Transport of residual endocytosed products into terminal lysosomes occurs slowly in rat liver endothelial cells



Receptor-mediated endocytosis of circulating collagen is a major physiological scavenger function of the liver endothelial cell and an important catabolic event in the complete turnover of this abundant connective tissue protein. In the present study, transport of collagen through the endocytic pathway was investigated in cultured liver endothelial cells. Collagen conjugated to fluorescein isothiocyanate, to allow detection of the ligand by fluorescence and immunoelectron microscopy, was found sequentially in three different organelles that compose the basic degradative endocytic pathway of eukaryotic cells: early endosomes, late endosomes, and terminal lysosomes. Early endosomes were identified as vesicles positive for early endosome antigen 1 (EEA1). Late endosomes were distinguished as structures positive for the late endosomal/lysosomal marker rat lysosomal membrane glycoprotein 120, but negative for EEA1 and lysosomally targeted BSA–gold. Lysosomes were defined by their content of BSA–gold, injected 24 hours before isolation of cells. Coated pits and coated vesicles mediated an extremely rapid internalization. Shortly after internalization and during the first 20 minutes, ligand was found in early endosomes. From 20 minutes on, ligand started to appear in late endosomes (23%), and by 2 hours the transfer was largely complete (82.5%). Only 2.5% of ligand was transferred to the lysosomes after 2 hours, and this number slowly increased to 21% and 53% after 6 and 16 hours, respectively. We conclude that 1) EEA1 is a useful marker for tracing early events of endocytosis in liver endothelial cells; 2) in contrast to the rapid internalization, transit of internalized ligand through early sorting endosomes generally takes from 20 minutes to 2 hours; and 3) exit from the late endosomes is very slow, requiring several hours.