Glucagon-like peptide-1, but not glucose-dependent insulinotropic peptide, regulates fasting glycemia and nonenteral glucose clearance in mice

Glucagon-like Peptide-1 receptor Tie2+ cells are essential for the cardioprotective actions of liraglutide in mice with experimental myocardial infarction

OBJECTIVES

Glucagon-like peptide-1 receptor (GLP-1R) agonists reduce the rates of major cardiovascular events, including myocardial infarction in people with type 2 diabetes, and decrease infarct size while preserving ventricular function in preclinical studies. Nevertheless, the precise cellular sites of GLP-1R expression that mediate the cardioprotective actions of GLP-1 in the setting of ischemic cardiac injury are uncertain.

METHODS

Publicly available single cell RNA sequencing (scRNA-seq) datasets on mouse and human heart cells were analyzed for Glp1r/GLP1R expression. Fluorescent activated cell sorting was used to localize Glp1r expression in cell populations from the mouse heart. The importance of endothelial and hematopoietic cells for the cardioprotective response to liraglutide in the setting of acute myocardial infarction (MI) was determined by inactivating the Glp1r in Tie2+ cell populations. Cardiac gene expression profiles regulated by liraglutide were examined using RNA-seq to interrogate mouse atria and both infarcted and non-infarcted ventricular tissue after acute coronary artery ligation.

RESULTS

In mice, cardiac Glp1r mRNA transcripts were exclusively detected in endocardial cells by scRNA-seq. In contrast, analysis of human heart by scRNA-seq localized GLP1R mRNA transcripts to populations of atrial and ventricular cardiomyocytes. Moreover, very low levels of GIPR, GCGR and GLP2R mRNA transcripts were detected in the human heart. Cell sorting and RNA analyses detected cardiac Glp1r expression in endothelial cells (ECs) within the atria and ventricle in the ischemic and non-ischemic mouse heart. Transcriptional responses to liraglutide administration were not evident in wild type mouse ventricles following acute MI, however liraglutide differentially regulated genes important for inflammation, cardiac repair, cell proliferation, and angiogenesis in the left atrium, while reducing circulating levels of IL-6 and KC/GRO within hours of acute MI. Inactivation of the Glp1r within the Tie2+ cell expression domain encompassing ECs revealed normal cardiac structure and function, glucose homeostasis and body weight in Glp1r^Tie2-/- mice. Nevertheless, the cardioprotective actions of liraglutide to reduce infarct size, augment ejection fraction, and improve survival after experimental myocardial infarction (MI), were attenuated in Glp1r^Tie2-/- mice.

CONCLUSIONS

These findings identify the importance of the murine Tie2+ endothelial cell GLP-1R as a target for the cardioprotective actions of GLP-1R agonists and support the importance of the atrial and ventricular endocardial GLP-1R as key sites of GLP-1 action in the ischemic mouse heart. Hitherto unexplored species-specific differences in cardiac GLP-1R expression challenge the exclusive use of mouse models for understanding the mechanisms of GLP-1 action in the normal and ischemic human heart.

NSAID-Induced Enteropathy Affects Regulation of Hepatic Glucose Production by Decreasing GLP-1 Secretion

BACKGROUND/AIM

Given their widespread use and their notorious effects on the lining of gut cells, including the enteroendocrine cells, we explored if chronic exposure to non-steroidal anti-inflammatory drugs (NSAIDs) affects metabolic balance in a mouse model of NSAID-induced enteropathy.

METHOD

We administered variable NSAIDs to C57Blk/6J mice through intragastric gavage and measured their energy balance, glucose hemostasis, and GLP-1 levels. We treated them with Exendin-9 and Exendin-4 and ran a euglycemic-hyperinsulinemic clamp.

RESULTS

Chronic administration of multiple NSAIDs to C57Blk/6J mice induces ileal ulcerations and weight loss in animals consuming a high-fat diet. Despite losing weight, NSAID-treated mice exhibit no improvement in their glucose tolerance. Furthermore, glucose-stimulated (glucagon-like peptide -1) GLP-1 is significantly attenuated in the NSAID-treated groups. In addition, Exendin-9-a GLP-1 receptor antagonist-worsens glucose tolerance in the control group but not in the NSAID-treated group. Finally, the hyper-insulinemic euglycemic clamp study shows that endogenous glucose production, total glucose disposal, and their associated insulin levels were similar among an ibuprofen-treated group and its control. Exendin-4, a GLP-1 receptor agonist, reduces insulin levels in the ibuprofen group compared to their controls for the same glucose exchange rates.

CONCLUSIONS

Chronic NSAID use can induce small intestinal ulcerations, which can affect intestinal GLP-1 production, hepatic insulin sensitivity, and consequently, hepatic glucose production.

GLP1 receptor agonism protects against acute olanzapine-induced hyperglycemia

Olanzapine is a second-generation antipsychotic (SGA) used in the treatment of schizophrenia and a number of off-label conditions. Although effective in reducing psychoses, acute olanzapine treatment causes hyperglycemia. Pharmacological agonists of the glucagon-like peptide 1 (GLP1) receptor have been shown to offset weight gain associated with chronic SGA administration. It is not known whether GLP1 receptor agonism would mitigate the acute metabolic side effects of SGAs. Within this context, we sought to determine whether pharmacological targeting of the GLP1 receptor would be sufficient to protect against acute olanzapine-induced impairments in glucose and lipid homeostasis. Male C57BL/6J mice were treated with olanzapine and/or the GLP1 receptor agonists liraglutide and exendin 4, and the blood glucose response was measured. We found that liraglutide or exendin 4 completely protected male mice against olanzapine-induced hyperglycemia in parallel with increases in circulating insulin (liraglutide, exendin 4) and reductions in glucagon (liraglutide only). In additional experiments, female mice, which are protected from acute olanzapine-induced hyperglycemia, displayed hyperglycemia, increases in glucagon, and reductions in insulin when treated with olanzapine and the GLP1 receptor antagonist exendin 9-39 compared with olanzapine treatment alone. Although in some instances the pharmacological targeting of the GLP1 receptor attenuated indexes of olanzapine-induced lipolysis, increases in liver triglyceride accumulation were not impacted. Our findings provide evidence that signaling through the GLP1 receptor can remarkably influence acute olanzapine-induced hyperglycemia, and from the standpoint of protecting against acute excursions in blood glucose, GLP1 receptor agonists should be considered as an adjunct treatment approach.NEW & NOTEWORTHY Antipsychotic drugs cause rapid perturbations in glucose and lipid metabolism. In the present study we have demonstrated that cotreatment with glucagon-like peptide 1 (GLP1) receptor agonists, such as liraglutide, protects against metabolic dysregulation caused by the antipsychotic drug olanzapine. These findings suggest that pharmacological targeting of the GLP1 receptor could be an effective adjunct approach to mitigate the harmful acute metabolic side effects of antipsychotic drugs.

Cellular models for beta-cell function and diabetes gene therapy

Diabetes is characterized by the destruction and/or relative dysfunction of insulin-secreting beta-cells in the pancreatic islets of Langerhans. Consequently, considerable effort has been made to understand the physiological processes governing insulin production and secretion in these cells and to elucidate the mechanisms involved in their deterioration in the pathogenesis of diabetes. To date, considerable research has exploited clonal beta-cell lines derived from rodent insulinomas. Such cell lines have proven to be a great asset in diabetes research, in vitro drug testing, and studies of beta-cell physiology and provide a sustainable, and in many cases, more practical alternative to the use of animals or primary tissue. However, selection of the most appropriate rodent beta cell line is often challenging and no single cell line entirely recapitulates the properties of human beta-cells. The generation of stable human beta-cell lines would provide a much more suitable model for studies of human beta-cell physiology and pathology and could potentially be used as a readily available source of implantable insulin-releasing tissue for cell-based therapies of diabetes. In this review, we discuss the history, development, functional characteristics and use of available clonal rodent beta-cell lines, as well as reflecting on recent advances in the generation of human-derived beta-cell lines, their use in research studies and their potential for cell therapy of diabetes.

Glucagon-like peptide-1 receptor regulates endoplasmic reticulum stress-induced apoptosis and the associated inflammatory response in chondrocytes and the progression of osteoarthritis in rat

Treatments for osteoarthritis (OA) are designed to restore chondrocyte function and inhibit cell apoptosis. Previous studies have shown that activation of the glucagon-like peptide-1 receptor (GLP-1R) leads to anti-inflammatory and anti-apoptotic effects. However, the role of GLP-1R in the pathological process of OA is unclear. In present work, we aimed to demonstrate the potential effect of GLP-1R on chondrocytes and elucidate its underlying mechanisms. We found that activation of GLP-1R with liraglutide could protect chondrocytes against endoplasmic reticulum stress and apoptosis induced by interleukin (IL)-1β or triglycerides (TGs). These effects were partially attenuated by GLP-1R small interfering RNA treatment. Moreover, inhibiting PI3K/Akt signaling abolished the protective effects of GLP-1R by increase the apoptosis activity and ER stress. Activating GLP-1R suppressed the nuclear factor kappa-B pathway, decreased the release of inflammatory mediators (IL-6, tumor necrosis factor α), and reduced matrix catabolism in TG-treated chondrocytes; these effects were abolished by GLP-1R knockdown. In the end, liraglutide attenuated rat cartilage degeneration in an OA model of knee joints in vivo. Our results indicate that GLP-1R is a therapeutic target for the treatment of OA, and that liraglutide could be a therapeutic candidate for this clinical application.

Do we know the true mechanism of action of the DPP-4 inhibitors?

The prevalence of type 2 diabetes is increasing, which is alarming because of its serious complications. Anti-diabetic treatment aims to control glucose homeostasis as tightly as possible in order to reduce these complications. Dipeptidyl peptidase-4 (DPP-4) inhibitors are a recent addition to the anti-diabetic treatment modalities, and have become widely accepted because of their good efficacy, their benign side-effect profile and their low hypoglycaemia risk. The actions of DPP-4 inhibitors are not direct, but rather are mediated indirectly through preservation of the substrates they protect from degradation. The two incretin hormones, glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide, are known substrates, but other incretin-independent mechanisms may also be involved. It seems likely therefore that the mechanisms of action of DPP-4 inhibitors are more complex than originally thought, and may involve several substrates and encompass local paracrine, systemic endocrine and neural pathways, which are discussed here.

Functional and Molecular Adaptations of Enteroendocrine L-Cells in Male Obese Mice Are Associated With Preservation of Pancreatic α-Cell Function and Prevention of Hyperglycemia

Glucose homeostasis depends on the coordinated secretion of glucagon, insulin, and Glucagon-like peptide (GLP)-1 by pancreas and intestine. Obesity, which is associated with an increased risk of developing insulin resistance and type 2 diabetes, affects the function of these organs. Here, we investigate the functional and molecular adaptations of proglucagon-producing cells in obese mice to better define their involvement in type 2 diabetes development. We used GLU-Venus transgenic male mice specifically expressing Venus fluorochrome in proglucagon-producing cells. Mice were subjected to 16 weeks of low-fat diet or high-fat diet (HFD) and then subdivided by measuring glycated hemoglobin (HbA1c) in 3 groups: low-fat diet mice and I-HFD (glucose-intolerant) mice with similar HbA1c and H-HFD (hyperglycemic) mice, which exhibited higher HbA1c. At 16 weeks, both HFD groups exhibited similar weight gain, hyperinsulinemia, and insulin resistance. However, I-HFD mice exhibited better glucose tolerance compared with H-HFD mice. I-HFD mice displayed functional and molecular adaptations of enteroendocrine L-cells resulting in increased intestinal GLP-1 biosynthesis and release as well as maintained pancreatic α- and β-cell functions. By contrast, H-HFD mice exhibited dysfunctional L, α- and β-cells with increased β- and L-cell numbers. Administration of the GLP-1R antagonist Exendin9-39 in I-HFD mice led to hyperglycemia and alterations of glucagon secretion without changes in insulin secretion. Our results highlight the cross-talk between islet and intestine endocrine cells and indicate that a compensatory adaptation of L-cell function in obesity plays an important role in preserving glucose homeostasis through the control of pancreatic α-cell functions.

Incretin hormone receptors are required for normal beta cell development and function in female mice

The incretin hormones, glucose dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1), potentiate insulin secretion and are responsible for the majority of insulin secretion that occurs after a meal. They may also, however, have a fundamental role in pancreatic beta cell development and function, independently of their role in potentiating insulin secretion after a meal. This has led to observations that a loss of GIP or GLP-1 action affects normal beta cell function, however each one of the incretin hormones may compensate when the action of the other is lost and therefore the overall impact of the incretin hormones on beta cell function is not known. We therefore utilized a mouse line deficient in both the GLP-1 and GIP receptor genes, the double incretin receptor knockout (DIRKO), to determine the consequences of a lifelong, complete lack of incretin hormone action on beta cell function, in vivo, in intact animals. We found that DIRKO mice displayed impaired glucose tolerance and insulin secretion in response to both oral glucose and mixed meal tolerance tests compared to wild-type mice. Assessment of beta cell function using the hyperglycemic clamp technique revealed an 80% decrease in first phase insulin response in DIRKO mice, but a normal second phase insulin secretion. A similar decline was seen when wild-type mice were given acute intravenous injection of glucose together with the GLP-1 receptor antagonist Ex9-39. Ex vivo assessments of the pancreas revealed significantly fewer islets in the pancreata of DIRKO mice despite no differences in total pancreatic mass. Insulin secretion from isolated islets of DIRKO mice was impaired to a similar extent to that seen during the hyperglycemic clamp. Insulin secretion in wild-type islets was impaired by acute treatment with Ex9-39 to a similar extent as the in vivo intravenous glucose tolerance tests. In conclusion, a loss of the action of both incretin hormones results in direct impairment of beta cell function both in vivo and in vitro in a process that appears to be independent of the intestinally secreted incretin hormones. We therefore conclude that the incretin hormones together significantly impact both beta-cell function and beta-cell development.

Therapeutic stimulation of GLP-1 and GIP protein with DPP-4 inhibitors for type-2 diabetes treatment

Dipeptidyl peptidase-4 (DPP-4) inhibition is a new treatment for type-2 diabetes. DPP-4 inhibition increases levels of active GLP-1. GLP-1 enhances insulin secretion and diminishes glucagon secretion, in this manner reducing glucose concentrations in blood. A number of DPP-4 inhibitors are under clinical development. However, the durability and long-term safety of DPP-4 inhibition remain to be established. These synthetic DPP-4 inhibitors are showing some side effects. Herbal medicines are alternative medicine over synthetic drugs that can relieve the patients. Various research studies have been carried all over the world to evaluate the efficacy of herbs in the treatment of Type II diabetes mellitus. For a long time type II diabetes mellitus has been treated orally with herbal medicines, because plant products are frequently prescribed due to their less toxicity than conventional medicines.

Incretins: their physiology and application in the treatment of diabetes mellitus

Therapies targeting the action of incretin hormones have been under close scrutiny in recent years. The incretin effect has been defined as postprandial enhancement of insulin secretion by gut-derived factors. Likewise, incretin mimetics and incretin effect amplifiers are the two different incretin-based treatment strategies developed for the treatment of diabetes. Although, incretin mimetics produce effects very similar to those of natural incretin hormones, incretin effect amplifiers act by inhibiting dipeptidyl peptidase-4 (DPP-4) enzyme to increase plasma concentration of incretins and their biologic effects. Because glucagon-like peptide-1 (GLP-1) is an incretin hormone with various anti-diabetic actions including stimulation of glucose-induced insulin secretion, inhibition of glucagon secretion, hepatic glucose production and gastric emptying, it has been evaluated as a novel therapeutic agent for the treatment of type 2 diabetes mellitus (T2DM). GLP-1 also manifests trophic effects on pancreas such as pancreatic beta cell growth and differentiation. Because DPP-4 is the enzyme responsible for the inactivation of GLP-1, DPP-4 inhibition represents another potential strategy to increase plasma concentration of GLP-1 to enhance the incretin effect. Thus, anti-diabetic properties of these two classes of drugs have stimulated substantial clinical interest in the potential of incretin-based therapeutic agents as a means to control glucose homeostasis in T2DM patients. Despite this fact, clinical use of GLP-1 mimetics and DPP-4 inhibitors have raised substantial concerns owing to possible side effects of the treatments involving increased risk for pancreatitis, and C-cell adenoma/carcinoma. Thus, controversial issues in incretin-based therapies under development are reviewed and discussed in this manuscript.

Long-term metabolic benefits of exenatide in mice are mediated solely via the known glucagon-like peptide 1 receptor

Glucagon-like peptide 1 receptors (GLP-1R) are expressed in multiple tissues and activation results in metabolic benefits including enhanced insulin secretion, slowed gastric emptying, suppressed food intake, and improved hepatic steatosis. Limited and inconclusive knowledge exists regarding whether the effects of chronic exposure to a GLP-1R agonist are solely mediated via this receptor. Therefore, we examined 3-mo dosing of exenatide in mice lacking a functional GLP-1R (Glp1r−/−). Exenatide (30 nmol·kg−1·day−1) was infused subcutaneously for 12 wk in Glp1r−/− and wild-type (Glp1r+/+) control mice fed a high-fat diet. Glycated hemoglobin A1c (HbA1c), plasma glucose, insulin, amylase, lipase, alanine aminotransferase (ALT), aspartate aminotransferase (AST), body weight, food intake, terminal hepatic lipid content (HLC), and plasma exenatide levels were measured. At the end of the study, oral glucose tolerance test (OGTT) and rate of gastric emptying were assessed. Exenatide produced no significant changes in Glp1r−/− mice at study end. In contrast, exenatide decreased body weight, food intake, and glucose in Glp1r+/+ mice. When compared with vehicle, exenatide reduced insulin, OGTT glucose AUC0–2h, ALT, and HLC in Glp1r+/+ mice. Exenatide had no effect on plasma amylase or lipase levels. Exenatide concentrations were approximately eightfold higher in Glp1r−/− versus Glp1r+/+ mice after 12 wk of infusion, whereas renal function was similar. These data support the concept that exenatide requires a functional GLP-1R to exert chronic metabolic effects in mice, and that novel “GLP-1” receptors may not substantially contribute to these changes. Differential exenatide plasma levels in Glp1r+/+ versus Glp1r−/− mice suggest that GLP-1R may play an important role in plasma clearance of exenatide and potentially other GLP-1-related peptides.

Fibroblast growth factor 21 (FGF21) and glucagon-like peptide 1 contribute to diabetes resistance in glucagon receptor-deficient mice

Mice genetically deficient in the glucagon receptor (Gcgr(-/-)) show improved glucose tolerance, insulin sensitivity, and α-cell hyperplasia. In addition, Gcgr(-/-) mice do not develop diabetes after chemical destruction of β-cells. Since fibroblast growth factor 21 (FGF21) has insulin-independent glucose-lowering properties, we investigated whether FGF21 was contributing to diabetes resistance in insulin-deficient Gcgr(-/-) mice. Plasma FGF21 was 25-fold higher in Gcgr(-/-) mice than in wild-type mice. FGF21 was found to be expressed in pancreatic β- and α-cells, with high expression in the hyperplastic α-cells of Gcgr(-/-) mice. FGF21 expression was also significantly increased in liver and adipose tissue of Gcgr(-/-) mice. To investigate the potential antidiabetic actions of FGF21 in insulin-deficient Gcgr(-/-) mice, an FGF21-neutralizing antibody was administered prior to oral glucose tolerance tests (OGTTs). FGF21 neutralization caused a decline in glucose tolerance in insulin-deficient Gcgr(-/-) mice during the OGTT. Despite this decline, insulin-deficient Gcgr(-/-) mice did not develop hyperglycemia. Glucagon-like peptide 1 (GLP-1) also has insulin-independent glucose-lowering properties, and an elevated circulating level of GLP-1 is a known characteristic of Gcgr(-/-) mice. Neutralization of FGF21, while concurrently blocking the GLP-1 receptor with the antagonist Exendin 9-39 (Ex9-39), resulted in significant hyperglycemia in insulin-deficient Gcgr(-/-) mice, while blocking with Ex9-39 alone did not. In conclusion, FGF21 acts additively with GLP-1 to prevent insulinopenic diabetes in mice lacking glucagon action.

The role of gastrointestinal hormones in hepatic lipid metabolism

Hepatocellular accumulation of free fatty acids (FFAs) in the form of triglycerides constitutes the metabolic basis for the development of nonalcoholic fatty liver disease (NAFLD). Recent data demonstrate that excess FFA hepatocyte storage is likely to lead to lipotoxicity and hepatocyte apoptosis. Hence, FFA-mediated hepatocyte injury is a key contributor to the pathogenesis of nonalcoholic steatohepatitis (NASH). Nonalcoholic steatohepatitis, obesity, type 2 diabetes, essential hypertension, and other common medical problems together comprise metabolic syndrome. Evidence suggests that peptide hormones from the L cells of the distal small intestine, which comprise the core of the enteroendocrine system (EES), play two key roles, serving either as incretins, or as mediators of appetite and satiety in the central nervous system. Recent data related to glucagon-like peptide-1 (GLP-1) and other known L-cell hormones have accumulated due to the increasing frequency of bariatric surgery, which increase delivery of bile salts to the hindgut. Bile acids are a key stimulus for the TGR5 receptor of the L cells. Enhanced bile-salt flow and subsequent EES stimulation may be central to elimination of hepatic steatosis following bariatric surgery. Although GLP-1 is a clinically relevant pharmacological analogue that drives pancreatic β-cell insulin output, GLP-1 analogues also have independent benefits via their effects on hepatocellular FFA metabolism. The authors also discuss recent data regarding the role of the major peptides released by the EES, which promote satiety and modulate energy homeostasis and utilization, as well as those that control fat absorption and intestinal permeability. Taken together, elucidating novel functions for EES-related peptides and pharmacologic development of peptide analogues offer potential far-ranging treatment for obesity-related human disease.

Use of glucagon-like peptide-1 agonists to improve islet graft performance

Human islet transplantation is an effective and promising therapy for type I diabetes. However, long-term insulin independence is both difficult to achieve and inconsistent. De novo or early administration of incretin-based drugs is being explored for improving islet engraftment. In addition to its glucose-dependent insulinotropic effects, incretins also lower postprandial glucose excursion by inhibiting glucagon secretion, delaying gastric emptying, and can protect beta-cell function. Incretin therapy has so far proven clinically safe and tolerable with little hypoglycemic risk. The present review aims to highlight the new frontiers in research involving incretins from both in vitro and in vivo animal studies in the field of islet transplant. It also provides an overview of the current clinical status of incretin usage in islet transplantation in the management of type I diabetes.

Differential effects of oxyntomodulin and GLP-1 on glucose metabolism

Glucagon-like peptide-1 (GLP-1) and oxyntomodulin (OXM) are peptide hormones secreted postprandially from the gut that stimulate insulin secretion in a glucose-dependent manner. OXM activates both the GLP-1 receptor (GLP1R) and the glucagon receptor (GCGR). It has been suggested that OXM acutely modulates glucose metabolism solely through GLP1R agonism. Because OXM activates the GLP1R with lower affinity than GLP-1, we generated a peptide analog (Q→E, OXMQ3E) that does not exhibit glucagon receptor agonist activity but retains the same affinity as OXM for GLP1R. We compared the effects of OXM and OXMQ3E in a glucose tolerance test and, to better characterize the effect on glucose metabolism, we performed controlled infusions of OXM or OXMQ3E during a hyperglycemic clamp performed in wild-type, Glp1r(-/-), and Gcgr(-/-) mice. Our findings show that OXM, but not OXMQ3E, activates the GCGR in vivo. Second, OXM and OXMQ3E improve glucose tolerance following an acute glucose challenge and during a hyperglycemic clamp in mice. Finally, OXM infusion during a glucose clamp reduces the glucose infusion rate (GIR) despite a simultaneous increase in insulin levels in Glp1r(-/-) mice, whereas OXM and OXMQ3E increase GIR to a similar extent in Gcgr(-/-) mice. In conclusion, activation of the GCGR seems to partially attenuate the acute beneficial effects on glucose and contributes to the insulinotropic action of oxyntomodulin.

Regulation of mouse intestinal L cell progenitors proliferation by the glucagon family of peptides

Glucagon like peptide-1 (GLP-1) and GLP-2 are hormones secreted by intestinal L cells that stimulate glucose-dependent insulin secretion and regulate intestinal growth, respectively. Mice with deletion of the glucagon receptor (Gcgr) have high levels of circulating GLP-1 and GLP-2. We sought to determine whether the increased level of the glucagon-like peptides is due to L cell hyperplasia. We found, first, that high levels of the glucagon-like peptides increase L cell number but does not affect the number of other intestinal epithelial cell types. Second, a large proportion of ileal L cells of Gcgr(-/-) mice coexpressed glucose-dependent insulinotropic peptide (GIP). Cells coexpressing GIP and GLP-1 are termed LK cells. Third, the augmentation in L cell number was due to a higher rate of proliferation of L cell progenitors rather than to the entrance of mature L cells into the cell cycle. Fourth, a high concentration of the glucagon-like peptides in the circulation augmented the mRNA levels of transcription factors expressed by late but not early enteroendocrine progenitors. Fifth, the administration of exendin 9-39, a GLP-1 receptor antagonist, resulted in a decrease in the rate of L cell precursor proliferation. Finally, we determined that L cells do not express the GLP-1 receptor, suggesting that the effect of GLP-1 is mediated by paracrine and/or neuronal signals. Our results suggest that GLP-1 plays an important role in the regulation of L cell number.

CCK stimulation of GLP-1 neurons involves α1-adrenoceptor-mediated increase in glutamatergic synaptic inputs

OBJECTIVE

Glucagon-like peptide 1 (GLP-1) is involved in the central regulation of food intake. It is produced within the brain by preproglucagon (PPG) neurons, which are located primarily within the brain stem. These neurons project widely throughout the brain, including to the appetite centers in the hypothalamus, and are believed to convey signals related to satiety. Previous work demonstrated that they are directly activated by leptin and electrical activity of the afferent vagus. Another satiety hormone, cholecystokinin (CCK), has also been linked to activation of brain stem neurons, suggesting that it might act partially via centrally projecting neurons from the nucleus tractus solitarius (NTS). The aim of this study was to investigate the neuronal circuitry linking CCK to the population of NTS-PPG neurons.

RESEARCH DESIGN AND METHODS

Transgenic mice expressing yellow fluorescent protein (Venus) under the control of the PPG promoter were used to identify PPG neurons in vitro and to record their electrical and pharmacological profile.

RESULTS

PPG neurons in the NTS were excited by CCK and epinephrine, but not by the melanocortin receptor agonist melanotan II. Both CCK and epinephrine acted to increase glutamatergic transmission to the PPG neurons, and this involved activation of α(1)-adrenergic receptors. Inhibition of adrenergic signaling abolished the excitatory action of CCK.

CONCLUSIONS

CCK activates NTS-PPG cells by a circuit involving adrenergic and glutamatergic neurons. NTS-PPG neurons integrate a variety of peripheral signals that indicate both long-term energy balance and short-term nutritional and digestional status to produce an output signal to feeding and autonomic circuits.

Site-specific PEGylation of exenatide analogues markedly improved their glucoregulatory activity

BACKGROUND AND PURPOSE

Exenatide is a 39-amino-acid peptide widely used to manage type 2 diabetes mellitus. However, it has a short plasma half-life and requires a twice daily injection regime. To overcome these drawbacks we used maleimide-polyethylene glycol to induce site-specific PEGylation.

EXPERIMENTAL APPROACH

The analogue PB-105 (ExC39) was produced by replacing cysteine at position 39 of exenatide to provide a free thiol group. PB-105 showed the same glucoregulatory activity as exenatide in mice. Site-specific PEGylation of PB-105 was performed to produce PB-110 (ExC39PEG5kDa), PB-106 (ExC39PEG20kDa), PB-107 (ExC39PEG30kDa) and PB-108 (ExC39PEG40kDa). Their effects on intracellular cAMP, acute glucoregulatory activity and pharmacokinetic profile were compared in mice and rats.

KEY RESULTS

PEGylation shifted the concentration-response curve of PB-105 to the right in a parallel, polyethylene glycol mass-dependent manner but with an inflexion point of at least 20 kDa. The activities of PB-107 and PB-108 but not PB-106 were reduced by 90% and 99%. PEGylation affected in vivo glucoregulatory activity in the same 'Inflexion-Shift' fashion at least at 20 kDa, but linearly increased plasma duration and systemic exposure without inflexion. PB-106 had a plasma t(1/2) approximately 10-fold that of PB-105, and exhibited superior glucoregulatory activity compared with PB-105 in normal and diabetic mice.

CONCLUSIONS AND IMPLICATIONS

Site-specific PEGylation of exenatide with a permanent amide linkage affects its activity in a new type of 'Inflexion-Shift' fashion. PB-106 is a putative new analogue for treating diabetes; it possesses no loss of in vitro activity, prolonged plasma duration and superior, improved in vivo glucoregulatory activity compared with exenatide.

Effect of endogenous GLP-1 on insulin secretion in type 2 diabetes

OBJECTIVE

The incretins glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) account for up to 60% of postprandial insulin release in healthy people. Previous studies showed a reduced incretin effect in patients with type 2 diabetes but a robust response to exogenous GLP-1. The primary goal of this study was to determine whether endogenous GLP-1 regulates insulin secretion in type 2 diabetes.

METHODS

Twelve patients with well-controlled type 2 diabetes and eight matched nondiabetic subjects consumed a breakfast meal containing D-xylose during fixed hyperglycemia at 5 mmol/l above fasting levels. Studies were repeated, once with infusion of the GLP-1 receptor antagonist, exendin-(9-39) (Ex-9), and once with saline.

RESULTS

The relative increase in insulin secretion after meal ingestion was comparable in diabetic and nondiabetic groups (44 +/- 4% vs. 47 +/- 7%). Blocking the action of GLP-1 suppressed postprandial insulin secretion similarly in the diabetic and nondiabetic subjects (25 +/- 4% vs. 27 +/- 8%). However, Ex-9 also reduced the insulin response to intravenous glucose (25 +/- 5% vs. 26 +/- 7%; diabetic vs. nondiabetic subjects), when plasma GLP-1 levels were undetectable. The appearance of postprandial ingested d-xylose in the blood was not affected by Ex-9.

CONCLUSIONS

These findings indicate that in patients with well-controlled diabetes, the relative effects of enteral stimuli and endogenous GLP-1 to enhance insulin release are retained and comparable with those in nondiabetic subjects. Surprisingly, GLP-1 receptor signaling promotes glucose-stimulated insulin secretion independent of the mode of glucose entry. Based on rates of D-xylose absorption, GLP-1 receptor blockade did not affect gastric emptying of a solid meal.

Neurogenin 3-specific dipeptidyl peptidase-2 deficiency causes impaired glucose tolerance, insulin resistance, and visceral obesity

The control of glucose metabolism is a complex process, and dysregulation at any level can cause impaired glucose tolerance and insulin resistance. These two defects are well-known characteristics associated with obesity and onset of type 2 diabetes. Here we introduce the N-terminal dipeptidase, DPP2, as a novel regulator of the glucose metabolism. We generated mice with a neurogenin 3 (NGN3)-specific DPP2 knockdown (kd) to explore a possible role of DPP2 in maintaining metabolic homeostasis. These mice spontaneously developed hyperinsulinemia, glucose intolerance, and insulin resistance by 4 months of age. In addition, we observed an increase in food intake in DPP2 kd mice, which was associated with a significant increase in adipose tissue mass and enhanced liver steatosis but no difference in body weight. In accordance with these findings, the mutant mice had a higher rate of respiratory exchange than the control littermates. This phenotype was exacerbated with age and when challenged with a high-fat diet. We report, for the first time, that DPP2 enzyme activity is essential for preventing hyperinsulinemia and maintaining glucose homeostasis. Interestingly, the phenotype of NGN3-DPP2 kd mice is opposite that of DPP4 knockout mice with regard to glucose metabolism, namely the former have normal glucagon-like peptide 1 levels but present with glucose intolerance, whereas the latter have increased glucagon-like peptide 1, which is accompanied by augmented glucose tolerance.

Pax6 haploinsufficiency causes abnormal metabolic homeostasis by down-regulating glucagon-like peptide 1 in mice

Heterozygosity for the Pax6 allele is associated with impaired glucose tolerance in humans. With a Pax6 mutant mouse model, we found many of the metabolic abnormalities were consistent with the effects of down-regulating the expression of glucagon-like peptide 1 (GLP-1). In addition to impaired glucose tolerance, adult heterozygous mutant mice (Pax6(m/+)) secreted less insulin responding to glucose and arginine administration compared with control mice. Moreover, Pax6(m/+) mice showed increased food intake compared with control mice, although they were resistant to diet-induced fat accumulation. Indeed, levels of circulating GLP-1 and intestinal transcription of Gcg/Proglucagon were dramatically reduced in Pax6(m/+) mice. Mutated Pax6 also failed to activate the Gcg/Proglucagon promoter by in vitro transfection assay. Finally, administering the GLP-1 receptor agonist exendin-4 to Pax6(m/+) mice largely reversed their abnormal food intake, glycemic excursion, and insulin secretion. Our studies suggested that disruption of metabolic homeostasis mainly caused by Pax6 haploinsufficiency was mainly mediated by down-regulation of GLP-1. Administration of exendin-4 may be a useful therapy in humans with a similar mutation.

The glucagon-like peptide-1 receptor regulates endogenous glucose production and muscle glucose uptake independent of its incretin action

Glucagon-like peptide-1 (GLP-1) diminishes postmeal glucose excursions by enhancing insulin secretion via activation of the beta-cell GLP-1 receptor (Glp1r). GLP-1 may also control glucose levels through mechanisms that are independent of this incretin effect. The hyperinsulinemic-euglycemic clamp (insulin clamp) and exercise were used to examine the incretin-independent glucoregulatory properties of the Glp1r because both perturbations stimulate glucose flux independent of insulin secretion. Chow-fed mice with a functional disruption of the Glp1r (Glp1r(-/-)) were compared with wild-type littermates (Glp1r(+/+)). Studies were performed on 5-h-fasted mice implanted with arterial and venous catheters for sampling and infusions, respectively. During insulin clamps, [3-(3)H]glucose and 2[(14)C]deoxyglucose were used to determine whole-body glucose turnover and glucose metabolic index (R(g)), an indicator of glucose uptake. R(g) in sedentary and treadmill exercised mice was determined using 2[(3)H]deoxyglucose. Glp1r(-/-) mice exhibited increased glucose disappearance, muscle R(g), and muscle glycogen levels during insulin clamps. This was not associated with enhanced muscle insulin signaling. Glp1r(-/-) mice exhibited impaired suppression of endogenous glucose production and hepatic glycogen accumulation during insulin clamps. This was associated with impaired liver insulin signaling. Glp1r(-/-) mice became significantly hyperglycemic during exercise. Muscle R(g) was normal in exercised Glp1r(-/-) mice, suggesting that hyperglycemia resulted from an added drive to stimulate glucose production. Muscle AMP-activated protein kinase phosphorylation was higher in exercised Glp1r(-/-) mice. This was associated with increased relative exercise intensity and decreased exercise endurance. In conclusion, these results show that the endogenous Glp1r regulates hepatic and muscle glucose flux independent of its ability to enhance insulin secretion.

Acute dipeptidyl peptidase-4 inhibition rapidly enhances insulin-mediated suppression of endogenous glucose production in mice

Pharmacological approaches that enhance incretin action for the treatment of type 2 diabetes mellitus have recently been developed, i.e. injectable glucagon-like peptide-1 receptor (GLP-1R) agonists with prolonged plasma half-lives and orally available inhibitors of dipeptidyl peptidase (DPP)-4, the main enzyme responsible for the rapid degradation of circulating glucagon-like peptide-1 and glucose-dependent insulinotropic peptide. The mechanism(s) underlying the glucose-lowering effect of these two pharmacotherapies differs and is not yet fully understood. Here we investigated whether acute GLP-1R activation (exendin-4) or DPP-4 inhibition (des-F-sitagliptin) modulates insulin action in mice using a hyperinsulinemic euglycemic clamp. A single iv bolus of des-F-sitagliptin (11 mg/kg) was administered to mice 15 min after the start of the clamp, and its effect was compared with a 50-ng bolus of exendin-4 or the same volume of saline. Despite matched levels of plasma glucose and insulin, within 15 min the glucose infusion rate required to maintain euglycemia was significantly greater after des-F-sitagliptin compared with saline or exendin-4. This difference was entirely due to enhancement of insulin-mediated suppression of endogenous glucose production by des-F-sitagliptin, with no difference in glucose disposal rate. These findings illustrate that DPP-4 inhibition modulates glucose homeostasis through pathways distinct from those used by GLP-1R agonists in mice.

Exendin-4 modulates diabetes onset in nonobese diabetic mice

Activation of the glucagon-like peptide-1 receptor (GLP-1R) is associated with expansion of beta-cell mass due to stimulation of cell proliferation and induction of antiapoptotic pathways coupled to beta-cell survival. Although the GLP-1R agonist Exenatide (exendin-4) is currently being evaluated in subjects with type 1 diabetes, there is little information available about the efficacy of GLP-1R activation for prevention of experimental type 1 diabetes. We examined the consequences of exendin-4 (Ex-4) administration (100 ng once daily and 2 microg twice daily) on diabetes onset in nonobese diabetic mice beginning at either 4 or 9 wk of age prior to the onset of diabetes. Ex-4 treatment for 26 wk (2 microg twice daily) initiated at 4 wk of age delayed the onset of diabetes (P = 0.007). Ex-4-treated mice also exhibited a significant reduction in insulitis scores, enhanced beta-cell mass, and improved glucose tolerance. Although GLP-1R mRNA transcripts were detected in spleen, thymus, and lymph nodes from nonobese diabetic mice, Ex-4 treatment was not associated with significant changes in the numbers of CD4+ or CD8+ T cells or B cells in the spleen. However, Ex-4 treatment resulted in an increase in the number of CD4+ and CD8+ T cells in the lymph nodes and a reduction in the numbers of CD4+CD25+Foxp3+ regulatory T cells in the thymus but not in lymph nodes. These findings demonstrate that sustained GLP-1R activation in the absence of concomitant immune intervention may be associated with modest but significant delay in diabetes onset in a murine model of type 1 diabetes.

Insulin action in the double incretin receptor knockout mouse

OBJECTIVE

The incretins glucagon-like peptide 1 and glucose-dependent insulinotropic polypeptide have been postulated to play a role in regulating insulin action, although the mechanisms behind this relationship remain obscure. We used the hyperinsulinemic-euglycemic clamp to determine sites where insulin action may be modulated in double incretin receptor knockout (DIRKO) mice, which lack endogenous incretin action.

RESEARCH DESIGN AND METHODS

DIRKO and wild-type mice were fed regular chow or high-fat diet for 4 months. Clamps were performed on 5-h-fasted, conscious, unrestrained mice using an arterial catheter for sampling.

RESULTS

Compared with wild-type mice, chow and high fat-fed DIRKO mice exhibited decreased fat and muscle mass associated with increased energy expenditure and ambulatory activity. Clamp rates of glucose infusion (GIR), endogenous glucose production (endoR(a)), and disappearance (R(d)) were not different in chow-fed wild-type and DIRKO mice, although insulin levels were lower in DIRKO mice. Liver Akt expression was decreased but Akt activation was increased in chow-fed DIRKO compared with wild-type mice. High-fat feeding resulted in fasting hyperinsulinemia and hyperglycemia in wild-type but not in DIRKO mice. GIR, suppression of endoR(a), and stimulation of R(d) were inhibited in high fat-fed wild-type mice but not in DIRKO mice. High-fat feeding resulted in impaired tissue glucose uptake (R(g)) in skeletal muscle of wild-type mice but not of DIRKO mice. Liver and muscle Akt activation was enhanced in high fat-fed DIRKO compared with wild-type mice.

CONCLUSIONS

In summary, DIRKO mice exhibit enhanced insulin action compared with wild-type mice when fed a regular chow diet and are protected from high-fat diet-induced obesity and insulin resistance.

The physiology of glucagon-like peptide 1

Glucagon-like peptide 1 (GLP-1) is a 30-amino acid peptide hormone produced in the intestinal epithelial endocrine L-cells by differential processing of proglucagon, the gene which is expressed in these cells. The current knowledge regarding regulation of proglucagon gene expression in the gut and in the brain and mechanisms responsible for the posttranslational processing are reviewed. GLP-1 is released in response to meal intake, and the stimuli and molecular mechanisms involved are discussed. GLP-1 is extremely rapidly metabolized and inactivated by the enzyme dipeptidyl peptidase IV even before the hormone has left the gut, raising the possibility that the actions of GLP-1 are transmitted via sensory neurons in the intestine and the liver expressing the GLP-1 receptor. Because of this, it is important to distinguish between measurements of the intact hormone (responsible for endocrine actions) or the sum of the intact hormone and its metabolites, reflecting the total L-cell secretion and therefore also the possible neural actions. The main actions of GLP-1 are to stimulate insulin secretion (i.e., to act as an incretin hormone) and to inhibit glucagon secretion, thereby contributing to limit postprandial glucose excursions. It also inhibits gastrointestinal motility and secretion and thus acts as an enterogastrone and part of the "ileal brake" mechanism. GLP-1 also appears to be a physiological regulator of appetite and food intake. Because of these actions, GLP-1 or GLP-1 receptor agonists are currently being evaluated for the therapy of type 2 diabetes. Decreased secretion of GLP-1 may contribute to the development of obesity, and exaggerated secretion may be responsible for postprandial reactive hypoglycemia.

Downregulation of GLP-1 and GIP receptor expression by hyperglycemia: possible contribution to impaired incretin effects in diabetes

Stimulation of insulin secretion by the incretin hormones glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP) has been found to be diminished in type 2 diabetes. We hypothesized that this impairment is due to a defect at the receptor level induced by the diabetic state, particularly hyperglycemia. Gene expression of incretin receptors, GLP-1R and GIPR, were significantly decreased in islets of 90% pancreatectomized (Px) hyperglycemic rats, with recovery when glucose levels were normalized by phlorizin. Perifused islets isolated from hyperglycemic Px rats showed reduced insulin responses to GLP-1 and GIP. To examine the acute effect of hyperglycemia on incretin receptor expression, a hyperglycemic clamp study was performed for 96 h with reduction of GLP-1 receptor expression but increase in GIP receptor expression. Similar findings were found when islets were cultured at high glucose concentrations for 48 h. The reduction of GLP-1 receptor expression by high glucose was prevented by dominant-negative protein kinase C (PKC)alpha overexpression, whereas GLP-1 receptor expression was reduced with wild-type PKCalpha overexpression. Taken together, GLP-1 and GIP receptor expression is decreased with chronic hyperglycemia, and this decrease likely contributes to the impaired incretin effects found in diabetes.

The role of gut hormones in glucose homeostasis

The gastrointestinal tract has a crucial role in the control of energy homeostasis through its role in the digestion, absorption, and assimilation of ingested nutrients. Furthermore, signals from the gastrointestinal tract are important regulators of gut motility and satiety, both of which have implications for the long-term control of body weight. Among the specialized cell types in the gastrointestinal mucosa, enteroendocrine cells have important roles in regulating energy intake and glucose homeostasis through their actions on peripheral target organs, including the endocrine pancreas. This article reviews the biological actions of gut hormones regulating glucose homeostasis, with an emphasis on mechanisms of action and the emerging therapeutic roles of gut hormones for the treatment of type 2 diabetes mellitus.

Extrapancreatic incretin receptors modulate glucose homeostasis, body weight, and energy expenditure

The incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) control glucose homeostasis through well-defined actions on the islet beta cell via stimulation of insulin secretion and preservation and expansion of beta cell mass. We examined the importance of endogenous incretin receptors for control of glucose homeostasis through analysis of Glp1r(-/-), Gipr(-/-), and double incretin receptor knockout (DIRKO) mice fed a high-fat (HF) diet. DIRKO mice failed to upregulate levels of plasma insulin, pancreatic insulin mRNA transcripts, and insulin content following several months of HF feeding. Both single incretin receptor knockout and DIRKO mice exhibited resistance to diet-induced obesity, preservation of insulin sensitivity, and increased energy expenditure associated with increased locomotor activity. Moreover, plasma levels of plasminogen activator inhibitor-1 and resistin failed to increase significantly in DIRKO mice after HF feeding, and the GIP receptor agonist [D-Ala(2)]GIP, but not the GLP-1 receptor agonist exendin-4, increased the levels of plasma resistin in studies of both acute and chronic administration. These findings extend our understanding of how endogenous incretin circuits regulate glucose homeostasis independent of the beta cell via control of adipokine secretion and energy expenditure.

Gene therapy of diabetes using a novel GLP-1/IgG1-Fc fusion construct normalizes glucose levels in db/db mice

Glucagon-like peptide (GLP-1), a major physiological incretin, plays numerous important roles in modulating blood glucose homeostasis and has been proposed for the treatment of type 2 diabetes. The major obstacles for using native GLP-1 as a therapeutic agent are that it must be delivered by a parenteral route and has a short half-life. In an attempt to develop a strategy to prolong the physiological t(1/2) and enhance the potency of GLP-1, a fusion protein consisting of active human GLP-1 and mouse IgG(1) heavy chain constant regions (GLP-1/Fc) was generated. A plasmid encoding an IgK leader peptide-driven secretable fusion protein of the active GLP-1 and IgG(1)-Fc was constructed for mammalian expression. This plasmid allows for expression of bivalent GLP-1 peptide ligands as a result of IgG-Fc homodimerization. In vitro studies employing purified GLP-1/Fc indicate that the fusion protein is functional and elevates cAMP levels in insulin-secreting INS-1 cells. In addition, it stimulates insulin secretion in a glucose concentration-dependent manner. Intramuscular gene transfer of the plasmid in db/db mice demonstrated that expression of the GLP-1/Fc peptide normalizes glucose tolerance by enhancing insulin secretion and suppressing glucagon release. This strategy of using a bivalent GLP-1/Fc fusion protein as a therapeutic agent is a novel approach for the treatment of diabetes.

The biology of incretin hormones

Gut peptides, exemplified by glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are secreted in a nutrient-dependent manner and stimulate glucose-dependent insulin secretion. Both GIP and GLP-1 also promote beta cell proliferation and inhibit apoptosis, leading to expansion of beta cell mass. GLP-1, but not GIP, controls glycemia via additional actions on glucose sensors, inhibition of gastric emptying, food intake and glucagon secretion. Furthermore, GLP-1, unlike GIP, potently stimulates insulin secretion and reduces blood glucose in human subjects with type 2 diabetes. This article summarizes current concepts of incretin action and highlights the potential therapeutic utility of GLP-1 receptor agonists and dipeptidyl peptidase-4 (DPP-4) inhibitors for the treatment of type 2 diabetes.

Release of transgenic human insulin from gastric g cells: a novel approach for the amelioration of diabetes

We explored the hypothesis that meal-regulated release of insulin from gastric G cells can be used for gene therapy for diabetes. We generated transgenic mice in which the coding sequence of human insulin has been knocked into the mouse gastrin gene. Insulin was localized specifically to antral G cells of G-InsKi mice by double immunofluorescence staining using antibodies against insulin and gastrin. Insulin extracted from antral stomach of G-InsKi mice decreased blood glucose upon injection into streptozotocin-diabetic mice. Intragastric administration of peptone, a known potent luminal stimulant of gastrin secretion, induced an increase in circulating levels of transgenic human insulin from 10.7 +/- 2 to 23.3 +/- 4 pm in G-InsKi mice. Although G cell-produced insulin decreased blood glucose in G-InsKi mice, it did not cause toxic hypoglycemia. Proton pump inhibitors, pharmacological agents that increase gastrin output, caused a further increase in the circulating levels of gastric insulin (41.5 +/- 2 pm). G cell-produced insulin was released into circulation in response to the same meal-associated stimuli that control release of gastrin. The most striking aspect of the results presented here is that in the presence of the G-InsKi allele, Ins2(Akita/+) mice exhibited a marked prolongation of life span. These results imply that G cell-derived transgenic insulin is beneficial in the amelioration of diabetes. We suggest that an efficient G cells-based insulin gene therapy can relieve diabetic patients from daily insulin injections and protect them from complications of insulin insufficiency while avoiding episodes of toxic hypoglycemia.

GIP and GLP-1 as incretin hormones: lessons from single and double incretin receptor knockout mice

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are gut-derived incretins secreted in response to nutrient ingestion. Both incretins potentiate glucose-dependent insulin secretion and enhance beta-cell mass through regulation of beta-cell proliferation, neogenesis and apoptosis. In contrast, GLP-1, but not GIP, inhibits gastric emptying, glucagon secretion, and food intake. Furthermore, human subjects with Type 2 diabetes exhibit relative resistance to the actions of GIP, but not GLP-1R agonists. The physiological importance of both incretins has been investigated through generation and analysis of incretin receptor knockout mice. Elimination of incretin receptor action in GIPR-/- or GLP-1R-/- mice produces only modest impairment in glucose homeostasis. Similarly, double incretin receptor knockout (DIRKO) mice exhibit normal body weight and normal levels of plasma glucagon and hypoglycemic responses to exogenous insulin. However, glucose-stimulated insulin secretion is significantly decreased following oral but not intraperitoneal glucose challenge in DIRKO mice and the glucose lowering actions of dipeptidyl peptidase-IV (DPP-IV) inhibitors are extinguished in DIRKO mice. Hence, incretin receptor signaling exerts physiologically relevant actions critical for glucose homeostasis, and represents a pharmacologically attractive target for development of agents for the treatment of Type 2 diabetes.

Double incretin receptor knockout (DIRKO) mice reveal an essential role for the enteroinsular axis in transducing the glucoregulatory actions of DPP-IV inhibitors

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) are gut-derived incretins that potentiate glucose clearance following nutrient ingestion. Elimination of incretin receptor action in GIPR(-/-) or GLP-1R(-/-) mice produces only modest impairment in glucose homeostasis, perhaps due to compensatory upregulation of the remaining incretin. We have now studied glucose homeostasis in double incretin receptor knockout (DIRKO) mice. DIRKO mice exhibit normal body weight and fail to exhibit an improved glycemic response after exogenous administration of GIP or the GLP-1R agonist exendin-4. Plasma glucagon and the hypoglycemic response to exogenous insulin were normal in DIRKO mice. Glycemic excursion was abnormally increased and levels of glucose-stimulated insulin secretion were decreased following oral but not intraperitoneal glucose challenge in DIRKO compared with GIPR(-/-) or GLP-1R(-/-) mice. Similarly, glucose-stimulated insulin secretion and the response to forskolin were well preserved in perifused DIRKO islets. Although the dipeptidyl peptidase-IV (DPP-IV) inhibitors valine pyrrolidide (Val-Pyr) and SYR106124 lowered glucose and increased plasma insulin in wild-type and single incretin receptor knockout mice, the glucose-lowering actions of DPP-IV inhibitors were eliminated in DIRKO mice. These findings demonstrate that glucose-stimulated insulin secretion is maintained despite complete absence of both incretin receptors, and they delineate a critical role for incretin receptors as essential downstream targets for the acute glucoregulatory actions of DPP-IV inhibitors.

Gut peptides in the treatment of diabetes mellitus

It has been known for at least one century that agents secreted from the intestine during meal absorption regulates glucose assimilation. Extensive research during the past three decades has identified two gut hormones, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP, also known as gastric inhibitory polypeptide) that are important in postprandial glucose metabolism. Both peptides are incretins; they are secreted during carbohydrate absorption and increase insulin secretion. Since they are potent insulin secretagogues, GIP and GLP-1 have received considerable attention as potential diabetes therapeutics. However, only GLP-1 exerts insulinotropic properties when administered to patients with Type 2 diabetes. Both GLP-1 and GIP are rapidly inactivated in the circulation by the enzyme dipeptidyl peptidase IV (DPP-IV). The application of GLP-1 into clinical practice has been delayed due to the need to develop compounds that overcome this rapid inactivation. Two approaches have been taken to utilise the insulinotropic and glucose-lowering actions of GLP-1 as an antidiabetic agent: the development of DPP-IV-resistant analogues and the inhibition of DPP-IV. This review focuses on the physiology of GLP-1 and GIP and the advances that have been made thus far in developing treatments based on these physiological incretins for Type 2 diabetes.

Enhancing incretin action for the treatment of type 2 diabetes

OBJECTIVE

To examine the mechanisms of action, therapeutic potential, and challenges inherent in the use of incretin peptides and dipeptidyl peptidase-IV (DPP-IV) inhibitors for the treatment of type 2 diabetes.

RESEARCH DESIGN AND METHODS

The scientific literature describing the biological importance of incretin peptides and DPP-IV inhibitors in the control of glucose homeostasis has been reviewed, with an emphasis on mechanisms of action, experimental diabetes, human physiological experiments, and short-term clinical studies in normal and diabetic human subjects.

RESULTS

Glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP) exert important effects on beta-cells to stimulate glucose-dependent insulin secretion. Both peptides also regulate beta-cell proliferation and cytoprotection. GLP-1, but not GIP, inhibits gastric emptying, glucagon secretion, and food intake. The glucose-lowering actions of GLP-1, but not GIP, are preserved in subjects with type 2 diabetes. However, native GLP-1 is rapidly degraded by DPP-IV after parenteral administration; hence, degradation-resistant, long-acting GLP-1 receptor (GLP-1R) agonists are preferable agents for the chronic treatment of human diabetes. Alternatively, inhibition of DPP-IV-mediated incretin degradation represents a complementary therapeutic approach, as orally available DPP-IV inhibitors have been shown to lower glucose in experimental diabetic models and human subjects with type 2 diabetes.

CONCLUSIONS

GLP-1R agonists and DPP-IV inhibitors have shown promising results in clinical trials for the treatment of type 2 diabetes. The need for daily injections of potentially immunogenic GLP-1-derived peptides and the potential for unanticipated side effects with chronic use of DPP-IV inhibitors will require ongoing scrutiny of the risk-benefit ratio for these new therapies as they are evaluated in the clinic.

Glucose-dependent insulinotropic polypeptide receptor null mice exhibit compensatory changes in the enteroinsular axis

The incretins glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are gut hormones that act via the enteroinsular axis to potentiate insulin secretion from the pancreas in a glucose-dependent manner. Both GLP-1 receptor and GIP receptor knockout mice (GLP-1R(-/-) and GIPR(-/-), respectively) have been generated to investigate the physiological importance of this axis. Although reduced GIP action is a component of type 2 diabetes, GIPR-deficient mice exhibit only moderately impaired glucose tolerance. The present study was directed at investigating possible compensatory mechanisms that take place within the enteroinsular axis in the absence of GIP action. Although serum total GLP-1 levels in GIPR knockout mice were unaltered, insulin responses to GLP-1 from pancreas perfusions and static islet incubations were significantly greater (40-60%) in GIPR(-/-) than in wild-type (GIPR(+/+)) mice. Furthermore, GLP-1-induced cAMP production was also elevated twofold in the islets of the knockout animals. Pancreatic insulin content and gene expression were reduced in GIPR(-/-) mice compared with GIPR(+/+) mice. Paradoxically, immunocytochemical studies showed a significant increase in beta-cell area in the GIPR-null mice but with less intense staining for insulin. In conclusion, GIPR(-/-) mice exhibit altered islet structure and topography and increased islet sensitivity to GLP-1 despite a decrease in pancreatic insulin content and gene expression.

Gastric inhibitory polypeptide analogues: do they have a therapeutic role in diabetes mellitus similar to that of glucagon-like Peptide-1?

Gastric inhibitory polypeptide (GIP, also called glucose-dependent insulinotropic polypeptide) and glucagon-like peptide-1 (GLP-1) are peptide hormones from the gut that enhance nutrient-stimulated insulin secretion (the 'incretin' effect). Judging from experiments in mice with targeted deletions of GIP and GLP-1 receptors, the incretin effect is essential for normal glucose tolerance. In patients with type 2 diabetes mellitus it turns out that the incretin effect is severely impaired or abolished. The explanation seems to be that both the secretion of GLP-1 and the effect of GIP are impaired (whereas both the secretion of GIP and the effect of GLP-1 are near normal). The impaired GLP-1 secretion is probably a consequence of diabetic metabolic disturbances. The known genetic variations in the GIP receptor sequence are not associated with type 2 diabetes mellitus, but a defective insulinotropic effect of GIP may be found in first degree relatives of the patients, suggesting a genetic background for the defect. The molecular nature of the defect is not known and given the close similarity of the two receptors and their signalling, the dissociation of their effects is remarkable. Whereas GLP-1 and its analogues are attractive as therapeutic agents for type 2 diabetes mellitus, analogues of GIP are unlikely to be effective. On the other hand, GIP seems to play an important role in lipid metabolism, promoting the disposal of ingested lipids, and mice with a targeted deletion of the GIP receptor do not become obese when exposed to a high-fat diet. Therefore, antagonistic analogues of GIP may be speculated to have a role in the pharmaceutical management of obesity.

Lower blood glucose, hyperglucagonemia, and pancreatic alpha cell hyperplasia in glucagon receptor knockout mice

Glucagon, the counter-regulatory hormone to insulin, is secreted from pancreatic alpha cells in response to low blood glucose. To examine the role of glucagon in glucose homeostasis, mice were generated with a null mutation of the glucagon receptor (Gcgr(-/-)). These mice display lower blood glucose levels throughout the day and improved glucose tolerance but similar insulin levels compared with control animals. Gcgr(-/-) mice displayed supraphysiological glucagon levels associated with postnatal enlargement of the pancreas and hyperplasia of islets due predominantly to alpha cell, and to a lesser extent, delta cell proliferation. In addition, increased proglucagon expression and processing resulted in increased pancreatic glucogen-like peptide 1 (GLP-1) (1-37) and GLP-1 amide (1-36 amide) content and a 3- to 10-fold increase in circulating GLP-1 amide. Gcgr(-/-) mice also displayed reduced adiposity and leptin levels but normal body weight, food intake, and energy expenditure. These data indicate that glucagon is essential for maintenance of normal glycemia and postnatal regulation of islet and alpha and delta cell numbers. Furthermore, the lean phenotype of Gcgr(-/-) mice suggests glucagon action may be involved in the regulation of whole body composition.

Glucagon-like peptide-1 receptor signaling modulates beta cell apoptosis

Glucagon-like peptide-1 (GLP-1) stimulates insulin secretion and augments beta cell mass via activation of beta cell proliferation and islet neogenesis. We examined whether GLP-1 receptor signaling modifies the cellular susceptibility to apoptosis. Mice administered streptozotocin (STZ), an agent known to induce beta cell apoptosis, exhibit sustained improvement in glycemic control and increased levels of plasma insulin with concomitant administration of the GLP-1 agonist exendin-4 (Ex-4). Blood glucose remained significantly lower for weeks after cessation of exendin-4. STZ induced beta cell apoptosis, which was significantly reduced by co-administration of Ex-4. Conversely, mice with a targeted disruption of the GLP-1 receptor gene exhibited increased beta cell apoptosis after STZ administration. Exendin-4 directly reduced cytokine-induced apoptosis in purified rat beta cells exposed to interleukin 1beta, tumor necrosis fator alpha, and interferon gamma in vitro. Furthermore, Ex-4-treated BHK-GLP-1R cells exhibited significantly increased cell viability, reduced caspase activity, and decreased cleavage of beta-catenin after treatment with cycloheximide in vitro. These findings demonstrate that GLP-1 receptor signaling directly modifies the susceptibility to apoptotic injury, and provides a new potential mechanism linking GLP-1 receptor activation to preservation or enhancement of beta cell mass in vivo.

Therapeutic potential of dipeptidyl peptidase IV inhibitors for the treatment of type 2 diabetes

Incretins are peptide hormones, exemplified by glucose-dependent insulinotropic peptide and glucagon-like peptide 1 that are released from the gut in response to nutrient ingestion and enhance glucose-stimulated insulin secretion. Incretin action is terminated due to N-terminal cleavage of the peptides by the aminopeptidase dipeptidyl peptidase IV (DPP-IV). Hence, inhibition of glucose-dependent insulinotropic peptide and glucagon-like peptide 1 degradation via reduction of DPP-IV activity represents an innovative strategy for enhancing incretin action in vivo. This review summarises the biology of incretin action, the structure, expression and pleiotropic biological activities of DPP-IV and provides an overview of the rationale, potential merits and theoretical pitfalls in the development of DPP-IV inhibitors for the treatment of type 2 diabetes.

Glucagon is required for early insulin-positive differentiation in the developing mouse pancreas

The embryonic pancreas is thought to develop from pluripotent endodermal cells that give rise to endocrine and exocrine cells. A key guidance mechanism for pancreatic development has previously been found to be epithelial-mesenchymal interaction. Interactions within the epithelium, however, have not been well studied. Glucagon is the earliest peptide hormone present at appreciable levels in the developing pancreatic epithelium (embryonic day [E]-9.5 in mouse). Insulin accumulation begins slightly later (E11 in mouse), followed by a rapid accumulation during the "second wave" of insulin differentiation ( approximately E15). Here we found that blocking early expression and function of glucagon, but not GLP-1, an alternate gene product of preproglucagon mRNA, prevented insulin-positive differentiation in early embryonic (E11) pancreas. These results suggest a novel concept and a key role for glucagon in the paracrine induction of differentiation of other pancreatic components in the early embryonic pancreas.

Local, exendin-(9-39)-insensitive, site of action of GLP-1 in canine ileum

Glucagon-like peptide-1 (GLP-1) modulates glucose levels following a meal, including by inhibition of gastric emptying and intestinal transport. Intra-arterial injection of GLP-1 into the gastric corpus, antrum, or pylorus of anesthetized dogs had no effect on the contractile activity of the resting or neurally activated stomach. GLP-1 injected intra-arterially inhibited intestinal segments when activated by enteric nerve stimulation but not by acetylcholine. Isolated ileum segments were perfused intra-arterially, instrumented with strain gauges to record circular muscle activity and with subserosal electrodes to stimulate enteric nerves. GLP-1 caused concentration-dependent inhibition of nerve-stimulated phasic but not tonic activity. This was absent during TTX-induced activity and partly prevented by N(G)-nitro-L-arginine. Exendin-(9-39), the GLP-1 antagonist, had no intrinsic activity and did not affect the actions of GLP-1. Capsaicin mimicked the effects of GLP-1 and may have reduced the effect of subsequent GLP-1. GLP-1 may mediate paracrine action on afferent nerves in the canine ileal mucosa using an unusual receptor.

Human glucagon gene promoter sequences regulating tissue-specific versus nutrient-regulated gene expression

The glucagon-like peptides (GLPs) are synthesized and secreted in a nutrient-dependent manner in rodents; however, the factors regulating human GLP-1 and GLP-2 biosynthesis remain unclear. To understand how nutrients regulate human proglucagon gene expression, we studied the expression of a human proglucagon promoter-growth hormone (GH) transgene in 1.6 human glucagon-GH transgenic mice. Fasting-refeeding significantly decreased and increased the levels of circulating mouse insulin and transgene-derived hGH (P < 0.05 fasting vs. refeeding) and decreased and upregulated, respectively, the levels of endogenous mouse proglucagon RNA in the ileum but not in the jejunum or colon. High-fiber feeding significantly increased the levels of glucose-stimulated circulating hGH and upregulated levels of mouse intestinal proglucagon gene expression in the jejunum, ileum, and colon (P < 0.05, 0 vs. 30% fiber diet). In contrast, neither fasting-refeeding nor a high-fiber diet upregulated the expression of the human proglucagon promoter-hGH transgene. These findings demonstrate that human proglucagon gene regulatory sequences specifying tissue-specific expression in gut endocrine cells are not sufficient for recognition of energy-derived signals regulating murine glucagon gene expression in enteroendocrine cells in vivo.

Minireview: the glucagon-like peptides

The glucagon-like peptides GLP-1 and GLP-2 are produced in enteroendocrine L cells of the small and large intestine and secreted in a nutrient-dependent manner. GLP-1 regulates nutrient assimilation via inhibition of gastric emptying and food intake. GLP-1 controls blood glucose following nutrient absorption via stimulation of glucose-dependent insulin secretion, insulin biosynthesis, islet proliferation, and neogenesis and inhibition of glucagon secretion. Experiments using GLP-1 antagonists and GLP-1 receptor-/- mice indicate that the glucoregulatory actions of GLP-1 are essential for glucose homeostasis. In the central nervous system, GLP-1 regulates hypothalamic-pituitary function and GLP-1-activated circuits mediate the CNS response to aversive stimulation. GLP-2 maintains the integrity of the intestinal mucosal epithelium via effects on gastric motility and nutrient absorption, crypt cell proliferation and apoptosis, and intestinal permeability. Both GLP-1 and GLP-2 are rapidly inactivated in the circulation as a consequence of amino-terminal cleavage by the enzyme dipeptidyl peptidase IV (DP IV). The actions of these peptides on nutrient absorption and energy homeostasis and the efficacy of GLP-1 and GLP-2 in animal models of diabetes and intestinal diseases, respectively, suggest that analogs of these peptides may be clinically useful for the treatment of human disease.