title: Apolipoprotein A-I but not high-density lipoproteins are internalised by RAW macrophages: roles of ATP-binding cassette transporter A1 and scavenger receptor BI.
abstract: Accumulation of lipid-loaded macrophages (foam cells) within the vessel wall is an early hallmark of atherosclerosis. High-density lipoproteins (HDL) and apolipoprotein A-I (apoA-I) can efficiently promote cholesterol efflux from macrophages. Therefore, the interaction of HDL and apoA-I with macrophages appears to be important in the initial steps of reverse cholesterol transport, i.e. the transport of excess cholesterol from foam cells to the liver. However, although several cellular apoA-I and HDL receptors and transporters have been identified, it is as yet controversial how these interactions lead to cholesterol efflux from foam cells. In this study, we show that RAW264.7 macrophages bind HDL and apoA-I in a compatible manner. Furthermore, cell surface biotinylation experiments revealed that apoA-I but not HDL is specifically internalised. Binding of HDL to macrophages is decreased by reducing the expression of scavenger receptor BI (SR-BI) with cyclic adenosine monophosphate (cAMP), acetylated low-density lipoprotein (acLDL) or RNA interference. In contrast, apoA-I cell association and internalisation is modulated in parallel with ATP-binding cassette transporter A1 (ABCA1) expression which is altered by stimulating cells with cAMP and acLDL or expressing short hairpin RNA (shRNA) against ABCA1. Consistent with this, cell surface trapping of ABCA1 with cyclosporin A (CsA) results in increased apoA-I binding but reduced internalisation. Furthermore, blocking apoA-I uptake inhibits cholesterol efflux to apoA-I but not to HDL. Taken together, these data suggest that apoA-I- but not HDL-mediated cholesterol efflux may involve retroendocytosis.
authors: Institute of Clinical Chemistry and Center for Integrative Human Biology, University of Zurich, University Hospital Zurich, Rämistrasse 100, 8091, Zurich, Switzerland.
authors: Iris
authors: I
authors: Lorenzi
authors: Arnold
authors: A
authors: von Eckardstein
authors: Clara
authors: C
authors: Cavelier
authors: Silvija
authors: S
authors: Radosavljevic
authors: Lucia
authors: L
authors: Rohrer
mesh_terms: D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D064286:ATP Binding Cassette Transporter 1; D018528:ATP-Binding Cassette Transporters / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D016632:Apolipoprotein A-I / Q000378:metabolism*; D050197:Atherosclerosis / Q000378:metabolism; D002460:Cell Line; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism*; D002784:Cholesterol / Q000378:metabolism*; D000242:Cyclic AMP / Q000378:metabolism; D016572:Cyclosporine / Q000494:pharmacology; D004705:Endocytosis / Q000187:drug effects; D005487:Foam Cells / Q000378:metabolism; D008075:Lipoproteins, HDL / Q000378:metabolism; D008077:Lipoproteins, LDL / Q000378:metabolism; D008264:Macrophages / Q000187:drug effects / Q000378:metabolism*; D051379:Mice; D034622:RNA Interference; D034741:RNA, Small Interfering / Q000378:metabolism; D051122:Scavenger Receptors, Class B / Q000235:genetics / Q000378:metabolism*; D014162:Transfection
chemical_list: C577836:ABCA1 protein, human; D064286:ATP Binding Cassette Transporter 1; D018528:ATP-Binding Cassette Transporters; D016632:Apolipoprotein A-I; D008075:Lipoproteins, HDL; D008077:Lipoproteins, LDL; D034741:RNA, Small Interfering; C454739:Scarb1 protein, mouse; D051122:Scavenger Receptors, Class B; C032629:acetyl-LDL; D015124:8-Bromo Cyclic Adenosine Monophosphate; D016572:Cyclosporine; D002784:Cholesterol; D000242:Cyclic AMP