Pathways for the uptake of fluorescently labeled liposomes by
alveolar type ii cells in culture.
Muller, W. J., K. Zen, A. B. Fisher, and H. Shuman.
Air Quality Sciences, Inc., 1337 Capital Circle D, Atlanta, GA
30067; and Institute for Environmental Medicine, University of
Pennsylvania Medical Center, 1 John Morgan Building, Philadelphia, PA
19104-6068
APStracts 2:0022L, 1995.
Isolated alveolar type II cells are shown in real time to internalize
C12-NBD-PC labeled liposomes in an intact manner using fluorescence
microscopy. Cells isolated from rat lungs, cultured on plastic for 24
hours are exposed at 37 oC 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 oC 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 actin and 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.
Received 9 June 1994; accepted in final form 8 February 1995.
APS Manuscript Number L164-4.
Article publication pending Am. J. Physiol. (Lung Cell. Mol.
Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 24 February 1995.