Phagocytic Activation Induces Formation of Platelet‐Activating Factor in Human Monocyte‐Derived Macrophages and in Macrophage‐Derived foam Cells

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Abstract
Monocyte‐derived macrophages and macrophage‐derived foam cells in arterial tissue may undergo phagocytic activation and thereby contribute to an inflammatory reaction. We have investigated the effect of phagocytic activation on the formation of platelet‐activating factor (1‐O‐alkyl‐2–acetyl‐sn‐glycero‐3‐phosphocholine, PAF‐acether, PAF), a proinflammatory phospholipid, in human monocyte‐derived macrophages (macrophages) and in cholesterol‐loaded macrophage foam cells (foam cells). Adherent human monocyte‐derived macrophages were transformed into foam cells upon incubation with acetylated low‐density lipoproteins (Ac‐LDL). Such foam cells characteristically displayed a markedly increased content of cholesteryl esters compared with macrophages (4.3±1.3 μg/μg DNA and 0.2±0.3 μg/μg DNA, n= 5, respectively). After phagocytic stimulation with serum‐opsonized zymosan (OPZ), both macrophages and foam cells synthesized PAF transiently with maximal production (0.5–1.1 pmol PAF/μg DNA, n= 5, corresponding to 4.0–8.8 pmol PAF/106 cells, as assessed by bioassay) occurring approximately 15 min after stimulation. A major fraction of the synthesized PAF remained cell‐associated; such PAF was composed mainly of the hexadecyl (16:0 PAF, ∼ 75%) and the octadecenyl (18:1 PAF) species and of trace amounts of octadecyl (18:0 PAF), as assessed by reverse‐phase liquid chromatography. Addition of exogenous 16:0 lyso‐PAF alone triggered PAF formation (0.9–1.7 pmol PAF/μg DNA, after 15 min of cellular stimulation); simultaneous cellular stimulation with OPZ and 16:0 lyso‐PAF increased PAF formation in an additive manner. Acetyltransferase, the enzyme which acetylates the precursor lyso‐PAF and transforms it into PAF, displayed elevated activity both in macrophages and in foam cells, attaining 83–240 pmol PAF formed · min−1· mg DNA−1 (n= 4); such elevated activity was not increased by OPZ‐stimulation. The activity of acetylhydrolase, the PAF‐degrading enzyme, was similar in macrophages and in foam cells, and varied between 120 pmol and 320 pmol PAF degraded · min−1· mg DNA−1 (n= 5). Cell‐associated acetylhydrolase activity was increased significantly by 40±15% (P < 0.003, n= 5) after 15–30 min of activation with OPZ compared with non‐stimulated cells and may account for the rapid decrease in cellular PAF content observed approximately 30 min after stimulation. These studies have established that metabolism of PAF in foam cells closely resembles that in macrophages, and thus PAF metabolism is largely independent of cellular cholesterol content. Moreover our data are consistent with the hypothesis that both macrophages and macrophage‐derived foam cells upon phagocytic‐activation constitute a significant transient source of PAF at inflammatory sites in the arterial intima where this phospholipidic mediator may exert potent proatherogenic and prothrombotic effects.

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