Investigation of a role for reduction in ferric iron uptake by mouse duodenum

Iron transport across biologic membranes

Iron is essential for life, but is toxic in excess. Nearly all organisms have therefore developed regulated mechanisms for efficient transport of iron into cells. This paper reviews the current understanding of iron transport, focusing on valuable lessons from studies of yeast iron transport and the discovery of the first mammalian transmembrane iron transporter.

Adaptive Response of Iron Absorption to Anemia, Increased Erythropoiesis, Iron Deficiency, and Iron Loading in β2-Microglobulin Knockout Mice

Recently, a novel gene of the major histocompatibility complex (MHC) class I family, HFE (HLA-H), has been found to be mutated in a large proportion of hereditary hemochromatosis (HH) patients. Further support for a causative role of HFE in this disease comes from the observation that β2-microglobulin knockout (β2m−/−) mice, that fail to express MHC class I products, develop iron overload. We have now used this animal model of HH to examine the capacity to adapt iron absorption in response to altered iron metabolism in the absence of β2m-dependent molecule(s). Mucosal uptake, mucosal transfer and retention of iron were measured in control and β2m−/−mice with altered iron metabolism. Mucosal uptake of Fe(III), but not of Fe(II), by the mutant mice was significantly higher when compared with B6 control mice. Mucosal transfer in the β2m−/−mice was higher, independent of the iron form tested. No significant differences were found in iron absorption between control and β2m−/− mice when anemia was induced either by repetitive bleeding or by hemolysis through phenylhydrazine treatment. However, iron absorption in mice made anemic by dietary deprivation of iron was significantly higher in the mutant mice. Furthermore, the β2m−/− mice manifested an impaired capacity to downmodulate iron absorption when dietary or parenterally iron-loaded. The expression of the defect in iron absorption in the β2m−/− mice is quantitative, with iron absorption being excessively high for the size of body iron stores. The higher iron absorption capacity in the β2m−/− mice may involve the initial step of ferric mucosal uptake and the subsequent step of mucosal transfer of iron to the plasma.

On the methods for studying the mechanisms and bioavailability of iron

Studies of the molecular mechanisms involved in the absorption and bioavailability of iron are important to attempts made worldwide to control the high incidence of iron-associated disorders. The ultimate objective of these studies is to develop methods that are relevant to iron bioavailability and interactions in humans. However, a comprehensive understanding of the chemical and physiologic mechanisms that influence iron bioavailability is necessary to achieve this goal. Initial studies using in vitro and animal models offer the potential for flexibility and manipulation of experimental variables that could provide valuable information toward the understanding and improvement of food iron bioavailability.

Regulation of mammalian iron metabolism: current state and need for further knowledge

Due to its character as an essential element for all forms of life, the biochemistry and physiology of iron has attracted very intensive interest for many decades. In more recent years, the ways that iron metabolism is regulated in mammalian and human organisms have been clarified, and many aspects of iron metabolism have been reviewed. In this article, some newer aspects concerning absorption and intracellular regulation of iron concentration are considered. These include a sorting of possible models for intestinal iron absorption, a description of ways for membrane passage of iron after release from transferrin during receptor-mediated endocytosis, a consideration of possible mechanisms for non-transferrin bound iron uptake and its regulation, and a review of recent knowledge on the properties of iron regulatory proteins and on regulation of iron metabolism by these proteins, changes of their own properties by non-iron-mediated influences, and regulatory events not mediated by these proteins. This somewhat heterogeneous collection of themes is a consequence of the intention to avoid repetition of the many aforementioned reviews already existing and to concentrate on newer findings generated within the last couple of years.

Cloning and characterization of a mammalian proton-coupled metal-ion transporter

Metal ions are essential cofactors for a wealth of biological processes, including oxidative phosphorylation, gene regulation and free-radical homeostasis. Failure to maintain appropriate levels of metal ions in humans is a feature of hereditary haemochromatosis, disorders of metal-ion deficiency, and certain neurodegenerative diseases. Despite their pivotal physiological roles, however, there is no molecular information on how metal ions are actively absorbed by mammalian cells. We have now identified a new metal-ion transporter in the rat, DCT1, which has an unusually broad substrate range that includes Fe2+, Zn2+, Mn2+, Co2+, Cd2+, Cu2+, Ni2+ and Pb2+. DCT1 mediates active transport that is proton-coupled and depends on the cell membrane potential. It is a 561-amino-acid protein with 12 putative membrane-spanning domains and is ubiquitously expressed, most notably in the proximal duodenum. DCT1 is upregulated by dietary iron deficiency, and may represent a key mediator of intestinal iron absorption. DCT1 is a member of the 'natural-resistance-associated macrophage protein' (Nramp) family and thus its properties provide insight into how these proteins confer resistance to pathogens.

Characterization of the FET4 protein of yeast. Evidence for a direct role in the transport of iron

The low affinity Fe2+ uptake system of Saccharomyces cerevisiae requires the FET4 gene. In this report, we present evidence that FET4 encodes the Fe2+ transporter protein of this system. Antibodies prepared against FET4 detected two distinct proteins with molecular masses of 63 and 68 kDa. In vitro synthesis of FET4 suggested that the 68-kDa form is the primary translation product, and the 63-kDa form may be generated by proteolytic cleavage of the full-length protein. Consistent with its role as an Fe2+ transporter, FET4 is an integral membrane protein present in the plasma membrane. The level of FET4 closely correlated with uptake activity over a broad range of expression levels and is itself regulated by iron. Furthermore, mutations in FET4 can alter the kinetic properties of the low affinity uptake system, suggesting a direct interaction between FET4 and its Fe2+ substrate. Mutations affecting potential Fe2+ ligands located in the predicted transmembrane domains of FET4 significantly altered the apparent Km and/or Vmax of the low affinity system. These mutations may identify residues involved in Fe2+ binding during transport.

The molecular mechanisms of the metabolism and transport of iron in normal and neoplastic cells

Iron uptake by mammalian cells is mediated by the binding of serum Tf to the TfR. Transferrin is then internalized within an endocytotic vesicle by receptor-mediated endocytosis and the Fe released from the protein by a decrease in endosomal pH. Apart from this process, several cell types also have other efficient mechanisms of Fe uptake from Tf that includes a process consistent with non-specific adsorptive pinocytosis and a mechanism that is stimulated by small-Mr Fe complexes. This latter mechanism appears to be initiated by hydroxyl radicals generated by the Fe complexes, and may play a role in Fe overload disease where a significant amount of serum non-Tf-bound Fe exists. Apart from Tf-bound Fe uptake, mammalian cells also possess a number of mechanisms that can transport Fe from small-Mr Fe complexes into the cell. In fact, recent studies have demonstrated that the membrane-bound Tf homologue, MTf, can bind and internalize Fe from 59Fe-citrate. However, the significance of this Fe uptake process and its pathophysiological relevance remain uncertain. Iron derived from Tf or small-Mr complexes is probably transported into mammalian cells in the Fe(II) state. Once Fe passes through the membrane, it then becomes part of the poorly characterized intracellular labile Fe pool. Iron in the labile Fe pool that is not used for immediate requirements is stored within the Fe-storage protein, ferritin. Cellular Fe uptake and storage are coordinately regulated through a feedback control mechanism mediated at the post-transcriptional level by cytoplasmic factors known as IRP1 and IRP2. These proteins bind to stem-loop structures known as IREs on the 3 UTR of the TfR mRNA and 5 UTR of ferritin and erythroid delta-aminolevulinic acid synthase mRNAs. Interestingly, recent work has suggested that the short-lived messenger molecule, NO (or its by-product, peroxynitrite), can affect cellular Fe metabolism via its interaction with IRP1. Moreover, NO can decrease Fe uptake from Tf by a mechanism separate to its effects on IRP1, and NO may also be responsible for activated macrophage-mediated Fe release from target cells. On the other hand, the expression of inducible NOS which produces NO, can be stimulated by Fe chelators and decreased by the addition of Fe salts, suggesting that Fe is involved in the control of NOS expression.

Comparative nutrition of iron and copper

The suggestion from nutritional studies with mammals of a link between iron and copper metabolism has been reinforced by recent investigations with yeast cells. Iron must be in the reduced ferrous (FeII) state for uptake by yeast cells, and reoxidation to ferric (FeIII) by a copper oxidase is part of the transport process. Thus, yeast cells deficient in copper are unable to absorb iron. In an analogous way, animals deficient in copper appear to be unable to move FeII out of cells, probably because it cannot be oxidized to FeIII. Invertebrate animals use copper and iron in ways very similar to vertebrates, with some notable exceptions. In the cases where vertebrates and invertebrates are similar, the latter may be useful models for vertebrate metabolism. In cases where they differ (e.g. predominance of serum ferritin in insects, oxygen transport by a copper protein in many arthropods, central importance of phenoloxidase, a copper enzyme in arthropods), the differences may represent processes that are exaggerated in invertebrates and thus more amenable to study in these organisms. On the other hand, they may represent processes unique to invertebrates, thus providing novel information on species diversity.

A duodenal mucosal abnormality in the reduction of Fe(III) in patients with genetic haemochromatosis

BACKGROUND

Previous in vitro studies have shown that the uptake of Fe(III) by freshly isolated duodenal mucosal biopsy specimens is increased in patients with genetic haemochromatosis. Moreover, in the mouse it has recently been found that reduction of Fe(III) to Fe(II) is a prerequisite for iron uptake by the proximal intestine.

AIMS/METHODS

This study used the in vitro technique to investigate the rates of reduction and uptake of 59Fe(III) by duodenal mucosal biopsy specimens obtained at endoscopy from treated and untreated patients with genetic haemochromatosis.

RESULTS

The rate of reduction of iron in the medium was proportional to the incubation time and was not caused by the release of reducing factors from the tissue fragments. Ferrozine, a specific Fe(II) chelator and ferricyanide, a non-permeable oxidising agent, inhibited uptake of 59Fe showing that reduction of Fe(III) precedes uptake. The rates (all values given as pmol/mg/min) of reduction (152 (49) v 92 (23)) and uptake (8.3 (4.0) v 3.6 (1.3), mean (SD)), were significantly increased in biopsy specimens from the untreated group (n = 6) compared with those from 10 control subjects (p < 0.04). Furthermore, the reduction and uptake rates were still increased in five patients in whom iron stores were normal after venesection treatment.

CONCLUSIONS

These results show that there is a persistent abnormality in the reduction and uptake of iron by the intestine in genetic haemochromatosis.

Relationship between duodenal cytosolic aconitase activity and iron status in the mouse

Cytosolic aconitase activity was assayed in duodenal mucosa from mice subjected to a variety of manipulations known to modulate duodenal iron status and duodenal iron absorption. No changes in cytosolic aconitase activity were observed 1 h after oral FeSO4 dosing or intramuscular desferrioxamine treatment. Three days of hypoxic exposure and two weeks treatment with intramuscular iron dextran also had no effect on cytosolic aconitase. Three weeks growth on an iron deficient diet significantly reduced cytosolic aconitase activity. In no situation was there any evidence for significant amounts of inactive aconitase which could be activated in vitro with FeSO4/cysteine. These data suggest that duodenal cytosolic aconitase is not sensitive to acute changes in mucosal iron levels and is generally much less sensitive to body iron status than is duodenal iron absorption. There is evidence that chronic iron depletion reduces cytosolic aconitase to a relatively small degree but generally activity is maintained, consistent with an important metabolic role for the enzyme.

Cellular mechanisms underlying the increased duodenal iron absorption in rats in response to phenylhydrazine-induced haemolytic anaemia

Haemolytic anaemia induced by phenylhydrazine (PZ) promotes iron absorption across rat small intestine. This present study investigates the role of the brush border potential difference (Vm) and mucosal reducing activity in the response. In addition, quantitative autoradiography was used to assess PZ-induced changes in the villus localization of brush border iron uptake. Iron transfer from duodenum to blood was increased significantly 5 days after treatment with PZ. Autoradiography showed that most brush border iron uptake occurred at the upper villus region and the maximal rate was increased fourfold by PZ. Duodenal villus length was increased in PZ-treated rats. PZ treatment did not influence mucosal reducing activity but Vm, measured using duodenal sheets, increased from -50 to -57 mV (P < 0.001) and this was due to a reduced brush border sodium permeability. Thus, an expanded absorptive surface and an enhanced electrical driving force for iron uptake across the duodenal brush border are important adaptations for increased iron absorption in PZ-induced haemolytic anaemia.

Characterization and partial purification of a ferrireductase from human duodenal microvillus membranes

Reduction of ferric iron in the presence of HuTu 80 cells or duodenal microvillus membranes (MVMs) was investigated. With both systems, NADH-dependent reduction of Fe3+/NTA (nitrilotriacetic acid) was demonstrated, using the ferrous iron chelator ferrozine. Uptake of Fe3+ from Fe3+/NTA by HuTu 80 cells was strongly inhibited by addition of ferrozine, indicating that Fe2+ is the substrate for the iron uptake system. With isolated plasma membranes it is shown that the reductase activity is sensitive to trypsin and incubation at 65 degrees C. The reductase activity could be extracted from the plasma membrane and partially purified by ammonium sulphate precipitation and isoelectric focusing. From the purification and inhibition characteristics we conclude that reduction of ferric iron on the surface of duodenal plasma membranes is catalysed by a membrane protein.

Transport of iron and other transition metals into cells as revealed by a fluorescent probe

Transport of nontransferrin-bound iron into cells is thought to be mediated by a facilitated mechanism involving either the trivalent form Fe(III) or the divalent form Fe(II) following reduction of Fe(III) at the cell surface. We have made use of the probe calcein, whose fluorescence is rapidly and stoichiometrically quenched by divalent metals such as Fe(II), Cu(II), Co(II), and Ni(II) and is minimally affected by variations in ionic strength, Ca(II) and Mg(II). Addition of Fe(II) salts to calcein-loaded human erythroleukemia K-562 cells elicited a slow quenching response that was markedly accelerated by the ionophore A-23187 and was reversed by membrane-permeant but not by impermeant chelators. These observations were confirmed by fluorescence imaging of cells. Other divalent metals such as Co(II), Ni(II), and Mn(II) permeated into cells at roughly similar rates, and their uptake, like that of Fe(II), was blocked by trifluoperazine, bepridil, and impermeant sulfhydryl-reactive organomercurials, indicating the operation of a common transport mechanism. This method could provide a versatile tool for studying the transport of iron and other transition metals into cells.

A genetic approach to elucidating eukaryotic iron metabolism

Studies of mutants of the yeast Saccharomyces cerevisiae have led to the identification of genes required for high affinity iron uptake. Reduction of iron (III) outside the cell is accomplished by means of reductases encoded by FRE1 and FRE2, homologues of the gp91-phox component of the oxygen reductase of human granulocytes. High affinity iron (II) transport from the exterior to the interior of the cell occurs by means of a transport system that has not been molecularly characterized. However, the transport process requires the activity of a copper-containing oxidase encoded by FET3. The amino acid sequence of this protein resembles other multi-copper oxidases, including mammalian ceruloplasmin. High affinity copper uptake mediated by the copper transport protein encoded by CTR1 is required to provide the FET3 protein with copper, and thus copper uptake is indirectly required for ferrous iron uptake. These genetic elements of yeast and their relationships may be conserved in complex eukaryotic organisms.

Extracellular ferrireductase activity of K562 cells is coupled to transferrin-independent iron transport

The reduction of Fe3+ to Fe2+ has been established to play a critical role in the uptake of iron by many organisms. Recently, a mechanism of iron transport in the absence of transferrin (Tf) was described for the human K562 cell line and a role for ferrireductase activity was implicated in this process as well [Inman, R. S., & Wessling-Resnick, M. (1993) J. Biol. Chem. 268, 8521-8528]. The present report characterizes the extracellular reduction of ferricyanide to ferrocyanide catalyzed by K562 cells. The observation that membrane-impermeant ferricyanide competitively inhibits Tf-independent assimilation of iron from 55Fe-nitriloacetic acid indicates that this ferrireductase activity is indeed coupled to the transport mechanism. From a series of initial rate experiments, the kinetic parameters for cell surface ferrireductase activity, Vmax = 0.102 nmol min-1 (10(6) cells)-1 and Km = 6.13 microM, were determined. Neither the Vmax nor the Km of this reaction is modulated by changes in extra- or intracellular iron levels; thus, similar to Tf-independent transport activity in K562 cells, the ferrireductase activity is not regulated in response to iron levels. Transmembrane oxidoreductase activity is also reportedly involved in the control of cellular growth; however, the K562 cell ferrireductase is unresponsive to insulin and is not inhibited by the antitumor drugs adriamycin, actinomycin D, or cis-platin, observations that fail to support a role for this particular activity in cell regulation. Rather, the K562 cell ferrireductase appears to be tightly coupled to the mechanism of Tf-independent transport as demonstrated by its sensitivity to Cd2+, a specific inhibitor of non-Tf iron uptake by K562 cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Etiologies, consequences, and treatment of iron overload

From a global perspective, severe systemic iron overload occurs predominantly in individuals affected by geographically specific genetic mutations that permit the daily absorption from the diet of more iron than is physiologically needed. Two main types of hereditary iron overload are well recognized: (1) HLA-linked hemochromatosis in populations derived from Europe and (2) iron overload complicating thalassaemia major and intermedia syndromes in Southeast Asia, the Middle East, and the Mediterranean. Another very common form of iron overload occurs in Africa and is clearly related to high dietary iron content; recent evidence suggests that a genetic predisposition may also contribute to the pathogenesis. Patients with iron overload may develop multiorgan system toxicity; aggressive therapy with phlebotomy or iron chelation to remove excess iron from the body prevents organ damage and prolongs life.

The fission yeast ferric reductase gene frp1+ is required for ferric iron uptake and encodes a protein that is homologous to the gp91-phox subunit of the human NADPH phagocyte oxidoreductase

We have identified a cell surface ferric reductase activity in the fission yeast Schizosaccharomyces pombe. A mutant strain deficient in this activity was also deficient in ferric iron uptake, while ferrous iron uptake was not impaired. Therefore, reduction is a required step in cellular ferric iron acquisition. We have cloned frp1+, the wild-type allele of the mutant gene. frp1+ mRNA levels were repressed by iron addition to the growth medium. Fusion of 138 nucleotides of frp1+ promoter sequences to a reporter gene, the bacterial chloramphenicol acetyltransferase gene, conferred iron-dependent regulation upon the latter when introduced into S. pombe. The predicted amino acid sequence of the frp1+ gene exhibits hydrophobic regions compatible with transmembrane domains. It shows similarity to the Saccharomyces cerevisiae FRE1 gene product and the gp91-phox protein, a component of the human NADPH phagocyte oxidoreductase that is deficient in X-linked chronic granulomatous disease.

Iron speciation at physiological pH in media containing ascorbate and oxygen

The stability of iron ascorbate solutions was investigated, under both anaerobic and aerobic conditions, with the Fe2+ and Fe3+ indicators, respectively ferrozine and mimosine, at different pH values. The species present under the differing conditions were investigated by electron paramagnetic resonance (EPR) and Mössbauer spectroscopy and by gel-filtration chromatography. At physiological pH (6.8-7.4) iron ascorbate solutions rapidly form mononuclear chelatable Fe3+ species as reflected by indicator studies and EPR. Mössbauer spectroscopy fails to detect any Fe2+ species. EPR studies show a time-dependent decrease in rhombic Fe3+, particularly in oxygenated buffers, consistent with a conversion to polynuclear Fe. O2 uptake studies show that the conversion of Fe2+ to Fe3+ in Fe-ascorbate solutions at pH > 7.0 was accompanied by rapid O2 consumption but preceded depletion of ascorbate. Addition of high concentrations of mannitol (50-200 mM) reduces the O2 consumption and partly stabilizes the rapidly chelatable Fe form. Gel filtration studies show that the oxidation of Fe-ascorbate solutions at pH 7.4 is accompanied by an increase in the apparent relative molecular mass of the Fe, presumably due to Fe polymer formation. These studies indicate that inherent instability of Fe-ascorbate solutions above neutral pH and clearly have important implications in the use of ascorbate in studies of Fe physiology.

The fission yeast ferric reductase gene frp1+ is required for ferric iron uptake and encodes a protein that is homologous to the gp91-phox subunit of the human NADPH phagocyte oxidoreductase

We have identified a cell surface ferric reductase activity in the fission yeast Schizosaccharomyces pombe. A mutant strain deficient in this activity was also deficient in ferric iron uptake, while ferrous iron uptake was not impaired. Therefore, reduction is a required step in cellular ferric iron acquisition. We have cloned frp1+, the wild-type allele of the mutant gene. frp1+ mRNA levels were repressed by iron addition to the growth medium. Fusion of 138 nucleotides of frp1+ promoter sequences to a reporter gene, the bacterial chloramphenicol acetyltransferase gene, conferred iron-dependent regulation upon the latter when introduced into S. pombe. The predicted amino acid sequence of the frp1+ gene exhibits hydrophobic regions compatible with transmembrane domains. It shows similarity to the Saccharomyces cerevisiae FRE1 gene product and the gp91-phox protein, a component of the human NADPH phagocyte oxidoreductase that is deficient in X-linked chronic granulomatous disease.