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 lipopro tein-cholesteryl ester (HDL-CE)-selective uptake pathway, the mechanisms by which the hydrophobic CE molecules are transferred from the HDL particle t o the plasma membrane have remained elusive, until the discovery that scave nger receptor BI (SR-BI) plays an important role. To elucidate the molecula r mechanism, we examined the quantitative relationships between the binding of HDL and the selective uptake of its CE in the murine adrenal Y1-BS1 cel l line. A comparison of concentration dependences shows that half-maximal h igh 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 sim ilar, 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 bein g preferred over the major HDL phospholipids, phosphatidylcholine and sphin gomyelin, Comparison of the rates of selective uptake of different classes of phospholipid in this system gives the ranking: phosphatidylserine > phos phatidylcholine approximate to phosphatidylinositol > sphingomyelin. The ra te of CE-selective uptake from donor particles is proportional to the amoun t of CE initially present in the particles, suggesting a mechanism in which CE moves down its concentration gradient from HDL particles docked on SR-B I into the cell plasma membrane. The activation energy for CE uptake from e ither 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 acces s 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 gr adient into the cell plasma membrane.