The gene encoding the iron regulatory peptide hepcidin is regulated by anemia, hypoxia, and inflammation

Regulation of systemic iron homeostasis: how the body responds to changes in iron demand

The iron that is required to meet the metabolic needs of cells and tissues is derived from the plasma. Plasma iron in turn reflects the release of iron from various body cells, principally the macrophages of the reticuloendothelial system, and the absorption of dietary iron by the proximal small intestine. This iron donation is highly regulated and the liver-derived peptide hepcidin has emerged as the key modulator of cellular iron export. Following its synthesis and secretion from the liver, circulating hepcidin reduces iron export into the plasma by binding to the iron efflux protein ferroportin1 on the surface of enterocytes, macrophages and other cell types and causing its internalization. The level of hepatic hepcidin expression is influenced by HFE, transferrin receptor 2 and hemojuvelin, and the signal transduction pathway(s) linking these proteins to hepcidin are only beginning to be revealed. Hemojuvelin has recently been shown to signal through the bone morphogenetic protein pathway, ultimately activating receptor SMAD/SMAD4 complexes to alter hepcidin transcription. Circulating differic transferrin has emerged as a possible upstream regulator of the liver-based hepcidin regulatory pathway. In addition to being regulated by body iron requirements, hepcidin expression can be modulated by pro-inflammatory cytokines such as interleukin-6. The continuing analysis of inherited disorders of iron metabolism combined with biochemical analysis of signal transduction pathways is essential to fully define this important regulatory system.

Molecular control of iron transport

The iron-regulatory hormone hepcidin is a 25-amino acid peptide that is synthesized in hepatocytes. Hepcidin binds to the cellular iron export channel ferroportin and causes its internalization and degradation and thereby decreases iron efflux from iron exporting tissues into plasma. By this mechanism, hepcidin inhibits dietary iron absorption, the efflux of recycled iron from splenic and hepatic macrophages, and the release of iron from storage in hepatocytes. Hepcidin synthesis is stimulated by plasma iron and iron stores and is inhibited by erythropoietic activity, ensuring that extracellular plasma iron concentrations and iron stores remain stable and the erythropoietic demand for iron is met. During inflammation, increased hepcidin concentrations cause iron sequestration in macrophages, resulting in hypoferremia and eventually anemia of inflammation. Hepcidin deficiency plays a central role in most iron overload disorders. The role of hepcidin abnormalities in anemias that are associated with renal disease and in resistance to erythropoietic therapies remains to be elucidated.

Hepcidin in Anemia and Inflammation in Chronic Kidney Disease

Maintaining the correct iron balance is crucial for health. Our understanding of the molecular control of iron metabolism has increased dramatically over the past 5 years due to the discovery of hepcidin. This is a circulating antimicrobial peptide mainly synthesized in the liver, which has been recently proposed as a factor regulating the uptake of dietary iron and its mobilization from macrophages and hepatic stores. Inflammation causes an increase of production of hepcidin, which is a potent mediator of anemia of chronic diseases. Anemia in chronic kidney disease is mainly due to erythropoietin deficiency but these patients often have a chronic inflammatory state. The aim of this review is to summarize the current knowledge dealing with a possible role of hepcidin in iron metabolism and its regulation, particularly in kidney disease. In addition, current methods of determination of hepcidin are reviewed.

Hepcidin ? central regulator of iron metabolism

The knowledge about mammalian iron metabolism has advanced dramatically over the past decades. Studies of genetics, biochemistry and molecular biology allowed us the identification and characterization of many of the molecules involved in regulation of iron homeostasis. Important progresses were made after the discovery in 2000 of a small peptide--hepcidin--that has been proved to play a central role in orchestration on iron metabolism also providing a link between iron metabolism and inflammation and innate immunity. Hepcidin directly interacts with ferroportin (FPN), the only known mammalian iron exporter, which is expressed by enterocytes, macrophages and hepatocytes. The direct hepcidin-FPN interaction allows an adaptative response from the body in situations that alter normal iron homeostasis (hypoxia, anemia, iron deficiency, iron overload, and inflammation).

Does hepcidin affect erythropoiesis in hemodialysis patients?

INTRODUCTION

Prohepcidin is the precursor of hepcidin, a liver-derived peptide involved in iron metabolism by blocking its intestinal absorption and its release by the reticuloendothelial system. Iron overload and inflammation increase hepcidin expression, whereas anemia and hypoxia suppress it. In the present study prohepcidin levels were determined in the serum of hemodialysis (HD) patients and its correlations with iron metabolism markers, C-reactive protein (CRP) and hematocrit (Hct) were assessed.

PATIENTS AND METHODS

Forty-sixHD patients and 22 healthy volunteers were enrolled in the study. Hct, serum prohepcidin, CRP, iron, ferritin, transferrin saturation and transferrin receptors were measured. The weekly erythropoietin dose, last-month intravenous iron dose and the patients' demographics were recorded.

RESULTS

In comparison to the healthy volunteers, the HD patients had higher serum ferritin, transferrin receptors and CRP, lower serum iron and similar transferrin saturation and prohepcidin levels. In the patient group prohepcidin levels were negatively correlated with Hct but not with any other of the examined parameters. Multiple linear regression analysis considering age, inflammation, iron adequacy, erythropoietin dose and prohepcidin levels revealed that prohepcidin was the predominant determinant of Hct.

CONCLUSIONS

Taking into account the low Hct levels in the HD patients of our study, it seems plausible that the prohepcidin levels assessed in this group are inappropriately high. These functionally high prohepcidin levels may be associated with the factors that inhibit erythropoiesis in HD patients. On the other hand, the absence of other expected correlations indicates that further studies are needed in order to definitely clarify this aspect.

Expression of iron regulatory genes in a rat model of hepatocellular carcinoma

BACKGROUND/AIMS

The altered iron metabolism in hepatocellular carcinomas (HCCs), characterized by the iron-deficient phenotype, is suggested to be of importance for tumour growth. However, the underlying molecular mechanisms remain poorly understood. We asked whether these iron perturbations would involve altered expression of genes controlling iron homeostasis.

METHODS

HCCs were induced in rats by the Solt and Farber protocol of chemical hepatocarcinogenesis, and to evaluate the effects of iron loading, one group of animals were supplemented with dietary iron during tumour progression. Tissue iron contents were determined, labelling indices of S-phase nuclei were calculated, and mRNA levels of iron-regulatory genes were quantitated. Protein levels of ferroportin1 were determined with Western blot.

RESULTS

HCCs displayed reduced amount of tissue iron and lack of histologically stainable iron. HCCs expressed significantly higher mRNA levels of genes involved in iron uptake (transferrin receptor-1, divalent metal ion transporter-1), ferroxidase activity (Ferritin-H), and iron extrusion (ferroportin1). The protein levels of ferroportin1 in iron-deficient HCCs were similar as in control livers, and did not increase in HCCs exposed to iron. Hepcidin mRNA levels were decreased in iron-deficient HCCs, rose in response to iron loading and correlated to the tissue iron content.

CONCLUSIONS

Taken together, the altered expressions of iron-regulatory genes in HCCs possibly reflect an increased demand for bioavailable iron and a high iron turnover in neoplastic cells.

Hepcidin--a peptide hormone at the interface of innate immunity and iron metabolism

Hepcidin is a cationic amphipathic peptide made in the liver, released into plasma and excreted in urine. Hepcidin is the homeostatic regulator of intestinal iron absorption, iron recycling by macrophages, and iron mobilization from hepatic stores, but it is also markedly induced during infections and inflammation. Under the influence of hepcidin, macrophages, hepatocytes, and enterocytes retain iron that would otherwise be released into plasma. Hepcidin acts by inhibiting the efflux of iron through ferroportin, the sole known iron exporter that is expressed in the small intestine, and in hepatocytes and macrophages. As befits an iron-regulatory hormone, hepcidin synthesis is increased by iron loading, and decreased by anemia and hypoxia. Hepcidin is also rapidly induced by cytokines, including IL-6. The resulting decrease in plasma iron levels eventually limits iron availability to erythropoiesis and contributes to the anemia associated with infection and inflammation. The decrease in extracellular iron concentrations due to hepcidin probably limits iron availability to invading microorganisms, thus contributing to host defense.

Downregulation of hepcidin and haemojuvelin expression in the hepatocyte cell-line HepG2 induced by thalassaemic sera

Beta-thalassaemia represents a group of diseases, in which ineffective erythropoiesis is accompanied by iron overload. In a mouse model of beta-thalassaemia, we observed that the liver expressed relatively low levels of hepcidin, which is a key factor in the regulation of iron absorption by the gut and of iron recycling by the reticuloendothelial system. It was hypothesised that, despite the overt iron overload, a putative plasma factor found in beta-thalassaemia might suppress liver hepcidin expression. Sera from beta-thalassaemia and haemochromatosis (C282Y mutation) patients were compared with those of healthy individuals regarding their capacity to induce changes the expression of key genes of iron metabolism in human HepG2 hepatoma cells. Sera from beta-thalassaemia major patients induced a major decrease in hepcidin (HAMP) and lipocalin2 (oncogene 24p3) (LCN2) expression, as well as a moderate decrease in haemojuvelin (HFE2) expression, compared with sera from healthy individuals. A significant correlation was found between the degree of downregulation of HAMP and HFE2 induced by beta-thalassaemia major sera (r = 0.852, P < 0.0009). Decreased HAMP expression was also found in HepG2 cells treated with sera from beta-thalassaemia intermedia patients. In contrast, the majority of sera from hereditary haemochromatosis patients induced an increase in HAMP expression, which correlated with transferrin (Tf) saturation (r = 0.765, P < 0.0099). Our results suggest that, in beta-thalassaemia, serum factors might override the potential effect of iron overload on HAMP expression, thereby providing an explanation for the failure to arrest excessive intestinal iron absorption in these patients.

Bone morphogenetic proteins 2, 4, and 9 stimulate murine hepcidin 1 expression independently of Hfe, transferrin receptor 2 (Tfr2), and IL-6

Recently, it has been suggested that hepcidin, a peptide involved in iron homeostasis, is regulated by bone morphogenetic proteins (BMPs), apparently by binding to hemojuvelin (Hjv) as a coreceptor and signaling through Smad4. We investigate the role of Hfe, Tfr2 (transferrin receptor 2), and IL-6 in BMP2-, BMP4-, and BMP9-stimulated up-regulation of murine hepcidin, because these molecules, like Hjv, are known to be involved in hepcidin signaling. We show that the BMP signaling pathway acts independently of Hfe, Tfr2, and IL-6: The response to BMP2, BMP4, and BMP9 is similar in isolated hepatocytes of wild-type, Hfe(-/-), IL-6(-/-), and Tfr2(m) mutant mice. The potency of different human BMPs in stimulating hepcidin transcription by murine primary hepatocytes is BMP9 > BMP4 > BMP2. However, in human HepG2 cells, BMP4 and BMP9 are equally potent, whereas BMP2 requires a higher dose to become an effective hepcidin activator. Moreover, all of the tested BMPs are more potent regulators of hepcidin than IL-6 and thus are the most potent known stimulators of hepcidin transcription.

New insights into the regulation of iron homeostasis

Hepcidin evolves as a potent hepatocyte-derived regulator of the body's iron distribution piloting the flow of iron via, and directly binding, to the cellular iron exporter ferroportin. The hepcidin-ferroportin axis dominates the iron egress from all cellular compartments that are critical to iron homeostasis, namely placental syncytiotrophoblasts, duodenal enterocytes, hepatocytes and macrophages of the reticuloendothelial system. The gene that encodes hepcidin expression (HAMP) is subject to regulation by proinflammatory cytokines, such as IL-6 and IL-1; excessive hepcidin production explains the relative deficiency of iron during inflammatory states, eventually resulting in the anaemia of inflammation. The haemochromatosis genes HFE, TfR2 and HJV potentially facilitate the transcription of HAMP. Disruption of each of the four genes leads to a diminished hepatic release of hepcidin consistent with both a dominant role of hepcidin in hereditary haemochromatosis and an upstream regulatory role of HFE, TfR2 and HJV on HAMP expression. The engineered generation of hepcidin agonists, mimetics or antagonists could largely broaden current therapeutic strategies to redirect the flow of iron.

Plasma Prohepcidin Positively Correlates with Hematocrit in Chronic Hemodialysis Patients

BACKGROUND/AIMS

Experimental studies have demonstrated interleukin-6 and iron load induce expression of hepcidin (an iron regulatory peptide), whereas anemia and erythropoietin (EPO) suppress its expression. We are the first to explore the relationships of plasma prohepcidin (the pro-hormone of hepcidin) in patients undergoing chronic hemodialysis (HD).

METHODS

We enrolled 71 chronic HD patients. During the preceding 3 months before enrollment, they all had steady weekly levels of haematocrit (Hct) and fixed subcutaneous doses of recombinant human EPO. Plasma levels of prohepcidin, proinflammatory cytokines, and EPO were determined by ELISA kits.

RESULTS

Of the patients, prohepcidin levels correlated positively with Hct, and negatively with interleukin-6 and EPO. Examined by a multivariate lineal regression method, we found Hct was the only significant predictor of plasma prohepcidin level. However, prohepcidin had no significant correlation with iron profiles.

CONCLUSION

Our findings suggest prohepcidin expression in chronic HD patients might be positively regulated by Hct.

Hepcidin: an important new regulator of iron homeostasis

Summary Hepcidin is an important and recently discovered regulator of iron homeostasis. There is strong evidence in support of an important role for hepcidin dysregulation in the pathogenesis of iron overload disorders, and possibly in the aetiology of the anaemia of chronic disease. Further research is needed into the physiology of hepcidin to elucidate the relative contributions of the liver and kidney to its production and metabolism. The study of the differential roles of prohepcidin and its metabolites as well as the significance of their serum and urine levels will enhance our understanding of their role in iron metabolism.

Role of interleukin-6 in hypoxic regulation of intestinal iron absorption

The regulation of intestinal iron absorption is not fully understood. Hepcidin, a liver-produced peptide, has recently been identified as a negative regulator of iron absorption in various conditions associated with altered iron metabolism (e.g. inflammation, anaemia, hypoxia). It is not clear whether these perturbants share a common signalling pathway. In this study, the importance of the cytokine interleukin-6 (IL-6) was investigated in the hypoxic mouse model. Hypoxia was associated with increased levels of circulating IL-6, decreased liver hepcidin mRNA and increased iron absorption (especially MT). A significant positive correlation existed between the total iron uptake and IL-6 levels in circulation. IL-6 per se, though inducing hepcidin mRNA, failed to affect basal iron absorption. The adaptive response to absorption following the hypoxic exposure was, however, more prominent if mice had been treated concurrently with IL-6. This enhancement in absorption occurred even though hepcidin mRNA was not significantly changed. Similar prominent responses were seen with both human and mouse IL-6. Anti-IL-6 antiserum normalised iron absorption in mice exposed to hypoxia, because of a reduction in the MT. These data indicate that IL-6 can influence iron absorption (especially MT) during the hypoxic exposure, but via a mechanism independent of hepcidin.

Is hepcidin a link between anemia, inflammation and liver function in hemodialyzed patients?

BACKGROUND

Hepcidin synthesis in hepatocytes is modulated in response to anemia, hypoxia or inflammation. A cross-sectional study was performed to assess hepcidin correlations with markers of iron status, erythropoietin therapy and markers of inflammation in hemodialyzed patients and in the healthy volunteers.

METHODS

Iron status, complete blood count, creatinine, albumin, lipids were assessed using standard laboratory methods. Hepcidin and high-sensitivity CRP were measured using commercially available kits.

RESULTS

Serum iron, TIBC, TSAT, erythrocyte count, Hb, Ht, platelet count, albumin, and cholesterol were lower, whereas ferritin and hepcidin were higher in hemodialyzed patients over controls. Hepcidin correlated positively with triglycerides, albumin, aspartate aminotransferase, lymphocyte count, ferritin and erythropoietin dose and negatively with erythrocyte count, Hb, and Ht in hemodialyzed patients. In multiple regression analysis, triglycerides (beta value was 0.28, p = 0.02) and albumin (beta value was -0.31, p = 0.006) were correlates of hepcidin in hemodialyzed patients.

CONCLUSIONS

Elevated hepcidin levels in hemodialyzed patients may be due to functional iron deficiency and anemia. Liver plays an important role in the synthesis of hepcidin. Low-grade inflammation, frequently found in hemodialyzed patients, might also contribute to elevated hepcidin concentration. Hypothesis that hepcidin might link anemia, inflammation and liver function in kidney disease should be further evaluated.

The evaluation of hyperferritinemia: an updated strategy based on advances in detecting genetic abnormalities

The number of new genes implicated in iron metabolism has dramatically increased during the last few years. Alterations of these genes may cause hyperferritinemia associated or not with iron overload. Correct assignment of the specific disorder of iron metabolism requires the identification of the causative gene mutation. Here, we propose a rational strategy that allows targeting the gene(s) to be screened for a diagnostic purpose. This strategy relies on the age of onset of the disease, the type of clinical symptoms, the biochemical profile (elevated or normal serum transferrin saturation (TfSat)), the presence or not of visceral iron excess, and the mode of inheritance (autosomal recessive or dominant). Then, two main entities can be differentiated: genetic (adult or juvenile) hemochromatosis characterized by elevated TfSat, and hereditary hyperferritinemias where TfSat is normal (or only slightly modified). Adult genetic hemochromatosis (GH) is related mainly to mutations of the HFE gene, and exceptionally to mutations of the TFR2 gene. Juvenile GH is a rare condition related principally to mutations of the HJV gene coding for hemojuvelin, and rarely to mutations of the HAMP gene coding for hepcidin. Hereditary hyperferritinemias are linked to mutations of three genes: the L-ferritin gene responsible for the hereditary hyperferritinemia cataract syndrome (without iron overload), the ferroportin gene leading to a dominant form of iron overload, and the ceruloplasmin (CP) gene corresponding to an iron overload syndrome with neurological symptoms. The proposed strategic approach may change with the identification of other genes involved in iron metabolism.

Iron release from macrophages after erythrophagocytosis is up-regulated by ferroportin 1 overexpression and down-regulated by hepcidin

Ferroportin 1 (FPN1) is transmembrane protein involved in iron homeostasis. In the duodenum, FPN1 localizes to the basolateral surface of enterocytes where it appears to export iron out of the cell and into the portal circulation. FPN1 is also abundantly expressed in reticuloendothelial macrophages of the liver, spleen, and bone marrow, suggesting that this protein serves as an iron exporter in cells that recycle iron from senescent red blood cells. To directly test the hypothesis that FPN1 functions in the export of iron after erythrophagocytosis, FPN1 was stably expressed in J774 mouse macrophages by using retroviral transduction, and release of 59Fe after phagocytosis of 59Fe-labeled rat erythrocytes was measured. J774 cells overexpressing FPN1 released 70% more 59Fe after erythrophagocytosis than control cells, consistent with a role in the recycling of iron from senescent red cells. Treatment of cells with the peptide hormone hepcidin, a systemic regulator of iron metabolism, dramatically decreased FPN1 protein levels and significantly reduced the efflux of 59Fe after erythrophagocytosis. Subsequent fractionation of the total released 59Fe into heme and nonheme compounds revealed that hepcidin treatment reduced the release of nonheme 59Fe by 50% and 25% from control and FPN1-overexpressing cells, respectively, but did not diminish efflux of 59Fe-heme. We conclude that FPN1 is directly involved in the export of iron during erythrocyte-iron recycling by macrophages.

The emerging role of the liver in iron metabolism

Iron is essential in health and well-being and its dysregulation is a common theme in disease. Recent advances in our understanding of the molecular biology underlying hemochromatosis and anemia has provided insight into the complex mechanisms implicated in iron metabolism. The proximal small bowel is the major site of iron absorption and, it is becoming increasingly clear that the regulation of this process involves the liver and, in particular, the hepatic antimicrobial peptide hepcidin. A number of studies have shown hepcidin to have an inhibitory function at the level of small bowel iron absorption, although its exact site of action remains to be elucidated. Clearly, identifying the target of hepcidin is of importance and is likely to lead to the development of therapeutic agents in the treatment of iron disorders.

Molecular cloning and expression of canine hepcidin

BACKGROUND

Hepcidin is a recently identified acute phase protein with antimicrobial and iron regulatory functions. It has been suggested that hepcidin may be the key mediator of anemia of chronic disease. Our research group is interested in developing a diagnostic assay to measure hepcidin in dogs.

OBJECTIVES

The objectives of this study were to clone and sequence the canine hepcidin gene and to gather preliminary data about tissue expression of hepcidin in dogs.

METHODS

RNA was extracted from fresh canine liver tissue and cDNA was generated and amplified. Standard reverse transcription polymerase chain reaction techniques were used with degenerate primers based on sequence homology between several other species. The amino acid (AA) sequence was compared with known sequences in other species. Tissue expression of canine hepcidin was determined by Western blot.

RESULTS

The canine hepcidin cDNA sequence encoded a highly conserved protein of 85 AAs with 8 cysteine residues at the C-terminal end. This protein was likely the precursor form (pro-hepcidin) of a smaller secreted peptide. Phylogenetic analysis showed that human hepcidin was more homologous with canine than with rodent hepcidin. In dogs, as in people, hepcidin was expressed most strongly in the liver. Western blotting showed a clear band of approximately 9 kDa, consistent with pro-hepcidin. Weak expression was also detected in canine kidney and lung tissues.

CONCLUSION

The results of this study establish the basis for future investigation involving canine hepcidin and suggest that the dog may be a suitable model for studying the role of hepcidin in human health and disease.

Anti-microbial peptides: from invertebrates to vertebrates

Gene-encoded anti-microbial peptides (AMPs) are widespread in nature, as they are synthesized by microorganisms as well as by multicellular organisms from both the vegetal and the animal kingdoms. These naturally occurring AMPs form a first line of host defense against pathogens and are involved in innate immunity. Depending on their tissue distribution, AMPs ensure either a systemic or a local protection of the organism against environmental pathogens. They are classified into three major groups: (i) peptides with an alpha-helical conformation (insect cecropins, magainins, etc.), (ii) cyclic and open-ended cyclic peptides with pairs of cysteine residues (defensins, protegrin, etc.), and (iii) peptides with an over-representation of some amino acids (proline rich, histidine rich, etc.). Most AMPs display hydrophobic and cationic properties, have a molecular mass below 25-30 kDa, and adopt an amphipathic structure (alpha-helix, beta-hairpin-like beta-sheet, beta-sheet, or alpha-helix/beta-sheet mixed structures) that is believed to be essential to their anti-microbial action. Interestingly, in recent years, a series of novel AMPs have been discovered as processed forms of large proteins. Despite the extreme diversity in their primary and secondary structures, all natural AMPs have the in vitro particularity to affect a large number of microorganisms (bacteria, fungi, yeast, virus, etc.) with identical or complementary activity spectra. This review focuses on AMPs forming alpha-helices, beta-hairpin-like beta-sheets, beta-sheets, or alpha-helix/beta-sheet mixed structures from invertebrate and vertebrate origins. These molecules show some promise for therapeutic use.