A fluorescent metal-sensor study provides evidence for iron transport by transcytosis in the intestinal epithelial cells

Iron transport across the intestinal epithelium is facilitated by the divalent metal transporter 1 (DMT1) on the brush border membrane (BBM). The fluorescent metal sensor calcein, which is hydrophilic, membrane-impermeable and quenched by chelation with iron, was used to test our hypothesis that intestinal iron absorption is through the endocytic processes and is involved in a pathway where BBM-derived vesicles fuse with basolateral membrane (BLM)-derived vesicles. To monitor the flux of iron via transcytosis, Caco-2 cells were employed as a polarized cell layer in Transwell chambers. When calcein was added to the basal chamber along with apo-transferrin (apo-Tf), calcein rapidly underwent endocytosis and co-localized with apo-Tf. Calcein was quenched by adding an iron-ascorbate complex and then restored by adding 2,2'-dipyridyl into the apical chamber. These results were confirmed by live-cell imaging. When hemin from the apical surface and calcein from the basal chamber were added to the Caco-2 cells, internalization of DMT1 and quenching of calcein were not observed until 2 h later. These results indicated that absorbed hemin required processing before hemin-derived iron was available to BLM-derived vesicles. These studies suggest that iron is transported in Caco-2 cells by transcytosis with apical-derived vesicles that are fused to BLM-derived vesicles.