An analysis of the role of a retroendocytosis pathway in ABCA1-mediated cholesterol efflux from macrophages

Loren E. Faulkner, Stacey E. Panagotopulos, Jacob D. Johnson, Laura A. Woollett, David Y. Hui, Scott R. Witting, J. Nicholas Maiorano, W. Sean Davidson

Research output: Contribution to journalArticle

51 Scopus citations

Abstract

The ATP binding cassette transporter A-1 (ABCA1) is critical for apolipoprotein-mediated cholesterol efflux, an important mechanism employed by macrophages to avoid becoming lipid-laden foam cells, the hallmark of early atherosclerotic lesions. It has been proposed that lipid-free apolipoprotein A-I (apoA-I) enters the cell and is resecreted as a lipidated particle via a retroendocytosis pathway during ABCA1-mediated cholesterol efflux from macrophages. To determine the functional importance of such a pathway, confocal microscopy was used to characterize the internalization of a fully functional apoA-I cysteine mutant containing a thiol-reactive fluorescent probe in cultured macrophages. ApoA-I was also endogenously labeled with 35S- methionine to quantify cellular uptake and to determine the metabolic fate of the internalized protein. It was found that apoA-I was specifically taken inside macrophages and that a small amount of intact apoA-I was resecreted from the cells. However, a majority of the label that reappeared in the media was degraded. We estimate that the mass of apoA-I retroendocytosed is not sufficient to account for the HDL produced by the cholesterol efflux reaction. Furthermore, we have demonstrated that lipid-free apoA-I-mediated cholesterol efflux from macrophages can be pharmacologically uncoupled from apoA-I internalization into cells. On the basis these findings, we present a model in which the ABCA1-mediated lipid transfer process occurs primarily at the membrane surface in macrophages, but still accounts for the observed specific internalization of apoA-I.

Original languageEnglish (US)
Pages (from-to)1322-1332
Number of pages11
JournalJournal of Lipid Research
Volume49
Issue number6
DOIs
StatePublished - Jun 1 2008

Keywords

  • Apolipoprotein A-I
  • ATP binding cassette transporter A-1
  • Cell biology
  • Confocal microscopy
  • Endocytosis
  • Macrophage
  • Protein expression

ASJC Scopus subject areas

  • Biochemistry
  • Cell Biology
  • Endocrinology

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