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.
Medical Research Council Laboratory for Molecular Cell Biology, University College London, United Kingdom.
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Cutler
D001692:Biological Transport / Q000187:drug effects; D002460:Cell Line; D016764:Cell Polarity; D003593:Cytoplasm / Q000378:metabolism; D004705:Endocytosis / Q000187:drug effects; D006801:Humans; D007413:Intestinal Mucosa / Q000378:metabolism*; D007437:Intrinsic Factor / Q000378:metabolism*; D007976:Leupeptins / Q000494:pharmacology; D014805:Vitamin B 12 / Q000378:metabolism*
D007976:Leupeptins; D007437:Intrinsic Factor; C032854:leupeptin; D014805:Vitamin B 12