Galanin is a potent inhibitor of glucagon-like peptide-1 secretion from rat ileum

The Molecular Determinants of Glucagon-like Peptide Secretion by the Intestinal L cell

The intestinal L cell secretes a diversity of biologically active hormones, most notably the glucagon-like peptides, GLP-1 and GLP-2. The highly successful introduction of GLP-1-based drugs into the clinic for the treatment of patients with type 2 diabetes and obesity, and of a GLP-2 analog for patients with short bowel syndrome, has led to the suggestion that stimulation of the endogenous secretion of these peptides may serve as a novel therapeutic approach in these conditions. Situated in the intestinal epithelium, the L cell demonstrates complex relationships with not only circulating, paracrine, and neural regulators, but also ingested nutrients and other factors in the lumen, most notably the microbiota. The integrated input from these numerous secretagogues results in a variety of temporal patterns in L cell secretion, ranging from minutes to 24 hours. This review combines the findings of traditional, physiological studies with those using newer molecular approaches to describe what is known and what remains to be elucidated after 5 decades of research on the intestinal L cell and its secreted peptides, GLP-1 and GLP-2.

The Enteroendocrine System in Obesity

The enteroendocrine system coordinates the physiological response to food intake by regulating rates of digestion, nutrient absorption, insulin secretion, satiation and satiety. Gut hormones with important anorexigenic and/or insulinotropic roles include glucagon-like peptide 1 (GLP-1), peptide YY (PYY_3-36), cholecystokinin (CCK) and glucose-dependent insulinotropic peptide (GIP). High BMI or obesogenic diets do not markedly disrupt this enteroendocrine system, which represents a critical target for inducing weight loss and treating co-morbidities in individuals with obesity.

In vivo, ex vivo and in vitro evidence for atropine-mediated attenuation of glucagon-like peptide-1 secretion: findings from a systematic review

Glucagon-like peptide-1 (GLP-1) is involved in postprandial glucose homeostasis. Secretion of which involves a cholinergic pathway. Anticholinergic agent like atropine could act as a competitive antagonist of acetylcholine at muscarinic receptors. This review explores studies that assess the role of atropine in GLP-1 secretion. We selected published original articles from PubMed, Science Direct, The Cochrane Library, Trip, Google and the reference lists of the selected articles. Reporting was done according to the PRISMA statement. Relevant standard and previously published tools were used to assess the risk of bias of the selected articles. Twelve articles out of 185 search results fulfilled the review criteria. Eight were in vivo studies (six animal and two human studies), three were ex vivo studies and one was an in vitro study. Animal studies had rats, mice, pigs and monkeys as the subjects. Human studies involved healthy men and women. Majority of the studies reported an atropine-mediated attenuation of GLP-1 secretion and postprandial secretion of GLP-1 was mainly affected. However, atropine failed to significantly affect GLP-1 secretion when dipeptidyl peptidase-4 (DPP-4) enzyme was inhibited.

Galanin inhibits GLP-1 and GIP secretion via the GAL1 receptor in enteroendocrine L and K cells

Background and Purpose

Galanin is a widely expressed neuropeptide, which in the gut is thought to modulate gastrointestinal motility and secretion. We aimed to elucidate the poorly characterised mechanisms underlying the inhibitory effect of galanin and the potential involvement of G‐protein coupled inwardly rectifying potassium, K_ir3, (GIRK) channels in glucagon‐like peptide 1 (GLP‐1) and glucose‐dependent insulinotropic polypeptide (GIP) secretion.

Experimental Approach

Purified murine L and K cells were analysed for expression of galanin receptors and GIRK subunits. Hormone secretion was measured from primary murine intestinal cultures. Intracellular cAMP was monitored in primary L cells derived from mice expressing the Epac2camps sensor under the control of the proglucagon promoter.

Key Results

Galanin receptor 1 (GAL₁, Galr1) and GIRK channel 1 (K_ir3.1, Kcnj3) and 4 (K_ir3.4, Kcnj5) mRNA expression was highly enriched in K and L cells. Galanin and a selective GAL₁ receptor agonist (M617) potently inhibited GLP‐1 and GIP secretion from primary small intestinal cultures. In L cells, galanin significantly inhibited the forskolin‐induced cAMP response. The GIRK1/4 activator ML297 significantly reduced glucose‐stimulated and IBMX‐stimulated GLP‐1 secretion but had no effect on GIP. The GIRK blocker tertiapin‐Q did not impair galanin‐mediated GLP‐1 inhibition.

Conclusions and Implications

Galanin, acting via the GAL₁ receptor and G_i‐coupled signalling in L and K cells, is a potent inhibitor of GLP‐1 and GIP secretion. Although GIRK1/4 channels are expressed in these cells, their activation does not appear to play a major role in galanin‐mediated inhibition of incretin secretion.

Pharmacology and physiology of gastrointestinal enteroendocrine cells

Gastrointestinal (GI) polypeptides are secreted from enteroendocrine cells (EECs). Recent technical advances and the identification of endogenous and synthetic ligands have enabled exploration of the pharmacology and physiology of EECs. Enteroendocrine signaling pathways stimulating hormone secretion involve multiple nutrient transporters and G protein-coupled receptors (GPCRs), which are activated simultaneously under prevailing nutrient conditions in the intestine following a meal. The majority of studies investigate hormone secretion from EECs in response to single ligands and although the mechanisms behind how individual signaling pathways generate a hormonal output have been well characterized, our understanding of how these signaling pathways converge to generate a single hormone secretory response is still in its infancy. However, a picture is beginning to emerge of how nutrients and full, partial, or allosteric GPCR ligands differentially regulate the enteroendocrine system and its interaction with the enteric and central nervous system. So far, activation of multiple pathways underlies drug discovery efforts to harness the therapeutic potential of the enteroendocrine system to mimic the phenotypic changes observed in patients who have undergone Roux-en-Y gastric surgery. Typically obese patients exhibit ∼30% weight loss and greater than 80% of obese diabetics show remission of diabetes. Targeting combinations of enteroendocrine signaling pathways that work synergistically may manifest with significant, differentiated EEC secretory efficacy. Furthermore, allosteric modulators with their increased selectivity, self-limiting activity, and structural novelty may translate into more promising enteroendocrine drugs. Together with the potential to bias enteroendocrine GPCR signaling and/or to activate multiple divergent signaling pathways highlights the considerable range of therapeutic possibilities available. Here, we review the pharmacology and physiology of the EEC system.

Gut chemosensing mechanisms

The enteroendocrine system is the primary sensor of ingested nutrients and is responsible for secreting an array of gut hormones, which modulate multiple physiological responses including gastrointestinal motility and secretion, glucose homeostasis, and appetite. This Review provides an up-to-date synopsis of the molecular mechanisms underlying enteroendocrine nutrient sensing and highlights our current understanding of the neuro-hormonal regulation of gut hormone secretion, including the interaction between the enteroendocrine system and the enteric nervous system. It is hoped that a deeper understanding of how these systems collectively regulate postprandial physiology will further facilitate the development of novel therapeutic strategies.

The role of small molecule GPR119 agonist, AS1535907, in glucose-stimulated insulin secretion and pancreatic β-cell function

AIM

AS1535907, a small molecule agonist of GPR119, was assessed for its glucose-stimulated insulin secretory activity and pancreatic β-cell function in type 2 diabetes.

METHODS

Both in vitro and in vivo tests were conducted using NIT-1 and HEK293 cell lines, male normal and db/db mice and isolated perfused rat pancreas preparations.

RESULTS

AS1535907 had an EC₅₀ value of 1.5 µM for human GPR119 transfected in HEK293 cells. AS1535907 enhanced insulin secretion in NIT-1 cells and in the perfused rat pancreas. A transient increase in the human insulin promoter activity was also observed in NIT-1 cells. First-phase insulin secretion was particularly more evident in the AS1535907-treated perfused rat pancreas than that in the nateglinide or glibenclamide-treated group. Oral glucose tolerance improved following a single dose of AS1535907 in normal and db/db mice. Subsequently, 2 weeks of multiple dosing significantly increased plasma insulin levels and decreased blood glucose levels in db/db mice. After 3 weeks of treatment in db/db mice, the numbers of insulin and proliferation cell nuclear antigen-positive cells and the islet area were significantly higher than those in the vehicle-treated mice. As compared with the vehicle, gene expression analysis revealed that AS1535907 significantly upregulated transcription factors (Nkx 2.2, Nkx 6.1, NeuroD and activin A), responsible for β-cell regulation and prohormone-converting enzyme 1 responsible for insulin biosynthesis.

CONCLUSION

These results suggest that AS1535907 can potentially regulate first-phase insulin secretion and exert a protective effect on pancreatic β-cell function via regulation of transcription factors.

A role for intestinal endocrine cell-expressed g protein-coupled receptor 119 in glycemic control by enhancing glucagon-like Peptide-1 and glucose-dependent insulinotropic Peptide release

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 beta-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 beta-cells. These data also suggest that combined stimulation of incretin hormone release and protection against incretin hormone degradation may be an effective antidiabetic strategy.

Neural regulation of glucagon-like peptide-1 secretion in pigs

Glucagon-like peptide (GLP)-1 is secreted rapidly from the intestine postprandially. We therefore investigated its possible neural regulation. With the use of isolated perfused porcine ileum, GLP-1 secretion was measured in response to electrical stimulation of the mixed, perivascular nerve supply and infusions of neuroactive agents alone and in combination with different blocking agents. Electrical nerve stimulation inhibited GLP-1 secretion, an effect abolished by phentolamine. Norepinephrine inhibited secretion, and phentolamine abolished this effect. GLP-1 secretion was stimulated by isoproterenol (abolished by propranolol). Acetylcholine stimulated GLP-1 secretion, and atropine blocked this effect. Dimethylphenylpiperazine stimulated GLP-1 secretion. In chloralose-anesthetized pigs, however, electrical stimulation of the vagal trunks at the level of the diaphragm had no effect on GLP-1 or GLP-2 and weak effects on glucose-dependent insulinotropic peptide and somatostatin secretion, although this elicited a marked atropine-resistant release of the neuropeptide vasoactive intestinal polypeptide to the portal circulation. Thus GLP-1 secretion is inhibited by the sympathetic nerves to the gut and may be stimulated by intrinsic cholinergic nerves, whereas the extrinsic vagal supply has no effect.

The effects of duodenal peptides on glucagon-like peptide-1 secretion from the ileum. A duodeno--ileal loop?

Secretion of the gut hormone glucagon-like peptide-1 (GLP-1) is stimulated by meal ingestion. The response is rapid, suggesting a stimulatory pathway elicited from the upper gastrointestinal area. In pigs, we have been unable to demonstrate a neural stimulatory pathway, but GLP-1 secretion is regulated by local somatostatin secretion. In search for an endocrine pathway, we studied the effect of a range of concentrations of cholecystokinin octapeptide (26-33) (CCK 8), gastric inhibitory peptide 1-42 (GIP), secretin, motilin, calcitonin gene-related peptide (CGRP), and the modified amino acid, 5-hydroxytryptamine (serotonin, 5-HT) on GLP-1 and somatostatin release from isolated perfused segments of porcine ileum.GLP-1 secretion was stimulated by 1 nM CCK 8 and 10 nM GIP, but suppressed by 1 nM motilin and 1 microM 5-HT. Secretin and CGRP had no effect. Somatostatin secretion was stimulated by CCK 8 at 1 and 10 nM, by GIP at 1 and 10 nM and by 10 nM CGRP. Secretin, 5-HT and motilin had no effect on somatostatin secretion. We conclude that CCK 8 and GIP 1-42 stimulated GLP-1 secretion, but only in concentrations greatly exceeding normal postprandial concentrations. Thus, we find it unlikely that endocrine agents from the duodenum regulate GLP-1 secretion in pigs.

Somatostatin restrains the secretion of glucagon-like peptide-1 and -2 from isolated perfused porcine ileum

Suspecting that paracrine inhibition might influence neuronal regulation of the endocrine L cells, we studied the role of somatostatin (SS) in the regulation of the secretion of the proglucagon-derived hormones glucagon-like peptide-1 and -2 (GLP-1 and GLP-2). This was examined using the isolated perfused porcine ileum stimulated with acetylcholine (ACh, 10(-6) M), neuromedin C (NC, 10(-8) M), and electrical nerve stimulation (NS) with or without alpha-adrenergic blockade (phentolamine 10(-5) M), and perfusion with a high-affinity monoclonal antibody against SS. ACh and NC significantly increased GLP secretion, whereas NS had little effect. SS immunoneutralization increased GLP secretion eight- to ninefold but had little influence on the GLP responses to ACh, NC, and NS. Basal SS secretion (mainly SS28) was unaffected by NS alone. Phentolamine + NS and NC abstract strongly stimulated release mainly of SS14, whereas ACh had little effect. Infused intravascularly, SS14 weakly and SS28 strongly inhibited GLP secretion. We conclude that GLP secretion is tonically inhibited by a local release of SS28 from epithelial paracrine cells, whereas SS14, supposedly derived from enteric neurons, only weakly influences GLP secretion.

Nitric oxide-mediated erectile effects of galantide but not galanin in vivo

The purpose of this study was to investigate the in vivo effects of intracavernosal injections of galanin and galantide (a specific galanin receptor antagonist) on penile erection in the anesthetized cat. Erectile responses to galanin and galantide were compared with responses to a standard triple drug combination [1.65 mg papaverine, 25 microg phentolamine, and 0.5 microg prostaglandin E(1) (PGE(1))]. Intracavernosal injections of galanin (3-100 nmol) and galantide (0. 1-3 nmol) induced penile erection in a dose-dependent manner. In terms of relative potency, galantide was approximately 100-fold more potent than galanin at increasing cavernosal pressure. The maximal increases in intracavernosal pressure in response to galanin and galantide were 83 and 95%, respectively, of the control triple drug combination. The total durations of erectile response caused by these peptides were significantly shorter (P<0.05) than those by the triple drug combination. The nitric oxide synthase inhibitor L-NAME (20 mg) significantly decreased the erectile response in the cat to galantide but not to galanin, while the K(+)(ATP) channel antagonist U-37883A (3 mg) had no effect on the erectile response to galanin nor galantide. The results of the present study demonstrate that galantide, a putative antagonist for the galanin receptor, has more potent agonist activity than galanin in increasing intracavernosal pressure in the cat. Moreover, these data suggest that galantide, but not galanin, causes penile erection by an NO/cGMP-dependent mechanism. This is the first study to demonstrate that galanin may play a role in the physiology of penile erection.

Calcitonin gene-related peptide potently stimulates glucagon-like peptide-1 release in the isolated perfused rat ileum

The post-prandial release of glucagon-like peptide-1 (GLP-1) from the distal gut appears to involve a neural reflex that arises from the proximal gut. The neuropeptide calcitonin gene-related peptide (CGRP)'s potent stimulatory effect on GLP-1 release was characterized, using the isolated vascularly perfused rat ileum. CGRP, but not its homolog amylin, induced a dose-dependent and sustained release of GLP-1. This effect was greatly reduced in the presence of CGRP(8-37), was abolished by galanin, potentiated by luminal glucose and unaffected by atropine. GIP enhanced, but did not potentiate, this effect. The results reveal how CGRP is involved in the complex regulation of GLP-1 release.