A novel iron responsive element in the 3'UTR of human MRCKalpha

Iron Homeostasis and Metabolism During Pregnancy: Exploring Innovative Drug Delivery Approaches for Treating Iron Deficiency Anemia in Pregnant Women

Pregnant women and small children are more prone to anemia. Even among the most affluent and educated portions of society, an estimated 50% of pregnant women, adolescent girls, and youngsters are anemic. This review recapitulates previous findings exploring advancements in anemia management in pregnant women. The published articles were searched using Google Scholar, Web of Science, Scopus, and Clinical Trials. Primary causes of anemia are an inadequate supply of dietary iron, deficiency of folate due to the lack of vegetable consumption, and thus a lack of vitamin B12, and a lack of dietary iron bioavailability from phytate and fiber-rich diets. When hemoglobin falls below 5 g/dL, the maternal mortality rate multiplies 8-10 times. Early detection and treatment of anemia during pregnancy may minimize maternal mortality, substantially decrease childhood and adolescent nutritional deficiency, and improve adult height. Maternal anemia decreases intrauterine growth, which increases the risk of premature delivery and low birth weight in babies. Intrauterine growth retardation coupled with a low birth weight leads to an inadequate growth trajectory throughout childhood, adolescence, and adulthood. Nano-delivery systems stand out as a promising avenue, utilizing nanotechnology to enhance the absorption of iron. These systems offer targeted delivery of iron supplements, overcoming challenges associated with conventional formulations. The exploration of nanotechnology in iron deficiency anemia treatment marks a significant stride toward developing advanced and tailored solutions for improving iron supplementation.

Iron inhibits glioblastoma cell migration and polarization

Glioblastoma is one of the deadliest malignancies facing modern oncology today. The ability of glioblastoma cells to diffusely spread into neighboring healthy brain makes complete surgical resection nearly impossible and contributes to the recurrent disease faced by most patients. Although research into the impact of iron on glioblastoma has addressed proliferation, there has been little investigation into how cellular iron impacts the ability of glioblastoma cells to migrate-a key question, especially in the context of the diffuse spread observed in these tumors. Herein, we show that increasing cellular iron content results in decreased migratory capacity of human glioblastoma cells. The decrease in migratory capacity was accompanied by a decrease in cellular polarization in the direction of movement. Expression of CDC42, a Rho GTPase that is essential for both cellular migration and establishment of polarity in the direction of cell movement, was reduced upon iron treatment. We then analyzed a single-cell RNA-seq dataset of human glioblastoma samples and found that cells at the tumor periphery had a gene signature that is consistent with having lower levels of cellular iron. Altogether, our results suggest that cellular iron content is impacting glioblastoma cell migratory capacity and that cells with higher iron levels exhibit reduced motility.

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

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

Iron Overload, Oxidative Stress, and Ferroptosis in the Failing Heart and Liver

Iron accumulation is a key mediator of several cytotoxic mechanisms leading to the impairment of redox homeostasis and cellular death. Iron overload is often associated with haematological diseases which require regular blood transfusion/phlebotomy, and it represents a common complication in thalassaemic patients. Major damages predominantly occur in the liver and the heart, leading to a specific form of cell death recently named ferroptosis. Different from apoptosis, necrosis, and autophagy, ferroptosis is strictly dependent on iron and reactive oxygen species, with a dysregulation of mitochondrial structure/function. Susceptibility to ferroptosis is dependent on intracellular antioxidant capacity and varies according to the different cell types. Chemotherapy-induced cardiotoxicity has been proven to be mediated predominantly by iron accumulation and ferroptosis, whereas there is evidence about the role of ferritin in protecting cardiomyocytes from ferroptosis and consequent heart failure. Another paradigmatic organ for transfusion-associated complication due to iron overload is the liver, in which the role of ferroptosis is yet to be elucidated. Some studies report a role of ferroptosis in the initiation of hepatic inflammation processes while others provide evidence about an involvement in several pathologies including immune-related hepatitis and acute liver failure. In this manuscript, we aim to review the literature to address putative common features between the response to ferroptosis in the heart and liver. A better comprehension of (dys)similarities is pivotal for the development of future therapeutic strategies that can be designed to specifically target this type of cell death in an attempt to minimize iron-overload effects in specific organs.

Iron Responsive Element-Mediated Responses to Iron Dyshomeostasis in Alzheimer's Disease

BACKGROUND

Iron trafficking and accumulation is associated with Alzheimer's disease (AD) pathogenesis. However, the role of iron dyshomeostasis in early disease stages is uncertain. Currently, gene expression changes indicative of iron dyshomeostasis are not well characterized, making it difficult to explore these in existing datasets.

OBJECTIVE

To identify sets of genes predicted to contain iron responsive elements (IREs) and use these to explore possible iron dyshomeostasis-associated gene expression responses in AD.

METHODS

Comprehensive sets of genes containing predicted IRE or IRE-like motifs in their 3' or 5' untranslated regions (UTRs) were identified in human, mouse, and zebrafish reference transcriptomes. Further analyses focusing on these genes were applied to a range of cultured cell, human, mouse, and zebrafish gene expression datasets.

RESULTS

IRE gene sets are sufficiently sensitive to distinguish not only between iron overload and deficiency in cultured cells, but also between AD and other pathological brain conditions. Notably, changes in IRE transcript abundance are among the earliest observable changes in zebrafish familial AD (fAD)-like brains, preceding other AD-typical pathologies such as inflammatory changes. Unexpectedly, while some IREs in the 3' untranslated regions of transcripts show significantly increased stability under iron deficiency in line with current assumptions, many such transcripts instead display decreased stability, indicating that this is not a generalizable paradigm.

CONCLUSION

Our results reveal IRE gene expression changes as early markers of the pathogenic process in fAD and are consistent with iron dyshomeostasis as an important driver of this disease. Our work demonstrates how differences in the stability of IRE-containing transcripts can be used to explore and compare iron dyshomeostasis-associated gene expression responses across different species, tissues, and conditions.

CD63 is Regulated by Iron via the IRE-IRP System and is Important for Ferritin Secretion by Extracellular Vesicles

CD63 is involved in EV secretion from cells and is shown herein to be regulated by iron via the IRE-IRP system.

Iron-loading increased secretion of CD63⁺ EVs containing iron-loaded ferritin.

Extracellular vesicles (EVs) transfer functional molecules between cells. CD63 is a widely recognized EV marker that contributes to EV secretion from cells. However, the regulation of its expression remains largely unknown. Ferritin is a cellular iron storage protein that can also be secreted by the exosome pathway, and serum ferritin levels classically reflect body iron stores. Iron metabolism–associated proteins such as ferritin are intricately regulated by cellular iron levels via the iron responsive element-iron regulatory protein (IRE-IRP) system. Herein, we present a novel mechanism demonstrating that the expression of the EV-associated protein CD63 is under the regulation of the IRE-IRP system. We discovered a canonical IRE in the 5′ untranslated region of CD63 messenger RNA that is responsible for regulating its expression in response to increased iron. Cellular iron loading caused a marked increase in CD63 expression and the secretion of CD63⁺ EVs from cells, which were shown to contain ferritin-H and ferritin-L. Our results demonstrate that under iron loading, intracellular ferritin is transferred via nuclear receptor coactivator 4 (NCOA4) to CD63⁺ EVs that are then secreted. Such iron-regulated secretion of the major iron storage protein ferritin via CD63⁺ EVs, is significant for understanding the local cell-to-cell exchange of ferritin and iron.

Mechanisms of cellular iron sensing, regulation of erythropoiesis and mitochondrial iron utilization

To maintain an adequate iron supply for hemoglobin synthesis and essential metabolic functions while counteracting iron toxicity, humans and other vertebrates have evolved effective mechanisms to conserve and finely regulate iron concentration, storage, and distribution to tissues. At the systemic level, the iron-regulatory hormone hepcidin is secreted by the liver in response to serum iron levels and inflammation. Hepcidin regulates the expression of the sole known mammalian iron exporter, ferroportin, to control dietary absorption, storage and tissue distribution of iron. At the cellular level, iron regulatory proteins 1 and 2 (IRP1 and IRP2) register cytosolic iron concentrations and post-transcriptionally regulate the expression of iron metabolism genes to optimize iron availability for essential cellular processes, including heme biosynthesis and iron-sulfur cluster biogenesis. Genetic malfunctions affecting the iron sensing mechanisms or the main pathways that utilize iron in the cell cause a broad range of human diseases, some of which are characterized by mitochondrial iron accumulation. This review will discuss the mechanisms of systemic and cellular iron sensing with a focus on the main iron utilization pathways in the cell, and on human conditions that arise from compromised function of the regulatory axes that control iron homeostasis.

Myotonic dystrophy kinase-related CDC42-binding kinase α, a new transferrin receptor type 2-binding partner, is a regulator of erythropoiesis

Efficient erythropoiesis relies on the expression of the transferrin receptor type 2 (TFR2). In erythroid precursors, TFR2 facilitates the export of the erythropoietin receptor (EPOR) to cell surface, which ensures the survival and proliferation of erythroblasts. Although TFR2 has a crucial role in erythropoiesis regulation, its mechanism of action remains to be clarified. To understand its role better, we aimed at identifying its protein partners by mass-spectrometry after immunoprecipitation in erythroid cells. Here we report the kinase MRCKα (myotonic dystrophy kinase-related CDC42-binding kinase α) as a new partner of both TFR2 and EPOR in erythroblasts. We show that MRCKα is co-expressed with TFR2, and TFR1 during terminal differentiation and regulates the internalization of the two types of transferrin receptors. The knockdown of MRCKα by shRNA in human primary erythroblasts leads to a decreased cell surface expression of both TFR1 and TFR2, an increased cell-surface expression of EPOR, and a delayed differentiation. Additionally, knockout of Mrckα in the murine MEDEP cells also leads to a striking delay in erythropoiesis, showcasing the importance of this kinase in both species. Our data highlight the importance of MRCKα in the regulation of erythropoiesis.

Differential translational control of 5' IRE-containing mRNA in response to dietary iron deficiency and acute iron overload

Iron regulatory proteins (IRPs) are iron-responsive RNA binding proteins that dictate changes in cellular iron metabolism in animal cells by controlling the fate of mRNAs containing iron responsive elements (IREs). IRPs have broader physiological roles as some targeted mRNAs encode proteins with functions beyond iron metabolism suggesting hierarchical regulation of IRP-targeted mRNAs. We observe that the translational regulation of IRP-targeted mRNAs encoding iron storage (L- and H-ferritins) and export (ferroportin) proteins have different set-points of iron responsiveness compared to that for the TCA cycle enzyme mitochondrial aconitase. The ferritins and ferroportin mRNA were largely translationally repressed in the liver of rats fed a normal diet whereas mitochondrial aconitase mRNA is primarily polysome bound. Consequently, acute iron overload increases polysome association of H- and L-ferritin and ferroportin mRNAs while mitochondrial aconitase mRNA showed little stimulation. Conversely, mitochondrial aconitase mRNA is most responsive in iron deficiency. These differences in regulation were associated with a faster off-rate of IRP1 for the IRE of mitochondrial aconitase in comparison to that of L-ferritin. Thus, hierarchical control of mRNA translation by IRPs involves selective control of cellular functions acting at different states of cellular iron status and that are critical for adaptations to iron deficiency or prevention of iron toxicity.

Overdosing on iron: Elevated iron and degenerative brain disorders

IMPACT STATEMENT

Brain degenerative disorders, which include some neurodevelopmental disorders and age-associated diseases, cause debilitating neurological deficits and are generally fatal. A large body of emerging evidence indicates that iron accumulation in neurons within specific regions of the brain plays an important role in the pathogenesis of many of these disorders. Iron homeostasis is a highly complex and incompletely understood process involving a large number of regulatory molecules. Our review provides a description of what is known about how iron is obtained by the body and brain and how defects in the homeostatic processes could contribute to the development of brain diseases, focusing on Alzheimer's disease and Parkinson's disease as well as four other disorders belonging to a class of inherited conditions referred to as neurodegeneration based on iron accumulation (NBIA) disorders. A description of potential therapeutic approaches being tested for each of these different disorders is provided.

Iron Regulatory Protein 2 Exerts its Oncogenic Activities by Suppressing TAp63 Expression

Iron regulatory protein 2 (IRP2) is a key regulator of iron homeostasis and is found to be altered in several types of human cancer. However, how IRP2 contributes to tumorigenesis remains to be elucidated. In this study, we sought to investigate the role of IRP2 in tumorigenesis and found that IRP2 promotes cell growth by repressing TAp63, a member of p53 tumor suppressor family. Specifically, we found that IRP2 overexpression decreased, whereas IRP2 deficiency increased, TAp63 expression. We also showed that the repression of TAp63 by IRP2 was independent of tumor suppressor p53. To uncover the molecular basis, we found that IRP2 stabilized TAp63 mRNA by binding to an iron response element in the 3'UTR of p63 mRNA. To determine the biological significance of this regulation, we showed that IRP2 facilitates cell proliferation, at least in part, via repressing TAp63 expression. Moreover, we found that IRP2 deficiency markedly alleviated cellular senescence in TAp63-deficient mouse embryo fibroblasts. Together, we have uncovered a novel regulation of TAp63 by IRP2 and our data suggest that IRP2 exerts its oncogenic activities at least in part by repressing TAp63 expression. IMPLICATIONS: We have revealed a novel regulation of TAp63 by IRP2 and our data suggest that IRP2 exerts its oncogenic activities, at least in part, by repressing TAp63 expression.

Evaluation of the iron regulatory protein-1 interactome

The interactions of iron regulatory proteins (IRPs) with mRNAs containing an iron-responsive element (IRE) is a major means through which intracellular iron homeostasis is maintained and integrated with cellular function. Although IRE-IRP interactions have been proposed to modulate the expression of a diverse number of mRNAs, a transcriptome analysis of the interactions that form within the native mRNA structure and cellular environment has not previously been described. An RNA-CLIP study is described here that identified IRP-1 interactions occurring within a primary cell line expressing physiologically relevant amounts of mRNA and protein. The study suggests that only a small subset of the previously proposed IREs interact with IRP-1 in situ. Identifying authentic IRP interactions is not only important to a greater understanding of iron homeostasis and its integration with cell biology but also to the development of novel therapeutics that can compensate for iron imbalances.

Methods for Studying Iron Regulatory Protein 1: An Important Protein in Human Iron Metabolism

Iron regulatory proteins 1 and 2 (IRP1 and IRP2) are two cytosolic proteins that maintain cellular iron homeostasis by regulating the expression of genes involved in iron metabolism. IRPs respond to cellular iron deficiency by binding to iron-responsive elements (IREs) found in the mRNAs of iron metabolism transcripts, enhancing iron import, and reducing iron storage, utilization, and export. IRP1, a bifunctional protein, exists in equilibrium between a [Fe₄S₄] cluster containing cytosolic aconitase, and an apoprotein that binds to IREs. At high cellular iron levels, this equilibrium is shifted more toward iron-sulfur cluster containing aconitase, whereas IRP2 undergoes proteasomal degradation by an E3 ubiquitin ligase complex that contains an F-box protein, FBXL5. Irp1^-/- mice develop polycythemia and pulmonary hypertension, whereas Irp2^-/- mice develop microcytic anemia and progressive neurodegeneration, indicating that Irp1 has important functions in the erythropoietic and pulmonary systems, and Irp2 has essential roles in supporting erythropoiesis and nervous system functions. Mice lacking both Irp1 and Irp2 die during embryogenesis, suggesting that functions of Irp1 and Irp2 are redundant. In this review, we will focus on the methods for studying IRP1 activities and function in cells and animals.

Identification of loci associated with susceptibility to subspecies () tissue infection in cattle

Johne's disease is a contagious bacterial infection of cattle caused by ssp. (). A previous genome-wide association analysis (GWAA) in Holstein cattle identified QTL on BTA3 and BTA9 that were highly associated (P < 5 × 10) and on BTA1, BTA16, and BTA21 that were moderately associated (P < 5 × 10) with Map tissue infection. The objectives of this study were to validate previous GWAA results in Jersey cattle ( = 57), Holstein cattle from the Pacific Northwest (PNW, = 205) and a combined Holstein population from the PNW and the Northeast (PNW + NE, = 423), and also identify new loci associated with tissue infection. DNA was genotyped using the Illumina BovineSNP50 BeadChip, and the PNW + NE data was also imputed to whole genome sequence level using Run4 of the 1000 Bull Genomes project with Beagle v 4.1 and FImpute. Cases were ileocecal node positive and controls were negative for by quantitative PCR (qPCR). Individuals were removed for SNP call rate < 90%, and SNP were removed for genotype call rate < 90% or minor allele frequency < 1%. For the Jersey, PNW, and PNW + NE, GWAA were conducted using an allelic dosage model. For the PNW and the PNW + NE, an additional efficient mixed-model association eXpedited (EMMAX) analysis was performed using additive, dominance and recessive models. Seven QTL on BTA22 were identified in the Jersey population with the most significant ( = 4.45 × 10) located at 21.7 megabases (Mb). Six QTL were associated in the PNW and the PNW + NE analyses, including a QTL previously identified on BTA16 in the NE population. The most significant locus for the PNW was located on BTA21 at 61 Mb ( = 8.61 × 10) while the most significant locus for the PNW + NE was on BTA12 at 90 Mb ( = 2.33 × 10). No additional QTL were identified with the imputed GWAA. Putative positional candidate genes were identified within 50 kb 5' and 3' of each QTL. Two positional candidate genes were identified in Jersey cattle, 1 identified in the PNW and 8 in the PNW + NE populations. Many identified positional candidate genes are involved in signal transduction, have immunological functions, or have putative functional relevance in entry into host cells. This study supported 2 previously identified SNP within a QTL on BTA16 and identified 16 new QTL, including 2 found in the PNW and the PNW+NE, associated with tissue infection.

Effect of iron on cholesterol 7α-hydroxylase expression in alcohol-induced hepatic steatosis in mice

Both iron and lipids are involved in the progression of alcoholic fatty liver disease (AFLD), but the interaction between iron and lipids in AFLD is unclear. Here, we tested the hypothesis that iron regulates the expression of genes involved in lipid metabolism through iron regulatory proteins (IRPs), which interact with the iron-responsive elements (IREs) in the untranslated regions (UTRs) of genes, resulting in lipid accumulation. Using "RNA structure software", we predicted the mRNA secondary structures of more than 100 genes involved in lipid metabolism to investigate whether the IRE structure exists in novel mRNAs. Cholesterol 7α-hydroxylase (Cyp7a1) has an IRE-like stem-loop, a noncanonical IRE structure, in its 3'-UTR. Cyp7a1 expression can be regulated by in vivo and in vitro iron treatment. In addition, the noncanonical IRE motif can efficiently bind both to IRP1 and IRP2. The results indicate that hepatic iron overloading in AFLD mice decreased Cyp7a1 expression and resulted in cholesterol accumulation, providing a new mechanism of iron-regulated gene transcription and translation through the interaction between iron and a noncanonical IRE structure in Cyp7a1 mRNA. This finding has significant implications in studying a proposed mechanism for the regulation of cholesterol homeostasis by an Fe/IRP/noncanonical IRE axis.

Frataxin and the molecular mechanism of mitochondrial iron-loading in Friedreich's ataxia

The mitochondrion is a major site for the metabolism of the transition metal, iron, which is necessary for metabolic processes critical for cell vitality. The enigmatic mitochondrial protein, frataxin, is known to play a significant role in both cellular and mitochondrial iron metabolism due to its iron-binding properties and its involvement in iron-sulfur cluster (ISC) and heme synthesis. The inherited neuro- and cardio-degenerative disease, Friedreich's ataxia (FA), is caused by the deficient expression of frataxin that leads to deleterious alterations in iron metabolism. These changes lead to the accumulation of inorganic iron aggregates in the mitochondrial matrix that are presumed to play a key role in the oxidative damage and subsequent degenerative features of this disease. Furthermore, the concurrent dys-regulation of cellular antioxidant defense, which coincides with frataxin deficiency, exacerbates oxidative stress. Hence, the pathogenesis of FA underscores the importance of the integrated homeostasis of cellular iron metabolism and the cytoplasmic and mitochondrial redox environments. This review focuses on describing the pathogenesis of the disease, the molecular mechanisms involved in mitochondrial iron-loading and the dys-regulation of cellular antioxidant defense due to frataxin deficiency. In turn, current and emerging therapeutic strategies are also discussed.

Common variable immunodeficiency associated with microdeletion of chromosome 1q42.1-q42.3 and inositol 1,4,5-trisphosphate kinase B (ITPKB) deficiency

Common variable immunodeficiency (CVID) is a heterogenous disorder characterized by hypogammaglobulinemia and impaired specific antibody response and increased susceptibility to infections, autoimmunity and malignancies. A number of gene mutations, including ICOS, TACI and BAFF-R, and CD19, CD20, CD21, CD81, MSH5 and LRBA have been described; however, they account for approximately 20–25% of total cases of CVID. In this study, we report a patient with CVID with an intrinsic microdeletion of chromosome 1q42.1-42.3, where gene for inositol 1,3,4, trisphosphate kinase β (ITPKB) is localized. ITPKB has an important role in the development, survival and function of B cells. In this subject, the expression of ITPKB mRNA as well as ITKPB protein was significantly reduced. The sequencing of ITPKB gene revealed three variants, two of them were missense variants and third was a synonymous variant; the significance of each of them in relation to CVID is discussed. This case suggests that a deficiency of ITPKB may have a role in CVID.

Iron regulatory proteins and their role in controlling iron metabolism

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

Involvement of the iron regulatory protein from Eisenia andrei earthworms in the regulation of cellular iron homeostasis

Iron homeostasis in cells is regulated by iron regulatory proteins (IRPs) that exist in different organisms. IRPs are cytosolic proteins that bind to iron-responsive elements (IREs) of the 5′- or 3′-untranslated regions (UTR) of mRNAs that encode many proteins involved in iron metabolism. In this study, we have cloned and described a new regulatory protein belonging to the family of IRPs from the earthworm Eisenia andrei (EaIRP). The earthworm IRE site in 5′-UTR of ferritin mRNA most likely folds into a secondary structure that differs from the conventional IRE structures of ferritin due to the absence of a typically unpaired cytosine that participates in protein binding. Prepared recombinant EaIRP and proteins from mammalian liver extracts are able to bind both mammalian and Eisenia IRE structures of ferritin mRNA, although the affinity of the rEaIRP/Eisenia IRE structure is rather low. This result suggests the possible contribution of a conventional IRE structure. When IRP is supplemented with a Fe-S cluster, it can function as a cytosolic aconitase. Cellular cytosolic and mitochondrial fractions, as well as recombinant EaIRP, exhibit aconitase activity that can be abolished by the action of oxygen radicals. The highest expression of EaIRP was detected in parts of the digestive tract. We can assume that earthworms may possess an IRE/IRP regulatory network as a potential mechanism for maintaining cellular iron homeostasis, although the aconitase function of EaIRP is most likely more relevant.

The actin-myosin regulatory MRCK kinases: regulation, biological functions and associations with human cancer

The contractile actin-myosin cytoskeleton provides much of the force required for numerous cellular activities such as motility, adhesion, cytokinesis and changes in morphology. Key elements that respond to various signal pathways are the myosin II regulatory light chains (MLC), which participate in actin-myosin contraction by modulating the ATPase activity and consequent contractile force generation mediated by myosin heavy chain heads. Considerable effort has focussed on the role of MLC kinases, and yet the contributions of the myotonic dystrophy-related Cdc42-binding kinases (MRCK) proteins in MLC phosphorylation and cytoskeleton regulation have not been well characterized. In contrast to the closely related ROCK1 and ROCK2 kinases that are regulated by the RhoA and RhoC GTPases, there is relatively little information about the CDC42-regulated MRCKα, MRCKβ and MRCKγ members of the AGC (PKA, PKG and PKC) kinase family. As well as differences in upstream activation pathways, MRCK and ROCK kinases apparently differ in the way that they spatially regulate MLC phosphorylation, which ultimately affects their influence on the organization and dynamics of the actin-myosin cytoskeleton. In this review, we will summarize the MRCK protein structures, expression patterns, small molecule inhibitors, biological functions and associations with human diseases such as cancer.

Overexpression of transferrin receptor CD71 and its tumorigenic properties in esophageal squamous cell carcinoma

Esophageal squamous cell carcinoma (ESCC) is the predominant type of esophageal cancer in endemic Asian regions. In the present study, we investigated the clinical implication and role of transferrin receptor CD71 in ESCC. CD71 has a physiological role in cellular iron intake and is implicated in the carcinogenesis of various types of tumors. In our cohort, more than a 2-fold upregulation of the CD71 transcript was detected in 61.5% of patients using quantitative polymerase chain reaction. Immunohistochemical analysis also showed strong membranous and cytoplasmic localization of CD71 in paraffin-embedded tumors. Staining parallel tumor sections with the proliferative marker Ki-67 revealed that the pattern of Ki-67 staining was associated with CD71 expression. Analysis of clinicopathological data indicated that CD71 overexpression can be used as an indicator for advanced T4 stage (p=0.0307). These data suggested a strong link between CD71 and ESCC. Subsequent in vitro assays using short interfering RNA (siRNA) to suppress CD71 expression confirmed the tumorigenic properties of CD71 in ESCC; cell growth inhibition and cell cycle arrest at S phase were observed in CD71-suppressed cells. The underlying mechanism involved activation of the MEK/ERK pathway. In summary, the present study provides evidence showing the tumorigenic properties of CD71 in ESCC with clinical correlations and suggests targeting CD71 as a strategy for the treatment of ESCC.

Nucleotide-specific recognition of iron-responsive elements by iron regulatory protein 1

IRP1 [iron regulatory protein (IRP) 1] is a bifunctional protein with mutually exclusive end-states. In one mode of operation, IRP1 binds iron-responsive element (IRE) stem-loops in messenger RNAs encoding proteins of iron metabolism to control their rate of translation. In its other mode, IRP1 serves as cytoplasmic aconitase to correlate iron availability with the energy and oxidative stress status of the cell. IRP1/IRE binding occurs through two separate interfaces, which together contribute about two-dozen hydrogen bonds. Five amino acids make base-specific contacts and are expected to contribute significantly to binding affinity and specificity of this protein:RNA interaction. In this mutagenesis study, each of the five base-specific amino acids was changed to alter binding at each site. Analysis of IRE binding affinity and translational repression activity of the resulting IRP1 mutants showed that four of the five contact points contribute uniquely to the overall binding affinity of the IRP1:IRE interaction, while one site was found to be unimportant. The stronger-than-expected effect on binding affinity of mutations at Lys379 and Ser681, residues that make contact with the conserved nucleotides G16 and C8, respectively, identified them as particularly critical for providing specificity and stability to IRP1:IRE complex formation. We also show that even though the base-specific RNA-binding residues are not part of the aconitase active site, their substitutions can affect the aconitase activity of holo-IRP1, positively or negatively.

Mammalian iron homeostasis in health and disease: uptake, storage, transport, and molecular mechanisms of action

Iron is a crucial factor for life. However, it also has the potential to cause the formation of noxious free radicals. These double-edged sword characteristics demand a tight regulation of cellular iron metabolism. In this review, we discuss the various pathways of cellular iron uptake, cellular iron storage, and transport. Recent advances in understanding the reduction and uptake of non-transferrin-bound iron are discussed. We also discuss the recent progress in the understanding of transcriptional and translational regulation by iron. Furthermore, we discuss recent advances in the understanding of the regulation of cellular and systemic iron homeostasis and several key diseases resulting from iron deficiency and overload. We also discuss the knockout mice available for studying iron metabolism and the related human conditions.

Mammalian iron metabolism and its control by iron regulatory proteins

Cellular iron homeostasis is maintained by iron regulatory proteins 1 and 2 (IRP1 and IRP2). IRPs bind to iron-responsive elements (IREs) located in the untranslated regions of mRNAs encoding protein involved in iron uptake, storage, utilization and export. Over the past decade, significant progress has been made in understanding how IRPs are regulated by iron-dependent and iron-independent mechanisms and the pathological consequences of IRP2 deficiency in mice. The identification of novel IREs involved in diverse cellular pathways has revealed that the IRP-IRE network extends to processes other than iron homeostasis. A mechanistic understanding of IRP regulation will likely yield important insights into the basis of disorders of iron metabolism. This article is part of a Special Issue entitled: Cell Biology of Metals.

Molecular insights into the regulation of iron metabolism during the prenatal and early postnatal periods

Molecular iron metabolism and its regulation are least well understood in the fetal and early postnatal periods of mammalian ontogenic development. The scope of this review is to summarize recent progress in uncovering the molecular mechanisms of fetal iron homeostasis, introduce the molecules involved in iron transfer across the placenta, and briefly explain the role of iron transporters in the absorption of this microelement during early postnatal life. These issues are discussed and parallels are drawn with the relatively well-established system for elemental and heme iron regulation in adult mammals. We conclude that detailed investigations into the regulatory mechanisms of iron metabolism at early stages of development are required in order to optimize strategies to prevent neonatal iron deficiency. We propose that newborn piglets represent a suitable animal model for studies on iron deficiency anemia in neonates.

The role of the 3' untranslated region in post-transcriptional regulation of protein expression in mammalian cells

The untranslated regions (UTRs) at the 3'end of mRNA transcripts contain important sequences that influence the fate of mRNA and thus proteosynthesis. In this review, we summarize the information known to date about 3'end processing, sequence characteristics including related binding proteins and the role of 3'UTRs in several selected signaling pathways to delineate their importance in the regulatory processes in mammalian cells. In addition to reviewing recent advances in the more well known aspects, such as cleavage and polyadenylation processes that influence mRNA stability and location, we concentrate on some newly emerging concepts of the role of the 3'UTR, including alternative polyadenylation sites in relation to proliferation and differentiation and the recognition of the multi-functional properties of non-coding RNAs, including miRNAs that commonly target the 3'UTR. The emerging picture is of a highly complex set of regulatory systems that include autoregulation, cooperativity and competition to fine tune proteosynthesis in context-dependent manners.

Siderophore-mediated iron trafficking in humans is regulated by iron

Siderophores are best known as small iron binding molecules that facilitate microbial iron transport. In our previous study we identified a siderophore-like molecule in mammalian cells and found that its biogenesis is evolutionarily conserved. A member of the short chain dehydrogenase family of reductases, 3-hydroxy butyrate dehydrogenase (BDH2) catalyzes a rate-limiting step in the biogenesis of the mammalian siderophore. We have shown that depletion of the mammalian siderophore by inhibiting expression of bdh2 results in abnormal accumulation of cellular iron and mitochondrial iron deficiency. These observations suggest that the mammalian siderophore is a critical regulator of cellular iron homeostasis and facilitates mitochondrial iron import. By utilizing bioinformatics, we identified an iron-responsive element (IRE; a stem-loop structure that regulates genes expression post-transcriptionally upon binding to iron regulatory proteins or IRPs) in the 3'-untranslated region of the human BDH2 (hBDH2) gene. In cultured cells as well as in patient samples we now demonstrate that the IRE confers iron-dependent regulation on hBDH2 and binds IRPs in RNA electrophoretic mobility shift assays. In addition, we show that the hBDH2 IRE associates with IRPs in cells and that abrogation of IRPs by RNAi eliminates the iron-dependent regulation of hBDH2 mRNA. The key physiologic implication is that iron-mediated post-transcriptional regulation of hBDH2 controls mitochondrial iron homeostasis in human cells. These observations provide a new and an unanticipated mechanism by which iron regulates its intracellular trafficking.

Reactive oxygen species (ROS) homeostasis and redox regulation in cellular signaling

Reactive oxygen species (ROS) are generated during mitochondrial oxidative metabolism as well as in cellular response to xenobiotics, cytokines, and bacterial invasion. Oxidative stress refers to the imbalance due to excess ROS or oxidants over the capability of the cell to mount an effective antioxidant response. Oxidative stress results in macromolecular damage and is implicated in various disease states such as atherosclerosis, diabetes, cancer, neurodegeneration, and aging. Paradoxically, accumulating evidence indicates that ROS also serve as critical signaling molecules in cell proliferation and survival. While there is a large body of research demonstrating the general effect of oxidative stress on signaling pathways, less is known about the initial and direct regulation of signaling molecules by ROS, or what we term the "oxidative interface." Cellular ROS sensing and metabolism are tightly regulated by a variety of proteins involved in the redox (reduction/oxidation) mechanism. This review focuses on the molecular mechanisms through which ROS directly interact with critical signaling molecules to initiate signaling in a broad variety of cellular processes, such as proliferation and survival (MAP kinases, PI3 kinase, PTEN, and protein tyrosine phosphatases), ROS homeostasis and antioxidant gene regulation (thioredoxin, peroxiredoxin, Ref-1, and Nrf-2), mitochondrial oxidative stress, apoptosis, and aging (p66Shc), iron homeostasis through iron-sulfur cluster proteins (IRE-IRP), and ATM-regulated DNA damage response.

Mitochondrial mayhem: the mitochondrion as a modulator of iron metabolism and its role in disease

The mitochondrion plays vital roles in various aspects of cellular metabolism, ranging from energy transduction and apoptosis to the synthesis of important molecules such as heme. Mitochondria are also centrally involved in iron metabolism, as exemplified by disruptions in mitochondrial proteins that lead to perturbations in whole-cell iron processing. Recent investigations have identified a host of mitochondrial proteins (e.g., mitochondrial ferritin; mitoferrins 1 and 2; ABCBs 6, 7, and 10; and frataxin) that may play roles in the homeostasis of mitochondrial iron. These mitochondrial proteins appear to participate in one or more processes of iron storage, iron uptake, and heme and iron-sulfur cluster synthesis. In this review, we present and critically discuss the evidence suggesting that the mitochondrion may contribute to the regulation of whole-cell iron metabolism. Further, human diseases that arise from a dysregulation of these mitochondrial molecules reveal the ability of the mitochondrion to communicate with cytosolic iron metabolism to coordinate whole-cell iron processing and to fulfill the high demands of this organelle for iron. This review highlights new advances in understanding iron metabolism in terms of novel molecular players and diseases associated with its dysregulation.

The importance of the stem cell marker prominin-1/CD133 in the uptake of transferrin and in iron metabolism in human colon cancer Caco-2 cells

As the pentaspan stem cell marker CD133 was shown to bind cholesterol and to localize in plasma membrane protrusions, we investigated a possible function for CD133 in endocytosis. Using the CD133 siRNA knockdown strategy and non-differentiated human colon cancer Caco-2 cells that constitutively over-expressed CD133, we provide for the first time direct evidence for a role of CD133 in the intracellular accumulation of fluorescently labeled extracellular compounds. Assessed using AC133 monoclonal antibody, CD133 knockdown was shown to improve Alexa488-transferrin (Tf) uptake in Caco-2 cells but had no impact on FITC-dextran or FITC-cholera-toxin. Absence of effect of the CD133 knockdown on Tf recycling established a role for CD133 in inhibiting Tf endocytosis rather than in stimulating Tf exocytosis. Use of previously identified inhibitors of known endocytic pathways and the positive impact of CD133 knockdown on cellular uptake of clathrin-endocytosed synthetic lipid nanocapsules supported that CD133 impact on endocytosis was primarily ascribed to the clathrin pathway. Also, cholesterol extraction with methyl-β-cyclodextrine up regulated Tf uptake at greater intensity in the CD133^high situation than in the CD133^low situation, thus suggesting a role for cholesterol in the inhibitory effect of CD133 on endocytosis. Interestingly, cell treatment with the AC133 antibody down regulated Tf uptake, thus demonstrating that direct extracellular binding to CD133 could affect endocytosis. Moreover, flow cytometry and confocal microscopy established that down regulation of CD133 improved the accessibility to the TfR from the extracellular space, providing a mechanism by which CD133 inhibited Tf uptake. As Tf is involved in supplying iron to the cell, effects of iron supplementation and deprivation on CD133/AC133 expression were investigated. Both demonstrated a dose-dependent down regulation here discussed to the light of transcriptional and post-transciptional effects. Taken together, these data extend our knowledge of the function of CD133 and underline the interest of further exploring the CD133-Tf-iron network.

Iron regulatory protein-1 and -2: transcriptome-wide definition of binding mRNAs and shaping of the cellular proteome by iron regulatory proteins

Iron regulatory proteins (IRPs) 1 and 2 are RNA-binding proteins that control cellular iron metabolism by binding to conserved RNA motifs called iron-responsive elements (IREs). The currently known IRP-binding mRNAs encode proteins involved in iron uptake, storage, and release as well as heme synthesis. To systematically define the IRE/IRP regulatory network on a transcriptome-wide scale, IRP1/IRE and IRP2/IRE messenger ribonucleoprotein complexes were immunoselected, and the mRNA composition was determined using microarrays. We identify 35 novel mRNAs that bind both IRP1 and IRP2, and we also report for the first time cellular mRNAs with exclusive specificity for IRP1 or IRP2. To further explore cellular iron metabolism at a system-wide level, we undertook proteomic analysis by pulsed stable isotope labeling by amino acids in cell culture in an iron-modulated mouse hepatic cell line and in bone marrow-derived macrophages from IRP1- and IRP2-deficient mice. This work investigates cellular iron metabolism in unprecedented depth and defines a wide network of mRNAs and proteins with iron-dependent regulation, IRP-dependent regulation, or both.

Nitrosyl iron complexes--synthesis, structure and biology

Nitrosyl complexes of iron are formed in living species in the presence of nitric oxide. They are considered a form in which NO can be stored and stabilized within a living cell. Upon entering a topic in bioinorganic chemistry the researcher faces a wide spectrum of issues concerning synthetic methods, the structure and chemical properties of the complex on the one hand, and its biological implications on the other. The aim of this review is to present the newest knowledge on nitrosyl iron complexes, summarizing the issues that are important for understanding the nature of nitrosyl iron complexes, their possible interactions, behavior in vitro and in vivo, handling of the preparations etc. in response to the growing interest in these compounds. Herein we focus mostly on the dinitrosyl iron complexes (DNICs) due to their prevailing occurrence in NO-treated biological samples. This article reviews recent knowledge on the structure, chemical properties and biological action of DNICs and some mononitrosyls of heme proteins. Synthetic methods are also briefly reviewed.

Regulation of cellular iron metabolism

Iron is an essential but potentially hazardous biometal. Mammalian cells require sufficient amounts of iron to satisfy metabolic needs or to accomplish specialized functions. Iron is delivered to tissues by circulating transferrin, a transporter that captures iron released into the plasma mainly from intestinal enterocytes or reticuloendothelial macrophages. The binding of iron-laden transferrin to the cell-surface transferrin receptor 1 results in endocytosis and uptake of the metal cargo. Internalized iron is transported to mitochondria for the synthesis of haem or iron–sulfur clusters, which are integral parts of several metalloproteins, and excess iron is stored and detoxified in cytosolic ferritin. Iron metabolism is controlled at different levels and by diverse mechanisms. The present review summarizes basic concepts of iron transport, use and storage and focuses on the IRE (iron-responsive element)/IRP (iron-regulatory protein) system, a well known post-transcriptional regulatory circuit that not only maintains iron homoeostasis in various cell types, but also contributes to systemic iron balance.

Iron regulatory proteins: from molecular mechanisms to drug development

Eukaryotic cells require iron for survival but, as an excess of poorly liganded iron can lead to the catalytic production of toxic radicals that can damage cell structures, regulatory mechanisms have been developed to maintain appropriate cell and body iron levels. The interactions of iron responsive elements (IREs) with iron regulatory proteins (IRPs) coordinately regulate the expression of the genes involved in iron uptake, use, storage, and export at the post-transcriptional level, and represent the main regulatory network controlling cell iron homeostasis. IRP1 and IRP2 are similar (but not identical) proteins with partially overlapping and complementary functions, and control cell iron metabolism by binding to IREs (i.e., conserved RNA stem-loops located in the untranslated regions of a dozen mRNAs directly or indirectly related to iron metabolism). The discovery of the presence of IREs in a number of other mRNAs has extended our knowledge of the influence of the IRE/IRP regulatory network to new metabolic pathways, and it has been recently learned that an increasing number of agents and physiopathological conditions impinge on the IRE/IRP system. This review focuses on recent findings concerning the IRP-mediated regulation of iron homeostasis, its alterations in disease, and new research directions to be explored in the near future.

The role of iron in the proliferation of Drosophila l(2) mbn cells

Iron is essential for life and is needed for cell proliferation and cell cycle progression. Iron deprivation results first in cell cycle arrest and then in apoptosis. The Drosophila tumorous larval hemocyte cell line l(2) mbn was used to study the sensitivity and cellular response to iron deprivation through the chelator desferrioxamine (DFO). At a concentration of 10 μM DFO or more the proliferation was inhibited reversibly, while the amount of dead cells did not increase. FACS analysis showed that the cell cycle was arrested in G1/S-phase and the transcript level of cycE was decreased to less than 50% of control cells. These results show that iron chelation in this insect tumorous cell line causes a specific and coordinated cell cycle arrest.

Mitochondrial iron trafficking and the integration of iron metabolism between the mitochondrion and cytosol

The mitochondrion is well known for its key role in energy transduction. However, it is less well appreciated that it is also a focal point of iron metabolism. Iron is needed not only for heme and iron sulfur cluster (ISC)-containing proteins involved in electron transport and oxidative phosphorylation, but also for a wide variety of cytoplasmic and nuclear functions, including DNA synthesis. The mitochondrial pathways involved in the generation of both heme and ISCs have been characterized to some extent. However, little is known concerning the regulation of iron uptake by the mitochondrion and how this is coordinated with iron metabolism in the cytosol and other organelles (e.g., lysosomes). In this article, we discuss the burgeoning field of mitochondrial iron metabolism and trafficking that has recently been stimulated by the discovery of proteins involved in mitochondrial iron storage (mitochondrial ferritin) and transport (mitoferrin-1 and -2). In addition, recent work examining mitochondrial diseases (e.g., Friedreich's ataxia) has established that communication exists between iron metabolism in the mitochondrion and the cytosol. This finding has revealed the ability of the mitochondrion to modulate whole-cell iron-processing to satisfy its own requirements for the crucial processes of heme and ISC synthesis. Knowledge of mitochondrial iron-processing pathways and the interaction between organelles and the cytosol could revolutionize the investigation of iron metabolism.

SIREs: searching for iron-responsive elements

The iron regulatory protein/iron-responsive element regulatory system plays a crucial role in the post-transcriptional regulation of gene expression and its disruption results in human disease. IREs are cis-acting regulatory motifs present in mRNAs that encode proteins involved in iron metabolism. They function as binding sites for two related trans-acting factors, namely the IRP-1 and -2. Among cis-acting RNA regulatory elements, the IRE is one of the best characterized. It is defined by a combination of RNA sequence and structure. However, currently available programs to predict IREs do not show a satisfactory level of sensitivity and fail to detect some of the functional IREs. Here, we report an improved software for the prediction of IREs implemented as a user-friendly web server tool. The SIREs web server uses a simple data input interface and provides structure analysis, predicted RNA folds, folding energy data and an overall quality flag based on properties of well characterized IREs. Results are reported in a tabular format and as a schematic visual representation that highlights important features of the IRE. The SIREs (Search for iron-responsive elements) web server is freely available on the web at http://ccbg.imppc.org/sires/index.html

Human MRCKalpha is regulated by cellular iron levels and interferes with transferrin iron uptake

Myotonic dystrophy kinase-related Cdc42-binding kinase alpha (MRCKalpha, formally known as CDC42BPA) is a serine/threonine kinase that can regulate actin/myosin assembly and activity. Recently, it has been shown that it possesses a functional iron responsive element (IRE) in the 3'-untranslated region (UTR) of its mRNA, suggesting that it may be involved in iron metabolism. Here we report that MRCKalpha protein expression is also regulated by iron levels; MRCKalpha colocalizes with transferrin (Tf)-loaded transferrin receptors (TfR), and attenuation of MRCKalpha expression by a short hairpin RNA silencing construct leads to a significant decrease in Tf-mediated iron uptake. Our results thus indicate that MRCKalpha takes part in Tf-iron uptake, probably via regulation of Tf-TfR endocytosis/endosome trafficking that is dependent on the cellular cytoskeleton. Regulation of the MRCKalpha activity by intracellular iron levels could thus represent another molecular feedback mechanism cells could use to finely tune iron uptake to actual needs.

Systemic iron homeostasis and the iron-responsive element/iron-regulatory protein (IRE/IRP) regulatory network

The regulation and maintenance of systemic iron homeostasis is critical to human health. Iron overload and deficiency diseases belong to the most common nutrition-related pathologies across the globe. It is now well appreciated that the hormonal hepcidin/ferroportin system plays an important regulatory role for systemic iron metabolism. We review recent data that uncover the importance of the cellular iron-responsive element/iron-regulatory protein (IRE/IRP) regulatory network in systemic iron homeostasis. We also discuss how the IRE/IRP regulatory system communicates with the hepcidin/ferroportin system to connect the control networks for systemic and cellular iron balance.

An iron responsive element-like stem-loop regulates alpha-hemoglobin-stabilizing protein mRNA

Hemoglobin production during erythropoiesis is mechanistically coupled to the acquisition and metabolism of iron. We discovered that iron regulates the expression of alpha-hemoglobin-stabilizing protein (AHSP), a molecular chaperone that binds and stabilizes free alpha-globin during hemoglobin synthesis. In primates, the 3'-untranslated region (UTR) of AHSP mRNA contains a nucleotide sequence resembling iron responsive elements (IREs), stem-loop structures that regulate gene expression post-transcriptionally by binding iron regulatory proteins (IRPs). The AHSP IRE-like stem-loop deviates from classical consensus sequences and binds IRPs poorly in electrophoretic mobility shift assays. However, in cytoplasmic extracts, AHSP mRNA co-immunoprecipitates with IRPs in a fashion that is dependent on the stem-loop structure and inhibited by iron. Moreover, this interaction enhances AHSP mRNA stability in erythroid and heterologous cells. Our findings demonstrate that IRPs can regulate mRNA expression through non-canonical IREs and extend the repertoire of known iron-regulated genes. In addition, we illustrate a new mechanism through which hemoglobin may be modulated according to iron status.

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

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

The functional duality of iron regulatory protein 1

Iron homeostasis in animal cells is controlled post-transcriptionally by the iron regulatory proteins IRP1 and IRP2. IRP1 can assume two different functions in the cell, depending on conditions. During iron scarcity or oxidative stress, IRP1 binds to mRNA stem-loop structures called iron responsive elements (IREs) to modulate the translation of iron metabolism genes. In iron-rich conditions, IRP1 binds an iron-sulfur cluster to function as a cytosolic aconitase. This functional duality of IRP1 connects the translational control of iron metabolizing proteins to cellular iron levels. The recently determined structures of IRP1 in both functional states reveal the large-scale conformational changes required for these mutually exclusive roles, providing new insights into the mechanisms of IRP1 interconversion and ligand binding.

Iron-regulatory proteins: molecular biology and pathophysiological implications

Iron is required for key cellular functions, and there is a strong link between iron metabolism and important metabolic processes, such as cell growth, apoptosis and inflammation. Diseases that are directly or indirectly related to iron metabolism represent major health problems. Iron-regulatory proteins (IRPs) 1 and 2 are key controllers of vertebrate iron metabolism and post-transcriptionally regulate expression of the major iron homeostasis genes. Here we discuss how dysregulation of the IRP system can result from both iron-related and unrelated effectors and explain how this can have important pathological consequences in several human disorders.

Evolution of the iron-responsive element

An RNA hairpin structure referred to as the iron-responsive element (IRE) and iron regulatory proteins (IRPs) are key players in the control of iron metabolism in animal cells. They regulate translation initiation or mRNA stability, and the IRE is found in a variety of mRNAs, such as those encoding ferritin, transferrin receptor (Tfr), erythroid aminolevulinic acid synthase (eALAS), mitochondrial aconitase (mACO), ferroportin, and divalent metal transporter 1 (DMT1). We have studied the evolution of the IRE by considering all mRNAs previously known to be associated with this structure and by computationally examining its occurrence in a large variety of eukaryotic organisms. More than 100 novel sequences together with approximately 50 IREs that were previously reported resulted in a comprehensive view of the phylogenetic distribution of this element. A comparison of the different mRNAs shows that the IREs of eALAS and mACO are found in chordates, those of ferroportin and Tfr1 are found in vertebrates, and the IRE of DMT1 is confined to mammals. In contrast, the IRE of ferritin occurs in a majority of metazoa including lower metazoa such as sponges and Nematostella (sea anemone). These findings suggest that the ferritin IRE represents the ancestral version of this type of translational control and that during the evolution of higher animals the IRE structure was adopted by other genes. On the basis of primary sequence comparison between different organisms, we suggest that some of these IREs developed by "convergent evolution" through stepwise changes in sequence, rather than by recombination events.

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.

Iron regulation and the cell cycle: identification of an iron-responsive element in the 3'-untranslated region of human cell division cycle 14A mRNA by a refined microarray-based screening strategy

Iron regulatory proteins (IRPs) 1 and 2 post-transcriptionally control mammalian iron homeostasis by binding to iron-responsive elements (IREs), conserved RNA stem-loop structures located in the 5'- or 3'-untranslated regions of genes involved in iron metabolism (e.g. FTH1, FTL, and TFRC). To identify novel IRE-containing mRNAs, we integrated biochemical, biocomputational, and microarray-based experimental approaches. IRP/IRE messenger ribonucleoproteins were immunoselected, and their mRNA composition was analyzed using an IronChip microarray enriched for genes predicted computationally to contain IRE-like motifs. Among different candidates, this report focuses on a novel IRE located in the 3'-untranslated region of the cell division cycle 14A mRNA. We show that this IRE motif efficiently binds both IRP1 and IRP2. Differential splicing of cell division cycle 14A produces IRE- and non-IRE-containing mRNA isoforms. Interestingly, only the expression of the IRE-containing mRNA isoforms is selectively increased by cellular iron deficiency. This work describes a new experimental strategy to explore the IRE/IRP regulatory network and uncovers a previously unrecognized regulatory link between iron metabolism and the cell cycle.