Binding of [3H]estradiol- and [3H]H1285-receptor complexes to rabbit uterine chromatin

Estrogen receptor interaction with specific histones. Binding to genomic DNA and an estrogen response element

Chromosomal proteins that impart high affinity and specificity to the binding of the estrogen receptor (ER) to DNA are termed estrogen receptor binding factors (ERBFs). Certain partially purified chromosomal protein fractions obtained from rabbit uterine chromatin by extraction with various molarities of GdnHCl when reconstituted to double-stranded DNA demonstrated high affinity binding for the ER. We report the purification and characterization of ERBFs in the chromosomal protein fraction extracted with 4 M GdnHCl (CP4) after large scale purification. These protein fractions were further purified by CL-Sepharose 6B column chromatography which resolved fractions from CP4 that recognized the ER bound by estrogen only or antiestrogen only. Thus, these hydrophobic chromosomal proteins enhanced the binding of the ER to reconstituted chromatin. To further investigate the interaction of ERBFs with ER, gel mobility shift assays were performed. The highly purified CP4 fraction with ERBF activity in the binding assay with reconstituted chromatin caused an increase in the formation of the retarded ER-estrogen responsive element (ERE) band. Thus, chromatin contains specific ERBFs for ER bound by estrogen which enhance the binding of ER to genomic DNA and a target ERE sequence. Further purification of the CL-Sepharose fraction with ERBF activity was achieved by preparative SDS-PAGE. ERBF activity was attributed to proteins with approximate molecular weights of 16,000, 13,000, and 12,000 and a pl of > 9.0. Peptides were partially sequenced by Edman degradation and were found to have identity with histones H2B and H4. A 17 kDa protein without ERBF activity was identified as H3. Since these histones were not readily extracted from chromatin with 3 M NaCl or 1-3 M GdnHCl, we postulate that some ERBFs may be histone variants or modified histones that display a very high affinity for DNA and ER.

Nuclear matrix acceptor binding sites for steroid hormone receptors: a candidate nuclear matrix acceptor protein

Steroid/nuclear-hormone receptors are ligand-activated transcription factors that have been localized to the nuclear matrix. The classic model of hormone action suggests that, following activation, these receptors bind to specific "steroid response elements" on the DNA, then interact with other factors in the transcription initiation complex. However, evidence demonstrates the existence of specific chromatin proteins that act as accessory factors by facilitating the binding of the steroid receptors to the DNA. One such protein, the "receptor binding factor (RBF)-1", has been purified and shown to confer specific, high-affinity binding of the progesterone receptor to the DNA. Interestingly, the RBF-1 is localized to the nuclear matrix. Further, the RBF-1 binds specifically to a sequence of the c-myc proto-oncogene that has the appearance of a nuclear matrix attached region (MAR). These results, and other findings reviewed here, suggest that the nuclear matrix is involved intimately in steroid hormone-regulated gene expression.

The ubiquitous nature of the progesterone receptor binding factor-1 (RBF-1) in avian tissues

The avian oviduct receptor binding factor-1 (RBF-1) is a 10 kDa nuclear matrix protein that was originally identified through its ability to effect high affinity interaction of activated progesterone receptor (PR) with chromatin. In the present study, the RBF-1 is shown to not be restricted to reproductive tissues (e.g., oviduct) but present in all avian tissues examined by Western blot analysis with a monoclonal antibody prepared against purified RBF-1. The heart and pancreas had the highest and lowest RBF-1 levels, respectively; the concentration ranging by approximately 50-fold in these tissues. The 10 kDa size of the RBF-1 detected in all tissues suggests no significant tissue-specific differences in the protein. This was consistent with the finding that purified hepatic and oviductal RBF-1 have identical amino-terminal sequence. Using a recently isolated cDNA to RBF-1, the levels of RBF-1 mRNA were found to correlate well with the ubiquitous presence of the protein as well as tissue-specific differences in concentration. The presence of RBF-1 in non-progesterone responsive tissues suggests the possibility that RBF-1 may not be specifically involved in PR-DNA interactions but may play a more diverse role, possibly involving other steroid receptors such as the glucocorticoid receptor.

Immunohistochemical localization of the avian progesterone receptor and its candidate receptor binding factor (RBF-1)

An avian oviduct nuclear matrix protein in the 6-10 kDa size range has been implicated to function in the cell-free nuclear binding of the avian oviduct progesterone receptor (PR). This protein, termed the receptor binding factor-1 (RBF-1), has been purified and partially characterized [Schuchard et al.: Biochemistry 30:4535-4542, 1991]. This paper describes the immunohistochemical co-localization of the RBF-1 and PR in the avian oviduct cell nuclei and rat reproductive cell nuclei using antibodies directed specifically against the RBF-1 and activated PR. In the undifferentiated oviduct, the immunoreactivities for both PR and RBF-1 were co-localized in the nuclei of only epithelial cells, but not the stromal cells or smooth muscle cells. In the partially differentiated oviduct of estrogen treated chicks, the immunoreactivity co-localized in the nuclei of not only epithelial but also glandular and stromal cells. Staining for the PR, but not RBF-1, was detected in the smooth muscle cells. The intensity of the PR but not the RBF-1 staining was markedly down-regulated in these cells at 2 and 6 h after treatment of the animals with progesterone (P). However, the band patterns for RBF-1 in the Western blots did show qualitative changes which may reflect P-induced posttranslational modifications which alter the epitope on the RBF-1. Interestingly, immunohistochemical analysis of several reproductive tissues of the rat showed that certain cell types in the uterus, ovary, and prostate displayed strong positive nuclear staining for an RBF-1-like antigen(s).(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of the Ah receptor to receptor binding factors in chromatin

Dioxin induces biological responses through interaction with a specific intracellular receptor, the Ah receptor, and the subsequent interaction of the Ah receptor with chromatin. We report the binding of the Ah receptor, partially purified from rabbit liver, to receptor binding factors in chromatin. Rabbit liver chromatin proteins (CP) were isolated by adsorption of chromatin to hydroxylapatite followed by sequential extraction with 1-8 M GdnHCl. To assay for receptor binding a portion of each CP fraction was reconstituted to rabbit double-stranded DNA using a reverse gradient dialysis of 7.5 to 0 M GdnHCl. These reconstituted nucleoacidic proteins were then examined for binding to [3H]-2,3,7,8-tetrachlorodibenzo-p-dioxin ([3H]TCDD)-receptor complexes by the streptomycin filter assay. Prior to the binding assay, [3H]TCDD-receptor complexes were partially purified by step elution from DEAE-cellulose columns. CP fractions 2, 5, and 7 were found to bind to the Ah receptor with high affinity. Scatchard analysis yielded Kd values in the nanomolar range. Competition with 2-fold excess unlabeled TCDD-receptor complexes was demonstrated, and binding was reduced markedly when the receptor was prepared in the presence of 10 mM molybdate. Such chromatin receptor binding factors (RBFs) may participate in the interaction of receptor with specific DNA sequences resulting in modulation of specific gene expression.

Purification of a nuclear protein (receptor binding factor-1) associated with the chromatin acceptor sites for the avian oviduct progesterone receptor

The specific high affinity binding of the avian oviduct progesterone receptor (PR) to target cell nuclei and chromatin has been shown to involve DNA complexed with specific chromatin acceptor proteins. One of these chromatin acceptor proteins has been partially purified and found to be a small hydrophobic protein with a broad pI of 5.0-6.0 [Goldberger and Spelsberg (1988), Biochem. 27, 2103-2109]. Using western immunoblots with anti-RBF-1 polyclonal antibodies to monitor the purification, a 10 kD candidate acceptor protein, termed the Receptor Binding Factor-1 (RBF-1), has been purified to apparent homogeneity. RBF-1 has an amino acid composition consistent with a hydrophobic protein having an acidic pI and a unique N-terminal sequence. Two-dimensional polyacrylamide gel electrophoresis and high-performance capillary electrophoresis support the purity of a protein congruent to 10 kD in size, having an acidic pI, but with evidence of several differently charged isoforms. Phosphatase treatment provides evidence that charge heterogeneity may result from variable phosphorylation states. A role of this factor as a candidate "acceptor protein" in the chromatin acceptor sites for the avian oviduct PR is proposed.

Effects of antiestrogen versus antiprogestin on transformed and nontransformed steroid receptors

In order to determine if different physicochemical properties exist among antihormone-receptor complexes, we have compared the interaction of the antiprogestin RU486 with progesterone receptor (PR) versus the triphenylethylene antiestrogen H1285 (4-(N,N-diethyl-aminoethoxy)-4'-methoxy-alpha-(p-hydroxyphenyl-alp ha'- ethylstilbene] with estrogen receptor (ER) from rabbit uterine tissue. Contrary to other reports, we observed no difference in the sedimentation properties of transformed PR (4S) when bound by the antagonist RU486 versus the progesterone agonist R5020 in either cytosol or DEAE partially-purified receptor preparations analyzed on sucrose gradients containing 0.3 M KCl. In addition, we found no difference in the sedimentation properties of these receptor preparations in the presence of 10 mM sodium molybdate: the nontransformed RU486-PR and nontransformed R5020-PR both sedimented as a 6S species. These same results were obtained when the receptor preparation and gradient analysis were performed in the absence of monothioglycerol. Likewise, there was no change in the sedimentation properties of the transformed PR when the receptor, partially purified in the absence of molybdate, was analyzed on sucrose gradients containing 10 mM sodium molybdate to prevent receptor alteration during centrifugation. From DNA-cellulose assays performed with partially purified PR in the absence of molybdate we determined that the 4S form of R5020-PR and RU486-PR is transformed receptor; whereas in the presence of molybdate, the 6S species is nontransformed. In contrast, we found a different pattern of sedimentation when comparing transformed antiestrogen-receptor complexes with transformed estrogen-receptor complexes. In this case, transformed H1285-ER sedimented as 6S and estradiol-ER sedimented as 4S. We conclude from these experiments that these two antihormones, RU486 and H1285, may have different mechanisms of action in their antagonism of steroid hormone action. Antiestrogen stabilizes the salt-transformed ER as a dimer while antiprogestin appears to permit dissociation of the oligomeric form of the receptor to the monomeric form.

Differences in the form of the salt-transformed estrogen receptor when bound by estrogen versus antiestrogen

Our laboratory has previously reported that antiestrogen binding to molybdate-stabilized non-transformed estrogen receptor results in a larger form of the receptor in 0.3 M KCl when compared with estrogen bound receptor. Estradiol promoted the formation of monomers in the presence of 0.3 M KCl whereas antiestrogen appeared to promote dimer formation. We have extended these studies examining the rabbit uterine salt-transformed estrogen receptor partially purified by DEAE-cellulose chromatography. We previously demonstrated that estrogen receptor prepared in this way bound to different sites on partially deproteinized chromatin subfractions or reconstituted chromosomal protein/DNA fractions when the receptor was complexed with estrogen vs antiestrogen. Analysis of these receptor preparations indicated that DEAE-cellulose step-elution resulted in a peak fraction which sedimented as a single 5.9S peak in 5-20% sucrose density gradients containing 0.3 M KCl for receptor bound by the antiestrogens H1285 and trans-hydroxytamoxifen. However, receptor bound by estradiol sedimented as 4.5S. These receptor complexes bound DNA-cellulose indicating that these partially purified receptors were transformed. DEAE rechromatography or agarose gel filtration of the partially purified antiestrogen-receptor complexes resulted in significant dissociation of the larger complex into monomers. Incubations of 5.9S antiestrogen-receptor complexes with antibodies against nontransformed steroid receptor-associated proteins (the 59 and 90 kDa proteins) did not result in the interaction of this larger antiestrogen-receptor complex with these antibodies (obtained from L. E. Faber and D. O. Toft, respectively). Our results support the concept that antiestrogen binding induces a different receptor conformation which could affect monomer-dimer equilibrium, thus rendering the antiestrogen-receptor complex incapable of inducing complete estrogenic responses in target tissues.

Nuclear acceptor sites for estrogen-receptor complexes in the liver of the turtle, Chrysemys picta. I. Sexual differences, species specificity and hormonal dependency

Hepatic estrogen receptors (ERs) of the female turtle, Chrysemys picta, when complexed with [3H]estradiol ([3H]E2), were shown to bind specifically to liver chromatin isolated from the same species. The binding of the [3H]E2 receptor complex to chromatin requires both the steroid ligand and the receptor protein. Maximal binding occurred within 60-70 min of incubation at 4 degrees C in a Tris buffer containing 0.1 M KCl. The binding of the [3H]E2 receptor complex to intact chromatin was saturable, whereas the binding to turtle or calf thymus DNA remained linear. Scatchard analyses revealed more estrogen receptor binding sites on hepatic chromatin isolated from female turtles than that prepared from the males (binding capacities: female chromatin = 67.9 +/- 6.8 fmol/mg DNA equivalent; male chromatin = 28.5 +/- 2.5 fmol/mg DNA equivalent). Furthermore, the [3H]E2 receptor complex was bound with a higher affinity to female chromatin than to male chromatin (association constants: female chromatin = 11.7 +/- 2.7 X 10(10) M-1; male chromatin = 2.5 +/- 0.7 X 10(10) M-1). In contrast to turtle hepatic [3H]E2 receptors, ERs in rat liver or mouse uterine cytosol exhibited little binding affinity for hepatic chromatin isolated from the turtle. Tissue specificity was demonstrated in the interaction of the [3H]E2 receptor complex and chromatin; high affinity, saturable binding of the [3H]E2 receptor complex was only observed on chromatin isolated from the liver but not on those prepared from the heart, kidney and muscle. A 3- to 4-fold increase in the number of hepatic chromatin [3H]E2 receptor binding sites was observed in 21-day ovariectomized or hypophysectomized female (capacities = 209.3 +/- 6.1 and 270 +/- 10.1 fmol/mg DNA equivalent, respectively). It is postulated that [3H]E2 receptor binding sites on the chromatin of intact females are partially 'masked', and removal of a gonadal and/or pituitary factor(s) unveils additional binding sites on the female chromatin. This paper is first to report the presence of high affinity, species- and tissue-specific acceptor sites on the liver chromatin of a reptilian species. The fact that the levels and properties of these acceptor sites are dependent on the sex and hormonal state of the animal suggests that they may play a role in the regulation of hepatic estrogen responsiveness and vitellogenesis in this species.

Nuclear acceptor sites for steroid hormone receptors: comparisons of steroids and antisteroids

The respective chromatin binding sites (acceptor sites) for both the avian oviduct progesterone receptor (PR) and the rabbit uterine estrogen receptor (ER) reported by two separate laboratories are compared. Support for a saturable, high affinity binding to the chromatin acceptor sites by both receptors is described. Nonradiolabelled PR or ER compete with their homologous radiolabelled receptors for binding. However, there is no competition between the heterologous ER and PR for the nuclear binding sites. In both receptor systems, evidence for a receptor dependent, receptor specific binding, which mimics the binding measured in vivo, is reported. With both the PR and ER, evidence for extensive masking in chromatin of many of the acceptor sites is described. The PR and ER acceptor sites appear to be composed of specific acceptor proteins bound to DNA. The dissociation of these proteins from DNA causes a loss of specific binding. Reannealing of these same chromatin protein fractions, but not other protein fractions, back to the DNA reinstates the specific PR binding. Antibodies against the PR acceptors proteins are described. These antibodies block PR but not ER binding to the chromatin acceptor sites, supporting the steroid receptor specific acceptor sites. In the rabbit uterine system, the ER acceptor proteins dissociate from chromatin in three distinct fractions according to their affinity for the DNA. Each of these fractions contains acceptor activity in that each can be reannealed to the DNA to reconstitute specific ER binding sites, i.e. those which are saturable, high affinity, and receptor dependent. Interestingly, the antiestrogen receptor complexes do not bind to one fraction of acceptor proteins which does bind the native estrogen receptor complex. This ER acceptor protein fraction contains two regions of acceptor activity in the molecular mass range of 50,000 and 12,000 daltons. The latter species is in the size range of the PR acceptor proteins. This difference in binding to chromatin acceptor sites by the estrogen receptor, complexed with an estrogen or antiestrogen, may explain the differential regulation of gene expression and the differential biological response to antiestrogens compared to the native estrogens.

Differential extraction of estradiol- and tamoxifen-receptor complexes from hypothalamic cell nuclei

These experiments compared nuclear estradiol- and tamoxifen-receptor complexes from brain by measuring the ability of a series of ionic agents and DNA intercalators to release estrogen receptors from hypothalamic cell nuclei following in vivo administration of the agonist or antagonist. Over the concentration ranges tested, intercalators released 50-80% of the total estrogen receptors from hypothalamic cell nuclei in a concentration-independent fashion whereas extraction of nuclear receptors by the ionic agents was concentration dependent. In general, brain cell nuclear estrogen receptors were extracted more easily by both ionic agents and intercalators following injections of tamoxifen than of estradiol. It was also found that the polyanion heparin releases a small population of nuclear estrogen receptors that are not extracted by 0.5 M KCl, suggesting the existence of KCl-resistant chromatin acceptor sites in hypothalamic nuclei. These data support the hypothesis that estrogen agonists and antagonists promote different conformational changes when they bind neural estrogen receptors, resulting in the formation of ligand-receptor complexes with different biological activity.

Binding of glucocorticoid receptors to mammary chromatin acceptor sites

We have recently characterized the interaction of mouse mammary estrogen receptors (ER) with mammary chromatin acceptor sites and demonstrated that ER from estrogen resistant lactating mammary glands do not bind to chromatin. In this study we have characterized the chromatin binding of the glucocorticoid receptor from mouse mammary glands isolated from nulliparous and lactating mice in order to better understand the relationship between receptor binding to chromatin and steroidogenic sensitivity of the tissue. Mammary chromatin was linked covalently to cellulose and deproteinized sequentially by 0-8 M Gdn-HCl. Binding to intact chromatin as well as to chromatin deproteinized by Gdn-HCl was determined using partially purified [3H]dexamethasone labelled glucocorticoid-receptor complexes (GR) obtained by fractionation on DEAE-cellulose columns. The binding of [3H]GR from mammary glands of nulliparous mice to chromatin fractions from the same tissue revealed maximal binding activity (acceptor sites) on chromatin previously extracted with 5-6 M Gdn-HCl. Binding of [3H]GR was of high affinity (Kd = 0.2 nM) and saturable. A simultaneous comparison of the chromatin binding patterns for [3H]ER and [3H]GR isolated from mammary glands of nulliparous mice revealed that the chromatin subfractions obtained with 4-6 M Gdn-HCl extraction contained acceptor sites for both [3H]ER and [3H]GR; however, while the [3H]ER bound to a 4.5 M and a 5.5 M site, the [3]GR bound a 5 M and a 6 M site. Competition experiments supported the steroid receptor specificity of the chromatin acceptor sites. Thus, the 4-6 M chromatin fractions contain distinct acceptor sites for the glucocorticoid receptor and for the estrogen receptor. In addition our studies reveal that the binding patterns of [3H]GR isolated from mammary glands of nulliparous and lactating mice to their homologous chromatin is essentially similar. Thus, in contrast to estrogen receptors, glucocorticoid receptors from lactating mammary glands are able to effectively bind to chromatin acceptor sites which supports our previous suggestion that the estrogenic insensitivity of lactating mouse mammary glands may at least be in part due to the impeded interaction of ER with chromatin acceptor sites.

Characterization of the chromatin acceptor sites for the avian oviduct progesterone receptor using monoclonal antibodies

Monoclonal antibodies (MAb) against the chromatin acceptor sites for the avian oviduct progesterone receptor were prepared with highly purified hen oviduct acceptor proteins reconstituted to hen DNA. Addition of the MAbs to a cell-free assay blocked progesterone receptor from chick oviduct (PRov) binding to native-like acceptor sites on nucleoacidic protein (NAP) representing a partially deproteinized chromatin, which has been shown to be enriched in these binding sites. However, the antibodies do not block PRov binding to pure DNA, nor do they affect the receptor itself. Estrogen receptor binding to NAP was not inhibited, supporting a receptor specificity of the PRov acceptor sites as reported previously from direct competition studies. These data support earlier studies showing that (1) the reconstituted PRov acceptor sites resemble the native sites, (2) the acceptor sites are receptor specific, and (3) the PRov binding sites of NAP are different from those of pure DNA. While some animal-species specificity in the PRov binding inhibition was observed, no tissue specificity was seen. Direct binding of the antibodies to native acceptor sites was demonstrated in an enzyme-linked immunosorbent assay (ELISA) system. The antibodies showed little recognition of free acceptor protein or DNA alone, indicating specificity for the protein-DNA complex. A partial evolutionary conservation of the nuclear acceptor sites for PRov was shown by the fact that about 50% of the inhibition seen with hen NAP was obtained with NAPs from several other species, and this partial cross-reactivity of the MAbs with the same NAPs from other animal species was also seen in the ELISA.

Nuclear acceptor sites for sex steroid hormone receptors in chromatin

The steroid receptor interactions in vitro with specific acceptor sites composed of acceptor protein-DNA complexes fulfill many of the criteria of a physiologically significant binding system. Chromatin acceptor sites for many steroid receptors (especially for the progesterone and estrogen receptor) are specific since they are saturable and competitive with unlabelled receptors, have high affinity for the receptor, distinguish between functional and nonfunctional receptors and demonstrate target tissue specificity. Pure DNA as acceptor sites does not display many of these properties. Therefore, it is clear that certain chromatin proteins provide the necessary specificity for the acceptor sites for the steroid receptors. For the progesterone receptor in the chick oviduct, these nuclear sites appear to contain specific chromosomal proteins as well as specific DNA sequences. The substitution of other chromosomal proteins or the genomic DNAs from evolutionarily distant organisms results in a loss of the specific nuclear binding. The nuclear acceptor sites appear to be resistant to the DNase activity which is not characteristic of transcriptionally active domains of the genome. Further studies using the ovalbumin gene sequences from genomic clones also indicate that none of the sequences within this domain and the 3-k flanking regions appear to contain the specific acceptor sequences. These observations have led to development of a model suggesting that the steroid receptors bind to acceptor sites distant from the structural genes the steroids ultimately regulate. Neighboring these acceptor sites are regulatory genes which code for regulatory substances which in turn (as secondary messengers) regulate at great distances the expression of the structural gene. This model might better fit the sex steroids which require 1-2 h to measurably alter gene transcription, as opposed to the glucocorticoids which more rapidly alter gene expression.

Monoclonal antibodies against putative nuclear acceptor sites of the avian oviduct progesterone receptor

Evidence from this and other laboratories has suggested that the nuclear binding sites (acceptor sites) for steroid receptors on chromatin involves chromatin protein-DNA complexes. A saturable high affinity receptor-dependent nuclear binding to these sites by isolated steroid receptor complexes has been reported. Addition of nonradiolabelled progesterone receptor from the chicken oviduct (PRov) successfully competes for the [3H]PRov binding to these acceptor sites in isolated chromatin or in nucleoacidic protein (NAP), a partially deproteinized chromatin enriched in these binding sites. This competition does not occur with pure DNA. This laboratory has isolated and enriched the chromatin proteins (acceptor proteins) involved in the nuclear acceptor sites for the avian oviduct PRov. Monoclonal antibodies against the nuclear acceptor sites for the PRov have been prepared using highly purified hen oviduct acceptor proteins reconstituted to hen DNA. Addition of the MAbs to a cell-free assay blocks PR binding to native oviduct chromatin as well as to NAP. However, the antibodies do not block PR binding to pure DNA nor do they affect the receptor itself. A partial animal species specifically was observed with the Ab inhibition of the PR binding, whereas no tissue specificity was seen. Direct binding of the antibodies to native acceptor sites was demonstrated using an ELISA system. The antibodies showed little recognition of free acceptor protein or DNA alone, indicating specificity for the protein-DNA complex. The partial evolutionary conservation of the nuclear acceptor sites for PR, as shown by the inhibition of PRov binding, was further supported by the partial crossreactivity of the MAbs with the NAPs from the same animal species using the ELISA. These data support earlier studies using PR binding assays showing that: 1) the reconstituted PR acceptor sites resemble the native sites; 2) the sites on whole chromatin and on NAP are similar; 3) the PR binding sites of chromatin and NAP are different from those of pure DNA; and 4) the nuclear acceptor sites for PR are different from those of the estrogen receptor. These results support a receptor specificity of the PR acceptor sites as reported previously using direct receptor competition studies.

Nuclease resistance and the enrichment of native nuclear acceptor sites for the avian oviduct progesterone receptor

High-affinity nucleoprotein acceptor sites for the avian oviduct progesterone receptor (PR) have been enriched by a combination of nuclease digestion and centrifugation. These enriched binding elements exhibited markedly enhanced PR binding on a per mass DNA basis compared to chromatin (20- to 25-fold) or dehistonized chromatin (4- to 5-fold). Electrophoretic analysis of the nuclease-resistant DNA showed that there is a set of DNA fragments of 100-150 base pairs that are protected from digestion. Excessive digestion resulted in smaller DNA fragments and a loss of PR binding activity. The PR binding was saturable using a crude receptor preparation and displayed a competition with the same receptor preparation that was labeled with nonradioactive progesterone. The enhanced binding was also demonstrable using highly purified receptor preparations that exhibit two classes of binding sites both of which are of high affinity and saturable as assessed by Scatchard analyses. These two high-affinity classes of binding sites are shown to be competed by unlabeled purified PR. The nuclease resistance of these nucleoprotein acceptor sites from chromatin is a property similar to the nuclear matrix binding sites suggesting a relationship between these two classes of nuclear acceptor sites.

Proteins that mask the nuclear binding sites of the avian oviduct progesterone receptor

The binding of a steroid receptor to specific nuclear sites (i.e., nuclear acceptor sites) represents the immediate event preceding the steroid regulation of gene transcription. How the same steroid receptor regulates different genes in different tissues is unknown. Since a major fraction of the nuclear acceptor sites for a variety of steroid receptors has been reported to be masked in the chromatins of a variety of tissues, the differential expression of the nuclear acceptor sites may explain this regulation of different genes. In the avian oviduct, the removal of a subfraction of chromosomal non-histone proteins, termed CP-2, results in the unmasking of the nuclear acceptor sites for the progesterone receptor (PR). Further, the extent of masking of these nuclear acceptor sites for PR has been reported to vary during cytodifferentiation of the avian oviduct. This paper describes a method for the reconstitution of the masking of PR nuclear acceptor sites in the avian oviduct chromatin using a partially purified chromosomal protein fraction (CP-2b). The reannealling of the CP-2b fraction to unmasked avian oviduct chromatin (termed nucleoacidic protein or NAP) results in the "remasking" of about the same number of nuclear acceptor sites for PR as found in intact chromatin. Because some of the PR acceptor sites on the NAP cannot be remasked, these sites either must be protected from masking or not be recognized by the masking proteins. The masking activity apparently involves only protein(s) because the unmasking of acceptor sites can be achieved with protease but not ribonuclease activities and because the dissociated masking activity is destroyed only by proteases. The masking appears to be reversible because the reconstituted masked sites can again be unmasked. Preliminary purification and characterization of the masking activity in fraction CP-2b by molecular sieve chromatography indicate a heterogeneity of size with the activity eluting in a molecular weight range of from 60 000 to greater than 150 000. Whether the masking proteins prevent the binding of the progesterone receptor by directly binding the acceptor sites or by binding neighboring domains to condense the chromatin is unknown. It is speculated that the masking of acceptor sites may be responsible in part for determining the tissue-specific gene expression induced by steroids and/or may play a role in the unresponsiveness of certain human tumors containing steroid receptors.

Estrogen and antiestrogen binding to rat uterine and pituitary estrogen receptor: evidence for at least two physicochemical forms of the estrogen receptor

Our laboratory has previously reported that calf uterine cytosol prepared in buffer containing 10 mM molybdate and chromatographed on DEAE-Sephadex contains two forms of the unactivated estrogen receptor, Peak I and Peak II; however, cytosol receptor bound to the high-affinity antiestrogen, H1285 (4-(N,N-diethylaminoethoxy)-4'-methoxy-alpha- (p-hydroxyphenyl)-alpha'-ethylstilbene), eluted only as Peak I. We have extended these studies to the rat uterus and pituitary in order to determine the organ and species specificity of this phenomenon. Cytosol prepared in Tris-molybdate buffer from immature and adult rat uteri or pituitaries was labelled with 10 nM [3H]estradiol or [3H]H1285 and chromatographed on QAE-Sephadex. Uterine estrogen receptors bound to either [3H]estradiol or [3H]H1285 eluted from QAE-Sephadex as a large Peak I (approximately 0.21 M KCl) and a smaller Peak II (approximately 0.25 M KCl). Analyses of these partially purified estrogen receptor fractions using high-salt sucrose density gradients showed that Peak I [3H]estradiol-receptor complexes sedimented predominantly as a lighter form (4.0S). In contrast, Peak I [3H]H1285-receptor complexes sedimented primarily as a heavier form (5.5S) often accompanied by a smaller lighter form (4.0S). Peak II [3H]estradiol- and [3H]H1285-receptor complexes sedimented as the heavier form (5.3-5.5S). These data suggest a monomer-dimer relationship between estrogen receptor forms with antiestrogen binding favoring the formation of the dimeric form. Further analysis of these Peak I receptor complexes by gel filtration chromatography yielded molecular forms of approx 70 KDaltons for [3H]estradiol-receptor complexes and 73 KDaltons and 165 KDaltons for [3H]H1285-receptor complexes, supporting the monomer-dimer concept. Data from experiments with the pituitary also suggest that H1285 causes the formation of the dimeric receptor form whereas estradiol interaction with the receptor results only in the monomeric form. These differences in estrogen receptor forms when bound by estrogen versus antiestrogen may be related to the different biological responses induced by these ligands.

Binding of [3H]triamcinolone acetonide-receptor complexes to chromatin from the B-cell leukemia line, BCL1

The binding characteristics of partially purified glucocorticoid receptor complexes from hormone sensitive, non-differentiating BCL1 cells to sequentially deproteinized BCL1 chromatin-cellulose was investigated. [3H]Triamcinolone acetonide (TA)-receptor complexes were purified (approx. 30-fold) from DEAE-cellulose columns by salt elution which allowed receptor activation only in the absence of molybdate. Addition of 10 mM molybdate completely blocked salt activation. The binding pattern of the activated [3H]TA-receptor complexes to chromatin-cellulose extracted with 0-8 M guanidine hydrochloride revealed three regions of increased binding activity (acceptor sites), at 2, 5 and 7 M guanidine hydrochloride. Acceptor site binding was markedly reduced for chromatin extracted with 3, 6 and 8 M guanidine hydrochloride. Non-activated receptor complexes demonstrated very low binding to deproteinized chromatin. It was also shown that chromatin binding required glucocorticoid receptors and that free ligand or ligand bound to other proteins did not bind significantly to chromatin. In addition, binding of [3H]TA-receptor complexes to partially deproteinized chromatin was competable by unlabeled TA-receptor complexes. Scatchard analysis demonstrated that chromatin from non-differentiating BCL1 cells possesses multiple, high-affinity binding sites which differ in their affinity for the glucocorticoid receptor. Partially deproteinized chromatin from lipopolysaccharide-stimulated BCL1 cells demonstrated a different pattern of receptor binding, i.e., receptor binding was significantly greater to chromatin previously extracted with 6-8 M guanidine hydrochloride. These results suggest that differentiation alters the state of chromatin and the interaction of non-histone protein/DNA acceptor sites with glucocorticoid receptors. These alterations may play a role in the acquisition of hormone resistance.

Interaction of the antiestrogen [3H]H1285 with the two forms of the molybdate-stabilized calf uterine estrogen receptor

The high affinity antiestrogen [3H]H1285 bound to the cytosol calf uterine estrogen receptor dissociated very slowly (t 1/2 approx 30 h at 20 degrees C) and did not demonstrate a change in dissociation rate in the presence of molybdate, which is characteristic of [3H]estradiol-receptor complexes. [3H]H1285-Receptor complexes sediment at approx 6S on 5-20% sucrose density gradients containing 0.3M KCl with or without 10 mM molybdate. This is in contrast to [3H]estradiol-receptor complexes which sedimented at approx 4.5S without molybdate and at approx 6S with molybdate. These results suggest a physicochemical difference in the estrogen receptor when occupied by antiestrogens versus estrogens. We recently reported that the cytoplasmic uterine estrogen receptor, when bound by estradiol and prepared in 10 mM molybdate, eluted from DEAE-Sephadex columns as Peak I (0.21 M KCl) & Peak II (0.25 M KCl). However, [3H]H1285 bound to the estrogen receptor eluted only as one peak at 0.21 M KCl, also suggesting that the initial interaction of antiestrogens with the estrogen receptor is different. We have extended these studies and report that H1285 can compete with [3H]estradiol for binding to both forms of the estrogen receptor and [3H]H1285 can bind to both forms if the unoccupied receptor is first separated by DEAE-Sephadex chromatography. However, if the receptor is first bound by unlabeled H1285, eluted from the column and post-labeled by exchange with [3H]estradiol, only one peak is measured. Thus, it appears that H1285 binding alters the properties of the receptor such that all receptor components seem to elute as one form. These partially purified [3H]H1285-receptor complexes obtained from DEAE-Sephadex columns sedimented as 5.5S in sucrose density gradients in contrast to the sedimentation values for the [3H]estradiol-receptor components eluting as Peak I (4.5S) and Peak II (6.3S). These differences in the physicochemical characteristics of the estrogen receptor when bound by estrogen versus antiestrogens may be related to some of the biological response differences induced by these ligands.