Fasting glucose measurement as a potential first step screening for glucose metabolism abnormalities in women with anovulatory polycystic ovary syndrome.

STUDY QUESTION: Is routine screening by oral glucose tolerance test (OGTT) needed for all women with polycystic ovary syndrome (PCOS)? SUMMARY ANSWER: Screening for glucose metabolism abnormalities of PCOS patients by an OGTT could potentially be limited to patients who present with a fasting glucose concentration between 6.1 and 7.0 mmol/l only. WHAT IS KNOWN ALREADY: Women with PCOS are at increased risk of developing diabetes. This study proposes a stepwise screening strategy for (pre)diabetes for PCOS patients based on risk stratification by fasting plasma glucose. STUDY DESIGN, SIZE, DURATION: A cross-sectional study of 226 women diagnosed with anovulatory PCOS. PARTICIPANTS AND SETTING: A consecutive series of 226 patients, diagnosed with PCOS at the University Medical Centre Utrecht, the Netherlands, were screened for glucose metabolism abnormalities by OGTT (75 g glucose load). MAIN RESULTS AND ROLE OF CHANCE: The majority of the 226 women (mean age: 29.6 ± 4.3 years; BMI: 27.3 ± 6.7 kg/m(2); 81% Caucasian) presented with a normal OGTT (169 women (75%)). Of the 57 (25%) women presenting with mild to moderate glucose abnormalities, 53 (93%) could be identified by fasting glucose concentrations only. Diabetes was diagnosed in a total of eight women (3.5%). In six women, the diagnosis was based on fasting glucose >7.0 mmol/l. The other two cases of diabetes initially presented with fasting glucose between 6.1 and 7.0 mmol/l and were diagnosed by OGTT assessment. No women diagnosed with diabetes presented with fasting glucose levels below 6.1 mmol/l. We therefore conclude that all diabetes patients could potentially be found by initial fasting glucose assessment followed by OGTT only in patients with fasting glucose between 6.1 and 7.0 mmol/l. LIMITATIONS, REASONS FOR CAUTION: Before general implementation can be advised, this screening algorithm should be validated in a prospective study of a similar or greater number of PCOS women. WIDER IMPLICATIONS OF THE FINDINGS: Our study comprised of a mostly Caucasian (81%) population, therefore generalization to other ethnic populations should be done with caution. STUDY FUNDING/COMPETING INTEREST(S): No external finance was involved in this study. B.C.J.M.F. has received fees and grant support from the following companies (in alphabetic order); Andromed, Ardana, Ferring, Genovum, Merck Serono, MSD, Organon, Pantharei Bioscience, PregLem, Schering, Schering Plough, Serono and Wyeth. A.J.G. has received fees from Abbott, Bayer Schering and IBSA. T.W.H. has received fees from Merck, Sharpe & Dohme, GlaxoSmithKline, NovoNordisk and Eli Lilly. The authors declare complete independence from funders. CLINICAL TRIAL REGISTRATION NUMBER: NCT00821379.

Induction of insulin resistance by the adipokines resistin, leptin, plasminogen activator inhibitor-1 and retinol binding protein 4 in human megakaryocytes.

BACKGROUND: In normal platelets, insulin inhibits agonist-induced Ca(2+) mobilization by raising cyclic AMP. Platelet from patients with type 2 diabetes are resistant to insulin and show increased Ca(2+) mobilization, aggregation and procoagulant activity. We searched for the cause of this insulin resistance. DESIGN AND METHODS: Platelets, the megakaryocytic cell line CHRF-288-11 and primary megakaryocytes were incubated with adipokines and with plasma from individuals with a disturbed adipokine profile. Thrombin-induced Ca(2+) mobilization and signaling through the insulin receptor and insulin receptor substrate 1 were measured. Abnormalities induced by adipokines were compared with abnormalities found in platelets from patients with type 2 diabetes. RESULTS: Resistin, leptin, plasminogen activator inhibitor-1 and retinol binding protein 4 left platelets unchanged but induced insulin resistance in CHRF-288-11 cells. Interleukin-6, tumor necrosis factor-α and visfatin had no effect. These results were confirmed in primary megakaryocytes. Contact with adipokines for 2 hours disturbed insulin receptor substrate 1 Ser(307)-phosphorylation, while contact for 72 hours caused insulin receptor substrate 1 degradation. Plasma with a disturbed adipokine profile also made CHRF-288-11 cells insulin-resistant. Platelets from patients with type 2 diabetes showed decreased insulin receptor substrate 1 expression. CONCLUSIONS: Adipokines resistin, leptin, plasminogen activator-1 and retinol binding protein 4 disturb insulin receptor substrate 1 activity and expression in megakaryocytes. This might be a cause of the insulin resistance observed in platelets from patients with type 2 diabetes.

Endocrine autoimmune disease: genetics become complex.

The endocrine system is a frequent target in pathogenic autoimmune responses. Type 1 diabetes and autoimmune thyroid disease are the prevailing examples. When several diseases cluster together in one individual, the phenomenon is called autoimmune polyglandular syndrome. Progress has been made in understanding the genetic factors involved in endocrine autoimmune diseases. Studies on monogenic autoimmune diseases such as autoimmune polyglandular syndrome type 1, immunodysregulation, polyendocrinopathy, enteropathy, X-linked and primary immune deficiencies helped uncover the role of key regulators in the preservation of immune tolerance. Alleles of the major histocompatibility complex have been known to contribute to the susceptibility to most forms of autoimmunity for more than 3 decades. Furthermore, sequencing studies revealed three non-major histocompatibility complex loci and some disease specific loci, which control T lymphocyte activation or signalling. Recent genome-wide association studies (GWAS) have enabled acceleration in the identification of novel (non-HLA) loci and hence other relevant immune response pathways. Interestingly, several loci are shared between autoimmune diseases, and surprisingly some work in opposite direction. This means that the same allele which predisposes to a certain autoimmune disease can be protective in another. Well powered GWAS in type 1 diabetes has led to the uncovering of a significant number of risk variants with modest effect. These studies showed that the innate immune system may also play a role in addition to the adaptive immune system. It is anticipated that next generation sequencing techniques will uncover other (rare) variants. For other autoimmune disease (such as autoimmune thyroid disease) GWAS are clearly needed.

Adipose tissue dysfunction in obesity, diabetes, and vascular diseases.

The classical perception of adipose tissue as a storage place of fatty acids has been replaced over the last years by the notion that adipose tissue has a central role in lipid and glucose metabolism and produces a large number of hormones and cytokines, e.g. tumour necrosis factor-alpha, interleukin-6, adiponectin, leptin, and plasminogen activator inhibitor-1. The increased prevalence of excessive visceral obesity and obesity-related cardiovascular risk factors is closely associated with the rising incidence of cardiovascular diseases and type 2 diabetes mellitus. This clustering of vascular risk factors in (visceral) obesity is often referred to as metabolic syndrome. The close relationship between an increased quantity of visceral fat, metabolic disturbances, including low-grade inflammation, and cardiovascular diseases and the unique anatomical relation to the hepatic portal circulation has led to an intense endeavour to unravel the specific endocrine functions of this visceral fat depot. The objective of this paper is to describe adipose tissue dysfunction, delineate the relation between adipose tissue dysfunction and obesity and to describe how adipose tissue dysfunction is involved in the development of diabetes mellitus type 2 and atherosclerotic vascular diseases. First, normal physiology of adipocytes and adipose tissue will be described.

HHEX gene polymorphisms are associated with type 2 diabetes in the Dutch Breda cohort.

Recently, the hematopoietically expressed homeobox (HHEX) gene, encoding a transcription factor, was identified in a large genome-wide scan in French individuals as a type 2 diabetes (T2D)-susceptibility locus. We aimed to check whether this finding could be replicated in a Dutch T2D cohort. Two common variants (rs7923837 and rs1111875) located near the HHEX gene were genotyped in 501 unrelated T2D patients and in 920 healthy controls. The major alleles of both variants were overrepresented in T2D cases compared with controls (66.7 vs 64.1%, P=0.16 for rs7923837 and 64.6 vs 60.4%, P=0.027 for rs1111875). For both polymorphisms, the risk for T2D was significantly increased in carriers of the major alleles (rs7923837: odds ratio (OR): 1.57, 95% confidence interval (CI): 1.08-2.27, P=0.017 and rs1111875: OR: 1.68, 95% CI: 1.19-2.35, P=0.003). The haplotype analysis did not reveal a risk haplotype that provided stronger evidence for association with T2D than each variant individually. Assuming a dominant genetic model, the population-attributable risks for diabetes due to the at-risk alleles of rs7923837 and rs1111875 were estimated to be 33 and 36%, respectively. These data provide evidence that variants near the HHEX gene contribute to the risk of T2D in a Dutch population.

Identification of TUB as a novel candidate gene influencing body weight in humans.

Previously, we identified a locus on 11p influencing obesity in families with type 2 diabetes. Based on mouse studies, we selected TUB as a functional candidate gene and performed association studies to determine whether this controls obesity. We analyzed the genotypes of 13 single nucleotide polymorphisms (SNPs) around TUB in 492 unrelated type 2 diabetic patients with known BMI values. One SNP (rs1528133) was found to have a significant effect on BMI (1.54 kg/m(2), P = 0.006). This association was confirmed in a population enriched for type 2 diabetes, using 750 individuals who were not selected for type 2 diabetes. Two SNPs in linkage disequilibrium with rs1528133 and mapping to the 3' end of TUB, rs2272382, and rs2272383 also affected BMI by 1.3 kg/m2 (P = 0.016 and P = 0.010, respectively). Combined analysis confirmed this association (P = 0.005 and P = 0.002, respectively). Moreover, comparing 349 obese subjects (BMI >30 kg/m(2)) from the combined cohort with 289 normal subjects (BMI <25 kg/m(2)) revealed that the protective alleles have a lower frequency in obese subjects (odds ratio 1.32 [95% CI 1.04-1.67], P = 0.022). Altogether, data from the tubby mouse as well as these data suggest that TUB could be an important factor in controlling the central regulation of body weight in humans.

Loss of endothelial glycocalyx during acute hyperglycemia coincides with endothelial dysfunction and coagulation activation in vivo.

Hyperglycemia is associated with increased susceptibility to atherothrombotic stimuli. The glycocalyx, a layer of proteoglycans covering the endothelium, is involved in the protective capacity of the vessel wall. We therefore evaluated whether hyperglycemia affects the glycocalyx, thereby increasing vascular vulnerability. The systemic glycocalyx volume was estimated by comparing the distribution volume of a glycocalyx permeable tracer (dextran 40) with that of a glycocalyx impermeable tracer (labeled erythrocytes) in 10 healthy male subjects. Measurements were performed in random order on five occasions: two control measurements, two measurements during normoinsulinemic hyperglycemia with or without N-acetylcysteine (NAC) infusion, and one during mannitol infusion. Glycocalyx measurements were reproducible (1.7 +/- 0.2 vs. 1.7 +/- 0.3 l). Hyperglycemia reduced glycocalyx volume (to 0.8 +/- 0.2 l; P < 0.05), and NAC was able to prevent the reduction (1.4 +/- 0.2 l). Mannitol infusion had no effect on glycocalyx volume (1.6 +/- 0.1 l). Hyperglycemia resulted in endothelial dysfunction, increased plasma hyaluronan levels (from 70 +/- 6 to 112 +/- 16 ng/ml; P < 0.05) and coagulation activation (prothrombin activation fragment 1 + 2: from 0.4 +/- 0.1 to 1.1 +/- 0.2 nmol/l; d-dimer: from 0.27 +/- 0.1 to 0.55 +/- 0.2 g/l; P < 0.05). Taken together, these data indicate a potential role for glycocalyx perturbation in mediating vascular dysfunction during hyperglycemia.

Platelet inhibition by insulin is absent in type 2 diabetes mellitus.

OBJECTIVE: ADP-induced P2y12 signaling is crucial for formation and stabilization of an arterial thrombus. We demonstrated recently in platelets from healthy subjects that insulin interferes with Ca2+ increases induced by ADP-P2y1 contact through blockade of the G-protein Gi, and thereby with P2y12-mediated suppression of cAMP. METHODS AND RESULTS: Here we show in patients with type 2 diabetes mellitus (DM2) that platelets have lost responsiveness to insulin leading to increased adhesion, aggregation, and procoagulant activity on contact with collagen. Using Ser473 phosphorylation of protein kinase B as output for insulin signaling, a 2-fold increase is found in insulin-stimulated normal platelets, but in DM platelets there is no significant response. In addition, DM2 platelets show increased P2y12-mediated suppression of cAMP and decreased P2y12 inhibition by the receptor antagonist AR-C69931MX. CONCLUSIONS: The loss of responsiveness to insulin together with increased signaling through P2y12 might explain the hyperactivity of platelets in patients with DM2.

A Genome-Wide Association Study of IVGTT-Based Measures of First-Phase Insulin Secretion Refines the Underlying Physiology of Type 2 Diabetes Variants.

Understanding the physiological mechanisms by which common variants predispose to type 2 diabetes requires large studies with detailed measures of insulin secretion and sensitivity. Here we performed the largest genome-wide association study of first-phase insulin secretion, as measured by intravenous glucose tolerance tests, using up to 5,567 individuals without diabetes from 10 studies. We aimed to refine the mechanisms of 178 known associations between common variants and glycemic traits and identify new loci. Thirty type 2 diabetes or fasting glucose-raising alleles were associated with a measure of first-phase insulin secretion at P < 0.05 and provided new evidence, or the strongest evidence yet, that insulin secretion, intrinsic to the islet cells, is a key mechanism underlying the associations at the HNF1A, IGF2BP2, KCNQ1, HNF1B, VPS13C/C2CD4A, FAF1, PTPRD, AP3S2, KCNK16, MAEA, LPP, WFS1, and TMPRSS6 loci. The fasting glucose-raising allele near PDX1, a known key insulin transcription factor, was strongly associated with lower first-phase insulin secretion but has no evidence for an effect on type 2 diabetes risk. The diabetes risk allele at TCF7L2 was associated with a stronger effect on peak insulin response than on C-peptide-based insulin secretion rate, suggesting a possible additional role in hepatic insulin clearance or insulin processing. In summary, our study provides further insight into the mechanisms by which common genetic variation influences type 2 diabetes risk and glycemic traits.

Type 2 diabetes: principles of pathogenesis and therapy.

Type 2 diabetes mellitus has become an epidemic, and virtually no physician is without patients who have the disease. Whereas insulin insensitivity is an early phenomenon partly related to obesity, pancreas beta-cell function declines gradually over time already before the onset of clinical hyperglycaemia. Several mechanisms have been proposed, including increased non-esterified fatty acids, inflammatory cytokines, adipokines, and mitochondrial dysfunction for insulin resistance, and glucotoxicity, lipotoxicity, and amyloid formation for beta-cell dysfunction. Moreover, the disease has a strong genetic component, but only a handful of genes have been identified so far: genes for calpain 10, potassium inward-rectifier 6.2, peroxisome proliferator-activated receptor gamma, insulin receptor substrate-1, and others. Management includes not only diet and exercise, but also combinations of anti-hyperglycaemic drug treatment with lipid-lowering, antihypertensive, and anti platelet therapy.

Altered homeostatic adaptation of first- and second-phase beta-cell secretion in the offspring of patients with type 2 diabetes: studies with a minimal model to assess beta-cell function.

We adapted a minimal model to assess beta-cell function during a hyperglycemic glucose clamp and to uncover peculiar aspects of the relationship among beta-cell function, plasma glucose, and insulin sensitivity (IS) in offspring of Caucasian patients with type 2 diabetes (OfT2D). We pooled two data sets of OfT2D (n = 69) and control subjects (n = 45) with normal glucose regulation. Plasma C-peptide was measured during a hyperglycemic clamp ( approximately 10 mmol/l) to quantify model-based first-phase secretion and glucose sensitivity of second-phase secretion (beta). IS was quantified during the hyperglycemic clamp. In the pooled data, first-phase secretion was linearly and negatively related to fasting plasma glucose, but not IS; OfT2D lay on a distinct line shifted to the left of the control subjects. In contrast, beta was negatively related to IS, and OfT2D lay on a distinct line shifted more and more to the left of the control subjects, as IS was worse. Thus, in OfT2D lower beta-cell adaptive responses exist between ambient glycemia and first-phase insulin secretion and between IS and second-phase secretion. Under conditions leading to decreased insulin sensitivity, these disturbed relationships may lead to progression to diabetes in OfT2D.

Variations in insulin secretion in carriers of the E23K variant in the KIR6.2 subunit of the ATP-sensitive K(+) channel in the beta-cell.

An association between type 2 diabetes and genetic variation in the KIR6.2 gene has been reported in several populations. Based on in vitro studies with cell lines expressing the Glu(23)Lys (E23K) mutation, it was recently suggested that this mutation might result in altered insulin secretion. We have examined glucose-stimulated insulin secretion in relation to this KIR6.2 gene variant in two independent Dutch cohorts. Subjects with normal (n = 65) or impaired (n = 94) glucose tolerance underwent 3-h hyperglycemic clamps at 10 mmol/l glucose. We did not observe significant differences in first- or second-phase insulin secretion between carriers and noncarriers of the gene variant in either of the study populations (all P > 0.45). Furthermore, we found no evidence for a significant interaction with disease-associated gene variants in the sulfonylurea receptor (SUR1) gene. We conclude that the E23K mutation in the KIR6.2 gene is not associated with detectable alterations in glucose-stimulated insulin secretion in two independent populations from the Netherlands.

Disturbances in beta-cell function in impaired fasting glycemia.

In a cross-sectional study, we assessed beta-cell function and insulin sensitivity index (ISI) with hyperglycemic clamps (10 mmol/l) in 24 subjects with impaired fasting glycemia (IFG, fasting plasma glucose [FPG] between 6.1 and 7.0 mmol/l), 15 type 2 diabetic subjects (FPG >7.0 mmol/l), and 280 subjects with normal fasting glycemia (NFG, FPG <6.1 mmol/l). First-phase insulin release (0-10 min) was lower in IFG (geometric mean 541 pmol/l.10 min; 95% confidence interval [CI] 416-702 pmol/l.10 min) and in type 2 diabetes (geometric mean 376 pmol/l.10 min; 95% CI 247-572 pmol/l.10 min) than NFG (geometric mean 814 pmol/l.10 min; 95% CI 759-873 pmol/l.10 min) (P < 0.001). Second-phase insulin secretion (140-180 min) was also lower in IFG (geometric mean 251 pmol/l; 95% CI 198-318 pmol/l; P = 0.026) and type 2 diabetes (geometric mean 157 pmol/l; 95% CI 105-235 pmol/l; P < 0.001) than NFG (geometric mean 295 pmol/l; 95% CI 276-315 pmol/l). IFG and type 2 diabetic subjects had a lower ISI (0.15 plus minus 0.02 and 0.16 plus minus 0.02 micromol/kg fat-free mass [FFM]/min/pmol/l, respectively) than NFG (0.24 plus minus 0.01 micromol/kg FFM/min/pmol/l, P < 0.05). We found a stepwise decline in first-phase (and second-phase) secretion in NFG subjects with progressive decline in oral glucose tolerance (P < 0.05). IFG subjects with impaired glucose tolerance (IGT) had lower first-phase secretion than NFG subjects with IGT (P < 0.02), with comparable second-phase secretion and ISI. NFG and IFG subjects with a diabetic glucose tolerance (2-h glucose >11.1 mmol/l) had a lower ISI than their respective IGT counterparts (P < 0.05). We conclude that the early stages of glucose intolerance are associated with disturbances in beta-cell function, while insulin resistance is seen more markedly in later stages.

Variations in insulin secretion in carriers of gene variants in IRS-1 and -2.

Associations between type 2 diabetes (and/or parameters contributing to glucose homeostasis) and genetic variation in the genes encoding insulin receptor substrate (IRS)-1 and -2 have been reported in several populations. Recently, it has been reported that the Gly(972)Arg variant in IRS-1 was associated with reduced insulin secretion during hyperglycemic clamps in German subjects with normal glucose tolerance. We have examined glucose-stimulated insulin secretion in relation to gene variants in the IRS-1 (Gly(972)Arg) and IRS-2 (Gly(1057)Asp) genes in two Dutch cohorts. Subjects with normal (n = 64) or impaired (n = 94) glucose tolerance underwent 3-h hyperglycemic clamps at 10 mmol/l glucose. All subjects were genotyped for the IRS-1 and IRS-2 variants by PCR-RFLP--based methods. We did not observe any significant difference in both first- and second-phase insulin secretion between carriers and noncarriers of both gene variants, nor was there evidence for an association with other diabetes-related parameters. We conclude that the common gene variants in IRS-1 and IRS-2 are not associated with altered glucose-stimulated insulin secretion in two populations from the Netherlands.

Genetic factors and insulin secretion: gene variants in the IGF genes.

IGFs are important regulators of pancreatic beta-cell development, growth, and maintenance. Mutations in the IGF genes have been found to be associated with type 2 diabetes, myocardial infarction, birth weight, and obesity. These associations could result from changes in insulin secretion. We have analyzed glucose-stimulated insulin secretion using hyperglycemic clamps in carriers of a CA repeat in the IGF-I promoter and an ApaI polymorphism in the IGF-II gene. Normal and impaired glucose-tolerant subjects (n = 237) were independently recruited from three different populations in the Netherlands and Germany to allow independent replication of associations. Both first- and second-phase insulin secretion were not significantly different between the various IGF-I or IGF-II genotypes. Remarkably, noncarriers of the IGF-I CA repeat allele had both a reduced insulin sensitivity index (ISI) and disposition index (DI), suggesting an altered balance between insulin secretion and insulin action. Other diabetes-related parameters were not significantly different for both the IGF-I and IGF-II gene variant. We conclude that gene variants in the IGF-I and IGF-II genes are not associated with detectable variations in glucose-stimulated insulin secretion in these three independent populations. Further studies are needed to examine the exact contributions of the IGF-I CA repeat alleles to variations in ISI and DI.

The sulfonylurea glyburide induces impairment of glucagon and growth hormone responses during mild insulin-induced hypoglycemia.

OBJECTIVE: The sulfonylurea (SU) glyburide may cause severe and prolonged episodes of hypoglycemia. We aimed at investigating the impact of glyburide on glucose counterregulatory hormones during stepwise hypoglycemic clamp studies. RESEARCH DESIGN AND METHODS: We performed stepwise hypoglycemic clamp studies in 16 healthy volunteers (7 women and 9 men aged 44 +/- 10 years). We investigated counterregulatory hormonal and symptom responses at arterialized venous plasma glucose levels (PG) of 3.8, 3.2, and 2.6 mmol/l, comparing 10 mg glyburide orally and placebo in a double-blind, randomized crossover fashion. RESULTS: The increase in plasma glucagon with time from PG = 3.8 onward was smaller for glyburide than for placebo (P = 0.014). Plasma glucagon area under the curve (AUC)(60-180) was lower after glyburide than after placebo (1,774 +/- 715 vs. 2,161 +/- 856 pmol. l(-1). min, P = 0.014). From PG = 3.8 onward, plasma growth hormone (GH) levels with placebo were nearly two times (1.9 [95% CI 1.2-2.9]) as high as with glyburide (P = 0.011). AUC(60-180) for GH was lower after glyburide than after placebo (geometric mean [range] 665 [356-1,275] and 1,058 [392-1,818] mU. l(-1). min, respectively; P = 0.04). No significant differences were observed for plasma cortisol, epinephrine and norepinephrine, or incremental symptom scores. CONCLUSIONS: The SU glyburide induces multiple defects in glucose counterregulatory hormonal responses, notably decreases in both glucagon and GH release.

Early disturbances in insulin secretion in the development of type 2 diabetes mellitus.

After a short description of normal glucose homeostasis, recent findings in relation to insulin release in three groups with a high risk of future development of type 2 diabetes are described. Hyperglycemic clamps in subjects with impaired glucose tolerance (IGT) clearly indicate that pancreatic beta cell function is decreased, in addition to the decreased insulin sensitivity. In women with former gestational diabetes mellitus (GDM), insulin release is also lower than in controls. In Caucasian first-degree relatives (FDRs) with normal glucose tolerance, various studies have shown that beta cell function is lower than in controls, while on the average insulin sensitivity is normal. This implies that beta cell function is disturbed earlier in subjects at risk of developing diabetes than is often appreciated. In the near future, the genetic studies currently underway will presumably unravel the pathogenesis of disturbances both in insulin secretion and in insulin action, in type 2 diabetes mellitus.

The CTRB1/2 locus affects diabetes susceptibility and treatment via the incretin pathway.

The incretin hormone glucagon-like peptide 1 (GLP-1) promotes glucose homeostasis and enhances ß-cell function. GLP-1 receptor agonists (GLP-1 RAs) and dipeptidyl peptidase-4 (DPP-4) inhibitors, which inhibit the physiological inactivation of endogenous GLP-1, are used for the treatment of type 2 diabetes. Using the Metabochip, we identified three novel genetic loci with large effects (30-40%) on GLP-1-stimulated insulin secretion during hyperglycemic clamps in nondiabetic Caucasian individuals (TMEM114; CHST3 and CTRB1/2; n = 232; all P ≤ 8.8 × 10(-7)). rs7202877 near CTRB1/2, a known diabetes risk locus, also associated with an absolute 0.51 ± 0.16% (5.6 ± 1.7 mmol/mol) lower A1C response to DPP-4 inhibitor treatment in G-allele carriers, but there was no effect on GLP-1 RA treatment in type 2 diabetic patients (n = 527). Furthermore, in pancreatic tissue, we show that rs7202877 acts as expression quantitative trait locus for CTRB1 and CTRB2, encoding chymotrypsinogen, and increases fecal chymotrypsin activity in healthy carriers. Chymotrypsin is one of the most abundant digestive enzymes in the gut where it cleaves food proteins into smaller peptide fragments. Our data identify chymotrypsin in the regulation of the incretin pathway, development of diabetes, and response to DPP-4 inhibitor treatment.