High Density Lipoprotein Phospholipid Composition Is a Major Determinant of the Bi-directional Flux and Net Movement of Cellular Free Cholesterol Mediated by Scavenger Receptor BI*

The role of high density lipoprotein (HDL) phospholipid in scavenger receptor BI (SR-BI)-mediated free cholesterol flux was examined by manipulating HDL₃phosphatidylcholine and sphingomyelin content. Both phosphatidylcholine and sphingomyelin enrichment of HDL enhanced the net efflux of cholesterol from SR-BI-expressing COS-7 cells but by two different mechanisms. Phosphatidylcholine enrichment of HDL increased efflux, whereas sphingomyelin enrichment decreased influx of HDL cholesterol. Although similar trends were observed in control (vector-transfected) COS-7 cells, SR-BI overexpression amplified the effects of phosphatidylcholine and sphingomyelin enrichment of HDL 25- and 2.8-fold, respectively. By using both phosphatidylcholine-enriched and phospholipase A₂-treated HDL to obtain HDL with a graded phosphatidylcholine content, we showed that SR-BI-mediated cholesterol efflux was highly correlated (r ² = 0.985) with HDL phosphatidylcholine content. The effects of varying HDL phospholipid composition on SR-BI-mediated free cholesterol flux were not correlated with changes in either the K _d orB _max values for high affinity binding to SR-BI. We conclude that SR-BI-mediated free cholesterol flux is highly sensitive to HDL phospholipid composition. Thus, factors that regulate cellular SR-BI expression and the local modification of HDL phospholipid composition will have a large impact on reverse cholesterol transport.