Single incretin receptor knockout mice do not compensate by increasing glucose-stimulated secretion of the remaining incretin hormone
Katrine D. GalsgaardJorge VergaraSara L. JepsenAlice BazzichiHannelouise KissowMark M. SmitsJens J. Holst
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Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are incretin hormones. Lack of GLP-1 receptor signaling has been reported to be compensated for by increased GIP secretion and action. Conversely GLP-1 sensitivity has been reported to be increased in GIP receptor knockout ( Gipr -/- ) mice. This suggests a compensatory adaptation to loss of incretin signaling via increased action/secretion of the remaining incretin hormone. We assessed glucose-stimulated GIP and GLP-1 secretion during oral glucose tolerance tests (OGTTs) and in isolated perfused intestines of GLP-1 receptor knockout ( Glp-1r -/- ) mice and their wild-type littermates ( Glp-1r +/+ ) and in Gipr -/- mice and their wild-type littermates ( Gipr +/+ ). Sensitivity to GIP and GLP-1 was assessed in isolated perfused pancreases of Glp-1r -/- and Glp-1r +/+ mice and Gipr -/- and Gipr +/+ mice, respectively. We found similar GIP responses in Glp-1r -/- and Glp-1r +/+ mice and similar GLP-1 responses in Gipr -/- and Gipr +/+ mice during the OGTTs and in the isolated perfused intestines. Insulin responses to GIP and GLP-1 were similar in Glp-1r -/- and Glp-1r +/+ mice and in Gipr -/- and Gipr +/+ mice, respectively. Our results do not support the existence of a compensatory adaptation to loss of single incretin signaling via increased glucose-stimulated secretion of, or sensitivity to, the remaining incretin hormone.Keywords:
Incretin
Gastric inhibitory polypeptide
Knockout mouse
Abstract The hormonal factors implicated as transmitters of signals from the gut to pancreatic β‐cells are referred to as incretins. Gastric inhibitory polypeptide ( GIP ) and glucagon‐like peptide‐1 ( GLP ‐1) are incretins. In addition to the insulinotropic effects, we have shown, using the GIP receptor and GLP ‐1 receptor‐deficient mice, that GIP and GLP ‐1 have direct actions on adipocytes and the kidney, respectively. Because GIP receptors and GLP ‐1 receptors are differentially expressed in a tissue‐specific manner, GIP and GLP ‐1 have specific physiological activities, and further comprehensive characterization of the extrapancreatic actions of GIP and GLP ‐1 is anticipated, as dipeptidyl peptidase IV inhibitors activate both GIP and GLP ‐1 signaling.
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Gastric inhibitory polypeptide
Dipeptidyl peptidase
Gastrointestinal hormone
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One billion people live with obesity. The most promising medications for its treatment are incretin-based therapies, based on enteroendocrine peptides released in response to oral nutrients, specifically glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP). The mechanisms by which GLP-1 receptor agonism cause weight reduction are becoming increasingly understood. However, the mechanisms by which GIP receptor-modulating medications cause weight loss remain to be clarified.
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Gastric inhibitory polypeptide
Enteroendocrine cell
Gastrointestinal hormone
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Aims/Introduction: Gastric inhibitory polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are major incretins that potentiate insulin secretion from pancreatic β-cells. The factors responsible for incretin secretion have been reported in Caucasian subjects, but have not been thoroughly evaluated in Japanese subjects. We evaluated the factors associated with incretin secretion during oral glucose tolerance test (OGTT) in Japanese subjects with normal glucose tolerance (NGT). We measured plasma GIP and GLP-1 levels during OGTT in 17 Japanese NGT subjects and evaluated the factors associated with GIP and GLP-1 secretion using simple and multiple regression analyses. GIP secretion (AUC-GIP) was positively associated with body mass index (P < 0.05), and area under the curve (AUC) of C-peptide (P < 0.05) and glucagon (P < 0.01), whereas GLP-1 secretion (AUC-GLP-1) was negatively associated with AUC of plasma glucose (P < 0.05). The insulinogenic index was most strongly associated with GIP secretion (P < 0.05); homeostasis model assessment β-cell was the most the strongly associated factor in GLP-1 secretion (P < 0.05) among the four indices of insulin secretion and insulin sensitivity. Several distinct factors might be associated with GIP and GLP-1 secretion during OGTT in Japanese subjects. (J Diabetes Invest, doi: 10.1111/j.2040-1124.2010.00078.x, 2011).
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Gastric inhibitory polypeptide
Homeostasis
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In type-2 diabetes, the overall incretin effect is reduced. The present investigation was designed to compare insulinotropic actions of exogenous incretin hormones (gastric inhibitory peptide [GIP] and glucagon-like peptide 1 [GLP-1] [7-36 amide]) in nine type-2 diabetic patients (fasting plasma glucose 7.8 mmol/liter; hemoglobin A1c 6.3 +/- 0.6%) and in nine age- and weight-matched normal subjects. Synthetic human GIP (0.8 and 2.4 pmol/kg.min over 1 h each), GLP-1 [7-36 amide] (0.4 and 1.2 pmol/kg.min over 1 h each), and placebo were administered under hyperglycemic clamp conditions (8.75 mmol/liter) in separate experiments. Plasma GIP and GLP-1 [7-36 amide] concentrations (radioimmunoassay) were comparable to those after oral glucose with the low, and clearly supraphysiological with the high infusion rates. Both GIP and GLP-1 [7-36 amide] dose-dependently augmented insulin secretion (insulin, C-peptide) in both groups (P < 0.05). With GIP, the maximum effect in type-2 diabetic patients was significantly lower (by 54%; P < 0.05) than in normal subjects. With GLP-1 [7-36 amide] type-2 diabetic patients reached 71% of the increments in C-peptide of normal subjects (difference not significant). Glucagon was lowered during hyperglycemic clamps in normal subjects, but not in type-2 diabetic patients, and further by GLP-1 [7-36 amide] in both groups (P < 0.05), but not by GIP. In conclusion, in mild type-2 diabetes, GLP-1 [7-36 amide], in contrast to GIP, retains much of its insulinotropic activity. It also lowers glucagon concentrations.
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Gastric inhibitory polypeptide
Amide
Glucose clamp technique
C-peptide
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Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are important incretin hormones. They are released from the gut after meal ingestion and potentiate glucose-stimulated insulin secretion. Their release after meal ingestion and oral glucose are well established and have been characterized previously. During recent years, knowledge of other regulatory aspects that potentially may affect GIP and GLP-1 secretion after meal ingestion have also begun to emerge. Here, the results of human studies on these novel aspects of meal- and nutrient-stimulated incretin hormone secretion are reviewed.The human literature was revisited by identifying articles in PubMed using key words GIP, GLP-1, secretion, meal, and nutrients.The results show that all macronutrients individually stimulate GIP and GLP-1 secretion. However, there was no synergistic action when given in combination. A pre-load 30 min before a meal augments the GIP and GLP-1 response. GIP and GLP-1 secretion have a diurnal variation with a higher response to an identical meal in the morning than in the afternoon. There is no difference in GIP and GLP-1 secretion whether a meal is ingested slowly or rapidly. GIP and GLP-1 secretion after dinner are the same whether or not breakfast and lunch have been ingested. The temperature of the food may be of importance for the incretin hormone response.These novel findings have increased our knowledge on the regulation of the complexity of the incretin system and are also important knowledge when designing future studies.
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Gastric inhibitory polypeptide
Gastrointestinal hormone
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The incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are degraded by dipeptidyl peptidase IV (DPP IV), thereby losing insulinotropic activity. DPP IV inhibition reduces exogenous GLP-1 degradation, but the extent of endogenous incretin protection has not been fully assessed, largely because suitable assays which distinguish between intact and degraded peptides have been unavailable. Using newly developed assays for intact GLP-1 and GIP, the effect of DPP IV inhibition on incretin hormone metabolism was examined. Conscious dogs were given NVP-DPP728, a specific DPP IV inhibitor, at a dose that inhibited over 90% of plasma DPP IV for the first 90 min following treatment. Total and intact incretin concentrations increased (P<0.0001) following a mixed meal, but on control days (vehicle infusion), intact peptide concentrations were lower (P<0.01) than total peptide concentrations (22.6 +/- 1.2% intact GIP; 10.1 +/- 0.4% intact GLP-1). Following inhibitor treatment, the proportion of intact peptide increased (92.5 +/- 4.3% intact GIP, P<0.0001; 99.0 +/- 22.6% intact GLP-1, P<0.02). Active (intact) incretins increased after NVP-DPP728 (from 4797 +/- 364 to 10 649 +/- 106 pM x min for GIP, P<0.03; from 646 +/- 134 to 2822 +/- 528 pM x m in for GLP-1, P<0.05). In contrast, total incretins fell (from 21 632 +/- 654 to 12 084 +/- 1723 pM x min for GIP, P<0.002; from 5145 +/- 677 to 3060 +/- 601 pM x min for GLP-1, P<0.05). Plasma glucose, insulin and glucagon concentrations were unaltered by the inhibitor. We have concluded that DPP IV inhibition with NVP-DPP728 prevents N-terminal degradation of endogenous incretins in vivo, resulting in increased plasma concentrations of intact, biologically active GIP and GLP-1. Total incretin secretion was reduced by DPP IV inhibition, suggesting the possibility of a feedback mechanism.
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Dipeptidyl peptidase
Gastric inhibitory polypeptide
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Gastric inhibitory polypeptide (GIP) and glucagon-like peptide-1-(7-36) amide (GLP-1) are glucose-dependent insulinotropic gut hormones that may explain the greater insulin secretory response with oral compared to i.v. glucose (incretin effect). To study their individual and combined contributions, in eight healthy volunteers, on separate occasions, synthetic human GIP (1 pmol/kg.min) and/or GLP-1 (0.3 pmol/kg.min) or placebo were infused i.v. (-30 to 120 min), while at 0 min, a glucose infusion "isoglycemic" to the profile after an oral glucose load of 50 g/400 mL was started. After the administration of 50 g oral glucose, immunoreactive GIP rose several-fold to 337 +/- 43 pmol/L, while there was only a transient (10-30 min) and moderate increment in immunoreactive GLP-1 (from basal, 25-30, to 41 +/- 4 pmol/L). Isoglycemic i.v. glucose infusions led to smaller B-cell responses (estimated incretin effect, 41 +/- 5%). With single infusions of GIP or GLP-1 (circulating concentrations, 464 +/- 73 and 54 +/- 3 pmol/L, respectively), B-cell responses were significantly augmented compared to i.v. glucose alone and were no longer significantly different from those after oral glucose. The combination of GIP and GLP-1 led to B-cell responses that were significantly higher than those with either hormone alone (additive mode of cooperation). Plasma GIP concentrations were similar after endogenous secretion (oral glucose) and i.v. infusion, while exogenously administered GLP-1 led to plasma levels that were maintained at an elevated level for a longer period during exogenous infusion than after stimulation by oral glucose. When, in seven volunteers, a lower dose (0.15 pmol/kg.min) of GLP-1 was infused during isoglycemic glucose infusion experiments only for the duration of elevated plasma levels in the oral glucose challenges (0-30 min), a significant, but transient, increment in insulin and C-peptide concentrations was observed, which was equivalent to 26 +/- 10% of the estimated incretin effect. Therefore, in conclusion, circulating GIP seems to make a major contribution to the incretin effect after oral glucose, and GLP-1 appears to mediate a smaller proportion. GIP and GLP-1 can interact in an additive manner in normal man.
Gastric inhibitory polypeptide
Incretin
Gastrointestinal hormone
Basal (medicine)
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Incretin
Gastric inhibitory polypeptide
Basal (medicine)
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Incretin
Gastric inhibitory polypeptide
Gastrointestinal hormone
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Monounsaturated fatty acids, such as oleic acid (OA), and certain milk proteins, especially whey protein (WP), have insulinotropic effects and can reduce postprandial glycemia. This effect may involve the incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). To explore this, we examined the release and inactivation of GIP and GLP-1 after administration of glucose with or without OA or WP through gastric gavage in anesthetized C57BL/6J mice. Insulin responses to glucose (75 mg) were 3-fold augmented by addition of WP (75 mg; P < 0.01), which was associated with enhanced oral glucose tolerance (P < 0.01). The insulin response to glucose was also augmented by addition of OA (34 mg; P < 0.05) although only 1.5-fold and with no associated increase in glucose elimination. The slope of the glucose-insulin curve was increased by OA (1.7-fold; P < 0.05) and by WP (4-fold; P < 0.01) compared with glucose alone, suggesting potentiation of glucose-stimulated insulin release. WP increased GLP-1 secretion (P < 0.01), whereas GIP secretion was unaffected. OA did not affect GIP or GLP-1 secretion. Nevertheless, WP increased the levels of both intact GIP and intact GLP-1 (both P < 0.01), and OA increased the levels of intact GLP-1 (P < 0.05). WP inhibited dipeptidyl peptidase IV activity in the proximal small intestine by 50% (P < 0.05), suggesting that luminal degradation of WP generates small fragments, which are substrates for dipeptidyl peptidase IV and act as competitive inhibitors. We therefore conclude that fat and protein may serve as exogenous regulators of secretion and inactivation of the incretin hormones with beneficial influences on glucose metabolism.
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Gastric inhibitory polypeptide
Dipeptidyl peptidase
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