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ATP-Binding Cassette Transporter A1 Modulates Apolipoprotein A-I Transcytosis Through Aortic Endothelial Cells

From the Institute of Clinical Chemistry, University Hospital of Zurich and Center for Integrative Human Physiology, University of Zurich, Switzerland.

Correspondence to Arnold von Eckardstein, Institute of Clinical Chemistry, University Hospital of Zurich and Center for Integrative Human Biology, University of Zurich, Raemistrasse 100, CH-8091 Zurich, Switzerland. E-mail arnold.voneckardstein{at}usz.ch

High-density lipoproteins and their major protein constituent apolipoprotein A-I (apoA-I) possess diverse atheroprotective properties. Most of them must be exerted within the arterial wall. Actually, high-density lipoproteins are the most abundant lipoproteins within the arterial intima. We have recently reported that apoA-I is transcytosed through aortic endothelial cells. In the present study, we evaluate the role of ATP-binding cassette transporter A1 (ABCA1) and scavenger receptor BI (SR-BI) in this process. Using pharmacological interventions and RNA interference, we investigated whether ABCA1 and SR-BI modulate apoA-I binding, internalization and transcytosis in endothelial cells. Upregulation of ABCA1 with oxysterols increased apoA-I binding and internalization. Trapping ABCA1 on the cell surface with cyclosporin A enhanced apoA-I binding but decreased its internalization and transcytosis. In addition, apoA-I binding, internalization, and transcytosis were reduced by at least 50% after silencing ABCA1 but not after knocking down SR-BI. The integrity of the endothelial cell monolayer was affected neither by cyclosporin A treatment nor by ABCA1 silencing, as controlled by measuring inulin permeability. Finally, in ABCA1-GFP–expressing cells, fluorescently labeled apoA-I colocalized intracellularly with ABCA1-GFP. However, apoA-I–containing vesicles did not colocalize with the late endosome marker LAMP-1 (lysosome-associated membrane protein-1). In conclusion, ABCA1, but not SR-BI, modulates the transcytosis of apoA-I through endothelial cells.

Key Words: high-density lipoproteins • endothelium • apolipoprotein A-I • transcytosis • ABCA1

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