Inhibition of bone morphogenetic protein signaling attenuates anemia associated with inflammation

Hemojuvelin and bone morphogenetic protein (BMP) signaling in iron homeostasis

Mutations in hemojuvelin (HJV) are the most common cause of the juvenile-onset form of the iron overload disorder hereditary hemochromatosis. The discovery that HJV functions as a co-receptor for the bone morphogenetic protein (BMP) family of signaling molecules helped to identify this signaling pathway as a central regulator of the key iron hormone hepcidin in the control of systemic iron homeostasis. This review highlights recent work uncovering the mechanism of action of HJV and the BMP-SMAD signaling pathway in regulating hepcidin expression in the liver, as well as additional studies investigating possible extra-hepatic functions of HJV. This review also explores the interaction between HJV, the BMP-SMAD signaling pathway and other regulators of hepcidin expression in systemic iron balance.

Physiology of iron metabolism

A revolution occurred during the last decade in the comprehension of the physiology as well as in the physiopathology of iron metabolism. The purpose of this review is to summarize the recent knowledge that has accumulated, allowing a better comprehension of the mechanisms implicated in iron homeostasis. Iron metabolism is very fine tuned. The free molecule is very toxic; therefore, complex regulatory mechanisms have been developed in mammalian to insure adequate intestinal absorption, transportation, utilization, and elimination. 'Ironomics' certainly will be the future of the understanding of genes as well as of the protein-protein interactions involved in iron metabolism.

Hepcidin-dependent and hepcidin-independent regulation of erythropoiesis in a mouse model of anemia of chronic inflammation

Increased hepcidin antimicrobial peptide correlates with hypoferremia and anemia in various disease states, but its requirement for anemia of inflammation has not been adequately demonstrated. Anemia of inflammation is usually described as normocytic and normochromic, while diseases associated with over expression of hepcidin, alone, are often microcytic and hypochromic. These differences in erythrocyte parameters suggest anemia in many inflammatory states may not be fully explained by hepcidin-mediated iron sequestration. We used turpentine-induced sterile abscesses to model chronic inflammation in mice with targeted disruption of Hepcidin 1 [Hepc1 (-/-)] or its positive regulator, Interleukin-6 [IL-6 (-/-)], to determine whether these genes are required for features characteristic of anemia of inflammation. Although hemoglobin levels did not decline in Hepc1 (-/-) mice with sterile abscesses, erythrocyte numbers were significantly reduced compared to untreated Hepc1 (-/-) mice. In contrast, both hemoglobin concentration and erythrocyte number declined significantly in wild type and IL-6 (-/-) mice with sterile abscesses. Both Hepc1 (-/-) and IL-6 (-/-) mice had increased erythrocyte mean cell volume and mean cell hemoglobin following sterile abscesses, while wild types had no change. Thus, IL-6 (-/-) mice with sterile abscesses exhibit an intermediate phenotype between wild type and Hepc1 (-/-). Our results demonstrate the requirement of Hepc1 for the development of anemia in this rodent model. Simultaneously, our results demonstrate hepcidin-independent effects of inflammation on the suppression of erythropoiesis. Our results suggest chronic anemia associated with inflammation may benefit from interventions protecting erythrocyte number in addition to anti-hepcidin interventions aimed at enhancing iron availability.

Hepcidin antagonists for potential treatments of disorders with hepcidin excess

The discovery of hepcidin clarified the basic mechanism of the control of systemic iron homeostasis. Hepcidin is mainly produced by the liver as a propeptide and processed by furin into the mature active peptide. Hepcidin binds ferroportin, the only cellular iron exporter, causing the internalization and degradation of both. Thus hepcidin blocks iron export from the key cells for dietary iron absorption (enterocytes), recycling of hemoglobin iron (the macrophages) and the release of storage iron from hepatocytes, resulting in the reduction of systemic iron availability. The BMP/HJV/SMAD pathway is the major regulator of hepcidin expression that responds to iron status. Also inflammation stimulates hepcidin via the IL6/STAT3 pathway with a support of an active BMP/HJV/SMAD pathway. In some pathological conditions hepcidin level is inadequately elevated and reduces iron availability in the body, resulting in anemia. These conditions occur in the genetic iron refractory iron deficiency anemia and the common anemia of chronic disease (ACD) or anemia of inflammation. Currently, there is no definite treatment for ACD. Erythropoiesis-stimulating agents and intravenous iron have been proposed in some cases but they are scarcely effective and may have adverse effects. Alternative approaches aimed to a pharmacological control of hepcidin expression have been attempted, targeting different regulatory steps. They include hepcidin sequestering agents (antibodies, anticalins, and aptamers), inhibitors of BMP/SMAD or of IL6/STAT3 pathway or of hepcidin transduction (siRNA/shRNA) or ferroportin stabilizers. In this review we summarized the biochemical interactions of the proteins involved in the BMP/HJV/SMAD pathway and its natural inhibitors, the murine and rat models with high hepcidin levels currently available and finally the progresses in the development of hepcidin antagonists, with particular attention to the role of heparins and heparin sulfate proteoglycans in hepcidin expression and modulation of the BMP6/SMAD pathway.

Black soyabean seed coat extract regulates iron metabolism by inhibiting the expression of hepcidin

Hepcidin, a key regulator of Fe homeostasis, is an ideal drug target for treating patients with Fe disorders such as haemochromatosis, anaemia of chronic inflammation and Fe-deficiency anaemia. However, whether (and how) traditional Chinese black foods (e.g., black soyabeans) target hepcidin and improve Fe-deficiency anaemia remains unclear. Herein, we report that black soyabean seed coat extract (BSSCE) can potently inhibit the in vitro and in vivo expression of hepcidin. In the present study, in cells treated with 200 μg/ml BSSCE, hepcidin expression was found to be reduced to only 6% of the control levels (P<0.01). An AIN-76A diet containing 2% BSSCE was fed to 8-week-old male C57BL/6 mice for 0, 1, 7, 15 or 30 d; importantly, compared with the day 0 group, the day 7 group exhibited nearly a 50% decrease in hepatic hepcidin expression (P<0.01), a 35% decrease in splenic Fe concentrations (P<0.05) and a 135% increase in serum Fe concentrations (P<0.05). Mechanistically, the effect of BSSCE on hepcidin expression was mediated via a reduction in the phosphorylation levels of mothers against decapentaplegic homolog proteins (Smad)1/5/8. Consequently, the mice in the day 30 group exhibited large increases in erythrocyte counts (111% v. day 0, P<0.01), Hb concentrations (109%, P<0.01) and haematocrit values (108%, P<0.01). In conclusion, these results indicate that black soyabean extract regulates Fe metabolism by inhibiting the expression of hepcidin. This finding can be used to optimise the intervention of patients with hepcidin-related diseases, including Fe-deficiency anaemia.

The type I BMP receptor Alk3 is required for the induction of hepatic hepcidin gene expression by interleukin-6

Increased IL-6 production induces, via STAT3 phosphorylation, hepatic transcription of the gene encoding the iron-regulatory hormone, hepcidin, leading to development of anemia of chronic disease (ACD). Inhibition of bone morphogenetic protein (BMP) signaling prevents the induction of hepcidin gene expression by IL-6 and ameliorates ACD. Using mice with hepatocyte-specific deficiency of Alk2 or Alk3, we sought to identify the BMP type I receptor that participates in IL-6-mediated induction of hepcidin gene expression. Mice were injected with adenovirus specifying IL-6 (Ad.IL-6) or control adenovirus. Seventy-two hours later, serum iron concentrations and hepatic levels of STAT3 phosphorylation and hepcidin messenger RNA were measured. Additional mice were injected with recombinant murine IL-6 (mIL-6) or vehicle, and hepatic hepcidin gene expression was measured 4 hours later. Deficiency of Alk2 or Alk3 did not alter the ability of Ad.IL-6 injection to induce hepatic STAT3 phosphorylation. Ad.IL-6 increased hepatic hepcidin messenger RNA levels and decreased serum iron concentrations in Alk2- but not Alk3-deficient mice. Similarly, administration of mIL-6 induced hepatic hepcidin gene expression in Alk2- but not Alk3-deficient mice. These results demonstrate that the ability of IL-6 to induce hepatic hepcidin gene expression and reduce serum iron concentrations is dependent on the BMP type I receptor Alk3.

Systemic iron homeostasis

The iron hormone hepcidin and its receptor and cellular iron exporter ferroportin control the major fluxes of iron into blood plasma: intestinal iron absorption, the delivery of recycled iron from macrophages, and the release of stored iron from hepatocytes. Because iron losses are comparatively very small, iron absorption and its regulation by hepcidin and ferroportin determine total body iron content. Hepcidin is in turn feedback-regulated by plasma iron concentration and iron stores, and negatively regulated by the activity of erythrocyte precursors, the dominant consumers of iron. Hepcidin and ferroportin also play a role in host defense and inflammation, and hepcidin synthesis is induced by inflammatory signals including interleukin-6 and activin B. This review summarizes and discusses recent progress in molecular characterization of systemic iron homeostasis and its disorders, and identifies areas for further investigation.

The iron cycle in chronic kidney disease (CKD): from genetics and experimental models to CKD patients

Iron is essential for most living organisms but iron excess can be toxic. Cellular and systemic iron balance is therefore tightly controlled. Iron homeostasis is dysregulated in chronic kidney disease (CKD) and contributes to the anemia that is prevalent in this patient population. Iron supplementation is one cornerstone of anemia management in CKD patients, but has not been rigorously studied in large prospective randomized controlled trials. This review highlights important advances from genetic studies and animal models that have provided key insights into the molecular mechanisms governing iron homeostasis and its disturbance in CKD, and summarizes how these findings may yield advances in the care of this patient population.

Expression of iron-related proteins in the duodenum is up-regulated in patients with chronic inflammatory disorders

Mechanisms responsible for derangements in Fe homeostasis in chronic inflammatory conditions are not entirely clear. The aim of the present study was to test the hypothesis that inflammation affects the expression of Fe-related proteins in the duodenum and monocytes of patients with chronic inflammatory disorders, thus contributing to dysregulated Fe homeostasis. Duodenal mucosal samples and peripheral blood monocytes obtained from patients with chronic inflammatory disorders, namely ulcerative colitis (UC), Crohn's disease (CD) and rheumatoid arthritis, were used for gene and protein expression studies. Hb levels were significantly lower and serum C-reactive protein levels were significantly higher in patients in the disease groups. The gene expression of several Fe-related proteins in the duodenum was significantly up-regulated in patients with UC and CD. In patients with UC, the protein expression of divalent metal transporter 1 and ferroportin, which are involved in the absorption of dietary non-haem Fe, was also found to be significantly higher in the duodenal mucosa. The gene expression of the duodenal proteins of interest correlated positively with one another and negatively with Hb. In patients with UC, the gene expression of Fe-related proteins in monocytes was found to be unaffected. In a separate group of patients with UC, serum hepcidin levels were found to be significantly lower than those in the control group. In conclusion, the expression of Fe-related proteins was up-regulated in the duodenum of patients with chronic inflammatory conditions in the present study. The effects appeared to be secondary to anaemia and the consequent erythropoietic drive.

A low-molecular-weight compound K7174 represses hepcidin: possible therapeutic strategy against anemia of chronic disease

Hepcidin is the principal iron regulatory hormone, controlling the systemic absorption and remobilization of iron from intracellular stores. The expression of the hepcidin gene, HAMP, is increased in patients with anemia of chronic disease. Previously, the synthetic compound K7174 was identified through chemical screening as a novel inhibitor of the adhesion of monocytes to cytokine-stimulated endothelial cells. K7174 also ameliorated anemia induced by inflammatory cytokines in mice, which suggests a possible involvement of hepcidin regulation. The present study was performed to assess the impact of K7174 on hepcidin expression in a human hematoma cell line and in mice in vivo. We first demonstrated that K7174 treatment in HepG2 cells significantly decreased HAMP expression. Then, we conducted microarray analysis to determine the molecular mechanism by which K7174 inhibits HAMP expression. Transcriptional profiling confirmed the downregulation of HAMP. Surprisingly, we found that K7174 strongly induced GDF15, known as a negative regulator of HAMP expression. Western blotting analysis as well as ELISA confirmed the induction of GDF15 by K7174 treatment. Furthermore, K7174-mediated HAMP suppression was rescued by the silencing of GDF15 expression. Interestingly, we found that K7174 also upregulates CEBPB. Promoter analysis and chromatin immunoprecipitation analysis revealed that CEBPB could contribute to K7174-mediated transcriptional activation of GDF15. Subsequently, we also examined whether K7174 inhibits hepcidin expression in mice. Quantitative RT-PCR analysis with liver samples from K7174-treated mice demonstrated significant upregulation of Gdf15 and downregulation of Hamp expression, as compared to control mice. Furthermore, serum hepcidin concentration was also significantly decreased in K7174-treated mice. In conclusion, K7174 inhibits hepcidin expression partly by inducing GDF15. K-7174 may be a potential therapeutic option to treat anemia of chronic disease.

Regulation of systemic iron homeostasis

PURPOSE OF REVIEW

The circulating peptide hepcidin modulates systemic iron balance by limiting the absorption of dietary iron and the release of iron from macrophage stores. Recent studies conducted in humans, animal models, and tissue culture systems have enhanced our understanding of the molecular mechanisms by which hepcidin levels are altered in response to iron stores, inflammation, and erythropoietic activity.

RECENT FINDINGS

The bone morphogenetic protein (BMP) type I receptors ALK2 and ALK3 play key, nonredundant roles in mediating hepcidin synthesis through the BMP signaling pathway. Actions of the hereditary hemochromatosis proteins HFE and transferrin receptor 2 may intersect with the BMP pathway. Hepcidin induction in response to inflammation requires cooperative BMP signaling. A variety of innate immune and infectious stimuli induce hepcidin expression. The hypoxia inducible factor pathway appears to suppress hepcidin indirectly through the capacity of erythropoietin to stimulate erythropoiesis.

SUMMARY

Study of the molecular mechanisms underlying the regulation of hepcidin synthesis has revealed complex biology. Improved understanding of the signaling pathways involved in hepcidin regulation may contribute to improved therapeutic outcomes for patients with genetic and acquired disorders that impact systemic iron balance.

Out of balance--systemic iron homeostasis in iron-related disorders

Iron is an essential element in our daily diet. Most iron is required for the de novo synthesis of red blood cells, where it plays a critical role in oxygen binding to hemoglobin. Thus, iron deficiency causes anemia, a major public health burden worldwide. On the other extreme, iron accumulation in critical organs such as liver, heart, and pancreas causes organ dysfunction due to the generation of oxidative stress. Therefore, systemic iron levels must be tightly balanced. Here we focus on the regulatory role of the hepcidin/ferroportin circuitry as the major regulator of systemic iron homeostasis. We discuss how regulatory cues (e.g., iron, inflammation, or hypoxia) affect the hepcidin response and how impairment of the hepcidin/ferroportin regulatory system causes disorders of iron metabolism.

Screening identifies the Chinese medicinal plant Caulis Spatholobi as an effective HAMP expression inhibitor

Hepcidin, the pivotal regulator of iron metabolism, plays a critical role in multiple diseases including anemia of chronic disease and hemochromatosis. Recent studies have focused on identifying antagonists of hepcidin. We hypothesized that bioactive extracts from Chinese medicinal plants may be efficacious in the inhibition of expression of the hepcidin-encoding gene (HAMP) product, hepcidin. To test this, we measured the level of hepcidin expression in cultured cells treated with 16 different medicinal plant extracts, all of which are used to treat anemia-related disorders in traditional Chinese medicine. Among the extracts tested, that of Caulis Spatholobi (CS; also called Jixueteng, the stem of Spatholobus suberectus Dunn) showed the most potent inhibitory effect on HAMP expression in the Huh7 cell line and was therefore selected for further mechanistic study. In cells treated with 400 μg/mL of extract, phosphorylated mothers against decapentaplegic homolog proteins 1/5/8 levels were 80% less than those of controls (P < 0.001), and the inhibitory effect on interleukin-6-induced HAMP expression (65% inhibition) was weaker than the strong inhibition on bone morphogenetic protein 6-induced HAMP expression (97% inhibition). Seven-week-old C57BL/6 female mice were fed an AIN-76A diet containing 10.8% dried CS and then analyzed on d 0, 5, 10, or 15. On d 5, there was a 60% decrease in hepatic HAMP expression (P < 0.05), an 18% decrease in hepatic iron concentration, and a 100% increase in serum iron concentration (P < 0.05) compared with the d 0 group. In conclusion, we identify the extract of CS as a novel, potent HAMP expression inhibitor, which may be further modified and optimized to become a dietary supplement or a therapeutic option for the amelioration of hepcidin-overexpression-related diseases, including iron deficiency anemia.

Iron regulation by hepcidin

Hepcidin is a key hormone that is involved in the control of iron homeostasis in the body. Physiologically, hepcidin is controlled by iron stores, inflammation, hypoxia, and erythropoiesis. The regulation of hepcidin expression by iron is a complex process that requires the coordination of multiple proteins, including hemojuvelin, bone morphogenetic protein 6 (BMP6), hereditary hemochromatosis protein, transferrin receptor 2, matriptase-2, neogenin, BMP receptors, and transferrin. Misregulation of hepcidin is found in many disease states, such as the anemia of chronic disease, iron refractory iron deficiency anemia, cancer, hereditary hemochromatosis, and ineffective erythropoiesis, such as β-thalassemia. Thus, the regulation of hepcidin is the subject of interest for the amelioration of the detrimental effects of either iron deficiency or overload.

Augmentation of Smad-dependent BMP signaling in neural crest cells causes craniosynostosis in mice

Craniosynostosis describes conditions in which one or more sutures of the infant skull are prematurely fused, resulting in facial deformity and delayed brain development. Approximately 20% of human craniosynostoses are thought to result from gene mutations altering growth factor signaling; however, the molecular mechanisms by which these mutations cause craniosynostosis are incompletely characterized, and the causative genes for diverse types of syndromic craniosynostosis have yet to be identified. Here, we show that enhanced bone morphogenetic protein (BMP) signaling through the BMP type IA receptor (BMPR1A) in cranial neural crest cells, but not in osteoblasts, causes premature suture fusion in mice. In support of a requirement for precisely regulated BMP signaling, this defect was rescued on a Bmpr1a haploinsufficient background, with corresponding normalization of Smad phosphorylation. Moreover, in vivo treatment with LDN-193189, a selective chemical inhibitor of BMP type I receptor kinases, resulted in partial rescue of craniosynostosis. Enhanced signaling of the fibroblast growth factor (FGF) pathway, which has been implicated in craniosynostosis, was observed in both mutant and rescued mice, suggesting that augmentation of FGF signaling is not the sole cause of premature fusion found in this model. The finding that relatively modest augmentation of Smad-dependent BMP signaling leads to premature cranial suture fusion suggests an important contribution of dysregulated BMP signaling to syndromic craniosynostoses and potential strategies for early intervention.

Development of an ALK2-biased BMP type I receptor kinase inhibitor

The bone morphogenetic protein (BMP) signaling pathway has essential functions in development, homeostasis, and the normal and pathophysiologic remodeling of tissues. Small molecule inhibitors of the BMP receptor kinase family have been useful for probing physiologic functions of BMP signaling in vitro and in vivo and may have roles in the treatment of BMP-mediated diseases. Here we describe the development of a selective and potent inhibitor of the BMP type I receptor kinases, LDN-212854, which in contrast to previously described BMP receptor kinase inhibitors exhibits nearly 4 orders of selectivity for BMP versus the closely related TGF-β and Activin type I receptors. In vitro, LDN-212854 exhibits some selectivity for ALK2 in preference to other BMP type I receptors, ALK1 and ALK3, which may permit the interrogation of ALK2-mediated signaling, transcriptional activity, and function. LDN-212854 potently inhibits heterotopic ossification in an inducible transgenic mutant ALK2 mouse model of fibrodysplasia ossificans progressiva. These findings represent a significant step toward developing selective inhibitors targeting individual members of the highly homologous BMP type I receptor family. Such inhibitors would provide greater resolution as probes of physiologic function and improved selectivity against therapeutic targets.

A new class of small molecule inhibitor of BMP signaling

Growth factor signaling pathways are tightly regulated by phosphorylation and include many important kinase targets of interest for drug discovery. Small molecule inhibitors of the bone morphogenetic protein (BMP) receptor kinase ALK2 (ACVR1) are needed urgently to treat the progressively debilitating musculoskeletal disease fibrodysplasia ossificans progressiva (FOP). Dorsomorphin analogues, first identified in zebrafish, remain the only BMP inhibitor chemotype reported to date. By screening an assay panel of 250 recombinant human kinases we identified a highly selective 2-aminopyridine-based inhibitor K02288 with in vitro activity against ALK2 at low nanomolar concentrations similar to the current lead compound LDN-193189. K02288 specifically inhibited the BMP-induced Smad pathway without affecting TGF-β signaling and induced dorsalization of zebrafish embryos. Comparison of the crystal structures of ALK2 with K02288 and LDN-193189 revealed additional contacts in the K02288 complex affording improved shape complementarity and identified the exposed phenol group for further optimization of pharmacokinetics. The discovery of a new chemical series provides an independent pharmacological tool to investigate BMP signaling and offers multiple opportunities for pre-clinical development.

Synthesis and structure-activity relationships of a novel and selective bone morphogenetic protein receptor (BMP) inhibitor derived from the pyrazolo[1.5-a]pyrimidine scaffold of dorsomorphin: the discovery of ML347 as an ALK2 versus ALK3 selective MLPCN probe

A structure-activity relationship of the 3- and 6-positions of the pyrazolo[1,5-a]pyrimidine scaffold of the known BMP inhibitors dorsomorphin, 1, LDN-193189, 2, and DMH1, 3, led to the identification of a potent and selective compound for ALK2 versus ALK3. The potency contributions of several 3-position substituents were evaluated with subtle structural changes leading to significant changes in potency. From these studies, a novel 5-quinoline molecule was identified and designated an MLPCN probe molecule, ML347, which shows >300-fold selectivity for ALK2 and presents the community with a selective molecular probe for further biological evaluation.

A hepcidin lowering agent mobilizes iron for incorporation into red blood cells in an adenine-induced kidney disease model of anemia in rats

BACKGROUND

Anemia is a common complication of chronic kidney disease (CKD) that negatively impacts the quality of life and is associated with numerous adverse outcomes. Excess levels of the iron regulatory hormone hepcidin are thought to contribute to anemia in CKD patients by decreasing iron availability from the diet and from body stores. Adenine treatment in rats has been proposed as an animal model of anemia of CKD with high hepcidin levels that mirrors the condition in human patients.

METHODS

We developed a modified adenine-induced kidney disease model with a higher survival rate than previously reported models, while maintaining persistent kidney disease and anemia. We then tested whether the small molecule bone morphogenetic protein (BMP) inhibitor LDN-193189, which was previously shown to lower hepcidin levels in rodents, mobilized iron into the plasma and improved iron-restricted erythropoiesis in this model.

RESULTS

Adenine-treated rats exhibited increased hepatic hepcidin mRNA, decreased serum iron, increased spleen iron content, low hemoglobin (Hb) and inappropriately low erythropoietin (EPO) levels relative to the degree of anemia. LDN-193189 administration to adenine-treated rats lowered hepatic hepcidin mRNA, mobilized stored iron into plasma and increased Hb content of reticulocytes.

CONCLUSIONS

Our data suggest that hepcidin lowering agents may provide a new therapeutic strategy to improve iron availability for erythropoiesis in CKD.

Identification and characterization of a novel murine allele of Tmprss6

Mutagenesis screens can establish mouse models of utility for the study of critical biological processes such as iron metabolism. Such screens can produce mutations in novel genes or establish novel alleles of known genes, both of which can be useful tools for study. In order to identify genes of relevance to hematologic as well as other phenotypes, we performed N-ethyl-N-nitrosourea mutagenesis in C57BL/6J mice. An anemic mouse was identified and a putative mutation was characterized by mapping, sequencing and in vitro activity analysis. The mouse strain was backcrossed for ten generations then phenotypically characterized with respect to a previously established null mouse strain. Potential modifying loci were identified by quantitative trait locus analysis. Mapping and sequencing in an anemic mouse termed hem8 identified an I286F substitution in Tmprss6, a serine protease essential for iron metabolism; this substitution impaired in vitro protease activity. After backcrossing to C57BL6/J for ten generations, the hem8(-/-) strain exhibited a phenotype similar in some but not all aspects to that of Tmprss6(-/-) mice. The hem8 and Tmprss6-null mutations were allelic. Both hem8(-/-) and Tmprss6(-/-) mice responded similarly to pharmacological modulators of bone morphogenetic protein signaling, a key regulator of iron metabolism. Quantitative trait locus analysis in the hem8 strain identified potential modifying loci on chromosomes 2, 4, 7 and 10. In conclusion, the hem8 mouse model carries a novel allele of Tmprss6. Potential uses for this strain in the study of iron metabolism are discussed.

Iron and hepcidin: a story of recycling and balance

To avoid iron deficiency and overload, iron availability is tightly regulated at both the cellular and systemic levels. The liver peptide hepcidin controls iron flux to plasma from enterocytes and macrophages through degradation of the cellular iron exporter ferroportin. The hepcidin-ferroportin axis is essential to maintaining iron homeostasis. Genetic inactivation of proteins of the hepcidin-activating pathway causes iron overload of varying severity in human and mice. Hepcidin insufficiency and increased iron absorption are also characteristic of anemia due to ineffective erythropoiesis in which, despite high total body iron, hepcidin is suppressed by the high erythropoietic activity, worsening both iron overload and anemia in a vicious cycle. Hepcidin excess resulting from genetic inactivation of a hepcidin inhibitor, the transmembrane protease serine 6 (TMPRSS6) leads to a form of iron deficiency refractory to oral iron. Increased hepcidin explains the iron sequestration and iron-restricted erythropoiesis of anemia associated with chronic inflammatory diseases. In mice, deletion of TMPRSS6 in vivo has profound effects on the iron phenotype of hemochromatosis and beta-thalassemia. Hepcidin manipulation to restrict iron is a successful strategy to improve erythropoiesis in thalassemia, as shown clearly in preclinical studies targeting TMPRSS6; attempts to control anemia of chronic diseases by antagonizing the hepcidin effect are ongoing. Finally, the metabolic pathways identified from iron disorders are now being explored in other human pathologic conditions, including cancer.

Administration of recombinant erythropoietin alone does not improve the phenotype in iron refractory iron deficiency anemia patients

Mutations in transmembrane protease, serine 6 (TMPRSS6) cause iron refractory iron deficiency anemia (IRIDA). Parenteral iron administration may slightly improve hemoglobin level but is troublesome for patients. Optimal treatment has yet to be determined. We identified five patients from four independent families displaying the IRIDA picture with truncating biallelic mutations in TMPRSS6, one of which is novel. Liver iron determined by superconducting quantum interference device biosusceptometry ranged from 390 to 720 µg Fe/g wet weight (normal range 100-500; n = 3). Intestinal iron absorption (12 and 32 %, normal range 10-50; n = 2) and 59Fe erythrocyte incorporation after ingestion of 59Fe (57 and 38 %, normal range 70-90; n = 2) were inadequately low for iron-deficient anemic individuals. Baseline serum erythropoietin was elevated or borderline high in four patients. Administration of recombinant human erythropoietin (rhEPO) at up to 273 and 188 U/kg body weight/week alone did not improve anemia or result in a decrease of urinary hepcidin in two individuals. In conclusion, the ability of exogenous rhEPO to increase hemoglobin level appears to be impaired in IRIDA.

Anaemia in inflammatory rheumatic diseases

Anaemia is frequently observed in patients with inflammatory rheumatic diseases. Depending on its severity, anaemia negatively affects cardiovascular performance, physical activity and the quality of life of patients. However, anaemia is considered to be a symptom of the underlying inflammatory disease and, thus, neglected as a complex medical condition that warrants specific diagnosis and treatment. Although inflammation-induced alterations in iron homeostasis and erythropoiesis have a dominant role in the pathogenesis of this type of anaemia, multiple other factors such as chronic blood loss, haemolysis, disease and treatment-associated adverse effects or vitamin deficiencies can also take part in the development of anaemia. Accordingly, the prevalence of anaemia is positively associated with the severity of the underlying disease. This Review will summarize epidemiological data on anaemia in inflammatory rheumatic diseases, along with a detailed description of underlying pathophysiological pathways, available diagnostic tools and practical diagnostic strategies. Discussion of established and newly emerging treatment regimens, as well as the need for further research in this clinically relevant field, will also be included.

Management of anemia of inflammation in the elderly

Anemia of any degree is recognized as a significant independent contributor to morbidity, mortality, and frailty in elderly patients. Among the broad types of anemia in the elderly a peculiar role seems to be played by the anemia associated with chronic inflammation, which remains the most complex form of anemia to treat. The origin of this nonspecific inflammation in the elderly has not yet been clarified. It seems more plausible that the oxidative stress that accompanies ageing is the real cause of chronic inflammation of the elderly and that the same oxidative stress is actually a major cause of this anemia. The erythropoietic agents have the potential to play a therapeutic role in this patient population. Despite some promising results, rHuEPO does not have a specific indication for the treatment of anemia in the elderly. Moreover, concerns about their side effects have spurred the search for alternatives. Considering the etiopathogenetic mechanisms of anemia of inflammation in the elderly population, an integrated nutritional/dietetic approach with nutraceuticals that can manipulate oxidative stress and related inflammation may prevent the onset of this anemia and its negative impact on patients' performance and quality of life.

Downregulation of ferroportin 1 expression in hFOB1.19 osteoblasts by hepcidin

The present study was designed to address the relationship between iron homeostasis and bone metabolism. Cultured hFOB 1.19 osteoblasts were incubated with selected concentrations of hepcidin (50, 100, and 200 nmol/L) for 20 h, harvested for extraction of total RNA and proteins, and the expression of ferroportin 1 was analyzed by RT-PCR and western blotting. The results showed the presence of ferroportin 1 expression in cultured hFOB 1.19 cells. Furthermore, the ferroportin 1 had a similar expression pattern in hFOB cells as in hepatocytes and enterocytes and was downregulated by hepcidin. Our data indicate that osteoblasts are target cells for hepcidin, suggest that hepcidin may have many more targets than previously recognized, and support the role of hepcidin in the development of osteoporosis.

Mechanisms of anemia in CKD

Anemia is a common feature of CKD associated with poor outcomes. The current management of patients with anemia in CKD is controversial, with recent clinical trials demonstrating increased morbidity and mortality related to erythropoiesis stimulating agents. Here, we examine recent insights into the molecular mechanisms underlying anemia of CKD. These insights hold promise for the development of new diagnostic tests and therapies that directly target the pathophysiologic processes underlying this form of anemia.

Induction of activin B by inflammatory stimuli up-regulates expression of the iron-regulatory peptide hepcidin through Smad1/5/8 signaling

Anemia is very common in patients suffering from infections or chronic inflammation and can add substantially to the morbidity of the underlying disease. It is mediated by excessive production of the iron-regulatory peptide hepcidin, but the signaling pathway responsible for hepcidin up-regulation in the inflammatory context is still not understood completely. In the present study, we show that activin B has an unexpected but crucial role in the induction of hepcidin by inflammation. There is a dramatic induction of Inhbb mRNA, encoding the activin β(B)-subunit, in the livers of mice challenged with lipopolysaccharide, slightly preceding an increase in Smad1/5/8 phosphorylation and Hamp mRNA. Activin B also induces Smad1/5/8 phosphorylation in human hepatoma-derived cells and, synergistically with IL-6 and STAT-3 signaling, up-regulates hepcidin expression markedly, an observation confirmed in mouse primary hepatocytes. Pretreatment with a bone morphogenic protein type I receptor inhibitor showed that the effect of activin B on hepcidin expression is entirely attributable to its effect on bone morphogenetic protein signaling, most likely via activin receptor-like kinase 3. Activin B is therefore a novel and specific target for the treatment of anemia of inflammation.

Late stage erythroid precursor production is impaired in mice with chronic inflammation

BACKGROUND

We and others have shown previously that over-expression of hepcidin antimicrobial peptide, independently of inflammation, induces several features of anemia of inflammation and chronic disease, including hypoferremia, sequestration of iron stores and iron-restricted erythropoiesis. Because the iron-restricted erythropoiesis evident in hepcidin transgenic mice differs from the normocytic, normochromic anemia most often observed in anemia of inflammation, we tested the hypothesis that chronic inflammation may contribute additional features to anemia of inflammation which continue to impair erythropoiesis following the acute phase of inflammation in which hepcidin is active.

DESIGN AND METHODS

We compared erythropoiesis and iron handling in mice with turpentine-induced sterile abscesses with erythropoiesis and iron handling in hepcidin transgenic mice. We compared erythrocyte indices, expression of genes in the hepcidin regulatory pathway, tissue iron distribution, expression of heme and iron transport genes in splenic macrophages, the phenotype of erythroid maturation and chloromethyl dichlorodihydrofluorescein diacetate, acetyl ester fluorescence.

RESULTS

Mice with sterile abscesses exhibited an intense, acute inflammatory phase followed by a mild to moderate chronic inflammatory phase. We found that erythrocytes in mice with sterile abscesses were normocytic and normochromic in contrast to those in hepcidin transgenic mice. We also observed that although hypoferremia resolved in the late phases of inflammation, erythropoiesis remained suppressed, with evidence of inefficient maturation of erythroid precursors in the bone marrow of mice with sterile abscesses. Finally, we observed increased oxidative stress in erythroid progenitors and circulating erythrocytes of mice with sterile abscesses which was not evident in hepcidin transgenic mice.

CONCLUSIONS

Our results suggest that chronic inflammation inhibits late stages of erythroid production in the turpentine-induced sterile abscess model and induces features of impaired erythropoiesis which are distinct from those in hepcidin transgenic mice.

Targeting the hepcidin-ferroportin axis to develop new treatment strategies for anemia of chronic disease and anemia of inflammation

Anemia of chronic disease (ACD) or anemia of inflammation is prevalent in patients with chronic infection, autoimmune disease, cancer, and chronic kidney disease. ACD is associated with poor prognosis and lower quality of life. Management of ACD using intravenous iron and erythropoiesis stimulating agents are ineffective for some patients and are not without adverse effects, driving the need for new alternative therapies. Recent advances in our understanding of the molecular mechanisms of iron regulation reveal that increased hepcidin, the iron regulatory hormone, is a key factor in the development of ACD. In this review, we will summarize the role of hepcidin in iron homeostasis, its contribution to the pathophysiology of ACD, and novel strategies that modulate hepcidin and its target ferroportin for the treatment of ACD.

Inhibition of bone morphogenetic protein signaling reduces vascular calcification and atherosclerosis

OBJECTIVE

The expression of bone morphogenetic proteins (BMPs) is enhanced in human atherosclerotic and calcific vascular lesions. Although genetic gain- and loss-of-function experiments in mice have supported a causal role of BMP signaling in atherosclerosis and vascular calcification, it remains uncertain whether BMP signaling might be targeted pharmacologically to ameliorate both of these processes.

METHODS AND RESULTS

We tested the impact of pharmacological BMP inhibition on atherosclerosis and calcification in LDL receptor-deficient (LDLR-/-) mice. LDLR-/- mice fed a high-fat diet developed abundant vascular calcification within 20 weeks. Prolonged treatment of LDLR-/- mice with the small molecule BMP inhibitor LDN-193189 was well-tolerated and potently inhibited development of atheroma, as well as associated vascular inflammation, osteogenic activity, and calcification. Administration of recombinant BMP antagonist ALK3-Fc replicated the antiatherosclerotic and anti-inflammatory effects of LDN-193189. Treatment of human aortic endothelial cells with LDN-193189 or ALK3-Fc abrogated the production of reactive oxygen species induced by oxidized LDL, a known early event in atherogenesis. Unexpectedly, treatment of mice with LDN-193189 lowered LDL serum cholesterol by 35% and markedly decreased hepatosteatosis without inhibiting HMG-CoA reductase activity. Treatment with BMP2 increased, whereas LDN-193189 or ALK3-Fc inhibited apolipoprotein B100 secretion in HepG2 cells, suggesting that BMP signaling contributes to the regulation of cholesterol biosynthesis.

CONCLUSION

These results definitively implicate BMP signaling in atherosclerosis and calcification, while uncovering a previously unidentified role for BMP signaling in LDL cholesterol metabolism. BMP inhibition may be helpful in the treatment of atherosclerosis and associated vascular calcification.

Hepcidin regulation by innate immune and infectious stimuli

Hepcidin controls the levels and distribution of iron, an element whose availability can influence the outcome of infections. We investigated hepcidin regulation by infection-associated cytokines, pathogen-derived molecules, and whole pathogens in vitro and in vivo. We found that IL-22, an effector cytokine implicated in responses to extracellular infections, caused IL-6-independent hepcidin up-regulation in human hepatoma cells, suggesting it might represent an additional inflammatory hepcidin agonist. Like IL-6, IL-22 caused phosphorylation of STAT3 and synergized with BMP6 potentiating hepcidin induction. In human leukocytes, IL-6 caused potent, transient hepcidin up-regulation that was augmented by TGF-β1. Pathogen-derived TLR agonists also stimulated hepcidin, most notably the TLR5 agonist flagellin in an IL-6-dependent manner. In contrast, leukocyte hepcidin induction by heat-killed Candida albicans hyphae was IL-6-independent, but partially TGF-β-dependent. In a murine acute systemic candidiasis model, C albicans strongly stimulated hepcidin, accompanied by a major reduction in transferrin saturation. Similarly, hepcidin was up-regulated with concomitant lowering of serum iron during acute murine Influenza A/PR/8/34 virus (H1N1) infection. This intracellular pathogen also stimulated hepcidin expression in leukocytes and hepatoma cells. Together, these results indicate that hepcidin induction represents a component of the innate immune response to acute infection, with the potential to affect disease pathogenesis.

Hereditary hemochromatosis and transferrin receptor 2

BACKGROUND

Multicellular organisms regulate the uptake of calories, trace elements, and other nutrients by complex feedback mechanisms. In the case of iron, the body senses internal iron stores, iron requirements for hematopoiesis, and inflammatory status, and regulates iron uptake by modulating the uptake of dietary iron from the intestine. Both the liver and the intestine participate in the coordination of iron uptake and distribution in the body. The liver senses inflammatory signals and iron status of the organism and secretes a peptide hormone, hepcidin. Under high iron or inflammatory conditions hepcidin levels increase. Hepcidin binds to the iron transport protein, ferroportin (FPN), promoting FPN internalization and degradation. Decreased FPN levels reduce iron efflux out of intestinal epithelial cells and macrophages into the circulation. Derangements in iron metabolism result in either the abnormal accumulation of iron in the body, or in anemias. The identification of the mutations that cause the iron overload disease, hereditary hemochromatosis (HH), or iron-refractory iron-deficiency anemia has revealed many of the proteins used to regulate iron uptake.

SCOPE OF THE REVIEW

In this review we discuss recent data concerning the regulation of iron homeostasis in the body by the liver and how transferrin receptor 2 (TfR2) affects this process.

MAJOR CONCLUSIONS

TfR2 plays a key role in regulating iron homeostasis in the body.

GENERAL SIGNIFICANCE

The regulation of iron homeostasis is important. One third of the people in the world are anemic. HH is the most common inherited disease in people of Northern European origin and can lead to severe health complications if left untreated. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

Perturbation of hepcidin expression by BMP type I receptor deletion induces iron overload in mice

Bone morphogenetic protein (BMP) signaling induces hepatic expression of the peptide hormone hepcidin. Hepcidin reduces serum iron levels by promoting degradation of the iron exporter ferroportin. A relative deficiency of hepcidin underlies the pathophysiology of many of the genetically distinct iron overload disorders, collectively termed hereditary hemochromatosis. Conversely, chronic inflammatory conditions and neoplastic diseases can induce high hepcidin levels, leading to impaired mobilization of iron stores and the anemia of chronic disease. Two BMP type I receptors, Alk2 (Acvr1) and Alk3 (Bmpr1a), are expressed in murine hepatocytes. We report that liver-specific deletion of either Alk2 or Alk3 causes iron overload in mice. The iron overload phenotype was more marked in Alk3- than in Alk2-deficient mice, and Alk3 deficiency was associated with a nearly complete ablation of basal BMP signaling and hepcidin expression. Both Alk2 and Alk3 were required for induction of hepcidin gene expression by BMP2 in cultured hepatocytes or by iron challenge in vivo. These observations demonstrate that one type I BMP receptor, Alk3, is critically responsible for basal hepcidin expression, whereas 2 type I BMP receptors, Alk2 and Alk3, are required for regulation of hepcidin gene expression in response to iron and BMP signaling.

Pharmacologic inhibition of hepcidin expression reverses anemia of chronic inflammation in rats

Anemia of chronic inflammation (ACI) is the most frequent anemia in hospitalized patients and is associated with significant morbidity. A major underlying mechanism of ACI is the retention of iron within cells of the reticuloendothelial system (RES), thus making the metal unavailable for efficient erythropoiesis. This reticuloendothelial iron sequestration is primarily mediated by excess levels of the iron regulatory peptide hepcidin down-regulating the functional expression of the only known cellular iron export protein ferroportin resulting in blockade of iron egress from these cells. Using a well-established rat model of ACI, we herein provide novel evidence for effective treatment of ACI by blocking endogenous hepcidin production using the small molecule dorsomorphin derivative LDN-193189 or the protein soluble hemojuvelin-Fc (HJV.Fc) to inhibit bone morphogenetic protein-Smad mediated signaling required for effective hepcidin transcription. Pharmacologic inhibition of hepcidin expression results in mobilization of iron from the RES, stimulation of erythropoiesis and correction of anemia. Thus, hepcidin lowering agents are a promising new class of pharmacologic drugs to effectively combat ACI.

The hepcidin-ferroportin system as a therapeutic target in anemias and iron overload disorders

The review summarizes the current understanding of the role of hepcidin and ferroportin in normal iron homeostasis and its disorders. The various approaches to therapeutic targeting of hepcidin and ferroportin in iron-overload disorders (mainly hereditary hemochromatosis and β-thalassemia) and iron-restrictive anemias (anemias associated with infections, inflammatory disorders, and certain malignancies, anemia of chronic kidney diseases, and iron-refractory iron-deficiency anemia) are also discussed.

Unraveling mechanisms regulating systemic iron homeostasis

Systemic iron balance must be tightly regulated to prevent the deleterious effects of iron deficiency and iron overload. Hepcidin, a circulating hormone that is synthesized by the liver, has emerged as a key regulator of systemic iron homeostasis. Hepcidin inhibits the absorption of dietary iron from the intestine and the release of iron derived from red blood cells from macrophages. Therefore, variation in hepcidin levels modifies the total amount of iron stored in the body and the availability of iron for erythropoiesis. The production of hepcidin by the liver is modulated by multiple physiological stimuli, including iron loading, inflammation, and erythropoietic activity. Investigation of the functions of the gene products mutated in inherited iron disorders using tissue-culture systems and animal models has provided valuable insights into the mechanisms by which these hepcidin responses are mediated. This review focuses on recent advances in our understanding of the molecular mechanisms underlying the regulation of systemic iron homeostasis.