Arcuate Glucagon-Like Peptide 1 Receptors Regulate Glucose Homeostasis but Not Food Intake
Darleen A. Sandoval 1 ,
Didier Bagnol 2 ,
Stephen C. Woods 1 ,
David A. D'Alessio 1 and
Randy J. Seeley 1
1 ...Departments of Psychiatry and Medicine, University of Cincinnati, Cincinnati, Ohio
2 Arena Pharmaceuticals, San Diego, California
Corresponding author: Dr. Darleen Sandoval, darleen.sandoval{at}uc.edu
Abstract
OBJECTIVE— Glucagon-like peptide-1 (GLP-1) promotes glucose homeostasis through regulation of islet hormone secretion, as well as hepatic
and gastric function. Because GLP-1 is also synthesized in the brain, where it regulates food intake, we hypothesized that
the central GLP-1 system regulates glucose tolerance as well.
RESEARCH DESIGN AND METHODS— We used glucose tolerance tests and hyperinsulinemic-euglycemic clamps to assess the role of the central GLP-1 system on glucose
tolerance, insulin secretion, and hepatic and peripheral insulin sensitivity. Finally, in situ hybridization was used to examine
colocalization of GLP-1 receptors with neuropeptide tyrosine and pro-opiomelanocortin neurons.
RESULTS— We found that central, but not peripheral, administration of low doses of a GLP-1 receptor antagonist caused relative hyperglycemia
during a glucose tolerance test, suggesting that activation of central GLP-1 receptors regulates key processes involved in
the maintenance of glucose homeostasis. Central administration of GLP-1 augmented glucose-stimulated insulin secretion, and
direct administration of GLP-1 into the arcuate, but not the paraventricular, nucleus of the hypothalamus reduced hepatic
glucose production. Consistent with a role for GLP-1 receptors in the arcuate, GLP-1 receptor mRNA was found to be expressed
in 68.1% of arcuate neurons that expressed pro-opiomelanocortin mRNA but was not significantly coexpressed with neuropeptide
tyrosine.
CONCLUSIONS— These data suggest that the arcuate GLP-1 receptors are a key component of the GLP-1 system for improving glucose homeostasis
by regulating both insulin secretion and glucose production.
Footnotes
Published ahead of print at http://diabetes.diabetesjournals.org on 16 May 2008.
Readers may use this article as long as the work is properly cited, the use is educational and not for profit, and the work
is not altered. See http://creativecommons.org/licenses/by-nc-nd/3.0/ for details.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore
be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Accepted May 8, 2008.
Received January 2, 2008.
DIABETES
We recently showed that activation of G protein-coupled receptor 119 (GPR119) (also termed glucose dependent insulinotropic receptor) improves glucose homeostasis via direct cAMP-mediated enhancement ...of glucose-dependent insulin release in pancreatic β-cells. Here we show that GPR119 also stimulates incretin hormone release and thus may regulate glucose homeostasis by this additional mechanism. GPR119 mRNA was found to be expressed at significant levels in intestinal subregions that produce glucose-dependent insulinotropic peptide and glucagon-like peptide (GLP)-1. Furthermore, in situ hybridization studies indicated that most GLP-1-producing cells coexpress GPR119 mRNA. In GLUTag cells, a well-established model of intestinal L-cell function, the potent GPR119 agonist AR231453 stimulated cAMP accumulation and GLP-1 release. When administered in mice, AR231453 increased active GLP-1 levels within 2 min after oral glucose delivery and substantially enhanced total glucose-dependent insulinotropic peptide levels. Blockade of GLP-1 receptor signaling with exendin(9–39) reduced the ability of AR231453 to improve glucose tolerance in mice. Conversely, combined administration of AR231453 and the DPP-4 inhibitor sitagliptin to wild-type mice significantly amplified both plasma GLP-1 levels and oral glucose tolerance, relative to either agent alone. In mice lacking GPR119, no such enhancement was seen. Thus, GPR119 regulates glucose tolerance by acting on intestinal endocrine cells as well as pancreatic β-cells. These data also suggest that combined stimulation of incretin hormone release and protection against incretin hormone degradation may be an effective antidiabetic strategy.
The RFamide neuropeptide 26RFa was first isolated from the brain of the European green frog on the basis of cross‐reactivity with antibodies raised against bovine neuropeptide FF (NPFF). 26RFa and ...its N‐terminally extended form glutamine RF‐amide peptide (QRFP) have been identified as cognate ligands of the former orphan receptor GPR103, now renamed glutamine RF‐amide peptide receptor (QRFP receptor). The 26RFa/QRFP precursor has been characterized in various mammalian and non‐mammalian species. In the brain of mammals, including humans, 26RFa/QRFP mRNA is almost exclusively expressed in hypothalamic nuclei. The 26RFa/QRFP transcript is also present in various organs especially in endocrine glands. While humans express only one QRFP receptor, two isoforms are present in rodents. The QRFP receptor genes are widely expressed in the CNS and in peripheral tissues, notably in bone, heart, kidney, pancreas and testis. Structure–activity relationship studies have led to the identification of low MW peptidergic agonists and antagonists of QRFP receptor. Concurrently, several selective non‐peptidic antagonists have been designed from high‐throughput screening hit optimization. Consistent with the widespread distribution of QRFP receptor mRNA and 26RFa binding sites, 26RFa/QRFP exerts a large range of biological activities, notably in the control of energy homeostasis, bone formation and nociception that are mediated by QRFP receptor or NPFF2. The present report reviews the current knowledge concerning the 26RFa/QRFP‐QRFP receptor system and discusses the potential use of selective QRFP receptor ligands for therapeutic applications.
Herpes simplex virus 1 (HSV-1) infects the cornea and caused blinding ocular disease. In the present study, we evaluated whether and how a novel engineered version of fibroblast growth factor-1 ...(FGF-1), designated as TTHX1114, would reduce the severity of HSV-1-induced and recurrent ocular herpes in the mouse model. The efficacy of TTHX1114 against corneal keratopathy was assessed in B6 mice following corneal infection with HSV-1, strain McKrae. Starting day one post infection (PI), mice received TTHX1114 for 14 days. The severity of primary stromal keratitis and blepharitis were monitored up to 28 days PI. Inflammatory cell infiltrating infected corneas were characterized up to day 21 PI. The severity of recurrent herpetic disease was quantified in latently infected B6 mice up to 30 days post-UVB corneal exposure. The effect of TTHX1114 on M1 and M2 macrophage polarization was determined
in vivo
in mice and
in vitro
on primary human monocytes-derived macrophages. Compared to HSV-1 infected non-treated mice, the infected and TTHX1114 treated mice exhibited significant reduction of primary and recurrent stromal keratitis and blepharitis, without affecting virus corneal replication. The therapeutic effect of TTHX1114 was associated with a significant decrease in the frequency of M1 macrophages infiltrating the cornea, which expressed significantly lower levels of pro-inflammatory cytokines and chemokines. This polarization toward M2 phenotype was confirmed
in vitro
on human primary macrophages. This pre-clinical finding suggests use of this engineered FGF-1 as a novel immunotherapeutic regimen to reduce primary and recurrent HSV-1-induced corneal disease in the clinic.
To further understand the functions of the orexin/hypocretin system, we examined the expression and regulation of the orexin/hypocretin receptor (OX1R and OX2R) mRNA in the brain by using ...quantitative in situ hybridization. Expression of OX1R and OX2R mRNA exhibited distinct distribution patterns. Within the hypothalamus, expression for the OX1R mRNA was largely restricted in the ventromedial (VMH) and dorsomedial hypothalamic nuclei, while high levels of OX2R mRNA were contained in the paraventricular nucleus, VMH, and arcuate nucleus as well as in mammilary nuclei. In the amygdala, OX1R mRNA was expressed throughout the amygdaloid complex with robust labeling in the medial nucleus, while OX2R mRNA was only present in the posterior cortical nucleus of amygdala. High levels of OX2R mRNA were also observed in the ventral tegmental area. Moreover, both OX1R and OX2R mRNA were observed in the hippocampus, some thalamic nuclei, and subthalamic nuclei. Furthermore, we analyzed the effect of fasting on levels of OX1R and OX2R mRNA in the hypothalamic and amygdaloid subregions. After 20 h of fasting, levels of OX1R mRNA were significantly increased in the VMH and the medial division of amygdala. An initial decrease (14 h) and a subsequent increase (20 h) in OX1R mRNA levels after fasting were observed in the dorsomedial hypothalamic nucleus and lateral division of amygdala. Levels of OX2R mRNA were augmented in the arcuate nucleus, but remained unchanged in the dorsomedial hypothalamic nucleus, paraventricular hypothalamic nucleus, and amygdala following fasting. The time-dependent and region-specific regulatory patterns of OX1R and OX2R suggest that they may participate in distinct neural circuits under the condition of food deprivation.
Agouti-related protein (AGRP) is a recently discovered orexigenic neuropeptide that inhibits the binding and action of alpha-melanocyte-stimulating hormone derived from proopiomelanocortin (POMC) at ...the melanocortin 3 receptor (MC3R) and melanocortin 4 receptor (MC4R) and has been proposed to function primarily as an endogenous melanocortin antagonist. To better understand the interplay between the AGRP and melanocortin signaling systems, we compared their nerve fiber distributions with each other by immunohistochemistry and their perikarya distribution with MC3R and MC4R by double in situ hybridization. Although deriving from distinct cell groups, AGRP and melanocortin terminals project to identical brain areas. Both AGRP and melanocortin neurons selectively express the MC3R, which provides a neuroanatomical basis for a dual-input circuit with biological amplification and feedback inhibition. These studies highlight a broader complexity in POMC-mediated behavior in the brain.
Central serotonin systems have long been associated with the control of feeding behavior and the modulation of behavioral effects of psychostimulants. 5-HT
receptors are present in hypothalamic ...centers such as the arcuate nucleus (ARC), controlling homeostatic regulation of food intake, as well as in the ventral tegmental area (VTA), a region involved in motivation aspects in multiple behaviors, including feeding. In the present study, we investigated whether the 5-HT
receptors control amphetamine-evoked locomotor activity and regulate food consumption. Localized microinjections into the VTA or the ARC were used to assess the effects of a highly selective 5-HT
receptor agonist, AR231630, on the locomotor stimulant effect of amphetamine as well as on food intake. AR231630 injected into the VTA, but not into the ARC, dose-dependently reduced locomotor activity elicited by amphetamine. Unexpectedly, intra-ARC injection of AR231630 did not reduce food intake even at the dose of 10 μg, whereas intra-VTA injection of the same dose of AR231630 did. In addition, we showed that pretreatment with the selective 5-HT
receptor antagonist SB242084 infused into the VTA partially prevented hypophagia induced by peripheral administration of AR231630. We can conclude that 5-HT
receptor in the VTA, but not in the ARC, participates in both homeostatic and hedonic food intake and brain reward function.
Central serotonin systems have long been associated with the control of feeding behavior and the modulation of behavioral effects of psychostimulants. 5-HT2C receptors are present in hypothalamic ...centers such as the arcuate nucleus (ARC), controlling homeostatic regulation of food intake, as well as in the ventral tegmental area (VTA), a region involved in motivation aspects in multiple behaviors, including feeding. In the present study, we investigated whether the 5-HT2C receptors control amphetamine-evoked locomotor activity and regulate food consumption. Localized microinjections into the VTA or the ARC were used to assess the effects of a highly selective 5-HT2C receptor agonist, AR231630, on the locomotor stimulant effect of amphetamine as well as on food intake. AR231630 injected into the VTA, but not into the ARC, dose-dependently reduced locomotor activity elicited by amphetamine. Unexpectedly, intra-ARC injection of AR231630 did not reduce food intake even at the dose of 10 μg, whereas intra-VTA injection of the same dose of AR231630 did. In addition, we showed that pretreatment with the selective 5-HT2C receptor antagonist SB242084 infused into the VTA partially prevented hypophagia induced by peripheral administration of AR231630. We can conclude that 5-HT2C receptor in the VTA, but not in the ARC, participates in both homeostatic and hedonic food intake and brain reward function.
Pancreatic β-cell dysfunction is a hallmark event in the pathogenesis of type 2 diabetes. Injectable peptide agonists of the glucagon-like peptide 1 (GLP-1) receptor have shown significant promise as ...antidiabetic agents by virtue of their ability to amplify glucose-dependent insulin release and preserve pancreatic β-cell mass. These effects are mediated via stimulation of cAMP through β-cell GLP-1 receptors. We report that the Gαs-coupled receptor GPR119 is largely restricted to insulin-producing β-cells of pancreatic islets. Additionally, we show here that GPR119 functions as a glucose-dependent insulinotropic receptor. Unlike receptors for GLP-1 and other peptides that mediate enhanced glucose-dependent insulin release, GPR119 was suitable for the development of potent, orally active, small-molecule agonists. The GPR119-specific agonist AR231453 significantly increased cAMP accumulation and insulin release in both HIT-T15 cells and rodent islets. In both cases, loss of GPR119 rendered AR231453 inactive. AR231453 also enhanced glucose-dependent insulin release in vivo and improved oral glucose tolerance in wild-type mice but not in GPR119-deficient mice. Diabetic KK/Ay mice were also highly responsive to AR231453. Orally active GPR119 agonists may offer significant promise as novel antihyperglycemic agents acting in a glucose-dependent fashion.
PDF
