title: Surfactant protein A and lipid are internalized via the coated-pit pathway by type II pneumocytes.
abstract: Surfactant protein (SP) A and SP-A-mediated lipid uptake by isolated type II cells were investigated with biochemical and morphological methods. Inhibition of coated-pit function by potassium depletion severely reduced both SP-A and SP-A-mediated lipid internalization. Lipid uptake in the absence of SP-A was not affected. With confocal laser scanning microscopy and immunoelectron microscopy, SP-A and lipid predominantly (60%) colocalized in intracellular vesicles carrying early endosomal markers (EEA1) 5 min after endocytosis but were negative for the late endosomal or lysosomal marker LAMP-1. As estimated by subcellular fractionation, at this time point, 23% of the internalized SP-A and 45% of internalized lipid were localized within light (<0.38 M sucrose) fractions, which contain lamellar bodies and are positive for EEA1. The remaining label was predominantly found within EEA1-positive and plasma membrane-containing subfractions (> or = 1 M sucrose). We suggest that in isolated type II cells in vitro, SP-A and lipid are taken up together via the coated-pit pathway and that at early time points, both components reside in the same early endosomal compartment.
authors.affiliation: Clinic of Neonatology, University Children's Hospital Charité, Humboldt-Universität Berlin, 10098 Berlin, Germany. paul_a.stevens@charite.de
authors.forename: P A
authors.initials: PA
authors.lastname: Stevens
authors.forename: H
authors.initials: H
authors.lastname: Wissel
authors.forename: S
authors.initials: S
authors.lastname: Zastrow
authors.forename: D
authors.initials: D
authors.lastname: Sieger
authors.forename: K P
authors.initials: KP
authors.lastname: Zimmer
mesh_terms: D015060:1,2-Dipalmitoylphosphatidylcholine / Q000493:pharmacokinetics*; D000818:Animals; D015703:Antigens, CD / Q000378:metabolism; D002788:Cholesterol Esters / Q000493:pharmacokinetics; D003034:Coated Pits, Cell-Membrane / Q000378:metabolism* / Q000648:ultrastructure; D004705:Endocytosis / Q000502:physiology; D011992:Endosomes / Q000378:metabolism / Q000648:ultrastructure; D004734:Energy Metabolism / Q000502:physiology; D008081:Liposomes / Q000378:metabolism; D051907:Lysosomal Membrane Proteins; D008297:Male; D008562:Membrane Glycoproteins / Q000378:metabolism; D016253:Microscopy, Immunoelectron; D011188:Potassium / Q000494:pharmacology; D014176:Protein Biosynthesis; D011510:Proteolipids / Q000493:pharmacokinetics*; D037662:Pulmonary Surfactant-Associated Protein A; D037661:Pulmonary Surfactant-Associated Proteins; D011663:Pulmonary Surfactants / Q000493:pharmacokinetics*; D051381:Rats; D020545:Respiratory Mucosa / Q000166:cytology* / Q000378:metabolism*; D012756:Sheep; D013347:Subcellular Fractions / Q000378:metabolism; D014316:Tritium
chemical_list: D015703:Antigens, CD; D002788:Cholesterol Esters; D008081:Liposomes; D051907:Lysosomal Membrane Proteins; D008562:Membrane Glycoproteins; D011510:Proteolipids; D037662:Pulmonary Surfactant-Associated Protein A; D037661:Pulmonary Surfactant-Associated Proteins; D011663:Pulmonary Surfactants; D014316:Tritium; D015060:1,2-Dipalmitoylphosphatidylcholine; D011188:Potassium