title: Apolipoprotein A-I-stimulated apolipoprotein E secretion from human macrophages is independent of cholesterol efflux.
abstract: Apolipoprotein A-I (apoA-I)-mediated cholesterol efflux involves the binding of apoA-I to the plasma membrane via its C terminus and requires cellular ATP-binding cassette transporter (ABCA1) activity. ApoA-I also stimulates secretion of apolipoprotein E (apoE) from macrophage foam cells, although the mechanism of this process is not understood. In this study, we demonstrate that apoA-I stimulates secretion of apoE independently of both ABCA1-mediated cholesterol efflux and of lipid binding by its C terminus. Pulse-chase experiments using (35)S-labeled cellular apoE demonstrate that macrophage apoE exists in both relatively mobile (E(m)) and stable (E(s)) pools, that apoA-I diverts apoE from degradation to secretion, and that only a small proportion of apoA-I-mobilized apoE is derived from the cell surface. The structural requirements for induction of apoE secretion and cholesterol efflux are clearly dissociated, as C-terminal deletions in recombinant apoA-I reduce cholesterol efflux but increase apoE secretion, and deletion of central helices 5 and 6 decreases apoE secretion without perturbing cholesterol efflux. Moreover, a range of 11- and 22-mer alpha-helical peptides representing amphipathic alpha-helical segments of apoA-I stimulate apoE secretion whereas only the C-terminal alpha-helix (domains 220-241) stimulates cholesterol efflux. Other alpha-helix-containing apolipoproteins (apoA-II, apoA-IV, apoE2, apoE3, apoE4) also stimulate apoE secretion, implying a positive feedback autocrine loop for apoE secretion, although apoE4 is less effective. Finally, apoA-I stimulates apoE secretion normally from macrophages of two unrelated subjects with genetically confirmed Tangier Disease (mutations C733R and c.5220-5222delTCT; and mutations A1046D and c.4629-4630insA), despite severely inhibited cholesterol efflux. We conclude that apoA-I stimulates secretion of apoE independently of cholesterol efflux, and that this represents a novel, ABCA-1-independent, positive feedback pathway for stimulation of potentially anti-atherogenic apoE secretion by alpha-helix-containing molecules including apoA-I and apoE.
authors.affiliation: Macrophage Biology Group, Centre for Vascular Research, University of New South Wales, Sydney 2052, Australia.
authors.forename: Maaike
authors.initials: M
authors.lastname: Kockx
authors.forename: Kerry-Anne
authors.initials: KA
authors.lastname: Rye
authors.forename: Katharina
authors.initials: K
authors.lastname: Gaus
authors.forename: Carmel M
authors.initials: CM
authors.lastname: Quinn
authors.forename: Janelle
authors.initials: J
authors.lastname: Wright
authors.forename: Timothy
authors.initials: T
authors.lastname: Sloane
authors.forename: Dimitri
authors.initials: D
authors.lastname: Sviridov
authors.forename: Ying
authors.initials: Y
authors.lastname: Fu
authors.forename: David
authors.initials: D
authors.lastname: Sullivan
authors.forename: John R
authors.initials: JR
authors.lastname: Burnett
authors.forename: Stephan
authors.initials: S
authors.lastname: Rust
authors.forename: Gerd
authors.initials: G
authors.lastname: Assmann
authors.forename: G M
authors.initials: GM
authors.lastname: Anantharamaiah
authors.forename: Mayakonda N
authors.initials: MN
authors.lastname: Palgunachari
authors.forename: Sissel Lund
authors.initials: SL
authors.lastname: Katz
authors.forename: Michael C
authors.initials: MC
authors.lastname: Phillips
authors.forename: Roger T
authors.initials: RT
authors.lastname: Dean
authors.forename: Wendy
authors.initials: W
authors.lastname: Jessup
authors.forename: Leonard
authors.initials: L
authors.lastname: Kritharides
mesh_terms: D064286:ATP Binding Cassette Transporter 1; D018528:ATP-Binding Cassette Transporters / Q000378:metabolism; D016632:Apolipoprotein A-I / Q000378:metabolism*; D001057:Apolipoproteins E / Q000378:metabolism*; D019921:Biotinylation; D015153:Blotting, Western; D002499:Centrifugation, Density Gradient; D002784:Cholesterol / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D004797:Enzyme-Linked Immunosorbent Assay; D005091:Exons; D006801:Humans; D007963:Leukocytes, Mononuclear / Q000378:metabolism; D050356:Lipid Metabolism; D008075:Lipoproteins, HDL / Q000378:metabolism; D008264:Macrophages / Q000378:metabolism*; D009000:Monocytes / Q000378:metabolism; D009154:Mutation; D010455:Peptides / Q000737:chemistry; D010743:Phospholipids / Q000378:metabolism; D017434:Protein Structure, Tertiary; D012323:RNA Processing, Post-Transcriptional; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013395:Sucrose / Q000494:pharmacology; D013631:Tangier Disease / Q000235:genetics; D013997:Time Factors; D014461:Ultracentrifugation
chemical_list: C577836:ABCA1 protein, human; D064286:ATP Binding Cassette Transporter 1; D018528:ATP-Binding Cassette Transporters; D016632:Apolipoprotein A-I; D001057:Apolipoproteins E; D008075:Lipoproteins, HDL; D010455:Peptides; D010743:Phospholipids; D011994:Recombinant Proteins; D013395:Sucrose; D002784:Cholesterol