Combined activities of secretory phospholipases and eosinophil lysophospholipases induce pulmonary surfactant dysfunction by phospholipid hydrolysis

BackgroundSurfactant dysfunction is implicated in small airway closure in asthma. Increased activity of secretory phospholipase A2 (sPLA2) in the airways is associated with asthma exacerbations. Phosphatidylcholine, the principal component of pulmonary surfactant that maintains small airway patency, is hydrolyzed by sPLA2. The lysophosphatidylcholine product is the substrate for eosinophil lysophospholipases.ObjectiveTo determine whether surfactant phospholipid hydrolysis by the combined activities of sPLA2s and eosinophil lysophospholipases induces surfactant dysfunction.MethodsThe effect of these enzymes on surfactant function was determined by capillary surfactometry. Thin layer chromatography was used to correlate enzyme-induced changes in surfactant phospholipid composition and function. Phosphatidylcholine and its hydrolytic products were measured by using mass spectrometry.ResultsEosinophils express a 25-kd lysophospholipase and group IIA sPLA2. Phospholipase A2 alone induced only a small decrease in surfactant function, and 25-kd lysophospholipase alone degraded lysophosphatidylcholine but had no effect on surfactant function. The combined actions of sPLA2 and lysophospholipase produced dose-dependent and time-dependent losses of surfactant function, concomitant with hydrolysis of phosphatidylcholine and lysophosphatidylcholine. Lysates of AML14.3D10 eosinophils induced surfactant dysfunction, indicating these cells express all the necessary lipolytic activities. In contrast, lysates of blood eosinophils required exogenous phospholipase A2 to induce maximal surfactant dysfunction.ConclusionThe combined activities of sPLA2s and eosinophil lysophospholipases are necessary to degrade surfactant phospholipids sufficiently to induce functional losses in surfactant activity as reported in asthma.Clinical implicationsThe phospholipases and lysophospholipases expressed by eosinophils or other airway cells may represent novel therapeutic targets for blocking surfactant degradation, dysfunction, and peripheral airway closure in asthma.