L. Ostini

Parenteral vs. oral iron: influence on hepcidin signaling pathways through analysis of Hfe/Tfr2-null mice

Treatment for iron deficiency anemia can involve iron supplementation via dietary or parenteral routes that result in different cellular iron distributions. The effect of the administered iron on the iron regulatory system and hepcidin in the liver has not been well studied. Hepcidin, the liver-expressed central iron-regulatory peptide, is itself regulated through the bone morphogenetic protein (BMP)/SMAD signaling pathway. Specifically, Bmp6 expression is upregulated in response to iron and induces hepcidin through phosphorylation of Smad1/5/8. The hemochromatosis-associated proteins Hfe and transferrin receptor 2 (Tfr2) are known upstream regulators of hepcidin, although their precise roles are still unclear. To investigate the mechanisms of this regulation and the roles of the Hfe and Tfr2, we subjected wild-type, Hfe(-/-), Tfr2(-/-), and Hfe(-/-)/Tfr2(-/-) mice to iron loading via dietary or parenteral routes. Systematic analysis demonstrated that Tfr2 is required for effective upregulation of Bmp6 in response to hepatocyte iron, but not nonparenchymal iron. Hfe is not required for Bmp6 upregulation, regardless of iron localization, but rather, is required for efficient downstream transmission of the regulatory signal. Our results demonstrate that Hfe and Tfr2 play separate roles in the regulatory responses to iron compartmentalized in different cell types and further elucidates the regulatory mechanisms controlling iron homeostasis.

Iron loading and oxidative stress in the Atm−/− mouse liver

Ataxia-Telangiectasia (A-T) is an autosomal recessive disorder resulting in a myriad of abnormalities, including progressive neurodegeneration and cancer predisposition. At the cellular level, A-T is a disease of chronic oxidative stress (OS) causing damage to proteins, lipids, and DNA. OS is contributed to by pro-oxidative transition metals such as iron that catalyze the conversion of weakly reactive oxygen species to highly reactive hydroxyl radicals. Iron-associated OS has been linked to neurodegeneration in Alzheimers and Parkinsons diseases and development of lymphoid tumors (which afflict ∼30% of A-T patients). To investigate iron regulation in A-T, iron indexes, regulatory genes, and OS markers were studied in livers of wild-type and Ataxia telangiectasia mutated (Atm) null mice on control or high-iron diets. Atm(-/-) mice had increased serum iron, hepatic iron, and ferritin and significantly higher Hepcidin compared with wild-type mice. When challenged with the high-iron diet, Bmp6 and Hfe expression was significantly increased. Atm(-/-) mice had increased protein tyrosine nitration and significantly higher Heme Oxygenase (decycling) 1 levels that were substantially increased by a high-iron diet. Ferroportin gene expression was significantly increased; however, protein levels were unchanged. We demonstrate that Atm(-/-) mice have a propensity to accumulate iron that is associated with a significant increase in hepatic OS. The iron-induced increase in hepcidin peptide in turn suppresses ferroportin protein levels, thus nullifying the upregulation of mRNA expression in response to increased OS. Our results suggest that increased iron status may contribute to the chronic OS seen in A-T patients and development of disease pathology.

Hepatic Iron Deposition Does Not Predict Extrahepatic Iron Loading in Mouse Models of Hereditary Hemochromatosis

Hereditary hemochromatosis is characterized by tissue iron loading and associated organ damage. However, the phenotype can be highly variable. The relationship between iron loading of different organs and the temporal nature of its deposition is still not well understood. We examined the progression of tissue iron loading in three mouse models to advance our understanding of the natural history of iron deposition in hereditary hemochromatosis. Wild-type, Hfe(-/-), Tfr2(-/-), and Hfe(-/-)/Tfr2(-/-) mice were analyzed at 3, 5, 10, 26, and 52 weeks, respectively. Hepatic, splenic, cardiac, and pancreatic iron concentrations were determined. Expression of both iron-regulatory and fibrosis genes was determined by quantitative real-time PCR in livers and hearts of 52-week-old mice. In all models, hepatic iron increased rapidly, plateauing before 10 weeks at different levels, depending on the genotype. Iron deposition in the pancreas and heart occurred after maximal iron loading of the liver was reached and was most marked in the Hfe(-/-)/Tfr2(-/-) mice. Although a significant positive correlation was identified between pancreatic and cardiac iron in all models at 52 weeks, there was no correlation between hepatic and either pancreatic or cardiac iron. There is variability in the timing and extent of tissue iron loading within a genotype, suggesting that hepatic iron levels in hereditary hemochromatosis may not accurately predict the iron content of other organs.

G80S-linked ferroportin disease: Classical ferroportin disease in an Asian family and reclassification of the mutant as iron transport defective

BACKGROUND & AIMS Hereditary iron overload associated with mutations in the ferroportin gene produces a dichotomy of phenotypes resulting from either increase or decrease in iron efflux capacity. In this study, we examined the molecular basis of iron overload in a family of Vietnamese origin, characterized the molecular and cellular defect, and correlated it with the clinical and pathological phenotype. METHODS We analyzed the ferroportin gene by DNA sequencing. The molecular characterization was performed by immunofluorescence microscopy analysis of transfected cells. We analyzed ferritin levels, in cells expressing wild-type and mutant ferroportin, to define the nature of the molecular defect in iron transport. RESULTS We identified a G to A nucleotide change at position 238 in the ferroportin gene leading to the G80S substitution. Cellular analysis of the mutant protein indicates that this amino acid change does not affect the localization of the protein but does affect its ability to transport iron. CONCLUSIONS The G80S mutation results in a mutated ferroportin associated with iron overload and is predicted to be defective in iron export.

Functional analysis of matriptase-2 mutations and domains: insights into the molecular basis of iron-refractory iron deficiency anemia

Mutations in the TMPRSS6 gene are associated with severe iron-refractory iron deficiency anemia resulting from an overexpression of hepcidin, the key regulator of iron homeostasis. The matriptase (MT)-2 protein (encoded by the TMPRSS6 gene) regulates hepcidin expression by cleaving hemojuvelin [HJV/hemochromatosis type 2 (HFE2)], a bone morphogenetic protein (BMP) coreceptor in the hepcidin regulatory pathway. We investigated the functional consequences of five clinically associated TMPRSS6 variants and the role of MT-2 protein domains by generating epitope-tagged mutant and domain-swapped MT-2-MT-1 (encoded by the ST14 gene) chimeric constructs and expressing them in HepG2/C3A cells. We developed a novel cell culture immunofluorescence assay to assess the effect of MT-2 on cell surface HJV expression levels, compatible with HJV cleavage. The TMPRSS6 variants Y141C, I212T, G442R, and C510S were retained intracellularly and were unable to inhibit BMP6 induction of hepcidin. The R271Q variant, although it has been associated with iron-refractory iron deficiency anemia, appears to remain functional. Analysis of the chimeric constructs showed that replacement of sperm protein, enterokinase, and agrin (SEA), low-density-lipoprotein receptor class A (LDLRA), and protease (PROT) domains from MT-2 with those from MT-1 resulted in limited cell surface localization, while the complement C1r/C1s, Uegf, Bmp1 (CUB) domain chimera retained localization at the cell surface. The SEA domain chimera was able to reduce cell surface HJV expression, while the CUB, LDLRA, and PROT domain chimeras were not. These studies suggest that the SEA and LDLRA domains of MT-2 are important for trafficking to the cell surface and that the CUB, LDLRA, and PROT domains are required for cleavage of HJV.

The dynamics of hepcidin-ferroportin internalization and consequences of a novel ferroportin disease mutation

The hepcidin-ferroportin axis underlies the pathophysiology of many iron-associated disorders and is a key target for the development of therapeutics for treating iron-associated disorders. The aims of this study were to investigate the dynamics of hepcidin-mediated ferroportin internalization and the consequences of a novel disease-causing mutation on ferroportin function. Specific reagents for ferroportin are limited; we developed and characterized antibodies against the largest extracellular loop of ferroportin and developed a novel cell-based assay for studying hepcidin-ferroportin function. We show that hepcidin-mediated ferroportin internalization is a rapid process and could be induced using low concentrations of hepcidin. Targeted next-generation sequencing utilizing an iron metabolism gene panel developed in our group identified a novel ferroportin p.D84E variant in a patient with iron overload. Wild-type and mutant ferroportin constructs were generated, transfected into HEK293 cells and analysed using an all-in-one flow-cytometry-based assay to study the effects on hepcidin-mediated internalization and iron transport. Consistent with the classical phenotype of ferroportin disease, the p.D84E mutation results in an inability to transport iron and hepcidin insensitivity. These results validate a recently proposed 3D-structural model of ferroportin and highlight the significance of this variant in the structure and function of ferroportin. Our novel ferroportin antibody and assay will be valuable tools for investigating the regulation of hepcidin/ferroportin function and the development of novel approaches for the therapeutic modulation of iron homeostasis.

Evaluation of a bone morphogenetic protein 6 variant as a cause of iron loading

BackgroundAtypical iron overload without variation in the five clinically associated hereditary hemochromatosis genes is now recognized; however, their etiology remains unknown. Since the identification of iron overload in the bone morphogenetic protein 6 (Bmp6) knockout mouse, the search has been on for clinically pathogenic variants in the BMP6 gene. A recent report proposes that variants in the pro-peptide region of BMP6 are the underlying cause of several cases of iron overload. We performed targeted next-generation sequencing on three cases of atypical iron overload with Asian ethnicity and identified a p.Q118dup (aka p.E112indelEQ, p.Q115dup, p.Q118_L119insQ) variant in BMP6. The purpose of this study was to characterize the molecular function of the identified BMP6 variant. Molecular characterization by immunofluorescence microscopy and Western blotting of transfected cells, bioinformatics, and population analyses was performed.ResultsIn contrast to reports for other BMP6 pro-peptide variants in this region, our data indicates that this variant does not affect the function of the mature BMP6 protein.ConclusionsOur data suggest that assignment of disease causation in clinical cases of iron overload to pro-peptide variants in BMP6 should thus be treated with caution and requires biological characterization.

Isolated Hepatic Iron Deficiency Despite Abundant Systemic Iron in Mdr2-/- Mice: Integrity of the Biliary Transport System Is Important in Liver Iron Homeostasis

Introduction: The liver is central to the metabolism of both iron and cholesterol. Cholesterol is synthesised and further metabolised to bile acids in the liver and the liver plays an important role in regulation of iron metabolism. It is also the organ in which excess iron is stored. Clinically, links have been noted between lipid and iron metabolism, with approximately one - third of patients with non - alcoholic fatty liver disease exhibiting altered iron parameters. On a molecular level, we have previously reported that wild - type mice fed iron - deficient, normal or iron - loaded diets exhibited increased hepatic cholesterol and increased hepatic gene expression of enzymes in the cholesterol biosynthesis pathway with increasing hepatic iron burden. In the genetic disorder, haemochromatosis, the liver can become overloaded with iron; however, clinical studies have suggested that lipid metabolism may not be perturbed in haemochromatosis. Methods and Materials: We investigated hepatic cholesterol metabolis m in three mouse models of hereditary haemochromatosis: Hfe - / - , Tfr2 Y245X single mutant and Hfe - / - x Tfr2 Y245X double mutant animals as well as wild - type controls. Mice were fed normal mouse chow and sacrificed at 10 weeks of age. Hepatic gene expression, total cholesterol and non – haem iron were measured. Liver non - haem iron was similar in Hfe - / - and Tfr2 Y245X mice (16.6±0.8 and 17±1 μmol Fe /g liver, respectively) and significantly higher in the double mutant animals (22.4±0.7 μmol Fe /g liver ; P<0.004) than either of the single mutant mice. Results: Only one group of genes increased significantly with increasing hepatic iron: those involved in cholesterol transport. Gene expression of apolipoproteins A4, C1, C2, C3 and E increased significantly with increasing hepatic iron as did expression of VLDL receptor. In contrast to our findings in wild - type mice, gene expression of cholesterol biosynthetic enzymes did not increase significantly with liver iron burden and there were no differences in hepatic cholesterol between the groups of mutant mice. We also measured expression of genes involved in cholesterol regulation, which similarly, did not increase with increasing hepatic iron. Approximately 50% of cholesterol synthesised in the liver is directed to bile acid synthesis; however, gene expression of bile acid pathway enzymes did not change with respect to hepatic iron burden. Conclusion: These results suggest that iron - associated cholesterol regulation may be ameliorated by the genetic changes which occur in haemochromatosis.Poster presented at Fifth Congress of the International BioIron Society that took place in University College London (London, United Kingdom) during 14-18th April 2013.

Blunted hepcidin response to inflammation in the absence of Hfe and Tfr2

Matrix metalloproteinases and their inhibitors are altered in a time-course model of irinotecaninduced mucositis N AL-DASOOQI, R GIBSON, J BOWEN, R LOGAN, A STRINGER, D KEEFE Department of Medicine, University of Adelaide, Department of Medical Oncology, Royal Adelaide Hospital, School of Medical Sciences, University of Adelaide, Division of Surgical Pathology, SA Pathology, Oral Pathology, School of Dentistry, Faculty of Health Sciences, University of Adelaide, Cancer Council South Australia, Eastwood, Australia

G80S-linked ferroportin disease: classical ferroportin disease in an Australian family of Asian descent and reclassification of the mutant as iron transport defective

Matrix metalloproteinases and their inhibitors are altered in a time-course model of irinotecaninduced mucositis N AL-DASOOQI, R GIBSON, J BOWEN, R LOGAN, A STRINGER, D KEEFE Department of Medicine, University of Adelaide, Department of Medical Oncology, Royal Adelaide Hospital, School of Medical Sciences, University of Adelaide, Division of Surgical Pathology, SA Pathology, Oral Pathology, School of Dentistry, Faculty of Health Sciences, University of Adelaide, Cancer Council South Australia, Eastwood, Australia