Plasma membrane-specific phospholipase A 1 activation by nitrogen dioxide in pulmonary artery endothelial cells

Abstract

Nitrogen dioxide (NO 2), an environmental oxidant, alters the plasma membrane structure and function of pulmonary artery endothelial cells through peroxidative injury. Because peroxidative injury can activate membrane phospholipases and alter phospholipid composition of membranes, we evaluated the effects of NO 2 exposure on phospholipase A 1 (PLA 1), phospholipase A 2 (PLA 2), and diacylglycerol lipase (DG lipase) activities in pulmonary artery endothelial cell plasma, mitochondrial, and microsomal membranes. We also evaluated the effect of NO 2 exposure on the phospholipid composition of plasma membranes of these cells. Exposure to 5 ppm NO 2 for 48 hr resulted in a significant ( p < 0.01) increase in PLA 1 activity in plasma membranes but not in mitochondrial or microsomal membranes of pulmonary artery endothelial cells, whereas PLA 2 and DG lipase activities were comparable to controls in all membranes. As a result of PLA 1 activation, the total phospholipid content of the plasma membranes of NO 2-exposed cells was significantly ( p < 0.01) reduced compared to controls. Phosphatidylethanolamine (PE) content was reduced ( p < 0.05), whereas lyso-PE (LPE), a product of PLA 1 hydrolysis of PE, as well as phosphatidylserine (PS) contents were increased ( p < 0.01 for both LPE and PS) in the plasma membranes of NO 2-exposed cells. Incorporation of exogenous PS into pulmonary artery endothelial cells mimicked the stimulatory effect of NO 2 on PLA 1 activity. These results demonstrate that NO 2 specifically reacts with the plasma membrane component of pulmonary artery endothelial cells, causing specific activation of PLA 1. The NO 2-induced increase of PS in the plasma membranes appears to be responsible for the specific activation of PLA 1 in pulmonary artery endothelial cells.