An analysis of the role of a retroendocytosis pathway in ATP-binding cassette transporter (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, and W. Sean Davidson

Pathology and Laboratory Medicine, University of Cincinnati, Cincinnati, OH 45237-0507

Corresponding Author: Sean.Davidson{at}uc.edu

The ATP-binding cassette transporter A1 (ABCA1)1 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 which reappeared in the media was degraded. We estimate that the mass of apoA-I retroendocytosed is not sufficient to account for the high-density lipoprotein 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. Based on 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.

Add to CiteULike CiteULike Add to Complore Complore Add to Connotea Connotea Add to Del.icio.us Del.icio.us Add to Digg Digg Add to Reddit Reddit Add to Technorati Technorati What's this?

This article has been cited by other articles:

A. Mulya, J.-Y. Lee, A. K. Gebre, E. Y. Boudyguina, S.-K. Chung, T. L. Smith, P. L. Colvin, X.-C. Jiang, and J. S. Parks
Initial interaction of apoA-I with ABCA1 impacts in vivo metabolic fate of nascent HDL
J. Lipid Res., November 1, 2008; 49(11): 2390 - 2401.
[Abstract] [Full Text] [PDF]

R. Lu, R. Arakawa, C. Ito-Osumi, N. Iwamoto, and S. Yokoyama
ApoA-I Facilitates ABCA1 Recycle/Accumulation to Cell Surface by Inhibiting Its Intracellular Degradation and Increases HDL Generation
Arterioscler. Thromb. Vasc. Biol., October 1, 2008; 28(10): 1820 - 1824.
[Abstract] [Full Text] [PDF]