Effects of long-term oral administration of arachidonic acid and docosahexaenoic acid on the immune functions of young rats

Abstract

Natural killer (NK) cells have many functional activities, including cytotoxicity and the capacity to produce cytokines and chemokines. NK cell activity is regulated partly by eicosanoids, which are produced from arachidonic acid (ARA) and eicosapentaenoic (EPA) acid. In this study, we investigated the effects of long-term therapy with ARA or docosahexaenoic acid (DHA) on the cytotoxic effects of the NK cells of young rats, which were fed on a nonfish oil diet for two generations. Control oil, ARA (240 mg/kg BW/day) or DHA (240 mg/kg BW/day) were orally administrated to the rats for 13 weeks before determining the cytotoxic activity of NK cells from the spleen against YAC-1 mouse lymphoma cell line, as well as the plasma levels of docosanoids or eicosanoids and inflammatory cytokines. Long-term ARA administration significantly suppressed the cytotoxic activity of NK cells. Moreover, ARA administration significantly increased the plasma levels of ARA, prostaglandin (PG) E2, and PGD2. However, DHA administration did not produce any different effects compared with those in the control rats. Furthermore, the inflammatory cytokine levels were not affected by the administration of ARA or DHA. These results suggest that long-term ARA administration has an inhibitory effect on the tumor cytotoxicity of NK cells in rat spleen lymphocytes owing to the enhanced synthesis of PGE2 and PGD2 from ARA because of the elevated plasma ARA levels in young rats.

Figure 1

Cytotoxicity of natural killer (NK) cells in rat spleen lymphocytes against YAC-1 cells. The cytotoxicity levels in YAC-1 cells caused by NK cells in spleen lymphocytes after 24 h coculture in the y-axis. The ratio of the mixture of lymphocytes and YAC-1 is presented on the x-axis. The relative cytotoxicity levels are expressed as the mean ± S.E. (n = 6). * significant at p < 0.05 vs. rat lymphocytes in the control group using a one-way ANOVA followed by Dunnett’s test.

Figure 2

Plasma levels of free PUFAs and PGs in control, ARA, or DHA oil-treated rats. Plasma samples were subjected to LC-MS/MS lipidomic analysis. The graph presents the concentrations of: (A) ARA, (B) EPA, (C) DHA, (D) PGE₂, (E) PGD₂, and (F) PGF_2α in the rat plasma. The values are expressed as the mean ± S.E. (n = 8). Samples were analyzed using a one-way ANOVA followed by a Bonferroni post hoc test. a, b Different letters indicate statistical differences at p < 0.05.

Figure 3

Plasma levels of ARA, EPA, and DHA metabolites generated by LOXs. Plasma samples were subjected to LC-MS/MS lipidomic analysis. The values are expressed as the mean ± S.E. (n = 8) percentages relative to the control. The graph presents the plasma levels of the metabolites from ARA, EPA, and DHA: (A) 5-HETE, (B) 12-HETE, (C) 15-HETE, (D) 5-HEPE, (E) 12-HEPE, (F) 15-HEPE, (G) RvE2, (H) 10-HDoHE, (I) 14-HDoHE, (J) 17-HDoHE, and K) PD1. Samples were analyzed using a one-way ANOVA followed by a Bonferroni post hoc test. a, b Different letters indicate statistical differences at p < 0.05.