Precocious induction of tryptophan dioxygenase by glucocorticoid in suckling rats and correlation with change in glucocorticoid receptor

When young rats (less than 14 days old) were treated once a day for 2 days with 100 micrograms/100 g body weight of dexamethasone, their liver cytosol showed a sharp new peak of glucocorticoid binding protein (peak C) eluted with 0.14 M NaCl on DEAE-cellulose chromatography. When the young rats were given a single injection of the hormone, the chromatogram showed a dominant peak of binding protein (peak B), eluted with 0.07 M NaCl, which was similar to that in untreated rats. The appearance of peak C on two treatments of young rats with hormone was confirmed by both in vivo and in vitro labeling and also studies on the nuclear fraction. Peaks B and C were specific hormone-binding proteins as shown with excess unlabeled hormone. The appearance of peak C was concomitant with the precocious induction of tryptophan dioxygenase in the liver of the young rats, and pretreatment with two injections of dexamethasone were necessary for maximal enzyme induction. On the other hand, in adult rats a single injection of dexamethasone (of 20 micrograms/100 g body weight or more) was enough to cause the appearance of peak C and induce tryptophan dioxygenase activity maximally; an additional injection of the hormone did not change the chromatographic pattern of the specific binding or the enzyme activity. For this effect in young rats, dexamethasone could not be replaced by other hormones such as glucagon, growth hormone, thyroid hormones, insulin, sex steroids or short-acting glucocorticoid.     

Mechanism of glucocorticoid regulation of the intestinal tight junction barrier

A defective intestinal epithelial tight junction (TJ) barrier has been proposed as an important pathogenic factor contributing to the intestinal inflammation of Crohn's disease. Glucocorticoids are first-line therapeutic agents for the treatment of moderate to severe Crohn's disease. Glucocorticoid treatment has been shown to induce retightening of the intestinal TJ barrier defect in Crohn's disease patients. However, the mechanisms that mediate the glucocorticoid therapeutic action on intestinal TJ barrier function remain unknown. The aim of this study was to elucidate the mechanism of glucocorticoid modulation of the intestinal epithelial TJ barrier using an in vitro model system. Filter-grown Caco-2 intestinal epithelial cells were used as an in vitro model to examine the effects of glucocorticoids on basal intestinal epithelial TJ barrier function and on TNF-alpha-induced disruption of the TJ barrier. Glucocorticoids (prednisolone and dexamethasone) did not have a significant effect on baseline Caco-2 TJ barrier function but prevented the TNF-alpha-induced increase in Caco-2 TJ permeability. The glucocorticoid protective effect against the TNF-alpha-induced increase in Caco-2 TJ permeability required activation of the glucocorticoid receptor (GR) complex. The activation of the GR complex resulted in GR complex binding to the glucocorticoid response element (GRE) site on DNA and activation of a GR-responsive promoter. Glucocorticoids inhibited the TNF-alpha-induced increase in myosin light chain kinase (MLCK) protein expression, a key process mediating the TNF-alpha increase in intestinal TJ permeability. The glucocorticoid inhibition of the TNF-alpha-induced increase in MLCK protein expression was due to the binding of the GR complex to a GRE binding site on the MLCK promoter region suppressing the TNF-alpha-induced activation. Glucocorticoids inhibit the TNF-alpha-induced increase in Caco-2 TJ permeability. The prednisolone protective action was mediated by binding of activated GR complex to the GRE site on the MLCK promoter, suppressing the TNF-alpha-induced increase in MLCK gene activity, protein expression, and subsequent opening of the intestinal TJ barrier.     

Somatostatin increases glucocorticoid binding and signaling in macrophages by blocking the calpain-specific cleavage of Hsp 90

Somatostatin has direct anti-inflammatory actions and participates in the anti-inflammatory actions of glucocorticoids, but the mechanisms underlying this regulation remain poorly understood. The objective of this study was to evaluate whether somatostatin increases glucocorticoid responsiveness by up-regulating glucocorticoid receptor (GR) expression and signaling. Somatostatin promoted a time- and dose-dependent increase in [(3)H]dexamethasone binding to RAW 264.7 macrophages. Cell exposure to 10 nM somatostatin for 18 h promoted a 2-fold increase in the number of GR sites per cell without significant modification of the affinity. Analysis of GR heterocomplex components demonstrated that somatostatin increased the level of heat shock protein (Hsp) 90, whereas the level of GR remained almost unchanged. The increase in Hsp 90 was associated with a decrease in the cleavage of its carboxyl-terminal domain. Evidence for the involvement of calpain inhibition in this process was obtained by the demonstration that 1) somatostatin induced a dose-dependent decrease in calpain activity and 2) calpain inhibitors, calpain inhibitor I and calpeptin, both abolished the cleavage of Hsp 90 and induced a dose-dependent increase in [(3)H]dexamethasone binding. Increases in glucocorticoid binding after somatostatin treatment were associated with similar increases in the ability of GR to transactivate a minimal promoter containing two glucocorticoid response elements (GRE) and to interfere with the activation of nuclear factor-kappaB (NF-kappaB). Thus, the present findings indicate that somatostatin increases glucocorticoid binding and signaling by limiting the calpain-specific cleavage of GR-associated Hsp 90. This mechanism may represent a novel target for intervention to increase glucocorticoid responsiveness.     

Age-dependent regulation of glucocorticoid receptors in the liver of male rats

Specific binding of [3H]dexamethasone to cytosol and the activation of bound hormone-receptor complexes were studied in the liver of immature (3 weeks old) and mature (26 weeks old) Long-Evans male rats. The concentration of specific binding sites was significantly higher (33%) in the liver of immature rats as compared to mature, while dissociation constants (Kd) remain unaltered at both ages. Heat activation (for 45 min at 25 degrees C) significantly enhances the binding of [3H]dexamethasone-receptor complexes to DNA-cellulose and purified nuclei at both the ages, with a greater magnitude in mature rats. Cross mixing experiments (i.e., binding of activated cytosol from mature rats to nuclei of immature and vice-versa) show receptor specificity. Ca2+ activation (20 mM Ca2+ for 45 min at 0 degree C) also enhances the nuclear and DNA-cellulose binding at both the ages, but to a similar extent. Differences in the number of specific binding sites and some of the physiochemical properties of glucocorticoid receptors presented here between immature and mature rats may underlie the functional changes in tissue response with age.     

Expression of the mouse glucocorticoid receptor and its role during development

Genes encoding enzymes involved in gluconeogenesis are activated in liver shortly after birth by the synergistic effect of glucagon and glucocorticoids. This induction is achieved by the synergistic action of hormone responsive and liver-specific enhancer elements. In the case of glucocorticoids, this enhancer is composed of a glucocorticoid-response element (GRE) and a number of cell-specific hepatocyte nuclear factor 3 (HNF-3) binding sites. The GRE binds the ligand-activated glucocorticoid receptor (GR) which is ubiquitously expressed and the HNF-3 element binds a cell-specific protein factor. To further understand the role of cell-specific glucocorticoid signalling in the perinatal period and earlier during development we have studied the expression of the mouse GR gene. The gene has been cloned and fully characterized. Expression of the gene is controlled by at least three promoters, one of which is only active in T-lymphocytes. Expression of GR mRNA has been detected back to day 9.5 of mouse development. The role of GR during mouse development has been further analysed by disruption of the GR gene in vivo by homologous recombination in mouse embryonic stem cells.     

Assembly of a glucocorticoid receptor complex prior to DNA binding enhances its specific interaction with a glucocorticoid response element

Gel retardation analysis with full- and half-palindromic sequences using partially purified glucocorticoid receptor (GR) resulted in GR-glucocorticoid response element (GRE) species of identical mobilities, suggesting that formation of the dimeric GR protein complex is not catalyzed by DNA binding. These results are in contrast to the behavior of the isolated DNA binding domain of the glucocorticoid receptor where dimerization occurred on the GRE. Density gradient centrifugation of cytosolic GR resulted in two forms, a 4 S peak characteristic of the monomeric GR and a fraction which sediments at 6 S which is consistent with the observed size of the dimeric GR. These two forms were found to differ in their ability to bind to specific DNA sequences with the 6 S species having a higher affinity for a GRE. Taken together our results are consistent with a two-step model for hormone-induced transformation of GR: dissociation of the multimeric untransformed complex and dimerization of the GR to yield a high affinity DNA binding species.     

The responses of rat liver glucocorticoid receptors and genes for tyrosine aminotransferase, alpha-2-macroglobulin and gamma-fibrinogen to adrenalectomy-, dexamethasone- and inflammation-induced changes in the levels of glucocorticoids and proinflammatory cytokines

The responses of liver glucocorticoid receptor (GR) and genes coding for a glucocorticoid-inducible tyrosine aminotransferase (TAT) and two acute-phase proteins (APP) [alpha2-macroglobulin (alpha2-M) and gamma-fibrinogen (Fb)] to changes in glucocorticoid (GC) and proinflammatory (AP) cytokine contents have been examined in rats after single or combined treatments with turpentine oil, dexamethasone (Dex) and adrenalectomy. Activation of two APP genes in turpentine-induced inflammation was accompanied by an increase in the level of GR mRNA and a preferential translocation of GR-GC complexes to the nucleoplasm, while the expression of TAT remained unaltered. Dex alone caused a decrease in the levels of GR and Fb mRNAs, activation of TAT and alpha2-M genes, a decrease in the affinity of hormone binding sites and redistribution of translocated GR-Dex complexes within the nuclei. Inflammation potentiated the effect which Dex alone exerted on the GR content and the number of GR binding sites but counteracted its influence on the affinity of GR binding sites and nuclear distribution of GR-Dex complexes. Adrenalectomy promoted a fall in TAT mRNA, no changes in the GR and Fb mRNA, a decrease in the affinity of GR hormone binding sites and redistribution of GR-hormone complexes within the nuclei. The AP cytokines released in response to inflammation exerted a counteracting effect on the adrenalectomy-induced changes in the affinity of hormone binding sites and nuclear distribution of GR-hormone complexes. They potentiated a fall of TAT mRNA but promoted full expression of the Fb gene. These results argue strongly for the influence of AP cytokines on the functional state of the GR and GC signaling pathways.     

Alcohol dysregulates corticotropin-releasing-hormone (CRH) promoter activity by interfering with the negative glucocorticoid response element (nGRE)

EtOH exposure in male rats increases corticotropin-releasing hormone (CRH) mRNA in the paraventricular nucleus of the hypothalamus (PVN), a brain region responsible for coordinating stress and anxiety responses. In this study we identified the molecular mechanisms involved in mediating these effects by examining the direct effects of EtOH on CRH promoter activity in a neuronal cell line derived from the PVN (IVB). In addition, we investigated the potential interactions of EtOH and glucocorticoids on the CRH promoter by concomitantly treating cells with EtOH and the glucocorticoid receptor (GR) antagonist RU486, and by sequentially deleting GR binding sites within glucocorticoid response element (GRE) on the CRH promoter. Cells were transiently transfected with a firefly luciferase reporter construct containing 2.5 kb of the rat wild type (WT) or mutated CRH promoter. Our results showed that EtOH treatment induced a biphasic response in CRH promoter activity. EtOH exposure for 0.5 h significantly decreased promoter activity compared to vehicle treated controls, whereas promoter activity was significantly increased after 2.0 h of EtOH exposure. Treatment with RU486, or deletion of the GR binding sites 1 and 2 within the GRE, abolished the EtOH-induced increase in the promoter activity, however did not affect EtOH-induced decrease in CRH promoter activity at an earlier time point. Overall, our data suggest that alcohol exposure directly regulates CRH promoter activity by interfering with the normal feedback mechanisms of glucocorticoids mediated by GR signaling at the GRE site of the CRH promoter.     

Endocrine-mediated parallel changes in hepatic glucocorticoid and prolactin receptors

The objective of this study was to determine whether in female rat liver any relationship existed between prolactin and glucocorticoid receptors after hormonal manipulation. Bromocryptine (CB-154) treatment of adult SD female rats (80-100 days old) for 48 h decreased prolactin binding to hepatic membranes 49% and dexamethasone binding in hepatic cytosol 40% below control values. Administration of rat prolactin along with bromocriptine prevented these changes. In another study, prolactin binding to hepatic membranes increased 53% and dexamethasone binding in hepatic cytosol increased 113% above sham-control values, 3 days after adrenalectomy. On the other hand, hydrocortisone treatment of sham-operated rats reduced prolactin binding by 57% and dexamethasone binding by 76%. Scatchard analyses of the prolactin or dexamethasone binding data indicated that these manipulations changed the number of prolactin or dexamethasone binding sites rather than their apparent affinity constants. In vitro treatment of rat whole liver homogenate with various doses (10(-9) - 10(-5) M) of dexamethasone and corticosterone for 15 min at 22 degrees C resulted in a dose-dependent decrease in prolactin binding activity. However, direct addition of dexamethasone to a hepatic 15 000 X g to 100 000 X g membrane preparation exhibited no significant effects on prolactin binding. In conclusion, these studies show that (a) there is a parallel in vivo modulation of rat liver prolactin and glucocorticoid receptors under various experimental conditions and (b) in vitro exposure of whole liver homogenate to glucocorticoids inhibits the prolactin binding activity.     

Accessory factors facilitate the binding of glucocorticoid receptor to the phosphoenolpyruvate carboxykinase gene promoter

Glucocorticoid induction of the phosphoenolpyruvate carboxykinase (PEPCK) gene requires a glucocorticoid response unit (GRU) comprised of two non-consensus glucocorticoid receptor (GR) binding sites, GR1 and GR2, and at least three accessory factor elements (gAF1-3). DNA-binding accessory proteins are commonly required for the regulation of genes whose products play an important role in metabolism, development, and a variety of defense responses, but little is known about why they are necessary. Quantitative, real time homogenous assays of cooperative protein-DNA interactions in complex media (e.g. nuclear extracts) have not previously been reported. Here we perform quantitative, real time equilibrium and stopped-flow fluorescence anisotropy measurements of protein-DNA interactions in nuclear extracts to demonstrate that GR binds to the GR1-GR2 elements poorly as compared with a palindromic or consensus glucocorticoid response element (GRE). Inclusion of either the gAF1 or gAF2 element with GR1-GR2, however, creates a high affinity binding environment for GR. GR can undergo multiple rounds of binding and dissociation to the palindromic GRE in less than 100 ms at nanomolar concentrations. The dissociation rate of GR is differentially slowed by the gAF1 or gAF2 elements that bind two functionally distinct accessory factors, COUP-TF/HNF4 and HNF3, respectively.     

Identification and characterization of glucocorticoid receptor-binding sites in the human genome

Glucocorticoids regulate gene expression via binding of the ligand-activated glucocorticoid receptor (GR) to glucocorticoid-responsive elements (GRE). To identify GR-binding sites, we developed a modified yeast one-hybrid system which enables rapid and efficient identification of genomic targets for DNA-binding proteins. The human GR expression vector was transformed into yeast cells containing a library of human genomic fragments cloned upstream of the reporter gene URA3. The genomic fragments with GR-binding sites were identified by growth of yeast clones in media lacking uracil but containing dexamethasone. DNA fragments were recovered by colony-direct PCR and GRE sequences were predicted by in silico analysis. Using electrophoretic mobility shift assay and fluorescence correlation spectroscopy, we demonstrated that 314 predicted GREs could directly interact with recombinant human GR proteins. In addition, when the genomic fragments were inserted in front of the heterologous SV40 promoter, at least 150 fragments could function as GREs in HEK293 cells. Furthermore, we identified four functional regulatory polymorphisms which may influence individual variation in sensitivity to glucocorticoids. These results provide insights into the molecular mechanisms underlying the physiological and pathological actions of glucocorticoid.