Aims/hypothesis Metformin is widely used for the treatment of type 2 diabetes. Although it reduces hepatic glucose production, clinical studies show that metformin may reduce plasma dipeptidyl peptidase-4 activity and increase circulating levels of glucagon-like peptide 1 (GLP-1). We examined whether metformin exerts glucoregulatory actions via modulation of the incretin axis. Methods Metformin action was assessed in Glp1r

OBJECTIVEClinical reports link use of the glucagon-like peptide-1 receptor (GLP-1R) agonists exenatide and liraglutide to pancreatitis. However, whether these agents act on the exocrine pancreas is poorly understood.RESEARCH DESIGN AND METHODSWe assessed whether the antidiabetic agents exendin (Ex)-4, liraglutide, the dipeptidyl peptidase-4 inhibitor sitagliptin, or the biguanide metformin were associated with changes in expression of genes associated with the development of experimental pancreatitis. The effects of Ex-4 when administered before or after the initiation of caerulein-induced experimental pancreatitis were determined. The importance of endogenous GLP-1R signaling for gene expression in the exocrine pancreas and the severity of pancreatitis was assessed in Glp1r−/− mice.RESULTSAcute administration of Ex-4 increased expression of egr-1 and c-fos in the exocrine pancreas. Administration of Ex-4 or liraglutide for 1 week increased pancreas weight and induced expression of mRNA transcripts encoding the anti-inflammatory proteins pancreatitis-associated protein (PAP) (RegIIIβ) and RegIIIα. Chronic Ex-4 treatment of high-fat–fed mice increased expression of PAP and reduced pancreatic expression of mRNA transcripts encoding for the proinflammatory monocyte chemotactic protein-1, tumor necrosis factor-α, and signal transducer and activator of transcription-3. Sitagliptin and metformin did not significantly change pancreatic gene expression profiles. Ex-4 administered before or after caerulein did not modify the severity of experimental pancreatitis, and levels of pancreatic edema and serum amylase were comparable in caerulein-treated Glp1r−/− versus Glp1r+/+ mice.CONCLUSIONSThese findings demonstrate that GLP-1 receptor activation increases pancreatic mass and selectively modulates the expression of genes associated with pancreatitis. However, activation or genetic elimination of GLP-1R signaling does not modify the severity of experimental pancreatitis in mice.

The glucagon-like peptide-1 (GLP-1) receptor and the glucose-dependent insulinotropic polypeptide (GIP) receptor transduce nutrient-stimulated signals to control beta cell function. Although the GLP-1 receptor (GLP-1R) is a validated drug target for diabetes, the importance of the GIP receptor (GIPR) for the function of beta cells remains uncertain. We demonstrate that mice with selective ablation of GIPR in beta cells (MIP-Cre:Gipr(Flox/Flox); Gipr(-/-βCell)) exhibit lower levels of meal-stimulated insulin secretion, decreased expansion of adipose tissue mass and preservation of insulin sensitivity when compared to MIP-Cre controls. Beta cells from Gipr(-/-βCell) mice display greater sensitivity to apoptosis and markedly lower islet expression of T cell-specific transcription factor-1 (TCF1, encoded by Tcf7), a protein not previously characterized in beta cells. GIP, but not GLP-1, promotes beta cell Tcf7 expression via a cyclic adenosine monophosphate (cAMP)-independent and extracellular signal-regulated kinase (ERK)-dependent pathway. Tcf7 (in mice) or TCF7 (in humans) levels are lower in islets taken from diabetic mice and in humans with type 2 diabetes; knockdown of TCF7 in human and mouse islets impairs the cytoprotective responsiveness to GIP and enhances the magnitude of apoptotic injury, whereas restoring TCF1 levels in beta cells from Gipr(-/-βCell) mice lowers the number of apoptotic cells compared to that seen in MIP-Cre controls. Tcf7(-/-) mice show impaired insulin secretion, deterioration of glucose tolerance with either aging and/or high-fat feeding and increased sensitivity to beta cell injury relative to wild-type (WT) controls. Hence the GIPR-TCF1 axis represents a potential therapeutic target for preserving both the function and survival of vulnerable, diabetic beta cells.

Glucagon-like peptide-1 (GLP-1) circulates at low levels and acts as an incretin hormone, potentiating glucose-dependent insulin secretion from islet β cells. GLP-1 also modulates gastric emptying and engages neural circuits in the portal region and CNS that contribute to GLP-1 receptor-dependent (GLP-1R-dependent) regulation of glucose homeostasis. To elucidate the importance of pancreatic GLP-1R signaling for glucose homeostasis, we generated transgenic mice that expressed the human GLP-1R in islets and pancreatic ductal cells (Pdx1-hGLP1R:Glp1r -/-mice). Transgene expression restored GLP-1R-dependent stimulation of cAMP and Akt phosphorylation in isolated islets, conferred GLP-1R-dependent stimulation of β cell proliferation, and was sufficient for restoration of GLP-1-stimulated insulin secretion in perifused islets. Systemic GLP-1R activation with the GLP-1R agonist exendin-4 had no effect on food intake, hindbrain c-fos expression, or gastric emptying but improved glucose tolerance and stimulated insulin secretion in Pdx1-hGLP1R:Glp1r -/-mice. i.c.v. GLP-1R blockade with the antagonist exendin (9-39) impaired glucose tolerance in WT mice but had no effect in Pdx1-hGLP1R:Glp1r -/-mice. Nevertheless, transgenic expression of the pancreatic GLP-1R was sufficient to normalize both oral and i.p. glucose tolerance in Glp1r -/-mice. These findings illustrate that low levels of endogenous GLP-1 secreted from gut endocrine cells are capable of augmenting glucoregulatory activity via pancreatic GLP-1Rs independent of communication with neural pathways.

OBJECTIVE-We examined whether chronic administration of a glucagon-like peptide 1 (GLP-1) receptor agonist exendin-4 (Ex-4), a glucose-dependent insulinotropic polypeptide (GIP) receptor agonist D-Ala 2 -GIP (DA-GIP), or a dipeptidyl peptidase-4 (DPP-4) inhibitor (DPP-4i) des-fluoro-sitagliptin produced comparable antidiabetic actions in high fat-fed mice.RESEARCH DESIGN AND METHODS-High fat-fed mice were administered twice-daily injections of Ex-4, DA-GIP, vehicle (saline), or vehicle with the addition of des-fluoro-sitagliptin (DPP-4i) in food to produce sustained inhibition of DPP-4 activity.RESULTS AND CONCLUSIONS-Mice treated with vehicle alone or DA-GIP exhibited progressive weight gain, whereas treatment with Ex-4 or DPP-4i prevented weight gain. Although Ex-4 improved oral glucose tolerance and insulin-to-glucose ratios after an intraperitoneal glucose tolerance test (IPGTT), DPP-4i had no significant effect after IPGTT but improved glucose excursion and insulin levels after an oral glucose tolerance test. The extent of improvement in glycemic control was more sustained with continuous DPP-4 inhibition, as evidenced by loss of glucose control evident 9 h after peptide administration and a significant reduction in A1C observed with DPP-4i but not with DA-GIP or Ex-4 therapy. DA-GIP, but not Ex-4 or DPP-4i, was associated with impairment in insulin sensitivity and increased levels of plasma leptin and resistin. Although none of the therapies increased ␤-cell mass, only Ex-4 -treated mice exhibited increased pancreatic mRNA transcripts for Irs2, Egfr, and Gck. These findings highlight significant differences between pharmacological administration of incretin receptor agonists and potentiation of endogenous GLP-1 and GIP via DPP-4 inhibition. Diabetes 57: [190][191][192][193][194][195][196][197][198] 2008 T he gastrointestinal tract plays a key role in the control of food digestion, nutrient absorption, and energy assimilation (1). It is increasingly recognized that specialized enteroendocrine cells, distributed along the length of the small and large intestine, contribute to the control of energy homeostasis by secreting hormones important for regulation of satiety, gut motility, and pancreatic islet function. Two of these gut hormones, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1), subserve important roles as incretins, gut-derived peptides that augment insulin secretion from the ␤-cell after enteral nutrient ingestion (2). The interest in the actions of these incretin hormones has increased considerably in recent years following clinical evidence suggesting that incretinbased therapies may be useful for the treatment of human subjects with type 2 diabetes (3,4).GLP-1 receptor (GLP-1R) agonists control glycemia not only via stimulation of insulin secretion but also through inhibition of glucagon secretion and reduction of the rate of gastric emptying. Moreover, chronic administration of GLP-1R agonists may improve ␤-cell function in preclinical studies via enhanc...

Disordered glucagon secretion contributes to the symptoms of diabetes, and reduced glucagon action is known to improve glucose homeostasis. In mice, genetic deletion of the glucagon receptor (Gcgr) results in increased levels of the insulinotropic hormone glucagon-like peptide 1 (GLP-1), which may contribute to the alterations in glucose homeostasis observed in Gcgr -/-mice. Here, we assessed the contribution of GLP-1 receptor (GLP-1R) signaling to the phenotype of Gcgr -/-mice by generating Gcgr -/-Glp1r -/-mice. Although insulin sensitivity was similar in all genotypes, fasting glucose was increased in Gcgr -/-Glp1r -/-mice. Elimination of the Glp1r normalized gastric emptying and impaired intraperitoneal glucose tolerance in Gcgr -/-mice. Unexpectedly, deletion of Glp1r in Gcgr -/-mice did not alter the improved oral glucose tolerance and increased insulin secretion characteristic of that genotype. Although Gcgr -/-Glp1r -/-islets exhibited increased sensitivity to the incretin glucose-dependent insulinotropic polypeptide (GIP), mice lacking both Glp1r and the GIP receptor (Gipr) maintained preservation of the enteroinsular axis following reduction of Gcgr signaling. Moreover, Gcgr -/-Glp1r -/-islets expressed increased levels of the cholecystokinin A receptor (Cckar) and G protein-coupled receptor 119 (Gpr119) mRNA transcripts, and Gcgr -/-Glp1r -/-mice exhibited increased sensitivity to exogenous CCK and the GPR119 agonist AR231453. Our data reveal extensive functional plasticity in the enteroinsular axis via induction of compensatory mechanisms that control nutrient-dependent regulation of insulin secretion.

Background/Objectives:Dietary guidelines for the past 20 years have recommended that dietary fat should be minimized. In contrast, recent studies have suggested that there could be some potential benefits for reducing carbohydrate intake in favor of increased fat. It has also been suggested that low-carbohydrate diets be recommended for people with type 2 diabetes. However, whether such diets can improve glycemic control will likely depend on their ability to improve β-cell function, which has not been studied. The objective of the study was to assess whether a low-carbohydrate and therefore high-fat diet (LCHFD) is beneficial for improving the endogenous insulin secretory response to glucose in prediabetic New Zealand Obese (NZO) mice.Methods:NZO mice were maintained on either standard rodent chow or an LCHFD from 6 to 15 weeks of age. Body weight, food intake and blood glucose were assessed weekly. Blood glucose and insulin levels were also assessed after fasting and re-feeding and during an oral glucose tolerance test. The capacity of pancreatic β-cells to secrete insulin was assessed in vivo with an intravenous glucose tolerance test. β-Cell mass was assessed in histological sections of pancreata collected at the end of the study.Results:In NZO mice, an LCHFD reduced plasma triglycerides (P=0.001) but increased weight gain (P<0.0001), adipose tissue mass (P=0.0015), high-density lipoprotein cholesterol (P=0.044) and exacerbated glucose intolerance (P=0.013). Although fasting insulin levels tended to be higher (P=0.08), insulin secretory function in LCHFD-fed mice was not improved (P=0.93) nor was β-cell mass (P=0.75).Conclusions:An LCHFD is unlikely to be of benefit for preventing the decline in β-cell function associated with the progression of hyperglycemia in type 2 diabetes.

In type 2 diabetes, increased endogenous glucose production (EGP) as a result of elevated gluconeogenesis contributes to hyperglycemia. An increase in glycerol gluconeogenesis has led to the suggestion that, in obese human subjects with type 2 diabetes, there may be an increase in the activity of the gluconeogenic enzyme fructose-1,6-bisphosphatase (FBPase). The aim of this study was to generate transgenic mice that overexpress human liver FBPase in the liver and assess the consequences to whole-body and hepatic glucose metabolism. FBPase transgenic mice had significantly higher levels of transgene expression in the liver and, as a result, had increased FBPase protein and enzyme activity levels in the liver. This resulted in an increase in the rate of glycerol conversion to glucose but not in EGP. The increased expression of FBPase in the liver did not result in any significant differences compared with littermate control mice in insulin or glucose tolerance. Therefore, it appears that, on its own, an increase in FBPase does not lead to impaired regulation of EGP and hence does not affect whole-body glucose metabolism. This suggests that, for EGP to be increased, other factors associated with obesity are also required.