On the effect of peroxisomal beta-oxidation and carnitine palmitoyltransferase activity by eicosapentaenoic acid in liver and heart from rats

Down-regulation in muscle and liver lipogenic genes: EPA ethyl ester treatment in lean and overweight (high-fat-fed) rats

The precise mechanisms by which omega-3 fatty acids improve fat metabolism are not completely understood. This study was designed to determine the effects of eicosapentaenoic acid (EPA) ethyl ester administration on the expression levels of several muscle, liver and adipose tissue genes involved in lipogenesis and fatty acid oxidation pathways. Male Wistar rats fed a standard diet (control animals) or a high-fat diet were treated daily by oral gavage with EPA ethyl ester (1g/kg) for 5 weeks. The high-fat diet caused a very significant increase in plasma cholesterol (P<.01) levels, which was reverted by EPA (P<.001). A significant decrease in circulating triglyceride levels (P<.05) was also observed in EPA-treated groups. EPA administration induced a significant down-regulation in some lipogenic genes such as muscle acetyl CoA carboxylase beta (ACC beta) (P<.05) and liver fatty acid synthase (FAS) (P<.05). Furthermore, a decrease in glucokinase (GK) gene expression was observed in EPA-treated animals fed a control diet (P<.01), whereas a significant increase in GK mRNA levels was found in groups fed a high-fat diet. On the other hand, no alterations in genes involved in beta-oxidation, such acetyl CoA synthase 4 (ACS4), acetyl CoA synthase 5 (ACS5) or acetyl CoA oxidase (ACO), were found in EPA-treated groups. Surprisingly and opposite to the expectations, a very significant decrease in the expression levels of liver PPARalpha (P<.01) was observed after EPA treatment. These findings show the ability of EPA ethyl ester treatment to down-regulate some genes involved in fatty acid synthesis without affecting the transcriptional activation of beta-oxidation-related genes.

The effect of supplementation with n-3 fatty acids on the physical performance in subjects with spinal cord injury

A global physical evaluation was performed in 21 males with spinal cord injury (SCI), at the beginning and at three and six months of omega-3 fatty acid (FA) supplementation. A significant increase in the proportion of eicosapentaenoic acid and docosahexanoic acid in plasma was observed in response to the supplementation (p<0.05). After six months of FA supplementation, strength endurance time increased from 127.7+/-19.0 s to 215.2+/-45.6 s in the right arm, and from 139+/-27.6 s to 237.7+/-48.7 s, in the left arm. The time to perform 20 repetitions of 70% maximum workload showed a reduction of 41% between the first and the third test. The time taken to cover a 90 meter long track, with a 6% slope, was reduced from 66.9+/-8.0 s to 59.3+/-6.7 s, at the end of the study (p<0.05). In conclusion, omega-3 FA supplementation could contribute to improve the functional capabilities in SCI subjects.

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.

Stimulation of acyl-CoA oxidase by α-linolenic acid-rich perilla oil lowers plasma triacylglycerol level in rats

The effect of dietary polyunsaturated fatty acids (PUFA) on hepatic peroxisomal oxidation was investigated with respect to the postprandial triacylglycerol levels. Male Sprague--Dawley rats were fed semipurified diets containing either 1% (w/w) corn oil, or 10% each of beef tallow, corn oil, perilla oil, and fish oil for 4 weeks and 4 days. Hepatic and plasma triacylglycerol levels were reduced in rats fed fish and perilla oil diets compared with corn oil and beef tallow diets. The peroxisomal beta-oxidation, catalase activity, and acyl-CoA oxidase (AOX) activity were markedly increased by fish oil feeding. To a lesser extent, perilla oil elevated AOX activity in a 4-day feeding although the effect gradually decreased in a 4-week feeding. Similarly, the mRNA levels were increased in rats fed fish and perilla oils. AOX activity was negatively correlated with postprandial triacylglycerol levels. In addition, the stimulation of AOX was highly associated with the content of long chain n-3 PUFA such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in hepatic microsome. These effects were evident within 4 days of initiating feeding. Therefore, alpha-linolenic perilla oil exerts a similar effect to fish oil in stimulating hepatic activity and gene expression of AOX by enriching long chain n-3 PUFA in hepatic membrane fraction, which can partly account for the reduction of postprandial triglyceridemia.

Effect of 18∶1n−9, 20∶5n−3, and 22∶6n−3 on lipid accumulation and secretion by atlantic salmon hepatocytes

We have studied the effects of dietary FA on the accumulation and secretion of [3H]glycerolipids by salmon hepatocytes in culture. Atlantic salmon were fed diets supplemented with either 100% soybean oil (SO) or 100% fish oil (FO), and grew from an initial weight of 113 +/- 5 g to a final weight of 338 +/- 19 g. Hepatocytes were isolated from both dietary groups and incubated with [3H]glycerol in an FA-free medium; a medium supplemented with 0.75 mM of one of three FA-18:1 n-9, 20:5n-3, or 22:6n-3--or a medium supplemented with 0.75 mM of the sulfur-substituted FA analog tetradecylthioacetic acid (TTA), which cannot undergo beta-oxidation. Incubations were allowed to proceed for 1, 2, 6, or 24 h. The rate of the secretion of radioactive glycerolipids with no FA added was 36% lower from hepatocytes isolated from fish fed the FO diet than it was from hepatocytes isolated from fish fed the SO diet. Hepatocytes incubated with 18:1 n-9 secreted more [3H]TAG than when incubated with no FA, whereas hepatocytes incubated with 20:5n-3 or TTA secreted less labeled TAG than when incubated with no FA. This observation was independent of the feeding group. Hepatocytes incubated with 22:6n-3 secreted the highest amounts of total [3H]glycerolipids compared with the other treatments, owing to increased secretion of phospholipids and mono- and diacylglycerols (MDG). In contrast, the same amounts of [3H]TAG were secreted from these cells as from cells incubated in an FA-free medium. The lipid-lowering effect of FO is thus independent of 22:6n-3, showing that 20:5n-3 is the FA that is responsible for the lipid-lowering effect. The ratio of TAG to MDG in lipids secreted from hepatocytes to which 20:5n-3 or TTA had been added was lower than that in lipids secreted from hepatocytes incubated with 18:1 n-9 or 22:6n-3, suggesting that the last step in TAG synthesis was inhibited. Morphometric measurements revealed that hepatocytes incubated with 20:5n-3 accumulated significantly more cellular lipid than cells treated with 18:1n-9, 22:6n-3, TTA, or no treatment. The area occupied by mitochondria was also greater in these cells. The present study shows that dietary FO reduces TAG secretion from salmon hepatocytes and that 20:5n-3 mediates this effect.

Identification of a novel putative non-selenocysteine containing phospholipid hydroperoxide glutathione peroxidase (NPGPx) essential for alleviating oxidative stress generated from polyunsaturated fatty acids in breast cancer cells

A dramatic reduction in the expression of a novel phospholipid hydroperoxide glutathione peroxidase (PHGPx), which incorporates cysteine instead of selenocysteine in the conserved catalytic motif was observed in a microarray analysis using cDNAs amplified from mRNA of Brca1-null mouse embryonic fibroblasts. This non-selenocysteine PHGPx named NPGPx is a cytoplasmic protein with molecular mass of approximately 22 kDa and has little detectable glutathione peroxidase activity in vitro. Ectopic expression of NPGPx in Brca1-null cells that were sensitive to oxidative stress induced by hydrogen peroxide conferred a similar resistance level to that of the wild-type cells, suggesting the importance of this protein in reducing oxidative stress. Expression of NPGPx was found in many tissues, including developing mammary gland. However, the majority of breast cancer cell lines studied (11 of 12) expressed very low or undetectable levels of NPGPx irrespective of BRCA1 status. Re-expression of NPGPx in breast cancer lines, MCF-7 and HCC1937, which have very little or no endogenous NPGPx, induced resistance to eicosapentaenoic acid (an omega-3 type of polyunsaturated fatty acid)-mediated cell death. Conversely, inhibition of the expression of NPGPx by the specific small interfering RNA in HS578T breast cancer cells that originally express substantial amounts of endogenous NPGPx increased their sensitivity to eicosapentaenoic acid-mediated cell death. Thus, NPGPx plays an essential role in breast cancer cells in alleviating oxidative stress generated from polyunsaturated fatty acid metabolism.

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.

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.

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.

Metabolic flux analysis of cultured hepatocytes exposed to plasma

Hepatic metabolism can be investigated using metabolic flux analysis (MFA), which provides a comprehensive overview of the intracellular metabolic flux distribution. The characterization of intermediary metabolism in hepatocytes is important for all biotechnological applications involving liver cells, including the development of bioartificial liver (BAL) devices. During BAL operation, hepatocytes are exposed to plasma or blood from the patient, at which time they are prone to accumulate intracellular lipids and exhibit poor liver-specific functions. In a prior study, we found that preconditioning the primary rat hepatocytes in culture medium containing physiological levels of insulin, as opposed to the typical supraphysiological levels found in standard hepatocyte culture media, reduced lipid accumulation during subsequent plasma exposure. Furthermore, supplementing the plasma with amino acids restored hepatospecific functions. In the current study, we used MFA to quantify the changes in intracellular pathway fluxes of primary rat hepatocytes in response to low-insulin preconditioning and amino acid supplementation. We found that culturing hepatocytes in medium containing lower physiological levels of insulin decreased the clearance of glucose and glycerol with a concomitant decrease in glycolysis. These findings are consistent with the general notion that low insulin, especially in the presence of high glucagon levels, downregulates glycolysis in favor of gluconeogenesis in hepatocytes. The MFA model shows that, during subsequent plasma exposure, low-insulin preconditioning upregulated gluconeogenesis, with lactate as the primary precursor in unsupplemented plasma, with a greater contribution from deaminated amino acids in amino acid-supplemented plasma. Concomitantly, low-insulin preconditioning increased fatty acid oxidation, an effect that was further enhanced by amino acid supplementation to the plasma. The increase in fatty acid oxidation reduced intracellular triglyceride accumulation. Overall, these findings are consistent with the notion that the insulin level in medium culture presets the metabolic machinery of hepatocytes such that it directly impacts on their metabolic behavior during subsequent plasma culture.

Antioxidants in exercise nutrition

Physical exercise may be associated with a 10- to 20-fold increase in whole body oxygen uptake. Oxygen flux in the active peripheral skeletal muscle fibres may increase by as much as 100- to 200-fold during exercise. Studies during the past 2 decades suggest that during strenuous exercise, generation of reactive oxygen species (ROS) is elevated to a level that overwhelms tissue antioxidant defence systems. The result is oxidative stress. The magnitude of the stress depends on the ability of the tissues to detoxify ROS, that is, antioxidant defences. Antioxidants produced by the body act in concert with their exogenous, mainly dietary, counterparts to provide protection against the ravages of reactive oxygen as well as nitrogen species. Antioxidant supplementation is likely to provide beneficial effects against exercise-induced oxidative tissue damage. While universal recommendations specifying types and dosages of antioxidants are difficult to make, it would be prudent for competitive athletes routinely engaged in strenuous exercise to seek an estimate of individual requirement. A new dimension in oxidant biology has recently unfolded. Although excessive oxidants may cause damage to tissues, lower levels of oxidants in biological cells may act as messenger molecules enabling the function of numerous physiological processes. It is plausible that some exercise-induced beneficial effects are actually oxidant-mediated. Such developments call for an even more careful analysis of the overall significance of types and amounts of antioxidants in diet. While these complexities pose significant challenges, experts agree that if used prudently, oxidants and antioxidants may serve as potent therapeutic tools. Efforts to determine individual needs of athletes and a balanced diet rich in antioxidant supplements are highly recommended.

Effects of dietary alpha-linolenic acid on the conversion and oxidation of 13C-alpha-linolenic acid

The effects of a diet rich in alpha-linolenic acid vs. one rich in oleic acid on the oxidation of uniformly labeled 13C-alpha-linolenic acid and its conversion into longer-chain polyunsaturates (LCP) were investigated in vivo in healthy human subjects. Volunteers received a diet rich in oleic acid (n = 5) or a diet rich in alpha-linolenic acid (n = 7; 8.3 g/d) for 6 wk before and during the study. After 6 wk, subjects were given 45 mg of 13C-alpha-linolenic acid dissolved in olive oil. Blood samples were collected at t = 0, 5, 11, 24, 96, and 336 h. Breath was sampled and CO2 production was measured each hour for the first 12 h. The mean (+/- SEM) maximal absolute amount of 13C-eicosapentaenoic acid (EPA) in plasma total lipids was 0.04 +/- 0.01 mg in the alpha-linolenic acid group, which was significantly lower (P = 0.01) than the amount of 0.12 +/- 0.03 mg 13C-EPA in the oleic acid group. Amounts of 13C-docosapentaenoic acid (DPA) and 13C-docosahexaenoic acid (DHA) tended to be lower as well. The mean proportion of labeled alpha-linolenic acid (ALA) recovered as 13CO2 in breath after 12 h was 20.4% in the ALA and 15.7% in the oleic acid group, which was not significantly different (P = 0.12). The cumulative recovery of 13C from 13C-ALA in breath during the first 12 h was negatively correlated with the maximal amounts of plasma 13C-EPA (r = -0.58, P = 0.047) and 13C-DPA (r = -0.63, P = 0.027), but not of 13C-DHA (r = -0.49, P = 0.108). In conclusion, conversion of 13C-ALA into its LCP may be decreased on diets rich in ALA, while oxidation of 13C-ALA is negatively correlated with its conversion into LCP. In a few pilot samples, low 13C enrichments of n-3 LCP were observed in a diet rich in EPA/DHA as compared to oleic acid.

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.

Multiple mechanisms for polyunsaturated fatty acid regulation of hepatic gene transcription

Dietary polyunsaturated fatty acids (PUFA) have profound effects on hepatic gene transcription leading to significant changes in lipid metabolism. Highly unsaturated n-3 PUFA suppress the transcription of genes encoding specific lipogenic enzymes and induce the expression of genes encoding specific enzymes involved in peroxisomal and microsomal fatty acid oxidation. Our studies have shown that fatty acid effects on hepatic gene expression may involve at least three distinct pathways. One pathway involves peroxisome proliferator-activated receptor (PPARalpha), a fatty acid activated nuclear receptor. PPARalpha is required for the PUFA induction of mRNAs encoding enzymes involved in fatty acid oxidation. However, PPARalpha is not required for PUFA suppression of mRNAs encoding proteins involved in lipogenesis. A second pathway involves prostanoids. In cultured 3T3-L1 adipocytes, cyclooxygenase derived 20:4 n-6 metabolites, like PGE2, suppress mRNAs encoding proteins involved in lipogenesis. However, in hepatic parenchymal cells, 20:4 n-6 suppression of lipogenic gene expression does not require a cyclooxygenase. Nevertheless, PGE2 and PGF2alpha suppress hepatic lipogenic gene expression. 20:4 n-6 cyclooxygenase products can arise from non-parenchymal cells and through a paracrine control process act on a G-protein linked receptor signaling cascade to suppress lipogenic gene expression. The fact that n-3 and n-6 PUFA suppression of lipogenic gene expression does not require PPARalpha or cyclooxygenase activity indicates the presence of a third pathway for the control of hepatic gene transcription. These studies indicate that the pleiotropic effects of PUFA on hepatic lipid metabolism cannot be attributed to a single regulatory mechanism.

Comparative effects of alpha- and gamma-linolenic acids on rat liver fatty acid oxidation

It has been reported that both n-3 and n-6 octadecatrienoic acids can increase hepatic fatty acid oxidation activity. It remains unclear, however, whether different enzymes in fatty acid oxidation show a similar response to n-3 and n-6 octadecatrienoic acids. The activity of hepatic fatty acid oxidation enzymes in rats fed an oil mixture rich in alpha-linolenic acid (18:3n-3) and borage oil rich in gamma-linolenic acid (18:3n-6) was therefore compared to that in rats fed an oil mixture rich in linoleic acid (18:2n-6) and a saturated fat (palm oil) in this study. Linseed oil served as the source of 18:3n-3 for the oil mixture rich in this octadecatrienoic acid and contained 30.6% 18:3n-3 but not 18:3n-6. Borage oil contained 25.7% 18:3n-6 and 4.5% 18:3n-3. Groups of seven rats each were fed diets containing 15% various fats for 15 d. The oxidation rate of palmitoyl-CoA in the peroxisomes was higher in rats fed a fat mixture rich in 18:3n-3 (3.03 nmol/min/mg protein) and borage oil (2.89 nmol/min/mg protein) than in rats fed palm oil (2.08 nmol/min/mg protein) and a fat mixture rich in 18:2n-6 (2.15 nmol/min/mg protein). The mitochondrial palmitoyl-CoA oxidation rate was highest in rats fed a fat mixture rich in 18:3n-3 (1.93 nmol/min/mg protein), but no significant differences in this parameter were seen among the other groups (1.25-1.46 nmol/min/mg protein). Compared to palm oil and fat mixtures rich in 18:2n-6, a fat mixture rich in 18:3n-3 and borage oil significantly increased the hepatic activity of carnitine palmitoyltransferase and acyl-CoA oxidase. Compared to palm oil and a fat mixture rich in 18:2n-6, a fat mixture rich in 18:3n-3, but not fats rich in 18:3n-6, significantly decreased 3-hydroxyacyl-CoA dehydrogenase activity. Compared to palm oil and a fat mixture rich in 18:2n-6, borage oil profoundly decreased mitochondrial acyl-CoA dehydrogenase activity, but a fat mixture rich in 18:3n-3 increased it. 2,4-Dienoyl-CoA reductase activity was significantly lower in rats fed palm oil than in other groups. Compared to other fats, borage oil significantly increased delt3,delta2-enoyl-CoA isomerase activity. Activity was also significantly higher in rats fed 18:2n-6 oil than in those fed palm oil. It was confirmed that both dietary 18:3n-6 and 18:3n-3 increased fatty acid oxidation activity in the liver. These two dietary octadecatrienoic acids differ considerably, however, in how they affect individual fatty acid oxidation enzymes.

On the effect of 2-deuterium- and 2-methyl-eicosapentaenoic acid derivatives on triglycerides, peroxisomal beta-oxidation and platelet aggregation in rats

A series of 2-substituted eicosapentaenoic acid (EPA) derivatives (as ethyl esters) have been synthesized and evaluated as hypolipidemic and antithrombotic agents in feeding experiments in rats. Repeated administration of purified 2-methyl-eicosapentaenoic acid and its deuterium analogues (all as ethyl esters) to rats resulted in a decrease in plasma triglycerides and high density lipoprotein cholesterol. The 2-methyl-EPA analogues were, apparently, four times more potent than EPA in inducing the triglyceride lowering effect. The 2-deuterium-2-methyl-EPA decreased plasma cholesterol level to approximately 40%. A moderate enlargement of the liver was observed in 2-methyl-EPA treated rats. This was accompanied with an acute reduction in the liver content of triglycerides and a stimulation of peroxisomal beta-oxidation and fatty acyl-CoA oxidase activity. The results suggest that the triglyceride-lowering effect of 2-methyl-EPA may be due to a reduced supply of fatty acids for hepatic triglyceride biosynthesis because of increased fatty acid oxidation. Platelet aggregation with ADP and A23187 was performed ex vivo in platelet-rich plasma, after administration of different doses of the EPA-derivatives for five days. EPA and 2,2-dideuterium EPA had no effect on ADP-induced aggregation, while 2-deuterium-, 2-methyl- and 2-deuterium-2-methyl EPA produced a biphasic effect, i.e. potentiation and inhibition at low (250 mg/day kg body weight) and higher doses (600-1300 mg/day kg body weight), respectively. A23187-induced platelet aggregation was affected in a similar way by feeding the 2-substituted EPA derivatives, except that 2-deuterium-2-methyl EPA had no effect relative to EPA itself and that the inhibition was far greater than that for ADP-induced aggregation (approximately 100% inhibition with 600 mg 2-methyl-EPA/day kg body weight). The ranking order of the EPA-derivatives to affect platelet aggregation and to cause hypolipidemia was different, suggesting different mechanisms. Our observations suggest that the effects of the EPA derivatives on platelet aggregation could be related to the degree of bulkiness around C2 and that an asymmetric substitution at C2 caused inhibition of platelet aggregation while a symmetric substitution did not. It is suggested that the bulky, asymmetric derivatives inhibit platelet aggregation by altering platelet membrane phospholipid packing.

Dose-Response Effect of Dietary Docosahexaenoic Acid on Fatty Acid Profiles of Serum and Tissue Lipids in Rats

The dose-response effects of dietary docosahexaenoic acid (22:6n-3, DHA) on the fatty acid profiles of total lipids of rat serum and tissues were investigated. Rats were fed diets containing graded levels of purified DHA at 0, 1.0, 3.4, and 8.7% of total energy in the diets for 2 weeks. It was found that each tissue had its own peculiar composition of fatty acids which differed markedly from that of circulating serum lipid. This composition was basically influenced dose-dependently by the dietary lipids with graded levels of DHA, but to different degrees in different tissues. Those of brain were most resistant and of heart most susceptible to dietary DHA.

Dietary fat, genes, and human health

These studies show that a macronutrient like dietary fat plays an important role in gene expression. In the cases presented here, dietary fat regulates gene expression leading to changes in carbohydrate and lipid metabolism. The interesting outcome of these studies is the finding that the molecular targets for dietary fat action did not converge with the principal targets for hormonal regulation of gene transcription, like hormone receptors. Instead, PUFA-RF targets elements that play key ancillary roles in gene transcription. This is important because it shows how PUFA can interfere with hormone regulation of a specific gene without having generalized effect on overall hormonal control, i.e. PUFA effects are promoter-specific. How PUFA-RF interferes with gene transcription will require the isolation and characterization of PUFA-RF along with the tissue-specific factors targeted by PUFA-RF. A different story emerges when fatty acids activate PPAR. Based on the studies presented here and elsewhere, long chain-highly unsaturated fatty acids (like 20:5,n-3 and 22:6, n-3) or high levels of fat activate PPAR. PPAR directly activates genes like AOX, but also inhibits transcription of genes like S14, FAS, apolipoprotein CIII, transferrin. For S14, the mechanism of inhibition involves sequestration of RXR, a critical factor for T3 receptor binding to DNA. Thus, PPAR can have generalized effects on T3 action or on other nuclear receptors, like vit. D (VDR) and retinoic acid (RAR) receptors, that require RXR for action. For apolipoprotein CIII and transferrin, PPAR/RXR heterodimers compete for HNF-4 binding sites (DR + 1). In addition to HNF-4, COUP-TF, ARP-1 and RXR all bind the DR + 1 type motif. These factors are important for tissue-specific regulation of gene transcription. PPAR can potentially interfere with the transcription of multiple genes through disruption of nuclear receptor signaling leading to changes in phenotype. Clearly, more studies are required to assess the role PPAR plays in the fatty acid regulation of gene transcription and its contribution to chronic disease. Finally, it is clear that dietary fat has the potential to affect gene expression through multiple pathways. Depending on the gene examined, PUFA might augment or abrogate gene transcription which leads to specific phenotypic changes altering metabolism, differentiation or cell growth. These effects can be beneficial to the organism, such as the n-3 PUFA-mediated suppression of serum triglycerides or detrimental, like the saturated and n-6 PUFA-mediated promotion of insulin resistance. How such effects contribute to the onset or progression of specific neoplasia is unclear. However, studies in metabolism might provide important clues for this connection.

Enhanced hepatic fatty acid oxidation and upregulated carnitine palmitoyltransferase II gene expression by methyl 3-thiaoctadeca-6,9,12,15-tetraenoate in rats

This study reports the effects of a novel polyunsaturated 3-thia fatty acid, methyl 3-thiaoctadeca-6,9,12,15-tetraenoate on serum lipids and key enzymes in hepatic fatty acid metabolism compared to a saturated 3-thia fatty acid, tetradecylthioacetic acid. Palmitic acid treated rats served as controls. Fatty acids were administered by gavage in daily doses of 150 mg/kg body weight for 10 days. The aim of the present study was: (a) To investigate the effect of a polyunsaturated 3-thia fatty acid ester, methyl 3-thiaoctadeca-6,9,12,15-tetraenoate on plasma lipids in normolipidemic rats: (b) to verify whether the lipid-lowering effect could be consistent with enhanced fatty acid oxidation: and (c) to study whether decreased activity of esterifying enzymes and diversion to phospholipid synthesis is a concerted mechanism in limiting the availability of free fatty acid as a substrate for hepatic triglyceride formation. Repeated administration of the polyunsaturated 3-thia fatty acid ester for 10 days resulted in a reduction of plasma triglycerides (40%), cholesterol (33%) and phospholipids (20%) compared to controls. Administration of polyunsaturated and saturated 3-thia fatty acids (daily doses of 150 mg/kg body weight) reduced levels of lipids to a similar extent and followed about the same time-course. Both mitochondrial and peroxisomal fatty acid oxidation increased (1.4-fold- and 4.2-fold, respectively) and significantly increased activities of carnitine palmitoyltransferase (CPT) (1.6-fold), 2,4-dienoyl-CoA reductase (1.2-fold) and fatty acyl-CoA oxidase (3.0-fold) were observed in polyunsaturated 3-thia fatty acid treated animals. This was accompanied by increased CPT-II mRNA (1.7-fold). 2,4-dienoyl-CoA reductase mRNA (2.9-fold) and fatty acyl-CoA oxidase mRNA (1.7-fold). Compared to controls, the hepatic triglyceride biosynthesis was retarded as indicated by a decrease in liver triglyceride content (40%). The activities of glycerophosphate acyltransferase, acyl-CoA: 1,2-diacylglycerol acyltransferase and CTP:phosphocholine cytidylyltransferase were increased. The cholesterol lowering effect was accompanied by a reduction in HMG-CoA reductase activity (80%) and acyl-CoA:cholesterol acyltransferase activity (33%). In hepatocytes treated with methyl 3-thiaoctadeca-6,9,12,15-tetraenoate, fatty acid oxidation was increased 1.8-fold compared to controls. The results suggest that treatment with methyl 3-thiaoctadeca-6,9,12,15-tetraenoate reduces plasma triglycerides by a decrease in the availability of fatty acid substrate for triglyceride biosynthesis via enhanced fatty acid oxidation, most likely attributed to the mitochondrial fatty acid oxidation. It is hypothesized that decreased phosphatidate phosphohydrolase activity may be an additive mechanism which contribute whereby 3-thia fatty acids reduce triglyceride formation in the liver. The cholesterol-lowering effect of the polyunsaturated 3-thia fatty acid ester may be due to changes in cholesterol/cholesterol ester synthesis as 60% of this acid was observed in the hepatic cholesterol ester fraction.

Polyunsaturated fatty acid suppression of hepatic fatty acid synthase and S14 gene expression does not require peroxisome proliferator-activated receptor alpha

Dietary polyunsaturated fatty acids (PUFA) induce hepatic peroxisomal and microsomal fatty acid oxidation and suppress lipogenic gene expression. The peroxisome proliferator-activated receptor alpha (PPARalpha) has been implicated as a mediator of fatty acid effects on gene transcription. This report uses the PPARalpha-deficient mouse to examine the role of PPARalpha in the PUFA regulation of mRNAs encoding hepatic lipogenic (fatty acid synthase (FAS) and the S14 protein (S14)), microsomal (cytochrome P450 4A2 (CYP4A2)), and peroxisomal (acyl-CoA oxidase (AOX)) enzymes. PUFA ingestion induced mRNAAOX (2.3-fold) and mRNACYP4A2 (8-fold) and suppressed mRNAFAS and mRNAS14 by >/=80% in wild type mice. In PPARalpha-deficient mice, PUFA did not induce mRNAAOX or mRNACYP4A2, indicating a requirement for PPARalpha in the PUFA-mediated induction of these enzymes. However, PUFA still suppressed mRNAFAS and mRNAS14 in the PPARalpha-deficient mice. Studies in rats provided additional support for the differential regulation of lipogenic and peroxisomal enzymes by PUFA. These studies provide evidence for two distinct pathways for PUFA control of hepatic lipid metabolism. One requires PPARalpha and is involved in regulating peroxisomal and microsomal enzymes. The other pathway does not require PPARalpha and is involved in the PUFA-mediated suppression of lipogenic gene expression.

Eicosapentaenoic acid and sulphur substituted fatty acid analogues inhibit the proliferation of human breast cancer cells in culture

Numerous studies have shown dietary fatty acids to influence the progression of several types of cancers. The purpose of the present investigation was to examine the influence of various types of fatty acids, including omega-3 fatty acids and a new class of hypolipidemic peroxisome proliferating fatty acid analogues, namely the 3-thia fatty acids, on MCF-7 human breast cancer cell growth. 3-thia fatty acids represent non-beta-oxidizable fatty acid analogues in which a sulphur atom substitutes for the beta-methylene group (3-position) in the saturated and unsaturated fatty acids. The effects of increasing concentrations of palmitic acid, tetradecylthioacetic acid (a 3-thia fatty acid), eicosapentaenoic acid, docosahexaenoic acid, and two 3-thia polyunsaturated fatty acids on the proliferation of MCF-7 cells, maintained in serum-free culture, were studied. At the highest concentration of fatty acid used (64 microM) tetradecylthioacetic acid was found to be the most effective of all fatty acids tested in inhibiting cell growth, whilst palmitic acid and docosahexaenoic acid had no significant effect on cell growth. Thus, of the two dietary polyunsaturated omega-3 fatty acids eicosapentaenoic acid and docosahexaenoic acid, only eicosapentaenoic acid possesses an inhibitory effect on the proliferation of MCF-7 cells. In all cases the inhibitory effect of the fatty acid was found to be reversible. Tetradecylthioacetic acid has been shown to be a potent peroxisome proliferator. It was, therefore, hypothesized that tetradecylthioacetic acid may inhibit the human MCF-7 cell growth by increasing the level of oxidative stress within the cell. However, use of agents which modify the cell's protective apparatus against oxidative stress had no influence on the inhibitory effect of tetradecylthioacetic acid. These experiments indicate that tetradecylthioacetic acid inhibits cell growth by mechanisms which may be independent of oxidative status.

Activity of hepatic fatty acid oxidation enzymes in rats fed alpha-linolenic acid

The activity of hepatic fatty acid oxidation enzymes in rats fed linseed and perilla oils rich in alpha-linolenic acid (alpha-18:3) was compared to that in rats fed safflower oil rich in linoleic acid (18:2) and a saturated fat (palm oil). Palm and safflower oils were essentially devoid of alpha-18:3. The palmitoyl-CoA oxidation rates both in mitochondrial and peroxisomal pathways in liver homogenates were significantly higher in rats fed linseed oil than in those fed palm and safflower oils. Among rats fed diets containing palm oil, safflower oil, fat mixtures composed of safflower and perilla oils (2:1, w/w and 1:2, w/w), and perilla oil, mitochondrial and peroxisomal fatty oxidation rates increased with increasing dietary levels of perilla oil. Compared to palm and safflower oils, dietary alpha-18:3 either in the form of linseed or perilla oils profoundly increased the activity of carnitine palmitoyltransferase, acyl-CoA oxidase, 3-ketoacyl-CoA thiolase, and 2,4-dienoyl-CoA reductase. Smaller but significant increases by dietary alpha-18:3 of the activity of acyl-CoA dehydrogenase, enoyl-CoA hydratase, and delta 3, delta 2-enoyl-CoA isomerase were also observed. Unexpectedly, dietary alpha-18:3 greatly reduced the activity of 3-hydroxy-acyl-CoA dehydrogenase. Compared to palm oil, dietary polyunsaturated fats significantly reduced the activity of fatty acid synthetase and glucose-6-phosphate dehydrogenase to the same levels. The activity of pyruvate kinase was significantly higher in rats fed palm oil than in those fed polyunsaturated fats. The extent of reduction was more prominent with polyunsaturated fats containing alpha-18:3 than with safflower oil devoid of alpha-18:3. Thus, compared to linoleic acid and saturated fatty acids, dietary alpha-18:3 caused characteristic changes in the activity of hepatic enzymes in fatty acid and glucose metabolism in rats.

Eicosapentaenoic acid, but not docosahexaenoic acid, increases mitochondrial fatty acid oxidation and upregulates 2,4-dienoyl-CoA reductase gene expression in rats

The aim of the present study was to investigate whether eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA) was responsible for the triglyceride-lowering effect of fish oil. In rats fed a single dose of EPA as ethyl ester (EPA-EE), the plasma concentration of triglycerides was decreased at 8 h after acute administration. This was accompanied by an increased hepatic fatty acid oxidation and mitochondrial 2,4-dienoyl-CoA reductase activity. The steady-state level of 2,4-dienoyl-CoA reductase mRNA increased in parallel with the enzyme activity. An increased hepatic long-chain acyl-CoA content, but a reduced amount of hepatic malonyl-CoA, was obtained at 8 h after acute EPA-EE treatment. On EPA-EE supplementation, both EPA (20:5n-3) and docosapentaenoic acid (DPA, 22:5n-3) increased in the liver, whereas the hepatic DHA (22:6n-3) concentration was unchanged. On DHA-EE supplementation retroconversion to EPA occurred. No statistically significant differences were found, however, for mitochondrial enzyme activities, malonyl-CoA, long-chain acyl-CoA, plasma lipid levels, and the amount of cellular fatty acids between DHA-EE treated rats and their controls at any time point studied. In cultured rat hepatocytes, the oxidation of [1-14C]palmitic acid was reduced by DHA, whereas it was stimulated by EPA. In the in vivo studies, the activities of phosphatidate phosphohydrolase and acetyl-CoA carboxylase were unaffected after acute EPA-EE and DHA-EE administration, but the fatty acyl-CoA oxidase, the rate-limiting enzyme in peroxisomal fatty acid oxidation, was increased after feeding these n-3 fatty acids. The hypocholesterolemic properties of EPA-EE may be due to decreased 3-hydroxy-3-methylglutaryl-CoA reductase activity. Furthermore, replacement of the ordinary fatty acids, i.e., the monoenes (16:1n-7, 18:1n-7, and 18:1n-9) with EPA and some conversion to DPA concomitant with increased fatty acid oxidation is probably the mechanism leading to changed fatty acid composition. In contrast, DHA does not stimulate fatty acid oxidation and, consequently, no such displacement mechanism operates. In conclusion, we have obtained evidence that EPA, and not DHA, is the fatty acid primarily responsible for the triglyceride-lowering effect of fish oil in rats.

n-3 fatty acids and lipoproteins: comparison of results from human and animal studies

The impact of n-3 fatty acids (FA) on blood lipoprotein levels has been examined in many studies over the last 15 yr in both animals and humans. Studies in humans first demonstrated the potent triglyceride-lowering effect of n-3 FA, and these were followed up with animal studies to unravel the mechanism of action. This paper reviews the reported effects of n-3 FA on blood lipoproteins in 72 placebo-controlled human trials, at least 2 wk in length and providing 7 or less g of n-3 FA/day. Trials in normolipidemic subjects (triglycerides < 2.0 mM; 177 mg/dL) were compared to those in hypertriglyceridemic patients (triglycerides > or = 2.0 mM). In the healthy subjects, mean triglyceride levels decreased by 25% (P < 0.0001), and total cholesterol (C) levels increased by 2% (P < 0.009) due to the combined increases in low density lipoprotein (LDL)-C (4%, P < 0.02) and high density lipoprotein (HDL)-C (3%, P < 0.008). In the patients, triglyceride levels decreased by 28% (P < 0.0001), LDL-C rose by 7% (P < 0.0001), but neither total C nor HDL-C changed significantly. Although the effect on triglyceride levels is also observed in rats and swine, it is rarely seen in mice, rabbits, monkeys, dogs, and hamsters. Whereas n-3 FA have only a minor impact on lipoprotein C levels in humans, they often markedly lower both total C and HDL-C levels in animals, especially monkeys. These differences are not widely appreciated and must be taken into account when studying the effects of n-3 FA on lipoprotein metabolism.

Chronic administration of eicosapentaenoic acid and docosahexaenoic acid as ethyl esters reduced plasma cholesterol and changed the fatty acid composition in rat blood and organs

Fish oils rich in n-3 fatty acids have been shown to decrease plasma lipid levels, but the underlying mechanism has not yet been elucidated. This investigation was performed in order to further clarify the effects of purified ethyl esters of eicosapentaenoic acid (EPA-EE) and docosahexaenoic acid (DHA-EE) on lipid metabolism in rats. The animals were fed EPA-EE, DHA-EE, palmitic acid, or corn oil (1 g/kg/d) by orogastric intubation along with a chow background diet for three months. At the end the animals were sacrificed. Plasma and liver lipids were measured, as well as lipid-related enzyme activities and mRNA levels. The fatty acid composition of plasma and different tissues was also determined. This study shows that, compared to the corn oil control, EPA-EE and DHA-EE lowered plasma cholesterol level, whereas only EPA-EE lowered the amount of plasma triacylglycerol. In liver peroxisomes, both EE preparations increased fatty acyl-CoA oxidase FAO activities, and neither altered 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase activities. In liver microsomes, EPA-EE raised HMG-CoA reductase and acyl-CoAicholesterol acyltransferase activities, whereas DHA-EE lowered the former and did not affect the latter. Neither product altered mRNA levels for HMG-CoA reductase, low density lipoprotein-receptor, or low density lipoprotein-receptor related protein. EPA-EE lowered plasma triacylglycerol, reflecting lowered very low density lipoprotein secretion, thus the cholesterol lowering effect in EPA-EE-treated rats may be secondary to the hypotriacylglycerolemic effect. An inhibition of HMG-CoA reductase activity in DHA-EE treated rats may contribute to the hypocholesterolemic effect. The present study reports that 20:5n-3, and not 22:6n-3, is the fatty acid primarily responsible for the triacylglycerol lowering effect of fish oil. Finally, 20:5n-3 was not converted to 22:6n-3, whereas retroconversion of 22:6n-3 to 20:5n-3 was observed.

Effect of long-chain mono-unsaturated and n-3 polyunsaturated fatty acids on postprandial blood and liver lipids in rats

The effects on blood and liver lipids after feeding rats with concentrated fractions from fish oil consisting of mono-unsaturated fatty acids (80% C20:1 and 22:1) or n-3 polyunsaturated fatty acids (85% C20:5 and 22:6 n-3) were examined. Mono-unsaturated fat had no effects on plasma triacylglycerol, total cholesterol phospholipids or unesterified fatty acid as compared to controls (lard). However, n-3 polyunsaturated fatty acid-fed animals showed a significant decrease in plasma triacylglycerol (74%), phospholipids (40%) and unesterified fatty acids (52%). The concentrated fractions had no effects on liver lipids. While the n-3 diet increased peroxisomal beta-oxidation 2.5-fold, there was only a slight increase with the mono-unsaturated diet. The fatty acid composition in plasma and liver phospholipids was changed with the various diets; 20:4 n-6 was significantly reduced in plasma and liver with the mono-unsaturated diet, and with the n-3 diet in liver. The mono-unsaturated diet, and especially the n-3 diet, increased the 20:5 n-3 level in both plasma and liver. Our results indicate that long-chain mono-unsaturated fatty acids in fish oil do not change the levels of plasma lipids. The beneficial role of fish oil on the level of blood lipids, may therefore be mostly attributed to the effects of long-chain n-3 fatty acids. However, the low 20:4 n-6 and high 20:5 n-3 levels in plasma and liver phospholipids with the concentrated mono-unsaturated fatty acid diet may be of importance for a favourable haemostatic balance with regard to cardiovascular diseases.

Eicosapentaenoic acid causes transient accumulation of lipids in rat myocardium

Rats were given eicosapentaenoic acid (EPA) or palmitic acid (PALM) up to 15 days, and control animals were given carboxymethylcellulose. All suspensions which were given by gastric intubation contained tocopherol. Heart triacylglycerols, heart cholesterol and heart phospholipids significantly increased after one day of EPA treatment, but they were normalized within 15 days. Both after 2 and 10 days of treatment with palmitic acid the heart triacylglycerols were significantly greater than control. The heart cholesterol and heart phospholipids were significantly greater than control after 10 days of treatment with palmitic acid. Total carnitine palmitoyltransferase (CPT) activity in heart was significantly greater in rats treated with EPA for 15 days compared to control, but treatment with palmitic acid had no effect. The fatty acyl-CoA oxidase activity was greater in rats treated with EPA for 15 days and palmitic acid for 10 days compared to control. The fractional volume of lipid droplets in myocardial cells was calculated from electronmicrographs and was 0.112 +/- 0.016% after 1 day of EPA treatment compared to 0.035 +/- 0.016% in the control group. After 5 and 15 days the fractional volume was the same as control. The fractional volume of lipid droplets in rats treated with palmitic acid for 10 days was 0.120 +/- 0.023%. Treatment with EPA caused an immediate accumulation of lipids and lipid droplets in the rat heart which after few days normalized in parallel with an increased activity of total CPT in the heart.

Quantification in vivo of the effects of different types of dietary fat on the loci of control involved in hepatic triacylglycerol secretion

Polyunsaturated fatty acids (PUFA) have been suggested to exert their hypotriglyceridaemic effect through several possible mechanisms that would be expected to decrease the rate of hepatic very-low-density-lipoprotein-triacylglycerol secretion. We have quantified the role played in vivo by changes in the pattern of partitioning of (i) acyl-CoA between oxidation and esterification, (ii) diacylglycerol between synthesis of triacylglycerol and of the major phospholipids, and (iii) triacylglycerol between secretion and storage within the liver, in response to two dietary levels of n-6 and n-3 PUFA. In order to achieve this we used the technique of selective labelling of hepatic fatty acids in vivo. Compared with a predominantly saturated fatty acid diet, both n-6 and n-3 PUFA intake resulted in a decrease in the proportion of acyl moieties that were secreted by the liver, through an increased diversion of acyl-CoA towards oxidation and a lower fractional rate of secretion of newly synthesized triacylglycerol. In addition, a diet rich in n-3 fatty acids resulted not only in a greater magnitude of these effects but also in a doubling of the partitioning of diacylglycerol towards phospholipid labelling. It is shown that the overall 50% reduction achieved by fish oil feeding in the proportion of acyl groups that were secreted by the liver was distributed over all three branch points. The contribution of each of these adaptations was quantified. The application of such an approach, i.e. the localization and in vivo quantification of the importance of loci of control, in studies on dietary and pharmacological agents that affect lipaemia, is discussed.

Coordinate induction of hepatic fatty acyl-CoA oxidase and P4504A1 in rat after activation of the peroxisome proliferator-activated receptor (PPAR) by sulphur-substituted fatty acid analogues

1. In the liver of rat fed a single dose of 3-thia fatty acids, 3-dithiahexadecanedioic acid (3-thiadicarboxylic acid) and tetradecylthioacetic acid, steady-state levels of P4504A1 and fatty acyl-CoA oxidase mRNAs increased in parallel. The increases were significant 8 h after administration, reaching a maximum after 12 h and decreased from 12 to 24 h after administration. 2. The corresponding enzyme activities of P4504A1 and fatty acyl-CoA oxidase were also induced in a parallel manner by the 3-thia fatty acids. The enzyme activities were significantly increased 12 h after administration and increased further after 24 h. This may reflect a possible effect of the 3-thia fatty acids not only on mRNA levels, but also on the translation and degradation rate of the two enzymes. 3. Repeated administration of 3-thia fatty acids resulted in an increase of the specific P4504A1 protein accompanied with an increased lauric acid hydroxylase activity. The correlation between induction of P4504A1 and fatty acyl-CoA oxidase mRNAs and their enzyme activities may reflect a coordinated rather than a causative induction mechanism, and that these genes respond to a common signal. This suggests that the increased P450 activity may not be responsible or be a prerequisite for fatty acyl-CoA oxidase induction. 4. Since the peroxisome proliferator-activated receptor (PPAR) plays a role in mediating the induction of fatty acyl-CoA oxidase, we analysed the activation of PPAR by fatty acids and sulphur-substituted analogues utilizing a chimera between the N-terminal and DNA-binding domain of the glucocorticoid receptor and the putative ligand-binding domain of PPAR. Arachidonic acid activated this chimeric receptor in Chinese hamster ovary cells. Inhibitors of P450 did not affect the activation of PPAR by arachidonic acid. Furthermore, dicarboxylic acids including 1,12-dodecanedioic acid or 1,16-hexadecanedioic acid only weakly activated the chimera. 3-Thidicarboxylic acid, however, was a much more effective activator than the non-sulphur-substituted analogues. In conclusion, the data suggest that the most likely mechanism of the induction process is fatty acid-induced activation of PPAR, which then leads to a coordinated induction of P4504A1 and fatty acyl-CoA oxidase.

Effect of dietary n-3 and n-6 polyunsaturated fatty acids on lipid-metabolizing enzymes in obese rat liver

This study was designed to examine whether n-3 and n-6 polyunsaturated fatty acids at a very low dietary level (about 0.2%) would alter liver activities in respect to fatty acid oxidation. Obese Zucker rats were used because of their low level of fatty acid oxidation, which would make increases easier to detect. Zucker rats were fed diets containing different oil mixtures (5%, w/w) with the same ratio of n-6/n-3 fatty acids supplied either as fish oil or arachidonic acid concentrate. Decreased hepatic triacylglycerol levels were observed only with the diet containing fish oil. In mitochondrial outer membranes, which support carnitine palmitoyltransferase I activity, cholesterol content was similar for all diets, while the percentage of 22:6n-3 and 20:4n-6 in phospholipids was enhanced about by 6 and 3% with the diets containing fish oil and arachidonic acid, respectively. With the fish oil diet, the only difference found in activities related to fatty acid oxidation was the lower sensitivity of carnitine palmitoyltransferase I to malonyl-CoA inhibition. With the diet containing arachidonic acid, peroxisomal fatty acid oxidation and carnitine palmitoyltransferase I activity were markedly depressed. Compared with the control diet, the diets enriched in fish oil and in arachidonic acid gave rise to a higher specific activity of aryl-ester hydrolase in microsomal fractions. We suggest that slight changes in composition of n-3 or n-6 polyunsaturated fatty acids in mitochondrial outer membranes may alter carnitine palmitoyltransferase I activity.

Modulation of plasma and hepatic oxidative status and changes in plasma lipid profile by n-3 (EPA and DHA), n-6 (corn oil) and a 3-thia fatty acid in rats

This manuscript describes changes in plasma lipid profiles and parameters of oxidative status in the plasma and liver of rats fed 5 different fatty acids: 95% eicosapentaenoic acid, 92% docosahexaenoic acid (DHA), corn oil (n-6), 1-mono-(carboxymethylthio)-tetradecane (CMTTD) and palmitic acid (controls) for 3 months. At the given doses both EPA and the 3-thia fatty acid, CMTTD, caused a significant decrease in plasma triglycerides, phospholipids, free fatty acids and cholesterol. DHA decreased plasma free fatty acids and cholesterol, while corn oil feeding reduced only plasma free fatty acids. Plasma and hepatic vitamin E levels were significantly decreased in EPA, DHA and CMTTD fed rats, but remained unchanged in corn oil fed rats. Plasma glutathione was noted to decrease after EPA and DHA feeding but remained unchanged in other groups. However, hepatic glutathione content was increased in EPA, DHA and CMTTD fed rats, whereas cysteine levels were noted to decrease. As hepatic levels of cysteinylglycine remained unchanged, increased rate of cellular glutathione synthesis rather than its decreased degradation is likely to contribute to the increased hepatic glutathione content in EPA, DHA and CMTTD fed rats. Except for reduction in the levels of plasma lipid peroxidation caused by CMTTD, no significant changes were noted between the different treatment groups. Hepatic lipid peroxidation was elevated only in rats given DHA. Furthermore, our results show that EPA and DHA cause minimal imbalance of the peroxisomal H2O2 metabolising enzymes as compared to CMTTD. In addition, contrary to the potent peroxisome proliferator compound CMTTD which decreased the activities of glutathione transferase and glutathione peroxidase, EPA and DHA increased the activities of these detoxification enzymes.

Lipoprotein, lecithin:cholesterol acyl transferase and acetyl CoA carboxylase in stroke-prone spontaneously hypertensive rats fed a diet high in eicosapentaenoic acid

To investigate the effect of dietary eicosapentaenoic acid (EPA) on plasma lipoprotein levels, lecithin:cholesterol acyltransferase (LCAT) activity and liver acetyl CoA carboxylase activity, highly concentrated EPA (78%) purified from sardine oil was fed to stroke-prone spontaneously hypertensive rats (SHRSP) for 30 days. Significantly (P < 0.05) lower systolic blood pressure and plasma total cholesterol were observed in rats fed an EPA diet. In addition, higher HDL cholesterol and lower VLDL cholesterol levels were found in rats fed the EPA diet as compared with rats fed the control diet. However, no significant change of plasma LDL cholesterol was observed in rats between the two dietary groups. EPA supplementation increased the activity of plasma LCAT in rats. In addition, rats fed an EPA diet had lower liver total lipids and adipose tissue weights. However, higher liver acetyl CoA carboxylase activity was observed in rats fed the EPA diet. Results from the present study suggest that dietary EPA might stimulate the plasma lipoprotein metabolism and also alter lipogenesis in the liver of SHRSP rats.

Impact of cytochrome P450 system on lipoprotein metabolism. Effect of abnormal fatty acids (3-thia fatty acids)

Fatty acid omega-hydroxylation, peroxisomal and mitochondrial fatty acid oxidation and related lipid-metabolizing enzymes are constitutive activities of mammalian cells. The past 5 years have witnessed an increased interest in the modulation of these pathways and functions by a new group of abnormal fatty acids (sulfur-substituted fatty acid analogs), due to the metabolic and nutritional aspects related to human health and disease, and possible treatment of certain inherited peroxisomal and mitochondrial disorders. The purpose of this review is to present an overview of current knowledge in the field and to provide an account of recent developments, particularly with respect to the chemical nature of the biologically active factors and their possible mechanism of action.

Structural and metabolic requirements for activators of the peroxisome proliferator-activated receptor

Fatty acids have recently been demonstrated to activate peroxisome proliferator-activated receptors (PPARs) but specific structural requirements of fatty acids to produce this response have not yet been determined. Importantly, it has hitherto not been possible to show specific binding of these compounds to PPAR. To test whether a common PPAR binding metabolite might be formed, we tested the effects of long-chain omega-3 polyunsaturated fatty acids, differentially beta-oxidizable fatty acids and inhibitors of fatty acid metabolism. We determined the activation of a reporter gene by a chimaeric receptor encompassing the DNA binding domain of the glucocorticoid receptor and the ligand binding domain of PPAR. The omega-3 unsaturated fatty acids were slightly more potent PPAR activators in vitro than saturated fatty acids. The peroxisomal proliferation-inducing, non-beta-oxidizable, tetradecylthioacetic acid activated PPAR to the same extent as the strong peroxisomal proliferator WY 14,643, whereas the homologous beta-oxidizable tetradecylthiopropionic acid was only as potent as a non-substituted fatty acid. Cyclooxygenase inhibitors, radical scavengers or cytochrome P450 inhibitors did not affect activation of PPAR. In conclusion, beta-oxidation is apparently not required for the formation of the PPAR-activating molecule and this moiety might be a fatty acid, its ester with CoA, or a further derivative of the activated fatty acid prior to beta-oxidation of the acyl-CoA ester. These data should aid understanding of signal transduction via PPAR and the identification of a receptor ligand.