Glucagon-like peptide 1 and gastric inhibitory polypeptide: potential applications in type 2 diabetes mellitus

A Quantitative Systems Pharmacology Model of the Incretin Hormones GIP and GLP1, Glucagon, Glucose, Insulin, and the Small Molecule DPP-4 Inhibitor, Linagliptin

In the current study, we established a comprehensive quantitative systems pharmacology (QSP) model using linagliptin as the model drug, where drug disposition, drug intervention on dipeptidyl peptidase-4 (DPP-4), glucose-dependent insulinotropic peptide (GIP), Glucagon-like peptide-1 (GLP-1), glucagon, glucose, and insulin are integrated together with the cross talk and feedback loops incorporated among the whole glycemic control system. In the final linagliptin QSP model, the complicated disposition of linagliptin was characterized by a 2-compartment pharmacokinetic (PK) model with an enterohepatic cycling (EHC) component as well as target-mediated drug disposition (TMDD) processes occurring in both tissues and plasma, and the inhibitory effect of linagliptin on DPP-4 was determined by the linagliptin-DPP-4 complex in the central compartment based on target occupancy principle. The integrated GIP-GLP1-glucagon-glucose-insulin system contains five indirect response models as the "skeleton" structure with 12 feedback loops incorporated within the glucose control system. Our model adequately characterized the substantial nonlinear PK of linagliptin, time course of DPP-4 inhibition, as well as the kinetics of GIP, GLP-1, glucagon, and glucose simultaneously in humans. Our model provided valuable insights on linagliptin pharmacokinetics/pharmacodynamics and complicated glucose homeostasis. Since the glucose regulation modeling framework within the QSP model is "drug-independent", our model can be easily adopted by others to evaluate the effect of other DPP-4 inhibitors on the glucose control system. In addition, our QSP model, which contains more components than other reported glucose regulation models, can potentially be used to evaluate the effect of combination antidiabetic therapy targeting different components of glucose control system.

Weight Regain After Gastric Bypass: Influence of Gut Hormones

BACKGROUND

The Roux-en-Y gastric bypass (RYGB) is the gold standard bariatric operation. However, a major concern in late follow-up is the substantial weight regain. Understanding the role of gastrointestinal hormone secretion in this situation is relevant.

METHODS

The aim of the present study was to evaluate the influence of gastrointestinal hormones comparing postprandial secretion of ghrelin, glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide 1 (GLP-1), and leptin between patients with weight regain and those with favorable weight control. Twenty-four patients with follow-up from 27 to 59 months were divided into two groups according to sustained weight loss: group A (14 patients) had sustained weight losses, and group B (10 patients) had significant weight regain. Basal serum levels of ghrelin, GIP, GLP-1, and leptin after fasting and 30, 60, 90, and 120 min after a standard meal were measured.

RESULTS

There was no difference in the ghrelin secretion. There was a difference in the GIP secretion, with a higher percentage increase in 30 min in group A (330% × 192.2%; p = 0.01). There were also differences in the GLP-1 secretion, with higher increases in absolute (p = 0.03) and percentage values after 30 min in group A (124% × 46.5%; p = 0.01). There was also a difference between baseline leptin values, with higher levels in group B (p = 0.02).

CONCLUSIONS

The secretion of gut hormones in patients with weight regain after RYGB is different from that in patients with satisfactory weight outcome. After meal stimulation, reduced levels of GIP and GLP-1 may indicate the influence of gut hormones in the process of weight regain.

The Role of Vildagliptin in the Management of Type 2 Diabetes Mellitus

Objective:

To highlight the role of incretin hormones in the management of type 2 diabetes mellitus with a focus on vildagliptin, a dipeptidyl peptidase IV (DPP IV) inhibitor currently in development.

Data Sources:

Searches were conducted in MEDLINE (1950–April 2007) and International Pharmaceutical Abstracts (1970–April 2007) using the key words vildagliptin, LAF237, and dipeptidyl peptidase IV inhibitor. Additional data were obtained from abstracts presented at the American Diabetes Association Scientific Sessions (2003–2006) and from the manufacturer.

Study Selection and Data Extraction:

Articles pertaining to the pharmacology, pharmacokinetics, safety, and efficacy of vildagliptin for the treatment of type 2 diabetes were reviewed for inclusion. When available, human trials were included over animal studies.

Data Synthesis:

Reduced incretin effect is thought to be associated with type 2 diabetes. Glucagon-like peptide-1 (GLP-1), an incretin hormone, stimulates postprandial insulin release; however, it is rapidly degraded by DPP IV. Studies evaluating the use of vildagliptin in patients with type 2 diabetes found significant decreases in DPP IV and increased GLP-1 activity 45 minutes after dosing. Glucagon levels were reduced, with little to no change in insulin levels. With vildagliptin doses ranging from 25 mg daily to 100 mg twice daily, researchers observed consistent reductions in fasting plasma glucose, 4 hour postprandial glucose, and hemoglobin A1c. Similar benefits were seen when vildagliptin was used in combination with metformin. Vildagliptin was well tolerated after 12 weeks; however, incidences of hypoglycemia increased with longer study duration. Optimal results with minimal adverse effects were achieved with 25 mg twice daily and 50 mg once daily doses.

Conclusions:

Vildagliptin represents a safe and effective new approach to targeting GLP-1 deficiencies in patients with type 2 diabetes by inhibiting DPP IV.

Abundance and turnover of GLP-1 producing L-cells in ileal mucosa are not different in patients with and without type 2 diabetes

INTRODUCTION

The gastrointestinal hormone GLP-1 is released from enteroendocrine L-cells and augments postprandial insulin secretion. In patients with type 2 diabetes, the incretin effect is markedly diminished. It is unclear, whether this is due to a reduction in the abundance of L-cells in the intestine.

METHODS

Ileal tissue samples from 10 patients with and 10 patients without diabetes that underwent surgery for the removal of colon tumors were included. Tissue sections were stained for GLP-1, Ki67, TUNEL and chromogranin A.

RESULTS

The number of L-cells was not different between patients with and without diabetes in either crypts (1.81±0.21% vs. 1.49±0.24%, respectively; p=0.31) or villi (1.07±0.16% vs. 0.83±0.10%, respectively; p=0.23). L-cell number was higher in crypts than in villi (p<0.0001). L-cell replication was detected rarely and not different between the groups. L-cell apoptosis was similar in patients with and without diabetes in both crypts (7.84±2.77% vs. 8.65±3.77%, p=0.85) and villi (4.48±2.89% vs. 8.62±4.64%, p=0.42). Chromogranin A staining was found in a subset of L-cells only.

CONCLUSIONS

Intestinal L-cell density is higher in crypts than in villi. Chromogranin A is not a prerequisite for GLP-1 production. L-cell density and turnover are not different between patients with and without diabetes. Thus, alterations in the number of GLP-1 producing cells do not explain the reduced incretin effect in patients with type 2 diabetes.

In vitro and in vivo insulinotropic properties of the multifunctional frog skin peptide hymenochirin-1B: a structure-activity study

Hymenochirin-1b (Hym-1B; IKLSPETKDNLKKVLKGAIKGAIAVAKMV.NH2) is a cationic, α-helical amphibian host-defense peptide with antimicrobial, anticancer, and immunomodulatory properties. This study investigates the abilities of the peptide and nine analogues containing substitutions of Pro(5), Glu(6), and Asp(9) by either L-lysine or D-lysine to stimulate insulin release in vitro using BRIN-BD11 clonal β cells or isolated mouse islets and in vivo using mice fed a high-fat diet to produce obesity and insulin resistance. Hym-1B produced a significant and concentration-dependent increase in the rate of insulin release from BRIN-BD11 cells without cytotoxicity at concentrations up to 1 µM with a threshold concentration of 1 nM. The threshold concentrations for the analogues were: [P5K], [E6K], [D9K], [P5K, E6K] and [E6K, D9k] 0.003 nM, [E6K, D9K] and [D9k] 0.01 nM, [P5K, D9K] 0.1 nM and [E6k] 0.3 nM. All peptides displayed cytotoxicity at concentrations ≥1 µM except the [P5K] and [D9k] analogues which were non-toxic at 3 µM. The potency and maximum rate of insulin release from mouse islets produced by the [P5K] peptide were significantly greater than produced by Hym-1B. Neither Hym-1B nor the [P5K] analogue at 1 µM concentration had an effect on membrane depolarization or intracellular Ca(2+). The [P5K] analogue (1 µM) produced a significant increase in cAMP concentration in BRIN-BD11 cells and stimulated GLP-1 secretion from GLUTag cells. Down-regulation of the protein kinase A pathway by overnight incubation with forskolin completely abolished the insulin-releasing effects of [P5K]hym-1B. Intraperitoneal administration of the [P5K] and [D9k] analogues (75 nmol/kg body weight) to high-fat-fed mice with insulin resistance significantly enhanced glucose tolerance with a concomitant increase in insulin secretion. We conclude that [P5K]hym-1B and [D9k]hym-1B show potential for development into anti-diabetic agents.

Upper gastrointestinal motility and symptoms in individuals with diabetes, prediabetes and normal glucose tolerance

AIMS/HYPOTHESIS

Type 2 diabetes has been associated with upper gastrointestinal motility dysfunction, but the relationship with diabetes duration and glucose control is less well understood. Gastric emptying, oesophageal motility and gastrointestinal symptoms were examined in volunteers with diabetes, prediabetes (impaired fasting glucose [IFG] or impaired glucose tolerance [IGT]) and normal glucose tolerance (NGT).

METHODS

The study included 41 patients with type 2 diabetes, 17 individuals with IFG/IGT and 31 individuals with NGT. A gastric emptying breath test and high-resolution oesophageal manometry were performed. Gastrointestinal symptoms were assessed using questionnaires.

RESULTS

Gastric emptying was delayed in individuals with IFG/IGT (p < 0.05) but was normal in the diabetic group. Amongst the diabetic patients, gastric emptying rate was fastest in those with longer diabetes duration and the highest HbA1c levels (p < 0.001). Oesophageal motility variables were similar between the groups. However, the lower oesophagus resting pressure was reduced in patients with longer diabetes duration (p = 0.01). Abdominal pain/discomfort was more frequent amongst patients with diabetes (p = 0.04) but was unrelated to gastric emptying. Significant associations between various oesophageal motility variables and gastrointestinal symptoms were observed.

CONCLUSIONS/INTERPRETATION

Gastric emptying and oesophageal motility are not generally altered in patients with type 2 diabetes. In more advanced disease stages, however, gastric emptying and oesophageal motility may be disturbed, probably as a consequence of autonomic neuropathy. Delayed gastric emptying in IFG/IGT individuals might be secondary to acute hyperglycaemia. Determination of gastric emptying and oesophageal manometry should be considered for the diagnostic workup of patients with diabetes and gastrointestinal symptoms.

[Gastrointestinal hormones in food intake control]

The discovery of gut hormones regulating the energy balance has aroused great interest in the scientific community. Some of these hormones modulate appetite and satiety, acting on the hypothalamus or the solitary tract nucleus in the brainstem. In general, the endocrine signals generated in the gut have direct or indirect (through the autonomous nervous system) anorexigenic effects. Only ghrelin, a gastric hormone, has been consistently associated with the initiation of food intake and is regarded as the main orexigenic signal both in animal models and humans. In this review, we provide a brief description of the major gastrointestinal hormones implicated in the regulation of food intake. Given the increased importance of food intake disturbances, especially obesity, a better understanding of the underlying mechanisms of action of the gastrointestinal hormones might contribute to the development of new molecules that could increase the therapeutic arsenal for treating obesity and its associated comorbidities.

Effect of renal impairment on the pharmacokinetics of the GLP-1 analogue liraglutide

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

* Patients with Type 2 diabetes are likely to have or to develop renal impairment, which affects the pharmacokinetics of some antidiabetic treatments. * Whether dosing of the once-daily human glucagon-like peptide-1 analogue liraglutide should be modified in patients with renal impairment has not previously been studied.

WHAT THIS STUDY ADDS

* Renal dysfunction was not found to increase the exposure of liraglutide. * Hence, no dose adjustment is expected to be required in patients with Type 2 diabetes and renal impairment treated with liraglutide.

AIMS

To investigate whether dose adjustment of the once-daily human glucagon-like peptide-1 analogue liraglutide is required in patients with varying stages of renal impairment.

METHODS

A cohort of 30 subjects, of whom 24 had varying degrees of renal impairment and six had normal renal function, were given a single dose of liraglutide, 0.75 mg subcutaneously, and completed serial blood sampling for plasma liraglutide measurements for pharmacokinetic estimation.

RESULTS

No clear trend for change in pharmacokinetics was evident across groups with increasing renal dysfunction. While the between-group comparisons of the area under the liraglutide concentration-curve (AUC) did not demonstrate equivalence [estimated ratio AUC(severe)/AUC(healthy) 0.73, 90% confidence interval (CI) 0.57, 0.94; and AUC (continuous ambulatory peritoneal dialysis)(CAPD)/AUC(healthy) 0.74, 90% CI 0.56, 0.97], the regression analysis of log(AUC) for subjects with normal renal function and mild-to-severe renal impairment showed no significant effect of decreasing creatinine clearance on the pharmacokinetics of liraglutide. The expected AUC ratio between the two subjects with the lowest and highest creatinine clearance in the study was estimated to be 0.88 (95% CI 0.58, 1.34) (NS). Degree of renal impairment did not appear to be associated with an increased risk of adverse events.

CONCLUSIONS

This study indicated no safety concerns regarding use of liraglutide in patients with renal impairment. Renal dysfunction was not found to increase exposure of liraglutide, and patients with Type 2 diabetes and renal impairment should use standard treatment regimens of liraglutide. There is, however, currently limited experience with liraglutide in patients beyond mild-stage renal disease.

Dose-dependent improvement in glycemia with once-daily liraglutide without hypoglycemia or weight gain: A double-blind, randomized, controlled trial in Japanese patients with type 2 diabetes

AIMS

To evaluate dose-response efficacy and safety of once-daily human GLP-1 analog liraglutide in Japanese subjects with type 2 diabetes.

METHODS

Patients (226, treated with diet with/without OADs, mean HbA(1c) 8.30%, mean BMI 23.9kg/m(2)) were randomized after OAD discontinuation and washout to receive liraglutide 0.1, 0.3, 0.6 or 0.9mg once daily, or placebo in double-blind, parallel-group design for 14 weeks.

RESULTS

Liraglutide dose levels reduced HbA(1c) versus placebo (by 0.79%, 1.22%, 1.64% and 1.85%, respectively; p<0.0001 for linear contrast). Liraglutide 0.9mg/day resulted in 75% of patients achieving HbA(1c) <7.0% and 57% achieving HbA(1c) <6.5%. There were no major or minor hypoglycemic events. Liraglutide also reduced, with significant dose-response (each p<0.0001 for linear contrast) versus placebo: fasting plasma glucose (up to 2.5mmol/L), postprandial (0-3h) glucose excursion (up to 12.8mmol/(Lh)); and increased postprandial insulin secretion (up to 23.0microU/(mLh)) and beta-cell function as evaluated by HOMA-beta (up to around 20.0(microU/mL)/(mg/dL)). Body weight was unchanged; no development of liraglutide antibodies was detected.

CONCLUSIONS

Liraglutide was highly effective and well tolerated at doses up to 0.9mg/day in Japanese patients with type 2 diabetes, allowing glycemic control without weight gain or hypoglycemia.

Orlistat in the prevention of diabetes in the obese patient

There has been an increase in the concern about preventing type 2 diabetes mellitus (T2DM), a disease with great and increasing prevalence. The prevalence of obesity, physical inactivity, Western processed diet, important risk factors for the development of T2DM, are also rising. Free fatty acids are increased in obesity and reduce insulin clearance and increase hepatic glucose production. Implementation of a healthy lifestyle has been show to slow the progression of impaired glucose tolerance to T2DM. Orlistat is an inhibitor of lipase activity, with proved efficacy in body weight reduction and long-term management of obesity and more favorable effects on carbohydrate metabolism and it was prospectively shown in XENDOS study that orlistat promoted long-term weight loss and prevented T2DM onset in obese individuals with normal and impaired glucose tolerance at baseline over four years. This benefit could be associated to the weight loss itself, to the limited absorption of lipids and reduction of plasma free fatty acids, to increased production of incretins or to modulation of secretion of cytokines by adipocytes, all effects secondary to orlistat treatment. A proposed strategy is to identify subjects at highest risk to receive a drug intervention, using lifestyle interventions alone at the community level.

Crystal structure of the incretin-bound extracellular domain of a G protein-coupled receptor

Incretins, endogenous polypeptide hormones released in response to food intake, potentiate insulin secretion from pancreatic beta cells after oral glucose ingestion (the incretin effect). This response is signaled by the two peptide hormones glucose-dependent insulinotropic polypeptide (GIP) (also known as gastric inhibitory polypeptide) and glucagon-like peptide 1 through binding and activation of their cognate class 2 G protein-coupled receptors (GPCRs). Because the incretin effect is lost or significantly reduced in patients with type 2 diabetes mellitus, glucagon-like peptide 1 and GIP have attracted considerable attention for their potential in antidiabetic therapy. A paucity of structural information precludes a detailed understanding of the processes of hormone binding and receptor activation, hampering efforts to develop novel pharmaceuticals. Here we report the crystal structure of the complex of human GIP receptor extracellular domain (ECD) with its agonist, the incretin GIP(1-42). The hormone binds in an alpha-helical conformation in a surface groove of the ECD largely through hydrophobic interactions. The N-terminal ligand residues would remain free to interact with other parts of the receptor. Thermodynamic data suggest that binding is concomitant with structural organization of the hormone, resulting in a complex mode of receptor-ligand recognition. The presentation of a well structured, alpha-helical ligand by the ECD is expected to be conserved among other hormone receptors of this class.

Peptide Drug Delivery Strategies for the Treatment of Diabetes

Drug delivery strategies for diabetes have included a wide range of scientific and engineering approaches, including molecular design, formulation and device design. Molecular engineering has resulted in modified pharmacokinetics, such as rapid-acting or slow-release analogs of insulin. Long-acting insulin formulations are designed to meet the body's basal needs, whereas rapid-acting insulin formulations are designed to cover mealtime glucose spikes. Furthermore, the discovery of new therapeutic biomolecules, which like insulin need to be injected, will drive the need for more flexible and universally applicable delivery systems. Formulation design, such as particle engineering, can be used to modify pharmacokinetic profiles. In general, suspension formulations of insulin commonly demonstrate reduced solubility and result in sustained release. Similarly, depot injections can result in precipitation of insulin at the site of injection, again resulting in lower solubility and sustained release. Particle engineering also has been applied to pulmonary formulations for delivery to the deep lung. The creation of novel drug delivery methods for the treatment of diabetes should remove barriers to insulin therapy and increase patient acceptance and compliance. Eliminating routine injections with needle-free injectors, insulin pumps, inhalation, buccal sprays, intra-nasal delivery, and transdermal patches may offer increasingly attractive alternatives.

[Incretins as new therapeutic targets of type 2 diabetes]

The epidemic characteristics of type 2 diabetes mellitus (DM) pose a formidable challenge in terms of healthcare, given the tremendous impact it has on the healthcare resources needed not only to treat it, but also to prevent and treat the associated cardiovascular complications. This makes up the number 1 cause of DM-associated morbidity-mortality in addition to its social and personal impact. We currently have a growing number of available treatment tools that make it possible to achieve the target glycemic control in most of our patients, albeit unfortunately, only temporarily in a good many of them, because of the progressive nature of the disease. Furthermore, current therapy often entails undesirable effects, such as weight gain or the emergence of hypoglycemias that limit their optimization. Recently, a new class of drugs has been incorporated into the treatment of DM - incretin mimetics. These new drugs act in very much the same way as the intestinal hormones that are naturally secreted following the intake of nutrients, called incretins (e.g., glucagon like peptide-1 [GLP-1]), with the added advantage that these molecules are resistant to enzymatic degradation by the DPP-IV enzyme. This provides them with a half-life that makes ambulatory treatment possible, unlike natural incretins whose half-life is too short to make them viable as treatment. The incretin mimetics bind to GLP-1 receptors, increasing glucose-dependent secretion of insulin and decreasing glucose-dependent posprandial secretion of glucagon, slowing gastric emptying, and reducing food intake. All these mechanisms have a significant impact on glucose homeostasis and a beneficial effect on body weight. Moreover, studies in experimental models suggest that these new molecules might have a promising effect on pancreatic beta cell function and mass. Exenatide is the first incretin mimetic available to date. Efficacy and safety data of this drug show it as a therapeutic option for the treatment of type 2 DM.

Understanding interactions of gastric inhibitory polypeptide (GIP) with its G-protein coupled receptor through NMR and molecular modeling

Gastric inhibitory polypeptide (GIP, or glucose-dependent insulinotropic polypeptide) is a 42-amino acid incretin hormone moderating glucose-induced insulin secretion. Antidiabetic therapy based on GIP holds great promise because of the fact that its insulinotropic action is highly dependent on the level of glucose, overcoming the sideeffects of hypoglycemia associated with the current therapy of Type 2 diabetes. The truncated peptide, GIP(1-30)NH2, has the same activity as the full length native peptide. We have studied the structure of GIP(1-30)NH2 and built a model of its G-protein coupled receptor (GPCR). The structure of GIP(1-30)NH2 in DMSO-d6 and H2O has been studied using 2D NMR (total correlation spectroscopy (TOCSY), nuclear overhauser effect spectroscopy (NOESY), double quantum filtered-COSY (DQF-COSY), 13C-heteronuclear single quantum correlation (HSQC) experiments, and its conformation built by MD simulations with the NMR data as constraints. The peptide in DMSO-d6 exhibits an alpha-helix between residues Ile12 and Lys30 with a discontinuity at residues Gln19 and Gln20. In H2O, the alpha-helix starts at Ile7, breaks off at Gln19, and then continues right through to Lys30. GIP(1-30)NH2 has all the structural features of peptides belonging to family B1 GPCRs, which are characterized by a coil at the N-terminal and a long C-terminal alpha-helix with or without a break. A model of the seven transmembrane (TM) helices of the GIP receptor (GIPR) has been built on the principles of comparative protein modeling, using the crystal structure of bovine rhodopsin as a template. The N-terminal domain of GIPR has been constructed from the NMR structure of the N-terminal of corticoptropin releasing factor receptor (CRFR), a family B1 GCPR. The intra and extra cellular loops and the C-terminal have been modeled from fragments retrieved from the PDB. On the basis of the experimental data available for some members of family B1 GPCRs, four pairs of constraints between GIP(1-30)NH2 and its receptor were used in the FTDOCK program, to build the complete model of the GIP(1-30)NH2:GIPR complex. The model can rationalize the various experimental observations including the potency of the truncated GIP peptide. This work is the first complete model at the atomic level of GIP(1-30)NH2 and of the complex with its GPCR.

Comparison of the subchronic antidiabetic effects of DPP IV–resistant GIP and GLP-1 analogues in obese diabetic (ob/ob) mice

Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are the two key incretin hormones released from the gastrointestinal tract that regulate blood glucose homeostasis through potent insulin secretion. The rapid degradation of GIP and GLP-1 by the ubiquitous enzyme dipeptidyl peptidase IV (DPP IV) renders both peptides noninsulinotropic. However, DPP IV stable agonists, such as N-AcGIP and (Val8)GLP-1, have now been developed. The present study has examined and compared the metabolic effects of subchronic administration of daily i.p. injections of N-AcGIP, (Val8) GLP-1 and a combination of both peptides (all at 25 nmol/kg bw) in obese diabetic (ob/ob) mice. Initial in vitro experiments confirmed the potent insulinotropic properties of N-AcGIP and (Val8)GLP-1 in the clonal pancreatic BRIN BD11 cell line. Subchronic administration of N-AcGIP, (Val8)GLP-1 or combined peptide administration had no significant effects on the body weight, food intake and plasma insulin concentrations. However, all treatment groups had significantly (p < 0.05) decreased plasma glucose levels and improved glucose tolerance by day 14. The effectiveness of the peptide groups was similar, and glucose concentrations were substantially reduced following injection of insulin to assess insulin sensitivity compared to control. These results provide evidence for an improvement of glucose homeostasis following treatment with enzyme-resistant GIP and GLP-1 analogues.

GIP(Lys16PAL) and GIP(Lys37PAL): novel long-acting acylated analogues of glucose-dependent insulinotropic polypeptide with improved antidiabetic potential

Glucose-dependent insulinotropic polypeptide (GIP) is a physiological insulin releasing peptide. We have developed two novel fatty acid derivatized GIP analogues, which bind to serum albumin and demonstrate enhanced duration of action in vivo. GIP(Lys(16)PAL) and GIP(Lys(37)PAL) were resistant to dipeptidyl peptidase IV (DPP IV) degradation. In vitro studies demonstrated that GIP analogues retained their ability to activate the GIP receptor through production of cAMP and to stimulate insulin secretion. Intraperitoneal administration of GIP analogues to obese diabetic (ob/ob) mice significantly decreased the glycemic excursion and elicited increased and prolonged insulin responses compared to native GIP. A protracted glucose-lowering effect was observed 24 h following GIP(Lys(37)PAL) administration. Once a day injection for 14 days decreased nonfasting glucose, improved glucose tolerance, and enhanced the insulin response to glucose. These data demonstrate that fatty acid derivatized GIP peptides represent a novel class of long-acting stable GIP analogues for therapy of type 2 diabetes.

Glucagon-like peptide-1-based therapies for the treatment of type 2 diabetes mellitus

The 'incretin effect' describes the phenomenon of an enhanced insulin response following oral ingestion of glucose compared with that after intravenous administration of glucose, leading to identical postprandial plasma glucose excursions. It accounts for up to 60% of the postprandial insulin secretion, but is diminished in patients with type 2 diabetes mellitus. Gastrointestinal hormones that promote the incretin effect are called incretins. Glucagon-like peptide-1 (GLP-1) is an important incretin. Under hyperglycemic conditions in humans, it stimulates insulin secretion and normalizes blood glucose levels. GLP-1 does not stimulate insulin secretion at normal glucose levels; therefore, it does not cause hypoglycemia. Furthermore, it inhibits glucagon secretion and delays gastric emptying. In vitro and animal data have demonstrated that GLP-1 increases beta-cell mass by stimulating islet cell neogenesis and by inhibiting the apoptosis of islet cells. The improvement of beta-cell function due to GLP-1 can be indirectly observed from the increased insulin secretory capacity of humans receiving such treatment. GLP-1 may represent an attractive therapeutic method for patients with type 2 diabetes because of its multiple effects, including the simulation of satiety in the CNS by acting as a transmitter or by crossing the blood brain barrier. Native GLP-1 is degraded rapidly upon intravenous or subcutaneous administration and is therefore not feasible for routine therapy. Long-acting GLP-1 analogs (e.g. liraglutide) and exendin-4 (exenatide) that are resistant to degradation, called 'incretin mimetics', are being investigated in clinical trials. Dipeptidyl peptidase-IV inhibitors (e.g. vildagliptin, sitagliptin, and saxagliptin) that inhibit the enzyme responsible for incretin degradation are also being studied.

Human duodenal enteroendocrine cells: source of both incretin peptides, GLP-1 and GIP

Among the products of enteroendocrine cells are the incretins glucagon-like peptide-1 (GLP-1, secreted by L cells) and glucose-dependent insulinotropic peptide (GIP, secreted by K cells). These are key modulators of insulin secretion, glucose homeostasis, and gastric emptying. Because of the rapid early rise of GLP-1 in plasma after oral glucose, we wished to definitively establish the absence or presence of L cells, as well as the relative distribution of the incretin cell types in human duodenum. We confirmed the presence of proglucagon and pro-GIP genes, their products, and glucosensory molecules by tissue immunohistochemistry and RT-PCR of laser-captured, single duodenal cells. We also assayed plasma glucose, incretin, and insulin levels in subjects with normal glucose tolerance and type 2 diabetes for 120 min after they ingested 75 g of glucose. Subjects with normal glucose tolerance (n=14) had as many L cells (15+/-1), expressed per 1,000 gut epithelial cells, as K cells (13+/-1), with some containing both hormones (L/K cells, 5+/-1). In type 2 diabetes, the number of L and L/K cells was increased (26+/-2; P<0.001 and 9+/-1; P < 0.001, respectively). Both L and K cells contained glucokinase and glucose transporter-1, -2, and -3. Newly diagnosed type 2 diabetic subjects had increased plasma GLP-1 levels between 20 and 80 min, concurrently with rising plasma insulin levels. Significant coexpression of the main incretin peptides occurs in human duodenum. L and K cells are present in equal numbers. New onset type 2 diabetes is associated with a shift to the L phenotype.

A comparison of the cellular and biological properties of DPP-IV-resistant N-glucitol analogues of glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide

AIM

The two major incretin hormones--glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP)--are being actively researched by the pharmaceutical industry because of their glucose-lowering and potential anti-diabetic properties. Unfortunately, the inactivation of GLP-1 and GIP in the circulation brought about by dipeptidyl-peptidase-IV (DPP-IV) degradation makes their biological actions short-lived. This study directly compares the cellular and biological properties of GLP-1, GIP and their N-terminally modified counterparts, with glucitol extension at positions His7 and Tyr1 respectively, to confer DPP-IV resistance.

METHODS

Using both the glucose-responsive pancreatic beta-cell line, BRIN BD11, and the obese diabetic (ob/ob) mouse, we assessed adenosine 3',5'-cyclic monophosphate (cAMP) production and insulinotropic action in vitro as well as in vivo glucose-lowering and insulin-releasing actions.

RESULTS

The results reveal that glycation of the N-terminus of GLP-1 or GIP stabilized both peptides against DPP-IV degradation. However, N-glucitol-GLP-1 displayed reduced cAMP production, insulinotropic activity and glucose-lowering potency, compared to native GLP-1. By contrast, N-glucitol-GIP exhibited substantially improved biological activities, compared to native GIP, and possessed similar or enhanced in vivo potency to GLP-1. N-terminal extension by means of glucitol addition is more beneficial to bioactivity of GIP than it is to GLP-1.

CONCLUSIONS

N-terminal glycation generates a super GIP agonist, which possesses acute in vivo glucose-lowering and insulinotropic actions superior to native GLP-1. Therefore, N-glucitol-GIP is a particularly attractive potential candidate molecule for drug therapy of type 2 diabetes.

New therapeutic strategies for the treatment of type 2 diabetes mellitus based on incretins

Orally ingested glucose leads to a greater insulin response compared to intravenously administered glucose leading to identical postprandial plasma glucose excursions, a phenomenon referred to as the "incretin effect". The incretin effect comprises up to 60% of the postprandial insulin secretion and is diminished in type 2 diabetes. One of the very important gastrointestinal hormones promoting this effect is glucagon-like peptide 1 (GLP-1). It only stimulates insulin secretion and normalizes blood glucose in humans under hyperglycemic conditions, therefore it does not cause hypoglycemia. Other important physiological actions of GLP-1 are the inhibition of glucagon secretion and gastric emptying. It further acts as a neurotransmitter in the hypothalamus stimulating satiety. In vitro and animal data demonstrated that GLP-1 increases beta-cell mass by stimulating islet cell neogenesis and by inhibiting apoptosis of islets. In humans, the improvement of beta-cell function can be indirectly observed from the increased insulin secretory capacity after GLP-1 infusions. GLP-1 represents an attractive therapeutic principle for type 2 diabetes. However, native GLP-1 is degraded rapidly upon exogenous administration and is therefore not feasible for routine therapy. The first long-acting GLP-1 analog ("incretin mimetic") Exenatide (Byetta) has just been approved for type 2 diabetes therapy. Other compounds are being investigated in clinical trials (e.g. liraglutide, CJC1131). Dipeptidyl-peptidase IV inhibitors (DPP-IV inhibitors; e.g. Vildagliptin, Sitagliptin) that inhibit the enzyme responsible for incretin degradation are also under study.

Dipeptidyl peptidase IV inhibitors: how do they work as new antidiabetic agents?

A number of new approaches to diabetes therapy are currently undergoing clinical trials, including those involving stimulation of the pancreatic beta-cell with the gut-derived insulinotropic hormones (incretins), GIP and GLP-1. The current review focuses on an approach based on the inhibition of dipeptidyl peptidase IV (DP IV), the major enzyme responsible for degrading the incretins in vivo. The rationale for this approach was that blockade of incretin degradation would increase their physiological actions, including the stimulation of insulin secretion and inhibition of gastric emptying. It is now clear that both GIP and GLP-1 also have powerful effects on beta-cell differentation, mitogenesis and survival. By potentiating these pleiotropic actions of the incretins, DP IV inhibition can therefore preserve beta-cell mass and improve secretory function in diabetics.

Glucagon-like peptide-1 as a treatment option for type 2 diabetes and its role in restoring beta-cell mass

The "incretin effect" describes the enhanced insulin response from orally ingested glucose compared with intravenous glucose leading to identical postprandial plasma glucose excursions. It makes up to 60% of the postprandial insulin secretion but is diminished in type 2 diabetes. Gastrointestinal hormones promoting the incretin effect are called incretins. Glucagon-like peptide- 1 (GLP-1) is an important incretin. In vitro and animal data have demonstrated that GLP-1 increases beta-cell mass by stimulating islet cell neogenesis and by inhibiting apoptosis of islets. The improvement of beta-cell function can be indirectly observed from the increased insulin secretory capacity of humans receiving GLP-1 or incretin mimetics that act like GLP-1. Furthermore, GLP-1 inhibits glucagon secretion and rarely causes hypoglycemia. It may represent an attractive therapeutic method for type 2 diabetes because of its multiple effects, including a slowing of gastric emptying and the simulation of satiety by acting as a transmitter in the CNS. Native GLP-1 is degraded rapidly upon intravenous or subcutaneous administration and is therefore not feasable for routine therapy. Long-acting GLP-1 analogs (e.g., Liraglutide [Novo Nordisk, Copenhagen, Denmark]) and exenadin-4 (Exenatide [Eli Lilly, Indianapolis, IN]) that are resistant to degradation, called "incretin mimetics," are being investigated in clinical trials. Dipeptidyl peptidase IV inhibitors (e.g., Vildagliptin [Novartis, Basel, Switzerland]) that inhibit the enzyme responsible for incretin degradation are also under study.

Inhibition of dipeptidyl peptidase IV activity by oral metformin in Type 2 diabetes

AIMS

Glucagon-like peptide-1 (GLP-1) and gastric inhibitory polypeptide (GIP) are important insulinotropic hormones that enhance the insulin secretory response to feeding. Their potential for treating Type 2 diabetes is limited by short biological half-life owing to degradation by dipeptidyl peptidase IV (DPP IV). We investigated the acute effects of metformin on DPP IV activity in Type 2 diabetes to elucidate inhibition of DPP IV as a possible mechanism of action.

METHODS

Eight fasting subjects with Type 2 diabetes (5M/3F, age 53.1+/-4.2 years, BMI 36.8+/-1.8 kg/m2, glucose 8.9+/-1.2 mmol/l, HbA1c 7.8+/-0.6%) received placebo or metformin 1 g orally 1 week apart in a random, crossover design.

RESULTS

Following metformin, DPP IV activity was suppressed compared with placebo (AUC0-6 h 3230+/-373 vs. 5764+/-504 nmol ml/l, respectively, P=0.001). Circulating glucose, insulin and total GLP-1 were unchanged. Metformin also concentration-dependently inhibited endogenous DPP IV activity in vitro in plasma from Type 2 diabetic subjects.

CONCLUSION

Oral metformin effectively inhibits DPP IV activity in Type 2 diabetic patients, suggesting that the drug may have potential for future combination therapy with incretin hormones.

Therapeutic assessment of glucagon-like peptide-1 agonists compared with dipeptidyl peptidase IV inhibitors as potential antidiabetic drugs

The most prevalent form of diabetes is non-insulin-dependent or Type 2 diabetes. Innovative strategies to enhance insulin secretion and thereby improve glucose tolerance in patients with this type of diabetes are currently under preclinical and clinical investigation. These therapies include the applications of incretin hormones; gut hormones released postprandially that stimulate insulin secretion in pancreatic beta-cells. Because incretin actions are rapidly terminated by N-terminal cleavage of these peptide hormones by the amino-peptidase dipeptidyl peptidase IV (DPP IV, CD26), the utility of DPP IV inhibitors for the treatment of Type 2 diabetes is also under investigation. This review compares the therapeutic potential and possible side effects of metabolically stable analogues/peptide agonists of the incretin glucagon-like peptide-1 (GLP-1) with the application of DPP IV inhibitors that reduce the rate of endogenous degradation of GLP-1 and other incretins. GLP-1 analogues have been shown to be highly efficacious in the treatment of Type 2 diabetes, with minimal side effects. Of particular importance is the fact that they do not induce hypoglycaemia. However, they are currently available only in an injectable form. In contrast, DPP IV inhibitors have the clear advantage of oral application resulting in better patient compliance. Furthermore, they also potentiate the actions of other incretins normally degraded by the action of DPP IV. However, they possess more potential side effects. Taken together, both approaches offer promising new drugs for the treatment of Type 2 diabetes.

Glucagon-like peptide-1: regulation of insulin secretion and therapeutic potential

Glucagon-like peptide-1 (GLP-1) is an intestinally derived insulinotropic hormone currently under investigation for use as a novel therapeutic agent in the treatment of type 2 diabetes. One of several important effects of GLP-1 is on nutrient-induced pancreatic hormone release and is mediated by binding to a specific G-protein coupled receptor resulting in the activation of adenylate cyclase and an increase in cAMP generation. In the beta-cell, cAMP binds and modulates activities of both protein kinase A and cAMP-regulated guanine nucleotide exchange factor II, thereby enhancing glucose-dependent insulin secretion. The stimulatory action of GLP-1 on insulin secretion involves interaction with a plethora of signal transduction processes including ion channel activity, intracellular Ca(2+) handling and exocytosis of the insulin-containing granules. In this review we focus principally on recent advances in our understanding on the cellular mechanisms proposed to underlie GLP-1's insulinotropic effect and attempt to incorporate this knowledge into a working model for the control of insulin secretion. Lastly, this review discusses the applicability of GLP-1 as a therapeutic agent for the treatment of type 2 diabetes.

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

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

NMR structure of the glucose-dependent insulinotropic polypeptide fragment, GIP(1–30)amide

Glucose-dependent insulinotropic polypeptide is an incretin hormone that stimulates insulin secretion and reduces postprandial glycaemic excursions. The glucose-dependent action of GIP on pancreatic beta-cells has attracted attention towards its exploitation as a potential drug for type 2 diabetes. Use of NMR or X-ray crystallography is vital to determine the three-dimensional structure of the peptide. Therefore, to understand the basic structural requirements for the biological activity of GIP, the solution structure of the major biologically active fragment, GIP(1-30)amide, was investigated by proton NMR spectroscopy and molecular modelling. The structure is characterised by a full length alpha-helical conformation between residues F(6) and A(28). This structural information could play an important role in the design of therapeutic agents based upon GIP receptor agonists.

Stimulation of insulin secretion by intravenous bolus injection and continuous infusion of gastric inhibitory polypeptide in patients with type 2 diabetes and healthy control subjects

A reduced insulinotropic effect of gastric inhibitory polypeptide (GIP) is a characteristic of patients with type 2 diabetes. It was the aim of this study to determine the response of insulin secretion to different GIP doses administered by intravenous bolus injection and via continuous infusion in both healthy subjects and patients with type 2 diabetes. Eight patients with type 2 diabetes and eight healthy subjects participated in a 240-min hyperglycemic clamp (140 mg/dl) with intravenous infusion of placebo, GIP at a low dose, and GIP at a high dose, each administered continuously over 60 min. Boluses of placebo, 20 pmol GIP/kg, and 80 pmol GIP/kg were injected intravenously at 0, 60, and 120 min, respectively. Capillary and venous blood was drawn for glucose, insulin, C-peptide, and GIP. Plasma insulin and C-peptide concentrations were lower in patients than in control subjects during all infusion periods. GIP bolus administration evoked a significant increase in plasma insulin levels in both patients with type 2 diabetes and healthy subjects. In contrast, the continuous GIP infusion led to a weak increase in insulin secretion in both healthy subjects and type 2 diabetic patients. The dose-response relationship for the increase in insulin secretion after GIP bolus administration was similar in both groups, although at different degrees of beta-cell function. The stimulation of insulin secretion by GIP is stronger after its bolus administration than during continuous infusion. Even though the insulin secretory capacity is generally impaired in patients with type 2 diabetes, the relative sensitivity of insulin secretion to a bolus administration of GIP is almost preserved. Therefore, the existence of a specific GIP receptor defect in type 2 diabetes appears unlikely.

Metabolic stability, receptor binding, cAMP generation, insulin secretion and antihyperglycaemic activity of novel N-terminal Glu9-substituted analogues of glucagon-like peptide-1

Glucagon-like peptide-1(7-36)amide (GLP-1) is an incretin hormone with therapeutic potential for type 2 diabetes. Rapid removal of the N-terminal dipeptide, His7-Ala8, by the ubiquitous enzyme dipeptidyl peptidase IV (DPP IV) curtails the biological activity of GLP-1. Chemical modifications or substitutions of GLP-1 at His7 or Ala8 improve resistance to DPP-IV action, but this often reduces potency. Little attention has focused on the metabolic stability and functional activity of GLP-1 analogues with amino acid substitution at Glu9, adjacent to the DPP IV cleavage site. We generated three novel Glu9-substituted GLP-1 analogues, (Pro9)GLP-1, (Phe9)GLP-1 and (Tyr9)GLP-1 and show for the first time that Glu9 of GLP-1 is important in DPP IV degradation, since replacing this amino acid, particularly with proline, substantially reduced susceptibility to degradation. All three novel GLP-1 analogues showed similar or slightly enhanced insulinotropic activity compared with native GLP-1 despite a moderate 4-10-fold reduction in receptor binding and cAMP generation. In addition, (Pro9)GLP-1 showed significant ability to moderate the plasma glucose excursion and increase circulating insulin concentrations in severely insulin resistant obese diabetic (ob/ob) mice. These observations indicate the importance of Glu9 for the biological activity of GLP-1 and susceptibility to DPP IV-mediated degradation.

Intravenous glucagon-like peptide 1 normalizes blood glucose after major surgery in patients with type 2 diabetes

OBJECTIVE

Hyperglycemia is a major risk factor for a poor outcome after major surgery in patients with type 2 diabetes. Intensive insulin treatment aiming at normoglycemia can markedly improve the survival of critically ill patients, but the broad clinical application is limited by its practicability and the risk of hypoglycemia. Therefore, the glucose-lowering effect of the incretin hormone glucagon-like peptide 1 (GLP-1) was investigated in patients with type 2 diabetes after major surgery.

DESIGN

Randomised clinical study.

SETTING

A surgical unit of a university hospital.

PATIENTS AND MEASUREMENTS

Eight patients with type 2 diabetes (five men, three women; age, 49+/-15 yrs; body mass index, 28+/-3 kg/m; glycosylated hemoglobin, 8.0%+/-1.9%), who had undergone major surgical procedures, were studied between the second and the eighth postoperative day with the intravenous administration of GLP-1 (1.2 pmol x kg x min) and placebo over 8 hrs, each administered in randomized order in the fasting state. C-reactive protein concentrations of 4.9+/-4.2 mg/dL indicated a systemic inflammation. Blood was drawn in 30-min intervals for glucose (glucose oxidase), insulin, C-peptide, glucagon, and GLP-1 (specific immunoassays). Statistics were done with repeated-measures analysis of variance and Duncan's post hoc tests.

MAIN RESULTS

During the intravenous infusion of GLP-1, plasma glucose concentrations were significantly lowered, reaching the normoglycemic fasting glucose range within 150 mins, but they remained elevated during placebo infusion (p <.001). The GLP-1 infusion led to a significant increase of insulin secretion (p <.001 for insulin and C-peptide) and a suppression of glucagon secretion (p =.041). No hypoglycemic events were recorded during the experiments.

CONCLUSIONS

As far as can be concluded on the basis of our data with the infusion of GLP-1 over 8 hrs in eight patients, GLP-1 can be used to reduce glucose concentrations in patients with type 2 diabetes after major surgery.

Secretion, degradation, and elimination of glucagon-like peptide 1 and gastric inhibitory polypeptide in patients with chronic renal insufficiency and healthy control subjects

Glucagon-like peptide 1 (GLP-1) and gastric inhibitory polypeptide (GIP) are important factors in the pathogenesis of type 2 diabetes and have a promising therapeutic potential. Alterations of their secretion, in vivo degradation, and elimination in patients with chronic renal insufficiency (CRI) have not yet been characterized. Ten patients with CRI (aged 47 +/- 15 years, BMI 24.5 +/- 2.2 kg/m(2), and serum creatinine 2.18 +/- 0.86 mg/dl) and 10 matched healthy control subjects (aged 44 +/- 12 years, BMI 24.9 +/- 3.4 kg/m(2), and serum creatinine 0.89 +/- 0.10 mg/dl) were included. On separate occasions, an oral glucose tolerance test (75 g), an intravenous infusion of GLP-1 (0.5 pmol. kg(-1). min(-1) over 30 min), and an intravenous infusion of GIP (1.0 pmol. kg(-1). min(-1) over 30 min) were performed. Venous blood samples were drawn for the determination of glucose (glucose oxidase), insulin, C-peptide, GLP-1 (total and intact), and GIP (total and intact; specific immunoassays). Plasma levels of GIP (3-42) and GLP-1 (9-36 amide) were calculated. Statistics were performed using repeated-measures and one-way ANOVA. After the oral glucose load, plasma concentrations of intact GLP-1 and intact GIP reached similar levels in both groups (P = 0.31 and P = 0.87, respectively). The concentrations of GIP (3-42) and GLP-1 (9-36 amide) were significantly higher in the patients than in the control subjects (P = 0.0021 and P = 0.027, respectively). During and after the exogenous infusion, GLP-1 (9-36 amide) and GIP (3-42) reached higher plasma concentrations in the CRI patients than in the control subjects (P < 0.001 and P = 0.0033, respectively), whereas the plasma levels of intact GLP-1 and GIP were not different between the groups (P = 0.29 and P = 0.27, respectively). Plasma half-lives were 3.4 +/- 0.6 and 2.3 +/- 0.4 min for intact GLP-1 (P = 0.13) and 5.3 +/- 0.8 and 3.3 +/- 0.4 min for the GLP-1 metabolite (P = 0.029) for CRI patients vs. healthy control subjects, respectively. Plasma half-lives of intact GIP were 6.9 +/- 1.4 and 5.0 +/- 1.2 min (P = 0.31) and 38.1 +/- 6.0 and 22.4 +/- 3.0 min for the GIP metabolite (P = 0.032) for CRI patients vs. healthy control subjects, respectively. Insulin concentrations tended to be lower in the patients during all experiments, whereas C-peptide levels tended to be elevated. These data underline the importance of the kidneys for the final elimination of GIP and GLP-1. The initial dipeptidyl peptidase IV-mediated degradation of both hormones is almost unaffected by impairments in renal function. Delayed elimination of GLP-1 and GIP in renal insufficiency may influence the pharmacokinetics and pharmacodynamics of dipeptidyl peptidase IV-resistant incretin derivatives to be used for the treatment of patients with type 2 diabetes.

Diastereoselective synthesis and configuration-dependent activity of (3-substituted-cycloalkyl)glycine pyrrolidides and thiazolidides as dipeptidyl peptidase IV inhibitors

A diastereoselective synthesis was used to prepare a series of (3-substituted-cyclopentyl and -cyclohexyl)glycine pyrrolidides and thiazolidides. The three chiral centers were generated in an unambiguous, stereochemically defined manner. Inhibitory activity was dependent on the configuration at each stereocenter and on the nature of the 3-substituent. In the cyclopentylglycine pyrrolidide series, high potency against dipeptidyl peptidase IV and good selectivity could be achieved.

Receptor gene expression of glucagon-like peptide-1, but not glucose-dependent insulinotropic polypeptide, in rat nodose ganglion cells

We previously reported that afferent signals of the rat hepatic vagus increased upon intraportal appearance of insulinotropic hormone glucagon-like peptide-1(7-36) amide (GLP-1), but not glucose-dependent insulinotropic polypeptide (GIP). To obtain molecular evidence for the vagal chemoreception of GLP-1, the concept derived from those electrophysiological observations, receptor gene expressions of GLP-1 and GIP in the rat nodose ganglion were examined by means of reverse transcriptase-mediated polymerase chain reaction (RT-PCR) and Northern blot analysis. Gene expression of the GLP-1 receptor was clearly detected by both RT-PCR and Northern blot analysis. In situ hybridization study confirmed that the expression occurs in neuronal cells of the ganglion. As to the GIP receptor, RT-PCR amplified the gene transcript faintly though Northern blot analysis failed to detect any messages. However, semi-quantitative RT-PCR revealed that the ratio of the gene expression level of the GIP receptor to that of the GLP-1 receptor was less than 1:250, indicating that receptor gene expression of GIP is practically negligible in the ganglion. Additionally, an equal level of GLP-1 receptor gene expressions between left- and right-side ganglia was evidenced by semi-quantitative RT-PCR, implying possible extrahepatic occurrence of vagal GLP-1 reception in addition to the reception through the hepatic vagus (originating from the left-side ganglion). The present results offer, for the first time, the molecular basis for the vagal chemoreception of GLP-1 via its specific receptor.

[Ser2]- and [SerP2] incretin analogs: comparison of dipeptidyl peptidase IV resistance and biological activities in vitro and in vivo

Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP; also known as gastric inhibitory polypeptide) are incretin hormones that reduce postprandial glycemic excursions via enhancing insulin release but are rapidly inactivated by enzymatic N-terminal truncation. As such, efforts have been made to improve their plasma stability by synthetic modification or by inhibition of the responsible protease, dipeptidyl peptidase (DP) IV. Here we report a parallel comparison of synthetic GIP and GLP-1 with their Ser2- and Ser(P)2-substituted analogs, examining receptor binding and activation, metabolic stability, and biological effects in vivo. Both incretins and their Ser2-substituted analogs showed similar EC50s (0.16-0.52 nm) and IC50s (4.3-8.1 nm) at their respective cloned receptors. Although both phosphoserine 2-modified (Ser(PO3H2); Ser(P)) peptides were able to stimulate maximal cAMP production and fully displace receptor-bound tracer, they showed significantly right-shifted concentration-response curves and binding affinities. Ser2-substituted analogs were moderately resistant to DP IV cleavage, whereas [Ser(P)2]GIP and [Ser(P)2] GLP-1 showed complete resistance to purified DP IV. It was shown that the Ser(P) forms were dephosphorylated in serum and thus in vivo act as precursor forms of Ser2-substituted analogs. When injected subcutaneously into conscious Wistar rats, all peptides reduced glycemic excursions (rank potency: [Ser(P)2]incretins > or = [Ser2] incretins > native hormones). Insulin determinations indicated that the reductions in postprandial glycemia were at least in part insulin-mediated. Thus it has been shown that despite having low in vitro bioactivity using receptor-transfected cells, in vivo potency of [Ser(P)2] incretins was comparable with or greater than that of native or [Ser2]peptides. Hence, Ser(P)2-modified incretins present as novel glucose-lowering agents.