A storm in the niche- Iron, oxidative stress and haemopoiesis.pdf

Contents lists available at ScienceDirectBlood Reviewsjournal homepage: storm in the niche:Iron,oxidative stress and haemopoiesisFederica Piloa,Emanuele Angeluccib,aHematology and Transplant Center,Ospedale Oncologico di Riferimento Regionale Armando Businco,Azienda Ospedaliera Brotzu,Cagliari,Italy.bHematology and Transplant Center,Ospedale Policlinico San Martino,Genova,ItalyA R T I C L E I N F OKeywords:IronOxidative stressHaematopoietic stem cellsClonal evolutionA B S T R A C TIron,although essential,is harmful in high amounts.Oxidative stress as a result of excess reactive oxygen species(ROS)and a prooxidative/antioxidative imbalance between ROS production and elimination,play a key role incellular damage.There is evidence to support the role of ROS in the pathogenesis of a range of diseases includingthe myelodysplastic syndromes(MDS)and leukaemia.Oxidative stress seems to affect the self-renewal,pro-liferation and differentiation of haematopoietic stem cells and impair cell growth.Three aspects of these de-fective haemopoietic mechanisms may be associated with the activities of ROS:clonal evolution,haematologicalimprovement and recovery of haemopoiesis after haematopoietic stem cell transplantation(HSCT).This reviewaims to provide haematologists with an overview of results from in vitro and murine models and preliminaryclinical evidence on the diagnostic,prognostic and therapeutic implications of the complex interactions betweenthe haemopoietic niche,iron,oxidative stress and inadequate haemopoiesis.1.IntroductionIron is fundamental for many cellular functions such as the cell cycleof growth and replication,metabolism and DNA synthesis/repair andiron-requiring proteins such as Fe-sulphur cluster proteins that areabundant in mitochondria 1.However,the ability to gain and loseelectrons gives iron the possibility of participating in potentiallyharmful free radical-generating reactions,producing the hydroxyl ra-dical(OH)a reactive oxygen species(ROS)based on the Fenton re-action 2.The hydroxyl radical,the most reactive chemical species inbiological systems,when present in excess,can not only damage lipids,proteins,and mitochondria but also cause oxidative DNA damage in-cluding DNA base modifications and DNA strand breaks 3,4,all ofwhich can be mutagenic 5.ROS levels are regulated by pro-oxidantand anti-oxidant systems and a correct balance between these twomechanisms is essential for cellular life.The modern history of redox biology began with the discovery byMcCord and Fridovich in 1969 of superoxide dismutase an enzymethat promotes ROS production 6.Later in 1985 the term oxidativestress was coined by Sies and Cadenas 7.Greater knowledge of theeffects of ROS activities has shed light on the role of iron and oxidativestress in cellular lifecycle and has revealed the dark side of iron anessential but potentially toxic element.There is now a substantial body of literature supporting the role ofROS in the pathogenesis of many diseases and in particular those re-latedtocellproliferationanddifferentiationsuchasthemyelodysplastic syndromes(MDS)and acute myeloid leukaemia(AML)8,9.Iron overload,caused by ineffective erythropoiesis,increasedgastrointestinal iron intake and transfusion dependency allow us toconsider the MDS as the ideal disease model to try to understand thecomplex relationship between the haemopoietic niche,iron,oxidativestress and inadequate haemopoiesis.This review aims to provide hae-matologists with an overview of results from in vitro and murinemodels and preliminary clinical evidence on the diagnostic,prognosticand therapeutic implications of the complex interactions between thehaemopoietic niche,iron,oxidative stress and inadequate haemopoi-esis.2.Cellular activities of ROSIron from gastrointestinal intake and blood transfusions is typicallybound to transferrin when it circulates in the plasma.This complex isinternalized by different tissue cells through the transferrin receptorand the iron-transferrin complex is then split and the free iron form,called labile cellular iron(LCI),is used by the cell for vital functionssuch as mitochondrial energy production.Excess iron is generally ac-cumulated as ferritin and hemosiderin 10.Under normal physiolo-gical conditions mitochondrial respiration is the main source of ROSwithin the cell during adenosine triphosphate(ATP)production.Othermajor sources of ROS in vivo are the enzymatic activity as nicotinamideadenine dinucleotide phosphate(NADPH)oxidase(NOX)9 or acti-vated phagocytic cells 11.If the level of LCI increases it is responsiblehttp:/dx.doi.org/10.1016/j.blre.2017.08.005Corresponding author.E-mail addresses:pilofedericaG(F.Pilo),emnangtin.it(E.Angelucci).Blood Reviews 32(2018)29350268-960X/2017 Elsevier Ltd.All rights reserved.Tfor potentially dangerous free radical-generating reactions,with theproduction of the hydroxyl radical and ultimately leading to cellulardeath and consequent tissue damage 12.Essentially,when the transferrin saturation excess is more than6070%,non-transferrin bound iron(NTBI)and its subcomponent la-bile plasma iron(LPI)appear in the serum 13.LPI ability to enter thecells through alternative channels(other than transferrin receptor ca-nals),results in an increase in LCI levels.When ROS production exceedsthe antioxidant enzyme systems,an excessive accumulation of ROSoccurs,leading to intracellular oxidative stress 14.This biochemicalmodel seems to be at the basis of liver,heart,endocrine gland tissuedamage.However,it has only recently been considered that bonemarrow,and consequently haemopoiesis,could be another importanttarget for iron-mediated damage 15.There are in vitro data showingthat in haematopoietic stem cells in the MDS and AML and other tu-mours the constitutional ROS balance is often defective 16,17.Aninteresting in vitro study,showed that various molecules involved incell metabolism(such as kinases,phosphatases or transcription factors)are considered crucial regulators of ROS levels and at the same timeredox sensor molecules 14.In other words an oxidative abnormality,in one or more of the different signals in which these molecules areinvolved(mostly present in tumours),may alter the key metabolicpathways related to stem cell fate,changing processes that regulate cellcycle progression,apoptosis,quiescence or differentiation 14.Thebasic idea is that cellular toxicity does not result directly from storageiron,but from the disruption of the dynamic balance that exists be-tween storage iron pool and functional iron pool 18.3.The haematological niche in normal conditionsHaematopoietic stem and progenitors cell reside within the so calledhaemopoietic niche,defined as cellular and molecular microenviron-ments that collaborate through cell mechanisms to maintain and reg-ulate stem cell functions,ensuring stem cell growth,proliferation anddifferentiation.The haematological niche is ideally divided in an os-teoblastic marrow compartment and a vascular marrow compartment.Haematopoietic stem cell growth is a two-phase process a quiescentphase(when the cell cycle is in G0 phase)and an activated phase(whenthe cell cycle is in G1-G2-S-M phase).At this particular point of the cellcycle,the haematopoietic stem cell can choose to return to the quies-cent phase or proceed from the activated phase to proliferation anddifferentiation(self-renewal).Haematopoietic stem cells are foundmainly adjacent to sinusoids when in the activated phase and near theosteoblastic cells compartment when in the quiescent phase 9.Endothelial cells,mesenchymal stromal cells,macrophages andperivascular stromal cells promote haematopoietic stem cell self-re-newal by producing stem cell factor(SCF),CXCL12 and other reg-ulating factors.It is likely that other cells also contribute to this niche,probably including cells near bone surfaces in trabecular rich areas.Allthe support elements(endothelial cells,mesenchymal stromal cells,macrophages,osteoclast/osteoblast)are essential to haematopoieticstem cell growth.The cross-talking between these elements guaranteesa correct haematopoietic stem cell growth 19.Recent studies suggest that ROS plays a role in haematopoietic stemcell state and function and it has been well described how ROS levelsare essential to maintain the self-renewal of stem cells 11.Haema-topoietic stem cells in the quiescent phase need low ROS levels and lowNOX enzyme expression,but when enter the activated phase,it is ne-cessary they to proceed from the periosteal hypoxic area to the sinusoidarea following an oxygen gradient.This increase in ROS(associatedwith the oxygen gradient)is essential for the activated phase and ele-vated ROS levels appear to drive haematopoietic stem cells out ofquiescence and reduce self-renewal capacity.The stem cell distancesitself from the osteoblastic interface of the niche where the micro-environment encourages quiescence and goes close to the vascular sideof the niche,through oxygen gradient,where the microenvironmentfavours proliferation and differentiation.This migration of haemato-poietic stem cells seems to be governed by ROS levels and the inter-action of chemokines 9.It is important to underline that ROS play adecisive physiological role in haematopoietic stem cell self-renewal,migration,maturation and differentiation.4.The haematological niche in oxidative stress conditionsLudin and colleagues 11 showed in vitro how oxidative stressinfluences the fate of haematopoietic stem cells by compromising mi-gration,development,self-renewal and cell cycle status.The hypoxicconditions 20 and several environmental factors(HIF1,COX2,PGE2,CCXCR4,CXCL12)21 participate to maintain low ROS levels.How-ever extremely low ROS levels in haemopoietic stem cells can causedefects in their differentiation ability leading to impaired repopulationcapacity 22.On the other hand increases in ROS levels,drives stemcell differentiation to short term repopulating cells and further on tomyeloid differentiation 23,24.Exceedingly high ROS levels,as mayoccur during important oxidative stress conditions such as chronic in-flammation or iron overload,can promote stem cell exhaustion andsubsequent apoptosis 25.The overall message of these studies is thathaematopoietic stem cells quiescent and active state is a balance be-tween ROS levels too much or too little ROS seems be a determiningfactor in the fate of many pathways critical to cell survival and pro-liferation 9.In essence ROS balance may determinate stem cell des-tiny(Fig.1).Similar effects have been described with osteogenic progenitors anddifferentiation of mesenchymal stem and progenitor cells 26,27.Bu-lycheva and colleagues described the direct connection between hae-matopoietic stem and progenitor cells(HSPCs)and osteoblast/osteo-clast activity in the haematopoietic niche,introducing the concept ofosteo-haematology.They described how a disturbed microenvironment(including osteogenic elements)might affect haematopoietic stem cellgrowth modifying the cross talking between the haematopoietic nichecomponents 28.In a myelodysplastic mouse model,they reporteddecreased osteoblasts and osteoclasts number and decreased bone for-mation rate.In particular,they identified iron overload as responsiblefor osteoblast inhibition and increases in osteoclasts.They concludedthat oxidative stress is involved in the pathogenesis of bone loss duringiron excess and that oxidative stress may affect the relationship be-tween haematopoietic cells and the microenvironment in MDS 28.Taken together these results suggest it is reasonable to consider thatiron overload and the consequent increases in ROS levels could impairhaematopoietic stem cell self-renewal,proliferation and differentiationby distressing oxygen gradient and impairing the microenvironmentcell growth,including osteogenesis.5.Possible clinical implications of ROS activity in thehaemopoietic systemThree important aspects of haematopoiesis may be associated withaltered levels of ROS:clonal evolution,haematological improvementand haematopoietic cell transplantation engraftment(Fig.2).Wesummarize and link the in vitro and in vivo evidence supporting therole of ROS in these three different circumstances.5.1.Clonal evolution5.1.1.In vitro resultsInadequate ROS homeostasis,resulting in oxidative stress and ge-netic instability in haematopoietic stem cells and myeloid progenitors,has been linked with myeloid malignancy 2932.The notion that ROSmay drive stem cell dysfunction with age draws precedence from thefree radical theory of ageing,first described by Harman in 1972 33.Cellular ageing is associated with reduced organ function,increasedoxidative stress,genomic mutations and increased incidence of certainF.Pilo,E.AngelucciBlood Reviews 32(2018)293530diseases,in particular cancer.Haematopoietic stem cells ageing drivenby both cell intrinsic and extrinsic factors is linked to impaired HSC self-renewal and regeneration,associated immune remodelling and in-creased incidence of leukaemia 34,35.MDS are essentially a clonaldisease of the elderly,characterized by increased genetic instability36 and as such are an ideal model to explore the role of iron in theclonal evolution phenomenon.Serial acquisition of mutations in anexpanded clone can lead first to the myelodysplastic phenotype andthen,through additional mutations to the leukaemia phenotype3739.In these patients,excess iron may contribute to inadequateROS homeostasis and the genomic instability of the pre-leukemic cloneand could be one of the possible cause of clonal evolution towards AML40.Interestingly,the relationship between iron and clonal evolution hasbeen confirmed in a murine model 41.Six mice were treated withtotal body irradiation(3 Gy)and a high concentration of iron dextran(5 mg)and compared with controls(only irradiation).Three mice fromeach group were sacrificed after three months.Mice with iron overloadhad expansion of the splenic white pulp and the LinCD45+haema-topoetic population in bone marrow.The surviving mice from eachcohort continued to be observed long-term and one of the three in theiron overload group died 8 months after treatment.Post-mortem ex-amination showed severe hepatomegaly and splenomegaly,massivesplenic and hepatic infiltration by leukaemic blasts and extensive boneFig.1.ROS balance and hemopoietic stem cell destiny.Bone marrowROS activityimplicationsHematological improvementBone marrow transplantationengrafmentClonal evolutionFig.2.Process in which ROS activity could be involved.F.Pilo,E.AngelucciBlood Reviews 32(2018)293531marrow necrosis,fibrosis,and substantial blast accumulation.Sub-sequent experiments to identify the underlying process indicated sus-tained DNA damage(DNA double-strand breaks)as a result of exposureto excess iron 41.Other models showed how increased ROS levelsstimulate leukaemogenesis through the regulation of redox-sensitivetranscription factors(Nrf2,Bach1,NF-kB,HIF1)42.Furthermore,excess production of ROS in AML drives,growth factor-independentpro