Induction of alveolar epithelial injury by phospholipase A₂

Severe damage to the alveolar type I epithelial cell is a characteristic morphological feature of lung injury due to numerous causes. It is postulated that excess phospholipase A₂ (PLA₂) activity might be responsible for these changes, as one of the naturally occurring products of this enzyme, lysophosphatidylcholine (lysoPC) has been shown to cause selective injury to the type I pneumonocyte when it is instilled into the lower air spaces of the lung. To further investigate this potential mechanism of type I epithelial cell toxicity, we have measured the epithelial permeability-surface area product (PS) for [¹⁴C]sucrose as well as whole-lung lysoPC content at several times after instilling PLA₂ (Naja naja venom) into either the air spaces or the perfusate of an isolated hamster lung preparation. As a molar percentage of total phospholipids, the normal hamster lung contains ~1.5% lysoPC, and this value is not affected by fluid filling of the air spaces or perfusion of the excised lung for periods up to 90 min. When 0.15 U/ml PLA₂ is instilled into the air spaces, lung lysoPC content increases to ~2.5% and there are barely detectable increases in [¹⁴C]sucrose PS. With air space PLA₂ concentrations of 0.30 U/ml, lysoPC content increases to between 4 and 5%, [¹⁴C]sucrose PS increases by greater than a factor of 10, and flooding of the alveolar spaces occurs. Ultrastructural studies of similarly treated lungs show widespread but selective damage to the type I epithelial cells. These same biochemical and functional changes are not seen when the same concentrations of PLA₂ are added to the lung perfusate. Analyses of the fatty acyl substituents of the lysoPC generated after lung exposure to PLA₂, as well as studies of the phospholipid composition of bronchoalveolar lavage fluid exposed to PLA₂ in vitro, suggest that the dipalmitoyl phosphatidylcholine contained within the surfactant material serves as a preferred substrate for this form of the enzyme. The lysoPC generated in this reaction is presumed to be an important proximal mediator of injury to the type I epithelial cells.