Dramatic protective effect of ligand against thermal degradation on mineraloand glucocorticoid receptors of rat kidney

Structural determinants of ligand binding to the mineralocorticoid receptor

The first and critical step in the mechanism of aldosterone action is its binding to the mineralocorticoid receptor (MR), a member of the nuclear receptor superfamily. Over the last 40 years, numerous studies have attempted to determine the structural determinants of ligand-binding to MR. An initial set of data showed that hsp90 is bound to the receptor via specific regions and maintains it in a ligand-binding competent state. Site-directed mutagenesis and functional studies guided by a 3D model of the MR ligand-binding domain (LBD) made it possible to identify the residues responsible for the high affinity and selectivity for aldosterone, and to characterize the mechanisms of MR activation and inactivation. The recent determination of the X-ray crystal structures of the LBD of the wild-type MR and MR(S810L), which is responsible for a familial form of hypertension, has made it possible to elucidate the peculiar mechanism of activation of MR(S810L) and established a clear structure/activity relationship for steroidal and non-steroidal MR antagonists.

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)

Stability study on renal type I mineralocorticoid receptor

The purpose of this work is to review stability and activation properties of type I receptor, in order to explain the reasons for its extreme in vitro instability. We demonstrate that the treatment of rat kidney cytosol with H2O2 prevents aldosterone binding, DNA/steroid-receptor complex interactions, and prevents the receptor thermal inactivation. In contrast, exogenous sulfhydryl reducing reagents are necessary to insure maximum binding of mineralocorticoid receptor and DNA/steroid-receptor interaction. However, the presence of beta-mercaptoethanol in thermal induced incubations reverts the H2O2 protection. We also demonstrate that contaminations with free or sequestered iron are harmful for both, receptor binding capacity (in a reversible form) and for hormone-receptor/DNA binding properties (in a partially reversible form). We propose a sulfhydryl oxidative mechanism for type I mineralocorticoid receptor inactivation in which iron contaminants might accelerate this process by oxidative catalysis. We also demonstrate that when thiol groups are blocked by specific reagents such as N-ethyl-maleimide or dithionitrobenzoic acid, type I sites loose binding capacity, but the protein is protected from oxidation as well as inactivation.

Expression of mineralocorticoid type I and glucocorticoid type II receptors in astrocyte glia as a function of time in culture

In the present study we have examined the expression of mineralocorticoid Type I and glucocorticoid Type II receptors in astrocyte glia maintained in culture for different periods of time. Cytosolic mineralocorticoid Type I receptors were labeled with [3H]aldosterone (ALDO) in the presence of a 500-fold molar excess of the potent Type II receptor ligand RU 28362. [3H]Dexamethasone (DEX) was used to label cytosolic Type II receptors. Both Type I and Type II receptor binding was saturable in astrocyte glia that had been maintained in culture for 20 and 30 days following final plating (i.e. 20- and 30-day-old cultures). Scatchard analysis of [3H]ALDO binding revealed a single class of Type I receptors, with dissociation constants (Kd) of 0.45 +/- 0.13 nM and 0.53 +/- 0.07 nM, respectively, in 20- and 30-day-old cultures. The number of Type I receptors in 30-day-old cultures was nearly half that found in 20-day-old cultures (22.06 vs 42.64 fmol/mg protein). Linear Scatchard plots were also obtained for [3H]DEX binding to cytosol prepared from 20- and 30-day-old cultures. There were no significant differences in the Kd or Bmax values for [3H]DEX binding in 20- or 30-day-old cultures, i.e. 2.06 +/- 0.15 nM and 247.36 +/- 18.16 fmol/mg protein for 20-day-old cells and 2.3 +/- 0.74 nM and 261.02 +/- 3.08 fmol/mg protein for 30-day-old cells. These Bmax values are more than double the Bmax value for [3H]DEX binding observed in our previous studies in 10-day-old astrocyte glial cultures. Switching cultured astrocyte glial from serum-supplemented to serum-free medium had no significant effects on the Kd values of Type I or Type II receptors in all the cultures tested. However, treatment with serum-free medium increased the number of Type I receptors in 30-day-old cultures to a level similar to that found in 20-day-old cultures. Taken together, these binding data suggest that Type I and Type II receptors are expressed differently in astrocyte glia as a function of time in culture.

The action of calcitonin on the TM4 Sertoli cell line and on rat Sertoli cell-enriched cultures

The effects of synthetic salmon calcitonin on primary Sertoli cell-enriched cultures and on an established cell line (TM4 cells, derived from immature mouse Sertoli cells) were studied. Synthetic salmon calcitonin stimulated the conversion of [3H]adenine to [3H]cyclic AMP in both cell systems. In addition, this peptide stimulated the secretion of rABP in primary Sertoli cell-enriched cultures prepared from rat testis. Calcitonin also increased the total concentration of both androgen and estrogen receptors in TM4 cells. Because cAMP analogs decreased androgen and estrogen receptor concentrations, the effect of calcitonin on sex steroid receptors may not be mediated by its effect on cyclic AMP in these cells. The possibility that the action of synthetic salmon calcitonin on the receptors might be mediated by a change in cellular Ca2+ was investigated. Lowering extracellular Ca2+ concentrations from 1.5 mM to less than 0.01 mM markedly reduced the concentration of androgen and estrogen receptors; restoration of Ca2+ to 1.5 mM returned receptor levels to normal. When the receptor concentrations were decreased by lowering extracellular Ca2+ concentrations to 0.5 mM, treatment with the calcium ionophore, A23187, restored receptor levels to normal. Although the calcium channel blocker, verapamil, decreased receptor levels, calcitonin partially counteracted its effect. Trifluoperazine, an inhibitor of calmodulin, also diminished androgen and estrogen receptor, levels in the cytosol of TM4 cells. It was concluded that calcitonin stimulates the formation of cyclic AMP and the secretion of rABP by Sertoli cells. This peptide also increases the concentration of androgen and estrogen receptors, possibly by a mechanism that is, in part, Ca2+ -mediated. These results, along with those on Leydig cells, suggest that calcitonin could be a regulator of testicular function.

Effects of polyhydric and monohydric compounds on the stability of type I receptors for adrenal steroids in brain cytosol

We have shown previously that unoccupied type I receptors for adrenal steroids in brain cytosol lose their capacity to bind [3H]aldosterone ([3H]ALDO) in a time- and temperature-dependent manner. Based on reports that sugars and polyvalent alcohols are capable of stabilizing a variety of globular proteins, we attempted in the present study to stabilize type I receptors by including polyhydric compounds in our brain cytosol preparations. However, contrary to expectations, adjusting cytosol to a 10% (g/dl) concentration of ethylene glycol, glycerol, erythritol, xylitol, ribitol, or sorbitol failed to stabilize these receptors at 0 degree C and in fact produced a slight reduction in [3H]ALDO binding capacity. The magnitude of this reduction was greater when cytosol was incubated for 2 h at 22 degrees C prior to incubation with [3H]ALDO. In contrast to these results, when brain cytosol was adjusted to a 10% (g/dl) concentration of the monohydric compound, ethanol, a significant increase in [3H]ALDO binding to type I receptors was found. Under identical conditions, methanol and propanol failed to have a significant effect on the binding capacity of these receptors. When cytosol was aged for 2 h at 22 degrees C, all three of these monohydric compounds produced a marked loss in the [3H]ALDO binding capacity of type I receptors. An investigation of various doses of ethanol at 0 degree C on the subsequent binding of [3H]ALDO yielded an inverse U-shaped curve with 10% ethanol producing the highest level of specific binding, as reflected by an increase in maximal binding in Scatchard plots, and 40% ethanol producing a complete loss in type I receptor binding capacity.(ABSTRACT TRUNCATED AT 250 WORDS)

Application of high-performance liquid chromatofocusing to the study of prostatic androgen receptors. Effect of stabilizing agents on the heterogeneity of receptor structure

Cytosolic androgen receptors from normal rat prostate were analyzed by high-performance liquid chromatofocusing. Two ion-exchange columns, AX-300 and AX-500, and two equilibration systems, Tris-HCl and imidazole-HCl, were used. pH gradients ranged between 8.3 and 3.5 for Tris-HCl and from 7.7 to 3.5 for imidazole-HCl. In the absence of sodium molybdate and inhibitors of proteolytic enzyme, six specific radioactive fractions (pH: 7.9, 7.7, 7.0, 5.1, 4.7 and 4.4) were eluted from AX-300 equilibrated with Tris-HCl in a ratio of 28:20:52 for acidic, intermediary and basic forms, respectively; similar results were obtained with AX-500 although this column was less effective in resolving basic forms of the receptor. The buffering capacity of imidazole-HCl was lower than that of Tris-HCl, resulting in a steeper elution pH profile. The resolution between acidic and basic forms was thus diminished and only four specific radioactive fractions at pH 7.2, 7.1, 6.5 and 3.6, were observed on AX-500 in a ratio of 23:10:67 for acidic, intermediary and basic forms. In the presence of sodium molybdate, two acidic fractions were found with Tris-HCl at pH 4.3 and 4.7 (47%) on AX-300, whereas the radioactivity of fractions at pH 7.0 and 5.1 (32%) was considerably lowered and intermediary forms remained unchanged (21%). With imidazole-HCl on AX-500, the peak at pH 7.2 disappeared and the acidic form shifted from pH 3.6 to 4.3. In the presence of inhibitors of proteolytic enzyme and sodium molybdate, specifically bound radioactivity was found mostly in a broad acidic fraction (75%) at pH 4.5 on columns equilibrated with Tris-HCl; radioactivity at pH 7.6 disappeared completely but a small amount (15%) remained at pH 7.9. In imidazole-HCl, a main radioactive fraction was eluted at pH 7.1 and two other fractions were collected at pH 6.8 and 4.3 respectively. In conclusion, multiple forms of the rat prostate androgen receptor were evinced by high-performance liquid chromatofocusing. Tris-HCl proved to be a more efficient equilibration system than imidazole-HCl for the resolution of rat prostate cytosolic binding proteins. Under the experimental conditions used, sodium molybdate and inhibitors of proteolytic enzyme greatly favored the acidic form to the detriment of the intermediary and basic entities.

Characterization of the in vitro stability of the rat hepatic receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

The in vitro stability of the Ah receptor from rat hepatic cytosol was evaluated by [3H]TCDD binding studies, gel filtration, and sucrose density gradient ultracentrifugation. Thermal inactivation of unoccupied receptor followed first-order kinetics between 5 and 40 degrees C, with an estimated Ea for inactivation of approximately 35 kcal/mol. Protease inhibitors did not reduce and dilution slightly increased the inactivation rate at 20 degrees C. Recovery and 20 degrees C stability decreased with increasing ionic strength. The TCDD-receptor complex was less susceptible to degradation at 20 degrees C, even in the presence of 0.4 M KCl. Specific binding was markedly pH dependent, with maximum recovery at 7.6. Analysis of the pH curve suggested that cysteine sulfhydryl groups may be involved in TCDD binding. Dithiothreitol (1 mM) maximized recovery and 20 degrees C stability, and addition of the thiol largely reactivated binding sites lost from cytosol prepared without it. Removal of low molecular weight components of cytosol by gel filtration resulted in a rapid 20 degrees C inactivation rate that could not be lessened by dithiothreitol. Glycerol (10% v/v) and EDTA (1.5 mM) maximized recovery of specific binding, but both decreased 20 degrees C stability in a concentration-dependent manner. Calcium chloride (4 mM) increased stability at 20 degrees C by approximately 20%, and retarded the characteristic shift in sedimentation coefficient from approximately 9 to approximately 6 S in high-salt sucrose gradients. The fact that sodium molybdate (20 mM) decreased recovery and 20 degrees C stability when dithiothreitol was present but slightly increased stability in its absence suggested an antagonism between the two compounds. Molybdate mitigated the inactivation induced by 0.4 M KCl, an effect which may be related to the observation of dual peaks in molybdate-containing high-salt sucrose gradients. These data indicate that thermal inactivation of the unoccupied rat hepatic Ah receptor primarily may be due to physical rather than enzymatic processes; (ii) sulfhydryl oxidation, removal of low molecular weight cytosolic components, and high ionic strength result in rapid rates of inactivation at 20 degrees C; and (iii) the large degree of protection conferred by TCDD binding implies a very tight ligand-receptor interaction, and as such accords with TCDDs extraordinary potency and persistence in producing its toxic and biochemical effects.

Affinity of corticosteroids for mineralocorticoid and glucocorticoid receptors of the rabbit kidney: effect of steroid substitution

Corticosteroid derivatives coupled in the C3, C7 or C17 position with a long aliphatic chain were synthesized in order to select a suitable ligand for the preparation of a biospecific affinity adsorbent for mineralocorticoid receptor purification. The affinity of these derivatives for mineralocorticoid receptors (MR) and glucocorticoid receptors (GR) was explored in rabbit kidney cytosol. In this model, aldosterone bound to a single class of receptors with high affinity (Kd 1 nM) and mineralocorticoid specificity. RU26988, a highly specific ligand for GR, did not compete for these sites. The C7 and C17 positions were found to be of crucial importance in the steroid's interaction with the mineralocorticoid receptors, since the linkage of a long side chain in these positions induced complete loss of affinity. Hence, deoxycorticosterone no longer bound to MR after 17 beta substitution with a 9-carbon aliphatic chain. This loss of affinity was not observed for glucocorticoids. The 17 beta nonylamide derivative of dexamethasone still competed for GR. Increasing the length of the C7 side of the spirolactone SC26304 suppressed its affinity for MR. Finally, C3 was an appropriate position for steroid substitution. The 3-nonylamide of carboxymethyloxime deoxycorticosterone bound to MR but not to GR, and therefore constitutes a suitable ligand for the preparation of a mineralocorticoid adsorbent.

Chemical differentiation of type I and type II receptors for adrenal steroids in brain cytosol

Studies outlined here compare the properties of mineralocorticoid (Type I) and glucocorticoid (Type II) receptors in cytosol from adrenalectomized mouse brain. Pretreating cytosol with dextran-coated charcoal (DCC) produced a 4.7-fold increase in the subsequent macromolecular binding of the mineralocorticoid, [3H]aldosterone (20 nM ALDO, in the presence of a 50-fold molar excess of the highly specific synthetic glucocorticoid, RU 26988), whereas it produced a 55% decrease in the binding of the glucocorticoid, [3H]triamcinolone acetonide (20 nM TA). Scatchard analyses revealed that DCC pretreatment had no effect on the affinity or maximal binding of Type I receptors for [3H]ALDO (in the presence of a 0-, 50- or 500-fold excess of RU 26988), whereas it produced a 3- to 6-fold increase in the Kd, and an 8-43% decrease in the maximal binding, of Type II receptors for [3H]TA and [3H]dexamethasone. Optimal stability of unoccupied Type I receptors at 0 degree C was found to be achieved in buffers containing glycerol, but lacking molybdate. Although the addition of molybdate was found to reduce the loss in Type I receptor binding observed after incubating unlabelled cytosol at 12 or 22 degrees C, this stabilization was accompanied by a concentration-dependent reduction in the binding of [3H]ALDO at 0 degree C. Scatchard analyses showed that this reduction was due to a shift in the maximal binding, and not the affinity, of the Type I receptors for [3H]ALDO. The presence or absence of dithiothreitol in cytosol appeared to have little effect on the stability of Type I receptors. In contrast to our finding for Type I receptors, it was possible to stabilize the binding capacity of unoccupied Type II receptors, even after 2-4 h at 12 or 22 degrees C, if the glycerol containing buffers were supplemented with both molybdate and dithiothreitol. In summary, these results indicate distinct chemical differences between Type I and Type II receptors for adrenal steroids.

Cytoplasmic and nuclear uptake of aldosterone in toad bladder: a mathematical modeling approach

The mechanism of aldosterone uptake in the epithelial cells of toad bladder was studied using mathematical modeling. Two complementary approaches were used. The first involved analysis of cytosolic aldosterone binding at steady state according to models defined by the sum of independent noninteractive binding sites. The best model describing the experimental data corresponded to two specific binding sites with mean dissociation constant values of 0.20 and 60 nM for types 1 and 2, respectively. The second approach was based on the analysis of cytoplasmic and nuclear aldosterone uptake kinetics at 25 and 0 degrees C in intact bladder. Two models (A and B) were studied. They both implied the existence of two types of aldosterone binding sites as precursors of the corresponding chromatin bound complexes. In model A, nuclear translocation of the two types of receptors was assumed to obey first-order kinetics. In model B, the translocation process for type 1 sites involved a time lag leading to delayed binding to chromatin. Both models were found to fit the experimental data satisfactorily. The fit obtained for model B appeared to be better at low aldosterone concentrations.

Effects of metal ions and chelating agents on in vitro stability of glucocorticoid receptors in brain cytosol

In vitro studies in a variety of tissues and cell types suggest that glucocorticoid receptor binding capacity is not static and that binding sites are subject to up- and down-regulatory mechanisms. The interpretation of such studies, however, is often complicated by factors affecting the stability of the receptor. This situation is particularly acute in the absence of ligand because of the increased lability of the unoccupied receptor. Studies reported here investigate effects of various metal ions and chelating agents on the stability of unoccupied [3H]dexamethasone binding sites in whole mouse brain cytosol. Variation in the ionic strength of cytosol, as created by the additions of various monovalent cations (Li+, Na+, K+, Rb+ and Cs+), was found to be an important factor affecting the increased stability of the receptor in vitro. Additions of divalent (Mg++, Ca++, Ba++, and Mn++) and trivalent (La , Cr and Al ) cations to cytosol, however, were generally found to produce a dose-dependent decrease in the stability of the unoccupied receptor. Additions of the chelating agents EDTA, EGTA and 1,10-phenanthroline to cytosol, resulted in differential, and sometimes complex, dose-dependent effects on receptor stability. The complex effects of various combinations of cations and the chelator EDTA were also investigated.

Sulfhydryl groups of aldosterone receptors from swine kidney

Specific [3H]aldosterone binding activity in swine kidney cytosol was inactivated by pretreatment of the cytosol with monoiodoacetamide (pH 8.5), N-ethylmaleimide (pH 7.0), or 5,5'-dithiobis(2-nitrobenzoate) (pH 7.5). Dithiothreitol restored the specific binding activity inactivated by the nitrobenzoate, but not that inactivated with ethylmaleimide. Incubation of the cytosol with aldosterone prior to pretreatment with ethylmaleimide protected the receptors from inactivation. The rank order of steroids for the protection was: aldosterone greater than hydrocortisone greater than or equal to dexamethazone = progesterone greater than triamcinolone greater than estradiol. The initial velocity of the specific hormone binding could be determined by the binding reaction for 60 sec at 30 degrees. Double reciprocal plots of the initial velocity versus the hormone concentration with or without the nitrobenzoate showed a typical pattern of competition between the hormone and the inactivator. The results indicated the presence of functional sulfhydryl groups on the hormone binding sites of aldosterone receptors.

Modulation of aldosterone receptors in rat kidney: effects of steroid treatment and potassium diet

The numbers of type I and type II aldosterone receptors in the kidney cytosol of adrenalectomized rats were estimated after animals were treated with various steroids, or fed with high or low potassium diets. Oestradiol and 5 beta-pregnane-3,20 dione, which exhibited no affinity for aldosterone receptors, did not modify the levels of type I or type II receptors. Cortisol, corticosterone, progesterone and spirolactones, which all competed with aldosterone for both types of receptor, reduced the number of type I sites, as does aldosterone itself. Steroid treatment has no appreciable effect on type II receptors. We conclude that type I receptors are modulated by steroids able to bind to aldosterone receptors and that steroid-receptor interaction is an essential step in the receptor modulation process. The effects of potassium on aldosterone receptor modulation were tested in adrenalectomized rats on hypo- or hyperkalaemic diets. No change in receptor levels was observed in the rats on a low potassium diet, but the number of type I receptors increased in animals on a high potassium diet. However, the effects of potassium on receptor modulation were of lesser magnitude than those of aldosterone agonists and antagonists.

Characterization of spironolactone binding sites distinct from aldosterone receptors in rat kidney homogenates

The binding of [3H]spironolactone to kidney homogenates from adrenalectomized rats was studied by dextran-charcoal absorption methods. [3H]Spironolactone binds with high affinity and low capacity (KD = 12.9 +/- 0.6 nM; Bmax = 93.4 +/- 3.8 fmoles/mg protein) at low temperatures (0 degrees-2 degrees). Its hormone specificity, as measured by relative binding affinity (RBA) is spironolactone greater than prorenone greater than methyltrienolone greater than testosterone greater than progesterone greater than aldosterone greater than dexamethasone. In the same tissue preparation, specific spironolactone binding sites and classical mineralocorticoid receptor sites labelled with [3H]aldosterone differ in their thermal stability, binding parameters and hormone specificities, whereas their tissue distributions are similar. In conclusion, [3H]spironolactone binds specifically to kidney homogenates from adrenalectomized rats and these binding sites, apparently, are different from the classical mineralocorticoid receptors. The theoretical and practical aspects of this finding are discussed.

Corticosteroid receptors in the kidney of chick embryo. III. Nature, properties, and ontogeny of aldosterone receptor

An aldosterone receptor in the cytosol from kidney of chick embryos which had a sedimentation coefficient of 8.2 S and a molecular weight higher than 100,000 was identified. Kinetic analysis at 4 degrees revealed a rapid association of the hormone to the receptor that followed second-order reaction kinetics and a dissociation of pseudo-first-order reaction kinetics. The association (ka) and dissociation (kd) rate constants were, respectively, 4.94 X 10(5) M-1 sec-1 and 8.33 X 10(-6) sec-1. From their ratio a KA value of 5.9 X 10(10) M-1 was calculated. In a series of experiments performed with kidneys of 17-day-old embryos, the KA at equilibrium, obtained from the Scatchard plot, was 3.1 +/- 1.2 X 10(8) M-1, whereas the Nmax was 172 +/- 14 fmol/mg protein. Competition studies with various steroids demonstrated that corticosterone had an affinity for the receptor close to that of aldosterone, thus suggesting a degree of resemblance of the mineralo- and glucocorticoid receptors in the chick embryo. However, the profiles of the binding affinities and capacities during the embryogenesis showed that the aldosterone-binding sites had a pattern completely different from that of the glucocorticoid receptor, indicating that the two receptors are most likely separate entities.

The characteristics of renal mineralocorticoid receptors in glycyrrhizinic acid or deoxycorticosterone-induced hypertensive rats

The relationship between blood pressure and the characteristics of renal mineralocorticoid receptors was studied in glycyrrhizinic acid (GR) or deoxycorticosterone (DOC) induced hypertensive rats. The apparent maximum binding (Bmax) of aldosterone to renal mineralocorticoid receptors was 3.1 +/- 0.2 X 10(-13) mol/mg cytosol protein and the dissociation constant (Kd) was 1.6 +/- 0.5 nM. GR treatment reduced the concentration of cytosol mineralocorticoid receptors (Bmax) but did not affect the Kd. In unilaterally adreno-nephrectomized rats, GR induced hypertension and hypokalemia as seen in DOC treated rats. After the discontinuation of GR, blood pressure was normalized with concomitant recovery of free cytosol mineralocorticoid receptors in 14 weeks. On the contrary, in DOC treated rats, the characteristics of mineralocorticoid receptors in kidney were already normal one week after the cessation of DOC treatment. However, blood pressure remained high up to 15 weeks. These findings suggest that the persistence of hypertension after GR discontinuation might be caused by a long-standing effect of GR on renal mineralocorticoid receptor mechanisms.

Mineralocorticoid receptors in the epithelial cells of human colon and ileum

Specific binding of [3H]aldosterone to the cytosolic fraction of epithelial cells was studied in the human colon and terminal ileum. Analysis of [3H]aldosterone binding to the epithelial cells of ascending colon, caecum and ileum as a function of [3H]aldosterone concentration revealed only one class of specific receptors with an affinity constant of about 2 nmol/l. [3H]aldosterone binding was approximately the same in the sigmoid, descending and transverse colon and in the caecum, but slightly lower in the ascending colon and ileum. The specificity of the [3H]aldosterone binding was the same along the colon. The relative order of potency in inhibiting [3H]aldosterone binding was: aldosterone = SC 26304 = dexamethasone much greater than dihydrotestosterone greater than estradiol = RU 26988.

Differential effects of chelating agents on cytosolic glucocorticoid receptor stability and nuclear binding in vitro

The effect of several metal chelators (EDTA, EGTA, and 1,10 phenanthroline) on rat liver glucocorticoid receptor properties in vitro was investigated. At 4 degrees C 10 mM EDTA (unlike 10 mM EGTA and 10 mM 1,10 phenanthroline) had a significant stabilizing effect on unbound hepatic glucocorticoid receptors. At higher temperature (25 degrees C) 10 mM EGTA appeared to act as a chemical stabilizer of unbound receptors. 1,10 Phenanthroline had no stabilizing effect at either temperature. Scatchard analysis indicated that the alteration in receptor binding after incubation at 4 and 25 degrees C in the presence and absence of chelating agents was due to a change in the number of steroid binding sites rather than perturbation of receptor affinity. Unlike results obtained with unbound receptors, all three chelating agents appeared to enhance prebound glucocorticoid-receptor complex inactivation. Interestingly these chelating reagents also significantly altered glucocorticoid-receptor complex binding to isolated nuclei.

Binding and antimineralocorticoid activities of spirolactones in toad bladder

The role of the soluble pool (cytoplasmic or cytosolic) of [3H]-aldosterone binding sites in the toad bladder was assessed by the use of two spirolactones, prorenone and spironolactone as a reference drug. Prorenone fulfills all the criteria for a specific competitive antagonist of aldosterone for its effect on Na+ transport. Compared with spironolactone (Ki approximately equal to 1 microM), prorenone was about eightfold less potent (Ki approximately equal to 8 microM). Competition for [3H]aldosterone binding sites by spironolactone and prorenone revealed an order of potency (spironolactone greater than prorenone) that corresponded to their antagonist activities in the Na+ transport assay. There was a linear correlation between the effects of the two spirolactones on the aldosterone-stimulated Na+ transport and their ability to displace [3H]aldosterone from its binding sites in the soluble pool. Finally [3H]prorenone binding sites were detected in the soluble pool but an insignificant number of antagonist-receptor complexes were found associated with the nuclear pool. Our study indicates that the aldosterone binding sites of the soluble pool are indeed mineralocorticoid receptors, which are probably the first intracellular mediators leading to an increased Na+ reabsorption.

Aldosterone binding sites in aortic cell cultures from spontaneously hypertensive rats

Spontaneously hypertensive rats and rats made hypertensive by deoxycorticosterone-salt treatment have in common increased Na+ and K+ permeability and transport in their aortic cells. These changes may be important factors in the development of the hypertensive state and may be mediated by mineralocorticoid binding to intracellular sites in the aorta. Therefore, we examined 3H-aldosterone binding in aortic cell cultures from spontaneously hypertensive rats and normotensive Wistar-Kyoto rats. Vascular corticoid binding sites in the two strains were compared by Scatchard analysis of Kd and Bmax, pH and temperature stability, and subcellular binding. By all of these criteria normotensive rats. These results indicate that the underlying genetic defect in spontaneous hypertension is not an intrinsic cellular defect which alters mineralocorticoid binding in the aorta.

Studies on high affinity binding of mineralo- and glucocorticoids in rabbit aorta cytosol

High affinity, specific binding-sites to mineralocorticoids and glucocorticoids, with characteristics of steroid receptors, have been found in rabbit aorta cytosol. Binding parameters (dissociation constants and number of binding sites per mg of cytosol protein) were determined from Scatchard plots, after statistical treatment of the data with the aid of a computer program, for the following tritiated steroids: 11-desoxycorticosterone (DOC), aldosterone (Aldo), progesterone (Prog), corticosterone (BK), cortisol (FK) and dexamethasone (Dex). The specificity of binding was then examined by means of steroid competition studies. The results of these experiments indicate that three different types of high-affinity binding sites to adrenal steroids are present in aorta cytosol: Type A, with the highest affinity for DOC; Type B, with the highest affinity for FK; Type C, with the highest affinity for Dex. In accordance with the relative competitive potencies of various steroids for these binding sites, Type A is designated as the "arterial mineralocorticoid binder", clearly differing in its binding characteristics from the cytoplasmic mineralocorticoid binders in known target tissues to these steroids (e.g. the renal receptor), while Type C is designated as the "arterial glucocorticoid binder", closely resembling the classical glucocorticoid receptor in known target tissues to glucocorticoids. Type B exhibited some of the binding characteristics of transcortin and may represent a modified, intracellular transcortin. While Types B and C are present also in the cytosol of inferior vena cava. Type A was only in the aorta. The role of these binders is not known at present. Arguments are presented in favor of a hypothesis that the Type A (mineralocorticoid) binder represents an arterial wall; and that, under certain conditions, this action leads to an increased contractility of arterial and arteriolar smooth muscles, increased peripheral resistance and hypertension.

Interactions of antiglucocorticoids with glucocorticoid receptors

Six known or potential antiglucocorticoids were used as steroid probes in a competitive binding assay to elucidate the binding specificity of the agonist sites of four different rat glucocorticoid receptors. The object was to determine whether the binding of this class of steroids was particularly sensitive to subtle differences between these sites. Cytosolic extracts of heart, pancreas, kidney and liver were evaluated. The order of competitive potency for the first three preparations was found to be medroxyprogesterone greater than deoxycorticosterone greater than progesterone greater than R-5020 greater than cortexolone greater than 17-hydroxyprogesterone. The order for the glucocorticoid receptor of liver, on the other hand, was R-5020 greater than progesterone greater than deoxycorticosterone greater than medroxyprogesterone greater than cortexolone greater than 17-hydroxyprogesterone. Although this partial reversal of specificity could reflect a difference in the agonist site of the liver receptor, mixing experiments, in which cytosolic extracts of liver were incubated with kidney cytosol extracts, demonstrated that the liver cytosol contained an additional factor that could change the apparent specificity of the kidney glucocorticoid receptor. This factor was stable at 0 degrees C for at least 18 h, heat-labile and non-dialyzable. These results suggest that the liver receptor's specificity may actually be the same as the other three, but appears to be different in this type of assay because of this additional factor. It is similar that their agonist sites are most likely identical.

Binding of aldosterone to cytoplasmic and nuclear receptors of the rabbit kidney

The binding characteristics of [3H]aldosterone ([3H]A) in the rabbit kidney were compared with those described in other species, and experimental conditions suitable for both the isolation of renal tubules and the retention of aldosterone receptors were examined. Specific binding of [3H]A to cytoplasmic (C) and nuclear (N) receptors was determined on kidney slices incubated at +30 degrees C in rabbits fed a low K+-high Na+ diet. Results indicate the presence of at least two binding sites in N fractions, one of high affinity (Kd (I)) 1.8 X 10(-8) M) and low capacity (Nmax (I) 4.6 X 10(-14) mol/mg DNA), called type I sites, and the other of lower affinity (Kd (II) 8.9 X 10(-8) M) and more numerous sites (Nmax (II) 13.1 X 10(-14) mol/mg DNA). Competition studies showed that type I sites have higher affinity for [3H]A than for other steroids. Incubation at +37 degrees C resulted in a loss of type I sites. In adrenalectomized rabbits the C binding was improved but not the N binding (Kd (I) 1.2 X 10(-8) M; Kd (II) 5.6 X 10(-8) M). In rabbits treated with aminoglutethimide, an inhibitor of steroid synthesis, the N binding was lower than in other series. It was concluded that 1) [3H]A binding in the rabbit kidney does not differ from that described in other species; 2) a low K+-high Na+ diet is a satisfactory condition for study of [3H]A binding; and 3) incubation at +30 degrees C, a temperature necessary for microdissection procedures, is suitable for aldosterone binding studies.

Kinetics of the glucocorticoid hormone-receptor interaction. False association constants determined in slowly equilibrating systems

The usual way of in vitro determination of association constants for hormone-receptor complexes is criticized. It is shown that if incubation time is short, relative to the half-life of the hormone-receptor complex, the value of the apparent Ka is proportional to the time of incubation. No sign of lack of equilibrium is apparent from the Scatchard plots. The case of rapidly denaturing receptor molecule is also discussed, with similar conclusions. Although terminology and examples are taken from the field of the glucocorticoid receptor research, all deductions are valid for other systems with similar association (and denaturation or monomolecular transformation) mechanisms and kinetic parameters.

Glucocorticoid receptors

Glucocorticoid receptors are found in most mammalian tissues and have been studied in detail in a number of tissue culture systems. With cells that have not been exposed to steroids, the receptors are found in the cytoplasmic fraction from which they can be isolated and studied. Methods for studying glucocorticoid receptors depend on their high-affinity specific binding of radioactive steroids. The reversible interaction is intracellular. It follows Michaelian kinetics, at least in cell-free cytosol, and involves a thermodynamically homogeneous population of about 10 000 sites per cell. The receptor is an asymmetric, slightly acidic protein of about 100 000 daltons. It is very labile, especially in the unbound form. Binding activity depends on the integrity of thiol groups and perhaps on phosphorylation of amino acid residues. Although indirect, the evidence is overwhelmingly convincing that this protein is the physiologic glucocorticoid receptor. The time-kinetics of binding and dissociation are consistent with the sequence of events in glucocorticoid action. Various steroid analogs display binding characteristics predictable from their glucocorticoid activity. Loss of the binding protein from certain cultured cell lines is accompanied by unresponsiveness to glucocorticoids. The extensive tissue distribution of receptors parallels the extensive role of glucocorticoids in regulation. Finally, there is a strong correlation between nuclear binding of receptors and nuclear effects of the steroid. The glucocorticoid receptor can be distinguished from other glucocorticoid-binding proteins, based on their steroid specificity and physicochemical properties. There is no clear-cut demonstration that the receptor differs from tissue to tissue, and it is in fact very similar in various species. Unlike in other systems, receptor concentration does not seem to be regulated by its ligand or by other hormones. However, certain cases of hypo- as well as hypersensitivity to glucocorticoids appear to result from changes at the receptor level. The data indicate that the receptor can exist in inactive and active forms. The former predominate in the absence of steroid or when an angatonist is bound. Glucocorticoid agonists bind the active form, allowing it to be "activated" and subsequently bound to the nucleus. All of the receptors in isolated cytosol do not appear to be available for immediate occupancy by an agonist and this may be due to the time required for conversion of the receptors from inactive to active forms. The correlations between receptor binding and the glucocorticoid response indicate that the receptor is a rate-limiting factor in the magnitude and kinetics of the response, and this finding has important implications regarding mechanisms.

Occupancy of aldosterone binding sites in rat kidney cytosol

The binding sites for aldosterone and a potent aldosterone antagonist (SC-26304) were studied in kidney cytosol from adrenalectomized rats. Preformed cytosol and kidney slices were incubated with 3H-labeled steroids in a wide range of concentrations. The recovery and characteristics of the binding sites were affected by the incubation and homogenization conditions. High-affinity, Type I mineralocorticoid binding was reduced by more than 95% when cytosol was incubated at 25 degrees C in the presence of calcium. Tissue dilution also affected the binding sites. SC-26304 was bound to high- and low-affinity receptors, similar to the binding of aldosterone. The physiologic response to aldosterone could result from binding to either or both sets of sites. Some of the physiologic responses to spirolactones could represent antagonism of the binding of aldosterone to either or both sites. A convenient method is presented for describing the relative occupancy of several different sites by any particular steroid.