Characterization and properties of pre beta-HDL particles formed by ABCA1-mediated cellular lipid efflux to apoA-I.
The contribution of ABCA1-mediated efflux of cellular phospholipid (PL) and cholesterol to human apolipoprotein A-I (apoA-I) to the formation of pre beta 1-HDL (or lipid-poor apoA-I) is not well defined. To explore this issue, we characterized the nascent HDL particles formed when lipid-free apoA-I was incubated with fibroblasts in which expression of the ABCA1 was upregulated. After a 2 h incubation, the extracellular medium contained small apoA-I/PL particles (pre beta 1-HDL; diameter = 7.5 +/- 0.4 nm). The pre beta 1-HDL (or lipid-poor apoA-I) particles contained a single apoA-I molecule and three to four PL molecules and one to two cholesterol molecules. An apoA-I variant lacking the C-terminal alpha-helix did not form such particles when incubated with the cell, indicating that this helix is critical for the formation of lipid-poor apoA-I particles. These pre beta 1-HDL particles were as effective as lipid-free apoA-I molecules in mediating both the efflux of cellular lipids via ABCA1 and the formation of larger, discoidal HDL particles. In conclusion, pre beta 1-HDL is both a product and a substrate in the ABCA1-mediated reaction to efflux cellular PL and cholesterol to apoA-I. A monomeric apoA-I molecule associated with three to four PL molecules (i.e., lipid-poor apoA-I) has similar properties to the lipid-free apoA-I molecule.
Duong
Phu T
PT
Division of Gastroenterology, Hepatology, and Nutrition, Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, PA 19104-4318, USA.
Weibel
Ginny L
GL
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*; D016632:Apolipoprotein A-I / Q000235:genetics / Q000378:metabolism*; D002478:Cells, Cultured; D005347:Fibroblasts / Q000378:metabolism; D005786:Gene Expression Regulation; D053442:High-Density Lipoproteins, Pre-beta / Q000737:chemistry / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008168:Lung / Q000378:metabolism*; D010743:Phospholipids / Q000378:metabolism*; D012867:Skin / Q000378:metabolism*
C577836:ABCA1 protein, human; D064286:ATP Binding Cassette Transporter 1; D018528:ATP-Binding Cassette Transporters; D016632:Apolipoprotein A-I; D053442:High-Density Lipoproteins, Pre-beta; D010743:Phospholipids