Cellular responses to surface binding and internalization of concanavalin A. An electron microscopic investigation on the problem of membrane cycling.

Monolayer cultures of normal and diethylnitrosamine-transformed rat liver cells were labeled in situ with Con A-HRP or ferritin-conjugated Con A. Ligand-induced redistribution with simultaneous internalization of labeled membrane areas occurred in normal as well as in transformed cells when they were reincubated with PBS at 37 degrees C for different periods of time (from 5 min up to 3 hrs). Compared to normal cells, these afore mentioned processes were accelerated in transformed cells. Internalization in normal and transformed cells resulted in a recycling of labeled plasma membrane areas in the Golgi region with the label being finally accumulated in elements which correspond mostly, but not exclusively, to GERL. Then formation of phagolysosomes and multivesiculated bodies occurred whose labeled content was exocytized after fusion with the plasma membrane. This suggested that the internalized plasma membrane areas were at least partly degraded. The relabeling of some parts of the plasma membrane by extruded lysosomal content indicates that at least some Con A molecules are still biological active. Membrane internalization by endocytosis after binding of Con A obviously causes an increased of membrane biogenesis and exocytosis, thus compensating for membrane removal. This is suggested by the vacuolization and enlargement of unlabeled (not in recycling involved) Golgi apparatus. It may indicate a differential functional role of the Golgi apparatus in membrane turnover in the same cell. The fusion of phagolysosomes with the plasma membrane and the insertion of phagolysosomal membrane into the plasma membrane might be another compensatory mechanism.