Thioredoxin activation of iron regulatory proteins. Redox regulation of RNA binding after exposure to nitric oxide

Inflaming the Brain with Iron

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

Cellular Dynamics of Transition Metal Exchange on Proteins: A Challenge but a Bonanza for Coordination Chemistry

Transition metals interact with a large proportion of the proteome in all forms of life, and they play mandatory and irreplaceable roles. The dynamics of ligand binding to ions of transition metals falls within the realm of Coordination Chemistry, and it provides the basic principles controlling traffic, regulation, and use of metals in cells. Yet, the cellular environment stands out against the conditions prevailing in the test tube when studying metal ions and their interactions with various ligands. Indeed, the complex and often changing cellular environment stimulates fast metal-ligand exchange that mostly escapes presently available probing methods. Reducing the complexity of the problem with purified proteins or in model organisms, although useful, is not free from pitfalls and misleading results. These problems arise mainly from the absence of the biosynthetic machinery and accessory proteins or chaperones dealing with metal / metal groups in cells. Even cells struggle with metal selectivity, as they do not have a metal-directed quality control system for metalloproteins, and serendipitous metal binding is probably not exceptional. The issue of metal exchange in biology is reviewed with particular reference to iron and illustrating examples in patho-physiology, regulation, nutrition, and toxicity.

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

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

Iron regulatory proteins and their role in controlling iron metabolism

Cellular iron homeostasis is regulated by post-transcriptional feedback mechanisms, which control the expression of proteins involved in iron uptake, release and storage. Two cytoplasmic proteins with mRNA-binding properties, iron regulatory proteins 1 and 2 (IRP1 and IRP2) play a central role in this regulation. Foremost, IRPs regulate ferritin H and ferritin L translation and thus iron storage, as well as transferrin receptor 1 (TfR1) mRNA stability, thereby adjusting receptor expression and iron uptake via receptor-mediated endocytosis of iron-loaded transferrin. In addition splice variants of iron transporters for import and export at the plasma-membrane, divalent metal transporter 1 (DMT1) and ferroportin are regulated by IRPs. These mechanisms have probably evolved to maintain the cytoplasmic labile iron pool (LIP) at an appropriate level. In certain tissues, the regulation exerted by IRPs influences iron homeostasis and utilization of the entire organism. In intestine, the control of ferritin expression limits intestinal iron absorption and, thus, whole body iron levels. In bone marrow, erythroid heme biosynthesis is coordinated with iron availability through IRP-mediated translational control of erythroid 5-aminolevulinate synthase mRNA. Moreover, the translational control of HIF2α mRNA in kidney by IRP1 coordinates erythropoietin synthesis with iron and oxygen supply. Besides IRPs, body iron absorption is negatively regulated by hepcidin. This peptide hormone, synthesized and secreted by the liver in response to high serum iron, downregulates ferroportin at the protein level and thereby limits iron absorption from the diet. Hepcidin will not be discussed in further detail here.

Thioredoxins, glutaredoxins, and peroxiredoxins--molecular mechanisms and health significance: from cofactors to antioxidants to redox signaling

Thioredoxins (Trxs), glutaredoxins (Grxs), and peroxiredoxins (Prxs) have been characterized as electron donors, guards of the intracellular redox state, and "antioxidants". Today, these redox catalysts are increasingly recognized for their specific role in redox signaling. The number of publications published on the functions of these proteins continues to increase exponentially. The field is experiencing an exciting transformation, from looking at a general redox homeostasis and the pathological oxidative stress model to realizing redox changes as a part of localized, rapid, specific, and reversible redox-regulated signaling events. This review summarizes the almost 50 years of research on these proteins, focusing primarily on data from vertebrates and mammals. The role of Trx fold proteins in redox signaling is discussed by looking at reaction mechanisms, reversible oxidative post-translational modifications of proteins, and characterized interaction partners. On the basis of this analysis, the specific regulatory functions are exemplified for the cellular processes of apoptosis, proliferation, and iron metabolism. The importance of Trxs, Grxs, and Prxs for human health is addressed in the second part of this review, that is, their potential impact and functions in different cell types, tissues, and various pathological conditions.

Thioredoxin 1-mediated post-translational modifications: reduction, transnitrosylation, denitrosylation, and related proteomics methodologies

Despite the significance of redox post-translational modifications (PTMs) in regulating diverse signal transduction pathways, the enzymatic systems that catalyze reversible and specific oxidative or reductive modifications have yet to be firmly established. Thioredoxin 1 (Trx1) is a conserved antioxidant protein that is well known for its disulfide reductase activity. Interestingly, Trx1 is also able to transnitrosylate or denitrosylate (defined as processes to transfer or remove a nitric oxide entity to/from substrates) specific proteins. An intricate redox regulatory mechanism has recently been uncovered that accounts for the ability of Trx1 to catalyze these different redox PTMs. In this review, we will summarize the available evidence in support of Trx1 as a specific disulfide reductase, and denitrosylation and transnitrosylation agent, as well as the biological significance of the diverse array of Trx1-regulated pathways and processes under different physiological contexts. The dramatic progress in redox proteomics techniques has enabled the identification of an increasing number of proteins, including peroxiredoxin 1, whose disulfide bond formation and nitrosylation status are regulated by Trx1. This review will also summarize the advancements of redox proteomics techniques for the identification of the protein targets of Trx1-mediated PTMs. Collectively, these studies have shed light on the mechanisms that regulate Trx1-mediated reduction, transnitrosylation, and denitrosylation of specific target proteins, solidifying the role of Trx1 as a master regulator of redox signal transduction.

Iron homeostasis and methionine-centred redox cycle in nasal polyposis

Nasal polyposis is a multifactorial disease with a strong inflammatory component. Its pathogenesis is often associated with ROS production catalysed by redox-active iron. This study aimed to characterize the roles of iron homeostasis and redox status in the pathogenesis of polyposis. Nasal polyps (NP) from asthmatics and non-asthmatics and turbinates from controls and NP-patients were analysed for ferritin, ferritin-bound iron (FBI) and levels of methionine-centred redox cycle proteins. The ferritin content in both NPs was significantly higher than in adjacent turbinates. No differences in FBI were observed between both NP groups and both turbinates groups, while in NPs it was significantly higher. In NP-turbinates the highest levels of redox proteins were observed. In conclusion, re-distribution of iron occurs upon the development of NP. While FBI is elevated in NPs, the adjacent turbinate remain iron-poor and low-inflammatory, suggesting the formation of virtual boundary between these tissues.

Cysteine oxidation regulates the RNA-binding activity of iron regulatory protein 2

Iron regulatory protein 2 (IRP2) is an RNA-binding protein that regulates the posttranscriptional expression of proteins required for iron homeostasis such as ferritin and transferrin receptor 1. IRP2 RNA-binding activity is primarily regulated by iron-mediated proteasomal degradation, but studies have suggested that IRP2 RNA binding is also regulated by thiol oxidation. We generated a model of IRP2 bound to RNA and found that two cysteines (C512 and C516) are predicted to lie in the RNA-binding cleft. Site-directed mutagenesis and thiol modification show that, while IRP2 C512 and C516 do not directly interact with RNA, both cysteines are located within the RNA-binding cleft and must be unmodified/reduced for IRP2-RNA interactions. Oxidative stress induced by cellular glucose deprivation reduces the RNA-binding activity of IRP2 but not IRP2-C512S or IRP2-C516S, consistent with the formation of a disulfide bond between IRP2 C512 and C516 during oxidative stress. Decreased IRP2 RNA binding is correlated with reduced transferrin receptor 1 mRNA abundance. These studies provide insight into the structural basis for IRP2-RNA interactions and reveal an iron-independent mechanism for regulating iron homeostasis through the redox regulation of IRP2 cysteines.

Aconitate hydratase of mammals under oxidative stress

Data on the structure, functions, regulation of activity, and expression of cytosolic and mitochondrial aconitate hydratase isoenzymes of mammals are reviewed. The role of aconitate hydratase and structurally similar iron-regulatory protein in maintenance of homeostasis of cell iron is described. Information on modifications of the aconitate hydratase molecule and changes in expression under oxidative stress is generalized. The role of aconitate hydratase in the pathogenesis of some diseases is considered.

Zinc deficiency-induced iron accumulation, a consequence of alterations in iron regulatory protein-binding activity, iron transporters, and iron storage proteins

One consequence of zinc deficiency is an elevation in cell and tissue iron concentrations. To examine the mechanism(s) underlying this phenomenon, Swiss 3T3 cells were cultured in zinc-deficient (D, 0.5 microM zinc), zinc-supplemented (S, 50 microM zinc), or control (C, 4 microM zinc) media. After 24 h of culture, cells in the D group were characterized by a 50% decrease in intracellular zinc and a 35% increase in intracellular iron relative to cells in the S and C groups. The increase in cellular iron was associated with increased transferrin receptor 1 protein and mRNA levels and increased ferritin light chain expression. The divalent metal transporter 1(+)iron-responsive element isoform mRNA was decreased during zinc deficiency-induced iron accumulation. Examination of zinc-deficient cells revealed increased binding of iron regulatory protein 2 (IRP2) and decreased binding of IRP1 to a consensus iron-responsive element. The increased IRP2-binding activity in zinc-deficient cells coincided with an increased level of IRP2 protein. The accumulation of IRP2 protein was independent of zinc deficiency-induced intracellular nitric oxide production but was attenuated by the addition of the antioxidant N-acetylcysteine or ascorbate to the D medium. These data support the concept that zinc deficiency can result in alterations in iron transporter, storage, and regulatory proteins, which facilitate iron accumulation.

Dual Effect of Pyruvate in Isolated Nerve Terminals: Generation of Reactive Oxygen Species and Protection of Aconitase

Generation of reactive oxygen species (ROS) in synaptosomes was investigated in the presence of different substrates. When pyruvate was used as a substrate an increased rate of hydrogen peroxide formation was detected by the Amplex Red fluorescent assay, but aconitase, which is known to be a highly sensitive enzyme to ROS was not inhibited. In contrast, pyruvate exerted a partial protection on aconitase against a time-dependent inactivation that occurred when synaptosomes were incubated in the absence of substrates. Disruption of synaptosomal membranes with Triton X-100 prevented the protective effect of pyruvate. It is suggested that citrate and/or isocitrate formed in the metabolism of pyruvate could be responsible for a partial protection of aconitase. Therefore while pyruvate could have a prooxidant effect it could also exert a protective effect on the aconitase.

Nitric oxide inhibition decreases bleomycin-detectable iron in spleen, bone marrow cells and heart but not in liver in exercise rats

The possible role of nitric oxide on the exercise-induced changes in bleomycin-detectable iron (BDI) in the liver, spleen, bone marrow cells and heart was investigated. Female Sprague-Dawley rats were randomly assigned to four groups: S1 (Sedentary), S2 (Sedentary + L-NAME [N-nitro-L-arginine methyl ester]), E1 (Exercise) and E2 (Exercise + L-NAME). Animals in the E1 and E2 swam for 2 h/day for 3 months. L-NAME in the drinking water (1 mg/ml) was administrated to rats in the S2 and E2 groups for the same period. At the end of the 3rd month, nitrite and nitrate (NOx), BDI and non-heme iron (NHI) contents in the liver, spleen, bone marrow cells and heart were measured. The ratio of BDI/NHI was calculated. The exercise induced a significant increase in NOx and BDI contents and/or BDI/NHI ratio in the spleen, bone morrow cells and heart. Treatment with L-NAME, an inhibitor of NOS, led to a significant decrease in NOx and an increase in BDI levels and BDI/NHI ratios in these tissues. The correlative analysis showed that there is significantly positive correlation between NOx levels and BDI contents and/or BDI/NHI ratios in the spleen, bone marrow cells and heart. These results suggest that the increased nitric oxide might be one of the reasons leading to the increased BDI levels in these tissues in the exercised rats. In contrast to the above tissues, in the liver, exercise led to a significant decrease rather than increase in BDI levels and BDI/NHI ratios with a significant increase in NOx contents. Treatment with L-NAME led to a significant increase in BDI levels and BDI/NHI ratios and a decrease in NOx contents in the tissue. These findings plus the results reported by others imply that nitric oxide might have an inhibitory effect on BDI in the liver.

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

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

Identification of novel molecular candidates for fatty liver in the hyperlipidemic mouse model, HcB19

The inbred HcB19 mouse strain expresses a truncated form of thioredoxin interacting protein and is phenotypically characterized by fatty liver and elevated plasma triglycerides and VLDL. Recently, these mice have been proposed as an animal model for familial combined hyperlipidemia. The aim of the present study was identification of hepatic proteins specifically associated with the presence of fatty liver. Eighteen differential proteins were detected in whole-liver homogenate from HcB19, or the parental strain C3H, using 2D electrophoresis, and 11 of those were successfully identified by mass spectrometry. Five of the identified differential proteins were mitochondrial, two peroxisomal, two cytosolic, and two secretory. Four differential proteins were novel in the fatty liver proteome [i.e., aconitase, succinate dehydrogenase, propionyl CoA carboxylase alpha chain (PCCA), and 3-hydroxyanthranilate 3,4 dioxygenase (3HAAO)]. Of these, PCCA and 3HAAO are of particular interest because of their known functions in nicotinic acid metabolism (3HAAO) and ketogenesis (PCCA). We have newly identified several differential proteins in the hepatic proteome of mice with fatty liver, including PCCA and 3HAAO, and confirmed differential expression of previously reported proteins. These individual proteins, PCCA and 3HAAO, can be important in development of fatty liver or in the expression of hyperlipidemia.

New thioredoxin targets in the unicellular photosynthetic eukaryote Chlamydomonas reinhardtii

Proteomics were used to identify the proteins from the eukaryotic unicellular green alga Chlamydomonas reinhardtii that can be reduced by thioredoxin. These proteins were retained specifically on a thioredoxin affinity column made of a monocysteinic thioredoxin mutant able to form mixed disulfides with its targets. Of a total of 55 identified targets, 29 had been found previously in higher plants or Synechocystis, but 26 were new targets. Biochemical tests were performed on three of them, showing a thioredoxin-dependent activation of isocitrate lyase and isopropylmalate dehydrogenase and a thioredoxin-dependent deactivation of catalase that is redox insensitive in Arabidopsis. In addition, we identified a Ran protein, a previously uncharacterized nuclear target in a photosynthetic organism. The metabolic and evolutionary implications of these findings are discussed.

Proteomic analysis of thioredoxin-targeted proteins in Escherichia coli

Thioredoxin, a ubiquitous and evolutionarily conserved protein, modulates the structure and activity of proteins involved in a spectrum of processes, such as gene expression, apoptosis, and the oxidative stress response. Here, we present a comprehensive analysis of the thioredoxin-linked Escherichia coli proteome by using tandem affinity purification and nanospray microcapillary tandem mass spectrometry. We have identified a total of 80 proteins associated with thioredoxin, implicating the involvement of thioredoxin in at least 26 distinct cellular processes that include transcription regulation, cell division, energy transduction, and several biosynthetic pathways. We also found a number of proteins associated with thioredoxin that either participate directly (SodA, HPI, and AhpC) or have key regulatory functions (Fur and AcnB) in the detoxification of the cell. Transcription factors NusG, OmpR, and RcsB, not considered to be under redox control, are also associated with thioredoxin.

Peroxynitrite and nitric oxide differently target the iron-sulfur cluster and amino acid residues of human iron regulatory protein 1

Iron regulatory protein 1 (IRP1) is a redox-sensitive protein which exists in two active forms in the cytosol of eukaryotic cells. Holo-IRP1 containing a [4Fe-4S] cluster exhibits aconitase activity which catalyzes the isomerization of citrate and isocitrate. The cluster-free protein (apo-IRP1) is a transregulator binding to specific mRNA, and thus post-transcriptionally modulating the expression of genes involved in iron metabolism. The resonance Raman (RR) spectra of human recombinant holo-IRP1 (rhIRP1) excited at 457.9 nm show that the 395 cm(-1) band, attributed to a terminal Fe-S stretching mode of the cluster, is replaced by a 405 cm(-1) band, consistent with the conversion of the [4Fe-4S](2+) center to a [3Fe-4S](+) center, upon exposure to peroxynitrite. This conclusion was confirmed by electron paramagnetic resonance (EPR) data and correlated with the loss of aconitase activity. In another series of experiments, the RR spectra also revealed the presence of additional bands at 818 and 399 cm(-1) when rhIRP1 was treated with a peroxynitrite synthesized by a different procedure. These bands correspond to those of 3-nitrotyrosine, and they indicate nitration of at least one tyrosine residue in rhIRP1. This was further confirmed by Western blot analysis with an anti-nitrotyrosine antibody. In contrast, the reaction of rhIRP1 with NO in the absence of oxygen revealed full mRNA binding activity of the protein, without nitration of tyrosines. These results strongly suggest that NO mainly acts as a regulator of IRP1 whereas peroxynitrite is likely to disrupt the IRP1/IRE regulatory pathway.

Nitric oxide and changes of iron metabolism in exercise

Accumulated data imply that exercise itself might not lead to a true iron deficiency or 'sport anaemia' in a healthy athlete who has adequate iron intake. The higher prevalence of iron deficiency anaemia in younger female athletes might be not due to exercise itself, but probably results from dietary choices, inadequate iron intake and menstruation. These factors can also induce iron deficiency or anaemia in the general population. However, exercise does affect iron metabolism, leading to low or sub-optimal iron status. The underlying mechanism is unknown. In this review, recent advances in the study of the effect of exercise on iron metabolism and nitric oxide, and the relationship between nitric oxide and iron status in exercise are discussed. A hypothesis that increased production of nitric oxide might contribute to sub-optimal iron status in exercise is proposed.

NO sensing by FNR: regulation of the Escherichia coli NO-detoxifying flavohaemoglobin, Hmp

Nitric oxide (NO) is a signalling and defence molecule of major importance in biology. The flavohaemoglobin Hmp of Escherichia coli is involved in protective responses to NO. Because hmp gene transcription is repressed by the O(2)-responsive regulator FNR, we investigated whether FNR also senses NO. The [4Fe-4S](2+) cluster of FNR is oxygen labile and controls protein dimerization and site-specific DNA binding. NO reacts anaerobically with the Fe-S cluster of purified FNR, generating spectral changes consistent with formation of a dinitrosyl-iron-cysteine complex. NO-inactivated FNR can be reconstituted, suggesting physiological relevance. FNR binds at an FNR box within the hmp promoter (P(hmp)). FNR samples inactivated by either O(2) or NO bind specifically to P(hmp), but with lower affinity. Dose-dependent up-regulation of P(hmp) in vivo by NO concentrations of pathophysiological relevance is abolished by fnr mutation, and NO also modulates expression from model FNR-regulated promoters. Thus, FNR can respond to not only O(2), but also NO, with major implications for global gene regulation in bacteria. We propose an NO-mediated mechanism of hmp regulation by which E.coli responds to NO challenge.

Nitric oxide and peroxynitrite activate the iron regulatory protein-1 of J774A.1 macrophages by direct disassembly of the Fe-S cluster of cytoplasmic aconitase

Posttranscriptional regulation of iron homeostasis involves, among other factors, a reversible conversion of the Fe-S enzyme cytoplasmic aconitase to a mRNA-binding iron regulatory protein (IRP-1) that lacks an Fe-S cluster. Previous studies have shown that aconitase/IRP-1 may be a target of *NO or peroxynitrite (ONOO(-)), formed after reaction of *NO with superoxide anion (O(2)(*-)); however, the mechanisms and consequences of such interactions have remained uncertain. In this study, recombinant aconitase/IRP-1 was exposed to SIN-1, whose thermal decomposition releases *NO and O(2)(*-). Results showed that SIN-1 was able to induce concomitant inactivation of aconitase and activation of IRP-1, attributable to cluster disassembly induced by ONOO(-). SIN-1 was used also in lysates of J774A.1 mouse macrophages grown under control conditions, or subjected to iron loading or starvation by treatment with hemin or desferrioxamine, respectively. Three lines of evidence confirmed that ONOO(-) activated IRP-1 by removing iron from the Fe-S cluster of cytoplasmic aconitase. First, IRP-1 activation was accompanied by iron release and loss of aconitase activity. Second, aconitase activity was recovered by reassembling Fe-S clusters with cysteine and ferrous ammonium sulfate. Third, iron release and IRP-1 activation were observed in lysates from control or iron-loaded macrophages, containing increasing levels of Fe-S clusters, but not in lysates from iron-starved macrophages, in which aconitase had already undergone cluster disassembly and switched to IRP-1. *NO was less efficient than ONOO(-) in attacking the Fe-S cluster of cytoplasmic aconitase; in fact, SIN-1-dependent iron release and IRP-1 activation were diminished by superoxide dismutase, which scavenged O(2)(*-) before it reacted with *NO to form ONOO(-). Under comparable conditions, however, both *NO and ONOO(-) inactivated an IRP-2 unable to assemble an Fe-S cluster. These results indicate that *NO and ONOO(-) may activate IRP-1 by attacking the Fe-S cluster of cytoplasmic aconitase, while also inactivating the cluster-deficient IRP-2. Such divergent actions offer clues to explain links between iron homeostasis and reactive nitrogen species in macrophages involved in inflammation or other pathophysiologic conditions.

Repair of nitric oxide-modified ferredoxin [2Fe-2S] cluster by cysteine desulfurase (IscS)

Iron-sulfur proteins are among the sensitive targets of the nitric oxide cytotoxicity. When Escherichia coli cells are exposed to nitric oxide, iron-sulfur clusters are modified forming protein-bound dinitrosyl iron complexes. Such modified protein dinitrosyl iron complexes are stable in vitro but are efficiently repaired in aerobically growing E. coli cells even without any new protein synthesis. Here we show that cysteine desulfurase encoded by the gene iscS of E. coli can directly convert the ferredoxin dinitrosyl iron complex to the ferredoxin [2Fe-2S] cluster in the presence of L-cysteine in vitro. A reassembly of the [2Fe-2S] cluster in the ferredoxin dinitrosyl iron complex does not require any addition of iron or other protein components. Furthermore, a complete removal of the dinitrosyl iron complex from ferredoxin prevents reassembly of the [2Fe-2S] cluster in the protein. The results suggest that cysteine desulfurase (IscS) together with L-cysteine can efficiently repair the nitric oxide-modified ferredoxin [2Fe-2S] cluster and that the iron center in the dinitrosyl iron complex may be recycled for the reassembly of iron-sulfur clusters in proteins.

Overexpressed thioredoxin compensates Fanconi anemia related chromosomal instability

The cause of the molecular defect of Fanconi anemia (FA) remains unknown. Cells from patients with FA exert an elevated spontaneous chromosomal instability which is further triggered by mitomycin C. The induced lability is reduced by overexpression of thioredoxin which is not the case for spontaneous instability. However, both are eliminated by overexpression of thioredoxin cDNA with an added nuclear localization signal. This implies that thioredoxin is lacking in the nuclei of FA cells. The total thioredoxin content in all FA cells tested is reduced. The resultant lack of nuclear thioredoxin can be the explanation for the major symptomatology in FA. Since thioredoxin is known to be the reactive cofactor of ribonucleotid reductase its shortcoming reduces the supply of deoxyribonucleotides thus hindering the DNA and replication repair with resultant chromosomal breaks. Furthermore, depression of tyrosine hydroxylase, the key enzyme of melanine synthesis, could be the basis for the pathognomotic 'café au lait' spots of FA. The observation of thioredoxin reduction in FA cells permits insight into the molecular phathophysiology of FA.

Detection of a [3Fe-4S] cluster intermediate of cytosolic aconitase in yeast expressing iron regulatory protein 1. Insights into the mechanism of Fe-S cluster cycling

Interconversion of iron regulatory protein 1 (IRP1) with cytosolic aconitase (c-aconitase) occurs via assembly/disassembly of a [4Fe-4S] cluster. Recent evidence implicates oxidants in cluster disassembly. We investigated H(2)O(2)-initiated Fe-S cluster disassembly in c-aconitase expressed in Saccharomyces cerevisiae. A signal for [3Fe-4S] c-aconitase was detected by whole-cell EPR of aerobically grown, aco1 yeast expressing wild-type IRP1 or a S138A-IRP1 mutant (IRP1(S138A)), providing the first direct evidence of a 3Fe intermediate in vivo. Exposure of yeast to H(2)O(2) increased this 3Fe c-aconitase signal up to 5-fold, coincident with inhibition of c-aconitase activity. Untreated yeast expressing IRP1(S138D) or IRP1(S138E), which mimic phosphorylated IRP1, failed to give a 3Fe signal. H(2)O(2) produced a weak 3Fe signal in yeast expressing IRP1(S138D). Yeast expressing IRP1(S138D) or IRP1(S138E) were the most sensitive to inhibition of aconitase-dependent growth by H(2)O(2) and were more responsive to changes in media iron status. Ferricyanide oxidation of anaerobically reconstituted c-aconitase yielded a strong 3Fe EPR signal with wild-type and S138A c-aconitases. Only a weak 3Fe signal was obtained with S138D c-aconitase, and no signal was obtained with S138E c-aconitase. This, paired with loss of c-aconitase activity, strongly argues that the Fe-S clusters of these phosphomimetic c-aconitase mutants undergo more complete disassembly upon oxidation. Our results demonstrate that 3Fe c-aconitase is an intermediate in H(2)O(2)-initiated Fe-S cluster disassembly in vivo and suggest that cluster assembly/disassembly in IRP1 is a dynamic process in aerobically growing yeast. Further, our results support the view that phosphorylation of IRP1 can modulate its response to iron through effects on Fe-S cluster stability and turnover.

Exercise decreases cytosolic aconitase activity in the liver, spleen, and bone marrow in rats

Effects of strenuous exercise on cytosolic aconitase activity (CAA) were investigated in this study. Female Sprague-Dawley rats were randomly assigned to four groups: S1 (Sedentary), S2 (Sedentary + L-NAME [N-nitro-l-arginine methyl ester]), E1 (Exercise), and E2 (Exercise + L-NAME). Rats in the E1 and E2 groups swam for 2 h/day for 3 months. L-NAME (an inhibitor of NOS) in drinking water (1 mg/ml) was administered to rats in the S2 and E2 groups for the same period. At the end of the third month, the CAA in the liver, spleen, and bone marrow cells was measured. In the exercise group (E1), CAA in the liver, spleen, and bone marrow cells was 19.99 +/- 1.49, 1.61 +/- 0.13, and 0.59 +/- 0.09 mU/mg protein, respectively. These values were significantly lower than the corresponding sedentary values in the S1 group (33.96 +/- 1.38, 3.96 +/- 0.19, and 3.20 +/- 0.18 mU/mg protein) (P < 0.01, 0.001, and 0.001, respectively). The treatment of L-NAME led to a significant increase in tissue CAA in the sedentary rats (S2). Also, the significantly higher CAA in the liver, spleen, and bone marrow cells was found in the exercised rats treated with L-NAME (E2) (29.50 +/- 1.27, 2.89 +/- 0.25, and 1.34 +/- 0.20 mU/mg) than without L-NAME (E1) (P < 0.01, 0.01, 0.05, respectively). However, the values in the E2 group were still significantly lower than those in the S1 group (P < 0.05, 0.01, and 0.001, respectively). This indicates that L-NAME treatment can partly recover the decreased CA in tissues in the exercised rats. These results provide evidence for the existence of the increased activity of IRP1 (iron regulatory protein 1) that is probably induced by the increased nitric oxide production in the strenuously exercised rats.

Iron regulatory proteins and the molecular control of mammalian iron metabolism

Mammalian iron homeostasis is maintained through the concerted action of sensory and regulatory networks that modulate the expression of proteins of iron metabolism at the transcriptional and/or post-transcriptional levels. Regulation of gene transcription provides critical developmental, cell cycle, and cell-type-specific controls on iron metabolism. Post-transcriptional control through the action of iron regulatory protein 1 (IRP1) and IRP2 coordinate the use of messenger RNA-encoding proteins that are involved in the uptake, storage, and use of iron in all cells of the body. IRPs may also provide a link between iron availability and cellular citrate use. Multiple factors, including iron, nitric oxide, oxidative stress, phosphorylation, and hypoxia/reoxygenation, influence IRP function. Recent evidence indicates that there is diversity in the function of the IRP system with respect to the response of specific IRPs to the same effector, as well as the selectivity with which IRPs modulate the use of specific messenger RNA.

Inactivation of iron responsive element-binding capacity and aconitase function of iron regulatory protein-1 of skin cells by ultraviolet A

The ultraviolet-A (UVA) component of sunlight produces in cutaneous cells a highly toxic oxidative stress mediated by redox cycling reactions of Fe ions. A tight regulation of cell iron uptake and storage by iron regulatory proteins (IRP) of keratinocytes and fibroblasts avoids these damaging reactions. We report here that about 40 J/cm2 of UVA are required to inactivate half of the binding capacity of apo-IRP-1 to iron responsive elements (IRE) of RNA whereas 15 J/cm2 already inhibit half of the holo-IRP-1 aconitase activity. No increase in the holo-IRP-1 activity is observed during the apo-IRP-1 photoinactivation suggesting that UVA does not trigger a shift between these two forms. As opposed to holo-IRP-1, which contains a 4Fe-4S cluster, apo-IRP-1 has no UVA chromophore. Thus it should be inactivated indirectly by reactive oxygen species generated by the UVA-induced endogenous photo-oxidative stress. The apo-IRP-1 photoinactivation is weakly prevented by the lipophilic oxyradical scavenger vitamin E but not by the hydrophilic azide anion, a singlet oxygen quencher or by diethyldithiocarbamate, a superoxide dismutase inhibitor. However, full protection against photoinactivation of the apo form is observed after incubation with N-acetylcysteine but the latter only partially protects the aconitase function of the holo-IRP-1 from photoinactivation. The marked difference in the kinetics of photoinactivation of the apo and holo forms, the light dose-independent effect of the sulfhydril group reagent, 2-mercaptoethanol and the partial protection brought by the ferric ion complexing agent desferrioxamine suggest that the photochemistry of the 4Fe-4S cluster of the holo form plays little, if any, role in the photoinactivation of the apo-IRP-1/IRE interaction. It is concluded that the apo/holo equilibrium is irreversibly destroyed by UVA irradiation.

A tribute to sulfur

Recent progress in a number of areas of biochemistry and biology has drawn attention to the critical importance of sulfur in the biosynthesis of vital cofactors and active sites in proteins, and in the complex reaction mechanisms often involved. This brief review is intended as a broad overview of this currently rapidly moving field of sulfur biochemistry, for those who are interested or are involved in one or the other aspect of it, a synopsis by one who has stumbled into this field from several directions in the course of time. Only for iron are metal-sulfur relationships discussed in detail, as the iron-sulfur subfield is one of the most active areas.

Regulation of mammalian iron homeostasis

Iron homeostasis is regulated with respect to uptake, storage and utilization. Newer work is presented that defines proteins responsible for iron transport, sequestration and sensing, and that addresses their regulation at the cellular and organismal levels by ambient iron concentrations, demand for erythropoiesis, body iron burden, and redox stimuli.

Down-regulation of iron regulatory protein 1 gene expression by nitric oxide

Iron regulatory protein 1 (IRP1) is an RNA binding protein that posttranscriptionally modulates the expression of mRNAs coding for proteins involved in iron metabolism. It has long been held that its RNA binding activity is regulated posttranslationally by the insertion/extrusion of a 4Fe-4S cluster, without changes in IRP1 levels. However, the question of a possible regulation of the expression of this protein has remained open. In the present study we analyzed the modulation of IRP1 expression in murine macrophages. We showed that activation by IFN-gamma and/or lipopolysaccharide, which induces IRP1 RNA binding activity via nitric oxide (NO), results simultaneously in a reduction in IRP1 protein levels, as determined by Western blot analyses. IRP1 expression decreased time-dependently to about 40% of control levels after 16 h. Down-regulation of IRP1 protein levels was correlated with the amount of NO produced and was partially abolished by the NO synthase (NOS) inhibitor N-monomethyl-l-arginine. No changes in IRP1 levels could be detected in stimulated peritoneal macrophages from NOS2 knockout (NOS2(-/-)) mice, unlike wild-type mice. Converse modulation of IRP1 RNA binding activity and IRP1 levels could be reproduced by exogenous NO and also was observed in nonmacrophage cells cocultured with NO-producing macrophages. We also analyzed IRP1 mRNA levels by Northern blotting and found a decrease in IRP1 mRNA expression after stimulation with IFN-gamma plus lipopolysaccharide, which was abrogated in the presence of N-monomethyl-l-arginine. This is evidence that IRP1 is regulated by a physiological stimulus other than posttranslationally.

The aconitase function of iron regulatory protein 1. Genetic studies in yeast implicate its role in iron-mediated redox regulation

Iron regulatory proteins (IRP) are sequence-specific RNA-binding proteins that mediate iron-responsive gene regulation in animals. IRP1 is also the cytosolic isoform of aconitase (c-aconitase). This latter activity could complement a mitochondrial aconitase mutation (aco1) in Saccharomyces cerevisiae to restore glutamate prototrophy. In yeast, the c-aconitase activity of IRP1 was responsive to iron availability in the growth medium. Although IRP1 expression rescued aco1 yeast from glutamate auxotrophy, cells remained growth-limited by glutamate, displaying a slow-growth phenotype on glutamate-free media. Second site mutations conferring enhanced cytosolic aconitase-dependent (ECA) growth were recovered. Relative c-aconitase activity was increased in extracts of strains harboring these mutations. One of the ECA mutations was found to be in the gene encoding cytosolic NADP(+)-dependent isocitrate dehydrogenase (IDP2). This mutation, an insertion of a Ty delta element into the 5' region of IDP2, markedly elevates expression of Idp2p in glucose media. Our results demonstrate the physiological significance of the aconitase activity of IRP1 and provide insight into the role of c-aconitase with respect to iron and cytoplasmic redox regulation.

Direct nitric oxide signal transduction via nitrosylation of iron-sulfur centers in the SoxR transcription activator

Nitric oxide (NO) has diverse roles in intercellular communication and (at higher levels) in immune-mediated cell killing. NO reacts with many cellular targets, with cell-killing effects correlated to inactivation of key enzymes through nitrosylation of their iron-sulfur centers. SoxR protein, a redox-sensitive transcription activator dependent on the oxidation state of its binuclear iron-sulfur ([2Fe-2S]) centers, is also activated in Escherichia coli on exposure to macrophage-generated NO. We show here that SoxR activation by NO occurs through direct modification of the [2Fe-2S] centers to form protein-bound dinitrosyl-iron-dithiol adducts, which we have observed both in intact bacterial cells and in purified SoxR after NO treatment. Functional activation through nitrosylation of iron-sulfur centers contrasts with the inactivation typically caused by this modification. Purified, nitrosylated SoxR has transcriptional activity similar to that of oxidized SoxR and is relatively stable. In contrast, nitrosylated SoxR is short-lived in intact cells, indicative of mechanisms that actively dispose of nitrosylated iron-sulfur centers.

Nitrogen monoxide activates iron regulatory protein 1 RNA-binding activity by two possible mechanisms: effect on the [4Fe-4S] cluster and iron mobilization from cells

The iron-regulatory protein 1 (IRP1) regulates the expression of several molecules involved in iron (Fe) metabolism by reversibly binding to iron-responsive elements (IREs) in the untranslated regions (UTR) of particular mRNA transcripts. Several studies have indicated that nitrogen monoxide (NO) may influence IRP1 RNA-binding activity by a direct effect on the [4Fe-4S] cluster of the protein. It has also been suggested that NO may act indirectly on IRP1 by affecting the intracellular Fe pools that regulate the function of this protein [Pantopoulous et al. (1996) Mol. Cell. Biol. 16, 3781-3788]. There is also the possibility that NO may S-nitrosate sulfhydryl groups that are crucial for mRNA binding and decrease IRP1 activity by this mechanism. We have examined the effect of a variety of NO donors [e.g., S-nitroso-N-acetylpenicillamine (SNAP), spermine-NONOate (SperNO), and S-nitrosoglutathione (GSNO)] on IRP1 RNA-binding activity in both LMTK(-) fibroblast lysates and whole cells. In cell lysates, the effects of NO at increasing RNA-binding activity were only observed when cells were made Fe-replete. Under these circumstances, IRP1 contains an [4Fe-4S] cluster that was susceptible to NO. In contrast, when lysates were prepared from cells treated with the Fe chelator desferrioxamine (DFO), NO had no effect on the RNA-binding activity of IRP1. The lack of effect of NO under these conditions was probably because this protein does not have an [4Fe-4S] cluster. In contrast to the NO generators above, sodium nitroprusside (SNP) decreased IRP1 RNA binding when cells were incubated with this compound. However, SNP had no effect on IRP1 RNA-binding activity in lysates, suggesting that the decrease after incubation of cells with SNP was not due to S-nitrosation of critical sulfhydryl groups. Apart from the direct effect of NO on IRP1 in Fe-replete cells, we have shown that NO generated by SNAP, SperNO, and GSNO could also mobilize Fe from cells. When NO generation was induced in RAW 264.7 macrophages, an increase in IRP1 RNA-binding activity occurred but there was only a small increase in Fe release. Our results suggest that NO could activate IRP1 RNA-binding by two possible mechanisms: (1) its direct effect on the [4Fe-4S] cluster and (2) mobilization of (59)Fe from cells resulting in Fe depletion, which then increases IRP1 RNA-binding activity.

Redox control of protein degradation

This review summarizes evidence that most of cell protein degradation is maintained by pathways transferring energy from glucose to reduction of enzymic and nonenzymic proteins (redox-responsive). In contrast, a major subcomponent of proteolysis is simultaneously independent of the cell redox network (redox-unresponsive). Thus far, direct and indirect redox-responsive proteolytic effector mechanisms characterized by various investigators include: several classes of proteases, some peptide protease inhibitors, substrate conjugation systems, substrate redox and folding status, cytoskeletal-membrane kinesis, metal homeostasis, and others. The present focus involves redox control of sulfhydryl proteases and proteolytic pathways of mammalian muscle; however, other mechanisms, cell types, and species are also surveyed. The diversity of redox-responsive catabolic mechanisms reveals that the machinery of protein turnover evolved with fundamental dependencies upon the cell redox network, as observed in many species. The net redox status of a reversible proteolytic effector mechanism represents the balance between combined oxidative inactivating influences versus reductive activating influences. Similar to other proteins, redox-responsive proteolytic effectors appear to be oxidized by mixed disulfide formation, nitrosation, reactive oxygen species, and associations or reactions with metal ions and various pro-oxidative metabolites. Systems reducing the proteolytic machinery include major redox enzyme chains, such as thioredoxins or glutaredoxins, and perhaps various reductive metabolites, including glutathione and dihydrolipoic acid. Much of mammalian intracellular protein degradation is reversibly responsive to noninjurious experimental intervention in the reductive energy supply-demand balance. Proteolysis is reversibly inhibited by diamide or dehydroascorbic acid; and such antiproteolytic actions are strongly dependent on the cell glucose supply. However, gross redox-responsive proteolysis is not accompanied by ATP depletion or vice versa. Redox-responsive proteolysis includes Golgi-endoplasmic reticulum degradation, lysosomal degradation, and some amount of extravesicular degradation, all comprising more than half of total cell proteolysis. Speculatively, redox-dependent proteolysis exhibits features expected of a controlling influence coordinating distinct proteolytic processes under some intracellular conditions.

Nitric oxide inhibits c-Jun DNA binding by specifically targeted S-glutathionylation

This study addresses potential molecular mechanisms underlying the inhibition of the transcription factor c-Jun by nitric oxide. We show that in the presence of the physiological sulfhydryl glutathione nitric oxide modifies the two cysteine residues contained in the DNA binding module of c-Jun in a selective and distinct way. Although nitric oxide induced the formation of an intermolecular disulfide bridge between cysteine residues in the leucine zipper site of c-Jun monomers, this same radical directed the covalent incorporation of stoichiometric amounts of glutathione to a single conserved cysteine residue in the DNA-binding site of the protein. We found that covalent dimerization of c-Jun apparently did not affect its DNA binding activity, whereas the formation of a mixed disulfide with glutathione correlated well with the inhibition of transcription factor binding to DNA. Furthermore, we provide experimental evidence that nitric oxide-induced S-glutathionylation and inhibition of c-Jun involves the formation of S-nitrosoglutathione. In conclusion, our results support the reversible formation of a mixed disulfide between glutathione and c-Jun as a potential mechanism by which nitrosative stress may be transduced into a functional response at the level of transcription.