Effect of different carbon sources on the biosynthesis of polyunsaturated fatty acids of alpha-linolenic acid family in culture of minimal deviation hepatoma 7288 C cells

The questionable role of a microsomal delta8 acyl-coA-dependent desaturase in the biosynthesis of polyunsaturated fatty acids

Several experimental approaches were used to determine whether rat liver and testes express an acyl-CoA-dependent delta8 desaturase. When [1-14C]5,11,14-eicosatrienoic acid was injected via the tail vein, or directly into testes, it was incorporated into liver and testes phospholipids, but it was not metabolized to other labeled fatty acids. When [1-14C]11,14-eicosadienoic acid was injected, via the tail vein or directly into testes, or incubated with microsomes from both tissues, it was only metabolized to 5,11,14-eicosatrienoic acid. When ethyl 5,5,11,11,14,14-d6-5,11,14-eicosatrienoate was fed to rats maintained on a diet devoid of fat, it primarily replaced esterified 5,8,11-eicosatrienoic acid, but not arachidonic acid. No labeled linoleate or arachidonate were detected. Dietary ethyl linoleate and ethyl 19,19,20,20-d4-1,2-13C-11,14-eicosadienoate were about equally effective as precursors of esterified arachidonate. The doubly labeled 11,14-eicosadienoate was metabolized primarily by conversion to 17,17,18,18-d4-9,12-ocatdecadienoic acid, followed by its conversion to yield esterified arachidonate, with a mass four units greater than endogenous arachidonate. In addition, the doubly labeled substrate gave rise to a small amount of arachidonate, six mass units greater than endogenous arachidonate. No evidence was obtained, with the radiolabeled substrates, for the presence of a delta8 desaturase. However, the presence of an ion, six mass units greater than endogenous arachidonate when doubly labeled 11,14-eicosadienoate was fed, suggests that a small amount of the substrate may have been metabolized by the sequential use of delta8 and delta5 desaturases.

Desaturation and chain elongation of n - 3 and n - 6 polyunsaturated fatty acids in the human CaCo-2 cell line

Human CaCo-2 cells were incubated with [14C]linoleic (18:2(n - 6)), [14C]linolenic (18:3(n - 3)) and [3H]eicosapentaenoic acid (20:5(n - 3)), and the interconversion of the radioactive fatty acids to higher homologues and their acylation into triacylglycerols (TG) and phospholipids were examined. An active conversion of [14C]18:3 to [14C]20:5 and [14C]docosapentaenoic acid (22:5(n - 3)) and of [3H]20:5 to [3H]22:5, but not to [3H]docosahexaenoic acid (22:6(n - 3)) was observed. In relation to the amounts that had been incorporated into cellular phospholipids and TG, the interconversion of [14C]18:3 clearly exceeded that of [14C]18:2. Addition of 10-100 microM 18:2 or 10-50 microM arachidonic acid (20:4(n - 6)) increased the percent interconversion of [14C]18:2 to [14C]20:4. E.g., addition of 50 microM 20:4 increased the formation of [14C]20:4 from 4.4 +/- 0.1% to 5.9 +/- 0.8%, decreased the incorporation into phospholipids from 64.8 +/- 6.3% to 31.4 +/- 1.2% and increased the incorporation into TG from 8.8 +/- 0.4% to 28.8 +/- 1.1%. In contrast, addition of 10-100 microM 18:3 or 20:5 significantly decreased the interconversion of both [14C]18:2 and [14C]18:3. E.g., addition of 50 microM 20:5 decreased the formation of [14C]20:4 from [14C]18:2 from 4.4 +/- 0.1% to 0.9 +/- 0.1%, whereas the effects on the acylation reactions were very similar to those of 20:4. 20:5 also decreased the formation of interconversion products from [14C]18:3. 18:2 and 20:4 caused a smaller decrease in the formation of [14C]20:5 and actually increased percent conversion to [14C]22:5. The percent conversion of [3H]20:5 to [3H]22:5 was also increased by the addition of 50-100 microM unlabeled 20:5. [14C]18:2 and [14C]18:3 were predominantly incorporated into phosphatidylcholine (PC) whereas more of the radioactive 20:4, 20:5 and 22:5 was incorporated into phosphatidylethanolamine (PE). An active fatty acid interconversion catalyzed by delta 6 and delta 5 desaturases thus occurs in the human CaCo-2 cell line, whereas conversion of 20:5(n - 3) to 22:6(n - 3) could not be demonstrated. The desaturation-elongation pathway has a preference for 18:3(n - 3) and is subjected to an efficient feedback regulation by 20:5(n - 3). Formation of 22:5 increases with available 20:5 mass and by the presence of other polyunsaturated fatty acids competing with 20:5 for acylation into phospholipids.

Phospholipid fatty acid composition of various mouse tissues after feeding alpha-linolenate (18:3n-3) or eicosatrienoate (20:3n-3)

The selective incorporation of dietary alpha-linolenate (18:3n-3) and its elongation product, eicosatrienoate (20:3n-3), into various phospholipids (PL) of mouse liver, spleen, kidney, and heart, was examined in a two-week feeding trial by assessing mol % changes in associated fatty acids. Mice were fed fat-free AIN 76A diets modified with either 2 wt% safflower oil (control); 1% safflower and 1% linolenate; or 1% safflower and 1% eicosatrienoate. After linolenate or eicosatrienoate feeding, 20:4n-6 was reduced by 36-50% in liver phosphatidylcholine (PC) and in liver and spleen phosphatidylethanolamine (PE). Linolenate was minimally incorporated into PL, but was desaturated and elongated to 20:5n-3, 22:5n-3, and 22:6n-3, with notable differences in the quantity of these n-3 derivatives associated with different tissues and PL. Eicosatrienoate was uniquely incorporated into the cardiolipin (CL) pool of all organs. There was also considerable retroconversion of 20:3n-3 to 18:3n-3 (PC,PE). Dietary eicosatrienoate may therefore affect metabolism in diverse ways--20:3n-3, which is retroconverted to 18:3n-3, may provide substrate for 20:5n-3 and 22:6n-3 syntheses, whereas intact 20:3n-3 may be incorporated into the CL pool. Acyl modifications of CL are known to affect the activity of key innermitochondrial enzymes, such as cytochrome c oxidase.

Desaturation and chain elongation of essential fatty acids in isolated liver cells from rat and rainbow trout

Isolated hepatocytes from rainbow trout and rat were incubated with 14C-labeled linoleic acid, linolenic acid, dihomogammalinolenic acid or eicosapentaenoic acid. The most striking difference in the desaturase activity was the lower level of delta 5 desaturase in trout than in rat. No delta 4 desaturation of 22:4(n-6) to 22:5(n-6) was observed in either of the two species, while the conversion of 22:5(n-3) to 22:6(n-3) was significant in both groups and highest in rainbow trout. The chain-elongating activity was remarkably similar in the two species, except for the "dead-end" elongation which was distinctly more important in fish.

Uptake and metabolism of eicosa-8, 11, 14-trienoic acid in normal hepatocytes and HTC cells

The incorporation and conversion of eicosa-8,11,14-trienoic acid to arachidonic acid were studied in HTC cells (7288 c hepatoma cells) and isolated hepatocytes of rat. The cells were incubated at different concentrations of the acid during 0 to 6 hours. Both kinds of cells were able to take up the acid. However, the HTC cells have a greater avidity for the eicosatrienoic acid than normal liver cells. The incorporation of the acid modified the fatty acid composition of the cells and caused a decrease in the amount of saturated and monoenoic acid. In both cells eicosatrienoic acid was converted to arachidonic acid. However, in HTC cells arachidonic acid level was low and not correlated with the amount of the eicosatrienoic acid incorporated. This fact is apparently not due to an impairment in delta 5 desaturation activity since this enzyme is active in both cells. It would be possible that arachidonic acid production in malignant cells would be also interrupted in another metabolic pathway after delta 6 desaturation step. The strikingly low amount of arachidonic acid biosynthesized in HTC cells compared to normal hepatocytes could be interpreted as a consequence of a lower availability of eicosatrienoic acid for the microsomal desaturation system in malignant cells, in addition to the low delta 6 desaturase activity.

Effect of fatty acids of omega 6 series on the biosynthesis of arachidonic acid in HTC cells

The influence of the preincubation of HTC cells with fatty acids of omega 6 series and columbinic acid (5t, 9c, 12c 18:3) on the biosynthesis of arachidonic acid was studied. The cells were incubated on a chemically defined medium with or without the addition of unlabeled linoleic, gamma-linolenic, eicosatrienoic, arachidonic, docosatetraenoic, docosapentaenoic and columbinic acids. After 24 hr of preincubation in the presence of the aforementioned fatty acids, [1-14C]eicosa-8,11,14-trienoic acid was added to the culture medium as the only lipidic source. Twenty-four hours later the synthesis of arachidonic acid and the fatty acid composition of the cells were determined. At 20 microM concentration the omega 6 fatty acids studied except docosapentaenoic acid produced an increase on the biosynthesis of arachidonic acid compared to the cells incubated in the absence of unlabeled fatty acids in the medium. The fatty acids added to the culture medium were incorporated into the cells and modified their fatty acid composition. Columbinic acid, with a similar structure to linoleic acid, also produced a significant increase on the conversion of eicosatrienoic acid to arachidonic acid. These results would suggest that the effect of both, linoleic and columbinic acids, may be adscribed to their configuration and not necessarily to their transformation in higher homologs, since columbinic acid is unable to be desaturated.

Linolenic acid desaturation and chain elongation and rapid turnover of phospholipid n - 3 fatty acids in isolated rat liver cells

The desaturation and chain elongation of [1-14C]linolenic acid was studied in isolated liver cells from rats fed a diet deficient in essential fatty acids. 14C-labelled 18:4, 20:3, 20:4, 20:5, 22:5 and 22:6, all n - 3 fatty acids, were formed. In the presence of lactate relatively large amounts of 20:5, 22:5 and 22:6 were formed. 20:5 was mainly present in phospholipids, 22:5 and 22:6 were present in both phospholipids and triacylglycerols. (+)-Decanoylcarnitine and (-)-hydroxycitrate decreased the formation of 20:5, 22:5 and 22:6 and increased the recovery of 18:4. The unchanged 18:3 substrate was also initially rapidly incorporated both in the phospholipids and in the triacylglycerol fraction. During long incubation periods, continued after nearly all the [14C]linolenic acid substrate had been metabolized either by esterification or by oxidation, the phospholipid content of labelled 18:3 and 18:4 decreased while the content of 20:5, 22:5 and 22:6 increased markedly, suggesting a remodeling of the phospholipid n - 3 fatty acid content by a series of deacylations-reacylations. The n - 3 fatty acid pattern in the triacylglycerol fraction changed little. 22:5 and 22:6 appeared in the VLDL fraction secreted by the isolated liver cells.

Effect of different carbon sources on delta 5 desaturation activity of HTC cells

The effect of different carbon sources on delta 5 desaturation activity in Minimal Deviation Hepatoma 7288 c (HTC) cells was studied. When HTC cells were incubated in the absence of carbon sources in the medium (fasted cells) the conversion of [1-14C]eicosa-8,11,14-trienoic acid to arachidonic acid increased significantly compared to those cells incubated in S-77 medium (fed cells). This activity was not modified if fasted cells were refed with glucose. However, the activity decreased significantly by the addition of lactalbumin hydrolysate to fasted cells. This effect can not be attributed to individual aminoacids since the addition of them to an aminoacid free medium produced no changes on delta 5 desaturation activity. The incubation of HTC cells with S-77 medium supplemented with glucose, pyruvate or glucose plus pyruvate, showed similar delta 5 desaturation activity to those cells incubated in S-77 medium alone.

Comparison of C18-, C20- and C22-unsaturated fatty acids in reducing fatty acid synthesis in isolated rat hepatocytes

C18-, C20- and C22-unsaturated acids were tested for inhibition of fatty acid synthesis in hepatocytes isolated from essential fatty acid-deficient rats. Fatty acid synthesis was measured by incorporation of radioactivity from [1-14C-A1-acetate or 3H2O into fatty acids. C20-polyunsaturated fatty acids included arachidonic acid (20 : 4 (n-6)) and 4 other fatty acids formed from linoleic acid (18 : 2 (n-6)) or linolenic acid (18 : 3 (n-3)). These were (11,14)-icosadienoic acid (20 : 2 (n-6)), (8,11,14)-icosatrienoic acid (20 : 3 (n-6)), (11,14,17)-icosatrienoic acid (20 : 3 (n-3)) and (5,8,11,14,17)-icosapentaenoic acid (20 : 5 (n-3). All of these have essential fatty acid activity. The fatty acid (5,8,11)-icosatrienoic acid (20 : 3 (n-9)) was also tested. This fatty acid is formed from oleic acid (18 : 1 (n-9)) and is not an essential fatty acid or a prostaglandin precursor. C20-unsaturated fatty acids and (22 : 6 (n-3)) were as effective as stearic acid in inhibiting fatty acid synthesis and were more inhibitory than their precursor C18-unsaturated fatty acids. These results are evidence that C20-unsaturated fatty acids of the linoleic and linolenic acid series can act in short-term inhibition as well as in adaptive inhibition of fatty acid synthesis (Bloch, K. and Vance, D. (1977) Annu. Rev. Biochem. 46, 263--298). The effectiveness of (5,8,11)-icosatrienoic acid indicates that short-term inhibition by C20-unsaturated fatty acids is not limited to those fatty acids which have essential fatty acid activity.

Effect of "fasting" and different concentrations of glucose on alpha-linolenic acid metabolism in HTC cells. Correlation with the ultrastructural study

HTC cells are able to convert alpha-linolenic acid into higher homologs by desaturating and elongating reactions. When the cells were cultured in a Krebs Ringer bicarbonate solution ("fasted cells") a decrease in both biosynthetic reactions took place. "Refeeding" the cells with Swim's 77 medium without glucose enhanced the biosynthesis of polyunsaturated fatty acids from alpa-linolenic acid family, but when glucose was added to the medium, alpha-linolenic acid was preferably elongated rather than converted into eicose-pentaenoic acid. The ultrastructural study revealed HTC cells with a simple cytoplasmic organization, showing little evidence of their origin from hepatocytes. The cells cultured in a complete medium appeared well preserved and were similar to those "fasted" for 12 hours and "refed" for another 12 hours using Swim's 77 medium without serum. The amount of glucose in the medium plays a role in preserving the cell structure. This effect does not occur if glucose is added in the absence of aminoacids and vitamins.