Mechanisms controlling cellular and systemic iron homeostasis.

In mammals, hundreds of proteins use iron in a multitude of cellular functions, including vital processes such as mitochondrial respiration, gene regulation and DNA synthesis or repair. Highly orchestrated regulatory systems control cellular and systemic iron fluxes ensuring sufficient iron delivery to target proteins is maintained, while limiting its potentially deleterious effects in iron-mediated oxidative cell damage and ferroptosis. In this Review, we discuss how cells acquire, traffick and export iron and how stored iron is mobilized for iron-sulfur cluster and haem biogenesis. Furthermore, we describe how these cellular processes are fine-tuned by the combination of various sensory and regulatory systems, such as the iron-regulatory protein (IRP)-iron-responsive element (IRE) network, the nuclear receptor co-activator 4 (NCOA4)-mediated ferritinophagy pathway, the prolyl hydroxylase domain (PHD)-hypoxia-inducible factor (HIF) axis or the nuclear factor erythroid 2-related factor 2 (NRF2) regulatory hub. We further describe how these pathways interact with systemic iron homeostasis control through the hepcidin-ferroportin axis to ensure appropriate iron fluxes. This knowledge is key for the identification of novel therapeutic opportunities to prevent diseases of cellular and/or systemic iron mismanagement.

Using Biotinylated Iron-Responsive Element to Analyze the Activity of Iron Regulatory Proteins.

Iron regulatory proteins (IRP1 and IRP2) are the master regulators of mammalian iron homeostasis. They bind to the iron-responsive elements (IREs) of the transcripts of iron-related genes to regulate their expression, thereby maintaining cellular iron availability. The primary method to measure the IRE-binding activity of IRPs is the electrophoresis mobility shift assay (EMSA). This method is particularly useful for evaluating IRP1 activity, since IRP1 is a bifunctional enzyme and its protein levels remain similar during conversion between the IRE-binding protein and cytosolic aconitase forms. Here, we exploited a method of using a biotinylated-IRE probe to separate IRE-binding IRPs followed by immunoblotting to analyze the IRE-binding activity. This method allows for the successful measurement of IRP activity in cultured cells and mouse tissues under various iron conditions. By separating IRE-binding IRPs from the rest of the lysates, this method increases the specificity of IRP antibodies and verifies whether a band represents an IRP, thereby revealing some previously unrecognized information about IRPs. With this method, we showed that the S711-phosphorylated IRP1 was found only in the IRE-binding form in PMA-treated Hep3B cells. Second, we found a truncated IRE-binding IRP2 isoform that is generated by proteolytic cleavage on sites in the 73aa insert region of the IRP2 protein. Third, we found that higher levels of SDS, compared to 1-2% SDS in regular loading buffer, could dramatically increase the band intensity of IRPs in immunoblots, especially in HL-60 cells. Fourth, we found that the addition of SDS or LDS to cell lysates activated protein degradation at 37 °C or room temperature, especially in HL-60 cell lysates. As this method is more practical, sensitive, and cost-effective, we believe that its application will enhance future research on iron regulation and metabolism.

Root-to-shoot iron partitioning in Arabidopsis requires IRON-REGULATED TRANSPORTER1 (IRT1) protein but not its iron(II) transport function.

IRON-REGULATED TRANSPORTER1 (IRT1) is the root high-affinity ferrous iron (Fe) uptake system and indispensable for the completion of the life cycle of Arabidopsis thaliana without vigorous Fe supplementation. Here we provide evidence supporting a second role of IRT1 in root-to-shoot partitioning of Fe. We show that irt1 mutants overaccumulate Fe in roots, most prominently in the cortex of the differentiation zone in irt1-2, compared to the wild type. Shoots of irt1-2 are severely Fe-deficient according to Fe content and marker transcripts, as expected. We generated irt1-2 lines producing IRT1 mutant variants carrying single amino-acid substitutions of key residues in transmembrane helices IV and V, Ser206 and His232, which are required for transport activity in yeast. Root short-term 55 Fe uptake rates were uninformative concerning IRT1-mediated transport. Overall irt1-like concentrations of the secondary substrate Mn suggested that the transgenic Arabidopsis lines also remain incapable of IRT1-mediated root Fe uptake. Yet, IRT1S206A partially complements rosette dwarfing and leaf chlorosis of irt1-2, as well as root-to-shoot Fe partitioning and gene expression defects of irt1-2, all of which are fully complemented by wild-type IRT1. Taken together, these results suggest a regulatory function for IRT1 in root-to-shoot Fe partitioning that does not require Fe transport activity of IRT1. Among the genes of which transcript levels are partially dependent on IRT1, we identify MYB DOMAIN PROTEIN10, MYB DOMAIN PROTEIN72 and NICOTIANAMINE SYNTHASE4 as candidates for effecting IRT1-dependent Fe mobilization in roots. Understanding the biological functions of IRT1 will help to improve Fe nutrition and the nutritional quality of agricultural crops.

Prospective role and immunotherapeutic targets of sideroflexin protein family in lung adenocarcinoma: evidence from bioinformatics validation.

As lung cancer remains the leading cause of cancer deaths globally, characterizing the tumor molecular profiles is crucial to tailoring treatments for individuals at advanced stages. Cancer cells exhibit strong dependence on iron for their proliferation, and several iron-regulatory proteins have been proposed as either oncogenes or tumor suppressive genes. This study aims to evaluate the prospective therapeutic and prognostic values of the sideroflexin (SFXN) gene family, whose functions involve mitochondrial iron metabolism, in lung adenocarcinoma (LUAD). Differential expression analysis using TIMER and UALCAN tools was first employed to compare SFXNs expression levels between normal and LUAD tissues. Next, SFXNs' prognostic values, biological significance, and potential as immunotherapy candidates were examined from GEPIA, cBioPortal, MetaCore, Cytoscape, and TIMER databases. It was found that all members of SFXN family, except SFXN3, were differentially expressed in LUAD compared to normal samples and within different stages of LUAD. Survival analysis then revealed SFXN1 to be related to worse overall survival outcome in patients with LUAD. Furthermore, several correlations between expression of SFXN1 and immune infiltration cells were discovered. To conclude, our study provides evidence of SFXN family gene's relevance to the prognosis and immunotherapeutic targets of LUAD.

Multimodal approaches for the improvement of the cellular folding of a recombinant iron regulatory protein in E. coli.

BACKGROUND: During the recombinant protein expression, most heterologous proteins expressed in E. coli cell factories are generated as insoluble and inactive aggregates, which prohibit E. coli from being employed as an expression host despite its numerous advantages and ease of use. The yeast mitochondrial aconitase protein, which has a tendency to aggregate when expressed in E. coli cells in the absence of heterologous chaperones GroEL/ES was utilised as a model to investigate how the modulation of physiological stimuli in the host cell can increase protein solubility. The presence of folding modulators such as exogenous molecular chaperones or osmolytes, as well as process variables such as incubation temperature, inducer concentrations, growth media are all important for cellular folding and are investigated in this study. This study also investigated how the cell's stress response system activates and protects the proteins from aggregation. RESULTS: The cells exposed to osmolytes plus a pre-induction heat shock showed a substantial increase in recombinant aconitase activity when combined with modulation of process conditions. The concomitant GroEL/ES expression further assists the folding of these soluble aggregates and increases the functional protein molecules in the cytoplasm of the recombinant E. coli cells. CONCLUSIONS: The recombinant E. coli cells enduring physiological stress provide a cytosolic environment for the enhancement in the solubility and activity of the recombinant proteins. GroEL/ES-expressing cells not only aided in the folding of recombinant proteins, but also had an effect on the physiology of the expression host. The improvement in the specific growth rate and aconitase production during chaperone GroEL/ES co-expression is attributed to the reduction in overall cellular stress caused by the expression host's aggregation-prone recombinant protein expression.

Iron, Neuroinflammation and Neurodegeneration.

Disturbance of the brain homeostasis, either directly via the formation of abnormal proteins or cerebral hypo-perfusion, or indirectly via peripheral inflammation, will activate microglia to synthesise a variety of pro-inflammatory agents which may lead to inflammation and cell death. The pro-inflammatory cytokines will induce changes in the iron proteins responsible for maintaining iron homeostasis, such that increased amounts of iron will be deposited in cells in the brain. The generation of reactive oxygen and nitrogen species, which is directly involved in the inflammatory process, can significantly affect iron metabolism via their interaction with iron-regulatory proteins (IRPs). This underlies the importance of ensuring that iron is maintained in a form that can be kept under control; hence, the elegant mechanisms which have become increasingly well understood for regulating iron homeostasis. Therapeutic approaches to minimise the toxicity of iron include N-acetyl cysteine, non-steroidal anti-inflammatory compounds and iron chelation.

Expression patterns of iron regulatory proteins after intense light exposure in a cone-dominated retina.

Iron is implicated in ocular diseases such as in age-related macular degeneration. Light is also considered as a pathological factor in this disease. Earlier, two studies reported the influence of constant light environment on the pattern of expressions of iron-handling proteins. Here, we aimed to see the influence of light in 12-h light-12-h dark (12L:12D) cycles on the expression of iron-handling proteins in chick retina. Chicks were exposed to 400 lx (control) and 5000 lx (experimental) light at 12L:12D cycles and sacrificed at variable timepoints. Retinal ferrous ion (Fe2+) level, ultrastructural changes, lipid peroxidation level, immunolocalization and expression patterns of iron-handling proteins were analysed after light exposure. Both total Fe2+ level (p = 0.0004) and lipid peroxidation (p = 0.002) significantly increased at 12-, 48- and 168-h timepoint (for Fe2+) and 48- and 168-h timepoint (for lipid peroxidation), and there were degenerative retinal changes after 168 h of light exposure. Intense light exposure led to an increase in the levels of transferrin and transferrin receptor-1 (at 168-h) and ferroportin-1, whereas the levels of ferritins, hephaestin, (at 24-, 48- and 168-h timepoint) and ceruloplasmin (at 168-h timepoint) were decreased. These changes in iron-handling proteins after light exposure are likely due to a disturbance in the iron storage pool evident from decreased ferritin levels, which would result in increased intracellular Fe2+ levels. To counteract this, Fe2+ is released into the extracellular space, an observation supported by increased expression of ferroportin-1. Ceruloplasmin was able to convert Fe2+ into Fe3+ until 48 h of light exposure, but its decreased expression with time (at 168-h timepoint) resulted in increased extracellular Fe2+ that might have caused oxidative stress and retinal cell damage.

Cell-type-specific insights into iron regulatory processes.

Despite its essential role in many biological processes, iron is toxic when in excess due to its propensity to generate reactive oxygen species. To prevent diseases associated with iron deficiency or iron loading, iron homeostasis must be tightly controlled. Intracellular iron content is regulated by the Iron Regulatory Element-Iron Regulatory Protein (IRE-IRP) system, whereas systemic iron availability is adjusted to body iron needs chiefly by the hepcidin-ferroportin (FPN) axis. Here, we aimed to review advances in the field that shed light on cell-type-specific regulatory mechanisms that control or modify systemic and local iron balance, and how shifts in cellular iron levels may affect specialized cell functions.

Iron-responsive-like elements and neurodegenerative ferroptosis.

A set of common-acting iron-responsive 5'untranslated region (5'UTR) motifs can fold into RNA stem loops that appear significant to the biology of cognitive declines of Parkinson's disease dementia (PDD), Lewy body dementia (LDD), and Alzheimer's disease (AD). Neurodegenerative diseases exhibit perturbations of iron homeostasis in defined brain subregions over characteristic time intervals of progression. While misfolding of Aß from the amyloid-precursor-protein (APP), alpha-synuclein, prion protein (PrP) each cause neuropathic protein inclusions in the brain subregions, iron-responsive-like element (IRE-like) RNA stem-loops reside in their transcripts. APP and αsyn have a role in iron transport while gene duplications elevate the expression of their products to cause rare familial cases of AD and PDD. Of note, IRE-like sequences are responsive to excesses of brain iron in a potential feedback loop to accelerate neuronal ferroptosis and cognitive declines as well as amyloidosis. This pathogenic feedback is consistent with the translational control of the iron storage protein ferritin. We discuss how the IRE-like RNA motifs in the 5'UTRs of APP, alpha-synuclein and PrP mRNAs represent uniquely folded drug targets for therapies to prevent perturbed iron homeostasis that accelerates AD, PD, PD dementia (PDD) and Lewy body dementia, thus preventing cognitive deficits. Inhibition of alpha-synuclein translation is an option to block manganese toxicity associated with early childhood cognitive problems and manganism while Pb toxicity is epigenetically associated with attention deficit and later-stage AD. Pathologies of heavy metal toxicity centered on an embargo of iron export may be treated with activators of APP and ferritin and inhibitors of alpha-synuclein translation.

SMAD family member 3 (SMAD3) and SMAD4 repress HIF2α-dependent iron-regulatory genes.

Hypoxia-inducible factor 2α (HIF2α) directly regulates a battery of genes essential for intestinal iron absorption. Interestingly, iron deficiency and overload disorders do not result in increased intestinal expression of glycolytic or angiogenic HIF2α target genes. Similarly, inflammatory and tumor foci can induce a distinct subset of HIF2α target genes in vivo These observations indicate that different stimuli activate distinct subsets of HIF2α target genes via mechanisms that remain unclear. Here, we conducted a high-throughput siRNA-based screen to identify genes that regulate HIF2α's transcriptional activity on the promoter of the iron transporter gene divalent metal transporter-1 (DMT1). SMAD family member 3 (SMAD3) and SMAD4 were identified as potential transcriptional repressors. Further analysis revealed that SMAD4 signaling selectively represses iron-absorptive gene promoters but not the inflammatory or glycolytic HIF2α or HIF1α target genes. Moreover, the highly homologous SMAD2 did not alter HIF2α transcriptional activity. During iron deficiency, SMAD3 and SMAD4 expression was significantly decreased via proteasomal degradation, allowing for derepression of iron target genes. Several iron-regulatory genes contain a SMAD-binding element (SBE) in their proximal promoters; however, mutation of the putative SBE on the DMT1 promoter did not alter the repressive function of SMAD3 or SMAD4. Importantly, the transcription factor forkhead box protein A1 (FOXA1) was critical in SMAD4-induced DMT1 repression, and DNA binding of SMAD4 was essential for the repression of HIF2α activity, suggesting an indirect repressive mechanism through DNA binding. These results provide mechanistic clues to how HIF signaling can be regulated by different cellular cues.

Role of a 21-kDa iron-regulated protein IrpA in the uptake of ferri-exochelin by Mycobacterium smegmatis.

AIMS: To characterize the 21-kDa iron-regulated cell wall protein in Mycobacterium smegmatis co-expressed with the siderophores mycobactin, exochelin and carboxymycobactin upon iron limitation. METHODS AND RESULTS: Mycobacterium smegmatis, grown in the presence of 0·02 µg Fe ml-1 (low iron) produced high levels of all the three siderophores, which were repressed in bacteria supplemented with 8 µg Fe ml-1 (high iron). Exochelin, the major extracellular siderophore was the first to rise and was expressed at high levels during log phase of growth. Carboxymycobactin, a minor component in log phase iron-starved M. smegmatis continued to rise when cultured for longer periods, reaching levels greater than exochelin. Iron-starved bacteria expressed a 21-kDa iron-regulated protein (IrpA) that was identified as Clp protease subunit (MSMEG_3671) and characterized as a receptor for ferri-exochelin. CONCLUSIONS: Ferri-exochelin is the preferred siderophore in M. smegmatis and this ferri-exochelin: IrpA machinery is absent in Mycobacterium tuberculosis. SIGNIFICANCE AND IMPACT OF THE STUDY: Exochelin machinery is functional in M. smegmatis and the carboxymycobactin-mycobactin machinery is the sole iron uptake system in M. tuberculosis. The absence of the ferri-exochelin: IrpA system in the pathogen signifies the importance of the carboxymycobactin-mycobactin system machinery in M. tuberculosis.

Structure of human Fe-S assembly subcomplex reveals unexpected cysteine desulfurase architecture and acyl-ACP-ISD11 interactions.

In eukaryotes, sulfur is mobilized for incorporation into multiple biosynthetic pathways by a cysteine desulfurase complex that consists of a catalytic subunit (NFS1), LYR protein (ISD11), and acyl carrier protein (ACP). This NFS1-ISD11-ACP (SDA) complex forms the core of the iron-sulfur (Fe-S) assembly complex and associates with assembly proteins ISCU2, frataxin (FXN), and ferredoxin to synthesize Fe-S clusters. Here we present crystallographic and electron microscopic structures of the SDA complex coupled to enzyme kinetic and cell-based studies to provide structure-function properties of a mitochondrial cysteine desulfurase. Unlike prokaryotic cysteine desulfurases, the SDA structure adopts an unexpected architecture in which a pair of ISD11 subunits form the dimeric core of the SDA complex, which clarifies the critical role of ISD11 in eukaryotic assemblies. The different quaternary structure results in an incompletely formed substrate channel and solvent-exposed pyridoxal 5'-phosphate cofactor and provides a rationale for the allosteric activator function of FXN in eukaryotic systems. The structure also reveals the 4'-phosphopantetheine-conjugated acyl-group of ACP occupies the hydrophobic core of ISD11, explaining the basis of ACP stabilization. The unexpected architecture for the SDA complex provides a framework for understanding interactions with acceptor proteins for sulfur-containing biosynthetic pathways, elucidating mechanistic details of eukaryotic Fe-S cluster biosynthesis, and clarifying how defects in Fe-S cluster assembly lead to diseases such as Friedreich's ataxia. Moreover, our results support a lock-and-key model in which LYR proteins associate with acyl-ACP as a mechanism for fatty acid biosynthesis to coordinate the expression, Fe-S cofactor maturation, and activity of the respiratory complexes.

Expression of iron regulatory proteins in full-term swine placenta.

In swine, even though the pregnant sows were with iron abundance, the inborn iron reserve of piglets was compromised. This indicates the insufficiency of molecular machinery involved in local placental iron flux. Here, we investigated the expression of iron regulatory proteins like hepcidin and ferroportin and also their association with iron reserve, inflammation and oxidative stress in placenta of full-term pregnant sows (n = 6). Amplification and sequencing of placental DNA confirmed the presence of hepcidin (MN579557) and ferroportin (MN565887) sequences and their 100% identity with existing GenBank data. Real-time amplification of placental mRNA revealed significant higher expression of hepcidin (p < .05) than ferroportin. Western blot analysis of placental tissues revealed specific bands for both hepcidin (~8 kDa) and ferroportin (~62 kDa) molecules. Immunohistochemistry revealed the immunoreactivity for both proteins in the cytoplasm and membrane of trophoblastic cells of the placenta. Hepcidin and ferroportin expressions were positively associated with placental non-haem iron reserve (p < .0001; p = .033), lipid peroxidation (p = .0060; p < .0001) and reactive oxygen species level (p = .0092; p = .0292). Hepcidin expression was positively associated with interleukin - 6 (p = .0002) and interferon gamma (p < .0001) expressions but ferroportin expression was negatively associated with interleukin-6 (p = .0005), interleukin-1ß (p = .0226) and interferon gamma (p = .0059) expressions. This indicates hepcidin and ferroportin may have a role in controlling the local placental iron flux by acting as a molecular bridge between iron trafficking and inflammation.

Distinct TP53 Mutation Types Exhibit Increased Sensitivity to Ferroptosis Independently of Changes in Iron Regulatory Protein Activity.

The tumor suppressor gene TP53 is the most commonly mutated gene in human cancer. In addition to loss of tumor suppressor functions, mutations in TP53 promote cancer progression by altering cellular iron acquisition and metabolism. A newly identified role for TP53 in the coordination of iron homeostasis and cancer cell survival lies in the ability for TP53 to protect against ferroptosis, a form of iron-mediated cell death. The purpose of this study was to determine the extent to which TP53 mutation status affects the cellular response to ferroptosis induction. Using H1299 cells, which are null for TP53, we generated cell lines expressing either a tetracycline inducible wild-type (WT) TP53 gene, or a representative mutated TP53 gene from six exemplary "hotspot" mutations in the DNA binding domain (R273H, R248Q, R282W, R175H, G245S, and R249S). TP53 mutants (R273H, R248Q, R175H, G245S, and R249S) exhibited increased sensitivity ferroptosis compared to cells expressing WT TP53. As iron-mediated lipid peroxidation is critical for ferroptosis induction, we hypothesized that iron acquisition pathways would be upregulated in mutant TP53-expressing cells. However, only cells expressing the R248Q, R175H, and G245S TP53 mutation types exhibited statistically significant increases in spontaneous iron regulatory protein (IRP) RNA binding activity following ferroptosis activation. Moreover, changes in the expression of downstream IRP targets were inconsistent with the observed differences in sensitivity to ferroptosis. These findings reveal that canonical iron regulatory pathways are bypassed during ferroptotic cell death. These results also indicate that induction of ferroptosis may be an effective therapeutic approach for tumor cells expressing distinct TP53 mutation types.

Structure of the Human ACP-ISD11 Heterodimer.

In recent years, the mammalian mitochondrial protein complex for iron-sulfur cluster assembly has been the focus of important studies. This is partly because of its high degree of relevance in cell metabolism and because mutations of the involved proteins are the cause of several human diseases. Cysteine desulfurase NFS1 is the key enzyme of the complex. At present, it is well-known that the active form of NFS1 is stabilized by the small protein ISD11. In this work, the structure of the human mitochondrial ACP-ISD11 heterodimer was determined at 2.0 Å resolution. ACP-ISD11 forms a cooperative unit stabilized by several ionic interactions, hydrogen bonds, and apolar interactions. The 4'-phosphopantetheine-acyl chain, which is covalently bound to ACP, interacts with several residues of ISD11, modulating together with ACP the foldability of ISD11. Recombinant human ACP-ISD11 was able to interact with the NFS1 desulfurase, thus yielding an active enzyme, and the NFS1/ACP-ISD11 core complex was activated by frataxin and ISCU proteins. Internal motions of ACP-ISD11 were studied by molecular dynamics simulations, showing the persistence of the interactions between both protein chains. The conformation of the dimer is similar to that found in the context of the (NFS1/ACP-ISD11)2 supercomplex core, which contains the Escherichia coli ACP instead of the human variant. This fact suggests a sequential mechanism for supercomplex consolidation, in which the ACP-ISD11 complex may fold independently and, after that, the NFS1 dimer would be stabilized.

Iron and the heart: A paradigm shift from systemic to cardiomyocyte abnormalities.

Iron is a key micronutrient for the human body and participates in biological processes, such as oxygen transport, storage, and utilization. Iron homeostasis plays a crucial role in the function of the heart and both iron deficiency and iron overload are harmful to the heart, which is partly mediated by increased oxidative stress. Iron enters the cardiomyocyte through the classic pathway, by binding to the transferrin 1 receptor (TfR1), but also through other routes: T-type calcium channel (TTCC), divalent metal transporter 1 (DMT1), L-type calcium channel (LTCC), Zrt-, Irt-like Proteins (ZIP) 8 and 14. Only one protein, ferroportin (FPN), extrudes iron from cardiomyocytes. Intracellular iron is utilized, stored bound to cytoplasmic ferritin or imported by mitochondria. This cardiomyocyte iron homeostasis is controlled by iron regulatory proteins (IRP). When the cellular iron level is low, expression of IRPs increases and they reduce expression of FPN, inhibiting iron efflux, reduce ferritin expression, inhibiting iron storage and augment expression of TfR1, increasing cellular iron availability. Such cellular iron homeostasis explains why the heart is very susceptible to iron overload: while cardiomyocytes possess redundant iron importing mechanisms, they are equipped with only one iron exporting protein, ferroportin. Furthermore, abnormalities of iron homeostasis have been found in heart failure and coronary artery disease, however, no clear picture is emerging yet in this area. If we better understand iron homeostasis in the cardiomyocyte, we may be able to develop better therapies for a variety of heart diseases to which abnormalities of iron homeostasis may contribute.

Hepcidin and iron regulatory proteins coordinately regulate ferroportin 1 expression in the brain of mice.

Iron plays an essential role in various cellular metabolic processes of the body. Maintenance of cellular iron homeostasis is particularly important for keeping the normal functions of the cells. Ferroportin 1 (FPN1) is the currently only known iron exporter on the cell membrane. It has been indicated that the regulation of FPN1 in response to the alteration of iron level mainly involves two processes, posttranscriptional repression by iron regulatory proteins (IRPs) and posttranslational degradation by hepcidin, the major iron-sensing hormone. However, whether there is any communication between the two types of regulations or which one plays dominant role has not been reported. In our study with IRP2-/- mice, we found that knockout of IRP2 increased FPN1 expression in the cerebral cortex of IRP2-/- mice, whereas the upregulation of FPN1 was more significant in IRP1/IRP2 dual knockdown fibroblasts. Interestingly, we found that the knockout of IRP2 severely affected the regulation effect of hepcidin on FPN1 in mouse brain. FPN1 level decreased dramatically in the brain of wild-type mice injected with hepcidin, but it did not decrease much in IRP2 knockout mice. Further investigation disclosed that the compromised hepcidin-FPN1 regulation in IRP2-/- cells was directly dependent on the existence of iron-responsive element (IRE) in FPN1 messenger RNA. These results indicate that IRPs and hepcidin coordinately regulate the FPN1 level in mice. This study will provide a more comprehensive understanding of the regulatory mechanisms of FPN1 expression.

Regulating ferroportin-1 and transferrin receptor-1 expression: A novel function of hydrogen sulfide.

Hydrogen sulfide (H2 S) has a significant effect on the regulation of interleukin-6 (IL-6) and signal transducer and activator of transcription 3 (STAT3) activities, while IL-6 directly regulates hepcidin expression via STAT3. We therefore hypothesized that H 2 S has a role in body iron homeostasis by regulating the expression of iron transport proteins via the IL-6/STAT3/Hepcidin pathway. Here, we investigated the effects of two H 2 S donors sodium hydrosulfide and GYY4137 on the expression of ferroportin-1 (Fpn1), transferrin receptor-1 (TfR1), hepcidin, IL-6 and pSTAT3 in the spleen of mice in vivo and peritoneal macrophage in vitro. We also examined the effects of H 2 S on serum iron, transferrin saturation, and ferritin light chain contents in the spleen, and on nitrite content, nuclear factor erythroid 2-related factor-2 (Nrf2) and iron regulatory protein 1 (IRP1) in the macrophages. We demonstrated that H 2 S regulates the expression of TfR1 and Fpn1 in the spleen in vivo and in peritoneal macrophages in vitro predominantly via the IL-6/pSTAT3/hepcidin pathway, under the conditions of inflammation induced by lipopolysaccharides. We also provide evidence that under uninflamed conditions, the regulation of Fpn1 and TfR1 expression by H 2 S, both in vivo and in vitro, are mediated by the nitric oxide (NO)/Nrf2 and iron regulatory protein/iron responsive element pathways, respectively, which are independent of IL-6/pSTAT3/hepcidin signals. These findings show that H 2 S is a key player in iron homeostasis under not only the inflamed conditions but also uninflamed conditions.

Delta-aminolevulinic acid synthase 2 expression in combination with iron as modifiers of disease severity in erythropoietic protoporphyria.

Deficiency in ferrochelatase (FECH), the last enzyme in the heme biosynthetic pathway, leads to an accumulation of protoporphyrin IX (PPIX) that causes a severely painful phototoxic reaction of the skin in patients with erythropoietic protoporphyria (EPP). Besides phototoxicity of the skin, EPP patients often present with symptoms of iron deficiency in form of a microcytic and hypochromic anemia with low serum iron and ferritin. In addition, elevated aminolevulinic acid synthase 2 (ALAS2) both at the mRNA and protein levels have been observed among EPP patients. ALAS is the first enzyme in the pathway and exists in two isoforms, whereby the isoform 2 (ALAS2) is expressed exclusively in erythropoiesis. The mRNA of ALAS2 contains an iron response element (IRE) at its 5'UTR. When iron is limited, iron response element binding protein 2 (IRP2) binds to the IRE of ALAS2 mRNA and suppresses its translation. In this study, we demonstrated that iron deprivation increased the amount of ALAS2 mRNA as well as the ratio of ALAS2 to FECH mRNAs in cultured erythroleukemic K562 cells. At the protein level, however, iron deprivation in the cell line caused reductions in both enzymes as shown by the Western blot analysis. A comparable increase in the ratio of ALAS2 to FECH mRNAs was also found in EPP patients indicating an imbalance in heme biosynthesis. As iron cannot be completely missing from an organism, we assume that in EPP patients, a certain amount of ALAS2 mRNA is translated despite a partial deficiency of FECH. The increase in ALAS2 enzyme contributes to the accumulation in PPIX in the patients. Targeted inhibition of ALAS2 could therefore be a treatment option for EPP.

The actin-binding protein profilin 2 is a novel regulator of iron homeostasis.

Cellular iron homeostasis is controlled by the iron regulatory proteins (IRPs) 1 and 2 that bind cis-regulatory iron-responsive elements (IRE) on target messenger RNAs (mRNA). We identified profilin 2 (Pfn2) mRNA, which encodes an actin-binding protein involved in endocytosis and neurotransmitter release, as a novel IRP-interacting transcript, and studied its role in iron metabolism. A combination of electrophoretic mobility shift assay experiments and bioinformatic analyses led to the identification of an atypical and conserved IRE in the 3' untranslated region of Pfn2 mRNA. Pfn2 mRNA levels were significantly reduced in duodenal samples from mice with intestinal IRP ablation, suggesting that IRPs exert a positive effect on Pfn2 mRNA expression in vivo. Overexpression of Pfn2 in HeLa and Hepa1-6 cells reduced their metabolically active iron pool. Importantly, Pfn2-deficient mice showed iron accumulation in discrete areas of the brain (olfactory bulb, hippocampus, and midbrain) and reduction of the hepatic iron store without anemia. Despite low liver iron levels, hepatic hepcidin expression remained high, likely because of compensatory activation of hepcidin by mild inflammation. Splenic ferroportin was increased probably to sustain hematopoiesis. Overall, our results indicate that Pfn2 expression is controlled by the IRPs in vivo and that Pfn2 contributes to maintaining iron homeostasis in cell lines and mice.