The protective role of surfactant protein A in oxidative and CP9-induced lung injury
MetadataShow full item record
The lung is continuously exposed to oxidant toxins and gases, foreign material, and infectious agents, and has developed specific defense systems under evolutionary selection pressures. As shown in this thesis, lung surfactant protein (SP)-A plays important roles in pulmonary defense against oxidants and infectious microbes. Experiments here demonstrate that exposure of freshly-isolated alveolar type II pneumocytes to hydrogen peroxide (H 2 O 2 ) in vitro led to a marked decrease in the rate of radiolabeled choline incorporation into phosphatidlycholine at 1 hr and substantial increases in apoptosis at 8 and 20 hr. Addition of SP-A partially reversed the detrimental effects of H 2 O 2 on choline incorporation and apoptosis in isolated type II cells. The protective effects of SP-A were significantly greater when it was given 15 min before H 2 O 2 exposure, consistent with measurements showing that this protein had direct antioxidant ability in rapidly clearing H 2 O 2 from solution. The protective effects of SP-A against oxidant injury to type II cells were greatly reduced when the protein was given 15 min after H 2 O 2 administration. SP-A is also shown in this thesis to protect lung cells from injury from CP9 (a pathogenic human extra-intestinal E. coli ). CP9 induced significant TNF-α release from isolated alveolar macrophages (0.7 ± 0.2% of total freshly-isolated lung cells) at 2 hr, significant GRO release from isolated type II cells (94.8 ± 0.7%) at 8 hr, and increased the apoptotic rate of all isolated cells at 2, 5, and 8 hr. SP-A mitigated these in vitro effects of CP9, perhaps in part through activation of a survival signal following binding of SP-A to the SP-A receptor (SPAR) on the surface of type II cells and/or macrophages. Additional in vivo experiments showed that CP9 pneumonia in rats was mitigated by appropriate supplementation with exogenous SP-A (1.8 mg/kg) based on measurements of CP9 clearance, the arterial partial oxygen pressure, and lavage protein content. Taken together, the studies of this thesis reinforce the perspective that SP-A has important biological roles in protecting the lungs against oxidants and bacteria in addition to its roles in lung surfactant biophysics.