Contribution of Hfe expression in macrophages to the regulation of hepatic hepcidin levels and iron loading

Liver macrophages in health and disease

Single-cell and spatial transcriptomic technologies have revealed an underappreciated heterogeneity of liver macrophages. This has led us to rethink the involvement of macrophages in liver homeostasis and disease. Identification of conserved gene signatures within these cells across species and diseases is enabling the correct identification of specific macrophage subsets and the generation of more specific tools to track and study the functions of these cells. Here, we discuss what is currently known about the definitions of these different macrophage populations, the markers that can be used to identify them, how they are wired within the liver, and their functional specializations in health and disease.

Oral iron supplementation after antibiotic exposure induces a deleterious recovery of the gut microbiota

Background

Oral iron supplementation is commonly prescribed for anemia and may play an important role in the gut microbiota recovery of anemic individuals who received antibiotic treatment. This study aims to investigate the effects of iron supplementation on gut microbiota recovery after antibiotics exposure.

Results

Mice were subjected to oral antibiotic treatment with neomycin and metronidazole and were fed diets with different concentrations of iron. The composition of the gut microbiota was followed throughout treatment by 16S rRNA sequencing of DNA extracted from fecal samples. Gut microbiota functions were inferred using PICRUSt2, and short-chain fatty acid concentration in fecal samples was assessed by liquid-chromatography mass spectrometry. Iron supplementation after antibiotic exposure shifted the gut microbiota composition towards a Bacteroidetes phylum-dominant composition. At the genus level, the iron-supplemented diet induced an increase in the abundance of Parasutterella and Bacteroides, and a decrease of Bilophila and Akkermansia. Parasutterella excrementihominis, Bacteroides vulgatus, and Alistipes finegoldii, were more abundant with the iron excess diet. Iron-induced shifts in microbiota composition were accompanied by functional modifications, including an enhancement of the biosynthesis of primary bile acids, nitrogen metabolism, cyanoamino acid metabolism and pentose phosphate pathways. Recovery after antibiotic treatment increased propionate levels independent of luminal iron levels, whereas butyrate levels were diminished by excess iron.

Conclusions

Oral iron supplementation after antibiotic therapy in mice may lead to deleterious changes in the recovery of the gut microbiota. Our results have implications on the use of oral iron supplementation after antibiotic exposure and justify further studies on alternative treatments for anemia in these settings.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-021-02320-0.

Oligosaccharides increase the genotoxic effect of colibactin produced by pks+ Escherichia coli strains

BACKGROUND

Colibactin is a genotoxin that induces DNA double-strand breaks that may lead to carcinogenesis and is produced by Escherichia coli strains harboring the pks island. Human and animal studies have shown that colibactin-producing gut bacteria promote carcinogenesis and enhance the progression of colorectal cancer through cellular senescence and chromosomal abnormalities. In this study, we investigated the impact of prebiotics on the genotoxicity of colibactin-producing E. coli strains Nissle 1917 and NC101.

METHODS

Bacteria were grown in medium supplemented with 20, 30 and 40 mg/mL of prebiotics inulin or galacto-oligosaccharide, and with or without 5 μM, 25 μM and 125 μM of ferrous sulfate. Colibactin expression was assessed by luciferase reporter assay for the clbA gene, essential for colibactin production, in E. coli Nissle 1917 and by RT-PCR in E. coli NC101. The human epithelial colorectal adenocarcinoma cell line, Caco-2, was used to assess colibactin-induced megalocytosis by methylene blue binding assay and genotoxicity by γ-H2AX immunofluorescence analysis.

RESULTS

Inulin and galacto-oligosaccharide enhanced the expression of clbA in pks+ E. coli. However, the addition of 125 μM of ferrous sulfate inhibited the expression of clbA triggered by oligosaccharides. In the presence of either oligosaccharide, E. coli NC101 increased dysplasia and DNA double-strand breaks in Caco-2 cells compared to untreated cells.

CONCLUSION

Our results suggest that, in vitro, prebiotic oligosaccharides exacerbate DNA damage induced by colibactin-producing bacteria. Further studies are necessary to establish whether oligosaccharide supplementation may lead to increased colorectal tumorigenesis in animal models colonized with pks+ E. coli.

Acute invariant NKT cell activation triggers an immune response that drives prominent changes in iron homeostasis

Iron homeostasis is an essential biological process that ensures the tissue distribution of iron for various cellular processes. As the major producer of hepcidin, the liver is central to the regulation of iron metabolism. The liver is also home to many immune cells, which upon activation may greatly impact iron metabolism. Here, we focus on the role of invariant natural killer T (iNKT) cells, a subset of T lymphocytes that, in mice, is most abundant in the liver. Activation of iNKT cells with the prototypical glycosphingolipid antigen, α-galactosylceramide, resulted in immune cell proliferation and biphasic changes in iron metabolism. This involved an early phase characterized by hypoferremia, hepcidin induction and ferroportin suppression, and a second phase associated with strong suppression of hepcidin despite elevated levels of circulating and tissue iron. We further show that these changes in iron metabolism are fully dependent on iNKT cell activation. Finally, we demonstrate that the biphasic regulation of hepcidin is independent of NK and Kupffer cells, and is initially driven by the STAT3 inflammatory pathway, whereas the second phase is regulated by repression of the BMP/SMAD signaling pathway. These findings indicate that iNKT activation and the resulting cell proliferation influence iron homeostasis.

Curcumin induces mild anemia in a DSS-induced colitis mouse model maintained on an iron-sufficient diet

Anemia is frequently encountered in patients with inflammatory bowel disease (IBD), decreasing the quality of life and significantly worsening the prognosis of the disease. The pathogenesis of anemia in IBD is multifactorial and results mainly from intestinal blood loss in inflamed mucosa and impaired dietary iron absorption. Multiple studies have proposed the use of the polyphenolic compound curcumin to counteract IBD pathogenesis since it has significant preventive and therapeutic properties as an anti-inflammatory agent and very low toxicity, even at high dosages. However, curcumin has been shown to possess properties consistent with those of an iron-chelator, such as the ability to modulate proteins of iron metabolism and decrease spleen and liver iron content. Thus, this property may further contribute to the development and severity of anemia of inflammation and iron deficiency in IBD. Herein, we evaluate the effects of curcumin on systemic iron balance in the dextran sodium sulfate (DSS) model of colitis in C57Bl/6 and BALB/c mouse strains that were fed an iron-sufficient diet. In these conditions, curcumin supplementation caused mild anemia, lowered iron stores, worsened colitis and significantly decreased overall survival, independent of the mouse strain. These findings suggest that curcumin usage as an anti-inflammatory supplement should be accompanied by monitoring of erythroid parameters to avoid exacerbation of iron deficiency anemia in IBD.

Liver HFE protein content is posttranscriptionally decreased in iron-deficient mice and rats

Although the relationship between hereditary hemochromatosis and mutations in the HFE gene was discovered more than 20 years ago, information on the in vivo regulation of HFE protein expression is still limited. The purpose of the study was to determine the response of liver HFE protein content to iron deficiency in mice and rats by immunoblotting. Attempts to visualize the HFE protein in whole liver homogenates were unsuccessful; however, HFE could be detected in liver microsomes or in plasma membrane-enriched fractions. Five-week-old male C57BL/6 mice fed an iron-deficient diet for 4 wk presented with a significant decrease in liver iron content and liver Hamp expression, as well as with a significant decrease in liver HFE protein content. Rats fed an iron-deficient diet for 4 wk also displayed significant decrease in liver Hamp expression and liver HFE protein content. These results suggest that the downregulation of HFE-dependent signaling may contribute to decreased Hamp gene expression in states of prolonged iron deficiency. It has recently been proposed that HFE protein could be a potential target of matriptase-2, a hepatocyte protease mutated in iron-refractory iron deficiency anemia. However, immunoblot analysis of HFE protein in the livers from Tmprss6-mutated mask mice did not show evidence of matriptase-2-dependent HFE protein cleavage. In addition, no indication of HFE protein cleavage was seen in iron-deficient rats, whereas the full-length matriptase-2 protein content in the same animals was significantly increased. These results suggest that HFE is probably not a major physiological target of matriptase-2. NEW & NOTEWORTHY Feeding of iron-deficient diet for 4 wk decreased liver HFE protein content in both mice and rats, suggesting that decreased HFE-dependent signaling may contribute to hepcidin downregulation in iron deficiency. There was no difference in HFE protein band appearance between matriptase-2-mutated mask mice and wild-type mice, indicating that HFE is probably not a major physiological substrate of matriptase-2-mediated protease activity in vivo.

MyD88 Regulates the Expression of SMAD4 and the Iron Regulatory Hormone Hepcidin

The myeloid differentiation primary response gene 88 (MyD88) is an adaptive protein that is essential for the induction of inflammatory cytokines through almost all the Toll-like receptors (TLRs). TLRs recognize molecular patterns present in microorganisms called pathogen-associated molecular patterns. Therefore, MyD88 plays an important role in innate immunity since its activation triggers the first line of defense against microorganisms. Herein, we describe the first reported role of MyD88 in an interconnection between innate immunity and the iron-sensing pathway (BMP/SMAD4). We found that direct interaction of MyD88 with SMAD4 protein activated hepcidin expression. The iron regulatory hormone hepcidin is indispensable for the intestinal regulation of iron absorption and iron recycling by macrophages. We show that MyD88 induces hepcidin expression in a manner dependent on the proximal BMP responsive element on the hepcidin gene (HAMP) promoter. We identified the Toll/interleukin-1 receptor (TIR) domain of MyD88 as the domain of interaction with SMAD4. Furthermore, we show that BMP6 stimulation, which activates SMAD6 expression, also induces MyD88 proteosomal degradation as a negative feedback mechanism to limit hepcidin induction. Finally, we report that the MyD88 gain-of-function L265P mutation, frequently encountered in B-cell lymphomas such as Waldenström’s macroglobulinemia, enhances hepcidin expression and iron accumulation in B cell lines. Our results reveal a new potential role for MyD88 in the SMAD signaling pathway and iron homeostasis regulation.

Haemochromatosis

Haemochromatosis is defined as systemic iron overload of genetic origin, caused by a reduction in the concentration of the iron regulatory hormone hepcidin, or a reduction in hepcidin-ferroportin binding. Hepcidin regulates the activity of ferroportin, which is the only identified cellular iron exporter. The most common form of haemochromatosis is due to homozygous mutations (specifically, the C282Y mutation) in HFE, which encodes hereditary haemochromatosis protein. Non-HFE forms of haemochromatosis due to mutations in HAMP, HJV or TFR2 are much rarer. Mutations in SLC40A1 (also known as FPN1; encoding ferroportin) that prevent hepcidin-ferroportin binding also cause haemochromatosis. Cellular iron excess in HFE and non-HFE forms of haemochromatosis is caused by increased concentrations of plasma iron, which can lead to the accumulation of iron in parenchymal cells, particularly hepatocytes, pancreatic cells and cardiomyocytes. Diagnosis is noninvasive and includes clinical examination, assessment of plasma iron parameters, imaging and genetic testing. The mainstay therapy is phlebotomy, although iron chelation can be used in some patients. Hepcidin supplementation might be an innovative future approach.

MyD88 Adaptor Protein Is Required for Appropriate Hepcidin Induction in Response to Dietary Iron Overload in Mice

Iron homeostasis is tightly regulated to provide virtually all cells in the body, particularly red blood cells, with this essential element while defending against its toxicity. The peptide hormone hepcidin is central to the control of the amount of iron absorbed from the diet and iron recycling from macrophages. Previously, we have shown that hepcidin induction in macrophages following Toll-like receptor (TLR) stimulation depends on the presence of myeloid differentiation primary response gene 88 (MyD88). In this study, we analyzed the regulation of iron metabolism in MyD88^−/− mice to further investigate MyD88 involvement in iron sensing and hepcidin induction. We show that mice lacking MyD88 accumulate significantly more iron in their livers than wild-type counterparts in response to dietary iron loading as they are unable to appropriately control hepcidin levels. The defect was associated with inappropriately low levels of Smad4 protein and Smad1/5/8 phosphorylation in liver samples found in the MyD88^−/− mice compared to wild-type mice. In conclusion, our results reveal a previously unknown link between MyD88 and iron homeostasis, and provide new insights into the regulation of hepcidin through the iron-sensing pathway.

Hepcidin in Iron Homeostasis: Diagnostic and Therapeutic Implications in Type 2 Diabetes Mellitus Patients

The prevalence of type 2 diabetes is increasing in epidemic proportions worldwide. Evidence suggests body iron overload is frequently linked and observed in patients with type 2 diabetes. Body iron metabolism is based on iron conservation and recycling by which only a part of the daily need is replaced by duodenal absorption. The principal liver-produced peptide called hepcidin plays a fundamental role in iron metabolism. It directly binds to ferroportin, the sole iron exporter, resulting in the internalization and degradation of ferroportin. However, inappropriate production of hepcidin has been shown to play a role in the pathogenesis of type 2 diabetes mellitus and its complications, based on the regulation and expression in iron-abundant cells. Underexpression of hepcidin results in body iron overload, which triggers the production of reactive oxygen species simultaneously thought to play a major role in diabetes pathogenesis mediated both by β-cell failure and insulin resistance. Increased hepcidin expression results in increased intracellular sequestration of iron, and is associated with the complications of type 2 diabetes. Besides, hepcidin concentrations have been linked to inflammatory cytokines, matriptase 2, and chronic hepatitis C infection, which have in turn been reported to be associated with diabetes by several approaches. Either hepcidin-targeted therapy alone or as adjunctive therapy with phlebotomy, iron chelators, or dietary iron restriction may be able to alter iron parameters in diabetic patients. Therefore, measuring hepcidin may improve differential diagnosis and the monitoring of disorders of iron metabolism.

Increased Plasmodium falciparum Parasitemia in Non-splenectomized Saimiri sciureus Monkeys Treated with Clodronate Liposomes

A major constraint in the study of Plasmodium falciparum malaria, including vaccine development, lies on the parasite's strict human host specificity and therefore the shortage of animal experimental models able to harbor human plasmodia. The best experimental models are neo-tropical primates of the genus Saimiri and Aotus, but they require splenectomy to reduce innate defenses for achieving high and consistent parasitemias, an important limitation. Clodronate-liposomes (CL) have been successfully used to deplete monocytes/macrophages in several experimental models. We investigated whether a reduction in the numbers of phagocytic cells by CL would improve the development of P. falciparum parasitemia in non-splenectomized Saimiri sciureus monkeys. Depletion of S. sciureus splenocytes after in vitro incubation with CL was quantified using anti-CD14 antibodies and flow cytometry. Non-infected and P. falciparum-infected S. sciureus were injected intravenously twice a week with either CL at either 0.5 or 1 mL (5 mg/mL) or phosphate buffered saline (PBS). Animals were monitored during infection and treated with mefloquine. After treatment and euthanasia, spleen and liver were collected for histological analysis. In vitro CL depleted S. sciureus splenic monocyte/macrophage population in a dose- and time-dependent manner. In vivo, half of P. falciparum-infected S. sciureus treated with CL 0.5 mL, and two-thirds of those treated with CL 1 mL developed high parasitemias requiring mefloquine treatment, whereas all control animals were able to self-control parasitemia without the need for antimalarial treatment. CL-treated infected S. sciureus showed a marked decrease in the degree of splenomegaly despite higher parasitemias, compared to PBS-treated animals. Histological evidence of partial monocyte/macrophage depletion, decreased hemozoin phagocytosis and decreased iron recycling was observed in both the spleen and liver of CL-treated infected S. sciureus. CL is capable of promoting higher parasitemia in P. falciparum-infected S. sciureus, associated with evidence of partial macrophage depletion in the spleen and liver. Macrophage depletion by CL is therefore a practical and viable alternative to surgical splenectomy in this experimental model.

Dietary Heme Induces Gut Dysbiosis, Aggravates Colitis, and Potentiates the Development of Adenomas in Mice

Dietary heme can be used by colonic bacteria equipped with heme-uptake systems as a growth factor and thereby impact on the microbial community structure. The impact of heme on the gut microbiota composition may be particularly pertinent in chronic inflammation such as in inflammatory bowel disease (IBD), where a strong association with gut dysbiosis has been consistently reported. In this study we investigated the influence of dietary heme on the gut microbiota and inferred metagenomic composition, and on chemically induced colitis and colitis-associated adenoma development in mice. Using 16S rRNA gene sequencing, we found that mice fed a diet supplemented with heme significantly altered their microbiota composition, characterized by a decrease in α-diversity, a reduction of Firmicutes and an increase of Proteobacteria, particularly Enterobacteriaceae. These changes were similar to shifts seen in dextran sodium sulfate (DSS)-treated mice to induce colitis. In addition, dietary heme, but not systemically delivered heme, contributed to the exacerbation of DSS-induced colitis and facilitated adenoma formation in the azoxymethane/DSS colorectal cancer (CRC) mouse model. Using inferred metagenomics, we found that the microbiota alterations elicited by dietary heme resulted in non-beneficial functional shifts, which were also characteristic of DSS-induced colitis. Furthermore, a reduction in fecal butyrate levels was found in mice fed the heme supplemented diet compared to mice fed the control diet. Iron metabolism genes known to contribute to heme release from red blood cells, heme uptake, and heme exporter proteins, were significantly enriched, indicating a shift toward favoring the growth of bacteria able to uptake heme and protect against its toxicity. In conclusion, our data suggest that luminal heme, originating from dietary components or gastrointestinal bleeding in IBD and, to lesser extent in CRC, directly contributes to microbiota dysbiosis. Thus, luminal heme levels may further exacerbate colitis through the modulation of the gut microbiota and its metagenomic functional composition. Our data may have implications in the development of novel targets for therapeutic approaches aimed at lowering gastrointestinal heme levels through heme chelation or degradation using probiotics and nutritional interventions.

The liver in regulation of iron homeostasis

The liver is one of the largest and most functionally diverse organs in the human body. In addition to roles in detoxification of xenobiotics, digestion, synthesis of important plasma proteins, gluconeogenesis, lipid metabolism, and storage, the liver also plays a significant role in iron homeostasis. Apart from being the storage site for excess body iron, it also plays a vital role in regulating the amount of iron released into the blood by enterocytes and macrophages. Since iron is essential for many important physiological and molecular processes, it increases the importance of liver in the proper functioning of the body's metabolism. This hepatic iron-regulatory function can be attributed to the expression of many liver-specific or liver-enriched proteins, all of which play an important role in the regulation of iron homeostasis. This review focuses on these proteins and their known roles in the regulation of body iron metabolism.

Human Leukocyte Antigen (HLA) and Immune Regulation: How Do Classical and Non-Classical HLA Alleles Modulate Immune Response to Human Immunodeficiency Virus and Hepatitis C Virus Infections?

The genetic factors associated with susceptibility or resistance to viral infections are likely to involve a sophisticated array of immune response. These genetic elements may modulate other biological factors that account for significant influence on the gene expression and/or protein function in the host. Among them, the role of the major histocompatibility complex in viral pathogenesis in particular human immunodeficiency virus (HIV) and hepatitis C virus (HCV), is very well documented. We, recently, added a novel insight into the field by identifying the molecular mechanism associated with the protective role of human leukocyte antigen (HLA)-B27/B57 CD8⁺ T cells in the context of HIV-1 infection and why these alleles act as a double-edged sword protecting against viral infections but predisposing the host to autoimmune diseases. The focus of this review will be reexamining the role of classical and non-classical HLA alleles, including class Ia (HLA-A, -B, -C), class Ib (HLA-E, -F, -G, -H), and class II (HLA-DR, -DQ, -DM, and -DP) in immune regulation and viral pathogenesis (e.g., HIV and HCV). To our knowledge, this is the very first review of its kind to comprehensively analyze the role of these molecules in immune regulation associated with chronic viral infections.

The hemochromatosis protein HFE 20 years later: An emerging role in antigen presentation and in the immune system

Introduction

Since its discovery, the hemochromatosis protein HFE has been primarily defined by its role in iron metabolism and homeostasis, and its involvement in the genetic disease termed hereditary hemochromatosis (HH). While HH patients are typically afflicted by dysregulated iron levels, many are also affected by several immune defects and increased incidence of autoimmune diseases that have thereby implicated HFE in the immune response. Growing evidence has supported an immunological role for HFE with recent studies describing HFE specifically as it relates to MHC I antigen presentation.

Methods/Results

Here, we present a comprehensive overview of the relationship between iron metabolism, HFE, and the immune system to better understand the origin and cause of immune defects in HH patients. We further describe the role of HFE in MHC I antigen presentation and its potential to impair autoimmune responses in homeostatic conditions, a mechanism which may be exploited by tumors to evade immune surveillance.

Conclusion

Overall, this increased understanding of the role of HFE in the immune response sets the stage for better treatment and management of HH and other iron‐related diseases, as well as of the immune defects related to this condition.

The effect of iron status on vascular health

The effect of increased iron stores on the progression of atherosclerosis and endothelial health remains inconclusive. This study was designed to evaluate the relationship between hemochromatosis genotypes, serum ferritin levels and presymptomatic vascular abnormalities in a cohort of healthy subjects. Carotid intima-media thickness (CIMT) and brachial flow-mediated vasodilation (FMD) were assessed by high-resolution ultrasound in 907 male (47 ± 10 years) participants enrolled in the Firefighters and their Endothelium (FATE) study. Analyses of the hemochromatosis C282Y, H63D and S65C alleles were simultaneously determined by a single nucleotide polymorphism (SNP) primer extension method. It was found that brachial FMD was not related to serum ferritin or hemochromatosis genotype status. The presence of a hemochromatosis-associated genotype ( n = 18) or heterozygosity for the C282Y genotype ( n = 98) was not associated with an increased mean CIMT. After adjustment for conventional risk factors, serum ferritin was also not associated with mean CIMT. In conclusion, neither ferritin nor a hemochromatosis genotype was related to brachial endothelial function or carotid atherosclerosis. The present study does not support the hypothesis that mild to moderately increased iron stores are associated with enhanced atherosclerosis risk.

Metabolic reprogramming and cell fate regulation in alcoholic liver disease

Alcoholic liver disease (ALD) should be defined as a life-style metabolic disease. Its pathogenesis is driven by altered cell fate of both parenchymal and non-parenchymal liver cell types, contributing to different pathologic spectra. A critical turning point in progression of ALD is chronic alcoholic steatohepatitis (ASH) or alcoholic neutrophilic hepatitis (AH), which markedly predisposes patients to most devastating ALD sequela, cirrhosis and liver cancer.

RESULTS

Our research identifies the pivotal roles of unique metabolic reprogramming in M1 activation of hepatic macrophages (HM) and myofibroblastic activation (MF) of hepatic stellate cells (HSC) in the genesis of inflammation and fibrosis, the two key histological features of chronic ASH and neutrophilic AH. For M1 HM activation, heightened proinflammatory iron redox signaling in endosomes or caveosomes results from altered iron metabolism and storage, promoting IKK/NF-kB activation via interactive activation of p21ras, TAK1, and PI3K. For MF cell fate regulation of HSC, activation of the morphogen Wnt pathway caused by the nuclear protein NECDIN or the single-pass trans-membrane protein DLK1, reprograms lipid metabolism via MeCP2-mediated epigenetic repression of the key HSC quiescence gene Ppar-γ.

CONCLUSIONS

The findings from these studies re-enforce the importance of metabolic reprogramming in cell fate regulation required for the pathogenesis of ALD.

The WT hemochromatosis protein HFE inhibits CD8⁺ T-lymphocyte activation

MHC class I (MHC I) antigen presentation is a ubiquitous process by which cells present endogenous proteins to CD8(+) T lymphocytes during immune surveillance and response. Hereditary hemochromatosis protein, HFE, is involved in cellular iron uptake but, while structurally homologous to MHC I, is unable to bind peptides. However, increasing evidence suggests a role for HFE in the immune system. Here, we investigated the impact of HFE on CD8(+) T-lymphocyte activation. Using transient HFE transfection assays in a model of APCs, we show that WT HFE (HFEWT ), but not C282Y-mutated HFE, inhibits secretion of MIP-1β from antigen-specific CD8(+) T lymphocytes. HFEWT expression also resulted in major decreases in CD8(+) T-lymphocyte activation as measured by 4-1BB expression. We further demonstrate that inhibition of CD8(+) T-lymphocyte activation was independent of MHC I surface levels, β2-m competition, HFE interaction with transferrin receptor, antigen origin, or epitope affinity. Finally, we identified the α1-2 domains of HFEWT as being responsible for inhibiting CD8(+) T-lymphocyte activation. Our data imply a new role for HFEWT in altering CD8(+) T-lymphocyte reactivity, which could modulate antigen immunogenicity.

Molecular basis of HFE-hemochromatosis

Iron-overload disorders owing to genetic misregulation of iron acquisition are referred to as hereditary hemochromatosis (HH). The most prevalent genetic iron overload disorder in Caucasians is caused by mutations in the HFE gene, an atypical MHC class I molecule. Recent studies classified HFE/Hfe-HH as a liver disease with the primarily failure in the production of the liver iron hormone hepcidin in hepatocytes. Inadequate hepcidin expression signals for excessive iron absorption from the diet and iron deposition in tissues causing multiple organ damage and failure. This review focuses on the molecular actions of the HFE/Hfe and hepcidin in maintaining systemic iron homeostasis and approaches undertaken so far to combat iron overload in HFE/Hfe-HH. In the light of the recent investigations, novel roles of extra-hepatocytic Hfe are discussed raising a question to the relevance of the multipurpose functions of Hfe for the understanding of HH-associated pathologies.

Associations of common variants in HFE and TMPRSS6 with iron parameters are independent of serum hepcidin in a general population: a replication study

BACKGROUND

Genome-wide association studies have convincingly shown that single nucleotide polymorphisms (SNPs) in HFE and TMPRSS6 are associated with iron parameters. It was commonly thought that these associations could be explained by the intermediate effect on hepcidin concentration. A recent study in an isolated Italian population, however, concluded that these associations were not exclusively dependent on hepcidin values. We report here the second study to investigate the role of hepcidin in the associations between common variants in HFE and TMPRSS6 with iron parameters.

METHODS

We extracted 101 SNPs in HFE and TMPRSS6 from genome-wide imputed SNP data of 1832 individuals from the general population (Nijmegen Biomedical Study). Single locus and haplotype associations with serum iron parameters and hepcidin were studied using linear regression analyses.

RESULTS

We found that HFE rs1800562 and TMPRSS6 rs855791 are the main determinants of HFE and TMPRSS6 related variation in serum iron, ferritin, transferrin saturation, and total iron binding capacity. These SNPs are associated with the ratios hepcidin/ferritin (p<1×10(-5)) and hepcidin/transferrin saturation (p<1×10(-3)), but not with serum hepcidin (p>0.2). Adjustment for hepcidin or the ratio hepcidin/ferritin did not decrease the strength of the SNP-iron parameter associations.

CONCLUSIONS

Our results do not support an intermediate role for hepcidin in the SNP-iron parameter associations, which confirms previous findings, and indicate a pleiotropic SNP effect on the hepcidin ratios and the iron parameters. Taken together, this suggests that there might be other, yet unknown, serum hepcidin independent mechanisms which play a role in the association of HFE and TMPRSS6 variants with serum iron parameters.

Toll-like receptor signal adaptor protein MyD88 is required for sustained endotoxin-induced acute hypoferremic response in mice

Hypoferremia, associated with immune system activation, involves a marked reduction in the levels of circulating iron, coupled with iron sequestration within macrophages. Toll-like receptor (TLR) signaling plays an important role in the development of the hypoferremic response, but how downstream signaling events affect genes involved in iron metabolism is incompletely understood. We investigated the involvement of MyD88-dependent (MyD88) and MyD88-independent (TRIF) TLR signaling in the development of hypoferremia. Using MyD88-deficient and TRIF-deficient mice, we show that MyD88 and TRIF signaling pathways are critical for up-regulation by lipopolysaccharide (LPS) of the iron regulator hepcidin. In addition, MyD88 signaling is required for the induction of lipocalin 2 secretion and iron sequestration in the spleen. Activation of TLR4 and TLR3 signaling through LPS and polyinosinic:polycytidylic acid [poly(I:C)] treatments resulted in rapid down-regulation of HFE protein [encoded by the hemochromatosis gene (Hfe)] and ferroportin [encoded by solute carrier family 40 (iron-regulated transporter), member 1 (Slc40a1)] expression in the spleen, independent of MyD88 or TRIF signaling and proinflammatory cytokine production. However, lack of MyD88 signaling significantly impaired the hypoferremic response triggered by LPS, indicating that ferroportin and HFE protein down-regulation alone are insufficient to maintain hypoferremia. The extent of the hypoferremic response was found to be limited by initial, basal iron levels. Together, these results suggest that targeting specific TLR signaling pathways by affecting the function of adaptor molecules may provide new strategies to counteract iron sequestration within macrophages during inflammation.

Systemic inflammation and liver injury following hemorrhagic shock and peripheral tissue trauma involve functional TLR9 signaling on bone marrow-derived cells and parenchymal cells

Hemorrhagic shock due to trauma (HS/T) induces an inflammatory response that can contribute to end-organ injury. The pathways involved in the initiation and propagation of HS/T-induced inflammation are incompletely understood. Here, we hypothesized that the DNA sensor TLR9 would have a role in inflammatory signaling after HS/T. Using mice expressing a nonfunctional, mutant form of TLR9, we identified a role of TLR9 in driving the initial cytokine response and liver damage in a model of hemorrhagic shock and bilateral femur fracture. Circulating DNA levels were found to correlate with the degree of tissue damage. Experiments using chimeric mice show that TLR9 on both bone marrow-derived cells and parenchymal cells are important for the TLR9-mediated liver and tissue damage, as well as systemic inflammation after HS/T. These data suggest that release of DNA may be a driver of the inflammatory response to severe injury as well as a marker of the extent of tissue damage. One of the sensors of DNA in the setting of HS/T seems to be TLR9.

Tmprss6 is a genetic modifier of the Hfe-hemochromatosis phenotype in mice

The hereditary hemochromatosis protein HFE promotes the expression of hepcidin, a circulating hormone produced by the liver that inhibits dietary iron absorption and macrophage iron release. HFE mutations are associated with impaired hepatic bone morphogenetic protein (BMP)/SMAD signaling for hepcidin production. TMPRSS6, a transmembrane serine protease mutated in iron-refractory iron deficiency anemia, inhibits hepcidin expression by dampening BMP/SMAD signaling. In the present study, we used genetic approaches in mice to examine the relationship between Hfe and Tmprss6 in the regulation of systemic iron homeostasis. Heterozygous loss of Tmprss6 in Hfe(-/-) mice reduced systemic iron overload, whereas homozygous loss caused systemic iron deficiency and elevated hepatic expression of hepcidin and other Bmp/Smad target genes. In contrast, neither genetic loss of Hfe nor hepatic Hfe overexpression modulated the hepcidin elevation and systemic iron deficiency of Tmprss6(-/-) mice. These results indicate that genetic loss of Tmprss6 increases Bmp/Smad signaling in an Hfe-independent manner that can restore Bmp/Smad signaling in Hfe(-/-) mice. Furthermore, these results suggest that natural genetic variation in the human ortholog TMPRSS6 might modify the clinical penetrance of HFE-associated hereditary hemochromatosis, raising the possibility that pharmacologic inhibition of TMPRSS6 could attenuate iron loading in this disorder.

The ethnospecific distribution of the HFE haplotypes for IVS2(+4)t/c, IVS4(-44)t/c, and IVS5(-47)g/a in populations of Russia and possible effects of these single-nucleotide polymorphisms in splicing

AIM

The aim of this work was a haplotype analysis of the major mutations (C282Y, H63D, S65C) and IVS2(+4)t/c, IVS4(-44)t/c, and IVS5(-47)a/g polymorphisms of the hemochromatosis HFE gene in populations inhabiting the territories of Russia (Russians, Finno-Ugrians, Central Asians, and Arctic Mongoloids).

METHOD

The hemochromatosis gene (HFE) alleles were detected using the polymerase chain reaction/restriction fragment length polymorphism method.

RESULTS

Of the eight possible intronic haplotype variants, the TTG, TTA, CTA, and CCA were identified. The HFE alleles with the different haplotype variants were distributed in an ethnospecific manner among the populations. Our finding was that every one of the C282Y, H63D, and S65C mutations was in linkage disequilibrium only with one of the intronic haplotype variants: TTG, CTA, and CCA, respectively. The data from context analysis of DNA regions where the examined single-nucleotide polymorphisms are located suggested their involvement in splicing.

CONCLUSIONS

Different genotypes of the HFE gene occur at different frequencies among populations of Russia. Carriers of the specific genotype variants may potentially express distinct sets of alternative HFE mRNAs.

Enhanced erythropoiesis in Hfe-KO mice indicates a role for Hfe in the modulation of erythroid iron homeostasis

In hereditary hemochromatosis, mutations in HFE lead to iron overload through abnormally low levels of hepcidin. In addition, HFE potentially modulates cellular iron uptake by interacting with transferrin receptor, a crucial protein during erythropoiesis. However, the role of HFE in this process was never explored. We hypothesize that HFE modulates erythropoiesis by affecting dietary iron absorption and erythroid iron intake. To investigate this, we used Hfe-KO mice in conditions of altered dietary iron and erythropoiesis. We show that Hfe-KO mice can overcome phlebotomy-induced anemia more rapidly than wild-type mice (even when iron loaded). Second, we evaluated mice combining the hemochromatosis and β-thalassemia phenotypes. Our results suggest that lack of Hfe is advantageous in conditions of increased erythropoietic activity because of augmented iron mobilization driven by deficient hepcidin response. Lastly, we demonstrate that Hfe is expressed in erythroid cells and impairs iron uptake, whereas its absence exclusively from the hematopoietic compartment is sufficient to accelerate recovery from phlebotomy. In summary, we demonstrate that Hfe influences erythropoiesis by 2 distinct mechanisms: limiting hepcidin expression under conditions of simultaneous iron overload and stress erythropoiesis, and impairing transferrin-bound iron uptake by erythroid cells. Moreover, our results provide novel suggestions to improve the treatment of hemochromatosis.

Iron metabolism in macrophages from HFE hemochromatosis patients

HFE-linked hereditary hemochromatosis is a common form of iron-overload disease in European populations. We studied the role of HFE in macrophage iron metabolism. Patients under venesection treatment had higher EPO levels and drastically reduced levels of transferrin receptor (TfRC and TfR2) mRNA, and also decreased levels of HAMP mRNA in macrophages cultured in autologous serum. Macrophages from C282Y/C282Y patients cultured either in autologous serum or in FBS with or without iron supplementation, had elevated CYBRD1 (cytochrome b reductase 1), SLC40A1 (ferroportin) and FTL (ferritin L) mRNA levels. Those incubated with holo-Tf also showed lower levels of TfRC and TfR2 mRNA. Iron flux from C282Y/C282Y macrophages incubated with a low concentration of non-transferrin-bound iron (NTBI) was similar to that from wild-type macrophages, but incubation with holo-Tf or high NTBI did not trigger a continuous increase in the cytosolic calcein-chelatable iron pool in C282Y/C282Y macrophages conversely to wild-type cells. All culture conditions revealed a high level of intracellular ferritin in C282Y/C282Y macrophages compared to wild-type cells. These results suggest that the non-functional C282Y form of HFE may alter the balance between cytosolic calcein-chelatable iron and sequestered iron, thereby disrupting the iron uptake and release equilibrium in cells involved in iron storage.

Iron metabolism and ineffective erythropoiesis in beta-thalassemia mouse models

beta-thalassemia is a disease associated with decreased beta-globin production leading to anemia, ineffective erythropoiesis, and iron overload. New mechanisms associated with modulation of erythropoiesis and iron metabolism have recently been discovered in thalassemic mice, improving our understanding of the pathophysiology of this disease. These discoveries have the potential to be translated into clinically-relevant therapeutic options to reduce ineffective erythropoiesis and iron overload. A new generation of therapies based on limiting ineffective erythropoiesis, iron absorption, and the correction of iron maldistribution could be on the way, possibly complementing and improving the current standard of patient care.

Iron metabolism gene expression in human skeletal muscle

Little is known about iron metabolism in skeletal muscle while hepatic iron metabolism is well understood. The aim of this study is to compare the iron metabolism gene expression profile in skeletal muscle and the liver in humans. Muscle and hepatic biopsies from six normal individuals were acquired. Twelve genes involved in iron metabolism( import, storage, export) were selected to be studied. Reverse transcriptase polymerase chain reaction (RT-PCR) was performed in order to determine the expression profile in skeletal muscle and compare it to the one from the liver. Semi-quantification of the gene expression in the studied tissues was performed by densitometric analysis (DA). The results were expressed relative to the percentage of the β-actin gene. Fine analysis was performed by real-time PCR (q-PCR) quantification for the genes that their expression presented a difference of more than 20% in the 2 tissues in the first applied densitometric analysis. Most of the studied genes, HJV, TFR1, HFE, DMT1, DMT1nonIRE, NGAL, HEPH, IREG1, FTH1 were well expressed (>70% of β-actin) in skeletal muscle . HAMP, CP, and TFR2 were absent or minimally expressed (<10% of β-actin) in skeletal muscle while they were well expressed in liver. HJV and Heph were found to have higher expression in skeletal muscle (SM) compared to liver (L) (SM/L=2.65 ± 1.1(p<0.05) and SM/L=1.5 ± 0.06(p<0.05 respectively in q-PCR). The relative expressions of the studied genes in both tissues and their relative contribution in iron homeostasis in different pathways are discussed.

Hepcidin expression does not rescue the iron-poor phenotype of Kupffer cells in Hfe-null mice after liver transplantation

BACKGROUND & AIMS

Hemochromatosis is a common hereditary disease caused by mutations in HFE and characterized by increased absorption of iron in the intestine. However, the intestine does not appear to be the site of mutant HFE activity in the disease; we investigated the role of the liver-the source of the iron regulatory hormone hepcidin-in pathogenesis in mice.

METHODS

We exchanged livers between Hfe wild-type (+/+) and Hfe null (-/-) mice by orthotopic liver transplantation (OLT) and assessed histopathology, serum and tissue iron parameters, and hepatic hepcidin messenger RNA expression.

RESULTS

At 6-8 months after OLT, Hfe(-/-) mice that received Hfe(-/-) livers maintained the hemochromatosis phenotype: iron accumulation in hepatocytes but not Kupffer cells (KC), increased transferrin levels, and low levels of iron in the spleen. Hfe(+/+) mice that received Hfe(-/-) livers had increased levels of iron in serum and liver and low levels of iron in spleen. However, they did not develop the iron-poor KCs that characterize hemochromatosis: KCs appeared iron rich, although hepatic hepcidin expression was low. Transplantation of Hfe(+/+) livers into Hfe(-/-) mice prevented hepatic iron accumulation but did not return spleen and plasma levels of iron to normal; KCs still appeared to be iron poor, despite normal hepcidin expression.

CONCLUSIONS

In Hfe(-/-) mice, transplantation of livers from Hfe(+/+) mice reversed the iron-loading phenotype associated with hemochromatosis (regardless of Hfe expression in intestine). However, KCs still had low levels of iron that were not affected by hepatic hepcidin expression. These findings indicate an independent, iron-modifying effect of HFE in KCs.

Is the iron donor lipocalin 2 implicated in the pathophysiology of hereditary hemochromatosis?

Under normal conditions, iron is taken up by the cells through the transferrin-mediated pathway. However, in hereditary hemochromatosis, a common iron-overloading disorder associated with mutations in the HFE gene, iron in plasma exceeds transferrin-binding capacity, and non-transferrin-bound iron (NTBI) appears in the circulation of patients with iron overload. NTBI can be taken up by hepatocytes through a transferrin-independent pathway. Lipocalin 2 (Lcn2), a secreted protein of the lipocalin family, has emerged as the mediator of an alternative, transferrin-independent pathway for cellular iron delivery. To evaluate the importance of Lcn2 in the pathogenesis of hepatic iron loading in Hfe knockout mice, we generated HfeLcn2 double-deficient mice. Our studies revealed that deletion of Lcn2 in Hfe-knockout mice does not influence hepatic iron accumulation in Hfe(-/-) mice, or their response to iron loading, as the phenotype of HfeLcn2(-/-) mice remained indistinguishable from that of Hfe(-/-) mice.

CONCLUSION

Lcn2 is not essential for iron delivery to hepatocytes in hemochromatosis.

Contribution of STAT3 and SMAD4 pathways to the regulation of hepcidin by opposing stimuli

Hepcidin, a key regulator of iron metabolism, is a small antimicrobial peptide produced by the liver that regulates intestinal iron absorption and iron recycling by macrophages. Hepcidin is stimulated when iron stores increase and during inflammation and, conversely, is inhibited by hypoxia and augmented erythropoiesis. In many pathologic situations, such as in the anemia of chronic disease (ACD) and iron-loading anemias, several of these factors may be present concomitantly and may generate opposing signaling to regulate hepcidin expression. Here, we address the question of dominance among the regulators of hepcidin expression. We show that erythropoiesis drive, stimulated by erythropoietin but not hypoxia, down-regulates hepcidin in a dose-dependent manner, even in the presence of lipopolysaccharide (LPS) or dietary iron-loading, which may act additively. These effects are mediated through down-regulation of phosphorylation of Stat3 triggered by LPS and of Smad1/5/8 induced by iron. In conclusion, hepcidin expression levels in the presence of opposing signaling are determined by the strength of the individual stimuli rather than by an absolute hierarchy among signaling pathways. Our findings also suggest that erythropoietic drive can inhibit both inflammatory and iron-sensing pathways, at least in part, via the suppression of STAT3 and SMAD4 signaling in vivo.

Function of the hemochromatosis protein HFE: Lessons from animal models

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

Hepatic macrophage iron aggravates experimental alcoholic steatohepatitis

One prime feature of alcoholic liver disease (ALD) is iron accumulation in hepatic macrophages/Kupffer cells (KC) associated with enhanced NF-kappaB activation. Our recent work demonstrates a peroxynitrite-mediated transient rise in intracellular labile iron (ILI) as novel signaling for endotoxin-induced IKK and NF-kappaB activation in rodent KC. The present study investigated the mechanism of KC iron accumulation and its effects on ILI response in experimental ALD. We also tested ILI response in human blood monocytes. Chronic alcohol feeding in rats results in increased expression of transferrin (Tf) receptor-1 and hemochromatosis gene (HFE), enhanced iron uptake, an increase in nonheme iron content, and accentuated ILI response for NF-kappaB activation in KC. Ex vivo treatment of these KC with an iron chelator abrogates the increment of iron content, ILI response, and NF-kappaB activation. The ILI response is evident in macrophages derived from human blood monocytes by PMA treatment but not in vehicle-treated monocytes, and this differentiation-associated phenomenon is essential for maximal TNF-alpha release. PMA-induced macrophages load iron dextran and enhance ILI response and TNF-alpha release. These effects are reproduced in KC selectively loaded in vivo with iron dextran in mice and more importantly aggravate experimental ALD. Our results suggest enhanced iron uptake as a mechanism of KC iron loading in ALD and demonstrate the ILI response as a function acquired by differentiated macrophages in humans and as a priming mechanism for ALD.

Hemochromatosis gene polymorphisms, mitochondrial haplogroups, and peripheral lipoatrophy during antiretroviral therapy

BACKGROUND

Antiretroviral therapy (ART)-associated lipoatrophy involves mitochondrial dysfunction. Iron metabolism impacts mitochondrial function and oxidative stress. Mitochondrial haplogroups and hemochromatosis gene (HFE) polymorphisms have been associated with ART-induced neuropathy. We assessed relationships between these variants and lipoatrophy.

METHODS

The AIDS Clinical Trials Group 384 study randomized ART-naive individuals to receive didanosine-stavudine or zidovudine-lamivudine, combined with efavirenz and/or nelfinavir. Substudy A5005s evaluated fat distribution by dual-energy X-ray absorptiometry (DEXA). We characterized HFE polymorphisms 845G>A and 187C>G and European mitochondrial haplogroups in A5005s participants who consented to genetic analyses.

RESULTS

Among 96 participants (58% were white, and 10% were female) with baseline and 48 or 64 week DEXA data, the median limb fat change was -8.8% (interquartile range, -28.7% to +15.6%). HFE 187C/G heterozygotes (n = 23) had less limb fat loss than 187C/C homozygotes (n = 71) (+6.1% vs. -12.5%; P = .02) and were less likely to develop lipoatrophy after adjustment for age, sex, race, and ART randomization (odds ratio, 0.31; 95% confidence interval, 0.10-0.95; P = .04). Among non-Hispanic white participants, median limb fat change was +26.1% among 5 participants with mitochondrial haplogroup J, compared with -9.7% among 49 participants with other mitochondrial haplogroups (P = .07).

CONCLUSIONS

HFE 187C>G and, possibly, mitochondrial haplogroup J gave relative protection against lipoatrophy during ART in A5005s. These associations should be replicated in other studies.

Anemia upregulates lipocalin 2 in the liver and serum

Lipocalin 2 (Lcn2), a mammalian protein that is expressed and secreted in various pathologic states, binds siderophores, which are high-affinity iron chelators. Besides its role in limiting iron availability to pathogens in the setting of bacterial infection, Lcn2:siderophore complexes can also deliver iron to cells. In this study, we examined Lcn2 regulation in the liver of mice in situations of increased iron utilization, namely, during anemia. Anemia induced by phlebotomy, iron deprivation, or phenylhydrazine treatment was associated with upregulation of Lcn2 gene expression in the liver and elevation of serum Lcn2 protein levels. We further explored the participation of several factors known to co-occur during anemia, including hypoxia, changes in iron levels, and erythropoietic drive, in the regulation of Lcn2 by anemia. We found that hypoxia, but not iron or erythropoietin, caused an induction of Lcn2 expression. The upregulation of Lcn2 levels by anemia and hypoxia, which is not directly mediated by iron or erythropoietin, suggests a possible physiological role for Lcn2 during increased iron utilization and mobilization from stores.

Hfe acts in hepatocytes to prevent hemochromatosis

Hereditary hemochromatosis (HH) is a prevalent, potentially fatal disorder of iron metabolism hallmarked by intestinal hyperabsorption of iron, hyperferremia, and hepatic iron overload. In both humans and mice, type I HH is associated with mutations in the broadly expressed HFE/Hfe gene. To identify where Hfe acts to prevent HH, we generated mice with tissue-specific Hfe ablations. This work demonstrates that local Hfe expression in hepatocytes serves to maintain physiological iron homeostasis, answering a long-standing question in medicine and explaining earlier clinical observations.

Molecular regulation of hepatic expression of iron regulatory hormone hepcidin

Iron is a micronutrient that is an essential component that drives many metabolic reactions. Too little iron leads to anemia and too much iron increases the oxidative stress of body tissues leading to inflammation, cell death, and system organ dysfunction, including cancer. Maintaining normal iron balance is achieved by rigorous control of the amount absorbed by the intestine, that released from macrophages following erythrophagocytosis of effete red cells and by either release or uptake from hepatocytes. Hepcidin is a recently characterized molecule that appears to play a key role in the regulation of iron efflux from enterocytes, macrophages, and hepatocytes. It is produced by hepatocytes under basal conditions, in response to alterations in increased iron stores or reduced requirement for erythropoiesis and by inflammation. The proteins that regulate hepcidin expression are presently being defined, albeit that our present understanding is still far from complete. This review focuses on the molecules which regulate hepcidin expression. The subsequent characterization of these proteins using molecular, cellular, and physiological approaches also is discussed along with inflammatory signals and receptors involved in hepcidin expression.

Repression of repulsive guidance molecule C during inflammation is independent of Hfe and involves tumor necrosis factor-alpha

Genetic iron overload, or hemochromatosis, can be caused by mutations in HFE, hemojuvelin, and hepcidin genes. Hepcidin, a negative regulator of intestinal iron absorption, is found to be inappropriately low in both patients and in animal models, indicating that proper control of basal hepcidin levels requires both hemojuvelin and HFE. In mice, repulsive guidance molecule c (Rgmc, the hemojuvelin mouse ortholog) and hepcidin levels are transcriptionally regulated during inflammation. Here, we report that basal Rgmc levels in Hfe-deficient mice are normal and that these mice retain the ability to suppress Rgmc expression after lipopolysaccharide (LPS) challenge. Thus, Rgmc regulation by LPS is Hfe-independent. The response of Rgmc to LPS involves signaling through toll-like receptor 4 (Tlr4), because Tlr4-deficient mice do not show altered Rgmc expression after LPS administration. We further show that tumor necrosis factor-alpha, but not interleukin-6, is sufficient to cause Rgmc down-regulation by LPS. These results contrast with previous data demonstrating that hepcidin levels are directly regulated by interleukin-6 but not by tumor necrosis factor-alpha. The regulation of iron-related genes by different cytokines may allow for time-dependent control of iron metabolism changes during inflammation and may be relevant to chronic inflammation, infections, and cancer settings, leading to the development of anemia of chronic disease.

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.

Localisation of proteins of iron metabolism in the human placenta and liver

Two anatomical sites that are important in human iron metabolism are the liver and placenta. Liver macrophages recycle iron from erythrocytes, and the placenta transfers iron from the mother to the fetus. The cellular distribution of proteins involved in iron transport in these two sites was studied. Transferrin receptor-1 (TfR1) and Ferroportin (FPN) expression was found on the placental syncytiotrophoblast (STB) and were polarised such that TfR1 was on the apical maternal-facing membrane and FPN was on the basal fetal-facing membrane, consistent with unidirectional iron transport from mother to fetus. Ferritin was strongly expressed in the stroma, suggesting that fetal tissue can store and accumulate iron. HFE was on some parts of the basal STB and, where present, HFE clearly colocalised with FPN but not TfR1. In the stroma, both HFE and FPN were present on CD68+ Hofbauer macrophage cells. In liver, the location of HFE is controversial. Using four mouse monoclonals and two polyclonal sera we showed that the pattern of HFE expression mirrored the distribution of CD68+ macrophage Kupffer cells. FPN was also most strongly expressed by CD68+ Kupffer cells. These findings contribute to understanding how iron is transported and stored in the human placenta and liver.

Distinct requirements for Hfe in basal and induced hepcidin levels in iron overload and inflammation

Hepcidin is a negative regulator of iron absorption produced mainly by the liver in response to changes in iron stores and inflammation, and its levels have been shown to regulate the intestinal basolateral iron transporter ferroportin1 (Fp1). Hereditary hemochromatosis patients and Hfe-deficient mice show inappropriate expression of hepcidin but, in apparent contradiction, still retain the ability to regulate iron absorption in response to alterations of iron metabolism. To further understand the molecular relationships among Hfe, hepcidin, and Fp1, we investigated hepcidin and Fp1 regulation in Hfe-deficient mice (Hfe-/- and beta2m-/-) in response to iron deprivation, iron loading, and acute inflammation. We found that whereas basal hepcidin levels were manifestly dependent on the presence of Hfe and on the mouse background, all Hfe-deficient mice were still able to regulate hepcidin in situations of altered iron homeostasis. In the liver, Fp1 was modulated in opposite directions by iron and LPS, and its regulation in Hfe-deficient mice was similar to that observed in wild-type mice. In addition, we found that iron-deprived mice were able to mount a robust response after LPS challenge and that Toll-like receptor 4 (TLR-4)-deficient mice fail to regulate hepcidin expression in response to LPS. In conclusion, these results suggest that although Hfe is necessary for the establishment of hepcidin basal levels, it is dispensable for hepcidin regulation through both the iron-sensing and inflammatory pathways, and hepatic Fp1 regulation is largely independent of hepcidin and Hfe. The inflammatory pathway overrides the iron-sensing pathway and is TLR-4 dependent.

Heritability of serum iron, ferritin and transferrin saturation in a genetically isolated population, the Erasmus Rucphen Family (ERF) Study

BACKGROUND

Iron has been implicated in the pathogenesis of various disorders. Mutations in the HFE gene are associated with an increase in serum iron parameters. The aim of this study was to estimate the heritability in serum iron parameters explained by HFE.

METHODS

Ninety families (980 subjects) were included in the present analysis. Heritability estimation was conducted using the variance component method. The likelihood ratio test was used to compare models. Phenotypic and genetic correlations between serum iron parameters were calculated.

RESULTS

The heritability (h(2) +/- SE) estimates were 0.23 +/- 0.07 (p < 0.0001) for iron, 0.29 +/- 0.09 (p < 0.0001) for ferritin and 0.28 +/- 0.07 (p < 0.0001) for transferrin saturation while adjusting for age, age(2) and sex. The HFE genotypes explained between 2 to 6% of the sex and age-adjusted variance in serum iron, ferritin and transferrin saturation. There was a high genetic correlation between serum iron parameters, suggesting pleiotropy between these traits.

CONCLUSION

A substantial proportion of the variance of iron, ferritin and transferrin saturation can be explained by additive genetic effects, independent of sex and age. The HFE genotypes explained a considerable proportion of serum iron parameters and may be an important factor in the complex iron network.

A compartmental model of iron regulation in the mouse

A simple compartmental model is developed for investigating the mechanism of iron homeostasis. In contrast to previous mathematical models of iron metabolism, the liver is included as a key site of iron regulation. Compartments for free iron in blood, diferric transferrin (Tf) in blood, hepatocytes, red blood cells, and macrophages are included, and their roles in iron regulation are explored. The function of hepcidin in regulating iron absorption is modeled through an inverse relationship between hepatocyte transferrin receptor 2 (TfR2) levels and the rate of iron export processes mediated by ferroportin (Fpn). Simulations of anemia and erythropoiesis stimulation support the idea that the iron demands of the erythroid compartment can be communicated through diferric Tf. The iron-responsive element of Fpn is found to be important for stabilizing intracellular iron stores in response to changing iron demands and allowing proper iron regulation through diferric Tf. The contribution of iron dysregulation to the pathogenesis of iron overload disorders is also investigated. It is shown that the characteristics of HFE hemochromatosis can be reproduced by increasing the setpoint of iron absorption in the duodenum to a level where the system cannot downregulate iron absorption to meet the iron excretion rate.

Regulation of iron acquisition and iron distribution in mammals

Both cellular iron deficiency and excess have adverse consequences. To maintain iron homeostasis, complex mechanisms have evolved to regulate cellular and extracellular iron concentrations. Extracellular iron concentrations are controlled by a peptide hormone hepcidin, which inhibits the supply of iron into plasma. Hepcidin acts by binding to and inducing the degradation of the cellular iron exporter, ferroportin, found in sites of major iron flows: duodenal enterocytes involved in iron absorption, macrophages that recycle iron from senescent erythrocytes, and hepatocytes that store iron. Hepcidin synthesis is in turn controlled by iron concentrations, hypoxia, anemia and inflammatory cytokines. The molecular mechanisms that regulate hepcidin production are only beginning to be understood, but its dysregulation is involved in the pathogenesis of a spectrum of iron disorders. Deficiency of hepcidin is the unifying cause of hereditary hemochromatoses, and excessive cytokine-stimulated hepcidin production causes hypoferremia and contributes to anemia of inflammation.