Dysfunction of pulmonary surfactant mediated by phospholipid oxidation is cholesterol-dependent

Pulmonary surfactant forms a cohesive film at the alveolar air-lung interface, lowering surface tension, and thus reducing the work of breathing and preventing atelectasis. Surfactant function becomes impaired during inflammation due to degradation of the surfactant lipids and proteins by free radicals. In this study, we examine the role of reactive nitrogen (RNS) and oxygen (ROS) species on surfactant function with and without physiological cholesterol levels (5-10%). Surface activity was assessed in vitro in a captive bubble surfactometer (CBS). Surfactant chemistry, monolayer fluidity and thermodynamic behavior were also recorded before and after oxidation. We report that physiologic amounts of cholesterol combined with oxidation results in severe impairment of surfactant function. We also show that surfactant polyunsaturated phospholipids are the most susceptible to oxidative alteration. Membrane thermodynamic experiments showed significant surfactant film stiffening after free radical exposure in the presence of cholesterol. These results point to a previously unappreciated role for cholesterol in amplifying defects in surface activity caused by oxidation of pulmonary surfactant, a finding that may have implications for treating several lung diseases.