A pathway-dependent on ApoE, ApoAI, and ABCA1 determines formation of buoyant high-density lipoprotein by macrophage foam cells

Objective - ABCA1-dependent and ABCA1-independent pathways may operate in high-density lipoprotein formation by macrophages secreting apolipoprotein (apo) E. We examined the impact of ABCA1 on apoE-mediated efflux from cholesterol-enriched macrophages. Methods and Results - Without acceptors, wild-type, ABCA1(-/-), and apoE(-/-) macrophages released 5.7% +/- 0.3%, 1.8% +/- 0.1%, and 2.3% +/- 0.2% of their cholesterol, and the LXR agonist, TO-901317, enhanced efflux by 137%, 10%, and 20%. Although similar amounts of apoE were secreted from ABCA1(-/-) and wild-type cells, apoE from ABCA1(-/-) cells was only partially phospholipidated and floated at density >1.21g/ mL, whereas apoE from wild-type cells floated at density of 1.09 to 1.17g/mL and paralleled the density of cholesterol. With apoAI, LXR stimulation increased efflux by 139% and 86% from wild-type and apoE(-/-) cells, resulting in a large difference in efflux (29.5% +/- 0.2% versus 17.0% +/- 0.5%). The density of apoE and cholesterol from wild-type cells did not change with apoAI, and most apoAI floated at density >= 1.17g/ mL. In apoE(-/-) cells, apoAI and cholesterol floated at similar density, but the peak fraction only contained 4 mu g cholesterol/mg protein versus 18 in WT cells. Conclusions - Macrophage apoE requires ABCA1 for formation of high-density lipoprotein. ApoAI facilitates association of apoE with more buoyant high-density lipoprotein, suggesting that apoE, plasma apoAI, and ABCA1 operate together to optimize mobilization of macrophage cholesterol, a process critical to limiting plaque development.