Glucocorticoid receptors

Pharmacological Basis of Differences in Dose Response, Dose Equivalence, and Duration of Action of Inhaled Corticosteroids

INTRODUCTION

Asthma treatment guidelines classify inhaled corticosteroid (ICS) regimens as low, medium, or high dose. However, efficacy and safety are not independently assessed accordingly. Moreover, differences in ICS duration of action are not considered when a dose regimen is selected. We investigated the efficacy and safety implications of these limitations for available ICS molecules.

METHODS

Published pharmacodynamic and pharmacokinetic parameters were used, alongside physiological and pharmacological principles, to estimate the efficacy and safety of available ICS molecules. Extent and duration of glucocorticoid receptor (GR) occupancy in the lung (efficacy) and cortisol suppression (systemic exposure and safety) were estimated.

RESULTS

Some ICS regimens (e.g., fluticasone furoate, fluticasone propionate, and ciclesonide) rank high for efficacy but low for systemic exposure, contrary to how ICS dose equivalence is currently viewed. Differences in dose-response relationships for efficacy and systemic exposure were unique for each ICS regimen and reflected in their therapeutic indices. Notably, even low doses of most ICSs can generate high GR occupancy (≥ 90%) across the entire dose interval at steady state, which may explain previously reported difficulties in obtaining dose responses within the clinical dose range and observations that most clinical benefit typically occurs at low doses. The estimated post dose duration of lung GR occupancy for ICS molecules was categorized as 4-6 h (short), 14-16 h (medium), 25-40 h (long), or > 80 h (ultra-long), suggesting potentially large differences in anti-inflammatory duration of action.

CONCLUSION

In a real-world clinical setting where there may be poor adherence to prescribed therapy, our findings suggest a significant therapeutic advantage for longer-acting ICS molecules in patients with asthma.

A Model of Glucocorticoid Receptor Interaction With Coregulators Predicts Transcriptional Regulation of Target Genes

Regulatory factors that control gene transcription in multicellular organisms are assembled in multicomponent complexes by combinatorial interactions. In this context, nuclear receptors provide well-characterized and physiologically relevant systems to study ligand-induced transcription resulting from the integration of cellular and genomic information in a cell- and gene-specific manner. Here, we developed a mathematical model describing the interactions between the glucocorticoid receptor (GR) and other components of a multifactorial regulatory complex controlling the transcription of GR-target genes, such as coregulator peptides. We support the validity of the model in relation to gene-specific GR transactivation with gene transcription data from A549 cells and in vitro real time quantification of coregulator-GR interactions. The model accurately describes and helps to interpret ligand-specific and gene-specific transcriptional regulation by the GR. The comprehensive character of the model allows future insight into the function and relative contribution of the molecular species proposed in ligand- and gene-specific transcriptional regulation.

Nuclear receptors in transgenerational epigenetic inheritance

Nuclear Receptors are ligand-activated transcription factors that translate information about the lipid environment into specific genetic programs, a property that renders them good candidates to be mediators of rapid adaptation changes of a species. Lipid-based morphogens, endocrine hormones, fatty acids and xenobiotics might act through this class of transcription factors making them regulators able to fine-tune physiological processes. Here we review the basic concepts and current knowledge on the process whereby small molecules act through nuclear receptors and contribute to transgenerational changes. Several molecules shown to cause transgenerational changes like phthalates, BPA, nicotine, tributylin bind and activate nuclear receptors like ERs, androgen receptors, glucocorticoid receptors or PPARγ. A specific subset of observations involving nuclear receptors has focused on the effects of environmental stress or maternal behaviour on the development of transgenerational traits. While these effects do not involve environmental ligands, they change the expression levels of Estrogen and glucocorticoid receptors of the second generation and consequently initiate an altered genetic program in the second generation. In this review we summarize the available literature about the role of nuclear receptors in transgenerational inheritance.

Chronic stress alters concentrations of corticosterone receptors in a tissue-specific manner in wild house sparrows (Passer domesticus)

The physiological stress response results in release of glucocorticoid hormones such as corticosterone (CORT). Whereas short-term activation of this response helps animals cope with environmental stressors, chronic activation can result in negative effects including metabolic dysregulation and reproductive failure. However, there is no consensus hormonal profile of a chronically-stressed animal, suggesting researchers may need to look beyond hormone titers to interpret the impacts of chronic stress. In this study, we brought wild house sparrows (Passer domesticus) into captivity. We then compared glucocorticoid and mineralocorticoid receptor concentrations in sparrows exposed either to a standardized chronic stress protocol (n=26) or to standard husbandry conditions (controls; n=20). We used radioligand binding assays to quantify receptors in whole brain, liver, kidneys, spleen, gonads, gastrocnemius and pectoralis muscle, omental and subcutaneous fat, and bib and back skin. In most tissues, CORT receptors did not differ between controls and stressed animals, although we found marginal increases in receptor density in kidney and testis in stressed birds at some time points. Only in pectoralis muscle was there a robust effect of chronic stress, with both receptor types higher in stressed animals. Increased pectoralis sensitivity to CORT with chronic stress may be part of the underlying mechanism for muscle wasting in animals administered exogenous CORT. Furthermore, the change in pectoralis was not paralleled by gastrocnemius receptors. This difference may help explain previous reports of a greater effect of CORT on pectoralis than on other muscle types, and indicate that birds use this muscle as a protein reserve.

Pharmacological characterization of intracellular glucocorticoid receptors in nine tissues from house sparrow (Passer domesticus)

Glucocorticoid hormones play a key role in the stress response, but plasma concentrations vary based on physiological, environmental, or social parameters. However, hormone titers alone do not determine organismal response. To enhance our understanding of glucocorticoid actions we can examine 'downstream' factors in the organismal stress response, measuring glucocorticoid receptors across target tissues. Here, we characterized intracellular binding sites for CORT (corticosterone, the avian glucocorticoid) in house sparrow (Passer domesticus) brain, liver, skeletal muscle, spleen, fat, testes, ovary, kidney and skin. We used radioligand binding assays to identify total capacity, relative density and affinity for CORT of intracellular receptors in each tissue. Most evidence supported two binding sites similar to mammalian low-affinity glucocorticoid receptor (GR) and a high-affinity mineralocorticoid receptor (MR) for brain, liver, kidney and testes, and only a GR-like receptor for muscle, spleen, fat, ovary and skin. However, kidney data were somewhat more complicated, possibly hinting at a mineralocorticoid function for CORT and/or GR in birds. In all tissues, GR and MR affinities were close to published house sparrow values (K(d)~6 nM for GR, and ~0.2 nM for MR). Taken together, these data show that CORT receptor distribution appears to be as widespread in birds as it is in mammals, and suggest that independent regulation of peripheral receptors in different target tissues may play a role in CORT's diverse physiological effects.

The road less traveled: new views of steroid receptor action from the path of dose-response curves

Conventional studies of steroid hormone action proceed via quantitation of the maximal activity for gene induction at saturating concentrations of agonist steroid (i.e., A(max)). Less frequently analyzed parameters of receptor-mediated gene expression are EC(50) and PAA. The EC(50) is the concentration of steroid required for half-maximal agonist activity and is readily determined from the dose-response curve. The PAA is the partial agonist activity of an antagonist steroid, expressed as percent of A(max) under the same conditions. Recent results demonstrate that new and otherwise inaccessible mechanistic information is obtained when the EC(50) and/or PAA are examined in addition to the A(max). Specifically, A(max), EC(50), and PAA can be independently regulated, which suggests that novel pathways and factors may preferentially modify the EC(50) and/or PAA with little effect on A(max). Other approaches indicate that the activity of receptor-bound factors can be altered without changing the binding of factors to receptor. Finally, a new theoretical model of steroid hormone action not only permits a mechanistically based definition of factor activity but also allows the positioning of when a factor acts, as opposed to binds, relative to a kinetically defined step. These advances illustrate some of the benefits of expanding the mechanistic studies of steroid hormone action to routinely include EC(50) and PAA.

Cell signaling: what is the signal and what information does it carry?

This paper reviews key findings from quantitative study of the yeast pheromone response system. Most come from single cell experiments that quantify molecular events the system uses to operate. After induction, signal propagation is relatively slow; peak activity takes minutes to reach the nucleus. At each measurement point along the transmission chain, signal rises, overshoots, peaks, and declines toward steady state. At at least one measurement point, this decline depends on negative feedback. The system senses and relays percent receptor occupancy, and one effect of the feedback is to maximize precision of this transmitted information. Over time, the system constantly adjusts quantitative behaviors to convey extracellular ligand concentration faithfully. These behaviors and mechanisms that control them are likely to be general for metazoan signaling systems.

Negative feedback that improves information transmission in yeast signalling

Haploid Saccharomyces cerevisiae yeast cells use a prototypic cell signalling system to transmit information about the extracellular concentration of mating pheromone secreted by potential mating partners. The ability of cells to respond distinguishably to different pheromone concentrations depends on how much information about pheromone concentration the system can transmit. Here we show that the mitogen-activated protein kinase Fus3 mediates fast-acting negative feedback that adjusts the dose response of the downstream system response to match the dose response of receptor-ligand binding. This 'dose-response alignment', defined by a linear relationship between receptor occupancy and downstream response, can improve the fidelity of information transmission by making downstream responses corresponding to different receptor occupancies more distinguishable and reducing amplification of stochastic noise during signal transmission. We also show that one target of the feedback is a previously uncharacterized signal-promoting function of the regulator of G-protein signalling protein Sst2. Our work suggests that negative feedback is a general mechanism used in signalling systems to align dose responses and thereby increase the fidelity of information transmission.

Specific activation of the glucocorticoid receptor and modulation of signal transduction pathways in human lens epithelial cells

PURPOSE

Prolonged use of glucocorticoids (GCs) can lead to cataract formation. Lens GC responses have been difficult to elucidate. A previous study showed the presence of the glucocorticoid receptor (GR) in immortalized and primary human lens epithelial cells (hLECs) and GC-induced changes in gene expression. This study demonstrates specific GR activation and identifies the biological effect of GC-induced changes in gene expression in hLECs.

METHODS

HLE B-3 (B-3) and primary cultures of hLECs were transfected with pGRE.Luc and treated with or without dexamethasone (Dex), RU-486, spironolactone, or vehicle. mRNA and protein expression were examined by real-time PCR and Western blot analysis, respectively. Cell proliferation and apoptosis were examined by WST-1 and flow cytometry, respectively.

RESULTS

Dex treatment of B-3 and primary cultures demonstrated specific GR, but not mineralocorticoid receptor (MR), activation and phosphorylation. Pathway analysis revealed GC-induced changes in expression of MAPK regulators. Increased expression of GILZ mRNA and MKP-1 mRNA and protein was observed in immortalized and donor hLECs. This corresponded with a decrease in the phosphorylated forms of RAF, ERK, p38, and AKT, but not in JNK. No net change in LEC proliferation or apoptosis was observed with Dex treatment.

CONCLUSIONS

GC treatment of hLECs activates the GR to modulate the expression of MAPK and PI3K/AKT regulators. This is the first demonstration of GC signaling in hLECs. GCs, MAPK, and PI3K/AKT are involved in cell processes implicated in steroid-induced cataractogenesis. The absence of a net change in cell activity with acute steroid treatment is consistent with the possibility that chronic treatment leads to prolonged modulation of these pathways and steroid-induced cataract.

Fast glucocorticoid actions on brain: back to the future

Rapid, non-transcriptionally mediated, effects of glucocorticoids affect many behaviors as well as inhibition of function in the hypothalamo-pituitary-adrenal axis. In this short review, it is argued that the fast glucocorticoid actions which are mediated by membrane receptors are an ancient type of sterol/steroid-mediated effect, and that these may be the primordial glucocorticoid receptors. Although the fast feedback actions of the glucocorticoids enjoyed study in the middle of the last century, new results and the availability of new techniques suggest that it is again time for a concerted effort to be made to understand the mechanism(s) of these rapid effects.

Heterogeneous nuclear ribonuclear protein U associates with YAP and regulates its co-activation of Bax transcription

Although initially described as a cytosolic scaffolding protein, YAP (Yes-associated protein of 65 kDa) is known to associate with multiple transcription factors in the nucleus. Using affinity chromatography and mass spectrometry, we show that YAP interacts with heterogeneous nuclear ribonuclear protein U (hnRNP U), an RNA- and DNA-binding protein enriched in the nuclear matrix that also plays a role in the regulation of gene expression. hnRNP U interacts specifically with the proline-rich amino terminus of YAP, a region of YAP that is not found in the related protein TAZ. Although hnRNP U and YAP localize to both the nucleus and the cytoplasm, YAP does not translocate to the nucleus in an hnRNP U-dependent manner. Furthermore, hnRNP U and YAP only interact in the nucleus, suggesting that the association between the two proteins is regulated. Co-expression of hnRNP U attenuates the ability of YAP to increase the activity of a p73-driven Bax-luciferase reporter plasmid. In contrast, hnRNP U has no effect when co-expressed with a truncated YAP protein lacking the hnRNP U-binding site. Because YAP is distinguished from the homologue TAZ by its proline-rich amino terminus, the YAP-hnRNP U interaction may uniquely regulate the nuclear function(s) of YAP. The YAP-hnRNP U interaction provides another mechanism of YAP transcriptional regulation.

Effect of bilateral adrenalectomy on VIP receptor/effector system in rat intestinal epithelial cells

The number of vasoactive intestinal peptide (VIP) receptors and the efficiency of VIP in the stimulation of cyclic AMP accumulation in rat jejunal epithelial cells increased after bilateral adrenalectomy. However, this condition increased neither receptor affinity nor VIP potency. In addition, jejunal VIP levels followed a parallel increase. These changes reversed to control conditions after glucocorticoid replacement with dexamethasone indicating that adrenalectomy modifies the intestinal VIP receptor/effector system and suggest a relationship between corticosteroids and VIP in the functions of intestinal epithelium.

Influence of dexamethasone on nocturnal melatonin production in healthy adult subjects

There is no conclusive evidence supporting an interaction between the pineal gland and the hypothalamic-pituitary-adrenal axis. In this study, 11 healthy adults (six women, five men; aged 18-47 years) received a placebo the first night and 1 mg dexamethasone the next night at either 1800 or 2300 h. Administration of 1 mg of dexamethasone was followed by an attenuation of the nocturnal production of melatonin in 9 of 11 subjects. A significant reduction was found between melatonin plasma levels before and after dexamethasone at 0400 h (P less than 0.01, t test for dependent groups). It is suggested that dexamethasone affects nocturnal production of melatonin by means of mechanisms within the pineal gland.

Glucocorticoid receptors in murine erythroleukaemic cells

Glucocorticoid receptors in murine erythroleukaemic cells were studied in relation to hexamethylene bisacetamide (HMBA) induced differentiation. Specific binding of dexamethasone was measured. A single class of saturable, high affinity binding sites was demonstrated in intact cells; with cell homogenates or fractions binding was low and could not be reliably quantified. Receptor binding in whole cell suspensions was lower in cells which had been treated with HMBA (36.5 +/- 8.2 pmol/g protein) than in untreated controls (87.9 +/- 23.6 pmol/g protein); dissociation constants were similar in treated (2.7 nM) and untreated cells (2.5 nM). Dexamethasone, hydrocortisone, corticosterone and progesterone competed with tritium-labelled dexamethasone for receptor binding sites; cortisone, deoxycorticosterone and oestradiol had little effect.

Receptor binding of corticosterone in some rat brain structures following neonatal blockade of the hypophyseoadrenal system

Administration of hydrocortisone to rats during the first five postnatal days leads to the blockade of the hypophyseoadrenal system and results in a decrease in the number of corticosterone receptors in the hypophysis, hypothalamus, and hippocampus. Such a decrease in the receptor binding of corticosterone in the brain structures involved in the regulation of the hypophyseoadrenal system by a feedback mechanism is due to a change in the number of true glucocorticoid receptors.

Mechanisms of glucocorticoid hormone action

This report summarizes our studies, in context with the results of other laboratories, of the molecular mechanisms of glucocorticoid hormone action. The receptors for these steroids are comprised of single polypeptide chains of about 90,000 molecular weight. Binding of agonist steroids to the receptor induces a conformational change to an active receptor form that is followed by a second change in the glucocorticoid-receptor complex, termed activation, that alters the charge of the complex and results in its binding to specific sites on the DNA termed glucocorticoid regulatory elements (GREs). The GRE on the human metallothionein-IIA gene is located in the 5'-flanking DNA. It can function independently of the gene's promoter, and when ligated upstream from the herpes simplex virus (HSV) thymidine kinase (TK) gene promoter, can activate it. The binding of the glucocorticoid-receptor complex to the GRE probably alters chromatin structure over a limited span to facilitate RNA polymerase action. The regulation by glucocorticoids of growth hormone gene expression is more complex. The steroid appears to elicit both transcriptional and posttranscriptional influences that are also affected by thyroid hormone. Also the glucocorticoid influences appear to be exerted in part through DNA structures located downstream from the transcriptional initiation site. A GRE has been defined in intron A of the hGH gene through gene transfer and DNA binding experiments. Finally, gene transfer experiments suggest that pituitary-specific factors influence the ability of glucocorticoids to affect GH gene expression.

Total and unbound cortisol-, progesterone-, oestrone- and transcortin-binding activities in sera from patients with myocardial infarction: evidence for differential responses of good and bad prognostic cases

Day-of-admission sera from myocardial infarction patients (MI) and patients with cardiopathies other than MI (non-MI) were analysed for total and unbound cortisol (F), progesterone (P4), oestrone (E1), and corticosteroid binding activities (CBG). The MI who survived (n = 28) showed high increases of F, P4 and E1 compared to healthy controls. By contrast, the MI who died within 10 days of admission (n = 6) had unchanged F and less increased P4 and E1 than survivors. The non-MI (n = 6) had higher F and E1 than controls but normal P4. The unbound steroids were increased in all patients: however, the MI who died showed much smaller rises than survivors (P less than 0.001 for unbound F and E1 increases in survivors vs. deceased). The CBG activity was in all MI lower than in normals (P less than 0.001) but unchanged in non-MI. These results are discussed in terms of the potential significance of unbound plasma steroids as predictors of MI severity.

Steroid hormone-receptor activity in the presence of a mycotoxic phomopsin toxin and the sesquiterpene ivalin toxin

The influences of two toxins, phomopsin and ivalin, which are reported to exhibit carcinogenic and antitumor activities, respectively, were studied on steroid hormone receptor binding. A mycotoxic carcinogenic fraction A (phomopsin) was isolated from Phomopsis leptostromiformis. The antitumor sesquiterpene lactone ivalin was obtained from the "vomiting bush" Geigeria. Competitive binding analyses were conducted with radiolabeled steroid ligands and unlabeled toxins. No effect of these toxins was observed on either the binding capacity or on the rate of steroid association of [3H]-estradiol-17 beta, [3H]promegestone (R5020), and [3H]dexamethasone to their respective receptors in cytosol of human breast cancer and rat liver. The concentrations of phomopsin and ivalin varied between 0.85 nM to 14 micron in the competitive binding assays. These data suggest the carcinogenic and antitumor activities of these toxins do not involve association with steroid hormone receptors.

Modulation of glucocorticoid receptor activity by cyclic nucleotides and its implications on the regulation of human skin fibroblast growth and protein synthesis

The modulation of glucocorticoid receptor activity by cyclic nucleotides was studied in cultured human skin fibroblasts. The receptors appeared to be activated in the presence of dibutyryl-cAMP and inactivated by dibutyryl-cGMP. Significantly, the cGMP content of the fibroblasts increased during cell growth, with a concomitant decrease in the glucocorticoid receptor activity, while when the cells reached early confluency the decrease in cGMP content was accompanied by an increase in cAMP and increased activity of the glucocorticoid receptors. In addition, cortisol induced (2'-5')oligoadenylate synthetase in these cells and raised the cellular (2'-5')oligoadenylate concentrations. This resulted in a decrease in both DNA and protein synthesis activity in the cells, a response which correlated with the (2'-5')oligoadenylate concentration. The combination of cortisol and dibutyryl-cAMP had a synergetic stimulatory effect on the (2'-5')oligoadenylate concentration and a synergetic inhibitory effect on protein synthesis. In conclusion, it is demonstrated here that cyclic nucleotides can modulate glucocorticoid receptor activity in cultured human skin fibroblasts, and thus these compounds may indirectly affect cellular metabolism by regulating the cellular responses to glucocorticoids.

Kinetic deuterium isotope effects in glucocorticoid receptor activation

Activation and deactivation of the chick thymus glucocorticoid receptor protein was studied in ordinary and heavy water by DNA-cellulose binding of the tritiated triamcinolone acetonide-receptor complex. Activation was significantly slower in heavy water if it was promoted by incubation at elevated temperature in buffers of low ionic strength. In the presence of 300 mM KC1 or after separation from the low molecular weight cytosol constituents, the complex was activated at the same rate in both solvents. Deactivation (time dependent loss of DNA-binding capacity) was much faster in ordinary than in heavy water regardless of gel filtration or the presence of KC1. A model of receptor activation-deactivation was constructed on the basis of these data that accounts for the observed kinetic deuterium isotope effects and reveals some submolecular details of the process.

Cortisol decreases the cellular concentration of translatable procollagen mRNA species in cultured human skin fibroblasts

The effect of cortisol on the cellular concentration of translatable procollagen mRNAs was studied in cultured human skin fibroblasts. Cortisol selectively decreased the amount of procollagen mRNAs, in comparison to the total mRNA activity, when the cells were grown in enriched medium conditions, i.e., with 10% newborn calf serum. The selective decrease was first observed after 6 h exposure to 1 microM cortisol. In depleted medium conditions, i.e., with 2% newborn calf serum, the initial response was a stimulatory one, followed after about 12 h by a decrease in the procollagen mRNA activity. The results suggest that the selective inhibitory effect of cortisol on the cellular concentration of translatable procollagen mRNA species needs an optimal serum concentration. Furthermore, the results give support to the hypothesis that the decrease in the procollagen mRNA concentration after cortisol administration is a secondary response, preceded by the induction of some intracellular regulation system.

Regulation of mouse haptoglobin synthesis

A cloned line of mouse hepatoma cells (Hepa-1) responded to treatment with dexamethasone by a 30-80-fold increase in synthesis and secretion of functional haptoglobin. Under the same conditions, the production of albumin was only slightly elevated whereas that of alpha 1-fetoprotein was reduced by 50%. The hormone concentration for half-maximal stimulation of haptoglobin synthesis was between 1 and 2 X 10(-8) M. The time course of induction is characteristic for a glucocorticoid-regulated protein. Cell-free translation of RNA indicated an increase in the amount of functional haptoglobin mRNA that can account for the change in the protein production. To correlate our findings on Hepa-1 cells with those on nontransformed liver cells, we tested the hormonal response of isolated hepatocytes in tissue culture. Haptoglobin was first synthesized and secreted by hepatocytes from 17-19-d-old fetuses. But neither prenatal nor adult hepatocytes showed a dexamethasone-dependent increase in haptoglobin synthesis. However, when several independent clones of hybrid cells formed from adult mouse hepatocytes and rat hepatoma cells were treated with dexamethasone, the synthesis of mouse haptoglobin was in all cases elevated. It appears that haptoglobin expression in mouse liver cells is potentially sensitive to glucocorticoids, but this modulation is manifested only in transformed cells and their derivatives.

Glucagon regulation of amino acid transport in hepatocytes: effect of cell enucleation

Glucagon and cAMP analogs stimulate amino acid transport in freshly isolated hepatocytes by inducing the synthesis of new transport proteins. The role of the cell nucleus in the glucagon regulation of amino acid transport has been studied in rat hepatocytes enucleated by centrifugation through a discontinuous Ficoll gradient in the presence of cytochalasin B. Enucleated hepatocytes take up alpha-aminoisobutyric acid (AIB) through a Na+-dependent transport component with kinetic properties similar to those found in intact hepatocytes. Cytoplasts prepared from glucagon-stimulated cells retain the increase AIB transport induced by the hormone in the intact cells. The direct addition of glucagon to cytoplasts has no effect on AIB transport, in spite of the fact that the cytoplasts exhibit a higher capacity to bind glucagon than their nucleated counterparts. These data indicate that the nucleus is required for the glucagon stimulation of amino acid transport in isolated hepatocytes.

Evidence for glucocorticoid target cells in the rat optic nerve. Hormone binding and glycerolphosphate dehydrogenase induction

Biochemical evidence suggests that neuroglia are responsive to glucocorticoids, yet previous studies of glucocorticoid localization have typically failed to demonstrate significant uptake by neuroglial cells. To further investigate this problem, we measured glycerol-3-phosphate dehydrogenase (GPDH) activity and glucocorticoid receptor binding capacity in normal rat optic nerves and in those undergoing Wallerian (axonal) degeneration. Binding studies were also performed on hippocampus and anterior pituitary for comparison purposes. Normal optic nerve preparations possessed a high level of GPDH activity that was glucocorticoid-inducible and that increased further following axonal degeneration. Antibody inactivation experiments demonstrated the presence of more enzyme molecules in the degenerating nerve preparations. correlative immunocytochemical studies found GPDH-positive reaction product only in morphologically identified oligodendrocytes, a result that is consistent with the previously reported localization of this enzyme in rat brain. Optic nerve cytosol fractions displayed substantial high-affinity binding of both dexamethasone (DEX) and corticosterone (CORT) that, like GPDH, was elevated approximately two fold in degenerating nerves. Finally, in vivo accumulation of [3H]DEX and [3H]CORT by optic nerve and other myelinated tracts was examined using nuclear isolation and autoradiographic methods. Although neither steroid was found to be heavily concentrated by these tissues in vivo, a small preference for DEX was observed in the nuclear uptake experiments. These results are discussed in terms of the hypothesis that glial cells are targets for glucocorticoid hormones.

Evidence for glucocorticoid target cells in the rat optic nerve. Physiochemical characterization of cytosol binding sites

Cytosolic dexamethasone (DEX) binding sites were studied in the Wallerian-degenerating rat optic nerve (ON), a tissue that is rich in neuroglial cells but devoid of neuronal perikarya and processes. For comparison, hippocampal (HI) and anterior pituitary (AP) cytosols were studied in parallel. Binding sites in these three tissues were found to be quite similar in almost all respects. The sites have a high affinity for DEX (Kp = 2.5-3.5 nM), are present at a high concentration (Bmax = 360-365 fmol/mg cytosol protein), and possess a binding specificity typical of glucocorticoid receptors in other organs. Most experiments supported the assumption of single DEX-binding species in each tissue. Saturation analyses consistently yielded linear Scatchard plots over the range of DEX concentrations tested. Density gradient centrifugation in each case revealed a single peak with a sedimentation coefficient of 7-8S at low ionic strength and 4-4.5S in the presence of 0.3 M KCl. Isoelectric focusing similarly localized most of the binding in each cytosol to a single large peak with an isoelectric point of approximately 6.0. Dissociation rate determinations, on the other hand, suggested the possibility or two different binding sites in each tissue. These studies show that glucocorticoid binders present in cells of the ON possess the same characteristics as the cytoplasmic receptors found in HI, AP, and other recognized glucocorticoid target tissues.

Effect of dissociative methods on cortisol binding and glutamyltransferase inducibility in chick embryo retina

The ability of the isolated embryonic chick retina (12 days) to bind a steroid (cortisol) decreases when the tissue is dissociated; the extent of this decrease depends upon the method of dissociation. Trypsin and mechanical dissociation decreased cortisol binding slightly; papain dissociation essentially eliminated it. Cortisol binding decreased with time in culture in both whole retina and monolayer cultures; this decrease may reflect, in part, a similar development decrease in ovo. Inducibility of glutamine synthetase in whole retinas and retinal monolayers prepared with either trypsin or papain also decreased with time in culture. For whole and trypsin-dissociated retinas, the drop in inducibility correlates with the drop in cortisol-binding capacity. This was not the case for monolayer cultures prepared by papain dissociation.

Taurocholate inhibits the glucocorticoid-induced rise of 3-hydroxy-3-methylglutaryl-CoA reductase in primary culture of hepatocytes

The influence of taurocholate, the major bile acid of the rat, on 3-hydroxy-3-methylglutaryl-CoA reductase [mevalonate: NADP+ oxidoreductase (acylating CoA); EC 1.1.1.34], the regulatory enzyme of cholesterol synthesis, was studied in primary cultures of rat hepatocytes. The basal activity of the enzyme was not altered by adding up to 10 microM taurocholate to the culture medium. On the contrary, 1nM to 10 microM taurocholate caused a dose-dependent inhibition of enzyme activity within 6 h if added simultaneously with 10 microM dexamethasone. Because this glucocorticoid causes a cycloheximide-sensitive rise of 3-hydroxy-3-methylglutaryl-CoA reductase activity in this system the results are taken as evidence that bile salts inhibit the synthesis of the enzyme. The induction of tyrosine transaminase (L-tyrosine:2-oxoglutarate aminotransferase; EC 2.6.1.5) by dexamethasone was not influenced by taurocholate, which demonstrates that the glucocorticoid sensitivity of the cells was not impaired by the bile salt. It is concluded that there is a direct control of hepatic cholesterol synthesis by bile salts.