Docosahexaenoic acid shows no triglyceride-lowering effects but increases the peroxisomal fatty acid oxidation in liver of rats

Effect of dietary docosahexaenoic acid on biosynthesis of docosahexaenoic acid from alpha-linolenic acid in young rats

Docosahexaenoic acid (DHA), a crucial nervous system n-3 PUFA, may be obtained in the diet or synthesized in vivo from dietary alpha-linolenic acid (LNA). We addressed whether DHA synthesis is regulated by the availability of dietary DHA in artificially reared rat pups, during p8 to p28 development. Over 20 days, one group of rat pups was continuously fed deuterium-labeled LNA (d5-LNA) and no other n-3 PUFA (d5-LNA diet), and a second group of rat pups was fed a d5-LNA diet with unlabeled DHA (d5-LNA + DHA diet). The rat pups were then euthanized, and the total amount of deuterium-labeled docosahexaenoic acid (d5-DHA) (synthesized DHA) as well as other n-3 fatty acids present in various body tissues, was quantified. In the d5-LNA + DHA group, the presence of dietary DHA led to a marked decrease (3- to 5-fold) in the total amount of d5-DHA that accumulated in all tissues that we examined, except in adipose. Overall, DHA accretion from d5-DHA was generally diminished by availability of dietary preformed DHA, inasmuch as this was found to be the predominant source of tissue DHA. When preformed DHA was unavailable, d5-DHA and unlabeled DHA were preferentially accreted in some tissues along with a net loss of unlabeled DHA from other organs.

Hypolipidaemic effects of fenofibrate and fasting in the herbivorous grass carp ( Ctenopharyngodon idella) fed a high-fat diet

We investigated whether the hypolipidaemic effect of fenofibrate and fasting observed in most omnivorous mammals may also apply to herbivorous fish. Grass carp (Ctenopharyngodon idella) fed a high-fat (8 %) diet exhibited a marked increase in blood lipids and body fat after 6 weeks. They were then treated with fenofibrate (100 mg/kg body weight) in the same high-fat diet for 2 weeks, followed by fasting for 1 week. Plasma lipid concentration, body fat amount, fatty acid composition, plasma thiobarbituric acid-reactive substances and some parameters related to hepatic fatty acid oxidation were measured, and liver samples were stained for histological examination. Fenofibrate treatment decreased TAG and cholesterol concentrations in plasma, total lipids of the whole body and liver, and EPA and DHA contents in tissues. Further, a mobilisation of mesenteric fat concomitant with an increase in hepatic peroxisomal fatty acid oxidation and lipid peroxidation was observed. Compared with fenofibrate treatment, fasting decreased body weight and plasma TAG, but not plasma cholesterol. It also reduced the fat content of the whole body and increased the EPA and DHA contents in the liver and other tissues. Fatty acid oxidation was stimulated by fasting in mitochondria, but not in peroxisomes. These data suggest that fenofibrate and fasting regulate the lipid metabolism in grass carp through different metabolic pathways. The grass carp is moderately responsive to a fibrate derivative in comparison with the well-known excess responsiveness of the rat model, and so it could be used for the study of lipid abnormalities as a herbivorous model.

Fatty acid consumption and metabolic syndrome components: the GOCADAN study

Fatty acids (FAs) have been related to changes in glucose and lipid metabolism. In this article, the authors assess the association between intake of specific FAs and components of the metabolic syndrome (MS) in adult Eskimos. A total of 691 Inupiat Eskimos (325 men and 366 women), aged 34 to 75 years, were examined as part of the Genetics of Coronary Artery Disease in Alaska Natives (GOCADAN) study. The investigation included a physical examination, blood pressure measurements, blood sampling under fasting conditions, 2-hour oral glucose tolerance test, and a personal interview including a validated food frequency questionnaire. Components of MS were defined according to the Third Report of the National Cholesterol Education Program Adult Treatment Panel criteria. Consumption of individual FAs showed associations with MS components. Long-chain omega-3 FAs, from fish and sea mammals, were associated with lower blood pressure, serum triglycerides, and 2-hour glucose and higher high-density lipoprotein cholesterol, fasting insulin, and homeostasis model assessment. Saturated fat consumption was associated with higher triglyceride levels and blood pressure. Trans-FA consumption was associated with higher blood pressure. Consumption of long-chain omega-3 FAs from marine sources may improve certain MS components, and thus may reduce risk for cardiovascular disease. High consumption of saturated FAs and trans-FAs may have an adverse effect on MS.

An emerging risk factor for obesity: does disequilibrium of polyunsaturated fatty acid metabolism contribute to excessive adipose tissue development?

A positive energy balance (energy intake>energy expenditure), in which total fat intake plays an important role, is commonly regarded as a major factor contributing to obesity. Adipose tissue development, i.e. both size (hypertrophy) and number (hyperplasia), is stimulated by high dietary fat intake during early postnatal development, a susceptibility that now appears to continue well into adulthood. Recent human and animal studies suggest that by altering rates of adipocyte differentiation and proliferation, differences in the composition of dietary fat may also contribute to adipose tissue development. At least in rodent models, the relative intake of n-6 to n-3 PUFA is clearly emerging as a new factor in this development. In these models, higher linoleate intake raises tissue arachidonic acid, which increases prostacyclin production and, in turn, stimulates signalling pathways implicated in adipogenesis. Signalling pathways stimulated by arachidonic acid probably include phospholipase and/or cyclo-oxygenase activation and may be linked as much to relatively low intake of n-3 PUFA as to excessive dietary linoleate. One factor potentially contributing to oversight about the apparent role of dietary n-6 PUFA (especially excess dietary linoleate) in adipose tissue development is the historical overestimation of linoleate requirements and the enthusiasm for higher intake of 'essential fatty acids'. More research is needed to address whether disequilibration of dietary PUFA intake contributes to the risk of obesity in humans.

Fenofibrate modulates cardiac and hepatic metabolism and increases ischemic tolerance in diet-induced obese mice

Peroxisome proliferator-activated receptors (PPARs) play an important role in the transcriptional regulation of lipid utilization and storage in several organs, including liver and heart. Our working hypothesis is that treatment of obesity/hyperlipedemia with the PPARalpha ligand fenofibrate leads to drainage of plasma lipids by the liver, resulting in reduced myocardial lipid supply, reduced myocardial fatty acid oxidation and improved myocardial tolerance to ischemic stress. Thus, we investigated changes in substrate utilization in heart and liver, as well as post-ischemic functional recovery in hearts from diet-induced obese (DIO) mice following long-term (11-12 weeks) treatment with fenofibrate. The present study shows that DIO mice express increased plasma lipids and glucose, as well as increased myocardial fatty acid oxidation and a concomitant decrease in glucose oxidation. The lipid-lowering effect of fenofibrate was associated with increased hepatic mitochondrial and peroxisomal fatty acid oxidation, as indicated by a more than 30% increase in hepatic palmiotyl-CoA oxidation and more than a 10-fold increase in acyl-CoA oxidase (ACO) activity. In line with an adaptation to the reduced myocardial lipid supply, isolated hearts from fenofibrate-treated DIO mice showed increased glucose oxidation and decreased fatty acid oxidation, as well as reduced ACO activity. Fenofibrate treatment also prevented the diet-induced decrease in cardiac function and improved post-ischemic functional recovery. We also found that, while fenofibrate treatment markedly increased the expression of PPARalpha target genes in the liver, there were no such changes in the heart. These data demonstrate that fenofibrate results in a direct activation of PPARalpha in the liver with increased hepatic drainage of plasma lipids, while the cardiac effect of the compound most likely is secondary to its lipid-lowering effect.

Eicosapentaenoic acid and docosahexaenoic acid from fish oils: differential associations with lipid responses

Fish-oil supplementation can reduce circulating triacylglycerol (TG) levels and cardiovascular risk. This study aimed to assess independent associations between changes in platelet eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) and fasting and postprandial (PP) lipoprotein concentrations and LDL oxidation status, following fish-oil intervention. Fifty-five mildly hypertriacylglycerolaemic (TG 1·5–4·0 mmol/l) men completed a double-blind placebo controlled cross over study, where individuals consumed 6 g fish oil (3 g EPA+DHA) or 6 g olive oil (placebo)/d for two 6-week intervention periods, with a 12-week wash-out period in between. Fish-oil intervention resulted in a significant increase in the platelet phospholipid EPA (+491 %,P<0·001) and DHA (+44 %,P<0·001) content and a significant decrease in the arachidonic acid (-10 %,P<0·001) and γ-linolenic acid (-24 %,P<0·001) levels. A 30 % increase inex vivoLDL oxidation (P<0·001) was observed. In addition, fish oil resulted in a significant decrease in fasting and PP TG levels (P<0·001), PP non-esterified fatty acid (NEFA) levels, and in the percentage LDL as LDL-3 (P=0·040), and an increase in LDL-cholesterol (P=0·027). In multivariate analysis, changes in platelet phospholipid DHA emerged as being independently associated with the rise in LDL-cholesterol, accounting for 16 % of the variability in this outcome measure (P=0·030). In contrast, increases in platelet EPA were independently associated with the reductions in fasting (P=0·046) and PP TG (P=0·023), and PP NEFA (P=0·015), explaining 15–20 % and 25 % of the variability in response respectively. Increases in platelet EPA+DHA were independently and positively associated with the increase in LDL oxidation (P=0·011). EPA and DHA may have differential effects on plasma lipids in mildly hypertriacylglycerolaemic men.

Tetradecylselenoacetic acid, a PPAR ligand with antioxidant, antiinflammatory, and hypolipidemic properties

OBJECTIVE

Antioxidants protect against oxidative stress and inflammation, which, in combination with hyperlipidemia, are important mediators of atherogenesis. Here we present a selenium-substituted fatty acid, tetradecylselenoacetic acid (TSA), which is hypothesized to have antioxidant, antiinflammatory, and hypolipidemic properties.

METHODS AND RESULTS

We show that TSA exerts antioxidant properties by delaying the onset of oxidation of human low density lipoprotein (LDL), by reducing the uptake of oxidized LDL in murine macrophages, and by increasing the mRNA level of superoxide dismutase in rat liver. TSA also showed antiinflammatory effects by suppressing the release of interleukin (IL)-2 and -4, and by increasing the release of IL-10 in human blood leukocytes. In addition, TSA decreased the plasma triacylglycerol level and increased the mitochondrial fatty acid beta-oxidation in rat liver. In pigs, TSA seemed to reduce coronary artery intimal thickening after percutaneous coronary intervention. In HepG2 cells TSA activated all peroxisome proliferator-activated receptors (PPARs) in a dose-dependent manner.

CONCLUSIONS

Our data suggest that TSA exert potent antioxidant, antiinflammatory, and hypolipidemic properties, potentially involving PPAR-related mechanisms. Based on these effects, it is tempting to hypothesize that TSA could be an interesting antiatherogenic approach to atherosclerotic disorders.

Dietary soya protein concentrate enriched with isoflavones reduced fatty liver, increased hepatic fatty acid oxidation and decreased the hepatic mRNA level of VLDL receptor in obese Zucker rats

Casein-based diets containing a low (LDI) or high (HDI) dose of soya protein concentrate enriched with isoflavones were fed to obese Zucker rats for 6 weeks. HDI feeding, but not LDI feeding, reduced the fatty liver and decreased the plasma levels of alanine transaminase and aspartate transaminase. This was accompanied by increased activities of mitochondrial and peroxisomal beta-oxidation, acetyl-CoA carboxylase, fatty acid synthase and glycerol-3-phosphate acyltransferase in liver and increased triacylglycerol level in plasma. The decreased fatty liver and the increased plasma triacylglycerol level appeared not to be caused by an increased secretion of VLDL, as HDI decreased the hepatic mRNA levels of apo B and arylacetamide deacetylase. However, the gene expression of VLDL receptor was markedly decreased in liver, but unchanged in epididymal white adipose tissue and skeletal muscle of rats fed HDI, indicating that the liver may be the key organ for the reduced clearance of triacylglycerol-rich lipoproteins from plasma after HDI feeding. The n-3/n-6, 20:4n-6/18:2n-6 and (20:5n-3+22:6n-3)/18:3n-3 ratios were increased in liver triacylglycerol by HDI. The phospholipids in liver of rats fed HDI contained a low level of 20:4n-6 and a high level of 20:5n-3, favouring the production of anti-inflammatory eicosanoids. When obese Zucker rats were fed soya protein, this also resulted in reduced fatty liver, possibly through reduced clearance of VLDL by the liver. We conclude that the isoflavone-enriched soya concentrate as well as soya protein may be promising dietary supplements for treatment of non-alcoholic fatty liver.

Why do omega-3 fatty acids lower serum triglycerides?

PURPOSE OF REVIEW

Fish oils rich in n-3 fatty acids reduce serum triglyceride levels. This well known effect has been shown to be caused by decreased very low-density lipoprotein triglyceride secretion rates in kinetic studies in humans. Animal studies have explored the biochemical mechanisms underlying this effect. Triglyceride synthesis could be reduced by n-3 fatty acids in three general ways: reduced substrate (i.e. fatty acids) availability, which could be secondary to increase in beta-oxidation, decreased free fatty acids delivery to the liver, decreased hepatic fatty acids synthesis; increased phospholipid synthesis; or decreased activity of triglyceride-synthesizing enzymes (diacylgylcerol acyltranferase or phosphatidic acid phosphohydrolase).

RECENT FINDINGS

Rarely were experimental conditions used in rat studies physiologically relevant to the human situation in which 1.2% energy as n-3 fatty acids lowers serum triglyceride levels. Nevertheless, the most consistent effect of n-3 fatty acids feeding in rats is to decrease lipogenesis. Increased beta-oxidation was frequently, but not consistently, reported with similar numbers of studies reporting increased mitochondrial compared with peroxisomal oxidation. Inhibition of triglyceride-synthesizing enzymes was only occasionally noted.

SUMMARY

As the vast majority of studies fed unphysiologically high doses of n-3 fatty acids, these findings in rats must be considered tentative, and the mechanism by which n-3 fatty acids reduce triglyceride levels in humans remains speculative.

Causes and prevention of tamoxifen-induced accumulation of triacylglycerol in rat liver

Tamoxifen can induce hepatic steatosis in women. In this study, we wanted to elucidate the mechanism behind the tamoxifen-induced accumulation of triacylglycerol in liver in female rats, and we hoped to prevent this development by combination treatment with the modified fatty acid tetradecylthioacetic acid (TTA). The increased hepatic triacylglycerol level after tamoxifen treatment was accompanied by decreased acetyl-coenzyme A carboxylase (ACC) and FAS activities, increased glycerol-3-phosphate acyltransferase (GPAT) activity, and a tendency to increased diacylglycerol acyltransferase (DGAT) activity. The activities and mRNA levels of enzymes involved in beta-oxidation, ketogenesis, and uptake of lipids from liver were unaffected by tamoxifen, whereas the uptake of lipoproteins was unchanged and the uptake of fatty acids was decreased. Combination treatment with tamoxifen and TTA (Tam+TTA) normalized the hepatic triacylglycerol level and increased the activities of ACC, FAS, GPAT, and DGAT compared with tamoxifen-treated rats. The activities and mRNA levels of enzymes involved in beta-oxidation, ketogenesis, and uptake of lipids were increased after Tam+TTA treatment. In conclusion, tamoxifen increased the hepatic triacylglycerol level, probably as a result of increased triacylglycerol biosynthesis combined with unchanged beta-oxidation. The tamoxifen-induced accumulation of triacylglycerol was prevented by cotreatment with TTA, through mechanisms of increased mitochondrial and peroxisomal beta-oxidation.

Dietary sesamin and docosahexaenoic and eicosapentaenoic acids synergistically increase the gene expression of enzymes involved in hepatic peroxisomal fatty acid oxidation in rats

The interaction of sesamin, one of the most abundant lignans in sesame seed, and highly purified docosahexaenoic acid (DHA) or eicosapentaenoic acid (EPA) in the form of ethyl ester in affecting hepatic fatty acid oxidation was examined in rats. In the first experiment, 3 groups of rats were fed with purified experimental diets free of n-3 fatty acid ethyl ester and containing 0%, 0.2%, and 0.4% sesamin (1:1 mixture of sesamin and episesamin), and 2 groups of animals were fed with a 2% DHA ethyl ester diet containing either 0% or 0.2% sesamin. In the second trial, 4 groups of rats were fed with either a 0% or a 2% EPA ethyl ester diet containing 0% or 0.2% sesamin. After 15 days of feeding, DHA and EPA ethyl esters added to a sesamin-free diet little affected the activity and messenger RNA (mRNA) levels of various enzymes involved in fatty acid oxidation. Sesamin increased the activity levels of various hepatic enzymes involved in fatty acid oxidation irrespective of the presence or absence of n-3 fatty acid ethyl ester in diets. However, the diet containing sesamin and DHA or EPA ethyl ester in combination increased many of these parameters synergistically. In particular, the peroxisomal palmitoyl-coenzyme A oxidation rate and acyl-coenzyme A oxidase activity level were much higher in rats fed with sesamin and DHA or EPA in combination than in animals fed with a diet free of n-3 fatty acid ethyl ester and containing sesamin. Analyses of mRNA levels revealed that a diet simultaneously containing sesamin and n-3 fatty acid ethyl ester increased the gene expression of various enzymes involved in peroxisomal fatty acid oxidation in a synergistic manner. However, the combination of sesamin and n-3 fatty acid ethyl esters was ineffective in causing a synergistic increase in mRNA levels of enzymes of mitochondrial fatty acid oxidation, microsomal cytochrome P-450 IV A1, and cytosolic liver-type fatty acid-binding protein. It was concluded that sesamin and DHA or EPA ethyl ester synergistically increased hepatic fatty acid oxidation primarily through up-regulation of the gene expression of peroxisomal fatty acid oxidation enzymes. The results essentially reproduced those observed in our previous study with a diet containing both fish oil and sesamin despite the fact that DHA and EPA ethyl esters were much less effective than fish oil in increasing hepatic fatty acid oxidation.

Omega-3 fatty acids improve glucose tolerance and components of the metabolic syndrome in Alaskan Eskimos: the Alaska Siberia project

OBJECTIVES

To test the hypothesis that the unusually low prevalences of insulin resistance (IR), metabolic syndrome (MS) and diabetes (DM) in Alaskan Eskimos, compared to American Indians, is related to the traditional Eskimo diet, high in C20-C22 omega-3 fatty acids (FAs). To determine if the relatively low blood pressures, low serum triglycerides and high HDL cholesterol levels in Eskimos result from high omega-3 FA consumption.

STUDY DESIGN

Cross-sectional study.

METHODS

We measured plasma FA concentrations in 447 Norton Sound Eskimos (35-74 years of age) and screened for DM, CHD and associated risk factors. A dietary assessment (24-hr recall) was obtained for comparison the day before the blood sampling.

RESULTS

Plasma omega-3 FA concentrations were highly correlated with dietary omega-3 FAs and HDL levels and inversely correlated with plasma levels of insulin, 2-h insulin (OGTT), HOMI-IR, 2-h glucose (OGTT), triglyceride levels and diastolic blood pressure.

CONCLUSIONS

High consumption of omega-3 FAs positively affects components of the MS, insulin sensitivity and glucose tolerance. This finding suggests that high consumption of C20-C22 omega-3 FAs protects against the development of the MS and glucose intolerance.

The metabolic effects of thia fatty acids in rat liver depend on the position of the sulfur atom

The effects on oxidation and composition of fatty acids in rat liver were compared after administration of fatty acids with sulfur substituted in different positions. It has been hypothesized that drugs with hydrophobic backbone have lipid-lowering effects because they are not easily catabolized by mitochondrial beta-oxidation. Thia fatty acids cannot be beta-oxidized when sulfur is in 3-position, but beta-oxidation is possible when sulfur is positioned further from the carboxyl group. To investigate whether catabolism of thia fatty acids would affect their ability to influence lipid metabolism, a series of thia fatty acids were synthesized and administered by oral gavage to male Wistar rats (300 mg/kg bodyweight/day for 7 days). Depending on the position of the sulfur atom and the chain length, the thia fatty acids were beta-oxidized, desaturated and/or elongated, and the accumulated amounts were lower as the sulfur atom were positioned further from the carboxyl group. All thia fatty acids led to high peroxisomal beta-oxidation of endogenous fatty acids, whereas the mitochondrial beta-oxidation was high when sulfur was in 3-position, low when sulfur was in 4-position and similar to controls when sulfur was in 5- or 7-position. The changes in hepatic fatty acid composition were more pronounced when sulfur was positioned close to the carboxyl group. In conclusion, both the position of the sulfur atom and the chain length appear to determine the catabolic fate of thia fatty acids, and the non-beta-oxidizable thia fatty acids were most potent in regulating oxidation and composition of endogenous fatty acids in rat liver.

Fish consumption: recommendations versus advisories, can they be reconciled?

Consumption of at least two servings of fish per week is recommended by the American Heart Association (AHA) to achieve cardio-protective effects. However, some fish are contaminated with methylmercury, which may counteract the positive effect of the omega-3 fatty acids, and numerous governments have issued advisories for certain fish species. These mixed messages may be a source of confusion to the consumer and to the health professional. This paper reviews whether it is possible to follow the AHA recommendation for fish consumption while avoiding the risks associated with consuming mercury in amounts in excess of government thresholds.

Omega-3 fatty acid supplementation prevents hepatic steatosis in a murine model of nonalcoholic fatty liver disease

Prolonged use of total parenteral nutrition can lead to nonalcoholic fatty liver disease, ranging from hepatic steatosis to cirrhosis and liver failure. It has been demonstrated that omega-3 fatty acids are negative regulators of hepatic lipogenesis and that they can also modulate the inflammatory response in mice. Furthermore, they may attenuate hepatic steatosis even in leptin-deficient ob/ob mice. We hypothesized that omega-3 fatty acid supplementation may protect the liver against hepatic steatosis in a murine model of parenteral nutrition in which all animals develop steatosis and liver enzyme disturbances. For testing this hypothesis, groups of mice received a fat-free, high-carbohydrate liquid diet ad libitum for 19 d with enteral or i.v. supplementation of an omega-3 fatty acid emulsion or a standard i.v. lipid emulsion. Control mice received food alone or the fat-free, high-carbohydrate diet without lipid supplementation. Mice that received the fat-free, high-carbohydrate diet only or supplemented with a standard i.v. lipid emulsion developed severe liver damage as determined by histology and magnetic resonance spectroscopy as well as elevation of serum liver function tests. Animals that received an i.v. omega-3 fatty acid emulsion, however, showed only mild deposits of fat in the liver, whereas enteral omega-3 fatty acids prevented hepatic pathology and led to normalization of liver function tests. In conclusion, whereas standard i.v. lipid emulsions fail to improve dietary-induced steatotic injury to the liver, i.v. supplementation of omega-3 fatty acids partially and enteral supplementation completely protects the liver against such injury.

Tryptophan-NAD+ pathway metabolites as putative biomarkers and predictors of peroxisome proliferation

The present study was designed to provide further information about the relevance of raised urinary levels of N-methylnicotinamide (NMN), and/or its metabolites N-methyl-4-pyridone-3-carboxamide (4PY) and N-methyl-2-pyridone-3-carboxamide (2PY), to peroxisome proliferation by dosing rats with known peroxisome proliferator-activated receptor alpha (PPARalpha) ligands [fenofibrate, diethylhexylphthalate (DEHP) and long-chain fatty acids (LCFA)] and other compounds believed to modulate lipid metabolism via PPARalpha-independent mechanisms (simvastatin, hydrazine and chlorpromazine). Urinary NMN was correlated with standard markers of peroxisome proliferation and serum lipid parameters with the aim of establishing whether urinary NMN could be used as a biomarker for peroxisome proliferation in the rat. Data from this study were also used to validate a previously constructed multivariate statistical model of peroxisome proliferation (PP) in the rat. The predictive model, based on 1H nuclear magnetic resonance (NMR) spectroscopy of urine, uses spectral patterns of NMN, 4PY and other endogenous metabolites to predict hepatocellular peroxisome count. Each treatment induced pharmacological (serum lipid) effects characteristic of their class, but only fenofibrate, DEHP and simvastatin increased peroxisome number and raised urinary NMN, 2PY and 4PY, with simvastatin having only a transient effect on the latter. These compounds also reduced mRNA expression for aminocarboxymuconate-semialdehyde decarboxylase (ACMSDase, EC 4.1.1.45), the enzyme believed to be involved in modulating the flux of tryptophan through this pathway, with decreasing order of potency, fenofibrate (-10.39-fold) >DEHP (-3.09-fold) >simvastatin (-1.84-fold). Of the other treatments, only LCFA influenced mRNA expression of ACMSDase (-3.62-fold reduction) and quinolinate phosphoribosyltransferase (QAPRTase, EC 2.4.2.19) (-2.42-fold) without any change in urinary NMN excretion. Although there were no correlations between urinary NMN concentration and serum lipid parameters, NMN did correlate with peroxisome count (r2=0.63) and acyl-CoA oxidase activity (r2=0.61). These correlations were biased by the large response to fenofibrate compared to the other treatments; nevertheless the data do indicate a relationship between the tryptophan-NAD+ pathway and PPARalpha-dependent pathways, making this metabolite a potentially useful biomarker to detect PP. In order to strengthen the observed link between the metabolites associated with the tryptophan-NAD+ pathway and more accurately predict PP, other urinary metabolites were included in a predictive statistical model. This statistical model was found to predict the observed PP in 26/27 instances using a pre-determined threshold of 2-fold mean control peroxisome count. The model also predicted a time-dependent increase in peroxisome count for the fenofibrate group, which is important when considering the use of such modelling to predict the onset and progression of PP prior to its observation in samples taken at autopsy.

The hypotriglyceridemic effect of dietary n-3 FA is associated with increased beta-oxidation and reduced leptin expression

To study the mechanisms responsible for the hypotriglyceridemic effect of marine oils, we monitored the effects of high dietary intake of n-3 PUFA on hepatic and muscular beta-oxidation, plasma leptin concentration, leptin receptor gene expression, and in vivo insulin action. Two groups of male Wistar rats were fed either a high-fat diet [28% (w/w) of saturated fat] or a high-fat diet containing 10% n-3 PUFA and 18% saturated fat for 3 wk. The hypotriglyceridemic effect of n-3 PUFA was accompanied by increased hepatic oxidation of palmitoyl-CoA (125%, P < 0.005) and palmitoyl-L-carnitine (480%, P < 0.005). These findings were corroborated by raised carnitine palmitoyltransferase-2 activity (154%, P < 0.001) and mRNA levels (91%, P < 0.01) as well as by simultaneous elevation of hepatic peroxisomal acyl-CoA oxidase activity (144%, P < 0.01) and mRNA content (82%, P < 0.05). In contrast, hepatic carnitine palmitoyltransferase-1 activity remained unchanged despite a twofold increased mRNA level after n-3 PUFA feeding. Skeletal muscle FA oxidation was less affected by dietary n-3 PUFA, and the stimulatory effect was found only in peroxisomes. Dietary intake of n-3 PUFA was followed by increased acyl-CoA oxidase activity (48%, P < 0.05) and mRNA level (83%, P < 0.05) in skeletal muscle. The increased FA oxidation after n-3 PUFA supplementation of the high-fat diet was accompanied by lower plasma leptin concentration (-38%, P < 0.05) and leptin mRNA expression (-66%, P < 0.05) in retroperitoneal adipose tissue, and elevated hepatic mRNA level for the leptin receptor Ob-Ra (140%, P < 0.05). Supplementation of the high-fat diet with n-3 PUFA enhanced in vivo insulin sensitivity, as shown by normalization of the glucose infusion rate during euglycemic hyperinsulinemic clamp. Our results indicate that the hypotriglyceridemic effect of dietary n-3 PUFA is associated with stimulation of FA oxidation in the liver and to a smaller extent in skeletal muscle. This may ameliorate dyslipidemia, tissue lipid accumulation, and insulin action, in spite of decreased plasma leptin level and leptin mRNA in adipose tissue.

Profiling of hepatic gene expression in rats treated with fibric acid analogs

Peroxisome proliferator-activated receptors (PPARs) are a group of nuclear receptors whose ligands include fatty acids, eicosanoids and the fibrate class of drugs. In humans, fibrates are used to treat dyslipidemias. In rodents, fibrates cause peroxisome proliferation, a change that might explain the observed hepatomegaly. In this study, rats were treated with multiple dose levels of six fibric acid analogs (including fenofibrate) for up to two weeks. Pathological analysis identified hepatocellular hypertrophy as the only sign of hepatotoxicity, and only one compound at the highest dose caused any significant increase in serum ALT or AST activity. RNA profiling revealed that the expression of 1288 genes was related to dose or length of treatment and correlated with hepatocellular hypertrophy. This gene list included expression changes that were consistent with increased mitochondrial and peroxisomal beta-oxidation, increased fatty acid transport, increased hepatic uptake of LDL-cholesterol, decreased hepatic uptake of glucose, decreased gluconeogenesis and decreased glycolysis. These changes are likely linked to many of the clinical benefits of fibrate drugs, including decreased serum triglycerides, decreased serum LDL-cholesterol and increased serum HDL-cholesterol. In light of the fact that all six compounds stimulated similar or identical changes in the expression of this set of 1288 genes, these results indicate that hepatomegaly is due to PPARalpha activation, although signaling through other receptors (e.g. PPARgamma, RXR) or through non-receptor pathways cannot be excluded.

Effect of n-3 fatty acids on serum lipid levels and hepatic fatty acid metabolism in BALB/c.KOR-Apoeshl mice deficient in apolipoprotein E expression

N-3 fatty acids exert a potent serum lipid-lowering effect in rodents mainly by affecting hepatic fatty acid oxidation and synthesis. However, it has been observed that fish oil and docosahexaenoic acid ethyl ester do not lower serum lipid levels in apolipoprotein E (apoE)-knockout (Apoetm1Unc) mice generated by gene targeting. To test the hypothesis that apoE expression is required for n-3 fatty acid-dependent regulation of serum lipid levels and hepatic fatty acid metabolism, we examined the effect of fish oil and n-3 fatty acid ethyl esters on the activity and gene expression of hepatic enzymes involved in fatty acid oxidation and synthesis using an alternative apoE-deficient mouse model with the BALB/c genetic background (BALB/c.KOR-Apoeshl). ApoE-deficient mice were fed diets containing 9.4% palm oil, fish oil, or 5.4% palm oil and 1% EPA plus 3% DHA ethyl esters for 15 days. In contrast to the reported data on apoE-knockout mice, fish oil and n-3 fatty acid ethyl esters greatly decreased serum triacylglycerol, cholesterol, and phospholipid levels in the Apoeshl mice. The decreases were greater with fish oil than with ethyl esters. The alterations by dietary n-3 fatty acids of serum lipid levels were accompanied by parallel changes in the activity and mRNA levels of enzymes involved in hepatic fatty acid oxidation and synthesis. The reason for the discrepancy between the results of the current study and previous studies is unknown. However, our study at least indicates that a lack of apoE expression does not necessarily accompany deficits in the n-3 fatty acid-dependent regulation of serum lipid levels and hepatic fatty acid metabolism.

β-Oxidation of 18∶3n−3 in atlantic salmon (Salmo salarL.) hepatocytes treated with different fatty acids

To study whether Atlantic salmon beta-oxidation was affected by dietary FA composition, an in vitro study with primary hepatocytes was undertaken. Isolated hepatocyte cultures were stimulated with either 16:0, 18:1n-9, 18:2n-6, 18:3n-3, 20:5n-3, or 22:6n-3 in triplicate for 24 h. In addition, a control was included where no FA stimulation was performed, also in triplicate. After stimulation, radiolabeled [1-14C] 18:3n-3 was added and the cells were incubated for 2 h at 20 degrees C. The cells were then harvested, and radioactivity was determined in the acid-soluble part of the cells and medium, i.e., the end products of the beta-oxidation pathway. Specific beta-oxidation activity was significantly higher in hepatocytes stimulated with 18:3n-3. Further, when taking into account the amount of radiolabeled [1-14C]18:3n-3 taken up by the cells--the relative amount of beta-oxidized [1-14C]18:3n-3 of the total FA taken up by the hepatocytes-no significant differences were found. Thus, the regulation of beta-oxidation activity in the primary Atlantic salmon hepatocytes seems to be at the level of FA uptake and transport into the cell. This in vitro study shows that the catabolism processes in salmon hepatocytes are affected by the FA available and probably already regulated at the level of FA uptake.

Divergent effects of eicosapentaenoic and docosahexaenoic acid ethyl esters, and fish oil on hepatic fatty acid oxidation in the rat

The physiological activity of fish oil, and ethyl esters of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) affecting hepatic fatty acid oxidation was compared in rats. Five groups of rats were fed various experimental diets for 15 days. A group fed a diet containing 9.4% palm oil almost devoid of n-3 fatty acids served as a control. The test diets contained 4% n-3 fatty acids mainly as EPA and DHA in the form of triacylglycerol (9.4% fish oil) or ethyl esters (diets containing 4% EPA ethyl ester, 4% DHA ethyl ester, and 1% EPA plus 3% DHA ethyl esters). The lipid content of diets containing EPA and DHA ethyl esters was adjusted to 9.4% by adding palm oil. The fish oil diet and ethyl ester diets, compared to the control diet containing 9.4% palm oil, increased activity and mRNA levels of hepatic mitochondrial and peroxisomal fatty acid oxidation enzymes, though not 3-hydroxyacyl-CoA dehydrogenase activity. The extent of the increase was, however, much greater with the fish oil than with EPA and DHA ethyl esters. EPA and DHA ethyl esters, compared to the control diet, increased 3-hydroxyacyl-CoA dehydrogenase activity, but fish oil strongly reduced it. It is apparent that EPA and DHA in the form of ethyl esters cannot mimic the physiological activity of fish oil at least in affecting hepatic fatty acid oxidation in rat.

A review of the safety of DHA45-oil

Polyunsaturated fatty acids (PUFAs), such as docosahexaenoic acid (DHA), are natural constituents of the human diet; however, dietary intakes of these fatty acids are below recommended values. The main dietary source of DHA is fatty fish, with lesser amounts provided by shellfish, marine mammals, and organ meats. The addition to traditional food products of refined oils produced by marine microalgae represents potential sources of supplemental dietary DHA. DHA45-oil is manufactured through a multi-step fermentation and refining process using a non-toxigenic and non-pathogenic marine protist. Comprising approximately 45% DHA, and lesser concentrations of palmitic acid and docosapentaenoic acid, DHA45-oil is intended for use in foods as a dietary source of DHA. The safety of DHA45-oil was evaluated in various genotoxicity and acute, subchronic, and reproductive toxicity studies. DHA45-oil produced negative results in genotoxicity assays and demonstrated a low acute oral toxicity in mice and rats. Dietary administration of DHA45-oil to rats in subchronic and one-generation reproductive studies produced results consistent with those observed in oral studies using high concentrations of omega-3 PUFAs from fish or other microalgal-derived oils. The results of these studies, as well as those of various published metabolic, toxicological, and clinical studies with DHA-containing oils, support the safety of DHA45-oil as a potential dietary source of DHA.

A fish oil diet produces different degrees of suppression of apoB and triglyceride secretion in human apoB transgenic mouse strains

Human apolipoprotein B (apoB) transgenic (HuBTg) mouse strains were used to assess genetic effects on the response to fish oil (FO), a source of n-3 fatty acids. A congenic HuBTg strain of the C57BL/6 (B6) background and six F1 HuBTg strains were fed a FO for 2 weeks. Different responses of plasma lipid levels to FO were observed among these strains. In particular, plasma apoB levels changed minimally in FO-fed male B6 HuBTg mice, but increased markedly ( approximately 40%) in FO-fed male FVB/NJ (FVB) x B6 F1 HuBTg mice. These strain differences were determined mainly by hepatic apoB secretion rates and were likely regulated by posttranscriptional mechanisms. In addition, plasma triglyceride (TG) levels were reduced (14%) in FO-fed B6 mice, but not in FVB x B6 mice. These strain differences were determined mainly by TG secretion rates, but were not due to differences in hepatic lipogenesis. Hepatic mRNA levels of acyl-CoA oxidase, reflective of peroxisomal beta-oxidation rate, were increased in FO-fed B6 but not in FVB x B6 mice, which could account for the difference in TG secretion rates. In summary, differential effects of FO on plasma apoB and TG levels in B6 and FVB x B6 HuBTg mice were associated with strain differences in hepatic apoB and TG secretion and in peroxisomal beta-oxidation.

Cholesterol synthesis in mice is suppressed but lipofuscin formation is not affected by long-term feeding of n-3 fatty acid-enriched oils compared with lard and n-6 fatty acid-enriched oils

Hypocholesterolemic activity of dietary polyunsaturated fatty acids is observed after relatively short-term but not long-term feedings, and their long-term feedings are suspected to accelerate aging through tissue accumulation of lipid peroxides and age pigments (lipofuscin). To define the long-term effects of fats and oils in more detail, female mice were fed a conventional basal diet supplemented with lard (Lar), high-linoleic (n-6) safflower oil (Saf), rapeseed oil (Rap), high-alpha-linolenic (n-3) perilla oil (Per), or a mixture of ethyl docosahexaenoate and soybean oil (DHA/Soy) from 17 weeks to 71 weeks of age. The DHA/Soy and Per groups had decreased serum cholesterol levels compared with the Lar and Saf groups, but the difference between the Lar and Saf groups was not significant. The 3-hydroxy-3-methyglutary-CoA (HMG-CoA) reductase activity in the liver was also significantly lower in the Per and DHA/Soy groups. However, no significant difference in lipofuscin contents in the brain and liver was observed among the 5 dietary groups, despite significant differences in peroxidizability indices of the dietary and/or tissue lipids. These results indicate that n-3 fatty acid-rich oils are hypocholesterolemic by suppressing hepatic HMG-CoA reductase activity compared with animal fats and high-linoleic (n-6) oil, but tissue lipofuscin contents are not affected by a long-term feeding of fats and oils with different degree of unsaturation in mice.

Effects of fish oil in parenteral nutrition

OBJECTIVE

Fish oil is a rich source of omega-3 fatty acids (FAs), especially eicosapentaenoic acid and docosahexaenoic acid. The existing data suggest that eicosapentaenoic acid and docosahexaenoic acid are the active agents in fish oil. A number of clinical trials have shown that dietary fish oil supplementation has antiatherogenic properties and immunomodulation effects. Fish oils are not used widely in parenteral nutrition because fish oil emulsions have not been commercially available until very recently. Studies concerning the use of fish oil in parenteral route are rare.

METHODS

We reviewed the effect of parenteral fish oil infusion on lipid metabolism and immune response in normal and disease conditions.

RESULTS

Studies showed that the main effects of parenteral infusion of fish oil are: 1) incorporation of omega-3 FAs into cellular membranes of many cell populations that consequently influence the disease process of some disease conditions, 2) an effect on eicosanoid metabolism leading to a decrease in platelet aggregation and thrombosis, 3) amelioration of the severity of diet-induced hepatic steatosis, 4) less accumulation of lipid peroxidation products in liver tissue, and 5) immunomodulation effects and therapeutic benefits in animal disease models or various disease conditions of humans. Most of these studies suggested that parenteral infusion of omega-3 FAs have clinical beneficial effects comparable to those of dietary administration. However, different effects of omega-3 and omega-6 FAs in some situations has been reported. For example, plasma triacylglycerol levels were not lowered after fish oil infusion in normal or diabetic rats when compared with those of safflower oil or soybean oil infusion. The reason for the difference remain unclear.

CONCLUSION

The metabolic and immunologic effects of parenteral use of omega-3 FAs requires further evaluation, especially in some disease conditions.

Omega-3 fatty acid supplementation accelerates chylomicron triglyceride clearance

Omega-3 fatty acids (FAs) reduce postprandial triacylglycerol (TG) concentrations. This study was undertaken to determine whether this effect was due to reduced production or increased clearance of chylomicrons. Healthy subjects (n = 33) began with a 4-week, olive oil placebo (4 g/d) run-in period. After a 4-week wash-out period, subjects were randomized to supplementation with 4 g/d of ethyl esters of either safflower oil (SAF), eicosapentaenoic acid (EPA), or docosahexaenoic acid (DHA) for 4 weeks. Results for EPA and DHA were similar, and therefore the data were combined into one omega-3 FA group. Omega-3 FA supplementation reduced the postprandial TG and apolipoprotein B (apo B)-48 and apoB-100 concentrations by 16% (P = 0.08), 28% (P < 0.001), and 24% (P < 0.01), respectively. Chylomicron TG half-lives in the fed state were reduced after omega-3 FA treatment (6.0 +/- 0.5 vs. 5.1 +/- 0.4 min; P < 0.05), but not after SAF (6.9 +/- 0.7 vs. 7.1 +/- 0.7 min). Omega-3 FA supplementation decreased chylomicron particle sizes (mean diameter; 293 +/- 44 vs. 175 +/- 25 nm; P < 0.01) and increased preheparin lipoprotein lipase (LPL; 0.6 +/- 0.1 vs. 0.9 +/- 0.1 micromol/h/ml; P < 0.05) activity during the fed state, but had no effect on postheparin LPL or hepatic lipase activities. The results suggest that omega-3 FA supplementation accelerates chylomicron TG clearance by increasing LPL activity, and that EPA and DHA are equally effective.

Diets rich in polyunsaturated fatty acids: effect on hepatic metabolism in rats

OBJECTIVE

We investigated the effect of diets rich in omega-6 and omega-3 fatty acids on hepatic metabolism.

METHODS

Male Wistar rats, just weaned, were fed ad libitum for 8 wk with one of the following diets: rat chow (C), rat chow containing 15% (w/w) soybean oil (S), rat chow containing 15% (w/w) fish oil (F), and rat chow containing 15% soy bean and fish oil (SF; 5:1, w/w). Casein was added to the fatty diets to achieve the same content of protein (20%) as the control chow. The rats were killed by decapitation, and the hepatic tissue was removed and weighed. Tissue lipid, glycogen, and protein content, in vivo lipogenesis rate, and adenosine triphosphate citrate lyase and malic enzyme activities were evaluated. Plasma total lipids, triacylglycerol, and cholesterol concentrations were assessed.

RESULTS

Body weight gain was higher in F and SF than in C and S rats. Liver weight, lipid content, and lipogenesis rate increased in F and SF rats, although adenosine triphosphate citrate lyase activity decreased. Glycogen concentration decreased in S, F, and SF rats compared with C rats. Plasma total lipids and triacylglycerol concentrations were lower in F and SF than in C rats. Total and high-density lipoprotein cholesterol (HDL-C) plasma levels decreased in F rats, with maintenance of the total:HDL-C ratio. In SF rats, an increase in HDL-C led to a lower total:HDL-C ratio.

CONCLUSIONS

These results indicated that an enrichment of the diet with omega-3 polyunsaturated fatty acids produces hypolipidemia but may cause changes in liver metabolism that favor lipid deposition. They also suggested that the addition of a small amount of eicosapentaenoic and docosahexaenoic polyunsaturated fatty acids to an omega-6-rich diet further improve the circulating lipid profile, in comparison with an omega-3-rich diet, but it does not prevent excess liver lipid accumulation.

An increase in peroxisomal fatty acid oxidation is not sufficient to prevent tissue lipid accumulation in hHTg rats

We observed earlier that increased skeletal muscle lipid content in the hereditary hypertriglyceridemic (hHTg) rat is accompanied by a decline in plasma leptin. Leptin has recently been shown to enhance peripheral insulin sensitivity by decreasing the tissue triglyceride accumulation, possibly through regulation of fatty acid oxidation and lipogenesis. Thus, to test the hypothesis that insulin resistance and increased skeletal muscle lipid accumulation in hHTg rats are due to a defect in lipid catabolism, we measured mitochondrial and peroxisomal fatty acid oxidation and malonyl-CoA and acetyl-CoA carboxylase-2 content in skeletal muscles of these animals. In addition, we investigated possible molecular mechanisms responsible for the lower leptin levels in hHTg rats by measuring leptin and leptin-receptor (Ob-Ra) mRNA levels. We found the following: (1) in spite of a higher skeletal muscle malonyl-CoA content and an increased sensitivity of carnitine palmitoyltransferase-1 to malonyl-CoA, carnitine palmitoyltransferase-1 activity in muscle of hHTg rats was normal; (2) increased peroxisomal fatty acid oxidation did not seem to be sufficient to prevent the tissue lipid accumulation in these animals; (3) both lower leptin production by white adipose tissue and increased leptin uptake seem to be responsible for lower circulating leptin levels and therefore lower fatty acid catabolism.

Comparative bioavailability of dietary alpha-linolenic and docosahexaenoic acids in the growing rat

Animal and human studies have indicated that developing mammals fed only alpha-linolenic acid (18:3n-3) have lower docosahexaenoic acid (22:6n-3) content in brain and tissue phospholipids when compared with mammals fed 18:3n-3 plus 22:6n-3. The aim of this study was to test the hypothesis that low bioavailability of dietary 18:3n-3 to be converted to 22:6n-3 could partly explain this difference in fatty acid accretion. For that purpose, we determined the partitioning of dietary 18:3n-3 and 22:6n-3 between total n-3 fatty acid body accumulation, excretion, and disappearance (difference between the intake and the sum of total n-3 fatty acids accumulated and excreted). This was assessed using the quantitative method of whole-body fatty acid balance in growing rats fed the same amount of a 5% fat diet supplying either 18:3n-3 or 22:6n-3 at a level of 0.45% of dietary energy (i.e., 200 mg/100 g diet). We found that 58.9% of the total amount of 18:3n-3 ingested disappeared, 0.4% was excreted in feces, 21.2% accumulated as 18:3n-3 (50% in total fats and 46% in the carcass-skin compartment), and 17.2% accumulated as long-chain derivatives (14% as 22:6n-3 and 3.2% as 20:5n-3 + 22:5n-3). Similar results were obtained from the docosahexaenoate balance (as % of the total amount ingested): disappearance, 64.5%; excretion, 0.5%; total accumulation, 35% with 30.1% as 22:6n-3. Thus, rats fed docosahexaenoate accumulated a twofold higher amount of 22:6n-3, which was mainly deposited in the carcass-skin compartment (68%). Similar proportions of disappearance of dietary 18:3n-3 and 22:6n-3 lead us to speculate that these two n-3 polyunsaturated fatty acids were beta-oxidized in the same amount.

The biochemistry of hypo- and hyperlipidemic fatty acid derivatives: metabolism and metabolic effects

A selection of amphipatic hyper- and hypolipidemic fatty acid derivatives (fibrates, thia- and branched chain fatty acids) are reviewed. They are probably all ligands for the peroxisome proliferation activation receptor (PPARalpha) which has a low selectivity for its ligands. These compounds give hyper- or hypolipidemic responses depending on their ability to inhibit or stimulate mitochondrial fatty acid oxidation in the liver. The hypolipidemic response is explained by the following metabolic effects: Lipoprotein lipase is induced in liver where it is normally not expressed. Apolipoprotein CIII is downregulated. These two effects in liver lead to a facilitated (re)uptake of chylomicrons and VLDL, thus creating a direct transport of fatty acids from the gut to the liver. Fatty acid metabolizing enzymes in the liver (CPT-I and II, peroxisomal and mitochondrial beta-oxidation enzymes, enzymes of ketogenesis, and omega-oxidation enzymes) are induced and create an increased capacity for fatty acid oxidation. The increased oxidation of fatty acids "drains" fatty acids from the body, reduces VLDL formation, and ultimately explains the antiadiposity and improved insulin sensitivity observed after administration of peroxisome proliferators.

Comparative effects of perilla and fish oils on the activity and gene expression of fatty acid oxidation enzymes in rat liver

The activity and mRNA level of hepatic enzymes in fatty acid oxidation and synthesis were compared in rats fed diets containing either 15% saturated fat (palm oil), safflower oil rich in linoleic acid, perilla oil rich in alpha-linolenic acid or fish oil rich in eicosapentaenoic (EPA) and docosahexaenoic acids (DHA) for 15 days. The mitochondrial fatty acid oxidation rate was 50% higher in rats fed perilla and fish oils than in the other groups. Perilla and fish oils compared to palm and safflower oils approximately doubled and more than tripled, respectively, peroxisomal fatty acid oxidation rate. Compared to palm and safflower oil, both perilla and fish oils caused a 50% increase in carnitine palmitoyltransferase I activity. Dietary fats rich in n-3 fatty acids also increased the activity of other fatty acid oxidation enzymes except for 3-hydroxyacyl-CoA dehydrogenase. The extent of the increase was greater with fish oil than with perilla oil. Interestingly, both perilla and fish oils decreased the activity of 3-hydroxyacyl-CoA dehydrogenase measured using short- and medium-chain substrates. Compared to palm and safflower oils, perilla and fish oils increased the mRNA level of many mitochondrial and peroxisomal enzymes. Increases were generally greater with fish oil than with perilla oil. Fatty acid synthase, glucose-6-phosphate dehydrogenase, and pyruvate kinase activity and mRNA level were higher in rats fed palm oil than in the other groups. Among rats fed polyunsaturated fats, activities and mRNA levels of these enzymes were lower in rats fed fish oil than in the animals fed perilla and safflower oils. The values were comparable between the latter two groups. Safflower and fish oils but not perilla oil, compared to palm oil, also decreased malic enzyme activity and mRNA level. Examination of the fatty acid composition of hepatic phospholipid indicated that dietary alpha-linolenic acid is effectively desaturated and elongated to form EPA and DHA. Dietary perilla oil and fish oil therefore exert similar physiological activity in modulating hepatic fatty acid oxidation, but these dietary fats considerably differ in affecting fatty acid synthesis.

cDNA cloning and analysis of tissue-specific expression of mouse peroxisomal straight-chain acyl-CoA oxidase

Straight-chain acyl-CoA oxidase is the first and rate limiting enzyme in the peroxisomal beta-oxidation pathway catalysing the desaturation of acyl-CoAs to 2-trans-enoyl-CoAs, thereby producing H2O2. To study peroxisomal beta-oxidation we cloned and characterized the cDNA of mouse peroxisomal acyl-CoA oxidase. It consists of 3778 bp, including a 1983-bp ORF encoding a polypeptide of 661 amino-acid residues. Like the rat and human homologue the C-terminus contains an SKL motif, an import signal present in several peroxisomal matrix proteins. Sequence analysis revealed high amino-acid homology with rat (96%) and human (87%) acyl-CoA oxidase in addition to minor homology ( approximately 40%) with other related proteins, such as rabbit trihydroxy-cholestanoyl-CoA oxidase, human branched chain acyl-CoA oxidase and rat trihydroxycoprostanoyl-CoA oxidase. Acyl-CoA oxidase mRNA and protein expression were most abundant in liver followed by kidney, brain and adipose tissue. During mouse brain development acyl-CoA oxidase mRNA expression was highest during the suckling period indicating that peroxisomal beta-oxidation is most critical during this developmental stage. Comparing tissue mRNA levels of peroxisome proliferator-activated receptor alpha and acyl-CoA oxidase, we noticed a constant relationship in all tissues investigated, except heart and adipose tissue in which much more, and respectively, much less, peroxisome proliferator-activated receptor alpha mRNA in proportion to acyl-CoA oxidase mRNA was found. Our data show that acyl-CoA oxidase is an evolutionary highly conserved enzyme with a distinct pattern of expression and indicate an important role in lipid metabolism.

Dietary docosahexaenoic acid suppresses inflammation and immunoresponses in contact hypersensitivity reaction in mice

This study was designed to examine the immunomodulatory effects of dietary docosahexaenoic acid (DHA) in the absence of eicosapentaenoic acid (EPA). We investigated the effects of feeding dietary DHA ethyl ester (DHA-Et) (97% pure) at levels of 4.8 wt% of the total diet and of feeding EPA ethyl ester (EPA-Et) (99% pure) at 4.8 wt% on the inflammatory response in the challenge phase of the contact hypersensitivity reaction (CHR) in the ears of mice sensitized with 2,4-dinitro-1-fluorobenzene (DNFB). The effect of DHA-Et on T lymphocytes at the CHR site was examined using anti-CD4 antibodies. Furthermore, we examined the cytokines formed at the CHR site on the mRNA level. It was found that 24 h after the challenge, DHA-Et but not EPA-Et reduced the ear swelling. Infiltration of inflammatory cells, in particular, CD4-positive T lymphocytes, into the ears in the challenge phase of CHR was observed. DHA-Et reduced the infiltration of CD4-positive T lymphocytes into the ears. DHA-Et also decreased the expression of interferon-gamma, interleukin (IL)-6, IL-1beta, and IL-2 mRNA in ears. These observations suggest that DHA, but not EPA, may exert an antiinflammatory and immunosuppressive effect. The immunosuppressive effectiveness of fish oil may be attributed mainly to DHA.

Effect of dietary alpha-linolenic acid on the activity and gene expression of hepatic fatty acid oxidation enzymes

The activities of hepatic fatty acid oxidation enzymes in rats fed linseed and perilla oils rich in alpha-linolenic acid (alpha-18:3) were compared with those in the animals fed safflower oil rich in linoleic acid (18:2) and saturated fats (coconut or palm oil). Mitochondrial and peroxisomal palmitoyl-CoA (16:0-CoA) oxidation rates in the liver homogenates were significantly higher in rats fed linseed and perilla oils than in those fed saturated fats and safflower oil. The fatty oxidation rates increased as dietary levels of alpha-18:3 increased. Dietary alpha-18:3 also increased the activity of fatty acid oxidation enzymes except for 3-hydroxyacyl-CoA dehydrogenase. Unexpectedly, dietary alpha-18:3 caused great reduction in the activity of 3-hydroxyacyl-CoA dehydrogenase measured with short- and medium-chain substrates but not with long-chain substrate. Dietary alpha-18:3 significantly increased the mRNA levels of hepatic fatty acid oxidation enzymes including carnitine palmitoyltransferase I and II, mitochondrial trifunctional protein, acyl-CoA oxidase, peroxisomal bifunctional protein, mitochondrial and peroxisomal 3-ketoacyl-CoA thiolases, 2, 4-dienoyl-CoA reductase and delta3, delta2-enoyl-CoA isomerase. Fish oil rich in very long-chain n-3 fatty acids caused similar changes in hepatic fatty acid oxidation. Regarding the substrate specificity of beta-oxidation pathway, mitochondrial and peroxisomal beta-oxidation rate of alpha-18:3-CoA, relative to 16:0- and 18:2-CoAs, was higher irrespective of the substrate/albumin ratios in the assay mixture or dietary fat sources. The substrate specificity of carnitine palmitoyltransferase I appeared to be responsible for the differential mitochondrial oxidation rates of these acyl-CoA substrates. Dietary fats rich in alpha-18:3-CoA relative to safflower oil did not affect the hepatic activity of fatty acid synthase and glucose 6-phosphate dehydrogenase. It was suggested that both substrate specificities and alterations in the activities of the enzymes in beta-oxidation pathway play a significant role in the regulation of the serum lipid concentrations in rats fed alpha-18:3.

Effects of eicosapentaenoic acid and docosahexaenoic acid on plasma membrane fluidity of aortic endothelial cells

We investigated the relative effects of n-3 eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3) on the plasma membrane fluidity of endothelial cells (EC) cultured from the thoracic aorta by determining fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH) and its cationic derivative trimethylamino-DPH (TMA-DPH). Fluidity assessed by TMA-DPH demonstrated no significant differences in plasma membranes of vehicle (dimethyl sulfoxide; DMSO)-, EPA-, and DHA-treated EC. Plasma membrane fluidity assessed by DPH polarization, however, was significantly higher in the order of DHA > EPA > DMSO. Total cholesterol content decreased significantly by 28.4 and 15.9% in the plasma membranes of DHA- and EPA-treated cells, respectively. Total phospholipid content remained unaltered in the plasma membranes of the three groups of cells; however, the molar ratio of total cholesterol to phospholipid decreased significantly only in the membranes of DHA-treated EC. The unsaturation index in the plasma membranes of EPA- and DHA-treated cells increased by 35.7 and 64.3%, respectively, compared with that in the plasma membranes of control cells. The activities of catalase and glutathione peroxidase in the whole-cell homogenates, and levels of lipid peroxides in either the whole-cell homogenates or in plasma membrane fractions were not altered in EPA- or DHA-treated EC. These results indicate that the influence of DHA is greater than that of EPA in increasing plasma membrane fluidity of vascular EC. We speculate that the greater effect of DHA compared to EPA is due to its greater ability to decrease membrane cholesterol content or the cholesterol/phospholipid molar ratio, or both, and also to its greater ability in elevating the unsaturation index in the plasma membranes of EC.

Methylated eicosapentaenoic acid and tetradecylthioacetic acid: effects on fatty acid metabolism

We introduced methyl or ethyl groups to the 2- or 3-position of the eicosapentaenoic acid (EPA) molecule to investigate whether the branching of EPA could influence its hypolipidemic effect in rats. The most effective branching involved two methyl groups in the 2-position and one methyl group in the 3-position. These EPA derivatives increased hepatic mitochondrial and peroxisomal beta-oxidation and decreased plasma lipids concomitant with suppressed acetyl-coenzyme A (CoA) carboxylase (EC 6.4.1.2) and fatty acid synthase (EC 2.3.1.85) activities. This was followed by elevated activities of camitine O-palmitoyltransferase (EC 2.3.1.21) and possibly 2,4-dienoyl-CoA reductase (EC 1.3.1.34), as well as induced mRNA levels of these enzymes and fatty acyl-CoA oxidase. The fatty acid composition in liver changed, with an increased 18:1 n-9 content, whereas the expression of delta9-desaturase remained unchanged. We investigated the flux of fatty acids in cultured hepatocytes, and found that oxidation of [1-14C]-labeled palmitic acid increased but the secretion of palmitic acid-labeled triglycerides decreased after addition of 2-methyl-EPA. The fatty acyl-CoA oxidase (EC 1.3.3.6) activity in these cells remained unchanged. A significant negative correlation was obtained between palmitic acid oxidation and palmitic acid-labeled synthesized triglycerides. To investigate whether the hypolipidemic effect occurred independently of induced peroxisomal beta-oxidation, we fed rats 2-methyl-tetradecylthioacetic acid. This compound increased the peroxisomal but not the mitochondrial beta-oxidation, and the plasma lipid levels were unchanged. In conclusion, EPA methylated in the 2- or 3-position renders it more potent as a hypolipidemic agent. Furthermore, this study supports the hypothesis that the mitochondrion is the primary site for the hypolipidemic effect.

Sesamin, a sesame lignan, is a potent inducer of hepatic fatty acid oxidation in the rat

The effects of sesamin, one of the most abundant lignans in sesame seed, on hepatic fatty acid oxidation were examined in rats that were fed experimental diets containing various amounts (0%, 0.1%, 0.2%, and 0.5%) of sesamin (a 1:1 mixture of sesamin and episesamin) for 15 days. Dietary sesamin dose-dependently increased both mitochondrial and peroxisomal palmitoyl-coenzyme A (CoA) oxidation rates. Mitochondrial activity almost doubled in rats on the 0.5% sesamin diet. Peroxisomal activity increased more than 10-fold in rats fed a 0.5% sesamin diet in relation to rats on the sesamin-free diet. Dietary sesamin greatly increased the hepatic activity of fatty acid oxidation enzymes, including carnitine palmitoyltransferase, acyl-CoA dehydrogenase, acyl-CoA oxidase, 3-hydroxyacyl-CoA dehydrogenase, enoyl-CoA hydratase, and 3-ketoacyl-CoA thiolase. Dietary sesamin also increased the activity of 2,4-dienoyl-CoA reductase and delta3,delta2-enoyl-CoA isomerase, enzymes involved in the auxiliary pathway for beta-oxidation of unsaturated fatty acids dose-dependently. Examination of hepatic mRNA levels using specific cDNA probes showed a sesamin-induced increase in the gene expression of mitochondrial and peroxisomal fatty acid oxidation enzymes. Among these various enzymes, peroxisomal acyl-CoA oxidase and bifunctional enzyme gene expression were affected most by dietary sesamin (15- and 50-fold increase by the 0.5% dietary level). Sesamin-induced alterations in the activity and gene expression of carnitine palmitoyltransferase I and acyl-CoA oxidase were in parallel with changes in the mitochondrial and peroxisomal palmitoyl-CoA oxidation rate, respectively. In contrast, dietary sesamin decreased the hepatic activity and mRNA abundance of fatty acid synthase and pyruvate kinase, the lipogenic enzymes. However, this lignan increased the activity and gene expression of malic enzyme, another lipogenic enzyme. An alteration in hepatic fatty acid metabolism may therefore account for the serum lipid-lowering effect of sesamin in the rat.

Eicosapentaenoic and docosahexaenoic acid affect mitochondrial and peroxisomal fatty acid oxidation in relation to substrate preference

Decreased triacylglycerol synthesis within hepatocytes due to decreased diacylglycerol acyltransferase (DGAT) activity has been suggested to be an important mechanism by which diets rich in fish oil lower plasma triacylglycerol levels. New findings suggest that eicosapentaenoic acid (EPA), and not docosahexaenoic acid (DHA), lowers plasma triacylglycerol by increased mitochondrial fatty acid oxidation and decreased availability of fatty acids for triacylglycerol synthesis. To contribute to the understanding of the triacylglycerol-lowering mechanism of fish oil, the different metabolic properties of EPA and DHA were studied in rat liver parenchymal cells and isolated rat liver organelles. EPA-CoA was a poorer substrate than DHA-CoA for DGAT in isolated rat liver microsomes, and in the presence of EPA, a markedly lower value for the triacyl[3H]glycerol/diacyl[3H]glycerol ratio was observed. The distribution of [1-14C]palmitic acid was shifted from incorporation into secreted glycerolipids toward oxidation in the presence of EPA (but not DHA) in rat liver parenchymal cells. [1-14C]EPA was oxidized to a much greater extent than [1-14C]DHA in rat liver parenchymal cells, isolated peroxisomes, and especially in purified mitochondria. As the oxidation of EPA was more effective and sensitive to the CPT-I inhibitor, etomoxir, when measured in a combination of both mitochondria and peroxisomes, we hypothesized that both are involved in EPA oxidation, whereas DHA mainly is oxidized in peroxisomes. In rats, EPA treatment lowered plasma triacylglycerol and increased hepatic mitochondrial fatty acid oxidation and carnitine palmitoyltransferase (CPT)-I activity in both the presence and absence of malonyl-CoA. Whereas only EPA treatment increased the mRNA levels of CPT-I, DHA treatment increased the mRNA levels of peroxisomal fatty acyl-CoA oxidase and fatty acid binding protein more effectively than EPA treatment. In conclusion, EPA and DHA affect cellular organelles in relation to their substrate preference. The present study strongly supports the hypothesis that EPA, and not DHA, lowers plasma triacylglycerol by increased mitochondrial fatty acid oxidation.

Selectivity of fatty acids on lipid metabolism and gene expression

Triacylglycerols represent the main form of storage for a wide spectrum of fatty acids. Their utilization first involves mobilization from adipose tissue through lipolysis. The release of individual fatty acids from adipose tissue is selective in vitro and in vivo in animal studies and also in human subjects. Generally, fatty acids are more readily mobilized from fat cells when they are short-chain and unsaturated. This selectivity could affect the storage of individual fatty acids in adipose tissue, and their subsequent supply to tissues. The nature of the dietary fats could affect lipid homeostasis and body fat deposition. Dietary fish oil influences adipose tissue development in a site-specific manner as a function of diet and feeding period. A diet high in n-3 polyunsaturated fatty acids (PUFA) results in a preferential partitioning of ingested energy towards oxidation at the expense of storage. Fatty acids are important mediators of gene expression in the liver. Indeed, genes encoding both glycolytic and lipogenic enzymes and key metabolic enzymes involved in fatty acid oxidation are regulated by dietary PUFA. White adipose tissue could also be a target for PUFA control of gene expression. The treatment of pre-adipose cells by fatty acids induces the expression of numerous genes that encode proteins involved in fatty acid metabolism. The mechanisms of PUFA-mediated repression of gene expression in adipocytes seem to be different, at least partly, from those described in liver. Tissue-specific and site-specific factors are possibly involved in the specific effect of PUFA on gene expression, although other mechanisms cannot be excluded.