N-Benzyl-linoleamide, a Constituent of Lepidium meyenii (Maca), Is an Orally Bioavailable Soluble Epoxide Hydrolase Inhibitor That Alleviates Inflammatory Pain

Lepidium meyenii (maca), a plant indigenous to the Peruvian Andes, recently has been utilized globally for claimed health or recreational benefits. The search for natural products that inhibit soluble epoxide hydrolase (sEH), with therapeutically relevant potencies and concentrations, led to the present study on bioactive amide secondary metabolites found in L. meyenii, the macamides. Based on known and suspected macamides, 19 possible macamides were synthesized and characterized. The majority of these amides displayed excellent inhibitory potency (IC50 ≈ 20-300 nM) toward the recombinant mouse, rat, and human sEH. Quantitative analysis of commercial maca products revealed that certain products contain known macamides (1-5, 8-12) at therapeutically relevant total concentrations (≥3.29 mg/g of root), while the inhibitory potency of L. meyenii extracts directly correlates with the sum of concentration/IC50 ratios of macamides present. Considering both its in vitro efficacy and high abundance in commercial products, N-benzyl-linoleamide (4) was identified as a particularly relevant macamide that can be utilized for in vivo studies. Following oral administration in the rat, compound 4 not only displayed acceptable pharmacokinetic characteristics but effectively reduced lipopolysaccharide-induced inflammatory pain. Inhibition of sEH by macamides provides a plausible biological mechanism of action to account for several beneficial effects previously observed with L. meyenii treatments.

Incorporation of cis-9,trans-11 or trans-10,cis-12 conjugated linoleic acid into plasma and cellular lipids in healthy men

This study investigated the incorporation of cis-9,trans-11 conjugated linoleic acid (c9,t11 CLA) and trans-10,cis-12-CLA (t10,c12 CLA) into plasma and peripheral blood mononuclear cell (PBMC) lipids when consumed as supplements highly enriched in these isomers. Healthy men (n = 49, age 31 +/- 8 years) consumed one, two, and four capsules containing approximately 600 mg of either c9,t11 CLA or t10,c12 CLA per capsule for sequential 8 week periods followed by a 6 week washout before consuming the alternative isomer. Both isomers were incorporated in a dose-dependent manner into plasma phosphatidylcholine (PC) (c9,t11 CLA r = 0.779, t10,c12 CLA r = 0.738; P < 0.0001) and cholesteryl ester (CE) (c9,t11 CLA r = 0.706, t10,c12 CLA r = 0.788; P < 0.0001). Only t10,c12 CLA was enriched in plasma nonesterified fatty acids. Both c9,t11 CLA and t10,c12 CLA were incorporated linearly into PBMC total lipids (r = 0.285 and r = 0.273, respectively; P < 0.0005). The highest concentrations of c9,t11 CLA and t10,c12 CLA in PBMC lipids were 3- to 4-fold lower than those in plasma PC and CE. These data suggest that the level of intake is a major determinant of plasma and PBMC CLA content, although PBMCs appear to incorporate both CLA isomers less readily.

Growth and fatty acid metabolism of human breast cancer (MCF-7) xenografts in nude rats: impact of constant light-induced nocturnal melatonin suppression

The nocturnal melatonin (MLT) surge is a relevant oncostatic signal for a variety of experimental malignancies. Population studies support the hypothesis that exposure to light at night may represent a new risk factor for breast cancer possibly through the suppression of pineal MLT production and/or circadian disruption. We tested the ability of constant light exposure to suppress MLT production in female nude rats and stimulate the growth of tissue-isolated MCF-7 human breast cancer xenografts via increased tumor linoleic acid (LA) metabolism. Rats maintained on an alternating light/dark cycle (L:D group) exhibited a robust circadian MLT rhythm that was abolished following constant light exposure. During the exposure of animals bearing tissue-isolated human MCF-7 breast cancer xenografts to constant light, the rate of tumor growth markedly increased relative to the L:D group. Tumor LA uptake and its metabolism to the mitogen 13-hydroxyoctadecadienoic acid (13-HODE) were also substantially higher under constant light conditions. This is the first biological evidence for a potential link between constant light exposure and increased human breast oncogenesis involving MLT suppression and stimulation of tumor LA metabolism.

Conjugated linoleic acids: all the same or to everyone its own function?

Conjugated linoleic acid (CLA) is a generic term referring to a mixture of geometrical and positional isomers of linoleic acid in which up to 16 members have been identified. Many potentially beneficial health effects have been ascribed to these fatty acids when consumed as a mixture, and where generally 2 isomers dominate, e.g. the 9c, 11t-isomer, the so-called rumenic acid, and the 10t, 12c-isomer: anti-carcinogenic, immune modulator, anti-atherosclerotic, and anti-obesity among the most spectacular. The question arises as to whether the pleiotropic biological activity is supported by one or several of the isomers. Recent studies using pure individual isomers have started to elucidate this issue, but many others are required to ascribe a respective role to each CLA isomer (the main ones as well as the minor ones), such as those occurring in some complex mixtures already commercially available, or even in foodstuff. The aim of the present study was to focus on the CLA-isomer specific effects depicted in the literature up to now.

Perturbation of lipid metabolism by linoleic acid hydroperoxide in CaCo-2 cells

Dietary hydroperoxides are being discussed as potential health hazards contributing to oxidative stress-related diseases. However, how food-born hydroperoxides could exert systemic effects remains elusive in view of the limited chances to be absorbed. Therefore, the metabolic fate of 13-HPODE (13-hydroperoxy octadecadienoic acid), 13-HODE (13-hydroxy octadecadienoic acid) and linoleic acid (LA) was investigated in a CaCo-2 cell monolayer as a model of the intestinal epithelium. [1-14C]-13-HPODE, up to a non-cytotoxic concentration of 100 microM, did not cross the CaCo-2 cell monolayer unreduced if applied to the luminal side. The [1 -14C]-HPODE-derived radioactivity was preferentially recovered from intracellular and released diacylglycerols (DG), phospholipids (PL) and cholesterol esterified with oxidized fatty acids (oxCE). A similar distribution pattern was obtained with 13-HODE. In contrast, LA is preferentially incorporated into triacylglycerols (TG), cholesteryl esters (CE) and PL (but mainly released as TG). 13-HPODE dose-dependently decreased the incorporation of LA into released TG, while LA accumulated in cellular and released DGs, effects similarily exerted by 13-HODE. We concluded that food-born hydroperoxy fatty acids are instantly reduced by the gastrointestinal glutathione peroxidase, which was previously shown to persist in selenium deficiency. Accordingly, modulation of the glutathione peroxidases by selenium deprivation/repletion did not modify the disturbance of the lipid metabolism by 13-HPODE. Thus, hydroperoxy fatty acids disturb intestinal lipid metabolism by being esterified as hydroxy fatty acids into complex lipids, and may render lipoproteins synthesized thereof susceptible to further oxidative modifications.

Preferential incorporation of trans, trans-conjugated linoleic acid isomers into the liver of suckling rats

The present study was designed to compare the conjugated linoleic acid (CLA) isomeric distribution pattern in the liver of suckling rats in relation to those in the milk and maternal diet. Silver-ion HPLC was used to separate individual CLA isomers. It was found that the isomeric distribution pattern in the milk was very similar to that in the maternal dietary fat. However, the CLA isomeric distribution patterns in the liver phospholipids (PL) and triacylglycerols were different from those in the diet and milk. In the liver PL, total cisltrans isomers accounted for 63.6-63.9% of total CLA, which was in contrast to the values of 88.1-89.1% in the milk and diet. In the liver PL, total transltrans isomers were 20.6-20.8% of the total CLA isomers whereas they were only 2.6-3.7% in the milk and diet. It is concluded that trans/trans-CLA were preferentially incorporated into the liver whereas for the incorporation of cis/trans-CLA there was partial discrimination.

Milk fat conjugated linoleic acid (CLA) inhibits growth of human mammary MCF-7 cancer cells

The relationship between growth and the antioxidant enzyme defence system in human MCF-7 (breast) cancer cells treated with bovine milk fat enriched with conjugated linoleic acid (CLA) was studied. Milk enriched in CLA was obtained from cows on pasture supplemented with full fat rapeseeds and full fat soyabeans (1). Cell number decreased up to 90% (p < 0.05) and lipid peroxidation increased 15-fold (p < 0.05) following incubation of MCF-7 cells for 8 days with increasing levels of milk fat yielding CLA concentrations between 16.9 and 22.6 ppm. Growth suppression and prooxidant effects of milk fat CLA were independent of the variable composition of the milk fat samples, suggesting that CLA was the active ingredient in milk fat responsible for the cytotoxic effect. Mixtures containing isomers of CLA (c9, t11-, t10, c12-, c11, t13- and minor amounts of other isomers) and linoleic acid (LA) at similar concentrations to the milk fat samples were as effective at inhibiting growth and stimulating peroxidation of MCF-7 cells as the milk fatty acids. Incubation of the cells with the c9, t11 CLA isomer (20 ppm) or the mixture of CLA isomers (20 ppm) for 8 days resulted in a 60% decrease (p < 0.05) in viability compared with untreated controls and was significantly (p < 0.05) more effective than incubation with the t10, c12 CLA isomer (20 ppm), which caused only a 15% decrease in cell numbers under similar conditions. A 25% increase (p < 0.05) in cell proliferation occurred when LA (20 ppm) alone was incubated with MCF-7 cells for 8 days. 14C-CLA was preferentially incorporated into the phospholipid fraction of the MCF-7 cell lipids in a dose-dependent manner and CLA accumulated in cell membranes more efficiently when the cells were incubated in the presence of milk fat than the c9, t11 synthetic CLA isomer. Superoxide dismutase (SOD), catalase and glutathione peroxidase (GPx) activities were induced in MCF-7 cells exposed to milk fat (containing 16.9-22.6 ppm CLA) over 8 days. The data indicate that milk fat triglyceride-bound CLA, consisting primarily of the c9, t11 isomer, was cytotoxic towards MCF-7 cells.

Gastrointestinal glutathione peroxidase prevents transport of lipid hydroperoxides in CaCo-2 cells

BACKGROUND & AIMS

Gastrointestinal glutathione peroxidase (GI-GPx), 1 of the 4 types of selenium-dependent glutathione peroxidases, is expressed exclusively in the gastrointestinal system and has therefore been suggested to function as a barrier against the absorption of dietary hydroperoxides.

METHODS

The selenium-dependent expression of GI-GPx and cytosolic GPx (cGPx) was analyzed by Western blotting. Transport of 13-hydroperoxy octadecadienoic acid (13-HPODE) was investigated in a CaCo-2 cell monolayer modulated in GI-GPx and cGPx by selenium restriction or repletion. Localization of GI-GPx in rat intestine was visualized by immunohistochemistry.

RESULTS

Low but significant GI-GPx levels were detected in selenium-deficient CaCo-2 cells and in the gastrointestinal tract of selenium-deficient rats, whereas cGPx was completely absent. Selenium supplementation of CaCo-2 cells resulted in a 5-fold increase of GI-GPx protein, whereas total GPx activity increased by a factor of 13, with most of the GPx activity under selenium-adequate conditions being cGPx. Irrespective of the selenium status, 13-HPODE did not reach the basolateral side of an intact CaCo-2 cell monolayer. Depending on the selenium status, hydroperoxides damaged the monolayer as evidenced by loss of transepithelial resistance and paracellular diffusion of lucifer yellow. Only under these conditions was unmetabolized 13-HPODE detectable at the basolateral side.

CONCLUSIONS

Low GI-GPx levels, as present in selenium deficiency, suffice to prevent transport of 13-HPODE. GI-GPx may thus function as a barrier against hydroperoxide absorption. cGPx contributes to balance major oxidative challenge.

Lymphatic delivery and in vitro pancreatic lipase hydrolysis of glycerol esters of conjugated linoleic acids in rats

We examined the intestinal delivery of conjugated linoleic acids (CLA) given in their triacylglycerol form in the mesenteric lymph of rats. Emulsions containing a mixture of the trilinolein/triester of CLA (9:1) and a tri-[1-(14)C]-linoleyl-sn-glycerol tracer were administered by force-feeding. Lymph was collected over two time periods (0-6 and 6-24 h), and the apparent recovery of CLA was determined relative to that of [1-(14)C]-18:2(n-6). A mixture of CLA-triester/trilinolein (1:9), trilinolein or CLA-triester was separately subjected to pancreatic lipase hydrolysis in vitro to determine whether the lymphatic recovery of CLA was correlated with the initial step of digestion. Lymphatic recovery of CLA was similar to that of 18:2(n-6) (95.6+/-9.0% of the linoleic acid recovery), and isomer repartition was similar in lymph and in the oil fed, indicating that all the CLA isomers were equally absorbed by the enterocytes. Unexpectedly, the in vitro release of CLA into the absorbable forms (free fatty acids and 2-monoacyl-sn-glycerol) was consistently lower than that of 18:2(n-6). Moreover, the 9c, 11t-isomer of CLA was also released faster into the absorbable forms than its 10t,12c homolog (P = 0.05). We cannot ascribe a distinct cellular accumulation or a difference in the biological effects of different CLA isomers on the ground of a selective intestinal absorbability. Also, the physiological conditions prevailing in vivo in the digestive tract are likely to overcome the relative resistance of CLA ester bonds to pancreatic lipase hydrolysis and allow a lymphatic recovery of CLA similar to that of linoleic acid.

Effect of dietary docosahexaenoic acid on desaturation and uptake in vivo of isotope-labeled oleic, linoleic, and linolenic acids by male subjects

The effect of dietary docosahexaenoic acid (22:6n-3, DHA) on the metabolism of oleic, linoleic, and linolenic acids was investigated in male subjects (n = 6) confined to a metabolic unit and fed diets containing 6.5 or <0.1 g/d of DHA for 90 d. At the end of the diet period, the subjects were fed a mixture of deuterated triglycerides containing 18:1n-9[d6], 18:2n-6[d2], and 18:3n-3[d4]. Blood samples were drawn at 0, 2, 4, 6, 8, 12, 24, 48, and 72 h. Methyl esters of plasma total lipids, triglycerides, phospholipids, and cholesterol esters were analyzed by gas chromatography-mass spectrometry. Chylomicron triglyceride results show that the deuterated fatty acids were equally well absorbed and diet did not influence absorption. Compared to the low-DHA diet (LO-DHA), clearance of the labeled fatty acids from chylomicron triglycerides was modestly higher for subjects fed the high DHA diet (HI-DHA). DHA supplementation significantly reduced the concentrations of most n-6[d2] and n-3[d4] long-chain fatty acid (LCFA) metabolites in plasma lipids. Accumulation of 20:5n-3[d4] and 22:6n-3[d4] was depressed by 76 and 88%, respectively. Accumulations of 20:3n-6[d2] and 20:4n-6[d2] were both decreased by 72%. No effect of diet was observed on acyltransferase selectivity or on uptake and clearance of 18:1n-9[d6], 18:2n-6[d2], and 18:3n-3[d4]. The results indicate that accumulation of n-3 LCFA metabolites synthesized from 18:3n-3 in typical U.S. diets would be reduced from about 120 to 30 mg/d by supplementation with 6.5 g/d of DHA. Accumulation of n-6 LCFA metabolites synthesized from 18:2n-6 in U.S. diets is estimated to be reduced from about 800 to 180 mg/d. This decrease is two to three times the amount of n-6 LCFA in a typical U.S. diet. These results support the hypothesis that health benefits associated with DHA supplementation are the combined result of reduced accretion of n-6 LCFA metabolites and an increase in n-3 LCFA levels in tissue lipids.

Distributions of conjugated linoleic acid (CLA) isomers in tissue lipid classes of pigs fed a commercial CLA mixture determined by gas chromatography and silver ion-high-performance liquid chromatography

Pigs were fed a commercial conjugated linoleic acid (CLA) mixture, prepared by alkali isomerization of sunflower oil, at 2% of the basal diet, from 61.5 to 106 kg live weight, and were compared to pigs fed the same basal diet with 2% added sunflower oil. The total lipids from liver, heart, inner back fat, and omental fat of pigs fed the CLA diet were analyzed for the incorporation of CLA isomers into all the tissue lipid classes. A total of 10 lipid classes were isolated by three-directional thin-layer chromatography and analyzed by gas chromatography (GC) on long capillary columns and by silver-ion high-performance liquid chromatography (Ag+-HPLC); cholesterol was determined spectrophotometrically. Only trace amounts (<0.1%; by GC) of the 9,11-18:2 cis/trans and trans,trans isomers were observed in pigs fed the control diet. Ten and twelve CLA isomers in the diet and in pig tissue lipids were separated by GC and Ag+- HPLC, respectively. The relative concentration of all the CLA isomers in the different lipid classes ranged from 1 to 6% of the total fatty acids. The four major cis/trans isomers (18.9% 11 cis,13 trans-18:2; 26.3% 10 trans,12 cis-18:2; 20.4% 9 cis,11 trans-18:2; and 16.1% 8 trans, 10 cis-18:2) constituted 82% of the total CLA isomers in the dietary CLA mixture, and smaller amounts of the corresponding cis,cis (7.4%) and trans,trans (10.1%) isomers were present. The distribution of CLA isomers in inner back fat and in omental fat of the pigs was similar to that found in the diet. The liver triacylglycerols (TAG), free fatty acids (FFA), and cholesteryl esters showed a similar pattern to that found in the diet. The major liver phospholipids showed a marked increase of 9 cis,11 trans-18:2, ranging from 36 to 54%, compared to that present in the diet. However, liver diphosphatidylglycerol (DPG) showed a high incorporation of the 11 cis,13 trans-18:2 isomer (43%). All heart lipid classes, except TAG, showed a high content of 11 cis,13 trans-18:2, which was in marked contrast to results in the liver. The relative proportion of 11 cis,13 trans-18:2 ranged from 30% in the FFA to 77% in DPG. The second major isomer in all heart lipids was 9 cis,11 trans-18:2. In both liver and heart lipids the relative proportions of both 10 trans,12 cis-18:2 and 8 trans, 10 cis-18:2 were significantly lower compared to that found in the diet. The FFA in liver and heart showed the highest content of trans,trans isomers (31 to 36%) among all the lipid classes. The preferential accumulation of the 11 cis,13 trans-18:2 into cardiac lipids, and in particular the major phospholipid in the inner mitochondrial membrane, DPG, in both heart and liver, appears unique and may be of concern. The levels of 11 cis,13 trans-18:2 naturally found in foods have not been established.

Permeability and metabolic properties of a trophoblast cell line (HRP-1) derived from normal rat placenta

The HRP-1 cell line is derived from normal rat placenta and appears morphologically similar to and retains characteristic expression of cellular markers of labyrinthine trophoblast cells. In this study, monolayers of HRP-1 cells grown on permeable supports were evaluated as a potential in vitro system to study trophoblast transport and metabolism. The cell line was shown to express and retain functional activity of the predominant placental cytochrome P450 isozyme, CYP1A1. Additionally, the HRP-1 cells retain functional activity of angiotensin I converting enzyme and carboxypeptidase N-like enzyme, peptidases characteristic of the trophoblast. The permeation of several hydrophilic, inert markers across the HRP-1 monolayers was observed to be dependent on effective molecular size and to be passive in nature. Functional asymmetry of the HRP-1 cells was illustrated by the predominant permeation of linoleic acid in the apical-to-basolateral direction across the monolayers. Transferrin passage across HRP-1 monolayers was concentration-dependent, was bidirectional, and could be inhibited by unlabeled transferrin, features typical of the trophoblast transport system for transferrin. Collectively, these properties suggest that the HRP-1 cell line may provide a useful tool for evaluating some of the permeability and metabolic properties of the trophoblast.

Dietary linoleic acid intake controls the arterial blood plasma concentration and the rates of growth and linoleic acid uptake and metabolism in hepatoma 7288CTC in Buffalo rats

In this study, we tested the hypothesis that dietary linoleic acid intake controls the arterial blood plasma linoleic acid concentration and the rates of tumor growth and linoleic acid metabolism in vivo. Seven groups of young male Buffalo rats (11-21 rats/group) were given free access to semipurified diets containing different amounts of corn and/or olive oils. Four other groups (7-11 rats/group) were 30% energy-restricted. Each experiment included periods for rat growth and plasma lipid stabilization (6 wk), measurement of mean daily arterial blood plasma fatty acid concentrations (3 wk), surgical implantation of a subcutaneous tissue-isolated hepatoma 7288CTC, tumor growth and harvest (2-4 wk). Linoleic + arachidonic acid (P = 0.007) and oleic acid (P = 0.002) concentrations in arterial blood plasma were increased as dietary intake of linoleic and oleic acids was increased, respectively. In rats given free access to food, tumor growth was directly dependent on the plasma concentrations of linoleic (P < 0.001) and arachidonic acids (P = 0.04). Tumor growth in energy-restricted rats was dependent only on the linoleic acid concentration (P = 0.008). Energy restriction itself caused a growth inhibition independent of plasma linoleic acid. The linoleic acid and total fatty acid concentrations of tumor triacylglycerols were directly dependent on the plasma linoleic acid concentration in rats given free access to food (P = 0.009). Hepatoma 7288CTC (both in vivo and during perfusion in situ) supported a dose-dependent conversion (P < 0.001) of plasma linoleic acid to the mitogen, 13-hydroxy-9, 11-octadecadienoic acid. We conclude that increased arterial blood plasma linoleic acid concentrations, caused by increased dietary intakes, specifically stimulate growth, lipid storage and linoleic acid metabolism in hepatoma 7288CTC in vivo.

Nutrient uptake into undifferentiated and differentiated HT-29 cells in culture

HT-29 human colon carcinoma cells in culture have many characteristics of enterocytes, and these cells have been used by others to study intestinal drug and nutrient transport and metabolism. When grown in glucose-containing medium, HT-29 cells are largely undifferentiated (HT-29glu), but when grown in the absence of glucose but in the presence of galactose (HT-29gal), the population of cells is mostly differentiated. This study was undertaken with HT-29glu and HT-29gal cells to study the uptake of palmitic acid (16:0), linoleic acid (18:2), and cholesterol. The relationship between concentration and uptake of 16:0, 18:2, and cholesterol was linear in HT-29glu and HT-29gal cells, with the relative values of the slopes of this relationship being 18:2 > > 16:0 > > cholesterol. The rates of uptake of these lipids were at least three times higher in HT-29gal than in HT-29glu cells. In HT-29glu cells, the relative rates of uptake of the sugars at 32 mM were D-glucose = galactose > fructose > > alpha-methylglucose. Uptake of these sugars was much greater in HT-29gal than in HT-29glu cells. When 100 microM forskolin was added to the incubation medium for 7 days post-confluency, which stimulates the activity of adenylate cyclase and thereby increases the intracellular synthesis of cAMP, there was no effect on the uptake of the lipids or the sugars in either HT-29glu or HT-29gal cells. Thus, (i) differentiated HT-29gal cells transport larger amounts of lipids and sugars than do undifferentiated HT-29glu cells; (ii) forskolin has no effect on the uptake of lipids or sugars in these cells. This human cell culture system may be useful to study the in vitro transport of lipids, to establish the role of cell differentiation on these uptake processes, and to determine the potential role of selected intracellular signals.

The majority of dietary linoleate in growing rats is beta-oxidized or stored in visceral fat

On a quantitative, whole-body basis, little is known about the amount of linoleate that is converted to arachidonate or the partitioning of linoleate and its longer-chain derivatives among lean and fat tissues. The aim of the present study was to examine linoleate balance and organ partitioning in rats consuming a low but adequate level of linoleate. Weanling male Sprague-Dawley rats were given free access to a semipurified diet containing 2.3% of energy as linoleate. Food intake, fecal output and body weight gain were measured for 26 d. Whole-body fatty acid balance analysis showed that 75.5% of the linoleate consumed disappeared (apparently by beta-oxidation), 18.7% was accumulated as linoleate, 3.0% was converted to (n-6) longer-chain polyunsaturated fatty acids, and 1.2% was excreted in the feces Visceral fat contained 64% of the accumulated linoleate, and 23% was in lean tissues. Comparable values for alpha-linolenate were as follows: disappearance (84.9%), accumulation (10.9%), excretion in the feces (2.2%), and conversion to (n-3) longer-chain polyunsaturated fatty acids (1.4%). Visceral fat contained 67% of the accumulated alpha-linolenate, and 23% was in lean tissues. Visceral fat also accumulated 26% of newly synthesized (n-6) longer-chain polyunsaturated fatty acids and 31% of the (n-3) longer-chain polyunsaturated fatty acids. Thus, only 6.5% of dietary linoleate consumed at a low but adequate level for rats appeared in lean tissues as linoleate or its fatty acid metabolites; the rest was beta-oxidized or stored in fat, mostly in visceral fat. These results lead us to speculate whether losses through beta-oxidation contribute to the recommended intake for linoleate in growing rats.

Comparative effects of linoleic acid and linoleic acid hydroperoxide on growth and morphology of bovine retinal pigment epithelial cells in vitro

PURPOSE

Outer segments of the photoreceptor rods that are phagocytized by the retinal pigment epithelial (RPE) cells contain a high proportion of polyunsaturated fatty acids (PUFA). PUFA are susceptible to lipid peroxidation. We hypothesized that the resulting peroxides could injure RPE cells leading to retinal degeneration. Accordingly, we compared the effects of linoleic acid (LA) and its hydroperoxide (LHP) on the growth and morphology of RPE cells using laser scanning microscopy and transmission microscopy.

METHODS

We counted the number of RPE cells after incubation for 24 and 48 hrs with concentrations of LA or LHP of 0.035, 0.175, and 0.35 mM. To observe the actin filaments, cultured RPE cells were stained with rhodamine phalloidin. The cells were prefixed with 2% glutaraldehyde and postfixed in 1% osmium tetroxide. Specimens were embedded in Epon 812 after dehydration, and the ultrathin sections were doubly stained with 2% uranyl acetate and 2% lead acetate for examination by transmission electron microscopy.

RESULTS

Exposure to LA or LHP produced dose-dependent damage to RPE cells with a significantly greater effects of LHP than LA. After incubation for 24 hrs with 0.35 mM LA, the number of vacuoles in RPE cells exceeded that observed in control RPE cells by 365 nm laser microscopy. Exposure to 0.35 mM LHP for 24 hrs produced a pycnotic nucleus, with diffuse and granular autofluorescences observed in and around it. Exposure of RPE cells to 0.35 mM LA for 24 hrs showed that the LA incorporated into the lysosomes was digested and released extracellularly from lysosomes via exocytotic vesicles. However, such exposure to LHP damaged the RPE cells, including the membranes in the pinocytotic vesicles. The packed membranes resembled myelin.

CONCLUSIONS

While the LA incorporated into the lysosomes was released extracellularly, LHP persisted in the RPE cells, being observed as autofluorescent lipofuscin-like materials. LHP was cytotoxic, and caused damage to the membranes of pinocytotic vesicles and lysosomes.

Impaired gut lipid absorptive capacity after trauma-hemorrhage and resuscitation

Although the barrier function of the intestinal mucosa is impaired after hemorrhage, it remains unclear whether this is associated with a deficit in mucosal function. The aim of this study, therefore, was to determine whether trauma-hemorrhage affects the in vivo lipid absorptive capacity of the gut and, if so, to characterize the uptake process of free fatty acids in isolated enterocytes. To study this, rats were anesthetized, a laparotomy was performed (i.e., trauma was induced), and various blood vessels were cannulated. For in vivo lipid absorption, the main intestinal lymph vessel was cannulated and a jejunostomy feeding tube was inserted. The animals were bled to and maintained at a mean arterial pressure of 40 mmHg until 40% of shed blood volume was returned in the form of Ringer lactate. They were then resuscitated with four times the volume of maximal bleed out with Ringer lactate. The in vivo and in vitro lipid absorptive capacities were assessed by measuring lymph triglyceride output after a fat load and by determining the linoleic acid uptake rates on isolated enterocytes, respectively. The results show that the in vivo lipid absorption capacity of the gut is severely depressed after trauma-hemorrhage and resuscitation. Similarly, in enterocytes isolated from hemorrhaged rats, fatty acid uptake capacity, as reflected by the decreased maximal uptake rates, was significantly reduced: 1.2 +/- 0.2 and 2.6 +/- 0.6 nmol.min-1 x 10(6) cells-1 for hemorrhaged and sham, respectively. Thus gut lipid absorptive function is depressed after trauma-hemorrhage and resuscitation, which is at least partially due to the depressed uptake mechanism of the enterocyte.

Turnover of ethyl-linoleate in rat plasma and its distribution in various organs

The fate of [14C]ethyl-linoleate (EthLin) after its intravenous administration was investigated in pentobarbital-anesthetized rats. The disappearance of [14C]EthLin from the plasma was very rapid and followed quite closely a biexponential function of time. Fitting of the experimental data to a two-compartmental mammillary model revealed that the labeled compounds are eliminated from the plasma with a half-life of < 1 min during the early time following the intravenous injection and that a large portion of the EthLin is hydrolyzed instantly to linoleic acid and ethanol. About 9-11% of the plasma [14C]EthLin or its breakdown products are irreversibly cleared from the plasma compartment each minute. Most of the 14C-labeled compounds that originated in the plasma were recovered in the rat liver and lungs and to a lesser extent in the heart, spleen, and kidneys. Two hr after the [14C]EthLin administration, approximately 2.5-5.5% of the total radioactivity in the various organs was still associated with EthLin. Such accumulations, although relatively small, indicate that fatty acid ethyl esters (FAEEs) may be taken up from the plasma. Thus, some of the FAEEs that are formed in certain organs may spillover to the circulating blood where much of it would be hydrolyzed to free fatty acids, but reuptake from the plasma may still account, to some extent, to FAEE-induced damage in chronic alcohol abusers.

Fatty acid uptake and metabolism in Hep G2 human-hepatoma cells

The aim of this work was to study the fatty acid metabolism of the human-hepatoma cell line Hep G2. The cultured cells were incubated with either a saturated (palmitic, stearic) or a polyunsaturated (linoleic, alpha-linolenic, eicosatrienoic n-6) radioactive fatty acid. The fatty acids were incorporated into all the basic lipid classes as well as into the main phospholipid subclasses in the cellular membranes. All the fatty acids tested provided a source of carbon for lower members of the saturated fatty-acid family or for cholesterol through beta-oxidation and a new cycle of de novo synthesis. Moreover, all radioactive fatty-acid precursors, whether saturated or unsaturated, were anabolized to higher derivatives within their own family. In the case of saturated fatty acids, palmitic and stearic, they were readily monodesaturated to their corresponding products, thus demonstrating the presence of a delta 9 desaturase. Linoleate and alpha-linolenate were both desaturated and elongated to all the subsequent members of their respective n-6 and n-3 families. These latter observations provide evidence for the incidence of desaturation at the 6 and 5 positions along with the existence of an elongating capacity for fatty acids of all families and chain lengths. In addition, the cellular steady-state fatty-acid profile was seen to be significantly different from the spectrum of exogenous fatty acids available in the growth medium. We conclude that the Hep G2 human-hepatoma line represents an appropriate and relevant experimental model system for investigating the fatty-acid metabolism of adult human liver in vivo.

Linoleic acid uptake by isolated enterocytes: influence of alpha-linolenic acid on absorption

In a previous study we showed that intestinal uptake of alpha-linolenic acid (18:3n-3) was carrier-mediated and we suggested that a plasma membrane fatty acid protein was involved in the transport of long-chain fatty acids. To further test this hypothesis, the mechanism of linoleic acid (18:2n-6) uptake by isolated intestinal cells was examined using a rapid filtration method and 20 mM sodium taurocholate as solubilizing agent. Under these experimental conditions transport of [1-14C]linoleic acid monomers in the concentration range of 2 to 2220 nM was saturable with a Vm of 5.1 +/- 0.6 nmol/mg protein/min and a Km of 183 +/- 7 nM. Experiments carried out in the presence of metabolic inhibitors, such as 2,4-dinitrophenol and antimycin A, suggested that an active, carrier-mediated mechanism was involved in the intestinal uptake of this essential fatty acid. The addition of excess unlabeled linoleic acid to the incubation medium led to a 89% decrease in the uptake of [1-14C]linoleic acid, while D-glucose did not compete for transport into the cell. Other long-chain polyunsaturated fatty acids added to the incubation mixture inhibited linoleic acid uptake by more than 80%. The presence of alpha-linolenic acid (18:3n-3) in the incubation medium caused the competitive inhibition (Ki = 353 nM) of linoleic acid uptake. The data are compatible with the hypothesis that intestinal uptake of both linoleic, and alpha-linolenic acid is mediated by a membrane carrier common to long-chain fatty acids.

Conjugated linoleic acid (9,11- and 10,12-octadecadienoic acid) is produced in conventional but not germ-free rats fed linoleic acid

Conjugated linoleic acid (CLA) is an anticarcinogen in several model animal systems. Conjugated linoleic acid occurs naturally in food and is present at higher concentrations in products from ruminant animals. Given that certain rumen microorganisms produce CLA from free linoleic acid, we studied the effect of feeding free or esterified linoleic acid on tissue CLA concentrations using conventional and germ-free rats. Conventional rats were fed a 5% (wt/wt) corn oil control diet alone or supplemented with 5% free linoleic acid or 8.63% corn oil (equivalent to 5% linoleic acid in triglyceride). Germ-free rats were fed autoclavable nonpurified diet alone or supplemented with 5% free linoleic acid. Analyses of CLA concentrations were performed on lipids extracted from liver, lung, kidney, skeletal muscle and abdominal adipose tissue, and on liver phospholipid and neutral lipid fractions. Tissue CLA concentrations were higher in conventional rats fed free linoleic acid (the major isomers were cis-9, trans-11 and trans-9, cis-11) than in control animals. Conjugated linoleic acid concentrations in free linoleic acid-fed rats were maximal at 4 wk, and levels were 5-10 times higher than those of controls. Elevated CLA concentrations were also observed in liver phospholipid and neutral lipid fractions. In contrast, CLA concentrations in the tissues of germ-free rats were not affected by diet. Feeding the corn oil-fortified diet to conventional rats did not increase CLA concentration in the tissues.(ABSTRACT TRUNCATED AT 250 WORDS)

Linoleic acid passage through the blood-brain barrier and a possible effect of age

It has already been established that the blood-brain barrier is readily crossed by unsaturated fatty acids, while saturated fatty acid transport appears to be protein mediated. When the passage of the fatty acids is tested in vivo by using perfusion buffers containing both linoleate and palmitate in different concentrations, linoleate is able to decrease the palmitate passage, while palmitate increases the linoleate passage. These results could be related to the effect of two fatty acids on the ratio between the fatty acids bound to the serum albumin and the free fatty acid pool, which is only available for transport through membranes. However, on the basis of some results obtained with aged rats, the possibility that a relationship may exist between palmitate and linoleate during their passage through the BBB is discussed. Moreover, it seems likely that in aged rats a moderate modification for fatty acids takes place in the BBB.

[Linoleic acid and the immune system. Controversies about lipid emulsions]

The selection of a given lipidic function for nutritional backup requires not only knowledge of the metabolism of the different existing lipidic emulsions and of their specific therapeutic indications, but also of their contraindications and controversies because, apart from their calorific value, the contribution of liposoluble vitamins and their function in preventing essential fatty acid deficiencies, we know that they are powerful metabolic modulators. This in associated with the fact that manipulation of dietary lipids (enteral or parenteral) can affect and modulate the response to the disease, attack or infection by improving or impairing the different immune functions. This review is focused on the scientific publications which have examined the varying effects of lipidic emulsions, in quantity and in quality (particularly linoleic acid) on the immune system, on the fatty acid composition of the cellular membranes and on the production of and prostaglandins and leukotrienes. An update is given of the known interrelation between lipids and immunity, with appraisal of triglycerides and long-medium -- and short-chain fatty acids, mixtures of medium -- and long-chain triglycerides, the proportions between infinity-3/infinity-6, and structured lipids.

Effects of cytochalasin, colchicine, and ethylenediaminetetraacetic acid on linoleic acid transport across rat jejunal enterocytes

The involvement and the site of interference of the cytoskeleton in the transport of linoleic acid across the rat jejunum was investigated by administration of microfilamentous and microtubular altering agents such as cytochalasin, colchicine, and ethylenediaminetetraacetic acid (EDTA). An isolated jejunal segment was perfused with a buffer containing labeled linoleic acid, and portal blood and perfusate samples were collected concomitantly at 5-min intervals and assayed for their radioactivity. At the end of the perfusion, the amount of radioactivity retained in the intestine was also determined. The results were analyzed by using a three-compartment physical model that allows the determination of mucosal and serosal permeability coefficients, from which changes in the permeability of the mucosal and serosal membranes were assessed. Cytochalasin decreased the permeability of the mucosal membrane to linoleic acid, but not that of the serosal membrane. The administration of colchicine, EDTA, or cytochalasin + colchicine increased the permeability of the serosal membrane but did not affect the mucosal membrane.

Absorption and lymphatic transport of peroxidized lipids by rat small intestine in vivo: role of mucosal GSH

The absorption and lymphatic transport of peroxidized MaxEPA fish oil was studied using the lymph fistula rat to determine the role of mucosal glutathione (GSH) in intestinal metabolism of luminal lipid hydroperoxides. Decreasing intestinal GSH concentrations with buthionine sulfoximine (BSO, 1.15 +/- 0.20 nmol/g), diethyl maleate (DEM, 0.93 +/- 0.26 nmol/g), phorone (1.46 +/- 0.14 nmol/g), or 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU, 1.54 +/- 0.18 nmol/g) compared with control (2.60 +/- 0.38 nmol/g) resulted in higher luminal recovery of the infused lipid hydroperoxide (% of infused dose): BSO (87.8 +/- 4.8%), DEM (86.1 +/- 1.3%), phorone (78.1 +/- 2.1%), and BCNU (71.7 +/- 4.8%) compared with control (52.8 +/- 4.3%). These results suggest that decreased elimination of luminal peroxidized lipids is associated with decreased tissue GSH. Treatment of rats with BSO, DEM, phorone, or BCNU resulted in dramatic increases in appearance of peroxidized lipids in lymph over 6-h lipid infusion (54.7 +/- 3.7, 57.7 +/- 4.6, 46.4 +/- 2.7, and 42.1 +/- 3.9 nmol, respectively) compared with control (20.5 +/- 3.4 nmol). The results are consistent with decreased intracellular metabolism of absorbed hydroperoxides and enhanced transport into lymph under GSH-deficient conditions. The current findings suggest that the function of the mucosal GSH peroxidase/oxidized glutathione (GSSG) reductase system may play an important role in intestinal handling of luminal lipid hydroperoxides. A compromised function of this detoxication mechanism in GSH-deficient states can significantly alter the metabolic fate of dietary peroxidized lipids.

The effect of three intravenous fat emulsions containing different concentrations of linoleic and alpha-linolenic acids on the plasma total fatty acid profile of neonates

We determined the fatty acid profile of total plasma lipids in infants who received one of three intravenous fat emulsions that differed primarily in their linoleic and alpha-linolenic acid content: (I) a safflower oil emulsion, (II) a 50:50 mixture of safflower and soybean oils, or (III) a soybean oil emulsion. After 2 weeks of fat therapy, oleic acid, expressed as a percentage of total plasma lipid fatty acids, decreased in all groups, but less so in group III (p less than 0.01). The linoleic acid percentage increased in all groups, but group I had the greatest increase (p less than 0.05). Group II patients had higher percentages of the linoleic acid metabolites, dihomo-gamma-linolenic acid (II greater than I, p less than 0.05; II greater than III, p less than 0.01) and arachidonic acid (II greater than III, p less than 0.05). Group II patients also had higher levels of alpha-linolenic acid (II greater than I, p less than 0.05) and its metabolite, eicosapentaenoic acid (II greater than I, p less than 0.05). Another alpha-linolenic acid metabolite, docosahexaenoic acid, however, increased in group III, remained stable in group II, and decreased in group I (III and II greater than I, p less than 0.05). We conclude that the content of linoleic acid and alpha-linolenic acid in intravenous fat emulsions results in statistically significant changes in the fatty acid profile of total plasma lipids in infants receiving total parenteral nutrition.

The use of two in-situ simultaneously-perfused placentas to investigate the effect of carrier proteins on linoleic and palmitic acid transfer in the guinea pig

By perfusing two placentas concurrently in five animals we have found that linoleic acid is transferred across the guinea-pig placenta more readily than palmitic acid. In the in-situ perfusion this transfer is modulated by the type of albumin used in the perfusate. Removing the associated lipid from the perfusate albumin reduced the amount of non-esterified fatty acid (NEFA) transfer rather than enhanced it. Many researchers use fat free albumin as a protein carrier in in vivo and in-vitro systems, our results indicate that this may not be the most appropriate substance to use.

Histamine release from rat mast cells induced by metabolic activation of polyunsaturated fatty acids into free radicals

Polyunsaturated fatty acids (PUFA: arachidonic and linoleic acid) release histamine from isolated purified rat serosal mast cells only in the presence of oxidizing systems such as phenobarbital-induced rat liver microsomes, prostaglandin-H-synthetase (PHS) or soybean lipoxygenase. The release of mast cell histamine by activated PUFA has a long time-course and the electron microscopical features are consistent with an exocytotic secretion in the case of arachidonic acid and cell lysis in the case of linoleic acid. The phenomenon is associated with a significant increase in malonyldialdehyde (MDA) and conjugated diene generation, suggesting a relationship between histamine release and membrane lipid peroxidation. The secretion of histamine was inhibited by anti-free radical interventions such as D-mannitol, reduced glutathione and alpha-tocopherol. Some cyclooxygenase and lipoxygenase inhibitors, cimetidine and carnitine derivatives, are differentially active in the inhibition of mast cell histamine release by activated arachidonic acid. These results suggest that free radical derivatives of PUFA, generated by metabolic activation, trigger mast cell histamine release.

Effects of pectin on fatty acid and glucose absorption and on thickness of unstirred water layer in rat and human intestine

Effects of pectin, a soluble dietary fiber, on fatty acid and glucose absorption were studied in vivo in rats and humans by perfusing the intestine with linoleic acid and glucose solutions with and without pectin. Linoleic acid and glucose absorption decreased with increasing concentrations of pectin. The reduction in linoleic acid absorption was not caused by binding of linoleic acids by pectin or impaired micelle formation due to binding of bile acid by pectin. The unstirred water layer expanded with increasing concentrations of pectin. These results suggest that enlargement of the unstirred water layer is closely associated with the reduction of absorption of fatty acid and glucose ingested with pectin.

Mechanisms of linoleic acid uptake by rabbit small intestinal brush border membrane vesicles

We examined the initial transport of a long-chain unsaturated fatty acid, linoleic acid, by brush border membrane vesicles isolated from rabbit small intestine. This preparation allowed us to examine the transport of linoleic acid across the brush border membrane without the effect of the unstirred water layer or cytosol binding proteins. Linoleic acid was solubilized in a 2 mM taurocholate solution which did not compromise the functional integrity of the vesicles. Linoleic acid uptake in the range of 1 to 100 microM followed passive diffusion kinetics. Time course study showed that linoleic acid uptake reached maximal levels during the initial 15 seconds. Although the amount of linoleic acid accumulated in the vesicles diminished over the next 30 minutes, the molar quantity was still twentyfold higher than that of D-glucose (6.5 vs 0.33 nmol/mg protein). Uptake of D-glucose by the vesicles demonstrated typical osmotic responsiveness. We found no osmotic effect on linoleic acid uptake. Hypotonic lysis of membrane vesicles loaded with linoleic acid released 40% of the fatty acid. We concluded that a major portion of the accumulated fatty acid was bound to or incorporated into the membrane itself while ca. 40% did traverse the membrane and accumulated in the intravesicular space as nonmicellar aggregates. The known inhibitors of anion transport, diisothiocyanatostilbene and isothiocyanatostilbene did not change the transport of linoleic acid. We conclude that, in the absence of an unstirred layer or cytosol proteins, linoleic acid transport at up to 100 microM concentration is passive with rapid accumulation both by the cell membrane and the lumen of vesicles.

Selective channelling of arachidonic and linoleic acids into glycerolipids of rat hepatocytes in primary culture

Rat hepatocytes in primary culture were incubated with a mixture of linoleic and arachidonic acid at various total fatty acid/serum albumin molar ratios. Mixed fatty acids were taken up at the same rate and distributed with the same pattern as fatty acids added separately. The rates of total uptake, incorporation into hepatocyte and secreted triacylglycerols and beta-oxidation were linearly related to the fatty acid/albumin ratios, whereas the rate of incorporation into phospholipids was saturable. Neither the uptake rate nor the distribution of both fatty acids considered together varied with the arachidonic acid/linoleic acid molar ratio. Changes in this ratio and in the uptake rate led to significant variations in the respective fate of the fatty acids. The preferential channelling of arachidonic acid versus linoleic acid into beta-oxidation and phosphatidylinositol was greatest at a low uptake rate and then decreased as the uptake rose. Conversely, the preferential channelling of arachidonic acid versus linoleic acid into phosphatidylcholine, but not phosphatidylethanolamine, increased with the uptake rate. Moreover, both arachidonic acid and linoleic acid were preferentially incorporated into the 1-palmitoyl molecular species of phosphatidylcholine and phosphatidylethanolamine at a low uptake rate, and of phosphatidylcholine at a high uptake rate. This could be related to the synthesis of biliary phosphatidylcholine, of which 1-palmitoyl-2-linoleoyl and 1-palmitoyl-2-arachidonoyl are the main molecular species. Linoleic and arachidonic acid were selectively distributed into distinct metabolic pools of triacylglycerol, the intrahepatocyte pool which preferentially incorporated linoleic acid at a low uptake rate and the secreted pool in which the relative enrichment of arachidonic acid increased with the uptake rate. This strengthens the central role of hepatic secretion in the supply of arachidonic acid to peripheral tissues.

Alteration of uptake and distribution of eicosanoid precursor fatty acids by aspirin

Aspirin is an important drug in the treatment of numerous disorders, especially rheumatic diseases. Its several mechanisms of action include inhibition of prostaglandin production by acetylation of prostaglandin synthetase. To explore further the modulatory effect of aspirin on eicosanoid production, we examined its effect on uptake and incorporation of fatty acids into phospholipids of human peripheral blood monocytes. Aspirin ingestion by normal volunteers inhibited uptake of arachidonic acid and linoleic acid in monocytes cultured for 3 days. Similar inhibition was observed when cultured normal human peritoneal macrophages were treated with aspirin for 3 days. In contrast, monocytes cultured for 12 days from both normal volunteers who had ingested aspirin and normal cells treated with aspirin in vitro for the first 3 days of a 12-day culture period expressed an increased uptake of both arachidonic and linoleic acids. Similarly, incorporation of fatty acid into phosphatidylcholine was depressed in 3-day cultured cells but was increased in 12-day cultured cells. Thus, aspirin, whether administered in vivo or added in vitro, modulates cellular uptake and incorporation of eicosanoid precursor fatty acids and their insertion into membrane phospholipids in cultured human monocytes and macrophages.

Selective incorporation of n-3 and n-6 fatty acids in essential fatty acid deficient rats in response to short-term oil feeding

Essential fatty acid deficient male Sprague Dawley rats were fed for 7 days a fat-free semi-synthetic diet supplemented with 10% by weight of different oil supplements. The oil supplement was a mixture of olive, safflower and linseed oils prepared at different proportions so the dietary n-9/n-6/n-3 ratios were approximate 2/1/1, 1/2/1, 1/1/2, and 1/1/1. The fatty acid compositions of plasma and liver lipids were then examined. Our results show polyunsaturated n-6 and n-3 fatty acids were selectively incorporated into plasma and liver phospholipids, and also into plasma cholesteryl esters. A preferential incorporation of n-6 over n-3 fatty acids into plasma cholesteryl esters and phospholipids was also observed.