Post-transcriptional control via iron-responsive elements: the impact of aberrations in hereditary disease

Lysosomal proteolysis is the primary degradation pathway for cytosolic ferritin and cytosolic ferritin degradation is necessary for iron exit

Cytosolic ferritins sequester and store iron, consequently protecting cells against iron-mediated free radical damage. However, the mechanisms of iron exit from the ferritin cage and reutilization are largely unknown. In a previous study, we found that mitochondrial ferritin (MtFt) expression led to a decrease in cytosolic ferritin. Here we showed that treatment with inhibitors of lysosomal proteases largely blocked cytosolic ferritin loss in both MtFt-expressing and wild-type cells. Moreover, cytosolic ferritin in cells treated with inhibitors of lysosomal proteases was found to store more iron than did cytosolic ferritins in untreated cells. The prevention of cytosolic ferritin degradation in MtFt-expressing cells significantly blocked iron mobilization from the protein cage induced by MtFt expression. These studies also showed that blockage of cytosolic ferritin loss by leupeptin resulted in decreased cytosolic ferritin synthesis and prolonged cytosolic ferritin stability, potentially resulting in diminished iron availability. Lastly, we found that proteasomes were responsible for cytosolic ferritin degradation in cells pretreated with ferric ammonium citrate. Thus, the current studies suggest that cytosolic ferritin degradation precedes the release of iron in MtFt-expressing cells; that MtFt-induced cytosolic ferritin decrease is partially preventable by lysosomal protease inhibitors; and that both lysosomal and proteasomal pathways may be involved in cytosolic ferritin degradation.

A single nucleotide polymorphism in the 3'UTR of the SNCA gene encoding alpha-synuclein is a new potential susceptibility locus for Parkinson disease

In Parkinson disease, the second most common neurodegenerative disorder in humans, increased alpha-synuclein (SNCA) levels are pathogenic, as evidenced by gene copy number mutations and increased alpha-synuclein levels detected in some familial and sporadic PD cases, respectively. Gene expression can be regulated at the post-transcriptional level by elements in the 3' untranslated region (3'UTR) of mRNAs. The goal of this study was to determine whether the 3'UTR of human SNCA can affect gene expression. Comparative sequence analysis revealed very high conservation across the entire 3'UTR of human SNCA over millions of years, suggesting the presence of multiple functionally important domains. EST and RT-PCR analyses showed that four different polyadenylation events occur in the 3'UTR of human SNCA. Finally, using luciferase assays, we examined the effect of the minor allele of five naturally occurring single nucleotide polymorphisms (SNPs) in the 3'UTR of SNCA on gene expression. The minor allele of SNP rs17016074 increased luciferase expression by 32% in a transient transfection assay in SHSY5Y neuroblastoma cells. Understanding the role of the 3'UTR of human SNCA and identifying functionally important naturally occurring SNPs using reporter assays can complement disease association studies in humans, uncovering potential susceptibility or protective polymorphisms in Parkinson disease. Our findings demonstrate that the 3'UTR of human SNCA, as a whole, and rs17016074, in particular, are loci of potential clinical importance for Parkinson disease.

Comparison between the HCV IRES domain IV RNA structure and the Iron Responsive Element

Background

Serum ferritin and hepatic iron concentrations are frequently elevated in patients who are chronically infected with the hepatitis C virus (HCV), and hepatic iron concentration has been used to predict response to interferon therapy, but these correlations are not well understood. The HCV genome contains an RNA structure resembling an iron responsive element (IRE) in its internal ribosome entry site (IRES) structural domain IV (dIV). An IRE is a stem loop structure used to control the expression of eukaryotic proteins involved in iron homeostasis by either inhibiting ribosomal binding or protecting the mRNA from nuclease degradation. The HCV structure, located within the binding site of the 40S ribosomal subunit, might function as an authentic IRE or by an IRE-like mechanism.

Results

Electrophoretic mobility shift assays showed that the HCV IRES domain IV structure does not interact with the iron regulatory protein 1 (IRP1) in vitro. Systematic HCV IRES RNA mutagenesis suggested that IRP1 cannot accommodate the shape of the wild type HCV IRES dIV RNA structure.

Conclusion

The HCV IRES dIV RNA structure is not an authentic IRE. The possibility that this RNA structure is responsible for the observed correlations between intracellular iron concentration and HCV infection parameters through an IRE-like mechanism in response to some other cellular signal remains to be tested.

Effect of metal ions on HIF-1α and Fe homeostasis in human A549 cells

Several metals are carcinogenic but little is known about the mechanisms by which they cause cancer. A pathway that may contribute to metal ion induced carcinogenesis is by hypoxia signaling, which involves a disruption of cellular iron homeostasis by competition with iron transporters or iron-regulated enzymes. To examine the involvement of iron in the hypoxia signaling activity of these metal ions we investigated HIF-1alpha protein stabilization, IRP-1 activity, and ferritin protein levels in human lung carcinoma A459 cells exposed to various agents in serum- and iron-free salt-glucose medium (SGM) or in normal complete medium. We also studied the effects of excess exogenous iron on these responses induced by nickel ion exposure. Our results show the following: (1) SGM enhanced metals-induced HIF-1alpha stabilization and IRP-1 activation (e.g., nickel and cobalt ions). (2) If SGM was reconstituted with a slight excess level (25 microM of FeSO(4)) of iron, this enhancing ability was significantly decreased. (3) The effect of a high level of exogenous iron (500 microM of FeSO(4)) on metal-induced hypoxia and iron metabolism was highly dependent on the order of addition. If treatment with the Fe and metal ions was simultaneous (co-treatment), the effects of nickel ion exposure were overwhelmed, since the added Fe reversed HIF-1alpha stabilization, decreased IRP-1 activity, and increased ferritin level. Pre-treatment with iron was not able to reverse the responses caused by nickel ion exposure. These results imply that it is important to consider the available iron concentration and suitable exposure design when studying metal-induced hypoxia or metal-induced disruption of Fe homeostasis.

In vivo tumor growth is inhibited by cytosolic iron deprivation caused by the expression of mitochondrial ferritin

Mitochondrial ferritin (MtFt) is a mitochondrial iron-storage protein whose function and regulation is largely unknown. Our previous results have shown that MtFt overexpression markedly affects intracellular iron homeostasis in mammalian cells. Using tumor xenografts, we examined the effects of MtFt overexpression on tumor iron metabolism and growth. The expression of MtFt dramatically reduced implanted tumor growth in nude mice. Mitochondrial iron deposition in MtFt-expressing tumors was directly observed by transmission electron microscopy. A cytosolic iron starvation phenotype in MtFt-expressing tumors was revealed by increased RNA-binding activity of iron regulatory proteins, and concomitantly both an increase in transferrin receptor levels and a decrease in cytosolic ferritin. MtFt overexpression also led to decreases in total cellular heme content and heme oxygenase-1 levels. In addition, elevated MtFt in tumors was also associated with a decrease in total aconitase activity and lower frataxin protein level. In conclusion, our study shows that high MtFt levels can significantly affect tumor iron homeostasis by shunting iron into mitochondria; iron scarcity resulted in partially deficient heme and iron-sulfur cluster synthesis. It is likely that deprivation of iron in the cytosol is the cause for the significant inhibition of xenograft tumor growth.

Hereditary hyperferritinemia cataract syndrome in three unrelated families of western Greek origin caused by the C39 > G mutation of L-ferritin IRE

Hereditary hyperferritinemia-cataract syndrome (HHCS) is a well-characterized autosomal dominant disease caused by mutations in the iron responsive element (IRE) of ferritin L-chain (FTL) mRNA. Mutations in the IRE result in reduced binding of the trans-acting iron regulatory proteins (IRPs) and hence in upregulation of ferritin L-chain synthesis. The disease is characterized by increased L-ferritin in serum and tissues and early onset of bilateral cataracts. Iron metabolism is normal, and there is no tissue iron overload. At least 25 nucleotide substitutions and deletions in the L-ferritin IRE have been described in families with HHCS, originating from diverse European, Australian and North American populations. We studied the molecular pathogenesis of HHCS in three unrelated kinderships of western Greek origin, with 19 affected members. We identified a relatively rare C39G mutation located in the hexanucleotide loop of L-ferritin IRE. Computational analysis of mRNA folding of mutant FTL IRE predicted that the C39 > G mutation leads to a rearrangement of base pairing in this critical region, which is likely to modify the IRP binding affinity. All subjects with HHCS were heterozygotes for the same C39G mutation. Clinical and laboratory phenotypes were described. Moreover, there was evidence of an association between this FTL IRE stem-loop mutation and very high ferritin levels. Our findings broaden the list of populations where HHCS has been described.

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.

Hereditary hyperferritinaemia-cataract syndrome: a challenging diagnosis for the hepatogastroenterologist

Hereditary hyperferritinaemia-cataract syndrome (HHCS) is a relatively rare disorder with an autosomal dominant trait. It can be caused by various mutations within the iron responsive element (IRE) of the L-ferritin gene. These mutations result in an increased translation of L-ferritin mRNA and consequently the accumulation of L-ferritin in different fluids and tissues. HHCS patients present with an isolated hyperferritinaemia in the absence of any sign of iron overload. Early onset bilateral cataract, probably due to accumulation of ferritin crystals in the lens, is the only presenting clinical manifestation. Internists, especially gastrohepatologists, should be aware of this syndrome and differentiate it from haemochromatosis which is much more frequent, in order to avoid unnecessary imaging procedures, liver biopsies and an eventual venesection therapy, which will only lead to microcytic anaemia. In the present paper we report the first cases with HHCS diagnosed in Belgium. At diagnosis, the seven known affected members of three different families had ferritin levels between 603 and 3432 microg/l (normal < 150 microg/l), and this in combination with normal iron and transferrin values. All of them were known with early-onset bilateral cataract and our postulated diagnosis of HHCS was confirmed after genetic sequencing of the L-ferritin gene, which showed a C39U point mutation in the first family, and an A40G point mutation in the IRE loop segment in the two other families. The other investigated members of the three families had normal ferritin values, no history of early-onset cataract and genetic analyses could not reveal a mutation in the IRE of their L-ferritin gene. In current clinical practice, gastroenterologists should remember that elevated ferritin levels in the absence of documented iron overload is not haemochromatosis.

Alteration at translational but not transcriptional level of transferrin receptor expression following manganese exposure at the blood-CSF barrier in vitro

Manganese exposure alters iron homeostasis in blood and cerebrospinal fluid (CSF), possibly by acting on iron transport mechanisms localized at the blood-brain barrier and/or blood-CSF barrier. This study was designed to test the hypothesis that manganese exposure may change the binding affinity of iron regulatory proteins (IRPs) to mRNAs encoding transferrin receptor (TfR), thereby influencing iron transport at the blood-CSF barrier. A primary culture of choroidal epithelial cells was adapted to grow on a permeable membrane sandwiched between two culture chambers to mimic blood-CSF barrier. Trace (59)Fe was used to determine the transepithelial transport of iron. Following manganese treatment (100 microM for 24 h), the initial flux rate constant (K(i)) of iron was increased by 34%, whereas the storage of iron in cells was reduced by 58%, as compared to controls. A gel shift assay demonstrated that manganese exposure increased the binding of IRP1 and IRP2 to the stem loop-containing mRNAs. Consequently, the cellular concentrations of TfR proteins were increased by 84% in comparison to controls. Assays utilizing RT-PCR, quantitative real-time reverse transcriptase-PCR, and nuclear run off techniques showed that manganese treatment did not affect the level of heterogeneous nuclear RNA (hnRNA) encoding TfR, nor did it affect the level of nascent TfR mRNA. However, manganese exposure resulted in a significantly increased level of TfR mRNA and reduced levels of ferritin mRNA. Taken together, these results suggest that manganese exposure increases iron transport at the blood-CSF barrier; the effect is likely due to manganese action on translational events relevant to the production of TfR, but not due to its action on transcriptional, gene expression of TfR. The disrupted protein-TfR mRNA interaction in the choroidal epithelial cells may explain the toxicity of manganese at the blood-CSF barrier.

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.

New insights into erythropoiesis: the roles of folate, vitamin B12, and iron

Erythropoiesis is the process in which new erythrocytes are produced. These new erythrocytes replace the oldest erythrocytes (normally about one percent) that are phagocytosed and destroyed each day. Folate, vitamin B12, and iron have crucial roles in erythropoiesis. Erythroblasts require folate and vitamin B12 for proliferation during their differentiation. Deficiency of folate or vitamin B12 inhibits purine and thymidylate syntheses, impairs DNA synthesis, and causes erythroblast apoptosis, resulting in anemia from ineffective erythropoiesis. Erythroblasts require large amounts of iron for hemoglobin synthesis. Large amounts of iron are recycled daily with hemoglobin breakdown from destroyed old erythrocytes. Many recently identified proteins are involved in absorption, storage, and cellular export of nonheme iron and in erythroblast uptake and utilization of iron. Erythroblast heme levels regulate uptake of iron and globin synthesis such that iron deficiency causes anemia by retarded production rates with smaller, less hemoglobinized erythrocytes.

Hereditary Causes of Disturbed Iron Homeostasis in the Central Nervous System

Iron is essential for oxidation-reduction catalysis and bioenergetics; however, unless appropriately shielded, this metal plays a crucial role in the formation of toxic oxygen radicals that can attack all biological molecules. Organisms are equipped with specific proteins designed for iron acquisition, export and transport, and storage, as well as with sophisticated mechanisms that maintain the intracellular labile iron pool at an appropriate level. Despite these homeostatic mechanisms, organisms often face the threat of either iron deficiency or iron overload. This review describes several hereditary iron-overloading conditions that are confined to the brain. Recently, a mutation in the L-subunit of ferritin has been described that causes the formation of aberrant L-ferritin with an altered C-terminus. Individuals with this mutation in one allele of L-ferritin have abnormal aggregates of ferritin and iron in the brain, primarily in the globus pallidus. Patients with this dominantly inherited late-onset disease present with symptoms of extrapyramidal dysfunction. Mice with a targeted disruption of a gene for iron regulatory protein 2 (IRP2), a translational repressor of ferritin, misregulate iron metabolism in the intestinal mucosa and the central nervous system. Significant amounts of ferritin and iron accumulate in white matter tracts and nuclei, and adult IRP2-deficient mice develop a movement disorder consisting of ataxia, bradykinesia, and tremor. Mutations in the frataxin gene are responsible for Friedreich's ataxia, the most common of the inherited ataxias. Frataxin appears to regulate mitochondrial iron-sulfur cluster formation, and the neurologic and cardiac manifestations of Friedreich's ataxia are due to iron-mediated mitochondrial toxicity. Patients with Hallervorden-Spatz syndrome, an autosomal recessive, progressive neurodegenerative disorder, have mutations in a novel pantothenate kinase gene (PANK2). The cardinal feature of this extrapyramidal disease is pathologic iron accumulation in the globus pallidus. The defect in PANK2 is predicted to cause the accumulation of cysteine, which binds iron and causes oxidative stress in the iron-rich globus pallidus. Finally, aceruloplasminemia is an autosomal recessive disorder of iron metabolism caused by loss-of-function mutations in ceruloplasmin gene that leads to misregulation of both systemic and central nervous system iron trafficking. Affected individuals suffer from extrapyramidal signs, cerebellar ataxia, progressive neurodegeneration of retina, and diabetes mellitus. Excessive iron depositions are found in the brain, liver, pancreas, and other parenchymal cells, but plasma iron concentrations are decreased. These conditions are not common, but awareness about them is important for differential diagnosis of various neurodegenerative disorders.

S-nitrosylation of IRP2 regulates its stability via the ubiquitin-proteasome pathway

Nitric oxide (NO) is an important signaling molecule that interacts with different targets depending on its redox state. NO can interact with thiol groups resulting in S-nitrosylation of proteins, but the functional implications of this modification are not yet fully understood. We have reported that treatment of RAW 264.7 cells with NO caused a decrease in levels of iron regulatory protein 2 (IRP2), which binds to iron-responsive elements present in untranslated regions of mRNAs for several proteins involved in iron metabolism. In this study, we show that NO causes S-nitrosylation of IRP2, both in vitro and in vivo, and this modification leads to IRP2 ubiquitination followed by its degradation in the proteasome. Moreover, mutation of one cysteine (C178S) prevents NO-mediated degradation of IRP2. Hence, S-nitrosylation is a novel signal for IRP2 degradation via the ubiquitin-proteasome pathway.

Complex II defect via down-regulation of iron-sulfur subunit induces mitochondrial dysfunction and cell cycle delay in iron chelation-induced senescence-associated growth arrest

Mitochondria play a pivotal role as an ATP generator in aerobically growing cells, and their defects have long been implicated in the cellular aging process, although its detailed underlying mechanisms remain unclear. Recently, we found that, in the cellular senescent process of Chang cells induced by desferroxamine mesylate, an iron chelator, a significant decrease of intracellular ATP level was accompanied by decline in complex II activity, which preceded acquisition of the senescent phenotype. In the present study, we investigated the mechanism of how the mitochondrial ATP productivity was damaged by iron chelation and how complex II defect was involved in the senescent arrest. The ATP loss was irreversible and accompanied by sustained collapse of mitochondrial membrane potential (Delta psi m), but the ATP loss itself did not seem to be essential in progression to the senescent arrest. The Delta psi m disruption was due to decreased mitochondrial respiration, which was primarily associated with the defective complex II activity. Furthermore, we found that the declined activity of complex II was mainly due to down-regulation of protein expression of the iron-sulfur subunit, which was associated with the irreversibility of the arrest. Finally, we demonstrated that specific inhibition of complex II with 2-thenoyltrifluoroacetone induced overall delay of the cell cycle, suggesting that the delayed arrest by desferroxamine mesylate might be in part due to inhibition of complex II activity. Taken together, our results suggest that complex II might be considered as one of the primary factors to regulate mitochondrial respiratory function by responding to the cellular iron level, thereby influencing cellular growth.

Pushing the limits of the scanning mechanism for initiation of translation

Selection of the translational initiation site in most eukaryotic mRNAs appears to occur via a scanning mechanism which predicts that proximity to the 5' end plays a dominant role in identifying the start codon. This "position effect" is seen in cases where a mutation creates an AUG codon upstream from the normal start site and translation shifts to the upstream site. The position effect is evident also in cases where a silent internal AUG codon is activated upon being relocated closer to the 5' end. Two mechanisms for escaping the first-AUG rule--reinitiation and context-dependent leaky scanning--enable downstream AUG codons to be accessed in some mRNAs. Although these mechanisms are not new, many new examples of their use have emerged. Via these escape pathways, the scanning mechanism operates even in extreme cases, such as a plant virus mRNA in which translation initiates from three start sites over a distance of 900 nt. This depends on careful structural arrangements, however, which are rarely present in cellular mRNAs. Understanding the rules for initiation of translation enables understanding of human diseases in which the expression of a critical gene is reduced by mutations that add upstream AUG codons or change the context around the AUG(START) codon. The opposite problem occurs in the case of hereditary thrombocythemia: translational efficiency is increased by mutations that remove or restructure a small upstream open reading frame in thrombopoietin mRNA, and the resulting overproduction of the cytokine causes the disease. This and other examples support the idea that 5' leader sequences are sometimes structured deliberately in a way that constrains scanning in order to prevent harmful overproduction of potent regulatory proteins. The accumulated evidence reveals how the scanning mechanism dictates the pattern of transcription--forcing production of monocistronic mRNAs--and the pattern of translation of eukaryotic cellular and viral genes.

Control of eukaryotic protein synthesis by upstream open reading frames in the 5'-untranslated region of an mRNA

Control of gene expression is achieved at various levels. Translational control becomes crucial in the absence of transcription, such as occurs in early developmental stages. One of the initiating events in translation is that the 40 S subunit of the ribosome binds the mRNA at the 5'-cap structure and scans the 5'-untranslated region (5'-UTR) for AUG initiation codons. AUG codons upstream of the main open reading frame can induce formation of a translation-competent ribosome that may translate and (i) terminate and re-initiate, (ii) terminate and leave the mRNA, resulting in down-regulation of translation of the main open reading frame, or (iii) synthesize an N-terminally extended protein. In the present review we discuss how upstream AUGs can control the expression of the main open reading frame, and a comparison is made with other elements in the 5'-UTR that control mRNA translation, such as hairpins and internal ribosome entry sites. Recent data indicate the flexibility of controlling translation initiation, and how the mode of ribosome entry on the mRNA as well as the elements in the 5'-UTR can accurately regulate the amount of protein synthesized from a specific mRNA.

Nitrogen monoxide-mediated control of ferritin synthesis: implications for macrophage iron homeostasis

Intracellular iron homeostasis is regulated posttranscriptionally by iron regulatory proteins 1 and 2 (IRP1 and IRP2). In the absence of iron in the labile pool, IRPs bind to specific nucleotide sequences called iron responsive elements (IREs), which are located in the 5' untranslated region of ferritin mRNA and the 3' untranslated region of transferrin receptor mRNA. IRP binding to the IREs suppresses ferritin translation and stabilizes transferrin receptor mRNA, whereas the opposite scenario develops in iron-replete cells. Binding of IRPs to the IREs is also affected by nitrogen monoxide (NO), but there are conflicting reports regarding the effect of NO on ferritin synthesis. In this study, we demonstrated that a short exposure of RAW 264.7 cells (a macrophage cell line) to the NO+ donor, sodium nitroprusside (SNP), resulted in a dramatic increase in ferritin synthesis. The SNP-mediated increase of ferritin synthesis could be blocked by MG132, an inhibitor of proteasome-dependent protein degradation, which also prevented the degradation of IRP2 caused by SNP treatment. Moreover, treatment of RAW 264.7 cells with IFN-gamma and lipopolysaccharide caused IRP2 degradation and stimulated ferritin synthesis, changes that could be prevented by specific inhibitors of inducible nitric oxide synthase. Furthermore, the SNP-mediated increase in ferritin synthesis was associated with a significant enhancement of iron incorporation into ferritin. These observations indicate that NO+-mediated modulation of IRP2 plays an important role in controlling ferritin synthesis and iron metabolism in murine macrophages.

Brain iron pathways and their relevance to Parkinson's disease

A central role of iron in the pathogenesis of Parkinson's disease (PD), due to its increase in substantia nigra pars compacta dopaminergic neurons and reactive microglia and its capacity to enhance production of toxic reactive oxygen radicals, has been discussed for many years. Recent transcranial ultrasound findings and the observation of the ability of iron to induce aggregation and toxicity of alpha-synuclein have reinforced the critical role of iron in the pathogenesis of nigrostriatal injury. Presently the mechanisms involved in the disturbances of iron metabolism in PD remain obscure. In this review we summarize evidence from recent studies suggesting disturbances of iron metabolism in PD at possibly different levels including iron uptake, storage, intracellular metabolism, release and post-transcriptional control. Moreover we outline that the interaction of iron with other molecules, especially alpha-synuclein, may contribute to the process of neurodegeneration. Because many neurodegenerative diseases show increased accumulation of iron at the site of neurodegeneration, it is believed that maintenance of cellular iron homeostasis is crucial for the viability of neurons.

Iron and its sensitive balance in the cell

Iron is vital in life because it is an important component of molecules that undergoes redox reactions or transport oxygen. However, the existence of two stable and inter-convertible forms of iron, iron(III) and iron(II), makes possible one electron being transferred to or captured from other species to form radicals. In particular, superoxide and hydroxyl radicals may be formed in these reactions, both with capacity of attacking other molecules. DNA is one important target and a vast literature exists showing that attack of hydroxyl radical to DNA leads to cell death cellular necrosis, apoptosis, mutation and malignant transformation. Therefore, a fine balance must exist at various levels of an organism to maintain iron concentration in a narrow range, above and below which deleterious effects of distinct nature occur. This review will deal with the formation of oxygen reactive species in iron participating reactions, defenses in the organism against these species, the different mechanisms of iron homeostasis and iron deficiency and iron overload related diseases.

The roles of iron in health and disease

Iron is vital for almost all living organisms by participating in a wide variety of metabolic processes, including oxygen transport, DNA synthesis, and electron transport. However, iron concentrations in body tissues must be tightly regulated because excessive iron leads to tissue damage, as a result of formation of free radicals. Disorders of iron metabolism are among the most common diseases of humans and encompass a broad spectrum of diseases with diverse clinical manifestations, ranging from anemia to iron overload and, possibly, to neurodegenerative diseases. The molecular understanding of iron regulation in the body is critical in identifying the underlying causes for each disease and in providing proper diagnosis and treatments. Recent advances in genetics, molecular biology and biochemistry of iron metabolism have assisted in elucidating the molecular mechanisms of iron homeostasis. The coordinate control of iron uptake and storage is tightly regulated by the feedback system of iron responsive element-containing gene products and iron regulatory proteins that modulate the expression levels of the genes involved in iron metabolism. Recent identification and characterization of the hemochromatosis protein HFE, the iron importer Nramp2, the iron exporter ferroportin1, and the second transferrin-binding and -transport protein transferrin receptor 2, have demonstrated their important roles in maintaining body's iron homeostasis. Functional studies of these gene products have expanded our knowledge at the molecular level about the pathways of iron metabolism and have provided valuable insight into the defects of iron metabolism disorders. In addition, a variety of animal models have implemented the identification of many genetic defects that lead to abnormal iron homeostasis and have provided crucial clinical information about the pathophysiology of iron disorders. In this review, we discuss the latest progress in studies of iron metabolism and our current understanding of the molecular mechanisms of iron absorption, transport, utilization, and storage. Finally, we will discuss the clinical presentations of iron metabolism disorders, including secondary iron disorders that are either associated with or the result of abnormal iron accumulation.

Regulation of ferritin mRNA translation in primary erythroblasts: exogenous c-Kit plus EpoR signaling mimics v-ErbA oncoprotein activity

In general, translation efficiency of ferritin mRNAs is modulated by variations in iron supply. In primary avian erythroblasts undergoing short-term proliferation, however, ferritin heavy chain (ferH) mRNA is repressed at all iron levels. Yet, expression of v-ErbA oncoprotein is sufficient to reinduce ferH mRNA utilization at physiological iron concentrations. Since overexpression of the receptor tyrosine kinase c-Kit and erythropoietin receptor (EpoR) stimulates long-term proliferation of primary erythroblasts like v-ErbA, we analyzed the impact of cooperation between c-Kit and EpoR on the regulation of iron storage. Whereas endogenous c-Kit in combination with exogenous EpoR had no significant effect, ectopic overexpression of both receptors abolished translational repression of ferH mRNA upon iron administration. Thus, high-intensity signaling through c-Kit plus EpoR pathways mimics the v-ErbA-mediated regulatory phenotype.

Identification of iron responsive genes by screening cDNA libraries from suppression subtractive hybridization with antisense probes from three iron conditions

The goal of the present study is to identify genes that respond to iron availability. Suppression subtraction hybridization (SSH) was used to generate cDNA libraries from iron loaded and control human astrocytoma cells (SW1088). The cDNA libraries were screened with antisense cDNA probes obtained from mRNA isolated from astrocytoma cells exposed to three conditions: (i) normal media (control), (ii) deferox-amine treated (iron deficient) or (iii) iron loaded. The screening of the cDNA libraries with antisense probes from the three conditions enhanced the screening efficiency and decreased the number of false positives. Positive clones were identified and sequenced. The genes of interest were further analyzed by determining changes in hybridization signal on northern blots from astrocytoma cells exposed to iron or deferoxamine over different time intervals. Our analysis identified cDNAs corresponding to known iron responsive genes such as L-chain ferritin, but also revealed a number of mRNAs with novel sequences and mRNAs previously not known to be responsive to iron such as one of the ABC transporters and Thy-1 glycoprotein. Thus our results suggest that the expression of a number of genes may be influenced by changes in iron availability.