Central (visceral) obesity is more closely associated with insulin resistance, type 2 diabetes, and cardiovascular disease than is peripheral [subcutaneous (sc)] obesity, but the underlying mechanism for this pathophysiological difference is largely unknown. To understand the molecular basis of this difference, we sequenced 10,437 expressed sequence tags (ESTs) from a human omental fat cDNA library and discovered a novel visceral fat depot-specific secretory protein, which we have named omentin. Omentin ESTs were more abundant than many known adipose genes, such as perilipin, adiponectin, and leptin in the cDNA library. Protein sequence analysis indicated that omentin mRNA encodes a peptide of 313 amino acids, containing a secretory signal sequence and a fibrinogen-related domain. Northern analysis demonstrated that omentin mRNA was predominantly expressed in visceral adipose tissue and was barely detectable in sc fat depots in humans and rhesus monkeys. Quantative real-time PCR showed that omentin mRNA was expressed in stromal vascular cells, but not fat cells, isolated from omental adipose tissue, with >150-fold less in sc cell fractions. Accordingly, omentin protein was secreted into the culture medium of omental, but not sc, fat explants. Omentin was detectable in human serum by Western blot analysis. Addition of recombinant omentin in vitro did not affect basal but enhanced insulin-stimulated glucose uptake in both sc (47%, n = 9, P = 0.003) and omental (∼30%, n = 3, P < 0.05) human adipocytes. Omentin increased Akt phosphorylation in the absence and presence of insulin. In conclusion, omentin is a new adipokine that is expressed in omental adipose tissue in humans and may regulate insulin action.

Central obesity and the accumulation of visceral fat are risk factors for the development of type 2 diabetes and cardiovascular disease. Omentin is a protein expressed and secreted from visceral but not subcutaneous adipose tissue that increases insulin sensitivity in human adipocytes. To determine the impact of obesity-dependent insulin resistance on the regulation of two omentin isoforms, gene expression and plasma levels were measured in lean, overweight, and obese subjects. Omentin 1 was shown to be the major circulating isoform in human plasma. Lean subjects had significantly higher plasma omentin 1 levels than obese and overweight subjects. In addition, higher plasma omentin 1 levels were detected in women compared with men. Plasma omentin 1 levels were inversely correlated with BMI, waist circumference, leptin levels, and insulin resistance as measured by homeostasis model assessment and positively correlated with adiponectin and HDL levels. Both omentin 1 and omentin 2 gene expression were decreased with obesity and were highly correlated with each other in visceral adipose tissue. In summary, decreased omentin levels are associated with increasing obesity and insulin resistance. Therefore, omentin levels may be predictive of the metabolic consequences or co-morbidities associated with obesity.

Apolipoprotein C-III (apoC-III) inhibits triglyceride hydrolysis and has been implicated in coronary artery disease. Through a genome-wide association study, we have found that about 5% of the Lancaster Amish are heterozygous carriers of a null mutation (R19X) in the gene encoding apoC-III (APOC3) and, as a result, express half the amount of apoC-III present in noncarriers. Mutation carriers compared to noncarriers had lower fasting and postprandial serum triglycerides, higher levels of HDL-cholesterol and lower levels of LDL-cholesterol. Subclinical atherosclerosis, as measured by coronary artery calcification, was less common in carriers than noncarriers, suggesting that lifelong deficiency of apoC-III has a cardioprotective effect.Elevated plasma levels of low density lipoprotein cholesterol (LDL-C) and triglycerides (TG) are important contributors to premature coronary heart disease (CHD) (1-3), and genetic variants causing low LDL-C are associated with reduced risk of CHD (4). Recently, nonfasting TG was found to be an independent CHD risk factor (5,6), in one study showing higher predictive power than fasting TG (FTG), the traditional measure, likely because of the atherogenic remnant lipoproteins generated during absorption and clearance of dietary fat (5).To identify genetic factors contributing to FTG and post-prandial TG (ppTG) dietary response, we performed a single high fat feeding intervention and genome-wide association study (GWAS) in 809 Old Order Amish individuals as part of the Heredity and Phenotype Intervention (HAPI) Heart Study (7). Characteristics of these participants are shown in Table S1. These individuals were fed a milkshake containing 782 kcal/m 2 body surface area with 77.6% of these calories from fat and had blood drawn for lipid levels 0, 1, 2, 3, 4 and 6 hours after the intervention. The Affymetrix GeneChip® Human Mapping 500K Array Set was used for genotyping leukocyte DNA from these 809 participants. Traits were normalized and * This manuscript has been accepted for publication in Science. This version has not undergone final editing. Please refer to the complete version of record at http://www.sciencemag.org/cgi/content/full/322/5908/1702. Their manuscript may not be reproduced or used in any manner that does not fall within the fair use provisions of the Copyright Act without the prior, written permission of AAAS. analyses accounting for sex and sex-specific age and age 2 , body mass index (BMI) and relatedness among participants were performed as described in the Methods (8).Results of the GWAS of FTG and ppTG (as estimated by the incremental area under the curve, iAUCTG (8)), transformed by their natural logarithm (ln), are shown in Table S2 and Figure S1. The strongest evidence for association with both ln-FTG (p = 3.8 × 10 −14 ) and ln-iAUCTG (p = 2.8 × 10 −10 ) occurred on chromosome 11q23 at single nucleotide polymorphism (SNP) rs10892151, which had a minor allele frequency (MAF) of 0.028 (A allele; Table S2). SNP rs10892151 is located within an intron of th...

OBJECTIVE-Accumulation of intracellular lipid droplets (LDs) in non-adipose tissues is recognized as a strong prognostic factor for the development of insulin resistance in obesity. LDs are coated with perilipin, adipose differentiation-related protein, tail interacting protein of 47 kd (PAT) proteins that are thought to regulate LD turnover by modulating lipolysis. Our hypothesis is that PAT proteins modulate LD metabolism and therefore insulin resistance. RESEARCH DESIGN AND METHODS-We used a cell culture model (murine AML12 loaded with oleic acid) and small interfering RNA to directly assess the impact of PAT proteins on LD accumulation, lipid metabolism, and insulin action. PAT proteins associated with excess fat deposited in livers of dietinduced obese (DIO) mice were also measured.RESULTS-Cells lacking PAT proteins exhibited a dramatic increase in LD size and a decrease in LD number. Further, the lipolytic rate increased by ϳ2-to 2.5-fold in association with increased adipose triglyceride lipase (ATGL) at the LD surface. Downregulation of PAT proteins also produced insulin resistance, as indicated by decreased insulin stimulation of Akt phosphorylation (P Ͻ 0.001). Phosphoinositide-dependent kinase-1 and phosphoinositide 3-kinase decreased, and insulin receptor substrate-1 307 phosphorylation increased. Increased lipids in DIO mice livers were accompanied by changes in PAT composition but also increased ATGL, suggesting a relative PAT deficiency.CONCLUSIONS-These data establish an important role for PAT proteins as surfactant at the LD surface, packaging lipids in smaller units and restricting access of lipases and thus preventing insulin resistance. We suggest that a deficiency of PAT proteins relative to the quantity of ectopic fat could contribute to cellular dysfunction in obesity and type 2 diabetes. Diabetes

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BACKGROUND Lipolysis regulates energy homeostasis through the hydrolysis of intracellular triglycerides and the release of fatty acids for use as energy substrates or lipid mediators in cellular processes. Genes encoding proteins that regulate energy homeostasis through lipolysis are thus likely to play an important role in determining susceptibility to metabolic disorders. METHODS We sequenced 12 lipolytic-pathway genes in Old Order Amish participants whose fasting serum triglyceride levels were at the extremes of the distribution and identified a novel 19-bp frameshift deletion in exon 9 of LIPE, encoding hormone-sensitive lipase (HSL), a key enzyme for lipolysis. We genotyped the deletion in DNA from 2738 Amish participants and performed association analyses to determine the effects of the deletion on metabolic traits. We also obtained biopsy specimens of abdominal subcutaneous adipose tissue from 2 study participants who were homozygous for the deletion (DD genotype), 10 who were heterozygous (ID genotype), and 7 who were noncarriers (II genotype) for assessment of adipose histologic characteristics, lipolysis, enzyme activity, cytokine release, and messenger RNA (mRNA) and protein levels. RESULTS Carriers of the mutation had dyslipidemia, hepatic steatosis, systemic insulin resistance, and diabetes. In adipose tissue from study participants with the DD genotype, the mutation resulted in the absence of HSL protein, small adipocytes, impaired lipolysis, insulin resistance, and inflammation. Transcription factors responsive to peroxisome-proliferator–activated receptor γ (PPAR-γ) and downstream target genes were down-regulated in adipose tissue from participants with the DD genotype, altering the regulation of pathways influencing adipogenesis, insulin sensitivity, and lipid metabolism. CONCLUSIONS These findings indicate the physiological significance of HSL in adipocyte function and the regulation of systemic lipid and glucose homeostasis and underscore the severe metabolic consequences of impaired lipolysis. (Funded by the National Institutes of Health and others).

The cDNAs for two putative glucose transporters from mouse 3T3-L1 adipocytes were isolated and sequenced. One of these cDNAs encodes the murine homolog of the human hepG2/erythrocyte glucose transporter, termed GT1. GT1 mRNA is most abundant in mouse brain and is expressed in both 3T3-L1 preadipocytes and adipocytes. The other cDNA encodes a glucose transporter-like protein, termed GT2, that has a unique amino acid sequence and tissue distribution. GT2 cDNA encodes a protein with 63% amino acid sequence identity and a similar structural organization to GT1. GT2 mRNA is found at high levels in mouse skeletal muscle, heart, and adipose tissue, all of which exhibit insulinstimulated glucose uptake. GT2 mRNA is absent from 3T3-L1 preadipocytes but is induced dramatically during differentiation into adipocytes. This increase in mRNA content correlates closely with the acquisition of insulin-stimulated glucose uptake. We propose that GT2 is an insulin-regulated glucose transporter.

Insulin rapidly represses expression of the gene encoding the insulin-responsive glucose transporter (GLUT4) in 3T3-L1 mouse adipocytes. Upon exposure to the hormone the cellular level of GLUT4 mRNA falls (ti,2 2.5 hr)to 20-30% of its initial level within 10 hr. This is followed by a similar decrease in the level of GLUT4 protein. Downregulation ofGLUT4 mRNA is a result of both rapid repression of transcription of the GLUT4 gene and an increased rate of turnover of the GLUT4 message. As a consequence of prolonged exposure to insulin, 3T3-L1 adipocytes lose their capacity for acute stimulation of hexose uptake by insulin. These findings provide an explanation for the resistance of glucose uptake to Insulin in adipose tissue observed in non-insulindependent (type 2) diabetes mellitus, particularly that associated with hyperinsulinemia and obesity.In mammalian cells the facilitated diffusion of glucose across the plasma membrane is mediated by a diverse group of transporters, each with different properties (1-3). The five facilitative glucose transporters identified and sequenced to date exhibit a unique pattern of expression among different cell types (3). One of these transporters, GLUT4, which is expressed only in adipocytes, heart, and skeletal muscle, is responsible for the acute stimulation of glucose uptake by insulin that is observed only in these cell types (3-6). It has been established (7,8) that this acute regulation ofGLUT4 by insulin involves rapid translocation of the transporter from intracellular vesicles to the plasma membrane, although changes in the transporter's intrinsic activity may also occur (9, 10).In addition to the acute regulation of GLUT4 activity by insulin, expression of the GLUT4 gene appears to be hormonally and metabolically regulated. For example, both fasting and insulin-dependent (including streptozotocininduced) diabetes, which alter blood levels of insulin and its counterregulatory hormones, drastically lower GLUT4 mRNA and protein levels in adipose tissue (11-14). Recent findings suggest that this effect may be mediated, at least in part, by cAMP, which was shown to repress transcription of the GLUT4 gene in 3T3-L1 adipocytes (15). This view is supported by the fact that the level of cAMP in adipose tissue increases in the starvation-induced state (16). Non-insulindependent (type 2) diabetes, which is usually accompanied by hyperinsulinemia and peripheral insulin resistance, also leads to reduced levels of GLUT4 message and protein in adipose tissue (17), most likely by a different mechanism (see Discussion). To gain insight into the mechanism of insulin resistance in adipose tissue in type 2 diabetes accompanied by hyperinsulinemia, we investigated the possibility that insulin itself might repress expression of the GLUT4 gene.Previous results from other laboratories (18-21), although not in complete agreement, suggested that the level of GLUT4 protein in adipocytes falls upon prolonged exposure to insulin. Using 3T3-L1 adipocytes as a model system, we demonstrate th...