The distal location of the iron responsive region of the hepcidin promoter

Bone morphogenic proteins in iron homeostasis

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

Selenoprotein P neutralizes lipopolysaccharide and participates in hepatic cell endoplasmic reticulum stress response

Low serum selenium or selenoprotein P (SePP) levels have been repetitively observed in severe sepsis. The role of SePP in sepsis is incompletely characterized. To test the hypothesis that lipopolysaccharide (LPS) interacts with SePP, we investigated the interaction between LPS and the histidine-rich (His-rich) regions of SePP. We demonstrate that both purified SePP and synthetic peptides corresponding to the His-rich motifs neutralized LPS. In addition, we used a hepatocyte model to study the fate of SePP in response to LPS or endoplasmic reticulum (ER) stress. Our findings indicate that ER stress increases the cellular level of SePP and promotes its nuclear localization.

The SMAD Pathway Is Required for Hepcidin Response During Endoplasmic Reticulum Stress

Hepcidin, the iron hormone, is regulated by a number of stimulatory and inhibitory signals. The cAMP responsive element binding protein 3-like 3 (CREB3L3) mediates hepcidin response to endoplasmic reticulum (ER) stress. In this study we asked whether hepcidin response to ER stress also requires the small mother against decapentaplegic (SMAD)-1/5/8 pathway, which has a major role in hepcidin regulation in response to iron and other stimuli. We analyzed hepcidin mRNA expression and promoter activity in response to ER stressors in HepG2 cells in the presence of the bone morphogenetic protein (BMP) type I receptor inhibitor LDN-193189, mutated hepcidin promoter or small interfering RNA against different SMAD proteins. We then used a similar approach in vivo in wild-type, Smad1/5, or Creb3l3^-/- animals undergoing ER stress. In vitro, LDN-193189 prevented hepcidin mRNA induction by different ER stressors. Seemingly, mutation of a BMP-responsive element in the hepcidin promoter prevented ER stress-mediated up-regulation. Moreover, in vitro silencing of SMAD proteins by small interfering RNA, in particular SMAD5, blunted hepcidin response to ER stress. On the contrary, hepcidin induction by ER stress was maintained when using antibodies against canonical BMP receptor ligands. In vivo, hepcidin was induced by ER stress and prevented by LDN-193189. In addition, in Smad1/5 knockout mice, ER stress was unable to induce hepcidin expression. Finally, in Creb3l3 knockout mice, in response to ER stress, SMAD1/5 were correctly phosphorylated and hepcidin induction was still appreciable, although to a lesser extent as compared with the control mice. In conclusion, our study indicates that hepcidin induction by ER stress involves the central regulatory SMAD1/5 pathway.

Regulation of Hepcidin by Erythropoiesis: The Story So Far

Hepcidin is the master regulator of systemic iron homeostasis, facilitating iron balance by controlling intestinal iron absorption and recycling. Hepcidin levels are suppressed when erythropoiesis is stimulated, for example following acute blood loss, appropriately enhancing cellular iron export to the plasma to support production of new red blood cells. However, persistent increased and ineffective erythropoiesis, for example in thalassemia, results in sustained elevations in iron absorption, which cause iron overload with associated organ toxicities. The ligands, receptors, and canonical pathways by which iron loading and inflammation upregulate hepcidin expression have been largely established. However, although several mechanisms have been proposed, the means by which erythropoiesis causes hepcidin suppression have been unclear. The erythroid-derived hormone erythroferrone appears to be a convincing candidate for the link between increased erythropoiesis and hepcidin suppression. If confirmed to be clinically and physiologically relevant in humans, potentiation or inhibition of erythroferrone activity could be a crucial pharmaceutical strategy.

Bone morphogenetic proteins as regulators of iron metabolism

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor-beta (TGF-β) superfamily of signaling molecules. In addition to protean roles in embryonic development, germ-line specification, and cellular differentiation, a central role in iron homeostasis has recently been demonstrated for certain BMPs. Specifically, BMP6 serves to relate hepatic iron stores to the hepatocellular expression of the iron-regulatory hormone hepcidin. This regulation occurs via cellular SMAD-signaling molecules and is strongly modulated by the BMP coreceptor hemojuvelin (HJV). Mutations in certain genes influencing signaling to hepcidin via the BMP/SMAD pathway are associated with human disorders of iron metabolism, such as hereditary hemochromatosis and iron-refractory iron-deficiency anemia. Evidence suggests that signals in addition to iron stores influence hepcidin expression via the BMP/SMAD pathway. This review summarizes the details of BMP/SMAD signaling, with a particular focus on its role in iron homeostasis and iron-related diseases.

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

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

A multi-scale model of hepcidin promoter regulation reveals factors controlling 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 signaling cascades. Deregulation of either pathway causes iron-related diseases such as hemochromatosis or anemia of inflammation. We quantitatively analyzed how BMP6 and IL6 control hepcidin expression. Transcription factor (TF) phosphorylation and reporter gene expression were measured under co-stimulation conditions, and the promoter was perturbed by mutagenesis. Using mathematical modeling, we systematically analyzed potential mechanisms of cooperative and competitive promoter regulation by the transcription factors, and experimentally validated the model predictions. Our results reveal that hepcidin cross-regulation primarily occurs by combinatorial transcription factor binding to the promoter, whereas signaling crosstalk is insignificant. We find that the presence of two BMP-responsive elements enhances the steepness of the promoter response towards the iron-sensing BMP signaling axis, which promotes iron homeostasis in vivo. IL6 co-stimulation reduces the promoter sensitivity towards the BMP signal, because the SMAD and STAT transcription factors compete for recruiting RNA polymerase to the transcription start site. This may explain why inflammatory signals disturb iron homeostasis in anemia of inflammation. Taken together, our results reveal why the iron homeostasis circuit is sensitive to perturbations implicated in disease.

The nutritional iron uptake is tightly regulated because the body has limited capacity of iron excretion. Mammals maintain iron homeostasis by a negative feedback loop, in which the peptide hepcidin senses the iron blood level and controls iron resorption. Molecular perturbations in the homeostasis loop lead to iron-related diseases such as hemochromatosis or anemia of inflammation. Quantitative studies are required to understand the dynamics of the iron homeostasis circuitry in health and disease. We investigated how the biological activity of hepcidin is regulated by combining experiments with mathematical modeling. We present a multi-scale model that describes the signaling network and the gene promoter controlling hepcidin expression. Possible scenarios of hepcidin regulation were systematically tested against experimental data, and interpreted using a network model of iron metabolism in vivo. The analysis showed that the presence of multiple redundant regulatory elements in the hepcidin gene promoter facilitates homeostasis, because changes in iron blood levels are sensed with high sensitivity. We further suggest that inflammatory signals establish molecular competition at the hepcidin promoter, thereby reducing its iron sensitivity and leading to a loss of homeostasis in anemia of inflammation. We conclude that quantitative insights into hepcidin expression regulation explain features of systemic iron homeostasis.

17β-Estradiol inhibits iron hormone hepcidin through an estrogen responsive element half-site

Interaction of estrogen with iron at the systemic level is long suspected, but direct evidence linking the two is limited. In the present study, we examined the effects of 17β-estradiol (E2) on hepcidin, a key negative regulator of iron absorption from the liver. We found that transcription of hepcidin was suppressed by E2 treatment in human liver HuH7 and HepG2 cells, and this down-regulation was blocked by E2 antagonist ICI 182780. Chromatin immunoprecipitation, deletion, and EMSA detected a functional estrogen responsive element half-site that is located between -2474 and -2462 upstream from the start of transcription of the hepcidin gene. After cloning the human hepcidin promoter into the pGL3Luc-Reporter vector, luciferase activity was also down-regulated by E2 treatment in HepG2 cells. E2 reduced hepcidin mRNA in wild-type mice as well as in hemochromatosis Fe gene knockout mice. In summary, our data suggest that hepcidin inhibition by E2 is to increase iron uptake, a mechanism to compensate iron loss during menstruation. This mechanism may also contribute to increased iron stores in oral contraceptive users.

High hepcidin level accounts for the nigral iron accumulation in acute peripheral iron intoxication rats

Hepcidin is considered to be a circulatory hormone and a major mechanism regulating iron homeostasis. Our previous publication revealed that acute iron intoxication induced iron deposit and dopaminergic neuron degeneration in the substantia nigra (SN) of a rat model. However, whether and how hepcidin functions in this nigral iron accumulation has not been elucidated. In the present study, we observed a decreased of FPN1 protein level in the SN triggered by peripheral iron overload within 4 h, which correlated with a high hepcidin level. To further investigate the role of intracellular hepcidin under iron overload circumstances, we assessed the expression of hepcidin mRNA and FPN1 protein in vitro. We observed that hepcidin mRNA level was up-regulated and FPN1 protein level was down-regulated in MES23.5 dopaminergic cells in a period of 4h incubation with iron. Both in pCMV-XL4-hepcidin transfected and hepcidin-treated cells, decreased FPN1 protein levels were observed. Our data provide direct evidence that the role for intracellular hepcidin generated in the SN is particularly relevant to restrict iron release by down-regulation FPN1 expression in this region, thus an important contributor to the abnormal iron deposit occurred at an early stage in conditions of peripheral iron intoxication.

Iron-sensing proteins that regulate hepcidin and enteric iron absorption

The human body cannot actively excrete excess iron. As a consequence, iron absorption must be strictly regulated to ensure adequate iron uptake and prevent toxic iron accumulation. Iron absorption is controlled chiefly by hepcidin, the iron-regulatory hormone. Produced by the liver and secreted into the circulation, hepcidin regulates iron metabolism by inhibiting iron release from cells, including duodenal enterocytes, which mediate the absorption of dietary iron. Hepcidin production increases in response to iron loading and decreases in iron deficiency. Such regulation of hepcidin expression serves to modulate iron absorption to meet body iron demand. This review discusses the proteins that orchestrate hepatic hepcidin production and iron absorption by the intestine. Emphasis is placed on the proteins that directly sense iron and how they coordinate and fine-tune the molecular, cellular, and physiologic responses to iron deficiency and overload.

Two to tango: regulation of Mammalian iron metabolism

Disruptions in iron homeostasis from both iron deficiency and overload account for some of the most common human diseases. Iron metabolism is balanced by two regulatory systems, one that functions systemically and relies on the hormone hepcidin and the iron exporter ferroportin, and another that predominantly controls cellular iron metabolism through iron-regulatory proteins that bind iron-responsive elements in regulated messenger RNAs. We describe how the two distinct systems function and how they "tango" together in a coordinated manner. We also highlight some of the current questions in mammalian iron metabolism and discuss therapeutic opportunities arising from a better understanding of the underlying biological principles.

Suppression of the hepcidin-encoding gene Hamp permits iron overload in mice lacking both hemojuvelin and matriptase-2/TMPRSS6

Hepcidin, the master regulator of enteric iron absorption, is controlled by the opposing effects of pathways activated in response to iron excess or iron attenuation. Iron excess is regulated through a pathway involving the cell surface receptor hemojuvelin (HFE2) that stimulates expression of the hepcidin encoding gene (HAMP). Iron attenuation is countered through a pathway involving the hepatocyte-specific plasma membrane protease matriptase-2 encoded by TMPRSS6, leading to suppression of HAMP expression. The non-redundant function of hemojuvelin and matriptase-2 has been deduced from the phenotype imparted by mutations of HFE2 and TMPRSS6, which cause iron excess and iron deficiency respectively. Hemojuvelin is positioned to be the ideal substrate for matriptase-2. To examine the relationship between hemojuvelin and matriptase-2 in vivo, we crossed mice lacking the protease domain of matriptase-2 with mice lacking hemojuvelin. Mice lacking functional matriptase-2 and hemojuvelin exhibited low Hamp (Hamp1) expression, high serum and liver iron, and high transferrin saturation. Surprisingly, the double mutant mice also exhibited lower levels of iron in the heart compared to hemojuvelin-deficient mice, demonstrating a possible cardioprotective effect resulting from the loss of matriptase-2. This phenotype is consistent with hemojuvelin being a major substrate for matriptase-2/TMPRSS6 protease activity.

Role of STAT1, NF-kappaB, and C/EBPbeta in the macrophage transcriptional regulation of hepcidin by mycobacterial infection and IFN-gamma

Hepcidin is an antimicrobial peptide involved in regulating iron homeostasis. It is induced by iron overload and decreased by hypoxia and anemia. Hepcidin regulates iron metabolism by inhibiting iron absorption by the duodenum and by inhibiting macrophage iron recycling. Hepcidin is induced in hepatocytes during the acute-phase response by IL-6. Previously, we have shown that hepcidin is not induced in macrophages by IL-6 but is induced by the synergistic interaction of IFN-gamma and Mycobacterium tuberculosis infection. In the present study, we examined the pathways involved in inducing macrophage hepcidin expression. We show that TLRs TLR2 and TLR4 and the transcription factor STAT1 are required for induction of hepcidin mRNA. Hepcidin promoter activity is also synergistically induced in RAW264.7 macrophages by IFN-gamma and M. tuberculosis. NF-kappaB and C/CEBP binding sites are required for promoter activity. Binding of NF-kappaB (p50/p65) to the NF-kappaB site and STAT1 and C/EBPbeta to the C/CEBP site was confirmed by EMSA. Knockdown of STAT1 and C/EBPbeta expression in RAW264.7 cells with siRNA plasmids inhibited hepcidin promoter activity induced by IFN-gamma and M. tuberculosis. Together, these studies demonstrate that macrophage hepcidin expression is induced by the activation of STAT1 and NF-kappaB and the induction of C/EBPbeta expression.

Regulation of hepcidin and iron-overload disease

Hepcidin, a 25-amino-acid antimicrobial peptide, is the central regulator of iron homeostasis. Hepcidin transcription is upregulated by inflammatory cytokines, iron, and bone morphogenetic proteins and is downregulated by iron deficiency, ineffective erythropoiesis, and hypoxia. The iron transporter ferroportin is the cognate receptor of hepcidin and is destroyed as a result of interaction with the peptide. Except for inherited defects of ferroportin and hepcidin itself, all forms of iron-storage disease appear to arise from hepcidin dysregulation. Studies using multiple approaches have begun to delineate the molecular mechanisms that regulate hepcidin expression, particularly at the transcriptional level. Knowledge of the regulation of hepcidin by inflammation, iron, erythropoiesis, and hypoxia will lead to an understanding of the pathogenesis of primary hemochromatosis, secondary iron overload, and anemia of inflammatory disease.

Cross-talk between the mitogen activated protein kinase and bone morphogenetic protein/hemojuvelin pathways is required for the induction of hepcidin by holotransferrin in primary mouse hepatocytes

BACKGROUND

The circulating hormone hepcidin plays a central role in iron homeostasis. Our goal was to establish an ex vivo iron-sensing model and to characterize the molecular mechanisms linking iron to hepcidin.

DESIGN AND METHODS

Murine hepatocytes were isolated by the collagenase method, either from wild type or HFE knockout mice, and cultured 42 h without serum before treatments.

RESULTS

After 42 h of serum-free culture, hepcidin gene expression was undetectable in the hepatocytes. Hepcidin gene expression could, however, be re-activated by an additional 24 h of incubation with 10% serum. Interestingly, addition of 30 microM holotransferrin consistently increased serum-dependent hepcidin levels 3- to 5-fold. The effects of serum and serum+holotransferrin were direct, transcriptional, independent of de novo protein synthesis and required the presence of bone morphogenetic protein. Transferrin receptor-2 activation by its ligand holotransferrin led to extracellular signal regulated kinase (ERK)/mitogen activated protein kinase pathway stimulation and the ERK specific inhibitor U0-126 blunted holotransferrin-mediated induction of hepcidin. ERK activation by holotransferrin provoked increased levels of phospho-Smad1/5/8 highlighting cross-talk between the bone morphogenetic protein/hemojuvelin and ERK1/2 pathways. Finally, we demonstrated, using hepatocytes isolated from Hfe(-/-) mice, that HFE was not critical for the hepcidin response to holotransferrin but important for basal hepcidin expression.

CONCLUSIONS

We demonstrate that hepatocytes are liver iron-sensor cells and that transferrin receptor-2, by signaling through the ERK1/2 pathway, and bone morphogenetic protein/hemojuvelin, by signaling through the Smad pathways, coordinately regulate the iron-sensing machinery linking holotransferrin to hepcidin.

Bone morphogenetic protein (BMP)-responsive elements located in the proximal and distal hepcidin promoter are critical for its response to HJV/BMP/SMAD

The hemochromatosis proteins HFE, transferrin receptor 2 (TfR2) and hemojuvelin (HJV, HFE2) positively control expression of the major iron regulatory hormone hepcidin. HJV is a bone morphogenetic protein (BMP) co-receptor that enhances the cellular response to BMP cytokines via the phosphorylation of SMAD proteins. In this study, we show that two highly conserved and sequence-identical BMP-responsive elements located at positions -84/-79 (BMP-RE1) and -2,255/-2,250 (BMP-RE2) of the human hepcidin promoter are critical for both the basal hepcidin mRNA expression and the hepcidin response to BMP-2 and BMP-6. While BMP-RE1 and BMP-RE2 show additive effects in responding to HJV-mediated BMP signals, only BMP-RE1 that is located in close proximity to a previously identified STAT-binding site is important for the hepcidin response to IL-6. These data identify a missing link between the HJV/BMP signaling pathways and hepcidin transcription, and further define the connection between inflammation and BMP-dependent hepcidin promoter activation. As such, they provide important new information furthering our understanding of disorders of iron metabolism and the anemia of inflammation.

Function of the hemochromatosis protein HFE: Lessons from animal models

Hereditary hemochromatosis (HH) is caused by chronic hyperabsorption of dietary iron. Progressive accumulation of excess iron within tissue parenchymal cells may lead to severe organ damage. The most prevalent type of HH is linked to mutations in the HFE gene, encoding an atypical major histocompatibility complex classImolecule. Shortly after its discovery in 1996, the hemochromatosis protein HFE was shown to physically interact with transferrin receptor 1 (TfR1) and impair the uptake of transferrin-bound iron in cells. However, these findings provided no clue why HFE mutations associate with systemic iron overload. It was later established that all forms of HH result from misregulation of hepcidin expression. This liver-derived circulating peptide hormone controls iron efflux from duodenal enterocytes and reticuloendothelial macrophages by promoting the degradation of the iron exporter ferroportin. Recent studies with animal models of HH uncover a crucial role of HFE as a hepatocyte iron sensor and upstream regulator of hepcidin. Thus, hepatocyte HFE is indispensable for signaling to hepcidin, presumably as a constituent of a larger iron-sensing complex. A working model postulates that the signaling activity of HFE is silenced when the protein is bound to TfR1. An increase in the iron saturation of plasma transferrin leads to displacement of TfR1 from HFE and assembly of the putative iron-sensing complex. In this way, iron uptake by the hepatocyte is translated into upregulation of hepcidin, reinforcing the concept that the liver is the major regulatory site for systemic iron homeostasis, and not merely an iron storage depot.

Secretion of bioactive hepcidin-25 by liver cells correlates with its gene transcription and points towards synergism between iron and inflammation signaling pathways

Hepcidin is a small liver-derived peptide central in the regulation of systemic iron homeostasis. Although the gene regulation has been extensively studied at transcriptional level, the corresponding effects on the production of bioactive peptide are largely unknown. We therefore applied a proteomics-based approach by combining immunocapture with time-of-flight mass spectrometry to characterize hepcidin-25 produced by hepatocyte-derived cell lines. Similar to its transcriptional regulation, mature hepcidin-25 was strongly secreted upon stimulation with BMPs and IL-6. The immunocaptured peptide down-modulated iron-exporter ferroportin on the monocyte/macrophage surface. Further mass spectrometry-based analyses indicated that hepcidin-25 in its bioactive conformation was very stable in serum and urine and not converted into its smaller isoforms. Hepcidin-25 was processed in the Golgi apparatus from its precursor, while the unprocessed prohepcidin was secreted only when furin-like protease activity was intracellularly inhibited. Furthermore, the amounts of hepatocytic secretion of hepcidin-25 are highly correlated with the gene transcript levels. An unexpected observation was the synergistic effect of BMPs and IL-6 on hepcidin-25 secretion, which points towards cross-talk between iron and inflammatory stimuli. The study underscores hepcidin-25 quantification as a valuable tool to unravel regulatory pathways in iron metabolism.

Iron regulation and erythropoiesis

PURPOSE OF REVIEW

The peptide hormone hepcidin regulates iron metabolism in response to erythropoietic demand, iron stores and inflammation. Major advances have been made in understanding the regulation of hepcidin production, and consequently the availability of iron for erythropoiesis.

RECENT FINDINGS

It is becoming clear that the bone morphogenetic protein (BMP) pathway plays a major role in setting the baseline hepcidin level and, with the assistance of BMP2/4 and hemochromatosis-related proteins hemojuvelin, HFE and transferrin receptor 2, also regulates hepcidin expression in response to iron. Regulation of hepcidin in anemias has now been linked to increased erythropoietic activity and is likely mediated by factor(s) secreted by erythroid precursors. GDF-15 was identified as a candidate for one of the erythroid factors suppressing hepcidin. Tissue hypoxia may also directly contribute to hepcidin suppression in anemias. Regulation of hepcidin by inflammation may include multiple cytokines and the Toll-like receptors pathways. Although it has not yet been shown that increased hepcidin is indispensible for the development of anemia of inflammation, transgenic overexpression of hepcidin was sufficient to replicate its key features.

SUMMARY

Regulation of hepcidin and iron availability for erythropoiesis has revealed unexpected pathways and much complexity. The renaissance of the study of iron regulation continues to reward researchers with interesting biology and potential therapeutic targets.

The serine protease TMPRSS6 is required to sense iron deficiency

Hepcidin, a liver-derived protein that restricts enteric iron absorption, is the key regulator of body iron content. Several proteins induce expression of the hepcidin-encoding gene Hamp in response to infection or high levels of iron. However, mechanism(s) of Hamp suppression during iron depletion are poorly understood. We describe mask: a recessive, chemically induced mutant mouse phenotype, characterized by progressive loss of body (but not facial) hair and microcytic anemia. The mask phenotype results from reduced absorption of dietary iron caused by high levels of hepcidin and is due to a splicing defect in the transmembrane serine protease 6 gene Tmprss6. Overexpression of normal TMPRSS6 protein suppresses activation of the Hamp promoter, and the TMPRSS6 cytoplasmic domain mediates Hamp suppression via proximal promoter element(s). TMPRSS6 is an essential component of a pathway that detects iron deficiency and blocks Hamp transcription, permitting enhanced dietary iron absorption.

Hemojuvelin regulates hepcidin expression via a selective subset of BMP ligands and receptors independently of neogenin

Hemojuvelin (HJV) is a coreceptor for bone morphogenetic protein (BMP) signaling that regulates hepcidin expression and iron metabolism. However, the precise combinations of BMP ligands and receptors used by HJV remain unknown. HJV has also been demonstrated to bind to neogenin, but it is not known whether this interaction has a role in regulating hepcidin expression. In the present study, we show that BMP-2, BMP-4, and BMP-6 are endogenous ligands for HJV in hepatoma-derived cell lines, and that all 3 of these ligands are expressed in human liver. We demonstrate in vitro that HJV selectively uses the BMP type II receptors ActRIIA and BMPRII, but not ActRIIB, and HJV enhances utilization of ActRIIA by BMP-2 and BMP-4. Interestingly, ActRIIA is the predominant BMP type II receptor expressed in human liver. While HJV can use all 3 BMP type I receptors (ALK2, ALK3, and ALK6) in vitro, only ALK2 and ALK3 are detected in human liver. Finally, we show that HJV-induced BMP signaling and hepcidin expression are not altered by neogenin overexpression or by inhibition of endogenous neogenin expression. Thus, HJV-mediated BMP signaling and hepcidin regulation occur via a distinct subset of BMP ligands and BMP receptors, independently of neogenin.