DGAT1 promoter polymorphism associated with alterations in body mass index, high density lipoprotein levels and blood pressure in Turkish women

The negative adipogenesis regulator Dlk1 is transcriptionally regulated by Ifrd1 (TIS7) and translationally by its orthologue Ifrd2 (SKMc15)

Delta-like homolog 1 (Dlk1), an inhibitor of adipogenesis, controls the cell fate of adipocyte progenitors. Experimental data presented here identify two independent regulatory mechanisms, transcriptional and translational, by which Ifrd1 (TIS7) and its orthologue Ifrd2 (SKMc15) regulate Dlk1 levels. Mice deficient in both Ifrd1 and Ifrd2 (dKO) had severely reduced adipose tissue and were resistant to high-fat diet-induced obesity. Wnt signaling, a negative regulator of adipocyte differentiation, was significantly upregulated in dKO mice. Elevated levels of the Wnt/β-catenin target protein Dlk1 inhibited the expression of adipogenesis regulators Pparg and Cebpa, and fatty acid transporter Cd36. Although both Ifrd1 and Ifrd2 contributed to this phenotype, they utilized two different mechanisms. Ifrd1 acted by controlling Wnt signaling and thereby transcriptional regulation of Dlk1. On the other hand, distinctive experimental evidence showed that Ifrd2 acts as a general translational inhibitor significantly affecting Dlk1 protein levels. Novel mechanisms of Dlk1 regulation in adipocyte differentiation involving Ifrd1 and Ifrd2 are based on experimental data presented here.

Identification of Bcl2 as a Stably Expressed qPCR Reference Gene for Human Colon Cancer Cells Treated with Cottonseed-Derived Gossypol and Bioactive Extracts and Bacteria-Derived Lipopolysaccharides

Cottonseed contains many bioactive molecules including plant polyphenols. Cottonseed value might be increased by providing high-value bioactive polyphenols for improving nutrition and health. However, there was a lack of molecular evidence for cottonseed bioactivity in mammalian cells. One widely used method for evaluating the bioactivity of natural products is quantitative real-time-PCR (qPCR). The selection of stably expressed internal reference genes is a crucial task of qPCR assay for data analysis. The rationale for reference gene selection is that a lower standard deviation of the cycle of threshold (Cq) among the treatments indicates a more stable expression of the gene. The objective of this study was to select reference genes in human colon cancer cells (COLO 205) treated with cottonseed-derived gossypol and bioactive extracts along with bacterial endotoxin lipopolysaccharides (LPS). SYBR Green qPCR was used to analyze the mRNA levels of a wide range of biomarkers involved in glucose transport, lipid biosynthesis, inflammatory response, and cancer development. qPCR data (10,560 Cq values) were generated from 55 genes analyzed from 64 treatments with triplicate per treatment for each gene. The data showed that B-cell lymphoma 2 (Bcl2) mRNA was the most stable among the 55 mRNAs analyzed in the human colon cancer cells. Glyceraldehyde 3 phosphate dehydrogenase (Gapdh) and ribosome protein L32 (Rpl32) mRNAs were not good qPCR references for the colon cancer cells. These observations were consistent regardless of the treatment comparison between gossypol and LPS, glanded and glandless seed extracts, seed coat and kernel extracts, or treatment for 8 and 24 h. These results suggest that Bcl2 is a preferable reference gene for qPCR assays in human colon cancer cells treated with cottonseed-derived gossypol and bioactive extracts as well as LPS. The extensive qPCR results firmly support the conclusion that the Bcl2 gene is stably expressed at the mRNA level in the human colon cancer cells regardless of the treatment, suggesting that Bcl2 gene expression is not regulated at the mRNA level but at the post-transcriptional level. These results should facilitate studies designated to evaluate bioactivity on gene expression regulation by cottonseed molecules and other natural and synthetic molecules for nutrition and health uses.

Cottonseed extracts regulate gene expression in human colon cancer cells

Cotton plant provides economically important fiber and cottonseed, but cottonseed contributes 20% of the crop value. Cottonseed value could be increased by providing high value bioactive compounds and polyphenolic extracts aimed at improving nutrition and preventing diseases because plant polyphenol extracts have been used as medicinal remedy for various diseases. The objective of this study was to investigate the effects of cottonseed extracts on cell viability and gene expression in human colon cancer cells. COLO 225 cells were treated with ethanol extracts from glanded and glandless cottonseed followed by MTT and qPCR assays. Cottonseed extracts showed minor effects on cell viability. qPCR assay analyzed 55 mRNAs involved in several pathways including DGAT, GLUT, TTP, IL, gossypol-regulated and TTP-mediated pathways. Using BCL2 mRNA as the internal reference, qPCR analysis showed minor effects of ethanol extracts from glanded seed coat and kernel and glandless seed coat on mRNA levels in the cells. However, glandless seed kernel extract significantly reduced mRNA levels of many genes involved in glucose transport, lipid biosynthesis and inflammation. The inhibitory effects of glandless kernel extract on gene expression may provide a useful opportunity for improving nutrition and healthcare associated with colon cancer. This in turn may provide the potential of increasing cottonseed value by using ethanol extract as a nutrition/health intervention agent.

Gossypol decreased cell viability and down-regulated the expression of a number of genes in human colon cancer cells

Plant polyphenol gossypol has anticancer activities. This may increase cottonseed value by using gossypol as a health intervention agent. It is necessary to understand its molecular mechanisms before human consumption. The aim was to uncover the effects of gossypol on cell viability and gene expression in cancer cells. In this study, human colon cancer cells (COLO 225) were treated with gossypol. MTT assay showed significant inhibitory effect under high concentration and longtime treatment. We analyzed the expression of 55 genes at the mRNA level in the cells; many of them are regulated by gossypol or ZFP36/TTP in cancer cells. BCL2 mRNA was the most stable among the 55 mRNAs analyzed in human colon cancer cells. GAPDH and RPL32 mRNAs were not good qPCR references for the colon cancer cells. Gossypol decreased the mRNA levels of DGAT, GLUT, TTP, IL families and a number of previously reported genes. In particular, gossypol suppressed the expression of genes coding for CLAUDIN1, ELK1, FAS, GAPDH, IL2, IL8 and ZFAND5 mRNAs, but enhanced the expression of the gene coding for GLUT3 mRNA. The results showed that gossypol inhibited cell survival with decreased expression of a number of genes in the colon cancer cells.

Substrate channeling in the glycerol-3-phosphate pathway regulates the synthesis, storage and secretion of glycerolipids

The successive acylation of glycerol-3-phosphate (G3P) by glycerol-3-phosphate acyltransferases and acylglycerol-3-phosphate acyltransferases produces phosphatidic acid (PA), a precursor for CDP-diacylglycerol-dependent phospholipid synthesis. PA is further dephosphorylated by LIPINs to produce diacylglycerol (DG), a substrate for the synthesis of triglyceride (TG) by DG acyltransferases and a precursor for phospholipid synthesis via the CDP-choline and CDP-ethanolamine (Kennedy) pathways. The channeling of fatty acids into TG for storage in lipid droplets and secretion in lipoproteins or phospholipids for membrane biogenesis is dependent on isoform expression, activity and localization of G3P pathway enzymes, as well as dietary and hormonal and tissue-specific factors. Here, we review the mechanisms that control partitioning of substrates into lipid products of the G3P pathway.

Role of DGAT enzymes in triacylglycerol metabolism

The esterification of a fatty acyl moiety to diacylglycerol to form triacylglycerol (TAG) is catalysed by two diacylglycerol O-acyltransferases (DGATs) encoded by genes belonging to two distinct gene families. The enzymes are referred to as DGAT1 and DGAT2 in order of their identification. Both proteins are transmembrane proteins localized in the endoplasmic reticulum. Their membrane topologies are however significantly different. This difference is hypothesized to give the two isozymes different abilities to interact with other proteins and organelles and access to different pools of fatty acids, thereby creating a distinction between the enzymes in terms of their role and contribution to lipid metabolism. DGAT1 is proposed to have dual topology contributing to TAG synthesis on both sides of the ER membrane and esterifying only the pre-formed fatty acids. There is evidence to suggest that DGAT2 translocates to the lipid droplet (LD), associates with other proteins, and synthesizes cytosolic and luminal apolipoprotein B associated LD-TAG from both endogenous and exogenous fatty acids. The aim of this review is to differentiate between the two DGAT enzymes by comparing the genes that encode them, their proposed topologies, the proteins they interact with, and their roles in lipid metabolism.

How lipid droplets "TAG" along: Glycerolipid synthetic enzymes and lipid storage

Triacylglycerols (TAG) serve as the predominant form of energy storage in mammalian cells, and TAG synthesis influences conditions such as obesity, fatty liver, and insulin resistance. In most tissues, the glycerol 3-phosphate pathway enzymes are responsible for TAG synthesis, and the regulation and function of these enzymes is therefore important for metabolic homeostasis. Here we review the sites and regulation of glycerol-3-phosphate acyltransferase (GPAT), acylglycerol-3-phosphate acyltransferase (AGPAT), lipin phosphatidic acid phosphatase (PAP), and diacylglycerol acyltransferase (DGAT) enzyme action. We highlight the critical roles that these enzymes play in human health by reviewing Mendelian disorders that result from mutation in the corresponding genes. We also summarize the valuable insights that genetically engineered mouse models have provided into the cellular and physiological roles of GPATs, AGPATs, lipins and DGATs. Finally, we comment on the status and feasibility of therapeutic approaches to metabolic disease that target enzymes of the glycerol 3-phosphate pathway. This article is part of a Special Issue entitled: Recent Advances in Lipid Droplet Biology edited by Rosalind Coleman and Matthijs Hesselink.

JTT-553, a novel Acyl CoA:diacylglycerol acyltransferase (DGAT) 1 inhibitor, improves glucose metabolism in diet-induced obesity and genetic T2DM mice

Type 2 diabetes mellitus (T2DM) arises primarily due to lifestyle factors and genetics. A number of lifestyle factors are known to be important in the development of T2DM, including obesity. JTT-553, a novel Acyl CoA:diacylglycerol acyltransferase 1 inhibitor, reduced body weight depending on dietary fat in diet-induced obesity (DIO) rats in our previous study. Here, the effect of JTT-553 on glucose metabolism was evaluated using body weight reduction in T2DM mice. JTT-553 was repeatedly administered to DIO and KK-A(y) mice. JTT-553 reduced body weight gain and fat weight in both mouse models. In DIO mice, JTT-553 decreased insulin, non-esterified fatty acid (NEFA), total cholesterol (TC), and liver triglyceride (TG) plasma concentrations in non-fasting conditions. JTT-553 also improved insulin-dependent glucose uptake in adipose tissues and glucose intolerance in DIO mice. In KK-A(y) mice, JTT-553 decreased glucose, NEFA, TC and liver TG plasma concentrations in non-fasting conditions. JTT-553 also decreased glucose, insulin, and TC plasma concentrations in fasting conditions. In addition, JTT-553 decreased TNF-α mRNA levels and increased GLUT4 mRNA levels in adipose tissues in KK-A(y) mice. These results suggest that JTT-553 improves insulin resistance in adipose tissues and systemic glucose metabolism through reductions in body weight.

Effects of phenotypic and genotypic factors on the lipid responses to niacin in Chinese patients with dyslipidemia

The acyl-CoA:diacylglycerol acyltransferase (DGAT) enzymes DGAT1 and DGAT2 catalyze the final step in triglycerides biosynthesis. This study examined the relationships of baseline phenotypes and the common polymorphisms in DGAT1 and DGAT2 with the lipid responses to niacin.Lipid responses in Chinese patients with dyslipidemia treated with the extended release (ER) niacin/laropiprant combination 1000/20 mg for 4 weeks and then 2000/40 mg for 8 weeks (n = 121, the primary study) or with ER niacin 1500 mg for at least 4 weeks (n = 68, the replication study) were analyzed according to genotypes of DGAT1 rs7003945 T>C and DGAT2 rs3060 T>C polymorphisms.Treatment with ER niacin improved all lipid parameters in both studies. Absolute and percentage changes in lipids were related to their baseline levels, particularly for low-density lipoprotein cholesterol (LDL-C). The DGAT2 rs3060 T>C polymorphism was associated with lower baseline LDL-C, apoB, high-density lipoprotein cholesterol (HDL-C), and apoAI in patients on statin therapy in the primary study. Subjects with the DGAT2 rs3060 T>C variant had less reduction in LDL-C in the primary study and smaller changes in triglyceride and HDL-C in the replication study but these associations became non-significant after adjusting for baseline lipid values. The DGAT1 rs7003945 T>C polymorphism was not related to lipid baseline values or changes in either study. Concomitant statin therapy and lower body weight were also associated with greater reduction in LDL-C.Baseline lipid levels were the main determinants of lipid responses especially for LDL-C. The DGAT2 rs3060 polymorphism might influence the lipid responses depending on baseline phenotype, but this association did not persist after adjustment for the baseline lipid levels.

Multiphasic triacylglycerol dynamics in the intact heart during acute in vivo overexpression of CD36

Cardiac triacylglycerol (TAG) stores buffer the intracellular availability of long chain fatty acid (LCFA) that act as nuclear receptor ligands, substrate for lipotoxic derivatives, and high energy-yield fuel. The kinetic characteristics of TAG turnover and homeostatic mechanisms linking uptake and storage dynamics in hearts have until now remained elusive. This work examines TAG pool dynamics in the intact beating heart, under normal conditions and in response to acute gene expression-induced changes in CD36. Dynamic mode (13)C NMR elucidated multiple kinetic processes in (13)C-palmitate incorporation into TAG: an initial, saturable exponential component and a slower linear rate. Although previous work indicates the linear component to reflect TAG turnover, we hypothesized the saturable exponential to reflect transport of LCFA across the sarcolemma. Thus, we overexpressed the LCFA transporter CD36 through cardiac-specific adenoviral infection in vivo. Within 72 h, CD36 expression was increased 40% in intact hearts, accelerating the exponential phase relative to PBS-infused hearts. TAG turnover also increased with elevations in adipose triglyceride lipase (ATGL) and a modest increase in diacylglycerol acyltransferase 1 (DGAT1), without a significant expansion of the intracellular lipid pools. The results demonstrate a dynamic system of reciprocal gene regulation that couples saturable LCFA uptake across the sarcolemma to TAG synthesis/lipolysis rates.

Different domains of Phytophthora sojae effector Avr4/6 are recognized by soybean resistance genes Rps4 and Rps6

At least 12 avirulence genes have been genetically identified and mapped in Phytophthora sojae, an oomycete pathogen causing root and stem rot of soybean. Previously, the Avr4 and Avr6 genes of P. sojae were genetically mapped within a 24 kb interval of the genome. Here, we identify Avr4 and Avr6 and show that they are actually a single gene, Avr4/6, located near the 24-kb region. Avr4/6 encodes a secreted protein of 123 amino acids with an RXLR-dEER protein translocation motif. Transient expression of Avr4/6 in soybean leaves revealed that its gene product could trigger a hypersensitive response (HR) in the presence of either Rps4 or Rps6. Silencing Avr4/6 in P. sojae stable transformants abolished the avirulence phenotype exhibited on both Rps4 and Rps6 soybean cultivars. The N terminus of Avr4/6, including the dEER motif, is sufficient to trigger Rps4-dependent HR while its C terminus is sufficient to trigger Rps6-mediated HR. Compared with alleles from avirulent races, alleles of Avr4/6 from virulent races possess nucleotide substitutions in the 5' untranslated region of the gene but not in the protein-coding region.

Beyond triglyceride synthesis: the dynamic functional roles of MGAT and DGAT enzymes in energy metabolism

Monoacyglycerol acyltransferases (MGATs) and diacylglycerol acyltransferases (DGATs) catalyze two consecutive steps of enzyme reactions in the synthesis of triacylglycerols (TAGs). The metabolic complexity of TAG synthesis is reflected by the presence of multiple isoforms of MGAT and DGAT enzymes that differ in catalytic properties, subcellular localization, tissue distribution, and physiological functions. MGAT and DGAT enzymes play fundamental roles in the metabolism of monoacylglycerol (MAG), diacylglycerol (DAG), and triacylglycerol (TAG) that are involved in many aspects of physiological functions, such as intestinal fat absorption, lipoprotein assembly, adipose tissue formation, signal transduction, satiety, and lactation. The recent progress in the phenotypic characterization of mice deficient in MGAT and DGAT enzymes and the development of chemical inhibitors have revealed important roles of these enzymes in the regulation of energy homeostasis and insulin sensitivity. Consequently, selective inhibition of MGAT or DGAT enzymes by synthetic compounds may provide novel treatment for obesity and its related metabolic complications.

Genetic-epidemiological evidence on genes associated with HDL cholesterol levels: a systematic in-depth review

High-density lipoprotein (HDL) particles exhibit multiple antiatherogenic effects. They are key players in the reverse cholesterol transport which shuttles cholesterol from peripheral cells (e.g. macrophages) to the liver or other tissues. This complex process is thought to represent the basis for the antiatherogenic properties of HDL particles. The amount of cholesterol transported in HDL particles is measured as HDL cholesterol (HDLC) and is inversely correlated with the risk for coronary artery disease: an increase of 1mg/dL of HDLC levels is associated with a 2% and 3% decrease of the risk for coronary artery disease in men and women, respectively. Genetically determined conditions with high HDLC levels (e.g. familial hyperalphalipoproteinemia) often coexist with longevity, and higher HDLC levels were found among healthy elderly individuals. HDLC levels are under considerable genetic control with heritability estimates of up to 80%. The identification and characterization of genetic variants associated with HDLC concentrations can provide new insights into the background of longevity. This review provides an extended overview on the current genetic-epidemiological evidence from association studies on genes involved in HDLC metabolism. It provides a path through the jungle of association studies which are sometimes confusing due to the varying and sometimes erroneous names of genetic variants, positions and directions of associations. Furthermore, it reviews the recent findings from genome-wide association studies which have identified new genes influencing HDLC levels. The yet identified genes together explain only a small amount of less than 10% of the HDLC variance, which leaves an enormous room for further yet to be identified genetic variants. This might be accomplished by large population-based genome-wide meta-analyses and by deep-sequencing approaches on the identified genes. The resulting findings will probably result in a re-drawing and extension of the involved metabolic pathways of HDLC metabolism.

Thematic review series: glycerolipids. DGAT enzymes and triacylglycerol biosynthesis

Triacylglycerols (triglycerides) (TGs) are the major storage molecules of metabolic energy and FAs in most living organisms. Excessive accumulation of TGs, however, is associated with human diseases, such as obesity, diabetes mellitus, and steatohepatitis. The final and the only committed step in the biosynthesis of TGs is catalyzed by acyl-CoA:diacylglycerol acyltransferase (DGAT) enzymes. The genes encoding two DGAT enzymes, DGAT1 and DGAT2, were identified in the past decade, and the use of molecular tools, including mice deficient in either enzyme, has shed light on their functions. Although DGAT enzymes are involved in TG synthesis, they have distinct protein sequences and differ in their biochemical, cellular, and physiological functions. Both enzymes may be useful as therapeutic targets for diseases. Here we review the current knowledge of DGAT enzymes, focusing on new advances since the cloning of their genes, including possible roles in human health and diseases.

Alleles of the bovine DGAT1 variable number of tandem repeat associated with a milk fat QTL at chromosome 14 can stimulate gene expression

A quantitative trait locus (QTL) affecting milk fat percentage has been mapped to the centromeric end of the bovine chromosome 14 (BTA14). This genomic area includes the DGAT1 gene, which encodes acyl-CoA:diacylglycerol acyltransferase 1, the key enzyme of triglyceride biosynthesis. Genetic and biochemical studies led to the identification of the nonconservative DGAT1-K232A polymorphism as a causal mutation for the QTL. In addition to this, another polymorphism in the 5'-regulatory region of this gene, the DGAT1 variable number of tandem repeat (VNTR), also showed a strong association with milk fat percentage. This promoter VNTR polymorphism affects the number of potential Sp1 binding sites and therefore might have an impact on DGAT1 expression and also milk fat content. Hence, the DGAT1 VNTR polymorphism might be another causal mutation for the BTA14 QTL. However, evidence for Sp1 binding to this polymorphic site and for the capability of DGAT1 VNTR alleles to stimulate gene expression was lacking. In the current work Sp1-VNTR interactions were analyzed by EMSA. In addition, effects of DGAT1 VNTR alleles on gene expression were measured with reporter gene analyses. Conclusions from the results are that 1) the DGAT1 VNTR sequence is indeed a target for Sp1 binding; 2) DGAT1 VNTR alleles can stimulate gene expression in vitro and probably in vivo as well; and 3) although the stimulating effects of the different DGAT1 VNTR alleles did not show significant differences in vitro, their effects on transcription might be different in the chromatin context existing in vivo.

Molecular Cloning and Single Nucleotide Polymorphism Detection of Buffalo DGAT1 Gene

We report molecular cloning and single nucleotide polymorphism detection of the buffalo DGAT1 gene. Diacylglycerol acyltransferase (DGAT1) is considered the key enzyme in controlling the rate of synthesis of triglycerides. The DGAT1 gene was recently identified as a strong functional candidate gene affecting milk yield and composition in cattle. A full-length buffalo DGAT1 genomic DNA was amplified by iterative PCR based on homolog cloning. The buffalo DGAT1 gene comprises 17 exons and spans approximately 8.3 kb. The genomic structures of DGAT1 are highly conserved among mammal species. The deduced protein of buffalo DGAT1 contains 489 amino acids, showing high-sequence similarity with mammal homologs. Through PCR-SSCP analysis and sequencing, seven polymorphic positions were detected in the complete genomic region of buffalo DGAT1, and their frequencies were observed from a collection of 117 buffalo. The SNP (C/T) detected at position 11785 in exon 17 creates a substitution change for the amino acid sequence, resulting in an Ala residue (GCG) transition to a Val residue (GTG) in position 484 of buffalo DGAT1 protein. Information provided in this study will be useful in further studies to determine the role DGAT1 plays in the regulation of milk fat synthesis and quality improvement for milk in buffalo.

Obesity and polymorphisms in genes regulating human adipose tissue

Obesity is the result of an imbalance between food intake and energy expenditure resulting in the storing of energy as fat. Adipose tissue contains the largest store of energy in the body and plays important roles in regulating energy partitioning. Developments in genomics, in particular microarray-based expression profiling, have provided scientists with a number of new candidate genes whose expression in adipose tissue is regulated by obesity. Integrating expression profiles with genome-wide linkage and/or association analyses is a promising strategy to identify new genes underlying susceptibility to obesity. This article provides a comprehensive review of adipose-tissue-expressed genes implicated in predisposition to human obesity. The authors consider the following genes of particular interest: peroxisome proliferator-activated receptor gamma and, potentially, INSIG2 acting in adipogenesis; the adrenoreceptors beta 2 and 3, as well as hormone-sensitive lipase acting on lipolysis; uncoupling protein 2 acting in mitochondria energy expenditure; and among secreted molecules the cytokine tumor necrosis factor alpha and the hormone leptin. With the rapid development in genome research, we predict that additional alleles in genes regulating adipose tissue function will be established as risk factors for common obesity in the coming years. This has important implications for the prevention of obesity and may also offer new therapeutic targets.

The biology and chemistry of hyperlipidemia

Coronary arterial diseases are responsible for more deaths than all other associated causes combined. Elevated serum cholesterol levels leading to atherosclerosis can cause coronary heart disease (CHD). Reduction in serum cholesterol levels reduces the risk for CHD, substantially. Medicinal chemists all around the world have been designing, synthesizing, and evaluating a variety of new bioactive molecules for lowering lipid levels. This review summarizes the disorders associated with elevation of lipids in blood and the current strategies to control them. The emphasis has been laid in particular on the new potential biological targets and the possible treatments as well as the current ongoing research status in the field of lipid lowering agents.

Medical sequencing at the extremes of human body mass

Body weight is a quantitative trait with significant heritability in humans. To identify potential genetic contributors to this phenotype, we resequenced the coding exons and splice junctions of 58 genes in 379 obese and 378 lean individuals. Our 96-Mb survey included 21 genes associated with monogenic forms of obesity in humans or mice, as well as 37 genes that function in body weight-related pathways. We found that the monogenic obesity-associated gene group was enriched for rare nonsynonymous variants unique to the obese population compared with the lean population. In addition, computational analysis predicted a greater fraction of deleterious variants within the obese cohort. Together, these data suggest that multiple rare alleles contribute to obesity in the population and provide a medical sequencing-based approach to detect them.

Apolipoprotein A-V: a potential modulator of plasma triglyceride levels in Turks

The apolipoprotein A-V gene (APOA5) plays an important role in determining plasma triglyceride levels. We studied the effects of APOA5 polymorphisms on plasma triglyceride levels in Turks, a population with low levels of HDL cholesterol and a high prevalence of coronary artery disease. We found 15 polymorphisms, three of which were novel. Seven haplotype-tagging single nucleotide polymorphisms (SNPs) were chosen and genotyped in approximately 3,000 subjects. The rare alleles of the -1464T>C, -1131T>C, S19W, and 1259T>C SNPs were significantly associated with increased triglyceride levels (19-86 mg/dl; P < 0.05) and had clear gene-dose effects. Haplotype analysis of the nine common APOA5 haplotypes revealed significant effects on triglyceride levels (P < 0.001). Detailed analysis of haplotypes clearly showed that the -1464T>C polymorphism had no effect by itself but was a marker for the -1131T>C, S19W, and 1259T>C polymorphisms. The -1131T>C and 1259T>C polymorphisms were in a strong but incomplete linkage disequilibrium and appeared to have independent effects. Thus, the APOA5 -1131T>C, S19W, and 1259T>C rare alleles were associated with significant increases in plasma triglyceride levels. At least one of these alleles was present in approximately 40% of the Turks. Similar associations were observed for -1131T>C and S19W in white Americans living in San Francisco, California.

Evidence for multiple alleles at the DGAT1 locus better explains a quantitative trait locus with major effect on milk fat content in cattle

A quantitative trait locus (QTL) for milk fat percentage has been mapped consistently to the centromeric region of bovine chromosome 14 (BTA14). Two independent studies have identified the nonconservative mutation K232A in the acylCoA-diacylglycerol-acyltransferase 1 (DGAT1) gene as likely to be causal for the observed variation. Here we provide evidence for additional genetic variability at the same QTL that is associated with milk fat percentage variation within the German Holstein population. Namely, we show that alleles of the DGAT1 promoter derived from the variable number of tandem repeat (VNTR) polymorphism are associated with milk fat content in animals homozygous for the allele 232A at DGAT1. Our results present another example for more than two trait-associated alleles being involved in a major gene effect on a quantitative trait. The segregation of multiple alleles affecting milk production traits at the QTL on BTA14 has to be considered whenever marker-assisted selection programs are implemented in dairy cattle. Due to the presence of a potential transcription factor binding site in the 18mer element of the VNTR, the variation in the number of tandem repeats of the 18mer element might be causal for the variability in the transcription level of the DGAT1 gene.

Genetics of Variation in HDL Cholesterol in Humans and Mice

Plasma high-density lipoprotein cholesterol (HDL-C) concentrations are genetically determined to a great extent, and quantitative trait locus (QTL) analysis has been used to identify chromosomal regions containing genes regulating HDL-C levels. We discuss new genes found to participate in HDL metabolism. We also summarize 37 mouse and 30 human QTLs for plasma HDL-C levels, finding that all but three of the mouse QTLs have been confirmed by a second cross or a homologous human QTL, that the mouse QTL map is almost saturated because 92% of recently reported QTLs are repeats of those already found, and that 28 of the 30 human QTLs are located in regions homologous to mouse QTLs. This high degree of concordance between mouse and human QTLs suggests that the underlying genes may be the same. Strategies to more rapidly identify genes underlying mouse and human QTLs for HDL-C include focusing on the mouse and using mouse–human homologies, combining crosses, and haplotyping to narrow the region. Sequence analysis and expression studies can distinguish candidate genes consistent across multiple mouse crosses, and testing the candidate genes in human association studies can provide additional evidence for the candidacy of a gene. Together these strategies can accelerate the pace of finding genes that regulate HDL.

Inhibition of triglyceride synthesis as a treatment strategy for obesity: lessons from DGAT1-deficient mice

Because the ability to make triglycerides is essential for the accumulation of adipose tissue, inhibition of triglyceride synthesis may ameliorate obesity and its related medical consequences. Acyl coenzyme A (CoA):diacylglycerol acyltransferase 1 (DGAT1) is 1 of 2 DGAT enzymes that catalyze the final reaction in the known pathways of mammalian triglyceride synthesis. Mice lacking DGAT1 are resistant to obesity and have increased sensitivity to insulin and leptin. DGAT1-deficient mice are also resistant to diet-induced hepatic steatosis. The effects of DGAT1 deficiency on energy and glucose metabolism result in part from the altered secretion of adipocyte-derived factors. Although complete DGAT1 deficiency causes alopecia and impairs development of the mammary gland, these abnormalities are not observed in mice with partial DGAT1 deficiency. These findings suggest that pharmacological inhibition of DGAT1 may be a feasible therapeutic strategy for human obesity and type 2 diabetes.

Leptin and the control of metabolism: role for stearoyl-CoA desaturase-1 (SCD-1)

The incidence of obesity has increased sharply in recent years, making it one of the most urgent public health concerns worldwide. The hormone leptin is the central mediator in a negative feedback loop regulating energy homeostasis. Leptin administration leads to reduced food intake, increased energy expenditure, and weight loss. Leptin also mediates unique metabolic effects, specifically depleting lipid from liver and other peripheral tissues. While elucidation of leptin's role has permitted a more detailed view of the biology underlying energy homeostasis, most obese individuals are leptin resistant. A more complete understanding of the molecular components of the leptin pathway is necessary to develop effective treatment for obesity and the Metabolic Syndrome. We review here studies on the identification of one such component, stearoyl-CoA desaturase-1 (SCD-1), as a gene specifically repressed by leptin and discuss the role of this process in mediating the metabolic effects of leptin. Data indicate that pharmacologic manipulation of SCD-1 may be of benefit in the treatment of obesity, diabetes, hepatic steatosis, and other components of the Metabolic Syndrome.

Role of the DGAT gene C79T single-nucleotide polymorphism in French obese subjects

Acyl-coenzyme A, diacylglycerol acyltransferase (DGAT), is a key enzyme involved in adipose-cell triglyceride storage. A 79-bp T-to-C single-nucleotide polymorphism (SNP) on the 3' region of the DGAT transcriptional site has been reported to increase promoter activity and is associated with higher BMI in Turkish women. To validate the possible role of this genetic variant in obesity, as well as the variant's possible cellular-functional significance, we performed an association study between the T79C change and several obesity-related phenotypes in 1357 obese French adults and children. The prevalence of the T79C SNP was similar between obese adults and children when each group was compared with the controls. (CC genotype carrier frequencies were 0.25 to 0.29 in the obese groups and 0.21 in controls; p > 0.05.) In each of the obese adult and child groups studied, the T79C variant was not found to be associated with any of the obesity-related phenotypes tested. Although the T79C SNP of the DGAT gene was studied in several groups of white subjects, the association between this SNP and obesity-related phenotypes, previously described, was not confirmed in our population.

A predominant role of acyl-CoA:monoacylglycerol acyltransferase-2 in dietary fat absorption implicated by tissue distribution, subcellular localization, and up-regulation by high fat diet

Acyl-CoA:monoacylglycerol acyltransferase-2 (MGAT2) catalyzes the synthesis of diacylglycerol and differs from the MGAT1 and MGAT3 in tissue distribution at the mRNA level. In addition to the small intestine, MGAT2 mRNA is also expressed at high levels in human liver, the lower gastrointestinal tract, and the mouse kidney, but the physiological significance of such expression has not yet been studied. Using an affinity-purified antibody, the present study investigated the expression of murine MGAT2 protein along the intestinal tract, determined its subcellular localization, and studied its regulation by diet and in db/db mouse. Results demonstrate a high level of MGAT2 expression in the small intestine in a proximal-to-distal gradient that correlated well with both MGAT enzyme activity and fat absorption pattern. In contrast, MGAT2 protein was not detectable in other sections of the digestive tract, including stomach, cecum, colon, and rectum, or other mouse tissues such as kidney, liver, and adipocytes. Immunohistological studies provided direct evidence that the enzyme is expressed not only in the villi, but also in the crypt regions of the small intestine, which suggests that MGAT2 expression occurs prior to the maturation of enterocytes. MGAT2 is localized in the endoplasmic reticulum (ER) in both MGAT2-transfected COS-7 and Caco-2 cells, indicating that the ER is the primary site for dietary fat re-synthesis. MGAT2 expression appeared not to be affected by diabetes in the db/db mouse, however, the total intestinal MGAT activity was significantly enhanced. Finally, an up-regulation of both MGAT2 protein expression and MGAT activity was observed in mice fed a high fat diet, implicating a role of MGAT2 in diet-induced obesity. Taken together, our data suggest a predominant role of MGAT2 in dietary fat absorption.

The DGAT1 K232A mutation is not solely responsible for the milk production quantitative trait locus on the bovine chromosome 14

The gene, acyl-CoA:diacylglycerol acyltransferase1 (DGAT1), was recently identified as the one underlying the quantitative trait locus (QTL) for milk production traits in the centromeric region of the bovine chromosome 14. Until now, 2 alleles, the lysine variant (increasing fat yield, fat and protein percentage) and the alanine variant (increasing protein and milk yield), were postulated at DGAT1. This study investigated whether the diallelic DGAT1 polymorphism is responsible for all the genetic variation at the centromeric region of this chromosome for milk, fat, and protein yield and fat and protein percentage. A statistical model was applied to a granddaughter design to analyze 16 German Holstein families. The model included the diallelic DGAT1 effect and the QTL transition probability estimated for each chromosomal position by a multiple marker approach. Because the regression coefficient of this probability was corrected for the diallelic DGAT1 polymorphism, it represented a putative conditional QTL effect. The effect of the DGAT1 gene was always highly significant. The conditional QTL effect was significant genomewise for fat percentage at the proximal end of the chromosome and for protein percentage at a more distal chromosomal region. Additional chromosomewise significance was found for fat and protein yield. Our results suggest an additional source of genetic variance on this chromosome for these traits; either one or more additional alleles segregating at DGAT1 that were not previously detected, a second quantitative trait locus affecting these traits, or both.

Assessment of the gene content of the chromosomal regions flanking bovine DGAT1

As a first step towards verifying the candidate status of DGAT1 as the causal gene for milk fat percentage in cattle, we constructed a bovine BAC contig spanning 576 kb of the chromosomal region containing DGAT1. High content of NotI sites (21 within the contig) indicated that the region is gene-rich. Twenty-three genes neighboring DGAT1 were mapped, including two bovine cDNA sequences that have no orthologous sequences within the NCBI sequence databases. On average, 2015 bp for each of the 23 neighboring genes were sequenced and entered into EMBL. Likewise, 10 new STS markers were established by BAC-end sequencing. Within the genes and STS markers, 55 polymorphisms were discovered. These will form the basis of future linkage disequilibrium studies to test whether any genes neighboring DGAT1 are associated with variation in milk fat percentage, thereby testing the candidate status of DGAT1.

The role of acyl-CoA:diacylglycerol acyltransferase (DGAT) in energy metabolism

Acyl-CoA:diacylglycerol acyltransferase (DGAT, EC2.3.1.20), a key enzyme in triglyceride (TG) biosynthesis, not only participates in lipid metabolism but also influences metabolic pathways of other fuel molecules. Changes in the expression and/or activity levels of DGAT may lead to changes in systemic insulin sensitivity and energy homeostasis. The synthetic role of DGAT in adipose tissue, the liver, and the intestine, sites where endogenous levels of DGAT activity and TG synthesis are high, is relatively clear. Less clear is whether DGAT plays a mediating or preventive role in the development of ectopic lipotoxicity in tissues such as muscle and the pancreas, when their supply of free fatty acids (FFAs) exceeds their needs. Future studies with tissue-specific overexpression and/or knockout in these animal models would be expected to shed additional light on these issues.

Posttranscriptional control of the expression and function of diacylglycerol acyltransferase-1 in mouse adipocytes

Acyl-CoA:diacylglycerol acyltransferase-1 (DGAT1) catalyzes the final step of triglyceride synthesis in mammalian cells. Data obtained from DGAT1-knockout mice have indicated that this enzyme plays an important role in energy homeostasis. We investigated the regulation of the expression and function of DGAT1 in mouse 3T3-L1 cell as a model for mammalian adipocytes. We demonstrated that the DGAT1 protein level increased by approximately 90-fold following differentiation of 3T3-L1 into mature adipocytes, a change that was accompanied by approximately 7-fold increase in DGAT1 mRNA. On the other hand, forced overexpression of DGAT1 mRNA by >20-fold via a recombinant adenovirus only resulted in approximately 2-fold increase in DGAT1 protein in mature adipocytes and little increase in preadipocytes. These results indicated that gene expression of DGAT1 in adipocytes is subjected to rigorous posttranscriptional regulation, which is modulated significantly by the differentiation status of 3T3-L1 cells. Protein stability is not a significant factor in the control of DGAT1 expression. The steady-state levels of DGAT1 were unaffected by blockage of proteolytic pathways by ALLN. However, translational control was suggested by sequence analysis of the 5'-untranslated region of human DGAT1 (hDGAT1) mRNA. We found that the level of DGAT1 activity was predominantly a function of the steady-state level of DGAT1 protein. No significant functional changes were observed when the conserved tyrosine phosphorylation site in hDGAT1 was mutated by a single base pair substitution. Despite only a approximately 2-fold increase in DGAT1 protein caused by recombinant viral transduction, a proportionate increase in cellular triglyceride synthesis resulted without affecting the triglyceride lipolysis rate, leading to >2-fold increase in intracellular triglyceride accumulation. No change in adipocyte morphology or in the expression levels of lipoprotein lipase, proxisomal proliferation-activating receptor-gamma, and aP2 was evident secondary to DGAT1 overexpression at different stages in 3T3-L1 differentiation. These data suggest that dysregulation of DGAT1 may play a role in the development of obesity, and manipulation of the steady-state level of DGAT1 protein may offer a potential means to treat or prevent obesity.