Iron regulatory proteins and the molecular control of mammalian iron metabolism

Distinct translation regulation by two alternative 5'UTRs of a stress-responsive protein--dPrx I

Translation efficiency is often regulated in part by 5'-untranslated region (5'UTR). Sequence analysis of an evolutionarily conserved stress-responsive protein, Drosophila Peroxiredoxin I (dPrx I), found the transcript to have two alternative 5'UTRs that lead to an identical coding sequence: namely Ia and Ib. Although both isoforms coexisted in Drosophila cells, the Ia isoform appeared to be dominant. Furthermore, reporter assay found that Ia enhanced translation in steady-state cells while Ib increased translation in cells under oxidative stress. Together, our data suggest that the two alternative 5'UTRs of dPrx I may be involved in a translational regulatory mechanism that responds to cellular oxidative stress.

Addition of calcite reduces iron's bioavailability in the Pennsylvania coals--potential use of calcite for the prevention of coal workers' lung diseases

In the present study, a hypothesis was tested that the addition of calcite into the Pennsylvania coals may reduce levels of bioavailable iron (BAI), an important component in the mixed coal dust that may contribute to coal workers' lung diseases. Predetermined proportions of calcite (0, 1, 2, 5, 10% w/w) were added into three PA coals. After suspending the mixtures in an aqueous phosphate solution (10 mM, pH 4.5), which mimics the phagolysosomal conditions of the cells, levels of pH as well as calcium ions (Ca2+) in the coals were increased in a calcite concentration-dependent manner. In contrast, levels of BAI (both Fe2+ and Fe3+) were decreased. The inhibitory effects of calcite on the bioavailability of iron in human lung epithelial A549 cells and primary rat alveolar macrophages (AMs) were also examined. It was found that levels of low-molecular-weight (LMW) iron were significantly decreased in both A549 cells and AMs treated with the 10% calcite-PA coal mixture compared to those treated with the PA coal alone, while calcite itself had no effect on intracellular LMW iron. Calcite also showed a significant inhibitory effect on PA coal-induced ferritin synthesis in A549 cells. Reverse-transcription polymerase chain reaction (RT-PCR) studies revealed that the iron-containing PA coal downregulated levels of transferrin receptor (TfR) mRNA in A549 cells, which was partially restored by the addition of calcite. Our results indicate that calcite can inhibit the bioavailability of iron in the iron-containing PA coals.

The restless legs syndrome

The restless legs syndrome (RLS) is one of the commonest neurological sensorimotor disorders at least in the Western countries and is often associated with periodic limb movements (PLM) during sleep leading to severe insomnia. However, it remains largely underdiagnosed and its underlying pathogenesis is presently unknown. Women are more affected than men and early-onset disease is associated with familial cases. A genetic origin has been suggested but the mode of inheritance is unknown. Secondary causes of RLS may share a common underlying pathophysiology implicating iron deficiency or misuse. The excellent response to dopaminegic drugs points to a central role of dopamine in the pathophysiology of RLS. Iron may also represent a primary factor in the development of RLS, as suggested by recent pathological and brain imaging studies. However, the way dopamine and iron, and probably other compounds, interact to generate the circadian pattern in the occurrence of RLS and PLM symptoms remains unknown. The same is also the case for the level of interaction of the two compounds within the central nervous system (CNS). Recent electrophysiological and animals studies suggest that complex spinal mechanisms are involved in the generation of RLS and PLM symptomatology. Dopamine modulation of spinal reflexes through dopamine D3 receptors was recently highlighted in animal models. The present review suggests that RLS is a complex disorder that may result from a complex dysfunction of interacting neuronal networks at one or several levels of the CNS and involving numerous neurotransmitter systems.

Serum ferritin in stroke: a marker of increased body iron stores or stroke severity?

To evaluate the effect of body iron stores on the vulnerability of the brain to ischemia, a focal permanent brain ischemia was induced by photothrombotic occlusion of cortical vessels in rats with or without chronic treatment with iron dextran (25 mg iron/kg, every other day for 20 days, intraperitoneally). Iron dextran induced systemic iron overload as evidenced by high ferritin (Ft) ( x 5) and total iron levels ( x 3) in serum as well as increased Ft expression in the liver and heart. Conversely, neither serum free iron levels nor Ft expression in the brain were changed by iron dextran. Finally, infarct volume was not modified by iron dextran. In addition, induction of ischemia in rats treated with FeCl(3) (560 microg iron/kg, intravenously) as a means of increasing serum free iron levels during the ischemic period did not enlarge infarct volume. We then explored the effect of brain ischemia itself on serum Ft by measuring serum Ft before and after induction of brain ischemic insults with different neurologic outcomes in rats (brain embolization with microspheres, photothrombotic occlusion of cortical vessels, four-vessel occlusion). Serum Ft levels were found higher at day 1 after ischemia than before ischemia only in rats subjected to the most severe insult (brain embolization). In conclusion, our study showed that increased body iron stores do not increase the vulnerability of the brain to ischemia and that brain ischemia, if severe, results in the elevation of serum Ft levels.

Altered body iron distribution and microcytosis in mice deficient in iron regulatory protein 2 (IRP2)

Iron regulatory protein 2 (IRP2)-deficient mice have been reported to suffer from late-onset neurodegeneration by an unknown mechanism. We report that young adult Irp2-/- mice display signs of iron mismanagement within the central iron recycling pathway in the mammalian body, the liver-bone marrow-spleen axis, with altered body iron distribution and compromised hematopoiesis. In comparison with wild-type littermates, Irp2-/- mice are mildly microcytic with reduced serum hemoglobin levels and hematocrit. Serum iron and transferrin saturation are unchanged, and hence microcytosis is not due to an overt decrease in systemic iron availability. The liver and duodenum are iron loaded, while the spleen is iron deficient, associated with a reduced expression of the iron exporter ferroportin. A reduction in transferrin receptor 1 (TfR1) mRNA levels in the bone marrow of Irp2-/- mice can plausibly explain the microcytosis by an intrinsic defect in erythropoiesis due to a failure to adequately protect TfR1 mRNA against degradation. This study links a classic regulator of cellular iron metabolism to systemic iron homeostasis and erythropoietic TfR1 expression. Furthermore, this work uncovers aspects of mammalian iron metabolism that can or cannot be compensated for by the expression of IRP1. (Blood. 2005;106: 2580-2589)

Subcellular localization of iron regulatory proteins to Golgi and ER membranes

Interaction between iron regulatory proteins and iron responsive elements on certain mRNAs is at the core of regulation of intracellular iron homeostasis. Previous results suggested that in cultured cells iron regulatory proteins (IRPs) exist in cytosolic and microsomal subcellular locations and that this distribution is affected by cellular iron status. In this study, we tested the hypothesis that the membrane-associated fractions of iron regulatory proteins are specifically in the endoplasmic reticulum and Golgi membranes. Confocal microscopy revealed that IRP1 could be co-localized to the endoplasmic reticulum and the Golgi apparatus. To examine the intracellular distribution of IRPs biochemically, we used rats fed normal or iron-deficient diets. As expected, the IRPs were found predominantly in the cytosolic fraction. However, subfractionation of crude microsomal preparations revealed IRP1 in the Golgi apparatus. In animals fed an iron-deficient diet, IRP1 was found in the Golgi apparatus and the endoplasmic reticulum. To identify the mechanisms and factors involved in the localization of iron regulatory proteins in the cytosol and membrane fractions, cells were treated with a phorbol ester, a protein kinase C inhibitor (chelerythrine), hydrogen peroxide, interleukin-1beta, and 1,2-bis-(o-aminophenoxy)-ethane-N,N,-N'N'-tetraacetic acid tetraacetoxy-methyl ester. The results indicate that iron-regulatory-protein-binding activity in the membrane fraction can be altered by cell stress or iron status and that phosphorylation plays a role in the translocation. As a result of this study we propose a novel model for intracellular distribution of IRPs and identify differences between the two iron regulatory proteins.

Divergent modulation of iron regulatory proteins and ferritin biosynthesis by hypoxia/reoxygenation in neurones and glial cells

Ferritin, the main iron storage protein, exerts a cytoprotective effect against the iron-catalyzed production of reactive oxygen species, but its role in brain injury caused by hypoxia/reoxygenation is unclear. Ferritin expression is regulated mainly at post-transcriptional level by iron regulatory proteins (IRP1 and IRP2) that bind specific RNA sequences (IREs) in the 5'untranslated region of ferritin mRNA. Here, we show that hypoxia decreases IRP1 binding activity in glial cells and enhances it in cortical neurons. These effects were reversed by reoxygenation in both cell types. In glial cells there was an early increase of ferritin synthesis during hypoxia and reoxygenation. Conversely, in cortical neurons, ferritin synthesis increased during the late phase of reoxygenation. Steady-state analysis of ferritin mRNA levels suggested that ferritin synthesis is regulated mainly post-transcriptionally by IRPs in glioma cells, both transcriptionally and post-transcriptionally in type-1 astrocytes, and mainly at transcriptional level in an IRP-independent way in neurons. The different regulation of ferritin expression may account for the different vulnerability of neurons and glial cells to the injury elicited by oxygen and glucose deprivation (OGD)/reoxygenation. The greater vulnerability of cortical neurons to hypoxia-reoxygenation was strongly attenuated by the exogenous administration of ferritin during OGD/reoxygenation, suggesting the possible cytoprotective role exerted by this iron-segregating protein.

Presence of the iron exporter ferroportin at the plasma membrane of macrophages is enhanced by iron loading and down-regulated by hepcidin

Ferroportin, the only mammalian iron exporter identified to date, is highly expressed in duodenal enterocytes and in macrophages. Several lines of evidence indicate that in enterocytes the iron export mediated by ferroportin occurs and is regulated at the basolateral cell surface, where the transporter is strongly expressed. By contrast, in macrophages, ferroportin has been shown in intracellular vesicles. We used a high-affinity antibody to specify the localization of endogenous ferroportin expressed in primary culture of bone marrow-derived macrophages, in both basal and induced conditions. Our observations indicate that ferroportin is expressed in vesicular compartments that can reach the plasma membrane of macrophages. Of importance, when ferroportin expression was up-regulated through iron treatment or erythrophagocytosis, ferroportin expression was strongly enhanced at the plasma membrane of macrophages. Moreover, hepcidin dramatically reduced macrophage ferroportin protein levels. At the subcellular level, hepcidin was shown to induce rapid internalization and degradation of the macrophage iron exporter. These data are consistent with a direct iron export by ferroportin through the plasma membrane of macrophages and strongly support an efficient posttranscriptional down-regulation of ferroportin by hepcidin in these cells.

Intracellular labile iron pools as direct targets of iron chelators: a fluorescence study of chelator action in living cells

The primary targets of iron chelators used for treating transfusional iron overload are prevention of iron ingress into tissues and its intracellular scavenging. The present study was aimed at elucidating the capacity of clinically important iron chelators such as deferiprone (DFP), desferrioxamine, and ICL670 to (a) gain direct access to intracellular iron pools of key cells of iron accumulation (macrophages, hepatocytes, and cardiomyocyte cell lines); (b) chelate the labile iron present in discrete cell compartments/organelles; and (c) prevent labile iron involvement in the generation of reactive oxidant species. Chelation of cytosolic and organellar cell iron was visualized dynamically and quantitatively in living cells by fluorescence microscopic imaging of fluorescent metallosensors (used as iron-quenched complexes of calceins) targeted to either cytosol, endosome-lysosomes, or mitochondria. The rate and extent of fluorescence recovery provided an in situ measure of the accessibility of chelators to particular cell sites/organelles. Complementary, fluorogenic redox probes associated with cell compartments enabled identification of chelator-sensitive, localized reactive oxidant production. Our studies indicate that chelation by desferrioxamine is slow and is enhanced in cells with relatively high endocytic activities, while ICL670 and DFP readily enter most cells and efficiently reach the major intracellular sites of iron accumulation.

The influence of high iron diet on rat lung manganese absorption

Individuals chronically exposed to manganese are at high risk for neurotoxic effects of this metal. A primary route of exposure is through respiration, although little is known about pulmonary uptake of metals or factors that modify this process. High dietary iron levels inversely affect intestinal uptake of manganese, and a major goal of this study was to determine if dietary iron loading could increase lung non-heme iron levels and alter manganese absorption. Rats were fed a high iron (1% carbonyl iron) or control diet for 4 weeks. Lung non-heme iron levels increased approximately 2-fold in rats fed the high iron diet. To determine if iron-loading affected manganese uptake, 54Mn was administered by intratracheal (it) instillation or intravenous (iv) injection for pharmacokinetic studies. 54Mn absorption from the lungs to the blood was lower in it-instilled rats fed the 1% carbonyl iron diet. Pharmacokinetics of iv-injected 54Mn revealed that the isotope was cleared more rapidly from the blood of iron-loaded rats. In situ analysis of divalent metal transporter-1 (DMT1) expression in lung detected mRNA in airway epithelium and bronchus-associated lymphatic tissue (BALT). Staining of the latter was significantly reduced in rats fed the high iron diet. In situ analysis of transferrin receptor (TfR) mRNA showed staining in BALT alone. These data demonstrate that manganese absorption from the lungs to the blood can be modified by iron status and the route of administration.

Biogenesis of iron-sulfur proteins in plants

Iron-sulfur (Fe-S) clusters are ubiquitous prosthetic groups required to sustain fundamental life processes. The assembly of Fe-S clusters and insertion into polypeptides in vivo has recently become an area of intense research. Many of the genes involved are conserved in bacteria, fungi, animals and plants. Plant cells can carry out both photosynthesis and respiration - two processes that require significant amounts of Fe-S proteins. Recent findings now suggest that both plastids and mitochondria are capable of assembling Fe-S proteins using assembly machineries that differ in biochemical properties, genetic make-up and evolutionary origin.

Iron-regulated transcription and capsule formation in the fungal pathogen Cryptococcus neoformans

Cryptococcus neoformans is the leading cause of fungal meningitis in humans. Production of a polysaccharide capsule is a key virulence property for the fungus and capsule synthesis is regulated by iron levels. Given that iron acquisition is an important aspect of virulence for many pathogens, we employed serial analysis of gene expression (SAGE) to examine the transcriptome under iron-limiting and iron-replete conditions. Initially, we demonstrated by SAGE and Northern analysis that iron limitation results in an elevated transcript level for the CAP60 gene that is required for capsule production. We also identified genes encoding putative components for iron transport and homeostasis, including the FTR1 (iron permease) gene, with higher transcript levels in the low-iron condition. An FTR1 disruption mutant grows more slowly than wild-type cells in low-iron medium, and shows delayed growth and altered capsule regulation in iron-replete medium. Iron deprivation also resulted in elevated SAGE tags for putative extracellular mannoproteins and the GPI8 gene encoding a glycosylphosphatidylinositol (GPI) transamidase. The GPI8 gene appears to be essential while disruption of the CIG1 gene encoding a mannoprotein resulted in impaired growth in low-iron medium and altered capsule response to the iron-replete condition. Additionally, we found that iron-replete conditions led to elevated transcripts for genes for iron storage, nitrogen metabolism, glycolysis, mitochondrial function, lipid metabolism and calmodulin-calcineurin signalling. Overall, these studies provide the first view of the C. neoformans transcriptional response to different iron levels.

The pathway for IRP2 degradation involving 2-oxoglutarate-dependent oxygenase(s) does not require the E3 ubiquitin ligase activity of pVHL

Iron regulatory protein 2 (IRP2), a posttranscriptional regulator of iron metabolism, is subjected to iron-dependent degradation by the proteasome. Recent experiments proposed a mechanism involving 2-oxoglutarate-dependent oxygenases. Enzymes of this class, such as prolyl-4-hydroxylases, mediate the oxygen and iron-dependent degradation of the hypoxia inducible factor HIF-1alpha, which requires the E3 ubiquitin ligase activity of pVHL. Considering that the pathways for IRP2 and HIF-1alpha degradation share remarkable similarities, we investigated whether pVHL may also be involved in the degradation of IRP2. We show here that IRP2 can interact with pVHL in co-transfection/co-immunoprecipitation assays. Furthermore, pVHL is able to promote the ubiquitination and the decay of transfected IRP2. However, the iron-dependent degradation of endogenous IRP2 is not impaired in VHL-deficient cell lines, suggesting that pVHL is not a necessary component of this pathway.

Secretion of ferritin by iron-laden macrophages and influence of lipoproteins

Increasing evidence supports a role of cellular iron in the initiation and development of atherosclerosis. We and others reported earlier that iron-laden macrophages are associated with LDL oxidation, angiogenesis, nitric oxide production and apoptosis in atherosclerotic processes. Here we have further studied perturbed iron metabolism in macrophages, their interaction with lipoproteins and the origin of iron accumulation in human atheroma. In both early and advanced human atheroma lesions, hemoglobin and ferritin accumulation correlated with the macrophage-rich areas. Iron uptake into macrophages, via transferrin receptors or scavenger receptor-mediated erythrophagocytosis, increased cellular iron and accelerated ferritin synthesis at both mRNA and protein levels. The binding activity of iron regulatory proteins was enhanced by desferrioxamine (DFO) and decreased by hemin and iron compounds. Iron-laden macrophages exocytosed both iron and ferritin into the culture medium. Exposure to oxidized low-density lipoprotein (oxLDL, >or=50 microg/mL) resulted in <20% apoptosis of iron-laden human macrophages, but cells remained impermeable after a 24 h period and an increased excretion of ferritin could be observed by immunostaining techniques. Exposure to high-density lipoprotein (HDL) significantly decreased ferritin excretion from these cells. We conclude: (i) erythrophagocytosis and hemoglobin catabolism by macrophages contribute to ferritin accumulation in human atherosclerotic lesions and; (ii) iron uptake into macrophages leads to increased synthesis and secretion of ferritin; (iii) oxidized LDL and HDL have different effects on these processes.

Iron accumulation and iron-regulatory protein activity in human hepatoma (HepG2) cells

Iron may populate distinct hepatocellular iron pools that differentially regulate expression of proteins such as ferritin and transferrin receptor (TfR) through iron-regulatory mRNA-binding proteins (IRPs), and may additionally regulate uptake and accumulation of non-transferrin-bound iron (NTBI). We examined iron-regulatory protein (IRP) binding activity and ferritin/TfR expression in human hepatoma (HepG2) cells exposed to iron at different levels for different periods. Several iron-dependent RNA-binding activities were identified, but only IRP increased with beta-mercaptoethanol. With exposures between 0 and 20 microg/ml iron, decreases in IRP binding accompanied large changes in TfR and ferritin expression, while chelation of residual iron with deferoxamine (DFO) caused a large increase in IRP binding with little additional effect on TfR or ferritin expression. Cellular iron content increased beyond 4 days of exposure to iron at 20 microg/ml, when IRP binding, TfR, and ferritin had all reached stable levels. However, iron content of the cells plateaued by 7 days, or decreased with 24 h exposure to very high concentrations (>50 microg/ml) of iron. These results indicate that iron-replete HepG2 cells exhibit a narrow range of maximal responsiveness of the IRP-regulatory mechanism, whose functional response is blunted both by excessive iron exposure and by removal of iron from a chelatable pool. HepG2 cells are able to limit iron accumulation upon higher or prolonged exposure to NTBI, apparently independent of the IRP mechanism.

CuZnSOD deficiency leads to persistent and widespread oxidative damage and hepatocarcinogenesis later in life

Mice deficient in CuZn superoxide dismutase (CuZnSOD) showed no overt abnormalities during development and early adulthood, but had a reduced lifespan and increased incidence of neoplastic changes in the liver. Greater than 70% of Sod1-/- mice developed liver nodules that were either nodular hyperplasia or hepatocellular carcinoma (HCC). Cross-sectional studies with livers collected from Sod1-/- and age-matched +/+ controls revealed extensive oxidative damage in the cytoplasm and, to a lesser extent, in the nucleus and mitochondria from as early as 3 months of age. A marked reduction in cytosolic aconitase, increased levels of 8-oxo dG and F2-isoprostanes, and a moderate reduction in glutathione peroxidase activities and porin levels were observed in all age groups of Sod1-/- mice examined. There were also age-related reductions in Mn superoxide dismutase activities and carbonic anhydrase III. Parallel to the biochemical changes, there were progressive increases in the DNA repair enzyme APEX1, the cell cycle control proteins cyclin D1 and D3, and the hepatocyte growth factor receptor Met. Increased cell proliferation in the presence of persistent oxidative damage to macromolecules likely contributes to hepatocarcinogenesis later in life.

Development of a fluorescent reporter to assess iron regulatory protein activity in living cells

Through the insertion of an iron responsive element (IRE) into a pd2ECFP vector, we demonstrate a noninvasive method for determining alterations in iron regulatory protein (IRP) activity that results in changes in protein translation in living cells. This construct takes advantage of the specifically iron-dependent interaction between IRPs that bind IREs on mRNAs to posttranscriptionally regulate protein expression in a manner similar to ferritin production. In this report, we demonstrate, using HEK-293 cells, that an IRE-driven fluorescent reporter can be used to observe changes in cellular iron status that are sufficient to alter protein synthesis. When iron availability was decreased, there was less cyan fluorescent protein (CFP) expression, suggesting that IRPs bind to the IRE and block protein translation. Conversely, exposing the cells to iron increased CFP fluorescence. This construct has advantages over traditionally used dyes and existing IRE driven constructs because it can be used to repeatedly study iron-influenced protein production over extended periods of time. The future applications of this construct include investigation of how mutations in cells may impact cellular iron metabolism and how various types of exogenously applied trophic, stress, and therapeutic agents may impact cellular iron metabolism.

Molecular analysis of iron overload in beta2-microglobulin-deficient mice

Beta2-microglobulin knockout (beta2m-/-) mice represent an instructive model of spontaneous iron overload resembling genetic hemochromatosis. The mechanism of iron accumulation in this mouse model may be more complex than involving the MHC class I-like protein HFE. We report that beta2m-deficient mice, like Hfe-/- mice, lack the adaptive hepatic hepcidin mRNA increase to iron overload. The inverse correlation of hepatic iron levels and hepcidin mRNA expression in six beta2m-/- mice underlines the importance of hepcidin in regulating body iron stores. In contrast to Hfe-/- mice, beta2m-deficient mice display increased expression of the duodenal iron transporters DMT1 and ferroportin 1. This result implicates a broader role of beta2m in mammalian iron metabolism, suggesting that (an) additional beta2m-interacting protein(s) could be involved in controlling iron homeostasis, and highlighting the emerging connection of iron metabolism with the immune system.

Iron regulatory protein 1 as a sensor of reactive oxygen species

Iron regulatory proteins (IRP1 and 2) function as translational regulators that coordinate the cellular iron metabolism of eukaryotes by binding to the mRNA of target genes such as the transferrin receptor or ferritin. In addition to IRP2, IRP1 serves as sensor of reactive oxygen species (ROS). As iron and oxygen are essential but potentially toxic constituents of most organisms, ROS-mediated modulation of IRP1 activity may be an important regulatory element in dissecting iron homeostasis and oxidative stress. The responses of IRP1 towards reactive oxygen species are compartment-specific and rather complex: H2O2 activates IRP1 via a signaling cascade that leads to upregulation of the transferrin receptor and cellular iron accumulation. Contrary, superoxide inactivates IRP1 by a direct chemical attack being limited to the intracellular compartment. In particular, activation of IRP1 by H2O2 has established a new regulatory link between inflammation and iron metabolism with new clinical implications. This mechanism seems to contribute to the anemia of chronic disease and inflammation-mediated iron accumulation in tissues. In addition, the cytotoxic side effects of redox-cycling anticancer drugs such as doxorubicin may involve H2O2-mediated IRP1 activation. These molecular insights open up new therapeutic strategies for the clinical management of chronic inflammation and drug-mediated cardiotoxicity.

The emerging role of the liver in iron metabolism

Iron is essential in health and well-being and its dysregulation is a common theme in disease. Recent advances in our understanding of the molecular biology underlying hemochromatosis and anemia has provided insight into the complex mechanisms implicated in iron metabolism. The proximal small bowel is the major site of iron absorption and, it is becoming increasingly clear that the regulation of this process involves the liver and, in particular, the hepatic antimicrobial peptide hepcidin. A number of studies have shown hepcidin to have an inhibitory function at the level of small bowel iron absorption, although its exact site of action remains to be elucidated. Clearly, identifying the target of hepcidin is of importance and is likely to lead to the development of therapeutic agents in the treatment of iron disorders.

A Northwestern blotting approach for studying iron regulatory element-binding proteins

At least two proteins binding to iron regulatory elements (IRE) in mRNA are known, designated as iron regulatory proteins (IRP) 1 and 2. Their binding activity is widely studied by electrophoretic mobility shift assays (EMSA), which resolves one or two bands depending on the species. We used Northwestern blotting to resolve this EMSA complex into four components, and identified two other IRE-binding peptides present in HepG2 cell extracts. We designate these six peptide bands A to F on Northwestern blots, ranging in apparent molecular weight from 111 to 37 kDa. Band C is lost when cells are preloaded with iron or when leupeptin (but not several other protease inhibitors) is included in the extraction buffer. Band E is also lost with leupeptin but increases with iron loading. Binding of all bands is sensitive to iron in vitro. Two-dimensional electrophoresis reveals additional processing, especially indicating charge variants of band C. Northwestern bands A and B both react with an antibody to IRP-1 on parallel Western blots. We conclude that cellular processing can produce multiple IRE-binding species that may be involved in a more complex regulation of iron metabolism than generally appreciated. The Northwestern approach should facilitate studies of processing and binding requirements of proteins and peptides that recognize the IRE sequence.

Iron uptake of the normoxic, anoxic and postanoxic microglial cell line RAW 264.7

Iron is one of the trace elements playing a key role in the normal brain metabolism. An excess of free iron on the other hand is catalyzing the iron-mediated oxygen radical production. Such a condition might be a harmful event leading perhaps to serious tissue damage and degeneration. Therefore, during evolution a complex iron sequestering apparatus developed, minimizing the amount of redox-reactive free iron. However, this system might be severely disturbed under pathophysiological conditions including hypoxia or anoxia. Since little is known about the non-transferrin-mediated iron metabolism of the brain during anoxia/reoxygenation, we tested the ability of the microglial cell line RAW 264.7 to take up iron independently of transferrin under various oxygen concentrations. Microglial cells are thought to be the major player in the maintenance of the extracellular homeostasis in the brain. Therefore, we investigated the iron metabolism of microglial cells employing radiolabeled ferric chloride. We tested the uptake of iron under normoxic, anoxic and postanoxic conditions. Furthermore, the amount of ferritin was measured by immunoblotting. We were able to show that iron enters the microglial cell line in the absence of extracellular transferrin under normoxic, anoxic and postanoxic conditions. Interestingly, the amount of ferritin is decreasing in the early reoxygenation phase. Therefore, we concluded that microglia is able to contribute to the brain iron homeostasis under anoxic and postanoxic conditions.

Alexa Fluor 488 as an iron sensing molecule and its application in PEBBLE nanosensors

Molecular Probes' Alexa Fluor dyes are generally used for biological labeling because of their ideal fluorescent properties, but here we detail Alexa Fluor 488's nanomolar sensitivity to free iron. Furthermore, the dye has been encapsulated into a polymer nanosphere by a microemulsion method, producing <100 nm particles. These nanosensors, PEBBLEs (Probe Encapsulated By Biologically Localized Embedding) have micromolar sensitivity and are non-responsive to other metal ions of biological interest.

Bacterial iron sources: from siderophores to hemophores

Iron is an essential element for most organisms, including bacteria. The oxidized form is insoluble, and the reduced form is highly toxic for most macromolecules and, in biological systems, is generally sequestrated by iron- and heme-carrier proteins. Thus, despite its abundance on earth, there is practically no free iron available for bacteria whatever biotope they colonize. To fulfill their iron needs, bacteria have multiple iron acquisition systems, reflecting the diversity of their potential biotopes. The iron/heme acquisition systems in bacteria have one of two general mechanisms. The first involves direct contact between the bacterium and the exogenous iron/heme sources. The second mechanism relies on molecules (siderophores and hemophores) synthesized and released by bacteria into the extracellular medium; these molecules scavenge iron or heme from various sources. Recent genetic, biochemical, and crystallographic studies have allowed substantial progress in describing molecular mechanisms of siderophore and hemophore interactions with the outer membrane receptors, transport through the inner membrane, iron storage, and regulation of genes encoding biosynthesis and uptake proteins.

Nickel decreases cellular iron level and converts cytosolic aconitase to iron-regulatory protein 1 in A549 cells

Nickel (Ni) compounds are well-established carcinogens and are known to initiate a hypoxic response in cells via the stabilization and transactivation of hypoxia-inducible factor-1 alpha (HIF-1alpha). This change may be the consequence of nickel's interference with the function of several Fe(II)-dependent enzymes. In this study, the effects of soluble nickel exposure on cellular iron homeostasis were investigated. Nickel treatment decreased both mitochondrial and cytosolic aconitase (c-aconitase) activity in A549 cells. Cytosolic aconitase was converted to iron-regulatory protein 1, a form critical for the regulation of cellular iron homeostasis. The increased activity of iron-regulatory protein 1 after nickel exposure stabilized and increased transferrin receptor (Tfr) mRNA and antagonized the iron-induced ferritin light chain protein synthesis. The decrease of aconitase activity after nickel treatment reflected neither direct interference with aconitase function nor obstruction of [4Fe-4S] cluster reconstitution by nickel. Exposure of A549 cells to soluble nickel decreased total cellular iron by about 40%, a decrease that likely caused the observed decrease in aconitase activity and the increase of iron-regulatory protein 1 activity. Iron treatment reversed the effect of nickel on cytosolic aconitase and iron-regulatory protein 1. To assess the mechanism for the observed effects, human embryonic kidney (HEK) cells over expressing divalent metal transporter-1 (DMT1) were compared to A549 cells expressing only endogenous transporters for inhibition of iron uptake by nickel. The inhibition data suggest that nickel can enter via DMT1 and compete with iron for entry into the cell. This disturbance of cellular iron homeostasis by nickel may have a great impact on the ability of the cell to regulate a variety of cell functions, as well as create a state of hypoxia in cells under normal oxygen tension. These effects may be very important in how nickel exerts phenotypic selection pressure to convert a normal initiated cell into a cancer cell.

Dihydrolipoic acid lowers the redox activity of transition metal ions but does not remove them from the active site of enzymes

Alpha-lipoic acid (LA) and its reduced form, dihydrolipoic acid (DHLA), have been suggested to chelate transition metal ions and, hence, mitigate iron- and copper-mediated oxidative stress in biological systems. However, it remains unclear whether LA and DHLA chelate transition metal ions in a redox-inactive form, and whether they remove metal ions from the active site of enzymes. Therefore, we investigated the effects of LA and DHLA on iron- or copper-catalyzed oxidation of ascorbate, a sensitive assay for the redox activity of these metal ions. We found that DHLA, but not LA, significantly inhibited ascorbate oxidation mediated by Fe(III)-citrate, suggesting that reduced thiols are required for iron binding. DHLA also strongly inhibited Cu(II)(histidine)(2)-mediated ascorbate oxidation in a concentration-dependent manner, with complete inhibition at a DHLA:Cu(II) molar ratio of 3:1. In contrast, no inhibition of copper-catalyzed ascorbate oxidation was observed with LA. To investigate whether LA and DHLA remove copper or iron from the active site of enzymes, Cu,Zn superoxide dismutase and the iron-containing enzyme aconitase were used. We found that neither LA nor DHLA, even at high, millimolar concentrations, altered the activity of these enzymes. Our results suggest that DHLA chelates and inactivates redox-active transition metal ions in small-molecular, biological complexes without affecting iron- or copper-dependent enzyme activities.

Friedreich ataxia: the oxidative stress paradox

Friedreich ataxia (FRDA) results from a generalized deficiency of mitochondrial and cytosolic iron-sulfur protein activity initially ascribed to mitochondrial iron overload. Recent in vitro data suggest that frataxin is necessary for iron incorporation in Fe-S cluster (ISC) and heme biosynthesis. In addition, several reports suggest that continuous oxidative damage resulting from hampered superoxide dismutases (SODs) signaling participates in the mitochondrial deficiency and ultimately the neuronal and cardiac cell death. This has led to the use of antioxidants such as idebenone for FRDA therapy. To further discern the role of oxidative stress in FRDA pathophysiology, we have tested the potential effect of increased antioxidant defense using an MnSOD mimetic (MnTBAP) and Cu,ZnSOD overexpression on the murine FRDA cardiomyopathy. Surprisingly, no positive effect was observed, suggesting that increased superoxide production could not explain by itself the FRDA cardiac pathophysiology. Moreover, we demonstrate that complete frataxin-deficiency neither induces oxidative stress in neuronal tissues nor alters the MnSOD expression and induction in the early step of the pathology (neuronal and cardiac) as previously suggested. We show that cytosolic ISC aconitase activity of iron regulatory protein-1 progressively decreases, whereas its apo-RNA binding form increases despite the absence of oxidative stress, suggesting that in a mammalian system the mitochondrial ISC assembly machinery is essential for cytosolic ISC biogenesis. In conclusion, our data demonstrate that in FRDA, mitochondrial iron accumulation does not induce oxidative stress and we propose that, contrary to the general assumption, FRDA is a neurodegenerative disease not associated with oxidative damage.

Cytoprotective efficacy and mechanisms of the liposoluble iron chelator 2,2'-dipyridyl in the rat photothrombotic ischemic stroke model

We examined the efficacy of the liposoluble iron chelator 2,2'-dipyridyl (DP) in reducing histological damage in rats submitted to cerebral ischemia and the mechanisms involved in the potential cytoprotection. For this purpose, DP (20 mg/kg, i.p.) was administered 15 min before and 1 h after induction of cortical photothrombotic vascular occlusion in rat. Histological studies were performed to assess infarct volume (at days 1 and 3 postischemia) and astromicroglial activation (at day 3 postischemia). Damage to endothelial and neuronal cells was evaluated at day 1 postischemia by quantitative measurements of Evans Blue extravasation and N-acetylaspartate levels, respectively. Cerebral blood flow was recorded in the ischemic core by laser-Doppler flowmetry within the 15 min to 2 h period after photothrombosis. At 4-h postischemia, radical oxygen species (ROS) production was evaluated by measuring brain glutathione concentrations. The cortical expression of the proteins heme oxygenase-1 (HO-1) and hypoxia-inducible factor-1alpha (HIF-1alpha) was analyzed by Western blotting at day 1 postischemia. Infarct volume and ischemic damage to endothelial and neuronal cells were significantly reduced by DP treatment. This cytoprotection was associated with a reduction in ROS production, perfusion deficits, and astrocytic activation. DP treatment also resulted in significant changes in HO-1 (+100%) and HIF-1alpha (-50%) protein expression at the level of the ischemic core. These results report the efficacy of the liposoluble iron chelator DP in reducing histological damage induced by permanent focal ischemia.

Aluminum triggers decreased aconitase activity via Fe-S cluster disruption and the overexpression of isocitrate dehydrogenase and isocitrate lyase: a metabolic network mediating cellular survival

Although aluminum is known to be toxic to most organisms, its precise biochemical interactions are not fully understood. In the present study, we demonstrate that aluminum promotes the inhibition of aconitase (Acn) activity via the perturbation of the Fe-S cluster in Pseudomonas fluorescens. Despite the significant decrease in citrate isomerization activity, cellular survival is assured by the overexpression of isocitrate lyase and isocitrate dehydrogenase (IDH)-NADP+. 13C NMR spectroscopic studies, Blue Native PAGE, and Western blot analyses indicated that although the decrease in Acn activity is concomitant with the increase of aluminum in the culture, the amount of Acn expressed is not sensitive to the concentration of the trivalent metal. A 6-fold decrease in Acn activity and no discernable change in protein content in aluminum-stressed cultures were observed. The addition of Fe(NH4)2(SO4)2 in a reducing environment led to a significant recovery in Acn activity. This enzymatic activity reverted to normal levels when aluminum-stressed cells were transferred to either a control or an iron-supplemented medium. The overexpression of the two isocitrate-metabolizing enzymes isocitrate lyase and IDH-NADP+ appears to mitigate the deficit in Acn activity. The levels of these enzymes are dependent on the aluminum content of the culture and appear to be under transcriptional control. Hence, the regulation of the enzymes involved in the homeostasis of isocitrate constitutes a pivotal component of the global metabolic strategy that ensures the survival of this organism in an aluminum citrate environment.

Overexpression of mitochondrial ferritin causes cytosolic iron depletion and changes cellular iron homeostasis

Cytosolic ferritin sequesters and stores iron and, consequently, protects cells against iron-mediated free radical damage. However, the function of the newly discovered mitochondrial ferritin (MtFt) is unknown. To examine the role of MtFt in cellular iron metabolism, we established a cell line that stably overexpresses mouse MtFt under the control of a tetracycline-responsive promoter. The overexpression of MtFt caused a dose-dependent iron deficiency in the cytosol that was revealed by increased RNA-binding activity of iron regulatory proteins (IRPs) along with an increase in transferrin receptor levels and decrease in cytosolic ferritin. Consequently, the induction of MtFt resulted in a dramatic increase in cellular iron uptake from transferrin, most of which was incorporated into MtFt. The induction of MtFt caused a shift of iron from cytosolic ferritin to MtFt. In addition, iron inserted into MtFt was less available for chelation than that in cytosolic ferritin and the expression of MtFt was associated with decreased mitochondrial and cytosolic aconitase activities, the latter being consistent with the increase in IRP-binding activity. In conclusion, our results indicate that overexpression of MtFt causes a dramatic change in intracellular iron homeostasis and that shunting iron to MtFt likely limits its availability for active iron proteins.

Proteomic studies of human and other vertebrate muscle proteins

This review summarizes results of some systemic studies of muscle proteins of humans and some other vertebrates. The studies, started after introduction of two-dimensional gel electrophoresis of O'Farrell, were significantly extended during development of proteomics, a special branch of functional genomics. Special attention is paid to analysis of characteristic features of strategy for practical realization of the systemic approach during three main stages of these studies: pre-genomic, genomic (with organizational registration of proteomics), and post-genomic characterized by active use of structural genomics data. Proteomic technologies play an important role in detection of changes in isoforms of various muscle proteins (myosins, troponins, etc.). These changes possibly reflecting tissue specificity of gene expression may underline functional state of muscle tissues under normal and pathological conditions, and such proteomic analysis is now used in various fields of medicine.

Iron, brain ageing and neurodegenerative disorders

There is increasing evidence that iron is involved in the mechanisms that underlie many neurodegenerative diseases. Conditions such as neuroferritinopathy and Friedreich ataxia are associated with mutations in genes that encode proteins that are involved in iron metabolism, and as the brain ages, iron accumulates in regions that are affected by Alzheimer's disease and Parkinson's disease. High concentrations of reactive iron can increase oxidative-stress induced neuronal vulnerability, and iron accumulation might increase the toxicity of environmental or endogenous toxins. By studying the accumulation and cellular distribution of iron during ageing, we should be able to increase our understanding of these neurodegenerative disorders and develop new therapeutic strategies.

Role of oxidative modifications in atherosclerosis

This review focuses on the role of oxidative processes in atherosclerosis and its resultant cardiovascular events. There is now a consensus that atherosclerosis represents a state of heightened oxidative stress characterized by lipid and protein oxidation in the vascular wall. The oxidative modification hypothesis of atherosclerosis predicts that low-density lipoprotein (LDL) oxidation is an early event in atherosclerosis and that oxidized LDL contributes to atherogenesis. In support of this hypothesis, oxidized LDL can support foam cell formation in vitro, the lipid in human lesions is substantially oxidized, there is evidence for the presence of oxidized LDL in vivo, oxidized LDL has a number of potentially proatherogenic activities, and several structurally unrelated antioxidants inhibit atherosclerosis in animals. An emerging consensus also underscores the importance in vascular disease of oxidative events in addition to LDL oxidation. These include the production of reactive oxygen and nitrogen species by vascular cells, as well as oxidative modifications contributing to important clinical manifestations of coronary artery disease such as endothelial dysfunction and plaque disruption. Despite these abundant data however, fundamental problems remain with implicating oxidative modification as a (requisite) pathophysiologically important cause for atherosclerosis. These include the poor performance of antioxidant strategies in limiting either atherosclerosis or cardiovascular events from atherosclerosis, and observations in animals that suggest dissociation between atherosclerosis and lipoprotein oxidation. Indeed, it remains to be established that oxidative events are a cause rather than an injurious response to atherogenesis. In this context, inflammation needs to be considered as a primary process of atherosclerosis, and oxidative stress as a secondary event. To address this issue, we have proposed an "oxidative response to inflammation" model as a means of reconciling the response-to-injury and oxidative modification hypotheses of atherosclerosis.

Selective inhibition of the citrate-to-isocitrate reaction of cytosolic aconitase by phosphomimetic mutation of serine-711

Iron-regulatory protein 1 (IRP1) is a dual-function protein with mutually exclusive roles as a posttranscriptional regulator of animal-cell iron metabolism or as the cytosolic isoform of the iron-sulfur enzyme aconitase (c-acon). Much effort has focused on the role of IRP1 in posttranscriptional gene regulation and in factors that influence its interconversion with c-acon, but little is known about the metabolic function and regulation of c-acon. The role of PKC-dependent phosphorylation of S711 on IRP1/c-acon function was examined. Phosphorylation state-specific antibodies revealed that S711 is phosphorylated by PKC in vitro and in human embryonic kidney cells treated with a PKC activator. In aco1 yeast, the phosphomimetic mutants S711D and S711E exhibited severely impaired aconitase function, whereas S711A and S711T were unaffected relative to the WT protein. Aconitase activity in yeast extracts displayed a similar pattern when assayed for capacity to convert citrate to isocitrate: WT, S711A, and S711T were active, but S711D and S711E activity was undetectable. In contrast, when measured by the conversion of isocitrate to cis-aconitate, S711D and S711E displayed substantial activity, indicating that phosphorylation impairs the citrate but not isocitrate mode of aconitase function. This possibility was confirmed in vivo by demonstrating that S711D and S711E specifically antagonized the requirement for isocitrate in two metabolic scenarios. Iron-responsive element RNA-binding affinity was unaffected by S711 mutations. Our results show that S711 is a target of phosphorylation capable of conferring distinct effects on c-acon function potentially dictating changes in cytosolic citrate/isocitrate metabolism.

Endogenous nitration of iron regulatory protein-1 (IRP-1) in nitric oxide-producing murine macrophages: further insight into the mechanism of nitration in vivo and its impact on IRP-1 functions

Iron regulatory protein-1 (IRP-1) is a bifunctional [4Fe-4S] protein that functions as a cytosolic aconitase or as a trans-regulatory factor controlling iron homeostasis at a post-transcriptional level. Because IRP-1 is a sensitive target protein for nitric oxide (NO), we investigated whether this protein is nitrated in inflammatory macrophages and whether this post-transcriptional modification changes its activities. RAW 264.7 macrophages were first stimulated with interferon-gamma and lipopolysaccharide (IFN-gamma/LPS) and then triggered by phorbol 12-myristate 13-acetate (PMA) in order to promote co-generation of NO* and O*2-.. IRP-1 was isolated by immunoprecipitation and analyzed for protein-bound nitrotyrosine by Western blotting. We show that nitration of endogenous IRP-1 in NO-producing macrophages boosted to produce O*2- was accompanied by aconitase inhibition and impairment of its capacity to bind the iron-responsive element (IRE) of ferritin mRNA. Lost IRE-binding activity was not recovered by exposure of IRP-1 to 2% 2-mercaptoethanol and was not due to protein degradation. Inclusion of cis-aconitate with cell extract to stabilize the [4Fe-4S] cluster of holo-IRP-1 rendered protein insensitive to nitration by peroxynitrite, suggesting that loss of [Fe-S] cluster and subsequent change of conformation are prerequisites for tyrosine nitration. IRP-1 nitration was strongly reduced when IFN-gamma/LPS/PMA-stimulated cells were incubated with myeloperoxidase inhibitors, which points to the contribution of the nitrite/H2O2/peroxidase pathway to IRP-1 nitration in vivo. Interestingly, under these conditions, IRP-1 recovered full IRE binding as assessed by treatment with 2% 2-mercaptoethanol. Peroxidase-mediated nitration of critical tyrosine residues, by holding IRP-1 in an inactive state, may constitute, in activated macrophages, a self-protecting mechanism against iron-induced toxicity.

Expression of epithelial cell iron-related genes upon infection by Neisseria meningitidis

Infection by the obligate human pathogens Neisseria meningitidis (MC) and Neisseria gonorrhoeae (GC) reduces the expression of host epithelial cell transferrin receptor 1 (TfR-1) (Bonnah et al., 2000, Cellular Microbiology 2: 207-218). In addition, the rate and pattern of TfR-1 cycling is altered, leading to diminished uptake of Tf-iron by infected host cells. As Tf-iron is important for maintaining iron homeostasis in the eukaryotic cell, these findings raised the possibility that Neisseria infection might affect further pathways of epithelial cell iron metabolism. We used a specialized cDNA microarray platform, the 'IronChip', to investigate the expression of genes involved in iron transport, storage and regulation. We show that mRNA expression of several host genes involved in iron homeostasis is altered. Surprisingly, the general mRNA expression profile of infected cells closely resembled that of uninfected cells grown in an iron-limited environment. An important exception to this profile is TfR-1, the mRNA level of which is strongly reduced. Low TfR-1 expression may be explained in part by decreased activity of the iron-regulatory proteins (IRPs) in MC-infected cells, which may result in the destabilization of TfR-1 mRNA. Intriguingly, low IRP activity contrasts with the decrease in H-ferritin protein levels in infected cells. This finding suggests that low IRP activity may be responsible in part for the decrease in TfR-1 mRNA levels. A discussion of these novel findings in relation to MC infection and virulence is provided.

The iron-heart disease connection: is it dead or just hiding?

More than 20 years ago it was proposed that differences in body iron stores may account for differential heart disease prevalence in men and women. In 1992 Finnish investigators presented intriguing supportive evidence for this proposal by showing that middle-aged men with normal but elevated serum ferritin, a biomarker of body iron stores, were at increased risk for acute myocardial infarction. During the past dozen years many studies have followed leading to intense debate on the role of iron in the development of heart disease. A meta-analysis of prospective studies found, however, no support for the iron-heart disease hypothesis. As new disease associations with iron status continue to be explored, consideration needs to be given to the lessons learned from these studies. Moreover, additional efforts need to be made to find a simple, high throughput and more satisfactory measure of the forms of iron in the body that are liable to do damage.

Does cellular iron dysregulation play a causative role in Parkinson's disease?

Selective dopaminergic cell loss in Parkinson's disease is correlated with increased levels of cellular iron. It is still hotly debated as to whether the increase in iron is an upstream event which acts to promote neurodegeneration via formation of oxidative stress or whether iron accumulates as a by-product of the neuronal cell loss. Here we review evidence for loss of iron homeostasis as a causative factor in disease-associated neurodegeneration and the primary players which may be involved. A series of recent studies suggest that iron regulatory proteins (IRPs) coordinate both cellular iron levels and energy metabolism, both of which are disrupted in Parkinson's disease (PD) and may in turn contribute to increased levels of oxidative stress associated with the disease. Iron has also been recently been implicated in promotion of alpha-synuclein aggregation either directly or via increasing levels of oxidative stress suggesting an important role for it in Lewy body formation, another important hallmark of the disease.

Induction of ferritin expression by oxalomalate

Ferritin is a ubiquitous protein required for intracellular iron storage; its biosynthesis is mainly regulated by iron-regulatory proteins (IRP1 and IRP2) at post-transcriptional level. This regulation prevents iron excess from promoting the formation of reactive oxygen species (ROS). IRP1 is regulated by such factors as intracellular iron levels, the oxidants H(2)O(2) and NO. We recently demonstrated that oxalomalate (OMA, alpha-hydroxy-beta-oxalosuccinic acid), a competitive inhibitor of aconitase, which is an enzyme of the citric acid cycle, remarkably decreases the binding activity of IRP1. The aim of the present study was to investigate whether this molecule could affect the expression of ferritin. The RNA-binding activity of IRP1, evaluated by gel retardation assay, decreased after treatment of several cell lines with 5 mM OMA, with a maximal decrease of about 3-fold after 6 h. This effect remained almost constant up to 48 h after which it returned to basal levels. Intracellular ferritin levels, determined by Western blot analysis, increased in correlation with the OMA-induced decrease of IRP1 binding activity. Furthermore, treatment of cells with OMA caused a rise in ferritin mRNA levels. Interestingly, in cells exposed to iron challenge, OMA-induced overexpression of ferritin prevented formation of ROS and cellular lipid peroxidation. These data show that an inhibitor of aconitase, OMA, besides being involved in energetic metabolism, is able to control ferritin expression, probably through molecular mechanisms of either post-transcriptional regulation or transcriptional modulation, with advantageous consequences for the cell.

Nitric oxide and iron metabolism in exercised rat with L-arginine supplementation

The present study was designed to investigate whether L-arginine (Arg) supplementation in exercise affects nitric oxide (NO) synthesis in tissues and thus iron metabolism. Rats were assigned to one of four groups: EG (Exercise), SG (Sedentary), EAG (Exercise + Arg), and SAG (Sedentary + Arg). Both EG and EAG swam 2 h/day for 3 months. Both SAG and EAG received 3% Arg supplementation in their drinking water. The results showed that Arg supplementation in exercise (EAG) significantly increased nitrite and nitrate (NOx) concentration in the kidney and BMC, rather than in the liver, spleen and heart. Arg supplementation significantly increased both nonheme iron (NHI) and catalytic iron (CI) content in the kidney, to the extent that the ratio of CI/NHI or storage iron (SI)/NHI was not significantly affected, and significantly decreased NHI content and increased CI content in BMC, to the extent that SI content or SI/NHI was significantly decreased. These findings suggest that Arg supplementation in exercise, possibly through increasing NO synthesis, may change CI formation in the kidney and BMC, and affect iron storage in BMC rather than in the kidney.

Influence of gestational age and fetal iron status on IRP activity and iron transporter protein expression in third-trimester human placenta

Placental iron transport during the last trimester of pregnancy determines the iron endowment of the neonate. Iron transport is a function of the major iron transport proteins: transferrin receptor-1 (TfR-1) and ferroportin-1 (FPN-1). The mRNAs for TfR-1 and, potentially, FPN-1 are posttranscriptionally regulated by iron regulatory protein (IRP)-1 and IRP-2. We assessed the effect of gestational age and fetal iron status on IRP-1- and IRP-2-binding activity and on the localization and protein expression of TfR-1 and FPN-1 protein at 24-40 wk of gestation in 21 placentas obtained from iron-sufficient nonanemic mothers. Gestational age had no effect on cord serum ferritin concentration, IRP-2 RNA-binding activity, transporter protein location, and TfR-1 or FPN-1 protein expression. IRP-1 activity remained constant until full term, when it decreased (P = 0.01). Placental ferritin (r = 0.76, P < 0.001) and FPN-1 (r = 0.44, P < 0.05) expression increased with gestational age. Fetal iron status, as indexed by cord serum ferritin concentration, was inversely related to placental IRP-1 (r = -0.66, P < 0.001) and IRP-2 (r = -0.42, P = 0.05) activities. Placental ferritin protein expression correlated better with IRP-1 (r = -0.45, P = 0.04) than with IRP-2 (r = -0.35, P = 0.10) activity. Placental TfR-1 and FPN-1 protein expression was independent of fetal or placental iron status and IRP activities. Iron status had no effect on transport protein localization. We conclude that, toward the end of the third trimester of iron-sufficient human pregnancy, the placenta accumulates ferritin and potentially increases placental-fetal iron delivery through increased FPN-1 expression. IRP-1 may have a more dominant role than IRP-2 activity in regulating ferritin expression.

Iron uncouples oxidative phosphorylation in brain mitochondria isolated from vitamin E-deficient rats

Few, if any, studies have examined the effect of vitamin E deficiency on brain mitochondrial oxidative phosphorylation. The latter was studied using brain mitochondria isolated from control and vitamin E-deficient rats (13 months of deficiency) after exposure to iron, an inducer of oxidative stress. Mitochondria were treated with iron (2 to 50 microM) added as ferrous ammonium sulfate. Rates of state 3 and state 4 respiration, respiratory control ratios, and ADP/O ratios were not affected by vitamin E deficiency alone. However, iron uncoupled oxidative phosphorylation in vitamin E-deficient mitochondria, but not in controls. In vitamin E-deficient mitochondria, iron decreased ADP/O ratios and markedly stimulated state 4 respiration; iron had only a modest effect on these parameters in control mitochondria. Thus, vitamin E may have an important role in sustaining oxidative phosphorylation. Low concentrations of iron (2 to 5 microM) oxidized mitochondrial tocopherol that exists in two pools. The release of iron in brain may impair oxidative phosphorylation, which would be exacerbated by vitamin E deficiency. The results are important for understanding the pathogenesis of human brain disorders known to be associated with abnormalities in mitochondrial function as well as iron homeostasis (e.g., Parkinson's disease).

Iron metabolism and the IRE/IRP regulatory system: an update

Cellular iron homeostasis is accomplished by the coordinated regulated expression of the transferrin receptor and ferritin, which mediate iron uptake and storage, respectively. The mechanism is posttranscriptional and involves two cytoplasmic iron regulatory proteins, IRP1 and IRP2. Under conditions of iron starvation, IRPs stabilize the transferrin receptor and inhibit the translation of ferritin mRNAs by binding to "iron responsive elements" (IREs) within their untranslated regions. The IRE/IRP system also controls the expression of additional IRE-containing mRNAs, encoding proteins of iron and energy metabolism. The activities of IRP1 and IRP2 are regulated by distinct posttranslational mechanisms in response to cellular iron levels. Thus, in iron-replete cells, IRP1 assembles a cubane iron-sulfur cluster, which prevents IRE binding, while IRP2 undergoes proteasomal degradation. IRP1 and IRP2 also respond, albeit differentially, to iron-independent signals, such as hydrogen peroxide, hypoxia, or nitric oxide. Basic principles of the IRE/IRP system and recent advances in understanding the regulation and the function of IRP1 and IRP2 are discussed.

Parkin and relatives: the RBR family of ubiquitin ligases

Mutations in the parkin gene cause autosomal-recessive juvenile parkinsonism. Parkin encodes a ubiquitin-protein ligase characterized by having the RBR domain, composed of two RING fingers plus an IBR/DRIL domain. The RBR family is defined as the group of genes whose products contain an RBR domain. RBR family members exist in all eukaryotic species for which significant sequence data is available, including animals, plants, fungi, and several protists. The integration of comparative genomics with structural and functional data allows us to conclude that RBR proteins have multiple roles, not only in protein quality control mechanisms, but also as indirect regulators of transcription. A recently formulated hypothesis, based on a case of gene fusion, suggested that RBR proteins may be often part of cullin-containing ubiquitin ligase complexes. Recent data on Parkin protein agrees with that hypothesis. We discuss the involvement of RBR proteins in several neurodegenerative diseases and cancer.

The nature of extracellular iron influences iron acquisition by Mycobacterium tuberculosis residing within human macrophages

We have reported that Mycobacterium tuberculosis residing within the phagosomes of human monocyte-derived macrophages (MDM) can acquire Fe from extracellular transferrin (TF) and sources within the MDM. In the lung, Fe is also bound to lactoferrin (LF) and low-molecular-weight chelates. We therefore investigated the ability of intraphagosomal M. tuberculosis to acquire Fe from these sources. M. tuberculosis acquired 30-fold and 3-fold more Fe from LF and citrate, respectively, compared to TF, in spite of similar MDM-associated Fe. M. tuberculosis infection decreased MDM-associated Fe relative to uninfected MDM as follows: TF (38.7%), citrate (21.1%), and LF (15.3%). M. tuberculosis Fe acquisition from extracellular chelates (exogenous source) and from endogenous MDM Fe initially acquired from the three chelates (endogenous source) was compared. M. tuberculosis Fe acquisition was similar from exogenous and endogenous sources supplied as Fe-TF. In contrast, there was much greater intracellular M. tuberculosis Fe uptake from LF and citrate from the exogenous than endogenous source. Gamma interferon (IFN-gamma) reduced MDM Fe uptake from each chelate by approximately 50% and augmented the M. tuberculosis-induced decrease in MDM Fe uptake from exogenous TF, but not from LF or citrate. IFN-gamma minimally decreased intracellular M. tuberculosis Fe acquisition from exogenous Fe-TF but significantly increased Fe uptake from LF and citrate. Intraphagosomal M. tuberculosis Fe acquisition from both exogenous and endogenous MDM sources, and the effect of IFN-gamma on this process, is influenced by the nature of the extracellular Fe chelate. M. tuberculosis has developed efficient mechanisms of acquiring Fe from a variety of Fe chelates that it likely encounters within the human lung.

Thy1 expression in the brain is affected by iron and is decreased in Restless Legs Syndrome

Thy-1 is a cell adhesion molecule that plays a regulatory role in the vesicular release of neurotransmitters. The objective of this study is to examine the relationship between iron status and Thy1 expression in neuronal systems of varying complexity. Pheochromocytoma cell (PC12) cells were used to explore whether there was a direct relation between cellular iron status and Thy1 expression. Iron chelation significantly decreased expression of Thy1 in PC12 cells in a dose and time dependent manner. Transferrin receptor expression was increased with iron chelation demonstrating that a global decrease in protein synthesis could not account for the Thy1 changes. We also examined brain homogenates from adult rats that were nursed by dams on an iron deficient (ID) diet and found a significant decrease in Thy1 compared to control rats. Finally, the substantia nigra from individuals with Restless Legs Syndrome reportedly has lower than normal amounts of iron. Therefore, we examined this brain region from individuals with the clinical diagnosis of primary Restless Legs syndrome (RLS) and found the concentration of Thy1 was less than half that of controls. The results of these studies support the novel concept that there is a relationship between Thy1 and iron and point to a novel mechanism by which iron deficiency can affect brain function. They also indicate a possible mechanism by which iron deficiency compromises dopaminergic transmission in RLS, providing a potentially important link between decreased brain iron and the responsiveness to levodopa and iron supplementation treatment in RLS.

Effects of sham air and cigarette smoke on A549 lung cells: implications for iron-mediated oxidative damage

Inhalation of airborne pollution particles that contain iron can result in a variety of detrimental changes to lung cells and tissues. The lung iron burden can be substantially increased by exposure to cigarette smoke, and cigarette smoke contains iron particulates, as well as several environmental toxins, that could influence intracellular iron status. We are interested in the effects of environmental contaminants on intracellular iron metabolism. We initiated our studies using lung A549 type II epithelial cells as a model, and we evaluated the effects of iron dose and smoke treatment on several parameters of intracellular iron metabolism. We show that iron at a physiological dose stimulates ferritin synthesis without altering the transferrin receptor (TfR) mRNA levels of these cells. This is mediated primarily by a reduction of iron regulatory protein 2. Higher doses of iron reduce iron regulatory protein-1 binding activity and are accompanied by a reduction in TfR mRNA. Thus, for A549 cells, different mechanisms influencing IRP-IRE interaction allow ferritin translation in the presence of TfR mRNA to provide for iron needs and yet prevent excessive iron uptake. More importantly, we report that smoke treatment diminishes ferritin levels and increases TfR mRNA of A549 cells. Ferritin serves as a cytoprotective agent against oxidative stress. These data suggest that exposure of lung cells to low levels of smoke as are present in environmental pollutants could result in reduced cytoprotection by ferritin at a time when iron uptake is sustained, thus enhancing the possibility of lung damage by iron-mediated oxidative stress.

Rat duodenal IRP1 activity and iron absorption in iron deficiency and after HO perfusion

BACKGROUND

Iron regulatory protein 1 (IRP1), a post-transcriptional regulator of iron metabolism, is activated in the duodenum of iron-deficient animals, which is associated with increased iron absorption. In cell cultures IRP1 was also activated by iron-independent signals, such as H(2)O(2). Here we investigate whether luminal perfusion of rat duodenum with H(2)O(2) activates duodenal IRP1 and modulates duodenal iron absorption.

METHODS

Duodena from iron-adequate Sprague-Dawley rats were luminally perfused with H(2)O(2). Iron regulatory protein-1 activity was determined in duodenal mucosa or in villus and crypt preparations by an electrophoretic mobility shift assay. Duodenal (59)Fe absorption was measured in isolated, perfused duodenal segments ex vivo and in ligated loops in vivo. (59)Fe uptake from the blood side was assessed after i.v. injection of (59)Fe-nitrilotriacetic acid.

RESULTS

Similar to iron deficiency, the perfusion with 0-50 mM of H(2)O(2) increases duodenal IRP1 activity along the entire crypt villus-axis in a dose-dependent manner. After H(2)O(2) treatment, IRP1 remains activated for 12-24 h in the tips and for 72 h in the crypts. In iron-deficiency, IRP activation correlates with increased (59)Fe absorption. However, the H(2)O(2) treatment fails to stimulate any increase in (59)Fe uptake, without promoting damage of mucosal architecture or impairing glucose and water transport.

CONCLUSION

Duodenal (59)Fe uptake is not affected by the H(2)O(2)-mediated activation of IRP1.