High density lipoprotein stimulates sterol translocation between intracellular and plasma membrane pools in human monocyte-derived macrophages

Binding of high density lipoprotein (HDL) to its receptor on cultured fibroblasts and aortic endothelial cells was previously shown to facilitate sterol efflux by initiation of translocation of intracellular sterol to the plasma membrane. After cholesterol-loaded human monocyte-derived macrophages were incubated with either 3H]mevalonolactone or lipoprotein-associated 3H]cholesteryl ester to radiolabel intracellular pools of sterol, incubation with HDL3 led to stimulation of 3H-labeled sterol translocation from intracellular sites to the cell surface which preceeded maximum 3H-labeled sterol efflux. A similar pattern was demonstrated for macrophages that were preloaded with cholesterol derived from either low density lipoprotein (LDL), acetyl-LDL, or phospholipase C-modified LDL. However, in macrophages that were not loaded with cholesterol, HDL3 stimulated net movement of 3H-labeled sterol from the plasma membrane into intracellular compartments, the opposite direction from that seen for cholesterol-loaded cells. A similar influx pattern was found in nonloaded macrophages and fibroblasts that were labeled with trace amounts of exogenous 3H]cholesterol. Cholesterol translocation from intracellular pools to the cell surface of cholesterol-loaded macrophages appeared to be stimulated by receptor binding of HDL, since chemical modification of HDL with tetranitromethane (TNM), which abolishes its receptor binding, reduced its ability to stimulate 3H-labeled sterol translocation and efflux. In nonloaded cells, however, the ability of HDL3 to stimulate sterol efflux and movement of sterol from the plasma membrane into intracellular pools was unaffected by TNM modification. Thus, binding of HDL to its receptor on cholesterol-loaded macrophages appears to promote translocation of intracellular cholesterol to the plasma membrane followed by cholesterol efflux into the medium. However, in nonloaded macrophages, HDL stimulates sterol movement from the plasma membrane into intracellular pools by a receptor-independent process.