Stimulated erythropoiesis with secondary iron loading leads to a decrease in hepcidin despite an increase in bone morphogenetic protein 6 expression

A Decrease in Maternal Iron Levels Is the Predominant Factor Suppressing Hepcidin during Pregnancy in Mice

In order to supply adequate iron during pregnancy, the levels of the iron regulatory hormone hepcidin in the maternal circulation are suppressed, thereby increasing dietary iron absorption and storage iron release. Whether this decrease in maternal hepcidin is caused by changes in factors known to regulate hepcidin expression, or by other unidentified pregnancy factors, is not known. To investigate this, we examined iron parameters during pregnancy in mice. We observed that hepatic iron stores and transferrin saturation, both established regulators of hepcidin production, were decreased in mid and late pregnancy in normal and iron loaded dams, indicating an increase in iron utilization. This can be explained by a significant increase in maternal erythropoiesis, a known suppressor of hepcidin production, by mid-pregnancy, as indicated by an elevation in circulating erythropoietin and an increase in spleen size and splenic iron uptake. Iron utilization increased further in late pregnancy due to elevated fetal iron demand. By increasing maternal iron levels in late gestation, we were able to stimulate the expression of the gene encoding hepcidin, suggesting that the iron status of the mother is the predominant factor influencing hepcidin levels during pregnancy. Our data indicate that pregnancy-induced hepcidin suppression likely occurs because of reductions in maternal iron reserves due to increased iron requirements, which predominantly reflect stimulated erythropoiesis in mid-gestation and increased fetal iron requirements in late gestation, and that there is no need to invoke other factors, including novel pregnancy factor(s), to explain these changes.

[Expression of iron-regulating erythroid factors in different types of erythropoiesis disorders]

目的

研究Erythroferrone（ERFE）等铁代谢红系调节因子（iron-regulatory erythroid factor）在不同类型红系造血异常疾病中的表达情况。

方法

采用ELISA方法检测2016年1月至2019年11月共47例真性红细胞增多症（PV）、纯红细胞再生障碍（PRCA）、自身免疫性溶血性贫血（AIHA）和骨髓增生异常综合征（MDS）患者血浆ERFE、生长分化因子15（GDF15）、生长分化因子11（GDF11）和扭转原肠胚形成同系物（TWSG1）的表达，分析铁代谢调节因子与红系造血异常类型及旺盛程度（以骨髓有核红细胞比例反映）的适配性。

结果

血浆GDF15表达水平在PV、PRCA、AIHA、MDS各组依次为266.01（112.40,452.37）、110.63（81.41,220.42）、52.11（32.61,171.66）、276.53（132.16,525.70）ng/L，均显著高于正常对照组的37.45（19.65,57.72）ng/L（P值均<0.01）。不同类型红系造血异常患者血浆TWSG1表达水平与正常对照组比较差异均无统计学意义（P值均>0.05）。血浆GDF11表达水平仅在PV组患者中明显高于正常对照组[74.75（10.95,121.32）ng/L对36.90（3.38,98.34）ng/L，P<0.01]，而PRCA、AIHA、MDS 3组患者与正常对照组比较差异无统计学意义（P>0.05）。PV组血浆ERFE水平为129.63（47.02, 170.03）ng/L，AIHA组血浆ERFE水平最高为121.76（68.12,343.11）ng/L，二者均明显高于正常对照组的43.23（35.18,65.41）ng/L（P值均<0.01）；PRCA组、MDS组血浆ERFE水平分别为48.92（44.59，84.83）、40.47（26.97,72.87）ng/L，与正常对照组比较差异无统计学意义（P值均>0.05）。骨髓有核红细胞比例与ERFE（r＝0.458，P＝0.001）呈正相关，而与GDF15（r＝−0.163，P＝0.274）、GDF11（r＝0.120，P＝0.421）、TWSG1（r＝−0.166，P＝0.269）无明显相关性。

结论

铁代谢红系调节因子在不同红系造血异常疾病的表达谱不尽一致，ERFE与红系造血旺盛程度相关度最高。

Targeting the hepcidin-ferroportin pathway in anaemia of chronic kidney disease

AIMS

Erythropoiesis-stimulating agents used to treat anaemia in patients with chronic kidney disease (CKD) have been associated with cardiovascular adverse events. Hepcidin production, controlled by bone morphogenic protein 6 (BMP6), regulates iron homeostasis via interactions with the iron transporter, ferroportin. High hepcidin levels are thought to contribute to increased iron sequestration and subsequent anaemia in CKD patients. To investigate alternative therapies to erythropoiesis-stimulating agents for CKD patients, monoclonal antibodies, LY3113593 and LY2928057, targeting BMP6 and ferroportin respectively, were tested in CKD patients.

METHODS

Preclinical in vitro/vivo data and clinical data in healthy subjects and CKD patients were used to illustrate the translation of pharmacological properties of LY3113593 and LY2928057, highlighting the novelty of targeting these nodes within the hepcidin-ferroportin pathway.

RESULTS

LY2928057 bound ferroportin and blocked interactions with hepcidin, allowing iron efflux, leading to increased serum iron and transferrin saturation levels and increased hepcidin in monkeys and humans. In CKD patients, LY2928057 led to slower haemoglobin decline and reduction in ferritin (compared to placebo). Serum iron increase was (mean [90% confidence interval]) 1.98 [1.46-2.68] and 1.36 [1.22-1.51] fold-relative to baseline following LY2928057 600 mg and LY311593 150 mg respectively in CKD patients. LY3113593 specifically blocked BMP6 binding to its receptor and produced increases in iron and transferrin saturation and decreases in hepcidin preclinically and clinically. In CKD patients, LY3113593 produced an increase in haemoglobin and reduction in ferritin (compared to placebo).

CONCLUSION

LY3113593 and LY2928057 pharmacological effects (serum iron and ferritin) were translated from preclinical-to-clinical development. Such interventions may lead to new CKD anaemia treatments.

What can we learn from ineffective erythropoiesis in thalassemia?

Erythropoiesis is a dynamic process regulated at multiple levels to balance proliferation, differentiation and survival of erythroid progenitors. Ineffective erythropoiesis is a key feature of various diseases, including β-thalassemia. The pathogenic mechanisms leading to ineffective erythropoiesis are complex and still not fully understood. Altered survival and decreased differentiation of erythroid progenitors are both critical processes contributing to reduced production of mature red blood cells. Recent studies have identified novel important players and provided major advances in the development of targeted therapeutic approaches. In this review, β-thalassemia is used as a paradigmatic example to describe our current knowledge on the mechanisms leading to ineffective erythropoiesis and novel treatments that may have the potential to improve the clinical phenotype of associated diseases in the future.

Smad1/5 is required for erythropoietin-mediated suppression of hepcidin in mice

Anemia suppresses liver hepcidin expression to supply adequate iron for erythropoiesis. Erythroferrone mediates hepcidin suppression by anemia, but its mechanism of action remains uncertain. The bone morphogenetic protein (BMP)-SMAD signaling pathway has a central role in hepcidin transcriptional regulation. Here, we explored the contribution of individual receptor-activated SMADs in hepcidin regulation and their involvement in erythroferrone suppression of hepcidin. In Hep3B cells, SMAD5 or SMAD1 but not SMAD8, knockdown inhibited hepcidin (HAMP) messenger RNA (mRNA) expression. Hepatocyte-specific double-knockout Smad1^fl/fl;Smad5^fl/fl;Cre⁺ mice exhibited ∼90% transferrin saturation and massive liver iron overload, whereas Smad1^fl/fl;Smad5^fl/wt;Cre⁺ mice or Smad1^fl/wt;Smad5^fl/fl;Cre⁺ female mice with 1 functional Smad5 or Smad1 allele had modestly increased serum and liver iron, and single-knockout Smad5^fl/fl;Cre⁺ or Smad1^fl/fl;Cre⁺ mice had minimal to no iron loading, suggesting a gene dosage effect. Hamp mRNA was reduced in all Cre⁺ mouse livers at 12 days and in all Cre⁺ primary hepatocytes. However, only double-knockout mice continued to exhibit low liver Hamp at 8 weeks and failed to induce Hamp in response to Bmp6 in primary hepatocyte cultures. Epoetin alfa (EPO) robustly induced bone marrow erythroferrone (Fam132b) mRNA in control and Smad1^fl/fl;Smad5^fl/fl;Cre⁺ mice but suppressed hepcidin only in control mice. Likewise, erythroferrone failed to decrease Hamp mRNA in Smad1^fl/fl;Smad5^fl/fl;Cre⁺ primary hepatocytes and SMAD1/SMAD5 knockdown Hep3B cells. EPO and erythroferrone reduced liver Smad1/5 phosphorylation in parallel with Hamp mRNA in control mice and Hep3B cells. Thus, Smad1 and Smad5 have overlapping functions to govern hepcidin transcription. Moreover, erythropoietin and erythroferrone target Smad1/5 signaling and require Smad1/5 to suppress hepcidin expression.

Hepcidin suppression in β-thalassemia is associated with the down-regulation of atonal homolog 8

Atonal homolog 8 (ATOH8) is defined as a positive regulator of hepcidin transcription, which links erythropoietic activity with iron-sensing molecules. In the present study, we investigated the association between hepcidin and ATOH8 expression in β-thalassemia. We found that inhibition of hepcidin expression in β-thalassemia is correlated with reduced ATOH8 expression. Hepatic hepcidin 1 (Hamp1) and Atoh8 mRNA expression were down-regulated in β-thalassemic mice. Hepcidin (HAMP) and ATOH8 mRNA expression were consistently suppressed in Huh7 cells cultured in medium supplemented with β-thalassemia patient serum. The Huh7 cells, which were transfected with ATOH8-FLAG expression plasmid and cultured in the supplemented medium, exhibited increased levels of ATOH8 mRNA, ATOH8-FLAG protein, pSMAD1,5,8, and HAMP mRNA. Interestingly, over-expression of ATOH8 reversed the effects of hepcidin suppression induced by the β-thalassemia patient sera. In conclusion, hepcidin suppression in β-thalassemia is associated with the down-regulation of ATOH8 in response to anemia. We, therefore, suggest that ATOH8 is an important transcriptional regulator of hepcidin in β-thalassemia.

Transforming growth factor 15 increased in severe aplastic anemia patients

OBJECTIVES

The patients with severe aplastic anemia (SAA) usually rely on red cell transfusion which lead to secondary iron overload. Transforming growth differentiation factor-15 (GDF-15) plays an important role in erythropoiesis and iron regulation. In this study, we investigated the level of GDF-15 and other indexes of iron metabolism in SAA patients to explore the correlation with GDF-15 and iron overload in SAA.

METHODS

The levels of serum GDF-15, hepcidin (Hepc), and erythropoietin (EPO) were determined by ELISA. The levels of serum iron (SI), ferritin, TIBC, and transferrin saturation (TS) were measured by an auto analyzer. Iron staining of bone marrow cells was used for testing extracellular and intracellular iron.

RESULTS

The GDF-15 level in the experimental group was higher than that of the case-control group and normal control group (all p < 0.05). The Hepc level in the experimental group and case-control group were both higher than that of healthy controls (all p < 0.05). The Hepc level was significantly lower in the experimental group patients who had excessive GDF-15 (r = -0.766, p = 0.000). There was a positive correlation between the level of GDF15 and EPO in the experimental group (r = 0.68, p < 0.000). The level of GDF15 in SAA patients was positively correlated with SI levels (r = 0.537, p = 0.008), TS levels (r = 0.466, p = 0.025), and sideroblasts (%) (r = 0.463, p = 0.026). Moreover, there was a positive correlation between GDF-15 level and blood transfusion-dependent time (r = 0.739, p = 0.000).

DISCUSSION

Our data indicated that GDF-15 plays an important role in iron metabolism in SAA. GDF-15 might be a novel target for SAA therapy.

Erythropoietin administration increases splenic erythroferrone protein content and liver TMPRSS6 protein content in rats

Erythroferrone (ERFE) and TMPRSS6 are important proteins in the regulation of iron metabolism. The objective of the study was to examine splenic ERFE and liver TMPRSS6 synthesis in rats treated with a combination of iron and erythropoietin (EPO). EPO was administered to female Wistar rats at 600U/day for four days, iron-pretreated rats received 150mg of iron before EPO treatment. Content of ERFE and TMPRSS6 proteins was determined by commercial antibodies. Iron pretreatment prevented the EPO-induced decrease in hepcidin expression. Content of phosphorylated SMAD 1,5,8 proteins was decreased in the liver by both EPO and iron plus EPO treatment. Fam132b expression in the spleen was increased both by EPO and iron plus EPO treatments; these treatments also significantly induced splenic Fam132a expression. ERFE protein content in the spleen was increased both by EPO and iron plus EPO to a similar extent. EPO administration increased TMPRSS6 content in the plasma membrane-enriched fraction of liver homogenate; in iron-pretreated rats, this increase was abolished. The results confirm that iron pretreatment prevents the EPO-induced decrease in liver Hamp expression. This effect probably occurs despite high circulating ERFE levels, since EPO-induced ERFE protein synthesis is not influenced by iron pretreatment.

Characterization of Putative Erythroid Regulators of Hepcidin in Mouse Models of Anemia

Iron is crucial for many biological functions, but quantitatively the most important use of iron is in the production of hemoglobin in red blood cell precursors. The amount of iron in the plasma, and hence its availability for hemoglobin synthesis, is determined by the liver-derived iron regulatory hormone hepcidin. When the iron supply to erythroid precursors is limited, as often occurs during stimulated erythropoiesis, these cells produce signals to inhibit hepatic hepcidin production, thereby increasing the amount of iron that enters the plasma. How stimulated erythropoiesis suppresses hepcidin production is incompletely understood, but erythroferrone, Gdf15 and Twsg1 have emerged as candidate regulatory molecules. To further examine the relationship between erythropoiesis and the candidate erythroid regulators, we have studied five mouse models of anemia, including two models of β-thalassemia (Hbb^th3/+ and RBC14), the hemoglobin deficit mouse (hbd), dietary iron deficient mice and mice treated with phenylhydrazine to induce acute hemolysis. Hematological parameters, iron status and the expression of Erfe (the gene encoding erythroferrone), Gdf15 and Twsg1 in the bone marrow and spleen were examined. Erfe expression was the most consistently upregulated of the candidate erythroid regulators in all of the mouse models examined. Gene expression was particularly high in the bone marrow and spleen of iron deficient animals, making erythroferrone an ideal candidate erythroid regulator, as its influence is strongest when iron supply to developing erythroid cells is limited. Gdf15 expression was also upregulated in most of the anemia models studied although the magnitude of the increase was generally less than that of Erfe. In contrast, very little regulation of Twsg1 was observed. These results support the prevailing hypothesis that erythroferrone is a promising erythroid regulator and demonstrate that Erfe expression is stimulated most strongly when the iron supply to developing erythroid cells is compromised.

Iron deficiency or anemia of inflammation? : Differential diagnosis and mechanisms of anemia of inflammation

Iron deficiency and immune activation are the two most frequent causes of anemia, both of which are based on disturbances of iron homeostasis. Iron deficiency anemia results from a reduction of the body’s iron content due to blood loss, inadequate dietary iron intake, its malabsorption, or increased iron demand. Immune activation drives a diversion of iron fluxes from the erythropoietic bone marrow, where hemoglobinization takes place, to storage sites, particularly the mononuclear phagocytes system in liver and spleen. This results in iron-limited erythropoiesis and anemia. This review summarizes current diagnostic and pathophysiological concepts of iron deficiency anemia and anemia of inflammation, as well as combined conditions, and provides a brief outlook on novel therapeutic options.

Hepcidin independent iron recycling in a mouse model of β-thalassaemia intermedia

In conditions such as β-thalassaemia, stimulated erythropoiesis can reduce the expression of the iron regulatory hormone hepcidin, increasing both macrophage iron release and intestinal iron absorption and leading to iron loading. However, in certain conditions, sustained elevation of erythropoiesis can occur without an increase in body iron load. To investigate this in more detail, we made use of a novel mouse strain (RBC14), which exhibits mild β-thalassaemia intermedia with minimal iron loading. We compared iron homeostasis in RBC14 mice to that of Hbb^th3/+ mice, a more severe model of β-thalassaemia intermedia. Both mouse strains showed a decrease in plasma iron half-life, although the changes were less severe in RBC14 mice. Despite this, intestinal ferroportin and serum hepcidin levels were unaltered in RBC14 mice. In contrast, Hbb^th3/+ mice exhibited reduced serum hepcidin and increased intestinal ferroportin. However, splenic ferroportin levels were increased in both mouse strains. These data suggest that in low-grade chronic haemolytic anaemia, such as that seen in RBC14 mice, the increased erythroid iron requirements can be met through enhanced macrophage iron release without the need to increase iron absorption, implying that hepcidin is not the sole regulator of macrophage iron release in vivo.

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.

Vitamin A deficiency modulates iron metabolism independent of hemojuvelin (Hfe2) and bone morphogenetic protein 6 (Bmp6) transcript levels

BACKGROUND

Considering that vitamin A deficiency modulates hepcidin expression and consequently affects iron metabolism, we evaluated the effect of vitamin A deficiency in the expression of genes involved in the hemojuvelin (HJV)-bone morphogenetic protein 6 (BMP6)-small mothers against decapentaplegic protein (SMAD) signaling pathway.

METHODS

Male Wistar rats were treated: control AIN-93G diet (CT), vitamin A-deficient diet (VAD), iron-deficient diet (FeD), vitamin A- and iron-deficient diet (VAFeD), or 12 mg all-trans retinoic acid (atRA)/kg diet.

RESULTS

Vitamin A deficiency (VAD) increased hepatic Bmp6 and Hfe2 mRNA levels and down-regulated hepatic Hamp, Smad7, Rarα, and intestinal Fpn1 mRNA levels compared with the control. The FeD rats showed lower hepatic Hamp, Bmp6, and Smad7 mRNA levels compared with those of the control, while in the VAFeD rats only Hamp and Smad7 mRNA levels were lower than those of the control. The VAFeD diet up-regulated intestinal Dmt1 mRNA levels in relation to those of the control. The replacement of retinyl ester by atRA did not restore hepatic Hamp mRNA levels; however, the hepatic Hfe2, Bmp6, and Smad7 mRNA levels were similar to the control. The atRA rats showed an increase of hepatic Rarα mRNA levels and a reduction of intestinal Dmt1 mRNA and Fpn1 levels compared with those of the control.

CONCLUSIONS

The HJV-BMP6-SMAD signaling pathway that normally activates the expression of hepcidin in iron deficiency is impaired by vitamin A deficiency despite increased expression of liver Bmp6 and Hfe2 mRNA levels and decreased expression of Smad7 mRNA. This response may be associated to the systemic iron deficiency and spleen iron retention promoted by vitamin A deficiency.

New strategies to target iron metabolism for the treatment of beta thalassemia

Iron is one of the most abundant elements in the Earth and a fundamental component of enzymes and other proteins that participate in a wide range of biological processes. As the human body has no mechanisms to eliminate the excess of iron, its metabolism needs to be tightly controlled in order to avoid all the sequelae associated with high iron levels. Iron overload is the main cause of morbidity and mortality in beta thalassemia. The master regulator of iron homeostasis, hepcidin, is chronically repressed in this disorder, leading to increased intestinal iron absorption and consequent iron overload. Many groups have focused on obtaining a better understanding of the pathways involved in iron regulation. New molecules have recently been synthesized and used in animal models of dysregulated iron metabolism, demonstrating their ability to target and reduce iron load. Antisense oligonucleotides, as well as lipid nanoparticle-formulated small interfering RNAs and minihepcidins peptides, are novel agents that have already proved to be efficient in modulating iron metabolism in mouse models and are therefore promising candidates for the treatment of patients affected by iron disorders.

New insights into iron regulation and erythropoiesis

PURPOSE OF REVIEW

Iron homeostasis and erythropoiesis regulate each other to ensure optimal delivery of oxygen and iron to cells and tissues. Defining the mechanisms of this crosstalk is important for understanding the pathogenesis of common conditions associated with disordered iron metabolism and erythropoiesis.

RECENT FINDINGS

Stress erythropoiesis causes suppression of hepcidin to increase iron availability for hemoglobin synthesis. The erythroid hormone erythroferrone (ERFE) was identified as the mediator of this process. ERFE and additional candidates (TWSG1 and GDF15) may also mediate hepcidin suppression in ineffective erythropoiesis. Several mechanisms by which iron regulates erythropoiesis were also recently identified. Iron deficiency suppresses erythropoietin production via the IRP1-HIF2α axis to prevent excessive iron usage by erythropoiesis during systemic iron restriction. Iron restriction also directly impairs erythroid maturation by inhibiting aconitase, and this can be reversed by the administration of the aconitase product isocitrate. Another novel target is GDF11, which is thought to autoinhibit erythroid maturation. GDF11 traps show promising pharmacologic activity in models of both ineffective erythropoiesis and iron-restricted anemia.

SUMMARY

This review summarizes exciting advances in understanding the mechanisms of iron and erythropoietic regulation, and development of novel therapeutic tools for disorders resulting from dysregulation of iron metabolism or erythropoiesis.

Increased hepcidin in transferrin-treated thalassemic mice correlates with increased liver BMP2 expression and decreased hepatocyte ERK activation

Iron overload results in significant morbidity and mortality in β-thalassemic patients. Insufficient hepcidin is implicated in parenchymal iron overload in β-thalassemia and approaches to increase hepcidin have therapeutic potential. We have previously shown that exogenous apo-transferrin markedly ameliorates ineffective erythropoiesis and increases hepcidin expression in Hbb(th1/th1) (thalassemic) mice. We utilize in vivo and in vitro systems to investigate effects of exogenous apo-transferrin on Smad and ERK1/2 signaling, pathways that participate in hepcidin regulation. Our results demonstrate that apo-transferrin increases hepcidin expression in vivo despite decreased circulating and parenchymal iron concentrations and unchanged liver Bmp6 mRNA expression in thalassemic mice. Hepatocytes from apo-transferrin-treated mice demonstrate decreased ERK1/2 pathway and increased serum BMP2 concentration and hepatocyte BMP2 expression. Furthermore, hepatocyte ERK1/2 phosphorylation is enhanced by neutralizing anti-BMP2/4 antibodies and suppressed in vitro in a dose-dependent manner by BMP2, resulting in converse effects on hepcidin expression, and hepatocytes treated with MEK/ERK1/2 inhibitor U0126 in combination with BMP2 exhibit an additive increase in hepcidin expression. Lastly, bone marrow erythroferrone expression is normalized in apo-transferrin treated thalassemic mice but increased in apo-transferrin injected wild-type mice. These findings suggest that increased hepcidin expression after exogenous apo-transferrin is in part independent of erythroferrone and support a model in which apo-transferrin treatment in thalassemic mice increases BMP2 expression in the liver and other organs, decreases hepatocellular ERK1/2 activation, and increases nuclear Smad to increase hepcidin expression in hepatocytes.

The dietary flavonoid myricetin regulates iron homeostasis by suppressing hepcidin expression

Hepcidin, a master regulator of iron homeostasis, is a promising target in treatment of iron disorders such as hemochromatosis, anemia of inflammation and iron-deficiency anemia. We previously reported that black soybean seed coat extract could inhibit hepcidin expression. Based on this finding, we performed a screen in cultured cells in order to identify the compounds in black soybeans that inhibit hepcidin expression. We found that the dietary flavonoid myricetin significantly inhibited the expression of hepcidin both in vitro and in vivo. Treating cultured cells with myricetin decreased both HAMP mRNA levels and promoter activity by reducing SMAD1/5/8 phosphorylation. This effect was observed even in the presence of bone morphogenic protein-6 (BMP6) and interleukin-6 (IL-6), two factors that stimulate hepcidin expression. Furthermore, mice that were treated with myricetin (either orally or systemically) had reduced hepatic hepcidin expression, decreased splenic iron levels and increased serum iron levels. Notably, myricetin-treated mice increased red blood cell counts and hemoglobin levels. In addition, pretreating mice with myricetin prevented LPS-induced hypoferremia. We conclude that myricetin potently inhibits hepcidin expression both in vitro and in vivo, and this effect is mediated by altering BMP/SMAD signaling. These experiments highlight the feasibility of identifying and characterizing bioactive phytochemicals to suppress hepcidin expression. These results also suggest that myricetin may represent a novel therapy for treating iron deficiency-related diseases.

The pathophysiology of transfusional iron overload

The pathophysiologic consequences of transfusional iron overload (TIO) as well as the benefits of iron chelation therapy are best described in thalassemia major, although TIO is increasingly seen in other clinical settings. These consequences broadly reflect the levels and distribution of excess storage iron in the heart, endocrine tissues, and liver. TIO also increases the risk of infection, due to increased availability of labile iron to microorganisms. The authors suggest that extrahepatic iron distribution, and hence toxicity, is influenced by balance between generation of nontransferrin-bound iron from red cell catabolism and the utilization of transferrin iron by the erythron.

Peripheral expression of hepcidin gene in Egyptian β-thalassemia major

Iron overload is the major cause of morbidity and mortality in transfusion dependent β-thalassemia major patients. There is a sophisticated balance of body iron metabolism of storage and transport which is regulated by several factors including the peptide hepcidin. Hepcidin is the main iron regulatory molecule; it is secreted mainly by the liver and other tissues including monocytes and lymphocytes. Expression of hepcidin in such cells is unclear and has been studied in few reports with controverted result. Peripheral expression of hepcidin was measured using quantitative real time PCR (qRT-PCR) in 50 β-thalassemia major patients, in addition to 20 healthy volunteers as a control group. Hepcidin levels in β-thalassemia major patients showed statistically significant decrease in comparison to the control group, and was correlated to cardiac iron stores (T2*). However, hepcidin level was not different among the patients according to the HCV status or whether splenectomized or not. In conclusion; peripheral expression of hepcidin, in iron overloaded β-thalassemia major patients, is a reflection of hepatic expression. It can be used as a molecular predictor for the severity of cardiac iron overload and can be used as a future target for therapy in β-thalassemia major patients.

Effect of iron overload on furin expression in wild-type and β-thalassemic mice

Furin is a proprotein convertase enzyme. In the liver, it cleaves prohepcidin to form active hepcidin-25, which regulates systemic iron homeostasis. Hepcidin deficiency is a component of several iron overload disorders, including β-thalassemia. Several studies have identified factors that repress hepcidin gene transcription in iron overload. However, the effect of iron overload on furin, a post-translational regulator of hepcidin, has never been evaluated. The present study aimed to investigate the changes in furin and related factors in parenteral iron-overloaded mice, including those with β-thalassemia. Wild-type (WT) and β-thalassemia intermedia (th3/+) C57BL/6 mice were intraperitoneally injected with 9 doses of iron dextran (1 g iron/kg body weight) over 2 weeks. In the iron overload condition, our data demonstrated a significant Furin mRNA reduction in WT and th3/+ mice. In addition, the liver furin protein level in iron-overloaded WT mice was significantly reduced by 70% compared to control WT mice. However, the liver furin protein in iron-overloaded th3/+ mice did not show a significant reduction compared to control th3/+ mice. The hepcidin gene (hepcidin antimicrobial peptide gene, Hamp1) expression was increased in iron-overloaded WT and th3/+ mice. Surprisingly, the liver hepcidin protein level and total serum hepcidin were not increased in both WT and th3/+ mice with iron overload, regardless of the increase in Hamp1 mRNA. In conclusion, we demonstrate furin downregulation in conjunction with Hamp1 mRNA-unrelated pattern of hepcidin protein expression in iron-overloaded mice, particularly the WT mice, suggesting that, not only the amount of hepcidin but also the furin-mediated physiological activity may be decreased in severe iron overload condition.

Circulating factors are involved in hypoxia-induced hepcidin suppression

Hepcidin transcription is strongly down-regulated under hypoxic conditions, however whether hypoxia inhibits hepcidin directly or indirectly is still unknown. We investigated the time course of hypoxia-mediated hepcidin down-regulation in vivo in healthy volunteers exposed to hypobaric hypoxia at high altitude and, based on the hypothesis that circulating factors are implicated in hepcidin inhibition, we analyzed the effect of sera of these volunteers exposed to normoxia and hypoxia on hepcidin expression in Huh-7 cell lines. Hypoxia led to a significant hepcidin down-regulation in vivo that was almost complete within 72h of exposure and followed erythropoietin induction. This delay in hepcidin down-regulation suggests the existence of soluble factor/s regulating hepcidin production. We then stimulated HuH-7 cells with normoxic and hypoxic sera to analyze the effects of sera on hepcidin regulation. Hypoxic sera had a significant inhibitory effect on hepcidin promoter activity assessed by a luciferase assay, although the amount of such decrease was not as relevant as that observed in vivo. Cellular mRNA analysis showed that a number of volunteers' sera inhibited hepcidin expression, concurrently with ID1 inhibition, suggesting that inhibitory factor(s) may act through the SMAD-pathway.

Molecular liaisons between erythropoiesis and iron metabolism

Although most circulating iron in blood plasma is destined for erythropoiesis, the mechanisms by which erythropoietic demand modulates the iron supply ("erythroid regulators") remain largely unknown. Iron absorption, plasma iron concentrations, and tissue iron distribution are tightly controlled by the liver-produced hormone hepcidin. During the last decade, much progress has been made in elucidating hepcidin regulation by iron and inflammation. This review discusses the less understood mechanisms and mediators of hepcidin suppression in physiologically and pathologically stimulated erythropoiesis.

The regulation of iron transport

Iron is an essential nutrient, but its concentration and distribution in the body must be tightly controlled due to its inherent toxicity and insolubility in aqueous solution. Living systems have successfully overcome these potential limitations by evolving a range of iron binding proteins and transport systems that effectively maintain iron in a nontoxic and soluble form for much, if not all, of its time within the body. In the circulation, iron is transported to target organs bound to the serum iron binding protein transferrin. Individual cells modulate their uptake of transferrin-bound iron depending on their iron requirements, using both transferrin receptor 1-dependent and independent pathways. Once inside the cell, iron can be chaperoned to sites of need or, if in excess, stored within ferritin. Iron is released from cells by the iron export protein ferroportin1, which requires the ferroxidase activity of ceruloplasmin or hephestin to load iron safely onto transferrin. The regulation of iron export is controlled predominantly at the systemic level by the master regulator of iron homeostasis hepcidin. Hepcidin, in turn, responds to changes in body iron demand, making use of a range of regulatory mechanisms that center on the bone morphogenetic protein signaling pathway. This review provides an overview of recent advances in the field of iron metabolism and outlines the key components of the iron transport and regulation systems.

Bmp6 expression can be regulated independently of liver iron in mice

The liver is the primary organ for storing iron and plays a central role in the regulation of body iron levels by secretion of the hormone Hamp1. Although many factors modulate Hamp1 expression, their regulatory mechanisms are poorly understood. Here, we used conditional knockout mice for the iron exporter ferroportin1 (Fpn1) to modulate tissue iron in specific tissues in combination with iron-deficient or iron-rich diets and transferrin (Tf) supplementation to investigate the mechanisms underlying Hamp1 expression. Despite liver iron overload, expression of bone morphogenetic protein 6 (Bmp6), a potent-stimulator of Hamp1 expression that is expressed under iron-loaded conditions, was decreased. We hypothesized that factors other than liver iron must play a role in controlling Bmp6 expression. Our results show that erythropoietin and Tf-bound iron do not underlie the down-regulation of Bmp6 in our mice models. Moreover, Bmp6 was down-regulated under conditions of high iron demand, irrespective of the presence of anemia. We therefore inferred that the signals were driven by high iron demand. Furthermore, we also confirmed previous suggestions that Tf-bound iron regulates Hamp1 expression via Smad1/5/8 phosphorylation without affecting Bmp6 expression, and the effect of Tf-bound iron on Hamp1 regulation appeared before a significant change in Bmp6 expression. Together, these results are consistent with novel mechanisms for regulating Bmp6 and Hamp1 expression.

Liver iron modulates hepcidin expression during chronically elevated erythropoiesis in mice

The liver-derived peptide hepcidin controls the balance between iron demand and iron supply. By inhibiting the iron export activity of ferroportin, hepcidin modulates iron absorption and delivery from the body's stores. The regulation of hepcidin, however, is not completely understood and includes a variety of different signals. We studied iron metabolism and hepcidin expression in mice constitutively overexpressing erythropoietin (Epo) (Tg6 mice), which leads to excessive erythropoiesis. We observed a very strong down-regulation of hepcidin in Tg6 mice that was accompanied by a strong increase in duodenal expression of ferroportin and divalent metal tranporter-1, as well as enhanced duodenal iron absorption. Despite these compensatory mechanisms, Tg6 mice displayed marked circulating iron deficiency and low levels of iron in liver, spleen, and muscle. To elucidate the primary signal affecting hepcidin expression during chronically elevated erythropoiesis, we increased iron availability by either providing iron (thus further increasing the hematocrit) or reducing erythropoiesis-dependent iron consumption by means of splenectomy. Both treatments increased liver iron and up-regulated hepcidin expression and the BMP6/SMAD pathway despite continuously high plasma Epo levels and sustained erythropoiesis. This suggests that hepcidin expression is not controlled by erythropoietic signals directly in this setting. Rather, these results indicate that iron consumption for erythropoiesis modulates liver iron content, and ultimately BMP6 and hepcidin. Analysis of the BMP6/SMAD pathway targets showed that inhibitor of DNA binding 1 (ID1) and SMAD7, but not transmembrane serine protease 6 (TMPRSS6), were up-regulated by increased iron availability and thus may be involved in setting the upper limit of hepcidin.

CONCLUSION

We provide evidence that under conditions of excessive and effective erythropoiesis, liver iron regulates hepcidin expression through the BMP6/SMAD pathway.

The transcription factor ATOH8 is regulated by erythropoietic activity and regulates HAMP transcription and cellular pSMAD1,5,8 levels

ATOH8 has previously been shown to be an iron-regulated transcription factor, however its role in iron metabolism is not known. ATOH8 expression in HEK293 cells resulted in increased endogenous HAMP mRNA levels as well as HAMP promoter activity. Mutation of the E-box or SMAD response elements within the HAMP promoter significantly reduced the effects of ATOH8, indicating that ATOH8 activates HAMP transcription directly as well as through bone morphogenic protein (BMP) signalling. In support of the former, Chromatin immunoprecipitation assays provided evidence that ATOH8 binds to E-box regions within the HAMP promoter while the latter was supported by the finding that ATOH8 expression in HEK293 cells led to increased phosphorylated SMAD1,5,8 levels. Liver Atoh8 levels were reduced in mice under conditions associated with increased erythropoietic activity such as hypoxia, haemolytic anaemia, hypotransferrinaemia and erythropoietin treatment and increased by inhibitors of erythropoiesis. Hepatic Atoh8mRNA levels increased in mice treated with holo transferrin, suggesting that Atoh8 responds to changes in plasma iron. ATOH8 is therefore a novel transcriptional regulator of HAMP, which is responsive to changes in plasma iron and erythroid activity and could explain how changes in erythroid activity lead to regulation of HAMP.

Serum hepcidin and growth differentiation factor-15 (GDF-15) levels in polycythemia vera and essential thrombocythemia

OBJECTIVES

Hepcidin plays a regulatory role in systemic iron homeostasis. GDF-15 has been found to be expressed from matured erythroblasts and very high levels of GDF-15 suppresses hepcidin secretion. In this study, we evaluated hepcidin and GDF-15 levels in polycythemia vera (PV) and essential thrombocythemia (ET).

METHODS

The study included 29 patients and 21 healthy controls. The patient group included 13 patients with ET and 16 patients with PV. Serum hepcidin and GDF-15 levels were measured at the time of diagnosis, before the initiation of any therapy.

RESULTS

Hepcidin levels did not differ significantly in patients with chronic myeloproliferative disease (CMPD) and healthy controls. However, GDF-15 levels were significantly increased in patients with CMPD (P = 0.038). No difference could be found between patients with PV and ET in terms of hepcidin and GDF-15 levels. Patients with JAK2-V617F mutation had increased GDF-15 levels when compared with patients without this mutation (P: 0.006).

CONCLUSIONS

The levels of GDF-15 were higher in CMPD, which are characterized by increased erythropoiesis, and this effect was more pronounced particularly in individuals with JAK2-V617F mutation. Hepcidin levels were not suppressed despite the increased erythroid activity and GDF-15 levels may be protective against the clinical complications of the disease such as thrombosis. This study revealed that, hepcidin levels were not suppressed despite increased erythroid activity and high GDF-15 levels in CMPD. We hypothesized that, this may be an attempt to prevent further amplification of erythropoietic activity by reducing iron utilization.

Transfusion suppresses erythropoiesis and increases hepcidin in adult patients with β-thalassemia major: a longitudinal study

β-Thalassemia major causes ineffective erythropoiesis and chronic anemia and is associated with iron overload due to both transfused iron and increased iron absorption, the latter mediated by suppression of the iron-regulatory hormone hepcidin. We sought to determine whether, in β-thalassemia major, transfusion-mediated inhibition of erythropoiesis dynamically affects hepcidin. We recruited 31 chronically transfused patients with β-thalassemia major and collected samples immediately before and 4 to 8 days after transfusion. Pretransfusion hepcidin was positively correlated with hemoglobin and ferritin and inversely with erythropoiesis. The hepcidin-ferritin ratio indicated hepcidin was relatively suppressed given the degree of iron loading. Posttransfusion, hemoglobin and hepcidin increased, and erythropoietin and growth differentiation factor-15 decreased. By multiple regression, pre- and posttransfusion hepcidin concentrations were both associated positively with hemoglobin, inversely with erythropoiesis, and positively with ferritin. Although men and women had similar pretransfusion hemoglobin, men had significantly increased erythropoiesis and lower hepcidin, received a lower transfusion volume per liter blood volume, and experienced a smaller posttransfusion reduction in erythropoiesis and hepcidin rise. Age of blood was not associated with posttransfusion hemoglobin or ferritin change. Hepcidin levels in patients with β-thalassemia major dynamically reflect competing influences from erythropoiesis, anemia, and iron overload. Measurement of these indices could assist clinical monitoring.

Hepcidin regulates intrarenal iron handling at the distal nephron

Hepcidin, the key regulatory hormone of iron homeostasis, and iron carriers such as transferrin receptor1 (TFR1), divalent metal transporter1 (DMT1), and ferroportin (FPN) are expressed in kidney. Whether hepcidin plays an intrinsic role in the regulation of renal iron transport is unknown. Here, we analyzed the renal handling of iron in hemochromatosis Hepc(-/-) and Hjv(-/-) mouse models, as well as in phenylhydrazine (PHZ)-treated mice. We found a marked medullary iron deposition in the kidneys of Hepc(-/-) mice, and iron leak in the urine. The kidneys of Hepc(-/-) mice exhibited a concomitant decrease in TFR1 and increase in ferritin and FPN expression. Increased FPN abundance was restricted to the thick ascending limb (TAL). DMT1 protein remained unaffected despite a significant decrease of its mRNA level, suggesting that DMT1 protein is stabilized in the absence of hepcidin. Treatment of kidney sections from Hepc(-/-) mice with hepcidin decreased DMT1 protein, an effect confirmed in renal cell lines where hepcidin markedly decreased (55)Fe transport. In the kidneys of Hjv(-/-) mice exhibiting low hepcidin expression, the iron overload was similar to that in the kidneys of Hepc(-/-) mice. However, in PHZ mice, iron accumulation resulting from hemoglobin leak was detected in the proximal tubule. Thus, kidneys exhibit a tissue-specific handling of iron that depends on the extra iron source. Hepcidin may control the expression of iron transporters to prevent renal iron overload.

Increased iron loading induces Bmp6 expression in the non-parenchymal cells of the liver independent of the BMP-signaling pathway

Bone morphogenetic protein 6 (BMP6) is an essential cytokine for the expression of hepcidin, an iron regulatory hormone secreted predominantly by hepatocytes. Bmp6 expression is upregulated by increased iron-levels in the liver. Both hepatocytes and non-parenchymal liver cells have detectable Bmp6 mRNA. Here we showed that induction of hepcidin expression in hepatocytes by dietary iron is associated with an elevation of Bmp6 mRNA in the non-parenchymal cells of the liver. Consistently, incubation with iron-saturated transferrin induces Bmp6 mRNA expression in isolated hepatic stellate cells, but not in hepatocytes. These observations suggest an important role of the non-parenchymal liver cells in regulating iron-homeostasis by acting as a source of Bmp6.

Reducing TMPRSS6 ameliorates hemochromatosis and β-thalassemia in mice

β-Thalassemia and HFE-related hemochromatosis are 2 of the most frequently inherited disorders worldwide. Both disorders are characterized by low levels of hepcidin (HAMP), the hormone that regulates iron absorption. As a consequence, patients affected by these disorders exhibit iron overload, which is the main cause of morbidity and mortality. HAMP expression is controlled by activation of the SMAD1,5,8/SMAD4 complex. TMPRSS6 is a serine protease that reduces SMAD activation and blocks HAMP expression. We identified second generation antisense oligonucleotides (ASOs) targeting mouse Tmprss6. ASO treatment in mice affected by hemochromatosis (Hfe(-/-)) significantly decreased serum iron, transferrin saturation and liver iron accumulation. Furthermore, ASO treatment of mice affected by β-thalassemia (HBB(th3/+) mice, referred to hereafter as th3/+ mice) decreased the formation of insoluble membrane-bound globins, ROS, and apoptosis, and improved anemia. These animals also exhibited lower erythropoietin levels, a significant amelioration of ineffective erythropoiesis (IE) and splenomegaly, and an increase in total hemoglobin levels. These data suggest that ASOs targeting Tmprss6 could be beneficial in individuals with hemochromatosis, β-thalassemia, and related disorders.