Complete loss of iron regulatory proteins 1 and 2 prevents viability of murine zygotes beyond the blastocyst stage of embryonic development

Iron regulatory protein two facilitates ferritinophagy and DNA damage/repair through guiding ATG9A trafficking

Trace elemental iron is an essential nutrient that participates in diverse metabolic processes. Dysregulation of cellular iron homeostasis, both iron deficiency and iron overload, is detrimental and tightly associated with disease pathogenesis. IRPs-IREs system is located at the center for iron homeostasis regulation. Additionally, ferritinophagy, the autophagy-dependent ferritin catabolism for iron recycling, is emerging as a novel mechanism for iron homeostasis regulation. It is still unclear whether IRPs-IREs system and ferritinophagy are synergistic or redundant in determining iron homeostasis. Here we report that IRP2, but not IRP1, is indispensable for ferritinophagy in response to iron depletion. Mechanistically, IRP2 ablation results in compromised AMPK activation and defective ATG9A endosomal trafficking, leading to the decreased engulfment of NCOA4-ferritin complex by endosomes and the subsequent dysregulated endosomal microferritinophagy. Moreover, this defective endosomal microferritinophagy exacerbates DNA damage and reduces colony formation in IRP2-depleted cells. Collectively, this study expands the physiological function of IRP2 in endosomal microferritinophagy and highlights potential crosstalk between IRPs-IREs and ferritinophagy in manipulating iron homeostasis.

Advance in Iron Metabolism, Oxidative Stress and Cellular Dysfunction in Experimental and Human Kidney Diseases

Kidney diseases pose a significant global health issue, frequently resulting in the gradual decline of renal function and eventually leading to end-stage renal failure. Abnormal iron metabolism and oxidative stress-mediated cellular dysfunction facilitates the advancement of kidney diseases. Iron homeostasis is strictly regulated in the body, and disturbance in this regulatory system results in abnormal iron accumulation or deficiency, both of which are associated with the pathogenesis of kidney diseases. Iron overload promotes the production of reactive oxygen species (ROS) through the Fenton reaction, resulting in oxidative damage to cellular molecules and impaired cellular function. Increased oxidative stress can also influence iron metabolism through upregulation of iron regulatory proteins and altering the expression and activity of key iron transport and storage proteins. This creates a harmful cycle in which abnormal iron metabolism and oxidative stress perpetuate each other, ultimately contributing to the advancement of kidney diseases. The crosstalk of iron metabolism and oxidative stress involves multiple signaling pathways, such as hypoxia-inducible factor (HIF) and nuclear factor erythroid 2-related factor 2 (Nrf2) pathways. This review delves into the functions and mechanisms of iron metabolism and oxidative stress, along with the intricate relationship between these two factors in the context of kidney diseases. Understanding the underlying mechanisms should help to identify potential therapeutic targets and develop novel and effective therapeutic strategies to combat the burden of kidney diseases.

Iron, Copper, and Selenium: Cancer's Thing for Redox Bling

Cells require micronutrients for numerous basic functions. Among these, iron, copper, and selenium are particularly critical for redox metabolism, and their importance is heightened during oncogene-driven perturbations in cancer. In this review, which particularly focuses on iron, we describe how these micronutrients are carefully chaperoned about the body and made available to tissues, a process that is designed to limit the toxicity of free iron and copper or by-products of selenium metabolism. We delineate perturbations in iron metabolism and iron-dependent proteins that are observed in cancer, and describe the current approaches being used to target iron metabolism and iron-dependent processes.

Mechanisms controlling cellular and systemic iron homeostasis

In mammals, hundreds of proteins use iron in a multitude of cellular functions, including vital processes such as mitochondrial respiration, gene regulation and DNA synthesis or repair. Highly orchestrated regulatory systems control cellular and systemic iron fluxes ensuring sufficient iron delivery to target proteins is maintained, while limiting its potentially deleterious effects in iron-mediated oxidative cell damage and ferroptosis. In this Review, we discuss how cells acquire, traffick and export iron and how stored iron is mobilized for iron-sulfur cluster and haem biogenesis. Furthermore, we describe how these cellular processes are fine-tuned by the combination of various sensory and regulatory systems, such as the iron-regulatory protein (IRP)-iron-responsive element (IRE) network, the nuclear receptor co-activator 4 (NCOA4)-mediated ferritinophagy pathway, the prolyl hydroxylase domain (PHD)-hypoxia-inducible factor (HIF) axis or the nuclear factor erythroid 2-related factor 2 (NRF2) regulatory hub. We further describe how these pathways interact with systemic iron homeostasis control through the hepcidin-ferroportin axis to ensure appropriate iron fluxes. This knowledge is key for the identification of novel therapeutic opportunities to prevent diseases of cellular and/or systemic iron mismanagement.

Cardiac iron metabolism during aging - Role of inflammation and proteolysis

Iron is the most abundant trace element in the human body. Since iron can switch between its 2-valent and 3-valent form it is essential in various physiological processes such as energy production, proliferation or DNA synthesis. Especially high metabolic organs such as the heart rely on iron-associated iron-sulfur and heme proteins. However, due to switches in iron oxidation state, iron overload exhibits high toxicity through formation of reactive oxygen species, underlining the importance of balanced iron levels. Growing evidence demonstrates disturbance of this balance during aging. While age-associated cardiovascular diseases are often related to iron deficiency, in physiological aging cardiac iron accumulates. To understand these changes, we focused on inflammation and proteolysis, two hallmarks of aging, and their role in iron metabolism. Via the IL-6-hepcidin axis, inflammation and iron status are strongly connected often resulting in anemia accompanied by infiltration of macrophages. This tight connection between anemia and inflammation highlights the importance of the macrophage iron metabolism during inflammation. Age-related decrease in proteolytic activity additionally affects iron balance due to impaired degradation of iron metabolism proteins. Therefore, this review accentuates alterations in iron metabolism during aging with regards to inflammation and proteolysis to draw attention to their implications and associations.

Ferrodifferentiation regulates neurodevelopment via ROS generation

Iron is important for life, and iron deficiency impairs development, but whether the iron level regulates neural differentiation remains elusive. In this study, with iron-regulatory proteins (IRPs) knockout embryonic stem cells (ESCs) that showed severe iron deficiency, we found that the Pax6- and Sox2-positive neuronal precursor cells and Tuj1 fibers in IRP1-/-IRP2-/- ESCs were significantly decreased after inducing neural differentiation. Consistently, in vivo study showed that the knockdown of IRP1 in IRP2-/- fetal mice remarkably affected the differentiation of neuronal precursors and the migration of neurons. These findings suggest that low intracellular iron status significantly inhibits neurodifferentiation. When supplementing IRP1-/-IRP2-/- ESCs with iron, these ESCs could differentiate normally. Further investigations revealed that the underlying mechanism was associated with an increase in reactive oxygen species (ROS) production caused by the substantially low level of iron and the down-regulation of iron-sulfur cluster protein ISCU, which, in turn, affected the proliferation and differentiation of stem cells. Thus, the appropriate amount of iron is crucial for maintaining normal neural differentiation that is termed ferrodifferentiation.

Generation of Red Blood Cells from Human Pluripotent Stem Cells-An Update

Red blood cell (RBC) transfusion is a lifesaving medical procedure that can treat patients with anemia and hemoglobin disorders. However, the shortage of blood supply and risks of transfusion-transmitted infection and immune incompatibility present a challenge for transfusion. The in vitro generation of RBCs or erythrocytes holds great promise for transfusion medicine and novel cell-based therapies. While hematopoietic stem cells and progenitors derived from peripheral blood, cord blood, and bone marrow can give rise to erythrocytes, the use of human pluripotent stem cells (hPSCs) has also provided an important opportunity to obtain erythrocytes. These hPSCs include both human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs). As hESCs carry ethical and political controversies, hiPSCs can be a more universal source for RBC generation. In this review, we first discuss the key concepts and mechanisms of erythropoiesis. Thereafter, we summarize different methodologies to differentiate hPSCs into erythrocytes with an emphasis on the key features of human definitive erythroid lineage cells. Finally, we address the current limitations and future directions of clinical applications using hiPSC-derived erythrocytes.

Ferroptosis in life: To be or not to be

Ferroptosis is a novel type of programmed cell death, characterized by a dysregulated iron metabolism and accumulation of lipid peroxides. It features the alteration of mitochondria and aberrant accumulation of excessive iron as well as loss of the cysteine-glutathione-GPX4 axis. Eventually, the accumulated lipid peroxides result in lethal damage to the cells. Ferroptosis is induced by the overloading of iron and the accumulation of ROS and can be inhibited by the activation of the GPX4 pathway, FS1-CoQ10 pathway, GCH1-BH4 pathway, and the DHODH pathway, it is also regulated by the oncogenes and tumor suppressors. Ferroptosis involves various physiological and pathological processes, and increasing evidence indicates that ferroptosis play a critical role in cancers and other diseases. It inhibits the proliferation of malignant cells in various types of cancers and inducing ferroptosis may become a new method of cancer treatment. Many inhibitors targeting the key factors of ferroptosis such as SLC7A11, GPX4, and iron overload have been developed. The application of ferroptosis is mainly divided into two directions, i.e. to avoid ferroptosis in healthy cells and selectively induce ferroptosis in cancers. In this review, we provide a critical analysis of the concept, and regulation pathways of ferroptosis and explored its roles in various diseases, we also summarized the compounds targeting ferroptosis, aiming to promote the speed of clinical use of ferroptosis induction in cancer treatment.

Dysregulation of Iron Homeostasis in the Central Nervous System and the Role of Ferroptosis in Neurodegenerative Disorders

Significance: Iron accumulation occurs in the central nervous system (CNS) in a variety of neurological conditions as diverse as spinal cord injury, stroke, multiple sclerosis, Parkinson's disease, and others. Iron is a redox-active metal that gives rise to damaging free radicals if its intracellular levels are not controlled or if it is not properly sequestered within cells. The accumulation of iron occurs due to dysregulation of mechanisms that control cellular iron homeostasis. Recent Advances: The molecular mechanisms that regulate cellular iron homeostasis have been revealed in much detail in the past three decades, and new advances continue to be made. Understanding which aspects of iron homeostasis are dysregulated in different conditions will provide insights into the causes of iron accumulation and iron-mediated tissue damage. Recent advances in iron-dependent lipid peroxidation leading to cell death, called ferroptosis, has provided useful insights that are highly relevant for the lipid-rich environment of the CNS. Critical Issues: This review examines the mechanisms that control normal cellular iron homeostasis, the dysregulation of these mechanisms in neurological disorders, and more recent work on how iron can induce tissue damage via ferroptosis. Future Directions: Quick and reliable tests are needed to determine if and when ferroptosis contributes to the pathogenesis of neurological disorders. In addition, there is need to develop better druggable agents to scavenge lipid radicals and reduce CNS damage for neurological conditions for which there are currently few effective treatments. Antioxid. Redox Signal. 37, 150-170.

Disruption of cellular iron homeostasis by IREB2 missense variants causes severe neurodevelopmental delay, dystonia and seizures

Altered brain iron homeostasis can contribute to neurodegeneration by interfering with the delivery of the iron needed to support key cellular processes, including mitochondrial respiration, synthesis of myelin and essential neurotransmitters. Intracellular iron homeostasis in mammals is maintained by two homologous ubiquitously expressed iron-responsive element-binding proteins (IRP1 and IRP2). Using exome sequencing, two patients with severe neurodegenerative disease and bi-allelic mutations in the gene IREB2 were first identified and clinically characterized in 2019. Here, we report the case of a 7-year-old male patient with compound heterozygous missense variants in IREB2, whose neurological features resembled those of the two previously reported IRP2-deficient patients, including a profound global neurodevelopmental delay and dystonia. Biochemical characterization of a lymphoblast cell line derived from the patient revealed functional iron deficiency, altered post-transcriptional regulation of iron metabolism genes and mitochondrial dysfunction. The iron metabolism abnormalities of the patient cell line were reversed by lentiviral-mediated restoration of IREB2 expression. These results, in addition to confirming the essential role of IRP2 in the regulation of iron metabolism in humans, expand the scope of the known IRP2-related neurodegenerative disorders and underscore that IREB2 pathological variants may impact the iron-responsive element-binding activity of IRP2 with varying degrees of severity. The three severely affected patients identified so far all suffered from complete loss of function of IRP2, raising the possibility that individuals with significant but incomplete loss of IRP2 function may develop less severe forms of the disease, analogous to other human conditions that present with a wide range of phenotypic manifestations.

Effect of Fe3+ on Na,K-ATPase: Unexpected activation of ATP hydrolysis

Iron is a key element in cell function; however, its excess in iron overload conditions can be harmful through the generation of reactive oxygen species (ROS) and cell oxidative stress. Activity of Na,K-ATPase has been shown to be implicated in cellular iron uptake and iron modulates the Na,K-ATPase function from different tissues. In this study, we determined the effect of iron overload on Na,K-ATPase activity and established the role that isoforms and conformational states of this enzyme has on this effect. Total blood and membrane preparations from erythrocytes (ghost cells), as well as pig kidney and rat brain cortex, and enterocytes cells (Caco-2) were used. In E1-related subconformations, an enzyme activation effect by iron was observed, and in the E2-related subconformations enzyme inhibition was observed. The enzyme's kinetic parameters were significantly changed only in the Na⁺ curve in ghost cells. In contrast to Na,K-ATPase α2 and α3 isoforms, activation was not observed for the α1 isoform. In Caco-2 cells, which only contain Na,K-ATPase α1 isoform, the FeCl₃ increased the intracellular storage of iron, catalase activity, the production of H₂O₂ and the expression levels of the α1 isoform. In contrast, iron did not affect lipid peroxidation, GSH content, superoxide dismutase and Na,K-ATPase activities. These results suggest that iron itself modulates Na,K-ATPase and that one or more E1-related subconformations seems to be determinant for the sensitivity of iron modulation through a mechanism in which the involvement of the Na, K-ATPase α3 isoform needs to be further investigated.

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.

Paralogous synthetic lethality underlies genetic dependencies of the cancer-mutated gene STAG2

STAG2, a component of the mitotically essential cohesin complex, is highly mutated in several different tumour types, including glioblastoma and bladder cancer. Whereas cohesin has roles in many cancer-related pathways, such as chromosome instability, DNA repair and gene expression, the complex nature of cohesin function has made it difficult to determine how STAG2 loss might either promote tumorigenesis or be leveraged therapeutically across divergent cancer types. Here, we have performed whole-genome CRISPR-Cas9 screens for STAG2-dependent genetic interactions in three distinct cellular backgrounds. Surprisingly, STAG1, the paralog of STAG2, was the only negative genetic interaction that was shared across all three backgrounds. We also uncovered a paralogous synthetic lethal mechanism behind a genetic interaction between STAG2 and the iron regulatory gene IREB2 Finally, investigation of an unusually strong context-dependent genetic interaction in HAP1 cells revealed factors that could be important for alleviating cohesin loading stress. Together, our results reveal new facets of STAG2 and cohesin function across a variety of genetic contexts.

Role of Iron Metabolism-Related Genes in Prenatal Development: Insights from Mouse Transgenic Models

Iron is an essential nutrient during all stages of mammalian development. Studies carried out over the last 20 years have provided important insights into cellular and systemic iron metabolism in adult organisms and led to the deciphering of many molecular details of its regulation. However, our knowledge of iron handling in prenatal development has remained remarkably under-appreciated, even though it is critical for the health of both the embryo/fetus and its mother, and has a far-reaching impact in postnatal life. Prenatal development requires a continuous, albeit quantitatively matched with the stage of development, supply of iron to support rapid cell division during embryogenesis in order to meet iron needs for erythropoiesis and to build up hepatic iron stores, (which are the major source of this microelement for the neonate). Here, we provide a concise overview of current knowledge of the role of iron metabolism-related genes in the maintenance of iron homeostasis in pre- and post-implantation development based on studies on transgenic (mainly knock-out) mouse models. Most studies on mice with globally deleted genes do not conclude whether underlying in utero iron disorders or lethality is due to defective placental iron transport or iron misregulation in the embryo/fetus proper (or due to both). Therefore, there is a need of animal models with tissue specific targeted deletion of genes to advance the understanding of prenatal iron metabolism.

Advances in understanding the crosstalk between mother and fetus on iron utilization

A full-term pregnancy comes with significant demand for iron. Not meeting this demand has adverse effects on maternal health and on the intrauterine and postnatal development of the infant. In the infant, some of these adverse effects cannot be reversed by postnatal iron supplementation, highlighting the need to tackle iron deficiency in utero. Achieving this requires sound understanding of the pathways that govern iron transfer at the fetomaternal interface. Two pathways are emerging as key players in this context; the hepcidin/ferroportin axis pathway and the iron regulatory protein (IRPs) pathway. In late gestation, suppression of maternal hepcidin, by as yet unknown factors, is required for increasing iron availability to the growing fetus. In the placenta, the rate of iron uptake by transferrin receptor TfR1 at the apical/maternal side and of iron release by ferroportin FPN at the basal/fetal side is controlled by IRP1. In fetal hepatocytes, build up of fetal iron stores requires post-translational inhibition of FPN by the cell-autonomous action of hepcidin. In the fetal liver, FPN is also subject to additional control at the transcriptional level, possibly by the action of hypoxia-inducible factor HIF2α. The rates of apical iron uptake and basal iron release in the placenta are modulated according to iron availability in the maternal blood and the placenta's own needs. This placental modulation ensures that the amount of iron delivered to the fetal circulation is maintained within a normal range, even in the face of mild maternal iron deficiency or overload. However, when maternal iron deficiency or overload are extreme, placental modulation is not sufficient to maintain normal iron supply to the fetus, resulting in fetal iron deficiency and overload respectively. Thus, the rate of iron transfer at the fetomaternal interface is subject to several regulatory signals operating simultaneously in the maternal liver, the placenta and the fetal liver. These regulatory signals act in concert to maintain normal iron supply to the fetus within a wide range of maternal iron states, but fail to do so when maternal iron deficiency or overload are extreme. The limitations of existing experimental models must be overcome if we are to gain better understanding of the role of these regulatory signals in normal and complicated pregnancy. Ultimately, that understanding could help identify better markers of fetal iron demand and underpin novel iron replacement strategies to treat maternal and fetal iron deficiency.

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.

RNAi-mediated silencing of ferritin genes in the brown planthopper Nilaparvata lugens affects survival, growth and female fecundity

BACKGROUND

The brown planthopper (BPH), Nilaparvata lugens, is the most destructive rice insect pest. To exploit potential target genes for applications in transgenic rice to control this sap-sucking insect pest, three ferritin genes were functionally characterized in this study.

RESULTS

In this study, three ferritin genes, that is, ferritin 1 Heavy Chain (NlFer1), ferritin 2 Light Chain (NlFer2) and soma ferritin (Nlsoma-Fer), were identified from BPH. Tissue-specific analyses showed that all three genes were highly expressed in the gut. Although double-stranded RNA injection-mediated RNA inference (RNAi) of Nlsoma-Fer expression resulted in only < 14% mortality in BPH, knockdown of NlFer1 or NlFer2 led to retarded growth and 100% mortality in young nymphs, and downregulation of NlFer1 and NlFer2 in newly emerged female adults caused undeveloped ovaries and severely inhibited oocyte growth, resulting in extremely low fecundity and a zero hatching rate. Knockdown of NlFer1 and NlFer2 caused similar phenotypes in BPH, indicating that they function together, as in many other animals. The results demonstrated that NlFer1 and NlFer2 were essential for BPH development and reproduction. BPHs showed high sensitivity to both dsNlFer1 and dsNlFer2, and injection of only 0.625 ng dsNlFer1 per BPH resulted in 100% mortality. Additionally, the effectiveness of feeding dsNlFer1 and dsNlFer2 to BPH nymphs was further proven.

CONCLUSION

NlFer1 and NlFer2 are essential for BPH development and reproduction, and the insect is highly sensitive to their depletion, suggesting that the two gut-highly-expressed genes are promising candidates for application in RNAi-based control of this destructive pest.

Effects of maternal iron status on placental and fetal iron homeostasis

Iron deficiency is common worldwide and is associated with adverse pregnancy outcomes. The increasing prevalence of indiscriminate iron supplementation during pregnancy also raises concerns about the potential adverse effects of iron excess. We examined how maternal iron status affects the delivery of iron to the placenta and fetus. Using mouse models, we documented maternal homeostatic mechanisms that protect the placenta and fetus from maternal iron excess. We determined that under physiological conditions or in iron deficiency, fetal and placental hepcidin did not regulate fetal iron endowment. With maternal iron deficiency, critical transporters mediating placental iron uptake (transferrin receptor 1 [TFR1]) and export (ferroportin [FPN]) were strongly regulated. In mice, not only was TFR1 increased, but FPN was surprisingly decreased to preserve placental iron in the face of fetal iron deficiency. In human placentas from pregnancies with mild iron deficiency, TFR1 was increased, but there was no change in FPN. However, induction of more severe iron deficiency in human trophoblast in vitro resulted in the regulation of both TFR1 and FPN, similar to what was observed in the mouse model. This placental adaptation that prioritizes placental iron is mediated by iron regulatory protein 1 (IRP1) and is important for the maintenance of mitochondrial respiration, thus ultimately protecting the fetus from the potentially dire consequences of generalized placental dysfunction.

Cellular iron sensing and regulation: Nuclear IRP1 extends a classic paradigm

The classic view is that iron regulatory proteins operate at the post-transcriptional level. Iron Regulatory Protein 1 (IRP1) shifts between an apo-form that binds mRNAs and a holo-form that harbors a [4Fe4S] cluster. The latter form is not considered relevant to iron regulation, but rather thought to act as a non-essential cytosolic aconitase. Recent work in Drosophila, however, shows that holo-IRP1 can also translocate to the nucleus, where it appears to downregulate iron metabolism genes, preparing the cell for a decline in iron uptake. The shifting of IRP1 between states requires a functional mitoNEET pathway that includes a glycogen branching enzyme for the repair or disassembly of IRP1's oxidatively damaged [3Fe4S] cluster. The new findings add to the notion that glucose metabolism is modulated by iron metabolism. Furthermore, we propose that ferritin ferroxidase activity participates in the repair of the IRP1 [3Fe4S] cluster leading to the hypothesis that cytosolic ferritin directly contributes to cellular iron sensing.

Mdm2 is a target and mediator of IRP2 in cell growth control

Iron is an essential element to all living organisms and plays a vital role in many cellular processes, such as DNA synthesis and energy production. The Mdm2 oncogene is an E3 ligase and known to promote tumor growth. However, the role of Mdm2 in iron homeostasis is not certain. Here, we showed that Mdm2 expression was increased by iron depletion but decreased by iron repletion. We also showed that Iron Regulatory Protein 2 (IRP2) mediated iron-regulated Mdm2 expression. Specifically, Mdm2 expression was increased by ectopic IRP2 but decreased by knockdown or knockout of IRP2 in human cancer cells as well as in mouse embryonic fibroblasts. In addition, we showed that IRP2-regulated Mdm2 expression was independent of tumor suppressor p53. Mechanistically, we found that IRP2 stabilized Mdm2 transcript via binding to an iron response element (IRE) in the 3'UTR of Mdm2 mRNA. Finally, we showed that Mdm2 is required for IRP2-mediated cell proliferation and Mdm2 expression is highly associated with IRP2 in both the normal and cancerous liver tissues. Together, we uncover a novel regulation of Mdm2 by IRP2 via mRNA stability and that the IRP2-Mdm2 axis may play a critical role in cell growth.

A combination of bovine serum albumin with insulin-transferrin-sodium selenite and/or epidermal growth factor as alternatives to fetal bovine serum in culture medium improves bovine embryo quality and trophoblast invasion by induction of matrix metalloproteinases

This study investigated the use of bovine serum albumin (BSA) plus insulin-transferrin-sodium selenite (ITS) and/or epidermal growth factor (EGF) as alternatives to fetal bovine serum (FBS) in embryo culture medium. The developmental ability and quality of bovine embryos were determined by assessing their cell number, lipid content, gene expression and cryotolerance, as well as the invasion ability of trophoblasts. The percentage of embryos that underwent cleavage and formed a blastocyst was higher (P<0.01) in medium containing ITS plus EGF and BSA than in medium containing FBS. Culture with ITS plus EGF and BSA also increased the hatching ability of blastocysts and the total cell number per blastocyst. Furthermore, the beneficial effects of BAS plus ITS and EGF on embryos were associated with a significantly reduced intracellular lipid content, which increased their cryotolerance. An invasion assay confirmed that culture with ITS plus EGF and BSA significantly improved the invasion ability of trophoblasts. Real-time quantitative polymerase chain reaction analysis showed that the mRNA levels of matrix metalloproteinase-2 (MMP2) and MMP9, acyl-CoA synthetase long-chain family member 3, acyl-coenzyme A dehydrogenase long-chain and hydroxymethylglutaryl-CoA reductase significantly increased upon culture with ITS plus EGF and BSA. Moreover, protein expression levels of matrix metalloproteinase-2 and -9 increased (P<0.01) in medium supplemented with ITS plus EGF and BSA compared with medium supplemented with FBS. Taken together, these data suggest that supplementation of medium with ITS plus EGF and BSA improves invitro bovine embryo production, cryotolerance and invasion ability of trophoblasts.

Iron overload: Effects on cellular biochemistry

Iron is an essential element for human life. However, it is a pro-oxidant agent capable of reacting with hydrogen peroxide. An iron overload can cause cellular changes, such as damage to the plasma membrane leading to cell death. Effects of iron overload in cellular biochemical processes include modulating membrane enzymes, such as the Na, K-ATPase, impairing the ionic transport and inducing irreversible damage to cellular homeostasis. To avoid such damage, cells have an antioxidant system that acts in an integrated manner to prevent oxidative stress. In addition, the cells contain proteins responsible for iron transport and storage, preventing its reaction with other substances during absorption. Moreover, iron is associated with cellular events coordinated by iron-responsive proteins (IRPs) that regulate several cellular functions, including a process of cell death called ferroptosis. This review will address the biochemical aspects of iron overload at the cellular level and its effects on important cellular structures.

The indispensable role of mammalian iron sulfur proteins in function and regulation of multiple diverse metabolic pathways

In recent years, iron sulfur (Fe–S) proteins have been identified as key players in mammalian metabolism, ranging from long-known roles in the respiratory complexes and the citric acid cycle, to more recently recognized roles in RNA and DNA metabolism. Fe–S cofactors have often been missed because of their intrinsic lability and oxygen sensitivity. More Fe–S proteins have now been identified owing to detection of their direct interactions with components of the Fe–S biogenesis machinery, and through use of informatics to detect a motif that binds the co-chaperone responsible for transferring nascent Fe–S clusters to domains of recipient proteins. Dissection of the molecular steps involved in Fe–S transfer to Fe–S proteins has revealed that direct and shielded transfer occurs through highly conserved pathways that operate in parallel in the mitochondrial matrix and in the cytosolic/nuclear compartments of eukaryotic cells. Because Fe–S clusters have the unusual ability to accept or donate single electrons in chemical reactions, their presence renders complex chemical reactions possible. In addition, Fe–S clusters may function as sensors that interconnect activity of metabolic pathways with cellular redox status. Presence in pathways that control growth and division may enable cells to regulate their growth according to sufficiency of energy stores represented by redox capacity, and oxidation of such proteins could diminish anabolic activities to give cells an opportunity to restore energy supplies. This review will discuss mechanisms of Fe–S biogenesis and delivery, and methods that will likely reveal important roles of Fe–S proteins in proteins not yet recognized as Fe–S proteins.

Regulation of cellular iron metabolism: Iron-dependent degradation of IRP by SCFFBXL5 ubiquitin ligase

Because of essentiality and toxicity of iron in our body, iron metabolism is tightly regulated in cells. In mammalian cells, iron regulatory protein 1 and 2 (IRP1 and IRP2) are the central regulators of cellular iron metabolism. IRPs regulate iron metabolism by interacting with the RNA stem-loop structures, iron-responsive elements (IREs), found on the transcripts encoding proteins involved in iron metabolism only in iron depleted condition. It is also well-known that the ubiquitin system plays central roles in cellular iron regulation because both IRPs having the IRE binding activity are recognized and ubiquitinated by the SCF^FBXL5 ubiquitin ligase in condition of iron-replete. FBXL5, which is a substrate recognition subunit of SCF^FBXL5, senses iron availability via its hemerythrin-like domain. In this small article, current understanding of the roles of SCF^FBXL5-mediated degradation of IRPs played in cellular iron metabolism is discussed.

Brain Iron Metabolism and Regulation

With the development of research, more and more evidences suggested that mutations in the genes associated with brain iron metabolism induced diseases in the brain. Brain iron metabolism disorders might be one cause of neurodegenerative diseases. This review mainly summarizes the normal process of iron entry into the brain across the blood-brain barrier, and the distribution and transportation of iron among neurons and glial cells, as well as the underlying regulation mechanisms. To understand the mechanisms of iron metabolism in the brain will provide theoretical basis to prevent and cure brain diseases related to iron metabolism disorders.

Perturbation of Iron Metabolism by Cisplatin through Inhibition of Iron Regulatory Protein 2

Cisplatin is classically known to exhibit anticancer activity through DNA damage in the nucleus. Here we found a mechanism by which cisplatin affects iron metabolism, leading to toxicity and cell death. Cisplatin causes intracellular iron deficiency through direct inhibition of the master regulator of iron metabolism, iron regulatory protein 2 (IRP2) with marginal effects on IRP1. Cisplatin, but not carboplatin or transplatin, binds human IRP2 at Cys512 and Cys516 and impairs IRP2 binding to iron-responsive elements of ferritin and transferrin receptor-1 (TfR1) mRNAs. IRP2 inhibition by cisplatin caused ferritin upregulation and TfR1 downregulation leading to sustained intracellular iron deficiency. Cys512/516Ala mutant IRP2 made cells more resistant to cisplatin. Furthermore, combination of cisplatin and the iron chelator desferrioxamine enhanced cytotoxicity through augmented iron depletion in culture and xenograft mouse model. Collectively, cisplatin is an inhibitor of IRP2 that induces intracellular iron deficiency.

Hepcidin and iron regulatory proteins coordinately regulate ferroportin 1 expression in the brain of mice

Iron plays an essential role in various cellular metabolic processes of the body. Maintenance of cellular iron homeostasis is particularly important for keeping the normal functions of the cells. Ferroportin 1 (FPN1) is the currently only known iron exporter on the cell membrane. It has been indicated that the regulation of FPN1 in response to the alteration of iron level mainly involves two processes, posttranscriptional repression by iron regulatory proteins (IRPs) and posttranslational degradation by hepcidin, the major iron-sensing hormone. However, whether there is any communication between the two types of regulations or which one plays dominant role has not been reported. In our study with IRP2^-/- mice, we found that knockout of IRP2 increased FPN1 expression in the cerebral cortex of IRP2^-/- mice, whereas the upregulation of FPN1 was more significant in IRP1/IRP2 dual knockdown fibroblasts. Interestingly, we found that the knockout of IRP2 severely affected the regulation effect of hepcidin on FPN1 in mouse brain. FPN1 level decreased dramatically in the brain of wild-type mice injected with hepcidin, but it did not decrease much in IRP2 knockout mice. Further investigation disclosed that the compromised hepcidin-FPN1 regulation in IRP2^-/- cells was directly dependent on the existence of iron-responsive element (IRE) in FPN1 messenger RNA. These results indicate that IRPs and hepcidin coordinately regulate the FPN1 level in mice. This study will provide a more comprehensive understanding of the regulatory mechanisms of FPN1 expression.

p53 tumor suppressor and iron homeostasis

Iron is an essential nutrient for all living organisms and plays a vital role in many fundamental biochemical processes, such as oxygen transport, energy metabolism, and DNA synthesis. Due to its capability to produce free radicals, iron has deleterious effects and thus, its level needs to be tightly controlled in the body. Deregulation of iron metabolism is known to cause diseases, including anemia by iron deficiency and hereditary hemochromatosis by iron overload. Interestingly, dysregulated iron metabolism occurs frequently in tumor cells and contributes to tumorigenesis. In this review, we will discuss the role of p53 tumor suppressor in iron homeostasis.

A novel splicing site IRP1 somatic mutation in a patient with pheochromocytoma and JAK2V617F positive polycythemia vera: a case report

Background

The role of the hypoxia signaling pathway in the pathogenesis of pheochromocytoma/paraganglioma (PPGL)-polycythemia syndrome has been elucidated. Novel somatic mutations in hypoxia-inducible factor type 2A (HIF2A) and germline mutations in prolyl hydroxylase type 1 and type 2 (PHD1 and PHD2) have been identified to cause upregulation of the hypoxia signaling pathway and its target genes including erythropoietin (EPO) and its receptor (EPOR). However, in a minority of patients presenting with this syndrome, the genetics and molecular pathogenesis remain unexplained. The aim of the present study was to uncover novel genetic causes of PPGL-polycythemia syndrome.

Case presentation

A female presented with a history of JAK2^V617F positive PV, diagnosed in 2007, and right adrenal pheochromocytoma diagnosed and resected in 2011. Her polycythemia symptoms and hematocrit levels continued to worsen from 2007 to 2011, with an increased frequency of phlebotomies. Postoperatively, until early 2013, her hematocrit levels remained normalized. Following this, the hematocrit levels ranged between 46.4 and 48.9% [35–45%]. Tumor tissue from the patient was further tested for mutations in genes related to upregulation of the hypoxia signaling pathway including iron regulatory protein 1 (IRP1), which is a known regulator of HIF-2α mRNA translation. Functional studies were performed to investigate the consequences of these mutations, especially their effect on the HIF signaling pathway and EPO. Indel mutations (c.267-1_267delGGinsTA) were discovered at the exon 3 splicing site of IRP1. Minigene construct and splicing site analysis showed that the mutation led to a new splicing site and a frameshift mutation of IRP1, which caused a truncated protein. Fluorescence in situ hybridization analysis demonstrated heterozygous IRP1 deletions in tumor cells. Immunohistochemistry results confirmed the truncated IRP1 and overexpressed HIF-2α, EPO and EPOR in tumor cells.

Conclusions

This is the first report which provides direct molecular genetic evidence of association between a somatic IRP1 loss-of-function mutation and PHEO and secondary polycythemia. In patients diagnosed with PHEO/PGL and polycythemia with negative genetic testing for mutations in HIF2A, PHD1/2, and VHL, IRP1 should be considered as a candidate gene.

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.

The actin-binding protein profilin 2 is a novel regulator of iron homeostasis

Pfn2 mRNA has a functional and conserved IRE in the 3′ untranslated region. Pfn2 knockout mice display an iron phenotype with iron accumulation in specific areas of the brain and depletion of liver iron stores.

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

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

Transferrin receptor mRNA interactions contributing to iron homeostasis

The transferrin receptor is the primary means of iron importation for most mammalian cells and understanding its regulatory mechanisms is relevant to hematologic, oncologic, and other disorders in which iron homeostasis is perturbed. The 3' UTR of the transferrin receptor mRNA contains an instability element that is protected from degradation during iron depletion through interactions of iron regulatory proteins (IRPs) with five iron-responsive elements (IREs). The structural features required for degradation and the site of IRP binding required for in situ protection remain unclear. An RNA-CLIP strategy is described here that identifies the predominant site of IRP-1 interaction within a nontransformed primary cell line. This approach avoided complications associated with the use of elevated concentrations of protein and/or mRNA and detected interactions within the native environment of the mRNA. A compensatory mutagenesis strategy indicates that the instability element at minimum consists of three non-IRE stem-loops that can function additively, suggesting that they are not forming one highly interdependent structure. Although the IREs are not essential for instability, they enhance instability when IRP interactions are inhibited. These results are supportive of a mechanism for a graded response to the intracellular iron resulting from a progressive loss of IRP protection.

RNA-Binding Proteins in Trichomonas vaginalis: Atypical Multifunctional Proteins

Iron homeostasis is highly regulated in vertebrates through a regulatory system mediated by RNA-protein interactions between the iron regulatory proteins (IRPs) that interact with an iron responsive element (IRE) located in certain mRNAs, dubbed the IRE-IRP regulatory system. Trichomonas vaginalis, the causal agent of trichomoniasis, presents high iron dependency to regulate its growth, metabolism, and virulence properties. Although T. vaginalis lacks IRPs or proteins with aconitase activity, possesses gene expression mechanisms of iron regulation at the transcriptional and posttranscriptional levels. However, only one gene with iron regulation at the transcriptional level has been described. Recently, our research group described an iron posttranscriptional regulatory mechanism in the T. vaginalis tvcp4 and tvcp12 cysteine proteinase mRNAs. The tvcp4 and tvcp12 mRNAs have a stem-loop structure in the 5'-coding region or in the 3'-UTR, respectively that interacts with T. vaginalis multifunctional proteins HSP70, α-Actinin, and Actin under iron starvation condition, causing translation inhibition or mRNA stabilization similar to the previously characterized IRE-IRP system in eukaryotes. Herein, we summarize recent progress and shed some light on atypical RNA-binding proteins that may participate in the iron posttranscriptional regulation in T. vaginalis.

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.

Beyond soluble transferrin receptor: old challenges and new horizons

Disturbances of iron metabolism are a frequent challenge in outpatient and inpatient care. Although several established biomarkers are commonly used by clinicians for differential diagnosis, the discrimination between latent or classic iron deficiency, anaemia of chronic disease or a combination of functional iron deficiency (iron-restricted erythropoiesis) with anaemia of chronic disease in patients affected by inflammatory disease can be demanding. Soluble transferrin receptor (sTfR) is a cleaved monomer of transferrin receptor 1 and correlates positively with tissue iron deficiency as well as with stimulated erythropoiesis. The ratio between sTfR and ferritin in combination with reticulocyte haemoglobin content further helps to identify different states of iron deficiency. In this review, we will focus on biological aspects of iron metabolism and sTfR, established clinical applications and limitations of sTfR and derived indices, and prospects of future research and applications.

Iron misregulation and neurodegenerative disease in mouse models that lack iron regulatory proteins

Iron regulatory proteins 1 and 2 (IRP1 and IRP2) are two cytosolic proteins that maintain cellular iron homeostasis by binding to RNA stem loops known as iron responsive elements (IREs) that are found in the untranslated regions of target mRNAs that encode proteins involved in iron metabolism. IRPs modify the expression of iron metabolism genes, and global and tissue-specific knockout mice have been made to evaluate the physiological significance of these iron regulatory proteins (Irps). Here, we will discuss the results of the studies that have been performed with mice engineered to lack the expression of one or both Irps and made in different strains using different methodologies. Both Irp1 and Irp2 knockout mice are viable, but the double knockout (Irp1(-/-)Irp2(-/-)) mice die before birth, indicating that these Irps play a crucial role in maintaining iron homeostasis. Irp1(-/-) mice develop polycythemia and pulmonary hypertension, and when these mice are challenged with a low iron diet, they die early of abdominal hemorrhages, suggesting that Irp1 plays an essential role in erythropoiesis and in the pulmonary and cardiovascular systems. Irp2(-/-) mice develop microcytic anemia, erythropoietic protoporphyria and a progressive neurological disorder, indicating that Irp2 has important functions in the nervous system and erythropoietic homeostasis. Several excellent review articles have recently been published on Irp knockout mice that mainly focus on Irp1(-/-) mice (referenced in the introduction). In this review, we will briefly describe the phenotypes and physiological implications of Irp1(-/-) mice and discuss the phenotypes observed for Irp2(-/-) mice in detail with a particular emphasis on the neurological problems of these mice.

The crossroads of iron with hypoxia and cellular metabolism. Implications in the pathobiology of pulmonary hypertension

The pathologic hallmark of pulmonary arterial hypertension (PAH) is pulmonary vascular remodeling, characterized by endothelial cell proliferation, smooth muscle hypertrophy, and perivascular inflammation, ultimately contributing to increased pulmonary arterial pressures. Several recent studies have observed that iron deficiency in patients with various forms of PAH is associated with worsened clinical outcome. Iron plays a key role in many cellular processes regulating the response to hypoxia, oxidative stress, cellular proliferation, and cell metabolism. Given the potential importance of iron supplementation in patients with the disease and the broad cellular functions of iron, we review its role in processes that pertain to PAH.

Iron in Eukarya

Iron is essential for the normal physiological function of all organisms. In humans it is required for a plethora of biochemical roles including the transport of oxygen in the blood and energy production in the mitochondria. However, iron is also highly cytotoxic when present at high levels as it readily participates in oxidation-reduction reactions that lead to the generation of reactive oxygen species. One unique feature of iron biology is the lack of excretory mechanisms to remove excess iron from the body. Therefore, the concerted action of several genes and proteins working together to regulate the movement of iron across cell membranes, its storage in peripheral tissues and its physiological utilization in the body is essential for maintaining iron homeostasis. Humans are exposed to iron in a number of chemical forms (haem or non-haem; ferric or ferrous). This chapter will describe how humans acquire iron from their diet; the subsequent delivery of iron to its sites of utilization and storage; and how iron is recycled from effete erythrocytes for re-use in metabolism. Mutations in a number of the genes controlling iron metabolism have been identified and study of the pathological consequences of these mutations has allowed us to gain a greater understanding of how the body senses changes in iron status and coordinates its transport, storage and utilization to maintain homeostasis.

The IRP/IRE system in vivo: insights from mouse models

Iron regulatory proteins 1 and 2 (IRP1 and IRP2) post-transcriptionally control the expression of several mRNAs encoding proteins of iron, oxygen and energy metabolism. The mechanism involves their binding to iron responsive elements (IREs) in the untranslated regions of target mRNAs, thereby controlling mRNA translation or stability. Whereas IRP2 functions solely as an RNA-binding protein, IRP1 operates as either an RNA-binding protein or a cytosolic aconitase. Early experiments in cultured cells established a crucial role of IRPs in regulation of cellular iron metabolism. More recently, studies in mouse models with global or localized Irp1 and/or Irp2 deficiencies uncovered new physiological functions of IRPs in the context of systemic iron homeostasis. Thus, IRP1 emerged as a key regulator of erythropoiesis and iron absorption by controlling hypoxia inducible factor 2α (HIF2α) mRNA translation, while IRP2 appears to dominate the control of iron uptake and heme biosynthesis in erythroid progenitor cells by regulating the expression of transferrin receptor 1 (TfR1) and 5-aminolevulinic acid synthase 2 (ALAS2) mRNAs, respectively. Targeted disruption of either Irp1 or Irp2 in mice is associated with distinct phenotypic abnormalities. Thus, Irp1(-/-) mice develop polycythemia and pulmonary hypertension, while Irp2(-/-) mice present with microcytic anemia, iron overload in the intestine and the liver, and neurologic defects. Combined disruption of both Irp1 and Irp2 is incombatible with life and leads to early embryonic lethality. Mice with intestinal- or liver-specific disruption of both Irps are viable at birth but die later on due to malabsorption or liver failure, respectively. Adult mice lacking both Irps in the intestine exhibit a profound defect in dietary iron absorption due to a "mucosal block" that is caused by the de-repression of ferritin mRNA translation. Herein, we discuss the physiological function of the IRE/IRP regulatory system.

The physiological functions of iron regulatory proteins in iron homeostasis - an update

Iron regulatory proteins (IRPs) regulate the expression of genes involved in iron metabolism by binding to RNA stem-loop structures known as iron responsive elements (IREs) in target mRNAs. IRP binding inhibits the translation of mRNAs that contain an IRE in the 5′untranslated region of the transcripts, and increases the stability of mRNAs that contain IREs in the 3′untranslated region of transcripts. By these mechanisms, IRPs increase cellular iron absorption and decrease storage and export of iron to maintain an optimal intracellular iron balance. There are two members of the mammalian IRP protein family, IRP1 and IRP2, and they have redundant functions as evidenced by the embryonic lethality of the mice that completely lack IRP expression (Irp1^-/-/Irp2^-/- mice), which contrasts with the fact that Irp1^-/- and Irp2^-/- mice are viable. In addition, Irp2^-/- mice also display neurodegenerative symptoms and microcytic hypochromic anemia, suggesting that IRP2 function predominates in the nervous system and erythropoietic homeostasis. Though the physiological significance of IRP1 had been unclear since Irp1^-/- animals were first assessed in the early 1990s, recent studies indicate that IRP1 plays an essential function in orchestrating the balance between erythropoiesis and bodily iron homeostasis. Additionally, Irp1^-/- mice develop pulmonary hypertension, and they experience sudden death when maintained on an iron-deficient diet, indicating that IRP1 has a critical role in the pulmonary and cardiovascular systems. This review summarizes recent progress that has been made in understanding the physiological roles of IRP1 and IRP2, and further discusses the implications for clinical research on patients with idiopathic polycythemia, pulmonary hypertension, and neurodegeneration.

Hypoxia-inducible factors link iron homeostasis and erythropoiesis

Iron is required for efficient oxygen transport, and hypoxia signaling links erythropoiesis with iron homeostasis. Hypoxia induces a highly conserved signaling pathway in cells under conditions of low levels of O2. One component of this pathway, hypoxia-inducible factor (HIF), is a transcription factor that is highly active in hypoxic cells. The first HIF target gene characterized was EPO, which encodes erythropoietin-a glycoprotein hormone that controls erythropoiesis. In the past decade, there have been fundamental advances in our understanding of how hypoxia regulates iron levels to support erythropoiesis and maintain systemic iron homeostasis. We review the cell type-specific effects of hypoxia and HIFs in adaptive response to changes in oxygen and iron availability as well as potential uses of HIF modulators for patients with iron-related disorders.

Iron and intracerebral hemorrhage: from mechanism to translation

Intracerebral hemorrhage (ICH) is a leading cause of morbidity and mortality around the world. Currently, there is no effective medical treatment available to improve functional outcomes in patients with ICH due to its unknown mechanisms of damage. Increasing evidence has shown that the metabolic products of erythrocytes are the key contributor of ICH-induced secondary brain injury. Iron, an important metabolic product that accumulates in the brain parenchyma, has a detrimental effect on secondary injury following ICH. Because the damage mechanism of iron during ICH-induced secondary injury is clear, iron removal therapy research on animal models is effective. Although many animal and clinical studies have been conducted, the exact metabolic pathways of iron and the mechanisms of iron removal treatments are still not clear. This review summarizes recent progress concerning the iron metabolism mechanisms underlying ICH-induced injury. We focus on iron, brain iron metabolism, the role of iron in oxidative injury, and iron removal therapy following ICH, and we suggest that further studies focus on brain iron metabolism after ICH and the mechanism for iron removal therapy.

Multi-copper oxidases and human iron metabolism

Multi-copper oxidases (MCOs) are a small group of enzymes that oxidize their substrate with the concomitant reduction of dioxygen to two water molecules. Generally, multi-copper oxidases are promiscuous with regards to their reducing substrates and are capable of performing various functions in different species. To date, three multi-copper oxidases have been detected in humans—ceruloplasmin, hephaestin and zyklopen. Each of these enzymes has a high specificity towards iron with the resulting ferroxidase activity being associated with ferroportin, the only known iron exporter protein in humans. Ferroportin exports iron as Fe²⁺, but transferrin, the major iron transporter protein of blood, can bind only Fe³⁺ effectively. Iron oxidation in enterocytes is mediated mainly by hephaestin thus allowing dietary iron to enter the bloodstream. Zyklopen is involved in iron efflux from placental trophoblasts during iron transfer from mother to fetus. Release of iron from the liver relies on ferroportin and the ferroxidase activity of ceruloplasmin which is found in blood in a soluble form. Ceruloplasmin, hephaestin and zyklopen show distinctive expression patterns and have unique mechanisms for regulating their expression. These features of human multi-copper ferroxidases can serve as a basis for the precise control of iron efflux in different tissues. In this manuscript, we review the biochemical and biological properties of the three human MCOs and discuss their potential roles in human iron homeostasis.