BIPOLAR FUNCTIONAL EXPRESSION OF TRANSCOBALAMIN-II RECEPTOR IN HUMAN INTESTINAL EPITHELIAL CACO-2 CELL

Transcobalamin II (TC II) receptor is expressed in the apical and baso lateral membranes of human intestinal mucosa and in post-confluent hum an intestinal epithelial Caco-2 cells with a 6-7-fold enrichment in ba solateral membranes. Caco-2 cells grown on culture inserts bound (at 5 degrees C) 30 and 180 fmol of the ligand, TC II-[Co-57]cobalamin (Cbl ), to the apical and the basolateral surfaces, respectively. Within 5 h at 37 degrees C, all apically bound Cbl was internalized and subsequ ently 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 [Co-57]Cbl or I-125-TC II. In contrast, following basolateral internalization of the Ligand, both chloroquine and leupeptin inhibited She intracellula r degradation of I-125-TC II which resulted in secretion of 60-65% of TC II-Cbl complex into the basolateral medium. When I-125-TC II-Cbl wa s orally administered to rats, intact labeled TC II was detected in th e portal blood 4 and 8 h later. These studies suggest that TC II-Cbl i s processed when presented to the (a) apical/luminal side by a hithert o unrecognized non-lysosomal pathway in which both TC II and Cbl are t ranscytosed and (b) basolateral side by the lysosomal pathway in which TC II is degraded and the released Cbl is utilized.