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

Sex-specific regulation of miR-22 and ERα in white adipose tissue of obese dam's female offspring impairs the early postnatal development of functional beige adipocytes in mice

During inguinal adipose tissue (iWAT) ontogenesis, beige adipocytes spontaneously appear between postnatal 10 (P10) and P20 and their ablation impairs iWAT browning capacity in adulthood. Since maternal obesity has deleterious effects on offspring iWAT function, we aimed to investigate its effect in spontaneous iWAT browning in offspring. Female C57BL/6 J mice were fed a control or obesogenic diet six weeks before mating. Male and female offspring were euthanized at P10 and P20 or weaned at P21 and fed chow diet until P60. At P50, mice were treated with saline or CL316,243, a β3-adrenoceptor agonist, for ten days. Maternal obesity induced insulin resistance at P60, and CL316,243 treatment effectively restored insulin sensitivity in male but not female offspring. This discrepancy occurred due to female offspring severe browning impairment. During development, the spontaneous iWAT browning and sympathetic nerve branching at P20 were severely impaired in female obese dam's offspring but occurred normally in males. Additionally, maternal obesity increased miR-22 expression in the iWAT of male and female offspring during development. ERα, a target and regulator of miR-22, was concomitantly upregulated in the male's iWAT. Next, we evaluated miR-22 knockout (KO) offspring at P10 and P20. The miR-22 deficiency does not affect spontaneous iWAT browning in females and, surprisingly, anticipates iWAT browning in males. In conclusion, maternal obesity impairs functional iWAT development in the offspring in a sex-specific way that seems to be driven by miR-22 levels and ERα signaling. This impacts adult browning capacity and glucose homeostasis, especially in female offspring.

Insight into the regulatory networks underlying the high lipid perennial ryegrass growth under different irradiances

Under favourable conditions, perennial ryegrass (Lolium perenne) engineered to accumulated high lipid (HL) carbon sink in their leaves was previously shown to also enhance photosynthesis and growth. The greater aboveground biomass was found to be diminished in a dense canopy compared to spaced pots. Besides, the underlying genetic regulatory network linking between leaf lipid sinks and these physiological changes remains unknown. In this study, we demonstrated that the growth advantage was not displayed in HL Lolium grown in spaced pots under low lights. Under standard lights, analysis of differentiating transcripts in HL Lolium reveals that the plants had elevated transcripts involved in lipid metabolism, light capturing, photosynthesis, and sugar signalling while reduced expression of genes participating in sugar biosynthesis and transportation. The plants also had altered several transcripts involved in mitochondrial oxidative respiration and redox potential. Many of the above upregulated or downregulated transcript levels were found to be complemented by growing the plants under low light. Overall, this study emphasizes the importance of carbon and energy homeostatic regulatory mechanisms to overall productivity of the HL Lolium through photosynthesis, most of which are significantly impacted by low irradiances.

An alternative angiosperm DGAT1 topology and potential motifs in the N-terminus

The highly variable cytoplasmic N-terminus of the plant diacylglycerol acyltransferase 1 (DGAT1) has been shown to have roles in oligomerization as well as allostery; however, the biological significance of the variation within this region is not understood. Comparing the coding sequences over the variable N-termini revealed the Poaceae DGAT1s contain relatively high GC compositional gradients as well as numerous direct and inverted repeats in this region. Using a variety of reciprocal chimeric DGAT1s from angiosperms we show that related N-termini had similar effects (positive or negative) on the accumulation of the recombinant protein in Saccharomyces cerevisiae. When expressed in Camelina sativa seeds the recombinant proteins of specific chimeras elevated total lipid content of the seeds as well as increased seed size. In addition, we combine N- and C-terminal as well as internal tags with high pH membrane reformation, protease protection and differential permeabilization. This led us to conclude the C-terminus is in the ER lumen; this contradicts earlier reports of the cytoplasmic location of plant DGAT1 C-termini.

Acyl-CoA:diacylglycerol acyltransferase: Properties, physiological roles, metabolic engineering and intentional control

Acyl-CoA:diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) catalyzes the last reaction in the acyl-CoA-dependent biosynthesis of triacylglycerol (TAG). DGAT activity resides mainly in membrane-bound DGAT1 and DGAT2 in eukaryotes and bifunctional wax ester synthase-diacylglycerol acyltransferase (WSD) in bacteria, which are all membrane-bound proteins but exhibit no sequence homology to each other. Recent studies also identified other DGAT enzymes such as the soluble DGAT3 and diacylglycerol acetyltransferase (EaDAcT), as well as enzymes with DGAT activities including defective in cuticular ridges (DCR) and steryl and phytyl ester synthases (PESs). This review comprehensively discusses research advances on DGATs in prokaryotes and eukaryotes with a focus on their biochemical properties, physiological roles, and biotechnological and therapeutic applications. The review begins with a discussion of DGAT assay methods, followed by a systematic discussion of TAG biosynthesis and the properties and physiological role of DGATs. Thereafter, the review discusses the three-dimensional structure and insights into mechanism of action of human DGAT1, and the modeled DGAT1 from Brassica napus. The review then examines metabolic engineering strategies involving manipulation of DGAT, followed by a discussion of its therapeutic applications. DGAT in relation to improvement of livestock traits is also discussed along with DGATs in various other eukaryotic organisms.

Role of diacylglycerol O-acyltransferase (DGAT) isoforms in bovine hepatic fatty acid metabolism

Fatty acid accumulation in hepatocytes induced by high concentrations of fatty acids due to lipolysis and the associated oxidative damage they cause occur most frequently after calving. Because of their role in esterification of fatty acids, diacylglycerol acyltransferase isoforms (DGAT1 and DGAT2) could play a role in the susceptibility of dairy cows to develop fatty liver. To gain mechanistic insights, we performed in vivo and in vitro analyses using liver biopsies or isolated primary hepatocytes. The in vivo study (n = 5 cows/group) involved healthy cows [average liver triacylglycerol (TAG) = 0.78%; 0.58 to 0.93%, ratio of triglyceride weight to wet liver weight] or cows diagnosed with fatty liver (average TAG = 7.60%; 5.31 to 10.54%). In vitro, hepatocytes isolated from 3 healthy female calves (1 d old, 44 to 53 kg) were challenged with (fatty acids) or without (control) a 1.2 mM mixture of fatty acids in an attempt to induce metabolic stress. Furthermore, hepatocytes were treated with DGAT1 inhibitor or DGAT2 inhibitor for 2 h followed by a challenge with (DGAT1 inhibitor + fatty acids or DGAT2 inhibitor + fatty acids) or without (DGAT1 inhibitor or DGAT2 inhibitor) the 1.2 mM mixture of fatty acids for 12 h. Data analysis of liver biopsies was compared using a 2-tailed unpaired Student's t-test. Data from calf hepatocyte treatment comparisons were assessed by one-way ANOVA, and multiplicity for each experiment was adjusted by the Holm's procedure. Data indicated that both fatty liver and in vitro challenge with fatty acids were associated with greater mRNA and protein abundance of SREBF1, FASN, DGAT1, and DGAT2. In contrast, mRNA and protein abundance of CPT1A and very low-density lipoprotein synthesis-related proteins MTTP and APOB were markedly lower. However, compared with fatty acid challenge alone, DGAT1 inhibitor + fatty acids led to greater mRNA and protein abundance of CPT1A and APOB, and greater mRNA abundance of SREBF1 and MTTP. Furthermore, this treatment led to lower mRNA abundance of FASN and DGAT2 and TAG concentrations. Compared with fatty acid challenge alone, DGAT2 inhibitor + fatty acids led to greater mRNA and protein abundance of CPT1A, MTTP, and APOB, and lower mRNA and protein abundance of SREBF1 and FASN. In addition, compared with control and fatty acids, there was greater protein abundance of GRP78 and PERK in both DGAT1 and DGAT2 inhibitor with or without fatty acids. Furthermore, compared with control and fatty acids, reactive oxygen species concentrations in the DGAT1 inhibitor with or without fatty acid group was greater. Overall, data suggested that DGAT1 is particularly relevant in the context of hepatocyte TAG synthesis from exogenous fatty acids. Disruption of both DGAT1 and DGAT2 altered lipid homeostasis, channeling fatty acids toward oxidation and generation of reactive oxygen species. Both DGAT isoforms play a role in promoting fatty acid storage into TAG and lipid droplets to protect hepatocytes from oxidative damage.

Effects of Dietary Protein Concentration on Lipid Metabolism Gene Expression and Fatty Acid Composition in 18-23-Month-Old Hanwoo Steers

The present study evaluated the influence of dietary protein level on growth performance, fatty acid composition, and the expression of lipid metabolic genes in intramuscular adipose tissues from 18- to 23-month-old Hanwoo steers, representing the switching point of the lean-to-fat ratio. Forty steers with an initial live weight of 486 ± 37 kg were assigned to one of two treatment groups fed either a concentrate diet with 14.5% CP and or with 17% CP for 6 months. Biopsy samples of intramuscular tissue were collected to analyze the fatty acid composition and gene expression at 23 months of age. Throughout the entire experimental period, all steers were restrained twice daily to allow individual feeding. Growth performance, blood metabolites, and carcass traits, according to ultrasonic measurements, were not affected by the experimental diets. The high-protein diet significantly increased the expression of intramuscular PPARα (p < 0.1) and LPL (p < 0.05) but did not affect genes involved in fatty acid uptake (CD36 and FABP4) nor lipogenesis (ACACA, FASN, and SCD). In addition, it downregulated intramuscular VLCAD (p < 0.01) related to lipogenesis but also GPAT1 (p = 0.001), DGAT2 (p = 0.016), and SNAP23 (p = 0.057), which are involved in fatty acid esterification and adipocyte size. Hanwoo steers fed a high-protein diet at 18-23 months of age resulted in a relatively lower lipid turnover rate than steers fed a low-protein diet, which could be responsible for shortening the feeding period.

Exploring novel single nucleotide polymorphisms and haplotypes of the diacylglycerol O-acyltransferase 1 (DGAT1) gene and their effects on protein structure in Iranian buffalo

BACKGROUND

Diacylglycerol O-acyltransferase 1 (DGAT1) plays a key role in the synthesis of triglycerides. Recent studies have shown that a transition mutation resulting in substitutions of guanine by adenine in the DGAT1 gene in cattle has considerable effects on milk yield and composition. Currently, there is no systematic research reporting on the utilization of this gene segment in Iranian buffalo (Bubalus bubalis).

OBJECTIVE

In this study, the genetic differentiation of three indigenous Iranian buffalo populations was investigated in the region spanning exon 3 to exon 17 of the DGAT1 gene.

METHODS

A total of 200 buffaloes were genotyped, all the samples were sequenced directly in both directions with forward and reverse sequencing primers.

RESULTS

Sequence analysis showed novel SNPs compared to the reference GenBank sequence (DQ886485) at nucleotide positions g.6097A>G, g.7036C>T, g.7338G>A, g.7710C>T, g.8087C>T, g.8259G>A, g.8275G>A, g.8367C>T, and g.8426C>T. No polymorphisms were found within exon 8. Therefore, the K232A position was thought to be a conserved and fixed region for high milk fat content (K allele) in Bos indicus and all buffalo breeds. Comparison with Indian buffalo revealed three exonic SNPs, one of which was nonsynonymous. A unique 22 bp insertion was observed in intron 10 of DGAT1. Linkage disequilibrium analysis allowed the identification of nine haplotypes among the sampled animals. To our knowledge, this is the first report of sequencing analysis of the DGAT1 gene in Iranian buffalo.

CONCLUSION

Our results suggest that genetic diversity exists and could be useful in examining the association between the DGAT1 gene and milk production traits in buffalo.

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.

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.

Diacylglycerol acyltransferase 2 links glucose utilization to fatty acid oxidation in the brown adipocytes

Brown adipose tissue uptake of glucose and fatty acids is very high during nonshivering thermogenesis. Adrenergic stimulation markedly increases glucose uptake, de novo lipogenesis, and FA oxidation simultaneously. The mechanism that enables this concerted response has hitherto been unknown. Here, we find that in primary brown adipocytes and brown adipocyte-derived cell line (IMBAT-1), acute inhibition and longer-term knockdown of DGAT2 links the increased de novo synthesis of fatty acids from glucose to a pool of TAG that is simultaneously hydrolyzed, providing FA for mitochondrial oxidation. DGAT1 does not contribute to this pathway, but uses exogenous FA and glycerol to synthesize a functionally distinct pool of TAG to which DGAT2 also contributes. The DGAT2-dependent channelling of 14C from glucose into TAG and CO2 was reproduced in β3-agonist-stimulated primary brown adipocytes. Knockdown of DGAT2 in IMBAT-1 affected the mRNA levels of UCP1 and genes important in FA activation and esterification. Therefore, in β3-agonist activated brown adipocytes, DGAT2 specifically enables channelling of de novo synthesized FA into a rapidly mobilized pool of TAG, which is simultaneously hydrolyzed to provide substrates for mitochondrial fatty acid oxidation.

Nutrition, insulin resistance and dysfunctional adipose tissue determine the different components of metabolic syndrome

Obesity is an excessive accumulation of body fat that may be harmful to health. Today, obesity is a major public health problem, affecting in greater or lesser proportion all demographic groups. Obesity is estimated by body mass index (BMI) in a clinical setting, but BMI reports neither body composition nor the location of excess body fat. Deaths from cardiovascular diseases, cancer and diabetes accounted for approximately 65% of all deaths, and adiposity and mainly abdominal adiposity are associated with all these disorders. Adipose tissue could expand to inflexibility levels. Then, adiposity is associated with a state of low-grade chronic inflammation, with increased tumor necrosis factor-α and interleukin-6 release, which interfere with adipose cell differentiation, and the action pattern of adiponectin and leptin until the adipose tissue begins to be dysfunctional. In this state the subject presents insulin resistance and hyperinsulinemia, probably the first step of a dysfunctional metabolic system. Subsequent to central obesity, insulin resistance, hyperglycemia, hypertriglyceridemia, hypoalphalipoproteinemia, hypertension and fatty liver are grouped in the so-called metabolic syndrome (MetS). In subjects with MetS an energy balance is critical to maintain a healthy body weight, mainly limiting the intake of high energy density foods (fat). However, high-carbohydrate rich (CHO) diets increase postprandial peaks of insulin and glucose. Triglyceride-rich lipoproteins are also increased, which interferes with reverse cholesterol transport lowering high-density lipoprotein cholesterol. In addition, CHO-rich diets could move fat from peripheral to central deposits and reduce adiponectin activity in peripheral adipose tissue. All these are improved with monounsaturated fatty acid-rich diets. Lastly, increased portions of ω-3 and ω-6 fatty acids also decrease triglyceride levels, and complement the healthy diet that is recommended in patients with MetS.

Phosphorylation and function of DGAT1 in skeletal muscle cells

Aberrant intramuscular triacylglycerol (TAG) storage in human skeletal muscle is closely related to insulin insensitivity. Excessive lipid storage can induce insulin resistance of skeletal muscle, and under severe conditions, lead to type 2 diabetes. The balance of interconversion between diacylglycerol and TAG greatly influences lipid storage and utilization. Diacylglycerol O-acyltransferase 1 (DGAT1) plays a key role in this process, but its activation and phosphorylation requires further dissection. In this study, 12 putative conserved phosphorylation sites of DGAT1 were identified by examining amino acid conservation of DGAT1 in different species. Another 12 putative phosphorylation sites were also found based on bioinformatics predictions and previous reports. Meanwhile, several phosphorylation sites of DGAT1 were verified by phosphorylation mass spectrometry analysis of purified DGAT1 from mouse myoblast C2C12 cells. Using single point mutations, a regulatory role of 3 putative phosphorylation sites was dissected. Finally, using truncation mutations, a potential domain of DGAT1 that was involved in the regulation of enzymatic activity was revealed. This study provides useful information for further understanding DGAT1 activity regulation.

Antidiabetic property of Symplocos cochinchinensis is mediated by inhibition of alpha glucosidase and enhanced insulin sensitivity

The study is designed to find out the biochemical basis of antidiabetic property of Symplocos cochinchinensis (SC), the main ingredient of ‘Nisakathakadi’ an Ayurvedic decoction for diabetes. Since diabetes is a multifactorial disease, ethanolic extract of the bark (SCE) and its fractions (hexane, dichloromethane, ethyl acetate and 90% ethanol) were evaluated by in vitro methods against multiple targets relevant to diabetes such as the alpha glucosidase inhibition, glucose uptake, adipogenic potential, oxidative stress, pancreatic beta cell proliferation, inhibition of protein glycation, protein tyrosine phosphatase-1B (PTP-1B) and dipeptidyl peptidase-IV (DPP-IV). Among the extracts, SCE exhibited comparatively better activity like alpha glucosidase inhibition (IC₅₀ value-82.07±2.10 µg/mL), insulin dependent glucose uptake (3 fold increase) in L6 myotubes, pancreatic beta cell regeneration in RIN-m5F (3.5 fold increase) and reduced triglyceride accumulation (22% decrease) in 3T3L1 cells, protection from hyperglycemia induced generation of reactive oxygen species in HepG2 cells (59.57% decrease) with moderate antiglycation and PTP-1B inhibition. Chemical characterization by HPLC revealed the superiority of SCE over other extracts due to presence and quantity of bioactives (beta-sitosterol, phloretin 2′glucoside, oleanolic acid) in addition to minerals like magnesium, calcium, potassium, sodium, zinc and manganese. So SCE has been subjected to oral sucrose tolerance test to evaluate its antihyperglycemic property in mild diabetic and diabetic animal models. SCE showed significant antihyperglycemic activity in in vivo diabetic models. We conclude that SC mediates the antidiabetic activity mainly via alpha glucosidase inhibition, improved insulin sensitivity, with moderate antiglycation and antioxidant activity.

Novel SNPs and INDEL polymorphisms in the 3'UTR of DGAT1 gene: in silico analyses and a possible association

Diacylglycerol-O-acyltransferase (DGAT1) gene encodes the rate-limiting enzyme of triglyceride synthesis. A polymorphism in this gene, DGAT1 K232A, has been associated with milk production and composition in taurine breeds. However, this polymorphism is not a good tool for ascertaining the effects of this QTL in Bos indicus (Zebu), since the frequency of the DGAT1 232A allele is too low in these breeds. We sequenced the 3'-untranslated region of DGAT1 gene in a sample of bulls of the breeds Guzerá (Bos indicus) and Holstein (Bos taurus) and, using in silico analysis, we searched for genetic variation, evolutionary conservation, regulatory elements, and possible substitution effects. Six single nucleotide (SNPs) and one insertion-deletion (INDEL) polymorphisms were found in the Guzerá bulls. Additionally, we developed a preliminary association study, using this INDEL polymorphism as a genetic marker. A significant association was detected (P ≤ 0.05) between the INDEL (DGAT1 3'UTR INDEL) and the breeding values (BV) for protein, fat, and milk yields over a 305-day lactation period. The DGAT1 3' UTR INDEL genotype I/I (I, for insertion) was associated with lower BVs (-38.77 kg for milk, -1.86 kg for fat, and -1.48 kg for protein yields), when compared to the genotype I/D (D, for deletion). I/D genotype was lower D/D genotype (-34.98 kg milk, -1.73 kg fat, and -1.09 kg protein yields). This study reports the first polymorphism of DGAT1 3'UTR in the Guzerá breed, as well as its association with BV for milk protein, fat, and milk yields.

Identification of novel single nucleotide polymorphisms in the DGAT1 gene of buffaloes by PCR-SSCP

Diacylglycerol O-acyltransferase 1 (DGAT1) is a microsomal enzyme that catalyzes the final step of triglyceride synthesis. The DGAT1 gene is a strong functional candidate for determining milk fat content in cattle. In this work, we used PCR-SSCP (polymerase chain reaction-single-strand conformation polymorphism) and DNA sequencing to examine polymorphism in the region spanning exon 7 to exon 9 of the DGAT1 gene in Murrah and Pandharpuri buffaloes. Three alleles (A, B and C) and four novel single-nucleotide polymorphisms were identified in the buffalo DGAT1 gene. The frequencies of the alleles differed between the two buffalo breeds, with allele C being present in Murrah but not in Pandharpuri buffalo. The allele variation detected in this work may influence DGAT1 expression and function. The results described here could be useful in examining the association between the DGAT1 gene and milk traits in buffalo.

High fatty acid availability after exercise alters the regulation of muscle lipid metabolism

We previously reported that a single exercise session protects against fatty acid (FA)-induced insulin resistance, perhaps in part through augmented intramyocellular triacylglycerol (IMTG) synthesis. The aim of this study was to examine the effect of elevated FA availability after exercise on factors regulating IMTG metabolism. After exercise (90 minutes, 65% peak oxygen uptake), 7 healthy women (body mass index, 23 ± 1 kg/m(2)) were infused overnight (16 hours) with either a lipid and heparin solution (LIPID, 0.11 g fat per kilogram per hour) or saline (SALINE). We measured resting FA oxidation (indirect calorimetry) and obtained a skeletal muscle biopsy sample the next morning. The 4-fold increase in overnight plasma FA concentration during LIPID increased IMTG by approximately 30% during LIPID vs SALINE. This was accompanied by an approximately 25% greater membrane-associated abundance of the FA transporter FAT/CD36 (P < .01) and an approximately 8% increase in the activity of the IMTG synthesis enzyme glycerol-3-phosphate acyltransferase (GPAT, P < .01). In contrast, resting FA oxidation was not affected. We also found no difference in the protein abundance of GPAT1 and diacylglycerol acyltransferase-1, diacylglycerol acyltransferase activity, or the abundance of the lipid droplet coat proteins (perilipins 2, 3, 4, and 5) between treatments. Our findings suggest that augmented capacity for FA flux into muscle (ie, via membrane-associated FAT/CD36), perhaps together with a slight yet significant increase in activity of a key IMTG synthesis enzyme (GPAT), may enhance IMTG storage when FA availability is high after exercise. The importance of the absence of a change in perilipin protein abundance despite increased muscle lipid storage remains to be determined.

Identification of diacylglycerol acyltransferase inhibitors from Rosa centifolia petals

Diacylglycerol acyltransferase (DGAT) catalyzes the final step of triacylglycerol (TAG) synthesis, and is considered as a potential target to control hypertriglyceridemia or other metabolic disorders. In this study, we found that the extract of rose petals suppressed TAG synthesis in cultured cells, and that the extract showed DGAT inhibitory action in a dose-dependent manner. Fractionation of the rose extract revealed that the DGAT inhibitory substances in the extract were ellagitannins; among them rugosin B, and D, and eusupinin A inhibited DGAT activity by 96, 82, and 84% respectively, at 10 μM. These substances did not inhibit the activities of other hepatic microsomal enzymes, glucose-6-phosphatase and HMG-CoA reductase, or pancreatic lipase, suggesting that ellagitannins inhibit DGAT preferentially. In an oral fat load test using mice, postprandial plasma TAG increase was suppressed by rose extract; TAG levels 2 h after the fat load were significantly lower in mice administered a fat emulsion containing rose extract than in control mice (446.3 ± 33.1 vs 345.3 ± 25.0 mg/dL, control vs rose extract group; P < 0.05). These results suggest that rose ellagitannins or rose extract could be beneficial in controlling lipid metabolism and used to improve metabolic disorders.

High muscle lipid content in obesity is not due to enhanced activation of key triglyceride esterification enzymes or the suppression of lipolytic proteins

The mechanisms underlying alterations in muscle lipid metabolism in obesity are poorly understood. The primary aim of this study was to compare the abundance and/or activities of key proteins that regulate intramyocellular triglyceride (IMTG) concentration in the skeletal muscle obtained from obese (OB; n = 8, BMI 38 ± 1 kg/m(2)) and nonobese (NOB; n = 9, BMI 23 ± 1 kg/m(2)) women. IMTG concentration was nearly twofold greater in OB vs. NOB subjects (75 ± 15 vs. 40 ± 8 μmol/g dry wt, P < 0.05). In contrast, the activity and protein abundance of key enzymes that regulate the esterification of IMTG (i.e., glycerol-3-phosphate acyltransferase and diacylglycerol acyltransferase) were not elevated. We also found no differences between groups in muscle adipose triglyceride lipase and hormone-sensitive lipase (HSL) protein abundance and no differences in phosphorylation of specific sites known to affect HSL activity. However, we did find the elevated IMTG in obesity to be accompanied by a greater abundance of the fatty acid transporter FAT/CD36 in the membrane fraction of muscle from OB vs. NOB subjects (P < 0.05), suggestive of an elevated fatty acid transport capacity. Additionally, protein abundance of the lipid-trafficking protein perilipin 3 was lower (P < 0.05) in muscle from OB vs. NOB when expressed relative to IMTG content. Our findings indicate that the elevated IMTG content found in obese women was not due to an upregulation of key lipogenic proteins or to the suppression of lipolytic proteins. The impact of a low perilipin protein abundance relative to the amount of IMTG in obesity remains to be clarified.

Paradoxical coupling of triglyceride synthesis and fatty acid oxidation in skeletal muscle overexpressing DGAT1

OBJECTIVE

Transgenic expression of diacylglycerol acyltransferase-1 (DGAT1) in skeletal muscle leads to protection against fat-induced insulin resistance despite accumulation of intramuscular triglyceride, a phenomenon similar to what is known as the "athlete paradox." The primary objective of this study is to determine how DGAT1 affects muscle fatty acid oxidation in relation to whole-body energy metabolism and insulin sensitivity.

RESEARCH DESIGN AND METHODS

We first quantified insulin sensitivity and the relative tissue contributions to the improved whole-body insulin sensitivity in muscle creatine kisase (MCK)-DGAT1 transgenic mice by hyperinsulinemic-euglycemic clamps. Metabolic consequences of DGAT1 overexpression in skeletal muscles were determined by quantifying triglyceride synthesis/storage (anabolic) and fatty acid oxidation (catabolic), in conjunction with gene expression levels of representative marker genes in fatty acid metabolism. Whole-body energy metabolism including food consumption, body weights, oxygen consumption, locomotor activity, and respiration exchange ratios were determined at steady states.

RESULTS

MCK-DGAT1 mice were protected against muscle lipoptoxicity, although they remain susceptible to hepatic lipotoxicity. While augmenting triglyceride synthesis, DGAT1 overexpression also led to increased muscle mitochondrial fatty acid oxidation efficiency, as compared with wild-type muscles. On a high-fat diet, MCK-DGAT1 mice displayed higher basal metabolic rates and 5-10% lower body weights compared with wild-type littermates, whereas food consumption was not different.

CONCLUSIONS

DGAT1 overexpression in skeletal muscle led to parallel increases in triglyceride synthesis and fatty acid oxidation. Seemingly paradoxical, this phenomenon is characteristic of insulin-sensitive myofibers and suggests that DGAT1 plays an active role in metabolic "remodeling" of skeletal muscle coupled with insulin sensitization.

DGAT1 expression increases heart triglyceride content but ameliorates lipotoxicity

Intracellular lipid accumulation in the heart is associated with cardiomyopathy, yet the precise role of triglyceride (TG) remains unclear. With exercise, wild type hearts develop physiologic hypertrophy. This was associated with greater TG stores and a marked induction of the TG-synthesizing enzyme diacylglycerol (DAG) acyltransferase 1 (DGAT1). Transgenic overexpression of DGAT1 in the heart using the cardiomyocyte- specific alpha-myosin heavy chain (MHC) promoter led to approximately a doubling of DGAT activity and TG content and reductions of approximately 35% in cardiac ceramide, 26% in DAG, and 20% in free fatty acid levels. Cardiac function assessed by echocardiography and cardiac catheterization was unaffected. These mice were then crossed with animals expressing long-chain acyl-CoA synthetase via the MHC promoter (MHC-ACS), which develop lipotoxic cardiomyopathy. MHC-DGAT1XMHC-ACS double transgenic male mice had improved heart function; fractional shortening increased by 74%, and diastolic function improved compared with MHC-ACS mice. The improvement of heart function correlated with a reduction in cardiac DAG and ceramide and reduced cardiomyocyte apoptosis but increased fatty acid oxidation. In addition, the survival of the mice was improved. Our study indicates that TG is not likely to be a toxic lipid species directly, but rather it is a feature of physiologic hypertrophy and may serve a cytoprotective role in lipid overload states. Moreover, induction of DGAT1 could be beneficial in the setting of excess heart accumulation of toxic lipids.

Phasing of muscle gene expression with fasting-induced recovery growth in Atlantic salmon

BACKGROUND

Many fish species experience long periods of fasting in nature often associated with seasonal reductions in water temperature and prey availability or spawning migrations. During periods of nutrient restriction, changes in metabolism occur to provide cellular energy via catabolic processes. Muscle is particularly affected by prolonged fasting as myofibrillar proteins act as a major energy source. To investigate the mechanisms of metabolic reorganisation with fasting and refeeding in a saltwater stage of Atlantic salmon (Salmo salar L.) we analysed the expression of genes involved in myogenesis, growth signalling, lipid biosynthesis and myofibrillar protein degradation and synthesis pathways using qPCR.

RESULTS

Hierarchical clustering of gene expression data revealed three clusters. The first cluster comprised genes involved in lipid metabolism and triacylglycerol synthesis (ALDOB, DGAT1 and LPL) which had peak expression 3-14d after refeeding. The second cluster comprised ADIPOQ, MLC2, IGF-I and TALDO1, with peak expression 14-32d after refeeding. Cluster III contained genes strongly down regulated as an initial response to feeding and included the ubiquitin ligases MuRF1 and MAFbx, myogenic regulatory factors and some metabolic genes.

CONCLUSION

Early responses to refeeding in fasted salmon included the synthesis of triacylglycerols and activation of the adipogenic differentiation program. Inhibition of MuRF1 and MAFbx respectively may result in decreased degradation and concomitant increased production of myofibrillar proteins. Both of these processes preceded any increase in expression of myogenic regulatory factors and IGF-I. These responses could be a necessary strategy for an animal adapted to long periods of food deprivation whereby energy reserves are replenished prior to the resumption of myogenesis.

The genetics of neutral lipid biosynthesis: an evolutionary perspective

The storage of fatty acids and fatty alcohols in the form of neutral lipids such as triacylglycerol (TAG), cholesteryl ester (CE), and wax ester (WE) serves to provide reservoirs for membrane formation and maintenance, lipoprotein trafficking, lipid detoxification, evaporation barriers, and fuel in times of stress or nutrient deprivation. This ancient process likely originated in actinomycetes and has persisted in eukaryotes, albeit by different molecular mechanisms. A surfeit of neutral lipids is strongly, perhaps causally, related to several human diseases such as diabetes mellitus, obesity, atherosclerosis and nonalcoholic fatty liver disease. Therefore, understanding the metabolic pathways of neutral lipid synthesis and the roles of the enzymes involved may facilitate the development of new therapeutic interventions for these syndromes.

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.

Paraoxonase 2 attenuates macrophage triglyceride accumulation via inhibition of diacylglycerol acyltransferase 1

This study questioned the role of paraoxonase 2 (PON2) in attenuation of macrophage lipids accumulation. Mouse peritoneal macrophages (MPMs) harvested from PON2-deficient mice versus control C57BL/6 mice, look like foam cells and were larger in size and filled with lipid droplets. Macrophage triglyceride (but not cholesterol) content, biosynthesis rate, and microsomal acyl-CoA:diacylglycerol acyltransferase 1 (DGAT1) activity (not mRNA and protein) in PON2-deficient versus control MPM were all significantly increased by 4.6-, 3.6-, and 4.4-fold, respectively. Similarly, microsomal DGAT1 activity and cellular triglyceride content were significantly decreased in human PON2-transfected cells as well as upon incubation of PON2-deficient MPM with recombinant PON2. In all the above experimental systems, PON2 also decreased macrophage oxidative state. Incubation of PON2-deficient MPM with the free radicals generator 2,2'-amidinopropane hydrochloride increased cellular oxidative stress and DGAT1 activity by 2.2- and 3.4-fold, respectively, whereas incubation of microsomes from PON2-deficient MPM with superoxide dismutase decreased DGAT1 activity by 40%. We thus conclude that PON2 attenuates macrophage triglyceride accumulation and foam cell formation via inhibition of microsomal DGAT1 activity, which appears to be sensitive to oxidative state.

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.

Diacylglycerol acyltransferases: Potential roles as pharmacological targets

Triglyceride (TG) synthesis occurs in many cell-types, but only the adipocyte is specialised for TG storage. The increased incidence of obesity and its attendant pathologies have increased interest in pharmacological strategies aimed at inhibition of triglyceride synthesis. In the liver this would also appear to offer the advantages of the prevention of steatosis and/or dyslipidaemia. The two major enzymes that have DGAT activity appear to have specialised functions, that are most evident in triglyceride-secreting tissues. The presence of triglyceride in non-adipose cells can lead to (through lipolysis), or be a marker for, undesirable complications such as insulin resistance, or can be indicative of simultaneously high capacities for triglyceride synthesis, lipolysis and oxidation of fatty acids as in highly aerobic, trained muscle. Consequently, inhibition of triglyceride synthesis may not be a straightforward strategy, either in terms of its achievement pharmacologically or in its anticipated outcomes. The metabolic complexities of triglyceride synthesis, with particular reference to the diacylglycerol acyltransferases (DGATs) are considered in this short review.

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.

Cruciferous indole-3-carbinol inhibits apolipoprotein B secretion in HepG2 cells

The cardioprotective effect of consuming cruciferous vegetables may be attributed to a number of unique indole-based compounds. We investigated the potential role and mechanism of action of an indole-based compound, indole-3-carbinol (I-3-C), on apolipoprotein B-100 (apoB) production using HepG2 cells. I-3-C reduced apoB secretion into the media dose dependently by 56% at 100 micromol/L. Relative to the untreated control cells, no change in the density of the secreted lipoproteins was noted. Significant decreases in cellular lipid synthesis, including triglycerides (TG) and cholesterol esters (CE), were observed in cells treated with I-3-C, indicating that limited lipid availability is a major factor in the regulation of apoB secretion. The decrease in TG synthesis was associated with significantly decreased diacylglycerol acyltransferase-1 and -2 activity and reduced fatty acid synthase (FASN) gene expression. The decreased CE synthesis was associated with significantly decreased acyl CoA:cholesterol acyltransferase gene expression and activity. The effect on FASN was shown to be mediated by sterol regulatory element binding protein-1, an important transcription factor involved in fatty acid synthesis. Further investigative work revealed that LDL uptake and fatty acid oxidation were not involved in the I-3-C-mediated reduction of apoB secretion. The results indicate that plant indoles have beneficial effects on lipid synthesis that could contribute to their potential cardioprotective effect.

Upregulation of myocellular DGAT1 augments triglyceride synthesis in skeletal muscle and protects against fat-induced insulin resistance

Increased fat deposition in skeletal muscle is associated with insulin resistance. However, exercise increases both intramyocellular fat stores and insulin sensitivity, a phenomenon referred to as "the athlete's paradox". In this study, we provide evidence that augmenting triglyceride synthesis in skeletal muscle is intrinsically connected with increased insulin sensitivity. Exercise increased diacylglycerol (DAG) acyltransferase (DGAT) activity in skeletal muscle. Channeling fatty acid substrates into TG resulted in decreased DAG and ceramide levels. Transgenic overexpression of DGAT1 in mouse skeletal muscle replicated these findings and protected mice against high-fat diet-induced insulin resistance. Moreover, in isolated muscle, DGAT1 deficiency exacerbated insulin resistance caused by fatty acids, whereas DGAT1 overexpression mitigated the detrimental effect of fatty acids. The heightened insulin sensitivity in the transgenic mice was associated with attenuated fat-induced activation of DAG-responsive PKCs and the stress mediator JNK1. Consistent with these changes, serine phosphorylation of insulin receptor substrate 1 was reduced, and Akt activation and glucose 4 membrane translocation were increased. In conclusion, upregulation of DGAT1 in skeletal muscle is sufficient to recreate the athlete's paradox and illustrates a mechanism of exercise-induced enhancement of muscle insulin sensitivity. Thus, increasing muscle DGAT activity may offer a new approach to prevent and treat insulin resistance and type 2 diabetes mellitus.

Regulation of triglyceride metabolism. I. Eukaryotic neutral lipid synthesis: "Many ways to skin ACAT or a DGAT"

Esterification of sterols, fatty acids and other alcohols into biologically inert forms conserves lipid resources for many cellular functions. Paradoxically, the accumulation of neutral lipids such as cholesteryl ester or triglyceride, is linked to several major disease pathologies. In a remarkable example of genetic expansion, there are at least eleven acyltransferase reactions that lead to neutral lipid production. In this review, we speculate that the complexity and apparent redundancy of neutral lipid synthesis may actually hasten rather than impede the development of novel, isoform-specific, therapeutic interventions for acne, type 2 diabetes, obesity, hyperlipidemia, fatty liver disease, and atherosclerosis.

Whole-body insulin resistance in the absence of obesity in FVB mice with overexpression of Dgat1 in adipose tissue

Insulin resistance is often associated with obesity. We tested whether augmentation of triglyceride synthesis in adipose tissue by transgenic overexpression of the diacylglycerol aclytransferase-1 (Dgat1) gene causes obesity and/or alters insulin sensitivity. Male FVB mice expressing the aP2-Dgat1 had threefold more Dgat1 mRNA and twofold greater DGAT activity levels in adipose tissue. After 30 weeks of age, these mice had hyperglycemia, hyperinsulinemia, and glucose intolerance on a high-fat diet but were not more obese than wild-type littermates. Compared with control littermates, Dgat1 transgenic mice were both insulin and leptin resistant and had markedly elevated plasma free fatty acid levels. Adipocytes from Dgat1 transgenic mice displayed increased basal and isoproterenol-stimulated lipolysis rates and decreased gene expression for fatty acid uptake. Muscle triglyceride content was unaffected, but liver mass and triglyceride content were increased by 20 and 300%, respectively. Hepatic insulin signaling was suppressed, as evidenced by decreased phosphorylation of insulin receptor-beta (Tyr(1,131)/Tyr(1,146)) and protein kinase B (Ser473). Gene expression data suggest that the gluconeogenic enzymes, glucose-6-phosphatase and phosphoenolpyruvate carboxykinase, were upregulated. Thus, adipose overexpression of Dgat1 gene in FVB mice leads to diet-inducible insulin resistance, which is secondary to redistribution of fat from adipose tissue to the liver in the absence of obesity.

LXR and PPAR activators stimulate cholesterol sulfotransferase type 2 isoform 1b in human keratinocytes

Liver X receptors (LXRs) and peroxisome proliferator-activated receptors (PPARs) are potent regulators of keratinocyte proliferation, differentiation, and epidermal permeability barrier homeostasis. Cholesterol sulfotransferase type 2B isoform 1b (SULT2B1b) is a key enzyme in the synthesis of cholesterol sulfate (CS), a critical regulator of keratinocyte differentiation and desquamation, as well as a mediator of barrier homeostasis. In this study, we assessed the effect of activators of LXR, PPARalpha, PPARbeta/delta, and PPARgamma on SULT2B1b gene expression and enzyme activity in cultured human keratinocytes (CHKs). Our results demonstrate that PPAR and LXR activators increase SULT2B1b mRNA levels, with the most dramatic effect (a 26-fold increase) induced by the PPARgamma activator ciglitazone. Ciglitazone upregulates SULT2B1b mRNA in a dose- and time-dependent manner. Moreover, the stimulation of SULT2B1b gene expression by LXR and PPAR activators occurs in both undifferentiated and differentiated CHKs. The upregulation of SULT2B1b mRNA by ciglitazone appears to occur at a transcriptional level, because the degradation of SULT2B1b is not accelerated by ciglitazone. In addition, cycloheximide almost completely blocks the ciglitazone-induced increase in SULT2B1b mRNA, suggesting that the transcription of SULTB1b mRNA is dependent on new protein synthesis. Finally, LXR and PPAR activators also increased the activity of cholesterol sulfotransferase. Thus, LXR and PPAR activators regulate the expression of SULT2B1b, the key enzyme in the synthesis of CS, which is a potent regulator of epidermal differentiation and corneocyte desquamation.

Mitochondrial glycerol-3-phosphate acyltransferase-1 directs the metabolic fate of exogenous fatty acids in hepatocytes

Because excess triacylglycerol (TAG) in nonadipose tissues is closely associated with the development of insulin resistance, interest has increased in the metabolism of long-chain acyl-CoAs toward beta-oxidation or the synthesis and storage of TAG. To learn whether a mitochondrial isoform of glycerol-3-phosphate acyltransferase (mtGPAT1) competes with carnitine palmitoyltransferase I (CPT I) for acyl-CoAs and whether it contributes to the formation of TAG, we overexpressed rat mtGPAT1 13-fold in primary hepatocytes obtained from fasted rats. When 100, 250, or 750 microM oleate was present, both TAG mass and the incorporation of [14C]oleate into TAG increased more than twofold in hepatocytes overexpressing mtGPAT1 compared with vector controls. Although the incorporation of [14C]oleate into CO2 and acid-soluble metabolites increased with increasing amounts of oleate in the media, these metabolites were approximately 40% lower in the Ad-mtGPAT1 infected cells, consistent with competition for acyl-CoAs between CPT I and mtGPAT1. A 50-60% decrease was also observed in [14C]oleate incorporation into cholesteryl ester. With increasing amounts of exogenous oleate, [14C]TAG secretion increased appropriately in vector control-infected hepatocytes, suggesting that the machinery for VLDL-TAG biogenesis and secretion was unaffected. Despite the marked increases in TAG synthesis and storage in the Ad-mtGPAT1 cells, however, the Ad-mtGPAT1 cells secreted the same amount of [14C]TAG as the vector control cells. Thus, in isolated hepatocytes, mtGPAT1 may synthesize a cytosolic pool of TAG that cannot be secreted.

Increased diacylglycerol acyltransferase activity is associated with triglyceride accumulation in tissues of diet-induced insulin-resistant hyperlipidemic hamsters

Over-accumulation of triglyceride (TG) in insulin-sensitive tissues is associated with the development of insulin resistance. We investigated whether enhanced de novo lipogenesis via diacylglycerol acyltransferase (DGAT) may contribute to the over-accumulation of TG in various tissues (liver, adipose, muscle, and intestine) using 2 well-characterized hyperlipidemic, insulin-resistant hamster models. In general, a marked increase in TG accumulation was noted in most tissues. Interestingly, the increase in TG accumulation corresponded to an increase in microsomal DGAT activity which ranged from 114% to 575% in all of the examined tissues (n = 7 per group). To delineate the mechanism for the increase in DGAT activity, we measured the expression of DGAT-1 and DGAT-2 messenger RNA by relative reverse transcriptase polymerase chain reaction (RT-PCR). In general, DGAT gene expression changed with DGAT-1 changing the most in the liver and adipose tissue, whereas DGAT-2 showed responses mainly in muscle and intestine. The increases in messenger RNA expression were not remarkable (averaging 35%; n = 4 per group) indicating that posttranscriptional mechanism(s) may play a larger role in regulating DGAT activity. In summary, the data suggest that elevated DGAT activity/expression and the subsequent increase in de novo lipogenesis could in part induce the insulin-resistant state.

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.

Inhibitory activity of diacylglycerol acyltransferase (DGAT) and microsomal triglyceride transfer protein (MTP) by the flavonoid, taxifolin, in HepG2 cells: potential role in the regulation of apolipoprotein B secretion

The purpose of the present study was to examine the role of taxifolin, a plant flavonoid, on several aspects involving apolipoprotein B (apoB) secretion and triglyceride (TG) availability in HepG2 cells. Taxifolin was shown by ELISA to markedly reduce apoB secretion under basal and lipid-rich conditions up to 63% at 200 micromol/L. As to the mechanism underlying this effect, we examined whether taxifolin exerted its effect by limiting TG availability in the microsomal lumen essential for lipoprotein assembly. Taxifolin was shown to inhibit microsomal TG synthesis by 37% and its subsequent transfer into the lumen (-26%). The reduction in synthesis was due to a decrease in diacylglycerol acyltransferase (DGAT) activity (-35%). The effect on DGAT activity was found to be non-competitive and non-transcriptional in nature. Both DGAT-1 and DGAT-2 mRNA expression remained essentially unchanged suggesting the point of regulation may be at the post-transcriptional level. Evidence is accumulating that microsomal triglyceride transfer protein (MTP) is also involved in determining the amount of lumenal TG available for lipoprotein assembly and secretion. Taxifolin was shown to inhibit this enzyme by 41%. Whether the reduction in TG accumulation in the microsomal lumen is predominantly due to DGAT and/or MTP activity remains to be addressed. In summary, taxifolin reduced apoB secretion by limiting TG availability via DGAT and MTP activity.

Obesity‐related leptin regulates Alzheimer's Aβ

Abeta peptide is the major proteinateous component of the amyloid plaques found in the brains of Alzheimer's disease (AD) patients and is regarded by many as the culprit of the disorder. It is well documented that brain lipids are intricately involved in Abeta-related pathogenic pathways. An important modulator of lipid homeostasis is the pluripotent peptide leptin. Here we demonstrate leptin's ability to modify Abeta levels in vitro and in vivo. Similar to methyl-beta-cyclodextrin, leptin reduces beta-secretase activity in neuronal cells possibly by altering the lipid composition of membrane lipid rafts. This phenotype contrasts treatments with cholesterol and etomoxir, an inhibitor of carnitine-palmitoyl transferase-1. Conversely, inhibitors of acetyl CoA carboxylase and fatty acid synthase mimicked leptin's action. Leptin was also able to increase apoE-dependent Abeta uptake in vitro. Thus, leptin can modulate bidirectional Abeta kinesis, reducing its levels extracellularly. Most strikingly, chronic administration of leptin to AD-transgenic animals reduced the brain Abeta load, underlying its therapeutic potential.

Regulation of diacylglycerol acyltransferase in developing seeds of castor

We have previously reported the cloning of castor diacylglycerol acyltransferase (RcDGAT) based on its homology to other plant type 1 diacylglycerol acyltransferases (DGATs). To elucidate the physiological role of the RcDGAT, we have investigated the regulation of RcDGAT expression in developing seeds of castor. The RcDGAT transcript appeared at 12 d after pollination (DAP), reached the highest level at 26 DAP, and declined rapidly after that. However, the RcDGAT protein started to accumulate at 26 DAP, reached its peak at 47 DAP, then remained at this high level until 54 DAP. The significant difference between the expression of mRNA and protein indicates that gene expression of RcDGAT in maturing castor seeds is controlled at the posttranscriptional level. We found that DGAT activity measured in microsomal membranes isolated from seed at different stages of development was parallel to RcDGAT protein level, suggesting DGAT activity is mainly a function of the level of RcDGAT protein. We monitored the triacylglycerol (TG) composition and content during seed development. Compared with the overall rate of TG accumulation, DGAT activity appeared coincidently with the onset of lipid accumulation at 26 DAP; the highest DGAT activity occurred during the rapid phase of lipid accumulation at 40 DAP; and a decline in DGAT activity coincided with a decline in the accumulation rate of TG after 40 DAP. The ricinoleate-containing TG content was very low (only about 7%) in oil extracted from seeds before 19 DAP; however, it increased up to about 77% of the oil at 26 DAP. The relative amount of triricinolein in oil at 26 DAP was 53 times higher than that at 19 DAP, and it was about 76% of the amount present in oil from mature castor seeds. The close correlation between profiles of RcDGAT activity and oil accumulation confirms the role of RcDGAT in castor oil biosynthesis.

Regulation of the enzymes of hepatic microsomal triacylglycerol lipolysis and re-esterification by the glucocorticoid dexamethasone

Hepatic VLDL (very-low-density lipoprotein) assembly is a complex process that is largely regulated by the provision of lipid for apolipoprotein B assembly. Intracellular stored TAG (triacylglycerol) undergoes an initial lipolysis followed by re-esterification of the lipolytic products to form TAG prior to their incorporation into a VLDL particle. TGH (TAG hydrolase) is a lipase that hydrolyses intracellular TAG within the hepatocyte. We have utilized both dexamethasone-injected mouse and primary hepatocyte models to address whether stimulation of TAG biosynthesis by the synthetic glucocorticoid, dexamethasone, altered hepatic lipolysis and re-esterification and the provision of stored TAG for lipoprotein secretion. Dexamethasone treatment resulted in decreased TGH expression, primarily due to a dexamethasone-induced decrease in TGH mRNA stability. The expression and activities of diacylglycerol acyltransferases 1 and 2 were stimulated by dexamethasone. The combination of reduced intracellular TAG lipolysis and increased TAG biosynthesis contributed to the accumulation of TAG within the livers of dexamethasone-injected mice. The rate of hepatic TAG secretion in dexamethasonetreated mice was maintained at similar levels as in control mice. Our data demonstrate that stimulation of de novo TAG synthesis by dexamethasone increased the proportion of secreted TAG that was derived from de novo sources, while the utilization of stored TAG for secretion was reduced. The results show that, during markedly increased TAG synthesis, some TAGs are diverted from the cytosolic storage pool and are utilized directly for VLDL assembly within the endoplasmic reticulum lumen.

Chronological changes in metabolism and functions of cultured adipocytes: a hypothesis for cell aging in mature adipocytes

The growth and aging of 3T3-L1 adipocytes were investigated in a synchronized tissue-culture system. We systematically characterized several major aspects of adipocyte metabolism and functions as variables of cell age. We found that terminal differentiation of 3T3-L1 cells is followed by a near-linear hypertrophic growth (increase in triglyceride content) of the cultured adipocytes throughout a 20-day study period. However, three metabolically and functionally distinct stages are recognized. The first stage overlaps with differentiation and is represented by small immature adipocytes. The second stage is characterized by fully mature adipocytes that show peaked overall metabolic activities. The third stage is marked by cell aging, with deterioration in every major aspect of the cell's functionality except for the function of net energy storage, which is preserved even in aged adipocytes. Compared with young mature adipocytes, older cells are increasingly insulin resistant, have decreased glucose uptake and fuel consumption, and show impaired glycerokinase-mediated fatty acid reesterification. Moreover, aged adipocytes show reduced gene expression for adiponectin and leptin, each of which is important in systemic regulation of energy metabolism. The characterization of these cell age-dependent changes in adipocyte functionality provides a model for understanding dynamic changes at the tissue level and suggests that adipose tissue is modifiable via adipocyte aging.

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.