Prostaglandin E2 limits arachidonic acid availability and inhibits leukotriene B4 synthesis in rat alveolar macrophages by a nonphospholipase A2 mechanism

Prostaglandin E2 (PGE2), a potent mediator of inflammation released in large amounts by endotoxin-stimulated alveolar macrophages (AM), has been shown to inhibit leukotriene B4 (LTB4) release by activated neutrophils. We investigated the hypothesis that LTB4 synthesis by AM can be regulated by PGE2 and performed experiments to determine the biochemical site of regulation. AM obtained from Sprague-Dawley rats were preincubated with PGE2 before stimulation with the calcium ionophore A23187. LTB4, platelet-activating factor (PAF), lysoPAF, (AA), and 5-hydroxyeicosatetraenoic acid were isolated from AM cells and supernatants, then quantified by gas chromatography/electron capture negative ion mass spectrometry. Stimulated AM released 30.50 +/- 4.52 ng LTB4/10(6) cells and were inhibited by PGE2 in a dose dependent manner. PGE2 (1 microM) inhibited LTB4 synthesis by 37% and decreased release of both 5-hydroxyeicosatetraenoic acid and arachidonic acid by stimulated AM, but did not alter synthesis of PAF or lysoPAF. These mass measurements suggest that PGE2 does not affect the activity of phospholipase A2, PAF acetyltransferase, leukotriene A4 hydrolase. We conclude that PGE2 attenuates LTB4 production in alveolar macrophages by altering the activity of lipases other than phospholipase A2. PGE2-mediated inhibition of LTB4 synthesis by AM may regulate the initiation of lung inflammation.