Abstract
The internalization of cationized ferritin (CF) was studied in isolated pancreatic acinar cells in vitro. Horseradish peroxidase (HRP) was used in conjunction with CF to compare internalization of soluble-phase and membrane-bound tracers. The mode of internalization of CF was dependent upon tracer concentration and origin of the plasma membrane (apical vs. lateral-basal). At the lower tracer concentrations (0.19 and 0.38 mg/ml), internalization from the apical cell surface occurred via small vesicles. The tracer then appeared in multivesicular bodies, in tubules, and in irregular membrane-bound structures. After 15 min, CF particles were seen in many small vesicles near the Golgi apparatus, but not in the Golgi saccules. In contrast, at the lateral-basal cell surface the CF particles tended to form clusters. These clusters were more pronounced at higher CF concentrations (0.76 and 1.5 mg/ml) and were associated with elongated cellular processes, which seemed to engulf CF accumulations in a phagocytic manner. Once internalized, CF was found primarily in large irregular structures which appeared to migrate slowly toward the nucleus, reaching a juxtanuclear position after approximately 30 min. CF was observed in lysosomes after 30-45 min and by 90 min most of the CF was confined to large vacuoles and to trimetaphosphatase-positive lysosomes. Similar routes were observed when cells were double-labeled with CF and HRP, where endocytic structures showed co-localization of both tracers. The results of this study indicate the importance of the Golgi region in the intracellular sorting of internalized apical membrane. Furthermore, this work confirms the presence of distinct endocytic pathways at the apical and lateral-basal cell surfaces.