RNA-dependent release of androgen and other steroidreceptor complexes from DNA

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.

Membrane estrogen receptors: genomic actions and post transcriptional regulation

The primary cellular location of the nuclear estrogen receptor II (nER II) is the plasma membrane. A number of reports that have appeared in the recent past indicate that plasma membrane localized estrogen receptor alpha (ERalpha) also exists. Whether the membrane localized ERalpha represents the receptor that binds to the estrogen responsive element (ERE) remains to be known. The mechanisms that underlie the internalization of nER II (non-activated estrogen receptor, deglycosylated) have been identified to a certain extent. The question remains: is the primary location of the ERalpha also the plasma membrane? If that is the case, it will be a challenging task to identify the molecular events that underlie the plasma membrane-to-nucleus movement of ERalpha. The internalization mechanisms for the two 66kDa plasma membrane ERs, following hormone binding, appear to be distinct and without any overlaps. Interestingly, while the major gene regulatory role for ERalpha appears to be at the level of transcription, the nER II has its major functional role in post transcriptional mechanisms. The endoplasmic reticulum associated anchor protein-55 (ap55) that was recently reported from the author's laboratory needs a closer look. It is a high affinity estrogen binding protein that anchors the estrogen receptor activation factor (E-RAF) in an estrogen-mediated event. It will be interesting to examine whether ap55 bears any structural similarity with either ERalpha or ERbeta.

Nuclear estrogen receptor II (nER-II) is involved in the estrogen-dependent ribonucleoprotein transport in the goat uterus I. Localization of nER-II in snRNP

Exposure of goat uterine nuclei to estradiol in vitro results in an immediate exit of ribonucleoproteins (RNP) from the nuclei to the medium. This RNP exit appears to be mediated by an estrogen receptor localized in small nuclear ribonucleoproteins containing U1 and U2 snRNA. Available evidence indicates that the estrogen receptor involved is not the ERalpha, but an alternative form, which is also a 66 kDa protein. This is the nuclear estrogen receptor II (nER-II) that has no DNA-binding capacity. The transport is estrogen-specific since non-estrogenic steroids do not stimulate the transport of the RNP where the receptor is localized.

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.

Mechanisms of progesterone receptor export from nuclei: role of nuclear localization signal, nuclear export signal, and ran guanosine triphosphate

Steroid hormone receptors are, in most cases, mainly nuclear proteins that undergo a continuous nucleocytoplasmic shuttling. The mechanism of the nuclear export of these proteins remains largely unknown. To approach this problem experimentally in vivo, we have prepared cell lines permanently coexpressing the wild-type nuclear progesterone receptor (PR) and a cytoplasmic receptor mutant deleted of its nuclear localization signal (NLS) [(deltaNLS)PR]. Each receptor species was deleted from the epitope recognized by a specific monoclonal antibody, thus allowing separated observation of the two receptor forms in the same cells. Administration of hormone provoked formation of heterodimers during nucleocytoplasmic shuttling and import of (deltaNLS)PR into the nucleus. Washing out of the hormone allowed us to follow the export of (deltaNLS)PR into the cytoplasm. Microinjection of BSA coupled to a NLS inhibited the export of (deltaNLS)PR. On the contrary, microinjection of BSA coupled to a nuclear export signal (NES) was without effect. Moreover, leptomycin B, which inhibits NES-mediated export, was also without effect. tsBN2 cells contain a thermosensitive RCC1 protein (Ran GTP exchange protein). At the nonpermissive temperature, the nuclear export of (deltaNLS)PR could be observed, whereas the export of NES-BSA was suppressed. Microinjection of GTPgammaS confirmed that the export of (deltaNLS)PR was not dependent on GTP hydrolysis. These experiments show that the nuclear export of PR is not NES mediated but probably involves the NLS. It does not involve Ran GTP, and it is not dependent on the hydrolysis of GTP. The nucleocytoplasmic shuttling of steroid hormone receptors thus appears to utilize mechanisms different from those previously described for some viral, regulatory, and heterogeneous ribonuclear proteins.

The Ernst Schering Poster Award. Intracellular traffic of steroid hormone receptors

The signal responsible for the nuclear localization of the progesterone receptor has been characterized. It is a complex signal. The study of the mechanism of this nuclear localization has revealed that the receptor continuously shuttles between nucleus and the cytoplasm. The receptor diffuses into the cytoplasm and is constantly and actively transported back into the nucleus. The same phenomenon exists for estradiol and glucocorticoid receptors. The mechanism of entry of proteins into the nucleus is well documented, whereas the mechanism of their outward movement to the cytoplasm is not understood. We have grafted different nuclear localization signals (NLSs) onto beta-galactosidase and have studied the traffic of this protein using heterokaryons and microinjection experiments. We have demonstrated that the same NLSs are involved in both the inward and the outward movement of proteins through the nuclear membrane. These results suggest that the nucleocytoplasmic shuttling may be a general phenomenon for nuclear proteins that could possibly undergo modifications in the cytoplasm and exert some biological activities there. These conclusions also imply that at least part of the cellular machinery involved in the nuclear import of proteins may function bidirectionally. Using these techniques, we have shown that the two major antiprogestins, RU486 and ZK98299, act at the same distal level of hormone action.

Polyamine-induced changes in the sedimentation profile and DNA binding of aryl hydrocarbon receptor

Aryl hydrocarbon (Ah) receptor mediates the toxic action of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). We studied the effects of polyamines--putrescine, spermidine and spermine--on the physical chemical properties of Ah receptor from A431 cells. Spermidine and spermine caused the precipitation of 9S oligomeric receptor with a gradual decrease in the receptor peak during density gradient sedimentation. RNase A treatment transformed the 9S Ah receptor to a 6S form and DNA binding increased by 2-fold. Following partial purification of transformed Ah receptor by preparative sucrose gradient centrifugation, it lost the ability to bind to DNA, but addition of spermidine increased DNA binding in a concentration-dependent manner. These data show that polyamines modulate the structure and DNA binding of Ah receptor and suggest that cellular polyamine levels might be important in the tissue specific toxicity of TCDD.

Cytoplasmic-nuclear trafficking of progesterone receptor. In vivo study of the mechanism of action of antiprogestins

The signal responsible for the nuclear localization of the progesterone receptor has been characterized. The study of the mechanism of this nuclear localization has revealed that the receptor continuously shuttles between the nucleus and the cytoplasm. The receptor diffuses into the cytoplasm and is constantly and actively transported back into the nucleus. Preliminary evidence suggests that the same mechanism exists for estradiol and glucocorticoid receptors. Experiments designed to study the traffic of steroid hormone receptors have been applied to the determination of the molecular mechanism of action of antisteroids. Using these techniques, we have shown that two major antiprogestins, RU486 and ZK98299, act at the same point in the cell as the hormone.

Cytoplasmic-nuclear trafficking of steroid hormone receptors

The nuclear localization of most steroid hormone receptors reflects a dynamic process: the receptor constantly diffuses out of the nucleus and is reimported by an active mechanism. The outward movement from the nucleus of the receptors and of other nuclear proteins is also mediated by the nuclear localization signals.

Purification and characterization of a macromolecular-translocation inhibitor III of activated glucocorticoid-receptor-complex binding to nuclei from rat liver

Macromolecular-translocation inhibitors (MTI) of binding of the activated glucocorticoid-receptor complex (GRC) to nuclei from rat liver are separated into at least three components (MTI-I-III) by DEAE-cellulose column chromatography [Okamoto, K., Isohashi, F., Horiuchi, M. & Sakamoto, Y. (1982) Biochem. Biophys. Res. Commun. 108, 1655-1660]. In this study, we have purified MTI-III from the livers of adrenalectomized rats to apparent homogeneity, as determined by SDS/PAGE. The purification procedure consisted of DEAE-cellulose chromatography, acid treatment and sequential chromatographies using gel filtration, S-Sepharose and Mono S columns. The purified protein had a molecular mass of approximately 69 kDa, as estimated by SDS/PAGE, and the molecular mass of the inhibitor was approximately 68 kDa, as estimated by gel filtration. Thus, MTI-III exerts its inhibitory activity as a monomer. The sedimentation coefficient of MTI-III was approximately 3.7 S. Purified MTI-III was fairly stable at 4 degrees C but at higher temperatures, especially above 25 degrees C, it was rapidly inactivated. Under low-salt conditions, MTI-III was associated with activated GRC (4.2 S) and the resulting complex was detected on sucrose density gradients as a larger species (6.8 S). Initial treatment of nuclei or DNA-cellulose with MTI-III did not alter their abilities to bind activated GRC. These results indicate that MTI-III acts through an interaction with GRC.

Post-transcriptional regulation of mouse renal cytochrome P450 2E1 by testosterone

Our previous studies demonstrated that the sex-related difference in renal metabolism of N-nitrosodimethylamine in C3H/HeJ mouse was due to the sexual dichotomy of cytochrome P450 2E1 (P450 2E1) and that renal P450 2E1 in female mouse was inducible by testosterone. The present study demonstrates that the sex-related difference in renal P450 2E1 and the testosterone-mediated regulation also occurred in other mouse strains studied. The time- and dose-responses in the testosterone-mediated regulation of P450 2E1 were characterized. 19-Nortestosterone, an analog of testosterone, was shown to have an effect on the regulation of mouse renal P450 2E1 similar to that of testosterone. Testicular feminized mice (Tfm, a mouse strain devoid of functional androgen receptors) had about only one-tenth the renal P450 2E1 mRNA level as the wild-type male mice and testosterone treatment of the Tfm mice had no effect on the level of renal P450 2E1 mRNA. The result suggests that androgen receptor plays an important role in the sex- and testosterone-related regulation of mouse renal P450 2E1. To study the mechanism of the sex-related and testosterone-mediated regulation of P450 2E1 mRNA in mouse kidney, the transcription rate of the P450 2E1 gene was measured by a nuclear run-on transcription assay. Although the kidneys of male and testosterone-treated female mice had much higher steady-state levels of P450 2E1 mRNA, their transcription rates of the P450 2E1 gene were not higher than the kidneys of untreated female mice. This result suggests that the sex- and testosterone-related regulation of mouse renal P450 2E1 is predominantly at the post-transcriptional level.

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.

The binding of 3H-labelled androgen-receptor complexes to hypothalamic chromatin of neonatal mice: effect of sex and androgenization

The binding of 3H-labelled androgen-receptor complexes, prepared by (NH4)2SO4 precipitation from the 105,000 g supernatant of hypothalamic cytosol, to hypothalamic chromatin of neonatal mice covalently coupled to cellulose was measured in vitro. Saturation binding was also determined after extraction of histones and the masking of acidic proteins with high molarities of guanidine hydrochloride. This investigation showed the presence of high-affinity, low-capacity acceptor sites for [3H]-testosterone-receptor complexes in male hypothalamic chromatin (Kd value = 0.39 x 10(-10) M and binding sites of 41 fmol per mg of DNA). Acceptor activity seems to be associated with the acidic protein fraction of chromatin. No specific acceptor sites of similar nature were found in chromatin taken from the hypothalami of female mice. On the basis of these results, it is suggested that the androgen-unresponsiveness of female mice is related to the absence of acceptors for the androgen-receptor in female mice hypothalami.

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.

Translational regulation of myelin protein synthesis by steroids

Various steroids, including glucocorticoids, were observed to exert a direct effect on the rates of translation of several myelin-protein specific transcripts in a cell-free, reticulocyte lysate system. Hydrocortisone caused a twofold stimulation in the translation of mRNAs of myelin basic protein and proteolipid protein. It inhibited the translation of 2',3'-cyclic nucleotide 3'-phosphodiesterase mRNA by 50%, and had no effect on the translation of a number of other mRNAs. The data suggest that steroid-mediated translational regulation may serve as a novel mechanism to modulate the expression of myelin protein genes at the translational level.

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.

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.

Suppression of steroid receptor-chromatin interaction by inhibitors present in ammonium sulfate-fractionated rat testicular androgen receptor preparations

Tritiated androgen receptor complex was partially purified by ammonium sulfate precipitation at 15 to 37% saturation from rat testicular or prostatic cytosols. A greater amount of prostatic tritiated androgen receptor complex bound to rat Sertoli cell chromatin than tritiated testicular androgen receptor complex. However, the testicular androgen receptor complex was more potent in competing for binding of either tritiated androgen receptor complex preparation to the Sertoli cell chromatin. Combining equal amounts of tritiated testicular and prostatic androgen receptor complexes resulted in lower binding than that observed with tritiated prostatic androgen receptor complex alone, while doubling the concentration of tritiated prostatic androgen receptor complex led to a 2-fold increase in binding. It is conceivable that the partially purified testicular androgen receptor complex preparations contain factor(s) that inhibit the binding interaction of tritiated androgen receptor complex with chromatin, while there is no evidence for the presence of such inhibitors in tritiated prostatic androgen receptor complex preparations. Fractionation of testicular cytosol on DEAE-cellulose yielded a flow-through fraction with inhibitory activity and a bound fraction containing tritiated androgen receptor complex that was eluted with 0.3 M KCl and also showed inhibitory activity. These factors also inhibited the binding of tritiated estradiol receptor complex to chromatin. They were non-dialyzable and their inhibitory effect was abolished after heating at 60 C for 30 minutes. It is concluded that ammonium sulfate-fractionated rat testicular androgen receptor complex preparations contain inhibitors of steroid receptor-chromatin interaction. The inhibitory factors can be fractionated into two distinct fractions by DEAE-cellulose column chromatography. They are nondialyzable and heat-labile. The precise chemical nature of the postulated inhibitor(s) remains to be determined.

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.

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.

Properties of an intermediate-sized androgen receptor: association with RNA

This study identifies an intermediate-sized androgen receptor and characterizes its relationship with the 9.1S and 4.4S receptor forms. Under low ionic conditions, at 2-4 degrees C, there exists a 9.1S (+/- 0.17) (n = 30) oligomeric form which does not bind to DNA. Under high ionic conditions, this form dissociates to a 4.4S (+/- 0.08) (n = 18) monomeric form. When the salt concentration is lowered, the 4.4S monomer converts to a species with an intermediate sedimentation coefficient of 7.7S (+/- 0.15) (n = 17) which binds to DNA. Unlike the 9.1S oligomer the 7.7S form is not maintained by sodium molybdate under high ionic conditions but rather dissociates to the 4.4S monomer. To determine whether these forms were associated with RNA, the 7.7S form was incubated with RNase A and analyzed by density gradient centrifugation. The 7.7S form was digested fully by RNase to the 4.4S monomer. The 7.7S form demonstrated a buoyant density of 1.2459 +/- 0.014 g/cm3 (n = 6) in metrizamide gradients, suggesting a ribonucleoprotein component. The sedimentation coefficient of the 9.1S form was unaffected by RNase. These data suggest that the intermediate 7.7S receptor form is composed of 4.4S monomer associated with a ribonucleoprotein molecule(s).

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)

Testicular cytosol factors inhibit the binding of steroid-receptor complexes to chromatin

Testicular and prostatic androgen-receptor complexes as well as uterine estradiol-receptor complexes, partially purified by ammonium sulfate precipitation (15-37%), were bound to germ cell chromatin. At equivalent concentrations, less testicular androgen-receptor complexes bound to chromatin than did the other two steroid-receptor complexes. Addition of a partially purified testicular androgen-receptor preparation with prostatic androgen-receptor or uterine estradiol-receptor preparation to the binding interaction mixture reduced the binding of either of the latter two steroid-receptor complexes to chromatin. These data suggest the presence of inhibitory factor(s) in the testicular receptor preparations. Testicular cytosols were fractionated by ammonium sulfate precipitation into fractions A (15-37% saturation), B (37-50%) and C (50-75%). All fractions inhibit binding of these steroid-receptor complexes to chromatin. Fractions A and B appear to be heat labile, while fraction C was more stable. Further fractionation of A and C fractions on DEAE cellulose yielded A1 and C1 (filtrates) as well as A2 and C2 (0.3 M NaCl eluents), respectively. Subfractions A1, A2, and C2 contained inhibitory factors for the binding of steroid-receptor complexes to chromatin while C1 showed no effect. These data demonstrated that testicular cytosol contains a variety of inhibitory factors which affect the binding of both androgen-receptor and estradiol-receptor complexes to chromatin.

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.

Steroid receptor activation: the glucocorticoid receptor as a model system

The glucocorticoid receptor has been used as a model for steroid receptor activation. Because of recent evidence for the essentially nuclear location of the unoccupied receptors of 1,25-dihydroxycholecalciferol and 17 beta-estradiol, the significance of the activation mechanism converting unactivated receptor complexes to DNA-binding forms is unclear for some receptors. Up to now the weight of evidence favors a cytoplasmic location of the unactivated glucocorticoid receptor. In this article we describe studies on the nature of the activation mechanism and of regulatory factors.

Stabilization of glucocorticoid receptor association with RNA by a low molecular weight factor from rat liver cytosol

A small (Mr less than 500) anionic, heat-stable molecule has been identified in rat liver cytosol which prevents the RNase-induced decrease in the glucocorticoid receptor sedimentation properties which we described previously. This factor, which can be removed by dialysis, molecular exclusion chromatography, or ultrafiltration, functions as a true stabilizer of the RNA-glucocorticoid receptor association, and not as a RNase inhibitor. Preliminary characterization shows that the factor is not a protein, nucleic acid, or nucleotide, is not absorbed by activated dextran-charcoal, and is unaffected by extraction with organic solvents. This factor prevents activation of the glucocorticoid receptor by dilution. The relationship of this stabilization factor to a low molecular weight activation inhibitor described by others is discussed.

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.

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.

An approach to the mechanism of the potent antiglucocorticoid: 17 beta-hydroxy-11 beta-4-dimethylaminophenyl-17 alpha-propynyl-estra-4,9-dien-3-one

The effects of a new synthetic steroid, 17 beta-hydroxy-11 beta-4-dimethyl-aminophenyl-17 alpha-propynyl-estra-4,9-dien-3-one, classified under reference R38486 in the Roussel-Uclaf nomenclature [1], were investigated in two established rat hepatoma cell lines in order to gain information on the mechanism of action. The induction of tyrosine aminotransferase (TAT) and alanine aminotransferase (AAT) was totally abolished when R38486 was added with dexamethasone either on a 1-1 basis or on a 10-fold excess depending on the differentiation state of the cell. Binding studies showed a high affinity for the glucocorticoid receptor; however our "whole cell" study with [3H] R38486 indicates that only a low amount of antagonist-receptor complexes was translocated into the nucleus. Nuclear fractionation experiments showed that R38486, like the other antagonists studied, was located in the chromatin fraction where it may exert some definite role. Our observations based on whole cell experiments using physiological doses of glucocorticoid analogs indicate that the binding of activated antiglucocorticoid-receptor complexes to nuclear acceptor sites represents a physiologically significant process. Moreover the differences in the nuclear binding of antagonist-receptor- as compared to agonist-receptor-complexes may set off the machinery of antagonistic action.

RNA-containing nuclear binding sites for glucocorticoid-receptor complexes

RNase A treatment of HeLa cell nuclei causes a time- and concentration-dependent release of dexamethasone-receptor complexes. If nuclei are incubated in the absence of enzyme, only 60% of RNase-releasable complexes can be detected. Sucrose density gradient analysis of nuclear extracts shows that receptor complexes released by RNase treatment sediment at 3.6 S, whereas complexes obtained from untreated nuclei sediment between 7 and 3.6 S. Our results show that a fraction of dexamethasone-receptor complexes retained by HeLa cell nuclei is located in binding sites involving RNA.

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.