Reconstituted HDL (rHDL), is of interest as a potential novel therapy for atherosclerosis due to its ability to promote free cholesterol (FC) mobilization after intravenous administration. We performed studies to identify the underlying molecular mechanisms by which rHDL promote FC mobilization from whole body in vivo and macrophages in vitro.
Methods and results
Wild type (WT), SR-BI-KO, ABCA1-KO and ABCG1-KO mice received either rHDL or PBS intravenously. Blood was drawn before and at several time points after injection for apoA-I, phosphatidylcholine and FC measurement. In WT mice, serum FC peaked at 20 min and rapidly returned towards baseline levels by 24 h. Unexpectedly, ABCA1-KO and ABCG1-KO mice did not differ from WT mice regard to the kinetics of FC mobilization. In contrast, in SR-BI-KO mice the increase in FC level at 20 min was only 10% of that in control mice (p<0.01). Bone marrow-derived macrophages from WT, SR-BI-KO, ABCA1-KO and ABCG1-KO mice were incubated in vitro with rHDL and cholesterol efflux determined. Efflux from SR-BI KO and ABCA1 KO macrophages was not different from WT macrophages. In contrast, efflux from ABCG1-KO macrophages was ∼ 50% lower as compared with WT macrophages (p<0.001).
The bulk mobilization of FC observed in circulation after rHDL administration is primarily mediated by SR-BI. However, cholesterol mobilization from macrophages to rHDL is primarily mediated by ABCG1.