A Novel Efflux–Recapture Process Underlies the Mechanism of High-Density Lipoprotein Cholesteryl Ester-Selective Uptake Mediated by the Low-Density Lipoprotein Receptor–Related Protein

Abstract

Objective— To determine the mechanism of low-density lipoprotein (LDL) receptor-related protein (LRP)-mediated selective uptake of high-density lipoprotein (HDL)-derived cholesteryl esters (CE). Methods and Results— Apolipoprotein E (apoE) and heparin sulfate proteoglycans are required for LRP-mediated selective uptake in adipocytes. Furthermore, 2-deoxyglucose and NaN 3 abolish this process, indicating that cellular energy is required. LRP-mediated selective uptake is also abolished by monensin or when clathrin-mediated internalization is inhibited (using hypotonic, K + -free medium or hyperosmolar sucrose), clearly implicating receptor endocytosis. The receptor-associated protein (RAP), an inhibitor of ligand binding to LRP, reduced the transport of CE into an intracellular compartment but not into the plasma membrane. Remarkably, the CE that is ultimately transported by LRP first enters the plasma membrane then undergoes apoE-mediated CE efflux before being recaptured and internalized by LRP. Conclusion— According to this “efflux-recapture” model, LRP contributes to selective uptake because it recovers CE that would normally be lost by efflux mediated by apoE.