The influence of apolipoprotein structure on the efflux of cellular free cholesterol to high density lipoprotein

Academic Article

Abstract

  • The influence of apolipoprotein conformation on the ability of high density lipoprotein (HDL) to remove cellular free cholesterol (FC) has not been studied in detail. To address the effects of amphipathic α-helix structure on cellular FC efflux, three class A helical peptides and apolipoprotein (apo) AI were complexed to dimyristoyl phosphatidylcholine (DMPC) to make discoidal complexes that were used as acceptors of cell cholesterol. The peptides consisted of an 18-amino acid, amphipathic, α- helical peptide with the sequence DWLKAFYDKVAEKLKEAF (18A), a dimer of 18A covalently linked by a proline residue (37pA), and acetyl-18A-amide (Ac-18A- NH2) that has a higher α-helix content than the unblocked 18A molecule. The three peptides strongly mimic the lipid-binding characteristics of the amphipathic segments of apolipoproteins and form discoidal complexes with DMPC that are similar in diameter (11-12 nm) to those formed by human apoAI when reconstituted at a 2.5:1 (w:w) phospholipid to protein ratio. The abilities of these complexes to remove radiolabeled FC were compared in experiments using cultured mouse L-cell fibroblasts; efflux of FC from both the plasma membrane and the lysosomal pools was examined. For each of the acceptors, the removal of cholesterol from the plasma membrane and lysosomal pools was equally efficient. All four discoidal complexes were equally efficient cell membrane FC acceptors when compared at saturating acceptor concentrations of >200 μg of DMPC/ml of medium. However, at the same lipid concentration, protein-free DMPC small unilamellar vesicles (SUV) were significantly less efficient. The initial rates of FC removal from cells at saturating concentrations of acceptor particles (V(max)) were 12, 10, 10, and 11% per h, respectively, for the complexes containing either 18A, Ac-18A- NH2, 37pA, or apoAI, but only 1% cellular FC per h for the DMPC SUV. The 10- fold higher V(max) for the apoprotein/peptide-containing acceptors was likely due to a reversible interaction of apoprotein or peptide with the plasma membrane that changed the lipid packing characteristics in such a way as to increase the rate of FC desorption from the cell surface. This interaction required amphipathic α-helical segments, but it was not affected by the length, number, or lipid-binding affinity of the helices. Furthermore, the efflux efficiency was not dependent on the amino acid sequence of the helical segments which suggests that this interaction is not mediated by a specific cell surface binding site. At lower acceptor concentrations of <100 μg of DMPC/ml of medium, differences in efflux efficiency were seen between the peptide-containing particles as a group and the apoAI-containing particle demonstrating that α-helix structure can affect the ability of acceptor particles to either sequester or retain cholesterol molecules.
  • Author List

  • Davidson WS; Lund-Katz S; Johnson WJ; Anantharamaiah GM; Palgunachari MN; Segrest JP; Rothblat GH; Phillips MC
  • Start Page

  • 22975
  • End Page

  • 22982
  • Volume

  • 269
  • Issue

  • 37