Binding, uptake, and localization of surfactant protein B in isolated rat alveolar type II cells.
This study reports the ability of rat alveolar type II cells to internalize mature bovine surfactant protein B (SP-B) in vitro. Isolated type II cells were incubated with labeled SP-B, and binding and internalization were studied biochemically and morphologically. Biochemical analyses demonstrated a time-dependent association of 125I-labeled SP-B with type II cells; binding steadily increased through 4 h and then remained constant through 20 h of incubation. The association of [3H]SP-B with type II cells was characterized via light and electron microscopic autoradiography. Significant quantities of [3H]SP-B were found at the plasma membrane, in the endocytic pathway, and in lamellar bodies. The pathway of SP-B internalization was not altered by the presence of whole rat surfactant; however, the quantity of SP-B internalized into lamellar bodies was increased. 3[H]SP-B was not associated with coated pits and colocalized with horseradish peroxidase (HRP), consistent with receptor-independent internalization. Cell-associated SP-B was not degraded and was detected in lamellar bodies undergoing exocytosis. These results suggest that SP-B may follow a recycling pathway similar to that previously reported for surfactant phospholipids.
Breslin
J S
JS
Division of Pulmonary Biology, Children's Hospital Medical Center, Cincinnati, Ohio.
Weaver
T E
TE
D000818:Animals; D001345:Autoradiography; D002462:Cell Membrane / Q000378:metabolism / Q000648:ultrastructure; D002478:Cells, Cultured; D004705:Endocytosis; D007457:Iodine Radioisotopes; D007700:Kinetics; D008297:Male; D008854:Microscopy, Electron; D015388:Organelles / Q000378:metabolism / Q000648:ultrastructure; D011510:Proteolipids / Q000032:analysis / Q000378:metabolism*; D011650:Pulmonary Alveoli / Q000166:cytology / Q000378:metabolism* / Q000648:ultrastructure; D011663:Pulmonary Surfactants / Q000032:analysis / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D013347:Subcellular Fractions / Q000378:metabolism / Q000648:ultrastructure; D014316:Tritium
D007457:Iodine Radioisotopes; D011510:Proteolipids; D011663:Pulmonary Surfactants; D014316:Tritium