Electron microscopic evidence for an asialoglycoprotein receptor on Kupffer cells: localization of lectin-mediated endocytosis.

Direct evidence is given for the presence of an N-acetyl-D-galactosamine-specific lectin on the Kupffer cell surface by visualization of ligand binding in electron microscopy. When freshly isolated Kupffer cells are incubated with asialofetuin adsorbed onto colloidal gold particles (ASF-gold), binding and endocytosis of ligand are seen. Recognition of ASF-gold by Kupffer cells is completely abolished in the presence of N-acetyl-D-galactosamine (25 mM) or EGTA (3 mM), but is not significantly reduced by N-acetyl-D-glucosamine or D-mannose (25 mM). ASF particles are endocytosed via the coated pit/vesicle pathway and appear to be transported to the secondary lysosomes by coated vesicles, as shown by the occurrence of coated areas in the secondary lysosome membrane. These observations demonstrate the presence of an asialoglycoprotein receptor on Kupffer cells; therefore, the hepatocyte is not the only cell in the rat liver with D-galactose receptor activity.