No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Altered hepatic BMP signaling pathway in human HFE hemochromatosis
Abstract
Human hemochromatosis (HC) has been associated with the common C282Y polymorphism of HFE or rare pathogenic mutations of TfR2, HJV, FPN and HAMP. All forms of human HC seem to be caused by low or inadequate levels of hepcidin, the iron hormone. We and others have recently shown that Hfe(-/-) mice exhibit an impairment in the bone morphogenetic protein (BMP) signaling pathway controlling hepcidin. However, all data indicating the central role of BMPs in hepcidin regulation and an impaired BMP/SMAD signaling in HC have been collected in mice. In this study we investigated whether also in humans the expression of BMP signaling targets, SMAD7 and Id1, are associated with liver iron concentration (LIC) and whether such regulation is disrupted in HFE-HC. We correlated the mRNA expression, assessed by RT-PCR, of HAMP, SMAD7 and Id1 with LIC in liver biopsies from patients with normal iron status, HFE-HC or non-HC hepatic iron overload. We found that in human liver, not only HAMP, but also SMAD7 and Id1 mRNA significantly correlate with the extent of hepatic iron burden. However, this correlation is lost in patients with HFE-HC, but maintained in subjects with non-hemochromatotic iron overload. These data indicate that in human HFE-HC a disrupted BMP/SMAD signaling in the liver is key in the pathogenesis of the disease.
... 80,81 In these disorders, excess iron deposits in organs such as the liver, heart, pancreas, and other endocrine glands leading to tissue injury and dysfunction, which are the leading cause of morbidity and mortality in these diseases. 82 Hereditary hemochromatosis is caused by mutations in the genes encoding hepcidin (HAMP) itself, or other proteins involved in hepcidin regulation by iron, including homeostatic iron regulator (HFE), transferrin receptor 2 (TFR2), and HJV, which appear to exert their effects through functional interactions with the BMP-SMAD signaling pathway (discussed in more detail below 39,61,79,[83][84][85][86][87][88][89][90][91]. β-thalassemia is an inherited hemoglobin disorder caused by mutations in the HBB gene encoding β-globin, which causes ineffective erythropoiesis as a result of decreased β-globin production. ...
... Mutation or ablation of HFE or TFR2 leads to a reduction, whereas overexpression leads to an increase, in liver SMAD1,5,8 phosphorylation and BMP-SMAD1,5,8 target transcript expression in mice and/or humans. [86][87][88][89][90]97 Coimmunoprecipitation studies using overexpressed proteins in cell culture systems have also demonstrated that HFE and TFR2 can interact with HJV. 172 HFE has also been shown to interact with ALK3, which is reported to stabilize ALK3 by inhibiting its ubiquitination and proteasomal degradation. ...
Iron homeostasis is tightly regulated to balance the iron requirement for erythropoiesis and other vital cellular functions, while preventing cellular injury from iron excess. The liver hormone hepcidin is the master regulator of systemic iron balance by controlling the degradation and function of the sole known mammalian iron exporter ferroportin. Liver hepcidin expression is coordinately regulated by several signals that indicate the need for more or less iron, including plasma and tissue iron levels, inflammation, and erythropoietic drive. Most of these signals regulate hepcidin expression by modulating the activity of the bone morphogenetic protein (BMP)‐SMAD pathway, which controls hepcidin transcription. Genetic disorders of iron overload and iron deficiency have identified several hepatocyte membrane proteins that play a critical role in mediating the BMP‐SMAD and hepcidin regulatory response to iron. However, the precise molecular mechanisms by which serum and tissue iron levels are sensed to regulate BMP ligand production and promote the physical and/or functional interaction of these proteins to modulate SMAD signaling and hepcidin expression remain uncertain. This critical commentary will focus on the current understanding and key unanswered questions regarding how the liver senses iron levels to regulate BMP‐SMAD signaling and thereby hepcidin expression to control systemic iron homeostasis.
... Here, HJV is essential for amplifying the strength of BMP6/SMAD signaling [19, 20]. Interestingly, HFE inactivation has also been associated with defective SMAD signaling in humans [21, 22] and mice [23, 24] , indicating a crosstalk between the pathways involving HFE and HJV. To examine whether Hfe and Hjv operate in conjunction or in parallel for iron signaling to hepcidin, we crossed isogenic Hfe − / − and Hjv − / − mice and analyzed iron metabolism in the progeny. ...
... Moreover, recent studies cast doubt on the previously reported HFE/TfR2 interaction [14, 15]. Since the lack of functional Hfe is associated with impaired Smad signaling21222324, both Hjv and Hfe can be viewed as co-factors in this pathway for iron-dependent activation of hepcidin. This notion is also supported by genetic data showing that induction of hepcidin via hepatocyte-specific transgenic overexpression of Hfe is abolished in an Hjv − / − [34], but not Tfr2 − / − [14] background. ...
... Here, HJV is essential for amplifying the strength of BMP6/SMAD signaling [19, 20]. Interestingly, HFE inactivation has also been associated with defective SMAD signaling in humans [21, 22] and mice [23, 24] , indicating a crosstalk between the pathways involving HFE and HJV. To examine whether Hfe and Hjv operate in conjunction or in parallel for iron signaling to hepcidin, we crossed isogenic Hfe − / − and Hjv − / − mice and analyzed iron metabolism in the progeny. ...
... Moreover, recent studies cast doubt on the previously reported HFE/TfR2 interaction [14, 15]. Since the lack of functional Hfe is associated with impaired Smad signaling21222324, both Hjv and Hfe can be viewed as co-factors in this pathway for iron-dependent activation of hepcidin. This notion is also supported by genetic data showing that induction of hepcidin via hepatocyte-specific transgenic overexpression of Hfe is abolished in an Hjv − / − [34], but not Tfr2 − / − [14] background. ...
... Here, HJV is essential for amplifying the strength of BMP6/SMAD signaling [19,20]. Interestingly, HFE inactivation has also been associated with defective SMAD signaling in humans [21,22] and mice [23,24], indicating a crosstalk between the pathways involving HFE and HJV. ...
... Since the lack of functional Hfe is associated with impaired Smad signaling [21][22][23][24], both Hjv and Hfe can be viewed as co-factors in this pathway for iron-dependent activation of hepcidin. This notion is also supported by genetic data showing that induction of hepcidin via hepatocyte-specific transgenic overexpression of Hfe is abolished in an Hjv − / − [34], but not Tfr2 − / − [14] background. ...
Functional inactivation of HFE or hemojuvelin (HJV) is causatively linked to adult or juvenile hereditary hemochromatosis, respectively. Systemic iron overload results from inadequate expression of hepcidin, the iron regulatory hormone. While HJV regulates hepcidin by amplifying bone morphogenetic protein (BMP) signaling, the role of HFE in the hepcidin pathway remains incompletely understood. We investigated the pathophysiological implications of combined Hfe and Hjv ablation in mice. Isogenic Hfe (-)/(-) and Hjv (-)/(-) mice were crossed to generate double Hfe (-)/(-) Hjv (-)/(-) progeny. Wild-type control and mutant mice of all genotypes were analyzed for serum, hepatic, and splenic iron content, expression of iron metabolism proteins, and expression of hepcidin and Smad signaling in the liver, in response to a standard or an iron-enriched diet. As expected, Hfe (-)/(-) and Hjv (-)/(-) mice developed relatively mild or severe iron overload, respectively, which corresponded to the degree of hepcidin inhibition. The double Hfe (-)/(-) Hjv (-)/(-) mice exhibited an indistinguishable phenotype to single Hjv (-)/(-) counterparts with regard to suppression of hepcidin, serum and hepatic iron overload, splenic iron deficiency, tissue iron metabolism, and Smad signaling, under both dietary regimens. We conclude that the hemochromatotic phenotype caused by disruption of Hjv is not further aggravated by combined Hfe/Hjv deficiency. Our results provide genetic evidence that Hfe and Hjv operate in the same pathway for the regulation of hepcidin expression and iron metabolism.
Combined disruption of Hfe and Hjv phenocopies single Hjv deficiency. Single Hjv(-)/(-) and double Hfe(-)/(-)Hjv(-)/(-) mice exhibit comparable iron overload. Hfe and Hjv regulate hepcidin via the same pathway.
... Interestingly, in Hfe knockout mice and in patients with HFE mutations, phosphorylated Smad1/5/8 and Id1 levels relative to BMP6 or iron levels in livers are reduced compared with controls. 16,19,[26][27][28][29] This suggests that HFE may regulate hepcidin expression through the BMP pathway in liver cells. However, the mechanism by which HFE interacts with the BMP pathway to regulate hepcidin expression remains to be defined. ...
... Interestingly, defective BMP/Smad signaling has been observed in Hfe and Tfr2 knockout mice 16,19,26,27 and in HFE-HH patients. 28,29 These data implicate the BMP pathway in HFE-and TFR2-regulated hepcidin expression. In the present study, we provide direct biochemical evidence that HFE regulates hepcidin expression through the BMP pathway. ...
Mutations in HFE are the most common cause of hereditary hemochromatosis (HH). HFE mutations result in reduced expression of hepcidin, a hepatic hormone, which negatively regulates iron absorption from the duodenum and iron release from macrophages. However, the mechanism by which HFE regulates hepcidin expression in hepatocytes is not well understood. It is known that the bone morphogenetic protein (BMP) pathway plays a central role in controlling hepcidin expression in the liver. Here we show that HFE overexpression increased Smad1/5/8 phosphorylation and hepcidin expression, whereas inhibition of BMP signaling abolished HFE-induced hepcidin expression in Hep3B cells. HFE was found to associate with ALK3, inhibiting ALK3 ubiquitination and proteasomal degradation and increasing ALK3 protein expression and accumulation on the cell surface. The 2 HFE mutants associated with HH, HFE C282Y and HFE H63D, regulated ALK3 protein ubiquitination and trafficking differently, but both failed to increase ALK3 cell-surface expression. Deletion of Hfe in mice resulted in a decrease in hepatic ALK3 protein expression. Our results provide evidence that HFE induces hepcidin expression via the BMP pathway: HFE interacts with ALK3 to stabilize ALK3 protein and increase ALK3 expression at the cell surface.
... Name of a gene mutated in the most frequent HH subtype. MHC class1-like protein involved in iron sensing; sensitizes cells to BMP stimuli; activator of hepcidin transcription [53][54][55][56][57][58][59]. ...
... Two sequence motifs (BMP-responsive elements) are critical for hepcidin promoter activity mediated by HJV, BMP6, and SMAD4. Patients with HFE-deficiency and mice with Hfe or Tfr2-deficiency show an attenuation of BMP signaling, which suggests that these proteins control the efficiency of the BMP signaling pathway ( Figure 3) [55,75]. Biochemical evidence supports these data by demonstrating that HJV, HFE, and TfR2 form a membrane associated complex in human hepatoma cells [57]. ...
Iron is an essential element in our daily diet. Most iron is required for the de novo synthesis of red blood cells, where it plays a critical role in oxygen binding to hemoglobin. Thus, iron deficiency causes anemia, a major public health burden worldwide. On the other extreme, iron accumulation in critical organs such as liver, heart, and pancreas causes organ dysfunction due to the generation of oxidative stress. Therefore, systemic iron levels must be tightly balanced. Here we focus on the regulatory role of the hepcidin/ferroportin circuitry as the major regulator of systemic iron homeostasis. We discuss how regulatory cues (e.g., iron, inflammation, or hypoxia) affect the hepcidin response and how impairment of the hepcidin/ferroportin regulatory system causes disorders of iron metabolism.
... 45 Hepcidin exerts control over iron homeostasis by decreasing intestinal iron absorption and macrophage iron release in response to excessive accumulation of iron in vital organs. Hepatic iron overload increases hepcidin transcript levels in normal mice 46 and humans. 46 Thus, we hypothesized that genistein might increase Hepcidin expression by enhancing hepatocyte iron uptake. ...
... Hepatic iron overload increases hepcidin transcript levels in normal mice 46 and humans. 46 Thus, we hypothesized that genistein might increase Hepcidin expression by enhancing hepatocyte iron uptake. We found that genistein failed to increase either transferrin-bound or nontransferrin-bound iron uptake. ...
Unlabelled:
Hepcidin, a peptide hormone that decreases intestinal iron absorption and macrophage iron release, is a potential drug target for patients with iron overload syndromes because its levels are inappropriately low in these individuals. Endogenous stimulants of Hepcidin transcription include bone morphogenic protein 6 (BMP6) and interleukin-6 (IL-6) by effects on mothers against decapentaplegic homolog (Smad)4 or signal transducer and activator of transcription (Stat)3, respectively. We conducted a small-scale chemical screen in zebrafish embryos to identify small molecules that modulate hepcidin expression. We found that treatment with the isoflavone, genistein, from 28-52 hours postfertilization in zebrafish embryos enhanced Hepcidin transcript levels, as assessed by whole-mount in situ hybridization and quantitative real-time reverse-transcriptase polymerase chain reaction. Genistein's stimulatory effect was conserved in human hepatocytes: Genistein treatment of HepG2 cells increased both Hepcidin transcript levels and promoter activity. We found that genistein's effect on Hepcidin expression did not depend on estrogen receptor signaling or increased cellular iron uptake, but was impaired by mutation of either BMP response elements or the Stat3-binding site in the Hepcidin promoter. RNA sequencing of transcripts from genistein-treated hepatocytes indicated that genistein up-regulated 68% of the transcripts that were up-regulated by BMP6; however, genistein raised levels of several transcripts involved in Stat3 signaling that were not up-regulated by BMP6. Chromatin immunoprecipitation and ELISA experiments revealed that genistein enhanced Stat3 binding to the Hepcidin promoter and increased phosphorylation of Stat3 in HepG2 cells.
Conclusion:
Genistein is the first small-molecule experimental drug that stimulates Hepcidin expression in vivo and in vitro. These experiments demonstrate the feasibility of identifying and characterizing small molecules that increase Hepcidin expression. Genistein and other candidate molecules may subsequently be developed into new therapies for iron overload syndromes.
... For example, although liver iron content (LIC) is correlated with hepatic Bmp6 mRNA levels in mice [17][18][19] , suggesting that tissue iron levels regulate BMP-SMAD pathway activity by regulating ligand expression, it is unknown whether circulating iron levels also regulate hepatic BMP6 mRNA, or whether circulating iron sensitizes hepatocytes to increase SMAD1/5/8 phosphorylation in response to tonic BMP6 levels. Additionally, although recent studies suggest that HFE and possibly TFR2 may regulate hepcidin expression through an interaction with the BMP6-SMAD signaling pathway 18,[20][21][22][23][24] , several studies provide indirect evidence suggesting that hepatic MAPK signaling, more specifically the ERK1/2 kinases, may also be involved in hepcidin regulation by iron mediated by TFR2 and/or HFE. 21,[25][26][27] Notably, cross-talk between the canonical SMAD signaling pathway and the MAPK pathway is well-described (reviewed in 28 ). ...
... Recent studies suggest a role for inhibitory SMAD7 in hepcidin regulation and iron homeostasis 10,17,[23][24] . Inhibitory SMADs function as feedback inhibitors of the BMP/TGFβ pathway by interacting with type I receptors to block their phosphorylation or to promote receptor dephosphorylation or degradation. ...
Unlabelled:
The bone morphogenetic protein 6 (BMP6)-SMAD signaling pathway is a central regulator of hepcidin expression and systemic iron balance. However, the molecular mechanisms by which iron is sensed to regulate BMP6-SMAD signaling and hepcidin expression are unknown. Here we examined the effects of circulating and tissue iron on Bmp6-Smad pathway activation and hepcidin expression in vivo after acute and chronic enteral iron administration in mice. We demonstrated that both transferrin saturation and liver iron content independently influence hepcidin expression. Although liver iron content is independently positively correlated with hepatic Bmp6 messenger RNA (mRNA) expression and overall activation of the Smad1/5/8 signaling pathway, transferrin saturation activates the downstream Smad1/5/8 signaling cascade, but does not induce Bmp6 mRNA expression in the liver. Hepatic inhibitory Smad7 mRNA expression is increased by both acute and chronic iron administration and mirrors overall activation of the Smad1/5/8 signaling cascade. In contrast to the Smad pathway, the extracellular signal-regulated kinase 1 and 2 (Erk1/2) mitogen-activated protein kinase (Mapk) signaling pathway in the liver is not activated by acute or chronic iron administration in mice.
Conclusion:
Our data demonstrate that the hepatic Bmp6-Smad signaling pathway is differentially activated by circulating and tissue iron to induce hepcidin expression, whereas the hepatic Erk1/2 signaling pathway is not activated by iron in vivo.
... Hepcidin guards iron homeostasis by decreasing intestinal iron absorption and macrophage iron release in response to organs iron overload [32]. Hepatic iron overload increases hepcidin transcript levels in mice [15] and humans [33]. Thus, we hypothesized that Hcy might increase hepcidin expression, which would be attributable to hepatic iron deposition in hyperhomocysteinemia subjects. ...
... (31) Consistent with these findings, altered BMP signaling has been found in patients with type 1 HH. (32,33) There remains a need for further in vivo and patient data to provide meaningful insights into the mechanisms by which HFE, TFR1, and TFR2 regulate hepcidin expression. The most common variants in the HFE gene are the C282Y and H63D mutations. ...
Hepcidin, a peptide hormone produced by hepatocytes, is the central regulator of systemic iron homeostasis through its interaction with ferroportin, the major cellular iron export protein. Hepcidin binding to ferroportin results in reduced iron export from macrophages and intestinal absorptive cells, leading to decreased serum iron levels. Hepcidin expression is influenced by several factors that include serum and liver iron stores, erythropoiesis, hypoxia, inflammation, and infection. Erythropoietic drive and hypoxia suppress hepcidin expression and promote red cell production. In contrast, inflammation and infection are associated with increased hepcidin production to sequester iron intracellularly as a means of depriving microorganisms of iron. Chronic inflammation may up‐regulate hepcidin expression through the interleukin‐6 (IL‐6)–Janus kinase 2 (JAK2)–signal transducer and activator of transcription 3 (STAT3) pathway. The bone morphogenetic protein (BMP)–mothers against decapentaplegic homolog (SMAD) pathway is a major positive driver of hepcidin expression in response to either increased circulating iron in the form of transferrin or iron loading in organs. Hereditary hemochromatosis (HH) consists of several inherited disorders that cause inappropriately reduced hepcidin expression in response to body iron stores, leading to increased iron absorption from a normal diet. The most common form of HH is due to a mutation in the HFE gene, which causes a failure in the hepatocyte iron–sensing mechanism, leading to reduced hepcidin expression; the clinical manifestations of HFE‐HH include increased serum transferrin–iron saturation and progressive iron loading in the liver and other tissues over time among patients who express the disease phenotype. In this article, we review the physiologic mechanisms and cellular pathways by which hepcidin expression is regulated, and the different forms of HH resulting from various mutations that cause hepcidin deficiency. We also review other drivers of hepcidin expression and the associated pathophysiologic consequences.
... The protein encoded by this gene is an HLA class I-like membrane protein that associates with β2-microglobulin and transferrin receptors on the cell surface [9]. HFE may reduce the affinity of transferrin receptor 1 for transferrin, [10] but its major role is in regulating hepcidin expression, likely via the bone morphogenetic protein (BMP)-SMAD signaling pathway [11]. ...
Genetic variants associated with iron homeostasis have been identified, but their association with iron-related indices and variables among different ethnic populations remains controversial. We aimed to explore the genotype frequency and allelic distribution of three iron-metabolism related variants in homeostatic iron regulator gene (HFE; rs1800562 G/A), transmembrane protease, Serine-6 gene (TMPRSS6; rs855791 A/G), and BTB domain-containing protein-9 gene (BTBD9; rs9357271 C/T) among a sample of the Middle Eastern blood donors and to detect the association of these variants on blood indices, and serum hepcidin/ferritin levels. Real-Time TaqMan genotyping assay for the specified variants was applied for 197 unrelated blood donors. Complete blood picture and serum hepcidin/ferritin levels were assessed. All participants were carriers of rs1800562*G/G genotype for HFE. The frequency of A/A and A/G genotypes of TMPRSS6 rs855791 variant was 55% and 45%, and for C/C, C/T, and T/T of BTBD9 rs9357271, were 15%, 43%, and 42%, respectively. Minor allele frequencies of rs855791*G and rs9357271*C were 0.23 and 0.37. The GGC genotype combination (for HFE/TMPRSS6/BTBD9, respectively) was more frequent in male participants. Higher serum hepcidin and hepcidin/ferritin ratio were observed in TMPRSS6 (A/G) carriers. While subjects with BTBD9 C/T and TT genotypes had lower serum ferritin values and higher levels of hepcidin and hepcidin/ferritin ratio compared with C/C genotype. No significant associations were found with any other blood parameters.
In conclusion, TMPRSS6 rs855791 (A/G) and BTBD9 rs9357271 (C/T) variants were prevalent in the present blood donor population and may influence the serum hepcidin and/or ferritin levels.
... and Hfe-deficient mice, and observed a decrease in the BMP signaling pathway [10][11][12][13], leading to a moderate iron overload phenotype [14]. Hepatocyte-specific Alk3 deficiency caused severe iron overload with increased serum iron, transferrin saturation and tissue iron content compared to control mice [15]. ...
Hepcidin deficiency leads to iron overload by increased dietary iron uptake and iron release from storage cells. The most frequent mutation in Hfe leads to reduced hepcidin expression and thereby causes iron overload. Recent findings suggested that HFE activates hepcidin expression predominantly via the BMP type I receptor ALK3. Here, we investigated whether HFE exclusively utilizes ALK3 or other signaling mechanisms also. We generated mice with double deficiency of Hfe and hepatocyte-specific Alk3 and compared the iron overload phenotypes of these double knockout mice to single hepatocyte-specific Alk3 deficient or Hfe knockout mice. Double Hfe-/-/hepatic Alk3fl/fl;Alb-Cre knockouts develop a similar iron overload phenotype compared to single hepatocyte-specific Alk3 deficient mice hallmarked by serum iron levels, tissue iron content and hepcidin levels of similar grades. HFE protein levels were increased in Alk3fl/fl;Alb-Cre mice compared to Alk3fl/fl mice, which was caused by iron overload – and not by Alk3 deficiency. The data provide evidence by genetic means that 1. HFE exclusively uses the BMP type I receptor ALK3 to induce hepcidin expression and 2. HFE protein expression is induced by iron overload, which further emphasizes the iron sensing function of HFE.
... However, liver SMAD1/5/8 signaling is further blunted in both the double endothelial Bmp2/Hfe KO mice and double Bmp6/B2m KO mice in parallel with hepcidin, suggesting that the BMP2-and BMP6-independent effect of HFE is still SMAD1/5/8 dependent, presumably by potentiating signaling by another BMP ligand or ligands [34,62]. Importantly, hemochromatosis patients with HFE mutations also exhibit evidence of disrupted BMP-SMAD signaling [63,64]. However, although HFE might potentiate maximal hepcidin induction by BMPs, HFE is not required for hepcidin induction by BMPs, and exogenous BMP6 can still induce hepcidin and ameliorate iron overload in Hfe −/− mice [65]. ...
The bone morphogenetic protein (BMP)-SMAD signaling pathway plays a central role in regulating hepcidin, which is the master hormone governing systemic iron homeostasis. Hepcidin is produced by the liver and acts on the iron exporter ferroportin to control iron absorption from the diet and iron release from body stores, thereby providing adequate iron for red blood cell production, while limiting the toxic effects of excess iron. BMP6 and BMP2 ligands produced by liver endothelial cells bind to BMP receptors and the coreceptor hemojuvelin (HJV) on hepatocytes to activate SMAD1/5/8 signaling, which directly upregulates hepcidin transcription. Most major signals that influence hepcidin production, including iron, erythropoietic drive, and inflammation, intersect with the BMP-SMAD pathway to regulate hepcidin transcription. Mutation or inactivation of BMP ligands, BMP receptors, HJV, SMADs or other proteins that modulate the BMP-SMAD pathway result in hepcidin dysregulation, leading to iron-related disorders, such as hemochromatosis and iron refractory iron deficiency anemia. Pharmacologic modulators of the BMP-SMAD pathway have shown efficacy in pre-clinical models to regulate hepcidin expression and treat iron-related disorders. This review will discuss recent insights into the role of the BMP-SMAD pathway in regulating hepcidin to control systemic iron homeostasis.
... HFE was previously proposed to stimulate hepcidin through a pathway that intersects with the BMP-SMAD signaling cascade downstream or independent of BMP6. (23)(24)(25)32,33) To determine if HFE-mediated hepcidin regulation is dependent on BMP2, we generated littermate mice with a single endothelial KO of and analyzed their iron phenotype. Hamp mRNA levels were significantly reduced in single endothelial Bmp2 KO mice compared to controls, whereas Hamp mRNA was only reduced in Hfe KO mice when analyzed relative to liver iron content (Fig. 4A). ...
Background and aims:
Bone morphogenetic proteins BMP2 and BMP6 play key roles in systemic iron homeostasis by regulating production of the iron hormone hepcidin. The homeostatic iron regulator (HFE) also regulates hepcidin through a mechanism that intersects with the BMP-mothers against decapentaplegic homolog 1/5/8 (SMAD1/5/8) pathway. However, the relative roles of BMP2 compared with BMP6 and whether HFE regulates hepcidin through a BMP2-dependent mechanism remain uncertain.
Approach and results:
We therefore examined the iron phenotype of mice deficient for both Bmp2 and Bmp6 or both Bmp2 and Hfe compared with single knockout (KO) mice and littermate controls. Eight-week-old double endothelial Bmp6/Bmp2 KO mice exhibited a similar degree of hepcidin deficiency, serum iron overload, and tissue iron overload compared with single KO mice. Notably, dietary iron loading still induced liver SMAD5 phosphorylation and hepcidin in double Bmp6/endothelial Bmp2 KO mice, although no other BMP ligand mRNAs were increased in the livers of double KO mice, and only Bmp6 and Bmp2 mRNA were induced by dietary iron loading in wild-type mice. In contrast, double Hfe/endothelial Bmp2 KO mice exhibited reduced hepcidin and increased extrahepatic iron loading compared to single Hfe or endothelial Bmp2 KO mice. Liver phosphorylated SMAD5 and the SMAD1/5/8 target inhibitor of DNA binding 1 (Id1) mRNA were also reduced in double Hfe/endothelial Bmp2 KO compared with single endothelial Bmp2 KO female mice. Finally, hepcidin and Id1 mRNA induction by homodimeric BMP2, homodimeric BMP6, and heterodimeric BMP2/6 were blunted in Hfe KO primary hepatocytes.
Conclusions:
These data suggest that BMP2 and BMP6 work collaboratively to regulate hepcidin expression, that BMP2-independent and BMP6-independent SMAD1/5/8 signaling contributes a nonredundant role to hepcidin regulation by iron, and that HFE regulates hepcidin at least in part through a BMP2-independent but SMAD1/5/8-dependent mechanism.
... 94 Both HFE and TFR2 stimulate hepcidin expression through a functional interaction with the SMAD signaling pathway (Figure 2), as demonstrated by blunted SMAD signaling and hepcidin expression in mice and/or patients with mutations in these genes. [95][96][97][98][99][100] In vitro overexpression systems demonstrated that HFE and TFR2 bind to each other 101 and to HJV, 91 and these protein interactions are thought to be one mechanism by which HFE and TFR2 influence hepcidin production. HFE was also demonstrated to bind ALK3 and increase its stability by interfering with ubiquitination and degradation. ...
The liver orchestrates systemic iron balance by producing and secreting hepcidin. Known as the iron hormone, hepcidin induces degradation of the iron exporter ferroportin to control iron entry into the bloodstream from dietary sources, iron recycling macrophages, and body stores. Under physiologic conditions, hepcidin production is reduced by iron deficiency and erythropoietic drive to increase the iron supply when needed to support red blood cell production and other essential functions. Conversely, hepcidin production is induced by iron loading and inflammation to prevent the toxicity of iron excess and limit its availability to pathogens. The inability to appropriately regulate hepcidin production in response to these physiologic cues underlies genetic disorders of iron overload and deficiency, including hereditary hemochromatosis and iron-refractory iron deficiency anemia. Moreover, excess hepcidin suppression in the setting of ineffective erythropoiesis contributes to iron-loading anemias such as b-thalassemia, whereas excess hepcidin induction contributes to iron-restricted erythropoiesis and anemia in chronic inflammatory diseases. These diseases have provided key insights into understanding the mechanisms by which the liver senses plasma and tissue iron levels, the iron demand of erythrocyte precursors, and the presence of potential pathogens and, importantly, how these various signals are integrated to appropriately regulate hepcidin production. This review will focus on recent insights into how the liver senses body iron levels and coordinates this with other signals to regulate hepcidin production and systemic iron homeostasis.
... 11,12 In response to iron overload or inflammation, human hepatocytes secrete the iron-regulatory peptide hormone hepcidin. [14][15][16] Hepcidin binds ferroportin, resulting in the internalization and degradation of both proteins 17 ( Figure 1). Hepcidin appears to be the key regulator of iron homeostasis, because loss-of-function mutations in genes that regulate Hepcidin expression, for example, Transferrin receptor 2, HFE, Hemojuvelin, or in Hepcidin itself, have each been associated with hereditary iron overload syndromes. ...
The anemia of chronic disease is an old disease concept, but contemporary research in the role of proinflammatory cytokines and iron biology has shed new light on the pathophysiology of the condition. Recent epidemiologic studies have connected the anemia of chronic disease with critical illness, obesity, aging, and kidney failure, as well as with the well-established associations of cancer, chronic infection, and autoimmune disease. Functional iron deficiency, mediated principally by the interaction of interleukin-6, the iron regulatory hormone hepcidin, and the iron exporter ferroportin, is a major contributor to the anemia of chronic disease. Although anemia is associated with adverse outcomes, experimental models suggest that iron sequestration is desirable in the setting of severe infection. Experimental therapeutic approaches targeting interleukin-6 or the ferroportin-hepcidin axis have shown efficacy in reversing anemia in either animal models or human patients, although these agents have not yet been approved for the treatment of the anemia of chronic disease.
... Several studies indicate that HFE and TfR2 are required for the normal activation of BMP6-induced signals, specifically the phosphorylation of SMAD 1/5/8, that are involved in increasing hepcidin expression. [26][27][28][29] Recent experiments suggest that TfR2, but not HFE, may have some role in the up-regulation of BMP6 by hepatocyte iron, and that TfR2 and HFE may interact with HJV to facilitate hepcidin induction. 27,30 The importance of HFE, TfR2, HJV, as well as hepcidin and FPN, in iron homeostasis is well illustrated by the fact that mutations in genes encoding these proteins are associated with the development of hemochromatosis, a clinically significant iron overload syndrome in humans. ...
Anemia is a frequent complication of many inflammatory disorders, including inflammatory bowel disease. Although the pathogenesis of this problem is multifactorial, a key component is the abnormal elevation of the hormone hepcidin, the central regulator of systemic iron homeostasis. Investigations over the last decade have resulted in important insights into the role of hepcidin in iron metabolism and the mechanisms that lead to hepcidin dysregulation in the context of inflammation. These insights provide the foundation for novel strategies to prevent and treat the anemia associated with inflammatory diseases.
... This observation was not confirmed in patients with iron deposits free of the HFE mutation. [38] D'Alessio et al [39] observed the interaction between HFE, TfR2, and HJV in iron sensing, and found that HFE, TfR2, and HJV form a multi-protein membrane complex on the surface of hepatocytes to regulate hepcidin expression. ...
Background:
The liver, as the main iron storage compartment and the place of hepcidin synthesis, is the central organ involved in maintaining iron homeostasis in the body. Excessive accumulation of iron is an important risk factor in liver disease progression to cirrhosis and hepatocellular carcinoma. Here, we review the literature on the molecular pathogenesis of iron overload and its clinical consequences in chronic liver diseases.
Data sources:
PubMed was searched for English-language articles on molecular genesis of primary and secondary iron overload, as well as on their association with liver disease progression. We have also included literature on adjuvant therapeutic interventions aiming to alleviate detrimental effects of excessive body iron load in liver cirrhosis.
Results:
Excess of free, unbound iron induces oxidative stress, increases cell sensitivity to other detrimental factors, and can directly affect cellular signaling pathways, resulting in accelerated liver disease progression. Diagnosis of liver cirrhosis is, in turn, often associated with the identification of a pathological accumulation of iron, even in the absence of genetic background of hereditary hemochromatosis. Iron depletion and adjuvant therapy with antioxidants are shown to cause significant improvement of liver functions in patients with iron overload. Phlebotomy can have beneficial effects on liver histology in patients with excessive iron accumulation combined with compensated liver cirrhosis of different etiology.
Conclusion:
Excessive accumulation of body iron in liver cirrhosis is an important predictor of liver failure and available data suggest that it can be considered as target for adjuvant therapy in this condition.
... Hepcidin guards iron homeostasis by decreasing intestinal iron absorption and macrophage iron release in response to organs iron overload [32]. Hepatic iron overload increases hepcidin transcript levels in mice [15] and humans [33]. Thus, we hypothesized that Hcy might increase hepcidin expression, which would be attributable to hepatic iron deposition in hyperhomocysteinemia subjects. ...
... Primary iron overload occurs when there is a genetic disorder in the iron regulatory system (Feder et al., 1996). For instance, the hereditary hemochromatosis was characterized by abnormal iron metabolism, leading an excessively iron absorption and store in the body (Bolondi et al., 2010). Iron overload can also occur in the ineffective erythropoiesis, sideroblastic anemia, primary liver disease, thalassemia, dyserythropoietic anemia, and Sickle cell disease, hematological diseases that needing chronic blood transfusions, a condition known as secondary iron overload (Weatherall and Clegg, 1996;Porter, 2009). ...
Iron plays a critical role in a mammal's physiological processes. However, iron tissue deposits have been shown to act as endocrine disrupters. Studies that evaluate the effect of acute iron overload on hypothalamic-pituitary-gonadal (HPG) axis health are particularly sparse. This study demonstrates that acute iron overload leads to HPG axis abnormalities, including iron accumulation and impairment in reproductive tract morphology. Female rats were treated with iron-dextran (Fe rats) to assess their HPG morphophysiology. The increasing serum iron levels due to iron-dextran treatment were positively correlated with higher iron accumulation in the HPG axis and uterus of Fe rats than in control rats. An increase in the production of superoxide anions was observed in the pituitary, uterus and ovary of Fe rats. Morphophysiological reproductive tract abnormalities, such as abnormal ovarian follicular development and the reduction of serum estrogen levels, were observed in Fe rats. In addition, a significant negative correlation was obtained between ovary superoxide anion and serum estrogen levels. Together, these data provide in vivo evidence that acute iron overload is toxic for the HPG axis, a finding that may be associated with the subsequent development of the risk of reproductive dysfunction.
... These mutations thus lead to increased duodenal iron absorption and macrophage iron release resulting in elevated serum iron levels and liver iron overload. Recently, the pathogenesis of HH was linked to the bone morphogenetic proteins (BMPs), a group of cytokines that belongs to the TGF-ß superfamily [54]. BMPs initiate intracellular signalling by binding to complexes of type I and type II serine/threonine kinase receptors, which when activated phosphorylate intracellular SMAD proteins. ...
Since the development of advanced mathematical modelling techniques in biology, thermodynamics (and therefore equilibrium statistical mechanics) has played a key role in mathematically quantifying biological activities. We use this underlying notion of thermodynamic ‘micro-states’ to attempt to uncover how the hormone hepcidin under the influence of two major signalling pathways maintains systemic iron homeostasis. Systemic iron homeostasis involves a negative feedback circuit in which the expression level of the peptide hormone hepcidin depends on and controls the iron blood levels. Hepcidin expression is regulated by the BMP6/SMAD and IL6/STAT signalling cascades. Deregulation of either pathway causes iron storage diseases such as hemochromatosis or anaemia of inflammation (AI). We quantitatively analyzed how BMP and IL6 control hepcidin expression in human hepatoma (HuH7) cells. We used data from our experimental collaborators who measured transcription factor phosphorylation and reporter gene expression under co-stimulation conditions and perturbed the promoter by mutagenesis. We applied statistical data analysis and mathematical modelling to reveal possible biological mechanisms that control hepcidin expression at the promoter level. Specifically we develop a thermodynamic modelling framework that is able to simulate and predict possible molecular mechanisms that might underlie iron homeostasis by hepcidin. Our results reveal that hepcidin cross- regulation primarily occurs by combinatorial transcription factor binding to the promoter, whereas signalling crosstalk is insignificant. We find that the presence of two BMP-responsive elements in the promoter ensures high sensitivity towards the iron-sensing BMP signalling axis, which promotes iron homeostasis in vivo. IL6 stimulation reduces the promoter sensitivity to the BMP signal that may explain the disturbance of iron homeostasis in AI. We get to understand why the iron homeostasis circuit is sensitive to certain perturbations implicated in disease. Taken together, our work reveals how mathematical quantification and modelling can aid in understanding biological phenomenon that underlies gene expression.
... While iron is essential for many of the body's processes, excess iron produces oxidative stress, leading to vascular and organ damage. Iron overload can be caused by genetic mutations to iron regulatory genes (primary iron overload, i.e. hemochromatosis) or by chronic blood transfusions during the treatment of hemoglobinopathies such as thalassemia (secondary iron overload) [1][2][3]. Despite current treatments, iron-mediated cardiomyopathy remains the leading cause of death in thalassemia [4]. ...
Iron cardiomyopathy is the leading cause of death in iron overload. Men have twice the mortality rate of women, though the cause is unknown. In hemojuvelin-knockout mice, a model of the disease, males load more cardiac iron than females. We postulated that sex differences in cardiac iron import cause differences in cardiac iron concentration. RT-PCR was used to measure mRNA of cardiac iron transporters in hemojuvelin-knockout mice. No sex differences were discovered among putative importers of non-transferrin bound iron (L-type and T-type calcium channels, ZRT/IRT-like protein 14 zinc channels). Transferrin-bound iron transporters were also analyzed; these are controlled by the iron regulatory element/iron regulatory protein (IRE/IRP) system. There was a positive relationship between cardiac iron and ferroportin mRNA in both sexes, but it was significantly steeper in females (p<0.05). Transferrin receptor 1 and divalent metal transporter 1 were more highly expressed in females than males (p<0.01 and p<0.0001, respectively), consistent with their lower cardiac iron levels, as predicted by IRE/IRP regulatory pathways. Light-chain (L) ferritin showed a positive correlation with cardiac iron that was nearly identical in males and females (R(2)=0.41, p<0.01 and R(2)=0.56, p<0.05, respectively), while heavy-chain (H) ferritin was constitutively expressed in both sexes. This represents the first report of IRE/IRP regulatory pathways in the heart. Transcriptional regulation of ferroportin was suggested in both sexes, creating a potential mechanism for differential set points for iron export. Constitutive H-ferritin expression suggests a logical limit to cardiac iron buffering capacity at levels known to produce heart failure in humans.
... It has been proposed that HFE and TFR2 may form a "supercomplex" with HJV to stimulate hepcidin expression via the BMP-SMAD pathway. Studies supporting this model have demonstrated that liver BMP-SMAD signaling is impaired in mice and humans with HFE and/or TFR2 mutations, suggesting an interaction at some level between HFE, TFR2 and the BMP-SMAD pathway (Corradini et al., , 2011bKautz et al., 2009;Wallace et al., 2009;Bolondi et al., 2010;Ryan et al., 2010). Recently, it was published in an overexpression tissue culture system using tagged proteins that HFE and TFR2 can form a complex with HJV (D' Alessio et al., 2012). ...
Mutations in hemojuvelin (HJV) are the most common cause of the juvenile-onset form of the iron overload disorder hereditary hemochromatosis. The discovery that HJV functions as a co-receptor for the bone morphogenetic protein (BMP) family of signaling molecules helped to identify this signaling pathway as a central regulator of the key iron hormone hepcidin in the control of systemic iron homeostasis. This review highlights recent work uncovering the mechanism of action of HJV and the BMP-SMAD signaling pathway in regulating hepcidin expression in the liver, as well as additional studies investigating possible extra-hepatic functions of HJV. This review also explores the interaction between HJV, the BMP-SMAD signaling pathway and other regulators of hepcidin expression in systemic iron balance.
... There is some evidence from in vitro overexpression systems that HFE, TFR2 and HJV can interact with each other [56][57][58], but it is uncertain whether this occurs in vivo. HFE and TFR2 do appear to intersect with the BMP-SMAD pathway at some level, since mice and human patients with HFE and TFR2 mutations exhibit impairment in liver SMAD signaling [59][60][61][62][63][64]. However, the functions of HFE and TFR2 in regulating hepcidin are not entirely overlapping given the differential severity of the iron overload phenotype in mice and patients with HFE mutations alone, TFR2 mutations alone and double HFE/TFR2 mutations [61,64,65]. ...
Iron is essential for most living organisms but iron excess can be toxic. Cellular and systemic iron balance is therefore
tightly controlled. Iron homeostasis is dysregulated in chronic kidney disease (CKD) and contributes to the anemia that is
prevalent in this patient population. Iron supplementation is one cornerstone of anemia management in CKD patients, but has
not been rigorously studied in large prospective randomized controlled trials. This review highlights important advances from
genetic studies and animal models that have provided key insights into the molecular mechanisms governing iron homeostasis
and its disturbance in CKD, and summarizes how these findings may yield advances in the care of this patient population.
... Primary iron overload occurs when there is a deleterious mutation in the iron regulatory system (e.g. HFE or HJV) (8,9). This disease is characterized by a blunted hepcidin response, resulting in high plasma iron and high transferrin saturation (10,11). ...
Iron cardiomyopathy is the leading cause of death in transfusional iron overload, and men have twice the mortality of women. Because the prevalence of cardiac iron overload increases rapidly during the second decade of life, we postulated that there are steroid-dependent sex differences in cardiac iron uptake. To test this hypothesis, we manipulated sex steroids in mice with constitutive iron absorption (homozygous hemojuvelin knockout); this model mimics the myocyte iron deposition observed in humans. At 4 weeks of age, female mice were ovariectomized (OVX) and male mice were castrated (OrchX). Female mice received an estrogen implant (OVX + E) or a cholesterol control (OVX), whereas male mice received an implant containing testosterone (OrchX + T), dihydrotestosterone (OrchX + DHT), estrogen (OrchX + E), or cholesterol (OrchX). All animals received a high-iron diet for 8 weeks. OrchX, OVX, and OVX + E mice all had similar cardiac iron loads. However, OrchX + E males had a significant increase in cardiac iron concentration compared with OrchX mice (P < 0.01), whereas the OrchX + T and OrchX + DHT groups only trended higher (P < 0.06 and P < 0.15, respectively). Hormone treatments did not impact liver iron concentration in either sex. When data were pooled across hormone therapies, liver iron concentration was 25% greater in males than females (P < 0.01). In summary, we found that estrogen increased cardiac iron loading in male mice, but not in females. Male mice loaded 25% more hepatic iron than female mice regardless of the hormone treatment.
... This likely leads to plasma and tissue iron deficiency, with mild impairment of erythropoiesis. miR-122 levels are not regulated as a result of iron accumulation in the liver of HH patients, but more likely as a consequence of the signalling activities reduced by a lack of HFE which is known to attenuate BMP/Smad signalling in HH patients and its respective murine disease model [92] . ...
Primary liver cancer is a global disease that is on the increase. Hepatocellular carcinoma (HCC) accounts for most primary liver cancers and has a notably low survival rate, largely attributable to late diagnosis, resistance to treatment, tumour recurrence and metastasis. MicroRNAs (miRNAs/miRs) are regulatory RNAs that modulate protein synthesis. miRNAs are involved in several biological and pathological processes including the development and progression of HCC. Given the poor outcomes with current HCC treatments, miRNAs represent an important new target for therapeutic intervention. Several studies have demonstrated their role in HCC development and progression. While many risk factors underlie the development of HCC, one process commonly altered is iron homeostasis. Iron overload occurs in several liver diseases associated with the development of HCC including Hepatitis C infection and the importance of miRNAs in iron homeostasis and hepatic iron overload is well characterised. Aberrant miRNA expression in hepatic fibrosis and injury response have been reported, as have dysregulated miRNA expression patterns affecting cell cycle progression, evasion of apoptosis, invasion and metastasis. In 2009, miR-26a delivery was shown to prevent HCC progression, highlighting its therapeutic potential. Several studies have since investigated the clinical potential of other miRNAs with one drug, Miravirsen, currently in phase II clinical trials. miRNAs also have potential as biomarkers for the diagnosis of HCC and to evaluate treatment efficacy. Ongoing studies and clinical trials suggest miRNA-based treatments and diagnostic methods will have novel clinical applications for HCC in the coming years, yielding improved HCC survival rates and patient outcomes.
... 27 Collectively, these data demonstrate that BMP-SMAD signaling is an important regulatory pathway for hepcidin expression and thus iron metabolism. In Hfe knockout mice 28,29 , and in patients with HFE-associated HH 30,31 , the induction of Bmp6 mRNA by iron is intact, but Smad1,5,8 signaling to hepcidin is impaired. Impaired Bmp6 signaling to hepcidin has also been reported in murine models of Tfr2-associated HH 15,32 The goal of this study was to investigate the Bmp6-Smad-hepcidin signaling pathway in the Tfr2 and Hfe mutant mouse models of HH, under standard iron diets and with dietary iron loading. ...
HFE and transferrin receptor 2 (TFR2) are each necessary for the normal relationship between body iron status and liver hepcidin expression. In murine Hfe and Tfr2 knockout models of hereditary hemochromatosis (HH), signal transduction to hepcidin via the bone morphogenetic protein 6 (Bmp6)/Smad1,5,8 pathway is attenuated. We examined the effect of dietary iron on regulation of hepcidin expression via the Bmp6/Smad1,5,8 pathway using mice with targeted disruption of Tfr2, Hfe, or both genes.
Hepatic iron concentrations and messenger RNA expression of Bmp6 and hepcidin were compared with wild-type mice in each of the HH models on standard or iron-loading diets. Liver phospho-Smad (P-Smad)1,5,8 and Id1 messenger RNA levels were measured as markers of Bmp/Smad signaling.
Whereas Bmp6 expression was increased, liver hepcidin and Id1 expression were decreased in each of the HH models compared with wild-type mice. Each of the HH models also showed attenuated P-Smad1,5,8 levels relative to liver iron status. Mice with combined Hfe/Tfr2 disruption were most affected. Dietary iron loading increased hepcidin and Id1 expression in each of the HH models. Compared with wild-type mice, HH mice demonstrated attenuated (Hfe knockout) or no increases in P-Smad1,5,8 levels in response to dietary iron loading.
These observations show that Tfr2 and Hfe are each required for normal signaling of iron status to hepcidin via the Bmp6/Smad1,5,8 pathway. Mice with combined loss of Hfe and Tfr2 up-regulate hepcidin in response to dietary iron loading without increases in liver Bmp6 messenger RNA or steady-state P-Smad1,5,8 levels.
... Phosphorylated SMAD1/5/8 form a heterodimeric complex with SMAD4 in the cytoplasm that translocates to the nucleus to activate target genes including hepcidin. 2,3 Inactivation of genes that directly (HJV, BMP6) or indirectly (HFE, TfR2) regulate BMP-SMAD signaling, impairs hepcidin production, and causes iron overload in humans 4 and mice. [5][6][7][8][9] Hepcidin is also an antimicrobial peptide, 10 and its expression is induced by infection/inflammation. ...
Hepcidin is an antimicrobial peptide that controls systemic iron homeostasis. Hepcidin binding to its receptor ferroportin reduces iron availability, thus controlling microbial growth. In parallel it triggers an anti-inflammatory response in macrophages. Hepcidin is transcriptionally regulated by iron, through the bone morphogenetic protein-son of mothers against decapentaplegic (BMP-SMAD) pathway and by inflammation, through IL6-mediated STAT3 signaling. To investigate the mechanisms linking iron and inflammation, we treated C57BL/6 iron-deficient mice with a sublethal dose of lipopolysaccharide (LPS) and analyzed their inflammatory response in comparison with controls. We show that iron-deprived mice have a proinflammatory condition, exacerbated by LPS treatment leading to increased IL6 and TNFα mRNA in liver and spleen macrophages, and increased serum IL6 (482.29 ± 205.59 pg/mL) versus controls (69.01 ± 17.52 pg/mL; P < .05). Hepcidin was undetectable in iron-deficient mice but pretreatment with hepcidin normalized their response to LPS. Tmprss6(-/-) mice, characterized by iron deficiency and high hepcidin, show a blunted inflammatory response when challenged with LPS. Our data support a model in which the lack of hepcidin is responsible of the high inflammatory response to LPS in iron deficiency. The proinflammatory status associated with chronic iron deficiency could explain the resistance to infection seen in this condition.
... 11 As TS increases, HFE dissociates from TFR1 and binds to TFR2 to possibly convey the necessary signal downstream to stimulate hepcidin synthesis. 12,13 Some studies support the premise that TFR2 and HFE interact with the BMP6-SMAD pathway, because this signaling pathway is impaired in Tfr2 and/or Hfe null mice 9,14-16 as well as in subjects with HFE-associated HH, 17,18 whereas others report no interaction. 5 TFR2 and HFE may also signal independently of each other, because disruption of both Tfr2 and Hfe in mice causes a more severe iron overload phenotype. ...
Hepcidin is the master regulator of systemic iron homeostasis. The bone morphogenetic protein (BMP) signaling pathway is a critical regulator of hepcidin expression in response to iron and erythropoietic drive. Although endothelial-derived BMP6 and BMP2 ligands have key functional roles as endogenous hepcidin regulators, both iron and erythropoietic drive still regulate hepcidin in mice lacking either ligand or both. Here, we utilize mice with an inactivating Bmp5 mutation (Bmp5se), either alone or together with a global or endothelial Bmp6 knockout, to investigate the functional role of BMP5 in hepcidin and systemic iron homeostasis regulation. We show that Bmp5se mutant mice exhibit hepcidin deficiency at 10 days of age, blunted hepcidin induction to oral iron gavage, and mild liver iron loading when fed a low- or high-iron diet. Loss of 1 or 2 functional Bmp5 alleles also leads to more iron loading in Bmp6 heterozygous mice and more profound hemochromatosis in global or endothelial Bmp6 knockout mice. Moreover, double Bmp5 and Bmp6 mutant mice fail to induce hepcidin in response to chronic dietary iron loading. Finally, erythroferrone (ERFE) binds directly to BMP5 and inhibits BMP5 induction of hepcidin in vitro. Whereas erythropoietin (EPO) suppresses hepcidin in Bmp5se mutant mice, EPO fails to suppress hepcidin in double Bmp5 and Bmp6 mutant males. Together, these data demonstrate that BMP5 has a functional role in hepcidin and iron homeostasis regulation, particularly under conditions where BMP6 is limited.
Cambridge Core - Medicine: General Interest - Anemia - edited by Edward J. Benz, Jr.
Anaemia of inflammation (AI) is the second most common cause of anaemia worldwide, after iron deficiency. This pathology is commonly observed in patients with chronic infections, malignancy, trauma, and inflammatory disorders. Typically, AI is normocytic normochromic usually mild anaemia produced by hypoferremia resulted from reticulo endothelial sequestration of iron and interruption of intestinal iron absorption. The pathogenesis of AI is mediated by inflammatory cytokines and hepcidin, a liver-derived poly- peptide regulator of iron homeostasis, acting together to suppress erythropoiesis and shorten erythrocyte survival in blood. The routine diagnostic methods described in this review may be useful, but there will remain a group of anemic states with high suspicion of AI that cannot be easily classified. Recently, different immunoassays have been developed to evaluate hepcidin and inflammatory cytokines levels, which will improve the diagnosis. AI models have been developed in mouse taking into account the proposed pathophysiology and could be useful for evaluating different types of treatment. In fact, inhibitors of hepcidin, and various inflammatory modulators show promise in a near future.
Bone morphogenetic proteins (BMPs)/growth and differentiation factors (GDFs) are involved in a wide variety of embryologic, developmental, and physiologic processes. One important area of physiology that requires BMP signaling is the homeostatic regulation of iron in the body. Iron is an essential nutrient that is critical for several fundamental cellular processes including oxygen delivery to tissues and generation of adenosine triphosphate (ATP) in mitochondria. However, excess iron can lead to the generation of reactive oxygen species (ROS) that are highly damaging to cells, and insufficient iron is the major source of anemia worldwide. Therefore, the homeostatic regulation of total body iron content is an important physiologic process that must be exquisitely controlled to prevent the pathologic states of iron excess or iron deficiency. BMP signaling in the liver by the BMP ligands and receptors including the co-receptor hemojuvelin/RGMc regulates the expression of the iron hormone hepcidin to maintain iron homeostasis.
In the presence of iron, the iron-responsive element (IRE)- iron regulatory protein (IRP) interaction is blocked, allowing translation of the mRNA, which explains the initial observations in rat liver. With the demonstration that the binding of the IRPs had different effects depending on the localization of the IRE, it was concluded that the IRE/IRP system constituted a general post-transcriptional regulatory mechanism for controlling cellular iron homeostasis. Over the past twenty years it has become clear that the major regulator of systemic iron homeostasis relies on the regulatory circuit of the hormone hepcidin and the cellular iron exporter ferroportin (FPN1). However, hypoxia-inducible factors (HIFs) also control the transcription of numerous genes involved in maintaining iron metabolism in equilibrium. The interplay between the three regulatory systems, namely the IRE/IRP system, the HIF system and the ultimate key orchestrator of systemic iron homeostasis, the hepcidin/ferroportin system, is slowly beginning to be understood.
: A classic Girl Scout song says, "Make new friends/but keep the old/One is silver/and the other gold." This review focuses on the past decade of discovery in the field of iron homeostasis, which has identified "new friends" or key modifiers of the critical systemic iron regulator, hepcidin antimicrobial peptide. The foundation for these discoveries has been the identification of mutated genes in well-characterized cohorts of patients with inherited hemochromatosis from across the globe. Transgenic mouse models of iron overload and iron-restricted anemia have also contributed to understanding molecular pathophysiology in ways that could never be accomplished in human subjects alone. The majority of these newly discovered molecules coordinate signaling through the bone morphogenetic protein pathway of ligands, receptors and coreceptors, intracellular signaling and transcription. The discovery of these proteins and their interactions with "old friends," such as the 1st known hereditary hemochromatosis gene product, HFE and transferrin receptor, has opened the field of iron homeostasis to include regulatory networks involving signal transduction pathways, in particular, the mitogen-activated protein kinase and Smad pathways. These newly discovered partnerships have also made way for opportunities to develop novel therapeutics for the treatment of iron regulatory disorders, including hemochromatosis.
Iron can be deleterious in excess because of its ability to generate reactive oxygen species. Consequently, the appropriate maintenance of organismal iron balance is crucial. Disturbances in iron metabolism are the root cause of many prevalent human diseases. Hereditary hemochromatosis is a genetic disorder resulting in a chronic increase of iron uptake from the diet leading to disease. Although first mentioned in the medical literature by Trousseau in 1865, the genetic and pathophysiological basis of this disorder was until recently poorly understood. The vast majority of cases are caused by mutations in HFE, the classic hemochromatosis gene. More recently, genetic studies have implicated hemochromatosis disease-causing mutations in hemojuvelin (HJV), hepcidin (HAMP), transferrin receptor 2 (TFR2) and ferroportin (SLC40A1). Research involving these five genetically distinct causes of hemochromatosis has increased our knowledge of the underlying pathogenic mechanisms of the disease and has led to a better general understanding of iron metabolism. Current thought places hepcidin, a soluble 25 amino-acid peptide, as the master regulator of iron homeostasis. Hepcidin acts as a ligand for the iron transporter ferroportin, negatively regulating iron release from red blood cell-recycling macrophages or dietary iron uptake in the duodenum. Perturbance of the hepcidin-ferroportin regulatory apparatus in hemochromatosis leads to dysregulated dietary iron uptake eventuating in disease. Recent work has augmented our understanding of how iron regulates hepcidin expression. Hemojuvelin, a bone morphogenetic protein (BMP) co-receptor, plays a central role in the regulation of hepcidin expression through a SMAD-mediated signaling pathway. Both HFE and TFR2 are thought to play essential roles in the sensing of total body iron needs, but the mechanism by which these molecules affect HJV/BMP/SMAD-mediated hepcidin control is not completely elucidated. Recent insights into the molecular basis of hereditary hemochromatosis and iron metabolism will be reviewed here.
Inherited metabolic diseases that affect the liver are a frequent cause of liver failure in children, but other disorders more commonly cause liver failure in adulthood where they may present with chronic liver disease and, less frequently, with acute liver failure. The identification of the underlying genetic defect for many of these inherited disorders has improved our understanding of their pathophysiology and impacted on the indications for and timing of liver transplant, yielding better outcomes. Screening for disease and genetic counseling of family members may help prevent adverse outcomes in relatives of affected individuals. Timely liver transplantation offers correction of the inherited metabolic defect and restores liver function when medical therapy is not possible or when complications of liver disease arise. Some inherited metabolic diseases have their defect based in the liver and lead not to liver disease, but to other end organ damage. Earlier detection of these disorders may prevent pathological injury by treatment of the underlying disease or by pre-emptive liver transplant. In some instances where damage of other organs has already occurred, dual organ transplant with liver and another organ may be needed. Improvement in the technical aspects of performing liver transplantation and posttransplant care has led to better outcomes for those with inherited metabolic disorders of the liver.
The inhibitory Smad7 acts as a critical suppressor of hepcidin, the major regulator of systemic iron homeostasis. In this study we define the mRNA expression of the two functionally related Smad proteins, Smad6 and Smad7, within pathways known to regulate hepcidin levels. Using mouse models for hereditary hemochromatosis (Hfe-, TfR2-, Hfe/TfR2-, Hjv- and hepcidin1-deficient mice) we show that hepcidin, Smad6 and Smad7 mRNA expression is coordinated in such a way that it correlates with the activity of the Bmp/Smad signaling pathway rather than with liver iron levels. This regulatory circuitry is disconnected by iron treatment of Hfe-/- and Hfe/TfR2 mice that significantly increases hepatic iron levels as well as hepcidin, Smad6 and Smad7 mRNA expression but fails to augment pSmad1/5/8 levels. This suggests that additional pathways contribute to the regulation of hepcidin, Smad6 and Smad7 under these conditions which do not require Hfe.
The hereditary hemochromatosis-associated membrane proteins HFE, TfR2, and HJV are required for adequate hepatic expression of the iron hormone hepcidin. While the genetic interactions are clear, it remains elusive how bone morphogenetic protein co-receptor HJV functions together with HFE and TfR2 to activate hepcidin transcription via the BMP-SMAD signaling pathway. Here, we investigate whether HFE, TfR2, and HJV physically interact on the surface of hepatocytes.
We explore protein-protein interactions by glycerol gradient sedimentation assays and co-immunoprecipitation analyses in transfected HuH7 hepatoma-derived cells.
Our data demonstrate that HFE and TfR2 bind HJV in a non-competitive manner. Co-immunoprecipitation analyses provide direct experimental evidence that HFE, TfR2, and HJV form a multi-protein membrane complex. Our experiments show that like TfR2, HJV competes with TfR1 for binding to HFE, indicating that the expression of TfR2 and HJV may be critical for iron sensing. We identify residues 120-139 of the TfR2 extra-cellular domain as the critical amino acids required for the binding of both HFE and HJV. Interestingly, RGMA, a central nervous system homolog, can substitute for HJV in the complex and promote hepcidin transcription, implicating RGMA in the local control of hepcidin in the CNS.
Taken together, our findings provide a biochemical basis for hepcidin control by HFE, TfR2, and HJV.
Iron homeostasis is maintained at the cellular and systemic levels to assure adequate iron supply while preventing iron overload. The identification of genes mutated in patients with iron-related disorders or animal models with imbalances of iron homeostasis gave insight into the molecular mechanisms underlying processes critical for balancing iron levels, such as iron uptake, storage, export, and monitoring of available iron. MicroRNAs control genes involved in some of these processes adding an additional level of complexity to the regulation of iron metabolism. This review summarizes recent advances how miRNAs regulate iron homeostasis.
Bone morphogenetic proteins (BMPs) belong to the TGF-β superfamily and were first discovered as potent bone homeostasis regulators for their ability to induce endochondral bone formation, ectopic bone formation and fracture repair. A preeminent role of BMP signaling in developmental control of cell type specification, differentiation and organogenesis is also well established. More recently, a role for BMPs in adult tissue homeostasis has started being revealed. Thus, new studies show that BMPs regulate many cellular processes such as proliferation, apoptosis, differentiation and migration in many tissues and organs. As a consequence, dysregulation of BMP activity can have pathological consequences, and there is mounting evidence for the involvement of BMPs in different human diseases. In this review, we have focused on summarizing the present knowledge on the relevance of BMPs in liver physiology and pathophysiology, from the well-recognized role in liver development to the emerging contribution to the function and dysfunction of the adult liver. While no doubts seem to rise about the regulatory activities of BMPs on metabolic pathways in the liver, potential pro- and anti-fibrogenic and tumorigenic actions will likely be a matter of debate during coming years. Collectively, the work here presented provides the basis to consider BMPs as potential targets of intervention in liver diseases.
Hepcidin is a key player in the regulation of iron homeostasis. Several pathological conditions associated with iron overload are attributed to the depressed expression of hepcidin and are often associated with bone diseases including osteoporosis. Hepcidin was suggested to have anti-osteoporosis effects by preventing iron overload. We recently observed that hepcidin could increase intracellular calcium concentration in cultured osteoblast cells. The present study was designed to elucidate the source of the increased intracellular calcium following hepcidin activation. Cultured hFOB1.19 cells were used to test whether there was a dose dependent effect of hepcidin on increasing intracellular calcium. After finding the optimal concentration in increasing intracellular calcium, Cultured hFOB1.19 cells were then divided into three groups: (1) control group, (2) and (3) groups pretreated with either nimodipine (2 × 10(-5)mol/L) or EDTA (2 × 10(-3)mol/L) for 10 min before incubation with hepcidin (100 nmol/L). All cells were stimulated with hepcidin for 60 min and then stained with fluo-3/AM for 40 min before the intracellular calcium was observed using flow cytometry (FCM). As compared with controls, hepcidin treatment significantly increased intracellular calcium concentration. This effect was blocked by nimodipine and EDTA pretreatments which suggested that hepcidin-mediated calcium inflow was mainly through L-type Ca(2+) channels and that the release of intracellular calcium store was not significant. Hepcidin increases of intracellular calcium may be related to its anti-osteoporosis effect but this hypothesis needs further investigation.
Hereditary hemochromatosis is a genetic disorder of iron overload. Over the past 15 years, significant advances have been made in understanding the molecular pathogenesis of this disorder. First, genetic studies linked this disorder to mutations in several genes, including HFE, transferrin receptor 2 ( TFR2), hepcidin ( HAMP), ferroportin ( SLC40A1), and hemojuvelin ( HFE2). Recent progress has generated significant insight into the function of these molecules in systemic iron homeostasis, and has revealed that despite the genetic and phenotypic diversity of hereditary hemochromatosis, there are common pathogenic mechanisms underlying this disease. The common downstream mechanism of iron overload in hereditary hemochromatosis is abnormal regulation of the hepcidin-ferroportin axis, leading to a failure to prevent excess iron from entering the circulation. Recent data are starting to unravel the molecular mechanisms by which iron regulates hepcidin production, and has demonstrated a key role for the bone morphogenetic protein-hemojuvelin-SMAD signaling pathway in this process. Future studies will be needed to more fully understand the molecular mechanisms of iron sensing and the roles of HFE and TFR2 in this process. Here, the authors review the current state of knowledge on the molecular pathogenesis of hereditary hemochromatosis.
Bone morphogenetic protein (BMP) signals coordinate developmental patterning and have essential physiological roles in mature organisms. Here we describe the first known small-molecule inhibitor of BMP signaling—dorsomorphin, which we identified in a screen for compounds that perturb dorsoventral axis formation in zebrafish. We found that dorsomorphin selectively inhibits the BMP type I receptors ALK2, ALK3 and ALK6 and thus blocks BMP-mediated SMAD1/5/8 phosphorylation, target gene transcription and osteogenic differentiation. Using dorsomorphin, we examined the role of BMP signaling in iron homeostasis. In vitro, dorsomorphin inhibited BMP-, hemojuvelin- and interleukin 6–stimulated expression of the systemic iron regulator hepcidin, which suggests that BMP receptors regulate hepcidin induction by all of these stimuli. In vivo, systemic challenge with iron rapidly induced SMAD1/5/8 phosphorylation and hepcidin expression in the liver, whereas treatment with dorsomorphin blocked SMAD1/5/8 phosphorylation, normalized hepcidin expression and increased serum iron levels. These findings suggest an essential physiological role for hepatic BMP signaling in iron-hepcidin homeostasis.
Impaired regulation of hepcidin expression in response to iron loading appears to be the pathogenic mechanism for hereditary hemochromatosis. Iron normally induces expression of the BMP6 ligand, which, in turn, activates the BMP/Smad signaling cascade directing hepcidin expression. The molecular function of the HFE protein, involved in the most common form of hereditary hemochromatosis, is still unknown. We have used Hfe-deficient mice of different genetic backgrounds to test whether HFE has a role in the signaling cascade induced by BMP6. At 7 weeks of age, these mice have accumulated iron in their liver and have increased Bmp6 mRNA and protein. However, in contrast to mice with secondary iron overload, levels of phosphorylated Smads 1/5/8 and of Id1 mRNA, both indicators of BMP signaling, are not significantly higher in the liver of these mice than in wild-type livers. As a consequence, hepcidin mRNA levels in Hfe-deficient mice are similar or marginally reduced, compared with 7-week-old wild-type mice. The inappropriately low levels of Id1 and hepcidin mRNA observed at weaning further suggest that Hfe deficiency triggers iron overload by impairing hepatic Bmp/Smad signaling. HFE therefore appears to facilitate signal transduction induced by the BMP6 ligand.
Expression of hepcidin, a key regulator of intestinal iron absorption, can be induced in vitro by several bone morphogenetic proteins (BMPs), including BMP2, BMP4 and BMP9 (refs. 1,2). However, in contrast to BMP6, expression of other BMPs is not regulated at the mRNA level by iron in vivo, and their relevance to iron homeostasis is unclear. We show here that targeted disruption of Bmp6 in mice causes a rapid and massive accumulation of iron in the liver, the acinar cells of the exocrine pancreas, the heart and the renal convoluted tubules. Despite their severe iron overload, the livers of Bmp6-deficient mice have low levels of phosphorylated Smad1, Smad5 and Smad8, and these Smads are not significantly translocated to the nucleus. In addition, hepcidin synthesis is markedly reduced. This indicates that Bmp6 is critical for iron homeostasis and that it is functionally nonredundant with other members of the Bmp subfamily. Notably, Bmp6-deficient mice retain their capacity to induce hepcidin in response to inflammation. The iron burden in Bmp6 mutant mice is significantly greater than that in mice deficient in the gene associated with classical hemochromatosis (Hfe), suggesting that mutations in BMP6 might cause iron overload in humans with severe juvenile hemochromatosis for which the genetic basis has not yet been characterized.
Juvenile hemochromatosis is an iron-overload disorder caused by mutations in the genes encoding the major iron regulatory hormone hepcidin (HAMP) and hemojuvelin (HFE2). We have previously shown that hemojuvelin is a co-receptor for bone morphogenetic proteins (BMPs) and that BMP signals regulate hepcidin expression and iron metabolism. However, the endogenous BMP regulator(s) of hepcidin in vivo is unknown. Here we show that compared with soluble hemojuvelin (HJV.Fc), the homologous DRAGON.Fc is a more potent inhibitor of BMP2 or BMP4 but a less potent inhibitor of BMP6 in vitro. In vivo, HJV.Fc or a neutralizing antibody to BMP6 inhibits hepcidin expression and increases serum iron, whereas DRAGON.Fc has no effect. Notably, Bmp6-null mice have a phenotype resembling hereditary hemochromatosis, with reduced hepcidin expression and tissue iron overload. Finally, we demonstrate a physical interaction between HJV.Fc and BMP6, and we show that BMP6 increases hepcidin expression and reduces serum iron in mice. These data support a key role for BMP6 as a ligand for hemojuvelin and an endogenous regulator of hepcidin expression and iron metabolism in vivo.
Hereditary haemochromatosis (HH), which affects some 1 in 400 and has an estimated carrier frequency of 1 in 10 individuals of Northern European descent, results in multi-organ dysfunction caused by increased iron deposition, and is treatable if detected early. Using linkage-disequilibrium and full haplotype analysis, we have identified a 250-kilobase region more than 3 megabases telomeric of the major histocompatibility complex (MHC) that is identical-by-descent in 85% of patient chromosomes. Within this region, we have identified a gene related to the MHC class I family, termed HLA-H, containing two missense alterations. One of these is predicted to inactivate this class of proteins and was found homozygous in 83% of 178 patients. A role of this gene in haemochromatosis is supported by the frequency and nature of the major mutation and prior studies implicating MHC class I-like proteins in iron metabolism.
Haemochromatosis is a genetic disease associated with progressive iron overload, and is common among populations of northern European origin. HLA-H is a recently reported candidate gene for this condition. Two mutations have been identified, a substitution of cysteine for tyrosine at amino acid 282 (C282Y, nucleotide 845) and of histidine for aspartate at amino acid 63 (H63D, nucleotide 187). Over 90% of UK haemochromatosis patients are homozygous for the C282Y mutation. We have examined 5956 chromosomes (2978 people) for the presence of HLA-H C282Y and H63D by PCR followed by restriction enzyme analysis. We have found world wide allele frequencies of 1.9% for C282Y and 8.1% for H63D. The highest frequencies were 10% for C282Y in 90 Irish chromosomes and 30.4% for H63D in 56 Basque chromosomes. C282Y was most frequent in northern European populations and absent from 1042 African chromosomes, 484 Asian chromosomes, and 644 Australasian chromosomes. The distribution of the C282Y mutation coincides with that of populations in which haemochromatosis has been reported and is consistent with the theory of a north European origin for the mutation. The H63D polymorphism is more widely distributed and its connection with haemochromatosis remains unclear.
Considering that the development of hepatic lesions related to iron overload diseases might be a result of abnormally expressed hepatic genes, we searched for new genes up-regulated under the condition of iron excess. By suppressive subtractive hybridization performed between livers from carbonyl iron-overloaded and control mice, we isolated a 225-base pair cDNA. By Northern blot analysis, the corresponding mRNA was confirmed to be overexpressed in livers of experimentally (carbonyl iron and iron-dextran-treated mice) and spontaneously (beta(2)-microglobulin knockout mice) iron-overloaded mice. In addition, beta(2)-microglobulin knockout mice fed with a low iron content diet exhibited a decrease of hepatic mRNA expression. The murine full-length cDNA was isolated and was found to encode an 83-amino acid protein presenting a strong homology in its C-terminal region to the human antimicrobial peptide hepcidin. In addition, we cloned the corresponding rat and human orthologue cDNAs. Both mouse and human genes named HEPC are constituted of 3 exons and 2 introns and are located on chromosome 7 and 19, respectively, in close proximity to USF2 gene. In mouse and human, HEPC mRNA was predominantly expressed in the liver. During both in vivo and in vitro studies, HEPC mRNA expression was enhanced in mouse hepatocytes under the effect of lipopolysaccharide. Finally, to analyze the intracellular localization of the predicted protein, we used the green fluorescent protein chimera expression vectors. The murine green fluorescent protein-prohepcidin protein was exclusively localized in the nucleus. When the putative nuclear localization signal was deleted, the resulting protein was addressed to the cytoplasm. Taken together, our data strongly suggest that the product of the new liver-specific gene HEPC might play a specific role during iron overload and exhibit additional functions distinct from its antimicrobial activity.
Hereditary hemochromatosis (HH) is a very common disorder characterized by iron overload and multi-organ damage. Several genes involved in iron metabolism have been implicated in the pathology of HH (refs. 1-4). We report that a mutation in the gene encoding Solute Carrier family 11, member A3 (SLC11A3), also known as ferroportin, is associated with autosomal dominant hemochromatosis.
Animal models indicate that the antimicrobial peptide hepcidin (HAMP; OMIM 606464) is probably a key regulator of iron absorption in mammals. Here we report the identification of two mutations (93delG and 166C-->T) in HAMP on 19q13 in two families with a new type of juvenile hemochromatosis.
Experimental data suggest the antimicrobial peptide hepcidin as a central regulator in iron homeostasis. In this study, we characterized the expression of human hepcidin in experimental and clinical iron overload conditions, including hereditary hemochromatosis. Using quantitative reverse transcriptase-polymerase chain reaction (RT-PCR), we determined expression of hepcidin and the most relevant iron-related genes in liver biopsies from patients with hemochromatosis and iron-stain-negative control subjects. Regulation of hepcidin mRNA expression in response to transferrin-bound iron, non-transferrin-bound iron, and deferoxamine was analyzed in HepG2 cells. Hepcidin expression correlated significantly with serum ferritin levels in controls, whereas no significant up-regulation was observed in patients with hemochromatosis despite iron-overload conditions and high serum ferritin levels. However, patients with hemochromatosis showed an inverse correlation between hepcidin transcript levels and the serum transferrin saturation. Moreover, we found a significant correlation between hepatic transcript levels of hepcidin and transferrin receptor-2 irrespective of the iron status. In vitro data indicated that hepcidin expression is down-regulated in response to non-transferrin-bound iron. In conclusion, the presented data suggest a close relationship between the transferrin saturation and hepatic hepcidin expression in hereditary hemochromatosis. Although the causality is not yet clear, this interaction might result from a down-regulation of hepcidin expression in response to significant levels of non-transferrin-bound iron.
Juvenile hemochromatosis is an early-onset autosomal recessive disorder of iron overload resulting in cardiomyopathy, diabetes and hypogonadism that presents in the teens and early 20s (refs. 1,2). Juvenile hemochromatosis has previously been linked to the centromeric region of chromosome 1q (refs. 3-6), a region that is incomplete in the human genome assembly. Here we report the positional cloning of the locus associated with juvenile hemochromatosis and the identification of a new gene crucial to iron metabolism. We finely mapped the recombinant interval in families of Greek descent and identified multiple deleterious mutations in a transcription unit of previously unknown function (LOC148738), now called HFE2, whose protein product we call hemojuvelin. Analysis of Greek, Canadian and French families indicated that one mutation, the amino acid substitution G320V, was observed in all three populations and accounted for two-thirds of the mutations found. HFE2 transcript expression was restricted to liver, heart and skeletal muscle, similar to that of hepcidin, a key protein implicated in iron metabolism. Urinary hepcidin levels were depressed in individuals with juvenile hemochromatosis, suggesting that hemojuvelin is probably not the hepcidin receptor. Rather, HFE2 seems to modulate hepcidin expression.
The hepatic peptide hepcidin is the key regulator of iron metabolism in mammals. Recent evidence indicates that certain forms of hereditary hemochromatosis are caused by hepcidin deficiency. Juvenile hemochromatosis is associated with hepcidin or hemojuvelin mutations, and these patients have low or absent urinary hepcidin. Patients with C282Y HFE hemochromatosis also have inappropriately low hepcidin levels for the degree of iron loading. The relationship between the hemochromatosis due to transferrin receptor 2 (TFR2) mutations and hepcidin was unknown. We measured urinary hepcidin levels in 10 patients homozygous for TFR2 mutations, all with increased transferrin saturation. Urinary hepcidin was low or undetectable in 8 of 10 cases irrespective of the previous phlebotomy treatments. The only 2 cases with normal hepcidin values had concomitant inflammatory conditions. Our data indicate that TFR2 is a modulator of hepcidin production in response to iron.
Hereditary hemochromatosis is an iron-overload disorder resulting from mutations in proteins presumed to be involved in the maintenance of iron homeostasis. Mutations in hemojuvelin (HJV) cause severe, early-onset juvenile hemochromatosis. The normal function of HJV is unknown. Juvenile hemochromatosis patients have decreased urinary levels of hepcidin, a peptide hormone that binds to the cellular iron exporter ferroportin, causing its internalization and degradation. We have disrupted the murine Hjv gene and shown that Hjv-/- mice have markedly increased iron deposition in liver, pancreas, and heart but decreased iron levels in tissue macrophages. Hepcidin mRNA expression was decreased in Hjv-/- mice. Accordingly, ferroportin expression detected by immunohistochemistry was markedly increased in both intestinal epithelial cells and macrophages. We propose that excess, unregulated ferroportin activity in these cell types leads to the increased intestinal iron absorption and plasma iron levels characteristic of the juvenile hemochromatosis phenotype.
Iron homeostasis plays a critical role in many physiological processes, notably synthesis of heme proteins. Dietary iron sensing and inflammation converge in the control of iron absorption and retention by regulating the expression of hepcidin, a regulator of the iron exporter ferroportin. Human mutations in the glycosylphosphatidylinositol-anchored protein hemojuvelin (HJV; also known as RGMc and HFE2) cause juvenile hemochromatosis, a severe iron overload disease, but the way in which HJV intersects with the iron regulatory network has been unclear. Here we show that, within the liver, mouse Hjv is selectively expressed by periportal hepatocytes and also that Hjv-mutant mice exhibit iron overload as well as a dramatic decrease in hepcidin expression. Our findings define a key role for Hjv in dietary iron sensing and also reveal that cytokine-induced inflammation regulates hepcidin expression through an Hjv-independent pathway.
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.
Hepcidin production is homeostatically regulated by iron stores, anemia and hypoxia. We evaluated the effect of iron overload and of ineffective erythropoeisis on hepcidin expression in patients with thalassemia major. Liver hepcidin mRNA levels correlated with hemoglobin concentration and inversely correlated with serum transferrin receptor, erythropoietin and non-transferrin-bound iron. They did not correlate with indices of iron load. Urinary hepcidin levels were disproportionably suppressed in regards to iron burden. We conclude that hepcidin expression is regulated mainly by increased erythropoietic activity rather than by iron load and that hepcidin plays a central regulatory role in iron circulation and iron toxicity in patients with thalassemia.
Although hepcidin expression was shown to be induced by the BMP/Smad signaling pathway, it is not yet known how iron regulates this pathway and what its exact molecular targets are. We therefore assessed genome-wide liver transcription profiles of mice of 2 genetic backgrounds fed iron-deficient, -balanced, or -enriched diets. Among 1419 transcripts significantly modulated by the dietary iron content, 4 were regulated similarly to the hepcidin genes Hamp1 and Hamp2. They are coding for Bmp6, Smad7, Id1, and Atoh8 all related to the Bmp/Smad pathway. As shown by Western blot analysis, variations in Bmp6 expression induced by the diet iron content have for functional consequence similar changes in Smad1/5/8 phosphorylation that leads to formation of heteromeric complexes with Smad4 and their translocation to the nucleus. Gene expression variations induced by secondary iron deficiency or iron overload were compared with those consecutive to Smad4 and Hamp1 deficiency. Iron overload developed by Smad4- and Hamp1-deficient mice also increased Bmp6 transcription. However, as shown by analysis of mice with liver-specific disruption of Smad4, activation of Smad7, Id1, and Atoh8 transcription by iron requires Smad4. This study points out molecules that appear to play a critical role in the control of systemic iron balance.
Hepcidin is a peptide hormone secreted by the liver in response to iron loading and inflammation. Decreased hepcidin leads
to tissue iron overload, whereas hepcidin overproduction leads to hypoferremia and the anemia of inflammation. Ferroportin
is an iron exporter present on the surface of absorptive enterocytes, macrophages, hepatocytes, and placental cells. Here
we report that hepcidin bound to ferroportin in tissue culture cells. After binding, ferroportin was internalized and degraded,
leading to decreased export of cellular iron. The posttranslational regulation of ferroportin by hepcidin may thus complete
a homeostatic loop: Iron regulates the secretion of hepcidin, which in turn controls the concentration of ferroportin on the
cell surface.
Abnormal hepcidin regulation is central to the pathogenesis of HFE hemochromatosis. Hepatic bone morphogenetic protein 6 (BMP6)-SMAD signaling is a main regulatory mechanism controlling hepcidin expression, and this pathway was recently shown to be impaired in Hfe knockout (Hfe(-/-)) mice. To more definitively determine whether HFE regulates hepcidin expression through an interaction with the BMP6-SMAD signaling pathway, we investigated whether hepatic Hfe overexpression activates the BMP6-SMAD pathway to induce hepcidin expression. We then investigated whether excess exogenous BMP6 administration overcomes the BMP6-SMAD signaling impairment and ameliorates hemochromatosis in Hfe(-/-) mice.
The BMP6-SMAD pathway and the effects of neutralizing BMP6 antibody were examined in Hfe transgenic mice (Hfe Tg) compared with wild-type (WT) mice. Hfe(-/-) and WT mice were treated with exogenous BMP6 and analyzed for hepcidin expression and iron parameters.
Hfe Tg mice exhibited hepcidin excess and iron deficiency anemia. Hfe Tg mice also exhibited increased hepatic BMP6-SMAD target gene expression compared with WT mice, whereas anti-BMP6 antibody administration to Hfe Tg mice improved the hepcidin excess and iron deficiency. In Hfe(-/-) mice, supraphysiologic doses of exogenous BMP6 improved hepcidin deficiency, reduced serum iron, and redistributed tissue iron to appropriate storage sites.
HFE interacts with the BMP6-SMAD signaling pathway to regulate hepcidin expression, but HFE is not necessary for hepcidin induction by BMP6. Exogenous BMP6 treatment in mice compensates for the molecular defect underlying Hfe hemochromatosis, and BMP6-like agonists may have a role as an alternative therapeutic strategy for this disease.
Unlabelled:
Hereditary hemochromatosis (HH) is a common inherited iron overload disorder. The vast majority of patients carry the missense Cys282Tyr mutation of the HFE gene. Hepcidin, the central regulator of iron homeostasis, is deficient in HH, leading to unchecked iron absorption and subsequent iron overload. The bone morphogenic protein (BMP)/small mothers against decapentaplegic (Smad) signaling cascade is central to the regulation of hepcidin. Recent data from HH mice models indicate that this pathway may be defective in the absence of the HFE protein. Hepatic BMP/Smad signaling has not been characterized in a human HFE-HH cohort to date. Hepatic expression of BMP/Smad-related genes was examined in 20 HFE-HH males with significant iron overload, and compared to seven male HFE wild-type controls using quantitative real-time reverse transcription polymerase chain reaction. Hepatic expression of BMP6 was appropriately elevated in HFE-HH compared to controls (P = 0.02), likely related to iron overload. Despite this, no increased expression of the BMP target genes hepcidin and Id1 was observed, and diminished phosphorylation of Smad1/Smad5/Smad8 protein relative to iron burden was found upon immunohistochemical analysis, suggesting that impaired BMP signaling occurs in HFE-HH. Furthermore, Smad6 and Smad7, inhibitors of BMP signaling, were up-regulated in HFE-HH compared to controls (P = 0.001 and P = 0.018, respectively).
Conclusion:
New data arising from this study suggest that impaired BMP signaling underlies the hepcidin deficiency of HFE-HH. Moreover, the inhibitory Smads, Smad6, and Smad7 are identified as potential disruptors of this signal and, hence, contributors to the pathogenesis of this disease.
In the late 1800s, hemochromatosis was considered an odd autoptic finding. More than a century later, it was finally recognized as a hereditary, multi-organ disorder associated with a polymorphism that is common among white people: a 845G-->A change in HFE that results in C282Y in the gene product. Hemochromatosis is now a well-defined syndrome characterized by normal iron-driven erythropoiesis and the toxic accumulation of iron in parenchymal cells of liver, heart, and endocrine glands. It can be caused by mutations that affect any of the proteins that limit the entry of iron into the blood. In mice, deletion of the iron hormone hepcidin and any of 8 genes that regulate its biology, including Hfe, transferrin receptor 2 (Tfr2), and hemojuvelin (Hjv) (which all sense the accumulation of iron that hepcidin corrects) or ferroportin (Fpn) (the cellular iron exporter down-regulated by hepcidin), cause iron overload but not organ disease. In humans, loss of TfR2, HJV, and hepcidin itself or FPN mutations result in full-blown hemochromatosis. Unlike these rare instances, in white people, homozygotes for C282Y polymorphism in HFE are numerous, but they are only predisposed to hemochromatosis; complete organ disease develops in a minority, when these individuals abuse alcohol or from other unidentified modifying factors. HFE gene testing can be used to diagnose hemochromatosis, but analyses of liver histology and clinical features are still required to identify patients with rare, non-HFE forms of the disease. The role of hepcidin in the pathogenesis of hemochromatosis reveals its similarities to endocrine diseases such as diabetes and indicates new approaches to diagnosis and management of this common disorder in iron metabolism.
Hepcidin is the master regulatory hormone of systemic iron metabolism. Hepcidin deficiency causes common iron overload syndromes whereas its overexpression is responsible for microcytic anemias. Hepcidin transcription is activated by the bone morphogenetic protein (BMP) and the inflammatory JAK-STAT pathways, whereas comparatively little is known about how hepcidin expression is inhibited. By using high-throughput siRNA screening we identified SMAD7 as a potent hepcidin suppressor. SMAD7 is an inhibitory SMAD protein that mediates a negative feedback loop to both transforming growth factor-beta and BMP signaling and that recently was shown to be coregulated with hepcidin via SMAD4 in response to altered iron availability in vivo. We show that SMAD7 is coregulated with hepcidin by BMPs in primary murine hepatocytes and that SMAD7 overexpression completely abolishes hepcidin activation by BMPs and transforming growth factor-beta. We identify a distinct SMAD regulatory motif (GTCAAGAC) within the hepcidin promoter involved in SMAD7-dependent hepcidin suppression, demonstrating that SMAD7 does not simply antagonize the previously reported hemojuvelin/BMP-responsive elements. This work identifies a potent inhibitory factor for hepcidin expression and uncovers a negative feedback pathway for hepcidin regulation, providing insight into a mechanism how hepcidin expression may be limited to avoid iron deficiency.
The BMP signaling pathway controls a number of cell processes during development and in adult tissues. At the cellular level, ligands of the BMP family act by binding a hetero-tetrameric signaling complex, composed of two type I and two type II receptors. BMP ligands make use of a limited number of receptors, which in turn activate a common signal transduction cascade at the intracellular level. A complex regulatory network is required in order to activate the signaling cascade at proper times and locations, and to generate specific downstream effects in the appropriate cellular context. One such regulatory mechanism is the repulsive guidance molecule (RGM) family of BMP co-receptors. This article reviews the current knowledge regarding the structure, regulation, and function of RGMs, focusing on known and potential roles of RGMs in physiology and pathophysiology.
Mutations in HFE are the most common cause of the iron-overload disorder hereditary hemochromatosis. Levels of the main iron regulatory hormone, hepcidin, are inappropriately low in hereditary hemochromatosis mouse models and patients with HFE mutations, indicating that HFE regulates hepcidin. The bone morphogenetic protein 6 (BMP6)-SMAD signaling pathway is an important endogenous regulator of hepcidin expression. We investigated whether HFE is involved in BMP6-SMAD regulation of hepcidin expression.
The BMP6-SMAD pathway was examined in Hfe knockout (KO) mice and in wild-type (WT) mice as controls. Mice were placed on diets of varying iron content. Hepcidin induction by BMP6 was examined in primary hepatocytes from Hfe KO mice; data were compared with those of WT mice.
Liver levels of Bmp6 messenger RNA (mRNA) were higher in Hfe KO mice; these were appropriate for the increased hepatic levels of iron in these mice, compared with WT mice. However, levels of hepatic phosphorylated Smad 1/5/8 protein (an intracellular mediator of Bmp6 signaling) and Id1 mRNA (a target gene of Bmp6) were inappropriately low for the body iron burden and Bmp6 mRNA levels in Hfe KO, compared with WT mice. BMP6 induction of hepcidin expression was reduced in Hfe KO hepatocytes compared with WT hepatocytes.
HFE is not involved in regulation of BMP6 by iron, but does regulate the downstream signals of BMP6 that are triggered by iron.
Haemochromatosis is a common recessive disorder characterized by progressive iron overload, which may lead to severe clinical complications. Most patients are homozygous for the C282Y mutation in HFE on 6p (refs 1-5). A locus for juvenile haemochromatosis (HFE2) maps to 1q (ref. 7). Here we report a new locus (HFE3) on 7q22 and show that a homozygous nonsense mutation in the gene encoding transferrin receptor-2 (TFR2) is found in people with haemochromatosis that maps to HFE3.
Members of the transforming growth factor-beta (TGF-beta) superfamily bind to two different serine/threonine kinase receptors, i.e. type I and type II receptors. Upon ligand binding, type I receptors specifically activate intracellular Smad proteins. R-Smads are direct substrates of type I receptors; Smads 2 and 3 are specifically activated by activin/nodal and TGF-beta type I receptors, whereas Smads 1, 5 and 8 are activated by BMP type I receptors. Nearly 30 proteins have been identified as members of the TGF-beta superfamily in mammals, and can be classified based on whether they activate activin/TGF-beta-specific R-Smads (AR-Smads) or BMP-specific R-Smads (BR-Smads). R-Smads form complexes with Co-Smads and translocate into the nucleus, where they regulate the transcription of target genes. AR-Smads bind to various proteins, including transcription factors and transcriptional co-activators or co-repressors, whereas BR-Smads interact with other proteins less efficiently than AR-Smads. Id proteins are induced by BR-Smads, and play important roles in exhibiting some biological effects of BMPs. Understanding the mechanisms of TGF-beta superfamily signalling is thus important for the development of new ways to treat various clinical diseases in which TGF-beta superfamily signalling is involved.
The mechanisms responsible for disturbed iron homoeostasis in hereditary haemochromatosis are poorly understood. However, results of some studies indicate a link between hepcidin, a liver-derived peptide, and intestinal iron absorption, suggesting that this molecule could play a part in hepatic iron overload. To investigate this possible association, we studied the hepatic expression of the gene for hepcidin (HAMP) and a gene important in iron transport (IREG1) in patients with haemochromatosis, in normal controls, and in Hfe-knockout mice.
We extracted total RNA from the liver tissue of 27 patients with HFE-associated haemochromatosis, seven transplant donors (controls), and Hfe-knockout mice. HAMP and IREG1 mRNA concentrations were examined by ribonuclease protection assays and expressed relative to the housekeeping gene GAPD.
There was a significant decrease in HAMP expression in untreated patients compared with controls (5.4-fold, 95% CI 3.3-7.5; p<0.0001) despite significantly increased iron loading. Similarly, we noted a decrease in Hamp expression in iron-loaded Hfe-knockout mice. Hepatic IREG1 expression was greatly upregulated in patients with haemochromatosis (1.8-fold, 95% CI 1.5-2.2; p=0.002). There was a significant correlation between hepatic iron concentration and expression of HAMP (r=0.59, p=0.02) and IREG1 (r=0.67, p=0.007) in untreated patients.
Lack of HAMP upregulation in HFE-associated haemochromatosis despite significant hepatic iron loading indicates that HFE plays an important part in the regulation of hepcidin expression in response to iron overload. Our results imply that the liver is important in the pathophysiology of HFE-associated haemochromatosis. Furthermore, the increase in hepatic IREG1 expression in haemochromatosis suggests that IREG1 could function to facilitate the removal of excess iron from the liver.
In HFE-related hereditary hemochromatosis an inappropriately low hepatic expression of the iron-regulatory peptide hepcidin (encoded by HAMP) has been suggested to cause iron overload. The aim of the present study was to evaluate whether the hepatic expression of HAMP in relation to iron stores requires HFE or might involve other important iron-related genes including HJV (encoding hemojuvelin) and TFR2 (encoding transferrin receptor-2).
Using quantitative RT-PCR, the iron-dependent hepatic expression patterns of HAMP, HJV, and TFR2 were evaluated in human and murine HFE-related hemochromatosis.
The overall level of hepatic HAMP expression in human and murine HFE-related hemochromatosis is impaired but can still be modulated by iron stores. Moreover, we demonstrate an HFE-independent correlation between the expression of HAMP and TFR2 in mouse and human livers. On the other hand, a strong correlation between the hepatic expression of HAMP and HJV was only found in hemochromatosis patients and Hfe-deficient mice.
The central pathogenetic step in HFE-related hemochromatosis is an impaired basal expression of HAMP rather than a lack of HAMP upregulation in response to iron stores. An HFE-independent pathway that seems to involve TFR2 and HJV can regulate HAMP expression under conditions of iron overload.
Hereditary hemochromatosis, characterized by iron overload in multiple organs, is one of the most common genetic disorders among Caucasians. Hepcidin, which is synthesized in the liver, plays important roles in iron overload syndromes. Here, we show that a Cre-loxP-mediated liver-specific disruption of SMAD4 results in markedly decreased hepcidin expression and accumulation of iron in many organs, which is most pronounced in liver, kidney, and pancreas. Transcript levels of genes involved in intestinal iron absorption, including Dcytb, DMT1, and ferroportin, are significantly elevated in the absence of hepcidin. We demonstrate that ectopic overexpression of SMAD4 activates the hepcidin promoter and is associated with epigenetic modification of histone H3 to a transcriptionally active form. Moreover, transcriptional activation of hepcidin is abrogated in SMAD4-deficient hepatocytes in response to iron overload, TGF-beta, BMP, or IL-6. Our study uncovers a novel role of TGF-beta/SMAD4 in regulating hepcidin expression and thus intestinal iron transport and iron homeostasis.
Mutations in hemojuvelin disrupt its ability to stimulate expression of the iron regulatory peptide hepcidin and result in the severe iron loading disorder juvenile hemochromatosis. A new study shows that hemojuvelin acts through the multifunctional bone morphogenetic protein pathway to modulate hepcidin levels, providing new insights into communication within a key physiological pathway.
Systemic iron balance is regulated by hepcidin, a peptide hormone secreted by the liver. By decreasing cell surface expression of the iron exporter ferroportin, hepcidin decreases iron absorption from the intestine and iron release from reticuloendothelial stores. Hepcidin excess has been implicated in the pathogenesis of anemia of chronic disease, while hepcidin deficiency has a key role in the pathogenesis of the iron overload disorder hemochromatosis. We have recently shown that hemojuvelin is a coreceptor for bone morphogenetic protein (BMP) signaling and that BMP signaling positively regulates hepcidin expression in liver cells in vitro. Here we show that BMP-2 administration increases hepcidin expression and decreases serum iron levels in vivo. We also show that soluble hemojuvelin (HJV.Fc) selectively inhibits BMP induction of hepcidin expression in vitro and that administration of HJV.Fc decreases hepcidin expression, increases ferroportin expression, mobilizes splenic iron stores, and increases serum iron levels in vivo. These data support a role for modulators of the BMP signaling pathway in treating diseases of iron overload and anemia of chronic disease.
Helicobacter pylori (Hp) infection causes a chronic gastric inflammation, which can lead to peptic ulceration and cancer. The inflammatory response is multifactorial and is characterized by exaggerated Th1 cytokine production. How the Th1 response is induced and maintained in the stomach of Hp-infected patients remains unclear. Transforming growth factor (TGF)-beta 1 negatively regulates Th1 cell development, and TGF-beta 1-deficient mice spontaneously develop gastritis. Here, we examined TGF-beta 1 signaling in Hp-associated gastritis.
Gastric biopsy specimens taken from patients with or without Hp infection were analyzed for the content of activated TGF-beta1 by ELISA and Smad3 and 7 expression by Western blotting. Induction of Smad7 by interferon (IFN)-gamma was examined in normal gastric mucosal biopsy specimens, whereas the effect of Smad7 inhibition on the ongoing Th1 response was analyzed in Hp-colonized biopsy specimens.
Activated TGF-beta 1 was abundant in the mucosa of controls and Hp-infected patients, with no significant difference between the 2 groups. Despite this, in whole biopsy specimens and isolated mucosal cells from Hp-infected patients, there was defective TGF-beta 1-associated Smad3 phosphorylation, which was associated with high expression of the inhibitor Smad7. Blocking Smad7 with antisense oligonucleotides restored TGF-beta 1 signaling in biopsy specimens from Hp-infected patients and concomitantly reduced interferon-gamma and T-bet. Smad7 was inducible in normal gastric biopsy specimens by interferon-gamma through a STAT1-dependent mechanism, and neutralization of interferon-gamma in biopsy specimens from Hp-infected patients reduced Smad7 expression.
These data suggest that, in Hp-infected gastric mucosa, interferon-gamma induces the expression of Smad7, which then prevents endogenous TGF-beta 1 from down-regulating the ongoing tissue-damaging Th1 response.
Hemojuvelin is essential for dietary iron sensing, and its mutation leads to severe iron overloadPubMed: 16075058] Bolondi et al. Page 6 Blood Cells Mol Dis Author manuscript; available in PMC 2012 March 06. NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript 29
- V Niederkofler
- R Salie
- Arber
Niederkofler V, Salie R, Arber S. Hemojuvelin is essential for dietary iron sensing, and its mutation leads to severe iron overload. J Clin Invest. 2005; 115:2180–6. [PubMed: 16075058] Bolondi et al. Page 6 Blood Cells Mol Dis. Author manuscript; available in PMC 2012 March 06. NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript 29. Torrance, JD.; Bothwell, TH. Tissue iron stores. In: Cook, JD., editor. Methods in Hematology. Churchill Livingstone; New York, NY: 1980. p. 104
Iron regulates phosphorylation of Smad1
- L Kautz
- D Meynard
- A Monnier
L. Kautz, D. Meynard, A. Monnier, et al., Iron regulates phosphorylation of Smad1/5/8 and gene expression of Bmp6, Smad7, Id1, and Atoh8 in the mouse liver, Blood 112 (2008) 1503–1509.
Modulation of bone morphogenetic protein signaling in vivo regulates systemic iron balance
- Jl Babitt
- Fw Huang
- Y Xia
Babitt JL, Huang FW, Xia Y, et al. Modulation of bone morphogenetic protein signaling in vivo
regulates systemic iron balance. J Clin Invest. 2007; 117:1933–1939. [PubMed: 17607365]
BMP6 is a key endogenous regulator of hepcidin expression and iron metabolism
- B Andriopoulos
- Jr
- E Corradini
- Y Xia
Andriopoulos B Jr, Corradini E, Xia Y, et al. BMP6 is a key endogenous regulator of hepcidin
expression and iron metabolism. Nat Genet. 2009; 41:482–7. [PubMed: 19252486]