Acute and long-term effects of Roux-en-Y gastric bypass on glucose metabolism in subjects with Type 2 diabetes and normal glucose tolerance

Our aim was to study the potential mechanisms responsible for the improvement in glucose control in Type 2 diabetes (T2D) within days after Roux-en-Y gastric bypass (RYGB). Thirteen obese subjects with T2D and twelve matched subjects with normal glucose tolerance (NGT) were examined during a liquid meal before (Pre), 1 wk, 3 mo, and 1 yr after RYGB. Glucose, insulin, C-peptide, glucagon-like peptide-1 (GLP-1), glucose-dependent-insulinotropic polypeptide (GIP), and glucagon concentrations were measured. Insulin resistance (HOMA-IR), β-cell glucose sensitivity (β-GS), and disposition index (D(β-GS): β-GS × 1/HOMA-IR) were calculated. Within the first week after RYGB, fasting glucose [T2D Pre: 8.8 ± 2.3, 1 wk: 7.0 ± 1.2 (P < 0.001)], and insulin concentrations decreased significantly in both groups. At 129 min, glucose concentrations decreased in T2D [Pre: 11.4 ± 3, 1 wk: 8.2 ± 2 (P = 0.003)] but not in NGT. HOMA-IR decreased by 50% in both groups. β-GS increased in T2D [Pre: 1.03 ± 0.49, 1 wk: 1.70 ± 1.2, (P = 0.012)] but did not change in NGT. The increase in DI(β-GS) was 3-fold in T2D and 1.5-fold in NGT. After RYGB, glucagon secretion was increased in response to the meal. GIP secretion was unchanged, while GLP-1 secretion increased more than 10-fold in both groups. The changes induced by RYGB were sustained or further enhanced 3 mo and 1 yr after surgery. Improvement in glycemic control in T2D after RYGB occurs within days after surgery and is associated with increased insulin sensitivity and improved β-cell function, the latter of which may be explained by dramatic increases in GLP-1 secretion.     

Reduction of hepatic insulin clearance after oral glucose ingestion is not mediated by glucagon-like peptide 1 or gastric inhibitory polypeptide in humans

Changes in hepatic insulin clearance can occur after oral glucose or meal ingestion. This has been attributed to the secretion and action of gastric inhibitory polypeptide (GIP) and glucagon-like peptide (GLP)-1. Given the recent availability of drugs based on incretin hormones, such clearance effects may be important for the future treatment of type 2 diabetes. Therefore, we determined insulin clearance in response to endogenously secreted and exogenously administered GIP and GLP-1. Insulin clearance was estimated from the molar C-peptide-to-insulin ratio calculated at basal conditions and from the respective areas under the curve after glucose, GIP, or GLP-1 administration. Oral glucose administration led to an approximately 60% reduction in the C-peptide-to-insulin ratio (P < 0.0001), whereas intravenous glucose administration had no effect (P = 0.09). The endogenous secretion of GIP or GLP-1 was unrelated to the changes in insulin clearance. The C-peptide-to-insulin ratio was unchanged after the intravenous administration of GIP or GLP-1 in the fasting state (P = 0.27 and P = 0.35, respectively). Likewise, infusing GLP-1 during a meal course did not alter insulin clearance (P = 0.87). An inverse nonlinear relationship was found between the C-peptide-to-insulin ratio and the integrated insulin levels after oral and during intravenous glucose administration. Insulin clearance is reduced by oral but not by intravenous glucose administration. Neither GIP nor GLP-1 has significant effects on insulin extraction. An inverse relationship between insulin concentrations and insulin clearance suggests that the secretion of insulin itself determines the rate of hepatic insulin clearance.     

GIP and GLP-1 as incretin hormones: lessons from single and double incretin receptor knockout mice

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are gut-derived incretins secreted in response to nutrient ingestion. Both incretins potentiate glucose-dependent insulin secretion and enhance beta-cell mass through regulation of beta-cell proliferation, neogenesis and apoptosis. In contrast, GLP-1, but not GIP, inhibits gastric emptying, glucagon secretion, and food intake. Furthermore, human subjects with Type 2 diabetes exhibit relative resistance to the actions of GIP, but not GLP-1R agonists. The physiological importance of both incretins has been investigated through generation and analysis of incretin receptor knockout mice. Elimination of incretin receptor action in GIPR-/- or GLP-1R-/- mice produces only modest impairment in glucose homeostasis. Similarly, double incretin receptor knockout (DIRKO) mice exhibit normal body weight and normal levels of plasma glucagon and hypoglycemic responses to exogenous insulin. However, glucose-stimulated insulin secretion is significantly decreased following oral but not intraperitoneal glucose challenge in DIRKO mice and the glucose lowering actions of dipeptidyl peptidase-IV (DPP-IV) inhibitors are extinguished in DIRKO mice. Hence, incretin receptor signaling exerts physiologically relevant actions critical for glucose homeostasis, and represents a pharmacologically attractive target for development of agents for the treatment of Type 2 diabetes.     

The biology of incretin hormones

Gut peptides, exemplified by glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are secreted in a nutrient-dependent manner and stimulate glucose-dependent insulin secretion. Both GIP and GLP-1 also promote β cell proliferation and inhibit apoptosis, leading to expansion of β cell mass. GLP-1, but not GIP, controls glycemia via additional actions on glucose sensors, inhibition of gastric emptying, food intake and glucagon secretion. Furthermore, GLP-1, unlike GIP, potently stimulates insulin secretion and reduces blood glucose in human subjects with type 2 diabetes. This article summarizes current concepts of incretin action and highlights the potential therapeutic utility of GLP-1 receptor agonists and dipeptidyl peptidase-4 (DPP-4) inhibitors for the treatment of type 2 diabetes.     

Duodenal-jejunal bypass protects GK rats from {beta}-cell loss and aggravation of hyperglycemia and increases enteroendocrine cells coexpressing GIP and GLP-1

Dramatic improvement of type 2 diabetes is commonly observed after bariatric surgery. However, the mechanisms behind the alterations in glucose homeostasis are still elusive. We examined the effect of duodenal-jejunal bypass (DJB), which maintains the gastric volume intact while bypassing the entire duodenum and the proximal jejunum, on glycemic control, β-cell mass, islet morphology, and changes in enteroendocrine cell populations in nonobese diabetic Goto-Kakizaki (GK) rats and nondiabetic control Wistar rats. We performed DJB or sham surgery in GK and Wistar rats. Blood glucose levels and glucose tolerance were monitored, and the plasma insulin, glucagon-like peptide-1 (GLP-1), and glucose-dependent insulinotropic polypeptide (GIP) levels were measured. β-Cell area, islet fibrosis, intestinal morphology, and the density of enteroendocrine cells expressing GLP-1 and/or GIP were quantified. Improved postprandial glycemia was observed from 3 mo after DJB in diabetic GK rats, persisting until 12 mo after surgery. Compared with the sham-GK rats, the DJB-GK rats had an increased β-cell area and a decreased islet fibrosis, increased insulin secretion with increased GLP-1 secretion in response to a mixed meal, and an increased population of cells coexpressing GIP and GLP-1 in the jejunum anastomosed to the stomach. In contrast, DJB impaired glucose tolerance in nondiabetic Wistar rats. In conclusion, although DJB worsens glucose homeostasis in normal nondiabetic Wistar rats, it can prevent long-term aggravation of glucose homeostasis in diabetic GK rats in association with changes in intestinal enteroendocrine cell populations, increased GLP-1 production, and reduced β-cell deterioration.     

Prkar1a in the regulation of insulin secretion

The incidence of type 2 diabetes mellitus (T2DM) is rapidly increasing worldwide with significant consequences on individual quality of life as well as economic burden on states' healthcare costs. While origins of the pathogenesis of T2DM are poorly understood, an early defect in glucose-stimulated insulin secretion (GSIS) from pancreatic β-cells is considered a hallmark of T2DM 1.Upon a glucose stimulus, insulin is secreted in a biphasic manner with an early first-phase burst of insulin, which is followed by a second, more sustained phase of insulin output 2. First phase insulin secretion is diminished early in T2DM as well is in subjects who are at risk of developing T2DM 3 4 5 6.An effective treatment of T2DM with incretin hormone glucagon-like peptide-1 (GLP-1) or its long acting peptide analogue exendin-4 (E4), restores first-phase and augments second-phase glucose stimulated insulin secretion. This effect of incretin action occurs within minutes of GLP-1/E4 infusion in T2DM humans. An additional important consideration is that incretin hormones augment GSIS only above a certain glucose threshold, which is slightly above the normal glucose range. This ensures that incretin hormones stimulate GSIS only when glucose levels are high, while they are ineffective when insulin levels are below a certain threshold 7 8.Activation of the GLP-1 receptor, which is highly expressed on pancreatic β-cells, stimulates 2 -distinct intracellular signaling pathways: a) the cAMP-protein kinase A branch and b) the cAMP-EPAC2 (EPAC=exchange protein activated by cAMP) branch. While the EPAC2 branch is considered to mediate GLP-1 effects on first-phase GSIS, the PKA branch is necessary for the former branch to be active 9 10. However, how these 2 branches interplay and converge and how their effects on insulin secretion and insulin vesicle exocytosis are coordinated is poorly understood.Thus, at the outset of our studies we have a poorly understood intracellular interplay of cAMP-dependent signaling pathways, which - when stimulated - restore glucose-dependent first phase and augment second phase insulin secretion in the ailing β-cells of T2DM.     

Glucose-Induced Glucagon-Like Peptide 1 Secretion Is Deficient in Patients with Non-Alcoholic Fatty Liver Disease

Background & Aims

The incretins glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are gastrointestinal peptide hormones regulating postprandial insulin release from pancreatic β-cells. GLP-1 agonism is a treatment strategy in Type 2 diabetes and is evaluated in Non-alcoholic fatty liver disease (NAFLD). However, the role of incretins in its pathophysiology is insufficiently understood. Studies in mice suggest improvement of hepatic steatosis by GLP-1 agonism. We determined the secretion of incretins after oral glucose administration in non-diabetic NAFLD patients.

Methods

N = 52 patients (n = 16 NAFLD and n = 36 Non-alcoholic steatohepatitis (NASH) patients) and n = 50 matched healthy controls were included. Standardized oral glucose tolerance test was performed. Glucose, insulin, glucagon, GLP-1 and GIP plasma levels were measured sequentially for 120 minutes after glucose administration.

Results

Glucose induced GLP-1 secretion was significantly decreased in patients compared to controls (p<0.001). In contrast, GIP secretion was unchanged. There was no difference in GLP-1 and GIP secretion between NAFLD and NASH subgroups. All patients were insulin resistant, however HOMA2-IR was highest in the NASH subgroup. Fasting and glucose-induced insulin secretion was higher in NAFLD and NASH compared to controls, while the glucose lowering effect was diminished. Concomitantly, fasting glucagon secretion was significantly elevated in NAFLD and NASH.

Conclusions

Glucose-induced GLP-1 secretion is deficient in patients with NAFLD and NASH. GIP secretion is contrarily preserved. Insulin resistance, with hyperinsulinemia and hyperglucagonemia, is present in all patients, and is more severe in NASH compared to NAFLD. These pathophysiologic findings endorse the current evaluation of GLP-1 agonism for the treatment of NAFLD.     

Insulin contributes to fine-tuning of the pancreatic beta-cell response to glucagon-like peptide-1

Glucagon-like peptide-1 (GLP-1) stimulates insulin secretion from pancreatic β-cells in a glucose-dependent manner. However, factors other than glucose that regulate the β-cell response to GLP-1 remain poorly understood. In this study, we examined the possible involvement of insulin and receptor tyrosine kinase signaling in regulation of the GLP-1 responsiveness of β-cells. Pretreatment of β-cells with HNMPA, an insulin receptor inhibitor, and AG1478, an epidermal growth factor receptor inhibitor, further increased the cAMP level and Erk phosphorylation in the presence of exendin-4 (exe-4), a GLP-1 agonist. When β-cells were exposed to a high concentration of glucose (25 mM), which stimulates insulin secretion, exe-4-induced cAMP formation declined gradually as exposure time was increased. This decreased cAMP formation was not observed in the presence of HNMPA. HNMPA was able to further increase the exe-4-induced insulin secretion when β-cells were exposed to high glucose for 18 h. Treatment of β-cells with insulin significantly decreased exe-4-induced cAMP formation in a dose-dependent manner. Lowering the phospho-Akt level by HNMPA or LY294002, a PI3K inhibitor, further augmented exe-4-induced cAMP formation and Erk phosphorylation. These results suggest that insulin contributes to fine-tuning of the β-cell response to GLP-1.     

Effect of glucagon-like peptide-1 on insulin secretion

The insulinotropic actions of two forms of glucagon-like peptide 1 (GLP-1) containing 31 and 37 amino acid residues on perfused rat pancreas were compared with that of gastric inhibitory polypeptide (GIP), hitherto the most potent intestinal insulinotropic polypeptide known. The smaller form, C-terminally amidated GLP-1-(7-36), strongly enhanced insulin secretion stimulated by 11.1 mM D-glucose at a concentration as low as 0.1 nM. Comparable effects of GIP and GLP-1-(1-37) on insulin secretion were observed at concentrations of 1.0 nM and 10.0 nM, respectively. At the doses tested, neither GLP-1s nor GIP had any effect on insulin secretion induced by 3.3 mM D-glucose. At a concentration of 1.0 nM, GLP-1-(7-36 amide) also enhanced insulin secretion induced by 5 mM L-arginine whereas at concentrations of up to 10.0 nM, GLP-1-(1-37) did not. The results show that the smaller form of GLP-1 is more strongly insulinotropic than GIP. These findings suggest that the smaller GLP-1 may have a physiologically more important role as a modulator of insulin release.     

Secretion of incretin hormones (GIP and GLP-1) and incretin effect after oral glucose in first-degree relatives of patients with type 2 diabetes

AIMS/HYPOTHESIS: Since insulin secretion in response to exogenous gastric inhibitory polypeptide (GIP) is diminished not only in patients with type 2 diabetes, but also in their normal glucose-tolerant first-degree relatives, it was the aim to investigate the integrity of the entero-insular axis in such subjects. METHODS: Sixteen first-degree relatives of patients with type 2 diabetes (4 male, 12 female, age 50+/-12 years, BMI 26.1+/-3.8 kg/m(2)) and 10 matched healthy controls (negative family history, 6 male, 4 female, 45+/-13 years, 26.1+/-4.2 kg/m(2)) were examined with an oral glucose load (75 g) and an "isoglycaemic" intravenous glucose infusion. Blood was drawn over 240 min for plasma glucose (glucose oxidase), insulin, C-peptide, GIP and glucagon-like peptide 1 (GLP-1; specific immunoassays). RESULTS: The pattern of glucose concentrations could precisely be copied by the intravenous glucose infusion (p=0.99). Insulin secretion was stimulated significantly more by oral as compared to intravenous glucose in both groups (p<0.0001). The percent contribution of the incretin effect was similar in both groups (C-peptide: 61.9+/-5.4 vs. 64.4+/-5.8%; p=0.77; insulin: 74.2+/-3.3 vs. 75.8+/-4.9; p=0.97; in first-degree relatives and controls, respectively). The individual responses of GIP and GLP-1 secretion were significantly correlated with each other (p=0.0003). The individual secretion of both GIP and GLP-1 was identified as a strong predictor of the integrated incremental insulin secretory responses as well as of the incretin effect. CONCLUSION/INTERPRETATION: Despite a lower insulin secretory response to exogenous GIP, incretin effects are similar in first-degree relatives of patients with type 2 diabetes and control subjects. This may be the result of a B cell secretory defect that affects stimulation by oral and intravenous glucose to a similar degree. Nevertheless, endogenous secretion of GIP and GLP-1 is a major determinant of insulin secretion after oral glucose.     

Glucose-dependent insulinotropic polypeptide receptor null mice exhibit compensatory changes in the enteroinsular axis

The incretins glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are gut hormones that act via the enteroinsular axis to potentiate insulin secretion from the pancreas in a glucose-dependent manner. Both GLP-1 receptor and GIP receptor knockout mice (GLP-1R(-/-) and GIPR(-/-), respectively) have been generated to investigate the physiological importance of this axis. Although reduced GIP action is a component of type 2 diabetes, GIPR-deficient mice exhibit only moderately impaired glucose tolerance. The present study was directed at investigating possible compensatory mechanisms that take place within the enteroinsular axis in the absence of GIP action. Although serum total GLP-1 levels in GIPR knockout mice were unaltered, insulin responses to GLP-1 from pancreas perfusions and static islet incubations were significantly greater (40-60%) in GIPR(-/-) than in wild-type (GIPR(+/+)) mice. Furthermore, GLP-1-induced cAMP production was also elevated twofold in the islets of the knockout animals. Pancreatic insulin content and gene expression were reduced in GIPR(-/-) mice compared with GIPR(+/+) mice. Paradoxically, immunocytochemical studies showed a significant increase in beta-cell area in the GIPR-null mice but with less intense staining for insulin. In conclusion, GIPR(-/-) mice exhibit altered islet structure and topography and increased islet sensitivity to GLP-1 despite a decrease in pancreatic insulin content and gene expression.     

Physiology of GIP--a lesson from GIP receptor knockout mice.

A much greater insulin response is observed after oral glucose load than after intravenous injection of glucose. The hormonal factor(s) implicated as transmitters of signals from the gut to pancreatic beta-cells was referred to incretin; gastric inhibitory polypeptide or glucose-dependent insulinotropic polypeptide (GIP) is identified as one of the incretins. GIP exerts its effects by binding to its specific receptor, the GIP receptor, which is expressed in various tissues including pancreatic islets, adipose tissue, and brain. However, the physiological role of GIP has been generally thought to stimulate insulin secretion from pancreatic beta-cells, and the other actions of GIP have received little attention. We have bred and characterized mice with a targeted mutation of the GIP receptor gene. From these studies, we now know that GIP not only mediates early insulin secretion by acting on pancreatic beta-cells, but also links overnutrition to obesity by acting on adipocytes.     

Overview of incretin hormones.

Incretins are hormones released by nutrients from the GI tract. They amplify glucose-induced insulin release. By raising circulating incretin levels, oral glucose provokes a higher insulin response than that resulting from intravenous glucose. The two most important incretin hormones are glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). In patients with type 2 diabetes, the incretin effect is decreased, mainly due to loss of the GIP-regulated second phase of insulin secretion, and because of a decreased secretion of GLP-1. In addition to its insulinotropic effect, GLP-1 inhibits glucagon release, prolongs gastric emptying, and leads to decreases in body-weight, all of which explain the marked antidiabetogenic effect of this incretin hormone.     

Role of incretin hormones in the regulation of insulin secretion in diabetic and nondiabetic humans

The available evidence suggests that about two-thirds of the insulin response to an oral glucose load is due to the potentiating effect of gut-derived incretin hormones. The strongest candidates for the incretin effect are glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1). In patients with type 2 diabetes, however, the incretin effect is lost or greatly impaired. It is hypothesized that this loss explains an important part of the impaired insulin secretion in patients. Further analysis of the incretin effects in patients has revealed that the secretion of GIP is near normal, whereas the secretion of GLP-1 is decreased. On the other hand, the insulintropic effect of GLP-1 is preserved, whereas the effect of GIP is greatly reduced, mainly because of a complete loss of the normal GIP-induced potentiation of second-phase insulin secretion. These two features, therefore, explain the incretin defect of type 2 diabetes. Strong support for the hypothesis that the defect plays an important role in the insulin deficiency of patients is provided by the finding that administration of excess GLP-1 to patients may completely restore the glucose-induced insulin secretion as well as the beta-cells' sensitivity to glucose. Because of this, analogs of GLP-1 or GLP-1 receptor activations are currently being developed for diabetes treatment, so far with very promising results.     

Differential Stimulation of Insulin Secretion by GLP-1 and Kisspeptin-10

β-cells in the pancreatic islet respond to elevated plasma glucose by secreting insulin to maintain glucose homeostasis. In addition to glucose stimulation, insulin secretion is modulated by numerous G-protein coupled receptors (GPCRs). The GPCR ligands Kisspeptin-10 (KP) and glucagon-like peptide-1 (GLP-1) potentiate insulin secretion through G_q and G_s-coupled receptors, respectively. Despite many studies, the signaling mechanisms by which KP and GLP-1 potentiate insulin release are not thoroughly understood. We investigated the downstream signaling pathways of these ligands and their affects on cellular redox potential, intracellular calcium activity ([Ca²⁺]_i), and insulin secretion from β-cells within intact murine islets. In contrast to previous studies performed on single β-cells, neither KP nor GLP-1 affect [Ca²⁺]_i upon stimulation with glucose. KP significantly increases the cellular redox potential, while no effect is observed with GLP-1, suggesting that KP and GLP-1 potentiate insulin secretion through different mechanisms. Co-treatment with KP and the G_βγ-subunit inhibitor gallein inhibits insulin secretion similar to that observed with gallein alone, while co-treatment with gallein and GLP-1 does not differ from GLP-1 alone. In contrast, co-treatment with the G_βγ activator mSIRK and either KP or GLP-1 stimulates insulin release similar to mSIRK alone. Neither gallein nor mSIRK alter [Ca²⁺]_i activity in the presence of KP or GLP-1. These data suggest that KP likely alters insulin secretion through a G_βγ-dependent process that stimulates glucose metabolism without altering Ca²⁺ activity, while GLP-1 does so, at least partly, through a G_α-dependent pathway that is independent of both metabolism and Ca²⁺.     

Two incretin hormones GLP-1 and GIP: comparison of their actions in insulin secretion and β cell preservation

Gastric inhibitory polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are the two primary incretin hormones secreted from the intestine upon ingestion of glucose or nutrients to stimulate insulin secretion from pancreatic β cells. GIP and GLP-1 exert their effects by binding to their specific receptors, the GIP receptor (GIPR) and the GLP-1 receptor (GLP-1R), which belong to the G-protein coupled receptor family. Receptor binding activates and increases the level of intracellular cAMP in pancreatic β cells, thereby stimulating insulin secretion glucose-dependently. In addition to their insulinotropic effects, GIP and GLP-1 have been shown to preserve pancreatic β cell mass by inhibiting apoptosis of β cells and enhancing their proliferation. Due to such characteristics, incretin hormones have been gaining mush attention as attractive targets for treatment of type 2 diabetes, and indeed incretin-based therapeutics have been rapidly disseminated worldwide. However, despites of plethora of rigorous studies, molecular mechanisms underlying how GIPR and GLP-1R activation leads to enhancement of glucose-dependent insulin secretion are still largely unknown. Here, we summarize the similarities and differences of these two incretin hormones in secretion and metabolism, their insulinotropic actions and their effects on pancreatic β cell preservation. We then try to discuss potential of GLP-1 and GIP in treatment of type 2 diabetes.     

Enhanced GLP-1- and sulfonylurea-induced insulin secretion in islets lacking leptin signaling

We have previously reported that the absence of leptin signaling in β-cells enhances glucose-stimulated insulin secretion and improves glucose tolerance in vivo. To investigate the relevance of β-cell leptin signaling in the context of postprandial or therapeutic insulin secretion, we examined the cross talk between leptin and glucagon-like peptide (GLP)-1 and sulfonylurea actions. Single and size-matched islets isolated from control or pancreas-specific leptin receptor knockout (pancreas-ObR-KO) mice were treated either with GLP-1 or with glibenclamide. Leptin suppressed GLP-1-stimulated intracellular Ca(2+) concentrations ([Ca(2+)](i)) increase that paralleled the decrease in insulin secretion in controls. In contrast, and as expected, the ObR-KO islets were nonresponsive to leptin, and instead, showed a 2.8-fold greater GLP-1-stimulated [Ca(2+)](i) increase and a 1.7-fold greater insulin secretion. Phosphorylation of cAMP-responsive element binding protein was enhanced, and phosphodiesterase enzymatic activity was suppressed in MIN6 β-cells with ObR knockdown compared with controls. The ObR-KO islets also showed significantly higher glibenclamide-induced insulin secretion compared with control islets, whereas [Ca(2+)](i) was similar to the controls. These data support enhanced insulinotropic effects of glucose, GLP-1, and sulfonylureas in the islets lacking leptin signaling with potential therapeutic implications.     

A VGF-derived peptide attenuates development of type 2 diabetes via enhancement of islet β-cell survival and function

Deterioration of functional islet β-cell mass is the final step in progression to Type 2 diabetes. We previously reported that overexpression of Nkx6.1 in rat islets has the dual effects of enhancing glucose-stimulated insulin secretion (GSIS) and increasing β-cell replication. Here we show that Nkx6.1 strongly upregulates the prohormone VGF in rat islets and that VGF is both necessary and sufficient for Nkx6.1-mediated enhancement of GSIS. Moreover, the VGF-derived peptide TLQP-21 potentiates GSIS in rat and human islets and improves glucose tolerance in vivo. Chronic injection of TLQP-21 in prediabetic ZDF rats preserves islet mass and slows diabetes onset. TLQP-21 prevents islet cell apoptosis by a pathway similar to that used by GLP-1, but independent of the GLP-1, GIP, or VIP receptors. Unlike GLP-1, TLQP-21 does not inhibit gastric emptying or increase heart rate. We conclude that TLQP-21 is a targeted agent for enhancing islet β-cell survival and function.     

Ultra-structural study of insulin granules in pancreatic β-cells of db/db mouse by scanning transmission electron microscopy tomography

Insulin granule trafficking is a key step in the secretion of glucose-stimulated insulin from pancreatic β-cells. The main feature of type 2 diabetes (T2D) is the failure of pancreatic β-cells to secrete sufficient amounts of insulin to maintain normal blood glucose levels. In this work, we developed and applied tomography based on scanning transmission electron microscopy (STEM) to image intact insulin granules in the β-cells of mouse pancreatic islets. Using three-dimensional (3D) reconstruction, we found decreases in both the number and the grey level of insulin granules in db/db mouse pancreatic β-cells. Moreover, insulin granules were closer to the plasma membrane in diabetic β-cells than in control cells. Thus, 3D ultra-structural tomography may provide new insights into the pathology of insulin secretion in T2D.     

Xenin-25 Potentiates Glucose-dependent Insulinotropic Polypeptide Action via a Novel Cholinergic Relay Mechanism

The intestinal peptides GLP-1 and GIP potentiate glucose-mediated insulin release. Agents that increase GLP-1 action are effective therapies in type 2 diabetes mellitus (T2DM). However, GIP action is blunted in T2DM, and GIP-based therapies have not been developed. Thus, it is important to increase our understanding of the mechanisms of GIP action. We developed mice lacking GIP-producing K cells. Like humans with T2DM, “GIP/DT” animals exhibited a normal insulin secretory response to exogenous GLP-1 but a blunted response to GIP. Pharmacologic doses of xenin-25, another peptide produced by K cells, restored the GIP-mediated insulin secretory response and reduced hyperglycemia in GIP/DT mice. Xenin-25 alone had no effect. Studies with islets, insulin-producing cell lines, and perfused pancreata indicated xenin-25 does not enhance GIP-mediated insulin release by acting directly on the β-cell. The in vivo effects of xenin-25 to potentiate insulin release were inhibited by atropine sulfate and atropine methyl bromide but not by hexamethonium. Consistent with this, carbachol potentiated GIP-mediated insulin release from in situ perfused pancreata of GIP/DT mice. In vivo, xenin-25 did not activate c-fos expression in the hind brain or paraventricular nucleus of the hypothalamus indicating that central nervous system activation is not required. These data suggest that xenin-25 potentiates GIP-mediated insulin release by activating non-ganglionic cholinergic neurons that innervate the islets, presumably part of an enteric-neuronal-pancreatic pathway. Xenin-25, or molecules that increase acetylcholine receptor signaling in β-cells, may represent a novel approach to overcome GIP resistance and therefore treat humans with T2DM.