Activation of iron regulatory protein-1 by oxidative stress in vitro

Advances in Ferritin Physiology and Possible Implications in Bacterial Infection

Due to its advantageous redox properties, iron plays an important role in the metabolism of nearly all life. However, these properties are not only a boon but also the bane of such life forms. Since labile iron results in the generation of reactive oxygen species by Fenton chemistry, iron is stored in a relatively safe form inside of ferritin. Despite the fact that the iron storage protein ferritin has been extensively researched, many of its physiological functions are hitherto unresolved. However, research regarding ferritin's functions is gaining momentum. For example, recent major discoveries on its secretion and distribution mechanisms have been made as well as the paradigm-changing finding of intracellular compartmentalization of ferritin via interaction with nuclear receptor coactivator 4 (NCOA4). In this review, we discuss established knowledge as well as these new findings and the implications they may have for host-pathogen interaction during bacterial infection.

A Bioinformatics-Assisted Review on Iron Metabolism and Immune System to Identify Potential Biomarkers of Exercise Stress-Induced Immunosuppression

The immune function is closely related to iron (Fe) homeostasis and allostasis. The aim of this bioinformatics-assisted review was twofold; (i) to update the current knowledge of Fe metabolism and its relationship to the immune system, and (ii) to perform a prediction analysis of regulatory network hubs that might serve as potential biomarkers during stress-induced immunosuppression. Several literature and bioinformatics databases/repositories were utilized to review Fe metabolism and complement the molecular description of prioritized proteins. The Search Tool for the Retrieval of Interacting Genes (STRING) was used to build a protein-protein interactions network for subsequent network topology analysis. Importantly, Fe is a sensitive double-edged sword where two extremes of its nutritional status may have harmful effects on innate and adaptive immunity. We identified clearly connected important hubs that belong to two clusters: (i) presentation of peptide antigens to the immune system with the involvement of redox reactions of Fe, heme, and Fe trafficking/transport; and (ii) ubiquitination, endocytosis, and degradation processes of proteins related to Fe metabolism in immune cells (e.g., macrophages). The identified potential biomarkers were in agreement with the current experimental evidence, are included in several immunological/biomarkers databases, and/or are emerging genetic markers for different stressful conditions. Although further validation is warranted, this hybrid method (human-machine collaboration) to extract meaningful biological applications using available data in literature and bioinformatics tools should be highlighted.

Utilization of Pharmacological Ascorbate to Enhance Hydrogen Peroxide-Mediated Radiosensitivity in Cancer Therapy

Interest in the use of pharmacological ascorbate as a treatment for cancer has increased considerably since it was introduced by Cameron and Pauling in the 1970s. Recently, pharmacological ascorbate has been used in preclinical and early-phase clinical trials as a selective radiation sensitizer in cancer. The results of these studies are promising. This review summarizes data on pharmacological ascorbate (1) as a safe and efficacious adjuvant to cancer therapy; (2) as a selective radiosensitizer of cancer via a mechanism involving hydrogen peroxide; and (3) as a radioprotector in normal tissues. Additionally, we present new data demonstrating the ability of pharmacological ascorbate to enhance radiation-induced DNA damage in glioblastoma cells, facilitating cancer cell death. We propose that pharmacological ascorbate may be a general radiosensitizer in cancer therapy and simultaneously a radioprotector of normal tissue.

Inflaming the Brain with Iron

Iron accumulation and neuroinflammation are pathological conditions found in several neurodegenerative diseases, including Alzheimer's disease (AD) and Parkinson's disease (PD). Iron and inflammation are intertwined in a bidirectional relationship, where iron modifies the inflammatory phenotype of microglia and infiltrating macrophages, and in turn, these cells secrete diffusible mediators that reshape neuronal iron homeostasis and regulate iron entry into the brain. Secreted inflammatory mediators include cytokines and reactive oxygen/nitrogen species (ROS/RNS), notably hepcidin and nitric oxide (·NO). Hepcidin is a small cationic peptide with a central role in regulating systemic iron homeostasis. Also present in the cerebrospinal fluid (CSF), hepcidin can reduce iron export from neurons and decreases iron entry through the blood-brain barrier (BBB) by binding to the iron exporter ferroportin 1 (Fpn1). Likewise, ·NO selectively converts cytosolic aconitase (c-aconitase) into the iron regulatory protein 1 (IRP1), which regulates cellular iron homeostasis through its binding to iron response elements (IRE) located in the mRNAs of iron-related proteins. Nitric oxide-activated IRP1 can impair cellular iron homeostasis during neuroinflammation, triggering iron accumulation, especially in the mitochondria, leading to neuronal death. In this review, we will summarize findings that connect neuroinflammation and iron accumulation, which support their causal association in the neurodegenerative processes observed in AD and PD.

Altered nutrient status reprograms host inflammation and metabolic health via gut microbiota

The metabolism of macro- and micronutrients is a complex and highly regulated biological process. An imbalance in the metabolites and their signaling networks can lead to nonresolving inflammation and consequently to the development of chronic inflammatory-associated diseases. Therefore, identifying the accumulated metabolites and altered pathways during inflammatory disorders would not only serve as "real-time" markers but also help in the development of nutritional therapeutics. In this review, we explore recent research that has delved into elucidating the effects of carbohydrate/calorie restriction, protein malnutrition, lipid emulsions and micronutrient deficiencies on metabolic health and inflammation. Moreover, we describe the integrated stress response in terms of amino acid starvation and lipemia and how this modulates new age diseases such as inflammatory bowel disease and atherosclerosis. Lastly, we explain the latest research on metaflammation and inflammaging. This review focuses on multiple signaling pathways, including, but not limited to, the FGF21-β-hydroxybutryate-NLRP3 axis, the GCN2-eIF2α-ATF4 pathway, the von Hippel-Lindau/hypoxia-inducible transcription factor pathway and the TMAO-PERK-FoxO1 axis. Additionally, throughout the review, we explain how the gut microbiota responds to altered nutrient status and also how antimicrobial peptides generated from nutrient-based signaling pathways can modulate the gut microbiota. Collectively, it must be emphasized that metabolic starvation and inflammation are strongly regulated by both environmental (i.e., nutrition, gut microbiome) and nonenvironmental (i.e., genetics) factors, which can influence the susceptibility to inflammatory disorders.

Aconitases: Non-redox Iron-Sulfur Proteins Sensitive to Reactive Species

Mammalian aconitases (mitochondrial and cytosolic isoenzymes) are unique iron-sulfur cluster-containing proteins in which the metallic center participates in the catalysis of a non-redox reaction. Within the cubane iron-sulfur cluster of aconitases only three of the four iron ions have cysteine thiolate ligands; the fourth iron ion (Fe_α) is solvent exposed within the active-site pocket and bound to oxygen atoms from either water or substrates to be dehydrated. The catalyzed reaction is the reversible isomerization of citrate to isocitrate with an intermediate metabolite, cis-aconitate. The cytosolic isoform of aconitase is a moonlighting enzyme; when intracellular iron is scarce, the complete disassembly of the iron-sulfur cluster occurs and apo-aconitase acquires the function of an iron responsive protein and regulates the translation of proteins involved in iron metabolism. In the late 1980s and during the 1990s, cumulative experimental evidence pointed out that aconitases are main targets of reactive oxygen and nitrogen species such as superoxide radical (O₂^•-), hydrogen peroxide (H₂O₂), nitric oxide (^•NO), and peroxynitrite (ONOO^-). These intermediates are capable of oxidizing the cluster, which leads to iron release and consequent loss of the catalytic activity of aconitase. As the reaction of the Fe-S cluster with O₂^•- is fast (∼10⁷ M^-1 s^-1), quite specific, and reversible in vivo, quantification of active aconitase has been used to evaluate O₂^•- formation in cells. While ^•NO is modestly reactive with aconitase, its reaction with O₂^•- yields ONOO^-, a strong oxidant that readily leads to the disruption of the Fe-S cluster. In the case of cytosolic aconitase, it has been seen that H₂O₂ and ^•NO promote activation of iron responsive protein activity in cells. Proteomic advances in the 2000s confirmed that aconitases are main targets of reactive species in cellular models and in vivo, and other post-translational oxidative modifications such as protein nitration and carbonylation have been detected. Herein, we (1) outline the particular structural features of aconitase that make these proteins specific targets of reactive species, (2) characterize the reactions of O₂^•-, H₂O₂, ^•NO, and ONOO^- and related species with aconitases, (3) discuss how different oxidative post-translational modifications of aconitase impact the different functions of aconitases, and (4) argue how these proteins might function as redox sensors within different cellular compartments, regulating citrate concentration and efflux from mitochondria, iron availability in the cytosol, and cellular oxidant production.

Chemical, Biological and Medical Controversies Surrounding the Fenton Reaction

A critical evaluation is made of the role of the Fenton reaction (Fe2+ + H2O2 → Fe3+ + •OH + OH-) in the promotion of oxidative damage in mammalian systems. Following a brief, historical overview of the Fenton reaction, including the formulation of the Haber–Weiss cycle as a mechanism for the catalysis of hydroxyl radical production, an appraisal is made of the biological relevance of the reaction today, following recognition of the important role played by nitric oxide and its congers in the promotion of biomolecular damage. In depth coverage is then given of the evidence (largely from EPR studies) for and against the hydroxyl radical as the active oxidant produced in the Fenton reaction and the role of metal chelating agents (including those of biological importance) and ascorbic acid in the modulation of its generation. This is followed by a description of the important developments that have occurred recently in the molecular and cellular biology of iron, including evidence for the presence of ‘free’ iron that is available in vivo for the Fenton reaction. Particular attention here is given to the role of the iron-regulatory proteins in the modulation of cellular iron status and how their functioning may become dysregulated during oxidative and nitrosative stress, as well as in hereditary haemochromatosis, a common disorder of iron metabolism. Finally, an assessment is made of the biological relevance of ascorbic acid in the promotion of hydroxyl radical generation by the Fenton reaction in health and disease.

Regulation of the iron regulatory proteins by reactive nitrogen and oxygen species

Iron regulatory proteins 1 and 2 (IRP1 and IRP2) are RNA binding proteins that posttranscriptionally regulate the expression of mRNAs coding for proteins involved in the maintenance of iron and energy homeostasis. The RNA binding activities of the IRPs are regulated by changes in cellular iron. Thus, the IRPs are considered iron sensors and the principle regulators of cellular iron homeostasis. The mechanisms governing iron regulation of the IRPs are well described. Recently, however, much attention has focused on the regulation of IRPs by reactive nitrogen and oxygen species (RNS, ROS). Here we focus on summarizing the iron-regulated RNA binding activities of the IRPs, as well as the recent findings of IRP regulation by RNS and ROS. The recent observations that changes in oxygen tension regulate both IRP1 and IRP2 RNA binding activities will be addressed in light of ROS regulation of the IRPs.

FHC, an NS4B-interacting Protein, Enhances Classical Swine Fever Virus Propagation and Acts Positively in Viral Anti-apoptosis

Classical swine fever virus (CSFV), the etiological agent of classical swine fever, causes enormous economic loss to the pig industry. Ferritin heavy chain (FHC) is a notable anti-apoptotic protein, and existing evidence suggests that CSFV cannot induce apoptosis of host cells, however, the role of FHC in CSFV replication remains unclear. In the present study, we found that recombinant lentivirus-mediated knockdown or overexpression of FHC inhibited or enhanced CSFV replication, respectively, indicating a positive role for FHC in CSFV proliferation. Furthermore, interaction between the CSFV NS4B protein and FHC was confirmed by glutathione S-transferase (GST) pull-down, co-immunoprecipitation (co-IP) and confocal imaging assays. In addition, both CSFV replication and NS4B expression upregulated expression of FHC, which counteracts apoptosis by modulating cellular reactive oxygen species (ROS). These results suggest that FHC, an NS4B-interacting protein, enhances CSFV replication and has a positive role in viral anti-apoptosis by regulating ROS accumulation. This work may provide a new perspective for understanding the mechanism of CSFV pathogenesis.

Oxidative stress and the homeodynamics of iron metabolism

Iron and oxygen share a delicate partnership since both are indispensable for survival, but if the partnership becomes inadequate, this may rapidly terminate life. Virtually all cell components are directly or indirectly affected by cellular iron metabolism, which represents a complex, redox-based machinery that is controlled by, and essential to, metabolic requirements. Under conditions of increased oxidative stress—i.e., enhanced formation of reactive oxygen species (ROS)—however, this machinery may turn into a potential threat, the continued requirement for iron promoting adverse reactions such as the iron/H2O2-based formation of hydroxyl radicals, which exacerbate the initial pro-oxidant condition. This review will discuss the multifaceted homeodynamics of cellular iron management under normal conditions as well as in the context of oxidative stress.

Involvement of the iron regulatory protein from Eisenia andrei earthworms in the regulation of cellular iron homeostasis

Iron homeostasis in cells is regulated by iron regulatory proteins (IRPs) that exist in different organisms. IRPs are cytosolic proteins that bind to iron-responsive elements (IREs) of the 5′- or 3′-untranslated regions (UTR) of mRNAs that encode many proteins involved in iron metabolism. In this study, we have cloned and described a new regulatory protein belonging to the family of IRPs from the earthworm Eisenia andrei (EaIRP). The earthworm IRE site in 5′-UTR of ferritin mRNA most likely folds into a secondary structure that differs from the conventional IRE structures of ferritin due to the absence of a typically unpaired cytosine that participates in protein binding. Prepared recombinant EaIRP and proteins from mammalian liver extracts are able to bind both mammalian and Eisenia IRE structures of ferritin mRNA, although the affinity of the rEaIRP/Eisenia IRE structure is rather low. This result suggests the possible contribution of a conventional IRE structure. When IRP is supplemented with a Fe-S cluster, it can function as a cytosolic aconitase. Cellular cytosolic and mitochondrial fractions, as well as recombinant EaIRP, exhibit aconitase activity that can be abolished by the action of oxygen radicals. The highest expression of EaIRP was detected in parts of the digestive tract. We can assume that earthworms may possess an IRE/IRP regulatory network as a potential mechanism for maintaining cellular iron homeostasis, although the aconitase function of EaIRP is most likely more relevant.

Role of iron in ischemia-induced neurodegeneration: mechanisms and insights

Iron is an important micronutrient for neuronal function and survival. It plays an essential role in DNA and protein synthesis, neurotransmission and electron transport chain due to its dual redox states. On the contrary, iron also catalyses the production of free radicals and hence, causes oxidative stress. Therefore, maintenance of iron homeostasis is very crucial and it involves a number of proteins in iron metabolism and transport that maintain the balance. In ischemic conditions large amount of iron is released and this free iron catalyzes production of more free radicals and hence, causing more damage. In this review we have focused on the iron transport and maintenance of iron homeostasis at large and also the effect of imbalance in iron homeostasis on retinal and brain tissue under ischemic conditions. The understanding of the proteins involved in the homeostasis imbalance will help in developing therapeutic strategies for cerebral as well retinal ischemia.

Oxidative stress and hepatic Nox proteins in chronic hepatitis C and hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the most common liver cancer and a leading cause of cancer-related mortality in the world. Hepatitis C virus (HCV) is a major etiologic agent of HCC. A majority of HCV infections lead to chronic infection that can progress to cirrhosis and, eventually, HCC and liver failure. A common pathogenic feature present in HCV infection, and other conditions leading to HCC, is oxidative stress. HCV directly increases superoxide and H2O2 formation in hepatocytes by elevating Nox protein expression and sensitizing mitochondria to reactive oxygen species generation while decreasing glutathione. Nitric oxide synthesis and hepatic iron are also elevated. Furthermore, activation of phagocytic NADPH oxidase (Nox) 2 of host immune cells is likely to exacerbate oxidative stress in HCV-infected patients. Key mechanisms of HCC include genome instability, epigenetic regulation, inflammation with chronic tissue injury and sustained cell proliferation, and modulation of cell growth and death. Oxidative stress, or Nox proteins, plays various roles in these mechanisms. Nox proteins also function in hepatic fibrosis, which commonly precedes HCC, and Nox4 elevation by HCV is mediated by transforming growth factor β. This review summarizes mechanisms of oncogenesis by HCV, highlighting the roles of oxidative stress and hepatic Nox enzymes in HCC.

SIRT3 regulates cellular iron metabolism and cancer growth by repressing iron regulatory protein 1

Iron metabolism is essential for many cellular processes including oxygen transport, respiration and DNA synthesis, and many cancer cells exhibit dysregulation in iron metabolism. Maintenance of cellular iron homeostasis is regulated by iron regulatory proteins (IRPs), which control the expression of iron-related genes by binding iron-responsive elements (IREs) of target mRNAs. Here, we report that mitochondrial SIRT3 regulates cellular iron metabolism by modulating IRP1 activity. SIRT3 loss increases reactive oxygen species production, leading to elevated IRP1 binding to IREs. As a consequence, IRP1 target genes, such as the transferrin receptor (TfR1), a membrane-associated glycoprotein critical for iron uptake and cell proliferation, are controlled by SIRT3. Importantly, SIRT3 deficiency results in a defect in cellular iron homeostasis. SIRT3 null cells contain high levels of iron and lose iron-dependent TfR1 regulation. Moreover, SIRT3 null mice exhibit higher levels of iron and TfR1 expression in the pancreas. We found that the regulation of iron uptake and TfR1 expression contribute to the tumor suppressive activity of SIRT3. Indeed, SIRT3 expression is negatively correlated with TfR1 expression in human pancreatic cancers. SIRT3 overexpression decreases TfR1 expression by inhibiting IRP1 and represses proliferation in pancreatic cancer cells. Our data uncover a novel role of SIRT3 in cellular iron metabolism through IRP1 regulation, and suggest that SIRT3 functions as a tumor suppressor, in part, by modulating cellular iron metabolism.

F-box and leucine-rich repeat protein 5 (FBXL5): sensing intracellular iron and oxygen

Though essential for many vital biological processes, excess iron results in the formation of damaging reactive oxygen species (ROS). Therefore, iron metabolism must be tightly regulated. F-box and leucine-rich repeat protein 5 (FBXL5), an E3 ubiquitin ligase subunit, regulates cellular and systemic iron homeostasis by facilitating iron regulatory protein 2 (IRP2) degradation. FBXL5 possesses an N-terminal hemerythrin (Hr)-like domain that mediates its own differential stability by switching between two different conformations to communicate cellular iron availability. In addition, the FBXL5-Hr domain also senses O2 availability, albeit by a distinct mechanism. Mice lacking FBXL5 fail to sense intracellular iron levels and die in utero due to iron overload and exposure to damaging levels of oxidative stress. By closely monitoring intracellular levels of iron and oxygen, FBLX5 prevents the formation of conditions that favor ROS formation. These findings suggest that FBXL5 is essential for the maintenance of iron homeostasis and is a key sensor of bioavailable iron. Here, we describe the iron and oxygen sensing mechanisms of the FBXL5 Hr-like domain and its role in mediating ROS biology.

Mammalian iron metabolism and its control by iron regulatory proteins

Cellular iron homeostasis is maintained by iron regulatory proteins 1 and 2 (IRP1 and IRP2). IRPs bind to iron-responsive elements (IREs) located in the untranslated regions of mRNAs encoding protein involved in iron uptake, storage, utilization and export. Over the past decade, significant progress has been made in understanding how IRPs are regulated by iron-dependent and iron-independent mechanisms and the pathological consequences of IRP2 deficiency in mice. The identification of novel IREs involved in diverse cellular pathways has revealed that the IRP-IRE network extends to processes other than iron homeostasis. A mechanistic understanding of IRP regulation will likely yield important insights into the basis of disorders of iron metabolism. This article is part of a Special Issue entitled: Cell Biology of Metals.

Interaction of iron regulatory protein-1 (IRP-1) with ATP/ADP maintains a non-IRE-binding state

In its aconitase-inactive form, IRP-1 (iron regulatory protein-1)/cytosolic aconitase binds to the IRE (iron-responsive element) of several mRNAs to effect post-transcriptional regulation. We have shown previously that IRP-1 has ATPase activity and that binding of ATP suppresses the IRP-1/IRE interaction. In the present study, we characterize the binding activity further. Binding is observed with both [alpha-32P]ATP and [alpha-32P]ADP, but not with [gamma-32P]ATP. Recombinant IRP-1 binds approximately two molecules of ATP, and positive co-operativity is observed with a Hill coefficient of 1.67+/-0.36 (EC50=44 microM) commencing at 1 microM ATP. Similar characteristics are observed with both apoprotein and the aconitase form. On binding, ATP is hydrolysed to ADP, and similar binding parameters and co-operativity are seen with ADP, suggesting that ATP hydrolysis is not rate limiting in product formation. The non-hydrolysable analogue AMP-PNP (adenosine 5'-[beta,gamma-imido]triphosphate) does not induce co-operativity. Upon incubation of IRP-1 with increasing concentrations of ATP or ADP, the protein migrates more slowly on agarose gel electrophoresis, and there is a shift in the CD spectrum. In this new state, adenosine nucleotide binding is competed for by other nucleotides (CTP, GTP and AMP-PNP), although ATP and ADP, but not the other nucleotides, partially stabilize the protein against spontaneous loss of aconitase activity when incubated at 37 degrees C. A mutant IRP-1(C437S) lacking aconitase activity shows only one ATP-binding site and lacks co-operativity. It has increased IRE-binding capacity and lower ATPase activity (Km=75+/-17 nmol/min per mg of protein) compared with the wild-type protein (Km=147+/-48 nmol/min per mg of protein). Under normal cellular conditions, it is predicted that ATP/ADP will maintain IRP-1 in a non-IRE-binding state.

Two to tango: regulation of Mammalian iron metabolism

Disruptions in iron homeostasis from both iron deficiency and overload account for some of the most common human diseases. Iron metabolism is balanced by two regulatory systems, one that functions systemically and relies on the hormone hepcidin and the iron exporter ferroportin, and another that predominantly controls cellular iron metabolism through iron-regulatory proteins that bind iron-responsive elements in regulated messenger RNAs. We describe how the two distinct systems function and how they "tango" together in a coordinated manner. We also highlight some of the current questions in mammalian iron metabolism and discuss therapeutic opportunities arising from a better understanding of the underlying biological principles.

Iron regulatory protein 2 is involved in brain copper homeostasis

Trace metal homeostasis is tightly controlled in the brain, as even a slight dysregulation may severely impact normal brain function. This is especially apparent in Alzheimer's disease, where brain homeostasis of trace metals such as copper and iron is dysregulated. As it is known that iron and copper metabolism are linked, we wanted to investigate if a common mechanism could explain the increase in iron and decrease in copper seen in Alzheimer's disease brain. Amyloid-beta protein precursor (AbetaPP) has been implicated in copper efflux from the brain. Furthermore, it was shown that iron regulatory proteins (IRP), which regulate iron homeostasis, can block AbetaPP mRNA translation. In a correlative study we have therefore compared brain regional copper levels and AbetaPP expression in mice with a targeted deletion of IRP2-/-. Compared with controls, six week old IRP2-/- mice had significantly less brain copper in the parietal cortex, hippocampus, ventral striatum, thalamus, hypothalamus, and whole brain, while AbetaPP was significantly upregulated in the hippocampus (p < 0.05) and showed a trend toward upregulation in the thalamus (p < 0.1). This is the first study to demonstrate that iron regulatory proteins affect brain copper levels, which has significant implications for neurodegenerative diseases.

Nitric oxide and frataxin: two players contributing to maintain cellular iron homeostasis

BACKGROUND

Nitric oxide (NO) is a signalling and physiologically active molecule in animals, plants and bacteria. The specificity of the molecular mechanism(s) involved in transducing the NO signal within and between cells and tissues is still poorly understood. NO has been shown to be an emerging and potent signal molecule in plant growth, development and stress physiology. The NO donor S-nitrosoglutathion (GSNO) was shown to be a biologically active compound in plants and a candidate for NO storage and/or mobilization between plant tissues and cells. NO has been implicated as a central component in maintaining iron bioavailavility in plants.

SCOPE AND CONCLUSIONS

Iron is an essential nutrient for almost all organisms. This review presents an overview of the functions of NO in iron metabolism in animals and discusses how NO production constitutes a key response in plant iron sensing and availability. In plants, NO drives downstream responses to both iron deficiency and iron overload. NO-mediated improvement of iron nutrition in plants growing under iron-deficient conditions represents a powerful tool to cope with soils displaying low iron availability. An interconversion between different redox forms based on the iron and NO status of the plant cells might be the core of a metabolic process driving plant iron homeostasis. Frataxin, a recently identified protein in plants, plays an important role in mitochondria biogenesis and in maintaining mitochondrial iron homeostasis. Evidence regarding the interaction between frataxin, NO and iron from analysis of frataxin knock-down Arabidopsis thaliana mutants is reviewed and discussed.

Molecular control of the cytosolic aconitase/IRP1 switch by extramitochondrial frataxin

The inability to produce normal levels of the mitochondrial protein frataxin causes the hereditary degenerative disorder Friedreich's Ataxia (FRDA), a syndrome characterized by progressive gait instability, cardiomyopathy and high incidence of diabetes. Frataxin is an iron-binding protein involved in the biogenesis of iron-sulfur clusters (ISC), prosthetic groups allowing essential cellular functions such as oxidative phosphorylation, enzyme catalysis and gene regulation. Although several evidence suggest that frataxin acts as an iron-chaperone within the mitochondrial compartment, we have recently demonstrated the existence of a functional extramitochondrial pool of mature frataxin in various human cell types. Here, we show that a similar proteolytic process generates both mature mitochondrial and extramitochondrial frataxin. To address the physiological function of human extramitochondrial frataxin, we searched for ISC-dependent interaction partners. We demonstrate that the extramitochondrial form of frataxin directly interacts with cytosolic aconitase/iron regulatory protein-1 (IRP1), a bifunctional protein alternating between an enzymatic and a RNA-binding function through the 'iron-sulfur switch' mechanism. Importantly, we found that the cytosolic aconitase defect and consequent IRP1 activation occurring in FRDA cells are reversed by the action of extramitochondrial frataxin. These results provide new insight into the control of cytosolic aconitase/IRP1 switch and expand current knowledge about the molecular pathogenesis of FRDA.

IF/TA-related metabolic changes--proteome analysis of rat renal allografts

BACKGROUND

Chronic allograft nephropathy, now more specifically termed interstitial fibrosis and tubular atrophy without evidence of any specific aetiology (IF/TA), is still an important cause of late graft loss. There is no effective therapy for IF/TA, in part due to the disease's multifactorial nature and its incompletely understood pathogenesis.

METHODS

We used a differential in-gel electrophoresis and mass spectrometry technique to study IF/TA in a renal transplantation model. Dark Agouti (DA) kidneys were allogeneically transplanted to Wistar-Furth (DA-WF, aTX) rats. Syngeneic grafts (DA-DA, sTX) served as controls. Nine weeks after transplantation, blood pressure, renal function and electrolytes were studied, in addition to real-time PCR, western blot analysis, histology and immunohistochemistry.

RESULTS

In contrast to sTX, the aTX developed IF/TA-dependent renal damage. Ten differentially regulated proteins were identified by 2D gel analysis and mass spectrometry, whereupon five proteins are mainly related to oxidative stress (aldo-keto reductase, peroxiredoxin-1, NAD(+)-dependent isocitrate dehydrogenase, iron-responsive element-binding protein-1 and serum albumin), two participate in cytoskeleton organization (l-plastin and ezrin) and three are assigned to metabolic functions (creatine kinase, ornithine aminotransferase and fructose-1,6-bisphosphatase).

CONCLUSION

The proteins related to IF/TA and involved in oxidative stress, cytoskeleton organization and metabolic functions may correspond with novel therapeutic targets.

Metal ion physiopathology in neurodegenerative disorders

Metal dyshomeostasis in the brain (BMD) has often been proposed as a possible cause for several neurodegenerative disorders (NDs). Nevertheless, the precise nature of the biochemical mechanisms of metal involvement in NDs is still largely unknown. Mounting evidence suggests that normal aging itself is characterized by, among other features, a significant degree of metal ion dysmetabolism in the brain. This is probably the result of a progressive deterioration of the metal regulatory systems and, at least in some cases, of life-long metal exposure and brain accumulation. Although alterations of metal metabolism do occur to some extent in normal aging, they appear to be highly enhanced under various neuropathological conditions, causing increased oxidative stress and favoring abnormal metal-protein interactions. Intriguingly, despite the fact that most common NDs have a distinct etiological basis, they share striking similarities as they are all characterized by a documented brain metal impairment. This review will primarily focus on the alterations of metal homeostasis that are observed in normal aging and in Alzheimer's disease. We also present a brief survey on BMD in other NDs (Amyotrophic Lateral Sclerosis, Parkinson's, and Prion Protein disease) in order to highlight what represents the most reliable evidence supporting a crucial involvement of metals in neurodegeneration. The opportunities for metal-targeted pharmacological strategies in the major NDs are briefly outlined as well.

Chlamydia trachomatis alters iron-regulatory protein-1 binding capacity and modulates cellular iron homeostasis in HeLa-229 cells

Chlamydia trachomatis (CT) is the leading cause of diseases related to reproductive health and iron plays important role in chlamydial pathogenesis. Iron homeostasis in chlamydia-infected cells is not clear thus far. This study shows that expression of the transferrin receptor (TfR) is downregulated, whereas expression of the ferritin heavy chain is upregulated in CT-infected HeLa-229 cells. Expression of iron-regulatory protein (IRP)-1 predominates over IRP-2 in infected cells. In infected cells, attenuated binding activity of IRP-iron responsive elements (IREs) is observed using the electrophoretic mobility-shift assay. These results suggest that iron homeostasis is modulated in CT-infected HeLa cells at the interface of acquisition and commensal use of iron.

Overexpression of mitochondrial ferritin sensitizes cells to oxidative stress via an iron-mediated mechanism

Mitochondrial ferritin (MtFt) is a newly identified H-ferritin-like protein expressed only in mitochondria. Previous studies have shown that its overexpression markedly affects intracellular iron homeostasis and rescues defects caused by frataxin deficiency. To assess how MtFt exerts its function under oxidative stress conditions, MtFt overexpressing cells were treated with tert-butyl-hydroperoxide (tBHP), and the effects of MtFt expression on cell survival and iron homeostasis were examined. We found that MtFt expression was associated with decreased mitochondrial metabolic activity and reduced glutathione levels as well as a concomitant increase in reactive oxygen species levels and apoptosis. Moreover, mechanistic studies demonstrated that tBHP treatment led to a prolonged decrease in cytosolic ferritins levels in MtFt-expressing cells, while ferritin levels recovered to basal levels in control counterparts. tBHP treatment also resulted in elevated transferrin receptors, followed by more iron acquisition in MtFt expressing cells. The high molecular weight desferrioxamine, targeting to lysosomes, as well as the hydrophobic iron chelator salicylaldehyde isonicotinoyl hydrazone significantly attenuated tBHP-induced cell damage. In conclusion, the current study indicates that both the newly acquired iron from the extracellular environment and internal iron redistribution from ferritin degradation may be responsible for the increased sensitivity to oxidative stress in MtFt-expressing cells.

The role of reactive oxygen and nitrogen species in cellular iron metabolism

The catalytic role of iron in the Haber-Weiss chemistry, which results in propagation of damaging reactive oxygen species (ROS), is well established. In this review, we attempt to summarize the recent evidence showing the reverse: That reactive oxygen and nitrogen species can significantly affect iron metabolism. Their interaction with iron-regulatory proteins (IRPs) seems to be one of the essential mechanisms of influencing iron homeostasis. Iron depletion is known to provoke normal iron uptake via IRPs, superoxide and hydrogen peroxide are supposed to cause unnecessary iron uptake by similar mechanism. Furthermore, ROS are able to release iron from iron-containing molecules. On the contrary, nitric oxide (NO) appears to be involved in cellular defense against the iron-mediated ROS generation probably mainly by inducing iron removal from cells. In addition, NO may attenuate the effect of superoxide by mutual reaction, although the reaction product-peroxynitrite-is capable to produce highly reactive hydroxyl radicals.

Role of iron in the pathogenesis of cysteamine-induced duodenal ulceration in rats

Cysteamine induces perforating duodenal ulcers in rats within 24-48 h. This reducing aminothiol generates hydrogen peroxide in the presence of transition metals (e.g., ferric iron), producing oxidative stress, which may contribute to organ-specific tissue damage. Since most intestinal iron absorption takes place in the proximal duodenum, we hypothesized that cysteamine may disrupt regulation of mucosal iron transport, and iron may facilitate cysteamine-induced duodenal ulceration. We show here that cysteamine-induced ulceration was aggravated by pretreatment of rats with Fe(3+) or Fe(2+) compounds, which elevated iron concentration in the duodenal mucosa. In contrast, feeding rats an iron-deficient diet was associated with a 4.6-fold decrease in ulcer formation, accompanied by a 34% decrease (P < 0.05) in the duodenal mucosal iron concentration. Administration of deferoxamine inhibited ulceration by 65%. We also observed that the antiulcer effect of H2 receptor antagonist cimetidine included a 35% decrease in iron concentration in the duodenal mucosa. Cysteamine-induced duodenal ulcers were also decreased in iron-deficient Belgrade rats (P < 0.05). In normal rats, cysteamine administration increased the iron concentration in the proximal duodenal mucosa by 33% in the preulcerogenic stage but at the same time decreased serum iron (P < 0.05). Cysteamine also enhanced activation of mucosal iron regulatory protein 1 and increased the expression of divalent metal transporter 1 mRNA and protein. Transferrin receptor 1 protein expression was also increased, although mucosal ferroportin and ferritin remained almost unchanged. These results indicate an expansion of the intracellular labile iron pool in the duodenal mucosa, increasing its susceptibility to oxidative stress, and suggest a role for iron in the pathogenesis of organ-specific tissue injury such as duodenal ulcers.

Redox regulation of hepatitis C in nonalcoholic and alcoholic liver

Hepatitis C virus (HCV) is an RNA virus of the Flaviviridae family that is estimated to have infected 170 million people worldwide. HCV can cause serious liver disease in humans, such as cirrhosis, steatosis, and hepatocellular carcinoma. HCV induces a state of oxidative/nitrosative stress in patients through multiple mechanisms, and this redox perturbation has been recognized as a key player in HCV-induced pathogenesis. Studies have shown that alcohol synergizes with HCV in the pathogenesis of liver disease, and part of these effects may be mediated by reactive species that are generated during hepatic metabolism of alcohol. Furthermore, reactive species and alcohol may influence HCV replication and the outcome of interferon therapy. Alcohol consumption has also been associated with increased sequence heterogeneity of the HCV RNA sequences, suggesting multiple modes of interaction between alcohol and HCV. This review summarizes the current understanding of oxidative and nitrosative stress during HCV infection and possible combined effects of HCV, alcohol, and reactive species in the pathogenesis of liver disease.

Excess capacity of the iron regulatory protein system

Iron regulatory proteins (IRP1 and IRP2) are master regulators of cellular iron metabolism. IRPs bind to iron-responsive elements (IREs) present in the untranslated regions of mRNAs encoding proteins of iron storage, uptake, transport, and export. Because simultaneous knockout of IRP1 and IRP2 is embryonically lethal, it has not been possible to use dual knockouts to explore the consequences of loss of both IRP1 and IRP2 in mammalian cells. In this report, we describe the use of small interfering RNA to assess the relative contributions of IRP1 and IRP2 in epithelial cells. Stable cell lines were created in which either IRP1, IRP2, or both were knocked down. Knockdown of IRP1 decreased IRE binding activity but did not affect ferritin H and transferrin receptor 1 (TfR1) expression, whereas knockdown of IRP2 marginally affected IRE binding activity but caused an increase in ferritin H and a decrease in TfR1. Knockdown of both IRPs resulted in a greater reduction of IRE binding activity and more severe perturbation of ferritin H and TfR1 expression compared with single IRP knockdown. Even though the knockdown of IRP-1, IRP-2, or both was efficient, resulting in nondetectable protein and under 5% of wild type levels of mRNA, all stable knockdowns retained an ability to modulate ferritin H and TfR1 appropriately in response to iron challenge. However, further knockdown of IRPs accomplished by transient transfection of small interfering RNA in stable knockdown cells completely abolished the response of ferritin H and TfR1 to iron challenge, demonstrating an extensive excess capacity of the IRP system.

Glutathione depletion in hippocampal cells increases levels of H and L ferritin and glutathione S-transferase mRNAs

Glutathione plays an essential role in maintaining cellular redox balance, protecting cells from oxidative stress and detoxifying xenobiotic compounds. Glutathione depletion has been implicated in neurodegenerative disorders, including Alzheimer's and Parkinson's diseases. Cells of neuronal origin are acutely sensitive to glutathione depletion, providing an avenue for studying the mechanisms invoked for neuronal survival in response to oxidant challenge. We investigated the changes in mRNA profile in HT22 hippocampal cells following administration of homocysteic acid (HCA), a glutathione-depleting drug. We report that HCA treatment of HT22 murine hippocampal cells increases the levels of the mRNAs encoding at least three proteins involved in protection from oxidant injury, the mRNAs encoding heavy (H) and light (L) ferritin and glutathione S-transferase (GST).

Inhibition of an iron-responsive element/iron regulatory protein-1 complex by ATP binding and hydrolysis

Iron regulatory protein-1 binding to the iron-responsive element of mRNA is sensitive to iron, oxidative stress, NO, and hypoxia. Each of these agents changes the level of intracellular ATP, suggesting a link between iron levels and cellular energy metabolism. Furthermore, restoration of iron regulatory protein-1 aconitase activity after NO removal has been shown to require mitochondrial ATP. We demonstrate here that the iron-responsive element-binding activity of iron regulatory protein is ATP-dependent in HepG2 cells. Iron cannot decrease iron regulatory protein binding activity in cell extracts if they are simultaneously treated with an uncoupler of oxidative phosphorylation. Physiologic concentrations of ATP inhibit iron-responsive element/iron regulatory protein binding in cell extracts and binding of iron-responsive element to recombinant iron regulatory protein-1. ADP has the same effect, in contrast to the nonhydrolyzable analog adenosine 5'-(beta,gamma-imido)triphosphate, indicating that in order to inhibit iron regulatory protein-1 binding activity, ATP must be hydrolyzed. Indeed, recombinant iron regulatory protein-1 binds ATP with a Kd of 86+/-17 microM in a filter-binding assay, and can be photo-crosslinked to azido-ATP. Upon binding, ATP is hydrolyzed. The kinetic parameters [Km=5.3 microM, Vmax=3.4 nmol.min(-1).(mg protein)(-1)] are consistent with those of a number of other ATP-hydrolyzing proteins, including the RNA-binding helicases. Although the iron-responsive element does not itself hydrolyze ATP, its presence enhances iron regulatory protein-1's ATPase activity, and ATP hydrolysis results in loss of the complex in gel shift assays.

Effect of hypoxia on the binding and subcellular distribution of iron regulatory proteins

Iron regulatory proteins 1 and 2 (IRP1, IRP2) are key determinants of uptake and storage of iron by the liver, and are responsive to oxidative stress and hypoxia potentially at the level of both protein concentration and mRNA-binding activity. We examined the effect of hypoxia (1% O(2)) on IRP1 and IRP2 levels (Western blots) and mRNA-binding activity (gel shift assays) in human hepatoma HepG2 cells, and compared them with HEK 293 cells, a renal cell line known to respond to hypoxia. Total IRP binding to an iron responsive element (IRE) mRNA probe was increased several fold by hypoxia in HEK 293 cells, maximally at 4-8 h. An earlier and more modest increase (1.5- to 2-fold, peaking at 2 h and then declining) was seen in HepG2 cells. In both cell lines, IRP1 made a greater contribution to IRE-binding activity than IRP2. IRP1 protein levels were increased slightly by hypoxia in HEK 293 but not in HepG2 cells. IRP1 was distributed between cytosolic and membrane-bound fractions, and in both cells hypoxia increased both the amount and IRE-binding activity of the membrane-associated IRP1 fraction. Further density gradient fractionation of HepG2 membranes revealed that hypoxia caused an increase in total membrane IRP1, with a shift in the membrane-bound fraction from Golgi to an endoplasmic reticulum (ER)-enriched fraction. Translocation of IRP to the ER has previously been shown to stabilize transferrin receptor mRNA, thus increasing iron availability to the cell. Iron depletion with deferoxamine also caused an increase in ER-associated IRP1. Phorbol ester caused serine phosphorylation of IRP1 and increased its association with the ER. The calcium ionophore ionomycin likewise increased ER-associated IRP1, without affecting total IRE-binding activity. We conclude that IRP1 is translocated to the ER by multiple signals in HepG2 cells, including hypoxia, thereby facilitating its role in regulation of hepatic gene expression.

Bacillus subtilis aconitase is required for efficient late-sporulation gene expression

Bacillus subtilis aconitase, encoded by the citB gene, is homologous to the bifunctional eukaryotic protein IRP-1 (iron regulatory protein 1). Like IRP-1, B. subtilis aconitase is both an enzyme and an RNA binding protein. In an attempt to separate the two activities of aconitase, the C-terminal region of the B. subtilis citB gene product was mutagenized. The resulting strain had high catalytic activity but was defective in sporulation. The defect was at a late stage of sporulation, specifically affecting expression of sigmaK-dependent genes, many of which are important for spore coat assembly and require transcriptional activation by GerE. Accumulation of gerE mRNA and GerE protein was delayed in the aconitase mutant strain. Pure B. subtilis aconitase bound to the 3' untranslated region of gerE mRNA in in vitro gel mobility shift assays, strongly suggesting that aconitase RNA binding activity may stabilize gerE mRNA in order to allow efficient GerE synthesis and proper timing of spore coat assembly.

The ins and outs of iron homeostasis

Iron is an essential element that is toxic when it accumulates in excess. Intricate regulatory mechanisms have evolved to maintain iron homeostasis within cells and between different tissues of complex organisms. This review discusses the proteins involved in iron transport and storage and their regulation in health and disease.

Molecular control of vertebrate iron homeostasis by iron regulatory proteins

Both deficiencies and excesses of iron represent major public health problems throughout the world. Understanding the cellular and organismal processes controlling iron homeostasis is critical for identifying iron-related diseases and in advancing the clinical treatments for such disorders of iron metabolism. Iron regulatory proteins (IRPs) 1 and 2 are key regulators of vertebrate iron metabolism. These RNA binding proteins post-transcriptionally control the stability or translation of mRNAs encoding proteins involved in iron homeostasis thereby controlling the uptake, utilization, storage or export of iron. Recent evidence provides insight into how IRPs selectively control the translation or stability of target mRNAs, how IRP RNA binding activity is controlled by iron-dependent and iron-independent effectors, and the pathological consequences of dysregulation of the IRP system.

Alcohol metabolism-mediated oxidative stress down-regulates hepcidin transcription and leads to increased duodenal iron transporter expression

Patients with alcoholic liver disease frequently exhibit iron overload in association with increased hepatic fibrosis. Even moderate alcohol consumption elevates body iron stores; however, the underlying molecular mechanisms are unknown. Hepcidin, a circulatory peptide synthesized in the liver, is a key mediator of iron metabolism. Ethanol metabolism significantly down-regulated both in vitro and in vivo hepcidin mRNA and protein expression. 4-Methylpyrazole, a specific inhibitor of the alcohol-metabolizing enzymes, abolished the effects of ethanol on hepcidin. However, ethanol did not alter the expression of transferrin receptor1 and ferritin or the activation of iron regulatory RNA-binding proteins, IRP1 and IRP2. Mice maintained on 10-20% ethanol for 7 days displayed down-regulation of liver hepcidin expression without changes in liver triglycerides or histology. This was accompanied by elevated duodenal divalent metal transporter1 and ferroportin protein expression. Injection of hepcidin peptide negated the effect of ethanol on duodenal iron transporters. Ethanol down-regulated hepcidin promoter activity and the DNA binding activity of CCAAT/enhancer-binding protein alpha (C/EBPalpha) but not beta. Interestingly, the antioxidants vitamin E and N-acetylcysteine abolished both the alcohol-mediated down-regulation of C/EBPalpha binding activity and hepcidin expression in the liver and the up-regulation of duodenal divalent metal transporter 1. Collectively, these findings indicate that alcohol metabolism-mediated oxidative stress regulates hepcidin transcription via C/EBPalpha, which in turn leads to increased duodenal iron transport.

Extracellular H2O2 and not superoxide determines the compartment-specific activation of transferrin receptor by iron regulatory protein 1

Iron regulatory protein 1 (IRP1) functions as translational regulator that plays a central role in coordinating the cellular iron metabolism by binding to the mRNA of target genes such as the transferrin receptor (TfR)--the major iron uptake protein. Reactive oxygen species such as H2O2 and O2*- that are both co-released by inflammatory cells modulate IRP1 in opposing directions. While H2O2--similar to iron depletion--strongly induces IRP1 via a signalling cascade, O2*- inactivates the mRNA binding activity by a direct chemical attack. These findings have raised the question of whether compartmentalization may be an important mechanism for isolating these biological reactants when released from inflammatory cells during the oxygen burst cascade. To address this question, we studied cytosolic IRP1 and its downstream target TfR in conjunction with a tightly controlled biochemical modulation of extracellular O2*- and H2O2 levels mimicking the oxygen burst cascade of inflammatory cells. We here demonstrate that IRP1 activity and expression of TfR are solely dependent on H2O2 when co-released O2*- with from xanthine oxidase. Our findings confirm that extracellular H2O2 determines the functionality of the IRP1 cluster and its downstream targets while the reactivity of O2*- is limited to its compartment of origin.

Endocytic delivery of lipocalin-siderophore-iron complex rescues the kidney from ischemia-reperfusion injury

Neutrophil gelatinase-associated lipocalin (Ngal), also known as siderocalin, forms a complex with iron-binding siderophores (Ngal:siderophore:Fe). This complex converts renal progenitors into epithelial tubules. In this study, we tested the hypothesis that Ngal:siderophore:Fe protects adult kidney epithelial cells or accelerates their recovery from damage. Using a mouse model of severe renal failure, ischemia-reperfusion injury, we show that a single dose of Ngal (10 microg), introduced during the initial phase of the disease, dramatically protects the kidney and mitigates azotemia. Ngal activity depends on delivery of the protein and its siderophore to the proximal tubule. Iron must also be delivered, since blockade of the siderophore with gallium inhibits the rescue from ischemia. The Ngal:siderophore:Fe complex upregulates heme oxygenase-1, a protective enzyme, preserves proximal tubule N-cadherin, and inhibits cell death. Because mouse urine contains an Ngal-dependent siderophore-like activity, endogenous Ngal might also play a protective role. Indeed, Ngal is highly accumulated in the human kidney cortical tubules and in the blood and urine after nephrotoxic and ischemic injury. We reveal what we believe to be a novel pathway of iron traffic that is activated in human and mouse renal diseases, and it provides a unique method for their treatment.

Endocytic delivery of lipocalin-siderophore-iron complex rescues the kidney from ischemia-reperfusion injury

Neutrophil gelatinase-associated lipocalin (Ngal), also known as siderocalin, forms a complex with iron-binding siderophores (Ngal:siderophore:Fe). This complex converts renal progenitors into epithelial tubules. In this study, we tested the hypothesis that Ngal:siderophore:Fe protects adult kidney epithelial cells or accelerates their recovery from damage. Using a mouse model of severe renal failure, ischemia-reperfusion injury, we show that a single dose of Ngal (10 microg), introduced during the initial phase of the disease, dramatically protects the kidney and mitigates azotemia. Ngal activity depends on delivery of the protein and its siderophore to the proximal tubule. Iron must also be delivered, since blockade of the siderophore with gallium inhibits the rescue from ischemia. The Ngal:siderophore:Fe complex upregulates heme oxygenase-1, a protective enzyme, preserves proximal tubule N-cadherin, and inhibits cell death. Because mouse urine contains an Ngal-dependent siderophore-like activity, endogenous Ngal might also play a protective role. Indeed, Ngal is highly accumulated in the human kidney cortical tubules and in the blood and urine after nephrotoxic and ischemic injury. We reveal what we believe to be a novel pathway of iron traffic that is activated in human and mouse renal diseases, and it provides a unique method for their treatment.

Ferritin in atherosclerosis

Iron, an essential element for many important cellular functions in all living organisms, can catalyze the formation of potentially toxic free radicals. Excessive iron is sequestered by ferritin in a nontoxic and readily available form in a cell. Ferritin is composed of 24 subunits of different proportions of two functionally distinct subunits: ferritin H and L. The former is involved in ferroxidase activity necessary for iron uptake and oxidation of ferrous iron, while the latter is involved in nucleation of the iron core. The expression of ferritin is under delicate control and is regulated at both the transcriptional and posttranscriptional levels by iron, cytokines and oxidative stress. Elevated ferritin levels are associated with an increased risk of atherosclerotic coronary artery disease (CAD), the leading cause of death and illness in developed countries. Serum ferritin levels are a good indicator of iron stores in the body. In fact, epidemiological studies have suggested that elevated serum ferritin levels are associated with an increased risk of CAD and myocardial infarction (MI), though inconsistent results were obtained in some other studies. Moreover, recent proteomics and molecular biology studies have shown that ferritin levels in arteries are increased in diseased tissues, which further supports the link of ferritin to CAD/MI. Future studies will determine whether increased ferritin levels can serve as a distinct biomarker for the incidence of CAD/MI and distinguish whether increased ferritin levels are a cause of CAD or a consequence of the disease process.

Iron metabolism and toxicity

Iron is an essential nutrient with limited bioavailability. When present in excess, iron poses a threat to cells and tissues, and therefore iron homeostasis has to be tightly controlled. Iron's toxicity is largely based on its ability to catalyze the generation of radicals, which attack and damage cellular macromolecules and promote cell death and tissue injury. This is lucidly illustrated in diseases of iron overload, such as hereditary hemochromatosis or transfusional siderosis, where excessive iron accumulation results in tissue damage and organ failure. Pathological iron accumulation in the liver has also been linked to the development of hepatocellular cancer. Here we provide a background on the biology and toxicity of iron and the basic concepts of iron homeostasis at the cellular and systemic level. In addition, we provide an overview of the various disorders of iron overload, which are directly linked to iron's toxicity. Finally, we discuss the potential role of iron in malignant transformation and cancer.

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.

Distribution of ferritin in the rat hippocampus after kainate-induced neuronal injury

A gradual increase in iron occurs in the lesioned hippocampus after neuronal injury induced by the excitotoxin kainate, and the present study was carried out to investigate whether this increase in iron might be associated with changes in expression of the iron binding protein, ferritin. An increase in ferritin immunoreactivity was observed in glial cells of the hippocampus, as early as three days after intracerebroventricular injections of kainate. The number of ferritin positive cells peaked four weeks after the kainate injection, and decreased eight and twelve weeks after injection. They were found to be mostly microglia and oligodendrocytes by double immunofluorescence labeling with glial markers. A number of ferritin-labeled endothelial cells were also observed via electron microscopy. The decline in ferritin immunoreactivity four weeks after the injection of kainate is accompanied by an increase in the number of ferric and ferrous iron positive cells in the lesioned tissue. A substantial non-overlap between ferritin and iron-containing cells was observed. In particular, spherical ferric or ferrous iron-laden cells in the degenerating hippocampus were unlabeled for ferritin for long time periods after the kainate injection. An increase in iron, together with a reduced expression of iron binding proteins such as ferritin at long time intervals after kainate lesions, could result in a relative decrease in ferritin-induced ferroxidase activity and the presence of some of the iron in the ferrous form. It is postulated that this may contribute to chronic neuronal injury, following acute kainate-induced neurodegeneration.

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.

Modulation of DMT1 activity by redox compounds

Iron(II) exacerbates the effects of oxidative stress via the Fenton reaction. A number of human diseases are associated with iron accumulation including ischemia-reperfusion injury, inflammation and certain neurodegenerative diseases. The functional properties and localization in plasma membrane of cells and endosomes suggest an important role for the divalent metal transporter DMT1 (also known as DCT1 and Nramp2) in iron transport and cellular iron homeostasis. Although iron metabolism is strictly controlled and the activity of DMT1 is central in controlling iron homeostasis, no regulatory mechanisms for DMT1 have been so far identified. Our studies show that the activity of DMT1 is modulated by compounds that affect its redox status. We also show that both iron and zinc are transported by DMT1 when expressed in Xenopus laevis oocytes. Radiotracer uptake and electrophysiological measurements revealed that H(2)O(2) and Hg(2+) treatments result in substantial inhibition of DMT1. These findings may have a profound relevance from a physiological and pathophysiological standpoint.

Transferrin receptor induction in Toxoplasma gondii-infected HFF is associated with increased iron-responsive protein 1 activity and is mediated by secreted factors

Infection with the apicomplexan parasite Toxoplasma gondii results in a significant alteration of the host-cell transcriptional profile. We have previously shown that the transferrin receptor (TfR) is specifically up-regulated in T. gondii-infected human fibroblasts but not in host cells infected with the bacterial pathogens Salmonella Typhimurium and Chlamydia trachomatis. In this report, we describe the prerequisites and physiological conditions that are associated with this pathogen-specific gene induction. Band-shift assays revealed that T. gondii infection resulted in increased activity in the iron response protein IRP1, which, in this state, stabilizes TfR mRNA from degradation. Although T. gondii depends on host-cell iron as demonstrated by sensitivity to deferoxamine, a parasite-induced iron starvation is not responsible for TfR up-regulation. The increased iron availability due to treatment with holotransferrin and FeNTA did not prevent TfR induction nor was the transferrin-independent iron-transporter NRAMP2 up-regulated in infected host cells. In addition, inhibition of parasite replication by drug treatment did not prevent TfR up-regulation. Instead, TfR induction was sensitive to cycloheximide and could be induced by treatment with conditioned media from infected human fibroblasts. Together our findings suggest that the T. gondii-specific TfR up-regulation is not due to a direct interaction of parasitic factors with the iron-uptake machinery of the host cell but is instead mediated indirectly as a result of secreted host cell- or parasite-derived factors.