Receptor-mediated uptake and 'retroendocytosis' of high-density lipoproteins by cholesterol-loaded human monocyte-derived macrophages: possible role in enhancing reverse cholesterol transport.

Human monocyte-derived macrophages (MDM) are cholesterol-loaded, and the rates of uptake, degradation and resecretion of high-density lipoproteins are measured and compared to the rates in control cells. Results show the binding activity of these lipoproteins is upregulated in cholesterol-loaded cells; the bound and internalized lipoproteins are not degraded to any appreciable extent but primarily resecreted as a larger particle. The enhancement of binding activity for high-density lipoproteins is arrested when cycloheximide is added to the medium, suggesting that protein synthesis is involved. Preliminary evidence also indicates that HDL3 (without apoE) after internalisation is converted intracellularly to a larger apoE-containing HDL2-like particles. Thus, MDM appears to possess specific receptors for HDL3 without apoE that may function to facilitate HDL-mediated removal of excess cholesterol from cells.