Roles of glucose‐dependent insulinotropic polypeptide in diet‐induced obesity

Glucose‐dependent insulinotropic polypeptide (GIP) and glucagon‐like peptide‐1 (GLP‐1) are incretins that play an important role in glucose metabolism, by increasing glucose‐induced insulin secretion from pancreatic β‐cells and help regulate bodyweight. Although they show a similar action on glucose‐induced insulin secretion, two incretins are distinct in various aspects. GIP is secreted from enteroendocrine K cell mainly expressed in the upper small intestine, and GLP‐1 is secreted from enteroendocrine L cells mainly expressed in the lower small intestine and colon by the stimulation of various nutrients. The mechanism of GIP secretion induced by nutrients, especially carbohydrates, is different from that of GLP‐1 secretion. GIP promotes fat deposition in adipose tissue, and contributes to fat‐induced obesity. In contrast, GLP‐1 participates in reducing bodyweight by suppressing food consumption and/or slowing gastric emptying. There is substantial evidence that GIP and GLP‐1 might differently contribute to bodyweight control. Although meal contents influence both glycemic and weight control, we do not fully understand whether incretin actions differ depending on the contents of the meal and what kind of signaling is involved in its context. We focus on the molecular mechanism of GIP secretion induced by nutrients, as well as the roles of GIP in weight changes caused by various diets.     

Regular low‐calorie sweetener consumption is associated with increased secretion of glucose‐dependent insulinotropic polypeptide

Low‐calorie sweeteners (LCSs) are widely used for weight control despite limited evidence of their effectiveness and studies linking LCS consumption with incident obesity. We tested the hypothesis that regular LCS consumption is associated with higher postprandial glucose‐dependent insulinotropic polypeptide (GIP) secretion, which has been linked to obesity. We used data from participants in the Baltimore Longitudinal Study of Aging who had completed a diet diary, had at least one visit during which they underwent an oral glucose tolerance test (OGTT), and had no diabetes. Of 232 participants, 166 contributed 1, 39 contributed 2, and 27 contributed 3 visits, and 96 (41%) reported using LCS. Plasma OGTT samples were analysed for glucose, insulin and GIP. Fasting glucose, insulin and GIP levels were no different between LCS users and non‐users. The association of LCS use with 2‐hour OGTT responses after adjustment for covariates was non‐significant for glucose (P = .98) and insulin (P = .18), but significant for greater increase in GIP in LCS users (P = .037). Regular consumption of LCSs was associated with greater increases in GIP secretion after food intake, which may potentially lead to weight gain through the lipogenic properties of GIP.     

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)     

Clinical endocrinology and metabolism. Glucose-dependent insulinotropic polypeptide/gastric inhibitory polypeptide

The 42 amino acid polypeptide glucose-dependent insulinotropic polypeptide/gastric inhibitory polypeptide (GIP) is released from intestinal K-cells in response to nutrient ingestion. Based on animal studies, the peptide was initially assumed to act as an endogenous inhibitor of gastric acid secretion. Later it was found that GIP is capable of augmenting glucose-stimulated insulin secretion, and subsequent studies provided evidence that, in humans, the peptide predominantly acts as an incretin hormone. A role for GIP in the regulation of lipid homeostasis and in the development of obesity has been inferred from different animal studies. While GIP strongly stimulates insulin release in healthy humans, the peptide has almost completely lost its insulinotropic effect in patients with type 2 diabetes. This is different from the actions of glucagon-like peptide 1, which stimulates insulin secretion even in the later stages of type 2 diabetes. This suggests that a diminished insulinotropic effect of GIP may contribute to the pathogenesis of type 2 diabetes. This review will summarize the actions of GIP in human physiology and discuss its role in the pathogenesis of type 2 diabetes, as well as the therapeutic options derived from these findings.     

Gastric inhibitory polypeptide

The available data show that GIP is at present the strongest candidate for the insulin-secreting factor of the gut named incretin. Its release is triggered by the absorption of ingested nutrients. GIP acts on the B-cells of the pancreas by potentiating glucose-induced insulin secretion. The role of GIP as an enterogastrone is less well established. The release of GIP from the gut cells seems to be regulated by the composition and the amount of the ingested food, by the rate of absorption of nutrients by neural factors (vagal), and by feedback control mediated by insulin. In addition, the adaptation of the intestine to individual eating habits influences the response of the GIP cells. It is suggested that an overactive enteroinsular axis, i.e. enhanced GIP secretion, participates in the development of the hyperinsulinaemia of obesity and maturity onset diabetes mellitus. In gastrointestinal diseases accompanied by malabsorption the GIP response is diminished. In gastrointestinal disorders with rapid gastric emptying (duodenal ulcer) or with accelerated passage of the nutrients through the intestine, hypersecretion of GIP and insulin occurs. This may be significant for the reactive hypoglycaemia of these conditions.     

Gastric inhibitory polypeptide is the major insulinotropic factor in K(ATP) null mice

OBJECTIVE: ATP-sensitive K(+) (K(ATP)) channels in pancreatic beta-cells are crucial in the regulation of glucose-induced insulin secretion. Recently, K(ATP) channel-deficient mice were generated by genetic disruption of Kir6.2, the pore-forming component of K(ATP) channels, but the mice still showed a significant insulin response after oral glucose loading in vivo. Gastric inhibitory polypeptide (GIP) is a physiological incretin that stimulates insulin release upon ingestion of nutrients. To determine if GIP is the insulinotropic factor in insulin secretion in K(ATP) channel-deficient mice, we generated double-knockout Kir6.2 and GIP receptor null mice and compared them with Kir6.2 knockout mice. METHODS: Double-knockout mice were generated by intercrossing Kir6.2-knockout mice with GIP receptor-knockout mice. An oral glucose tolerance test, insulin tolerance test and batch incubation study of pancreatic islets were performed on double-knockout mice and Kir6.2-knockout mice. RESULTS: Fasting glucose and insulin levels were similar in both groups. After oral glucose loading, blood glucose levels of double-knockout mice became elevated compared with Kir6.2-knockout mice, especially at 15 min (345+/-10 mg/dl vs 294+/-20 mg/dl, P<0.05) and 30 min (453+/-20 mg/dl vs 381+/-26 mg/dl, P<0.05). The insulin response was almost completely lost in double-knockout mice, although insulin secretion from isolated islets was stimulated by another incretin, glucagon-like peptide-1 in the double-knockout mice. Double-knockout mice and Kir6.2-knockout mice were similarly insulin sensitive as assessed by the insulin tolerance test. CONCLUSION: GIP is the major insulinotropic factor in the secretion of insulin in response to glucose load in K(ATP) channel-deficient mice.     

Glucose‐dependent insulinotropic polypeptide: from pathophysiology to therapeutic opportunities in obesity‐associated disorders

Glucose‐dependent insulinotropic polypeptide (GIP) is a hormone secreted from the intestinal K‐cells with established insulin‐releasing actions. However, the GIP receptor is widely distributed in peripheral organs, including the adipose tissue, gut, bone and brain, where GIP modulates energy intake, cell metabolism and proliferation, and lipid and glucose metabolism, eventually promoting lipid and glucose storage. In diabetes and obesity, the incretin effect of GIP is blunted, while the extrapancreatic tissues keep a normal sensitivity to this hormone. As GIP levels are normal or elevated in obesity and diabetes, mounting evidence from chemical or genetic GIP deletion in animal models of obesity‐related diabetes suggests that GIP may have a pro‐obesogenic action and that a strategy antagonizing GIP action may be beneficial in these conditions, clearing triglyceride deposits from adipose tissue, liver and muscle, and restoring normal insulin sensitivity. Emerging evidence also suggests that the metabolic benefits of bypass surgery are mediated, at least in part, by surgical removal of GIP‐secreting K‐cells in the upper small intestine.     

Glucose‐dependent insulinotropic polypeptide and glucagon‐like peptide‐1: Incretin actions beyond the pancreas

Glucose‐dependent insulinotropic polypeptide (GIP) and glucagon‐like peptide‐1 (GLP‐1) are the two primary incretin hormones secreted from the intestine on ingestion of various nutrients to stimulate insulin secretion from pancreatic β‐cells glucose‐dependently. GIP and GLP‐1 undergo degradation by dipeptidyl peptidase‐4 (DPP‐4), and rapidly lose their biological activities. The actions of GIP and GLP‐1 are mediated by their specific receptors, the GIP receptor (GIPR) and the GLP‐1 receptor (GLP‐1R), which are expressed in pancreatic β‐cells, as well as in various tissues and organs. A series of investigations using mice lacking GIPR and/or GLP‐1R, as well as mice lacking DPP‐4, showed involvement of GIP and GLP‐1 in divergent biological activities, some of which could have implications for preventing diabetes‐related microvascular complications (e.g., retinopathy, nephropathy and neuropathy) and macrovascular complications (e.g., coronary artery disease, peripheral artery disease and cerebrovascular disease), as well as diabetes‐related comorbidity (e.g., obesity, non‐alcoholic fatty liver disease, bone fracture and cognitive dysfunction). Furthermore, recent studies using incretin‐based drugs, such as GLP‐1 receptor agonists, which stably activate GLP‐1R signaling, and DPP‐4 inhibitors, which enhance both GLP‐1R and GIPR signaling, showed that GLP‐1 and GIP exert effects possibly linked to prevention or treatment of diabetes‐related complications and comorbidities independently of hyperglycemia. We review recent findings on the extrapancreatic effects of GIP and GLP‐1 on the heart, brain, kidney, eye and nerves, as well as in the liver, fat and several organs from the perspective of diabetes‐related complications and comorbidities.     

Peptide YY inhibits the insulinotropic action of gastric inhibitory polypeptide

Peptide YY (PYY) is released from the gut after ingestion of fat or after a meal. The purpose of this investigation was to examine the effect of PYY on gastric inhibitory polypeptide (GIP)-stimulated insulin release in conscious dogs with gastric and duodenal fistulas. In control experiments, 6 dogs received GIP (400 pmol/kg, i.v., for 1 h) and glucose (0.6 g/kg, i.v., for 1 h); the integrated insulin response over a 1-h period was 142 +/- 32.7 ng-60 min/ml. The plasma GIP levels achieved by this procedure were similar to those observed by intraduodenal infusion of Lipomul (2 ml/min), suggesting that the dose of GIP used was within the physiologic range. Intravenous infusion of three different doses of PYY (100, 200, or 400 pmol/kg.h) caused a significant inhibition of insulin release stimulated by GIP + glucose; the integrated insulin response was reduced to 105, 88, and 79 ng-60 min/ml, respectively. On the other hand, PYY (400 pmol/kg.h) had no effect on insulin secretion induced by intravenous glucose (0.6 g/kg.h) alone. These results indicate that PYY specifically inhibits the insulinotropic action of GIP and that PYY may play a negative-feedback regulatory role in the enteroinsular axis.     

Potential of a glucagon‐like peptide‐1 receptor/glucose‐dependent insulinotropic polypeptide receptor/glucagon receptor triagonist for the treatment of obesity and type 2 diabetes

Triagonists of GLP‐1R/ GIPR /GCGR, including SAR441255, bind to each receptor and induce specific effects through each receptor signaling pathway, thus result in weight loss and glycemic control in obese T2D animal models.

Type 2 diabetes, which often accompanies obesity, is one of the major health‐threatening diseases worldwide. Incretin‐based therapies, such as dipeptidyl peptidase‐4 inhibitors and glucagon‐like peptide‐1 receptor (GLP‐1R) agonists, are used for the treatment of type 2 diabetes. The GLP‐1R agonists have been shown to improve glycemic control and reduce bodyweight through its various actions, including glucose‐dependent insulin secretion, suppression of glucagon secretion, and inhibition of gastric emptying and food intake ¹ .Recently, dual agonists of GLP‐1R and glucose‐dependent insulinotropic polypeptide receptor (GIPR), and those of GLP‐1R and glucagon receptor (GCGR) have been developed. One of the dual agonists of GLP‐1R and GIPR, named tirzepatide, has been shown to bind and activate both GLP‐1R and GIPR in vitro and in vivo, its efficacy for the treatment of type 2 diabetes has been proven in a phase III trial, and it has recently been approved by the US Food and Drug Administration ^{2 , 3} . In the phase III trial, once‐weekly injection of trizepatide (5, 10 or 15 mg) for 40 weeks reduced glycated hemoglobin (HbA1c) from baseline by 1.87, 1.89 or 2.07%, respectively, and reduced bodyweight from baseline by 7.0, 7.8 or 9.5 kg, respectively, in type 2 diabetes patients. The most frequent adverse events with trizepatide were gastrointestinal events ³ . In the clinical trial that compared trizepatide (5, 10 or 15 mg) with once‐weekly semaglutide (1 mg) in type 2 diabetes patients, trizepatide showed non‐inferior and superior reductions in HbA1c levels and in bodyweight ⁴ . For both treatments, gastrointestinal events were the most common adverse events ⁴ . The possible additional effects of GIP signaling in type 2 diabetes patients might depend on its augmentation of insulin secretion during hyperglycemic states ⁵ .The dual agonists of GLP‐1R and GCGR specific for mice or monkeys have been developed and were investigated their effects in each species. In obese and diabetic cynomolgus monkeys induced by high‐fat diet feeding, the monkey‐specific dual agonist of GLP‐1R and GCGR reduced total energy intake, decreased bodyweight and improved glucose tolerance ⁶ . However, in another study using obese diabetic monkeys, glycemic control was worse when they were co‐administered with both GLP‐1R and GCGR agonists compared with that treated with an GLP‐1R agonist alone ⁷ . Therefore, the significance of the activation of GCGR signaling has not yet been fully clarified.In 2015, Finan et al. ⁸ created a triagonist that activates GLP‐1R, GIPR and GCGR by selecting amino acids from the three peptide hormones, GLP‐1, GIP and glucagon, and reported that the triagonist reduced bodyweight and improved glucose control in rodent models of obesity and diabetes.More recently, Bossart et al. ⁹ developed a novel GLP‐1R, GIPR and GCGR triagonist, SAR441255. SAR441255 was confirmed to efficiently bind to and stimulate all three receptors in in vitro studies. In a diet‐induced obesity mouse model, subcutaneous injection of SAR441255 (0.3, 1, 3, 10 or 30 μg/kg) showed a dose‐dependent effect on bodyweight (+9.7%, +6.9%, +5.8%, −4.8% and −14.1%, respectively) compared with injection of vehicle (+11.5%). Non‐fasted blood glucose levels were significantly lower in mice treated with SAR441255 at doses of ≥1 μg/kg when compared with vehicle‐treated controls. In obese diabetic cynomolgus monkeys, treatment with SAR441255 (11 μg/kg) or a monkey‐specific GLP‐1R/GCGR dual agonist (4 μg/kg) ⁶ significantly reduced the bodyweight by −12.6% or −8.1%, respectively, and decreased the HbA1c levels by −1.37% or −1.85%, respectively. Fasting plasma glucose and alanine aminotransferase levels were significantly reduced with SAR441255, but not with the dual agonist (Figure 1).Open in a separate windowFigure 1Possible mechanisms of glucagon‐like peptide‐1 receptor (GLP‐1R)/glucose dependent insulinotropic polypeptide receptor (GIPR)/glucagon receptor (GCGR) triagonist in weight loss and glycemic control in obese type 2 diabetes animal models. Triagonists of GLP‐1R/GIPR/GCGR, including SAR441255, bind to each receptor and induce specific effects through each receptor signaling pathway, thus resulting in weight loss and glycemic control in obese type 2 diabetes animal models. Potential effects of each receptor signaling that contribute to weight loss and/or better glycemic control are listed below each receptor.To further understand engagement of the different receptors in vivo, receptor occupancy of SAR441255 at the GLP‐1 and the GCG receptors was studied with the use of positron emission tomography imaging after radiotracer administration in lean cynomolgus monkeys. A subcutaneous injection of 11 μg/kg SAR411255 together with radiotracers specific for GLP‐1R and GCGR showed the significant signals of both receptors in the liver and pancreas of monkeys, suggesting the binding of SAR411255 to both receptors existing in these organs. Cardiovascular safety of SAR411255 was further confirmed in lean cynomolgus monkeys.Finally, a phase I study with SAR411255 in lean to overweight healthy participants was carried out to assess the safety, tolerability, pharmacokinetics and pharmacodynamics. After subcutaneous administration, SAR441255 concentration reached the median maximum serum concentration by 3.0–3.5 h, and was eliminated with a mean elimination terminal half‐life of 3.5–6.1 h. After administration of single doses (80 and 150 mg) of SAR441255 in the fasting state, maximal reduction in blood glucose levels were observed at 1 h post‐dose. At this time point, three of six participants who received the 80 mg dose and all six participants who received the 150 mg dose showed hypoglycemia (<70 mg/dL) without any clinical signs or symptoms. No further low blood glucose values were observed in either dose group. Blood glucose levels subsequently returned to baseline levels within 1–2 h. In contrast, fasting insulin and C‐peptide levels were relatively stable, and showed no correlation with the glucose levels. Therefore, the mechanisms of this early phase hypoglycemia have not yet been clarified. After the mixed‐meal test 3 h after the SAR441255 administration (80 and 150 mg), a dose‐dependent reduction in postprandial plasma glucose was observed. Because insulin and C‐peptide levels were also reduced during mixed‐meal test, postprandial plasma glucose reduction was suggested to be due to inhibition of gastric emptying, as observed with GLP‐1R engagement.Gastrointestinal disorders (nausea, vomiting, dry mouth and mouth ulceration) were the most frequent treatment‐emergent adverse events after treatment with SAR441255. All events were mild in severity, and occurred between 3 and 4 h after SAR441255 administration.To clarify the effects of SAR441255 on GIPR and GCGR in humans, specific biomarkers for each receptor were measured. As expected, a biomarker for GIPR activation, C‐telopeptide of cross‐linked type I collagen, which is a marker of bone turnover, was significantly reduced by >50% in participants who received SAR441255 administration (80 and 150 mg). Likely, plasma amino acids levels, which are sensitive biomarkers of GCGR activation, were reduced after SAR441255 administration in these individuals.As aforementioned, the dual agonists of GLP‐1R and GIPR might have comparable or even more stronger effects in reductions of HbA1c and bodyweight in obese type 2 diabetes patients ⁴ ; the significance of the activation of GCGR signaling should be evaluated to confirm the benefits of the triagonists of GLP‐1R, GIPR and GCGR. In this regard, the effects of glucagon to enhance energy expenditure partly through enhancement of fat and glucose oxidation could cover some of the benefits ⁷ , but further investigations should be necessary. In addition, it has recently been reported that GLP‐1R agonists have clinically significant cardiovascular benefits in type 2 diabetes patients ¹ . Therefore, it should be clarified if the dual or triagonists also have comparable cardiovascular benefits, as observed in GLP‐1R agonists in type 2 diabetes patients.Because triagonists have been shown to have better profiles in weight loss and glycemic control in animal models of obese type 2 diabetes compared with GLP‐1R agonists and dual agonists of GLP‐1R and GIPR or GLP‐1R and GCGR ^{8 , 9} , clinical studies that investigate the efficacy in type 2 diabetes patients should be of great interest.     

Gastric inhibitory polypeptide (GIP)

Radioimmunoassayable gastric inhibitory polypeptide was measured in extracts of canine antrum, duodenum, jejunum, and ileum. The highest GIP concentrations were found in the duodenum (347±53 ng/g) and jejunum (300±68 ng/g). An immunochemical similarity was demonstrable between porcine GIP and canine GIP. Dogs prepared with Mann-Bollman fistulae were given an amino acid (AA) mixture or medium-chain triglycerides (MTC) by intraduodenal perfusion. With AA, a peak mean serum concentration of 672±106 pg/ml was reached 15 min after starting the perfusion. MCT resulted in a peak mean serum GIP concentration of 504±55 pg/ml 30 min after beginning the perfusion. When compared to results previously reported from this laboratory, AA and MCT are not as potent as corn oil (long-chain triglyceride) or glucose in stimulating GIP release. We conclude: (1) Immunoassayable GIP concentrations are highest in the canine proximal small intestine. (2) AA and MCT are weak stimulants of GIP release in the dog.This study was supported in part by the Bremer Foundation, 521532-7308, the National Institute of Arthritis and Metabolic Disease 5T01-AM05118-19, and General Clinical Research Center Program, National Institutes of Health RR-34, and National Cancer Institute 5F22-CA00776-02.     

Gastric inhibitory polypeptide in obesity and diabetes mellitus

Gastric inhibitory polypeptide (GIP) concentrations may be influenced by obesity, diabetes, and glucagon deficiency and be under feedback inhibition by insulin. To assess these factors, insulin-dependent diabetic, totally pancreatectomized diabetic, and lean and obese noninsulin-dependent diabetic patients were studied twice, once during partial insulin withdrawal and again when euglycemia was achieved before and after mixed meal ingestion, using an artificial endocrine pancreas. The results were compared to those from weight-matched lean and obese nondiabetic subjects. No significant differences in postprandial GIP responses were found between lean and obese nondiabetic subjects. Despite basal and postprandial hyperglycemia, the GIP responses to the mixed meal were not significantly different between insulin-deficient (insulin-dependent and totally pancreatectomized) patients and lean nondiabetic subjects. In addition, there were no significant differences in postprandial GIP responses between insulin-dependent and totally pancreatectomized patients. In contrast, lean and obese noninsulin-dependent diabetic patients had reduced GIP responses compared to weight-matched nondiabetic subjects (mean +/- SE, 37.9 +/- 5.4 vs. 67.1 +/- 10.8 ng ml-1 240 min-1, respectively; P less than 0.05). This difference was entirely due to the reduced GIP responses in obese noninsulin-dependent diabetic patients compared to those in obese nondiabetic subjects (32.1 +/- 7.9 vs. 76.9 +/- 18.2 ng ml-1 240 min-1, respectively; P less than 0.05); the postprandial GIP responses were not significantly different between lean noninsulin-dependent diabetic patients and lean nondiabetic subjects. Insulin infusion by an artificial endocrine pancreas resulted in postprandial insulin and glucose profiles that approximated those of nondiabetics, but did not significantly alter GIP responses to the mixed meal (48.2 +/- 5.5 ng ml-1 240 min-1) in the 18 diabetic patients compared to results obtained with sc insulin treatment (42.2 +/- 5.2 ng ml-1 240 min-1). In conclusion, postprandial GIP responses are normal in obese nondiabetic subjects and insulin-deficient diabetic patients and are blunted in obese, but not in lean, noninsulin-dependent diabetic patients. In addition, GIP does not appear to be under feedback inhibition by insulin or influenced by glucagon deficiency in diabetes.     

Role of G protein-coupled receptor kinases in glucose-dependent insulinotropic polypeptide receptor signaling

The glucose-dependent insulinotropic polypeptide receptor (GIPR) is a member of class II G protein-coupled receptors. Recent studies have suggested that desensitization of the GIPR might contribute to impaired insulin secretion in type II diabetic patients, but the molecular mechanisms of GIPR signal termination are unknown. Using HEK L293 cells stably transfected with GIPR complementary DNA (L293-GIPR), the mechanisms of GIPR desensitization were investigated. GIP dose dependently increased intracellular cAMP levels in L293-GIPR cells, but this response was abolished (65%) by cotransfection with G protein-coupled receptor kinase 2 (GRK2), but not with GRK5 or GRK6. Beta-arrestin-1 transfection also induced a significantly decrease in GIP-stimulated cAMP production, and this effect was greater with cotransfection of both GRK2 and beta-arrestin-1 than with either alone. In betaTC3 cells, expression of GRK2 or beta-arrestin-1 attenuated GIP-induced insulin release and cAMP production, whereas glucose-stimulated insulin secretion was not affected. GRK2 and beta-arrestin-1 messenger RNAs were identified by Northern blot analysis to be expressed endogenously in betaTC3 and L293 cells. Overexpression of GRK2 enhanced agonist-induced GIPR phosphorylation, but receptor endocytosis was not affected by cotransfection with GRKs or beta-arrestin-1. These results suggest a potential role for GRK2/beta-arrestin-1 system in modulating GIP-mediated insulin secretion in pancreatic islet cells. Furthermore, GRK-mediated receptor phosphorylation is not required for endocytosis of the GIPR.