Biosynthesis of docosahexaenoic acid in human cells: evidence that two different delta 6-desaturase activities may exist

The polypyrimidine tract binding protein regulates desaturase alternative splicing and PUFA composition

The Δ6 desaturase, encoded by FADS2, plays a crucial role in omega-3 and omega-6 fatty acid synthesis. These fatty acids are essential components of the central nervous system, and they act as precursors for eicosanoid signaling molecules and as direct modulators of gene expression. The polypyrimidine tract binding protein (PTB or hnRNP I) is a splicing factor that regulates alternative pre-mRNA splicing. Here, PTB is shown to bind an exonic splicing silencer element and repress alternative splicing of FADS2 into FADS2 AT1. PTB and FADS2AT1 were inversely correlated in neonatal baboon tissues, implicating PTB as a major regulator of tissue-specific FADS2 splicing. In HepG2 cells, PTB knockdown modulated alternative splicing of FADS2, as well as FADS3, a putative desaturase of unknown function. Omega-3 fatty acids decreased by nearly one half relative to omega-6 fatty acids in PTB knockdown cells compared with controls, with a particularly strong decrease in eicosapentaenoic acid (EPA) concentration and its ratio to arachidonic acid (ARA). This is a rare demonstration of a mechanism specifically altering the cellular omega-3 to omega-6 fatty acid ratio without any change in diet/media. These findings reveal a novel role for PTB, regulating availability of membrane components and eicosanoid precursors for cell signaling.

Alternative transcripts of fatty acid desaturase (FADS) genes

Alternative splicing is a major mechanism for increasing the range of products encoded by the genome. We recently reported positive identification of the first alternative transcripts (AT) of fatty acid desaturase 3 (FADS3) and FADS2 in fetal and neonatal baboons. FADS3, a putative polyunsaturated fatty acid (PUFA) desaturase gene with no known function, has 7 AT that are expressed in at least twelve organs in an apparently constitutive manner. At least five of seven AT are expressed in several mammals and the chicken. FADS2, catalyzing 6 and 8 desaturation and having multiple PUFA substrates, has one AT that is missing two exons and portions of two others. Semi-quantitative expression estimates reveal at least 20-fold differential expression of FADS2 AT1 among neonatal baboon organs compared to 2-fold in the same organs for the classically spliced (CS) FADS2 transcript. Expression of four of the FADS3 AT, those with missing putatively active domains, is highly correlated among organs, suggesting coordinated coexpression. AT may serve as templates to generate protein isoforms or as signaling molecules, and their widespread detection and expression patterns suggest that they play an important role in PUFA biosynthesis.

Do FADS genotypes enhance our knowledge about fatty acid related phenotypes?

Several physiological processes, such as visual and cognitive development in early life, are dependent on the availability of long-chain polyunsaturated fatty acids (LC-PUFAs). Furthermore, the concentration of LC-PUFAs in phospholipids has been associated with numerous complex diseases like cardiovascular disease, atopic disease and metabolic syndrome. The level and composition of LC-PUFAs in the human body is mainly dependent on their dietary intake or on the intake of fatty acid precursors, which are endogenously elongated and desaturated to physiologically active LC-PUFAs. The delta-5 and delta-6 desaturase are the most important enzymes in this reaction cascade. In the last few years, several studies have reported an association between single nucleotide polymorphisms (SNPs) in the two desaturase encoding genes (FADS1 and FADS2) and the concentration of omega-6 and omega-3 fatty acids. This shows that beside nutrition, genetic factors play an important role in the regulation of LC-PUFAs as well. This review focuses on current knowledge of the impact of FADS genotypes on LC-PUFA and lipid metabolism and discusses their influence on infant intellectual development, neurological conditions, metabolic disease as well as cardiovascular disease.

Relationship between abnormal sperm morphology induced by dietary zinc deficiency and lipid composition in testes of growing rats

The present study investigated the effect of dietary Zn deficiency during sexual maturation on sperm integrity and testis phospholipid fatty acid composition. Male weanling Sprague-Dawley rats were randomised into four dietary groups for 3 weeks: Zn control (ZC; 30 mg Zn/kg); Zn marginally deficient (ZMD; 9 mg Zn/kg); Zn deficient (ZD; < 1 mg Zn/kg); pair fed (PF; 30 mg Zn/kg) to the ZD group. Morphology of cauda epididymal sperm and lipid profiles of testis phospholipids were analysed. The rats fed the ZD diet had a lower testis weight (P < 0.02). Seminal vesicles and prostate weight were also lower in the ZD and PF groups. Rats fed the ZD diet, but not the ZMD diet, had 34-35 % more abnormal spermatozoa and 24 % shorter sperm tail length than the ZC and PF rats (P < 0.001). Testis cholesterol concentration was higher in the ZD rats compared with the ZC and PF rats (P < 0.04). Testes were highly enriched with n-6 fatty acids by showing n-6 : n-3 fatty acid ratios of 27:1 in phosphatidylcholine (PC) and 23:1 in phosphatidylethanolamine (PE). The dominant fatty acid in testes was docosapentaenoic acid (22 : 5n-6), comprising 15 and 24 % of PC and PE, respectively. This fatty acid was significantly lower in the ZD rats, whereas 18 : 2n-6 was higher compared with the rats in the other diet groups. These results demonstrate that severe Zn deficiency adversely affects sperm integrity and modulates testis fatty acid composition by interrupting essential fatty acid metabolism. This suggests that Zn deficiency-associated abnormal testicular function is perhaps preceded by altered membrane fatty acid composition, especially of a major fatty acid, 22 : 5n-6.

Multi-targeted therapy of cancer by omega-3 fatty acids

Omega-3 (n-3) and omega-6 (n-6) polyunsaturated fatty acids (PUFAs) are essential fatty acids necessary for human health. Currently, the Western diet contains a disproportionally high amount of n-6 PUFAs and low amount of n-3 PUFAs, and the resulting high n-6/n-3 ratio is thought to contribute to cardiovascular disease, inflammation, and cancer. Studies in human populations have linked high consumption of fish or fish oil to reduced risk of colon, prostate, and breast cancer, although other studies failed to find a significant association. Nonetheless, the available epidemiological evidence, combined with the demonstrated effects of n-3 PUFAs on cancer in animal and cell culture models, has motivated the development of clinical interventions using n-3 PUFAs in the prevention and treatment of cancer, as well as for nutritional support of cancer patients to reduce weight loss and modulate the immune system. In this review, we discuss the rationale for using long-chain n-3 PUFAs in cancer prevention and treatment and the challenges that such approaches pose in the design of clinical trials.

Dietary α-linolenic acid and health-related outcomes: a metabolic perspective

α-Linolenic acid (αLNA; 18: 3n-3) is essential in the human diet, probably because it is the substrate for the synthesis of longer-chain, more unsaturatedn-3 fatty acids, principally EPA (20: 5n-3) and DHA (22: 6n-3), which confer important biophysical properties on cell membranes and so are required for tissue function. The extent to which this molecular transformation occurs in man is controversial. The present paper reviews the recent literature on the metabolism of αLNA in man, including the use of dietary αLNA in β-oxidation, recycling of carbon by fatty acid synthesisde novoand conversion to longer-chain PUFA. Sex differences in αLNA metabolism and the possible biological consequences are discussed. Increased consumption of EPA and DHA in fish oil has a number of well-characterised beneficial effects on health. The present paper also reviews the efficacy of increased αLNA consumption in increasing the concentrations of EPA and DHA in blood and cell lipid pools, and the extent to which such dietary interventions might be protective against CVD and inflammation. Although the effects on CVD risk factors and inflammatory markers are variable, where beneficial effects have been reported these are weaker than have been achieved from increasing consumption of EPA+DHA or linoleic acid. Overall, the limited capacity for conversion to longer-chainn-3 fatty acids, and the lack of efficacy in ameliorating CVD risk factors and inflammatory markers in man suggests that increased consumption of αLNA may be of little benefit in altering EPA+DHA status or in improving health outcomes compared with other dietary interventions.

20:5n-3 but not 22:6n-3 is a preferred substrate for synthesis of n-3 very-long- chain fatty acids (C24-C36) in retina

The objective of this study was to determine if 20:5n-3 or 22:6n-3 is the primary precursor of very-long-chain fatty acids (VLCFAs; C24-C36) synthesized in retina. Rats were fed semisynthetic, nutritionally complete diet containing 20% (w/w) fat with 3% (w/w) of 22:6n-3. After 6 weeks feeding, the vitreal fluid of each eye was injected with [3H]20:5n-3 or [3H]22:6n-3. Rats were then maintained under constant light (330 lux) or dark conditions for 48 hr. After 48 hr in vivo metabolism, the amount of label present in individual fatty acids was determined in major phospholipids in retina. For [3H]22:6n-3, 90% of total incorporation remained in 22:6n-3, whereas for [3H]20:5n-3 the label was actively incorporated into pentaenoic and hexaenoic VLCFAs up to 34 carbon chain length. 22:5n-3 derived from [3H]20:5n-3 was among the most highly labeled fatty acids. These observations suggest that 22:6n-3 is incorporated directly into retinal phospholipids without further metabolism, whereas 20:5n-3 and 22:5n-3 are metabolically active precursors for synthesis of VLCFAs.

Changes of the transcriptional and fatty acid profiles in response to n-3 fatty acids in SH-SY5Y neuroblastoma cells

Synthesis of docosahexaenoic acid (DHA) from its metabolic precursors contributes to membrane incorporation of this FA within the central nervous system. Although cultured neural cells are able to produce DHA, the membrane DHA contents resulting from metabolic conversion do not match the high values of those resulting from supplementation with preformed DHA. We have examined whether the DHA precursors down-regulate the incorporation of newly formed DHA within human neuroblastoma cells. SH-SY5Y cells were incubated with gradual doses of alpha-linolenic acid (alpha-LNA), EPA, or docosapentaenoic acid (DPA), and the incorporation of DHA into ethanolamine glycerophospholipids was analyzed as a reflection of synthesizing activity. The incorporation of EPA, DPA, and preformed DHA followed a dose-response saturating curve, whereas that of DHA synthesized either from alpha-LNA, EPA, or DPA peaked at concentrations of precursors below 15-30 microM and sharply decreased with higher doses. The mRNA encoding for six FA metabolism genes were quantified using real-time PCR. Two enzymes of the peroxisomal beta-oxidation, L-bifunctional protein and peroxisomal acyl-CoA oxidase, were expressed at lower levels than fatty acyl-CoA ligase 3 (FACL3) and delta6-desaturase (delta6-D). The delta6-D mRNA slightly increased between 16 and 48 h of culture, and this effect was abolished in the presence of 70 microM EPA. In contrast, the EPA treatment resulted in a time-dependent increase of FACL3 mRNA. The terminal step of DHA synthesis seems to form a "metabolic bottleneck," resulting in accretion of EPA and DPA when the precursor concentration exceeds a specific threshold value. We conclude that the critical precursor- concentration window of responsiveness may originate from the low basal expression level of peroxisomal enzymes.

In vivo conversion of 18- and 20-C essential fatty acids in rats using the multiple simultaneous stable isotope method

An important question for mammalian nutrition is the relative efficiency of C18 versus C20 essential fatty acids (EFAs) for supporting the tissue composition of n-3 and n-6 pathway end products. One specific question is whether C22 EFAs are made available to tissues more effectively by dietary alpha-linolenic acid (18:3n-3) and linoleic acid (18:2n-6) or by dietary eicosapentaenoic acid (20:5n-3) and dihomo-gamma-linolenic acid (20:3n-6). To address this question in a direct manner, four stable isotope compounds were given simultaneously in a novel paradigm. A single oral dose of a mixture of 2H5-18:3n-3, 13C-U-20:5n-3, 13C-U-18:2n-6, and 2H5-20:3n-6 was administered to rats given a defined diet. There was a preferential in vivo conversion of arachidonic acid (20:4n-6) to docosatetraenoic acid (22:4n-6) and of 22:4n-6 to n-6 docosapentaenoic acid (22:5n-6) when the substrates originated from the C18 precursors. However, when the end products docosahexaenoic acid (22:6n-3) or 22:5n-6 were expressed as the total amount in the plasma compartment divided by the dosage, this parameter was 11-fold greater for 20:5n-3 than for 18:3n-3 and 14-fold greater for 20:3n-6 than for 18:2n-6. Thus, on a per dosage basis, the total amounts of n-3 and n-6 end products accreted in plasma were considerably greater for C20 EFA precursors relative to C18.

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.

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.

Essential fatty acid synthesis and its regulation in mammals

The tissue content of highly unsaturated fatty acids (HUFA) such as arachidonic acid and docosahexaenoic acid is maintained in a narrow range by feedback regulation of synthesis. Delta-6 desaturase (D6D) catalyzes the first and rate-limiting step of the HUFA synthesis. Recent identification of a human case of D6D deficiency underscores the importance of this pathway. Sterol regulatory element binding protein-1c (SREBP-1c) is a key transcription factor that activates transcription of genes involved with fatty acid synthesis. We recently identified sterol regulatory element (SRE) that is required for activation of the human D6D gene by SREBP-1c. Moreover, the same SRE also mediates the suppression of the D6D gene by HUFA. The identification of SREBP-1c as a key regulator of D6D suggests that the major physiological function of SREBP-1c in liver may be the regulation of phospholipid synthesis rather than triglyceride synthesis. Peroxisome proliferators (PP) induce fatty acid oxidation enzymes and desaturases in rodent liver. However, the induction of desaturases by PP is slower than the induction of oxidation enzymes. This delayed induction may be a compensatory reaction to the increased demand of HUFA caused by increased HUFA oxidation and peroxisome proliferation in PP administration. Recent studies have demonstrated a critical role of peroxisomal beta-oxidation in DHA synthesis, and identified acyl CoA oxidase and D-bifunctional protein as the key enzymes.

Delayed induction of delta-6 and delta-5 desaturases by a peroxisome proliferator

Delta-6 desaturase (D6D) is the key enzyme for the synthesis of highly unsaturated fatty acids (HUFA) such as arachidonic acid (AA) and docosahexaenoic acid (DHA) in mammals. Transcription of D6D gene is activated by both sterol regulatory element binding protein-1c (SREBP-1c) and peroxisome proliferators (PP). This response of D6D is paradoxical because SREBP-1c transactivates genes for fatty acid synthesis in liver, while PP induce enzymes for fatty acid oxidation. We hypothesized that the induction of D6D gene by PP is a compensatory response to the increased HUFA demand caused by peroxisome proliferation and induction of fatty acid oxidation. We investigated the time-course effects of a PP, Wy14643, on the induction of HUFA metabolizing genes and HUFA profile in rat liver. The mRNA of fatty acid oxidation enzymes in the Wy14643 fed group became significantly higher than controls at 4 h and reached maximum within 28 h. In contrast, the mRNA of delta-6 and delta-5 desaturases in the Wy14643 group was not significantly higher than control at 4 h and took >28 h to reach the maximum. Despite the induction of HUFA synthetic pathway, the concentration of end products (AA and DHA) remained unchanged throughout the 4-day period in liver phospholipids and non-esterified fatty acids. Taken together, this study supports our hypothesis and suggests that peroxisome proliferation and induction of fatty acid oxidation enzymes are the major mechanisms of the induction of HUFA synthesis by PP.

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 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.

Specific changes in n -6 fatty acid metabolism in patients with chronic intestinal failure

BACKGROUND AND AIMS

In patients presenting severe malabsorption, essential fatty acid (EFA) deficiency can be corrected by intravenous lipids, but EFA abnormalities persist. The purpose of this study was to evaluate the role of large resection of the small bowel or malabsorption on plasma phospholipid EFA profile.

METHODS

The plasma phospholipid EFA composition was measured by gas chromatography in home parenteral nutrition patients with (n=13) or without small bowel resection (n=7) and in 14 healthy subjects.

RESULTS

The two groups of patients had the same nutritional status and comparable amounts of intravenous fat. In both groups, plasma fatty acid concentrations were significantly different from those observed in healthy subjects without EFA deficiency. Among them: a decrease in 18:2n -6, 22:5n -3, 22:6n -3 and an increase in 18:3n -3, 20:4n -6, 22:4n -6. Moreover, arachidonic acid to linoleic acid ratio was higher in both groups of patients, suggesting a stimulation of the elongation and desaturation of 18:2n -6. In multiple linear regression, 18:2n -6 and 20:4n -6 levels were not associated with the small bowel length, only 22:6n -3 concentration was correlated with small bowel length.

CONCLUSIONS

The patients with chronic intestinal failure on home parenteral nutrition presented specific change in their EFA and an increase in the n -6 fatty acid pathway. This could be related to the severe malabsorption.

Activity and mRNA abundance of Delta-5 and Delta-6 fatty acid desaturases in two human cell lines

We analyzed fatty acid biosynthesis in Chang and ZR-75-1 cells. Both cell lines could desaturate and further elongate substrates for Delta-5 desaturase. ZR-75-1 but not Chang cells showed Delta-6 desaturation of 18:2n-6, 18:3n-3, 24:4n-6 and 24:5n-3. In both cell lines, the mRNA abundance can be related to Delta-5 or Delta-6 fatty acid desaturase activities. These results suggest that desaturase genes could have, at least in part, independent control mechanisms and that Delta-6 desaturase impairment is not specific to any particular step of the fatty acid metabolic pathways, which may diminish the rationale for the existence of at least two distinct enzymes.

Chain separation of monounsaturated fatty acid methyl esters by argentation thin-layer chromatography

A technique for separating methyl esters of monounsaturated fatty acids by argentation chromatography using silver nitrate-impregnated TLC plates is described. Monounsaturated fatty acid methyl esters are separated from polyunsaturated and saturated fatty acid methyl esters and the monounsaturated fatty methyl esters are resolved according to chain length. cis isomers are well resolved from the corresponding trans isomers. R(F) values for individual monounsaturated fatty acids are very reproducible. The potential of the technique in metabolic studies is demonstrated in the chain elongation of [14C]-18:1(n-9) and delta-9 desaturation of [14C]-18:0 by human skin fibroblasts. Recoveries of individual [14C]-fatty acids for scintillation counting exceed 94%.

High-precision isotope ratio mass spectrometry and stable isotope precursors for tracer studies in cell culture

The use of stable isotope-labeled tracers is demonstrated in an in vitro system with analysis by high-precision isotope ratio mass spectrometry (IRMS), using n-3 long-chain polyunsaturated fatty acid (LCP) biosynthesis from [U-(13)C]18:3n-3 (18:3n-3*) in Y79 human retinoblastoma cells as a model system. The cells were cultured as a suspension in RPMI 1640 medium supplemented with 15% fetal calf serum at 37 degrees C with 5% CO(2) in air. They were harvested by sedimentation and cell lipids were extracted to determine the presence of 18:3n-3* metabolites using gas chromatography-combustion (GCC)-IRMS. As the dose of 18:3n-3* was systematically increased from treatment to treatment, the atom percent excess and the amounts of biosynthesized LCP* increased, while the percentage dose in each n-3 LCP* remained constant. Cultures incubated with 0.5 micromol (10 microM) of albumin-bound 18:3n-3, composed of 18:3n-3* diluted 1/60 or 1/100 with natural abundance 18:3n-3, yielded products with enrichments about 1.5 at.% excess (delta(13)C(PDB) < 1500 per thousand), which is optimal for high-precision measurements. Kinetics in Y79 cells incubated with 18:3n-3* showed that n-3 LCP* incorporation increased over time; 18:3n-3*, 20:5n-3*, 22:5n-3*, and 22:6n-3* were detected at all time points with the 1/60 dilution. These data document experimental parameters for optimal stable isotope use and IRMS detection for in vitro tracer methodology.

Phospholipid incorporation and metabolic conversion of n-3 polyunsaturated fatty acids in the Y79 retinoblastoma cell line

The metabolic conversion of n-3 fatty acids was studied in the human Y79 retinoblastoma cell line. Cultured cells were exposed to increasing concentrations of either 18:3n-3, 22:5n-3, or 22:6n-3, and their phospholipid fatty acid composition was analyzed after 72 hr. Cells internalized the supplemental fatty acids and proceeded to their metabolic conversion. Supplemental 22:6n-3 was directly esterified into cell phospholipids, at levels typical for normal neural retinas (41% by weight of phosphatidylethanolamine fatty acids, and 24% of phosphatidylcholine fatty acids). In contrast, 18:3n-3 was mainly converted to 20:5n-3 and 22:5n-3, both of which appeared in cell phospholipids after exposure to low external concentrations of 18:3n-3 (10 microg/ml). Y79 cells can proceed to the metabolic conversion of 18:3n-3 through elongation and Delta6- and Delta5-desaturation. When cells were exposed to high external concentrations of 18:3n-3 (30 microg/ml), the supplemental fatty acid was directly incorporated, and its relative content increased in both phospholipid classes to the detriment of all other n-3 fatty acids. Cells cultured in the presence of 22:5n-3 did not incorporate 22:6n-3 into their phospholipids but did incorporate 20:5n-3 and 22:5n-3. The data suggest that Y79 cells can proceed to the microsomal steps of n-3 metabolism, involving elongation, desaturation, and chain shortening of 22C fatty acids. Although Y79 cells avidly used supplemental 22:6n-3 for phospholipid incorporation at levels typical for normal photoreceptor cells, they failed to match such levels through metabolic conversion of n-3 parent fatty acids. The terminal step of the very long-chain polyunsaturated fatty acid synthesis, consisting in Delta6-desaturation followed by peroxisomal chain shortening of 24C-fatty acids, could be rate-limiting in Y79 cells.

cDNA cloning, genomic structure, and chromosomal localization of three members of the human fatty acid desaturase family

The insertion of double bonds into specific positions of fatty acids is achieved by the action of distinct desaturase enzymes. Here we report the cloning and characterization of three members of the fatty acid desaturase (FADS) gene family in humans. Initially identified as cDNA fragments by direct cDNA selection within a defined 1.4-Mb region in 11q12-q13.1, full-length fatty acid desaturase-1 (FADS1) and fatty acid desaturase-2 (FADS2) transcripts were obtained by EST sequence assembly. A third member, fatty acid desaturase-3 (FADS3), was identified in silico revealing 62 and 70% nucleotide sequence identity with FADS1 and FADS2, respectively. The three genes are clustered within 92 kb of genomic DNA located 2 kb telomeric to FEN1 and 50 kb centromeric to VMD2 and are likely to have arisen evolutionarily from gene duplication as they share a remarkably similar exon/intron organization. Protein database searches identified FADS1, FADS2, and FADS3 as fusion products composed of an N-terminal cytochrome b5-like domain and a C-terminal multiple membrane-spanning desaturase portion.

Cloning, expression, and fatty acid regulation of the human delta-5 desaturase

Arachidonic (20:4(n-6)), eicosapentaenoic (20:5(n-3)), and docosahexaenoic (22:6(n-3)) acids are major components of brain and retina phospholipids, substrates for eicosanoid production, and regulators of nuclear transcription factors. One of the two rate-limiting steps in the production of these polyenoic fatty acids is the desaturation of 20:3(n-6) and 20:4(n-3) by Delta-5 desaturase. This report describes the cloning and expression of the human Delta-5 desaturase, and it compares the structural characteristics and nutritional regulation of the Delta-5 and Delta-6 desaturases. The open reading frame of the human Delta-5 desaturase encodes a 444-amino acid peptide which is identical in size to the Delta-6 desaturase and which shares 61% identity with the human Delta-6 desaturase. The Delta-5 desaturase contains two membrane-spanning domains, three histidine-rich regions, and a cytochrome b(5) domain that all align perfectly with the same domains located in the Delta-6 desaturase. Expression of the open reading frame in Chinese hamster ovary cells instilled the ability to convert 20:3(n-6) to 20:4(n-6). Northern analysis revealed that many human tissues including skeletal muscle, lung, placenta, kidney, and pancreas expressed Delta-5 desaturase mRNA, but Delta-5 desaturase was most abundant in the liver, brain, and heart. However, in all tissues, the abundance of Delta-5 desaturase mRNA was much lower than that observed for the Delta-6 desaturase. When rats were fed a diet containing 10% safflower oil or menhaden fish oil, the level of hepatic mRNA for Delta-5 and Delta-6 desaturase was only 25% of that found in the liver of rats fed a fat-free diet or a diet containing triolein. Finally, a BLAST and Genemap search of the human genome revealed that the Delta-5 and Delta-6 desaturase genes reside in reverse orientation on chromosome 11 and that they are separated by <11,000 base pairs.

Early neurological and neuropsychological development of the preterm infant and polyunsaturated fatty acids supply

OBJECTIVES

To compare the early neurological maturation of premature newborns (PT) fed breast milk (BM) or a formula containing only 18-carbon polyunsaturated fatty acids (PUFA) (A) or enriched with long chain (LC) PUFA (B).

METHODS

PT enrolled the 2nd day of enteral feeding (D0) were fed BM (n = 15; 4 dropped out) or randomly assigned to A (n = 11; 2) or B (n = 14; 1) for at least 30 days (D30). Auditory and visual evoked potentials (EPs) and nerve conduction velocity (NCV) and plasma and red blood cell (RBC) phospholipid composition were determined at D0 and D30. No difference was found between groups for the D0-D30 changes in EP parameters. The maturation of motor NCV was slower in the B group than in the two other groups. In plasma, the changes were higher in B than in the BM and A groups for linoleic acid (P < 0.05), in BM versus B group for arachidonic acid (P < 0.02). In RBC, formula groups displayed higher linoleic acid level than the BM group (P < 0.05). No difference was found between groups for the changes in arachidonic and docosahexaenoic acids.

CONCLUSIONS

A balanced supply of n-6 and n-3 PUFA without addition of LC-PUFA allowed an adequate early maturation of the central nerve system. The effects of LC-PUFA on the maturation of NCV remain to be confirmed.

Fatty acid desaturation: effect of alphafetoprotein on alpha-linolenic acid conversion by fetal rat hepatocytes

Freshly isolated fetal hepatocytes transformed 4.3, 8.5 and 19.2 pmol/min/10(6) cells of stearic, linoleic and alpha-linolenic acids, respectively, complexed to albumin or alpha-fetoprotein (AFP), to more unsaturated derivatives. Thus, fetal hepatocytes displayed high elongase and delta9, delta6, delta5-desaturase activities, as well as an ability to synthesize hexaene derivatives. Desaturase activities decreased when the time of culture of fetal hepatocytes (previous to incubation with the substrate) was prolonged, being practically undetectable after 24 h of culture. However, the rate of fatty acid uptake remained nearly constant. When AFP was used as the carrier the amount of hexaene fatty acid derivatives of alpha-linolenic acid recovered in cells was reduced up to 50% by albumin. This effect was associated with an increase of radioactivity found in the culture medium of hepatocytes incubated with AFP compared to albumin. Both observations taken together could be explained by an efflux of hexaene derivatives from cells caused by AFP.

Cloning, expression, and nutritional regulation of the mammalian Delta-6 desaturase

Arachidonic acid (20:4(n-6)) and docosahexaenoic acid (22:6(n-3)) have a variety of physiological functions that include being the major component of membrane phospholipid in brain and retina, substrates for eicosanoid production, and regulators of nuclear transcription factors. The rate-limiting step in the production of 20:4(n-6) and 22:6(n-3) is the desaturation of 18:2(n-6) and 18:3(n-3) by Delta-6 desaturase. In this report, we describe the cloning, characterization, and expression of a mammalian Delta-6 desaturase. The open reading frames for mouse and human Delta-6 desaturase each encode a 444-amino acid peptide, and the two peptides share an 87% amino acid homology. The amino acid sequence predicts that the peptide contains two membrane-spanning domains as well as a cytochrome b5-like domain that is characteristic of nonmammalian Delta-6 desaturases. Expression of the open reading frame in rat hepatocytes and Chinese hamster ovary cells instilled in these cells the ability to convert 18:2(n-6) and 18:3(n-3) to their respective products, 18:3(n-6) and 18:4(n-3). When mice were fed a diet containing 10% fat, hepatic enzymatic activity and mRNA abundance for hepatic Delta-6 desaturase in mice fed corn oil were 70 and 50% lower than in mice fed triolein. Finally, Northern analysis revealed that the brain contained an amount of Delta-6 desaturase mRNA that was several times greater than that found in other tissues including the liver, lung, heart, and skeletal muscle. The RNA abundance data indicate that prior conclusions regarding the low level of Delta-6 desaturase expression in nonhepatic tissues may need to be reevaluated.

Analysis of the putative role of 24-carbon polyunsaturated fatty acids in the biosynthesis of docosapentaenoic (22:5n-6) and docosahexaenoic (22:6n-3) acids

The recent literature on the putative involvement of a single cycle of peroxisomal beta-oxidation of 24:5n-6 and 24:6n-3 polyunsaturated fatty acids in the biosynthesis of the respective docosapentaenoic (22:5n-6) and docosahexaenoic (22:6n-3) fatty acids is critically reviewed. Present evidence suggests that in vitro data in support of the above proposition is an artifact of a low 2,4-dienoyl-CoA reductase activity due to depletion of NADPH resulting from incubation conditions. Kinetic studies with radiolabeled precursors in cell cultures have shown lower initial rates of labeling of 24:6n-3 than that of 22:6n-3, indicating that 24:6n-3 is an elongation product of 22:6n-3 rather than its precursor. Analysis of other literature data supports the proposal that 22:5n-6 and 22:6n-3 are synthesized in mitochondria via channeled carnitine-dependent pathways involving separate n-6- and n-3-specific desaturases. It is proposed that impaired peroxisomal function in some peroxisomal disorders is a secondary consequence of defective mitochondrial synthesis of 22:6n-3; moreover, some disorders of peroxisomal beta-oxidation show normal or increased 22:5n-6 concentrations, indicating that 22:5n-6 is synthesized by independent desaturases without peroxisomal involvement.

Effects of docosahexaenoic (22:6n-3), tetracosapentaenoic (24:5n-3) and tetracosahexaenoic (24:6n-3) acids on the desaturation and elongation of n-3 polyunsaturated fatty acids in trout liver microsomes

The effects of long chain n-3 polyunsaturated fatty acids (PUFA) on the desaturation and elongation systems involved in the conversion of 18:3n-3 to 24:6n-3 were investigated. Microsomes were prepared from the livers of rainbow trout and incubated with 14C-labelled 18:3n-3 and cofactors required for elongation and/or desaturation in the presence of 22:6n-3, 24:5n-3 or 24:6n-3. The formation of 24:6n-3 was significantly inhibited in the presence of 50 microM 22:6n-3, 24:5n-3 or 24:6n-3, whereas the amount of radiolabelled 20:5n-3 formed was inhibited by only 24:5n-3 or 24:6n-3 at the same concentration. When malonyl-CoA was omitted from the incubation system to allow the measurement of desaturation in the absence of elongation, the Delta6 desaturation of 14C-18:3n-3 to 14C-18:4n-3 was inhibited by approximately 25% in the presence of 24:5n-3 or 24:6n-3 but was not affected by 22:6n-3. The Delta5 desaturation of 14C-20:4n-3 was not affected by the presence of any of the long chain PUFA and no significant effect of 18:3n-3, 22:6n-3 or 24:6n-3 on the Delta6 desaturation of 24:5n-3 to 24:6n-3 was observed. To permit the measurement of individual elongation reactions, KCN was included in the incubation medium to inhibit desaturation and 14C-labelled 18:3n-3, 18:4n-3, 20:4n-3, 20:5n-3 and 22:5n-3 were examined as substrates. 18:4n-3 and 22:5n-3 were more extensively used for elongation than 18:3n-3, 20:4n-3 and 20:5n-3. The presence of 22:6n-3, 24:5n-3 or 24:6n-3 in the incubation system had no effect on any of the specific elongations of any of the substrates examined. It is concluded that, in the conversion of 18:3n-3 to 24:6n-3 by trout liver microsomes, the Delta6 desaturation of 18:3n-3 may be subjected to direct feedback inhibition and that 24:5n-3 may be preferred over 18:3n-3 as a substrate for Delta6 desaturation.

Pathophysiological effects of dietary essential fatty acid balance on neural systems

Dietary fatty acid balance has been revealed to affect neural functions as well as chronic diseases such as cancer, cerebro- and cardiovascular diseases, and allergic hyper-reactivity. In this review, we focused on the pathophysiological effects of n-6 and n-3 fatty acids on brain functions. Long-term n-3 fatty acid deficiency in the presence of n-6 fatty acids has been shown to affect learning behavior, drug sensitivity and retinal functions. Some membrane enzymes and ion channel functions have been shown in experimental animals to be regulated by membrane fatty acid modifications. We also summarized the effects of these fatty acids in diets on human psychotic aspects and brain diseases. Although biochemical mechanisms remain to be elucidated, investigations on the effect of dietary fatty acids on neural networks may provide an important clue to clarify complex brain functions.

Diabetes-induced and age-related changes in fatty acid proportions of plasma lipids in rats

Diabetes-induced and age-related proportional changes in plasma fatty acids of triglycerides (TG), phospholipids (PL), and cholesteryl esters (CE) were investigated using streptozotocin-induced diabetic and control rats. Among n-6 fatty acids from diabetic rat plasma, increased proportions of 18:2n-6 and 20:3n-6 in all three lipid classes and of 18:3n-6 in PL at 1-3 months old and in TG at 3-5 months old were observed. The proportions of 20:4n-6 decreased in both PL and CE, but were unchanged in diabetic TG. Among the n-3 fatty acids, in the early stage, diabetes caused increases in the proportions of 18:3n-3 in PL and CE and of 20:5n-3 and 22:6n-3 in TG, while 22:5n-3 was decreased later in the disease course. These results suggest reduced delta 5-desaturase activities on 20:3n-6 but not on 20:4n-3, while delta 6-desaturase activity on 18:2n-6 was essentially unaffected. Furthermore, the reduction in delta 9-desaturase activity in diabetic rats may well explain the decreases in the proportions of 16:1n-7 and 18:1n-7. However, the proportion of 18:1n-9, another product of delta 9-desaturase, was significantly increased in CE and PL as compared to the controls. Thus, there was a discrepancy between our results and those of earlier studies with respect to the n-9, n-6, and n-3 fatty acid proportions of plasma lipids in diabetic rats. We also investigated age-related changes in the proportions of plasma fatty acids. Although rather small, age-related changes were evident in both diabetic and control rats.

Regulation of the biosynthesis of 4,7,10,13,16-docosapentaenoic acid

It is now established that fatty acid 7,10,13,16-22:4 is metabolized into 4,7,10,13,16-22:5 as follows: 7,10,13,16-22:4-->9,12,15, 18-24:4-->6,9,12,15,18-24:5-->4,7,10,13,16-22:5. Neither C24 fatty acid was esterified to 1-acyl-sn-glycero-3-phosphocholine (1-acyl-GPC) by microsomes, whereas the rates of esterification of 4, 7,10,13,16-22:5, 7,10,13,16-22:4 and 5,8,11,14-20:4 were respectively 135, 18 and 160 nmol/min per mg of microsomal protein. About four times as much acid-soluble radioactivity was produced when peroxisomes were incubated with [3-14C]9,12,15,18-24:4 compared with 6,9,12,15,18-24:5. Only [1-14C]7,10,13,16-22:4 accumulated when [3-14C]9,12,15,18-24:4 was the substrate, but both 4,7,10,13,16-22:5 and 2-trans-4,7,10,13,16-22:6 were produced from [3-14C]6,9,12,15, 18-24:5. When the two C24 fatty acids were incubated with peroxisomes, microsomes and 1-acyl-GPC there was a decrease in the production of acid-soluble radioactivity from [3-14C]6,9,12,15, 18-24:5, but not from [3-14C]9,12,15,18-24:4. The preferential fate of [1-14C]4,7,10,13,16-22:5, when it was produced, was to move out of peroxisomes for esterification into the acceptor, whereas only small amounts of 7,10,13,16-22:4 were esterified. By using 2H-labelled 9,12,15,18-24:4 it was shown that, when 7,10,13,16-22:4 was produced, its primary metabolic fate was degradation to yield esterified arachidonate. Collectively, the results show that an inverse relationship exists between rates of peroxisomal beta-oxidation and of esterification into 1-acyl-GPC by microsomes. Most importantly, when a fatty acid is produced with its first double bond at position 4, it preferentially moves out of peroxisomes for esterification to 1-acyl-GPC by microsomes, rather than being degraded further via a cycle of beta-oxidation that requires NADPH-dependent 2,4-dienoyl-CoA reductase.

Studies to determine if rat liver contains multiple chain elongating enzymes

According to the revised pathways of polyunsaturated fatty acid biosynthesis three, rather than two acids, must be chain elongated for converting linoleate and linolenate, respectively, to 22:5(n-6) and 22:6(n-3) (Sprecher et al. (1995) J. Lipid Res. 36, 2471-2477). The present study was undertaken to determine whether microsomes contained chain-length specific chain-elongating enzymes and, secondly, whether reaction rates for any of these reactions might be rate limiting in the synthesis of 24:5(n-6) and 24:6(n-3), which are the immediate precursors of 22:5(n-6) and 22:6(n-3). Rates of total chain elongation products produced from both 18:4(n-3) and 20:5(n-3) were about 3 nmol/min/mg of microsomal protein while only about 0.5 nmol/min/mg of 24:5(n-3) plus 24:6(n-3) was synthesized from 22:5(n-3). The rate of 24:5(n-3) synthesis was similar to that for the desaturation of 24:5(n-3), at position 6, to yield 24:6(n-3) (Geiger et al. (1993) Biochim. Biophys. Acta 1170, 137-142). The results suggest that the last chain elongation step in unsaturated fatty acid biosynthesis may be equally regulatory in governing the synthesis of fatty acids as is desaturation at position 6. When an enzyme saturating level of [1-(14)C]18:4(n-3) was incubated with increasing amounts of 18:3(n-6) there was a decrease in the production [1-(14)C]20:4(n-3). In a similar way it was observed that 18:4(n-3) inhibited the chain elongation of [1-(14)C]18:3(n-6). Identical cross-over inhibitory studies, using 20:4(n-6) and 20:5(n-3), as well as 22:4(n-6) and 22:5(n-3) also suggested that microsomes contain chain length specific chain-elongating enzymes. This conclusion was further supported by the finding that neither 20:5(n-3) or 22:5(n-3) inhibited the chain elongation of [1-(14)C]18:4(n-3). However, 18:4(n-3), and to a lesser degree, 22:5(n-3) did inhibit the chain elongation of [1-(14)C]20:5(n-3). This latter finding suggests that 18:4(n-3) and 20:5(n-3) might interact with the enzyme that chain elongates 20:5(n-3) to depress its ability to synthesize 22:5(n-3). Our results are most consistent with the presence of multiple chain-elongating enzymes, but a more definitive answer requires the purification of these membrane-bound proteins. In addition our results suggest that the channeling of acids between enzymes in the endoplasmic reticulum may play an important role in regulating the biosynthesis of unsaturated fatty acids.

On the molecular etiology of decreased arachidonic (20:4n-6), docosapentaenoic (22:5n-6) and docosahexaenoic (22:6n-3) acids in Zellweger syndrome and other peroxisomal disorders

Alterations in the metabolism of arachidonic (20:4n-6), docosapentaenoic (22:5n-6), and docosahexaenoic (22:6n-3) acids and other polyunsaturated fatty acids in Zellweger syndrome and other peroxisomal disorders are reviewed. Previous proposals that peroxisomes are necessary for the synthesis of 22:6n-3 and 22:5n-6 are critically examined. The data suggest that 22:6n-3 is biosynthesized in mitochondria via a channelled carnitine-dependent pathway involving an n-3-specific delta-4 desaturase, while 20:4n-6, 20:5n-3 and 22:5n-6 are synthesized by both mitochondrial and microsomal systems; these pathways are postulated to be interregulated as compensatory-redundant systems. Present evidence suggests that 22:6n-3-containing phospholipids may be required for the biochemical events involved in successful neuronal migration and developmental morphogenesis, and as structural cofactors for the functional assembly and integration of a variety of membrane enzymes, receptors, and other proteins in peroxisomes and other subcellular organelles. A defect in the mitochondrial desaturation pathway is proposed to be a primary etiologic factor in the clinicopathology of Zellweger syndrome and other related disorders. Several implications of this proposal are examined relating to effects of pharmacological agents which appear to inhibit steps in this pathway, such as some hypolipidemics (fibrates), neuroleptics (phenothiazines and phenytoin) and prenatal alcohol exposure.