Abstract

The intracellular pathways taken by galactose-terminal glycoproteins were examined following endocytosis by the asialoglycoprotein receptor in monolayers of the human hepatoma cell line, Hep G2. In addition to a pathway leading to lysosomal degradation, single cohort kinetics revealed that up to 28% of surface-bound and internalized 125I-asialoorosomucoid (ASOR) eventually returned undegraded to the extracellular medium over 6 hr in the presence or absence of free ASOR in the exocytosis medium. This reappearance of ligand in the exocytosis medium represented a constant fraction of surface bound and internalized 125I-ASOR, and followed pseudo-first order kinetics with t1/2 = 84 min (long transit pool). Under conditions of enhanced ligand-receptor dissociation (incubation with 100 mM N-acetylgalactosamine (GalNAc), at least 50% of initially internalized 125I-ASOR returned to the cell surface as ligand-receptor complexes, followed by dissociation of free ligand into the exocytosis medium. This rapid transit pool of ligand also displayed pseudo-first order kinetics with t1/2 = 24 min. Exocytosis of 125I-Gal-cytochrome c, a synthesized ligand displaying rapid dissociation from the asialoglycoprotein receptor (ASGP-R), paralleled the kinetics of the rapid transit pool of 125I-ASOR (t1/2 = 28 min). Furthermore, in addition to spontaneous dissociation from ASPG-R following return to the cell surface, studies conducted in saponin-permeabilized monolayers support the return of free intracellular 125I-Gal-cytochrome c to the cell surface during exocytosis. The rapid transit pool of ligand was insensitive to inhibition by 10 mM sodium azide or 0.1 mM primaquine. In contrast, the long transit pool destined for exocytosis was inhibited 50% by 10 mM sodium azide, but insensitive to inhibition by 0.1 mM primaquine. These data suggest that, following internalization by the ASGP-R, a major pathway of ligand movement includes the rapid return of ligand-receptor complexes and/or free ligand to the cell surface. Return of ligand-receptor complexes or free ligand to the cell surface occurs prior to an acidic sorting compartment, can involve multiple cycles of return to the cell surface, and may involve passage through other nonlysosomal intracellular organelles.