Novel role of a triglyceride-synthesizing enzyme: DGAT1 at the crossroad between triglyceride and cholesterol metabolism

Acyl-CoA:diacylglycerol acyltransferase 1 (DGAT1) is a key enzyme in triacylglycerol (TG) biosynthesis. Here we show that genetic deficiency and pharmacological inhibition of DGAT1 in mice alters cholesterol metabolism. Cholesterol absorption, as assessed by acute cholesterol uptake, was significantly decreased in the small intestine and liver upon DGAT1 deficiency/inhibition. Ablation of DGAT1 in the intestine (I-DGAT1^−/−) alone is sufficient to cause these effects. Consequences of I-DGAT1 deficiency phenocopy findings in whole-body DGAT1^−/− and DGAT1 inhibitor-treated mice. We show that deficiency/inhibition of DGAT1 affects cholesterol metabolism via reduced chylomicron size and increased trans-intestinal cholesterol excretion. These effects are independent of cholesterol uptake at the apical surface of enterocytes but mediated through altered dietary fatty acid metabolism. Our findings provide insight into a novel role of DGAT1 and identify a pathway by which intestinal DGAT1 deficiency affects whole-body cholesterol homeostasis in mice. Targeting intestinal DGAT1 may represent a novel approach for treating hypercholesterolemia.

Cardiomyocyte-specific loss of diacylglycerol acyltransferase 1 (DGAT1) reproduces the abnormalities in lipids found in severe heart failure

Diacylglycerol acyltransferase 1 (DGAT1) catalyzes the final step in triglyceride synthesis, the conversion of diacylglycerol (DAG) to triglyceride. Dgat1(-/-) mice exhibit a number of beneficial metabolic effects including reduced obesity and improved insulin sensitivity and no known cardiac dysfunction. In contrast, failing human hearts have severely reduced DGAT1 expression associated with accumulation of DAGs and ceramides. To test whether DGAT1 loss alone affects heart function, we created cardiomyocyte-specific DGAT1 knock-out (hDgat1(-/-)) mice. hDgat1(-/-) mouse hearts had 95% increased DAG and 85% increased ceramides compared with floxed controls. 50% of these mice died by 9 months of age. The heart failure marker brain natriuretic peptide increased 5-fold in hDgat1(-/-) hearts, and fractional shortening (FS) was reduced. This was associated with increased expression of peroxisome proliferator-activated receptor α and cluster of differentiation 36. We crossed hDgat1(-/-) mice with previously described enterocyte-specific Dgat1 knock-out mice (hiDgat1(-/-)). This corrected the early mortality, improved FS, and reduced cardiac ceramide and DAG content. Treatment of hDgat1(-/-) mice with the glucagon-like peptide 1 receptor agonist exenatide also improved FS and reduced heart DAG and ceramide content. Increased fatty acid uptake into hDgat1(-/-) hearts was normalized by exenatide. Reduced activation of protein kinase Cα (PKCα), which is increased by DAG and ceramides, paralleled the reductions in these lipids. Our mouse studies show that loss of DGAT1 reproduces the lipid abnormalities seen in severe human heart failure.

11β-Hydroxysteroid dehydrogenase type 1 (11β-HSD1) inhibitors still improve metabolic phenotype in male 11β-HSD1 knockout mice suggesting off-target mechanisms

The enzyme 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) is a target for novel type 2 diabetes and obesity therapies based on the premise that lowering of tissue glucocorticoids will have positive effects on body weight, glycemic control, and insulin sensitivity. An 11β-HSD1 inhibitor (compound C) inhibited liver 11β-HSD1 by >90% but led to only small improvements in metabolic parameters in high-fat diet (HFD)-fed male C57BL/6J mice. A 4-fold higher concentration produced similar enzyme inhibition but, in addition, reduced body weight (17%), food intake (28%), and glucose (22%). We hypothesized that at the higher doses compound C might be accessing the brain. However, when we developed male brain-specific 11β-HSD1 knockout mice and fed them the HFD, they had body weight and fat pad mass and glucose and insulin responses similar to those of HFD-fed Nestin-Cre controls. We then found that administration of compound C to male global 11β-HSD1 knockout mice elicited improvements in metabolic parameters, suggesting "off-target" mechanisms. Based on the patent literature, we synthesized another 11β-HSD1 inhibitor (MK-0916) from a different chemical series and showed that it too had similar off-target body weight and food intake effects at high doses. In summary, a significant component of the beneficial metabolic effects of these 11β-HSD1 inhibitors occurs via 11β-HSD1-independent pathways, and only limited efficacy is achievable from selective 11β-HSD1 inhibition. These data challenge the concept that inhibition of 11β-HSD1 is likely to produce a "step-change" treatment for diabetes and/or obesity.

11-Dehydrocorticosterone causes metabolic syndrome, which is prevented when 11β-HSD1 is knocked out in livers of male mice

Metabolic syndrome is growing in importance with the rising levels of obesity, type 2 diabetes, and insulin resistance. Metabolic syndrome shares many characteristics with Cushing's syndrome, which has led to investigation of the link between excess glucocorticoids and metabolic syndrome. Indeed, increased glucocorticoids from intracellular regeneration by 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) drives insulin resistance and increases adiposity, but these metabolic changes are assumed to be due to increased circulating glucocorticoids. We hypothesized that increasing the substrate for 11β-HSD1 (11-dehydrocorticosterone, 11-DHC) would adversely affect metabolic parameters. We found that chronic administration of 11-DHC to male C57BL/6J mice resulted in increased circulating glucocorticoids, and down-regulation of the hypothalamic-pituitary-adrenal axis. This elevated 11β-HSD1-derived corticosterone led to increased body weight gain and adiposity and produced marked insulin resistance. Surprisingly liver-specific 11β-HSD1 knockout (LKO) mice given 11-DHC did not show any of the adverse metabolic effects seen in wild-type mice. This occurred despite the 11-DHC administration resulting in elevated circulating corticosterone, presumably from adipose tissue. Mice with global deletion of 11β-HSD1 (global knockout) were unaffected by treatment with 11-DHC, having no increase in circulating corticosterone and exhibiting no signs of metabolic impairment. Taken together, these data show that in the absence of 11β-HSD1 in the liver, mice are protected from the metabolic effects of 11-DHC administration, even though circulating glucocorticoids are increased. This implies that liver-derived intratissue glucocorticoids, rather than circulating glucocorticoids, contribute significantly to the development of metabolic syndrome and suggest that local action within hepatic tissue mediates these effects.

Diacylglycerol acyltransferase-1 inhibition enhances intestinal fatty acid oxidation and reduces energy intake in rats

Acyl CoA:diacylglycerol acyltransferase-1 (DGAT-1) catalyzes the final step in triacylglycerol (TAG) synthesis and is highly expressed in the small intestine. Because DGAT-1 knockout mice are resistant to diet-induced obesity, we investigated the acute effects of intragastric (IG) infusion of a small molecule diacylglycerol acyltransferase-1 inhibitor (DGAT-1i) on eating, circulating fat metabolites, indirect calorimetry, and hepatic and intestinal expression of key fat catabolism enzymes in male rats adapted to an 8 h feeding-16 h deprivation schedule. Also, the DGAT-1i effect on fatty acid oxidation (FAO) was investigated in enterocyte cell culture models. IG DGAT-1i infusions reduced energy intake compared with vehicle in high-fat diet (HFD)-fed rats, but scarcely in chow-fed rats. IG DGAT-1i also blunted the postprandial increase in serum TAG and increased β-hydroxybutyrate levels only in HFD-fed rats, in which it lowered the respiratory quotient and increased intestinal, but not hepatic, protein levels of Complex III of the mitochondrial respiratory chain and of mitochondrial hydroxymethylglutaryl-CoA synthase. Finally, the DGAT-1i enhanced FAO in CaCo2 (EC50 = 0.3494) and HuTu80 (EC50 = 0.00762) cells. Thus, pharmacological DGAT-1 inhibition leads to an increase in intestinal FAO and ketogenesis when dietary fat is available. This may contribute to the observed eating-inhibitory effect.

ACAT-selective and Nonselective DGAT1 Inhibition

Acyl-coenzyme A: cholesterol O-Acyltransferase (ACAT) and Acyl-coenzyme A: diacylglycerol O-acyltransferase (DGAT) enzymes play important roles in synthesizing neutral lipids, and inhibitors of these enzymes have been investigated as potential treatments for diabetes and other metabolic diseases. Administration of a Acyl-coenzyme A: diacylglycerol O-acyltransferase 1 (DGAT1) inhibitor with very limited cellular selectivity over ACAT resulted in significant adrenocortical degenerative changes in dogs. These changes included macrosteatotic vacuolation associated with adrenocyte cell death in the zonae glomerulosa and fasciculata and minimal to substantial mixed inflammatory cell infiltration and were similar to those described previously for some ACAT inhibitors in dogs. In the mouse, similar but only transient adrenocortical degenerative changes were seen as well as a distinctive nondegenerative reduction in cortical fine vacuolation. In the marmoset, only the distinctive nondegenerative reduction in cortical fine vacuolation was observed, suggesting that the dog, followed by the mouse, is the most sensitive species for cortical degeneration. Biochemical analysis of adrenal cholesterol and cholesteryl ester indicated that the distinctive reduction in cortical fine vacuolation correlated with a significant reduction in cholesteryl ester in the mouse and marmoset, whereas no significant reduction in cholestryl ester, but an increase in free cholesterol was observed in dogs. Administration of a DGAT1 inhibitor with markedly improved selectivity over ACAT to the marmoset and the mouse resulted in no adrenal pathology at exposures sufficient to cause substantial DGAT1 but not ACAT inhibition, thereby implicating ACAT rather than DGAT1 inhibition as the probable cause of the observed adrenal changes. Recognizing that the distinctive nondegenerative reduction in cortical fine vacuolation in the mouse could be used as a histopathological biomarker for an in vivo model of the more severe changes observed in dogs, the mouse has subsequently been used as a model to select DGAT1 inhibitors free of adrenocortical toxicity.

Design and synthesis of a novel series of cyclohexyloxy-pyridyl derivatives as inhibitors of diacylglycerol acyl transferase 1

Design and synthesis of a novel series of cyclohexyloxy-pyridyl inhibitors of diacylglycerol acyl transferase 1.

Optimisation of biphenyl acetic acid inhibitors of diacylglycerol acetyl transferase 1 – the discovery of AZD2353

Focus on ligand efficiency, ligand lipophilicity efficiency, and conformational restriction led to the discovery of AZD2353.

Intestinal DGAT1 deficiency reduces postprandial triglyceride and retinyl ester excursions by inhibiting chylomicron secretion and delaying gastric emptying

Acyl CoA:diacylglycerol acyltransferase (DGAT) 1 catalyzes the final step of triglyceride (TG) synthesis. We show that acute administration of a DGAT1 inhibitor (DGAT1i) by oral gavage or genetic deletion of intestinal Dgat1 (intestine-Dgat1(-/-)) markedly reduced postprandial plasma TG and retinyl ester excursions by inhibiting chylomicron secretion in mice. Loss of DGAT1 activity did not affect the efficiency of retinol esterification, but it did reduce TG and retinoid accumulation in the small intestine. In contrast, inhibition of microsomal triglyceride transfer protein (MTP) reduced chylomicron secretion after oral fat/retinol loads, but with accumulation of dietary TG and retinoids in the small intestine. Lack of intestinal accumulation of TG and retinoids in DGAT1i-treated or intestine-Dgat1(-/-) mice resulted, in part, from delayed gastric emptying associated with increased plasma levels of glucagon-like peptide (GLP)-1. However, neither bypassing the stomach through duodenal oil injection nor inhibiting the receptor for GLP-1 normalized postprandial TG or retinyl esters excursions in the absence of DGAT1 activity. In summary, intestinal DGAT1 inhibition or deficiency acutely delayed gastric emptying and inhibited chylomicron secretion; however, the latter occurred when gastric emptying was normal or when lipid was administered directly into the small intestine. Long-term hepatic retinoid metabolism was not impacted by DGAT1 inhibition.

DGAT1 deficiency decreases PPAR expression and does not lead to lipotoxicity in cardiac and skeletal muscle

Diacylglycerol (DAG) acyl transferase 1 (Dgat1) knockout ((-/-)) mice are resistant to high-fat-induced obesity and insulin resistance, but the reasons are unclear. Dgat1(-/-) mice had reduced mRNA levels of all three Ppar genes and genes involved in fatty acid oxidation in the myocardium of Dgat1(-/-) mice. Although DGAT1 converts DAG to triglyceride (TG), tissue levels of DAG were not increased in Dgat1(-/-) mice. Hearts of chow-diet Dgat1(-/-) mice were larger than those of wild-type (WT) mice, but cardiac function was normal. Skeletal muscles from Dgat1(-/-) mice were also larger. Muscle hypertrophy factors phospho-AKT and phospho-mTOR were increased in Dgat1(-/-) cardiac and skeletal muscle. In contrast to muscle, liver from Dgat1(-/-) mice had no reduction in mRNA levels of genes mediating fatty acid oxidation. Glucose uptake was increased in cardiac and skeletal muscle in Dgat1(-/-) mice. Treatment with an inhibitor specific for DGAT1 led to similarly striking reductions in mRNA levels of genes mediating fatty acid oxidation in cardiac and skeletal muscle. These changes were reproduced in cultured myocytes with the DGAT1 inhibitor, which also blocked the increase in mRNA levels of Ppar genes and their targets induced by palmitic acid. Thus, loss of DGAT1 activity in muscles decreases mRNA levels of genes involved in lipid uptake and oxidation.

DGAT1 inhibitors as anti-obesity and anti-diabetic agents

Since 2008, significant advances have been made in understanding the role of diacylglycerol acyl transferase-1 (DGAT1) in disease states such as diabetes and obesity. Gene deletion and overexpression studies have provided important new insights into the function of DGAT1, as have the first reports from preclinical models of small-molecule inhibitor effects, which are discussed in this review in relation to the phenotypes of DGAT knockout and overexpression models. The progress of medicinal chemistry efforts has resulted in a new generation of DGAT1 inhibitors that have progressed into clinical development, with the leading compound LCQ-908 (Novartis AG) now in phase II clinical trials. This exciting progress has led researchers to anticipate that an understanding of the human pharmacology of DGAT1 inhibitors, as well as their potential as therapeutic agents for the treatment of diabetes and obesity, will be achieved in the next few years.

Discovery of a potent, selective, and orally efficacious pyrimidinooxazinyl bicyclooctaneacetic acid diacylglycerol acyltransferase-1 inhibitor

Inhibition of DGAT-1 is increasingly seen as an attractive mechanism with the potential for treatment of obesity and other elements of the metabolic syndrome. We report here a bicyclooctaneacetic acid derivative in the pyrimidinooxazine structural class of DGAT-1 inhibitors that has good potency, selectivity, and pharmacokinetic characteristics across a variety of species. This compound is an effective inhibitor of DGAT-1 in both intestinal and adipose tissue, which results in a reduction in body weight or body weight gain following oral administration in both mouse and rat models of dietary-induced obesity.

An assessment of the in vivo efficacy of the glycogen phosphorylase inhibitor GPi688 in rat models of hyperglycaemia

BACKGROUND AND PURPOSE

Studies in cultured hepatocytes demonstrate glycogen synthase (GS) activation with glycogen phosphorylase (GP) inhibitors. The current study investigated whether these phenomena occurred in vivo using a novel GP inhibitor.

EXPERIMENTAL APPROACH

An allosteric GP inhibitor, GPi688, was evaluated against both glucagon-mediated hyperglycaemia and oral glucose challenge-mediated hyperglycaemia to determine the relative effects against GP and GS in vivo.

KEY RESULTS

In rat primary hepatocytes, GPi688 inhibited glucagons-mediated glucose output in a concentration dependent manner. Additionally GP activity was reduced and GS activity increased seven-fold. GPi688 inhibited glucagon-mediated hyperglycaemia in both Wistar (65%) & obese Zucker (100%) rats and demonstrated a long duration of action in the Zucker rat. The in vivo efficacy in the glucagon challenge model could be predicted by the equation; % glucagon inhibition=56.9+34.3[log ([free plasma]/rat IC50)], r=0.921). GPi688 also reduced the blood glucose of obese Zucker rats after a 7 h fast by 23%. In an oral glucose tolerance test in Zucker rats, however, GPi688 was less efficacious (7% reduction) than a glycogen synthase kinase-3 (GSK-3) inhibitor (22% reduction), despite also observing activation (by 45%) of GS in vivo.

CONCLUSIONS AND IMPLICATIONS

Although GP inhibition can inhibit hyperglycaemia mediated by increased glucose production, the degree of GS activation induced by allosteric GP inhibitors in vivo, although discernible, is insufficient to increase glucose disposal. The data suggests that GP inhibitors might be more effective clinically against fasting rather than prandial hyperglycaemic control.

Glucagon challenge in the rat: A robust method for the in vivo assessment of Glycogen phosphorlyase inhibitor efficacy

INTRODUCTION

Glycogen phosphorlyase inhibitors (GPi) act on the glycogenolytic pathway decreasing hepatic glucose output, making them potential candidates for Type 2 diabetes treatment. We established a robust in vivo method to assess GPis efficacy utilising glucagon-stimulated glycogenolysis.

METHODS

Blood glucose was monitored in both male AP Wistar and AP Zucker rats using tail prick samples pre- and post intraperitoneal or subcutaneous glucagon administration. The effect of glycogen phosphorylase inhibitors GPi296 (6-60 mg kg(-1) po) and DAB (5 mg kg(-1) po) upon glucose response to subcutaneous glucagon were examined in both strains.

RESULTS

In the Wistar rat glucagon induced dose related increases in blood glucose, with the maximum increase occurring 20 min post dose (4.0+/-0.88 mmol l(-1), intraperitoneal; and 2.8+/-0.72 mmol l(-1), subcutaneous, ns). Intraperitoneal glucagon administration produced shorter duration blood glucose elevation than observed with the subcutaneous route of administration. In the Zucker rat, no differences were observed between the 10 and 13 week old rats in response to glucagon (3-200 microg kg(-1) subcutaneous). The maximum blood glucose increase was lower in the Wistar rat compared to the Zucker rats (2.9+/-0.20 vs 7.7+/-1.22 mmol l(-1), P<0.0000018). GPi296 and DAB both produced similar inhibition in each strain.

DISCUSSION

Subcutaneous glucagon administration induced more sustained increases in blood glucose than intraperitoneal administration. Blood glucose response to glucagon was higher in the Zucker rat compared to the Wistar rat; there was no difference in inhibition mediated by either GPi296 or DAB between the two strains. We believe that subcutaneous glucagon administration produces a robust model for the assessment of GPis in either rat strain.

Inhibition of 11beta-hydroxysteroid dehydrogenase type 1 reduces food intake and weight gain but maintains energy expenditure in diet-induced obese mice

AIMS/HYPOTHESIS

The 11beta-hydroxysteroid dehydrogenase type-1 inhibitor BVT.2733 lowers blood glucose and insulin in mutant mouse models of obesity and diabetes. Its effects on energy balance and body composition, and their contribution to improved glucose homeostasis have received little attention.

MATERIALS AND METHODS

BVT.2733 (100 mg/kg, orally) was given twice daily to lean and diet-induced obese mice for 16 or 17 days. A group of obese mice was pair-fed to the amounts consumed by BVT.2733-treated mice.

RESULTS

In both obese and lean mice, BVT.2733 reduced food intake and weight gain, but increased water intake. Pair-feeding caused almost as great a decrease in body weight as BVT.2733. Energy expenditure was 38+/-8% higher in the BVT.2733-treated obese mice than in the pair-fed mice. Terminal plasma corticosterone was raised, lean body weight reduced and percentage fat unchanged in the pair-fed mice (control, 47.8+/-2.6%; pair-fed, 47.1+/-1.9%), whereas BVT.2733 did not reduce lean mass, but did reduce percentage fat (40.9+/-2.0%). BVT.2733 but not pair-feeding reduced both the glucose tolerance AUC and the plasma insulin concentration 30 min after giving glucose.

CONCLUSIONS/INTERPRETATION

BVT.2733 reduced food intake but prevented a concomitant reduction in lean body mass and energy expenditure. The latter effects may have contributed to improved glucose tolerance.

Interleukin-6 is an afferent signal to the hypothalamo-pituitary-adrenal axis during local inflammation in mice

The cytokines IL-1 and IL-6 are able to induce prostaglandin (PG)-dependent activation of the hypothalamo-pituitary-adrenal axis (HPAA) and are thought to play key roles in immune-neuroendocrine interactions during inflammation. The present study shows that inflammation induced by im injection of turpentine (TPS) in the hind limb of mice causes an increase in the plasma concentration of IL-6, but not that of IL-1 alpha or IL-1 beta, together with a prolonged (>18-h) activation of the HPAA. IL-6 plays a causal role in the TPS-induced elevation in HPAA activity, because the sustained (8-18 h) increases in 1) plasma corticosterone, 2) plasma ACTH, and 3) induction of c-Fos in the hypothalamic paraventricular nucleus are all markedly blunted in IL-6-deficient (IL-6(-/-)) mice. Peripheral administration of a neutralizing IL-6 antiserum inhibited the plasma corticosterone response of normal (C57BL/6) mice to hind limb inflammation to an extent similar to that seen in IL-6(-/-) mice, suggesting that the IL-6 responsible for the increased HPAA activity is produced, or acts, on the blood side of the blood-brain barrier. We also show that IL-6 in the circulation is induced almost exclusively at the local inflammatory site, where IL-1 beta is produced. Induction of IL-6 and activation of the HPAA are dependent upon prior activation of an IL-1 type I receptor, as both are inhibited in type I IL-1 receptor-deficient mice. Furthermore, hind limb inflammation induced cyclooxygenase-2 protein expression around the cerebrovasculature of normal (IL-6(+/+)), but not IL-6(-/-), mice. Based on these data, we propose that IL-6 is produced at the local inflammatory site under the control of IL-1 beta and is the circulating afferent signal that is in part responsible for elevated HPAA activity, possibly acting via eicosanoid production within the cerebrovasculature.

Proopiomelanocortin-derived peptides in rat cerebrospinal fluid and hypothalamic extracts: evidence that secretion is regulated with respect to energy balance

Regulation of proopiomelanocortin (POMC) is an important means of controlling the central melanocortin system. It has never been established whether the spectrum of POMC-derived peptides synthesized and secreted from the hypothalamus is altered in response to changes in energy homeostasis in vivo. To monitor secretion, we analyzed peptide content of rat cerebrospinal fluid. Strikingly, both the POMC precursor and ACTH were readily detected. Moreover, levels of both were lower in samples from obese Zucker rats (fa/fa) vs. lean Zucker rats (+/+, fa/+) and from fasted vs. fed rats, whereas alpha MSH could not be detected. POMC levels were also decreased in hypothalamic extracts from obese and fasted animals. In contrast, despite being the most predominant peptide in extracts, alpha MSH levels were not significantly changed in any of the rat models. The ratio of precursor to derived peptides in cerebrospinal fluid was significantly higher in obese vs. lean and fed vs. fasted rats, indicating that secretion of POMC-derived peptides is differentially down-regulated during negative energy balance. In contrast to peptide analysis, we found that POMC gene expression was not significantly decreased in fasted rat hypothalami. We conclude that regulation of peptide secretion is an important mechanism by which the POMC system is controlled.

Rat anterior pituitary folliculostellate cells are targets of interleukin-1beta and a major source of intrapituitary follistatin

Folliculostellate cells of the anterior pituitary are postulated to be an important source of factors, such as follistatin, that regulate pituitary function by intercellular communication. To gain further insight into the function of this cell type, folliculostellate cells were enriched from cultured rat anterior pituitary cells, and an immortalized cell line designated FS/D1h was established and characterized. These FS/D1h cells express S100 immunoreactivity and produce IL-6 but not pituitary hormones such as GH, ACTH, FSH, and LH. Importantly, FS/D1h cells express large amounts of follistatin mRNA and secrete the protein, as quantified indirectly by the amount of [(125)I]activin A immunoprecipitated with a follistatin antiserum. The FS/D1h cells also express alpha, betaA, and betaB inhibin/activin subunit mRNAs, but whether they produce the corresponding activins and inhibins has not been determined. The response of FS/D1h cells to agents thought to modulate folliculostellate cell function was evaluated. IL-1beta (0.005-5 nM) stimulated the secretion of follistatin and increased mRNA expression. In parallel, IL-6 secretion was stimulated. Dexamethasone, pituitary adenylate cyclase-activating polypeptide(1-27), and lipopolysaccharide but not testosterone, 12-O-tetradecanoylphorbol-13-acetate, or forskolin also increased follistatin secretion. Surprisingly, activin had no effect on follistatin mRNA levels, despite the fact that FS/D1h cells express ActRII, ActRIIB, and ALK-4 (ActRIB). Activin, on the other hand, induced Smad7 mRNA accumulation and exerted an antiproliferative effect on FS/D1h cells. Altogether, these observations support the possibility that follistatin originating from folliculostellate cells participates in mediating the effects of IL-1beta, glucocorticoids, and other agents on the response of pituitary cells to activins.

Discovery and optimization of a series of carbazole ureas as NPY5 antagonists for the treatment of obesity

The hypothesis that antagonists of the neuropeptide Y5 receptor would provide safe and effective appetite suppressants for the treatment of obesity has prompted vigorous research to identify suitable compounds. We discovered a series of acylated aminocarbazole derivatives (e.g., 3a) that are potent and selective Y5 antagonists, representing interesting starting points but suffering from poor bioavailability and concerns about potential toxicity as a consequence of the embedded aminocarbazole fragment. It proved relatively easy to improve the drug metabolism and pharmacokinetic (DMPK) properties by variation of the side chain (as in 4a) but difficult to eliminate the aminocarbazole fragment. For compounds in this series to have the potential to be drugs, we believed that both the compound itself and the component aniline must be free of mutagenic activity. Parallel structure-activity relationship studies looking at the effects of ring substitution have proved that it is possible by incorporation of a 4-methyl substituent to produce carbazole ureas with potent Y5 activity, comprised of carbazole anilines that in themselves are devoid of mutagenic activity in the Ames test. Compound 4o (also known as NPY5RA-972) is highly selective with respect to Y1, Y2, and Y4 receptors (and also to a diverse range of unrelated receptors and enzymes), with an excellent DMPK profile including central nervous system penetration. NPY5RA-972 (4o) is a highly potent Y5 antagonist in vivo but does not block neuropeptide Y-induced feeding nor does it reduce feeding in rats, suggesting that the Y5 receptor alone has no significant role in feeding in these models.

Selective antagonism of the NPY Y5 receptor does not have a major effect on feeding in rats

Neuropeptide Y (NPY) is thought to play a key role in stimulating feeding, thus making NPY receptors attractive appetite suppressant drug targets for treating obesity. Because the orexigenic effects of NPY have been ascribed to actions at the NPY Y5 receptor, we have determined the role of this receptor in feeding in rats, using a small molecule antagonist of this receptor. NPY5RA-972 is a selective and potent (<10 nmol/l) NPY Y5 receptor antagonist. This compound is central nervous system (CNS) penetrant, and an oral dose of 10 mg/kg NPY5RA-972 to rats produced concentrations in cerebrospinal fluid that greatly exceeded the in vitro IC(50) (inhibitory concentration 50%). Indeed, at doses to rats as low as 1 mg/kg, NPY5RA-972 inhibited feeding induced by intracerebroventricular (ICV) administration of a selective NPY Y5 agonist ([cPP(1-7),NPY(19-23),Ala(31),Aib(32),Gln(34)]-hPP). However, in the dose range 1-10 mg/kg, NPY5RA-972 had no significant effect on food intake in Wistar rats induced to feed by either ICV NPY or 24 h fasting or in free-feeding Wistar or obese Zucker rats. Chronic administration of NPY5RA-972 (10 mg/kg twice daily) had no effect on food intake or body weight in either free-feeding Wistar rats or dietary obese rats. These data indicate that NPY5RA-972 is a potent, selective, orally active, and CNS-penetrant antagonist of the NPY Y5 receptor that prevents feeding driven by activation of this receptor. The data obtained with this antagonist indicate that the NPY Y5 receptor is not a major regulator of feeding in the rat.

Pro-opiomelanocortin processing in the hypothalamus: impact on melanocortin signalling and obesity

Bioactive peptides derived from the prohormone, pro-opiomelanocortin (POMC), are generated in neurons of the hypothalamus and act as endogenous ligands for the melanocortin-4 receptor (MC4R), a key molecule underlying appetite control and energy homeostasis. It is therefore important to understand many aspects of POMC gene regulation in the brain, as pharmacological manipulation of POMC expression/processing could be a potential strategy to combat obesity. Most studies that have analysed POMC gene expression in the hypothalamus have focused on gene transcription experiments. Ultimately, however, factors that regulate post-translational processing and secretion of peptides will have most bearing on melanocortin signalling. This article focuses on (a) current evidence that POMC is involved in obesity, (b) how POMC transcription is regulated in the hypothalamus, (c) the mechanism by which proteolytic processing of POMC is controlled in the hypothalamus and what peptides are produced and (d) which POMC-derived peptides are the most potent ligands at the melanocortin receptor in vitro and in vivo. It seems that post-translational cleavage of POMC in the hypothalamus may be regulated with respect to energy requirement. We predict that further research into hypothalamic POMC processing, and the proteolytic enzymes involved, may yield important new clues on how flux through the MC4R pathway is regulated.

Circulating interleukin-6 mediates the febrile response to localised inflammation in rats

Interleukin (IL)-6 is an important mediator of the host response to disease and has been proposed, largely based upon circumstantial evidence, as the principal endogenous circulating pyrogen responsible for activating CNS mechanisms in fever during infection and inflammation. In the present investigation, we studied the role of peripheral IL-6 in fever and its relationship with IL-1, itself an important endogenous pyrogen and a potent stimulus of IL-6 production. Injection of lipopolysaccharide (LPS) into a sterile, subcutaneous air pouch (i.po.) in rats evoked an increase in body temperature which peaked at 3 h, and which was abolished in animals pretreated (intraperitoneally) with IL-6 antiserum. The increase in body temperature was accompanied by a significant elevation in concentrations of (immunoreactive) IL-1 and IL-6 at the site of inflammation (pouch), but only IL-6 in the circulation and cerebrospinal fluids. We propose that much of the circulating IL-6 originates at the site of inflammation, since injection of human recombinant (hr)IL-6 (i.po.) was detected (10 min after the injection) in the plasma using an ELISA specific for human IL-6. However, despite the relatively high concentration of IL-6 injected (25 microg kg-1, i.po.), this cytokine had no effect on body temperature when injected alone, but did induce fever when co-injected with a non-pyrogenic dose (when given alone) of IL-1beta, and exacerbated the fever to a pyrogenic dose of IL-1beta. The results from the present study demonstrate that IL-6 is a circulating endogenous pyrogen during LPS-induced fever, which acts in concert with IL-1beta at the local site of inflammation, before entering the circulation. Circulating IL-6 can then activate CNS mechanisms resulting in the development of the febrile response during disease.

Hypothalamic control of feeding

Our understanding of the hypothalamic control of energy homeostasis has increased greatly since the discovery of leptin, the adipose cell derived protein. Recent studies have identified several new hypothalamic neuropeptides that affect food intake and energy balance. By studying these molecules and their neuronal systems, receptors and interactions, we are beginning to unravel the circuitry between peripheral adipogenic signals and hypothalamic effector pathways.

Sprague-Dawley rats obtained from different vendors exhibit distinct adrenocorticotropin responses to inflammatory stimuli

The purpose of this work was to compare the plasma adrenocorticotropin (ACTH), corticosterone and interleukin-6 (IL-6) responses that rats of the outbred Sprague-Dawley strain obtained from two different vendors: Charles River (CR) and Harlan (HSD). Basal plasma ACTH and IL-6 concentrations were similar in rats from either vendor (HSD or CR), while CR animals exhibited slightly elevated corticosterone levels in late afternoon. Inflammatory stimuli such as lipopolysaccharide (LPS) (1 microgram/kg, i.v.) or turpentine (50 microliter/100 g, i.m.) which induce the production of endogenous cytokines, produced a significantly larger ACTH response in CR, compared to HSD rats, while the overall corticosterone responses were comparable in both rat groups. This could probably not be accounted for by a greater ACTH responsiveness in CR rats per se because CR and HSD rats showed similar peak ACTH responses to electrofootshock. Furthermore, in contrast to when the stimulus was one that induced endogenous cytokine production, the administration of exogenous interleukin-1beta (IL-1beta, 200 ng/kg, i.v.) produced a 2-fold greater rise in plasma ACTH concentrations in HSD rats compared to CR rats. The plasma IL-6 responses to the inflammatory stimuli showed a similar pattern to ACTH, with LPS and turpentine tending to pruduce greater IL-6 responses in CR rats, though these differences were not statistically significant. In contrast HSD rats had a significantly greater IL-6 response to IL-1beta than did CR rats. Collectively, these results show that Sprague-Dawley rats obtained from different commercial sources can differ in immune-neuroendocrine responses to inflammatory stimuli.

CRF type I receptor-deficient mice exhibit a pronounced pituitary-adrenal response to local inflammation

Recent studies indicate that the regulation of adrenocorticotropin (ACTH) secretion by corticotropin-releasing factor (CRF) is mediated predominantly by the type I CRF receptor (CRF-R1). Indeed, CRF-R1-deficient (CRF-R1 -/-) mice show marked impairment of the pituitary-adrenal axis. However, the plasma ACTH concentrations of unstressed CRF-R1 -/- mice are similar to those in wild-type mice. We show here that arginine vasopressin (AVP) is a major ACTH secretagogue in CRF-R1 -/- mice in resting conditions, since administration of anti-AVP serum, but not anti-CRF serum, markedly reduced (by 60%) resting plasma ACTH concentrations in these mutants. We also investigated the pituitary-adrenal response to turpentine-induced local inflammation in CRF-R1 -/- mice. Administration of turpentine into the hind-limb of CRF-R1 -/- mice produced a slightly (15-25%) smaller swelling of the limb, but a 10 fold greater rise in plasma IL-6 levels, compared to CRF-R1 +/+ controls. Turpentine-induced local inflammation produced pronounced elevations in the plasma concentrations of both ACTH and corticosterone in both CRF-R1 -/- and wild-type mice, but ACTH secretion could be inhibited by anti-CRF and anti-AVP sera only in wild-type mice. These data indicate that resting ACTH secretion in CRF-R1 -/- mice is in part attributable to AVP-dependent mechanisms. Furthermore, while in normal mice the pituitary-adrenal response to local inflammation is mediated largely via CRF-dependent mechanisms, mice deficient in CRF-R1 are still able to mount a pituitary-adrenal response via mechanisms that do not depend critically on either CRF or AVP action.

Effects of prenatal exposure to alcohol on the release of adenocorticotropic hormone, corticosterone, and proinflammatory cytokines

Prenatal alcohol exposure has been shown to produce hyperresponsiveness of the hypothalamic-pituitary-adrenal (HPA) axis to immune challenges. Because cytokines, which are released in response to immune challenges, are known to activate the HPA axis, this study determined whether altered release of cytokines contribute to the HPA hyperresponsiveness to immune challenges observed after prenatal alcohol exposure. Pregnant dams were exposed to alcohol vapors (6-7 hr daily) between days 7 and 18 of gestation. At postnatal days 45 and 60, control (C) and prenatal alcohol-exposed (E) offspring were subjected to three different types of immune challenges: injections of interleukin-1beta or endotoxin (lipopolysaccharide), or turpentine-induced tissue injury. We observed the expected higher plasma adrenocorticotropic hormone and corticosterone levels in E compared with C rats, and this HPA hyperresponsiveness was greater in E females compared with E males. Plasma tumor necrosis factor-alpha or interleukin-6 responses were comparable in the C and E groups. Females exhibited significantly higher corticosterone, tumor necrosis factor-alpha, and interleukin-6 responses than males. These results indicate that (1) prenatal alcohol exposure produces HPA hyperresponsiveness to immune challenges; (2) prenatal alcohol treatment does not influence the release of cytokines to immune challenges; and (3) there are gender differences in the secretory pattern of corticosterone and cytokines to immune challenges. Therefore, these data do not support the hypothesis that cytokines play a role in the hyperresponsiveness of the HPA axis to immune challenges observed after prenatal alcohol exposure.

Urocortin is not a significant regulator of intermittent electrofootshock-induced adrenocorticotropin secretion in the intact male rat

Urocortin (Ucn) is a newly identified mammalian member of the CRF family of peptides. Ucn activates CRF receptors (both CRF-R1 and CRF-R2) with greater potency than CRF itself, suggesting that Ucn may play an endogenous role in eliciting (at least some) CRF receptor-mediated events. Because the most characterized physiological function of CRF receptors is the activation of pituitary ACTH secretion, we have compared the effects and potential endogenous roles of CRF and Ucn in regulating plasma ACTH concentrations in intact male rats. Synthetic rat Ucn injected i.v. (0.09-9.0 nmol/kg) elicited ACTH secretion in a dose-dependent manner, causing greater ACTH secretion than CRF at each dose tested. The increases in plasma ACTH concentrations produced by CRF or Ucn were virtually abolished by pretreatment with the CRF receptor antagonist, astressin (3 mg/kg), and were partially attenuated (by 27-37%) by an antiarginine vasopressin serum. These data indicate that both Ucn and CRF elicit ACTH secretion via CRF receptor-dependent mechanisms, and that the ACTH-releasing activities of both CRF and Ucn are potentiated by endogenous arginine vasopressin. Intravenous administration of rabbit anti-Ucn serum, which inhibited ACTH secretion produced by Ucn, but not CRF, had no statistically significant effect on either resting (midday) plasma ACTH concentrations or the rise in ACTH levels elicited by 30 min of intermittent electrofootshocks. By contrast, treatment with a rabbit anti-CRF serum that specifically inhibited the ACTH response to CRF lowered plasma concentrations in control unstressed rats and largely prevented the plasma ACTH response to electrofootshocks. These data indicate that although Ucn is a more potent ACTH secretagogue than CRF in the intact male rat, it is not a major endogenous regulator of pituitary ACTH secretion under basal (midday) conditions or during acute footshock stress.

Regulation of the hypothalamic-pituitary-adrenal axis by cytokines: actions and mechanisms of action

Glucocorticoids are hormone products of the adrenal gland, which have long been recognized to have a profound impact on immunologic processes. The communication between immune and neuroendocrine systems is, however, bidirectional. The endocrine and immune systems share a common "chemical language," with both systems possessing ligands and receptors of "classical" hormones and immunoregulatory mediators. Studies in the early to mid 1980s demonstrated that monocyte-derived or recombinant interleukin-1 (IL-1) causes secretion of hormones of the hypothalamic-pituitary-adrenal (HPA) axis, establishing that immunoregulators, known as cytokines, play a pivotal role in this bidirectional communication between the immune and neuroendocrine systems. The subsequent 10-15 years have witnessed demonstrations that numerous members of several cytokine families increase the secretory activity of the HPA axis. Because this neuroendocrine action of cytokines is mediated primarily at the level of the central nervous system, studies investigating the mechanisms of HPA activation produced by cytokines take on a more broad significance, with findings relevant to the more fundamental question of how cytokines signal the brain. This article reviews published findings that have documented which cytokines have been shown to influence hormone secretion from the HPA axis, determined under what physiological/pathophysiological circumstances endogenous cytokines regulate HPA axis activity, established the possible sites of cytokine action on HPA axis hormone secretion, and identified the potential neuroanatomic and pharmacological mechanisms by which cytokines signal the neuroendocrine hypothalamus.

Influence of carbon monoxide, and its interaction with nitric oxide, on the adrenocorticotropin hormone response of the normal rat to a physico-emotional stress

We determined whether the gas carbon monoxide (CO) altered the adrenocorticotropin hormone (ACTH) response to mild inescapable electrofootshocks, and whether it interacted with nitric oxide (NO). Peripheral injection of the NO synthase (NOS) inhibitor Nwnitro-L-arginine-methylester (L-NAME), a compound which readily crosses the blood-brain barrier, produced the expected blunting of the ACTH response to the shocks. This effect was mimicked by other arginine analogues such as L-nitroarginine (L-NNA) and NG-methyl-L-arginine (NMMA). The subcutaneous (s.c.) administration of the heme oxygenase (HO) blockers tin mesoporphyrin (SnMP) or tin protoporphyrin (SnPP) significantly decreased brain HO levels, indicating that both compounds had penetrated the brain. Blood pressure showed a modest increase in response to SnMP, and no change after SnPP. SnMP and SnPP both decreased shock-induced ACTH release, though the magnitude of this effect was slightly less than that of L-NAME. The influence of SnPP was further augmented in rats with concomitant blockade of NO formation, which suggests that both NO and CO are necessary for the full response of this axis to electrofootshocks. Finally, the ability of SnPP to significantly blunt the expression of the mRNA for the immediate early gene NGFI-B in the paraventricular nucleus (PVN) of rats exposed to shocks, indicates that the influence of CO was exerted on hypothalamic neuronal activity. Collectively, our results show that NO and CO exert a stimulatory effect on the HPA axis response to mild electrofootshocks, and that at least part of this influence takes place on hypothalamic neurons and/or their afferents.

Interleukin-1beta regulates pituitary follistatin and inhibin/activin betaB mRNA levels and attenuates FSH secretion in response to activin-A

Activins and follistatins regulate all levels of the reproductive axis, including the pituitary where they stimulate and inhibit FSH production, respectively. Gonadotropes are known to express inhibin/activin betaB and activin-B (betaBbetaB) functions as an autocrine modulator of FSH production. By contrast, the mRNA for the activin-binding protein, follistatin, is present in most pituitary cells and folliculo-stellate cells may be the major source of the protein secreted by the anterior pituitary. Interleukin-1beta (IL-1beta) is one of several cytokines known to also influence the reproductive axis. IL-1beta inhibits the hypothalamo-pituitary-gonadal (HPG) axis by suppressing GnRH and gonadal steroid production. Because several pituitary cell types, including follistatin-producing folliculo-stellate cells, are targets of IL-1beta, cytokine effects on gonadotrope function were evaluated using cultured rat anterior pituitary cells. Activin-A (0.01 to 1 nM; 24h) increased basal FSH secretion approximately 2-fold. IL-1beta (0.005 to 0.5 nM) by itself had no effect on basal FSH secretion. However, IL-1beta attenuated FSH secretion in response to all concentrations of activin-A. These results suggest that the cytokine might stimulate the local production of a factor, such as follistatin, that antagonizes the action of activin-A. RNase protection analysis indicated that IL-1beta (0.005 to 5 nM) stimulated follistatin and inhibin/activin betaB mRNA accumulation in a time-dependent manner. These in vitro effects of IL-1beta were blocked by the specific IL-1 receptor antagonist (IL-lra) and were not mimicked by either rhIL-6 or lipopolysaccharide (LPS). Treatment of intact male rats with LPS (50 microg, i.v.), which increases plasma IL-1beta and induces IL-1beta expression in many tissues, including the pituitary, produced similar time-dependent increases in pituitary follistatin and inhibin/activin subunit mRNA levels. These results suggest that IL-1beta can modulate gonadotrope responses to activins by influencing the local balance of activin-B and follistatin within the pituitary.

Mechanisms of hypothalamic-pituitary-adrenal axis stimulation by immune signals in the adult rat

Immune stimulation increases the activity of the HPA axis, a phenomenon directly or indirectly mediated through cytokines. We have used two models, the peripheral administration of endotoxin (LPS) or turpentine-induced tissue injury to show that corticotropin-releasing factor (CRF) and vasopressin (VP), hypothalamic peptides released by cytokines, play a dominant role in the increased ACTH measured in these two paradigms. In turn, CRF and VP synthesis and/or release is modulated by catecholamines, prostaglandins (PGs), and nitric oxide (NO). These secretagogues are produced in the periphery and/or the central nervous system (CNS) in response to increased cytokine levels and act on CRF/VP neurons and nerve terminals. Finally, endotoxemia and local tissue inflammation may upregulate brain levels of tumor necrosis factor alpha, interleukin-1 beta, and/or interleukin-6, providing yet another mechanism through which the occurrence of systemic inflammation is conveyed to the brain. The relative importance of brain or peripheral intermediates appears to depend on the site at which cytokine levels are increased. We have shown, for example, that peripheral, but not brain, PGs are important in mediating the neuroendocrine influence of blood-borne cytokines, while PGs in the CNS play a role in situations characterized by elevated brain immune proteins. NO, on the other hand, restrains the response of the HPA axis to circulating, but not brain cytokines. These results illustrate the complexity of the mechanisms involved in the stimulation of the HPA axis and suggest that their specific involvement depends on the type, intensity, and duration of immune stimulation.

Cerebral interleukin-6 is neuroprotective during permanent focal cerebral ischemia in the rat

Interleukin-6 (IL-6) is a neurotrophic cytokine expressed in both neurons and glia. The present study shows that cerebral ischemia produced by permanent occlusion of the middle cerebral artery (MCAO) produces a dramatic increase in IL-6 bioactivity in the ischemic hemisphere within 2 hours of MCAO (167 +/- 55 IU versus sham: 50 +/- 35 IU), with further increases at 8 hours (3,456 +/- 1,162 IU) and 24 hours (6,088 +/- 1,772 IU). In a separate series of experiments, intracerebroventricular injection of recombinant IL-6 (3,100 or 31,000 IU) significantly reduced ischemic brain damage after MCAO (to 52% and 65% of controls, respectively). The large increase in endogenous IL-6 bioactivity in response to ischemia, together with the marked neuroprotection produced by exogenous IL-6 suggest that this cytokine is an important endogenous inhibitor of neuronal death during cerebral ischemia.

Intracerebroventricular passive immunization. II. Intracerebroventricular infusion of neuropeptide antisera can inhibit neuropeptide signaling in peripheral tissues

The findings of the preceding article suggest that intracerebroventricular (i.c.v.) administration of small amounts (5 microl) of antisera to rats may produce effectual immunoneutralization of peptides in blood/tissues outside of the central nervous system (CNS). In the present work we sought to test this hypothesis by determining the titers of corresponding antibodies in jugular venous plasma after i.c.v. infusion of three different antisera: a sheep anti-CRF, a rabbit anti-CRF, and a rabbit anti-GnRH. For all antisera tested, corresponding antibodies were detected in systemic plasma within 30 min of i.c.v. infusion of 5 microl antiserum. By 8 h, blood levels of the corresponding antibodies were similar whether the antisera had been infused i.c.v. or i.v. When the dilutions of antibodies equivalent to those in systemic blood 1-24 h after i.c.v. infusion of 5 microl antiserum were employed in rat anterior pituitary cell culture assays, they proved effective at inhibiting CRF- or GnRH-induced hormone secretion. Furthermore, in rats pretreated i.c.v. with 5 microl anti-CRF (at -4 h), pituitary ACTH secretion induced by i.v. CRF (0.3 nmol/kg) was reduced by 88%. Collectively, these data demonstrate that shortly after i.c.v. infusion of neuropeptide antisera, the levels of corresponding antibodies found in systemic blood are sufficient to inhibit neuropeptide signaling within peripheral tissues. As i.c.v. passive immunization procedures have been used extensively in the investigation of the biological roles of neuropeptides within the CNS, these findings indicate a critical reevaluation of the peripheral vs. CNS functions of neuropeptides.

Intracerebroventricular passive immunization. I. The effect of intracerebroventricular administration of an antiserum to tumor necrosis factor-alpha on the plasma adrenocorticotropin response to lipopolysaccharide in rats

The present study tested the hypothesis that the cytokine tumor necrosis factor-alpha (TNF-alpha) is an important central nervous system mediator of the rat hypothalamo-pituitary-adrenal (HPA) axis response to the i.v. administration of lipopolysaccharide (LPS; 5 microg/kg). LPS produced a rapid (within 30 min) rise in plasma TNF-alpha levels, which preceded elevations in plasma ACTH (commencing at 45 min). Despite a lack of detectable TNF-alpha biological activity in the brain 30 min to 2 h after LPS administration, intracerebroventricular (i.c.v.) pretreatment (-20 h) with 5 microl anti-TNF-alpha antiserum significantly delayed the onset of the plasma ACTH response to LPS, suggesting that TNF-alpha acts within the brain. However, we also noted that the i.c.v. infusion of anti-TNF-alpha 20 h earlier produced experimentally significant concentrations of the same anti-TNF-alpha antibodies in systemic blood. This suggested the possibility that the effect of this antiserum was due to its leakage to the periphery. Indeed, 5 microl anti-TNF-alpha administered i.v. at -20 h produced an inhibition of the ACTH response to LPS that was temporally and quantitatively similar to that produced by i.c.v. anti-TNF-alpha. Intracerebroventricular administration of anti-TNF-alpha immediately before LPS produced only low systemic blood levels of corresponding anti-TNF-alpha antibodies and did not significantly alter the plasma ACTH response, whereas i.v. administration of anti-TNF-alpha immediately before LPS was clearly effective. Collectively, these results show that 1) biologically active levels of TNF-alpha in systemic plasma and the ensuing ACTH responses to LPS were always temporally and qualitatively related; and 2) even though i.c.v. administration of anti-TNF-alpha could inhibit the HPA axis response to LPS, this was apparent only when substantial amounts of anti-TNF-alpha antibodies had reached systemic blood. We, therefore, conclude that at the dose of LPS used in this study (5 microg/kg), TNF-alpha is an important mediator of the HPA axis response to LPS by an action within the periphery, but probably not within the brain.

Corticotropin-releasing factor (CRF) and endocrine responses to stress: CRF receptors, binding protein, and related peptides

Corticotropin-releasing factor (CRF) is a 41-amino acid neuropeptide, which is recognized as a critical mediator of complimentary, stress-related endocrine, autonomic, and behavioral responses in mammalian species. CRF belongs to a family of structurally related peptides including frogskin sauvagine and fish urotensin I. The effects of CRF and related peptides are mediated by two distinct receptors, which differ in their anatomical distribution, as well as in their pharmacological characteristics. In addition, CRF is bound with high affinity by a CRF binding protein (CRF-BP), which is a putative inhibitor of CRF action. CRF is probably not the sole endogenous ligand for CRF receptors or the CRF-BP, since a second mammalian member of the CRF family, urocortin, has recently been identified. This article describes recent findings with respect to CRF, its receptors, binding protein, and CRF-related peptides, which provide further insights into the role and mechanisms of CRF action in stress responses.

Inhibition of tumor necrosis factor-alpha action within the CNS markedly reduces the plasma adrenocorticotropin response to peripheral local inflammation in rats

The present study tested the hypothesis that the cytokine tumor necrosis factor-alpha (TNF-alpha) is an important CNS mediator of the hypothalamo-pituitary-adrenal (HPA) axis response to local inflammation in the rat. Recombinant murine TNF-alpha administered directly into the cerebroventricles of normal rats produced a dose-dependent increase in plasma adrenocorticotropin (ACTH) concentration. Local inflammation induced by the intramuscular injection of turpentine (50 microl/100 gm body weight) also produced an increase in plasma ACTH, peaking at 160-200 pg/ml at 7.5 hr after injection (compared with 10-30 pg/ml in controls). Intracerebroventricular pretreatment with either 5 microl of anti-TNF-alpha antiserum or 1-50 microg of soluble TNF receptor construct (rhTNFR:Fc) reduced the peak of the ACTH response to local inflammation by 62-72%. In contrast, intravenous treatment with the same doses of anti-TNF-alpha or rhTNFR:Fc had no significant effect on the ACTH response to local inflammation. Although these data indicated an action of TNF-alpha specifically within the brain, no increase in brain TNF-alpha protein (measured by bioassay) or mRNA (assessed using either in situ hybridization histochemical or semi-quantitative RT-PCR procedures) was demonstrable during the onset or peak of HPA activation produced by local inflammation. Furthermore, increased passage of TNF-alpha from blood to brain seems unlikely, because inflammation did not affect plasma TNF-alpha biological activity. Collectively these data demonstrate that TNF-alpha action within the brain is critical to the elaboration of the HPA axis response to local inflammation in the rat, but they indicate that increases in cerebral TNF-alpha synthesis are not a necessary accompaniment.

Endotoxin decreases corticotropin-releasing factor receptor 1 messenger ribonucleic acid levels in the rat pituitary

Bacterial endotoxins produce profound activation of the hypothalamo-pituitary-adrenal axis, mediated by stimulation of hypothalamic CRF neurons. Although a number of studies have described direct pituitary actions of inflammatory mediators, the effects of inflammatory stimuli on the sensitivity of corticotropes to CRF remain to be elucidated. The aim of this study was to determine the effects of inflammatory stress on the CRF receptor 1 (CRF-R1) messenger RNA (mRNA) levels in the rat pituitary. The systemic injection of endotoxin [lipopolysaccharide (LPS); 50 microg/kg, i.v.] increased plasma concentrations of ACTH and corticosterone. Ribonuclease protection analysis of total RNA isolated from individual whole pituitaries indicated that LPS produced a significant decrease in CRF-R1 mRNA that was evident by 2 h after injection (to 57% of control) and more marked by 6 h (to 38% of control). To evaluate whether the decrease in CRF-R1 mRNA was dependent upon increased exposure to CRF and/or vasopressin (AVP), LPS was injected with an anti-CRF antiserum, a CRF receptor antagonist (Astressin), or anti-AVP antiserum. A strong inhibition of the ACTH response to LPS was produced by pretreatment with anti-CRF antiserum, Astressin, or anti-AVP antiserum. However, these treatments had no effect on the decrease in CRF-R1 mRNA produced by LPS, indicating that neither CRF nor AVP are obligatory mediators of this pituitary response. The hypothesis that LPS might have direct pituitary effects on CRF-R1 mRNA levels was tested in vitro. Indeed, decreases in CRF-R1 mRNA to 43% and 53% of the control level were observed in rat anterior pituitary cell cultures that were treated with either LPS itself or the inflammatory mediator interleukin-1beta, respectively. Collectively, these results show that CRF receptor mRNA levels in the pituitary of the rat are markedly reduced by systemic LPS treatment and that this decrease is not dependent upon increased exposure of the pituitary to CRF or AVP, but may involve direct effects within the pituitary of either LPS itself or ensuing cytokine production.

Febrile response to tissue inflammation involves both peripheral and brain IL-1 and TNF-alpha in the rat

We investigated the role and interaction between tumor necrosis factor (TNF)-alpha, interleukin (IL)-1, and IL-6 in the development of fever and their involvement in brain and systemic pathways in response to localized tissue inflammation caused by injection of turpentine (TPS) in the rat. Intramuscular injection of 10 microl TPS caused significant increases in body temperature, of up to 2 degrees C, compared with saline-treated animals. Fevers were maximal 7-8 h after injection and were preceded by significant increases in plasma bioactive IL-6. No changes in circulating bioactive IL-1 or TNF-alpha were detected. Systemic injection of IL-1 receptor antagonist (IL-1ra, 2 mg/kg i.p.) or anti-TNF-alpha antiserum (0.4 ml i.v.) almost completely abolished the febrile responses to TPS over 8 h and markedly inhibited the rise in plasma IL-6 bioactivity measured 6 h after TPS. To test the involvement of brain cytokines, anti-TNF-alpha antiserum and IL-1ra were injected intracerebroventricularly. Injections of anti-TNF-alpha antiserum (3 microl/rat i.c.v.) or IL-1ra (400 microg/kg i.c.v.) significantly (P < 0.01 and P < 0.05, respectively) inhibited fever induced by TPS. These data suggest that both localized peripheral and brain IL-1 and TNF-alpha are involved directly in the pyrogenic response to inflammation. The results indicate that, in the periphery, IL-1 and TNF-alpha cause increased production of IL-6, the most likely candidate as a circulating endogenous pyrogen.

Inhibition of gonadotropin-induced testosterone secretion by the intracerebroventricular injection of interleukin-1 beta in the male rat

The intracerebroventricular (icv) injection of the proinflammatory cytokine interleukin (IL)-1 beta is known to significantly decrease plasma LH levels in the male rat, thereby lowering testosterone (T) secretion. We show here that central administration of this cytokine (20-80 ng) also inhibits T secretion in response to human CG (hCG), an effect that is apparent already when IL-1 beta is injected 15 min before hCG. This phenomenon is independent of LH secretion because lowering LH levels with the potent GnRH antagonist Azaline B neither mimics nor affects the suppressive influence of icv IL-1 beta on the hCG-induced T secretory response. Elevations in plasma corticosterone levels do not seem to play a role either, because icv IL-1 beta is able to blunt hCG-induced T secretion in animals whose corticosterone has been removed by adrenalectomy or reduced by the administration of antibodies to CRF. Furthermore, the observation that icv IL-1 beta inhibits the T response to hCG before elevations in plasma IL-6 concentrations are detectable, and that central treatment with the cytokine is more effective than iv treatment, indicates that circulating levels of neither IL-1 beta nor IL-6 are important mediators of this effect. Collectively, these results lead us to propose that IL-1 beta of central origin influences neural pathways linking the brain and the testes, resulting in decreased testicular responses to hCG.

Selective inhibitors of nitric oxide synthase (NOS) implicate a constitutive isoform of NOS in the regulation of interleukin-1-induced ACTH secretion in rats

Nitric oxide synthase (NOS) exists in at least three distinct isoforms: an inducible NOS (NOS II), and two forms which are constitutively expressed-brain NOS (NOS I) and endothelial NOS (NOS III). We have previously shown that the NOS inhibitor, N(ω)-nitro-L-arginine methyl ester hydrochloride (L-NAME), markedly potentiates and prolongs the increase in plasma adrenocorticotropin (ACTH) concentrations produced by the intravenous injection of interleukin-1β (IL-1β) in the rat. However, the mechanism of action of L-NAME is unknown. The purpose of the present study was to determine the effects on IL-1β-induced ACTH secretion in the rat, of several NOS inhibitors, whose selectivity for the different NOS isoforms has been well characterized, and which lack the muscarinic receptor antagonist properties that have been reported for L-NAME. Subcutaneous (sc) pretreatment with L-NAME (50-300 μmol/kg) produced the expected pronounced exacerbation of the ACTH response to IL-1β. This effect was mimicked by N(G)-nitro-L-arginine, which preferentially inhibits constitutive forms of NOS. In contrast, aminoguanidine, a selective inducible NOS inhibitor at doses up to 3×1.8 mmol/kg, was without effect, suggesting that it is a constitutive form of NOS that regulates the ACTH response to IL-1β. Selective inhibition of brain NOS using either 7-nitro-indazole (administered intraperitoneally) or L-NAME (administered intracerebroventricularly) did not significantly alter ACTH concentrations after IL-1β. Collectively, these data indicate that NO restrains the ACTH response to IL-1β, and that the NO responsible for this effect is generated by a constitutive, most probably endothelial, isoform of NOS.

Urocortin, a corticotropin-releasing factor-related mammalian peptide, inhibits edema due to thermal injury in rats

Urocortin is a recently characterized mammalian peptide which appears to be an endogenous ligand for corticotropin-releasing factor (CRF) receptors, in particular CRF receptor type 2. The effect of rat urocortin on protein extravasation and edema, produced by immersion of the paws of anesthetized rats in 58 degrees C water for 5 min, was compared to that of rat/human CRF. Urocortin administered i.v. dose-dependently inhibited the leakage of Evans blue dye into the skin of the footpad and the increase in paw weight, with a potency 6.6-6.7 times greater than CRF. alpha-Helical CRF-(9-41), a more potent antagonist of type 2 than type 1 CRF receptors, completely reversed the inhibition of edema produced by either CRF or urocortin, at a dose (200 nmol/kg) that did not affect adrenocorticotropin secretion induced by either peptide. These data indicate that urocortin is a potent inhibitor of heat-induced edema, and that this action is mediated by CRF receptors, most likely CRF receptor type 2.

Corticotropin-releasing factor, vasopressin, and prostaglandins mediate, and nitric oxide restrains, the hypothalamic-pituitary-adrenal response to acute local inflammation in the rat

The present study determined the plasma ACTH and corticosterone responses of the rat to acute local inflammation induced by the im injection of a small volume of turpentine. In response to tissue injury, ACTH and corticosterone concentrations rose rapidly, peaked at 1 h, and returned toward basal values by 3 h after turpentine injection. As acute inflammation developed, plasma interleukin-6 bioactivity increased significantly, and ACTH and corticosterone levels exhibited a secondary rise. These secondary responses were maximum 6-12 h after turpentine administration, persisted for 20-28 h, and were statistically significant regardless of the normal circadian variations in ACTH and corticosterone secretion. Injection of neutralizing anti-CRF antiserum 7 h after turpentine produced a complete reversal, whereas antiarginine vasopressin (anti-AVP) caused a partial (approximately 40%) inhibition, of inflammation-induced ACTH secretion. The cyclooxygenase inhibitor, ibuprofen (10 mg/kg, iv), like CRF antiserum, rapidly and completely reversed turpentine-induced ACTH secretion. In contrast, the nitric oxide synthase inhibitor, Nw-nitro-L-arginine methyl ester (30 mg/kg, iv), produced a significant enhancement of the ACTH response within 30 min of its injection. Measurement of plasma interleukin-6 bioactivity and fever showed that neither anti-CRF, anti-AVP, ibuprofen, nor Nw-nitro-L-arginine methyl ester acutely influenced the local inflammatory process itself, suggesting that these agents interacted directly with the hypothalamo-pituitary-adrenal axis. These data demonstrate that the ACTH response to local inflammation is mediated by synergistic actions of CRF and AVP, and that both stimulatory (PGs) and inhibitory (nitric oxide) intermediates regulate this response.

Regulation of the HPA axis by cytokines

Cytokines are a group of polypeptide mediators, classically associated with the regulation of immunity and inflammation. However, these peptides regulate not only local immune/inflammatory responses, but also elicit many CNS-mediated responses which accompany such immune/inflammatory reactions. This article reviews the evidence that interleukin (IL)-1, IL-6, and tumor necrosis factor alpha (TNFalpha) produce hypothalamo-pituitary-adrenal (HPA) axis activation in response to various threats to homeostasis. To aid such an examination, and to gain insights into the potential mechanisms by which these cytokines influence the HPA axis, experimental findings are discussed within a framework of criteria. If a particular cytokine plays a significant role in the regulation of the HPA axis in response to a particular pathophysiology, then necessarily: (1) receptors for that cytokine should be present within tissues associated with the HPA axis; (2) administration of that cytokine should elicit HPA activation; (3) the HPA axis should be exposed to that cytokine; and (4) inhibition of the action of that cytokine should prevent HPA activation. The evidence discussed indicates that some, if not all, of these criteria are met for each of IL-1, IL-6, and TNFalpha. However, the extensive interactions between different cytokines, the broad spectrum of pathophysiologies associated with increased cytokine production (including inflammatory and non-inflammatory stresses), and the number of tissues/cells capable of either synthesizing or responding to cytokines, suggest that multiple mechanisms mediate the influence of cytokines on the HPA axis.