Rationale: Macrophages cannot limit the uptake of lipids and rely on cholesterol efflux mechanisms for maintaining cellular cholesterol homeostasis. Important mediators of macrophage cholesterol efflux are ATP-binding cassette transporter 1 (ABCA1), which mediates the efflux of cholesterol to lipid-poor apolipoprotein AI, and scavenger receptor class B type I (SR-BI), which promotes efflux to mature high-density lipoprotein.
Objective:The aim of the present study was to increase the insight into the putative synergistic roles of ABCA1 and SR-BI in foam cell formation and atherosclerosis. ABCA1 is a full-size ABC-transporter that facilitates cholesterol efflux to lipid-poor apolipoprotein (apo)AI. 2,3 Totalbody ABCA1 knockout mice and Tangier disease patients with dysfunctional ABCA1 display a virtual absence of high-density lipoprotein (HDL), showing the essential role for ABCA1 in HDL metabolism. 3 Targeted inactivation of ABCA1 in bone marrow-derived cells in mice leads to increased atherosclerotic lesion formation, 4,5 whereas overexpression of ABCA1 inhibits the progression of atherosclerosis. 6 Macrophages lacking ABCA1, however, still have substantial ability to efflux cholesterol to HDL despite impaired efflux to lipid-poor apoAI, suggesting that macrophages have additional pathways via which cellular cholesterol can be exported. In addition to ABCA1, macrophages also express the ABC half-transporter ABCG1. In contrast to ABCA1, ABCG1 facilitates cellular cholesterol efflux from macrophages to mature HDL but not to lipid-free apolipopro-
Methods and Results: