Exendin(9-39)amide is an antagonist of glucagon-like peptide-1(7-36)amide in humans

Indirect effects of glucagon-like peptide-1 receptor agonist exendin-4 on the peripheral circadian clocks in mice

Circadian clocks in peripheral tissues are powerfully entrained by feeding. The mechanisms underlying this food entrainment remain unclear, although various humoral and neural factors have been reported to affect peripheral clocks. Because glucagon-like peptide-1 (GLP-1), which is rapidly secreted in response to food ingestion, influences multiple humoral and neural signaling pathways, we suggest that GLP-1 plays a role in the food entrainment of peripheral clocks. To test this, we compared the effects of exendin-4, a GLP-1 receptor agonist, on mRNA expression of the clock genes (Clock, Bmal1, Nr1d1, Per1, Per2, and Cry1) with those of refeeding. In addition, we investigated whether exendin-4 could affect the rhythms of the peripheral clocks. In male C57BL/6J mice, although refeeding rapidly (within 2 h) altered mRNA levels of Per1 and Per2 in the liver and that of Per1 in adipose tissue, a single i.p. injection of exendin-4 did not cause such changes. However, unlike the GLP-1 receptor antagonist exendin-(9-39), exendin-4 significantly influenced Per1 mRNA levels in the liver at 12 h after injection. Moreover, pretreatment with exendin-4 affected the rapid-feeding-induced change in Per1 not only in the liver, but also in adipose tissue, without effect on food intake. Furthermore, during light-phase restricted feeding, repeated dosing of exendin-4 at the beginning of the dark phase profoundly influenced both the food intake and daily rhythms of clock gene expression in peripheral tissues. Thus, these results suggest that exendin-4 modulates peripheral clocks via multiple mechanisms different from those of refeeding.

Functional activity of murine intestinal mucosal cells is regulated by the glucagon-like peptide-1 receptor

To determine whether the glucagon-like peptide-1 receptor (GLP-1r) plays a role in the regulation of intestinal functional activity, we analyzed the distribution of the GLP-1r in mouse tissues and tested if tissues expressing the receptor respond to exendin-4 and exendin (9-39) amide, a GLP-1r agonist and antagonist respectively. In ileum, Glp1r mRNA level was two fold higher in extracts from epithelial cells than non-epithelial tissues. By immunohistochemistry, the receptor was localized to the mucosal cell layer of villi of ileum and colon, to the myenteric and submucosal plexus and to Paneth cells. Intravenous administration of exendin-4 to CD-1 mice induced expression of the immediate early gene c-fos in mucosal cells but not in cells of the enteric plexuses or in L cells of ileum. The induction of c-fos was inhibited by the voltage-gated sodium channel blocker tetrodotoxin. Exendin-4 also increased c-fos expression in ileal segments in vitro, suggesting that this action of the analog was independent of an extrinsic input. The induction of c-fos expression by exendin-4 was inhibited by exendin (9-39) amide, indicating that the action of exendin-4 was mediated by activation of the receptor. Our findings indicate that the GLP-1r is involved in ileal enterocyte and Paneth cell function, that the GLP-1 analog activates c-fos expression in the absence of an extrinsic input and that some of the actions of the receptor is/are mediated by voltage-gated Na channels.

Physiology of incretins and loss of incretin effect in type 2 diabetes and obesity

An important role in the regulation of glucose homeostasis is played by incretins, which are gut-derived hormones released in response to nutrient ingestion. In humans, the major incretin hormones are glucagon-like peptide (GLP)-1 and glucose-dependent insulinotropic polypeptide (GIP), and together they fully account for the incretin effect (that is, higher insulin release in response to an oral glucose challenge compared to an equal intravenous glucose load). Studies have shown that GLP-1 and GIP levels and actions may be perturbed in disease states, and the loss of incretin effect is likely to contribute importantly to the postprandial hyperglycaemia in type 2 diabetes. However, the specific cause-effect relationship between disease and incretins is still unclear. This review focuses on several key studies elucidating the association of defective incretin action with obesity and T2DM and the effects of metformin and other anti-diabetic agents on the incretin system.

Exaggerated glucagon-like peptide 1 response is important for improved β-cell function and glucose tolerance after Roux-en-Y gastric bypass in patients with type 2 diabetes

β-Cell function improves in patients with type 2 diabetes in response to an oral glucose stimulus after Roux-en-Y gastric bypass (RYGB) surgery. This has been linked to the exaggerated secretion of glucagon-like peptide 1 (GLP-1), but causality has not been established. The aim of this study was to investigate the role of GLP-1 in improving β-cell function and glucose tolerance and regulating glucagon release after RYGB using exendin(9-39) (Ex-9), a GLP-1 receptor (GLP-1R)-specific antagonist. Nine patients with type 2 diabetes were examined before and 1 week and 3 months after surgery. Each visit consisted of two experimental days, allowing a meal test with randomized infusion of saline or Ex-9. After RYGB, glucose tolerance improved, β-cell glucose sensitivity (β-GS) doubled, the GLP-1 response greatly increased, and glucagon secretion was augmented. GLP-1R blockade did not affect β-cell function or meal-induced glucagon release before the operation but did impair glucose tolerance. After RYGB, β-GS decreased to preoperative levels, glucagon secretion increased, and glucose tolerance was impaired by Ex-9 infusion. Thus, the exaggerated effect of GLP-1 after RYGB is of major importance for the improvement in β-cell function, control of glucagon release, and glucose tolerance in patients with type 2 diabetes.

Direct effects of exendin-(9,39) and GLP-1-(9,36)amide on insulin action, β-cell function, and glucose metabolism in nondiabetic subjects

Exendin-(9,39) is a competitive antagonist of glucagon-like peptide-1 (GLP-1) at its receptor. However, it is unclear if it has direct and unique effects of its own. We tested the hypothesis that exendin-(9,39) and GLP-1-(9,36)amide have direct effects on hormone secretion and β-cell function as well as glucose metabolism in healthy subjects. Glucose containing [3-(3)H]glucose was infused to mimic the systemic appearance of glucose after a meal. Saline, GLP-1-(9,36)amide, or exendin-(9,39) at 30 pmol/kg/min (Ex 30) or 300 pmol/kg/min (Ex 300) were infused in random order on separate days. Integrated glucose concentrations were slightly but significantly increased by exendin-(9,39) (365 ± 43 vs. 383 ± 35 vs. 492 ± 49 vs. 337 ± 50 mmol per 6 h, saline, Ex 30, Ex 300, and GLP-1-[9,36]amide, respectively; P = 0.05). Insulin secretion did not differ among groups. However, insulin action was lowered by exendin-(9,39) (25 ± 4 vs. 20 ± 4 vs. 18 ± 3 vs. 21 ± 4 10(-4) dL/kg[min per μU/mL]; P = 0.02), resulting in a lower disposition index (DI) during exendin-(9,39) infusion (1,118 ± 118 vs. 816 ± 83 vs. 725 ± 127 vs. 955 ± 166 10(-14) dL/kg/min(2) per pmol/L; P = 0.003). Endogenous glucose production and glucose disappearance did not differ significantly among groups. We conclude that exendin-(9,39), but not GLP-1-(9,36)amide, decreases insulin action and DI in healthy humans.

Liraglutide, a long-acting GLP-1 mimetic, and its metabolite attenuate inflammation after intracerebral hemorrhage

The inflammatory response plays a pivotal role in propagating injury of intracerebral hemorrhage (ICH). Glucagon-like-peptide-1 (GLP-1) is a hormone with antidiabetic effect and may also have antiinflammatory properties. Despite consensus that the glucoregulatory action is mediated by the GLP-1 receptor (GLP-1R), mechanisms in the brain remain unclear. We investigated the effect of a long-acting GLP-1 analog, liraglutide, and its truncated metabolite, GLP-1(9-36)a from dipeptidyl peptidase-4 (DPP-4) cleavage in ICH-induced brain injury. Primary outcomes were cerebral edema formation, neurobehavior, and inflammatory parameters. GLP-1(9-36)a, GLP-1R inhibitor, adenosine monophosphate-activated protein kinase (AMPK) phosphorylation inhibitor and DPP-4 inhibitor were administered to examine the mechanisms of action. Liraglutide suppressed neuroinflammation, prevented brain edema and neurologic deficit following ICH, which were partially reversed by GLP-1R inhibitor and AMPK phosphorylation inhibitor. Liraglutide-mediated AMPK phosphorylation was unaffected by GLP-1R inhibitor, and was found to be induced by GLP-1(9-36)a. GLP-1(9-36)a showed salutary effects on primary outcomes that were reversed by AMPK phosphorylation inhibitor but not by GLP-1R inhibitor. Liraglutide and DPP-4 inhibitor co-administration reversed liraglutide-mediated AMPK phosphorylation and antiinflammatory effects. Liraglutide exerted duals actions and the antiinflammatory effects are partially mediated by its metabolite in a phosphorylated AMPK-dependent manner. Therapies that inhibit GLP-1 degradation may weaken the metabolite-mediated effects.

Comparison of Bariatric Surgical Procedures for Diabetes Remission: Efficacy and Mechanisms

Bariatric surgery induces a mean weight loss of 15-30% of initial body weight (depending on the procedure), as well as a 45-95% rate of diabetes remission. Procedures that induce greater weight loss are associated with higher rates of diabetes remission. Improvements in glucose homeostasis after bariatric surgery are likely mediated by a combination of caloric restriction (followed by weight loss) and the effects of altered gut anatomy on the secretion of glucoregulatory gut hormones.

GLP-1 receptor antagonist exendin-(9-39) elevates fasting blood glucose levels in congenital hyperinsulinism owing to inactivating mutations in the ATP-sensitive K+ channel

Infants with congenital hyperinsulinism owing to inactivating mutations in the K(ATP) channel (K(ATP)HI) who are unresponsive to medical therapy will require pancreatectomy to control the hypoglycemia. In preclinical studies, we showed that the GLP-1 receptor antagonist exendin-(9-39) suppresses insulin secretion and corrects fasting hypoglycemia in SUR-1(-/-) mice. The aim of this study was to examine the effects of exendin-(9-39) on fasting blood glucose in subjects with K(ATP)HI. This was a randomized, open-label, two-period crossover pilot clinical study. Nine subjects with K(ATP)HI received either exendin-(9-39) or vehicle on two different days. The primary outcome was blood glucose; secondary outcomes were insulin, glucagon, and GLP-1. In all subjects, mean nadir blood glucose and glucose area under the curve were significantly increased by exendin-(9-39). Insulin-to-glucose ratios were significantly lower during exendin-(9-39) infusion compared with vehicle. Fasting glucagon and intact GLP-1 were not affected by treatment. In addition, exendin-(9-39) significantly inhibited amino acid-stimulated insulin secretion in pancreatic islets isolated from neonates with K(ATP)HI. Our findings have two important implications: 1) GLP-1 and its receptor play a role in the regulation of fasting glycemia in K(ATP)HI; and 2) the GLP-1 receptor may be a therapeutic target for the treatment of children with K(ATP)HI.

Possible role of GLP-1 and its agonists in the treatment of type 1 diabetes mellitus

Unfortunately, the only approved medical treatment for type 1 diabetes mellitus (DM) is insulin, despite the fact that tight control cannot be reached without some serious side effects such as hypoglycemia and weight gain. More and more importance is now shifted towards developing new drugs that can reach a better glycemic control with lesser side effects. Some of these promising drugs are the glucagon-like peptides 1 (GLP-1) and their agonists, which have been FDA approved for the treatment of type 2 DM. The purpose of this article is to review all of the relevant literature on the potential role of GLP-1 in the treatment of type 1 DM. The major source of data acquisition included Medline search strategies, using the words "type 1 diabetes mellitus" and "GLP-1." Articles published in the last 20 years were screened. GLP-1 increases insulin secretion in humans with existing beta cells; it also decreases glucagon secretion, and blunts appetite. Of note, new animal studies demonstrate a role in beta cell-proliferation and decreased apoptosis. Because of all the effects mentioned above, GLP-1 seems to be a promising drug for type 1 DM treatment, but more studies are still needed before solid conclusions can be drawn.

The role of endogenous incretin secretion as amplifier of glucose-stimulated insulin secretion in healthy subjects and patients with type 2 diabetes

In order to quantify the role of incretins in first- and second-phase insulin secretion (ISR) in type 2 diabetes mellitus (T2DM), a double-blind, randomized study with 12 T2DM subjects and 12 healthy subjects (HS) was conducted using the hyperglycemic clamp technique together with duodenal nutrition perfusion and intravenous infusion of the glucagon-like peptide 1 (GLP-1) receptor antagonist exendin(9-39). Intravenous glucose alone resulted in a significantly greater first- and second-phase ISR in HS compared with T2DM subjects. Duodenal nutrition perfusion augmented both first- and second-phase ISR but first-phase ISR more in T2DM subjects (approximately eight- vs. twofold). Glucose-related stimulation of ISR contributed only 20% to overall ISR. Infusion with exendin(9-39) significantly reduced first- and second-phase ISR in both HS and T2DM subjects. Thus, both GLP-1 and non-GLP-1 incretins contribute to the incretin effect. In conclusion, both phases of ISR are impaired in T2DM. In particular, the responsiveness to glucose in first-phase ISR is blunted. GLP-1 and glucose-dependent insulinotropic polypeptide (GIP) secretions are unaltered. The absolute incretin effect is reduced in T2DM; its relative importance, however, appears to be increased, highlighting its role as an important amplifier of first-phase ISR in T2DM.

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

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

Biophysical and pharmacological properties of glucagon-like peptide-1 in rats under isoflurane anesthesia

BACKGROUND

Glucagon-like peptide-1 (GLP-1) increases insulin secretion and has an important role in maintaining glucose homeostasis. In this study, we evaluated the biophysical and pharmacological properties of GLP-1 by performing in vivo and in vitro experiments to determine the applicability of GLP-1 in glycemic control in rats under isoflurane anesthesia.

METHODS

Levels of portal GLP-1, insulin, and glucose and dipeptidyl peptidase-4 activity were measured in the basal fasting state and after gastric glucose load before, during, and after exposure to 30% O(2) in air (control) or 1.4% isoflurane in a mixture of 30% O(2) and air. The direct effects of isoflurane on GLP-1 secretion were assessed in human enteroendocrine NCI-H716 cells. Insulin release from isolated pancreatic islets was measured using a radioimmunoassay. Single pancreatic β-cell membrane potentials were recorded using whole-cell current-clamp patches perforated by β-escin.

RESULTS

In fasting rats, inhalation of isoflurane led to a decrease in the basal levels of GLP-1 but did not affect insulin and glucose levels. Levels of GLP-1, insulin, and glucose increased after gastric administration of glucose in control rats. However, isoflurane attenuated the glucose-induced increase in GLP-1 and insulin levels and increased plasma glucose levels. In contrast, isoflurane did not affect dipeptidyl peptidase-4 activity before or after gastric glucose loading. Isoflurane (0.35 mM) inhibited GLP-1 release in NCI-H716 cells; this finding was similar to that observed in in vivo studies. In perifusion experiments, isoflurane (0.35 mM) inhibited glucose-induced insulin release, whereas exogenous GLP-1 (10 nM) enhanced insulin release. Importantly, combined administration of isoflurane and GLP-1 enhanced both phases of glucose-induced insulin release to an extent similar to that achieved with GLP-1 alone. Whole-cell patches showed that exposure to GLP-1 (10 nM) led to nearly complete restoration of glucose-stimulated depolarization that had been suppressed by isoflurane (0.35 mM).

CONCLUSIONS

GLP-1 secretion is impaired during isoflurane anesthesia. However, our study showed that the insulinotropic action of GLP-1 was not affected by isoflurane. Furthermore, exposure to GLP-1 increased the membrane activity of pancreatic β-cells, preventing isoflurane-induced impairment of glucose-induced insulin secretion. These results support the hypothesis that GLP-1-based therapy may be a useful approach for achieving intraoperative glycemic control.

Chitosan-based therapeutic nanoparticles for combination gene therapy and gene silencing of in vitro cell lines relevant to type 2 diabetes

Glucagon like peptide 1 (GLP-1), a blood glucose homeostasis modulating incretin, has been proposed for the treatment of type 2 diabetes mellitus (T2DM). However, native GLP-1 pharmacokinetics reveals low bioavailability due to degradation by the ubiquitous dipeptydil peptidase IV (DPP-IV) endoprotease. In this study, the glucosamine-based polymer chitosan was used as a cationic polymer-based in vitro delivery system for GLP-1, DPP-IV resistant GLP-1 analogues and siRNA targeting DPP-IV mRNA. We found chitosans to form spherical nanocomplexes with these nucleic acids, generating two distinct non-overlapping size ranges of 141-283 nm and 68-129 nm for plasmid and siRNA, respectively. The low molecular weight high DDA chitosan 92-10-5 (degree of deacetylation, molecular weight and N:P ratio (DDA-Mn-N:P)) showed the highest plasmid DNA transfection efficiency in HepG2 and Caco-2 cell lines when compared to 80-10-10 and 80-80-5 chitosans. Recombinant native GLP-1 protein levels in media of transfected cells reached 23 ng/L while our DPP-IV resistant analogues resulted in a fivefold increase of GLP-1 protein levels (115 ng/L) relative to native GLP-1, and equivalent to the Lipofectamine positive control. We also found that all chitosan-DPP-IV siRNA nanocomplexes were capable of DPP-IV silencing, with 92-10-5 being significantly more effective in abrogating enzymatic activity of DPP-IV in media of silenced cells, and with no apparent cytotoxicity. These results indicate that specific chitosan formulations may be effectively used for the delivery of plasmid DNA and siRNA in a combination therapy of type 2 diabetes.

Long-term dipeptidyl-peptidase 4 inhibition reduces atherosclerosis and inflammation via effects on monocyte recruitment and chemotaxis

BACKGROUND

Dipeptidyl-peptidase 4 (DPP-4) inhibitors are increasingly used to accomplish glycemic targets in patients with type II diabetes mellitus. Because DPP-4 is expressed in inflammatory cells, we hypothesized that its inhibition will exert favorable effects in atherosclerosis.

METHODS AND RESULTS

Male LDLR(-/-) mice (6 weeks) were fed a high-fat diet or normal chow diet for 4 weeks and then randomized to vehicle or alogliptin, a high-affinity DPP-4 inhibitor (40 mg · kg(-1) · d(-1)), for 12 weeks. Metabolic parameters, blood pressure, vascular function, atherosclerosis burden, and indexes of inflammation were obtained in target tissues, including the vasculature, adipose, and bone marrow, with assessment of global and cell-specific inflammatory pathways. In vitro and in vivo assays of DPP-4 inhibition (DPP-4i) on monocyte activation/migration were conducted in both human and murine cells and in a short-term ApoE(-/-) mouse model. DPP-4i improved markers of insulin resistance and reduced blood pressure. DPP-4i reduced visceral adipose tissue macrophage content (adipose tissue macrophages; CD11b(+), CD11c(+), Ly6C(hi)) concomitant with upregulation of CD163. DPP-4 was highly expressed in bone marrow-derived CD11b(+) cells, with DPP-4i downregulating proinflammatory genes in these cells. DPP-4i decreased aortic plaque with a striking reduction in plaque macrophages. DPP-4i prevented monocyte migration and actin polymerization in in vitro assays via Rac-dependent mechanisms and prevented in vivo migration of labeled monocytes to the aorta in response to exogenous tumor necrosis factor-α and DPP-4.

CONCLUSION

DPP-4i exerts antiatherosclerotic effects and reduces inflammation via inhibition of monocyte activation/chemotaxis. These findings have important implications for the use of this class of drugs in atherosclerosis.

Contributions of fat and protein to the incretin effect of a mixed meal

BACKGROUND

The relative contributions of fat and protein to the incretin effect are still largely unknown.

OBJECTIVE

This study assessed the incretin effects elicited by a mixed meal, and by its fat and protein components alone, with the use of a hyperglycemic clamp combined with oral nutrients.

DESIGN

Eight healthy volunteers were studied over 6 h after ingestion of a sandwich containing 1) dried meat, butter, and white bread; 2) dried meat alone; 3) butter alone; or 4) no meal (fasting control). Meals were ingested during a hyperglycemic clamp, and the incretin effect was calculated as the increment in plasma insulin after food intake relative to the concentrations observed during the control study.

RESULTS

A significant augmentation of postprandial insulin secretion, independent of plasma glycemia, occurred after ingestion of the mixed nutrients and the lipid component of the mixed meal (203 ± 20.7% and 167.4 ± 22.9% of control, respectively; both P < 0.05), whereas the protein component did not induce a significant incretin effect (129.0 ± 7.9% of control; P = 0.6)

CONCLUSIONS

Fat ingestion, in an amount typical of a standard meal, increases insulin secretion during physiologic hyperglycemia and thus contributes to the incretin effect. In contrast, ingestion of protein typical of normal meals does not contribute to the augmentation of postprandial insulin secretion. This trial was registered at clinicaltrials.gov as NCT00869453.

The role of dysregulated glucagon secretion in type 2 diabetes

Excessive production of glucose by the liver contributes to fasting and postprandial hyperglycaemia, hallmarks of type 2 diabetes. A central feature of this pathologic response is insufficient hepatic insulin action, due to a combination of insulin resistance and impaired β-cell function. However, a case can be made that glucagon also plays a role in dysregulated hepatic glucose production and abnormal glucose homeostasis. Plasma glucagon concentrations are inappropriately elevated in diabetic individuals, and α-cell suppression by hyperglycaemia is blunted. Experimental evidence suggests that this contributes to greater rates of hepatic glucose production in the fasting state and attenuated reduction after meals. Recent studies in animal models indicate that reduction of glucagon action can have profound effects to mitigate hyperglycaemia even in the face of severe hypoinsulinaemia. While there are no specific treatments for diabetic patients yet available that act specifically on the glucagon signalling pathway, newer agents including glucagon-like peptide-1 (GLP-1) receptor agonists and dipeptidyl peptidase-4 (DPP-4) inhibitors reduce plasma glucagon and this is thought to contribute to their action to lower blood glucose. The α-cell and glucagon receptor remain tempting targets for novel diabetes treatments, but it is important to understand the magnitude of benefit new strategies would provide as preclinical models suggest that chronic interference with glucagon action could entail adverse effects as well.

Gastric bypass surgery enhances glucagon-like peptide 1-stimulated postprandial insulin secretion in humans

OBJECTIVE

Gastric bypass (GB) surgery is associated with postprandial hyperinsulinemia, and this effect is accentuated in postsurgical patients who develop recurrent hypoglycemia. Plasma levels of the incretin glucagon-like peptide 1 (GLP-1) are dramatically increased after GB, suggesting that its action contributes to alteration in postprandial glucose regulation. The aim of this study was to establish the role of GLP-1 on insulin secretion in patients with GB.

RESEARCH DESIGN AND METHODS

Twelve asymptomatic individuals with previous GB (Asym-GB), 10 matched healthy nonoperated control subjects, and 12 patients with recurrent hypoglycemia after GB (Hypo-GB) had pre- and postprandial hormone levels and insulin secretion rates (ISR) measured during a hyperglycemic clamp with either GLP-1 receptor blockade with exendin-(9-39) or saline.

RESULTS

Blocking the action of GLP-1 suppressed postprandial ISR to a larger extent in Asym-GB individuals versus control subjects (33 ± 4 vs.16 ± 5%; P = 0.04). In Hypo-GB patients, GLP-1 accounted for 43 ± 4% of postprandial ISR, which was not significantly higher than that in Asym-GB subjects (P = 0.20). Glucagon was suppressed similarly by hyperglycemia in all groups but rose significantly after the meal in surgical individuals but remained suppressed in nonsurgical subjects. GLP-1 receptor blockade increased postprandial glucagon in both surgical groups.

CONCLUSIONS

Increased GLP-1-stimulated insulin secretion contributes significantly to hyperinsulinism in GB subjects. However, the exaggerated effect of GLP-1 on postprandial insulin secretion in surgical subjects is not significantly different in those with and without recurrent hypoglycemia.

Peripheral and central GLP-1 receptor populations mediate the anorectic effects of peripherally administered GLP-1 receptor agonists, liraglutide and exendin-4

The long-acting glucagon-like peptide-1 receptor (GLP-1R) agonists, exendin-4 and liraglutide, suppress food intake and body weight. The mediating site(s) of action for the anorectic effects produced by peripheral administration of these GLP-1R agonists are not known. Experiments addressed whether food intake suppression after i.p. delivery of exendin-4 and liraglutide is mediated exclusively by peripheral GLP-1R or also involves direct central nervous system (CNS) GLP-1R activation. Results showed that CNS delivery [third intracerebroventricular (3(rd) ICV)] of the GLP-1R antagonist exendin-(9-39) (100 μg), attenuated the intake suppression by i.p. liraglutide (10 μg) and exendin-4 (3 μg), particularly at 6 h and 24 h. Control experiments show that these findings appear to be based neither on the GLP-1R antagonist acting as a nonspecific competing orexigenic signal nor on blockade of peripheral GLP-1R via efflux of exendin-(9-39) to the periphery. To assess the contribution of GLP-1R expressed on subdiaphragmatic vagal afferents to the anorectic effects of liraglutide and exendin-4, food intake was compared in rats with complete subdiaphragmatic vagal deafferentation and surgical controls after i.p. delivery of the agonists. Both liraglutide and exendin-4 suppressed food intake at 3 h, 6 h, and 24 h for controls; for subdiaphragmatic vagal deafferentation rats higher doses of the GLP-1R agonists were needed for significant food intake suppression, which was observed at 6 h and 24 h after liraglutide and at 24 h after exendin-4.

CONCLUSION

Food intake suppression after peripheral administration of exendin-4 and liraglutide is mediated by activation of GLP-1R expressed on vagal afferents as well as direct CNS GLP-1R activation.

The Current Status of Exenatide Once Weekly

Type 2 diabetes mellitus (T2DM) is a chronic, progressive metabolic disorder that is associated with long-term microvascular (retinopathy, neuropathy and nephropathy) and macrovascular (myocardial infarction, stroke, peripheral arterial disease) complications. Both the prevalence of T2DM and the cost of its long-term complications have driven the focus and emphasis on treatments aimed at reducing hyperglycemia and controlling hypertension and dyslipidemia while minimizing hypoglycemia and weight gain. Exenatide twice daily, the first GLP-1R agonist approved by the US Food and Drug Administration (FDA) and European Medicines Agency (EMEA), has been shown to reduce hemoglobin A1C, lower fasting and postprandial plasma blood glucose concentrations as well as reduce body weight without causing significant hypoglycemia. However, its current formulation requires twice daily subcutaneous injections and does not provide continuous GLP-1R activation. Therefore, a long-acting release form of exenatide has been developed for use as a once-weekly injection, providing for convenient administration and continuous GLP-1R activation. This review covers the currently published data on this new formulation including mechanism of action, pharmacokinetics, efficacy and comparison trials to other commonly used anti-diabetic agents.

Antidiabetic actions of endogenous and exogenous GLP-1 in type 1 diabetic patients with and without residual β-cell function

OBJECTIVE

To investigate the effect of exogenous as well as endogenous glucagon-like peptide 1 (GLP-1) on postprandial glucose excursions and to characterize the secretion of incretin hormones in type 1 diabetic patients with and without residual β-cell function.

RESEARCH DESIGN AND METHODS

Eight type 1 diabetic patients with (T1D+), eight without (T1D-) residual β-cell function, and eight healthy matched control subjects were studied during a mixed meal with concomitant infusion of GLP-1 (1.2 pmol/kg/min), saline, or exendin 9-39 (300 pmol/kg/min). Before the meal, half dose of usual fast-acting insulin was injected. Plasma glucose (PG), glucagon, C-peptide, total GLP-1, intact glucose-dependent insulinotropic polypeptide (GIP), free fatty acids, triglycerides, and gastric emptying rate (GE) by plasma acetaminophen were measured.

RESULTS

Incretin responses did not differ between patients and control subjects. Infusion of GLP-1 decreased peak PG by 45% in both groups of type 1 diabetic patients. In T1D+ patients, postprandial PG decreased below fasting levels and was indistinguishable from control subjects infused with saline. In T1D- patients, postprandial PG remained at fasting levels. GLP-1 infusion reduced GE and glucagon levels in all groups and increased fasting C-peptide in T1D+ patients and control subjects. Blocking endogenous GLP-1 receptor action increased endogenous GLP-1 secretion in all groups and increased postprandial glucose, glucagon, and GE in T1D+ and T1D- patients. The insulinogenic index (the ratio of insulin to glucose) decreased in T1D+ patients during blockade of endogenous GLP-1 receptor action.

CONCLUSIONS

Type 1 diabetic patients have normal incretin responses to meals. In type 1 diabetic patients, exogenous GLP-1 decreases peak postprandial glucose by 45% regardless of residual β-cell function. Endogenous GLP-1 regulates postprandial glucose excursions by modulating glucagon levels, GE, and β-cell responsiveness to glucose. Long-term effects of GLP-1 in type 1 diabetic patients should be investigated in future clinical trials.

Inhibition of DPP-4 with vildagliptin improved insulin secretion in response to oral as well as "isoglycemic" intravenous glucose without numerically changing the incretin effect in patients with type 2 diabetes

BACKGROUND AND AIMS

Dipeptidyl peptidase-4 (DPP-4) inhibitors block the degradation of glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide. The aim of the present study was to quantitatively assess the incretin effect after treatment with the DPP-4 inhibitor vildagliptin (V) or placebo (P) in patients with type 2 diabetes.

MATERIALS AND METHODS

Twenty-one patients (three women, 18 men) with type 2 diabetes previously treated with metformin (mean age, 59 yr; body mass index, 28.6 kg/m(2); glycosylated hemoglobin, 7.3%) were studied in a two-period crossover design. They received 100 mg V once daily or P for 13 d in randomized order. The incretin effect was measured on d 12 (75-g oral glucose) and d 13 ("isoglycemic" iv glucose) based on insulin and C-peptide determinations and insulin secretion rates (ISR).

RESULTS

V relative to P treatment significantly increased intact incretin concentrations after oral glucose and insulin secretory responses to both oral glucose and isoglycemic iv glucose (e.g. AUC(ISR oral), by 32.7%, P = 0.0006; AUC(ISR iv), by 33.1%, P = 0.01). The numerical incretin effect was not changed (IE(ISR), V vs. P, 35.7 ± 4.9 and 34.6 ± 4.0%, P = 0.80).

CONCLUSIONS

DPP-4 inhibition augmented insulin secretory responses both after oral glucose and during isoglycemic iv glucose infusions, with no net change in the incretin effect. Thus, slight variations in basal incretin levels may be more important than previously thought. Or, DPP-4 inhibitor-induced change in the incretin-related environment of islets may persist overnight, augmenting insulin secretory responses to iv glucose as well. Alternatively, yet unidentified mediators of DPP-4 inhibition may have caused these effects.

Involvement of endogenous glucagon-like peptide-1 in regulation of gastric motility and pancreatic endocrine secretion

OBJECTIVE

To study the role of endogenous glucagon-like peptide-1 (GLP-1) on gastric emptying rates of a solid meal as well as postprandial hormone secretion and glucose disposal.

MATERIAL AND METHODS

In nine healthy subjects, gastric emptying of a 310-kcal radio-labelled solid meal and plasma concentrations of insulin, glucagon and glucose were measured during infusion of saline or the GLP-1 receptor antagonist exendin(9-39)amide (Ex(9-39)) at 300 pmol·kg(-1)·min(-1).

RESULTS

Ex(9-39) infusion had no effect on the total gastric emptying curve, but changed the intra-gastric distribution of the meal. During infusion of Ex(9-39), more content stayed in the upper stomach (79.1 ± 2.5% of total during Ex(9-39) compared to 66.6 ± 5.7% during saline at 5 min). During Ex(9-39) infusion, higher concentrations of plasma glucagon were measured both before (after 40 min of Ex(9-39) infusion the glucagon level was 15.1 ± 0.7 pmol·L(-1) compared to 5.4 ± 1.4 during saline) and after the meal, and postprandial GLP-1 levels increased. Basal insulin and glucose levels were not affected by Ex(9-39), but the postprandial rise of insulin and glucose enhanced during Ex(9-39).

CONCLUSIONS

Endogenous GLP-1 is involved in the regulation of gastric motility in relation to meal intake and also in the regulation of postprandial insulin and glucose levels. Furthermore, endogenous GLP-1 seems to tonically restrain glucagon secretion.

A novel human-based receptor antagonist of sustained action reveals body weight control by endogenous GLP-1

Ex-4 (9-39)a is a well characterized GLP-1 receptor antagonist that suffers from two notable limitations, its nonhuman amino acid sequence and its relatively short in vivo duration of action. Comparable N-terminal shortening of human GLP-1 lessens agonism but does not provide a high potency antagonist. Through a series of GLP-1/Ex-4 hybrid peptides, the minimal structural changes required to generate a pure GLP-1-based antagonist were identified as Glu16, Val19, and Arg20, yielding an antagonist of approximately 3-fold greater in vitro potency compared with Ex-4 (9-39)a. The structural basis of antagonism appears to result from stabilization of the α helix combined with enhanced electrostatic and hydrophobic interactions with the extracellular domain of the receptor. Site-specific acylation of the human-based antagonist yielded a peptide of increased potency as a GLP-1 receptor antagonist and 10-fold greater selectivity relative to the GIP receptor. The acylated antagonist demonstrated sufficient duration of action to maintain inhibitory activity when administered as a daily subcutaneous injection. The sustained pharmacokinetics and enhanced human sequence combine to form an antagonist optimized for clinical study. Daily administration of this antagonist by subcutaneous injection to diet-induced obese mice for 1 week caused a significant increase in food intake, body weight, and glucose intolerance, demonstrating endogenous GLP-1 as a relevant hormone in mammalian energy balance in the obese state.

Incretin-based therapies for type 2 diabetes mellitus: properties, functions, and clinical implications

The incretin hormones, glucose-dependent insulinotropic polypeptide (GIP) and glucagonlike peptide-1 (GLP-1), which are secreted by cells of the gastrointestinal tract in response to meal ingestion, exercise important glucoregulatory effects, including the glucose-dependent potentiation of insulin secretion by pancreatic β-cells. Research on the defective incretin action in type 2 diabetes mellitus suggests that the observed loss of insulinotropic activity may be due primarily to a decreased responsiveness of β-cells to GIP. GLP-1 does retain efficacy, albeit not at physiologic levels. Accordingly, augmentation of GLP-1 is a logical therapeutic strategy to ameliorate this deficiency, although the short metabolic half-life of the native hormone renders direct infusion impractical. GLP-1 receptor agonists that resist degradation by the enzyme dipeptidyl peptidase-4 (DPP-4) and have protracted-action kinetics have been developed, and DPP-4 inhibitors that slow the enzymatic cleavage of native GLP-1 provide alternative approaches to enhancing incretin-mediated glucose control. However, GLP-1 receptor agonists and DPP-4 inhibitors are premised on highly divergent mechanisms of action. DPP-4 is ubiquitously expressed in many tissues and is involved in a wide range of physiologic processes in addition to its physiologic influence on incretin hormone biological activity. GLP-1 receptor agonists provide a pharmacologic level of GLP-1 receptor stimulation, whereas DPP-4 inhibitors appear to increase levels of circulating GLP-1 to within the physiologic range. This article examines the physiology of the incretin system, mechanistic differences between GLP-1 receptor agonists and DPP-4 inhibitors used as glucose-lowering agents in the treatment of type 2 diabetes, and the implications of these differences for treatment. The results of recent head-to-head trials are reviewed, comparing the effects of incretin-based therapies on a range of clinical parameters, including glycemia, β-cell function, weight, and cardiovascular function.

Add-on therapies to metformin for type 2 diabetes

IMPORTANCE OF THE FIELD

As of 2010, approximately 285 million people worldwide have diabetes; that number is estimated to increase to 439 million by 2030. The majority of these individuals (> 90%) have type 2 diabetes, a chronic and progressive disease.

AREAS COVERED IN THIS REVIEW

Metformin monotherapy is a safe and effective option. However, its effects on glycemia are typically of limited durability. Progressive loss of β-cell function and failure of metformin monotherapy to control glucose adequately prompt the addition of other oral antidiabetic drugs, such as sulfonylureas and thiazolidinediones, which have their own limitations. Evidence suggests that incretin-based agents can successfully achieve glycemic control while potentially providing cardiovascular and β-cell-function benefits.

WHAT THE READER WILL GAIN

Knowledge of the available clinical evidence on the incretin-based therapies and other pharmacotherapeutic options for patients with type 2 diabetes who fail first-line therapy with metformin, through an analysis of improved glycemic parameters and overall risk:benefit profiles.

TAKE HOME MESSAGE

Traditional oral antidiabetic agents, recommended as first- and second-line therapies in patients with type 2 diabetes inadequately controlled with diet/exercise or monotherapy, have limited durability of effect and are associated with an increased risk of adverse events. Glucagon-like peptide-1 receptor agonists and dipeptidyl peptidase-4 inhibitors provide glycemic control and are promising additions to the pharmacotherapeutic armamentarium.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

The contribution of enteroinsular hormones to the pathogenesis of type 2 diabetes mellitus

The gastrointestinal hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), termed incretins, are essential regulators of normal glucose homeostasis. Research indicates that the incretin effect is impaired in type 2 diabetes, and this seems to be a consequence rather than a cause of type 2 diabetes. This review describes the defects in the incretin system seen in diabetic patients and discusses the potential roles of GIP and GLP-1 in the pathogenesis of type 2 diabetes. In addition, new information on clinical applications that exploit the enteroinsular axis to control blood glucose is discussed.

Taspoglutide: a long acting human glucagon-like polypeptide-1 analogue

BACKGROUND

Taspoglutide (R1583/BIM51077) is a new anti diabetic drug from Hoffmann-La Roche. The compound is to be administered as a subcutaneous injection once weekly and is also effective given bi-weekly. It is a long acting 10% formulation of (Aib 8-35) human glucagon-like polypeptide-1 (7 - 36 amides) with 93% homology with the native polypeptide. It activates the glucagon-like polypeptide-1 receptor. Phase III trials are currently in process.

OBJECTIVE

To provide a critical review of taspoglutide based on available published data.

METHODS

Information provided from the search on Internet has been reviewed. A clinical interpretation is given on a background of practical experience as an investigator in a clinical trial with taspoglutide.

RESULTS

Search on PubMed, EMBASE and Google gave hits on six clinical studies investigating taspoglutide of which the largest accounted for > 50% of the total study population. In addition, some animal studies were identified. Significant improvement on glucose control as well as several metabolic parameters has been shown with taspoglutide.

DISCUSSION

Data from the clinical trials are interpreted in a medical context. The prospects of taspoglutide in the treatment of diabetes type 2 and metabolic syndrome are discussed.

CONCLUSION

Taspoglutide is a new activator of the glucagon-like polypeptide-1 receptor. It is effective when injected once weekly and less effective when injected bi-weekly. In addition to its anti diabetic properties, taspoglutide has favorable effects on body weight and significantly reduces three of five diagnostic criteria for metabolic syndrome, namely glucose, waist circumference and fasting triglyceride.

Incretin-based therapies: new treatments for type 2 diabetes in the new millennium

The advent of 'incretin-based therapies' - GLP-1 agonists and dipeptidyl-peptidase-4 inhibitors - which result in improvements in glycemic control comparable to those with existing oral hypoglycemic agents, and potentially improve cardiovascular and pancreatic beta-cell function, represents a major therapeutic advance in the management of type 2 diabetes. Gastrointestinal adverse effects occur commonly with GLP-1 agonists, and rarely with DPP-4 inhibitors, but are dose-dependent and usually transient. The low risk of hypoglycemia, and beneficial or neutral effects on body weight, render GLP-1 agonists and DPP-4 inhibitors suitable alternatives to insulin secretagogues and insulin in overweight and elderly patients. Incretin-based therapies also improve quality of life in patients with type 2 diabetes, and may be cost-effective in the long term.

GLP-1 regulates gastroduodenal motility involving cholinergic pathways

The gut-born incretin hormone glucagon-like peptide-1 (GLP-1) delays gastric emptying. To elucidate the mechanisms by which GLP-1 affects gastroduodenal motility and glycaemia, we studied the effects of exendin(9-39), a potent GLP-1 receptor antagonist, on gastroduodenal motility and pancreatic hormones. In this randomized, double-blind, placebo-controlled, four-arm, cross-over trial, 10 healthy volunteers were studied during the interdigestive period followed by duodenal perfusion of a mixed liquid meal (250 kcal). On four separate days, exendin(9-39), atropine, exendin(9-39) + atropine or saline were infused intravenously. Antro-pyloro-duodenal and fundic motility were assessed. The compliance of the proximal stomach was determined by isobaric distensions. During fasting, exendin(9-39) did not influence proximal gastric volume, pyloric tone, and duodenal contractility. Exendin(9-39) significantly increased antral waves only in the absence of atropine. During duodenal meal perfusion, exendin(9-39) significantly reduced proximal gastric volume accommodation, abbreviated postprandial antral inhibition, reduced the postprandial increase in pyloric tone, and reduced gastric compliance. Atropine abolished the effects of exendin(9-39) on gastric volume accommodation but did not affect its effects on postprandial antroduodenal motility and on gastric compliance. Exendin(9-39) increased fasting and postprandial glycaemia and plasma glucagon but not insulin concentrations. Atropine did not affect GLP-1 secretion. Cholinergic mechanisms mediate the effects of GLP-1 on postprandial gastric accommodation but not on antro-pyloro-duodenal motility. GLP-1 reduces fasting and postprandial glycaemia, in part by reducing glucagon secretion.

Dose-dependent effects of cholecystokinin-8 on antropyloroduodenal motility, gastrointestinal hormones, appetite, and energy intake in healthy men

CCK mediates the effects of nutrients on gastrointestinal motility and appetite. Intravenously administered CCK stimulates pyloric pressures, increases plasma PYY, and suppresses ghrelin, all of which may be important in the regulation of appetite and energy intake. The dose-related effects of exogenous CCK on gastrointestinal motility and gut hormone release, and the relationships between these effects and those on energy intake, are uncertain. We hypothesized that 1) intravenous CCK-8 would have dose-dependent effects on antropyloroduodenal (APD) pressures, plasma PYY and ghrelin concentrations, appetite, and energy intake and 2) the suppression of energy intake by CCK-8 would be related to the stimulation of pyloric motility. Ten healthy men (age 26 +/- 2 yr) were studied on four separate occasions in double-blind, randomized fashion. APD pressures, plasma PYY and ghrelin, and appetite were measured during 120-min intravenous infusions of 1) saline ("control") or 2) CCK-8 at 0.33 ("CCK0.33"), 3) 0.66 ("CCK0.66"), or 4) 2.0 ("CCK2.0") ng.kg(-1).min(-1). After 90 min, energy intake at a buffet meal was quantified. CCK-8 dose-dependently stimulated phasic and tonic pyloric pressures and plasma PYY concentrations (r > 0.70, P < 0.05) and reduced desire to eat and energy intake (r > -0.60, P < 0.05) without inducing nausea. There were relationships between basal pyloric pressure and isolated pyloric pressure waves (IPPW) with plasma CCK (r > 0.50, P < 0.01) and between energy intake with IPPW (r = -0.70, P < 0.05). Therefore, our study demonstrates that exogenous CCK-8 has dose-related effects on APD motility, plasma PYY, desire to eat, and energy intake and suggests that the suppression of energy intake is related to the stimulation of IPPW.

Regulation of islet hormone release and gastric emptying by endogenous glucagon-like peptide 1 after glucose ingestion

BACKGROUND

Exogenous administration of glucagon-like peptide (GLP)-1 improves glucose tolerance by stimulation of insulin secretion, inhibition of glucagon secretion, and delay of gastric emptying. It is not known which of these effects is involved in the action of endogenous GLP-1 to control blood glucose. To determine the role of endogenous GLP-1 on islet cell function and gastric emptying independent of variable glycemia, we clamped blood glucose before and during glucose ingestion with and without GLP-1 receptor blockade with exendin-[9-39] (Ex-9).

METHODS

There were 10 healthy subjects that participated in two experiments each, one a control and one with infusion of 750 pm/kg . min Ex-9. Subjects consumed 75 g oral glucose solution mixed with d-xylose and (13)C-glucose while their blood glucose levels were held fixed at approximately 8.9 mmol/liter.

RESULTS

Plasma insulin levels during hyperglycemia alone were similar in the two studies (control, 282.5 +/- 42 vs. Ex-9, 263.8 +/- 59 pmol/liter) but were reduced by approximately 30% by Ex-9 after glucose ingestion (control, 1154 +/- 203 vs. Ex-9, 835 +/- 120 pmol/liter; P < 0.05). Blocking the action of endogenous GLP-1 caused an approximate 80% increase in postprandial glucagon concentrations. The appearance of ingested d-xylose in the blood was not affected by Ex-9, suggesting that postprandial secretion of GLP-1 has only minimal effects on gastric emptying of oral glucose.

CONCLUSIONS

These findings indicate that GLP-1 is an incretin in healthy humans at modestly supraphysiological blood glucose levels. The primary effect of GLP-1 to regulate oral glucose tolerance is mediated by effects on islet hormones and not on gastric emptying.

Exenatide once weekly versus twice daily for the treatment of type 2 diabetes: a randomised, open-label, non-inferiority study

BACKGROUND

Exenatide is an incretin mimetic that shares glucoregulatory properties with glucagon-like peptide 1 (GLP-1), and improves glycaemic control, with progressive bodyweight reductions, when administered twice a day in patients with type 2 diabetes. We compared the efficacy of a once-weekly formulation of exenatide to that of a twice daily dose.

METHODS

A 30-week, randomised, non-inferiority study compared a long-acting release formulation of exenatide 2 mg administered once weekly to 10 mug exenatide administered twice a day, in 295 patients with type 2 diabetes (haemoglobin A(1c) [HbA(1c)] 8.3% [SD 1.0], mean fasting plasma glucose 9 [SD 2] mmol/L, weight 102 [SD 20] kg, diabetes duration 6.7 [SD 5.0] years). The patients were naive to drug therapy, or on one or more oral antidiabetic agents. The primary endpoint was the change in HbA(1c) at 30 weeks. This study is registered with ClinicalTrials.gov, number NCT00308139.

FINDINGS

At 30 weeks, the patients given exenatide once a week had significantly greater changes in HbA(1c) than those given exenatide twice a day (-1.9 [SE 0.1%] vs -1.5 [0.1%], 95% CI -0.54% to -0.12%; p=0.0023). A significantly greater proportion of patients receiving treatment once a week versus twice a day achieved target HbA(1c) levels of 7.0% or less (77%vs 61% of evaluable patients, p=0.0039).

INTERPRETATION

Exenatide once weekly resulted in significantly greater improvements in glycaemic control than exenatide given twice a day, with no increased risk of hypoglycaemia and similar reductions in bodyweight.

Exenatide can reduce glucose independent of islet hormones or gastric emptying

Exenatide is a long-acting glucagon-like peptide-1 (GLP-1) mimetic used in the treatment of type 2 diabetes. There is increasing evidence that GLP-1 can influence glycemia not only via pancreatic (insulinotropic and glucagon suppression) and gastric-emptying effects, but also via an independent mechanism mediated by portal vein receptors. The aim of our study was to investigate whether exenatide has an islet- and gastric-independent glycemia-reducing effect, similar to GLP-1. First, we administered mixed meals, with or without exenatide (20 microg sc) to dogs. Second, to determine whether exenatide-induced reduction in glycemia is independent of slower gastric emptying, in the same animals we infused glucose intraportally (to simulate meal test glucose appearance) with exenatide, exenatide + the intraportal GLP-1 receptor antagonist exendin-(9-39), or saline. Exenatide markedly decreased postprandial glucose: net 0- to 135-min area under the curve = +526 +/- 315 and -536 +/- 197 mg.dl(-1).min(-1) with saline and exenatide, respectively (P < 0.05). Importantly, the decrease in plasma glucose occurred without a corresponding increase in postprandial insulin but was accompanied by delayed gastric emptying and lower glucagon. Significantly lower glycemia was induced by intraportal glucose infusion with exenatide than with saline (92 +/- 1 vs. 97 +/- 1 mg/dl, P < 0.001) in the absence of hyperinsulinemia or glucagon suppression. The exenatide-induced lower glycemia was partly reversed by intraportal exendin-(9-39): 95 +/- 3 and 92 +/- 3 mg/dl with exenatide + antagonist and exenatide, respectively (P < 0.01). Our results suggest that, similar to GLP-1, exenatide lowers glycemia via a novel mechanism independent of islet hormones and slowing of gastric emptying. We hypothesize that receptors in the portal vein, via a neural mechanism, increase glucose clearance independent of islet hormones.

Islet cell function: alpha and beta cells--partners towards normoglycaemia

Under normal conditions, insulin and glucagon are counter-regulatory hormones whose balanced action exhibits a relationship that ensures normoglycaemia. Elevated glucose levels following a meal stimulate pancreatic islet beta cells to secrete insulin and islet alpha cells to downregulate production of glucagon. With declining glucose and insulin levels, alpha-cell production of glucagon is increased to stimulate hepatic glucose production, preventing fasting hypoglycaemia. In type 2 diabetes mellitus (T2DM), beta-cell insulin response to glucose is blunted, including absence of early acute response, and alpha-cell response to glucose is impaired, resulting in absolute or relative hyperglucagonaemia and inappropriate hepatic glucose output that contributes to fasting hyperglycaemia. These changes are associated with structural and functional changes in pancreatic islets, including reduced beta-cell mass and reduced beta-cell:alpha-cell ratio. The role of the incretin hormone glucagon-like peptide-1 (GLP-1) in regulating glucose-dependent beta-cell insulin production and glucose-dependent alpha-cell glucagon production has been used to develop GLP-1-based therapies. These therapies may reduce the imbalances among insulin and glucagon that characterise T2DM, resulting in improved glycaemic control.

Overview of glucagon-like peptide-1 analogs and dipeptidyl peptidase-4 inhibitors for type 2 diabetes

CONTEXT

Impairment of incretin activity is now recognized as integral to the metabolic derangement underlying type 2 diabetes. Glucoregulatory agents that target the incretin system have recently been developed, and the place of these drugs in the treatment of type 2 diabetes can be assessed based on a growing body of clinical data.

EVIDENCE ACQUISITION

A PubMed search was conducted to identify clinical studies of incretin therapies in type 2 diabetes. Article reference lists were also searched for relevant information, and supplemental material such as conference abstracts, drug prescribing information, and treatment guidelines were included as appropriate.

EVIDENCE SYNTHESIS

Two classes of therapies target the incretin system. The first, glucagon-like peptide-1 (GLP-1) agonists (exemplified by exenatide and liraglutide), have demonstrated considerable efficacy in clinical trials, reducing hemoglobin A1c (HbA1c) by up to 1.3%, decreasing fasting and postprandial glucose concentrations, reducing weight by approximately 3.0 kg, and improving cardiovascular risk factors. The second class, the dipeptidyl peptidase-4 inhibitors (such as sitagliptin and vildagliptin) rely on production of endogenous GLP-1 and act by reducing its turnover. The dipeptidyl peptidase-4 (DPP-4) inhibitors produce modest reductions in HbA1c (< 1%) compared with GLP-1 agonists and are generally weight-neutral. Neither GLP-1 agonists nor DPP-4 inhibitors cause hypoglycemia unless used with other agents known to increase risk.

CONCLUSIONS

GLP-1 agonists and DPP-4 inhibitors provide a valuable new treatment option for patients with type 2 diabetes and may be associated with a wider range of therapeutic benefits than older drugs.

Exendin-(9-39) corrects fasting hypoglycemia in SUR-1-/- mice by lowering cAMP in pancreatic beta-cells and inhibiting insulin secretion

Congenital hyperinsulinism is a disorder of pancreatic beta-cell function characterized by failure to suppress insulin secretion in the setting of hypoglycemia, resulting in brain damage or death if untreated. Loss-of-function mutations in the K(ATP) channel (composed of two subunits: Kir6.2 and SUR-1) are responsible for the most common and severe form of congenital hyperinsulinism. Most patients are unresponsive to available medical therapy and require palliative pancreatectomy. Similar to the human condition, the SUR-1(-/-) mouse is hypoglycemic when fasted and hyperglycemic when glucose-loaded. We have previously reported that the glucagon-like peptide-1 receptor antagonist exendin-(9-39) raises fasting blood glucose in normal mice. Here we examine the effect of exendin-(9-39) on fasting blood glucose in SUR-1(-/-) mice. Mice were randomized to receive exendin-(9-39) or vehicle. Fasting blood glucose levels in SUR-1(-/-) mice treated with exendin-(9-39) were significantly higher than in vehicle-treated mice and not different from wild-type littermates. Exendin-(9-39) did not further worsen glucose tolerance and had no effect on body weight and insulin sensitivity. Isolated islet perifusion studies demonstrated that exendin-(9-39) blocked amino acid-stimulated insulin secretion, which is abnormally increased in SUR-1(-/-) islets. Furthermore, cAMP content in SUR-1(-/-) islets was reduced by exendin-(9-39) both basally and when stimulated by amino acids, whereas cytosolic calcium levels were not affected. These findings suggest that cAMP plays a key role in K(ATP)-independent insulin secretion and that the GLP-1 receptor is constitutively active in SUR-1(-/-) beta-cells. Our findings indicate that exendin-(9-39) normalizes fasting hypoglycemia in SUR-1(-/-) mice via a direct effect on insulin secretion, thereby raising exendin-(9-39) as a potential therapeutic agent for K(ATP) hyperinsulinism.

GLP-1 (9-36) amide, cleavage product of GLP-1 (7-36) amide, is a glucoregulatory peptide

OBJECTIVE

Glucagon-like peptide-1 (GLP-1) (7-36) amide is a glucoregulatory hormone with insulinotropic and insulinomimetic actions. We determined whether the insulinomimetic effects of GLP-1 are mediated through its principal metabolite, GLP-1 (9-36) amide (GLP-1m).

METHODS AND PROCEDURES

Glucose turnover during two, 2-h, euglycemic clamps was measured in 12 lean and 12 obese (BMI <25 or >30 kg/m(2)) male and female subject volunteers with normal oral glucose tolerance test. Saline or GLP-1m were infused from 0 to 60 min in each study. Additionally, seven lean and six obese subjects underwent a third clamp in which the GLP-1 receptor (GLP-1R) antagonist, exendin (9-39) amide was infused from -60 to 60 min with GLP-1m from 0 to 60 min.

RESULTS

No glucose infusion was required in lean subjects to sustain euglycemia (glucose clamp) during saline or GLP-1m infusions. However, in obese subjects glucose infusion was necessary during GLP-1m infusion alone in order to compensate for a marked (>50%) inhibition of hepatic glucose production (HGP). Plasma insulin levels remained constant in lean subjects but rose significantly in obese subjects after termination of the peptide infusions. During GLP-1R blockade, infusion of glucose was immediately required upon starting GLP-1m infusions in all subjects due to a more dramatic reduction in HGP, as well as a delayed and modest insulinotropic response.

DISCUSSION

We conclude that GLP-1m potently inhibits HGP and is a weak insulinotropic agent. These properties are particularly apparent and pronounced in obese but only become apparent in lean subjects during GLP-1 (7-36) receptor blockade. These previously unrecognized antidiabetogenic actions of GLP-1m may have therapeutic usefulness.

Glucose homeostasis and the gastrointestinal tract: insights into the treatment of diabetes

The gastrointestinal tract is increasingly viewed as a critical organ in glucose metabolism because of its role in delivering glucose to the circulation and in secreting multiple glucoregulatory hormones that, in concert with insulin and glucagon, regulate glucose homeostasis. Under normal conditions, a complex interplay of these hormones acts to maintain plasma glucose within a narrow range despite large variations in the availability of glucose, particularly during transition from the fasting to fed state. In the fed state, the rate at which nutrients are passed from the stomach to the duodenum, termed gastric emptying rate, is a key determinant of postprandial glucose flux. In patients with diabetes, the regulation of glucose metabolism is disrupted resulting in fasting and postprandial hyperglycaemia. Elucidation of the role of the gastrointestinal tract, gut-derived glucoregulatory peptides and gastric emptying rate offers a new perspective on glucose homeostasis and the respective importance of these factors in the diabetes state. This review will highlight the importance of the gastrointestinal tract in playing a key role in glucose homeostasis, particularly in the postprandial period, and the role of established or new therapies that either leverage or modify gastrointestinal function to improve glycaemic state.

Glucagon-like peptide-1 (GLP-1) receptors expressed on nerve terminals in the portal vein mediate the effects of endogenous GLP-1 on glucose tolerance in rats

Glucagon-like peptide-1 (GLP-1) is an intestinal hormone that is secreted during meal absorption and is essential for normal glucose homeostasis. However, the relatively low plasma levels and rapid metabolism of GLP-1 raise questions as to whether direct endocrine action on target organs, such as islet cells, account for all of its effects on glucose tolerance. Recently, an alternative neural pathway initiated by sensors in the hepatic portal region has been proposed to mediate GLP-1 activity. We hypothesized that visceral afferent neurons in the portal bed express the GLP-1 receptor (GLP-1r) and regulate glucose tolerance. Consistent with this hypothesis, GLP-1r mRNA was present in the nodose ganglia, and nerve terminals innervating the portal vein contained the GLP-1r. Rats given an intraportal infusion of the GLP-1r antagonist, [des-His(1),Glu(9)] exendin-4, in a low dose, had glucose intolerance, with a 53% higher glucose excursion compared with a vehicle-infused control group. Infusion of [des-His(1),Glu(9)] exendin-4 at an identical rate into the jugular vein had no effect on glucose tolerance, demonstrating that this dose of GLP-1r antagonist did not affect blood glucose due to spillover into the systemic circulation. These studies demonstrate that GLP-1r are present on nerve terminals in the hepatic portal bed and that GLP-1 antagonism localized to this region impairs glucose tolerance. These data are consistent with an important component of neural mediation of GLP-1 action.

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.

Intravenous CCK-8, but not GLP-1, suppresses ghrelin and stimulates PYY release in healthy men

We have investigated the effects of exogenous CCK-8 and GLP-1, alone and in combination, on ghrelin and PYY secretion. Nine healthy males were studied on four occasions. Plasma ghrelin and PYY concentrations were measured during 150 min intravenous infusions of: (i) isotonic saline, (ii) CCK-8 at 1.8 pmol/kg/min, (iii) GLP-1 at 0.9 pmol/kg/min or (iv) CCK-8 and GLP-1 combined. CCK-8 markedly suppressed ghrelin and stimulated PYY when compared with control between t=0-120 min (P<0.001 for both). GLP-1 had no effect on ghrelin, but decreased PYY slightly at 120 min (P<0.05). During infusion of CCK-8+GLP-1, there was comparable suppression of ghrelin (P<0.001), but the stimulation of PYY was less (P<0.001), than that induced by CCK-8, between t=20-120 min. In conclusion, in healthy subjects, in the doses evaluated, exogenous CCK-8 suppresses ghrelin and stimulates PYY, and exogenous GLP-1 has no effect on ghrelin and attenuates the effect of CCK-8 on PYY.

The incretin system: glucagon-like peptide-1 receptor agonists and dipeptidyl peptidase-4 inhibitors in type 2 diabetes

Glucagon-like peptide 1 (GLP-1) is a gut-derived incretin hormone that stimulates insulin and suppresses glucagon secretion, inhibits gastric emptying, and reduces appetite and food intake. Therapeutic approaches for enhancing incretin action include degradation-resistant GLP-1 receptor agonists (incretin mimetics), and inhibitors of dipeptidyl peptidase-4 (DPP-4) activity (incretin enhancers). Clinical trials with the incretin mimetic exenatide (two injections per day or long-acting release form once weekly) and liraglutide (one injection per day) show reductions in fasting and postprandial glucose concentrations, and haemoglobin A1c (HbA1c) (1-2%), associated with weight loss (2-5 kg). The most common adverse event associated with GLP-1 receptor agonists is mild nausea, which lessens over time. Orally administered DPP-4 inhibitors, such as sitagliptin and vildagliptin, reduce HbA1c by 0.5-1.0%, with few adverse events and no weight gain. These new classes of antidiabetic agents, and incretin mimetics and enhancers, also expand beta-cell mass in preclinical studies. However, long-term clinical studies are needed to determine the benefits of targeting the incretin axis for the treatment of type 2 diabetes.

Gastrointestinal satiety signals in humans — Physiologic roles for GLP-1 and PYY ?

The present review summarizes the appetite suppressing effects of PYY and GLP-1 in the regulation of food intake in humans. Current evidence supports a role for gastrointestinal peptides as regulators of satiety. The regulation of satiety is, however, complex and it is not surprising that multiple control systems exist. It is interesting to note that nutrients in the small intestine such as hydrolysis products of fat stimulate the release of satiety peptides such as GLP-1 or PYY that serve as satiety signals. Both peptides, released from L-cells from the gastrointestinal tract by the local action of digested food, exert various regulatory functions: stimulation of insulin secretion and inhibition of glucagon secretion as typical actions of GLP-1, inhibition of gastric emptying, and inhibition of appetite for both GLP-1 and PYY. The review focuses on the question, whether the two peptides are true endocrine factors that act as physiologic, hormonal regulators of appetite.