Docosahexaenoic acid synthesis from alpha-linolenic acid is inhibited by diets high in polyunsaturated fatty acids

The conversion of the plant-derived omega-3 (n-3) α-linolenic acid (ALA, 18:3n-3) to the long-chain eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3) can be increased by ALA sufficient diets compared to ALA deficient diets. Diets containing ALA above an optimal level result in no further increase in DHA levels in animals and humans. The present study evaluates means of maximizing plasma DHA accumulation by systematically varying both linoleic acid (LA, 18:2n-6) and ALA dietary level. Weanling rats were fed one of 54 diets for three weeks. The diets varied in the percentage of energy (en%) of LA (0.07–17.1 en%) and ALA (0.02–12.1 en%) by manipulating both the fat content and the balance of vegetable oils. The peak of plasma phospholipid DHA (>8% total fatty acids) was attained as a result of feeding a narrow dietary range of 1–3 en% ALA and 1–2 en% LA but was suppressed to basal levels (～2% total fatty acids) at dietary intakes of total polyunsaturated fatty acids (PUFA) above 3 en%. We conclude it is possible to enhance the DHA status of rats fed diets containing ALA as the only source of n-3 fatty acids but only when the level of dietary PUFA is low (<3 en%).     

Individual effects of dietary EPA and DHA on the functioning of the isolated working rat heart

The aim of this study was to evaluate the effects of dietary pure eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) on the physiology of the heart in normoxic conditions and during postischemic reperfusion. These effects were compared with those of dietary n-6 polyunsaturated fatty acids (PUFA). Rats were fed a diet containing either sunflower seed oil (75 g x kg(-1), SSO group), or a mixture of EPA (20:5 n-3) ethyl ester and SSO (10:90, EPA group), or a mixture of DHA (22:6 n-3) ethyl ester and SSO (10:90, DHA group), or a mixture of EPA + DHA ethyl esters and SSO (4.2:5.8:90, e+D group) for 6 weeks. The hearts were then perfused according to the working mode. The perfusion was maintained either in normoxic conditions or stopped for 17 min (global zero-flow ischemia) and restored for 33 min (reperfusion). The aortic and coronary flows, aortic developed pressure, and electrocardiogram were continuously monitored. When rats were fed a diet containing either EPA and (or) DHA, the n-6/n-3 PUFA ratio of cardiac phospholipids decreased. The proportion of arachidonic acid was reduced more with DHA than dietary EPA. In the EPA group, the percentage of DHA was lower than in the DHA group, but the percentage of EPA and docosapentaenoic acid (22:5 n-3) was higher. These changes in membrane fatty acid composition altered the cardiac function. In normoxic conditions, the coronary flow was higher in the SSO group than in the DHA and EPA groups. The heart rate was lower in the DHA and e+D groups than in the EPA and SSO groups. The aortic flow, cardiac output, and aortic developed pressure were not affected. During postischemic reperfusion, the recovery of aortic flow, coronary flow, and aortic developed pressure was similar in the four groups. A slightly improved recovery of cardiac function was noticed in the EPA group, but the difference was not significant. Feeding rats 5% fish oil + 5% SSO instead of 10% SSO for 8 weeks increased the incorporation of EPA in cardiac phospholipids and favored the recovery (+120%) of aortic flow during postischemic reperfusion. In conclusion, the beneficial effect of dietary fish oil on the recovery of cardiac pump activity during reperfusion was not observed with DHA or EPA alone. It appears to be positively related to the accumulation of EPA in membrane phospholipids. The dietary conditions favouring EPA accumulation remain to be determined.     

Liver conversion of docosahexaenoic and arachidonic acids from their 18-carbon precursors in rats on a DHA-free but α-LNA-containing n-3 PUFA adequate diet

The long-chain polyunsaturated fatty acids (PUFAs), eicosapentaenoic acid (EPA, 20:5n-3), docosahexaenoic acid (DHA, 22:6n-3), and arachidonic acid (AA, 20:4n-6), are critical for health. These PUFAs can be synthesized in liver from their plant-derived precursors, α-linolenic acid (α-LNA, 18:3n-3) and linoleic acid (LA, 18:2n-6). Vegetarians and vegans may have suboptimal long-chain n-3 PUFA status, and the extent of the conversion of α-LNA to EPA and DHA by the liver is debatable. We quantified liver conversion of DHA and other n-3 PUFAs from α-LNA in rats fed a DHA-free but α-LNA (n-3 PUFA) adequate diet, and compared results to conversion of LA to AA. [U-(13)C]LA or [U-(13)C]α-LNA was infused intravenously for 2h at a constant rate into unanesthetized rats fed a DHA-free α-LNA adequate diet, and published equations were used to calculate kinetic parameters. The conversion coefficient k(?) of DHA from α-LNA was much higher than for AA from LA (97.2×10(-3) vs. 10.6×10(-3)min(-1)), suggesting that liver elongation-desaturation is more selective for n-3 PUFA biosynthesis on a per molecule basis. The net daily secretion rate of DHA, 20.3μmol/day, exceeded the reported brain DHA consumption rate by 50-fold, suggesting that the liver can maintain brain DHA metabolism with an adequate dietary supply solely of α-LNA. This infusion method could be used in vegetarians or vegans to determine minimal daily requirements of EPA and DHA in humans.     

Inhibition of Δ-6 desaturase reduces fatty acid re-esterification in 3T3-L1 adipocytes independent of changes in n3-PUFA cellular content

Δ-6 desaturase (D6D) is a key enzyme in the synthesis of long-chain polyunsaturated fatty acids (LC-PUFA). Evidence suggests that reduced D6D activity not only disrupts LC-PUFA production, but also impacts whole body lipid handling and body weight; however, the mechanisms remain largely unexplored. Therefore, we investigated the effect of D6D inhibition on the regulation of lipid accumulation in 3T3-L1 adipocytes with and without changes in n-3 PUFA content. 3T3-L1 cells were treated with a D6D inhibitor (SC-26196) in the presence or absence of α-linolenic acid (ALA) throughout differentiation. We found that D6D inhibition blocked the conversion of ALA to eicosapentaenoic acid (EPA) and docosapentaenoic acid (DPAn-3) when ALA was supplemented, while no changes in n-3 PUFA content were observed in cells treated with the D6D inhibitor alone. D6D inhibited cells had reduced triacylglycerol (TAG) accumulation despite an EPA/DPA deficiency. In addition, analyses of cellular protein markers, as well as non-esterified fatty acids and glycerol release in medium, suggested an increase in lipolysis and a decrease in fatty acid re-esterification in D6D-inhibited cells, independent of n-3 PUFA changes. To provide further evidence, we treated cells with the D6D inhibitor in the presence or absence of EPA and compared them with ALA-treated cells. Although EPA further reduced TAG content, the reduced markers of fatty acid re-esterification were not affected by ALA or EPA. Collectively, this study provides new insight showing that D6D inhibition reduces TAG accumulation and fatty acid re-esterification in adipocytes independent of changes in n-3 PUFA cellular content.     

Linoleic and α-linolenic acid as precursor and inhibitor for the synthesis of long-chain polyunsaturated fatty acids in liver and brain of growing pigs

Studies suggested that in human adults, linoleic acid (LA) inhibits the biosynthesis of n-3 long-chain polyunsaturated fatty acids (LC-PUFA), but their effects in growing subjects are largely unknown. We used growing pigs as a model to investigate whether high LA intake affects the conversion of n-3 LC-PUFA by determining fatty acid composition and mRNA levels of Δ5- and Δ6 desaturase and elongase 2 and -5 in liver and brain. In a 2 × 2 factorial arrangement, 32 gilts from eight litters were assigned to one of the four dietary treatments, varying in LA and α-linolenic acid (ALA) intakes. Low ALA and LA intakes were 0.15 and 1.31, and high ALA and LA intakes were 1.48 and 2.65 g/kg BW0.75 per day, respectively. LA intake increased arachidonic acid (ARA) in liver. ALA intake increased eicosapentaenoic acid (EPA) concentrations, but decreased docosahexaenoic acid (DHA) (all P < 0.01) in liver. Competition between the n-3 and n-6 LC-PUFA biosynthetic pathways was evidenced by reductions of ARA (>40%) at high ALA intakes. Concentration of EPA (>35%) and DHA (>20%) was decreased by high LA intake (all P < 0.001). Liver mRNA levels of Δ5- and Δ6 desaturase were increased by LA, and that of elongase 2 by both ALA and LA intakes. In contrast, brain DHA was virtually unaffected by dietary LA and ALA. Generally, dietary LA inhibited the biosynthesis of n-3 LC-PUFA in liver. ALA strongly affects the conversion of both hepatic n-3 and n-6 LC-PUFA. DHA levels in brain were irresponsive to these diets. Apart from Δ6 desaturase, elongase 2 may be a rate-limiting enzyme in the formation of DHA.     

NGF blocks polyunsaturated fatty acids biosynthesis in n-3 fatty acid-supplemented PC12 cells

Regulation of polyunsaturated fatty acid (PUFA) biosynthesis in proliferating and NGF-differentiated PC12 pheochromocytoma cells deficient in n-3 docosahexaenoic acid (DHA 22:6n-3) was studied. A dose- and time-dependent increase in eicosapentaenoic acid (EPA, 20:5n-3), docosapentaenoic acid (DPA, 22:5n-3) and DHA in phosphatidylethanolamine (PtdEtn) and phosphatidylserine (PtdSer) glycerophospholipids (GPL) via the elongation/desaturation pathway following alpha-linolenic acid (ALA, 18:3n-3) supplements was observed. That was accompanied by a marked reduction of eicosatrienoic acid (Mead acid 20:3n-9), an index of PUFA deficiency. EPA supplements were equally effective converted to 22:5n-3 and 22:6n-3. On the other hand, supplements of linoleic acid (LNA, 18:2n-6) were not effectively converted into higher n-6 PUFA intermediates nor did they impair elongation/desaturation of ALA. Co-supplements of DHA along with ALA did not interfere with 20:5n-3 biosynthesis but reduced further elongation to 22-hydrocarbon PUFA intermediates. A marked decrease in the newly synthesized 22:5n-3 and 22:6n-3 following ALA or EPA supplements was observed after nerve growth factor (NGF)-induced differentiation. NGF also inhibited the last step in 22:5n-6 formation from LNA. These results emphasize the importance of overcoming n-3 PUFA deficiency and raise the possibility that growth factor regulation of the last step in PUFA biosynthesis may constitute an important feature of neuronal phenotype acquisition.     

Comparative effects of well-balanced diets enriched in α-linolenic or linoleic acids on LC-PUFA metabolism in rat tissues

The intake of the essential fatty acid precursor α-linolenic acid (ALA) contributes to ensure adequate n-3 long-chain polyunsaturated fatty acid (LC-PUFA) bioavailability. Conversely, linoleic acid (LA) intake may compromise tissue n-3 PUFA status as its conversion to n-6 LC-PUFA shares a common enzymatic pathway with the n-3 family. This study aimed to measure dietary ALA and LA contribution to LC-PUFA biosynthesis and tissue composition. Rats were fed with control or experimental diets moderately enriched in ALA or LA for 8 weeks. Liver Δ6- and Δ5-desaturases were analyzed and FA composition was determined in tissues (red blood cells, liver, brain and heart). Hepatic Δ6-desaturase activity was activated with both diets, and Δ5-desaturase activity only with the ALA diet. The ALA diet led to higher n-3 LC-PUFA composition, including DHA in brain and heart. The LA diet reduced n-3 content in blood, liver and heart, without impacting n-6 LC-PUFA composition. At levels relevant with human nutrition, increasing dietary ALA and reducing LA intake were both beneficial in increasing n-3 LC-PUFA bioavailability in tissues.     

α-亚麻酸对高脂模型大鼠组织脂肪酸代谢的影响

研究不同ALA含量油脂对高脂模型大鼠组织脂肪酸代谢的影响.60只雄性Wistar大鼠分为正常组、高脂组、花生油组、13％、27％和55％ ALA含量油脂组,除正常组和高脂组外,其余各组在饲喂高脂饲料的同时采用灌胃方式连续给予2 mL/kg.bw剂量的受试油.试验6周后分别测定大鼠各组织脂肪酸组成.结果表明,高脂饮食能够降低大鼠各组织n-3脂肪酸含量,但摄入不同ALA油脂可显著增加组织n-3脂肪酸含量,并具有一定的剂量效应关系;但ALA及其代谢产物EPA、DPA和DHA的累积具有组织特异性,其中肾和心组织中ALA累积高于血浆、脑及肝组织,肝和脑组织中EPA和DPA含量增加较显著,而肾和心组织中EPA含量不变,各组织DHA含量增加不显著.不同ALA油脂组C18:3(n-6)和C20:3 (n-6)差异不显著,但与花生油组相比,其血浆、脑和肾组织C20:4含量显著降低.因此,富含ALA含量的油脂能够增加组织中ALA及其代谢产物在组织中的含量,提高其在脑组织中的分布比例,这可能是ALA具有心血管保护作用和促进脑生长发育的作用机制之一.     

Correlations between blood and tissue omega-3 LCPUFA status following dietary ALA intervention in rats

The aim of this study was to assess relationships between the fatty acid contents of plasma and erythrocyte phospholipids and those in liver, heart, brain, kidney and quadriceps muscle in rats. To obtain a wide range of tissue omega-3 (n-3) long chain polyunsaturated fatty acids (LCPUFA) we subjected weanling rats to dietary treatment with the n-3 LCPUFA precursor, alpha linolenic acid (ALA, 18:3 n-3) for 3 weeks. With the exception of the brain, we found strong and consistent correlations between the total n-3 LCPUFA fatty acid content of both plasma and erythrocyte phospholipids with fatty acid levels in all tissues. The relationships between eicosapentaenoic acid (EPA, 20:5 n-3) and docosapentaenoic acid (DPA, 22:5 n-3) content in both blood fractions with levels in liver, kidney, heart and quadriceps muscle phospholipids were stronger than those for docosahexaenoic acid (DHA, 22:6 n-3). The strong correlations between the EPA+DHA (the Omega-3 Index), total n-3 LCPUFA and total n-3 PUFA contents in both plasma and erythrocyte phospholipids and tissues investigated in this study suggest that, under a wide range of n-3 LCPUFA values, plasma and erythrocyte n-3 fatty acid content reflect not only dietary PUFA intakes but also accumulation of endogenously synthesised n-3 LCPUFA, and thus can be used as a reliable surrogate for assessing n-3 status in key peripheral tissues.     

Dietary ALA,EPA and DHA have distinct effects on oxylipin profiles in female and male rat kidney,liver and serum

There is much data on the effects of dietary n-3 fatty acids on tissue fatty acid compositions, but comparable comprehensive data on their oxygenated metabolites (oxylipins) is limited. The effects of providing female and male rats with diets high in α-linolenic acid (ALA), EPA or DHA for 6 weeks on oxylipins and fatty acids in kidney, liver and serum were therefore examined. The oxylipin profile generally reflected fatty acids, but it also revealed unique effects of individual n-3 fatty acids that were not apparent from fatty acid data alone. Dietary ALA increased renal and serum DHA oxylipins even though DHA itself did not increase, while dietary EPA did not increase DHA oxylipins in kidney or liver, suggesting that high EPA may inhibit this conversion. Oxylipin data generally corroborated fatty acid data that indicated that DHA can be retroconverted to EPA and that further retroconversion to ALA is limited. Dietary n-3 fatty acids decreased n-6 fatty acids and their oxylipins (except linoleic acid and its oxylipins), in order of effectiveness of DHA > EPA > ALA, with some exceptions: several arachidonic acid oxylipins modified at carbon 15 were not lower in all three sites, and EPA had a greater effect on 12-hydroxy-eicosatetraenoic acid and its metabolites in the liver. Oxylipins were predominantly higher in males, which was not reflective of fatty acids. Tissue-specific oxylipin profiles, therefore, provide further information on individual dietary n-3 fatty acid and sex effects that may help explain their unique physiological effects and have implications for dietary recommendations.     

Marine fish oil is more potent than plant-based n-3 polyunsaturated fatty acids in the prevention of mammary tumors

Marine-derived n-3 polyunsaturated fatty acids (PUFAs), such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), have been shown to inhibit mammary carcinogenesis. However, evidence regarding plant-based α-linolenic acid (ALA), the major n-3 PUFA in the Western diet, remains equivocal. The objective of this study was to examine the effect of lifelong exposure to plant- or marine-derived n-3 PUFAs on pubertal mammary gland and tumor development in MMTV-neu(ndl)-YD5 mice. It is hypothesized that lifelong exposure to n-3 PUFA reduces terminal end buds during puberty leading to delayed tumor onset, volume and multiplicity. It is further hypothesized that plant-derived n-3 PUFAs will exert dose-dependent effects. Harems of MMTV-FVB males were bred with wild-type females and fed either a (1) 10% safflower (10% SF, n-6 PUFA, control), (2) 10% flaxseed (10% FS), (3) 7% safflower plus 3% flaxseed (3% FS) or (4) 7% safflower plus 3% menhaden (3% FO) diet. Female offspring were maintained on parental diets. Compared to SF, 10% FS and 3% FO reduced (P<.05) terminal end buds at 6 weeks and tumor volume and multiplicity at 20 weeks. A dose-dependent reduction of tumor volume and multiplicity was observed in mice fed 3% and 10% FS. Antitumorigenic effects were associated with altered HER2, pHER-2, pAkt and Ki-67 protein expression. Compared to 10% SF, 3% FO significantly down-regulated expression of genes involved in eicosanoid synthesis and inflammation. From this, it can be estimated that ALA was 1/8 as potent as EPA+DHA. Thus, marine-derived n-3 PUFAs have greater potency versus plant-based n-3 PUFAs.     

Compartmental modeling to quantify alpha-linolenic acid conversion after longer term intake of multiple tracer boluses

To estimate in vivo alpha-linolenic acid (ALA; C18:3n-3) conversion, 29 healthy subjects consumed for 28 days a diet providing 7% of energy from linoleic acid (C18:2n-6) and 0.4% from ALA. On day 19, subjects received a single bolus of 30 mg of uniformly labeled [(13)C]ALA and for the next 8 days 10 mg twice daily. Fasting plasma phospholipid concentrations of (12)C- and (13)C-labeled ALA, eicosapentaenoic acid (EPA; C20:5n-3), docosapentaenoic acid (DPA; C22:5n-3), and docosahexaenoic acid (DHA; C22:6n-3) were determined on days 19, 21, 23, 26, 27, and 28. To estimate hepatic conversion of n-3 fatty acids, a tracer model was developed based on the averaged (13)C data of the participants. A similar tracee model was solved using the averaged (12)C values, the kinetic parameters derived from the tracer model, and mean ALA consumption. ALA incorporation into plasma phospholipids was estimated by solving both models simultaneously. It was found that nearly 7% of dietary ALA was incorporated into plasma phospholipids. From this pool, 99.8% was converted into EPA and 1% was converted into DPA and subsequently into DHA. The limited incorporation of dietary ALA into the hepatic phospholipid pool contributes to the low hepatic conversion of ALA into EPA. A low conversion of ALA-derived EPA into DPA might be an additional obstacle for DHA synthesis.     

Quantitative contributions of diet and liver synthesis to docosahexaenoic acid homeostasis

Dietary requirements for maintaining brain and heart docosahexaenoic acid (DHA, 22:6n-3) homeostasis are not agreed on, in part because rates of liver DHA synthesis from circulating α-linolenic acid (α-LNA, 18:3n-3) have not been quantified. These rates can be estimated using intravenous radiotracer- or heavy isotope-labeled α-LNA infusion. In adult unanesthetized male rats, such infusion shows that liver synthesis–secretion rates of DHA from α-LNA markedly exceed brain and heart DHA synthesis rates and the brain DHA consumption rate, and that liver but not heart or brain synthesis is upregulated when dietary n-3 PUFA content is reduced. These rate differences reflect much higher expression of DHA-synthesizing enzymes in liver, and upregulation of liver but not heart or brain enzyme expression by reduced dietary n-3 PUFA content. A noninvasive intravenous [U?¹³C]α-LNA infusion method that produces steady-state liver tracer metabolism gives exact liver DHA synthesis–secretion rates and could be extended for human studies.     

Two weeks of docosahexaenoic acid (DHA) supplementation increases synthesis-secretion kinetics of n-3 polyunsaturated fatty acids compared to 8 weeks of DHA supplementation

Docosahexaenoic acid (DHA, 22:6n-3) must be consumed in the diet or synthesized from n-3 polyunsaturated fatty acid (PUFA) precursors. However, the effect of dietary DHA on the metabolic pathway is not fully understood. Presently, 21-day-old Long Evans rats were weaned onto one of four dietary protocols: 1) 8 weeks of 2% ALA (ALA), 2) 6 weeks ALA followed by 2 weeks of 2% ALA + 2% DHA (DHA), 3) 4 weeks ALA followed by 4 weeks DHA and 4) 8 weeks of DHA. After the feeding period, ²H₅-ALA and ¹³C₂₀-eicosapentaenoic acid (EPA, 20:5n-3) were co-infused and blood was collected over 3 h for determination of whole-body synthesis-secretion kinetics. The synthesis-secretion coefficient (ml/min, means ± SEM) for EPA (0.238±0.104 vs. 0.021±0.001) and DPAn-3 (0.194±0.060 vs. 0.020±0.008) synthesis from plasma unesterified ALA, and DPAn-3 from plasma unesterified EPA (2.04±0.89 vs. 0.163±0.025) were higher (P<.05) after 2 weeks compared to 8 weeks of DHA feeding. The daily synthesis-secretion rate (nmol/d) of DHA from EPA was highest after 4 weeks of DHA feeding (843±409) compared to no DHA (70±22). Liver gene expression of ELOVL2 and FADS2 were lower (P<.05) after 4 vs. 8 weeks of DHA. Higher synthesis-secretion kinetics after 2 and 4 weeks of DHA feeding suggests an increased throughput of the PUFA metabolic pathway. Furthermore, these findings may lead to novel dietary strategies to maximize DHA levels while minimizing dietary requirements.     

Elongase reactions as control points in long-chain polyunsaturated fatty acid synthesis

Background

Δ6-Desaturase (Fads2) is widely regarded as rate-limiting in the conversion of dietary α-linolenic acid (18:3n-3; ALA) to the long-chain omega-3 polyunsaturated fatty acid docosahexaenoic acid (22:6n-3; DHA). However, increasing dietary ALA or the direct Fads2 product, stearidonic acid (18:4n-3; SDA), increases tissue levels of eicosapentaenoic acid (20:5n-3; EPA) and docosapentaenoic acid (22:5n-3; DPA), but not DHA. These observations suggest that one or more control points must exist beyond ALA metabolism by Fads2. One possible control point is a second reaction involving Fads2 itself, since this enzyme catalyses desaturation of 24:5n-3 to 24:6n-3, as well as ALA to SDA. However, metabolism of EPA and DPA both require elongation reactions. This study examined the activities of two elongase enzymes as well as the second reaction of Fads2 in order to concentrate on the metabolism of EPA to DHA.

Methodology/Principal Findings

The substrate selectivities, competitive substrate interactions and dose response curves of the rat elongases, Elovl2 and Elovl5 were determined after expression of the enzymes in yeast. The competitive substrate interactions for rat Fads2 were also examined. Rat Elovl2 was active with C₂₀ and C₂₂ polyunsaturated fatty acids and this single enzyme catalysed the sequential elongation reactions of EPA→DPA→24:5n-3. The second reaction DPA→24:5n-3 appeared to be saturated at substrate concentrations not saturating for the first reaction EPA→DPA. ALA dose-dependently inhibited Fads2 conversion of 24:5n-3 to 24:6n-3.

Conclusions

The competition between ALA and 24:5n-3 for Fads2 may explain the decrease in DHA levels observed after certain intakes of dietary ALA have been exceeded. In addition, the apparent saturation of the second Elovl2 reaction, DPA→24:5n-3, provides further explanations for the accumulation of DPA when ALA, SDA or EPA is provided in the diet. This study suggests that Elovl2 will be critical in understanding if DHA synthesis can be increased by dietary means.     

N-3 polyunsaturated fatty acid consumption produces neurobiological effects associated with prevention of depression in rats after the forced swimming test

Epidemiological data and clinical trials suggest that n-3 polyunsaturated fatty acids (PUFA) eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) have preventive and therapeutic effects on depression; however, the underlying mechanism remains elusive. The present study aimed to examine the behavioral effects and antidepressant mechanism of n-3 PUFA using a forced swimming test. Eleven-week-old male Sprague-Dawley rats were fed an American Institute of Nutrition-93M diet containing 0%, 0.5% or 1% EPA and DHA relative to the total energy intake in their diet for 12 weeks (n=8 per group). Total dietary intake, body weight and hippocampus weights were not significantly different among groups. The groups administered 0.5% and 1% EPA+DHA diets had significantly higher levels of n-3 PUFA in their brain phospholipids compared to those in the control group. The immobility time was significantly decreased and the climbing time was significantly increased in the 0.5% and 1% EPA+DHA groups compared with those in the 0% EPA+DHA group. Plasma serotonin concentration and hippocampus c-AMP response element binding protein (CREB) expression were significantly increased in the 0.5% and 1% EPA+DHA groups compared with those in the 0% EPA+DHA group. Conversely, interleukin (IL)-6 expression was significantly reduced in the 0.5% and 1% EPA+DHA groups compared with that in the 0% EPA+DHA group. However, there were no dose-dependent effects of n-3 PUFA and no significant differences in expressions of IL-1β, tumor necrosis factor-α, brain-derived neurotrophic factor or phosphorylated CREB. In conclusion, long-term intake of EPA+DHA induced antidepressant-like effects in rats and overexpression of CREB via decreased IL-6 expression.     

Distinct regulation of plasma LDL cholesterol by eicosapentaenoic acid and docosahexaenoic acid in high fat diet-fed hamsters: Participation of cholesterol ester transfer protein and LDL receptor

Despite established anti-atherogenic action, previous reports have shown that fish oils or n-3 poly-unsaturated fatty acid (PUFA) increase plasma LDL-C in animals and humans. However, which component of n-3 PUFAs and what mechanisms contribute to this increase are unclear. We investigated the effects of the major components of n-3 PUFA, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), on plasma LDL-C in high fat diet-fed hamsters. While LDL-C increased significantly with n-3 PUFA oil and DHA, EPA had no effect on LDL-C. Interestingly, a positive correlation was found between plasma cholesterol ester transfer protein (CETP) activity and LDL-C. Only DHA increased plasma CETP activity and significantly decreased LDL receptor expression in the liver. Our data suggest that DHA, not EPA, is a major factor in the LDL-C increasing effect of n-3 PUFA oil. These differential effects on LDL-C may arise from differences in plasma CETP activity and LDL receptor expression.     

Effects of ALA,EPA and DHA in high-carbohydrate,high-fat diet-induced metabolic syndrome in rats

We compared the cardiovascular, hepatic and metabolic responses to individual dietary n-3 fatty acids (α-linolenic acid, ALA; eicosapentaenoic acid, EPA; and docosahexaenoic acid, DHA) in a high-carbohydrate, high-fat diet-induced model of metabolic syndrome in rats. Additionally, we measured fatty acid composition of plasma, adipose tissue, liver, heart and skeletal muscle in these rats. The same dosages of ALA and EPA/DHA produced different physiological responses to decrease the risk factors for metabolic syndrome. ALA did not reduce total body fat but induced lipid redistribution away from the abdominal area and favorably improved glucose tolerance, insulin sensitivity, dyslipidemia, hypertension and left ventricular dimensions, contractility, volumes and stiffness. EPA and DHA increased sympathetic activation, reduced the abdominal adiposity and total body fat and attenuated insulin sensitivity, dyslipidemia, hypertension and left ventricular stiffness but not glucose tolerance. However, ALA, EPA and DHA all reduced inflammation in both the heart and the liver, cardiac fibrosis and hepatic steatosis. These effects were associated with complete suppression of stearoyl-CoA desaturase 1 activity. Since the physiological responses to EPA and DHA were similar, it is likely that the effects are mediated by DHA with EPA serving as a precursor. Also, ALA supplementation increased DHA concentrations but induced different physiological responses to EPA and DHA. This result strongly suggests that ALA has independent effects in metabolic syndrome, not relying on its metabolism to DHA.     

Low dietary fish-oil threshold for myocardial membrane n-3 PUFA enrichment independent of n-6 PUFA intake in rats

Long chain n-3 PUFA docosahexaenoic acid (DHA) is important for heart and brain function. Investigations of biologically plausible mechanisms using animal models associate cardioprotection with DHA incorporation into myocardial membranes that are largely derived from supra-physiological fish oil (FO) intake. We measured the incorporation of DHA into myocardial membranes of rats from low dietary FO intake within human dietary range and quantitatively assessed the influence of dietary n-6 PUFA. With rats fed diets containing 0.16%–5% FO, equal to 0.12%–8.7% energy (%en) as eicosapentaenoic acid (EPA) and DHA (EPA+DHA), and either 1.5%en or 7.5%en n-6 PUFA (linoleic acid) for four weeks, dietary n-6:n-3 PUFA ratios ranged from 74 to 0.3. Myocardial DHA concentration increased in a log-linear fashion with a dietary threshold of 0.019%en as EPA+DHA and half maximal dietary [EPA+DHA] equal to 0.29%en (95% CI, 0.23–0.35). Dietary linoleic acid intake did not influence myocardial DHA. Myocardial membranes are sensitive to absolute dietary intake of long chain n-3 PUFA at low %en in the rat, equivalent to a human intake of one meal of fatty fish per week or less. The dietary ratio of n-6:n-3 PUFA has no influence on long chain n-3 PUFA cellular incorporation from dietary fish oil.     

Anti-inflammatory effects of α-linolenic acid in M1-like macrophages are associated with enhanced production of oxylipins from α-linolenic and linoleic acid

Chronic inflammation, mediated in large part by proinflammatory macrophage populations, contributes directly to the induction and perpetuation of metabolic diseases, including obesity, insulin resistance and type 2 diabetes. Polyunsaturated fatty acids (PUFAs) can have profound effects on inflammation through the formation of bioactive oxygenated metabolites called oxylipins. The objective of this study was to determine if exposure to the dietary omega-3 PUFA α-linolenic acid (ALA) can dampen the inflammatory properties of classically activated (M1-like) macrophages derived from the human THP-1 cell line and to examine the accompanying alterations in oxylipin secretion. We find that ALA treatment leads to a reduction in lipopolysaccharide (LPS)-induced interleukin (IL)-1β, IL-6 and tumor necrosis factor-α production. Although ALA is known to be converted to longer-chain PUFAs eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), DHA oxylipins were reduced overall by ALA treatment, as was LPS-induced secretion of EPA oxylipins. In contrast, we observed profound increases in oxylipins directly derived from ALA. Lipoxygenase products of linoleic acid were also dramatically increased, and LPS-induced production of AA oxylipins, particularly prostaglandin D2, was reduced. These results suggest that ALA may act to dampen the inflammatory phenotype of M1-like macrophages by a unique set of mechanisms distinct from those used by the long-chain omega-3 fatty acids EPA and DHA. Thus, there is strong rationale for investigating the functions of ALA oxylipins and lesser-known LA oxylipins since they hold promise as anti-inflammatory agents.