Specific Binding of ApoA-I, Enhanced Cholesterol Efflux, and Altered Plasma Membrane Morphology in Cells Expressing ABC1

Abstract

Mutations of the ABC1 transporter have been identified as the defect in Tangier disease, characterized by low HDL and cholesterol ester accumulation in macrophages. A full-length mouse ABC1 cDNA was used to investigate the mechanisms of lipid efflux to apoA-I or HDL in transfected 293 cells. ABC1 expression markedly increased cellular cholesterol and phospholipid efflux to apoA-I but had only minor effects on lipid efflux to HDL. The increased lipid efflux appears to involve a direct interaction between apoA-I and ABC1, because ABC1 expression substantially increased apoA-I binding at the cell surface, and chemical cross-linking and immunoprecipitation analysis showed that apoA-I binds directly to ABC1. In contrast to scavenger receptor BI (SR-BI), another cell surface molecule capable of facilitating cholesterol efflux, ABC1 preferentially bound lipid-free apoA-I but not HDL. Immunofluorescence confocal microscopy showed that ABC1 is primarily localized on the cell surface. In the absence of apoA-I, cells overexpressing ABC1 displayed a distinctive morphology, characterized by plasma membrane protrusions and resembling echinocytes that form when there are excess lipids in the outer membrane hemileaflet. The studies provide evidence for a direct interaction between ABC1 and apoA-I, but not HDL, indicating that free apoA-I is the metabolic substrate for ABC1. Plasma membrane ABC1 may act as a phospholipid/cholesterol flippase, providing lipid to bound apoA-I, or to the outer membrane hemileaflet. Mutations of the ABC1 transporter have been identified as the defect in Tangier disease, characterized by low HDL and cholesterol ester accumulation in macrophages. A full-length mouse ABC1 cDNA was used to investigate the mechanisms of lipid efflux to apoA-I or HDL in transfected 293 cells. ABC1 expression markedly increased cellular cholesterol and phospholipid efflux to apoA-I but had only minor effects on lipid efflux to HDL. The increased lipid efflux appears to involve a direct interaction between apoA-I and ABC1, because ABC1 expression substantially increased apoA-I binding at the cell surface, and chemical cross-linking and immunoprecipitation analysis showed that apoA-I binds directly to ABC1. In contrast to scavenger receptor BI (SR-BI), another cell surface molecule capable of facilitating cholesterol efflux, ABC1 preferentially bound lipid-free apoA-I but not HDL. Immunofluorescence confocal microscopy showed that ABC1 is primarily localized on the cell surface. In the absence of apoA-I, cells overexpressing ABC1 displayed a distinctive morphology, characterized by plasma membrane protrusions and resembling echinocytes that form when there are excess lipids in the outer membrane hemileaflet. The studies provide evidence for a direct interaction between ABC1 and apoA-I, but not HDL, indicating that free apoA-I is the metabolic substrate for ABC1. Plasma membrane ABC1 may act as a phospholipid/cholesterol flippase, providing lipid to bound apoA-I, or to the outer membrane hemileaflet. high density lipoprotein apolipoprotein A-I ATP binding cassette transporter 1 cholesterol ester transfer protein cholesterol ester dithiobis(succinimidylpropionate) scavenger receptor BI polymerase chain reaction base pair(s) Dulbecco's modified Eagle's medium phosphate-buffered saline bovine serum albumin phosphatidyl choline Epidemiological studies have demonstrated a strong inverse correlation between the levels of plasma HDL1 and the risk of coronary heart disease (1Castelli W.P. Garrison R.J. Wilson P.W. Abbott R.D. Kalousdian S. Kannel W.B. JAMA. 1986; 256: 2835-2838Crossref PubMed Scopus (2091) Google Scholar). It has been proposed that HDL promotes reverse cholesterol transport by facilitating transfer of cholesterol from peripheral tissues to the liver for disposal (2Tall A.R. J. Clin. Invest. 1990; 86: 379-384Crossref PubMed Scopus (586) Google Scholar). However, the molecular mechanisms for transfer of cholesterol from peripheral cells to HDL are incompletely understood. A breakthrough in our understanding of this process came recently from studies of Tangier disease and familial HDL deficiency, in which the molecular defect was shown to be mutations in the ATP binding cassette transporter 1 (ABC1) gene (3Brooks-Wilson A. Marcil M. Clee S.M. Zhang L.H. Roomp K. van Dam M., Yu, L. Brewer C. Collins J.A. Molhuizen H.O. Loubser O. Ouelette B.F. Fichter K. Ashbourne-Excoffon K.J. Sensen C.W. Scherer S. Mott S. Denis M. Martindale D. Frohlich J. Morgan K. Koop B. Pimstone S. Kastelein J.J. Hayden M.R. et al.Nat. Genet. 1999; 22: 336-345Crossref PubMed Scopus (1509) Google Scholar, 4Bodzioch M. Orso E. Klucken J. Langmann T. Bottcher A. Diederich W. Drobnik W. Barlage S. Buchler C. Porsch-Ozcurumez M. Kaminski W.E. Hahmann H.W. Oette K. Rothe G. Aslanidis C. Lackner K.J. Schmitz G. Nat. Genet. 1999; 22: 347-351Crossref PubMed Scopus (1349) Google Scholar, 5Rust S. Rosier M. Funke H. Real J. Amoura Z. Piette J.C. Deleuze J.F. Brewer H.B. Duverger N. Denefle P. Assmann G. Nat. Genet. 1999; 22: 352-355Crossref PubMed Scopus (1269) Google Scholar). Tangier disease is a rare recessive genetic disorder characterized by extremely low HDL levels, accumulation of cholesterol esters in macrophages, and, in some cases, premature coronary heart diseases (6Schaefer E.J. Zech L.A. Schwartz D.E. Brewer Jr., H.B. Ann. Intern. Med. 1980; 93: 261-266Crossref PubMed Scopus (175) Google Scholar). ABC1 is a 240-kDa protein belonging to a large family of conserved transmembrane proteins that transport a wide variety of substrates, including ions, drugs, peptides, and lipids, across cell membranes (7Croop J.M. Methods Enzymol. 1998; 292: 101-116Crossref PubMed Scopus (53) Google Scholar). The finding that fibroblasts from Tangier disease patients have a marked defect in efflux of cholesterol and phospholipids to apoA-I (8Francis G.A. Knopp R.H. Oram J.F. J. Clin. Invest. 1995; 96: 78-87Crossref PubMed Scopus (373) Google Scholar, 9Rogler G. Trumbach B. Klima B. Lackner K.J. Schmitz G. Arterioscler. Thromb. Vasc. Biol. 1995; 15: 683-690Crossref PubMed Scopus (113) Google Scholar, 10Walter M. Gerdes U. Seedorf U. Assmann G. Biochem. Cell Biol. 1994; 205: 850-856Google Scholar) suggests that ABC1 mediates or regulates the efflux of cellular cholesterol and phospholipids to apoA-I. Another membrane protein that binds HDL and mediates HDL cholesterol ester (HDL CE) uptake (11Acton S. Rigotti A. Landschulz K.T. Xu S. Hobbs H.H. Krieger M. Science. 1996; 271: 518-520Crossref PubMed Scopus (2011) Google Scholar) and cellular unesterified cholesterol efflux (12Ji Y. Jian B. Wang N. Sun Y. Moya M.L. Phillips M.C. Rothblat G.H. Swaney J.B. Tall A.R. J. Biol. Chem. 1997; 272: 20982-20985Abstract Full Text Full Text PDF PubMed Scopus (636) Google Scholar, 13Jian B. de la Llera-Moya M. Ji Y. Wang N. Phillips M.C. Swaney J.B. Tall A.R. Rothblat G.H. J. Biol. Chem. 1998; 273: 5599-5606Abstract Full Text Full Text PDF PubMed Scopus (263) Google Scholar) is scavenger receptor class B type I (SR-BI). SR-BI is primarily expressed in liver and steroidogenic tissues and mediates HDL CE and free cholesterol uptake (11Acton S. Rigotti A. Landschulz K.T. Xu S. Hobbs H.H. Krieger M. Science. 1996; 271: 518-520Crossref PubMed Scopus (2011) Google Scholar, 14Wang N. Weng W. Breslow J.L. Tall A.R. J. Biol. Chem. 1996; 271: 21001-21004Abstract Full Text Full Text PDF PubMed Scopus (192) Google Scholar). SR-BI also has been reported to be expressed in atherosclerotic lesion macrophages (15Murao K. Terpstra V. Green S.R. Kondratenko N. Steinberg D. Quehenberger O. J. Biol. Chem. 1997; 272: 17551-17557Abstract Full Text Full Text PDF PubMed Scopus (288) Google Scholar, 16Hirano K. Yamashita S. Nakagawa Y. Ohya T. Matsuura F. Tsukamoto K. Okamoto Y. Matsuyama A. Matsumoto K. Miyagawa J. Matsuzawa Y. Circ. Res. 1999; 85: 108-116Crossref PubMed Scopus (144) Google Scholar, 17Chinetti G. Gbaguidi F.G. Griglio S. Mallat Z. Antonucci M. Poulain P. Chapman J. Fruchart J.C. Tedgui A. Najib-Fruchart J. Staels B. Circulation. 2000; 101: 2411-2417Crossref PubMed Scopus (390) Google Scholar). However, the function of SR-BI in macrophages is yet to be defined. In the present study, we characterized the ABC1 cDNA in human embryonic kidney 293 (HEK 293) cells. In contrast to the published ABC1 cDNA (GenBank™ accession numbers X75926 and NM005502), we found an additional 60-amino acid at the amino terminus of ABC1 that is required for it to mediate cellular cholesterol and phospholipid efflux. We also demonstrate, by binding and chemical cross-linking studies, that apoA-I directly interacts with ABC1. Human apoA-I was commercially obtained (BioDesign). Anti-FLAG antibodies were from Sigma (St. Louis, MO) and anti-SR-BI antibodies were from Novus Biologicals (Littleton, CO). Dithiobis(succinimidylpropionate) (DSP) was from Pierce (Rockford, IL). Murine ABC1 cDNA was obtained by reverse transcription-PCR using murine RAW cell total RNA and cloned into a pcDNA3.1 vector (Invitrogen, CA). The 5′-rapid amplification of cDNA ends PCR of human ABC1 mRNA of THP1 cells, isolation, and characterization of novel ABC1 transcripts, and isolation and determination of human ABC1 promoter and exon/intron structures of human ABC1 gene have been described (18Costet P. Luo Y. Wang N. Tall A.R. J. Biol. Chem. 2000; 275: 28240-28245Abstract Full Text Full Text PDF PubMed Scopus (853) Google Scholar). The pcDNA3.1/mABC1Δ contained a murine ABC1 cDNA from bp 254 to 6869 according to the published mouse ABC1 mRNA sequence (GenBank™ accession number X75926). pcDNA3.1/mABC1 had the murine ABC1 cDNA from bp 49 to 6869. Compared with ABC1Δ, this results in a putative extra amino-terminal 60-amino acid sequence (MACWPQLRLLLWKNLTFRRRQTCQLLLEVAWPLF- IFLILISVRLSWPPYEQHECHFPNKA). ABC1 with the FLAG epitope incorporated at its carboxyl terminus was constructed using PCR and primers encoding the amino acid sequence of the FLAG epitope (DYKDDDDK). HEK 293 cells were cultured in DMEM media plus 10% fetal bovine serum and antibiotics. 1 day before transfection, the cells were plated on 6- or 24-well plates coated with collagen. The next day, cells at about 95% confluence were transfected using LipofectAMINE 2000 (Life Technologies, Inc., Rockville, MD) and corresponding plasmid constructs. For lipid efflux assays, cells were labeled with 0.5 μCi/ml 1,2-[3H]cholesterol or 1 μCi/ml [methyl-3H]choline on the same day as cell transfection. 24 h after transfection and labeling, the cells were washed three times with PBS, incubated at 37 °C for 2 h with DMEM media plus 0.2% fatty acid free bovine albumin (DMED/BSA). The media were then replaced with fresh DMED/BSA in the presence or absence of the indicated amount of apoA-I or HDL3 and incubated at 37 °C for 4 h. The media were collected and counted for by in cholesterol efflux or with and then counted in phospholipid efflux were with and 0.2% in cholesterol efflux or with in phospholipid efflux and in cells was or HDL3 from human plasma were with by to a of apoA-I or HDL3 on 24-well plates were incubated on for 2 h in with labeled apoA-I or HDL3 in the presence or absence of a excess of were then washed times with were with and 0.2% was with a modified and bound were by on plates were incubated at 37 °C for 1 h with 1 apoA-I or HDL3 in in the presence or absence of excess of were then on for and washed three times with was before in and to with PBS, and was were incubated at for 1 the medium was and the cells were washed with were at 4 °C with and 1 For the cell were at in a for The was of and of were to and the was at 4 °C for h. a of or of anti-SR-BI was to the cell and incubated by at 4 °C was then and was for 2 h by the The were and the was washed with and with The bound proteins were from the by and with in the presence or absence of The proteins were for by to a for and were with for and then incubated with in for 2 with PBS, cells were incubated with in 4 in at for or was used as the and with cells were by confocal microscopy as described Wang N. Tall A.R. J. Clin. Invest. 2000; PubMed Scopus Google Scholar). studies to the promoter of the human ABC1 we found a of the cDNA with the published cDNA (GenBank™ accession numbers X75926 and (18Costet P. Luo Y. Wang N. Tall A.R. J. Biol. Chem. 2000; 275: 28240-28245Abstract Full Text Full Text PDF PubMed Scopus (853) Google Scholar). contained a with a strong sequence M. J. Biol. PubMed Scopus Google Scholar) and is with the published amino F. S. G. 1994; PubMed Scopus Google Scholar). The strong of this amino-terminal amino acid sequence of ABC1 with the amino-terminal amino acid of and ABC1 family N. Sun H. A. A. B. L. D. A. A. P. Y. J. M. M. Nat. Genet. 1997; 15: PubMed Scopus Google Scholar, N. J.M. A. Y. A. M. M. Science. 1997; PubMed Scopus Google Scholar, 1997; PubMed Scopus Google and the of a with a between and the H. J. S. G. 1997; PubMed Scopus Google that this identified is the for ABC1. this we plasmid ABC1 with or the putative amino-terminal amino expression a FLAG epitope was incorporated into the carboxyl terminus of ABC1 or cholesterol efflux by apoA-I was in HEK 293 cells transfected with the expression constructs. of apoA-I in the cells only increased cholesterol efflux In apoA-I markedly increased cholesterol efflux in cells. The of cholesterol efflux present apoA-I increased efflux in in was as as ABC1 in cholesterol efflux, that the FLAG epitope not with the function of ABC1 and analysis showed that with a not In expression of had on cholesterol efflux, indicating that the amino-terminal amino of ABC1 (18Costet P. Luo Y. Wang N. Tall A.R. J. Biol. Chem. 2000; 275: 28240-28245Abstract Full Text Full Text PDF PubMed Scopus (853) Google Scholar) are for for the expression of ABC1Δ, we also transfected HEK 293 cells with Immunofluorescence microscopy the expression of in the transfected cells not this to cholesterol efflux. studies have that and cholesterol efflux are in fibroblasts from Tangier disease patients (8Francis G.A. Knopp R.H. Oram J.F. J. Clin. Invest. 1995; 96: 78-87Crossref PubMed Scopus (373) Google Scholar, J.A. H. Brewer Jr., H.B. Arterioscler. Thromb. Vasc. Biol. 1997; PubMed Scopus Google Scholar). we the of apoA-I and HDL to mediate cholesterol efflux in or cells. markedly increased cholesterol efflux, in a process that was at low apoA-I 2 The for cholesterol efflux in cells was cellular cholesterol efflux in the cells However, this was because ABC1 expression only increased cholesterol efflux by results that apoA-I is the for cholesterol expression markedly apoA-I but not lipid efflux. the was to that in 1 that a of apoA-I was that HDL3 was cells were transfected and labeled with For efflux, media in the presence or absence of apoA-I or HDL3 were and incubated with cells for 4 h at 37 or cells were to from free and We also and phosphatidyl choline efflux. ABC1 transfection substantially increased apoA-I efflux 2 However, efflux was only increased in cells to the ABC1 binding of to the cell surface, and to HEK 293 cells was ABC1 transfection markedly increased the binding of apoA-I to cells at 4 °C However, to the cell surface not in cells to the In contrast to ABC1, SR-BI expression in HEK 293 cells was found to apoA-I and HDL binding as reported S. M. Rigotti A. Krieger M. J. Res. 1997; Full Text PDF PubMed Google Scholar, de la Llera-Moya M. S. S. Phillips M.C. J. Biol. Chem. 2000; 275: Full Text Full Text PDF PubMed Scopus Google Scholar). The increased binding of apoA-I in HEK 293 cells overexpressing ABC1 be to direct between apoA-I and ABC1, or it be to in the cell membrane from ABC1 we chemical cross-linking studies using or and the the isolation of the by we transfected cells with the which was from the ABC1 cDNA in cellular cholesterol efflux was with from the of cells and as a at an molecular of when with the cells not that apoA-I directly binds to ABC1. The of the suggests that ABC1 is In the absence of or in the presence of excess of apoA-I this high molecular was as the of and the by the as a high molecular in at the of apoA-I a that apoA-I binds directly to SR-BI de la Llera-Moya M. S. S. Phillips M.C. J. Biol. Chem. 2000; 275: Full Text Full Text PDF PubMed Scopus Google we found that bound and to SR-BI and as a high molecular is with the that SR-BI in de la Llera-Moya M. S. S. Phillips M.C. J. Biol. Chem. 2000; 275: Full Text Full Text PDF PubMed Scopus Google Scholar). In contrast to apoA-I, HDL3 was not with from cells with the lipoprotein were bound and to SR-BI results that apoA-I directly interacts with ABC1, HDL Immunofluorescence confocal microscopy was used to the of ABC1 with in HEK 293 cells. microscopy a strong cell surface indicating that ABC1 is localized primarily on the cellular plasma were also a structures with FLAG ABC1 at as reported E. C. Kaminski W.E. Bottcher A. G. Drobnik W. A. O. Diederich W. Langmann T. T. Rothe G. Lackner K.J. G. Schmitz G. Nat. Genet. 2000; PubMed Scopus Google Scholar). we found that cells had from the cell strong for ABC1 4 of cells displayed In the was to when apoA-I was transfection not not the transfection because the expression of protein was in the presence of apoA-I. In this we have demonstrated that ABC1 cellular cholesterol and phospholipid efflux. ABC1 expression binding of apoA-I, but not to the cell surface. The chemical cross-linking and immunoprecipitation analysis that apoA-I directly interacts with ABC1. Compared with HDL, apoA-I is the for cholesterol and phospholipid efflux. ABC1 is primarily localized on the cellular plasma It is that the direct interaction between apoA-I and ABC1 at the cell surface is required for the lipid efflux by cholesterol and phospholipid efflux by ABC1 expression has been reported by but in studies the for an additional amino at the amino terminus was not used ABC1 cDNA to in this E. C. Kaminski W.E. Bottcher A. G. Drobnik W. A. O. Diederich W. Langmann T. T. Rothe G. Lackner K.J. G. Schmitz G. Nat. Genet. 2000; PubMed Scopus Google the of the ABC1 cDNA used by another was not M.R. Wang Schwartz K. J.J. Oram J.F. J. Clin. Invest. 1999; PubMed Scopus Google Scholar). to cholesterol efflux the of this We the of The of this amino-terminal 60-amino acid sequence has also been recently by sequence analysis Biochem. Cell Biol. 2000; 271: studies were The binding of apoA-I to fibroblasts from Tangier disease patients has been reported to be in some studies (8Francis G.A. Knopp R.H. Oram J.F. J. Clin. Invest. 1995; 96: 78-87Crossref PubMed Scopus (373) Google Scholar) but not in J.A. H. Brewer Jr., H.B. Arterioscler. Thromb. Vasc. Biol. 1997; PubMed Scopus Google Scholar). The of ABC1 mutations and the for Tangier disease In to apoA-I, cholesterol efflux by as and are in Tangier cells J.A. H. Brewer Jr., H.B. Arterioscler. Thromb. Vasc. Biol. 1997; PubMed Scopus Google Scholar). on has been as to a receptor binds apoA-I in the process of lipid efflux J.A. H. Brewer Jr., H.B. Arterioscler. Thromb. Vasc. Biol. 1997; PubMed Scopus Google Scholar). that apoA-I directly binds to ABC1. In ABC1 to the cell surface binding of HDL3 and cholesterol efflux. that direct interaction of lipid-free with ABC1 is for the of cholesterol efflux. The inverse of ABC1 and SR-BI to apoA-I and HDL3 has for this and studies de la Llera-Moya M. S. S. Phillips M.C. J. Biol. Chem. 2000; 275: Full Text Full Text PDF PubMed Scopus Google Scholar) have shown the binding of apoA-I and HDL to studies that lipid-free apoA-I or HDL binds to SR-BI HDL T. Xu S. Krieger J. Krieger M. J. Biol. Chem. 2000; 275: Full Text Full Text PDF PubMed Scopus Google Scholar). the binding of HDL is with the density of the lipoprotein that SR-BI binds to low HDL to the of CE uptake T. Xu S. Krieger J. Krieger M. J. Biol. Chem. 2000; 275: Full Text Full Text PDF PubMed Scopus Google Scholar). In the that ABC1 binds lipid-free apoA-I with high but binds HDL3 it is HDL, lipid-free apoA-I to cholesterol efflux by SR-BI it binds SR-BI (12Ji Y. Jian B. Wang N. Sun Y. Moya M.L. Phillips M.C. Rothblat G.H. Swaney J.B. Tall A.R. J. Biol. Chem. 1997; 272: 20982-20985Abstract Full Text Full Text PDF PubMed Scopus (636) Google Scholar). it is that in apoA-I from liver and and interacts with ABC1 on the cell surface, phospholipids and cholesterol from peripheral cells. of lipids on apoA-I may the for ABC1 and from the lipoprotein are into HDL by lipids from peripheral cells, by phospholipid transfer lipid transfer from lipoprotein A. Biochem. 1995; PubMed Scopus Google Scholar) and by cholesterol The HDL to liver and preferentially binds process may be by on T. F. L. V. W. T. D. Tall A.R. U. S. A. 1999; 96: PubMed Scopus Google Scholar). uptake of HDL CE by the and have for In to of apoA-I by the liver and and also in the of HDL to free apoA-I. The of in may be to the increased of HDL and of free apoA-I from HDL T. L. A. E. Breslow J.L. Tall A.R. J. Clin. Invest. 1995; 96: PubMed Scopus Google Scholar). In to apoA-I, also may function as for lipid efflux, in contrast to ABC1 E. C. Kaminski W.E. Bottcher A. G. Drobnik W. A. O. Diederich W. Langmann T. T. Rothe G. Lackner K.J. G. Schmitz G. Nat. Genet. 2000; PubMed Scopus Google apoA-I not cholesterol esters in macrophages Weng W. Breslow J.L. J. Clin. Invest. 1996; PubMed Scopus Google Scholar). apoA-I, appears to be a serum from has markedly increased cholesterol efflux in cells only after with a that expression of ABC1 N. V. J.L. M. Duverger N. Rothblat G.H. N. Arterioscler. Thromb. Vasc. Biol. 2000; PubMed Scopus Google Scholar). the in N. G. J.M. F. G. Fruchart J.C. A. Denefle P. Science. 1996; 273: PubMed Scopus Google Scholar). In the serum from not cholesterol efflux from cells N. V. J.L. M. Duverger N. Rothblat G.H. N. Arterioscler. Thromb. Vasc. Biol. 2000; PubMed Scopus Google a cell with a high of SR-BI expression (12Ji Y. Jian B. Wang N. Sun Y. Moya M.L. Phillips M.C. Rothblat G.H. Swaney J.B. Tall A.R. J. Biol. Chem. 1997; 272: 20982-20985Abstract Full Text Full Text PDF PubMed Scopus (636) Google Scholar). that apoA-I in and lipid efflux. The low for cholesterol efflux in cells ABC1 and the direct binding of apoA-I to ABC1 a high of ABC1 for apoA-I. In the plasma of free apoA-I and lipid HDL has been to be J. J. Res. Full Text PDF PubMed Google Scholar, P. A. A. U. L. Full Text PDF PubMed Scopus Google Scholar). that the of apoA-I is not for cholesterol efflux. However, the of lipid-free apoA-I to ABC1 in atherosclerotic is to be have markedly increased apoA-I and Breslow J.L. U. S. A. 1994; PubMed Scopus Google that apoA-I is a in reverse cholesterol The of ABC1 expression cholesterol in macrophages T. Klucken J. M. G. G. Kaminski W.E. Schmitz G. Biochem. Cell Biol. 1999; Scholar) be another to excess of cellular However, this is by liver and to which are not in cells (18Costet P. Luo Y. Wang N. Tall A.R. J. Biol. Chem. 2000; 275: 28240-28245Abstract Full Text Full Text PDF PubMed Scopus (853) Google Scholar). suggests that ABC1 expression is also for reverse cholesterol transport from cells. overexpressing ABC1 a resembling are cells plasma membrane which form of phospholipids or to the outer of plasma membrane U. S. A. PubMed Scopus Google Scholar). also form plasma membrane or of phospholipids A. A. PubMed Scopus Google Scholar, Jr., K.J. PubMed Scopus Google Scholar). The distinctive and of by of apoA-I that from an increased of phospholipid and in the outer of the plasma with an of an transporter that as a phospholipid at the membranes J.J. E. van L. van van et Full Text PDF PubMed Scopus Google Scholar, S. P. 1994; Full Text PDF PubMed Scopus Google results that ABC1 at the plasma membrane as a lipid the ABC1 is a transporter of transmembrane and with binding A microscopy suggests that the of a of about in with a large of about in J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar). The is at the of the membrane but at the In an to the lipid the of the is also A is that lipids the protein by in the of the and the is then across the with the of ATP S. P. 1994; Full Text PDF PubMed Scopus Google Scholar). ABC1 a to the lipid-free apoA-I the of the transporter and The of the the binding and cholesterol efflux by In our studies provide evidence for a direct interaction between apoA-I and ABC1 but not between HDL and ABC1, that lipid apoA-I or lipid as the for lipid efflux. The of free apoA-I, as as the expression of ABC1 in cells, may be for reverse cholesterol We D. and for providing the plasmid for this