Beta very low density lipoprotein and clathrin-coated vesicles co-localize to microvilli in pigeon monocyte-derived macrophages.

Macrophages derived from blood monocytes are key in the development of atherosclerosis, as monocyte migration into the intima and accumulation of cholesterol leads to foam cell formation. To investigate the relationship between lipoprotein binding and the distribution of clathrin-coated endocytic vesicles, monocyte-derived macrophages were exposed in vitro to beta very low density lipoprotein (beta VLDL), conjugated to colloidal gold, and later were processed for immuno-electron microscopy to localize clathrin-coated vesicles. The immunolocalization was done in conjunction with either cryosectioning or whole mount intermediate voltage electron microscopy. Preferential binding of beta VLDL on small membrane ruffles and microvilli was quantitatively verified. Clathrin-coated vesicles were distributed throughout the cell; however, clusters of microvilli were associated with both a high concentration of coated vesicles and lipoprotein. Small membrane ruffles were not associated with clathrin-coated vesicles. These data support our hypothesis that endocytosis of beta VLDL near microvilli involves coated vesicles, whereas endocytosis of beta VLDL near ruffles is not mediated by coated endocytic vesicles. Furthermore, the association of coated vesicles with microvilli but not membrane ruffles may be important in understanding ligand trafficking within the cell. Given the distribution of coated vesicles within the cell, it is possible that the site of lipoprotein binding may determine the mechanism of entry into the cell and the metabolic effects of the internalized ligand.