Reduced sulfhydryl groups are required for activation of uterine progesterone receptor. Possible involvement of an inhibitor of activation

Modulation of nuclear receptor function by cellular redox poise

Nuclear receptors (NRs) are ligand-responsive transcription factors involved in diverse cellular processes ranging from metabolism to circadian rhythms. This review focuses on NRs that contain redox-active thiol groups, a common feature within the superfamily. We will begin by describing NRs, how they regulate various cellular processes and how binding ligands, corepressors and/or coactivators modulate their activity. We will then describe the general area of redox regulation, especially as it pertains to thiol-disulfide interconversion and the cellular systems that respond to and govern this redox equilibrium. Lastly, we will discuss specific examples of NRs whose activities are regulated by redox-active thiols. Glucocorticoid, estrogen, and the heme-responsive receptor, Rev-erb, will be described in the most detail as they exhibit archetypal redox regulatory mechanisms.

Human breast tumors containing non-DNA-binding immunoreactive (67 kDa) estrogen receptor

Evidence to date indicates that structurally abnormal estrogen receptor (variant ER) can be detected in some human breast tumors. Based on in vitro ability to bind DNA sequences containing the cognate estrogen response element (ERE), these variant receptors may be categorized into DNA-binding ER (Type-1 variants) and non-DNA-binding ER (Type-2 variants). To look for Type-2 variants of normal size (67 kDa ER) that lack the ability to form immunoreactive ER-ERE complexes, a panel of 40 cryopreserved primary breast tumors were extracted and analyzed by enzyme immunoassay (ER-EIA), gel-shift, and Western blot techniques. For the 33 tumor extracts containing > or = 10 fmol/mg ER (by ER-EIA), the amount of 67 kDa ER detectable by D75 anti-ER monoclonal antibody under fully denatured and reduced assay conditions (Western blotting) did not correlate well with the presence or intensity of D75 immunoreactive ER-ERE bands seen under native conditions by gel-shift assay. Overall, 30% (10 of 33) of these extracts containing 67 kDa ER failed to produce immunoreactive ER-ERE complexes, with this frequency varying from over 40% in tumor samples with lower ER content (10-49 fmol/mg) to 11% in tumor samples with the highest ER content (> 100 fmol/mg). These results indicate that Type-2 variant receptors characterized as non-DNA-binding 67 kDa ER may be present in a significant fraction of ER-positive primary breast tumors; preliminary evidence suggests that further study of abnormalities in ER tertiary or quaternary structure, such as those produced by intracellular oxidation of ER thiol groups, is warranted.

A model for the determination of the 3D-spatial distribution of the functions of the hormone-binding domain of receptors that bind 3-keto-4-ene steroids

A method of comparing the hydrophobic clusters of proteins (hydrophobic cluster analysis, HCA) has revealed that the 3D-folding pattern of the hormone-binding domain (HBD) of steroid hormone receptors (SHRs) may have an unexpectedly high degree of analogy with the known 3D-crystal structures of proteins belonging to the serine proteinase inhibitor (SERPIN) superfamily, e.g. alpha 1-antitrypsin and ovalbumin. The present paper briefly reviews some of the biochemical evidence that supports the structural validity of the SERPIN model and shows how the model can be used to establish hypothetical 3D-locations for functions attributed to different amino-acids or peptide sequences of the HBD: i.e. heat-shock protein binding, transcription activation, phosphorylation, steroid binding, but also ATP-binding. Indeed, the model has enabled the identification of a Rossmann-fold in SHRs that might bind ATP. Visualization of all these functions should help to interpret the chain of concerted events induced by steroid binding.

The heat-stable cytosolic factor that promotes glucocorticoid receptor binding to DNA is neither thioredoxin nor ribonuclease

Treatment of rat liver cytosol containing temperature-transformed [3H]dexamethasone-bound receptors at 0 degree C with the sulfhydryl modifying reagent methyl methanethiosulfonate (MMTS) inhibits the DNA-binding activity of the receptor, and DNA-binding activity is restored after addition of dithiothreitol (DTT). However, transformed receptors that are treated with MMTS and then separated from low Mr components of cytosol by passage through a column of Sephadex G-50 have very little DNA-binding activity when DTT is added to regenerate sulfhydryl moities. The receptors will bind to DNA if whole liver cytosol or boiled liver cytosol is added in addition to DTT. The effect of boiled cytosol is mimicked by purified rat thioredoxin or bovine RNase A in a manner that does not reflect the reducing activity of the former or the catalytic activity of the latter. This suggests that the reported ability of each of these heat-stable peptides to stimulate DNA binding by glucocorticoid receptors is not a biologically relevant action. We suggest that stimulation of DNA binding of partially purified receptors by boiled cytosol does not constitute a reconstitution of a complete cytosolic system in which the dissociated receptor must associate with a specific heat-stable accessory protein required for DNA binding, as has been suggested in the "two-step" model of receptor transformation recently proposed by Schmidt et al. (Schmidt T.J., Miller-Diener, A., Webb M.L. and Litwack G. (1985) J. biol. Chem. 260, 16255-16262).

Transformation of glucocorticoid and progesterone receptors to the DNA-binding state

This brief review explores some recent observations relating to the structure of untransformed glucocorticoid and progesterone receptors and the mechanism by which the receptors are transformed to the DNA-binding state. In their molybdatestabilized, untransformed state, progesterone and glucocorticoid receptors exist as a heteromeric 8-9S complex containing one unit of steroid binding phosphoprotein and one or two units of the 90 kD heat shock protein hsp90. When the receptors are transformed, the steroid-binding protein dissociates from hsp90. In cytosol preparations, temperature-mediated dissociation proceeds much more rapidly in the presence of hormone. The dissociated receptor binds to DNA with high affinity, regardless of whether it is in the hormone-bound or the hormone-free state. These observations raise the possibility that the primary, and perhaps the only, role for the hormone is to promote dissociation of the receptor-hsp90 complex. Molybdate, vanadate, and tungstate inhibit receptor transformation to the DNA-binding form, an effect that appears to reflect the ability of these transition metal oxyanions to stabilize the complex between the steroid receptor and hsp90. By promoting the formation of disulfide bonds, hydrogen peroxide also stabilizes the glucocorticoid receptor-hsp90 complex and prevents receptor transformation. A small, heat-stable factor present in all cytosol preparations inhibits receptor transformation, and, when the factor is removed, glucocorticoid receptors are rapidly transformed. This ubiquitous factor has the physical properties of a metal anion, and it is proposed that molybdate and vanadate affect steroid receptor complexes by interacting with a metal anion-binding site that is normally occupied by this endogenous receptor-stabilizing factor.

Characterization of nuclear acceptor sites for mammalian progesterone receptor: comparison with the chick oviduct system

We performed binding studies with hamster uterine progesterone receptor (Rp) and various chromatin-cellulose preparations to determine whether Rp acceptor sites exist in mammalian uterus analogous to those observed in the chick oviduct. Chick and hamster Rp acceptor site assays were done according to the method of Spelsberg et al. [(1983) Recent Prog. Horm. Res. 39, 463-513]. Hamster Rp bound to hamster uterine chromatin-cellulose, NAP-cellulose (a 4 M guanidine hydrochloride-extracted fraction) and DNA-cellulose in a manner similar to that observed in the chick oviduct. Hamster Rp binding was tissue specific as evidenced by higher Rp binding to target-tissue vs non-target-tissue chromatin. The greatest degree of Rp binding occurred in the NAP fraction, and the higher level of binding seen in NAP-chromatin as compared with that in crude chromatin may be attributed to extraction of "masking proteins" which inhibit Rp-chromosomal protein interactions. When guanidine hydrochloride at greater than 4 M was used to extract crude chromatin, Rp binding decreased, indicating that the Rp acceptor sites were removed or denatured. These findings demonstrate the existence of Rp acceptor sites in the mammalian uterus which are similar to avian oviduct acceptor sites, suggesting that such sites may play a role in mediating Rp-induced gene expression.

A possible role for dephosphorylation in glucocorticoid receptor transformation

Addition of bovine intestinal alkaline phosphatase to mouse AtT-20 cell cytosol increases the rate of glucocorticoid receptor transformation, as evidenced by a change in sedimentation rate from 9.1S to 5.2S. Acid phosphatases are completely ineffective in this regard. Alkaline phosphatase-promoted receptor transformation is both time- and dose-dependent. A variety of phosphatase inhibitors are effective in inhibiting this process, the most potent being transition metal oxyanions such as molybdate, tungstate, and arsenate. The ability of the various inhibitors to suppress alkaline phosphatase-promoted receptor transformation does not correspond well with their potencies for inhibiting para-nitrophenyl phosphate hydrolysis. However, a better correspondence between the inhibition of endogenous receptor transformation and total cytosolic phosphatase activity is observed, and both sodium fluoride and glucose-1-phosphate inhibit endogenous receptor transformation. The protease inhibitors phenyl-methylsulfonyl fluoride and antipain have no effect on receptor transformation. Surprisingly, leupeptin is effective in inhibiting alkaline phosphatase-promoted receptor transformation. Although this raises the possibility of a contaminating protease activity in the alkaline phosphatase enzyme preparation, treatment of covalently affinity-labeled receptor with the enzyme shows no proteolysis of the receptor or any other non-specifically labeled cytosolic protein. Thus, it is possible that a novel action of leupeptin, unrelated to its protease-inhibitory activity, may be involved in the suppression of receptor transformation. The studies presented here suggest that dephosphorylation of some component in cytosol is involved in the destabilization of receptor subunit interactions, resulting in glucocorticoid receptor transformation.

Characterization of progesterone binding to nuclear receptors in rat placenta

Exchange assays have been validated to study several forms of the progesterone receptor found to occur in nuclei of rat placenta after extraction with high salt. One form was solubilized by the extraction procedure (KCl extractable Rpn) and another form remained attached to nuclear structures (KCl resistant Rpn). Specific binding of progesterone was optimized in both forms using buffered media containing 0.01 M Tris, 30%-glycerol (v/v), 0.2 mM leupeptin, and 1 mM dithiothreitol (TDGL), pH 7.8, at 0-4 degrees C for 18-24 h. At 0-4 degrees C the nuclear receptors were stable and degradation was negligible even after 44 h of in vitro incubation. The binding reaction between progesterone and receptor demonstrated mass action principles of ligand exchange throughout this interval. Saturation analysis indicated the presence of a single binding moiety of high affinity (app Kd = 2.9-3.2 nM) for both forms of the receptor. However, the nuclear progesterone receptor was thermolabile and after a 10 min exposure to 30 degrees C no longer complexed ligand. At an intermediate incubation temperature of 22 degrees C the binding reaction was stable for about 30 min. The KCl resistant binding sites were markedly more thermolabile. Addition of 10 mM Na molybdate protected all forms of the nuclear progesterone receptor from thermal denaturation and extended the life of the complex 3-4-fold. The dissociation rate constant of progesterone-nuclear receptor complex in each preparation was 6-8 X 10(5) s-1 resulting in a half-life of about 3 h. The KCl resistant and extractable binding sites were sensitive to blockade by 1 mM N-ethylmaleimide which was reversed by co-incubation with a 2-fold molar excess of dithiothreitol. This suggested that reduced sulfhydryl groups located on or near the surface of the ligand binding domain of the receptor were necessary to bind hormone. These studies showed that the interactions between ligand and the KCl resistant and extractable receptor sites found in rat placenta were of high affinity, saturable, and heat sensitive. Thus, these binding moieties exhibited physicochemical behavior very similar to each other and to the placental receptor which has previously been partially purified from the cytosol. The conclusion is made that all of the nuclear receptor binding sites for progesterone are structurally identical. Thus, the distinctive physicochemical properties responsible for KCl resistant and extractable forms of the nuclear progesterone receptor must reside in other domains of the receptor molecule.

The effect of dithiothreitol on the kinetics of dissociation of dexamethasone from the non-transformed mammary cytosolic glucocorticoid receptor

Sulfhydryl reducing agents such as dithiothreitol are required for maximum binding of dexamethasone to the mammary cytosolic glucocorticoid receptor, but little is known concerning the effects of dithiothreitol on the kinetics of the binding reaction. In this report we have examined the influence of dithiothreitol on the dissociation kinetics of dexamethasone from the non-transformed glucocorticoid-receptor complex at 0-4 degrees C under various experimental conditions. Without dithiothreitol, the rate of dissociation of dexamethasone remains essentially the same (t1/2 approximately 17 h) regardless of the method chosen to monitor dissociation. With dithiothreitol, however, there is a marked acceleration in the rate of dissociation of receptor-bound dexamethasone when an excess of unlabeled dexamethasone is used to study dissociation (t1/2 approximately 5 h) but not when dissociation is investigated by removal of free labeled dexamethasone by charcoal adsorption (t1/2 approximately 21 h); dithiothreitol also accelerates the observed rate of dissociation when a combination of these methods is used. An acceleration in the rate of receptor-bound dexamethasone is also observed when an excess of the synthetic progestin, R5020, is used in the dissociation assay. The possible reasons and importance underlying these findings have been discussed.

Covalent chromatography by thiol-disulfide interchange of the highly-purified non-transformed rat liver glucocorticoid-receptor

Highly-purified non-transformed rat liver [3H]triamcinolone acetonide-receptor complex was shown to be covalently adsorbed on activated thiol sepharose 4B, a reactive sulfhydryl matrice. Elution by mercaptoethanol in excess and inhibition of binding by previous treatment of the complex with N-ethylmaleimide clearly demonstrated the specificity of the binding by thiol disulfide interchange. The transformed [3H]triamcinolone acetonide-receptor complex, partially purified by DNA-cellulose chromatography, was also retained on activated thiol sepharose 4B. The physicochemical characteristics of both the transformed and non-transformed glucocorticoid receptor complexes eluted from the covalent chromatography column were studied by HP size exclusion chromatography on a TSK G 3000 SW column and were found to be identical to those of the starting complexes. These results provide direct evidence for accessible sulfhydryl groups on the glucocorticoid receptor complex surface, probably distinct from the steroid binding essential sulfhydryl group.

A mini-column method for routine measurement of human prostatic androgen receptors

The complex heterogeneous nature of the human prostate gland is such that it is advisable to know the histological characteristics of each sample used for androgen receptor (AR) measurement. Adequate size of sample for AR determination is thus a problem if specimens provided during routine transurethral prostatectomy are to be used for both estimation of AR and histological examination. We present a simple method suitable for these small specimens in which [3H]R 1881 bound to AR is separated from free steroid on mini-columns of controlled-pore glass beads. Data obtained indicate a single class of binding sites of high affinity and low capacity with steroid specificity typical of an androgen receptor. The assay is suitable for samples as small as 20 mg wet weight and is linear using 25-125 microliter cytosol (correlation coefficient 0.995). Intra-assay variation is 6.8% and interassay variation 25.8% (n = 22) over 4 months. A single saturating concentration of steroid measures 97% of AR calculated by Scatchard analysis. Inclusion of high salt (0.4 M KNO3) and 10 mM dithiothreitol in incubation buffer at pH 8.4 are essential; inclusion of 10 mM sodium molybdate in the homogenisation buffer improves measurement. A comparison of AR measured in histologically similar samples obtained by a transurethral resectoscope (TUR) and a cold punch resectoscope (CPR) taken in juxtaposition demonstrated no difference in receptor content. Although carcinomatous samples contained significantly higher receptors levels than benign samples, no differences were observed between TUR and CPR specimens.

Evidence for the involvement of essential sulphydryl group in rat hepatic lactogenic receptor but not in somatogenic receptor

Incorporation of p-chloromercuribenzene sulphonate (PCMBS) (1mM) in the assay medium for rat hepatic lactogenic receptor produced complete inhibition of binding of [125I]oPRL to the membrane. However, the presence of the thiol-reactive agent produced no effect on the binding of [125I]bGH to rat hepatic somatogenic receptor. Pretreatment of rat hepatic membrane with PCMBS inhibited the binding of [125I]oPRL but not that of [125I]bGH. The lactogenic receptor binding inhibition by PCMBS pretreatment was both concentration- and time-dependent, with complete inhibition at 0.5 mM for 60 min at 0 degrees C. Scatchard analysis of [125I]oPRL binding to membrane at 50% inhibition by PCMBS (0.11 mM) revealed that the binding capacity was decreased rather than the binding affinity. Furthermore, the inhibition of lactogenic receptor binding by PCMBS could be reversed by dithioerythritol (DTE) treatment. Following 80% inhibition by 0.2 mM PCMBS, full recovery of receptor binding was achieved at 6 mM DTE for 60 min at 0 degrees C. The 'recovered' membrane showed no difference from the control membrane in terms of binding capacity and affinity.

Zinc concentration and progestin receptor in human benign hyperplastic prostate

The effect of cationic zinc on the specific binding of the synthetic progestin, promegestone (17 alpha-21-dimethyl-19-nor-pregna-4,9-diene-3,20-dione, R5020), has been investigated in cytosols from human benign hyperplastic prostate. Zinc was analyzed using flameless atomic absorption spectrometry. Progestin specific binding was assayed using sucrose density gradient centrifugation in a vertical tube rotor. The concentration of endogenous zinc in benign hyperplastic prostate cytosols was 0.53 mM +/- 0.26 (range 0.15 mM to 0.88 mM). No significant relationship was observed between zinc concentration in cytosols and the amount of progestin specific binding in the 7-8S peak in sucrose density gradients. Addition of excess zinc to cytosols (1.6 mM) in the presence or absence of monothioglycerol had no effect on the total amount of 7-8S progestin specific binding observed. Removal of free zinc by gel filtration of cytosols on Sephadex G-25 did not change the amount of 7-8S cytosolic receptor binding observed. The addition of the chelating agents, 2,2',2''-tripyridine, 9,10-phenanthroline or EGTA, after the gel filtration step had no apparent effect on the 7-8S progestin specific binding. The anionic chelator, EDTA, inhibited the 7-8S specific binding in a dose dependent manner. However, this inhibition was not reversed by the addition of excess zinc (6 mM) and was observed at very high concentrations of EDTA. This suggests that the effect of EDTA was due to its anionic effect rather than to its effect as a chelator. We conclude that cationic zinc in concentrations up to 2 mM does not change the amount of 7-8S progestin specific receptor binding in cytosols from BPH tissue.

Human androgen insensitivity mutation does not alter oligonucleotide recognition by the androgen receptor-DHT complex

We studied the binding of dihydrotestosterone-receptor complexes (DHT-R) from human genital skin fibroblasts to oligodeoxyribonucleotides and DNA. Following incubation of fibroblasts with 2 nM [3H]DHT (45 min, 37 degrees C), DHT-R were prepared as total fibroblast sonicates (sonication of cells in 0.5 M KCl), intact fibroblast cytosol (100000 X g supernatant) or intact fibroblast nuclear extract (sonication of nuclei in 0.5 M KCl). DHT-R were also prepared by incubation of fractionated fibroblast cytosol with 4 nM [3H]DHT (4 h, 0 degrees C). Optimal conditions were established for binding of DHT-R from total fibroblast sonicates to oligo-dT cellulose: 60 min, 0 degrees C, low salt (0.05-0.10 M KCl), linearity with DHT-R concentration, and nucleotide saturation. With total fibroblast sonicates the rank order of DHT-R binding was oligo-dT approximately equal to -dG greater than DNA greater than -dC greater than or equal to -dA approximately equal to -dI. Intact fibroblast cytosol displayed a similar preference of DHT-R binding to oligo-dT and -dG but the binding was quantitatively higher than for total fibroblast sonicates, the binding for fractionated fibroblast cytosolic DHT-R formed at 0 degrees C being quantitatively lower. However, binding of DHT-R from cytosol (0 degrees C) to DNA-cellulose was equal to that for DHT-R from cytosol (37 degrees C). Binding of DHT-R from intact fibroblast nuclear extracts was lower than for total fibroblast sonicates. Preparations from cells of patients with receptor-negative, complete androgen insensitivity lacked both DHT-R formation and specific oligonucleotide binding. Binding of oligonucleotides to DHT-R from cells of patients with receptor-positive, complete androgen insensitivity could not be distinguished from that of normal cells. These results suggest: (a) androgen receptor-steroid complexes recognize and bind to certain preferred deoxyribonucleotides; (b) various factors affect the quantitative binding of DHT-R from different cellular preparations to deoxyribonucleotides; and (c) neither qualitative nor quantitative abnormalities for DHT-R of complete androgen-insensitive patients were detectable from oligonucleotide or DNA binding.

Resolution of the effects of sulfhydryl-blocking reagents on hormone- and DNA-binding activities of the chick oviduct progesterone receptor

The differential effects of sulfhydryl (SH)-blocking agents on hormone and DNA binding by the chick oviduct progesterone receptor were investigated. Previous studies have demonstrated inhibition of steroid-receptor interaction by SH-blocking agents and protection against inhibition by bound hormone. The present results indicate that the SH group required for steroid binding is within or near the hormone-binding site itself, and that a second SH group (or groups) is involved in the binding of receptor to DNA. Three findings relate to the site of action of SH-blocking agents on hormone binding. First, glycerol decreased the rate of hormone dissociation and the rate of hormone displacement by mercurial reagents by 75 to 90%. Second, mercurial reagents displaced [3H]progesterone bound to the mero-receptor, a Mr 23,000 proteolytic fragment containing the hormone-binding site, but not the site of interaction with DNA. Third, hormone displacement was still present after a 10,000-fold purification of the progesterone receptor. Mercurial reagents also inhibited binding of progesterone receptor to DNA, whereas the SH-alkylating agents N-ethylmaleimide and iodoacetamide had no effect. It is likely that distinct sulfhydryl groups are required for steroid receptor interaction with hormone and with DNA, since brief treatment with mercurial reagents blocked DNA binding, but caused only a slight displacement of bound hormone. The SH group required for hormone binding probably lies within or near the hormone-binding site, is sensitive to mercurials, alkylating agents, and 5,5'-dithiobis(2-nitrobenzoate) (DTNB), and is protected by bound hormone. The SH group required for DNA binding, in contrast, is sensitive to mercurials but not to alkylating agents, is only partially sensitive to DTNB, and is not protected by bound hormone.

Subunit dissociation as a possible mechanism of glucocorticoid receptor activation

For the elucidation of the mechanism of steroid hormone receptor activation, the hydrodynamic properties of the unactivated and activated forms of the nonproteolyzed glucocorticoid receptor from the mouse AtT-20 pituitary tumor cell line were determined. The unactivated, molybdate-stabilized receptor has the following properties: sedimentation coefficient = 9 S; Rs = 8.3 nm; Mr = 317 000; f/f0 = 1.70; axial ratio (prolate ellipsoid) = 14. The activated monomeric receptor has a sedimentation coefficient of 3.2 S, a Stokes radius of 6 nm, a molecular weight of 81 000, a frictional ratio of 1.93, and an axial ratio (prolate ellipsoid) of 18. A receptor species of intermediate size was detected when the analysis was performed in buffer containing both 0.3 M KCl and 20mM Na2MoO4. Its characteristics are as follows: sedimentation coefficient = 5 S; Rs = 8.3 nm; Mr = 176 000; f/f0 = 2.06; axial ratio (prolate ellipsoid) = 22. A preliminary study seemed to indicate that this is an activated form of the receptor. On the basis of the molecular weights, it is likely that the unactivated receptor is a tetramer of identical hormone-binding subunits (Mr = 81 000) while the intermediate form is a homodimer. Alternatively, non-hormone-binding components (receptor-binding factors) may be involved in forming the multimeric, nonactivated receptor complex. In either case, the dissociation of a multimeric, nonactivated receptor into subunits appears to be a possible mechanism of receptor activation. Finally, the addition of high concentrations of 1-thioglycerol promoted activation. Thus, sulfhydryl groups may be involved in receptor subunit interaction.