APOTRANSFERRIN AND HOLOTRANSFERRIN UNDERGO DIFFERENT ENDOCYTIC CYCLESIN INTESTINAL EPITHELIA (CACO-2) CELLS

Previous studies have demonstrated that diferric transferrin and apotr ansferrin compete for the binding to basolateral transferrin receptors and that transferrin-mediated iron uptake by Caco-2 cells is inhibite d by apotransferrin to a larger extent than that predicted solely by r eceptor competition, This inhibition can have important implications i n determining the net exchange of iron between intestinal cells and th e basolateral milieu. Accordingly, we further characterized the endocy tic cycles of apotransferrin and diferric transferrin in Caco-2 cells. We found that after internalization both apotransferrin and diferric transferrin recycled to the cell exterior, but that apotransferrin had a protracted endocytic cycle. Confocal microscopy studies revealed a different cellular distribution of apotransferrin and diferric transfe rrin; both were found in a compartment close to the basal membrane, bu t apotransferrin reached as well regions closer to the apical membrane . Moreover, the intracellular distribution of transferrin receptors wa s influenced by the iron load of transferrin; cells incubated with apo transferrin presented a more apical distribution of transferrin recept ors than cells incubated with diferric transferrin. These results indi cate for the first time that the endocytic cycle of transferrin recept ors in intestinal epithelial cells is determined by the iron content o f transferrin. They explain also the marked inhibitory effect of apotr ansferrin on transferrin-mediated iron uptake by Caco-2 cells, since i ncubation of cells with apotransferrin resulted in the actual sequestr ation of the receptor in the cell interior.