EXPRESSION OF COBALAMIN TRANSPORT PROTEINS AND COBALAMIN TRANSCYTOSIS BY COLON ADENOCARCINOMA CELLS

Human colon adenocarcinoma (Caco-2) cells express both intrinsic factor-cobalamin receptor and transcobalamin II (TC II). The expression of these activities began to rise by day 6 and reached peak levels between 10 and 15 days in culture. The postconfluent Caco-2 cell membranes bound approximately 30-35 fmol of intrinsic factor (IF)[Co-57]Cbl/mg protein. The size of the mature receptor expressed in the apical brush border had a relative molecular mass of 230 kDa. The intracellular form of TC II had a M(r) of 43, 5 higher than the secreted form of TC II. TC II was secreted unidirectionally via the basolateral direction when Caco-2 cells were grown on culture inserts. When grown on culture inserts, the Caco-2 cells were polarized (electrical resistance > 200-OMEGA/cm2) and transcytosed [Co-57]Cbl bound to IF from apical-to-basal but not from basal-to-apical direction. Under these conditions, [Co-57]Cbl complexed to haptocorrin was not transported. These cells also transcytosed free [Co-57]Cbl, although less efficiently. The [Co-57]Cbl transcytosed using either IF[Co-57]Cbl or free [Co-57]Cbl as ligands was bound exclusively to TC II. Intracellular [Co-57]Cbl decreased during transcytosis with a slow (t 1/2 = 4 h) transfer of [Co-57]Cbl from IF to TC II. These results show that the transport of Cbl in Caco-2 cells is very similar to the human enterocyte system.