ABCA1 and ABCG1 Synergize to Mediate Cholesterol Export to ApoA-I

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Abstract
Objective— To study the acceptor specificity for human ABCG1 (hABCG1)-mediated cholesterol efflux. Methods and Results— Cells overexpressing hABCG1 were created in Chinese Hamster Ovary (CHO-K1) cells and characterized in terms of lipid composition. hABCG1 expressed in these cells formed homodimers and was mostly present intracellularly. Cholesterol efflux from hABCG1 cells to HDL 2 and HDL 3 was increased but not to lipid-free apolipoproteins. A range of phospholipid containing acceptors apart from high-density lipoprotein (HDL) subclasses were also efficient in mediating ABCG1-dependent export of cholesterol. Importantly, a buoyant phospholipid-containing fraction generated from incubation of lipid-free apoA-I with macrophages was nearly as efficient as HDL 2 . The capacity of acceptors to induce ABCG1-mediated efflux was strongly correlated with their total phospholipid content, suggesting that acceptor phospholipids drive ABCG1-mediated efflux. Most importantly, acceptors for ABCG1-mediated cholesterol export could be generated from incubation of cells with lipid-free apoA-I through the action of ABCA1 alone. Conclusions— These results indicate a synergistic relationship between ABCA1 and ABCG1 in peripheral tissues, where ABCA1 lipidates any lipid-poor/free apoA-I to generate nascent or pre–β-HDL. These particles in turn may serve as substrates for ABCG1-mediated cholesterol export.