Localization of the iron transport proteins Mobilferrin and DMT-1 in the duodenum: the surprising role of mucin.

There are two pathways for inorganic iron uptake in the intestine, the ferric pathway, mediated by the key protein mobilferrin, and the ferrous pathway, mediated by DMT-1. Previous studies reported that the amount of DMT-1 increased in the intestinal mucosa in iron deficiency and the increase was seen in the apical portion of the villus of the duodenal mucosa. Mobilferrin did not quantitatively increase but became localized at the cell membrane. However, studies on fresh tissue have not previously been performed and the localization to the microvillae has not been demonstrated. In order to more definitively localize these proteins immunofluorescent and electron microscopic studies were undertaken. Samples were also subjected to biochemical analysis and Western analysis. In iron-deficient animals both DMT-1 and Mobilferrin were concentrated in the apical surface of the villae. Electron microscopy revealed that the majority of this increase in the amount of these proteins near the luminal surface was due to increased binding of the proteins to mucin in vesicles near the surface. A significant portion of the iron transport proteins was localized in the goblet cells and outside the cell in the luminal mucin, as demonstrated by immunofluorescence, electron microscopy, and isolation of the mucin by cesium chloride gradient centrifugation and Western analysis. A new model for the transport of metal ions was suggested. The metal transport proteins travel from vesicles inside the cell out to the lumen mucin. This increases the surface area and allows a greater portion of the lumen contents to be exposed to the binding proteins. Once the metal is bound to the externalized protein it is internalized into the cell. This explains many of the unique properties of the iron-binding proteins and suggests that it may be a more general model for the absorption of other nutrients.

DMT1: a mammalian transporter for multiple metals.

DMT1 has four names, transports as many as eight metals, may have four or more isoforms and carries out its transport for multiple purposes. This review is a start at sorting out these multiplicities. A G185R mutation results in diminished gastrointestinal iron uptake and decreased endosomal iron exit in microcytic mice and Belgrade rats. Comparison of mutant to normal rodents is one analytical tool. Ectopic expression is another. Antibodies that distinguish the isoforms are also useful. Two mRNA isoforms differ in the 3' UTR: +IRE DMT1 has an IRE (Iron Responsive Element) but -IRE DMT1 lacks this feature. The +/-IRE proteins differ in the distal 18 or 25 amino acid residues after shared identity for the proximal 543 residues. A major function is serving as the apical iron transporter in the lumen of the gut. The +IRE isoform appears to have that role. Another role is endosomal exit of iron. Some evidence indicts the -IRE isoform for this function. In our ectopic expression assay for metal uptake, four metals--Fe2+, Mn2+, Ni2+ and Co2+--respond to the normal DMT1 cDNA but not the G185R mutant. Two metals did not--Cd2+ and Zn2+--and two--Cu2+ and Pb2+--remain to be tested. In competition experiments in the same assay, Cd2+, Cu2+ and Pb2+ inhibit Mn2+ uptake but Zn2+ did not. In rodent mutants, Fe and Mn appear more dependent on DMT1 than Cu and Zn. Experiments based on ectopic expression, specific antibodies that inhibit metal uptake and labeling data indicate that Fe3+ uptake depends on a different pathway in multiple cells. Two isoforms localize differently in a number of cell types. Unexpectedly, the -IRE isoform is in the nuclei of cells with neuronal properties. While the function of -IRE DMT1 in the nucleus is speculative, one may safely infer that this localization identifies new role(s) for this multifunctional transporter. Management of toxic challenges is another function related to metal homeostasis. Airways represent a gateway tissue for metal entry. Preliminary evidence using specific PCR primers and antibodies specific to the two isoforms indicates that -IRE mRNA and protein increase in response to exposure to metal in lungs and in a cell culture model; the +IRE form is unresponsive. Thus the -IRE form could be part of a detoxification system in which +IRE DMT1 does not participate. How does iron status affect other metals' toxicity? In the case of Mn, iron deficiency may enhance cellular responses.

Cellular location of proteins related to iron absorption and transport.

K562 erythroleukemia cells and IEC6 rat cells were examined using confocal microscopy and antibodies raised against DMT-1 (Nramp-2, DCT-1), transferrin receptor (CD71), beta(3) integrin (CD61), mobilferrin (calreticulin), and Hephaestin. The cellular location of each of these proteins was identified by immunofluorescence in both saponin-permeabilized and non-permeabilized cells. Fluorescent reactivity was observed on or near the cell surface of each of these proteins, suggesting that they might participate in surface membrane transport of iron. Fluorescence was observed in the region of the cytoplasm with each antibody to include beta(3) integrin and transferrin receptor. It was pronounced in cells incubated with mobilferrin, Hephaestin, and DMT-1 antibodies. Speckled nuclear fluorescence was observed in cells incubated with anti-DMT-1. While these observations are descriptive, they demonstrate that there are significant concentrations of DMT-1, mobilferrin, and Hephaestin in the cytoplasmic region of cells. This suggests that there may be intracellular roles for these proteins in addition to their serving to transit iron across the cell surface membrane.

The ferrireductase paraferritin contains divalent metal transporter as well as mobilferrin.

Inorganic iron can be transported into cells in the absence of transferrin. Ferric iron enters cells utilizing an integrin-mobilferrin-paraferritin pathway, whereas ferrous iron uptake is facilitated by divalent metal transporter-1 (DMT-1). Immunoprecipitation studies using antimobilferrin antibody precipitated the previously described large-molecular-weight protein complex named paraferritin. It was previously shown that paraferritin functions as an intracellular ferrireductase, reducing ferric iron to ferrous iron utilizing NADPH as the energy source. It functions in the pathway for the cellular uptake of ferric iron. This multipeptide protein contains a number of active peptides, including the ferric iron binding protein mobilferrin and a flavin monooxygenase. The immunoprecipitates and purified preparations of paraferritin also contained DMT-1. This identifies DMT-1 as one of the peptides constituting the paraferritin complex. Since paraferritin functions to reduce newly transported ferric iron to ferrous iron and DMT-1 can transport ferrous iron, these findings suggest a role for DMT-1 in conveyance of iron from paraferritin to ferrochelatase, the enzyme utilizing ferrous iron for the synthesis of heme in the mitochondrion.