Surfactant apoprotein Mr = 26,000-36,000 enhances uptake of liposomes by type II cells.
The alveolar type II cell which synthesizes and secretes surfactant also plays a major role in the reuptake of surfactant lipids. In a recent in vivo study we found that the subfractions of natural surfactant that contained the surfactant protein with molecular weights of 26,000-36,000 (SP-26-36) were preferentially taken up into lamellar bodies of type II cells to a greater extent than were fractions that did not contain SP-26-36. Because the subfractions of natural surfactant in that study differed in other properties than the presence or absence of SP-26-36, the current study was undertaken to determine whether purified SP-26-36 enhanced the uptake of surfactant-like lipids by freshly isolated type II cells. SP-26-36 increased the uptake of label in radioactive surfactant-like lipids by up to 10-fold, and the effect of SP-26-36 was dependent on time, protein concentration, and temperature. The enhancement was inhibited by heat-treating the protein, by a polyclonal antibody against SP-26-36, and by metabolic inhibitors. The distribution of radioactivity in cell-associated phospholipids differed if cells were incubated with or without SP-26-36. If SP-26-36 was present during the incubation, greater than 96% of the radioactivity remained associated with phosphatidylcholine. In the absence of SP-26-36, only 85% of the radioactivity remained associated with phosphatidylcholine and 7% of the label appeared in phosphatidylglycerol. We hypothesize that SP-26-36 may act as a ligand to direct surfactant lipids to type II cells, perhaps to different metabolic pathways, and to regulate recycling and surfactant homeostasis.
D000818:Animals; D001059:Apoproteins / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D008081:Liposomes / Q000378:metabolism*; D008297:Male; D008856:Microscopy, Fluorescence; D008970:Molecular Weight; D011650:Pulmonary Alveoli / Q000166:cytology*; D037661:Pulmonary Surfactant-Associated Proteins; D011663:Pulmonary Surfactants / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D013997:Time Factors
D001059:Apoproteins; D008081:Liposomes; D037661:Pulmonary Surfactant-Associated Proteins; D011663:Pulmonary Surfactants; C046616:pulmonary surfactant apoprotein