Iron regulatory proteins are essential for intestinal function and control key iron absorption molecules in the duodenum

HIF-1 regulates iron homeostasis in Caenorhabditis elegans by activation and inhibition of genes involved in iron uptake and storage

Caenorhabditis elegans ftn-1 and ftn-2, which encode the iron-storage protein ferritin, are transcriptionally inhibited during iron deficiency in intestine. Intestinal specific transcription is dependent on binding of ELT-2 to GATA binding sites in an iron-dependent enhancer (IDE) located in ftn-1 and ftn-2 promoters, but the mechanism for iron regulation is unknown. Here, we identify HIF-1 (hypoxia-inducible factor -1) as a negative regulator of ferritin transcription. HIF-1 binds to hypoxia-response elements (HREs) in the IDE in vitro and in vivo. Depletion of hif-1 by RNA interference blocks transcriptional inhibition of ftn-1 and ftn-2 reporters, and ftn-1 and ftn-2 mRNAs are not regulated in a hif-1 null strain during iron deficiency. An IDE is also present in smf-3 encoding a protein homologous to mammalian divalent metal transporter-1. Unlike the ftn-1 IDE, the smf-3 IDE is required for HIF-1–dependent transcriptional activation of smf-3 during iron deficiency. We show that hif-1 null worms grown under iron limiting conditions are developmentally delayed and that depletion of FTN-1 and FTN-2 rescues this phenotype. These data show that HIF-1 regulates intestinal iron homeostasis during iron deficiency by activating and inhibiting genes involved in iron uptake and storage.

Due to its presence in proteins involved in hemoglobin synthesis, DNA synthesis, and mitochondrial respiration, eukaryotic cells require iron for survival. Excess iron can lead to oxidative damage, while iron deficiency reduces cell growth and causes cell death. Dysregulation of iron homeostasis in humans caused by iron deficiency or excess leads to anemia, diabetes, and neurodegenerative disorders. All organisms have thus developed mechanisms to sense, acquire, and store iron. We use Caenorhabditis elegans as a model organism to study mechanisms of iron regulation. Our previous studies show that the iron-storage protein ferritin (FTN-1, FTN-2) is transcriptionally inhibited in intestine during iron deficiency, but the mechanisms regulating iron regulation are not known. Here, we find that hypoxia-inducible factor 1 (HIF-1) transcriptionally inhibits ftn-1 and ftn-2 during iron deficiency. We also show that HIF-1 activates the iron uptake gene smf-3. Transcriptional activation and inhibition by HIF-1 is dependent on an iron enhancer in the promoters of these genes. HIF-1 is a known transcriptional activator, but its role in transcriptional inhibition is not well understood. Our data show that HIF-1 regulates iron homeostasis by activating and inhibiting iron uptake and storage genes, and they provide insight into HIF-1 transcriptional inhibition.

Differential expression of iron metabolism proteins in diabetic and diabetic iron-supplemented rat liver

Diabetes mellitus is associated with altered iron homeostasis that can potentially effect reactive oxygen species generation and contribute to diabetes-related complications. We investigated, by quantitative polymerase chain reaction, whether the expression of liver hepcidin, ferritin, and TfR-1 is altered in diabetes. Rats in the control (C) group received a standard diet; control iron (CI) group received a standard diet supplemented with iron; diabetic (D) group received an injection of streptozotocin; and diabetic iron (DI) group received streptozotocin and the diet with iron. Animals of the D group showed higher levels of serum iron, increased concentration of carbonyl protein, and a decrease in antioxidant status. Group D rats showed increased hepatic expression of Trf-1 compared to the other groups. Iron supplementation reversed this increase. Hepcidin mRNA was 81% higher in DI than in C and CI rats. The results suggest that diabetes, with or without excess iron, can cause perturbations in iron status, hepcidin and Trf-1 expression.

Knockout mouse models of iron homeostasis

Murine models have made valuable contributions to our understanding of iron metabolism. Investigation of mice with inherited forms of anemia has led to the discovery of novel proteins involved in iron homeostasis. A growing number of murine models are being developed to investigate mitochondrial iron metabolism. Mouse strains are available for the major forms of hereditary hemochromatosis. Findings in murine models support the concept that the pathogenesis of nearly all forms of hereditary hemochromatosis involves inappropriately low expression of hepcidin. The availability of mice with floxed iron-related genes allows the study of the in vivo consequences of cell-selective deletion of these genes.

The role of ubiquitination in hepcidin-independent and hepcidin-dependent degradation of ferroportin

The iron exporter ferroportin (Fpn) is essential to transfer iron from cells to plasma. Systemic iron homeostasis in vertebrates is regulated by the hepcidin-mediated internalization of Fpn. Here, we demonstrate a second route for Fpn internalization; when cytosolic iron levels are low, Fpn is internalized in a hepcidin-independent manner dependent upon the E3 ubiquitin ligase Nedd4-2 and the Nedd4-2 binding protein Nfdip-1. Retention of cell-surface Fpn through reductions in Nedd4-2 results in cell death through depletion of cytosolic iron. Nedd4-2 is also required for internalization of Fpn in the absence of ferroxidase activity as well as for the entry of hepcidin-induced Fpn into the multivesicular body. C. elegans lacks hepcidin genes, and C. elegans Fpn expressed in mammalian cells is not internalized by hepcidin but is internalized in response to iron deprivation in a Nedd4-2-dependent manner, supporting the hypothesis that Nedd4-2-induced internalization of Fpn is evolutionarily conserved.

Iron regulatory protein-1 and -2: transcriptome-wide definition of binding mRNAs and shaping of the cellular proteome by iron regulatory proteins

Iron regulatory proteins (IRPs) 1 and 2 are RNA-binding proteins that control cellular iron metabolism by binding to conserved RNA motifs called iron-responsive elements (IREs). The currently known IRP-binding mRNAs encode proteins involved in iron uptake, storage, and release as well as heme synthesis. To systematically define the IRE/IRP regulatory network on a transcriptome-wide scale, IRP1/IRE and IRP2/IRE messenger ribonucleoprotein complexes were immunoselected, and the mRNA composition was determined using microarrays. We identify 35 novel mRNAs that bind both IRP1 and IRP2, and we also report for the first time cellular mRNAs with exclusive specificity for IRP1 or IRP2. To further explore cellular iron metabolism at a system-wide level, we undertook proteomic analysis by pulsed stable isotope labeling by amino acids in cell culture in an iron-modulated mouse hepatic cell line and in bone marrow-derived macrophages from IRP1- and IRP2-deficient mice. This work investigates cellular iron metabolism in unprecedented depth and defines a wide network of mRNAs and proteins with iron-dependent regulation, IRP-dependent regulation, or both.

Hypoxia inducible factor-2 α is translationally repressed in response to dietary iron deficiency in Sprague-Dawley rats

Iron regulatory proteins (IRP) regulate cellular iron metabolism by binding to iron-responsive elements (IRE) located in untranslated regions of mRNA-encoding proteins of iron metabolism. Recently, IRE have been identified in mRNA-encoding proteins with previously uncharacterized roles in iron metabolism, thus expanding the role of IRP beyond the regulation of cellular iron homeostasis. The mRNA for HIF 2-α contains an IRE and undergoes iron-dependent regulation in vitro, though the translational regulation of HIF-2α in vivo remains unknown. To examine HIF-2α translational regulation in vivo, we evaluated the effects of iron deficiency on the regulation of hepatic IRP activity and HIF-2α translation. Rats were fed either a control (C; 50 mg Fe/kg diet) or iron-deficient (ID; <5 mg Fe/kg diet) diet or were pair-fed (PF) the C diet for 21 d. In ID rats, there was a 2-fold increase in IRP activity compared to the PF group (P < 0.05), which was reflected by a 30-40% increase in HIF-2α repression (P < 0.05). In agreement with a decrease in translation, the levels of HIF-2α proteins were also decreased. The relative abundance of HIF-2α mRNA did not differ between treatment groups. Taken together, these results suggest that the translation of HIF-2α in the liver is regulated in part by the action of IRP in response to dietary iron deficiency and provide evidence that IRP may assist in coordinating the cellular response to alterations in iron and oxygen status associated with iron deficiency anemia.

Interactions between ferroportin and hephaestin in rat enterocytes are reduced after iron ingestion

BACKGROUND & AIMS

Ferroportin (Fpn) is a multiple transmembrane protein required for iron export into the systemic circulation, in cooperation with hephaestin (Heph). Despite the importance of Fpn in iron transport, there is controversy about its topology and functional state upon interaction with Heph.

METHODS

The topology of Fpn was determined using monospecific antisera against its different epitopes, in sheets of cells from duodenum that were or were not permeabilized with detergent. Immunoprecipitation and blue native polyacrylamide gel electrophoresis, followed by immunoblot analysis, were used to determine the extent of interactions between Fpn and Heph. Antisera against the intracellular, C-termini of divalent metal transporter (Dmt1) and Heph served as controls.

RESULTS

Immunofluorescence analysis with antisera against amino acids 172-193 of Fpn (anti-Fpn 172) detected Fpn only in permeabilized cells, whereas anti-Fpn 232 (amino acids 232-249), anti-Fpn 370 (amino acids 370-420), and anti-Fpn C (the C-terminus) detected Fpn in nonpermeabilized and permeabilized cells. Immunoprecipitation studies showed that Fpn and Heph coprecipitated with either anti-Fpn or anti-Heph. Blue native polyacrylamide gel electrophoresis studies revealed that a fraction of Fpn comigrates with Heph; the apparent interaction decreases after iron ingestion.

CONCLUSIONS

Studies with antisera to different epitopes of Fpn indicate that the topology of Fpn is consistent with an 11-transmembrane model, with the C-terminus exposed on the cell surface. Reduced interactions between Fpn and Heph after iron ingestion indicate that this is a regulatory mechanism for limiting further iron absorption.

Regulation of cellular iron metabolism

Iron is an essential but potentially hazardous biometal. Mammalian cells require sufficient amounts of iron to satisfy metabolic needs or to accomplish specialized functions. Iron is delivered to tissues by circulating transferrin, a transporter that captures iron released into the plasma mainly from intestinal enterocytes or reticuloendothelial macrophages. The binding of iron-laden transferrin to the cell-surface transferrin receptor 1 results in endocytosis and uptake of the metal cargo. Internalized iron is transported to mitochondria for the synthesis of haem or iron–sulfur clusters, which are integral parts of several metalloproteins, and excess iron is stored and detoxified in cytosolic ferritin. Iron metabolism is controlled at different levels and by diverse mechanisms. The present review summarizes basic concepts of iron transport, use and storage and focuses on the IRE (iron-responsive element)/IRP (iron-regulatory protein) system, a well known post-transcriptional regulatory circuit that not only maintains iron homoeostasis in various cell types, but also contributes to systemic iron balance.

Iron absorption in hepcidin1 knockout mice

Hepcidin, the Fe-regulatory peptide, has been shown to inhibit Fe absorption and reticuloendothelial Fe recycling. The present study was conducted to explore the mechanism of in vivo Fe regulation through genetic disruption of hepcidin1 and acute effects of hepcidin treatment in hepcidin1 knockout (Hepc1-/-) and heterozygous mice. Hepcidin1 disruption resulted in significantly increased intestinal Fe uptake. Hepcidin injection inhibited Fe absorption in both genotypes, but the effects were more evident in the knockout mice. Hepcidin administration was also associated with decreased membrane localisation of ferroportin in the duodenum, liver and, most significantly, in the spleen of Hepc1-/- mice. Hypoferraemia was induced in heterozygous mice by hepcidin treatment, but not in Hepc1-/- mice, 4 h after injection. Interestingly, Fe absorption and serum Fe levels in Hepc1-/- and heterozygous mice fed a low-Fe diet were not affected by hepcidin injection. The present study demonstrates that hepcidin deficiency causes increased Fe absorption. The effects of hepcidin were abolished by dietary Fe deficiency, indicating that the response to hepcidin may be influenced by dietary Fe level or Fe status.

Iron handling in hippocampal neurons: activity-dependent iron entry and mitochondria-mediated neurotoxicity

The characterization of iron handling in neurons is still lacking, with contradictory and incomplete results. In particular, the relevance of non-transferrin-bound iron (NTBI), under physiologic conditions, during aging and in neurodegenerative disorders, is undetermined. This study investigates the mechanisms underlying NTBI entry into primary hippocampal neurons and evaluates the consequence of iron elevation on neuronal viability. Fluorescence-based single cell analysis revealed that an increase in extracellular free Fe(2+) (the main component of NTBI pool) is sufficient to promote Fe(2+) entry and that activation of either N-methyl-d-aspartate receptors (NMDARs) or voltage operated calcium channels (VOCCs) significantly potentiates this pathway, independently of changes in intracellular Ca(2+) concentration ([Ca(2+) ](i) ). The enhancement of Fe(2+) influx was accompanied by a corresponding elevation of reactive oxygen species (ROS) production and higher susceptibility of neurons to death. Interestingly, iron vulnerability increased in aged cultures. Scavenging of mitochondrial ROS was the most powerful protective treatment against iron overload, being able to preserve the mitochondrial membrane potential and to safeguard the morphologic integrity of these organelles. Overall, we demonstrate for the first time that Fe(2+) and Ca(2+) compete for common routes (i.e. NMDARs and different types of VOCCs) to enter primary neurons. These iron entry pathways are not controlled by the intracellular iron level and can be harmful for neurons during aging and in conditions of elevated NTBI levels. Finally, our data draw the attention to mitochondria as a potential target for the treatment of the neurodegenerative processes induced by iron dysmetabolism.

Molecular mechanism of intestinal iron absorption

Iron is an essential trace metal in the human diet because of its role in a number of metabolic processes including oxygen transport. In the diet, iron is present in two fundamental forms, heme and non-heme iron. This article presents a brief overview of the molecular mechanisms of intestinal iron absorption and its regulation. While many proteins that orchestrate iron transport pathway have been identified, a number of key factors that control the regulation of iron absorption still remain to be elucidated. This review also summarizes new and emerging information about iron metabolic regulators that coordinate regulation of intestinal iron absorption.

Iron regulatory proteins: from molecular mechanisms to drug development

Eukaryotic cells require iron for survival but, as an excess of poorly liganded iron can lead to the catalytic production of toxic radicals that can damage cell structures, regulatory mechanisms have been developed to maintain appropriate cell and body iron levels. The interactions of iron responsive elements (IREs) with iron regulatory proteins (IRPs) coordinately regulate the expression of the genes involved in iron uptake, use, storage, and export at the post-transcriptional level, and represent the main regulatory network controlling cell iron homeostasis. IRP1 and IRP2 are similar (but not identical) proteins with partially overlapping and complementary functions, and control cell iron metabolism by binding to IREs (i.e., conserved RNA stem-loops located in the untranslated regions of a dozen mRNAs directly or indirectly related to iron metabolism). The discovery of the presence of IREs in a number of other mRNAs has extended our knowledge of the influence of the IRE/IRP regulatory network to new metabolic pathways, and it has been recently learned that an increasing number of agents and physiopathological conditions impinge on the IRE/IRP system. This review focuses on recent findings concerning the IRP-mediated regulation of iron homeostasis, its alterations in disease, and new research directions to be explored in the near future.

Iron-sensing proteins that regulate hepcidin and enteric iron absorption

The human body cannot actively excrete excess iron. As a consequence, iron absorption must be strictly regulated to ensure adequate iron uptake and prevent toxic iron accumulation. Iron absorption is controlled chiefly by hepcidin, the iron-regulatory hormone. Produced by the liver and secreted into the circulation, hepcidin regulates iron metabolism by inhibiting iron release from cells, including duodenal enterocytes, which mediate the absorption of dietary iron. Hepcidin production increases in response to iron loading and decreases in iron deficiency. Such regulation of hepcidin expression serves to modulate iron absorption to meet body iron demand. This review discusses the proteins that orchestrate hepatic hepcidin production and iron absorption by the intestine. Emphasis is placed on the proteins that directly sense iron and how they coordinate and fine-tune the molecular, cellular, and physiologic responses to iron deficiency and overload.

Intestinal ferritin H is required for an accurate control of iron absorption

To maintain appropriate body iron levels, iron absorption by the proximal duodenum is thought to be controlled by hepcidin, a polypeptide secreted by hepatocytes in response to high serum iron. Hepcidin limits basolateral iron efflux from the duodenal epithelium by binding and downregulating the intestinal iron exporter ferroportin. Here, we found that mice with an intestinal ferritin H gene deletion show increased body iron stores and transferrin saturation. As expected for iron-loaded animals, the ferritin H-deleted mice showed induced liver hepcidin mRNA levels and reduced duodenal expression of DMT1 and DcytB mRNA. In spite of these feedback controls, intestinal ferroportin protein and (59)Fe absorption were increased more than 2-fold in the deleted mice. Our results demonstrate that hepcidin-mediated regulation alone is insufficient to restrict iron absorption and that intestinal ferritin H is also required to limit iron efflux from intestinal cells.

Iron regulatory proteins secure mitochondrial iron sufficiency and function

Mitochondria supply cells with ATP, heme, and iron sulfur clusters (ISC), and mitochondrial energy metabolism involves both heme- and ISC-dependent enzymes. Here, we show that mitochondrial iron supply and function require iron regulatory proteins (IRP), cytosolic RNA-binding proteins that control mRNA translation and stability. Mice lacking both IRP1 and IRP2 in their hepatocytes suffer from mitochondrial iron deficiency and dysfunction associated with alterations of the ISC and heme biosynthetic pathways, leading to liver failure and death. These results uncover a major role of the IRPs in cell biology: to ensure adequate iron supply to the mitochondrion for proper function of this critical organelle.

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.

Iron metabolism: microbes, mouse, and man

Recent advances in research on iron metabolism have revealed the identity of a number of genes, signal transduction pathways, and proteins involved in iron regulation in mammals. The emerging paradigm is a coordination of homeostasis within a network of classical iron metabolic pathways and other cellular processes such as cell differentiation, growth, inflammation, immunity, and a host of physiologic and pathologic conditions. Iron, immunity, and infection are intricately linked and their regulation is fundamental to the survival of mammals. The mutual dependence on iron by the host and invading pathogenic organisms elicits competition for the element during infection. While the host maintains mechanisms to utilize iron for its own metabolism exclusively, pathogenic organisms are armed with a myriad of strategies to circumvent these measures. This review explores iron metabolism in mammalian host, defense mechanisms against pathogenic microbes and the competitive devices of microbes for access to iron.

Iron and porphyrin trafficking in heme biogenesis

Iron is an essential element for diverse biological functions. In mammals, the majority of iron is enclosed within a single prosthetic group: heme. In metazoans, heme is synthesized via a highly conserved and coordinated pathway within the mitochondria. However, iron is acquired from the environment and subsequently assimilated into various cellular pathways, including heme synthesis. Both iron and heme are toxic but essential cofactors. How is iron transported from the extracellular milieu to the mitochondria? How are heme and heme intermediates coordinated with iron transport? Although recent studies have answered some questions, several pieces of this intriguing puzzle remain unsolved.

SIREs: searching for iron-responsive elements

The iron regulatory protein/iron-responsive element regulatory system plays a crucial role in the post-transcriptional regulation of gene expression and its disruption results in human disease. IREs are cis-acting regulatory motifs present in mRNAs that encode proteins involved in iron metabolism. They function as binding sites for two related trans-acting factors, namely the IRP-1 and -2. Among cis-acting RNA regulatory elements, the IRE is one of the best characterized. It is defined by a combination of RNA sequence and structure. However, currently available programs to predict IREs do not show a satisfactory level of sensitivity and fail to detect some of the functional IREs. Here, we report an improved software for the prediction of IREs implemented as a user-friendly web server tool. The SIREs web server uses a simple data input interface and provides structure analysis, predicted RNA folds, folding energy data and an overall quality flag based on properties of well characterized IREs. Results are reported in a tabular format and as a schematic visual representation that highlights important features of the IRE. The SIREs (Search for iron-responsive elements) web server is freely available on the web at http://ccbg.imppc.org/sires/index.html

Regulation of divalent metal transporter 1 (DMT1) non-IRE isoform by the microRNA Let-7d in erythroid cells

BACKGROUND

Divalent metal transporter 1 (DMT1) is a widely expressed metal-iron transporter gene encoding four variant mRNA transcripts, differing for alternative promoter at 5' (DMT1 1A and 1B) and alternative splicing at 3' UTR, differing by a specific sequence either containing or lacking an iron regulatory element (+IRE and -IRE, respectively). DMT1-IRE might be the major DMT1 isoform expressed in erythroid cells, although its regulation pathways are still unknown.

DESIGN AND METHODS

The microRNA (miRNA) Let-7d (miR-Let-7d) was selected by the analysis of four miRNAs, predicted to target the DMT1-IRE gene in CD34(+) hematopoietic progenitor cells, in K562 and in HEL cells induced to erythroid differentiation. Using a luciferase reporter assay we demonstrated the inhibition of DMT1-IRE by miR-Let-7d in K562 and HEL cells. The function of miR-Let-7d in erythroid cells was evaluated by the flow cytometry analysis of erythroid differentiation markers, by benzidine staining and by iron flame atomic absorption for the evaluation of iron concentration in the endosomes from K562 cells over-expressing miR-Let-7d.

RESULTS

We show that in erythroid cells, DMT1-IRE expression is under the regulation of miR-Let-7d. DMT1-IRE and miR-Let-7d are inversely correlated with CD34(+) cells, K562 and HEL cells during erythroid differentiation. Moreover, overexpression of miR-Let-7d decreases the expression of DMT1-IRE at the mRNA and protein levels in K562 and HEL cells. MiR-Let-7d impairs erythroid differentiation of K562 cells by accumulation of iron in the endosomes.

CONCLUSIONS

Overall, these data suggest that miR-Let-7d participates in the finely tuned regulation of iron metabolism by targeting DMT1-IRE isoform in erythroid cells.

Tumorigenic properties of iron regulatory protein 2 (IRP2) mediated by its specific 73-amino acids insert

Iron regulatory proteins, IRP1 and IRP2, bind to mRNAs harboring iron responsive elements and control their expression. IRPs may also perform additional functions. Thus, IRP1 exhibited apparent tumor suppressor properties in a tumor xenograft model. Here we examined the effects of IRP2 in a similar setting. Human H1299 lung cancer cells or clones engineered for tetracycline-inducible expression of wild type IRP2, or the deletion mutant IRP2_Δ73 (lacking a specific insert of 73 amino acids), were injected subcutaneously into nude mice. The induction of IRP2 profoundly stimulated the growth of tumor xenografts, and this response was blunted by addition of tetracycline in the drinking water of the animals, to turnoff the IRP2 transgene. Interestingly, IRP2_Δ73 failed to promote tumor growth above control levels. As expected, xenografts expressing the IRP2 transgene exhibited high levels of transferrin receptor 1 (TfR1); however, the expression of other known IRP targets was not affected. Moreover, these xenografts manifested increased c-MYC levels and ERK1/2 phosphorylation. A microarray analysis identified distinct gene expression patterns between control and tumors containing IRP2 or IRP1 transgenes. By contrast, gene expression profiles of control and IRP2_Δ73-related tumors were more similar, consistently with their growth phenotype. Collectively, these data demonstrate an apparent pro-oncogenic activity of IRP2 that depends on its specific 73 amino acids insert, and provide further evidence for a link between IRPs and cancer biology.

Regulatory mechanisms of intestinal iron absorption-uncovering of a fast-response mechanism based on DMT1 and ferroportin endocytosis

Knowledge on the intestinal iron transport process and the regulation of body iron stores has greatly increased during the last decade. The liver, through the sensing of circulating iron, is now recognized as the central organ in this regulation. High iron levels induce the synthesis of hepcidin, which in turn decreases circulating iron by inhibiting its recycling from macrophages and its absorption at the intestine. Another mechanism for the control of iron absorption by the enterocyte is an active Iron Responsive Element (IRE)/Iron Regulatory Protein (IRP) system. The IRE/IRP system regulates the expression of iron uptake and storage proteins thus regulating iron absorption. Similarly, increasing evidence points to the transcriptional regulation of both divalent metal transporter 1 (DMT1) and ferroportin expression. A new mechanism of regulation related to a phenomenon called the mucosal block is starting to be unveiled. The mucosal block describes the ability of an initial dose of ingested iron to block absorption of a second dose given 2-4 h later. Here, we review the mechanisms involved in the expression of DMT1 and ferroportin, and present recent evidence on the molecular components and cellular processes involved in the mucosal block response. Our studies indicate that mucosal block is a fast-response endocytic mechanism destined to decrease intestinal iron absorption during a high ingest of iron.

Heme controls ferroportin1 (FPN1) transcription involving Bach1, Nrf2 and a MARE/ARE sequence motif at position -7007 of the FPN1 promoter

BACKGROUND

Macrophages of the reticuloendothelial system play a key role in recycling iron from hemoglobin of senescent or damaged erythrocytes. Heme oxygenase 1 degrades the heme moiety and releases inorganic iron that is stored in ferritin or exported to the plasma via the iron export protein ferroportin. In the plasma, iron binds to transferrin and is made available for de novo red cell synthesis. The aim of this study was to gain insight into the regulatory mechanisms that control the transcriptional response of iron export protein ferroportin to hemoglobin in macrophages.

DESIGN AND METHODS

Iron export protein ferroportin mRNA expression was analyzed in RAW264.7 mouse macrophages in response to hemoglobin, heme, ferric ammonium citrate or protoporphyrin treatment or to siRNA mediated knockdown or overexpression of Btb And Cnc Homology 1 or nuclear accumulation of Nuclear Factor Erythroid 2-like. Iron export protein ferroportin promoter activity was analyzed using reporter constructs that contain specific truncations of the iron export protein ferroportin promoter or mutations in a newly identified MARE/ARE element.

RESULTS

We show that iron export protein ferroportin is transcriptionally co-regulated with heme oxygenase 1 by heme, a degradation product of hemoglobin. The protoporphyrin ring of heme is sufficient to increase iron export protein ferroportin transcriptional activity while the iron released from the heme moiety controls iron export protein ferroportin translation involving the IRE in the 5'untranslated region. Transcription of iron export protein ferroportin is inhibited by Btb and Cnc Homology 1 and activated by Nuclear Factor Erythroid 2-like involving a MARE/ARE element located at position -7007/-7016 of the iron export protein ferroportin promoter.

CONCLUSIONS

This finding suggests that heme controls a macrophage iron recycling regulon involving Btb and Cnc Homology 1 and Nuclear Factor Erythroid 2-like to assure the coordinated degradation of heme by heme oxygenase 1, iron storage and detoxification by ferritin, and iron export by iron export protein ferroportin.

Up-regulation of divalent metal transporter 1 in 6-hydroxydopamine intoxication is IRE/IRP dependent

Iron plays a key role in Parkinson's disease (PD). Increased iron content of the substantia nigra (SN) has been found in PD patients, and divalent metal transporter 1 (DMT1) has been shown to be up-regulated in the SN of both MPTP-induced PD models and PD patients. However, the mechanisms underlying DMT1 up-regulation are largely unknown. In the present study, we observed that in the SN of 6-hydroxydopamine (6-OHDA)-induced PD rats, DMT1 with the iron responsive element (IRE, DMT1+IRE), but not DMT1 without IRE (DMT1-IRE), was up-regulated, suggesting that increased DMT1+IRE expression might account for nigral iron accumulation in PD rats. This possibility was further assessed in an in vitro study using 6-OHDA-treated and DMT1+IRE-over-expressing MES23.5 cells. In 6-OHDA-treated MES23.5 cells, increased iron regulatory protein (IRP) 1 and IRP2 expression was observed, while silencing of IRPs dramatically diminished 6-OHDA-induced DMT1+IRE up-regulation. Pretreatment with N-acetyl-L-cysteine fully suppressed IRPs up-regulation by inhibition of 6-OHDA-induced oxidative stress. Increased DMT1+IRE expression resulted in increased iron influx by MES23.5 cells. Our data provide direct evidence that DMT1+IRE up-regulation can account for IRE/IRP-dependent 6-OHDA-induced iron accumulation initiated by 6-OHDA-induced intracellular oxidative stress and that increased levels of intracellular iron result in aggravated oxidative stress. The results of this study provide novel evidence supporting the use of anti-oxidants in the treatment of PD, with the goal of inhibiting iron accumulation by regulation of DMT1 expression.

Multiple determinants within iron-responsive elements dictate iron regulatory protein binding and regulatory hierarchy

Iron regulatory proteins (IRPs) are iron-regulated RNA binding proteins that, along with iron-responsive elements (IREs), control the translation of a diverse set of mRNA with 5' IRE. Dysregulation of IRP action causes disease with etiology that may reflect differential control of IRE-containing mRNA. IREs are defined by a conserved stem-loop structure including a midstem bulge at C8 and a terminal CAGUGH sequence that forms an AGU pseudo-triloop and N19 bulge. C8 and the pseudo-triloop nucleotides make the majority of the 22 identified bonds with IRP1. We show that IRP1 binds 5' IREs in a hierarchy extending over a ninefold range of affinities that encompasses changes in IRE binding affinity observed with human L-ferritin IRE mutants. The limits of this IRE binding hierarchy are predicted to arise due to small differences in binding energy (e.g., equivalent to one H-bond). We demonstrate that multiple regions of the IRE stem not predicted to contact IRP1 help establish the binding hierarchy with the sequence and structure of the C8 region displaying a major role. In contrast, base-pairing and stacking in the upper stem region proximal to the terminal loop had a minor role. Unexpectedly, an N20 bulge compensated for the lack of an N19 bulge, suggesting the existence of novel IREs. Taken together, we suggest that a regulatory binding hierarchy is established through the impact of the IRE stem on the strength, not the number, of bonds between C8 or pseudo-triloop nucleotides and IRP1 or through their impact on an induced fit mechanism of binding.

Alternative splicing of the Menkes copper Atpase (Atp7a) transcript in the rat intestinal epithelium

The intestinal Menkes copper Atpase (Atp7a) gene is strongly induced by iron deficiency in the rat intestine. We sought to develop an in vitro model to understand the mechanism of this induction by performing molecular studies in native rat intestine and in intestinal epithelial (IEC-6) cells. IEC-6 cells express Atp7a, and induction was noted with iron deprivation. 5' Rapid amplification of cDNA ends and PCR experiments revealed three splice variants in rat intestine and IEC-6 cells; all variants were strongly induced during iron deprivation (five- to sevenfold). The splice variants presumably encode proteins that would either contain the extreme NH(2) terminus of the protein (containing copper binding domain 1) or not. We thus hypothesized that more than one version of Atp7a protein exists. Antibodies against this NH(2)-terminal region of the protein were developed (named N-term) and used along with previously reported antibodies (against more COOH-terminal regions, termed 54-10) to perform immunoblotting and immunolocalization studies. Results with the 54-10 antiserum revealed an Atp7a protein variant of approximately 190 kDa that localized to the trans-Golgi network of IEC-6 cells and trafficked to the plasma membrane with copper loading. Using the N-term antiserum, however, we noted protein of approximately 97 and 64 kDa. The 97-kDa protein was cytosolic and nuclear, whereas the 64-kDa protein was nuclear specific. Immunolocalization analyses with the N-term antiserum showed strong staining of nuclei in IEC-6 and Caco-2 cells and in rat intestine. We conclude that novel Atp7a protein variants may exist in rat and human intestinal epithelial cells, with different intracellular locations and potentially distinct physiological functions.

Mammalian iron transport

Iron is essential for basic cellular processes but is toxic when present in excess. Consequently, iron transport into and out of cells is tightly regulated. Most iron is delivered to cells bound to plasma transferrin via a process that involves transferrin receptor 1, divalent metal-ion transporter 1 and several other proteins. Non-transferrin-bound iron can also be taken up efficiently by cells, although the mechanism is poorly understood. Cells can divest themselves of iron via the iron export protein ferroportin in conjunction with an iron oxidase. The linking of an oxidoreductase to a membrane permease is a common theme in membrane iron transport. At the systemic level, iron transport is regulated by the liver-derived peptide hepcidin which acts on ferroportin to control iron release to the plasma.

Iron delocalisation in the pathogenesis of malarial anaemia

There is consensus that the pathophysiology of malaria-associated anaemia is multifactorial, but the precise mechanisms behind many of the haematological changes during malaria remain unclear. In this review, we attempt to build a composite picture of the pathophysiology of malarial anaemia using evidence from experimental, human and animal studies. We propose that cytokine- and hepcidin-mediated iron delocalisation, a principal mechanism in the anaemia of inflammation, plays an important role in the aetiology of malarial anaemia, and can explain some of the clinical and laboratory findings. These mechanisms interact with other aetiological determinants, such as dietary iron and micronutrient supply, helminth load, other infections and genetic variation, in determining the severity and associated features of anaemia. We suggest that iron delocalisation as a mechanism for malarial anaemia could be exploited for the development of alternative therapeutic strategies for post-malaria anaemia.

Are there common biochemical and molecular mechanisms controlling manganism and parkisonism

Over the past several decades there has been considerable progress in our basic knowledge as to the mechanisms and factors regulating Mn toxicity. The disorder known as manganism is associated with the preferential accumulation of Mn in the globus pallidus of the basal ganglia which is generally considered to be the major and initial site of injury. Because the area of the CNS comprising the basal ganglia is very complex and dependent on the precise function and balance of several neurotransmitters, it is not surprising that symptoms of manganism often overlap with that of Parkinson's disease. The fact that neurological symptoms and onset of Mn toxicity are quite broad and can vary unpredictably probably reflects specific genetic variance of the physiological and biochemical makeup within the basal ganglia in any individual. Differences in response to Mn overexposure are, thus, likely due to underlying genetic variability which ultimately presents in deviations in both susceptibility as well as the characteristics of the neurological lesions and symptoms expressed. Although chronic exposure to Mn is not the initial causative agent provoking Parkinsonism, there is evidence suggesting that persistent exposure can predispose an individual to acquire dystonic movements associated with Parkinson's disease. As noted in this review, there appears to be common threads between the two disorders, as mutations in the genes, parkin and ATP13A2, associated with early onset of Parkinsonism, may also predispose an individual to develop Mn toxicity. Mutations in both genes appear to effect transport of Mn into the cell. These genetic difference coupled with additional environmental or nutritional factors must also be considered as contributing to the severity and onset of manganism.

Hepcidin decreases iron transporter expression in vivo in mouse duodenum and spleen and in vitro in THP-1 macrophages and intestinal Caco-2 cells

Hepcidin is thought to control iron metabolism by interacting with the iron efflux transporter ferroportin. In macrophages, there is compelling evidence that hepcidin directly regulates ferroportin protein expression. However, the effects of hepcidin on intestinal ferroportin levels are less conclusive. In this study, we compared the effects of hepcidin on iron transporter expression in the spleen and duodenum of mice treated with hepcidin over a 24- to 72-h period and observed a marked decrease in the expression of ferroportin in both duodenal enterocytes and splenic macrophages following treatment. Changes in transporter protein expression were associated with significant decreases in duodenal iron transport and serum iron. In THP-1 macrophages, ferroportin protein levels were decreased by 300 and 1000 nmol/L hepcidin. In contrast, ferroportin protein expression was unaltered in intestinal Caco-2 cells following exposure to hepcidin. However, iron efflux from Caco-2 cells was significantly inhibited in the presence of hepcidin, suggesting that the peptide could block ferroportin function in these cells. We conclude that hepcidin regulates the release of iron from both enterocytes and macrophages. However, taken together with our previous work, it is apparent that macrophages are more sensitive than enterocytes to a hepcidin challenge.

Regulation of intestinal iron absorption: the mucosa takes control?

Two studies (Shah et al., 2009; Mastrogiannaki et al., 2009) show that the hypoxia inducible factor HIF-2alpha is a major player in regulating iron absorption by directly controlling the transcription of iron transporters in the intestine in response to changes in mucosal iron or oxygen levels. The HIF-2alpha mechanism has major effects on iron metabolism which can override the well-known hepcidin-ferroportin regulatory axis.

A ferroportin transcript that lacks an iron-responsive element enables duodenal and erythroid precursor cells to evade translational repression

Ferroportin (FPN1), the sole characterized mammalian iron exporter, has an iron-responsive element (IRE) in its 5' untranslated region, which ensures that its translation is repressed by iron regulatory proteins (IRPs) in iron-deficient conditions to maintain cellular iron content. However, here we demonstrate that duodenal epithelial and erythroid precursor cells utilize an alternative upstream promoter to express a FPN1 transcript, FPN1B, which lacks the IRE and is not repressed in iron-deficient conditions. The FPN1B transcript encodes ferroportin with an identical open reading frame and contributes significantly to ferroportin protein expression in erythroid precursors and likely also in the duodenum of iron-starved animals. The identification of FPN1B reveals how FPN1 expression can bypass IRP-dependent repression in intestinal iron uptake, even when cells throughout the body are iron deficient. In erythroid precursor cells, we hypothesize that FPN1B expression enhances real-time sensing of systemic iron status and facilitates restriction of erythropoiesis in response to low systemic iron.

A general map of iron metabolism and tissue-specific subnetworks

Iron is required for survival of mammalian cells. Recently, understanding of iron metabolism and trafficking has increased dramatically, revealing a complex, interacting network largely unknown just a few years ago. This provides an excellent model for systems biology development and analysis. The first step in such an analysis is the construction of a structural network of iron metabolism, which we present here. This network was created using CellDesigner version 3.5.2 and includes reactions occurring in mammalian cells of numerous tissue types. The iron metabolic network contains 151 chemical species and 107 reactions and transport steps. Starting from this general model, we construct iron networks for specific tissues and cells that are fundamental to maintaining body iron homeostasis. We include subnetworks for cells of the intestine and liver, tissues important in iron uptake and storage, respectively, as well as the reticulocyte and macrophage, key cells in iron utilization and recycling. The addition of kinetic information to our structural network will permit the simulation of iron metabolism in different tissues as well as in health and disease.

HIF-2alpha, but not HIF-1alpha, promotes iron absorption in mice

HIF transcription factors (HIF-1 and HIF-2) are central mediators of cellular adaptation to hypoxia. Because the resting partial pressure of oxygen is low in the intestinal lumen, epithelial cells are believed to be mildly hypoxic. Having recently established a link between HIF and the iron-regulatory hormone hepcidin, we hypothesized that HIFs, stabilized in the hypoxic intestinal epithelium, may also play critical roles in regulating intestinal iron absorption. To explore this idea, we first established that the mouse duodenum, the site of iron absorption in the intestine, is hypoxic and generated conditional knockout mice that lacked either Hif1a or Hif2a specifically in the intestinal epithelium. Using these mice, we found that HIF-1alpha was not necessary for iron absorption, whereas HIF-2alpha played a crucial role in maintaining iron balance in the organism by directly regulating the transcription of the gene encoding divalent metal transporter 1 (DMT1), the principal intestinal iron transporter. Specific deletion of Hif2a led to a decrease in serum and liver iron levels and a marked decrease in liver hepcidin expression, indicating the involvement of an induced systemic response to counteract the iron deficiency. This finding may provide a basis for the development of new strategies, specifically in targeting HIF-2alpha, to improve iron homeostasis in patients with iron disorders.

Conditional inactivation of glucocorticoid receptor gene in dopamine-beta-hydroxylase cells impairs chromaffin cell survival

Glucocorticoid hormones (GCs) have been thought to determine the fate of chromaffin cells from sympathoadrenal progenitor cells. The analysis of mice carrying a germ line deletion of the glucocorticoid receptor (GR) gene has challenged these previous results because the embryonic development of adrenal chromaffin cells is largely unaltered. In the present study, we have analyzed the role of GC-dependent signaling in the postnatal development of adrenal chromaffin cells by conditional inactivation of the GR gene in cells expressing dopamine-beta-hydroxylase, an enzyme required for the synthesis of noradrenaline and adrenaline. These mutant mice are viable, allowing to study whether in the absence of GC signaling further development of the adrenal medulla is affected. Our analysis shows that the loss of GR leads not only to the loss of phenylethanolamine-N-methyl-transferase expression and, therefore, to inhibition of adrenaline synthesis, but also to a dramatic reduction in the number of adrenal chromaffin cells. We provide evidence that increased apoptotic cell death is the main consequence of GR loss. These findings define the essential role of GCs for survival of chromaffin cells and underscore the specific requirement of GCs for adrenergic chromaffin cell differentiation and maintenance.

Iron absorption and metabolism

PURPOSE OF REVIEW

Intestinal iron absorption is an essential physiological process that is regulated by the liver-derived peptide hepcidin. This review will describe recent advances in hepcidin biology and enterocyte iron transport.

RECENT FINDINGS

Hepcidin acts as a repressor of iron absorption and its expression in turn reflects a range of systemic cues, including iron status, hypoxia, erythropoiesis and inflammation. These act through proteins on the hepatocyte plasma membrane such as HFE, hemojuvelin and transferrin receptor 2 to alter transcription of the hepcidin gene. Bone morphogenetic protein-SMAD signaling provides a key pathway of hepcidin activation, whereas the membrane-bound serine protease matriptase-2 and the erythroid factor growth differentiation factor 15 have emerged as important negative regulators of hepcidin expression. At the enterocyte itself, the recent demonstration of a chaperone for delivering iron to ferritin and new data on iron release from the hepcidin target ferroportin are helping to define the pathway of iron movement across the intestinal epithelium.

SUMMARY

Disturbances in the hepcidin regulatory pathway underlie a range of iron metabolism disorders, from iron deficiency to iron loading, and there is considerable promise that the exciting recent advances in understanding hepcidin action will be translated into improved diagnostic and therapeutic modalities in the near future.

Intestinal hypoxia-inducible transcription factors are essential for iron absorption following iron deficiency

Iron deficiency and iron overload are among the most prevalent nutritional disorders worldwide. Duodenal cytochrome b (DcytB) and divalent metal transporter 1 (DMT1) are regulators of iron absorption. Their expression is increased during high systemic requirements for iron, but the molecular mechanisms that regulate DcytB and DMT1 expression are undefined. Hypoxia-inducible factor (HIF) signaling was induced in the intestine following acute iron deficiency in the duodenum, resulting in activation of DcytB and DMT1 expression and an increase in iron uptake. DcytB and DMT1 were demonstrated as direct HIF-2alpha target genes. Genetic disruption of HIF signaling in the intestine abolished the adaptive induction of iron absorption following iron deficiency, resulting in low systemic iron and hematological defects. These results demonstrate that HIF signaling in the intestine is a critical regulator of systemic iron homeostasis.

E7 oncoprotein of novel human papillomavirus type 108 lacking the E6 gene induces dysplasia in organotypic keratinocyte cultures

The genome organization of the novel human papillomavirus type 108 (HPV108), isolated from a low-grade cervical lesion, deviates from those of other HPVs in lacking an E6 gene. The three related HPV types HPV103, HPV108, and HPV101 were isolated from cervicovaginal cells taken from normal genital mucosa (HPV103) and low-grade (HPV108) and high-grade cervical (HPV101) intraepithelial neoplasia (Z. Chen, M. Schiffman, R. Herrero, R. DeSalle, and R. D. Burk, Virology 360:447-453, 2007, and this report). Their unusual genome organization, against the background of considerable phylogenetic distance from the other HPV types usually associated with lesions of the genital tract, prompted us to investigate whether HPV108 E7 per se is sufficient to induce the above-mentioned clinical lesions. Expression of HPV108 E7 in organotypic keratinocyte cultures increases proliferation and apoptosis, focal nuclear polymorphism, and polychromasia. This is associated with irregular intra- and extracellular lipid accumulation and loss of the epithelial barrier. These alterations are linked to HPV108 E7 binding to pRb and inducing its decrease, an increase in PCNA expression, and BrdU incorporation, as well as increased p53 and p21(CIP1) protein levels. A delay in keratin K10 expression, increased expression of keratins K14 and K16, and loss of the corneal proteins involucrin and loricrin have also been noted. These modifications are suggestive of infection by a high-risk papillomavirus.

Molecular mechanisms of normal iron homeostasis

Humans possess elegant control mechanisms to maintain iron homeostasis by coordinately regulating iron absorption, iron recycling, and mobilization of stored iron. Dietary iron absorption is regulated locally by hypoxia inducible factor (HIF) signaling and iron-regulatory proteins (IRPs) in enterocytes and systematically by hepatic hepcidin, the central iron regulatory hormone. Hepcidin not only controls the rate of iron absorption but also determines iron mobilization from stores through negatively modulating the function of ferroportin, the only identified cellular iron exporter to date. The regulation of hepatic hepcidin is accomplished by the coordinated activity of multiple proteins through different signaling pathways. Recent studies have greatly expanded the knowledge in the understanding of hepcidin expression and regulation by the bone morphogenetic protein (BMP) signaling, the erythroid factors, and inflammation. In this review, we mainly focus on the roles of recently identified proteins in the regulation of iron homeostasis.

Divalent metal transporter 1 (DMT1) contributes to neurodegeneration in animal models of Parkinson's disease

Dopaminergic cell death in the substantia nigra (SN) is central to Parkinson's disease (PD), but the neurodegenerative mechanisms have not been completely elucidated. Iron accumulation in dopaminergic and glial cells in the SN of PD patients may contribute to the generation of oxidative stress, protein aggregation, and neuronal death. The mechanisms involved in iron accumulation also remain unclear. Here, we describe an increase in the expression of an isoform of the divalent metal transporter 1 (DMT1/Nramp2/Slc11a2) in the SN of PD patients. Using the PD animal model of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intoxication in mice, we showed that DMT1 expression increases in the ventral mesencephalon of intoxicated animals, concomitant with iron accumulation, oxidative stress, and dopaminergic cell loss. In addition, we report that a mutation in DMT1 that impairs iron transport protects rodents against parkinsonism-inducing neurotoxins MPTP and 6-hydroxydopamine. This study supports a critical role for DMT1 in iron-mediated neurodegeneration in PD.

Cell-autonomous and systemic context-dependent functions of iron regulatory protein 2 in mammalian iron metabolism

Mice with total and constitutive iron regulatory protein 2 (IRP2) deficiency exhibit microcytosis and altered body iron distribution with duodenal and hepatic iron loading and decreased iron levels in splenic macrophages. To explore cell-autonomous and systemic context-dependent functions of IRP2 and to assess the systemic consequences of local IRP2 deficiency, we applied Cre/Lox technology to specifically ablate IRP2 in enterocytes, hepatocytes, or macrophages, respectively. This study reveals that the hepatic and duodenal manifestations of systemic IRP2 deficiency are largely explained by cell-autonomous functions of IRP2. By contrast, IRP2-deficient macrophages from otherwise IRP2-sufficient mice do not display the abnormalities of macrophages from systemically IRP2-deficient animals, suggesting that these result from IRP2 disruption in other cell type(s). Mice with enterocyte-, hepatocyte-, or macrophage-specific IRP2 deficiency display normal red blood cell and plasma iron parameters, supporting the notion that the microcytosis in IRP2-deficient mice likely reflects an intrinsic defect in hematopoiesis. This work defines the respective roles of IRP2 in the determination of critical body iron parameters such as organ iron loading and erythropoiesis.

Systemic iron homeostasis and the iron-responsive element/iron-regulatory protein (IRE/IRP) regulatory network

The regulation and maintenance of systemic iron homeostasis is critical to human health. Iron overload and deficiency diseases belong to the most common nutrition-related pathologies across the globe. It is now well appreciated that the hormonal hepcidin/ferroportin system plays an important regulatory role for systemic iron metabolism. We review recent data that uncover the importance of the cellular iron-responsive element/iron-regulatory protein (IRE/IRP) regulatory network in systemic iron homeostasis. We also discuss how the IRE/IRP regulatory system communicates with the hepcidin/ferroportin system to connect the control networks for systemic and cellular iron balance.

Iron homeostasis: recently identified proteins provide insight into novel control mechanisms

Iron is an essential nutrient required for a variety of biochemical processes. It is a vital component of the heme in hemoglobin, myoglobin, and cytochromes and is also an essential cofactor for non-heme enzymes such as ribonucleotide reductase, the limiting enzyme for DNA synthesis. When in excess, iron is toxic because it generates superoxide anions and hydroxyl radicals that react readily with biological molecules, including proteins, lipids, and DNA. As a result, humans possess elegant control mechanisms to maintain iron homeostasis by coordinately regulating iron absorption, iron recycling, and mobilization of stored iron. Disruption of these processes causes either iron-deficient anemia or iron overload disorders. In this minireview, we focus on the roles of recently identified proteins in the regulation of iron homeostasis.

Iron homeostasis: fitting the puzzle pieces together

Here, recent insights into iron homeostasis are highlighted. Three studies demonstrate the importance of the IRE-IRP system for enterocytes in balancing extracellular iron demand against cellular iron requirements, show that the hemochromatosis protein HFE exerts its iron-regulatory activity principally in hepatocytes by modulating the production of hepcidin, and provide strong support for a proposed mechanism of transcriptional regulation of hepcidin through a signaling cascade initiated by holotransferrin displacing HFE from transferrin receptor 1.