Molecular genetics and control of iron metabolism in hemochromatosis

Background and Objectives. Hereditary hemochromatosis (HC) is an inborn error of iron metabolism that leads to progressive iron overload. Considerable advances in the knowledge of molecular events in iron metabolism have been recently obtained. These molecular findings, the cloning of the gene responsible for HC (HFE gene) and the results of preliminary studies on the HFE protein prompted us to review this topic. information Sources. The material examined in this review article includes papers and abstracts published in the Journals covered by the Science Citation Index(C) and Medline(C). The authors have been working in the field of HC for several years and have contributed eleven of the quoted papers. State of Art and Perspective. HC is now recognized as the genetic disease characterized by the homozygosity for the Cys --> Tyr substitution at position 282 (C282Y) of the HFE protein. The mutation abolishes the association of the HFE protein with beta(2)-microglobulin (beta(2)M), making the complex unable to gain the cell surface. Thus HC is an example of abnormal trafficking of the corresponding proteins. It Is clear by the analysis of its structure that HFE protein is not an iron transporter itself, but has a regulatory role in iron metabolism. Its peculiar localization in the crypt cells of the small intestine suggests an important role in iron trafficking at this level. However, other proteins are involved in iron uptake, as the recently cloned Nramp2, the first iron transporter discovered in mammalians. Nramp2 has a recognized role both in the intestinal iron uptake and in the iron transport within the erythroblast. The relationships between HFE and Nramp2 are still unexplored. The recent association of HFE gene with transferrin receptor (TfR) in trophoblast cells opens new possibilities on its role in cellular iron uptake. The existence of other forms of genetic iron overload suggests that the scenario of iron proteins is not yet fully defined. Further studies in this field will contribute to our knowledge of iron metabolism regulation in humans. (C)1998, Ferrata Storti Foundation

New and old players in the hepcidin pathway

The identification of the liver peptide hepcidin at the beginning of the new millennium opened a new era in our understanding of iron metabolism.[1][1]–[3][2] Hepcidin is the main regulator of intestinal iron absorption and macrophage iron release, thus ultimately of the iron available fo

Furin-mediated release of soluble hemojuvelin: a new link between hypoxia and iron homeostasis

The liver peptide hepcidin regulates iron absorption and recycling. Hemojuvelin (HJV) has a key role in hepcidin regulation, and its inactivation causes severe iron overload both in humans and in mice. Membrane HJV (m-HJV) acts as a coreceptor for bone morphogenetic proteins (BMPs), whereas soluble HJV (s-HJV) may down-regulate hepcidin in a competitive way interfering with BMP signaling. s-HJV is decreased by iron in vitro and increased by iron deficiency in vivo. However, the mechanisms regulating the 2 HJV isoforms remain unclear. Here we show that s-HJV originates from a furin cleavage at position 332–335. s-HJV is reduced in the cleavage mutant R335Q as well as in cells treated with a furin inhibitor, and increased in cells overexpressing exogenous furin, but not in cells overexpressing an inactive furin variant. Furin is up-regulated by iron deficiency and hypoxia in association with the stabilization of HIF-1α. Increased s-HJV in response to HIF-1α occurs during differentiation of murine muscle cells expressing endogenous Hjv. Our data are relevant to the mechanisms that relate iron metabolism to the hypoxic response. The release of s-HJV might be a tissue-specific mechanism, signaling the local iron requests of hypoxic skeletal muscles independently of the oxygen status of the liver

Hemojuvelin N-terminal mutants reach the plasma membrane but do not activate the hepcidin response

Background Hemojuvelin is a glycosylphosphatidylinositol-anchored protein, expressed in liver, skeletal muscle and heart. As a co-receptor of bone morphogenetic protein, membrane hemojuvelin positively modulates the iron regulator hepcidin. Mutations of the gene encoding for hemojuvelin cause juvenile hemochromatosis, characterized by hepcidin deficiency and severe iron overload. We have previously shown that several hemojuvelin variants do not efficiently reach the plasma membrane, whereas a few N-terminal mutants localize to the plasma membrane. Design and Methods We studied hemojuvelin mutants of N-terminus (C80R, S85P, G99V, ΔRGD) and GDPH-consensus site for autoproteolysis (A168D, F170S, D172E) transiently expressed in HeLa cells, using electron microscopy, morphometric analysis and binding assays at different time points. Hepcidin activation by wild-type and mutant forms of hemojuvelin was assessed in Hep3B cells transfected with a hepcidin-promoter luciferase-reporter construct. Results S85P, G99V and ΔRGD were localized to plasma membrane 36 hours after transfection, but less efficiently exported than the wild-type protein at earlier (24–30 hours) times. Morphometric analysis clearly documented delayed export and endoplasmic reticulum retention of G99V. C80R was exported without delay. GDPH variants were partially retained in the endoplasmic reticulum and Golgi apparatus, but showed impaired plasma membrane localization. In the hepcidin promoter assay only wild type hemojuvelin was able to activate hepcidin. Conclusions The delayed export and retention in the endoplasmic reticulum of some N-terminal mutants could contribute to the pathogenesis of juvenile hemochromatosis, reducing a prompt response of bone morphogenetic protein. However, independently of their plasma membrane export, all hemojuvelin mutants tested showed no or minimal hepcidin activation

Two to Tango: Regulation of Mammalian Iron Metabolism

Disruptions in iron homeostasis from both iron deficiency and overload account for some of the most common human diseases. Iron metabolism is balanced by two regulatory systems, one that functions systemically and relies on the hormone hepcidin and the iron exporter ferroportin, and another that predominantly controls cellular iron metabolism through iron-regulatory proteins that bind iron-responsive elements in regulated messenger RNAs. We describe how the two distinct systems function and how they “tango” together in a coordinated manner. We also highlight some of the current questions in mammalian iron metabolism and discuss therapeutic opportunities arising from a better understanding of the underlying biological principles

New TFR2 mutations in young Italian patients with hemochromatosis.

This work describes the identification of two subjects with young-age iron overload carrying new causative mutations in transferrin receptor-2 gene. One was compound heterozygous (Asn411del/Ala444Thr) and the second was homozygous for a mutation affecting RNA splicing (IVS17+5636G>A). Another mutation (His33Asn) and a polymorphism were found in a group of 50 controls

Iron causes lipid oxidation and inhibits proteasome function in multiple myeloma cells: A proof of concept for novel combination therapies

Adaptation to import iron for proliferation makes cancer cells potentially sensitive to iron toxicity. Iron loading impairs multiple myeloma (MM) cell proliferation and increases the efficacy of the proteasome inhibitor bortezomib. Here, we defined the mechanisms of iron toxicity in MM.1S, U266, H929, and OPM-2 MM cell lines, and validated this strategy in preclinical studies using Vk*MYC mice as MM model. High-dose ferric ammonium citrate triggered cell death in all cell lines tested, increasing malondialdehyde levels, the by-product of lipid peroxidation and index of ferroptosis. In addition, iron exposure caused dose-dependent accumulation of polyubiquitinated proteins in highly iron-sensitive MM.1S and H929 cells, suggesting that proteasome workload contributes to iron sensitivity. Accordingly, high iron concentrations inhibited the proteasomal chymotrypsin-like activity of 26S particles and of MM cellular extracts in vitro. In all MM cells, bortezomib-iron combination induced persistent lipid damage, exacerbated bortezomib-induced polyubiquitinated proteins accumulation, and triggered cell death more efficiently than individual treatments. In Vk*MYC mice, addition of iron dextran or ferric carboxymaltose to the bortezomib-melphalan-prednisone (VMP) regimen increased the therapeutic response and prolonged remission without causing evident toxicity. We conclude that iron loading interferes both with redox and protein homeostasis, a property that can be exploited to design novel combination strategies including iron supplementation, to increase the efficacy of current MM therapies

Molecular Basis of Phenylketonuria in Italy

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A Portuguese patient homozygous for the -25G>A mutation of the HAMP promoter shows evidence of steady-state transcription but fails to up-regulate hepcidin levels by iron.

Blood. 2005 Oct 15;106(8):2922-3. A Portuguese patient homozygous for the -25G>A mutation of the HAMP promoter shows evidence of steady-state transcription but fails to up-regulate hepcidin levels by iron. Porto G, Roetto A, Daraio F, Pinto JP, Almeida S, Bacelar C, Nemeth E, Ganz T, Camaschella C. PMID: 16204153 [PubMed - indexed for MEDLINE]Free Article Publication Types, MeSH Terms, SubstancesPublication Types: Letter Research Support, Non-U.S. Gov't MeSH Terms: Antimicrobial Cationic Peptides/genetics* Antimicrobial Cationic Peptides/urine Glycine/genetics* Hemochromatosis/genetics Homozygote* Humans Iron/pharmacology* Mutation/genetics Portugal Promoter Regions, Genetic/genetics* Transcription, Genetic/genetics* Up-Regulation/drug effects* Substances: Antimicrobial Cationic Peptides hepcidin Glycine Iron LinkOut - more resourcesFull Text Sources: HighWire Press EBSCO Other Literature Sources: COS Scholar Universe Medical: Genetics Home Reference - HAMP Gene - Genetics Home Reference Molecular Biology Databases: IRON - HSDB GLYCINE - HSD

Construction of a YAC contig covering human chromosome 6p22

A contig covering human chromosome 6p22 that consists of 134 YAC clones aligned based on the presence/absence of 52 DNA markers is presented. This contig overlaps with the 6p23 contig at its telomeric end and with the 6p21.3 contig at its centromeric end. The order of loci within the contig resolves the relative positions of several genetically mapped markers. Among the additional markers used here, there are eight novel PCR assays. The 12 known genes and anonymous ESTs located within the contig establish a first step toward a transcriptional map of this region. The instability of YAC clones observed during this work is also discussed. (C) 1996 Academic Press, Inc