Binding and uptake of pulmonary surfactant protein (SP-A) by pulmonary type II epithelial cells.
A glycoprotein of Mr 26-36,000 (SP-A) is an abundant phospholipid-associated protein in pulmonary surfactant. SP-A enhances phospholipid reuptake and inhibits secretion by Type II epithelial cells in vitro. We have used two electron microscopic cytochemical methods to demonstrate selective binding and uptake of SP-A by rat pulmonary Type II epithelial cells. Using an immunogold bridging technique, we showed that SP-A binding was selective for Type II cell surfaces. Binding was dose dependent and saturable, reaching maximal binding at approximately 10 ng/ml. On warming to 23 degrees C, SP-A binding sites were clustered in coated pits on the cell surface. To characterize the internalization and intracellular routing of SP-A, we used the biotinyl ligand-avidin-gold technique. Biotinyl SP-A was bound by rat Type II epithelial cells as described above. On warming, biotinyl SP-A was seen in association with coated vesicles and was subsequently located in endosomes and multivesicular bodies. Biotinyl SP-A-gold complexes were seen in close approximation to lamellar bodies 10-60 min after warming. Binding of biotinyl SP-A was inhibited by competition with unlabeled SP-A. These results support the concept that Type II epithelial cells bind and internalize SP-A by receptor-mediated endocytosis. This newly described uptake system may play a role in the recycling of surfactant components or mediate the actions of SP-A on surfactant phospholipid secretion.
Ryan
R M
RM
University of Cincinnati College of Medicine, Department of Pediatrics, Ohio 45267-0541.
Morris
R E
RE
Rice
W R
WR
Ciraolo
G
G
Whitsett
J A
JA
D000818:Animals; D001710:Biotin; D004848:Epithelium / Q000378:metabolism; D006651:Histocytochemistry / Q000379:methods; D008168:Lung / Q000166:cytology / Q000378:metabolism*; D008297:Male; D008854:Microscopy, Electron / Q000379:methods; D011663:Pulmonary Surfactants / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains
D011663:Pulmonary Surfactants; D001710:Biotin