Ethanol increases rat liver tryptophan oxygenase: Evidence for corticosterone mediation
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Corticosterone
Antiglucocorticoid
The mechanism of action of the synthetic glucocorticoid antagonist, RU 38486, has yet to be completely elucidated. Although RU 38486 is a potent antiglucocorticoid in vivo, several studies have indicated that it has some agonist activities in vitro, such as high-affinity steroid binding to the receptor, activation, and DNA binding. Nevertheless, these in vitro postbinding events do not lead to any known gene expression. To understand the action of the glucocorticoid antagonist RU 38486, we studied glucocorticoid receptor localization on a mouse melanoma cell line (B16C3) by indirect immunofluorescent staining techniques, using monoclonal antibodies to the glucocorticoid receptor. Our data in intact cells suggest that, unlike glucocorticoid agonists such as triamcinolone acetonide, and similar to the glucocorticoid antagonist cortexolone, RU 38486-bound receptors do not translocate to the nucleus and hence do not allow for transcription of glucocorticoid-regulated genes to occur. Passage through the nuclear membrane may be a rate-limiting step in the action of glucocorticoid antagonists, and translocation may in itself be an important regulatory mechanism of steroid hormone action.
Antiglucocorticoid
Mechanism of Action
Hormone response element
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Exposure of the Fu5 rat hepatoma cell line to glucocorticoids, such as dexamethasone and hydrocortisone, suppressed the growth rate and final density of cells grown in the presence of serum. This hormonal effect was proportional to receptor occupancy and affinity and, in addition, the glucocorticoid antagonist RU38486 prevented this response. Two classes of dexamethasone-resistant variants that failed to be growth inhibited were recovered from ethyl methylsulfonate-mutagenized populations by continuous culture in the presence of 1 microM dexamethasone. The first class, represented by the EDR3 subclone, was completely glucocorticoid unresponsive and failed to express receptor transcripts. The second class, represented by the EDR1, EDR5, and EDR7 subclones, possessed significant levels of glucocorticoid receptor but were only partially glucocorticoid responsive when stimulated with saturating levels of hormone. Introduction of functional glucocorticoid receptor genes into both classes of dexamethasone-resistant variants by a recombinant retrovirus expression vector restored glucocorticoid responsiveness and suppression of cell growth. A hypersensitive variant (BDS1), recovered by bromodeoxyuridine selection, was fully glucocorticoid responsive, and its inhibition of proliferation was more acutely regulated by dexamethasone. Taken together, our results established that the inhibition of proliferation in Fu5 rat hepatoma cells represents a new glucocorticoid response that requires the expression of a functional glucocorticoid receptor.
Antiglucocorticoid
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Transient transfections of steroid receptors have yielded much of the data used to construct the current models of steroid hormone action. These experiments invariably examine the ability of receptors to regulate transcription when occupied by saturating concentrations of steroid. We now report that other induction properties of a transiently transfected gene are not constant but vary with the concentration of transiently transfected glucocorticoid receptors. Thus, the percentage of maximal induction seen with subsaturating concentrations of glucocorticoid could be dramatically increased, and an antiglucocorticoid could be converted into a partial glucocorticoid, simply by increasing the concentration of glucocorticoid receptors. This behavior was observed in HeLa cells, containing endogenous receptors, or in CV-1 cells, containing almost no endogenous receptor, with either homologous or heterologous receptors. These increases were relatively insensitive to the concentration of reporter gene, suggesting the titration of some transcription factor(s) involved in regulating the position of the glucocorticoid dose-response curve and the agonist activity of an antiglucocorticoid. This property of transfected glucocorticoid receptors required a full-length, functionally active receptor but was retained, albeit reduced in magnitude, in the absence of binding to a glucocorticoid response element. Furthermore, this phenomenon was specific in that the A form of the human progesterone receptor had no effect under the same conditions. These variations in induction properties of antiglucocorticoids and of subsaturating concentrations of glucocorticoid, in a manner that was proportional to the amount of transfected receptor, reveal processes that are not operative with saturating concentrations of glucocorticoid. These variations also demonstrate that caution should be exercised in making mechanistic conclusions based solely on experiments conducted with saturating concentrations of glucocorticoid. Transient transfections of steroid receptors have yielded much of the data used to construct the current models of steroid hormone action. These experiments invariably examine the ability of receptors to regulate transcription when occupied by saturating concentrations of steroid. We now report that other induction properties of a transiently transfected gene are not constant but vary with the concentration of transiently transfected glucocorticoid receptors. Thus, the percentage of maximal induction seen with subsaturating concentrations of glucocorticoid could be dramatically increased, and an antiglucocorticoid could be converted into a partial glucocorticoid, simply by increasing the concentration of glucocorticoid receptors. This behavior was observed in HeLa cells, containing endogenous receptors, or in CV-1 cells, containing almost no endogenous receptor, with either homologous or heterologous receptors. These increases were relatively insensitive to the concentration of reporter gene, suggesting the titration of some transcription factor(s) involved in regulating the position of the glucocorticoid dose-response curve and the agonist activity of an antiglucocorticoid. This property of transfected glucocorticoid receptors required a full-length, functionally active receptor but was retained, albeit reduced in magnitude, in the absence of binding to a glucocorticoid response element. Furthermore, this phenomenon was specific in that the A form of the human progesterone receptor had no effect under the same conditions. These variations in induction properties of antiglucocorticoids and of subsaturating concentrations of glucocorticoid, in a manner that was proportional to the amount of transfected receptor, reveal processes that are not operative with saturating concentrations of glucocorticoid. These variations also demonstrate that caution should be exercised in making mechanistic conclusions based solely on experiments conducted with saturating concentrations of glucocorticoid.
Antiglucocorticoid
Steroid hormone
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The effect of corticosterone on transcriptional activity in regenerating hepatocytes of rat after partial hepatectomy (PH) was studied by using nucleolar organizer region associated proteins (AgNOR) staining as marker. The results showed that: during 0-24 h after PH, the numbers of AgNOR decreased whatever in sham adrenalectomized rats, adrenalectomized rats or adrenalectomized rats treated with corticosterone. The numbers of AgNOR in sham operated rats peaked at 36 h after PH, and then restored the pre PH level at 48 h. AgNORs increased constantly 24-48 h after PH in adrenalectomized rats. After the treatment of corticosterone at a dose of 10 or 20 or 40 mg/kg body weight respectively in the adrenalectomized rats, it was found that at 36 h and 48 h post PH, the more corticosterone used, the less the AgNORs appeared and the faster the ranger of AgNORs declined. RU486, the glucocorticoid receptor antagonist, was given subcutaneously at a dose of 10 mg/kg body weight 12 h before PH in order to investigate extensively the action of endogenous corticosterone. The similar results were observed in the adrenalectomized rats. These findings suggested that transcriptional activity in regenerating hepatocytes of rat was inhibited by corticosterone. This phenomenon appeared mainly at 24 h following PH via the binding of corticosterone to its receptor.
Corticosterone
Antiglucocorticoid
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Corticosterone
Antiglucocorticoid
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Corticosterone
Antiglucocorticoid
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An analysis of receptor mechanisms of glucocorticoid and antiglucocorticoid effect was made. The limiting link in glucocorticoid and antiglucocorticoid realization are specific cytoplasmic glucocorticoid receptors (GR) which function is controlled by heat shock protein (HSP) of 90 kD molecular weight. Under the influence of glucocorticoids (G), GR are released from GR-HSP complex, forming GR-G complexes. The latter are translocated into cell nucleus activate the function of genetic apparatus, change biosynthesis of specific enzymes realizing intracellular glucocorticoid effect Receptor mechanism of antiglucocorticoid effect is realized via competition of steroid and non-steroid drugs with glucocorticoids for binding sites on GR or pharmacological stabilization of GR-HSP complex, decreasing 4SGR release, 4S GR-G forming, and 4SGR-G translocation into cell nucleus. New data about GR chemical structure according to which GR contain 3 functional domains, characterized by regulatory DNA-binding and ligand-binding activity promote researches of antiglucocorticoids. It promoted synthesis of a new most active receptor antiglucocorticoid RU-486 (19-norsteroid), that inhibits ovalbumin and conalbumin synthesis induced by glucocorticoids.
Antiglucocorticoid
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Dexamethasone is frequently used in the therapy of brain tumor patients. We investigated the effect of dexamethasone on the proliferation of three short-term and four established human glioma cell lines in vitro, using a microculture tetrazolium assay to determine growth rates. In one short-term culture and in one established cell line dexamethasone consistently stimulated the proliferation in a concentration-dependent way. The proliferation was maximally enhanced at a concentration of approximately 0.1 microM. In these two cell lines a relatively high level of glucocorticoid receptors was present, whereas low levels of glucocorticoid receptors were found in the other cell lines. In addition, we demonstrated that the stimulatory effects of dexamethasone on the proliferation of the glioma cell lines can be antagonized by the antiglucocorticoid RU38486. The results demonstrate unequivocally that the glucocorticoid receptor plays a role in the growth stimulating effect of dexamethasone.
Antiglucocorticoid
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