Apolipoprotein A-I is necessary for the in vivo formation of high density lipoprotein competent for scavenger receptor BI-mediated cholesteryl ester-selective uptake

The severe depletion of cholesteryl ester (CE) in steroidogenic cells of apoA-I-/- mice suggests that apolipoprotein (apo) A-I plays a specific role in the high density lipoprotein (HDL) CE-selective uptake process mediated by scavenger receptor BI (SR-BI) in vivo. The nature of this role, however, is unclear because a variety of apolipoproteins bind to SR-BI expressed in transfected cells. In this study the role of apoA-I-/- in SR-BI-mediated HDL CE-selective uptake was tested via analyses of the biochemical properties of apoA-I-/- HDL and its interaction with SR-BI on adrenocortical cells, hepatoma cells, and cells expressing a transfected SR-BI. apoA-I-/- HDL are large heterogeneous particles with a core consisting predominantly of CE and a surface enriched in phospholipid, free cholesterol, apoA-II, and apoE. Functional analysis showed apoA-I-/- HDL to bind to SR-BI with the same or higher affinity as compared with apoA-I+/+ HDL, but apoA-I-/- HDL showed a 2-3-fold decrease in the V-max for CE transfer from the HDL particle to adrenal cells. These results indicate that the absence of apoA-I results in HDL particles with a reduced capacity for SR-BI-mediated CE-selective uptake. The reduced V-max illustrates that HDL properties necessary for binding to SR-BI are distinct from those properties necessary for the transfer of HDL CE from the core of the HDL particle to the plasma membrane. The reduced V-max for HDL CE-selective uptake likely contributes to the severe reduction in CE accumulation in steroidogenic cells of apoA-I-/- mice.