Decreased prostaglandin production by cholesterol-rich macrophages

The regulation of prostaglandin production by macrophages enriched in cholesterol was examined. Mouse peritoneal macrophages were incubated for 18 h with 25 micrograms/ml of human acetyl-LDL (low density lipoprotein) and trace amounts of labeled arachidonic acid. After cholesterol enrichment, the cells were incubated with phorbol 12-myristate 13-acetate (PMA), calcium ionophore, or zymosan to stimulate endogenous arachidonic acid metabolism. A high performance liquid chromatography profile of the eicosanoids released revealed no qualitative differences between unmodified and modified macrophages. Cholesterol-rich cells, however, released less prostacyclin (PGI2) and prostaglandin E2 (PGE2) compared to unmodified cells, and products from the lipoxygenase pathway became the predominant metabolites. A decrease in the synthesis of PGI2 and PGE2 by cholesterol-rich macrophages was confirmed by radioimmunoassay and radiolabeled experiments. The activity of prostaglandin synthetase was modestly increased in the cholesterol-modified macrophages compared to controls. As an estimation of phospholipase activity, the release of labeled arachidonic acid from membrane phospholipids, however, was significantly decreased in cholesterol-rich macrophages. The phosphatidylinositol fraction was particularly resistant to arachidonate release in response to calcium ionophore and PMA in the modified cells. The measurement of membrane phospholipid fatty acid composition before and after calcium ionophore supported the observation that less arachidonate was released by cholesterol-enriched cells in response to the ionophore. Based on these observations, we propose that prostaglandin synthesis from endogenous arachidonate stores is decreased in the cholesterol-rich macrophage. A decrease in agonist-induced activation of the phospholipase activity is proposed as a mechanism for this effect.