Surfactant protein-A plays an important role in lung surfactant clearance: evidence using the surfactant protein-A gene-targeted mouse

Previous studies with the isolated perfused rat lung showed that both clathrin- and actin-mediated pathways are responsible for endocytosis of dipalmitoyl-phosphatidylcholine (DPPC)-labeled liposomes by granular pneumocytes in the intact lung. Using surfactant protein-A (SP-A) gene-targeted mice, we examined the uptake of [H-3] DPPC liposomes by isolated mouse lungs under basal and secretagogue-stimulated conditions. Unilamellar liposomes composed of [H-3] DPPC: phosphatidylcholine: cholesterol: egg phosphatidylglycerol ( 10: 5: 3: 2 mol fraction) were instilled into the trachea of anesthetized mice, and the lungs were perfused ( 2 h). Uptake was calculated as percentage of instilled disintegrations per minute in the postlavaged lung. Amantadine, an inhibitor of clathrin and, thus, receptor-mediated endocytosis via clathrin- coated pits, decreased basal [H-3] DPPC uptake by 70% in SP-A +/+ but only by 20% in SP-A -/- lung, data compatible with an SP-A/receptor-regulated lipid clearance pathway in the SP-A +/+ mice. The nonclathrin, actin-dependent process was low in the SP-A +/+ lung but accounted for 55% of liposome endocytosis in the SP-A -/- mouse. With secretagogue (8-bromoadenosine 3', 5'-cyclic monophosphate) treatment, both clathrin- and actin-dependent lipid clearance were elevated in the SP-A +/+ lungs while neither pathway responded in the SP-A -/- lungs. Binding of iodinated SP-A to type II cells isolated from both genotypes of mice was similar indicating a normal SP-A receptor status in the SP-A -/- lung. Inclusion of SP-A with instilled liposomes served to "rescue" the SP-A -/- lungs by reestablishing secretagogue-dependent enhancement of liposome uptake. These data are compatible with a major role for receptor-mediated endocytosis of DPPC by granular pneumocytes, a process critically dependent on SP- A.