Apolipoprotein A-I-stimulated apolipoprotein E secretion from human macrophages is independent of cholesterol efflux.

Academic Article

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.
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    Keywords

  • ATP Binding Cassette Transporter 1, ATP-Binding Cassette Transporters, Apolipoprotein A-I, Apolipoproteins E, Biotinylation, Blotting, Western, Centrifugation, Density Gradient, Cholesterol, Electrophoresis, Polyacrylamide Gel, Enzyme-Linked Immunosorbent Assay, Exons, Humans, Leukocytes, Mononuclear, Lipid Metabolism, Lipoproteins, HDL, Macrophages, Monocytes, Mutation, Peptides, Phospholipids, Protein Structure, Tertiary, RNA Processing, Post-Transcriptional, Recombinant Proteins, Reverse Transcriptase Polymerase Chain Reaction, Sucrose, Tangier Disease, Time Factors, Ultracentrifugation
  • Digital Object Identifier (doi)

    Author List

  • Kockx M; Rye K-A; Gaus K; Quinn CM; Wright J; Sloane T; Sviridov D; Fu Y; Sullivan D; Burnett JR
  • Start Page

  • 25966
  • End Page

  • 25977
  • Volume

  • 279
  • Issue

  • 25