Effect of docosahexaenoic acid on the generation of platelet-activating factor by eosinophilic leukemia cells, Eol-1

Human eosinophilic leukemia (Eol-1) cells were examined for their ability to generate platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) (PAF) and the effect of supplementation of docosahexaenoic acid (C_22:6n-3) on the PAF synthesis was explored in relation to the fatty acid composition of phospholipids and the liberation of arachidonic acid (C_20:4n-6 AA). Alhough undifferentiated cells did not produce PAF, the exposure of IFN-γ differentiated Eol-1 to generate PAF in response to the Ca-ionophore. In addition, the IFN-γ-treated cells acquired the ability to release free fatty acids, approximately 55% of which was found to be AA. When DHA was supplemented into the culture of Eol-1 for 24 h, PAF production decreased by 40 to 50% at concentrations of 3 to 10 μM. On the other hand, supplementation of 10 μM eicosapentaenoic acid (C_20:5n-3) did not significantly decrease PAF production. With the supplementation of 10 μM DHA, DHA levels in phospholipid subclasses, including alkylacylglycerophosphocholine, were greatly increased with concurrent decreases in other unsaturated-fatty acids. In these cells, the liberation of AA in response to an ionophore was decreased by 55%. Even when DHA was enriched in phospholipids, DHA release in response to ionophore stimulation was almost negligible, indicating that the DHA moiety of phospholipids is not susceptible to the action of phospholipase A₂. Furthermore, DHA supplementation appeared to attenuate phospholipase A₂ reaction by some unknown mechanism because the decrease in AA release was much more than that for the AA level in phospholipids. Acetyl-CoA:1-alkylGPC acetyltransferase activity of stimulated cell lysate was also reduced by DHA supplementation but the reduction was much less when compared with that of PAF synthesis or AA release. These results implicated that enrichment of DHA attenuates enzymic reactions for PAF synthesis, mainly the initial reaction catalyzed by AA-specific phos-pholipase, and thereby reduces PAF synthesis in Eol-1.