion of the untransformed receptor (8.0 nm) to a 6.0-6.5-nm receptor form not previously observed in either hypotonic or hypertonic buffers. In the presence of both 6 M urea and 0.4 M KCl, the untransformed estrogen-receptor complex was converted to a smaller receptor form intermediate in apparent size (4.5-5.0 nm) to that observed in 6 M urea or 0.4 M KCl alone. The formation of this 4.5-5.0-nm receptor form was partially estrogen dependent as determined by parallel analyses of unliganded receptor in urea/KCl buffer. The urea-induced change in apparent size (8 nm to 6.0-6.5 nm) at low ionic strength was accompanied by little or no detectable change in net surface charge as determined by chromatofocusing but a complete exposure of the DNA-binding site as evidenced by nearly quantitative interaction with DNA-agarose.(ABSTRACT TRUNCATED AT 250 WORDS)

Detection of estrogen receptor mRNA in human uterus

We transcribed a cDNA clone of the human estrogen receptor (ER) with T7 RNA polymerase. The 32P-cRNA transcript complementary to ER mRNA was hybridized to poly(A)+ RNA from human uterus and revealed a single band of approximately 4.2 kilobases. No hybridization was seen with the cRNA probe of the opposite orientation. Hybridization of total RNA from calf and rat uterus yielded a single band at approximately 3.8 kilobases for both species. Total RNA from rat spleen did not hybridize. A 35S-labeled cRNA probe was prepared for in situ hybridization of ER mRNA in human uterus and spleen. Autoradiographic signal was present over endometrial epithelium, stromal cells of the lamina propria, and smooth muscle cells of the myometrium but was absent from sections of spleen. The ER mRNA hybridization label was located over cytoplasm and nuclei of uterine target cells.

A system for the partial purification of the human androgen receptor following reversible denaturation

Androgen receptors from normal human foreskins were partially purified by sequential phosphocellulose chromatography and affinity chromatography resulting in a 28,000-fold purification and an 81% recovery. SDS-electrophoresis of the partially purified receptor preparation demonstrated that binding activity could be recovered and showed two peaks of specific binding mol. wt 35,000-55,000 and 85,000-105,000). This method demonstrates that androgen receptors can withstand harsh denaturation conditions and should prove to be a valuable tool for purifying the human androgen receptor.

Application of a protein-blotting procedure to the study of human glucocorticoid receptor interactions with DNA

To exert their effects, glucocorticoid receptor complexes interact selectively with DNA sequences known as glucocorticoid regulatory elements. We have studied the interaction between human glucocorticoid receptors and mouse mammary tumor virus (MMTV) DNA by means of a procedure that permits analysis after immobilization of the receptor on nitrocellulose. Proteins from crude cytosolic or nuclear extracts were electrophoresed on NaDodSO4/PAGE gels, soaked in a urea buffer to remove NaDodSO4, transferred to nitrocellulose, and probed with nick-translated MMTV [32P]DNA in a 5% nonfat dry milk buffer, which minimizes nonselective DNA-protein interactions. We present evidence that MMTV [32P]DNA interacts selectively with the glucocorticoid receptor. These data include comigration of [3H]dexamethasone mesylate-labeled band and bound MMTV [32P]DNA on gel electrophoresis systems; localization of DNA-binding activity in the cytosol of cells incubated with steroid at 0 degrees C and in the nucleus and cytosol of cells incubated at 37 degrees C; binding of the MMTV DNA to highly purified receptor; and absence of MMTV DNA binding activity in extracts from cells whose receptor has been down-regulated. Furthermore, glucocorticoid receptors analyzed under these conditions exhibit selective binding to DNA fragments that contain glucocorticoid regulatory elements.

Avian and mammalian receptors for 1,25-dihydroxyvitamin D3: in vitro translation to characterize size and hormone-dependent regulation

In vitro translation of cellular poly(A)+ RNA coupled with immunoprecipitation was developed as a technique for characterizing 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] receptors and assessing receptor mRNA activity. Cell-free translation of poly(A)+ RNA isolated from chicken intestine revealed two immunoprecipitable forms of avian receptor at 60 kDa and 58 kDa. These two species were identical in electrophoretic mobility to those detected directly in intestinal cytosol by immunoblot analysis. Liver, a tissue devoid of 1,25-(OH)2D3 binding activity, contained no apparent translatable receptor mRNA. 1,25-(OH)2D3 receptors were also synthesized in vitro employing poly(A)+ RNA obtained from several cultured mammalian cell lines. Selective immunoprecipitation revealed a single form of receptor at 54 kDa in mouse fibroblasts (3T6) and pig kidney cells (LLC-PK1) and a 52-kDa species in human breast carcinoma (T47D). Each of these in vitro translated mammalian 1,25-(OH)2D3 receptors migrated identically with its cellular counterpart that was synthesized in vivo employing metabolic labeling of cell protein with [35S]methionine. In vitro translation of poly(A)+ RNA derived from mouse 3T6 cells treated with 1,25-(OH)2D3 for 24-48 hr disclosed a 5-fold increase in receptor mRNA activity over untreated control cells. These results are consistent with the conclusions that 1,25-(OH)2D3 receptors are protein species ranging from 52 to 60 kDa and that, though their functional and immunological domains have been evolutionarily conserved, an inverse relationship apparently exists between phylogenetic status and receptor mass. The data also support the hypothesis that the presence of 1,25-(OH)2D3 leads to a significant increase in receptor mRNA activity in 3T6 cells, indicative of receptor autoregulation.

Evidence for a single steroid-binding protein in the rabbit progesterone receptor

The rabbit uterine progesterone receptor copurifies as two molecular weight (Mr) forms of about 105,000 and 78,000. To investigate whether these are different proteins, we have used protease digestion, reversible denaturation, and photoaffinity labeling in studies on the steroid-binding domain of the receptor. Digestion of the Mr 105,000 and 78,000 forms, photoaffinity labeled with [3H]R5020, with Staphylococcus aureus V8 protease revealed identical peptide fragments of Mr 43,000, 39,000, and 27,000-30,000. When receptor in cytosol was denatured, separated by electrophoresis, and then reconstituted, [3H]progesterone bound specifically to a single form at about Mr 105,000. After partial purification, the reversible denaturation procedure revealed both the larger and the smaller progesterone-binding species similar to the photoaffinity-labeled species in this preparation. Receptor in uterine cytosol prepared under mild conditions appeared as a predominant large molecular weight form on photoaffinity labeling with [17 alpha-methyl-3H]R5020, [6,7-3H]R5020, or [3H]RU27987. Further purification of this cytosol showed the generation of a smaller labeled species. These results from three different approaches reinforce the view that the rabbit progesterone receptor contains a single steroid-binding protein.

Further characterization of a steroid receptor-active protease from the mature rabbit epididymis

The nucleomyofibrillar fraction of mature rabbit epididymides contains a salt-extractable and leupeptin-sensitive protease that alters the sedimentation coefficient of cytosolic steroid receptors. We refer to this modification as receptor conversion. The substrate used in these studies was cytosolic estrogen receptor obtained from frozen rabbit uteri. The unactivated form of the receptor exists as an oligomer under hypotonic (0.01 M KCl) conditions (S20,w congruent to 9.6, Stokes radius (Rs) congruent to 7.4 nm, Mr congruent to 320,000) and dissociates under hypertonic (0.4 M KCl) conditions to yield the steroid-binding monomer (S20,w congruent to 4.7, Rs congruent to 5.1 nm, Mr congruent to 104,000). According to analysis under hypotonic conditions, the epididymal protease disrupts the oligomeric architecture of the receptor and reduces the size of the steroid-binding monomer (S20,w congruent to 3.2, Rs congruent to 3.0 nm, Mr congruent to 42,000). The epididymal protease had no detectable effect on the structure of the proteins used as standards for the ultracentrifugal or gel filtration analyses. Although inhibited by leupeptin, the epididymal enzyme is not a typical thiol protease since it was unaffected by thiol-blocking agents (iodoacetamide and N-ethylmaleimide), and was partially inhibited by thiol-reducing agents (monothioglycerol and dithiothreitol). Calcium and magnesium ions alone, or in combination with ATP, had no effect on the activity of the protease. However, both cations selectively suppressed recovery of the oligomeric receptor form. These results, in conjunction with those from previous studies, serve to distinguish the epididymal protease from receptor-active proteases described in extracts of other animal tissues. Molybdate, at a concentration of 50 mM, blocked receptor conversion. The ability of the receptor to be stabilized by molybdate was lost following conversion. Finally, the epididymal protease appears to remove a portion of the estrogen receptor that is necessary for nucleotide-binding.

Estradiol receptor has proteolytic activity that is responsible for its own transformation

We have investigated the effect of various protease inhibitors and substrates on the hormone- and temperature-dependent binding of partially purified estradiol-receptor complex to isolated nuclei. Only serine protease substrates and inhibitors significantly depressed estradiol receptor transformation. At 20 degrees C, we observed 50% inhibition with about 3 microM aprotinin or with 1.4 mM diisopropyl fluorophosphate. Aprotinin also blocked those size and charge modifications of receptor that are characteristic of the transformation process. The estradiol receptor was able to bind to aprotinin-agarose only under transforming conditions; i.e., the interaction was hormone- and temperature-dependent and inhibited by molybdate. Diisopropyl fluorophosphate, a covalent reagent for serine esterases, competitively inhibited the binding and specifically eluted the estradiol-receptor complex that had been bound to aprotinin-agarose. These results indicate that estradiol receptor transformation is due to the effect of a serine protease and that the receptor itself is endowed with this catalytic activity, which is triggered by the steroid.

Sequence and expression of human estrogen receptor complementary DNA

The mechanism by which the estrogen receptor and other steroid hormone receptors regulate gene expression in eukaryotic cells is not well understood. In this study, a complementary DNA clone containing the entire translated portion of the messenger RNA for the estrogen receptor from MCF-7 human breast cancer cells was sequenced and then expressed in Chinese hamster ovary (CHO-K1) cells to give a functional protein. An open reading frame of 1785 nucleotides in the complementary DNA corresponded to a polypeptide of 595 amino acids and a molecular weight of 66,200, which is in good agreement with published molecular weight values of 65,000 to 70,000 for the estrogen receptor. Homogenates of transformed Chinese hamster ovary cells containing a protein that bound [3H]estradiol and sedimented as a 4S complex in salt-containing sucrose gradients and as an 8 to 9S complex in the absence of salt. Interaction of this receptor-[3H]estradiol complex with a monoclonal antibody that is specific for primate ER confirms the identity of the expressed complementary DNA as human estrogen receptor. Amino acid sequence comparisons revealed significant regional homology among the human estrogen receptor, the human glucocorticoid receptor, and the putative v-erbA oncogene product. This suggests that steroid receptor genes and the avian erythroblastosis viral oncogene are derived from a common primordial gene. The homologous region, which is rich in cysteine, lysine, and arginine, may represent the DNA-binding domain of these proteins.

Identification of the porcine intestinal 1,25-dihydroxyvitamin D3 receptor on sodium dodecyl sulfate/polyacrylamide gels by renaturation and immunoblotting

Identification of the porcine 1,25-dihydroxyvitamin D3 receptor protein on NaDodSO4/polyacrylamide slab gels was accomplished by two separate techniques: (i) assay of the specific binding activity of tritiated 1,25-dihydroxyvitamin D3 to protein eluted from NaDodSO4/polyacrylamide gels and renatured and (ii) immunoblotting of the partially purified receptor using two anti-receptor monoclonal antibodies. The porcine receptor preparation used in these studies was isolated from a crude nuclear extract of intestinal mucosa followed by chromatography on DNA-cellulose, ammonium sulfate precipitation, gel filtration HPLC, and DEAE-Sepharose chromatography. These receptor fractions were then electrophoresed on NaDodSO4/polyacrylamide gels. The receptor was eluted from the gel, renatured, and assayed for its ability to bind tritiated 1,25-dihydroxyvitamin D3. The renatured receptor appears as a single peak of specific tritiated 1,25-dihydroxyvitamin D3 binding activity. This binding activity corresponds to a band on a silver-stained gel that correlates with the receptor peak eluted from the DEAE-Sepharose column. It also corresponds to the highest molecular weight species identified on an immunoblot with anti-receptor monoclonal antibodies. The 1,25-dihydroxyvitamin D3 receptor protein has a molecular weight of 55,000 as deduced from its migration on NaDodSO4/polyacrylamide gels.