Transgenic expression of group V, but not group X, secreted phospholipase A₂ in mice leads to neonatal lethality because of lung dysfunction

In an effort to elucidate the functions of secreted phospholipase A ₂ (sPLA₂) enzymes in vivo, we generated transgenic (Tg) mice for group V sPLA₂ (sPLA₂-V) and group X sPLA ₂ (sPLA₂-X), which act potently on phosphatidylcholine in vitro. We found that sPLA₂-V Tg mice died in the neonatal period because of respiratory failure. The lungs of sPLA₂-V Tg mice exhibited atelectasis with thickened alveolar walls and narrow air spaces, accompanied by infiltration of macrophages and only modest changes in eicosanoid levels. This severe pulmonary defect in sPLA₂-V Tg mice was attributable to marked reduction of the lung surfactant phospholipids, phosphatidylcholine and phosphatidylglycerol. Given that the expression of sPLA₂-V is greatly elevated in human lungs with severe inflammation, our present results raise the intriguing possibility that this isozyme may contribute to ongoing surfactant hydrolysis often observed in the lungs of patients with respiratory distress syndrome. In contrast, sPLA₂-X Tg neonates displayed minimal abnormality of the respiratory tract with normal alveolar architecture and surfactant composition. This unexpected result was likely because sPLA₂-X protein existed as an inactive zymogen in most tissues. The active form of sPLA₂-X was detected in tissues with inflammatory granulation in sPLA₂-X Tg mice. These results suggest that sPLA₂-X mostly remains inactive under physiological conditions and that its proteolytic activation occurs during inflammation or other as yet unidentified circumstances in vivo.