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

Methods to identify and characterize iron-sulfur oligopeptides in water

Iron-sulfur clusters are ubiquitous cofactors that mediate central biological processes. However, despite their long history, these metallocofactors remain challenging to investigate when coordinated to small (≤ six amino acids) oligopeptides in aqueous solution. In addition to being often unstable in vitro, iron-sulfur clusters can be found in a wide variety of forms with varied characteristics, which makes it difficult to easily discern what is in solution. This difficulty is compounded by the dynamics of iron-sulfur peptides, which frequently coordinate multiple types of clusters simultaneously. To aid investigations of such complex samples, a summary of data from multiple techniques used to characterize both iron-sulfur proteins and peptides is provided. Although not all spectroscopic techniques are equally insightful, it is possible to use several, readily available methods to gain insight into the complex composition of aqueous solutions of iron-sulfur peptides.

FBXL5 Regulates IRP2 Stability in Iron Homeostasis via an Oxygen-Responsive [2Fe2S] Cluster

Cellular iron homeostasis is dominated by FBXL5-mediated degradation of iron regulatory protein 2 (IRP2), which is dependent on both iron and oxygen. However, how the physical interaction between FBXL5 and IRP2 is regulated remains elusive. Here, we show that the C-terminal substrate-binding domain of FBXL5 harbors a [2Fe2S] cluster in the oxidized state. A cryoelectron microscopy (cryo-EM) structure of the IRP2-FBXL5-SKP1 complex reveals that the cluster organizes the FBXL5 C-terminal loop responsible for recruiting IRP2. Interestingly, IRP2 binding to FBXL5 hinges on the oxidized state of the [2Fe2S] cluster maintained by ambient oxygen, which could explain hypoxia-induced IRP2 stabilization. Steric incompatibility also allows FBXL5 to physically dislodge IRP2 from iron-responsive element RNA to facilitate its turnover. Taken together, our studies have identified an iron-sulfur cluster within FBXL5, which promotes IRP2 polyubiquitination and degradation in response to both iron and oxygen concentrations.

Direct Fe2+ sensing by iron-responsive messenger RNA:repressor complexes weakens binding

Fe(2+) is now shown to weaken binding between ferritin and mitochondrial aconitase messenger RNA noncoding regulatory structures ((iron-responsive element) (IRE)-RNAs) and the regulatory proteins (IRPs), which adds a direct role of iron to regulation that can complement the well known regulatory protein modification and degradative pathways related to iron-induced mRNA translation. We observe that the K(d) value increases 17-fold in 5'-untranslated region IRE-RNA:repressor complexes; Fe(2+), is studied in the absence of O(2). Other metal ions, Mn(2+) and Mg(2+) have similar effects to Fe(2+) but the required Mg(2+) concentration is 100 times greater than for Fe(2+) or Mn(2+). Metal ions also weaken ethidium bromide binding to IRE-RNA with no effect on IRP fluorescence, using Mn(2+) as an O(2)-resistant surrogate for Fe(2+), indicating that metal ions bound IRE-RNA but not IRP: Fe(2+) decreases IRP repressor complex stability of ferritin IRE-RNA 5-10 times compared with 2-5 times for mitochondrial aconitase IRE-RNA, over the same concentration range, suggesting that differences among IRE-RNA structures contribute to the differences in the iron responses observed in vivo. The results show the IRE-RNA:repressor complex literally responds to Fe(2+), selectively for each IRE-mRNA.

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-independent phosphorylation of iron regulatory protein 2 regulates ferritin during the cell cycle

Iron regulatory protein 2 (IRP2) is a key iron sensor that post-transcriptionally regulates mammalian iron homeostasis by binding to iron-responsive elements (IREs) in mRNAs that encode proteins involved in iron metabolism (e.g. ferritin and transferrin receptor 1). During iron deficiency, IRP2 binds IREs to regulate mRNA translation or stability, whereas during iron sufficiency IRP2 is degraded by the proteasome. Here, we identify an iron-independent IRP2 phosphorylation site that is regulated by the cell cycle. IRP2 Ser-157 is phosphorylated by Cdk1/cyclin B1 during G(2)/M and is dephosphorylated during mitotic exit by the phosphatase Cdc14A. Ser-157 phosphorylation during G(2)/M reduces IRP2 RNA-binding activity and increases ferritin synthesis, whereas Ser-157 dephosphorylation during mitotic exit restores IRP2 RNA-binding activity and represses ferritin synthesis. These data show that reversible phosphorylation of IRP2 during G(2)/M has a role in modulating the iron-independent expression of ferritin and other IRE-containing mRNAs during the cell cycle.

Molecular control of vertebrate iron homeostasis by iron regulatory proteins

Both deficiencies and excesses of iron represent major public health problems throughout the world. Understanding the cellular and organismal processes controlling iron homeostasis is critical for identifying iron-related diseases and in advancing the clinical treatments for such disorders of iron metabolism. Iron regulatory proteins (IRPs) 1 and 2 are key regulators of vertebrate iron metabolism. These RNA binding proteins post-transcriptionally control the stability or translation of mRNAs encoding proteins involved in iron homeostasis thereby controlling the uptake, utilization, storage or export of iron. Recent evidence provides insight into how IRPs selectively control the translation or stability of target mRNAs, how IRP RNA binding activity is controlled by iron-dependent and iron-independent effectors, and the pathological consequences of dysregulation of the IRP system.

A high-capacity RNA affinity column for the purification of human IRP1 and IRP2 overexpressed in Pichia pastoris

Regulated expression of proteins involved in mammalian iron metabolism is achieved in part through the interaction of the iron regulatory proteins IRP1 and IRP2 with highly conserved RNA stem-loop structures, known as iron-responsive elements (IREs), that are located within the 5' or 3' untranslated regions of regulated transcripts. As part of an effort to determine the structures of the IRP-IRE complexes using crystallographic methods, we have developed an efficient process for obtaining functionally pure IRP1 and IRP2 that relies upon the improved overexpression (>10 mg of soluble IRP per liter of culture) of each human IRP in the yeast Pichia pastoris and large-scale purification using RNA affinity chromatography. Despite the utility of RNA affinity chromatography in the isolation of RNA-binding proteins, current methods for preparing RNA affinity matrices produce columns of low capacity and limited stability. To address these limitations, we have devised a simple method for preparing stable, reusable, high-capacity RNA affinity columns. This method utilizes a bifunctional linker to covalently join a 5'-amino tethered RNA with a thiol-modified Sepharose, and can be used to load 150 nmole or more of RNA per milliliter of solid support. We demonstrate here the use of an IRE affinity column in the large-scale purification of IRP1 and IRP2, and suggest that the convenience of this approach will prove attractive in the analysis of other RNA-binding proteins.

Iron metabolism in mammalian cells

Most living things require iron to exist. Iron has many functions within cells but is rarely found unbound because of its propensity to catalyze the formation of toxic free radicals. Thus the regulation of iron requirements by cells and the acquisition and uptake of iron into tissues in multicellular organisms is tightly regulated. In humans, understanding iron transport and utility has recently been advanced by a "great conjunction" of molecular genetics in simple organisms, identifying genes involved in genetic diseases of metal metabolism and by the application of traditional cell physiology approaches. We are now able to approach a rudimentary understanding of the "iron cycle" within mammals. In the future, this information will be applied toward modulating the outcome of therapies designed to overcome diseases involving metals.

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.

An optical method for the detection of oxidative stress using protein-RNA interaction

The cytosolic 4Fe-4S protein aconitase can be converted under the influence of reactive oxygen species into an iron-regulatory protein (IRP1). Therefore, the IRP1 level is considered as an indirect marker of oxidative stress. An experimental approach is presented here to detect the concentration of this marker protein by surface plasmon resonance. The optical method exploits the natural binding affinity of IRP1 to an iron-responsive element (IRE) which was in vitro transcribed with a linker sequence and subsequently immobilized on a BIACORE sensor chip. The detection was found to be reproducible and sensitive in the range 20-200 nM IRP. Conditions of the binding process, such as pH and thiol concentration, were characterized. Feasibility of the method to detect and quantify IRP1 in physiological media was demonstrated.

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.

The roles of iron in health and disease

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

Role of Saccharomyces cerevisiae ISA1 and ISA2 in iron homeostasis

The budding yeast Saccharomyces cerevisiae contains two homologues of bacterial IscA proteins, designated Isa1p and Isa2p. Bacterial IscA is a product of the isc (iron-sulfur cluster) operon and has been suggested to participate in Fe-S cluster formation or repair. To test the function of yeast Isa1p and Isa2p, single or combinatorial disruptions were introduced in ISA1 and ISA2. The resultant isaDelta mutants were viable but exhibited a dependency on lysine and glutamate for growth and a respiratory deficiency due to an accumulation of mutations in mitochondrial DNA. As with other yeast genes proposed to function in Fe-S cluster assembly, mitochondrial iron concentration was significantly elevated in the isa mutants, and the activities of the Fe-S cluster-containing enzymes aconitase and succinate dehydrogenase were dramatically reduced. An inspection of Isa-like proteins from bacteria to mammals revealed three invariant cysteine residues, which in the case of Isa1p and Isa2p are essential for function and may be involved in iron binding. As predicted, Isa1p is targeted to the mitochondrial matrix. However, Isa2p is present within the intermembrane space of the mitochondria. Our deletion analyses revealed that Isa2p harbors a bipartite N-terminal leader sequence containing a mitochondrial import signal linked to a second sequence that targets Isa2p to the intermembrane space. Both signals are needed for Isa2p function. A model for the nonredundant roles of Isa1p and Isa2p in delivering iron to sites of the Fe-S cluster assembly is discussed.

Ironing-out mechanisms of neuronal injury under hypoxic-ischemic conditions and potential role of iron chelators as neuroprotective agents

Iron is the most abundant transition metal in the brain, where it functions as an important cofactor in a host of vital metabolic processes and plays an absolutely essential role in cell viability. Free iron is also very toxic when present in high concentrations, thus placing this essential metal at the core of neurotoxic injury in a number of neurological disorders. The pivotal role of iron in cellular homeostasis, including its latent toxicity, necessitates a tight regulation of iron metabolism. Oxygen and iron appear to play an important role in iron homeostasis. They appear to exert their homeostatic role by modulating the proteins involved in a complex interplay between iron sensing, transport, and storage. These key regulatory proteins include ferritin (intracellular storage), transferrin (extracellular transport), transferrin receptor, and iron regulatory protein (sensor of intracellular iron concentration). The interplay of iron and oxygen is most intriguing in the setting of stroke, where hypoxia and free iron appear to interact in causing the subsequent neuronal death.

Regulation of Iron Metabolism in Murine J774 Macrophages: Role of Nitric Oxide–Dependent and –Independent Pathways Following Activation With Gamma Interferon and Lipopolysaccharide

To elucidate the pathways by which nitric oxide (NO) influences macrophage iron metabolism, the uptake, release, and intracellular distribution of iron in the murine macrophage cell line J774 has been investigated, together with transferrin receptor (TfR) expression and iron-regulatory protein (IRP1 and IRP2) activity. Stimulation of macrophages with interferon-γ (IFN-γ) and/or lipopolysaccharide (LPS) decreased Fe uptake from transferrin (Tf), and there was a concomitant downregulation of TfR expression. These effects were mediated by NO-dependent and NO-independent mechanisms. Addition of the NO synthase (NOS) inhibitor N-monomethyl arginine (NMMA) partially restored Fe uptake but either had no effect on or downregulated TfR expression, which suggests that NO by itself is able to affect iron availability. Analysis of the intracellular distribution of incorporated iron revealed that in IFN-γ/LPS-activated macrophages there was a decreased amount and proportion of ferritin-bound iron and a compensatory increase in insoluble iron, which probably consists mainly of iron bound to intracellular organelles. Finally, although NO released by IFN-γ/LPS-activated macrophages increased the iron-responsive element (IRE)-binding activity of both IRP1 and IRP2, IFN-γ treatment decreased IRP2 activity in an NO-independent manner. This study demonstrates that the effect of IFN-γ and/or LPS on macrophage iron metabolism is complex, and is not entirely due to either NO-or to IRP-mediated mechanisms. The overall effect is to decrease iron uptake, but not its utilization.

Human cytoplasmic aconitase (Iron regulatory protein 1) is converted into its [3Fe-4S] form by hydrogen peroxide in vitro but is not activated for iron-responsive element binding

Iron regulatory protein 1 (IRP1) regulates the synthesis of proteins involved in iron homeostasis by binding to iron-responsive elements (IREs) of messenger RNA. IRP1 is a cytoplasmic aconitase when it contains a [4Fe-4S] cluster and an RNA-binding protein after complete removal of the metal center by an unknown mechanism. Human IRP1, obtained as the pure recombinant [4Fe-4S] form, is an enzyme as efficient toward cis-aconitate as the homologous mitochondrial aconitase. The aconitase activity of IRP1 is rapidly lost by reaction with hydrogen peroxide as the [4Fe-4S] cluster is quantitatively converted into the [3Fe-4S] form with release of a single ferrous ion per molecule. The IRE binding capacity of IRP1 is not elicited with H(2)O(2). Ferrous sulfate (but not other more tightly coordinated ferrous ions, such as the complex with ethylenediamine tetraacetic acid) counteracts the inhibitory action of hydrogen peroxide on cytoplasmic aconitase, probably by replenishing iron at the active site. These results cast doubt on the ability of reactive oxygen species to directly increase IRP1 binding to IRE and support a signaling role for hydrogen peroxide in the posttranscriptional control of proteins involved in iron homeostasis in vivo.

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.

Hypoxia post-translationally activates iron-regulatory protein 2

Iron-regulatory proteins 1 and 2 (IRP1 and IRP2) are RNA-binding proteins that post-transcriptionally regulate the expression of mRNAs that code for proteins involved in the maintenance of iron and energy homeostasis. Here we show that hypoxia differentially regulates the RNA binding activities of IRP1 and IRP2 in human 293 and in mouse Hepa-1 cells. In contrast to IRP1, where hypoxic exposure decreases IRP1 RNA binding activity, hypoxia increases IRP2 RNA binding activity. The hypoxic increase in IRP2 RNA binding activity results from increased IRP2 protein levels. Cobalt, which mimics hypoxia by activation of hypoxia-inducible factor 1 (HIF-1), also increases IRP2 protein levels; however, cobalt-induced IRP2 lacks RNA binding activity. Addition of a reductant to cobalt-treated extracts restored IRP2 RNA binding activity. Hypoxic activation of IRP2 is not because of an increase in transcriptional activation by HIF-1, because IRP2 accumulates in Hepa-1 cells lacking a functional HIF-1beta subunit, nor is it because of an increase in IRP2 mRNA stability. Rather, our data indicate that hypoxia increases IRP2 levels by a post-translational mechanism involving protein stability. Differential regulation of IRP1 and IRP2 during hypoxia may regulate specific IRP target mRNAs whose expression is required for hypoxic adaptation. Furthermore, these data imply mechanistic parallels between the hypoxia-induced post-transcriptional regulation of IRP2 and HIF-1alpha.

Preferential activation of iron regulatory protein-2 in cell lines as a result of higher sensitivity to iron

Iron regulatory proteins (IRP)-1 and 2 are cytoplasmic mRNA-binding proteins that control intracellular iron homeostasis by regulating the translation of ferritin mRNA and stability of transferrin receptor mRNA in an iron-dependent fashion. Although structurally and functionally similar, the two IRP are different in their mode of regulation, pattern of tissue expression and modulation by multiple factors, such as bioradicals. In the present study RNA bandshift assays demonstrated that IRP-2, but not IRP-1, activity was higher in cultured cells than in tissues. Increased expression of IRP-2 in cell lines was not related to immortalization and differentiation but seemed associated to cell proliferation, although not closely dependent on cell growth rate. As a growing cell consumes more iron than its quiescent counterpart, we assessed the iron status of cell lines and found that ferritin content was lower than in tissues. Analysis of IRP activity in cell lines supplemented with heme or non-heme iron and in livers of iron-loaded and iron-deficient rats indicated that IRP-2 responds more promptly than IRP-1 to modulations of iron content. We propose that enhanced IRP-2 activity in cultured cells could be due to a proliferation-dependent, relative iron deficiency that is sensed first by IRP-2.

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.

Loops and bulge/loops in iron-responsive element isoforms influence iron regulatory protein binding. Fine-tuning of mRNA regulation?

A family of noncoding mRNA sequences, iron-responsive elements (IREs), coordinately regulate several mRNAs through binding a family of mRNA-specific proteins, iron regulatory proteins (IRPs). IREs are hairpins with a constant terminal loop and base-paired stems interrupted by an internal loop/bulge (in ferritin mRNA) or a C-bulge (in m-aconitase, erythroid aminolevulinate synthase, and transferrin receptor mRNAs). IRP2 binding requires the conserved C-G base pair in the terminal loop, whereas IRP1 binding occurs with the C-G or engineered U-A. Here we show the contribution of the IRE internal loop/bulge to IRP2 binding by comparing natural and engineered IRE variants. Conversion of the internal loop/bulge in the ferritin-IRE to a C-bulge, by deletion of U, decreased IRP2 binding by >95%, whereas IRP1 binding changed only 13%. Moreover, IRP2 binding to natural IREs with the C-bulge was similar to the DeltaU6 ferritin-IRE: >90% lower than the ferritin-IRE. The results predict mRNA-specific variation in IRE-dependent regulation in vivo and may relate to previously observed differences in iron-induced ferritin and m-aconitase synthesis in liver and cultured cells. Variations in IRE structure and cellular IRP1/IRP2 ratios can provide a range of finely tuned, mRNA-specific responses to the same (iron) signal.

Involvement of heme in the degradation of iron-regulatory protein 2

Iron-regulatory proteins (IRPs) recognize and bind to specific RNA structures called iron-responsive elements. Mediation of these binding interactions by iron and iron-containing compounds regulates several post-transcriptional events relevant to iron metabolism. There are two known IRPs, IRP1 and IRP2, both of which can respond to iron fluxes in the cell. There is ample evidence that IRP1 is converted by iron to cytoplasmic aconitase in vivo. It has also been shown that, under certain conditions, a significant fraction of IRP1 is degraded in cells exposed to iron or heme. Studies have shown that the degradation of IRP1 that is induced by iron can be inhibited by either desferrioxamine mesylate (an iron chelator) or succinyl acetone (an inhibitor of heme synthesis), whereas the degradation induced by heme cannot. This suggests that heme rather than iron is responsible for this degradation. Several laboratories have shown that IRP2 is also degraded in cells treated with iron salts. We now show evidence suggesting that this IRP2 degradation may be mediated by heme. Thus, in experiments analogous to those used previously to study IRP1, we find that IRP2 is degraded in rabbit fibroblast cells exposed to heme or iron salts. However, as shown earlier with IRP1, both desferrioxamine mesylate and succinyl acetone will inhibit the degradation of IRP2 induced by iron but not that induced by heme.

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.

Converse modulation of IRP1 and IRP2 by immunological stimuli in murine RAW 264.7 macrophages

Iron regulatory proteins (IRP1 and IRP2) are two cytoplasmic RNA-binding proteins that control iron metabolism in mammalian cells. Both IRPs bind to specific sequences called iron-responsive elements (IREs) located in the 3' or 5' untranslated regions of several mRNAs, in particular mRNA encoding ferritin and transferrin receptor. In this study, we followed in parallel the in vivo regulation of the two IRPs in physiologically stimulated macrophages. We show that stimulation of mouse RAW 264.7 macrophage-like cells increased IRP1 IRE binding activity 4-fold, whereas IRP2 activity decreased 2-fold 8 h after interferon-gamma/lipopolysaccharide treatment. Decrease in IRP2 was not due to nitric oxide (NO) production and did not require de novo protein synthesis. Our data therefore indicate that the two IRPs can be conversely regulated in response to the same stimulus. In addition, the effect of endogenously produced NO on IRP1 was further characterized in an activated macrophage/target cell system. We show that NO acts as an intercellular signal to increase IRP1 activity in adjacent cells. As the effect was detectable within 1 h and did not require de novo protein synthesis, this result supports a direct action of NO on IRP1.

Regulation of iron regulatory protein 1 during hypoxia and hypoxia/reoxygenation

Given the important relationship between O2 and iron (Fenton chemistry) a study was undertaken to characterize the effects of hypoxia, as well as subsequent reoxygenation, on the iron-regulatory proteins 1 and 2 (IRP1 and IRP2) in a rat hepatoma cell line. IRP1 and IRP2 are cytosolic RNA-binding proteins that bind RNA stem-loops located in the 5'- or 3'-untranslated regions of specific mRNAs encoding proteins that are involved in iron homeostasis. In cells exposed to hypoxia, IRP1 RNA binding was decreased approximately 2. 8-fold after a 6-h exposure to 3% O2. Hypoxic inactivation of IRP1 was abolished when cells were pretreated with the iron chelator desferrioxamine, indicating a role for iron in inactivation. IRP1 inactivation was reversible since re-exposure of hypoxically-treated cells to 21% O2 increased RNA binding activity approximately 7-fold after 21 h with an increase in activity seen as early as 1-h post-reoxygenation. IRP1 protein levels were unaffected during hypoxia as well as during reoxygenation. Whereas the protein synthesis inhibitor cycloheximide did not block IRP1 inactivation during hypoxia, it completely blocked IRP1 reactivation during subsequent reoxygenation. Reactivation of IRP1 during reoxygenation was also partially blocked by the phosphatase inhibitor okadaic acid. Finally, reactivated IRP1 was found to be resistant to inactivation by exogenous iron known to down-regulate its activity during normoxia. These data demonstrate that IRP1 RNA binding activity is post-translationally regulated during hypoxia and hypoxia/reoxygenation. Regulation of IRP1 by changing oxygen tension may provide a novel mechanism for post-transcriptionally regulating gene expression under these stresses.

Cadmium and calcium-dependent c-fos expression in mesangial cells

Cadmium is a carcinogenic metal known to increase the expression of several protooncogenes in a variety of cells. although the underlying mechanisms are unknown. Renal mesangial cells are smooth muscle cells in which Ca2+ signaling pathways regulate the induction of c-fos through both cAMP-dependent and mitogen-activated protein kinase- (MAPK-) dependent pathways. We report that c-fos is induced in these cells by both protein kinase C- (PKC-) dependent (phorbol ester, platelet-derived growth factor), and independent (serum, ionomycin) mechanisms. In all cases, prevention of an increase in cytosolic [Ca2+] with the chelator BAPTA prevented this induction. CdCl2 (10 microM) caused an accumulation of c-fos mRNA over 30 min that was sustained for at least 8 h. Cycloheximide inhibits turnover of c-fos mRNA and shows a synergistic effect with Cd2+ on transcript levels. Together with a similar half life of the transcript whether accumulated in response Cd2+ or induced by phorbol ester, this suggests induction of c-fos by Cd2+ rather than an effect of Cd2+ on transcript stability. Cadmium increased MAPK activity by 5 min; this was sustained for at least 8 h, consistent with the time course of c-fos mRNA accumulation. The MAPK kinase inhibitor PD98059 caused a marked decrease in the induction of c-fos by Cd2+, but did not eliminate the phenomenon completely. Although Cd2+ has been reported to activate PKC in vitro, no effect was found on PKC activity in Cd2+ -treated cells, indicating the activation of MAPK by Cd2+ is through an unidentified PKC-independent pathway. We conclude that Cd2+ can cause a sustained induction of c-fos in part through sustained activation of MAPK, that contrasts with the transient activation of these species in response to physiological mitogenic stimuli.

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.

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.