Characterization of two distinct pathways of endocytosis of ricin by rat liver endothelial cells.

We have studied the characteristics of internalization and intracellular transport of ricin via two distinct pathways in rat liver endothelial cells (EC), i.e., via binding to mannose receptors and surface galactosyl-residues, respectively. Treatments that inhibit endocytosis from coated pits, i.e., hyperosmolarity and acidification of the cytoplasm, decreased uptake via mannose receptors much more than uptake via galactosyl-residues, indicating that mannose receptors are largely internalized from coated pits, whereas internalization via galactosyl-residues is to a significant extent independent of coated pits. Uptake of ricin via mannose receptors was strongly inhibited by NH4Cl and monensin, and accordingly, NH4Cl protected the cells against ricin intoxication via mannose receptors. On the other hand, uptake via galactosyl-residues was not significantly inhibited by NH4Cl or monensin, and NH4Cl even sensitized the cells to intoxication via this pathway. Brefeldin A, which did not affect ricin uptake, protected the cells against ricin intoxication via either pathway. Protein synthesis in the EC was efficiently inhibited by ricin, even after very short periods of uptake at low ricin concentrations. The onset of protein synthesis inhibition was more rapid upon internalization of ricin via mannose receptors than via galactosyl-residues. Also, ricin internalized via mannose receptors was more efficiently transported from endosomes to lysosomes than ricin internalized via galactosyl-residues. Partial blocking of the galactosyl-binding sites of ricin caused a reduction in the extent of recycling of ricin from endosomes to the cell surface (retroendocytosis), indicating that binding of ricin to membrane galactosyl-residues, which is relatively stable at the slightly acidic pH of endosomes, is an important determinant of the intracellular handling of ricin. We suggest that the observed difference in the transport from endosomes to lysosomes between the two internalization pathways is related to the different stability of the two binding mechanisms at endosomal pH.