Enhancement of scavenger receptor class B type I-mediated selective cholesteryl ester uptake from apoA-I^-/- high density lipoprotein (HDL) by apolipoprotein A-I requires HDL reorganization by lecithin cholesterol acyltransferase

The severe depletion of cholesteryl ester (CE) in adrenocortical cells of apoA-I^-/- mice suggests that apolipoprotein (apo) A-I plays an important role in the high density lipoprotein (HDL) CE selective uptake process mediated by scavenger receptor BI (SR-BI) in vivo. A recent study showed that apoA-I^-/- HDL binds to SR-BI with the same affinity as apoA-I^+/+ HDL, but apoA-I^-/- HDL has a decreased V_max for CE transfer from the HDL particle to adrenal cells. The present study was designed to determine the basis for the reduced selective uptake of CE from apoA-I^-/- HDL. Variations in apoA-I^-/- HDL particle diameter, free cholesterol or phospholipid content, or the apoE or apoA-II content of apoA-I^-/- HDL had little effect on HDL CE selective uptake into Y1-BS1 adrenal cells. Lecithin cholesterol acyltransferase treatment alone or addition of apoA-I to apoA-I^-/- HDL alone also had little effect. However, addition of apoA-I to apoA-I^-/- HDL in the presence of lecithin cholesterol acyltransferase reorganized the large heterogeneous apoA-I^-/- HDL to a more discrete particle with enhanced CE selective uptake activity. These results show a unique role for apoA-I in HDL CE selective uptake that is distinct from its role as a ligand for HDL binding to SR-BI. These data suggest that the conformation of apoA-I at the HDL surface is important for the efficient transfer of CE to the cell.