Mechanism of scavenger receptor class B type I-mediated selective uptake of cholesteryl esters from high density lipoprotein to adrenal cells

Despite extensive studies and characterizations of the high density lipoprotein-cholesteryl ester (HDL-CE)-selective uptake pathway, the mechanisms by which the hydrophobic CE molecules are transferred from the HDL particle to the plasma membrane have remained elusive, until the discovery that scavenger receptor BI (SR-BI) plays an important role. To elucidate the molecular mechanism, we examined the quantitative relationships between the binding of HDL and the selective uptake of its CE in the murine adrenal Y1-BS1 cell line. A comparison of concentration dependences shows that half-maximal high affinity cell association of HDL occurs at 8.7 +/- 4.7 mu g/ml and the K-m of HDL-CE-selective uptake is 4.5 +/- 1.5 mu g/ml. These values are similar, and there is a very high correlation between these two processes (r(2) = 0.98), suggesting that they are Linked. An examination of lipid uptake from reconstituted HDL particles of defined composition and size shows that there is a non-stoichiometric uptake of HDL lipid components, with CE being preferred over the major HDL phospholipids, phosphatidylcholine and sphingomyelin, Comparison of the rates of selective uptake of different classes of phospholipid in this system gives the ranking: phosphatidylserine > phosphatidylcholine approximate to phosphatidylinositol > sphingomyelin. The rate of CE-selective uptake from donor particles is proportional to the amount of CE initially present in the particles, suggesting a mechanism in which CE moves down its concentration gradient from HDL particles docked on SR-BI into the cell plasma membrane. The activation energy for CE uptake from either HDL3 or reconstituted HDL is about 9 kcal/mol, indicating that HDL CE uptake occurs via a non-aqueous pathway. HDL binding to SR-BI allows access of CE molecules to a "channel" formed by the receptor from which water is excluded and along which HDL-CE molecules move down their concentration gradient into the cell plasma membrane.