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Journal of Lipid Research, Vol. 49, 1006-1014, May 2008
Copyright © 2008 by American Society for Biochemistry and Molecular Biology

Characterization and properties of preβ-HDL particles formed by ABCA1-mediated cellular lipid efflux to apoA-I^*

Phu T. Duong, Ginny L. Weibel, Sissel Lund-Katz, George H. Rothblat and Michael C. Phillips¹

Division of Gastroenterology, Hepatology, and Nutrition, Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, PA 19104-4318

^* This work was supported by National Institutes of Health Grant HL-22633.

Published, JLR Papers in Press, February 5, 2008.

¹ To whom correspondence should be addressed. e-mail: phillipsmi{at}email.chop.edu

The contribution of ABCA1-mediated efflux of cellular phospholipid (PL) and cholesterol to human apolipoprotein A-I (apoA-I) to the formation of preβ1-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 (preβ1-HDL; diameter = 7.5 ± 0.4 nm). The preβ1-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 -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 preβ1-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, preβ1-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.

Supplementary key words phospholipids • cholesterol • lipoprotein • reverse cholesterol transport • fibroblasts • high density lipoprotein • ATP binding cassette transporter A1 • apolipoprotein A-I

Abbreviations: apoA-I, apolipoprotein A-I; DMPC, dimyristoyl phosphatidylcholine; FC, free (unesterified) cholesterol; LpA-I, lipoprotein A-I; MLV, multilamellar vesicle; PC, phosphatidylcholine; PL phospholipid

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