Submitted on December 6, 2005
Revised on January 6, 2006
Accepted on January 17, 2006
Characterization of nascent HDL particles and microparticles formed by ABCA1-mediated efflux of cellular lipids to ApoA-I
Phu T Duong, Heidi L. Collins, Margaret Nickel, Sissel Lund-Katz, George H. Rothblat, and Michael C. Phillips
GI/Nutrition, Children's Hospital of Philadelphia, Philadelphia, PA 19104-4318
Corresponding Author: phillipsmi{at}email.chop.edu
The nascent HDL created by ABCA1-mediated efflux of cellular phospholipid (PL) and free (unesterified) cholesterol (FC) to apoA-I has not been defined. To address this issue, we characterized the lipid particles released when J774 mouse macrophages and human skin fibroblasts in which ABCA1 is activated are incubated with human apoA-I. In both cases, three types of nascent HDL containing 2, 3 or 4 molecules of apoA-I per particle are formed. With J774 cells, the predominant species have hydrodynamic diameters of about 9 and 12 nm. These discoidal HDL particles have different FC contents and PL compositions, and the presence of acidic PL causes them to exhibit a-electrophoretic mobility. The results are consistent with ABCA1 located in more than one membrane microenvironment being responsible for the production of the heterogeneous HDL. Activation of ABCA1 also leads to the release of apoA-I-free plasma membrane vesicles (microparticles). These larger, spherical particles released from J774 cells have the same PL composition as the 12 nm HDL and contain CD14 and ganglioside GM1, consistent with the origin being plasma membrane raft domains. The various HDL particles and microparticles are created concurrently and there is not a precursor-product relationship between them. Importantly, a large fraction of the cellular FC effluxed from these cells by ABCA1 is located in microparticles. Collectively, the results show that the products of apoA-I/ABCA1 interaction include discoidal HDL particles containing different numbers of apoA-I molecules. The cellular phospholipids and cholesterol incorporated into these nascent HDL particles originate from different cell membrane domains.
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