Glucagon-like peptide 1 receptor signaling influences topography of islet cells in mice

In vivo functional profiling and structural characterisation of the human Glp1r A316T variant

Glucagon-like peptide-1 receptor (GLP-1R) agonists are a highly effective therapy class for type 2 diabetes (T2D) and obesity, yet there are variable patient responses. Variation in the human Glp1r gene leading to altered receptor structure, signal transduction, and function might be directly linked to therapeutic responses in patients. A naturally occurring, low-frequency, gain-of-function missense variant, rs10305492 G>A (A316T), protects against T2D and cardiovascular disease. Here we employ CRISPR/Cas9 technology to generate a humanised knock-in mouse model bearing the homozygous Glp1r A316T substitution. Human Glp1r A316T/A316T mice displayed lower fasting blood glucose levels and improved glucose tolerance, as well as increased plasma insulin levels and insulin secretion responses, even under metabolic stress. They also exhibited alterations in islet cytoarchitecture and β-cell identity indicative of compensatory mechanisms under a high-fat, high-sucrose (HFHS) diet challenge. Across all models investigated, the human Glp1r A316T variant exhibited characteristics of constitutive activation but blunted incretin-induced responses. Our results are further supported by cryo-EM analysis and molecular dynamics (MD) simulations of the GLP-1R A316T structure, demonstrating that the A316T Glp1r variant governs basal receptor activity and pharmacological responses to GLP-1R-targeting anti-diabetic therapies, highlighting the importance of the precise molecular characterisation of human Glp1r variants to predict individual therapy responses.

β Cell GLP-1R Signaling Alters α Cell Proglucagon Processing after Vertical Sleeve Gastrectomy in Mice

Bariatric surgery, such as vertical sleeve gastrectomy (VSG), causes high rates of type 2 diabetes remission and remarkable increases in postprandial glucagon-like peptide-1 (GLP-1) secretion. GLP-1 plays a critical role in islet function by potentiating glucose-stimulated insulin secretion; however, the mechanisms remain incompletely defined. Therefore, we applied a murine VSG model to an inducible β cell-specific GLP-1 receptor (GLP-1R) knockout mouse model to investigate the role of the β cell GLP-1R in islet function. Our data show that loss of β cell GLP-1R signaling decreases α cell GLP-1 expression after VSG. Furthermore, we find a β cell GLP-1R-dependent increase in α cell expression of the prohormone convertase required for the production of GLP-1 after VSG. Together, the findings herein reveal two concepts. First, our data support a paracrine role for α cell-derived GLP-1 in the metabolic benefits observed after VSG. Second, we have identified a role for the β cell GLP-1R as a regulator of α cell proglucagon processing.

Gut: A key player in the pathogenesis of type 2 diabetes?

The gut regulates glucose and energy homeostasis; thus, the presence of ingested nutrients into the gut activates sensing mechanisms that affect both glucose homeostasis and regulate food intake. Increasing evidence suggest that gut may also play a key role in the pathogenesis of type 2 diabetes which may be related to both the intestinal microbiological profile and patterns of gut hormones secretion. Intestinal microbiota includes trillions of microorganisms but its composition and function may be adversely affected in type 2 diabetes. The intestinal microbiota may be responsible of the secretion of molecules that may impair insulin secretion/action. At the same time, intestinal milieu regulates the secretion of hormones such as GLP-1, GIP, ghrelin, gastrin, somatostatin, CCK, serotonin, peptide YY, GLP-2, all of which importantly influence metabolism in general and in particular glucose metabolism. Thus, the aim of this paper is to review the current evidence on the role of the gut in the pathogenesis of type 2 diabetes, taking into account both hormonal and microbiological aspects.

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.

Cord serum dipeptidyl-peptidase 4 activity in gestational diabetes

BACKGROUND

Tissue-specific dipeptidyl-peptidase 4 (DPP4) dysregulation has been described in adults with diabetes mellitus. The DPP4 -incretin system has not been studied in foetal life. In this study, DPP4 activity and glucagon-like peptide-1 (GLP-1) levels were assessed in cord blood of neonates born to women with gestational diabetes mellitus (GDM) and nondiabetic controls.

MATERIAL AND METHODS

This study has been conducted in two Hungarian and one Austrian centres.

PATIENTS

A total of 568 pregnant women were enrolled in the study after their OGTT between the 24th and 28th gestational week. Cord blood samplings with DPP4 activity and GLP-1 level measurements were possible in 270 (DPP4: 159 control, 111 GDM) and 112 (GLP-1: 72 control, 40 GDM) cases. OGTT (24-28th gestational week) and cord blood sampling at delivery were performed. Cord serum DPP4 activity was determined in a continuous monitoring microplate-based kinetic assay, and cord plasma GLP-1 was measured using a fluorescence ELISA method.

RESULTS

Cord serum DPP4 activity was lower in GDM [mean (95% CI): 28.07 U/L (26.32-29.82 U/L)] than in controls [31.61 U/L (29.93-33.29 U/L), MWU P = 0.0015]. Cord plasma active GLP-1 levels were close to the lower detection limit and were not altered in GDM (control: mean = 3.43 pM, 95% CI: 3.04-3.82 pM, GDM: mean = 3.61 pM, 95% CI: 2.96-4.28 pM - MWU test P = 0.6).

CONCLUSIONS

Decreased cord serum DPP4 activity in gestational diabetes mellitus might be the result of an adaptive foetal response or an early dysregulation in the entero-insular axis with consequences beyond the incretin system. Cord plasma GLP-1 levels may reflect the missing oral intake with a limited glucose sensing of L cells via the circulation in foetal life.

Incretin receptor null mice reveal key role of GLP-1 but not GIP in pancreatic beta cell adaptation to pregnancy

Islet adaptations to pregnancy were explored in C57BL6/J mice lacking functional receptors for glucagon-like peptide 1 (GLP-1) and gastric inhibitory polypeptide (GIP). Pregnant wild type mice and GIPRKO mice exhibited marked increases in islet and beta cell area, numbers of medium/large sized islets, with positive effects on Ki67/Tunel ratio favouring beta cell growth and enhanced pancreatic insulin content. Alpha cell area and glucagon content were unchanged but prohormone convertases PC2 and PC1/3 together with significant amounts of GLP-1 and GIP were detected in alpha cells. Knockout of GLP-1R abolished these islet adaptations and paradoxically decreased pancreatic insulin, GLP-1 and GIP. This was associated with abolition of normal pregnancy-induced increases in plasma GIP, L-cell numbers, and intestinal GIP and GLP-1 stores. These data indicate that GLP-1 but not GIP is a key mediator of beta cell mass expansion and related adaptations in pregnancy, triggered in part by generation of intra-islet GLP-1.

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.

The ductal origin of structural and functional heterogeneity between pancreatic islets

Islets form in the pancreas after the first endocrine cells have arisen as either single cells or small cell clusters in the epithelial cords. These cords constitute the developing pancreas in one of its earliest recognizable stages. Islet formation begins at the time the cords transform into a branching ductal system, continues while the ductal system expands, and finally stops before the exocrine tissue of ducts and acini reaches its final expansion. Thus, islets continuously arise from founder cells located in the branching and ramifying ducts. Islets arising from proximal duct cells locate between the exocrine lobules, develop strong autonomic and sensory innervations, and pass their blood to efferent veins (insulo-venous efferent system). Islets arising from cells of more distal ducts locate within the exocrine lobules, respond to nerve impulses ending at neighbouring blood vessels, and pass their blood to the surrounding acini (insulo-acinar portal system). Consequently, the section of the ductal system from which an islet arises determines to a large extent its future neighbouring tissue, architecture, properties, and functions. We note that islets interlobular in position are frequently found in rodents (rats and mice), whereas intralobularly-located, peripheral duct islets prevail in humans and cattle. Also, we expound on bovine foetal Laguesse islets as a prominent foetal type of type 1 interlobular neuro-insular complexes, similar to neuro-insular associations frequently found in rodents. Finally, we consider the probable physiological and pathophysiological implications of the different islet positions within and between species.

Minireview: Signal bias, allosterism, and polymorphic variation at the GLP-1R: implications for drug discovery

The glucagon-like peptide-1 receptor (GLP-1R) controls the physiological responses to the incretin hormone glucagon-like peptide-1 and is a major therapeutic target for the treatment of type 2 diabetes, owing to the broad range of effects that are mediated upon its activation. These include the promotion of glucose-dependent insulin secretion, increased insulin biosynthesis, preservation of β-cell mass, improved peripheral insulin action, and promotion of weight loss. Regulation of GLP-1R function is complex, with multiple endogenous and exogenous peptides that interact with the receptor that result in the activation of numerous downstream signaling cascades. The current understanding of GLP-1R signaling and regulation is limited, with the desired spectrum of signaling required for the ideal therapeutic outcome still to be determined. In addition, there are several single-nucleotide polymorphisms (used in this review as defining a natural change of single nucleotide in the receptor sequence; clinically, this is viewed as a single-nucleotide polymorphism only if the frequency of the mutation occurs in 1% or more of the population) distributed within the coding sequence of the receptor protein that have the potential to produce differential responses for distinct ligands. In this review, we discuss the current understanding of GLP-1R function, in particular highlighting recent advances in the field on ligand-directed signal bias, allosteric modulation, and probe dependence and the implications of these behaviors for drug discovery and development.

GLP-1R and amylin agonism in metabolic disease: complementary mechanisms and future opportunities

The discoveries of the incretin hormone glucagon-like peptide-1 (GLP-1) and the β-cell hormone amylin have translated into hormone-based therapies for diabetes. Both classes of molecules also exhibit weight-lowering effects and have been investigated for their anti-obesity potential. In the present review, we explore the mechanisms underlying the physiological and pharmacological actions of GLP-1 and amylin agonism. Despite their similarities (e.g. both molecular classes slow gastric emptying, decrease glucagon and inhibit food intake), there are important distinctions between the central and/or peripheral pathways that mediate their effects on glycaemia and energy balance. We suggest that understanding the similarities and differences between these molecules holds important implications for the development of novel, combination-based therapies, which are increasingly the norm for diabetes/metabolic disease. Finally, the future of GLP-1- and amylin agonist-based therapeutics is discussed.

Anorexigenic effects of GLP-1 and its analogues

GLP-1 receptors are expressed in the brain, especially in the regions responsible for the regulation of food intake, and intracerebroventricular injection of GLP-1 results in inhibition of food intake. Peripheral administration of GLP-1 dose-dependently enhances satiety and reduces food intake in normal and obese subjects as well as in type 2 diabetic patients. So far, the mechanisms by which GLP-1 exerts its effects are not completely clear. Interactions with neurons in the gastrointestinal tract or possibly direct access to the brain through the blood-brain barrier as observed in rats are possible and discussed in this chapter as well as a novel hypothesis based on the finding that GLP-1 is also expressed in taste cells. Finally, the role of GLP-1 receptor agonists as a possible treatment option in obesity is discussed as well as the role of GLP-1 in the effects of bariatric surgery on adiposity and glucose homeostasis.

New gene targets for glucagon-like peptide-1 during embryonic development and in undifferentiated pluripotent cells

In humans, glucagon-like peptide (GLP-1) functions during adult life as an incretin hormone with anorexigenic and antidiabetogenic properties. Also, the therapeutic potential of GLP-1 in preventing the adipocyte hyperplasia associated with obesity and in bolstering the maintenance of human mesenchymal stem cell (hMSC) stores by promoting the proliferation and cytoprotection of hMSC seems to be relevant. Since these observations suggest a role for GLP-1 during developmental processes, the aim of the present work was to characterize GLP-1 in early development as well as its gene targets in mouse embryonic stem (mES) cells. Mouse embryos E6, E8, and E10.5 and pluripotent mES were used for the inmunodetection of GLP-1 and GLP-1 receptor. Quantitative real-time PCR was used to determine the expression levels of GLP-1R in several tissues from E12.5 mouse embryos. Additionally, GLP-1 gene targets were studied in mES by multiple gene expression analyses. GLP-1 and its receptors were identified in mES and during embryonic development. In pluripotent mES, GLP-1 modified the expression of endodermal, ectodermal, and mesodermal gene markers as well as sonic hedgehog, noggin, members of the fibroblast and hepatic growth factor families, and others involved in pancreatic development. Additionally, GLP-1 promoted the expression of the antiapoptotic gene bcl2 and at the same time reduced proapoptotic caspase genes. Our results indicate that apart from the effects and therapeutic benefits of GLP-1 in adulthood, it may have additional gene targets in mES cells during embryonic life. Furthermore, the pathophysiological implications of GLP-1 imbalance in adulthood may have a counterpart during development.

Postnatal development of numbers and mean sizes of pancreatic islets and beta-cells in healthy mice and GIPR(dn) transgenic diabetic mice

The aim of this study was to examine postnatal islet and beta-cell expansion in healthy female control mice and its disturbances in diabetic GIPR^dn transgenic mice, which exhibit an early reduction of beta-cell mass. Pancreata of female control and GIPR^dn transgenic mice, aged 10, 45, 90 and 180 days were examined, using state-of-the-art quantitative-stereological methods. Total islet and beta-cell volumes, as well as their absolute numbers increased significantly until 90 days in control mice, and remained stable thereafter. The mean islet volumes of controls also increased slightly but significantly between 10 and 45 days of age, and then remained stable until 180 days. The total volume of isolated beta-cells, an indicator of islet neogenesis, and the number of proliferating (BrdU-positive) islet cells were highest in 10-day-old controls and declined significantly between 10 and 45 days. In GIPR^dn transgenic mice, the numbers of islets and beta-cells were significantly reduced from 10 days of age onwards vs. controls, and no postnatal expansion of total islet and beta-cell volumes occurred due to a reduction in islet neogenesis whereas early islet-cell proliferation and apoptosis were unchanged as compared to control mice. Insulin secretion in response to pharmacological doses of GIP was preserved in GIPR^dn transgenic mice, and serum insulin to pancreatic insulin content in response to GLP-1 and arginine was significantly higher in GIPR^dn transgenic mice vs. controls. We could show that the increase in islet number is mainly responsible for expansion of islet and beta-cell mass in healthy control mice. GIPR^dn transgenic mice show a disturbed expansion of the endocrine pancreas, due to perturbed islet neogenesis.

Identification of 3-aminomethyl-1,2-dihydro-4-phenyl-1-isoquinolones: a new class of potent, selective, and orally active non-peptide dipeptidyl peptidase IV inhibitors that form a unique interaction with Lys554

The design, synthesis, and structure-activity relationships of a new class of potent and orally active non-peptide dipeptidyl peptidase IV (DPP-4) inhibitors, 3-aminomethyl-1,2-dihydro-4-phenyl-1-isoquinolones, are described. We hypothesized that the 4-phenyl group of the isoquinolone occupies the S1 pocket of the enzyme, the 3-aminomethyl group forms an electrostatic interaction with the S2 pocket, and the introduction of a hydrogen bond donor onto the 6- or 7-substituent provides interaction with the hydrophilic region of the enzyme. Based on this hypothesis, intensive research focused on developing new non-peptide DPP-4 inhibitors has been carried out. Among the compounds designed in this study, we identified 2-[(3-aminomethyl-2-(2-methylpropyl)-1-oxo-4-phenyl-1,2-dihydro-6-isoquinolinyl)oxy]acetamide (35a) as a potent, selective, and orally bioavailable DPP-4 inhibitor, which exhibited in vivo efficacy in diabetic model rats. Finally, X-ray crystallography of 35a in a complex with the enzyme validated our hypothesized binding mode and identified Lys554 as a new target-binding site available for DPP-4 inhibitors.

Gastrointestinal hormones and the regulation of β-cell mass

Type 2 diabetes occurs due to a relative deficit in β-cell mass or function. Glucagon-like peptide 1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), cholecystokinin (CCK), and gastrin are gastrointestinal hormones that are secreted in response to nutrient intake, regulating digestion, insulin secretion, satiety, and β-cell mass. In this review, we focus upon β-cell mass regulation. β-cell mass expands through β-cell proliferation and islet neogenesis; β-cell mass is lost via apoptosis. GLP-1 and GIP are well-studied gastrointestinal hormones and influence β-cell proliferation, apoptosis, and islet neogenesis. CCK regulates β-cell apoptosis and mitogenesis, and gastrin stimulates islet neogenesis. GLP-1 and GIP bind to G protein-coupled receptors and regulate β-cell mass via multiple signaling pathways. The protein kinase A pathway is central to this process because it directly regulates proliferative and anti-apoptotic genes and transactivates several signaling cascades, including Akt and mitogen-activated protein kinases. However, the signaling pathways downstream of G protein-coupled CCK receptors that influence β-cell mass remain unidentified. Gastrointestinal hormones integrate nutrient signals from the gut to the β-cell, regulating insulin secretion and β-cell mass adaptation.

Cholecystokinin is up-regulated in obese mouse islets and expands beta-cell mass by increasing beta-cell survival

An absolute or functional deficit in beta-cell mass is a key factor in the pathogenesis of diabetes. We model obesity-driven beta-cell mass expansion by studying the diabetes-resistant C57BL/6-Leptin(ob/ob) mouse. We previously reported that cholecystokinin (Cck) was the most up-regulated gene in obese pancreatic islets. We now show that islet cholecystokinin (CCK) is up-regulated 500-fold by obesity and expressed in both alpha- and beta-cells. We bred a null Cck allele into the C57BL/6-Leptin(ob/ob) background and investigated beta-cell mass and metabolic parameters of Cck-deficient obese mice. Loss of CCK resulted in decreased islet size and reduced beta-cell mass through increased beta-cell death. CCK deficiency and decreased beta-cell mass exacerbated fasting hyperglycemia and reduced hyperinsulinemia. We further investigated whether CCK can directly affect beta-cell death in cell culture and isolated islets. CCK was able to directly reduce cytokine- and endoplasmic reticulum stress-induced cell death. In summary, CCK is up-regulated by islet cells during obesity and functions as a paracrine or autocrine factor to increase beta-cell survival and expand beta-cell mass to compensate for obesity-induced insulin resistance.

Loss of enteroendocrine cells in mice alters lipid absorption and glucose homeostasis and impairs postnatal survival

At least 10 enteroendocrine cell types have been identified, and the peptide hormones they secrete have diverse functions that include regulation of glucose homeostasis, food intake, and gastric emptying. Mice lacking individual enteroendocrine hormones, their receptors, or combinations of these have shed light on the role of these hormones in the regulation of energy homeostasis. However, because enteroendocrine hormones have partially overlapping functions, these loss-of-function studies produced only minor phenotypes, and none of the enteroendocrine hormones was shown to be essential for life. To examine the effect of loss of all enteroendocrine cells and hormones on energy homeostasis, we generated mice with intestinal-specific ablation of the proendocrine transcription factor neurogenin 3 (referred to herein as Ngn3Deltaint mice). Ngn3Deltaint mice were deficient for all enteroendocrine cells and hormones, and died with a high frequency during the first week of life. Mutant mice were growth retarded and had yellowish stool suggestive of steatorrhea. Subsequent analyses revealed that Ngn3Deltaint mice had impaired lipid absorption, reduced weight gain, and improved glucose homeostasis. Furthermore, intestinal epithelium of the mutant mice showed an enlarged proliferative crypt compartment and accelerated cell turnover but no changes to goblet and Paneth cell numbers. Enterocytes had shorter microvilli, but the expression of the main brush border enzymes was unaffected. Our data help unravel the role of enteroendocrine cells and hormones in lipid absorption and maintenance of the intestinal epithelium.

Glucagon-like peptide 1 receptor stimulation as a means of neuroprotection

Glucagon-like peptide 1 (GLP-1) is a relatively recently discovered molecule originating in the so-called L-cells of the intestine. The peptide has insulinotrophic properties and it is this characteristic that has predominantly been investigated. This has led to the use of the GLP-1-like peptide exendin-4 (EX-4), which has a much longer plasma half-life than GLP-1 itself, being used in the treatment of type II diabetes. The mode of action of this effect appears to be a reduction in pancreatic apoptosis, an increase in beta cell proliferation or both. Thus, the effects of GLP-1 receptor stimulation are not based upon insulin replacement but an apparent repair of the pancreas. Similar data suggest that the same effects may occur in other peripheral tissues. More recently, the roles of GLP-1 and EX-4 have been studied in nervous tissue. As in the periphery, both peptides appear to promote cellular growth and reduce apoptosis. In models of Alzheimer's disease, Parkinson's disease and peripheral neuropathy, stimulation of the GLP-1 receptor has proved to be highly beneficial. In the case of Parkinson's disease this effect is evident after the neurotoxic lesion is established, suggesting real potential for therapeutic use. In the present review we examine the current status of the GLP-1 receptor and its potential as a therapeutic target.

Pleiotropic actions of the incretin hormones

The insulin secretory response to a meal results largely from glucose stimulation of the pancreatic islets and both direct and indirect (autonomic) glucose-dependent stimulation by incretin hormones released from the gastrointestinal tract. Two incretins, Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), have so far been identified. Localization of the cognate G protein-coupled receptors for GIP and GLP-1 revealed that they are present in numerous tissues in addition to the endocrine pancreas, including the gastrointestinal, cardiovascular, central nervous and autonomic nervous systems (ANSs), adipose tissue, and bone. At these sites, the incretin hormones exert a range of pleiotropic effects, many of which contribute to the integration of processes involved in the regulation of food intake, and nutrient and mineral processing and storage. From detailed studies at the cellular and molecular level, it is also evident that both incretin hormones act via multiple signal transduction pathways that regulate both acute and long-term cell function. Here, we provide an overview of current knowledge relating to the physiological roles of GIP and GLP-1, with specific emphasis on their modes of action on islet hormone secretion, β-cell proliferation and survival, central and autonomic neuronal function, gastrointestinal motility, and glucose and lipid metabolism. However, it is emphasized that despite intensive research on the various body systems, in many cases there is uncertainty as to the pathways by which the incretins mediate their pleiotropic effects and only a rudimentary understanding of the underlying cellular mechanisms involved, and these are challenges for the future.

Differential importance of glucose-dependent insulinotropic polypeptide vs glucagon-like peptide 1 receptor signaling for beta cell survival in mice

BACKGROUND & AIMS

Glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) activate pathways involved in beta cell survival and proliferation in vitro; we compared the relative importance of exogenous and endogenous GIP receptor (GIPR) and GLP-1 receptor (GLP-1R) activation for beta cell cytoprotection in mice.

METHODS

The effects of incretin hormone receptor signaling on beta cell regeneration and survival were assessed in mice following administration of streptozotocin in the absence or presence of the GIPR agonist [D-Ala(2)]-GIP (D-GIP), the GLP-1R agonist exendin-4, or the dipeptidyl peptidase-4 inhibitor sitagliptin. Beta cell survival was assessed in Gipr(-/-) mice given streptozotocin and by gene expression profiling of RNA from islets isolated from Glp1r(-/-) and Gipr(-/-) mice. The antiapoptotic actions of sitagliptin were assessed in wild-type and dual incretin receptor knockout (DIRKO) mice.

RESULTS

Administration of exendin-4 for 7 or 60 days improved blood glucose and insulin levels, reduced islet cell apoptosis, and increased pancreatic insulin content and beta cell mass. In contrast, D-GIP was less effective at improving these parameters under identical experimental conditions. Furthermore, Gipr(-/-) mice did not exhibit increased sensitivity to streptozotocin-induced diabetes. Sitagliptin reduced hemoglobin A(1c) levels and increased plasma and pancreatic levels of insulin after streptozotocin administration to wild-type mice. Sitagliptin reduced the levels of activated caspase-3 in wild-type islets but not in beta cells from DIRKO mice.

CONCLUSIONS

There are functionally important differences in the pharmacologic and physiologic roles of incretin receptors in beta cells. GLP-1R signaling exerts more robust control of beta cell survival, relative to GIPR activation or dipeptidylpeptidase-4 inhibition in mice in vivo.

DsAAV8-mediated expression of glucagon-like peptide-1 in pancreatic beta-cells ameliorates streptozotocin-induced diabetes

Glucagon-like peptide-1 (GLP-1) is an incretin hormone that performs a wide array of well-characterized antidiabetic actions, including stimulation of glucose-dependent insulin secretion, upregulation of insulin gene expression and improvements in beta-cell survival. GLP-1-receptor agonists have been developed for treatment of diabetes; however, the short biological half-lives of these peptide-based therapeutics requires that frequent injections be administered to maintain sufficient circulating levels. Thus, novel methods of delivering GLP-1 remain an important avenue of active research. It has recently been demonstrated that self-complimentary, double-stranded, adeno-associated virus serotype-8 (DsAAV8) can efficiently transduce pancreatic beta-cells in vivo, resulting in long-term transgene expression. In this study, we engineered a DsAAV8 vector containing a GLP-1 transgene driven by the mouse insulin-II promoter (MIP). Biological activity of the GLP-1 produced from this transgene was assessed using a luciferase-based bioassay. DsAAV8-MIP-GLP-1 was delivered via intraperitoneal injection and beta-cell damage induced by multiple low dose streptozotocin (STZ) administration. Glucose tolerance was assessed following intraperitoneal glucose injections and beta-cell proliferation measured by PCNA expression. Expression of GLP-1 in Min6 beta-cells resulted in glucose-dependent secretion of biologically active GLP-1. Intraperitoneal delivery of DsAAV8-MIP-GLP-1 to mice led to localized GLP-1 expression in beta-cells and protection against development of diabetes induced by multiple low-dose STZ administration. This protection was associated with significant increase in beta-cell proliferation. Results from this study indicate that expression and secretion of GLP-1 from beta-cells in vivo via DsAAV8 represents a novel therapeutic strategy for treatment of diabetes.

Clinical review: The extrapancreatic effects of glucagon-like peptide-1 and related peptides

CONTEXT

Glucagon-like peptide-1 (GLP-1) 7-36 amide, an insulinotropic hormone released from the intestinal L cells in response to nutrient ingestion, has been extensively reviewed with respect to beta-cell function. However GLP-1 receptors are abundant in many other tissues. Thus, the function of GLP-1 is not limited to the islet cells, and it has regulatory actions on many other organs.

EVIDENCE ACQUISITION

A review of published, peer-reviewed medical literature (1987 to September 2008) on the extrapancreatic actions of GLP-1 was performed.

EVIDENCE SYNTHESIS

The extrapancreatic actions of GLP-1 include inhibition of gastric emptying and gastric acid secretion, thereby fulfilling the definition of GLP-1 as an enterogastrone. Other important extrapancreatic actions of GLP-1 include a regulatory role in hepatic glucose production, the inhibition of pancreatic exocrine secretion, cardioprotective and cardiotropic effects, the regulation of appetite and satiety, and stimulation of afferent sensory nerves. The primary metabolite of GLP-1, GLP-1 (9-36) amide, or GLP-1m, is the truncated product of degradation by dipeptidyl peptidase-4. GLP-1m has insulinomimetic effects on hepatic glucose production and cardiac function. Exendin-4 present in the salivary gland of the reptile, Gila monster (Heloderma suspectum), is a high-affinity agonist for the mammalian GLP-1 receptor. It is resistant to degradation by dipeptidyl peptidase-4, and therefore has a prolonged half-life.

CONCLUSION

GLP-1 and its metabolite have important extrapancreatic effects particularly with regard to the cardiovascular system and insulinomimetic effects with respect to glucose homeostasis. These effects may be particularly important in the obese state. GLP-1, GLP-1m, and exendin-4 therefore have potential therapeutic roles because of their diffuse extrapancreatic actions.

Role of furin in granular acidification in the endocrine pancreas: identification of the V-ATPase subunit Ac45 as a candidate substrate

Furin is a proprotein convertase which activates a variety of regulatory proteins in the constitutive exocytic and endocytic pathway. The effect of genetic ablation of fur was studied in the endocrine pancreas to define its physiological function in the regulated secretory pathway. Pdx1-Cre/loxP furin KO mice show decreased secretion of insulin and impaired processing of known PC2 substrates like proPC2 and proinsulin II. Both secretion and PC2 activity depend on granule acidification, which was demonstrated to be significantly decreased in furin-deficient beta cells by using the acidotrophic agent 3-(2,4-dinitroanilino)-3'amino-N-methyldipropylamine (DAMP). Ac45, an accessory subunit of the proton pump V-ATPase, was investigated as a candidate substrate. Ac45 is highly expressed in islets of Langerhans and furin was able to cleave Ac45 ex vivo. Furthermore, the exact cleavage site was determined. In addition, reduced regulated secretion and proinsulin II processing could be obtained in the insulinoma cell line betaTC3 by downregulation of either furin or Ac45. Together, these data establish an important role for furin in regulated secretion, particularly in intragranular acidification most likely due to impaired processing of Ac45.

Diets influence the diabetic phenotype of transgenic mice expressing a dominant negative glucose-dependent insulinotropic polypeptide receptor (GIPRdn)

Transgenic mice overexpressing a dominant negative glucose-dependent insulinotropic polypeptide receptor (GIPR(dn)) have recently been shown to develop diabetes mellitus due to disturbed postnatal development of the endocrine pancreas. In this study, the effects of feeding a high fibre/low calorie diet on the diabetic phenotype of GIPR(dn) transgenic mice were examined. Transgenic and control animals received either a conventional breeding diet (BD) or a high fibre diet (HF). Both fasting and postprandial blood glucose levels and HbA1C levels were largely elevated in transgenic mice vs. controls (p<0.05), irrespective of the diet fed. Food and water intake and the daily urine volume of GIPR(dn) transgenic mice were higher than that of control mice (p<0.05). Transgenic animals receiving the HF diet showed significantly lower blood glucose and HbA1C levels as well as less food and water intake than transgenic mice fed BD. The 365-day survival of transgenic mice was significantly lower than that of control mice. Transgenic animals fed the HF diet lived significantly longer than their counterparts receiving BD. GIPR(dn) transgenic mice develop a severe diabetic phenotype which can be ameliorated by a HF diet, thereby resembling some aspects of the pathophysiology of human type 2 diabetes mellitus.

Biology of incretins: GLP-1 and GIP

This review focuses on the mechanisms regulating the synthesis, secretion, biological actions, and therapeutic relevance of the incretin peptides glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). The published literature was reviewed, with emphasis on recent advances in our understanding of the biology of GIP and GLP-1. GIP and GLP-1 are both secreted within minutes of nutrient ingestion and facilitate the rapid disposal of ingested nutrients. Both peptides share common actions on islet beta-cells acting through structurally distinct yet related receptors. Incretin-receptor activation leads to glucose-dependent insulin secretion, induction of beta-cell proliferation, and enhanced resistance to apoptosis. GIP also promotes energy storage via direct actions on adipose tissue, and enhances bone formation via stimulation of osteoblast proliferation and inhibition of apoptosis. In contrast, GLP-1 exerts glucoregulatory actions via slowing of gastric emptying and glucose-dependent inhibition of glucagon secretion. GLP-1 also promotes satiety and sustained GLP-1-receptor activation is associated with weight loss in both preclinical and clinical studies. The rapid degradation of both GIP and GLP-1 by the enzyme dipeptidyl peptidase-4 has led to the development of degradation-resistant GLP-1-receptor agonists and dipeptidyl peptidase-4 inhibitors for the treatment of type 2 diabetes. These agents decrease hemoglobin A1c (HbA1c) safely without weight gain in subjects with type 2 diabetes. GLP-1 and GIP integrate nutrient-derived signals to control food intake, energy absorption, and assimilation. Recently approved therapeutic agents based on potentiation of incretin action provide new physiologically based approaches for the treatment of type 2 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.

The multiple faces of glucagon-like peptide-1--obesity, appetite, and stress: what is next? A review

By itself, glucagon-like peptide-1(GLP-1) appears to be an excellent drug for appetite control and the treatment of obesity. Unfortunately, few enzymes, such as IV dipeptidyl peptidase and renal excretin, degrade and render GLP-1 inactive within minutes. A receptor agonist, exendin-4, with a longer biological half-life than GLP-1, has been tried. Subcutaneous injection of exendin-4 or continuous IV injection of GLP-1 warrants further research and investigation.

Mechanisms of action of glucagon-like peptide 1 in the pancreas

Glucagon-like peptide 1 (GLP-1) is a hormone that is encoded in the proglucagon gene. It is mainly produced in enteroendocrine L cells of the gut and is secreted into the blood stream when food containing fat, protein hydrolysate, and/or glucose enters the duodenum. Its particular effects on insulin and glucagon secretion have generated a flurry of research activity over the past 20 years culminating in a naturally occurring GLP-1 receptor (GLP-1R) agonist, exendin 4 (Ex-4), now being used to treat type 2 diabetes mellitus (T2DM). GLP-1 engages a specific guanine nucleotide-binding protein (G-protein) coupled receptor (GPCR) that is present in tissues other than the pancreas (brain, kidney, lung, heart, and major blood vessels). The most widely studied cell activated by GLP-1 is the insulin-secreting beta cell where its defining action is augmentation of glucose-induced insulin secretion. Upon GLP-1R activation, adenylyl cyclase (AC) is activated and cAMP is generated, leading, in turn, to cAMP-dependent activation of second messenger pathways, such as the protein kinase A (PKA) and Epac pathways. As well as short-term effects of enhancing glucose-induced insulin secretion, continuous GLP-1R activation also increases insulin synthesis, beta cell proliferation, and neogenesis. Although these latter effects cannot be currently monitored in humans, there are substantial improvements in glucose tolerance and increases in both first phase and plateau phase insulin secretory responses in T2DM patients treated with Ex-4. This review will focus on the effects resulting from GLP-1R activation in the pancreas.

Improving function and survival of pancreatic islets by endogenous production of glucagon-like peptide 1 (GLP-1)

Glucagon-like peptide 1 (GLP-1) is a hormone that has received significant attention as a therapy for diabetes because of its ability to stimulate insulin biosynthesis and release and to promote growth and survival of insulin-producing beta cells. While GLP-1 is produced from the proglucagon precursor by means of prohormone convertase (PC) 1/3 activity in enteroendocrine L cells, the same precursor is differentially processed by PC2 in pancreatic islet alpha cells to release glucagon, leaving GLP-1 trapped within a larger fragment with no known function. We hypothesized that we could induce GLP-1 production directly within pancreatic islets by means of delivery of PC1/3 and, further, that this intervention would improve the viability and function of islets. Here, we show that adenovirus-mediated expression of PC1/3 in alpha cells increases islet GLP-1 secretion, resulting in improved glucose-stimulated insulin secretion and enhanced survival in response to cytokine treatment. PC1/3 expression in alpha cells also improved performance after islet transplantation in a mouse model of type 1 diabetes, possibly by enhancing nuclear Pdx1 and insulin content of islet beta cells. These results demonstrate a unique strategy for liberating GLP-1 from directly within the target organ and highlight the potential for up-regulating islet GLP-1 production as a means of treating diabetes.

Exenatide: a GLP-1 receptor agonist as novel therapy for Type 2 diabetes mellitus

Exenatide is a glucagon-like peptide 1 receptor agonist, which has recently received FDA approval in the US for the treatment of Type 2 diabetes. Exenatide is an incretin mimetic that improves glycaemic control in patients with diabetes through acute mechanisms, such as glucose-dependent stimulation of insulin secretion, suppression of inappropriate glucagon secretion and slowing of gastric emptying, as well as chronic mechanisms that include enhancement of beta-cell mass in rodent studies and weight loss and inhibition of food intake in humans. This article reviews the mechanisms of exenatide action, as well as its efficacy in the treatment of Type 2 diabetes.

Proglucagon-derived peptides: mechanisms of action and therapeutic potential

Glucagon is used for the treatment of hypoglycemia, and glucagon receptor antagonists are under development for the treatment of type 2 diabetes. Moreover, glucagon-like peptide (GLP)-1 and GLP-2 receptor agonists appear to be promising therapies for the treatment of type 2 diabetes and intestinal disorders, respectively. This review discusses the physiological, pharmacological, and therapeutic actions of the proglucagon-derived peptides, with an emphasis on clinical relevance of the peptides for the treatment of human disease.

Regulation of pancreatic beta-cell mass

Beta-cell mass regulation represents a critical issue for understanding diabetes, a disease characterized by a near-absolute (type 1) or relative (type 2) deficiency in the number of pancreatic beta cells. The number of islet beta cells present at birth is mainly generated by the proliferation and differentiation of pancreatic progenitor cells, a process called neogenesis. Shortly after birth, beta-cell neogenesis stops and a small proportion of cycling beta cells can still expand the cell number to compensate for increased insulin demands, albeit at a slow rate. The low capacity for self-replication in the adult is too limited to result in a significant regeneration following extensive tissue injury. Likewise, chronically increased metabolic demands can lead to beta-cell failure to compensate. Neogenesis from progenitor cells inside or outside islets represents a more potent mechanism leading to robust expansion of the beta-cell mass, but it may require external stimuli. For therapeutic purposes, advantage could be taken from the surprising differentiation plasticity of adult pancreatic cells and possibly also from stem cells. Recent studies have demonstrated that it is feasible to regenerate and expand the beta-cell mass by the application of hormones and growth factors like glucagon-like peptide-1, gastrin, epidermal growth factor, and others. Treatment with these external stimuli can restore a functional beta-cell mass in diabetic animals, but further studies are required before it can be applied to humans.

Role of glucagon-like peptide-1 in the pathogenesis and treatment of diabetes mellitus

Glucagon-like peptide-1 (GLP-1) is an incretin hormone secreted from enteroendocrine L cells in response to ingested nutrients. The first recognized and most important action of GLP-1 is the potentiation of glucose-stimulated insulin secretion in beta-cells, mediated by activation of its seven transmembrane domain G-protein-coupled receptor. In addition to its insulinotropic actions, GLP-1 exerts islet-trophic effects by stimulating replication and differentiation and by decreasing apoptosis of beta-cells. The GLP-1 receptor is expressed in a variety of other tissues important for carbohydrate metabolism, including pancreatic alpha-cells, hypothalamus and brainstem, and proximal intestinal tract. GLP-1 also appears to exert important actions in liver, muscle and fat. Thus, GLP-1 suppresses glucagon secretion, promotes satiety, delays gastric emptying and stimulates peripheral glucose uptake. The impaired GLP-1 secretion observed in type 2 diabetes suggests that GLP-1 plays a role in the pathogenesis of this disorder. Thus, because of its multiple actions, GLP-1 is an attractive therapeutic target for the treatment of type 2 diabetes, and major interest has resulted in the development of a variety of GLP-1 receptor agonists for this purpose. Ongoing clinical trials have shown promising results and the first analogs of GLP-1 are expected to be available in the near future.

Growth factors and beta cell replication

Recent studies have demonstrated that human islet allograft transplantation can be a successful therapeutic option in the treatment of patients with Type I diabetes. However, this impressive recent advance is accompanied by a very important constraint. There is a critical paucity of pancreatic islets or pancreatic beta cells for islet transplantation to become a large-scale therapeutic option in patients with diabetes. This has prompted many laboratories around the world to invigorate their efforts in finding ways for increasing the availability of beta cells or beta cell surrogates that potentially could be transplanted into patients with diabetes. The number of studies analyzing the mechanisms that govern beta cell proliferation and growth in physiological and pathological conditions has increased exponentially during the last decade. These studies exploring the role of growth factors, intracellular signaling molecules and cell cycle regulators constitute the substrate for future strategies aimed at expanding human beta cells in vitro and/or in vivo after transplantation. In this review, we describe the current knowledge on the effects of several beta cell growth factors that have been shown to increase beta cell proliferation and expand beta cell mass in vitro and/or in vivo and that they could be potentially deployed in an effort to increase the number of patients transplanted with islets. Furthermore, we also analyze in this review recent studies deciphering the relevance of these specific islet growth factors as physiological and pathophysiological regulators of beta cell proliferation and islet growth.

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.

Overexpression of a dominant negative GIP receptor in transgenic mice results in disturbed postnatal pancreatic islet and beta-cell development

The expression of a dominant negative glucose-dependent insulinotropic polypeptide receptor (GIPRdn) under the control of the rat pro-insulin gene promoter induces severe diabetes mellitus in transgenic mice. This study aims to gain further insight into the effect of the expression of a dominant negative GIPR on glucose homeostasis and postnatal development of the endocrine pancreas. The diabetic phenotype of GIPRdn transgenic animals was first observed between 14 and 21 days of age (urine glucose>1000 mg/dl). After onset of diabetes, serum glucose was significantly higher and insulin values were significantly lower in GIPRdn transgenic mice vs. non-transgenic littermate controls. Morphometric studies of pancreatic islets and their endocrine cell types were carried out at 10, 30 and 90 days of age. The total islet and total beta-cell volume of transgenic mice was severely reduced as compared to control mice, irrespective of the age at sampling (p<0.05). The total volume of isolated insulin positive cells that were not contained within established islets was significantly reduced in transgenic mice, indicating disturbed islet neogenesis. These findings demonstrate in vivo evidence that intact signaling of G-protein coupled receptors is involved in postnatal islet and beta-cell development and neogenesis of the pancreatic islets.

Harnessing the therapeutic potential of glucagon-like peptide-1: a critical review

Glucagon-like peptide-1 (GLP-1) is synthesized from proglucagon in enteroendocrine cells and regulates glucose homeostasis via multiple complementary actions on appetite, gastrointestinal motility and islet hormone secretion. GLP-1 is secreted from the distal gut in response to food ingestion, and levels of circulating GLP-1 may be diminished in patients with type 2 diabetes mellitus. GLP-1 administration stimulates glucose-dependent insulin secretion, inhibits glucagon secretion, and lowers blood glucose in normal and diabetic rodents and in humans. GLP-1 exerts additional glucose-lowering actions in patients with diabetes mellitus already treated with metformin or sulfonylurea therapy. GLP-1 inhibits gastric emptying in healthy individuals and those with diabetes mellitus, and excess GLP-1 administration may cause nausea or vomiting in susceptible individuals. Chronic GLP-1 treatment of normal or diabetic rodents is associated with bodyweight loss and GLP-1 agonists transiently inhibit food intake and may prevent bodyweight gain in humans. The potential for GLP-1 therapy to prevent deterioration of beta-cell function is exemplified by studies demonstrating that GLP-1 analogs stimulate proliferation and neogenesis of beta-cells, leading to expansion of beta-cell mass in diabetic rodents. The rapid N-terminal inactivation of bioactive GLP-1 by dipeptidyl peptidase-IV (DPP-IV) limits the utility of the native peptide for the treatment of patients with diabetes mellitus, and has fostered the development of more potent and stable protease-resistant GLP-1 analogs which exhibit longer durations of action. The importance of DPP-IV for glucose control is illustrated by the phenotype of rodents with genetic inactivation of DPP-IV which exhibit reduced glycemic excursion and increased levels of circulating GLP-1 in vivo. Inhibitors of DPP-IV potentiate incretin action by preventing degradation of GLP-1 and glucose-dependent insulinotropic peptide, and lower blood glucose in normal rodents and in experimental models of diabetes mellitus. Hence, orally available DPP-IV inhibitors also represent a new class of therapeutic agents that enhance incretin action for the treatment of patients with type 2 diabetes mellitus.

Glucagon-like peptide 1(GLP-1) in biology and pathology

Post-translational proteolytic processing of the preproglucagon gene in the gut results in the formation of glucagon-like peptide 1 (GLP-1). Owing to its glucose-dependent insulinotropic effect, this hormone was postulated to primarily act as an incretin, i.e. to augment insulin secretion after oral glucose or meal ingestion. In addition, GLP-1 decelerates gastric emptying and suppresses glucagon secretion. Under physiological conditions, GLP-1 acts as a part of the 'ileal brake', meaning that is slows the transition of nutrients into the distal gut. Animal studies suggest a role for GLP-1 in the development and growth of the endocrine pancreas. In light of its multiple actions throughout the body, different therapeutic applications of GLP-1 are possible. Promising results have been obtained with GLP-1 in the treatment of type 2 diabetes, but its potential to reduce appetite and food intake may also allow its use for the treatment of obesity. While rapid in vivo degradation of GLP-1 has yet prevented its broad clinical use, different pharmacological approaches aiming to extend the in vivo half-life of GLP-1 or to inhibit its inactivation are currently being evaluated. Therefore, antidiabetic treatment based on GLP-1 may become available within the next years. This review will summarize the biological effects of GLP-1, characterize its role in human biology and pathology, and discuss potential clinical applications as well as current clinical studies.

beta-Cell Pdx1 expression is essential for the glucoregulatory, proliferative, and cytoprotective actions of glucagon-like peptide-1

Glucagon-like peptide-1 (GLP-1) regulates energy intake, gastrointestinal motility, and nutrient disposal. The relative importance of the islet beta-cell for GLP-1 actions remains unclear. We determined the role of the islet beta-cell and the pancreatic duodenal homeobox-1 (Pdx1) transcription factor for GLP-1 receptor (GLP-1R)-dependent actions through analysis of mice with beta-cell-specific inactivation of the Pdx1 gene (beta-cell(Pdx1-/-) mice). The GLP-1R agonist exendin-4 (Ex-4) reduced glycemic excursion following intraperitoneal (i.p.) glucose challenge in control littermates (beta-cell(Pdx1+/+) mice) but not in beta-cell(Pdx1-/-) mice. Similarly, Ex-4 failed to increase levels of plasma insulin, pancreatic insulin content, and pancreatic insulin mRNA transcripts in beta-cell(Pdx1-/-) mice. Furthermore, Ex-4 significantly increased beta-cell proliferation and reduced beta-cell apoptosis in beta-cell(Pdx1+/+) mice but not in beta-cell(Pdx1-/-) mice. Moreover, Ex-4 increased the levels of insulin and amylin mRNA transcripts and augmented glucose-stimulated insulin secretion in islets from beta-cell(Pdx1+/+) mice but not in beta-cell(Pdx1-/-) islets. Surprisingly, Ex-4 failed to reduce levels of plasma glucagon in beta-cell(Pdx1-/-) mice. These findings demonstrate that Pdx1 expression is essential for integrating GLP-1R-dependent signals regulating alpha-cell glucagon secretion and for the growth, differentiated function, and survival of islet beta-cells.

Clinical endocrinology and metabolism. Glucagon-like peptide-1 and glucagon-like peptide-2

The glucagon-like peptides (glucagon-like peptide-1 (GLP-1) and glucagon-like peptide-2 (GLP-2)) are released from enteroendocrine cells in response to nutrient ingestion. GLP-1 enhances glucose-stimulated insulin secretion and inhibits glucagon secretion, gastric emptying and feeding. GLP-1 also has proliferative, neogenic and antiapoptotic effects on pancreatic beta-cells. More recent studies illustrate a potential protective role for GLP-1 in the cardiovascular and central nervous systems. GLP-2 is an intestinal trophic peptide that stimulates cell proliferation and inhibits apoptosis in the intestinal crypt compartment. GLP-2 also regulates intestinal glucose transport, food intake and gastric acid secretion and emptying, and improves intestinal barrier function. Thus, GLP-1 and GLP-2 exhibit a diverse array of metabolic, proliferative and cytoprotective actions with important clinical implications for the treatment of diabetes and gastrointestinal disease, respectively. This review will highlight our current understanding of the biology of GLP-1 and GLP-2, with an emphasis on both well-characterized and more novel therapeutic applications of these peptides.

The major glucagon-like peptide-1 metabolite, GLP-1-(9–36)-amide, does not affect glucose or insulin levels in mice

Glucagon-like peptide-1 (GLP-1), a future treatment for type 2 diabetes, is efficiently degraded by the enzyme dipeptidyl peptidase IV (DPP IV), yielding the major metabolite GLP-1-(9-36)-amide. In this study, we examined the potential glucose lowering effect of GLP-1-(9-36)-amide in mice and found that GLP-1-(9-36)-amide (3 and 10 nmol/kg) did not affect insulin secretion or glucose elimination when administered intravenously together with glucose (1 g/kg). This was observed both in normal mice and in transgenic mice having a complete disruption of the signalling from the GLP-1 receptor. Furthermore, after blocking insulin secretion, using diazoxide (25 mg/kg), no effect on insulin-independent glucose disposal of GLP-1-(9-36)-amide was observed. Therefore, GLP-1-(9-36)-amide does not affect glucose disposal in mice either in the presence or absence of intact GLP-1-receptors or in the presence or absence of stimulated insulin levels. This suggests that the GLP-1 metabolite is not involved in the regulation of glucose homeostasis.

Glucagon-like peptide 1: evolution of an incretin into a treatment for diabetes

Glucagon-like peptide 1 (GLP-1) is a product of proglucagon that is secreted by specialized intestinal endocrine cells after meals. GLP-1 is insulinotropic and plays a role in the incretin effect, the augmented insulin response observed when glucose is absorbed through the gut. GLP-1 also appears to regulate a number of processes that reduce fluctuations in blood glucose, such as gastric emptying, glucagon secretion, food intake, and possibly glucose production and glucose uptake. These effects, in addition to the stimulation of insulin secretion, suggest a broad role for GLP-1 as a mediator of postprandial glucose homeostasis. Consistent with this role, the most prominent effect of experimental blockade of GLP-1 signaling is an increase in blood glucose. Recent data also suggest that GLP-1 is involved in the regulation of beta-cell mass. Whereas other insulinotropic gastrointestinal hormones are relatively ineffective in stimulating insulin secretion in persons with type 2 diabetes, GLP-1 retains this action and is very effective in lowering blood glucose levels in these patients. There are currently a number of products in development that utilize the GLP-1-signaling system as a mechanism for the treatment of diabetes. These compounds, GLP-1 receptor agonists and agents that retard the metabolism of native GLP-1, have shown promising results in clinical trials. The application of GLP-1 to clinical use fulfills a long-standing interest in adapting endogenous insulinotropic hormones to the treatment of diabetes.

Glucagon-like peptide-1 regulates proliferation and apoptosis via activation of protein kinase B in pancreatic INS-1 beta cells

AIMS/HYPOTHESIS

The incretin hormone glucagon-like peptide-1 augments islet cell mass in vivo by increasing proliferation and decreasing apoptosis of the beta cells. However, the signalling pathways that mediate these effects are mostly unknown. Using a clonal rat pancreatic beta cell line (INS-1), we examined the role of protein kinase B in mediating beta-cell growth and survival stimulated by glucagon-like peptide-1.

METHODS

Immunoblot analysis was used to detect active (phospho-) and total protein kinase B. Proliferation was assessed using (3)H-thymidine incorporation, while apoptosis was quantitated using 4'-6-diamidino-2-phenylindole staining and APO percentage apoptosis assay. Kinase-dead and wild-type protein kinase B was introduced into cells using adenoviral vectors.

RESULTS

Glucagon-like peptide-1 rapidly activated protein kinase B in INS-1 cells (by 2.7+/-0.7-fold, p<0.05). This effect was completely abrogated by inhibition, with wortmannin, of the upstream activator of protein kinase B, phosphatidylinositol-3-kinase. Glucagon-like peptide-1 also stimulated INS-1 cell proliferation in a dose-dependent manner (by 1.8+/-0.5-fold at 10(-7) mol/l, p<0.01), and inhibited staurosporine-induced apoptosis (by 69+/-12%, p<0.05). Both of these effects were also prevented by wortmannin treatment. Ablation of protein kinase B by adenovirus-mediated overexpression of the kinase-dead form of protein kinase Balpha prevented protein kinase B phosphorylation and completely abrogated both cellular proliferation ( p<0.05) and protection from drug-induced cellular death ( p<0.01) induced by glucagon-like peptide-1.

CONCLUSIONS/INTERPRETATION

These results identify protein kinase B as an essential mediator linking the glucagon-like peptide-1 signal to the intracellular machinery that modulates beta-cell growth and survival.

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.

Enteroinsular signaling: perspectives on the role of the gastrointestinal hormones glucagon-like peptide 1 and glucose-dependent insulinotropic polypeptide in normal and abnormal glucose metabolism

PURPOSE OF REVIEW

The gastrointestinal hormones glucagon-like peptide 1 and glucose-dependent insulinotropic polypeptide are emerging as essential regulators of insulin secretion and glucose homeostasis. These peptides, termed incretins, are the key intermediaries in a system that links the absorption of nutrients in the gut with important metabolic processes in substrate assimilation. New findings indicate that the enteroinsular system mediated by the incretins is relevant to both the pathophysiology and treatment of diabetes.

RECENT FINDINGS

Important advances have been made in the understanding of mechanisms fundamental to incretin function such as their release from the intestine during meals, their actions on beta-cell secretion, and extrapancreatic effects. In addition, the regulation of islet growth by glucagon-like peptide 1 and glucose-dependent insulinotropic polypeptide is a novel area with considerable support from recent studies. Abnormalities of incretin function are present in patients with diabetes and current research has implicated specific defects of both glucagon-like peptide 1 and glucose-dependent insulinotropic polypeptide action in diabetes. Finally, several pharmacological applications of the incretin signaling pathways are under active investigation for the treatment of diabetes.

SUMMARY

With the intensified research of the last several years the physiologic importance of the incretins has been clarified. Enteroinsular signaling is an essential component of the metabolic processes that govern carbohydrate, and likely other nutrient metabolism. As a pathophysiology of the incretins emerges, glucagon-like peptide 1 and glucose-dependent insulinotropic polypeptide will have increasing clinical relevance. This is currently exemplified by the development of therapeutics for diabetes that work through the incretin signaling pathways.

1-[[(3-hydroxy-1-adamantyl)amino]acetyl]-2-cyano-(S)-pyrrolidine: a potent, selective, and orally bioavailable dipeptidyl peptidase IV inhibitor with antihyperglycemic properties

Dipeptidyl peptidase IV (DPP-IV) inhibition has the potential to become a valuable therapy for type 2 diabetes. The synthesis and structure-activity relationship of a new DPP-IV inhibitor class, N-substituted-glycyl-2-cyanopyrrolidines, are described as well as the path that led from clinical development compound 1-[2-[5-cyanopyridin-2-yl)amino]ethylamino]acetyl-2-cyano-(S)-pyrrolidine (NVP-DPP728, 8c) to its follow-up, 1-[[(3-hydroxy-1-adamantyl) amino]acetyl]-2-cyano-(S)-pyrrolidine (NVP-LAF237, 12j). The pharmacological profile of 12j in obese Zucker fa/fa rats along with pharmacokinetic profile comparison of 8c and 12j in normal cynomolgus monkeys is discussed. The results suggest that 12j is a potent, stable, selective DPP-IV inhibitor possessing excellent oral bioavailability and potent antihyperglycemic activity with potential for once-a-day administration.