How Hematopoietic Stem Cells Respond to Irradiation: Similarities and Differences between Low and High Doses of Ionizing Radiations

•Low doses of irradiation do not induce DNA damage.•One common feature of high- versus low-doses of irradiation is oxidative stress.•Low doses of irradiation provoke HSC self-renewal defects through increases in ROS.•Antioxidants protect HSCs against the effects of low doses of irradiation. In this review, we will specifically address the newest insights on the effect of low doses of ionizing radiations on the hematopoietic stem cells, which are prone to long-term deleterious effects. Impact of high doses of irradiation on hematopoietic cells has been widely studied over the years, in line with the risk of accidental or terrorist exposure to irradiation and with a particular attention to the sensitivity of the hematopoietic system. Recently, more studies have focused on lower doses of irradiation on different tissues, due to the increasing exposure caused by medical imaging, radiotherapy or plane travelling for instance. Hence, we will delineate similarities and discrepancies in HSC response to high and low doses of irradiation from these studies. In this review, we will specifically address the newest insights on the effect of low doses of ionizing radiations on the hematopoietic stem cells, which are prone to long-term deleterious effects. Impact of high doses of irradiation on hematopoietic cells has been widely studied over the years, in line with the risk of accidental or terrorist exposure to irradiation and with a particular attention to the sensitivity of the hematopoietic system. Recently, more studies have focused on lower doses of irradiation on different tissues, due to the increasing exposure caused by medical imaging, radiotherapy or plane travelling for instance. Hence, we will delineate similarities and discrepancies in HSC response to high and low doses of irradiation from these studies. The scientific understanding of radiation effects is a constantly evolving field. However, it can sometimes be tricky to define doses of radiation, given the different units in which it can be expressed. The "absorbed dose" is expressed in Gray (Gy), while the "equivalent dose" and the "efficient dose" are expressed in Sievert (Sv). Although both correspond to the same units in the International Units System (J/kg), the relationship between these parameters implies several multiplicative factors taking into account the type of radiation, the organ sensitivity or the species [1International Commission on Radiological Protection (ICRP)Recommandations 2007 de la Commission internationale de protection radiologique. ICRP Publication 103. Author, Ottawa, ON2007Google Scholar]. For laboratory studies, the unit commonly used is the Gy, while in epidemiology studying human exposure, it is the Sv, which makes it uneasy to compare them. Moreover, it also depends on the type of radiation, its energy or linear energy transfer (LET), and its relative biological effectiveness (RBE) [2Radiation Safety for Radiation Workers Manual. Madison: University of Wisconsin;Google Scholar]. Despite this, effects of high doses of irradiation (HDIR) are quite well characterized. Counteracting these effects involves the activation of pathways of the DNA repair response. In humans, the most sensitive tissue is the hematopoietic system: after exposure to irradiation, changes in blood composition might be evidenced with irradiation doses as low as 1 Gy. Death can occur starting at 2 Gy and is certain above 7 Gy in 4 to 6 weeks as a result of hematological failure [3Shao L Luo Y Zhou D. Hematopoietic stem cell injury induced by ionizing radiation.Antioxid Redox Signal. 2014; 20: 1447-1462Crossref PubMed Scopus (185) Google Scholar]. At higher doses, other organs (intestines, skin) are also touched. It has long been stated, in particular for HDIR, that irradiation implies either a linear dose-dependent effect or threshold effect, relying mainly on DNA damage. However, debates regarding the effects of low doses of irradiation (LDIR) remain, the definition of a low dose itself being unclear. Doses inferior to 500 mGy, sometimes up to 1 Gy, are often considered low doses, and we refer to that in what follows. Recently, more concerns over the potential danger of LDIR (below the threshold considered dangerous by the authorities and therefore authorized for medical imaging, for instance) have arisen because of the increased risk of developing leukemia and brain tumors in children who have undergone several computed tomography (CT) scans, with total received doses ranging from 0 to 50 mGy in the bone marrow (BM) and 0 to 350 mGy in the brain [4Pearce MS. Patterns in paediatric CT use: An international and epidemiological perspective.J Med Imaging Radiat Oncol. 2011; 55: 107-109Crossref PubMed Scopus (26) Google Scholar,5Pearce MS Salotti JA Little MP et al.Radiation exposure from CT scans in childhood and subsequent risk of leukaemia and brain tumours: a retrospective cohort study.Lancet. 2012; 380: 499-505Abstract Full Text Full Text PDF PubMed Scopus (2459) Google Scholar]. It became a European priority (RISK-IR European network) to characterize how LDIR affects these organs, considering their sensitivity, in particular for the blood system and hematopoietic stem cells. Hematopoietic stem cells (HSCs) are responsible for the sustainability of the blood system of an individual lifelong. For such long-term maintenance, they physiologically exhibit intrinsic properties such as self-renewal and multipotency, which allows them to differentiate into all blood cell lineages while keeping a pool of HSCs. However, HSCs are very sensitive to any stress. The smallest perturbation in the system, such as infection (for review, see [6Passegué E Ernst P. IFN-α wakes up sleeping hematopoietic stem cells.Nat Med. 2009; 15: 612-613Crossref PubMed Scopus (32) Google Scholar]), blood loss [7Cheshier SH Prohaska SS Weissman IL. The effect of bleeding on hematopoietic stem cell cycling and self-renewal.Stem Cells Dev. 2007; 16: 707-717Crossref PubMed Scopus (67) Google Scholar], chemical or physical agents [8Gardner RV Lerner C Astle CM Harrison DE. 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For murine HSCs, the accepted phenotype for a population with long-term reconstitution potential is Lin–Sca1+cKit+(LSK) CD150+CD48–CD34– [11Kiel MJ Yilmaz OH Iwashita T Yilmaz OH Terhorst C Morrison SJ. SLAM family receptors distinguish hematopoietic stem and progenitor cells and reveal endothelial niches for stem cells.Cell. 2005; 121: 1109-1121Abstract Full Text Full Text PDF PubMed Scopus (2360) Google Scholar,12Wilson A Laurenti E Oser G et al.Hematopoietic stem cells reversibly switch from dormancy to self-renewal during homeostasis and repair.Cell. 2008; 135: 1118-1129Abstract Full Text Full Text PDF PubMed Scopus (1332) Google Scholar]. For human HSCs, the purest population is Lin–CD34+CD38lowCD45RA–CD90+ [13Doulatov S Notta F Eppert K Nguyen LT Ohashi PS Dick JE. 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Maintenance of the hematopoietic stem cell pool by CXCL12–CXCR4 chemokine signaling in bone marrow stromal cell niches.Immunity. 2006; 25: 977-988Abstract Full Text Full Text PDF PubMed Scopus (1641) Google Scholar] ligand of CXCR4, thrombopoietin (TPO) [24Yoshihara H Arai F Hosokawa K et al.Thrombopoietin/MPL signaling regulates hematopoietic stem cell quiescence and interaction with the osteoblastic niche.Cell Stem Cell. 2007; 1: 685-697Abstract Full Text Full Text PDF PubMed Scopus (552) Google Scholar], angiopoietin [25Arai F Hirao A Ohmura M et al.Tie2/angiopoietin-1 signaling regulates hematopoietic stem cell quiescence in the bone marrow niche.Cell. 2004; 118: 149-161Abstract Full Text Full Text PDF PubMed Scopus (1511) Google Scholar], BMP4 [26Goldman DC Bailey AS Pfaffle DL Al Masri A Christian JL Fleming WH BMP4 regulates the hematopoietic stem cell niche.Blood. 2009; 114: 4393-4401Crossref PubMed Scopus (91) Google Scholar]) are involved in the retention of HSCs in their niches, therefore contributing to their maintenance. 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Finally, actors involved in irradiation-induced stress responses also play a role in maintaining HSC stemness under steady-state conditions, as well as after exposure to irradiation or other genotoxic stresses. In this review, we address the similar and different effects of high and low doses of irradiation (HDIR and LDIR) with respect to the steady-state regulation of HSC properties, focusing on components involved in the DNA-repair response (DRR) and reactive oxygen species (ROS) pathways. The DRR comprises several pathways depending on the DNA damage generated (for review, see [38Biechonski S Yassin M Milyavsky M. DNA-damage response in hematopoietic stem cells: an evolutionary trade-off between blood regeneration and leukemia suppression.Carcinogenesis. 2017; 38: 367-377Crossref PubMed Scopus (19) Google Scholar]). 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In competitive transplantation experiments, HSC mutants exhibit a loss of functionality. In most knockdown mouse models, the number of HSCs is drastically decreased compared with the number in wild-type (WT) mice. In particular, atm–/– mice exhibit a decreased frequency of HSCs with a defect in their reconstitution capacity. ROS levels are increased in atm–/– compared with WT HSCs. When mice are treated, in drinking water, with N-acetylcysteine (NAC), an ROS scavenger, ROS levels of atm–/– HSCs decrease while their reconstitution potential, in particular their long-term reconstitution capacity, is restored [49Ito K Hirao A Arai F et al.Regulation of oxidative stress by ATM is required for self-renewal of haematopoietic stem cells.Nature. 2004; 431: 997-1002Crossref PubMed Scopus (945) Google Scholar]. Interestingly, NAC treatment can prevent activation of the p38MAPK signaling pathway [50Ito K Hirao A Arai F et al.Reactive oxygen species act through p38 MAPK to limit the lifespan of hematopoietic stem cells.Nat Med. 2006; 12: 446-451Crossref PubMed Scopus (1049) Google Scholar]. The same effects can be observed after treatment with SB203580, a p38MAPK inhibitor [50Ito K Hirao A Arai F et al.Reactive oxygen species act through p38 MAPK to limit the lifespan of hematopoietic stem cells.Nat Med. 2006; 12: 446-451Crossref PubMed Scopus (1049) Google Scholar]. These defects in atm–/– HSCs can be linked to HSC exhaustion as atm–/– HSCs are less quiescent than WT HSCs, with increased expression of genes involved in cell cycle progression (p16ink4a and p19arf). Interestingly, NAC treatment, as well as SB203580 treatment, also induced a decrease in expression of these genes and prevented atm–/– HSC exhaustion. Together, these data indicate that under steady-state conditions, ATM contributes to protecting HSC quiescence and self-renewal potential by controlling ROS levels and p38MAPK activation [49Ito K Hirao A Arai F et al.Regulation of oxidative stress by ATM is required for self-renewal of haematopoietic stem cells.Nature. 2004; 431: 997-1002Crossref PubMed Scopus (945) Google Scholar,50Ito K Hirao A Arai F et al.Reactive oxygen species act through p38 MAPK to limit the lifespan of hematopoietic stem cells.Nat Med. 2006; 12: 446-451Crossref PubMed Scopus (1049) Google Scholar]. Under steady-state conditions and independently of ATM, ROS involvement in loss of self-renewal potential is well documented. HSCs have a very specific metabolic activity linked to the hypoxic environment in which they reside. Moreover, HSC intrinsic metabolism is correlated with a low level of ROS [51Bigarella CL Liang R Ghaffari S. 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In WT mice, ROS levels increase on serial transplantation [50Ito K Hirao A Arai F et al.Reactive oxygen species act through p38 MAPK to limit the lifespan of hematopoietic stem cells.Nat Med. 2006; 12: 446-451Crossref PubMed Scopus (1049) Google Scholar,59Yahata T Takanashi T Muguruma Y et al.Accumulation of oxidative DNA damage restricts the self-renewal capacity of human hematopoietic stem cells.Blood. 2011; 118: 2941-2950Crossref PubMed Scopus (218) Google Scholar], and NAC treatment of donor mice as well as recipient mice contributes to the reduction of ROS levels and to the protection of HSC reconstitution capacity during serial transplantations [50Ito K Hirao A Arai F et al.Reactive oxygen species act through p38 MAPK to limit the lifespan of hematopoietic stem cells.Nat Med. 2006; 12: 446-451Crossref PubMed Scopus (1049) Google Scholar,60Hu L Cheng H Gao Y et al.Antioxidant N-acetyl-L-cysteine increases engraftment of human hematopoietic stem cells in immune-deficient mice.Blood. 2014; 124: e45-e48Crossref PubMed Scopus (48) Google Scholar]. Downstream effectors induced by ROS are equally involved in the maintenance of stemness. For instance, inhibiting the degradation of p53 [61Liu Y Elf SE Miyata Y et al.p53 regulates hematopoietic stem cell quiescence.Cell Stem Cell. 2009; 4: 37-48Abstract Full Text Full Text PDF PubMed Scopus (403) Google Scholar] or inducing activation of p38MAPK [62Karigane D Kobayashi H Morikawa T et al.p38alpha activates purine metabolism to initiate hematopoietic stem/progenitor cell cycling in response to stress.Cell Stem Cell. 2016; 19: 192-204Abstract Full Text Full Text PDF PubMed Scopus (60) Google Scholar] impairs the functions of HSC, mimicking the effects of elevated ROS. However, important regulators of ROS such as NRF2 [63Venugopal R Jaiswal AK. 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Moreover, HSCs exhibiting the highest self-renewal potential were reported to be mainly quiescent, in the G0 state, and even dormant, dividing only a few times in an entire lifetime [12Wilson A Laurenti E Oser G et al.Hematopoietic stem cells reversibly switch from dormancy to self-renewal during homeostasis and repair.Cell. 2008; 135: 1118-1129Abstract Full Text Full Text PDF PubMed Scopus (1332) Google Scholar]. As mentioned above, increased ROS levels induce exit of quiescence of HSCs, and these features are regulated by different members of the Cdkn family, such as p21 [35Cheng T Rodrigues N Shen H et al.Hematopoietic stem cell quiescence maintained by p21cip1/waf1.Science. 2000; 287: 1804-1808Crossref PubMed Scopus (1056) Google Scholar] and p27 and p57 [36Zou P Yoshihara H Hosokawa K et al.p57(Kip2) and p27(Kip1) cooperate to maintain hematopoietic stem cell quiescence through interactions with Hsc70.Cell Stem Cell. 2011; 9: 247-261Abstract Full Text Full Text PDF PubMed Scopus (200) Google Scholar,37Matsumoto A Takeishi S Kanie T et al.p57 is required for quiescence and maintenance of adult hematopoietic stem cells.Cell Stem Cell. 2011; 9: 262-271Abstract Full Text Full Text PDF PubMed Scopus (220) Google Scholar], which are themselves targets of effectors previously cited, such as p53 [65Insinga A Cicalese A Faretta M et al.DNA damage in stem cells activates p21, inhibits p53, and induces symmetric self-renewing divisions.Proc Natl Acad Sci USA. 2013; 110: 3931-3936Crossref PubMed Scopus (100) Google Scholar]. Radiosensitivity is usually linked to three specificities of a cell: its division speed, the length of its dividing future, and its undifferentiated state [61Liu Y Elf SE Miyata Y et al.p53 regulates hematopoietic stem cell quiescence.Cell Stem Cell. 2009; 4: 37-48Abstract Full Text Full Text PDF PubMed Scopus (403) Google Scholar]. Usually acute radiation lethality arises from anemia or infection caused by hematopoietic failure and pancytopenia. Long-term delayed effects such as leukemia are also predominant after nonlethal irradiation, as witnessed in a significant proportion of the Hiroshima and Nagasaki bombing survivors [66Dainiak N. Hematologic consequences of exposure to ionizing radiation.Exp Hematol. 2002; 30: 513-528Abstract Full Text Full Text PDF PubMed Scopus (263) Google Scholar]. They occur when DNA of surviving immature cells is damaged. The probability of developing a malignancy usually increases with the dose. The mechanisms by which these short- and long-term effects occur are today quite well characterized (Table 1) in hematopoietic stem cells as well as in many other cell types, in several species, and have already been reviewed elsewhere [3Shao L Luo Y Zhou D. Hematopoietic stem cell injury induced by ionizing radiation.Antioxid Redox Signal. 2014; 20: 1447-1462Crossref PubMed Scopus (185) Google Scholar]. Mainly, ionizing radiation produces free radicals and ROS and nitrogen oxide species (NOS) [67Azzam EI Jay-Gerin JP Pain D. Ionizing radiation-induced metabolic oxidative stress and prolonged cell injury.Cancer Lett. 2012; 327: 48-60Crossref PubMed Scopus (758) Google Scholar], which in turn induce mitochondrial and DNA damage, mostly double-strand breaks (DSBs) but also oxidative DNA damage, and eventually lead to apoptosis or senescence if needed. In response to this damage, different pathways, such as the NHEJ, HR, base excision repair (BER), or nucleotide excision repair (NER), and ATM pathways, are activated either to r