A two-step mechanism for free cholesterol and phospholipid efflux from human vascular cells to apolipoprotein A-1.

Smooth muscle and endothelial cells in vivo are quiescent yet exposed to high levels of lipoprotein lipids. Phospholipid (PL) and free cholesterol (FC) efflux maintain homeostasis. Smooth muscle cells (SMC) expressed high levels of ABC-1 transporter mRNA, and glyburide-dependent PL and FC efflux to apolipoprotein A-1 (apo A-1), the major protein of high-density lipoprotein. FC efflux was inhibited by vanadate and okadaic acid, while PL efflux was not. Phosphatidylcholine was the major PL transferred by both cell types. Stimulation of phosphatidylserine efflux, redistributed within the membrane by this transporter, was only minimally increased. Umbilical vein and aortic endothelial cells expressed little ABC-1 mRNA, nor did these cells promote either PL or FC efflux in response to the presence of apo A-1. To investigate the mechanism of ABC-1-dependent lipid efflux from these cells, apo A-1 was preincubated in the presence of unlabeled SMC or fibroblasts, and the conditioned medium was then transferred to endothelial cells. This medium catalyzed the efflux of FC but not of PL from endothelial cells. Such FC efflux was resistant to glyburide but inhibited by okadaic acid and vanadate. The data suggest that ABC-1-dependent PL efflux precedes FC efflux to apo A-1 and that the complex of apo A-1 and PL is a much better acceptor of FC than apo A-1 itself. Inhibition of FC but not PL efflux by vanadate and okadaic acid suggests these transfers involve different mechanisms.