Debin Lan

The mechanisms responsible for the inverse relationship between plasma high-density lipoprotein (HDL) levels and atherosclerotic cardiovascular disease are poorly understood. The ATP-binding cassette transporter A1 (ABCA1) mediates efflux of cellular cholesterol to lipid-poor apolipoproteins but not to HDL particles that constitute the bulk of plasma HDL. We show that two ABC transporters of unknown function, ABCG1 and ABCG4, mediate isotopic and net mass efflux of cellular cholesterol to HDL. In transfected 293 cells, ABCG1 and ABCG4 stimulate cholesterol efflux to both smaller (HDL-3) and larger (HDL-2) subclasses but not to lipid-poor apoA-I. Treatment of macrophages with an liver X receptor activator results in up-regulation of ABCG1 and increases cholesterol efflux to HDL. RNA interference reduced the expression of ABCG1 in liver X receptor-activated macrophages and caused a parallel decrease in cholesterol efflux to HDL. These studies indicate that ABCG1 and ABCG4 promote cholesterol efflux from cells to HDL. ABCG1 is highly expressed in macrophages and probably mediates cholesterol efflux from macrophage foam cells to the major HDL fractions, providing a mechanism to explain the relationship between HDL levels and atherosclerosis risk.

ATP-binding cassette transporter 1 (ABCA1), the defective transporter in Tangier disease, binds and promotes cellular cholesterol and phospholipid efflux to apolipoprotein I (apoA-I). Based on a high degree of sequence homology between ABCA1 and ABCA7, a transporter of unknown function, we investigated the possibility that ABCA7 might be involved in apolipoprotein binding and lipid efflux. Similarly to cells expressing ABCA1, HEK293 cells overexpressing ABCA7 showed specific binding and cross-linking of lipid-poor apoA-I. ABCA7 expression increased cellular phosphatidylcholine and sphingomyelin efflux to apoA-I in a manner similar to ABCA1 but had no effect on cholesterol efflux. Western analysis showed a high protein level of ABCA7 in mouse spleen, lung, adrenal, and brain but low expression in liver. In contrast to ABCA1, ABCA7 showed moderate basal mRNA and protein levels in macrophages and lymphocytes but no induction by liver X receptor activation. These studies show that ABCA7 has the ability to bind apolipoproteins and promote efflux of cellular phospholipids without cholesterol, and they suggest a possible role of ABCA7 in cellular phospholipid metabolism in peripheral tissues.

The protein deacetylase, sirtuin 1 (SIRT1), is a proposed master regulator of exercise-induced mitochondrial biogenesis in skeletal muscle, primarily via its ability to deacetylate and activate peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α). To investigate regulation of mitochondrial biogenesis by SIRT1 in vivo, we generated mice lacking SIRT1 deacetylase activity in skeletal muscle (mKO). We hypothesized that deacetylation of PGC-1α and mitochondrial biogenesis in sedentary mice and after endurance exercise would be impaired in mKO mice. Skeletal muscle contractile characteristics were determined in extensor digitorum longus muscle ex vivo. Mitochondrial biogenesis was assessed after 20 days of voluntary wheel running by measuring electron transport chain protein content, enzyme activity, and mitochondrial DNA expression. PGC-1α expression, nuclear localization, acetylation, and interacting protein association were determined following an acute bout of treadmill exercise (AEX) using co-immunoprecipitation and immunoblotting. Contrary to our hypothesis, skeletal muscle endurance, electron transport chain activity, and voluntary wheel running-induced mitochondrial biogenesis were not impaired in mKO versus wild-type (WT) mice. Moreover, PGC-1α expression, nuclear translocation, activity, and deacetylation after AEX were similar in mKO versus WT mice. Alternatively, we made the novel observation that deacetylation of PGC-1α after AEX occurs in parallel with reduced nuclear abundance of the acetyltransferase, general control of amino-acid synthesis 5 (GCN5), as well as reduced association between GCN5 and nuclear PGC-1α. These findings demonstrate that SIRT1 deacetylase activity is not required for exercise-induced deacetylation of PGC-1α or mitochondrial biogenesis in skeletal muscle and suggest that changes in GCN5 acetyltransferase activity may be an important regulator of PGC-1α activity after exercise.

Although many organic anion transport protein (Oatp) family members have PDZ consensus binding sites at their C termini, the functional significance is unknown. In the present study, we utilized rat Oatp1a1 (NM_017111) as a prototypical member of this family to examine the mechanism governing its subcellular trafficking. A peptide corresponding to the C-terminal 16 amino acids of rat Oatp1a1 was used to affinity-isolate interacting proteins from rat liver cytosol. Protein mass fingerprinting identified PDZK1 as the major interacting protein. This was confirmed by immunoprecipitation of an Oatp1a1-PDZK1 complex from cotransfected 293T cells as well as from native rat liver membrane extracts. Oatp1a1 bound predominantly to the first and third PDZ binding domains of PDZK1, whereas the high density lipoprotein receptor, scavenger receptor B type I binds to the first domain. Although it is possible that PDZK1 forms a complex with these two integral membrane proteins, this did not occur, suggesting that as yet undescribed factors lead to selectivity in the interaction of these protein ligands with PDZK1. Oatp1a1 protein expression was near normal in PDZK1 knock-out mouse liver. However, it was located predominantly in intracellular structures, in contrast to its normal basolateral plasma membrane distribution. Plasma disappearance of the Oatp1a1 ligand [35S]sulfobromophthalein was correspondingly delayed in knock-out mice. These studies show a critical role for oligomerization of Oatp1a1 with PDZK1 for its proper subcellular localization and function. Because its ability to transport substances into the cell requires surface expression, this must be considered in any assessment of physiologic function.

ABCA7 is homologous to ABCA1 and has recently been shown in cell culture to bind apolipoprotein A-I (apoA-I) and to promote the efflux of phospholipids. However, it is not known if ABCA7 promotes lipid efflux in vivo. When expressed in HEK293 cells, both human and mouse ABCA7 promoted phospholipid efflux to apoA-I but no detectable cholesterol efflux. However, genetic knockdown of ABCA7 in mouse peritoneal macrophages did not affect phospholipid or cholesterol efflux to apoA-I. Moreover, in ABCA1-knockout macrophages, there was no detectable apoA-I-stimulated phospholipid efflux, inconsistent with a residual role of ABCA7. In contrast to plasma membrane localization of ABCA7 in transfected embryonic kidney cells, immunofluorescence microscopy of endogenous ABCA7 in macrophages showed a predominantly intracellular localization of the protein. Strikingly, immunofluorescence studies of adult mouse kidney revealed an apical brush border membrane localization of ABCA7 in the proximal tubule, suggesting that ABCA7 may come in contact with apoA-I in the glomerular filtrate.Although ABCA7 does not contribute to apolipoprotein-mediated lipid efflux in resting macrophages, its cell surface location in the kidney suggests that it could serve such a role in tissue microenvironments. ABCA7 is homologous to ABCA1 and has recently been shown in cell culture to bind apolipoprotein A-I (apoA-I) and to promote the efflux of phospholipids. However, it is not known if ABCA7 promotes lipid efflux in vivo. When expressed in HEK293 cells, both human and mouse ABCA7 promoted phospholipid efflux to apoA-I but no detectable cholesterol efflux. However, genetic knockdown of ABCA7 in mouse peritoneal macrophages did not affect phospholipid or cholesterol efflux to apoA-I. Moreover, in ABCA1-knockout macrophages, there was no detectable apoA-I-stimulated phospholipid efflux, inconsistent with a residual role of ABCA7. In contrast to plasma membrane localization of ABCA7 in transfected embryonic kidney cells, immunofluorescence microscopy of endogenous ABCA7 in macrophages showed a predominantly intracellular localization of the protein. Strikingly, immunofluorescence studies of adult mouse kidney revealed an apical brush border membrane localization of ABCA7 in the proximal tubule, suggesting that ABCA7 may come in contact with apoA-I in the glomerular filtrate. Although ABCA7 does not contribute to apolipoprotein-mediated lipid efflux in resting macrophages, its cell surface location in the kidney suggests that it could serve such a role in tissue microenvironments. HDL-cholesterol levels are inversely correlated with the risk of coronary heart disease (1Rhoads G.G. Gulbrandsen C.L. Kagan A. Serum lipoproteins and coronary heart disease in a population study of Hawaii Japanese men.N. Engl. J. Med. 1976; 294: 293-298Crossref PubMed Scopus (670) Google Scholar). HDL particles are capable of picking up excess cholesterol in peripheral tissues and thus promote the flux of excess cholesterol to the liver for excretion into the bile. This process has been termed “reverse cholesterol transport” and is considered the major reason for the antiatherogenic properties of HDL (2Glomset J.A. High-density lipoproteins in human health and disease.Adv. Intern. Med. 1980; 25: 91-116PubMed Google Scholar). Heart efflux of phospholipids and cholesterol to lipid-poor apolipoprotein A-I (apoA-I) constitutes the first crucial step in the formation of nascent HDL particles. Patients with Tangier disease show a defect in this lipidation of apoA-I, resulting in drastically reduced plasma HDL levels (3Francis G.A. Knopp R.H. Oram J.F. Defective removal of cellular cholesterol and phospholipids by apolipoprotein A-I in Tangier disease.J. Clin. Invest. 1995; 96: 78-87Crossref PubMed Scopus (369) Google Scholar, 4Remaley A.T. Schumacher U.K. Stonik J.A. Farsi B.D. Nazih H. Brewer Jr, H.B. Decreased reverse cholesterol transport from Tangier disease fibroblasts. Acceptor specificity and effect of brefeldin on lipid efflux.Arterioscler. Thromb. Vasc. Biol. 1997; 17: 1813-1821Crossref PubMed Scopus (190) Google Scholar). ABCA1 has been identified as the defective molecule in Tangier disease (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. Tangier disease is caused by mutations in the gene encoding ATP-binding cassette transporter 1.Nat. Genet. 1999; 22: 352-355Crossref PubMed Scopus (1249) Google Scholar, 6Bodzioch 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. The gene encoding ATP-binding cassette transporter 1 is mutated in Tangier disease.Nat. Genet. 1999; 22: 347-351Crossref PubMed Scopus (1328) Google Scholar, 7Brooks-Wilson A. Marcil M. Clee S.M. Zhang L–H. Roomp K. Dam M. van Yu L. Brewer C. Collins J.A. Molhuizen H.O.F. Loubser O. Ouelette B.F.F. Fichter K. Ashbourne-Excoffon K.J.D. Sensen C.W. Scherer S. Mott S. Denis M. Martindale D. Frohlich J. Morgan K. Koop B. Pimstone S. Kastelein J.J.P. Genest Jr., J. Hayden M.R. Mutations in ABC1 in Tangier disease and familial high-density lipoprotein deficiency.Nat. Genet. 1999; 22: 336-345Crossref PubMed Scopus (1481) Google Scholar). ABCA1 is capable of binding lipid-poor apolipoproteins and promotes the efflux of phospholipids and free cholesterol (8Wang N. Silver D.L. Costet P. Tall A.R. Specific binding of apoA-I, enhanced cholesterol efflux, and altered plasma membrane morphology in cells expressing ABC1.J. Biol. Chem. 2000; 275: 33053-33058Abstract Full Text Full Text PDF PubMed Scopus (496) Google Scholar). ABCA7 and ABCA4, a retina-specific protein, are the closest homologs of ABCA1. ABCA7 is a 220 kDa protein that is widely expressed in a variety of tissues, including macrophages, erythrocytes, platelets, brain, lung, adrenal gland, kidney, spleen, thymus, lymph node, testis, keratinocytes, and pancreatic islets (9Sasaki M. Shoji A. Kubo Y. Nada S. Yamaguchi A. Cloning of rat ABCA7 and its preferential expression in platelets.Biochem. Biophys. Res. Commun. 2003; 304: 777-782Crossref PubMed Scopus (31) Google Scholar, 10Kaminski W.E. Orso E. Diederich W. Klucken J. Drobnik W. Schmitz G. Identification of a novel human sterol-sensitive ATP-binding cassette transporter (ABCA7).Biochem. Biophys. Res. Commun. 2000; 273: 532-538Crossref PubMed Scopus (120) Google Scholar, 11Broccardo C. Osorio J. Luciani M.F. Schriml L.M. Prades C. Shulenin S. Arnould I. Naudin L. Lafargue C. Rosier M. Jordan B. Mattei M.G. Dean M. Denefle P. Chimini G. Comparative analysis of the promoter structure and genomic organization of the human and mouse ABCA7 gene encoding a novel ABCA transporter.Cytogenet. Cell Genet. 2001; 92: 264-270Crossref PubMed Scopus (34) Google Scholar, 12Wang N. Lan D. Gerbod-Giannone M. Linsel-Nitschke P. Jehle A.W. Chen W. Martinez L.O. Tall A.R. ATP-binding cassette transporter A7 (ABCA7) binds apolipoprotein A-I and mediates cellular phospholipid but not cholesterol efflux.J. Biol. Chem. 2003; 278: 42906-42912Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar). We have recently shown that mouse ABCA7 binds apoA-I and promotes the efflux of phosphatidylcholine and sphingomyelin to apoA-I and apoE in ABCA7-transfected cells. In contrast to our findings, Abe-Dohmae and coworkers (13Abe-Dohmae S. Ikeda Y. Matsuo M. Hayashi M. Okuhira K. Ueda K. Yokoyama S. Human ABCA7 supports apolipoprotein-mediated release of cellular cholesterol and phospholipid to generate high density lipoprotein.J. Biol. Chem. 2004; 279: 604-611Abstract Full Text Full Text PDF PubMed Scopus (141) Google Scholar) reported increased efflux of both cholesterol and phospholipids to apoA-I and apoA-II after overexpressing the human ABCA7 protein and suggested that there might be a species difference in lipid efflux specificity. The in vivo role of ABCA7 remains poorly understood. Although expressed in macrophages, it is not known if ABCA7 contributes to lipid efflux in these cells. The goals of the present study were to compare the lipid efflux specificity of human versus mouse ABCA7 and to assess a potential role of ABCA7 in lipid efflux in macrophages and other cell types. Full-length cDNA for mouse ABCA7 and mouse ABCA1 was cloned as described previously (12Wang N. Lan D. Gerbod-Giannone M. Linsel-Nitschke P. Jehle A.W. Chen W. Martinez L.O. Tall A.R. ATP-binding cassette transporter A7 (ABCA7) binds apolipoprotein A-I and mediates cellular phospholipid but not cholesterol efflux.J. Biol. Chem. 2003; 278: 42906-42912Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar). Full-length cDNA for human ABCA7, transcript variant 1, was obtained by reverse transcription PCR amplification of the 5′ end of human ABCA7 from human RNA isolated from Jurkat cells and linking of the PCR product to the 5′ end of a human expressed sequence tag clone (gi 21847676; Image-clone 6280653). Both full-length cDNAs were introduced into the pcDNA3.1Hygro+ vector (Invitrogen). The sequences of both inserts were confirmed by sequencing. HEK293 cells were obtained from the ATCC and maintained in Dulbecco's modified Eagle's medium (Gibco) supplemented with 10% (v/v) heat-inactivated fetal calf serum (Invitrogen) under a humidified atmosphere of 5% carbon dioxide and 95% air at 37°C. For transient transfection of HEK293 cells, cells on 12- or 24-well collagen-coated plates were transfected with various plasmid constructs at the indicated DNA concentrations with Lipofectamine 2000 (Invitrogen) at 37°C overnight (∼20 h). To estimate transfection efficiency, a construct expressing green fluorescent protein was routinely used in the experiment to visually monitor for transfection efficiency. The transfection efficiency of HEK293 cells was in the range of 50–80% of cells. Although transfection efficiency did vary from experiment to experiment, we found that the variation within the same experiment was small. The assays were carried out as described (8Wang N. Silver D.L. Costet P. Tall A.R. Specific binding of apoA-I, enhanced cholesterol efflux, and altered plasma membrane morphology in cells expressing ABC1.J. Biol. Chem. 2000; 275: 33053-33058Abstract Full Text Full Text PDF PubMed Scopus (496) Google Scholar). Generally, HEK293 cells were labeled by culturing for 24 h in 10% FBS/Dulbecco's modified Eagle's medium containing either 2 μCi/ml [3H]cholesterol for cholesterol efflux or 2 μCi/ml [3H]choline for phospholipid efflux. The next day, cells were washed with fresh medium before or after treatment as indicated and then apoA-I was added as acceptor and incubated for the indicated period before the medium and cells were collected for analysis. Phospholipid and cholesterol efflux were expressed as the percentage of the radioactivity released from the cells into the medium relative to the total radioactivity in cells plus medium. For cholesterol mass efflux, the collected media were extracted with hexane-isopropanol (3:2, v/v) with β-sitosterol (5 μg per sample) added as the internal standard. The recovered lipid fractions were dried under nitrogen gas, 100 μl of chloroform was added, and the samples were subjected to gas-liquid chromatographic analysis. For immunoblot analysis of mouse ABCA7, ABCA1-Flag, human ABCA7-Flag, or mouse ABCA7-Flag, the transfected HEK293 cells were lysed in radioimmune precipitation assay buffer [10 mM Tris-HCl, pH 7.3, 1 mM MgCl2, 1.0% Nonidet P-40, 0.5% sodium deoxycholate, and 5 mM EDTA in the presence of protease inhibitors (0.5 μg/ml leupeptin, 1 μg/ml aprotinin, and 1 μg/ml pepstatin A); Roche Applied Science]. Postnuclear supernatants containing the indicated amounts of protein were subjected to Western analysis. Rabbit polyclonal antibody raised against the C terminus of mouse ABCA7 has been described previously (12Wang N. Lan D. Gerbod-Giannone M. Linsel-Nitschke P. Jehle A.W. Chen W. Martinez L.O. Tall A.R. ATP-binding cassette transporter A7 (ABCA7) binds apolipoprotein A-I and mediates cellular phospholipid but not cholesterol efflux.J. Biol. Chem. 2003; 278: 42906-42912Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar). M2 anti-Flag mouse monoclonal antibody and mouse monoclonal anti-β-actin antibody were purchased from Sigma. Bands were detected by chemiluminescence, and the relative intensities were determined by densitometry (ImageQuaNT 2.2; Amersham Biosciences). Small interfering RNA (siRNA) oligonucleotides derived from the mouse ABCA7 sequence were obtained from and used to ABCA7 expression in mouse peritoneal The sequence was by the from and for ABCA7. RNA oligonucleotides were obtained from were transfected with the (Invitrogen) at a RNA of of the cells was 24 h after transfection by the lipid efflux assay 24 h ABCA1-knockout were by O. and macrophages isolated from and were used for the ABCA7 were by G. of the resulting in the of was confirmed by and PCR and be in a were with for and then incubated with in for 2 with cells were incubated with antibody in and in at for was used as a For mouse kidney were with antibody was used at a of To compare the lipid efflux specificity of human ABCA7, mouse ABCA7, and mouse we a of both phospholipid and cholesterol efflux from cells transfected with the cDNA caused a in phospholipid efflux to apoA-I. amounts of apoA-I, human and mouse ABCA7 a in phospholipid efflux. HEK293 cells did not show efflux of phospholipids or cholesterol and no detectable endogenous expression of ABCA7 by Western an antibody against human ABCA7 not are with (8Wang N. Silver D.L. Costet P. Tall A.R. Specific binding of apoA-I, enhanced cholesterol efflux, and altered plasma membrane morphology in cells expressing ABC1.J. Biol. Chem. 2000; 275: 33053-33058Abstract Full Text Full Text PDF PubMed Scopus (496) Google Scholar, 12Wang N. Lan D. Gerbod-Giannone M. Linsel-Nitschke P. Jehle A.W. Chen W. Martinez L.O. Tall A.R. ATP-binding cassette transporter A7 (ABCA7) binds apolipoprotein A-I and mediates cellular phospholipid but not cholesterol efflux.J. Biol. Chem. 2003; 278: 42906-42912Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar). The of the cholesterol efflux assays carried out in were Although ABCA1 showed a of cholesterol efflux to apoA-I, human and mouse ABCA7 showed a in efflux at the of apoA-I This was in cells not and cholesterol efflux at high concentrations of apoA-I. In to the cholesterol efflux we determined mass efflux of cholesterol by The mass efflux thus obtained confirmed the obtained in the with the cells mass efflux of cholesterol to apoA-I. Abe-Dohmae and (13Abe-Dohmae S. Ikeda Y. Matsuo M. Hayashi M. Okuhira K. Ueda K. Yokoyama S. Human ABCA7 supports apolipoprotein-mediated release of cellular cholesterol and phospholipid to generate high density lipoprotein.J. Biol. Chem. 2004; 279: 604-611Abstract Full Text Full Text PDF PubMed Scopus (141) Google Scholar) reported cholesterol efflux by ABCA7 after and 24 h of with apoA-I. In our previously we and could not cholesterol efflux (12Wang N. Lan D. Gerbod-Giannone M. Linsel-Nitschke P. Jehle A.W. Chen W. Martinez L.O. Tall A.R. ATP-binding cassette transporter A7 (ABCA7) binds apolipoprotein A-I and mediates cellular phospholipid but not cholesterol efflux.J. Biol. Chem. 2003; 278: 42906-42912Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar). To this difference in we carried out a of and cholesterol efflux from to 24 h In contrast to the in cholesterol efflux to apoA-I by no efflux of cholesterol to apoA-I was for either human or mouse ABCA7 transfected cells at For there was a in cholesterol radioactivity in the medium at 24 but for human and mouse ABCA7 this was not increased by the of apoA-I. of cholesterol radioactivity in the medium. human mouse ABCA7 promoted cholesterol efflux to apoA-I. The studies ABCA7 and ABCA1 for protein expression levels (3Francis G.A. Knopp R.H. Oram J.F. Defective removal of cellular cholesterol and phospholipids by apolipoprotein A-I in Tangier disease.J. Clin. Invest. 1995; 96: 78-87Crossref PubMed Scopus (369) Google Scholar, 12Wang N. Lan D. Gerbod-Giannone M. Linsel-Nitschke P. Jehle A.W. Chen W. Martinez L.O. Tall A.R. ATP-binding cassette transporter A7 (ABCA7) binds apolipoprotein A-I and mediates cellular phospholipid but not cholesterol efflux.J. Biol. Chem. 2003; 278: 42906-42912Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar, S. Ikeda Y. Matsuo M. Hayashi M. Okuhira K. Ueda K. Yokoyama S. Human ABCA7 supports apolipoprotein-mediated release of cellular cholesterol and phospholipid to generate high density lipoprotein.J. Biol. Chem. 2004; 279: 604-611Abstract Full Text Full Text PDF PubMed Scopus (141) Google Scholar). we that ABCA7 protein was expressed at levels ABCA1 amounts of plasmid were the of properties to high levels of Cell surface protein of both was by assay and that the plasma membrane concentrations of both ABCA1 and ABCA7 were to total protein concentrations not a plasmid DNA of for mouse and for mouse ABCA1-Flag, expression levels for both were by Western immunoblot anti-Flag antibody When the phospholipid efflux at these plasmid DNA concentrations was to protein expression the phospholipid efflux was for both ABCA1 and ABCA7 show that at protein both ABCA7 and ABCA1 are capable of phospholipid efflux. To the role of phospholipid efflux in macrophages, we next carried out genetic knockdown of ABCA7 in mouse peritoneal a construct to mouse ABCA7, we an of protein expression of with the construct used as a This knockdown was in the in ABCA7 protein there was no effect on either phospholipid or cholesterol efflux to apoA-I. We have recently obtained with a of of the mouse ABCA7 gene and confirmed of the vector by not showed and be in a no have been obtained to we to to were with macrophages from in efflux and protein expression macrophages from showed an of in ABCA7 protein levels with the with the obtained by there was no in phospholipid or cholesterol efflux to apoA-I in the ABCA7 macrophages with ABCA7 macrophages a to a potential role of ABCA7 in apolipoprotein-mediated lipid efflux in macrophages, we cells to if there was residual lipid efflux that could be to ABCA7. We have reported previously that no residual cholesterol efflux to apoA-I is detectable in peritoneal macrophages from N. Lan D. Chen W. Tall A.R. ATP-binding cassette and cellular cholesterol efflux to high-density 2004; PubMed Scopus Google Scholar) residual phospholipid efflux could be detected in macrophages, that ABCA7 does not contribute to lipid efflux in resting To ABCA7 does not contribute to lipid efflux to apoA-I, we next the tissue and localization of ABCA7 in mouse peritoneal macrophages and other previously described (12Wang N. Lan D. Gerbod-Giannone M. Linsel-Nitschke P. Jehle A.W. Chen W. Martinez L.O. Tall A.R. ATP-binding cassette transporter A7 (ABCA7) binds apolipoprotein A-I and mediates cellular phospholipid but not cholesterol efflux.J. Biol. Chem. 2003; 278: 42906-42912Abstract Full Text Full Text PDF PubMed Scopus (157) Google ABCA7 be detected on the plasma membrane by immunofluorescence microscopy in HEK293 cells In we mouse peritoneal macrophages under the same we could not an ABCA7 at the plasma membrane and found the protein to be predominantly in the intracellular microscopy of mouse ABCA7 HEK293 cells and mouse peritoneal macrophages antibody was used as a of mouse kidney antibody or serum proximal is in both of the brush border membrane with the ABCA7 To other tissues or cell in ABCA7 might with apoA-I, we carried out of various tissues expressing ABCA7. variety of tissues were found to ABCA7 protein, including kidney, thymus, spleen, lymph node, brain, lung, and pancreatic lipid-poor apoA-I is present in the glomerular G.A. D. of of A-I in the Biol. Chem. Full Text PDF PubMed Google we considered the that ABCA7 might be present in the Strikingly, in the of the proximal tubule, ABCA7 was to the apical brush border the serum did not This that ABCA7 be at the cell surface in cell under and might phospholipid efflux to apoA-I at these In an to assess a lipid efflux role of ABCA7 in the proximal tubule, of cells were However, ABCA7 expression was reduced in the this was not We previously reported that mouse ABCA7 promotes phospholipid but not cholesterol efflux to apoA-I in transfected cells, with the of the transporter ABCA1 to promote the efflux of both cholesterol and phospholipids. We show that this lipid efflux specificity is human and mouse ABCA7 and be under a variety of and Although to a species difference mouse and human ABCA7 (13Abe-Dohmae S. Ikeda Y. Matsuo M. Hayashi M. Okuhira K. Ueda K. Yokoyama S. Human ABCA7 supports apolipoprotein-mediated release of cellular cholesterol and phospholipid to generate high density lipoprotein.J. Biol. Chem. 2004; 279: 604-611Abstract Full Text Full Text PDF PubMed Scopus (141) Google our that ABCA7 may as a lipid transporter with efflux specificity for phospholipids However, the in vivo role of ABCA7 remains and our that ABCA7 is to contribute to lipid efflux in macrophages, it could this role in tissue such as the The for the our and of Abe-Dohmae (13Abe-Dohmae S. Ikeda Y. Matsuo M. Hayashi M. Okuhira K. Ueda K. Yokoyama S. Human ABCA7 supports apolipoprotein-mediated release of cellular cholesterol and phospholipid to generate high density lipoprotein.J. Biol. Chem. 2004; 279: 604-611Abstract Full Text Full Text PDF PubMed Scopus (141) Google Scholar) are not the reported by this a preferential efflux of phospholipids with cholesterol in transfected cells. the of efflux was for ABCA1 and for ABCA7 1 in In these of cells with expression by cell analysis and found efflux of both cholesterol and phospholipids to apoA-I (13Abe-Dohmae S. Ikeda Y. Matsuo M. Hayashi M. Okuhira K. Ueda K. Yokoyama S. Human ABCA7 supports apolipoprotein-mediated release of cellular cholesterol and phospholipid to generate high density lipoprotein.J. Biol. Chem. 2004; 279: 604-611Abstract Full Text Full Text PDF PubMed Scopus (141) Google Scholar). is that the are to cell or high expression analysis used assays of cholesterol and phospholipid we used or mass cholesterol efflux by in our This not be a of efflux to lipid-poor apolipoproteins mass efflux of Although our studies that ABCA7 has a role as a phospholipid transporter and an with lipid-poor the in vivo of ABCA7 knockdown of ABCA7 in mouse peritoneal macrophages by to show effect on phospholipid or cholesterol to apoA-I. The knockdown reduced ABCA7 protein levels by it is that the expression was to lipid efflux. However, we found no cholesterol or phospholipid efflux to apoA-I in macrophages from inconsistent with a residual role of ABCA7. it remains that the of ABCA7 ABCA1 and that ABCA7 is present in excess in However, cell transfection in HEK293 cells in the of ABCA1 in lipid efflux this ABCA7 is to a role in lipid efflux, at in the resting The of an effect on lipid efflux could be by the of ABCA7 from the plasma of peritoneal macrophages, that no with apoA-I is is that ABCA7 has a cellular to membrane phospholipid This is detected in cellular as a of the of lipid-poor apolipoproteins to the cells. However, under the contact ABCA7 and apoA-I may be and to cellular or In the of macrophages or other cells that ABCA7, the transporter could be to the plasma membrane under such as cell or and could then with apoA-I. expression of ABCA7 has been detected in (12Wang N. Lan D. Gerbod-Giannone M. Linsel-Nitschke P. Jehle A.W. Chen W. Martinez L.O. Tall A.R. ATP-binding cassette transporter A7 (ABCA7) binds apolipoprotein A-I and mediates cellular phospholipid but not cholesterol efflux.J. Biol. Chem. 2003; 278: 42906-42912Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar) and (9Sasaki M. Shoji A. Kubo Y. Nada S. Yamaguchi A. Cloning of rat ABCA7 and its preferential expression in platelets.Biochem. Biophys. Res. Commun. 2003; 304: 777-782Crossref PubMed Scopus (31) Google Scholar) and both of cells a ABCA7 could come into contact with apoA-I and phospholipid efflux in these cell types. by we identified the apical brush border membrane in the proximal of the kidney as location ABCA7 is at the cell studies have shown that apoA-I be at the and in the glomerular G.A. D. of of A-I in the Biol. Chem. Full Text PDF PubMed Google that a ABCA7 and apoA-I in the of the proximal Moreover, the kidney a major of apoA-I the cellular and are poorly understood. variety of including and ABCA7 are expressed in the kidney (12Wang N. Lan D. Gerbod-Giannone M. Linsel-Nitschke P. Jehle A.W. Chen W. Martinez L.O. Tall A.R. ATP-binding cassette transporter A7 (ABCA7) binds apolipoprotein A-I and mediates cellular phospholipid but not cholesterol efflux.J. Biol. Chem. 2003; 278: 42906-42912Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar, Y. K. S. of ABCA1 in lipid and kidney as as high-density lipoprotein cholesterol J. 2000; Full Text Full Text PDF PubMed Scopus Google Scholar, J. Buchler C. Orso E. Kaminski W.E. Porsch-Ozcurumez M. G. M. Diederich W. Drobnik W. Dean M. Schmitz G. the human of the is a of cholesterol and phospholipid 2000; PubMed Scopus Google suggesting that there may be major of HDL and apolipoproteins by these in the The H. B. and J. for