Transcytosis and processing of intrinsic factor-cobalamin in Caco-2 cells.

Intracellular events within enterocytes following receptor-mediated endocytosis of intrinsic factor-cobalamin (IF-Cbl) are poorly understood. We have examined the fate of IF and Cbl in filter-grown Caco-2 cells which express both IF receptors and transcobalamin II and which transcytose Cbl. Uptake of IF-Cbl from the apical surface leads to the intracellular accumulation of Cbl in a process that reaches an equilibrium between internalization and secretion only after a 20-h continuous incubation. Transcytosed Cbl is detectable in the basolateral medium 4 h after the onset of endocytosis. Cbl is released from the basolateral surface with the same kinetics irrespective of from which cell surface endocytosis of IF-Cbl took place. Following uptake, internalized IF is degraded with a half-time of 4 h. Leupeptin causes a partial block in the proteolysis of IF, an intracellular accumulation of Cbl bound to IF, and a decrease in transcytosis of Cbl. Finally, an analysis of intracellular Cbl during transcytosis shows that free Cbl is present within cells during transcytosis.