Review Article Dual Role of ROS as Signal and Stress Agents: Iron …downloads.hindawi.com/journals/omcl/2016/8629024.pdf · ROS switches from signaling to damage (Figure ). 2. The

Review ArticleDual Role of ROS as Signal and Stress Agents Iron Tips theBalance in favor of Toxic Effects

Elena Gammella Stefania Recalcati and Gaetano Cairo

Department of Biomedical Sciences for Health University of Milan 20133 Milan Italy

Correspondence should be addressed to Gaetano Cairo gaetanocairounimiit

Received 21 December 2015 Accepted 28 January 2016

Academic Editor Victor M Victor

Copyright copy 2016 Elena Gammella et al This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

Iron is essential for life while also being potentially harmful Therefore its level is strictly monitored and complex pathways haveevolved to keep iron safely bound to transport or storage proteins thereby maintaining homeostasis at the cellular and systemiclevels These sequestration mechanisms ensure that mildly reactive oxygen species like anion superoxide and hydrogen peroxidewhich are continuously generated in cells living under aerobic conditions keep their physiologic role in cell signalingwhile escapingiron-catalyzed transformation in the highly toxic hydroxyl radical In this review we describe the multifaceted systems regulatingcellular and body iron homeostasis and discuss how altered iron balance may lead to oxidative damage in some pathophysiologicalsettings

1 Introduction

In eukaryotes oxygen utilization by the mitochondriaNADPHoxidase enzymes cytochromes and so forth leads tothe generation of reactive oxygen species (ROS) unstable andreactive molecules formed by one-electron transfers from aredox donor tomolecular oxygenThefirst of such products isanion superoxide (O

2

minus) which can be converted to hydrogenperoxide (H

2O2) by superoxide dismutase enzymes Metal-

catalyzed oxidation of H2O2can then originate hydroxyl

radicals (HO∙) themost reactive ROS Redox balance that isregulated production of ROS is essential for normal cellularphysiology as deregulation in the production of oxidativespecies that is oxidative stress causes DNA damage lipidperoxidation and aberrant posttranslational modification ofproteins thus leading to injury cell death and disease [1]Conversely accumulating evidence indicates that physiolog-ical concentrations of ROS are necessary to support redoxsignaling events that are involved in important physiologicalfunctions and adaptive cell responses such as chemotaxishormone synthesis immune response cytoskeletal remod-eling and calcium homeostasis [2] To be considered assignaling biological messengers ROS should meet precisespatial and regulatory criteria that is they should be

produced enzymatically and their levels should be regulatedby intracellular molecular mechanisms [3 4] Therefore O

2

minus

andH2O2 particularlyH

2O2that ismore stable thanO

2

minus andcan cross membranes are considered signaling moleculesbecause their levels under normal conditions remain undera physiological threshold and their synthesis is enzymaticallyregulated whereas HO∙ is deemed to be a highly toxic reac-tant leading to permanent modifications of target moleculesthat can impact cellular function and life In the cell rapidand efficient conversion of H

2O2to HO∙ through the Fenton

reaction requires the presence of transition metals that iscopper and ironWhile copper is amore efficient catalyst ironis much more abundant and thus it is the actual key player inthe so-called metal-catalyzed oxidative reactions [5]

While metal-dependent ROS production has histori-cally been associated with necrosis a form of passive celldeath characterized by dissolution of cellular structures newevidence suggests a role in regulated necrosis In fact aform of regulated cell death termed ferroptosis because itrequires iron has been recently identified [6] Interestinglyferroptosis is induced following inhibition of cystine importand downstream glutathione synthesis leading to the accu-mulation of intracellular ROS and lipid peroxidation [7]Further evidence for the role of iron-mediated disruption

Hindawi Publishing CorporationOxidative Medicine and Cellular LongevityVolume 2016 Article ID 8629024 9 pageshttpdxdoiorg10115520168629024

2 Oxidative Medicine and Cellular Longevity

Signaling Damage

FeFe

FeH2O2

H2O2 H2O2 rarr HO∙

Figure 1 Dual role of ROS as signaling and toxic molecules irontips the scale for damage By triggering Fenton chemistry increasediron (Fe) availability may change the role of H

2O2from a relatively

safe compound involved in cell signaling to a source of the highlytoxic HO∙

of cellular redox homeostasis in ferroptosis was providedby the essential regulatory function in ferroptotic cell deathof glutathione peroxidase [8] which is the only knownenzyme able to reduce phospholipid hydroperoxides andthus one of the most important antioxidant enzymes andby the demonstration that transferrin-mediated iron importis essential for ferroptosis [9] Moreover in hepatocellularcarcinoma cells knockdown of H ferritin one of the twosubunits that compose the iron storage protein ferritinpromoted ferroptosis in response to classical inducers [10]

During the slow and long process of oxygen accumulationthat led to the present 209 atmospheric oxygen contentinitiated about 15 billion years ago by photosynthesizingcyanobacteria all life forms evolved mechanisms to exploitthe chemical reactivity of iron for efficient aerobic reac-tions Indeed the number of iron-containing enzymes isstriking and includes proteins of vital physiological signif-icance involved in functions such as oxygen transport cellrespiration and DNA synthesis [11 12] Living organismswere also forced to concurrently evolve protection systemsagainst highly damaging HO∙ radicals produced by the iron-catalyzed conversion of superoxide and H

2O2 To this aim a

complex regulatory pathway formed by a variety of proteinsthat bind transport and store iron has been developed inorder to maintain an appropriate iron balance in both theindividual cells and the whole body [13 14] Over the lastyears the mechanisms by which iron homeostasis at the cel-lular and organismal levels is regulated have been elucidatedIn this review we have summarized recent advances in thecontrol of iron homeostasis and how changes in availability ofpoorly liganded ferrous iron are related to ROS productionMoreover we propose that whenever the efficiency of thenetwork controlling iron balance is compromised the role ofROS switches from signaling to damage (Figure 1)

2 The IREIRP Regulatory Pathway ControlsCellular Iron Homeostasis

The challenging task of maintaining intracellular iron levelssufficient for essential cellular functions including ROS-dependent cell signaling but as low as possible to avoid

ROS-mediated injury is controlled at multiple steps butprimarily accomplished by iron regulatory proteins (IRP1 andIRP2) which strictly control intracellular ironmetabolism byposttranscriptionally regulating the coordinated expressionof proteins involved in iron utilization (eg erythroid 5aminolevulinic acid synthase mitochondrial aconitase andDrosophila succinate dehydrogenase) uptake (transferrinreceptor (TfR1) and divalentmetal transporter (DMT1)) stor-age (H and L ferritin subunits) and export (ferroportin) [1516] (Figure 2) IRP1 and IRP2 recognize and bind conserved25ndash30 nucleotides-long RNA stem-loop structures namediron responsive elements (IREs) in the untranslated regions ofthe mRNAs coding for these proteins It has also been shownthat IRPs can bind the mRNAs for other proteins not directlyrelated to iron homeostasis [15 16]

The activity of IRP1 and IRP2 is dictated by the size ofthe cellular labile iron pool (LIP) a pool of iron in the low120583M range bound to low molecular weight compounds likecitrate or glutathione [17] which is in continuous equilibriumwith the sites of iron utilization or storage When cells areiron deficient and thus the size of the LIP is shrunk IRP1and IRP2 bind to IREs located in the 31015840 region of transcriptsand stabilize the mRNA for TfR1 and DMT1 thus increasingthe uptake of both transferrin-bound and unbound ironAt the same time binding to 51015840 IRE impairs translation ofmRNAs for ferroportin the only iron exporter and ferritinwhich sequesters iron in a catalytically inactive form Thiscoordinate regulation eventually expands the cellular LIPConversely when the LIP is large the IRE-binding activityof both IRPs is decreased resulting in efficient translation offerritin and ferroportin mRNAs and lower stability of TfR1and DMT1 mRNAs ultimately enhancing iron storage andrelease over uptake [15 16] (Figure 2)

Both IRPs are homologous to the mitochondrial TCAcycle enzyme aconitase that converts citrate to isocitrate usingan iron sulfur cluster (4Fendash4S) as a cofactor but only IRP1can assemble a cluster when sufficient iron is available thusfunctioning as cytosolic aconitase which is the prevailingform in most cells Conversely in conditions of iron defi-ciency the cluster is disassembled and the IRP1 apoformbinds IRE [15 16] IRP2 accumulates in iron-deficient cellswhere it binds IRE motifs with affinity and specificity similarto that of IRP1 whereas in iron-replete cells it is rapidly tar-geted for proteasomal degradation by an E3 ubiquitin ligasecomplex that comprises FBXL5 a recently identified proteincontaining an hemerythrin-like domain that is involved inits regulation according to iron (and oxygen) availability[18] Under conditions of iron scarcity the assembly of thedi-iron center in the hemerythrin-like domain is impairedand FBXL5 is polyubiquitinated and degraded by the pro-teasome thereby leading to IRP2 stabilization Converselywhen iron is abundant FBXL5 levels in the cell increasepromoting IRP2 (and apo-IRP1) proteasomal degradation[19 20]

The respective role and importance of IRPs have beenrevealed by studies involving gene deletion [21 22] Ablationof both IRP1 and IRP2 which are ubiquitously expressedis early embryonic lethal whereas single knockout mice areviable indicating that the two IRPs can compensate for each

Oxidative Medicine and Cellular Longevity 3

Abundant iron Scarce iron

FtFpn

TfR1

Tf

Tf

Tf

LIPIRP12

IRP12

IRP12IRE

IRE

IRE

Ft Fpn

TfR1

IRP12

IRP12

IRP12

Tf

TfTf

LIP

IRE

IRE

IRE

Figure 2 Simplified model of IRP-dependent regulation of intracellular iron homeostasis The IREIRP machinery posttranscriptionallycontrols the expression of the major proteins of intracellular iron import (transferrin receptor TfR1) export (ferroportin Fpn) and storage(ferritin Ft) Under conditions of iron excess in the labile iron pool (LIP) IRPs lose their RNA-binding capacity and hence TfR1 mRNA isdegraded (small arrow) while Fpn and Ft mRNAs are actively translated (big arrow and circle resp) The opposite occurs under conditionsof iron deficiency IRP1 and IRP2 binding to iron responsive elements (IREs) stabilize TfR1 mRNA (big arrow) and prevent the translation ofFpn and Ft mRNAs (small arrow and circle resp) this response increases iron availability

other and confirming the in vitro data which suggestedessential but largely overlapping functions However earlystudies did not show an apparent phenotype in IRP1 sin-gle knockout mice whereas hematopoietic defects and lateonset neurodegeneration were described in mice lackingIRP2 These findings showing a dominant role for IRP2in the regulation of iron homeostasis in mice were in linewith a study indicating that IRP2 is the major regulatorof intracellular iron metabolism in humans [23] Howeverrecent results showed that IRP1 predominantly binds specificIRE-containing mRNAs such as those coding for erythroidaminolevulinate synthase and hypoxia inducible factor 2120572(HIF2120572) [24]

Although iron is the major regulator of the IREIRPnetwork other signals and conditions can modulate theactivity of both IRPs including oxygen tension and nitricoxide (reviewed in [16]) Not surprisingly given the rela-tionship between ROS and iron IRPs are both targets andmodulators of free radical reactions and IRP activity isaltered under conditions of oxidative stress Despite reportsshowing IRP1 activation in cells exposed to H

2O2 whichmay

be a phenomenon more correlated to the signaling role ofH2O2rather than to oxidative stress it appears that IRP1

inactivation occurs in response to ROS both in vitro andin vivo [15 16] Overall since IRP2 is highly susceptible toROS-mediated downregulation it is possible to conclude thatinhibition of IRP1 and IRP2 binding activity with ensuingferritin induction and reduction of LIP may represent a pro-tective strategy to prevent amplification of oxidative injury(discussed in [25]) The role of ferritin as an antioxidantprotein is underscored by the multiplicity of mechanismsleading to its upregulation in response to oxidative challengein fact it has been demonstrated that stressful conditionstranscriptionally activate ferritin expression in various celltypes and ferritin overexpression protects from oxidativestress [26]

Readers can refer to recent excellent and comprehensivereviews for specific aspects of the IREIRP regulatory network[24 27ndash29]

3 Conditions in Which Disruption ofCellular Iron Homeostasis Leads toOxidative Damage

31 Neurodegenerative Disorders Abundant evidence showsthat a number of neurodegenerative disorders are character-ized by regional iron accumulation in particular areas of thecentral andor peripheral nervous systems [30 31] This isoften caused by cellular iron redistribution and may resultin iron-catalyzed Fenton chemistry For instance increasediron levels in specific regions of the brain are a hallmark ofParkinson disease [32]

Friedrichrsquos ataxia (FRDA) is a paradigmatic examplebecause the disruption of iron homeostasis in this disease hasbeen well defined [33] FRDA which is the most prevalentform of hereditary ataxia in Caucasians is characterized byprogressive degeneration of large sensory neurons in thecentral and peripheral nervous systems leading to neuro-logical impairment In addition to neurological symptomslike spinocerebellar and sensory ataxia FRDA patients alsosuffer from important nonneurological manifestations inparticular hypertrophic cardiomyopathy The disease resultsfrom loss of function mutations (most often triplet expan-sion) in the FXN gene that lead to decreased expression offrataxin a mitochondrial iron-binding protein that interactswith proteins involved in the mitochondrial Fe-S clusterbiogenesis [34] In patients frataxin deficiency results indisruption of Fe-S cluster biosynthesis severe mitochondrialiron overload a hallmark of Fe-S defects and increasedsensitivity to oxidative stress [35]

Although several studies provided evidence that ROSgenerated through Fenton reaction play a role in FRDAthe primary involvement and the importance of ROS in thepathophysiology of FRDA are still debated [33] Howeverthe protection afforded by mitochondrial ferritin which hasa strong antioxidant role [36] in yeast mammalian cellsand fibroblasts from FRDA patients was accompanied byreduced ROS level thus strongly indicating the involvementof toxic free radicals [37] Since mitochondrial ferritin plays


a protective role in several pathological conditions by seques-tering iron [36] these data suggest that iron chelators areof particular interest as therapeutic approaches for FRDAHowever FRDA and other regional sideroses (ie ironaccumulation in particular tissues or cell compartments) willrequire novel chelation modalities [38]

32 Role of Iron and ROS in Anthracycline CardiotoxicityThe role of iron in anthracycline cardiotoxicity is anotherilluminating example of the complex interplay and synergismbetween iron and free radicals as causative factors of apopto-sis or other forms of cell damage Doxorubicin (DOX) is anantineoplastic drug of the anthracyclines family which playsa recognized key role in the chemotherapy for several typesof cancer However anthracyclines have established risksof cardiotoxicity as their chronic administration inducescardiomyopathy and congestive heart failure [39] This dose-dependent side effect limits the clinical use of DOX in cancerpatients Multiple mechanisms of cardiotoxicity induced byDOX have been described but activation of the mitochon-drial intrinsic pathway of apoptosis seems to represent amajor response to anthracycline treatmentThe developmentof anthracycline-induced cardiomyopathy has been found todepend on drug metabolism [40] in fact in addition to therole of reductive activation of the quinone moiety of DOXdiscussed below a correlation exists between toxicity andmyocardial accumulation of anthracycline secondary alcoholmetabolites [39] Conversely anthracycline oxidative degra-dationmay serve as a salvage pathway for diminishing the lev-els and toxicity of DOX in cardiomyocytes (reviewed in [41])

The involvement of iron inDOX-induced cardiac damageis well established and cardiotoxicity induced by DOX mayoccur at lower cumulative doses under conditions of ironoverload [42] The adverse role of iron has been suggested byseveral lines of evidence in particular a number of studiesshowed the protecting efficacy of iron chelators both inpatients and in animal models while others demonstratedthat primary and secondary iron overload exacerbated thecardiotoxic effects of the drug but the underlying molecularmechanisms remain to be fully understood (see [39 41 42]for review) Iron has been proposed to act as a catalyst ofROS formation in reactions primed by DOX In fact DOXis a redox compound as NAD(P)H reductases catalyze one-electron reduction of the quinone moiety of the tetracyclinering to the semiquinone free radical which can regenerate theparent quinone reacting with molecular oxygen The latterreaction generates O

2

minus and its dismutation product H2O2

which can then be transformed into the more potent HO∙ byreactions catalyzed by iron In turn HO∙ can damage DNAand proteins and initiate membrane lipid peroxidation thusleading to cardiomyocyte death [42 43]

However since antioxidants did not offer protection inclinical settings the apparently obvious explanation for theaggravating role of iron in DOX cardiotoxicity based onincreased iron-catalyzed ROS formation has been calledinto question [39 41] In line with this view we showedthat DOX doses in the range of the plasma levels foundin patients undergoing chemotherapy were able to causeapoptotic death of cardiac-derived H9c2 myocytes in the

absence of ROS production [44] Moreover we providedevidence that activation of the HIF pathway contributes tothe cardioprotective effect of the iron chelator dexrazoxanethus suggesting that the protective capacity of iron chelatorsagainst DOX toxicity may be mediated by mechanisms notrelated to the prevention of ROS formation [45]

We also showed that anthracycline cardiotoxicity isrelated to ROS-dependent and ROS-independent disruptionof cardiac iron homeostasis due to targeted interaction ofDOX with IRP1 which leads to a ldquonullrdquo IRP1 devoid ofboth its functions and hence it is unable to sense iron levelsand to regulate iron homeostasis Moreover DOX triggersIRP2 degradation which may serve as a protective role byfavoring iron sequestration in newly formed ferritin [2546] Indeed it has been shown that ferritin is induced inH9c2 cardiomyocytes [47] and mouse hearts [48] exposedto DOX and protects cardiac cells against iron toxicityMoreover ferritin H chain plays an important role in thepreventive effect of metformin against DOX cardiotoxicityin isolated cardiomyocytes [49] DOX treatment also resultsin iron storage by inducing mechanisms leading to higheraccumulation of iron into ferritin [50] Overall these resultssuggest that the role of iron in anthracycline-dependentcardiotoxicity may extend beyond the formation of ROS

On the other hand recent results regarding mitochon-drial ferritin (FtMt) a ferritin type particularly expressedin mitochondria-rich tissues including the heart whereit prevents iron-mediated oxidative damage [51] reinforcethe idea that ROS are involved in the mechanisms linkingiron and anthracycline cardiotoxicity FtMt expression wasinduced in the heart of mice exposed to DOX [52] and FtMt-deficient mice exposed to DOX are more sensitive to ROS-mediated heart damage and death [53] In additionmicewithheart-specific deletion or overexpression of ABCB8 whichexports iron out of the mitochondria were more sensitive orresistant respectively to DOX cardiotoxicity [52]

The importance of methodological aspects introducessome cautionary issues that should be taken into accountwhen considering the discrepancies reported above regard-ing the pathophysiological relevance of iron-mediated ROSproduction in DOX toxicity In fact one should keep inmind that the model systems in which the mechanisms ofDOX cardiotoxicity have been characterized have inherentlimitations in representing the human chronic cardiomyopa-thy Moreover the dual role of ROS in signaling events andcell damage should be considered when evaluating if ironcontributes to chronic cardiomyopathy by mechanisms notrelated to its ability to generate HO∙

4 Systemic Iron Metabolism

Given the dual role of iron elegant control mechanisms haveevolved to maintain appropriate body iron levels by meansof a complex network of transporters storage moleculesand regulators Intestinal iron absorption and iron recyclingin reticuloendothelial cells are coordinately orchestrated inorder to maintain iron levels in the circulation adequate forthe needs of the various tissues and organs but insufficient toactivate dangerous ROS production [13 14]


Transferrin iron

Duodenum

Red blood cells

Liver

Hepcidin

Fpn

Reticuloendothelial cells

Hepcidin

Fpn

Fpn

Hepcidin

Figure 3 Hepcidin regulates transferrin-mediated body iron traffic The interaction of hepcidin with ferroportin inhibits the flow of ironinto plasma and thereby regulates the transferrin-mediated distribution of iron in the body from sites of iron absorption (duodenum) andrecycling (reticuloendothelial cells in spleen and liver) to tissues where it is used (eg for the synthesis of hemoglobin in red blood cells) orstored (eg liver)

The task of keeping circulating iron in a safe but readilyavailable form is performed by transferrin (Tf) a proteinsynthesized and secreted by the liver that binds up to twoferric iron atoms with high affinity and is the major irontransport protein Under physiological conditions Tf-boundiron is the main source of iron for the majority of tissuesprimarily for bone marrow erythroid precursors whichconsume iron for hemoglobin biosynthesis (Figure 3)

Tf incorporates iron coming from two major sourcesdietary iron (both inorganic and heme iron) absorbed in theduodenum to compensate for daily iron loss and iron derivedfrom destruction of old and effete erythrocytes by reticuloen-dothelial cells in the spleen and liver (Figure 3) After reduc-tion by the reductaseDcytB dietary iron is transported acrossthe apical membrane of absorptive epithelial cells by DMT1[54] In intestinal enterocytes most iron is then exported tothe blood at the basolateral surface by ferroportin assistedby the function of oxidases (circulating ceruloplasmin andmembrane-bound hephaestin) that convert ferrous iron toferric iron and thus permit the incorporation of iron into TfAlthough various transporters have been identified [54] themechanisms of intestinal uptake and release of heme iron areless clearly understood but the majority of heme is degradedin the enterocytes and iron is released by ferroportin asferroportin-deficient mice are not viable [55]

Spleen and liver macrophages specialized in recyclingiron obtained from the phagocytosis and destruction ofsenescent erythrocytes are the main iron supplier for hemo-globin synthesis The major pathway of heme iron recy-cling involves hemoglobin degradation by cytosolic hemeoxygenase-1 and export of heme-derived iron into the circu-lation by ferroportin [56]

Recent studies in mice with disrupted IRP1 andor IRP2in the entire organismor specific tissues have shown that IRPsare important regulators also of systemic homeostasis [22]but hepcidin a peptide hormone produced and secreted by

the liver can be considered the key regulator of body ironbalance [57 58]That hepcidin is most important regulator ofbody iron homeostasis which is also indicated by a numberof studies showing that disruption of hepcidin regulationis involved in a variety of disorders associated with irondeficiency (eg anemia) or overload (eg siderosis) Inparticular inadequate hepcidin levels in relation to body ironstores characterize most hereditary iron overload diseases[59] Hepcidin controls plasma iron concentration and bodyiron balance by regulating the expression of ferroportin theonly known cellular iron exporter (Figure 3) The binding ofhepcidin to ferroportin on the plasma membrane inducesits internalization and degradation thereby blocking ironrelease [60] Hepcidin expression is regulated at multiplelevels the expression of hepcidin is induced by iron overloadinflammatory stimuli or endoplasmic reticulum stress [5758 61] this mechanism stops the efflux of unwanted iron inthe circulation by negatively modulating iron absorption byenterocytes iron recycling by reticuloendothelial cells andiron mobilization from hepatic stores Conversely increasederythropoietic activity under conditions of iron deficiencyanemia and hypoxia represses hepcidin thereby leading tohigher iron availability for new erythrocytes synthesis [2062] Among the positive regulators of hepcidin iron exertsits effect through the BMPsSMAD dependent pathwaywhile inflammatory cytokines especially IL-6 activate theJAK2-STAT3 signaling cascade Erythroferrone producedby erythroid precursors in the marrow and the spleen inresponse to erythropoietin seems to be themajor inhibitor ofhepcidin production when erythropoiesis is stimulated [63]

41 Iron Overload Conditions Two mechanisms protect thecells from the damaging effects of excess body iron thepartial saturation of Tf at the systemic level and the IRP-dependent regulated expression of TfR1 at the cellular levelUnder normal conditions Tf saturation is around 30 and


Tf saturation Tf saturation

HeartJoints

Liver

Tissue damage

NTBI

Normal condition Iron overload

NTBI

NTBI

NTBI

Glands

Figure 4 Mechanisms of iron overload mediated tissue damage Whenever the iron-binding capacity of transferrin (Tf) is exceeded non-transferrin-bound iron (NTBI) forms penetrates into the cells and undergoes redox cycling ultimately leading to cell injury and organdamage

the protein can therefore bind excess iron entering thecirculation thus functioning as a protective shield againstHO∙ production However under conditions of heavy sys-temic iron overload (primarily due to insufficient hepcidinproduction) the iron-binding capacity of Tf is exceededandnon-transferrin-bound iron (NTBI) is formed (Figure 4)Despite the downregulation of TfR1 cells become iron-loaded because NTBI that is iron associated with variouslow molecular weight plasma components like citrate phos-phates and proteins lacks a regulated uptake system andhence is able to penetrate into the cells [38] The consequentdecrease in IRP binding activity leads to efficient ferritinmRNA translation to promote iron storage but ultimatelythe high capacity of ferritin is exceeded as this unabatedflux of NTBI continues NTBI is clearly involved in tissuesiderosis but the incomplete characterization of the ironspecies involved prevents a clear understanding of how itenters the cell under different pathological conditions NTBIenters into cells mainly through divalent cation transporterssuch asDMT1 andZIP14which require previous reduction toferrous iron [64 65] orCa2+ channels either L-type orT-typeparticularly in cardiomyocytes [66] NTBI toxicity withincells derives from the fact that iron in the expanded LIP isno longer protected from redox cycling hence it acceleratesthe catalysis of reactions that produce HO∙ which then causelipid peroxidation and organelle damage and ultimately celldeath (Figure 4) In particular mitochondria appear to beprimarily affected by iron-mediated HO∙ production In factit has been demonstrated that mitochondria rapidly acquireNTBI and this along with their high generation of ROS[67] props up oxidative damage Apparently the variety ofenzymatic and nonenzymatic antioxidant defense systems isinadequate to prevent metal-catalyzed oxidative injury onceiron in the LIP is increased [66] Although production ofhydroxyl radical and lipid peroxidation are important in theinitiation of iron overload pathology additional mechanismsinvolving apoptosis and fibrosis can account for its complexpathophysiology that leads to organ failure [58]

The most important clinical conditions involving pri-mary and secondary iron overload leading to iron-mediatedtissue damage are genetic hemochromatosis and transfu-sional siderosis respectively Hereditary hemochromatosislinked to mutations in HFE a MHC class I-like proteinthat is a necessary component of the iron-sensing machin-ery controlling hepcidin expression is the most commongenetic disease in Caucasians and presents a multisysteminvolvement although iron overload first affects the liver inhemochromatosis patients endocrine abnormalities cardiacproblems and arthropathy are also common [68] Loss ofHFE function leads to inappropriately low hepcidin produc-tion and unneeded iron release in the bloodstream fromthe duodenum and reticuloendothelial system ConsequentNTBI formation and iron deposition in parenchymal cellslead to oxidative damage and determine the clinical featuresof hemochromatosis [69]

Secondary iron overload is mainly observed in associa-tion with transfusion-dependent diseases [70] Since ourbody lacks any regulated mechanism to effectively excreteexcess iron long-term blood transfusion inescapably resultsin iron overload in patients Transfusional iron over-load affects particularly patients with inherited hemoglo-binopathies such as 120573 thalassemia which is the secondaryiron overload condition more closely linked to tissue ironoverload [71 72] However the adverse effects of iron over-load are also found in patients with a variety of conditions(eg Blackfan-Diamond anemia aplastic anemia sideroblas-tic anemia myelodysplasia etc)

With continued transfusion reticuloendothelial cells canno longer safely store all the excess iron which thus entersthe circulation in amounts that exceed the binding capacityof Tf and NTBI develops [69] It should be noted that inconditions such as 120573 thalassemia hepcidin levels are paradox-ically low because erythropoiesis-dependent downregulationprevails over the upregulation associated with body ironlevels therefore high intestinal iron absorption contributesto iron overload [20 62]While in hemochromatosis patients


excess iron is removed by phlebotomy the treatment oftransfusional iron overload is mainly based on iron chelationtherapy [61] Iron chelators can be considered as antioxidantsbut not all the possible molecules able to bind iron can beregarded as safe antioxidants Since all the six iron atoms haveto be bound to the chelator to form a stable complex incom-plete iron-chelate complexes (eg iron-EDTA) can undergoredox cycling and generate harmful free radicals Thereforefor efficient and safe scavenging of excess iron chelatingmolecules andor dosages should be carefully evaluated toraise the thermodynamic stability of the iron-ligand complex

5 Conclusions

Presently it is not completely clear as to why in someconditions ROS are associated with cell damage and scav-enging high ROS levels improves metabolic homeostasiswhereas in other settings ROS exert signaling functionsimportant for essential cell activities Possible explanationsfor these distinct biological specificities of ROS action includedifferences in the amount sources duration and localizationof ROS production in addition certainly the higher ROSreactivity is the greater toxicity is while signaling capacityis diminished Therefore H

2O2 which is relatively stable

and diffusible is indeed a mild oxidant but is suitable forsignaling However in the presence of ferrous iron H

2O2

can generate the highly reactive and toxic HO∙ Thereforewe propose that increased availability of iron not bound toproteins specifically evolved to transport or store this essen-tial metal can make the critical difference between the twoopposite functions of ROS The view that iron plays a mainrole in the scenario leading ROS to become signal or stressagents is supported by the sophisticated systems regulatingintracellular and systemic iron homeostasis that we havesummarized in this review Additional evidence indicatingthe contribution of iron emerges from the number of studiesshowing the occurrence of ROS-mediated tissue damagewhenever the control of iron metabolism is disrupted con-ditions of which we selectively highlighted a few illustrativeexamples Improved understanding of the complex interplaybetween iron metabolism and redox homeostasis will clarifythese pathways and their relevance in pathophysiology Inconsideration of the disappointing results of antioxidant ther-apy for a variety of diseases [73] targeting iron will possiblyrepresent a more advanced therapeutic approach aimed atpreventing the harmful effects of ROS while permitting theirphysiological role in cell signaling

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

This work was supported by grants from Ministero dellrsquoIs-truzione dellrsquoUniversita e della Ricerca and Ministero dellaSalute

References

[1] B Halliwell and J M C Gutteridge ldquoOxygen toxicity oxygenradicals transitionmetals anddiseaserdquoBiochemical Journal vol219 no 1 pp 1ndash14 1984

[2] D Trachootham W Lu M A Ogasawara N R-D Valle andP Huang ldquoRedox regulation of cell survivalrdquo Antioxidants andRedox Signaling vol 10 no 8 pp 1343ndash1374 2008

[3] T Finkel ldquoSignal transduction by mitochondrial oxidantsrdquoTheJournal of Biological Chemistry vol 287 no 7 pp 4434ndash44402012

[4] G Shadel and T Horvath ldquoMitochondrial ROS signaling inorganismal homeostasisrdquoCell vol 163 no 3 pp 560ndash569 2015

[5] B Harwell ldquoBiochemistry of oxidative stressrdquo BiochemicalSociety Transactions vol 35 no 5 pp 1147ndash1150 2007

[6] S J Dixon KM LembergM R Lamprecht et al ldquoFerroptosisan iron-dependent form of nonapoptotic cell deathrdquo Cell vol149 no 5 pp 1060ndash1072 2012

[7] S J Dixon D Patel M Welsch et al ldquoPharmacologicalinhibition of cystine-glutamate exchange induces endoplasmicreticulum stress and ferroptosisrdquo eLife vol 2014 no 3 2014

[8] J P Friedmann Angeli M Schneider B Proneth et al ldquoInac-tivation of the ferroptosis regulator Gpx4 triggers acute renalfailure inmicerdquoNature Cell Biology vol 16 no 12 pp 1180ndash11912014

[9] M Gao P Monian N Quadri R Ramasamy and X JiangldquoGlutaminolysis and transferrin regulate ferroptosisrdquoMolecularCell vol 59 no 2 pp 298ndash308 2015

[10] X Sun Z Ou R Chen et al ldquoActivation of the p62-Keap1-NRF2 pathway protects against ferroptosis in hepatocellularcarcinoma cellsrdquo Hepatology vol 63 no 1 pp 173ndash184 2016

[11] G Cairo F Bernuzzi and S Recalcati ldquoA precious metal ironan essential nutrient for all cellsrdquo Genes amp Nutrition vol 1 no1 pp 25ndash39 2006

[12] A D Sheftel A B Mason and P Ponka ldquoThe long history ofiron in the Universe and in health and diseaserdquo Biochimica etBiophysica Acta (BBA)mdashGeneral Subjects vol 1820 no 3 pp161ndash187 2012

[13] N C Andrews ldquoForging a field the golden age of iron biologyrdquoBlood vol 112 no 2 pp 219ndash230 2008

[14] MWHentzeMUMuckenthaler BGaly andCCamaschellaldquoTwo to tango regulation of Mammalian iron metabolismrdquoCell vol 142 no 1 pp 24ndash38 2010

[15] G Cairo and S Recalcati ldquoIron-regulatory proteins molecularbiology and pathophysiological implicationsrdquoExpert Reviews inMolecular Medicine vol 9 no 33 pp 1ndash13 2007

[16] S Recalcati GMinotti and G Cairo ldquoIron regulatory proteinsfrom molecular mechanisms to drug developmentrdquo Antioxi-dants and Redox Signaling vol 13 no 10 pp 1593ndash1616 2010

[17] R C Hider and X Kong ldquoIron speciation in the cytosol anoverviewrdquo Dalton Transactions vol 42 no 9 pp 3220ndash32292013

[18] T Moroishi M Nishiyama Y Takeda K Iwai and K INakayama ldquoThe FBXL5-IRP2 axis is integral to control of ironmetabolism in vivordquo Cell Metabolism vol 14 no 3 pp 339ndash3512011

[19] J C Ruiz and R K Bruick ldquoF-box and leucine-rich repeatprotein 5 (FBXL5) sensing intracellular iron and oxygenrdquoJournal of Inorganic Biochemistry vol 133 pp 73ndash77 2014

[20] S Recalcati E Gammella and G Cairo ldquoNew perspectiveson the molecular basis of the interaction between oxygen


homeostasis and iron metabolismrdquo Hypoxia vol 3 pp 93ndash1032015

[21] M U Muckenthaler B Galy and MW Hentze ldquoSystemic ironhomeostasis and the iron-responsive elementiron-regulatoryprotein (IREIRP) regulatory networkrdquoAnnual Review of Nutri-tion vol 28 pp 197ndash213 2008

[22] NWilkinson andK Pantopoulos ldquoThe IRPIRE system in vivoinsights frommouse modelsrdquo Frontiers in Pharmacology vol 5article 176 2014

[23] S Recalcati A Alberghini A Campanella et al ldquoIron regula-tory proteins 1 and 2 in human monocytes macrophages andduodenum expression and regulation in hereditary hemochro-matosis and iron deficiencyrdquo Haematologica vol 91 no 3 pp303ndash310 2006

[24] D-L ZhangMCGhosh andTA Rouault ldquoThephysiologicalfunctions of iron regulatory proteins in iron homeostasismdashanupdaterdquo Frontiers in Pharmacology vol 5 article 124 2014

[25] G Cairo S Recalcati A Pietrangelo and G Minotti ldquoTheiron regulatory proteins targets and modulators of free radicalreactions and oxidative damagerdquo Free Radical Biology andMedicine vol 32 no 12 pp 1237ndash1243 2002

[26] P Arosio F Carmona R Gozzelino F Maccarinelli and MPoli ldquoThe importance of eukaryotic ferritins in iron handlingand cytoprotectionrdquo Biochemical Journal vol 472 no 1 pp 1ndash15 2015

[27] C P Anderson M Shen R S Eisenstein and E A LeiboldldquoMammalian ironmetabolism and its control by iron regulatoryproteinsrdquo Biochimica et Biophysica Acta (BBA)mdashMolecular CellResearch vol 1823 no 9 pp 1468ndash1483 2012

[28] K Pantopoulos S K Porwal A Tartakoff and L DevireddyldquoMechanisms of mammalian iron homeostasisrdquo Biochemistryvol 51 no 29 pp 5705ndash5724 2012

[29] L C Kuhn ldquoIron regulatory proteins and their role in control-ling iron metabolismrdquo Metallomics vol 7 no 2 pp 232ndash2432015

[30] T A Rouault ldquoIron metabolism in the CNS implications forneurodegenerative diseasesrdquo Nature Reviews Neuroscience vol14 no 8 pp 551ndash564 2013

[31] G Isaya ldquoMitochondrial iron-sulfur cluster dysfunction inneurodegenerative diseaserdquo Frontiers in Pharmacology vol 5article 29 2014

[32] L Zecca M B H Youdim P Riederer J R Connor and R RCrichton ldquoIron brain ageing and neurodegenerative disordersrdquoNature Reviews Neuroscience vol 5 no 11 pp 863ndash873 2004

[33] A Martelli and H Puccio ldquoDysregulation of cellular ironmetabolism in Friedreich ataxia from primary iron-sulfurcluster deficit to mitochondrial iron accumulationrdquo Frontiers inPharmacology vol 5 article 130 2014

[34] V Campuzano L Montermini M D Molto et al ldquoFriedreichrsquosataxia autosomal recessive disease caused by an intronic GAAtriplet repeat expansionrdquo Science vol 271 no 5254 pp 1423ndash1427 1996

[35] R A Vaubel and G Isaya ldquoIron-sulfur cluster synthesis ironhomeostasis and oxidative stress in Friedreich ataxiardquoMolecularand Cellular Neuroscience vol 55 pp 50ndash61 2013

[36] P Arosio and S Levi ldquoCytosolic and mitochondrial ferritins inthe regulation of cellular iron homeostasis and oxidative dam-agerdquo Biochimica et Biophysica Acta (BBA)mdashGeneral Subjectsvol 1800 no 8 pp 783ndash792 2010

[37] A Campanella G Isaya H A OrsquoNeill et al ldquoThe expression ofhuman mitochondrial ferritin rescues respiratory function in

frataxin-deficient yeastrdquoHumanMolecular Genetics vol 13 no19 pp 2279ndash2288 2004

[38] Z I Cabantchik ldquoLabile iron in cells and body fluids physiol-ogy pathology and pharmacologyrdquo Frontiers in Pharmacologyvol 5 article 45 2014

[39] G Minotti P Menna E Salvatorelli G Cairo and L GiannildquoAnthracyclines molecular advances and pharmacologic devel-opments in antitumor activity and cardiotoxicityrdquo Pharmaco-logical Reviews vol 56 no 2 pp 185ndash229 2004

[40] P Menna S Recalcati G Cairo and G Minotti ldquoAn introduc-tion to the metabolic determinants of anthracycline cardiotox-icityrdquo Cardiovascular Toxicology vol 7 no 2 pp 80ndash85 2007

[41] E Gammella S Recalcati I Rybinska P Buratti and GCairo ldquoIron-induced damage in cardiomyopathy oxidative-dependent and independent mechanismsrdquo Oxidative Medicineand Cellular Longevity vol 2015 Article ID 230182 10 pages2015

[42] G Minotti G Cairo and E Monti ldquoRole of iron in anthra-cycline cardiotoxicity new tunes for an old songrdquo The FASEBJournal vol 13 no 2 pp 199ndash212 1999

[43] M Sterba O Popelova A Vavrova et al ldquoOxidative stressredox signaling andmetal chelation in anthracycline cardiotox-icity and pharmacological cardioprotectionrdquo Antioxidants andRedox Signaling vol 18 no 8 pp 899ndash929 2013

[44] F Bernuzzi S Recalcati A Alberghini and G Cairo ldquoReac-tive oxygen species-independent apoptosis in doxorubicin-treatedH9c2 cardiomyocytes role for heme oxygenase-1 down-modulationrdquo Chemico-Biological Interactions vol 177 no 1 pp12ndash20 2009

[45] R D Spagnuolo S Recalcati L Tacchini and G Cairo ldquoRoleof hypoxia-inducible factors in the dexrazoxane-mediated pro-tection of cardiomyocytes from doxorubicin-induced toxicityrdquoBritish Journal of Pharmacology vol 163 no 2 pp 299ndash312 2011

[46] G Minotti R Ronchi E Salvatorelli P Menna and G CairoldquoDoxorubicin irreversibly inactivates iron regulatory proteins1 and 2 in cardiomyocytes evidence for distinct metabolicpathways and implications for iron-mediated cardiotoxicity ofantitumor therapyrdquo Cancer Research vol 61 no 23 pp 8422ndash8428 2001

[47] G Corna P Santambrogio G Minotti and G Cairo ldquoDox-orubicin paradoxically protects cardiomyocytes against iron-mediated toxicity role of reactive oxygen species and ferritinrdquoThe Journal of Biological Chemistry vol 279 no 14 pp 13738ndash13745 2004

[48] G Corna B Galy M W Hentze and G Cairo ldquoIRP1-independent alterations of cardiac iron metabolism indoxorubicin-treated micerdquo Journal of Molecular Medicine vol84 no 7 pp 551ndash560 2006

[49] M C Asensio-Lopez J Sanchez-Mas D A Pascual-Figal etal ldquoInvolvement of ferritin heavy chain in the preventive effectof metformin against doxorubicin-induced cardiotoxicityrdquo FreeRadical Biology and Medicine vol 57 pp 188ndash200 2013

[50] J C Kwok and D R Richardson ldquoExamination of the mech-anism(s) involved in doxorubicin-mediated iron accumulationin ferritin studies using metabolic inhibitors protein synthesisinhibitors and lysosomotropic agentsrdquo Molecular Pharmacol-ogy vol 65 no 1 pp 181ndash195 2004

[51] J Drysdale P Arosio R Invernizzi et al ldquoMitochondrialferritin a new player in ironmetabolismrdquoBlood CellsMoleculesamp Diseases vol 29 no 3 pp 376ndash383 2002


[52] Y Ichikawa M Ghanefar M Bayeva et al ldquoCardiotoxicity ofdoxorubicin is mediated through mitochondrial iron accumu-lationrdquo The Journal of Clinical Investigation vol 124 no 2 pp617ndash630 2014

[53] F Maccarinelli E Gammella M Asperti et al ldquoMice lack-ing mitochondrial ferritin are more sensitive to doxorubicin-mediated cardiotoxicityrdquo Journal of Molecular Medicine vol 92no 8 pp 859ndash869 2014

[54] S Gulec G J Anderson and J F Collins ldquoMechanistic andregulatory aspects of intestinal iron absorptionrdquoThe AmericanJournal of PhysiologymdashGastrointestinal and Liver Physiologyvol 307 no 4 pp G397ndashG409 2014

[55] J E Levy O Jin Y Fujiwara F Kuo andN C Andrews ldquoTrans-ferrin receptor is necessary for development of erythrocytes andthe nervous systemrdquoNature Genetics vol 21 no 4 pp 396ndash3991999

[56] E Gammella F Maccarinelli P Buratti S Recalcati andG Cairo ldquoThe role of iron in anthracycline cardiotoxicityrdquoFrontiers in Pharmacology vol 5 article 25 2014

[57] TGanz ldquoSystemic iron homeostasisrdquoPhysiological Reviews vol93 no 4 pp 1721ndash1741 2013

[58] A Pietrangelo ldquoPathogens metabolic adaptation and humandiseasesmdashan iron-thrifty genetic modelrdquo Gastroenterology vol149 no 4 pp 834ndash838 2015

[59] A Pietrangelo ldquoHepcidin in human iron disorders therapeuticimplicationsrdquo Journal of Hepatology vol 54 no 1 pp 173ndash1812011

[60] E Nemeth M S Tuttle J Powelson et al ldquoHepcidin regulatescellular iron efflux by binding to ferroportin and inducing itsinternalizationrdquo Science vol 306 no 5704 pp 2090ndash20932004

[61] C Camaschella ldquoIron-deficiency anemiardquo The New EnglandJournal of Medicine vol 373 no 5 pp 485ndash486 2015

[62] A Kim and E Nemeth ldquoNew insights into iron regulation anderythropoiesisrdquo Current Opinion in Hematology vol 22 no 3pp 199ndash205 2015

[63] L Kautz G Jung E V Valore S Rivella E Nemeth andT Ganz ldquoIdentification of erythroferrone as an erythroidregulator of iron metabolismrdquo Nature Genetics vol 46 no 7pp 678ndash684 2014

[64] J P Liuzzi F Aydemir H Nam M D Knutson and RJ Cousins ldquoZip14 (Slc39a14) mediates non-transferrin-boundiron uptake into cellsrdquo Proceedings of the National Academy ofSciences of the United States of America vol 103 no 37 pp13612ndash13617 2006

[65] S Ludwiczek I Theurl M U Muckenthaler et al ldquoCa2+channel blockers reverse iron overload by a newmechanism viadivalent metal transporter-1rdquoNatureMedicine vol 13 no 4 pp448ndash454 2007

[66] G Y Oudit H Sun M G Trivieri et al ldquoL-type Ca2+ channelsprovide a major pathway for iron entry into cardiomyocytes iniron-overload cardiomyopathyrdquo Nature Medicine vol 9 no 9pp 1187ndash1194 2003

[67] P Venditti L Di Stefano and S Di Meo ldquoMitochondrialmetabolism of reactive oxygen speciesrdquoMitochondrion vol 13no 2 pp 71ndash82 2013

[68] A Pietrangelo ldquoHereditary hemochromatosis pathogenesisdiagnosis and treatmentrdquo Gastroenterology vol 139 no 2 pp393e1ndash408e2 2010

[69] G Sebastiani and K Pantopoulos ldquoDisorders associated withsystemic or local iron overload from pathophysiology toclinical practicerdquoMetallomics vol 3 no 10 pp 971ndash986 2011

[70] R E Fleming and P Ponka ldquoIron overload in human diseaserdquoThe New England Journal of Medicine vol 366 no 4 pp 348ndash359 2012

[71] J B Porter and M Garbowski ldquoThe pathophysiology of trans-fusional iron overloadrdquo HematologyOncology Clinics of NorthAmerica vol 28 no 4 pp 683ndash701 2014

[72] V Berdoukas T D Coates and Z I Cabantchik ldquoIron andoxidative stress in cardiomyopathy in thalassemiardquo Free RadicalBiology and Medicine vol 88 pp 3ndash9 2015

[73] J M C Gutteridge and B Halliwell ldquoAntioxidants moleculesmedicines and mythsrdquo Biochemical and Biophysical ResearchCommunications vol 393 no 4 pp 561ndash564 2010

Submit your manuscripts athttpwwwhindawicom

Stem CellsInternational

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014


MEDIATORSINFLAMMATION

of


Behavioural Neurology

EndocrinologyInternational Journal of



Disease Markers


BioMed Research International

OncologyJournal of



Oxidative Medicine and Cellular Longevity


PPAR Research

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Immunology ResearchHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

ObesityJournal of



Computational and Mathematical Methods in Medicine

OphthalmologyJournal of


Diabetes ResearchJournal of



Research and TreatmentAIDS


Gastroenterology Research and Practice


Parkinsonrsquos Disease

Evidence-Based Complementary and Alternative Medicine

Volume 2014Hindawi Publishing Corporationhttpwwwhindawicom


Signaling Damage

FeFe

FeH2O2

H2O2 H2O2 rarr HO∙

Figure 1 Dual role of ROS as signaling and toxic molecules irontips the scale for damage By triggering Fenton chemistry increasediron (Fe) availability may change the role of H

2O2from a relatively

safe compound involved in cell signaling to a source of the highlytoxic HO∙

of cellular redox homeostasis in ferroptosis was providedby the essential regulatory function in ferroptotic cell deathof glutathione peroxidase [8] which is the only knownenzyme able to reduce phospholipid hydroperoxides andthus one of the most important antioxidant enzymes andby the demonstration that transferrin-mediated iron importis essential for ferroptosis [9] Moreover in hepatocellularcarcinoma cells knockdown of H ferritin one of the twosubunits that compose the iron storage protein ferritinpromoted ferroptosis in response to classical inducers [10]

During the slow and long process of oxygen accumulationthat led to the present 209 atmospheric oxygen contentinitiated about 15 billion years ago by photosynthesizingcyanobacteria all life forms evolved mechanisms to exploitthe chemical reactivity of iron for efficient aerobic reac-tions Indeed the number of iron-containing enzymes isstriking and includes proteins of vital physiological signif-icance involved in functions such as oxygen transport cellrespiration and DNA synthesis [11 12] Living organismswere also forced to concurrently evolve protection systemsagainst highly damaging HO∙ radicals produced by the iron-catalyzed conversion of superoxide and H

2O2 To this aim a

complex regulatory pathway formed by a variety of proteinsthat bind transport and store iron has been developed inorder to maintain an appropriate iron balance in both theindividual cells and the whole body [13 14] Over the lastyears the mechanisms by which iron homeostasis at the cel-lular and organismal levels is regulated have been elucidatedIn this review we have summarized recent advances in thecontrol of iron homeostasis and how changes in availability ofpoorly liganded ferrous iron are related to ROS productionMoreover we propose that whenever the efficiency of thenetwork controlling iron balance is compromised the role ofROS switches from signaling to damage (Figure 1)

2 The IREIRP Regulatory Pathway ControlsCellular Iron Homeostasis

The challenging task of maintaining intracellular iron levelssufficient for essential cellular functions including ROS-dependent cell signaling but as low as possible to avoid

ROS-mediated injury is controlled at multiple steps butprimarily accomplished by iron regulatory proteins (IRP1 andIRP2) which strictly control intracellular ironmetabolism byposttranscriptionally regulating the coordinated expressionof proteins involved in iron utilization (eg erythroid 5aminolevulinic acid synthase mitochondrial aconitase andDrosophila succinate dehydrogenase) uptake (transferrinreceptor (TfR1) and divalentmetal transporter (DMT1)) stor-age (H and L ferritin subunits) and export (ferroportin) [1516] (Figure 2) IRP1 and IRP2 recognize and bind conserved25ndash30 nucleotides-long RNA stem-loop structures namediron responsive elements (IREs) in the untranslated regions ofthe mRNAs coding for these proteins It has also been shownthat IRPs can bind the mRNAs for other proteins not directlyrelated to iron homeostasis [15 16]

The activity of IRP1 and IRP2 is dictated by the size ofthe cellular labile iron pool (LIP) a pool of iron in the low120583M range bound to low molecular weight compounds likecitrate or glutathione [17] which is in continuous equilibriumwith the sites of iron utilization or storage When cells areiron deficient and thus the size of the LIP is shrunk IRP1and IRP2 bind to IREs located in the 31015840 region of transcriptsand stabilize the mRNA for TfR1 and DMT1 thus increasingthe uptake of both transferrin-bound and unbound ironAt the same time binding to 51015840 IRE impairs translation ofmRNAs for ferroportin the only iron exporter and ferritinwhich sequesters iron in a catalytically inactive form Thiscoordinate regulation eventually expands the cellular LIPConversely when the LIP is large the IRE-binding activityof both IRPs is decreased resulting in efficient translation offerritin and ferroportin mRNAs and lower stability of TfR1and DMT1 mRNAs ultimately enhancing iron storage andrelease over uptake [15 16] (Figure 2)

Both IRPs are homologous to the mitochondrial TCAcycle enzyme aconitase that converts citrate to isocitrate usingan iron sulfur cluster (4Fendash4S) as a cofactor but only IRP1can assemble a cluster when sufficient iron is available thusfunctioning as cytosolic aconitase which is the prevailingform in most cells Conversely in conditions of iron defi-ciency the cluster is disassembled and the IRP1 apoformbinds IRE [15 16] IRP2 accumulates in iron-deficient cellswhere it binds IRE motifs with affinity and specificity similarto that of IRP1 whereas in iron-replete cells it is rapidly tar-geted for proteasomal degradation by an E3 ubiquitin ligasecomplex that comprises FBXL5 a recently identified proteincontaining an hemerythrin-like domain that is involved inits regulation according to iron (and oxygen) availability[18] Under conditions of iron scarcity the assembly of thedi-iron center in the hemerythrin-like domain is impairedand FBXL5 is polyubiquitinated and degraded by the pro-teasome thereby leading to IRP2 stabilization Converselywhen iron is abundant FBXL5 levels in the cell increasepromoting IRP2 (and apo-IRP1) proteasomal degradation[19 20]

The respective role and importance of IRPs have beenrevealed by studies involving gene deletion [21 22] Ablationof both IRP1 and IRP2 which are ubiquitously expressedis early embryonic lethal whereas single knockout mice areviable indicating that the two IRPs can compensate for each



FtFpn

TfR1

Tf

Tf

Tf

LIPIRP12

IRP12

IRP12IRE

IRE

IRE

Ft Fpn

TfR1

IRP12

IRP12

IRP12

Tf

TfTf

LIP

IRE

IRE

IRE




2O2 whichmay












2











Transferrin iron

Duodenum

Red blood cells

Liver

Hepcidin

Fpn


Hepcidin

Fpn

Fpn

Hepcidin










HeartJoints

Liver

Tissue damage

NTBI


NTBI

NTBI

NTBI

Glands








5 Conclusions




2O2




Acknowledgments


References



















































































of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of


















FtFpn

TfR1

Tf

Tf

Tf

LIPIRP12

IRP12

IRP12IRE

IRE

IRE

Ft Fpn

TfR1

IRP12

IRP12

IRP12

Tf

TfTf

LIP

IRE

IRE

IRE




2O2 whichmay












2











Transferrin iron

Duodenum

Red blood cells

Liver

Hepcidin

Fpn


Hepcidin

Fpn

Fpn

Hepcidin










HeartJoints

Liver

Tissue damage

NTBI


NTBI

NTBI

NTBI

Glands








5 Conclusions




2O2




Acknowledgments


References



















































































of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of




















2











Transferrin iron

Duodenum

Red blood cells

Liver

Hepcidin

Fpn


Hepcidin

Fpn

Fpn

Hepcidin










HeartJoints

Liver

Tissue damage

NTBI


NTBI

NTBI

NTBI

Glands








5 Conclusions




2O2




Acknowledgments


References



















































































of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of

















Transferrin iron

Duodenum

Red blood cells

Liver

Hepcidin

Fpn


Hepcidin

Fpn

Fpn

Hepcidin










HeartJoints

Liver

Tissue damage

NTBI


NTBI

NTBI

NTBI

Glands








5 Conclusions




2O2




Acknowledgments


References



















































































of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of


















HeartJoints

Liver

Tissue damage

NTBI


NTBI

NTBI

NTBI

Glands








5 Conclusions




2O2




Acknowledgments


References



















































































of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of


















5 Conclusions




2O2




Acknowledgments


References



















































































of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of














































































of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of












































of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of





















of






Disease Markers



OncologyJournal of





PPAR Research



Journal of

ObesityJournal of
















Documents

Review Article Dual Role of ROS as Signal and Stress Agents: Iron …downloads.hindawi.com/journals/omcl/2016/8629024.pdf · ROS switches from signaling to damage (Figure ). 2. The