title: Plasma membrane delivery, endocytosis and turnover of transcobalamin receptor in polarized human intestinal epithelial cells.
abstract: Cells that are metabolically active and in a high degree of differentiation and proliferation require cobalamin (Cbl: vitamin B(12)) and they obtain it from the circulation bound to transcobalamin (TC) via the transcobalamin receptor (TC-R). This study has investigated the plasma membrane dynamics of TC-R expression in polarized human intestinal epithelial Caco-2 cells using techniques of pulse-chase labelling, domain-specific biotinylation and cell fractionation. Endogenously synthesized TC-R turned over with a half-life (T(1/2)) of 8 h following its delivery to the basolateral plasma membrane (BLM). The T(1/2) of BLM delivery was 15 min and TC-R delivered to the BLM was endocytosed and subsequently degraded by leupeptin-sensitive proteases. However, about 15% of TC-R endocytosed from the BLM was transcytosed (T(1/2), 45 min) to the apical membranes (BBM) where it underwent endocytosis and was degraded. TC-R delivery to both BLM and BBM was inhibited by Brefeldin A and tunicamycin, but not by wortmannin or leupeptin. Colchicine inhibited TC-R delivery to BBM, but not BLM. At steady state, apical TC-R was associated with megalin and both these proteins were enriched in an intracellular compartment which also contained Rab5 and transferrin receptor. These results indicate that following rapid delivery to both plasma membrane domains of Caco-2 cells, TC-R undergoes constitutive endocytosis and degradation by leupeptin-sensitive proteases. TC-R expressed in apical BBM complexes with megalin during its transcytosis from the BLM.
authors.affiliation: Division of Gastroenterology and Hepatology, Department of Medicine, Medical College of Wisconsin and Veterans Administration Medical Center, Milwaukee, Wisconsin 53295, USA.
authors.forename: Santanu
authors.initials: S
authors.lastname: Bose
authors.forename: Seema
authors.initials: S
authors.lastname: Kalra
authors.forename: Raghunatha R
authors.initials: RR
authors.lastname: Yammani
authors.forename: Rajiv
authors.initials: R
authors.lastname: Ahuja
authors.forename: Bellur
authors.initials: B
authors.lastname: Seetharam
mesh_terms: D019921:Biotinylation; D020126:Brefeldin A / Q000494:pharmacology; D018938:Caco-2 Cells; D002458:Cell Fractionation; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism*; D016764:Cell Polarity; D003078:Colchicine / Q000494:pharmacology; D004705:Endocytosis / Q000187:drug effects; D011992:Endosomes / Q000378:metabolism; D004847:Epithelial Cells / Q000187:drug effects / Q000378:metabolism*; D006031:Glycosylation; D006056:Golgi Apparatus / Q000378:metabolism; D006207:Half-Life; D006801:Humans; D007413:Intestinal Mucosa / Q000187:drug effects / Q000378:metabolism* / Q000473:pathology; D007700:Kinetics; D026561:Low Density Lipoprotein Receptor-Related Protein-2 / Q000378:metabolism; D010447:Peptide Hydrolases / Q000378:metabolism; D011499:Protein Processing, Post-Translational / Q000187:drug effects; D021381:Protein Transport; D011956:Receptors, Cell Surface / Q000378:metabolism*; D011990:Receptors, Transferrin / Q000378:metabolism; D050257:Tubulin Modulators / Q000494:pharmacology; D014415:Tunicamycin / Q000494:pharmacology; D020696:rab5 GTP-Binding Proteins / Q000378:metabolism
chemical_list: D026561:Low Density Lipoprotein Receptor-Related Protein-2; D011956:Receptors, Cell Surface; D011990:Receptors, Transferrin; D050257:Tubulin Modulators; C028176:transcobalamin receptor; D014415:Tunicamycin; D020126:Brefeldin A; D010447:Peptide Hydrolases; D020696:rab5 GTP-Binding Proteins; D003078:Colchicine