Biogenesis and functions of mammalian iron-sulfur proteins in the regulation of iron homeostasis and pivotal metabolic pathways.

Fe-S cofactors are composed of iron and inorganic sulfur in various stoichiometries. A complex assembly pathway conducts their initial synthesis and subsequent binding to recipient proteins. In this minireview, we discuss how discovery of the role of the mammalian cytosolic aconitase, known as iron regulatory protein 1 (IRP1), led to the characterization of the function of its Fe-S cluster in sensing and regulating cellular iron homeostasis. Moreover, we present an overview of recent studies that have provided insights into the mechanism of Fe-S cluster transfer to recipient Fe-S proteins.

Aspirin down Regulates Hepcidin by Inhibiting NF-κB and IL6/JAK2/STAT3 Pathways in BV-2 Microglial Cells Treated with Lipopolysaccharide.

Aspirin down regulates transferrin receptor 1 (TfR1) and up regulates ferroportin 1 (Fpn1) and ferritin expression in BV-2 microglial cells treated without lipopolysaccharides (LPS), as well as down regulates hepcidin and interleukin 6 (IL-6) in cells treated with LPS. However, the relevant mechanisms are unknown. Here, we investigate the effects of aspirin on expression of hepcidin and iron regulatory protein 1 (IRP1), phosphorylation of Janus kinase 2 (JAK2), signal transducer and activator of transcription 3 (STAT3) and P65 (nuclear factor-κB), and the production of nitric oxide (NO) in BV-2 microglial cells treated with and without LPS. We demonstrated that aspirin inhibited hepcidin mRNA as well as NO production in cells treated with LPS, but not in cells without LPS, suppresses IL-6, JAK2, STAT3, and P65 (nuclear factor-κB) phosphorylation and has no effect on IRP1 in cells treated with or without LPS. These findings provide evidence that aspirin down regulates hepcidin by inhibiting IL6/JAK2/STAT3 and P65 (nuclear factor-κB) pathways in the cells under inflammatory conditions, and imply that an aspirin-induced reduction in TfR1 and an increase in ferritin are not associated with IRP1 and NO.

SIRT3 regulates cellular iron metabolism and cancer growth by repressing iron regulatory protein 1.

Iron metabolism is essential for many cellular processes, including oxygen transport, respiration and DNA synthesis, and many cancer cells exhibit dysregulation in iron metabolism. Maintenance of cellular iron homeostasis is regulated by iron regulatory proteins (IRPs), which control the expression of iron-related genes by binding iron-responsive elements (IREs) of target mRNAs. Here, we report that mitochondrial SIRT3 regulates cellular iron metabolism by modulating IRP1 activity. SIRT3 loss increases reactive oxygen species production, leading to elevated IRP1 binding to IREs. As a consequence, IRP1 target genes, such as the transferrin receptor (TfR1), a membrane-associated glycoprotein critical for iron uptake and cell proliferation, are controlled by SIRT3. Importantly, SIRT3 deficiency results in a defect in cellular iron homeostasis. SIRT3 null cells contain high levels of iron and lose iron-dependent TfR1 regulation. Moreover, SIRT3 null mice exhibit higher levels of iron and TfR1 expression in the pancreas. We found that the regulation of iron uptake and TfR1 expression contribute to the tumor-suppressive activity of SIRT3. Indeed, SIRT3 expression is negatively correlated with TfR1 expression in human pancreatic cancers. SIRT3 overexpression decreases TfR1 expression by inhibiting IRP1 and represses proliferation in pancreatic cancer cells. Our data uncover a novel role of SIRT3 in cellular iron metabolism through IRP1 regulation and suggest that SIRT3 functions as a tumor suppressor, in part, by modulating cellular iron metabolism.

The glycolytic shift in fumarate-hydratase-deficient kidney cancer lowers AMPK levels, increases anabolic propensities and lowers cellular iron levels.

Inactivation of the TCA cycle enzyme, fumarate hydratase (FH), drives a metabolic shift to aerobic glycolysis in FH-deficient kidney tumors and cell lines from patients with hereditary leiomyomatosis renal cell cancer (HLRCC), resulting in decreased levels of AMP-activated kinase (AMPK) and p53 tumor suppressor, and activation of the anabolic factors, acetyl-CoA carboxylase and ribosomal protein S6. Reduced AMPK levels lead to diminished expression of the DMT1 iron transporter, and the resulting cytosolic iron deficiency activates the iron regulatory proteins, IRP1 and IRP2, and increases expression of the hypoxia inducible factor HIF-1α, but not HIF-2α. Silencing of HIF-1α or activation of AMPK diminishes invasive activities, indicating that alterations of HIF-1α and AMPK contribute to the oncogenic growth of FH-deficient cells.

Leucine biosynthesis regulates cytoplasmic iron-sulfur enzyme biogenesis in an Atm1p-independent manner.

Fe-S clusters (ISCs) are versatile cofactors utilized by many mitochondrial, cytoplasmic, and nuclear enzymes. Whereas mitochondria can independently initiate and complete ISC synthesis, other cellular compartments are believed to assemble ISCs from putative precursors exported from the mitochondria via an ATP binding cassette (ABC) transporter conserved from yeast (Atm1p) to humans (ABCB7). However, the regulatory interactions between mitochondrial and extramitochondrial ISC synthesis are largely unknown. In yeast, we found that mitochondrial ISC synthesis is regulated by the leucine biosynthetic pathway, which among other proteins involves an abundant cytoplasmic [4Fe-4S] enzyme, Leu1p. Enzymatic blocks in the pathway (i.e. leu1Δ or leu2Δ gene deletion mutations) induced post-transcriptional up-regulation of core components of mitochondrial ISC biosynthesis (i.e. the sulfur donor Nfs1p, the iron donor Yfh1p, and the ISC scaffold Isu1p). In leu2Δ cells, transcriptional mechanisms also led to dramatic up-regulation of Leu1p with concomitant down-regulation of mitochondrial aconitase (Aco1p), a [4Fe-4S] enzyme in the tricarboxylic acid cycle. Accordingly, the leu2Δ deletion mutation exacerbated Aco1p inactivation in cells with mutations in Yfh1p. These data indicate that defects in leucine biosynthesis promote the biogenesis of enzymatically active Leu1p at the expense of Aco1p activity. Surprisingly, this effect is independent of Atm1p; previous reports linking the loss of Leu1p activity to Atm1p depletion were confounded by the fact that LEU2 was used as a selectable marker to create Atm1p-depleted cells, whereas a leu2Δ allele was present in Atm1p-repleted controls. Thus, still largely unknown transcriptional and post-transcriptional mechanisms control ISC distribution between mitochondria and other cellular compartments.

Lack of DNA helicase Pif1 disrupts zinc and iron homoeostasis in yeast.

The Saccharomyces cerevisiae gene PIF1 encodes a conserved eukaryotic DNA helicase required for both mitochondrial and nuclear DNA integrity. Our previous work revealed that a pif1Δ strain is tolerant to zinc overload. In the present study we demonstrate that this effect is independent of the Pif1 helicase activity and is only observed when the protein is absent from the mitochondria. pif1Δ cells accumulate abnormal amounts of mitochondrial zinc and iron. Transcriptional profiling reveals that pif1Δ cells under standard growth conditions overexpress aconitase-related genes. When exposed to zinc, pif1Δ cells show lower induction of genes encoding iron (siderophores) transporters and higher expression of genes related to oxidative stress responses than wild-type cells. Coincidently, pif1Δ mutants are less prone to zinc-induced oxidative stress and display a higher reduced/oxidized glutathione ratio. Strikingly, although pif1Δ cells contain normal amounts of the Aco1 (yeast aconitase) protein, they completely lack aconitase activity. Loss of Aco1 activity is also observed when the cell expresses a non-mitochondrially targeted form of Pif1. We postulate that lack of Pif1 forces aconitase to play its DNA protective role as a nucleoid protein and that this triggers a domino effect on iron homoeostasis resulting in increased zinc tolerance.

Polarization dictates iron handling by inflammatory and alternatively activated macrophages.

BACKGROUND: Macrophages play a key role in iron homeostasis. In peripheral tissues, they are known to polarize into classically activated (or M1) macrophages and alternatively activated (or M2) macrophages. Little is known on whether the polarization program influences the ability of macrophages to store or recycle iron and the molecular machinery involved in the processes. DESIGN AND METHODS: Inflammatory/M1 and alternatively activated/M2 macrophages were propagated in vitro from mouse bone-marrow precursors and polarized in the presence of recombinant interferon-γ or interleukin-4. We characterized and compared their ability to handle radioactive iron, the characteristics of the intracellular iron pools and the expression of molecules involved in internalization, storage and export of the metal. Moreover we verified the influence of iron on the relative ability of polarized macrophages to activate antigen-specific T cells. RESULTS: M1 macrophages have low iron regulatory protein 1 and 2 binding activity, express high levels of ferritin H, low levels of transferrin receptor 1 and internalize--albeit with low efficiency -iron only when its extracellular concentration is high. In contrast, M2 macrophages have high iron regulatory protein binding activity, express low levels of ferritin H and high levels of transferrin receptor 1. M2 macrophages have a larger intracellular labile iron pool, effectively take up and spontaneously release iron at low concentrations and have limited storage ability. Iron export correlates with the expression of ferroportin, which is higher in M2 macrophages. M1 and M2 cells activate antigen-specific, MHC class II-restricted T cells. In the absence of the metal, only M1 macrophages are effective. CONCLUSIONS: Cytokines that drive macrophage polarization ultimately control iron handling, leading to the differentiation of macrophages into a subset which has a relatively sealed intracellular iron content (M1) or into a subset endowed with the ability to recycle the metal (M2).

Chlamydia trachomatis alters iron-regulatory protein-1 binding capacity and modulates cellular iron homeostasis in HeLa-229 cells.

Chlamydia trachomatis (CT) is the leading cause of diseases related to reproductive health and iron plays important role in chlamydial pathogenesis. Iron homeostasis in chlamydia-infected cells is not clear thus far. This study shows that expression of the transferrin receptor (TfR) is downregulated, whereas expression of the ferritin heavy chain is upregulated in CT-infected HeLa-229 cells. Expression of iron-regulatory protein (IRP)-1 predominates over IRP-2 in infected cells. In infected cells, attenuated binding activity of IRP-iron responsive elements (IREs) is observed using the electrophoretic mobility-shift assay. These results suggest that iron homeostasis is modulated in CT-infected HeLa cells at the interface of acquisition and commensal use of iron.

A role for IOP1 in mammalian cytosolic iron-sulfur protein biogenesis.

The biogenesis of cytosolic iron-sulfur (Fe-S) proteins in mammalian cells is poorly understood. In Saccharomyces cerevisiae, there is a pathway dedicated to cytosolic Fe-S protein maturation that involves several essential proteins. One of these is Nar1, which intriguingly is homologous to iron-only hydrogenases, ancient enzymes that catalyze the formation of hydrogen gas in anaerobic bacteria. There are two orthologues of Nar1 in mammalian cells, iron-only hydrogenase-like protein 1 (IOP1) and IOP2 (also known as nuclear prelamin A recognition factor). We examined IOP1 for a potential role in mammalian cytosolic Fe-S protein biogenesis. We found that knockdown of IOP1 in both HeLa and Hep3B cells decreases the activity of cytosolic aconitase, an Fe-S protein, but not that of mitochondrial aconitase. Knockdown of IOP2, in contrast, had no effect on either. The decrease in aconitase activity upon IOP1 knockdown is rescued by expression of a small interference RNA-resistant version of IOP1. Upon loss of its Fe-S cluster, cytosolic aconitase is known to be converted to iron regulatory protein 1, and consistent with this, we found that IOP1 knockdown increases transferrin receptor 1 mRNA levels and decreases ferritin heavy chain protein levels. IOP1 knockdown also leads to a decrease in activity of xanthine oxidase, a distinct cytosolic Fe-S protein. Taken together, these results provide evidence that IOP1 is involved in mammalian cytosolic Fe-S protein maturation.

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

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

STAT5 proteins are involved in down-regulation of iron regulatory protein 1 gene expression by nitric oxide.

RNA-binding activity of IRP1 (iron regulatory protein 1) is regulated by the insertion/extrusion of a [4Fe-4S] cluster into/from the IRP1 molecule. NO (nitic oxide), whose ability to activate IRP1 by removing its [4Fe-4S] cluster is well known, has also been shown to down-regulate expression of the IRP1 gene. In the present study, we examine whether this regulation occurs at the transcriptional level. Analysis of the mouse IRP1 promoter sequence revealed two conserved putative binding sites for transcription factor(s) regulated by NO and/or changes in intracellular iron level: Sp1 (promoter-selective transcription factor 1) and MTF1 (metal transcription factor 1), plus GAS (interferon-gamma-activated sequence), a binding site for STAT (signal transducer and activator of transcription) proteins. In order to define the functional activity of these sequences, reporter constructs were generated through the insertion of overlapping fragments of the mouse IRP1 promoter upstream of the luciferase gene. Transient expression assays following transfection of HuH7 cells with these plasmids revealed that while both the Sp1 and GAS sequences are involved in basal transcriptional activity of the IRP1 promoter, the role of the latter is predominant. Analysis of protein binding to these sequences in EMSAs (electrophoretic mobility-shift assays) using nuclear extracts from mouse RAW 264.7 macrophages stimulated to synthesize NO showed a significant decrease in the formation of Sp1-DNA and STAT-DNA complexes, compared with controls. We have also demonstrated that the GAS sequence is involved in NO-dependent down-regulation of IRP1 transcription. Further analysis revealed that levels of STAT5a and STAT5b in the nucleus and cytosol of NO-producing macrophages are substantially lower than in control cells. These findings provide evidence that STAT5 proteins play a role in NO-mediated down-regulation of IRP1 gene expression.

Iron chelation in the biological activity of curcumin.

Curcumin is among the more successful chemopreventive compounds investigated in recent years, and is currently in human trials to prevent cancer. The mechanism of action of curcumin is complex and likely multifactorial. We have made the unexpected observation that curcumin strikingly modulates proteins of iron metabolism in cells and in tissues, suggesting that curcumin has properties of an iron chelator. Curcumin increased mRNA levels of ferritin and GSTalpha in cultured liver cells. Unexpectedly, however, although levels of GSTalpha protein increased in parallel with mRNA levels in response to curcumin, levels of ferritin protein declined. Since iron chelators repress ferritin translation, we considered that curcumin may act as an iron chelator. To test this hypothesis, we measured the effect of curcumin on transferrin receptor 1, a protein stabilized under conditions of iron limitation, as well as the ability of curcumin to activate iron regulatory proteins (IRPs). Both transferrin receptor 1 and activated IRP, indicators of iron depletion, increased in response to curcumin. Consistent with the hypothesis that curcumin acts as an iron chelator, mice that were fed diets supplemented with curcumin exhibited a decline in levels of ferritin protein in the liver. These results suggest that iron chelation may be an additional mode of action of curcumin.

Iron regulatory proteins 1 and 2 in human monocytes, macrophages and duodenum: expression and regulation in hereditary hemochromatosis and iron deficiency.

BACKGROUND AND OBJECTIVES: The functions of the iron regulatory proteins (IRP1 and IRP2), which control cellular iron homeostasis are similar but not identical. As an inappropriate up-regulation of total IRP activity has been found in the duodenum and monocytes of patients with hereditary hemochromatosis (HH), we investigated the respective roles of IRP1 and IRP2 in these settings. DESIGN AND METHODS: Specific antibodies were used in RNA-supershift, immunoblotting and immunohistochemistry assays to evaluate IRP1 and IRP2 separately in monocytes, macrophages and duodenum of control subjects, and patients with HH or iron-deficiency anemia. RESULTS: The activity of both IRP1 and IRP2 and the levels of IRP2 were: (i) higher in monocytes and macrophages of HH patients than in those of control subjects; (ii) increased in the duodenal samples of the patients with HH and iron-deficiency anemia. IRP2 levels increased when monocytes differentiated to macrophages. Under all of the examined conditions, IRP2 was induced to a greater extent. In the duodenum of HH and anemic patients, IRP1 was shifted from the aconitase form (present in controls) to the apoform, whereas the IRP1 in monocytes/macrophages was always in the apoform, in both the patients and controls. The RNA-bound fraction of IRP1 was small in all of the samples. Both IRP were expressed more in the villi than in the crypts of the duodenum, with no differences in localization or expression between the patients and controls. INTERPRETATION AND CONCLUSIONS: These findings of the first extensive investigation of the comparative expression of the two IRP in human tissues and blood cells indicate that IRP2 is the major regulator of intracellular iron homeostasis in humans.

Extracellular H2O2 and not superoxide determines the compartment-specific activation of transferrin receptor by iron regulatory protein 1.

Iron regulatory protein 1 (IRP1) functions as translational regulator that plays a central role in coordinating the cellular iron metabolism by binding to the mRNA of target genes such as the transferrin receptor (TfR)--the major iron uptake protein. Reactive oxygen species such as H2O2 and O2*- that are both co-released by inflammatory cells modulate IRP1 in opposing directions. While H2O2--similar to iron depletion--strongly induces IRP1 via a signalling cascade, O2*- inactivates the mRNA binding activity by a direct chemical attack. These findings have raised the question of whether compartmentalization may be an important mechanism for isolating these biological reactants when released from inflammatory cells during the oxygen burst cascade. To address this question, we studied cytosolic IRP1 and its downstream target TfR in conjunction with a tightly controlled biochemical modulation of extracellular O2*- and H2O2 levels mimicking the oxygen burst cascade of inflammatory cells. We here demonstrate that IRP1 activity and expression of TfR are solely dependent on H2O2 when co-released O2*- with from xanthine oxidase. Our findings confirm that extracellular H2O2 determines the functionality of the IRP1 cluster and its downstream targets while the reactivity of O2*- is limited to its compartment of origin.

Nickel decreases cellular iron level and converts cytosolic aconitase to iron-regulatory protein 1 in A549 cells.

Nickel (Ni) compounds are well-established carcinogens and are known to initiate a hypoxic response in cells via the stabilization and transactivation of hypoxia-inducible factor-1 alpha (HIF-1alpha). This change may be the consequence of nickel's interference with the function of several Fe(II)-dependent enzymes. In this study, the effects of soluble nickel exposure on cellular iron homeostasis were investigated. Nickel treatment decreased both mitochondrial and cytosolic aconitase (c-aconitase) activity in A549 cells. Cytosolic aconitase was converted to iron-regulatory protein 1, a form critical for the regulation of cellular iron homeostasis. The increased activity of iron-regulatory protein 1 after nickel exposure stabilized and increased transferrin receptor (Tfr) mRNA and antagonized the iron-induced ferritin light chain protein synthesis. The decrease of aconitase activity after nickel treatment reflected neither direct interference with aconitase function nor obstruction of [4Fe-4S] cluster reconstitution by nickel. Exposure of A549 cells to soluble nickel decreased total cellular iron by about 40%, a decrease that likely caused the observed decrease in aconitase activity and the increase of iron-regulatory protein 1 activity. Iron treatment reversed the effect of nickel on cytosolic aconitase and iron-regulatory protein 1. To assess the mechanism for the observed effects, human embryonic kidney (HEK) cells over expressing divalent metal transporter-1 (DMT1) were compared to A549 cells expressing only endogenous transporters for inhibition of iron uptake by nickel. The inhibition data suggest that nickel can enter via DMT1 and compete with iron for entry into the cell. This disturbance of cellular iron homeostasis by nickel may have a great impact on the ability of the cell to regulate a variety of cell functions, as well as create a state of hypoxia in cells under normal oxygen tension. These effects may be very important in how nickel exerts phenotypic selection pressure to convert a normal initiated cell into a cancer cell.

Down-regulation of iron regulatory protein 1 activities and expression in superoxide dismutase 1 knock-out mice is not associated with alterations in iron metabolism.

Iron and oxygen (O2) are intimately associated in many well characterized patho-physiological processes. These include oxidation of the [4Fe-4S] cluster of mitochondrial aconitase and inactivation of this Krebs cycle enzyme by the superoxide anion (O2*-), a product of the one-electron of reduction O2. In contrast to the apparent toxicity of this reaction, the biological consequences of O2*- -mediated inactivation of the cytosolic counterpart of mitochondrial aconitase, commonly known as iron regulatory protein 1 (IRP1), are not clear. Apart from its ability to convert citrate to iso-citrate, IRP1 in its apo-form binds to iron-responsive elements in the untranslated regions of mRNAs coding for proteins involved in iron metabolism, to regulate their synthesis and thus control the cellular homeostasis of this metal. Here, we show that in superoxide dismutase 1 (SOD1) knock-out mice, lacking Cu,Zn-SOD, an enzyme that acts to reduce the concentration of O2*- mainly in cytosol, not only is aconitase activity of IRP1 inhibited but the level of IRP1 is also strongly decreased. Despite such an evident alteration in IRP1 status, SOD1-deficient mice display a normal iron metabolism phenotype. Our findings clearly show that under conditions of O2*- -mediated oxidative stress, IRP1 is not essential for the maintenance of iron metabolism in mammals.

Influence of parenteral iron preparations on non-transferrin bound iron uptake, the iron regulatory protein and the expression of ferritin and the divalent metal transporter DMT-1 in HepG2 human hepatoma cells.

It is widely assumed that standard parenteral iron preparations are degraded in the reticuloendothelial cells and that the iron is subsequently incorporated into transferrin. Hepatocytes or other epithelial cells have been considered as not affected. We show that this picture should be carefully reconsidered. By using the human hepatoma cell line HepG2 we showed that the parenteral iron preparations ferric saccharate and ferric gluconate donated iron to the cells as efficiently as low molecular weight iron and stimulated non-transferrin bound iron uptake. This led to inactivation of the iron regulatory protein 1 and to an increase in the expression of ferritin and of the divalent metal transporter (DMT-1). Ferric dextran was only a weak stimulator of ferritin and DMT-1 expression. The observed changes in iron metabolism occurred at concentrations of parenteral iron that can also be found in the plasma of patients after i.v. infusion. We conclude that parenteral iron also influences the iron metabolism of non-reticuloendothelial cells like HepG2 cells. Further the increase in the expression of the transporter DMT-1 in HepG2 cells after iron treatment is in contrast to the regulation in the duodenum and may be involved in the upregulated uptake of potentially toxic non-transferrin bound iron from the circulation to store it in the non-toxic form of ferritin.