Bipolar functional expression of transcobalamin II receptor in human intestinal epithelial Caco-2 cells.

Transcobalamin II (TC II) receptor is expressed in the apical and basolateral membranes of human intestinal mucosa and in post-confluent human intestinal epithelial Caco-2 cells with a 6-7-fold enrichment in basolateral membranes. Caco-2 cells grown on culture inserts bound (at 5 degrees C) 30 and 180 fmol of the ligand, TC II-[57Co]cobalamin (Cbl), to the apical and the basolateral surfaces, respectively. Within 5 h at 37 degrees C, all apically bound Cbl was internalized and subsequently transcytosed as TC II-Cbl. In contrast, all basolateral surface-bound Cbl was internalized and retained by the cells, but transferred from TC II to other cellular proteins. Chloroquine or leupeptin had no effect on the apical to basolateral transcytosis of either [57Co]Cbl or 125I-TC II. In contrast, following basolateral internalization of the ligand, both chloroquine and leupeptin inhibited the intracellular degradation of 125I-TC II, which resulted in secretion of 60-65% of TC II-Cbl complex into the basolateral medium. When 125I-TC II-Cbl was orally administered to rats, intact labeled TC II was detected in the portal blood 4 and 8 h later. These studies suggest that TC II-Cbl is processed when presented to the (a) apical/luminal side by a hitherto unrecognized non-lysosomal pathway in which both TC II and Cbl are transcytosed and (b) basolateral side by the lysosomal pathway in which TC II is degraded and the released Cbl is utilized.