Harald Staiger

Fetuin-A Induces Cytokine Expression and Suppresses Adiponectin Production

Background The secreted liver protein fetuin-A (AHSG) is up-regulated in hepatic steatosis and the metabolic syndrome. These states are strongly associated with low-grade inflammation and hypoadiponectinemia. We, therefore, hypothesized that fetuin-A may play a role in the regulation of cytokine expression, the modulation of adipose tissue expression and plasma concentration of the insulin-sensitizing and atheroprotective adipokine adiponectin. Methodology and Principal Findings Human monocytic THP1 cells and human in vitro differenttiated adipocytes as well as C57BL/6 mice were treated with fetuin-A. mRNA expression of the genes encoding inflammatory cytokines and the adipokine adiponectin (ADIPOQ) was assessed by real-time RT-PCR. In 122 subjects, plasma levels of fetuin-A, adiponectin and, in a subgroup, the multimeric forms of adiponectin were determined. Fetuin-A treatment induced TNF and IL1B mRNA expression in THP1 cells (p<0.05). Treatment of mice with fetuin-A, analogously, resulted in a marked increase in adipose tissue Tnf mRNA as well as Il6 expression (27- and 174-fold, respectively). These effects were accompanied by a decrease in adipose tissue Adipoq mRNA expression and lower circulating adiponectin levels (p<0.05, both). Furthermore, fetuin-A repressed ADIPOQ mRNA expression of human in vitro differentiated adipocytes (p<0.02) and induced inflammatory cytokine expression. In humans in plasma, fetuin-A correlated positively with high-sensitivity C-reactive protein, a marker of subclinical inflammation (r = 0.26, p = 0.01), and negatively with total- (r = −0.28, p = 0.02) and, particularly, high molecular weight adiponectin (r = −0.36, p = 0.01). Conclusions and Significance We provide novel evidence that the secreted liver protein fetuin-A induces low-grade inflammation and represses adiponectin production in animals and in humans. These data suggest an important role of fatty liver in the pathophysiology of insulin resistance and atherosclerosis.

Polymorphisms within novel risk loci for type 2 diabetes determine β-cell function.

Background Type 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 Findings By 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/Significance The 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.

Variation in the FTO gene influences food intake but not energy expenditure.

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.

Muscle-Derived Angiopoietin-Like Protein 4 Is Induced by Fatty Acids via Peroxisome Proliferator–Activated Receptor (PPAR)-δ and Is of Metabolic Relevance in Humans

OBJECTIVE— Long-chain fatty acids (LCFAs) contribute to metabolic homeostasis in part via gene regulation. This studys objective was to identify novel LCFA target genes in human skeletal muscle cells (myotubes). RESEARCH DESIGN AND METHODS— In vitro methods included culture and treatment of human myotubes and C2C12 cells, gene array analysis, real-time RT-PCR, Western blotting, ELISA, chromatin immunoprecipitation, and RNA interference. Human subjects (two cohorts) were characterized by oral glucose tolerance test, hyperinsulinemic-euglycemic clamp, magnetic resonance imaging and spectroscopy, and standard blood analyses (glucose, insulin, C-peptide, and plasma lipids). RESULTS— We show here that ANGPTL4 (encoding angiopoietin-like protein 4) represents a prominent LCFA-responsive gene in human myotubes. LCFA activated peroxisome proliferator-activated receptor (PPAR)-δ, but not PPAR-α or -γ, and pharmacological activation of PPAR-δ markedly induced ANGPTL4 production and secretion. In C2C12 myocytes, knockdown of PPARD, but not of PPARG, blocked LCFA-mediated ANGPTL4 induction, and LCFA treatment resulted in PPAR-δ recruitment to the ANGPTL4 gene. In addition, pharmacological PPAR-δ activation induced LIPE (encoding hormone-sensitive lipase), and this response crucially depended on ANGPTL4, as revealed by ANGPTL4 knockdown. In a human cohort of 108 thoroughly phenotyped subjects, plasma ANGPTL4 positively correlated with fasting nonesterified fatty acids (P = 0.0036) and adipose tissue lipolysis (P = 0.0012). Moreover, in 38 myotube donors, plasma ANGPTL4 levels and adipose tissue lipolysis in vivo were reflected by basal myotube ANGPTL4 expression in vitro (P = 0.02, both). CONCLUSIONS— ANGPTL4 is produced by human myotubes in response to LCFA via PPAR-δ, and muscle-derived ANGPTL4 seems to be of systemic relevance in humans.

Individual Stearoyl-CoA Desaturase 1 Expression Modulates Endoplasmic Reticulum Stress and Inflammation in Human Myotubes and Is Associated With Skeletal Muscle Lipid Storage and Insulin Sensitivity In Vivo

OBJECTIVE Increased plasma levels of free fatty acids occur in obesity and type 2 diabetes and contribute to the development of insulin resistance. Saturated fatty acids (SFAs) such as palmitate especially have lipotoxic effects leading to endoplasmatic reticulum (ER) stress, inflammation, and insulin resistance. Stearoyl-CoA desaturase 1 (SCD1) plays a key role in preventing lipotoxic effects, as it converts SFAs to less harmful monounsaturated fatty acids. Here, we tested the hypothesis that individual differences in the regulation of SCD1 expression by palmitate exist and influence insulin sensitivity and the cellular response to palmitate. RESEARCH DESIGN AND METHODS Palmitate-induced gene expression was studied in primary human myotubes of 39 metabolically characterized individuals, as well as in an SCD1-overexpressing cell culture model. RESULTS SCD1 mRNA expression and inducibility by palmitate in cultured myotubes showed a broad interindividual variation, presumably due to inheritable characteristics of the donors. Overexpression of SCD1 prevented the inflammatory and ER stress response to palmitate exposure. In primary human myotubes, high SCD1 inducibility was associated with a low inflammatory (interleukin [IL]-6, IL-8, and chemokine [CXC motif] ligand 3 [CXCL3]) and ER stress (CCAAT/enhancer binding protein [C/EBP] homologous protein, activating transcription factor 3 [ATF3], and X-box binding protein 1 [XBP1]) response to palmitate exposure. Finally, palmitate-stimulated SCD1 mRNA expression, positively correlated with intramyocellular lipid (IMCL) content of the donors, was measured by 1H-magnetic resonance spectroscopy. After adjustment for IMCL, SCD1 expression and inducibility were positively correlated with insulin sensitivity. CONCLUSIONS We hypothesize that myocellular SCD1 inducibility by palmitate is an individual characteristic that modulates lipid storage, palmitate-induced inflammation, ER stress, and insulin resistance. This may describe individuals with increased capability of innoxious free fatty acid handling and benign triglyceride storage.

Saturated, but Not Unsaturated, Fatty Acids Induce Apoptosis of Human Coronary Artery Endothelial Cells via Nuclear Factor-κB Activation

High nonesterified fatty acid (NEFA) concentrations, as observed in the metabolic syndrome, trigger apoptosis of human umbilical vein endothelial cells. Since endothelial apoptosis may contribute to atherothrombosis, we studied the apoptotic susceptibility of human coronary artery endothelial cells (HCAECs) toward selected NEFAs and the underlying mechanisms. HCAECs were treated with single or combined NEFAs. Apoptosis was quantified by flow cytometry, nuclear factor κB (NFκB) activation by electrophoretic mobility shift assay, and secreted cytokines by enzyme-linked immunosorbent assay. Treatment of HCAECs with saturated NEFAs (palmitate and stearate) increased apoptosis up to fivefold (P < 0.05; n = 4). Unsaturated NEFAs (palmitoleate, oleate, and linoleate) did not promote apoptosis but prevented stearate-induced apoptosis (P < 0.05; n = 4). Saturated NEFA-induced apoptosis neither depended on ceramide formation nor on oxidative NEFA catabolism. However, NEFA activation via acyl-CoA formation was essential. Stearate activated NFκB and linoleate impaired stearate-induced NFκB activation. Pharmacological inhibition of NFκB and inhibitor of κB kinase (IKK) also blocked stearate-induced apoptosis. Finally, the saturated NEFA effect on NFκB was not attributable to NEFA-induced cytokine production. In conclusion, NEFAs display differential effects on HCAEC survival; saturated NEFAs (palmitate and stearate) are proapoptotic, and unsaturated NEFAs (palmitoleate, oleate, and linoleate) are antilipoapoptotic. Mechanistically, promotion of HCAEC apoptosis by saturated NEFA requires acyl-CoA formation, IKK, and NFκB activation.

Pancreatic fat is negatively associated with insulin secretion in individuals with impaired fasting glucose and/or impaired glucose tolerance: a nuclear magnetic resonance study.

The pathogenesis of type 2 diabetes is characterized by insulin resistance and β‐cell dysfunction. Pancreatic fat load may add to the development of β‐cell dysfunction. The aim was to thoroughly quantify the fat content of pancreas sections (caput, corpus, and cauda) and to compare the impact of pancreatic, intrahepatic, and visceral fat on insulin secretion in humans.

Circulating Palmitoleate Strongly and Independently Predicts Insulin Sensitivity in Humans

OBJECTIVE We investigated whether palmitoleate, which prevents insulin resistance in mice, predicts insulin sensitivity in humans. RESEARCH DESIGN AND METHODS The fasting fatty acid pattern in the plasma free fatty acid (FFA) fraction was determined in 100 subjects at increased risk for type 2 diabetes. Insulin sensitivity was estimated during an oral glucose tolerance test (OGTT) at baseline and after 9 months of lifestyle intervention and measured during the euglycemic-hyperinsulinemic clamp (n = 79). RESULTS Circulating palmitoleate (OGTT:F ratio = 8.2, P = 0.005; clamp:F ratio = 7.8, P = 0.007) but not total FFAs (OGTT:F ratio = 0.6, P = 0.42; clamp:F ratio = 0.7, P = 0.40) correlated positively with insulin sensitivity, independently of age, sex, and adiposity. High baseline palmitoleate predicted a larger increase in insulin sensitivity. For 1-SD increase in palmitoleate, the odds ratio for being in the highest versus the lowest tertile of adjusted change in insulin sensitivity was 2.35 (95% CI 1.16–5.35). CONCLUSIONS Circulating palmitoleate strongly and independently predicts insulin sensitivity, suggesting that it plays an important role in the pathophysiology of insulin resistance in humans.

Relationships of Circulating Sex Hormone–Binding Globulin With Metabolic Traits in Humans

OBJECTIVE Recent data suggested that sex hormone–binding globulin (SHBG) levels decrease when fat accumulates in the liver and that circulating SHBG may be causally involved in the pathogenesis of type 2 diabetes in humans. In the present study, we investigated mechanisms by which high SHBG may prevent development to diabetes. RESEARCH DESIGN AND METHODS Before and during a 9-month lifestyle intervention, total body and visceral fat were precisely measured by magnetic resonance (MR) tomography and liver fat was measured by 1H-MR spectroscopy in 225 subjects. Insulin sensitivity was estimated from a 75-g oral glucose tolerance test (ISOGTT) and measured by a euglycemic hyperinsulinemic clamp (ISclamp, n = 172). Insulin secretion was measured during the OGTT and an ivGTT (n = 172). RESULTS SHBG levels correlated positively with insulin sensitivity (ISOGTT, P = 0.037; ISclamp, P = 0.057), independently of age, sex, and total body fat. In a multivariate model, these relationships were also significant after additional adjustment for levels of the adipokine adiponectin and the hepatokine fetuin-A (ISOGTT, P = 0.0096; ISclamp, P = 0.029). Adjustment of circulating SHBG for liver fat abolished the relationships of SHBG with insulin sensitivity. In contrast, circulating SHBG correlated negatively with fasting glycemia, before (r = −0.17, P = 0.009) and after (r = −0.14, P = 0.04) adjustment for liver fat. No correlation of circulating SHBG with adjusted insulin secretion was observed (OGTT, P = 0.16; ivGTT, P = 0.35). The SNP rs1799941 in SHBG was associated with circulating SHBG (P ≤ 0.025) but not with metabolic characteristics (all P > 0.18). CONCLUSIONS Possible mechanisms by which high circulating SHBG prevents the development of type 2 diabetes involve regulation of fasting glycemia but not alteration of insulin secretory function.

Polymorphisms within the Novel Type 2 Diabetes Risk Locus MTNR1B Determine β-Cell Function

Background Very 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 Findings We 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/Significance In 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.