Regulation of iron metabolism by hepcidin

Changes in hepatic gene expression in dogs with experimentally induced nutritional iron deficiency

BACKGROUND AND OBJECTIVE

We investigated hepatic gene expression in dogs with experimentally induced nutritional iron deficiency (ID). Our hypothesis was that ID would result in decreased hepcidin gene expression, and possibly in altered expression of other genes associated with iron metabolism.

METHODS

Liver biopsies were collected from each of 3 dogs before induction of ID, at the point of maximal ID, and after resolution of ID. Using Affymetrix microarray technology and analytical tools specifically designed for microarray data, we identified genes that had at least a 2-fold change in expression in response to ID. Four genes were selected for further analysis by reverse transcriptase PCR (RT-PCR).

RESULTS

Dogs with ID had markedly decreased expression of the hepcidin gene (mean decrease of 40-fold for one probe and >100-fold for another probe) and increased expression of the transferrin receptor gene (mean increase of >7-fold). There was also mildly decreased expression of the "similar to calreticulin" gene and a gene of unknown function. Results of RT-PCR analysis were consistent with microarray findings.

CONCLUSION

Changes in hepcidin and transferrin receptor gene expression were consistent with the known biology of iron metabolism. The decrease in expression of a gene identified as "similar to calreticulin," while not statistically significant, was consistent with the findings of other investigators that suggest iron plays a role in calreticulin expression.

Attenuated inflammatory responses in hemochromatosis reveal a role for iron in the regulation of macrophage cytokine translation

Disturbances of iron homeostasis are associated with altered susceptibility to infectious disease, but the underlying molecular mechanisms are poorly understood. To study this phenomenon, we examined innate immunity to oral Salmonella infection in Hfe knockout (Hfe(-/-)) mice, a model of the human inherited disorder of iron metabolism type I hemochromatosis. Salmonella- and LPS-induced inflammatory responses were attenuated in the mutant animals, with less severe enterocolitis observed in vivo and reduced macrophage TNF-alpha and IL-6 secretion measured in vitro. The macrophage iron exporter ferroportin (FPN) was up-regulated in the Hfe(-/-) mice, and correspondingly, intramacrophage iron levels were lowered. Consistent with the functional importance of these changes, the abnormal cytokine production of the mutant macrophages could be reproduced in wild-type cells by iron chelation, and in a macrophage cell line by overexpression of FPN. The results of analyzing specific steps in the biosynthesis of TNF-alpha and IL-6, including intracellular concentrations, posttranslational stability and transcript levels, were consistent with reduced translation of cytokine mRNAs in Hfe(-/-) macrophages. Polyribosome profile analysis confirmed that elevated macrophage FPN expression and low intracellular iron impaired the translation of specific inflammatory cytokine transcripts. Our results provide molecular insight into immune function in type I hemochromatosis and other disorders of iron homeostasis, and reveal a novel role for iron in the regulation of the inflammatory response.

Erythroblastic islands: niches for erythropoiesis

Erythroblastic islands, the specialized niches in which erythroid precursors proliferate, differentiate, and enucleate, were first described 50 years ago by analysis of transmission electron micrographs of bone marrow. These hematopoietic subcompartments are composed of erythroblasts surrounding a central macrophage. A hiatus of several decades followed, during which the importance of erythroblastic islands remained unrecognized as erythroid progenitors were shown to possess an autonomous differentiation program with a capacity to complete terminal differentiation in vitro in the presence of erythropoietin but without macrophages. However, as the extent of proliferation, differentiation, and enucleation efficiency documented in vivo could not be recapitulated in vitro, a resurgence of interest in erythroid niches has emerged. We now have an increased molecular understanding of processes operating within erythroid niches, including cell-cell and cell-extracellular matrix adhesion, positive and negative regulatory feedback, and central macrophage function. These features of erythroblast islands represent important contributors to normal erythroid development, as well as altered erythropoiesis found in such diverse diseases as anemia of inflammation and chronic disease, myelodysplasia, thalassemia, and malarial anemia. Coupling of historical, current, and future insights will be essential to understand the tightly regulated production of red cells both in steady state and stress erythropoiesis.

Alpha+ -thalassemia protects against anemia associated with asymptomatic malaria: evidence from community-based surveys in Tanzania and Kenya

BACKGROUND

In hospital-based studies, alpha(+)-thalassemia has been found to protect against severe, life-threatening falciparum malaria. alpha(+)-Thalassemia does not seem to prevent infection or high parasite densities but rather limits progression to severe disease--in particular, severe malarial anemia. We assessed to what extent alpha(+)-thalassemia influences the association between mild, asymptomatic Plasmodium falciparum infection and hemoglobin concentration.

METHODS

The study was based on 2 community-based surveys conducted among afebrile children (0.5-8 years old; n=801) in Kenya and Tanzania.

RESULTS

Among children without inflammation (whole-blood C-reactive protein concentration <or=10 mg/L), P. falciparum infection was associated with only small reductions in hemoglobin concentration, and effects were similar across alpha-globin genotypes. By contrast, the reduction in hemoglobin concentration associated with P. falciparum infection accompanied by inflammation was larger and strongly depended on genotype (normal, -21.8 g/L; heterozygous, -16.7 g/L; and homozygous, -4.6 g/L). Relative to children with a normal genotype, this difference in effect was 5.1 g/L (95% confidence interval [CI], -1.0 to 11.1 g/L) for heterozygotes and 17.2 g/L (95% CI, 8.3 to 26.2 g/L) for homozygotes (estimates are adjusted for study site, age, height-for-age z score, and iron deficiency).

CONCLUSIONS

alpha(+)-Thalassemia limits the decline in hemoglobin concentration that is associated with afebrile infections, particularly those that are accompanied by inflammation.

Economic evaluation in Sweden of epoetin beta with intravenous iron supplementation in anaemic patients with lymphoproliferative malignancies not receiving chemotherapy

BACKGROUND AND OBJECTIVE

Functional iron deficiency is one reason for lack of response to erythropoietin treatment. Concomitant intravenous (IV) iron supplementation has the potential to improve response to erythropoietin, allowing a decrease in erythropoietin dose requirements. In a recent study of anaemic, iron-replete patients with lymphoproliferative malignancies (Leukemia, 21, 2007, 627), the haemoglobin (Hb) increase and response rate were significantly greater in patients receiving epoetin beta with concomitant IV iron compared with patients receiving epoetin beta without IV iron (P < 0.05). The present analysis aimed to investigate whether a combination of epoetin beta and IV iron is cost-effective compared with epoetin beta without IV iron.

METHODS

This analysis was performed from a Swedish societal perspective as a within-trial evaluation of overall costs (based on differences in drug costs and resource use between groups) and effect (differences in Hb increases) during 16 weeks' treatment with epoetin beta with or without concomitant IV iron.

RESULTS AND DISCUSSION

There was an improved response to epoetin beta with IV iron therapy and an almost 2-fold greater increase in Hb levels. Overall mean cost per patient in the epoetin beta with IV iron group was euro5558 and in the epoetin beta without IV iron group was euro6228. Thus, treatment with epoetin beta with IV iron resulted in overall cost savings of about 11% compared with epoetin beta without iron, mainly due to reduced erythropoietin dosages.

CONCLUSION

Epoetin beta with concomitant IV iron in anaemic patients with lymphoproliferative malignancies not receiving chemotherapy resulted in better outcomes at lower cost compared with epoetin beta without iron. This suggests that epoetin beta with IV iron is a dominant therapy from a Swedish perspective.

Serum prohepcidin is associated with soluble transferrin receptor-1 but not ferritin in healthy post-menopausal women

Hepcidin is a 25-amino-acid iron peptide hormone originated from its two precursors of prohepcidin (60-amino-acid) and preprohepcidin (84-amino-acid). Serum prohepcidin levels have been widely used to evaluate iron overload in clinical and preclinical studies. However, its usefulness is often questioned and its stepwise conversion mechanism remains largely unknown. Using New York University Women's Health Study subjects, we measured serum levels of prohepcidin with ELISA and hepcidin with mass spectrometry as well as ferritin and soluble transferrin receptor 1 (sTfR1) in 45 normal healthy post-menopausal women over a 1-year period with 2 samples per subject. We found that serum prohepcidin levels are correlated with the serum sTfR1 levels (r=0.45, p<0.01) but not to ferritin levels (r=0.08, p=0.60), suggesting that serum prohepcidin is not a biomarker of iron overload that was originally thought and designed for. Interestingly, serum hepcidin levels are associated with serum ferritin levels (r=0.64, p<0.0001) but not with sTfR1 levels (r=0.06, p=0.70), indicating that hepcidin is a measure of iron overload. Although hepcidin is a downstream product of prohepcidin, the amounts of hepcidin and prohepcidin are not related to each other (r=-0.007, p=0.90) under normal physiological conditions. The interrelationships between sTfR1 and prohepcidin or between ferritin and hepcidin suggest that ferritin- and sTfR1-sensed hepcidin conversion system exist in human body and maybe regulated at the post-translational level.

Effects of marginal iron overload on iron homeostasis and immune function in alveolar macrophages isolated from pregnant and normal rats

The effects of changes in macrophage iron status, induced by single or multiple iron injections, iron depletion or pregnancy, on both immune function and mRNA expression of genes involved in iron influx and egress have been evaluated. Macrophages isolated from iron deficient rats, or pregnant rats at day 21 of gestation, either supplemented with a single dose of iron dextran, 10 mg, at the commencement of pregnancy, or not, showed significant increases of macrophage ferroportin mRNA expression, which was paralleled by significant decreases in hepatic Hamp mRNA expression. IRP activity in macrophages was not significantly altered by iron status or the inducement of pregnancy +/- a single iron supplement. Macrophage immune function was significantly altered by iron supplementation and pregnancy. Iron supplementation, alone or combined with pregnancy, increased the activities of both NADPH oxidase and nuclear factor kappa B (NFkappaB). In contrast, the imposition of pregnancy reduced the ability of these parameters to respond to an inflammatory stimuli. Increasing iron status, if only marginally, will reduce the ability of macrophages to mount a sustained response to inflammation as well as altering iron homeostatic mechanisms.

The recombinant humanized anti-IL-6 receptor antibody tocilizumab, an innovative drug for the treatment of rheumatoid arthritis

BACKGROUND

IL-6 is a pro-inflammatory cytokine with multiple roles in the pathogenesis of rheumatoid arthritis (RA). Targeting IL-6 with the humanized anti IL-6 receptor antibody tocilizumab was effective in several placebo-controlled clinical studies in RA.

OBJECTIVES

To address how clinically efficacious blockade of IL-6 signalling with inteleukin-6 receptor antibody is in RA patients and what the potential mode of action explaining tocilizumab activity in RA treatment could be.

RESULTS/CONCLUSION

IL-6 induces autoantibody-producing plasma cells and effector T cells and is implicated in the development of clinical signs and symptoms, including increased synthesis of acute phase reactants, fatigue, anaemia and anorexia. Its effects also included significant improvements in American College of Rheumatology (ACR)20, ACR50 and ACR70 values, as well as in health-related quality of life measures, compared with controls. Tocilizumab also prevents radiographic progression of joint damage. Tocilizumab is generally well tolerated and efficacious in patients refractive to conventional DMARD therapies.

Lipopolysaccharide induces a significant increase in expression of iron regulatory hormone hepcidin in the cortex and substantia nigra in rat brain

Hepcidin plays an essential role in maintaining normal iron homeostasis outside the brain. This recently discovered iron regulation hormone is predominantly expressed in the liver, and regulated by iron and hypoxia. As an antimicrobial peptide, this hormone is also elevated during infections and inflammation. In this study we investigated the expression of hepcidin mRNA and protein in different brain regions, including the cortex, hippocampus, striatum, and substantia nigra, and the effects of lipopolysaccharide (LPS) on the expression of hepcidin using quantitative real-time RT-PCR and immunofluorescence analysis. Our data provided further evidence for the existence of hepcidin in all the regions we examined. We also demonstrated for the first time that LPS administration by iv injection can regulate the expression of hepcidin mRNA and protein not only in peripheral organs such as the liver, but also in the brain. LPS induced a significant increase in the expression of hepcidin mRNA and protein in the cortex and substantia nigra, but not in the hippocampus and striatum, indicating a regionally specific regulation of LPS on hepcidin in the brain. The relevant mechanisms and the functions of hepcidin in the brain remain to be elucidated.

Viral infection and iron metabolism

Fundamental cellular operations, including DNA synthesis and the generation of ATP, require iron. Viruses hijack cells in order to replicate, and efficient replication needs an iron-replete host. Some viruses selectively infect iron-acquiring cells by binding to transferrin receptor 1 during cell entry. Other viruses alter the expression of proteins involved in iron homeostasis, such as HFE and hepcidin. In HIV-1 and hepatitis C virus infections, iron overload is associated with poor prognosis and could be partly caused by the viruses themselves. Understanding how iron metabolism and viral infection interact might suggest new methods to control disease.

Fine tuning of hepcidin expression by positive and negative regulators

Hepcidin, the systemic regulator of iron homeostasis is activated by proteins responsible for hereditary hemochromatosis, bone morphogenetic proteins (BMPs), and inflammatory cytokines. Three recent publications now identify a novel hepcidin suppressor, the transmembrane serine protease TMPRSS6 (also known as matriptase-2), which is required to sense iron deficiency.

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.

Crusade for iron: iron uptake in unicellular eukaryotes and its significance for virulence

The effective acquisition of iron is a pre-requisite for survival of all organisms, especially parasites that have a high iron requirement. In mammals, iron homeostasis is meticulously regulated; extracellular free iron is essentially unavailable and host iron availability has a crucial role in the host-pathogen relationship. Therefore, pathogens use specialized and effective mechanisms to acquire iron. In this review, we summarize the iron-uptake systems in eukaryotic unicellular organisms with particular focus on the pathogenic species: Candida albicans, Tritrichomonas foetus, Trypanosoma brucei and Leishmania spp. We describe the diversity of their iron-uptake mechanisms and highlight the importance of the process for virulence.

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.

2-Oxoglutarate-dependent oxygenases control hepcidin gene expression

BACKGROUND/AIMS

Hepcidin is a liver-produced hormone that regulates systemic iron homeostasis. Hepcidin expression is stimulated upon iron overload or inflammation while iron deficiency, anemia and tissue hypoxia are negative regulators. We investigated the involvement of 2-oxoglutarate-dependent oxygenases, HIF-1 and other transcription factors in the hypoxic suppression of hepcidin.

METHODS

Northern blotting analysis and real time PCR were used to determine hepcidin mRNA levels in hepatoma cells and hepcidin promoter activity was measured using Huh7 cells transfected with suitable reporter constructs under various conditions.

RESULTS

Treatment of human cultured hepatoma cells with hypoxia or known inhibitors of 2-oxoglutarate-dependent oxygenases, such as the iron chelator desferrioxamine, cobalt or the 2-oxoglutarate analogue dimethyl-oxalylglycine significantly reduced hepcidin mRNA levels and down-regulated its gene promoter activity. This effect was not dependent on the HREs or other known putative response elements in the hepcidin promoter and was observed even under interleukin-6 treatment.

CONCLUSIONS

2-Oxoglutarate-dependent oxygenases are important to maintain high hepcidin mRNA expression in a HIF-1-independent manner. We suggest that modulation of oxygenase activity may be of therapeutic value in iron-related disorders.

Neogenin-mediated hemojuvelin shedding occurs after hemojuvelin traffics to the plasma membrane

HFE2 (hemochromatosis type 2 gene) is highly expressed in skeletal muscle and liver hepatocytes. Its encoded protein, hemojuvelin (HJV), is a co-receptor for the bone morphogenetic proteins 2 and 4 (BMP2 and BMP4) and enhances the BMP-induced hepcidin expression. Hepcidin is a central iron regulatory hormone predominantly secreted from hepatocytes. HJV also binds neogenin, a membrane protein widely expressed in many tissues. Neogenin is required for the processing and release of HJV from cells. The role that neogenin plays in HJV trafficking was investigated, using HepG2 cells, a human hepatoma cell line. Knockdown of endogenous neogenin markedly suppresses HJV release but has no evident effect on HJV trafficking to the plasma membrane. The addition of a soluble neogenin ectodomain to cells markedly inhibits HJV release, indicating that the HJV shedding is not processed before trafficking to the cell surface. At the plasma membrane it undergoes endocytosis in a dynamin-independent but cholesterol-dependent manner. The additional findings that HJV release is coupled to lysosomal degradation of neogenin and that cholesterol depletion by filipin blocks both HJV endocytosis and HJV release suggest that neogenin-mediated HJV release occurs after the HJV-neogenin complex is internalized from the cell surface.

Cancer cell iron metabolism and the development of potent iron chelators as anti-tumour agents

Cancer contributes to 50% of deaths worldwide and new anti-tumour therapeutics with novel mechanisms of actions are essential to develop. Metabolic inhibitors represent an important class of anti-tumour agents and for many years, agents targeting the nutrient folate were developed for the treatment of cancer. This is because of the critical need of this factor for DNA synthesis. Similarly to folate, Fe is an essential cellular nutrient that is critical for DNA synthesis. However, in contrast to folate, there has been limited effort applied to specifically design and develop Fe chelators for the treatment of cancer. Recently, investigations have led to the generation of novel di-2-pyridylketone thiosemicarbazone (DpT) and 2-benzoylpyridine thiosemicarbazone (BpT) group of ligands that demonstrate marked and selective anti-tumour activity in vitro and also in vivo against a wide spectrum of tumours. Indeed, administration of these compounds to mice did not induce whole body Fe-depletion or disturbances in haematological or biochemical indices due to the very low doses required. The mechanism of action of these ligands includes alterations in expression of molecules involved in cell cycle control and metastasis suppression, as well as the generation of redox-active Fe complexes. This review examines the alterations in Fe metabolism in tumour cells and the systematic development of novel aroylhydrazone and thiosemicarbazone Fe chelators for cancer treatment.

Hepcidin expression in mouse retina and its regulation via lipopolysaccharide/Toll-like receptor-4 pathway independent of Hfe

Hepcidin is a hormone central to the regulation of iron homeostasis in the body. It is believed to be produced exclusively by the liver. Ferroportin, an iron exporter, is the receptor for hepcidin. This transporter/receptor is expressed in Müller cells, photoreceptor cells and the RPE (retinal pigment epithelium) within the retina. Since the retina is protected by the retinal-blood barriers, we asked whether ferroportin in the retina is regulated by hepcidin in the circulation or whether the retina produces hepcidin for regulation of its own iron homeostasis. Here we show that hepcidin is expressed robustly in Müller cells, photoreceptor cells and RPE cells, closely resembling the expression pattern of ferroportin. We also show that bacterial LPS (lipopolysaccharide) is a regulator of hepcidin expression in Müller cells and the RPE, both in vitro and in vivo, and that the regulation occurs at the transcriptional level. The action of LPS on hepcidin expression is mediated by the TLR4 (Toll-like receptor-4). The upregulation of hepcidin by LPS occurs independent of Hfe (human leukocyte antigen-like protein involved in Fe homeostasis). The increase in hepcidin levels in retinal cells in response to LPS treatment is associated with a decrease in ferroportin levels. The LPS-induced upregulation of hepcidin and consequent down-regulation of ferroportin is associated with increased oxidative stress and apoptosis within the retina in vivo. We conclude that retinal iron homeostasis may be regulated in an autonomous manner by hepcidin generated within the retina and that chronic bacterial infection/inflammation of the retina may disrupt iron homeostasis and retinal function.

Mutations in TMPRSS6 cause iron-refractory iron deficiency anemia (IRIDA)

Iron deficiency is usually attributed to chronic blood loss or inadequate dietary intake. Here, we show that iron deficiency anemia refractory to oral iron therapy can be caused by germline mutations in TMPRSS6, which encodes a type II transmembrane serine protease produced by the liver that regulates the expression of the systemic iron regulatory hormone hepcidin. These findings demonstrate that TMPRSS6 is essential for normal systemic iron homeostasis in humans.

Interpreting indicators of iron status during an acute phase response--lessons from malaria and human immunodeficiency virus

Iron status is influenced by inflammation when the normal control of iron metabolism is reorganized by the primary mediators of the acute phase response, tumour necrosis factor-alpha and interleukin-1. The objective of this review is to show how indices of iron status, particularly haemoglobin, serum ferritin and soluble transferrin receptor concentrations relate to changes in the acute phase proteins during inflammation. The pattern of acute phase response after elective surgery, not preceded by infection, is used to demonstrate the time course of stimulation of the acute phase proteins. The changes in the concentrations of serum acute phase protein and markers of iron status during treatment for infection are used to demonstrate inter-relationships between the indicators. In many developing countries, asymptomatic malaria and human immunodeficiency virus (HIV) are common and may affect the interpretation of iron indicators during population assessments. Malaria produces an acute phase response and relationships between acute phase protein and indices of iron status indicate an influence of inflammation in both symptomatic and asymptomatic malaria, except when the parasitaemia is less than 1000/microL of blood when ferritin appears to be unaffected. HIV-1 impacts on haemopoiesis and anaemia. Anaemia increases in severity as the disease progresses and it is often a negative prognostic indicator. However, in individuals infected with HIV there may be an atypical acute phase response in the absence of opportunistic infections. Tentative conclusions are drawn concerning the inter-relationships between ferritin and the acute phase proteins, C-reactive protein and alpha-1-acid glycoprotein during an acute phase response.

Managing anemia in lymphoma and multiple myeloma

Anemia is common in cancer, and lymphoproliferative disease is no exception. Erythropoiesis-stimulating agents (ESA) have been used for renal anemia since 1986, and considerably later in cancer anemia. The first studies were published around 1993, but the use of ESA did not become common in cancer anemia until in the late 1990s. Cancer anemia is still under-treated. This review gives an overview of the use of ESA in hematologic malignancies. A background is given about this treatment in the cancer field generally. The pathophysiology of cancer anemia is described with special emphasis on the disturbances in iron metabolism. Functional iron deficiency has been shown to be both frequent and important as a hindrance for response to ESA treatment, and recent studies are reported in some detail, where the use of intravenous iron was shown to improve the response rate of ESA treatment.

Iron absorption in young Indian women: the interaction of iron status with the influence of tea and ascorbic acid

BACKGROUND

Ascorbic acid (AA) enhances and tea inhibits iron absorption. It is unclear whether iron status influences the magnitude of this effect.

OBJECTIVE

We evaluated the influence of the iron status of young women on iron absorption from a rice meal with or without added tea or AA.

DESIGN

Two stable-isotope iron absorption studies were made in 2 groups of 10 subjects with iron deficiency anemia (IDA) and 10 subjects who were iron replete (control subjects). In study 1, the reference rice meal was fed alone or with 1 or 2 cups of black tea. In study 2, the reference meal was fed alone or with AA (molar ratio to iron, 2:1 or 4:1). Iron absorption was measured by the erythrocyte incorporation of (57)Fe and (58)Fe labels at 14 d.

RESULTS

Mean fractional iron absorption from the reference rice meal was approximately 2.5 times as great in the IDA group as in the control group (P < 0.05). The consumption of 1 or 2 cups of tea decreased iron absorption in the control subjects by 49% (P < 0.05) or 66% (P < 0.01), respectively, and in the IDA group by 59% or 67% (P < 0.001 for both), respectively. AA (molar ratio to iron, 2:1 or 4:1) increased iron absorption by 270% or 343%, respectively, in control subjects and by 291% or 350%, respectively, in subjects with IDA (P < 0.001).

CONCLUSIONS

The inhibitory effect of tea and the enhancing effect of AA on iron absorption were similar in the 2 groups. Overall differences in iron absorption in the 2 groups, however, continued to be dictated by iron status.

Reticulocyte hemoglobin content

Under normal conditions, reticulocytes are the youngest erythrocytes released from the bone marrow into circulating blood. They mature for 1-3 days within the bone marrow and circulate for 1-2 days before becoming mature erythrocytes. Measurement of cellular hemoglobin concentration has long been reported by automated hematology analyzers as one of the red blood cell indices. The reticulocyte hemoglobin content (CHr or Ret-He) provides an indirect measure of the functional iron available for new red blood cell production over the previous 3-4 days. Measurement of reticulocyte hemoglobin content in peripheral blood samples is useful for diagnosis of iron deficiency in adults (Mast et al., Blood 2002;99:1489-1491) and children (Brugnara et al., JAMA 1999;281:2225-2230; Ullrich et al., JAMA 2005;294:924-930; Bakr and Sarette, Eur J Pediatr 2006;165:442-445). It provides an early measure of the response to iron therapy increasing within 2-4 days of the initiation of intravenous iron therapy (Brugnara et al., Blood 1994;83:3100-3101). Sequential measurements of reticulocyte hemoglobin content in patients with iron deficiency anemia provide a rapid means for assessing the erythropoietic response to iron replacement therapy (Brugnara et al., Blood 1994;83:3100-3101). It is also an early indicator or iron-restricted erythropoiesis in patients receiving erythropoietin therapy (Fishbane et al., Kidney Int 1997;52:217-222; Fishbane et al., Kidney Int 2001;60:2406-2411; Mittman et al., Am J Kidney Dis 1997;30:912-922; Tsuchiya et al., Clin Nephrol 2003;59:115-123; Chuang et al., Nephrol Dial Transplant 2003;18:370-377). Thus, reticulocyte hemoglobin content is a recent addition to an expanding list of biomarkers that can be used to differentiate iron deficiency from other causes of anemia.

Critical review of strategies to prevent and control iron deficiency in children

Iron deficiency is prevalent in infants and young children in developing countries and is associated with adverse developmental outcomes. The routine provision of additional iron by food fortification or the use of iron supplements is generally recommended. The wisdom of this approach in regions where the transmission of Plasmodium falciparum malaria is perennial and intense is now being questioned, because a large trial in Pemba, Tanzania, demonstrated an increased risk of serious morbidity among children under the age of 3 years who were given routine daily iron and folic acid supplements. However, the results of a concurrent substudy suggest that the untoward effects occurred in children who were not iron deficient, and that iron deficiency itself is associated with an increased risk of severe morbidity that can be reduced by iron and folic acid supplementation. There is an urgent need for additional research to confirm these observations, to establish the role, if any, of the concurrent folic acid supplementation, to evaluate the risk of alternative methods for delivering iron that, on theoretical grounds, could be safer, and to establish the programmatic feasibility of targeting iron fortificants or supplements to iron-deficient children. It is evident that a single strategy for ensuring adequate iron nutrition in young children in different parts of the world is no longer likely to be satisfactory. Moreover, integration with other health-related strategies, particularly malaria control programs, will be essential.

Iron metabolism in children: confounding factors

Characterization of iron metabolism in infants and children may be confounded by the diverse effects of developmental, genetic, and acquired influences on iron metabolism and laboratory measurements of iron status, especially in areas with intense perennial transmission of Plasmodium falciparum malaria. In the Pemba iron and folic acid supplementation trial, the coadministration of folic acid with iron is a further confounding factor. Because the design of the Pemba iron and folic acid supplementation study did not include a group that received iron supplementation without folic acid, the observed increase in serious adverse events cannot be ascribed unequivocally to iron alone, to folic acid alone, or to the combination of the two. In interpreting the results from the Pemba iron and folic acid supplementation trial, additional analyses of existing data from the trial and from earlier studies in the area could help clarify the roles of iron and folic acid.

Iron depletion limits intracellular bacterial growth in macrophages

Many intracellular pathogens infect macrophages and these pathogens require iron for growth. Here we demonstrate in vitro that the intracellular growth of Chlamydia psittaci, trachomatis, and Legionella pneumophila is regulated by the levels of intracellular iron. Macrophages that express cell surface ferroportin, the only known cellular iron exporter, limit the intracellular growth of these bacteria. Hepcidin is an antimicrobial peptide secreted by the liver in response to inflammation. Hepcidin binds to ferroportin mediating its internalization and degradation. Addition of hepcidin to infected macrophages enhanced the intracellular growth of these pathogens. Macrophages from flatiron mice, a strain heterozygous for a loss-of-function ferroportin mutation, showed enhanced intracellular bacterial growth independent of the presence of exogenous hepcidin. Macrophages, from wild-type or flatiron mice, incubated with the oral iron chelator deferriprone or desferasirox showed reduced intracellular bacterial growth suggesting that these chelators might be therapeutic in chronic intracellular bacterial infections.

Erythropoietin mediates hepcidin expression in hepatocytes through EPOR signaling and regulation of C/EBPalpha

Hepcidin is the principal iron regulatory hormone, controlling the systemic absorption and remobilization of iron from intracellular stores. Recent in vivo studies have shown that hepcidin is down-regulated by erythropoiesis, anemia, and hypoxia, which meets the need of iron input for erythrocyte production. Erythropoietin (EPO) is the primary signal that triggers erythropoiesis in anemic and hypoxic conditions. Therefore, a direct involvement of EPO in hepcidin regulation can be hypothesized. We report here the regulation of hepcidin expression by EPO, in a dose-dependent manner, in freshly isolated mouse hepatocytes and in the HepG2 human hepatocyte cell model. The effect is mediated through EPOR signaling, since hepcidin mRNA levels are restored by pretreatment with an EPOR-blocking antibody. The transcription factor C/EBPalpha showed a pattern of expression similar to hepcidin, at the mRNA and protein levels, following EPO and anti-EPOR treatments. Chromatin immunoprecipitation experiments showed a significant decrease of C/EBPalpha binding to the hepcidin promoter after EPO supplementation, suggesting the involvement of this transcription factor in the transcriptional response of hepcidin to EPO.

Increased iron content and RNA oxidative damage in skeletal muscle with aging and disuse atrophy

Muscle atrophy with aging or disuse is associated with deregulated iron homeostasis and increased oxidative stress likely inflicting damage to nucleic acids. Therefore, we investigated RNA and DNA oxidation, and iron homeostasis in gastrocnemius muscles. Disuse atrophy was induced in 6- and 32-month old male Fischer 344/Brown Norway rats by 14 days of hind limb suspension (HS). We show that RNA, but not DNA, oxidative damage increased 85% with age and 36% with HS in aged muscle. Additionally, non-heme iron levels increased 233% with aging and 83% with HS at old age, while staining for free iron was strongest in the smallest fibers. Simultaneously, the mRNA abundance of transferrin receptor-1 decreased by 80% with age and 48% with HS for young animals, while that of the hepcidin regulator hemojuvelin decreased 37% with age, but increased about 44% with disuse, indicating a dysregulation of iron homeostasis favoring increased intracellular free iron in atrophied muscles. RNA and DNA concentrations increased with age and were negatively correlated with muscle mass, whereas protein concentrations decreased with aging, indicating a preferential loss of protein compared to nucleic acids. Furthermore, xanthine oxidase activity increased with age, but not with HS, while mRNA abundance of the Y box-binding protein-1, which has been suggested to bind oxidized RNA, did not change with age or HS. These results suggest that RNA oxidation, possibly mediated by increased non-heme iron, might contribute to muscle atrophy due to disuse particularly in aged muscle.

The regulation of cellular iron metabolism

While iron is an essential trace element required by nearly all living organisms, deficiencies or excesses can lead to pathological conditions such as iron deficiency anemia or hemochromatosis, respectively. A decade has passed since the discovery of the hemochromatosis gene, HFE, and our understanding of hereditary hemochromatosis (HH) and iron metabolism in health and a variety of diseases has progressed considerably. Although HFE-related hemochromatosis is the most widespread, other forms of HH have subsequently been identified. These forms are not attributed to mutations in the HFE gene but rather to mutations in genes involved in the transport, storage, and regulation of iron. This review is an overview of cellular iron metabolism and regulation, describing the function of key proteins involved in these processes, with particular emphasis on the liver's role in iron homeostasis, as it is the main target of iron deposition in pathological iron overload. Current knowledge on their roles in maintaining iron homeostasis and how their dysregulation leads to the pathogenesis of HH are discussed.

Iron regulatory proteins are essential for intestinal function and control key iron absorption molecules in the duodenum

Iron regulatory proteins (IRPs) orchestrate the posttranscriptional regulation of critical iron metabolism proteins at the cellular level. Redundancy between IRP1 and IRP2 associated with embryonic lethality of doubly IRP-deficient mice has precluded the study of IRP function in vivo. Here we use Cre/Lox technology to generate viable organisms lacking IRP expression in a single tissue, the intestine. Mice lacking intestinal IRP expression develop intestinal malabsorption and dehydration postnatally and die within 4 weeks of birth. We demonstrate that IRPs control the expression of divalent metal transporter 1 (DMT1) mRNA and protein, a limiting intestinal iron importer. IRPs are also shown to be critically important to secure physiological levels of the basolateral iron exporter ferroportin. IRPs are thus essential for intestinal function and organismal survival and coordinate the synthesis of key iron metabolism proteins in the duodenum.

Recent advances in immunopathophysiology of interleukin-6: an innovative therapeutic drug, tocilizumab (recombinant humanized anti-human interleukin-6 receptor antibody), unveils the mysterious etiology of immune-mediated inflammatory diseases

Interleukin (IL)-6 cDNA was originally cloned as a terminal B cell differentiation factor into antibody-producing plasma cells. This revealed that it is a multifunctional cytokine that acts on a variety of cells. From the clinical viewpoint, it is especially important that IL-6 acts on hepatocytes to induce acute-phase reactants, including C-reactive protein, serum amyloid A protein, and fibrinogen, and to decrease serum albumin levels. Very recently, this cytokine has been found to enhance the synthesis of a peptide called hepcidin in the liver which regulates iron recycling, resulting in anemia due to hypofferemia. It has also been shown that IL-6 is responsible for various clinical symptoms, including the appearance of autoantibodies, fatigue, anemia, anorexia, fever, and increases in the erythrocyte sedimentation rate, all of which develop in patients with various chronic autoimmune inflammatory diseases. In practice, blocking the IL-6 signaling pathway with a recombinant humanized anti-IL-6 receptor antibody, tocilizumab (TCZ), has dramatically improved all the signs and symptoms of these patients. A study in mice demonstrated that IL-6 promotes the development of a new type of T-helper cells called Th17 cells that impact the pathogenesis of autoimmune diseases. This suggests that TCZ is not only an antiinflammatory agent but also might affect basic autoimmunity. In this review, recent advances in the immunobiology of interleukin-6 related to immune-mediated diseases are discussed.

Effect of penicillamine and zinc on iron metabolism in Wilson's disease

OBJECTIVE

The physiology of iron metabolism in Wilson's disease is largely unknown, and there is a paucity of data on the real presence and progression of iron accumulation. The purpose of this study was to assess the iron metabolism parameters, including hepatic iron concentration, in follow-up liver biopsies and serum, and urinary pro-hepcidin.

MATERIAL AND METHODS

Twenty-three Wilson's disease patients undergoing long-term treatment were enrolled in the study.

RESULTS

Hepatic iron content was significantly increased in penicillamine-treated patients compared with zinc-treated patients. Serum and urinary pro-hepcidin concentrations were significantly higher in Wilson's disease patients than in healthy volunteers, despite a normal biochemical pattern of iron metabolism.

CONCLUSIONS

Long-term penicillamine treatment seems to be responsible for a more marked iron accumulation in the liver. This observation may justify a revision of long-term Wilson's disease treatment modalities with penicillamine. The finding that serum and urinary pro-hepcidin is significantly increased in Wilson's disease patients compared with healthy volunteers suggests a role for hepcidin in iron metabolism in Wilson's disease, but this needs to be confirmed by a study of hepatic hepcidin expression in these patients.

Adherence to macrophages in erythroblastic islands enhances erythroblast proliferation and increases erythrocyte production by a different mechanism than erythropoietin

Erythroblasts adhere to central macrophages forming erythroblastic islands in hematopoietic tissues, but the function of these islands is not understood. Murine erythroblastic islands were reconstituted in vitro with macrophages and developmentally synchronous proerythroblasts. Erythroblasts cocultured with macrophages proliferated 3-fold greater than erythroblasts cultured alone. Direct contact with the macrophages was necessary for this enhanced erythroblast proliferation, which resulted from decreased transit time in the G(0)/G(1) phase of cell cycle. Increased erythroblast proliferation in erythroblastic islands occurred over a wide range of erythropoietin concentrations and was the result of a mechanism different from the antiapoptotic effect of erythropoietin. Erythroblasts adherent to macrophages had slightly delayed enucleation, but otherwise differentiation was similar to erythroblasts cultured alone or those that became nonadherent in cocultures. These results suggest a mechanism for the development of anemias associated with abnormal macrophage function and for reduced responsiveness of those anemias to erythropoietin therapy.

Different regulatory elements are required for response of hepcidin to interleukin-6 and bone morphogenetic proteins 4 and 9

Hepcidin is a major regulator of iron homeostasis. Hepcidin expression is upregulated by inflammatory cytokines, particularly interleukin (IL)-6 and even more potently by the bone morphogenetic proteins 2, 4 and 9 (BMP-2, BMP-4 and BMP-9). This study showed that the regulation of hepcidin expression by IL-6 and BMPs occurs through distinct regulatory elements. The induction of hepcidin by BMPs requires at least two regions of the Hamp1 promoter, one between 140-260 bp and the other between 1.6-2.0 kb upstream of the start of translation. Reporter constructs including 1.6-2.0 kb of the Hamp1 promoter were induced >16-fold by BMPs whereas a 260 bp reporter Hamp1 promoter construct was induced only two- to threefold. The distal 1.6-2.0 kb region appeared to contain several different BMP-responsive elements, as incremental lengthening of the promoter construct in this region produced gradual escalation of BMP-responsiveness. In contrast, the IL-6 response required only the proximal 260 bp Hamp1 promoter region. Furthermore, there were no regulatory elements located in the non-coding or coding regions of Hamp1 and activation of the Hamp1 promoter was absent or markedly reduced in cells of non-hepatic origin.

Pathophysiology of anemia and erythrocytosis

An increasing understanding of the process of erythropoiesis raises some interesting questions about the pathophysiology, diagnosis and treatment of anemia and erythrocytosis. The mechanisms underlying the development of many of the erythrocytoses, previously characterised as idiopathic, have been elucidated leading to an increased understanding of oxygen homeostasis. Characterisation of anemia and erythrocytosis in relation to serum erythropoietin levels can be a useful addition to clinical diagnostic criteria and provide a rationale for treatment with erythropoiesis stimulating agents (ESAs). Recombinant human erythropoietin as well as other ESAs are now widely used to treat anemias associated with a range of conditions, including chronic kidney disease, chronic inflammatory disorders and cancer. There is also heightened awareness of the potential abuse of ESAs to boost athletic performance in competitive sport. The discovery of erythropoietin receptors outside of the erythropoietic compartment may herald future applications for ESAs in the management of neurological and cardiac diseases. The current controversy concerning optimal hemoglobin levels in chronic kidney disease patients treated with ESAs and the potential negative clinical outcomes of ESA treatment in cancer reinforces the need for cautious evaluation of the pleiotropic effects of ESAs in non-erythroid tissues.

Macrophage iron, hepcidin, and atherosclerotic plaque stability

Hepcidin has emerged as the key hormone in the regulation of iron balance and recycling. Elevated levels increase iron in macrophages and inhibit gastrointestinal iron uptake. The physiology of hepcidin suggests an additional mechanism by which iron depletion could protect against atherosclerotic lesion progression. Without hepcidin, macrophages retain less iron. Very low hepcidin levels occur in iron deficiency anemia and also in homozygous hemochromatosis. There is defective retention of iron in macrophages in hemochromatosis and also evidently no increase in atherosclerosis in this disorder. In normal subjects with intact hepcidin responses, atherosclerotic plaque has been reported to have roughly an order of magnitude higher iron concentration than that in healthy arterial wall. Hepcidin may promote plaque destabilization by preventing iron mobilization from macrophages within atherosclerotic lesions; the absence of this mobilization may result in increased cellular iron loads, lipid peroxidation, and progression to foam cells. Marked downregulation of hepcidin (e.g., by induction of iron deficiency anemia) could accelerate iron loss from intralesional macrophages. It is proposed that the minimally proatherogenic level of hepcidin is near the low levels associated with iron deficiency anemia or homozygous hemochromatosis. Induced iron deficiency anemia intensely mobilizes macrophage iron throughout the body to support erythropoiesis. Macrophage iron in the interior of atherosclerotic plaques is not exempt from this process. Decreases in both intralesional iron and lesion size by systemic iron reduction have been shown in animal studies. It remains to be confirmed in humans that a period of systemic iron depletion can decrease lesion size and increase lesion stability as demonstrated in animal studies. The proposed effects of hepcidin and iron in plaque progression offer an explanation of the paradox of no increase in atherosclerosis in patients with hemochromatosis despite a key role of iron in atherogenesis in normal subjects.

Copper and iron disorders of the brain

Copper and iron are transition elements essential for life. These metals are required to maintain the brain's biochemistry such that deficiency or excess of either copper or iron results in central nervous system disease. This review focuses on the inherited disorders in humans that directly affect copper or iron homeostasis in the brain. Elucidation of the molecular genetic basis of these rare disorders has provided insight into the mechanisms of copper and iron acquisition, trafficking, storage, and excretion in the brain. This knowledge permits a greater understanding of copper and iron roles in neurobiology and neurologic disease and may allow for the development of therapeutic approaches where aberrant metal homeostasis is implicated in disease pathogenesis.

Hepcidin regulation: ironing out the details

Hepcidin is a peptide hormone secreted by the liver that plays a central role in the regulation of iron homeostasis. Increased hepcidin levels result in anemia while decreased expression is the causative feature in most primary iron overload diseases. Mutations in hemochromatosis type 2 (HFE2), which encodes the protein hemojuvelin (HJV), result in the absence of hepcidin and an early-onset form of iron overload disease. HJV is a bone morphogenetic protein (BMP) coreceptor and HJV mutants have impaired BMP signaling. In this issue of the JCI, Babitt and colleagues show that BMPs are autocrine hormones that induce hepcidin expression (see the related article beginning on page 1933). Administration of a recombinant, soluble form of HJV decreased hepcidin expression and increased serum iron levels by mobilizing iron from splenic stores. These results demonstrate that recombinant HJV may be a useful therapeutic agent for treatment of the anemia of chronic disease, a disorder resulting from high levels of hepcidin expression.