Diferric transferrin regulates transferrin receptor 2 protein stability

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.

Matriptase-2 regulates iron homeostasis primarily by setting the basal levels of hepatic hepcidin expression through a nonproteolytic mechanism

Matriptase-2 (MT2), encoded by TMPRSS6, is a membrane-anchored serine protease. It plays a key role in iron homeostasis by suppressing the iron-regulatory hormone, hepcidin. Lack of functional MT2 results in an inappropriately high hepcidin and iron-refractory iron-deficiency anemia. Mt2 cleaves multiple components of the hepcidin-induction pathway in vitro. It is inhibited by the membrane-anchored serine protease inhibitor, Hai-2. Earlier in vivo studies show that Mt2 can suppress hepcidin expression independently of its proteolytic activity. In this study, our data indicate that hepatic Mt2 was a limiting factor in suppressing hepcidin. Studies in Tmprss6-/- mice revealed that increases in dietary iron to ∼0.5% were sufficient to overcome the high hepcidin barrier and to correct iron-deficiency anemia. Interestingly, the increased iron in Tmprss6-/- mice was able to further upregulate hepcidin expression to a similar magnitude as in wild-type mice. These results suggest that a lack of Mt2 does not impact the iron induction of hepcidin. Additional studies of wild-type Mt2 and the proteolytic-dead form, fMt2S762A, indicated that the function of Mt2 is to lower the basal levels of hepcidin expression in a manner that primarily relies on its nonproteolytic role. This idea is supported by the studies in mice with the hepatocyte-specific ablation of Hai-2, which showed a marginal impact on iron homeostasis and no significant effects on iron regulation of hepcidin. Together, these observations suggest that the function of Mt2 is to set the basal levels of hepcidin expression and that this process is primarily accomplished through a nonproteolytic mechanism.

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.

Relationship between iron overload caused by abnormal hepcidin expression and liver disease: A review

Iron is essential to organisms, the liver plays a vital role in its storage. Under pathological conditions, iron uptake by the intestine or hepatocytes increases, allowing excess iron to accumulate in liver cells. When the expression of hepcidin is abnormal, iron homeostasis in humans cannot be regulated, and resulting in iron overload. Hepcidin also regulates the release of iron from siderophores, thereby regulating the concentration of iron in plasma. Important factors related to hepcidin and systemic iron homeostasis include plasma iron concentration, body iron storage, infection, inflammation, and erythropoietin. This review summarizes the mechanism and regulation of iron overload caused by hepcidin, as well as related liver diseases caused by iron overload and treatment.

Mitochondrial iron overload mediated by cooperative transfer of plasma membrane ATP5B and TFR2 to mitochondria triggers hepatic insulin resistance under PFOS exposure

In general populations, insulin resistance (IR) is related to perfluorooctane sulfonate (PFOS), a persistent organic pollutant. However, the underlying mechanism remains unclear. In this study, PFOS induced mitochondrial iron accumulation in the liver of mice and human hepatocytes L-O2. In the PFOS-treated L-O2 cells, mitochondrial iron overload preceded the occurrence of IR, and pharmacological inhibition of mitochondrial iron relieved PFOS-caused IR. Both transferrin receptor 2 (TFR2) and ATP synthase β subunit (ATP5B) were redistributed from the plasma membrane to mitochondria with PFOS treatment. Inhibiting the translocation of TFR2 to mitochondria reversed PFOS-induced mitochondrial iron overload and IR. In the PFOS-treated cells, ATP5B interacted with TFR2. Stabilizing ATP5B on the plasma membrane or knockdown of ATP5B disturbed the translocation of TFR2. PFOS inhibited the activity of plasma-membrane ATP synthase (ectopic ATP synthase, e-ATPS), and activating e-ATPS prevented the translocation of ATP5B and TFR2. Consistently, PFOS induced ATP5B/TFR2 interaction and redistribution of ATP5B and TFR2 to mitochondria in the liver of mice. Thus, our results indicated that mitochondrial iron overload induced by collaborative translocation of ATP5B and TFR2 was an up-stream and initiating event for PFOS-related hepatic IR, providing novel understandings of the biological function of e-ATPS, the regulatory mechanism for mitochondrial iron and the mechanism underlying PFOS toxicity.

Hepcidin mimetics in polycythemia vera: resolving the irony of iron deficiency and erythrocytosis

PURPOSE OF REVIEW

Development of hepcidin therapeutics has been a ground-breaking discovery in restoring iron homeostasis in several haematological disorders. The hepcidin mimetic, rusfertide, is in late-stage clinical development for treating polycythemia vera patients with a global phase 3 trial [NCT05210790] currently underway. Rusfertide serves as the first possible noncytoreductive therapeutic option to maintain haematocrit control and avoid phlebotomy in polycythemia vera patients. In this comprehensive review, we discuss the pathobiology of dysregulated iron metabolism in polycythemia vera, provide the rationale for targeting the hepcidin-ferroportin axis and elaborate on the preclinical and clinical trial evidence supporting the role of hepcidin mimetics in polycythemia vera.

RECENT FINDINGS

Recently, updated results from two phase 2 clinical trials [NCT04057040 & NCT04767802] of rusfertide (PTG300) demonstrate that the drug is highly effective in eliminating the need for therapeutic phlebotomies, normalizing haematological parameters, repleting iron stores and relieving constitutional symptoms in patients with polycythemia vera. In light of these findings, additional hepcidin mimetic agents are also being evaluated in polycythemia vera patients.

SUMMARY

Hepcidin agonists essentially serve as a 'chemical phlebotomy' and are poised to vastly improve the quality of life for phlebotomy requiring polycythemia vera patients.

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.

Functional role of endothelial transferrin receptor 1 in iron sensing and homeostasis

Systemic iron homeostasis is regulated by the hepatic hormone hepcidin to balance meeting iron requirements while limiting toxicity from iron excess. Iron-mediated induction of bone morphogenetic protein (BMP) 6 is a central mechanism for regulating hepcidin production. Liver endothelial cells (LECs) are the main source of endogenous BMP6, but how they sense iron to modulate BMP6 transcription and thereby hepcidin is uncertain. Here, we investigate the role of endothelial cell transferrin receptor 1 (TFR1) in iron uptake, BMP6 regulation, and systemic iron homeostasis using primary LEC cultures and endothelial Tfrc (encoding TFR1) knockout mice. We show that intracellular iron regulates Bmp6 expression in a cell-autonomous manner, and TFR1 mediates iron uptake and Bmp6 expression by holo-transferrin in primary LEC cultures. In addition, endothelial Tfrc knockout mice exhibit altered iron homeostasis compared with littermate controls when fed a limited iron diet, as evidenced by increased liver iron and inappropriately low Bmp6 and hepcidin expression relative to liver iron. However, endothelial Tfrc knockout mice have a similar iron phenotype compared to littermate controls when fed an iron-rich standard diet. Finally, ferritin and non-transferrin bound iron (NTBI) are additional sources of iron that mediate Bmp6 induction in primary LEC cultures via TFR1-independent mechanisms. Together, our data demonstrate a minor functional role for endothelial cell TFR1 in iron uptake, BMP6 regulation, and hepatocyte hepcidin regulation under iron limiting conditions, and suggest that ferritin and/or NTBI uptake by other transporters have a dominant role when iron availability is high.

Transferrin receptor 2 (Tfr2) genetic deletion makes transfusion-independent a murine model of transfusion-dependent β-thalassemia

β-thalassemia is a genetic disorder caused by mutations in the β-globin gene, and characterized by anemia, ineffective erythropoiesis and iron overload. Patients affected by the most severe transfusion-dependent form of the disease (TDT) require lifelong blood transfusions and iron chelation therapy, a symptomatic treatment associated with several complications. Other therapeutic opportunities are available, but none is fully effective and/or applicable to all patients, calling for the identification of novel strategies. Transferrin receptor 2 (TFR2) balances red blood cells production according to iron availability, being an activator of the iron-regulatory hormone hepcidin in the liver and a modulator of erythropoietin signaling in erythroid cells. Selective Tfr2 deletion in the BM improves anemia and iron-overload in non-TDT mice, both as a monotherapy and, even more strikingly, in combination with iron-restricting approaches. However, whether Tfr2 targeting might represent a therapeutic option for TDT has never been investigated so far. Here, we prove that BM Tfr2 deletion improves anemia, erythrocytes morphology and ineffective erythropoiesis in the Hbb^th1/th2 murine model of TDT. This effect is associated with a decrease in the expression of α-globin, which partially corrects the unbalance with β-globin chains and limits the precipitation of misfolded hemoglobin, and with a decrease in the activation of unfolded protein response. Remarkably, BM Tfr2 deletion is also sufficient to avoid long-term blood transfusions required for survival of Hbb^th1/th2 animals, preventing mortality due to chronic anemia and reducing transfusion-associated complications, such as progressive iron-loading. Altogether, TFR2 targeting might represent a promising therapeutic option also for TDT.

A crosstalk between hepcidin and IRE/IRP pathways controls ferroportin expression and determines serum iron levels in mice

The iron hormone hepcidin is transcriptionally activated by iron or inflammation via distinct, partially overlapping pathways. We addressed how iron affects inflammatory hepcidin levels and the ensuing hypoferremic response. Dietary iron overload did not mitigate hepcidin induction in lipopolysaccharide (LPS)-treated wild type mice but prevented effective inflammatory hypoferremia. Likewise, LPS modestly decreased serum iron in hepcidin-deficient Hjv^-/- mice, model of hemochromatosis. Synthetic hepcidin triggered hypoferremia in control but not iron-loaded wild type animals. Furthermore, it dramatically decreased hepatic and splenic ferroportin in Hjv^-/- mice on standard or iron-deficient diet, but only triggered hypoferremia in the latter. Mechanistically, iron antagonized hepcidin responsiveness by inactivating IRPs in the liver and spleen to stimulate ferroportin mRNA translation. Prolonged LPS treatment eliminated ferroportin mRNA and permitted hepcidin-mediated hypoferremia in iron-loaded mice. Thus, de novo ferroportin synthesis is a critical determinant of serum iron and finetunes hepcidin-dependent functional outcomes. Our data uncover a crosstalk between hepcidin and IRE/IRP systems that controls tissue ferroportin expression and determines serum iron levels. Moreover, they suggest that hepcidin supplementation therapy is more efficient when combined with iron depletion.

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.

Hepatocyte neogenin is required for hemojuvelin-mediated hepcidin expression and iron homeostasis in mice

Neogenin (NEO1) is a ubiquitously expressed multifunctional transmembrane protein. It interacts with hemojuvelin (HJV), a BMP coreceptor that plays a pivotal role in hepatic hepcidin expression. Earlier studies suggest that the function of HJV relies on its interaction with NEO1. However, the role of NEO1 in iron homeostasis remains controversial because of the lack of an appropriate animal model. Here, we generated a hepatocyte-specific Neo1 knockout (Neo1fl/fl;Alb-Cre+) mouse model that circumvented the developmental and lethality issues of the global Neo1 mutant. Results show that ablation of hepatocyte Neo1 decreased hepcidin expression and caused iron overload. This iron overload did not result from altered iron utilization by erythropoiesis. Replacement studies revealed that expression of the Neo1L1046E mutant that does not interact with Hjv, was unable to correct the decreased hepcidin expression and high serum iron in Neo1fl/fl;Alb-Cre+ mice. In Hjv-/- mice, expression of HjvA183R mutant that has reduced interaction with Neo1, also displayed a blunted induction of hepcidin expression. These observations indicate that Neo1-Hjv interaction is essential for hepcidin expression. Further analyses suggest that the Hjv binding triggered the cleavage of the Neo1 cytoplasmic domain by a protease, which resulted in accumulation of truncated Neo1 on the plasma membrane. Additional studies did not support that Neo1 functions by inhibiting Hjv shedding as previously proposed. Together, our data favor a model in which Neo1 interaction with Hjv leads to accumulation of cleaved Neo1 on the plasma membrane, where Neo1 acts as a scaffold to induce the Bmp signaling and hepcidin expression.

The effect of the flavonol rutin on serum and liver iron content in a genetic mouse model of iron overload

The flavonol rutin has been shown to possess antioxidant and iron chelating properties in vitro and in vivo. These dual properties are beneficial as therapeutic options to reduce iron accumulation and the generation of reactive oxygen species (ROS) resultant from excess free iron. The effect of rutin on iron metabolism has been limited to studies performed in wildtype mice either injected or fed high-iron diets. The effect of rutin on iron overload caused by genetic dysregulation of iron homoeostasis has not yet been investigated. In the present study we examined the effect of rutin treatment on tissue iron loading in a genetic mouse model of iron overload, which mirrors the iron loading associated with Type 3 hereditary haemochromatosis patients who have a defect in Transferrin Receptor 2 (TFR2). Male TFR2 knockout (KO) mice were administered rutin via oral gavage for 21 continuous days. Following treatment, iron levels in serum, liver, duodenum and spleen were assessed. In addition, hepatic ferritin protein levels were determined by Western blotting, and expression of iron homoeostasis genes by quantitative real-time PCR. Rutin treatment resulted in a significant reduction in hepatic ferritin protein expression and serum transferrin saturation. In addition, trends towards decreased iron levels in the liver and serum, and increased serum unsaturated iron binding capacity were observed. This is the first study to explore the utility of rutin as a potential iron chelator and therapeutic in an animal model of genetic iron overload.

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.

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.

Possible mechanisms by which silkworm faeces extract ameliorates adenine-induced renal anaemia in rats

ETHNOPHARMACOLOGICAL RELEVANCE

Silkworm faeces are the dry faeces of the insect Bombyx mori (Linnaeus) and have historically been used in traditional Chinese medicine to treat blood deficiency and rheumatic pain. Silkworm faeces extract (SFE) is derived from silkworm faeces.

AIM OF THE STUDY

Clinical observations of patients in the Department of Nephrology have shown that SFE effectively improves renal anaemia. However, the molecular mechanism remains unclear. This article mainly explores the regulatory effects of SFE on erythropoietin (EPO) and hepcidin to identify the molecular mechanism of SFE.

MATERIALS AND METHODS

A rat model of renal anaemia was established by feeding rats food containing 0.75% adenine. SFE was orally administered to the rats, while recombinant human erythropoietin (rhEPO) was used as a positive control drug. Haematological parameters and inflammation levels were compared between rats from each group, and pathological kidney sections from each rat were observed. The serum EPO and hepcidin levels were detected using enzyme-linked immunosorbent assay (ELISA) kits, while Western blot analyses were performed to detect the levels of proteins involved in the EPO-related hypoxia-inducible factor 2α (HIF-2α)/prolyl hydroxylase 2 (PHD2) signalling pathway and hepcidin-related BMP6/SMAD4 and interleukin-6 (IL-6)/STAT3 signalling pathways.

RESULTS

SFE significantly ameliorated haematological parameters, renal function, and inflammation levels in the rats. A mechanistic study showed that SFE promoted EPO expression by upregulating HIF-2α expression and inhibiting the expression of NF-κB and GATA2 both in vivo and in vitro. In particular, SFE inhibited PHD2 expression, resulting in a decrease in the enzymatic reaction of HIF-2α to increase EPO expression. Furthermore, SFE inhibited hepcidin expression by blocking the BMP6/SMAD4 and IL-6/STAT3 pathways.

CONCLUSIONS

SFE regulated iron metabolism by inhibiting hepcidin and simultaneously promoted EPO synthesis to improve renal anaemia in rats.

Transferrin Receptors in Erythropoiesis

Erythropoiesis is a highly dynamic process giving rise to red blood cells from hematopoietic stem cells present in the bone marrow. Red blood cells transport oxygen to tissues thanks to the hemoglobin comprised of α- and β-globin chains and of iron-containing hemes. Erythropoiesis is the most iron-consuming process to support hemoglobin production. Iron delivery is mediated via transferrin internalization by the endocytosis of transferrin receptor type 1 (TFR1), one of the most abundant membrane proteins of erythroblasts. A second transferrin receptor—TFR2—associates with the erythropoietin receptor and has been implicated in the regulation of erythropoiesis. In erythroblasts, both transferrin receptors adopt peculiarities such as an erythroid-specific regulation of TFR1 and a trafficking pathway reliant on TFR2 for iron. This review reports both trafficking and signaling functions of these receptors and reassesses the debated role of TFR2 in erythropoiesis in the light of recent findings. Potential therapeutic uses targeting the transferrin-TFR1 axis or TFR2 in hematological disorders are also discussed.

Erythropoietin regulation of red blood cell production: from bench to bedside and back

More than 50 years of efforts to identify the major cytokine responsible for red blood cell (RBC) production (erythropoiesis) led to the identification of erythropoietin (EPO) in 1977 and its receptor (EPOR) in 1989, followed by three decades of rich scientific discovery. We now know that an elaborate oxygen-sensing mechanism regulates the production of EPO, which in turn promotes the maturation and survival of erythroid progenitors. Engagement of the EPOR by EPO activates three interconnected signaling pathways that drive RBC production via diverse downstream effectors and simultaneously trigger negative feedback loops to suppress signaling activity. Together, the finely tuned mechanisms that drive endogenous EPO production and facilitate its downstream activities have evolved to maintain RBC levels in a narrow physiological range and to respond rapidly to erythropoietic stresses such as hypoxia or blood loss. Examination of these pathways has elucidated the genetics of numerous inherited and acquired disorders associated with deficient or excessive RBC production and generated valuable drugs to treat anemia, including recombinant human EPO and more recently the prolyl hydroxylase inhibitors, which act partly by stimulating endogenous EPO synthesis. Ongoing structure-function studies of the EPOR and its essential partner, tyrosine kinase JAK2, suggest that it may be possible to generate new "designer" drugs that control selected subsets of cytokine receptor activities for therapeutic manipulation of hematopoiesis and treatment of blood cancers.

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.

Low expression of transferrin receptor 2 predict poor prognosis in gastric cancer patients

This study was to detect the expression level of transferrin receptor 2 (TfR2) in gastric cancer and to analyze its value in predicting the prognosis of gastric cancer patients. Real-time PCR (RT-qPCR) was applied to detect the mRNA expression of TfR2 in gastric cancer tissues and paired adjacent nontumorous tissues. Immunohistochemistry (IHC) and western blotting were used to determine the protein expression of TfR2 in gastric cancer, and the relationship with the prognosis of gastric cancer patients was analyzed. The results were: (1) The mRNA and protein levels of TfR2 in gastric cancer tissues were remarkably lower than those in adjacent nontumorous gastric tissue (P < .05). (2) Clinicopathological analysis showed that the expression level of TfR2 was significantly correlated with TNM stage of patients (P = .047). (3) The results of univariate survival analysis showed that histological grade, T stage, N stage, M stage, Lauren's classification, and TfR2 expression level influenced the overall survival of gastric cancer patients. Multivariate survival analysis showed that low TfR2 expression (HR = 1.671, 95%CI: 1.006-2.774, P = .047), poor differentiation (HR = 2.123, 95%CI: 1.188-3.795, P = .011), and M1 stage (HR = 8.541, 95%CI: 3.416-21.353, P < .001) were independent risk factors affecting the overall survival of gastric cancer patients. In conclusion, TfR2 exhibited low expression in gastric cancer tissue, and is an independent prognostic factor of gastric cancer patients.

Evaluation of the level of selected iron-related proteins/receptors in the liver of rats during separate/combined vanadium and magnesium administration

BACKGROUND

The current knowledge about the effects of vanadium (V) on iron (Fe)-related proteins and Fe homeostasis (which is regulated at the systemic, organelle, and cellular levels) is still insufficient.

OBJECTIVE

This fact and our earlier results prompted us to conduct studies with the aim to explain the mechanism of anemia accompanied by a rise in hepatic and splenic Fe deposition in rats receiving sodium metavanadate (SMV) separately and in combination with magnesium sulfate (MS).

RESULTS

We demonstrated for the first time that SMV (0.125 mg V/mL) administered to rats individually and in conjunction with MS (0.06 mg Mg/mL) for 12 weeks did not cause significant differences in the hepatic hepcidin (Hepc) and hemojuvelin (HJV) concentrations, compared to the control. In comparison with the control, there were no significant changes in the concentration of transferrin receptor 1 (TfR1) in the liver of rats treated with SMV and MS alone (in both cases only a downward trend of 14% and 15% was observed). However, a significant reduction in the hepatic TfR1 level was found in rats receiving SMV and MS simultaneously. In turn, the concentration of transferrin receptor 2 (TfR2) showed an increasing trend in the liver of rats treated with SMV and/or MS.

CONCLUSIONS

The experimental data suggest that the pathomechanism of the SMV-induced anemia is not associated with the effect of V on the concentration of Hepc in the liver, as confirmed by the unaltered hepatic HJV and TfR1 levels. Therefore, further studies are needed in order to check whether anemia that developed in the rats at the SMV administration (a) results from the inhibitory effect of V on erythropoietin (EPO) production, (b) is related to the effect of V on the induction of matriptase-2 (TMPRSS6) expression, or (c) is associated with the influence of this metal on haem synthesis.

Low iron promotes megakaryocytic commitment of megakaryocytic-erythroid progenitors in humans and mice

The mechanisms underlying thrombocytosis in patients with iron deficiency anemia remain unknown. Here, we present findings that support the hypothesis that low iron biases the commitment of megakaryocytic (Mk)-erythroid progenitors (MEPs) toward the Mk lineage in both human and mouse. In MEPs of transmembrane serine protease 6 knockout (Tmprss6-/-) mice, which exhibit iron deficiency anemia and thrombocytosis, we observed a Mk bias, decreased labile iron, and decreased proliferation relative to wild-type (WT) MEPs. Bone marrow transplantation assays suggest that systemic iron deficiency, rather than a local role for Tmprss6-/- in hematopoietic cells, contributes to the MEP lineage commitment bias observed in Tmprss6-/- mice. Nontransgenic mice with acquired iron deficiency anemia also show thrombocytosis and Mk-biased MEPs. Gene expression analysis reveals that messenger RNAs encoding genes involved in metabolic, vascular endothelial growth factor, and extracellular signal-regulated kinase (ERK) pathways are enriched in Tmprss6-/- vs WT MEPs. Corroborating our findings from the murine models of iron deficiency anemia, primary human MEPs exhibit decreased proliferation and Mk-biased commitment after knockdown of transferrin receptor 2, a putative iron sensor. Signal transduction analyses reveal that both human and murine MEP have lower levels of phospho-ERK1/2 in iron-deficient conditions compared with controls. These data are consistent with a model in which low iron in the marrow environment affects MEP metabolism, attenuates ERK signaling, slows proliferation, and biases MEPs toward Mk lineage commitment.

Extrahepatic deficiency of transferrin receptor 2 is associated with increased erythropoiesis independent of iron overload

Transferrin receptor 2 (TFR2) is a transmembrane protein expressed mainly in hepatocytes and in developing erythroid cells and is an important focal point in systemic iron regulation. Loss of TFR2 function results in a rare form of the iron-overload disease hereditary hemochromatosis. Although TFR2 in the liver has been shown to be important for regulating iron homeostasis in the body, TFR2's function in erythroid progenitors remains controversial. In this report, we analyzed TFR2-deficient mice in the presence or absence of iron overload to distinguish between the effects caused by a high iron load and those caused by loss of TFR2 function. Analysis of bone marrow from TFR2-deficient mice revealed a reduction in the early burst-forming unit-erythroid and an expansion of late-stage erythroblasts that was independent of iron overload. Spleens of TFR2-deficient mice displayed an increase in colony-forming unit-erythroid progenitors and in all erythroblast populations regardless of iron overload. This expansion of the erythroid compartment coincided with increased erythroferrone (ERFE) expression and serum erythropoietin (EPO) levels. Rescue of hepatic TFR2 expression normalized hepcidin expression and the total cell count of the bone marrow and spleen, but it had no effect on erythroid progenitor frequency. On the basis of these results, we propose a model of TFR2's function in murine erythropoiesis, indicating that deficiency in this receptor is associated with increased erythroid development and expression of EPO and ERFE in extrahepatic tissues independent of TFR's role in the liver.

Effect of stimulated erythropoiesis on liver SMAD signaling pathway in iron-overloaded and iron-deficient mice

Expression of hepcidin, the hormone regulating iron homeostasis, is increased by iron overload and decreased by accelerated erythropoiesis or iron deficiency. The purpose of the study was to examine the effect of these stimuli, either alone or in combination, on the main signaling pathway controlling hepcidin biosynthesis in the liver, and on the expression of splenic modulators of hepcidin biosynthesis. Liver phosphorylated SMAD 1 and 5 proteins were determined by immunoblotting in male mice treated with iron dextran, kept on an iron deficient diet, or administered recombinant erythropoietin for four consecutive days. Administration of iron increased liver phosphorylated SMAD protein content and hepcidin mRNA content; subsequent administration of erythropoietin significantly decreased both the iron-induced phosphorylated SMAD proteins and hepcidin mRNA. These results are in agreement with the recent observation that erythroferrone binds and inactivates the BMP6 protein. Administration of erythropoietin substantially increased the amount of erythroferrone and transferrin receptor 2 proteins in the spleen; pretreatment with iron did not influence the erythropoietin-induced content of these proteins. Erythropoietin-treated iron-deficient mice displayed smaller spleen size in comparison with erythropoietin-treated mice kept on a control diet. While the erythropoietin-induced increase in splenic erythroferrone protein content was not significantly affected by iron deficiency, the content of transferrin receptor 2 protein was lower in the spleens of erythropoietin-treated mice kept on iron-deficient diet, suggesting posttranscriptional regulation of transferrin receptor 2. Interestingly, iron deficiency and erythropoietin administration had additive effect on hepcidin gene downregulation in the liver. In mice subjected both to iron deficiency and erythropoietin administration, the decrease of hepcidin expression was much more pronounced than the decrease in phosphorylated SMAD protein content or the decrease in the expression of the SMAD target genes Id1 and Smad7. These results suggest the existence of another, SMAD-independent pathway of hepcidin gene downregulation.

Transferrin and transferrin receptors update

In vertebrates, transferrin (Tf) safely delivers iron through circulation to cells. Tf-bound iron is incorporated through Tf receptor (TfR) 1-mediated endocytosis. TfR1 can mediate cellular uptake of both Tf and H-ferritin, an iron storage protein. New World arenaviruses, which cause hemorrhagic fever, and Plasmodium vivax use TfR1 for entry into host cells. Human TfR2, another receptor for Tf, is predominantly expressed in hepatocytes and erythroid precursors, and holo-Tf dramatically upregulates its expression. TfR2 forms a complex with hemochromatosis protein, HFE, and serves as a component of the iron sensing machinery in hepatocytes. Defects in TfR2 cause systemic iron overload, hemochromatosis, through down-regulation of hepcidin. In erythroid cells, TfR2 forms a complex with the erythropoietin receptor and regulates erythropoiesis. TfR2 facilitates iron transport from lysosomes to mitochondria in erythroblasts and dopaminergic neurons. Administration of apo-Tf, which scavenges free iron, has been explored for various clinical conditions including atransferrinemia, iron overload, and tissue ischemia. Apo-Tf has also been shown to ameliorate anemia in animal models of β-thalassemia. In this review, I provide an update and summary on our knowledge of mammalian Tf and its receptors.

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.

Transferrin receptor 2 controls bone mass and pathological bone formation via BMP and Wnt signaling

Transferrin receptor 2 (Tfr2) is mainly expressed in the liver and controls iron homeostasis. Here, we identify Tfr2 as a regulator of bone homeostasis that inhibits bone formation. Mice lacking Tfr2 display increased bone mass and mineralization independent of iron homeostasis and hepatic Tfr2. Bone marrow transplantation experiments and studies of cell-specific Tfr2 knockout mice demonstrate that Tfr2 impairs BMP-p38MAPK signaling and decreases expression of the Wnt inhibitor sclerostin specifically in osteoblasts. Reactivation of MAPK or overexpression of sclerostin rescues skeletal abnormalities in Tfr2 knockout mice. We further show that the extracellular domain of Tfr2 binds BMPs and inhibits BMP-2-induced heterotopic ossification by acting as a decoy receptor. These data indicate that Tfr2 limits bone formation by modulating BMP signaling, possibly through direct interaction with BMP either as a receptor or as a co-receptor in a complex with other BMP receptors. Finally, the Tfr2 extracellular domain may be effective in the treatment of conditions associated with pathological bone formation.

Transferrin receptor 1 controls systemic iron homeostasis by fine-tuning hepcidin expression to hepatocellular iron load

Transferrin receptor 1 (Tfr1) mediates uptake of circulating transferrin-bound iron to developing erythroid cells and other cell types. Its critical physiological function is highlighted by the embryonic lethal phenotype of Tfr1-knockout (Tfrc^-/-) mice and the pathologies of several tissue-specific knockouts. We generated Tfrc^Alb-Cre mice bearing hepatocyte-specific ablation of Tfr1 to explore implications in hepatocellular and systemic iron homeostasis. Tfrc^Alb-Cre mice are viable and do not display any apparent liver pathology. Nevertheless, their liver iron content (LIC) is lower compared with that of control Tfrc^fl/fl littermates as a result of the reduced capacity of Tfr1-deficient hepatocytes to internalize iron from transferrin. Even though liver Hamp messenger RNA (mRNA) and serum hepcidin levels do not differ between Tfrc^Alb-Cre and Tfrc^fl/fl mice, Hamp/LIC and hepcidin/LIC ratios are significantly higher in the former. Importantly, this is accompanied by modest hypoferremia and microcytosis, and it predisposes Tfrc^Alb-Cre mice to iron-deficiency anemia. Tfrc^Alb-Cre mice appropriately regulate Hamp expression following dietary iron manipulations or holo-transferrin injection. Holo-transferrin also triggers proper induction of Hamp mRNA, ferritin, and Tfr2 in primary Tfrc^Alb-Cre hepatocytes. We further show that these cells can acquire ⁵⁹Fe from ⁵⁹Fe-transferrin, presumably via Tfr2. We conclude that Tfr1 is redundant for basal hepatocellular iron supply but essential for fine-tuning hepcidin responses according to the iron load of hepatocytes. Our data are consistent with an inhibitory function of Tfr1 on iron signaling to hepcidin via its interaction with Hfe. Moreover, they highlight hepatocellular Tfr1 as a link between cellular and systemic iron-regulatory pathways.

Liver iron sensing and body iron homeostasis

The liver orchestrates systemic iron balance by producing and secreting hepcidin. Known as the iron hormone, hepcidin induces degradation of the iron exporter ferroportin to control iron entry into the bloodstream from dietary sources, iron recycling macrophages, and body stores. Under physiologic conditions, hepcidin production is reduced by iron deficiency and erythropoietic drive to increase the iron supply when needed to support red blood cell production and other essential functions. Conversely, hepcidin production is induced by iron loading and inflammation to prevent the toxicity of iron excess and limit its availability to pathogens. The inability to appropriately regulate hepcidin production in response to these physiologic cues underlies genetic disorders of iron overload and deficiency, including hereditary hemochromatosis and iron-refractory iron deficiency anemia. Moreover, excess hepcidin suppression in the setting of ineffective erythropoiesis contributes to iron-loading anemias such as β-thalassemia, whereas excess hepcidin induction contributes to iron-restricted erythropoiesis and anemia in chronic inflammatory diseases. These diseases have provided key insights into understanding the mechanisms by which the liver senses plasma and tissue iron levels, the iron demand of erythrocyte precursors, and the presence of potential pathogens and, importantly, how these various signals are integrated to appropriately regulate hepcidin production. This review will focus on recent insights into how the liver senses body iron levels and coordinates this with other signals to regulate hepcidin production and systemic iron homeostasis.

The Functional Versatility of Transferrin Receptor 2 and Its Therapeutic Value

Iron homeostasis is a tightly regulated process in all living organisms because this metal is essential for cellular metabolism, but could be extremely toxic when present in excess. In mammals, there is a complex pathway devoted to iron regulation, whose key protein is hepcidin (Hepc), which is a powerful iron absorption inhibitor mainly produced by the liver. Transferrin receptor 2 (Tfr2) is one of the hepcidin regulators, and mutations in TFR2 gene are responsible for type 3 hereditary hemochromatosis (HFE3), a genetically heterogeneous disease characterized by systemic iron overload. It has been recently pointed out that Hepc production and iron regulation could be exerted also in tissues other than liver, and that Tfr2 has an extrahepatic role in iron metabolism as well. This review summarizes all the most recent data on Tfr2 extrahepatic role, taking into account the putative distinct roles of the two main Tfr2 isoforms, Tfr2α and Tfr2β. Representing Hepc modulation an effective approach to correct iron balance impairment in common human diseases, and with Tfr2 being one of its regulators, it would be worthwhile to envisage Tfr2 as a therapeutic target.

Hemochromatosis: pathophysiology, evaluation, and management of hepatic iron overload with a focus on MRI

Hereditary hemochromatosis (HH) is an autosomal recessive disorder that occurs in approximately 1 in 200-250 individuals. Mutations in the HFE gene lead to excess iron absorption. Excess iron in the form of non-transferrin-bound iron (NTBI) causes injury and is readily uptaken by cardiomyocytes, pancreatic islet cells, and hepatocytes. Symptoms greatly vary among patients and include fatigue, abdominal pain, arthralgias, impotence, decreased libido, diabetes, and heart failure. Untreated hemochromatosis can lead to chronic liver disease, fibrosis, cirrhosis, and hepatocellular carcinoma (HCC). Many invasive and noninvasive diagnostic tests are available to aid in diagnosis and treatment. MRI has emerged as the reference standard imaging modality for the detection and quantification of hepatic iron deposition, as ultrasound (US) is unable to detect iron overload and computed tomography (CT) findings are nonspecific and influenced by multiple confounding variables. If caught and treated early, HH disease progression can significantly be altered. Area covered: The data on Hemochromatosis, iron overload, and MRI were gathered by searching PubMed. Expert commentary: MRI is a great tool for diagnosis and management of iron overload. It is safe, effective, and a standard protocol should be included in diagnostic algorithms of future treatment guidelines.

Crossing the Iron Gate: Why and How Transferrin Receptors Mediate Viral Entry

Because both the host and pathogen require iron, the innate immune response carefully orchestrates control over iron metabolism to limit its availability during times of infection. Nutritional iron deficiency can impair host immunity, while iron overload can cause oxidative stress to propagate harmful viral mutations. An emerging enigma is that many viruses use the primary gatekeeper of iron metabolism, the transferrin receptor, as a means to enter cells. Why and how this iron gate is a viral target for infection are the focus of this review.

Hemochromatosis: Evaluation of the dietary iron model and regulation of hepcidin

Our knowledge of iron homeostasis has increased steadily over the last two decades; much of this has been made possible through the study of animal models of iron-related disease. Analysis of transgenic mice with deletions or perturbations in genes known to be involved in systemic or local regulation of iron metabolism has been particularly informative. The effect of these genes on iron accumulation and hepcidin regulation is traditionally compared with wildtype mice fed a high iron diet, most often a 2% carbonyl iron diet. Recent studies have indicated that a very high iron diet could be detrimental to the health of the mice and could potentially affect homeostasis of other metals, for example zinc and copper. We analyzed mice fed a diet containing either 0.25%, 0.5%, 1% or 2% carbonyl iron for two weeks and compared them with mice on a control diet. Our results indicate that a 0.25% carbonyl iron diet is sufficient to induce maximal hepatic hepcidin response. Importantly these results also demonstrate that in a chronic setting of iron administration, the amount of excess hepatic iron may not further influence hepcidin regulation and that expression of hepcidin plateaus at lower hepatic iron levels. These studies provide further insights into the regulation of this important hormone.

Transferrin Receptors TfR1 and TfR2 Bind Transferrin through Differing Mechanisms

Hereditary hemochromatosis (HH), a disease marked by chronic iron overload from insufficient expression of the hormone hepcidin, is one of the most common genetic diseases. One form of HH (type III) results from mutations in transferrin receptor-2 (TfR2). TfR2 is postulated to be a part of signaling system that is capable of modulating hepcidin expression. However, the molecular details of TfR2's role in this system remain unclear. TfR2 is predicted to bind the iron carrier transferrin (Tf) when the iron saturation of Tf is high. To better understand the nature of these TfR-Tf interactions, a binding study with the full-length receptors was conducted. In agreement with previous studies with truncated forms of these receptors, holo-Tf binds to the TfR1 homologue significantly stronger than to TfR2. However, the binding constant for Tf-TfR2 is still far above that of physiological holo-Tf levels, inconsistent with the hypothetical model, suggesting that other factors mediate the interaction. One possible factor, apo-Tf, only weakly binds TfR2 at serum pH and thus will not be able to effectively compete with holo-Tf. Tf binding to a TfR2 chimera containing the TfR1 helical domain indicates that the differences in the helical domain account for differences in the on rate of Tf, and nonconserved inter-receptor interactions are necessary for the stabilization of the complex. Conserved residues at one possible site of stabilization, the apical arm junction, are not important for TfR1-Tf binding but are critical for the TfR2-Tf interaction. Our results highlight the differences in Tf interactions with the two TfRs.

Disorders of Iron Metabolism and Anemia in Chronic Kidney Disease

Dysregulated iron homeostasis plays a central role in the development of anemia of chronic kidney disease (CKD) and is a major contributor toward resistance to treatment with erythropoiesis-stimulating agents. Understanding the underlying pathophysiology requires an in-depth understanding of normal iron physiology and regulation. Recent discoveries in the field of iron biology have greatly improved our understanding of the hormonal regulation of iron trafficking in human beings and how its alterations lead to the development of anemia of CKD. In addition, emerging evidence has suggested that iron homeostasis interacts with bone and mineral metabolism on multiple levels, opening up new avenues of investigation into the genesis of disordered iron metabolism in CKD. Building on recent advances in our understanding of normal iron physiology and abnormalities in iron homeostasis in CKD, this review characterizes how anemia related to disordered iron metabolism develops in the setting of CKD. In addition, this review explores our emerging recognition of the connections between iron homeostasis and mineral metabolism and their implications for the management of altered iron status and anemia of CKD.

Iron modulation of erythropoiesis is associated with Scribble-mediated control of the erythropoietin receptor

Iron deficiency causes resistance in erythroid progenitors against proliferative but not survival signals of erythropoietin. Khalil et al. link this response to the down-regulation of Scribble, an orchestrator of receptor trafficking and signaling. With iron deprivation, transferrin receptor 2 drives Scribble degradation, reconfiguring erythropoietin receptor function.

Iron-restricted human anemias are associated with the acquisition of marrow resistance to the hematopoietic cytokine erythropoietin (Epo). Regulation of Epo responsiveness by iron availability serves as the basis for intravenous iron therapy in anemias of chronic disease. Epo engagement of its receptor normally promotes survival, proliferation, and differentiation of erythroid progenitors. However, Epo resistance caused by iron restriction selectively impairs proliferation and differentiation while preserving viability. Our results reveal that iron restriction limits surface display of Epo receptor in primary progenitors and that mice with enforced surface retention of the receptor fail to develop anemia with iron deprivation. A mechanistic pathway is identified in which erythroid iron restriction down-regulates a receptor control element, Scribble, through the mediation of the iron-sensing transferrin receptor 2. Scribble deficiency reduces surface expression of Epo receptor but selectively retains survival signaling via Akt. This mechanism integrates nutrient sensing with receptor function to permit modulation of progenitor expansion without compromising survival.

Matriptase-2 suppresses hepcidin expression by cleaving multiple components of the hepcidin induction pathway

Systemic iron homeostasis is maintained by regulation of iron absorption in the duodenum, iron recycling from erythrocytes, and iron mobilization from the liver and is controlled by the hepatic hormone hepcidin. Hepcidin expression is induced via the bone morphogenetic protein (BMP) signaling pathway that preferentially uses two type I (ALK2 and ALK3) and two type II (ActRIIA and BMPR2) BMP receptors. Hemojuvelin (HJV), HFE, and transferrin receptor-2 (TfR2) facilitate this process presumably by forming a plasma membrane complex with BMP receptors. Matriptase-2 (MT2) is a protease and key suppressor of hepatic hepcidin expression and cleaves HJV. Previous studies have therefore suggested that MT2 exerts its inhibitory effect by inactivating HJV. Here, we report that MT2 suppresses hepcidin expression independently of HJV. In Hjv^-/- mice, increased expression of exogenous MT2 in the liver significantly reduced hepcidin expression similarly as observed in wild-type mice. Exogenous MT2 could fully correct abnormally high hepcidin expression and iron deficiency in MT2^-/- mice. In contrast to MT2, increased Hjv expression caused no significant changes in wild-type mice, suggesting that Hjv is not a limiting factor for hepcidin expression. Further studies revealed that MT2 cleaves ALK2, ALK3, ActRIIA, Bmpr2, Hfe, and, to a lesser extent, Hjv and Tfr2. MT2-mediated Tfr2 cleavage was also observed in HepG2 cells endogenously expressing MT2 and TfR2. Moreover, iron-loaded transferrin blocked MT2-mediated Tfr2 cleavage, providing further insights into the mechanism of Tfr2's regulation by transferrin. Together, these observations indicate that MT2 suppresses hepcidin expression by cleaving multiple components of the hepcidin induction pathway.

The liver in regulation of iron homeostasis

The liver is one of the largest and most functionally diverse organs in the human body. In addition to roles in detoxification of xenobiotics, digestion, synthesis of important plasma proteins, gluconeogenesis, lipid metabolism, and storage, the liver also plays a significant role in iron homeostasis. Apart from being the storage site for excess body iron, it also plays a vital role in regulating the amount of iron released into the blood by enterocytes and macrophages. Since iron is essential for many important physiological and molecular processes, it increases the importance of liver in the proper functioning of the body's metabolism. This hepatic iron-regulatory function can be attributed to the expression of many liver-specific or liver-enriched proteins, all of which play an important role in the regulation of iron homeostasis. This review focuses on these proteins and their known roles in the regulation of body iron metabolism.

Iron homeostasis: An anthropocentric perspective

The regulation of iron metabolism in biological systems centers on providing adequate iron for cellular function while limiting iron toxicity. Because mammals cannot excrete iron, mechanisms have evolved to control iron acquisition, storage, and distribution at both systemic and cellular levels. Hepcidin, the master regulator of iron homeostasis, controls iron flows into plasma through inhibition of the only known mammalian cellular iron exporter ferroportin. Hepcidin is feedback-regulated by iron status and strongly modulated by inflammation and erythropoietic demand. This review highlights recent advances that have changed our understanding of iron metabolism and its regulation.

The mutual control of iron and erythropoiesis

BACKGROUND

Iron is essential for hemoglobin synthesis during terminal erythropoiesis. To supply adequate iron the carrier transferrin is required together with transferrin receptor endosomal cycle and normal mitochondrial iron utilization. Iron and iron protein deficiencies result in different types of anemia. Iron-deficiency anemia is the commonest anemia worldwide due to increased requirements, malnutrition, chronic blood losses and malabsorption. Mutations of transferrin, transferrin receptor cycle proteins, enzymes of the first step of heme synthesis and iron sulfur cluster biogenesis lead to rare anemias, usually accompanied by iron overload. Hepcidin plays an indirect role in erythropoiesis by controlling plasma iron. Inappropriately high hepcidin levels characterize the rare genetic iron-refractory iron-deficiency anemia (IRIDA) and the common anemia of chronic disease. Iron modulates both effective and ineffective erythropoiesis: iron restriction reduces heme and alpha-globin synthesis that may be of benefit in thalassemia.

MATERIAL AND METHODS

This review relies on the analysis of the most recent literature and personal data.

RESULTS

Erythropoiesis controls iron homeostasis, by releasing erythroferrone that inhibits hepcidin transcription to increase iron acquisition in iron deficiency, hypoxia and EPO treatment. Erythroferrone, produced by EPO-stimulated erythropoiesis, inhibits hepcidin only when the activity of BMP/SMAD pathway is low, suggesting that EPO somehow modulates the latter signaling. Erythroblasts sense circulating iron through the second transferrin receptor (TFR2) that, in animal models, modulates the sensitivity of the erythroid cells to EPO.

DISCUSSION

The advanced knowledge of the regulation of systemic iron homeostasis and erythropoiesis-mediated hepcidin regulation is leading to the development of targeted therapies for anemias and iron disorders.

Normal systemic iron homeostasis in mice with macrophage-specific deletion of transferrin receptor 2

Iron is an essential element, since it is a component of many macromolecules involved in diverse physiological and cellular functions, including oxygen transport, cellular growth, and metabolism. Systemic iron homeostasis is predominantly regulated by the liver through the iron regulatory hormone hepcidin. Hepcidin expression is itself regulated by a number of proteins, including transferrin receptor 2 (TFR2). TFR2 has been shown to be expressed in the liver, bone marrow, macrophages, and peripheral blood mononuclear cells. Studies from our laboratory have shown that mice with a hepatocyte-specific deletion of Tfr2 recapitulate the hemochromatosis phenotype of the global Tfr2 knockout mice, suggesting that the hepatic expression of TFR2 is important in systemic iron homeostasis. It is unclear how TFR2 in macrophages contributes to the regulation of iron metabolism. We examined the role of TFR2 in macrophages by analysis of transgenic mice lacking Tfr2 in macrophages by crossing Tfr2(f/f) mice with LysM-Cre mice. Mice were fed an iron-rich diet or injected with lipopolysaccharide to examine the role of macrophage Tfr2 in iron- or inflammation-mediated regulation of hepcidin. Body iron homeostasis was unaffected in the knockout mice, suggesting that macrophage TFR2 is not required for the regulation of systemic iron metabolism. However, peritoneal macrophages of knockout mice had significantly lower levels of ferroportin mRNA and protein, suggesting that TFR2 may be involved in regulating ferroportin levels in macrophages. These studies further elucidate the role of TFR2 in the regulation of iron homeostasis and its role in regulation of ferroportin and thus macrophage iron homeostasis.

The use of hypotransferrinemic mice in studies of iron biology

The hypotransferrinemic (hpx) mouse is a model of inherited transferrin deficiency that originated several decades ago in the BALB/cJ mouse strain. Also known as the hpx mouse, this line is almost completely devoid of transferrin, an abundant serum iron-binding protein. Two of the most prominent phenotypes of the hpx mouse are severe anemia and tissue iron overload. These phenotypes reflect the essential role of transferrin in iron delivery to bone marrow and regulation of iron homeostasis. Over the years, the hpx mouse has been utilized in studies on the role of transferrin, iron and other metals in a variety of organ systems and biological processes. This review summarizes the lessons learned from these studies and suggests possible areas of future exploration using this versatile yet complex mouse model.

Regulation of cell surface transferrin receptor-2 by iron-dependent cleavage and release of a soluble form

Transferrin receptor-2 is a transmembrane protein whose expression is restricted to hepatocytes and erythroid cells. Transferrin receptor-2 has a regulatory function in iron homeostasis, since its inactivation causes systemic iron overload. Hepatic transferrin receptor-2 participates in iron sensing and is involved in hepcidin activation, although the mechanism remains unclear. Erythroid transferrin receptor-2 associates with and stabilizes erythropoietin receptors on the erythroblast surface and is essential to control erythrocyte production in iron deficiency. We identified a soluble form of transferrin receptor-2 in the media of transfected cells and showed that cultured human erythroid cells release an endogenous soluble form. Soluble transferrin receptor-2 originates from a cleavage of the cell surface protein, which is inhibited by diferric transferrin in a dose-dependent manner. Accordingly, the shedding of the transferrin receptor-2 variant G679A, mutated in the Arginine-Glycine-Aspartic acid motif and unable to bind diferric transferrin, is not modulated by the ligand. This observation links the process of transferrin receptor-2 removal from the plasma membrane to iron homeostasis. Soluble transferrin receptor-2 does not affect the binding of erythropoietin to erythropoietin receptor or the consequent signaling and partially inhibits hepcidin promoter activation only in vitro. Whether it is a component of the signals released by erythropoiesis in iron deficiency remains to be investigated. Our results indicate that membrane transferrin receptor-2, a sensor of circulating iron, is released from the cell membrane in iron deficiency.

CD81 promotes both the degradation of transferrin receptor 2 (TfR2) and the Tfr2-mediated maintenance of hepcidin expression

Mutations in transferrin receptor 2 (TfR2) cause a rare form of the hereditary hemochromatosis, resulting in iron overload predominantly in the liver. TfR2 is primarily expressed in hepatocytes and is hypothesized to sense iron levels in the blood to positively regulate the expression of hepcidin through activation of the BMP signaling pathway. Hepcidin is a peptide hormone that negatively regulates iron egress from cells and thus limits intestinal iron uptake. In this study, a yeast two-hybrid approach using the cytoplasmic domain of TfR2 identified CD81 as an interacting protein. CD81 is an abundant tetraspanin in the liver. Co-precipitations of CD81 with different TfR2 constructs demonstrated that both the cytoplasmic and ecto-transmembrane domains of TfR2 interact with CD81. Knockdown of CD81 using siRNA significantly increased TfR2 levels by increasing the half-life of TfR2, indicating that CD81 promotes degradation of TfR2. Previous studies showed that CD81 is targeted for degradation by GRAIL, an ubiquitin E3 ligase. Knockdown of GRAIL in Hep3B-TfR2 cells increased TfR2 levels, consistent with inhibition of CD81 ubiquitination. These results suggest that down-regulation of CD81 by GRAIL targets TfR2 for degradation. Surprisingly, knockdown of CD81 decreased hepcidin expression, implying that the TfR2/CD81 complex is involved in the maintenance of hepcidin mRNA. Moreover, knockdown of CD81 did not affect the stimulation of hepcidin expression by BMP6 but increased both the expression of ID1 and SMAD7, direct targets of BMP signaling pathway, and the phosphorylation of ERK1/2, indicating that the CD81 regulates hepcidin expression differently from the BMP and ERK1/2 signaling pathways.

Non-HFE hemochromatosis: pathophysiological and diagnostic aspects

Rare genetic iron overload diseases are an evolving field due to major advances in genetics and molecular biology. Genetic iron overload has long been confined to the classical type 1 hemochromatosis related to the HFE C282Y mutation. Breakthroughs in the understanding of iron metabolism biology and molecular mechanisms led to the discovery of new genes and subsequently, new types of hemochromatosis. To date, four types of hemochromatosis have been identified: HFE-related or type1 hemochromatosis, the most frequent form in Caucasians, and four rare types, named type 2 (A and B) hemochromatosis (juvenile hemochromatosis due to hemojuvelin and hepcidin mutation), type 3 hemochromatosis (related to transferrin receptor 2 mutation), and type 4 (A and B) hemochromatosis (ferroportin disease). The diagnosis relies on the comprehension of the involved physiological defect that can now be explored by biological and imaging tools, which allow non-invasive assessment of iron metabolism. A multidisciplinary approach is essential to support the physicians in the diagnosis and management of those rare diseases.

The role of hepatic transferrin receptor 2 in the regulation of iron homeostasis in the body

Fine-tuning of body iron is required to prevent diseases such as iron-overload and anemia. The putative iron sensor, transferrin receptor 2 (TfR2), is expressed in the liver and mutations in this protein result in the iron-overload disease Type III hereditary hemochromatosis (HH). With the loss of functional TfR2, the liver produces about 2-fold less of the peptide hormone hepcidin, which is responsible for negatively regulating iron uptake from the diet. This reduction in hepcidin expression leads to the slow accumulation of iron in the liver, heart, joints, and pancreas and subsequent cirrhosis, heart disease, arthritis, and diabetes. TfR2 can bind iron-loaded transferrin (Tf) in the bloodstream, and hepatocytes treated with Tf respond with a 2-fold increase in hepcidin expression through stimulation of the bone morphogenetic protein (BMP)-signaling pathway. Loss of functional TfR2 or its binding partner, the original HH protein, results in a loss of this transferrin-sensitivity. While much is known about the trafficking and regulation of TfR2, the mechanism of its transferrin-sensitivity through the BMP-signaling pathway is still not known.