TRANSCYTOSIS AND PROCESSING OF INTRINSIC FACTOR-COBALAMIN IN CACO-2 CELLS

Intracellular events within enterocytes following receptor-mediated en docytosis 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 trans cytose Cbl. Uptake of IF-Cbl from the apical surface leads to the intr acellular accumulation of Cbl in a process that reaches an equilibrium between internalization and secretion only after a 20-h continuous in cubation. 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 i s degraded with a half-time of 4 h. Leupeptin causes a partial block i n the proteolysis of IF, an intracellular accumulation of Cbl bound to IF, and a decrease in transcytosis of Cbl. Finally, an analysis of in tracellular Cbl during transcytosis shows that free Cbl is present wit hin cells during transcytosis.