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

Restoration of metal homeostasis: a potential strategy against neurodegenerative diseases

Metal homeostasis is critical to normal neurophysiological activity. Metal ions are involved in the development, metabolism, redox and neurotransmitter transmission of the central nervous system (CNS). Thus, disturbance of homeostasis (such as metal deficiency or excess) can result in serious consequences, including neurooxidative stress, excitotoxicity, neuroinflammation, and nerve cell death. The uptake, transport and metabolism of metal ions are highly regulated by ion channels. There is growing evidence that metal ion disorders and/or the dysfunction of ion channels contribute to the progression of neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and multiple sclerosis (MS). Therefore, metal homeostasis-related signaling pathways are emerging as promising therapeutic targets for diverse neurological diseases. This review summarizes recent advances in the studies regarding the physiological and pathophysiological functions of metal ions and their channels, as well as their role in neurodegenerative diseases. In addition, currently available metal ion modulators and in vivo quantitative metal ion imaging methods are also discussed. Current work provides certain recommendations based on literatures and in-depth reflections to improve neurodegenerative diseases. Future studies should turn to crosstalk and interactions between different metal ions and their channels. Concomitant pharmacological interventions for two or more metal signaling pathways may offer clinical advantages in treating the neurodegenerative diseases.

Chelators for Treatment of Iron and Copper Overload: Shift from Low-Molecular-Weight Compounds to Polymers

Iron and copper are essential micronutrients needed for the proper function of every cell. However, in excessive amounts, these elements are toxic, as they may cause oxidative stress, resulting in damage to the liver and other organs. This may happen due to poisoning, as a side effect of thalassemia infusion therapy or due to hereditary diseases hemochromatosis or Wilson's disease. The current golden standard of therapy of iron and copper overload is the use of low-molecular-weight chelators of these elements. However, these agents suffer from severe side effects, are often expensive and possess unfavorable pharmacokinetics, thus limiting the usability of such therapy. The emerging concepts are polymer-supported iron- and copper-chelating therapeutics, either for parenteral or oral use, which shows vivid potential to keep the therapeutic efficacy of low-molecular-weight agents, while avoiding their drawbacks, especially their side effects. Critical evaluation of this new perspective polymer approach is the purpose of this review article.

Measuring Trans-Plasma Membrane Electron Transport by C2C12 Myotubes

Trans-plasma membrane electron transport (tPMET) plays a role in protection of cells from intracellular reductive stress as well as protection from damage by extracellular oxidants. This process of transporting electrons from intracellular reductants to extracellular oxidants is not well defined. Here we present spectrophotometric assays by C2C12 myotubes to monitor tPMET utilizing the extracellular electron acceptors: water-soluble tetrazolium salt-1 (WST-1) and 2,6-dichlorophenolindophenol (DPIP or DCIP). Through reduction of these electron acceptors, we are able to monitor this process in a real-time analysis. With the addition of enzymes such as ascorbate oxidase (AO) and superoxide dismutase (SOD) to the assays, we can determine which portion of tPMET is due to ascorbate export or superoxide production, respectively. While WST-1 was shown to produce stable results with low background, DPIP was able to be re-oxidized after the addition of AO and SOD, which was demonstrated with spectrophotometric analysis. This method demonstrates a real-time, multi-well, quick spectrophotometric assay with advantages over other methods used to monitor tPMET, such as ferricyanide (FeCN) and ferricytochrome c reduction.

The therapeutic potential of iron-targeting gallium compounds in human disease: From basic research to clinical application

Gallium, group IIIa metal, shares certain chemical characteristics with iron which enable it to function as an iron mimetic that can disrupt iron-dependent tumor cell growth. Gallium may also display antimicrobial activity by disrupting iron homeostasis in certain bacteria and fungi. Gallium's action on iron homeostasis leads to inhibition of ribonucleotide reductase, mitochondrial function, and changes in proteins of iron transport and storage. In addition, gallium induces an increase in mitochondrial reactive oxygen species in cells which triggers downstream upregulation of metallothionein and hemoxygenase-1. Early clinical trials evaluated the efficacy of the simple gallium salts, gallium nitrate and gallium chloride. However, newer gallium-ligands such as Tris(8-quinolinolato)gallium(III) (KP46) and gallium maltolate have been developed and are undergoing clinical evaluation. Additional gallium-ligands that demonstrate antitumor activity in preclinical studies have emerged. Their mechanisms of action and their spectrum of antitumor activity may extend beyond the earlier generations of gallium compounds and warrant further investigation. This review will focus on the evolution and potential of gallium-based therapeutics.

Involvement of Trichoderma asperellum strain T6 in regulating iron acquisition in plants

Iron (Fe) deficiency is a major plant nutritional disorder in many parts of the world, particularly in areas with saline soils. Among the numerous root-associated microbes that are beneficial for plant nutrient uptake, Trichoderma spp. are the most effective rhizosphere fungi for enhancing plant growth and plant resistance to biotic and abiotic stresses. To investigate the potential mechanisms of action of Trichoderma on insoluble Fe in the soil, which is difficult for plants to absorb and utilize, a high siderophore-producing strain of Trichoderma T6, was isolated from the rhizosphere of cucumber plants. The strain was identified as T. asperellum based on the morphological features and molecular phylogenetic analyses. Applying strain T6 to sterile soil could increase soil levels of Fe(2+) and siderophores, as well as increase Fe(2+) and Fe(3+)-chelate reductase (FCR) activity in cucumber tissues. Purified siderophore eluent (PSE) increased plant growth, thus confirming its role in plant growth promotion. Moreover, extracellular Fe(3+) reducing activity and three kinds of organic acids were detected in the culture filtrate of strain T6. These results indicate that strain T6 influences plant Fe absorption in several ways. Siderophore-based Fe chelation is effective in providing Fe to plants, organic acids, and Fe(3+) reducing enzymes may participate in the solubilization and reduction of insoluble Fe(3+) to Fe(2+).

Effect of subchronic exposure to arsenic on levels of essential trace elements in mice brain and its gender difference

The interactions of toxic metals with essential metals may result in disturbances in the homeostasis of essential elements. However, there are few reports about toxic effect of arsenic (As) on the levels of essential trace elements in the central nervous system. To investigate whether subchronic exposure to As disturbs levels of main essential trace elements in the brain of mice and whether the gender difference in the response to As are altered, the concentrations of As, Iron (Fe), copper (Cu), selenium (Se), zinc (Zn) and Chromium (Cr) in the cerebrum and cerebellum of mice exposed to As subchronically were examined by inductively coupled plasma-mass spectrometry (ICP-MS). The gender difference in the changed levels of these essential trace elements was also statistically analyzed. The concentration of As was significantly higher in the cerebrum or cerebellum of mice exposed to As than that in control group (P < 0.05). It indicates that As can accumulate in brain of mice after subchronic exposure. The concentrations of Fe, Se and Cr in the cerebrum or cerebellum were significantly lower in mice exposed to As than those in control group (P < 0.05). On the contrary, the concentration of Cu in the cerebrum or cerebellum was significantly higher in mice exposed to As (P < 0.05). Our results indicate that subchronic exposure to As may decrease the levels of Fe, Se and Cr or increase the level of Cu in the brain of mice. Moreover, the significant gender difference was found relative to the effect of As on concentration of Se in cerebrum and concentrations of Cu and Se in cerebellum of mice. Therefore, more experiments are required to further understand mechanisms whereby As interacts with essential elements in brain and induces the gender difference.

Zip14 is a complex broad-scope metal-ion transporter whose functional properties support roles in the cellular uptake of zinc and nontransferrin-bound iron

Recent studies have shown that overexpression of the transmembrane protein Zrt- and Irt-like protein 14 (Zip14) stimulates the cellular uptake of zinc and nontransferrin-bound iron (NTBI). Here, we directly tested the hypothesis that Zip14 transports free zinc, iron, and other metal ions by using the Xenopus laevis oocyte heterologous expression system, and use of this approach also allowed us to characterize the functional properties of Zip14. Expression of mouse Zip14 in RNA-injected oocytes stimulated the uptake of (55)Fe in the presence of l-ascorbate but not nitrilotriacetic acid, indicating that Zip14 is an iron transporter specific for ferrous ion (Fe(2+)) over ferric ion (Fe(3+)). Zip14-mediated (55)Fe(2+) uptake was saturable (K(0.5) ≈ 2 μM), temperature-dependent (apparent activation energy, E(a) = 15 kcal/mol), pH-sensitive, Ca(2+)-dependent, and inhibited by Co(2+), Mn(2+), and Zn(2+). HCO(3)(-) stimulated (55)Fe(2+) transport. These properties are in close agreement with those of NTBI uptake in the perfused rat liver and in isolated hepatocytes reported in the literature. Zip14 also mediated the uptake of (109)Cd(2+), (54)Mn(2+), and (65)Zn(2+) but not (64)Cu (I or II). (65)Zn(2+) uptake also was saturable (K(0.5) ≈ 2 μM) but, notably, the metal-ion inhibition profile and Ca(2+) dependence of Zn(2+) transport differed from those of Fe(2+) transport, and we propose a model to account for these observations. Our data reveal that Zip14 is a complex, broad-scope metal-ion transporter. Whereas zinc appears to be a preferred substrate under normal conditions, we found that Zip14 is capable of mediating cellular uptake of NTBI characteristic of iron-overload conditions.

Mammalian iron transport

Iron is essential for basic cellular processes but is toxic when present in excess. Consequently, iron transport into and out of cells is tightly regulated. Most iron is delivered to cells bound to plasma transferrin via a process that involves transferrin receptor 1, divalent metal-ion transporter 1 and several other proteins. Non-transferrin-bound iron can also be taken up efficiently by cells, although the mechanism is poorly understood. Cells can divest themselves of iron via the iron export protein ferroportin in conjunction with an iron oxidase. The linking of an oxidoreductase to a membrane permease is a common theme in membrane iron transport. At the systemic level, iron transport is regulated by the liver-derived peptide hepcidin which acts on ferroportin to control iron release to the plasma.

Duodenal cytochrome B expression stimulates iron uptake by human intestinal epithelial cells

Duodenal cytochrome B (Dcytb) is localized principally in the apical membrane of the enterocyte. It is thought to act as a ferric reductase that furnishes Fe(II), the specific and selective iron species transported by divalent metal transporter 1 (DMT1) in the duodenal enterocytes. Expression of both genes is strongly iron regulated and is thought to be required for transcellular iron trafficking in concert in response to physiological requirements. We tested this hypothesis by expressing Dcytb in Caco-2 cells, a human cell line model often used to mimic intestinal enterocytes. Iron uptake (59Fe) was significantly higher in Dcytb-transfected Caco-2 cells than in cells transfected with empty vector as a control. Fe(III) reductase activity of Dcytb was measured with ferrozine, a strong chelator of Fe(II) species. Cells expressing Dcytb exhibited enhanced ferric reductase activity as well as increased 59Fe uptake compared with cells transfected with empty vector as a control. Ferrozine blocked iron uptake and preincubation of cells with dehydroascorbate (to increase cellular ascorbate levels) stimulated iron uptake. Cotransfection of Dcytb and DMT1 resulted in an additive increase in iron uptake by the cells. The results confirm Dcytb can act as a ferric reductase that stimulates iron uptake in Caco-2 cells.

Non-transferrin iron reduction and uptake are regulated by transmembrane ascorbate cycling in K562 cells

K562 erythroleukemia cells import non-transferrin-bound iron (NTBI) by an incompletely understood process that requires initial iron reduction. The mechanism of NTBI ferrireduction remains unknown but probably involves transplasma membrane electron transport. We here provide evidence for a novel mechanism of NTBI reduction and uptake by K562 cells that utilizes transplasma membrane ascorbate cycling. Incubation of cells with dehydroascorbic acid, but not ascorbate, resulted in (i) accumulation of intracellular ascorbate that was blocked by the glucose transporter inhibitor, cytochalasin B, and (ii) subsequent release of micromolar concentrations of ascorbate into the external medium via a route that was sensitive to the anion channel inhibitor, 4,4'-diisothiocyanatostilbene-2,2'-disulfonate. Ascorbate-deficient control cells demonstrated low levels of ferric citrate reduction. However, incubation of the cells with dehydroascorbic acid resulted in a dose-dependent stimulation of both iron reduction and uptake from radiolabeled [(55)Fe]ferric citrate. This stimulation was abrogated by ascorbate oxidase treatment, suggesting dependence on direct chemical reduction by ascorbate. These results support a novel model of NTBI reduction and uptake by K562 cells in which uptake is preceded by reduction of iron by extracellular ascorbate, the latter of which is subsequently regenerated by transplasma membrane ascorbate cycling.

Iron homeostasis

Iron is needed by all mammalian cells but is toxic in excess. Specialized transport mechanisms conduct iron across cellular membranes. These are regulated to ensure homeostasis both systemically in living organisms and within individual cells. Over the past decade, major advances have been made in identifying and characterizing the proteins involved in the transport, handling, and homeostatic regulation of iron. Molecular understanding of these processes has provided important insights into the pathophysiology of human iron disorders.

Identification of a ferrireductase required for efficient transferrin-dependent iron uptake in erythroid cells

The reduction of iron is an essential step in the transferrin (Tf) cycle, which is the dominant pathway for iron uptake by red blood cell precursors. A deficiency in iron acquisition by red blood cells leads to hypochromic, microcytic anemia. Using a positional cloning strategy, we identified a gene, six-transmembrane epithelial antigen of the prostate 3 (Steap3), responsible for the iron deficiency anemia in the mouse mutant nm1054. Steap3 is expressed highly in hematopoietic tissues, colocalizes with the Tf cycle endosome and facilitates Tf-bound iron uptake. Steap3 shares homology with F(420)H(2):NADP(+) oxidoreductases found in archaea and bacteria, as well as with the yeast FRE family of metalloreductases. Overexpression of Steap3 stimulates the reduction of iron, and mice lacking Steap3 are deficient in erythroid ferrireductase activity. Taken together, these findings indicate that Steap3 is an endosomal ferrireductase required for efficient Tf-dependent iron uptake in erythroid cells.

Divalent metal-ion transporter DMT1 mediates both H+ -coupled Fe2+ transport and uncoupled fluxes

The H(+) -coupled divalent metal-ion transporter DMT1 serves as both the primary entry point for iron into the body (intestinal brush-border uptake) and the route by which transferrin-associated iron is mobilized from endosomes to cytosol in erythroid precursors and other cells. Elucidating the molecular mechanisms of DMT1 will therefore increase our understanding of iron metabolism and the etiology of iron overload disorders. We expressed wild type and mutant DMT1 in Xenopus oocytes and monitored metal-ion uptake, currents and intracellular pH. DMT1 was activated in the presence of an inwardly directed H(+) electrochemical gradient. At low extracellular pH (pH(o)), H(+) binding preceded binding of Fe(2+) and its simultaneous translocation. However, DMT1 did not behave like a typical ion-coupled transporter at higher pH(o), and at pH(o) 7.4 we observed Fe(2+) transport that was not associated with H(+) influx. His(272) --> Ala substitution uncoupled the Fe(2+) and H(+) fluxes. At low pH(o), H272A mediated H(+) uniport that was inhibited by Fe(2+). Meanwhile H272A-mediated Fe(2+) transport was independent of pH(o). Our data indicate (i) that H(+) coupling in DMT1 serves to increase affinity for Fe(2+) and provide a thermodynamic driving force for Fe(2+) transport and (ii) that His-272 is critical in transducing the effects of H(+) coupling. Notably, our data also indicate that DMT1 can mediate facilitative Fe(2+) transport in the absence of a H(+) gradient. Since plasma membrane expression of DMT1 is upregulated in liver of hemochromatosis patients, this H(+) -uncoupled facilitative Fe(2+) transport via DMT1 can account for the uptake of nontransferrin-bound plasma iron characteristic of iron overload disorders.

Slc11a2 is required for intestinal iron absorption and erythropoiesis but dispensable in placenta and liver

Solute carrier family 11, member 2 (SLC11A2) is the only transmembrane iron transporter known to be involved in cellular iron uptake. It is widely expressed and has been postulated to play important roles in intestinal iron absorption, erythroid iron utilization, hepatic iron accumulation, placental iron transfer, and other processes. Previous studies have suggested that other transporters might exist, but their physiological significance remained uncertain. To define the activities of Slc11a2 in vivo, we inactivated the murine gene that encodes it globally and selectively. We found that fetal Slc11a2 is not needed for materno-fetal iron transfer but that Slc11a2 activity is essential for intestinal non-heme iron absorption after birth. Slc11a2 is also required for normal hemoglobin production during the development of erythroid precursors. However, hepatocytes and most other cells must have an alternative, as-yet-unknown, iron uptake mechanism. We previously showed that Slc11a2 serves as the primary portal for intestinal iron entry in hemochromatosis. However, inactivation of murine Hfe ameliorates the phenotype of animals lacking Slc11a2.

Nontransferrin-bound iron uptake by hepatocytes is increased in the Hfe knockout mouse model of hereditary hemochromatosis

Hereditary hemochromatosis (HH) is an iron-overload disorder caused by a C282Y mutation in the HFE gene. In HH, plasma nontransferrin-bound iron (NTBI) levels are increased and NTBI is bound mainly by citrate. The aim of this study was to examine the importance of NTBI in the pathogenesis of hepatic iron loading in Hfe knockout mice. Plasma NTBI levels were increased 2.5-fold in Hfe knockout mice compared with control mice. Total ferric citrate uptake by hepatocytes isolated from Hfe knockout mice (34.1 +/- 2.8 pmol Fe/mg protein/min) increased by 2-fold compared with control mice (17.8 +/- 2.7 pmol Fe/mg protein/min; P <.001; mean +/- SEM; n = 7). Ferrous ion chelators, bathophenanthroline disulfonate, and 2',2-bipyridine inhibited ferric citrate uptake by hepatocytes from both mouse types. Divalent metal ions inhibited ferric citrate uptake by hepatocytes, as did diferric transferrin. Divalent metal transporter 1 (DMT1) mRNA and protein expression was increased approximately 2-fold by hepatocytes from Hfe knockout mice. We conclude that NTBI uptake by hepatocytes from Hfe knockout mice contributed to hepatic iron loading. Ferric ion was reduced to ferrous ion and taken up by hepatocytes by a pathway shared with diferric transferrin. Inhibition of uptake by divalent metals and up-regulation of DMT1 expression suggested that NTBI uptake was mediated by DMT1.

Transplasma membrane electron transport: enzymes involved and biological function

The notion of transmembrane electron transport is usually associated with mitochondria and chloroplasts. However, since the early 1970s, it has been known that this phenomenon also occurs at the level of the plasma membrane. Ever since, evidence has accumulated for the existence of a plethora of transplasma membrane electron transport enzymes. In this review, we discuss the various enzymes known, their molecular characteristics and their biological functions.

Accumulation of arsenic in tissues of iron-deficient rats

Toxic concentration of arsenic (400 microg/kg body weight/day) was administered orally for 8 weeks to both iron-supplemented and iron-deficient rats. The results showed greater amount of arsenic in both the duodenum and liver of iron-deficient rats, compared to iron-supplemented ones. Duodenum containing high concentration of arsenic in iron-deficient rats showed significant reduction of ferrireductase (the enzyme responsible for the conversion of ferric iron to ferrous one) activity. Our results suggest that anemic rats are more prone to develop arsenic poisoning following chronic ingestion of high content of arsenic.

Leishmania chagasi: uptake of iron bound to lactoferrin or transferrin requires an iron reductase

Leishmania chagasi can utilize iron bound to transferrin, lactoferrin, or other chelates. We investigated the mechanism of iron uptake. Promastigotes preferentially took up iron in a reduced rather than an oxidized form, suggesting that extracellular iron must be reduced prior to internalization. Similar to literature reports, a 70-kDa protein in promastigote membrane-containing microsomes bound to [125I]-labeled transferrin. However, [125I]lactoferrin and [125I]albumin also bound a similar 70-kDa protein, suggesting that binding might not be specific. Both total and fractionated promastigotes exhibited an NADPH-dependent iron reductase activity. In contrast to trypanosomes, which take up transferrin through a specific receptor, these data support a model in which a parasite-associated or secreted reductase reduces ferric to ferrous iron, decreasing its affinity for the extracellular chelate and allowing it to be readily internalized by the parasite. This could account for the ability of the parasite to utilize iron from multiple sources in diverse host environments. Index Descriptors and Abbreviations. Index descriptors: Cryptococcus neoformans, Histoplasma capsulatum, iron, iron reductase, lactoferrin, L. chagasi, leishmaniasis, nutrient acquisition, protozoan, Saccharomyces cerevisiae, Trypanosoma brucei, Trypanosoma cruzi, transferrin; Abbreviations used: DNA, deoxyribonucleic acid; DTT, dithiothreitol; HBSS, Hanks' balanced salt solution; MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide; NEM, N-ethylmaleimide; RNA, ribonucleic acid.

Copper repletion enhances apical iron uptake and transepithelial iron transport by Caco-2 cells

The influence of copper status on Caco-2 cell apical iron uptake and transepithelial transport was examined. Cells grown for 7-8 days in media supplemented with 1 microM CuCl(2) had 10-fold higher cellular levels of copper compared with control. Copper supplementation did not affect the integrity of differentiated Caco-2 cell monolayers grown on microporous membranes. Copper-repleted cells displayed increased uptake of iron as well as increased transport of iron across the cell monolayer. Northern blot analysis revealed that expression of the apical iron transporter divalent metal transporter-1 (DMT1), the basolateral transporter ferroportin-1 (Fpn1), and the putative ferroxidase hephaestin (Heph) was upregulated by copper supplementation, whereas the recently identified ferrireductase duodenal cytochrome b (Dcytb) was not. These results suggest that DMT1, Fpn1, and Heph are involved in the iron uptake process modulated by copper status. Although a clear role for Dcytb was not identified, an apical surface ferrireductase was modulated by copper status, suggesting that its function also contributes to the enhanced iron uptake by copper-repleted cells. A model is proposed wherein copper promotes iron depletion of intestinal Caco-2 cells, creating a deficiency state that induces upregulation of iron transport factors.

Iron and gallium increase iron uptake from transferrin by human melanoma cells: further examination of the ferric ammonium citrate-activated iron uptake process

Previously we showed that preincubation of cells with ferric ammonium citrate (FAC) resulted in a marked increase in Fe uptake from both (59)Fe-transferrin (Tf) and (59)Fe-citrate (D.R. Richardson, E. Baker, J. Biol. Chem. 267 (1992) 13972-13979; D.R. Richardson, P. Ponka, Biochim. Biophys. Acta 1269 (1995) 105-114). This Fe uptake process was independent of the transferrin receptor and appeared to be activated by free radicals generated via the iron-catalysed Haber-Weiss reaction. To further understand this process, the present investigation was performed. In these experiments, cells were preincubated for 3 h at 37 degrees C with FAC or metal ion solutions and then labelled for 3 h at 37 degrees C with (59)Fe-Tf. Exposure of cells to FAC resulted in Fe uptake from (59)Fe-citrate that became saturated at an Fe concentration of 2.5 microM, while FAC-activated Fe uptake from Tf was not saturable up to 25 microM. In addition, the extent of FAC-activated Fe uptake from citrate was far greater than that from Tf. These results suggest a mechanism where FAC-activated Fe uptake from citrate may result from direct interaction with the transporter, while Fe uptake from Tf appears indirect and less efficient. Preincubation of cells with FAC at 4 degrees C instead of 37 degrees C prevented its effect at stimulating (59)Fe uptake from (59)Fe-Tf, suggesting that an active process was involved. Previous studies by others have shown that FAC can increase ferrireductase activity that may enhance (59)Fe uptake from (59)Fe-Tf. However, there was no difference in the ability of FAC-treated cells compared to controls to reduce ferricyanide to ferrocyanide, suggesting no change in oxidoreductase activity. To examine if activation of this Fe uptake mechanism could occur by incubation with a range of metal ions, cells were preincubated with either FAC, ferric chloride, ferrous sulphate, ferrous ammonium sulphate, gallium nitrate, copper chloride, zinc chloride, or cobalt chloride. Stimulation of (59)Fe uptake from Tf was shown (in order of potency) with ferric chloride, ferrous sulphate, ferrous ammonium sulphate, and gallium nitrate. The other metal ions examined decreased (59)Fe uptake from Tf. The fact that redox-active Cu(II) ion did not stimulate Fe uptake while redox-inactive Ga(III) did, suggests a mechanism of transporter activation not solely dependent on free radical generation. Indeed, the activation of Fe uptake appears dependent on the presence of the Fe atom itself or a metal ion with atomic similarities to Fe (e.g. Ga).

Iron transport

Iron homeostasis is maintained by regulating its absorption: Under conditions of deficiency, assimilation is enhanced but iron uptake is otherwise limited to prevent toxicity due to overload. Iron deficiency remains the most important micronutrient deficiency worldwide, but increasing awareness of the genetic basis for iron-loading diseases points to iron overload as a major public health issue as well. Recent identification of mutant alleles causing iron uptake disorders in mice and humans provides new insights into the mechanisms involved in iron transport and its regulation. This article summarizes these discoveries and discusses their impact on our current understanding of iron transport and its regulation.

The aconitase function of iron regulatory protein 1. Genetic studies in yeast implicate its role in iron-mediated redox regulation

Iron regulatory proteins (IRP) are sequence-specific RNA-binding proteins that mediate iron-responsive gene regulation in animals. IRP1 is also the cytosolic isoform of aconitase (c-aconitase). This latter activity could complement a mitochondrial aconitase mutation (aco1) in Saccharomyces cerevisiae to restore glutamate prototrophy. In yeast, the c-aconitase activity of IRP1 was responsive to iron availability in the growth medium. Although IRP1 expression rescued aco1 yeast from glutamate auxotrophy, cells remained growth-limited by glutamate, displaying a slow-growth phenotype on glutamate-free media. Second site mutations conferring enhanced cytosolic aconitase-dependent (ECA) growth were recovered. Relative c-aconitase activity was increased in extracts of strains harboring these mutations. One of the ECA mutations was found to be in the gene encoding cytosolic NADP(+)-dependent isocitrate dehydrogenase (IDP2). This mutation, an insertion of a Ty delta element into the 5' region of IDP2, markedly elevates expression of Idp2p in glucose media. Our results demonstrate the physiological significance of the aconitase activity of IRP1 and provide insight into the role of c-aconitase with respect to iron and cytoplasmic redox regulation.

Plasma membrane NADH-oxidoreductase system: a critical review of the structural and functional data

The observation in the early 1970s that ferricyanide can replace transferrin as a growth factor highlighted the major role plasma membrane proteins can play within a mammalian cell. Ferricyanide, being impermeant to the cell, was assumed to act at the level of the plasma membrane. Since that time, several enzymes isolated from the plasma membrane have been described, which, using NADH as the intracellular electron donor, are capable of reducing ferricyanide. However, their exact modes of action, and their physiological substrates and functions have not been solved to date. Numerous hypotheses have been proposed for the role of such redox enzymes within the plasma membrane. Examples include the regulation of cell signaling, cell growth, apoptosis, proton pumping, and ion channels. All of these roles may be a result of the function of these enzymes as cellular redox sensors. The emergence of many diverse roles for ferricyanide utilizing redox enzymes present in the plasma membrane might also, in part, be due to the numerous redox enzymes present within the membrane; the poor molecular characterization of the enzymes may be the reason for some of the diverging results reported in the literature as various researchers may be working on different enzymes. Here we review the diverse proposals given for structure and function to the plasma membrane NADH-oxidoreductase system(s) with a specific focus on those enzyme activities which can couple ferricyanide and NADH. Although they are still ill-defined enzymes, evidence is rising that they are of utmost significance for cellular regulation.

Iron absorption and transport

Iron is vital for living organisms because it is essential for multiple metabolic processes to include oxygen transport, DNA synthesis, and electron transport. However, iron must be bound to proteins to prevent tissue damage from free radical formation. Thus, its concentrations in body organs must be regulated carefully. Intestinal absorption is the primary mechanism regulating iron concentrations in the body. Three pathways for intestinal iron uptake have been proposed and reported. These are the mobilferrin-integrin pathway, the divalent cation transporter 1 (DCT-1) [or natural resistance-associated macrophage protein (Nramp2)] pathway, and a separate pathway for uptake of heme by absorptive cells. Each of these pathways are incompletely described. However, studies with blocking antibodies, observations in rodents with disorders of iron metabolism, and studies in tissue culture cells suggest that the DCT-1 pathway is dominant in embryonic cells and is involved with cellular uptake of ferrous iron, whereas the mobilferrin-integrin pathway facilitates absorption of dietary inorganic ferric iron. Thus, there are separate pathways for cellular uptake of ferric and ferrous inorganic iron. Body iron can enter intestinal cells from plasma via basolateral membranes containing the classical transferrin receptor pathway with a high affinity for holotransferrin. This keeps the absorptive cell informed of the state of iron repletion of the host. Intestinal mucosal cell iron seems to exit the cell via a distinct apotransferrin receptor and a newly described protein named hephaestin. Unlike the absorptive surface of intestinal cells, most other cells possess transferrin receptors on their surfaces and the vast majority of iron entering these cells is transferrin associated. There seem to be 2 distinct pathways by which transferrin iron enters nonintestinal cells. In the classical clathrin-coated pitendosome pathway, iron accompanies transferrin into the cell to enter a vesicle, which releases the iron to the cytosol with acidification (high affinity, low capacity). Under physiological conditions, a second transferrin associated pathway (low affinity, high capacity) exists which has been named the transferrin receptor independent pathway (TRIP). How the TRIP delivers iron to cells is incompletely described. In addition, tissue culture studies show that nonintestinal cells can accept iron from soluble iron salts. This occurs via the mobilferrin-integrin and probably the DCT-1 pathways. Cellular uptake of iron from iron salts probably occurs in iron overloading disorders and may be responsible for free radical damage when the iron binding capacity of plasma is exceeded. Radioiron entering the cell via the heme and transferrin associated pathways can be found in isolates of mobilferrin/paraferritin and hemoglobin. This interaction probably occurs to permit NADPH dependent ferrireduction so iron can be used for synthesis of heme proteins. Production of heme from iron delivered via these routes indicates functional specificity for the pathways.

Chloramphenicol-induced mitochondrial dysfunction is associated with decreased transferrin receptor expression and ferritin synthesis in K562 cells and is unrelated to IRE-IRP interactions

Chloramphenicol is an antibiotic that consistently suppresses the bone marrow and induces sideroblastic anemia. It is also a rare cause of aplastic anemia. These toxicities are thought to be related to mitochondrial dysfunction, since chloramphenicol inhibits mitochondrial protein synthesis. We hypothesized that chloramphenicol-induced mitochondrial impairment alters the synthesis of ferritin and the transferrin receptor. After treating K562 erythroleukemia cells with a therapeutic dose of chloramphenicol (10 microg/ml) for 4 days, there was a marked decrease in cell surface transferrin receptor expression and de novo ferritin synthesis associated with significant decreases in cytochrome c oxidase activity, ATP levels, respiratory activity, and cell growth. Decreases in the transferrin receptor and ferritin were associated with reduced and unchanged message levels, respectively. The mechanism by which mitochondrial dysfunction alters these important proteins in iron homeostasis is not clear. A global decrease in synthetic processes seems unlikely, since the expression of the cellular adhesion proteins VLA4 and CD58 was not significantly decreased by chloramphenicol, nor were the message levels of beta-actin or ferritin. The alterations were not accompanied by changes in binding of the iron response protein (IRP) to the iron-responsive element (IRE), although cytosolic aconitase activity was reduced by 27% in chloramphenicol-treated cells. A disturbance in iron homeostasis due to alterations in the transferrin receptor and ferritin may explain the hypochromic-microcytic anemia and the accumulation of nonferritin iron in the mitochondria in some individuals after chloramphenicol therapy. Also, these studies provide evidence of a link between mitochondrial impairment and iron metabolism in K562 cells.

Metal ion transporters in mammals: structure, function and pathological implications

Despite the importance of metal ions in several catalytic functions, there has been, until recently, little molecular information available on the mechanisms whereby metal ions are actively taken up by mammalian cells. The classical concept for iron uptake into mammalian cells has been the endocytosis of transferrin-bound Fe3+ by the transferrin receptor. Studies with hypotransferrinaemic mice revealed that in the intestine mucosal transferrin is derived from the plasma and that its presence is not required in the intestinal lumen for dietary iron absorption. This suggests that, at least in the intestine, other non-receptor-mediated uptake systems exist. The molecular identification of metal ion transporters is of great importance, in particular since an increasing number of human diseases are thought to be related to disturbances in metal ion homeostasis, including metal ion overload and deficiency disorders (i.e. anaemia, haemochromatosis, Menkes disease, Wilson's disease), and neurodegenerative diseases (i.e. Alzheimer's, Friedreich's ataxia and Parkinson's diseases). Furthermore, susceptibilities to mycobacterial infections are caused by metal ion transporter defects. The pathological implications of disturbed metal ion homeostasis confirm the vital roles these metal ions play in the catalytic function of many enzymes, in gene regulation (zinc-finger proteins), and in free radical homeostasis. Recent insights have significantly advanced our knowledge of how metal ions are taken up or released by mammalian cells. The purpose of this review is to summarize these advances and to give an overview on the growing number of mammalian metal ion transporters.

Role of ascorbic acid in transferrin-independent reduction and uptake of iron by U-937 cells

The role of ascorbic acid in transferrin-independent ferric iron reduction and uptake was evaluated in cultured U-937 monocytic cells. Uptake of 55Fe by U-937 cells was doubled by 100 microM extracellular ascorbate, and by pre-incubation of cells with 100 microM dehydroascorbic acid, the two-electron-oxidized form of ascorbate. Reduction of extracellular ferric citrate also was enhanced by loading the cells with dehydroascorbic acid. Dehydroascorbic acid was taken up rapidly by the cells and reduced to ascorbate, such that the latter reached intracellular concentrations as high as 6 mM. However, some ascorbate did escape the cells and could be detected at concentrations of up to 1 microM in the incubation medium. Further, addition of ascorbate oxidase almost reversed the effects of dehydroascorbic acid on both 55Fe uptake and ferric citrate reduction. Thus, it is likely that extracellular ascorbate reduced ferric to ferrous iron, which was then taken up by the cells. This hypothesis also was supported by the finding that during loading with ferric citrate, only extracellular ascorbate increased the pool of intracellular ferrous iron that could be chelated with cell-penetrant ferrous iron chelators. In contrast to its inhibition of ascorbate-dependent ferric iron reduction, ascorbate oxidase was without effect on ascorbate-dependent reduction of extracellular ferricyanide. This indicates that the cells use different mechanisms for reduction of ferric iron and ferricyanide. Therefore, extracellular ascorbate derived from cells can enhance transferrin-independent iron uptake by reducing ferric to ferrous iron, but intracellular ascorbate neither contributes to this reduction nor modifies the redox status of intracellular free iron.

Is ascorbic acid an antioxidant for the plasma membrane?

Ascorbic acid, or vitamin C, is a primary antioxidant in plasma and within cells, but it can also interact with the plasma membrane by donating electrons to the alpha-tocopheroxyl radical and a trans-plasma membrane oxidoreductase activity. Ascorbate-derived reducing capacity is thus transmitted both into and across the plasma membrane. Recycling of alpha-tocopherol by ascorbate helps to protect membrane lipids from peroxidation. However, neither the mechanism nor function of the ascorbate-dependent oxidoreductase activity is known. This activity has typically been studied using extracellular ferricyanide as an electron acceptor. Whereas an NADH:ferricyanide reductase activity is evident in open membranes, ascorbate is the preferred electron donor within cells. The oxidoreductase may be a single membrane-spanning protein or may only partially span the membrane as part of a trans-membrane electron transport chain composed of a cytochrome or even hydrophobic antioxidants such as alpha-tocopherol or ubiquinol-10. Further studies are needed to elucidate the structural components, mechanism, and physiological significance of this activity. Proposed functions for the oxidoreductase include stimulation of cell growth, reduction of the ascorbate free radical outside cells, recycling of alpha-tocopherol, reduction of lipid hydroperoxides, and reduction of ferric iron prior to iron uptake by a transferrin-independent pathway.

Iron metabolism

The understanding of iron metabolism at the molecular level has been enormously expanded in recent years by new findings about the functioning of transferrin, the transferrin receptor and ferritin. Other recent developments include the discovery of the hemochromatosis gene HFE, identification of previously unknown proteins involved in iron transport, divalent metal transporter 1 and stimulator of Fe transport, and expanded insights into the regulation and expression of proteins involved in iron metabolism. Interactions among principal participants in iron transport have been uncovered, although the complexity of such interactions is still incompletely understood. Correlated efforts involving techniques and concepts of crystallography, spectroscopy and molecular biology applied to cellular processes have been, and should continue to be, particularly revealing.

Activation of an iron uptake mechanism from transferrin in hepatocytes by small-molecular-weight iron complexes: implications for the pathogenesis of iron-overload disease

The liver is one of the principal sites of iron overload in diseases such as hemochromatosis and beta thalassemia. Hence, much effort has been invested in examining the mechanisms of Fe uptake by hepatocytes. In the present study we have examined the effect of small molecular weight (M(r)) Fe complexes on Fe uptake from iron 59-labeled transferrin (Tf) and 59Fe-labeled citrate by primary cultures of hepatocytes. This was important to assess because Fe-citrate and saturated diferric Tf coexist in the serum of patients with untreated Fe overload. Preincubation of hepatocytes with the low-M(r) Fe complex ferric ammonium citrate (FAC; 25 microg/mL; (Fe) = 4.4 microg/mL) followed by incubation with 59Fe-Tf or 59Fe-citrate ((Fe) = 0.25 to 25 micromol/L) resulted in the marked stimulation of 59Fe uptake. For example, at a physiologically relevant Tf-Fe concentration of 25 micromol/L, there was an 8-fold increase in 59Fe uptake by cells incubated with FAC compared to control cells. In contrast, at Tf-Fe concentrations of 0.25 to 2.5 micromol/L, 59Fe uptake in FAC-treated cells was only 1-fold to 3-fold greater than that in the corresponding controls. These data suggest that the FAC-activated Fe uptake process predominates at physiologically relevant Tf concentrations above the saturation of the Tf receptor (TfR). This is the first study to demonstrate that preincubation of hepatocytes with Iow-M(r)Fe complexes can markedly increase Fe uptake from diferric Tf. In conclusion, these results may help to explain the loading of hepatocytes with Fe that occurs in Fe-overload disease despite marked down-regulation of the TfR.

Nitric oxide reduces nontransferrin-bound iron transport in HepG2 cells

Nitric oxide (NO) donors S-nitroso-N-acetylpenicillamine (SNAP) and sodium nitroprusside (SNP) modulate iron regulatory protein (IRP) activity and may, therefore, affect iron uptake through transferrin receptor expression. However, iron also enters the cell as nontransferrin-bound iron (NTBI), and the aim of this study was to evaluate the effects of NO donors on NTBI transport in HepG2 cells, a model of liver physiology. Incubation with SNP and SNAP led to a time- and concentration-dependent reduction in Fe3+ and Fe2+ uptake, thus indicating an effect on the transporter rather than on the reductase. In terms of Fe2+ uptake, no variations in the Michaelis-Menten constant (Km) and a reduction in maximum uptake (Vmax) (50, 33, and 16.6 fmol/microgram protein/min in control, SNP-, and SNAP-treated cells, respectively) were detected, which suggested a decrease in the number of putative NTBI transport protein(s). Gel shift assays showed that IRP activity was reduced by SNP and slightly increased by SNAP. Northern blot analysis of transferrin receptor messenger RNA (mRNA) levels showed variations similar to those observed for IRPs, but both NO donors increased L-ferritin mRNA levels and had no effect on the stimulator of Fe transport (SFT) mRNA. In conclusion, NO donors significantly reduce NTBI transport in HepG2 cells, an effect that seems to be IRP and SFT independent. Moreover, the reduction in NTBI uptake after NO treatment suggests that this form of iron may play a minor role in the increased hepatic iron stores observed in inflammation or that other liver cells are more involved in this pathological condition.

Ceruloplasmin ferroxidase activity stimulates cellular iron uptake by a trivalent cation-specific transport mechanism

The balance required to maintain appropriate cellular and tissue iron levels has led to the evolution of multiple mechanisms to precisely regulate iron uptake from transferrin and low molecular weight iron chelates. A role for ceruloplasmin (Cp) in vertebrate iron metabolism is suggested by its potent ferroxidase activity catalyzing conversion of Fe2+ to Fe3+, by identification of yeast copper oxidases homologous to Cp that facilitate high affinity iron uptake, and by studies of "aceruloplasminemic" patients who have extensive iron deposits in multiple tissues. We have recently shown that Cp increases iron uptake by cultured HepG2 cells. In this report, we investigated the mechanism by which Cp stimulates cellular iron uptake. Cp stimulated the rate of non-transferrin 55Fe uptake by iron-deficient K562 cells by 2-3-fold, using a transferrin receptor-independent pathway. Induction of Cp-stimulated iron uptake by iron deficiency was blocked by actinomycin D and cycloheximide, consistent with a transcriptionally induced or regulated transporter. Cp-stimulated iron uptake was completely blocked by unlabeled Fe3+ and by other trivalent cations including Al3+, Ga3+, and Cr3+, but not by divalent cations. These results indicate that Cp utilizes a trivalent cation-specific transporter. Cp ferroxidase activity was required for iron uptake as shown by the ineffectiveness of two ferroxidase-deficient Cp preparations, copper-deficient Cp and thiomolybdate-treated Cp. We propose a model in which iron reduction and subsequent re-oxidation by Cp are essential for an iron uptake pathway with high ion specificity.

Iron loading into ferritin by an intracellular ferroxidase

An intracellular, membrane-bound enzyme exhibiting both p-phenylenediamine oxidase activity and ferrous iron oxidase activity was isolated with the plasma membrane fraction of horse heart and studied for its ability to load iron into ferritin. The ferroxidase activity of the tissue oxidase was stimulated approximately twofold by horse spleen apoferritin, and the iron was loaded into ferritin. The loading of iron into ferritin by the tissue oxidase was inhibited by anti-horse serum ceruloplasmin antibody. The stoichiometry of iron oxidation and oxygen consumption during iron loading into ferritin by the tissue-derived oxidase and serum ceruloplasmin were 3.6 +/- 0.2 and 3.9 +/- 0.2, respectively. These data provide evidence that an enzyme analogous to ceruloplasmin is present on the plasma membrane of horse heart and that this ferroxidase is capable of catalyzing the loading of iron into ferritin. The implications of these data on the present models for the uptake and storage of iron by cells are discussed.

Non-transferrin iron uptake by trophoblast cells in culture. Significance of a NADH-dependent ferrireductase

This study shows that trophoblast cells in culture are able to take up 59Fe from both Fe(III)nitrilotriacetate (NTA) and Fe-ascorbate. Fe in the presence of ascorbate is assumed to be Fe(III) in equilibrium with Fe(II). Kinetic parameters for non-transferrin iron uptake are determined from initial rate experiments, yielding Vmax=366 pmol/mg protein/5 min and Km=0.96 microM for Fe(III)NTA and Vmax=4043 pmol/mg protein/5 min and Km= 1.3 microM for Fe-ascorbate. Since trophoblast cells in culture reduce extracellular Fe(III)CN, and uptake of 59Fe from Fe-ascorbate is higher than that from Fe(III)NTA, it is suggested that reduction of Fe(III) precedes uptake. Uptake of 59Fe from both Fe-ascorbate and Fe(III)NTA is inhibited by Fe(II)chelator ferrozine and membrane-impermeable Fe(III)CN, further supporting this hypothesis. Studies with microvillous membrane vesicles (MMV) and basal membrane vesicles (BMV) reveal the presence of a NADH-dependent ferrireductase. Reduction of Fe(III)CN follows Michaelis-Menten kinetics, both with respect to [NADH] and [Fe]. NADPH is ineffective as electron donor. The rate of Fe(III)CN reduction by BMV is 2.5 times higher compared to MMV, while Km values for Fe(III)CN and NADH are not significantly different. These results reveal that a transmembrane NADH-dependent ferrireductase plays a role in uptake of non-transferrin iron. The possibility that this enzyme system is involved in iron transfer across the basal membrane is discussed.

Mechanisms of ferric citrate uptake by human hepatoma cells

The mechanisms of uptake of non-transferrin-bound iron by human hepatoma cells (HuH7) were investigated using 59Fe-citrate and [14C]citrate. The amount of iron associated with the cells increased linearly with time, whereas citrate uptake reached a plateau after 45 min, resulting in a cellular accumulation of iron over citrate. The cells displayed high-affinity membrane binding sites for citrate with maximum binding of 118 +/- 17 pmol citrate/mg protein and a dissociation constant of 21 +/- 2 microM (n = 3). Iron uptake was saturable with a maximum uptake rate of 1.95 +/- 0.43 pmol . mg protein-1 . min-1 and an apparent Michaelis constant of 1.1 +/- 0.1 microM. Nonradioactive ferric citrate and citrate inhibited 59Fe uptake to a similar degree. This suggests that the uptake of citrate-bound iron is dependent on either binding to specific citrate binding sites or the concentration of unbound iron. The uptake of iron was inhibited by ferricyanide (>100 microM) and ferrous iron chelators but stimulated by ferrocyanide and ascorbate, suggesting that the iron is reduced from Fe3+ to Fe2+ and transported into the cell by an iron carrier-mediated step.

Neuroleptic drug-stimulated iron uptake by synaptosome preparations of rat cerebral cortex

Neuroleptic-induced tardive dyskinesia has been linked to impaired iron homeostasis in the central nervous system attributed to increased iron levels. A chlorpromazine stimulatory effect upon iron uptake from 55Fe-citrate and 55Fe-transferrin by cortical synaptosome preparations of rats was recently demonstrated. The present work extends this study to other neuroleptic drugs such as thioridazine, haloperidol, clozapine and fluphenazine. Like chlorpromazine, thioridazine showed a stimulatory effect upon iron uptake from both iron donors whereas fluphenazine highly increased uptake from 55Fe-citrate but not from 55Fe-transferrin. Haloperidol and clozapine had no effect. Stimulation of iron uptake by neuroleptics is probably related to their property of calmodulin antagonism, since calmidazolium also stimulated synaptosomal iron uptake from both donors. Calmidazolium-stimulated uptake from 55Fe-citrate was approx. 5-fold when compared to control samples while uptake from 55Fe-transferrin was 250% higher. The results are in agreement with the iron uptake magnitude observed with the different drugs for the two iron donors used and the reported Ki values of neuroleptic drugs for calmodulin antagonism evaluated by the inhibition of 3',5'-monophosphate phosphodiesterase activity. Moreover, vanadate, an inhibitor of protein phosphorylation and KCl-promoted membrane depolarization, greatly inhibited iron uptake from 55Fe-citrate by both chlorpromazine-treated and untreated synaptosome preparations.

Ascorbate stimulates ferricyanide reduction in HL-60 cells through a mechanism distinct from the NADH-dependent plasma membrane reductase

The impermeable oxidant ferricyanide is reduced by the plasma membrane redox system of HL-60 cells. The rate of reduction is strongly enhanced by ascorbate or dehydroascorbate. The aim of this study was to determine the mechanism by which ascorbate and dehydroascorbate accelerate ferricyanide reduction in HL-60 cells. Addition of ascorbate or dehydroascorbate to cells in the presence of ferricyanide led to the intracellular accumulation of ascorbate. Control experiments showed that extracellular ascorbate was rapidly converted to dehydroascorbate in the presence of ferricyanide. These data suggest that intracellular ascorbate originates from extracellular dehydroascorbate. Accumulation of ascorbate was prevented by inhibitors of dehydroascorbate transport into the cell. These compounds also strongly inhibited ascorbate-stimulated ferricyanide reduction in HL-60 cells. Thus, it is concluded that the stimulation of ferricyanide reduction is dependent on intracellular accumulation of ascorbate. Changing the alpha-tocopherol content of the cells had no effect on the ascorbate-stimulated ferricyanide reduction, showing that a nonenzymatic redox system utilizing alpha-tocopherol was not involved. p-Chloromercuribenzenesulfonic acid strongly affected ferricyanide reduction in the absence of ascorbate, whereas the stimulated reaction was much less responsive to this compound. Thus, it appears that at least two different membrane redox systems are operative in HL-60 cells, both capable of reducing ferricyanide, but through different mechanisms. The first system is the ferricyanide reductase, which uses NADH as its source for electrons, whereas the novel system proposed in this paper relies on ascorbate.

The surface of rat hepatocytes can transfer iron from stable chelates to external acceptors

The chelator diethylenetriaminepentaacetate (DTPA) forms a stable complex with iron that does not donate iron to transferrin under physiological conditions, i.e., pH above 7 and isotonic milieu. It does, however, deliver iron to hepatocytes. This uptake is initiated by a mobilization of the metal from the complex by the cell surface. When an external chelator is added simultaneously, it can bind the iron and inhibit its accumulation by the cells. This is shown here with the impermeant siderophore conjugate hydroxyethyl-starch coupled desferrioxamine, as well as with apotransferrin. We also demonstrate exchange of iron between DTPA and holo-transferrin, or at least movement from the chelator to the protein, which may have lost its iron to the cell in advance, providing new binding sites for mobilized iron. The efficient hepatocyte iron donor lactoferrin greatly stimulates iron uptake from DTPA, apparently by binding iron and transferring it into the cells by endocytosis. Ferritin is unable to do this; therefore, the mobilization of iron is not caused by a reducing activity at the cell surface, because iron is readily transferred from DTPA to ferritin by the reductant ascorbic acid. The transfer process is dependent on the temperature, the time, and the amount of cells present, and is partly inhibited by sulfhydryl reagents. We conclude that this activity represents a hitherto unidentified first step in the movement of iron through the cell membrane and may be relevant for transferrin-bound, as well as for non-transferrin-bound, iron uptake by hepatocytes.

Influence of copper depletion on iron uptake mediated by SFT, a stimulator of Fe transport

We recently identified a novel factor involved in cellular iron assimilation called SFT or Stimulator of Fe Transport (Gutierrez, J. A., Yu, J., Rivera, S., and Wessling-Resnick, M. (1997) J. Cell Biol. 149, 895-905). When stably expressed in HeLa cells, SFT was found to stimulate the uptake of both transferrin- and nontransferrin-bound Fe (iron). Assimilation of nontransferrin-bound Fe by HeLa cells stably expressing SFT was time- and temperature-dependent; both the rate and extent of uptake was enhanced relative to the activity of control nontransfected cells. Although the apparent Km for Fe uptake was unaffected by expression of SFT (5.6 versus 5.1 microM measured for control), the Vmax of transport was increased from 7.0 to 14.7 pmol/min/mg protein. Transport mediated by SFT was inhibitable by diethylenetriaminepentaacetic acid and ferrozine, Fe3+- and Fe2+-specific chelators. Because cellular copper status is known to influence Fe assimilation, we investigated the effects of Cu (copper) depletion on SFT function. After 4 days of culture in Cu-deficient media, HeLa cell Cu,Zn superoxide dismutase activity was reduced by more than 60%. Both control cells and cells stably expressing SFT displayed reduced Fe uptake as well; levels of transferrin-mediated import fell by approximately 80%, whereas levels of nontransferrin-bound Fe uptake were approximately 50% that of Cu-replete cells. The failure of SFT expression to stimulate Fe uptake above basal levels in Cu-depleted cells suggests a critical role for Cu in SFT function. A current model for both transferrin- and nontransferrin-bound Fe uptake involves the function of a ferrireductase that acts to reduce Fe3+ to Fe2+, with subsequent transport of the divalent cation across the membrane bilayer. SFT expression did not enhance levels of HeLa cell surface reductase activity; however, Cu depletion was found to reduce endogenous activity by 60%, suggesting impaired ferrireductase function may account for the influence of Cu depletion on SFT-mediated Fe uptake. To test this hypothesis, the ability of SFT to directly mediate Fe2+ import was examined. Although expression of SFT enhanced Fe2+ uptake by HeLa cells, Cu depletion did not significantly reduce this activity. Thus, we conclude that a ferrireductase activity is required for SFT function in Fe3+ transport and that Cu depletion reduces cellular iron assimilation by affecting this activity.

Nramp2 is mutated in the anemic Belgrade (b) rat: evidence of a role for Nramp2 in endosomal iron transport

The Belgrade (b) rat has an autosomal recessively inherited, microcytic, hypochromic anemia associated with abnormal reticulocyte iron uptake and gastrointestinal iron absorption. The b reticulocyte defect appears to be failure of iron transport out of endosomes within the transferrin cycle. Aspects of this phenotype are similar to those reported for the microcytic anemia (mk) mutation in the mouse. Recently, mk has been attributed to a missense mutation in the gene encoding the putative iron transporter protein Nramp2. To investigate the possibility that Nramp2 was also mutated in the b rat, we established linkage of the phenotype to the centromeric portion of rat chromosome 7. This region exhibits synteny to the chromosomal location of Nramp2 in the mouse. A polymorphism within the rat Nramp2 gene cosegregated with the b phenotype. A glycine-to-arginine missense mutation (G185R) was present in the b Nramp2 gene, but not in the normal allele. Strikingly, this amino acid alteration is the same as that seen in the mk mouse. Functional studies of the protein encoded by the b allele of rat Nramp2 demonstrated that the mutation disrupted iron transport. These results confirm the hypothesis that Nramp2 is the protein defective in the Belgrade rat and raise the possibility that the phenotype shared by mk and b animals is unique to the G185R mutation. Furthermore, the phenotypic characteristics of these animals indicate that Nramp2 is essential both for normal intestinal iron absorption and for transport of iron out of the transferrin cycle endosome.

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.

Functional expression cloning and characterization of SFT, a stimulator of Fe transport

A stimulator of Fe transport (SFT) was identified by functional expression cloning in Xenopus oocytes. SFT-mediated transport has properties defined for transferrin-independent Fe uptake, but its cytolocalization in recycling endosomes and the observed stimulation of transferrin-bound Fe assimilation indicate a key role in intracellular Fe membrane transport as well. SFT has six predicted transmembranous domains and a functionally important RExxE motif that resembles domains involved in yeast Fe transport and Fe-binding by ferritin L-chains. The observation that SFT oligomerizes, along with other structural and mechanistic features, suggests it may be a member of either the ATP-binding cassette or cation diffusion facilitator families. The 3' untranslated region of SFT contains a translation inhibitory element and inhibition of SFT expression in Xenopus oocytes was found to be relieved by coinjection of transcripts from other defined cDNAs that are also described in this report. SFT is the first component of the mammalian Fe membrane transport machinery to be identified.

Nitric oxide prevents oxidative damage produced by tert-butyl hydroperoxide in erythroleukemia cells via nitrosylation of heme and non-heme iron. Electron paramagnetic resonance evidence

We studied protective effects of NO against tert-butylhydroperoxide (t-BuOOH)-induced oxidations in a subline of human erythroleukemia K562 cells with different intracellular hemoglobin (Hb) concentrations. t-BuOOH-induced formation of oxoferryl-Hb-derived free radical species in cells was demonstrated by low temperature EPR spectroscopy. Intensity of the signals was proportional to Hb concentrations and was correlated with cell viability. Peroxidation of phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, and cardiolipin metabolically labeled with oxidation-sensitive cis-parinaric acid was induced by t-BuOOH. An NO donor, (Z)-1-[N-(3-ammoniopropyl)-N-(n-propyl)amino]-diazen-1-iu m-1, 2-diolate], produced non-heme iron dinitrosyl complexes and hexa- and pentacoordinated Hb-nitrosyl complexes in the cells. Nitrosylation of non-heme iron centers and Hb-heme protected against t-BuOOH-induced: (a) formation of oxoferryl-Hb-derived free radical species, (b) peroxidation of cis-parinaric acid-labeled phospholipids, and (c) cytotoxicity. Since NO did not inhibit peroxidation induced by an azo-initiator of peroxyl radicals, 2, 2'-azobis(2,4-dimethylvaleronitrile), protective effects of NO were due to formation of iron-nitrosyl complexes whose redox interactions with t-BuOOH prevented generation of oxoferryl-Hb-derived free radical species.