Effect of cholesterol depletion on exocytosis of alveolar type II cells.
Alveolar epithelial type II cells secrete lung surfactant via exocytosis. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE) are implicated in this process. Lipid rafts, the cholesterol- and sphingolipid-rich microdomains, may offer a platform for protein organization on the cell membrane. We tested the hypothesis that lipid rafts organize exocytotic proteins in type II cells and are essential for the fusion of lamellar bodies, the secretory granules of type II cells, with the plasma membrane. The lipid rafts, isolated from type II cells using 1% Triton X-100 and a sucrose gradient centrifugation, contained the lipid raft markers, flotillin-1 and -2, whereas they excluded the nonraft marker, Na+-K+ ATPase. SNAP-23, syntaxin 2, and VAMP-2 were enriched in lipid rafts. When type II cells were depleted of cholesterol, the association of SNAREs with the lipid rafts was disrupted and the formation of fusion pore was inhibited. Furthermore, the cholesterol-depleted plasma membrane had less ability to fuse with lamellar bodies, a process mediated by annexin A2. The secretagogue-stimulated secretion of lung surfactant from type II cells was also reduced by methyl-beta-cyclodextrin. When the raft-associated cell surface protein, CD44, was cross-linked using anti-CD44 antibodies, the CD44 clusters were observed. Syntaxin 2, SNAP-23, and annexin A2 co-localized with the CD44 clusters, which were cholesterol dependent. Our results suggested that lipid rafts may form a functional platform for surfactant secretion in alveolar type II cells, and raft integrity was essential for the fusion between lamellar bodies with the plasma membrane.
Chintagari
Narendranath Reddy
NR
Department of Physiological Sciences, Oklahoma State University, OK 74078, USA.
Jin
Nili
N
Wang
Pengcheng
P
Narasaraju
Telugu Akula
TA
Chen
Jiwang
J
Liu
Lin
L
D000818:Animals; D017306:Annexin A2 / Q000378:metabolism; D002478:Cells, Cultured; D002784:Cholesterol / Q000172:deficiency / Q000378:metabolism*; D005089:Exocytosis; D018960:Hyaluronan Receptors / Q000276:immunology / Q000378:metabolism; D008297:Male; D008561:Membrane Fusion; D021962:Membrane Microdomains / Q000737:chemistry / Q000276:immunology / Q000378:metabolism*; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D011650:Pulmonary Alveoli / Q000378:metabolism*; D011663:Pulmonary Surfactants / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D020133:Reverse Transcriptase Polymerase Chain Reaction; D050600:SNARE Proteins / Q000032:analysis / Q000378:metabolism*; D022142:Secretory Vesicles / Q000378:metabolism; D013997:Time Factors; D033921:Vesicular Transport Proteins / Q000032:analysis / Q000378:metabolism; D047392:beta-Cyclodextrins
D017306:Annexin A2; D018960:Hyaluronan Receptors; D008565:Membrane Proteins; D011663:Pulmonary Surfactants; D050600:SNARE Proteins; C498858:Snap23 protein, rat; D033921:Vesicular Transport Proteins; D047392:beta-Cyclodextrins; C106354:flotillins; C108732:methyl-beta-cyclodextrin; D002784:Cholesterol