Relationships between renal cytoplasmic and nuclear aldosterone-receptors

Structural and molecular determinants of mineralocorticoid receptor signalling

During the past decades, the mineralocorticoid receptor (MR) has evolved from a much-overlooked member of the steroid hormone receptor family to an important player, not only in volume and electrolyte homeostasis but also in pathological changes occurring in an increasing number of tissues, especially the renal and cardiovascular systems. Simultaneously, a wealth of information about the structure, interaction partners and chromatin requirements for genomic signalling of steroid hormone receptors became available. However, much of the information for the MR has been deduced from studies of other family members and there is still a lack of knowledge about MR-specific features in ligand binding, chromatin remodelling, co-factor interactions and general MR specificity-conferring mechanisms that can completely explain the differences in pathophysiological function between MR and its closest relative, the glucocorticoid receptor. This review aims to give an overview of the current knowledge of MR structure, signalling and co-factors modulating its activity.

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)

5 alpha-Dihydro-11-deoxycorticosterone as a mineralocorticoid agonist and antagonist: evidence for a weak mineralocorticoid as an antagonist of potent mineralocorticoids

To evaluate the possibility that 5 alpha-dihydro-11-deoxycorticosterone (5 alpha-DH-DOC), a weak mineralocorticoid, is an antagonist of a more potent mineralocorticoid, aldosterone, 0.25 microgram aldosterone was injected into adrenalectomized rats simultaneously with 200-800 micrograms 5 alpha-DH-DOC and urinary Na/K ratio and Na and K excretion were evaluated. Urinary Na/K ratio and Na excretion were significantly lower than those of control rats regardless of whether rats were treated with 0.25 microgram aldosterone alone or 400-800 micrograms 5 alpha-DH-DOC alone. Urinary Na/K ratio and Na excretion of rats given a combination of 0.25 microgram aldosterone plus 400-800 micrograms 5 alpha-DH-DOC were significantly higher than those of rats given 0.25 microgram aldosterone alone. None of the treatment caused significant changes in urinary K excretion. The results demonstrate that 5 alpha-DH-DOC, a weak mineralocorticoid, is an antagonist of the sodium-retaining action of a more potent mineralocorticoid, aldosterone. Progesterone which has weak mineralocorticoid activity is also known as an antagonist of more potent mineralocorticoids. The results of the present study demonstrate further evidence that weak mineralocorticoids may work as antagonists of more potent mineralocorticoids.

Cellular mechanisms of action of mineralocorticoid hormones

Mineralocorticoid hormones are a subset of steroid hormones that act primarily in epithelial tissues to regulate ion transport of Na+, K+ and H+. Cellular specificity is conferred by receptors which act in the nucleus to stimulate gene expression. Transcription and subsequent translation result in the production of new proteins which mediate the physiologic effects. The mechanisms involved in receptor specificity and localization, in regulation of gene activation, and in expression of transport effects are reviewed. The cellular actions of mineralocorticoids fit well with the general model of steroid hormone action but considerable questions remain at each step in the process.

Corticosteroid receptors in cells derived from rat brain microvessels: mRNA identification and aldosterone binding

B7 is a cell clone derived from rat brain microvessels. Expression of an amiloride-sensitive cationic channel has been recently established in these cells. In this study, the polymerase chain reaction (PCR) was used to amplify definite segments of mineralocorticoid and glucocorticoid receptor mRNA in B7 cells. Aldosterone binding was also characterized. Two classes of sites were detected. Aldosterone exhibited a high affinity for type I sites [dissociation constant (Kd) approximately 0.3 nM] and a lower one for type II sites (Kd approximately 20 nM). RU 28362, a highly specific glucocorticoid agonist, did not compete for type I sites. RU 28362 and dexamethasone were better competitors for type II sites than aldosterone. The sedimentation coefficients of aldosterone type I and type II complexes were approximately 9S. These characteristics are close to the one exhibited by aldosterone type I and type II receptors in rat kidney and other target tissues. In intact B7 cells, aldosterone binding expressed as number of acceptor sites per cell was higher (approximately 41,000 for type II and 8,800 for type I) than in the soluble cellular extract (approximately 18,000 for type II and 1,000 for type I).

Anti-aldosteronergic effect of torasemide

The diuretic actions of torasemide and furosemide were studied in normotensive rats and in deoxycorticosterone acetate (DOCA)-saline-loaded hypertensive rats. Torasemide (0.3-3 mg/kg) and furosemide (3-30 mg/kg) had a dose-dependent and significant diuretic action in normotensive rats. Potassium retention was only observed in the case of torasemide. Torasemide also had a dose-dependent and significant diuretic action in DOCA-saline-loaded hypertensive rats, whereas furosemide did not. Higher doses of torasemide (10 mg/kg) and furosemide (100 mg/kg) increased both plasma renin activity and aldosterone concentration in normotensive rats in a similar manner. In vivo aldosterone receptor binding was determined to test the possible anti-aldosteronergic effect of torasemide. Torasemide inhibited the binding of aldosterone to its receptor in the cytoplasmic fraction of rat kidney in a dose-dependent manner, while furosemide produced no effect. These results suggest strongly that an anti-aldosteronergic action of torasemide contributes to producing less kaliuresis.

Steroid hormone receptors

In the three decades since the original discovery of receptors for steroid hormones, much has been learned about the biochemical processes by which these regulatory agents exert their effects in target tissues. The intracellular receptor proteins are potential transcription factors, needed for optimal gene expression in hormone-dependent cells. They are present in an inactive form until association with the hormone converts them to a functional state that can react with target genes. Transformation of the receptor protein to the nuclear binding form appears to involve the removal of both macromolecular and micromolecular factors that act to keep the receptor form reacting with DNA. Much of the native receptor is present in the nucleus, loosely bound and readily extractable, but for some and possibly all steroid hormones, some receptor is in the cytoplasm, perhaps in equilibrium with a nuclear pool. Methods have been developed for the stabilization, purification, and characterization of receptor proteins, and through cloning and sequencing of their cDNAs, primary structures for these receptors are now known. This has led to the recognition of structural similarities among the family of receptors for the different steroid hormones and to the identification of regions in the protein molecule responsible for the various aspects of their function. Monoclonal antibodies recognizing specific molecular domains are available for most receptors. Despite the knowledge that has been acquired, many important questions remain unsolved. How does association with the steroid remove factors keeping the receptor protein in its native state, and how does binding of the transformed receptor to the response element in the promoter region enhance gene transcription? Once it has converted the receptor to the nuclear binding state, is there a further role for the steroid in modulating transcription? Still not entirely clear is the involvement of phosphorylation and/or dephosphorylation in hormone binding, receptor transformation, and transcriptional activation. Less vital to basic understanding but important in the overall picture is whether the native receptors for gonadal hormones are entirely confined to the nucleus or whether there is an intracellular distribution equilibrium. With the effort now being devoted to this field, and with the application of new experimental techniques, especially those of molecular biology, our understanding of receptor function is progressing rapidly. The precise mechanism of steroid hormone action should soon be completely established.

A model of antiglucocorticoid action for designing a potent glucocorticoid antagonist

We have previously shown that the biological efficacy of an antiglucocorticoid is directly related to its affinity for the glucocorticoid receptor in whole cells at 37 degrees C. We have also shown that RU 486-receptor complexes differ from other antiglucocorticoid-receptor complexes in so far as their affinity is as high at 37 degrees C in whole cells as at 0 degree C in a cell-free system, whereas a decrease by a factor of 5-10 is observed with the other antagonists. The aim of the present paper was to evaluate the contributions of temperature and cellular integrity (or the biological events linked to temperature and cellular integrity) to the affinity of a steroid for its receptor for the purpose of determining the parameters favorable to high affinity, which is the prerequisite of a potent antagonist. We provide evidence showing that: (1) an increase in temperature has an unfavorable effect on the affinity of a glucocorticoid for its receptor (4-6-fold decrease between 0 and 37 degrees C), (2) RU 486, like an agonist, forms a complex with the cytosolic glucocorticoid receptor, which satisfies the criteria for an "activated" complex under "in vitro activating treatment", (3) these biological post-binding events (either agonistic or otherwise nature), which change the nature of the complexes, contribute to compensating for the negative effect of rising temperatures on their apparent dissociation constant. We conclude that potent antiglucocorticoids must have a chemical structure allowing them to induce biological post-binding events, such as receptor activation, but in an abortive form which thus effectively "traps" the receptor in a non-functional state.

Type I receptors in parotid, colon, and pituitary are aldosterone selective in vivo

Previous in vivo studies have demonstrated that type I receptors in the rat kidney are aldosterone selective, whereas those in the hippocampus do not appear to discriminate between aldosterone and corticosterone. We have injected mature rats with [3H]aldosterone or [3H]corticosterone plus 100-fold excess of RU 28362, with or without unlabeled aldosterone or corticosterone, and compared type I receptor occupancy in two classic mineralocorticoid target tissues (parotid and colon) and in the pituitary. Mature rats were killed 10-180 min after tracer administration; [3H]aldosterone was well taken up and retained in all tissues, whereas [3H]corticosterone was significantly retained only in the pituitary 10 min after tracer administration. To assess a possible role for corticosterone-binding globulin (CBG) in conferring aldosterone specificity on type I receptors, 10-day-old rats (with very low levels of CBG) were similarly injected. In the colon and parotid, [3H]aldosterone binding was at least an order of magnitude higher than that of corticosterone; in the pituitary aldosterone binding was approximately three times that of corticosterone. We interpret these data as evidence that in the parotid and colon type I receptors are aldosterone selective by a non-CBG-requiring mechanism, whereas in the pituitary there appear to be both aldosterone-selective and nonselective type I sites.

Characterization of steroid receptors in the gut and kidney of the frog (Rana catesbeiana) and in the gut of the turtle (Chrysemys picta)

Paraglucocorticoid- and paramineralocorticoid-binding cytosolic receptors (pGR, pMR) were demonstrated in the intestine and kidney of the frog, Rana catesbeiana and in the intestine of the turtle, Chrysemys picta, in the presence of sodium molybdate. These receptors were of high affinity and low capacity with the following binding parameters: pGR:Kd:frog intestine (FI), triamcinolone acetonide (TA): 3.3 nM, corticosterone (B): 3.4 nM; frog kidney (FK), TA:4.3 nM, B: 9.3 nM; turtle intestine (TI), TA: 4.8 nM; Nmax: FI, TA: 357, B: 371; FK, TA: 301, B: 157; TI, TA: 350 fmol/mg protein. pMR:Kd: FI, aldosterone: 0.9 and 90 nM (biphasic curves); FK, aldosterone: 0.6 and 36 nM (biphasic curves); Nmax: FI, 13 and 147 fmol/mg protein; FK, 78 and 109 fmol/mg protein. The receptor had the following ligand affinities: pGR: FI and FK: triamcinolone acetonide greater than DOC greater than 11 beta-hydroxyprogesterone greater than progesterone greater than corticosterone greater than cortisol greater than aldosterone greater than 11-dehydrocorticosterone greater than 17 alpha-hydroxyprogesterone greater than cortisone; TI: triamcinolone acetonide greater than corticosterone greater than progesterone greater than DOC greater than cortisol greater than aldosterone; pMR: FI and FK: corticosterone greater than 11 beta-hydroxyprogesterone greater than aldosterone greater than triamcinoline acetonide = cortisol greater than DOC greater than 11-dehydrocorticosterone greater than progesterone greater than 17 alpha-hydroxyprogesterone greater than cortisone. Androgens, estrogens or 18-hydroxycorticosterone did not compete for binding in either tissue. The heat activated frog receptors did not bind to naked DNA, though the turtle receptor did. It was possible to show that cytosol receptor-ligand complexes from all tissues were bound by nuclear acceptor sites. On linear sucrose gradients, the FI TA-receptor complex sediments with a single peak (7.5S), the FK TA-receptor complex gave two peaks (8.0 and 4.4S) and the TI TA-receptor complex showed a single peak (9.0S). The hydrodynamic parameters of the pGR's were determined by gel exclusion on Sephacel S-300. The following results were obtained: Mr: FI, 265, 80, 40 kDa (multiple proteins); FK, 280, 60, 20 kDa (multiple proteins); TI, 366 kDa; Rs: FI, 6.9, 3.9 nm; FK, 6.9, 2.9 nm; TI, 7.6 nm; f/f0: FI, 1.6; FK, 1.6; TI, 1.6. It is suggested on the basis of the binding and hydrodynamic parameters that non-mammalian epithelia corticosterone receptors have undergone biochemical evolution from one class of vertebrates to another.

Mineralocorticoid specificity of renal type I receptors: in vivo binding studies

We have injected rats with [3H]aldosterone or [3H]corticosterone, plus 100-fold excess of the highly specific glucocorticoid RU 28362, with or without excess unlabeled aldosterone or corticosterone and compared type I receptor occupancy in kidney and hippocampus. Thirty minutes after subcutaneous injection [3H]aldosterone was well retained in renal papilla-inner medulla, renal cortex-outer medulla, and hippocampus; in contrast, [3H]corticosterone was well retained only in hippocampus. Competition studies for [3H]aldosterone binding sites showed corticosterone to be a poor competitor in the kidney compared with hippocampus. Time-course studies, with rats killed 10-180 min after tracer administration, showed very low uptake/retention of [3H]corticosterone by kidney; in hippocampus [3H]corticosterone retention was similar to that of [3H]aldosterone in kidney, and retention of [3H]aldosterone by hippocampus was much more prolonged than of either tracer in any other tissue. Studies in 10-day-old rats, with very low levels of corticosteroid binding globulin (CBG), showed a high degree of aldosterone selectivity in both zones of the kidney, whereas [3H]aldosterone and [3H]corticosterone were equivalently bound in hippocampus. We interpret these data as evidence for a mechanism unrelated to extravascular CBG conferring mineralocorticoid specificity on renal type I receptors and propose two models derived from our findings consistent with such differential selectivity.

Characterization of glucocorticoid receptors in whole and cellular subfractions of embryonic chick spinal cord

Steroid-specific binding sites for tritiated corticosterone have been localized, via autoradiography, to motoneurons in the lateral motor columns of the adult rat spinal cord. Binding sites in adult rat spinal cord have been characterized biochemically and shown to possess the properties of a putative glucocorticoid receptor. The presence of receptors for glucocorticoids in embryonic chick spinal cord was determined and their characterization undertaken as a prelude to the study of the functions under regulation by glucocorticoids during development. Assay conditions were defined and binding of [3H]dexamethasone [( 3H]Dex) to cytosols of 6- and 10-day embryonic spinal cord and cellular subfractions of 6-day spinal cord determined. Saturable, high-affinity binding of [3H]Dex to cytosols prepared from both whole 6- and 10-day spinal cords and cells of all 3 cellular subfractions of 6-day spinal cords was observed. The binding component in 10-day cytosols was (1) proteinaceous, as binding was eliminated by heating cytosols, and (2) a macromolecular species, as it displayed a sedimentation coefficient of 8S upon centrifugation in sucrose gradients. The putative receptor displayed binding specific for glucocorticoids in a competition assay, with the exception that some inhibition of binding by the androgen ligand, methyltrienolone (R1881) was observed. The binding affinity decreased as the values for KD increased from 3.4 +/- 0.9 nM on day 6 to 15.7 +/- 1.8 nM on day 10, while the values for Bmax increased from 270 to 855 fmol/mg protein over the same period.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of the synthesis of thymidine kinase by aldosterone in the kidney of the immature male rat

Thymidine kinase activity was studied in kidneys from immature male rats after administration of aldosterone. Kinetic studies showed that the enzyme activity reached its maximum level 24 h after aldosterone injection. That increase was specific for aldosterone and could be related to the synthesis of new molecules of enzyme.

A study of [3H]aldosterone binding by nuclear and cytoplasmic receptors of the rat kidney with different content of aldosterone in the organism

The distribution of specific mineralocorticoid receptors in rat kidney cells was found to depend on aldosterone concentration. With increasing aldosterone concentrations the number of specific receptors for aldosterone in the cytoplasm decreased and their quantity in the nuclei increased. This was evidently due to their enhanced transport from the cytoplasm to the nuclei. It was found that aldosterone (its complex with the receptor) attached to non-histone proteins of chromatin and that the structural integrity of DNA is needed to provide the binding of this complex to the cell nuclei. The quantity of aldosterone acceptor sites in the kidney cell nuclei remained constant when aldosterone concentrations in the organism changed, and, hence, the functional states of the target organ changed too.

Activation of cytosol aldosterone receptors in rat kidney

Cytosolic aldosterone-protein complexes are isolated from rat kidney slices after incubation with [3H]aldosterone and dexamethasone. Activated and unactivated forms of the complex are characterized by gel electrophoresis and hydroxyapatite chromatography after incubation at 4 degrees C and 25 degrees C respectively. It is found that the activated form reaches a maximum after 30 min at 25 degrees C and can be separated as an homogeneous peak by electrophoresis. Intermediate forms can also be identified. In the presence of 10 mM ATP, activation immediately occurs at 4 degrees C and is almost complete. In the presence of 10 mM molybdate, the activation is strongly enhanced and the increase in activated form may be about fifteen-fold whether molybdate is added during kidney homogenization or just before incubation at 25 degrees C. On the other hand molybdate reduces to one third the binding of the aldosterone-receptor complexes to nuclei. In the presence of the steroid RU 26988 which is a pure glucocorticoid, experiments done on aldosterone-receptors complexes and their binding to nuclei are confirmed. This proves that aldosterone is specific for mineralocorticoid sites. The general pattern of the mineralocorticoid receptor activation is discussed and its resemblance to the case of other steroid hormones is emphasized.

Autoradiographic study of nuclear localization of aldosterone binding sites in intact renal cells: lack of temperature dependency

We examined by autoradiography on intact target cells the nucleocytoplasmic distribution of aldosterone-receptor complexes and the thermodependency of the nuclear translocation process. Autoradiographs (dry films) were performed on cortical collecting tubules isolated by microdissection, after incubation of rabbit kidney pyramids with [3H]aldosterone (2 X 10(-9)M) in the presence or absence of an excess unlabelled aldosterone (X100). Paired experiments were done at 30 degrees C (1 h) or 4 degrees C (2 h). In both cases, the specific labelling was exclusively nuclear. Values were higher at 30 degrees C (14.5 +/- 1.5 specific silver grains per 100 micron2) than at 4 degrees C (5.4 +/- 0.6), with no concommitant cytoplasmic labelling (4 degrees C:0.7 +/- 0.3; 30 degrees C: -0.04 +/- 0.6). At 30 degrees C, addition of unlabelled spirolactone (X200) prevented the observed autoradiographic nuclear accumulation of aldosterone-receptor complexes, suggesting that cytoplasmic binding preceded the nuclear entry. The results suggest that, in intact cells, almost all aldosterone receptor complexes accumulate in nuclei, and that this process does not depend on temperature. In parallel biochemical-binding series we found the classical distribution of aldosterone receptor complexes both in cytoplasm and nuclei, and the classical thermodependency of nuclear translocation. The present autoradiographic results, together with similar observations reported for sex steroids (Martin P.M. and Sheridan P. J., J. steroid Biochem. 16 (1982) 215-229), question the classical model of thermodependent nuclear translocation, based on biochemical experiments.

Autoradiographic study of thermo-dependent nucleo-cytoplasmic distribution of aldosterone binding sites in intact target cells

Autoradiographic studies of [3H]aldosterone [( 3H-A] and [3H]dexamethasone binding sites in intact target cells (isolated collecting tubules of rabbit and rat kidney) revealed an almost exclusive nuclear localization of the hormone-receptor complexes. In the present work we compared the nucleo-cytoplasmic repartition of [3H]A-receptor complexes studied in parallel by biochemical and autoradiographic methods. In addition, the thermo-dependency of the nuclear translocation was examined. Kidney pyramids were incubated in vitro with [3H]A (2 X 10(-9) M) in the presence or absence of a 100-fold excess unlabelled A, at 30 degrees C for 1 h or 4 degrees C for 2 h. Then tissue was processed for isolation of nuclear and cytoplasmic fractions, on the one hand, or for obtention of microdissected tubular segments on which autoradiographs on dry films were performed. Autoradiographs showed that the specific labelling was almost exclusively nuclear without significant cytoplasmic labelling, at both 30 or 4 degrees C. This indicates that almost all binding sites migrated rapidly into nuclei, and that this translocation did not depend on temperature. In contrast, parallel biochemical experiments yielded classical results, that is, at 30 degrees C, the presence of specific binding sites in both cytoplasm and nuclei with a predominance in cytoplasm. At 4 degrees C, the cytoplasmic binding was unchanged, but nuclear binding was drastically reduced, indicating thermodependency of nuclear translocation, when studied by biochemical methods including cell disruption. Autoradiographic results thus questioned the classical notion of thermo-dependent nuclear translocation of aldosterone-receptor complexes, based on results obtained by biochemical methods.

Corticosteroid receptors in the kidney of chick embryo. I. Nature and properties of corticosterone receptor

Cytosol from kidney of chick embryo (age 16-18 days) contained a corticosterone-binding site with the features of a putative receptor. This receptor was a thermolabile protein, readily digested by proteolytic enzymes, with a sedimentation coefficient of 7-8 S and with an apparent molecular weight greater than 100,000. Simultaneous studies with transcortin (CBG) revealed several differences between the renal- and serum-binding protein pertaining to the effect of temperature, the sedimentation coefficient, the charcoal "stripping" and, finally, the binding and competition of various steroids for the two proteins. Kinetic analysis showed a rapid association (10 min), which followed second-order reaction kinetics, and a dissociation of pseudo-first-order reaction kinetics with a t1/2 of 168 min at 0 degrees. The analysis of the Scatchard plot showed the presence of a single class of binding sites with an association constant (KA) of 1.3 X 10(8)M-1 and a binding capacity (nmax) of 500-700 fmol/mg protein. We obtained similar results when we used dexamethasone as a ligand. The association (ka) and dissociation (kd) rate constants were respectively 2.9 X 10(6)M-1 sec-1 and 6.86 X 10(-5) sec-1. From their ratio a KA value of 4.2 X 10(10) M-1 was obtained. Studies with various steroids demonstrated that only dexamethasone and, to a lesser degree, progesterone competed for the binding site. These data showed that the kidney of chick embryo possessed one type of receptor for the glucocorticoids, which was similar to the type II described in rat kidney.

Kinetics of subcellular distribution in rat intestine of 1,25-dihydroxycholecalciferol administered in vivo. Evidence for concentration within 5 min into purified nuclei

To better understand the initial steps in the induction of intestinal Ca2+ transport by 1,25-dihydroxycholecalciferol [1,25(OH)2D3], we studied the early subcellular localization of 1,25(OH)2D3 in rat intestine. Vitamin D-deficient rats received 300 pmol of 1,25(OH)2[3H]D3 intravenously at 5 min to 4h before being killed. Cells homogenized in buffer of I = 90 mmol/litre were fractionated by centrifugation into a crude nuclear pellet, purified nuclei, Golgi and basal-lateral membranes, cytosol and a post-nuclear pellet. Nuclear purification was established by biochemical and morphological criteria and gave a yield of 32 +/- 2% (mean +/- S.E.M.; n = 21). Although re-establishment of Ca2+ uptake by Golgi is one of the earliest reported intestinal responses to 1,25(OH)2D3, no direct localization of 1,25(OH)2D3 to Golgi was detected. Purified nuclei had the highest specific radioactivity at all times studied, with nuclear localization detectable at 5 min and peak nuclear uptake at 1 h. Relative specific radioactivity of nuclei to cytosol increased from 5 min to 30 min, at which time equilibrium between cytosol and nucleus appeared to be attained. Nuclear uptake occurred in all cells from villus to crypt. Of total nuclear binding 10% was resistant to high ionic strength buffer (I = 365 mmol/litre); peak nuclear uptake was observed at 30 min in this buffer. This tight binding may represent the active fraction of 1,25(OH)2D3. These results indicate that localization of 1,25(OH)2D3 to rat intestinal nuclei precedes the observed Golgi-membrane effects and suggest the existence of high-affinity nuclear 1,25(OH)2D3-binding sites.

Molecular action of aldosterone

Aldosterone stimulates the reabsorption of sodium across epithelial cells of various target tissues. The initial events in the molecular action of the mineralocorticoid are the following: (1) Diffusion of the steroid across the cellular (baso-lateral, serosal) plasma membrane into the cytoplasmic compartment. (2) Binding of the steroid to a receptor protein specific for the class of steroid and activation of this steroid-receptor-complex. (3) Translocation of the activated aldosterone-receptor complex to the nucleus and stimulation of RNA synthesis (including the synthesis of messenger RNA and ribosomal RNA). (4) Translation of the steroid-induced messenger RNAs at the ribosomal level into the aldosterone-induced proteins (AIP) within the cytoplasmic compartment. Whereas these induction steps are uniformly accepted, the mechanisms by which the AIPs increase the activity of a rate-limiting step in the sodium transport process are still object of debate. In this paper we discuss the initial events in the mode of action of aldosterone and the biochemical and physiological approaches to the aldosterone-induced proteins with special reference to the "sodium permease", the "energy", and the "sodium pump" theory. Our analysis shows that despite serious efforts by multiple laboratories, the first AIP with an established relationship to the mineralocorticoid actions of aldosterone is yet to be identified.

Factors influencing association of glucocorticoid receptor-steroid complexes with nuclei, chromatin, and DNA: interpretation of binding data

Attempts to reconstruct, in a test tube, the steroid-hormone system of a responsive cell are fraught with enumerable difficulties. In this chapter I have attempted to point out some of the factors that affect receptor-steroid complexes and their interactions with acceptors. In most cases there is a quantitative influence of these factors on the level of steroid complex binding to acceptors. In some cases, selected experimental designs that neglect these factors and methods of presenting the observed data may lead to artifactual conclusions. Several of these problems should disappear when the prospect of pure receptor-steroid complexes [127, 147, 150, 181, 247, 248] becomes a common occurrence. Nevertheless much has already been learned about the interactions of complexes with acceptors, which in turn have been used to help formulate models of steroid-hormone action.

Binding of aldosterone to cytoplasmic and nuclear receptors of the urinary bladder epithelium of Bufo marinus

Binding of aldosterone to cytoplasmic and nuclear sites in urinary bladder epithelia of Bufo marinus (Dominican variant) is saturable and dependent upon steroid concentration. Scatchard analysis of specific cytoplasmic binding yielded a maximal binding capacity (N) of 14.5 X 10(-14) mol/mg protein and an apparent equilibrium dissociation constant (Kd) of 1.4 X 10(-8) M. Since Scatchard analysis of specific nuclear binding was complex, this binding was resolved by a computer-generated cirve-fitting technique which analyzed total aldosterone bound. Nuclear binding was resolved into three sites: a nonsaturable site that was linearly dependent upon aldosterone concentration, and two saturable sites (types I and II). Type I sites had relatively low capacity for aldosterone (N = 31 +/- 1 X 10(-14) mol/mg DNA) and high affinity (Kd = 2.5 +/- 0.5 X 10(-9 M); tffininty (Kd = 8.6 +/- 1.7 X 10(-7) M). Competition for [3H]aldosterone binding by dexamethasone, corticosterone, cortisol, progesterone, testosterone, and 17 beta-estradiol demonstrated that type I nuclear sites have higher affinity for aldosterone than for other steroids. The findings are consistent with the inference that the type I site is the mineralocorticoid receptor.

Aldosterone receptor occupancy and sodium transport in the urinary bladder of Bufo marinus

The concentration dependence of binding of [3H]aldosterone to cytoplasmic and nuclear receptors was evaluated in urinary bladder epithelial cells of Colombian toads. One class of specific sites (sensitive to displacement by excess aldosterone) was detected in the cytosol. However, two classes of specific nuclear [3H]aldosterone binding sites were evident. In the nucleus, high-affinity (Kd = 2.7 X 10(-9) M), low-capacity (N = 15 X 10(-14) mol/mg DNA) sites (type I) were completely saturated at approximately 4 X 10(-8) M aldosterone, a concentration which gave a maximal increase in short-circuit current (SCC). Occupancy of low-affinity (Kd = 4.6 X 10(-7) M), high-capacity sites (N = 150 X 10(-14) mol/mg DNA) (type II) occurred at higher steroid concentrations and did not correlate with further increase in SCC. Binding parameters of Colombian and Dominican variants of Bufo marinus were compared. In both variants, the SCC increase elicited by aldosterone correlated with accumulation of type I complexes in the nucleus, but the relationship was markedly nonlinear. Various alternatives were considered as the basis for a curvilinear dependence of the increment in Na+ transport on abundance of nuclear type I complexes.

Effect of aldosterone on incorporation of amino acids into renal medullary proteins

Studies on the effects of pretreatment with aldosterone on the incorporation of 3H leucine or 3H methionine into proteins in renal slices were carried out in Joklik-modified minimal essential medium. Administration of aldosterone (2microgram/100 g body wt) to adrenalectomized rats increased 3H leucine incorporation into trichloroacetic acid insoluble fractions of crude homogenates of cortical slices by 15.5 +/- 0.4% and of medullary slices by 53.5 +/- 1.3%. No increase in isotope incorporation was observed in slices of renal papilla or spleen prepared from the same rats. Aldosterone had no effect on the 3H-leucine content of the trichloroacetic acid-soluble fractions of all three renal zones and the spleen. The dose of aldosterone that elicited a half-maximal increase in 3H-methionine incorporation into proteins of renal medullary slices (0.45 microgram of aldosterone/100 g body wt) was indistinguishable from that needed to elicit a halt-maximal increase in the urinary K+/Na+ ratio (0.35 microgram of aldosterone/100 g body wt). Dexamethasone, a potent glucocorticoid, at a dose of 0.8 microgram/100 g body wt did not augment 3H-leucine incorporation into renal medullary proteins but was effective at 8 microgram/100 g body wt. Spirolactone (SC-26304), a potent anti-mineralocorticoid, abolished the effect of aldosterone on amino acid incorporation into medullary proteins when administered at a 100-fold higher dosage [i.e., 80 microgram (per 100 g body wt)]. These results imply that the action of aldosterone on amino acid incorporation is mediated by the mineralocorticoid rather than the glucocorticoid pathway, presumably the mineralocorticoid receptors. Moreover, pretreatment of the rats with actinomycin D (70--80 microgram/100 g body wt) erased the effect of aldosterone (0.8 microgram/100 g body wt) on amino acid incorporation into medullary proteins. In paired experiments with 3H and 35S methionine, aldosterone (0.8 microgram/100 g body wt) increased methionine incorporation into trichloroacetic acid precipitable proteins of subcellular fractions of the renal medulla. The effect of aldosterone on incorporation of methionine into medullary cytosol proteins was analyzed further by polyacrylamide gel electrophoresis at pH 8.3 in tris-glycine buffer. The gel profiles indicate that aldosterone significantly increased methionine incorporation into at least one protein (independent of the isotope) with a molecular weight of approximately 31,000. This increase was inhibited by either pretreatment of the rat with actinomycin D (70--80 microgram/100 g body wt or SC-26304 (80 microgram/100 g body wt). Dexamethasone (0.8 microgram/100 g body wt) did not increase incorporation of methinine into the medullary cytosol proteins resolved by polyacrylamide gel electrophoresis.

Interaction of glucocorticoid receptors from lymphoid cell lines with their nuclear acceptor sites

Procedures have been developed which provide simple means of determining binding constants of steroid receptors for glucocorticoids in mouse lymphoid cell lines and of characterizing the interaction of the steroid--receptor complex with the nucleus. An average of 70% of the steroid--receptor complexes is found associated with the nuclear fraction in three investigated cell lines, whereas 30% of the steroid--receptor complexes is found in the cytosol fraction. This distribution of the steroid-receptor complex within the cell is independent of whether steroid uptake of the cells is performed at low or at high steroid concentration. Part of the binding of the steroid receptor to the nuclear fraction is sensitive to high ionic strength and to high pH. A larger fraction of the steroid--receptor complex binding to the nuclear fraction is insensitive to high ionic strength and pH when the steroid uptake is performed at low steroid concentrations than when performed at high steroid concentrations. Steroid--receptor complex is released from the nuclear fraction by DNAase treatment but not by RNAase treatment. The possible correlation between the sensitivity to ionic strength and pH and the specificity of the binding is discussed.