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

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.

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.

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.

Structure and function of the glucocorticoid receptor

The glucocorticoid receptor protein is a transcriptional regulatory protein that interacts with specific enhancer sequences. A stoichiometric analysis of the interaction indicates an equimolar relationship between the receptor protein and the specific enhancer sequences. The activity of the receptor protein is itself regulated by the binding of glucocorticoids. The two functional domains (DNA-binding and steroid-binding) are adjacent and lie within the C-terminal half of the receptor protein. The N-terminal half of the protein appears to modulate the interaction with DNA but does not have any role in the binding of the steroid. The functional domains have also been defined at the sequence level.