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

Mendelian inheritance of anemia due to disturbed iron homeostasis

Genetic disorders that affect proteins involved in maintaining iron balance may lead to Mendelian anemias. They may be classified as defects of intestinal iron absorption, iron transport in the circulation, iron uptake and utilization by maturing erythroid cells, iron recycling by macrophages and systemic regulation of iron homeostasis. All these Mendelian anemias are rare disorders, prevalently recessive, characterized by microcytic and hypochromic red blood cells. Advances in our knowledge of iron metabolism and its systemic regulation on one side have facilitated the identification of novel iron related anemias, while on the other the study of the affected patients and of the corresponding animal models have contributed to our understanding of iron trafficking and regulation.

Predicted loss and gain of function mutations in ACO1 are associated with erythropoiesis

Hemoglobin is the essential oxygen-carrying molecule in humans and is regulated by cellular iron and oxygen sensing mechanisms. To search for novel variants associated with hemoglobin concentration, we performed genome-wide association studies of hemoglobin concentration using a combined set of 684,122 individuals from Iceland and the UK. Notably, we found seven novel variants, six rare coding and one common, at the ACO1 locus associating with either decreased or increased hemoglobin concentration. Of these variants, the missense Cys506Ser and the stop-gained Lys334Ter mutations are specific to eight and ten generation pedigrees, respectively, and have the two largest effects in the study (EffectCys506Ser = -1.61 SD, CI95 = [-1.98, -1.35]; EffectLys334Ter = 0.63 SD, CI95 = [0.36, 0.91]). We also find Cys506Ser to associate with increased risk of persistent anemia (OR = 17.1, P = 2 × 10-14). The strong bidirectional effects seen in this study implicate ACO1, a known iron sensing molecule, as a major homeostatic regulator of hemoglobin concentration.

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.

Rli1/ABCE1 Recycles Terminating Ribosomes and Controls Translation Reinitiation in 3'UTRs In Vivo

To study the function of Rli1/ABCE1 in vivo, we used ribosome profiling and biochemistry to characterize its contribution to ribosome recycling. When Rli1 levels were diminished, 80S ribosomes accumulated both at stop codons and in the adjoining 3'UTRs of most mRNAs. Frequently, these ribosomes reinitiated translation without the need for a canonical start codon, as small peptide products predicted by 3'UTR ribosome occupancy in all three reading frames were confirmed by western analysis and mass spectrometry. Eliminating the ribosome-rescue factor Dom34 dramatically increased 3'UTR ribosome occupancy in Rli1 depleted cells, indicating that Dom34 clears the bulk of unrecycled ribosomes. Thus, Rli1 is crucial for ribosome recycling in vivo and controls ribosome homeostasis. 3'UTR translation occurs in wild-type cells as well, and observations of elevated 3'UTR ribosomes during stress suggest that modulating recycling and reinitiation is involved in responding to environmental changes.

Expression and properties of the mitochondrial and cytosolic forms of aconitase in maize scutellum

Aconitase (EC 4.2.1.3) catalyzes the reversible interconversion of citrate, cis-aconitate, and D-isocitrate. It operates in mitochondria and cytosol. We investigated the expression of two aconitase genes (Aco1 and Aco4) and activities of the mitochondrial and cytosolic forms in maize (Zea mays L.) scutellum during germination. Both forms were isolated and purified. The cytosolic form had a higher pH optimum (8.0), twice higher affinity to citrate (K(m) 9.5 mM), and slightly lower affinity to D,L-isocitrate (K(m) 1.7 mM) as compared to the mitochondrial form (optimum pH 7.5, K(m) with citrate 21 mM, and K(m) with isocitrate 1.5 mM). The highest activity of both forms of aconitase was observed on the 4th day of germination; then the activity and expression of the cytosolic form sharply decreased, while the mitochondrial form decreased more slowly. The mitochondrial aconitase was more strongly inhibited by H2O2 (half-inhibition at 35 μM) than the cytosolic form (60 μM). Aconitase activity was not detected in the glyoxysomal fraction beyond the cross-contamination level. It is suggested that the mitochondrial form operates in the tricarboxylic acid cycle, whereas the cytosolic form participates in the reactions of the glyoxylate cycle taking place outside the glyoxysome.

Dom34 rescues ribosomes in 3' untranslated regions

Ribosomes that stall before completing peptide synthesis must be recycled and returned to the cytoplasmic pool. The protein Dom34 and cofactors Hbs1 and Rli1 can dissociate stalled ribosomes in vitro, but the identity of targets in the cell is unknown. Here, we extend ribosome profiling methodology to reveal a high-resolution molecular characterization of Dom34 function in vivo. Dom34 removes stalled ribosomes from truncated mRNAs, but, in contrast, does not generally dissociate ribosomes on coding sequences known to trigger stalling, such as polyproline. We also show that Dom34 targets arrested ribosomes near the ends of 3' UTRs. These ribosomes appear to gain access to the 3' UTR via a mechanism that does not require decoding of the mRNA. These results suggest that ribosomes frequently enter downstream noncoding regions and that Dom34 carries out the important task of rescuing them.

Nucleotide-specific recognition of iron-responsive elements by iron regulatory protein 1

IRP1 [iron regulatory protein (IRP) 1] is a bifunctional protein with mutually exclusive end-states. In one mode of operation, IRP1 binds iron-responsive element (IRE) stem-loops in messenger RNAs encoding proteins of iron metabolism to control their rate of translation. In its other mode, IRP1 serves as cytoplasmic aconitase to correlate iron availability with the energy and oxidative stress status of the cell. IRP1/IRE binding occurs through two separate interfaces, which together contribute about two-dozen hydrogen bonds. Five amino acids make base-specific contacts and are expected to contribute significantly to binding affinity and specificity of this protein:RNA interaction. In this mutagenesis study, each of the five base-specific amino acids was changed to alter binding at each site. Analysis of IRE binding affinity and translational repression activity of the resulting IRP1 mutants showed that four of the five contact points contribute uniquely to the overall binding affinity of the IRP1:IRE interaction, while one site was found to be unimportant. The stronger-than-expected effect on binding affinity of mutations at Lys379 and Ser681, residues that make contact with the conserved nucleotides G16 and C8, respectively, identified them as particularly critical for providing specificity and stability to IRP1:IRE complex formation. We also show that even though the base-specific RNA-binding residues are not part of the aconitase active site, their substitutions can affect the aconitase activity of holo-IRP1, positively or negatively.

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

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

Transcriptional upregulation of four genes of the lysine biosynthetic pathway by homocitrate accumulation in Penicillium chrysogenum: homocitrate as a sensor of lysine-pathway distress

The lysine biosynthetic pathway has to supply large amounts of alpha-aminoadipic acid for penicillin biosynthesis in Penicillium chrysogenum. In this study, we have characterized the P. chrysogenum L2 mutant, a lysine auxotroph that shows highly increased expression of several lysine biosynthesis genes (lys1, lys2, lys3, lys7). The L2 mutant was found to be deficient in homoaconitase activity since it was complemented by the Aspergillus nidulans lysF gene. We have cloned a gene (named lys3) that complements the L2 mutation by transformation with a P. chrysogenum genomic library, constructed in an autonomous replicating plasmid. The lys3-encoded protein showed high identity to homoaconitases. In addition, we cloned the mutant lys3 allele from the L2 strain that showed a G(1534) to A(1534) point mutation resulting in a Gly(495) to Asp(495) substitution. This mutation is located in a highly conserved region adjacent to two of the three cysteine residues that act as ligands to bind the iron-sulfur cluster required for homoaconitase activity. The L2 mutant accumulates homocitrate. Deletion of the lys1 gene (homocitrate synthase) in the L2 strain prevented homocitrate accumulation and reverted expression levels of the four lysine biosynthesis genes tested to those of the parental prototrophic strain. Homocitrate accumulation seems to act as a sensor of lysine-pathway distress, triggering overexpression of four of the lysine biosynthesis genes.

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.

Iron-dependent degradation of apo-IRP1 by the ubiquitin-proteasome pathway

Iron regulatory protein 1 (IRP1) controls the translation or stability of several mRNAs by binding to "iron-responsive elements" within their untranslated regions. In iron-replete cells, IRP1 assembles a cubane iron-sulfur cluster (ISC) that inhibits RNA-binding activity and converts the protein to cytosolic aconitase. We show that the constitutive IRP1(C437S) mutant, which fails to form an ISC, is destabilized by iron. Thus, exposure of H1299 cells to ferric ammonium citrate reduced the half-life of transfected IRP1(C437S) from approximately 24 h to approximately 10 h. The iron-dependent degradation of IRP1(C437S) involved ubiquitination, required ongoing transcription and translation, and could be efficiently blocked by the proteasomal inhibitors MG132 and lactacystin. Similar results were obtained with overexpressed wild-type IRP1, which predominated in the apo-form even in iron-loaded H1299 cells, possibly due to saturation of the ISC assembly machinery. Importantly, inhibition of ISC biogenesis in HeLa cells by small interfering RNA knockdown of the cysteine desulfurase Nfs1 sensitized endogenous IRP1 for iron-dependent degradation. Collectively, these data uncover a mechanism for the regulation of IRP1 abundance as a means to control its RNA-binding activity, when the ISC assembly pathway is impaired.

Structure of dual function iron regulatory protein 1 complexed with ferritin IRE-RNA

Iron regulatory protein 1 (IRP1) binds iron-responsive elements (IREs) in messenger RNAs (mRNAs), to repress translation or degradation, or binds an iron-sulfur cluster, to become a cytosolic aconitase enzyme. The 2.8 angstrom resolution crystal structure of the IRP1:ferritin H IRE complex shows an open protein conformation compared with that of cytosolic aconitase. The extended, L-shaped IRP1 molecule embraces the IRE stem-loop through interactions at two sites separated by approximately 30 angstroms, each involving about a dozen protein:RNA bonds. Extensive conformational changes related to binding the IRE or an iron-sulfur cluster explain the alternate functions of IRP1 as an mRNA regulator or enzyme.

Of two cytosolic aconitases expressed in Drosophila, only one functions as an iron-regulatory protein

In mammalian cells, iron homeostasis is largely regulated by post-transcriptional control of gene expression through the binding of iron-regulatory proteins (IRP1 and IRP2) to iron-responsive elements (IREs) contained in the untranslated regions of target mRNAs. IRP2 is the dominant iron sensor in mammalian cells under normoxia, but IRP1 is the more ancient protein in evolutionary terms and has an additional function as a cytosolic aconitase. The Caenorhabditis elegans genome does not contain an IRP2 homolog or identifiable IREs; its IRP1 homolog has aconitase activity but does not bind to mammalian IREs. The Drosophila genome offers an evolutionary intermediate containing two IRP1-like proteins (IRP-1A and IRP-1B) and target genes with IREs. Here, we used purified recombinant IRP-1A and IRP-1B from Drosophila melanogaster and showed that only IRP-1A can bind to IREs, although both proteins possess aconitase activity. These results were also corroborated in whole-fly homogenates from transgenic flies that overexpress IRP-1A and IRP-1B in their fat bodies. Ubiquitous and muscle-specific overexpression of IRP-1A, but not of IRP-1B, resulted in pre-adult lethality, underscoring the importance of the biochemical difference between the two proteins. Domain-swap experiments showed that multiple amino acid substitutions scattered throughout the IRP1 domains are synergistically required for conferring IRE binding activity. Our data suggest that as a first step during the evolution of the IRP/IRE system, the ancient cytosolic aconitase was duplicated in insects with one variant acquiring IRE-specific binding.

Iron accumulation in alcoholic liver diseases

Increased hepatic iron is one of the important key factors which contribute alcohol toxicity of liver due to the production of reactive oxygen species. In patients with alcoholic liver diseases (ALD), liver iron is increased and the resulted lipid metabolite 4-hydroxynonenal-protein adduct was also increased. In general, iron is deposited in both parenchymal cells and and Kupffer cells in ALD. However, in patients with mild ALD, the parenchymal iron deposition is dominant rather than reticuloendothelial iron deposition, while the latter iron deposition is domimant in severe ALD, possibly due to endotoxemia and overproduction of inflammatory cytokines. We speculated that a parenchymal iron deposition in mild ALD is an important factor to trigger hepatocytes injury by ethanol, and the possible cause of parencynal iron deposition may be an increase of cellular iron uptake via serum transferrin in hepatocytes after ethanol exposure. By immuno-histochemical study of biopsied liver samples, the expression of transferrin receptor 1 (TfR1), which mediates cellular iron uptake by serum transferrin was increased. This increase of TfR1 by ethanol is confirmed by in vitro experiment using HepG2 cells and primary rat hepatocytes culture. Fe-labeled transferrin incorporation (but not transferrin non-bound iron (NTBI)) into the cells is also increased, suggesting that the increased TfR1 is functional. The increase of TfR1 expression is partially due to the increased activity of iron regulatory protein (IRP) by oxidative stress of ethanol metabolism. Thus, the post-transcriptional regulation of iron uptake by ethanol is involved in the hepatocyte iron accumulation. Another possibility is an increase of intestinal iron absorption. Our recent finding regarding the increase of pro-hepcidin serum in alcoholic patients with high serum ferritin support this assumption.

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.

Molecular pharmacology of the interaction of anthracyclines with iron

Although anthracyclines such as doxorubicin are widely used antitumor agents, a major limitation for their use is the development of cardiomyopathy at high cumulative doses. This severe adverse side effect may be due to interactions with cellular iron metabolism, because iron loading promotes anthracycline-induced cell damage. On the other hand, anthracycline-induced cardiotoxicity is significantly alleviated by iron chelators (e.g., desferrioxamine and dexrazoxane). The molecular mechanisms by which anthracyclines interfere with cellular iron trafficking are complex and still unclear. Doxorubicin can directly bind iron and can perturb iron metabolism by interacting with multiple molecular targets, including the iron regulatory proteins (IRP) 1 and 2. The RNA-binding activity of these molecules regulates synthesis of the transferrin receptor 1 and ferritin, which are crucial proteins involved in iron uptake and storage, respectively. At present, it is not clear whether doxorubicin affects IRP1-RNA-binding activity by intracellular formation of doxorubicinol and/or by generation of the doxorubicin-iron(III) complex. Furthermore, doxorubicin prevents the mobilization of iron from ferritin by a mechanism that may involve lysosomal degradation of this protein. Prevention of iron mobilization from ferritin would probably disturb vital cellular functions as a result of inhibition of essential iron-dependent proteins, such as ribonucleotide reductase. This review discusses the molecular interactions of anthracyclines with iron metabolism and the development of cardioprotective strategies such as iron chelators.

Induction of transferrin receptor by ethanol in rat primary hepatocyte culture

BACKGROUND

It is not uncommon for alcoholics to have iron accumulation in the liver, a condition that may contribute to the development of alcoholic liver disease. Recently, we reported that the expression of transferrin receptor, which mediates cellular iron uptake, was increased in hepatocytes in patients with alcoholic liver disease. To elucidate the mechanism of the iron accumulation in hepatocytes in such disease, we examined whether ethanol exposure induced the transferrin receptor expression and increased the cellular iron uptake.

METHODS

Rat primary hepatocytes were isolated and cultured in the presence of 20 micromol/liter of iron and 25 mmol/liter of ethanol.

RESULTS

Ethanol exposure to the hepatocytes demonstrated an ~2-fold increase in transferrin receptor expression for 24 hr, shown by Western blot analysis and S-methionine metabolic labeling, 19% increase in Fe-transferrin uptake by hepatocytes, and 20% increase in activity of iron regulatory protein examined by band shift assay.

CONCLUSION

Ethanol exposure induced the transferrin receptor expression, partially through the activation of iron regulatory protein, and increased the transferrin-bound iron uptake in rat hepatocyte cultures. The induction of transferrin receptor by ethanol might be one of the mechanisms of iron accumulation in the hepatocytes in alcoholic liver disease.

Induction of Transferrin Receptor by Ethanol in Rat Primary Hepatocyte Culture

BACKGROUND

It is not uncommon for alcoholics to have iron accumulation in the liver, a condition that may contribute to the development of alcoholic liver disease. Recently, we reported that the expression of transferrin receptor, which mediates cellular iron uptake, was increased in hepatocytes in patients with alcoholic liver disease. To elucidate the mechanism of the iron accumulation in hepatocytes in such disease, we examined whether ethanol exposure induced the transferrin receptor expression and increased the cellular iron uptake.

METHODS

Rat primary hepatocytes were isolated and cultured in the presence of 20 micromol/liter of iron and 25 mmol/liter of ethanol.

RESULTS

Ethanol exposure to the hepatocytes demonstrated an ~2-fold increase in transferrin receptor expression for 24 hr, shown by Western blot analysis and S-methionine metabolic labeling, 19% increase in Fe-transferrin uptake by hepatocytes, and 20% increase in activity of iron regulatory protein examined by band shift assay.

CONCLUSION

Ethanol exposure induced the transferrin receptor expression, partially through the activation of iron regulatory protein, and increased the transferrin-bound iron uptake in rat hepatocyte cultures. The induction of transferrin receptor by ethanol might be one of the mechanisms of iron accumulation in the hepatocytes in alcoholic liver disease.

Nitric oxide-mediated modulation of iron regulatory proteins: implication for cellular iron homeostasis

Iron regulatory proteins (IRP1 and IRP2) control the synthesis of transferrin receptors (TfR) and ferritin by binding to iron-responsive elements (IREs) that are located in the 3' untranslated region (UTR) and the 5' UTR of their respective mRNAs. Cellular iron levels affect binding of IRPs to IREs and consequently expression of TfR and ferritin. Moreover, NO(.), a redox species of nitric oxide that interacts primarily with iron, can activate IRP1 RNA-binding activity resulting in an increase in TfR mRNA levels and a decrease in ferritin synthesis. We have shown that treatment of RAW 264.7 cells (a murine macrophage cell line) with NO(+) (nitrosonium ion, which causes S-nitrosylation of thiol groups) resulted in a rapid decrease in RNA-binding of IRP2, followed by IRP2 degradation, and these changes were associated with a decrease in TfR mRNA levels and a dramatic increase in ferritin synthesis. Moreover, we demonstrated that stimulation of RAW 264.7 cells with lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma) increased IRP1 binding activity, whereas RNA-binding of IRP2 decreased and was followed by a degradation of this protein. Furthermore, the decrease of IRP2 binding/protein levels was associated with a decrease in TfR mRNA levels and an increase in ferritin synthesis in LPS/IFN-gamma-treated cells, and these changes were prevented by inhibitors of inducible nitric oxide synthase. These results suggest that NO(+)-mediated degradation of IRP2 plays a major role in iron metabolism during inflammation.

Unexpected anthracycline-mediated alterations in iron-regulatory protein-RNA-binding activity: the iron and copper complexes of anthracyclines decrease RNA-binding activity

Anthracyclines are effective antineoplastic agents. However, the interaction of these drugs with iron (Fe) is an important cause of myocardial toxicity, limiting their therapeutic use (J Lab Clin Med 122:245-251, 1993). To overcome this limitation, it is crucial to understand how anthracyclines interact with the Fe metabolism of myocardial and neoplastic cells. Iron-regulatory proteins (IRPs) play vital roles in regulating cellular Fe metabolism via their mRNA-binding activity. We showed that doxorubicin (DOX) and its analogs interfere with tumor and myocardial cell Fe metabolism by affecting the RNA-binding activity of IRPs. Unexpectedly, experiments with the free radical scavengers, catalase, superoxide dismutase, ebselen, and Mn(III) tetrakis (4-benzoic acid) porphyrin complex, suggested that the effects of DOX on IRP-RNA-binding activity were not due to anthracycline-mediated free radical production. In contrast to previous studies, we showed that the DOX metabolite, doxorubicinol, had no effect on IRP-RNA-binding activity. Rather, the anthracycline-Fe and -copper (Cu) complexes decreased IRP-RNA-binding activity, indicating that formation of anthracycline-metal complexes may affect cellular Fe metabolism. In addition, anthracyclines prevented the response of IRPs to the depletion of intracellular Fe by chelators. This information may be useful in designing novel therapeutic strategies against tumor cells by combining chelators and anthracyclines. Interestingly, the effect of DOX on primary cultures of cardiomyocytes was similar to that observed using neoplastic cells, and particularly notable was the decrease in IRP2-RNA-binding activity. Our results add significant new information regarding the effects of anthracyclines on Fe metabolism that may lead to the design of more effective treatments.

Conditional derepression of ferritin synthesis in cells expressing a constitutive IRP1 mutant

Iron regulatory protein 1 (IRP1), a major posttranscriptional regulator of cellular iron and energy metabolism, is controlled by an iron-sulfur cluster switch. Cysteine-437 is critical for coordinating the cluster, and its replacement yields mutants that do not respond to iron perturbations and constitutively bind to cognate mRNA iron-responsive elements (IREs). The expression of IRP1(C437S) in cells has been associated with aberrations in iron homeostasis and toxicity. We have established clones of human lung (H1299) and breast (MCF7) cancer cells that express high levels of IRP1(C437S) in a tetracycline-inducible manner. As expected, IRP1(C437S) stabilizes transferrin receptor mRNA and inhibits translation of ferritin mRNA in both cell types by binding to their respective IREs. However, H1299 transfectants grown at high densities are able to overcome the IRP1(C437S)-mediated inhibition in ferritin synthesis. The mechanism involves neither alteration in ferritin mRNA levels nor utilization of alternative transcription start sites to eliminate the IRE or relocate it in less inhibitory downstream positions. The derepression of ferritin mRNA translation occurs under conditions where global protein synthesis appears to be impaired, as judged by a significant enrichment in the expression of the underphosphorylated form of the translational regulator 4E-BP1. Collectively, these data document an example where ferritin mRNA translation evades control of the IRE-IRP system. The physiological implications of this response are reflected in protection against iron-mediated toxicity, oxidative stress, and apoptosis.

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.

Alterations in cellular Ca(2+) and free iron pool by sulfur amino acid deprivation: the role of ferritin light chain down-regulation in prooxidant production

Deficiency of sulfur amino acids occurs in certain pathophysiological states such as protein-calorie malnutrition. Sulfur amino acid deprivation (SAAD) increases oxidative stress through a decrease in GSH. Ferritin expression is induced by oxidative stress, which confers resistance to oxidative insults. The effects of SAAD on the changes in cellular Ca(2+) and free iron pool, prooxidant production and the ferritin light chain (FLC) expression were comparatively evaluated in Hepa1c1c7 and Raw264.7 cells. [Ca(2+)](i) was rapidly increased by SAAD. Sulfhydryl-containing compounds prevented the increase in [Ca(2+)](i) in cells under SAAD, supporting the role of redox-state in the regulation of [Ca(2+)](i). Thapsigargin or Ca(2+)-free medium inhibited the increase in [Ca(2+)](i), showing that Ca(2+) originated from endoplasmic reticulum as well as from extracellular source. Inhibition of Ca(2+) mobilization decreased the fluorescence of Phen Green SK inside cells, representing the inhibition of free iron release. Both inhibition of Ca(2+) mobilization and iron chelation decreased dichlorofluorescein oxidation, indicating the possibility that the increase in [Ca(2+)](i) affected that in cellular free iron and prooxidant production. FLC protein level was immunochemically detectable in Raw264.7 cells, but not in Hepa1c1c7 cells. SAAD alone (or in combination with FeSO(4)) down-regulated FLC protein expression, while SAAD increased the FLC mRNA level in both Hepa1c1c7 and Raw264.7 cells. Calcium or iron chelators prevented increases in the FLC mRNA. These results provided evidence that changes in cellular Ca(2+) and iron pool by SAAD increased cellular oxidative stress and that the down-regulation of FLC protein by SAAD would further enhance prooxidant production in spite of the increase in FLC mRNA.

Mitochondrial aconitase binds to the 3' untranslated region of the mouse hepatitis virus genome

Mouse hepatitis virus (MHV), a member of the Coronaviridae, contains a polyadenylated positive-sense single-stranded genomic RNA which is 31 kb long. MHV replication and transcription take place via the synthesis of negative-strand RNA intermediates from a positive-strand genomic template. A cis-acting element previously identified in the 3' untranslated region binds to trans-acting host factors from mouse fibroblasts and forms at least three RNA-protein complexes. The largest RNA-protein complex formed by the cis-acting element and the lysate from uninfected mouse fibroblasts has a molecular weight of about 200 kDa. The complex observed in gel shift assays has been resolved by second-dimension sodium dodecyl sulfate-polyacrylamide gel electrophoresis into four proteins of approximately 90, 70, 58, and 40 kDa after RNase treatment. Specific RNA affinity chromatography also has revealed the presence of a 90-kDa protein associated with RNA containing the cis-acting element bound to magnetic beads. The 90-kDa protein has been purified from uninfected mouse fibroblast crude lysates. Protein microsequencing identified the 90-kDa protein as mitochondrial aconitase. Antibody raised against purified mitochondrial aconitase recognizes the RNA-protein complex and the 90-kDa protein, which can be released from the complex by RNase digestion. Furthermore, UV cross-linking studies indicate that highly purified mitochondrial aconitase binds specifically to the MHV 3' protein-binding element. Increasing the intracellular level of mitochondrial aconitase by iron supplementation resulted in increased RNA-binding activity in cell extracts and increased virus production as well as viral protein synthesis at early hours of infection. These results are particularly interesting in terms of identification of an RNA target for mitochondrial aconitase, which has a cytoplasmic homolog, cytoplasmic aconitase, also known as iron regulatory protein 1, a well-recognized RNA-binding protein. The binding properties of mitochondrial aconitase and the functional relevance of RNA binding appear to parallel those of cytoplasmic aconitase.

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.

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.

Effects of interferon-gamma and lipopolysaccharide on macrophage iron metabolism are mediated by nitric oxide-induced degradation of iron regulatory protein 2

Iron regulatory proteins (IRP-1 and IRP-2) control the synthesis of transferrin receptors (TfR) and ferritin by binding to iron-responsive elements, which are located in the 3'-untranslated region and the 5'-untranslated region of their respective mRNAs. Cellular iron levels affect binding of IRPs to iron-responsive elements and consequently expression of TfR and ferritin. Moreover, NO(*), a redox species of nitric oxide that interacts primarily with iron, can activate IRP-1 RNA binding activity resulting in an increase in TfR mRNA levels. Recently we found that treatment of RAW 264.7 cells (a murine macrophage cell line) with NO(+) (nitrosonium ion, which causes S-nitrosylation of thiol groups) resulted in a rapid decrease in RNA binding of IRP-2 followed by IRP-2 degradation, and these changes were associated with a decrease in TfR mRNA levels (Kim, S., and Ponka, P. (1999) J. Biol. Chem. 274, 33035-33042). In this study, we demonstrated that stimulation of RAW 264.7 cells with lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma) increased IRP-1 binding activity, whereas RNA binding of IRP-2 decreased and was followed by a degradation of this protein. Moreover, the decrease of IRP-2 binding/protein levels was associated with a decrease in TfR mRNA levels in LPS/IFN-gamma-treated cells, and these changes were prevented by inhibitors of inducible nitric oxide synthase. Furthermore, LPS/IFN-gamma-stimulated RAW 264.7 cells showed increased rates of ferritin synthesis. These results suggest that NO(+)-mediated degradation of IRP-2 plays a major role in iron metabolism during inflammation.

A developmentally regulated aconitase related to iron-regulatory protein-1 is localized in the cytoplasm and in the mitochondrion of Trypanosoma brucei

Mitochondrial energy metabolism and Krebs cycle activities are developmentally regulated in the life cycle of the protozoan parasite Trypanosoma brucei. Here we report cloning of a T. brucei aconitase gene that is closely related to mammalian iron-regulatory protein 1 (IRP-1) and plant aconitases. Kinetic analysis of purified recombinant TbACO expressed in Escherichia coli resulted in a K(m) (isocitrate) of 3 +/- 0.4 mM, similar to aconitases of other organisms. This was unexpected since an arginine conserved in the aconitase protein family and crucial for substrate positioning in the catalytic center and for activity of pig mitochondrial aconitase (Zheng, L., Kennedy, M. C., Beinert, H., and Zalkin, H. (1992) J. Biol. Chem. 267, 7895-7903) is substituted by leucine in the TbACO sequence. Expression of the 98-kDa TbACO was shown to be lowest in the slender bloodstream stage of the parasite, 8-fold elevated in the stumpy stage, and increased a further 4-fold in the procyclic stage. The differential expression of TbACO protein contrasted with only minor changes in TbACO mRNA, indicating translational or post-translational mechanisms of regulation. Whereas animal cells express two distinct compartmentalized aconitases, mitochondrial aconitase and cytoplasmic aconitase/IRP-1, TbACO accounts for total aconitase activity in trypanosomes. By cell fractionation and immunofluorescence microscopy, we show that native as well as a transfected epitope-tagged TbACO localizes in both the mitochondrion (30%) and in the cytoplasm (70%). Together with phylogenetic reconstructions of the aconitase family, this suggests that animal IRPs have evolved from a multicompartmentalized ancestral aconitase. The possible functions of a cytoplasmic aconitase in trypanosomes are discussed.

Control of transferrin receptor expression via nitric oxide-mediated modulation of iron-regulatory protein 2

Cellular iron storage and uptake are coordinately regulated post-transcriptionally by cytoplasmic factors, iron-regulatory proteins 1 and 2 (IRP-1 and IRP-2). When iron in the intracellular transit pool is scarce, IRPs bind to iron-responsive elements (IREs) in the 5'-untranslated region of the ferritin mRNA and 3'-untranslated region of the transferrin receptor (TfR) mRNA. Such binding inhibits translation of ferritin mRNA and stabilizes the mRNA for TfR, whereas the opposite scenario develops when iron in the transit pool is plentiful. However, we (Richardson, D. R., Neumannova, V., Nagy, E., and Ponka, P. (1995) Blood 86, 3211-3219) and others reported that the binding of IRPs to IREs can also be modulated by nitric oxide (NO). In this study, we showed that a short exposure of RAW 264.7 cells (a murine macrophage cell line) to the NO(+) donor, sodium nitroprusside (SNP), caused a significant decrease in IRP-2 binding to the IREs followed by IRP-2 degradation and that these changes occurred without affecting IRP-1 binding. The SNP-mediated degradation of IRP-2 in RAW 264.7 cells could be prevented by MG-132 or lactacystin, known inhibitors of proteasome-dependent protein degradation. A SNP-mediated decrease in IRP-2 binding and levels was associated with a dramatic decrease in TfR mRNA levels and an increase in ferritin synthesis. Importantly, the proteasome inhibitor MG-132 prevented the SNP-mediated decrease in TfR mRNA levels. These observations suggest that IRP-2 can play an important role in controlling transferrin receptor expression.

Regulation of genes of iron metabolism by the iron-response proteins

Iron is an essential nutrient, yet excess iron can be toxic to cells. The uptake of iron by mammalian cells is post-transcriptionally regulated by the interaction of iron-response proteins (IRP1 and IRP2) with iron-response elements (IREs) found in the mRNAs of genes of iron metabolism, such as ferritin, the transferrin receptor, erythroid aminolevulinic acid synthase, and mitochondrial aconitase. The IRPs are RNA binding proteins that bind to the IRE (found in the mRNAs of the regulated genes) in an iron- dependent manner. Binding of IRPs to the IREs leads to changes in the expression of the regulated genes and subsequent changes in the uptake, utilization, or storage of intracellular iron. Recent work has demonstrated that the binding of the IRPs to the IREs can also be modulated by changes in the redox state or oxidative stress level of the cell. These findings provide an important link between iron metabolism and states of oxidative stress.

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.

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.

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

Iron regulatory proteins (IRP1 and IRP2) are redox-sensitive RNA-binding proteins that modulate the expression of several genes encoding key proteins of iron metabolism. IRP1 can also exist as an aconitase containing a [4Fe-4S] cluster bound to three cysteines at the active site. We previously showed that biosynthesis of nitric oxide (NO) induces the transition of IRP1 from aconitase to apoprotein able to bind RNA. This switch is also observed when cytosolic extracts are exposed to NO donors. However, the activation of IRP1 under these conditions is far from maximal. In this study we examined the capacity of physiological reducing systems to cooperate with NO in the activation of IRP1. Cytosolic extracts from the macrophage cell line RAW 264.7 or purified IRP1 were incubated with NO donors and subsequently exposed to glutathione or to thioredoxin (Trx), a strong protein disulfide reductase. Trx was the most effective, inducing a 2-6-fold enhancement of the RNA binding activity of NO-treated IRP1. Furthermore, the effect of NO on IRP1 from cytosolic extracts was abolished in the presence of anti-Trx antibodies. We also studied the combined effect of NO and Trx on IRP2, which exhibits constitutive RNA binding activity. We observed an inhibition of IRP2 activity following exposure to NO donors which was restored by Trx. Collectively, these results point to a crucial role of Trx as a modulator of IRP activity in situations of NO production.

Effect of reactive oxygen species on iron regulatory protein activity

Iron may be important in catalyzing excessive production of reactive oxygen species (ROS). Cellular iron homeostasis is regulated by iron regulatory proteins (IRPs), which bind to iron-responsive elements (IRE) of mRNAs for ferritin and transferrin receptor (TfR) modulating iron uptake and sequestration, respectively. Although iron is the main regulator of IRP activity, IRP is also influenced by other factors, including the redox state. Therefore, IRP might be sensitive to pathophysiological alterations of redox state caused by ROS. However, previous studies have produced diverging evidence on the effect of oxidative injury on IRP. Results obtained in an animal model close to a pathophysiological condition, such as ischemia reperfusion of the liver as well as in a cell-free system involving an enzymatic source of O2 and H2O2, indicate that IRP is downregulated by oxidative stress. In fact, IRP activity is inhibited at early times of post-ischemic reperfusion. Moreover, the concerted action of O2 and H2O2 produced by xanthine oxidase in a cell-free system caused a remarkable inhibition of IRP activity. IRP seems a direct target of ROS; in fact, in vivo inhibition can be prevented by the antioxidant N-acetylcysteine and by interleukin-1 receptor antagonist. In addition, modulation of iron levels of the cell-free assay did not affect the downregulation imposed by xanthine oxidase. Conceivably, downregulation of IRP activity by O2 and H2O2 may facilitate iron sequestration into ferritin, thus limiting the pro-oxidant challenge of iron.

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.

Regulation of mammalian iron metabolism: current state and need for further knowledge

Due to its character as an essential element for all forms of life, the biochemistry and physiology of iron has attracted very intensive interest for many decades. In more recent years, the ways that iron metabolism is regulated in mammalian and human organisms have been clarified, and many aspects of iron metabolism have been reviewed. In this article, some newer aspects concerning absorption and intracellular regulation of iron concentration are considered. These include a sorting of possible models for intestinal iron absorption, a description of ways for membrane passage of iron after release from transferrin during receptor-mediated endocytosis, a consideration of possible mechanisms for non-transferrin bound iron uptake and its regulation, and a review of recent knowledge on the properties of iron regulatory proteins and on regulation of iron metabolism by these proteins, changes of their own properties by non-iron-mediated influences, and regulatory events not mediated by these proteins. This somewhat heterogeneous collection of themes is a consequence of the intention to avoid repetition of the many aforementioned reviews already existing and to concentrate on newer findings generated within the last couple of years.

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

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

Redox modulation of iron regulatory proteins by peroxynitrite

Expression of several proteins of higher eukaryotes is post-transcriptionally regulated by interaction of iron-responsive elements (IREs) on their mRNAs and iron regulatory proteins (IRP1 and IRP2). IRP1 is a redox-sensitive iron-sulfur protein whose regulatory activity is modulated by iron depletion, synthesis of nitric oxide, or oxidative stress. IRP2 is closely related to IRP1, but it does not possess a [Fe-S] cluster. IRP2 is also regulated by intracellular iron level, but it is assumed that regulation is achieved by accelerated turn-over. In this report, the effect of peroxynitrite, a strong oxidant produced when nitric oxide and O-2 are biosynthesized simultaneously, on the RNA binding activity of IRP1 and IRP2 was investigated in vitro. Macrophage cytosolic extracts were exposed directly to a bolus addition of peroxynitrite or to SIN-1, which releases a continuous flux of peroxynitrite. Under these two experimental conditions, IRP1 lost its aconitase activity but did not gain increased capacity to bind IRE. However, addition of low amounts of the disulfide-reducing agent 2-ME during the binding assay revealed formation of a complex between IRP1 and IRE. Substrates of aconitase, which bind to the cluster of IRP1, prevented this effect, pointing to the [Fe-S] cluster as the target of peroxynitrite. Moreover, single mutation of the redox active Cys437 precluded oxidation of human recombinant IRP1 by SIN-1. Collectively, these results imply that peroxynitrite predisposes IRP1 to bind IREs under a suitable reducing environment. It is assumed that in addition to disrupting the cluster peroxynitrite also promotes disulfide bridge(s) between proximal cysteine residues in the vicinity of the IRE-binding domain, in particular Cys437. When exposed to peroxynitrite, IRP2 lost its spontaneous IRE binding activity, which was restored by further exposure to 2-mercaptoethanol, thus showing that peroxynitrite can also regulate IRP2 by a post-translational event.

The iron-sulfur cluster of iron regulatory protein 1 modulates the accessibility of RNA binding and phosphorylation sites

Iron regulatory protein 1 (IRP1) modulates iron metabolism by binding to mRNAs encoding proteins involved in the uptake, storage, and metabolic utilization of iron. Iron regulates IRP1 function by promoting assembly of an iron-sulfur cluster in the apo or RNA binding form, thereby converting it to the active holo or cytoplasmic aconitase form. In continuing our studies on phosphoregulation of IRP1 by protein kinase C (PKC), we noted that the purified apoprotein was more efficiently phosphorylated than was the form partially purified from liver cytosol by chromatography on DEAE-Sepharose which had characteristics of the [3Fe-4S] form of the protein. RNA binding measurements revealed a 20-fold increase in RNA binding affinity and a 4-5-fold higher rate of phosphorylation after removal of the Fe-S cluster from the highly purified [4Fe-4S] form. Phosphorylation of apo-IRP1 by PKC was specifically inhibited by IRE-containing RNA. The RNA binding form had a more open structure as judged by its much greater sensitivity to limited cleavage by a number of proteases. N-Terminal sequencing of chymotryptic peptides of apo-IRP1 demonstrated an increased accessibility to proteolysis of sites (residues 132 and 504) near or within the putative cleft of the protein, including regions that are thought to be involved in RNA binding (residues 116-151) and phosphoregulation (Ser 138). Enhanced cleavage was also observed in the proposed hinge linker region (residue 623) on the surface of the protein opposite from the cleft. Taken together, our results indicate that significant structural changes occur in IRP1 during cluster insertion or removal that affect the accessibility to RNA binding and phosphorylation sites.

Interplay between NO and [Fe-S] clusters: relevance to biological systems

In mammalian cells, nitric oxide (NO) synthesis results in the inactivation of several mitochondrial iron-sulfur enzymes involved in ATP synthesis that correlates with the appearance of complexes of the [(NO)2Fe(SR)2] type detectable by electron paramagnetic resonance spectroscopy. More specifically, the activity of two [Fe-S] enzymes was followed during the course of NO synthase expression:mitochondrial aconitase, which catalyzes citrate:isocitrate conversion in the Krebs cycle, and cytoplasmic aconitase, or iron regulatory protein (IRP), a trans-regulator that controls expression at the posttranscriptional level of proteins involved in iron metabolism. In response to physiological stimuli, the synthesis of NO leads to inhibition of enzymatic activities of both mitochondrial and cytoplasmic aconitases, whereas the RNA binding activity of IRP is increased. Coordination of the diffusible gas NO with [Fe-S] clusters is thought to result in impairment of metabolic functions. Here it is proposed that the interplay between NO (or some NO-derived molecule) and [Fe-S] clusters at critical catalytic or allosteric sites is crucial in the response to environmental signals within cells.

Interaction between iron-regulatory proteins and their RNA target sequences, iron-responsive elements

In this chapter, we have focused on the biochemistry of IRP-1 and the features which distinguish it from the related RNA-binding protein, IRP-2. IRP-1 is the cytoplasmic isoform of the enzyme aconitase, and, depending on iron status, may switch between enzymatic and RNA-binding activities. IRP-1 and IRP-2 are trans-acting regulators of mRNAs involved in iron uptake, storage and utilisation. The finding of an IRE in the citric acid cycle enzymes, mitochondrial aconitase and succinate dehydrogenase, suggests that the IRPs may also influence cellular energy production. These two proteins appear to bind RNAs with different but overlapping specificity, suggesting that they may regulate the stability or translation of as yet undefined mRNA targets, possibly extending their regulatory function beyond that of iron homeostasis. The interaction between the IRPs and the IRE represents one of the best characterised model systems for posttranscriptional gene control, and given that each IRP can also recognise its own unique set of RNAs, the search for new in vivo mRNA targets is expected to provide yet more surprises and insights into the fate of cytoplasmic mRNAs.

Regulation of iron metabolism in eukaryotes

Iron metabolism is regulated in cells to ensure that iron supplies are adequate and nontoxic. The expression of iron metabolism is regulated primarily by posttranscriptional mechanisms. Ferritin, eALAS, SDHb of Drosophila, and mammalian mitochondrial aconitase are translationally regulated. The TfR is regulated at the level of mRNA stability. Iron regulatory proteins are regulated either by assembly or by disassembly of an iron-sulfur cluster (IRP1) or by rapid degradation in the presence of iron (IRP2). The list of targets for IRP-mediated regulation is growing longer, and a range of possibilities for versatile regulation exists, as each IRP can bind to unique targets that differ from the consensus IRE. The reactivity of iron with oxygen and the creation of toxic by-products may be the evolutionary stimulus that produced this system of tight posttranscriptional gene regulation.

Expression and biochemical characterization of iron regulatory proteins 1 and 2 in Saccharomyces cerevisiae

Iron-regulatory proteins (IRPs) 1 and 2 are cytosolic RNA-binding proteins that bind to specific stem-loop structures, termed iron-responsive elements (IREs) that are located in the untranslated regions of specific mRNAs encoding proteins involved in iron metabolism. The binding of IRPs to IREs regulates either translation or stabilization of mRNA. Although IRP1 and IRP2 are similar proteins in that they are ubiquitously expressed and are negatively regulated by iron, they are regulated by iron by different mechanisms. IRP1, the well-characterized IRP in cells, is a dual-function protein exhibiting either aconitase activity when cellular iron is abundant or RNA-binding activity when cellular iron is scarce. In contrast, IRP2 lacks detectable aconitase activity and functions exclusively as an RNA-binding protein. To study and compare the biochemical characteristics of IRP1 and IRP2, we expressed wild-type and mutant rat IRP1 and IRP2 in the yeast Saccharomyces cerevisiae. IRP1 and IRP2 expressed in yeast bind the IRE RNA with high affinity, resulting in the inhibition of translation of an IRE-reporter mRNA. Mutant IRP2s lacking a 73 amino acid domain unique to IRP2 and a mutant IRP1 containing an insertion of this domain bound RNA, but lacked detectable aconitase activity, suggesting that the presence of this domain prevents aconitase activity. Like IRP1, the RNA-binding activity of IRP2 was sensitive to inactivation by N-ethylmaleimide (NEM) or 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB), indicating IRP2 contains a cysteine(s) that is (are) necessary for RNA binding. However, unlike IRP1, where reconstitution of the 4Fe-4S cluster resulted in a loss in RNA-binding activity, the RNA-binding activity of IRP2 was unaffected using the same iron treatment. These data suggested that IRP2 does not contain a 4Fe-4S cluster similar to the cluster in IRP1, indicating that they sense iron by different mechanisms.

Iron regulatory proteins 1 and 2

Iron uptake and storage in mammalian cells is at least partly regulated at a post-transcriptional level by the iron regulatory proteins (IRP-1 and IRP-2). These cytoplasmic regulators share 79% similarity in protein sequence and bind tightly to conserved mRNA stem-loops, named iron-responsive elements (IREs). The IRP:IRE interaction underlies the regulation of translation and stability of several mRNAs central to iron metabolism. The question of why the cell requires two such closely related regulatory proteins may be resolved as we learn more about the expression and regulation of these proteins. It is evident so far that, despite similarities, the IRPs differ in several important respects. They are coordinately regulated by cellular iron, but whereas IRP-1 is inactivated by high iron levels, IRP-2 is rapidly degraded. Further differences arise in their expression and RNA-binding specificity. The two proteins each recognise a large repertoire of IRE-like sequences, including a small group of exclusive RNA targets. These findings hint that IRP-1 and IRP-2 may bind preferentially to certain mRNAs in vivo, possibly extending their known functions beyond the regulation of intracellular iron homeostasis.

The ferritins: molecular properties, iron storage function and cellular regulation

The iron storage protein, ferritin, plays a key role in iron metabolism. Its ability to sequester the element gives ferritin the dual functions of iron detoxification and iron reserve. The importance of these functions is emphasised by ferritin's ubiquitous distribution among living species. Ferritin's three-dimensional structure is highly conserved. All ferritins have 24 protein subunits arranged in 432 symmetry to give a hollow shell with an 80 A diameter cavity capable of storing up to 4500 Fe(III) atoms as an inorganic complex. Subunits are folded as 4-helix bundles each having a fifth short helix at roughly 60 degrees to the bundle axis. Structural features of ferritins from humans, horse, bullfrog and bacteria are described: all have essentially the same architecture in spite of large variations in primary structure (amino acid sequence identities can be as low as 14%) and the presence in some bacterial ferritins of haem groups. Ferritin molecules isolated from vertebrates are composed of two types of subunit (H and L), whereas those from plants and bacteria contain only H-type chains, where 'H-type' is associated with the presence of centres catalysing the oxidation of two Fe(II) atoms. The similarity between the dinuclear iron centres of ferritin H-chains and those of ribonucleotide reductase and other proteins suggests a possible wider evolutionary linkage. A great deal of research effort is now concentrated on two aspects of ferritin: its functional mechanisms and its regulation. These form the major part of the review. Steps in iron storage within ferritin molecules consist of Fe(II) oxidation, Fe(III) migration and the nucleation and growth of the iron core mineral. H-chains are important for Fe(II) oxidation and L-chains assist in core formation. Iron mobilisation, relevant to ferritin's role as iron reserve, is also discussed. Translational regulation of mammalian ferritin synthesis in response to iron and the apparent links between iron and citrate metabolism through a single molecule with dual function are described. The molecule, when binding a [4Fe-4S] cluster, is a functioning (cytoplasmic) aconitase. When cellular iron is low, loss of the [4Fe-4S] cluster allows the molecule to bind to the 5'-untranslated region (5'-UTR) of the ferritin m-RNA and thus to repress translation. In this form it is known as the iron regulatory protein (IRP) and the stem-loop RNA structure to which it binds is the iron regulatory element (IRE). IREs are found in the 3'-UTR of the transferrin receptor and in the 5'-UTR of erythroid aminolaevulinic acid synthase, enabling tight co-ordination between cellular iron uptake and the synthesis of ferritin and haem. Degradation of ferritin could potentially lead to an increase in toxicity due to uncontrolled release of iron. Degradation within membrane-encapsulated "secondary lysosomes' may avoid this problem and this seems to be the origin of another form of storage iron known as haemosiderin. However, in certain pathological states, massive deposits of "haemosiderin' are found which do not arise directly from ferritin breakdown. Understanding the numerous inter-relationships between the various intracellular iron complexes presents a major challenge.