Interaction of high density lipoproteins with cholesteryl ester-laden macrophages: biochemical and morphological characterization of cell surface receptor binding, endocytosis and resecretion of high density lipoproteins by macrophages.

Morphological and biochemical experiments were carried out to investigate the interaction of human serum high density lipoproteins (HDL) with mouse peritoneal macrophages. It is demonstrated that resident mouse peritoneal macrophages express HDL receptors. Subsequent to receptor-mediated binding, HDL are internalized and intracellularly transported into endosomes. These endosomes do not fuse with the lysosomal compartment but interact with the margin of intracellular plasma lipid droplets. Macrophages do not degrade, but rather resecrete internalized HDL particles as described for the transferrin-receptor pathway. HDL binding to freshly isolated macrophages is saturable at a concentration of approximately 320 ng HDL-protein/mg cell protein and a Scatchard plot indicates the presence of some 130 000-190 000 receptors/cell with a Kd of approximately 9 X 10(-7) M. Binding of HDL on the macrophage surface is significantly enhanced in cholesterol-laden macrophages, whereas the increase in the rate of uptake and secretion is less pronounced. Within the HDL fraction the HDL2 subclass showed higher binding, uptake and secretion activity as compared with HDL3. From these experimental data we postulate that cholesterol uptake from macrophages is mediated by HDL particles which interact with these cells via a receptor-mediated retroendocytosis pathway.