Docosahexaenoic acid lowers phosphatidate level in human activated lymphocytes despite phospholipase D activation

N-3 polyunsaturated fatty acids from marine oil have been shown to decrease T cell-mediated immune function both in animals and humans, and to inhibit the mitogen-induced lymphoproliferative response when added to lymphocyte culture medium. As phosphatidic acid (PA) is a key mediator of the mitogenic process, the present study aims to investigate whether docosahexaenoic (DHA) and eicosapentaenoic (EPA) acids, the main n-3 fatty acids from fish oil, are able to alter the mitogen-induced synthesis of PA, when added to the culture medium of human peripheral blood mononuclear cells (PBMC). Incubation of PBMC in a medium containing 5 microM DHA bound to 5 microM human delipidated serum albumin induced a 2-fold increase in the basal PA mass whereas incubation with EPA, in the same conditions, had no effect. In contrast, both fatty acids markedly reduced the concanavalin A (ConA)-induced production of PA as compared with untreated cells. Paradoxically, phospholipase D (PLD) activity, evidenced by the synthesis of phosphatidylbutanol, was only detected in DHA-treated cells further stimulated by ConA, indicating that both DHA and ConA are required for PLD activation. Similarly, an increased diacylglycerol (DAG) mass was only observed in DHA-treated cells stimulated by ConA, whereas no modification occurred in control or EPA-treated cells stimulated or not by ConA. Furthermore, 1-butanol suppressed the ConA-induced increase of DAG mass observed in DHA-treated cells, indicating that phosphatidate was the source of the newly synthesized diacylglycerol. Altogether, these results show that, in concanavalin A-activated human peripheral blood mononuclear cells, docosahexaenoate stimulates both phospholipase D and phosphatidate phosphohydrolase activities, which ultimately results in an increased diacylglycerol production at the expense of phosphatidate.