Intestinal Epithelia (Caco-2) Cells Acquire Iron through the Basolateral Endocytosis of Transferrin

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Abstract
Although the absorption of iron through the intestinal epithelia is inversely related to body iron stores, the mechanisms by which the enterocytes sense body iron stores are unknown. Polarized enterocytes have transferrin receptors in their basolateral surface; hence, we tested the hypothesis that the endocytosis of circulating transferrin may be part of the body's iron sensing mechanism. Particularly, we evaluated the contribution of basolateral transferrin to iron content of intestinal cells, and we investigated what factors modulate this contribution. For this purpose, we used the intestinal cell line Caco-2 grown on porous filters. When the cells were simultaneously offered equimolar amounts of iron, from the apical medium as 55Fe-nitrilotriacetate and from the basolateral medium as 59Fe-transferrin, most of the internalized iron came from the basolateral endocytosis of 59Fe-transferrin. Experiments of transferrin binding and internalization showed that holotransferrin and apotransferrin had similar numbers of basolateral receptors. The receptors had associated constants for diferric transferrin, monoferric transferrin, and apotransferrin of 2.69, 1.71 and 0.26 × 107 L/mol, respectively. The binding of diferric transferrin or monoferric transferrin to receptors was competitively inhibited by apotransferrin. Caco-2 cells, but not K562 cells, showed inhibition of basolateral, transferrin-mediated iron uptake by apotransferrin. This inhibition should regulate the net basolateral uptake of iron mediated by the endocytosis of Fe-containing transferrins. We propose that the basolateral endocytosis of transferrin forms part of the system by which intestinal epithelia cells sense plasma iron concentrations.