Physical measurements of the liver glucocorticoid receptor

Glucocorticoid receptor homodimers and glucocorticoid-mineralocorticoid receptor heterodimers form in the cytoplasm through alternative dimerization interfaces

Steroid hormone receptors act to regulate specific gene transcription primarily as steroid-specific dimers bound to palindromic DNA response elements. DNA-dependent dimerization contacts mediated between the receptor DNA binding domains stabilize DNA binding. Additionally, some steroid receptors dimerize prior to their arrival on DNA through interactions mediated through the receptor ligand binding domain. In this report, we describe the steroid-induced homomeric interaction of the rat glucocorticoid receptor (GR) in solution in vivo. Our results demonstrate that GR interacts in solution at least as a dimer, and we have delimited this interaction to a novel interface within the hinge region of GR that appears to be both necessary and sufficient for direct binding. Strikingly, we also demonstrate an interaction between GR and the mineralocorticoid receptor in solution in vivo that is dependent on the ligand binding domain of GR alone and is separable from homodimerization of the glucocorticoid receptor. These results indicate that functional interactions between the glucocorticoid and mineralocorticoid receptors in activating specific gene transcription are probably more complex than has been previously appreciated.

Discrimination between NL1- and NL2-mediated nuclear localization of the glucocorticoid receptor

Glucocorticoid receptor (GR) cycles between a free liganded form that is localized to the nucleus and a heat shock protein (hsp)-immunophilin-complexed, unliganded form that is usually localized to the cytoplasm but that can also be nuclear. In addition, rapid nucleocytoplasmic exchange or shuttling of the receptor underlies its localization. Nuclear import of liganded GR is mediated through a well-characterized sequence, NL1, adjacent to the receptor DNA binding domain and a second, uncharacterized motif, NL2, that overlaps with the ligand binding domain. In this study we report that rapid nuclear import (half-life [t1/2] of 4 to 6 min) of agonist- and antagonist-treated GR and the localization of unliganded, hsp-associated GRs to the nucleus in G0 are mediated through NL1 and correlate with the binding of GR to pendulin/importin alpha. By contrast, NL2-mediated nuclear transfer of GR occurred more slowly (t1/2 = 45 min to 1 h), was agonist specific, and appeared to be independent of binding to importin alpha. Together, these results suggest that NL2 mediates the nuclear import of GR through an alternative nuclear import pathway. Nuclear export of GR was inhibited by leptomycin B, suggesting that the transfer of GR to the cytoplasm is mediated through the CRM1-dependent pathway. Inhibition of GR nuclear export by leptomycin B enhanced the nuclear localization of both unliganded, wild-type GR and hormone-treated NL1(-) GR. These results highlight that the subcellular localization of both liganded and unliganded GRs is determined, at least in part, by a flexible equilibrium between the rates of nuclear import and export.

Nucleocytoplasmic trafficking of steroid-free glucocorticoid receptor

Glucocorticoid receptor (GR) recycles between an inactive form complexed with heat shock proteins (hsps) and localized to the cytoplasm and a free liganded form that regulates specific gene transcription in the nucleus. We report here that, contrary to previous assumptions, association of GR into hsp-containing complexes is not sufficient to prevent the shuttling or trafficking of the GR across the nuclear membrane. Following the withdrawal of treatment with cortisol or the hormone antagonist RU486, GRs recycled rapidly into hsp-associated, hormone-responsive complexes. However, cortisol-withdrawn receptors redistributed to the cytoplasm very slowly (t(1)/(2) = 8-9 h) and RU486-withdrawn receptors not at all. Persistent localization of these GRs to the nucleus was not due to a gross defect in export, since in both instances the complexed nuclear GRs transferred efficiently between heterokaryon nuclei. Moreover, the addition of a nuclear retention signal to the N terminus of GR induced the transfer of naive receptor to the nucleus in the absence of steroid. These results suggest that the localization of GR to the cytoplasm is determined by fine control of the rates of transfer of GR across the nuclear membrane and/or by active retention that occurs independently from the association of GR with hsps.

A subclass of glutathione S-transferases as intracellular high-capacity and high-affinity steroid-binding proteins

The distribution of glucocorticoids incubated with rat liver cytosol preparations or administered in vivo to adrenalectomized rats was analysed by chromatographic procedures. Corticosterone or dexamethasone was co-eluted with Yb-type GSH S-transferases in anion-exchange and gel-permeation chromatography systems, and these glucocorticoids also were bound to Yb forms in analyses by immunoadsorbent and lysyl-GSH affinity matrices. Pretreatment of cytosol with lysyl-GSH to extract GSH S-transferases or incubation with excess bilirubin, which is expected to compete with steroids for binding to the protein, yielded preparations that were devoid of this major steroid-binding component. In mixtures of the multiple rat GSH S-transferases, corticosterone preferentially interacted with Yb forms rather than Ya and Yc subgroups. All of the multiple Yb forms resolved by chromatofocusing procedures retained the steroid-binding capacity. It is suggested that these abundant proteins can account for a considerable share of intracellular glucocorticoid binding and represent a high-affinity non-saturable binding component with potential to function in steroid-hormone metabolism and action.

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.

Comparison of the physical characteristics of the molybdate-stabilized glucocorticoid receptor from rat, pig and human tissues

The molybdate-stabilized glucocorticoid receptor in cytosol preparations from rat, pig and human thymus, rat liver and CEM-C7 human leukaemic lymphoblasts was characterized as its complex with [1,2,4-3H]triamcinolone acetonide, by gel filtration on Sephacryl S-300 and by ultracentrifugation on sucrose density gradients. At low ionic strength, the complex from all sources was detected as a species with a sedimentation coefficient of 9.1-9.6S and a Stokes radius of 5.70-5.80 nm (form I); this form is believed to represent the non-transformed, undegraded complex. Exposure to 0.4 M KCl in the presence of molybdate resulted, in all cases, in the generation of a smaller form (form II) with a sedimentation coefficient of 4.5-4.8S and a Stokes radius of 4.70-4.77 nm. Form II has been shown to possess properties characteristic of the transformed (activated) state. It is concluded that the glucocorticoid receptor shows pronounced interspecies and intertissue similarities, and that pig thymus may represent a convenient source of tissue for glucocorticoid receptor purification.

Characterization of the molybdate-stabilized glucocorticoid receptor from rat thymus

The untransformed glucocorticoid receptor of rat thymus cytosol was characterized in the form of its complex with [1,2,4-3H]triamcinolone acetonide by ion-exchange chromatography and by gel filtration and sucrose-density-gradient ultracentrifugation at different ionic strengths. Molybdate (10 mM) was present throughout all experimental procedures and prevented receptor inactivation and degradation as well as transformation. At low ionic strength the molybdate-stabilized steroid-receptor complex was detected as a single highly asymmetric entity with a Stokes radius of 5.85 nm, a sedimentation coefficient of 9.6 S and an apparent molecular weight of 236 000. This form was converted into a smaller, even more asymmetric, form in increasing proportion as the ionic strength was increased. In the presence of 0.4 M-KCl, the smaller form had a Stokes radius of 4.95 nm, a sedimentation coefficient of 4.6 S and an apparent molecular weight of 95 500. It is concluded that the glucocorticoid-receptor complex exists at low ionic strengths as a homodimer or as a heterodimer in which only one subunit possesses a steroid-binding site, and that the process of dissociation into subunits brought about by increasing the ionic strength is a process distinct from, but possibly preceding, the transformation phenomenon responsible for conferring DNA-binding properties on the complex.

In Vitro biosynthesis of adipocyte proteins having the characteristics of glucocorticoid receptors

A quantitative method for the measurement of putative glucocorticoid receptor biosynthesis in rat adipocytes is described. The method utilizes the incorporation of radioactive amino acids into newly synthesized putative receptor proteins and their subsequent separation from other labeled proteins by affinity chromatography. Dexamethasone and deoxycorticosterone-Sepharose are used as affinity adsorbants. Specific binding of radioactive putative receptors to these gels is time- and protein concentration-dependent, and is abolished by exposure of cells to cycloheximide, pretreatment of adipocyte cytosol preparations with unlabeled steroids or incubation of cytosols at 37C for 4 h. Specifically bound radioactivity, which represents about 10% of the radioactivity initially associated with affinity adsorbants can be quantitatively eluted under rigidly defined conditions including high ionic strength. Specifically eluted material, which comprises up to 50% of total eluted radioactivity sediments at 3.8 S in sucrose gradients containing 1 M KCl, and electrophoretically migrates on 0.1% SDS gels in single band with a molecular weight of about 50 000. The sedimentation coefficient is comparable to that of the native adipocyte cytosol receptor not subject to affinity chromatography (3.7 S). Under low ionic-strength conditions most of the native receptor sediments at 8 S. The molecular weight of 50 000 is in the range of those reported for glucocorticoid receptors of liver (45 000---66 000 for monomers). The properties of the protein or proteins measured in the present system are therefore consistent with the current state of knowledge regarding glucocorticoid receptors in adipocytes.

Metabolism and activation of chemical carcinogens

A great deal of effort has gone into research on the mechanisms of action of chemical carcinogens. The action of the host on the carcinogen represents one approach--to determine what metabolic products may be involved. Study of the action of the carcinogen on the host requires a thorough comparison of changes wrought in cellular constituents by the carcinogen. Meshing of these two approaches increases the potential to discover the detailed mechanisms involved. Despite the increase in knowledge, many crucial molecular events and their consequences in the process of carcinogenesis still remain a mystery.

An antiserum to the rat liver glucocorticoid receptor

A rabbit immunized with a highly purified preparation of rat liver [3H]triamcinolone-receptor complex developed antibodies to the receptor. Although precipitating reactions were not detected, complexes formed between IgG and the receptor could be detected by Staphylococcus aureus protein A-Sepharose and gel permeation chromatography. IgG was purified and covalently immobilized on Sepharose CL-4B; this affinity matrix adsorbed the ligand-free receptor and both activated and nonactivated forms of the [3H]triamcinolone-receptor complex. Rat liver cytosol proteins adsorbed by control and immune immunoglobulin-Sepharoses were eluted with 0.1 M acetic acid and analyzed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. A protein with molecular weight 78,000 was the major species eluted from immune immunglobulin-Sepharose, and it was not present in eluates from control columns. Rat transcortin, glucocorticoid binder IB, and an estrogen-binding protein from rat liver were not adsorbed by immune IgG-Sepharose. Mouse and hamster liver glucocorticoid receptors showed only limited adsorption. Thus, the antiserum does not crossreact with other major glucocorticoid-binding proteins and demonstrates species specificity.

Three-step purification of glucocorticoid receptors from rat liver

The 90 000-Mr glucocorticoid receptor was purified to homogeneity according to sodium dodecylsulfate/polyacrylamide gel electrophoresis. An affinity column containing either dexamethasone-17 beta-carboxylic acid or dexamethasone-21-methanesulfonate bound to the matrix with the help of a disulfide bond is used in this study. Using this affinity matrix, in a single step, 8700-fold purification of glucocorticoid receptor from rat liver cytosol could be achieved. Following the method of activation and DNA-cellulose chromatography the 90 000-Mr receptor subunit was purified to homogeneity.