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Alan R Tall – One of the best experts on this subject based on the ideXlab platform.

  • Abstract 523: Regulation of Pancreatic β-cell Gene Expression and Function by ABCA1 and ABCG1
    Arteriosclerosis Thrombosis and Vascular Biology, 2015
    Co-Authors: Liming Hou, Alan R Tall, Marit Westerterp, Fatiha Tabet, Blake J. Cochran, Philip J. Barter, Kerry-anne Rye

    Abstract:

    Background: Loss-of-function mutations in the ABCA1 gene cause beta cell dysfunction in humans. Insulin secretion is impaired and glucose metabolism is adversely affected in mice with conditional deletion of ABCA1 in beta cells and in ABCG1 knockout mice. These effects are exacerbated in mice with conditional beta cell ABCA1 deletion plus global ABCG1 deficiency. To identify key beta cell pathways regulated by ABCA1 and ABCG1, we generated mice with beta cell-specific deletion of ABCA1 and ABCG1 (ABCA1β-cell-/-/ABCG1βcell-/- DKO mice). Aim: Identification of genes in key beta cell metabolic and signal transduction pathways that are regulated by ABCA1 and ABCG1. Methods: Islets were isolated from 16 week old chow fed ABCA1β-cell-/-/ABCG1β-cell-/- DKO mice. mRNA was extracted and subjected to Affymetrix GeneChip Mouse Gene ST 2.0 analysis, followed by Partek Genomics Suite and Ingeunity Pathway analysis. Islet cholesterol levels were determined by HPLC. Plasma and HDL cholesterol levels were measured enzyma…

  • deletion of abca1 and ABCG1 impairs macrophage migration because of increased rac1 signaling
    Circulation Research, 2011
    Co-Authors: Tamara A Pagler, Mi Wang, Mousumi Mondal, Andrew J. Murphy, Marit Westerterp, Kathryn J Moore, Frederick R Maxfield, Alan R Tall

    Abstract:

    RATIONALE: Reduced plasma cholesterol and increased high-density lipoprotein (HDL) levels promote regression of atherosclerosis, in a process characterized by lipid unloading and emigration of macrophages from lesions. In contrast free cholesterol loading of macrophages leads to imbalanced Rac1/Rho activities and impaired chemotaxis. OBJECTIVE: To study the role of HDL and the ATP-binding cassette transporters ABCA1 and ABCG1 in modulating the chemotaxis of macrophages. METHODS AND RESULTS: Abca1(-/-)ABCG1(-/-) mouse macrophages displayed profoundly impaired chemotaxis both in a Transwell chamber assay and in the peritoneal cavity of wild-type (WT) mice. HDL reversed impaired chemotaxis in free cholesterol-loaded WT macrophages but was without effect in Abca1(-/-)ABCG1(-/-) cells, whereas cyclodextrin was effective in both. Abca1(-/-)ABCG1(-/-) macrophages had markedly increased Rac1 activity and increased association of Rac1 with the plasma membrane (PM). Their defective chemotaxis was reversed by a Rac1 inhibitor. To gain a better understanding of the role of transporters in PM cholesterol movement, we measured transbilayer PM sterol distribution. In WT macrophages, the majority of cholesterol was located on the inner leaflet, whereas on upregulation of transporters by liver X receptor activation, PM sterol was shifted to the outer leaflet, where it could be removed by HDL. Abca1(-/-)ABCG1(-/-) macrophages showed increased PM sterol content and defective redistribution of sterol to the outer leaflet. CONCLUSIONS: Deletion of ABCA1 and ABCG1 causes an increased cholesterol content on the inner leaflet of the PM, associated with increased Rac1 PM localization, activation, and impairment of migration. ABCA1 and ABCG1 facilitate macrophage chemotaxis by promoting PM transbilayer cholesterol movement and may contribute to the ability of HDL to promote regression of atherosclerosis.

  • abca1 and ABCG1 protect against oxidative stress induced macrophage apoptosis during efferocytosis
    Circulation Research, 2010
    Co-Authors: Laurent Yvancharvet, Carrie L Welch, Tamara A Pagler, Tracie A Seimon, Edward B Thorp, Joseph L Witztum, Ira Tabas, Alan R Tall

    Abstract:

    Rationale: Antiatherogenic effects of plasma high-density lipoprotein (HDL) include the ability to inhibit apoptosis of macrophage foam cells. The ATP-binding cassette transporters ABCA1 and ABCG1 have a major role in promoting cholesterol efflux from macrophages to apolipoprotein A-1 and HDL and are upregulated during the phagocytosis of apoptotic cells (efferocytosis).

    Objective: The goal of this study was to determine the roles of ABCA1 and ABCG1 in preserving the viability of macrophages during efferocytosis.

    Methods and Results: We show that despite similar clearance of apoptotic cells, peritoneal macrophages from Abca1 −/− ABCG1 −/−, ABCG1 −/−, and, to a lesser extent, Abca1 −/− mice are much more prone to apoptosis during efferocytosis compared to wild-type cells. Similar findings were observed following incubations with oxidized phospholipids, and the ability of HDL to protect against oxidized phospholipid-induced apoptosis was markedly reduced in Abca1 −/− ABCG1 −/− and ABCG1 −/− cells. These effects were independent of any role of ABCA1 and ABCG1 in mediating oxidized phospholipid efflux but were reversed by cyclodextrin-mediated cholesterol efflux. The apoptotic response observed in Abca1 −/− ABCG1 −/− macrophages after oxidized phospholipid exposure or engulfment of apoptotic cells was dependent on an excessive oxidative burst secondary to enhanced assembly of NADPH oxidase (NOX)2 complexes, leading to sustained Jnk activation which turned on the apoptotic cell death program. Increased NOX2 assembly required Toll-like receptors 2/4 and MyD88 signaling, which are known to be enhanced in transporter deficient cells in a lipid raft–dependent fashion.

    Conclusions: We identified a new beneficial role of ABCA1, ABCG1 and HDL in dampening the oxidative burst and preserving viability of macrophages following exposure to oxidized phospholipids and/or apoptotic cells.

Peter A. Edwards – One of the best experts on this subject based on the ideXlab platform.

  • dancing with the sterols critical roles for ABCG1 abca1 mirnas and nuclear and cell surface receptors in controlling cellular sterol homeostasis
    Biochimica et Biophysica Acta, 2012
    Co-Authors: Elizabeth J Tarling, Peter A. Edwards

    Abstract:

    Abstract ATP binding cassette (ABC) transporters represent a large and diverse family of proteins that transport specific substrates across a membrane. The importance of these transporters is illustrated by the finding that inactivating mutations within 17 different family members are known to lead to specific human diseases. Clinical data from humans and/or studies with mice lacking functional transporters indicate that ABCA1, ABCG1, ABCG4, ABCG5 and ABCG8 are involved in cholesterol and/or phospholipid transport. This review discusses the multiple mechanisms that control cellular sterol homeostasis, including the roles of microRNAs, nuclear and cell surface receptors and ABC transporters, with particular emphasis on recent findings that have provided insights into the role(s) of ABCG1. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945–2010).

  • ATP binding cassette transporter G1 (ABCG1) is an intracellular sterol transporter
    Proceedings of the National Academy of Sciences of the United States of America, 2011
    Co-Authors: Elizabeth J Tarling, Peter A. Edwards

    Abstract:

    Four members of the mammalian ATP binding cassette (ABC) transporter G subfamily are thought to be involved in transmembrane (TM) transport of sterols. The residues responsible for this transport are unknown. The mechanism of action of ABCG1 is controversial and it has been proposed to act at the plasma membrane to facilitate the efflux of cellular sterols to exogenous high-density lipoprotein (HDL). Here we show that ABCG1 function is dependent on localization to intracellular endosomes. Importantly, localization to the endosome pathway distinguishes ABCG1 and/or ABCG4 from all other mammalian members of this superfamily, including other sterol transporters. We have identified critical residues within the TM domains of ABCG1 that are both essential for sterol transport and conserved in some other members of the ABCG subfamily and/or the insulin-induced gene 2 (INSIG-2). Our conclusions are based on studies in which (i) biotinylation of peritoneal macrophages showed that endogenous ABCG1 is intracellular and undetectable at the cell surface, (ii) a chimeric protein containing the TM of ABCG1 and the cytoplasmic domains of the nonsterol transporter ABCG2 is both targeted to endosomes and functional, and (iii) ABCG1 colocalizes with multiple proteins that mark late endosomes and recycling endosomes. Mutagenesis studies identify critical residues in the TM domains that are important for ABCG1 to alter sterol efflux, induce sterol regulatory element binding protein-2 (SREBP-2) processing, and selectively attenuate the oxysterol-mediated repression of SREBP-2 processing. Our data demonstrate that ABCG1 is an intracellular sterol transporter that localizes to endocytic vesicles to facilitate the redistribution of specific intracellular sterols away from the endoplasmic reticulum (ER).

  • Differential expression and function of ABCG1 and ABCG4 during development and aging
    Journal of lipid research, 2009
    Co-Authors: Dragana D. Bojanic, Paul T. Tarr, Greg D. Gale, Desmond J. Smith, Dean Bok, Bryan Chen, Steven Nusinowitz, Anita Lövgren-sandblom, Ingemar Björkhem, Peter A. Edwards

    Abstract:

    ABCG1 and ABCG4 are highly homologous members of the ATP binding cassette (ABC) transporter family that regulate cellular cholesterol homeostasis. In adult mice, ABCG1 is known to be expressed in numerous cell types and tissues, whereas ABCG4 expression is limited to the central nervous system (CNS). Here, we show significant differences in expression of these two transporters during development. Examination of beta-galactosidase-stained tissue sections from ABCG1(-/-)LacZ and Abcg4(-/-)LacZ knockin mice shows that ABCG4 is highly but transiently expressed both in hematopoietic cells and in enterocytes during development. In contrast, ABCG1 is expressed in macrophages and in endothelial cells of both embryonic and adult liver. We also show that ABCG1 and ABCG4 are both expressed as early as E12.5 in the embryonic eye and developing CNS. Loss of both ABCG1 and ABCG4 results in accumulation in the retina and/or brain of oxysterols, in altered expression of liver X receptor and sterol-regulatory element binding protein-2 target genes, and in a stress response gene. Finally, behavioral tests show that Abcg4(-/-) mice have a general deficit in associative fear memory. Together, these data indicate that loss of ABCG1 and/or ABCG4 from the CNS results in changes in metabolic pathways and in behavior.

Lucia Rohrer – One of the best experts on this subject based on the ideXlab platform.

  • lipidation of apolipoprotein a i by atp binding cassette transporter abc a1 generates an interaction partner for ABCG1 but not for scavenger receptor bi
    Biochimica et Biophysica Acta, 2008
    Co-Authors: Iris Lorenzi, Arnold Von Eckardstein, Silvija Radosavljevic, Lucia Rohrer

    Abstract:

    Abstract The ATP-binding cassette transporters ABCA1 and ABCG1 as well as scavenger receptor BI (SR-BI) mediate the efflux of lipids from macrophages to apolipoprotein A-I (apoA-I) and high density lipoproteins (HDL). We used RNA interference in RAW264.7 macrophages to study the interactions of ABCA1, ABCG1, and SR-BI with lipid-free apoA-I, native and reconstituted HDL with apoA-I:phosphatidylcholine ratios of either 1:40 (rHDL1:40) or 1:100 (rHDL1:100). Knock-down of ABCA1 inhibits the cellular binding at 4 °C of lipid-free apoA-I but not of HDL whereas suppression of ABCG1 or SR-BI reduces the binding of HDL but not lipid-free apoA-I. The degree of lipidation influences the interactions of rHDL with ABCG1 and SR-BI. Knock-down of ABCG1 inhibits more effectively the binding and cholesterol efflux capacities of lipid-poorer rHDL1:40 whereas knock-down of SR-BI has a more profound effect on the binding and cholesterol efflux capacities of lipid-richer rHDL1:100. Moreover, knock-down of ABCG1 but not SR-BI interferes with the association of lipid-free apoA-I during prolonged incubation at 37 °C. Finally, knock-down of ABCG1 inhibits the binding of initially lipid-free apoA-I which has been preconditioned by cells with high ABCA1 activity. The gained ability of initially lipid-free apoA-I to interact with ABCG1 is accompanied by its shift from electrophoretic pre-β- to α-mobility. Taken together, these data suggest that the interaction of lipid-free apoA-I with ABCA1 generates a particle that immediately interacts with ABCG1 but not with SR-BI. Furthermore, the degree of lipidation influences the interaction of HDL with ABCG1 or SR-BI.

  • Lipidation of apolipoprotein A-I by ATP-binding cassette transporter (ABC) A1 generates an interaction partner for ABCG1 but not for scavenger receptor BI
    Biochimica et Biophysica Acta (BBA) – Molecular and Cell Biology of Lipids, 2008
    Co-Authors: Iris Lorenzi, Silvija Radosavljevic, Arnold Von Eckardstein, Lucia Rohrer

    Abstract:

    The ATP-binding cassette transporters ABCA1 and ABCG1 as well as scavenger receptor BI (SR-BI) mediate the efflux of lipids from macrophages to apolipoprotein A-I (apoA-I) and high density lipoproteins (HDL). We used RNA interference in RAW264.7 macrophages to study the interactions of ABCA1, ABCG1, and SR-BI with lipid-free apoA-I, native and reconstituted HDL with apoA-I:phosphatidylcholine ratios of either 1:40 (rHDL(1:40)) or 1:100 (rHDL(1:100)). Knock-down of ABCA1 inhibits the cellular binding at 4 degrees C of lipid-free apoA-I but not of HDL whereas suppression of ABCG1 or SR-BI reduces the binding of HDL but not lipid-free apoA-I. The degree of lipidation influences the interactions of rHDL with ABCG1 and SR-BI. Knock-down of ABCG1 inhibits more effectively the binding and cholesterol efflux capacities of lipid-poorer rHDL(1:40) whereas knock-down of SR-BI has a more profound effect on the binding and cholesterol efflux capacities of lipid-richer rHDL(1:100). Moreover, knock-down of ABCG1 but not SR-BI interferes with the association of lipid-free apoA-I during prolonged incubation at 37 degrees C. Finally, knock-down of ABCG1 inhibits the binding of initially lipid-free apoA-I which has been preconditioned by cells with high ABCA1 activity. The gained ability of initially lipid-free apoA-I to interact with ABCG1 is accompanied by its shift from electrophoretic pre-beta- to alpha-mobility. Taken together, these data suggest that the interaction of lipid-free apoA-I with ABCA1 generates a particle that immediately interacts with ABCG1 but not with SR-BI. Furthermore, the degree of lipidation influences the interaction of HDL with ABCG1 or SR-BI

  • lipid efflux by the atp binding cassette transporters abca1 and ABCG1
    Biochimica et Biophysica Acta, 2006
    Co-Authors: Clara Cavelier, Iris Lorenzi, Lucia Rohrer, Arnold Von Eckardstein

    Abstract:

    Plasma levels of high-density lipoproteins (HDL) and apolipoprotein A-I (apoA-I) are inversely correlated with the risk of cardiovascular disease. One major atheroprotective mechanism of HDL and apoA-I is their role in reverse cholesterol transport, i.e., the transport of excess cholesterol from foam cells to the liver for secretion. The ATP-binding cassette transporters ABCA1 and ABCG1 play a pivotal role in this process by effluxing lipids from foam cells to apoA-I and HDL, respectively. In the liver, ABCA1 activity is one rate-limiting step in the formation of HDL. In macrophages, ABCA1 and ABCG1 prevent the excessive accumulation of lipids and thereby protect the arteries from developing atherosclerotic lesions. However, the mechanisms by which ABCA1 and ABCG1 mediate lipid removal are still unclear. Particularly, three questions remain controversial and are discussed in this review: (1) Do apoA-I and HDL directly interact with ABCA1 and ABCG1, respectively? (2) Does cholesterol efflux involve retroendocytosis of apoA-I or HDL? (3) Which lipids are directly transported by ABCA1 and ABCG1?