CHARACTERIZATION OF RETROENDOCYTOSIS IN RAT-LIVER PARENCHYMAL-CELLS AND SINUSOIDAL ENDOTHELIAL-CELLS

After receptor-mediated endocytosis, internalized ligands may be recycled to the cell surface instead of being routed to lysosomes for degradation, a process termed retroendocytosis. We have investigated the kinetics and extent of retroendocytosis of neoglycoproteins after internalization via two carbohydrate-specific receptors in rat liver cells: galactose receptors in parenchymal cells (PC) and mannose receptors in sinusoidal endothelial cells (EC). Retroendocytosis in both cell types occurred with first-order kinetics, and the rate of recycling of internalized ligands was about 4 times higher in EC than in PC. As the length of the internalization pulse was increased, the extent of subsequent retroendocytosis decreased, indicating that retroendocytosis takes place from a relatively early stage in the endocytic pathway. Furthermore, as the degree of carbohydrate substitution of the neoglycoprotein ligands increased, the affinities of the receptors for the ligands and the extent of ligand retroendocytosis increased. In the EC, the relationship between degree of substitution and extent of retroendocytosis was not immediately apparent, as some of the neoglycoprotein ligands used may also bind to and be internalized by scavenger receptors on the EC, causing a decreased apparent retroendocytosis. However, when this interaction was inhibited, this relationship was restored. We conclude that retroendocytosis mainly occurs because of incomplete dissocation of ligands from receptors before receptor recycling to the cell surface and that the affinities of a receptor for its ligand at the cell surface and in the endosomal environment are major factors in determining the extent of retroendocytosis.