Two genetic loci participate in the regulation by iron of the gene for the human transferrin receptor

Efficacy and safety of deferoxamine in moderately ill COVID-19 patients: An open label, randomized controlled trial

BACKGROUND

Deferoxamine is a potent iron chelator that could remove iron from the virus, and severe acute respiratory syndrome coronavirus 2 requires iron to replication. Also, deferoxamine has antioxidant and cytokine-modulating effects. Therefore, we evaluated the efficacy and safety of deferoxamine in patients with moderate coronavirus disease 2019 (COVID-19).

METHODS

In this randomized controlled trial, patients with moderate COVID-19 were randomly assigned in a 1:1 ratio to the deferoxamine group (received a solution of 500 mg deferoxamine divided into 4 doses a day through a nebulizer for 7 days) and the control group. The main outcomes were viral clearance, oxygen saturation (SPO2), body temperature, and respiratory rate (RR). Intensive care unit admission, hospital length of stay, and hospital mortality were also assessed.

RESULTS

A total of 62 patients, with 30 in the deferoxamine group and 32 in the control group, were randomly assigned. There was no statistically significant improvement in viral clearance after the intervention ended in the deferoxamine group (36.7%) compared to the control group (34.4%). The results showed there was no significant difference between the analyzed groups in terms of SPO2, body temperature, RR, and the number of patients with a worse prognosis (SPO2 < 96%, temperature ≥ 37.5 °C, or RR ≥ 16/min) at the end of the study. There were no significant differences seen between the groups in terms of intensive care unit admission, hospital length of stay, hospital mortality, and the occurrence of adverse medication events during the follow-up period.

CONCLUSION

Deferoxamine had no significant impact on improving moderately ill patients with COVID-19. However, it was well-tolerated in the patients, and this intervention demonstrated a safe profile of adverse events.

Iron-dependent post transcriptional control of mitochondrial aconitase expression

Iron regulatory proteins (IRPs) control the translation of animal cell mRNAs encoding proteins with diverse roles. This includes the iron storage protein ferritin and the tricarboxylic cycle (TCA) enzyme mitochondrial aconitase (ACO2) through iron-dependent binding of IRP to the iron responsive element (IRE) in the 5' untranslated region (UTR). To further elucidate the mechanisms allowing IRPs to control translation of 5' IRE-containing mRNA differentially, we focused on Aco2 mRNA, which is weakly controlled versus the ferritins. Rat liver contains two classes of Aco2 mRNAs, with and without an IRE, due to alterations in the transcription start site. Structural analysis showed that the Aco2 IRE adopts the canonical IRE structure but lacks the dynamic internal loop/bulge five base pairs 5' of the CAGUG(U/C) terminal loop in the ferritin IREs. Unlike ferritin mRNAs, the Aco2 IRE lacks an extensive base-paired flanking region. Using a full-length Aco2 mRNA expression construct, iron controlled ACO2 expression in an IRE-dependent and IRE-independent manner, the latter of which was eliminated with the ACO23C3S mutant that cannot bind the FeS cluster. Iron regulation of ACO23C3S encoded by the full-length mRNA was completely IRE-dependent. Replacement of the Aco23C3S 5' UTR with the Fth1 IRE with base-paired flanking sequences substantially improved iron responsiveness, as did fusing of the Fth1 base-paired flanking sequences to the native IRE in the Aco3C3S construct. Our studies further define the mechanisms underlying the IRP-dependent translational regulatory hierarchy and reveal that Aco2 mRNA species lacking the IRE contribute to the expression of this TCA cycle enzyme.

Roquin is a major mediator of iron-regulated changes to transferrin receptor-1 mRNA stability

Transferrin receptor-1 (TfR1) has essential iron transport and proposed signal transduction functions. Proper TfR1 regulation is a requirement for hematopoiesis, neurological development, and the homeostasis of tissues including the intestine and muscle, while dysregulation is associated with cancers and immunodeficiency. TfR1 mRNA degradation is highly regulated, but the identity of the degradation activity remains uncertain. Here, we show with gene knockouts and siRNA knockdowns that two Roquin paralogs are major mediators of iron-regulated changes to the steady-state TfR1 mRNA level within four different cell types (HAP1, HUVEC, L-M, and MEF). Roquin is demonstrated to destabilize the TfR1 mRNA, and its activity is fully dependent on three hairpin loops within the TfR1 mRNA 3′-UTR that are essential for iron-regulated instability. We further show in L-M cells that TfR1 mRNA degradation does not require ongoing translation, consistent with Roquin-mediated instability. We conclude that Roquin is a major effector of TfR1 mRNA abundance.

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Roquin is a major mediator of iron-regulated TfR1 mRNA instability

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Roquin-mediated instability requires three stem loops within the TfR1 3′-UTR

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Iron-regulated TfR1 mRNA instability can occur in the absence of Regnase-1

The DMT1 IVS4+44C>A polymorphism and the risk of iron deficiency anemia in children with celiac disease

BACKGROUND

Iron deficiency anemia in celiac disease is related to impaired duodenal mucosal uptake, due to villous atrophy. Iron enters the enterocytes through an apical divalent metal transporter, DMT1. Different DMT1 transcripts have been identified, depending on the presence of an iron-responsive element that allows DMT1 up-regulation during iron starvation. An intronic DMT1 polymorphism, IVS4+44C>A, has been associated with metal toxicity, and the CC-carriers show high iron levels.

AIMS

This study investigates the association between DMT1 IVS4+44C>A and anemia in a cohort of 387 Italian celiac children, and the functional role of the polymorphism.

METHODS AND RESULTS

By association analysis, we found that DMT1 IVS4+44-AA genotype confers a four-fold risk of developing anemia, despite of atrophy degree. By analysis of mRNA from gastroesophageal biopsies, we found that total DMT1 is significantly upregulated in presence of mild, but not severe, atrophy, independently from IVS4+44C>A variant, and in normal but not in atrophic CC-biopsies. Moreover, we found that A-allele is associated to preferential expression of the DMT1 transcripts lacking the iron-responsive element, thus limiting the DMT1 overexpression that normally occurs to respond to iron starvation.

DISCUSSION

Possibly, the IVS4+44-AA-related dysregulation of the iron-induced changes in DMT1 expression is not able to impair iron absorption in physiological condition. However, if exacerbated by the concomitant massive loss of functional absorbing tissue paralleling worsened stages of villus atrophy, it might be ineffective in counteracting iron deficiency, despite of DMT1 overexpression.

CONCLUSION

We suggest, for the first time, that celiac disease may unmask the contribution of the DMT1 IVS4+44C>A polymorphism to the risk of anemia.

Hepcidin knockout mice spontaneously develop chronic pancreatitis owing to cytoplasmic iron overload in acinar cells

Iron is both an essential and a potentially toxic element, and its systemic homeostasis is controlled by the iron hormone hepcidin. Hepcidin binds to the cellular iron exporter ferroportin, causes its degradation, and thereby diminishes iron uptake from the intestine and the release of iron from macrophages. Given that hepcidin-resistant ferroportin mutant mice show exocrine pancreas dysfunction, we analysed pancreata of aging hepcidin knockout (KO) mice. Hepcidin and Hfe KO mice were compared with wild-type (WT) mice kept on standard or iron-rich diets. Twelve-month-old hepcidin KO mice were subjected to daily minihepcidin PR73 treatment for 1 week. Six-month-old hepcidin KO mice showed cytoplasmic acinar iron overload and mild pancreatitis, together with elevated expression of the iron uptake mediators DMT1 and Zip14. Acinar atrophy, massive macrophage infiltration, fatty changes and pancreas fibrosis were noted in 1-year-old hepcidin KO mice. As an underlying mechanism, 6-month-old hepcidin KO mice showed increased pancreatic oxidative stress, with elevated DNA damage, apoptosis and activated nuclear factor-κB (NF-κB) signalling. Neither iron overload nor pancreatic damage was observed in WT mice fed iron-rich diet or in Hfe KO mice. Minihepcidin application to hepcidin KO mice led to an improvement in general health status and to iron redistribution from acinar cells to macrophages. It also resulted in decreased NF-κB activation and reduced DNA damage. In conclusion, loss of hepcidin signalling in mice leads to iron overload-induced chronic pancreatitis that is not seen in situations with less severe iron accumulation. The observed tissue injury can be reversed by hepcidin supplementation. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Hepatocyte-targeted HFE and TFR2 control hepcidin expression in mice

Hereditary hemochromatosis is caused by mutations in the hereditary hemochromatosis protein (HFE), transferrin-receptor 2 (TfR2), hemojuvelin, hepcidin, or ferroportin genes. Hepcidin is a key iron regulator, which is secreted by the liver, and decreases serum iron levels by causing the down-regulation of the iron transporter, ferroportin. Mutations in either HFE or TfR2 lower hepcidin levels, implying that both HFE and TfR2 are necessary for regulation of hepcidin expression. In this study, we used a recombinant adeno-associated virus, AAV2/8, for hepatocyte-specific expression of either Hfe or Tfr2 in mice. Expression of Hfe in Hfe-null mice both increased Hfe and hepcidin mRNA and lowered hepatic iron and Tf saturation. Expression of Tfr2 in Tfr2-deficient mice had a similar effect, whereas expression of Hfe in Tfr2-deficient mice or of Tfr2 in Hfe-null mice had no effect on liver or serum iron levels. Expression of Hfe in wild-type mice increased hepcidin mRNA and lowered iron levels. In contrast, expression of Tfr2 had no effect on wild-type mice. These findings suggest that Hfe is limiting in formation of the Hfe/Tfr2 complex that regulates hepcidin expression. In addition, these studies show that the use of recombinant AAV vector to deliver genes is a promising approach for studying physiologic consequences of protein complexes.

Mammalian iron transport

Iron is essential for basic cellular processes but is toxic when present in excess. Consequently, iron transport into and out of cells is tightly regulated. Most iron is delivered to cells bound to plasma transferrin via a process that involves transferrin receptor 1, divalent metal-ion transporter 1 and several other proteins. Non-transferrin-bound iron can also be taken up efficiently by cells, although the mechanism is poorly understood. Cells can divest themselves of iron via the iron export protein ferroportin in conjunction with an iron oxidase. The linking of an oxidoreductase to a membrane permease is a common theme in membrane iron transport. At the systemic level, iron transport is regulated by the liver-derived peptide hepcidin which acts on ferroportin to control iron release to the plasma.

Liver iron transport

The liver plays a central role in iron metabolism. It is the major storage site for iron and also expresses a complex range of molecules which are involved in iron transport and regulation of iron homeostasis. An increasing number of genes associated with hepatic iron transport or regulation have been identified. These include transferrin receptors (TFR1 and 2), a ferrireductase (STEAP3), the transporters divalent metal transporter-1 (DMT1) and ferroportin (FPN) as well as the haemochromatosis protein, HFE and haemojuvelin (HJV), which are signalling molecules. Many of these genes also participate in iron regulatory pathways which focus on the hepatic peptide hepcidin. However, we are still only beginning to understand the complex interactions between liver iron transport and iron homeostasis. This review outlines our current knowledge of molecules of iron metabolism and their roles in iron transport and regulation of iron homeostasis.

The ferritin family of iron storage proteins

The ferritins are a family of proteins produced in a variety of amounts and types depending on the state of development of an animal, or the state of differentiation of a particular cell type, or the environment. Iron storage is the main function of the ferritins when iron is needed for intracellular use (housekeeping) for iron proteins such as ribonucleotide reductase, cytochromes, oxidases, nitrogenases, or photosynthetic reaction centers or for extracellular use by other cells (specialized). Under abnormal conditions, such as the breach of transferrin-receptor-controlled incorporation of iron, ferritin can also serve to detoxify excess intracellular iron. The structure of ferritin is very complex, consisting of a protein coat of 24 polypeptide subunits, approximately 20 kDa, which surrounds an inorganic phase of hydrous ferric oxide. The polypeptide subunits, bundles of four alpha helices, display remarkable conservation of sequence among plants and animals, which is probably related to the necessity of forming the hollow sphere pierced by 14 channels through which iron may pass. In spite of the conserved regions of sequence, there are multiple genes for ferritin polypeptide subunits within an organism; at the moment three distinct subunit types, H H'(or M), and L, have been identified which are expressed in a cell-specific fashion. How many different subunit types exist, the influence on function, and the number of genes required to encode them are currently being actively investigated. Not only does the protein coat of ferritin display variations, the inorganic phase of ferritin can vary as well. For instance, differences can occur in the number of Fe atoms (up to 4500), as well as in the phosphorus content and in the degree of hydration and order. Such observations have depended on the use of a variety of physical techniques such as X-ray diffraction, EXAFS, and Mössbauer spectroscopy. The same approaches, as well as EPR spectroscopy, have been used to monitor the path taken by Fe as it passes from mononuclear Fe(II) outside the protein coat to polynuclear Fe(III) inside the protein coat. Both mononuclear Fe(II) and Fe(III) have been observed, as well as dimeric Fe(II)-O-Fe(III), and Fe(III)-oxo bridged clusters attached to the protein. A possible protein site for the Fe(III) cluster is a groove on the inner surface of the dimeric interface, suggested by the structure and from the affect of natural cross-links between subunit pairs.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular control of vertebrate iron homeostasis by iron regulatory proteins

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

Inhibition of transferrin receptor 1 transcription by a cell density response element

TfR1 (transferrin receptor 1) mediates the uptake of transferrin-bound iron and thereby plays a critical role in cellular iron metabolism. Its expression is coupled to cell proliferation/differentiation and controlled in response to iron levels and other signals by transcriptional and post-transcriptional mechanisms. It is well established that TfR1 levels decline when cultured cells reach a high density and in the present study we have investigated the underlying mechanisms. Consistent with previous findings, we demonstrate that TfR1 expression is attenuated in a cell-density-dependent manner in human lung cancer H1299 cells and in murine B6 fibroblasts as the result of a marked decrease in mRNA content. This response is not associated with alterations in the RNA-binding activity of iron regulatory proteins that are indicative of a transcriptional mechanism. Reporter assays reveal that the human TfR1 promoters contains sequences mediating cell-density-dependent transcriptional inhibition. Mapping of the human and mouse TfR1 promoters identified a conserved hexa-nucleotide 5'-GAGGGC-3' motif with notable sequence similarity to a previously described element within the IGF-2 (insulin-like growth factor-2) promoter. We show that this motif is necessary for the formation of specific complexes with nuclear extracts and for cell-density-dependent regulation in reporter gene assays. Thus the TfR1 promoter contains a functional 'cell density response element' (CDRE).

Transferrin as a muscle trophic factor

Muscle cells grow by proliferation and protein accumulation. During the initial stages of development the participation of nerves is not always required. Myoblasts and satellite cells proliferate, fusing to form myotubes which further differentiate to muscle fibers. Myotubes and muscle fibers grow by protein accumulation and fusion with other myogenic cells. Muscle fibers finally reach a quasi-steady state which is then maintained for a long period. The mechanism of maintenance is not well understood. However, it is clear that protein metabolism plays a paramount role. The role played by satellite cells in the maintenance of muscle fibers is not known. Growth and maintenance of muscle cells are under the influence of various tissues and substances. Among them are Tf and the motor nerve, the former being the main object of this review and essential for both DNA and protein synthesis. Two sources of Tf have been proposed, i.e., the motor nerve and the tissue fluid. The first proposal is that the nervous trophic influence on muscle cells is mediated by Tf which is released from the nerve terminals. In this model, the sole source of Tf which is donated to muscle cells should be the nerve, and Tf should not be provided for muscle fiber at sites other than the synaptic region; otherwise, denervation atrophy would not occur, since Tf provided from TfR located at another site would cancel the effect of denervation. The second proposal is that Tf is provided from tissue fluid. This implies that an adequate amount of Tf is transferred from serum to tissue fluid; in this case TfR may be distributed over the entire surface of the cells. The trophic effects of the motor neuron have been studied in vivo, but its effects of myoblast proliferation have not been determined. There are few experiments on its effects on myotubes. Most work has been made on muscle fibers, where innervation is absolutely required for their maintenance. Without it, muscle fibers atrophy, although they do not degenerate. In contrast, almost all the work on Tf has been performed in vitro. Its effects on myoblast proliferation and myotube growth and maintenance have been established; myotubes degenerate following Tf removal. But its effects on mature muscle fibers in vivo are not well understood. Muscle fibers possess TfR all over on their cell surface and contain a variety of Fe-binding proteins, such as myoglobin. It is entirely plausible that muscle fibers require an amount of Tf, and that this is provided by TfR scattered on the cell surface.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of transferrin receptor 2 protein levels by transferrin

Transferrin receptor 2 (TfR2) plays a critical role in iron homeostasis because patients carrying disabling mutations in the TFR2 gene suffer from hemochromatosis. In this study, iron-responsive regulation of TfR2 at the protein level was examined in vitro and in vivo. HepG2 cell TfR2 protein levels were up-regulated after exposure to holotransferrin (holoTf) in a time- and dose-responsive manner. ApoTf or high-iron treatment with non–Tf-bound iron failed to elicit similar effects, suggesting that TfR2 regulation reflects interactions of the iron-bound ligand. Hepatic TfR2 protein levels also reflected an adaptive response to changing iron status in vivo. Liver TfR2 protein levels were down- and up-regulated in rats fed an iron-deficient and a high-iron diet, respectively. TfR2 was also up-regulated in Hfe-/- mice, an animal model that displays liver iron loading. In contrast, TfR2 levels were reduced in hypotransferrinemic mice despite liver iron overload, supporting the idea that regulation of the receptor is dependent on Tf. This idea is confirmed by up-regulation of TfR2 in β-thalassemic mice, which, like hypotransferrinemic mice, are anemic and incur iron loading, but have functional Tf. Based on these combined results, we hypothesize that TfR2 acts as a sensor of iron status such that receptor levels reflect Tf saturation.

Genetic regulation of cell function in response to iron overload or chelation

Iron influences many aspects of cell function on different biochemical levels. This review considers effects mediated through iron-dependent changes in gene expression in mammalian cells. Several classes of related genes are responsive to cellular iron levels, but no clear patterns readily account for the toxicity of iron overload or the consequences of removal of iron with chelating agents. Here we group some of the genes influenced by iron status into those related to iron metabolism, oxygen and oxidative stress, energy metabolism, cell cycle regulation, and tissue fibrosis. Iron excess and chelation do not generally result in a continuous or graded transcriptional response, but indicate operation of distinct mechanisms. An emerging concept is that iron signals through generation of reactive oxygen species to activate transcription factors such as NF-kappaB, whereas iron removal mimics hypoxia, perhaps by disrupting iron-based O(2) sensors and influencing gene expression through, e.g., the hypoxia-inducible factor, HIF-1. Heme and other metalloporphyrins have other distinct mechanisms for regulating transcription. Regulation of gene expression through iron-responsive elements in mRNAs coded by several genes is one of the best understood mechanisms of translational control.

Previously uncharacterized isoforms of divalent metal transporter (DMT)-1: implications for regulation and cellular function

Divalent metal transporter 1 (DMT1) mediates apical iron uptake into duodenal enterocytes and also transfers iron from the endosome into the cytosol after cellular uptake via the transferrin receptor. Hence, mutations in DMT1 cause systemic iron deficiency and anemia. DMT1 mRNA levels are increased in the duodenum of iron-deficient animals. This regulation has been observed for DMT1 mRNA harboring an iron-responsive element (IRE) in its 3' UTR, but not for a processing variant lacking a 3'UTR IRE, suggesting that the IRE regulates the expression of DMT1 mRNA in response to iron levels. Here, we show that iron regulation of DMT1 involves the expression of a previously unrecognized upstream 5' exon (exon 1A) of the human and murine DMT1 gene. The expression of this previously uncharacterized 5' exon is tissue-specific and particularly prevalent in the duodenum and kidney. It adds an in-frame AUG translation initiation codon extending the DMT1 ORF by a conserved sequence of 29-31 amino acids. In combination with the IRE- and non-IRE variants in the 3'UTR, our results reveal the existence of four DMT1 mRNA isoforms predicting the synthesis of four different DMT1 proteins. We show that two regulatory regions, the 5' promoter/exon 1A region and the IRE-containing terminal exon participate in iron regulation of DMT1 expression, which operate in a tissue-specific way. These results uncover an unexpected complexity of DMT1 expression and regulation, with implications for understanding the physiology, cell biology, and pathophysiology of mammalian iron metabolism.

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

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

Pathogenesis and treatment of anaemia of chronic disease

Anaemia of chronic disease (ACD), the most frequent anaemia among hospitalized patients, develops under chronic inflammatory disorders such as chronic infections, cancer or autoimmune diseases. A number of different pathways contribute to ACD, such as diversion of iron traffic, a diminished erythropoiesis, a blunted response to erythropoietin, erythrophagocytosis and bone marrow invasion by tumour cells and pathogens. Nevertheless, ACD is a reflection of an activated immune system and possibly results from an innovative defence strategy of the body in order to withdraw the essential growth factor iron from invading pathogens and to increase the efficacy of cell-mediated immunity. Diagnosis of ACD can be assessed by examination of chances in serum iron parameters with low to normal serum iron, transferrin saturation and transferrin concentrations on the one hand and normal to increased ferritin, zinc protoporphyrin IX and cytokine levels on the other side. Therapy of ACD includes the cure of the underlying the disease. Apart from this transfusions for rapid correction of haemoglobin levels, and human recombinant erythropoietin for prolonged therapy are used. However, response rates to recombinant erythropoietin are sometimes low. Iron alone should be strictly avoided due to its growth-promoting effect towards micro-organisms and tumour cells and because of it capacity to inhibit T-cell-mediated immune effector pathways. We urgently need prospective clinical trials to gain knowledge about the effects of anaemia correction and/or the use of erythropoietin towards the course of the underlying disease, to find out if a combination therapy with erythropoietin and iron may be beneficial in ACD and to define therapeutic end-points.

Nitric oxide-inducible expression of heme oxygenase-1 in human cells. Translation-independent stabilization of the mRNA and evidence for direct action of nitric oxide

Expression of heme oxygenase-1 (HO-1) in mammalian cells contributes to resistance to various types of free radical damage. Nitric oxide (NO) induces HO-1 in many cell types, but the specific contribution of transcriptional or post-transcriptional effects to this induction have remained unresolved. Here we show that the extent of HO-1 mRNA expression in IMR-90 and HeLa cells depends on the rate of NO delivery, and that the induction occurs more slowly in HeLa than in human fibroblast (IMR-90) cells. We used a specific NO scavenger (2-(4-carboxylphenyl)-4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide) that completely prevented the inducible expression of HO-1 by NO, pointing to direct signaling action of NO in this induction. By inhibiting transcription during the NO exposure, we have confirmed that NO treatment activates a mechanism that stabilizes HO-1 mRNA. The increase in the HO-1 mRNA half-life in IMR-90 cells was directly correlated with increasing rates of NO release. We also show here that the stabilization of the HO-1 message does not require de novo protein synthesis. Collectively, these results show that stabilization of HO-1 mRNA can be finely tuned to the NO exposure, and that the effect in human fibroblasts is mediated by a pre-existing protein.

Analysis of the expression of murine glutaryl-CoA dehydrogenase: in vitro and in vivo studies

Glutaric acidemia type I (GAI) is an autosomal recessive organic acidemia caused by a mutation in the gene encoding glutaryl-CoA dehydrogenase (GCD). Clinically, GAI is characterized by progressive dystonia, resulting from degeneration of neurons in the caudate and putamen nuclei of the striatum. In an attempt to understand the basis for the specific neuropathology in GAI, we have analyzed the expression of the murine GCD gene using both in vitro and in vivo approaches. Transfection studies mapped the mouse GCD promoter to a 500-bp region of DNA 5' of the translation start site. The promoter lacks a TATA consensus sequence, but includes possible binding sites for several transcription factors with roles in the regulation of nuclear genes encoding mitochondrial proteins. Western blot and RT/PCR analyses of mouse tissues demonstrated that GCD is ubiquitously expressed, with the highest levels of expression in liver and kidney, consistent with its role in amino acid oxidation. Expression in multiple regions of the brain was also detected by Western blotting. Based on these results we conclude that the specific neuropathology associated with GCD deficiency in GAI cannot be accounted for by its expression pattern.

Targeting mRNA to regulate iron and oxygen metabolism

A family of non-coding sequences in the mRNA (iso-IREs [iron-responsive elements]) regulate synthesis of key proteins in animal iron and oxidative metabolism such as ferritin and mitochondrial aconitase. Differential recognition between iso-IREs and iso-IRPs (iron regulatory proteins) regulates the translation or degradation of the IRE-containing mRNAs. IREs are hairpin loop structures with an internal loop/bulge or bulge that influence the binding of the iso-IRPs. The iso-IRPs have sequence homology to the aconitases and at least one IRP can be converted to an aconitase. Signals that target the iso-IRE/iso-IRP interactions in mRNA include environmental iron, O2, nitric oxide, H2O2, ascorbate, growth factors, and protein kinase C-dependent IRP phosphorylation. Iso-IRE structural specificity suggests a means of pharmacologically targeting mRNA function with chemicals such as Fe-bleomycin and other transition metal complexes that could be extended to other mRNAs with specific structures. With the iso-IRE/iso-IRP system, nature has evolved coordinated combinatorial control of iron and oxygen metabolism that may exemplify control of mRNAs in other metabolic pathways, viral reproduction, and oncogenesis.

HIF-1-mediated activation of transferrin receptor gene transcription by iron chelation

Treatment with iron chelators mimics hypoxic induction of the hypoxia inducible factor (HIF-1) which activates transcription by binding to hypoxia responsive elements (HRE). We investigated whether HIF-1 is involved in transcriptional activation of the transferrin receptor (TfR), a membrane protein which mediates cellular iron uptake, in response to iron deprivation. The transcription rate of the TfR gene in isolated nuclei was up-regulated by treatment of Hep3B human hepatoma cells with the iron chelator desferrioxamine (DFO). The role of HIF-1 in the activation of TfR was indicated by the following observations: (i) DFO-dependent activation of a luciferase reporter gene in transfected Hep3B cells was mediated by a fragment of the human TfR promoter containing a putative HRE sequence; (ii) mutation of this sequence prevented stimulation of luciferase activity; (iii) binding to this sequence of HIF-1alpha, identified by competition experiments and supershift assays, was induced by DFO. Furthermore, in mouse hepatoma cells unable to assemble functional HIF-1, inducibility of TfR transcription by DFO was lost and TfR mRNA up-regulation was reduced. These results, which show the role of HIF-1 in the control of TfR gene expression in conditions of iron depletion, give insights into the mechanisms of transcriptional regulation which concur with the well-characterized post-transcriptional control of TfR expression to expand the extent of response to iron deficiency.

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.

The transferrin receptor: role in health and disease

The transferrin receptor is a membrane glycoprotein whose only clearly defined function is to mediate cellular uptake of iron from a plasma glycoprotein, transferrin. Iron uptake from transferrin involves the binding of transferrin to the transferrin receptor, internalization of transferrin within an endocytic vesicle by receptor-mediated endocytosis and the release of iron from the protein by a decrease in endosomal pH. With the exception of highly differentiated cells, transferrin receptors are probably expressed on all cells but their levels vary greatly. Transferrin receptors are highly expressed on immature erythroid cells, placental tissue, and rapidly dividing cells, both normal and malignant. In proliferating nonerythroid cells the expression of transferrin receptors is negatively regulated post-transcriptionally by intracellular iron through iron responsive elements (IREs) in the 3' untranslated region of transferrin receptor mRNA. IREs are recognized by specific cytoplasmic proteins (IRPs; iron regulatory proteins) that, in the absence of iron in the labile pool, bind to the IREs of transferrin receptor mRNA, preventing its degradation. On the other hand, the expansion of the labile iron pool leads to a rapid degradation of transferrin receptor mRNA that is not protected since IRPs are not bound to it. However, some cells and tissues with specific requirements for iron probably evolved mechanisms that can override the IRE/IRP-dependent control of transferrin receptor expression. Erythroid cells, which are the most avid consumers of iron in the organism, use a transcriptional mechanism to maintain very high transferrin receptor levels. Transcriptional regulation is also involved in the receptor expression during T and B lymphocyte activation. Macrophages are another example of a cell type that shows 'unorthodox' responses in terms of IRE/IRP paradigm since in these cells elevated iron levels increase (rather than decrease) transferrin receptor mRNA and protein levels. Erythroid cells contain the highest mass of the total organismal transferrin receptors which are released from reticulocytes during their maturation to erythrocytes. Hence, plasma contains small amounts of transferrin receptors which represent a soluble fragment of the extracellular receptor domain. Measurements of serum transferrin receptor concentrations are clinically useful since their levels correlate with the total mass of immature erythroid cells.

Bacillus subtilis aconitase is an RNA-binding protein

The aconitase protein of Bacillus subtilis was able to bind specifically to sequences resembling the iron response elements (IREs) found in eukaryotic mRNAs. The sequences bound include the rabbit ferritin IRE and IRE-like sequences in the B. subtilis operons that encode the major cytochrome oxidase and an iron uptake system. IRE binding activity was affected by the availability of iron both in vivo and in vitro. In eukaryotic cells, aconitase-like proteins regulate translation and stability of iron metabolism mRNAs in response to iron availability. A mutant strain of B. subtilis that produces an enzymatically inactive aconitase that was still able to bind RNA sporulated 40x more efficiently than did an aconitase null mutant, suggesting that a nonenzymatic activity of aconitase is important for sporulation. The results support the idea that bacterial aconitases, like their eukaryotic homologs, are bifunctional proteins, showing aconitase activity in the presence of iron and RNA binding activity when cells are iron-deprived.

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

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

Regulation of 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.

Complex regulation of transferrin receptors during erythropoietin-induced differentiation of J2E erythroid cells--elevated transcription and mRNA stabilisation produce only a modest rise in protein content

The regulation of transferrin-receptor synthesis was studied in J2E erythroid cells induced to differentiate with erythropoietin. Nuclear run-on assays demonstrated that transcription of the transferrin-receptor gene rose markedly after erythropoietin treatment. In addition, transferrin-receptor mRNA was stabilised and this was associated with an increase in the activity of the RNA-binding protein IRP (iron regulatory protein). As a result of increased transcription and mRNA stabilisation, steady-state RNA levels increased 10-20-fold. However, despite these large increases in mRNA, translation only doubled; consequently, modest increases in total protein and surface transferrin receptors were observed. Moreover, this rise in transferrin receptors was transient, and correlated with a burst of proliferation shortly after erythropoietin treatment. The expected inverse relationship between transferrin receptors and ferritin did not occur during J2E maturation as translation of both ferritin subunits increased when transferrin-receptor mRNA levels rose. Analysis of mutant J2E clones incapable of synthesising haemoglobin revealed that surface transferrin-receptor levels were only 15-25% that of the parental erythroid line. We propose that the surface expression of transferrin receptors in J2E cells is governed by three factors: basal levels essential for normal growth in culture; elevated levels needed for haemoglobin synthesis; and a transient erythropoietin-induced increase that is required for the final burst of proliferation. It was concluded that the regulation of transferrin-receptor production in erythropoietin-stimulated J2E cells is complex and that there are several sites of control.

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 mouse gene for vascular endothelial growth factor. Genomic structure, definition of the transcriptional unit, and characterization of transcriptional and post-transcriptional regulatory sequences

We describe the genomic organization and functional characterization of the mouse gene encoding vascular endothelial growth factor (VEGF), a polypeptide implicated in embryonic vascular development and postnatal angiogenesis. The coding region for mouse VEGF is interrupted by seven introns and encompasses approximately 14 kilobases. Organization of exons suggests that, similar to the human VEGF gene, alternative splicing generates the 120-, 164-, and 188-amino acid isoforms, but does not predict a fourth VEGF isoform corresponding to human VEGF206. Approximately 1. 2 kilobases of 5'-flanking region have been sequenced, and primer extension analysis identified a single major transcription initiation site, notably lacking TATA or CCAT consensus sequences. The 5'-flanking region is sufficient to promote a 7-fold induction of basal transcription. The genomic region encoding the 3'-untranslated region was determined by Northern and nuclease mapping analysis. Investigation of mRNA sequences responsible for the rapid turnover of VEGF mRNA (mRNA half-life, <1 h) (Shima, D. T. , Deutsch, U., and D'Amore, P. A. (1995) FEBS Lett. 370, 203-208) revealed that the 3'-untranslated region was sufficient to trigger the rapid turnover of a normally long-lived reporter mRNA in vitro. These data and reagents will allow the molecular and genetic analysis of mechanisms that control the developmental and pathological expression of VEGF.

Defining a novel cis element in the 3'-untranslated region of mammalian ribonucleotide reductase component R2 mRNA: role in transforming growth factor-beta 1 induced mRNA stabilization

Ribonucleotide reductase R2 gene expression is elevated in BALB/c 3T3 fibroblasts treated with transforming growth factor beta 1. We investigated the possibility that the 3'-UTR of ribonucleotide reductase R2 mRNA contains regulatory information for TGF-beta 1 induced message stability. Using end-labeled RNA fragments in gel shift assays and UV cross-linking analyses, we detected in the 3'-UTR a novel 9 nucleotide (nt) cis element, 5'-GAGUUUGAG-3' site, which interacted specifically with a cytosolic protease sensitive factor to form a 75 kDa complex. The cis element protein binding activity was inducible and markedly up-regulated cross-link 4 h after TGF-beta 1 treatment of mouse BALB/c 3T3 cells. Other 3'-UTRs [IRE, GM-CSF, c-myc and homopolymer (U)] were poor competitors to the cis element with regard to forming the TGF-beta 1 dependent RNA-protein complex. However, the cis element effectively competed out the formation of the R2 3'-UTR protein complex. Cytosolic extracts from a variety of mammalian cell lines (monkey Cos7, several mouse fibrosarcomas and human HeLa S3) demonstrated similar TGF-beta 1 dependent RNA-protein band shifts as cell extract from BALB/c 3T3 mouse fibroblasts. Binding was completely prevented by several different mutations within the cis element, and by substitution mutagenesis, we were able to predict the consensus sequences, 5'-GAGUUUNNN-3' and 5'-NNNUUUGAG-3' for optimal protein binding. These results support a model in which the 9 nt region functions in cis to destabilize R2 mRNA in cells; and upon activation, a TGF-beta 1 responsive protein is induced and interacts with the 9 nt cis element in a mechanism that leads to stabilization of the mRNA. This appears to be the first example of a mRNA binding site that is involved in TGF-beta 1-mediated effects.

Nitric oxide signaling to iron-regulatory protein: direct control of ferritin mRNA translation and transferrin receptor mRNA stability in transfected fibroblasts

Iron-regulatory protein (IRP) is a master regulator of cellular iron homeostasis. Expression of several genes involved in iron uptake, storage, and utilization is regulated by binding of IRP to iron-responsive elements (IREs), structural motifs within the untranslated regions of their mRNAs. IRP-binding to IREs is controlled by cellular iron availability. Recent work revealed that nitric oxide (NO) can mimic the effect of iron chelation on IRP and on ferritin mRNA translation, whereas the stabilization of transferrin receptor mRNA following NO-mediated IRP activation could not be observed in gamma-interferon/lipopolysaccharide-stimulated murine macrophages. In this study, we establish the function of NO as a signaling molecule to IRP and as a regulator of mRNA translation and stabilization. Fibroblasts with undetectable levels of endogenous NO synthase activity were stably transfected with a cDNA encoding murine macrophage inducible NO synthase. Synthesis of NO activates IRE binding, which in turn represses ferritin mRNA translation and stabilizes transferrin receptor mRNA against targeted degradation. Furthermore, iron starvation and NO release are shown to be independent signals to IRP. The posttranscriptional control of iron metabolism is thus intimately connected with the NO pathways.

Defining a novel ribonucleotide reductase r1 mRNA cis element that binds to an unique cytoplasmic trans-acting protein

Ribonucleotide reductase is a highly regulated rate-limiting enzyme activity in DNA synthesis, responsible for reducing ribonucleotides to their deoxyribonucleotide forms. Using 3'-end labeled RNA and band-shift and UV cross-linking analyses, we have identified a cis-element, 5'-CAAACUUC-3', within the 3'-untranslated region of the mammalian ribonucleotide reductase R1 mRNA, which binds a cytoplasmic protein in BALB/c 3T3 mouse cells, to form a 57 kDa RNA-protein complex. Sequence-specific binding was observed, and binding was prevented by several different mutations within the cis-element. We suggest that this cis-trans interaction plays a role in R1 mRNA stability.

The bifunctional iron-responsive element binding protein/cytosolic aconitase: the role of active-site residues in ligand binding and regulation

The iron-responsive element binding protein/cytosolic aconitase functions as either an RNA binding protein that regulates the uptake, sequestration, and utilization of iron or an enzyme that interconverts citrate and isocitrate. These mutually exclusive functions are regulated by changes in cellular iron levels. By site-directed mutagenesis we show that (i) ligation of a [4Fe-4S] cluster is necessary to inactivate RNA binding and activate enzyme function in vivo, (ii) three of four arginine residues of the aconitase active site participate in RNA binding, and (iii) aconitase activity is not required for iron-mediated regulation of RNA binding.

Ferrochelatase, glutathione peroxidase and transferrin receptor mRNA synthesis and levels in mouse erythroleukemia cells

Mouse erythroleukemia Friend cells induced to undergo erythroid differentiation by treatment with hexamethylenebisacetamide (HMBA) were shown to increase cytoplasmic ferrochelatase mRNA, transferrin receptor (TfR) mRNA and glutathione peroxidase (GSHPx) mRNA levels. Inhibition of heme synthesis at the level of 5-aminolevulinic acid synthase by isonicotinic acid hydrazide (INH) and D,L-penicillamine (PA) or at the level of 5-aminolevulinic acid dehydratase by succinylacetone (SA) decreased the expression of ferrochelatase mRNA and TfR mRNA. In contrast with these mRNAs, the synthesis and the levels of glutathione peroxidase mRNA increased by the addition of these inhibitors of heme synthesis. The amount of iron in the intracellular low molecular mass iron pool detected in the post-mitochondrial supernatant of Friend cells treated with heme synthesis inhibitors was increased. On the other hand, iron levels in this pool declined with the preincubation of Friend cells with iron chelator pyridoxal isonicotinoylhydrazone (PIH). Further treatment with PIH or desferrioxamine (Desferal) increased the synthesis of TfR mRNA in induced Friend cells. The synthesis of ferrochelatase mRNA declined by the same treatment. The opposite was observed when the iron level in the low molecular mass intracellular nonheme iron pool was elevated by treatment with either diferric transferrin (Fe-Tf) or ferric pyridoxal isonicotinoylhydrazone (Fe-PIH). Exogenously supplied hemin stimulated the synthesis of ferrochelatase mRNA in uninduced Friend cells, while the synthesis of this mRNA in Friend cells taken on the fifth day after induction was inhibited by the addition of hemin.

Transferrin uptake by bone marrow macrophages is independent of the degree of iron saturation

The uptake of transferrin by macrophages was studied in relation to the degree of iron saturation. Rat bone marrow derived macrophages were incubated with transferrin labelled with 59Fe and 3H. At 37 degrees C the amount of 59Fe incorporated by macrophages was dependent on the time of incubation. 3H labelled transferrin was found degraded in the supernatants of the cell culture (material not precipitated by trichloroacetic acid) in a time dependent fashion. Taking into account the specific activity of 59Fe-3H labelled transferrin, we found that 95% of the transferrin uptake was degraded. This suggests that most of the uptake of transferrin was not mediated by a receptor-dependent mechanism, but by a phase fluid endocytosis. 3H-labelled apotransferrin appears in the supernatant of the cell culture at the same rate as 59Fe-3H labelled diferric transferrin, showing an identical uptake for the two types of transferrin. Uptake of apo- or diferric transferrin by macrophages was identical in relation to time of incubation and the amount of transferrin used. These studies suggest that most of the transferrin uptake by bone marrow macrophages (non-activated or non-elicited cells) is mediated by a non-receptor mechanism that is independent of the degree of transferrin saturation.

Reciprocal control of RNA-binding and aconitase activity in the regulation of the iron-responsive element binding protein: role of the iron-sulfur cluster

Several mechanisms of posttranscriptional gene regulation are involved in regulation of the expression of essential proteins of iron metabolism. Coordinate regulation of ferritin and transferrin receptor expression is produced by binding of a cytosolic protein, the iron-responsive element binding protein (IRE-BP) to specific stem-loop structures present in target RNAs. The affinity of this protein for its cognate RNA is regulated by the cell in response to changes in iron availability. The IRE-BP demonstrates a striking level of amino acid sequence identity to the iron-sulfur (Fe-S) protein mitochondrial aconitase. Moreover, the recombinant IRE-BP has aconitase function. The lability of the Fe-S cluster in mitochondrial aconitase has led us to propose that the mechanism by which iron levels are sensed by the IRE-BP involves changes in an Fe-S cluster in the IRE-BP. In this study, we demonstrate that procedures aimed at altering the IRE-BP Fe-S cluster in vitro reciprocally alter the RNA binding and aconitase activity of the IRE-BP. The changes in the RNA binding of the protein produced in vitro appear to match the previously described alterations of the protein in response to iron availability in the cell. Furthermore, iron manipulation of cells correlates with the activation or inactivation of the IRE-BP aconitase activity. The results are consistent with a model for the posttranslational regulation of the IRE-BP in which the Fe-S cluster is altered in response to the availability of intracellular iron and this, in turn, regulates the RNA-binding activity.

Coordination of cellular iron metabolism by post-transcriptional gene regulation

Maintenance of cellular iron homeostasis demands the coordination of iron uptake, intracellular storage, and utilization. Recent investigations suggest that a single genetic regulatory system orchestrates the expression of proteins with central importance for all three aspects of cellular iron metabolism at the level of mRNA stability and translation. Two components of this regulatory system have been defined: a cis-acting mRNA sequence/structure motif called "iron-responsive element" (IRE) and a specific trans-acting cytoplasmic binding protein, here referred to as "IRE-binding protein" (IRE-BP). As an early event in the regulatory cascade, cellular iron deprivation induces the IRE-binding activity of IRE-BP, whereas binding activity is reduced in iron-replete cells. IRE-BP is highly homologous to the iron-sulphur (Fe-S) protein aconitase which strongly suggests that IRE-BP is an Fe-S protein itself. Control over IRE-BP activity by the cellular iron status is exerted post-translationally and likely involves changes between (4Fe-4S) and (3Fe-4S) states of the postulated IRE-BP Fe-S cluster. In addition, post-translational regulation of IRE-BP activity via heme has been proposed. Subsequent to its activation, IRE-BP binds with high affinity to single IREs contained in the 5' untranslated regions (UTRs) of ferritin and erythroid 5-aminolevulinic acid synthase (eALAS) mRNAs. The binding represses translation of these proteins involved in iron storage and utilization, respectively. In contrast, iron uptake is largely regulated via multiple IREs in the 3' UTR of transferrin receptor (TfR) mRNA. TfR-IREs are required for the iron-sensitive control of TfR mRNA stability. IRE-BP binding stabilizes TfR gene transcripts against as yet undefined ribonucleases. As a result of these regulatory interactions, iron starvation induces the expression of TfR, thereby increasing iron uptake, and represses the synthesis of proteins involved in iron storage and utilization. As cellular iron levels rise, the homeostatic balance is maintained by lowering iron uptake and increasing iron storage in ferritin.

The iron regulatory region of ferritin mRNA is also a positive control element for iron-independent translation

The iron regulatory element (IRE) in the 5'-untranslated region of ferritin mRNA interacts with a specific regulator protein (P-90, IRE-BP, or FRP) to block translation. High cellular iron changes the IRE/P-90 interaction to relax the translational block and allow polyribosome formation. We now show that the IRE and base-paired flanking regions also enhance translation in the absence of P-90, explaining the high translational efficiency of deregulated ferritin mRNA observed previously. The effect of the IRE on translational efficiency was examined by comparing four sets of mRNAs: (1) +/- IRE in animal (frog) ferritin, regulated translationally by iron in vivo; (2) +/- animal IRE fused with plant (soybean) ferritin, regulated transcriptionally by iron in vivo; (3) repositioned IRE in animal ferritin; (4) mutated IRE in animal ferritin with G16A substitution, which decreases P-90 binding (negative control). The IRE region increased translational efficiency of both the animal ferritin and the heterologous IRE/soybean ferritin fusion mRNAs; the effect was observed in cell-free translation systems from either plants (wheat germ) or animals (rabbit reticulocyte). Repositioning the IRE further from the 5' cap eliminated positive control of translation. The single base mutation had no effect, indicating that positive and negative translational control involves different sections of the IRE region. Thus, the IRE region in ferritin mRNA encodes both positive translational control and, when combined with the regulator protein P-90, negative translational control.

A regulated RNA binding protein also possesses aconitase activity

A clone for the iron-responsive element (IRE)-binding protein (IRE-BP) has been transfected and expressed in mouse fibroblasts. The IRE-BP gene product binds IREs with high affinity and specificity. Amino acid alignments reveal that the IRE-BP is 30% identical to mitochondrial aconitase. The 18 active site residues of mitochondrial aconitase are identical to those in the IRE-BP, suggesting that the IRE-BP may possess aconitase activity. After purification of native IRE-BP and immunoaffinity purification of transfected and expressed IRE-BP, we demonstrate that the purified IRE-BP has aconitase activity.

Translation and the stability of mRNAs encoding the transferrin receptor and c-fos

Turnover of the full-length human transferrin receptor (TfR) mRNA is regulated by iron, and this regulation is mediated by the transcript's 3' untranslated region. Alterations in the sequence of the TfR mRNA regulatory region have been identified that render the mRNA unregulated by iron and intrinsically unstable. When cells expressing this unstable mRNA are treated with inhibitors of protein synthesis (cycloheximide or puromycin), the steady-state level of the encoded human TfR mRNA is increased due to a stabilization of the transcript. A similar set of observations has been made using a chimeric mRNA in which the rapid turnover determinant of the TfR mRNA is replaced by the (A+U)-rich region from the 3' untranslated region of c-fos mRNA. To distinguish between a labile protein participant in the degradation of these mRNAs and a requirement for their translation per se, we introduced a ferritin iron-responsive element into the 5' untranslated region of each of these mRNAs. The presence of the 5' iron-responsive element allowed us to use iron availability to alter the translation of the mRNAs in question without global effects on cellular protein synthesis. Although specific translation of these mRNAs could be inhibited by iron chelation to a degree comparable to that seen with cycloheximide (approximately 95% inhibition), no effects on mRNA turnover were observed. These data support a model in which a trans-acting labile protein is necessary for the turnover of these mRNAs rather than there being a requirement for the translation of the mRNAs themselves.