Diversity in the ability of cultured cells to elongate and desaturate essential (n-6 and n-3) fatty acids

FADS2 function at the major cancer hotspot 11q13 locus alters fatty acid metabolism in cancer

Dysregulation of fatty acid metabolism and de novo lipogenesis is a key driver of several cancer types through highly unsaturated fatty acid (HUFA) signaling precursors such as arachidonic acid. The human chromosome 11q13 locus has long been established as the most frequently amplified in a variety of human cancers. The fatty acid desaturase genes (FADS1, FADS2 and FADS3) responsible for HUFA biosynthesis localize to the 11q12-13.1 region. FADS2 activity is promiscuous, catalyzing biosynthesis of several unsaturated fatty acids by Δ6, Δ8, and Δ4 desaturation. Our main aim here is to review known and putative consequences of FADS2 dysregulation due to effects on the 11q13 locus potentially driving various cancer types. FADS2 silencing causes synthesis of sciadonic acid (5Z,11Z,14Z-20:3) in MCF7 cells and breast cancer in vivo. 5Z,11Z,14Z-20:3 is structurally identical to arachidonic acid (5Z,8Z,11Z,14Z-20:4) except it lacks the internal Δ8 double bond required for prostaglandin and leukotriene synthesis, among other eicosanoids. Palmitic acid has substrate specificity for both SCD and FADS2. Melanoma, prostate, liver and lung cancer cells insensitive to SCD inhibition show increased FADS2 activity and sapienic acid biosynthesis. Elevated serum mead acid levels found in hepatocellular carcinoma patients suggest an unsatisfied demand for arachidonic acid. FADS2 circular RNAs are at high levels in colorectal and lung cancer tissues. FADS2 circular RNAs are associated with shorter overall survival in colorectal cancer patients. The evidence thusfar supports an effort for future research on the role of FADS2 as a tumor suppressor in a range of neoplastic disorders.

A review on the role of fatty acids in colorectal cancer progression

Colorectal cancer (CRC) is the third leading cause of mortality in cancer patients. The role of fatty acids (FA) and their metabolism in cancer, particularly in CRC raises a growing interest. In particular, dysregulation of synthesis, desaturation, elongation, and mitochondrial oxidation of fatty acids are involved. Here we review the current evidence on the link between cancer, in particular CRC, and fatty acids metabolism, not only to provide insight on its pathogenesis, but also on the development of novel biomarkers and innovative pharmacological therapies that are based on FAs dependency of cancer cells.

α-linolenic acid interconversion is sufficient as a source of longer chain ω-3 polyunsaturated fatty acids in humans: An opinion

α-linolenic acid (αLNA) conversion into the functionally important ω-3 polyunsaturated fatty acids (PUFA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), has been regarded as inadequate for meeting nutritional requirements for these PUFA. This view is based on findings of small αLNA supplementation trials and stable isotope tracer studies that have been interpreted as indicating human capacity for EPA and, in particular, DHA synthesis is limited. The purpose of this review is to re-evaluate this interpretation. Markedly differing study designs, inconsistent findings and lack of trial replication preclude robust consensus regarding the nutritional adequacy of αLNA as a source of EPC and DHA. The conclusion that αLNA conversion in humans is constrained is inaccurate because it presupposes the existence of an unspecified, higher level of metabolic activity. Since capacity for EPA and DHA synthesis is the product of evolution it may be argued that the levels of EPA and DHA it maintains are nutritionally appropriate. Dietary and supra-dietary EPA plus DHA intakes confer health benefits. Paradoxically, such health benefits are also found amongst vegetarians who do not consume EPA and DHA, and for whom αLNA conversion is the primary source of ω-3 PUFA. Since there are no reported adverse effects on health or cognitive development of diets that exclude EPA and DHA, their synthesis from αLNA appears to be nutritionally adequate. This is consistent with the dietary essentiality of αLNA and has implications for developing sustainable nutritional recommendations for ω-3 PUFA.

The aromatase inhibitor letrozole restores FADS2 function in ER+ MCF7 human breast cancer cells

PURPOSE

Plasticity in fatty acid metabolism is increasingly recognized as a major feature influencing cancer progression and efficacy of treatments. Estrogen receptor positive MCF7 human breast cancer cells have long been known to have no FADS2-mediated Δ6-desaturase activity. Our objective was to examine the effect of estrogen and the "antiestrogen" aromatase inhibitor letrozole, on Δ5- and Δ6-desaturase synthesized fatty acids in vitro.

METHODS

Eicosa-11,14-dienoic acid (20:2n-6), a known substrate for both FADS1 and FADS2, was used as a sentinel of relative FADS2 and FADS1 activity. MCF7 cells and four additional estrogen responsive wild type cell lines (HepG2, SK-N-SH, Y79 and Caco2) were studied. FAME were quantified by GC-FID and structures identified by GCCACI-MS/MS.

RESULTS

In all five cell lines, estrogen caused a dose dependent decrease in sciadonic acid (5,11,14-20:3, ScA) via apparent inhibition of FADS1 activity, and had no effect on FADS2 catalyzed synthesis of dihomo-gamma linolenic acid (8,11,14-20:3; DGLA). In MCF7 cells, letrozole caused a dose dependent increase in FADS2-catalyzed DGLA synthesis, which plateaued in SK-N-SH cells.

CONCLUSION

Letrozole restores Δ6-desaturase mediated synthesis of the anti-inflammatory PGE1-precursor DGLA in vitro and is the first endocrine-active agent to have opposing effects on FADS1 and FADS2 catalyzed activities.

Polyunsaturated fatty acid biosynthesis pathway and genetics. implications for interindividual variability in prothrombotic, inflammatory conditions such as COVID-19✰,✰✰,★,★★

COVID-19 symptoms vary from silence to rapid death, the latter mediated by both a cytokine storm and a thrombotic storm. SARS-CoV (2003) induces Cox-2, catalyzing the synthesis, from highly unsaturated fatty acids (HUFA), of eicosanoids and docosanoids that mediate both inflammation and thrombosis. HUFA balance between arachidonic acid (AA) and other HUFA is a likely determinant of net signaling to induce a healthy or runaway physiological response. AA levels are determined by a non-protein coding regulatory polymorphisms that mostly affect the expression of FADS1, located in the FADS gene cluster on chromosome 11. Major and minor haplotypes in Europeans, and a specific functional insertion-deletion (Indel), rs66698963, consistently show major differences in circulating AA (>50%) and in the balance between AA and other HUFA (47-84%) in free living humans; the indel is evolutionarily selective, probably based on diet. The pattern of fatty acid responses is fully consistent with specific genetic modulation of desaturation at the FADS1-mediated 20:3→20:4 step. Well established principles of net tissue HUFA levels indicate that the high linoleic acid and low alpha-linoleic acid in populations drive the net balance of HUFA for any individual. We predict that fast desaturators (insertion allele at rs66698963; major haplotype in Europeans) are predisposed to higher risk and pathological responses to SARS-CoV-2 could be reduced with high dose omega-3 HUFA.

Fatty acid desaturase 2 (FADS2) but not FADS1 desaturates branched chain and odd chain saturated fatty acids

Branched chain fatty acids (BCFA) and linear chain/normal odd chain fatty acids (n-OCFA) are major fatty acids in human skin lipids, especially sebaceous gland (SG) wax esters. Skin lipids contain variable amounts of monounsaturated BCFA and n-OCFA, in some reports exceeding over 20% of total fatty acids. Fatty acid desaturase 2 (FADS2) codes for a multifunctional enzyme that catalyzes Δ4-, Δ6- and Δ8-desaturation towards ten unsaturated fatty acids but only one saturate, palmitic acid, converting it to 16:1n-10; FADS2 is not active towards 14:0 or 18:0. Here we test the hypothesis that FADS2 also operates on BCFA and n-OCFA. MCF-7 cancer cells stably expressing FADS1 or FADS2 along with empty vector control cells were incubated with anteiso-15:0, iso-16:0, iso-17:0, anteiso-17:0, iso-18:0, or n-17:0. BCFA were Δ6-desaturated by FADS2 as follows: iso-16:0 → iso-6Z-16:1, iso-17:0 → iso-6Z-17:1, anteiso-17:0 → anteiso-6Z-17:1 and iso-18:0 → iso-6Z-18:1. anteiso-15:0 was not desaturated in either FADS1 or FADS2 cells. n-17:0 was converted to both n-6Z-17:1 by FADS2 Δ6-desaturation and n-9Z-17:1 by SCD Δ9-desaturation. We thus establish novel FADS2-coded enzymatic activity towards BCFA and n-OCFA, expanding the number of known FADS2 saturated fatty acid substrates from one to six. Because of the importance of FADS2 in human skin, our results imply that dysfunction in activity of sebaceous FADS2 may play a role in skin abnormalities associated with skin lipids.

Is essential fatty acid interconversion an important source of PUFA in humans?

Humans can obtain pre-formed long-chain PUFA from the diet and are also able to convert essential fatty acids (EFA) to longer-chain PUFA. The metabolic pathway responsible for EFA interconversion involves alternating desaturation and carbon chain elongation reactions, and carbon chain shortening by peroxisomal β-oxidation. Studies using stable isotope tracers or diets supplemented with EFA show that capacity for PUFA synthesis is limited in humans, such that DHA (22 : 6n-3) synthesis in men is negligible. PUFA synthesis is higher in women of reproductive age compared with men. However, the magnitude of the contribution of hepatic PUFA synthesis to whole-body PUFA status remains unclear. A number of extra-hepatic tissues have been shown to synthesise PUFA or to express genes for enzymes involved in this pathway. The precise function of extra-hepatic PUFA synthesis is largely unknown, although in T lymphocytes PUFA synthesis is involved in the regulation of cell activation and proliferation. Local PUFA synthesis may also be important for spermatogenesis and fertility. One possible role of extra-hepatic PUFA synthesis is that it may provide PUFA in a timely manner to facilitate specific cell functions. If so, this may suggest novel insights into the effect of dietary PUFA and/or polymorphisms in genes involved in PUFA synthesis on health and tissue function.

A rare eicosanoid precursor analogue, sciadonic acid (5Z,11Z,14Z-20:3), detected in vivo in hormone positive breast cancer tissue

Numerous genetic alterations of HSA 11q13 are found frequently in several cancer types, including breast cancer (BC). The 11q13 locus harbors FADS2 encoding Δ6 desaturation which is not functional in several cancer cell lines, including hormone positive MCF7 BC cells. In vitro, the non-functional FADS2 activity unmasks 18:2n-6 elongation to 20:2n-6 and Δ5 desaturation by FADS1 to yield 5Z,11Z,14Z-20:3 (sciadonic acid) rather than 5Z,8Z,11Z,14Z-20:4 (arachidonic acid). In this pilot study we aimed to determine whether 5,11,14-20:3 appears in vivo in hormone positive human BC tissue. Fatty acids were profiled in surgically removed human breast tumor and adjacent normal tissue (n = 9). Sciadonic acid was detected in three of nine breast tumor samples and was below detect limits in normal breast tissue. The internal Δ8 double bond of arachidonic acid is required for normal eicosanoid synthesis but is missing in sciadonic acid. This pilot study demonstrates for the first time in vivo sciadonic acid in hormone positive BC tissue, warranting a larger survey study to further evaluate its appearance and the functional implications.

Metabolism and phospholipid assembly of polyunsaturated fatty acids in human bone marrow mesenchymal stromal cells

High arachidonic acid (20:4n-6) and low n-3 PUFA levels impair the capacity of cultured human bone marrow mesenchymal stromal cells (hBMSCs) to modulate immune functions. The capacity of the hBMSCs to modify PUFA structures was found to be limited. Therefore, different PUFA supplements given to the cells resulted in very different glycerophospholipid (GPL) species profiles and substrate availability for phospholipases, which have preferences for polar head group and acyl chains when liberating PUFA precursors for production of lipid mediators. When supplemented with 20:4n-6, the cells increased prostaglandin E2 secretion. However, they elongated 20:4n-6 to the less active precursor, 22:4n-6, and also incorporated it into triacylglycerols, which may have limited the proinflammatory signaling. The n-3 PUFA precursor, 18:3n-3, had little potency to reduce the GPL 20:4n-6 content, while the eicosapentaenoic (20:5n-3) and docosahexaenoic (22:6n-3) acid supplements efficiently displaced the 20:4n-6 acyls, and created diverse GPL species substrate pools allowing attenuation of inflammatory signaling. The results emphasize the importance of choosing appropriate PUFA supplements for in vitro hBMSC expansion and suggests that for optimal function they require an exogenous fatty acid source providing 20:5n-3 and 22:6n-3 sufficiently, but 20:4n-6 moderately, which calls for specifically designed optimal PUFA supplements for the cultures.

Loss of FADS2 function severely impairs the use of HeLa cells as an in vitro model for host response studies involving fatty acid effects

Scope

Established epithelial cell lines equipped with pattern recognition receptors such as the Toll-like receptor (TLR)-2 are common tools for immune response studies on invading pathogens, e.g. the obligate intracellular species of Chlamydia. Moreover, such models are widely used to elucidate fatty acid-mediated immune effects. In several transformed cell lines, however, unusual loss of metabolic functions was described. The cell lines A549 and HeLa are poorly characterized in this respect. Therefore, we comparatively assessed the metabolic capacity of A549 and HeLa prior to proposed application as invitro model for fatty acid effects on chlamydial infection.

Methodology/Principal Findings

We incubated both cell lines either with substrates (C18∶2n−6 or C18∶3n−3) or products (C18∶3n−6, C18∶4n−3) of fatty acid desaturase-2 (FADS2), and analysed the fatty acid profiles after 24 h and 72 h by gas chromatography. Based on these data, we suspected that the complete discontinuation of normal biosynthesis of long-chain polyunsaturated fatty acids (LC-PUFA) in HeLa was due to loss of FADS2 function. Consequently, prostaglandin E2 (PGE2) formation was less inducible by TLR2 stimulation in HeLa, likely as a result of not only insufficient supply of precursors but also weak cyclooxygenase-2 (COX-2) response. In accordance, Chlamydia infection rates were consistently lower in HeLa than in A549. Sequence analysis revealed no alteration within the FADS2 gene in HeLa. The FADS2 expression level, however, was significantly lower and, in contrast to A549, not regulated by C18∶2n−6. A549 exhibited regular fatty acid metabolism and enzyme functionality.

Conclusions/Significance

Our data show that HeLa cells considerably differ from A549 at several stages of fatty acid metabolism. The poor metabolic potential of HeLa, mainly concerning FADS2 upstream of COX-2 function, calls into question whether these cells represent a good model to unveil fatty acid or downstream eicosanoid effects in the course of intracellular bacterial infection.

Unsaturated fatty acids, desaturases, and human health

With the increasing concern for health and nutrition, dietary fat has attracted considerable attention. The composition of fatty acids in a diet is important since they are associated with major diseases, such as cancers, diabetes, and cardiovascular disease. The biosynthesis of unsaturated fatty acids (UFA) requires the expression of dietary fat-associated genes, such as SCD, FADS1, FADS2, and FADS3, which encode a variety of desaturases, to catalyze the addition of a double bond in a fatty acid chain. Recent studies using new molecular techniques and genomics, as well as clinical trials have shown that these genes and UFA are closely related to physiological conditions and chronic diseases; it was found that the existence of alternative transcripts of the desaturase genes and desaturase isoforms might affect human health and lipid metabolism in different ways. In this review, we provide an overview of UFA and desaturases associated with human health and nutrition. Moreover, recent findings of UFA, desaturases, and their associated genes in human systems are discussed. Consequently, this review may help elucidate the complicated physiology of UFA in human health and diseases.

FADS2 function loss at the cancer hotspot 11q13 locus diverts lipid signaling precursor synthesis to unusual eicosanoid fatty acids

BACKGROUND

Genes coding for the fatty acid desaturases (FADS1, 2, 3) localized at the cancer genomic hotspot 11q13 locus are required for the biosynthesis of 20 carbon polyunsaturated fatty acids (PUFA) that are direct eicosanoid precursors. In several cancer cell lines, FADS2 encoded Δ6 and Δ8 desaturation is not functional.

METHODOLOGY/PRINCIPAL FINDINGS

Analyzing MCF7 cell fatty acids with detailed structural mass spectrometry, we show that in the absence of FADS2 activity, the FADS1 product Δ5-desaturase operates to produce 5,11,14-20∶3 and 5,11,14,17-20∶4. These PUFA are missing the 8-9 double bond of the eicosanoid signaling precursors arachidonic acid (5,8,11,14-20∶4) and eicosapentaenoic acid (5,8,11,14,17-20∶5). Heterologous expression of FADS2 restores Δ6 and Δ8-desaturase activity and normal eicosanoid precursor synthesis.

CONCLUSIONS/SIGNIFICANCE

The loss of FADS2-encoded activities in cancer cells shuts down normal PUFA biosynthesis, deleting the endogenous supply of eicosanoid and downstream docosanoid precursors, and replacing them with unusual butylene-interrupted fatty acids. If recapitulated in vivo, the normal eicosanoid and docosanoid cell signaling milieu would be depleted and altered due to reduction and substitution of normal substrates with unusual substrates, with unpredictable consequences for cellular communication.

Determinants of DHA incorporation into tumor tissue during dietary DHA supplementation

Docosahexaenoic acid (DHA), upon incorporation into tumor tissue, has the potential to sensitize tumors to the effects of chemotherapy or radiation therapy. Although DHA has usually been supplied to tumor tissue in the diet, appropriate dietary conditions required to obtain optimal tumor levels have not been established. Hence, we studied mammary tumor tissue responses in rats fed various durations and doses of DHA. Rats fed a palm oil enriched diet (diet 0) were switched to diets providing either 0.8 g DHA/day (diet 1) or 1.5 g DHA/day (diet 2). Tumor tissue fatty acid composition was analysed at baseline (diet 0), at weeks 1, 4 and 9 during diet 1 and at week 4 during diet 2. Dietary DHA supplementation differentially increased DHA within phospholipids (PL) and triacylglycerol (TAG) fractions in tumors. DHA level equilibrated between 2 and 4 weeks in PL while DHA increase was more progressive in TAG and did not reach a steady state. A higher dose of DHA further increased DHA content in tumor PL and TAG (P = 0.018 and P < 0.001, respectively). DHA concentration in plasma PL was positively correlated with DHA in tumor PL (r = 0.72; P = 0.0003) and TAG (r = 0.64; P = 0.003). We conclude that dietary DHA supplementation enhances tumor content of DHA in a time- and dose-dependent manner, and that the DHA level in plasma PL could be used as a proxy for tumor DHA. These findings have implications for dietary DHA supplementations in cancer patients.

Acyl chain-dependent effect of lysophosphatidylcholine on endothelial prostacyclin production

Previously we identified palmitoyl-lysophosphatidylcholine (16:0 LPC), linoleoyl-LPC (18:2 LPC), arachidonoyl-LPC (20:4 LPC), and oleoyl-LPC (18:1 LPC) as the most prominent LPC species generated by the action of endothelial lipase (EL) on high-density lipoprotein. In the present study, the impact of those LPC on prostacyclin (PGI(2)) production was examined in vitro in primary human aortic endothelial cells (HAEC) and in vivo in mice. Although 18:2 LPC was inactive, 16:0, 18:1, and 20:4 LPC induced PGI(2) production in HAEC by 1.4-, 3-, and 8.3-fold, respectively. LPC-elicited 6-keto PGF1α formation depended on both cyclooxygenase (COX)-1 and COX-2 and on the activity of cytosolic phospholipase type IVA (cPLA2). The LPC-induced, cPLA2-dependent (14)C-arachidonic acid (AA) release was increased 4.5-fold with 16:0, 2-fold with 18:1, and 2.7-fold with 20:4 LPC, respectively, and related to the ability of LPC to increase cytosolic Ca(2+) concentration. In vivo, LPC increased 6-keto PGF(1α) concentration in mouse plasma with a similar order of potency as found in HAEC. Our results indicate that the tested LPC species are capable of eliciting production of PGI(2), whereby the efficacy and the relative contribution of underlying mechanisms are strongly related to acyl-chain length and degree of saturation.

Impact of linoleic acid on bovine oocyte maturation and embryo development

Linoleic acid (LA; 18:2 n-6) is the most abundant fatty acid in bovine follicular fluid, and it was previously reported that LA concentration significantly decreases when follicle size increases. This suggests that LA may have a role in the regulation of oocyte maturation. The present study investigated the effect of LA supplementation on bovine oocyte maturation and early embryo development in vitro. Treatment of cumulus-oocyte complexes (COCs) with LA significantly inhibited cumulus cell expansion and retarded development of the oocytes to the metaphase II (MII) stage in a dose-dependent manner. This effect was reversible, and the oocytes developed to the MII stage after extended culture in the absence of LA. Treatment of COCs with LA also resulted in a significantly lower percentage of cleaved embryos and blastocyst yield. Furthermore, COCs treated with LA had significant effects compared with controls in i) increasing prostaglandin E(2) concentration in the medium, ii) decreasing intracellular cAMP at 6 and 24 h of maturation and iii) decreasing phosphorylation of the MAPK1 and 3 at 24 h, and AKT at 6 h of maturation. In conclusion, LA supplementation to bovine oocytes during maturation altered the molecular mechanisms regulating oocyte maturation and resulted in decreased percentage of oocytes at MII stage and inhibition of the subsequent early embryo development. These data provide evidence for adverse effects of LA on oocyte development, which can be associated with dietary increased level of LA in the follicular fluid and the decline in fertility in farm animals and human.

Repression of cellular anaplerosis as the hypothesized mechanism of gamma-linolenic acid-induced toxicity to tumor cells

In in vitro cultures, the cell is virtually isolated and can no longer rely on mechanisms for physiological regulation of substrate availability found in tissues. More emphasis is laid on utilization of preponderant substrate in a proposed reciprocal relationship between glycolysis and free fatty acid (FFA) oxidation for energy. Supraphysiological concentrations of gamma-linolenic acid and some other polyunsaturated fatty acids (PUFAs) therefore suppress glycolysis but also inhibit FFA oxidation initiated through a cytochrome P450-mediated epoxidation of PUFA to inhibit fatty acid synthase (FAS) activity. FAS inhibition accumulates malonyl CoA which inhibits carnitine palmitoyl transferase I and prevents FFA oxidation. The cell is starved of energy and anabolic intermediates, leading to decreased proliferation or death for tumor cells. Tumor cells are more vulnerable to this induced toxicity due to possession of specific phenotypes of elevated expression for FAS and pyruvate kinase, type M2, both factors inducing tumor cell apoptosis on inhibition.

The Delta8-desaturase of Euglena gracilis: an alternate pathway for synthesis of 20-carbon polyunsaturated fatty acids

Desaturation of fatty acids is an important metabolic process. In mammals, 20-carbon and longer polyunsaturated fatty acids are not only incorporated into cellular membranes in a tissue-specific manner, but also serve as the precursors to synthesis of eicosanoid metabolic regulators. The processes of desaturation and elongation in human liver are well characterized, but an alternate Delta8 desaturation pathway that may be important in certain tissues or in cancer cells is less well examined. The Delta8-desaturase enzyme introduces a double bond at the 8-position in 20-carbon fatty acids that have an existing Delta11 unsaturation. We have isolated the first fatty acid Delta8-desaturase, from the protist Euglena gracilis, in order to explore this alternate pathway. A full-length cDNA was obtained after reverse transcription of mRNA purified from heterotrophically grown Euglena, followed by PCR amplification with primers degenerate to conserved histidine-rich regions of microsomal desaturases. The protein predicted from the cDNA sequence is highly homologous to Delta5 and Delta6 desaturases of Caenhorabditis elegans. When the cDNA was expressed in Saccharomyces cerevisiae, the yeast cultures readily desaturated appropriate 20-carbon fatty acids by inserting an additional double bond at the Delta8-position. The enzyme demonstrated a preference for substrates of metabolic significance, 20:3 Delta11,14,17 and 20:2 Delta11,14. Cloning of a Delta8 fatty acid desaturase offers the opportunity to examine an alternate pathway of long chain fatty acid biosynthesis.

Role of peroxisomal oxidation in the conversion of arachidonic acid to eicosatrienoic acid in human skin fibroblasts

Human skin fibroblasts converted [5,6,8,9,11,12,14,15-3H]arachidonic acid ([3H]20:4) to eicosatrienoic acid (20:3), but appreciable amounts of radiolabeled 20:3 were not detected in corresponding incubations with [1-(14)C]20:4. This indicates that the main pathway for synthesizing 20:3 from arachidonic acid in the fibroblast involves oxidative removal of the carboxyl group of arachidonic acid. Fibroblasts deficient in long-chain acyl coenzyme A dehydrogenase (LCAD) converted [3H]20:4 to [3H]20:3. However, Zellweger fibroblasts that are deficient in peroxisomal fatty acid oxidation did not, indicating that the oxidative removal of the carboxyl group occurs in the peroxisomes. [3H]Hexadecatrienoic acid (16:3) was the main product that accumulated when [3H]20:4 was incubated with normal, LCAD deficient, and very long-chain acyl coenzyme A dehydrogenase (VLCAD) deficient fibroblasts, but Zellweger fibroblasts did not form this product. Normal fibroblasts converted [3H]16:3 to radiolabeled 20:3 and arachidonic acid. These findings suggest that some of the 16:3 produced from arachidonic acid by peroxisomal beta-oxidation can be recycled and that this recycling process constitutes a novel pathway for the conversion of arachidonic acid to 20:3 in human fibroblasts.

Gamma-linolenic acid (GLA) is cytotoxic to 36B10 malignant rat astrocytoma cells but not to 'normal' rat astrocytes

This study compares the effect of gamma-linolenic acid (GLA) and its precursor linoleic acid (LA) on survival of 36B10 malignant rat astrocytoma cells and 'normal' rat astrocytes. GLA was cytotoxic to 36B10 cells but not to astrocytes. By contrast, LA supplementation did not affect the survival of either cell types. There were minor differences in the uptake, distribution and use of radiolabelled GLA and LA by the 36B10 cells and astrocytes. GLA and LA supplementation increased the total polyunsaturated fatty acid (PUFA) content of the cells indicating increased oxidative potential. However, elevated levels of 8-isoprostane, an indicator of increased oxidative stress, were only observed in the GLA supplemented 36B10 cells. Addition of the antioxidant trolox to GLA-enriched 36B10 cells blocked the cytotoxic effect. Further, GLA enhanced the radiation sensitivity of the astrocytoma cells but not the astrocytes; trolox blocked the GLA-mediated increase in astrocytoma cell radiosensitivity. LA did not affect the radiation response of either cell type. While cyclo-oxygenase inhibitors did not affect GLA cytotoxicity, they blocked the enhanced radiation response of GLA-supplemented cells. The lipoxygenase inhibitor NDGA did not affect the toxicity produced by GLA. Thus, GLA is toxic to the neoplastic astrocytoma cells but not to normal astrocytes.

Delta-6-desaturase and delta-5-desaturase in human Hep G2 cells are both fatty acid interconversion rate limiting and are upregulated under essential fatty acid deficient conditions

Essential fatty acids are interconverted by desaturases and elongases to eicosanoid precursors. In essential fatty acid deficiency (EFAD) an increased hepatic interconversion of linoleic acid (18:2) to arachidonic acid (20:4n-6) has been demonstrated in vivo. We now cultured Hep G2 cells under EFAD conditions. 20:3n-6 appeared in EFAD cells, but also in controls. After adding 14C-18:2 to the medium, interconversion products and their distribution in different lipids were studied by HPLC. When trace amounts 18:2 were incubated, 38% were converted by the EFAD cells after 21 h, vs 6% by controls. 20% was converted to 20:4 by EFAD cells vs 14% by controls. EFAD cells preferentially distributed more 18:2 and conversion products to neutral fats and to phosphatidyl ethanolamine, but less to cardiolipin than controls did, when incubated with trace amount 18:2, but not with 1 mM 18:2. A relative accumulation of radioactivity in 20:3 was observed. In conclusion; in EFAD Hep G2 cells delta-6- and delta-5-desaturase both were found to be upregulated and eicosanoid precursors were distributed more into phosphatidyl ethanolamine. Delta-5-desaturase had a rate limiting property as well as delta-6-desaturase.

Differential effects of polyunsaturated fatty acids on chemosensitivity of NIH3T3 cells and its transformants

Polyunsaturated fatty acids (PUFAs) have been suggested, on the basis of animal-model studies, to be related not only to cancer development but also to chemotherapeutic effects. Controversy persists, however, as to which types of PUFAs are beneficial in terms of chemosensitivity. In this study, we used the NIH3T3 cell line and its SIC(sigmoid colon cancer)-oncogene transformants to investigate the effects of PUFAs on the chemosensitivity of non-malignant and malignant cells in terms of cell proliferation. We also determined the fatty-acid composition of cells by high-performance liquid chromatography (HPLC). The results revealed that the sensitivity of SIC transformants to mitomycin C (MC) was lower than that of NIH3T3 cells cultured in 10% calf-serum DMEM without PUFA supplementation. When cells were cultured in DMEM supplemented with eicosapentaenoic acid (EPA) at a concentration (2 micrograms/ml) that does not influence cell proliferation, the sensitivity of SIC transformants to MC increased, whereas that of NIH3T3 cells decreased in comparison with the sensitivity of cells cultured without PUFA supplementation (p < 0.05). There was no difference between the 2 cell lines in the chemosensitivity of cells cultured in medium supplemented with arachidonic acid (ARA). The SIC transformants contained more stearic acid (C:18) and less lauric acid (C:12) than NIH3T3 cells cultured without PUFA. Culturing the cells in medium supplemented with EPA or ARA modified the cellular fatty-acid composition. EPA caused the relative combined percentage of lauric acid and myristic acid (C:14) in SIC transformants to decrease significantly, and the SIC transformants tended to accumulate additional EPA, in contrast to the NIH3T3 cells. We conclude that the alterations in fatty-acid composition in malignant transformants caused by exogenous EPA differ from those in non-malignant cells, and that these changes account for the increased chemosensitivity of malignant transformants. Although preliminary, these findings imply that EPA specifically enhances the chemosensitivity of malignant cells.

Diverse effects of essential (n-6 and n-3) fatty acids on cultured cells

Fatty acids (FAs) have long been recognized for their nutritional value in the absence of glucose, and as necessary components of cell membranes. However, FAs have other effects on cells that may be less familiar. Polyunsaturated FAs of dietary origin (n-6 and n-3) cannot be synthesized by mammals, and are termed 'essential' because they are required for the optimal biologic function of specialized cells and tissues. However, they do not appear to be necessary for normal growth and metabolism of a variety of cells in culture. The essential fatty acids (EFAs) have received increased attention in recent years due to their presumed involvement in cardiovascular disorders and in cancers of the breast, pancreas, colon and prostate. Many in vitro systems have emerged which either examine the role of EFAs in human disease directly, or utilize EFAs to mimic the in vivo cellular environment. The effects of EFAs on cells are both direct and indirect. As components of membrane phospholipids, and due to their varying structural and physical properties, EFAs can alter membrane fluidity, at least in the local environment, and affect any process that is mediated via the membrane. EFAs containing 20 carbons and at least three double bonds can be enzymatically converted to eicosanoid hormones, which play important roles in a variety of physiological and pathological processes. Alternatively, EFAs released into cells from phospholipids can act as second messengers that activate protein kinase C. Furthermore, susceptibility to oxidative damage increases with the degree of unsaturation, a complication that merits consideration because lipid peroxidation can lead to a variety of substances with toxic and mutagenic properties. The effects of EFAs on cultured cells are illustrated using the responses of normal and tumor human mammary epithelial cells. A thorough evaluation of EFA effects on commercially important cells could be used to advantage in the biotechnology industry by identifying EFA supplements that lead to improved cell growth and/or productivity.