Aims/hypothesis Variation within six novel genetic loci has been reported to confer risk of type 2 diabetes and may be associated with beta cell dysfunction. We investigated whether these polymorphisms are also associated with impaired proinsulin to insulin conversion. Methods We genotyped 1,065 German participants for single nucleotide polymorphisms rs7903146 in TCF7L2, rs7754840 in CDKAL1, rs7923837 and rs1111875 in HHEX, rs13266634 in SLC30A8, rs10811661 in CDKN2A/B and rs4402960 in IGF2BP2. All participants underwent an OGTT. Insulin, proinsulin and C-peptide concentrations were measured at 0, 30, 60, 90 and 120 min during the OGTT. Insulin secretion was estimated from C-peptide or insulin levels during the OGTT using validated indices. We used the ratio proinsulin/insulin during the OGTT as indicator of proinsulin conversion. Results In our cohort, we confirmed the significant association of variants in TCF7L2, CDKAL1 and HHEX with reduced insulin secretion during the OGTT (p<0.05 for all). Variation in SLC30A8, CDKN2A/B and IGF2BP2 was not associated with insulin secretion. The risk alleles of the variants in TCF7L2, CDKAL1 and SLC30A8 reduced proinsulin to insulin conversion (p<0.05 for all), whereas the risk alleles in HHEX, CDKN2A/B and IGF2BP2 were not associated with reduced proinsulin to insulin conversion (p>0.6). Conclusions/interpretation Diabetes-associated variants in TCF7L2 and CDKAL1 impair insulin secretion and conversion of proinsulin to insulin. However, both aspects of beta cell function are not necessarily linked, as impaired insulin secretion is specifically present in variants of HHEX and impaired proinsulin conversion is specifically present in a variant of SLC30A8.

Polymorphisms in the FTO (fat mass- and obesity-associated) gene are associated with obesity. The mechanisms how genetic variation in this gene influences body weight are unknown. Body weight is determined by energy intake/storage and energy expenditure. In this study, we investigated whether genetic variation in FTO influences energy expenditure or food intake in carefully phenotyped subjects. In 380 German subjects, insulin sensitivity was measured by a hyperinsulinemic euglycemic clamp. Lean body mass and body fat were quantified using the bioimpedance method. Indirect calorimetry was used to estimate the metabolic rate. Food intake was assessed using food diaries (mean 11+/-1 d) in 151 subjects participating in a lifestyle intervention program to prevent diabetes. All subjects were genotyped for the FTO single nucleotide polymorphism (SNP) rs8050136. The risk allele of SNP rs8050136 was associated with higher body fat-related parameters (all p< or =0.04, additive inheritance model). Energy expenditure was not affected by the SNP. However, the risk allele of rs8050136 was significantly associated with higher energy intake (p=0.01, dominant inheritance model) during dietary restriction. Our data suggest that the increased body weight in carriers of the risk allele of FTO SNP rs8050136 is a consequence of increased food intake, but not of impaired energy expenditure.

BackgroundType 2 diabetes arises when insulin resistance-induced compensatory insulin secretion exhausts. Insulin resistance and/or β-cell dysfunction result from the interaction of environmental factors (high-caloric diet and reduced physical activity) with a predisposing polygenic background. Very recently, genetic variations within four novel genetic loci (SLC30A8, HHEX, EXT2, and LOC387761) were reported to be more frequent in subjects with type 2 diabetes than in healthy controls. However, associations of these variations with insulin resistance and/or β-cell dysfunction were not assessed.Methodology/Principal FindingsBy genotyping of 921 metabolically characterized German subjects for the reported candidate single nucleotide polymorphisms (SNPs), we show that the major alleles of the SLC30A8 SNP rs13266634 and the HHEX SNP rs7923837 associate with reduced insulin secretion stimulated by orally or intravenously administered glucose, but not with insulin resistance. In contrast, the other reported type 2 diabetes candidate SNPs within the EXT2 and LOC387761 loci did not associate with insulin resistance or β-cell dysfunction, respectively.Conclusions/SignificanceThe HHEX and SLC30A8 genes encode for proteins that were shown to be required for organogenesis of the ventral pancreas and for insulin maturation/storage, respectively. Therefore, the major alleles of type 2 diabetes candidate SNPs within these genetic loci represent crucial alleles for β-cell dysfunction and, thus, might confer increased susceptibility of β-cells towards adverse environmental factors.

BackgroundVery recently, a novel type 2 diabetes risk gene, i.e., MTNR1B, was identified and reported to affect fasting glycemia. Using our thoroughly phenotyped cohort of subjects at an increased risk for type 2 diabetes, we assessed the association of common genetic variation within the MTNR1B locus with obesity and prediabetes traits, namely impaired insulin secretion and insulin resistance.Methodology/Principal FindingsWe genotyped 1,578 non-diabetic subjects, metabolically characterized by oral glucose tolerance test, for five tagging single nucleotide polymorphisms (SNPs) covering 100% of common genetic variation (minor allele frequency >0.05) within the MTNR1B locus (rs10830962, rs4753426, rs12804291, rs10830963, rs3781638). In a subgroup (N = 513), insulin sensitivity was assessed by hyperinsulinemic-euglycemic clamp, and in a further subgroup (N = 301), glucose-stimulated insulin secretion was determined by intravenous glucose tolerance test. After appropriate adjustment for confounding variables and Bonferroni correction for multiple comparisons, none of the tagging SNPs was reliably associated with measures of adiposity. SNPs rs10830962, rs4753426, and rs10830963 were significantly associated with higher fasting plasma glucose concentrations (p<0.0001) and reduced OGTT- and IVGTT-induced insulin release (p≤0.0007 and p≤0.01, respectively). By contrast, SNP rs3781638 displayed significant association with lower fasting plasma glucose levels and increased OGTT-induced insulin release (p<0.0001 and p≤0.0002, respectively). Moreover, SNP rs3781638 revealed significant association with elevated fasting- and OGTT-derived insulin sensitivity (p≤0.0021). None of the MTNR1B tagging SNPs altered proinsulin-to-insulin conversion.Conclusions/SignificanceIn conclusion, common genetic variation within MTNR1B determines glucose-stimulated insulin secretion and plasma glucose concentrations. Their impact on β-cell function might represent the prevailing pathomechanism how MTNR1B variants increase the type 2 diabetes risk.

OBJECTIVEKCNQ1 gene polymorphisms are associated with type 2 diabetes. This linkage appears to be mediated by altered β-cell function. In an attempt to study underlying mechanisms, we examined the effect of four KCNQ1 single nucleotide polymorphisms (SNPs) on insulin secretion upon different stimuli.RESEARCH DESIGN AND METHODSWe genotyped 1,578 nondiabetic subjects at increased risk of type 2 diabetes for rs151290, rs2237892, rs2237895, and rs2237897. All participants underwent an oral glucose tolerance test (OGTT); glucagon-like peptide (GLP)-1 and gastric inhibitory peptide secretion was measured in 170 participants. In 519 participants, a hyperinsulinemic-euglycemic clamp was performed, in 314 participants an intravenous glucose tolerance test (IVGTT), and in 102 subjects a hyperglycemic clamp combined with GLP-1 and arginine stimuli.RESULTSrs151290 was nominally associated with 30-min C-peptide levels during OGTT, first-phase insulin secretion, and insulinogenic index after adjustment in the dominant model (all P ≤ 0.01). rs2237892, rs2237895, and rs2237897 were nominally associated with OGTT-derived insulin secretion indexes (all P < 0.05). No SNPs were associated with β-cell function during intravenous glucose or GLP-1 administration. However, rs151290 was associated with glucose-stimulated gastric inhibitory polypeptide and GLP-1 increase after adjustment in the dominant model (P = 0.0042 and P = 0.0198, respectively). No associations were detected between the other SNPs and basal or stimulated incretin levels (all P ≥ 0.05).CONCLUSIONSCommon genetic variation in KCNQ1 is associated with insulin secretion upon oral glucose load in a German population at increased risk of type 2 diabetes. The discrepancy between orally and intravenously administered glucose seems to be explained not by altered incretin signaling but most likely by changes in incretin secretion.

Recent genome-wide association studies aiming to identify genes responsible for diabetes mellitus described a strong link between variations in the FTO (fat mass-and obesity-associated) gene and body weight (1,2). The association with obesity is replicable in most populations (3)(4)(5), but the mechanism how variants in FTO lead to obesity is still not completely understood.FTO was originally described in a mouse model and is involved in cell death programming (6). In heterozygote mice, the mutation results in fused toes. Second, the thymus is enlarged whereas other organs or body weight are similar to the wild type (7). With detection of the association of this gene locus with human obesity, the Gene Nomenclature Committee renamed the gene FTO. Recently, it has been shown that the FTO gene encodes a Fe(II)-and 2-oxoglutarate-dependent oxygenase putatively involved in DNA demethylation (8,9).In humans, the FTO gene is located on the chromosome 16q12.2. Duplication of this region results in mental retardation, obesity, dysmorphic facies, and digital anomalies (10). It is known that FTO is expressed in the hypothalamic region (11) and may play a role in regulation of central body weight (12,13).In addition, carriers of the FTO allele which is associated with increased BMI exhibit a reduced basal fat cell lipolysis (14), suggesting that the FTO gene may also play a role in the adipose tissue metabolism.In the initial studies, the higher diabetes incidence of the risk allele carriers depended only on the strong association with obesity (1). The influence of the FTO gene on body composition and different fat compartments is still largely unknown. We used a whole body imaging approach enabling quantification of body fat stores to test whether the FTO polymorphism influences body composition or ectopic lipid storage in the liver. Furthermore, variation in the FTO locus may also affect weight loss during lifestyle intervention (15,16). We, therefore, studied the effect of variations in FTO on weight loss and body composition in a lifestyle intervention program. SubjectS and MethodS cross-sectional analysisWe studied 1,466 nondiabetic subjects from the southern part of Germany who participated in the ongoing Tübingen Family Study for type 2 diabetes mellitus, which currently includes ~2,000 individuals (17). Polymorphisms in the fat mass-and obesity-associated (FTO) gene have been identified to be associated with obesity and diabetes in large genome-wide association studies. We hypothesized that variation in the FTO gene has an impact on whole body fat distribution and insulin sensitivity, and influences weight change during lifestyle intervention. To test this hypothesis, we genotyped 1,466 German subjects, with increased risk for type 2 diabetes, for single-nucleotide polymorphism rs8050136 in the FTO gene and estimated glucose tolerance and insulin sensitivity from an oral glucose tolerance test (OGTT). Distribution of fat depots was quantified using whole body magnetic resonance (MR) imaging and spectroscopy in 298 s...

SummaryA novel CC chemokine, HCC-1, was isolated from the hemoflhrate of patients with chronic renal failure. HCC-1 has a relative molecular mass of 8,673 and consists of 74 amino acids including four cysteines linked to disulfide bonds. HCC-1 cDNA was cloned from human bone marrow and shown to code for the mature protein plus a putative 19-residue leader sequence. Mature HCC-1 has sequence identity of 46% with macrophage inflammatory protein (MIP)-lc~ and MIP-][3, and 29-37% with the other human CC chemokines. Unlike MIP-I~x and the other CC chemokines, HCC-1 is expressed constitutively in several normal tissues (spleen, liver, skeletal and heart muscle, gut, and bone marrow), and is present at high concentrations (1-80 nM) in plasma. HCC-1 has weak activities on human monocytes and acts via receptors that also recognize MIP-lot. It induced intracellular Ca 2+ changes and enzyme release, but no chemotaxis, at concentrations of 100-1,000 nM, and was inactive on T lymphocytes, neutrophils, and eosinophil leukocytes. In addition, HCC-1 enhanced the proliferation of CD34 + myeloid progenitor ceils. It was as effective as MIP-lci, but about 100-fold less potent.H emofittration is a routine treatment for patients with chronic renal failure to remove substances that are normally cleared by the kidney. A filter membrane with a molecular mass cut-off of 20 kD is used, which virtually excludes plasma proteins (1, 2). Being available in large quantities, the hemoflltrate is an excellent source of biologically active human peptides that circulate in the blood. During the last 5 yr, a peptide bank was established from >200,000 liters of hemofiltrate, and several peptide hormones were purified (1). During the course of a systematic search for novel bioactive factors (3), we have identified a new member of the recently recognized family of chemotactic cytokines (chemokines). HCC-1 is structurally related to macrophage inflammatory protein (MIP)-loc Unlike MIP-lo~ and the other CC chemokines, HCC-1 is highly expressed in normal tissues and is present at high concentrations in human plasma. Materials and MethodsPurification. Peptides were extracted from batches of 2,000 liters of hemofiltrate by precipitation with 660 g/liter ammonium sulfate as described previously (2). The precipitate ("250 g) was dissolved in water (125 mg/ml). The peptides were precipitated again by addition of 4.5 vol of2-propanol, redissolved in 10 mM phosphate buffer, pH 3.0, and fractionated by cation exchange chromatography (Fractogel TSK SP-650 M, 6 • 20 cm; Merck, Darmstadt, Germany) with a 0-1.0-M NaC1 gradient in the same buffer. Fractions eluting at high salt concentrations were collected and further purified by preparative reverse-phase (lkP) HPLC (Parcosil RP C4, 300 ft,, 20-45 b~m, 3 • 12.5 cm; Biotek, Oestringen, Germany) using a gradient generated from 0.01 M HCI and 50% 2-propanol/30% methanol in 0.01 M HCI. Analytical R.P HPLC (Parcosil RP C4, 300 A, 5 btm, 1 • 12.5 cm, Biotek, Oestringen, Germany) was then performed using an acetonitrile/T...

The kinetics of insulin secretion and oxygen uptake in response to D-glucose and tolbutamide were compared in mouse pancreatic islets. In addition, the role of decreased ATP as a driving force for secretagogue-induced oxygen consumption was examined. D-glucose (10-30 mmol/l) triggered a biphasic insulin release which always coincided with a monophasic increase in islet oxygen uptake. In the presence of D-glucose (5-30 mmol/l), tolbutamide (3-500 mumol/l) consistently elicited an initial peak of insulin secretion which was followed by a continued decline. Tolbutamide-induced secretory profiles were accompanied by similar respiratory profiles. Oxygen consumption per ng of insulin released during the test phase was higher after elevation of the glucose concentration than after addition of tolbutamide. In conjunction with 5 or 10 mmol/l D-glucose, but not with 15 or 30 mmol/l D-glucose, tolbutamide (30-100 mumol/l) lowered islet ATP content significantly (p less than 0.02). Phosphocreatine was not found in isolated islets, although they contained substantial creatine kinase activity. It is concluded that the driving force for tolbutamide-induced oxygen uptake is a decrease in the phosphorylation potential caused by the work load imposed by stimulation of the secretion process. However, a major proportion of the respiratory response to glucose also results from enhancement of biosynthesis.