Incorporation of arachidonic, dihomogamma linolenic and eicosapentaenoic acids into cultured V79 cells

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

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

Metabolism of n-6 fatty acids by NIH-3T3 cells transfected with the ras oncogene

N-6 fatty acid metabolism was compared in NIH-3T3 cells and DT cells, which differ only in the presence of the v-Ki-ras oncogene. Non-dividing cells were incubated with [1-14C]-labelled fatty acids (18:2n-6, 18:3n-6, 20:3n-6 and 20:4n-6) at different time intervals (2-24 h) and concentration (0-120 microM). In both cells lines, the uptake of different fatty acids from the medium was similar and reached a maximum at 6-8 h. All fatty acids reached the same maximum level in DT cells, whereas, the relative uptake of added fatty acids by NIH-3T3 cells was different: 20:4n-6 > 20:3n-6 > 18:2n-6 = 18:3n-6. Throughout the incubation (2-24 h), desaturation and elongation of n-6 fatty acids was more active in DT cells than in NIH-3T3 cells. However, in both cell lines, incubated with different n-6 fatty acid precursors, the levels of radiolabelled 20:4n-6 were relatively constant. In DT cells, phosphatidylcholine was found to be the major fraction labelled with n-6 fatty acids precursors and those of endogenous synthesis, whereas, in NIH-3T3 cells the neutral lipid fraction, particularly triglycerides, was also strongly labelled. In concentration dependent studies, phospholipid labelling by fatty acids was saturable. At lower concentrations, especially in DT cells, phospholipids were labelled predominantly. As the concentration increased there was an overflow into the triglyceride fraction. Since the differences in fatty acid metabolism between the two cell lines cannot be related to the growth rate, it is suggested that they were a consequence of the expression of the v-Ki-ras oncogene.

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.

Effect of vitamin E on arachidonic acid peroxidation and its binding to Chinese hamster V79 cell DNA

Following 24 h incubation in standard culture medium (containing 2 microM of arachidonic acid, AA), and 10 and 20 microM supplemented AA, approx. 55, 40 and 33%, respectively, of the fatty acid was incorporated into Chinese hamster V79 cell lipids. The AA content of cells increased 5 to 7-fold with the 10 and 20 microM supplementations of AA and, there was a correspondingly marked decrease in the proportion of AA incorporated into phospholipids (94 vs. 50 and 32%), whereas an increased percentage of AA was recovered in triacylglycerols (1 vs. 30 and 50%) and free fatty acids (1 vs. 3 and 8%). AA at 12 and 22 microM induced a 50 and 80% increase, respectively, in cellular content of thiobarbituric acid reactive substances (TBARS), an indication of increased peroxidation of cell lipids. This formation of TBARS was inhibited by vitamin E but not by indomethacin or SKF-525. Binding of [3H]AA-derived counts to DNA increased in parallel to the levels of cellular lipid peroxidation. Vitamin E added to the culture medium at 10(-4) M was readily taken up by the cells within 48 h and significantly reduced both lipid peroxidation and binding of AA-derived counts to DNA, without affecting AA uptake. Cell vitamin E content was significantly reduced following 24 h incubation in the presence of 10 and 20 microM supplemented AA. This study indicates that products of lipid peroxidation can bind to DNA in cultured cells, and points to a potential cyto- and geno-toxic risk posed by increased cellular AA content relative to anti-oxidant defenses.

Ferrylmyoglobin-catalyzed linoleic acid peroxidation

The addition of linoleic acid (18:2) to a solution containing oxymyoglobin (MbIIO2), metmyoglobin (MbIII), or metmyoglobin-azide complex (MbIII-N3-) resulted in the formation of a common complex with identical absorption spectral properties. The addition of H2O2 to a MbIII/linoleic acid mixture revealed a spectral profile with lambda max at 530 nm and different from that observed in the reaction of MbIII with H2O2 and identical to that of ferrylmyoglobin. This was accompanied by a progressive decrease in the absorption in the visible region, indicating heme degradation during the lipid peroxidation process. The oxidation products of linoleic acid during the MbIII/18:2/H2O2 interaction were assessed by HPLC under anaerobic and aerobic conditions. In both instances, the chromatograms at lambda 234 nm revealed the formation of a main peak with a retention time of 11.1 min, which cochromatographed with a standard of 9-hydroperoxide of linoleic acid. The latter adduct was not degraded by the oxoferryl complex of myoglobin. The conclusions originating from this research are two-fold. On the one hand, the identical spectral properties exhibited by the product originating from the reaction of either MbIIO2 or MbIII with linoleic acid bridge the apparent discrepancy between the different reactivities of MbIIO2 and MbIII toward H2O2 and their ability to promote lipid peroxidation. On the other hand, the pattern of oxidation products of linoleic acid observed during the MbIII/H2O2 interaction, i.e., the formation of a 9-hydroperoxide adduct as a major product, points to a specific binding character and a regioselectivity of the oxoferryl complex in the oxidation of unsaturated fatty acids or a catalytic preference for decomposition of the various isomeric hydroperoxides over that of the 9-hydroperoxide.

Inhibition of arachidonic acid release by nordihydroguaiaretic acid and its antioxidant action in rat alveolar macrophages and Chinese hamster lung fibroblasts

The ability of nordihydroguaiaretic acid (NDGA) to inhibit arachidonic acid (AA) release from rat alveolar macrophages treated with t-butyl hydroperoxide (tBOOH) or from Chinese hamster lung fibroblasts (V79 cells) treated with linoleic acid hydroperoxide (LOOH) was examined. Treatment of alveolar macrophages with 100 microM tBOOH significantly increased arachidonic acid release and its conversion to metabolites. Pretreatment of macrophages with NDGA (greater than or equal to 2.5 microM) inhibited the release of AA and its subsequent metabolism following addition of tBOOH. Treatment of V79 cells with 1 microM LOOH stimulated the release of AA. Pretreatment with either 1 or 10 microM NDGA prior to the addition of LOOH inhibited the release of AA. A23187 (2 microM)-stimulated release of AA from V79 cells was less sensitive to NDGA inhibition. Pretreatment with 10 microM NDGA, but not with 1 microM NDGA, inhibited A23187-stimulated release of AA. PLA2-dependent hydrolysis of micelle preparations of disaturated phosphatidylcholine was not inhibited by NDGA. Previous studies have suggested that the addition of peroxides alters cells by inducing lipid peroxidation so that the action of phospholipases upon their membranes is enhanced. The results suggest that NDGA, a lipid-soluble antioxidant which traps free radicals, indirectly blocked the action of phospholipases upon cell membranes by inhibiting lipid peroxidation.

t-Butyl hydroperoxide stimulates alveolar macrophage biosynthesis of cyclooxygenase products

Rat alveolar macrophages, labeled with 3H-arachidonic acid, were treated with t-butyl hydroperoxide (tBOOH). Treatment of cells with 100 microM tBOOH led to a rapid increase in 12-hydroxyheptadecatrienoic acid (12-HHT) within 2.5 minutes. At 15 minutes, 12-HHT levels appeared to plateau as there was no further increase at 30 minutes. TxB2 levels increased in a similar manner to that found with 12-HHT; however, only the level at 15 minutes was statistically increased. TxB2 levels also appeared to plateau at 15 minutes. Indomethacin, at a concentration of 1 microM, significantly inhibited TxB2 and 12-HHT production by approximately 90%. Desferal, an iron chelator, had no effect on alterations of biosynthesis of cyclooxygenase products by macrophages treated with tBOOH. No evidence of lipoxygenase products was found. Thus, these results suggest that tBOOH rapidly and selectively stimulated arachidonic acid metabolism through the cyclooxygenase pathway in rat alveolar macrophages. The stimulation of cyclooxygenase activity was transient with a maximum rate observed at 100 microM tBOOH.

Induction of cytotoxicity and mutagenesis is facilitated by fatty acid hydroperoxidase activity in Chinese hamster lung fibroblasts (V79 cells)

The metabolic activation of benzo[a]pyrene and 7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene was studied in V79 Chinese hamster fibroblasts after supplementations with arachidonic acid or treatments with linoleic acid hydroperoxide. The extent of metabolic activation was estimated using cytotoxicity and mutagenesis as endpoints. Pretreatment of cells with arachidonic acid for 24 h resulted in significant elevations in the content of this fatty acid in cell phospholipids and increased prostaglandin synthesis. Arachidonic acid and linoleic acid hydroperoxide facilitated 7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene cytotoxicity and mutagenesis, and to a lesser extent increased the cytotoxicity and mutagenicity of benzo[a]pyrene. No other compounds tested were mutagenic under these conditions, however, linoleic acid hydroperoxide markedly increased their cytotoxicity. Arachidonic acid-facilitated toxicity and mutagenesis was inhibited by indomethacin, whereas no inhibition was seen when linoleic acid hydroperoxide was used. Nordihyroquairaretic acid abolished the cytotoxicity and mutagenesis facilitated by arachidonic acid and linoleic acid hydroperoxide. Our findings demonstrate that induction of cytotoxicity and mutagenesis following treatment of V79 cells with carcinogens may be limited by low levels of arachidonic acid in these cells. A peroxidatic mechanism is proposed, with limited substrate specificity, for the metabolic activation of chemicals in V79 cells.