title: ABCA1 redistributes membrane cholesterol independent of apolipoprotein interactions.
abstract: ATP binding cassette transporter A1 (ABCA1) mediates the transport of phospholipids and cholesterol from cells to lipid-poor HDL apolipoproteins. Cholesterol loading of cells induces ABCA1, implicating cholesterol as its major physiologic substrate. It is believed, however, that ABCA1 is primarily a phospholipid transporter and that cholesterol efflux occurs by diffusion to ABCA1-generated phospholipid-rich apolipoproteins. Here we show that overexpression of ABCA1 in baby hamster kidney cells in the absence of apolipoproteins redistributed membrane cholesterol to cell-surface domains accessible to treatment with the enzyme cholesterol oxidase. The cholesterol removed by apolipoprotein A-I (apoA-I), but not by HDL phospholipids, was derived exclusively from these domains. ABCA1 overexpression also increased cholesterol esterification, which was prevented by addition of apoA-I, suggesting that some of the cell-surface cholesterol not removed by apolipoproteins is transported to the intracellular esterifying enzyme acyl-CoA:cholesterol acyltransferase. ABCA1 expression was essential for cholesterol efflux even when apolipoproteins had already acquired phospholipids during prior exposure to ABCA1-expressing cells. These studies show that ABCA1 redistributes cholesterol to cell-surface domains, where it becomes accessible for removal by apolipoproteins, consistent with a direct role of ABCA1 in cholesterol transport.
authors.affiliation: Department of Medicine, University of Washington, Seattle, WA 98195-6426, USA.
authors.forename: Ashley M
authors.initials: AM
authors.lastname: Vaughan
authors.forename: John F
authors.initials: JF
authors.lastname: Oram
mesh_terms: D064286:ATP Binding Cassette Transporter 1; D018528:ATP-Binding Cassette Transporters / Q000096:biosynthesis / Q000502:physiology*; D000818:Animals; D016632:Apolipoprotein A-I / Q000737:chemistry; D001053:Apolipoproteins / Q000378:metabolism*; D001692:Biological Transport; D002462:Cell Membrane / Q000378:metabolism*; D002784:Cholesterol / Q000378:metabolism*; D006224:Cricetinae; D017077:Culture Media, Conditioned / Q000494:pharmacology; D006801:Humans; D015151:Immunoblotting; D050356:Lipid Metabolism; D008075:Lipoproteins, HDL / Q000737:chemistry; D015735:Mifepristone / Q000494:pharmacology; D010743:Phospholipids / Q000737:chemistry / Q000378:metabolism; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D013997:Time Factors; D014162:Transfection; D014357:Trypsin / Q000494:pharmacology
chemical_list: C577836:ABCA1 protein, human; D064286:ATP Binding Cassette Transporter 1; D018528:ATP-Binding Cassette Transporters; D016632:Apolipoprotein A-I; D001053:Apolipoproteins; D017077:Culture Media, Conditioned; D008075:Lipoproteins, HDL; D010743:Phospholipids; D015735:Mifepristone; D002784:Cholesterol; D014357:Trypsin