Mammalian Fe-S proteins: definition of a consensus motif recognized by the co-chaperone HSC20

Iron-sulfur (Fe-S) clusters are inorganic cofactors that are fundamental to several biological processes in all three kingdoms of life. In most organisms, Fe-S clusters are initially assembled on a scaffold protein, ISCU, and subsequently transferred to target proteins or to intermediate carriers by a dedicated chaperone/co-chaperone system. The delivery of assembled Fe-S clusters to recipient proteins is a crucial step in the biogenesis of Fe-S proteins, and, in mammals, it relies on the activity of a multiprotein transfer complex that contains the chaperone HSPA9, the co-chaperone HSC20 and the scaffold ISCU. How the transfer complex efficiently engages recipient Fe-S target proteins involves specific protein interactions that are not fully understood. This mini review focuses on recent insights into the molecular mechanism of amino acid motif recognition and discrimination by the co-chaperone HSC20, which guides Fe-S cluster delivery.

Role of hepcidin in murine brain iron metabolism

Brain iron homeostasis is maintained by a balance of both iron uptake and release, and accumulating evidence has revealed that brain iron concentrations increase with aging. Hepcidin, an iron regulatory hormone produced by hepatocytes in response to inflammatory stimuli, iron, and hypoxia, has been shown to be the long-sought hormone responsible for the regulation of body iron balance and recycling in mammals. In this study, we report that hepcidin is widely expressed in the murine brain. In cerebral cortex, hippocampus and striatum, hepcidin mRNA levels increased with aging. Injection of hepcidin into the lateral cerebral ventricle resulted in decreased Fpn1 protein levels in cerebral cortex, hippocampus, and striatum. Additionally, treatment of primary cultured neurons with hepcidin caused decreased neuronal iron release and Fpn1 protein levels. Together, our data provide further evidence that hepcidin may be involved in the regulation of brain iron metabolism.

Iron-dependent regulation of the divalent metal ion transporter

The first step in intestinal iron absorption is mediated by the H(+)-coupled Fe(2+) transporter called divalent cation transporter 1/divalent metal ion transporter 1 (DCT1/DMT1) (also known as natural resistance-associated macrophage protein 2). DCT1/DMT1 mRNA levels in the duodenum strongly increase in response to iron depletion. To study the mechanism of iron-dependent DCT1/DMT1 mRNA regulation, we investigated the endogenous expression of DCT1/DMT1 mRNA in various cell types. We found that only the iron responsive element (IRE)-containing form, which corresponds to one of two splice forms of DCT1/DMT1, is responsive to iron treatment and this responsiveness was cell type specific. We also examined the interaction of the putative 3'-UTR IRE with iron responsive binding proteins (IRP1 and IRP2), and found that IRP1 binds to the DCT1/DMT1-IRE with higher affinity compared to IRP2. This differential binding of IRP1 and IRP2 was also reported for the IREs of transferrin receptors, erythroid 5-aminolevulinate synthase and mitochondrial aconitase. We propose that regulation of DCT1/DMT1 mRNA by iron involves post-transcriptional regulation through the binding of IRP1 to the transporter's IRE, as well as other as yet unknown factors.

An IRP-like protein from Plasmodium falciparum binds to a mammalian iron-responsive element

This study cloned and sequenced the complementary DNA (cDNA) encoding of a putative malarial iron responsive element-binding protein (PfIRPa) and confirmed its identity to the previously identified iron-regulatory protein (IRP)-like cDNA from Plasmodium falciparum. Sequence alignment showed that the plasmodial sequence has 47% identity with human IRP1. Hemoglobin-free lysates obtained from erythrocyte-stage P falciparum contain a protein that binds a consensus mammalian iron-responsive element (IRE), indicating that a protein(s) with iron-regulatory activity was present in the lysates. IRE-binding activity was found to be iron regulated in the electrophoretic mobility shift assays. Western blot analysis showed a 2-fold increase in the level of PfIRPa in the desferrioxamine-treated cultures versus control or iron-supplemented cells. Malarial IRP was detected by anti-PfIRPa antibody in the IRE-protein complex from P falciparum lysates. Immunofluorescence studies confirmed the presence of PfIRPa in the infected red blood cells. These findings demonstrate that erythrocyte P falciparum contains an iron-regulated IRP that binds a mammalian consensus IRE sequence, raising the possibility that the malaria parasite expresses transcripts that contain IREs and are iron-dependently regulated.

Systemic iron metabolism: a review and implications for brain iron metabolism

Mammalian cells and organisms coordinate to regulate expression of numerous proteins involved in the uptake, sequestration, and export of iron. When cells in the systemic circulation are depleted of iron, they increase synthesis of the transferrin receptor and decrease synthesis of the iron sequestration protein, ferritin. In iron-depleted animals, expression of duodenal iron transporters markedly increases and intestinal iron uptake increases accordingly. The major proteins of iron metabolism in the systemic circulation are also expressed in the central nervous system. However, the mechanisms by which iron is transported and distributed throughout the central nervous system are not well understood. Iron accumulation in specific regions of the brain is observed in several neurodegenerative diseases. It is likely that misregulation of iron metabolism is important in the pathophysiology of several human neurodegenerative diseases.

Iron on the brain

Accumulations of iron are often detected in the brains of people suffering from neurodegenerative diseases. But it is often not known whether such accumulations contribute directly to disease progression. The identification of the genes mutated in two such disorders suggests that errors in iron metabolism do indeed have a key role.

MCV as a guide to phlebotomy therapy for hemochromatosis

BACKGROUND

A multitude of recommendations exist for laboratory assays to monitor the pace and endpoints of phlebotomy therapy for hemochromatosis. All of these recommendations rely on an assessment of storage iron to guide treatment, and none have been prospectively evaluated.

STUDY DESIGN AND METHODS

Nine consecutive patients underwent serial monitoring of Hb, MCV, transferrin saturation, and ferritin during weekly phlebotomy to deplete iron stores (induction therapy) and less frequent sessions to prevent iron reaccumulation (maintenance therapy). Changes in MCV and Hb were used to guide the pace of phlebotomy over a median of 7 years of follow-up.

RESULTS

During induction therapy, the MCV increased transiently because of reticulocytosis and then stabilized for a prolonged period before decreasing more sharply, which reflected iron-limited erythropoiesis. Iron depletion was achieved after a median of 38 phlebotomies and removal of 9.0 g of iron. Maintenance phlebotomy was targeted to maintain the MCV at 5 to 10 percent below prephlebotomy values and the Hb at >13 g per dL. Transferrin saturation fluctuated considerably during treatment, but remained below 35 percent during MCV-guided maintenance therapy. Ferritin values were not useful guides to the pace of phlebotomy. The median maintenance therapy phlebotomy interval was 7.5 weeks (range, 6-16), which corresponded to an average daily iron removal of 35 to 67 microg per kg. Most patients showed evidence of iron reaccumulation at phlebotomy intervals of 8 weeks or more.

CONCLUSION

The MCV is an inexpensive, precise, physiologic indicator of erythropoietic iron availability. When used in conjunction with the Hb, it is a clinically useful guide to the pace of phlebotomy therapy for hemochromatosis.

Targeted deletion of the gene encoding iron regulatory protein-2 causes misregulation of iron metabolism and neurodegenerative disease in mice

In mammalian cells, regulation of the expression of proteins involved in iron metabolism is achieved through interactions of iron-sensing proteins known as iron regulatory proteins (IRPs), with transcripts that contain RNA stem-loop structures referred to as iron responsive elements (IREs). Two distinct but highly homologous proteins, IRP1 and IRP2, bind IREs with high affinity when cells are depleted of iron, inhibiting translation of some transcripts, such as ferritin, or turnover of others, such as the transferrin receptor (TFRC). IRPs sense cytosolic iron levels and modify expression of proteins involved in iron uptake, export and sequestration according to the needs of individual cells. Here we generate mice with a targeted disruption of the gene encoding Irp2 (Ireb2). These mutant mice misregulate iron metabolism in the intestinal mucosa and the central nervous system. In adulthood, Ireb2(-/-) mice develop a movement disorder characterized by ataxia, bradykinesia and tremor. Significant accumulations of iron in white matter tracts and nuclei throughout the brain precede the onset of neurodegeneration and movement disorder symptoms by many months. Ferric iron accumulates in the cytosol of neurons and oligodendrocytes in distinctive regions of the brain. Abnormal accumulations of ferritin colocalize with iron accumulations in populations of neurons that degenerate, and iron-laden oligodendrocytes accumulate ubiquitin-positive inclusions. Thus, misregulation of iron metabolism leads to neurodegenerative disease in Ireb2(-/-) mice and may contribute to the pathogenesis of comparable human neurodegenerative diseases.

Regulation of intracellular iron metabolism in human erythroid precursors by internalized extracellular ferritin

Human erythroid precursors grown in culture possess membrane receptors that bind and internalize acid isoferritin. These receptors are regulated by the iron status of the cell, implying that ferritin iron uptake may represent a normal physiologic pathway. The present studies describe the fate of internalized ferritin, the mechanisms involved in the release of its iron, and the recognition of this iron by the cell. Normal human erythroid precursors were grown in a 2-phase liquid culture that supports the proliferation, differentiation, and maturation of erythroid precursors. At the stage of polychromatic normoblasts, cells were briefly incubated with (59)Fe- and/or (125)I-labeled acid isoferritin and chased. The (125)I-labeled ferritin protein was rapidly degraded and only 50% of the label remained in intact ferritin protein after 3 to 4 hours. In parallel, (59)Fe decreased in ferritin and increased in hemoglobin. Extracellular holoferritin uptake elevated the cellular labile iron pool (LIP) and reduced iron regulatory protein (IRP) activity; this was inhibited by leupeptin or chloroquine. Extracellular apoferritin taken up by the cell functioned as an iron scavenger: it decreased the level of cellular LIP and increased IRP activity. We suggest that the iron from extracellular is metabolized in a similar fashion by developing erythroid cells as is intracellular ferritin. Following its uptake, extracellular ferritin iron is released by proteolytic degradation of the protein shell in an acid compartment. The released iron induces an increase in the cellular LIP and participates in heme synthesis and in intracellular iron regulatory pathways.

Clinical severity and thermodynamic effects of iron-responsive element mutations in hereditary hyperferritinemia-cataract syndrome

Hereditary hyperferritinemia-cataract syndrome (HHCS) is a novel genetic disorder characterized by elevated serum ferritin and early onset cataract formation. The excessive ferritin production in HHCS patients arises from aberrant regulation of L-ferritin translation caused by mutations within the iron-responsive element (IRE) of the L-ferritin transcript. IREs serve as binding sites for iron regulatory proteins (IRPs), iron-sensing proteins that regulate ferritin translation. Previous observations suggested that each unique HHCS mutation conferred a characteristic degree of hyperferritinemia and cataract severity in affected individuals. Here we have measured the in vitro affinity of the IRPs for the mutant IREs and correlated decreases in binding affinity with clinical severity. Thermodynamic analysis of these IREs has also revealed that although some HHCS mutations lead to changes in the stability and secondary structure of the IRE, others appear to disrupt IRP-IRE recognition with minimal effect on IRE stability. HHCS is a noteworthy example of a human genetic disorder that arises from mutations within a protein-binding site of an mRNA cis-acting element. Analysis of the effects of these mutations on the energetics of the RNA-protein interaction explains the phenotypic variabilities of the disease state.

Measurements of iron status and survival in African iron overload

INTRODUCTION

Dietary iron overload is common in southern Africa and there is a misconception that the condition is benign. Early descriptions of the condition relied on autopsy studies, and the use of indirect measurements of iron status to diagnose this form of iron overload has not been clarified.

METHODS

The study involved 22 black subjects found to have iron overload on liver biopsy. Fourteen subjects presented to hospital with liver disease and were found to have iron overload on percutaneous liver biopsy. Eight subjects, drawn from a family study, underwent liver biopsy because of elevated serum ferritin concentrations suggestive of iron overload. Indirect measurements of iron status (transferrin saturation, serum ferritin) were performed on all subjects. Histological iron grade and hepatic iron concentration were used as direct measures of iron status.

RESULTS

There were no significant differences in either direct or indirect measurements of iron status between the two groups. In 75% of these subjects the hepatic iron concentration was greater than 350 micrograms/g dry weight, an extreme elevation associated with a high risk of fibrosis and cirrhosis. Serum ferritin was elevated in all subjects and the transferrin saturation was greater than 60% in 93% of the subjects. Hepatomegaly was present in 20 of the 22 cases and there was only a moderate derangement in liver enzymes except for a tenfold increase in the median gamma-glutamyl transpeptidase concentration. There was a strong correlation between serum ferritin and hepatic iron concentrations (r = 0.71, P = 0.006). After a median follow-up of 19 months, 6 (26%) of the subjects had died. The risk of mortality correlated significantly with both the hepatic iron concentration and the serum ferritin concentration.

CONCLUSIONS

Indirect measurements of iron status (serum ferritin concentration and transferrin saturation) are useful in the diagnosis of African dietary iron overload. When dietary iron overload becomes symptomatic it has a high mortality. Measures to prevent and treat this condition are needed.

Iron overload in urban Africans in the 1990s

BACKGROUND

In a previously described model, heterozygotes for an African iron loading locus develop iron overload only when dietary iron is high, but homozygotes may do so with normal dietary iron. If an iron loading gene is common, then homozygotes with iron overload will be found even in an urban population where traditional beer, the source of iron, is uncommon.

AIMS

To determine whether iron overload and the C282Y mutation characteristic of hereditary haemochromatosis are readily identifiable in an urban African population.

METHODS

Histological assessment, hepatocellular iron grading, and dry weight non-haem iron concentration were determined in post mortem tissue from liver, spleen, heart, lungs, and skin. DNA of subjects with elevated hepatic iron indexes was analysed for the C282Y mutation. Iron concentrations in other tissues were compared.

RESULTS

A moderate increase (>30 micromol/g) in hepatic iron concentrations was found in 31 subjects (23%; 95% confidence interval 15.9 to 30.1%), and they were considerably elevated (>180 micromol/g) in seven subjects (5.2%; 95% confidence interval 1.5 to 8.9%). Appreciably elevated hepatic iron concentrations were associated with heavy iron deposition in both hepatocytes and macrophages, and either portal fibrosis or cirrhosis. All were negative for the C282Y mutation. Very high concentrations were uncommon in subjects dying in hospital. Concentrations of iron in spleen, heart, lung, and skin were significantly higher in subjects with elevated hepatic iron.

CONCLUSIONS

Iron overload is readily identified among urban Africans and is associated with hepatic damage and iron loading of several tissues. The condition is unrelated to the genetic mutation found in hereditary haemochromatosis.

Iron and alcohol content of traditional beers in rural Zimbabwe

OBJECTIVES

To determine the concentrations of iron and alcohol in traditional beer, as well as how these may be related to the brewing process.

DESIGN

Cross sectional study.

SETTING/SUBJECTS

Rural communities living in four of Zimbabwe's nine provinces.

MAIN OUTCOME MEASURES

Ionic iron concentration and alcohol concentration in 94 different types of alcoholic beverages prepared in rural areas, and 18 commercially produced beers.

RESULTS

The commonest types of traditional beer were a seven day beverage called 'doro rematanda', a by-product of this seven day beer called 'muchaiwa,' and a one-day beverage called 'chikokiyana'. Methods of preparation were similar in the four provinces. Median (Q1, Q3) ionic iron concentrations were 52 (31 to 75) mg/L for the seven-day beer (n = 51), 24 (18 to 36) mg/L for muchaiwa (n = 30) and 21 (17 to 63) mg/L for chikokiyana (n = 13). In contrast, ionic iron concentrations in 12 samples of commercially prepared clear beers were 0.1 mg/L and in commercial opaque beer were 3.6 mg/L. Mean (SD) alcohol concentration in traditional beer was 4.1 g/100 ml (+/- 0.873) compared to 2.8 g/100 ml +/- 1.394) in the muchaiwa and 3.6 g/100 ml (+/- 1.445) in the one day brew, chikokiyana. Mean alcohol concentrations in the three commercial beers are reportedly 3.5 g/100 ml in the opaque beer (Scud), and 4.7 to 5.0 g/ml in clear beer (Zambezi and Castle lagers).

CONCLUSIONS

Several preparation methods lead to traditional fermented beverages with very high iron concentrations. Measures to prevent dietary iron overload should include all of these beverages in their scope.

Non-transferrin-bound iron and hepatic dysfunction in African dietary iron overload

BACKGROUND

Circulating iron is normally bound to transferrin. Non-transferrin-bound iron (NTBI) has been described in most forms of iron overload, but has not been studied in African dietary iron overload. This abnormal iron fraction is probably toxic, but this has not been demonstrated.

METHODS

High-pressure liquid chromatography was used to assay serum NTBI in 25 black African subjects with iron overload documented by liver biopsy and in 170 relatives and neighbours. Levels of NTBI were correlated with indirect measures of iron status and conventional liver function tests.

RESULTS

Non-transferrin-bound iron (> 2 micromol/L) was present in 43 people, 22 of patients of whom underwent liver biopsy and 21 relatives and neighbours. All but four of these had evidence of iron overload on the basis of either liver biopsy or elevated transferrin and serum ferritin concentrations. Among all 195 subjects, the presence of NTBI in serum was independently related to elevations in alanine and aspartate aminotransferase activity and bilirubin concentration. This relationship between serum NTBI and hepatic dysfunction was confirmed in the subgroup of 25 subjects with iron overload documented by liver biopsy. Non-transferrin-bound iron correlated significantly with elevations in alanine and aspartate aminotransferase activities after adjustment for hepatic iron grades, inflammation and diet.

CONCLUSIONS

Non-transferrin-bound iron was found to be commonly present in African patients with dietary iron overload and to correlate with transferrin saturation and serum ferritin concentration. The independent relationship between NTBI and elevated liver function tests suggests that it may be part of a pathway leading to hepatic injury.

Targeting of a human iron-sulfur cluster assembly enzyme, nifs, to different subcellular compartments is regulated through alternative AUG utilization

Iron-sulfur clusters are prosthetic groups that are required for the function of numerous enzymes in the cell, including enzymes important in respiration, photosynthesis, and nitrogen fixation. Here we report cloning of the human homolog of NifS, a cysteine desulfurase that is proposed to supply the inorganic sulfur in iron-sulfur clusters. In human cells, different forms of NifS that localize either to mitochondria or to the cytosol and nucleus are synthesized from a single transcript through initiation at alternative inframe AUGs, and initiation site selection varies according to the pH of the medium or cytosol. Thus, a novel form of translational regulation permits rapid redistribution of NifS proteins into different compartments of the cell in response to changes in metabolic status.

Cell-cycle arrest and inhibition of G1 cyclin translation by iron in AFT1-1(up) yeast

Although iron is an essential nutrient, it is also a potent cellular toxin, and the acquisition of iron is a highly regulated process in eukaryotes. In yeast, iron uptake is homeostatically regulated by the transcription factor encoded by AFT1. Expression of AFT1-1(up), a dominant mutant allele, results in inappropriately high rates of iron uptake, and AFT1-1(up) mutants grow slowly in the presence of high concentrations of iron. We present evidence that when Aft1-1(up) mutants are exposed to iron, they arrest the cell division cycle at the G1 regulatory point Start. This arrest is dependent on high-affinity iron uptake and does not require the activation of the DNA damage checkpoint governed by RAD9. The iron-induced arrest is bypassed by overexpression of a mutant G1 cyclin, cln3-2, and expression of the G1-specific cyclins Cln1 and Cln2 is reduced when yeast are exposed to increasing amounts of iron, which may account for the arrest. This reduction is not due to changes in transcription of CLN1 or CLN2, nor is it due to accelerated degradation of the protein. Instead, this reduction occurs at the level of Cln2 translation, a recently recognized locus of cell-cycle control in yeast.

Dietary iron overload as a risk factor for hepatocellular carcinoma in Black Africans

Although the iron-loading disease, hereditary hemochromatosis, has a strong causal association with hepatocellular carcinoma (HCC), the carcinogenic potential of dietary iron overload in Black Africans is not known. We investigated this potential by evaluating iron status, alcohol consumption, markers for hepatitis B (HBV) and C virus (HCV) infections, and exposure to dietary aflatoxin B1 in 24 rural patients with this tumor, 48 race-, sex-, and age-matched hospital-based controls, and 75 related or unrelated close family members of the cancer patients. Iron overload was defined as a raised serum ferritin concentration in combination with a transferrin saturation > or = 60%, and was confirmed histologically when possible. Among 24 patients and 48 hospital controls, the risk of developing HCC in the iron-loaded subjects was 10.6 (95% confidence limits of 1.5 and 76.8) relative to individuals with normal iron status, after adjusting for alcohol consumption, chronic HBV and HBC infections, and exposure to aflatoxin B1. The risk of HCC in subjects with HBV infection was 33.2 (7.2, 153.4) (odds ratio [95% confidence limits]), HCV infection 6.4 (0.3, 133.5), and alcohol consumption 2.0 (0.5, 8.2). Aflatoxin B1 exposure did not appear to increase the risk of HCC. The population attributable risk of iron overload in the development of HCC was estimated to be 29%. Among 20 cancer patients and 75 family members, the risk of developing HCC with iron overload was 4.1 (0.5, 32.2). We conclude that dietary iron overload may contribute to the development of HCC in Black Africans.

Iron-dependent oxidation, ubiquitination, and degradation of iron regulatory protein 2: implications for degradation of oxidized proteins

The ability of iron to catalyze formation of reactive oxygen species significantly contributes to its toxicity in cells and animals. Iron uptake and distribution is regulated tightly in mammalian cells, in part by iron regulatory protein 2 (IRP2), a protein that is degraded efficiently by the proteasome in iron-replete cells. Here, we demonstrate that IRP2 is oxidized and ubiquitinated in cells before degradation. Moreover, iron-dependent oxidation converts IRP2 into a substrate for ubiquitination in vitro. A regulatory pathway is described in which excess iron is sensed by its ability to catalyze site-specific oxidations in IRP2, oxidized IRP2 is ubiquitinated, and ubiquitinated IRP2 subsequently is degraded by the proteasome. Selective targeting and removal of oxidatively modified proteins may contribute to the turnover of many proteins that are degraded by the proteasome.

Serum transferrin receptors are decreased in the presence of iron overload

To test the hypothesis that the quantities of circulating transferrin receptors are reduced in iron overload, we studied serum transferrin receptors and indirect measures of iron status in 150 subjects from rural Zimbabwe. We found significant inverse correlations between serum concentrations of transferrin receptors and ferritin, the ratio of ferritin to aspartate aminotransferase, and transferrin saturation (r > or = 0.44; P < 0.001). The mean +/- SD concentration of serum transferrin receptors in 23 subjects classified as having iron overload (ferritin > 300 microg/L and transferrin saturation > 60%) was 1.55 +/- 0.61 mg/L, significantly lower than the 2.50 +/- 0.62 mg/L in 75 subjects with normal iron stores (ferritin 20-300 microg/L and transferrin saturation 15-55%; P < 0.0005) and the 2.83 +/- 1.14 mg/L in 8 subjects with iron deficiency (ferritin < 20 microg/L; P = 0.001). In keeping with the regulation of transferrin receptor expression at the cellular level, our findings suggest that serum transferrin receptors are decreased in the presence of iron overload.

Structure and dynamics of the iron responsive element RNA: implications for binding of the RNA by iron regulatory binding proteins

The iron responsive element (IRE) is a approximately 30 nucleotide RNA hairpin that is located in the 5' untranslated region of all ferritin mRNAs and in the 3' untranslated region of all transferrin receptor mRNAs. The IREs are bound by two related IRE-binding proteins (IRPs) which help control intracellular levels of iron by regulating the expression of both ferritin and transferrin receptor genes. Multi-dimensional NMR and computational approaches were used to study the structure and dynamics of the IRE RNA in solution. The NMR data are consistent with formation of A-form helical stem regions, a one-base internal bulge and a Watson-Crick C.G base-pair between the first and fifth nucleotides in the loop. A superposition of refined structures indicates that the conserved C in the internal bulge, and three residues in the six-nucleotide hairpin loop are quite dynamic in this RNA. The structural roles of the stems, the loop and the bulge in the function of the IRE RNA and in possible interactions with the iron regulatory protein are discussed.

Identification of a conserved and functional iron-responsive element in the 5'-untranslated region of mammalian mitochondrial aconitase

Iron-responsive elements (IREs) are RNA stem-loop motifs found in genes of iron metabolism. When cells are iron-depleted, iron regulatory proteins (IRPs) bind to IREs in the transcripts of ferritin, transferrin receptor, and erythroid amino-levulinic acid synthetase. Binding of IRPs to IRE motifs near the 5' end of the transcript results in attenuation of translation while binding to IREs in the 3'-untranslated region of the transferrin receptor results in protection from endonucleolytic cleavage. Iron deprivation results in activation of IRE binding activity, whereas iron replete cells lose IRE binding activation. Here, we report the identification of a conserved IRE in the 5'-untranslated region of the transcript of the citric acid cycle enzyme mitochondrial aconitase from four different mammalian species. The IRE in the transcript of mitochondrial aconitase can mediate in vitro translational repression of mitochondrial aconitase by IRPs. Furthermore, levels of mitochondrial aconitase are decreased in mice maintained on a low iron diet, whereas levels of mRNA remain unchanged. The decrease in levels of mitochondrial aconitase is likely due to activation of IRP binding and consequent attenuation of translation. Thus, expression of the iron-sulfur protein mitochondrial aconitase and function of the citric acid cycle may be regulated by iron levels in cells.

Iron-sulfur clusters as biosensors of oxidants and iron

Iron-sulfur clusters are prosthetic groups commonly found in proteins that participate in oxidation-reduction reactions and catalysis. Here, we focus on two proteins that contain iron-sulfur clusters, the fumarate nitrate reduction (FNR) protein of Escherichia coli and mammalian iron-responsive-element-binding protein 1 (IRP1), both of which function as direct sensors of oxygen and iron levels. Assembly and disassembly of iron-sulfur clusters is the key to sensing in these proteins and we speculate that iron-sulfur clusters might be found in other regulatory proteins that sense levels of iron and/or oxygen.

Differences in the RNA binding sites of iron regulatory proteins and potential target diversity

Posttranscriptional regulation of genes of mammalian iron metabolism is mediated by the interaction of iron regulatory proteins (IRPs) with RNA stem-loop sequence elements known as iron-responsive elements (IREs). There are two identified IRPs, IRP1 and IRP2, each of which binds consensus IREs present in eukaryotic transcripts with equal affinity. Site-directed mutagenesis of IRP1 and IRP2 reveals that, although the binding affinities for consensus IREs are indistinguishable, the contributions of arginine residues in the active-site cleft to the binding affinity are different in the two RNA binding sites. Furthermore, although each IRP binds the consensus IRE with high affinity, each IRP also binds a unique alternative ligand, which was identified in an in vitro systematic evolution of ligands by exponential enrichment procedure. Differences in the two binding sites may be important in the function of the IRE-IRP regulatory system.

The impact of oxidative stress on eukaryotic iron metabolism

The processes of iron uptake and distribution are highly regulated in mammalian cells. Expression of the transferrin receptor is increased when cells are iron-depleted, while expression of the iron sequestration protein ferritin is increased in cells that are iron-replete. Regulation of expression of proteins of iron uptake (transferrin receptor) and iron sequestration (ferritin) presumably ensures that levels of reactive free iron are not high in cells. Formation of reactive oxygen species occurs when free iron reacts with oxygen, and tight regulation of iron metabolism may enable cells to avoid engaging in destructive chemical reactions. Levels of intracellular iron are directly sensed by two iron sensing proteins. Iron regulatory protein 1 (IRP1) is a bifunctional protein; in cells that are iron-replete, IRP1 contains an iron-sulfur cluster and functions as cytosolic aconitase. In cells that are iron-depleted, IRP1 binds stem-loop structures in RNA transcripts known as iron responsive elements (IREs). Iron regulatory protein 2 (IRP2) binds similar stem-loop structures, but the mode of regulation of IRP2 is different in that IRP2 is rapidly degraded in iron-replete cells. The post-transcriptional regulation of genes of iron metabolism in mammalian cells ensures that cells have an adequate supply of iron, and also ensures that cells do not generate excess reactive oxygen species through the interaction of free iron and oxygen.

Requirements for iron-regulated degradation of the RNA binding protein, iron regulatory protein 2

Iron regulatory proteins (IRPs) regulate the expression of genes involved in iron metabolism whose transcripts contain RNA stem-loop motifs known as iron-responsive elements (IREs). When iron concentrations are low, IRPs bind to IREs in the 5' untranslated region (UTR) of transcripts where they repress translation, or the 3' UTR of transcripts where they inhibit degradation. The RNA binding activities of the homologous proteins IRP1 and IRP2 are both regulated post-translationally. The binding activity of IRP2 is regulated by the degradation of the protein when cells are iron-replete. Here, we demonstrate that a 73 amino acid sequence that corresponds to a unique exon in IRP2 contains a sequence required for rapid degradation in iron-replete cells. The deletion of this sequence eliminates the rapid turnover of IRP2, whereas the transfer of this sequence to the corresponding position in the homologous protein IRP1 confers the capacity for iron-dependent degradation upon IRP1. Site-directed mutagenesis has demonstrated that specific cysteines within the IRP2 exon are required for iron-dependent degradation. The degradation of IRP2 appears to be mediated by the proteasome in iron-replete cells. When degradation is prevented, the RNA binding activity of IRP2 is not regulated by iron concentration. Thus, degradation is required for the regulation of the RNA binding activity of IRP2.

Expression of a constitutive mutant of iron regulatory protein 1 abolishes iron homeostasis in mammalian cells

Iron regulatory proteins (IRPs) are iron-sensing proteins that bind to RNA stem-loop sequences known as iron-responsive elements (IREs) when cells are depleted of iron. Although IRPs have been shown to bind to IREs derived from ferritin and transferrin receptor (TfR) mRNAs in vitro, there has not been a direct demonstration of the impact of a recombinant IRP on the expression of endogenous IRE-containing transcripts. In this study, we evaluate the impact of expression of C437S, a mutant of IRP1 that binds IREs regardless of cellular iron status, on the regulation of biosynthesis of ferritin and TfR. Despite being made iron-replete, cells expressing C437S continue to synthesize and express high amounts of TfR, while the synthesis of ferritin is repressed. Thus, a single mutant IRP can prevent the usual homeostatic changes in ferritin and TfR biosynthesis. Cells expressing the mutant protein would therefore be predicted to be unable to defend against iron overload. Preliminary results show that cells treated with iron have diminished cell survival when C437S is expressed, and we have thus created a tissue culture model system for the study of iron toxicity.

Translational repressor activity is equivalent and is quantitatively predicted by in vitro RNA binding for two iron-responsive element-binding proteins, IRP1 and IRP2

Iron regulatory proteins (IRPs) bind to specific RNA stem-loop structures known as iron-responsive elements (IREs) which mediate the post-transcriptional regulation of many genes of iron metabolism. Most studies have focused on the role of IRP1, which has previously been shown to bind with high affinity to IREs and mediate repression of in vitro translation of ferritin mRNAs. More recently, a second IRP has been identified that is expressed in all tissues and that binds IREs (Rouault, T. A., Haile, D. H., Downey, W. E., Philpott, C. C., Tang, C., Samaniego, F., Chin, J., Paul, I., Orloff, D., Harford, J. B., and Klausner, R. D. (1992) BioMetals 5, 131-140; Henderson, B. R., Seiser, C., and Kuhn, L. C. (1993) J. Biol. Chem. 268, 27327-27334; Guo, B., Yu, Y., and Leibold, E. A. (1994) J. Biol. Chem. 269, 24252-24260; Samaniego, F., Chin, J., Iwai, K., Rouault, T. A., and Klausner, R. D. (1994) J. Biol. Chem. 269, 30904-30910). Here we report that purified recombinant IRP2 inhibits translation of ferritin mRNAs with a molar efficacy equal to that of recombinant IRP1. There is a quantitative correlation between binding to isolated RNA target motifs, as judged by gel retardation assays, and translational repressor function as assayed in an in vitro translation system. In contrast to IRP1, IRP2 is not inactivated for RNA binding by alkylation with N-ethylmaleimide or phenylmaleimide, and as we would therefore predict, IRP2 treated with N-ethylmaleimide remains an effective repressor of ferritin translation. As IRP1 and IRP2 clearly have equal capability of mediating translational repression in vitro, the contributions of both IRPs to overall regulation must be considered in describing the pathways of iron regulated gene expression in individual cells.

The iron-responsive element binding protein: a target for synaptic actions of nitric oxide

Molecular targets for the actions of nitric oxide (NO) have only been partially clarified. The dynamic properties of the iron-sulfur (Fe-S) cluster of the iron responsive-element binding protein (IRE-BP) suggested that it might serve as a target for NO produced in response to glutamatergic stimulation in neurons. In the present study, we demonstrate that N-methyl-D-aspartate, acting through NO, stimulates the RNA-binding function of the IRE-BP in brain slices while diminishing its aconitase activity. In addition, we demonstrate a selective localization of the IRE-BP in discrete neuronal structures, suggesting a potential role for this protein in the response of neurons to NO.

Molecular characterization of a second iron-responsive element binding protein, iron regulatory protein 2. Structure, function, and post-translational regulation

Several genes critical to the uptake, sequestration, and utilization of iron are regulated at the post-transcriptional level. The mRNAs encoded by these genes contain highly conserved stem-loop structures called iron-responsive elements (IREs). IREs function as the nucleic acid-binding sites for a cytosolic RNA-binding protein called the IRE-binding protein or IRE-BP. Binding of the IRE-BP to IREs is reversibly regulated by the iron status of the cell. The IRE-BP is highly conserved among human, rat, mouse, and rabbit, and it is identical to the cytosolic form of aconitase. In this study, we demonstrate that a distinct human gene encoding a protein which is 57% identical to the initially described IRE-BP, now referred to as iron regulatory protein 1 or IRP1, is also capable of binding to IREs with the same in vitro affinity and specificity the originally identified protein. This second gene product, which we call IRP2, is expressed in many tissues, but its mRNA abundance and tissue distribution are different from IRP1. In most cell lines tested, levels of IRP2 are inversely regulated by iron levels due to iron-dependent regulation of the half-life of the protein. In addition to changes in total amounts of IRP2, we demonstrate that the IRE binding activity of IRP2 can also vary up to 4-fold in the absence of any change in IRP2 protein levels. The possible reasons for the existence of a second IRP are discussed.

The bifunctional iron-responsive element binding protein/cytosolic aconitase: the role of active-site residues in ligand binding and regulation

The iron-responsive element binding protein/cytosolic aconitase functions as either an RNA binding protein that regulates the uptake, sequestration, and utilization of iron or an enzyme that interconverts citrate and isocitrate. These mutually exclusive functions are regulated by changes in cellular iron levels. By site-directed mutagenesis we show that (i) ligation of a [4Fe-4S] cluster is necessary to inactivate RNA binding and activate enzyme function in vivo, (ii) three of four arginine residues of the aconitase active site participate in RNA binding, and (iii) aconitase activity is not required for iron-mediated regulation of RNA binding.

Overexpression of iron-responsive element-binding protein and its analytical characterization as the RNA-binding form, devoid of an iron-sulfur cluster

The iron-responsive element-binding protein (IRE-BP) has been defined and identified as an RNA-binding protein found in iron-deprived eukaryotic cells. IRE-BP binds to stem-loop structures, iron-responsive elements (IREs), which are located in the untranslated regions of the mRNAs for several genes including ferritin, and the transferrin receptor. When bound, IRE-BP prevents ferritin translation and stabilizes the transferrin receptor transcript. When cells are iron replete, an iron-sulfur cluster is ligated to the IRE-BP, the protein loses RNA binding properties, and it acquires aconitase activity. Cytosolic aconitase from liver can be converted into the IRE-BP by oxidative removal of its Fe-S cluster. We describe here overexpression of IRE-BP in baculovirus-infected insect cells which yields IRE-BP devoid of an iron-sulfur cluster. We describe a one-step purification of the IRE-BP and a quantitative analysis of Fe, S2-, S0, protein, and enzyme activity on IRE-BP, as obtained in cell lysates, after purification, and after reconstitution to active aconitase. On the average not more than 3% of the over-expressed purified protein contained an intact Fe-S cluster, and it was demonstrated that that cluster was not lost during purification. Scatchard analysis of RNA-binding data was compatible with a single high-affinity RNA-binding form of the IRE-BP. Active aconitase could be reconstituted from the purified IRE-BP obtained from the expression system by addition of iron, thiol, and sulfide, and the characteristic epr spectrum of the 3Fe form of cytosolic aconitase was obtained after ferricyanide oxidation of the reconstituted material.

The iron-responsive element-binding protein: localization of the RNA-binding site to the aconitase active-site cleft

The iron-responsive element-binding protein (IRE-BP) binds to specific stem-loop RNA structures known as iron-responsive elements (IREs) present in a variety of cellular mRNAs (e.g., those encoding ferritin, erythroid 5-aminolevulinate synthase, and transferrin receptor). Expression of these genes is regulated by interaction with the IRE-BP. The IRE-BP is identical in sequence to cytosolic aconitase, and the function of the protein is determined by the presence or absence of an Fe-S cluster. The protein either functions as an active aconitase when the Fe-S cluster is present or as an RNA-binding protein when the protein lacks this cluster. Aconitase activity and IRE-binding activity are mutually exclusive, and interconversion between the two activities is determined by intracellular Fe concentrations. Mapping of the RNA-binding site of the IRE-BP by UV cross-linking studies defines a major contact site between IRE and protein in the active-site region. Modeling based on probable structural similarities between the previously crystallized mitochondrial aconitase and the IRE-BP predicts that these residues would be accessible to the IRE only were there a major change in the predicted conformation of the protein when cells are iron-depleted.

Identification and characterization of host factor interactions with cis-acting elements of rubella virus RNA

We have analyzed the function of cis-acting elements of rubella virus RNA and the components which interact with these elements in viral RNA replication. We demonstrated that the 5'- and 3'-terminal sequences from RV RNA promote translation and negative-strand RNA synthesis of chimeric chloroamphenicol acetyltransferase (CAT) RNAs. These sequences have a potential to form stem-loop (SL) structures and bind cellular proteins specifically in RNA gel-shift and UV cross-linking assays. The 5' end binding proteins were identified to be Ro/SSA-associated antigens by virtue of being recognized in an RNA complex by an autoimmune patient serum with Ro antigen type specificity. Purification and sequence analysis of the 3' end binding protein revealed that it is a homologue of human calreticulin. The role of host protein in RV replication is discussed.

Modification of a free Fe-S cluster cysteine residue in the active iron-responsive element-binding protein prevents RNA binding

The iron-responsive element-binding protein (IRE-BP) binds to specific RNA stem-loop structures called iron-responsive elements (IREs), which mediate the post-transcriptional regulation of a variety of genes involved in iron metabolism. The IRE-BP is cytosolic aconitase, and a [4Fe-4S] cubane cluster is required for aconitase activity but is associated with loss of IRE binding affinity. Chemical modification of the IRE-BP can abrogate RNA binding and the 3 cysteines predicted to coordinate the Fe-S cluster in the IRE-BP could be targets for modification. We report the expression of recombinant IRE-BP in which the three putative cluster cysteines (Cys-437, Cys-503, and Cys-506) have been mutated to serine residues. Replacement of any or all of these cysteine residues results in a complete loss of aconitase activity. While all of the mutants bind RNA, substitution of Cys-437 specifically renders the IRE-BP resistant to inactivation by low concentrations of N-ethylmaleimide or diamide. These results identify Cys-437 as the target of in vitro regulation of RNA binding in the IRE-BP and suggest that, in the RNA-binding form of the protein, Cys-437 is free and therefore available for modifications that inhibit RNA binding.

Cellular regulation of the iron-responsive element binding protein: disassembly of the cubane iron-sulfur cluster results in high-affinity RNA binding

The translation of ferritin mRNA and degradation of transferrin receptor mRNA are regulated by the interaction of an RNA-binding protein, the iron-responsive element binding protein (IRE-BP), with RNA stem-loop structures known as iron-responsive elements (IREs) contained within these transcripts. IRE-BP produced in iron-replete cells has aconitase (EC 4.2.1.3) activity. The protein shows extensive sequence homology with mitochondrial aconitase, and sequences of peptides prepared from cytosolic aconitase are identical with peptides of IRE-BP. As an active aconitase, IRE-BP is expected to have an Fe-S cluster, in analogy to other aconitases. This Fe-S cluster has been implicated as the region of the protein that senses intracellular iron levels and accordingly modifies the ability of the IRE-BP to interact with IREs. Expression of the IRE-BP in cultured cells has revealed that the IRE-BP functions either as an active aconitase, when the cells are iron-replete, or as an active RNA-binding protein, when the cells are iron-depleted. We compare properties of purified authentic cytosolic aconitase from beef liver with those of IRE-BP from tissue culture cells and establish that characteristics of the physiologically relevant form of the protein from iron-depleted cells resemble those of cytosolic aconitase apoprotein. We demonstrate that loss of the labile fourth iron atom of the Fe-S cluster results in loss of aconitase activity, but that more extensive cluster alteration is required before the IRE-BP acquires the capacity to bind RNA with the affinity seen in vivo. These results are consistent with a model in which the cubane Fe-S cluster is disassembled when intracellular iron is depleted.

Iron regulates the activity of the iron-responsive element binding protein without changing its rate of synthesis or degradation

The iron-responsive element binding protein (IRE-BP) interacts with specific sequence/structure motifs (iron-responsive elements) within the mRNAs encoding ferritin and the transferrin receptor and thereby post-transcriptionally regulates the expression of these two proteins involved in cellular iron homeostasis. The activity of the IRE-BP is itself regulated by iron such that when cells are treated with an iron source, the RNA binding activity is decreased. The expression of recombinant human IRE-BP in murine cells has been examined as have the expressions of the endogenous IRE-BP of both human and rabbit cells. In all cases, iron down-modulated the RNA binding activity of the IRE-BP, but in no instance was this decrease in activity accompanied by a decrease in the level of the protein as judged by quantitative Western blots. Moreover, the rate of synthesis of the IRE-BP and its rate of degradation have been found to be unaltered by iron manipulation of cells in culture. Consistent with IRE-BP regulation occurring post-translationally, the iron regulation of its activity was found to be unaffected by cycloheximide. These data are discussed in terms of a model of IRE-BP regulation involving the modification of the protein's iron-sulfur center.

Reciprocal control of RNA-binding and aconitase activity in the regulation of the iron-responsive element binding protein: role of the iron-sulfur cluster

Several mechanisms of posttranscriptional gene regulation are involved in regulation of the expression of essential proteins of iron metabolism. Coordinate regulation of ferritin and transferrin receptor expression is produced by binding of a cytosolic protein, the iron-responsive element binding protein (IRE-BP) to specific stem-loop structures present in target RNAs. The affinity of this protein for its cognate RNA is regulated by the cell in response to changes in iron availability. The IRE-BP demonstrates a striking level of amino acid sequence identity to the iron-sulfur (Fe-S) protein mitochondrial aconitase. Moreover, the recombinant IRE-BP has aconitase function. The lability of the Fe-S cluster in mitochondrial aconitase has led us to propose that the mechanism by which iron levels are sensed by the IRE-BP involves changes in an Fe-S cluster in the IRE-BP. In this study, we demonstrate that procedures aimed at altering the IRE-BP Fe-S cluster in vitro reciprocally alter the RNA binding and aconitase activity of the IRE-BP. The changes in the RNA binding of the protein produced in vitro appear to match the previously described alterations of the protein in response to iron availability in the cell. Furthermore, iron manipulation of cells correlates with the activation or inactivation of the IRE-BP aconitase activity. The results are consistent with a model for the posttranslational regulation of the IRE-BP in which the Fe-S cluster is altered in response to the availability of intracellular iron and this, in turn, regulates the RNA-binding activity.

An iron-sulfur cluster plays a novel regulatory role in the iron-responsive element binding protein

Post-transcriptional regulation of genes important in iron metabolism, ferritin and the transferrin receptor (TfR), is achieved through regulated binding of a cytosolic protein, the iron-responsive element binding protein (IRE-BP), to RNA stem-loop motifs known as iron-responsive elements (IREs). Binding of the IRE-BP represses ferritin translation and represses degradation of the TfR mRNA. The IRE-BP senses iron levels and accordingly modifies binding to IREs through a novel sensing mechanism. An iron-sulfur cluster of the IRE-BP reversibly binds iron; when cytosolic iron levels are depleted, the cluster becomes depleted of iron and the IRE-BP acquires the capacity to bind IREs. When cytosolic iron levels are replete, the IRE-BP loses RNA binding capacity, but acquires enzymatic activity as a functional aconitase. RNA binding and aconitase activity are mutually exclusive activities of the IRE-BP, and the state of the iron-sulfur cluster determines how the IRE-BP will function.

Sequence and expression of the murine iron-responsive element binding protein

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A regulated RNA binding protein also possesses aconitase activity

A clone for the iron-responsive element (IRE)-binding protein (IRE-BP) has been transfected and expressed in mouse fibroblasts. The IRE-BP gene product binds IREs with high affinity and specificity. Amino acid alignments reveal that the IRE-BP is 30% identical to mitochondrial aconitase. The 18 active site residues of mitochondrial aconitase are identical to those in the IRE-BP, suggesting that the IRE-BP may possess aconitase activity. After purification of native IRE-BP and immunoaffinity purification of transfected and expressed IRE-BP, we demonstrate that the purified IRE-BP has aconitase activity.

Specific binding of host cell proteins to the 3'-terminal stem-loop structure of rubella virus negative-strand RNA

At the 5' end of the rubella virus genomic RNA, there are sequences that can form a potentially stable stem-loop (SL) structure. The complementary negative-strand equivalent of the 5'-end SL structure of positive-strand rubella virus RNA [5' (+) SL structure] is thought to serve as a promoter for the initiation of positive-strand synthesis. We screened the negative-strand equivalent of the 5' (+) SL structure (64 nucleotides) and the adjacent region of the negative-strand RNA for their ability to bind to host cell proteins. Specific binding to the 64-nucleotide-long potential SL structure of three cytosolic proteins with relative molecular masses of 97, 79, and 56 kDa was observed by UV-induced covalent cross-linking. There was a significant increase in the binding of the 97-kDa protein from cells upon infection with rubella virus. Altering the SL structure by deleting sequences in either one of the two potential loops abolished the binding interaction. The 56-kDa protein also appeared to bind specifically to an SL derived from the 3' end of positive-strand RNA. The 3'-terminal structure of rubella virus negative-strand RNA shared the same protein-binding activity with similar structures in alphaviruses, such as Sindbis virus and eastern equine encephalitis virus. A possible role for the host proteins in the replication of rubella virus and alphaviruses is discussed.

Cloning of the cDNA encoding an RNA regulatory protein--the human iron-responsive element-binding protein

Iron-responsive elements (IREs) are stemloop structures found in the mRNAs encoding ferritin and the transferrin receptor. These elements participate in the iron-induced regulation of the translation of ferritin and the stability of the transferrin receptor mRNA. Regulation in both instances is mediated by binding of a cytosolic protein to the IREs. High-affinity binding is seen when cells are starved of iron and results in repression of ferritin translation and inhibition of transferrin receptor mRNA degradation. The IRE-binding protein (IRE-BP) has been identified as an approximately 90-kDa protein that has been purified by both affinity and conventional chromatography. In this report we use RNA affinity chromatography and two-dimensional gel electrophoresis to isolate the IRE-BP for protein sequencing. A degenerate oligonucleotide probe derived from a single peptide sequence was used to isolate a cDNA clone that encodes a protein containing 13 other sequenced peptides obtained from the IRE-BP. Consistent with previous characterization of the IRE-BP, the cDNA encodes a protein of 87 kDa with a slightly acidic pI, and the corresponding mRNA of approximately 3.6 kilobases is found in a variety of cell types. The encoded protein contains a nucleotide-binding consensus sequence and regions of cysteine and histidine clusters. This mRNA is encoded by a single gene on human chromosome 9, a finding consistent with previous localization by functional mapping. The protein contains no previously defined consensus motifs for either RNA or DNA binding. The simultaneous cloning of a different, but highly homologous, cDNA suggests that the IRE-BP is a member of a distinct gene family.