The scavenger receptor-BI (SR-BI) delivers sterols from circulating lipopro
teins to tissues, but the relative potency of individual lipoproteins and t
he transported cholesterol has not been studied in detail. In this study, w
e used Chinese hamster ovary cells that express recombinant mouse SR-BI but
have no functional low density lipoprotein (LDL) receptors (ldlA7-SRBI cel
ls) to compare the fate of lipids transferred from high or low density lipo
proteins to cells by SR-BI. HDL and LDL were equally effective in mediating
the transfer of [H-3]cholesterol to cells. Only 5% of the free cholesterol
transferred to cells was esterified, in direct contrast to the findings in
the cells that express I;DL receptors in which 50% of the transported chol
esterol was esterified. Almost all the free cholesterol transferred from li
poproteins to cells was rapidly excreted when the ldlA7-SRBI cells were swi
tched to media containing unlabeled lipoproteins. SR-BI expression was asso
ciated with an increase in selective cholesteryl ester uptake from both lip
oproteins, but HDL was a more effective donor. HDL and LDL were equally eff
ective in delivering cholesterol to the intracellular regulatory pool via S
R-EI. These data indicate that SR-BI is able to exchange cholesterol rapidl
y between lipoproteins and cell membranes and can mediate the uptake of cho
lesteryl esters from both classes of lipoproteins.