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Hepcidin in human iron disorders: Therapeutic implications

Open AccessPublished:August 31, 2010DOI:https://doi.org/10.1016/j.jhep.2010.08.004
      The discovery of hepcidin has triggered a virtual explosion of studies on iron metabolism and related disorders, the results of which have profoundly changed our view of human diseases associated with excess of iron, iron deficiency or iron misdistribution. Not only has new light been shed on the pathogenesis of these disorders, but therapeutic applications from these advances are now foreseen. The notion that hepcidin excess or deficiency may contribute to the dysregulation of iron homeostasis in hereditary and acquired iron disorders raises the possibility that hepcidin-lowering or enhancing agents may be an effective strategy for curing the main consequences of hepcidinopathies, anemia or iron overload, respectively. Experimental pre-clinical and clinical studies have shown that hepcidin antibodies, agonists or antagonists, cytokine receptor antibodies and small-molecules that modify hepcidin expression also reverse iron abnormalities in vivo, in a number of disease models. While future studies addressing safety and long-term efficacy of hepcidin-targeted treatments will clarify risks and benefits, a new era has begun based on the treatment of disorders of iron homeostasis through the modulation of its regulatory hormone, hepcidin.

      Abbreviations

      Introduction

      In 2000, a peptide was isolated from human blood ultrafiltrate and named LEAP-1 for liver expressed antimicrobial peptide [
      • Krause A.
      • Neitz S.
      • Magert H.J.
      • Schulz A.
      • Forssmann W.G.
      • Schulz-Knappe P.
      • et al.
      LEAP-1, a novel highly disulfide-bonded human peptide, exhibits antimicrobial activity.
      ]. In 2001, the same peptide was found in human urine [
      • Park C.H.
      • Valore E.V.
      • Waring A.J.
      • Ganz T.
      Hepcidin, a urinary antimicrobial peptide synthesized in the liver.
      ] and the cDNA of its murine homolog cloned from the mouse liver [
      • Pigeon C.
      • Ilyin G.
      • Courselaud B.
      • Leroyer P.
      • Turlin B.
      • Brissot P.
      • et al.
      A new mouse liver-specific gene, encoding a protein homologous to human antimicrobial peptide hepcidin, is overexpressed during iron overload.
      ] and renamed hepcidin. The 25mer peptide exerted some antibacterial and antifungal effects, and the protein itself was classified—along with the thionins and the defensins—as a member of the cysteine-rich, cationic, antimicrobial peptide family. At the time, we all thought we had just one more antimicrobial peptide to deal with. No one could predict how much that small peptide would impact the field of iron metabolism and related disorders. Pigeon et al. [
      • Pigeon C.
      • Ilyin G.
      • Courselaud B.
      • Leroyer P.
      • Turlin B.
      • Brissot P.
      • et al.
      A new mouse liver-specific gene, encoding a protein homologous to human antimicrobial peptide hepcidin, is overexpressed during iron overload.
      ] were the first to link hepcidin to iron, and shortly thereafter, knockout and transgenic mouse studies revealed that hepcidin is in fact the principal down-regulator of iron traffic directed toward the bloodstream from the external environment (i.e., the intestinal lumen or, for the fetus, maternal blood) and from the body storage sites [
      • Nicolas G.
      • Bennoun M.
      • Devaux I.
      • Beaumont C.
      • Grandchamp B.
      • Kahn A.
      • et al.
      Lack of hepcidin gene expression and severe tissue iron overload in upstream stimulatory factor 2 (USF2) knockout mice.
      ,
      • Courselaud B.
      • Pigeon C.
      • Inoue Y.
      • Inoue J.
      • Gonzalez F.J.
      • Leroyer P.
      • et al.
      C/EBPalpha regulates hepatic transcription of hepcidin, an antimicrobial peptide and regulator of iron metabolism. Cross-talk between C/EBP pathway and iron metabolism.
      ,
      • Nicolas G.
      • Bennoun M.
      • Porteu A.
      • Mativet S.
      • Beaumont C.
      • Grandchamp B.
      • et al.
      Severe iron deficiency anemia in transgenic mice expressing liver hepcidin.
      ]. Iron is a critical nutrient that is continuously recycled within the erythron compartment: the unavoidable daily losses are actively compensated by extracting 1–2 mg of iron from diet [
      • Bothwell T.H.
      The control of iron absorption.
      ]. We also know from pioneering iron kinetic studies performed in the 1950s that iron uptake, storage, and utilization are meticulously coordinated [
      • Bothwell T.H.
      • Hurtado A.V.
      • Donohue D.M.
      • Finch C.A.
      Erythrokinetics. IV. The plasma iron turnover as a measure of erythropoiesis.
      ]. It had been hypothesized that signals from storage compartments (mainly hepatocytes and macrophages) and utilization sites (primarily the bone marrow)—historically referred to as “store” and “erythroid” regulatory signals [
      • Finch C.A.
      Iron Balance in Man.
      ]—are transmitted to a central control site that would then dictate the amount of iron released into the bloodstream and delivered to the bone marrow. We now know that that the central control site of iron traffic is the liver and its effector is the hormone peptide hepcidin [
      • Pietrangelo A.
      Hemochromatosis: an endocrine liver disease.
      ].
      The discovery of hepcidin has triggered a virtual explosion of studies, the results of which have profoundly changed our view of diseases of iron metabolism and beyond. Not only has new light been shed on the pathogenesis of iron disorders, but new scenarios for diagnosis and treatment are now foreseen. The therapeutic implications of hepcidin discovery are the subject of this article.

      Hepcidin synthesis, function, and regulation

      Hepcidin is synthesized in the liver as a 84 amino acid pre-propeptide containing a typical N-terminal 24 amino acid endoplasmic reticulum targeting signal sequence, and a consensus furin cleavage site immediately preceding the C-terminal 25 amino acid bioactive peptide [
      • Park C.H.
      • Valore E.V.
      • Waring A.J.
      • Ganz T.
      Hepcidin, a urinary antimicrobial peptide synthesized in the liver.
      ,
      • Pigeon C.
      • Ilyin G.
      • Courselaud B.
      • Leroyer P.
      • Turlin B.
      • Brissot P.
      • et al.
      A new mouse liver-specific gene, encoding a protein homologous to human antimicrobial peptide hepcidin, is overexpressed during iron overload.
      ,
      • Valore E.V.
      • Ganz T.
      Posttranslational processing of hepcidin in human hepatocytes is mediated by the prohormone convertase furin.
      ]. In addition to prohepcidin and hepcidin-25, 22, and 20 carboxy terminal amino acid forms of hepcidin are found in the circulation and/or urine, but hepcidin-25 is the bioactive form of hepcidin, whereas the other forms have little or no biological activity.
      Hepcidin likely evolved in humans as part of the innate immune defense to prevent invading pathogens from using iron sources to grow and proliferate. In fact, it is the main circulating inhibitor of iron flux from enterocytes, hepatocytes, macrophages, and placental cells into the bloodstream. Hepcidin produces these effects by binding to ferroportin (FPN), the main cellular iron-exporter in mammals. Independently identified in 2000 by three different laboratories [
      • Abboud S.
      • Haile D.J.
      A novel mammalian iron-regulated protein involved in intracellular iron metabolism.
      ,
      • Donovan A.
      • Brownlie A.
      • Zhou Y.
      • Shepard J.
      • Pratt S.J.
      • Moynihan J.
      • et al.
      Positional cloning of zebrafish ferroportin1 identifies a conserved vertebrate iron exporter.
      ,
      • McKie A.T.
      • Marciani P.
      • Rolfs A.
      • Brennan K.
      • Wehr K.
      • Barrow D.
      • et al.
      A novel duodenal iron-regulated transporter, IREG1, implicated in the basolateral transfer of iron to the circulation.
      ], FPN (previously referred to as IREG1 or MTP1) is a multi-domain transmembrane protein encoded by the SLC40A1 gene. It is mainly expressed by the cells that play key roles in mammalian iron metabolism, including placental syncytiotrophoblasts, duodenal enterocytes, hepatocytes, and reticuloendothelial macrophages. As a result of its interaction with circulating hepcidin, FPN is internalized and degraded, thereby diminishing the cells’ ability to transfer iron to the plasma compartment [
      • Nemeth E.
      • Tuttle M.S.
      • Powelson J.
      • Vaughn M.B.
      • Donovan A.
      • Ward D.M.
      • et al.
      Hepcidin regulates cellular iron efflux by binding to ferroportin and inducing its internalization.
      ]. Through its N-terminus domain, hepcidin binds to the extracellular region of FPN, causing Jak2-mediated tyrosine phosphorylation at residues 302 and 303 in FPN’s cytosolic loop. FPN is then internalized, dephosphorylated, ubiquitinated, and ultimately degraded in the late endosome/lysosome compartment [
      • Nemeth E.
      • Preza G.C.
      • Jung C.L.
      • Kaplan J.
      • Waring A.J.
      • Ganz T.
      The N-terminus of hepcidin is essential for its interaction with ferroportin: structure-function study.
      ,
      • De Domenico I.
      • Ward D.M.
      • Langelier C.
      • Vaughn M.B.
      • Nemeth E.
      • Sundquist W.I.
      • et al.
      The molecular mechanism of hepcidin-mediated ferroportin down-regulation.
      ]. Later, if iron is needed in the bone marrow for hemoglobin synthesis, hepcidin production declines, FPN is re-expressed at the cell surface, and iron export to the bloodstream resumes. This negative-feedback mechanism keeps circulating iron levels within a range that will satisfy the body’s erythropoietic needs without provoking oxidative damage to body cells.
      Hepcidin responds to stimulatory and inhibitory signals. The former include inflammatory stimuli and iron load, but also stress signals arising within the cell, such as those related to endoplasmic reticulum (ER) stress [
      • Vecchi C.
      • Montosi G.
      • Zhang K.
      • Lamberti I.
      • Duncan S.A.
      • Kaufman R.J.
      • et al.
      ER stress controls iron metabolism through induction of hepcidin.
      ,
      • Oliveira S.J.
      • Pinto J.P.
      • Picarote G.
      • Costa V.M.
      • Carvalho F.
      • Rangel M.
      • et al.
      ER stress-inducible factor CHOP affects the expression of hepcidin by modulating C/EBPalpha activity.
      ] (Fig. 1). The latter mostly originate in the erythroid compartment and are probably hierarchically dominant. In fact, maintaining a constant source of iron for hemoglobin synthesis (around 20 mg a day) is a top priority for the human body, so when demand increases in the erythroid compartment, hepatic hepcidin output has to be decreased. Hypoxia, anemia, and iron deficiency all inhibit hepcidin synthesis [
      • Nicolas G.
      • Chauvet C.
      • Viatte L.
      • Danan J.L.
      • Bigard X.
      • Devaux I.
      • et al.
      The gene encoding the iron regulatory peptide hepcidin is regulated by anemia, hypoxia, and inflammation.
      ] and several putative mediators have been proposed: hypoxic inducible factor [
      • Peyssonnaux C.
      • Zinkernagel A.S.
      • Schuepbach R.A.
      • Rankin E.
      • Vaulont S.
      • Haase V.H.
      • et al.
      Regulation of iron homeostasis by the hypoxia-inducible transcription factors (HIFs).
      ], erythropoietin [
      • Pinto J.P.
      • Ribeiro S.
      • Pontes H.
      • Thowfeequ S.
      • Tosh D.
      • Carvalho F.
      • et al.
      Erythropoietin mediates hepcidin expression in hepatocytes through EPOR signaling and regulation of C/EBPalpha.
      ], or circulating factors derived from maturing erythroblasts in the bone marrow, such as growth differentiation factor 15 (GDF15) and twisted gastrulation protein homolog 1 (TWSG1) [
      • Tanno T.
      • Bhanu N.V.
      • Oneal P.A.
      • Goh S.H.
      • Staker P.
      • Lee Y.T.
      • et al.
      High levels of GDF15 in thalassemia suppress expression of the iron regulatory protein hepcidin.
      ,
      • Tanno T.
      • Porayette P.
      • Sripichai O.
      • Noh S.J.
      • Byrnes C.
      • Bhupatiraju A.
      • et al.
      Identification of TWSG1 as a second novel erythroid regulator of hepcidin expression in murine and human cells.
      ] (Fig. 1). The serine protease/matriptase 2 [
      • Du X.
      • She E.
      • Gelbart T.
      • Truksa J.
      • Lee P.
      • Xia Y.
      • et al.
      The serine protease TMPRSS6 is required to sense iron deficiency.
      ] (transmembrane serine protease 6 gene, TMPRSS6), inhibits hepcidin likely by cleaving hemojuvelin (HJV), a membrane-bound protein that promotes hepcidin signaling in hepatocytes (see below) (Fig. 1) [
      • Silvestri L.
      • Pagani A.
      • Nai A.
      • De Domenico I.
      • Kaplan J.
      • Camaschella C.
      The serine protease matriptase-2 (TMPRSS6) inhibits hepcidin activation by cleaving membrane hemojuvelin.
      ].
      Figure thumbnail gr1
      Fig. 1Signals and pathways controlling hepcidin expression in the liver. Hepcidin responds to stimulatory signals, such as iron and cytokines, or inhibitory signals, mainly linked to the erythroid activity. The iron-sensing process involves the local iron-induced production of bone morphogenic proteins (BMPs), such as BMP6, and the subsequent assembly of a membrane-associated hetero-tetrameric signaling complex, composed of two type I and two type II serine threonine kinase receptors. This activates a common signal transduction cascade involving the phosphorylation of intracellular receptor-activated Smad1, Smad5 and Smad8, which interact with Smad4 in the cytoplasm. The resulting complex then translocates to the nucleus, where it activates transcription of the hepcidin gene (see text for details). Neogenin, a membrane receptor for RGM, has been proposed to stabilize HJV, and participate in HJV shedding. The soluble form of HJV (sHJV), is thought to compete for BMP binding with its membrane-anchored counterpart (either by sequestering free ligands or by displacing HJV from its BMP-R binding site), thereby providing iron-sensitive modulation of hepcidin expression. SMAD7, which is stimulated by iron, seems to attenuate the signal for hepcidin activation. The serine protease/matriptase 2 (transmembrane serine protease 6, TMPRSS6), inhibits hepcidin expression, most likely by cleaving HJV. Normal HFE interacts with TfR1, and probably also with TfR2. Together, these three proteins might constitute a functional sensing unit responsible for conveying the iron signal to hepcidin. A key mediator of hepcidin response to inflammation is interleukin 6 (IL-6) which stimulates hepcidin transcription through STAT3, although a possible contribution of the BMP/SMD pathway has also been evoked. The C-AMP responsive element binding protein H (CREBH), has been recently implicated in the transcriptional activation of hepcidin during ER stress and seems also to partially contribute to hepcidin response to inflammatory stimuli in vivo. Inhibitory signals for hepcidin transcription include hypoxic inducible factor (HIF), erythropoietin (Epo), or circulating factors derived from maturing erythroblasts in the bone marrow, such as growth differentiation factor (GDF15) and twisted gastrulation protein homolog 1 (TWSG1) (see text for details).
      Inhibitory signals include those arising during infection and inflammatory states and iron itself. Inflammation readily induces hepcidin expression [
      • Pigeon C.
      • Ilyin G.
      • Courselaud B.
      • Leroyer P.
      • Turlin B.
      • Brissot P.
      • et al.
      A new mouse liver-specific gene, encoding a protein homologous to human antimicrobial peptide hepcidin, is overexpressed during iron overload.
      ,
      • Nicolas G.
      • Chauvet C.
      • Viatte L.
      • Danan J.L.
      • Bigard X.
      • Devaux I.
      • et al.
      The gene encoding the iron regulatory peptide hepcidin is regulated by anemia, hypoxia, and inflammation.
      ]. A key player in hepcidin activation during inflammation is interleukin 6 (IL-6) [
      • Nemeth E.
      • Rivera S.
      • Gabayan V.
      • Keller C.
      • Taudorf S.
      • Pedersen B.K.
      • et al.
      IL-6 mediates hypoferremia of inflammation by inducing the synthesis of the iron regulatory hormone hepcidin.
      ], which activates hepcidin transcription through STAT3 [
      • Wrighting D.M.
      • Andrews N.C.
      Interleukin-6 induces hepcidin expression through STAT3.
      ,
      • Verga Falzacappa M.V.
      • Vujic Spasic M.
      • Kessler R.
      • Stolte J.
      • Hentze M.W.
      • Muckenthaler M.U.
      STAT3 mediates hepatic hepcidin expression and its inflammatory stimulation.
      ,
      • Pietrangelo A.
      • Dierssen U.
      • Valli L.
      • Garuti C.
      • Rump A.
      • Corradini E.
      • et al.
      STAT3 is required for IL-6-gp130-dependent activation of hepcidin in vivo.
      ], although an intact BMP-SMAD signaling pathway seems to be also required for such activity [
      • Wang R.H.
      • Li C.
      • Xu X.
      • Zheng Y.
      • Xiao C.
      • Zerfas P.
      • et al.
      A role of SMAD4 in iron metabolism through the positive regulation of hepcidin expression.
      ,
      • Yu P.B.
      • Hong C.C.
      • Sachidanandan C.
      • Babitt J.L.
      • Deng D.Y.
      • Hoyng S.A.
      • et al.
      Dorsomorphin inhibits BMP signals required for embryogenesis and iron metabolism.
      ,
      • Babitt J.L.
      • Huang F.W.
      • Xia Y.
      • Sidis Y.
      • Andrews N.C.
      • Lin H.Y.
      Modulation of bone morphogenetic protein signaling in vivo regulates systemic iron balance.
      ,
      • Verga Falzacappa M.V.
      • Casanovas G.
      • Hentze M.W.
      • Muckenthaler M.U.
      A bone morphogenetic protein (BMP)-responsive element in the hepcidin promoter controls HFE2-mediated hepatic hepcidin expression and its response to IL-6 in cultured cells.
      ]. Pro-inflammatory cytokines and LPS also induce endoplasmic reticulum (ER) stress and activation of the C-AMP responsive element binding protein H (CREBH), recently involved in the regulation of acute phase proteins in the liver [
      • Zhang K.
      • Shen X.
      • Wu J.
      • Sakaki K.
      • Saunders T.
      • Rutkowski D.T.
      • et al.
      Endoplasmic reticulum stress activates cleavage of CREBH to induce a systemic inflammatory response.
      ]. CREBH has been recently implicated in the transcriptional activation of hepcidin during ER stress and also seems to partially contribute to hepcidin response to inflammatory stimuli in vivo [
      • Vecchi C.
      • Montosi G.
      • Zhang K.
      • Lamberti I.
      • Duncan S.A.
      • Kaufman R.J.
      • et al.
      ER stress controls iron metabolism through induction of hepcidin.
      ]. Hepcidin is also produced in monocytes/macrophages [
      • Liu X.B.
      • Nguyen N.B.
      • Marquess K.D.
      • Yang F.
      • Haile D.J.
      Regulation of hepcidin and ferroportin expression by lipopolysaccharide in splenic macrophages.
      ] and is induced in these cells by LPS and certain bacterial pathogens through Toll like receptors and possibly also the IL-6/STAT3 pathway. It might function through an autocrine mechanism to alter iron traffic locally during pathogen invasion [
      • Theurl I.
      • Theurl M.
      • Seifert M.
      • Mair S.
      • Nairz M.
      • Rumpold H.
      • et al.
      Autocrine formation of hepcidin induces iron retention in human monocytes.
      ]. Recently, a new function for hepcidin in modulating acute inflammatory responses has been proposed. De Domenico et al. [
      • De Domenico I.
      • Zhang T.Y.
      • Koening C.L.
      • Branch R.W.
      • London N.
      • Lo E.
      • et al.
      Hepcidin mediates transcriptional changes that modulate acute cytokine-induced inflammatory responses in mice.
      ] have shown that hepcidin-treatment of macrophages directly activates the transcription factor Stat3 through Jak2-mediated phosphorylation, resulting in a transcriptional anti-inflammatory response. Most notably, hepcidin pretreatment protected mice from a lethal dose of LPS and hepcidin-knockout mice could be rescued from LPS toxicity by injection of hepcidin [
      • De Domenico I.
      • Zhang T.Y.
      • Koening C.L.
      • Branch R.W.
      • London N.
      • Lo E.
      • et al.
      Hepcidin mediates transcriptional changes that modulate acute cytokine-induced inflammatory responses in mice.
      ].
      Iron is a main stimulus for hepcidin. “Iron-sensing” occurs through a complex multiprotein system at the hepatocyte plasma membrane (Fig. 1) that involves bone morphogenetic proteins (BMPs) and their receptors as well as a number of ancillary proteins. Members of the TGF-β superfamily [
      • Corradini E.
      • Babitt J.L.
      • Lin H.Y.
      The RGM/DRAGON family of BMP co-receptors.
      ], BMP ligands trigger intracellular signaling through downstream effectors, the Smad proteins, which translocate to the nucleus and activate the expression of various genes, including HAMP. The timing, location, and specific downstream effects of BMP signaling are controlled by a network of regulatory proteins, which includes a family of BMP co-receptors known as the repulsive guidance molecules (RGMs). The RGM providing specificity to the iron signal in the liver is RGMc, also known as hemojuvelin (HJV). In 2004, mutations involving the HJV gene were identified as the most common cause of juvenile hemochromatosis (HC) [
      • Papanikolaou G.
      • Samuels M.E.
      • Ludwig E.H.
      • MacDonald M.L.
      • Franchini P.L.
      • Dube M.P.
      • et al.
      Mutations in HFE2 cause iron overload in chromosome 1q-linked juvenile hemochromatosis.
      ]. The putative full-length protein (426 amino acids) encoded by HJV includes a C-terminal glycosylphosphatidylinositol anchor, which indicates that it can be present in either a soluble or cell-associated form. As shown in Fig. 1, the latter form is part of the BMP signaling complex whose activation results in the expression of hepcidin. The soluble form of HJV (sHJV), whose release (HJV shedding) is inhibited by increasing extracellular concentrations of iron, is thought to compete with its membrane-anchored counterpart for BMP binding [
      • Lin L.
      • Goldberg Y.P.
      • Ganz T.
      Competitive regulation of hepcidin mRNA by soluble and cell-associated hemojuvelin.
      ], although it is unclear whether and to what extent this process operates in vivo to regulate iron homeostasis (Fig. 1). Neogenin, a member of the DCC (deleted in colorectal cancer) family of tumor suppressor molecules has been identified as a receptor for RGMs, and appears to interact with HJV [
      • Zhang A.S.
      • West Jr., A.P.
      • Wyman A.E.
      • Bjorkman P.J.
      • Enns C.A.
      Interaction of hemojuvelin with neogenin results in iron accumulation in human embryonic kidney 293 cells.
      ,
      • Yang F.
      • West Jr., A.P.
      • Allendorph G.P.
      • Choe S.
      • Bjorkman P.J.
      Neogenin interacts with hemojuvelin through its two membrane-proximal fibronectin type III domains.
      ]. However, its role in HJV biology and hepcidin synthesis is still controversial [
      • Lee D.H.
      • Zhou L.J.
      • Zhou Z.
      • Xie J.X.
      • Jung J.U.
      • Liu Y.
      • et al.
      Neogenin inhibits HJV secretion and regulates BMP induced hepcidin expression and iron homeostasis.
      ]. Recently, it has been suggested that neogenin inhibits HJV shedding and formation of sHJV: the liver of neogenin-mutant mice exhibits reduced BMP signaling, low levels of hepcidin, and iron overload [
      • Xia Y.
      • Babitt J.L.
      • Sidis Y.
      • Chung R.T.
      • Lin H.Y.
      Hemojuvelin regulates hepcidin expression via a selective subset of BMP ligands and receptors independently of neogenin.
      ].
      Iron has been shown to increase hepatic BMP signaling [
      • Yu P.B.
      • Hong C.C.
      • Sachidanandan C.
      • Babitt J.L.
      • Deng D.Y.
      • Hoyng S.A.
      • et al.
      Dorsomorphin inhibits BMP signals required for embryogenesis and iron metabolism.
      ], while the administration of BMP increases hepcidin expression and reduces serum iron [
      • Wang R.H.
      • Li C.
      • Xu X.
      • Zheng Y.
      • Xiao C.
      • Zerfas P.
      • et al.
      A role of SMAD4 in iron metabolism through the positive regulation of hepcidin expression.
      ,
      • Babitt J.L.
      • Huang F.W.
      • Xia Y.
      • Sidis Y.
      • Andrews N.C.
      • Lin H.Y.
      Modulation of bone morphogenetic protein signaling in vivo regulates systemic iron balance.
      ,
      • Babitt J.L.
      • Huang F.W.
      • Wrighting D.M.
      • Xia Y.
      • Sidis Y.
      • Samad T.A.
      • et al.
      Bone morphogenetic protein signaling by hemojuvelin regulates hepcidin expression.
      ]. Since Smad7 [
      • Kautz L.
      • Meynard D.
      • Monnier A.
      • Darnaud V.
      • Bouvet R.
      • Wang R.H.
      • et al.
      Iron regulates phosphorylation of Smad1/5/8 and gene expression of Bmp6, Smad7, Id1, and Atoh8 in the mouse liver.
      ] is co-regulated with hepcidin by BMPs, and SMAD7 over-expression has been found to abrogate BMP-mediated activation of hepcidin, this protein might operate as an intracellular negative-feedback regulator of hepcidin synthesis [
      • Mleczko-Sanecka K.
      • Casanovas G.
      • Ragab A.
      • Breitkopf K.
      • Muller A.
      • Boutros M.
      • et al.
      SMAD7 controls iron metabolism as a potent inhibitor of hepcidin expression.
      ] (Fig. 1). Various BMPs have proved to be capable of stimulating hepcidin synthesis in vitro, but a particularly important role is emerging for BMP6 [
      • Andriopoulos Jr., B.
      • Corradini E.
      • Xia Y.
      • Faasse S.A.
      • Chen S.
      • Grgurevic L.
      • et al.
      BMP6 is a key endogenous regulator of hepcidin expression and iron metabolism.
      ,
      • Meynard D.
      • Kautz L.
      • Darnaud V.
      • Canonne-Hergaux F.
      • Coppin H.
      • Roth M.P.
      Lack of the bone morphogenetic protein BMP6 induces massive iron overload.
      ], which can physically interact with soluble HJV and increases hepcidin expression and reduces serum iron levels in mice [
      • Andriopoulos Jr., B.
      • Corradini E.
      • Xia Y.
      • Faasse S.A.
      • Chen S.
      • Grgurevic L.
      • et al.
      BMP6 is a key endogenous regulator of hepcidin expression and iron metabolism.
      ]. Notably, Bmp6-null mice exhibit an hemochromatosis-like phenotype characterized by reduced hepcidin expression and tissue iron overload. [
      • Andriopoulos Jr., B.
      • Corradini E.
      • Xia Y.
      • Faasse S.A.
      • Chen S.
      • Grgurevic L.
      • et al.
      BMP6 is a key endogenous regulator of hepcidin expression and iron metabolism.
      ,
      • Meynard D.
      • Kautz L.
      • Darnaud V.
      • Canonne-Hergaux F.
      • Coppin H.
      • Roth M.P.
      Lack of the bone morphogenetic protein BMP6 induces massive iron overload.
      ] These data point to BMP6 as an endogenous regulator of hepcidin expression and iron metabolism in vivo.
      The iron-sensing process also seems to involve two other proteins, whose loss, like that of HJV, has been linked to human HC: HFE and transferrin receptor 2 (TfR2). HFE is a major histocompatibility class-I-like protein that interacts with transferrin receptor 1 (TfR1) [
      • Feder J.N.
      • Penny D.M.
      • Irrinki A.
      • Lee V.K.
      • Lebrón J.A.
      • Watson N.
      • et al.
      The hemochromatosis gene product complexes with the transferrin receptor and lowers its affinity for ligand binding.
      ], a type II transmembrane glycoprotein that mediates uptake of transferrin-bound iron, particularly by erythroid cells. It now looks as if HFE or the TfR1-HFE complex might play some role in the regulation of hepcidin expression. The functional loss of HFE in mice [
      • Ahmad K.A.
      • Ahmann J.R.
      • Migas M.C.
      • Waheed A.
      • Britton R.S.
      • Bacon B.R.
      • et al.
      Decreased liver hepcidin expression in the Hfe knockout mouse.
      ] and humans [
      • Bridle K.R.
      • Frazer D.M.
      • Wilkins S.J.
      • Dixon J.L.
      • Purdie D.M.
      • Crawford D.H.
      • et al.
      Disrupted hepcidin regulation in HFE-associated haemochromatosis and the liver as a regulator of body iron homoeostasis.
      ,
      • Gehrke S.G.
      • Kulaksiz H.
      • Herrmann T.
      • Riedel H.D.
      • Bents K.
      • Veltkamp C.
      • et al.
      Expression of hepcidin in hereditary hemochromatosis: evidence for a regulation in response to the serum transferrin saturation and to non-transferrin-bound iron.
      ] has been shown to reduce hepcidin synthesis, but more recent findings indicate that hepcidin expression is clearly down-regulated by hepatocyte-specific deletion of Hfe [
      • Vujic Spasic M.
      • Kiss J.
      • Herrmann T.
      • Galy B.
      • Martinache S.
      • Stolte J.
      • et al.
      Hfe acts in hepatocytes to prevent hemochromatosis.
      ]. In fact, HFE loss seems to be associated with blunted signaling responses to BMP6, a key regulator of hepcidin, in vitro and in vivo [
      • Corradini E.
      • Garuti C.
      • Montosi G.
      • Ventura P.
      • Andriopoulos Jr., B.
      • Lin H.Y.
      • et al.
      Bone morphogenetic protein signaling is impaired in an HFE knockout mouse model of hemochromatosis.
      ,
      • Kautz L.
      • Meynard D.
      • Besson-Fournier C.
      • Darnaud V.
      • Al Saati T.
      • Coppin H.
      • et al.
      BMP/Smad signaling is not enhanced in Hfe-deficient mice despite increased Bmp6 expression.
      ]. This suggests that HFE might be necessary for an optimal response to low endogenous basal levels of BMPs, which is believed to act in an autocrine or paracrine fashion in the liver. TfR2, unlike TfR1, is highly expressed in the liver, and its expression is unaffected by the intracellular iron status. Moreover, functional loss of TfR2—in mice [
      • Kawabata H.
      • Fleming R.E.
      • Gui D.
      • Moon S.Y.
      • Saitoh T.
      • O’Kelly J.
      • et al.
      Expression of hepcidin is down-regulated in TfR2 mutant mice manifesting a phenotype of hereditary hemochromatosis.
      ] and in humans [
      • Nemeth E.
      • Roetto A.
      • Garozzo G.
      • Ganz T.
      • Camaschella C.
      Hepcidin is decreased in TFR2-Hemochromatosis.
      ]—is associated with decreased hepcidin expression. One possibility is that it operates in the pathway discussed above for HFE (or in a parallel pathway of its own), facilitating the BMP/SMAD signaling that activates hepcidin expression (Fig. 1). Another possibility is that TfR2, which is also able to interact with HFE [
      • Griffiths W.J.
      • Cox T.M.
      Co-localization of the mammalian hemochromatosis gene product (HFE) and a newly identified transferrin receptor (TfR2) in intestinal tissue and cells.
      ,
      • Goswami T.
      • Andrews N.C.
      Hereditary hemochromatosis protein, HFE, interaction with transferrin receptor 2 suggests a molecular mechanism for mammalian iron sensing.
      ,
      • Waheed A.
      • Britton R.S.
      • Grubb J.H.
      • Sly W.S.
      • Fleming R.E.
      HFE association with transferrin receptor 2 increases cellular uptake of transferrin-bound iron.
      ,
      • Chen J.
      • Chloupkova M.
      • Gao J.
      • Chapman-Arvedson T.L.
      • Enns C.A.
      HFE modulates transferrin receptor 2 levels in hepatoma cells via interactions that differ from transferrin receptor 1-HFE interactions.
      ], forms an iron-sensing complex that modulates hepcidin expression in response to blood levels of diferric transferrin [
      • Goswami T.
      • Andrews N.C.
      Hereditary hemochromatosis protein, HFE, interaction with transferrin receptor 2 suggests a molecular mechanism for mammalian iron sensing.
      ,
      • Schmidt P.J.
      • Toran P.T.
      • Giannetti A.M.
      • Bjorkman P.J.
      • Andrews N.C.
      The transferrin receptor modulates hfe-dependent regulation of hepcidin expression.
      ].
      In summary, hepcidin, a small hormone peptide, is constitutively produced by the liver to maintain plasma iron levels within a narrow physiologic range. To do so it senses a variety of physiologic and pathophysiologic stimuli, mainly the erythropoietic activity and inflammatory states, which tend to alter blood iron levels, and responds by inhibiting the main iron-exporter in mammals, ferroportin (Box 1).
      GDF15: growth differentiation factor 15; TWSG1: twisted gastrulation protein homolog 1.

      Targeting the hepcidin–ferroportin axis to cure human iron disorders

      The blood concentrations of iron must be kept constant as both iron deficiency or iron excess may be a cause of disability and disease (Table 1). Recently, it has been recognized iron also plays a role in other disorders that are associated with “iron-mis-distribution” [
      • Pietrangelo A.
      Iron chelation beyond transfusion iron overload.
      ], within certain cell types (e.g., macrophages in anemia of chronic disease) or even within cell organelles (mitochondria in Friedreich’s ataxia) [
      • Campuzano V.
      • Montermini L.
      • Molto M.D.
      • Pianese L.
      • Cossee M.
      • Cavalcanti F.
      • et al.
      Friedreich’s ataxia: autosomal recessive disease caused by an intronic GAA triplet repeat expansion.
      ]. Finally, conditions exist where iron preferentially accumulates in certain organs mostly following necro-inflammatory events or caused by locally altered iron traffic (e.g., the liver in chronic viral hepatitis or the brain in neurodegenerative disorders). In strict terms, the latter disorders may not all qualify as true iron overload states, as total body iron content may not be increased. Nevertheless, the impact on cell damage and organ disease may be extremely high even in the presence of mild iron overload, as any excess iron may fuel oxidative stress and affect signaling pathways important for the pathogenesis of that specific condition (e.g., chronic liver disorders). When considering the broad range of iron-related disorders, there is evidence for a pathogenic involvement of hepcidin/ferroportin in a number of them, either hereditary (such as HC, the ferroportin disease, IRIDA) or secondary to other causes (such as inflammation in anemia of chronic disease, ACD) or inefficient erythropoiesis in thalassemia intermedia and other hereditary anemias) (Table 1)(Fig. 2). Recently, experimental evidence has been provided for acquired factors, such as alcohol consumption [
      • Bridle K.
      • Cheung T.K.
      • Murphy T.
      • Walters M.
      • Anderson G.
      • Crawford D.G.
      • et al.
      Hepcidin is down-regulated in alcoholic liver injury: implications for the pathogenesis of alcoholic liver disease.
      ,
      • Harrison-Findik D.D.
      • Schafer D.
      • Klein E.
      • Timchenko N.A.
      • Kulaksiz H.
      • Clemens D.
      • et al.
      Alcohol metabolism-mediated oxidative stress down-regulates hepcidin transcription and leads to increased duodenal iron transporter expression.
      ] or chronic viral hepatitis [
      • Nishina S.
      • Hino K.
      • Korenaga M.
      • Vecchi C.
      • Pietrangelo A.
      • Mizukami Y.
      • et al.
      Hepatitis C virus-induced reactive oxygen species raise hepatic iron level in mice by reducing hepcidin transcription.
      ], being also able to decrease hepcidin transcription and cause hepatic iron overload.
      Table 1Human disorders associated with or caused by disturbances of iron metabolism.
      1Although the ferroportin disease is characterized by systemic iron overload, in the classic form of the disorder, iron excess may be exclusively found in the reticuloendothelial macrophage system. 2In Friedreich’s ataxia loss of frataxin leads to mitochondrial iron accumulation, cell damage and organ (mainly brain) disorders.
      Figure thumbnail gr2
      Fig. 2Hepcidin in the pathogenesis of human hereditary and acquired iron disorders. Abnormal hepcidin regulation is responsible for iron disturbances in paradigmatic human disorders characterized by hepcidin deficiency (HFE-hemochromatosis and thalassemia) or hepcidin excess (anemia of chronic disease or anemia of inflammation). The Figure depicts the liver, as the main site of hepcidin production, and the macrophage, a site for hepcidin activity highly expressing ferroportin, the hepcidin-target. HFE-Hemochromatosis: In HFE-HC, functional loss of HFE due to the C282Y mutation, leads to impaired BMP/SMAD signaling and hepcidin attenuated transcriptional activity (see text for details). The effect of inadequate levels of circulating hepcidin leads to unchecked iron-export by ferroportin in the macrophage, responsible for progressive plasma iron load, tissue iron accumulation and disease. Thalassemia: In hereditary anemias associated with inefficient erythropoiesis, such as thalassemia at early pre-transfusion stages, erythroid factors induced by inefficient erythropoiesis (such as GDF15 and TWSG1) inhibits hepcidin transcription, and cause, in analogy with HFE-HC, hepcidin deficiency and unrestricted iron release from macrophages toward the bloodstream. Anemia of inflammation: During inflammatory states or infection, pathogen by-products may lead to cytokine overproduction, particularly from macrophages and resident Kupffer cells in the liver. The latter may release cytokines, such as IL-6, that stimulate hepcidin production by the hepatocytes. Prolonged hyper-hepcidinemia may then lead to iron sequestration in macrophages, hypoferremia and iron-restricted anemia. In spite of low serum iron, tissue iron excess and inflammatory mediators lead to elevation of serum ferritin.

      Disorders potentially treatable with hepcidin agonists

      In general, based on what we have learned on hepcidin biology and regulation, we can predict that any genetic (or acquired) factor that decreases hepcidin synthesis/activity will lead to an unrestricted flux of iron into the bloodstream followed by tissue iron overload, with the potential for toxicity and damage. All these conditions may benefit from therapies based on hepcidin or hepcidin agonists (Table 2). Hereditary causes of hepcidin deficiency are now classified within the hereditary hemochromatosis syndromes, for simplicity, referred to as hemochromatosis (HC) [
      • Pietrangelo A.
      Hereditary hemochromatosis: pathogenesis, diagnosis and treatment.
      ] (Table 1). Human HC has been associated with the common C282Y polymorphism of HFE [
      • Feder J.N.
      • Gnirke A.
      • Thomas W.
      • Tsuchihashi Z.
      • Ruddy D.A.
      • Basava A.
      • et al.
      A novel MHC class I-like gene is mutated in patients with hereditary haemochromatosis.
      ] or rare pathogenic mutations of TfR2,[
      • Camaschella C.
      • Roetto A.
      • Cali A.
      • De Gobbi M.
      • Garozzo G.
      • Carella M.
      • et al.
      The gene TFR2 is mutated in a new type of haemochromatosis mapping to 7q22.
      ] HJV [
      • Papanikolaou G.
      • Samuels M.E.
      • Ludwig E.H.
      • MacDonald M.L.
      • Franchini P.L.
      • Dube M.P.
      • et al.
      Mutations in HFE2 cause iron overload in chromosome 1q-linked juvenile hemochromatosis.
      ], FPN [
      • Njajou O.T.
      • Vaessen N.
      • Joosse M.
      • Berghuis B.
      • van Dongen J.W.
      • Breuning M.H.
      • et al.
      A mutation in SLC11A3 is associated with autosomal dominant hemochromatosis.
      ] and HAMP itself [
      • Roetto A.
      • Papanikolaou G.
      • Politou M.
      • Alberti F.
      • Girelli D.
      • Christakis J.
      • et al.
      Mutant antimicrobial peptide hepcidin is associated with severe juvenile hemochromatosis.
      ]. All forms of human HC are characterized by low or inadequate levels of hepcidin [
      • Bridle K.R.
      • Frazer D.M.
      • Wilkins S.J.
      • Dixon J.L.
      • Purdie D.M.
      • Crawford D.H.
      • et al.
      Disrupted hepcidin regulation in HFE-associated haemochromatosis and the liver as a regulator of body iron homoeostasis.
      ,
      • Gehrke S.G.
      • Kulaksiz H.
      • Herrmann T.
      • Riedel H.D.
      • Bents K.
      • Veltkamp C.
      • et al.
      Expression of hepcidin in hereditary hemochromatosis: evidence for a regulation in response to the serum transferrin saturation and to non-transferrin-bound iron.
      ,
      • Nemeth E.
      • Roetto A.
      • Garozzo G.
      • Ganz T.
      • Camaschella C.
      Hepcidin is decreased in TFR2 hemochromatosis.
      ,
      • Kattamis A.
      • Papassotiriou I.
      • Palaiologou D.
      • Apostolakou F.
      • Galani A.
      • Ladis V.
      • et al.
      The effects of erythropoetic activity and iron burden on hepcidin expression in patients with thalassemia major.
      ]. In fact, HFE, TfR2, and HJV all appear to be independent but complementary regulators of hepcidin synthesis in the liver (Fig. 1). If the HAMP gene is intact and each of the three regulators is functioning as it should, the amount of iron transferred into the blood will be appropriate to the body’s needs, and excessive iron deposition in tissues will be avoided. Functional loss of one of the hepcidin regulators, depending on the different roles of such a regulator in hepcidin transcription, will result in a different effects on the actual level of circulating hormone and, consequently, on the extent of plasma iron loading, tissue iron excess, and the ultimate disease phenotype. The latter, will range from the severe early onset HJV- and HAMP-associated forms to the milder adult onset HFE and TfR2-associated forms. Fig. 2 shows the key pathogenic events associated with hepcidin deficiency in HFE-HC, the most common and paradigmatic form of HC.
      Table 2Hepcidin therapy in human iron disorders.
      BMP, bone morphogenetic proteins; SMAD, Sma and mothers against decapentaplegic homolog; EPO, erythropoietin; ESA, erythropoiesis stimulating agents; TMPRSS6, transmembrane serine protease 6 gene; IBD, inflammatory bowel disease; CKD, chronic kidney diseases.
      With this scheme in mind, HC, like diabetes, appears essentially as an endocrine disorder, caused by defective synthesis/activity of the iron hormone hepcidin; therefore, HC is a human disease that may benefit of hormonal (hepcidin) therapy. Iron overload in any form of HC can be readily managed in most patients by therapeutic phlebotomy [

      EASL. EASL Clinical Practice Guidelines on Hemochromatosis. J Hepatol 2010.

      ]. Phlebotomy can treat and revert many symptoms and complications of HC, and provide normal survival if initiated at early stages. However, patients with underlying anemias, severe heart disease or reduced venous access may not tolerate this form of treatment, and compliance may also be variable over time [

      EASL. EASL Clinical Practice Guidelines on Hemochromatosis. J Hepatol 2010.

      ]. Therefore, if phlebotomy is contraindicated because of severe anemia, cardiac failure, or poor tolerance, other therapeutic strategies, such as iron chelators can be considered. However, neither phlebotomy nor iron chelators represent true etiologic therapies of HC. Hormone replacement could be an option in the future, in severe forms of the disease or when no other strategies are possible. Long-acting hepcidin peptides (which overcome the problem of the short-lived hepcidin molecule in vivo) could be developed. Alternatively, the use of hepcidin agonists could be a valuable strategy aimed at either raising serum hepcidin levels, or favoring FPN internalization/degradation. The former, could be achieved by targeting the BMP/SMAD signaling pathway, the latter by using small-molecules able to trigger the internalization, ubiquitination or degradation of FPN. The first approach has already shown some potential in pre-clinical in vitro and in vivo settings. Supra-physiological doses of exogenous BMP6 in Hfe−/− mice were recently able to overcome the impairment in the BMP–SMAD signaling pathway and rescue hepcidin expression in Hfe−/− hepatocytes [
      • Corradini E.
      • Garuti C.
      • Montosi G.
      • Ventura P.
      • Andriopoulos Jr., B.
      • Lin H.Y.
      • et al.
      Bone morphogenetic protein signaling is impaired in an HFE knockout mouse model of hemochromatosis.
      ]. Moreover, in vivo administration of exogenous BMP6 for 10 days has improved hepcidin deficiency, and corrected serum and tissue iron abnormalities in Hfe−/− mice [
      • Corradini E.
      • Schmidt P.J.
      • Meynard D.
      • Garuti C.
      • Montosi G.
      • Chen S.
      • et al.
      BMP6 treatment compensates for the molecular defect and ameliorates hemochromatosis in Hfe knockout mice.
      ]. However, longer treatment with exogenous BMP6 were unsuccessful because of peritoneal calcifications, consistent with the known bone-inducing properties of BMP6 [
      • Corradini E.
      • Schmidt P.J.
      • Meynard D.
      • Garuti C.
      • Montosi G.
      • Chen S.
      • et al.
      BMP6 treatment compensates for the molecular defect and ameliorates hemochromatosis in Hfe knockout mice.
      ]. Thus, exogenous BMP6 administration in its current form is not a viable therapy. However, these data provide proof of concept that activators of the BMP6–SMAD signaling pathway can be used to treat HC if more specific therapies can be developed without bone inducing or other off-target effects.
      Similar strategies could be also applied to hereditary anemias associated with inefficient erythropoiesis, which, at early pre-transfusion stages, are characterized by low hepcidin levels [
      • Kearney S.L.
      • Nemeth E.
      • Neufeld E.J.
      • Thapa D.
      • Ganz T.
      • Weinstein D.A.
      • et al.
      Urinary hepcidin in congenital chronic anemias.
      ] (likely caused by erythroid factors overproduced in the bone marrow), increased iron absorption, and excess hepatic iron deposition in periportal hepatocytes (Fig. 2).

      Disorders potentially treatable with hepcidin antagonists

      Any genetic or acquired factor that causes increased hepcidin synthesis activity, will lead to decreased iron transfer into plasma and hypoferremia. If hepcidin stimulation does persist, iron-restricted erythropoiesis and anemia may follow. Three forms of anemia exist that are associated with hepcidin abnormalities; one common form: anemia of chronic disease (ACD); and two rare forms: iron-refractory iron deficiency anemia (IRIDA) and anemia associated with hepcidin-producing hepatic adenomas. All these conditions may benefit of therapies based on hepcidin antagonists (Table 2).
      Anemia of chronic diseases (ACD), or anemia of inflammation, develops in the setting of infections and inflammatory disorders, such as rheumatologic diseases, inflammatory bowel diseases, chronic kidney disease (CKD), infections, and malignancies [
      • Weiss G.
      Iron metabolism in the anemia of chronic disease.
      ]. In a number of these pathologic conditions, elevated hepcidin has been reported [
      • Nemeth E.
      Targeting the hepcidin–ferroportin axis in the diagnosis and treatment of anemias.
      ]. Even in disorders associated with mild chronic inflammation, such as obesity, iron deficiency has been associated with elevated hepcidin levels. In CKD, the cause of anemia is clearly multifactorial: inadequate production of erythropoietin (likely the most important factor in the pathogenesis of anemia, and the reason for treating CKD patients with erythropoietin stimulating agents, ESA) [
      • Babitt J.L.
      • Lin H.Y.
      Molecular mechanisms of hepcidin regulation: implications for the anemia of CKD.
      ]; shortened erythrocyte survival; erythropoiesis-inhibitory effects of accumulating uremic toxins; iron loss caused by blood trapping in the dialysis apparatus and repeated phlebotomy. Yet, some patients also have a functional iron deficiency and reticuloendothelial cell iron blockade, with low levels of circulating iron that limit erythropoiesis. This is likely due to high hepcidin levels induced by a chronic inflammatory state (Fig. 2) [
      • Zaritsky J.
      • Young B.
      • Wang H.J.
      • Westerman M.
      • Olbina G.
      • Nemeth E.
      • et al.
      Hepcidin–a potential novel biomarker for iron status in chronic kidney disease.
      ]. Beyond inflammation, another factor may contribute to increased hepcidin in CKD. Loss of kidney function could decrease hepcidin clearance and lead to the accumulation of hepcidin and the development of iron-restrictive anemia [
      • Nemeth E.
      Targeting the hepcidin–ferroportin axis in the diagnosis and treatment of anemias.
      ]. Hyper-hepcidinemia may explain cases of hyporesponsiveness to therapeutic erythropoietin in CKD [
      • Elliott J.
      • Mishler D.
      • Agarwal R.
      Hyporesponsiveness to erythropoietin: causes and management.
      ].
      IRIDA is a hereditary hypochromic, microcytic anemia, refractory to treatment with oral iron, and only partially responsive to parenteral iron. The molecular basis is increased hepcidin production due to mutations in the hepcidin inhibitor TMPRSS6 [
      • Melis M.A.
      • Cau M.
      • Congiu R.
      • Sole G.
      • Barella S.
      • Cao A.
      • et al.
      A mutation in the TMPRSS6 gene, encoding a transmembrane serine protease that suppresses hepcidin production, in familial iron deficiency anemia refractory to oral iron.
      ,
      • Finberg K.E.
      • Heeney M.M.
      • Campagna D.R.
      • Aydinok Y.
      • Pearson H.A.
      • Hartman K.R.
      • et al.
      Mutations in TMPRSS6 cause iron-refractory iron deficiency anemia (IRIDA).
      ] (Fig. 1). When TMPRSS6 is mutated, hepcidin expression is increased, chronically inhibiting iron absorption and resulting in the development of iron deficiency anemia. Another rare form of hepcidin-linked anemia is associated with hepatic adenomas. This picture was first reported in the setting of type 1a glycogen storage disease: the patients presented large hepatic adenomas overproducing hepcidin, iron-deficiency microcytic anemia unresponsive or partially responsive to iron [
      • Weinstein D.A.
      • Roy C.N.
      • Fleming M.D.
      • Loda M.F.
      • Wolfsdorf J.I.
      • Andrews N.C.
      Inappropriate expression of hepcidin is associated with iron refractory anemia: implications for the anemia of chronic disease.
      ]. Anemia was cured after tumor resection. A patient with a similar presentation in the setting of a large hepatic adenoma but no glycogen disease has been also reported [
      • Chung A.
      • Leo K.
      • Wong G.
      • Chuah K.
      • Ren J.
      • Lee C.
      Giant hepatocellular adenoma presenting with chronic iron deficiency anemia.
      ].
      Several experimental therapeutic approaches have been undertaken to target steps of hepcidin production or activity (Box 2). In a mouse model of ACD caused by injections of Brucella abortus, hepcidin antagonists (neutralizing monoclonal antibody to hepcidin) were effective in treating anemia [
      • Sasu B.J.
      • Cooke K.S.
      • Arvedson T.L.
      • Plewa C.
      • Ellison A.R.
      • Sheng J.
      • et al.
      Anti-hepcidin antibody treatment modulates iron metabolism and is effective in a mouse model of inflammation-induced anemia.
      ]. Other drugs can interfere with the iron–BMP–SMAD signalling pathway. Dorsomorphin, a small-molecule that selectively inhibits the BMP type I receptors ALK2, ALK3, and ALK6 and thus blocks BMP-mediated SMAD1/5/8 phosphorylation, was able to prevent hepcidin induction by iron in vivo [
      • Yu P.B.
      • Hong C.C.
      • Sachidanandan C.
      • Babitt J.L.
      • Deng D.Y.
      • Hoyng S.A.
      • et al.
      Dorsomorphin inhibits BMP signals required for embryogenesis and iron metabolism.
      ]. Soluble HJV, also acting as an antagonist of BMP signaling, decreased hepcidin baseline expression in mice and concurrently increased liver iron content [
      • Babitt J.L.
      • Huang F.W.
      • Xia Y.
      • Sidis Y.
      • Andrews N.C.
      • Lin H.Y.
      Modulation of bone morphogenetic protein signaling in vivo regulates systemic iron balance.
      ]. Furthermore, some existing anti-cytokine therapies may be acting by decreasing hepcidin production induced by cytokines during ACD. Tocilizumab, a humanized anti-interleukin-6 receptor antibody currently used in rheumatoid arthritis, was shown to improve anemia in monkey arthritis and patients with multicentric Castelman’s disease by suppressing IL-6-induced hepcidin expression [
      • Hashizume M.
      • Uchiyama Y.
      • Horai N.
      • Tomosugi N.
      • Mihara M.
      Tocilizumab, a humanized anti-interleukin-6 receptor antibody, improved anemia in monkey arthritis by suppressing IL-6-induced hepcidin production.
      ,
      • Kawabata H.
      • Tomosugi N.
      • Kanda J.
      • Tanaka Y.
      • Yoshizaki K.
      • Uchiyama T.
      Anti-interleukin 6 receptor antibody tocilizumab reduces the level of serum hepcidin in patients with multicentric Castleman’s disease.
      ]. Large pharmacological doses of erythropoietin (EPO) can sometimes overcome the resistance of ACD to erythropoietin [
      • Elliott J.
      • Mishler D.
      • Agarwal R.
      Hyporesponsiveness to erythropoietin: causes and management.
      ]. In fact, EPO injections in mice and humans resulted in suppression of hepcidin production [
      • Ashby D.R.
      • Gale D.P.
      • Busbridge M.
      • Murphy K.G.
      • Duncan N.D.
      • Cairns T.D.
      • et al.
      Erythropoietin administration in humans causes a marked and prolonged reduction in circulating hepcidin.
      ,
      • Huang H.
      • Constante M.
      • Layoun A.
      • Santos M.M.
      Contribution of STAT3 and SMAD4 pathways to the regulation of hepcidin by opposing stimuli.
      ], and this may be the mechanism by which high EPO levels overcome iron restriction. In line with this EPO effect, ESA administration inhibits hepcidin expression both in the CKD population and the general population. [
      • Weiss G.
      • Theurl I.
      • Eder S.
      • Koppelstaetter C.
      • Kurz K.
      • Sonnweber T.
      • et al.
      Serum hepcidin concentration in chronic haemodialysis patients: associations and effects of dialysis, iron and erythropoietin therapy.
      ]. Recently, heparin and heparin-derivatives have been reported to inhibit hepcidin transcription by interfering with the BMP signaling pathway (Paolo Arosio, Milan, personal communications).
      BMP: bone morphogenetic proteins; FPN: ferroportin; IL6: interleukin 6; EPO: erythropoietin; ESA: erythropoiesis stimulating agents; HIF: hypoxia inducible factors.

      Conclusions

      In recent years, we have witnessed a dramatic progress in iron metabolism research and our understanding of human iron disorders has enormously improved. We have recognized hepcidin as the key regulator of iron homeostasis and a central pathogenic factor in a number of human diseases. We are now seeing medical applications of these advances. The notion that hepcidin excess or decrease may contribute to the dysregulation of iron homeostasis in hereditary and acquired iron disorders raises the possibility that hepcidin-lowering or enhancing agents may be an effective strategy for curing forms of anemia or iron excess, respectively (Box 2). Some pre-clinical and clinical studies have shown that hepcidin antibodies, BMP agonists or antagonists, cytokine receptor antibodies, and small-molecules can modify hepcidin expression and reverse iron abnormalities in vivo, in a number of disease models. How far are we from using hepcidin therapies? Due to the more profound understanding of pathogenic mechanisms involved in hyper-hepcidinemia of inflammatory states and a growing interest of pharmaceutical and biotech companies in ACD, it is likely that hepcidin antagonists will be the first to hit the market. In fact, as discussed earlier, anti-IL6 antibodies inhibiting hepcidin in patients with rheumatologic disorders and ACD are already in place. However, well-designed clinical studies addressing safety and long-term efficacy are still needed in order to clarify the risks and benefits of hepcidin-targeted treatments. Yet, a new era has started in the treatment of human disorders associated with iron disturbances. From the naive and archaic approach of bleeding our patients to remove iron excess or re-filling iron stores by using all kind of iron supplements, we are now moving to the development of intelligent molecular therapies able to modify iron entrance into the body, iron release from tissues or even iron movements within cells and organelles. This may offer new tools to treat or complement the treatment of a number of human disorders associated with iron excess, deficiency or misdistribution.

      Conflict of interest

      The author declared that he does not have anything to disclose regarding funding or conflict of interest with respect to this manuscript. The author does not have a relationship with the manufacturers of the drugs involved either in the past or present and did not receive funding from the manufacturers to carry out his research.

      Acknowledgement

      Grant support: This paper was supported by the PRIN 2008 Grant from the Italian National Research Council and Telethon grant GGP10233 .

      References

        • Krause A.
        • Neitz S.
        • Magert H.J.
        • Schulz A.
        • Forssmann W.G.
        • Schulz-Knappe P.
        • et al.
        LEAP-1, a novel highly disulfide-bonded human peptide, exhibits antimicrobial activity.
        FEBS Lett. 2000; 480: 147-150
        • Park C.H.
        • Valore E.V.
        • Waring A.J.
        • Ganz T.
        Hepcidin, a urinary antimicrobial peptide synthesized in the liver.
        J Biol Chem. 2001; 276: 7806-7810
        • Pigeon C.
        • Ilyin G.
        • Courselaud B.
        • Leroyer P.
        • Turlin B.
        • Brissot P.
        • et al.
        A new mouse liver-specific gene, encoding a protein homologous to human antimicrobial peptide hepcidin, is overexpressed during iron overload.
        J Biol Chem. 2001; 276: 7811-7819
        • Nicolas G.
        • Bennoun M.
        • Devaux I.
        • Beaumont C.
        • Grandchamp B.
        • Kahn A.
        • et al.
        Lack of hepcidin gene expression and severe tissue iron overload in upstream stimulatory factor 2 (USF2) knockout mice.
        Proc Natl Acad Sci USA. 2001; 98: 8780-8785
        • Courselaud B.
        • Pigeon C.
        • Inoue Y.
        • Inoue J.
        • Gonzalez F.J.
        • Leroyer P.
        • et al.
        C/EBPalpha regulates hepatic transcription of hepcidin, an antimicrobial peptide and regulator of iron metabolism. Cross-talk between C/EBP pathway and iron metabolism.
        J Biol Chem. 2002; 277: 41163-41170
        • Nicolas G.
        • Bennoun M.
        • Porteu A.
        • Mativet S.
        • Beaumont C.
        • Grandchamp B.
        • et al.
        Severe iron deficiency anemia in transgenic mice expressing liver hepcidin.
        Proc Natl Acad Sci USA. 2002; 99: 4596-4601
        • Bothwell T.H.
        The control of iron absorption.
        Br J Haematol. 1968; 14: 453-456
        • Bothwell T.H.
        • Hurtado A.V.
        • Donohue D.M.
        • Finch C.A.
        Erythrokinetics. IV. The plasma iron turnover as a measure of erythropoiesis.
        Blood. 1957; 12: 409-427
        • Finch C.A.
        Iron Balance in Man.
        Nutr Rev. 1965; 23: 129-131
        • Pietrangelo A.
        Hemochromatosis: an endocrine liver disease.
        Hepatology. 2007; 46: 1291-1301
        • Valore E.V.
        • Ganz T.
        Posttranslational processing of hepcidin in human hepatocytes is mediated by the prohormone convertase furin.
        Blood Cells Mol Dis. 2008; 40: 132-138
        • Abboud S.
        • Haile D.J.
        A novel mammalian iron-regulated protein involved in intracellular iron metabolism.
        J Biol Chem. 2000; 275: 19906-19912
        • Donovan A.
        • Brownlie A.
        • Zhou Y.
        • Shepard J.
        • Pratt S.J.
        • Moynihan J.
        • et al.
        Positional cloning of zebrafish ferroportin1 identifies a conserved vertebrate iron exporter.
        Nature. 2000; 403: 776-781
        • McKie A.T.
        • Marciani P.
        • Rolfs A.
        • Brennan K.
        • Wehr K.
        • Barrow D.
        • et al.
        A novel duodenal iron-regulated transporter, IREG1, implicated in the basolateral transfer of iron to the circulation.
        Mol Cell. 2000; 5: 299-309
        • Nemeth E.
        • Tuttle M.S.
        • Powelson J.
        • Vaughn M.B.
        • Donovan A.
        • Ward D.M.
        • et al.
        Hepcidin regulates cellular iron efflux by binding to ferroportin and inducing its internalization.
        Science. 2004; 306: 2090-2093
        • Nemeth E.
        • Preza G.C.
        • Jung C.L.
        • Kaplan J.
        • Waring A.J.
        • Ganz T.
        The N-terminus of hepcidin is essential for its interaction with ferroportin: structure-function study.
        Blood. 2006; 107: 328-333
        • De Domenico I.
        • Ward D.M.
        • Langelier C.
        • Vaughn M.B.
        • Nemeth E.
        • Sundquist W.I.
        • et al.
        The molecular mechanism of hepcidin-mediated ferroportin down-regulation.
        Mol Biol Cell. 2007; 18: 2569-2578
        • Vecchi C.
        • Montosi G.
        • Zhang K.
        • Lamberti I.
        • Duncan S.A.
        • Kaufman R.J.
        • et al.
        ER stress controls iron metabolism through induction of hepcidin.
        Science. 2009; 325: 877-880
        • Oliveira S.J.
        • Pinto J.P.
        • Picarote G.
        • Costa V.M.
        • Carvalho F.
        • Rangel M.
        • et al.
        ER stress-inducible factor CHOP affects the expression of hepcidin by modulating C/EBPalpha activity.
        PLoS One. 2009; 4: e6618
        • Nicolas G.
        • Chauvet C.
        • Viatte L.
        • Danan J.L.
        • Bigard X.
        • Devaux I.
        • et al.
        The gene encoding the iron regulatory peptide hepcidin is regulated by anemia, hypoxia, and inflammation.
        J Clin Invest. 2002; 110: 1037-1044
        • Peyssonnaux C.
        • Zinkernagel A.S.
        • Schuepbach R.A.
        • Rankin E.
        • Vaulont S.
        • Haase V.H.
        • et al.
        Regulation of iron homeostasis by the hypoxia-inducible transcription factors (HIFs).
        J Clin Invest. 2007; 117: 1926-1932
        • Pinto J.P.
        • Ribeiro S.
        • Pontes H.
        • Thowfeequ S.
        • Tosh D.
        • Carvalho F.
        • et al.
        Erythropoietin mediates hepcidin expression in hepatocytes through EPOR signaling and regulation of C/EBPalpha.
        Blood. 2008; 111: 5727-5733
        • Tanno T.
        • Bhanu N.V.
        • Oneal P.A.
        • Goh S.H.
        • Staker P.
        • Lee Y.T.
        • et al.
        High levels of GDF15 in thalassemia suppress expression of the iron regulatory protein hepcidin.
        Nat Med. 2007; 13: 1096-1101
        • Tanno T.
        • Porayette P.
        • Sripichai O.
        • Noh S.J.
        • Byrnes C.
        • Bhupatiraju A.
        • et al.
        Identification of TWSG1 as a second novel erythroid regulator of hepcidin expression in murine and human cells.
        Blood. 2009; 114: 181-186
        • Du X.
        • She E.
        • Gelbart T.
        • Truksa J.
        • Lee P.
        • Xia Y.
        • et al.
        The serine protease TMPRSS6 is required to sense iron deficiency.
        Science. 2008; 320: 1088-1092
        • Silvestri L.
        • Pagani A.
        • Nai A.
        • De Domenico I.
        • Kaplan J.
        • Camaschella C.
        The serine protease matriptase-2 (TMPRSS6) inhibits hepcidin activation by cleaving membrane hemojuvelin.
        Cell Metab. 2008; 8: 502-511
        • Nemeth E.
        • Rivera S.
        • Gabayan V.
        • Keller C.
        • Taudorf S.
        • Pedersen B.K.
        • et al.
        IL-6 mediates hypoferremia of inflammation by inducing the synthesis of the iron regulatory hormone hepcidin.
        J Clin Invest. 2004; 113: 1271-1276
        • Wrighting D.M.
        • Andrews N.C.
        Interleukin-6 induces hepcidin expression through STAT3.
        Blood. 2006; 108: 3204-3209
        • Verga Falzacappa M.V.
        • Vujic Spasic M.
        • Kessler R.
        • Stolte J.
        • Hentze M.W.
        • Muckenthaler M.U.
        STAT3 mediates hepatic hepcidin expression and its inflammatory stimulation.
        Blood. 2007; 109: 353-358
        • Pietrangelo A.
        • Dierssen U.
        • Valli L.
        • Garuti C.
        • Rump A.
        • Corradini E.
        • et al.
        STAT3 is required for IL-6-gp130-dependent activation of hepcidin in vivo.
        Gastroenterology. 2007; 132: 294-300
        • Wang R.H.
        • Li C.
        • Xu X.
        • Zheng Y.
        • Xiao C.
        • Zerfas P.
        • et al.
        A role of SMAD4 in iron metabolism through the positive regulation of hepcidin expression.
        Cell Metab. 2005; 2: 399-409
        • Yu P.B.
        • Hong C.C.
        • Sachidanandan C.
        • Babitt J.L.
        • Deng D.Y.
        • Hoyng S.A.
        • et al.
        Dorsomorphin inhibits BMP signals required for embryogenesis and iron metabolism.
        Nat Chem Biol. 2008; 4: 33-41
        • Babitt J.L.
        • Huang F.W.
        • Xia Y.
        • Sidis Y.
        • Andrews N.C.
        • Lin H.Y.
        Modulation of bone morphogenetic protein signaling in vivo regulates systemic iron balance.
        J Clin Invest. 2007; 117: 1933-1939
        • Verga Falzacappa M.V.
        • Casanovas G.
        • Hentze M.W.
        • Muckenthaler M.U.
        A bone morphogenetic protein (BMP)-responsive element in the hepcidin promoter controls HFE2-mediated hepatic hepcidin expression and its response to IL-6 in cultured cells.
        J Mol Med. 2008; 86: 531-540
        • Zhang K.
        • Shen X.
        • Wu J.
        • Sakaki K.
        • Saunders T.
        • Rutkowski D.T.
        • et al.
        Endoplasmic reticulum stress activates cleavage of CREBH to induce a systemic inflammatory response.
        Cell. 2006; 124: 587-599
        • Liu X.B.
        • Nguyen N.B.
        • Marquess K.D.
        • Yang F.
        • Haile D.J.
        Regulation of hepcidin and ferroportin expression by lipopolysaccharide in splenic macrophages.
        Blood Cells Mol Dis. 2005; 35: 47-56
        • Theurl I.
        • Theurl M.
        • Seifert M.
        • Mair S.
        • Nairz M.
        • Rumpold H.
        • et al.
        Autocrine formation of hepcidin induces iron retention in human monocytes.
        Blood. 2008; 111: 2392-2399
        • De Domenico I.
        • Zhang T.Y.
        • Koening C.L.
        • Branch R.W.
        • London N.
        • Lo E.
        • et al.
        Hepcidin mediates transcriptional changes that modulate acute cytokine-induced inflammatory responses in mice.
        J Clin Invest. 2010; 120: 2395-2405
        • Corradini E.
        • Babitt J.L.
        • Lin H.Y.
        The RGM/DRAGON family of BMP co-receptors.
        Cytokine Growth Factor Rev. 2009; 20: 389-398
        • Papanikolaou G.
        • Samuels M.E.
        • Ludwig E.H.
        • MacDonald M.L.
        • Franchini P.L.
        • Dube M.P.
        • et al.
        Mutations in HFE2 cause iron overload in chromosome 1q-linked juvenile hemochromatosis.
        Nat Genet. 2004; 36: 77-82
        • Lin L.
        • Goldberg Y.P.
        • Ganz T.
        Competitive regulation of hepcidin mRNA by soluble and cell-associated hemojuvelin.
        Blood. 2005; 106: 2884-2889
        • Zhang A.S.
        • West Jr., A.P.
        • Wyman A.E.
        • Bjorkman P.J.
        • Enns C.A.
        Interaction of hemojuvelin with neogenin results in iron accumulation in human embryonic kidney 293 cells.
        J Biol Chem. 2005; 280: 33885-33894
        • Yang F.
        • West Jr., A.P.
        • Allendorph G.P.
        • Choe S.
        • Bjorkman P.J.
        Neogenin interacts with hemojuvelin through its two membrane-proximal fibronectin type III domains.
        Biochemistry. 2008; 47: 4237-4245
        • Lee D.H.
        • Zhou L.J.
        • Zhou Z.
        • Xie J.X.
        • Jung J.U.
        • Liu Y.
        • et al.
        Neogenin inhibits HJV secretion and regulates BMP induced hepcidin expression and iron homeostasis.
        Blood. 2010; 115: 3136-3145
        • Xia Y.
        • Babitt J.L.
        • Sidis Y.
        • Chung R.T.
        • Lin H.Y.
        Hemojuvelin regulates hepcidin expression via a selective subset of BMP ligands and receptors independently of neogenin.
        Blood. 2008; 111: 5195-5204
        • Babitt J.L.
        • Huang F.W.
        • Wrighting D.M.
        • Xia Y.
        • Sidis Y.
        • Samad T.A.
        • et al.
        Bone morphogenetic protein signaling by hemojuvelin regulates hepcidin expression.
        Nat Genet. 2006; 38: 531-539
        • Kautz L.
        • Meynard D.
        • Monnier A.
        • Darnaud V.
        • Bouvet R.
        • Wang R.H.
        • et al.
        Iron regulates phosphorylation of Smad1/5/8 and gene expression of Bmp6, Smad7, Id1, and Atoh8 in the mouse liver.
        Blood. 2008; 112: 1503-1509
        • Mleczko-Sanecka K.
        • Casanovas G.
        • Ragab A.
        • Breitkopf K.
        • Muller A.
        • Boutros M.
        • et al.
        SMAD7 controls iron metabolism as a potent inhibitor of hepcidin expression.
        Blood. 2010; 115: 2657-2665
        • Andriopoulos Jr., B.
        • Corradini E.
        • Xia Y.
        • Faasse S.A.
        • Chen S.
        • Grgurevic L.
        • et al.
        BMP6 is a key endogenous regulator of hepcidin expression and iron metabolism.
        Nat Genet. 2009; 41: 482-487
        • Meynard D.
        • Kautz L.
        • Darnaud V.
        • Canonne-Hergaux F.
        • Coppin H.
        • Roth M.P.
        Lack of the bone morphogenetic protein BMP6 induces massive iron overload.
        Nat Genet. 2009; 41: 478-481
        • Feder J.N.
        • Penny D.M.
        • Irrinki A.
        • Lee V.K.
        • Lebrón J.A.
        • Watson N.
        • et al.
        The hemochromatosis gene product complexes with the transferrin receptor and lowers its affinity for ligand binding.
        Proc Nat Acad Sci USA. 1998; 95: 1472-1477
        • Ahmad K.A.
        • Ahmann J.R.
        • Migas M.C.
        • Waheed A.
        • Britton R.S.
        • Bacon B.R.
        • et al.
        Decreased liver hepcidin expression in the Hfe knockout mouse.
        Blood Cells Mol Dis. 2002; 29: 361-366
        • Bridle K.R.
        • Frazer D.M.
        • Wilkins S.J.
        • Dixon J.L.
        • Purdie D.M.
        • Crawford D.H.
        • et al.
        Disrupted hepcidin regulation in HFE-associated haemochromatosis and the liver as a regulator of body iron homoeostasis.
        Lancet. 2003; 361: 669-673
        • Gehrke S.G.
        • Kulaksiz H.
        • Herrmann T.
        • Riedel H.D.
        • Bents K.
        • Veltkamp C.
        • et al.
        Expression of hepcidin in hereditary hemochromatosis: evidence for a regulation in response to the serum transferrin saturation and to non-transferrin-bound iron.
        Blood. 2003; 102: 371-376
        • Vujic Spasic M.
        • Kiss J.
        • Herrmann T.
        • Galy B.
        • Martinache S.
        • Stolte J.
        • et al.
        Hfe acts in hepatocytes to prevent hemochromatosis.
        Cell Metab. 2008; 7: 173-178
        • Corradini E.
        • Garuti C.
        • Montosi G.
        • Ventura P.
        • Andriopoulos Jr., B.
        • Lin H.Y.
        • et al.
        Bone morphogenetic protein signaling is impaired in an HFE knockout mouse model of hemochromatosis.
        Gastroenterology. 2009; 137: 1489-1497
        • Kautz L.
        • Meynard D.
        • Besson-Fournier C.
        • Darnaud V.
        • Al Saati T.
        • Coppin H.
        • et al.
        BMP/Smad signaling is not enhanced in Hfe-deficient mice despite increased Bmp6 expression.
        Blood. 2009; 114: 2515-2520
        • Kawabata H.
        • Fleming R.E.
        • Gui D.
        • Moon S.Y.
        • Saitoh T.
        • O’Kelly J.
        • et al.
        Expression of hepcidin is down-regulated in TfR2 mutant mice manifesting a phenotype of hereditary hemochromatosis.
        Blood. 2005; 105: 376-381
        • Nemeth E.
        • Roetto A.
        • Garozzo G.
        • Ganz T.
        • Camaschella C.
        Hepcidin is decreased in TFR2-Hemochromatosis.
        Blood. 2005; 105: 1803-1806
        • Griffiths W.J.
        • Cox T.M.
        Co-localization of the mammalian hemochromatosis gene product (HFE) and a newly identified transferrin receptor (TfR2) in intestinal tissue and cells.
        J Histochem Cytochem. 2003; 51: 613-624
        • Goswami T.
        • Andrews N.C.
        Hereditary hemochromatosis protein, HFE, interaction with transferrin receptor 2 suggests a molecular mechanism for mammalian iron sensing.
        J Biol Chem. 2006; 281: 28494-28498
        • Waheed A.
        • Britton R.S.
        • Grubb J.H.
        • Sly W.S.
        • Fleming R.E.
        HFE association with transferrin receptor 2 increases cellular uptake of transferrin-bound iron.
        Arch Biochem Biophys. 2008; 474: 193-197
        • Chen J.
        • Chloupkova M.
        • Gao J.
        • Chapman-Arvedson T.L.
        • Enns C.A.
        HFE modulates transferrin receptor 2 levels in hepatoma cells via interactions that differ from transferrin receptor 1-HFE interactions.
        J Biol Chem. 2007; 282: 36862-36870
        • Schmidt P.J.
        • Toran P.T.
        • Giannetti A.M.
        • Bjorkman P.J.
        • Andrews N.C.
        The transferrin receptor modulates hfe-dependent regulation of hepcidin expression.
        Cell Metab. 2008; 7: 205-214
        • Pietrangelo A.
        Iron chelation beyond transfusion iron overload.
        Am J Hematol. 2007; 82: 1142-1146
        • Campuzano V.
        • Montermini L.
        • Molto M.D.
        • Pianese L.
        • Cossee M.
        • Cavalcanti F.
        • et al.
        Friedreich’s ataxia: autosomal recessive disease caused by an intronic GAA triplet repeat expansion.
        Science. 1996; 271: 1423-1427
        • Bridle K.
        • Cheung T.K.
        • Murphy T.
        • Walters M.
        • Anderson G.
        • Crawford D.G.
        • et al.
        Hepcidin is down-regulated in alcoholic liver injury: implications for the pathogenesis of alcoholic liver disease.
        Alcohol Clin Exp Res. 2006; 30: 106-112
        • Harrison-Findik D.D.
        • Schafer D.
        • Klein E.
        • Timchenko N.A.
        • Kulaksiz H.
        • Clemens D.
        • et al.
        Alcohol metabolism-mediated oxidative stress down-regulates hepcidin transcription and leads to increased duodenal iron transporter expression.
        J Biol Chem. 2006; 281: 22974-22982
        • Nishina S.
        • Hino K.
        • Korenaga M.
        • Vecchi C.
        • Pietrangelo A.
        • Mizukami Y.
        • et al.
        Hepatitis C virus-induced reactive oxygen species raise hepatic iron level in mice by reducing hepcidin transcription.
        Gastroenterology. 2008; 134: 226-238
        • Pietrangelo A.
        Hereditary hemochromatosis: pathogenesis, diagnosis and treatment.
        Gastroenterology. 2010; 139: 393-408
        • Feder J.N.
        • Gnirke A.
        • Thomas W.
        • Tsuchihashi Z.
        • Ruddy D.A.
        • Basava A.
        • et al.
        A novel MHC class I-like gene is mutated in patients with hereditary haemochromatosis.
        Nat Genet. 1996; 13: 399-408
        • Camaschella C.
        • Roetto A.
        • Cali A.
        • De Gobbi M.
        • Garozzo G.
        • Carella M.
        • et al.
        The gene TFR2 is mutated in a new type of haemochromatosis mapping to 7q22.
        Nat Genet. 2000; 25: 14-15
        • Njajou O.T.
        • Vaessen N.
        • Joosse M.
        • Berghuis B.
        • van Dongen J.W.
        • Breuning M.H.
        • et al.
        A mutation in SLC11A3 is associated with autosomal dominant hemochromatosis.
        Nat Genet. 2001; 28: 213-214
        • Roetto A.
        • Papanikolaou G.
        • Politou M.
        • Alberti F.
        • Girelli D.
        • Christakis J.
        • et al.
        Mutant antimicrobial peptide hepcidin is associated with severe juvenile hemochromatosis.
        Nat Genet. 2003; 33: 21-22
        • Nemeth E.
        • Roetto A.
        • Garozzo G.
        • Ganz T.
        • Camaschella C.
        Hepcidin is decreased in TFR2 hemochromatosis.
        Blood. 2005; 105: 1803-1806
        • Kattamis A.
        • Papassotiriou I.
        • Palaiologou D.
        • Apostolakou F.
        • Galani A.
        • Ladis V.
        • et al.
        The effects of erythropoetic activity and iron burden on hepcidin expression in patients with thalassemia major.
        Haematologica. 2006; 91: 809-812
      1. EASL. EASL Clinical Practice Guidelines on Hemochromatosis. J Hepatol 2010.

        • Corradini E.
        • Schmidt P.J.
        • Meynard D.
        • Garuti C.
        • Montosi G.
        • Chen S.
        • et al.
        BMP6 treatment compensates for the molecular defect and ameliorates hemochromatosis in Hfe knockout mice.
        Gastroenterology. 2010; https://doi.org/10.1053/j.gastro.2010.07.044
        • Kearney S.L.
        • Nemeth E.
        • Neufeld E.J.
        • Thapa D.
        • Ganz T.
        • Weinstein D.A.
        • et al.
        Urinary hepcidin in congenital chronic anemias.
        Pediatr Blood Cancer. 2007; 48: 57-63
        • Weiss G.
        Iron metabolism in the anemia of chronic disease.
        Biochim Biophys Acta. 2009; 1790: 682-693
        • Nemeth E.
        Targeting the hepcidin–ferroportin axis in the diagnosis and treatment of anemias.
        Adv Hematol. 2010; 2010: 750643
        • Babitt J.L.
        • Lin H.Y.
        Molecular mechanisms of hepcidin regulation: implications for the anemia of CKD.
        Am J Kidney Dis. 2010; 55: 726-741
        • Zaritsky J.
        • Young B.
        • Wang H.J.
        • Westerman M.
        • Olbina G.
        • Nemeth E.
        • et al.
        Hepcidin–a potential novel biomarker for iron status in chronic kidney disease.
        Clin J Am Soc Nephrol. 2009; 4: 1051-1056
        • Elliott J.
        • Mishler D.
        • Agarwal R.
        Hyporesponsiveness to erythropoietin: causes and management.
        Adv Chronic Kidney Dis. 2009; 16: 94-100
        • Melis M.A.
        • Cau M.
        • Congiu R.
        • Sole G.
        • Barella S.
        • Cao A.
        • et al.
        A mutation in the TMPRSS6 gene, encoding a transmembrane serine protease that suppresses hepcidin production, in familial iron deficiency anemia refractory to oral iron.
        Haematologica. 2008; 93: 1473-1479
        • Finberg K.E.
        • Heeney M.M.
        • Campagna D.R.
        • Aydinok Y.
        • Pearson H.A.
        • Hartman K.R.
        • et al.
        Mutations in TMPRSS6 cause iron-refractory iron deficiency anemia (IRIDA).
        Nat Genet. 2008; 40: 569-571
        • Weinstein D.A.
        • Roy C.N.
        • Fleming M.D.
        • Loda M.F.
        • Wolfsdorf J.I.
        • Andrews N.C.
        Inappropriate expression of hepcidin is associated with iron refractory anemia: implications for the anemia of chronic disease.
        Blood. 2002; 100: 3776-3781
        • Chung A.
        • Leo K.
        • Wong G.
        • Chuah K.
        • Ren J.
        • Lee C.
        Giant hepatocellular adenoma presenting with chronic iron deficiency anemia.
        Am J Gastroenterol. 2006; 101: 2160-2162
        • Sasu B.J.
        • Cooke K.S.
        • Arvedson T.L.
        • Plewa C.
        • Ellison A.R.
        • Sheng J.
        • et al.
        Anti-hepcidin antibody treatment modulates iron metabolism and is effective in a mouse model of inflammation-induced anemia.
        Blood. 2010; 115: 3616-3624
        • Hashizume M.
        • Uchiyama Y.
        • Horai N.
        • Tomosugi N.
        • Mihara M.
        Tocilizumab, a humanized anti-interleukin-6 receptor antibody, improved anemia in monkey arthritis by suppressing IL-6-induced hepcidin production.
        Rheumatol Int. 2010; 30: 917-923
        • Kawabata H.
        • Tomosugi N.
        • Kanda J.
        • Tanaka Y.
        • Yoshizaki K.
        • Uchiyama T.
        Anti-interleukin 6 receptor antibody tocilizumab reduces the level of serum hepcidin in patients with multicentric Castleman’s disease.
        Haematologica. 2007; 92: 857-858
        • Ashby D.R.
        • Gale D.P.
        • Busbridge M.
        • Murphy K.G.
        • Duncan N.D.
        • Cairns T.D.
        • et al.
        Erythropoietin administration in humans causes a marked and prolonged reduction in circulating hepcidin.
        Haematologica. 2010; 95: 505-508
        • Huang H.
        • Constante M.
        • Layoun A.
        • Santos M.M.
        Contribution of STAT3 and SMAD4 pathways to the regulation of hepcidin by opposing stimuli.
        Blood. 2009; 113: 3593-3599
        • Weiss G.
        • Theurl I.
        • Eder S.
        • Koppelstaetter C.
        • Kurz K.
        • Sonnweber T.
        • et al.
        Serum hepcidin concentration in chronic haemodialysis patients: associations and effects of dialysis, iron and erythropoietin therapy.
        Eur J Clin Invest. 2009; 39: 883-890