In vivo and in vitro modulation of the mRNA-binding activity of iron-regulatory factor. Tissue distribution and effects of cell proliferation, iron levels and redox state

Differential processing and localization of human Nocturnin controls metabolism of mRNA and nicotinamide adenine dinucleotide cofactors

Nocturnin (NOCT) is a eukaryotic enzyme that belongs to a superfamily of exoribonucleases, endonucleases, and phosphatases. In this study, we analyze the expression, processing, localization, and cellular functions of human NOCT. We find that NOCT protein is differentially expressed and processed in a cell and tissue type-specific manner to control its localization to the cytoplasm or mitochondrial exterior or interior. The N terminus of NOCT is necessary and sufficient to confer import and processing in the mitochondria. We measured the impact of cytoplasmic NOCT on the transcriptome and observed that it affects mRNA levels of hundreds of genes that are significantly enriched in osteoblast, neuronal, and mitochondrial functions. Recent biochemical data indicate that NOCT dephosphorylates NADP(H) metabolites, and thus we measured the effect of NOCT on these cofactors in cells. We find that NOCT increases NAD(H) and decreases NADP(H) levels in a manner dependent on its intracellular localization. Collectively, our data indicate that NOCT can regulate levels of both mRNAs and NADP(H) cofactors in a manner specified by its location in cells.

Metabolic regulation of citrate and iron by aconitases: role of iron–sulfur cluster biogenesis

Iron and citrate are essential for the metabolism of most organisms, and regulation of iron and citrate biology at both the cellular and systemic levels is critical for normal physiology and survival. Mitochondrial and cytosolic aconitases catalyze the interconversion of citrate and isocitrate, and aconitase activities are affected by iron levels, oxidative stress and by the status of the Fe-S cluster biogenesis apparatus. Assembly and disassembly of Fe-S clusters is a key process not only in regulating the enzymatic activity of mitochondrial aconitase in the citric acid cycle, but also in controlling the iron sensing and RNA binding activities of cytosolic aconitase (also known as iron regulatory protein IRP1). This review discusses the central role of aconitases in intermediary metabolism and explores how iron homeostasis and Fe-S cluster biogenesis regulate the Fe-S cluster switch and modulate intracellular citrate flux.

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.

Caged-Iron Chelators a Novel Approach towards Protecting Skin Cells against UVA-Induced Necrotic Cell Death

Exposure of human skin cells to solar UVA radiation leads to an immediate dose-dependent increase of labile iron that subsequently promotes oxidative damage and necrotic cell death. Strong iron chelators have been shown to suppress cell damage and necrotic cell death by moderating the amount of labile iron pool (LIP), but chronic use would cause severe side effects owing to systemic iron depletion. Prodrugs that become activated in skin cells at physiologically relevant doses of UVA, such as "caged-iron chelators", may provide dose- and context-dependent release. Herein, we describe prototypical iron chelator compounds derived from salicylaldehyde isonicotinoyl hydrazone and pyridoxal isonicotinoyl hydrazone and demonstrate that the intracellular LIP and subsequent necrotic cell death of human skin fibroblasts is significantly decreased upon exposure to a combination of the prototypical compounds and physiologically relevant UVA doses. Iron regulatory protein bandshift and calcein fluorescence assays reveal decreased intracellular LIP following irradiation of caged-chelator-treated cells, but not in control samples where either UVA light, or caged-chelator is absent. Furthermore, flow cytometry shows that these compounds have no significant toxicity in the skin fibroblasts. This novel light-activated prodrug strategy may therefore be used to protect skin cells against the deleterious effects of sunlight.

Genetic ablations of iron regulatory proteins 1 and 2 reveal why iron regulatory protein 2 dominates iron homeostasis

The two iron regulatory proteins IRP1 and IRP2 bind to transcripts of ferritin, transferrin receptor and other target genes to control the expression of iron metabolism proteins at the post-transcriptional level. Here we compare the effects of genetic ablation of IRP1 to IRP2 in mice. IRP1-/- mice misregulate iron metabolism only in the kidney and brown fat, two tissues in which the endogenous expression level of IRP1 greatly exceeds that of IRP2, whereas IRP2-/- mice misregulate the expression of target proteins in all tissues. Surprisingly, the RNA-binding activity of IRP1 does not increase in animals on a low-iron diet that is sufficient to activate IRP2. In animal tissues, most of the bifunctional IRP1 is in the form of cytosolic aconitase rather than an RNA-binding protein. Our findings indicate that the small RNA-binding fraction of IRP1, which is insensitive to cellular iron status, contributes to basal mammalian iron homeostasis, whereas IRP2 is sensitive to iron status and can compensate for the loss of IRP1 by increasing its binding activity. Thus, IRP2 dominates post-transcriptional regulation 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.

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.

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.

Changes in gene expression with iron loading and chelation in cardiac myocytes and non-myocytic fibroblasts

Iron overload is associated with long-term cardiac iron accumulation and tissue changes such as fibrosis. To determine short-term iron-dependent changes in expression of genes associated with iron homeostasis and fibrosis we measured mRNA on Northern blots prepared from cultured rat neonatal cardiomyocytes and non-myocytes (fibroblasts) as a function of iron loading and chelation. Transferrin receptor mRNA was reduced in myocytes exposed to various concentrations of iron for 3 days and this decline was associated with a 63% decline in iron-response element (IRE) binding of iron regulatory protein-1, indicating that myocytes utilize IRE-dependent mechanisms to modulate gene expression. In myocytes iron caused a dose-dependent decline in mRNAs coding for transforming growth factor- beta(1)(TGF- beta(1)), biglycan, and collagen type I while plasminogen activator inhibitor-1 mRNA was unaffected by iron loading and decorin mRNA doubled. Total TGF- beta bioactivity was also decreased by iron loading. Thus, the effects of iron loading on genes related to cardiac fibrosis are gene-specific. Addition of deferoxamine for 1 day did not have any significant effect on any of these genes. Parallel changes in gene expression were exhibited by non-myocytes (fibroblasts), where chelation also decreased TGF- beta(1)mRNA and activity, and mRNA for collagen type I and biglycan, and collagen synthesis. In addition to these changes in transcripts associated with matrix formation the mRNA of the metabolic enzyme glyceraldehyde-3-phosphate dehydrogenase was unaffected by iron loading but doubled in both cell types upon treatment with deferoxamine. These findings suggest that in both cardiac myocytes and non-myocyte fibroblasts gene expression is coupled to intracellular iron pools by gene-specific and IRE-dependent and idependent mechanisms. This linkage may influence matrix deposition, a significant component of cardiac injury.

Impaired Ferritin mRNA Translation in Primary Erythroid Progenitors: Shift to Iron-Dependent Regulation by the v-ErbA Oncoprotein

In immortalized cells of the erythroid lineage, the iron-regulatory protein (IRP) has been suggested to coregulate biosynthesis of the iron storage protein ferritin and the erythroid delta-aminolevulinate synthase (eALAS), a key enzyme in heme production. Under iron scarcity, IRP binds to an iron-responsive element (IRE) located in ferritin and eALAS mRNA leaders, causing a block of translation. In contrast, IRP-IRE interaction is reduced under high iron conditions, allowing efficient translation. We show here that primary chicken erythroblasts (ebls) proliferating or differentiating in culture use a drastically different regulation of iron metabolism. Independently of iron administration, ferritin H (ferH) chain mRNA translation was massively decreased, whereas eALAS transcripts remained constitutively associated with polyribosomes, indicating efficient translation. Variations in iron supply had minor but significant effects on eALAS mRNA polysome recruitment but failed to modulate IRP-affinity to the ferH-IRE in vitro. However, leukemic ebls transformed by the v-ErbA/v-ErbB–expressing avian erythroblastosis virus showed an iron-dependent reduction of IRP mRNA-binding activity, resulting in mobilization of ferH mRNA into polysomes. Hence, we analyzed a panel of ebls overexpressing v-ErbA and/or v-ErbB oncoproteins as well as the respective normal cellular homologues (c-ErbA/TR, c-ErbB/EGFR). It turned out that v-ErbA, a mutated class II nuclear hormone receptor that arrests erythroid differentiation, caused the change in ferH mRNA translation. Accordingly, inhibition of v-ErbA function in these leukemic ebls led to a switch from iron-responsive to iron-independent ferH expression.

Impaired Ferritin mRNA Translation in Primary Erythroid Progenitors: Shift to Iron-Dependent Regulation by the v-ErbA Oncoprotein

In immortalized cells of the erythroid lineage, the iron-regulatory protein (IRP) has been suggested to coregulate biosynthesis of the iron storage protein ferritin and the erythroid delta-aminolevulinate synthase (eALAS), a key enzyme in heme production. Under iron scarcity, IRP binds to an iron-responsive element (IRE) located in ferritin and eALAS mRNA leaders, causing a block of translation. In contrast, IRP-IRE interaction is reduced under high iron conditions, allowing efficient translation. We show here that primary chicken erythroblasts (ebls) proliferating or differentiating in culture use a drastically different regulation of iron metabolism. Independently of iron administration, ferritin H (ferH) chain mRNA translation was massively decreased, whereas eALAS transcripts remained constitutively associated with polyribosomes, indicating efficient translation. Variations in iron supply had minor but significant effects on eALAS mRNA polysome recruitment but failed to modulate IRP-affinity to the ferH-IRE in vitro. However, leukemic ebls transformed by the v-ErbA/v-ErbB–expressing avian erythroblastosis virus showed an iron-dependent reduction of IRP mRNA-binding activity, resulting in mobilization of ferH mRNA into polysomes. Hence, we analyzed a panel of ebls overexpressing v-ErbA and/or v-ErbB oncoproteins as well as the respective normal cellular homologues (c-ErbA/TR, c-ErbB/EGFR). It turned out that v-ErbA, a mutated class II nuclear hormone receptor that arrests erythroid differentiation, caused the change in ferH mRNA translation. Accordingly, inhibition of v-ErbA function in these leukemic ebls led to a switch from iron-responsive to iron-independent ferH expression.

The redox state regulates RNA degradation in the chloroplast of Chlamydomonas reinhardtii

A Chlamydomonas reinhardtii chloroplast transformant, designated MU7, carrying a chimeric (rbcL promoter: beta-glucuronidase [GUS]: psaB 3' end) gene whose transcripts have been found previously to be unstable in light (half-life of 20 min in light as opposed to a half-life of 5 h in the dark), was used to study the role of electron transport and of the redox state in the degradation of chloroplast transcripts in the light. Blocking photosynthetic electron transport with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) prevented the light-dependent breakdown of the pool of GUS transcripts in MU7 cells. Diamide, an oxidizing agent, caused a measurable delay in the degradation of GUS transcripts in the light. The addition of dithiothreitol (DTT), a dithiol reductant, to MU7 cells in which GUS transcript levels were stabilized by DCMU induced degradation of GUS transcripts. Similarly, DTT induced a decrease in the levels of GUS transcripts when added to MU7 cells in the dark period of the light/dark cycle, a period in which GUS transcript levels normally increase. The levels of transcripts of endogenous chloroplast genes were affected by DCMU and DTT in the same direction as levels of GUS transcripts. The results suggest a regulatory role of the redox state in the degradation of chloroplast transcripts in C. reinhardtii.

Post-transcriptional control via iron-responsive elements: the impact of aberrations in hereditary disease

Tight regulation of iron metabolism is crucial to avoid formation of deleterious radicals and is mainly executed at the post-transcriptional level. The regulatory loops are exerted by trans-acting iron regulatory proteins (IRPs) and cis-acting stem-loop motifs, termed iron-responsive elements (IREs), located in the untranslated regions (UTRs) of target mRNAs. Iron scarcity induces binding of IRPs to a single IRE in the 5'-UTR of ferritin, eALAS, aconitase and SDHb mRNAs, which specifically suppresses translation initiation. Simultaneous interaction of IRPs with multiple IREs in the 3'-UTR of transferrin receptor (TfR) mRNA selectively causes its stabilization. The pattern is reverted under iron overload: IRP-mRNA binding affinity is reduced, which results in efficient protein synthesis of target transcripts harboring IREs in the 5'-UTR and rapid degradation of TfR mRNA. Although multiple evidences support this model, several studies reported massive alterations in the regulation of iron homeostasis under specific physiological conditions, raising the possibility for additional regulatory events. Intensive analysis of the palindromic IRE consensus sequence revealed the critical elements for the formation of a functional structure and demonstrated the consequences of IRE mutations in IRP binding. Recent investigations indicated the involvement of naturally occurring IRE mutations of the ferritin L subunit in the hyperferritinemia-cataract syndrome, a hereditary disorder. This review summarizes the apparent links between iron-dependent post-transcriptional control and its abnormalities, governed by the properties of a single mRNA stem-loop structure.

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

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

Ultraviolet A radiation induces immediate release of iron in human primary skin fibroblasts: the role of ferritin

In mammalian cells, the level of the iron-storage protein ferritin (Ft) is tightly controlled by the iron-regulatory protein-1 (IRP-1) at the posttranscriptional level. This regulation prevents iron acting as a catalyst in reactions between reactive oxygen species and biomolecules. The ultraviolet A (UVA) radiation component of sunlight (320-400 nm) has been shown to be a source of oxidative stress to skin via generation of reactive oxygen species. We report here that the exposure of human primary skin fibroblasts, FEK4, to UVA radiation causes an immediate release of "free" iron in the cells via proteolysis of Ft. Within minutes of exposure to a range of doses of UVA at natural exposure levels, the binding activity of IRP-1, as well as Ft levels, decreases in a dose-dependent manner. This decrease coincides with a significant leakage of the lysosomal components into the cytosol. Stabilization of Ft molecules occurs only when cells are pretreated with lysosomal protease inhibitors after UVA treatment. We propose that the oxidative damage to lysosomes that leads to Ft degradation and the consequent rapid release of potentially harmful "free" iron to the cytosol might be a major factor in UVA-induced damage to the skin.

The iron regulatory protein can determine the effectiveness of 5-aminolevulinic acid in inducing protoporphyrin IX in human primary skin fibroblasts

The level of endogenous photosensitiser, protoporphyrin IX (PPIX), can be enhanced in the cells by 5-aminolevulinic acid (ALA). We investigated the effect of critical parameters such as growth state of the cells and availability of intracellular iron in modulating the level of PPIX, in human primary cultured skin fibroblasts (FEK4) maintained either in exponentially growing or growth-arrested phase, following treatment with ALA. The addition of ALA to exponentially growing cells increased the level of PPIX 6-fold relative to control cells; however, in growth-arrested cells the same treatment increased the level of PPIX up to 34-fold. The simultaneous addition of the hydrophilic iron-chelator Desferal with ALA, boosted the level of PPIX up to 47-fold in growing cells and up to 42-fold in growth-arrested cells, suggesting that iron is limiting under the latter conditions. The strict dependence of PPIX enhancement on free available iron levels was examined by the level of activation of iron regulatory protein in band shift assays. This analysis revealed that the basal level of iron regulatory protein in growth-arrested cells was 6-fold higher than in growing cells, reflecting the influence of the free available iron pool in exponentially growing cells. Interestingly, the same ratio was found between the basal level concentration of PPIX in growing and growth-arrested cells. We propose that iron regulatory protein activation could serve as a marker for developing photodynamic therapy protocols because it identifies cells and tissues with a propensity to accumulate PPIX and it is therefore likely to predict the effectiveness of such therapies.

Inactivation of both RNA binding and aconitase activities of iron regulatory protein-1 by quinone-induced oxidative stress

Iron regulatory protein-1 (IRP-1) controls the expression of several mRNAs by binding to iron-responsive elements (IREs) in their untranslated regions. In iron-replete cells, a 4Fe-4S cluster converts IRP-1 to cytoplasmic aconitase. IRE binding activity is restored by cluster loss in response to iron starvation, NO, or extracellular H2O2. Here, we study the effects of intracellular quinone-induced oxidative stress on IRP-1. Treatment of murine B6 fibroblasts with menadione sodium bisulfite (MSB), a redox cycling drug, causes a modest activation of IRP-1 to bind to IREs within 15-30 min. However, IRE binding drops to basal levels within 60 min. Surprisingly, a remarkable loss of both IRE binding and aconitase activities of IRP-1 follows treatment with MSB for 1-2 h. These effects do not result from alterations in IRP-1 half-life, can be antagonized by the antioxidant N-acetylcysteine, and regulate IRE-containing mRNAs; the capacity of iron-starved MSB-treated cells to increase transferrin receptor mRNA levels is inhibited, and MSB increases the translation of a human growth hormone indicator mRNA bearing an IRE in its 5'-untranslated region. Nonetheless, MSB inhibits ferritin synthesis. Thus, menadione-induced oxidative stress leads to post-translational inactivation of both genetic and enzymatic functions of IRP-1 by a mechanism that lies beyond the "classical" Fe-S cluster switch and exerts multiple effects on cellular iron metabolism.

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

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

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

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

Regulatory signals in messenger RNA: determinants of nutrient–gene interaction and metabolic compartmentation

Nutrition has marked influences on gene expression and an understanding of the interaction between nutrients and gene expression is important in order to provide a basis for determining the nutritional requirements on an individual basis. The effects of nutrition can be exerted at many stages between transcription of the genetic sequence and production of a functional protein. This review focuses on the role of post-transcriptional control, particularly mRNA stability, translation and localization, in the interactions of nutrients with gene expression. The effects of both macronutrients and micronutrients on regulation of gene expression by post-transcriptional mechanisms are presented and the post-transcriptional regulation of specific genes of nutritional relevance (glucose transporters, transferrin, selenoenzymes, metallothionein, lipoproteins) is described in detail. The function of the regulatory signals in the untranslated regions of the mRNA is highlighted in relation to control of mRNA stability, translation and localization and the importance of these mRNA regions to regulation by nutrients is illustrated by reference to specific examples. The localization of mRNA by signals in the untranslated regions and its function in the spatial organization of protein synthesis is described; the potential of such mechanisms to play a key part in nutrient channelling and metabolic compartmentation is discussed. It is concluded that nutrients can influence gene expression through control of the regulatory signals in these untranslated regions and that the post-transcriptional regulation of gene expression by these mechanisms may influence nutritional requirements. It is emphasized that in studies of nutritional control of gene expression it is important not to focus only on regulation through gene promoters but also to consider the possibility of post-transcriptional control.

Lack of coordinate control of ferritin and transferrin receptor expression during rat liver regeneration

Transferrin receptor (TfR) and ferritin, key proteins of cellular iron metabolism, are coordinately and divergently controlled by cytoplasmic proteins (iron regulatory proteins, IRP-1 and IRP-2) that bind to conserved mRNA motifs called iron-responsive elements (IRE). IRP, in response to specific stimuli (low iron levels, growth and stress signals) are activated and prevent TfR mRNA degradation and ferritin mRNA translation by hindering ferritin mRNA binding to polysomes. We previously found that, in regenerating liver, IRP activation was accompanied by increased TfR mRNA levels, but not by reduced ferritin expression. The basis for this unexpected behavior was investigated in the present study. Liver regeneration triggered by carbon tetrachloride (CCl4) stimulated by four- to fivefold the synthesis of both L and H ferritin chains. This increase was accompanied with a transcriptionally regulated twofold rise in the amount of ferritin mRNAs. Moreover, polysome-associated ferritin transcripts were fourfold higher in CCl4-treated animals than in control animals. Because RNA bandshift assays showed a fourfold increase in IRP-2 binding activity after CCl4 administration, activated IRP in regenerating liver seemed unable to prevent ferritin mRNAs binding to polysomes. This was confirmed by direct demonstration in the wheat germ translation system that the efficiency of IRP as a translational repressor of a mRNA bearing an IRE motif in front of a reporter transcript is impaired in CCl4-treated rats in spite of an enhanced IRE-binding capacity. In conclusion, we show for the first time that the paradigm of coordinate and opposite control of ferritin and TfR by IRP is contradicted in liver regeneration. Under these circumstances, growth-dependent signals may activate ferritin gene transcription and at the same time hamper the ability of activated IRP-2 to repress translation of ferritin mRNAs, thus preserving for growing liver cells an essential iron-storage compartment.

Effect of reactive oxygen species on iron regulatory protein activity

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

Response of Monocyte Iron Regulatory Protein Activity to Inflammation: Abnormal Behavior in Genetic Hemochromatosis

In genetic hemochromatosis (GH), iron overload affects mainly parenchymal cells, whereas little iron is found in reticuloendothelial (RE) cells. We previously found that RE cells from GH patients had an inappropriately high activity of iron regulatory protein (IRP), the key regulator of intracellular iron homeostasis. Elevated IRP should reflect a reduction of the iron pool, possibly because of a failure to retain iron. A defect in iron handling by RE cells that results in a lack of feedback regulation of intestinal absorption might be the basic abnormality in GH. To further investigate the capacity of iron retention in RE cells of GH patients, we used inflammation as a model system as it is characterized by a block of iron release from macrophages. We analyzed the iron status of RE cells by assaying IRP activity and ferritin content after 4, 8, and 24 hours of incubation with lipopolysaccharide (LPS) and interferon-γ (IFN-γ). RNA-bandshift assays showed that in monocytes and macrophages from 16 control subjects, IRP activity was transiently elevated 4 hours after treatment with LPS and IFN-γ but remarkably downregulated thereafter. Treatment with NO donors produced the same effects whereas an inducible Nitric Oxide Synthase (iNOS) inhibitor prevented them, which suggests that the NO pathway was involved. Decreased IRP activity was also found in monocytes from eight patients with inflammation. Interestingly, no late decrease of IRP activity was detected in cytokine-treated RE cells from 12 GH patients. Ferritin content was increased 24 hours after treatment in monocytes from normal subjects but not in monocytes from GH patients. The lack of downregulation of IRP activity under inflammatory conditions seems to confirm that the control of iron release from RE cells is defective in GH.

Response of Monocyte Iron Regulatory Protein Activity to Inflammation: Abnormal Behavior in Genetic Hemochromatosis

In genetic hemochromatosis (GH), iron overload affects mainly parenchymal cells, whereas little iron is found in reticuloendothelial (RE) cells. We previously found that RE cells from GH patients had an inappropriately high activity of iron regulatory protein (IRP), the key regulator of intracellular iron homeostasis. Elevated IRP should reflect a reduction of the iron pool, possibly because of a failure to retain iron. A defect in iron handling by RE cells that results in a lack of feedback regulation of intestinal absorption might be the basic abnormality in GH. To further investigate the capacity of iron retention in RE cells of GH patients, we used inflammation as a model system as it is characterized by a block of iron release from macrophages. We analyzed the iron status of RE cells by assaying IRP activity and ferritin content after 4, 8, and 24 hours of incubation with lipopolysaccharide (LPS) and interferon-γ (IFN-γ). RNA-bandshift assays showed that in monocytes and macrophages from 16 control subjects, IRP activity was transiently elevated 4 hours after treatment with LPS and IFN-γ but remarkably downregulated thereafter. Treatment with NO donors produced the same effects whereas an inducible Nitric Oxide Synthase (iNOS) inhibitor prevented them, which suggests that the NO pathway was involved. Decreased IRP activity was also found in monocytes from eight patients with inflammation. Interestingly, no late decrease of IRP activity was detected in cytokine-treated RE cells from 12 GH patients. Ferritin content was increased 24 hours after treatment in monocytes from normal subjects but not in monocytes from GH patients. The lack of downregulation of IRP activity under inflammatory conditions seems to confirm that the control of iron release from RE cells is defective in GH.

Iron and gene expression: molecular mechanisms regulating cellular iron homeostasis

In recent years, specific post-transcriptional mechanisms in the cytoplasm of vertebrate cells have been elucidated that directly affect the stability and translation of mRNAs coding for central proteins in iron metabolism. This review shall focus primarily on these mechanisms. Other levels of control, either affecting gene transcription and/ or related to the function of iron-capturing substances and transmembrane transport, are also likely to exist and to influence the iron balance and utilization. They are, however, much less clear.

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

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

Regulation of expression of ferritin H-chain and transferrin receptor by protoporphyrin IX

The effect of protoporphyrin IX (hemin without iron) on the expression of transferrin receptor and ferritin was investigated in Friend leukemia cells. Cells treated with protoporphyrin IX exhibit enhanced transferrin-receptor expression and markedly reduced ferritin synthesis. Stimulation of transferrin-receptor expression is observed at both the mRNA and protein level. The effect on ferritin synthesis is mediated by translational inhibition of the mRNA, which, in contrast, is transcriptionally stimulated by protoporphyrin IX treatment. The regulation of transferrin receptor and ferritin in response to iron perturbations has been studied extensively and is mediated by the binding of iron-regulatory proteins (IRP) to the iron-responsive elements (IRE) present in the 3' and 5' untranslated regions of the transferrin-receptor and ferritin mRNA, respectively. To elucidate the molecular mechanisms underlying the effects of protoporphyrin IX on ferritin and transferrin-receptor expression, the role of the IRE sequence was investigated both in vivo by transfection experiments, with a construct containing the coding region for the chloramphenicol acetyltransferase (CAT) reporter gene under the translational control of the ferritin IRE, and in vitro by RNA band-shift assays. Whereas, examination of IRP binding to the IRE by in vitro assays suggests an apparent inactivation of IRP by protoporphyrin IX treatment, CAT assays indicate that protoporphyrin IX is able to induce in vivo a translational inhibition similar to that obtained by treatment with the iron chelator Desferal. This observation raises the possibility of different effects on the IRP activity exerted by porphyrin treatment in intact tissue-culture cells and in vitro. We conclude that translation of ferritin mRNA and degradation of transferrin-receptor mRNA are inhibited in intact tissue-culture cells by protoporphyrin IX through a mechanism similar to that exerted by iron chelation, thus involving depletion of the intracellular iron pool. These results can improve the understanding of the regulation of ferritin gene expression in some pathological conditions associated with disturbed heme synthesis.

Structure and function of the iron-responsive element from human ferritin L chain mRNA

We report the cloning and functional characterization of the iron responsive element (IRE) of human ferritin light (L) chain mRNA from a cDNA library of primary human T lymphocytes. Comparison of this palindromic cDNA element to the IRE predicted from the reported genomic sequence revealed significant differences, resulting in a stem-loop structure with lower stability than the IRE of the heavy (H) chain mRNA. Nevertheless, the L subunit IRE mediated efficient binding of the iron regulatory protein (IRP) in a manner comparable to that of human ferritin H chain mRNA in vitro. In accordance with previous observations on H form transcripts, the cis-acting regulatory IRE motif of human ferritin L chain mRNA was capable of repressing translation under iron deprivation but permitted mobilization of the transcripts into polysomes following iron repletion in vivo.

Inappropriately High Iron Regulatory Protein Activity in Monocytes of Patients With Genetic Hemochromatosis

In genetic hemochromatosis (GH), excess iron is deposited in parenchymal cells, whereas little iron is found in reticuloendothelial (RE) cells until the later stages of the disease. As iron absorption is inversely related to RE cells stores, a failure of RE to retain iron has been proposed as the basic defect in GH. In RE cells of GH subjects, we examined the activity of iron regulatory protein (IRP), a reliable indicator of the elusive regulatory labile iron pool, which modulates cellular iron homeostasis through control of ferritin (Ft) and transferrin receptor gene expression. RNA-bandshift assays showed a significant increase in IRP activity in monocytes from 16 patients with untreated GH compared with 28 control subjects (1.5-fold) and five patients with secondary hemochromatosis (SH) with similar iron burden (fourfold). In 17 phlebotomy-treated GH patients, IRP activity did not differ from that of control subjects. In both GH and SH monocyte-macrophages, Ft content increased by twofold and the L subunit-rich isoferritin profile was unchanged as compared with controls. IRP activity was still upregulated in vitro in monocyte-derived macrophages of GH subjects but, following manipulations of iron levels, was modulated normally. Therefore, the sustained activity of monocyte IRP found in vivo in monocytes of GH patients is not due to an inherent defect of its control, but is rather the expression of a critical abnormality of iron metabolism, eg, a paradoxical contraction of the regulatory iron pool. By preventing Ft mRNA translation, high IRP activity in monocytes may represent a molecular mechanism contributing to the inadequate Ft accumulation and insufficient RE iron storage in GH.

The molecular mechanisms of the metabolism and transport of iron in normal and neoplastic cells

Iron uptake by mammalian cells is mediated by the binding of serum Tf to the TfR. Transferrin is then internalized within an endocytotic vesicle by receptor-mediated endocytosis and the Fe released from the protein by a decrease in endosomal pH. Apart from this process, several cell types also have other efficient mechanisms of Fe uptake from Tf that includes a process consistent with non-specific adsorptive pinocytosis and a mechanism that is stimulated by small-Mr Fe complexes. This latter mechanism appears to be initiated by hydroxyl radicals generated by the Fe complexes, and may play a role in Fe overload disease where a significant amount of serum non-Tf-bound Fe exists. Apart from Tf-bound Fe uptake, mammalian cells also possess a number of mechanisms that can transport Fe from small-Mr Fe complexes into the cell. In fact, recent studies have demonstrated that the membrane-bound Tf homologue, MTf, can bind and internalize Fe from 59Fe-citrate. However, the significance of this Fe uptake process and its pathophysiological relevance remain uncertain. Iron derived from Tf or small-Mr complexes is probably transported into mammalian cells in the Fe(II) state. Once Fe passes through the membrane, it then becomes part of the poorly characterized intracellular labile Fe pool. Iron in the labile Fe pool that is not used for immediate requirements is stored within the Fe-storage protein, ferritin. Cellular Fe uptake and storage are coordinately regulated through a feedback control mechanism mediated at the post-transcriptional level by cytoplasmic factors known as IRP1 and IRP2. These proteins bind to stem-loop structures known as IREs on the 3 UTR of the TfR mRNA and 5 UTR of ferritin and erythroid delta-aminolevulinic acid synthase mRNAs. Interestingly, recent work has suggested that the short-lived messenger molecule, NO (or its by-product, peroxynitrite), can affect cellular Fe metabolism via its interaction with IRP1. Moreover, NO can decrease Fe uptake from Tf by a mechanism separate to its effects on IRP1, and NO may also be responsible for activated macrophage-mediated Fe release from target cells. On the other hand, the expression of inducible NOS which produces NO, can be stimulated by Fe chelators and decreased by the addition of Fe salts, suggesting that Fe is involved in the control of NOS expression.

Dietary iron intake modulates the activity of iron regulatory proteins and the abundance of ferritin and mitochondrial aconitase in rat liver

Iron regulatory protein 1 (IRP1) and IRP2 are cytoplasmic RNA binding proteins that coordinate cellular iron homeostasis in mammals. We investigated the effect of dietary iron intake on rat liver IRP activity in relation to the abundance of two targets of IRP action, ferritin and mitochondrial aconitase (m-aconitase). Rats were fed diets containing 2, 11, 20, 37 (control), 72 or 107 mg iron/kg diet for 3 wk. RNA binding activity of IRP1 and IRP2 was enhanced one- to twofold in rats fed 11 or 2 mg iron/kg diet compared with control rats. IRP RNA binding activity was inversely correlated to blood hemoglobin levels (r = -0.787; P < 0.0001). Compared with control rats, liver ferritin levels were depressed in rats fed 20 mg iron/kg diet and were undetectable in rats ingesting diets with 11 or 2 mg iron/kg diet. Ferritin concentrations were biphasically related to IRP RNA binding activity with the regulation of IRP occurring before the onset of ferritin accumulation. Iron deficiency caused up to a 50% decline in m-aconitase abundance. IRP RNA binding activity and m-aconitase abundance were inversely correlated (r = -0.751; P < 0.0001). Our results indicate that (1) liver IRP activity is responsive to a range of dietary iron levels, (2) there appears to be a differential effect of IRPs on ferritin and m-aconitase abundance, and (3) activation of IRPs may contribute to the alterations in energy metabolism in iron deficiency through an impairment of m-aconitase synthesis.

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

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

Iron metabolism: a comprehensive review

Despite its abundance in the earth's crust, iron deficiency is a serious health issue in many parts of the world. Although fundamental observations about iron metabolism and the significance of iron nutriture were first noted some time ago, the molecular mechanisms involved in iron metabolism are just now being defined.

Iron regulatory proteins 1 and 2

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

Molecular control of vertebrate iron metabolism: mRNA-based regulatory circuits operated by iron, nitric oxide, and oxidative stress

As an essential nutrient and a potential toxin, iron poses an exquisite regulatory problem in biology and medicine. At the cellular level, the basic molecular framework for the regulation of iron uptake, storage, and utilization has been defined. Two cytoplasmic RNA-binding proteins, iron-regulatory protein-1 (IRP-1) and IRP-2, respond to changes in cellular iron availability and coordinate the expression of mRNAs that harbor IRP-binding sites, iron-responsive elements (IREs). Nitric oxide (NO) and oxidative stress in the form of H2O2 also signal to IRPs and thereby influence cellular iron metabolism. The recent discovery of two IRE-regulated mRNAs encoding enzymes of the mitochondrial citric acid cycle may represent the beginnings of elucidating regulatory coupling between iron and energy metabolism. In addition to providing insights into the regulation of iron metabolism and its connections with other cellular pathways, the IRE/IRP system has emerged as a prime example for the understanding of translational regulation and mRNA stability control. Finally, IRP-1 has highlighted an unexpected role for iron sulfur clusters as post-translational regulatory switches.

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

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

Manduca sexta hemolymph ferritin: cDNA sequence and mRNA expression

A cDNA clone encoding a subunit of the tobacco hornworm Manduca sexta (Ms) hemolymph (serum) ferritin (Fer) has been identified and sequenced. The deduced amino acid (aa) sequence shows approx. 50% similarity to vertebrate Fer subunit sequences, and the nucleotide sequence contains a stem-loop structure in the 5' untranslated region that could serve as an iron-responsive element (IRE). The stem-loop of this putative IRE exhibits high identity to vertebrate IRE that play an essential role in the control of Fer synthesis. The Ms Fer subunit lacks one of the three Tyr residues required for the rapid biomineralization of iron shown in vertebrate heavy-chain Fer. In addition, aa residues that comprise the putative ferroxidase centers generally are not conserved, suggesting that the Ms Fer subunit more closely resembles the vertebrate light-chain subunit. Northern blot analyses indicate that the fer mRNA is expressed in the midgut, fat body and hemocytes, with the greatest expression in the midgut.

Role of retinoic acid and oxidative stress in embryonic stem cell death and neuronal differentiation

Embryonic stem (ES) cells are a suitable system to study events occurring during development. In the present work we show that the apoptotic program was activated in ES cells, either by simple removal of the reducing agent 2-mercaptoethanol (2-ME), or by addition of all trans-retinoic (ATRA) to embryoid bodies. In these two conditions, there was an increase in reactive oxygen species and antioxidants such as catalase, superoxide dismutase or phenol prevented ATRA-induced cell death. Neuronal differentiation was observed when undifferentiated ES cells were treated with ATRA in the absence of serum and the presence of 2-ME.

Relationship between duodenal cytosolic aconitase activity and iron status in the mouse

Cytosolic aconitase activity was assayed in duodenal mucosa from mice subjected to a variety of manipulations known to modulate duodenal iron status and duodenal iron absorption. No changes in cytosolic aconitase activity were observed 1 h after oral FeSO4 dosing or intramuscular desferrioxamine treatment. Three days of hypoxic exposure and two weeks treatment with intramuscular iron dextran also had no effect on cytosolic aconitase. Three weeks growth on an iron deficient diet significantly reduced cytosolic aconitase activity. In no situation was there any evidence for significant amounts of inactive aconitase which could be activated in vitro with FeSO4/cysteine. These data suggest that duodenal cytosolic aconitase is not sensitive to acute changes in mucosal iron levels and is generally much less sensitive to body iron status than is duodenal iron absorption. There is evidence that chronic iron depletion reduces cytosolic aconitase to a relatively small degree but generally activity is maintained, consistent with an important metabolic role for the enzyme.

Spectroscopic characterisation of an aconitase (AcnA) of Escherichia coli

A spectroscopic study of an aconitase, AcnA, from Escherichia coli is presented. The amino acid sequence of AcnA has 53% identity with mammalian cytosolic aconitase (c-aconitase) which is the translational regulator known as iron regulatory factor (IRF). In the [3Fe-4S](+)-containing, inactive state, AcnA displays an EPR signal which is not unlike the corresponding signal from mammalian mitochondrial aconitase (m-aconitase) but is even more similar to the signal from c-aconitase. This is perhaps related to the greater similarity of the AcnA amino acid sequence with c-aconitase. Magnetic circular dichroism (MCD) spectroscopy has revealed that the electronic structure of the [3Fe-4S] cluster of AcnA must be similar to, but not identical to that of m-aconitase. Whilst the [Fe-4S] clusters from both of these enzymes display some features in their MCD spectra common to [3Fe-4S] clusters in general, their spectra overall are unique and indicate that the Fea atom of the [4Fe-4S] form is not the only unusual feature of the [Fe-S] clusters of aconitases. Active [4Fe-4S]-containing AcnA can be reduced to yield an EPR signal due to a [4Fe-4S]+ cluster which is indistinguishable from the signals from the [4Fe-4S]+ cluster in the mammalian enzymes. However, in contrast to the mammalian enzymes, the EPR signals of the cluster in AcnA are not significantly perturbed upon the addition of substrate. Furthermore, the catalytic activity of [Fe-4S](2+)-containing AcnA is fivefold higher than that of m-aconitase. The mechanistic implications of these data are discussed. A novel S = 1/2 EPR signal with g approximately 2 was observed in AcnA upon treatment with EDTA. The species giving rise to this signal is proposed to be an intermediate in cluster deconstruction.

Nitric-oxide-mediated activation of iron-regulatory protein controls hepatic iron metabolism during acute inflammation

The molecular regulation of intracellular iron metabolism has been studied in the livers of rats undergoing an acute inflammatory reaction following turpentine injection. Treatment induced an increase in the steady-state level of the transferrin receptor (TfR) mRNA, peaking 18 h after treatment and returning to control levels 24 h after treatment, with no change in TfR gene transcription. RNA band-shift assays documented an activation of the cytoplasmic RNA-binding protein called the iron-regulatory protein (IRP), in parallel with a rise in the amount of TfR transcripts. A 2-3-fold increase in the amount of H and L ferritin subunit mRNAs was found 12-18 h after turpentine treatment. Surprisingly, higher accumulation of ferritin mRNAs did not result in appreciable differences in the liver ferritin content. This might be due to the concomitant rise of IRP activity, which is known to prevent ferritin mRNA translation. The absence of significant changes in the total iron and ferritin contents prompted us to investigate the role of nitric oxide (NO), an inflammatory mediator which is also known to modulate the activity of IRP. Northern-blot analysis showed a marked enhancement in the expression of the inducible form of nitric oxide synthase mRNA in turpentine-treated rats. Furthermore, the activation of IRP and the increase of the TfR mRNA content that occur in turpentine-treated rats were abolished by treatment with N5-nitro-L-arginine, a specific nitric oxide synthase inhibitor. The present data suggest that NO-mediated activation of IRP regulates alterations of hepatic iron homeostasis that occur in acute inflammation.

Superoxide radical and iron modulate aconitase activity in mammalian cells

Aconitase is a member of a family of iron-sulfur-containing (de)hydratases whose activities are modulated in bacteria by superoxide radical (O2-.)-mediated inactivation and iron-dependent reactivation. The inactivation-reactivation of aconitase(s) in cultured mammalian cells was explored since these reactions may impact important and diverse aconitase functions in the cytoplasm and mitochondria. Conditions which increase O2-. production including exposure to the redox-cycling agent phenazine methosulfate (PMS), inhibitors of mitochondrial ubiquinol-cytochrome c oxidoreductase, or hyperoxia inactivated aconitase in mammalian cells. Overproduction of mitochondrial Mn-superoxide dismutase protected aconitase from inactivation by PMS or inhibitors of ubiquinol-cytochrome c oxidoreductase, but not from normobaric hyperoxia. Aconitase activity was reactivated (t1/2 of 12 +/- 3 min) upon removal of PMS. The iron chelator deferoxamine impaired reactivation and increased net inactivation of aconitase by O2-.. The ability of ubiquinol-cytochrome c oxidoreductase-generated O2-. to inactivate aconitase in several cell types correlated with the fraction of the aconitase activity localized in mitochondria. Extracellular O2-. generated with xanthine oxidase did not affect aconitase activity nor did exogenous superoxide dismutase decrease aconitase inactivation by PMS. The results demonstrate a dynamic and cyclical O2-.-mediated inactivation and iron-dependent reactivation of the mammalian [4Fe-4S] aconitases under normal and stress conditions and provide further evidence for the membrane compartmentalization of O2-..

Molecular regulation of iron proteins

Cellular iron metabolism comprises pathways of iron-protein synthesis and degradation, iron uptake via transferrin receptor (TfR) or release to the extracellular space, as well as iron deposition into ferritin and remobilization from such stores. Different cell types, depending on their rate of proliferation and/or specific functions, show strong variations in these pathways and have to control their iron metabolism to cope with individual functions. Studies with cultured cells have revealed a specific cytoplasmic protein, called 'iron regulatory protein' (IRP) (previously known as IRE-BP or IRF), that plays a key role in iron homoeostasis by regulating coordinately the synthesis of TfR, ferritin, and erythroid 5-aminolevulinate synthase (eALAS). Present in all tissues analysed, IRP is identical with the [4Fe-4S] cluster containing cytoplasmic aconitase. Under conditions of iron chelation, IRP is an apo-protein which binds with high affinity to specific RNA stem-loop elements (IREs) located 5' of the initiation codon in ferritin and eALAS mRNA, and 3' in the untranslated region of TfR mRNA. At 5' sites IRF blocks mRNA translation, whereas 3' it inhibits TfR mRNA degradation. Both effects compensate for low intracellular iron concentrations. Under high iron conditions, IRP is converted to the holo-protein and dissociates from mRNA. This reverses the control towards less iron uptake and more iron storage. Iron can therefore be considered as a feedback regulator of its own metabolism. It has recently become evident that nitric oxide, produced by macrophages and other cell types in response to interferon-gamma, induces the IRE-binding activity of IRF. Moreover measurements of the RNA-binding activity of IRP in tissue extracts may provide valuable information on iron availability.

Control of cellular iron homeostasis by iron-responsive elements in vivo

It has recently been proposed that cellular iron homeostasis in mammalian cells is regulated at the post-transcriptional level by the reciprocal control of transferrin receptor and ferritin mRNA expression via an iron-regulatory factor. This iron-regulatory factor has been shown to be a cytoplasmic aconitase which can bind to iron-responsive elements in the corresponding mRNAs with greater or lesser affinity as a function of the iron status of the cell. In the present study, we show that in vivo the affinity of iron-regulatory factor for iron-responsive elements in liver reflects the long-term iron status of the tissue in animal models for iron overloading and iron deficiency, when combined with altered transferrin saturation and serum iron levels. In contrast hepatic iron overload achieved without altering such haematopoeitic indices, had a less pronounced effect. In both spleen and heart, the affinities of iron-regulatory factor changed in parallel with both altered iron status and haematological markers. In brain and duodenum, there were no consistent changes in iron-regulatory-factor activity with iron loading or depletion. Iron-regulatory-factor activity in kidney responded in an as yet unexplained manner.