Metabolism of highly unsaturated n-3 and n-6 fatty acids

Modulation of renal injury in pcy mice by dietary fat containing n−3 fatty acids depends on the level and type of fat

Low-fat diets and diets containing n-3 fatty acids (FA) slow the progression of renal injury in the male Han:Sprague-Dawley (SPRD)-cy rat model of polycystic kidney disease. To determine whether these dietary fat effects are similar in females and in another model of renal cystic disease, in this study we used both male and female pcy mice to examine the effects of fat level and type on disease progression. Adult pcy mice were fed 4, 10, or 20 g soybean oil/100 g diet for 130 d in study 1. In study 2, weanling pcy mice were fed high or low levels of fat rich in 18:2n-6 (corn oil, CO), 18:3n-3 (flaxseed oil/CO 4:1 g/g, FO), or 22:6n-3 (algal oil/CO 4:1 g/g, DO) for 8 wk. In adult pcy mice, low- compared with high-fat diets lowered kidney weights (2.4 +/- 0.2 vs. 3.1 +/- 0.2 g/100 g body weight, P = 0.006) and serum urea nitrogen (SUN) (9.6 +/- 0.6 vs. 11.9 +/- 0.6 mmol/L, P = 0.009), whereas in young pcy mice it reduced renal fibrosis volumes (0.44 +/- 0.04 vs. 0.62 +/- 0.04 mL/kg body weight, P < 0.0001). FO feeding in young pcy mice mitigated the detrimental effects of high fat on fibrosis while not altering kidney size, function, and oxidative damage when compared with the CO-fed mice. In contrast, DO- compared with CO-fed mice had higher kidney weights (2.64 +/- 0.07 vs. 2.24 +/- 0.08 g/100 g body weight, P = 0.005), SUN (9.4 +/- 0.57 vs. 7.0 +/- 0.62 mmol/L, P < 0.0001), and cyst volumes (7.9 +/- 0.28 vs. 6.2 +/- 0.30 mL/kg body weight, P < 0.0001) and similar levels of oxidative damage and fibrosis. The FA compositions of the diets were reflected in the kidneys: 18:2n-6, 18:3n-3, and 22:6n-3 were the highest in the CO, FO, and DO diets, respectively. Dietary effects on kidney disease progression were similar in males and females. A low-fat diet slows progression of renal injury in male and female pcy mice, consistent with findings in the male Han:SPRD-cy rat. Dietary fat type also influenced renal injury, with flaxseed oil diets rich in 18:3n-3 slowing early fibrosis progression compared with diets rich in 18:2n-6 or in 22:6n-3.

Alpha-linolenic acid metabolism in men and women: nutritional and biological implications

PURPOSE OF REVIEW

This review critically evaluates current knowledge of alpha-linolenic acid metabolism in adult humans based on the findings of studies using stable isotope tracers and on increased dietary alpha-linolenic acid intake. The relative roles of alpha-linolenic acid and of longer-chain polyunsaturated fatty acids in cell structure and function are discussed together with an overview of the major metabolic fates of alpha-linolenic acid. The extent of partitioning towards beta-oxidation and carbon recycling in humans is described. The use and limitations of stable isotope tracers to estimate alpha-linolenic acid desaturation and elongation are discussed. A consensus view of the extent of alpha-linolenic acid conversion to longer-chain fatty acids in humans is presented. The extent to which increasing dietary alpha-linolenic acid intake alters the concentrations of longer-chain n-3 fatty acids is described. The biological and nutritional implications of these findings are discussed.

RECENT FINDINGS

Conversion of alpha-linolenic acid to eicosapentaenoic acid is limited in men and further transformation to docosahexaenoic acid is very low. A lower proportion of alpha-linolenic acid is used as a substrate for beta-oxidation in women compared with men, while the fractional conversion to longer-chain fatty acids is greater, possibly due to the regulatory effects of oestrogen.

SUMMARY

Overall, alpha-linolenic acid appears to be a limited source of longer-chain n-3 fatty acids in man and so adequate intakes of preformed n-3 polyunsaturated fatty acids, in particular docosahexaenoic acid, may be important for maintaining optimal tissue function. Capacity to upregulate alpha-linolenic acid transformation in women may be important for meeting the demands of the fetus and neonate for docosahexaenoic acid.

Effects of n-3 polyunsaturated fatty acid supplementation in pregnancy on maternal and fetal erythrocyte fatty acid composition

OBJECTIVE

The aim of this study was to assess the effects of fish oil supplementation in pregnancy on maternal erythrocyte fatty acid composition at different stages of pregnancy and in the post-partum period, and on neonatal erythrocyte fatty acid composition.

DESIGN

A double-blind, randomised, placebo-controlled study.

SETTING

: Subiaco, Western Australia.

SUBJECTS

In all, 98 women booked for delivery at St John of God Hospital, Subiaco, were recruited from private rooms of obstetricians. In total, 83 women and their healthy full-term babies completed the study.

INTERVENTION

Women received either 4 g of fish oil (n=52) (56% docosahexaenoic acid (DHA) and 28% eicosapentaenoic acid (EPA) or placebo (olive oil) (n=46) per day from 20 weeks gestation until delivery.

MAIN OUTCOME MEASURES

Erythrocyte phospholipid fatty acids were measured in maternal peripheral blood at 20, 30 and 37 weeks of pregnancy and at 6 weeks post partum, and from cord blood collected at birth.

RESULTS

Compared to the control group, maternal EPA and DHA were significantly higher in the fish oil group at 30 and 37 weeks gestation, and remained elevated at 6 weeks post partum (P<0.001). The proportions of n-6 polyunsaturated (arachidonic acid, 22:3n-6 and 22:4n-6) were significantly lower in the fish oil supplemented group at the same time periods (P<0.001). Similarly, the proportions of EPA and DHA were significantly higher (P<0.001), and those of n-6 polyunsaturated fatty acids arachidonic acid, 20:3n-6, 22:3n-6 and 22:4n-6 were significantly lower (P<0.001), in erythrocytes from neonates in the fish oil group, compared to those in the control group.

CONCLUSION

Fish oil supplementation from 20 weeks of pregnancy until birth is an effective means of enhancing n-3 fatty acid status of both mothers and neonates. Furthermore, the changes in maternal erythrocyte fatty acid composition are retained until at least 6 weeks post partum. It is essential to assess the effects of concomitant decreases in arachidonic acid status before any dietary recommendations can be made.

SPONSORSHIP

The study was supported by grants from the NH & MRC and Raine Medical Research Foundation, Australia.

Association of cystic fibrosis with abnormalities in fatty acid metabolism

BACKGROUND

Patients with cystic fibrosis have altered levels of plasma fatty acids. We previously demonstrated that arachidonic acid levels are increased and docosahexaenoic acid levels are decreased in affected tissues from cystic fibrosis-knockout mice. In this study we determined whether humans with mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene have a similar fatty acid defect in tissues expressing CFTR.

METHODS

Fatty acids from nasal- and rectal-biopsy specimens, nasal epithelial scrapings, and plasma were analyzed from 38 subjects with cystic fibrosis and compared with results in 13 obligate heterozygotes, 24 healthy controls, 11 subjects with inflammatory bowel disease, 9 subjects with upper respiratory tract infection, and 16 subjects with asthma.

RESULTS

The ratio of arachidonic to docosahexaenoic acid was increased in mucosal and submucosal nasal-biopsy specimens (P<0.001) and rectal-biopsy specimens (P=0.009) from subjects with cystic fibrosis and pancreatic sufficiency and subjects with cystic fibrosis and pancreatic insufficiency, as compared with values in healthy control subjects. In nasal tissue, this change reflected an increase in arachidonic acid levels and a decrease in docosahexaenoic acid levels. In cells from nasal mucosa, the ratio of arachidonic to docosahexaenoic acid was increased in subjects with cystic fibrosis (P<0.001), as compared with healthy controls, with values in obligate heterozygotes intermediate between these two groups (P<0.001). The ratio was not increased in subjects with inflammatory bowel disease. Subjects with asthma and those with upper respiratory tract infection had values intermediate between those in subjects with cystic fibrosis and those in healthy control subjects.

CONCLUSIONS

These data indicate that alterations in fatty acids similar to those in cystic fibrosis-knockout mice are present in CFTR-expressing tissue from subjects with cystic fibrosis.

Beneficial effects of a diet rich in a mixture of n - 6/n - 3 essential fatty acids and of their metabolites on cyclosporine - nephrotoxicity

In this study we investigated the role of a mixture of n-6/n-3 essential fatty acids, in the cyclosporine model nephrotoxicity. Administration of cyclosporine in rats decreased creatinine clearance and provoked body weight loss, but it did not induce proteinuria and did not alter the urine volume. These changes were associated with decreased urinary ratios of prostaglandin E/thromboxane B and prostaglandin I/thromboxane B excretions. Light microscopic sections showed that 100% of the animals were affected by histological tubular lesions on their kidneys. Administration of cyclosporine to animals fed for 3 months on standard chow containing a mixture of n - 6/n - 3 essential fatty acids, restored creatinine clearance, augmented urine volume and prevented body weight loss. The improvement of renal function was accompanied by increased urinary ratios of prostaglandin E/thromboxane B and prostaglandin I/thromboxane B excretions. Light microscopic sections showed that only 40% of the animals demonstrated histological tubular lesions, of minor importance, to their kidneys. Our results suggest that the metabolites of arachidonic acid can play important role in the development of cyclosporine-nephrotoxicity because they increase the levels of thromboxane A and that the enhanced synthesis of prostaglandins (E) and (I) induced by a mixture of n - 6/n - 3 essential fatty acids, could play a beneficial role in the prevention of this renal dysfunction.

Docosahexaenoic acid (DHA) content of membranes determines molecular activity of the sodium pump: implications for disease states and metabolism

The omega-3 polyunsaturate, docosahexaenoic acid (DHA), plays a number of biologically important roles, particularly in the nervous system, where it is found in very high concentrations in cell membranes. In infants DHA is required for the growth and functional development of the brain, with a deficiency resulting in a variety of learning and cognitive disorders. During adulthood DHA maintains normal brain function and recent evidence suggests that reduced DHA intake in adults is linked with a number of neurological disorders including schizophrenia and depression. Here we report a high positive correlation between the molecular activity (ATP min(-1)) of individual Na(+)K(+)ATPase units and the content of DHA in the surrounding membrane bilayer. This represents a fundamental relationship underlying metabolic activity, but may also represent a link between reduced levels of DHA and neurological dysfunction, as up to 60% of energy consumption in the brain is linked to the Na(+)K(+)ATPase enzyme.

Identification of a very long chain polyunsaturated fatty acid Δ4-desaturase from the microalga Pavlova lutheri 1

Pavlova lutheri, a marine microalga, is rich in the very long chain polyunsaturated fatty acids (VLCPUFAs) eicosapentaenoic (20:5n-3) and docosahexaenoic (22:6n-3) acids. Using an expressed sequence tag approach, we isolated a cDNA designated Pldes1, and encoding an amino acid sequence showing high similarity with polyunsaturated fatty acid front-end desaturases. Heterologous expression in yeast demonstrated that PlDES1 desaturated 22:5n-3 and 22:4n-6 into 22:6n-3 and 22:5n-6 respectively, and was equally active on both substrates. Thus, PlDES1 is a novel VLCPUFA Delta4-desaturase. Pldes1 expression is four-fold higher during the mid-exponential phase of growth compared to late exponential and stationary phases.

Studies on the metabolic fate of n-3 polyunsaturated fatty acids

Several different processes involved in the metabolic fate of docosahexaenoic acid (DHA, C22:6n-3) and its precursor in the biosynthesis route, C24:6n-3, were studied. In cultured skin fibroblasts, the oxidation rate of [1-14C] 24:6n-3 was 2.7 times higher than for [1-14C]22:6n-3, whereas [1-14C]22:6n-3 was incorporated 7 times faster into different lipid classes than was [1-14C]24:6n-3. When determining the peroxisomal acyl-CoA oxidase activity, similar specific activities for C22:6(n-3)-CoA and C24:6(n-3)-CoA were found in mouse kidney peroxisomes. Thioesterase activity was measured for both substrates in mouse kidney peroxisomes as well as mitochondria, and C22:6(n-3)-CoA was hydrolyzed 1.7 times faster than C24:6(n-3)-CoA. These results imply that the preferred metabolic fate of C24:6(n-3)-CoA, after its synthesis in the endoplasmic reticulum (ER), is to move to the peroxisome, where it is beta-oxidized, producing C22:6(n-3)-CoA. This DHA-CoA then preferentially moves back, probably as free fatty acid, to the ER, where it is incorporated into membrane lipids.

Docosahexaenoic acid membrane content and mRNA expression of acyl-CoA oxidase and of peroxisome proliferator-activated receptor-delta are modulated in Y79 retinoblastoma cells differently by low and high doses of alpha-linolenic acid

The mRNA expression levels of acyl-CoA oxidase (AOX), a key enzyme in very-long-chain fatty acid peroxisomal oxidation, and of peroxisome proliferator-activated receptor-delta (PPAR-delta), a nuclear receptor possibly involved in the gene regulation of brain lipid metabolism, were determined in human Y79 retinoblastoma cells by using real-time quantitative polymerase chain reaction. Cells were dosed with alpha-linolenic acid (18:3n-3), the essential metabolic precursor of the n-3 polyunsaturated fatty acid series that normally gives rise through terminal peroxisomal oxidation to the synthesis of membrane docosahexaenoic acid (22:6n-3, or DHA). The AOX and PPAR-delta relative expression levels increased 2.3 and 3.4 times, respectively, upon dosing of cells with 7 microM 18:3n-3, whereas AOX cDNA abundance decreased by 50% upon dosing with 70 microM 18:3n-3. Concurrently, the DHA content increased by 23% in the membrane ethanolamine-phosphoglycerides from cells dosed with 7 microM 18:3n-3, whereas it decreased by 38% upon dosing with 70 microM 18:3n-3. The DHA's upstream precursors (20:5n-3 and 22:5n-3) both accumulated in cells dosed with 7 or 70 microM 18:3n-3. The 18:3n-3-induced changes in membrane phospholipid fatty acid composition support the hypothesis that the terminal peroxisomal step of n-3 conversion is rate limiting in the Y79 line. The concurrent 7 microM 18:3n-3-induced increase of mRNAs encoding for AOX and for PPAR-delta suggests that 18:3n-3 (or its metabolites) at low concentration could trigger its proper conversion to DHA, possibly through activation of PPAR-delta-mediated transcription of AOX. Decreased membrane DHA content and mRNA expression level of AOX in 70-microM 18:3n-3-dosed cells corroborated the relationship between AOX expression and DHA synthesis and suggested that simultaneous down-regulating events occurred at high concentrations of 18:3n-3.

In vivo approaches to quantifying and imaging brain arachidonic and docosahexaenoic acid metabolism

A novel in vivo fatty acid method has been developed to quantify and image brain metabolism of nutritionally essential polyunsaturated fatty acids (PUFAs). In unanesthetized rodents, a radiolabeled PUFA is injected intravenously, and its rate of incorporation into brain phospholipids is determined by chemical analysis or quantitative autoradiography. Results indicate that about 5% of brain arachidonic acid (20:4 n-6) and of docosahexaenoic acid (22:6 n-3) acid are lost daily by metabolism and are replaced from dietary sources through the plasma. Calculated turnover rates of PUFAs in brain phospholipids, due to deesterification by phospholipase A(2) (PLA(2)) followed by reesterification, are very rapid, consistent with active roles of PUFAs in signal transduction and other processes. Turnover rates of arachidonate and docosahexaenoate are independent of each other and probably are regulated by independent sets of enzymes. Brain incorporation of radiolabeled arachidonate can be imaged in response to drugs that bind to receptors coupled to PLA(2) through G proteins, thus measuring PLA(2)-initiated signal transduction. The in vivo fatty method is being extended for human studies using positron emission tomography.

Conversion of alpha-linolenic acid to palmitic, palmitoleic, stearic and oleic acids in men and women

The purpose of this study was to determine whether adult humans can recycle carbon from alpha-linolenic acid (18:3n-3) into saturated (SFA) and monounsaturated (MUFA) fatty acids. Six men and six women consumed 700 mg [U-13C]-18:3n-3. Blood was collected over 21 days and breath over 24h. [13C]-labelled SFA and MUFA were detected in plasma phosphatidylcholine (PC) and triacylglycerol (TAG). Total labelled fatty acid incorporation into SFA and MUFA was five- and 25-fold greater in PC than TAG in men and women, respectively. [13C]-16:0 was the major labelled fatty acid in both fractions. Total [13C] incorporation into SFA and MUFA was 20% greater in men than women, and related positively (r(2) = 0.35, P<0.05) to the fractional recovery of labelled 18:3n-3 as 13CO2 on breath. These results suggest that the extent of partitioning towards beta-oxidation and carbon recycling may regulate the availability of 18:3n-3 for conversion to longer-chain fatty acids.

Unsaturated fatty acid regulation of peroxisome proliferator-activated receptor alpha activity in rat primary hepatocytes

Peroxisome proliferator-activated receptors (PPARs alpha, beta, gamma1, and gamma2) are widely regarded as monitors of intracellular nonesterified fatty acid (NEFA) levels. As such, fatty acid binding to PPAR leads to changes in the transcription of many genes involved in lipid metabolism and storage. Although the composition of the intracellular NEFA pool is likely an important factor controlling PPAR activity, little information is available on factors affecting its composition. Accordingly, we have examined the effects of exogenous fatty acids on PPARalpha activity and NEFA pool composition in rat primary hepatocytes. Prior to the addition of fatty acids to primary hepatocytes, nonesterified unsaturated fatty acid levels are very low, representing </=0.5% of the total fatty acid in the cell. The relative abundance of putative PPARalpha ligands in the NEFA pool is 20:4n-6 = 18:2n-6 = 18:1n-9 > 22:6n-3 > 18:3n-3/6 = 20:5n-3. Of these fatty acids, only 20:5n-3 and 22:6n-3 consistently induced PPARalpha activity. Metabolic labeling of primary hepatocytes indicated that both 14C-18:1n-9 and 14C-20:5n-3 are rapidly assimilated into neutral and polar lipids. Although the addition of 18:1n-9 had no effect on NEFA pool composition, 20:5n-3 mass increased >15-fold within 90 min. Changes in NEFA pool 20:5n-3 mass correlated with dynamic changes in the PPARalpha-regulated transcript mRNACYP4A. Metabolic labeling also indicated that a significant fraction of 14C-20:5n-3 was elongated to 22:5n-3. Cells treated with 22:5n-3 or 22:6n-3 led to a significant accumulation of 20:5n-3 in the NEFA pool through a process that requires peroxisomal beta-oxidation and fatty acyl CoA thioesterase activity. Further analyses suggest that 20:5n-3 and 22:6n-3, but not 22:5n-3, are active ligands for PPARalpha. These studies suggest that basal fatty acid levels in the NEFA pool coupled with rates of fatty acid esterification, elongation, desaturation, peroxisomal beta-oxidation, and fatty acyl thioestease activity are important determinants controlling NEFA pool composition and PPARalpha activity.

Lipid requirements of infants: implications for nutrient composition of fortified complementary foods

Dietary lipids have traditionally been considered as solely part of the exchangeable energy supply. The main consideration in infant nutrition has been the amount of fat that can be tolerated and digested by infants and young children. The significance of the composition of dietary fat has received little attention. Presently, there is a growing interest in the quality of dietary lipid supply in early childhood as a major determinant of growth, infant development and long-term health. Thus, the selection of dietary lipids during the first years of life is now considered to be critically important for health and good nutrition throughout the life course. Over the past decades interest has focused on the role of essential lipids in central nervous system development and of fatty acids and cholesterol in lipoprotein metabolism throughout life. Lipids are structural components of all tissues and are indispensable for cell and plasma membrane synthesis. The brain, retina and other neural tissues are particularly rich in long-chain PUFA. Some (n-6) and (n-3) fatty acids are precursors for eicosanoid formation; these are powerful mediators of numerous cell and tissue functions. Recommendations for infant nutrition and implications of these for the nutrient composition of complementary foods are presented and discussed. There is more to fat than its role as a key fuel in energy metabolism and body energy storage; lipids are essential for tissue growth, cardiovascular health, brain development and function throughout the life course.

Effect of reduced maternal protein intake in pregnancy in the rat on the fatty acid composition of brain, liver, plasma, heart and lung phospholipids of the offspring after weaning

Reduced protein intake during pregnancy decreased maternal hepatic and plasma docosahexaenoic acid concentrations and impaired docosahexaenoic acid accumulation into fetal brain in the rat. The present study investigated whether restriction of maternal protein intake during pregnancy in the rat alters membrane phospholipid fatty acid composition in the offspring after weaning. Female rats (six per group) were mated and fed diets containing either 180 or 90 g protein/kg throughout pregnancy. Mothers were transferred to standard chow after delivery and the litters reduced to eight pups. Weaning was at 28 d and pups were killed 5 to 6 d later. Tissue weights or membrane total phosphatidylcholine (PC) and phosphatidylethanolamine (PE) concentrations in the offspring did not differ between dietary groups. There were significant differences between the 180 and 90 g/kg groups in liver, brain, lung and heart fatty acid composition that differed between tissues and phospholipid classes. For example, docosahexaenoic and arachidonic acid concentrations were 23 and 10 % lower respectively in hepatic PC, but not PE, in the 90 g/kg group. In brain, docosahexaenoic acid concentration was 17 % lower in PC, but not PE, while arachidonic acid content was 21 % greater in PE but unchanged in PC. The greatest differences were in unsaturated fatty acids, which suggests alterations to desaturase activities and/or the specificity of phospholipid biosynthesis. These results suggest that restricted maternal protein intake during pregnancy results in persistent alterations to membrane fatty acid content.

Beneficial effects of a diet rich in a mixture of n - 6/n - 3 essential fatty acids and of their metabolites on cyclosporine - nephrotoxicity

In this study we investigated the role of a mixture of n-6/n-3 essential fatty acids, in the cyclosporine model nephrotoxicity. Administration of cyclosporine in rats decreased creatinine clearance and provoked body weight loss, but it did not induce proteinuria and did not alter the urine volume. These changes were associated with decreased urinary ratios of prostaglandin E/thromboxane B and prostaglandin I/thromboxane B excretions. Light microscopic sections showed that 100% of the animals were affected by histological tubular lesions on their kidneys. Administration of cyclosporine to animals fed for 3 months on standard chow containing a mixture of n - 6/n - 3 essential fatty acids, restored creatinine clearance, augmented urine volume and prevented body weight loss. The improvement of renal function was accompanied by increased urinary ratios of prostaglandin E/thromboxane B and prostaglandin I/thromboxane B excretions. Light microscopic sections showed that only 40% of the animals demonstrated histological tubular lesions, of minor importance, to their kidneys. Our results suggest that the metabolites of arachidonic acid can play important role in the development of cyclosporine-nephrotoxicity because they increase the levels of thromboxane A and that the enchanced synthesis of prostaglandins (E) and (I) induced by a mixture of n - 6/n - 3 essential fatty acids, could play a beneficial role in the prevention of this renal dysfunction.

Effect of altered dietary n-3 fatty acid intake upon plasma lipid fatty acid composition, conversion of [13C]alpha-linolenic acid to longer-chain fatty acids and partitioning towards beta-oxidation in older men

The effect of increased dietary intakes of alpha-linolenic acid (ALNA) or eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) for 2 months upon plasma lipid composition and capacity for conversion of ALNA to longer-chain metabolites was investigated in healthy men (52 (SD 12) years). After a 4-week baseline period when the subjects substituted a control spread, a test meal containing [U-(13)C]ALNA (700 mg) was consumed to measure conversion to EPA, docosapentaenoic acid (DPA) and DHA over 48 h. Subjects were then randomised to one of three groups for 8 weeks before repeating the tracer study: (1) continued on same intake (control, n 5); (2) increased ALNA intake (10 g/d, n 4); (3) increased EPA+DHA intake (1.5 g/d, n 5). At baseline, apparent fractional conversion of labelled ALNA was: EPA 2.80, DPA 1.20 and DHA 0.04 %. After 8 weeks on the control diet, plasma lipid composition and [(13)C]ALNA conversion remained unchanged compared with baseline. The high-ALNA diet resulted in raised plasma triacylglycerol-EPA and -DPA concentrations and phosphatidylcholine-EPA concentration, whilst [(13)C]ALNA conversion was similar to baseline. The high-(EPA+DHA) diet raised plasma phosphatidylcholine-EPA and -DHA concentrations, decreased [(13)C]ALNA conversion to EPA (2-fold) and DPA (4-fold), whilst [(13)C]ALNA conversion to DHA was unchanged. The dietary interventions did not alter partitioning of ALNA towards beta-oxidation. The present results indicate ALNA conversion was down-regulated by increased product (EPA+DHA) availability, but was not up-regulated by increased substrate (ALNA) consumption. This suggests regulation of ALNA conversion may limit the influence of variations in dietary n-3 fatty acid intake on plasma lipid compositions.

Life, death and membrane bilayers

Membrane bilayers are essential elements of life, and the synthesis of the hydrocarbons that make up membrane bilayers may have preceded the appearance of life on Earth. Membrane-associated processes are significant components of metabolism, and the acyl composition of membrane bilayers is associated with metabolic activity in a predictable manner. This has resulted in the "membrane pacemaker" theory of metabolism, which proposes that the relative balance between monounsaturated and long-chain polyunsaturated acyl chains in membrane bilayers is a fundamental determinant of metabolic rate of a species. The omega-3 polyunsaturated docosahexaenoate is an especially important component of membranes in this regard. Whilst it is suggested that the physical properties of membrane polyunsaturates are important with respect to their influence on metabolic rate, it is their chemistry that is important in aging. Membrane acyl composition is related to maximum lifespan in mammals and birds, probably via their role in lipid peroxidation. Calorie restriction modifies acyl composition of membrane bilayers and is associated with decreased membrane lipid peroxidation and lifespan extension. The membrane pacemaker theory of metabolism has given birth to the membrane pacemaker hypothesis of aging, which will require further investigation.

Maternal fish oil supplementation in pregnancy reduces interleukin-13 levels in cord blood of infants at high risk of atopy

BACKGROUND AND OBJECTIVES

The epidemiological association between higher dietary n-3 polyunsaturated fatty acids (PUFA) and lower prevalence of asthma, has led to interest in the role of early dietary modification in allergic disease prevention. In this study we examined the effects of maternal n-3 (PUFA)-rich fish oil supplementation on cord blood (CB) IgE and cytokine levels in neonates at risk of developing allergic disease.

METHODS

In a randomized double-blind, placebo-controlled trial, 83 atopic pregnant women received either fish oil capsules (n = 40) containing 3.7 g n-3 PUFA/day or placebo capsules (n = 43) from 20 weeks gestation until delivery. CB cytokine levels (IL-4, IL-5, IL-6, IL-10, IL-12, IL-13, TNF-alpha and IFN-gamma) and total IgE levels were measured and compared between the two groups. Fatty acid composition of red cell membranes was analysed by gas chromatography and the relationships among PUFA, cytokine and IgE levels were examined.

RESULTS

Maternal fish oil supplementation resulted in a significant increase in n-3 PUFA levels (P < 0.001) in neonatal erythrocyte membranes. Neonates whose mothers had fish oil supplementation had significantly lower plasma IL-13 (P < 0.05) compared to the control group. There was also a significant inverse relationship between levels of n-3 PUFA in neonatal cell membranes and plasma IL-13. There was no difference in levels of IgE and the other cytokines measured.

CONCLUSIONS

This study provides preliminary evidence that increasing neonatal n-3 PUFA levels with maternal dietary supplementation can achieve subtle modification of neonatal cytokine levels. Further assessment of immune function and clinical follow-up of these infants will help determine if there are any significant effects on postnatal immune development and expression of allergic disease.

Effect of the delta6-desaturase inhibitor SC-26196 on PUFA metabolism in human cells

The objective of this study was to determine the effect of 2,2-diphenyl-5-(4-[[(1 E)-pyridin-3-yl-methylidene]amino]piperazin-1-yl)pentanenitrile (SC-26196), a delta6-desaturase inhibitor, on PUFA metabolism in human cells. SC-26196 inhibited the desaturation of 2 microM [1-14C] 18:2n-6 by 87-95% in cultured human skin fibroblasts, coronary artery smooth muscle cells, and astrocytes. By contrast, SC-26196 did not affect the conversion of [1-14C]20:3n-6 to 20:4 in the fibroblasts, demonstrating that it is selective for delta6-desaturase. The IC50 values for inhibition of the desaturation of 2 microM [1-14C] 18:3n-3 and [3-14C]24:5n-3 in the fibroblasts, 0.2-0.4 microM, were similar to those for the inhibition of [1-14C 18:2n-6 desaturation, and the rates of recovery of [1-14C]18:2n-6 and [3-14C]24:5n-3 desaturation after removal of SC-26196 from the culture medium also were similar. SC-26196 reduced the conversion of [3-14C]22:5n-3 and [3-14C]24:5n-3 to DHA by 75 and 84%, respectively, but it had no effect on the retroconversion of [3-14C]24:6n-3 to DHA. These results demonstrate that SC-26196 effectively inhibits the desaturation of 18- and 24-carbon PUFA and, therefore, decreases the synthesis of arachidonic acid, EPA, and DHA in human cells. Furthermore, they provide additional evidence that the conversion of 22:5n-3 to DHA involves delta6-desaturation.

Conversion of hexadecanoic acid to hexadecenoic acid by rat Delta 6-desaturase

A higher content of C16:1 n-10 has recently been reported in the preputial gland of mice with a targeted disruption of the gene encoding stearoyl-CoA desaturase 1 (SCD1-/- mice) when compared with wild-type mice. This result has provided the first physiological evidence for the presence and regulation of a palmitoyl-CoA Delta 6-desaturase in mammals. To investigate the putative involvement of the known Delta 6-desaturase (FADS2) in this process, COS-7 cells expressing rat Delta 6-desaturase were incubated with C16:0. Transfected cells were able to synthesize C16:1 n-10, while nontransfected cells did not produce any C16:1 n-10. Evidence is therefore presented that the rat Delta 6-desaturase, which acts on the 18- and 24-carbon fatty acids of the n-6 and n-3 series, is also able to catalyze palmitic acid Delta 6 -desaturation.

Differential modulation of Toll-like receptors by fatty acids: preferential inhibition by n-3 polyunsaturated fatty acids

Human subjects consuming fish oil showed a significant suppression of cyclooxygenase-2 (COX-2) expression in blood monocytes when stimulated in vitro with lipopolysaccharide (LPS), an agonist for Toll-like receptor 4 (TLR4). Results with a murine monocytic cell line (RAW 264.7) stably transfected with COX-2 promoter reporter gene also demonstrated that LPS-induced COX-2 expression was preferentially inhibited by docosahexaenoic acid (DHA, C22:6n-3) and eicosapentaenoic acid (EPA, C20:5n-3), the major n-3 polyunsaturated fatty acids (PUFAs) present in fish oil. Additionally, DHA and EPA significantly suppressed COX-2 expression induced by a synthetic lipopeptide, a TLR2 agonist. These results correlated with the preferential suppression of LPS- or lipopeptide-induced NF kappa B activation by DHA and EPA. The target of inhibition by DHA is TLR itself or its associated molecules, but not downstream signaling components. In contrast, COX-2 expression by TLR2 or TRL4 agonist was potentiated by lauric acid, a saturated fatty acid. These results demonstrate that inhibition of COX-2 expression by n-3 PUFAs is mediated through the modulation of TLR-mediated signaling pathways. Thus, the beneficial or detrimental effects of different types of dietary fatty acids on the risk of the development of many chronic inflammatory diseases may be in part mediated through the modulation of TLRs.

Different effects of n-6 and n-3 polyunsaturated fatty acids on the activation of rat smooth muscle cells by interleukin-1 beta

There is good evidence that the n-3 polyunsaturated fatty acids (PUFAs) in fish oil have antiinflammatory effects and reduce the pathogenesis of atherosclerosis. However, the mechanisms underlying these actions are largely unknown. This study was designed to investigate the effects of membrane incorporation of two major components of fish oil [eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)], on rat smooth muscle cells (SMCs) activation induced by interleukin-1 beta (IL1 beta). We compared their effects with those of n-6 arachidonic acid (AA). Expression of vascular cell adhesion molecule-1 and monocyte chemoattractant protein-1 adhesion molecules involved in SMCs migration was enhanced by AA, whereas EPA and DHA had no similar effects. We established that AA potentiates IL1 beta-induced expression of the type IIA secreted phospholipase A2 (sPLA2) gene, whereas EPA and DHA reduce this stimulation. EPA and DHA also abolished proinflammatory prostaglandin PGE2 production by inhibiting the IL1 beta-induced production of cyclooxygenase-2 (COX-2) mRNA. Much interest was then focused on three transcriptional factors implicated in inflammation control and especially in modulating rat sPLA2 and COX-2 gene transcription: nuclear factor-kappa B, CCAAT/enhancer binding protein beta, and E26 transformation-specific-1. electrophoretic mobility shift assay revealed that the binding activity of all three factors was increased by AA and reduced (or not affected) by n-3 PUFA. These results indicate that EPA and DHA act in opposition to AA by modulating various steps of the inflammatory process induced by IL1 beta, probably by reducing mitogen-activated protein kinase p42/p44 activity.

Hormonal modulation of delta6 and delta5 desaturases: case of diabetes

Animal biosynthesis of high polyunsaturated fatty acids from linoleic, alpha-linolenic and oleic acids is mainly modulated by the delta6 and delta5 desaturases through dietary and hormonal stimulated mechanisms. From hormones, only insulin activates both enzymes. In experimental diabetes mellitus type-1, the depressed delta6 desaturase is restored by insulin stimulation of the gene expression of its mRNA. However, cAMP or cycloheximide injection prevents this effect. The depression of delta6 and delta5 desaturases in diabetes is rapidly correlated by lower contents of arachidonic acid and higher contents of linoleic in almost all the tissues except brain. However, docosahexaenoic n-3 acid enhancement, mainly in liver phospholipids, is not explained yet. In experimental non-insulin dependent diabetes, the effect upon the delta6 and delta5 desaturases is not clear. From all other hormones glucagon, adrenaline, glucocorticoids, mineralocorticoids, oestriol, oestradiol, testosterone and ACTH depress both desaturases, and a few hormones: progesterone, cortexolone and pregnanediol are inactive.

The effects of dietary lipid and strain difference on polyunsaturated fatty acid composition and conversion in anadromous and landlocked salmon (Salmo salar L.) parr

Five experimental diets containing different proportions of olive, sunflower and linseed oils were used in a 55-day feeding trial on both anadromous and landlocked parr of Atlantic salmon (Salmo salar) of the same age, in order to study the effects of diet and strain on growth and fatty acid composition and absolute gains in fish whole body triacylglycerols (TAG) and phospholipids (PL). Growth rate was higher in landlocked than in anadromous parr, but not between the different diets. By contrast, the effect of diet on whole body fatty acid composition was much more pronounced than that of strain difference. The fatty acids deposition results establish significant (P<0.05) positive correlations and linear relationships between the percentage of several fatty acids (18:1n-9, 18:2n-6, 18:3n-3) in dietary lipids and their absolute gains in whole body TAG and PL of both stocks. They also indicate the selective deposition of 18:1n-9 compared with linoleic acid (LLA) and linolenic acid (LNA). Finally, the results suggest the occurrence of the conversion of LLA and LNA to long-chain polyunsaturated fatty acids, its stimulation by increased substrate availability, a significantly higher n-3 and n-6 polyunsaturated fatty acids conversion capacity in landlocked than in anadromous parr and a strong genetic influence on docosahexaenoic acid content in salmon parr PL.

Brain lipid metabolism in the cPLA2 knockout mouse

We examined brain phospholipid metabolism in mice in which the cytosolic phospholipase A(2) (cPLA(2,) Type IV, 85 kDa) was knocked out (cPLA(2)(-/-) mice). Compared with controls, these mice demonstrated altered brain concentrations of several phospholipids, reduced esterified linoleate, arachidonate, and docosahexaenoate in choline glycerophospholipid, and reduced esterified arachidonate in phosphatidylinositol. Unanesthetized cPLA(2)(-/-) mice had reduced rates of incorporation of unlabeled arachidonate from plasma and from the brain arachidonoyl-CoA pool into ethanolamine glycerophospholipid and choline glycerophospholipid, but elevated rates into phosphatidylinositol. These differences corresponded to altered turnover and metabolic loss of esterified brain arachidonate. These results suggests that cPLA(2) is necessary to maintain normal brain concentrations of phospholipids and of their esterified polyunsaturated fatty acids. Reduced esterified arachidonate and docosahexaenoate may account for the resistance of the cPLA(2)(-/-) mouse to middle cerebral artery occlusion, and should influence membrane fluidity, neuroinflammation, signal transduction, and other brain processes.

Comparison of fatty acid composition in major lipid classes of the dominant benthic invertebrates of the Yenisei river

The composition and content of fatty acids (FAs) in total lipids, triacylglycerols (TAG) and polar lipids (PL) in dominant groups of benthic invertebrates: gammarids (Gammaridae, Amphipoda), chironomid larvae (Chironomidae, Diptera), caddisfly larvae (Trichoptera) and mayfly larvae (Ephemeroptera) were studied in the Yenisei river. For the first time data on the FA composition of species belonging to Trichoptera (Insecta) are presented. The groups of aquatic insect larvae and gammarids weakly differed in total content of essential polyunsaturated fatty acids (PUFAs). Hence, the strong invasion of gammarids which occurred in the last decades in the Yenisei river should not result in a decrease in potential yield of essential PUFA in the ecosystem and corresponding decrease in food resource quality for fish in respect to PUFA content. Significant differences in biomarker FAs in TAG were found which correlated to specific food sources. Different levels of long-chain PUFA in PL of the invertebrates are discussed in relation to the genetic ability of particular taxa to form these FAs.

Acyl composition of muscle membranes varies with body size in birds

The acyl composition of phospholipids from pectoral muscle of eight species of birds, ranging in size from the 13 g zebra finch to the 34 kg emu, were measured and combined with recent published results for a 3 g hummingbird. This represents an approximately 11000-fold range in body mass. Muscle phospholipids, and thus muscle membrane bilayers, from birds had a relatively constant unsaturated acyl chain content of 62% but exhibited a significant allometric decline in unsaturation index (number of double bonds per 100 acyl chains) with increasing body mass. There was a significant allometric increase in the percentage of mono-unsaturates and a significant allometric decline in the percentage of n-3 polyunsaturates with increasing body mass,whilst there were no significant allometric trends in either percentage of n-6 or percentage of total polyunsaturates in bird muscle. The relative content of the highly polyunsaturated docosahexaenoic acid (22:6 n-3) showed the greatest scaling with body mass, having an allometric exponent of -0.28. The contribution of this n-3 polyunsaturate to the unsaturation index varied with body size, ranging from less than a 6%contribution in the emu to approximately 70% in the hummingbird. Such allometric variation in the acyl composition of bird muscle phospholipids is similar to that observed in mammals, although birds have fewer n-3 polyunsaturates and more n-6 polyunsaturates than do mammalian phospholipids. This allometric variation in phospholipid acyl composition is discussed with respect to both the metabolic intensity and lifespan of different sized bird species.

Dietary effects of arachidonate-rich fungal oil and fish oil on murine hepatic and hippocampal gene expression

BACKGROUND

The functions, actions, and regulation of tissue metabolism affected by the consumption of long chain polyunsaturated fatty acids (LC-PUFA) from fish oil and other sources remain poorly understood; particularly how LC-PUFAs affect transcription of genes involved in regulating metabolism. In the present work, mice were fed diets containing fish oil rich in eicosapentaenoic acid and docosahexaenoic acid, fungal oil rich in arachidonic acid, or the combination of both. Liver and hippocampus tissue were then analyzed through a combined gene expression- and lipid- profiling strategy in order to annotate the molecular functions and targets of dietary LC-PUFA.

RESULTS

Using microarray technology, 329 and 356 dietary regulated transcripts were identified in the liver and hippocampus, respectively. All genes selected as differentially expressed were grouped by expression patterns through a combined k-means/hierarchical clustering approach, and annotated using gene ontology classifications. In the liver, groups of genes were linked to the transcription factors PPARalpha, HNFalpha, and SREBP-1; transcription factors known to control lipid metabolism. The pattern of differentially regulated genes, further supported with quantitative lipid profiling, suggested that the experimental diets increased hepatic beta-oxidation and gluconeogenesis while decreasing fatty acid synthesis. Lastly, novel hippocampal gene changes were identified.

CONCLUSIONS

Examining the broad transcriptional effects of LC-PUFAs confirmed previously identified PUFA-mediated gene expression changes and identified novel gene targets. Gene expression profiling displayed a complex and diverse gene pattern underlying the biological response to dietary LC-PUFAs. The results of the studied dietary changes highlighted broad-spectrum effects on the major eukaryotic lipid metabolism transcription factors. Further focused studies, stemming from such transcriptomic data, will need to dissect the transcription factor signaling pathways to fully explain how fish oils and arachidonic acid achieve their specific effects on health.

Eicosapentaenoic and docosapentaenoic acids are the principal products of alpha-linolenic acid metabolism in young men*

The capacity for conversion of alpha-linolenic acid (ALNA) to n-3 long-chain polyunsaturated fatty acids was investigated in young men. Emulsified [U-13C]ALNA was administered orally with a mixed meal to six subjects consuming their habitual diet. Approximately 33 % of administered [13C]ALNA was recovered as 13CO2 on breath over the first 24 h. [13C]ALNA was mobilised from enterocytes primarily as chylomicron triacylglycerol (TAG), while [13C]ALNA incorporation into plasma phosphatidylcholine (PC) occurred later, probably by the liver. The time scale of conversion of [13C]ALNA to eicosapentaenoic acid (EPA) and docosapentaenoic acid (DPA) suggested that the liver was the principal site of ALNA desaturation and elongation, although there was some indication of EPA and DPA synthesis by enterocytes. [13C]EPA and [13C]DPA concentrations were greater in plasma PC than TAG, and were present in the circulation for up to 7 and 14 d, respectively. There was no apparent 13C enrichment of docosahexaenoic acid (DHA) in plasma PC, TAG or non-esterified fatty acids at any time point measured up to 21 d. This pattern of 13C n-3 fatty acid labelling suggests inhibition or restriction of DHA synthesis downstream of DPA. [13C]ALNA, [13C]EPA and [13C]DPA were incorporated into erythrocyte PC, but not phosphatidylethanolamine, suggesting uptake of intact plasma PC molecules from lipoproteins into erythrocyte membranes. Since the capacity of adult males to convert ALNA to DHA was either very low or absent, uptake of pre-formed DHA from the diet may be critical for maintaining adequate membrane DHA concentrations in these individuals.

Conversion of alpha-linolenic acid to eicosapentaenoic, docosapentaenoic and docosahexaenoic acids in young women

The extent to which women of reproductive age are able to convert the n-3 fatty acid alpha-linolenic acid (ALNA) to eicosapentaenoic acid (EPA), docosapentaenoic acid (DPA) and docosahexaenoic acid (DHA) was investigated in vivo by measuring the concentrations of labelled fatty acids in plasma for 21 d following the ingestion of [U-13C]ALNA (700 mg). [13C]ALNA excursion was greatest in cholesteryl ester (CE) (224 (sem 70) micromol/l over 21 d) compared with triacylglycerol (9-fold), non-esterified fatty acids (37-fold) and phosphatidylcholine (PC, 7-fold). EPA excursion was similar in both PC (42 (sem 8) micromol/l) and CE (42 (sem 9) micromol/l) over 21 d. In contrast both [13C]DPA and [13C]DHA were detected predominately in PC (18 (sem 4) and 27 (sem 7) micromol/l over 21 d, respectively). Estimated net fractional ALNA inter-conversion was EPA 21 %, DPA 6 % and DHA 9 %. Approximately 22 % of administered [13C]ALNA was recovered as 13CO2 on breath over the first 24 h of the study. These results suggest differential partitioning of ALNA, EPA and DHA between plasma lipid classes, which may facilitate targeting of individual n-3 fatty acids to specific tissues. Comparison with previous studies suggests that women may possess a greater capacity for ALNA conversion than men. Such metabolic capacity may be important for meeting the demands of the fetus and neonate for DHA during pregnancy and lactation. Differences in DHA status between women both in the non-pregnant state and in pregnancy may reflect variations in metabolic capacity for DHA synthesis.

Thermotropic behavior of major phospholipids from marine invertebrates: changes with warm-acclimation and seasonal acclimatization

The crystal-liquid crystal-isotropic melt phase transitions of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) from muscle tissue of five species (actinia Metridium senile fimbriatum, mussel Crenomytilus grayanus, sea-urchin Strongylocentrotus intermedius, starfish Distolasterias nipon and the ascidian Halocynthia aurantium) of marine invertebrates, collected in winter at 0 degrees C and then acclimated to 18.5 degrees C for 5 days, were studied by differential scanning calorimetry and polarising microscopy. To elevate temperature from 0 to 18.5 degrees C, we used the rate of 4.5 degrees C/h. Although phase transitions of both phospholipids from animals collected in summer occurred already at temperatures below -1.7 oC (minimal temperature of seawater in winter), compensatory mechanisms resulted in a decrease by 29-43 oC in the phase transition temperature of PE in winter. Thermotropic behavior of PCs changed in various trends. However, the total heat of their phase transitions always decreased in winter compared with summer. For all species, except the mussel, the time of warm-acclimation was insufficient to adjust the thermotropic behavior of either phospholipid. Nevertheless, the unsaturation index decreased to achieve summer values, due primarily to decreased proportions of eicosapentaenate and docosahexaenate. The accumulation of arachidonate, during warm-acclimation, might be connected to the signalling properties of n-6 eicosanoids. Absence of effective homeoviscous mechanisms suggests that most of the studied marine invertebrates have very limited capacity to survive an acute temperature elevation, e.g. at the appearance of thermal currents.

Polyunsaturated fatty acid synthesis: what will they think of next?

Polyunsaturated fatty acids have crucial roles in membrane biology and signaling processes in most living organisms. However, it is only recently that molecular genetic approaches have allowed detailed studies of the enzymes involved in their synthesis. New evidence has revealed a range of pathways in different organisms. These include a complex sequence for synthesis of docosahexaenoic acid (22:6) in mammals and a polyketide synthase pathway in marine microbes.

Identification and expression of mammalian long-chain PUFA elongation enzymes

In mammalian cells, Sprecher has proposed that the synthesis of long-chain PUFA from the 20-carbon substrates involves two consecutive elongation steps, a delta6-desaturation step followed by retroconversion (Sprecher, H., Biochim. Biophys. Acta 1486, 219-231, 2000). We searched the database using the translated sequence of human elongase ELOVL5, whose encoded enzyme elongates monounsaturated and polyunsaturated FA, as a query to identify the enzyme(s) involved in elongation of very long chain PUFA. The database search led to the isolation of two cDNA clones from human and mouse. These clones displayed deduced amino acid sequences that had 56.4 and 58% identity, respectively, to that of ELOVL5. The open reading frame of the human clone (ELOVL2) encodes a 296-amino acid peptide, whereas the mouse clone (Elovl2) encodes a 292-amino acid peptide. Expression of these open reading frames in baker's yeast, Saccharomyces cerevisiae, demonstrated that the encoded proteins were involved in the elongation of both 20- and 22-carbon long-chain PUFA, as determined by the conversion of 20:4n-6 to 22:4n-6, 22:4n-6 to 24:4n-6, 20:5n-3 to 22:5n-3, and 22:5n-3 to 24:5n-3. The elongation activity of the mouse Elovl2 was further demonstrated in the transformed mouse L cells incubated with long-chain (C20- and C22-carbon) n-6 and n-3 PUFA substrates by the significant increase in the levels of 24:4n-6 and 24:5n-3, respectively. This report demonstrates the isolation and identification of two mammalian genes that encode very long chain PUFA specific elongation enzymes in the Sprecher pathway for DHA synthesis.

The roles of anabolic and catabolic reactions in the synthesis and recycling of polyunsaturated fatty acids

Generally the biosynthesis and degradation of compounds take place in separate subcellular compartments. The synthesis of 22 carbon acids, with their first double bond at position 4, requires anabolic enzymes in the endoplasmic reticulum as well as peroxisomal beta-oxidation enzymes. Partial degradation-resynthesis cycles, using enzymes in these two subcellular compartments, may play an important role in determining what PUFA are available for membrane lipid biosynthesis.

Comparison of 20-, 22-, and 24-carbon n-3 and n-6 polyunsaturated fatty acid utilization in differentiated rat brain astrocytes

Astrocytes convert n-6 fatty acids primarily to arachidonic acid (20:4n-6), whereas n-3 fatty acids are converted to docosapentaenoic (22:5n-3) and docosahexaenoic (22:6n-3) acids. The utilization of 20-, 22- and 24-carbon n-3 and n-6 fatty acids was compared in differentiated rat astrocytes to determine the metabolic basis for this difference. The astrocytes retained 81% of the arachidonic acid ([(3)H]20:4n-6) uptake and retroconverted 57% of the docosatetraenoic acid ([3-(14)C]22:4n-6) uptake to 20:4n-6. By contrast, 68% of the eicosapentaenoic acid ([(3)H]20:5n-3) uptake was elongated, and only 9% of the [3-(14)C]22:5n-3 uptake was retroconverted to 20:5n-3. Both tetracosapentaenoic acid ([3-(14)C]24:5n-3) and tetracosatetraenoic acid ([3-(14)C]24:4n-6) were converted to docosahexaenoic acid (22:6n-3) and 22:5n-6, respectively. Therefore, the difference in the n-3 and n-6 fatty acid products formed is due primarily to differences in the utilization of their 20- and 22-carbon intermediates. This metabolic difference probably contributes to the preferential accumulation of docosahexaenoic acid in the brain.

Docosahexaenoic, arachidonic, palmitic, and oleic acids are differentially esterified into phospholipids of frog retina

Docosahexaenoic acid (22:6n-3) is highly enriched in the retina. To determine if retinal cells take up and metabolize fatty acids in a specific manner, retinas from Rana pipiens were incubated for 3 h with an equimolar mixture of tritiated 22:6n-3, arachidonic acid (20:4n-6), palmitic acid, and oleic acid. The radiolabeling of retinal lipids was determined and compared to the endogenous fatty acid content of the lipids. The results showed that in most, but not all, cases, the relative labeling with the four precursor fatty acids was similar to their relative abundance in each glycerolipid. Thus, during retinal glycerolipid synthesis, either through de novo or acyl exchange reactions, fatty acids are incorporated in proportions reflecting their steady-state mass levels. Since other studies with labeled glycerol have shown greater differences between early labeling patterns and molecular species mass, the final incorporation we report may be due primarily to acyl exchange reactions.

The acyl composition of mammalian phospholipids: an allometric analysis

Data concerning the acyl composition of tissue phospholipids from mammal species, ranging in size from the shrew (7 g) to cattle (370 kg), has been collated from the literature and analysed allometrically. Phospholipids from heart, skeletal muscle, liver and kidney exhibited similar allometric trends whereby phospholipids had a significant decrease in unsaturation index (number of double bonds per 100 acyl chains) as species body size increased whilst there was no change in the percent of unsaturated acyl chains. Whilst total polyunsaturate content did not change with body mass, both heart and skeletal muscle phospholipids showed a significant allometric decrease in the omega-3 polyunsaturate content. The content of the highly polyunsaturated docosahexaenoic acid (22:6 n-3) in phospholipids showed significant and substantial allometric decline with increasing body mass in all four tissues (exponents ranged from -0.19 in liver to -0.40 in skeletal muscle). Brain phospholipids showed no allometric trends in acyl composition and were highly polyunsaturated in all species. These trends are discussed in light of the hypothesis that the relative content of polyunsaturated acyl chains in membranes, and especially docosahexaenoate (22:6 n-3), can act as a membrane pacemaker for metabolic activity.

Recent studies on interactions between n-3 and n-6 polyunsaturated fatty acids in brain and other tissues

Recent literature provides a basis for understanding the behavioral, functional, and structural consequences of nutritional deprivation or disease-related abnormalities of n-3 polyunsaturated fatty acids. The literature suggests that these effects are mediated through competition between n-3 and n-6 polyunsaturated fatty acids at certain enzymatic steps, particularly those involving polyunsaturated fatty acid elongation and desaturation. One critical enzymatic site is a delta6-desaturase. On the other hand, an in-vivo method in rats, applied following chronic n-3 nutritional deprivation or chronic administration of lithium, indicates that the cycles of de-esterification/re-esterification of docosahexaenoic acid (22:6n-3) and arachidonic acid (20:4n-6) within brain phospholipids operate independently of each other, and thus that the enzymes regulating each of these cycles are not likely sites of n-3/n-6 competition.

The same rat Delta6-desaturase not only acts on 18- but also on 24-carbon fatty acids in very-long-chain polyunsaturated fatty acid biosynthesis

The recently cloned Delta6-desaturase is known to catalyse the first step in very-long-chain polyunsaturated fatty acid biosynthesis, i.e. the desaturation of linoleic and alpha-linolenic acids. The hypothesis that this enzyme could also catalyse the terminal desaturation step, i.e. the desaturation of 24-carbon highly unsaturated fatty acids, has never been elucidated. To test this hypothesis, the activity of rat Delta6-desaturase expressed in COS-7 cells was investigated. Recombinant Delta6-desaturase expression was analysed by Western blot, revealing a single band at 45 kDa. The putative involvement of this enzyme in the Delta6-desaturation of C(24:5) n-3 to C(24:6) n-3 was measured by incubating transfected cells with C(22:5) n-3. Whereas both transfected and non-transfected COS-7 cells were able to synthesize C(24:5) n-3 by elongation of C(22:5) n-3, only cells expressing Delta6-desaturase were also able to produce C(24:6) n-3. In addition, Delta6-desaturation of [1-(14)C]C(24:5) n-3 was assayed in vitro in homogenates from COS-7 cells expressing Delta6-desaturase or not, showing that Delta6-desaturase catalyses the conversion of C(24:5) n-3 to C(24:6) n-3. Evidence is therefore presented that the same rat Delta6-desaturase catalyses not only the conversion of C(18:3) n-3 to C(18:4) n-3, but also the conversion of C(24:5) n-3 to C(24:6) n-3. A similar mechanism in the n-6 series is strongly suggested.

Dietary polyunsaturated fatty acids and regulation of gene transcription

Dietary polyunsaturated fatty acids (PUFAs) are a source of energy and structural components for cells. PUFAs also have dramatic effects on gene expression by regulating the activity or abundance of four families of transcription factor, including peroxisome proliferator activated receptor (PPAR) (alpha, beta and gamma), liver X receptors (LXRs) (alpha and beta), hepatic nuclear factor-4 (HNF-4)alpha and sterol regulatory element binding proteins (SREBPs) 1 and 2. These transcription factors play a major role in hepatic carbohydrate, fatty acid, triglyceride, cholesterol and bile acid metabolism. Non-esterified fatty acids or fatty acid metabolites bind to and regulate the activity of PPARs, LXRs and HNF-4. In contrast, PUFAs regulate the nuclear abundance of SREBPs by controlling the proteolytic processing of SREBP precursors, or regulating transcription of the SREBP-1c gene or turnover of mRNA(SREBP-1c). The n3 and n6 PUFAs are feed-forward activators of PPARs, while these same fatty acids are feedback inhibitors of LXRs and SREBPs. Saturated fatty acyl coenzyme A thioesters activate HNF-4 alpha, while coenzyme A thioesters of PUFAs antagonize HNF-4 alpha action. Understanding how fatty acids regulate the activity and abundance of these and other transcription factors will likely provide insight into the development of novel therapeutic strategies for better management of whole body lipid and cholesterol metabolism.

Implications of dietary fatty acids during pregnancy on placental, fetal and postnatal development--a review

During pregnancy, the mother adapts her metabolism to support the continuous draining of substrates by the fetus. Her increase in net body weight (free of the conceptus) corresponds to the accumulation of fat depots during the first two-thirds of gestation, switching to an accelerated breakdown of these during the last trimester. Under fasting conditions, adipose tissue lipolytic activity is highly enhanced, and its products, free fatty acids (FFA) and glycerol, are mainly driven to maternal liver, where FFA are converted to ketone bodies and glycerol to glucose, which easily cross the placenta and sustain fetal metabolism. Lipolytic products reaching maternal liver are also used for triglyceride synthesis that are released in turn to the circulation, where together with an enhanced transfer of triglycerides among the different lipoprotein fractions, and a decrease in extrahepatic lipoprotein lipase activity, increase the content of triglycerides in all the lipoprotein fractions. Long chain polyunsaturated fatty acids (LCPUFA) circulate in maternal plasma associated to lipoprotein triglycerides, and in a minor proportion in the form of FFA. Despite the lack of a direct placental transfer of triglycerides, diffusion of their fatty acids to the fetus is ensured by means of lipoprotein receptors, lipoprotein lipase activity and intracellular lipase activities in the placenta. Maternal plasma FFA are also an important source of LCPUFA to the fetus, and their placental uptake occurs via a selective process of facilitated membrane translocation involving a plasma membrane fatty acid-binding protein. This mechanism together with a selective cellular metabolism determine the actual rate of placental transfer and its selectivity, resulting even in an enrichment of certain LCPUFA in fetal circulation as compared to maternal. The degree to which the fetus is capable of fatty acid desaturation and elongation is not clear, although both term and preterm infants can synthesize LCPUFA from parental essential fatty acids. Nutritional status of the mother during gestation is related to fetal growth, and excessive dietary intake of certain LCPUFA has inhibitory effects on Delta-5- and Delta-6-desaturases. This inhibition causes major declines in arachidonic acid levels, as directly found in pregnant and lactating rats fed a fish oil-rich diet as compared to olive oil. An excess in dietary PUFA may also enhance peroxidation and reduce antioxidant capacity. Thus, since benefit to risks of modifying maternal fat intake in pregnancy and lactation are not yet completely established, additional studies are needed before recommendations to increase LCPUFA intake in pregnancy are made.

Efficiency of conversion of alpha-linolenic acid to long chain n-3 fatty acids in man

Alpha-linolenic acid (18:3n-3) is the major n-3 (omega 3) fatty acid in the human diet. It is derived mainly from terrestrial plant consumption and it has long been thought that its major biochemical role is as the principal precursor for long chain polyunsaturated fatty acids, of which eicosapentaenoic (20:5n-3) and docosahexaenoic acid (22:6n-3) are the most prevalent. For infants, n-3 long chain polyunsaturated fatty acids are required for rapid growth of neural tissue in the perinatal period and a nutritional supply is particularly important for development of premature infants. For adults, n-3 long chain polyunsaturated fatty acid supplementation is implicated in improving a wide range of clinical pathologies involving cardiac, kidney, and neural tissues. Studies generally agree that whole body conversion of 18:3n-3 to 22:6n-3 is below 5% in humans, and depends on the concentration of n-6 fatty acids and long chain polyunsaturated fatty acids in the diet. Complete oxidation of dietary 18:3n-3 to CO2 accounts for about 25% of 18:3n-3 in the first 24 h, reaching 60% by 7 days. Much of the remaining 18:3n-3 serves as a source of acetate for synthesis of saturates and monounsaturates, with very little stored as 18:3n-3. In term and preterm infants, studies show wide variability in the plasma kinetics of 13C n-3 long chain polyunsaturated fatty acids after 13C-18:3n-3 dosing, suggesting wide variability among human infants in the development of biosynthetic capability to convert 18:3n-3 to 22:6n3. Tracer studies show that humans of all ages can perform the conversion of 18:3n-3 to 22:6n3. Further studies are required to establish quantitatively the partitioning of dietary 18:3n-3 among metabolic pathways and the influence of other dietary components and of physiological states on these processes.

Straight-chain acyl-CoA oxidase knockout mouse accumulates extremely long chain fatty acids from alpha-linolenic acid: evidence for runaway carousel-type enzyme kinetics in peroxisomal beta-oxidation diseases

Extremely long chain polyunsaturated fatty acids (ELCPs) with >24 carbons and four or more double bonds are normally found in excitatory tissues but have no known function, and are greatly increased in brain and other tissues of humans with peroxisomal disorders. Straight-chain acyl-CoA oxidase (AOX) catalyzes the first, rate-limiting step of peroxisomal beta-oxidation of very-long-chain saturated and unsaturated fatty acids. We have studied the polyunsaturated fatty acid metabolism of AOX knockout mice (AOX-/- as a model of human AOX deficiency (pseudo-neonatal adrenoleukodystrophy), and as a genetic tool to test the putative peroxisomal beta-oxidation involvement in polyunsaturated fatty acid synthesis. Liver lipids of 26-day-old weanling AOX-/- mice livers accumulate n-3 and n-6 ELCPs from C24 to C30 with 5 and 6 double bonds, have 356 +/- 66 microg/g docosahexaenoic acid (22:6n-3), similar to congenic (AOX -/* = AOX+/+ and AOX+/-) controls (401 +/- 96 microg/g), but increased 22:5n-6 (22.4 +/- 3.7 vs 6.4 +/- 1.5 microg/g). AOX+/* mice injected intraperitoneally at 23 days with [U-(13)C]-18:3n-3 show strong labeling of 22:6n-3 after 72 h, whereas AOX -/- mice display less labeling of 22:6n-3 but strong tracer incorporation into 24:6n-3, 26:6n-3, and 28:6n-3, after the same period. These data suggest that ELCPs are natural runaway elongation by-products of 22:6n-3 and 22:5n-6 synthesis, which are normally disposed of by peroxisomal beta-oxidation. Under conditions with impaired peroxisomal beta-oxidation, such as Zellweger syndrome and adrenoleukodystrophies, ELCPs accumulate due to increased synthesis and impaired disposal. Two mechanisms for the formation of these runaway elongation by-products and the involvement of secondary carnitine deficiency in this process are proposed: n-3 ELCPs are synthesized by a carnitine-dependent multifunctional mitochondrial docosahexaenoic acid synthase (mtDHAS) which normally synthesizes primarily 22:6n-3, while n-6 ELCPs are synthesized by independent elongation enzymes in the endoplasmic reticulum.