Role of adipose tissue GLP-1R expression in metabolic improvement after bariatric surgery in patients with type 2 diabetes.

We aimed to explore the relationship between GLP-1 receptor (GLP-1R) expression in adipose tissue (AT) and incretin secretion, glucose homeostasis and weight loss, in patients with morbid obesity and type 2 diabetes undergoing bariatric surgery. RNA was extracted from subcutaneous (SAT) and visceral (VAT) AT biopsies from 40 patients randomized to metabolic gastric bypass, sleeve gastrectomy or greater curvature plication. Biochemical parameters, fasting plasma insulin, glucagon and area under the curve (AUC) of GLP-1 following a standard meal test were determined before and 1 year after bariatric surgery. GLP-1R expression was higher in VAT than in SAT. GLP-1R expression in VAT correlated with weight (r = -0.453, p = 0.008), waist circumference (r = -0.494, p = 0.004), plasma insulin (r = -0.466, p = 0.007), and systolic blood pressure (BP) (r = -0.410, p = 0.018). At 1 year, GLP-1R expression in VAT was negatively associated with diastolic BP (r = -0.361, p = 0.039) and, following metabolic gastric bypass, with the increase of GLP-1 AUC, (R2 = 0.46, p = 0.038). Finally, GLP-1R in AT was similar independently of diabetes outcomes and was not associated with weight loss after surgery. Thus, GLP-1R expression in AT is of limited value to predict incretin response and does not play a role in metabolic outcomes after bariatric surgery.

Stimulation of proglucagon gene expression by human GPR119 in enteroendocrine L-cell line GLUTag.

GPR119 is a G protein-coupled receptor expressed on enteroendocrine L-cells that synthesize and secrete the incretin hormone glucagon-like peptide-1 (GLP-1). Although GPR119 agonists stimulate L-cell GLP-1 secretion, there is uncertainty concerning whether GLP-1 biosynthesis is under the control of GPR119. Here we report that GPR119 is functionally coupled to increased proglucagon (PG) gene expression that constitutes an essential first step in GLP-1 biosynthesis. Using a mouse L-cell line (GLUTag) that expresses endogenous GPR119, we demonstrate that PG gene promoter activity is stimulated by GPR119 agonist AS1269574. Surprisingly, transfection of GLUTag cells with recombinant human GPR119 (hGPR119) results in a constitutive and apparently ligand-independent increase of PG gene promoter activity and PG mRNA content. These constitutive actions of hGPR119 are mediated by cAMP-dependent protein kinase (PKA) but not cAMP sensor Epac2. Thus, the constitutive action of hGPR119 to stimulate PG gene promoter activity is diminished by: 1) a dominant-negative Gαs protein, 2) a dominant-negative PKA regulatory subunit, and 3) a dominant-negative A-CREB. Interestingly, PG gene promoter activity is stimulated by 6-Bn-cAMP-AM, a cAMP analog that selectively activates α and ß isoforms of type II, but not type I PKA regulatory subunits expressed in GLUTag cells. Finally, our analysis reveals that a specific inhibitor of Epac2 activation (ESI-05) fails to block the stimulatory action of 6-Bn-cAMP-AM at the PG gene promoter, nor is PG gene promoter activity stimulated by: 1) a constitutively active Epac2, or 2) cAMP analogs that selectively activate Epac proteins. Such findings are discussed within the context of ongoing controversies concerning the relative contributions of PKA and Epac2 to the control of PG gene expression.

[The physiology of incretins]. / Az inkretinek elettana.

The discovery of incretins-glucagon-like peptide (GLP)-1 and glucose-dependent insulinotrop peptide (GIP)-, clarification of their physiological properties as well as therapeutic application of incretin-based blood glucose lowering drugs opened new perspectives in the medical management of type 2 diabetes. New results of basic research investigations led to revaluation of the role of GIP in metabolic processes and a more established use of GLP-1 action. The article overviews the most relevant data of production and effects of incretins, as well as future possibilities of their therapeutic use.

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.

Early changes in incretin secretion after laparoscopic duodenal-jejunal bypass surgery in type 2 diabetic patients.

BACKGROUND: A stomach-preserving duodenal-jejunal bypass (DJB) has been used for the treatment of type 2 diabetes mellitus (DM) since Rubino et al. first reported a prospective trial. However, there has been no report on changes in incretin secretion after DJB. We aimed to investigate whether DJB changes incretin secretion in nonmorbidly obese type 2 diabetic patients. METHODS: The inclusion criteria in this prospective study were: patient age of 20-65 years, body mass index of <30 kg/m(2), a history of type 2 DM for ≤10 years, and fasting C-peptide ≥0.3 nmol/l. Six patients with type 2 DM without morbid obesity underwent DJB. Fasting plasma glucose and glycated hemoglobin (HbA1c) were measured. An oral glucose tolerance test (OGTT) was performed with measurement of glucagon-like peptide 1 (GLP-1), glucose-dependent insulinotropic peptide (GIP), insulin, C-peptide, and glucagon. The study subjects were followed up for 6 months. RESULTS: The body weight of patients declined after surgery. The area under the curve (AUC) of glucose, peak glucose levels during OGTT, and HbA1c also declined until 3 months postoperatively. The AUC of C-peptide and insulin tended to increase postoperatively. The AUC of glucagon had a significant increase at 6 months postoperatively. The AUC of active GLP-1 increased at 1 month and at 6 months postoperatively. There was no change in the AUC of total GIP. CONCLUSION: Our data suggest that DJB increases GLP-1 secretion in nonmorbidly obese type 2 diabetic patients. However, long-term data are needed to confirm this finding.

Incretin physiology and its role in type 2 diabetes mellitus.

Incretins are hormones that are released after ingestion of a meal and augment the secretion of insulin. Current research suggests that GLP-1 (glucagon-like peptide 1) is the most important. Their action is terminated by enzymes known as dipeptidyl peptidase-4 (DPP-4). The observation that the incretin response may be diminished in individuals with type 2 diabetes mellitus has led to advances in the management of this disease. Agents that act as incretin mimetics, such as exenatide and liraglutide, and DPP-4 inhibitors, such as sitagliptin phosphate and saxagliptin, improve glycated hemoglobin levels either as monotherapy or in combination with other agents. Importantly, these agents either lead to weight loss or are weight neutral and are associated with a low risk of hypoglycemia--properties that further contribute to their clinical utility.

The physiologic role of incretin hormones: clinical applications.

Treatment of patients with type 2 diabetes mellitus (T2DM) traditionally has involved a progression of phases, from conventional lifestyle interventions and monotherapy, to combination therapy involving oral agents, to insulin initiation and its use either alone or with oral pharmacotherapy. Currently, the need for antidiabetic therapies with fewer adverse effects (eg, weight gain, reduced rates of hypoglycemia) is unmet. In addition, most treatments fail to adequately control postprandial hyperglycemia. Traditional options have generally been directed at the "insulin demand" aspect and have targeted insulin secretion or insulin resistance in peripheral tissues. Only recently have agents been available to address the "glucose supply" aspect that leads to fasting hyperglycemia in patients with T2DM. Incretin-based therapies, however, address both aspects. Two classes of incretin-directed therapies are available and work by either increasing endogenous levels of glucagon-like peptide-1 (GLP-1) (ie, dipeptidyl peptidase-4 inhibitors) or by mimicking the activity of endogenous GLP-1 (ie, GLP-1 agonists). These therapies treat the key metabolic abnormalities associated with T2DM but do so with reduced rates of hypoglycemia and do not promote weight gain as compared with conventional therapies.

Gut microbiota fermentation of prebiotics increases satietogenic and incretin gut peptide production with consequences for appetite sensation and glucose response after a meal.

BACKGROUND: We have previously shown that gut microbial fermentation of prebiotics promotes satiety and lowers hunger and energy intake in humans. In rodents, these effects are associated with an increase in plasma gut peptide concentrations, which are involved in appetite regulation and glucose homeostasis. OBJECTIVE: Our aim was to examine the effects of prebiotic supplementation on satiety and related hormones during a test meal for human volunteers by using a noninvasive micromethod for blood sampling to measure plasma gut peptide concentrations. DESIGN: This study was a randomized, double-blind, parallel, placebo-controlled trial. A total of 10 healthy adults (5 men and 5 women) were randomly assigned to groups that received either 16 g prebiotics/d or 16 g dextrin maltose/d for 2 wk. Meal tolerance tests were performed in the morning to measure the following: hydrogen breath test, satiety, glucose homeostasis, and related hormone response. RESULTS: We show that the prebiotic treatment increased breath-hydrogen excretion (a marker of gut microbiota fermentation) by approximately 3-fold and lowered hunger rates. Prebiotics increased plasma glucagon-like peptide 1 and peptide YY concentrations, whereas postprandial plasma glucose responses decreased after the standardized meal. The areas under the curve for plasma glucagon-like peptide 1 and breath-hydrogen excretion measured after the meal (0-60 min) were significantly correlated (r = 0.85, P = 0.007). The glucose response was inversely correlated with the breath-hydrogen excretion areas under the curve (0-180 min; r = -0.73, P = 0.02). CONCLUSION: Prebiotic supplementation was associated with an increase in plasma gut peptide concentrations (glucagon-like peptide 1 and peptide YY), which may contribute in part to changes in appetite sensation and glucose excursion responses after a meal in healthy subjects.

Mining incretin hormone pathways for novel therapies.

The incretin hormones, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), are produced predominantly by enteroendocrine cells and have multiple blood glucose-lowering effects. Recent years have seen a surge of interest in understanding the basic physiology and pathophysiology of incretins and in applying this knowledge to the treatment of diabetes and obesity. Considerable gains have been made in elucidating the mechanisms controlling incretin secretion, and there is growing evidence to suggest that incretins might be involved in the rapid reversal of diabetes observed in gastric bypass patients. Here, we review these recent advances and outline the multiple strategies being pursued to exploit the potential therapeutic benefits of GIP and GLP-1.

WNT/beta-catenin increases the production of incretins by entero-endocrine cells.

AIMS/HYPOTHESIS: Glucose-dependent insulinotropic peptide (GIP) plays a pivotal role in the regulation of glucose homeostasis. Rates of diet-induced obesity, insulin resistance and type 2 diabetes are decreased when GIP signalling is disturbed in mice, suggesting that GIP plays a role in the onset of type 2 diabetes. WNT signalling is linked to type 2 diabetes and induces synthesis of the other incretin, glucagon-like peptide 1 (GLP-1). GLP-1 analogues improve treatment of type 2 diabetes patients in whom GLP-1 signalling is intact and have captured clinical attention. GIP levels are altered at the onset of type 2 diabetes and later on, while GIP signalling is impaired. Thus, GIP is not a candidate for treatment but might be an important target from a prevention perspective. Hypothesising that hypersecretion of GIP links altered WNT signalling to the onset of type 2 diabetes, we sought to determine whether WNT signalling induces GIP production by entero-endocrine cells. METHODS: RT-PCR and chromatin immunoprecipitation (ChIP) were used to study Gip gene induction. Gip promoter elements mediating WNT/lithium induction were identified (electrophoretic mobility shift assay, co-transfection of deletion mutants, ChIP). RESULTS: Lithium or WNT/beta-catenin signalling enhanced GIP production by entero-endocrine cells through a conserved site in the proximal Gip promoter. Lithium favours lymphoid enhancer factor-1/beta-catenin binding to Gip promoter and diminishes ChIP through T cell factor-4 and histone deacetylase 1. CONCLUSIONS/INTERPRETATION: Lithium and WNT are incretin inducers in general. This work provides a novel link between WNT signalling, obesity and diabetes.

Incretin mimetics and DPP-4 inhibitors: new approach to treatment of type 2 diabetes mellitus.

Type 2 diabetes constitutes the main bulk (85-90%) of diabetic population. It is a chronic metabolic disorder with progressive ?beta-cell dysfunction, impaired insulin actions and various other abnormalities. Insulin response of beta-cell is more after oral glucose or following meal than intravenous infusion of glucose. Gut related peptides, the incretin hormones released after meal following activation of the enteroinsular axis plays an important role in glucose homeostasis by pancreatic and extrapancreatic glucoregulatory effects and helps in preservation of beta-cell function. In type 2 diabetes, there is progressive decline of these incretins level, glucagons like peptide-1 (GLP-1) and glucose dependent insulinotropic polypeptide (GIP) with loss of beta-cell mass, beta-cell function and glycemic deterioration. These peptides are rapidly degraded by endogenous proteases, dipeptidyl peptides-4 (DPP-4) giving a very short half life of 2-3 minutes. Currently available anti-diabetic drugs do not address these arms of glucoregulatory dysfunction of type 2 diabetes. Modern therapeutic strategy should be targeted at preservation of beta-cell mass and function by exploiting the incretin hormones and enteroinsular axis. DPP-4 resistant incretin analogues/mimetics (e.g. exenatide, liraglutide) that have been developed by modifications/ substitutions in the polypeptide chain may be an effective alternative of the existing therapy of type-2 DM. DPP-4 inhibitors (e.g. sitagliptin, vindagliptin) prevent the degradation of endogenous GLP-1 and GIP, thereby potentiate their actions and help in glycemic control. Distinctive features of incretin mimetics are: their action is glucose dependent, do not produce hypoglycemia, help in preservation of beta-cell mass and function, help in weight reduction. DPP-4 inhibitors are weight neutral. Ongoing studies will reveal newer avenues and long term outcome of these molecules.

Secretion of insulinotropic proteins by commensal bacteria: rewiring the gut to treat diabetes.

Here, we show that commensal bacteria can stimulate intestinal epithelial cells to secrete insulin in response to glucose. Commensal strains were engineered to secrete the insulinotropic proteins GLP-1 and PDX-1. Epithelia stimulated by engineered strains and glucose secreted up to 1 ng ml(-1) of insulin with no significant background secretion.

Incretin and islet hormonal responses to fat and protein ingestion in healthy men.

Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) regulate islet function after carbohydrate ingestion. Whether incretin hormones are of importance for islet function after ingestion of noncarbohydrate macronutrients is not known. This study therefore examined integrated incretin and islet hormone responses to ingestion of pure fat (oleic acid; 0.88 g/kg) or protein (milk and egg protein; 2 g/kg) over 5 h in healthy men, aged 20-25 yr (n=12); plain water ingestion served as control. Both intact (active) and total GLP-1 and GIP levels were determined as was plasma activity of dipeptidyl peptidase-4 (DPP-4). Following water ingestion, glucose, insulin, glucagon, GLP-1, and GIP levels and DPP-4 activity were stable during the 5-h study period. Both fat and protein ingestion increased insulin, glucagon, GIP, and GLP-1 levels without affecting glucose levels or DPP-4 activity. The GLP-1 responses were similar after protein and fat, whereas the early (30 min) GIP response was higher after protein than after fat ingestion (P<0.001). This was associated with sevenfold higher insulin and glucagon responses compared with fat ingestion (both P<0.001). After protein, the early GIP, but not GLP-1, responses correlated to insulin (r(2)=0.86; P=0.0001) but not glucagon responses. In contrast, after fat ingestion, GLP-1 and GIP did not correlate to islet hormones. We conclude that, whereas protein and fat release both incretin and islet hormones, the early GIP secretion after protein ingestion may be of primary importance to islet hormone secretion.

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.