ApoB-containing lipoproteins in apoE-deficient mice are not metabolized by the class B scavenger receptor BI

The scavenger receptor class B type I (SR-BI) recognizes a broad variety of lipoprotein ligands, including HDL, LDL, and oxidized LDL. In this study, we investigated whether SR-BI plays a role in the metabolism of cholesterol-rich lipoprotein remnants that accumulate in apolipoprotein E (apoE)^-/- mice. These particles have an unusual apolipoprotein composition compared with conventional VLDL and LDL, containing mostly apoB-48 as well as substantial amounts of apoA-I and apoA-IV. To study SR-BI activity in vivo, the receptor was overexpressed in apoE^-/- mice by adenoviral vector-mediated gene transfer. An ∼10-fold increase in liver SR-BI expression resulted in no detectable alterations in VLDL-sized particles and a modest depletion of cholesterol in intermediate density lipoprotein/LDL-sized lipoprotein particles. This decrease was not attributable to altered secretion of apoB-containon lipoproteins in SR-BI-overexpressing mice. To directly assess whether SR-BI metabolizes apoE^-/- mouse lipoprotein remnants, in vitro assays were performed in both CHO cells and primary hepatocytes expressing high levels of SR-BI. This analysis showed a remarkable deficiency of these particles to serve as substrates for selective lipid uptake, despite high-affinity, high-capacity binding to SR-BI. Taken together, these data establish that SR-BI does not play a direct role in the metabolism of apoE^-/- mouse lipoprotein remnants.