Iron-sulphur clusters as genetic regulatory switches: the bifunctional iron regulatory protein-1

Mitochondrial Aconitase and Its Contribution to the Pathogenesis of Neurodegenerative Diseases

This survey reviews modern ideas on the structure and functions of mitochondrial and cytosolic aconitase isoenzymes in eukaryotes. Cumulative experimental evidence about mitochondrial aconitases (Aco2) as one of the main targets of reactive oxygen and nitrogen species is generalized. The important role of Aco2 in maintenance of homeostasis of the intracellular iron pool and maintenance of the mitochondrial DNA is discussed. The role of Aco2 in the pathogenesis of some neurodegenerative diseases is highlighted. Inactivation or dysfunction of Aco2 as well as mutations found in the ACO2 gene appear to be significant factors in the development and promotion of various types of neurodegenerative diseases. A restoration of efficient mitochondrial functioning as a source of energy for the cell by targeting Aco2 seems to be one of the promising therapeutic directions to minimize progressive neurodegenerative disorders.

The redox requirement and regulation during cell proliferation

The intracellular metabolic network comprises a variety of reduction-oxidation (redox) reactions that occur in a temporally and spatially distinct manner. In order to coordinate these redox processes, mammalian cells utilize a collection of electron-carrying molecules common to many redox reactions, including NAD, NADP, coenzyme Q (CoQ), and glutathione (GSH). This review considers the metabolic basis of redox regulation in the context of cell proliferation by analyzing how cells acquire and utilize electron carriers to maintain directional carbon flux, sustain reductive biosynthesis, and support antioxidant defense. Elucidating the redox requirement during cell proliferation can advance the understanding of human diseases such as cancer, and reveal effective therapeutic opportunities in the clinic.

Iron imbalance in neurodegeneration

Iron is an essential element for the development and functionality of the brain, and anomalies in its distribution and concentration in brain tissue have been found to be associated with the most frequent neurodegenerative diseases. When magnetic resonance techniques allowed iron quantification in vivo, it was confirmed that the alteration of brain iron homeostasis is a common feature of many neurodegenerative diseases. However, whether iron is the main actor in the neurodegenerative process, or its alteration is a consequence of the degenerative process is still an open question. Because the different iron-related pathogenic mechanisms are specific for distinctive diseases, identifying the molecular mechanisms common to the various pathologies could represent a way to clarify this complex topic. Indeed, both iron overload and iron deficiency have profound consequences on cellular functioning, and both contribute to neuronal death processes in different manners, such as promoting oxidative damage, a loss of membrane integrity, a loss of proteostasis, and mitochondrial dysfunction. In this review, with the attempt to elucidate the consequences of iron dyshomeostasis for brain health, we summarize the main pathological molecular mechanisms that couple iron and neuronal death.

ASS1 deficiency is associated with impaired neuronal differentiation in zebrafish larvae

Citrullinemia type 1 (CTLN1) is a rare autosomal recessive urea cycle disorder caused by deficiency of the cytosolic enzyme argininosuccinate synthetase 1 (ASS1) due to pathogenic variants in the ASS1 gene located on chromosome 9q34.11. Even though hyperammenomia is considered the major pathomechanistic factor for neurological impairment and cognitive dysfunction, a relevant subset of individuals presents with a neurodegenerative course in the absence of hyperammonemic decompensations. Here we show, that ASS1 deficiency induced by antisense-mediated knockdown of the zebrafish ASS1 homologue is associated with defective neuronal differentiation ultimately causing neuronal cell loss and consecutively decreased brain size in zebrafish larvae in vivo. Whereas ASS1-deficient zebrafish larvae are characterized by markedly elevated concentrations of citrulline - the biochemical hallmark of CTLN1, accumulation of L-citrulline, hyperammonemia or therewith associated secondary metabolic alterations did not account for the observed phenotype. Intriguingly, coinjection of the human ASS1 mRNA not only normalized citrulline concentration but also reversed the morphological cerebral phenotype and restored brain size, confirming conserved functional properties of ASS1 across species. The results of the present study imply a novel, potentially non-enzymatic (moonlighting) function of the ASS1 protein in neurodevelopment.

H2S regulation of iron homeostasis by IRP1 improves vascular smooth muscle cell functions

Either H2S or iron is essential for cellular processes. Abnormal metabolism of H2S and iron has increased risk for cardiovascular diseases. The aim of the present study is to examine the mutual interplay of iron and H2S signals in regulation of vascular smooth muscle cell (SMC) functions. Here we found that deficiency of cystathionine gamma-lyase (CSE, a major H2S-producing enzyme in vascular system) induced but NaHS (a H2S donor) administration attenuated iron accumulation in aortic tissues from angiotensin II-infused mice. In vitro, iron overload induced labile iron levels, promoted cell proliferation, disrupted F-actin filaments, and inhibited protein expressions of SMC-specific markers (αSMA and calponin) more significantly in SMCs from CSE knockout mice (KO-SMCs) than the cells from wild-type mice (WT-SMCs), which could be reversed by exogenously applied NaHS. In contrast, KO-SMCs were more vulnerable to iron starvation-induced cell death. Either iron overload or NaHS did not affect elastin level and gelatinolytic activity. We further found that H2S induced more aconitase activity of iron regulatory protein 1 (IRP1) but inhibited its RNA binding activity accompanied with increased protein levels of ferritin and ferriportin, which would contribute to the lower level of labile iron level inside the cells. In addition, iron was able to suppress CSE-derived H2S generation, while iron also non-enzymatically induced H2S release from cysteine. This study reveals the mutual interaction between iron and H2S signals in regulating SMC phenotypes and functions; CSE/H2S system would be a target for preventing iron metabolic disorder-related vascular diseases.

Iron Regulatory Protein 1 Inhibits Ferritin Translation Responding to OsHV-1 Infection in Ark Clams, Scapharca Broughtonii

Elemental iron is an indispensable prosthetic group of DNA replication relative enzymes. The upregulation of ferritin translation by iron regulatory proteins (IRP1) in host cells is a nutritional immune strategy to sequester available iron to pathogens. The efficient replication of Ostreid herpesvirus 1 (OsHV-1), a lethal dsDNA virus among bivalves, depends on available iron. OsHV-1 infection was found to trigger iron limitation in ark clams; however, it is still an enigma how OsHV-1 successfully conducted rapid replication, escaping host iron limitations. In this study, we identified the IRP1 protein (designated as SbIRP-1) in the ark clam (Scapharca broughtonii) and found it could bind to the iron-responsive element (IRE) of ferritin (SbFn) mRNA based on electrophoretic mobility shift assay (EMSA). Knockdown of SbIRP-1 expression (0.24 ± 1.82-fold of that in NC group, p < 0.01) by RNA interference resulted in the accumulation of SbFn in hemocytes (1.79 ± 0.01-fold, p < 0.01) post-24 h of enhanced RNA interference injection. During OsHV-1 infection, SbFn mRNA was significantly upregulated in hemocytes from 24 h to 60 h, while its protein level was significantly reduced from 24 h to 48 h, with the lowest value at 36 h post-infection (0.11 ± 0.01-fold, p < 0.01). Further analysis by RNA immunoprecipitation assays showed that OsHV-1 could enhance the binding of SbIRP-1 with the SbFn IRE, which was significantly increased (2.17 ± 0.25-fold, p < 0.01) at 36 h post-infection. Consistently, SbIRP-1 protein expression was significantly increased in hemocytes from 12 h to 48 h post OsHV-1 infection (p < 0.01). In conclusion, the results suggest that OsHV-1 infection could suppress post-transcriptional translation of SbFn through the regulation of SbIRP-1, which likely contributes to OsHV-1 evasion of SbFn-mediating host iron limitation.

Oxidative stress concept updated: Definitions, classifications, and regulatory pathways implicated

Reactive oxygen species were discovered in living organisms in the early 1950's and their action has been implicated in diverse biological processes. First formulated by H. Sies in 1985[57], the oxidative stress concept stimulated substantial interest in reactive oxygen species and it is now common that fundamental research in various biomedical fields includes mention of research on the involvement of oxidative stress. Such strong interest has resulted in the development of definitions and classifications of oxidative stress and much research progress in the field. Although we clearly understand the limitations of various definitions or classifications, such parameters may help to provide quantitative descriptions, compare related processes among different laboratories, and introduce some measurable parameters. This paper highlights recent advances in the areas of oxidative stress definitions and the classification of oxidative stresses. Such items are directly associated with our understanding of the molecular mechanisms involved in organismal responses to oxidative insults. The knowledge accumulated to date indicates that selective expression of specific genes is a central player in the adaptive response to oxidative stress and reversible oxidation of cysteine residues of sensor proteins is a key process regulating responses to oxidative stress.

Ferroptosis Is Regulated by Mitochondria in Neurodegenerative Diseases

BACKGROUND

Neurodegenerative diseases are characterized by a gradual decline in motor and/or cognitive function caused by the selective degeneration and loss of neurons in the central nervous system, but their pathological mechanism is still unclear. Previous research has revealed that many forms of cell death, such as apoptosis and necrosis, occur in neurodegenerative diseases. Research in recent years has noticed that there is a new type of cell death in neurodegenerative diseases: ferroptosis. An increasing body of literature provides evidence for an involvement of ferroptosis in neurodegenerative diseases.

SUMMARY

In this article, we review a new form of cell death in neurodegenerative diseases: ferroptosis. Ferroptosis is defined as an iron-dependent form of regulated cell death, which occurs through the lethal accumulation of lipid-based reactive oxygen species when glutathione-dependent lipid peroxide repair systems are compromised. Several salient and established features of neurodegenerative diseases (including lipid peroxidation and iron dyshomeostasis) are consistent with ferroptosis, which means that ferroptosis may be involved in the progression of neurodegenerative diseases. In addition, as the center of energy metabolism in cells, mitochondria are also closely related to the regulation of iron homeostasis in the nervous system. At the same time, neurodegenerative diseases are often accompanied by degeneration of mitochondrial activity. Mitochondrial damage has been found to be involved in lipid peroxidation and iron dyshomeostasis in neurodegenerative diseases. Key Messages: Based on the summary of the related mechanisms of ferroptosis, we conclude that mitochondrial damage may affect neurodegenerative diseases by regulating many aspects of ferroptosis, including cell metabolism, iron dyshomeostasis, and lipid peroxidation.

5-Aminolevulinate synthase catalysis: The catcher in heme biosynthesis

5-Aminolevulinate (ALA) synthase (ALAS), a homodimeric pyridoxal-5'-phosphate (PLP)-dependent enzyme, catalyzes the first step of heme biosynthesis in metazoa, fungi and α-proteobacteria. In this review, we focus on the advances made in unraveling the mechanism of the ALAS-catalyzed reaction during the past decade. The interplay between the PLP cofactor and the protein moiety determines and modulates the multi-intermediate reaction cycle of ALAS, which involves the decarboxylative condensation of two substrates, glycine and succinyl-CoA. Substrate binding and catalysis are rapid, and product (ALA) release dominates the overall ALAS kinetic mechanism. Interconversion between a catalytically incompetent, open conformation and a catalytically competent, closed conformation is linked to ALAS catalysis. Reversion to the open conformation, coincident with ALA dissociation, defines the slowest step of the reaction cycle. These findings were further substantiated by introducing seven mutations in the16-amino acid loop that gates the active site, yielding an ALAS variant with a greatly increased rate of catalytic turnover and heightened specificity constants for both substrates. Recently, molecular dynamics (MD) simulation analysis of various dimeric ALAS forms revealed that the seven active site loop mutations caused the proteins to adopt different conformations. In particular, the emergence of a β-strand in the mutated loop, which interacted with two preexisting β-strands to form an anti-parallel three-stranded β-sheet, conferred the murine heptavariant with a more stable open conformation and prompted faster product release than wild-type mALAS2. Moreover, the dynamics of the mALAS2 active site loop anti-correlated with that of the 35 amino acid C-terminal sequence. This led us to propose that this C-terminal extension, which is absent in prokaryotic ALASs, finely tunes mammalian ALAS activity. Based on the above results, we extend our previous proposal to include that discovery of a ligand inducing the mammalian C-terminal extension to fold offers a good prospect for the development of a new drug for X-linked protoporphyria and/or other porphyrias associated with enhanced ALAS activity and/or porphyrin accumulation.

Transcriptional Regulation of Gene Expression by Metalloproteins

FNR and SoxR are transcriptional regulators containing an iron–sulfur cluster. The iron–sulfur cluster in FNR acts as an oxygen sensor by reacting with oxygen. The structural change of the iron–sulfur cluster takes place when FNR senses oxygen, which regulates the transcriptional regulator activity of FNR through the change of the quaternary structure. SoxR contains the [2Fe–2S] cluster that regulates the transcriptional activator activity of SoxR. Only the oxidized SoxR containing the [2Fe–2S]2+ cluster is active as the transcriptional activator. CooA is a transcriptional activator containing a protoheme that acts as a CO sensor. CO is a physiological effector of CooA and regulates the transcriptional activator activity of CooA. In this review, the biochemical and biophysical properties of FNR, SoxR, and CooA are described.

Iron metabolism in erythroid cells and patients with congenital sideroblastic anemia

Sideroblastic anemias are anemic disorders characterized by the presence of ring sideroblasts in a patient's bone marrow. These disorders are typically divided into two types, congenital or acquired sideroblastic anemia. Recently, several genes were reported as responsible for congenital sideroblastic anemia; however, the relationship between the function of the gene products and ring sideroblasts is largely unclear. In this review article, we will focus on the iron metabolism in erythroid cells as well as in patients with congenital sideroblastic anemia.

Nitric oxide induces hypoxia ischemic injury in the neonatal brain via the disruption of neuronal iron metabolism

We have recently shown that increased hydrogen peroxide (H₂O₂) generation is involved in hypoxia–ischemia (HI)-mediated neonatal brain injury. H₂O₂ can react with free iron to form the hydroxyl radical, through Fenton Chemistry. Thus, the objective of this study was to determine if there was a role for the hydroxyl radical in neonatal HI brain injury and to elucidate the underlying mechanisms. Our data demonstrate that HI increases the deposition of free iron and hydroxyl radical formation, in both P7 hippocampal slice cultures exposed to oxygen–glucose deprivation (OGD), and the neonatal rat exposed to HI. Both these processes were found to be nitric oxide (NO) dependent. Further analysis demonstrated that the NO-dependent increase in iron deposition was mediated through increased transferrin receptor expression and a decrease in ferritin expression. This was correlated with a reduction in aconitase activity. Both NO inhibition and iron scavenging, using deferoxamine administration, reduced hydroxyl radical levels and neuronal cell death. In conclusion, our results suggest that increased NO generation leads to neuronal cell death during neonatal HI, at least in part, by altering iron homeostasis and hydroxyl radical generation.

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HI increases the deposition of free iron and hydroxyl radical formation in the neonatal brain.

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Both these processes are NO dependent.

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Increased iron deposition is mediated via increased TfR and decreased ferritin expression.

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These processes are involved in the neuronal cell death associated with neonatal HI.

Effect of mitochondrial complex I inhibition on Fe-S cluster protein activity

Iron-sulfur (Fe-S) clusters are small inorganic cofactors formed by tetrahedral coordination of iron atoms with sulfur groups. Present in numerous proteins, these clusters are involved in key biological processes such as electron transfer, metabolic and regulatory processes, DNA synthesis and repair and protein structure stabilization. Fe-S clusters are synthesized mainly in the mitochondrion, where they are directly incorporated into mitochondrial Fe-S cluster-containing proteins or exported for cytoplasmic and nuclear cluster-protein assembly. In this study, we tested the hypothesis that inhibition of mitochondrial complex I by rotenone decreases Fe-S cluster synthesis and cluster content and activity of Fe-S cluster-containing enzymes. Inhibition of complex I resulted in decreased activity of three Fe-S cluster-containing enzymes: mitochondrial and cytosolic aconitases and xanthine oxidase. In addition, the Fe-S cluster content of glutamine phosphoribosyl pyrophosphate amidotransferase and mitochondrial aconitase was dramatically decreased. The reduction in cytosolic aconitase activity was associated with an increase in iron regulatory protein (IRP) mRNA binding activity and with an increase in the cytoplasmic labile iron pool. Since IRP activity post-transcriptionally regulates the expression of iron import proteins, Fe-S cluster inhibition may result in a false iron deficiency signal. Given that inhibition of complex I and iron accumulation are hallmarks of idiopathic Parkinson's disease, the findings reported here may have relevance for understanding the pathophysiology of this disease.

Iron-dependent functions of mitochondria--relation to neurodegeneration

A number of neurodegenerative diseases are associated with iron dyshomeostasis and mitochondrial dysfunction. However, the pathomechanistic interplay between iron and mitochondria varies. This review summarises the physiological role of iron in mitochondria and subsequently exemplifies two neurodegenerative diseases with disturbed iron function in mitochondria: inherited Friedreich ataxia (FRDA) and idiopathic Parkinson disease (PD). In eukaryotes, mitochondria are main consumers of iron. The respiratory chain relies on iron-containing redox systems in the form of complexes I-III with iron-sulphur clusters and cytochromes with haem as prosthetic groups. The bifunctional enzyme aconitase is not only important in the citric acid cycle, but also functions as a key regulator of cell iron metabolism. Haem biosynthesis occurs partially in mitochondria as well as the biogenesis of iron-sulphur clusters that are co-factors in numerous iron-sulphur proteins. FRDA is characterised by a mutation of the frataxin gene, the protein of which serves as an iron chaperone in iron-sulphur cluster assembly. The lack of frataxin expression leads to defective iron-sulphur cluster biogenesis with decreased respiratory and aconitase activity. The resulting mitochondrial iron overload might fuel reactive oxygen species formation and contribute to clinical signs of oxidative stress. PD is typically associated with an increased iron content of the substantia nigra, the causes of which are largely unknown. Recent research demonstrated raised iron levels in individual dopaminergic neurons of the substantia nigra. Moreover, transferrin/transferrin receptor 2 mediated transport of iron into the mitochondria of these neurons was identified together with increased transferrin immunoreactivity. Resulting accumulation of iron into mitochondria might lead to oxidative stress damaging iron-sulphur cluster-containing proteins.

Redox control of the DNA damage-inducible protein DinG helicase activity via its iron-sulfur cluster

The Escherichia coli DNA damage-inducible protein DinG, a member of the superfamily 2 DNA helicases, has been implicated in the nucleotide excision repair and recombinational DNA repair pathways. Combining UV-visible absorption, EPR, and enzyme activity measurements, we demonstrate here that E. coli DinG contains a redox-active [4Fe-4S] cluster with a midpoint redox potential (E(m)) of -390 +/- 23 mV (pH 8.0) and that reduction of the [4Fe-4S] cluster reversibly switches off the DinG helicase activity. Unlike the [4Fe-4S] cluster in E. coli dihydroxyacid dehydratase, the DinG [4Fe-4S] cluster is stable, and the enzyme remains fully active after exposure to 100-fold excess of hydrogen peroxide, indicating that DinG could be functional under oxidative stress conditions. However, the DinG [4Fe-4S] cluster can be efficiently modified by nitric oxide (NO), forming the DinG-bound dinitrosyl iron complex with the concomitant inactivation of helicase activity in vitro and in vivo. Reassembly of the [4Fe-4S] cluster in NO-modified DinG restores helicase activity, indicating that the iron-sulfur cluster in DinG is the primary target of NO cytotoxicity. The results led us to propose that the iron-sulfur cluster in DinG may act as a sensor of intracellular redox potential to modulate its helicase activity and that modification of the iron-sulfur cluster in DinG and likely in other DNA repair enzymes by NO may contribute to NO-mediated genomic instability.

HBx modulates iron regulatory protein 1-mediated iron metabolism via reactive oxygen species

Hepatitis B virus X protein (HBx) is involved in viral metabolism and progression of liver disease. Iron metabolism plays a significant role in liver disease. In this report, to elucidate the relationship between iron metabolism and HBx, we established the Huh7 cell lines in which HBx was stably expressed (Huh7-HBx). In Huh7-HBx, we observed that transferrin receptor 1 (TfR1) expression decreased and ferritin heavy chain (FtH) expression increased as well as reactive oxygen species (ROS) level increased. We also found that these modulations were caused by the downregulation of iron regulatory protein 1 (IRP1). Furthermore, the levels of total iron and labile iron pool (LIP) were altered in Huh7-HBx. In addition, antioxidant N-acetylcystein (NaC) increased IRP1 expression by depleting HBx-induced ROS. We also confirmed these alterations of TfR1 and FtH in the primary hepatocytes of HBx transgenic mice and in HepG2.2.15 cells that constitutively replicate the intact HBV genome. In conclusion, these results suggest that HBx modulates iron metabolism via ROS leading to pathological status in liver diseases.

Heme as a magnificent molecule with multiple missions: heme determines its own fate and governs cellular homeostasis

Heme is a prosthetic group of various types of proteins, such as hemoglobin, myoglobin, cytochrome c, cytochrome p450, catalase and peroxidase. In addition, heme is involved in a variety of biological events by modulating the function or the state of hemoproteins. For example, protein synthesis is inhibited in erythroid cells under heme deficiency, as the consequence of the activation of heme-regulated inhibitor (HRI). Iron concentration in the cell is sensed and regulated by the heme-mediated oxidization and subsequent degradation of iron regulatory protein 2 (IRP2). Heme also binds to certain types of potassium channels, thereby inhibiting transmembrane K(+) currents. Importantly, heme determines its own fate; namely, heme regulates its synthesis and degradation through the feedback mechanisms, by which intracellular heme level is precisely maintained. Heme reduces heme synthesis by suppressing the expression of non-specific 5-aminolevulinate synthase (ALAS1) and stimulates heme breakdown by inducing heme oxygenase (HO)-1 expression. ALAS1 and HO-1 are the rate limiting enzymes in heme biosynthesis and catabolism, respectively. Accordingly, under the heme-rich condition, heme binds to cysteine-proline (CP) motifs of ALAS1 and those of transcriptional repressor Bach1, thereby leading to repression of mitochondrial transport of ALAS1 and induction of HO-1 transcription, respectively. Moreover, chemosensing functions of HO-2 containing CP motifs, another isozyme of HO, have been unveiled recently. In this review article, we summarize and update the pleiotropic effects of heme on various biological events and the regulatory network of heme biosynthesis and catabolism.

Crystal structure of human iron regulatory protein 1 as cytosolic aconitase

Iron regulatory proteins (IRPs) control the translation of proteins involved in iron uptake, storage and utilization by binding to specific noncoding sequences of the corresponding mRNAs known as iron-responsive elements (IREs). This strong interaction assures proper iron homeostasis in animal cells under iron shortage. Conversely, under iron-replete conditions, IRP1 binds a [4Fe-4S] cluster and functions as cytosolic aconitase. Regulation of the balance between the two IRP1 activities is complex, and it does not depend only on iron availability. Here, we report the crystal structure of human IRP1 in its aconitase form. Comparison with known structures of homologous enzymes reveals well-conserved folds and active site environments with significantly different surface shapes and charge distributions. The specific features of human IRP1 allow us to propose a tentative model of an IRP1-IRE complex that agrees with a range of previously obtained data.

Death-associated Protein Kinase as a Sensor of Mitochondrial Membrane Potential

We have investigated here the mechanism of dephosphorylation and activation of death-associated protein kinase (DAPK) and the role of lysosome in neuroblastoma cells (SH-SY5Y) treated with mitochondrial toxins, such as MPP(+) and rotenone. Mitochondrial respiratory chain inhibitors and uncouplers decreased mitochondrial membrane potential leading to DAPK dephosphorylation and activation. The class III phosphoinositide 3-kinase inhibitors attenuated DAPK dephosphorylation induced by mitochondrial toxins. Complex I inhibition by mitochondrial toxins (e.g. MPP(+)) resulted in mitochondrial swelling and lysosome reduction. Inhibition of class III phosphoinositide 3-kinase attenuated MPP(+)-induced lysosome reduction and cell death. The role of DAPK as a sensor of mitochondrial membrane potential in mitochondrial diseases was addressed.

Switching aconitase B between catalytic and regulatory modes involves iron-dependent dimer formation

In addition to being the major citric acid cycle aconitase in Escherichia coli the aconitase B protein (AcnB) is also a post-transcriptional regulator of gene expression. The AcnB proteins represent a distinct branch of the aconitase superfamily that possess a HEAT-like domain (domain 5). The HEAT domains of other proteins are implicated in protein:protein interactions. Gel filtration analysis has now shown that cell-free extracts contain high-molecular-weight species of AcnB. Furthermore, in vitro and in vivo protein interaction experiments have shown that AcnB forms homodimers. Addition of the iron chelator bipyridyl to cultures inhibited the dimer-dependent readout from an AcnB bacterial two-hybrid system. A similar response was observed with a catalytically inactive AcnB variant, AcnB(C769S), suggesting that the monomer-dimer transition is not mediated by the state of the AcnB iron-sulphur cluster. The iron-responsive interacting unit was accordingly traced to the N-terminal region (domains 4 and 5) of the AcnB protein, and not to domain 3 that houses the iron-sulphur cluster. Thus, it was shown that a polypeptide containing AcnB N-terminal domains 5 and 4 (AcnB5-4) interacts with a second AcnB5-4 to form a homodimer. AcnB has recently been shown to initiate a regulatory cascade controlling flagella biosynthesis in Salmonella enterica by binding to the ftsH transcript and inhibiting the synthesis of the FtsH protease. A plasmid encoding AcnB5-4 complemented the flagella-deficient phenotype of a S. enterica acnB mutant, and the isolated AcnB5-4 polypeptide specifically recognized and bound to the ftsH transcript. Thus, the N-terminal region of AcnB is necessary and sufficient for promoting the formation of AcnB dimers and also for AcnB binding to target mRNA. Furthermore, the relative effects of iron on these processes provide a simple iron-mediated dimerization mechanism for switching the AcnB protein between catalytic and regulatory roles.

Cap-dependent and cap-independent translation in eukaryotic systems

Unlike bacterial protein synthesis, eukaryotic protein synthesis has several mechanisms to initiate translation including cap-dependent initiation, re-initiation and internal initiation. While there is extensive biochemical characterization of the multiple steps in cap-dependent initiation, most of the information on the other two mechanisms is derived from studies on the nucleic acid sequences that influence their efficiency. However, even in the best of circumstances, both re-initiation and internal initiation are only 25% as efficient as cap-dependent initiation and more commonly, are only 1-10% as efficient. This general lack of efficiency leaves open possibilities for mis-interpretation/artifacts in vivo (cryptic promoters, alternate splicing) or in vitro (nuclease degradation). Two examples are cited from the author's laboratory as background for the development of a general set of guidelines to minimize errors and validate authenticity for internal initiation.

1-Methyl-4-phenylpyridinium-induced apoptosis in cerebellar granule neurons is mediated by transferrin receptor iron-dependent depletion of tetrahydrobiopterin and neuronal nitric-oxide synthase-derived superoxide

In this study, we investigated the molecular mechanisms of toxicity of 1-methyl-4-phenylpyridinium (MPP(+)), an ultimate toxic metabolite of a mitochondrial neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, that causes Parkinson-like symptoms in experimental animals and humans. We used rat cerebellar granule neurons as a model cell system for investigating MPP(+) toxicity. Results show that MPP(+) treatment resulted in the generation of reactive oxygen species from inhibition of complex I of the mitochondrial respiratory chain, and inactivation of aconitase. This, in turn, stimulated transferrin receptor (TfR)-dependent iron signaling via activation of the iron-regulatory protein/iron-responsive element interaction. MPP(+) caused a time-dependent depletion of tetrahydrobiopterin (BH(4)) that was mediated by H(2)O(2) and transferrin iron. Depletion of BH(4) decreased the active, dimeric form of neuronal nitric-oxide synthase (nNOS). MPP(+)-mediated "uncoupling" of nNOS decreased *NO and increased superoxide formation. Pretreatment of cells with sepiapterin to promote BH(4) biosynthesis or cell-permeable iron chelator and TfR antibody to prevent iron-catalyzed BH(4) decomposition inhibited MPP(+) cytotoxicity. Preincubation of cerebellar granule neurons with nNOS inhibitor exacerbated MPP(+)-induced iron uptake, BH(4) depletion, proteasomal inactivation, and apoptosis. We conclude that MPP(+)-dependent aconitase inactivation, Tf-iron uptake, and oxidant generation result in the depletion of intracellular BH(4), leading to the uncoupling of nNOS activity. This further exacerbates reactive oxygen species-mediated oxidative damage and apoptosis. Implications of these results in unraveling the molecular mechanisms of neurodegenerative diseases (Parkinson's and Alzheimer's disease) are discussed.

The use of antioxidants in Friedreich's ataxia treatment

Friedreich's ataxia is the most common recessive ataxia associated with life-threatening cardiomyopathy. It results from a loss of function of frataxin that ultimately leads to oxidative insult, particularly to neurons and cardiomyocytes. The disease is progressive, the oxidative insult being presumably subsequent to an abnormal iron/sulfur cluster synthesis that causes mitochondrial respiratory chain disease and impaired signalling of one antioxidant pathway. After a detailed in vitro study, idebenone, a short chain homologue of coenzyme Q(10) with potent antioxidant properties, was given to patients. The antioxidant had a dramatic and rapid effect on the cardiomyopathy in most patients. Although a subset of patients also report various improvements, implying that idebenone could have a broader spectrum of action including some neurological improvements, the antioxidant did not have noticeable effects on the ataxia. Several hypotheses on the mechanisms that could account for the contrasting effects of the antioxidant on clinical symptoms of Friedreich's ataxia are discussed in this review. The considerable difficulties still being encountered in ascertaining the effect of antioxidants on the course of the neurological condition are also considered.

Global transcription analysis of Krebs tricarboxylic acid cycle mutants reveals an alternating pattern of gene expression and effects on hypoxic and oxidative genes

To understand the many roles of the Krebs tricarboxylic acid (TCA) cycle in cell function, we used DNA microarrays to examine gene expression in response to TCA cycle dysfunction. mRNA was analyzed from yeast strains harboring defects in each of 15 genes that encode subunits of the eight TCA cycle enzymes. The expression of >400 genes changed at least threefold in response to TCA cycle dysfunction. Many genes displayed a common response to TCA cycle dysfunction indicative of a shift away from oxidative metabolism. Another set of genes displayed a pairwise, alternating pattern of expression in response to contiguous TCA cycle enzyme defects: expression was elevated in aconitase and isocitrate dehydrogenase mutants, diminished in alpha-ketoglutarate dehydrogenase and succinyl-CoA ligase mutants, elevated again in succinate dehydrogenase and fumarase mutants, and diminished again in malate dehydrogenase and citrate synthase mutants. This pattern correlated with previously defined TCA cycle growth-enhancing mutations and suggested a novel metabolic signaling pathway monitoring TCA cycle function. Expression of hypoxic/anaerobic genes was elevated in alpha-ketoglutarate dehydrogenase mutants, whereas expression of oxidative genes was diminished, consistent with a heme signaling defect caused by inadequate levels of the heme precursor, succinyl-CoA. These studies have revealed extensive responses to changes in TCA cycle function and have uncovered new and unexpected metabolic networks that are wired into the TCA cycle.

Structural changes associated with switching activities of human iron regulatory protein 1

Metazoan iron regulatory protein 1 is a dual activity protein, being either an aconitase or a regulatory factor binding to messenger RNA involved in iron homeostasis. Sequence comparisons and site-directed mutagenesis experiments have supported a structural relationship between mitochondrial aconitase and iron regulatory protein 1. The structural properties of human recombinant iron regulatory protein 1 have been probed in the present work. Although iron-free iron regulatory protein 1 displays a significantly larger radius of gyration measured by small-angle neutron scattering than calculated for mitochondrial aconitase, binding of either the [4Fe-4S] cluster needed for aconitase activity or of a RNA substrate turns iron regulatory protein 1 into a more compact molecule. These conformational changes are associated with the gain of secondary structural elements as indicated by circular dichroism studies. They likely involve alpha-helices covering the substrate binding cleft of cytosolic aconitase, and they suggest an induced fit mechanism of iron-responsive element recognition. These studies refine previously proposed models of the "iron-sulfur switch" driving the biological function of human iron regulatory protein 1, and they provide a structural framework to probe the relevance of the numerous cellular molecules proposed to affect its function.

In vitro serial passage of Staphylococcus aureus: changes in physiology, virulence factor production, and agr nucleotide sequence

Recently, we observed that Staphylococcus aureus strains newly isolated from patients had twofold-higher aconitase activity than a strain passaged extensively in vitro, leading us to hypothesize that aconitase specific activity decreases over time during in vitro passage. To test this hypothesis, a strain recovered from a patient with toxic shock syndrome was serially passaged for 6 weeks, and the aconitase activity was measured. Aconitase specific activity decreased 38% (P < 0.001) by the sixth week in culture. During serial passage, S. aureus existed as a heterogeneous population with two colony types that had pronounced (wild type) or negligible zones of beta-hemolytic activity. The cell density-sensing accessory gene regulatory (agr) system regulates beta-hemolytic activity. Surprisingly, the percentage of colonies with a wild-type beta-hemolytic phenotype correlated strongly with aconitase specific activity (rho = 0.96), suggesting a common cause of the decreased aconitase specific activity and the variation in percentage of beta-hemolytic colonies. The loss of the beta-hemolytic phenotype also coincided with the occurrence of mutations in the agrC coding region or the intergenic region between agrC and agrA in the derivative strains. Our results demonstrate that in vitro growth is sufficient to result in mutations within the agr operon. Additionally, our results demonstrate that S. aureus undergoes significant phenotypic and genotypic changes during serial passage and suggest that vigilance should be used when extrapolating data obtained from the study of high-passage strains.

Binding of IRE-BP to its cognate RNA sequence: SFM studies on a universal RNA backbone for the analysis of RNA-protein interaction

We have used an RNA consisting of the potato spindle tuber viroid (PSTVd) and 240 bp of double-stranded RNA derived from the GUS gene as a backbone for scanning force microscope (SFM) studies on RNA binding proteins. The in vitro transcribed RNA forms a rod-like structure of apparent 130 nm in length with a completely base paired central part flanked by the incompletely paired viroid helix with bulges on both sides. The termini of the molecule consist of loops such that no blunt or staggered RNA ends are exposed. Suitable, asymmetrical restriction sites in the construct allow for the insertion of sequences of interest, e. g. protein binding sites. We have inserted the IRE (iron responsive element) sequence into the construct and have used in vitro transcripts to study binding of IRE-BP. Relative binding frequencies show that 70% of the protein binds to the expected site in the molecule while only a slightly enhanced binding is observed at the termini. In the GUS-PSTVd-IRE backbone, the orientation of the molecule is easily determined by IRE-BP binding. It thus provides a versatile tool to study specific as well as preferential interaction of other proteins with sequences or structures inserted into a different part of the molecule.

Antioxidant mechanisms of nitric oxide against iron-catalyzed oxidative stress in cells

Three distinct antioxidant pathways are considered through which iron-catalyzed oxidative stress may be regulated by nitric oxide (NO). The first two pathways involve direct redox interactions of NO with iron catalytic sites and represent a fast response that may be considered an emergency mechanism to protect cells from the consequences of acute and intensive oxidative stress. These are (i) NO-induced nitrosylation at heme and non-heme iron catalytic sites that is capable of directly reducing oxoferryl-associated radicals, (ii) formation of nitrosyl complexes with intracellular "loosely" bound redox-active iron, and (iii) an indirect regulatory pathway that may function as an adaptive mechanism that becomes operational upon long-term exposure of cells to NO. In the latter pathway, NO down-regulates expression of iron-containing proteins to prevent their catalytic prooxidant reactions.

Iron-dependent changes in cellular energy metabolism: influence on citric acid cycle and oxidative phosphorylation

Iron modulates the expression of the critical citric acid cycle enzyme aconitase via a translational mechanism involving iron regulatory proteins. Thus, the present study was undertaken to investigate the consequences of iron perturbation on citric acid cycle activity, oxidative phosphorylation and mitochondrial respiration in the human cell line K-562. In agreement with previous data iron increases the activity of mitochondrial aconitase while it is reduced upon addition of the iron chelator desferrioxamine (DFO). Interestingly, iron also positively affects three other citric acid cycle enzymes, namely citrate synthase, isocitric dehydrogenase, and succinate dehydrogenase, while DFO decreases the activity of these enzymes. Consequently, iron supplementation results in increased formation of reducing equivalents (NADH) by the citric acid cycle, and thus in increased mitochondrial oxygen consumption and ATP formation via oxidative phosphorylation as shown herein. This in turn leads to downregulation of glucose utilization. In contrast, all these metabolic pathways are reduced upon iron depletion, and thus glycolysis and lactate formation are significantly increased in order to compensate for the decrease in ATP production via oxidative phosphorylation in the presence of DFO. Our results point to a complex interaction between iron homeostasis, oxygen supply and cellular energy metabolism in human cells.

Iron-regulatory proteins, iron-responsive elements and ferritin mRNA translation

Iron plays a central role in the metabolism of all cells. This is evident by its major contribution to many diverse functions, such as DNA replication, bacterial pathogenicity, photosynthesis, oxidative stress control and cell proliferation. In mammalian systems, control of intracellular iron homeostasis is largely due to posttranscriptional regulation of binding by iron-regulatory RNA-binding proteins (IRPs) to iron-responsive elements (IREs) within ferritin and transferrin receptor (TfR) mRNAs. the TfR transports iron into cells and the iron is subsequently stored within ferritin. IRP binding is under tight control so that it responds to changes in intracellular iron requirements in a coordinate manner by differentially regulating ferritin mRNA translational efficiency and TfR mRNA stability. Several different stimuli, as well as intracellular iron levels and oxidative stress, are capable of regulating these RNA-protein interactions. In this mini-review, we shall concentrate on the mechanisms underlying modulation of the interaction of IRPs and the ferritin IRE and its role in regulating ferritin gene expression.

Mechanisms of nitric oxide protection against tert-butyl hydroperoxide-induced cytotoxicity in iNOS-transduced human erythroleukemia cells

We studied nitric oxide-mediated protection against tert-butyl hydroperoxide (t-BuOOH)-induced cytotoxicity in a subline of human erythroleukemia K562 cells (K/VP.5) and in K/VP.5 cells transduced with a retroviral vector containing the human iNOS gene (K/VP. 5-iNOS). K/VP.5-iNOS cells were 2-fold less sensitive to the cytotoxic effects of t-BuOOH compared to K/VP.5 cells. A nitric oxide-donor, NOC-15 ((Z)-1-[N-(3-ammoniopropyl)-N-(n-propyl)amino]diazen-1- ium-1, 2-diolate), protected K/VP.5 cells against t-BuOOH-induced cytotoxicity and provided an additional increment of protection in K/VP.5-iNOS cells. Under conditions of excess t-BuOOH and deficiency of iron catalytic sites (hemoglobin, Hb) in K/VP.5-iNOS cells, the increase of intracellular Hb concentration is the main contributor to enhanced sensitivity of the cells to t-BuOOH-induced cytotoxicity (despite the effects of small amounts of endogenously produced nitric oxide). Protection against t-BuOOH-induced cytotoxicity in K/VP.5-iNOS cells was diminished by treatment with an iNOS inhibitor, L-N(G)-monomethylarginine (L-NMA), but was restored upon addition of NOC-15 to L-NMA-treated cells. Incubation of K/VP.5 cells with NOC-15 resulted in the production of dinitrosyl complexes of non-heme iron and hexacoordinated heme iron nitrosyl complexes based on low-temperature EPR spectra. In K/VP.5-iNOS cells, only a weak EPR signal of dinitrosyl complexes of non-heme iron was observed in the absence of NOC-15. In addition, no heme iron nitrosyl complexes were discernible in the EPR spectra from K/VP.5-iNOS cells. Upon addition of NOC-15 to K/VP.5-iNOS cells, the EPR signal of dinitrosyl complexes of non-heme iron was enhanced, and the EPR signal of nitrosylated heme iron became discernible. It was determined that levels of non-heme and heme (hemoglobin) iron were dramatically decreased in K/VP.5-iNOS cells compared to K/VP.5 cells, thus explaining the decreased intensities of EPR signals of nitrosylated species. In addition, t-BuOOH-induced oxoferryl-Hb-associated protein-centered free radical species as well as t-BuO(*) alkoxyl radicals were observed in these two cell lines. These t-BuOOH-induced radical species were greatly reduced in K/VP.5-iNOS cells compared to K/VP.5 cells, consistent with a reduction in heme iron levels in the iNOS-expressing cells. Most importantly, the combined action of NOC-15 and t-BuOOH resulted in complete elimination of both oxoferryl-associated radical EPR signals as well as those from dinitrosyl complexes of non-heme iron and nitrosylated heme iron in both K/VP.5-iNOS cells and K/VP.5 cells. We conclude that two independent pathways operate in erythroleukemia cells for nitric oxide-mediated protection against t-BuOOH-induced cytotoxicity. First, prolonged endogenous production of nitric oxide results in a decreased content of catalytic non-heme iron and heme iron sites through posttranscriptional regulation of iron homeostasis. Second, nitric oxide can chemically reduce t-BuOOH-induced oxoferryl and t-BuO(*) alkoxyl radicals.

Ligand-induced structural alterations in human iron regulatory protein-1 revealed by protein footprinting

Human iron regulatory protein-1 (IRP-1) is a bifunctional protein that regulates iron metabolism by binding to mRNAs encoding proteins involved in iron uptake, storage, and utilization. Intracellular iron accumulation regulates IRP-1 function by promoting the assembly of an iron-sulfur cluster, conferring aconitase activity to IRP-1, and hindering RNA binding. Using protein footprinting, we have studied the structure of the two functional forms of IRP-1 and have mapped the surface of the iron-responsive element (IRE) binding site. Binding of the ferritin IRE or of the minimal regulatory region of transferrin receptor mRNA induced strong protections against proteolysis in the region spanning amino acids 80 to 187, which are located in the putative cleft thought to be involved in RNA binding. In addition, IRE-induced protections were also found in the C-terminal domain at Arg-721 and Arg-728. These data implicate a bipartite IRE binding site located in the putative cleft of IRP-1. The aconitase form of IRP-1 adopts a more compact structure because strong reductions of cleavage were detected in two defined areas encompassing residues 149 to 187 and 721 to 735. Thus both ligands of apo-IRP-1, the IRE and the 4Fe-4S cluster, induce distinct but overlapping alterations in protease accessibility. These data provide evidences for structural changes in IRP-1 upon cluster formation that affect the accessibility of residues constituting the RNA binding site.

Thermodynamic influences on the fidelity of iron-sulphur cluster formation in proteins

In an organism, two thermodynamic factors are important in ensuring that homometallic [4Fe-4S] cubane clusters are formed in preference to clusters containing heterometals such as Zn or Cu. These are the electronic resonance stabilisation, which boosts the binding of Fe(II) within an Fe-S cluster relative to its normally low position in the Irving-Williams order, and attenuation of the cytoplasmic concentrations of competing metals such as Zn or Cu by specific ligands.

Roles of reactive oxygen species: signaling and regulation of cellular functions

Reactive oxygen species (ROS) are the side products (H2O2, O2.-, and OH.) of general metabolism and are also produced specifically by the NADPH oxidase system in most cell types. Cells have a very efficient antioxidant defense to counteract the toxic effect of ROS. The physiological significance of ROS is that ROS at low concentrations are able to mediate cellular functions through the same steps of intracellular signaling, which are activated by natural stimuli. Moreover, a variety of natural stimuli act through the intracellular formation of ROS that change the intracellular redox state (oxidation-reduction). Thus, the redox state is a part of intracellular signaling. As such, ROS are now considered signal molecules at nontoxic concentrations. Progress has been achieved in studying the oxidative activation of gene transcription in animal cells and bacteria. Changes in the redox state of intracellular thiols are considered to be an important mechanism that regulates cell functions.

Inactivation of both RNA binding and aconitase activities of iron regulatory protein-1 by quinone-induced oxidative stress

Iron regulatory protein-1 (IRP-1) controls the expression of several mRNAs by binding to iron-responsive elements (IREs) in their untranslated regions. In iron-replete cells, a 4Fe-4S cluster converts IRP-1 to cytoplasmic aconitase. IRE binding activity is restored by cluster loss in response to iron starvation, NO, or extracellular H2O2. Here, we study the effects of intracellular quinone-induced oxidative stress on IRP-1. Treatment of murine B6 fibroblasts with menadione sodium bisulfite (MSB), a redox cycling drug, causes a modest activation of IRP-1 to bind to IREs within 15-30 min. However, IRE binding drops to basal levels within 60 min. Surprisingly, a remarkable loss of both IRE binding and aconitase activities of IRP-1 follows treatment with MSB for 1-2 h. These effects do not result from alterations in IRP-1 half-life, can be antagonized by the antioxidant N-acetylcysteine, and regulate IRE-containing mRNAs; the capacity of iron-starved MSB-treated cells to increase transferrin receptor mRNA levels is inhibited, and MSB increases the translation of a human growth hormone indicator mRNA bearing an IRE in its 5'-untranslated region. Nonetheless, MSB inhibits ferritin synthesis. Thus, menadione-induced oxidative stress leads to post-translational inactivation of both genetic and enzymatic functions of IRP-1 by a mechanism that lies beyond the "classical" Fe-S cluster switch and exerts multiple effects on cellular iron metabolism.

Transition metal ion-catalyzed oxygen activation during pathogenic processes

Most pathological processes include the production of activated oxygen species augmented or attenuated by transition metal ions catalyzing one electron transitions. Inhalation of airborne particles, infections, ingestion of toxins or liberation from endogenous stores represent biological pathways for the induction of pathogenic processes by these metal ions. In this short review basic reactions involving transition metal ions operating during oxidative stress in certain diseases will be discussed.

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

Iron regulatory protein-1 (IRP-1), a central cytoplasmic regulator of cellular iron metabolism, is rapidly activated by oxidative stress to bind to mRNA iron-responsive elements. We have reconstituted the response of IRP-1 to extracellular H2O2 in a system derived from murine B6 fibroblasts permeabilized with streptolysin-O. This procedure allows separation of the cytosol from the remainder of the cells (cell pellet). IRP-1 in the cytosolic fraction fails to be directly activated by addition of H2O2. IRP-1 activation requires the presence of a nonsoluble, possibly membrane-associated component in the cell pellet. The streptolysin-O-based in vitro system faithfully recapitulates characteristic hallmarks of IRP-1 activation by H2O2 in intact cells. We show that the H2O2-mediated activation of IRP-1 is temperature dependent and sensitive to treatment with calf intestinal alkaline phosphatase (CIAP). Although IRP-1 activation is unaffected by addition of excess ATP or GTP to this in vitro system, it is negatively affected by the nonhydrolyzable nucleotide analogs adenylyl-imidodiphosphate and guanylyl-imidophosphate and completely blocked by ATP-gammaS and GTP-gammaS. The in vitro reconstitution of this oxidative stress-induced pathway has opened a different avenue for the biochemical dissection of the regulation of mammalian iron metabolism by oxidative stress. Our data show that H2O2 must be sensed to stimulate a pathway to activate IRP-1.

Regulation of ferritin synthesis and iron regulatory protein 1 by oxygen in mouse peritoneal macrophages

Ferritin is an intracellular iron storage protein whose synthesis is regulated post-transcriptionally by a mechanism that involves binding of cytoplasmic iron regulatory protein (IRP) to iron-responsive element (IRE) in the 5' untranslated region of ferritin mRNA. In this study, we have shown that in mouse peritoneal macrophages, the synthesis of ferritin was enhanced and the IRE binding activity of IRP-1 was diminished when the oxygen tension was decreased. Iron is known to induce ferritin synthesis and even in the presence of a low concentration of iron, synthesis of ferritin was enhanced and the activity of IRP-1 was decreased under hypoxia. The enhanced synthesis of ferritin under hypoxia was abolished by the addition of O2(-)-generating agents but not H2O2. The decreased activity of IRP-1 under hypoxia was reversed by adding O2(-)-generating agents. These data suggest that O2- generated in the cell is involved in alterations of ferritin synthesis and the activity of IRP-1 by oxygen.

Normal serum iron and ferritin concentrations in patients with Friedreich's ataxia

Friedreich's ataxia (FRDA) is caused by point mutations or trinucleotide repeat expansions in both alleles of the gene encoding frataxin. Studies of frataxin homologues in lower eukaryotes suggest that mitochondrial iron accumulation may underlie the pathophysiology of FRDA. To evaluate the possible role of iron-chelation therapy for FRDA, we measured serum iron and ferritin concentrations in 10 FRDA patients. The measurements were within normal limits, suggesting that iron-chelation therapy for FRDA may be problematic.

The secondary alcohol metabolite of doxorubicin irreversibly inactivates aconitase/iron regulatory protein-1 in cytosolic fractions from human myocardium

Anticancer therapy with doxorubicin (DOX) is limited by severe cardiotoxicity, presumably reflecting the intramyocardial formation of drug metabolites that alter cell constituents and functions. In a previous study, we showed that NADPH-supplemented cytosolic fractions from human myocardial samples can enzymatically reduce a carbonyl group in the side chain of DOX, yielding a secondary alcohol metabolite called doxorubicinol (DOXol). Here we demonstrate that DOXol delocalizes low molecular weight Fe(II) from the [4Fe-4S] cluster of cytoplasmic aconitase. Iron delocalization proceeds through the reoxidation of DOXol to DOX and liberates DOX-Fe(II) complexes as ultimate by-products. Under physiologic conditions, cluster disassembly abolishes aconitase activity and forms an apoprotein that binds to mRNAs, coordinately increasing the synthesis of transferrin receptor but decreasing that of ferritin. Aconitase is thus converted into an iron regulatory protein-1 (IRP-1) that causes iron uptake to prevail over sequestration, forming a pool of free iron that is used for metabolic functions. Conversely, cluster reassembly converts IRP-1 back to aconitase, providing a regulatory mechanism to decrease free iron when it exceeds metabolic requirements. In contrast to these physiologic mechanisms, DOXol-dependent iron release and cluster disassembly not only abolish aconitase activity, but also affect irreversibly the ability of the apoprotein to function as IRP-1 or to reincorporate iron within new Fe-S motifs. This damage is mediated by DOX-Fe(II) complexes and reflects oxidative modifications of -SH residues having the dual role to coordinate cluster assembly and facilitate interactions of IRP-1 with mRNAs. Collectively, these findings describe a novel mechanism of cardiotoxicity, suggesting that intramyocardial formation of DOXol may perturb the homeostatic processes associated with cluster assembly or disassembly and the reversible switch between aconitase and IRP-1. These results may also provide a guideline to design new drugs that mitigate the cardiotoxicity of DOX.

Iron binding and autoreduction by citrate: are these involved in signalling by iron regulatory protein-1?

Ferric ions bind to citrate and undergo an autoreduction to form a ferrous-citrate complex, greatly increasing the redox activity of the iron complex. Ferrous ions and citrate are also essential for the enzymic activity of aconitase. Aconitase, with its iron-sulphur cluster has a versatile structure which allows it to act as an iron regulatory protein (IRP-1). The purpose of this study was to see whether iron binding, and its autoreduction by citrate, could play a physiological signalling role in iron regulation. Significant amounts of ferrous ions were associated with citrate, when measured using ferrozine, however, these did not appear to activate iron-requiring aconitase.

In vivo kinetics of a redox-regulated transcriptional switch

SoxR is a transcription activator governing a cellular response to superoxide and nitric oxide in Escherichia coli. SoxR protein is a homodimer, and each monomer has a redox-active [2Fe-2S] cluster. Oxidation and reduction of the [2Fe-2S] clusters can reversibly activate and inactivate SoxR transcriptional activity. Here, we use electron paramagnetic resonance spectroscopy to follow the redox-switching process of SoxR protein in vivo. SoxR [2Fe-2S] clusters were in the fully reduced state during normal aerobic growth, but were completely oxidized after only 2-min aerobic exposure of the cells to superoxide-generating agents such as paraquat. The oxidized SoxR [2Fe-2S] clusters were rapidly re-reduced in vivo once the oxidative stress was removed. The in vivo kinetics of SoxR [2Fe-2S] cluster oxidation and reduction exactly paralleled the increase and decrease of transcription of soxS, the target gene for SoxR. The kinetic analysis also revealed that an oxidative stress-linked decrease in soxS mRNA stability contributes to the rapid attainment of a new steady state after SoxR activation. Such a redox stress-related change in soxS mRNA stability may represent a new level of biological control.

Differences in the regulation of iron regulatory protein-1 (IRP-1) by extra- and intracellular oxidative stress

We have studied the responses of iron regulatory protein-1 (IRP-1) to extra- and intracellular sources of reactive oxygen intermediates (ROIs). IRP-1 is a cytoplasmic RNA-binding protein that regulates iron metabolism following its activation by iron deficiency, nitric oxide, and administration of H2O2 or antimycin A, an inhibitor of the respiratory chain (Hentze, M. W., and Kühn, L. C. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 8175-8182). We show that 10 microM H2O2 suffice for complete IRP-1 activation within 60 min when H2O2 is generated extracellularly at steady-state. By contrast, rapid cellular H2O2 degradation necessitates a 5-10-fold higher bolus dose. To study IRP-1 responses to intracellular oxidative stress, mitochondrial respiration was inhibited with antimycin A (to generate oxidative stress by leakage of ROIs from complex III), or catalase was blocked with 3-amino-1,2,4-triazole (to diminish H2O2 degradation); in parallel, 2',7'-dichlorodihydrofluorescein diacetate was used as a redox-sensitive probe to monitor intracellular H2O2 levels by fluorescence-activated cell sorting. Catalase inhibition elevates intracellular H2O2, but surprisingly does not cause concomitant IRP-1 activation. Following antimycin A treatment, IRP-1 is activated, but the activation kinetics lag behind the rapid increase in detectable intracellular H2O2. IRP-1 is thus activated both by extra- and intracellular generation of ROIs. While extracellular H2O2 rapidly activates IRP-1 even without detectable increases in intracellular H2O2, intracellular H2O2 elevation is not sufficient for IRP-1 activation. IRP-1 thus represents a novel example of an H2O2-regulated protein that responds differentially to alterations of extra- and intracellular H2O2 levels. Our data also suggest that a direct attack on the 4Fe-4S cluster of IRP-1 by H2O2 (or an H2O2-derived reactive species) represents an unlikely explanation for IRP-1 activation by oxidative stress.