title: Pathways for uptake of fluorescently labeled liposomes by alveolar type II cells in culture.
abstract: Isolated alveolar type II cells are shown in real time to internalize 1-palmitoyl-2-[12-[(7 nitro-2-1,3-benzoxadiazol-4-yl)amino]dodecanoyl] phosphatidyl choline (C12-NBD-PC)-labeled liposomes in an intact manner with the use of fluorescence microscopy. Cells isolated from rat lungs, cultured on plastic for 24 h, are exposed at 37 degrees C to liposomes on the microscope stage, using a microperfusion system. Liposomes composed of a self-quenching concentration of fluorophore (25 mol% C12-NBD-PC) do not dequench at the level of the plasma membrane; rather, they appear to fuse with intracellular lipids primarily in vesicular structures consistent with lamellar bodies. Temperature and energy dependence of this uptake mechanism is demonstrated by the lack of uptake of maximally fluorescent liposomes (15 mol% C12-NBD-PC) at 4 degrees C or when depleted of ATP. Inhibition of the coated-pit endocytic pathway by hypertonic media reduces fluorescent lipid uptake by 50%, suggesting a separate, clathrin-independent endocytic pathway for intact internalization of liposomes by type II cells. Uptake is inhibited by cytochalasin D by 49%, suggesting a dependence on action, and by 88% with combined cytochalasin D and hypertonicity, or nearly the same level as ATP depletion. The evidence is consistent with the existence of two separate endocytic pathways for lipids in type II cells: one clathrin dependent and the other actin dependent.
authors.affiliation: Institute for Environmental Medicine, University of Pennsylvania Medical Center, Philadelphia 19104-6068, USA.
authors.forename: W J
authors.initials: WJ
authors.lastname: Muller
authors.forename: K
authors.initials: K
authors.lastname: Zen
authors.forename: A B
authors.initials: AB
authors.lastname: Fisher
authors.forename: H
authors.initials: H
authors.lastname: Shuman
mesh_terms: D015060:1,2-Dipalmitoylphosphatidylcholine / Q000031:analogs & derivatives; D009327:4-Chloro-7-nitrobenzofurazan / Q000031:analogs & derivatives / Q000493:pharmacokinetics; D000818:Animals; D002478:Cells, Cultured; D004705:Endocytosis; D005453:Fluorescence; D007425:Intracellular Membranes / Q000378:metabolism; D050356:Lipid Metabolism; D008081:Liposomes / Q000378:metabolism*; D015388:Organelles / Q000378:metabolism; D010713:Phosphatidylcholines / Q000493:pharmacokinetics; D011650:Pulmonary Alveoli / Q000166:cytology / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley
chemical_list: C078632:1-acyl-2-(12-((7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)dodecanoyl)phosphatidylcholine; D008081:Liposomes; D010713:Phosphatidylcholines; D015060:1,2-Dipalmitoylphosphatidylcholine; D009327:4-Chloro-7-nitrobenzofurazan