Distinct effects of SP-A and SP-B on endocytosis of SP-C by pulmonary epithelial cells

Binding and endocytosis of surfactant protein (SP) C were assessed in a mouse pulmonary adenocarcinoma cell line, MLE-12, and in isolated rat pulmonary type II epithelial cells. Binding and uptake of SP-C were detected using fluorescently labeled SP-C and dinitrophenyl-labeled SP-C (DNP-SP-C). Endocytosis of DNP-SP-C was visualized by immunocytochemistry and light microscopy. Endocytosis of DNP-SP-C occurred in MLE-12 cells, pulmonary type II epithelial cells, and NIH/3T3 cells, indicating that uptake of SP-C does not have an absolute requirement for a cell-specific receptor. After 30-60 min at 37 degrees C, DNP-SP-C was concentrated in large intracellular bodies in MLE-12 cells. Endocytosis of DNP-SP-C by MLE-12 cells or type II epithelial cells was decreased by SP-B or SP-B and SP-A together. SP-A alone did not inhibit DNP-SP-C uptake. Endocytosis of DNP-SP-C was inhibited by a 10-fold excess of lipid vesicles containing SP-C but not by a 10-fold excess of lipid alone. The inhibitory effect of SP-B on SP-C uptake may play a role in maintaining surface-active material at the air-liquid interface.