Roles of SP-A, SP-B, and SP-C in modulation of lipid uptake by pulmonary epithelial cells in vitro.
The effects of the surfactant proteins (SP)-A, SP-B, and SP-C on binding and endocytosis of fluorescently labeled lipid vesicles were studied in rat type II epithelial cells and in MLE-12 cells, a pulmonary adenocarcinoma cell line with alveolar cell characteristics. Incorporation of SP-C in lipid vesicles markedly stimulated binding to the cell membrane at 4 degrees C and endocytosis of lipids at 37 degrees C. SP-C enhanced lipid uptake in MLE-12 cells, type II cells, and NIH 3T3 cells. SP-B stimulated lipid uptake in MLE-12 cells, but to a lesser degree. SP-B decreased the amount of lipid uptake stimulated by SP-C, SP-A did not increase endocytosis of lipids by MLE-12 cells or type II cells, but aggregates of lipid were observed associated with the cell surface in the presence of SP-A. Maintenance of active surfactant in the lung may be achieved through the selective uptake and degradation of surfactant subfractions depleted in SP-A and SP-B.