Binding of glucocorticoid 21-oic acids and esters to molybdate-stabilized hepatic receptors

Steroid receptor interactions with heat shock protein and immunophilin chaperones

We have provided a historical perspective on a body of steroid receptor research dealing with the structure and physiological significance of the untransformed 9S receptor that has often confused both novice and expert investigators. The frequent controversies and equivocations of earlier studies were due to the fact that the native, hormone-free state of these receptors is a large multiprotein complex that resisted description for many years because of its unstable and dynamic nature. The untransformed 9S state of the steroid and dioxin receptors has provided a unique system for studying the function of the ubiquitous, abundant, and conserved heat shock protein, hsp90. The hormonal control of receptor association with hsp90 provided a method of manipulating the receptor heterocomplex in a manner that was physiologically meaningful. For several steroid receptors, binding to hsp90 was required for the receptor to be in a native hormone-binding state, and for all of the receptors, hormone binding promoted dissociation of the receptor from hsp90 and conversion of the receptor to the DNA-binding state. Although the complexes between tyrosine kinases and hsp90 were discovered earlier, the hormonal regulation or steroid receptor association with hsp90 permitted much more rapid and facile study of hsp90 function. The observations that hsp90 binds to the receptors through their HBDs and that these domains can be fused to structurally different proteins bringing their function under hormonal control provided a powerful linkage between the hormonal regulation of receptor binding to hsp90 and the initial step in steroid hormone action. Because the 9S receptor hsp90 heterocomplexes could be physically stabilized by molybdate, their protein composition could be readily studied, and it became clear that these complexes are multiprotein structures containing a number of unique proteins, such as FKBP51, FKBP52, CyP-40, and p23, that were discovered because of their presence in these structures. Further analysis showed that hsp90 itself exists in a variety of native multiprotein heterocomplexes independent of steroid receptors and other 'substrate' proteins. Cell-free systems can now be used to study the formation of receptor heterocomplexes. As we outlined in the scheme of Fig. 1, the multicomponent receptor-hsp90 heterocomplex assembly system is being reconstituted, and the importance of individual proteins, such as hsp70, p60, and p23, in the assembly process is becoming recognized. It should be noted that our understanding of the mechanism and purpose of steroid receptor heterocomplex assembly is still at an early stage. We can now speculate on the roles of receptor-associated proteins in receptor action, both as individuals and as a group, but their actual functions are still vague or unknown. We can make realistic models about the chaperoning and trafficking of steroid receptors, but we don't yet know how these processes occur, we don't know where chaperoning occurs in the cell (e.g. Is it limited to the cytoplasm? Is it a diffuse process or does chaperoning occur in association with structural elements?), and, with the exception of the requirement for hormone binding, we don't know the extent to which the hsp90-based chaperone system impacts on steroid hormone action. It is not yet clear how far the discovery of this hsp90 heterocomplex assembly system will be extended to the development of a general understanding of protein processing in the cell. Because this assembly system is apparently present in all eukaryotic cells, it probably performs an essential function for many proteins. The bacterial homolog of hsp90 is not an essential protein, but hsp90 is essential in eukaryotes, and recent studies indicate that the development of the cell nucleus from prokaryotic progenitors was accompanied by the duplication of genes for hsp90 and hsp70 (698). (ABSTRACT TRUNCATED)

In vivo plaque-forming cell suppression by methyl 20 beta-dihydroprednisolonate

Methyl 20 beta-dihydroprednisolonate, a new local antiinflammatory steroid, and its ester hydrolysis product, 20 beta-dihydroprednisolonic acid, were evaluated for potential immunosuppressive actions as determined by the splenic plaque-forming cell response assay. The parent compound, prednisolone, was used as a comparative agent. All three corticosteroid agents, when administered intraperitoneally or subcutaneously by separate injections given at the same time as antigen, demonstrated significant dose-related suppression of this immunologic parameter. In a croton oil-induced inflammation test (ear challenge in mice), methyl 20 beta-dihydroprednisolonate demonstrated significant antiinflammatory effects, but only in the ear to which it was applied. The comparative drug, prednisolone, significantly reduced inflammation in the ear to which it was applied and in the contralateral ear as well. The methyl ester derivative demonstrated effective antiinflammatory activity when applied topically in the oxazolone delayed-type hypersensitivity test (ear challenge in mice). The results seen in the plaque-forming cell response are in contrast to the generally noted lack of systemic glucocorticoid actions of the steroid acid ester derivatives reported in other studies.

Glucocorticoid effects on contact hypersensitivity and on the cutaneous response to ultraviolet light in the mouse

A single exposure to 254 nm ultraviolet irradiation (UV) can systemically suppress experimental sensitization to the simple allergen 2,4-dinitro, 1-chlorobenzene (DNCB) in the mouse. We show here that topical application at the site of irradiation of the 21-oic acid methyl ester derivative of the synthetic glucocorticoid triamcinolone acetonide (TAme) prevents UV suppression of sensitization. That is, mice painted with TAme at the site of UV exposure developed normal contact hypersensitivity (CH); mice exposed to UV only, like mice treated with the parent compound triamcinolone acetonide (TA), failed to be sensitized by DNCB applied to a distal site. TAme is inactivated rapidly by plasma esterases, so its effect is thought to be confined to the skin. Apparently, TAme blocked the cutaneous signal(s) for systemic suppression of CH. Histologically, irradiated skin exhibited mild inflammation and hyperproliferation, but these effects were greatly exaggerated and prolonged in the UV + TAme-treated skin, independent of sensitization at the distal site. The infiltrate consisted mostly of neutrophils and lacked the round cells characteristic of cell-mediated immunity. Apparently, normal immune suppression by UV prevented this vigorous reaction to irradiated skin. Applied together with DNCB. TAme blocked sensitization. It also prevented response to challenge by DNCB in previously sensitized animals. However, unlike the parent compound triamcinolone acetonide (TA), Budesonide or Beclomethasone diproprionate, each of which can penetrate the epidermis in active form, TAme had no effect on sensitization when applied at a distal site. Likewise, TAme did not affect plasma B (17-desoxycortisol) levels, whereas the other three compounds reduced plasma B tenfold, as expected of compounds causing adrenal-pituitary suppression. The results as a whole show that glucocorticoids can specifically inhibit cutaneous steps in induction of cell-mediated immunity or its suppression, and can, at the site of challenge, prevent its expression in CH.

Developmental changes of glucocorticoid receptors in the rat pancreas

Glucocorticoids are known to play a role in the maturation of the exocrine pancreas. The exact mechanism of glucocorticoid action in pancreatic ontogeny is, however, not clear. The present study characterized and quantitated the binding of [3H]dexamethasone to cytosol fractions from pancreata of rats at various ages. Trunk blood samples from these rats were also checked for levels of free and bound corticosterone. Specific and saturable bindings for dexamethasone were found in pancreatic cytosol fractions from newborn suckling and adult rats. Competition studies showed a preference for steroids with glucocorticoid activity. Specific binding was relatively low in pancreatic cytosol from newly born and 1-day old pups. A significant rise was seen after day 15. Cytosolic binding capacities were greatest from pancreata obtained from pups at weaning (3rd to 5th weeks). Values then declined toward the adult level. Scatchard analysis revealed a single class of binding sites with a dissociation constant (Kd) of 7.3 (+/- 1.1) X 10(-8) M and number of binding sites equalled to 1.29 (+/- 0.18) X 10(-13) mole/mg of cytosolic protein in adult rat pancreas. Pancreata from 25- and 15-day old rats had Kds of 3.4 (+/- 0.8) X 10(-8) M and 2.7 (+/- 0.7) X 10(-8) M with the number of binding sites equal to 1.77 (+/- 0.21) X 10(-13) mole/mg protein and 1.31 (+/- 0.16) X 10(-13) mole/mg protein respectively. Total plasma corticosterone concentration was low before day 10. It rose significantly by day 15, peaked at day 25, and then declined after weaning. About 5-15% of corticosterone during weaning and about 20-30% before and after weaning were in the free form. The peak level of dexamethasone binding corresponded to an increase in the plasma corticosterone level during weaning. This suggests a close relationship between plasma corticosterone levels and pancreatic glucocorticoid receptors. Both may, therefore, play a role in pancreatic development in the rat.

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.

Antiglucocorticoid steroids have increased agonist activity in those hepatoma cell lines that are more sensitive to glucocorticoids

FU5-5 rat hepatoma (Reuber H35) cells are hypersensitive in that the same percentages of full induction of tyrosine aminotransferase (TAT) occur at much lower concentrations of glucocorticoids than in the related HTC rat hepatoma (Morris) cells. Unexpectedly, these hypersensitive FU5-5 cells also exhibited more agonist activity with the affinity labeling antiglucocorticoids cortisol 21-mesylate and dexamethasone 21-mesylate than did HTC cells (Mercier et al., Endocrinology 112, 601-609 [1983]). In the present study, several other antiglucocorticoids (11-desoxycortisone, progesterone, dexamethasone oxetanone, and RU 486 in addition to dexamethasone 21-mesylate) and the antiandrogen cyproterone acetate were examined to see if chemically unreactive, reversible antisteroids also would exhibit an altered activity (i.e. increased agonist activity) in FU5-5 cells. Each antiglucocorticoid examined did display a 2-fold increased amount of agonist activity in FU5-5 cells, as compared to HTC cells; only RU 486 was predominantly an antagonist in FU5-5 cells but the potency of RU 486 was about 9-fold less than in HTC cells. Dexamethasone, and especially progesterone, was metabolized in FU5-5 and HTC cells. However, differential metabolism in FU5-5 vs HTC cells cannot account for the increased induction of TAT in FU5-5 cells since the amount of agonist activity seen for dexamethasone mesylate (or its metabolites) depended not on the cell type used but rather on the glucocorticoid inducible enzyme monitored, i.e. TAT or glutamine synthetase. The combined data suggest that the hypersensitivity of FU5-5 cells towards glucocorticoid induction of TAT may be linked with the ability of both reversible and irreversible antiglucocorticoids to display increased TAT agonist activity in FU5-5 cells. This behavior was somewhat steroid specific since the antiandrogen cyproterone acetate did not display increased TAT agonist activity in FU5-5 cells compared to HTC cells and was only 2-fold less effective as an antiglucocorticoid in FU5-5.

"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)

Binding of ester and amide epimers of 20 zeta-dihydroprednisolonic acid to cytosol receptors and their acute pharmacological activities in rats

The competitive binding of two new classes of anti-inflammatory steroids, the esters and amides derived from the epimers of 20 zeta-dihydroprednisolonic acid, to cytosol receptors from rat liver and thymus was studied. The relative inhibition of [3H]dexamethasone binding by the steroid derivatives was the same, irrespective of the receptor source, with the following order: dexamethasone greater than prednisolone greater than methyl 17,20 alpha-acetonidodihydroprednisolonate greater than methyl 17,20 beta-acetonidodihydroprednisolonate greater than N-propyl 20 alpha-dihydroprednisolonamide greater than methyl 20 alpha-dihydroprednisolonate greater than methyl 20 beta-dihydroprednisolonate greater than N-propyl 20 beta-dihydroprednisolonamide. The alpha-epimer of the steroids always showed a higher binding affinity than the corresponding beta-epimer. In an acute pharmacological study, prednisolone induced the suppression of plasma corticosterone and an increase in tyrosine aminotransferase activity and glycogen content of rat liver. The esters and amides had no effect on these parameters except in the case of the acetonide derivatives of the steroid acid esters which slightly increased liver glycogen content.

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.

Formation of a fluorescent glucocorticoid receptor-steroid complex in HTC cell cytosol

An intensely fluorescent rhodamine derivative of dexamethasone (i.e. Dex-C2-Rho) was synthesized. Dex-C2-Rho possessed high affinity for HTC cell glucocorticoid receptors in cell-free systems. Whole cell activity and receptor affinity of Dex-C2-Rho were both much lower, apparently due to problems with cell permeability and/or metabolism. A specific, fluorescent receptor-steroid complex at concentrations as low as 1 X 10(-9) M could readily be observed with crude HTC cell receptors after removal of the free Dex-C2-Rho. This appears to be the first report of a fluorescent glucocorticoid receptor-steroid complex.

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.

Exchange assay for androgen receptors in the presence of molybdate

Several reports have shown that sodium molybdate stabilizes steroid hormone receptors. We have utilized these observations to develop an exchange assay for the androgen receptor at elevated temperatures. Exchange was found to be complete after 30 min at 30 degrees C. Receptor degradation was negligible during this treatment. Scatchard analysis indicated that the dissociation constant of the androgen receptor was similar both in the absence (Kd = 3.9 nM) and presence (Kd = 2.9 nM) of molybdate. Steroid specificity of the androgen receptor was unaltered by this treatment. The exchange procedure was reproducible, with an interassay variation of 2.45% and intraassay variation less than 10.0%. Using this assay, highest concentrations of androgen binding were measured in androgen target tissues of the rat (Dunning R3327 tumor, prostate and seminal vesicle; 23.37, 20.20 and 19.84 fmol/mg protein respectively). Lower concentrations were observed in other tissues (lung, brain, heart, spleen, liver and kidney; 9.06, 5.63, 3.50, 2.42, 2.33 and 1.36 fmol/mg protein respectively). These results demonstrate that molybdate stabilization of the androgen receptor allows efficient steroid exchange without significant alteration of the receptor's steroid binding properties. Furthermore, this exchange assay can be used to obtain a reasonable measurement of receptor concentrations in different androgen target tissues.

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.

Influence of molybdate, ionic strength and pH on ligand binding to the glucocorticoid receptor

The addition of molybdate to rabbit liver cytosol increased significantly the affinity of the glucocorticoid receptor for [3H] dexamethasone without influencing the concentration of binding sites. This effect was concentration dependent. Analysis of the binding data by curve-fitting and Scatchard plot revealed the occurrence of a complex binding process in the presence of molybdate. The pH-dependence curve of the binding was shifted towards alkaline values by the oxyanion. Taken together, these data suggest that molybdate exerts its effects via an interaction with the receptor molecule.

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.

17 beta-carboxamide steroids: highly effective inhibitors of the phytohaemagglutinin mediated blastogenesis of normal human peripheral lymphocytes

Several novel 17 beta-carboxamide analogues of dexamethasone were synthesized. The common precursor, 9-fluoro-16 alpha-methyl-11 beta,17-dihydroxy-3-oxo-1,4-androstadiene-17 beta-carboxylic acid, did not bind to the glucocorticoid receptors of rat liver and human spleen tumours. In addition, no inhibition of the mitogen-induced blastogenesis of cultured human peripheral lymphocytes was observed. The 17 beta-carboxamide analogues, however, bound with similar affinities to the glucocorticoid receptors of both tissues. They inhibited the mitogen-induced blastogenesis of peripheral lymphocytes, showing the same potency and same order of binding affinity as the natural glucocorticoids.

Comparative study of the intraocular pressure effects of fluorometholone 0.1% versus dexamethasone 0.1%

The intraocular pressure effect of fluorometholone 0.1% was compared with that of dexamethasone 0.1% by performing corticosteroid provocative tests on 24 matched pairs of eyes. Fifteen of the 24 dexamethasone treated eyes, 62.5%, showed a change in intraocular pressure greater than 5 mmHg, with mean delta P = 8.58 mmHg and range 0 to +20 mmHg. Only 2 of the 24 fluorometholone treated eyes, 8.3%, showed a change in pressure greater than 5 mmHg, with mean delta P = 2.96 mmHg and range -2 to +14 mmHg. There was a highly statistically significant difference between the intraocular pressure effects of topical dexamethasone and fluorometholone (correlated t test, p less than 0.001). Fluorometholone would appear to be the topical steroid of choice for patients with glaucoma and other known steroid responders when topical steroid treatment is indicated.

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.

Anti-inflammatory steroids without pituitary-adrenal suppression

When two new steroids, methyl prednisolonate and methyl 20-dihydroprednisolonate, were applied locally their anti-inflammatory activities were nearly equivalent to those of the parent compound prednisolone in the cotton pellet granuloma bioassay. However, when these two derivatives were administered systemically, their anti-inflammatory activities were weaker than those of the parent compound. Furthermore, unlike the parent compound, these new anti-inflammatory steroids did not suppress pituitary-adrenal function or cause liver glycogen depletion in rats.

Activation of the glucocorticoid-receptor complex

A crucial step in the interaction of glucocorticoids with target cells is the activation step, which involves a conformational change in the cytoplasmic glucocorticoid-receptor protein complexes and facilitates their binding to the cell nucleus. Activation can be quantified by measuring the ability of glucocorticoid-receptor complexes to bind to polyanions, such as DNA-cellulose, and unactivated complexes can be separated from activated complexes by rapid ion exchange chromatography using diethylaminoethyl (DEAE)-Sephadex or DEAE-cellulose. Activation occurs in vivo under physiological conditions and the rate of activation of cytoplasmic glucocorticoid-receptor complexes can be enhanced in vitro by physical manipulations (elevated temperature, increased ionic strength, dilution). In vitro studies suggest that activation is a regulated process and a low molecular weight component termed modulator, which has been identified in rat hepatic cytosol, inhibits activation. Additional studies employing phosphatase inhibitors, such as molybdate, and purified calf intestinal alkaline phosphatase suggest that either the receptor protein or a regulatory component is dephosphorylated during activation. Results obtained with specific chemical probes suggest that activation results in the exposure of basic amino acid residues consisting minimally of lysine, arginine, and histidine. Pyridoxal 5'-phosphate, a specific probe for lysine residues, exerts dual effects on glucocorticoid-receptor complexes, since it stimulates the rate of activation and also inhibits the binding of previously activated complexes to nuclei or DNA-cellulose. The ability of 1,10-phenanthroline, a metal chelator, to inhibit the DNA-cellulose binding of activated complexes suggests that a metal ion(s) located at or near the DNA binding site may become exposed as a consequence of activation. Collectively, the results of these various experiments suggest that activation is a regulated biochemical phenomenon with physiological significance.