Effects of glucose-dependent insulinotropic polypeptide and glucagon-like peptide-I-(7-36) on insulin secretion

Incretin Hormones and Type 2 Diabetes-Mechanistic Insights and Therapeutic Approaches

Simple Summary

When we ingest a meal, our intestine secretes hormones that are released into the bloodstream. Amongst these hormones are the incretins hormones which stimulate the release of insulin from the pancreas which is essential for the regulation of in particular postprandial glucose concentrations. In patients with type 2 diabetes, the effect of the incretins is diminished. This is thought to contribute importantly to the pathophysiology of the disease. However, in pharmacological amounts, the incretins may still influence insulin secretion and metabolism. Much research has therefore been devoted to the development of incretin-based therapies for type 2 diabetes. These therapies include compounds that strongly resemble the incretins, hereby stimulating their effects as well as inhibitors of the enzymatic degradation of the hormones, thereby increasing the concentration of incretins in the blood. Both therapeutic approaches have been implemented successfully, but research is still ongoing aimed at the development of further optimized therapies.

Abstract

Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are secreted from the gut upon nutrient stimulation and regulate postprandial metabolism. These hormones are known as classical incretin hormones and are responsible for a major part of postprandial insulin release. The incretin effect is severely reduced in patients with type 2 diabetes, but it was discovered that administration of GLP-1 agonists was capable of normalizing glucose control in these patients. Over the last decades, much research has been focused on the development of incretin-based therapies for type 2 diabetes. These therapies include incretin receptor agonists and inhibitors of the incretin-degrading enzyme dipeptidyl peptidase-4. Especially the development of diverse GLP-1 receptor agonists has shown immense success, whereas studies of GIP monotherapy in patients with type 2 diabetes have consistently been disappointing. Interestingly, both GIP-GLP-1 co-agonists and GIP receptor antagonists administered in combination with GLP-1R agonists appear to be efficient with respect to both weight loss and control of diabetes, although the molecular mechanisms behind these effects remain unknown. This review describes our current knowledge of the two incretin hormones and the development of incretin-based therapies for treatment of type 2 diabetes.

Pancreatic Beta Cell G-Protein Coupled Receptors and Second Messenger Interactions: A Systems Biology Computational Analysis

Insulin secretory in pancreatic beta-cells responses to nutrient stimuli and hormonal modulators include multiple messengers and signaling pathways with complex interdependencies. Here we present a computational model that incorporates recent data on glucose metabolism, plasma membrane potential, G-protein-coupled-receptors (GPCR), cytoplasmic and endoplasmic reticulum calcium dynamics, cAMP and phospholipase C pathways that regulate interactions between second messengers in pancreatic beta-cells. The values of key model parameters were inferred from published experimental data. The model gives a reasonable fit to important aspects of experimentally measured metabolic and second messenger concentrations and provides a framework for analyzing the role of metabolic, hormones and neurotransmitters changes on insulin secretion. Our analysis of the dynamic data provides support for the hypothesis that activation of Ca²⁺-dependent adenylyl cyclases play a critical role in modulating the effects of glucagon-like peptide 1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP) and catecholamines. The regulatory properties of adenylyl cyclase isoforms determine fluctuations in cytoplasmic cAMP concentration and reveal a synergistic action of glucose, GLP-1 and GIP on insulin secretion. On the other hand, the regulatory properties of phospholipase C isoforms determine the interaction of glucose, acetylcholine and free fatty acids (FFA) (that act through the FFA receptors) on insulin secretion. We found that a combination of GPCR agonists activating different messenger pathways can stimulate insulin secretion more effectively than a combination of GPCR agonists for a single pathway. This analysis also suggests that the activators of GLP-1, GIP and FFA receptors may have a relatively low risk of hypoglycemia in fasting conditions whereas an activator of muscarinic receptors can increase this risk. This computational analysis demonstrates that study of second messenger pathway interactions will improve understanding of critical regulatory sites, how different GPCRs interact and pharmacological targets for modulating insulin secretion in type 2 diabetes.

Glicentin-related pancreatic polypeptide inhibits glucose-stimulated insulin secretion from the isolated pancreas of adult male rats

Peptides derived from the glucagon gene Gcg, for example, glucagon and glucagon‐like peptide 1 (GLP‐1), act as physiological regulators of fuel metabolism and are thus of major interest in the pathogenesis of diseases, such as type‐2 diabetes and obesity, and their therapeutic management. Glicentin‐related pancreatic polypeptide (GRPP) is a further, 30 amino acid Gcg‐derived peptide identified in human, mouse, rat, and pig. However, the potential glucoregulatory function of this peptide is largely unknown. Here, we synthesized rat GRPP (rGRPP) and a closely related peptide, rat GRPP‐like peptide (rGRPP‐LP), and investigated their actions in the liver and pancreas of adult male rats by employing isolated‐perfused organ preparations. Rat GRPP and rGRPP‐LP did not affect glucose output from the liver, but both elicited potent inhibition of glucose‐stimulated insulin secretion (GSIS) from the rat pancreas. This action is unlikely to be mediated by glucagon or GLP‐1 receptors, as rGRPP and rGRPP‐LP did not stimulate cyclic adenosine monophosphate (cAMP) production from the glucagon or GLP‐1 receptors, nor did they antagonize glucagon‐ or GLP‐1‐stimulated cAMP‐production at either receptor. GRPP and GRPP‐LP may be novel regulators of insulin secretion, acting through an as‐yet undefined receptor.

Glucose-dependent insulinotropic polypeptide signaling in pancreatic β-cells and adipocytes

Glucose‐dependent insulinotropic polypeptide (GIP) was the first incretin to be identified. In addition to stimulating insulin secretion, GIP plays regulatory roles in the maintenance, growth and survival of pancreatic islets, as well as impacting on adipocyte function. The current review focuses on the intracellular signaling pathways by which GIP contributes to the regulation of β‐cell secretion and survival, and adipocyte differentiation and lipogenesis. Studies on signaling underlying the insulinotropic actions of the incretin hormones have largely been carried out with glucagon‐like peptide‐1. They have provided evidence for contributions by both protein kinase A (PKA) and exchange protein directly activated by cyclic adenosine monophosphate (EPAC2), and their probable role in GIP signaling is discussed. Recent studies have shown that inhibition of the kinase apoptosis signal‐regulating kinase 1 (ASK1) by GIP plays a key role in reducing mitochondria‐induced apoptosis in β‐cells through protein kinase B (PKB)‐mediated pathways, and that GIP‐induced post‐translational modification of voltage‐ dependent K⁺ (Kv) channels also contributes to its prosurvival role. Through regulation of gene expression, GIP tips the balance between pro‐ and anti‐apoptotic members of the B‐cell lymphoma‐2 (Bcl‐2) protein family towards β‐cell survival. GIP also plays important roles in the differentiation of pre‐adipocytes to adipocytes, and in the regulation of lipoprotein lipase expression and lipogenesis. These events involve interactions between GIP, insulin and resistin signaling pathways. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2012.00196.x, 2012)

Glucose controls CREB activity in islet cells via regulated phosphorylation of TORC2

CREB is a cAMP- and calcium-responsive transcriptional activator that is required for islet beta cell proliferation and survival. Glucose and incretin hormones elicit beta cell insulin secretion and promote synergistic CREB activity by inducing the nuclear relocalization of TORC2 (also known as Crtc2), a coactivator for CREB. In islet cells under basal conditions when CREB activity is low, TORC2 is phosphorylated and sequestered in the cytoplasm by 14-3-3 proteins. In response to feeding stimuli, TORC2 is dephosphorylated, enters the nucleus, and binds to CREB located at target gene promoters. The dephosphorylation of TORC2 at Ser-171 in response to cAMP is insufficient to account for the dynamics of TORC2 localization and CREB activity in islet cells. Here, we identify Ser-275 of TORC2 as a 14-3-3 binding site that is phosphorylated under low glucose conditions and which becomes dephosphorylated by calcineurin in response to glucose influx. Dephosphorylation of Ser-275 is essential for both glucose and cAMP-mediated activation of CREB in beta cells and islets. Using a cell-based screen of 180 human protein kinases, we identified MARK2, a member of the AMPK family of Ser/Thr kinases, as a Ser-275 kinase that blocks TORC2:CREB activity. Taken together, these data provide the mechanistic underpinning for how cAMP and glucose cooperatively promote a transcriptional program critical for islet cell survival, and identifies MARK2 as a potential target for diabetes treatment.

Plasticity of the beta cell insulin secretory competence: preparing the pancreatic beta cell for the next meal

It is well established that the acute rise in plasma glucose and in the incretin hormones glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide-1 (7-36) amide (GLP-1), as occurs during a meal, is of pivotal importance in regulating the minute-to-minute output of insulin from pancreatic beta cells. In addition to this well studied acute effect, both glucose and incretin hormones have been recently observed to determine the future secretory responsiveness of the cells. Such plasticity of the insulin secretory competence would imply that glucose and incretins not only act during the present meal, but also help to prepare the beta cells to function during the subsequent meal. Evidence supporting this hypothesis is growing as a result of physiological studies of cultured beta cells (either primary cells or beta cell lines), as well as from an increasing number of large-scale gene expression studies, exploring transcriptional and post-transcriptional events in genes regulated by glucose and incretins. On the basis of this hypothesis, one can speculate that genetic or environmental disturbances of plasticity of the insulin secretory competence is one aspect of beta cell dysfunction that can contribute to the aetiology of type 2 diabetes.

Glucose-dependent insulinotropic peptide: differential effects on hepatic artery vs. portal vein endothelial cells

Glucose-dependent insulinotropic peptide (GIP) has been reported to have opposing effects on splanchnic blood flow. GIP infusion in dogs results in an increase in portal vein circulation but a drop in hepatic artery blood flow. In an effort to evaluate whether these different responses were related to intrinsic differences in GIP effects, we isolated canine hepatic artery (HAEC) and portal vein endothelial cells (PVEC). We report that there are differences in GIP activation of the signal transduction pathways in these two cell types. GIP stimulates secretion of endothelin-1 (ET-1), a potent vasoconstrictor, from HAEC (EC50 0.28 nM) but not from PVEC. This effect could be abolished by preventing a rise in intracellular calcium, demonstrating the calcium dependence of GIP-induced ET-1 secretion from HAEC. The GIP effect was specific, as a GIP receptor antagonist blocked it. In contrast, GIP stimulated nitric oxide production from PVEC (EC50 0.09 nM) but not from HAEC. Taken together, our data demonstrate distinct differences in GIP effects on HAEC from those on PVEC. We conclude that differences in GIP stimulation of ET-1 vs. nitric oxide production in different vascular beds may account for some of the observed differences in its physiological effects.

Incretins, insulin secretion and Type 2 diabetes mellitus

When glucose is taken orally, insulin secretion is stimulated much more than it is when glucose is infused intravenously so as to result in similar glucose concentrations. This effect, which is called the incretin effect and is estimated to be responsible for 50 to 70% of the insulin response to glucose, is caused mainly by the two intestinal insulin-stimulating hormones, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP). Their contributions have been confirmed in mimicry experiments, in experiments with antagonists of their actions, and in experiments where the genes encoding their receptors have been deleted. In patients with Type 2 diabetes, the incretin effect is either greatly impaired or absent, and it is assumed that this could contribute to the inability of these patients to adjust their insulin secretion to their needs. In studies of the mechanism of the impaired incretin effect in Type 2 diabetic patients, it has been found that the secretion of GIP is generally normal, whereas the secretion of GLP-1 is reduced, presumably as a consequence of the diabetic state. It might be of even greater importance that the effect of GLP-1 is preserved whereas the effect of GIP is severely impaired. The impaired GIP effect seems to have a genetic background, but could be aggravated by the diabetic state. The preserved effect of GLP-1 has inspired attempts to treat Type 2 diabetes with GLP-1 or analogues thereof, and intravenous GLP-1 administration has been shown to be able to near-normalize both fasting and postprandial glycaemic concentrations in the patients, perhaps because the treatment compensates for both the impaired secretion of GLP-1 and the impaired action of GIP. Several GLP-1 analogues are currently in clinical development and the reported results are, so far, encouraging.

Dipeptidyl peptidase IV-resistant [D-Ala(2)]glucose-dependent insulinotropic polypeptide (GIP) improves glucose tolerance in normal and obese diabetic rats

The therapeutic potential of glucose-dependent insulinotropic polypeptide (GIP) for improving glycemic control has largely gone unstudied. A series of synthetic GIP peptides modified at the NH(2)-terminus were screened in vitro for resistance to dipeptidyl peptidase IV (DP IV) degradation and potency to stimulate cyclic AMP and affinity for the transfected rat GIP receptor. In vitro experiments indicated that [D-Ala(2)]GIP possessed the greatest resistance to enzymatic degradation, combined with minimal effects on efficacy at the receptor. Thus, [D-Ala(2)]GIP(1--42) was selected for further testing in the perfused rat pancreas and bioassay in conscious Wistar and Zucker rats. When injected subcutaneously in normal Wistar, Fa/?, or fa/fa Vancouver Diabetic Fatty (VDF) Zucker rats, both GIP and [D-Ala(2)]GIP significantly reduced glycemic excursions during a concurrent oral glucose tolerance test via stimulation of insulin release. The latter peptide displayed greater in vivo effectiveness, likely because of resistance to enzymatic degradation. Hence, despite reduced bioactivity in diabetic models at physiological concentrations, GIP and analogs with improved plasma stability still improve glucose tolerance when given in supraphysiological doses, and thus may prove useful in the treatment of diabetic states.

Reduced insulinotropic effect of gastric inhibitory polypeptide in first-degree relatives of patients with type 2 diabetes

In patients with type 2 diabetes, gastric inhibitory polypeptide (GIP) has lost much of its insulinotropic activity. Whether this is similar in first-degree relatives of patients with type 2 diabetes is unknown. A total of 21 first-degree relatives, 10 patients with type 2 diabetes, and 10 control subjects (normal oral glucose tolerance) were examined. During a hyperglycemic "clamp" (140 mg/dl for 120 min), synthetic human GIP (2 pmol. kg(-1). min(-1)) was infused intravenously (30-90 min). With exogenous GIP, patients with type 2 diabetes responded with a lower increment (Delta) in insulin (P = 0.0003) and C-peptide concentrations (P < 0.0001) than control subjects. The GIP effects in first-degree relatives were diminished compared with control subjects (Delta insulin: P = 0.04; Delta C-peptide: P = 0.016) but significantly higher than in patients with type 2 diabetes (P < or = 0.05). The responses over the time course were below the 95% CI derived from control subjects in 7 (insulin) and 11 (C-peptide) of 21 first-degree relatives of patients with type 2 diabetes. In conclusion, a reduced insulinotropic activity of GIP is typical for a substantial subgroup of normoglycemic first-degree relatives of patients with type 2 diabetes, pointing to an early, possibly genetic defect.

GLP-1-induced alterations in the glucose-stimulated insulin secretory dose-response curve

The present study was undertaken to establish in normal volunteers the alterations in beta-cell responsiveness to glucose associated with a constant infusion of glucagon-like peptide-1 (GLP-1) or a pretreatment infusion for 60 min. A high-dose graded glucose infusion protocol was used to explore the dose-response relationship between glucose and insulin secretion. Studies were performed in 10 normal volunteers, and insulin secretion rates (ISR) were calculated by deconvolution of peripheral C-peptide levels by use of a two-compartmental model that utilized mean kinetic parameters. During the saline study, from 5 to 15 mM glucose, the relationship between glucose and ISR was linear. Constant GLP-1 infusion (0.4 pmol x kg(-1) x min(-1)) shifted the dose-response curve to the left, with an increase in the slope of this curve from 5 to 9 mM glucose from 71.0 +/- 12.4 pmol x min(-1) x mM(-1) during the saline study to 241.7 +/- 36.6 pmol x min(-1) x mM(-1) during the constant GLP-1 infusion (P < 0.0001). GLP-1 consistently stimulated a >200% increase in ISR at each 1 mM glucose interval, maintaining plasma glucose at <10 mM (P < 0.0007). Pretreatment with GLP-1 for 60 min resulted in no significant priming of the beta-cell response to glucose (P = 0.2). Insulin clearance rates were similar in all three studies at corresponding insulin levels. These studies demonstrate that physiological levels of GLP-1 stimulate glucose-induced insulin secretion in a linear manner, with a consistent increase in ISR at each 1 mM glucose interval, and that they have no independent effect on insulin clearance and no priming effect on subsequent insulin secretory response to glucose.

Identification of a bioactive domain in the amino-terminus of glucose-dependent insulinotropic polypeptide (GIP)

The incretins are a class of hormones released from the small bowel that act on the endocrine pancreas to potentiate insulin secretion in a glucose-dependent manner. Due to the requirement for an elevated glucose concentration for activity, the incretins, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1, have potential in the treatment of non-insulin-dependent diabetes mellitus. A series of synthetic peptide GIP fragments was generated for the purpose of elucidating the bioactive domain of the molecule. Peptides were screened for stimulation of cyclic AMP (cAMP) accumulation in Chinese hamster ovary cells transfected with the rat islet GIP receptor. Of the GIP fragments tested, GIP(1-14) and GIP(19-30) demonstrated the greatest cAMP-stimulating ability over the range of concentrations tested (up to 20 microM). In contrast, GIP fragments corresponding to amino acids 15-42, 15-30, 16-30 and 17-30 all demonstrated weak antagonism of GIP(1-42) activity. Competitive-binding displacement studies indicated that these peptides were low-affinity ligands for the GIP receptor. To examine biological activity in vivo, a bioassay was developed in the anesthetized rat. Intravenous infusion of GIP(1-42) (1 pmol/min/100 g) with a concurrent intraperitoneal glucose load (1 g/kg) significantly reduced circulating blood glucose excursions through stimulation of insulin release. Higher doses of GIP(1-14) and GIP(19-30) (100 pmol/min/100 g) also reduced blood glucose excursions.

Defective glucose-dependent insulinotropic polypeptide receptor expression in diabetic fatty Zucker rats

Glucose-dependent insulinotropic polypeptide (GIP) is a peptide hormone that is released postprandially from the small intestine and acts in concert with glucagon-like peptide (GLP)-1 to potentiate glucose-induced insulin secretion from the pancreatic beta-cell. In type 2 diabetes, there is a decreased responsiveness of the pancreas to GIP; however, the insulin response to GLP-1 remains intact. The literature suggests that the ineffectiveness of GIP in type 2 diabetes may be a result of chronic homologous desensitization of the GIP receptor. Yet, there has been no conclusive evidence suggesting that GIP levels are elevated in diabetes. The hypothesis of the present study is that one cause of decreased responsiveness to GIP in type 2 diabetes is an inappropriate expression of the GIP receptor in the pancreatic islet. This hypothesis was tested using a strain of diabetic fatty Zucker rats. The obese rats displayed basal GIP levels similar to the control animals; however, they were unresponsive to a GIP infusion (4 pmol.min(-1). kg(-1)), whereas the lean animals displayed a significant reduction in blood glucose (GIP levels, 50% control after 60 min, P < 0.05) as well as a significant increase in circulating insulin. GIP also potently stimulated first-phase insulin secretion from isolated perifused islets (10.3 +/- 3.0 x basal), and GIP and GLP-1 potentiated insulin secretion from the perfused pancreas (6 x control area under the curve [AUC]) from lean animals. GIP yielded no significant effect in the Vancouver diabetic fatty Zucker (VDF) rat pancreases, whereas GLP-1 elicited an eightfold increase of insulin secretion from the perfused VDF pancreas. Islets from lean animals subjected to static incubations with GIP showed a 2.2-fold increase in cAMP, whereas GIP failed to increase islet cAMP in the VDF islets. Finally, the expression of both GIP receptor mRNA and protein was decreased in islets from VDF rats. These data suggest that the decreased effectiveness of GIP in the VDF rat and in type 2 diabetes may be a result of a decreased receptor expression in the islet.