Scavenger receptor BI transfers major lipoprotein-associated phospholipidsinto the cells

The phospholipids of lipoproteins can be transferred to cells by an endocyt osis-independent uptake pathway. We analyzed the role of scavenger receptor BI (SR-BI) for the selective cellular phospholipid import. Human monocytes rapidly acquired the pyrene (py)-labeled phospholipids sphingomyelin (SM), phosphatidylcholine, and phosphatidylethanolamine from different donors (l ow and high density lipoproteins (LDL, HDL), lipid vesicles). The anti-SR-B I antibody directed against the extracellular loop of the membrane protein lowered the cellular import of the phospholipids by 40-80%. The phospholipi d transfer from the lipid vesicles into the monocytes was suppressed by LDL , I-IDL, and apoprotein AI, Transfection of BHK cells with the cDNA for hum an SR-BI enhanced the cellular import of the vesicle-derived py-phospholipi ds by 5-6-fold. In the case of the LDL donors, transfer of py-SM to the tra nsfected cells was stimulated to a greater extent than the uptake of the ot her py-phospholipids. Similar differences were not observed when the vesicl es and HDL were used as phospholipid donors, The concentration of LDL requi red for the half-maximal phospholipid import was close to the previously re ported apparent dissociation constant for LDL binding to SR-BI, The low act ivation energy of the SR-BI-mediated py-phospholipid import indicated that the transfer occurs entirely in a hydrophobic environment, Disruption of ce ll membrane caveolae by cyclodextrin treatment reduced the SR-BI-catalyzed incorporation of py-SM, suggesting that intact caveolae are necessary for t he phospholipid uptake. In conclusion, SR-BI mediates the selective import of the major lipoprotein-associated phospholipids into the cells, the trans fer efficiency being dependent on the structure of the donor lipoprotein.