Evidence for mechanisms of the hypotriglyceridemic effect of n-3 polyunsaturated fatty acids

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.

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.

Effects of fish oil on metabolic responses to oral fructose and glucose loads in healthy humans

This study examines the effect of the substitution of 6 g/day of fish oil in a saturated diet on glucose and fructose metabolism in healthy humans. Five subjects were submitted to two 3-wk controlled-diet periods (polyunsaturated/saturated = 0.21). During one period, 6 g/day of fat used for dressing were replaced by 6 g/day of fish oil [1.1 g/day of 20:5 (n-3) fatty acids and 0.7 g/day of 22:6 (n-3) fatty acids]. At the end of each period the subjects ingested a 1 g/kg fructose or glucose load 2 days apart. Plasma glucose fluxes were traced with the use of deuterated glucose and [U-13C]glucose. Substrate oxidation was measured by indirect calorimetry. Fish oil induced a 4% increase in basal and postload glycemia and a 40% decrease in insulinemia, whereas plasma C-peptide remained unaffected. Glucose fluxes were unaffected by fish oil, but carbohydrate (CHO) oxidation was reduced (fructose: 55.5 +/- 4.1 vs. 62.9 +/- 3.6 g/6 h; glucose: 36.7 +/- 4.7 vs. 50.5 +/- 4.7 g/6 h; all P < 0.05). Lipid oxidation was increased 35% by fish oil after both CHO loads. Nonoxidative glucose disposal was increased by fish oil (fructose: 9.4 +/- 2.5 vs. 2.9 +/- 1.1 g/6 h; glucose: 28.3 +/- 5.1 vs. 14.4 +/- 4.7 g/6 h; all P < 0.05). Fish oil could affect glucose transport and decrease CHO oxidation through the decrease in insulinemia and/or a specific effect on glycolytic pathway.

Dietary fish oil normalize dyslipidemia and glucose intolerance with unchanged insulin levels in rats fed a high sucrose diet

The aim of this study was to investigate the relationship between the lipid-lowering effects of fish oils and concomitant consequences on glucose tolerance and insulin sensitivity in an experimental animal model of hypertriglyceridemia induced by high sucrose intake. To achieve this goal, male Wistar rats were fed a semi-synthetic sucrose rich diet (SRD) (w/w: 62.3% sucrose, 8% corn oil, 17% protein) for 90 days. At the time, a well established and permanent hypertriglyceridemia accompanied by glucose intolerance was present. After that, one half of the animals continued on the SRD up to 120 days. The other half received an SRD in which the source of fat was substituted by cod liver oil (w/w 7% CLO plus 1% corn oil) from day 90 to 120 (SRD+CLO). Control rats were fed a semi-synthetic diet (CD) (w/w: 62.5% corn starch, 8% corn oil, 17% protein) throughout the 120 days experimental period. Results obtained after the experimental period show that the hypertriglyceridemia and glucose intolerance ensuing long term feeding normal rats with a sucrose-rich diet could be completely reversed mediating no change in circulating insulin levels by shifting the source of fat in the diet from corn oil to cod liver oil. These findings suggest that manipulation of dietary fats may play a role in the management of the lipid disorders associated with glucose intolerance and insulin resistance.

Effects of non-beta-oxidizable sulfur-substituted fatty acid analogues on synthesis and secretion of triacylglycerol and cholesterol in cultured rat hepatocytes

The mechanisms behind the hypolipidemic effect of two sulfur-substituted fatty acid analogues, 3-thiadicarboxylic acid and tetradecylthioacetic acid, have been investigated in cultured hepatocytes. There was a dose-dependent reduction in incorporation of [3H] water into triacylglycerol and diacylglycerol when tetradecylthioacetic acid was added to rat hepatocytes cultured in the presence of 200 muM oleic acid. Tetradecylthioacetic acid also increased the oxidation of [14C]palmitic acid compared to oleic acid, inhibited the incorporation of radiolabeled precursors into diacylglycerol to a greater extent than into triacylglycerol, and reduced the secretion of triacylglycerol more than its synthesis. A stimulation, rather than a reduction, in glycerolipid synthesis and secretion by oxidation of fatty acids and reduces the synthesis and secretion of glycerolipids. 3-Thiadicarboxylic acid reduces the synthesis and secretion of both glycerolipids and cholesterol to approximately the same extent without a concomitant increase in the oxidation of fatty acids.

Variations in composition of dietary fats affect hepatic uptake and metabolism of chylomicron remnants

The hepatic metabolism of [1-14C]oleate- and [1,2-3H]cholesterol-dual-labelled chylomicron remnants derived from olive, corn, palm and fish oil and butter fat was compared by adding each lipoprotein separately to the perfusate of isolated livers from rats fed on a normal diet. Labelled remnants from butter fat and fish oil were removed more rapidly from the perfusate than remnants derived from olive, corn and palm oil. The oxidation of labelled remnant fatty acid from olive oil, fish oil or butter fat was four to seven times greater than that from corn and palm oil. Labelled fatty acid in fish oil remnants was incorporated into phospholipid significantly more efficiently than the labelled fatty acid in olive, corn or palm oil remnants, with butter fat giving an intermediate value. For all the remnants, there was a significant amount of hydrolysis of labelled esterified cholesterol by the liver which was dependent on the magnitude of hepatic uptake of each type of remnant. The recovery of remnant [3H]cholesterol label in the bile was 50% less with palm oil remnants than with all the other remnants studied. The results indicate that the fatty acid composition of chylomicron remnants has a major impact on their uptake and metabolism by the liver.

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.

Evidence that dietary arachidonic acid increases circulating triglycerides

Male Syrian hamsters and male CD-1 mice were fed diets supplemented with ethyl esters of oleic, linoleic, arachidonic, and eicosapentaenoic acids (1.1-5%, w/w) for 3-4 wk. Plasma and serum triglycerides were significantly higher in the arachidonic acid-supplemented animals compared to those in the other supplementation groups. Changes in serum insulin and glucose levels did not appear to be related to the changes in circulating triglycerides observed in the arachidonic acid-supplemented group. These data indicate that dietary arachidonic acid elevates circulating triglyceride levels compared to other unsaturated fatty acids in hamsters and mice by unknown mechanisms.

Dietary n - 3 polyunsaturated fatty acids modify Syrian hamster platelet and macrophage phospholipid fatty acyl composition and eicosanoid synthesis: a controlled study

The aim of this study was to determine the effects of varying intakes of dietary n - 3 polyunsaturated fatty acids (PUFA) on the fatty acyl composition and arachidonic acid metabolite synthesis of platelets and macrophages in Syrian hamsters consuming diets that were strictly controlled for n - 6 PUFA content. Animals consumed highly controlled diets which were not supplemented with n - 3 PUFA (control) or supplemented with 0.4%, 0.8% or 2% (w/w) n - 3 fatty acids. The content of n - 3 PUFA in cellular phospholipids increased progressively with the intake of n - 3 PUFA, while n - 6 PUFA, including arachidonic acid, decreased despite the constant intake of 18:2(n - 6); this latter effect was more substantial in macrophages than in platelets. The synthesis by stimulated macrophages of prostaglandin E2, 6-keto-prostaglandin F1 alpha, thromboxane B2 and 11- and 15-hydroxyeicosatetraenoic acids decreased with the intake of 0.8% n - 3 PUFA to 30-50% of the control values. Little effect of diets on platelet aggregation and eicosanoid synthesis was observed reflecting the limited effect on platelet arachidonic acid content. The synthesis of 12-hydroxyeicosapentaenoic acid by stimulated platelets increased with n - 3 PUFA consumption in a dose-dependent fashion. Circulating triacylglycerols and HDL-cholesterol were decreased only in animals consuming 2% n - 3 PUFA. The strict control of n - 6 PUFA intake allows the determination of the effects of n - 3 PUFA intake on the measured parameters without confounding effects of other dietary lipids.

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.

Effect of dietary fish oil on blood levels of free fatty acids, ketone bodies and triacylglycerol in humans

Although the reduction of serum triacylglycerol concentrations by dietary fish oil is a well-known effect, the exact mechanism of this effect has not been previously studied in human subjects. Therefore, the aim of this study was (i) to examine the effect of short-term fish oil supplementation on blood concentrations of ketone bodies, free fatty acids and triacylglycerol in healthy humans and (ii) to verify whether the observed relationships between these variables would be consistent with reduced lipolysis and/or enhanced hepatic fatty acid oxidation after fish oil supplementation. Twenty subjects (21-23 years, normal liver function tests) were randomly divided into two groups to supplement their usual diet with either 30 g/d of fish oil (n = 11) or olive oil (n = 9). Venous blood samples were drawn after an overnight fast, before and after 1, 3 and 7 d of fish oil/olive oil supplementation. Blood concentrations of triacylglycerol and free fatty acids decreased consistently after fish oil supplementation; the reduction was already significant after one day of fish oil (P < 0.001 for triacylglycerol and P = 0.01 for free fatty acids). In contrast, neither of these blood values changed after olive oil supplementation (P > 0.10). No significant changes in glucose, insulin or ketone body levels were observed in either group after supplementation. After fish oil, but not after olive oil supplementation, the ratio of blood ketone body levels to free fatty acid levels increased significantly (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis of arachidonic acid metabolites by Syrian hamster platelets and peritoneal cells

In this study, the metabolism of arachidonic acid by hamster platelets and peritoneal macrophages was assessed. Peritoneal macrophages stimulated in vitro with the calcium ionophore A23187 or stimulated in vivo by intraperitoneal injections of opsonized zymosan produced prostaglandin E2, thromboxane B2 (TxB2) and 6-keto-prostaglandin F1 alpha, as determined by radioimmunoassays. Leukotriene B4 (LTB4), and 11- and 15-hydroxyeicosatetraenoic acids (HETE), which were identified by reverse-phase high-performance liquid chromatography coupled with diode array detection, were produced by peritoneal cells stimulated in vitro with A23187 but were not found in the peritoneal exudate following in vivo stimulation with opsonized zymosan. Synthesis of 11- and 15-HETE, but not LTB4, was inhibited by 1 microM indomethacin but not by 10 microM nordihydroguaiaretic acid, which did inhibit LTB4 synthesis. Washed hamster platelets were prepared and shown to synthesize TxB2, 12-HETE and 12-hydroxyheptadecatrienoic acid following stimulation with thrombin. This paper is the first to report on eicosanoid metabolism in tissues related to atherosclerosis, thrombosis and inflammation in hamsters.

Effect of various n-3/n-6 fatty acid ratio contents of high fat diets on rat liver and heart peroxisomal and mitochondrial beta-oxidation

The present work extends tissue investigations previously performed in rat gastric mucosa on lipid metabolism alterations caused by n-3 and n-6 fatty acid-enriched diets. Liver and heart tissues are here studied and demonstrated to undergo, upon exposure to high fat diets with various n-3/n-6 fatty acid ratio contents, biochemical and morphological changes which may be enumerated as follows: (1) Rat liver peroxisomal prostaglandin E2, fatty acid but not bile acid beta-oxidation rates are enhanced, especially upon the diet with the higher n-3/n-6 fatty acid ratio. Mitochondrial beta-oxidation rates are little or not affected by the high fat diets. (2) Rat liver carnitine acyltransferases are stimulated by the high fat diets, the more rich the n-3 fatty acid content, the more pronounced the stimulatory effect. (3) Rat heart peroxisomal and mitochondrial beta-oxidation rates were increased in animals receiving the n-3 fatty acid-enriched diet. At a low n-3/n-6 fatty acid ratio content of the diet, these oxidizing rate values were in control range. The carnitine acyltransferase activities were increased in rat heart to different extents, depending on the n-3/n-6 fatty acid ratio content of the diet. (4) Ultrastructural examination and morphometric determinations on hepatocytes from rats receiving the diets with the lowest and the highest n-3/n-6 fatty acid ratio contents disclose that in the latter case the numbers and fractional volumes of peroxisomes and mitochondria are significantly higher than in the former case.

Dependence of the plasma triacylglycerol-lowering effect of fish oil on insulin replacement in streptozotocin diabetic rats

Streptozotocin diabetic rats, with and without insulin replacement, and sham-injected controls were fed a high-fat (30% of energy) menhaden oil (MO) or corn oil (CO) diet for 2 weeks. After an overnight fast, plasma and livers were collected for analysis of insulin, glucose, triacylglycerol, cholesterol and glucose-6-phosphate dehydrogenase activity. Streptozotocin treatment resulted in decreased plasma insulin and elevated glucose. MO-feeding to insulin-replaced diabetic rats resulted in higher insulin and lower glucose levels compared to the respective CO-fed rats, suggesting decreased hepatic insulin extraction and greater peripheral utilization of glucose with MO. Plasma triacylglycerol and cholesterol, and hepatic glucose-6-phosphate dehydrogenase activity were reduced in MO-fed vs. CO-fed control rats. These effects of MO were prevented in the diabetic rats but were restored by insulin replacement. We conclude from our data that the presence of insulin is required to observe at least some of the effects of fish oil (FO). To explain our observations we propose that many of the effects of FO on hepatic metabolism are mediated by an inhibition of insulin action in the liver, thus providing a possible central mechanism for the regulation of hepatic lipid metabolism by dietary FO.