Gastric Inhibitory Polypeptide

Cardiovascular effects of GLP-1 receptor agonism

Glucagon-like peptide-1 (GLP-1) receptor agonists are extensively used in type 2 diabetic patients for the effective control of hyperglycemia. It is now clear from outcomes trials that this class of drugs offers important additional benefits to these patients due to reducing the risk of developing major adverse cardiac events (MACE). This risk reduction is, in part, due to effective glycemic control in patients; however, the various outcomes trials, further validated by subsequent meta-analysis of the outcomes trials, suggest that the risk reduction in MACE is also dependent on glycemic-independent mechanisms operant in cardiovascular tissues. These glycemic-independent mechanisms are likely mediated by GLP-1 receptors found throughout the cardiovascular system and by the complex signaling cascades triggered by the binding of agonists to the G-protein coupled receptors. This heterogeneity of signaling pathways underlying different downstream effects of GLP-1 agonists, and the discovery of biased agonists favoring specific signaling pathways, may have import in the future treatment of MACE in these patients. We review the evidence supporting the glycemic-independent evidence for risk reduction of MACE by the GLP-1 receptor agonists and highlight the putative mechanisms underlying these benefits. We also comment on the different signaling pathways which appear important for mediating these effects.

The early history of GIP 1969-2000: From enterogastrone to major metabolic hormone

This paper describes the early history of Gastric Inhibitory Polypeptide, better referred to simply as GIP, from its isolation by purification from a crude preparation of CCK-PZ (cholecystokinin/pancreozymin) to its recognition as a key play in the pathogenesis of obesity and other metabolic disorders far removed from the enterogastrone properties by which it was originally identified. Augmentation of glucose mediated insulin release, the incretin effect, was discovered soon after GIP was first isolated and only much later was its important role in the pathogenesis of obesity, through mechanism other than its insulin secretion, appreciated. Immunoassay - the method by which the concentration of GIP was measured in plasma until quite recently - was found to be flawed and to depend upon which specific epitope of the hormone an assay detected. This was especially true if it was an amino-acid sequence specific to porcine rather than human GIP. A further confounder was the discovery that much of the GIP measured by immunoassay was its biological antagonist produced by cleavage of its two N-terminal amino-acids in the circulation by the same dipeptidyl-peptidase as de-activates GLP-1. Potential use of synthetic agonistic and antagonistic GIP analogues in therapeutics was barely alluded to before year 2000.

The early history of GIP 1969-2000: From enterogastrone to major metabolic hormone

This paper describes the early history of Gastric Inhibitory Polypeptide, better referred to simply as GIP, from its isolation by purification from a crude preparation of CCK-PZ (cholecystokinin/pancreozymin) to its recognition as a key player in the pathogenesis of obesity and other metabolic disorders far removed from the enterogastrone properties by which it was originally identified. Augmentation of glucose mediated insulin release, the incretin effect, was discovered soon after GIP was first isolated and only much later was its important role in the pathogenesis of obesity, through mechanism other than insulin secretion, appreciated. Immunoassay - the only method by which the concentration of GIP was measured in plasma until quite recently - was found to be flawed and to depend upon which specific epitope of the hormone an assay detected. This was especially true if it was an amino-acid sequence specific to porcine rather than human GIP. A further confounder was the discovery that much of the GIP measured by immunoassay was its biological antagonist produced by cleavage of its two N-terminal amino-acids in the circulation by the same dipeptidyl-peptidase as de-activates GLP-1. Potential use of synthetic agonistic and antagonistic GIP analogues in therapeutics was barely alluded to before year 2000.

Does delayed gastric emptying shorten the H pylori eradication period? A double blind clinical trial

AIM: To evaluate the gastric emptying inhibitory effects of sugar and levodopa on H pylori eradication period.

METHODS: A total of 139 consecutive patients were randomized into 6 groups. The participants with peptic ulcer disease or non-ulcer dyspepsia non-responding to other medications who were also H pylori-positive patients either with positive rapid urease test (RUT) or positive histology were included. All groups were pretreated with omeprazole for 2 d and then treated with quadruple therapy regimen (omeprazole, bismuth, tetracycline and metronidazole); all drugs were given twice daily. Groups 1 and 2 were treated for 3 d, groups 3, 4 and 5 for 7 d, and group 6 for 14 d. Groups 1 to 4 received sugar in the form of 10% sucrose syrup. Levodopa was prescribed for groups 1 and 3. Patients in groups 2 and 4 were given placebo for levodopa and groups 5 and 6 received placebos for both sugar and levodopa. Upper endoscopy and biopsies were carried out before treatment and two months after treatment. Eradication of H pylori was assessed by RUT and histology 8 wk later.

RESULTS: Thirty patients were excluded. Per-protocol analysis showed successful eradication in 53% in group 1, 56% in group 2, 58% in group 3, 33.3% in group 4, 28% in group 5, and 53% in group 6. Eradication rate, patient compliance and satisfaction were not significantly different between the groups.

CONCLUSION: It seems that adding sugar or levodopa or both to anti H pylori eradication regimens may lead to shorter duration of treatment.

Incretins and the development of type 2 diabetes

The incretin hormones gastric inhibitory polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) are released in response to nutrient ingestion and potentiate glucose-stimulated insulin secretion from pancreatic beta cells. The augmentation of postprandial insulin secretion by such gastrointestinal hormones is called the incretin effect. The incretin effect is almost completely absent in patients with type 2 diabetes. This is due to 1) an approximate 15% reduction in postprandial GLP-1 secretion and 2) a near total loss of insulinotropic activity of GIP. This review article summarizes clinical studies on abnormalities in the secretion and insulinotropic effects of GIP and GLP-1 in patients with type 2 diabetes as well as in individuals at high risk. A significant proportion of first-degree relatives are characterized by a reduced insulinotropic response to exogenous GIP. Nevertheless, this phenomenon does not predispose to a more rapid deterioration in glucose tolerance or conversion to impaired glucose tolerance or diabetes. Therefore, although there are hints of early abnormalities in incretin secretion and action in prediabetic populations, it has not been proven that such phenomena are central to the pathogenesis of type 2 diabetes.

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

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

Typical Danish Caucasian type 2 diabetic patients do not commonly carry genetic variants in GIP and GLP-1 encoding regions of the proGIP and proglucagon genes

BACKGROUND

The enteroinsular-axis is abnormal in type 2 diabetics, which contributes to the diabetic phenotype. The effect of the incretin hormone gastric inhibitory polypeptide (GIP) and the secretion of the incretin hormone glucagon-like peptide-1 (GLP-1) are thus greatly diminished. The explanation for these changes could be changes in the structure of either of the hormones or their receptors. Thus, the aim of this study was to study the occurrence of genetic variants in the GIP and GLP-1 encoding regions of the proGIP and proglucagon genes in type 2 diabetic patients and matched control subjects.

METHODS AND RESULTS

Genomic DNA was extracted from buffy coats from 12 Caucasian type 2 diabetics and 12 healthy subjects, matched with respect to sex, age and BMI. The GIP and GLP-1 sequences were amplified using specific primers using the polymerase chain reaction (PCR). The amplified products were then sequenced. No germ-line mutations were identified in the GIP and the GLP-1 encoding regions of the proGIP and proglucagon genes in either the type 2 diabetic or the control subjects.

CONCLUSIONS

The perturbed incretin effect in type 2 diabetics is not commonly caused by genetic variants in either the GIP or the GLP-1 encoding genes in type 2 diabetics.

Upper gastrointestinal responses to intraduodenal nutrient in type 1 diabetes mellitus

BACKGROUND

Abnormal nutrient-related small-intestinal feedback may contribute to disordered gastric motility and upper gastrointestinal symptoms in patients with diabetes.

AIM

To evaluate the motor, sensory and incretin responses to intraduodenal nutrients in patients with type 1 diabetes and in controls.

METHODS

Eight type 1 diabetes patients (two with autonomic neuropathy) and nine controls were studied during euglycaemia. A manometric catheter was positioned across the pylorus, and nutrient was infused intraduodenally (90 kcal over 30 min). Blood glucose and plasma glucagon-like peptide 1 and gastric inhibitory polypeptide were measured, and sensations were assessed with visual analogue questionnaires.

RESULTS

During nutrient infusion, neither the number of antral waves nor the stimulation of phasic or basal pyloric pressures differed between patients and controls. Upper gut sensations and areas under the plasma incretin peptide curves did not differ between the groups.

CONCLUSIONS

During euglycaemia, the upper gastrointestinal motor, sensory and incretin peptide responses to small-intestinal nutrient are comparable in patients with relatively uncomplicated type 1 diabetes and in healthy subjects.

GLP-1 and GIP are colocalized in a subset of endocrine cells in the small intestine

BACKGROUND

The incretin hormones GIP and GLP-1 are thought to be produced in separate endocrine cells located in the proximal and distal ends of the mammalian small intestine, respectively.

METHODS AND RESULTS

Using double immunohistochemistry and in situ hybridization, we found that GLP-1 was colocalized with either GIP or PYY in endocrine cells of the porcine, rat, and human small intestines, whereas GIP and PYY were rarely colocalized. Thus, of all the cells staining positively for either GLP-1, GIP, or both, 55-75% were GLP-1 and GIP double-stained in the mid-small intestine. Concentrations of extractable GIP and PYY were highest in the midjejunum [154 (95-167) and 141 (67-158) pmol/g, median and range, respectively], whereas GLP-1 concentrations were highest in the ileum [92 (80-207) pmol/l], but GLP-1, GIP, and PYY immunoreactive cells were found throughout the porcine small intestine.

CONCLUSIONS

Our results provide a morphological basis to suggest simultaneous, rather than sequential, secretion of these hormones by postprandial luminal stimulation.

Inhibition of gastric inhibitory polypeptide signaling prevents obesity

Secretion of gastric inhibitory polypeptide (GIP), a duodenal hormone, is primarily induced by absorption of ingested fat. Here we describe a novel pathway of obesity promotion via GIP. Wild-type mice fed a high-fat diet exhibited both hypersecretion of GIP and extreme visceral and subcutaneous fat deposition with insulin resistance. In contrast, mice lacking the GIP receptor (Gipr(-/-)) fed a high-fat diet were clearly protected from both the obesity and the insulin resistance. Moreover, double-homozygous mice (Gipr(-/-), Lep(ob)/Lep(ob)) generated by crossbreeding Gipr(-/-) and obese ob/ob (Lep(ob)/Lep(ob)) mice gained less weight and had lower adiposity than Lep(ob)/Lep(ob) mice. The Gipr(-/-) mice had a lower respiratory quotient and used fat as the preferred energy substrate, and were thus resistant to obesity. Therefore, GIP directly links overnutrition to obesity and it is a potential target for anti-obesity drugs.

Plasma amino acid values and pancreatic beta-cell function in phenylketonuria

In 16 phenylketonuric (PKU) patients aged 5-12 years, plasma glucose, immunoreactive insulin (IRI), C-peptide (CP) and plasma amino acids were measured in basal conditions and under a standard oral glucose tolerance test (OGTT). The beta-cell response to OGTT was higher in PKU patients than in normal subjects as demonstrated by peak levels and areas under the curves of plasma concentrations of IRI and of CP. A significant correlation was observed between plasma phenylalanine values and both IRI and CP 'output' in PKU patients. Mean concentrations of branched chain amino acids and tyrosine in plasma decreased significantly during OGTT, while phenylalanine values increased in PKU subjects.

Stimulation of insulin release in isolated rat islets by GIP in physiological concentrations and its relation to islet cyclic AMP content

Several insulinotropic hormones have been shown to increase the level of cyclic AMP in isolated islets. This study was performed to investigate whether gastric inhibitory polypeptide (glucose-dependent insulin-releasing polypeptide) has a similar effect, in particular at concentrations close to the physiological level in blood. Collagenase isolated rat islets were maintained for 24 h in tissue culture (medium 199) and then incubated for 30 min for measurement of insulin release and cyclic AMP content. Glucose-induced (16.7 mmol/l) insulin release was enhanced by gastric inhibitory polypeptide 1-100 ng/ml (0.196-19.6 nmol/l) in a dose-related fashion. The cyclic AMP content was enhanced only by 100 ng/ml. However, when 0.1 mmol/l of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine was present, even 1 ng/ml of gastric inhibitory polypeptide increased both cyclic AMP content and insulin release. Such a concentration of the hormone can be measured in human blood after a meal. In contrast, in freshly isolated islets no effect of the hormone on glucose-induced insulin release or cyclic AMP content could be detected for concentrations ranging from 1 to 100 ng/ml. These findings demonstrate that the hormone sensitivity of isolated islets is markedly enhanced by short-term maintenance in tissue culture. The results suggest that an increase in cyclic AMP is seen in response to gastric inhibitory polypeptide and may be causally related to the insulinotropic effect of the hormone.

Jejunal secretory effect of intraduodenal food in humans. A comparison of mixed nutrients, proteins, lipids, and carbohydrates

The effect of the presence of food in the intestinal lumen on fluid transport by an intestinal loop isolated from nutrients is debatable and seems to be species dependent. The aim of the present study was to investigate this effect in humans. Fluid and ion transport by a 30-cm-long jejunal loop was measured by the perfusion of a plasmalike electrolyte solution below an occlusive balloon inflated at the angle of Treitz. At the same time, the duodenum was infused at the papilla by saline (control period) or one of the following solutions (test period): protein hydrolysate, starch hydrolysate, lipids, or mixed nutrients. The four solutions (pH 7; 300 mosmol/L; 540 kcal/L) were infused in 6 normal subjects in a randomized order. In 6 further subjects, two other loads of intraduodenal lipids (120 and 1080 kcal/L) were tested according to a similar protocol. Blood samples were taken serially for radioimmunoassays of gastrin, secretin, cholecystokinin, pancreatic polypeptide, gastric inhibitory polypeptide, vasoactive intestinal polypeptide, motilin, and somatostatin. Intraduodenal mixed nutrients, proteins, and lipids significantly reduced water and ion jejunal net absorption or induced a net secretion (without dose-effect relationship for lipids) and stimulated plasma cholecystokinin, pancreatic polypeptide, and gastric inhibitory polypeptide. Intraduodenal lipids also stimulated circulating levels of gastrin and vasoactive intestinal polypeptide. Intraduodenal sugars did not change jejunal fluid and ion transport and significantly increased plasma gastric inhibitory polypeptide. Covariance analysis showed transjejunal fluid movements to be linked with plasma levels of cholecystokinin. We conclude that an intraduodenal mixed meal exerts a secretory effect on a jejunal loop isolated from the nutrients and that this effect is due to the lipid and protein content of the meal; our data are compatible with a mediation of this phenomenon by cholecystokinin.