We have examined the regulation of apolipoprotein A-I (apoA-I) gene expression in response to glucose and insulin. In Hep G2 cells, endogenous apoA-I mRNA was suppressed by one-half or induced 2-fold following 48 h of exposure to high concentrations of glucose (22.4 mM) or insulin (100 microunits/ml), respectively, compared with control. Transcriptional activity of the rat apoA-I promoter (؊474 to ؊7) in Hep G2 cells paralleled endogenous mRNA expression, and this activity was dependent on the dose of glucose or insulin. Deletional analysis showed that a 50-base pair fragment spanning ؊425 to ؊376 of the promoter mediated the effects of both insulin and glucose. Within this DNA fragment there is a motif (؊411 to ؊404) that is homologous to a previously identified insulin response core element (IRCE). Mutation of this motif abolished not only the induction of the promoter by insulin but also abrogated its suppression by glucose. Electrophoretic mobility shift assay analysis of nuclear extracts from Hep G2 cells revealed IRCE binding activity that formed a duplex with radiolabeled probe. The IRCE binding activity correlated with insulin induction of apoA-I expression. In summary, our data show that glucose decreases and insulin increases apoA-I promoter activity. This effect appears to be mediated by a single cis-acting element.
To examine the effects of glucocorticoid (GC) on growth hormone (GH)-releasing hormone (GRH) receptor gene expression, a highly-sensitive and quantitative reverse-transcribed polymerase chain reaction (RT-PCR) method was used in this study. Rat anterior pituitary cells were isolated and cultured for 4 days. The cultured cells were treated with dexamethasone for 2, 6, and 24 h. GRH receptor mRNA levels were determined by competitive RT-PCR using a recombinant RNA as the competitor. Dexamethasone significantly increased GRH receptor mRNA levels at 5 nM after 6- and 24 h-incubations, and the maximal effect was found at 25 nM. The GC receptor-specific antagonist, RU 38486 completely eliminated the dexamethasone-induced enhancement of GRH receptor mRNA levels. Dexamethasone did not alter the mRNA levels of beta-actin and prolactin at 5 nM for 24 h, whereas GH mRNA levels were significantly increased by the same treatment. The GH response to GRH was significantly enhanced by the 24-h incubation with 5 nM dexamethasone. These findings suggest that GC stimulates GRH receptor gene expression through the ligand-activated GC receptors in the rat somatotrophs. The direct effects of GC on the GRH receptor gene could explain the enhancement of GRH-induced GH secretion.
The aim of the present study was to assess potential brain sites of stimulation by peripheral interleukin (IL)-6 of the hypothalamo-pituitary-adrenal (HPA) axis in the rat, using c-fos protein as a marker of cellular activation. Involvement of prostaglandins in IL-6-induced ACTH secretion and c-fos expression was also investigated. IL-6 was infused continuously (40 ng/min) for 90 min to conscious male rats. Blood samples were taken before the infusion and at 30 and 90 min for measurement of plasma ACTH. Expression of c-fos in the brain was examined by immunohistochemistry. Administration of IL-6 significantly elevated plasma ACTH levels at 30 min (495 ± 105 vs. 117 ± 17 pg/ml in controls, p < 0.05). Elevated levels were still present at 90 min (596 ± 139 vs. 113 ± 20 pg/ml in controls, p < 0.05). Infusion of IL-6 (3.6 µg/rat) markedly triggered c-fos expression in hypothalamic paraventricular (PVN) and supraoptic nuclei (SON), as well as in the central amygdaloid nucleus (CeA), the nucleus tractus solitarius and the locus coeruleus. Pretreatment with the cyclooxygenase inhibitor indomethacin (10 mg/kg, i.v.) suppressed the ACTH response induced by IL-6. The number of IL-6-induced immunoreactive cells in the PVN was significantly reduced by indomethacin pretreatment (p < 0.01), but the number of IL-6-induced c-fos-positive cells in the SON and CeA remained unchanged. These findings suggest that circulating IL-6 may exert central actions by acting directly or indirectly on brain neurons. In addition, the ability of IL-6 to activate the HPA axis may depend upon the release of prostaglandins, probably in the brain.
Utilizing immunohistochemistry for the c-fosprotein to detect neuronal activity, we examined the effects of continuous intravenous and intracerebroventricular infusion of interleukin (IL)-1βin the rat brain, and the involvement of prostaglandins (PGs) in IL-1β-induced c-fosexpression. Continuous intravenous infusion of IL-1β(10 ng/min) markedly augmented c-fosexpression in the paraventricular (PVN) and the supraoptic (SON) nuclei of the hypothalamus as well as in the central amygdaloid nucleus (CeA). The number of IL-1β-induced c-fos-positive cells in the PVN and SON was significantly lower in rats pretreated with indomethacin than in vehicle-treated rats. However, the number of IL1β-induced c-fos-positive cells in the CeA remained unchanged. c-fosprotein was induced after intracerebroventricular infusion of IL-1β(200 ng) in the PVN, SON and arcuate nuclei of the hypothalamus, and in the CeA. The induction of c-fosimmunoreactivity by central administration of IL-1βwas blocked by indomethacin (500µg/rat), except in the CeA. These findings suggest that PGs are involved in the complex transmission of signals from circulating or central IL-1βto hypothalamic neurons.
Although various pathophysiological effects of interleukin (IL) on the CRF-ACTH-adrenal axis and gonadotropin secretion have been studied extensively, the effects of IL on GH secretion still remain to be elucidated. We investigated the possible effects of IL on GH secretion in six groups of conscious rats. In four groups, IL was administered by continuous iv infusion and in the other two, by intracerebroventricular injection. Saline-treated rats served as controls for these groups. Sequential blood sampling was performed every 20 min in all groups, and the plasma GH concentration was determined by RIA. The expression of hypothalamic c-fos protein in a separate group was examined by immunohistochemistry. Continuous infusion of both IL-1 alpha and IL-1 beta (10 ng/min) significantly inhibited GH surges. The plasma IL-1 level was elevated to 2-3 ng/ml. Continuous iv infusion of IL-2 and IL-6 had no effect on GH secretion. The intracerebroventricular injection of both IL-1 alpha and IL-1 beta significantly inhibited GH surges, and the inhibitory effect was much greater for IL-1 beta than for IL-1 alpha. Continuous iv infusion of IL-1 beta markedly stimulated c-fos expression in specific hypothalamic nuclei, particularly in the paraventricular nucleus. These findings suggest that, in the rat, IL-1 inhibits GH secretion through its peripheral and central actions.
We have studied the effects of a cardiac sparing thyromimetic, CGS 23425, on postprandial levels of triglycerides, abundance of apolipoprotein B (apo B) protein and hepatic apo B mRNA expression in rats. When compared with control rats, triglyceride clearance was significantly accelerated by treatment with CGS 23425. A full return to baseline values was achieved within 8 h after ingesting a large quantity of fat, as compared to >24 h in control animals. The abundance of apo B-100 protein in CGS 23425-treated hyperlipidemic rats decreased in a dose-dependent manner, but levels of apo B-48 were not significantly affected. Like -triiodothyronine (-T 3 ), treatment with 30 µg/kg CGS 23425 for 6 or 9 days decreased the levels of apo B-100 protein by 80% and 40% respectively. This change was paralleled by a 27% reduction in hepatic apo B-100 mRNA. To investigate a potential mechanism of CGS 23425 action, we measured in vitro apo B mRNA editing activity in hepatocellular extract from control or CGS 23425-treated rats. Treatment with CGS 23425 increased activity of the hepatic apo B-100 editosome, apobec-1. In human hepatoma cells which lack apobec-1 activity, apo B-100 mRNA levels remained the same in cells treated with or without the agent. In summary, these observations show that CGS 23425 decreases the levels of apo B-100 in rats. This action of CGS 23425 involves apo B-100 mRNA editing activity.
Immunosuppressant agent FK506 has been reported to stimulate ACTH release from pituitary cells. We examined the effects of FK506 on GH release from the rat anterior pituitary cells and the effects of FK506 on hypothalamic GH- releasing hormone (GRH) and somatostatin (SS) gene expression in conscious male rats. In vitro experiments, the monolayer pituitary culture and reverse hemolytic plaque assay were employed to examine the GH release from the rat anterior pituitary cells. In in vivo experiments, the FK506 was administered for 7 days and then sequential blood sampling was performed every 20 min during 6 h in conscious rats. The hypothalamus was removed, and total RNA was extracted for Northern blot analysis. The FK506 significantly stimulated GH release from the rat anterior pituitary cells in a dose-dependent manner in vitro. In in vivo experiments, the area under the curve of GH surges was significantly increased in FK506-treated rats, although the peak height and the trough level of GH surges were not altered. Pituitary GH messenger RNA (mRNA) levels were significantly increased by the FK506 treatment. Hypothalamic GRH mRNA levels were significantly increased in FK506- treated rats, whereas hypothalamic SS mRNA levels were not altered. These findings indicate that FK506 stimulates GH secretion and gene expression of hypothalamic GRH in the rat.
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