Effects of RNA and ribonuclease on the binding of estrogen and glucocorticoid receptors from MCF-7 cells to DNA-cellulose

Science Signaling Podcast: 2 February 2010

A noncoding RNA competes with DNA for binding to the glucocorticoid receptor.

Noncoding RNA gas5 is a growth arrest- and starvation-associated repressor of the glucocorticoid receptor

The availability of nutrients influences cellular growth and survival by affecting gene transcription. Glucocorticoids also influence gene transcription and have diverse activities on cell growth, energy expenditure, and survival. We found that the growth arrest-specific 5 (Gas5) noncoding RNA, which is abundant in cells whose growth has been arrested because of lack of nutrients or growth factors, sensitized cells to apoptosis by suppressing glucocorticoid-mediated induction of several responsive genes, including the one encoding cellular inhibitor of apoptosis 2. Gas5 bound to the DNA-binding domain of the glucocorticoid receptor (GR) by acting as a decoy glucocorticoid response element (GRE), thus competing with DNA GREs for binding to the GR. We conclude that Gas5 is a "riborepressor" of the GR, influencing cell survival and metabolic activities during starvation by modulating the transcriptional activity of the GR.

Orphan receptor DAX-1 is a shuttling RNA binding protein associated with polyribosomes via mRNA

The DAX-1 (NR0B1) gene encodes an unusual member of the nuclear hormone receptor superfamily which acts as a transcriptional repressor. Mutations in the human DAX-1 gene cause X-linked adrenal hypoplasia congenita (AHC) associated with hypogonadotropic hypogonadism (HHG). We have studied the intracellular localization of the DAX-1 protein in human adrenal cortex and mouse Leydig tumor cells and found it to be both nuclear and cytoplasmic. A significant proportion of DAX-1 is associated with polyribosomes and is found complexed with polyadenylated RNA. DAX-1 directly binds to RNA, two domains within the protein being responsible for cooperative binding activity and specificity. Mutations in DAX-1 found in AHC-HHG patients significantly impair RNA binding. These findings reveal that DAX-1 plays multiple regulatory roles at the transcriptional and posttranscriptional levels.

Estradiol-induced down-regulation of estrogen receptor. Effect of various modulators of protein synthesis and expression

Incubation of MCF-7 cells with estradiol (E2) down-regulates estrogen receptor (ER) resulting in a progressive reduction of the capacity of cells to concentrate selectively [3H]E2. Scatchard plot analysis failed to detect any transformation of residual receptors into peptides of lower binding affinity. [3H]Estrone gave an identical ER disappearance pattern with an ER half-life comprised between 2 and 3 h. A similar value was established by incubating the cells with [3H]tamoxifenaziridine ([3H]TAZ) for 1 h before the addition of excessive unlabeled E2 which induced ER-down regulation and impeded any further labeling of the residual receptors. Submission of the [3H]TAZ labeled cell extracts to SDS-PAGE revealed no progressive emergence of low molecular weight cleavage products of the receptor (< 67 kDa). Two inhibitors of protein kinases, H-7 at 40 microM and H-89 at 20 microM, failed to block the E2-induced ER down-regulation. On the contrary, the protein phosphatases 1 and 2A inhibitor, okadaic acid, was effective with concentrations higher than 0.1 microM indicating that a dephosphorylation mechanism was involved in this phenomenon. Cycloheximide (CHX) also significantly reduced the receptor decrease at concentrations higher than 1 microM. G-C specific intercalating agents [actinomycin D (AMD) and chromomycin A3 at 1 microM] also prevented ER disappearance; ethidium bromide (EB) and quinacrine were ineffective. AMD and CHX operated immediately after their addition to the medium indicating an inhibitory action on the synthesis of an RNA and/or a peptide with high turnover rate involved in ER decline. Moreover, AMD produced its suppressive effects under conditions impeding any labeling of newly synthetized receptors (i.e. [3H]TAZ with an excess of unlabeled E2) rejecting the possibility of an increasing ER production which may partially hamper its disappearance. Finally, E2-induced ER mRNA down-regulation was similarly abolished by AMD while EB and CHX were devoid of effect.

The effects of various serine protease inhibitors on estrogen receptor steroid binding

Two serine protease inhibitors, phenylmethanesulfonyl fluoride (PMSF) and diisopropylfluorophosphate (DFP), were utilized to investigate the possible involvement of serine hydroxyl groups on 17 beta-estradiol binding to the rat estrogen receptor (ER). Single point saturation analysis and Scatchard analysis demonstrated that both 5 mM PMSF and 5 mM DFP were able to inhibit steroid binding to the ER after incubation at 37 degrees C, but neither were able to inhibit steroid binding of the nonactivated ER (0-4 degrees C). The reducing agent dithiothreitol (DTT) was used to differentiate between the interaction of PMSF with serine groups or with sulfhydryl groups of the receptor. When incubated in the presence of 5 mM PMSF, various concentrations of DTT up to 25 mM were not able to overcome the inhibition of this agent, indicating that there was no interaction of PMSF with sulfhydryl groups. Thus, these findings indicate that serine hydroxyl groups are involved in steroid binding of the rat ER.

Immunoelectron microscopic localization of estrogen receptor on pre-mRNA containing constituents of rat uterine cell nuclei

The localization and quantitative changes of estradiol receptor (ER) were studied by means of immunogold-electron microscope methods using a polyclonal antibody directed against an amino acid sequence representing the DNA binding site of ER, a monoclonal antibody against hnRNP core protein, and anti-DNA antibody. The uteri of normal rats in estrus and those of ovariectomized females were used. Ovariectomized rats were studied 21 days after surgery at different times after the injection of normal saline or estradiol-17 beta. The density of labeling was measured in interchromatin space, compact chromatin, nucleolus, cytoplasm, and background of epithelial cells, muscle cells, and fibroblasts. In the three types of cells ER was found mainly on extranucleolar ribonucleoprotein (RNP) fibrils. In epithelial and muscle cells the nucleolus was labeled but compact chromatin was not labeled. In epithelial cells there was a low but significant labeling of the cytoplasm. Fibroblasts exhibited a low labeling of the compact chromatin. Ovariectomy did not change these distributions. The estradiol injection increased labeling in all compartments of epithelial and muscle cells but decreased the labeling of compact chromatin of fibroblasts. These results show: (a) that ER is mainly nuclear but it is also present in the cytoplasm, (b) that ER binds to the nuclear particles containing newly synthesized RNA, and (c) that the binding to RNPs does not block the DNA binding domain of the ER.

Adrenocorticoid action in the spinal cord: some unique molecular properties of glucocorticoid receptors

1. Glucocorticoid hormones affect several functions of the spinal cord, such as synaptic transmission, biogenic amine content, lipid metabolism, and the activity of some enzymes (ornithine decarboxylase, glycerolphosphate dehydrogenase), indicating that this tissue is a target of adrenal hormones. 2. Corticosterone, the main glucocorticoid of the rat, is detected at all regional levels of the spinal cord, and cold stress increases this steroid, predominantly in the cervical regions. 3. Intracellular glucocorticoid receptors have been found in the spinal cord, with higher concentrations in the cervical and lumbar enlargements. Prima facie, these receptors presented biochemical, stereospecifical, and physicochemical properties similar to those of receptors found in other regions of the nervous system. The prevalent form in the spinal cord is the type II receptor, although type I is also present in small amounts. 4. The type II glucocorticoid receptor of the spinal cord shows an affinity lower (Kd 3.5 nM) than that of the hippocampal type II site (Kd 0.7 nM) when incubated with [3H]dexamethasone. This condition may impair the nuclear translocation of the spinal cord receptor. 5. Another peculiar property of spinal cord type II site is a greater affinity for DNA-cellulose binding than the hippocampal receptor during heat-induced transformation. Also, the spinal cord receptor shows resistance to the action of RNAse A, an enzyme which increases DNA-cellulose binding of the hippocampal receptor, indicating that both receptors may be structurally different. 6. Therefore, it is possible that a different subclass of type II, or "classical glucocorticoid receptor," is present in the spinal cord. This possibility makes the cord a useful system for studying diversity of glucocorticoid receptors of the nervous system, especially the relationship between receptor structure and function.

Particulate untransformed glucocorticoid-receptor complexes from HeLa cells crosslinked in vivo

When control HeLa cells were incubated at 2 degrees C in the presence of tritiated dexamethasone, most glucocorticoid-receptor complexes were found in cytosolic extracts as untransformed forms. Chemical crosslinking of intact HeLa cells resulted in the immobilization of 50% of the total cellular glucocorticoid-receptor complexes in the nuclear fraction. Under these conditions the redistribution of total protein, RNA and lactate dehydrogenase activity between cytosol and nuclei was negligible, indicating that glucocorticoid binding in the nuclear fraction was not due to a methodological artifact. High levels of glucocorticoid receptor were also found in the nuclear fraction of crosslinked cells which were not exposed to glucocorticoids. Nuclear receptor complexes could be released in soluble forms by DNase I and sonication. Evaluations of DNA binding and ionic properties of glucocorticoid receptors prepared from control and crosslinked cells maintained at 2 degrees C revealed that most of the hormone-receptor complex in cytosols and nuclear extracts behaved as untransformed forms. As opposed to glucocorticoid receptors prepared from control cells, heat treatment of extracts obtained from crosslinked cells did not result in increased DNA binding and changes in ionic properties of receptor complexes. I conclude that untransformed glucocorticoid receptors are present in both cytosol and nuclei of intact cells.

Glucocorticoid-receptor complexes are associated with small RNA in vitro

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

Androgen receptors: structures, mutations, antibodies and cellular dynamics

An overview of recent studies from this and other laboratories on the structures and intracellular dynamics of androgen receptors is presented. Human and rat androgen receptors are unique in that, aside from their DNA and androgen binding domains, they have amino terminal regions rich in oligo- and poly(amino acids) motifs as in some regulatory and homeotic genes. Point mutations that cause sequence changes or deletion of regions of androgen receptors appear to be responsible for some cases of androgen-insensitivity. Monoclonal antibodies produced against specific regions of the androgen receptor bind to androgen receptors but not other major steroid receptors. Androgen receptors in the human and rat prostate, and monkey seminal vesicle were localized to the nucleus of target cells in these tissues with these antibodies; androgen receptors also were found in the cytoplasm of some target cells. Actinomycin D and 3'-deoxyadenosine, inhibitors of transcription, RNA processing and nucleo-cytoplasmic transport of RNA, interfere with the intracellular dynamics of androgen receptors, suggesting as we have proposed previously that androgen receptors may function not only at the site of transcription but also are involved in posttranscriptional regulation of mRNA stability and utilization.

RNA-induced transformation of the estrogen receptor detected by a monoclonal antibody which recognizes the activated receptor

The effect of RNA and polyribonucleotides on the estrogen receptor from fetal guinea pig uterus was studied through the analysis of the sedimentation properties of this receptor and its interaction with the monoclonal antibody D547. Different exogenous RNAs (calf thymus RNA, yeast RNA and rabbit liver transfer RNA) were able to induce a transformation of the 9S native receptor to 4.5-7S sedimenting forms in low salt sucrose density gradients, as activating factors such as temperature and time do. This transformation was prevented by 20mM sodium molybdate. Moreover, the RNA treated receptor was partially recognized by the monoclonal antibody D547. This antibody, as was demonstrated previously, selectively reacts with the activated form of this receptor. When different homo-polyribonucleotides were tested, the effect depended on their composition. In contrast, DNA did not affect either the sedimentation properties of the receptor or its reaction with the antibody. These observations suggest that RNA induces a dissociation of the 9S receptor and that at least one of the resulting forms is the activated receptor. However, RNA and polyribonucleotides inhibited the receptor binding to DNA-cellulose apparently by competing with DNA. The data suggest a role of RNA in estrogen receptor activation.

Genes coding for RNA polymerase beta subunit in bacteria. Structure/function analysis

The nucleotide sequence of the rpoB gene of Salmonella typhimurium has been determined in this work. It was compared with known sequences of the gene from other sources and the conservative regions were detected. This allowed some interesting conclusions to be made about the distribution of the functional domains in bacterial RNA polymerase and about the three-dimensional structure of its beta subunit.

Tight binding of glucocorticoid-receptor complexes to histone-agarose

"Activated" glucocorticoid-receptor complexes purified about 3,000-fold from rat liver were found to bind to histone-agarose. Because of their tight binding, they could not be eluted from the column by high salt solution (3 M KCl) or low salt plus polyol buffer (50% ethylene glycol), but their binding could be disrupted by pyridoxal 5'-phosphate; more than 70% recovery of the "activated" receptor complexes was achieved with buffer containing 20 mM pyridoxal 5'-phosphate. This interaction of "activated" glucocorticoid-receptor complexes of rat liver with histone-agarose suggests a role of histones in the mechanism of action of steroid hormone.

Ribonucleic acid association with androgen receptor from rat submandibular gland

The transformed androgen receptor from rat submandibular gland converts to a faster sedimenting form (6-8S) on a glycerol gradient centrifugation after withdrawal of a transformation-inducing reagent (KCl or ATP). In this report, the association of cytosolic RNA with the transformed androgen receptor was investigated as a possible mechanism of molecular conversion of the androgen receptor. When the transformed and converted androgen receptors were treated with RNase A, these receptors sedimented at 4.5S in a low-salt glycerol gradient. Addition of RNA from rat submandibular gland to the RNase-Sepharose-treated transformed receptor caused a shift of receptor peak from 4.5S to 5.8S. RNA from rat submandibular gland, yeast RNA and E. coli rRNA inhibited DNA-cellulose binding of a RNase-treated transformed receptor in the absence of molybdate. These observations suggest that conversion from the transformed 4S androgen receptor to a 6-8S form resulted from the association of RNA(s) with the transformed receptor.

ATP-induced activation of purified rat hepatic glucocorticoid receptors

We have utilized unactivated rat hepatic glucocorticoid receptor complexes purified to near homogeneity by a three-step scheme which includes affinity chromatography, gel filtration and anion exchange chromatography, to demonstrate for the first time that ATP can interact directly with the receptor protein in stimulating activation. This stimulation is reflected by an increase in DNA-cellulose binding as well as by a shift in the elution profile of the purified receptor complexes from DEAE-cellulose. A concentration of 10 mM Na2MoO4 is able to block both of these effects. ATP stimulates activation in a dose-dependent manner (maximally at 10 mM), and elicits maximal activation within 30 min at 15 degrees C. There appears to be no nucleotide specificity since GTP, CTP and UTP, as well as ADP and GDP also stimulate activation. All of these observations closely parallel data obtained from similar activation experiments performed with crude rat hepatic receptors. ATP does not appear to stimulate activation of receptors (crude or purified) by initiating a phosphorylation reaction since hydrolysis-resistant analogues of ATP are also effective. Pyrophosphate (PPi) is as effective as ATP in promoting receptor activation, since it elicits similar increases in DNA-cellulose binding, shifts in elution patterns from DEAE-cellulose, and dose-response relationships. None of the compounds tested stimulate activation indirectly by pH or ionic strength effects. Despite the fact that high ATP concentrations (3-4-fold higher than those present in vivo) are necessary to stimulate maximal activation, a physiological role of ATP in directly regulating in vivo activation of glucocorticoid receptors cannot be ruled out.

Immunological difference between ribonuclease and temperature, time and salt-induced forms of the estrogen receptor detected by a monoclonal antibody

The effect of RNAase A on the activation of the estrogen receptor from fetal guinea pig uterus was studied by DNA-cellulose binding assay and immunorecognition of the estradiol-receptor complex by the monoclonal antibody D547 raised against the human estrogen receptor. After RNAase treatment at 4 degrees C or 25 degrees C the binding of the receptor to DNA-cellulose doubled. This stimulation was partially prevented by sodium molybdate. RNAase treatment did not modify the interaction of the receptor with the monoclonal antibody D547; this antibody, as was demonstrated previously, selectively recognizes the activated form of the receptor when activation has been induced by temperature, time or high salt concentrations. In addition, RNAase had little or no effect on the transformation of the 8-9 S receptor to more slowly sedimenting forms under low salt concentrations. These observations suggest that even if RNAase induces receptor activation, which can be inferred from the increase in its binding to DNA-cellulose, the conformational modifications of the receptor molecule involved in this process are apparently different from those induced by factors such as temperature, time or high-salt concentrations.

Transformation in vitro and covalent modification with biotin of steroid-affinity-purified rat-liver glucocorticoid-hormone-receptor complex

The molybdate-stabilized rat liver glucocorticoid receptor complex was purified 9000-fold with a 46% yield by steroid-affinity chromatography and DEAE-Sephacel ion-exchange chromatography. The purified glucocorticoid receptor was identified as a 90-92-kDa protein by SDS/polyacrylamide gel electrophoresis. Raising the temperature to 25 degrees C in the absence of molybdate resulted in increased binding of the receptor complex to DNA-cellulose or nuclei, similar to the effect on the cytosolic complex. The purified complex has a sedimentation coefficient of 9-10 S before and after heat treatment in the absence of molybdate. The appearance of smaller 3-4-S species was unrelated to the extent of DNA-cellulose binding of the complex. The process termed 'transformation', i.e. increasing the affinity for DNA, is not concomitant with subunit dissociation or loss of RNA. Highly purified glucocorticoid receptor could be covalently modified with biotin to retain its steroid-binding activity but with a 50% decrease in nuclear binding capacity. The biotin-modified complex reacts with streptavidin in solution without losing its steroid.

Sodium molybdate converts the RNA-associated transformed, oligomeric form of the glucocorticoid receptor into the transformed, monomeric form

The glucocorticoid receptor from rat liver cytosol prepared in 2 ml buffer/g tissue sedimented at approximately 10 S in low salt density gradient centrifugation without molybdate. When the receptor was heated at 25 degrees C, both approximately 10 S and approximately 7 S forms were seen in low salt gradient. The approximately 10 S form was not capable of binding to DNA-cellulose and was stabilized by sodium molybdate, namely it corresponded to untransformed receptor. The approximately 7 S form was capable of binding to DNA-cellulose and regarded as transformed receptor. On the other hand, partially-purified transformed receptor labeled with [3H]dexamethasone-21-mesylate sedimented at approximately 5 S, which migrated as a approximately 94 kDa species in SDS-polyacrylamide gel electrophoresis. The reconstitution analysis of this partially-purified approximately 5 S receptor and liver cytosol, showed the shift to approximately 7 S form. RNase A or T1 converted approximately 7 S transformed form into approximately 5 S but it did not affect approximately 10 S untransformed form. 5-20 mM sodium molybdate also shifted approximately 7 S to approximately 5 S. These results indicate that the approximately 7 S transformed form of the glucocorticoid receptor observed in low salt conditions might be an oligomer, probably including both approximately 5 S steroid-binding component and RNA/ribonucleoprotein, and that molybdate dissociates these interactions in a specific manner.

Transformation of glucocorticoid-receptor complex oligomers to DNA-binding forms in the absence of monomerization

The contention that transformation of steroid-receptor complexes is represented by dissociation of receptor oligomers was tested by comparing sedimentation and DNA binding properties of glucocorticoid-receptor complexes from HeLa cell cytosol under several conditions. Transformation of glucocorticoid-receptor complexes could be induced by heat, and/or salt treatment of cytosolic extracts, but not by dilution. Heat-induced transformation of receptor complexes was also confirmed by DEAE-cellulose chromatography. Analysis of cytosolic extracts showed that sedimentation and DNA binding properties of glucocorticoid-receptor complexes did not correlate. Both oligomeric and monomeric receptor complexes, in fact, were found to be either transformed, or untransformed, depending on the treatments cytosolic extracts underwent, before being subjected to analysis. We then concluded that release of glucocorticoid receptor monomers cannot account for their transformation to a DNA-binding form in vitro, and suggested that exposure of positive charges on the surface of receptors in the course of transformation occurs in some region of the glucocorticoid receptor which is not involved in interactions between the proteinaceous components of oligomers.

A comparison of the glucocorticoid receptor system in the spinal cord and hippocampus

Studied under in vivo conditions, uptake of [3H]corticosterone (CORT) by purified cell nuclei of the hippocampus was much higher than in the spinal cord, although the latter may contain in cytosol up to 50% of glucocorticoid receptors found in cytosol of hippocampus. Experiments were undertaken to explain these differences. First, the in vivo affinity of receptors for exogenous CORT was comparable in both tissues. Second, an inhibitor of translocation, although present, was not preferentially concentrated in the spinal cord as compared to the hippocampus. However, the sensitivity towards RNAase A, an enzyme that increased binding to DNA-cellulose (taken as a measure of increased affinity for nuclear components), was preserved in the hippocampus but absent in the cord. We discuss the possibility that refractoriness to RNAase A may play a role in the reduced nuclear uptake of [3H]CORT shown by the spinal cord in vivo, but also consider possible that heterogeneity of receptor types binding CORT in the spinal cord and hippocampus may account for the differences observed in both tissues.

Molybdate inhibits glucocorticoid-receptor complex binding to RNA

The binding of dexamethasone-receptor complexes to RNA was investigated by an assay system under cell-free conditions. By this gradient centrifugation assay, we found that, under low salt concentrations, dexamethasone-receptor complexes can bind to 18S RNA from HeLa cells. Molybdate, tungstate and methavanadate were able to inhibit dexamethasone-receptor complex binding to 18S RNA, whereas this was not the case when chloride, fluoride, or sulfate ions were present in our binding assays. Molybdate was also found to disrupt dexamethasone-receptor-18S RNA complexes once they were already formed. We concluded that interaction between dexamethasone-receptor complexes and RNA under cell-free conditions is affected by ions present in the medium.

The glucocorticoid receptor protein binds to transfer RNA

The glucocorticoid receptor from mouse AtT-20 cells exists in three forms: the untransformed receptor (9.1S; Mr of 319,000), a large oligomeric molecule that does not bind to DNA; the transformed receptor (4S; Mr of 96,000), which is formed by dissociation of untransformed receptor after steroid binding and which binds to DNA to modulate gene expression; and an intermediate size receptor (6S; Mr of 132,000), which also binds to DNA and contains a bound small RNA molecule. This RNA species has now been purified and identified as transfer RNA (tRNA). The three tRNA's for the basic amino acids accounted for about 78% of the total amino acid-accepting activity [arginine (52%), lysine (17%), and histidine (9%)], while the remaining 22% was represented by six other tRNA species. This tRNA-binding activity of the glucocorticoid receptor may reflect post-transcriptional mechanisms of regulating gene expression, such as alterations in the translational efficiency of or the modulation of the stability of hormone-induced proteins.

Development of structural organization of protein-synthesizing machinery from prokaryotes to eukaryotes

Though the mechanisms of protein biosynthesis are similar in the cells of prokaryotes and eukaryotes, the eukaryotic translational machinery in the cell is arranged more intricately. One of the most striking characteristic features of the eukaryotic translational machinery is that the eukaryotic proteins involved in the translational process, such as initiation factors, elongation factors and aminoacyl-tRNA synthetases, in contrast to their prokaryotic analogs, possess a non-specific affinity for RNA. Due to the RNA-binding ability, these eukaryotic proteins can be compartmentalized on polyribosomes. In addition to the proteins of the translational apparatus, several other eukaryotic RNA-binding proteins can be also compartmentalized on polyribosomes; these proteins include glycolytic enzymes, steroid hormone receptors and intermediate filament proteins. Thus, the eukaryotic polyribosome is an element of the cytoplasmic labile structure on which various proteins can be compartmentalized and, consequently, different biochemical pathways can be integrated.

Functions and dysfunctions of receptors for adrenal corticoids in the central nervous system

Glucocorticoids (GC) have several known effects on the function of the nervous system, and GC receptors have been identified in regions responding to hormonal action. In the spinal cord, GC receptors have been characterized in vitro, which share several biochemical properties in common with receptors in better studied areas such as the hippocampus. Moreover, enzymes which are induced by GC in the hippocampus, such as glycerolphosphate dehydrogenase and ornithine decarboxylase, are also under specific GC control in the spinal cord. Yet GC receptors in the latter tissue divert from those in hippocampus during some in vivo as well as in vitro studies. In vivo, uptake of [3H]corticosterone by purified cell nuclei was 5-8-fold higher in the hippocampus as compared to the cord. In vitro, a higher percentage of GC receptors previously transformed by heating, showed affinity towards DNA-cellulose in the spinal cord than in the hippocampus. The enzyme RNAse A effectively increased receptor binding to DNA-cellulose in hippocampus, whereas the cord was insensitive to its action. These results suggest that there is a "receptor dysfunction" in the spinal cord, the significance of which is poorly understood in terms of the accepted model of steroid hormone action.

Differential effects of molybdate on the hydrodynamic and DNA-binding properties of the non-activated and activated forms of the androgen receptor in calf uterus

Calf uterine cytosol contains an androgen receptor with a relative molecular mass of approx. 90,000. In this study we have analysed the structure and aggregation properties of the androgen receptor, using sucrose density gradient centrifugation on a vertical rotor (VTi65). In the presence of 10 mM NaCl the androgen receptor in whole cytosol sedimented at 8 S irrespective of the presence of molybdate. In 400 mM NaCl the receptor dissociated to a 4.3 S entity. In whole cytosol molybdate promoted a partial shift of the 4.3 S receptor into the aggregated 8 S state. The time of exposure of the receptor to molybdate and NaCl determined the proportion of receptor sedimentating at 8 S and 4.3 S. The DNA-binding form of the uterine androgen receptor when analysed under the conditions of the DNA-cellulose binding assay, sedimented at 6.5 S. Increasing concentrations of molybdate shifted its sedimentation coefficient gradually from 6.5 S to 4.5 S and in parallel reduced the DNA-binding capacity. Molybdate added to a partially purified, DNA-binding form of the androgen receptor did not promote receptor aggregation to faster sedimentating forms. This suggests that such preparations are devoid of an androgen receptor-aggregation factor. Indirect evidence for such a factor was obtained from reconstitution experiments with whole cytosol. Our results indicate that the DNA-binding form of the androgen receptor interacts with a cytosol factor to form the 8 S receptor complex. Molybdate has diverse effects: in the presence of the cytosol factor it stabilizes the 8S complex; in its absence molybdate prevents in a concentration-dependent way DNA-binding as well as reaggregation of the monomeric 4.3 S form.

Evidence for an association of a ribonucleic acid with the purified, unactivated glucocorticoid receptor

The unactivated glucocorticoid-receptor complex (GRC) was purified from rat hepatic cytosol (approximately 4000-fold by specific activity) by a procedure developed in our laboratory. Following elution of unactivated GRC from DEAE-cellulose with a 0.05-0.5 M potassium phosphate gradient, a second gradient of 0.5-1.0 M potassium chloride was started. This gradient eluted material at 0.6 M potassium chloride that incorporated [32P] in vivo and stained with ethidium bromide. A predominant ethidium bromide stained band of 100-110 nucleotides was observed. The presence of this material was dependent on the presence of highly purified GRC since presaturation of cytosol with 50 microM unlabeled triamcinolone acetonide (TA) precluded the appearance of this material. Experiments with partially purified GRC from CEM-C7 cells incubated in vivo with [14C]uridine indicated that the material eluted at 0.6 M potassium chloride incorporated [14C]uridine. Collectively, these data suggest that a RNA is associated with the purified, unactivated form of the GRC.

Studies with chymotrypsin and RNAase showing a heterooligomeric structure of the glucocorticoid receptor complex from rat liver which is stabilized by a low molecular weight factor

The glucocorticoid receptor from rat liver displays a differential sensitivity toward digestion by chymotrypsin and RNAase A that is dependent on its activation state. Unactivated (9-10 S) receptor is not digested by these enzymes, while activated 7-8 S receptor is. Chymotrypsin treatment yields an approx. 3 S form, while RNAase treatment yields a 4.9 S form that is distinct from the high-salt 4 S form. To firmly establish that the results are due to specific hydrolytic activities of the particular enzymes, we show that the chymotrypsin effect is inhibited by diisopropylfluorophosphate and not RNAasin, while the reverse is true for RNAase A. We further show that the differential sensitivity toward chymotrypsin is due to the association of a proteinase-resistant, heat-stable low molecular weight factor with the unactivated glucocorticoid receptor. When this factor is removed by warming, dialysis or molecular sieving of the receptor complex, the complex becomes sensitive to chymotrypsin. We also show that moderate chymotrypsin treatment yields a 6-7 S form of the receptor which is composed of, at least, RNA and the 4 S receptor. On the basis of these results, we propose that the 9-10 S receptor is composed of a low molecular weight stabilizing factor whose presence apparently alters the conformation of the complex such that the RNA and the RNA-binding site of the receptor are protected, a chymotrypsin-sensitive factor, RNA and the 4 S receptor itself.

Effects of bovine pancreatic ribonuclease A, S protein, and S peptide on activation of purified rat hepatic glucocorticoid-receptor complexes

Bovine pancreatic ribonuclease (RNase) A and S protein (enzymatically inactive proteolytic fragment of RNase A which contains RNA binding site) stimulate the activation, as evidenced by increasing DNA-cellulose binding, of highly purified rat hepatic glucocorticoid-receptor complexes. These effects are dose dependent with maximal stimulation of DNA-cellulose binding being detected at approximately 500 micrograms (50 units of RNase A/mL). RNase A and S protein do not enhance DNA-cellulose binding via their ability to interact directly with DNA or to increase nonspecific binding of receptors to cellulose. Neither S peptide (enzymatically inactive proteolytic fragment which lacks RNA binding site) nor cytochrome c, a nonspecific basic DNA binding protein, mimics these effects. RNase A and S protein do not stimulate the conformational change which is associated with activation and is reflected in a shift in the elution profile of receptor complexes from DEAE-cellulose. In contrast, these two proteins interact with previously heat-activated receptor complexes to further enhance their DNA-cellulose binding capacity and thus mimic the effects of an endogenous heat-stable cytoplasmic protein(s) which also function(s) during step 2 of in vitro activation [Schmidt, T. J., Miller-Diener, A., Webb, M. L., & Litwack, G. (1985) J. Biol. Chem. 260, 16255-16262]. Preadsorption of RNase A and S protein to an RNase affinity resin containing an inhibitory RNA analogue, or trypsin digestion of the RNA binding site within S protein, eliminates the subsequent ability of these two proteins to stimulate DNA-cellulose binding of the purified receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Ribonuclease-induced transformation of progesterone receptor from rabbit uterus

The effect of RNase on the transformation of progesterone receptor from rabbit uterus was studied by density-gradient centrifugation and DNA-cellulose binding assay. The 7S form of the receptor in crude cytosol was RNase sensitive, and converted to the 4S form after RNase treatment. This reaction was prevented by an RNase inhibitor and reversed by the addition of ribosomal RNA. RNase treatment also caused a two-fold increase in the DNA binding of cytosolic receptor, and reduced the time required for heat-induced transformation. However, sucrose-gradient-purified progesterone receptor (7S) did not undergo transformation by warming unless exogenous RNase was added, thereby suggesting that a cytosolic factor, which might be endogenous RNase, is necessary for the heat-induced transformation of progesterone receptor. Furthermore, degradation of the receptors which occurred after prolonged warming at 25 degrees C in the presence of RNase could be prevented by the addition of DNA-cellulose to the reaction mixture. These results indicate that RNA is associated with the 7S form of progesterone receptor, and that its hydrolysis by RNase might be involved in the transformation of this receptor.

"Defective" receptors in steroid-resistant conditions may be proteolytic artifacts

The specific question addressed in this report is whether the resistance to steroid treatment of certain tissues or tumors which appear to contain a normal quantity of steroid-binding sites may be due to structural defects in the receptors. This question may be seen as part of the more general question of whether there are intrinsic variations in the structures of receptors for a given class of steroids in different healthy tissues, in healthy vs. malignant tissues or in different types of tumors. Our experimental approach to these questions has involved the stabilization and precise physicochemical characterization of the receptors. To date, we have studied the estrogen and progestin receptors from human breast cancers and benign and malignant gynecologic specimens and the glucocorticoid receptors from several healthy and malignant rodent tissues and from normal human lymphocytes and various types of leukemic cells. Chromatographic and ultracentrifugal analyses in buffers of low ionic strength, containing 20 mM Na2MoO4 as the stabilizer, have revealed each of these receptors to be a large, oligomeric complex, characterized by remarkably similar values of the Stokes radius, sedimentation coefficient, molecular weight and axial ratio. In the absence of adequate stabilization, however, we found that the receptors for three classes of steroids in extracts of some healthy, steroid-responsive tissues, such as rat kidney and human uterine endometrium, are invariably degraded by endogenous proteinases. The extent of such cleavage is increased considerably by freezing the tissues prior to homogenization. Studies designed to distinguish the intact receptors from the products of proteolysis have included the characterization of receptors in cytosols prepared from mixtures of rat liver and kidney. The results strongly support the interpretation that the smaller size of the receptors detected in kidney cytosol reflects their cleavage by the more active proteinases in that tissue. The sizes and shapes of the receptors in cytosols from various tissues were found to be correlated with the activities of specific endopeptidases, assayed fluorometrically with peptidyl derivatives of 7-amino-4-methylcoumarin (AMC). These studies suggested that the receptors are vulnerable to cleavage by "lysine-specific" endopeptidases, detected with t-butyloxycarbonyl-L-valyl-L-leucyl-L-lysyl-AMC. An enzyme of this specificity was partially purified from rat kidney cytosol and tested for its ability to digest the glucocorticoid receptors from rat liver cytosol.(ABSTRACT TRUNCATED AT 400 WORDS)

Chymotrypsin treatment of glucocorticoid receptor attenuates RNA-dependent inhibition of DNA binding. Evidence for a distinct RNA-binding site

Certain types of RNA can prevent the association of the rat liver glucocorticoid receptor with DNA. This inhibition of receptor binding to DNA cannot be mitigated by increasing amounts of DNA, suggesting that the RNA is not merely acting as a competitive inhibitor. Treatment of partially purified receptor with low concentrations of chymotrypsin eliminates the inhibitory effects of some RNAs without negatively affecting the DNA-binding ability of the receptor. Potent inhibitors of the receptor-DNA association, such as poly(G) and poly(X), still inhibit DNA binding of the treated receptor, although to a lesser extent than the untreated controls. However, moderate inhibitors, such as tRNA and poly(U), no longer inhibit the receptor-DNA association at low concentrations. We take these findings to suggest that RNA inhibition of DNA binding is due to the interaction of the RNA at a distinct RNA-binding site. This site may serve as a regulator site for the receptor-DNA association.

Effect of ribonuclease on the physico-chemical properties of estrogen receptor

Estrogen receptors (ER) from rat and rabbit uterine cytosol were examined for their sensitivity to ribonuclease (RNase). After RNase treatment, a major part of rabbit uterine ER was converted from the 7S to 3-4S form, and its binding to DNA-cellulose was increased by 40%. Similar treatment on rat uterine ER showed a shift from 7S to 4.5S, and the DNA-cellulose binding was stimulated by 20%. Measurement of endogenous RNase levels showed that lower RNase concentration in rabbit uterine cytosol coincided with larger stimulation of DNA-cellulose binding by exogenous RNase. These results indicate that a major part of 7S ER is susceptible to RNase, and cleavage of bound RNA seems to uncover additional binding sites for DNA. In contrast to the general thinking that 4S to 5S transformation is essential for nuclear binding, we have observed that RNase-treated rat uterine ER did not undergo such a transformation by warming at 25 degrees C, while DNA-cellulose binding of the receptors increased. Thus, temperature activation could occur independent of 4S to 5S transformation.

Binding of activated progesterone receptor to microsomes

Specific binding of steroid hormones to microsomes has been reported for several tissues. In the hen oviduct, this receptor appears to be very similar to activated cytosolic receptor. The microsomal receptor is readily solubilized, and resembles the cytosolic receptor in all physico-chemical characteristics: sedimentation coefficient approximately 4 S, Stokes radius 5.5 nm, slow dissociation rate of the complex, adsorption to polyanions. It is precipitated by an antibody to the cytosolic receptor. Microsomes display saturable binding of cytosolic receptor, with a Bmax of approximately 300 fmol/mg protein. This binding is also observed using microsomes from non-target tissues, and is decreased by treatment with RNase. It seems likely that microsomal binding is due to the high affinity of activated cytosolic receptor for RNA.

RNase A effects on sedimentation and DNA binding properties of dexamethasone-receptor complexes from HeLa cell cytosol

Dexamethasone-receptor complexes from HeLa cell cytosol sediment at 7.4S in low salt sucrose gradients, and at 3.8S in high salt gradients. If cytosol is heated at 25 degrees C, receptor complexes sediment at 6.9S in low salt, and at 3.6S in high salt gradients. RNase A treatment at 25 degrees C, instead, results in receptor complexes which sediment in low salt gradients as two major forms at 6.5 and 4.8S. Receptor complexes from RNase A-treated cytosols sediment as their counterparts from untreated cytosols in high salt gradients. Although the shift in sedimentation properties of receptor complexes at 2 degrees C is induced by RNase A, and not by other low molecular weight basic proteins or RNase T1, the effect can be also obtained by inactive RNase A. The catalytically active enzyme, however, is required to observe 6.5 and 4.8S complexes after cytosol incubations at 25 degrees C. Placental ribonuclease inhibitor prevents the appearance of RNase A-induced receptor forms at 25 degrees C, but not at 2 degrees C. Moreover, this inhibitor can prevent the 7.4 to 6.9S shift in sedimentation coefficient of receptor complexes caused by cytosol heating. Dexamethasone-receptor complexes from HeLa cell cytosol show low levels of binding to DNA-cellulose, and heating at 25 degrees C is required to observe a six-fold increase in DNA binding levels. RNase A treatment of cytosols at 2 degrees C does not result in significant enhancement in receptor complex binding to DNA. If RNase A treatment is carried out at 25 degrees C, however, DNA binding levels of receptor complexes increased by 25% over the values observed with control heated cytosol. This effect cannot be observed if RNase T1 substitutes for RNase A. Placental ribonuclease inhibitor can prevent the temperature-dependent increase in DNA binding properties of dexamethasone-receptor complexes either in the presence or absence of exogenous RNase A. These findings indicate that exogenous RNases can perturb the structure of dexamethasone-receptor complexes without being involved in the transformation process.

Interaction of rat liver glucocorticoid receptor with heparin

When rat liver cytosol containing [3H]dexamethasone-glucocorticoid receptor complex is exposed to immobilized heparin (Sepharose-heparin; Seph-hep) the steroid receptor complex binds to the substituted Sepharose avidly [Kd = 3.5 (+/- 1.7) X 10(-10) M], and 80-90% of the receptor present is adsorbed to the solid phase after 40 min at 0 degree C. The binding is enhanced by Mn2+ (10 mM) and Mg2+, whereas Ca2+ and Sr2+ are ineffective. Sodium molybdate (10 mM) does not influence the reaction but enhances receptor stability. Moreover, binding of the receptor to Seph-hep is dependent on the ionic strength of the medium, because binding is totally reversed by 300 mM KCl. The bound [3H]dexamethasone-receptor complex can be recovered from Seph-hep with solutions (4 mg/mL) of heparin (95% release), dextran sulfate (88%), and chondroitin sulfate (63%); total calf liver RNA is less effective (9%), whereas dextran, D-glucosamine, N-acetyl-D-glucosamine, D-glucuronic acid, and sheared calf thymus DNA are totally ineffective (less than 3%). Both "native" and temperature "transformed" forms of the glucocorticoid receptor interact with immobilized heparin. These results strongly suggest that the receptor site that binds heparin is distinct from that binding DNA. An immediate application of this newly found ability of the glucocorticoid receptor to interact with heparin is the use of Seph-hep for affinity chromatography purification of the glucocorticoid receptor. A purification of 10-fold, with a recovery of 55-65%, can be achieved by using either 4 mg/mL heparin or 300 mM KCl to elute [3H]dexamethasone-receptor bound to the resin.

Structure and regulation of the glucocorticoid hormone receptor

The glucocorticoid receptor is an intracellular protein which possesses three distinct domains, one that binds agonist and antagonist steroids, one that binds DNA, and one that binds anti-receptor antibodies and is required for glucocorticoid modulation of gene expression. In intact cells, receptor number, affinity and activity can change in response to factors that bind to the receptor, or that act indirectly through ill-defined mechanisms which may include resumption or arrest of cell cycling and variations in intracellular calcium ion concentrations. Some of these factors appear to exert their effect by controlling critical receptor properties such as ATP-dependent phosphorylation, integrity of thiol groups, and exposure of key amino acid residues. Glucocorticoid agonists promote the 'transformation' of the receptor into the DNA-binding state, which is competent for modulating gene expression. Glucocorticoid antagonists are steroids that interact with the receptor but either fail to produce a stable complex or produce a stable but inefficient complex. Although substituent groups that confer agonist or antagonist activity to the steroid have been identified, the molecular determinants of this difference at the receptor level remain unknown. Most in vitro and in vivo data on receptor regulation can be accommodated by postulating the existence of an intracellular cycle that involves five states of the receptor. The active free receptor is phosphorylated, reduced, and presumably oligomeric (state A). Following binding of an agonist (state B), it can become transformed by dissociation into its subunits and dephosphorylation (state C). The transformed receptor then interacts with chromatin (state D). Dissociation of the steroid and oxidation of receptor thiol group(s) lead to the inactive receptor form (state E). Reduction and rephosphorylation of the receptor enable it to bind steroids again so that the cycle is closed.

An ATP-stimulated factor that enhances the nuclear binding of "activated" receptor-glucocorticoid complex

A macromolecular material that enhances the translocation, or binding, of already "activated" receptor-glucocorticoid complex to nuclei in the presence of 5 mM ATP was separated from the cytosol of rat liver by DEAE-cellulose column chromatography with about 0.025 M NaCl. The molecular weight of the material was about 93,000 +/- 4,900, as determined by agarose gel filtration. After incubation at 60 degrees C for 15 min, this material still had activity to increase the nuclear binding, but on boiling for 15 min it lost its activity.

Steroid receptor recycling and its possible role in the modulation of steroid hormone action

The fate of steroid-receptor complexes after their nuclear retention in target cells is not firmly established. Nuclear glucocorticoid- and androgen-receptor complexes could be recycled back to the cytosol in their responsive tissues, whereas this has not been clearly established for the case of progesterone and estrogen receptors. The models of steroid receptor recycling proposed so far involve release of chromatin-bound complexes into the cytosol, loss of steroid, and receptor inactivation. These receptors, however, can eventually be reactivated to a steroid binding form to reinitiate a cycle of steroid binding and further nuclear translocation. We propose that this model can represent a general aspect of steroid hormone action, provided that inactivation/reactivation processes occur in every steroid responsive system. A process involving a reversible receptor inactivation could play a major role in the control of steroid receptor recycling. It is proposed that a control on the extent of receptor available to steroid binding could result in a modulation of cellular responses to steroid hormones.

Separation and characterization of receptor-translocation inhibitors from AH 130 tumor cells

The objective of this investigation was to study the relationship between glucocorticoid resistance and macromolecular receptor-translocation inhibitors ( MTIs ). MTIs in various cytoplasmic preparations are known to inhibit the "activated" receptor-steroid complex association with isolated nuclei, chromatin, or DNA. It was found that the MTI in the cytosol of AH 130 tumor cells (glucocorticoid resistant cells) appeared to be about 5 times more inhibitory than crude MTI from rat liver. Another difference between these MTI preparations was that ATP decreased the inhibition by crude MTI from rat liver, but had little effect on that of MTI from the tumor cells. Both preparations gave three fractions of material with inhibitory activity on DEAE-cellulose chromatography. The first fraction (Peak I), eluted with about 0.1 M NaCl, was the largest fraction separated from the tumor cytosol, but a minor fraction of that from liver. In the presence of 5 mM ATP, Peak I from rat liver enhanced nuclear binding, but that from the tumor did not, suggesting that these fractions were qualitatively different. The other two fractions (Peak II and Peak III), eluted with about 0.2 M and 0.3 M NaCl, respectively, were comparable in the two preparations.

Binding of the rat liver 7-8 S dexamethasone receptor to deoxyribonucleic acid

The 7-8 S form of the [3H]dexamethasone (9 alpha-fluoro-11 beta,17,21-trihydroxy-16 alpha-methylpregna-1,4-diene-3, 20-dione) receptor from rat liver cytosol can be converted to the 3-4 S form by RNase treatment or high salt, suggesting a salt-sensitive association between the receptor protein and RNA. In DNA-cellulose column assays, the gradient-purified 3-4 S form bound DNA more efficiently than the 7-8 S form, though the 7-8 S form was also capable of binding to DNA-cellulose to a significant extent. Activated 7-8 S dexamethasone receptor could be released from its association with soluble DNA by treatment with DNase I. Sucrose gradient analysis showed that the released receptor sedimented as the 7-8 S form and was sensitive to RNase treatment, which induced a conversion to the 3-4 S form. Activated RNase-generated 3-4 S receptor again displayed a higher degree of binding to soluble DNA and was recovered in the 3-4 S form following DNase extraction. The fact that the 3-4 S form bound immobilized or soluble DNA more efficiently suggests that the associated RNA of the 7-8 S form interferes directly or indirectly with the receptor association with DNA. The observation that the receptor binds to DNA in its 7-8 S form suggests that the receptor complex is capable of binding RNA and DNA concurrently.

Increased binding to DNA-cellulose of the unactivated and activated glucocorticoid-receptor complex from mouse brain following sucrose density gradient ultracentrifugation

The binding to DNA-cellulose of both the unactivated and activated forms of the molybdate-stabilized glucocorticoid-receptor complex increases markedly after subjecting these preparations to sucrose density gradient ultracentrifugation. We speculate that this increase results from the removal of endogenous macromolecular factors which competitively inhibit glucocorticoid receptor binding to DNA and which may normally be involved in regulating the genomic responses of these steroids in brain.

Differential release of estradiol from immobilized estrogen receptors by p-sec-amylphenol: evidence for a functional microheterogeneity of estrogen receptors

3H-Estradiol-estrogen receptor complexes were adsorbed on a column of heparin-agarose and subjected to a gradient of increasing concentrations of p-sec-amylphenol. At least five peaks of released 3H-estradiol were observed--demonstrating the existence of subsets of heparin-immobilized receptors with different affinities for estradiol. This finding is presented as further evidence for a functional micro-heterogeneity among estrogen receptors. The origin of the observed differences in estrogen receptors and possible relevance of the findings to receptor-mediated responses are discussed.

Comparison of corticosteroid binder IB with the alpha-chymotrypsin- and RNase-treated hepatic glucocorticoid receptors

Rat liver and kidney cytosolic extracts contain the glucocorticoid receptor (binder II) and corticosteroid binder IB, both of which possess the steroid- and DNA-binding domains. Since it has been speculated that the smaller binder IB may be generated from binder II by proteolysis, the chymotrypsin-produced receptor fragment in rat liver cytosol has been compared with binder IB in terms of charge, size and DNA binding characteristics. The [3H]triamcinolone acetonide-receptor complex is converted to a smaller fragment by short term digestion (10 degrees C, 30 min) with 100 micrograms/ml alpha-chymotrypsin. Although the chymotrypsin fragment produced from previously heat-activated binder II and binder IB both exhibit DNA-binding capability, they differ in charge and size. Whereas the alpha-chymotrypsin-treated receptor has a Stokes radius of 30 A and elutes from DEAE-cellulose at 0.06 M potassium phosphate in a linear salt gradient, binder IB has a Stokes radius of 20 A and elutes in the buffer wash of the DEAE-cellulose column. Thus, while binder IB can be resolved from the heat-activated form of the [3H]TA-receptor on DEAE, the heat activated alpha-chymotrypsin product elutes from the anion exchange resin at the same ionic strength as intact activated binder II (i.e. at 0.05 M potassium phosphate), and the unactivated intact receptor elutes at about 0.20 M potassium phosphate. A more extended digestion with alpha-chymotrypsin (24 h, 0 degrees C) results in elimination of the DNA binding site without further reduction of the Stokes radius or change in the elution pattern from DEAE-cellulose. Furthermore, molybdate completely blocks formation of binder IB but does not inhibit the production of the receptor fragment by alpha-chymotrypsin. Treatment of the hepatic [3H]TA-receptor complex with RNase has no effect on the charge, size or DNA binding properties of the bound receptor. These results suggest that RNase does not activate the [3H]TA-receptor complex nor does it produce a IB-like component in the liver cytosol. The present results are consistent with the hypothesis that binder IB is formed in vitro by a process which may not involve proteolytic cleavage or RNase-induced modification of the glucocorticoid receptor (binder II).

Rapid analysis of estrogen receptor heterogeneity by chromatofocusing with high-performance liquid chromatography

Chromatofocusing principles have been utilized to develop a high-performance liquid chromatographic technique for the rapid and routine analysis of steroid receptor heterogeneity. Two anion-exchange columns (SynChropak AX-300 and AX-500) were compared for analytical and preparative chromatofocusing of estrogen receptor components. As many as ten different [125I]iodoestradiol-labeled binding proteins were identified in cytosols prepared from mammary gland and uterus. Estrogen receptors were well separated from other cytosolic proteins and recovery of activity routinely exceeded 90%. Parallel analyses of these cytosols to determine receptor size and shape indicated that HPLC chromatofocusing can be used effectively to study receptor isoforms with Stokes radii ranging from 30 A to greater than 70 A. In contrast to isoelectric focusing, this technique is compatible with the inclusion of a commonly used receptor-stabilizing agent, sodium molybdate. Inclusion of molybdate during chromatofocusing of molybdate-stabilized receptor allowed the identification of two acidic receptor species not previously reported.