The ATP-binding cassette transporter A1 (ABCA1) mediates the efflux of cellular unesterified cholesterol and phospholipid to lipid-poor apolipoprotein A-I. Chymase, a protease secreted by mast cells, selectively cleaves pre-beta-migrating particles from high density lipoprotein (HDL)3 and reduces the efflux of cholesterol from macrophages. To evaluate whether this effect is the result of reduction of ABCA1-dependent or -independent pathways of cholesterol efflux, in this study we examined the efflux of cholesterol to preparations of chymase- treated HDL3 in two types of cell: 1) in J774 murine macrophages endogenously expressing low levels of scavenger receptor class B, type I (SR-BI), and high levels of ABCA1 upon treatment with cAMP; and 2) in Fu5AH rat hepatoma cells endogenously expressing high levels of the SR-BI and low levels of ABCA1. Treatment of HDL3 with the human chymase resulted in rapid depletion of pre-beta-HDL and a concomitant decrease in the efflux of cholesterol and phospholipid (2-fold and 3-fold, respectively) from the ABCA1-expressing J774 cells. In contrast, efflux of free cholesterol from Fu5AH to chymase-treated and to untreated HDL3 was similar. Incubation of HDL3 with phospholipid transfer protein led to an increase in pre-beta-HDL contents as well as in ABCA1-mediated cholesterol efflux. A decreased cholesterol efflux to untreated HDL3 but not to chymase treated HDL3 was observed in ABCA1-expressing J774 with probucol, an inhibitor of cholesterol efflux to lipid-poor apoA-I. Similar results were obtained using brefeldin and gliburide, two inhibitors of ABCA1-mediated efflux. These results indicate that chymase treatment of HDL3 specifically impairs the ABCA1-dependent pathway without influencing either aqueous or SR-BI-facilitated diffusion and that this effect is caused by depletion of lipid-poor pre-beta-migrating particles in HDL3. Our results are compatible with the view that HDL3 promotes ABCA1-mediated lipid efflux entirely through its lipid-poor fraction with pre-beta mobility.
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