Effects of apolipoprotein A-I on ATP-binding cassette transporter A1-mediated efflux of macrophage phospholipid and cholesterol: formation of nascent high density lipoprotein particles.
The mechanism of formation of high density lipoprotein (HDL) particles by the action of ATP-binding cassette transporter A1 (ABCA1) is not defined completely. To address this issue, we monitored efflux to apoA-I of phosphatidylcholine (PC), sphingomyelin (SM), and unesterified (free) cholesterol (FC) from J774 macrophages, in which ABCA1 is up-regulated, and investigated the nature of the particles formed. The various apoA-I/lipid particles appearing in the extracellular medium were separated by gel filtration chromatography. The presence of apoA-I in the extracellular medium led to the simultaneous formation of more than one type of poorly lipidated apoA-I-containing particle: there were 9- and 12-nm diameter particles containing approximately 3:1 and 1:1 phospholipid/FC (mol/mol), respectively, which were present together with 6-nm monomeric apoA-I. Removal of the C-terminal alpha-helix (residues 223-243) of apoA-I reduced phospholipid and FC efflux and prevented formation of the 9- and 12-nm HDL particles; the apoA-I variant formed larger particles that eluted in the void volume. FC loading of the J774 cells also led to the formation of larger apoA-I-containing particles that were highly enriched in FC. Besides creating HDL particles, ABCA1 mediated release of larger (20-450-nm diameter) FC-rich particles that were not involved in HDL formation and that are probably membrane vesicles. These particles contained 1:1 PC/SM in contrast to the HDL particles, which contained 2:1 PC/SM. This is consistent with lipid raft and non-raft plasma membrane domains being involved primarily in ABCA1-mediated vesicle release and nascent HDL formation, respectively.
Liu
Lijuan
L
Gastrointestinal/Nutrition Division, Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-4318, USA.
Bortnick
Anna E
AE
Nickel
Margaret
M
Dhanasekaran
Padmaja
P
Subbaiah
Papasani V
PV
Lund-Katz
Sissel
S
Rothblat
George H
GH
Phillips
Michael C
MC
D064286:ATP Binding Cassette Transporter 1; D018528:ATP-Binding Cassette Transporters / Q000378:metabolism*; D000818:Animals; D016632:Apolipoprotein A-I / Q000378:metabolism / Q000502:physiology*; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured; D002784:Cholesterol / Q000378:metabolism*; D002850:Chromatography, Gel; D000242:Cyclic AMP / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D008075:Lipoproteins, HDL / Q000378:metabolism*; D008264:Macrophages / Q000378:metabolism*; D051379:Mice; D010713:Phosphatidylcholines / Q000378:metabolism; D010743:Phospholipids / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D013109:Sphingomyelins / Q000378:metabolism; D015854:Up-Regulation
C577836:ABCA1 protein, human; D064286:ATP Binding Cassette Transporter 1; D018528:ATP-Binding Cassette Transporters; D016632:Apolipoprotein A-I; D008075:Lipoproteins, HDL; D010713:Phosphatidylcholines; D010743:Phospholipids; D013109:Sphingomyelins; D002784:Cholesterol; D000242:Cyclic AMP