Secretion of the incretin hormones glucagon-like peptide-1 and gastric inhibitory polypeptide correlates with insulin secretion in normal man throughout the day

Physiology of incretins in health and disease

The incretin hormones, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), are gut peptides which are secreted by endocrine cells in the intestinal mucosa. Their plasma concentrations increase quickly following food ingestion, and carbohydrate, fat, and protein have all been shown to stimulate GLP-1 and GIP secretion. Although neural and hormonal mechanisms have also been proposed to regulate incretin hormone secretion, direct stimulation of the enteroendocrine cells by the presence of nutrients in the intestinal lumen is probably the most important factor in humans. The actions of the incretin hormones are crucial for maintaining normal islet function and glucose homeostasis. Furthermore, it is also now being recognized that incretin hormones may have other actions in addition to their glucoregulatory effects. Studies have shown that GLP-1 and GIP levels and actions may be perturbed in disease states, but interpretation of the precise relationship between disease and incretins is difficult. The balance of evidence seems to suggest that alterations in secretion and/or action of incretin hormones arise secondarily to the development of insulin resistance, glucose intolerance, and/or increases in body weight rather than being causative factors. However, these impairments may contribute to the deterioration of glycemic control in diabetic patients.

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.

Association between coefficients of variation of the R-R intervals on electrocardiograms and post-challenge hyperglycemia in patients with newly diagnosed type 2 diabetes

The aim of the present study was to examine whether there is a relationship between autonomic function and post‐challenge hyperglycemia in patients with type 2 diabetes. Subjects included 122 Japanese patients newly diagnosed with type 2 diabetes. Autonomic nerve function was assessed using coefficients of variation of the R‐R intervals on electrocardiograms (CVRR). Unlike anthropometry, insulin secretion and insulin resistance, age (r = −0.209, P < 0.021) and post‐challenge plasma glucose at 120 min (PG120; r = −0.219, P < 0.015) were the only variables significantly correlated with CVRR. Age was not significantly correlated with PG120. In multiple regression analyses, CVRR Z‐score, but not age, was significantly correlated with PG120. The present results suggest that autonomic function affects post‐challenge blood glucose levels independently of age. (J Diabetes Invest,doi: 10.1111/j.2040‐1124.2010.00098.x, 2011)

Increased postprandial glycaemia, insulinemia, and lipidemia after 10 weeks' sucrose-rich diet compared to an artificially sweetened diet: a randomised controlled trial

BACKGROUND

The importance of exchanging sucrose for artificial sweeteners on risk factors for developing diabetes and cardiovascular diseases is not yet clear.

OBJECTIVE

To investigate the effects of a diet high in sucrose versus a diet high in artificial sweeteners on fasting and postprandial metabolic profiles after 10 weeks.

DESIGN

Healthy overweight subjects were randomised to consume drinks and foods sweetened with either sucrose (∼2 g/kg body weight) (n = 12) or artificial sweeteners (n = 11) as supplements to their usual diet. Supplements were similar on the two diets and consisted of beverages (∼80 weight%) and solid foods (yoghurts, marmalade, ice cream, stewed fruits). The rest of the diet was free of choice and ad libitum. Before (week 0) and after the intervention (week 10) fasting blood samples were drawn and in week 10, postprandial blood was sampled during an 8-hour meal test (breakfast and lunch).

RESULTS

After 10 weeks postprandial glucose, insulin, lactate, triglyceride, leptin, glucagon, and GLP-1 were all significantly higher in the sucrose compared with the sweetener group. After adjusting for differences in body weight changes and fasting values (week 10), postprandial glucose, lactate, insulin, GIP, and GLP-1 were significantly higher and after further adjusting for differences in energy and sucrose intake, postprandial lactate, insulin, GIP, and GLP-1 levels were still significantly higher on the sucrose-rich diet.

CONCLUSION

A sucrose-rich diet consumed for 10 weeks resulted in significant elevations of postprandial glycaemia, insulinemia, and lipidemia compared to a diet rich in artificial sweeteners in slightly overweight healthy subjects.

Changes in glucose homeostasis after Roux-en-Y gastric bypass surgery for obesity at day three, two months, and one year after surgery: role of gut peptides

CONTEXT

Endocrine effects of gastric bypass (GBP) surgery for obesity on glucose homeostasis are not fully understood.

MAIN OBJECTIVE

The main objective of the study was to assess the changes in plasma glucose, insulin, glucagon-like peptide-1 (GLP-1), leptin, somatostatin, glucose-dependent insulinotropic peptide, enteroglucagon, and glucagon early after GBP.

METHOD

Twelve obese subjects (body mass index 45.3 ± 1.9 kg/m(2)) were subjected to a liquid meal without lipids before and 3 d, 2 months, and 1 yr after GBP. Plasma concentrations of glucose, insulin, leptin, and gut peptide hormones were assessed before and for 180 min after the meal. Satiety was measured with visual analog scales. The absorption rate of acetaminophen added to the liquid meal was measured. Insulin resistance was measured by the homeostasis model assessment of insulin resistance.

RESULTS

All subjects lost weight (body mass index 30.3 ± 1.8 kg/m(2) at 1 yr). Fasting glucose was significantly lower on d 3 (P < 0.05). There was a progressive decrease in the homeostasis model assessment of insulin resistance after 2 months postoperatively. Postprandially, there was a progressive rise of GLP-1 and enteroglucagon and a transient increase in pancreatic glucagon release over the study period. There was a leftward shift of the time course of plasma glucose and insulin. Somatostatin release was lower on d 3 (P < 0.05) but then unchanged. The absorption rate of acetaminophen was twice as fast after GBP compared with before surgery and did not change over time. Satiety scores increased markedly postoperatively.

CONCLUSION

Both enhanced insulin sensitivity and incretin hormones, such as GLP-1, contribute to the early control of glucose homeostasis. Progressively increasing postprandial levels of enteroglucagon (oxyntomodulin) and GLP-1 facilitate weight loss and enhance insulin effectiveness.

LY2189265, a long-acting glucagon-like peptide-1 analogue, showed a dose-dependent effect on insulin secretion in healthy subjects

AIM

To assess the safety, tolerability, pharmacokinetics, pharmacodynamics and potential immunogenicity of single, escalating subcutaneous injections of a once-weekly glucagon-like peptide-1 analogue in healthy subjects.

METHODS

This phase 1, three-period, crossover, double-blind, placebo-controlled study investigated single, escalating subcutaneous doses of LY2189265 (LY) ranging from 0.1 to 12 mg; approximately six subjects were randomized to each dose. Parameters of safety, including adverse events, were assessed. The pharmacokinetic profile was assessed over 14 days. Pharmacodynamic parameters (glucose and insulin concentrations) were measured following a step-glucose infusion (day 3) and as part of an oral glucose tolerance test (OGTT) (day 5).

RESULTS

LY was generally well tolerated with some increase in gastrointestinal symptoms with escalating doses. There were small dose-dependent increases in pulse rate with doses ≥1.0 mg and diastolic blood pressure with doses ≥3.0 mg. The half-life of LY was approximately 90 h, with C(max) occurring between 24 and 48 h in most subjects. Evidence of increase in glucose-dependent insulin secretion and suppression of serum glucose excursions were observed during an OGTT at all doses compared to placebo; no episodes of hypoglycaemia occurred. No subjects developed antibodies to LY2189265.

CONCLUSIONS

LY showed an acceptable safety profile and exhibited the expected glucagon-like peptide-1 pharmacological effects on glucose suppression and insulin secretion with a half-life that supports once-weekly dosing.

Incretin-based therapy: a powerful and promising weapon in the treatment of type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) is a progressive multisystemic disease that increases significantly cardiovascular morbidity and mortality. It is associated with obesity, insulin resistance, beta-cell dysfunction, and hyperglucagonemia, the combination of which typically leads to hyperglycemia. Incretin-based treatment modalities, and in particular glucagon-like peptide 1 (GLP-1) receptor agonists, are able to successfully counteract several of the underlying pathophysiological abnormalities of T2DM. The pancreatic effects of GLP-1 receptor agonists include glucose-lowering effects by stimulating insulin secretion and inhibiting glucagon release in a strictly glucose-dependent manner, increased beta-cell proliferation, and decreased beta-cell apoptosis. GLP-1 receptors are widely expressed throughout human body; thus, GLP-1-based therapies exert pleiotropic and multisystemic effects that extend far beyond pancreatic islets. A large body of experimental and clinical data have suggested a considerable protective role of GLP-1 analogs in the cardiovascular system (decreased blood pressure, improved endothelial and myocardial function, functional recovery of failing and ischemic heart, arterial vasodilatation), kidneys (increased diuresis and natriuresis), gastrointestinal tract (delayed gastric emptying, reduced gastric acid secretion), and central nervous system (appetite suppression, neuroprotective properties). The pharmacologic use of GLP-1 receptor agonists has been shown to reduce bodyweight and systolic blood pressure, and significantly improve glycemic control and lipid profile. Interestingly, weight reduction induced by GLP-1 analogs reflects mainly loss of abdominal visceral fat. The critical issue of whether the emerging positive cardiometabolic effects of GLP-1 analogs can be translated into better clinical outcomes for diabetic patients in terms of long-term hard endpoints, such as cardiovascular morbidity and mortality, remains to be elucidated with prospective, large-scale clinical trials.

Incretin hormone secretion over the day

The two incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are key factors in the regulation of islet function and glucose metabolism, and incretin-based therapy for type 2 diabetes has gained considerable interest during recent years. Regulation of incretin hormone secretion is less well characterized. The main stimulus for incretin hormone secretion is presence of nutrients in the intestinal lumen, and carbohydrate, fat as well as protein all have the capacity to stimulate GIP and GLP-1 secretion. More recently, it has been established that a diurnal regulation exists with incretin hormone secretion to an identical meal being greater when the meal is served in the morning compared to in the afternoon. Finally, whether incretin hormone secretion is altered in disease states is an area with, so far, controversial results in different studies, although some studies have demonstrated reduced incretin hormone secretion in type 2 diabetes. This review summarizes our knowledge on regulation of incretin hormone secretion and its potential changes in disease states.

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.

Liraglutide - overview of the preclinical and clinical data and its role in the treatment of type 2 diabetes

Type 2 diabetes is characterized by a progressive decline in glycaemic control. Many standard diabetes treatments, however, fail to achieve or maintain glycaemic control, and are often associated with an increased risk of hypoglycaemia and weight gain. Recently developed incretin-based therapies are a promising addition to the current armamentarium of diabetes treatments. Two types of incretin-based therapies are currently available: glucagon-like peptide (GLP)-1 receptor agonists (liraglutide and exenatide) and dipeptidyl peptidase-4 inhibitors (sitaglipin, vildagliptin and saxagliptin). This review aims to summarize the key efficacy and safety data of liraglutide, a once-daily human GLP-1 analogue. Extensive phase III clinical trials have shown liraglutide to improve glycaemic control with additional benefits on body weight, blood pressure and β-cell function. Liraglutide is also generally well tolerated with a low risk of hypoglycaemia. Liraglutide has recently been approved for marketing in Europe, Japan and the USA.

Initial combination therapy for type 2 diabetes mellitus: is it ready for prime time?

The increased prevalence of type 2 diabetes mellitus is primarily being driven by the increasing global rates of overweight/obesity. Given the magnitude of this epidemic, we can expect these metabolic abnormalities to play an increasing role in the development of cardiovascular disease. In a pathophysiologic sense, type 2 diabetes is a multiorgan, multifactorial condition, characterized by β-cell dysfunction, insulin resistance in peripheral tissues and the liver, defective incretin activity, and elevated levels of free fatty acids and proinflammatory mediators. Despite the considerable burden of disease associated with type 2 diabetes, most patients are not at, or are unable to achieve, recommended glycemic control guideline targets. In part, this is because of the relentlessly progressive nature of the disease, but it may also be attributable to the current diabetes treatment paradigm, which is characterized by ineffective lifestyle interventions, followed by monotherapy and frequent early treatment failure with prolonged periods of elevated glucose as a consequence of clinical inertia. Thus, it is most appropriate to rethink the current treatment paradigm for type 2 diabetes in the context of a more aggressive initial therapy; specifically with early initiation of combination therapy. Our current understanding of the complex pathophysiology of the disease and the progressive deterioration in glycemic control over time supports the philosophy of earlier intervention with a more comprehensive initial therapy. Thus, while control of hyperglycemia remains the paramount goal, focusing on the underlying pathophysiology of type 2 diabetes is increasingly becoming the therapeutic strategy, with the aim of potentially providing disease modification. Although this is a logical approach, it remains to be demonstrated that early combination therapy will result in disease modification in a clinical setting. Not surprisingly, the incretin-based therapies have gained a great deal of attention in the context of being a component of initial combination therapy, given their potential beneficial effects on β-cell function with lowered risk of weight gain and hypoglycemia.

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.

Differential incretin effects of GIP and GLP-1 on gastric emptying, appetite, and insulin-glucose homeostasis

BACKGROUND

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are major incretins with important effects on glucoregulatory functions. The aim of this study was to investigate effects of GIP and GLP-1 on gastric emptying and appetite after a mixed meal, and effects on insulin secretion and glucose disposal in humans.

METHODS

Randomized crossover single-blind study in 17 healthy volunteers receiving GIP (2 or 5 pmol kg(-1) min(-1), n = 8), GLP-1 (0.75 pmol kg(-1) min(-1), n = 9) or NaCl for 180 min with a radionuclide-labeled omelette and fruit punch (370 kcal). Outcome measures were gastric emptying rate, insulinogenic index, hunger, satiety, desire to eat, and prospective food consumption. Blood was analyzed for GIP, GLP-1, glucagon, C-peptide, peptide YY (PYY) and ghrelin.

KEY RESULTS

Glucose-dependent insulinotropic polypeptide 2 and 5 pmol kg(-1) min(-1) decreased gastric half-emptying time from 128.5 ± 34.0 min in controls to 93.3 ± 6.3 and 85.2 ± 11.0 min (P < 0.05). Glucose-dependent insulinotropic polypeptide 5 pmol kg(-1) min(-1) decreased postprandial glucose (P < 0.001) and insulin (P < 0.05) with increased insulinogenic index. Glucose-dependent insulinotropic polypeptide had no effects on hunger, desire to eat, satiety or prospective consumption. Glucagon-like peptide-1 0.75 pmol kg(-1) min(-1) increased half-emptying time from 76.6 ± 7.6 min to 329.4 ± 71.6 (P < 0.01). Glucagon-like peptide-1 decreased plasma glucose and insulin (both P < 0.05-0.001), and increased insulinogenic index markedly. Hunger, desire to eat and prospective consumption were decreased (P < 0.05), and satiety borderline increased (P < 0.06).

CONCLUSION & INFERENCES

The incretin effect of GIP and GLP-1 differs as GLP-1 exerts a strong glucoregulatory incretin through inhibition of gastric emptying, which GIP does not. Thus, GLP-1 as incretin mimetic may offer unique benefits in terms of weight loss in treatment of type 2 diabetes.

Individually timing high-protein preloads has no effect on daily energy intake, peptide YY and glucagon-like peptide-1

BACKGROUND/OBJECTIVES

Gut hormones have been shown to influence energy intake (EI). To our knowledge, no study has investigated the effects of dietary patterns aimed at optimizing fullness on EI, appetite and gut hormones.

SUBJECT/METHODS

To determine whether individually timing high-protein preloads would impact EI, appetite, and peptide YY and glucagon-like peptide-1 (GLP-1) levels. Ten men (body mass index = 25.5 ± 2.6 kg/m(2)) participated in a randomized crossover trial. The three conditions consisted of the self-selection of snacks (condition 1), or the consumption of a preload (300 kcal: 40% protein, 40% carbohydrates and 20% fat) at either 15 min (condition 2) or ∼ 50 min (individually set) (condition 3) before lunch and dinner. During each condition, a standardized breakfast was served, whereas lunch and dinner were self-selected from a five-item menu, and eaten ad libitum. Mealtime and daily EI were measured. Appetite, peptide YY and GLP-1 were sampled over 9 h.

RESULTS

No differences in daily EI were noted across conditions (1 = 3078 ± 720 kcal; 2 = 2929 ± 264 kcal; 3 = 2998 ± 437 kcal; not significant). For the most part, daily profiles as well as premeal levels of peptide YY and GLP-1 were not different between conditions. Desire to eat, hunger and prospective food consumption were found to be lowest during condition 1 (P < 0.05).

CONCLUSIONS

According to these results, it would seem that individually timing high-protein preloads does not reduce daily EI in healthy human subjects.

Synthesis and characterization of ester-based prodrugs of glucagon-like peptide 1

Peptides represent a rich natural source of potential medicines with one notable pharmaceutical limitation being their relatively short duration of action. A particularly good example of this phenomenon is glucagon-like peptide 1 (GLP), a hormone of appreciable interest for the treatment of type II diabetes. In the native form, GLP demonstrates an extremely short half-life in plasma and a relatively narrow therapeutic index with gastrointestinal adverse pharmacology. We envisioned a prodrug of GLP as a means to extend the duration of action and broaden the therapeutic index of this peptide hormone. We designed, synthesized, and characterized ester-based prodrugs of GLP that differentially convert to the parent drug under physiological conditions driven by their inherent chemical instability. In a set of dipeptide extended GLP-analogs we explored the rate of diketopiperazine (DKP) and diketomorpholine (DMP) formation, and the release of the active peptide. The rate of cleavage was observed to be a function of the conformation of the dipeptide promoiety and the strength of the cyclization nucleophile. Through the careful selection of chemical functionality, a set of GLP ester prodrugs of variable half-lives has been identified.

Effects of glucagon-like peptide 1 and oxyntomodulin on neuronal activity of ghrelin-sensitive neurons in the hypothalamic arcuate nucleus

Glucagon-like peptide 1 (GLP-1) and oxyntomodulin (OXM) are structurally related gastrointestinal hormones that are secreted in response to food intake. They reduce food intake and body weight and exert partly overlapping actions on glucose homeostasis and gastrointestinal function. The hypothalamic arcuate (ARC) nucleus is among the central structures expressing a high density of GLP-1 receptors (GLP-1R), which are known to be activated by both peptides. It was the aim of our electrophysiological studies to characterize the effects of GLP-1 and OXM on functionally defined ghrelin-sensitive ARC neurons. GLP-1 and OXM (10−7 M) exerted excitatory effects in about two-thirds of ghrelin-inhibited neurons and in approximately one-third of ghrelin-excited cells. In addition, a minor fraction of the ghrelin-excited cells was inhibited by both peptides. There was a high degree of cosensitivity to GLP-1 and OXM, and the effects of both hormones were blocked by the GLP-1R antagonist exendin(9–39). The GLP-1R-mediated excitations and inhibitions persisted under synaptic blockade, indicating a direct postsynaptic mode of action. Our results demonstrate that GLP-1 and OXM directly and similarly alter neuronal activity in the ARC, probably via a common GLP-1R-mediated mechanism. Ghrelin-antagonistic effects on neuronal activity, which might be implicated in ghrelin-antagonistic in vivo actions, resulting from GLP-1R stimulation (e.g., GLP-1R dependent supression of food intake), predominated in ghrelin-inhibited ARC neurons. However, a subset of ghrelin-excited ARC neurons showed responses to OXM or GLP-1, suggesting the existence of a common mode of action for these hormones; the functional relevance of this effect remains to be elucidated.

Secretion and dipeptidyl peptidase-4-mediated metabolism of incretin hormones after a mixed meal or glucose ingestion in obese compared to lean, nondiabetic men

CONTEXT

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are cleaved by dipeptidyl peptidase-4 (DPP-4); plasma activity of DPP-4 may be increased in obesity. The impact of this increase on incretin hormone secretion and metabolism is not known.

OBJECTIVE

The aim of the study was to assess incretin hormone secretion and degradation in lean and obese nondiabetic subjects.

DESIGN, SETTINGS, AND PARTICIPANTS

We studied the ingestion of a mixed meal (560 kcal) or oral glucose (2 g/kg) in healthy lean (n = 12; body mass index, 20-25 kg/m(2)) or obese (n = 13; body mass index, 30-35 kg/m(2)) males at a University Clinical Research Unit.

MAIN OUTCOME MEASURES

We measured the area under the curve of plasma intact (i) and total (t) GIP and GLP-1 after meal ingestion and oral glucose.

RESULTS

Plasma DPP-4 activity was higher in the obese subjects (38.5 +/- 3.0 vs. 26.7 +/- 1.6 mmol/min . microl; P = 0.002). Although GIP secretion (AUC(tGIP)) was not reduced in obese subjects after meal ingestion or oral glucose, AUC(iGIP) was lower in obese subjects (8.5 +/- 0.6 vs. 12.7 +/- 0.9 nmol/liter x 300 min; P < 0.001) after meal ingestion. GLP-1 secretion (AUC(tGLP-1)) was reduced in obese subjects after both meal ingestion (7.3 +/- 0.9 vs. 10.0 +/- 0.6 nmol/liter x 300 min; P = 0.022) and oral glucose (6.6 +/- 0.8 vs. 9.6 +/- 1.1 nmol/liter x 180 min; P = 0.035). iGLP-1 was reduced in parallel to tGLP-1.

CONCLUSIONS

1) Release and degradation of the two incretin hormones show dissociated changes in obesity: GLP-1 but not GIP secretion is lower after meal ingestion and oral glucose, whereas GIP but not GLP-1 metabolism is increased after meal ingestion. 2) Increased plasma DPP-4 activity in obesity is not associated with a generalized augmented incretin hormone metabolism.

Differential islet and incretin hormone responses in morning versus afternoon after standardized meal in healthy men

CONTEXT

The insulin response to meal ingestion is more rapid in the morning than in the afternoon. Whether this is explained by a corresponding variation in the incretin hormones is not known.

OBJECTIVE

Our objective was to assess islet and incretin hormones after meal ingestion in the morning vs. afternoon.

DESIGN, SETTINGS, AND PARTICIPANTS

Ingestion at 0800 and 1700 h of a standardized meal (524 kcal) in healthy lean males (n = 12) at a University Clinical Research Unit.

MAIN OUTCOME MEASURES

We assessed early (30-min) area under the curve (AUC30) of plasma levels of insulin and intact (i) and total (t) glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) after meal ingestion and made an estimation of beta-cell function by model analysis of glucose and C-peptide.

RESULTS

Peak glucose was lower in the morning than in the afternoon (6.1 +/- 0.2 vs. 7.4 +/- 0.3 mmol/liter, P = 0.001). AUC30(insulin) (4.9 +/- 0.6 vs. 2.8 +/- 0.4 nmol/liter . 30 min; P = 0.012), AUC30(tGLP-1) (300 +/- 40 vs. 160 +/- 30 pmol/liter . 30 min, P = 0.002), AUC30(iGIP) (0.7 +/- 0.1 vs. 0.3 +/- 0.1 nmol/liter . 30 min, P = 0.002), and AUC30(tGIP) (1.1 +/- 0.1 vs. 0.6 +/- 0.1 nmol/liter . min, P = 0.007) were all higher in the morning. AUC30(iGLP-1) (r = 0.68; P = 0.021) and AUC30(iGIP) (r = 0.78; P = 0.001) both correlated to AUC30(insulin). Model analysis of beta-cell function showed a higher first-hour potentiation factor in the morning (P = 0.009). This correlated negatively with the 60-min glucose level (r = -0.63; P < 0.001).

CONCLUSIONS

The early release of GLP-1 and GIP are more pronounced in the morning than in the afternoon. This may contribute to the more rapid early insulin response, more pronounced potentiation of beta-cell function, and lower glucose after the morning meal.

Liraglutide: A Once-Daily Incretin Mimetic for the Treatment of Type 2 Diabetes Mellitus

Objective:

To review the pharmacology, pharmacokinetics, efficacy, and safety of liraglutide, a glucagon-like peptide 1 (GLP-1) analog for the treatment of type 2 diabetes mellitus.

Data Sources:

A MEDLINE search (1966–May 2009) was conducted for English-language articles using the terms glucagon-like peptide 1, incretin mimetic, NN2211, and liraglutide. Abstracts presented at the American Diabetes Association and European Association for the Study of Diabetes annual meetings in 2006, 2007, and 2008 were also searched for relevant data.

Study Selection and Data Extraction:

Articles pertinent to the pharmacology, pharmacokinetics, efficacy, and safety of liraglutide were reviewed.

Data Synthesis:

Liraglutide is a GLP-1 analog with pharmacokinetic properties suitable for once-daily administration. Clinical trial data from large, controlled studies demonstrate the effectiveness of liraglutide in terms of hemoglobin A1c (A1C) reduction, reductions in body weight, and the drug's low risk for hypoglycemic events when used as monotherapy. Data also support benefits of liraglutide therapy on β-cell responsiveness to glucose, with animal and in vitro data indicating potential benefits in β-cell mass and neogenesis with liraglutide treatment. Liraglutide has been studied as monotherapy and in combination with metformin, glimepiride, and rosiglitazone for the treatment of type 2 diabetes. Additionally, comparative data with insulin glargine and exenatide therapy are available from Phase 3 trials providing practitioners valuable clinical data on which to base clinical decision making. Overall, liraglutide is well tolerated with dose-dependent nausea, vomiting, and diarrhea being the most commonly reported adverse events in clinical trials.

Conclusions:

Once-daily administration may provide a therapeutic advantage for liraglutide over twice-daily exenatide, with similar improvements in A1C and body weight observed when liraglutide was compared with exenatide. The glucose-dependent mechanism of insulin release with GLP-1 agonist therapy holds potential clinical significance in the management of postprandial hyperglycemic excursions, with minimal risk of hypoglycemia.

Gut hormones: implications for the treatment of obesity

Bariatric surgery is the only effective treatment for patients with morbid obesity. This is no solution to the present obesity pandemic however. Currently licensed non-surgical pharmaceuticals are of limited efficacy and alternatives are needed. Harnessing the body's own appetite-regulating signals is a desirable pharmacological strategy. The gastrointestinal tract has a prime role in sensing and signalling food intake to the brain. Gut hormones are key mediators of this information, including: peptide YY (PYY), pancreatic polypeptide (PP), glucagon-like peptide 1 (GLP-1), oxyntomodulin (OXM), ghrelin, amylin and cholecystokinin (CCK). This review summarises the latest knowledge regarding the physiological and pathophysiological role of gut hormones in regulating our food intake and how this knowledge could guide, or has guided, the development of weight-loss drugs. Up-to-date outcomes of clinical trials are evaluated and directions for the future suggested.

Hyperglycemia acutely lowers the postprandial excursions of glucagon-like Peptide-1 and gastric inhibitory polypeptide in humans

INTRODUCTION

Impaired secretion of glucagon-like peptide 1 (GLP-1) has been suggested to contribute to the deficient incretin effect in patients with type 2 diabetes. It is unclear whether this is a primary defect or a consequence of the hyperglycemia in type 2 diabetes. We examined whether acute hyperglycemia reduces the postprandial excursions of gastric inhibitory polypeptide (GIP) and GLP-1, and if so, whether this can be attributed to changes in gastric emptying.

PATIENTS AND METHODS

Fifteen nondiabetic individuals participated in a euglycemic clamp and a hyperglycemic clamp experiment, carried out over 285 min. A mixed meal was ingested after 45 min. Plasma concentrations of glucose, insulin, C-peptide, glucagon, triglycerides, GIP, and GLP-1 were determined, and gastric emptying was assessed using a (13)C-octanoate breath test.

RESULTS

Glucose levels were 160 +/- 1 mg/dl during the hyperglycemic clamp experiments and 83 +/- 3 mg/dl during the euglycemia (P < 0.0001). Glucose infusion rates were higher during hyperglycemia, but meal ingestion led to a decline in glucose requirements in both experiments (P < 0.0001). Insulin and C-peptide levels were higher during the hyperglycemic clamp experiments (P < 0.0001), whereas glucagon levels were higher during euglycemia (P < 0.0001). The postprandial increases in GIP and GLP-1 concentrations were 46 and 52% lower during the experiments with hyperglycemia (P = 0.0017 and P = 0.021). Hyperglycemia also elicited a significant delay in gastric emptying (P < 0.0001).

CONCLUSIONS

Hyperglycemia acutely reduces the postprandial levels of GIP and GLP-1, possibly through a deceleration of gastric emptying. This supports the concept that reduced incretin levels in some patients with type 2 diabetes are a consequence rather than a cause of type 2 diabetes.

Glucose and insulin responses to whole grain breakfasts varying in soluble fiber, beta-glucan: a dose response study in obese women with increased risk for insulin resistance

BACKGROUND

A high intake of whole grains containing soluble fiber has been shown to lower glucose and insulin responses in overweight humans and humans with type 2 diabetes.

AIM OF THE STUDY

We investigated the linearity of this response after consumption of 5 breakfast cereal test meals containing wheat and/or barley to provide varying amounts of soluble fiber, beta-glucan (0, 2.5, 5, 7.5 and 10 g).

METHODS

Seventeen normoglycemic, obese women at increased risk for insulin resistance consumed 5 test meals within a randomized cross-over design after consuming controlled diets for 2 days. Blood samples for glucose and insulin response were obtained prior to and 30, 60, 120 and 180 min after consuming the test meals.

RESULTS

Consumption of 10 g of beta-glucan significantly reduced peak glucose response at 30 min and delayed the rate of glucose response. Area under the curve for 2 h-postprandial glycemic response was not affected by beta-glucan content. However, peak and area under the curve of insulin responses were significantly affected by the beta-glucan amount in an inverse linear relationship.

CONCLUSION

These data suggest that acute consumption of 10 g of beta-glucan is able to induce physiologically beneficial effects on postprandial insulin responses in obese women at risk for insulin resistance.

Four weeks of near-normalisation of blood glucose improves the insulin response to glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide in patients with type 2 diabetes

OBJECTIVE

The incretin effect is attenuated in patients with type 2 diabetes mellitus, partly as a result of impaired beta cell responsiveness to glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). The aim of the present study was to investigate whether 4 weeks of near-normalisation of the blood glucose level could improve insulin responses to GIP and GLP-1 in patients with type 2 diabetes.

METHODS

Eight obese patients with type 2 diabetes with poor glycaemic control (HbA(1c) 8.6 +/- 1.3%), were investigated before and after 4 weeks of near-normalisation of blood glucose (mean blood glucose 7.4 +/- 1.2 mmol/l) using insulin treatment. Before and after insulin treatment the participants underwent three hyperglycaemic clamps (15 mmol/l) with infusion of GLP-1, GIP or saline. Insulin responses were evaluated as the incremental area under the plasma C-peptide curve.

RESULTS

Before and after near-normalisation of blood glucose, the C-peptide responses did not differ during the early phase of insulin secretion (0-10 min). The late phase C-peptide response (10-120 min) increased during GIP infusion from 33.0 +/- 8.5 to 103.9 +/- 24.2 (nmol/l) x (110 min)(-1) (p < 0.05) and during GLP-1 infusion from 48.7 +/- 11.8 to 126.6 +/- 32.5 (nmol/l) x (110 min)(-1) (p < 0.05), whereas during saline infusion the late-phase response did not differ before vs after near-normalisation of blood glucose (40.2 +/- 11.2 vs 46.5 +/- 12.7 [nmol/l] x [110 min](-1)).

CONCLUSIONS

Near-normalisation of blood glucose for 4 weeks improves beta cell responsiveness to both GLP-1 and GIP by a factor of three to four. No effect was found on beta cell responsiveness to glucose alone. CLINICALTRIALS.GOV ID NO.: NCT 00612950.

FUNDING

This study was supported by The Novo Nordisk Foundation, The Medical Science Research Foundation for Copenhagen.

Physiology of incretins (GIP and GLP-1) and abnormalities in type 2 diabetes

Incretin hormones are defined as intestinal hormones released in response to nutrient ingestion, which potentiate the glucose-induced insulin response. In humans, the incretin effect is mainly caused by two peptide hormones, glucose-dependent insulin releasing polypeptide (GIP), and glucagon-like peptide-1 (GLP-1). GIP is secreted by K cells from the upper small intestine while GLP-1 is mainly produced in the enteroendocrine L cells located in the distal intestine. Their effect is mediated through their binding with specific receptors, though part of their biological action may also involve neural modulation. GIP and GLP-1 are both rapidly degraded into inactive metabolites by the enzyme dipeptidyl-peptidase-IV (DPP-IV). In addition to its effects on insulin secretion, GLP-1 exerts other significant actions, including stimulation of insulin biosynthesis, inhibition of glucagon secretion, inhibition of gastric emptying and acid secretion, reduction of food intake, and trophic effects on the pancreas. As the insulinotropic action of GLP-1 is preserved in type 2 diabetic patients, this peptide was likely to be developed as a therapeutic agent for this disease.

Functional ontogeny of the proglucagon-derived peptide axis in the premature human neonate

BACKGROUND

The regulation of intestinal growth and development in human neonates is incompletely understood, which hinders the provision of nutrients enterally. The "hindgut" hormones glucagon-like peptides 1 and 2 have been shown to play an important role in the regulation of nutrient assimilation, intestinal growth, and function.

OBJECTIVE

Our goal was to investigate the production of glucagon-like peptides 1 and 2 in premature human infants and examine the effects of prematurity and feeding on hormone release.

PATIENTS AND METHODS

With informed consent, premature infants who were admitted to a tertiary neonatal intensive care nursery (gestational age: 28-32 weeks) were monitored with weekly determinations of postprandial glucagon-like peptide 1 and 2 levels. Comparison studies with groups of normal infants and adults were performed. Hormone levels were obtained by using specific radioimmunoassay for glucagon-like peptide 1 (1-36) and glucagon-like peptide 2 (1-33), modified for small sample volumes; accurate monitoring of enteral intake was performed at all of the sampling time points.

RESULTS

Forty-five infants with a mean gestational age of 29.6 +/- 1.9 weeks were studied; fasting levels of both glucagon-like peptides 1 and 2 were elevated. There was no correlation between gestational age and glucagon-like peptide 2 output. However, both glucagon-like peptide 1 and 2 levels were correlated with the caloric value of feeds.

CONCLUSIONS

The premature human neonate has significantly higher fasting levels of glucagon-like peptides 1 and 2 compared with adults; feeding increases these levels further. These findings suggest that the proglucagon-derived peptides may have a role in normal intestinal development and nutrient handling.

The physiology of glucagon-like peptide 1

Glucagon-like peptide 1 (GLP-1) is a 30-amino acid peptide hormone produced in the intestinal epithelial endocrine L-cells by differential processing of proglucagon, the gene which is expressed in these cells. The current knowledge regarding regulation of proglucagon gene expression in the gut and in the brain and mechanisms responsible for the posttranslational processing are reviewed. GLP-1 is released in response to meal intake, and the stimuli and molecular mechanisms involved are discussed. GLP-1 is extremely rapidly metabolized and inactivated by the enzyme dipeptidyl peptidase IV even before the hormone has left the gut, raising the possibility that the actions of GLP-1 are transmitted via sensory neurons in the intestine and the liver expressing the GLP-1 receptor. Because of this, it is important to distinguish between measurements of the intact hormone (responsible for endocrine actions) or the sum of the intact hormone and its metabolites, reflecting the total L-cell secretion and therefore also the possible neural actions. The main actions of GLP-1 are to stimulate insulin secretion (i.e., to act as an incretin hormone) and to inhibit glucagon secretion, thereby contributing to limit postprandial glucose excursions. It also inhibits gastrointestinal motility and secretion and thus acts as an enterogastrone and part of the "ileal brake" mechanism. GLP-1 also appears to be a physiological regulator of appetite and food intake. Because of these actions, GLP-1 or GLP-1 receptor agonists are currently being evaluated for the therapy of type 2 diabetes. Decreased secretion of GLP-1 may contribute to the development of obesity, and exaggerated secretion may be responsible for postprandial reactive hypoglycemia.

The regulation of the lymphatic secretion of glucagon-like peptide-1 (GLP-1) by intestinal absorption of fat and carbohydrate

Glucagon-like peptide-1 (GLP-1) is an important incretin produced in the L cells of the intestine. It is essential in the regulation of insulin secretion and glucose homeostasis. Systemic GLP-1 concentrations are typically low in rodents, so it can be difficult to assay physiological levels or detect changes in response to nutrients. We have established a method of assaying GLP-1 in response to nutrients using the intestinal lymph fistula model. Intraduodenal infusion of Intralipid (4.43 kcal/3 ml) induced a significant increase of lymphatic GLP-1 concentration compared with saline control at the peak of 30 min. (P < 0.001). Isocaloric and isovolumetric treatment with dextrin, a glucose polymer, also caused a significant fourfold increase in peak concentration at 60 min (P = 0.001). These findings indicate that intestinal lymph contains high concentrations of postprandial GLP-1. Second, they reveal that GLP-1 secretion into lymph occurs in response to both enteral carbohydrate and fat, but the response to dextrin occurs later than to Intralipid with peak times at 60 and 30 min, respectively. Third, the combination of Intralipid plus dextrin demonstrated an additive effect in the stimulation of GLP-1 with peak at 30 min. These results indicate that assessment of levels in lymph is a novel and powerful means of studying the secretion of GLP-1 and potentially other gastrointestinal hormones in vivo. Furthermore, the lymph fistula rat model provides insight into the gut hormone concentrations to which the neurons and cells in the lamina propria of the gut are likely exposed.

[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.

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 incretins: a link between nutrients and well-being

The glucoincretins, glucagon-like peptide-1 (GLP-1) and gastric inhibitory peptide (GIP), are intestinal peptides secreted in response to glucose or lipid intake. Data on isolated intestinal tissues, dietary treatments and knockout mice strongly suggest that GIP and GLP-1 secretion requires glucose and lipid metabolism by intestinal cells. However, incretin secretion can also be induced by non-digestible carbohydrates and involves the autonomic nervous system and endocrine factors such as GIP itself and cholecystokinin. The classical pharmacological approach and the recent use of knockout mice for the incretin receptors have shown that a remarkable feature of incretins is the ability to stimulate insulin secretion in the presence of hyperglycaemia only, hence avoiding any hypoglycaemic episode. This important role is the basis of ongoing clinical trials using GLP-1 analogues. Since most of the data concern GLP-1, we will focus on this incretin. In addition, GLP-1 is involved in glucose sensing by the autonomic nervous system of the hepato-portal vein controlling muscle glucose utilization and indirectly insulin secretion. GLP-1 has been shown to decrease glucagon secretion, food intake and gastric emptying, preventing excessive hyperglycaemia and overfeeding. Another remarkable feature of GLP-1 is its secretion by the brain. Recently, elegant data showed that cerebral GLP-1 is involved in cognition and memory. Experiments using knockout mice suggest that the lack of the GIP receptor prevents diet-induced obesity. Consequently, macronutrients controlling intestinal glucose and lipid metabolism would control incretin secretion and would consequently be beneficial for health. The control of incretin secretion represents a major goal for new therapeutic as well as nutrition strategies for treating and/or reducing the risk of hyperglycaemic syndromes, excessive body weight and thus improvement of well-being.

Incretins and other peptides in the treatment of diabetes

Glucagon-like peptide-1 (7-36) amide (GLP-1) is a gut hormone, released postprandially,which stimulates insulin secretion and insulin gene expression as well as pancreatic B-cell growth. Together with glucose-dependent insulinotropic polypeptide (GIP), it is responsible for the incretin effect which is the augmentation of insulin secretion following oral administration of glucose. Patients with Type 2 diabetes have greatly impaired or absent incretin-mediated insulin secretion which is mainly as a result of decreased secretion of GLP-1. However,the insulinotropic action of GLP-1 is preserved in patients with Type 2 diabetes,and this has encouraged attempts to treat Type 2 diabetic patients with GLP-1.GLP-1 also possesses a number of potential advantages over existing agents for the treatment of Type 2 diabetes. In addition to stimulating insulin secretion and promoting pancreatic B-cell mass, GLP-1 suppresses glucagon secretion,delays gastric emptying and inhibits food intake. Continuous intravenous and subcutaneous administration significantly improves glycaemic control and causes reductions in both HbA1c and body weight. However, GLP-1 is metabolized extremely rapidly in the circulation by the enzyme dipeptidyl peptidase IV(DPP-IV). This is the probable explanation for the short-lived effect of single doses of native GLP-1, making it an unlikely glucose-lowering agent. The DPP-IV resistant analogue, exenatide, has Food and Drug Administration (FDA) approval for the treatment of Type 2 diabetes and selective DPP-IV inhibitors are underdevelopment. Both approaches have demonstrated remarkable efficacy in animal models and human clinical studies. Both are well tolerated and appear to have advantages over current therapies for Type 2 diabetes, particularly in terms of the effects on pancreatic B-cell restoration and potential weight loss.

Liraglutide: a once-daily GLP-1 analogue for the treatment of Type 2 diabetes mellitus

The incretin hormones are intestinal peptides that enhance insulin secretion following ingestion of nutrients. Liraglutide is a glucagon-like peptide-1 receptor analogue, which is obtained by derivatising glucagon-like peptide-1 with a fatty acid, providing a compound with pharmacokinetic properties that are suitable for once-daily dosing. Liraglutide has demonstrated lasting improvement of HbA(1c )levels, weight reduction and improved beta-cell function in patients with Type 2 diabetes mellitus. Liraglutide is well tolerated; the adverse events that are most frequently reported being transient nausea and diarrhoea. This article reviews the mechanisms of action and efficacy of liraglutide for the treatment of Type 2 diabetes mellitus. This agent is presently in Phase III clinical development.

Glucagon like peptide-1 accelerates colonic transit via central CRF and peripheral vagal pathways in conscious rats

Glucagon like peptide-1 (7-36) (GLP-1), one of the gastrointestinal (GI) regulatory peptide, is known to act as a stress related brain neurotransmitter mediating GI function. Central administration of GLP-1 inhibits gastric emptying. However, little is known about the effect of central GLP-1 on colonic transit. Effects and mechanism of GLP-1 on colonic transit were investigated in conscious rats. Immediately after intracerebroventricular (icv)-injection of GLP-1, 51Cr was applied via the catheter positioned to the proximal colon. 90 min after 51Cr injection, rats were euthanized and the colon was removed and divided into 10 equal segments. The radioactivity of each segment was counted and the geometric center (GC) was calculated. Icv-injection of GLP-1 (0.3-3 nmol) dose-dependently accelerated colonic transit [(GC: 4.4+/-0.2 in controls, 7.8+/-0.5 in GLP-1 (3 nmol)]. In contrast, intraperitoneal (ip)-injection of GLP-1 (3 nmol) did not modify colonic transit. Icv-injection of GLP-1 (3 nmol)-induced acceleration of colonic transit was attenuated by vagotomy, atropine and hexamethonium, but not by guanethidine. Icv-injection of GLP-1 (3 nmol)-induced acceleration of colonic transit was abolished by corticotropin releasing factor (CRF) antagonist, astressin. Restraint stress-induced acceleration of colonic transit was abolished by a selective GLP-1 receptor antagonist, exendin. These results indicate that the endogenous GLP-1 is involved in mediating stress-induced alteration of colonic transit via a central CRF and peripheral cholinergic pathways in rats.

Effect of hormone therapy on the enteroinsular axis

OBJECTIVE

Menopause is associated with a decline in insulin response to glucose and with insulin resistance. It has been proven that hormone therapy (HT) improves carbohydrate metabolism in postmenopausal women. However, it is known that gastrointestinal hormones play a key role in the coordination of digestion and absorption of ingested nutrients and in the regulation of pancreatic endocrine function. Therefore, the aim of this study was to investigate the effect of HT on gastrointestinal hormones and carbohydrate metabolism in postmenopausal women.

DESIGN

The prospective study was performed in 90 healthy postmenopausal women (mean age 54.5 years, standard deviation 3.34 years), of whom 49 completed the study. They were randomized and treated either with continuous transdermal HT (0.05 mg 17[beta]-estradiol every 24 hours) combined with 5 mg oral dydrogesterone daily (group A, n = 25), or with oral HT (2 mg 17[beta]-estradiol semihydrate every 24 hours) combined with 10 mg dydrogesterone as a continuous therapy (group B, n = 8). The control group (group C, n = 16) received no HT. Both basal and meal-stimulated plasma concentrations of glucose, insulin, glucose-dependent insulinotropic peptide (GIP), glucagon-like peptide-1 (GLP-1), and cholecystokinin (CCK), as well as basal estrogen levels, were measured before HT and after 6 and 12 months of treatment. At the same time intervals, all the studied parameters were measured for group C.

RESULTS

After 12 months of the transdermal HT, a decrease in both fasting (P < 0.002) and postprandial (P < 0.05) plasma glucose levels was observed. Oral HT reduced only the fasting plasma glucose level in the 12th month of treatment (P < 0.05). Regardless of the route of administration, HT reduced postprandial plasma levels of insulin (oral HT: P < 0.05; transdermal HT: P < 0.02). Fasting plasma levels of GIP were reduced after 6 and 12 months of transdermal HT (P < 0.002 and P < 0.001, respectively). Moreover, levels of postprandial GIP were reduced after 6 and 12 months of transdermal HT (P < 0.002 in both cases). Fasting and postprandial GLP-1 levels were reduced by transdermal HT after 12 months of supplementation. Oral HT also decreased these levels, but not significantly. The observed differences may, however, be related not only to the route of administration, but also to the difference in the dose of estradiol. Regardless of the route of administration, HT did not influence plasma levels of CCK.

CONCLUSIONS

Hormone therapy significantly influences the enteroinsular axis in postmenopausal women and contributes to the normalization of plasma glucose levels.

Effects of gastric emptying on the postprandial ghrelin response

Distension and chemosensitization of the stomach are insufficient to induce a ghrelin response, suggesting that postgastric feedback is required. This postgastric feedback may be regulated through insulin. We investigated the relation between gastric emptying rate and the postprandial ghrelin response as well as the role of insulin and other hormones possibly mediating this response. Fifteen healthy men [BMI 21.6 kg/m2 (SD 1.9), age 20.5 yr (SD 2.5)] were studied in a single-blind, crossover design. Subjects received two treatments separated by 1 wk: 1) a dairy breakfast in combination with a 3-h intravenous infusion of glucagon-like peptide-1 (GLP-1), which delays gastric emptying, and 2) a dairy breakfast in combination with a 3-h intravenous infusion of saline. Blood samples were drawn before breakfast and during the infusion. Postprandial ghrelin (total) responses were lower following the saline infusion compared with the GLP-1 infusion (P < 0.05). Acetaminophen concentrations, an indirect measurement of gastric emptying rate, were inversely correlated with total ghrelin concentrations (saline r = -0.76; 95% CI = -0.90, -0.49, GLP-1 r = -0.47; 95% CI = -0.76, -0.04). Ghrelin concentrations were only weakly correlated with insulin concentrations (saline r = -0.36; 95% CI = -0.69, 0.09; GLP- 1 r = -0.42; 95% CI = -0.73, 0.03), but strongly inversely correlated with GIP concentrations (saline r = -0.74; 95% CI= -0.89, -0.45; GLP-1 r = -0.63; 95% CI = -0.84, -0.27). In conclusion, our results support the hypothesis that ghrelin requires postgastric feedback, which may not be regulated through insulin. Conversely, our data suggest a role of glucose-dependent insulinotropic polypeptide in ghrelin secretion.

Glucagon-related peptide 1 (GLP-1): hormone and neurotransmitter

The interest in glucagon-like petide-1 (GLP-1) and other pre-proglucagon derived peptides has risen almost exponentially since seminal papers in the early 1990s proposed to use GLP-1 agonists as therapeutic agents for treatment of type 2 diabetes. A wealth of interesting studies covering both normal and pathophysiological role of GLP-1 have been published over the last two decades and our understanding of GLP-1 action has widened considerably. In the present review, we have tried to cover our current understanding of GLP-1 actions both as a peripheral hormone and as a central neurotransmitter. From an initial focus on glycaemic control, GLP-1 research has been diverted to study its role in energy homeostasis, neurodegeneration, cognitive functions, anxiety and many more functions. With the upcoming introduction of GLP-1 agonists on the pharmaceutical venue, we have witnessed an outstanding example of how initial ideas from basic science laboratories have paved their way to become a novel therapeutic strategy to fight diabetes.

What do we know about the secretion and degradation of incretin hormones?

The incretin hormones, glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide-1 (GLP-1) are secreted from endocrine cells located in the intestinal mucosa, and act to enhance meal-induced insulin secretion. GIP and GLP-1 concentrations in the plasma rise rapidly after food ingestion, and the presence of unabsorbed nutrients in the intestinal lumen is a strong stimulus for their secretion. Nutrients can stimulate release of both hormones by direct contact with the K-cell (GIP) and L-cell (GLP-1), and this may be the most important signal. However, nutrients also stimulate GLP-1 and GIP secretion indirectly via other mechanisms. Incretin hormone secretion can be modulated neurally, with cholinergic muscarinic, beta-adrenergic and peptidergic (gastrin-releasing peptide, GRP) fibres generally having positive effects, while secretion is restrained by alpha-adrenergic and somatostatinergic fibres. Hormonal factors may also influence incretin hormone secretion. Somatostatin exerts a local inhibitory effect on the activity of both K- and L-cells via a paracrine mechanism, while, in rodents at least, GIP from the proximal intestine has a stimulatory effect on GLP-1 secretion, possibly mediated via a neural loop involving GRP. Once they have been released, both GLP-1 and GIP are subject to rapid degradation. The ubiquitous enzyme, dipeptidyl peptidase IV (DPP IV) cleaves N-terminally, removing a dipeptide and thereby inactivating both peptides, because the N-terminus is crucial for receptor binding. Subsequently, the peptides may be degraded by other enzymes and extracted in an organ-specific manner. The intact peptides are inactivated during passage across the hepatic bed and further metabolised by the peripheral tissues, while the kidney is important for the final elimination of the metabolites.

Treatment of type 2 diabetes mellitus with agonists of the GLP-1 receptor or DPP-IV inhibitors

Glucagon-like peptide-1 (GLP-1) is a peptide hormone from the gut that stimulates insulin secretion and protects beta-cells, inhibits glucagon secretion and gastric emptying, and reduces appetite and food intake. In agreement with these actions, it has been shown to be highly effective in the treatment of Type 2 diabetes, causing marked improvements in glycaemic profile, insulin sensitivity and beta-cell performance, as well as weight reduction. The hormone is metabolised rapidly by the enzyme dipeptidyl peptidase IV (DPP-IV) and, therefore, cannot be easily used clinically. Instead, resistant analogues of the hormone (or agonists of the GLP-1 receptor) are in development, along with DPP-IV inhibitors, which have been demonstrated to protect the endogenous hormone and enhance its activity. Agonists include both albumin-bound analogues of GLP-1 and exendin-4, a lizard peptide. Clinical studies with exendin have been carried out for > 6 months and have indicated efficacy in patients inadequately treated with oral antidiabetic agents. Orally active DPP-IV inhibitors, suitable for once-daily administration, have demonstrated similar efficacy. Diabetes therapy, based on GLP-1 receptor activation, therefore, appears very promising.

Biological actions of the incretins GIP and GLP-1 and therapeutic perspectives in patients with type 2 diabetes

Incretin hormones are defined as intestinal hormones released in response to nutrient ingestion, which potentiate the glucose-induced insulin response. In humans, the incretin effect is mainly caused by two peptide hormones, glucose-dependent insulin releasing polypeptide GIP, and glucagon-like peptide-1 GLP-1. GIP is secreted by K cells from the upper small intestine while GLP-1 is mainly produced in the enteroendocrine L cells located in the distal intestine. Their effect is mediated through their binding with specific receptors, though part of their biological action may also involve neural modulation. GIP and GLP-1 are both rapidly degraded into inactive metabolites by the enzyme dipeptidyl-peptidase-IV (DPP-IV). In addition to its effects on insulin secretion, GLP-1 exerts other significant actions, including stimulation of insulin biosynthesis, inhibition of glucagon secretion, inhibition of gastric emptying and acid secretion, reduction of food intake, and trophic effects on the pancreas. As the insulinotropic action of GLP-1 is preserved in type 2 diabetic patients, this peptide was a candidate as a therapeutic agent for this disease. A number of pharmacological strategies have been developed to provide continuous delivery of GLP-1 and to prevent degradation of GLP-1, including continuous administration of GLP-1, DPP-IV inhibitors and DPP-IV resistant GLP-1 analogues. Recent results of the most clinically advanced incretin mimetics confirmed their efficacy to improve glycemic control in type 2 diabetic patients. Further results are expected to confirm the efficacy/safety profile of these compounds, and to find their place in the therapeutic strategy of type 2 diabetes.

Entero-insular axis in children with anorexia nervosa

Entero-insular axis plays an important role in generating satiety signal. Thus disturbances in this axis may influence the course of anorexia nervosa. The aim of the study was analysis of the function of the hormonal part of the entero-insular axis in girls with anorexia nervosa. Thirteen girls with anorexia nervosa and in 10 healthy girls were studied. Each girl was subjected to oral glucose tolerance test and standard meal test. Blood was collected before stimulation and within 15, 30, 60, and 120 min thereafter. The concentrations of all peptides were determined by radioimmunoassay commercial kits. Fasted and postprandial levels of these peptides as well as integrated outputs were measured. Fasting insulin concentration was significantly higher in the group of girls with anorexia nervosa than in the control group (p<0.03). What more in girls with anorexia the integrated output of insulin was significantly lower in oral glucose tolerance test than after the meal (p<0.001). Also the integrated output of glucagon in both tests was higher in the group of girls with anorexia than in the control group. The mean output of pancreatic polypeptide and cholecystokinin in anorexia group was significantly higher (p<0.001 in both cases) than that in the control group but only after the test meal. The integrated outputs of gastric inhibitory peptide in both tests were significantly higher in anorectic girls than those in the control group (oral glucose tolerance test, p<0.02; meal test, p<0.001), However, mean values of the integrated output of glucagon-like peptide 1 in both tests were significantly higher in the control group than in the girls with anorexia (p<0.001 in each case). Highly significant correlation was found between glucose concentration and the concentrations of insulin, cholecystokinin, and gastric inhibitory peptide in both tests and for the both groups. In the anorectic girls, significant correlation between insulin concentration and the concentration of gastric inhibitory peptide was found after both stimulation tests and between insulin and cholecystokinin after oral glucose only.

CONCLUSION

the disturbed secretion of the hormones of entero-insular axis after the meal in anorectic girls may have negative influence on the course of anorexia nervosa. This disease has no effect on the incretin function of cholecystokinin, gastric inhibitory peptide and glucagon-like peptide 1.

The rise and rise of drug delivery

Drug delivery has typically focused on optimizing marketed compounds, improving their effectiveness or tolerability, and simplifying their administration. This role now includes the first biopharmaceuticals as well as more conventional drugs. As drug-delivery technologies come into play earlier in the development cycle, however, they can also enhance the screening and evaluation of new compounds and 'rescue' failed compounds, such as those with low solubility. In this article, we look back at how the burgeoning field of drug delivery came into being and describe approaches for future discovery and development.