The transferrin receptor modulates Hfe-dependent regulation of hepcidin expression

Iron deficiency and supplementation in heart failure

Non-anaemic iron deficiency (NAID) is a strategic target in cardiovascular medicine because of its association with a range of adverse effects in various conditions. Endeavours to tackle NAID in heart failure have yielded mixed results, exposing knowledge gaps in how best to define 'iron deficiency' and the handling of iron therapies by the body. To address these gaps, we harness the latest understanding of the mechanisms of iron homeostasis outside the erythron and integrate clinical and preclinical lines of evidence. The emerging picture is that current definitions of iron deficiency do not assimilate the multiple influences at play in patients with heart failure and, consequently, fail to identify those with a truly unmet need for iron. Additionally, current iron supplementation therapies benefit only certain patients with heart failure, reflecting differences in the nature of the unmet need for iron and the modifying effects of anaemia and inflammation on the handling of iron therapies by the body. Building on these insights, we identify untapped opportunities in the management of NAID, including the refinement of current approaches and the development of novel strategies. Lessons learned from NAID in cardiovascular disease could ultimately translate into benefits for patients with other chronic conditions such as chronic kidney disease, chronic obstructive pulmonary disease and cancer.

Can iron chelators ameliorate viral infections?

The redox reactivity of iron is a double-edged sword for cell functions, being either essential or harmful depending on metal concentration and location. Deregulation of iron homeostasis is associated with several clinical conditions, including viral infections. Clinical studies as well as in silico, in vitro and in vivo models show direct effects of several viruses on iron levels. There is support for the strategy of iron chelation as an alternative therapy to inhibit infection and/or viral replication, on the rationale that iron is required for the synthesis of some viral proteins and genes. In addition, abnormal iron levels can affect signaling immune response. However, other studies report different effects of viral infections on iron homeostasis, depending on the class and genotype of the virus, therefore making it difficult to predict whether iron chelation would have any benefit. This review brings general aspects of the relationship between iron homeostasis and the nonspecific immune response to viral infections, along with its relevance to the progress or inhibition of the inflammatory process, in order to elucidate situations in which the use of iron chelators could be efficient as antivirals.

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.

A Review of New Concepts in Iron Overload

Iron overload disorders are conditions that can lead to increased body iron stores and end-organ damage in affected organs. Increased iron deposition most commonly occurs in the liver, heart, endocrine system, joints, and pancreas. Iron overload disorders may be caused by genetic or acquired causes (transfusion, dyserythropoiesis, and chronic liver disease). The HFE gene C282Y homozygous mutation is the most common cause of hereditary hemochromatosis (HH). Other genes implicated in HH include TFR2, HAMP, HJV, and SLC40A1. In the past 2 decades, there have been major advances in the understanding of genetic iron overload disorders. Furthermore, new novel techniques to measure iron content in organs noninvasively, as well as new therapeutic options for the treatment of HH, are currently under development. This article focuses on the latest concepts in understanding, diagnosing, and managing genetic iron overload disorders, particularly HH.

Increased IL-6 Levels and the Upregulation of Iron Regulatory Biomarkers Contribute to the Progression of Japanese Encephalitis Virus Infection's Pathogenesis

Integrated analysis of iron regulatory biomarkers and inflammatory response could be an important strategy for Japanese encephalitis viral (JEV) infection disease management. In the present study, the inflammatory response was assessed by measuring serum Interleukin-6 (IL-6) levels using ELISA, and the transcription levels of iron homeostasis regulators were analyzed via RT-PCR. Furthermore, inter-individual variation in the transferrin gene was analyzed by PCR-RFLP and their association with clinical symptoms, susceptibility, severity, and outcomes was assessed through binary logistic regression and classification and regression tree (CART) analysis. Our findings revealed elevated levels of IL-6 in serum as well as increased expression of hepcidin (HAMP), transferrin (TF), and transferrin receptor-1 (TFR1) mRNA in JEV infection cases. Moreover, we found a genetic variation in TF (rs4481157) associated with clinical symptoms of meningoencephalitis. CART analysis indicates that individuals with the wild-type TF genotype are more susceptible to moderate JEV infection, while those with the homozygous type are in the high-risk group to develop a severe JEV condition. In summary, the study highlights that JEV infection induces alteration in both IL-6 levels and iron regulatory processes, which play pivotal roles in the development of JEV disease pathologies.

Low Serum Hepcidin Levels in Patients with Ulcerative Colitis - Implications for Treatment of Co-existent Iron-Deficiency Anemia

Ulcerative colitis (UC) is often associated with anemia. Hepcidin, the central regulator of iron homeostasis, is known to be induced by inflammation and suppressed by anemia. It is not clear how hepcidin is affected in those with UC, when both inflammation and anemia may co-exist.Such knowledge may hold implications for treatment. Hematological and iron-related parameters, C-reactive protein (CRP), growth differentiation factor 15 (GDF-15) and erythroferrone (ERFE) (erythroid regulators of hepcidin) levels were estimated in blood from those with UC and in control subjects. Values for hematological and iron-related parameters showed evidence of iron-deficiency and resultant anemia, in patients with UC. The presence of UC was significantly associated with inflammation. Serum levels of ERFE, but not of GDF-15, were significantly higher in patients with UC than in control patients, while hepcidin levels were significantly lower. Serum hepcidin concentrations in patients with UC correlated positively with serum iron, ferritin and GDF-15, and negatively with serum ERFE. The iron status and serum hepcidin levels in UC patients with co-existent anemia were significantly lower and serum ERFE values significantly higher than in those with UC without anemia. The effect of anemia on hepcidin predominated over that of inflammation in patients with UC, resulting in suppressed hepcidin levels. This effect is possibly mediated through erythroferrone. We suggest that a serum hepcidin-guided approach may be useful to guide use of oral iron supplements to treat co-existent iron-deficiency anemia in patients with UC and other chronic inflammatory diseases.

Normal and dysregulated crosstalk between iron metabolism and erythropoiesis

Erythroblasts possess unique characteristics as they undergo differentiation from hematopoietic stem cells. During terminal erythropoiesis, these cells incorporate large amounts of iron in order to generate hemoglobin and ultimately undergo enucleation to become mature red blood cells, ultimately delivering oxygen in the circulation. Thus, erythropoiesis is a finely tuned, multifaceted process requiring numerous properly timed physiological events to maintain efficient production of 2 million red blood cells per second in steady state. Iron is required for normal functioning in all human cells, the erythropoietic compartment consuming the majority in light of the high iron requirements for hemoglobin synthesis. Recent evidence regarding the crosstalk between erythropoiesis and iron metabolism sheds light on the regulation of iron availability by erythroblasts and the consequences of insufficient as well as excess iron on erythroid lineage proliferation and differentiation. In addition, significant progress has been made in our understanding of dysregulated iron metabolism in various congenital and acquired malignant and non-malignant diseases. Finally, we report several actual as well as theoretical opportunities for translating the recently acquired robust mechanistic understanding of iron metabolism regulation to improve management of patients with disordered erythropoiesis, such as anemia of chronic inflammation, β-thalassemia, polycythemia vera, and myelodysplastic syndromes.

Hepcidin and its multiple partners: Complex regulation of iron metabolism in health and disease

The peptide hormone hepcidin is central to the regulation of iron metabolism, influencing the movement of iron into the circulation and determining total body iron stores. Its effect on a cellular level involves binding ferroportin, the main iron export protein, preventing iron egress and leading to iron sequestration within ferroportin-expressing cells. Hepcidin expression is enhanced by iron loading and inflammation and suppressed by erythropoietic stimulation. Aberrantly increased hepcidin leads to systemic iron deficiency and/or iron restricted erythropoiesis as occurs in anemia of chronic inflammation. Furthermore, insufficiently elevated hepcidin occurs in multiple diseases associated with iron overload such as hereditary hemochromatosis and iron loading anemias. Abnormal iron metabolism as a consequence of hepcidin dysregulation is an underlying factor resulting in pathophysiology of multiple diseases and several agents aimed at manipulating this pathway have been designed, with some already in clinical trials. In this chapter, we assess the complex regulation of hepcidin, delineate the many binding partners involved in its regulation, and present an update on the development of hepcidin agonists and antagonists in various clinical scenarios.

Regulation of iron homeostasis by hepatocyte TfR1 requires HFE and contributes to hepcidin suppression in β-thalassemia

Transferrin receptor 1 (TfR1) performs a critical role in cellular iron uptake. Hepatocyte TfR1 is also proposed to influence systemic iron homeostasis by interacting with the hemochromatosis protein HFE to regulate hepcidin production. Here, we generated hepatocyte Tfrc knockout mice (Tfrcfl/fl;Alb-Cre+), either alone or together with Hfe knockout or β-thalassemia, to investigate the extent to which hepatocyte TfR1 function depends on HFE, whether hepatocyte TfR1 impacts hepcidin regulation by serum iron and erythropoietic signals, and its contribution to hepcidin suppression and iron overload in β-thalassemia. Compared with Tfrcfl/fl;Alb-Cre- controls, Tfrcfl/fl;Alb-Cre+ mice displayed reduced serum and liver iron; mildly reduced hematocrit, mean cell hemoglobin, and mean cell volume; increased erythropoietin and erythroferrone; and unchanged hepcidin levels that were inappropriately high relative to serum iron, liver iron, and erythroferrone levels. However, ablation of hepatocyte Tfrc had no impact on iron phenotype in Hfe knockout mice. Tfrcfl/fl;Alb-Cre+ mice also displayed a greater induction of hepcidin by serum iron compared with Tfrcfl/fl;Alb-Cre- controls. Finally, although acute erythropoietin injection similarly reduced hepcidin in Tfrcfl/fl;Alb-Cre+ and Tfrcfl/fl;Alb-Cre- mice, ablation of hepatocyte Tfrc in a mouse model of β-thalassemia intermedia ameliorated hepcidin deficiency and liver iron loading. Together, our data suggest that the major nonredundant function of hepatocyte TfR1 in iron homeostasis is to interact with HFE to regulate hepcidin. This regulatory pathway is modulated by serum iron and contributes to hepcidin suppression and iron overload in murine β-thalassemia.

Iron Dyshomeostasis in COVID-19: Biomarkers Reveal a Functional Link to 5-Lipoxygenase Activation

Coronavirus disease 2019 (COVID-19) is characterized by a broad spectrum of clinical symptoms. After acute infection, some subjects develop a post-COVID-19 syndrome known as long-COVID. This study aims to recognize the molecular and functional mechanisms that occur in COVID-19 and long-COVID patients and identify useful biomarkers for the management of patients with COVID-19 and long-COVID. Here, we profiled the response to COVID-19 by performing a proteomic analysis of lymphocytes isolated from patients. We identified significant changes in proteins involved in iron metabolism using different biochemical analyses, considering ceruloplasmin (Cp), transferrin (Tf), hemopexin (HPX), lipocalin 2 (LCN2), and superoxide dismutase 1 (SOD1). Moreover, our results show an activation of 5-lipoxygenase (5-LOX) in COVID-19 and in long-COVID possibly through an iron-dependent post-translational mechanism. Furthermore, this work defines leukotriene B4 (LTB4) and lipocalin 2 (LCN2) as possible markers of COVID-19 and long-COVID and suggests novel opportunities for prevention and treatment.

Iron and liver cancer: an inseparable connection

Iron is an essential element for all organisms. Iron-containing proteins play critical roles in cellular functions. The biological importance of iron is largely attributable to its chemical properties as a transitional metal. However, an excess of 'free' reactive iron damages the macromolecular components of cells and cellular DNA through the production of harmful free radicals. On the contrary, most of the body's excess iron is stored in the liver. Not only hereditary haemochromatosis but also some liver diseases with mild-to-moderate hepatic iron accumulation, such as chronic hepatitis C, alcoholic liver disease and nonalcoholic steatohepatitis, are associated with a high risk for liver cancer development. These findings have attracted attention to the causative and promotive roles of iron in the development of liver cancer. In the last decade, accumulating evidence regarding molecules regulating iron metabolism or iron-related cell death programmes such as ferroptosis has shed light on the relationship between hepatic iron accumulation and hepatocarcinogenesis. In this review, we briefly present the current molecular understanding of iron regulation in the liver. Next, we describe the mechanisms underlying dysregulated iron metabolism depending on the aetiology of liver diseases. Finally, we discuss the causative and promotive roles of iron in cancer development.

Engineering Peptide Inhibitors of the HFE-Transferrin Receptor 1 Complex

The protein HFE (homeostatic iron regulator) is a key regulator of iron metabolism, and mutations in HFE underlie the most frequent form of hereditary haemochromatosis (HH-type I). Studies have shown that HFE interacts with transferrin receptor 1 (TFR1), a homodimeric type II transmembrane glycoprotein that is responsible for the cellular uptake of iron via iron-loaded transferrin (holo-transferrin) binding. It has been hypothesised that the HFE/TFR1 interaction serves as a sensor to the level of iron-loaded transferrin in circulation by means of a competition mechanism between HFE and iron-loaded transferrin association with TFR1. To investigate this, a series of peptides based on the helical binding interface between HFE and TFR1 were generated and shown to significantly interfere with the HFE/TFR1 interaction in an in vitro proximity ligation assay. The helical conformation of one of these peptides, corresponding to the α1 and α2 helices of HFE, was stabilised by the introduction of sidechain lactam “staples”, but this did not result in an increase in the ability of the peptide to disrupt the HFE/TFR1 interaction. These peptides inhibitors of the protein–protein interaction between HFE and TFR1 are potentially useful tools for the analysis of the functional role of HFE in the regulation of hepcidin expression.

Discovery of a Transferrin Receptor 1-Binding Aptamer and Its Application in Cancer Cell Depletion for Adoptive T-Cell Therapy Manufacturing

The clinical manufacturing of chimeric antigen receptor (CAR) T cells includes cell selection, activation, gene transduction, and expansion. While the method of T-cell selection varies across companies, current methods do not actively eliminate the cancer cells in the patient's apheresis product from the healthy immune cells. Alarmingly, it has been found that transduction of a single leukemic B cell with the CAR gene can confer resistance to CAR T-cell therapy and lead to treatment failure. In this study, we report the identification of a novel high-affinity DNA aptamer, termed tJBA8.1, that binds transferrin receptor 1 (TfR1), a receptor broadly upregulated by cancer cells. Using competition assays, high resolution cryo-EM, and de novo model building of the aptamer into the resulting electron density, we reveal that tJBA8.1 shares a binding site on TfR1 with holo-transferrin, the natural ligand of TfR1. We use tJBA8.1 to effectively deplete B lymphoma cells spiked into peripheral blood mononuclear cells with minimal impact on the healthy immune cell composition. Lastly, we present opportunities for affinity improvement of tJBA8.1. As TfR1 expression is broadly upregulated in many cancers, including difficult-to-treat T-cell leukemias and lymphomas, our work provides a facile, universal, and inexpensive approach for comprehensively removing cancerous cells from patient apheresis products for safe manufacturing of adoptive T-cell therapies.

Iron-Induced Hepatocarcinogenesis—Preventive Effects of Nutrients

The liver is a primary organ that stores body iron, and plays a central role in the regulation of iron homeostasis. Hepatic iron overload (HIO) is a prevalent feature among patients with chronic liver diseases (CLDs), including alcoholic/nonalcoholic liver diseases and hepatitis C. HIO is suggested to promote the progression toward hepatocellular carcinoma because of the pro-oxidant nature of iron. Iron metabolism is tightly regulated by various factors, such as hepcidin and ferroportin, in healthy individuals to protect the liver from such deteriorative effects. However, their intrinsic expressions or functions are frequently compromised in patients with HIO. Thus, various nutrients have been reported to regulate hepatic iron metabolism and protect the liver from iron-induced damage. These nutrients are beneficial in HIO-associated CLD treatment and eventually prevent iron-mediated hepatocarcinogenesis. This mini-review aimed to discuss the mechanisms and hepatocarcinogenic risk of HIO in patients with CLDs. Moreover, nutrients that hold the potential to prevent iron-induced hepatocarcinogenesis are summarized.

The role of disrupted iron homeostasis in the development and progression of arthropathy

Arthropathy or joint disease leads to significant pain and disability irrespective of etiology. Clinical and experimental evidence point to the presence of considerable links between arthropathy and iron overload. Previous work has suggested that iron accumulation in the joints is often associated with increased oxidative stress, disrupted matrix metabolism, and cartilage degeneration. However, key issues regarding the role of iron overload in the pathogenesis of arthropathy remain ambiguous. For example, significant gaps in our knowledge of the primary cellular targets of iron overload-induced damage and the exact molecular mechanism through which disrupted iron homeostasis leads to joint damage still exist. The exact signaling pathway that links iron metabolism and cellular damage in arthropathy also remains largely unmapped. In this review, we focus on the relationship between iron overload and arthropathy with special emphasis on the adversarial relationship between iron that accumulates in the joints over time and cartilage homeostasis. A better understanding of the mechanisms and pathways underlying iron-induced cartilage degeneration may help in defining new prognostic markers and therapeutic targets in arthropathy.

AMP-activated protein kinase α1 phosphorylates PHD2 to maintain systemic iron homeostasis

BACKGROUND

Iron is essential for all mammalian life, and either a deficiency or excess of iron can cause diseases. AMP-activated protein kinase (AMPK) is a critical regulator of metabolic homeostasis; however, it has not been established whether AMPK regulates iron metabolism.

METHODS

Iron, hepcidin and ferroportin levels were examined in mice with global and hepatocyte-specific knockout of AMPKα1 and AMPKα2. Primary AMPKα1 or AMPKα2 deleted hepatocytes were isolated and cultured in hypoxia condition to explore PHD2, HIF and hydroxylated HIF1α levels. We performed immunoprecipitation, in vitro AMPK kinase assay and site-direct mutant assay to detect phosphorylation sites of PHD2. We also obtained liver tissues from patients with anaemia of chronic disease undergoing surgery, AMPKα1 and hydroxylated HIF1α levels were measured by immunohistochemical analysis.

RESULTS

We found that mice with global deficiency of AMPKα1, but not AMPKα2, exhibited hypoferraemia as well as iron sequestration in the spleen and liver. Hepatocyte-specific, but not myeloid-specific, ablation of AMPKα1 also reduced serum iron levels in association with increased hepcidin and decreased ferroportin protein levels. Mechanistically, AMPKα1 directly phosphorylated prolyl hydroxylase domain-containing (PHD)2 at serines 61 and 136, which suppressed PHD2-dependent hydroxylation of hypoxia-inducible factor (HIF)1α and subsequent regulation of hepatic hepcidin-related iron signalling. Inhibition of PHD2 hydroxylation ameliorated abnormal iron metabolism in hepatic AMPKα1-deficient mice. Furthermore, we found hepatic AMPKα/PHD2/HIFα/ hepcidin axes were highly clinically relevant to anaemia of chronic disease.

CONCLUSION

In conclusion, these observations suggest that hepatic AMPKα1 has an essential role in maintaining iron homeostasis by PHD2-dependent regulation of hepcidin, thus providing a potentially promising approach for the treatment of iron disturbances in chronic diseases.

Effects of Excess Iron on the Retina: Insights From Clinical Cases and Animal Models of Iron Disorders

Iron plays an important role in a wide range of metabolic pathways that are important for neuronal health. Excessive levels of iron, however, can promote toxicity and cell death. An example of an iron overload disorder is hemochromatosis (HH) which is a genetic disorder of iron metabolism in which the body’s ability to regulate iron absorption is altered, resulting in iron build-up and injury in several organs. The retina was traditionally assumed to be protected from high levels of systemic iron overload by the blood-retina barrier. However, recent data shows that expression of genes that are associated with HH can disrupt retinal iron metabolism. Thus, the effects of iron overload on the retina have become an area of research interest, as excessively high levels of iron are implicated in several retinal disorders, most notably age–related macular degeneration. This review is an effort to highlight risk factors for excessive levels of systemic iron build-up in the retina and its potential impact on the eye health. Information is integrated across clinical and preclinical animal studies to provide insights into the effects of systemic iron loading on the retina.

The Role of Estrogen Signaling in Cellular Iron Metabolism in Pancreatic β Cells

ABSTRACT

Several lines of evidence suggest that estrogen (17-β estradiol; E2) protects against diabetes mellitus and plays important roles in pancreatic β-cell survival and function. Mounting clinical and experimental evidence also suggest that E2 modulates cellular iron metabolism by regulating the expression of several iron regulatory genes, including hepcidin (HAMP), hypoxia-inducible factor 1-α, ferroportin (SLC40A1), and lipocalin (LCN2). However, whether E2 regulates cellular iron metabolism in pancreatic β cells and whether the antidiabetic effects of E2 can be, at least partially, attributed to its role in iron metabolism is not known. In this context, pancreatic β cells express considerable levels of conventional E2 receptors (ERs; mainly ER-α) and nonconventional G protein-coupled estrogen receptors and hence responsive to E2 signals. Moreover, pancreatic islet cells require significant amounts of iron for proper functioning, replication and survival and, hence, well equipped to manage cellular iron metabolism (acquisition, utilization, storage, and release). In this review, we examine the link between E2 and cellular iron metabolism in pancreatic β cells and discuss the bearing of such a link on β-cell survival and function.

Activation of STAT and SMAD Signaling Induces Hepcidin Re-Expression as a Therapeutic Target for β-Thalassemia Patients

Iron homeostasis is regulated by hepcidin, a hepatic hormone that controls dietary iron absorption and plasma iron concentration. Hepcidin binds to the only known iron export protein, ferroportin (FPN), which regulates its expression. The major factors that implicate hepcidin regulation include iron stores, hypoxia, inflammation, and erythropoiesis. When erythropoietic activity is suppressed, hepcidin expression is hampered, leading to deficiency, thus causing an iron overload in iron-loading anemia, such as β-thalassemia. Iron overload is the principal cause of mortality and morbidity in β-thalassemia patients with or without blood transfusion dependence. In the case of thalassemia major, the primary cause of iron overload is blood transfusion. In contrast, iron overload is attributed to hepcidin deficiency and hyperabsorption of dietary iron in non-transfusion thalassemia. Beta-thalassemia patients showed marked hepcidin suppression, anemia, iron overload, and ineffective erythropoiesis (IE). Recent molecular research has prompted the discovery of new diagnostic markers and therapeutic targets for several diseases, including β-thalassemia. In this review, signal transducers and activators of transcription (STAT) and SMAD (structurally similar to the small mothers against decapentaplegic in Drosophila) pathways and their effects on hepcidin expression have been discussed as a therapeutic target for β-thalassemia patients. Therefore, re-expression of hepcidin could be a therapeutic target in the management of thalassemia patients. Data from 65 relevant published experimental articles on hepcidin and β-thalassemia between January 2016 and May 2021 were retrieved by using PubMed and Google Scholar search engines. Published articles in any language other than English, review articles, books, or book chapters were excluded.

Coordination of iron homeostasis by bone morphogenetic proteins: Current understanding and unanswered questions

Iron homeostasis is tightly regulated to balance the iron requirement for erythropoiesis and other vital cellular functions, while preventing cellular injury from iron excess. The liver hormone hepcidin is the master regulator of systemic iron balance by controlling the degradation and function of the sole known mammalian iron exporter ferroportin. Liver hepcidin expression is coordinately regulated by several signals that indicate the need for more or less iron, including plasma and tissue iron levels, inflammation, and erythropoietic drive. Most of these signals regulate hepcidin expression by modulating the activity of the bone morphogenetic protein (BMP)-SMAD pathway, which controls hepcidin transcription. Genetic disorders of iron overload and iron deficiency have identified several hepatocyte membrane proteins that play a critical role in mediating the BMP-SMAD and hepcidin regulatory response to iron. However, the precise molecular mechanisms by which serum and tissue iron levels are sensed to regulate BMP ligand production and promote the physical and/or functional interaction of these proteins to modulate SMAD signaling and hepcidin expression remain uncertain. This critical commentary will focus on the current understanding and key unanswered questions regarding how the liver senses iron levels to regulate BMP-SMAD signaling and thereby hepcidin expression to control systemic iron homeostasis.

Soluble CD89 is a critical factor for mesangial proliferation in childhood IgA nephropathy

Childhood IgA nephropathy (IgAN) includes a wide spectrum of clinical presentations, from isolated hematuria to acute nephritis with rapid loss of kidney function. IgAN is an autoimmune disease and its pathogenesis involves galactose deficient (Gd) IgA1, IgG anti-Gd-IgA1 autoantibodies and the soluble IgA Fc receptor (CD89). However, implication of such factors, notably soluble CD89, in childhood IgAN pathogenesis remains unclear. Here, we studied these biomarkers in a cohort of 67 patients with childhood IgAN and 42 pediatric controls. While Gd-IgA1 was only moderately increased in patient plasma, levels of circulating IgA complexes (soluble CD89-IgA and IgG-IgA) and free soluble CD89 were markedly increased in childhood IgAN. Soluble CD89-IgA1 complexes and free soluble CD89 correlated with proteinuria, as well as histological markers of disease activity: mesangial, endocapillary hypercellularity and cellular crescents. Soluble CD89 was found in patient's urine but not in urine from pediatric controls. Mesangial deposits of soluble CD89 were detected in biopsies from patients with childhood IgAN. Serum chromatographic fractions containing covalently linked soluble CD89-IgA1 complexes or free soluble CD89 from patients induced mesangial cell proliferation in vitro in a soluble CD89-dependent manner. Recombinant soluble CD89 induced mesangial cell proliferation in vitro which was inhibited by free soluble recombinant CD71 (also a mesangial IgA receptor) or mTOR blockers. Interestingly, injection of recombinant soluble CD89 induced marked glomerular proliferation and proteinuria in mice expressing human IgA1. Thus, free and IgA1-complexed soluble CD89 are key players in mesangial proliferation. Hence, our findings suggest that soluble CD89 plays an essential role in IgAN pathogenesis making it a potential biomarker and therapeutic target.

The Role of Iron in Benign and Malignant Hematopoiesis

Significance: Iron is an essential element required for sustaining a normal healthy life. However, an excess amount of iron in the bloodstream and tissue generates toxic hydroxyl radicals through Fenton reactions. Henceforth, a balance in iron concentration is extremely important to maintain cellular homeostasis in both normal hematopoiesis and erythropoiesis. Iron deficiency or iron overload can impact hematopoiesis and is associated with many hematological diseases. Recent Advances: The mechanisms of action of key iron regulators such as erythroferrone and the discovery of new drugs, such as ACE-536/luspatercept, are of potential interest to treat hematological disorders, such as β-thalassemia. New therapies targeting inflammation-induced ineffective erythropoiesis are also in progress. Furthermore, emerging evidences support differential interactions between iron and its cellular antioxidant responses of hematopoietic and neighboring stromal cells. Both iron and its systemic regulator, such as hepcidin, play a significant role in regulating erythropoiesis. Critical Issues: Significant pre-clinical studies are on the way and new drugs targeting iron metabolism have been recently approved or are undergoing clinical trials to treat pathological conditions with impaired erythropoiesis such as myelodysplastic syndromes or β-thalassemia. Future Directions: Future studies should explore how iron regulates hematopoiesis in both benign and malignant conditions. Antioxid. Redox Signal. 35, 415-432.

Comprehensive transcriptome analysis of erythroid differentiation potential of olive leaf in haematopoietic stem cells

Anaemia is one of the leading causes of disability in young adults and is associated with increased morbidity and mortality in elderly. With a global target to reduce the disease burden of anaemia, recent researches focus on novel compounds with the ability to induce erythropoiesis and regulate iron homeostasis. We aimed to explore the biological events and potential polypharmacological effects of water-extracted olive leaf (WOL) on human bone marrow-derived haematopoietic stem cells (hHSCs) using a comprehensive gene expression analysis. HPLC analysis identifies six bioactive polyphenols in the WOL. Treatment with WOL for 12 days regulated gene expressions related to erythroid differentiation, oxygen homeostasis, iron homeostasis, haem metabolism and Hb biosynthesis in hHSCs. Functional clustering analysis reveals several major functions of WOL such as ribosomal biogenesis and mitochondrial translation machinery, glycolytic process, ATP biosynthesis and immune response. Additionally, the colonies of both primitive and mature erythroid progenitors, CFU-E and BFU-E, were significantly increased in WOL-treated hHSCs. The expressions of erythroid markers, CD47, glycophorin A (GYPA), and transferrin receptor (TFRC) and adult Hb subunits-HBA and HBB were also confirmed in immunofluorescent staining and flow cytometer analysis in WOL-treated hHSCs. It is well known that induction of lineage-specific differentiation, as well as the maturation of early haematopoietic precursors into fully mature erythrocytes, involves multiple simultaneous biological events and complex signalling networks. In this regard, our genome-wide transcriptome profiling with microarray study on WOL-treated hHSCs provides general insights into the multitarget prophylactic and/or therapeutic potential of WOL in anaemia and other haematological disorders.

Physiological and pathophysiological mechanisms of hepcidin regulation: clinical implications for iron disorders

The discovery of hepcidin has provided a solid foundation for understanding the mechanisms of systemic iron homeostasis and the aetiologies of iron disorders. Hepcidin assures the balance of circulating and stored iron levels for multiple physiological processes including oxygen transport and erythropoiesis, while limiting the toxicity of excess iron. The liver is the major site where regulatory signals from iron, erythropoietic drive and inflammation are integrated to control hepcidin production. Pathologically, hepcidin dysregulation by genetic inactivation, ineffective erythropoiesis, or inflammation leads to diseases of iron deficiency or overload such as iron-refractory iron-deficiency anaemia, anaemia of inflammation, iron-loading anaemias and hereditary haemochromatosis. In the present review, we discuss recent insights into the molecular mechanisms governing hepcidin regulation, how these pathways are disrupted in iron disorders, and how this knowledge is being used to develop novel diagnostic and therapeutic strategies.

Iron-loaded transferrin potentiates erythropoietin effects on erythroblast proliferation and survival: a novel role through transferrin receptors

Red blood cell production, or erythropoiesis, is a proliferative process that requires tight regulation. Erythropoietin (Epo) is a glycoprotein cytokine that plays a major role in erythropoiesis by triggering erythroid progenitors/precursors of varying sensitivity. The concentration of Epo in bone marrow is hypothesized to be suboptimal, and the survival of erythroid cells has been suggested to depend on Epo sensitivity. However, the key factors that control Epo sensitivity remain unknown. Two types of transferrin receptors (TfRs), TfR1 and TfR2, are known to play a role in iron uptake in erythroid cells. Here, we hypothesized that TfRs may additionally modulate Epo sensitivity during erythropoiesis by modulating Epo receptor (EpoR) signaling. Using an Epo-sensitive UT-7 (UT7/Epo) erythroid cell and human erythroid progenitor cell models, we report that iron-loaded transferrin, that is, holo-transferrin (holo-Tf), synergizes with suboptimal Epo levels to improve erythroid cell survival, proliferation, and differentiation. This is accomplished via the major signaling pathways of erythropoiesis, which include signal transducer and activator of transcription 5 (STAT5), mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK), and phosphoinositide-3-kinase (PI3K)/AKT. Furthermore, we found that this cooperation is improved by, but does not require, the internalization of TfR1. Interestingly, we observed that loss of TfR2 stabilizes EpoR levels and abolishes the beneficial effects of holo-Tf. Overall, these data reveal novel signaling properties of TfRs, which involve the regulation of erythropoiesis through EpoR signaling.

Hepcidin Signaling in Health and Disease: Ironing Out the Details

Hepcidin, a peptide hormone produced by hepatocytes, is the central regulator of systemic iron homeostasis through its interaction with ferroportin, the major cellular iron export protein. Hepcidin binding to ferroportin results in reduced iron export from macrophages and intestinal absorptive cells, leading to decreased serum iron levels. Hepcidin expression is influenced by several factors that include serum and liver iron stores, erythropoiesis, hypoxia, inflammation, and infection. Erythropoietic drive and hypoxia suppress hepcidin expression and promote red cell production. In contrast, inflammation and infection are associated with increased hepcidin production to sequester iron intracellularly as a means of depriving microorganisms of iron. Chronic inflammation may up-regulate hepcidin expression through the interleukin-6 (IL-6)-Janus kinase 2 (JAK2)-signal transducer and activator of transcription 3 (STAT3) pathway. The bone morphogenetic protein (BMP)-mothers against decapentaplegic homolog (SMAD) pathway is a major positive driver of hepcidin expression in response to either increased circulating iron in the form of transferrin or iron loading in organs. Hereditary hemochromatosis (HH) consists of several inherited disorders that cause inappropriately reduced hepcidin expression in response to body iron stores, leading to increased iron absorption from a normal diet. The most common form of HH is due to a mutation in the HFE gene, which causes a failure in the hepatocyte iron-sensing mechanism, leading to reduced hepcidin expression; the clinical manifestations of HFE-HH include increased serum transferrin-iron saturation and progressive iron loading in the liver and other tissues over time among patients who express the disease phenotype. In this article, we review the physiologic mechanisms and cellular pathways by which hepcidin expression is regulated, and the different forms of HH resulting from various mutations that cause hepcidin deficiency. We also review other drivers of hepcidin expression and the associated pathophysiologic consequences.

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

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

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

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

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

Tackling the unknowns in understanding and management of hospital acquired anemia

Hospital acquired anemia (HAA) has been a recognized entity for nearly 50 years. Despite multiple hypotheses, a mechanistic understanding is lacking, and targeted interventions have not yet yielded significantly impactful results. Known risk factors include advanced age, multiple co-morbidities, low bone marrow reserve, admission to the intensive care unit, and frequent phlebotomy. However, confounding variables in many studies continues to complicate the identification of additional risk factors. Improved understanding of iron metabolism, erythropoiesis, and the erythroid iron restriction response in the last few decades, as well as the recent demonstration of poor outcomes correlating with increased transfusion have refocused attention on HAA. While retrospective database studies provide ample correlative data between 1) HAA and poor outcomes; 2) reduction of phlebotomy volume and decrease in transfusion requirement; and 3) over-transfusion and increased mortality, no causal link between reduced phlebotomy volume, decreased rates of HAA, and improved mortality or other relevant outcomes have been definitely established. Here, we review the current state of knowledge and provide a summary of potential directions to understand and mitigate HAA. There are at present no clear guidelines on whether and when to evaluate hospitalized patients for underlying causes of anemia. We thus provide a guide for clinicians in general practice toward identifying patients at the highest risk for HAA, decreasing blood loss through phlebotomy to the greatest degree feasible, and evaluating and treating reversible causes of anemia in a targeted population.

Long-term phlebotomy successfully alleviated hepatic iron accumulation in a ferroportin disease patient with a mutation in SLC40A1: a case report

Background

Hereditary hemochromatosis is a heterogenous group of inherited iron-overload conditions that is characterized by increased intestinal absorption and deposition in vital organs. Hepcidin is a soluble regulator that acts to attenuate both intestinal iron absorption and iron release from reticuloendothelial macrophages through internalization of ferroportin-1, an iron exporter. Ferroportin disease is hereditary hemochromatosis which is affected by SLC40A1, a gene coding ferroportin-1, and phenotypically classified into two forms (classical and nonclassical). In nonclassical form, ferroportin mutations are responsible for a gain of function with full iron export capability but insensitivity to downregulation by hepcidin. Here, we report a case of nonclassical ferroportin disease.

Case presentation

A 46-year-old Japanese man showed elevated serum iron (284 μg/dl), ferritin (1722 ng/ml), transferrin saturation ratio (91.3%), and hepcidin-25 level (139.6 ng/ml). Magnetic resonance imaging (MRI) demonstrated a marked reduction in the signal intensity of the liver in T1- and T2-weighted images. The liver histology exhibited a large amount of iron that had accumulated predominantly in hepatocytes. We identified a heterozygous 1520A > G (p.H507R) mutation in the SLC40A1 gene. Phlebotomy (400 ml at a time) was monthly performed for 3 years in this patient. Importantly, the serum hepcidin level (1.0 ng/ml) was normal when the serum ferritin level was normal and hepatic iron accumulation was remarkably reduced after 3 years of phlebotomy.

Conclusions

The present case demonstrated for the first time that there was a correlation between hepatic iron levels as measured by MRI and serum hepcidin levels through long-term phlebotomy in a patient with ferroportin disease with the p.H507R mutation of in SLC40A1.

HFE and ALK3 act in the same signaling pathway

Hepcidin deficiency leads to iron overload by increased dietary iron uptake and iron release from storage cells. The most frequent mutation in Hfe leads to reduced hepcidin expression and thereby causes iron overload. Recent findings suggested that HFE activates hepcidin expression predominantly via the BMP type I receptor ALK3. Here, we investigated whether HFE exclusively utilizes ALK3 or other signaling mechanisms also. We generated mice with double deficiency of Hfe and hepatocyte-specific Alk3 and compared the iron overload phenotypes of these double knockout mice to single hepatocyte-specific Alk3 deficient or Hfe knockout mice. Double Hfe^-/-/hepatic Alk3^fl/fl;Alb-Cre knockouts develop a similar iron overload phenotype compared to single hepatocyte-specific Alk3 deficient mice hallmarked by serum iron levels, tissue iron content and hepcidin levels of similar grades. HFE protein levels were increased in Alk3^fl/fl;Alb-Cre mice compared to Alk3^fl/fl mice, which was caused by iron overload - and not by Alk3 deficiency. The data provide evidence by genetic means that 1. HFE exclusively uses the BMP type I receptor ALK3 to induce hepcidin expression and 2. HFE protein expression is induced by iron overload, which further emphasizes the iron sensing function of HFE.

Iron Metabolism in Oligodendrocytes and Astrocytes, Implications for Myelination and Remyelination

Iron is a key nutrient for normal central nervous system (CNS) development and function; thus, iron deficiency as well as iron excess may result in harmful effects in the CNS. Oligodendrocytes and astrocytes are crucial players in brain iron equilibrium. However, the mechanisms of iron uptake, storage, and efflux in oligodendrocytes and astrocytes during CNS development or under pathological situations such as demyelination are not completely understood. In the CNS, iron is directly required for myelin production as a cofactor for enzymes involved in ATP, cholesterol and lipid synthesis, and oligodendrocytes are the cells with the highest iron levels in the brain which is linked to their elevated metabolic needs associated with the process of myelination. Unlike oligodendrocytes, astrocytes do not have a high metabolic requirement for iron. However, these cells are in close contact with blood vessel and have a strong iron transport capacity. In several pathological situations, changes in iron homoeostasis result in altered cellular iron distribution and accumulation and oxidative stress. In inflammatory demyelinating diseases such as multiple sclerosis, reactive astrocytes accumulate iron and upregulate iron efflux and influx molecules, which suggest that they are outfitted to take up and safely recycle iron. In this review, we will discuss the participation of oligodendrocytes and astrocytes in CNS iron homeostasis. Understanding the molecular mechanisms of iron uptake, storage, and efflux in oligodendrocytes and astrocytes is necessary for planning effective strategies for iron management during CNS development as well as for the treatment of demyelinating diseases.

Bone morphogenic proteins in iron homeostasis

The bone morphogenetic protein (BMP)-SMAD signaling pathway plays a central role in regulating hepcidin, which is the master hormone governing systemic iron homeostasis. Hepcidin is produced by the liver and acts on the iron exporter ferroportin to control iron absorption from the diet and iron release from body stores, thereby providing adequate iron for red blood cell production, while limiting the toxic effects of excess iron. BMP6 and BMP2 ligands produced by liver endothelial cells bind to BMP receptors and the coreceptor hemojuvelin (HJV) on hepatocytes to activate SMAD1/5/8 signaling, which directly upregulates hepcidin transcription. Most major signals that influence hepcidin production, including iron, erythropoietic drive, and inflammation, intersect with the BMP-SMAD pathway to regulate hepcidin transcription. Mutation or inactivation of BMP ligands, BMP receptors, HJV, SMADs or other proteins that modulate the BMP-SMAD pathway result in hepcidin dysregulation, leading to iron-related disorders, such as hemochromatosis and iron refractory iron deficiency anemia. Pharmacologic modulators of the BMP-SMAD pathway have shown efficacy in pre-clinical models to regulate hepcidin expression and treat iron-related disorders. This review will discuss recent insights into the role of the BMP-SMAD pathway in regulating hepcidin to control systemic iron homeostasis.

Estrogen signaling differentially alters iron metabolism in monocytes in an Interleukin 6-dependent manner

The ability of monocytes to release or sequester iron affects their role in cancer and inflammation. Previous work has shown that while IL-6 upregulates hepcidin synthesis and enhances iron sequestration, E2 reduces hepcidin synthesis and increases iron release. Given that E2 upregulates IL-6 production in monocytes, it is likely that the exact effect of E2 on iron metabolism in monocytes is shaped by its effect on IL-6 expression. To address this issue, the expression of key iron regulatory proteins was assessed in E2-treated U937, HuT-78, THP-1 and Hep-G2 cells. Iron status was also evaluated in U937 cells treated with the ERα agonist PPT, the ER antagonist ICI-182780, dexamethasone + E2, IL-6 + E2 and in IL-6-silenced U937 cells. E2 treatment reduced hepcidin synthesis in HuT-78, THP-1 and Hep-G2 cells but increased hepcidin synthesis and reduced FPN expression in U937 cells. E2-treated U937 cells also showed reduced HIF-1α and FTH expression and increased TFR1 expression, which associated with increased labile iron content as compared with similarly treated Hep-G2 cells. While treatment of U937 cells with interleukin 6 (IL-6) resulted in increased expression of hepcidin, dexamethasone treatment resulted in reduced hepcidin synthesis relative to E2- or dexamethasone + E2-treated cells; IL-6 silencing also resulted in reduced hepcidin synthesis in U937 cells. Lastly, while iron depletion resulted in increased cell death in U937 cells, E2 treatment resulted in enhanced cell survival and reduced apoptosis. These findings suggest that E2 differentially alters iron metabolism in monocytes in an IL-6 dependent manner.

Auranofin mitigates systemic iron overload and induces ferroptosis via distinct mechanisms

Iron homeostasis is essential for health; moreover, hepcidin-deficiency results in iron overload in both hereditary hemochromatosis and iron-loading anemia. Here, we identified iron modulators by functionally screening hepcidin agonists using a library of 640 FDA-approved drugs in human hepatic Huh7 cells. We validated the results in C57BL/6J mice and a mouse model of hemochromatosis (Hfe^−/− mice). Our screen revealed that the anti-rheumatoid arthritis drug auranofin (AUR) potently upregulates hepcidin expression. Interestingly, we found that canonical signaling pathways that regulate iron, including the Bmp/Smad and IL-6/Jak2/Stat3 pathways, play indispensable roles in mediating AUR’s effects. In addition, AUR induces IL-6 via the NF-κB pathway. In C57BL/6J mice, acute treatment with 5 mg/kg AUR activated hepatic IL-6/hepcidin signaling and decreased serum iron and transferrin saturation. Whereas chronically treating male Hfe^−/− mice with 5 mg/kg AUR activated hepatic IL-6/hepcidin signaling, decreasing systemic iron overload, but less effective in females. Further analyses revealed that estrogen reduced the ability of AUR to induce IL-6/hepcidin signaling in Huh7 cells, providing a mechanistic explanation for ineffectiveness of AUR in female Hfe^−/− mice. Notably, high-dose AUR (25 mg/kg) induces ferroptosis and causes lipid peroxidation through inhibition of thioredoxin reductase (TXNRD) activity. We demonstrate the ferroptosis inhibitor ferrostatin significantly protects liver toxicity induced by high-dose AUR without comprising its beneficial effect on iron metabolism. In conclusion, our findings provide compelling evidence that TXNRD is a key regulator of ferroptosis, and AUR is a novel activator of hepcidin and ferroptosis via distinct mechanisms, suggesting a promising approach for treating hemochromatosis and hepcidin-deficiency related disorders.

Transferrin Receptor 1 Regulates Thermogenic Capacity and Cell Fate in Brown/Beige Adipocytes

Iron homeostasis is essential for maintaining cellular function in a wide range of cell types. However, whether iron affects the thermogenic properties of adipocytes is currently unknown. Using integrative analyses of multi-omics data, transferrin receptor 1 (Tfr1) is identified as a candidate for regulating thermogenesis in beige adipocytes. Furthermore, it is shown that mice lacking Tfr1 specifically in adipocytes have impaired thermogenesis, increased insulin resistance, and low-grade inflammation accompanied by iron deficiency and mitochondrial dysfunction. Mechanistically, the cold treatment in beige adipocytes selectively stabilizes hypoxia-inducible factor 1-alpha (HIF1α), upregulating the Tfr1 gene, and thermogenic adipocyte-specific Hif1α deletion reduces thermogenic gene expression in beige fat without altering core body temperature. Notably, Tfr1 deficiency in interscapular brown adipose tissue (iBAT) leads to the transdifferentiation of brown preadipocytes into white adipocytes and muscle cells; in contrast, long-term exposure to a low-iron diet fails to phenocopy the transdifferentiation effect found in Tfr1-deficient mice. Moreover, mice lacking transmembrane serine protease 6 (Tmprss6) develop iron deficiency in both inguinal white adipose tissue (iWAT) and iBAT, and have impaired cold-induced beige adipocyte formation and brown fat thermogenesis. Taken together, these findings indicate that Tfr1 plays an essential role in thermogenic adipocytes via both iron-dependent and iron-independent mechanisms.

Regulation of Iron Homeostasis and Related Diseases

The liver is the organ for iron storage and regulation; it senses circulating iron concentrations in the body through the BMP-SMAD pathway and regulates the iron intake from food and erythrocyte recovery into the bloodstream by secreting hepcidin. Under iron deficiency, hypoxia, and hemorrhage, the liver reduces the expression of hepcidin to ensure the erythropoiesis but increases the excretion of hepcidin during infection and inflammation to reduce the usage of iron by pathogens. Excessive iron causes system iron overload; it accumulates in never system and damages neurocyte leading to neurodegenerative diseases such as Parkinson's syndrome. When some gene mutations affect the perception of iron and iron regulation ability in the liver, then they decrease the expression of hepcidin, causing hereditary diseases such as hereditary hemochromatosis. This review summarizes the source and utilization of iron in the body, the liver regulates systemic iron homeostasis by sensing the circulating iron concentration, and the expression of hepcidin regulated by various signaling pathways, thereby understanding the pathogenesis of iron-related diseases.

Iron Metabolism in Cancer Progression

Iron is indispensable for cell metabolism of both normal and cancer cells. In the latter, several disruptions of its metabolism occur at the steps of tumor initiation, progression and metastasis. Noticeably, cancer cells require a large amount of iron, and exhibit a strong dependence on it for their proliferation. Numerous iron metabolism-related proteins and signaling pathways are altered by iron in malignancies, displaying the pivotal role of iron in cancer. Iron homeostasis is regulated at several levels, from absorption by enterocytes to recycling by macrophages and storage in hepatocytes. Mutations in HFE gene alter iron homeostasis leading to hereditary hemochromatosis and to an increased cancer risk because the accumulation of iron induces oxidative DNA damage and free radical activity. Additionally, the iron capability to modulate immune responses is pivotal in cancer progression. Macrophages show an iron release phenotype and potentially deliver iron to cancer cells, resulting in tumor promotion. Overall, alterations in iron metabolism are among the metabolic and immunological hallmarks of cancer, and further studies are required to dissect how perturbations of this element relate to tumor development and progression.

Iron metabolism and iron disorders revisited in the hepcidin era

Iron is biologically essential, but also potentially toxic; as such it is tightly controlled at cell and systemic levels to prevent both deficiency and overload. Iron regulatory proteins post-transcriptionally control genes encoding proteins that modulate iron uptake, recycling and storage and are themselves regulated by iron. The master regulator of systemic iron homeostasis is the liver peptide hepcidin, which controls serum iron through degradation of ferroportin in iron-absorptive enterocytes and iron-recycling macrophages. This review emphasizes the most recent findings in iron biology, deregulation of the hepcidin-ferroportin axis in iron disorders and how research results have an impact on clinical disorders. Insufficient hepcidin production is central to iron overload while hepcidin excess leads to iron restriction. Mutations of hemochro-matosis genes result in iron excess by downregulating the liver BMP-SMAD signaling pathway or by causing hepcidin-resistance. In iron-loading anemias, such as β-thalassemia, enhanced albeit ineffective ery-thropoiesis releases erythroferrone, which sequesters BMP receptor ligands, thereby inhibiting hepcidin. In iron-refractory, iron-deficiency ane-mia mutations of the hepcidin inhibitor TMPRSS6 upregulate the BMP-SMAD pathway. Interleukin-6 in acute and chronic inflammation increases hepcidin levels, causing iron-restricted erythropoiesis and ane-mia of inflammation in the presence of iron-replete macrophages. Our improved understanding of iron homeostasis and its regulation is having an impact on the established schedules of oral iron treatment and the choice of oral versus intravenous iron in the management of iron deficiency. Moreover it is leading to the development of targeted therapies for iron overload and inflammation, mainly centered on the manipulation of the hepcidin-ferroportin axis.

New diagnostic tools for delineating iron status

Recent advances in our understanding of iron metabolism regulation and crosstalk with erythropoiesis have provided insight into the pathophysiology of multiple disease conditions. For instance, the peptide hormone hepcidin is central to the regulation of iron metabolism. Its effect on cellular iron concentration involves binding ferroportin, the main iron export protein, resulting in its internalization and degradation and leading to iron sequestration within ferroportin-expressing cells. Furthermore, hepcidin regulation by erythropoiesis is attributed in large part to a bone marrow-derived hormone erythroferrone. Erythroferrone-induced hepcidin suppression in diseases of expanded hematopoiesis results in iron overload. Conversely, diseases, such as iron refractory iron deficiency anemia and anemia of chronic inflammation, are characterized by aberrantly increased hepcidin, resulting in iron sequestration and decreased circulating iron and eventually leading to iron-restricted erythropoiesis. Lastly, because iron functions in concert with erythropoietin to promote erythroid precursor survival, proliferation, and differentiation, iron deficiency anemia is a consequence not only of decreased hemoglobin synthesis in each cell but also, a decrease in erythropoietin responsiveness in the bone marrow. How to translate this new information to the clinical setting has not been fully elucidated. The purpose of this manuscript is to summarize current standard tools for identifying iron deficiency in anemic patients; explore the tools and context for evaluating novel markers, such as hepcidin, erythroferrone, and markers of the iron restriction response; and assess available evidence for how their use could increase our understanding of health outcomes in clinically challenging cases.

Iron Metabolism in the Peripheral Nervous System: The Role of DMT1, Ferritin, and Transferrin Receptor in Schwann Cell Maturation and Myelination

Iron is an essential cofactor for many cellular enzymes involved in myelin synthesis, and iron homeostasis unbalance is a central component of peripheral neuropathies. However, iron absorption and management in the PNS are poorly understood. To study iron metabolism in Schwann cells (SCs), we have created 3 inducible conditional KO mice in which three essential proteins implicated in iron uptake and storage, the divalent metal transporter 1 (DMT1), the ferritin heavy chain (Fth), and the transferrin receptor 1 (Tfr1), were postnatally ablated specifically in SCs. Deleting DMT1, Fth, or Tfr1 in vitro significantly reduce SC proliferation, maturation, and the myelination of DRG axons. This was accompanied by an important reduction in iron incorporation and storage. When these proteins were KO in vivo during the first postnatal week, the sciatic nerve of all 3 conditional KO animals displayed a significant reduction in the synthesis of myelin proteins and in the percentage of myelinated axons. Knocking out Fth produced the most severe phenotype, followed by DMT1 and, last, Tfr1. Importantly, DMT1 as well as Fth KO mice showed substantial motor coordination deficits. In contrast, deleting these proteins in mature myelinating SCs results in milder phenotypes characterized by small reductions in the percentage of myelinated axons and minor changes in the g-ratio of myelinated axons. These results indicate that DMT1, Fth, and Tfr1 are critical proteins for early postnatal iron uptake and storage in SCs and, as a consequence, for the normal myelination of the PNS.SIGNIFICANCE STATEMENT To determine the function of the divalent metal transporter 1, the transferrin receptor 1, and the ferritin heavy chain in Schwann cell (SC) maturation and myelination, we created 3 conditional KO mice in which these proteins were postnatally deleted in Sox10-positive SCs. We have established that these proteins are necessary for normal SC iron incorporation and storage, and, as a consequence, for an effective myelination of the PNS. Since iron is indispensable for SC maturation, understanding iron metabolism in SCs is an essential prerequisite for developing therapies for demyelinating diseases in the PNS.

Transferrin receptor 1-mediated iron uptake plays an essential role in hematopoiesis

Transferrin receptor 1 (Tfr1) mediates the endocytosis of diferric transferrin in order to transport iron, and Tfr1 has been suggested to play an important role in hematopoiesis. To study the role of Tfr1 in hematopoiesis, we generated hematopoietic stem cell (HSC) specific Tfr1 knockout mice. We found that Tfr1 conditional knockout mice reached full term but died within one week of birth. Further analyses revealed that Tfr1-deficient HSC had impaired development of all hematopoietic progenitors except thrombocytes and B lymphocytes. In addition, Tfr1-deficient cells had cellular iron deficiency, which blocked the proliferation and differentiation of hematopoietic precursor cells, attenuated the commitment of hematopoietic lineages, and reduced the regeneration potential of HSC. Notably, hemin rescued the colony-forming capacity of Tfr1-deficient HSC, whereas expressing a mutant Tfr1 that lacks the protein's iron-transporting capacity failed to rescue hematopoiesis. These findings provide direct evidence that Tfr1 is essential for hematopoiesis through binding diferric transferrin to supply iron to cells.

Characterization of Tfrc-mutant mice with microcytic phenotypes

To identify novel regulators of erythropoiesis, we performed independent forward genetic screens using the chemical mutagen ENU in mice. Among progeny displaying microcytic red-cell phenotypes, 7 independent mouse strains harboring mutations within the transferrin receptor gene Tfrc were identified. Six of the mutants, including the previously described red blood cell 6 (RBC6) strain, displayed reduced erythroblast CD71 expression and midgestation lethality of homozygotes (E12.5-E14.5), and 1 novel strain, RBC21, displayed a variable phenotype with sustained CD71 expression and late homozygous lethality (E18.5). Standard iron studies were normal in the RBC21 mutant, but intracellular ferritin was significantly reduced. The microcytic phenotype seen in the RBC21 strain was the result of impaired binding of transferrin to the receptor. Neither RBC6 nor RBC21 responded to iron replacement therapy. These studies describe how point mutations of the transferrin receptor can cause a microcytic anemia that does not respond to iron therapy and would not be detected by routine iron studies, such as serum ferritin.

Review article: iron disturbances in chronic liver diseases other than haemochromatosis - pathogenic, prognostic, and therapeutic implications

BACKGROUND

Disturbances in iron regulation have been described in diverse chronic liver diseases other than hereditary haemochromatosis, and iron toxicity may worsen liver injury and outcome.

AIMS

To describe manifestations and consequences of iron dysregulation in chronic liver diseases apart from hereditary haemochromatosis and to encourage investigations that clarify pathogenic mechanisms, define risk thresholds for iron toxicity, and direct management METHODS: English abstracts were identified in PubMed by multiple search terms. Full length articles were selected for review, and secondary and tertiary bibliographies were developed.

RESULTS

Hyperferritinemia is present in 4%-65% of patients with non-alcoholic fatty liver disease, autoimmune hepatitis, chronic viral hepatitis, or alcoholic liver disease, and hepatic iron content is increased in 11%-52%. Heterozygosity for the C282Y mutation is present in 17%-48%, but this has not uniformly distinguished patients with adverse outcomes. An inappropriately low serum hepcidin level has characterised most chronic liver diseases with the exception of non-alcoholic fatty liver disease, and the finding has been associated mainly with suppression of transcriptional activity of the hepcidin gene. Iron overload has been associated with oxidative stress, advanced fibrosis and decreased survival, and promising therapies beyond phlebotomy and oral iron chelation have included hepcidin agonists.

CONCLUSIONS

Iron dysregulation is common in chronic liver diseases other than hereditary haemochromatosis, and has been associated with liver toxicity and poor prognosis. Further evaluation of iron overload as a co-morbid factor should identify the key pathogenic disturbances, establish the risk threshold for iron toxicity, and promote molecular interventions.

Hepcidin-ferroportin axis in health and disease

Hepcidin is central to regulation of iron metabolism. Its effect on a cellular level involves binding ferroportin, the main iron export protein, resulting in its internalization and degradation and leading to iron sequestration within ferroportin-expressing cells. Aberrantly increased hepcidin leads to systemic iron deficiency and/or iron restricted erythropoiesis. Furthermore, insufficiently elevated hepcidin occurs in multiple diseases associated with iron overload. Abnormal iron metabolism as a consequence of hepcidin dysregulation is an underlying factor resulting in pathophysiology of multiple diseases and several agents aimed at manipulating this pathway have been designed, with some already in clinical trials. In this chapter, we present an overview of and rationale for exploring the development of hepcidin agonists and antagonists in various clinical scenarios.

Regulators of hepcidin expression

Iron, an essential nutrient, is required for many biological processes but is also toxic in excess. The lack of a mechanism to excrete excess iron makes it crucial for the body to regulate the amount of iron absorbed from the diet. This regulation is mediated by the hepatic hormone hepcidin. Hepcidin also controls iron release from macrophages that recycle iron and from hepatocytes that store iron. Hepcidin binds to the only known iron export protein, ferroportin, inducing its internalization and degradation and thus limiting the amount of iron released into the plasma. Important regulators of hepcidin, and therefore of systemic iron homeostasis, include plasma iron concentrations, body iron stores, infection and inflammation, hypoxia and erythropoiesis, and, to a lesser extent, testosterone. Dysregulation of hepcidin production contributes to the pathogenesis of many iron disorders: hepcidin deficiency causes iron overload in hereditary hemochromatosis and non-transfused β-thalassemia, whereas overproduction of hepcidin is associated with iron-restricted anemias seen in patients with chronic inflammatory diseases and inherited iron-refractory iron-deficiency anemia. The present review summarizes our current understanding of the molecular mechanisms and signaling pathways contributing to hepcidin regulation by these factors and highlights the issues that still need clarification.

Iron Regulation: Macrophages in Control

Macrophages are sentinel cells of the innate immune system and have important functions in development, tissue homeostasis, and immunity. These phylogenetically ancient cells also developed a variety of mechanisms to control erythropoiesis and the handling of iron. Red pulp macrophages in the spleen, Kupffer cells in the liver, and central nurse macrophages in the bone marrow ensure a coordinated metabolism of iron to support erythropoiesis. Phagocytosis of senescent red blood cells by macrophages in the spleen and the liver provide a continuous delivery of recycled iron under steady-state conditions and during anemic stress. Central nurse macrophages in the bone marrow utilize this iron and provide a cellular scaffold and niche to promote differentiation of erythroblasts. This review focuses on the role of the distinct macrophage populations that contribute to efficient iron metabolism and highlight important cellular and systemic mechanisms involved in iron-regulating processes.

High-Dose Deferoxamine Treatment Disrupts Intracellular Iron Homeostasis, Reduces Growth, and Induces Apoptosis in Metastatic and Nonmetastatic Breast Cancer Cell Lines

Mounting evidence suggest that iron overload enhances cancer growth and metastasis; hence, iron chelation is being increasingly used as part of the treatment regimen in patients with cancer. Now whether iron chelation depletes intracellular iron and/or disrupts intracellular iron homeostasis is yet to be fully addressed. MCF-7 and MDA-MB-231 breast cancer cells treated with increasing concentrations of the iron chelator deferoxamine were assessed for intracellular iron status, the expression of key proteins involved in iron metabolism, cell viability, growth potential, and apoptosis at different time points following treatment. Treatment with deferoxamine at 1, 5, or 10 μM for 24 or 48 hours, while not leading to significant changes in intracellular labile iron content, upregulated the expression of hepcidin, ferroportin, and transferrin receptors 1 and 2. In contrast, deferoxamine at 30, 100, or 300 μM for 24 hours induced a significant decrease in intracellular labile iron, which was associated with increased expression of hepcidin, ferritin, and transferrin receptors 1 and 2. At 48 hours, there was an increase in intracellular labile iron, which was associated with a significant reduction in hepcidin and ferritin expression and a significant increase in ferroportin expression. Although low-dose deferoxamine treatment resulted in a low to moderate decrease in MCF-7 cell growth, high-dose treatment resulted in a significant and precipitous decrease in cell viability and growth, which was associated with increased expression of phosphorylated Histone 2A family member X and near absence of survivin. High-dose deferoxamine treatment also resulted in a very pronounced reduction in wound healing and growth in MDA-MB-231 cells. These findings suggest that high-dose deferoxamine treatment disrupts intracellular iron homeostasis, reduces cell viability and growth, and enhances apoptosis in breast cancer cells. This is further evidence to the potential utility of iron chelation as an adjunctive therapy in iron-overloaded cancers.