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

Amyloid-β exposed astrocytes induce iron transport from endothelial cells at the blood-brain barrier by altering the ratio of apo- and holo-transferrin

Excessive brain iron accumulation is observed early in the onset of Alzheimer's disease, notably prior to widespread proteinopathy. These findings suggest that increases in brain iron levels are due to a dysregulation of the iron transport mechanism at the blood-brain barrier. Astrocytes release signals (apo- and holo-transferrin) that communicate brain iron needs to endothelial cells in order to modulate iron transport. Here we use iPSC-derived astrocytes and endothelial cells to investigate how early-disease levels of amyloid-β disrupt iron transport signals secreted by astrocytes to stimulate iron transport from endothelial cells. We demonstrate that conditioned media from astrocytes treated with amyloid-β stimulates iron transport from endothelial cells and induces changes in iron transport pathway proteins. The mechanism underlying this response begins with increased iron uptake and mitochondrial activity by the astrocytes, which in turn increases levels of apo-transferrin in the amyloid-β conditioned astrocyte media leading to increased iron transport from endothelial cells. These novel findings offer a potential explanation for the initiation of excessive iron accumulation in early stages of Alzheimer's disease. What's more, these data provide the first example of how the mechanism of iron transport regulation by apo- and holo-transferrin becomes misappropriated in disease that can lead to iron accumulation. The clinical benefit from understanding early dysregulation in brain iron transport in AD cannot be understated. If therapeutics can target this early process, they could possibly prevent the detrimental cascade that occurs with excessive iron accumulation.

Amyloid-β exposed astrocytes induce iron transport from endothelial cells at the blood-brain barrier by altering the ratio of apo- and holo-transferrin

Excessive brain iron accumulation is observed in early in the onset of Alzheimer's disease, notably prior to widespread proteinopathy. These findings suggest that increases in brain iron levels are due to a dysregulation of the iron transport mechanism at the blood-brain barrier. Astrocytes release signals (apo- and holo-transferrin) that communicate brain iron needs to endothelial cells in order to modulate iron transport. Here we use iPSC-derived astrocytes and endothelial cells to investigate how early-disease levels of amyloid-β disrupt iron transport signals secreted by astrocytes to stimulate iron transport from endothelial cells. We demonstrate that conditioned media from astrocytes treated with amyloid-β stimulates iron transport from endothelial cells and induces changes in iron transport pathway protein levels. The mechanism underlying this response begins with increased iron uptake and mitochondrial activity by the astrocytes which in turn increases levels of apo-transferrin in the amyloid-β conditioned astrocyte media leading to increased iron transport from endothelial cells. These novel findings offer a potential explanation for the initiation of excessive iron accumulation in early stages of Alzheimer's disease. What's more, these data provide the first example of how the mechanism of iron transport regulation by apo- and holo-transferrin becomes misappropriated in disease to detrimental ends. The clinical benefit from understanding early dysregulation in brain iron transport in AD cannot be understated. If therapeutics can target this early process, they could possibly prevent the detrimental cascade that occurs with excessive iron accumulation.

Significance Statement

Excessive brain iron accumulation is hallmark pathology of Alzheimer's disease that occurs early in the disease staging and before widespread proteinopathy deposition. This overabundance of brain iron has been implicated to contribute to disease progression, thus understandingthe mechanism of early iron accumulation has significant therapeutic potential to slow to halt disease progression. Here, we show that in response to low levels of amyloid-β exposure, astrocytes increase their mitochondrial activity and iron uptake, resulting in iron deficient conditions. Elevated levels of apo (iron free)-transferrin stimulate iron release from endothelial cells. These data are the first to propose a mechanism for the initiation of iron accumulation and the misappropriation of iron transport signaling leading to dysfunctional brain iron homeostasis and resultant disease pathology.

Rethinking IRPs/IRE system in neurodegenerative disorders: Looking beyond iron metabolism

Iron regulatory proteins (IRPs) and iron regulatory element (IRE) systems are well known in the progression of neurodegenerative disorders by regulating iron related proteins. IRPs are also regulated by iron homeostasis. However, an increasing number of studies have suggested a close relationship between the IRPs/IRE system and non-iron-related neurodegenerative disorders. In this paper, we reviewed that the IRPs/IRE system is not only controlled by iron ions, but also regulated by such factors as post-translational modification, oxygen, nitric oxide (NO), heme, interleukin-1 (IL-1), and metal ions. In addition, by regulating the transcription of non-iron related proteins, the IRPs/IRE system functioned in oxidative metabolism, cell cycle regulation, abnormal proteins aggregation, and neuroinflammation. Finally, by emphasizing the multiple regulations of IRPs/IRE system and its potential relationship with non-iron metabolic neurodegenerative disorders, we provided new strategies for disease treatment targeting IRPs/IRE system.

Inflaming the Brain with Iron

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

Redox-Sensing Iron-Sulfur Cluster Regulators

SIGNIFICANCE

Iron-sulfur cluster proteins carry out multiple functions, including as regulators of gene transcription/translation in response to environmental stimuli. In all known cases, the cluster acts as the sensory module, where the inherent reactivity/fragility of iron-sulfur clusters with small/redox-active molecules is exploited to effect conformational changes that modulate binding to DNA regulatory sequences. This promotes an often substantial reprogramming of the cellular proteome that enables the organism or cell to adapt to, or counteract, its changing circumstances. Recent Advances: Significant progress has been made recently in the structural and mechanistic characterization of iron-sulfur cluster regulators and, in particular, the O₂ and NO sensor FNR, the NO sensor NsrR, and WhiB-like proteins of Actinobacteria. These are the main focus of this review.

CRITICAL ISSUES

Striking examples of how the local environment controls the cluster sensitivity and reactivity are now emerging, but the basis for this is not yet fully understood for any regulatory family.

FUTURE DIRECTIONS

Characterization of iron-sulfur cluster regulators has long been hampered by a lack of high-resolution structural data. Although this still presents a major future challenge, recent advances now provide a firm foundation for detailed understanding of how a signal is transduced to effect gene regulation. This requires the identification of often unstable intermediate species, which are difficult to detect and may be hard to distinguish using traditional techniques. Novel approaches will be required to solve these problems.

Redox-sensing iron-sulfur cluster regulators

SIGNIFICANCE

Iron-sulfur cluster proteins carry out a wide range of functions, including as regulators of gene transcription/translation in response to environmental stimuli. In all known cases, the cluster acts as the sensory module, where the inherent reactivity/fragility of iron-sulfur clusters towards small/redox active molecules is exploited to effect conformational changes that modulate binding to DNA regulatory sequences. This promotes an often substantial re-programming of the cellular proteome that enables the organism or cell to adapt to, or counteract, its changing circumstances. Recent Advances. Significant progress has been made recently in the structural and mechanistic characterization of iron-sulfur cluster regulators and, in particular, the O2 and NO sensor FNR, the NO sensor NsrR, and WhiB-like proteins of Actinobacteria. These are the main focus of this review.

CRITICAL ISSUES

Striking examples of how the local environment controls the cluster sensitivity and reactivity are now emerging, but the basis for this is not yet fully understood for any regulatory family.

FUTURE DIRECTIONS

Characterization of iron-sulfur cluster regulators has long been hampered by a lack of high resolution structural data. Though this still presents a major future challenge, recent advances now provide a firm foundation for detailed understanding of how a signal is transduced to effect gene regulation. This requires the identification of often unstable intermediate species, which are difficult to detect and may be hard to distinguish using traditional techniques. Novel approaches will be required to solve these problems.

Iron regulatory proteins and their role in controlling iron metabolism

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

Iron regulatory protein 1 outcompetes iron regulatory protein 2 in regulating cellular iron homeostasis in response to nitric oxide

In mammals, iron regulatory proteins (IRPs) 1 and 2 posttranscriptionally regulate expression of genes involved in iron metabolism, including transferrin receptor 1, the ferritin (Ft) H and L subunits, and ferroportin by binding mRNA motifs called iron responsive elements (IREs). IRP1 is a bifunctional protein that mostly exists in a non-IRE-binding, [4Fe-4S] cluster aconitase form, whereas IRP2, which does not assemble an Fe-S cluster, spontaneously binds IREs. Although both IRPs fulfill a trans-regulatory function, only mice lacking IRP2 misregulate iron metabolism. NO stimulates the IRE-binding activity of IRP1 by targeting its Fe-S cluster. IRP2 has also been reported to sense NO, but the intrinsic function of IRP1 and IRP2 in NO-mediated regulation of cellular iron metabolism is controversial. In this study, we exposed bone marrow macrophages from Irp1(-/-) and Irp2(-/-) mice to NO and showed that the generated apo-IRP1 was entirely responsible for the posttranscriptional regulation of transferrin receptor 1, H-Ft, L-Ft, and ferroportin. The powerful action of NO on IRP1 also remedies the defects of iron storage found in IRP2-null bone marrow macrophages by efficiently reducing Ft overexpression. We also found that NO-dependent IRP1 activation, resulting in increased iron uptake and reduced iron sequestration and export, maintains enough intracellular iron to fuel the Fe-S cluster biosynthetic pathway for efficient restoration of the citric acid cycle aconitase in mitochondria. Thus, IRP1 is the dominant sensor and transducer of NO for posttranscriptional regulation of iron metabolism and participates in Fe-S cluster repair after exposure to NO.

Iron delocalisation in the pathogenesis of malarial anaemia

There is consensus that the pathophysiology of malaria-associated anaemia is multifactorial, but the precise mechanisms behind many of the haematological changes during malaria remain unclear. In this review, we attempt to build a composite picture of the pathophysiology of malarial anaemia using evidence from experimental, human and animal studies. We propose that cytokine- and hepcidin-mediated iron delocalisation, a principal mechanism in the anaemia of inflammation, plays an important role in the aetiology of malarial anaemia, and can explain some of the clinical and laboratory findings. These mechanisms interact with other aetiological determinants, such as dietary iron and micronutrient supply, helminth load, other infections and genetic variation, in determining the severity and associated features of anaemia. We suggest that iron delocalisation as a mechanism for malarial anaemia could be exploited for the development of alternative therapeutic strategies for post-malaria anaemia.

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.

The role of reactive oxygen and nitrogen species in cellular iron metabolism

The catalytic role of iron in the Haber-Weiss chemistry, which results in propagation of damaging reactive oxygen species (ROS), is well established. In this review, we attempt to summarize the recent evidence showing the reverse: That reactive oxygen and nitrogen species can significantly affect iron metabolism. Their interaction with iron-regulatory proteins (IRPs) seems to be one of the essential mechanisms of influencing iron homeostasis. Iron depletion is known to provoke normal iron uptake via IRPs, superoxide and hydrogen peroxide are supposed to cause unnecessary iron uptake by similar mechanism. Furthermore, ROS are able to release iron from iron-containing molecules. On the contrary, nitric oxide (NO) appears to be involved in cellular defense against the iron-mediated ROS generation probably mainly by inducing iron removal from cells. In addition, NO may attenuate the effect of superoxide by mutual reaction, although the reaction product-peroxynitrite-is capable to produce highly reactive hydroxyl radicals.

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.

Iron homeostasis in chronic inflammation

Inflammation induced anemia and resistance to erythropoietin are common features in patients with chronic kidney disease (CKD). Elevated levels of cytokines and enhanced oxidative stress, conditions associated with inflammatory states, are implicated in the development of anemia. Accumulating evidence suggests that activation of cytokine cascade and the associated acute-phase response, as it often occurs in patients with CKD, divert iron from erythropoiesis to storage sites within the reticuloendothelial system leading to functional iron deficiency and subsequently to anemia or resistance to erythropoietin. Other processes have also been shown to be involved in the pathogenesis of anemia provoked by the activated immune system including an inhibition of erythroid progenitor proliferation and differentiation, a suppression of erythropoietin production and a blunted response to erythropoietin. The present review concerns the underlying alterations in iron metabolism induced by chronic inflammation that result in anemia.

Role of iron in the nitric oxide-mediated fungicidal mechanism of IFN-gamma-activated murine macrophages against Paracoccidioides brasiliensis conidia

Iron is an essential growth element of virtually all microorganisms and its restriction is one of the mechanisms used by macrophages to control microbial multiplication. Paracoccidioides brasiliensis, the agent of paracoccidioidomycosis, an important systemic mycosis in Latin America, is inhibited in its conidia-to-yeast conversion in the absence of iron. We studied the participation of iron in the nitric oxide (NO)-mediated fungicidal mechanism against conidia. Peritoneal murine macrophages activated with 50U/mL of IFN-gamma or treated with 35 µM Deferoxamine (DEX) and infected with P. brasiliensis conidia, were co-cultured and incubated for 96 h in the presence of different concentrations of holotransferrin (HOLO) and FeS0(4). The supernatants were withdrawn in order to assess NO2 production by the Griess method. The monolayers were fixed, stained and observed microscopically. The percentage of the conidia-to-yeast transition was estimated by counting 200 intracellular propagules. IFN-gamma-activated or DEX-treated Mthetas presented marked inhibition of the conidia-to-yeast conversion (19 and 56%, respectively) in comparison with non-activated or untreated Mthetas (80%). IFN-gamma-activated macrophages produced high NO levels in comparison with the controls. Additionally, when the activated or treated-macrophages were supplemented with iron donors (HOLO or FeSO4), the inhibitory action was reversed, although NO production remained intact. These results suggest that the NO-mediated fungicidal mechanism exerted by IFN-gamma-activated macrophages against P. brasiliensis conidia, is dependent of an iron interaction.

An iron-induced nitric oxide burst precedes ubiquitin-dependent protein degradation for Arabidopsis AtFer1 ferritin gene expression

Ferritins play an essential role in iron homeostasis by sequestering iron in a bioavailable and non-toxic form. In plants, ferritin mRNAs are highly and quickly accumulated in response to iron overload. Such accumulation leads to a subsequent ferritin protein synthesis and iron storage, thus avoiding oxidative stress to take place. By combining pharmacological and imaging approaches in an Arabidopsis cell culture system, we have identified several elements in the signal transduction pathway leading to the increase of AtFer1 transcript level after iron treatment. Nitric oxide quickly accumulates in the plastids after iron treatment. This compound acts downstream of iron and upstream of a PP2A-type phosphatase to promote an increase of AtFer1 mRNA level. The AtFer1 gene transcription has been previously shown to be repressed under low iron conditions with the involvement of the cis-acting element iron-dependent regulatory sequence identified within the AtFer1 promoter sequence. We show here that the repressor is unlikely a transcription factor directly bound to the iron-dependent regulatory sequence; such a repressor is ubiquitinated upon iron treatment and subsequently degraded through a 26 S proteasome-dependent pathway.

Superoxide dismutase 1 modulates expression of transferrin receptor

Copper-zinc superoxide dismutase (SOD1) plays a protective role against the toxicity of superoxide, and studies in Saccharomyces cerevisiae and in Drosophila have suggested an additional role for SOD1 in iron metabolism. We have studied the effect of the modulation of SOD1 levels on iron metabolism in a cultured human glial cell line and in a mouse motoneuronal cell line. We observed that levels of the transferrin receptor and the iron regulatory protein 1 were modulated in response to altered intracellular levels of superoxide dismutase activity, carried either by wild-type SOD1 or by an SOD-active amyotrophic lateral sclerosis (ALS) mutant enzyme, G93A-SOD1, but not by a superoxide dismutase inactive ALS mutant, H46R-SOD1. Ferritin expression was also increased by wild-type SOD1 overexpression, but not by mutant SOD1s. We propose that changes in superoxide levels due to alteration of SOD1 activity affect iron metabolism in glial and neuronal cells from higher eukaryotes and that this may be relevant to diseases of the nervous system.

Nitric oxide inhibits the degradation of IRP2

Iron-regulatory protein 2 (IRP2), a posttranscriptional regulator of iron metabolism, undergoes proteasomal degradation in iron-replete cells, while it is stabilized in iron deficiency or hypoxia. IRP2 also responds to nitric oxide (NO), as shown in various cell types exposed to pharmacological NO donors and in gamma interferon/lipopolysaccharide-stimulated macrophages. However, the diverse experimental systems have yielded conflicting results on whether NO activates or inhibits IRP2. We show here that a treatment of mouse B6 fibroblasts or human H1299 lung cancer cells with the NO-releasing drug S-nitroso-N-acetyl-penicillamine (SNAP) activates IRP2 expression. Moreover, the exposure of H1299 cells to SNAP leads to stabilization of hemagglutinin (HA)-tagged IRP2, with kinetics analogous to those elicited by the iron chelator desferrioxamine. Similar results were obtained with IRP2(Delta)(73), a mutant lacking a conserved, IRP2-specific proline- and cysteine-rich domain. Importantly, SNAP fails to stabilize HA-tagged p53, suggesting that under the above experimental conditions, NO does not impair the capacity of the proteasome for protein degradation. Finally, by employing a coculture system of B6 and H1299 cells expressing NO synthase II or IRP2-HA cDNAs, respectively, we demonstrate that NO generated in B6 cells stabilizes IRP2-HA in target H1299 cells by passive diffusion. Thus, biologically synthesized NO promotes IRP2 stabilization without compromising the overall proteasomal activity. These results are consistent with the idea that NO may negatively affect the labile iron pool and thereby trigger responses to iron deficiency.

Protein S-nitrosylation: purview and parameters

S-nitrosylation, the covalent attachment of a nitrogen monoxide group to the thiol side chain of cysteine, has emerged as an important mechanism for dynamic, post-translational regulation of most or all main classes of protein. S-nitrosylation thereby conveys a large part of the ubiquitous influence of nitric oxide (NO) on cellular signal transduction, and provides a mechanism for redox-based physiological regulation.

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.

Detection of intracellular iron by its regulatory effect

Intracellular iron regulates gene expression by inhibiting the interaction of iron regulatory proteins (IRPs) with RNA motifs called iron-responsive elements (IREs). To assay this interaction in living cells we have developed two fluorescent IRE-based reporters that rapidly, reversibly, and specifically respond to changes in cellular iron status as well as signaling that modifies IRP activity. The reporters were also sufficiently sensitive to distinguish apo- from holotransferrin in the medium, to detect the effect of modifiers of the transferrin pathway such as HFE, and to detect the donation or chelation of iron by siderophores bound to the lipocalin neutrophil gelatinase-associated lipocalin (Ngal). In addition, alternative configurations of the IRE motif either enhanced or repressed fluorescence, permitting a ratio analysis of the iron-dependent response. These characteristics make it possible to visualize iron-IRP-IRE interactions in vivo.

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

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

Glutathione depletion increases nitric oxide-induced oxidative stress in primary rat hepatocyte cultures: involvement of low-molecular-weight iron

Various drugs and chemicals can cause a glutathione (GSH) depletion in the liver. Moreover, nitric oxide (NO) can be generated in response to physiological and pathological situations such as inflammation. The aim of this study was to estimate oxidative stress when primary rat hepatocytes were exposed to GSH depletion after NO production. For this purpose, cells were preincubated with lipopolysaccharide (LPS) and gamma-interferon (IFN) for 18 h in order to induce NO production by NO synthase and then L-buthionine sulfoximine (BSO), an inhibitor of GSH synthesis, was added for 5 h. In hepatocyte cultures preincubated with LPS and IFN before BSO addition, an increase in lipid peroxidation was noted. In those cells, an elevation of iron-bound NO and a decrease in free NO led us to suggest the involvement of low-molecular-weight iron (LMW iron) in the enhancement of oxidative stress. Indeed, addition of deferiprone, a chelator of LMW iron, reduced iron-bound NO levels and the extent of oxidative stress. Moreover, an important elevation of LMW iron levels was also observed. As both, N-acetylcysteine, a GSH precursor, and N(G)-monomethyl-L-arginine, a NO synthase inhibitor, totally inhibited the elevation of LMW iron and oxidative stress, a cooperative role could be attributed to NO production and GSH depletion.

Iron regulatory proteins as NO signal transducers

The iron regulatory proteins (IRPs) are an example of different proteins regulating the same metabolic process, iron uptake and metabolism. IRP1 is an iron-sulfur cluster-containing protein that can be converted from a cytosolic aconitase to an RNA binding posttranscriptional regulator in response to nitric oxide (NO). IRP2 lacks aconitase activity and its expression is decreased by NO signaling. In macrophages, NO is produced in response to such inflammatory ligands as interferon-gamma, which is expressed in response to mitogenic and antigenic stimuli, and lipopolysaccharide, a marker of bacterial invasion. Until recently, research results predict that the cellular response to increased NO production should be a decrease in ferritin synthesis, due to IRP1 binding to ferritin mRNA, and an increase in transferrin receptor biosynthesis, due to IRP1 binding to the transferrin mRNA. Surprisingly, however, macrophages exhibit decreased transferrin receptor concentration in response to inflammatory ligands. Bouton and Drapier discuss the physiological role and the mechanisms that may underlie this contradictory response.

Role of the ferroportin iron-responsive element in iron and nitric oxide dependent gene regulation

The newly described iron transporter, ferroportin (MTP1, IREG1), is expressed in a variety of tissues including the duodenum and cells of the mononuclear phagocyte system (MPS). In the MPS, ferroportin is hypothesized to be a major exporter of iron scavenged from senescent erythrocytes. Changes in iron metabolism, including the sequestration of iron in the MPS, are characteristic of both acute and chronic inflammation and these conditions induce changes in ferroportin expression. In a mouse model of acute inflammation, LPS administration is associated with reduced MPS ferroportin protein and mRNA expression. In addition, the ferroportin 5' UTR also has an iron-responsive element that binds to the iron-response proteins, but whether there is a role for this IRE in inflammation induced regulation of ferroportin has been unclear. A luciferase reporter gene under the control of the mouse ferroportin promoter and 5' UTR was used to determine if this 5' UTR conferred IRE-dependent regulation on this reporter gene. Stimulation of reporter gene transfected RAW 264.7 cells (a mouse macrophage cell line) with LPS resulted in IRE-dependent inhibition of luciferase production. Inhibitors of nitric oxide synthase abrogated the IRE-dependent effect of LPS. In addition, direct treatment of RAW 264.7 and with NO donor S-nitroso-N-acetylpenicillamine resulted in IRE-dependent down-regulation of luciferase expression. The effect of NO was consistent with IRP1/IRE mediated translation block. There are most likely both inflammation-mediated transcriptional and post-transcriptional (IRE-dependent) mechanisms for inhibiting ferroportin expression in MPS cells.

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.

Redox control of iron regulatory proteins

Iron regulatory proteins, IRP1 and IRP2, are cytoplasmic proteins of the iron-sulfur cluster isomerase family and serve as major post-transcriptional regulators of cellular iron metabolism. They bind to 'iron responsive elements' (IREs) of several mRNAs and thereby control their translation or stability. IRP1 and IRP2 respond to alterations in intracellular iron levels, but also to other signals such as nitric oxide (NO) and reactive oxygen species (ROS). The redox regulation of IRP1 and IRP2 provides direct links between the control of iron homeostasis and oxidative stress.

Recycling of RNA binding iron regulatory protein 1 into an aconitase after nitric oxide removal depends on mitochondrial ATP

Iron regulatory proteins (IRPs) control iron metabolism by specifically interacting with iron-responsive elements (IREs) on mRNAs. Nitric oxide (NO) converts IRP-1 from a [4Fe-4S] aconitase to a trans-regulatory protein through Fe-S cluster disassembly. Here, we have focused on the fate of IRE binding IRP1 from murine macrophages when NO flux stops. We show that virtually all IRP-1 molecules from NO-producing cells dissociated from IRE and recovered aconitase activity after re-assembling a [4Fe-4S] cluster in vitro. The reverse change in IRP-1 activities also occurred in intact cells no longer exposed to NO and did not require de novo protein synthesis. Likewise, inhibition of mitochondrial aconitase via NO-induced Fe-S cluster disassembly was also reversed independently of protein translation after NO removal. Our results provide the first evidence of Fe-S cluster repair of NO-modified aconitases in mammalian cells. Moreover, we show that reverse change in IRP-1 activities and repair of mitochondrial aconitase activity depended on energized mitochondria. Finally, we demonstrate that IRP-1 activation by NO was accompanied by both a drastic decrease in ferritin levels and an increase in transferrin receptor mRNA levels. However, although ferritin expression was recovered upon IRP-1-IRE dissociation, expression of transferrin receptor mRNA continued to rise for several hours after stopping NO flux.

Erythroid 5-aminolevulinate synthase, ferrochelatase and DMT1 expression in erythroid progenitors: differential pathways for erythropoietin and iron-dependent regulation

To determine whether erythropoietin (EPO) affects haem biosynthesis and iron transport, we studied the effects of EPO on the expression of erythroid 5-aminolevulinate synthase (eALAS), ferrochelatase and divalent metal transporter 1 (DMT-1) in human erythroid progenitor cells, and in the murine and human erythroid cell lines MEL and K562. Cytoplasmic e-ALAS mRNA levels were significantly increased after incubation of cells with EPO for at least 24 h, which could be the result of a transcriptional mechanism. In contrast, ferrochelatase or DMT-1 mRNA expression were not affected. Moreover, EPO also increased e-ALAS enzyme activity after only 4 h of stimulation, when mRNA levels were unchanged. The underlying mechanism was an effect of EPO on e-ALAS mRNA translation, which was under the control of iron regulatory proteins (IRP) 1 and 2. Thereby, EPO weakened the binding affinity of IRP-2 to the iron responsive element (IRE) within e-ALAS mRNA which resulted in the increased expression of e-ALAS IRE-controlled reporter gene constructs, following EPO stimulation. Our results show that EPO directly affected haem biosynthesis by stimulating the transcriptional and post-transcriptional expression of the key enzyme e-ALAS. These data provide new insights into the complex biochemical interaction between iron metabolism, haem biosynthesis and EPO biology.

Regulation of phagosomal iron release from murine macrophages by nitric oxide

The role of NO in macrophage iron turnover was studied in macrophages from inducible nitric oxide synthase (iNOS)-deficient mice. Interferon gamma/lipopolysaccharide (IFNgamma/LPS)-activated bone marrow-derived macrophages from iNOS-deficient mice, following phagocytosis of 59Fe-labelled transferrin-anti-transferrin immune complexes, showed reduced iron release compared with cells from wild-type iNOS littermates. Uptake of the complexes by macrophages was similar in iNOS-deficient and wild-type mice. Ferritin was up-regulated by IFNgamma/LPS treatment, but NO exercised a modest opposing down-regulatory effect. No effect of iNOS deficiency was seen when iron was taken up from iron citrate, which enters via a non-phagocytic route. These results suggest that NO plays a key role in regulating iron turnover in macrophages acquiring iron by phagocytosis of erythrocytes or cell debris, and thus the supply to peripheral tissues, such as to the bone marrow for erythropoiesis.

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.

Nitric oxide mediates iron-induced ferritin accumulation in Arabidopsis

Nitric oxide (NO) is a signaling molecule that plays a critical role in the activation of innate immune and inflammatory responses in animals. During the last few years, NO has also been detected in several plant species and the increasing number of reports on its function in plants have implicated NO as an important effector of growth, development and defense. Analogously to animals, NO has been recently shown to inhibit tobacco aconitase. This suggests that NO may elevate free iron levels in the cells by converting tobacco cytoplasmic aconitase into a mRNA binding protein that negatively regulates accumulation of ferritin. We investigated the possible role of NO as a regulator of ferritin levels in Arabidopsis and found that the NO-donor sodium nitroprusside (SNP) induces accumulation of ferritin both at mRNA and protein level. Iron is not necessary for this NO-mediated ferritin transcript accumulation, since SNP is still able to induce the accumulation of ferritin transcript in Arabidopsis suspension cultures pre-treated with the iron chelants DFO or ferrozine. However, NO is required for iron-induced ferritin accumulation, as the NO scavenger CPTIO prevents ferritin transcript accumulation in Arabidopsis suspension cultures treated with iron. The pathway is ser/thr phosphatase-dependent and necessitates protein synthesis; furthermore, NO mediates ferritin regulation through the IDRS sequence of the Atfer1 promoter responsible for transcriptional repression under low iron supply. NO, by acting downstream of iron in the induction of ferritin transcript accumulation is therefore a key signaling molecule for regulation of iron homeostasis in plants.

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.

Iron: deficiencies and requirements

A report from the World Health Organization estimates that 46% of the world's 5- to 14-year-old children are anemic. In addition, 48% of the world's pregnant women are anemic. A majority of these cases of anemia are due to iron deficiency. Our aim here is to review the latest data on iron regulatory mechanisms, iron sources and requirements. Human and animal studies have shown that amino acids and peptides influence iron absorption from the intestinal lumen. Inter-organ transport and uptake of nonheme iron is largely performed by the complex transferring-transferring receptor system. Moreover, the discovery of cytoplasmic iron regulatory proteins (IRPs) has provided a molecular framework from which we understand the coordination of cellular iron homeostasis in mammals. IRPs and the iron responsive elements (IREs) to which they bind allow mammals to make use of the essential properties of iron while reducing its potentially toxic effect. Physiologic iron requirements are three times higher in pregnancy than they are in menstruating women (approximately 1200 mg must be acquired from the body's iron store or from the diet by the end of pregnancy). The administration of iron supplements weekly instead of daily in humans has been proposed and is being actively investigated as a viable means of controlling iron deficiency in populations, including pregnant women.

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.

Interferon-gamma and lipopolysaccharide regulate the expression of Nramp2 and increase the uptake of iron from low relative molecular mass complexes by macrophages

The natural resistance associated macrophage protein 2 (Nramp2) is a transporter that is involved in iron (Fe) uptake from transferrin (Tf) and low molecular mass Fe complexes. Here we describe the effect of the inflammatory mediators interferon-gamma (IFN-gamma) and lipopolysaccharide (LPS) on the expression of Nramp2 mRNA and Fe uptake by cells of the macrophage lineage. After incubation of the RAW264.7 macrophage cell line with LPS there was a sevenfold increase in the expression of the 2.3 kb Nramp2 mRNA transcript when compared with the control, but little effect on the Nramp2 3.1 kb transcript. These results indicate differential regulation of the two transcripts. Treatment with LPS resulted in an increase in 59Fe uptake from 59Fe-nitrilotriacetic acid, while transferrin receptor (TfR) mRNA levels and 59Fe uptake from 59Fe-Tf were decreased. Paradoxically, at the same time, an increase in iron regulatory protein (IRP)1 RNA-binding activity was observed. Incubation with IFN-gamma (50 U.mL-1) resulted in a marked decrease in TfR mRNA levels but had no effect on Nramp2 mRNA expression. Exposure of RAW264.7 cells to both IFN-gamma and LPS resulted in a fourfold increase in the Nramp2 2.3-kb transcript and a four to fivefold decrease in the 3.1-kb transcript when compared with the control. Furthermore, there was a decrease in TfR mRNA levels despite an increase in IRP1 RNA-binding activity and a marked increase in inducible nitric oxide synthase mRNA expression. Hence, TfR and Nramp2 mRNA expression did not appear to be regulated in a concerted manner. Similar responses to those found above for RAW264.7 cells were also observed in the J774 macrophage cell line and also for primary cultures of mouse peritoneal macrophages. These results are of interest as the TfR and Nramp2 are thought to act together during Fe uptake from Tf. This is the first report to demonstrate regulation of the Nramp2 mRNA transcripts by inflammatory mediators.

Cellular and molecular aspects of iron and immune function

Fe plays a critical role in the immune system and defence against infection. However, many aspects of the way in which Fe influences these processes at the molecular and cellular level are unclear. The present review summarizes the role of Fe in lymphocyte activation and proliferation, and discusses how Fe is handled by macrophages.

Long-term consumption of a methionine-supplemented diet increases iron and lipid peroxide levels in rat liver

Methionine is a protective factor against various types of liver damage, but excessive dietary methionine is hepatotoxic. Because the mechanisms of L-methionine-related hepatotoxicity are poorly understood, the effect of long-term excessive L-methionine intake on the metabolism of iron and antioxidants was studied in rat liver to determine whether oxidative stress is involved. Wistar male rats were fed either an L-methionine-supplemented (16.0 g/kg) diet or a control diet for 1, 3, 6 and 9 mo. The growth rate of L-methionine-supplemented rats was significantly slower than that of controls. Iron, ferritin and thiobarbituric acid-reactive substances (TBARS) levels in the liver were greater in supplemented rats than in controls. Serum iron and transferrin levels were significantly lower in L-methionine-treated rats compared with controls. Serum ferritin did not differ between the two groups. Hepatic glutathione peroxidase activity, catalase activity and total glutathione concentrations were higher in rats fed the L-methionine-supplemented diet at 1 and 3 mo, but not at 6 and 9 mo. These results indicate that long-term consumption of excess L-methionine by rats may affect primarily iron metabolism rather than the antioxidant defense system and, consequently, induce an accumulation of iron.

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

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

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.

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.

Hypoxia alters iron-regulatory protein-1 binding capacity and modulates cellular iron homeostasis in human hepatoma and erythroleukemia cells

Ferritin and transferrin receptor expression is post-transcriptionally regulated by a conserved mRNA sequence termed the iron-responsive element (IRE), to which a transacting protein called the iron-regulatory protein (IRP) is bound. Our data demonstrate that hypoxia powerfully enhances IRE/IRP-1 binding in human cell lines. Using the human hepatoma cell line Hep3B as a model, we found that 16 h in a 1% oxygen atmosphere markedly increases IRE/IRP-1 binding as assessed by electromobility shift assay. Hypoxia also decreased cytosolic aconitase activity. The hypoxia-enhanced IRE/IRP-1 binding stabilized the transferrin receptor message, increased the cellular mRNA content by over 10-fold, and doubled surface receptor expression. Simultaneously, hypoxia suppressed ferritin message translation. Hypoxia's effect was most strikingly depicted by the absence of ferritin synthesis in cells challenged with inorganic iron. Our results contrast with previously reported data (Hanson, E. S., and Leibold, E. A. (1998) J. Biol. Chem. 273, 7588-7593) in which a 3% oxygen atmosphere reduced IRE/IRP-1 binding in rat hepatoma cells. We discuss some possible reasons for the differences. In aggregate with other investigations involving responses to hypoxia, iron, or nitric oxide, our data indicate that cellular iron metabolic responses are complex and that IRE/IRP-1 interactions vary between cell lines and perhaps between species.

Iron regulatory proteins, iron responsive elements and iron homeostasis

The discovery of iron regulatory proteins (IRPs) has provided a molecular framework from which to more fully understand the coordinate regulation of vertebrate iron metabolism. IRPs bind to iron responsive elements (IREs) in specific mRNAs and regulate their utilization. The targets of IRP action now appear to extend beyond proteins that function in the storage (ferritin) or cellular uptake (transferrin receptor) of iron to include those involved in other aspects of iron metabolism as well as in the tricarboxylic acid cycle. To date, it appears that IRPs modulate the utilization of six mammalian mRNAs. Current studies are aimed at defining the mechanisms responsible for the hierarchical regulation of these mRNAs by IRPs. In addition, much interest continues to focus on the signaling pathways through which IRP function is regulated. Multiple factors modulate the RNA binding activity of IRP1 and/or IRP2 including iron, nitric oxide, phosphorylation by protein kinase C, oxidative stress and hypoxia/reoxygenation. Because IRPs are key modulators of the uptake and metabolic fate of iron in cells, they are focal points for the modulation of cellular iron homeostasis in response to a variety of agents and circumstances.

Drosophila ferritin mRNA: alternative RNA splicing regulates the presence of the iron-responsive element

Several mRNAs encoding the same ferritin subunit of Drosophila melanogaster were identified. Alternative RNA splicing and utilisation of different polyadenylation sites were found to generate the transcripts. The alternative RNA splicing results in ferritin transcripts with four unique 5' untranslated regions. Only one of them contains an iron-responsive element. The iron-responsive element was found to bind in vitro specifically to human recombinant iron regulatory protein 1. Furthermore, the ferritin subunit mRNAs are differentially expressed during development. Our data provides the first molecular evidence that the presence of iron-responsive element in a ferritin mRNA is regulated by alternative RNA splicing.