Exogenous tetracosahexaenoic acid modifies the fatty acid composition of human primary T lymphocytes and Jurkat T cell leukemia cells contingent on cell type

Abstract

Tetracosahexaenoic acid (24:6ω-3) is an intermediate in the conversion of 18:3ω-3 to 22:6ω-3 in mammals. There is limited information about whether cells can assimilate and metabolize exogenous 24:6ω-3. This study compared the effect of incubation with 24:6ω-3 on the fatty acid composition of two related cell types, primary CD3⁺ T lymphocytes and Jurkat T cell leukemia, which differ in the integrity of the polyunsaturated fatty acid (PUFA) biosynthesis pathway. 24:6ω-3 was only detected in either cell type when cells were incubated with 24:6ω-3. Incubation with 24:6ω-3 induced similar increments in the amount of 22:6ω-3 in both cell types and modified the homeoviscous adaptations fatty acid composition induced by activation of T lymphocytes. The effect of incubation with 18:3ω-3 compared to 24:6ω-3 on the increment in 22:6ω-3 was tested in Jurkat cells because primary T cells cannot convert 18:3ω-3 to 22:6ω-3. The increment in the 22:6ω-3 content of Jurkat cells incubated with 24:6ω-3 was 19.5-fold greater than that of cells incubated with 18:3ω-3. Acyl-coA oxidase siRNA knockdown decreased the amount of 22:6ω-3 and increased the amount of 24:6ω-3 in Jurkat cells. These findings show exogenous 24:6ω-3 can be incorporated into primary human T lymphocytes and Jurkat cells and induces changes in fatty acid composition consistent with its conversion to 22:6ω-3 via a mechanism involving peroxisomal β-oxidation that is regulated independently from the integrity of the upstream PUFA synthesis pathway. One further implication is that consuming 24:6ω-3 may be an effective alternative means of achieving health benefits attributed to 20:5ω-3 and 22:6ω-3.

Keywords: desaturases; docosahexaenoic acid; fatty acid metabolism; general area; immunology; lipid biochemistry; mammalian lipid biochemistry; metabolism; n-3 fatty acids; polyunsaturated fatty acids (PUFA); specific lipids.

FIGURE 1

The ω‐3 polyunsaturated fatty acid biosynthesis pathway described in rat liver (Sprecher, 1999) plus the modifications found in human primary T lymphocytes (Robichaud et al., ; von Gerichten et al., 2021). (1) The first reaction in T lymphocytes is carbon chain elongation putatively by elongase‐5 (Sibbons et al., ; Robichaud et al., ; von Gerichten et al., 2021). (2) The first reaction in the hepatic pathway is Δ6 desaturation by the protein product of the FADS2 gene followed by chain elongation by elongase‐5. (3) The protein product of the FADS2 gene has Δ6 and Δ8 activities (Park et al., 2009), which are both expressed in Jurkat cells, while the Δ8 desaturase activity is predominant in T lymphocytes (Sibbons et al., 2018). (4) Desaturation at the Δ4 position is an alternative mechanism for 22:5ω‐3 synthesis in some cells 2:5ω‐3 synthesis in so (5) Elongase‐2 is not expressed in T lymphocytes (Robichaud et al., ; Sibbons et al., ; von Gerichten et al., 2021), therefore, truncating the pathway after synthesis of 22:5ω‐3. However, elongase‐2 is expressed in Jurkat cells (Sibbons et al., 2018). (6) The findings of (Moore et al., ; Voss et al., 1991) summarized by (Sprecher, 2000) suggest that the conversion of 24:6ω‐3 formed in the endoplasmic reticulum to 22:6ω‐3 involves translocation to peroxisomes and carbon chain shortening by one cycle of β‐oxidation.

FIGURE 2

Confirmation of 24:6ω‐3 and 22:6ω‐3 peak identities by gas chromatography (GC)‐mass spectrometry (MS). (a) Separation of 37 FAMEs standard mixture by GC–MS, indicating the retention time of 22:6ω‐3 methyl ester peak (*1). (b) The retention time of 24:6ω‐3 methyl ester (#2) standard detected by GC–MS. (c) Mass spectrum of the 22:6ω‐3 methyl ester standard peak (*1), (d) mass spectrum of the 24:6ω‐3 (THA) methyl ester standard peak (#1), (e) mass spectrum of the 22:6ω‐3 methyl ester peak (*2) from CD3⁺ T lymphocytes. (f) Mass spectrum of the putative 24:6ω‐3 methyl ester peak (#2) from CD3⁺ T lymphocytes.

FIGURE 3

The effect of treatment with ACOX1 siRNA on ACOX1 mRNA expression in Jurkat cells. Values are median (95% confidence interval, n = 6 culture replicates/treatment) acyl‐coA oxidase‐1 (ACOX) mRNA levels from individual Jurkat cell cultures treated for 48 h with either ACOX siRNA or nontargeted siRNA (NT). Statistical comparison was by the Mann–Whitney U test.