Cortisol 17 beta acid, transcortin, and the heterogeneity of rat brain glucocorticoid receptors

Corticosteroid receptors and 11beta-hydroxysteroid dehydrogenase isoforms in rat intestinal epithelia

To evaluate the potential roles that both receptors and enzymes play in corticosteroid regulation of intestinal function, we have determined glucocorticoid receptor (GR), mineralocorticoid receptor (MR), and 11beta-hydroxysteroid dehydrogenase (11beta-HSD) expression in intestinal epithelial cells. GR and MR mRNA and receptor binding were ubiquitously expressed in epithelial cells, with receptor levels higher in ileum and colon than jejunum and duodenum. RNase protection analysis showed that 11beta-HSD1 was not expressed in intestinal epithelial cells, and enzyme activity studies detected no 11-reductase activity. 11beta-HSD2 mRNA and protein were demonstrated in ileal and colonic epithelia; both MR and GR binding increased when enzyme activity was inhibited with carbenoxolone. Duodenal and jejunal epithelial cells showed very little 11beta-HSD2 mRNA and undetectable 11beta-HSD2 protein; despite minor (<7%) dehydrogenase activity in these cells, enzyme activity did not alter binding of corticosterone to either MR or GR. These findings demonstrate the ubiquitous but differential expression of MR and GR in intestinal epithelia and that 11beta-HSD2 modulates corticosteroid binding to both MR and GR in ileum and proximal and distal colon but not in duodenum or jejunum.

Decreased apparent affinity of corticosterone for colonic crypt glucocorticoid receptors is dependent on the cellular milieu and is distinct from corticosterone metabolism

[3H]Steroid binding in intact colonic crypt cells and cytosol was compared to determine if 11beta hydroxysteroid dehydrogenase (11betaHSD) activity modulates access of corticosterone (B) to both glucocorticoid receptors (GR) and mineralocorticoid receptors (MR). Cytosol from non-adrenalectomized rat colonic crypt cells showed no 11betaHSD activity, and B bound with high affinity to both MR (Kd=0.47+/-0.03 nM; Bmax=177+/-34 fmol/mg protein) and GR (Kd=4.5+/-0.3 nM; Bmax=279+/-40 fmol/mg protein). In contrast, intact colonic crypt cells incubated with 25-30 nM [3H]B for 90 min converted 62% of B to 11-dehydrocorticosterone, with little binding to MR (14+/-3 fmol/mg protein) and GR (22+/-5 fmol/mg protein). When 11betaHSD activity was inhibited with carbenoxolone, and the same concentration of [3H]B used, binding of [3H]B to MR increased 10-fold to 122+/-12 fmol/mg protein, not significantly different from MR levels in colonic crypt cytosol. In contrast, [3H]B binding to GR in intact cells increased only 1.6-fold to 36+/-9 fmol/mg protein, significantly less than to GR in cytosol (212+/-24 fmol/mg protein). Scatchard analysis showed both lower levels of GR and an apparently lower affinity for [3H]B in colonic crypt cells (Kd=31+/-3 nM; Bmax=130+/-21 fmol/mg protein) compared with cytosol (Kd=4.5+/-0.3 nM; Bmax=279+/-40 fmol/mg protein. [3H]Dexamethasone similarly showed an apparently lower affinity and capacity for GR (Kd=8.8+/-1.3 nM; Bmax=232+/-32 fmol/mg protein) in intact cells compared with cytosol (two separate determinations, Kd=2.6 and 2.9 nM; Bmax=369 and 300 fmol/mg protein). In contrast, [3H]aldosterone displayed similar affinity and capacity for MR in both intact cells (Kd=2.0 nM; Bmax=121 fmol/mg protein) and cytosol (Kd=1.5 and 1.4nM; Bmax=115 and 93 fmol/mg protein). These findings demonstrate not only that 11betaHSD modulates binding to both MR and GR in colonic crypt cells, but also that an additional mechanism(s) operating in whole cells but not in cytosol selectively reduces the affinity and capacity of colonic GR for both natural and synthetic ligands.

Differential regulation of adrenocorticoid receptors in the hippocampus and spinal cord of adrenalectomized rats

Using multiple polymerase chain reaction assay and cytosolic receptor binding assay we studied type I, mineralocorticoid receptor (MR), and type II, glucocorticoid receptor (GR), adrenocorticoid receptors expression in rat hippocampus and spinal cord, at various times after adrenalectomy: 12 hr, 24 hr, 3 days, and 1 week. Analysis of the data demonstrates that in hippocampus the expression of MR and GR mRNA was not significantly affected by adrenalectomy. On the contrary, Bmax of MR was significantly increased at each time post-surgery, with only slight modifications of Kd. Bmax and Kd for GR showed a significant increase after 3 days and 1 week. In the spinal cord, MR mRNA was increased 12 hr after adrenalectomy, reaching a maximum at 3 days. Bmax of MR was also significantly increased after 3 days, whereas its Kd remained unchanged for the entire duration of the the study. Both GR mRNA and binding parameters were poorly affected by adrenalectomy. The results of the present experiments demonstrate that the absence of adrenocortical hormones influences differentially MR and GR expression in hippocampus and spinal cord, suggesting the existence of various and independent mechanisms of regulation of adrenocorticoid receptor.

Corticosterone is a preferable ligand for measuring rat brain corticosteroid receptors: competition by RU 28362 and RU 26752 for dexamethasone binding in rat hippocampal cytosol

It is unclear whether in vitro corticosteroid receptor binding assays have used inappropriately high concentrations of synthetic corticosteroid competitors, thereby potentially introducing error into estimates of type I (mineralocorticoid) and type II (glucocorticoid) receptor binding. To determine more accurately the concentration of blockers necessary to discriminate between these two sites, we have derived Ki values for the competition of dexamethasone, RU 28362 and RU 26752 for [3H]corticosterone and [3H]dexamethasone binding in rat hippocampus. Non-specific binding of both radioligands was defined with unlabeled dexamethasone to exclude transcortin. The type II agonist RU 28362 competed for only a portion of [3H]corticosterone binding, exhibiting a Ki of 0.5 nM for this binding. In contrast, RU 28362 fully competed all binding of a saturating concentration of [3H]dexamethasone, even though [3H]dexamethasone also recognized type I receptors, defined as specific [3H]corticosterone binding in the presence of 80 nM RU 28362. RU 28362 competition for [3H]dexamethasone binding exhibited characteristics of a 2-site interaction, with Kis of 0.3 and 194 nM. The type I receptor antagonist RU 26752 competed less effectively for [3H]corticosterone and [3H]dexamethasone binding, but nonetheless competed fully within a 1000-fold concentration range. Even at a level less than 125 x its Ki for type I binding, RU 26752 still inhibited virtually all type II receptor binding by [3H]corticosterone. We conclude that type I and II receptors in rat brain are best distinguished using [3H]corticosterone as the labelling ligand, with cold RU 28362 and dexamethasone to eliminate binding to type II and transcortin sites, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Heterogeneity and properties of transformation of corticosteroid receptors in spinal cord and hippocampus

The central nervous system contains two classes of corticoid receptors, named types I and II following terminology accepted for the kidney. Phenotypically, type I sites are differentiated into a corticosterone (CORT)-preferring species (Ia) and a mineralocorticoid receptor (Ib). These populations were tentatively compared in the spinal cord and hippocampus. Using [3H]dexamethasone (DEX) and selective blockage of sites, we have observed that type II receptors were comparable in both tissues, while Ia was almost exclusive of the hippocampus. Saturation analysis using [3H]DEX demonstrated that type Ia was a low affinity receptor (Kd approximately equal to 2-5 nM) while type II was a higher affinity site (KdII less than KdI). Using [3H]CORT, or [3H]aldosterone (ALDO), as ligand, preferential labeling of type I sites was achieved, always showing higher concentrations in the hippocampus. Therefore, [3H]DEX seems a ligand of choice to visualize types Ia and II receptors. Another difference noted between the spinal cord and hippocampus, pertained to the sensitivity towards the enzyme RNAse A, which increases heat-induced transformation of the bound receptor, according to the results of DNA cellulose affinity chromatography. In these experiments, type I sites of both spinal cord and hippocampus, plus type II of hippocampus, showed sensitivity toward the enzyme, whereas type II of the spinal cord was refractory to RNAse A enhancement of transformation. These results indicate that the dynamics of transformation is different among receptors showing similar affinity and competition, suggesting further heterogeneity due to receptors themselves, or to tissue factors regulating their biochemical properties.

Investigation of the corticosteroid receptor system in rat hippocampus by ion exchange fast protein liquid chromatography

In order to study the receptor system for adrenocortical steroids, hippocampal cytosolic preparations--containing both type I and type II receptors--were subjected to anion exchange fast protein liquid chromatography (FPLC). With running buffer containing Tris, EDTA, and glycerol three peaks (1-3) were eluted from the column at 220, 400 and 560 mM NaCl respectively regardless of whether [3H]corticosterone or [3H]RU 28362 had been used as radiotracer. None of the peaks was caused by serum transcortin as revealed by control studies. However, the sequestering influence of transcortin on receptor binding of corticosterone could be demonstrated by the FPLC technique with mixtures containing serum and hippocampus cytosol. Competition experiments with cytosolic samples revealed that type I receptor was present only in peaks 2 and 3 while type II was found in all three peaks in variable amounts, depending on the presence of molybdate. When molybdate was added to the running buffer only two peaks (2 and 3) were eluted, both containing type I and type II receptors. Peak 1 was attributed to the activated type II receptor while peak 2 represented nonactivated receptors. The origin of peak 3 remains uncertain. The data indicate that molybdate must be present in the cytosolic preparation and in the running buffer to keep type II receptor in its nonactivated form. Type I receptor was probably not transformed into the activated form in the absence of molybdate but lost binding capacity and/or affinity for corticosterone.

Equivalent affinity of aldosterone and corticosterone for type I receptors in kidney and hippocampus: direct binding studies

In studies from several laboratories evidence has been adduced that renal Type I (mineralocorticoid) receptors and hippocampal "corticosterone-preferring" high affinity glucocorticoid receptors have similar high affinity for both aldosterone and corticosterone. In all these studies the evidence for renal mineralocorticoid receptors is indirect, inasmuch as the high concentrations of transcortin (CBG) in renal cytosol make studies with [3H]corticosterone as a probe difficult to interpret, given its high affinity for CBG. We here report direct binding studies, with [3H]aldosterone and [3H]corticosterone as probes, on hippocampal and renal cytosols from adrenalectomized rats, in which tracer was excluded from Type II dexamethasone binding glucocorticoid receptors with excess RU26988, and from CBG by excess cortisol 17 beta acid. In addition, we have compared the binding of [3H]aldosterone and [3H]corticosterone in renal cytosols from 10-day old rats, in which CBG levels in plasma and kidney are extremely low. Under conditions where neither tracer binds to type II sites or CBG, they label an equal number of sites (kidney 30-50 fmol/mg protein, hippocampus approximately 200 fmol/mg protein) with equal, high affinity (Kd 4 degrees C 0.3-0.5 nM). Thus direct tracer binding studies support the identity of renal Type I mineralocorticoid receptors and hippocampal Type I (high affinity, corticosterone preferring) glucocorticoid receptors.

In vivo competitive autoradiographic study of [3H]corticosterone and [3H]aldosterone binding sites within mouse brain hippocampus

The binding sites for [3H]corticosterone (3HB) and [3H]aldosterone (3HA) within the hippocampal area of the mouse brain have been studied by autoradiography in competition experiments. Excess unlabelled aldosterone (A) or corticosterone (B) both abolished the nuclear accumulation of radioactivity within neurons observed after injection of either 3HA or 3HB. Experiments where a subcutaneous injection of a "pure glucocorticoid' RU26988 was given before injection of 3HA alone showed a marked accumulation of radioactivity within neuronal nuclei of the hippocampus suggesting the presence of 3HA binding sites distinct from classical type II glucocorticoid receptors. In addition, when RU26988 was given before the injection of 3HA associated with a 30- or 100-fold excess of either A or B, the cell nuclear accumulation of radioactivity was no longer observed. These results showed that in our in vivo experimental conditions, B displayed the same ability as A to occupy 3HA binding sites, supporting the view that in mouse hippocampal neuronal nuclei, the aldosterone-binding and corticosterone-preferring sites represent the same molecular entity.