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Juvenile hemochromatosis
Journal of Hepatology 45 (2006) 892–894 www.elsevier.com/locate/jhep
Journal Club Special Section Editors: Peter R. Galle, Peter L.M. Jansen, Francesco Negro
Juvenile hemochromatosis Antonello Pietrangelo* Center for Hemochromatosis, Department of Internal Medicine, University of Modena and Reggio Emilia, Policlinico Via del Pozzo 71 41100 Modena, Italy
Bone morphogenetic protein signaling by hemojuvelin regulates hepcidin expression. Babitt JL, Huang FW, Wrighting DM, Xia Y, Sidis Y, Samad TA, Campagna JA, Chung RT, Schneyer AL, Woolf CJ, Andrews NC, Lin HY. Hepcidin is a key regulator of systemic iron homeostasis. Hepcidin deficiency induces iron overload, whereas hepcidin excess induces anemia. Mutations in the gene encoding hemojuvelin (HFE2, also known as HJV) cause severe iron overload and correlate with low hepcidin levels, suggesting that hemojuvelin positively regulates hepcidin expression. Hemojuvelin is a member of the repulsive guidance molecule (RGM) family, which also includes the bone morphogenetic protein (BMP) coreceptors RGMA and DRAGON (RGMB). Here, we report that hemojuvelin is a BMP coreceptor and that hemojuvelin mutants associated with hemochromatosis have impaired BMP signaling ability. Furthermore, BMP upregulates hepatocyte hepcidin expression, a process enhanced by hemojuvelin and blunted in Hfe2 / hepatocytes. Our data suggest a mechanism by which HFE2 mutations cause hemochromatosis: hemojuvelin dysfunction decreases BMP signaling, thereby lowering hepcidin expression. [Abstract reproduced by permission of Nat Genet 2006;38:531–539]
The rather vague term, ‘‘juvenile hemochromatosis’’, has been used for decades to refer to a distinct form of hereditary iron overload with a development
Available online 25 September 2006 * Tel.: +39 059 4222714; fax: +39 059 4224363. E-mail address: [email protected]
pattern resembling that of adult hemochromatosis but more rapidly progressive [1]. Because of the higher rate of iron loading associated with this disorder, cardiomyopathy and endocrinopathy (particularly hypogonadism) appear earlier than they do in adult hemochromatosis, and death before the age of 30 is not uncommon. Most juvenile-onset cases have been mapped to chromosome 1q, where the HJV (originally HFE2 or repulsive guidance molecule C, RmgC) gene has recently been identified [2]. Hemojuvelin, the product of HJV, is expressed in adult and fetal liver, heart and skeletal muscle [2], and shares considerable sequence similarity with RMGs involved in neural development. The putative full-length protein is 426 amino acids; it contains a C-terminal transmembrane domain characteristic of a glycosylphosphatidylinositol-linked membrane anchor (GPI-anchor), suggesting that it can be present in either a soluble or a cell-associated form. The function of hemojuvelin was elusive until recently, when it was reported that hepcidin levels are depressed in individuals with HJV mutations [2] and in HJV knock-out mice [3], and that HJV is a transcriptional regulator of hepcidin [4]. Hepcidin, a small peptide produced by the hepatocytes, is the master regulator of iron homeostasis: it normally down-regulates iron efflux from enterocytes, macrophages and placenta by binding and degrading the main iron exporter, ferroportin [5]. Lack of hepcidin, the product of the HAMP gene, would lead to the unrestricted release of iron from storage cells and cause circulatory and eventually tissue iron overload, that is, hemochromatosis. In fact, rare cases of juvenile hemochromatosis have been linked to HAMP mutations [6]. Despite all these new advancements, an unsolved issue still remained: how a RGM like protein
0168-8278/$32.00 Ó 2006 European Association for the Study of the Liver. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jhep.2006.09.003
A. Pietrangelo / Journal of Hepatology 45 (2006) 892–894
in the liver can control the expression of a major hepatocyte gene product, hepcidin? The first clue came recently from the finding that a CreloxP-mediated liver-specific disruption of SMAD4, a transcription factor involved in transforming growth factor-b (TGFb) superfamily signaling, resulted in markedly decreased hepcidin expression and accumulation of iron in many organs, particularly in the liver and pancreas [7]. SMAD4 is the terminal transcriptional effector of a signaling cascade initiated at the cell membrane by interaction of specific ligands with cell receptors. Bone morphogenic proteins (BMPs) represent a large subfamily of the (TGF-b) superfamily of ligands: signaling is initiated when BMP binds to complexes of two type I and two type II serine/ threonine kinase receptors. Constitutively active type II receptors phosphorylate type I receptors, which phosphorylate Smad proteins. Phosphorylated receptor-activated Smads form heteromeric complexes with common mediator Smad4, and the Smad complexes translocate to the nucleus where they modulate gene transcription. Regulation of this pathway occurs at multiple levels in order to generate specificity and to finely tune these signals. One key regulatory mechanism is the promotion or inhibition of ligand binding by coreceptors. Babitt et al. [8] had recently discovered that RGM family members, DRAGON (RGMB), and RGMA, function as coreceptors that enhance BMP signaling. Hemojuvelin shares 50–60% amino acid identity and key structural features with RGMA and DRAGON. Now, Babitt et al. have reported that hemojuvelin is indeed a coreceptor that enhances BMP signaling in the hepatocyte via the classical BMP pathway, involving BMP ligands, BMP receptors and BMP receptor-activated Smads [9]. They found that hemojuvelin enhances BMP signaling in hepatoma cells by selectively interacting with BMP-2 and that the signaling activity requires BMP type I receptors. They also demonstrate that BMP-2 positively regulates hepcidin expression at the transcriptional level and that hemojuvelin enhances hepcidin induction in response to BMP-2. Finally HJV / hepatocytes showed a significantly reduced induction of hepcidin expression in response to BMP-2 and livers of HJV / mice had reduced levels of phosphorylated Smad1/5/8, indicating that the absence of hemojuvelin results in lower hepatic BMP signaling in vivo. They therefore propose that hemojuvelin mediated BMP signaling is an important mechanism for regulating hepcidin expression and iron homeostasis. Loss of hemojuvelin function leads to decreased BMP signaling in liver cells, which then decreases hepcidin expression. Impaired regulation by hepcidin leads to ferroportin overactivity, thereby resulting in increased intestinal iron absorption, increased macrophage iron release, elevated serum iron and abnormal
893
tissue iron deposition. There are still important aspects that need to be addressed by future studies: what is the nature of the endogenous BMP/TGF-b superfamily ligand(s) through which hemojuvelin regulates hepatic hepcidin expression in vivo? do inflammatory mediators, which are known to activate hepcidin transcription, act independently of hemojuvelin to regulate hepcidin? Hemojuvelin is also highly expressed in skeletal and cardiac muscle. As recent data have suggested that human sera contain soluble hemojuvelin and that soluble hemojuvelin can inhibit the expression of hepcidin mRNA [4], can then soluble hemojuvelin bind and sequester BMP ligands to inhibit both BMP signaling and hepcidin expression? However, there is a burning question that has even more important implications. Since circulatory iron overload is a common finding in all forms of hemochromatosis and also adult onset forms of hemochromatosis due to HFE and transferrin receptor 2 (TfR2) mutations present low levels of hepcidin, a unifying pathogenic model was proposed where hepcidin deficiency is the central pathogenic factor for all forms of hemochromatosis [1]. The relative contributions of HJV, HFE and TFR2 to the modulatory activity of hepcidin expression may be different, with a more substantial role assigned to HJV based on the more severe iron overload phenotype associated with HJV mutations [1]. Now, do HFE and TfR2 participate at some level and to some extent to the signaling pathway reported above for hemojuvelin? While we can easily predict that most if not all these questions will be answered in the near future, the study by Babitt et al. [9] stands today as a breakthrough in science as it has solved the basic nature of the mysterious and elusive ‘‘juvenile hemochromatosis’’ and paved the way for new exciting discoveries that will have important reflections in liver cell biology and pathobiology.
References [1] Pietrangelo A. Hereditary hemochromatosis – a new look at an old disease. N Engl J Med 2004;350:2383–2397. [2] Papanikolaou G, Samuels ME, Ludwig EH, MacDonald ML, Franchini PL, Dube MP, et al. Mutations in HFE2 cause iron overload in chromosome 1q-linked juvenile hemochromatosis. Nat Genet 2004;36:77–82. [3] Huang FW, Pinkus JL, Pinkus GS, Fleming MD, Andrews NC. A mouse model of juvenile hemochromatosis. J Clin Invest 2005;115:2187–2191. [4] Lin L, Goldberg YP, Ganz T. Competitive regulation of hepcidin mRNA by soluble and cell-associated hemojuvelin. Blood 2005;106:2884–2889. [5] Nemeth E, Tuttle MS, Powelson J, Vaughn MB, Donovan A, Ward DM, et al. Hepcidin regulates cellular iron efflux by binding to ferroportin and inducing its internalization. Science 2004;306:2090–2093.
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A. Pietrangelo / Journal of Hepatology 45 (2006) 892–894
[6] 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. [7] Wang RH, 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.
[8] Babitt JL, Zhang Y, Samad TA, Xia Y, Tang J, Campagna JA, et al. Repulsive guidance molecule (RGMa), a DRAGON homologue, is a bone morphogenetic protein co-receptor. J Biol Chem 2005;280:29820–29827. [9] Babitt JL, Huang FW, Wrighting DM, Xia Y, Sidis Y, Samad TA, et al. Bone morphogenetic protein signaling by hemojuvelin regulates hepcidin expression. Nat Genet 2006;38:531–539.
Journal Club Special Section Editors: Peter R. Galle, Peter L.M. Jansen, Francesco Negro
Juvenile hemochromatosis Antonello Pietrangelo* Center for Hemochromatosis, Department of Internal Medicine, University of Modena and Reggio Emilia, Policlinico Via del Pozzo 71 41100 Modena, Italy
Bone morphogenetic protein signaling by hemojuvelin regulates hepcidin expression. Babitt JL, Huang FW, Wrighting DM, Xia Y, Sidis Y, Samad TA, Campagna JA, Chung RT, Schneyer AL, Woolf CJ, Andrews NC, Lin HY. Hepcidin is a key regulator of systemic iron homeostasis. Hepcidin deficiency induces iron overload, whereas hepcidin excess induces anemia. Mutations in the gene encoding hemojuvelin (HFE2, also known as HJV) cause severe iron overload and correlate with low hepcidin levels, suggesting that hemojuvelin positively regulates hepcidin expression. Hemojuvelin is a member of the repulsive guidance molecule (RGM) family, which also includes the bone morphogenetic protein (BMP) coreceptors RGMA and DRAGON (RGMB). Here, we report that hemojuvelin is a BMP coreceptor and that hemojuvelin mutants associated with hemochromatosis have impaired BMP signaling ability. Furthermore, BMP upregulates hepatocyte hepcidin expression, a process enhanced by hemojuvelin and blunted in Hfe2 / hepatocytes. Our data suggest a mechanism by which HFE2 mutations cause hemochromatosis: hemojuvelin dysfunction decreases BMP signaling, thereby lowering hepcidin expression. [Abstract reproduced by permission of Nat Genet 2006;38:531–539]
The rather vague term, ‘‘juvenile hemochromatosis’’, has been used for decades to refer to a distinct form of hereditary iron overload with a development
Available online 25 September 2006 * Tel.: +39 059 4222714; fax: +39 059 4224363. E-mail address: [email protected]
pattern resembling that of adult hemochromatosis but more rapidly progressive [1]. Because of the higher rate of iron loading associated with this disorder, cardiomyopathy and endocrinopathy (particularly hypogonadism) appear earlier than they do in adult hemochromatosis, and death before the age of 30 is not uncommon. Most juvenile-onset cases have been mapped to chromosome 1q, where the HJV (originally HFE2 or repulsive guidance molecule C, RmgC) gene has recently been identified [2]. Hemojuvelin, the product of HJV, is expressed in adult and fetal liver, heart and skeletal muscle [2], and shares considerable sequence similarity with RMGs involved in neural development. The putative full-length protein is 426 amino acids; it contains a C-terminal transmembrane domain characteristic of a glycosylphosphatidylinositol-linked membrane anchor (GPI-anchor), suggesting that it can be present in either a soluble or a cell-associated form. The function of hemojuvelin was elusive until recently, when it was reported that hepcidin levels are depressed in individuals with HJV mutations [2] and in HJV knock-out mice [3], and that HJV is a transcriptional regulator of hepcidin [4]. Hepcidin, a small peptide produced by the hepatocytes, is the master regulator of iron homeostasis: it normally down-regulates iron efflux from enterocytes, macrophages and placenta by binding and degrading the main iron exporter, ferroportin [5]. Lack of hepcidin, the product of the HAMP gene, would lead to the unrestricted release of iron from storage cells and cause circulatory and eventually tissue iron overload, that is, hemochromatosis. In fact, rare cases of juvenile hemochromatosis have been linked to HAMP mutations [6]. Despite all these new advancements, an unsolved issue still remained: how a RGM like protein
0168-8278/$32.00 Ó 2006 European Association for the Study of the Liver. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jhep.2006.09.003
A. Pietrangelo / Journal of Hepatology 45 (2006) 892–894
in the liver can control the expression of a major hepatocyte gene product, hepcidin? The first clue came recently from the finding that a CreloxP-mediated liver-specific disruption of SMAD4, a transcription factor involved in transforming growth factor-b (TGFb) superfamily signaling, resulted in markedly decreased hepcidin expression and accumulation of iron in many organs, particularly in the liver and pancreas [7]. SMAD4 is the terminal transcriptional effector of a signaling cascade initiated at the cell membrane by interaction of specific ligands with cell receptors. Bone morphogenic proteins (BMPs) represent a large subfamily of the (TGF-b) superfamily of ligands: signaling is initiated when BMP binds to complexes of two type I and two type II serine/ threonine kinase receptors. Constitutively active type II receptors phosphorylate type I receptors, which phosphorylate Smad proteins. Phosphorylated receptor-activated Smads form heteromeric complexes with common mediator Smad4, and the Smad complexes translocate to the nucleus where they modulate gene transcription. Regulation of this pathway occurs at multiple levels in order to generate specificity and to finely tune these signals. One key regulatory mechanism is the promotion or inhibition of ligand binding by coreceptors. Babitt et al. [8] had recently discovered that RGM family members, DRAGON (RGMB), and RGMA, function as coreceptors that enhance BMP signaling. Hemojuvelin shares 50–60% amino acid identity and key structural features with RGMA and DRAGON. Now, Babitt et al. have reported that hemojuvelin is indeed a coreceptor that enhances BMP signaling in the hepatocyte via the classical BMP pathway, involving BMP ligands, BMP receptors and BMP receptor-activated Smads [9]. They found that hemojuvelin enhances BMP signaling in hepatoma cells by selectively interacting with BMP-2 and that the signaling activity requires BMP type I receptors. They also demonstrate that BMP-2 positively regulates hepcidin expression at the transcriptional level and that hemojuvelin enhances hepcidin induction in response to BMP-2. Finally HJV / hepatocytes showed a significantly reduced induction of hepcidin expression in response to BMP-2 and livers of HJV / mice had reduced levels of phosphorylated Smad1/5/8, indicating that the absence of hemojuvelin results in lower hepatic BMP signaling in vivo. They therefore propose that hemojuvelin mediated BMP signaling is an important mechanism for regulating hepcidin expression and iron homeostasis. Loss of hemojuvelin function leads to decreased BMP signaling in liver cells, which then decreases hepcidin expression. Impaired regulation by hepcidin leads to ferroportin overactivity, thereby resulting in increased intestinal iron absorption, increased macrophage iron release, elevated serum iron and abnormal
893
tissue iron deposition. There are still important aspects that need to be addressed by future studies: what is the nature of the endogenous BMP/TGF-b superfamily ligand(s) through which hemojuvelin regulates hepatic hepcidin expression in vivo? do inflammatory mediators, which are known to activate hepcidin transcription, act independently of hemojuvelin to regulate hepcidin? Hemojuvelin is also highly expressed in skeletal and cardiac muscle. As recent data have suggested that human sera contain soluble hemojuvelin and that soluble hemojuvelin can inhibit the expression of hepcidin mRNA [4], can then soluble hemojuvelin bind and sequester BMP ligands to inhibit both BMP signaling and hepcidin expression? However, there is a burning question that has even more important implications. Since circulatory iron overload is a common finding in all forms of hemochromatosis and also adult onset forms of hemochromatosis due to HFE and transferrin receptor 2 (TfR2) mutations present low levels of hepcidin, a unifying pathogenic model was proposed where hepcidin deficiency is the central pathogenic factor for all forms of hemochromatosis [1]. The relative contributions of HJV, HFE and TFR2 to the modulatory activity of hepcidin expression may be different, with a more substantial role assigned to HJV based on the more severe iron overload phenotype associated with HJV mutations [1]. Now, do HFE and TfR2 participate at some level and to some extent to the signaling pathway reported above for hemojuvelin? While we can easily predict that most if not all these questions will be answered in the near future, the study by Babitt et al. [9] stands today as a breakthrough in science as it has solved the basic nature of the mysterious and elusive ‘‘juvenile hemochromatosis’’ and paved the way for new exciting discoveries that will have important reflections in liver cell biology and pathobiology.
References [1] Pietrangelo A. Hereditary hemochromatosis – a new look at an old disease. N Engl J Med 2004;350:2383–2397. [2] Papanikolaou G, Samuels ME, Ludwig EH, MacDonald ML, Franchini PL, Dube MP, et al. Mutations in HFE2 cause iron overload in chromosome 1q-linked juvenile hemochromatosis. Nat Genet 2004;36:77–82. [3] Huang FW, Pinkus JL, Pinkus GS, Fleming MD, Andrews NC. A mouse model of juvenile hemochromatosis. J Clin Invest 2005;115:2187–2191. [4] Lin L, Goldberg YP, Ganz T. Competitive regulation of hepcidin mRNA by soluble and cell-associated hemojuvelin. Blood 2005;106:2884–2889. [5] Nemeth E, Tuttle MS, Powelson J, Vaughn MB, Donovan A, Ward DM, et al. Hepcidin regulates cellular iron efflux by binding to ferroportin and inducing its internalization. Science 2004;306:2090–2093.
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A. Pietrangelo / Journal of Hepatology 45 (2006) 892–894
[6] 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. [7] Wang RH, 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.
[8] Babitt JL, Zhang Y, Samad TA, Xia Y, Tang J, Campagna JA, et al. Repulsive guidance molecule (RGMa), a DRAGON homologue, is a bone morphogenetic protein co-receptor. J Biol Chem 2005;280:29820–29827. [9] Babitt JL, Huang FW, Wrighting DM, Xia Y, Sidis Y, Samad TA, et al. Bone morphogenetic protein signaling by hemojuvelin regulates hepcidin expression. Nat Genet 2006;38:531–539.