Ectopic expression of GIP in pancreatic β-cells maintains enhanced insulin secretion in mice with complete absence of proglucagon-derived peptides

Glucose-Dependent Insulinotropic Polypeptide Inhibits AGE-Induced NADPH Oxidase-Derived Oxidative Stress Generation and Foam Cell Formation in Macrophages Partly via AMPK Activation

Glucose-dependent insulinotropic polypeptide (GIP) of the incretin group has been shown to exert pleiotropic actions. There is growing evidence that advanced glycation end products (AGEs), senescent macromolecules formed at an accelerated rate under chronic hyperglycemic conditions, play a role in the pathogenesis of atherosclerotic cardiovascular disease in diabetes. However, whether and how GIP could inhibit the AGE-induced foam cell formation of macrophages, an initial step of atherosclerosis remains to be elucidated. In this study, we address these issues. We found that AGEs increased oxidized low-density-lipoprotein uptake into reactive oxygen species (ROS) generation and Cdk5 and CD36 gene expressions in human U937 macrophages, all of which were significantly blocked by [D-Ala2]GIP(1-42) or an inhibitor of NADPH oxidase activity. An inhibitor of AMP-activated protein kinase (AMPK) attenuated all of the beneficial effects of [D-Ala2]GIP(1-42) on AGE-exposed U937 macrophages, whereas an activator of AMPK mimicked the effects of [D-Ala2]GIP(1-42) on foam cell formation, ROS generation, and Cdk5 and CD36 gene expressions in macrophages. The present study suggests that [D-Ala2]GIP(1-42) could inhibit the AGE-RAGE-induced, NADPH oxidase-derived oxidative stress generation in U937 macrophages via AMPK activation and subsequently suppress macrophage foam cell formation by reducing the Cdk5-CD36 pathway.

Blockade of glucagon increases muscle mass and alters fiber type composition in mice deficient in proglucagon-derived peptides

AIMS/INTRODUCTION

Glucagon is secreted from pancreatic α-cells and plays an important role in amino acid metabolism in liver. Various animal models deficient in glucagon action show hyper-amino acidemia and α-cell hyperplasia, indicating that glucagon contributes to feedback regulation between the liver and the α-cells. In addition, both insulin and various amino acids, including branched-chain amino acids and alanine, participate in protein synthesis in skeletal muscle. However, the effect of hyperaminoacidemia on skeletal muscle has not been investigated. In the present study, we examined the effect of blockade of glucagon action on skeletal muscle using mice deficient in proglucagon-derived peptides (GCGKO mice).

MATERIALS AND METHODS

Muscles isolated from GCGKO and control mice were analyzed for their morphology, gene expression and metabolites.

RESULTS

GCGKO mice showed muscle fiber hypertrophy, and a decreased ratio of type IIA and an increased ratio of type IIB fibers in the tibialis anterior. The expression levels of myosin heavy chain (Myh) 7, 2, 1 and myoglobin messenger ribonucleic acid were significantly lower in GCGKO mice than those in control mice in the tibialis anterior. GCGKO mice showed a significantly higher concentration of arginine, asparagine, serine and threonine in the quadriceps femoris muscles, and also alanine, aspartic acid, cysteine, glutamine, glycine and lysine, as well as four amino acids in gastrocnemius muscles.

CONCLUSIONS

These results show that hyperaminoacidemia induced by blockade of glucagon action in mice increases skeletal muscle weight and stimulates slow-to-fast transition in type II fibers of skeletal muscle, mimicking the phenotype of a high-protein diet.

Roles of glucose-dependent insulinotropic polypeptide in diet-induced obesity

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are incretins that play an important role in glucose metabolism, by increasing glucose-induced insulin secretion from pancreatic β-cells and help regulate bodyweight. Although they show a similar action on glucose-induced insulin secretion, two incretins are distinct in various aspects. GIP is secreted from enteroendocrine K cell mainly expressed in the upper small intestine, and GLP-1 is secreted from enteroendocrine L cells mainly expressed in the lower small intestine and colon by the stimulation of various nutrients. The mechanism of GIP secretion induced by nutrients, especially carbohydrates, is different from that of GLP-1 secretion. GIP promotes fat deposition in adipose tissue, and contributes to fat-induced obesity. In contrast, GLP-1 participates in reducing bodyweight by suppressing food consumption and/or slowing gastric emptying. There is substantial evidence that GIP and GLP-1 might differently contribute to bodyweight control. Although meal contents influence both glycemic and weight control, we do not fully understand whether incretin actions differ depending on the contents of the meal and what kind of signaling is involved in its context. We focus on the molecular mechanism of GIP secretion induced by nutrients, as well as the roles of GIP in weight changes caused by various diets.

High Protein Diet Feeding Aggravates Hyperaminoacidemia in Mice Deficient in Proglucagon-Derived Peptides

(1) Background: Protein stimulates the secretion of glucagon (GCG), which can affect glucose metabolism. This study aimed to analyze the metabolic effect of a high-protein diet (HPD) in the presence or absence of proglucagon-derived peptides, including GCG and GLP-1. (2) Methods: The response to HPD feeding for 7 days was analyzed in mice deficient in proglucagon-derived peptides (GCGKO). (3) Results: In both control and GCGKO mice, food intake and body weight decreased with HPD and intestinal expression of Pepck increased. HPD also decreased plasma FGF21 levels, regardless of the presence of proglucagon-derived peptides. In control mice, HPD increased the hepatic expression of enzymes involved in amino acid metabolism without the elevation of plasma amino acid levels, except branched-chain amino acids. On the other hand, HPD-induced changes in the hepatic gene expression were attenuated in GCGKO mice, resulting in marked hyperaminoacidemia with lower blood glucose levels; the plasma concentration of glutamine exceeded that of glucose in HPD-fed GCGKO mice. (4) Conclusions: Increased plasma amino acid levels are a common feature in animal models with blocked GCG activity, and our results underscore that GCG plays essential roles in the homeostasis of amino acid metabolism in response to altered protein intake.

Glucose-Dependent Insulinotropic Polypeptide Suppresses Foam Cell Formation of Macrophages through Inhibition of the Cyclin-Dependent Kinase 5-CD36 Pathway

Glucose-dependent insulinotropic polypeptide (GIP) has been reported to have an atheroprotective property in animal models. However, the effect of GIP on macrophage foam cell formation, a crucial step of atherosclerosis, remains largely unknown. We investigated the effects of GIP on foam cell formation of, and CD36 expression in, macrophages extracted from GIP receptor-deficient (Gipr^−/−) and Gipr^+/+ mice and cultured human U937 macrophages by using an agonist for GIP receptor, [D-Ala²]GIP(1–42). Foam cell formation evaluated by esterification of free cholesterol to cholesteryl ester and CD36 gene expression in macrophages isolated from Gipr^+/+ mice infused subcutaneously with [D-Ala²]GIP(1–42) were significantly suppressed compared with vehicle-treated mice, while these beneficial effects were not observed in macrophages isolated from Gipr^−/− mice infused with [D-Ala²]GIP(1–42). When macrophages were isolated from Gipr^+/+ and Gipr^−/− mice, and then exposed to [D-Ala²]GIP(1–42), similar results were obtained. [D-Ala²]GIP(1–42) attenuated ox-LDL uptake of, and CD36 gene expression in, human U937 macrophages as well. Gene expression level of cyclin-dependent kinase 5 (Cdk5) was also suppressed by [D-Ala²]GIP(1–42) in U937 cells, which was corelated with that of CD36. A selective inhibitor of Cdk5, (R)-DRF053 mimicked the effects of [D-Ala²]GIP(1–42) in U937 cells. The present study suggests that GIP could inhibit foam cell formation of macrophages by suppressing the Cdk5-CD36 pathway via GIP receptor.

Small Molecule-Induced Pancreatic β-Like Cell Development: Mechanistic Approaches and Available Strategies

Diabetes is a metabolic disease which affects not only glucose metabolism but also lipid and protein metabolism. It encompasses two major types: type 1 and 2 diabetes. Despite the different etiologies of type 1 and 2 diabetes mellitus (T1DM and T2DM, respectively), the defining features of the two forms are insulin deficiency and resistance, respectively. Stem cell therapy is an efficient method for the treatment of diabetes, which can be achieved by differentiating pancreatic β-like cells. The consistent generation of glucose-responsive insulin releasing cells remains challenging. In this review article, we present basic concepts of pancreatic organogenesis, which intermittently provides a basis for engineering differentiation procedures, mainly based on the use of small molecules. Small molecules are more auspicious than any other growth factors, as they have unique, valuable properties like cell-permeability, as well as a nonimmunogenic nature; furthermore, they offer immense benefits in terms of generating efficient functional beta-like cells. We also summarize advances in the generation of stem cell-derived pancreatic cell lineages, especially endocrine β-like cells or islet organoids. The successful induction of stem cells depends on the quantity and quality of available stem cells and the efficient use of small molecules.

Gut-Proglucagon-Derived Peptides Are Essential for Regulating Glucose Homeostasis in Mice

The importance of pancreatic versus intestinal-derived GLP-1 for glucose homeostasis is controversial. We detected active GLP-1 in the mouse and human pancreas, albeit at extremely low levels relative to glucagon. Accordingly, to elucidate the metabolic importance of intestinal proglucagon-derived peptides (PGDPs), we generated mice with reduction of Gcg expression within the distal (Gcg^DistalGut-/-) or entire (Gcg^Gut-/-) gut. Substantial reduction of gut Gcg expression markedly reduced circulating levels of GLP-1, and impaired glucose homeostasis, associated with increased levels of GIP, and accelerated gastric emptying. Gcg^DistalGut-/- mice similarly exhibited lower circulating GLP-1 and impaired oral glucose tolerance. Nevertheless, plasma levels of insulin remained normal following glucose administration in the absence of gut-derived GLP-1. Collectively, our findings identify the essential importance of gut-derived PGDPs for maintaining levels of circulating GLP-1, control of gastric emptying, and glucose homeostasis.

The role of GIP and pancreatic GLP-1 in the glucoregulatory effect of DPP-4 inhibition in mice

AIMS/HYPOTHESIS

Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are two peptides that function to promote insulin secretion. Dipeptidyl peptidase-4 (DPP-4) inhibitors increase the bioavailability of both GLP-1 and GIP but the dogma continues to be that it is the increase in GLP-1 that contributes to the improved glucose homeostasis. We have previously demonstrated that pancreatic rather than intestinal GLP-1 is necessary for improvements in glucose homeostasis in mice. Therefore, we hypothesise that a combination of pancreatic GLP-1 and GIP is necessary for the full effect of DPP-4 inhibitors on glucose homeostasis.

METHODS

We have genetically engineered mouse lines in which the preproglucagon gene (Gcg) is absent in the entire body (GcgRAΔNull) or is expressed exclusively in the intestine (GcgRAΔVilCre) or pancreas and duodenum (GcgRAΔPDX1Cre). These mice were used to examine oral glucose tolerance and GLP-1 and GIP responses to a DPP-4 inhibitor alone, or in combination with incretin receptor antagonists.

RESULTS

Administration of the DPP-4 inhibitor, linagliptin, improved glucose tolerance in GcgRAΔNull mice and control littermates and in GcgRAΔVilCre and GcgRAΔPDX1Cre mice. The potent GLP-1 receptor antagonist, exendin-[9-39] (Ex9), blunted improvements in glucose tolerance in linagliptin-treated control mice and in GcgRAΔPDX1Cre mice. Ex9 had no effect on glucose tolerance in linagliptin-treated GcgRAΔNull or in GcgRAΔVilCre mice. In addition to GLP-1, linagliptin also increased postprandial plasma levels of GIP to a similar degree in all genotypes. When linagliptin was co-administered with a GIP-antagonising antibody, the impact of linagliptin was partially blunted in wild-type mice and was fully blocked in GcgRAΔNull mice.

CONCLUSIONS/INTERPRETATION

Taken together, these data suggest that increases in pancreatic GLP-1 and GIP are necessary for the full effect of DPP-4 inhibitors on glucose tolerance.

Short-Term High-Starch, Low-Protein Diet Induces Reversible Increase in β-cell Mass Independent of Body Weight Gain in Mice

Long-term exposure to a high starch, low-protein diet (HSTD) induces body weight gain and hyperinsulinemia concomitantly with an increase in β-cell mass (BCM) and pancreatic islets number in mice; however, the effect of short-term exposure to HSTD on BCM and islet number has not been elucidated. In the present study, we investigated changes in body weight, plasma insulin levels, BCM and islet number in mice fed HSTD for 5 weeks followed by normal chow (NC) for 2 weeks. BCM and islet number were increased in mice fed HSTD for 5 weeks compared with those in mice fed NC. On the other hand, mice fed HSTD for 5 weeks followed by NC for 2 weeks (SN) showed decreased BCM and insulin levels, compared to mice fed HSTD for 7 weeks, and no significant differences in these parameters were observed between SN and the control NC at 7 weeks. No significant difference in body weight was observed among HSTD, NC and SN fed groups. These results suggest that a high-starch diet induces an increase in BCM in a manner independent of body weight gain, and that 2 weeks of NC feeding is sufficient for the reversal of the morphological changes induced in islets by HSTD feeding.

Functional adenosine triphosphate-sensitive potassium channel is required in high-carbohydrate diet-induced increase in β-cell mass

AIMS/INTRODUCTION

A high-carbohydrate diet is known to increase insulin secretion and induce obesity. However, whether or not a high-carbohydrate diet affects β-cell mass (BCM) has been little investigated.

MATERIALS AND METHODS

Both wild-type (WT) mice and adenosine triphosphate-sensitive potassium channel-deficient (Kir6.2KO) mice were fed normal chow or high-starch (ST) diets for 22 weeks. BCM and the numbers of islets were analyzed by immunohistochemistry, and gene expression levels in islets were investigated by quantitative real-time reverse transcription polymerase chain reaction. MIN6-K8 β-cells were stimulated in solution containing various concentrations of glucose combined with nifedipine and glimepiride, and gene expression was analyzed.

RESULTS

Both WT and Kir6.2KO mice fed ST showed hyperinsulinemia and body weight gain. BCM, the number of islets and the expression levels of cyclinD2 messenger ribonucleic acid were increased in WT mice fed ST compared with those in WT mice fed normal chow. In contrast, no significant difference in BCM, the number of islets or the expression levels of cyclinD2 messenger ribonucleic acid were observed between Kir6.2KO mice fed normal chow and those fed ST. Incubation of MIN6-K8 β-cells in high-glucose media or with glimepiride increased cyclinD2 expression, whereas nifedipine attenuated a high-glucose-induced increase in cyclinD2 expression.

CONCLUSIONS

These results show that a high-starch diet increases BCM in an adenosine triphosphate-sensitive potassium channel-dependent manner, which is mediated through upregulation of cyclinD2 expression.

Glucose-dependent insulinotropic polypeptide is required for moderate high-fat diet- but not high-carbohydrate diet-induced weight gain

Both high-fat (HFD) and high-carbohydrate (ST) diets are known to induce weight gain. Glucose-dependent insulinotropic polypeptide (GIP) is secreted mainly from intestinal K cells upon stimuli by nutrients such as fat and glucose, and it potentiates glucose-induced insulin secretion. GIP is well known to contribute to HFD-induced obesity. In this study, we analyzed the effect of ST feeding on GIP secretion and metabolic parameters to explore the role of GIP in ST-induced weight gain. Both wild-type (WT) and GIP receptor deficient ( GiprKO) mice were fed normal chow (NC), ST, or moderate (m)HFD for 22 wk. Body weight was measured, and then glucose tolerance tests were performed. Insulin secretion from isolated islets also was analyzed. WT mice fed ST or mHFD displayed weight gain concomitant with increased plasma GIP levels compared with WT mice fed NC. WT mice fed mHFD showed improved glucose tolerance due to enhanced insulin secretion during oral glucose tolerance tests compared with WT mice fed NC or ST. GiprKO mice fed mHFD did not display weight gain. On the other hand, GiprKO mice fed ST showed weight gain and did not display obvious glucose intolerance. Glucose-induced insulin secretion was enhanced during intraperitoneal glucose tolerance tests and from isolated islets in both WT and GiprKO mice fed ST compared with those fed NC. In conclusion, enhanced GIP secretion induced by mHFD-feeding contributes to increased insulin secretion and body weight gain, whereas GIP is marginally involved in weight gain induced by ST-feeding.

Endogenous GIP ameliorates impairment of insulin secretion in proglucagon-deficient mice under moderate beta cell damage induced by streptozotocin

AIMS/HYPOTHESIS

The action of incretin hormones including glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) is potentiated in animal models defective in glucagon action. It has been reported that such animal models maintain normoglycaemia under streptozotocin (STZ)-induced beta cell damage. However, the role of GIP in regulation of glucose metabolism under a combination of glucagon deficiency and STZ-induced beta cell damage has not been fully explored.

METHODS

In this study, we investigated glucose metabolism in mice deficient in proglucagon-derived peptides (PGDPs)-namely glucagon gene knockout (GcgKO) mice-administered with STZ. Single high-dose STZ (200 mg/kg, hSTZ) or moderate-dose STZ for five consecutive days (50 mg/kg × 5, mSTZ) was administered to GcgKO mice. The contribution of GIP to glucose metabolism in GcgKO mice was also investigated by experiments employing dipeptidyl peptidase IV (DPP4) inhibitor (DPP4i) or Gcg-Gipr double knockout (DKO) mice.

RESULTS

GcgKO mice developed severe diabetes by hSTZ administration despite the absence of glucagon. Administration of mSTZ decreased pancreatic insulin content to 18.8 ± 3.4 (%) in GcgKO mice, but ad libitum-fed blood glucose levels did not significantly increase. Glucose-induced insulin secretion was marginally impaired in mSTZ-treated GcgKO mice but was abolished in mSTZ-treated DKO mice. Although GcgKO mice lack GLP-1, treatment with DPP4i potentiated glucose-induced insulin secretion and ameliorated glucose intolerance in mSTZ-treated GcgKO mice, but did not increase beta cell area or significantly reduce apoptotic cells in islets.

CONCLUSIONS/INTERPRETATION

These results indicate that GIP has the potential to ameliorate glucose intolerance even under STZ-induced beta cell damage by increasing insulin secretion rather than by promoting beta cell survival.

The role of foregut exclusion in the deterioration of glucose and lipid metabolism induced by a high-fat diet

AIM

The small intestine may be involved in the improvement of glucose and lipid metabolism after bariatric surgery; however, the role of the foregut in metabolic changes remains unclear. This study used normal rats fed a high-fat diet (HFD) after bariatric surgery to determine the role of the foregut in glucose and lipid metabolism.

METHODS

Duodenum-jejunum bypass (DJB), gastrojejunostomy (GJ) and sham-operations were performed on Sprague-Dawley (SD) rats. Oral glucose tolerance, insulin sensitivity, β-cell function, lipid profile, glucose-stimulated glucose-dependent insulinotropic polypeptide (GIP) levels and glucagon-like peptide-1 (GLP-1) levels were measured. The rats were observed for 24 weeks post-surgery.

RESULTS

Food intake and body weight were similar between the groups during the study period (P>0.05). The DJB group exhibited better glucose and lipid metabolism than the other groups (P<0.05). Compared with the GJ group, the DJB group demonstrated superior oral glucose tolerance, insulin sensitivity and lipid profiles (P<0.05); β-cell function in the two groups was similar (P>0.05). The GIP levels were decreased in the DJB group and increased in the GJ group (P<0.05), and the GLP-1 levels were increased in the DJB and GJ groups (P>0.05).

CONCLUSIONS

We found that foregut exclusion can prevent disordered glucose and lipid metabolism. Additionally, decreased GIP secretion was associated with improvements in glucose tolerance and insulin sensitivity, particularly related to lipid metabolism. Increased GLP-1 benefited β-cell function; however, it could not reverse the disordered glucose and lipid metabolism induced by a HFD.

Carbohydrate-induced secretion of glucose-dependent insulinotropic polypeptide and glucagon-like peptide-1

Glucose‐dependent insulinotropic polypeptide (GIP) and glucagon‐like peptide‐1 (GLP‐1) are the incretin hormones secreted from enteroendocrine K‐cells and L‐cells, respectively, by oral ingestion of various nutrients including glucose. K‐cells, L‐cells and pancreatic β‐cells are glucose‐responsive cells with similar glucose‐sensing machinery including glucokinase and an adenosine triphosphate‐sensitive K⁺ channel comprising KIR6.2 and sulfonylurea receptor 1. However, the physiological role of the adenosine triphosphate‐sensitive K⁺ channel in GIP secretion in K‐cells and GLP‐1 secretion in L‐cells is not elucidated. Recently, it was reported that GIP and GLP‐1‐producing cells are present also in pancreatic islets, and islet‐derived GIP and GLP‐1 contribute to glucose‐induced insulin secretion from pancreatic β‐cells. In this short review, we focus on GIP and GLP‐1 secretion by monosaccharides, such as glucose or fructose, and the role of the adenosine triphosphate‐sensitive K⁺ channel in GIP and GLP‐1 secretion.

Mice Deficient in Proglucagon-Derived Peptides Exhibit Glucose Intolerance on a High-Fat Diet but Are Resistant to Obesity

Homozygous glucagon-GFP knock-in mice (Gcg^gfp/gfp) lack proglucagon derived-peptides including glucagon and GLP-1, and are normoglycemic. We have previously shown that Gcg^gfp/gfp show improved glucose tolerance with enhanced insulin secretion. Here, we studied glucose and energy metabolism in Gcg^gfp/gfp mice fed a high-fat diet (HFD). Male Gcg^gfp/gfp and Gcg^gfp/+ mice were fed either a normal chow diet (NCD) or an HFD for 15–20 weeks. Regardless of the genotype, mice on an HFD showed glucose intolerance, and Gcg^gfp/gfp mice on HFD exhibited impaired insulin secretion whereas Gcg^gfp/+ mice on HFD exhibited increased insulin secretion. A compensatory increase in β-cell mass was observed in Gcg^gfp/+mice on HFD, but not in Gcg^gfp/gfp mice on the same diet. Weight gain was significantly lower in Gcg^gfp/gfp mice than in Gcg^gfp/+mice. Oxygen consumption was enhanced in Gcg^gfp/gfp mice compared to Gcg^gfp/+ mice on an HFD. HFD feeding significantly increased uncoupling protein 1 mRNA expression in brown adipose and inguinal white adipose tissues of Gcg^gfp/gfp mice, but not of Gcg^gfp/+mice. Treatment with the glucagon-like peptide-1 receptor agonist liraglutide (200 mg/kg) improved glucose tolerance in Gcg^gfp/gfp mice and insulin content in Gcg^gfp/gfp and Gcg^gfp/+ mice was similar after liraglutide treatment. Our findings demonstrate that Gcg^gfp/gfp mice develop diabetes upon HFD-feeding in the absence of proglucagon-derived peptides, although they are resistant to diet-induced obesity.

Pancreatic Neuroendocrine Tumors in Mice Deficient in Proglucagon-Derived Peptides

Animal models with defective glucagon action show hyperplasia of islet α-cells, however, the regulatory mechanisms underlying the proliferation of islet endocrine cells remain largely to be elucidated. The Gcggfp/gfp mice, which are homozygous for glucagon/green fluorescent protein knock-in allele (GCGKO), lack all proglucagon-derived peptides including glucagon and GLP-1. The present study was aimed to characterize pancreatic neuroendocrine tumors (panNETs), which develop in the GCGKO mice. At 15 months of age, macroscopic GFP-positive tumors were identified in the pancreas of all the GCGKO mice, but not in that of the control heterozygous mice. The tumor manifested several features that were consistent with pancreatic neuroendocrine tumors (panNETs), such as organoid structures with trabecular and cribriform patterns, and the expression of chromogranin A and synaptophysin. Dissemination of GFP-positive cells was observed in the liver and lungs in 100% and 95%, respectively, of 15-month-old GCGKO mice. To elucidate the regulatory mechanism for tumor growth, PanNET grafts were transplanted into subrenal capsules in GCGKO and control mice. Ki-67 positive cells were identified in panNET grafts transplanted to GCGKO mice 1 month after transplantation, but not in those to control mice. These results suggest that humoral factors or conditions specific to GCGKO mice, are involved in the proliferation of panNETs. Taken together, GCGKO mice are novel animal model for studying the development, pathogenesis, and metastasis panNETs.

Physiology of proglucagon peptides: role of glucagon and GLP-1 in health and disease

The preproglucagon gene (Gcg) is expressed by specific enteroendocrine cells (L-cells) of the intestinal mucosa, pancreatic islet α-cells, and a discrete set of neurons within the nucleus of the solitary tract. Gcg encodes multiple peptides including glucagon, glucagon-like peptide-1, glucagon-like peptide-2, oxyntomodulin, and glicentin. Of these, glucagon and GLP-1 have received the most attention because of important roles in glucose metabolism, involvement in diabetes and other disorders, and application to therapeutics. The generally accepted model is that GLP-1 improves glucose homeostasis indirectly via stimulation of nutrient-induced insulin release and by reducing glucagon secretion. Yet the body of literature surrounding GLP-1 physiology reveals an incompletely understood and complex system that includes peripheral and central GLP-1 actions to regulate energy and glucose homeostasis. On the other hand, glucagon is established principally as a counterregulatory hormone, increasing in response to physiological challenges that threaten adequate blood glucose levels and driving glucose production to restore euglycemia. However, there also exists a potential role for glucagon in regulating energy expenditure that has recently been suggested in pharmacological studies. It is also becoming apparent that there is cross-talk between the proglucagon derived-peptides, e.g., GLP-1 inhibits glucagon secretion, and some additive or synergistic pharmacological interaction between GLP-1 and glucagon, e.g., dual glucagon/GLP-1 agonists cause more weight loss than single agonists. In this review, we discuss the physiological functions of both glucagon and GLP-1 by comparing and contrasting how these peptides function, variably in concert and opposition, to regulate glucose and energy homeostasis.

Current status of regeneration of pancreatic β-cells

Newly generated insulin‐secreting cells for use in cell therapy for insulin‐deficient diabetes mellitus require properties similar to those of native pancreatic β‐cells. Pancreatic β‐cells are highly specialized cells that produce a large amount of insulin, and secrete insulin in a regulated manner in response to glucose and other stimuli. It is not yet explained how the β‐cells acquire this complex function during normal differentiation. So far, in vitro generation of insulin‐secreting cells from embryonic stem cells, induced‐pluripotent stem cells and adult stem/progenitor‐like cells has been reported. However, most of these cells are functionally immature and show poor glucose‐responsive insulin secretion compared to that of native pancreatic β‐cells (or islets). Strategies to generate functional β‐cells or a whole organ in vivo have also recently been proposed. Establishing a protocol to generate fully functional insulin‐secreting cells that closely resemble native β‐cells is a critical matter in regenerative medicine for diabetes. Understanding the physiological processes of differentiation, proliferation and regeneration of pancreatic β‐cells might open the path to cell therapy to cure patients with absolute insulin deficiency.

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.

Pharmacology, physiology, and mechanisms of incretin hormone action

Incretin peptides, principally GLP-1 and GIP, regulate islet hormone secretion, glucose concentrations, lipid metabolism, gut motility, appetite and body weight, and immune function, providing a scientific basis for utilizing incretin-based therapies in the treatment of type 2 diabetes. Activation of GLP-1 and GIP receptors also leads to nonglycemic effects in multiple tissues, through direct actions on tissues expressing incretin receptors and indirect mechanisms mediated through neuronal and endocrine pathways. Here we contrast the pharmacology and physiology of incretin hormones and review recent advances in mechanisms coupling incretin receptor signaling to pleiotropic metabolic actions in preclinical studies. We discuss whether mechanisms identified in preclinical studies have potential translational relevance for the treatment of human disease and highlight controversies and uncertainties in incretin biology that require resolution in future studies.

Aristaless-related homeobox plays a key role in hyperplasia of the pancreas islet α-like cells in mice deficient in proglucagon-derived peptides

Defects in glucagon action can cause hyperplasia of islet α-cells, however, the underlying mechanisms remain largely to be elucidated. Mice homozygous for a glucagon-GFP knock-in allele (Gcg^gfp/gfp) completely lack proglucagon-derived peptides and exhibit hyperplasia of GFP-positive α-like cells. Expression of the transcription factor, aristaless-related homeobox (ARX), is also increased in the Gcg^gfp/gfp pancreas. Here, we sought to elucidate the role of ARX in the hyperplasia of α-like cells through analyses of two Arx mutant alleles (Arx^P355L/Y and Arx ^{[330insGCG]7/Y}) that have different levels of impairment of their function. Expression of Gfp and Arx genes was higher and the size and number of islets increased in the Gcg^gfp/gfp pancreas compared to and Gcg^gfp/+ pancreas at 2 weeks of age. In male Gcg^gfp/gfp mice that are hemizygous for the Arx^P355L/Y mutation that results in a protein with a P355L amino acid substitution, expression of Gfp mRNA in the pancreas was comparable to that in control Gcg^gfp/+Arx^+/Y mice. The increases in islet size and number were also reduced in these mice. Immunohistochemical analysis showed that the number of GFP-positive cells was comparable in Gcg^gfp/gfp Arx^P355L/Y and Gcg^gfp/+Arx^+/Y mice. These results indicate that the hyperplasia is reduced by introduction of an Arx mutation. Arx^P355L/Y mice appeared to be phenotypically normal; however, Arx ^{[330insGCG]7/Y} mice that have a mutant ARX protein with expansion of the polyalanine tract had a reduced body size and shortened life span. The number of GFP positive cells was further reduced in the Gcg^gfp/gfp Arx ^{[330insGCG]7/Y} mice. Taken together, our findings show that the function of ARX is one of the key modifiers for hyperplasia of islet α-like cells in the absence of proglucagon-derived peptides.