Disturbed α-Cell Function in Mice with β-Cell Specific Overexpression of Human Islet Amyloid Polypeptide

Exogenous administration of islet amyloid polypeptide (IAPP) has been shown to inhibit both insulin and glucagon secretion. This study examined α-cell function in mice with β-cell specific overexpression of human IAPP (hIAPP) after an oral protein gavage (75 mg whey protein/mouse). Baseline glucagon levels were higher in transgenic mice (41 ± 4.0 pg/mL, n = 6) than in wildtype animals (19 ± 5.1 pg/mL, n = 5, P = .015). In contrast, the glucagon response to protein was impaired in transgenic animals (21 ± 2.7 pg/mL in transgenic mice versus 38 ± 5.7 pg/mL in wildtype mice at 15 minutes; P = .027). Baseline insulin levels did not differ between the groups, while the insulin response, as the glucagon response, was impaired after protein challenge (P = .018). Glucose levels were not different between the groups and did not change significantly after protein gavage. Acetaminophen was given through gavage to the animals (2 mg/mouse) to estimate gastric emptying. The plasma acetaminophen profile was similar in the two groups of mice. We conclude that disturbances in glucagon secretion exist in mice with β-cell specific overexpression of human IAPP, which are not secondary to changes in gastric emptying. The reduced glucagon response to protein challenge may reflect a direct inhibitory influence of hIAPP on glucagon secretion

Chronic glucokinase activation reduces glycaemia and improves glucose tolerance in high-fat diet fed mice.

Glucokinase (GK) plays a key role in maintaining glucose homeostasis by promoting insulin secretion from pancreatic beta cells and increasing hepatic glucose uptake. Here we investigate the effects of acute and chronic GK activation on glucose tolerance and insulin secretion in mice with diet-induced insulin resistance. In the acute study, a small molecule GK activator (GKA71) was administered to mice fed a high-fat diet for 8weeks. In the long-term study, GKA71 was provided in the diet for 4weeks to high-fat diet-fed mice. Glucose tolerance was measured after intravenous glucose administration, and insulin secretion was measured both in vivo and in vitro. Acute GK activation efficiently improved glucose tolerance in association with increased insulin secretion after intravenous glucose both in control and high-fat fed mice. Chronic GK activation significantly reduced basal plasma glucose and insulin, and improved glucose tolerance despite reduced insulin secretion after intravenous glucose, suggesting improved insulin sensitivity. Isolated islets from chronically GKA71-treated mice displayed augmented insulin secretion at 8.3mmol/l glucose, without affecting glucose oxidation. High-fat diet fed mice had reduced glycogen and increased triglyceride in liver compared to control mice, and these parameters were not altered by long-term GK activation. We conclude that GK activation in high-fat diet-fed mice potently reduces glycaemia and improves glucose tolerance, with combined effect both to stimulate insulin secretion from islets and improve insulin sensitivity

G-protein-coupled receptors and islet function-Implications for treatment of type 2 diabetes.

Islet function is regulated by a number of different signals. A main signal is generated by glucose, which stimulates insulin secretion and inhibits glucagon secretion. The glucose effects are modulated by many factors, including hormones, neurotransmitters and nutrients. Several of these factors signal through guanine nucleotide-binding protein (G protein)-coupled receptors (GPCR). Examples of islet GPCR are GPR40 and GPR119, which are GPCR with fatty acids as ligands, the receptors for the incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), the receptors for the islet hormones glucagon and somatostatin, the receptors for the classical neurotransmittors acetylcholine (ACh; M(3) muscarinic receptors) and noradrenaline (beta(2)- and alpha(2)-adrenoceptors) and for the neuropeptides pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP; PAC(1) and VPAC(2) receptors), cholecystokinin (CCK(A) receptors) and neuropeptide Y (NPY Y1 receptors). Other islet GPCR are the cannabinoid receptor (CB(1) receptors), the vasopressin receptors (V1(B) receptors) and the purinergic receptors (P(2Y) receptors). The islet GPCR couple mainly to adenylate cyclase and to phospholipase C (PLC). Since important pharmacological strategies for treatment of type 2 diabetes are stimulation of insulin secretion and inhibition of glucagon secretion, islet GPCR are potential drug targets. This review summarizes knowledge on islet GPCR

Enterostatin in the Gastrointestinal Tract - Production and Possible Mechanism of Action

The presence of procolipase and enterostatin in the rat gastrointestinal tract has been investigated. Procolipase was identified, using in situ hybridization and immunohistochemistry, in exocrine cells of the pancreas as well as in chief cells of the fundus region of the stomach. Enterostatin is the activation peptide of procolipase and it has been found to act as a satiety signal for fat. Using antibodies directed towards the C-terminal part of enterostatin, the peptide was found to be present in endocrine cells in the antral part of the stomach and in the upper small intestine. Double immunostaining showed enterostatin and serotonin to co-localize in a subpopulation of enterochromaffine cells. Cloning of rat gastric procolipase from a rat stomach cDNA library, revealed a sequence identical to pancreatic procolipase. Gastric procolipase was found to be secreted into the gastric juice and cleaved into enterostatin and colipase. Pepsin and acid were found to be involved in the cleavage. The expression of gastric procolipase during high-fat feeding was reduced in contrast to pancreatic procolipase. Secretion of procolipase to the gastric juice was, however unaffected by high-fat feeding. The role of gastric procolipase may be to prepare lipase-catalyzed fat digestion in the intestine. Gastric enterostatin may be involved in the onset of early satiety. After purification of enterostatin from the rat gut and pancreas by various chromatographic steps including HPLC analysis, two forms of enterostatin were identified with the amino acid sequences APGPR and VPGPR, in agreement with data from the cloning of rat pancreatic procolipase. APGPR was the most abundant form of enterostatin, whereas only small amounts of VPGPR were identified. Chronic intraperitoneal treatment of rats with enterostatin (APGPR) for one week resulted in decreased fat intake and decreased body weight gain. Enterostatin displayed its biological effect in reducing fat intake when the dietary fat corresponded to 38% of the total energy intake. In situations where dietary fat was provided in lower amounts, enterostatin was ineffective. The physiological significance of enterostatin was illustrated by the observation that within a group of rats fed a high-fat diet, the amount of pancreatic colipase was inversely related to the fat intake. In the pursuit of a receptor for enterostatin, binding studies to brain membranes indicated a two-site binding model with one high-affinity and one low-affinity binding site. In conclusion, the data presented in this study support the hypothesis that enterostatin plays an important role as a physiological satiety signal produced in the gastrointestinal tract

The apj receptor is expressed in pancreatic islets and its ligand, apelin, inhibits insulin secretion in mice.

Apelin is the endogenous ligand of the G-protein coupled apj receptor. Apelin is expressed in the brain, the hypothalamus and the stomach and was recently shown also to be an adipokine secreted from the adipocytes. Although apelin has been suggested to be involved in the regulation of food intake, it is not known whether the peptide affects islet function and glucose homeostasis. We show here that the apj receptor is expressed in pancreatic islets and that intravenous administration of full-length apelin-36 (2 nmol/kg) inhibits the rapid insulin response to intravenous glucose (1 g/kg) by 35% in C57BL/6J mice. Thus, the acute (1-5 min) insulin response to intravenous glucose was 682 +/- 23 pmol/l after glucose alone (n = 17) and 445 +/- 58 pmol/l after glucose plus apelin-36 (n = 18; P=0.017). This was associated with impaired glucose elimination (the 5-20 min glucose elimination was 2.9 +/- 0.1%/min after glucose alone versus 2.3 +/- 0.2%/min after glucose plus apelin-36, P=0.008). Apelin (2 nmol/kg) also inhibited the insulin response to intravenous glucose in obese insulin resistant high-fat fed C57BL/6J mice (P=0.041). After 60 min incubation of isolated islets from normal mice, insulin secretion in the presence of 16.7 mmol/l glucose was inhibited by apelin-36 at 1 pmol/l, whereas apelin-36 did not significantly affect insulin secretion at 2.8 or 8.3 mmol/l glucose or after stimulation of insulin secretion by KCI. Islet glucose oxidation at 16.7 mmol/l was not affected by apelin-36. We conclude that the apj receptor is expressed in pancreatic islets and that apelin-36 inhibits glucose-stimulated insulin secretion both in vivo and in vitro. This may suggest that the islet beta-cells are targets for apelin-36

Temporal and dietary fat content-dependent islet adaptation to high-fat feeding-induced glucose intolerance in mice.

The high fat-fed mouse is an experimental model for studies of islet dysfunction as a mechanism for glucose intolerance and for evaluation of therapeutic targets. This model is, however, dynamic with a temporal and dietary fat content-dependent impact on islet function and glucose tolerance, the details of which are unknown. This study therefore examined the time course of changes in the insulin response to intravenous glucose (1 g/kg) in relation to glucose tolerance in female mice after 1, 3, 8, or 16 weeks of feeding with diets containing 11% fat (normal diet [ND]), 30% fat (medium-fat diet [MFD]), or 58% fat (high-fat diet [HFD]:, by energy). High-fat diet increased body weight and body fat content, whereas MFD did not. The insulin response (postglucose suprabasal mean 1- and 5-minute insulin) was impaired after 1 week on NIFD (481 +/- 33 pmol/L) or HFD (223 +/- 31 pmol/L) compared with ND (713 +/- 46 pmol/L, both P <.001). This was accompanied by impaired glucose elimination compared with ND (both P <.001). Over the 16-week study period, the insulin response adaptively increased in the groups fed with HFD and MFD, to be not significantly different from ND after 16 weeks. This compensation normalized glucose tolerance in MFD, whereas the glucose tolerance was still below normal in HFD. Insulin clearance, as judged by elimination of intravenous human insulin, was not altered in HFD, suggesting that the observed changes in insulin responses to glucose are due to changes in insulin secretion rather than to changes in insulin clearance. We conclude that time- and dietary fat dependent dynamic adaptive islet compensation evolves after introducing HFD in mice and that MFD-fed mice is a novel nonobese model of glucose intolerance. (c) 2007 Elsevier Inc. All rights reserved

The high-fat diet-fed mouse: a model for studying mechanisms and treatment of impaired glucose tolerance and type 2 diabetes.

This study characterizes the high-fat diet-fed mouse as a model for impaired glucose tolerance (IGT) and type 2 diabetes. Female C57BL/6J mice were fed a high-fat diet (58% energy by fat) or a normal diet (11% fat). Body weight was higher in mice fed the high-fat diet already after the first week, due to higher dietary intake in combination with lower metabolic efficiency. Circulating glucose increased after 1 week on high-fat diet and remained elevated at a level of approximately 1 mmol/l throughout the 12-month study period. In contrast, circulating insulin increased progressively by time. Intravenous glucose challenge revealed a severely compromised insulin response in association with marked glucose intolerance already after 1 week. To illustrate the usefulness of this model for the development of new treatment, mice were fed an orally active inhibitor of dipeptidyl peptidase-IV (LAF237) in the drinking water (0.3 mg/ml) for 4 weeks. This normalized glucose tolerance, as judged by an oral glucose tolerance test, in association with augmented insulin secretion. We conclude that the high-fat diet-fed C57BL/6J mouse model is a robust model for IGT and early type 2 diabetes, which may be used for studies on pathophysiology and development of new treatment

Role of VIP and PACAP in islet function

Vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are two closely related neuropeptides that are expressed in islets and in islet parasympathetic nerves. Both peptides bind to their common G-protein-coupled receptors, VPAC1 and VPAC2, and PACAP, in addition to the specific receptor PAC1, all three of which are expressed in islets. VIP and PACAP stimulate insulin secretion in a glucose-dependent manner and they both also stimulate glucagon secretion. This action is achieved through increased formation of cAMP after activation of adenylate cyclase and stimulation of extracellular calcium uptake. Deletion of PAC1 receptors or VPAC2 receptors results in glucose intolerance. These peptides may be of importance in mediating prandial insulin secretion and the glucagon response to hypoglycemia. Animal studies have also suggested that activation of the receptors, in particular VPAC2 receptors, may be used as a therapeutic approach for the treatment of type 2 diabetes. This review summarizes the current knowledge of the potential role of VIP and PACAP in islet function