Diferric transferrin regulates transferrin receptor 2 protein stability

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.

Interaction of the hereditary hemochromatosis protein HFE with transferrin receptor 2 is required for transferrin-induced hepcidin expression

The mechanisms that allow the body to sense iron levels in order to maintain iron homeostasis are unknown. Patients with the most common form of hereditary iron overload have mutations in the hereditary hemochromatosis protein HFE. They have lower levels of hepcidin than unaffected individuals. Hepcidin, a hepatic peptide hormone, negatively regulates iron efflux from the intestines into the blood. We report two hepatic cell lines, WIF-B cells and HepG2 cells transfected with HFE, where hepcidin expression responded to iron-loaded transferrin. The response was abolished when endogenous transferrin receptor 2 (TfR2) was suppressed or in primary hepatocytes lacking either functional TfR2 or HFE. Furthermore, transferrin-treated HepG2 cells transfected with HFE chimeras containing only the alpha3 and cytoplasmic domains could upregulate hepcidin expression. Since the HFE alpha3 domain interacts with TfR2, these results supported our finding that TfR2/HFE complex is required for transcriptional regulation of hepcidin by holo-Tf.

A novel transferrin/TfR2-mediated mitochondrial iron transport system is disrupted in Parkinson's disease

More than 80 years after iron accumulation was initially described in the substantia nigra (SN) of Parkinson's disease (PD) patients, the mechanisms responsible for this phenomenon are still unknown. Similarly, how iron is delivered to its major recipients in the cell - mitochondria and the respiratory complexes - has yet to be elucidated. Here, we report a novel transferrin/transferrin receptor 2 (Tf/TfR2)-mediated iron transport pathway in mitochondria of SN dopamine neurons. We found that TfR2 has a previously uncharacterized mitochondrial targeting sequence that is sufficient to import the protein into these organelles. Importantly, the Tf/TfR2 pathway can deliver Tf bound iron to mitochondria and to the respiratory complex I as well. The pathway is redox-sensitive and oxidation of Tf thiols to disulfides induces release from Tf of highly reactive ferrous iron, which contributes to free radical production. In the rotenone model of PD, Tf accumulates in dopamine neurons, with much of it accumulating in the mitochondria. This is associated with iron deposition in SN, similar to what occurs in PD. In the human SN, TfR2 is also found in mitochondria of dopamine neurons, and in PD there is a dramatic increase of oxidized Tf in SN. Thus, we have discovered a novel mitochondrial iron transport system that goes awry in PD, and which may provide a new target for therapeutic intervention.

Hepcidin modulation in human diseases: From research to clinic

By modulating hepcidin production, an organism controls intestinal iron absorption, iron uptake and mobilization from stores to meet body iron need. In recent years there has been important advancement in our knowledge of hepcidin regulation that also has implications for understanding the physiopathology of some human disorders. Since the discovery of hepcidin and the demonstration of its pivotal role in iron homeostasis, there has been a substantial interest in developing a reliable assay of the hormone in biological fluids. Measurement of hepcidin in biological fluids can improve our understanding of iron diseases and be a useful tool for diagnosis and clinical management of these disorders. We reviewed the literature and our own research on hepcidin to give an updated status of the situation in this rapidly evolving field.

Iron metabolism in the eye: a review

This review article covers all aspects of iron metabolism, which include studies of iron levels within the eye and the processes used to maintain normal levels of iron in ocular tissues. In addition, the involvement of iron in ocular pathology is explored. In each section there is a short introduction to a specific metabolic process responsible for iron homeostasis, which for the most part has been studied in non-ocular tissues. This is followed by a summary of our current knowledge of the process in ocular tissues.

Expression of hepcidin and other iron-related genes in type 3 hemochromatosis due to a novel mutation in transferrin receptor-2

Transferrin receptor-2 (TFR2) regulates hepatic hepcidin secretion and when mutated causes type-3 hemochromatosis. No functional study is available in humans. We studied a 47 year-old woman with hemochromatosis. TFR2 DNA and its hepatic transcript were directly sequenced. Hepatic expression of hepcidin and other iron-related genes were measured by qRT-PCR. Urinary hepcidin was measured at baseline and after an oral iron challenge (ferrous sulfate, 65 mg) by SELDI-TOF-MS. A novel homozygous TFR2 mutation was identified in the splicing donor site of intron 4 (c.614+4 A>G) causing exon 4 skipping. Hepcidin and hemojuvelin expression were markedly reduced. Urinary hepcidin was lower than normal and further decreased after iron challenge. This is the first description of iron-related gene expression profiles in a TFR2 mutated patient. The decreased hepatic and urinary expression of hepcidin and lack of acute response to iron challenge confirms the primary role of TFR2 in iron homeostasis.

Interacting signals in the control of hepcidin expression

The amount of iron in the plasma is determined by the regulated release of iron from most body cells, but macrophages, intestinal enterocytes and hepatocytes play a particularly important role in this process. This cellular iron efflux is modulated by the liver-derived peptide hepcidin, and this peptide is now regarded as the central regulator of body iron homeostasis. Hepcidin expression is influenced by systemic stimuli such as iron stores, the rate of erythropoiesis, inflammation, hypoxia and oxidative stress. These stimuli control hepcidin levels by acting through hepatocyte cell surface proteins including HFE, transferrin receptor 2, hemojuvelin, TMPRSS6 and the IL-6R. The surface proteins activate various cell signal transduction pathways, including the BMP-SMAD, JAK-STAT and HIF1 pathways, to alter transcription of HAMP, the gene which encodes hepcidin. It is becoming increasingly apparent that various stimuli can signal through multiple pathways to regulate hepcidin expression, and the interplay between positive and negative stimuli is critical in determining the net hepcidin level. The BMP-SMAD pathway appears to be particularly important and disruption of this pathway will abrogate the response of hepcidin to many stimuli.

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.

Regulation of iron metabolism-related genes in diethylnitrosamine-induced mouse liver tumors

BACKGROUND

It has been suggested that the altered iron metabolism in liver tumors, characterized by the iron-deficient phenotype, is of importance for tumor growth.

AIM

This study was performed to elucidate the mechanisms underlying iron deficiency in liver tumors by examining how the liver tumor development affects the expression of iron metabolism-related genes.

METHODS

Iron metabolism reference values were analyzed in the sera of diethylnitrosamine-induced hepatocellular adenoma-bearing mice. Expression of iron metabolism-related genes was analyzed in adenomas and surrounding non-tumor tissues, and a subgroup of adenoma-bearing mice loaded with iron 72h before sacrifice.

RESULTS

Iron content of the adenoma tissues was 2.0-2.5-fold lower compared to surrounding and age-matched control tissues. There was no significant difference in serum iron levels between the adenoma-bearing and control mice, while the adenoma-bearing mice exhibited a 2.4-fold lower level of serum transferrin saturation. Expression of iron metabolism-related genes was dysregulated in the adenomas. Iron loading affected protein expression similarly in the adenomas and surrounding tissues suggesting that iron-responsive regulation of the proteins was not impaired. However, the mRNA expression for ceruloplasmin and divalent metal transporter 1 (DMT1) IRE(+) in the adenomas was altered independently of iron status, and the dysregulation may contribute to diminished iron content.

CONCLUSION

These findings suggest that diethylnitrosamine-induced liver adenoma-bearing mice have abnormal iron metabolism and that dysregulation of iron metabolism-related genes contributes to iron deficiency in the adenomas.

Hereditary hemochromatosis in the post-HFE era

Following the discovery of the HFE gene in 1996 and its linkage to the iron overload disorder hereditary hemochromatosis (HH) there have been profound developments in our understanding of the pathogenesis of the biochemical and clinical manifestations of a number of iron overload disorders. This article provides an update of recent developments and key issues relating to iron homeostasis and inherited disorders of iron overload, with emphasis on HFE-related HH, and is based on the content of the American Association for the Study of Liver Diseases Single-Topic Conference entitled "Hemochromatosis: What has Happened After HFE?" which was held at the Emory Convention Center in Atlanta, September 7-9, 2007.

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.

Genetics of iron regulation and the possible role of iron in Parkinson's disease

Parkinson's disease (PD) is acknowledged as the second most common neurodegenerative disorder after Alzheimer's Disease. Older age may be the only unequivocal risk factor for PD although the male to female ratio is consistently greater than 1 in populations of European ancestry. Characteristic features of PD include dopaminergic neuron death in the substantia nigra (SN) pars compacta, accumulation of alpha-synuclein inclusions known as Lewy bodies in the SN, and brain iron accumulation beyond that observed in non-PD brains of a similar age. In this review article, we will provide an overview of human and animal studies investigating the contributions of iron in PD, a summary of human studies of iron-related genes in PD, a review of the literature on the genetics of iron metabolism, and some hypotheses on possible roles for iron in the pathogenic processes of PD including potential interactions between iron and other factors associated with Parkinson's disease.

Iron regulation and erythropoiesis

PURPOSE OF REVIEW

The peptide hormone hepcidin regulates iron metabolism in response to erythropoietic demand, iron stores and inflammation. Major advances have been made in understanding the regulation of hepcidin production, and consequently the availability of iron for erythropoiesis.

RECENT FINDINGS

It is becoming clear that the bone morphogenetic protein (BMP) pathway plays a major role in setting the baseline hepcidin level and, with the assistance of BMP2/4 and hemochromatosis-related proteins hemojuvelin, HFE and transferrin receptor 2, also regulates hepcidin expression in response to iron. Regulation of hepcidin in anemias has now been linked to increased erythropoietic activity and is likely mediated by factor(s) secreted by erythroid precursors. GDF-15 was identified as a candidate for one of the erythroid factors suppressing hepcidin. Tissue hypoxia may also directly contribute to hepcidin suppression in anemias. Regulation of hepcidin by inflammation may include multiple cytokines and the Toll-like receptors pathways. Although it has not yet been shown that increased hepcidin is indispensible for the development of anemia of inflammation, transgenic overexpression of hepcidin was sufficient to replicate its key features.

SUMMARY

Regulation of hepcidin and iron availability for erythropoiesis has revealed unexpected pathways and much complexity. The renaissance of the study of iron regulation continues to reward researchers with interesting biology and potential therapeutic targets.

Intracellular iron transport and storage: from molecular mechanisms to health implications

Maintenance of proper "labile iron" levels is a critical component in preserving homeostasis. Iron is a vital element that is a constituent of a number of important macromolecules, including those involved in energy production, respiration, DNA synthesis, and metabolism; however, excess "labile iron" is potentially detrimental to the cell or organism or both because of its propensity to participate in oxidation-reduction reactions that generate harmful free radicals. Because of this dual nature, elaborate systems tightly control the concentration of available iron. Perturbation of normal physiologic iron concentrations may be both a cause and a consequence of cellular damage and disease states. This review highlights the molecular mechanisms responsible for regulation of iron absorption, transport, and storage through the roles of key regulatory proteins, including ferroportin, hepcidin, ferritin, and frataxin. In addition, we present an overview of the relation between iron regulation and oxidative stress and we discuss the role of functional iron overload in the pathogenesis of hemochromatosis, neurodegeneration, and inflammation.

The genetics of essential metal homeostasis during development

The essential metals copper, zinc, and iron play key roles in embryonic, fetal, and postnatal development in higher eukaryotes. Recent advances in our understanding of the molecules involved in the intricate control of the homeostasis of these metals and the availability of natural mutations and targeted mutations in many of the genes involved have allowed for elucidation of the diverse roles of these metals during development. Evidence suggests that the ability of the embryo to control the homeostasis of these metals becomes essential at the blastocyst stage and during early morphogenesis. However, these metals play unique roles throughout development and exert pleiotropic, metal-specific, and often cell-specific effects on morphogenesis, growth, and differentiation. Herein, we briefly review the major players known to be involved in the homeostasis of each of these essential metals and their known roles in development.

The role of Hfe in transferrin-bound iron uptake by hepatocytes

HFE-related hereditary hemochromatosis results in hepatic iron overload. Hepatocytes acquire transferrin-bound iron via transferrin receptor (Tfr) 1 and Tfr1-independent pathways (possibly Tfr2-mediated). In this study, the role of Hfe in the regulation of hepatic transferrin-bound iron uptake by these pathways was investigated using Hfe knockout mice. Iron and transferrin uptake by hepatocytes from Hfe knockout, non-iron-loaded and iron-loaded wild-type mice were measured after incubation with 50 nM (125)I-Tf-(59)Fe (Tfr1 pathway) and 5 microM (125)I-Tf-(59)Fe (Tfr1-independent or putative Tfr2 pathway). Tfr1 and Tfr2 messenger RNA (mRNA) and protein expression were measured by real-time polymerase chain reaction and western blotting, respectively. Tfr1-mediated iron and transferrin uptake by Hfe knockout hepatocytes were increased by 40% to 70% compared with iron-loaded wild-type hepatocytes with similar iron levels and Tfr1 expression. Iron and transferrin uptake by the Tfr1-independent pathway was approximately 100-fold greater than by the Tfr1 pathway and was not affected by the absence of Hfe. Diferric transferrin increased hepatocyte Tfr2 protein expression, resulting in a small increase in transferrin but not iron uptake by the Tfr1-independent pathway.

CONCLUSION

Tfr1-mediated iron uptake is regulated by Hfe in hepatocytes. The Tfr1-independent pathway exhibited a much greater capacity for iron uptake than the Tfr1 pathway but it was not regulated by Hfe. Diferric transferrin up-regulated hepatocyte Tfr2 protein expression but not iron uptake, suggesting that Tfr2 may have a limited role in the Tfr1-independent pathway.

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.

HFE association with transferrin receptor 2 increases cellular uptake of transferrin-bound iron

Mutations in either HFE or transferrin receptor 2 (TfR2) cause decreased expression of the iron regulatory hormone hepcidin and hemochromatosis. HFE and TfR2 were recently discovered to form a stable complex at the cell membrane when co-expressed in heterologous cell lines. We analyzed the functional consequences of the co-expression of these proteins using transfected TRVb cells, a Chinese hamster ovary derived cell line without endogenous HFE or transferrin receptor. The co-expression of TfR2 in TRVb cells expressing HFE led to accelerated HFE biosynthesis and late-Golgi maturation, suggesting interaction prior to cell surface localization. The co-expression of HFE in cells expressing TfR2 led to increased affinity for diferric transferrin, increased transferrin-dependent iron uptake, and relative resistance to iron chelation. These observations indicate that HFE influences the functional properties of TfR2, and suggests a model in which the interaction of these proteins might influence signal transduction to hepcidin.

The transferrin receptor modulates Hfe-dependent regulation of hepcidin expression

Hemochromatosis is caused by mutations in HFE, a protein that competes with transferrin (TF) for binding to transferrin receptor 1 (TFR1). We developed mutant mouse strains to gain insight into the role of the Hfe/Tfr1 complex in regulating iron homeostasis. We introduced mutations into a ubiquitously expressed Tfr1 transgene or the endogenous Tfr1 locus to promote or prevent the Hfe/Tfr1 interaction. Under conditions favoring a constitutive Hfe/Tfr1 interaction, mice developed iron overload attributable to inappropriately low expression of the hormone hepcidin. In contrast, mice carrying a mutation that interferes with the Hfe/Tfr1 interaction developed iron deficiency associated with inappropriately high hepcidin expression. High-level expression of a liver-specific Hfe transgene in Hfe-/- mice was also associated with increased hepcidin production and iron deficiency. Together, these models suggest that Hfe induces hepcidin expression when it is not in complex with Tfr1.

The regulation of cellular iron metabolism

While iron is an essential trace element required by nearly all living organisms, deficiencies or excesses can lead to pathological conditions such as iron deficiency anemia or hemochromatosis, respectively. A decade has passed since the discovery of the hemochromatosis gene, HFE, and our understanding of hereditary hemochromatosis (HH) and iron metabolism in health and a variety of diseases has progressed considerably. Although HFE-related hemochromatosis is the most widespread, other forms of HH have subsequently been identified. These forms are not attributed to mutations in the HFE gene but rather to mutations in genes involved in the transport, storage, and regulation of iron. This review is an overview of cellular iron metabolism and regulation, describing the function of key proteins involved in these processes, with particular emphasis on the liver's role in iron homeostasis, as it is the main target of iron deposition in pathological iron overload. Current knowledge on their roles in maintaining iron homeostasis and how their dysregulation leads to the pathogenesis of HH are discussed.

Defective trafficking and localization of mutated transferrin receptor 2: implications for type 3 hereditary hemochromatosis

Transferrin receptor 2 (TfR2), a homologue of transferrin receptor 1 (TfR1), is a key molecule involved in the regulation of iron homeostasis. Mutations in TfR2 result in iron overload with similar features to HFE-associated hereditary hemochromatosis. The precise role of TfR2 in iron metabolism and the functional consequences of disease-causing mutations have not been fully determined. We have expressed wild-type and various mutant forms of TfR2 that are associated with human disease in a mouse liver cell line. Intracellular and surface analysis shows that all the TfR2 mutations analyzed cause the intracellular retention of the protein in the endoplasmic reticulum, whereas the wild-type protein is expressed in endocytic structures and at the cell surface. Our results indicate that the majority of mutations that cause type 3 hereditary hemochromatosis are a consequence of the defective localization of the protein.

Transferrin receptor 2 mediates uptake of transferrin-bound and non-transferrin-bound iron

BACKGROUND/AIMS

Transferrin receptor 2 appears to have dual roles in iron metabolism; one is signalling, the other is iron transport. It is sensitive to high levels of diferric transferrin, which is associated with disorders of iron overload. Also present in these disorders are increased levels of plasma non-transferrin-bound iron. This study sought to clarify the role of transferrin receptor 2 in the uptake of transferrin-bound and non-transferrin-bound iron.

METHODS

Variant Chinese Hamster Ovary (CHO) cells, transfected with transferrin receptor 2, were incubated with radio-labelled transferrin-bound or non-transferrin-bound iron. Competition studies were performed in the presence of unlabelled dimetallic transferrin; knockdown was performed using specific siRNA.

RESULTS

Cells expressing transferrin receptor 2 bound and internalised transferrin and transferrin-bound iron. Transferrin recycling occurred with an average cycling time of 11-15 min. Interestingly, the presence of transferrin receptor 2 was also associated with uptake of non-transferrin-bound iron which was inhibited by unlabelled transferrin-bound metals. Knockdown reduced transferrin-bound and non-transferrin-bound iron uptake by approximately 60%.

CONCLUSIONS

Transferrin receptor 2 mediates transferrin-bound iron uptake by receptor-mediated endocytosis. It is also involved in the uptake of non-transferrin-bound iron and the inhibition of non-transferrin-bound iron uptake by diferric transferrin in CHO cells.

Regulation of iron acquisition and storage: consequences for iron-linked disorders

Mammalian iron homeostasis must be meticulously regulated so that this essential element is available for use, but at the same time prevented from promoting the formation of toxic radicals. Controlling the entry of iron into blood plasma is the main mechanism by which iron stores in the body are physiologically manipulated and regulated. Defects in iron acquisition at the cellular and systemic levels lead to human disorders, which involve either iron overload or iron deficiency. Discoveries of iron transporters and insights into their regulation have provided important information about iron metabolism and genetic iron disorders.

Iron homeostasis and erythropoiesis

Erythrocytes require iron to perform their duty as oxygen carriers. Mammals have evolved a mechanism to maintain systemic iron within an optimal range that fosters erythroid development and function while satisfying other body iron needs. This chapter reviews erythroid iron uptake and utilization as well as systemic factors that influence iron availability. One of these factors is hepcidin, a circulating peptide hormone that maintains iron homeostasis. Elevated levels of hepcidin in the bloodstream effectively shut off iron absorption by disabling the iron exporter ferroportin. Conversely, low levels of circulating hepcidin allow ferroportin to export iron into the bloodstream. Aberrations in hepcidin expression or responsiveness to hepcidin result in disorders of iron deficiency and iron overload. It is clear that erythroid precursors communicate their iron needs to the liver to influence the production of hepcidin and thus the amount of iron available for use. However, the mechanism by which erythroid cells accomplish this remains unclear and is an area of active investigation.

The Regulation of Hepcidin and Its Effects on Systemic and Cellular Iron Metabolism

Systemic iron homeostasis depends on the regulated expression of hepcidin, a peptide hormone that negatively regulates iron egress from intestinal cells and macrophages by altering the expression of the cellular iron exporter ferroportin. In doing so, hepcidin can control both the total body iron by modulating intestinal iron absorption as well as promote iron available for erythropoiesis by affecting the efficiency with which macrophages recycle iron from effete red blood cells. This review focuses on the systemic and cellular physiology of hepcidin regulation in relation to iron stores, erythropoiesis, inflammation, and hypoxia and how hepcidin regulation and dysregulation contributes to normal iron homeostasis and iron metabolism disorders.

Expression of the CTCF-paralogous cancer-testis gene, brother of the regulator of imprinted sites (BORIS), is regulated by three alternative promoters modulated by CpG methylation and by CTCF and p53 transcription factors

BORIS, like other members of the ‘cancer/testis antigen’ family, is normally expressed in testicular germ cells and repressed in somatic cells, but is aberrantly activated in cancers. To understand regulatory mechanisms governing human BORIS expression, we characterized its 5′-flanking region. Using 5′ RACE, we identified three promoters, designated A, B and C, corresponding to transcription start sites at −1447, −899 and −658 bp upstream of the first ATG. Alternative promoter usage generated at least five alternatively spliced BORIS mRNAs with different half-lives determined by varying 5′-UTRs. In normal testis, BORIS is transcribed from all three promoters, but 84% of the 30 cancer cell lines tested used only promoter(s) A and/or C while the others utilized primarily promoters B and C. The differences in promoter usage between normal and cancer cells suggested that they were subject to differential regulation. We found that DNA methylation and functional p53 contributes to the negative regulation of each promoter. Moreover, reduction of CTCF in normally BORIS-negative human fibroblasts resulted in derepression of BORIS promoters. These results provide a mechanistic basis for understanding cancer-related associations between haploinsufficiency of CTCF and BORIS derepression, and between the lack of functional p53 and aberrant activation of BORIS.

Liver-gut axis in the regulation of iron homeostasis

The human body requires about 1-2 mg of iron per day for its normal functioning, and dietary iron is the only source for this essential metal. Since humans do not possess a mechanism for the active excretion of iron, the amount of iron in the body is determined by the amount absorbed across the proximal small intestine and, consequently, intestinal iron absorption is a highly regulated process. In recent years, the liver has emerged as a central regulator of both iron absorption and iron release from other tissues. It achieves this by secreting a peptide hormone called hepcidin that acts on the small intestinal epithelium and other cells to limit iron delivery to the plasma. Hepcidin itself is regulated in response to various systemic stimuli including variations in body iron stores, the rate of erythropoiesis, inflammation and hypoxia, the same stimuli that have been known for many years to modulate iron absorption. This review will summarize recent findings on the role played by the liver and hepcidin in the regulation of body iron absorption.

Liver iron transport

The liver plays a central role in iron metabolism. It is the major storage site for iron and also expresses a complex range of molecules which are involved in iron transport and regulation of iron homeostasis. An increasing number of genes associated with hepatic iron transport or regulation have been identified. These include transferrin receptors (TFR1 and 2), a ferrireductase (STEAP3), the transporters divalent metal transporter-1 (DMT1) and ferroportin (FPN) as well as the haemochromatosis protein, HFE and haemojuvelin (HJV), which are signalling molecules. Many of these genes also participate in iron regulatory pathways which focus on the hepatic peptide hepcidin. However, we are still only beginning to understand the complex interactions between liver iron transport and iron homeostasis. This review outlines our current knowledge of molecules of iron metabolism and their roles in iron transport and regulation of iron homeostasis.

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.

Transferrin receptor 2 is frequently expressed in human cancer cell lines

Different proteins ensure the fine control of iron metabolism at the level of various tissues. Among these proteins, it was discovered a second transferrin receptor (TfR2), that seems to play a key role in the regulation of iron homeostasis. Its mutations are responsible for type 3 hemochromatosis (Type 3 HH). Although TfR2 expression in normal tissues was restricted at the level of liver and intestine, we observed that TfR2 was frequently expressed in tumor cell lines. Particularly frequent was its expression in ovarian cancer, colon cancer and glioblastoma cell lines; less frequent was its expression in leukemic and melanoma cell lines. Interestingly, in these tumor cell lines, TfR2 expression was inversely related to that of receptor 1 for transferrin (TfR1). Experiments of in vitro iron loading or iron deprivation provided evidence that TfR2 is modulated in cancer cell lines according to cellular iron levels following two different mechanisms: (i) in some cells, iron loading caused a downmodulation of total TfR2 levels; (ii) in other cell types, iron loading caused a downmodulation of membrane-bound TfR2, without affecting the levels of total cellular TfR2 content. Iron deprivation caused in both conditions an opposite effect compared to iron loading. These observations suggest that TfR2 expression may be altered in human cancers and warrant further studies in primary tumors. Furthermore, our studies indicate that, at least in tumor cells, TfR2 expression is modulated by iron through different biochemical mechanisms, whose molecular basis remains to be determined.

Iron transferrin regulates hepcidin synthesis in primary hepatocyte culture through hemojuvelin and BMP2/4

The peptide hormone hepcidin is the principal regulator of systemic iron homeostasis. We examined the pathway by which iron stimulates the production of hepcidin. In humans who ingested 65 mg of iron, the increase in transferrin saturation preceded by hours the increase in urinary hepcidin excretion. Increases in urinary hepcidin concentrations were proportional to the increment in transferrin saturation. Paradoxically, in previous studies in primary hepatocytes and cell lines, hepcidin response to iron or iron transferrin was not observed. We now report that freshly isolated murine primary hepatocytes responded to holotransferrin but not apotransferrin by increasing hepcidin mRNA. Hepcidin increase was not due to contamination of the transferrin preparations by endotoxin, a potent pathologic stimulus of hepcidin synthesis. Using this culture system, we showed that holotransferrin concentrations regulate hepcidin mRNA concentrations through a hemojuvelin/BMP2/4-dependent pathway. Although BMP9 is known to be expressed in the liver and potently increased the basal concentrations of hepcidin mRNA, it did not interact with hemojuvelin, and interference with its signaling pathway did not affect iron regulation. Fresh primary hepatocytes constitute a sufficient system for the regulation of hepcidin by physiologic iron stimuli and will greatly facilitate studies of major disorders of iron homeostasis.

Iron homeostasis

Iron is needed by all mammalian cells but is toxic in excess. Specialized transport mechanisms conduct iron across cellular membranes. These are regulated to ensure homeostasis both systemically in living organisms and within individual cells. Over the past decade, major advances have been made in identifying and characterizing the proteins involved in the transport, handling, and homeostatic regulation of iron. Molecular understanding of these processes has provided important insights into the pathophysiology of human iron disorders.

The Cytoplasmic Domain of Transferrin Receptor 2 Dictates Its Stability and Response to Holo-transferrin in Hep3B Cells

Transferrin receptor 2 (TfR2) is a homolog of transferrin receptor 1 (TfR1), the receptor responsible for the uptake of iron-loaded transferrin (holo-Tf) into cells. Unlike the ubiquitous TfR1, TfR2 is predominantly expressed in the liver. Mutations in TfR2 gene cause a rare autosomal recessive form of the iron overload disease, hereditary hemochromatosis. Previous studies demonstrated that holo-Tf increases TfR2 levels by stabilizing TfR2 at the protein level. In this study we constructed two chimeras, one of which had the cytoplasmic domain of TfR2 and the remaining portion of TfR1 and the other with the cytoplasmic and transmembrane domain of TfR1 joined to the ectodomain of TfR2. Similar to TfR2, the levels of the chimera containing only the cytoplasmic domain of TfR2 increased in a time- and dose-dependent manner after the addition of holo-Tf to the medium. The half-life of the chimera increased 2.7-fold in cells exposed to holo-Tf like the endogenous TfR2 in HepG2 cells. Like TfR2 and unlike TfR1, the levels of the chimera did not respond to intracellular iron content. These results suggest that although holo-Tf binding to the ectodomain is necessary, the cytoplasmic domain of TfR2 is largely responsible for its stabilization by holo-Tf.

Transferrin receptor 2 is emerging as a major player in the control of iron metabolism

Our knowledge of mammalian iron metabolism has advanced dramatically over recent years. Iron is an essential element for virtually all living organisms. Its intestinal absorption and accurate cellular regulation is strictly required to ensure the coordinated synthesis of the numerous iron-containing proteins involved in key metabolic processes, while avoiding the uptake of excess iron that can lead to organ damage. A range of different proteins exist to ensure this fine control within the various tissues of the body. Among these proteins, transferrin receptor (TFR2) seems to play a key role in the regulation of iron homeostasis. Disabling mutations in TFR2 are responsible for type 3 hereditary hemochromatosis (Type 3 HH). This review describes the biological properties of this membrane receptor, with a particular emphasis paid to the structure, function and cellular localization. Although much information has been garnered on TFR2, further efforts are needed to elucidate its function in the context of the iron regulatory network.

Evidence that inhibition of hemojuvelin shedding in response to iron is mediated through neogenin

Hemojuvelin (HJV), encoded by the gene HFE2, is a critical upstream regulator of hepcidin expression. Hepcidin, the central iron regulatory hormone, is secreted from hepatocytes, whereas HFE2 is highly expressed in skeletal muscle and liver. Previous studies demonstrated that HJV is a GPI-anchored protein, binds the proteins neogenin and bone morphogenetic proteins (BMP2 and BMP4), and can be released from the cell membrane (shedding). In this study, we investigated the physiological significance and the underlying mechanism of HJV shedding. In acutely iron-deficient rats with markedly suppressed hepatic hepcidin expression, we detected an early phase increase of serum HJV with no significant change of either HFE2 mRNA or protein levels in gastrocnemius muscle. Studies in both C2C12 (a mouse myoblast cell line) and HepG2 (a human hepatoma cell line) cells showed active HJV shedding, implying that both skeletal muscle and liver could be the source of serum HJV. In agreement with the observations in iron-deficient rats, HJV shedding in these cell lines was down-regulated by holo-transferrin in a concentration-dependent manner. Our present study showing that knock-down of endogenous neogenin, a HJV receptor, in C2C12 cells suppresses HJV shedding and that overexpression of neogenin in HEK293 cells markedly enhances this process, suggests that membrane HJV shedding is mediated by neogenin. The finding that neither BMP4 nor its antagonist, noggin, was able to alter HJV shedding support the lack of involvement of BMP signaling pathway in this process.

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.

Hepcidin in Anemia and Inflammation in Chronic Kidney Disease

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

Iron absorption and hepatic iron uptake are increased in a transferrin receptor 2 (Y245X) mutant mouse model of hemochromatosis type 3

Hereditary hemochromatosis type 3 is an iron (Fe)-overload disorder caused by mutations in transferrin receptor 2 (TfR2). TfR2 is expressed highly in the liver and regulates Fe metabolism. The aim of this study was to investigate duodenal Fe absorption and hepatic Fe uptake in a TfR2 (Y245X) mutant mouse model of hereditary hemochromatosis type 3. Duodenal Fe absorption and hepatic Fe uptake were measured in vivo by 59Fe-labeled ascorbate in TfR2 mutant mice, wild-type mice, and Fe-loaded wild-type mice (2% dietary carbonyl Fe). Gene expression was measured by real-time RT-PCR. Liver nonheme Fe concentration increased progressively with age in TfR2 mutant mice compared with wild-type mice. Fe absorption (both duodenal Fe uptake and transfer) was increased in TfR2 mutant mice compared with wild-type mice. Likewise, expression of genes participating in duodenal Fe uptake (Dcytb, DMT1) and transfer (ferroportin) were increased in TfR2 mutant mice. Nearly all of the absorbed Fe was taken up rapidly by the liver. Despite hepatic Fe loading, hepcidin expression was decreased in TfR2 mutant mice compared with wild-type mice. Even when compared with Fe-loaded wild-type mice, TfR2 mutant mice had increased Fe absorption, increased duodenal Fe transport gene expression, increased liver Fe uptake, and decreased liver hepcidin expression. In conclusion, despite systemic Fe loading, Fe absorption and liver Fe uptake were increased in TfR2 mutant mice in association with decreased expression of hepcidin. These findings support a model in which TfR2 is a sensor of Fe status and regulates duodenal Fe absorption and liver Fe uptake.

Targeted disruption of the hepatic transferrin receptor 2 gene in mice leads to iron overload

BACKGROUND & AIMS

Transferrin receptor 2 (TfR2) plays a key role in the regulation of iron metabolism. Mutations of TfR2 in humans cause type 3 hereditary hemochromatosis. Although highly expressed in liver, several studies have reported TfR2 expression in other tissues. To determine the contribution of liver expressed TfR2 in iron homeostasis, we have generated and characterized a liver-specific TfR2-knockout (KO) mouse.

METHODS

Liver-specific TfR2-KO mice were generated by crossing TfR2-floxed mice with transgenic albumin-Cre mice. Tissue and serum from homozygous TfR2-floxed mice with and without albumin-Cre were analyzed. Serum transferrin saturation, hepatic, and splenic iron concentrations were determined. The expression of iron-related mRNA transcripts was analyzed by real-time PCR. Levels of the iron-related proteins TfR1, TfR2, ferritin, and prohepcidin were analyzed by immunoblotting.

RESULTS

Liver-specific TfR2-KO mice develop significant iron overload comparable to complete TfR2-KO mice. At all ages studied, transferrin saturation, hepatic iron concentration, and hepatic ferritin were significantly elevated. Hepatic TfR2 mRNA and protein were absent in the livers of liver-specific TfR2-KO mice, and TfR1 expression was reduced consistent with liver iron loading. At 5 weeks of age, hepcidin1 mRNA, and prohepcidin protein were decreased in liver-specific TfR2-KO compared to control mice.

CONCLUSIONS

The significant iron loading and modulation of expression of iron-related genes in liver-specific TfR2-KO mice demonstrates that the liver is the primary site for TfR2 expression and activity and that liver-expressed TfR2 is required for the regulation of hepcidin1.

Hepcidin ? central regulator of iron metabolism

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

Transferrin receptor 2: evidence for ligand-induced stabilization and redirection to a recycling pathway

Transferrin receptor 2 (TfR2) is a homologue of transferrin receptor 1 (TfR1), the protein that delivers iron to cells through receptor-mediated endocytosis of diferric transferrin (Fe(2)Tf). TfR2 also binds Fe(2)Tf, but it seems to function primarily in the regulation of systemic iron homeostasis. In contrast to TfR1, the trafficking of TfR2 within the cell has not been extensively characterized. Previously, we showed that Fe(2)Tf increases TfR2 stability, suggesting that trafficking of TfR2 may be regulated by interaction with its ligand. In the present study, therefore, we sought to identify the mode of TfR2 degradation, to characterize TfR2 trafficking, and to determine how Fe(2)Tf stabilizes TfR2. Stabilization of TfR2 by bafilomycin implies that TfR2 traffics to the lysosome for degradation. Confocal microscopy reveals that treatment of cells with Fe(2)Tf increases the fraction of TfR2 localizing to recycling endosomes and decreases the fraction of TfR2 localizing to late endosomes. Mutational analysis of TfR2 shows that the mutation G679A, which blocks TfR2 binding to Fe(2)Tf, increases the rate of receptor turnover and prevents stabilization by Fe(2)Tf, indicating a direct role of Fe(2)Tf in TfR2 stabilization. The mutation Y23A in the cytoplasmic domain of TfR2 inhibits its internalization and degradation, implicating YQRV as an endocytic motif.

Inhibition of transferrin iron release increases in vitro drug carrier efficacy

Transferrin (Tf) conjugates of CRM107 are currently being tested in clinical trials for treatment of malignant gliomas. However, the rapid cellular recycling of Tf limits its efficiency as a drug carrier. We have developed a mathematical model of the Tf/TfR trafficking cycle and have identified the Tf iron release rate as a previously unreported factor governing the degree of Tf cellular association. The release of iron from Tf is inhibited by replacing the synergistic carbonate anion with oxalate. Trafficking patterns for oxalate Tf and native Tf are compared by measuring their cellular association with HeLa cells. The amount of Tf associated with the cells is an average of 51% greater for oxalate Tf than for native Tf over a two hour period at Tf concentrations of 0.1 nM and 1 nM. Importantly, diphtheria toxin (DT) conjugates of oxalate Tf are more cytotoxic against HeLa cells than conjugates of native Tf. Conjugate IC(50) values were determined to be 0.06 nM for the oxalate Tf conjugate vs. 0.22 nM for the native Tf conjugate. Thus, we show that inhibition of Tf iron release improves the efficacy of Tf as a drug carrier through increased association with cells expressing TfR.

TfR2 localizes in lipid raft domains and is released in exosomes to activate signal transduction along the MAPK pathway

Transferrin receptor 2 (TfR2) possesses a YQRV motif similar to the YTRF motif of transferrin receptor 1 (TfR1) responsible for the internalization and secretion through the endosomal pathway. Raft biochemical dissection showed that TfR2 is a component of the low-density Triton-insoluble (LDTI) plasma membrane domain, able to co-immunoprecipitate with caveolin-1 and CD81, two structural raft proteins. In addition, subcellular fractionation experiments showed that TfR1, which spontaneously undergoes endocytosis and recycling, largely distributed to intracellular organelles, whereas TfR2 was mainly associated with the plasma membrane. Given the TfR2 localization in lipid rafts, we tested its capability to activate cell signalling. Interaction with an anti-TfR2 antibody or with human or bovine holotransferrin showed that it activated ERK1/ERK2 and p38 MAP kinases. Integrity of lipid rafts was required for MAPK activation. Co-localization of TfR2 with CD81, a raft tetraspanin exported through exosomes, prompted us to investigate exosomes released by HepG2 and K562 cells into culture medium. TfR2, CD81 and to a lesser extent caveolin-1, were found to be part of the exosomal budding vesicles. In conclusion, the present study indicates that TfR2 localizes in LDTI microdomains, where it promotes cell signalling, and is exported out of the cells through the exosome pathway, where it acts as an intercellular messenger.