Dorothy Hodgkin Lecture 2008. Gastric inhibitory polypeptide (GIP) revisited: a new therapeutic target for obesity-diabetes?

Duodenal enteroendocrine cells and GIP as treatment targets for obesity and type 2 diabetes

The duodenum is an important source of endocrine and paracrine signals controlling digestion and nutrient disposition, notably including the main incretin hormone glucose-dependent insulinotropic polypeptide (GIP). Bariatric procedures that prevent nutrients from contact with the duodenal mucosa are particularly effective interventions to reduce body weight and improve glycaemic control in obesity and type 2 diabetes. These procedures take advantage of increased nutrient delivery to more distal regions of the intestine which enhances secretion of the other incretin hormone glucagon-like peptide-1 (GLP-1). Preclinical experiments have shown that either an increase or a decrease in the secretion or action of GIP can decrease body weight and blood glucose in obesity and non-insulin dependent hyperglycaemia, but clinical studies involving administration of GIP have been inconclusive. However, a synthetic dual agonist peptide (tirzepatide) that exerts agonism at receptors for GIP and GLP-1 has produced marked weight-lowering and glucose-lowering effects in people with obesity and type 2 diabetes. This appears to result from chronic biased agonism in which the novel conformation of the peptide triggers enhanced signalling by the GLP-1 receptor through reduced internalisation while reducing signalling by the GIP receptor directly or via functional antagonism through increased internalisation and degradation.

Recent advances in peptide-based therapies for obesity and type 2 diabetes

Options for the treatment of type 2 diabetes mellitus (T2DM) and obesity have recently been expanded by the results of several large clinical trials with incretin-based peptide therapies. Most of these studies have been conducted with the glucagon-like peptide-1 (GLP-1) receptor agonist semaglutide, which is available as a once weekly subcutaneous injection and once daily tablet, and the once weekly injected dual agonist tirzepatide, which interacts with receptors for GLP-1 and glucose-dependent insulinotropic polypeptide (GIP). In individuals with T2DM these therapies have achieved reductions of glycated haemoglobin (HbA1c) by > 2% and lowered body weight by > 10%. In some studies, these agents tested in non-diabetic, obese individuals at much higher doses have lowered body weight by > 15%. Emerging evidence suggests these agents can also offer cardio-protective and potentially reno-protective effects. Other incretin-based peptide therapies in early clinical development, notably a triple GLP-1/GIP/glucagon receptor agonist (retatrutide) and a combination of semaglutide with the amylin analogue cagrilintide (CagriSema), have shown strong efficacy. Although incretin therapies can incur adverse gastrointestinal effects these are for most patients mild-to-moderate and transient but result in cessation of treatment in some cases. Thus, the efficacy of new incretin-based peptide therapies is enhancing the opportunity to control body weight and blood glucose and improve the treatment of T2DM and obesity.

Glucagon-Like Peptide-1 Receptor Agonists and Dual Glucose-Dependent Insulinotropic Polypeptide/Glucagon-Like Peptide-1 Receptor Agonists in the Treatment of Obesity/Metabolic Syndrome, Prediabetes/Diabetes and Non-Alcoholic Fatty Liver Disease-Current Evidence

The obesity pandemic is accompanied by increased risk of developing metabolic syndrome (MetS) and related conditions: non-alcoholic fatty liver disease (NAFLD)/non-alcoholic steatohepatitis (NASH), type 2 diabetes mellitus (T2DM) and cardiovascular (CV) disease (CVD). Lifestyle, as well as an imbalance of energy intake/expenditure, genetic predisposition, and epigenetics could lead to a dysmetabolic milieu, which is the cornerstone for the development of cardiometabolic complications. Glucagon-like peptide-1 (GLP-1) receptor agonists (RAs) and dual glucose-dependent insulinotropic polypeptide (GIP)/GLP-1 RAs promote positive effects on most components of the "cardiometabolic continuum" and consequently help reduce the need for polypharmacy. In this review, we highlight the main pathophysiological mechanisms and risk factors (RFs), that could be controlled by GLP-1 and dual GIP/GLP-1 RAs independently or through synergism or differences in their mode of action. We also address the evidence on the use of GLP-1 and dual GIP/GLP-1 RAs in the treatment of obesity, MetS and its related conditions (prediabetes, T2DM and NAFLD/NASH). In conclusion, GLP-1 RAs have already been established for the treatment of T2DM, obesity and cardioprotection in T2DM patients, while dual GIP/GLP-1 RAs appear to have the potential to possibly surpass them for the same indications. However, their use in the prevention of T2DM and the treatment of complex cardiometabolic metabolic diseases, such as NAFLD/NASH or other metabolic disorders, would benefit from more evidence and a thorough clinical patient-centered approach. There is a need to identify those patients in whom the metabolic component predominates, and whether the benefits outweigh any potential harm.

Effects of Roux-en-Y Gastric Bypass and Sleeve Gastrectomy on Non-Alcoholic Fatty Liver Disease: A 12-Month Follow-Up Study with Paired Liver Biopsies

Roux-en-Y gastric bypass (RYGB) improves, and can sometimes resolve, non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) but data based on histological assessment for the efficacy of sleeve gastrectomy (SG) in resolving NAFLD are sparse. Consequently, we aimed to compare the efficacy of RYGB vs. SG on NAFLD 12 months after surgery. In a prospective cohort study, 40 patients with obesity underwent bariatric surgery (16 RYGB and 24 SG). During surgery, a liver biopsy was taken and repeated 12 months later. NAFLD severity was evaluated using the NAFLD Activity Score (NAS) and Kleiner Fibrosis score. RYGB and SG patients were comparable at baseline. Mean (standard deviation, SD) NAS was 3.3 (0.9) in RYGB and 3.1 (1.4) in SG (p = 0.560) with similar degrees of steatosis, inflammation, and ballooning. Two RYGB patients, and six SG patients, had NASH (p = 0.439). Twelve months after surgery, NAS was significantly and comparably (p = 0.241) reduced in both RYGB (−3.00 (95% CI −3.79–−2.21), p < 0.001) and SG (−2.25 (95% CI −2.92–−1.59), p < 0.001) patients. RYGB patients had significantly more reduced (p = 0.007) liver steatosis (−0.91 (95% CI −1.47–−1.2) than SG patients (−0.33 (95% CI −0.54–−0.13) and greater improvement in the plasma lipid profile. Fibrosis declined non-significantly. NASH was resolved in seven of eight patients without a worsening of their fibrosis. RYGB and SG have similar beneficial effects on NAS and NASH without the worsening of fibrosis. RYGB is associated with a more pronounced reduction in liver steatosis.

Molecular mechanism of an antagonistic antibody against glucose-dependent insulinotropic polypeptide receptor

Glucose-dependent insulinotropic polypeptide (GIP) is an incretin hormone involved in regulating glucose and lipid metabolism. GIP receptor (GIPR) antagonism is believed to offer therapeutic potential for various metabolic diseases. Pharmacological intervention of GIPR, however, has limited success due to lack of effective antagonistic reagents. Previously we reported the discovery of two mouse anti-murine GIPR monoclonal antibodies (mAbs) with distinctive properties in rodent models. Here, we report the detailed structural and biochemical characterization of these two antibodies, mAb1 and mAb2. In vitro and in vivo characterizations demonstrated mAb2 is a full GIPR antagonistic antibody and mAb1 is a non-neutralizing GIPR binder. To understand the molecular basis of these two antibodies, we determined the co-crystal structures of GIPR extracellular domain in complex with mAb1 and with mAb2 at resolutions of 2.1 and 2.6 Å, respectively. While the non-neutralizing mAb1 binds to GIPR without competing with the ligand peptide, mAb2 not only partially occludes the ligand peptide binding, but also recognizes the GIPR C-terminal stalk region in a helical conformation that acts as a molecular mimic of the ligand peptide and locks GIPR in a novel auto-inhibited state. Furthermore, administration of mAb2 in diet-induced obesity mice for 7 weeks leads to both reduction in body weight gain and improvement of metabolic profiles. In contrast, mAb1 has no effect on body weight or other metabolic improvement. Together, our studies reveal the unique molecular mechanism of action underlying the superior antagonistic activity of mAb2 and signify the promising therapeutic potential of effective GIPR antagonism for the treatment of metabolic disorders.

GIP's involvement in the pathophysiology of type 2 diabetes

During the past four decades derangements in glucose-dependent insulinotropic polypeptide (GIP) biology has been viewed upon as contributing factors to various parts of the pathophysiology type 2 diabetes. This overview outlines and discusses the impaired insulin responses to GIP as well as the effect of GIP on glucagon secretion and the potential involvement of GIP in the obesity and bone disease associated with type 2 diabetes. As outlined in this review, it is unlikely that the impaired insulinotropic effect of GIP occurs as a primary event in the development of type 2 diabetes, but rather develops once the diabetic state is present and beta cells are unable to maintain normoglycemia. In various models, GIP has effects on glucagon secretion, bone and lipid homeostasis, but whether these effects contribute substantially to the pathophysiology of type 2 diabetes is at present controversial. The review also discusses the substantial uncertainty surrounding the translation of preclinical data relating to the GIP system and outline future research directions.

Recent advances of GIP and future horizons

Recently GIP-GLP-1 co-agonists with powerful effects on glycemic control and body weight in patients with type 2 diabetes have been described. While such effects are the expected ones from a glucagonlike peptide-1 receptor agonist, similar contributions from the GIP component of the co-agonist would be surprising and contrast to the existing literature. Conventionally, GIP is thought of as an important incretin hormone regulating postprandial insulin secretion in glucose tolerant individuals, but such effects are weak or absent in patients with type 2 diabetes, and GIP has been proposed to an obesity-promoting hormone, rather than the opposite. Recent studies with a GIP receptor antagonist suitable for human studies have confirmed these concepts regarding the actions of endogenous GIP and point to potential beneficial metabolic effects of GIP receptor antagonists rather than agonist in the treatment of obesity and type 2 diabetes. So how is it possible that apparently similar results can be obtained with GIP receptor agonists and antagonists? Maybe the explanation should be sought in GIP receptor dynamics, where the agonists clearly elicit beta-arrestin mediated receptor internalization, rendering the target tissues unresponsive, whereas antagonists block the internalization and increase receptor expression on the cell surfaces. This may explain that both antagonists and agonists show efficacy in obesity and type 2 diabetes.

The influence of Glucose-dependent Insulinotropic Polypeptide (GIP) on human adipose tissue and fat metabolism: Implications for obesity, type 2 diabetes and Non-Alcoholic Fatty Liver Disease (NAFLD)

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon like peptide (GLP-1) are the two incretin hormones secreted by the enteroendocrine system in response to nutrient ingestion. Compared with GLP-1, GIP is less well studied as a hormone or as a potential pharmacological treatment. Beyond its insulinotropic effects in the pancreas, GIP has important biological actions in many other tissues but its role in dietary fat metabolism and lipid storage in adipose tissue has been most studied. It is still unclear if such effects of GIP on adipose tissue/fat metabolism are protective or deleterious in the long term. Antagonising GIP actions through genetic and chemical disruption in mice models prevented diet induced obesity and improved insulin sensitivity. Whilst such effects of GIP antagonism are yet to be evaluated in humans, recent studies using combined GIP and GLP-1 agonists have shown weight reduction and improved glycaemic control in people with type 2 diabetes (T2D). Therapeutic manipulation of GIP physiology is intriguing in that both agonists and antagonists of GIP are being investigated to explore their potential weight-reducing and other metabolic benefits in people with obesity, T2D and non-alcoholic fatty liver disease (NAFLD). This review will discuss the physiological effects of GIP on fat metabolism in human adipose and other non-adipose tissues such as liver, pancreas, skeletal muscle and heart, describe where the actions of GIP may contribute to the pathophysiology of obesity, T2D and NAFLD and finally describe the therapeutic implications of GIP antagonism and agonism in these conditions.

GIP analogues and the treatment of obesity-diabetes

The potential application of glucose-dependent insulinotropic polypeptide (gastric inhibitory polypeptide, GIP) in the management of obesity and type 2 diabetes has been controversial. Initial interest in the therapeutic use of GIP was dampened by evidence that its insulinotropic activity was reduced in type 2 diabetes and by reports that it increased glucagon secretion and adipose deposition in non-diabetic individuals. Also, attention was diverted away from GIP by the successful development of glucagon-like peptide-1 (GLP-1) receptor agonists, and a therapeutic strategy for GIP became uncertain when evidence emerged that both inhibition and enhancement of GIP action could prevent or reverse obese non-insulin dependent forms of diabetes in rodents. Species differences in GIP receptor responsiveness complicated the extrapolation of evidence from rodents to humans, but initial clinical studies are investigating the effect of a GIP antagonist in non-diabetic individuals. A therapeutic role for GIP agonists was reconsidered when clinical studies noted that the insulinotropic effect of GIP was increased if near-normal glycaemia was re-established, and GIP was found to have little effect on glucagon secretion or adipose deposition in obese type 2 diabetes patients. This encouraged the development of designer peptides that act as GIP receptor agonists, including chimeric peptides that mimic the incretin partnership of GIP with GLP-1, where the two agents exert complementary and often additive effects to improve glycaemic control and facilitate weight loss. Polyagonist peptides that exert agonism at GIP, GLP-1 and glucagon receptors are also under investigation as potential treatments for obese type 2 diabetes.

Medium-chain triglyceride diet stimulates less GIP secretion and suppresses body weight and fat mass gain compared with long-chain triglyceride diet

Gastric inhibitory polypeptide (GIP) is an incretin secreted from enteroendocrine K cells and potentiates insulin secretion from pancreatic β-cells. GIP also enhances long-chain triglyceride (LCT) diet-induced obesity and insulin resistance. Long-term intake of medium-chain triglyceride (MCT) diet is known to induce less body weight and fat mass gain than that of LCT diet. However, the effect of MCT diet feeding on GIP secretion and the effect of GIP on body weight and fat mass under MCT diet-feeding condition are unknown. In this study, we evaluated the effect of single MCT oil administration on GIP secretion and compared the effect of long-term MCT and LCT diet on body weight and fat mass gain in wild-type (WT) and GIP-knockout (GIP KO) mice. Single administration of LCT oil induced GIP secretion but that of MCT oil did not in WT mice. Long-term intake of LCT diet induced GIP hypersecretion and significant body weight and fat mass gain compared with that of control fat (CF) diet in WT mice. In contrast, MCT diet did not induce GIP hypersecretion, and MCT diet-fed mice showed smaller increase in body weight and fat mass gain compared with CF diet-fed mice. In GIP KO mice, body weight and fat mass were markedly attenuated in LCT diet-fed mice but not in MCT diet-fed mice. Our results suggest that long-term intake of MCT diet stimulates less GIP secretion and suppresses body weight and fat mass gain compared with that of LCT diet.

High Glycemic Index Metabolic Damage - a Pivotal Role of GIP and GLP-1

When glucose-fructose dimers are supplied as the slowly digestible, completely absorbable, low glycemic index (GI) sugar isomaltulose, the detrimental effects of high GI sucrose are avoided. This difference requires the presence of intact glucose-induced insulinotropic peptide receptor (GIPR) and is mediated by the rapid uptake of glucose and the stimulation of GIP release from K cells in the upper small intestine. GIP promotes lipogenesis, fatty liver, insulin resistance, and postprandial inflammation, and reduces fat oxidation in skeletal muscle, partly by hypothalamic interference with energy partitioning and epigenetic programming. GIP is similarly required for the detrimental metabolic effects of other high GI carbohydrates. We therefore propose that the release of GIP in the upper small intestine is an important determinant of the metabolic quality of carbohydrates.

Rebirth of the Incretin Concept: Its conception and early development

This paper describes the resurrection of the Incretin Concept in the early 1960s. It began with the more or less simultaneous discovery by three groups working independently in London. Dupre demonstrated that secretin given intravenously with glucose increased its rate of disappearance from the blood, McIntyre and co-workers established that hyperglycaemia evoked by oral glucose stimulated more insulin secretion than comparable hyperglycaemia produced by intravenous glucose and Marks and Samols established the insulinotropic properties of glucagon. The concept evolved with the discovery by Samols and co-workers that oral glucose stimulated the release of immunoreactive glucagon-like substances from the gut mucosa and the subsequent isolation of glucagon immunoreactive compounds, most notably oxyntomodulin and glicentin, and of gastic inhibitory polypetide (GIP). It concluded with the isolation and characterisation of glucagon-like peptide 1 (7-36) amide.

Association of body mass index-related single nucleotide polymorphisms with psychiatric disease and memory performance in a Japanese population

OBJECTIVE

Obesity is a risk factor for psychiatric diseases. Recently, a number of single nucleotide polymorphisms (SNPs) have been shown to be related to body mass index (BMI). In this study, we investigated the association of BMI-related SNPs with psychiatric diseases and one of their endophenotypes, memory performance, in a Japanese population.

METHODS

The subjects were 1624 patients with one of three psychiatric diseases (799 patients with major depressive disorder, 594 with schizophrenia, and 231 with bipolar disorder) and 1189 healthy controls. Memory performance was assessed using the Wechsler Memory Scale - Revised (WMS-R). Genomic DNA was prepared from venous blood and used to genotype 23 BMI-related SNPs using the TaqMan 5'-exonuclease allelic discrimination assay. We then analysed the relationships between the SNPs and psychiatric disease and various subscales of the WMS-R.

RESULTS

Three SNPs (rs11142387, rs12597579, and rs6548238) showed significant differences in the genotype or allele frequency between patients with any psychiatric diseases and controls. Furthermore, six SNPs (rs11142387, rs12597579, rs2815752, rs2074356, rs4776970, and rs2287019) showed significant differences in at least one subscale of the WMS-R depending on the genotypes of the healthy controls. Interestingly, rs11142387 near the Kruppel-like factor 9 (KLF9) was significantly associated with psychiatric disease and poor memory function.

CONCLUSIONS

We identified three and six BMI-related SNPs associated with psychiatric disease and memory performance, respectively. In particular, carrying the A allele of rs11142387 near KLF9 was found to be associated with psychiatric disease and poor memory performance, which warrants further investigations.

Role of the Gut on Glucose Homeostasis: Lesson Learned from Metabolic Surgery

Purpose of Review

Bariatric surgery was initially intended to reduce weight, and only subsequently was the remission of type two diabetes (T2D) observed as a collateral event. At the moment, the term “metabolic surgery” is used to underline the fact that this type of surgery is performed specifically to treat diabetes and its metabolic complications, such as hyperlipidemia.

Recent Findings

Randomized, controlled studies have recently supported the use of bariatric surgery, and in particular of Roux-en-Y gastric bypass (RYGB) and biliopancreatic diversion (BPD) as an effective treatment for decompensated T2D. The lesson learned from these randomized and many other non-randomized clinical studies is that the stomach and the small intestine play a central role in glucose homeostasis. Bypassing the duodenum and parts of the jejunum exerts a substantial effect on insulin sensitivity and secretion. In fact, with BPD, nutrient transit bypasses duodenum, the entire jejunum and a small portion of the ileum, resulting in reversal of insulin sensitivity back to normal and reduction of insulin secretion, whereas RYGB has little effect on insulin resistance but increases insulin secretion. Hypotheses concerning the mechanism of action of metabolic surgery for diabetes remission vary from theories focusing on jejunal nutrient sensing, to incretin action, to the blunted secretion of putative insulin resistance hormone(s), to changes in the microbiota.

Summary

Whatever the mechanism, metabolic surgery has the undoubted merit of exposing the central role of the small intestine in insulin sensitivity and glucose homeostasis.

Glucose-dependent insulinotropic polypeptide promotes lipid deposition in subcutaneous adipocytes in obese type 2 diabetes patients: a maladaptive response

Glucose-dependent insulinotropic polypeptide (GIP) beyond its insulinotropic effects may regulate postprandial lipid metabolism. Whereas the insulinotropic action of GIP is known to be impaired in type 2 diabetes mellitus (T2DM), its adipogenic effect is unknown. We hypothesized that GIP is anabolic in human subcutaneous adipose tissue (SAT) promoting triacylglycerol (TAG) deposition through reesterification of nonesterified fatty acids (NEFA), and this effect may differ according to obesity status or glucose tolerance. Twenty-three subjects categorized into four groups, normoglycemic lean (n = 6), normoglycemic obese (n = 6), obese with impaired glucose regulation (IGR; n = 6), and obese T2DM (n = 5), participated in a double-blind, randomized, crossover study involving a hyperglycemic clamp with a 240-min GIP infusion (2 pmol·kg^-1·min^-1) or normal saline. Insulin, NEFA, SAT-TAG content, and gene expression of key lipogenic enzymes were determined before and immediately after GIP/saline infusions. GIP lowered NEFA concentrations in the obese T2DM group despite diminished insulinotropic activity (mean NEFA AUC_{0-4 h} ± SE, 41,992 ± 9,843 µmol·l^-1·min^-1 vs. 71,468 ± 13,605 with placebo, P = 0.039, 95% CI: 0.31-0.95). Additionally, GIP increased SAT-TAG in obese T2DM (1.78 ± 0.4 vs 0.86 ± 0.1-fold with placebo, P = 0.043, 95% CI: 0.1-1.8). Such effect with GIP was not observed in other three groups despite greater insulinotropic activity. Reduction in NEFA concentration with GIP correlated with adipose tissue insulin resistance for all subjects (Pearson, r = 0.56, P = 0.005). There were no significant gene expression changes in key SAT lipid metabolism enzymes. In conclusion, GIP appears to promote fat accretion and thus may exacerbate obesity and insulin resistance in T2DM.

Generation of a highly diverse panel of antagonistic chicken monoclonal antibodies against the GIP receptor

Raising functional antibodies against G protein-coupled receptors (GPCRs) is challenging due to their low density expression, instability in the absence of the cell membrane's lipid bilayer and frequently short extracellular domains that can serve as antigens. In addition, a particular therapeutic concept may require an antibody to not just bind the receptor, but also act as a functional receptor agonist or antagonist. Antagonizing the glucose-dependent insulinotropic polypeptide (GIP) receptor may open up new therapeutic modalities in the treatment of diabetes and obesity. As such, a panel of monoclonal antagonistic antibodies would be a useful tool for in vitro and in vivo proof of concept studies. The receptor is highly conserved between rodents and humans, which has contributed to previous mouse and rat immunization campaigns generating very few usable antibodies. Switching the immunization host to chicken, which is phylogenetically distant from mammals, enabled the generation of a large and diverse panel of monoclonal antibodies containing 172 unique sequences. Three-quarters of all chicken-derived antibodies were functional antagonists, exhibited high-affinities to the receptor extracellular domain and sampled a broad epitope repertoire. For difficult targets, including GPCRs such as GIPR, chickens are emerging as valuable immunization hosts for therapeutic antibody discovery.

Pharmacological Actions of Glucagon-Like Peptide-1, Gastric Inhibitory Polypeptide, and Glucagon

Glucagon family of peptide hormones is a group of structurally related brain-gut peptides that exert their pleiotropic actions through interactions with unique members of class B1 G protein-coupled receptors (GPCRs). They are key regulators of hormonal homeostasis and are important drug targets for metabolic disorders such as type-2 diabetes mellitus (T2DM), obesity, and dysregulations of the nervous systems such as migraine, anxiety, depression, neurodegeneration, psychiatric disorders, and cardiovascular diseases. The current review aims to provide a detailed overview of the current understanding of the pharmacological actions and therapeutic advances of three members within this family including glucagon-like peptide-1 (GLP-1), gastric inhibitory polypeptide (GIP), and glucagon.

The role of foregut exclusion in the deterioration of glucose and lipid metabolism induced by a high-fat diet

AIM

The small intestine may be involved in the improvement of glucose and lipid metabolism after bariatric surgery; however, the role of the foregut in metabolic changes remains unclear. This study used normal rats fed a high-fat diet (HFD) after bariatric surgery to determine the role of the foregut in glucose and lipid metabolism.

METHODS

Duodenum-jejunum bypass (DJB), gastrojejunostomy (GJ) and sham-operations were performed on Sprague-Dawley (SD) rats. Oral glucose tolerance, insulin sensitivity, β-cell function, lipid profile, glucose-stimulated glucose-dependent insulinotropic polypeptide (GIP) levels and glucagon-like peptide-1 (GLP-1) levels were measured. The rats were observed for 24 weeks post-surgery.

RESULTS

Food intake and body weight were similar between the groups during the study period (P>0.05). The DJB group exhibited better glucose and lipid metabolism than the other groups (P<0.05). Compared with the GJ group, the DJB group demonstrated superior oral glucose tolerance, insulin sensitivity and lipid profiles (P<0.05); β-cell function in the two groups was similar (P>0.05). The GIP levels were decreased in the DJB group and increased in the GJ group (P<0.05), and the GLP-1 levels were increased in the DJB and GJ groups (P>0.05).

CONCLUSIONS

We found that foregut exclusion can prevent disordered glucose and lipid metabolism. Additionally, decreased GIP secretion was associated with improvements in glucose tolerance and insulin sensitivity, particularly related to lipid metabolism. Increased GLP-1 benefited β-cell function; however, it could not reverse the disordered glucose and lipid metabolism induced by a HFD.

Pancreatic glucose-dependent insulinotropic polypeptide (GIP) (1-30) expression is upregulated in diabetes and PEGylated GIP(1-30) can suppress the progression of low-dose-STZ-induced hyperglycaemia in mice

AIMS/HYPOTHESIS

Glucose-dependent insulinotropic polypeptide (GIP) is a peptide hormone released from gut K cells. While the predominant form is GIP(1-42), a shorter form, GIP(1-30), is produced by pancreatic alpha cells and promotes insulin secretion in a paracrine manner. Here, we elucidated whether GIP(1-30) expression is modulated in mouse models of diabetes. We then investigated whether PEGylated GIP(1-30) can improve islet function and morphology as well as suppress the progression to hyperglycaemia in mice treated with low-dose streptozotocin (LD-STZ).

METHODS

We examined pancreatic GIP immunoreactivity in rodent diabetic models. We synthesised [D-Ala(2)]GIP(1-30) and modified the C-terminus with polyethylene glycol (PEG) to produce a dipeptidyl peptidase-4 (DPP-4)-resistant long-acting GIP analogue, [D-Ala(2)]GIP(1-30)-PEG. We performed i.p.GTT and immunohistochemical analysis in non-diabetic and LD-STZ diabetic mice, with or without administration of [D-Ala(2)]GIP(1-30)-PEG.

RESULTS

Pancreatic GIP expression was concomitantly enhanced with alpha cell expansion in rodent models of diabetes. Treatment with DPP-4 inhibitor decreased both the GIP- and glucagon-positive areas and preserved the insulin-positive area in LD-STZ diabetic mice. Body weight was not affected by [D-Ala(2)]GIP(1-30)-PEG in LD-STZ or non-diabetic mice. Treatment with GIP significantly ameliorated chronic hyperglycaemia and improved glucose excursions in LD-STZ mice. Treatment with GIP also reduced alpha cell expansion in the islets and suppressed plasma glucagon levels compared with non-treated LD-STZ mice. Additionally, [D-Ala(2)]GIP(1-30)-PEG preserved beta cell area via inhibition of apoptosis in LD-STZ mice.

CONCLUSIONS/INTERPRETATION

Our data suggest that GIP(1-30) expression is upregulated in diabetes, and PEGylated GIP(1-30) can suppress the progression to STZ-induced hyperglycaemia by inhibiting beta cell apoptosis and alpha cell expansion.

New perspectives on exploitation of incretin peptides for the treatment of diabetes and related disorders

The applicability of stable gut hormones for the treatment of obesity-related diabetes is now undisputable. This is based predominantly on prominent and sustained glucose-lowering actions, plus evidence that these peptides can augment insulin secretion and pancreatic islet function over time. This review highlights the therapeutic potential of glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), oxyntomodulin (OXM) and cholecystokinin (CCK) for obesity-related diabetes. Stable GLP-1 mimetics have already been successfully adopted into the diabetic clinic, whereas GIP, CCK and OXM molecules offer promise as potential new classes of antidiabetic drugs. Moreover, recent studies have shown improved therapeutic effects following simultaneous modulation of multiple receptor signalling pathways by combination therapy or use of dual/triple agonist peptides. However, timing and composition of injections may be important to permit interludes of beta-cell rest. The review also addresses the possible perils of incretin based drugs for treatment of prediabetes. Finally, the unanticipated utility of stable gut peptides as effective treatments for complications of diabetes, bone disorders, cognitive impairment and cardiovascular dysfunction is considered.

GIP increases adipose tissue expression and blood levels of MCP-1 in humans and links high energy diets to inflammation: a randomised trial

AIMS/HYPOTHESIS

Obesity is associated with elevated monocyte chemoattractant protein-1 (MCP-1), a proinflammatory chemokine related to diabetes and cardiovascular disease. Since obesity is triggered by energy dense diets, we hypothesised that nutrient induced intestinal hormones such as glucose-dependent insulinotropic peptide (GIP) may directly stimulate the release of chemokines from adipose tissue and induce low-grade inflammation.

METHODS

GIP effects on gene expression and secretion of inflammatory markers were studied by microarray analysis and PCR from human subcutaneous fat biopsies of slightly obese but healthy volunteers in the metabolic ward of German Institute of Human Nutrition, Department of Clinical Nutrition, Potsdam-Rehbrücke. To allocate the participants to the study arms they were numbered in order of their recruitment and then assigned to the groups by a random number generator. In a randomised, single-blind (participants) crossover design, the participants received GIP infusions in postprandial concentrations (2 pmol kg(-1) min(-1)) or saline (154 mmol/l NaCl) infusions for 240 min either alone, in combination with hyperinsulinaemic-euglycaemic (EU) or hyperinsulinaemic-hyperglycaemic (HC) clamps. Possible mechanisms of GIP effects were investigated in single and co-cultures of macrophage and adipocyte cell lines and in primary human monocytes, macrophages and adipocytes.

RESULTS

A total of 17 participants were randomised to the following groups: EU with GIP infusion (n = 9); EU with NaCl infusion (n = 9); HC with GIP infusion (n = 8); HC with NaCl infusion (n = 8); sole GIP infusion (n = 11) and sole placebo infusion (n = 11). All 17 individuals were analysed. The study is completed. In human subcutaneous adipose tissue (hSCAT), infusions of GIP significantly increased inflammatory chemokine and cytokine gene networks in transcriptomic microarray analyses. Particularly MCP-1 (180 ± 26%), MCP-2 (246 ± 58%) and IL-6 (234 ± 40%) mRNA levels in adipose tissue as well as circulating plasma concentrations of MCP-1 (165 ± 12 vs 135 ± 13 pg/ml; GIP vs saline after 240 min; p < 0.05 for all variables) in humans increased independently of circulating insulin or glucose plasma concentrations. GIP stimulation increased Mcp-1 mRNA-expression in co-cultures of differentiated 3T3L1-adipocytes and RAW 264.7 macrophages but not in the isolated cell lines. Similarly, GIP increased MCP-1 transcripts in co-cultures of primary human macrophages with human adipocytes. GIP receptor (GIPR) transcripts were present in primary monocytes and the different cell lines and induced activation of extracellular related kinase (ERK) as well as increases in cAMP, indicating functional receptors.

CONCLUSIONS/INTERPRETATION

Our findings suggest that the nutrient induced gut hormone GIP may initiate adipose tissue inflammation by triggering a crosstalk of adipocytes and macrophages involving MCP-1.

TRIAL REGISTRATION

ClinicalTrials.gov NCT00774488.

FUNDING

This work was supported by the German Research Foundation (DFG): grant No. Pf164/021002.

Postprandial oxidative stress and gastrointestinal hormones: is there a link?

Background

Abnormal postprandial elevation of plasma glucose and lipids plays an important role in the pathogenesis of diabetes and strongly predicts cardiovascular mortality. In patients suffering from type 2 diabetes (T2D) postprandial state is associated with oxidative stress, cardiovascular risk and, probably, with impairment of both secretion and the effect of gastrointestinal peptides. Evaluating postprandial changes of gastrointestinal hormones together with changes in oxidative stress markers may help to understand the mechanisms behind the postprandial state in diabetes as well as suggest new preventive and therapeutical strategies.

Methods

A standard meal test has been used for monitoring the postprandial concentrations of gastrointestinal hormones and oxidative stress markers in patients with T2D (n = 50) compared to healthy controls (n = 50). Blood samples were drawn 0, 30, 60, 120 and 180 minutes after the standard meal.

Results

Both basal and postprandial plasma concentrations of glucose and insulin proved to be significantly higher in patients with T2D, whereas plasma concentrations of ghrelin showed significantly lower values during the whole meal test. In comparison with healthy controls, both basal and postprandial concentrations of almost all other gastrointestinal hormones and lipoperoxidation were significantly increased while ascorbic acid, reduced glutathione and superoxide dismutase activity were decreased in patients with T2D. A positive relationship was found between changes in GIP and those of glucose and immunoreactive insulin in diabetic patients (p<0.001 and p<0.001, respectively) and between changes in PYY and those of glucose (p<0.01). There was a positive correlation between changes in GIP and PYY and changes in ascorbic acid in patients with T2D (p<0.05 and p<0.001, respectively).

Conclusion/Interpretation

Apart from a positive relationship of postprandial changes in GIP and PYY with changes in ascorbic acid, there was no direct link observed between gastrointestinal hormones and oxidative stress markers in diabetic patients.

Trial Registration

ClinicalTrials.gov NCT01572402

Chronic reduction of GIP secretion alleviates obesity and insulin resistance under high-fat diet conditions

Gastric inhibitory polypeptide (GIP) exhibits potent insulinotropic effects on β-cells and anabolic effects on bone formation and fat accumulation. We explored the impact of reduced GIP levels in vivo on glucose homeostasis, bone formation, and fat accumulation in a novel GIP-GFP knock-in (KI) mouse. We generated GIP-GFP KI mice with a truncated prepro-GIP gene. The phenotype was assessed in heterozygous and homozygous states in mice on a control fat diet and a high-fat diet (HFD) in vivo and in vitro. Heterozygous GIP-GFP KI mice (GIP-reduced mice [GIP(gfp/+)]) exhibited reduced GIP secretion; in the homozygous state (GIP-lacking mice [GIP(gfp/gfp)]), GIP secretion was undetectable. When fed standard chow, GIP(gfp/+) and GIP(gfp/gfp) mice showed mild glucose intolerance with decreased insulin levels; bone volume was decreased in GIP(gfp/gfp) mice and preserved in GIP(gfp/+) mice. Under an HFD, glucose levels during an oral glucose tolerance test were similar in wild-type, GIP(gfp/+), and GIP(gfp/gfp) mice, while insulin secretion remained lower. GIP(gfp/+) and GIP(gfp/gfp) mice showed reduced obesity and reduced insulin resistance, accompanied by higher fat oxidation and energy expenditure. GIP-reduced mice demonstrate that partial reduction of GIP does not extensively alter glucose tolerance, but it alleviates obesity and lessens the degree of insulin resistance under HFD conditions, suggesting a potential therapeutic value.

A novel long-acting glucose-dependent insulinotropic peptide analogue: enhanced efficacy in normal and diabetic rodents

AIM

Glucose-dependent insulinotropic peptide (GIP) is an incretin hormone that is released from intestinal K cells in response to nutrient ingestion. We aimed to investigate the therapeutic potential of the novel N- and C-terminally modified GIP analogue AC163794.

METHODS

AC163794 was synthesized by solid-phase peptide synthesis. Design involved the substitution of the C-terminus tail region of the dipeptidyl peptidase IV (DPP-IV)-resistant GIP analogue [d-Ala(2) ]GIP(1-42) with the unique nine amino acid tail region of exenatide. The functional activity and binding of AC163794 to the GIP receptor were evaluated in RIN-m5F β-cells. In vitro metabolic stability was tested in human plasma and kidney membrane preparations. Acute insulinotropic effects were investigated in isolated mouse islets and during an intravenous glucose tolerance test in normal and diabetic Zucker fatty diabetic (ZDF) rats. The biological actions of AC163794 were comprehensively assessed in normal, ob/ob and high-fat-fed streptozotocin (STZ)-induced diabetic mice. Acute glucoregulatory effects of AC163794 were tested in diet-induced obese mice treated subchronically with AC3174, the exendatide analogue [Leu(14) ] exenatide. Human GIP or [d-Ala(2) ]GIP(1-42) were used for comparison.

RESULTS

AC163794 exhibited nanomolar functional GIP receptor potency in vitro similar to GIP and [d-Ala(2) ]GIP(1-42). AC163794 was metabolically more stable in vitro and displayed longer duration of insulinotropic action in vivo versus GIP and [d-Ala(2) ]GIP(1-42). In diabetic mice, AC163794 improved HbA1c through enhanced insulinotropic action, partial restoration of pancreatic insulin content and improved insulin sensitivity with no adverse effects on fat storage and metabolism. AC163794 provided additional baseline glucose-lowering when injected to mice treated with AC3174.

CONCLUSIONS

These studies support the potential use of a novel GIP analogue AC163794 for the treatment of type 2 diabetes.

Preserved GLP-1 and exaggerated GIP secretion in type 2 diabetes and relationships with triglycerides and ALT

OBJECTIVE

To i) compare incretin responses to oral glucose and mixed meal of diabetic patients with the normoglycaemic population and ii) to investigate whether incretin responses are associated with hypertriglyceridaemia and alanine aminotransferase (ALT) as liver fat marker.

DESIGN

A population-based study.

METHODS

A total of 163 persons with normal glucose metabolism (NGM), 20 with intermediate hyperglycaemia and 20 with type 2 diabetes aged 40-65 years participated. Participants received a mixed meal and oral glucose load on separate occasions. Glucagon-like peptide 1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP) and glucagon profiles were analysed as total area under the curve (tAUC) and incremental area under the curve.

RESULTS

In diabetic patients compared with persons with NGM, we found increased GLP-1 secretion (tAUC per hour) following oral glucose (23.2 pmol/l (95% CI 17.7-28.7) vs 18.0 (95% CI 16.9-19.1), P<0.05) but not after the mixed meal. GIP secretion among diabetic patients was increased on both occasions (82.9 pmol/l (55.9-109.8) vs 47.1 (43.8-50.4) for oral glucose and 130.6 (92.5-168.7) vs 83.2 (77.5-88.9) for mixed meal, both P<0.05). After oral glucose, GLP-1 (tAUC per hour) was inversely related to fasting triglycerides. GIP (tAUC per hour) was positively related to fasting and postprandial triglycerides. Higher fasting GIP levels were related to higher fasting and postprandial triglyceride levels and ALT.

CONCLUSION

This study confirms that in type 2 diabetes, GLP-1 secretion is generally preserved and that GIP secretion is exaggerated. The mechanism underlying the divergent associations of GLP-1 and GIP metabolism with fat metabolism and liver fat accumulation warrants further study.

Structural and pharmacological characterization of novel potent and selective monoclonal antibody antagonists of glucose-dependent insulinotropic polypeptide receptor

Glucose-dependent insulinotropic polypeptide (GIP) is an endogenous hormonal factor (incretin) that, upon binding to its receptor (GIPr; a class B G-protein-coupled receptor), stimulates insulin secretion by beta cells in the pancreas. There has been a lack of potent inhibitors of the GIPr with prolonged in vivo exposure to support studies on GIP biology. Here we describe the generation of an antagonizing antibody to the GIPr, using phage and ribosome display libraries. Gipg013 is a specific competitive antagonist with equally high potencies to mouse, rat, dog, and human GIP receptors with a K_i of 7 nm for the human GIPr. Gipg013 antagonizes the GIP receptor and inhibits GIP-induced insulin secretion in vitro and in vivo. A crystal structure of Gipg013 Fab in complex with the human GIPr extracellular domain (ECD) shows that the antibody binds through a series of hydrogen bonds from the complementarity-determining regions of Gipg013 Fab to the N-terminal α-helix of GIPr ECD as well as to residues around its highly conserved glucagon receptor subfamily recognition fold. The antibody epitope overlaps with the GIP binding site on the GIPr ECD, ensuring competitive antagonism of the receptor. This well characterized antagonizing antibody to the GIPr will be useful as a tool to further understand the biological roles of GIP.

Dipeptidylpeptidase--IV, a key enzyme for the degradation of incretins and neuropeptides: activity and expression in the liver of lean and obese rats

Given the scarcity of donors, moderately fatty livers (FLs) are currently being considered as possible grafts for orthotopic liver transplantation (OLT), notwithstanding their poor tolerance to conventional cold preservation. The behaviour of parenchymal and sinusoidal liver cells during transplantation is being studied worldwide. Much less attention has been paid to the biliary tree, although this is considered the Achille's heel even of normal liver transplantation. To evaluate the response of the biliary compartment of FLs to the various phases of OLT reliable markers are necessary. Previously we demonstrated that Alkaline Phosphatase was scarcely active in bile canaliculi of FLs and thus ruled it out as a marker. As an alternative, dipeptidylpeptidase-IV (DPP-IV), was investigated. This ecto-peptidase plays an important role in glucose metabolism, rapidly inactivating insulin secreting hormones (incretins) that are important regulators of glucose metabolism. DPP-IV inhibitors are indeed used to treat Type II diabetes. Neuropeptides regulating bile transport and composition are further important substrates of DPP-IV in the enterohepatic axis. DPP-IV activity was investigated with an azo-coupling method in the liver of fatty Zucker rats (fa/fa), using as controls lean Zucker (fa/+) and normal Wistar rats. Protein expression was studied by immunofluorescence with the monoclonal antibody (clone 5E8). In Wistar rat liver, DPP-IV activity and expression were high in the whole biliary tree, and moderate in sinusoid endothelial cells, in agreement with the literature. Main substrates of DPP-IV in hepatocytes and cholangiocytes could be incretins GLP-1 and GIP, and neuropeptides such as vasoactive intestinal peptide (VIP) and substance P, suggesting that these substances are inactivated or modified through the biliary route. In lean Zucker rat liver the enzyme reaction and protein expression patterns were similar to those of Wistar rat. In obese rat liver the patterns of DPP-IV activity and expression in hepatocytes reflected the morphological alterations induced by steatosis as lipid-rich hepatocytes had scarce activity, located either in deformed bile canaliculi or in the sinusoidal and lateral domains of the plasma membrane. These findings suggest that bile canaliculi in steatotic cells have an impaired capacity to inactivate incretins and neuropeptides. Incretin and/or neuropeptide deregulation is indeed thought to play important roles in obesity and insulin-resistance. No alteration in enzyme activity and expression was found in the upper segments of the biliary tree of obese respect to lean Zucker and Wistar rats. In conclusion, this research demonstrates that DPP-IV is a promising in situ marker of biliary functionality not only of normal but also of fatty rats. The approach, initially devised to investigate the behaviour of the liver during the various phases of transplantation, appears to have a much higher potentiality as it could be further exploited to investigate any pathological or stressful conditions involving the biliary tract (i.e., metabolic syndrome and cholestasis) and the response of the biliary tract to therapy and/or to surgery.

Improvement in β-cell function after diet-induced weight loss is associated with decrease in pancreatic polypeptide in subjects with type 2 diabetes

OBJECTIVE

The aim of our study was to evaluate the effect of a lifestyle intervention program on β-cell function and to explore the role of gastrointestinal peptides in subjects with T2D.

METHODS

Subjects with T2D (n=74) received 24 weeks of intervention: 12 weeks of slimming diet (-500 kcal/day) and the subsequent 12 weeks of diet were combined with aerobic exercise. All subjects were examined at weeks 0, 12 and 24. β-cell function was assessed during standard meal tests. Insulin secretory rate (ISR) was calculated by C-peptide deconvolution, and β-cell function was quantified with a mathematical model. Plasma concentrations of gastrointestinal peptides were measured in a fasting state and during hyperinsulinemia induced by hyperinsulinemic isoglycemic clamp.

RESULTS

Mean weight loss was 5.03±4.38 kg (p<0.001) in weeks 0-12. Weight did not change significantly in weeks 12-24. Both insulin secretion at the reference level and glucose sensitivity increased in weeks 0-12 (by 33%±54% and by 26%±53%, respectively, p<0.001) and remained unchanged in weeks 12-24. Both fasting and hyperinsulinemic plasma concentrations of pancreatic polypeptide (PP) decreased in weeks 0-12 (p<0.05 for both) and did not change significantly in weeks 12-24. Changes in insulin secretion at the reference level correlated negatively with plasma concentrations of PP during hyperinsulinemia (r=-0.36; p<0.001). Changes in glucose sensitivity correlated negatively with changes in plasma concentrations of PP, both in fasting and during hyperinsulinemia (r=-0.2; p=0.01 for both). The correlations remained significant after adjustment for changes in body-mass-index.

CONCLUSIONS

After diet-induced weight loss, β-cell function improved in T2D subjects and remained unchanged after the addition of exercise. We demonstrate for the first time that these changes are associated with a decrease in PP secretion.

Chemoenzymatic reversible immobilization and labeling of proteins without prior purification

Site-specific chemical modification of proteins is important for many applications in biology and biotechnology. Recently, our laboratory and others have exploited the high specificity of the enzyme protein farnesyltransferase (PFTase) to site-specifically modify proteins through the use of alternative substrates that incorporate bioorthogonal functionality including azides and alkynes. In this study, we evaluate two aldehyde-containing molecules as substrates for PFTase and as reactants in both oxime and hydrazone formation. Using green fluorescent protein (GFP) as a model system, we demonstrate that the purified protein can be enzymatically modified with either analogue to yield aldehyde-functionalized proteins. Oxime or hydrazone formation was then employed to immobilize, fluorescently label, or PEGylate the resulting aldehyde-containing proteins. Immobilization via hydrazone formation was also shown to be reversible via transoximization with a fluorescent alkoxyamine. After characterizing this labeling strategy using pure protein, the specificity of the enzymatic process was used to selectively label GFP present in crude E. coli extract followed by capture of the aldehyde-modified protein using hydrazide-agarose. Subsequent incubation of the immobilized protein using a fluorescently labeled or PEGylated alkoxyamine resulted in the release of pure GFP containing the desired site-specific covalent modifications. This procedure was also employed to produce PEGylated glucose-dependent insulinotropic polypeptide (GIP), a protein with potential therapeutic activity for diabetes. Given the specificity of the PFTase-catalyzed reaction coupled with the ability to introduce a CAAX-box recognition sequence onto almost any protein, this method shows great potential as a general approach for selective immobilization and labeling of recombinant proteins present in crude cellular extract without prior purification. Beyond generating site-specifically modified proteins, this approach for polypeptide modification could be particularly useful for large-scale production of protein conjugates for therapeutic or industrial applications.

Role of Gut-Related Peptides and Other Hormones in the Amelioration of Type 2 Diabetes after Roux-en-Y Gastric Bypass Surgery

Bariatric surgery is currently the most effective and durable therapy for obesity. Roux-en-Y gastric bypass surgery, the most commonly performed procedure worldwide, causes substantial weight loss and improvement in several comorbidities associated with obesity, especially type 2 diabetes. Several mechanisms are proposed to explain the improvement in glucose metabolism after RYGB surgery: the caloric restriction and weight loss per se, the improvement in insulin resistance and beta cell function, and finally the alterations in the various gastrointestinal hormones and adipokines that have been shown to play an important role in glucose homeostasis. However, the timing, exact changes of these hormones, and the relative importance of these changes in the metabolic improvement postbariatric surgery remain to be further clarified. This paper reviews the various changes post-RYGB in adipokines and gut peptides in subjects with T2D.

The hypocaloric diet in type 2 diabetes - déjà vu

Dietary modification is considered a cornerstone in the management of diabetes superimposed upon which are pharmacological therapies as required. The value of hypocaloric diets in reducing and eliminating glycosuria was extolled in the pre-insulin era. A common feature of the nutrient balance of these diets was restriction in the availability of carbohydrate. Herein we review the use of diet as therapy in the past and discuss the rationale for hypocaloric dietary management of type 2 diabetes in the 21st century, drawing comparisons with bariatric surgery and considering why weight loss is particularly difficult for overweight and obese individuals with type 2 diabetes.

Weight-loss diets modify glucose-dependent insulinotropic polypeptide receptor rs2287019 genotype effects on changes in body weight, fasting glucose, and insulin resistance: the Preventing Overweight Using Novel Dietary Strategies trial

BACKGROUND

Glucose-dependent insulinotropic polypeptide [also known as gastric inhibitory polypeptide (GIP)] and its receptor (GIPR) may link overnutrition to obesity, insulin resistance, and type 2 diabetes. A GIPR variant rs2287019 was recently associated with obesity and glucose metabolism.

OBJECTIVE

We aimed to examine whether weight-loss diets that vary in fat content may modify the effect of this variant on changes in body weight, fasting glucose, and insulin resistance in the Preventing Overweight Using Novel Dietary Strategies (POUNDS LOST) trial.

DESIGN

We genotyped the GIPR rs2287019 in 737 overweight adults who were randomly assigned to 1 of 4 weight-loss diets that varied in macronutrient contents for 2 y. We assessed the percentage changes in body weight, fasting glucose, and insulin resistance (HOMA-IR) across genotypes by the low-fat and high-fat diets.

RESULTS

At 6 mo of diet intervention, the T allele of rs2287019 was associated with greater weight loss (β ± SE: -1.05 ± 0.56%; P = 0.06) and greater decreases in fasting glucose (β ± SE: -2.33 ± 0.86%; P = 0.006), fasting insulin (β ± SE: -8.76 ± 4.13%; P = 0.03), and HOMA-IR (β ± SE: -10.52 ± 4.39%; P = 0.01) in participants who were assigned to low-fat diets, whereas there was no significant genotype effect on changes in these traits in the group assigned to the high-fat diet (all P > 0.44; P-interaction = 0.08, 0.04, 0.10, and 0.07, respectively). After correction for multiple tests (significant P = 0.008), the genotype effect on changes in fasting glucose remained significant. Sensitivity analysis in white participants showed that the interactions were more evident on changes in insulin and HOMA-IR (P-interaction < 0.008).

CONCLUSION

The T allele of GIPR rs2287019 is associated with greater improvement of glucose homeostasis in individuals who choose a low-fat, high-carbohydrate, and high-fiber diet. The POUNDS LOST trial was registered at clinicaltrials.gov as NCT00072995.

Evaluation of the long-term effects of gastric inhibitory polypeptide–ovalbumin conjugates on insulin resistance, metabolic dysfunction, energy balance and cognition in high-fat-fed mice

The effects of active immunisation with gastric inhibitory polypeptide (GIP) or (proline3)GIP–ovalbumin conjugates on insulin resistance, metabolic dysfunction, energy expenditure and cognition were examined in high-fat-fed mice. Normal mice were injected (subcutaneously) once every 14 d for 98 d with GIP–ovalbumin conjugates, with transfer to a high-fat diet on day 21. Active immunisation resulted in GIP antibody generation and significantly (P < 0·01 to P < 0·001) reduced circulating non-fasting plasma insulin concentrations compared to high-fat control mice from day 70 onwards. The glycaemic responses to intraperitoneal glucose or nutrient ingestion were significantly improved in all treated mice, with corresponding stimulated plasma insulin levels depressed compared to high-fat controls. These changes were associated with substantially (P < 0·001) improved glucose-lowering responses to exogenous insulin and decreases of muscle and fat TAG, pancreatic insulin, circulating total and LDL-cholesterol levels (P < 0·01 to P < 0·001). Treatment with GIP–ovalbumin conjugates was not associated with alterations in energy expenditure, indirect calorimetry or aspects of cognitive function. The observed changes were almost identical in GIP and (Pro3)GIP immunised mice and were independent of any effects on food intake or body weight. Further tests established that coupling of GIP peptides to ovalbumin abolished any intrinsic insulin-releasing or glucose-lowering activity. These results suggest that induction of GIP-neutralising antibodies with GIP–ovalbumin conjugates is an effective means of countering the metabolic abnormalities induced by high-fat feeding and does not adversely have an impact on a marker of cognition function or energy expenditure.

Metabolomic linkage reveals functional interaction between glucose-dependent insulinotropic polypeptide and ghrelin in humans

The gastric peptide ghrelin promotes energy storage, appetite, and food intake. Nutrient intake strongly suppresses circulating ghrelin via molecular mechanisms possibly involving insulin and gastrointestinal hormones. On the basis of the growing evidence that glucose-dependent insulinotropic polypeptide (GIP) is involved in the control of fuel metabolism, we hypothesized that GIP and/or insulin, directly or via changes in plasma metabolites, might affect circulating ghrelin. Fourteen obese subjects were infused with GIP (2.0 pmol·kg(-1)·min(-1)) or placebo in the fasting state during either euglycemic hyperinsulinemic (EC) or hyperglycemic hyperinsulinemic clamps (HC). Apart from analysis of plasma ghrelin and insulin levels, GC-TOF/MS analysis was applied to create a hormone-metabolite network for each experiment. The GIP and insulin effects on circulating ghrelin were analyzed within the framework of those networks. In the HC, ghrelin levels decreased in the absence (19.2% vs. baseline, P = 0.028) as well as in the presence of GIP (33.8%, P = 0.018). Ghrelin levels were significantly lower during HC with GIP than with placebo, despite insulin levels not differing significantly. In the GIP network combining data on GIP-infusion, EC+GIP and HC+GIP experiments, ghrelin was integrated into hormone-metabolite networks through a connection to a group of long-chain fatty acids. In contrast, ghrelin was excluded from the network of experiments without GIP. GIP decreased circulating ghrelin and might have affected the ghrelin system via modification of long-chain fatty acid pools. These observations were independent of insulin and offer potential mechanistic underpinnings for the involvement of GIP in systemic control of energy metabolism.

Increased levels of serum glucose-dependent insulinotropic polypeptide as a novel risk factor for human colorectal adenoma

Obesity and insulin resistance are thought to be risk factors for colorectal adenoma. Glucose-dependent insulinotropic polypeptide (GIP) stimulates insulin secretion from the pancreas and promotes fat accumulation in adipocytes. The association between serum GIP and the risk of colorectal adenoma has not been examined previously. We investigated this association in 370 subjects who underwent total colonoscopy during thorough physical checkups between January and December 2008. We used a cross-sectional design and classified the subjects into a colorectal adenoma group and a control group without adenoma according to their endoscopic findings. Serum GIP concentrations in samples of venous blood obtained after an overnight fast were measured using a sandwich enzyme-linked immunosorbent assay kit. The mean levels of fasting GIP (34.9 ± 49.5 vs 25.0 ± 20.1 pg/mL, P = .04), triglyceride, glucose, and insulin and the values of the homeostasis model assessment of insulin resistance in the colorectal adenoma group were significantly higher than those in the control group. Multiple logistic regression analysis showed that the highest quartile of fasting GIP levels was associated with a significantly high risk of colorectal adenoma (odds ratio, 2.1; 95% confidence interval, 1.08-3.96; P = .01) in comparison with the lowest quartile. Quartile analysis demonstrated that increased levels of GIP were related to increased levels of fasting insulin and values of homeostasis model assessment β-cell. These results suggest that an increased level of fasting GIP is associated with an increased risk of colorectal adenoma.

Dual modulation of GIP and glucagon action by the low molecular weight compound 4-hydroxybenzoic acid 2-bromobenzylidene hydrazide

AIM

The presence of functional gastric inhibitory polypeptide (GIP) receptors on adipocytes and knowledge that GIP plays a key role in fat deposition suggests a beneficial effect of GIP receptor antagonism in obesity and insulin resistance. GIP receptor antagonists studied to date are peptidic GIP analogues that must be administered by injection.

METHODS

The present study has examined in vitro and in vivo metabolic actions of a low molecular weight GIP receptor modulator 4-hydroxybenzoic acid 2-bromobenzylidene hydrazide (4H2BH), suitable for oral administration.

RESULTS

4H2BH alone had no significant effect on cAMP production or insulin secretion from BRIN-BD11 cells. However, 4H2BH significantly inhibited GIP-mediated cAMP production and insulin secretion in vitro. 4H2BH also suppressed (p < 0.05 to p < 0.001) glucagon-induced elevations of cAMP generation and insulin secretion in BRIN-BD11 cells. However, 4H2BH had no effect on glucagon-like peptide-1 (GLP-1) mediated insulinotropic actions. Administration of 4H2BH to mice in combination with glucose and GIP significantly annulled the glucose-lowering actions of GIP. In agreement with this, 4H2BH completely annulled GIP-mediated insulin secretion. Combined injection of 4H2BH with glucagon also partially (p < 0.05 to p < 0.001) impaired glucagon-induced elevations in blood glucose and plasma insulin. 4H2BH had no effect on blood glucose or insulin levels when administered alone.

CONCLUSION

These results indicate that 4H2BH has a dual effect of inhibiting GIP and glucagon-mediated biological actions. Given that hyperglucagonaemia is also a cardinal feature of type 2 diabetes, 4H2BH and related low molecular weight compounds appear worthy of further evaluation for therapeutic potential in obesity diabetes.

GIP and bariatric surgery

Bariatric surgery is the most effective modality of achieving weight loss as well as the most effective treatment for type 2 diabetes mellitus (T2DM). Glucose-dependent insulinotropic polypeptide (GIP) is an incretin and is implicated in the pathogenesis of obesity and T2DM. Its role in weight loss and resolution of T2DM after bariatric surgery is very controversial. We have made an attempt to review the physiology of GIP and its role in weight loss and resolution of T2DM after bariatric surgery. We searched PubMed and included all relevant original articles (both human and animal) in the review. Whereas most human studies have shown a decrease in GIP post-malabsorptive bariatric surgery, the role of GIP in bariatric surgery done in animal experiments remains inconclusive.

GIP does not potentiate the antidiabetic effects of GLP-1 in hyperglycemic patients with type 2 diabetes

OBJECTIVE

The incretin glucagon-like peptide 1 (GLP-1) exerts insulinotropic activity in type 2 diabetic patients, whereas glucose-dependent insulinotropic polypeptide (GIP) no longer does. We studied whether GIP can alter the insulinotropic or glucagonostatic activity of GLP-1 in type 2 diabetic patients.

RESEARCH DESIGN AND METHODS

Twelve patients with type 2 diabetes (nine men and three women; 61 ± 10 years; BMI 30.0 ± 3.7 kg/m²; HbA(1c) 7.3 ± 1.5%) were studied. In randomized order, intravenous infusions of GLP-1(7-36)-amide (1.2 pmol · kg⁻¹ · min⁻¹), GIP (4 pmol · kg⁻¹ · min⁻¹), GLP-1 plus GIP, and placebo were administered over 360 min after an overnight fast (≥ 1 day wash-out period between experiments). Capillary blood glucose, plasma insulin, C-peptide, glucagon, GIP, GLP-1, and free fatty acids (FFA) were determined.

RESULTS

Exogenous GLP-1 alone reduced glycemia from 10.3 to 5.1 ± 0.2 mmol/L. Insulin secretion was stimulated (insulin, C-peptide, P < 0.0001), and glucagon was suppressed (P = 0.009). With GIP alone, glucose was lowered slightly (P = 0.0021); insulin and C-peptide were stimulated to a lesser degree than with GLP-1 (P < 0.001). Adding GIP to GLP-1 did not further enhance the insulinotropic activity of GLP-1 (insulin, P = 0.90; C-peptide, P = 0.85). Rather, the suppression of glucagon elicited by GLP-1 was antagonized by the addition of GIP (P = 0.008). FFA were suppressed by GLP-1 (P < 0.0001) and hardly affected by GIP (P = 0.07).

CONCLUSIONS

GIP is unable to further amplify the insulinotropic and glucose-lowering effects of GLP-1 in type 2 diabetes. Rather, the suppression of glucagon by GLP-1 is antagonized by GIP.

Role of the glucose-dependent insulinotropic polypeptide and its receptor in the central nervous system: therapeutic potential in neurological diseases

Glucose-dependent insulinotropic polypeptide (GIP) is a 42-amino acid hormone, secreted from the enteroendocrine K cells, which has insulin-releasing and extra-pancreatic actions. GIP and its receptor present a widespread distribution in the mammalian brain where they have been implicated with synaptic plasticity, neurogenesis, neuroprotection and behavioral alterations. This review attempts to provide a comprehensive picture of the role of GIP in the central nervous system and to highlight recent findings from our group showing its potential involvement in neurological illnesses including epilepsies, Parkinson's disease and Alzheimer's disease.

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.

Combination treatment with alogliptin and voglibose increases active GLP-1 circulation, prevents the development of diabetes and preserves pancreatic beta-cells in prediabetic db/db mice

AIM

Alogliptin, a dipeptidyl peptidase-4 (DPP-4) inhibitor, and voglibose, an alpha-glucosidase inhibitor, have different but complementary mechanisms of action on glucagon-like peptide-1 (GLP-1) regulation and glucose-lowering effects. The present study evaluated the chronic effects of combination treatment with alogliptin and voglibose in prediabetic db/db mice.

METHODS

Alogliptin (0.03%) and voglibose (0.001%) alone or in combination were administered in the diet to prediabetic db/db mice.

RESULTS

After 3 weeks, voglibose treatment increased GLP-1 secretion (voglibose alone, 1.6-fold; alogliptin plus voglibose, 1.5-fold), while it decreased plasma glucose-dependent insulinotropic polypeptide (GIP) (voglibose alone, -30%; alogliptin plus voglibose, -29%). Alogliptin, voglibose and combination treatment decreased plasma DPP-4 activity by 72, 15 and additively by 80%, respectively, and increased plasma active GLP-1 levels by 4.5-, 1.8- and synergistically by 9.1-fold respectively. Combination treatment increased plasma insulin by 3.6-fold (alogliptin alone, 1.3-fold; voglibose alone, 1.8-fold), decreased plasma glucagon by 30% (alogliptin alone, 11%; voglibose alone, 8%), and prevented the development of diabetes, much more effectively than either agent alone. After 4 weeks, alogliptin, voglibose and combination treatment increased pancreatic insulin content by 1.6-, 3.4- and synergistically by 8.5-fold respectively. Furthermore, combination treatment resulted in an increased expression of insulin, pancreatic and duodenal homeobox 1 (PDX1) and glucose transporter 2 (GLUT2), and maintenance of normal beta/alpha-cell distribution in the pancreatic islet.

CONCLUSIONS

Chronic treatment with alogliptin in combination with voglibose concurrently increased active GLP-1 circulation, increased insulin secretion, decreased glucagon secretion, prevented the onset of the disease, and preserved pancreatic beta-cells and islet structure in prediabetic db/db mice.

On the role of glucose-dependent insulintropic polypeptide in postprandial metabolism in humans

We investigated the role of glucose-dependent insulintropic polypeptide (GIP) in the regulation of gastric emptying (GE), appetite, energy intake (EI), energy expenditure (EE), plasma levels of triglycerides (TAG), and free fatty acids (FFA) in humans. First, 20 healthy males received intravenous infusion of GIP (0.8 pmol.kg(-1).min(-1)) or saline for 300 min during and after a fixed meal (protocol 1). GE was measured using paracetamol, appetite sensations using visual analog scales, EE using indirect calorimetry, and EI during a subsequent ad libitum meal (at 300 min). Next, 10 healthy males received intravenous infusions of Intralipid, glucose, or Intralipid plus glucose, with and without GIP (1.5 pmol.kg(-1).min(-1)) for 300 min (protocol 2). In protocol 1, GIP did not have any effect on GE, EI, EE, removal of TAG, or FFA and did not influence the subjective feeling of hunger, satiety, fullness or prospective food consumption compared with saline. In protocol 2, no difference was seen in the plasma TAG on Intralipid + GIP/saline and Intralipid + glucose + GIP/saline days. FFA concentrations were lower on Intralipid + glucose + GIP/saline days (P < 0.05) compared with Intralipid + GIP/saline days and on Intralipid + GIP day (P < 0.004) compared with Intralipid + saline day. Insulin increased on all GIP days compared with saline days (P < 0.05). In conclusion, while confirming its insulinotropic effects, these data suggest that GIP does not affect GE, appetite, energy intake, EE, or the clearance rate of the applied TAG formulation in humans. However, both insulin and GIP lower post-Intralipid FFA concentration, GIP probably via stimulation of insulin secretion, increasing FFA reesterification.

Glucagon-like peptide 1 and glucose-dependent insulinotropic polypeptide: new advances

PURPOSE OF REVIEW

This article highlights recent advances in our understanding of glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) physiology and their various sites of action beyond the incretin effect.

RECENT FINDINGS

Both GLP-1 and GIP stimulate insulin secretion in a glucose-dependent manner and are thus classified as incretins. Beyond glucose-dependent insulin secretion, the peptides have common actions on islet beta cells, leading beta-cell proliferation and resistance to apoptosis. However, the action of GLP-1 and GIP is not limited to the islet cells; they have regulatory functions in many organs. Recent evidence has suggested that GLP-1 has important beneficial effects in the cardiovascular system and central nervous system. GIP may play a role in promoting energy storage in humans, enhances bone formation via stimulation of osteoblast proliferation and inhibition of apoptosis and may play a role in central nervous system function.

SUMMARY

These new findings suggest further application of these hormones for the treatment of conditions such as cardiovascular disease and obesity.

New therapies for diabesity

Many patients with type 2 diabetes are obese (diabesity), and the two conditions together impose a particularly complex therapeutic challenge. Several differently acting agents are often required at the same time, encouraging development of more single-tablet combinations. Longer-acting (once daily and once weekly) injected agonists of glucagon-like peptide-1 are due to provide additional options to stimulate insulin secretion with weight loss and minimal risk of hypoglycemia. Further, dipeptidyl peptidase-4 inhibitors ("weight-neutral" insulinotropic agents) are also expected. Sodium-glucose cotransporter 2 inhibitors offer a new option to reduce hyperglycemia and facilitate weight loss by increasing the elimination of glucose in the urine. Selective peroxisome proliferator-activated receptor modulators are being studied to produce compounds with desired effects. Many other agents with antidiabetic and antiobesity activity are progressing in clinical development.

Endocrine and metabolic response to gastric bypass

PURPOSE OF REVIEW

Diabetes resolves in 80% of individuals undergoing successful Roux-en-Y gastric bypass. Absolute caloric restriction alone resulting from gastric anatomic changes indeed leads to weight loss; however, immediate effects in glycemic control often precede substantial weight loss typically associated with insulin sensitivity. One putative explanation relates to hormonal effects accompanying Roux-en-Y gastric bypass. We reviewed the existing and recent literature to investigate the hormonal changes accompanying Roux-en-Y gastric bypass.

RECENT FINDINGS

Changes in levels of five candidate enteric hormones have been recently associated with early postoperative glycemic control following Roux-en-Y gastric bypass; the strongest effects are seen with variations in glucagon-like peptide-1, glucose-dependent insulinotropic peptide and ghrelin.

SUMMARY

The unique hybridization of static anatomic restriction and dynamic absorptive bypass lends a duality to the beneficial effects of Roux-en-Y gastric bypass. This duality likely explains the short-term and long-term resolution of diabetes in patients undergoing Roux-en-Y gastric bypass.

Evidence for beneficial effects of compromised gastric inhibitory polypeptide action in obesity-related diabetes and possible therapeutic implications

Gastric inhibitory polypeptide (GIP) is a physiological gut peptide secreted from the intestinal K-cells with well documented insulin-releasing actions. However, the GIP receptor is widely distributed in peripheral organs, including the pancreas, gut, adipose tissue, heart, adrenal cortex and brain, suggesting that it may have other functions. The presence of functional GIP receptors on adipocytes and the key role played by GIP in lipid metabolism and fat deposition suggest a possible beneficial effect of compromised GIP action in obesity and insulin resistance. Several key observations in animal models of obesity-related diabetes with chemically or genetically mediated biological GIP deficiency support this concept. Thus, obese diabetic animals with compromised GIP action due to peptide-based GIP receptor antagonists, small molecular weight GIP receptor antagonists, vaccination against GIP, genetic knockout of GIP receptor or targeted K-cell destruction are protected against obesity and associated metabolic disturbances. In addition, by causing preferential oxidation of fat, blockade of GIP signalling clears triacylglycerol deposits from liver and muscle, thereby restoring mechanisms for suppression of hepatic glucose output and improving insulin sensitivity. Emerging evidence also suggests that rapid cure of diabetes in grossly obese patients undergoing bypass surgery is mediated, in part, by surgical removal of GIP-secreting K-cells in the upper small intestine.

Therapeutic potential for GIP receptor agonists and antagonists

Glucose-dependent insulinotropic polypeptide (GIP or gastric inhibitory polypeptide) is a 42-amino-acid hormone, secreted from the enteroendocrine K cells, which has insulin-releasing and extrapancreatic glucoregulatory actions. However, the unfavourable pharmacokinetic profile and the weak biological effects of native GIP limit its effectiveness for the treatment of type 2 diabetes. To overcome this, longer-acting GIP agonists exhibiting enzymatic stability and enhanced bioactivity have been generated and successfully tested in animal models of diabetes. Thus, GIP receptor agonists offer one of the newest classes of potential antidiabetic drug. GIP is also known to play a role in lipid metabolism and fat deposition. Accordingly, both genetic and chemical ablation of GIP signalling in mice with obesity-diabetes can protect against, or even reverse many of the obesity-associated metabolic disturbances. Strong parallels exist with the beneficial metabolic effects of Roux-en-Y gastric bypass in obese, insulin-resistant humans that surgically ablates GIP-secreting K cells. The purpose of this article is to highlight the therapeutic potential of GIP-based therapeutics in the treatment of type 2 diabetes and obesity.