The recent discovery of HFE, the MHC-Class-I-like gene mutated in up to 90% of patients with hereditary
haemochromatosis, and the gene encoding the Nramp2/
divalent metal transporter-1 (DMT-1) implicated in ferrous
iron transport holds promise for a greater understanding of human
iron metabolism. Since the
HFE protein can be crystallized as a ternary complex with the
transferrin receptor and
iron-saturated
transferrin, and DMT-1 expression is up-regulated in hereditary
haemochromatosis, these
proteins are likely to interact in a common pathway for human
iron homeostasis. To investigate the cellular interactions between the cognate
proteins encoded by these genes, we generated a panel of rabbit and avian
antisera from human HFE and DMT-1 derived
peptides. The
antibodies were characterized by ELISA reactions and Western immunoblotting. Immunohistochemical staining showed that DMT-1
protein localized to the brush border of human duodenum where it is predicted to serve as the principal transporter of ferrous
iron from the intestinal lumen. In the human cell lines, Caco-2 (small intestinal phenotype upon differentiation) and K562 (erythroleukaemic) HFE, in the presence of
iron-saturated
transferrin, co-localized with
transferrin receptors in an early endosome compartment using confocal immunofluorescence microscopy. This interaction may be critical in small-intestinal crypt cells which express HFE, where it may function to modulate their intrinsic
iron status thereby programming
iron absorption by DMT-1 in the mature enterocyte. In undifferentiated Caco-2 cells, DMT-1 localized to a discrete late endosome compartment distinct from that occupied by HFE where, in addition to brush-border
iron uptake, it may function to regulate the availability of
iron delivery to intracellular
iron pools. Disruption of the HFE gene as a result of mutations associated with hereditary
haemochromatosis may thus impair homeostatic mechanisms controlling
iron absorption within the small-intestine epithelium by a direct interaction with
transferrin receptors and by subsequent alteration of DMT-1 expression. Identification of the molecular interactions of HFE with DMT-1 and other key components of the
iron transport pathway has implications for a mechanistic understanding of the pathophysiology of human
iron storage diseases as well as the regulation of normal
iron balance.