Effects of in vitro exposure to arachidonic acid on TNF‐α production by murine peritoneal macrophages

Modifying the fatty acid composition of macrophages through diet can significantly alter some of their functions, such as tumoricidal capacity and tumor necrosis factor a (TNF‐α) production. The mechanism of that modification, however, is unknown. In this report, we provide evidence that fatty acids added to macrophages in culture can significantly alter macrophage TNF‐α production. For example when inflammatory macrophages were incubated with various doses of arachidonic acid [20:4(n‐6)] during activation with lipopolysaccharide (LPS), we observed a dose‐dependent decrease in the level of bioactive TNF‐α with complete inhibition at 2‐5 μΜ. This inhibition was specific for 20:4(n‐6) because in vitro treatment with other fatty acids, such as eicosapentaenoic [20:5(n‐3)] or docosahexaenoic [22:6(n‐3)] acids, had differential effects. The inhibitory action of 20:4(n‐6) did not involve toxicity because cell viability was not affected and in vitro interferon‐γ and lipopolysaccharide (LPS) activation of macrophages for killing of P815 tumor targets was not altered. Inhibition by 20:4(n‐6) occurred posttranscriptionally, and could be reversed when macrophages were treated with indomethacin during activation. Arachidonic acid treatment also significantly increased the production of immunoreactive prostaglandin E 2 (PGE2) by LPS‐treated and untreated macrophages. These results suggest that in vitro treatment of macrophages with 20:4(n‐6) may inhibit TNF‐α production through an alteration in the levels of PGE 2 at a posttranscriptional level. These results provide evidence that some dietary fats may affect macrophage activity through modification of eicosanoid synthesis.