Polyenoic acid metabolism in cultured human skin fibroblasts

Towards establishing dietary reference intakes for eicosapentaenoic and docosahexaenoic acids

There is considerable interest in the impact of (n-3) long-chain PUFA in mitigating the morbidity and mortality caused by chronic diseases. In 2002, the Institute of Medicine concluded that insufficient data were available to define Dietary Reference Intakes (DRI) for eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA), noting only that EPA and DHA could contribute up to 10% toward meeting the Adequate Intake for alpha-linolenic acid. Since then, substantial new evidence has emerged supporting the need to reassess this recommendation. Therefore, the Technical Committee on Dietary Lipids of the International Life Sciences Institute North America sponsored a workshop on 4-5 June 2008 to consider whether the body of evidence specific to the major chronic diseases in the United States--coronary heart disease (CHD), cancer, and cognitive decline--had evolved sufficiently to justify reconsideration of DRI for EPA+DHA. The workshop participants arrived at these conclusions: 1) consistent evidence from multiple research paradigms demonstrates a clear, inverse relation between EPA+DHA intake and risk of fatal (and possibly nonfatal) CHD, providing evidence that supports a nutritionally achievable DRI for EPA+DHA between 250 and 500 mg/d; 2) because of the demonstrated low conversion from dietary ALA, protective tissue levels of EPA+DHA can be achieved only through direct consumption of these fatty acids; 3) evidence of beneficial effects of EPA+DHA on cognitive decline are emerging but are not yet sufficient to support an intake level different from that needed to achieve CHD risk reduction; 4) EPA+DHA do not appear to reduce risk for cancer; and 5) there is no evidence that intakes of EPA+DHA in these recommended ranges are harmful.

Extremely limited synthesis of long chain polyunsaturates in adults: implications for their dietary essentiality and use as supplements

There is considerable interest in the potential impact of several polyunsaturated fatty acids (PUFAs) in mitigating the significant morbidity and mortality caused by degenerative diseases of the cardiovascular system and brain. Despite this interest, confusion surrounds the extent of conversion in humans of the parent PUFA, linoleic acid or α-linolenic acid (ALA), to their respective long-chain PUFA products. As a result, there is uncertainty about the potential benefits of ALA versus eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA). Some of the confusion arises because although mammals have the necessary enzymes to make the long-chain PUFA from the parent PUFA, in vivo studies in humans show that ≈5% of ALA is converted to EPA and <0.5% of ALA is converted to DHA. Because the capacity of this pathway is very low in healthy, nonvegetarian humans, even large amounts of dietary ALA have a negligible effect on plasma DHA, an effect paralleled in the ω6 PUFA by a negligible effect of dietary linoleic acid on plasma arachidonic acid. Despite this inefficient conversion, there are potential roles in human health for ALA and EPA that could be independent of their metabolism to DHA through the desaturation – chain elongation pathway.

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.

Incorporation and metabolic conversion of saturated and unsaturated fatty acids in SK-Hep1 human hepatoma cells in culture

We report here a study of the incorporation and metabolism of various long chain fatty acids in SK-Hep-1 cultured hepatoma cells. Medium supplementation with radiolabelled palmitic, stearic, linoleic, alpha-linolenic and eicosa-8,11,14-trienoic acids (1 microM, 24 H) resulted in an active uptake of each of these precursors by the cultures. Subsequent analysis of the cellular lipids indicated that they exhibit almost all the enzymic activities of polyunsaturated fatty acid metabolism that are characteristic of normal hepatic cells. With respect to the desaturation capacities of this cell line, although alpha-linolenic acid reacted more extensively than did linoleic acid and the conversion of 8,11,14-eicosatrienoic acid by the delta 5 specific enzyme was more avid than had been previously seen in normal rat or human liver: the saturated fatty acids constituted relatively poor substrates, being preferentially chain-elongated rather than (mono) desaturated at the delta 9 position. Analysis of the fatty acid profiles of total cellular lipids and of various lipid subclasses, however, revealed a relative paucity of essential fatty acids when compared with the abundance of endogenous monoenoic acids (particularly oleic). Of the total cellular fatty acids, 58% were present in the form of phospholipids; with 33% of the remaining 42% (i.e., the neutral lipids) being associated with triacylglycerol fraction. Within the total lipids, phosphatidyl-choline and phosphatidyl-ethanolamine were the major sites for the incorporation of all metabolic products derived from the incubated radiolabelled 16- and 18-carbon fatty acid precursors, whereas the phosphatidyl-inositol fraction was the predominant recipient of nascent arachidonic acid when the eicosatrienoate was the substrate. The express purpose of this investigation was to characterize the biochemical routes involved in the anabolism of various essential fatty acids in the human hepatocyte, through the use of cultured human hepatoma cells as an experimental model system. In view of the similarities between certain aspects of the polyunsaturated fatty acid metabolism of these cells and the corresponding properties of other mammalian hepatic or liver-derived tissues, the data presented here would thus constitute a significant beginning alone those lines. Moreover, considering the extreme difficulty in obtaining for such investigation relevant tissue samples from normal human sources, we regard these results- and the availability for use of this particular human hepatoma cell line-as important new developments in the effort to characterize a useful experimental model both for gaining immediate information and for designing future experiments.

Retroconversion and delta 4 desaturation of docosatetraenoate (22:4(n-6)) and docosapentaenoate (22:5(n-3)) by human cells in culture

This study has investigated the metabolic modification of [3-14C]docosatetraenoate (22:4(n-6)) and [3-14C]docosapentaenoate (22:5(n-3)) by human cells in culture. Fetal skin fibroblasts converted as much as 20% of the incorporated [14C]22:4(n-6) to [14C]20:4(n-6) within 6 h and 41% within 48 h. Retroconversion of incorporated [14C]22:5(n-3) was less than 13% at all time points. Chain shortening of [14C]22:4(n-6) was also 2-6-fold greater than that of [14C]22:5(n-3) in retinoblastoma and vascular endothelial cells. Fibroblasts, vascular endothelial cells and retinoblastoma cells all elongated substantially more [14C]eicosapentaenoate than [14C]arachidonate to the respective C22 fatty acids. Within 3-4 days, fibroblasts incubated with either [14C]20:5(n-3) or [14C]22:5(n-3) had the same ratio of radiolabeled C22:C20 fatty acids in cellular glycerolipids. By contrast, the cells incubated with [14C]22:4(n-6) or [14C]20:4(n-6) did not reach a common C22/C20 equilibrium by 5 days. Although fibroblasts were found to desaturate [14C]22:5(n-3), a substantial lag time was observed; [14C]22:6(n-3) was 2% at 48 h and 20% at 96 h. By contrast, synthesis of [14C]22:6(n-3) by retinoblastoma cells was 51% within 6 h and greater than 90% at 96 h. Desaturation of [14C]22:4(n-6) was observed in retinoblastoma cells, but not in fibroblasts. These results thus suggest that the ratio of C22C20 polyunsaturated fatty acids in cells is regulated by the relative rates of retroconversion and chain elongation, with the net effect of the two processes favoring C20 for n-6 and C22 for the n-3 fatty acids. Furthermore, although fibroblasts desaturate [14C]22:5(n-3), the process appears to be qualitatively different from that of retinoblastoma cells.

Effects of eicosapentaenoic acid and arachidonic acid on incorporation and metabolism of radioactive linoleic acid in cultured human fibroblasts

Effects of exogenous eicosapentaenoic acid, arachidonic acid and oleic acid on incorporation and metabolism of [14C] linoleic acid were examined in cultured human fibroblasts obtained from three donors of different ages. Eicosapentaenoic acid treatment (40 microM) inhibited incorporation of radioactive linoleic acid and actively reduced radioactivity of desaturation-elongation metabolites in phospholipids, predominantly in the phosphatidylethanolamine fraction. In contrast, radioactivities of the metabolites in triacylglycerols were significantly increased with arachidonic acid treatment (40 microM): eicosapentaenoic acid had a smaller effect or none. Oleic acid had virtually no effect. These effects were consistent in the three cell lines, but responses to treatment with the acids differed considerably among individual cells. The pool of linoleic acid metabolites in triacylglycerols may not be negligible. The exogenous fatty acids may influence both the transfer of lipids between the major lipid pools as well as the activities of the desaturation-elongation system.

Essential fatty acid pattern of glycerolipids in rat hepatocytes in primary culture and in coculture with rat liver epithelial cells

The linoleic acid content of phosphatidylethanolamine (PE), phosphatidylcholine (PC) and triglyceride (TG) rapidly fell in rat hepatocytes in primary culture up to four days and in coculture with liver epithelial cells up to eight days. At the same time, the level of polyunsaturated fatty acids (PUFA), especially arachidonic acid, remained constant in PE, slightly decreased in PC and dropped in TG. There was no variation of the nonessential PUFA, 20:3n-9. Linoleic acid supplementation of cultures 24 hr before the harvest induced a rise in the linoleic acid level of the three lipid classes. Arachidonic acid remained constant in TG and only slightly decreased in PE and PC at day 4 of primary culture and day 8 of coculture. The level of 20:3n-9 increased in PE and PC and much more in TG. This net increase in the arachidonic acid and 20:3n-9 levels in TG could not be explained only by a transfer from the phospholipid pools of PUFA because the phospholipid content of hepatocytes and PUFA levels of phospholipids did not vary under linoleic supplementation. The low percentage of arachidonic acid in epithelial cells rules out any participation of these cells in the increase of arachidonic acid in supplemented cocultures. Triglycerides may act as a storage pool for plasma PUFA up to four days of primary culture and eight days of coculture. Besides, coculture seems more potent than primary culture to maintain the phospholipid level, to spare the essential PUFA in PE and to increase the TG synthesis in response to linoleic acid supplementation.

Uptake of fatty acids and their mobilization from phospholipids in cultured monocyte-macrophages from rheumatoid arthritis patients

Prostaglandins (PG) and related eicosanoids which derive from essential fatty acids are important mediators and modulators of inflammation. Macrophages (M phi), which derive from peripheral blood monocytes (PBM), are prominent cells in the synovium of patients with rheumatoid arthritis (RA), and are a major source of synovial PGE2. In addition, fresh and cultured PBM from RA patients produce more PG than normal control cells. When allowed to mature in culture PBM exhibit many characteristics of macrophages (M-M phi). We examined uptake by M-M phi of eicosanoid precursor fatty acids (FA), their incorporation into cellular phospholipid (PL), and mobilization of FA after cell stimulation. Cultured M-M phi from treated and untreated RA patients (RA M-M phi) took up significantly more linoleic acid (LA), dihomogammalinolenic acid (DHLA) and arachidonic acid (AA) than M-M phi from normal volunteers (N M-M phi). The enhanced uptake of FA observed in 12-day cultures of RA M-M phi was similar to uptake seen in normal human peritoneal macrophages (PM phi). After uptake FA were incorporated mainly into phosphatidylcholine (PC). M-M phi from untreated RA patients incorporated a smaller proportion of [14C]LA into PC (37.0 +/- 12.7% of total PL label) than normal cells (86.0 +/- 4.2%), and a greater proportion of [3H]AA into PC (57.1 +/- 7.1%) than normals (23.9 +/- 6.9%). Stimulation of M-M phi with calcium ionophore A23187 resulted in significantly greater hydrolysis of LA and AA from PC in RA M-M phi from both treated and untreated patients than from PC in N M-M phi. The data indicate that M-M phi from RA patients mature more rapidly in vitro than M-M phi from controls as uptake of FA by RA M-M phi increases with duration of culture and by 12 days in culture equals uptake by normal human peritoneal M phi. Also, RA M-M phi exhibit differences from N M-M phi in uptake, PL distribution, and hydrolysis of eicosanoid precursor FA. Such changes in FA metabolism might influence cell function and inflammatory responses.

Arachidonate synthesis and uptake in isolated guinea-pig megakaryocytes and platelets

Arachidonic acid (20:4) synthesis and uptake were studied in guinea-pig megakaryocytes and platelets. Isolated megakaryocytes and platelets were incubated with [3H]20:4, 8,11,14-[14C]eicosatrienoic acid (gamma-20:3) and [14C]linoleic acid (18:2) and their lipids were analyzed for radioactivity. The study showed that there was 0.153 microM of unesterified 20:3 and 0.237 microM of 20:4 in guinea-pig plasma in nonfasting animals. At these concentrations, 42.6 pmol of gamma-20:3 and 53.3 pmol of 20:4 were taken up per h per 10(5) megakaryocytes in vitro. Megakaryocytes desaturated 27% of the gamma-20:3 to 20:4 but could not desaturate 18:2. Platelets could not desaturate gamma-20:3 or 18:2. In megakaryocytes, the distribution of 20:4 synthesized by the desaturation of gamma-20:3 and 20:4 taken up reflected the endogenous distribution of 20:4 in megakaryocyte phospholipids and 20:4 was predominantly incorporated into phosphatidylethanolamine (PE). In contrast, the distribution of [3H]20:4 taken up into platelets did not reflect the endogenous distribution. 20:4 was primarily incorporated into platelet phosphatidylcholine and phosphatidylinositol. The study showed that megakaryocytes but not platelets possess a delta 5 desaturase and can synthesize 20:4 from gamma-20:3. Neither cell was shown to have a delta 6 desaturase. Megakaryocytes appear to have the capacity to determine the composition of all pools of platelet 20:4 either by uptake or synthesis of 20:4. Platelets, most likely, have a limited capacity to alter structural pools of 20:4 contained primarily in PE and phosphatidylserine (PS).

Fatty acyl delta 6 desaturation activity of cultured human endothelial cells. Modulation by fetal bovine serum

Human endothelial cells from umbilical vein actively desaturate [14C]linoleate and synthesize icosatrienoate, arachidonate and docosatetraenoate. Desaturation and chain elongation of 9,12,15-[14 C]linolenate (n - 3) by these cells is more extensive than that of [14 C]linoleate. Both confluent primary monolayers and subconfluent subcultures exhibit greater fatty acyl CoA delta 6-desaturase activity when growth and incubation media contain 2.5% fetal bovine serum instead of 10%. Prior growth with 20% serum diminishes the extent of subsequent linoleate desaturation. Use of medium supplemented with 20-100 microM oleate results in up to 67% inhibition of [14 C]arachidonate synthesis. These results indicate that, despite previously published reports to the contrary, human vascular endothelial cells are similar to other normal mammalian cells in having fatty acyl delta 6-desaturase activity. Suppression of endogenous arachidonate synthesis by elevated levels of serum lipids may impair endothelial cell function.

The effects of unsaturated fatty acids on the synthesis of arachidonic acid in rat kidney cells

Cultured rat kidney cells absorbed exogenous linoleic acid (cic, cis-18:2n-6) and esterified it mostly into glycerophospholipids. As the concentration of 18:2 was increased (5-200 microM) the quantity absorbed increased linearly and the amount esterified in the triacylglycerol increased. The cells possessed active acyl delta 6-desaturase and elongase which facilely converted 18:2n-6 to 20:4n-6. At low intracellular concentrations of 18:2n-6 other unsaturated fatty acids, i.e., gamma-linolenic (18:3n-6), alpha-linolenic (18:3n-3), dihomo-gamma-linolenic (20:3n-6), and especially trans, trans-linoleic acid (trans, trans-18:2n- -6) at concentrations ranging from 25 to 200 microM depressed delta 6-desaturase activity. However, suppression of 20:4 synthesis even by trans, trans-18:2 was readily overcome by increasing the concentration of available cis, cis-18:2n-6.

Selective utilization of omega 6 and omega 3 polyunsaturated fatty acids by human skin fibroblasts

Incorporation of exogenous [14C] arachidonate by human skin fibroblasts was found to be significantly greater than that of either [14C]linoleate or alpha-[14C] linolenate. Arachidonate was preferentially esterified in the PI + PS and PE classes of phospholipids. Over 40% of the incorporated [14C] arachidonate was chain elongated in 24 hours. Cells were also grown in lipid-free medium to enhance PUFA desaturation and elongation and the utilization of various omega 6 and omega 3 metabolites examined. Whereas [14C] linoleate partitioned approximately 50:50 between PL and TAG, eicosatrienoate (20:3 omega 6) was selectively sequestered in TAG. Arachidonate and docosatetraenoate (22:4 omega 6) were preferentially incorporated into phospholipids; the PI + PS fraction was most highly enriched with arachidonate. Modification of alpha-[14C] linolenate was more extensive than that of [14C] linoleate. Docosapentaenoate (22:5 omega 3) was the major omega 3 [14C] PUFA of PI + PS and PE. Eicosapentaeonate was not selectively incorporated into phospholipids; within phospholipids the 20:5 omega 3 was primarily in PC. These results indicate that human skin fibroblasts exhibit acyl specificity in the esterification of polyunsaturated fatty acids, including preferential utilization of arachidonate rather than other prostaglandin precursors in the PI + PS fraction.

Enrichment of human platelet phospholipids with linoleic acid diminishes thromboxane release

We have investigated whether exposure of human platelets to elevated concentrations of linoleic acid, the principal dietary polyunsaturate, would influence platelet thromboxane A2 release. Platelets were incubated with albumin-bound linoleic acid at 30 degrees C for 24 h, with prostaglandin E1 added to prevent aggregation. The linoleic acid supplemented platelets released, on average, 50% less thromboxane A2 in response to stimulation with thrombin than corresponding control platelets. Other fatty acids were without appreciable effect. The inhibition of thrombin-stimulated thromboxane A2 release was dependent on the time and temperature of incubation, as well as on the concentration of added linoleic acid. Supplementation increased the amount of linoleic acid in the platelet phospholipids, but the arachidonic acid content of the phospholipids was reduced. [1-14C]Linoleic acid was not converted to arachidonic acid by the platelets. Linoleic acid was released exclusively from the inositol phosphoglycerides when the enriched platelets were stimulated with thrombin. The linoleate-enriched platelets converted less [1-14C]arachidonic acid to all prostaglandin products, suggesting that the platelet cyclooxygenase was partially inhibited.

Utilization of arachidonic and linoleic acids by cultured human endothelial cells

When cultured human umbilical vein endothelial cells are supplemented with linoleic acid, the arachidonic acid content of the cellular phospholipids is reduced approximately 35%. Most of the fatty acid compositional change occurs during the first 24 h. One factor responsible for this effect is the inability of the endothelial cells to convert appreciable amounts of linoleic to arachidonic acid, due to a fatty acid delta 6-desaturase deficiency. By contrast, these endothelial cultures contain delta 5- and delta 9-desaturase activity and are able to elongate long-chain polyunsaturated fatty acids. The other factor that contributes to the decrease in arachidonic acid is that high concentrations of linoleic acid reduce the incorporation of arachidonate into cellular phospholipids. Stearic acid, a long-chain saturate, does not produce any reduction, whereas eicosatrienoic acid is an even more effective inhibitor than linoleic acid. In spite of the fact that high concentrations of these polyunsaturates produced inhibition, the endothelial cells were found to efficiently incorporate exogenous arachidonic acid into cellular phospholipids and triglycerides. This may serve to compensate for the inability of these cells to synthesize arachidonic acid from linoleic acid. These findings suggest that the endothelium obtains arachidonic acid from an extracellular source, that this cannot be provided in the form of linoleic acid and, in fact, that high concentrations of linoleic acid actually may interfere with the ability of the endothelium to maintain an adequate supply of intracellular arachidonic acid.

Accumulation of N-3 polyunsaturated fatty acids cultured human Y79 retinoblastoma cells

The metabolism of the n-3 class of polyunsaturated fatty acids, which occur in relatively high quantities in neural tissues, was studied in human Y79 retinoblastoma cells. These cells contained low levels of n-3 polyunsaturates when grown in culture media supplemented with fetal bovine serum. The cells readily incorporated performed docosahexaenoic acid (22.6 n-3) into phospholipids, but human skin fibroblasts did this to a similar extent. When 10 to 30 mumol/ml linolenic acid (18:3 n-3) was added, the cell also accumulated 22:6 in phospholipids. The capacity to convert appreciable amounts of 18:3 to 22:6 appears to be a unique property of the retinoblastoma cells as compared with other continuously cultured cell lines. More 18:3 than linoleic acid (18:2 n-6) was incorporated into phospholipids by the retinoblastoma cultures, and 18:3 was channeled to a larger extent into the ethanolamine glycerophospholipid fraction. These findings indicate that retinoblastoma cells handle n-3 polyunsaturated fatty acids in a manner very similar to neural tissue in vivo. Based on the results obtained with this model system, it appears that three processes may contribute to the accumulation of 22:6 in retina and neural tissue: increased ability to incorporate 18:3, the capacity to convert 18:3 to 22:6, and channeling of 18:3 and its metabolites into ethanolamine glycerophospholipids.

The effect of dietary supplements of omega 3 polyunsaturated fatty acids on the fatty acid composition of platelets and plasma choline phosphoglycerides

1. The effects of dietary supplements of omega 3 polyunsaturated fatty acids on the fatty acid composition of platelets and plasma choline phosphoglycerides were studied in vegans and in omnivores. 2. A supplement of 18:3 omega 3 led to an increase in 20:5 omega 3 but was less effective than one of 20:5 omega 3 + 22:6 omega 3.

Retention of human skin fibroblast fatty acid modifications during maintenance culture

The fatty acid composition of cultured human skin fibroblasts was modified by adding either oleic or linoleic acid to the growth medium. After the cultures became confluent, they were washed and transferred to different maintenance media in order to determine the stability of the various fatty acyl modifications. Some changes in fatty acid composition occurred under all conditions. When the maintenance medium was supplemented with fatty acid, the cellular neutral lipid and phospholipid fatty acyl composition were altered markedly within 16 to 24 hr. If no supplemental fatty acid was available during the maintenance period, however, the modified fatty acyl compositions were sufficiently retained so that appreciable differences between the cells enriched with oleate and linoleate persisted for at least 48 to 72 hr. This considerable degree of stability occurred when either 10% delipidized fetal bovine serum or 10% fetal bovine serum containing its inherent lipids were present in the maintenance medium. Although the triglyceride content of the fatty acid-modified cells was quite labile, neither the cholesterol nor phospholipid content changed appreciably during culture in any of the maintenance media. Since the fatty acid compositional differences persisted during several days of maintenance under certain conditions, these modified cultures appear to be a useful experimental system for assessing the effect of lipid structure on fairly long-term cellular functions.

Selectivity in incorporation, utilization and retention of oleic and linoleic acids by human skin fibroblasts

Fetal human fibroblasts were grown in culture medium containing 10% fetal bovine serum supplemented with [1(-14)C]linoleate or [1(-14)C]oleate. At all concentrations of exogenous fatty acids, the incorporation of oleate was greater than that of linoleate. With increased medium fatty acid concentrations, linoleate in triacyglycerol (TAG) could be increased from 13 to 75% of the total incorporated; at each concentration, relatively more linoleate than oleate was in TAG. When the cells were exposed to exogenous oleate/linoleate mixtures, the composition of the mixture determined the extent of incorporation of both fatty acids. When the mixture was primarily linoleate, scarce oleate was used preferentially for phospholipids (PL); no such specificity for scarce linoleate was observed. Addition of exogenous fatty acids resulted in a shift of previously incorporated 14C fatty acids from phospholipid into TAG; retention of oleate in PL was greater than that of linoleate. Incorporation of oleate into phospholipids was also higher than that of linoleate from exogenous fatty acid mixtures which were 80% saturated. It is suggested that normal human fibroblasts have adapted to the low levels of exogenous polyunsaturated fatty acids in culture media by increased use of oleate in phospholipid. Even when the cells are supplemented with linoleate, the preferential use of oleate in phospholipid groups is retained.

Effect of fatty acid modification on prostacyclin production by cultured human endothelial cells

We have investigated whether changes in cellular fatty acid saturation can influence prostacyclin (PGI2) production by cultured human umbilical vein endothelial cells. As compared to control cells, those enriched with linoleic acid released 60--75% less PGI2 in response to thrombin or the calcium ionophore A23187. A similar but considerably smaller effect was observed when the cells were enriched with oleic or linolenic acid, but no reduction occurred with palmitic or linoelaidic acids. Some reduction in PGI2 release was noted as early as 1 h after exposure to linoleic acid. When the culture medium was supplemented with linoleic acid, the cell phospholipids contained four to five times more linoleate and 25--40% less arachidonate. These changes were most marked in the choline and serine plus inositol phosphoglyceride fractions. When the fatty acid composition of the cells enriched with linoleic acid was allowed to revert, there was a progressive increase in the capacity of the cells to release PGI2 in response to thrombin. The increase correlated with a reduction in linoleate content of the cell lipids, but there was no change in arachidonate content. This suggests that linoleic acid may act as an inhibitor of PGI2 production. The cultured endothelial cells were also able to produce PGI2 directly from added arachidonic acid. As the arachidonic acid concentration of the medium was raised, PGI2 formation by the linoleate-enriched cells increased relative to control cells, suggesting that the inhibition produced by linoleic acid may be competitive.