Fatty acid synthesis in adipose tissue incubated in tritiated water

Statin therapy inhibits fatty acid synthase via dynamic protein modifications

Statins are a class of drug widely prescribed for the prevention of cardiovascular disease, with pleiotropic cellular effects. Statins inhibit HMG-CoA reductase (HMGCR), which converts the metabolite HMG-CoA into mevalonate. Recent discoveries have shown HMG-CoA is a reactive metabolite that can non-enzymatically modify proteins and impact their activity. Therefore, we predicted that inhibition of HMGCR by statins might increase HMG-CoA levels and protein modifications. Upon statin treatment, we observe a strong increase in HMG-CoA levels and modification of only a single protein. Mass spectrometry identifies this protein as fatty acid synthase (FAS), which is modified on active site residues and, importantly, on non-lysine side-chains. The dynamic modifications occur only on a sub-pool of FAS that is located near HMGCR and alters cellular signaling around the ER and Golgi. These results uncover communication between cholesterol and lipid biosynthesis by the substrate of one pathway inhibiting another in a rapid and reversible manner.

Serine catabolism generates liver NADPH and supports hepatic lipogenesis

Carbohydrate can be converted into fat by de novo lipogenesis, a process upregulated in fatty liver disease. Chemically, de novo lipogenesis involves polymerization and reduction of acetyl-CoA, using NADPH as the electron donor. The feedstocks used to generate acetyl-CoA and NADPH in lipogenic tissues remain, however, unclear. Here we show using stable isotope tracing in mice that de novo lipogenesis in adipose is supported by glucose and its catabolism via the pentose phosphate pathway to make NADPH. The liver, in contrast, derives acetyl-CoA for lipogenesis from acetate and lactate, and NADPH from folate-mediated serine catabolism. Such NADPH generation involves the cytosolic serine pathway in liver running in the opposite direction to that observed in most tissues and tumours, with NADPH made by the SHMT1-MTHFD1-ALDH1L1 reaction sequence. SHMT inhibition decreases hepatic lipogenesis. Thus, liver folate metabolism is distinctively wired to support cytosolic NADPH production and lipogenesis. More generally, while the same enzymes are involved in fat synthesis in liver and adipose, different substrates are used, opening the door to tissue-specific pharmacological interventions.

De novo lipogenesis is essential for platelet production in humans

Acetyl-CoA carboxylase (ACC) catalyses the first step of de novo lipogenesis (DNL). Pharmacologic inhibition of ACC has been of interest for therapeutic intervention in a wide range of diseases. We demonstrate here that ACC and DNL are essential for platelet production in humans and monkeys, but in not rodents or dogs. During clinical evaluation of a systemically distributed ACC inhibitor, unexpected dose-dependent reductions in platelet count were observed. While platelet count reductions were not observed in rat and dog toxicology studies, subsequent studies in cynomolgus monkeys recapitulated these platelet count reductions with a similar concentration response to that in humans. These studies, along with ex vivo human megakaryocyte maturation studies, demonstrate that platelet lowering is a consequence of DNL inhibition likely to result in impaired megakaryocyte demarcation membrane formation. These observations demonstrate that while DNL is a minor quantitative contributor to global lipid balance in humans, DNL is essential to specific lipid pools of physiological importance.

Insulin signaling requires glucose to promote lipid anabolism in adipocytes

Adipose tissue is essential for metabolic homeostasis, balancing lipid storage and mobilization based on nutritional status. This is coordinated by insulin, which triggers kinase signaling cascades to modulate numerous metabolic proteins, leading to increased glucose uptake and anabolic processes like lipogenesis. Given recent evidence that glucose is dispensable for adipocyte respiration, we sought to test whether glucose is necessary for insulin-stimulated anabolism. Examining lipogenesis in cultured adipocytes, glucose was essential for insulin to stimulate the synthesis of fatty acids and glyceride-glycerol. Importantly, glucose was dispensable for lipogenesis in the absence of insulin, suggesting that distinct carbon sources are used with or without insulin. Metabolic tracing studies revealed that glucose was required for insulin to stimulate pathways providing carbon substrate, NADPH, and glycerol 3-phosphate for lipid synthesis and storage. Glucose also displaced leucine as a lipogenic substrate and was necessary to suppress fatty acid oxidation. Together, glucose provided substrates and metabolic control for insulin to promote lipogenesis in adipocytes. This contrasted with the suppression of lipolysis by insulin signaling, which occurred independently of glucose. Given previous observations that signal transduction acts primarily before glucose uptake in adipocytes, these data are consistent with a model whereby insulin initially utilizes protein phosphorylation to stimulate lipid anabolism, which is sustained by subsequent glucose metabolism. Consequently, lipid abundance was sensitive to glucose availability, both during adipogenesis and in Drosophila flies in vivo Together, these data highlight the importance of glucose metabolism to support insulin action, providing a complementary regulatory mechanism to signal transduction to stimulate adipose anabolism.

Development Defects of Hydroxy-Fatty Acid-Accumulating Seeds Are Reduced by Castor Acyltransferases

Researchers have long endeavored to produce modified fatty acids in easily managed crop plants where they are not natively found. An important step toward this goal has been the biosynthesis of these valuable products in model oilseeds. The successful production of such fatty acids has revealed barriers to the broad application of this technology, including low seed oil and low proportion of the introduced fatty acid and reduced seed vigor. Here, we analyze the impact of producing hydroxy-fatty acids on seedling development. We show that germinating seeds of a hydroxy-fatty acid-accumulating Arabidopsis (Arabidopsis thaliana) line produce chlorotic cotyledons and suffer reduced photosynthetic capacity. These seedlings retain hydroxy-fatty acids in polar lipids, including chloroplast lipids, and exhibit decreased fatty acid synthesis. Triacylglycerol mobilization in seedling development also is reduced, especially for lipids that include hydroxy-fatty acid moieties. These developmental defects are ameliorated by increased flux of hydroxy-fatty acids into seed triacylglycerol created through the expression of either castor (Ricinus communis) acyltransferase enzyme ACYL-COA:DIACYLGLYCEROL ACYLTRANSFERASE2 or PHOSPHOLIPID:DIACYLGLYCEROL ACYLTRANSFERASE1A. Such expression increases both the level of total stored triacylglycerol and the rate at which it is mobilized, fueling fatty acid synthesis and restoring photosynthetic capacity. Our results suggest that further improvements in seedling development may require the specific mobilization of triacylglycerol-containing hydroxy-fatty acids. Understanding the defects in early development caused by the accumulation of modified fatty acids and providing mechanisms to circumvent these defects are vital steps in the development of tailored oil crops.

Historical and contemporary stable isotope tracer approaches to studying mammalian protein metabolism

Over a century ago, Frederick Soddy provided the first evidence for the existence of isotopes; elements that occupy the same position in the periodic table are essentially chemically identical but differ in mass due to a different number of neutrons within the atomic nucleus. Allied to the discovery of isotopes was the development of some of the first forms of mass spectrometers, driven forward by the Nobel laureates JJ Thomson and FW Aston, enabling the accurate separation, identification, and quantification of the relative abundance of these isotopes. As a result, within a few years, the number of known isotopes both stable and radioactive had greatly increased and there are now over 300 stable or radioisotopes presently known. Unknown at the time, however, was the potential utility of these isotopes within biological disciplines, it was soon discovered that these stable isotopes, particularly those of carbon (13 C), nitrogen (15 N), oxygen (18 O), and hydrogen (2 H) could be chemically introduced into organic compounds, such as fatty acids, amino acids, and sugars, and used to "trace" the metabolic fate of these compounds within biological systems. From this important breakthrough, the age of the isotope tracer was born. Over the following 80 yrs, stable isotopes would become a vital tool in not only the biological sciences, but also areas as diverse as forensics, geology, and art. This progress has been almost exclusively driven through the development of new and innovative mass spectrometry equipment from IRMS to GC-MS to LC-MS, which has allowed for the accurate quantitation of isotopic abundance within samples of complex matrices. This historical review details the development of stable isotope tracers as metabolic tools, with particular reference to their use in monitoring protein metabolism, highlighting the unique array of tools that are now available for the investigation of protein metabolism in vivo at a whole body down to a single protein level. Importantly, it will detail how this development has been closely aligned to the technological development within the area of mass spectrometry. Without the dedicated development provided by these mass spectrometrists over the past century, the use of stable isotope tracers within the field of protein metabolism would not be as widely applied as it is today, this relationship will no doubt continue to flourish in the future and stable isotope tracers will maintain their importance as a tool within the biological sciences for many years to come. © 2016 The Authors. Mass Spectrometry Reviews Published by Wiley Periodicals, Inc. Mass Spec Rev.

Chemical Basis for Deuterium Labeling of Fat and NADPH

Much understanding of metabolism is based on monitoring chemical reactions in cells with isotope tracers. For this purpose, ¹³C is well suited due to its stable incorporation into biomolecules and minimal kinetic isotope effect. For redox reactions, deuterium tracing can provide additional information. To date, studies examining NADPH production with deuterated carbon sources have failed to account for roughly half of NADPH's redox active hydrogen. We show the missing hydrogen is the result of enzyme-catalyzed H-D exchange between water and NADPH. Though isolated NADPH does not undergo H-D exchange with water, such exchange is catalyzed by Flavin enzymes and occurs rapidly in cells. Correction for H-D exchange is required for accurate assessment of biological sources of NADPH's high energy electrons. Deuterated water (D₂O) is frequently used to monitor fat synthesis in vivo, but the chemical pathway of the deuterons into fat remains unclear. We show D₂O labels fatty acids primarily via NADPH. Knowledge of this route enables calculation, without any fitting parameters, of the mass isotopomer distributions of fatty acids from cells grown in D₂O. Thus, knowledge of enzyme-catalyzed H-D exchange between water and NADPH enables accurate interpretation of deuterium tracing studies of redox cofactor and fatty acid metabolism.

A high-fat diet suppresses de novo lipogenesis and desaturation but not elongation and triglyceride synthesis in mice

Intracellular lipids and their synthesis contribute to the mechanisms and complications of obesity-associated diseases. We describe an NMR approach that provides an abbreviated lipidomic analysis with concurrent lipid biosynthetic fluxes. Following deuterated water administration, positional isotopomer analysis by deuterium NMR of specific lipid species was used to examine flux through de novo lipogenesis (DNL), FA elongation, desaturation, and TG-glycerol synthesis. The NMR method obviated certain assumptions regarding sites of enrichment and exchangeable hydrogens required by mass isotope methods. The approach was responsive to genetic and pharmacological gain or loss of function of DNL, elongation, desaturation, and glyceride synthesis. BDF1 mice consuming a high-fat diet (HFD) or matched low-fat diet for 35 weeks were examined across feeding periods to determine how flux through these pathways contributes to diet induced fatty liver and obesity. HFD mice had increased rates of FA elongation and glyceride synthesis. However DNL was markedly suppressed despite insulin resistance and obesity. We conclude that most hepatic TGs in the liver of HFD mice were formed from the reesterification of existing or ingested lipids, not DNL.

Fatty acid synthesis is inhibited by inefficient utilization of unusual fatty acids for glycerolipid assembly

Degradation of unusual fatty acids through β-oxidation within transgenic plants has long been hypothesized as a major factor limiting the production of industrially useful unusual fatty acids in seed oils. Arabidopsis seeds expressing the castor fatty acid hydroxylase accumulate hydroxylated fatty acids up to 17% of total fatty acids in seed triacylglycerols; however, total seed oil is also reduced up to 50%. Investigations into the cause of the reduced oil phenotype through in vivo [(14)C]acetate and [(3)H]2O metabolic labeling of developing seeds surprisingly revealed that the rate of de novo fatty acid synthesis within the transgenic seeds was approximately half that of control seeds. RNAseq analysis indicated no changes in expression of fatty acid synthesis genes in hydroxylase-expressing plants. However, differential [(14)C]acetate and [(14)C]malonate metabolic labeling of hydroxylase-expressing seeds indicated the in vivo acetyl-CoA carboxylase activity was reduced to approximately half that of control seeds. Therefore, the reduction of oil content in the transgenic seeds is consistent with reduced de novo fatty acid synthesis in the plastid rather than fatty acid degradation. Intriguingly, the coexpression of triacylglycerol synthesis isozymes from castor along with the fatty acid hydroxylase alleviated the reduced acetyl-CoA carboxylase activity, restored the rate of fatty acid synthesis, and the accumulation of seed oil was substantially recovered. Together these results suggest a previously unidentified mechanism that detects inefficient utilization of unusual fatty acids within the endoplasmic reticulum and activates an endogenous pathway for posttranslational reduction of fatty acid synthesis within the plastid.

Phytosterols protect against diet-induced hypertriglyceridemia in Syrian golden hamsters

BACKGROUND

In addition to lowering LDL-C, emerging data suggests that phytosterols (PS) may reduce blood triglycerides (TG), however, the underlying mechanisms are not known.

METHODS

We examined the TG-lowering mechanisms of dietary PS in Syrian golden hamsters randomly assigned to a high fat (HF) diet or the HF diet supplemented with PS (2%) for 6 weeks (n = 12/group). An additional subset of animals (n = 12) was provided the HF diet supplemented with ezetimibe (EZ, 0.002%) as a positive control as it is a cholesterol-lowering agent with known TG-lowering properties.

RESULTS

In confirmation of diet formulation and compound delivery, both the PS and EZ treatments lowered (p < 0.05) intestinal cholesterol absorption (24 and 31%, respectively), blood non-HDL cholesterol (61 and 66%, respectively), and hepatic cholesterol (45 and 55%, respectively) compared with the HF-fed animals. Blood TG concentrations were lower (p < 0.05) in the PS (49%) and EZ (68%)-treated animals compared with the HF group. The TG-lowering response in the PS-supplemented group was associated with reduced (p < 0.05) intestinal SREBP1c mRNA (0.45 fold of HF), hepatic PPARα mRNA (0.73 fold of HF), hepatic FAS protein abundance (0.68 fold of HD), and de novo lipogenesis (44%) compared with the HF group. Similarly, lipogenesis was lower in the EZ-treated animals, albeit through a reduction in the hepatic protein abundance of ACC (0.47 fold of HF).

CONCLUSIONS

Study results suggest that dietary PS are protective against diet-induced hypertriglyceridemia, likely through multiple mechanisms that involve modulation of intestinal fatty acid metabolism and a reduction in hepatic lipogenesis.

De novo lipogenesis and cholesterol synthesis in humans with long-standing type 1 diabetes are comparable to non-diabetic individuals

BACKGROUND

Synthesis of lipid species, including fatty acids (FA) and cholesterol, can contribute to pathological disease. The purpose of this study was to investigate FA and cholesterol synthesis in individuals with type 1 diabetes, a group at elevated risk for vascular disease, using stable isotope analysis.

METHODS

Individuals with type 1 diabetes (n = 9) and age-, sex-, and BMI-matched non-diabetic subjects (n = 9) were recruited. On testing day, meals were provided to standardize food intake and elicit typical feeding responses. Blood samples were analyzed at fasting (0 and 24 h) and postprandial (2, 4, 6, and 8 hours after breakfast) time points. FA was isolated from VLDL to estimate hepatic FA synthesis, whereas free cholesterol (FC) and cholesteryl ester (CE) was isolated from plasma and VLDL to estimate whole-body and hepatic cholesterol synthesis, respectively. Lipid synthesis was measured using deuterium incorporation and isotope ratio mass spectrometry.

RESULTS

Fasting total hepatic lipogenesis (3.91 ± 0.90% vs. 5.30 ± 1.22%; P = 0.41) was not significantly different between diabetic and control groups, respectively, nor was synthesis of myristic (28.60 ± 4.90% vs. 26.66 ± 4.57%; P = 0.76), palmitic (12.52 ± 2.75% vs. 13.71 ± 2.64%; P = 0.65), palmitoleic (3.86 ± 0.91% vs. 4.80 ± 1.22%; P = 0.65), stearic (5.55 ± 1.04% vs. 6.96 ± 0.97%; P = 0.29), and oleic acid (1.45 ± 0.28% vs. 2.10 ± 0.51%; P = 0.21). Postprandial lipogenesis was also not different between groups (P = 0.38). Similarly, fasting synthesis of whole-body FC (8.2 ± 1.3% vs. 7.3 ± 0.8%/day; P = 0.88) and CE (1.9 ± 0.4% vs. 2.0 ± 0.3%/day; P = 0.96) and hepatic FC (8.2 ± 2.0% vs. 8.1 ± 0.8%/day; P = 0.72) was not significantly different between diabetic and control subjects.

CONCLUSIONS

Despite long-standing disease, lipogenesis and cholesterol synthesis was not different in individuals with type 1 diabetes compared to healthy non-diabetic humans.

New methodologies for studying lipid synthesis and turnover: looking backwards to enable moving forwards

Our ability to understand the pathogenesis of problems surrounding lipid accretion requires attention towards quantifying lipid kinetics. In addition, studies of metabolic flux should also help unravel mechanisms that lead to imbalances in inter-organ lipid trafficking which contribute to dyslipidemia and/or peripheral lipid accumulation (e.g. hepatic fat deposits). This review aims to outline the development and use of novel methods for studying lipid kinetics in vivo. Although our focus is directed towards some of the approaches that are currently reported in the literature, we include a discussion of the older literature in order to put "new" methods in better perspective and inform readers of valuable historical research. Presumably, future advances in understanding lipid dynamics will benefit from a careful consideration of the past efforts, where possible we have tried to identify seminal papers or those that provide clear data to emphasize essential points. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.

A low-protein, high-carbohydrate diet increases de novo fatty acid synthesis from glycerol and glycerokinase content in the liver of growing rats

We had previously shown that adipose tissue increased in rats fed a low-protein, high-carbohydrate (LPHC) diet (6% protein, 74% carbohydrate) without a simultaneous increase in the de novo fatty acids (FA) synthesis. In addition, impairment in insulin signaling in adipose tissues was observed in these rats. For this study, we hypothesized that the insulin signaling pathway is preserved in the livers from these rats, which contributes to an increase in liver lipogenesis and, consequently, an increase in the weight of the adipose tissue. We also hypothesized that glycerol from triacylglycerol is an important substrate for FA synthesis. Our results showed that administration of the LPHC diet induced an increase in the in vivo rate of total FA synthesis (150%) as well as FA synthesis from glucose (270%) in the liver. There were also increased rates of [U-¹⁴C]glycerol incorporation into glyceride-FA (15-fold), accompanied by increased glycerokinase content (30%) compared with livers of rats fed the control diet. The LPHC diet did not change the glycerol-3-phosphate generation from either glucose or glyceroneogenesis. There was an increase in the insulin sensitivity in liver from LPHC-fed rats, as evidenced by increases in IR(β) (35%) levels and serine/threonine protein kinase (AKT) levels (75%), and basal (95%) and insulin-stimulated AKT phosphorylation (105%) levels. The LPHC diet also induced an increase in the liver sterol regulatory element-binding protein-1c content (50%). In summary, these data confirmed the hypothesis that lipogenesis and insulin signaling are increased in the livers of LPHC-fed rats and that glycerol is important not only for FA esterification but also for FA synthesis.

Fatty acid ethanolamides modulate CD36-mRNA through dietary fatty acid manipulation in Syrian Golden hamsters

Fatty acids convert to fatty acid ethanolamides which associate with lipid signalling, fat oxidation, and energy balance; however, the extent to which dietary fatty acids manipulation can impact such control processes through fatty acid ethanolamides-related mechanisms remains understudied. The objective was to examine the impact of diets containing 6% corn oil, high oleic canola oil, docosahexaenoic acid + high oleic canola oil, and fish oil on plasma and organ levels of fatty acid ethanolamides, peroxisome proliferator-activated receptor-α regulatory targets, and lipid metabolism in Syrian Golden hamsters. After 29 days, in plasma, animals that were fed fish oil showed greater (p < 0.05) oleoylethanolamide and lower (p < 0.05) arachidonoylethanolamide and palmitoylethanolamide levels compared with other groups, while animals fed canola oil showed higher (p < 0.05) oleoylethanolamide levels in proximal intestine and liver than groups that were fed coin oil and fish oil. The canola oil group showed elevated (p < 0.01) fat oxidation (%) and over 3.0-fold higher (p < 0.05) hepatic-CD36 expression compared with the corn oil group. Hepatic-lipogenesis was lower (p < 0.05) in hamsters that were fed DHA-canola oil compared with the corn oil group. To conclude, dietary fatty acids produced shifts in plasma and organ levels of arachidonoylethanolamide, oleoylethanolamide, and palmitoylethanolamid, which were accompanied by changes in gene expression, lipogenesis, and energy expenditure, suggesting mechanisms through which dietary fatty acids influence disease risk.

Quantifying cholesterol synthesis in vivo using (2)H(2)O: enabling back-to-back studies in the same subject

The advantages of using (2)H(2)O to quantify cholesterol synthesis include i) homogeneous precursor labeling, ii) incorporation of (2)H via multiple pathways, and iii) the ability to perform long-term studies in free-living subjects. However, there are two concerns. First, the t(1/2) of tracer in body water presents a challenge when there is a need to acutely replicate measurements in the same subject. Second, assumptions are made regarding the number of hydrogens (n) that are incorporated during de novo synthesis. Our primary objective was to determine whether a step-based approach could be used to repeatedly study cholesterol synthesis a subject. We observed comparable changes in the (2)H-labeling of plasma water and total plasma cholesterol in African-Green monkeys that received five oral doses of (2)H(2)O, each dose separated by one week. Similar rates of cholesterol synthesis were estimated when comparing data in the group over the different weeks, but better reproducibility was observed when comparing replicate determinations of cholesterol synthesis in the same nonhuman primate during the respective dosing periods. Our secondary objective was to determine whether n depends on nutritional status in vivo; we observed n of ∼25 and ∼27 in mice fed a high-carbohydrate (HC) versus carbohydrate-free (CF) diet, respectively. We conclude that it is possible to acutely repeat studies of cholesterol synthesis using (2)H(2)O and that n is relatively constant.

A low-protein, high-carbohydrate diet increases the adipose lipid content without increasing the glycerol-3-phosphate or fatty acid content in growing rats

The amount of triacylglycerol (TAG) that accumulates in adipose tissue depends on 2 opposing processes: lipogenesis and lipolysis. We have previously shown that the weight and lipid content of epididymal (EPI) adipose tissue increases in growing rats fed a low-protein, high-carbohydrate (LPHC) diet for 15 days. The aim of this work was to study the pathways involved in lipogenesis and lipolysis, which ultimately regulate lipid accumulation in the tissue. De novo fatty acid synthesis was evaluated in vivo and was similar for rats fed an LPHC diet or a control diet; however, the LPHC-fed rats had decreased lipoprotein lipase activity in the EPI adipose tissue, which suggests that there was a decreased uptake of fatty acids from the circulating lipoproteins. The LPHC diet did not affect synthesis of glycerol-3-phosphate (G3P) via glycolysis or glyceroneogenesis. Glycerokinase activity - i.e., the phosphorylation of glycerol from the hydrolysis of endogenous TAG to form G3P - was also not affected in LPHC-fed rats. In contrast, adipocytes from LPHC animals had a reduced lipolytic response when stimulated by norepinephrine, even though the basal adipocyte lipolytic rate was similar for both of the groups. Thus, the results suggest that the reduction of lipolytic activity stimulated by norepinephrine seems essential for the TAG increase observed in the EPI adipose tissue of LPHC animals, probably by impairment of the process of activation of lipolysis by norepinephrine.

Consumption of plant sterols reduces plasma and hepatic triglycerides and modulates the expression of lipid regulatory genes and de novo lipogenesis in C57BL/6J mice

To investigate emerging clinical data suggesting a triglyceride (TAG)-lowering response to plant sterol (PS) therapy, we characterized changes in TAG metabolism in 16 C57BL/6J mice fed a basal control diet (CON) or the CON diet supplemented with 2% PS for 6 wk. PS consumption reduced (p<0.05) plasma (-28%) and hepatic (-30%) TAG concentrations compared with CON mice. PS consumption increased (p<0.05) hepatic lipogenic gene expression (sterol-regulatory-element-binding protein 1c, 2.4-fold of CON; fatty acid synthase, 6.5-fold of CON) and de novo lipogenesis (4.51+/-0.72 versus 2.82+/-0.61%/day) compared with CON. PS consumption increased (p<0.05) fecal palmitate and stearate excretion and reduced body weight gain compared with CON mice. Although no change in the transcription of intestinal fatty acid absorptive genes was observed, peroxisome proliferator-activated receptor alpha mRNA was reduced (p<0.05, 2.0-fold of CON) in the PS-fed mice. In conclusion, PS-fed C57BL/6J mice showed pronounced reductions in plasma and hepatic TAG concentrations despite increases in hepatic lipogenic gene expression and de novo lipogenesis. Interference with intestinal fatty acid/TAG metabolism as suggested by increased fecal fatty acid loss and reduced weight gain may be associated with the TAG-lowering response to PS consumption.

Diacylglycerol oil reduces body fat but does not alter energy or lipid metabolism in overweight, hypertriglyceridemic women

Diacylglycerol (DAG) may undergo differential metabolism compared with triacylglycerol (TAG) in humans, possibly resulting in decreased serum TAG concentration and TAG synthesis and increased energy expenditure (EE), thus reducing fat accumulation. Our objective was to examine the efficacy of DAG oil (Enova oil) consumption on serum lipid profiles, hepatic lipogenesis, EE, and body weight and composition compared with a control oil-blend composed of sunflower, safflower, and rapeseed oils at a 1:1:1 ratio. Twenty-six overweight (78.3 +/- 3.6 kg body weight and BMI 30.0 +/- 0.7 kg/m(2)) mildly hypertriglyceridemic (1.81 +/- 0.66 mmol/L) women underwent 2 treatment phases of 28 d separated by a 4-wk washout period using a randomized crossover design. They consumed 40 g/d of either DAG or control oil during treatment phases. The baseline, EE, fat oxidation, body composition, and lipid profiles did not differ between the DAG and control oil intervention periods. Relative to control oil, DAG oil did not alter endpoint postprandial EE, fat oxidation, serum lipid profiles, or hepatic lipogenesis. However, DAG oil consumption reduced (P < 0.05) accumulation of body fat within trunk, android, and gynoid regions at the endpoint compared with control oil, although neither DAG nor control oil altered any of these variables during the 4-wk intervention period compared with their respective baseline levels. We conclude that although DAG oil is not effective in lowing serum lipids over a 4-wk intervention, it may be useful for reducing adiposity.

Resveratrol inhibits fatty acid and triacylglycerol synthesis in rat hepatocytes

BACKGROUND

The putative role of resveratrol, a polyphenol present in grapes and other plants, in modulating dislypidemia, thus preventing cardiovascular diseases, is generally based on proliferating cell lines and in vivo studies in different pathological conditions. The aim of the present study was to investigate whether resveratrol plays a role on lipid biosynthesis in rat hepatocytes.

MATERIALS AND METHODS

The effect of resveratrol on total rate of fatty acid, cholesterol and complex lipid synthesis, assayed by the incorporation of [1-(14)C]acetate into these lipid fractions, was investigated in rat hepatocyte suspensions. Enzyme activities of acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS) as well as 3-hydroxy-3-methyl-glutaryl-CoA reductase (HMG-CoA-R), pace-setting steps of de novo fatty acid and cholesterol synthesis, respectively, were in situ measured in digitonin-permeabilized hepatocytes.

RESULTS

Resveratrol-treated hepatocytes exhibited a short-term (30 min) inhibition (IC(50) approximately 25 microm) of total fatty acid synthesis from [1-(14)C]acetate. Among neosynthesized fatty acids, palmitic acid formation was mainly reduced, thus suggesting that enzymatic step(s) of de novo fatty acid synthesis was affected by resveratrol. In digitonin-permeabilized hepatocytes, only ACC activity was noticeably reduced, while no change in FAS activity was observed. A noticeable resveratrol-induced reduction of label incorporation into triacylglycerols was also detected. Conversely, cholesterol synthesis and HMG-CoA-R activity were unaffected by resveratrol.

CONCLUSION

Results here reported show that in isolated hepatocytes from normal rats a resveratrol-induced short-term inhibition of fatty acid and triacylglycerol synthesis occurs. This finding may represent a potential mechanism contributing to the reported hypolipidemic effect of resveratrol.

The PPARgamma agonist rosiglitazone enhances rat brown adipose tissue lipogenesis from glucose without altering glucose uptake

We investigated the mechanisms whereby peroxisome proliferator-activated receptor-gamma (PPARgamma) agonism affects glucose and lipid metabolism in brown adipose tissue (BAT) by studying the impact of PPARgamma activation on BAT glucose uptake and metabolism, lipogenesis, and mRNA levels plus activities of enzymes involved in triacylglycerol (TAG) synthesis. Interscapular BAT of rats treated or not with rosiglitazone (15 mg*kg(-1).day(-1), 7 days) was evaluated in vivo for glucose uptake and lipogenesis and in vitro for glucose metabolism, gene expression, and activities of glycerolphosphate acyltransferase (GPAT), phosphatidate phosphatase-1 (PAP or lipin-1), and diacylglycerol acyltransferase (DGAT). Rosiglitazone increased BAT mass without affecting whole tissue glucose uptake. BAT glycogen content (-80%), its synthesis from glucose (-50%), and mRNA levels of UDP-glucose pyrophosphorylase (-40%), which generates UDP-linked glucose for glycogen synthesis, were all reduced by rosiglitazone. In contrast, BAT TAG-glycerol synthesis in vivo and glucose incorporation into TAG-glycerol in vitro were stimulated by the agonist along with the activities and mRNA levels of glycerol 3-phosphate-generating phosphoenolpyruvate carboxykinase and glycerokinase. Furthermore, rosiglitazone markedly increased the activities of GPAT and DGAT but not those of lipin-1-mediated PAP-1, enzymes involved in the sequential acylation of glycerol 3-phosphate and TAG synthesis. Because an adequate supply of fatty acids is essential for BAT nonshivering thermogenesis, the enhanced ability of BAT to synthesize TAG under PPARgamma activation may constitute an important mechanism by which lipid substrates are stored in preparation for an eventual thermogenic activation.

Depot-specific effects of the PPARgamma agonist rosiglitazone on adipose tissue glucose uptake and metabolism

We investigated mechanisms whereby peroxisome proliferator-activated receptor gamma (PPARgamma) agonism redistributes lipid from visceral (VF) toward subcutaneous fat (SF) by studying the impact of PPARgamma activation on VF and SF glucose uptake and metabolism, lipogenesis, and enzymes involved in triacylglycerol (TAG) synthesis. VF (retroperitoneal) and SF (inguinal) of rats treated or not for 7 days with rosiglitazone (15 mg/kg/day) were evaluated in vivo for glucose uptake and lipogenesis and in vitro for glucose metabolism, gene expression, and activities of glycerolphosphate acyltransferase (GPAT), phosphatidate phosphatase-1 (or lipin-1), and diacylglycerol acyltransferase. Rosiglitazone increased SF glucose uptake, GLUT4 mRNA, and insulin-stimulated glucose oxidation, conversion to lactate, glycogen, and the glycerol and fatty acid components of TAG. In VF, only glucose incorporation into TAG-glycerol was stimulated by rosiglitazone and less so than in SF (1.5- vs. 3-fold). mRNA levels of proteins involved in glycolysis, Krebs cycle, glycogen synthesis, and lipogenesis were markedly upregulated by rosiglitazone in SF and again less so in VF. Rosiglitazone activated TAG-glycerol synthesis in vivo (2.8- vs. 1.9-fold) and lipin activity (4.6- vs. 1.5-fold) more strongly in SF than VF, whereas GPAT activity was increased similarly in both depots. The preferential increase in glucose uptake and intracellular metabolism in SF contributes to the PPARgamma-mediated redistribution of TAG from VF to SF, which in turn favors global insulin sensitization.

Enigmatic effect of cellular ATP on fatty acid biosynthesis. Stimulation by moderate decrease and inhibition by increase of cellular ATP

The cellular ATP concentration was tested for its effect on fatty acid biosynthesis from glucose in hepatocytes. ATP was manipulated by adding increasing concentrations of cycloheximide, amytal, atractyloside, 2,4-dinitrophenol or adenosine. A slight decrease in cellular ATP coincided with a stimulation of fatty acid biosynthesis whereas a further lowering of cellular ATP resulted in a gradual inhibition. Increasing the cellular ATP level by titration with adenosine had the opposite effect. These results are in line with the suggestion that fatty acid biosynthesis from glucose is an energy-yielding process which is stimulated by a moderate drop in cellular ATP.

Oleic acid is a potent inhibitor of fatty acid and cholesterol synthesis in C6 glioma cells

Glial cells play a pivotal role in brain fatty acid metabolism and membrane biogenesis. However, the potential regulation of lipogenesis and cholesterologenesis by fatty acids in glial cells has been barely investigated. Here, we show that physiologically relevant concentrations of various saturated, monounsaturated, and polyunsaturated fatty acids significantly reduce [1-(14)C]acetate incorporation into fatty acids and cholesterol in C6 cells. Oleic acid was the most effective at depressing lipogenesis and cholesterologenesis; a decreased label incorporation into cellular palmitic, stearic, and oleic acids was detected, suggesting that an enzymatic step(s) of de novo fatty acid biosynthesis was affected. To clarify this issue, the activities of acetyl-coenzyme A carboxylase (ACC) and FAS were determined with an in situ digitonin-permeabilized cell assay after incubation of C6 cells with fatty acids. ACC activity was strongly reduced ( approximately 80%) by oleic acid, whereas no significant change in FAS activity was observed. Oleic acid also reduced the activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR). The inhibition of ACC and HMGCR activities is corroborated by the decreases in ACC and HMGCR mRNA abundance and protein levels. The downregulation of ACC and HMGCR activities and expression by oleic acid could contribute to the reduced lipogenesis and cholesterologenesis.

Thematic review series: Patient-Oriented Research. Recent advances in liver triacylglycerol and fatty acid metabolism using stable isotope labeling techniques

Isotopic measurement of biosynthetic rates of lipids in VLDL particles has long posed difficult technical problems. In this review, key methodologic issues and recent technical advances are discussed. A common problem for all biosynthetic measurements is the requirement to measure isotopic labeling of the true intracellular biosynthetic precursor pool. Two techniques that address this problem for lipid biosynthesis, and that are applicable to humans, have been developed-the combinatorial probability method (or mass isotopomer distribution analysis) and (2)H(2)O incorporation. The theoretical basis and practical application of these methods, both of which involve mass spectrometry, are described. Issues relevant to specific lipid components of VLDL, such as differences in the labeling of the various particle lipids (phospholipid, cholesterol, etc.), and the contribution of an intrahepatic cytosolic triacylglycerol (TG) storage pool to VLDL-TG are discussed. In summary, advances in stable isotope-mass spectrometric techniques now permit accurate measurement of liver-TG synthesis and flux. In vivo regulation of the synthesis, assembly, and secretion of VLDL-TG in humans is thereby accessible to direct investigation. Patient-oriented research in conditions such as dyslipidemia and hepatic steatosis is made feasible by these scientific advances.

Canavan disease and the role of N-acetylaspartate in myelin synthesis

Canavan disease (CD) is an autosomal-recessive neurodegenerative disorder caused by inactivation of the enzyme aspartoacylase (ASPA, EC 3.5.1.15) due to mutations. ASPA releases acetate by deacetylation of N-acetylaspartate (NAA), a highly abundant amino acid derivative in the central nervous system. CD results in spongiform degeneration of the brain and severe psychomotor retardation, and the affected children usually die by the age of 10. The pathogenesis of CD remains a matter of inquiry. Our hypothesis is that ASPA actively participates in myelin synthesis by providing NAA-derived acetate for acetyl CoA synthesis, which in turn is used for synthesis of the lipid portion of myelin. Consequently, CD results from defective myelin synthesis due to a deficiency in the supply of the NAA-derived acetate. The demonstration of the selective localization of ASPA in oligodendrocytes in the central nervous system (CNS) is consistent with the acetate deficiency hypothesis of CD. We have tested this hypothesis by determining acetate levels and studying myelin lipid synthesis in the ASPA gene knockout model of CD, and the results provided the first direct evidence in support of this hypothesis. Acetate supplementation therapy is proposed as a simple and inexpensive therapeutic approach to this fatal disease, and progress in our preclinical efforts toward this goal is presented.

Stable isotope methods for the in vivo measurement of lipogenesis and triglyceride metabolism1,2

Synthesis of fatty acids (via de novo lipogenesis) and triglycerides are important factors in fat accumulation and the efficiency of animal production. Recently, new stable isotope methods using heavy water (2H2O) have made possible the safe, and relatively easy, measurement of both of these processes in vivo in animals and humans over prolonged periods. These methods also provide information on the relative contribution of glycolysis and glyceroneogenesis to triglyceride synthesis under different physiological settings. The data suggest that numerous dietary factors, including nutrient composition and caloric content, may affect de novo lipogenesis. Significant differences in de novo lipogenesis have also been seen across species and in different tissues. The rates of triglyceride synthesis have been shown to be affected by diet and to differ significantly between different adipose depots, with metabolically active depots (e.g., visceral fat) having much more rapid triglyceride turnover than subcutaneous depots. Dietary fat and the peroxisome proliferator-activated-gamma agonist rosiglitazone have both been shown to influence triglyceride synthesis rates and to increase glyceroneogenesis. A significant portion of triglyceride synthesis is not related to triglyceride accumulation but rather is secondary to active lipolysis and reesterification. The application of these new techniques to animals other than rodents will undoubtedly enhance our understanding of adipose tissue biology and could lead to new methods for improving animal production.

Increased glyceroneogenesis in adipose tissue from rats adapted to a high-protein, carbohydrate-free diet: role of dietary fatty acids

We have previously shown in in vivo experiments that adipose tissue glyceroneogenesis is increased in rats adapted to a high-protein, carbohydrate-free (HP) diet. The objectives of the present study were (1) to verify if the increased glyceroneogenic activity is also observed in isolated adipocytes and (2) to investigate the role of preformed fatty acids in the production of the increased adipose tissue glyceroneogenesis. Control rats received a balanced diet, with the same lipid content of the HP diet. Glyceroneogenic activity was found to be higher in adipocytes from HP rats than in controls, as evidenced by increased rates of conversion of pyruvate and lactate to triacylglycerol (TAG)-glycerol. Administration of Triton WR 1339, which blocks the removal of TAG incorporated into circulating lipoproteins, to HP diet-adapted rats caused a significant reduction in the incorporation of 14C-pyruvate into TAG-glycerol by adipose tissue, which was accompanied by a marked inhibition of phosphoenolpyruvate carboxykinase activity, the key enzyme of glyceroneogenesis. The inhibitory effect of Triton on TAG-glycerol synthesis by adipose tissue was also observed in vivo, after administration of 3H2O. Adaptation to the HP diet induced a marked increase in the activity of retroperitoneal and epididymal fat LPL, which was restored to control values 24 hours after replacement of the HP diet by the balanced diet. The data suggest that in rats adapted to a carbohydrate-free diet, adipose tissue glyceroneogenesis is activated by an increased use of diet-derived fatty acids.

Short-term stimulation of lipogenesis by 3,5-L-diiodothyronine in cultured rat hepatocytes

Short-term effects of 3,5-l-diiodothyronine (T2) on lipid biosynthesis were studied in cultured hepatocytes from hypothyroid rats. A comparison with the effects of T3 was routinely carried out. After T2 addition to cell cultures, a distinct stimulation of fatty acid and cholesterol syntheses, measured as incorporation of [1-14C]acetate into these lipid fractions, was observed. The T2 dose-dependent effect on both metabolic pathways, already detectable at 10(-8)-10(-9) M, reached a 2-fold stimulation at 10(-5) M T2. At this concentration, the stimulatory effect was evident within 1 h of T2 addition to the hepatocytes and increased with time up to the length of the experimental period of 4 h. T2 stimulation of lipogenesis was also confirmed by incubating hepatocytes with [3H]H2O, used as an independent index of lipogenic activity. The effects of T2 are rather specific as 3,3',5,5'-tetraiodo-D-thyronine and 3,5-diiodo-L-tyrosine were practically ineffective on both fatty acid and cholesterol synthesis. Analysis of various lipid fractions showed that T2 addition to the cells produced a significant stimulation of the incorporation of newly synthesized fatty acids into both neutral and polar lipids. By comparing the effects induced by T2 with those seen in the presence of T3, it appeared that T2 was able to mimic T3 effects. Experiments conducted in the presence of cycloheximide, a protein synthesis inhibitor, indicated that the T2 stimulatory effect on fatty acid and cholesterol synthesis was essentially independent of protein synthesis.

Measurement of pulmonary surfactant disaturated-phosphatidylcholine synthesis in human infants using deuterium incorporation from body water

The aim of the study was to determine surfactant palmitate disaturated-phosphatidylcholine (DSPC-PA) synthesis in vivo in humans by the incorporation of deuterium from total body water into DSPC-PA under steady state condition. We studied three newborns and one infant (body weight (BW) 4.6 +/- 2.9 kg, gestational age 37.5 +/- 2 weeks, age 9 +/- 9 days) and four preterm newborns (BW 1.3 +/- 0.6 kg, gestational age 30.3 +/- 2.5 weeks, postnatal age 8.8 +/- 9.2 h). All infants were mechanically ventilated during the study and the four preterm infants received exogenous surfactant at the start of the study. We administered 0.44 g (2)H(2)O/kg BW as a bolus intravenously, followed by 0.0125 g (2)H(2)O/kg BW every 6 h to maintain deuterium enrichment at plateau over 2 days. Urine samples and tracheal aspirates (TA) were obtained prior to dosing and every 6 h thereafter. Isotopic enrichment curves of DSPC-PA from sequential TA and urine deuterium enrichments were analyzed by Gas Chromatography-Isotope Ratio-Mass Spectrometry (GC-IRMS) and normalized for Vienna Standard Mean Ocean Water. Enrichment data were used to measure DSPC-PA fractional synthesis rate (FSR) from the linear portion of the DSPC-PA enrichment rise over time, relative to plateau enrichment of urine deuterium. Secretion time (ST) was defined as the time lag between the start of the study and the appearance of DSPC-PA deuterium enrichment in TA. Data were given as mean +/- SD. All study infants reached deuterium-steady state in urine. DSPC-PA FSR was 6.5 +/- 2.8%/day (range 2.6-10.2). FSR for infants who did not receive exogenous surfactant was 5.7 +/- 3.5%/day (range 2.6-9.9%/day) and 7.3 +/- 2.1%/day (range 5.1-10.2%/day) in the preterms, whereas DSPC-PA ST was 10 +/- 10 h and 31 +/- 10 h respectively. Surfactant DSPC-PA synthesis can be measured in humans by the incorporation of deuterium from body water. This study is a simpler and less invasive method compared to previously published methods on surfactant kinetics by means of stable isotopes.

Effect of photoperiod on body mass, food intake and body composition in the field vole, Microtus agrestis

Many small mammals respond to seasonal changes in photoperiod by altering body mass and adiposity. These animals may provide valuable models for understanding the regulation of energy balance. Here, we present data on the field vole (Microtus agrestis) - a previously uncharacterised example of photoperiod-induced changes in body mass. We examined the effect of increased day length on body mass, food intake, apparent digestive efficiency, body composition, de novo lipogenesis and fatty acid composition of adipose tissue in cold-acclimated (8 degrees C) male field voles by transferring them from a short (SD, 8 h:16 h L:D) to long day photoperiod (LD, 16 h:8 h L:D). During the first 4 weeks of exposure to LD, voles underwent a substantial increase in body mass, after which the average difference between body masses of LD and SD voles stabilized at 7.5 g. This 24.8% increase in body mass reflected significant increases in absolute amounts of all body components, including dry fat mass, dry lean mass and body water mass. After correcting body composition and organ morphology data for the differences in body mass, only gonads (testes and seminal vesicles) were enlarged due to photoperiod treatment. To meet energetic demands of deposition and maintenance of extra tissue, voles adjusted their food intake to an increasing body mass and improved their apparent digestive efficiency. Consequently, although mass-corrected food intake did not differ between the photoperiod groups, the LD voles undergoing body mass increase assimilated on average 8.4 kJ day(-1) more than animals maintained in SD. The majority (73-77%) of the fat accumulated as adipose tissue had dietary origin. The rate of de novo lipogenesis and fatty acid composition of adipose tissue were not affected by photoperiod. The most important characteristics of the photoperiodic regulation of energy balance in the field vole are the clear delineation between phases where animals regulate body mass at two different levels and the rate at which animals are able to switch between different levels of energy homeostasis. Our data indicate that the field vole may provide an attractive novel animal model for investigation of the regulation of body mass and energy homeostasis at both organism and molecular levels.

Defective N-acetylaspartate catabolism reduces brain acetate levels and myelin lipid synthesis in Canavan's disease

Canavan's disease (CD) is a fatal, hereditary disorder of CNS development that has been linked to mutations in the gene for the enzyme aspartoacylase (ASPA) (EC 3.5.1.15). ASPA acts to hydrolyze N-acetylaspartate (NAA) into l-aspartate and acetate, but the connection between ASPA deficiency and the failure of proper CNS development is unclear. We hypothesize that one function of ASPA is to provide acetate for the increased lipid synthesis that occurs during postnatal CNS myelination. The gene encoding ASPA has been inactivated in the mouse model of CD, and here we show significant decreases in the synthesis of six classes of myelin-associated lipids, as well as reduced acetate levels, in the brains of these mice at the time of peak postnatal CNS myelination. Analysis of the lipid content of white matter from a human CD patient showed decreased cerebroside and sulfatide relative to normal white matter. These results demonstrate that myelin lipid synthesis is significantly compromised in CD and provide direct evidence that defective myelin synthesis, resulting from a deficiency of NAA-derived acetate, is involved in the pathogenesis of CD.

Brown adipose tissue glyceroneogenesis is activated in rats exposed to cold

We have previously found that glyceroneogenesis is very active in brown adipose tissue (BAT) and increases in fasted, diabetic and high-protein-diet-fed rats, situations of reduced thermogenic activity. To understand better the role of glyceroneogenesis in BAT glycerol-3-phosphate (G3P) generation, we investigated its activity during cold exposure (10 days at 4 degrees C), a condition in which, in contrast to the above situations, BAT thermogenesis is markedly activated. Rates of total (from all sources) BAT fatty acid (FA) synthesis and rates of incorporation of glucose carbon into BAT glyceride-FA and -glycerol in vivo were markedly increased by cold exposure. Cold exposure induced a marked increase in BAT glyceroneogenic activity, evidenced by (1) increased rates of non-glucose carbon incorporation into glyceride-glycerol in vivo and of [1-14C]-pyruvate incorporation into glyceride-glycerol in vitro, and (2) a threefold increase in phosphoenolpyruvate carboxykinase activity. Most of the glyceride-glycerol synthesized by BAT via glyceroneogenesis or from glucose was used to esterify preformed FA. This use was markedly increased by cold exposure, in parallel with a pronounced activation of BAT lipoprotein lipase activity. In conclusion, during cold exposure BAT glyceroneogenesis is markedly activated, contributing to increase the generation of G3P, which is mostly used to esterify preformed FA.

Using isotope tracers to study metabolism: application in mouse models

The application of isotope tracers for investigating metabolism in mice is discussed. To familiarize the reader, some basic principles regarding the use of tracer methods are outlined. Emphasis is placed on showing how investigators are using isotope tracers to study the regulation of carbohydrate, fat and/or protein turnover in vivo. Finally, some of the advantages of using labeled water (i.e., 2H(2)O and/or H(2)18O) to trace the kinetics of biological processes are considered. The background provided in this report should assist engineers in designing studies that enhance our understanding of conditions in which metabolism is altered (e.g., diabetes, cancer cachexia, failure to thrive and travel at zero-gravity).

Adipose tissue triglyceride turnover, de novo lipogenesis, and cell proliferation in humans measured with 2H2O

The turnover of adipose tissue components (lipids and cells) and the pathways of adipose lipid deposition have been difficult to measure in humans. We apply here a (2)H(2)O long-term labeling technique for concurrent measurement of adipose-triglyceride (TG) turnover, cell (DNA) proliferation, and de novo lipogenesis (DNL). Healthy subjects drank (2)H(2)O (70 ml/day) for 5-9 wk. Subcutaneous adipose tissue aspirates were taken (gluteal, thigh, and flank depots). Deuterium incorporation into TG glycerol (representing all-source TG synthesis), TG palmitate (representing DNL, by mass isotopomer distribution analysis), and DNA (representing cell proliferation) was measured by gas chromatography-mass spectrometry. Subjects tolerated the protocol well, and body (2)H(2)O enrichments were stable. Mean TG-glycerol fractional synthesis was 0.12 (i.e., 12%) with a range of 0.03-0.32 after 5 wk and 0.20 (range 0.08-0.49) after 9 wk (TG half-life 200-270 days). Label decay measurements 5-8 mo after discontinuing (2)H(2)O gave similar turnover estimates. Net lipolysis (TG turnover) was 50-60 g/day. DNL contribution to adipose-TG was 0.04 after 9 wk, representing approximately 20% of newly deposited TG. Cell proliferation was 0.10-0.17 after 9 wk (half-life 240-425 days). In summary, long-term (2)H(2)O administration to human subjects allows measurement of the dynamics of adipose tissue components. Turnover of all elements is slow, and DNL contributes approximately 20% of new TG.

Influence of diet on the modeling of adipose tissue triglycerides during growth

We have studied the accretion of lipids in growing mice. We measured the rates of synthesis and degradation of triglycerides in epididymal fat pads of mice maintained for 44 days on a low-fat, high-carbohydrate diet (I) or a high-fat, low-carbohydrate diet (II). 2H2O was added to the drinking water for 14 days. Rates of incorporation/washout of 2H to/from C1 of triglyceride-glycerol showed that triglyceride synthesis was greater than triglyceride degradation (net triglyceride balance was approximately 2.5 times greater in II than in I). The data also show that the contribution of de novo lipogenesis to triglyceride-bound palmitate was approximately 3 times greater in I than in II. This was consistent with a greater relative intake of carbohydrate in I vs. II. The rates of incorporation and washout of newly synthesized (2H-labeled) palmitate into and from triglycerides were also measured. Those data suggested a remodeling of triglyceride-bound fatty acids. On measuring the profile of triglyceride-bound fatty acids, we observed a decrease in the relative abundance of triglyceride-bound palmitate and stearate and an increase in triglyceride-bound oleate and linoleate. This was observed in I and II. In summary, diet substantially affects the deposition and modeling of triglycerides in adipose tissue during growth. 2H2O can be used to examine the mechanisms responsible for the accumulation of triglycerides, e.g., factors that affect 1) triglyceride synthesis and degradation and 2) the source of fatty acids that are used in esterification.

Measuring in vivo metabolism using heavy water

PURPOSE OF REVIEW

Our goal is to explain how 2H(2)O, a stable isotope of water, can be used to quantify metabolic flux in vivo. Although 2H(2)O was originally used more than 65 years ago, recent reports have described new applications. Investigators have also refined the original work. We discuss those points, emphasizing factors that should be considered when interpreting data. As our goal is to discuss why 2H(2)O is a novel tracer for studying metabolism, we do not comment on the interpretation(s) of physiological data.

RECENT FINDINGS

It is clear that 2H(2)O can now be used to address questions related to carbohydrate, lipid, protein and DNA synthesis. A particular advantage of using 2H(2)O is that investigators can study metabolism in free-living individuals. Using this novel tracer method, it is thus possible to elucidate new, highly relevant, knowledge regarding health and disease.

SUMMARY

As the use of 2H(2)O requires few assumptions, the application of this tracer should yield sound information regarding the regulation of biochemical reactions in vivo.

Dietary sodium restriction exacerbates age-related changes in rat adipose tissue and liver lipogenesis

To investigate the effects of prolonged dietary sodium restriction on lipid metabolism, male rats weighing 35 to 40 g (just weaned) were fed either a low-salt (LSD) or a normal salt diet (NSD) and used in metabolic experiments after 1, 2, or 3 months of diet consumption. After 2 and 3 months on the diet, LSD rats showed increased amounts of lipid in carcass and retroperitoneal tissue. In both LSD and NSD, extending the feeding period from 2 to 3 months resulted in a marked reduction in the in vivo rates of adipose tissue fatty acid synthesis that was accompanied by increases in liver lipogenesis and in the activity of adipose tissue lipoprotein lipase (LPL). However, these increases were more marked in LSD rats. Thus, in vivo rates of liver fatty synthesis and LPL activity in LSD rats, which were already higher (by about 35% and 20%, respectively) than in controls after 2 months, attained levels 50% higher than those in NSD animals after another month on the diet. Brown adipose tissue (BAT) thermogenic capacity, estimated after 2 and 3 months by the tissue temperature response to norepinephrine (NE) injection and by guanosine diphosphate (GDP) binding to BAT mitochondria, did not change in controls, but was significantly reduced in LSD rats. This raises the possibility that a decrease in overall energy expenditure, together with an LPL-induced increased uptake of preformed fatty acids from the circulation, may account for the excessive lipid accumulation in LSD rats. Taken together, the data indicate that prolonged dietary sodium restriction exacerbates normal, age-related changes in white and BAT metabolism.

Control of glyceroneogenic activity in rat brown adipose tissue

Brown adipose tissue (BAT) glyceroneogenesis was evaluated in rats either fasted for 48 h or with streptozotocin-diabetes induced 3 days previously or adapted for 20 days to a high-protein, carbohydrate-free (HP) diet, conditions in which BAT glucose utilization is reduced. The three treatments induced an increase in BAT glyceroneogenic activity, evidenced by increased rates of incorporation of [1-14C]pyruvate into triacylglycerol (TAG)-glycerol in vitro and a marked, threefold increase in the activity of BAT phosphoenolpyruvate carboxykinase (PEPCK). BAT glycerokinase activity was not significantly affected by fasting or diabetes. After unilateral BAT denervation of rats fed either the HP or a balanced diet, glyceroneogenesis activity increased in denervated pads, evidenced by increased rates of nonglucose carbon incorporation into TAG-glycerol in vivo (difference between 3H2O and [14C]glucose incorporations) and of [1-14C]pyruvate in vitro. PEPCK activity was not significantly affected by denervation. The data suggest that BAT glyceroneogenesis is not under sympathetic control but is sensitive to hormonal/metabolic factors. In situations of reduced glucose use there is an increase in BAT glyceroneogenesis that may compensate the decreased generation of glycerol-3-phosphate from the hexose.

From gallstones to genes: two hundred years of sterol research. A tribute to George J. Schroepfer Jr

The origins of cholesterol research can be traced to prerevolutionary France. The discovery of cholesterol as a single substance, present in human gallstones, owes much to the scientists of l'Académie Française, including Lavoisier, who contributed so much to the emergence of chemistry as a modern scientific discipline. Since that time, cholesterol probably has been the most intensively scrutinized natural product of all time, and it has been the subject of Nobel Prizes for several who have studied its structure, biosynthesis, and regulation. The pace of research into cholesterol shows no sign of diminishing, and recent discoveries have led to the recognition that the regulation of cholesterol metabolism is intimately linked with that of other metabolic pathways. Details of these interactions are only just emerging, but it is becoming apparent that under some circumstances it is difficult to reconcile, in a conventional manner, changes in regulatory gene expression with corresponding changes in pathway carbon flux. The present review includes some of our studies on the roles of the transcription factors sterol regulatory element-binding protein, liver X-receptor alpha, and peroxisome proliferator activated receptor a in the coordination of cholesterol and fatty acid synthesis and describes how some of the results obtained can best be interpreted from a Metabolic Control Analysis perspective of the regulation of pathway carbon fluxes.

De novo lipogenesis during controlled overfeeding with sucrose or glucose in lean and obese women

BACKGROUND

The results of previous studies suggest that de novo lipogenesis may play an important role in the etiology of obesity, particularly during overconsumption of different carbohydrates.

OBJECTIVE

We hypothesized that de novo lipogenesis would increase during overfeeding, would vary depending on the type of carbohydrate consumed, and would be greater in obese than in lean women.

DESIGN

De novo lipogenesis was measured during 96 h of overfeeding by 50% with either sucrose or glucose and during an energy balance treatment (control) in 8 lean and 5 obese women. De novo lipogenesis was determined by measuring the amount of deuterium incorporation into plasma triacylglycerols. Fat and carbohydrate balance were measured simultaneously by continuous whole-body calorimetry.

RESULTS

De novo lipogenesis did not differ significantly between lean and obese subjects, except with the control treatment, for which de novo lipogenesis was greater in the obese subjects. De novo lipogenesis was 2- to 3-fold higher after overfeeding by 50% than after the control treatment in all subjects. The type of carbohydrate overfeeding (sucrose or glucose) had no significant effect on de novo lipogenesis in either subject group. Estimated amounts of absolute VLDL production ranged from a minimum of 2 g/d (control) to a maximum of 10 g/d after overfeeding. This compares with a mean fat balance of approximately 275 g after 96 h of overfeeding. Individual subjects showed characteristic amounts of de novo lipogenesis, suggesting constitutive (possibly genetic) differences.

CONCLUSION

De novo lipogenesis increases after overfeeding with glucose and sucrose to the same extent in lean and obese women but does not contribute greatly to total fat balance.

Parameters related to lipid metabolism as markers of myelination in mouse brain

Myelination, during both normal development and with respect to disorders of myelination, is commonly studied by morphological and/or biochemical techniques that assay as their end-points the extent of myelination. The rate of myelination is potentially a more useful parameter, but it is difficult and time-consuming to establish, requiring a complete developmental study with labor-intensive methodology. We report herein development of methodology to assay the absolute rate of myelination at any desired time during development. This involves intraperitoneal injection of (3)H(2)O to label body water pools, followed by determination of label in the myelin-specific lipid, cerebroside. The absolute amount of cerebroside synthesized can then be calculated from the specific radioactivity of body water and knowledge of the number of hydrogens from water incorporated into cerebroside. During development, the rate of cerebroside synthesis correlated well with the rate of accumulation of the myelin-specific components, myelin basic protein and cerebroside. For purposes of control, we also tested other putative, albeit less quantitative, indices of the rate of myelination. Levels of mRNA for ceramide galactosyltransferase (rate-limiting enzyme in cerebroside synthesis) and for myelin basic protein did not closely correlate with myelination at all times. Cholesterol synthesis closely matched the rate of cholesterol accumulation but did not track well with myelination. Synthesis of fatty acids did not correlate well with accumulation of either fatty acids (phospholipids) or myelin markers. We conclude that measurement of cerebroside synthesis rates provides a good measure of the rate of myelination. This approach may be useful as an additional parameter for examining the effects of environmental or genetic alterations on the rate of myelination.

The conformational and biological analysis of a cyclic anti-obesity peptide from the C-terminal domain of human growth hormone

The three-dimensional solution structure of antiobesity drug (AOD), a 15-residue, disulfide-bonded, cyclic peptide, cyclo(6,13)-H2N-Leu-Arg-Ile-Val-Gln-Cys-Arg-Ser-Val-Glu-Gly-Ser-Cys-Gly-Phe-OH, derived from the C-terminal domain of the human growth hormone (hGH) (residues 177-191) was determined using two-dimensional 1H NMR spectroscopy. AOD stimulates lipolysis and inhibits lipogenesis, in vitro, in rodent, porcine and human adipose tissues. These biological effects suggest that AOD is a potential therapeutic candidate for the treatment of obesity. Conformational studies of AOD were conducted in aqueous solution and in water/dimethylsulfoxide mixtures. In general, spectral quality was superior in the water/ dimethylsulfoxide mixtures. The cyclic region of AOD in water/dimethylsulfoxide adopts type I beta-turns at residues Ser8-Val9-Glu10-Gly11 and Ser12-Cys13-Gly14-Phe15, each preceded by loop-like structures. Comparison of the conformation of this peptide with residues 177-191 in the native hGH protein X-ray crystal structure indicates that the synthetic peptide retains some structural similarity to the intact protein. This study provides evidence that the C-terminal region of hGH is a specific functional domain of the multifunctional hGH protein.

Perinatal feedings adversely affect lipogenic activities but not glucose handling in adult rats

The long-term effects of early under- and overfeeding on glucose metabolism and fat cell lipogenesis were studied. Newborn rats were reared in litter sizes of four, 10, and 16 pups. The amount of milk intake per pup varied inversely with litter sizes. A subgroup of pups from each group was studied at age 20 d, whereas another subgroup was weaned to an ad libitum feeding of standard rat chow and studied at 12 wk of age. There were no differences among groups in food intake on the basis of per gram body weight. Overfeeding during suckling resulted in fatter rats at weaning and in the adults. The higher fat contents in the adipose tissues and carcasses were associated with higher fatty acid synthase and lipogenic activities in the adipose tissues at weaning and 12 wk of age. Differences in plasma insulin and glucose levels among groups were observed only in the 20-d-old rats: basal insulin and glucose levels and 30-min postglucose insulin levels were highest in the overnourished and lowest in the undernourished rats. However, by 12 wk of age, there were no significant differences among groups in their basal insulin and glucose levels and after an oral dose of glucose. Our results suggest that overfeeding or underfeeding during the suckling period affects the glucose-insulin axis only temporarily and not permanently, but early overfeeding permanently enhances fatty acid synthase and lipogenic activities in adipose tissues, resulting in fatter adult rats.

De novo lipogenesis in adipose tissue of lean and obese women: application of deuterated water and isotope ratio mass spectrometry

OBJECTIVE

To evaluate the feasibility of using deuterated water and isotope ratio mass spectrometry to measure de novo fatty acid synthesis in adipose tissue, and to compare this parameter in obese and lean women.

SUBJECTS

Six lean and six obese premenopausal Caucasian women in the main study and three obese Pima Indians in a pilot study.

MEASUREMENTS

Deuterated water was administered orally twice daily for 14 days to create stable deuterium enrichment in body water, during which series of blood samples were collected to measure body water deuterium enrichment and deuterium incorporation into plasma total Triacylglycerol (TG) fatty acids and total cholesterol. Subcutaneous fat at different sites were sampled at the beginning and the end of deuterium administration to measure deuterium incorporation into TG fatty acids.

RESULTS

Fractional de novo synthesis rate of TG fatty acids in adipose tissue was 0. 014+/-0.005 and 0.014+/-0.007% in lean and obese Caucasian women, corresponding to 2+/-0.7 and 5.6+/-3.2 g (P=0.3) of fatty acids synthesized daily, respectively. Plasma TG fatty acids and cholesterol synthesis rates were comparable to those reported previously. A pilot study showed that de novo lipid synthesis in adipose tissue of obese Pima Indians was also quantitatively minor.

CONCLUSION

Human adipose tissue, like the liver, does not make a major contribution to whole body lipogenesis under eucaloric conditions. A combination of deuterated water and isotope ratio mass spectrometry is a useful research tool for studying accumulation of de novo synthesized lipids in human adipose tissue.