Total body irradiation selectively induces murine hematopoietic stem cell senescence

Sensitivity and dose dependency of radiation-induced injury in hematopoietic stem/progenitor cells in mice

We evaluated the sensitivity and dose dependency of radiation-induced injury in hematopoietic stem/progenitor cells. Adult C57BL/6 mice were daily exposed to 0, 2, 10, 50, and 250 mGy γ-ray for 1 month in succession, respectively. The damage of hematopoietic stem/progenitor cells in bone marrow were investigated within 2 hours (acute phase) or at 3 months (chronic phase) after the last exposure. Daily exposure to over 10 mGy γ-ray significantly decreased the number and colony-forming capacity of hematopoietic stem/progenitor cells at acute phase, and did not completely recover at chronic phase with 250 mGy exposure. Interestingly, the daily exposure to 10 or 50 mGy γ-ray decreased the formation of mixed types of colonies at chronic phase, but the total number of colonies was comparable to control. Immunostaining analysis showed that the formation of 53BP1 foci in c-kit⁺ stem/progenitor cells was significantly increased with daily exposure to 50 and 250 mGy at acute phase, and 250 mGy at chronic phase. Many genes involved in toxicity responses were up- or down-regulated with the exposures to all doses. Our data have clearly shown the sensitivity and dose dependency of radiation-induced injury in hematopoietic stem/progenitor cells of mice with daily exposures to 2 ~ 250 mGy γ-ray.

Low dose cranial irradiation-induced cerebrovascular damage is reversible in mice

Background

High-dose radiation-induced blood-brain barrier breakdown contributes to acute radiation toxicity syndrome and delayed brain injury, but there are few data on the effects of low dose cranial irradiation. Our goal was to measure blood-brain barrier changes after low (0.1 Gy), moderate (2 Gy) and high (10 Gy) dose irradiation under in vivo and in vitro conditions.

Methodology

Cranial irradiation was performed on 10-day-old and 10-week-old mice. Blood-brain barrier permeability for Evans blue, body weight and number of peripheral mononuclear and circulating endothelial progenitor cells were evaluated 1, 4 and 26 weeks postirradiation. Barrier properties of primary mouse brain endothelial cells co-cultured with glial cells were determined by measurement of resistance and permeability for marker molecules and staining for interendothelial junctions. Endothelial senescence was determined by senescence associated β-galactosidase staining.

Principle Findings

Extravasation of Evans blue increased in cerebrum and cerebellum in adult mice 1 week and in infant mice 4 weeks postirradiation at all treatment doses. Head irradiation with 10 Gy decreased body weight. The number of circulating endothelial progenitor cells in blood was decreased 1 day after irradiation with 0.1 and 2 Gy. Increase in the permeability of cultured brain endothelial monolayers for fluorescein and albumin was time- and radiation dose dependent and accompanied by changes in junctional immunostaining for claudin-5, ZO-1 and β-catenin. The number of cultured brain endothelial and glial cells decreased from third day of postirradiation and senescence in endothelial cells increased at 2 and 10 Gy.

Conclusion

Not only high but low and moderate doses of cranial irradiation increase permeability of cerebral vessels in mice, but this effect is reversible by 6 months. In-vitro experiments suggest that irradiation changes junctional morphology, decreases cell number and causes senescence in brain endothelial cells.

Kojic acid protects C57BL/6 mice from gamma-irradiation induced damage

The radioprotective effects of a single administration of kojic acid (KA) against ionizing radiation were evaluated via assessment of 30-day survival and alterations of peripheral blood parameters of adult C57BL/6 male mice. The 30-day survival rate of mice pretreated with KA (75 or 300 mg/kg body weight, KA75 or KA300) subcutaneously 27 h prior to a lethal dose (8 Gy, 153.52 cGy/min) of gamma irradiation was higher than that of mice irradiated alone (40% or 60% vs 0%). It was observed that the white blood cell (WBC) count/the red blood cell (RBC) count, haemoglobin content, haematocrit and platelet count of mice with or without KA pretreatment as exposed to a sub-lethal dose (4 Gy, 148.14 cGy/min) of gamma irradiation decreased maximally at day 4/day 8 post-irradiation. Although the initial WBC values were low in KA300 or WR-2721 (amifostine) groups, they significantly recovered to normal at day 19, whereas in the control group they did not. The results from the cytotoxicity and cell viability assays demonstrated that KA could highly protect Chinese hamster ovary (CHO) cells against ionizing radiation with low toxicity. In summary, KA provides marked radioprotective effects both in vivo and in vitro.

Space Radiation: The Number One Risk to Astronaut Health beyond Low Earth Orbit

Projecting a vision for space radiobiological research necessitates understanding the nature of the space radiation environment and how radiation risks influence mission planning, timelines and operational decisions. Exposure to space radiation increases the risks of astronauts developing cancer, experiencing central nervous system (CNS) decrements, exhibiting degenerative tissue effects or developing acute radiation syndrome. One or more of these deleterious health effects could develop during future multi-year space exploration missions beyond low Earth orbit (LEO). Shielding is an effective countermeasure against solar particle events (SPEs), but is ineffective in protecting crew members from the biological impacts of fast moving, highly-charged galactic cosmic radiation (GCR) nuclei. Astronauts traveling on a protracted voyage to Mars may be exposed to SPE radiation events, overlaid on a more predictable flux of GCR. Therefore, ground-based research studies employing model organisms seeking to accurately mimic the biological effects of the space radiation environment must concatenate exposures to both proton and heavy ion sources. New techniques in genomics, proteomics, metabolomics and other “omics” areas should also be intelligently employed and correlated with phenotypic observations. This approach will more precisely elucidate the effects of space radiation on human physiology and aid in developing personalized radiological countermeasures for astronauts.

Possible modulating impact of glutathione disulfide mimetic on physiological changes in irradiated rats

Glutathione disulfide mimetic (NOV-002) is a complex of oxidized glutathione (GSSG) formulated with cisplatin at approximately 1000:1 molar ratio. Cisplatin serves to stabilize GSSG but does not assert any therapeutic effect. The objective of this study is to evaluate the impact of NOV-002 on hematological suppression, excessive free radical damage and DNA fragmentation in splenocytes, and metabolite disorders in whole-body γ-irradiated rats. The obtained data revealed that rats treated with 25 mg kg(-1) NOV-002 injected intraperitoneally (i.p.) for 5 days after whole-body γ-irradiation (IR) at 6.5 Gy attenuated the decrease of red blood cells, platelets, total white blood cells, absolute lymphocytes and neutrophils counts, hematocrit value, and hemoglobin content. NOV-002 treatment inhibits serum advanced oxidation protein products, malondialdehyde concentrations as well as cholesterol, triglycerides, urea, and creatinine levels, while enhances glutathione content and superoxide dismutase activity and improves DNA fragmentation in splenocytes. These findings provide a better understanding of the NOV-002 modulating impact in whole-body γ-rays-induced hematological toxicities, oxidative stress, and biological disturbances in γ-irradiated rats and could enhance the tolerance to high doses of ionizing IR utilized in radiotherapy.

HSC senescence upon irradiation

In this issue of Blood, Shao et al report that a side effect of total body irradiation (TBI) is long-term bone marrow injury and thus dysfunctional hematopoiesis caused by the induction of hematopoietic stem cell (HSC) senescence. Interestingly and unexpectedly, however, this happens in a manner independent of the cell-cycle regulators Ink4a and Arf, which play a major role in senescence in other cell systems.

Hepatocyte senescence induced by radiation and partial hepatectomy in rat liver

PURPOSE

Exposure to radiation primes the liver for extensive replacement of the resident parenchymal cells by transplanted hepatocytes. The mechanisms underlying this repopulation remain to be clarified. In these studies, we examined the possible occurrence of cell senescence in vivo following radiation-associated preconditioning of the host liver.

MATERIALS AND METHODS

Fischer 344 rats underwent external-beam, computed-tomography-based partial liver irradiation. A single dose of 25 Gy was delivered to the right liver lobes (40% of liver mass). An additional group of animals received a 1/3 partial hepatectomy (removal of the left anterior lobe) four days after irradiation. Non-irradiated groups served as controls. All rats were sacrificed four weeks after the initial treatment.

RESULTS

The irradiated livers displayed several markers of cell senescence, including expression of senescence-associated-β-galactosidase (SA-β-gal), increase in cell size, and up-regulation of cyclin-dependent kinase inhibitors (CDK-I) p16 and p21. Furthermore, quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR) analysis revealed activation of the senescence-associated secretory phenotype (SASP), including the cytokines interleukin 6 (IL6) and 1α (IL1α). The senescence-related changes were more prominent in rats undergoing partial hepatectomy (PH) following irradiation (IR).

CONCLUSIONS

We conclude that priming with radiation for liver repopulation results in the induction of cell senescence and the up-regulation of a senescence-associated secretory phenotype. The latter can contribute to the extensive growth of transplanted cells in this system.

Bone Marrow Protein Oxidation in Response to Ionizing Radiation in C57BL/6J Mice

The bone marrow is one of the most radio-sensitive tissues. Accidental ionizing radiation exposure can damage mature blood cells and hematopoietic progenitor/stem cells, and mortality can result from hematopoietic insufficiency and infection. Ionizing radiation induces alterations in gene and protein expression in hematopoietic tissue. Here we investigated radiation effects on protein carbonylation, a primary marker for protein oxidative damage. C57BL/6 mice were either sham irradiated or exposed to 7.5 Gy ⁶⁰Co (0.6 Gy/min) total body irradiation. Bone marrow was obtained 24 h post-irradiation. Two dimensional (2-D) gel electrophoresis and Oxyblot immunodetection were used to discover carbonylated proteins, and peptide mass fingerprinting was performed for identification. 2D gels allowed the detection of 13 carbonylated proteins in the bone marrow; seven of these were identified, with two pairs of the same protein. Baseline levels of carbonylation were found in 78 kDa glucose-related protein, heat shock protein cognate 71 KDa, actin, chitinase-like protein 3 (CHI3L1), and carbonic anhydrase 2 (CAII). Radiation increased carbonylation in four proteins, including CHI3L1 and CAII, and induced carbonylation of one additional protein (not identified). Our findings indicate that the profile of specific protein carbonylation in bone marrow is substantially altered by ionizing radiation. Accordingly, protein oxidation may be a mechanism for reduced cell viability.

Administration of the resveratrol analogues isorhapontigenin and heyneanol-A protects mice hematopoietic cells against irradiation injuries

Ionizing radiation (IR) is known not only to cause acute bone marrow (BM) suppression but also to lead to long-term residual hematopoietic injury. These effects have been attributed to IR inducing the generation of reactive oxygen species (ROS) in hematopoietic cells. In this study, we examined if isorhapontigenin and heyneanol-A, two analogues of resveratrol, could mitigate IR-induced BM suppression. The results of cell viability assays, clonogenic assays, and competitive repopulation assays revealed that treatment with these compounds could protect mice BM mononuclear cells (BMMNC), hematopoietic progenitor cells, and hematopoietic stem cells from IR-induced BM suppression. Moreover, the expression of genes related to the endogenous cellular antioxidant system in hematopoietic cells was analyzed. The expression and activity of SOD2 and GPX1 were found to be decreased in irradiated BMMNC, and the application of the resveratrol analogues could ameliorate this damage. Our results suggest that in comparison with resveratrol and isorhapontigenin, treatment with heyneanol-A can protect hematopoietic cells from IR-induced damage to a greater degree; the protective effects of these compounds are probably the result of their antioxidant properties.

Purinergic P2Y₁₄ receptor modulates stress-induced hematopoietic stem/progenitor cell senescence

Purinergic receptors of the P2Y family are G protein-coupled surface receptors that respond to extracellular nucleotides and can mediate responses to local cell damage. P2Y-dependent signaling contributes to thrombotic and/or inflammatory consequences of tissue injury by altering platelet and endothelial activation and immune cell phagocytosis. Here, we have demonstrated that P2Y14 modifies cell senescence and cell death in response to tissue stress, thereby enabling preservation of hematopoietic stem/progenitor cell function. In mice, P2Y14 deficiency had no demonstrable effect under homeostatic conditions; however, radiation stress, aging, sequential exposure to chemotherapy, and serial bone marrow transplantation increased senescence in animals lacking P2Y14. Enhanced senescence coincided with increased ROS, elevated p16(INK4a) expression, and hypophosphorylated Rb and was inhibited by treatment with a ROS scavenger or inhibition of p38/MAPK and JNK. Treatment of WT cells with pertussis toxin recapitulated the P2Y14 phenotype, suggesting that P2Y14 mediates antisenescence effects through Gi/o protein-dependent pathways. Primitive hematopoietic cells lacking P2Y14 were compromised in their ability to restore hematopoiesis in irradiated mice. Together, these data indicate that P2Y14 on stem/progenitor cells of the hematopoietic system inhibits cell senescence by monitoring and responding to the extracellular manifestations of tissue stress and suggest that P2Y14-mediated responses prevent the premature decline of regenerative capacity after injury.

Mitigating the effects of Xuebijing injection on hematopoietic cell injury induced by total body irradiation with γ rays by decreasing reactive oxygen species levels

Hematopoietic injury is the most common side effect of radiotherapy. However, the methods available for the mitigating of radiation injury remain limited. Xuebijing injection (XBJ) is a traditional Chinese medicine used to treat sepsis in the clinic. In this study, we investigated the effects of XBJ on the survival rate in mice with hematopoietic injury induced by γ ray ionizing radiation (IR). Mice were intraperitoneally injected with XBJ daily for seven days after total body irradiation (TBI). Our results showed that XBJ (0.4 mL/kg) significantly increased 30-day survival rates in mice exposed to 7.5 Gy TBI. This effect may be attributable to improved preservation of white blood cells (WBCs) and hematopoietic cells, given that bone marrow (BM) cells from XBJ-treated mice produced more granulocyte-macrophage colony forming units (CFU-GM) than that in the 2 Gy/TBI group. XBJ also decreased the levels of reactive oxygen species (ROS) by increasing glutathione (GSH) and superoxide dismutase (SOD) levels in serum and attenuated the increased BM cell apoptosis caused by 2 Gy/TBI. In conclusion, these findings suggest that XBJ enhances the survival rate of irradiated mice and attenuates the effects of radiation on hematopoietic injury by decreasing ROS production in BM cells, indicating that XBJ may be a promising therapeutic candidate for reducing hematopoietic radiation injury.

Radiation sensitivity of human and murine peripheral blood lymphocytes, stem and progenitor cells

Immunodeficiency is a severe side effect of radiation therapy, notably at high radiation doses. It may also impact healthy individuals exposed to environmental ionizing radiation. Although it is believed to result from cytotoxicity of bone marrow cells and of immunocompetent cells in the peripheral blood, the response of distinct bone marrow and blood cell subpopulations following exposure to ionizing radiation is not yet fully explored. In this review, we aim to compile the knowledge on radiation sensitivity of immunocompetent cells and to summarize data from bone marrow and peripheral blood cells derived from mouse and human origin. In addition, we address the radiation response of blood stem and progenitor cells. The data indicate that stem cells, T helper cells, cytotoxic T cells, monocytes, neutrophils and, at a high degree, B cells display a radiation sensitive phenotype while regulatory T cells, macrophages, dendritic cells and natural killer cells appear to be more radioresistant. No conclusive data are available for basophil and eosinophil granulocytes. Erythrocytes and thrombocytes, but not their precursors, seem to be highly radioresistant. Overall, the data indicate considerable differences in radiosensitivity of bone marrow and blood normal and malignant cell populations, which are discussed in the light of differential radiation responses resulting in hematotoxicity and related clinical implications.

CD166 regulates human and murine hematopoietic stem cells and the hematopoietic niche

We previously showed that immature CD166(+) osteoblasts (OB) promote hematopoietic stem cell (HSC) function. Here, we demonstrate that CD166 is a functional HSC marker that identifies both murine and human long-term repopulating cells. Both murine LSKCD48(-)CD166(+)CD150(+) and LSKCD48(-)CD166(+)CD150(+)CD9(+) cells, as well as human Lin(-)CD34(+)CD38(-)CD49f(+)CD166(+) cells sustained significantly higher levels of chimerism in primary and secondary recipients than CD166(-) cells. CD166(-/-) knockout (KO) LSK cells engrafted poorly in wild-type (WT) recipients and KO bone marrow cells failed to radioprotect lethally irradiated WT recipients. CD166(-/-) hosts supported short-term, but not long-term WT HSC engraftment, confirming that loss of CD166 is detrimental to the competence of the hematopoietic niche. CD166(-/-) mice were significantly more sensitive to hematopoietic stress. Marrow-homed transplanted WT hematopoietic cells lodged closer to the recipient endosteum than CD166(-/-) cells, suggesting that HSC-OB homophilic CD166 interactions are critical for HSC engraftment. STAT3 has 3 binding sites on the CD166 promoter and STAT3 inhibition reduced CD166 expression, suggesting that both CD166 and STAT3 may be functionally coupled and involved in HSC competence. These studies illustrate the significance of CD166 in the identification and engraftment of HSC and in HSC-niche interactions, and suggest that CD166 expression can be modulated to enhance HSC function.

Inhibiting glycogen synthase kinase-3 mitigates the hematopoietic acute radiation syndrome in mice

Exposure to a nuclear accident or radiological attack can cause death from acute radiation syndrome (ARS), which results from radiation injury to vital organs such as the hematopoietic system. However, the U.S. Food and Drug Administration (FDA) has not approved any medical countermeasures for this specific purpose. With growing concern over nuclear terrorism, there is an urgent need to develop small molecule deliverables that mitigate mortality from ARS. One emerging modulator of hematopoietic stem/progenitor cell (HSPC) activity is glycogen synthase kinase-3 (GSK-3). The inhibition of GSK-3 has been shown to augment hematopoietic repopulation in mouse models of bone marrow transplantation. In this study, we performed an in vitro screen using irradiated bone marrow mononuclear cells (BM-MNCs) to test the effects of four GSK-3 inhibitors: CHIR99021; 6-Bromoindirubin-3'-oxime (BIO); SB415286; and SB216763. This screen showed that SB216763 significantly increased the frequency of c-Kit(+) Lin(-) Sca1(+) (KLS) cells and hematopoietic colony-forming cells in irradiated BM-MNCs. Importantly, administration of a single dose of SB216763 to C57BL/6J mice by subcutaneous injection 24 h after total-body irradiation significantly improved hematopoietic recovery and mitigated hematopoietic ARS. Collectively, our results demonstrate that the GSK-3 inhibitor SB216763 is an effective medical countermeasure against acute radiation injury of the hematopoietic system.

Hematopoietic stem cell injury induced by ionizing radiation

SIGNIFICANCE

Exposure to ionizing radiation (IR) as the result of nuclear accidents or terrorist attacks is a significant threat and a major medical concern. Hematopoietic stem cell (HSC) injury is the primary cause of death after accidental or intentional exposure to a moderate or high dose of IR. Protecting HSCs from IR should be a primary goal in the development of novel medical countermeasures against radiation.

RECENT ADVANCES

Significant progress has been made in our understanding of the mechanisms by which IR causes HSC damage. The mechanisms include (i) induction of HSC apoptosis via the p53-Puma pathway; (ii) promotion of HSC differentiation via the activation of the G-CSF/Stat3/BATF-dependent differentiation checkpoint; (iii) induction of HSC senescence via the ROS-p38 pathway; and (iv) damage to the HSC niche.

CRITICAL ISSUES

Induction of apoptosis in HSCs and hematopoietic progenitor cells is primarily responsible for IR-induced acute bone marrow (BM) injury. Long-term BM suppression caused by IR is mainly attributable to the induction of HSC senescence. However, the promotion of HSC differentiation and damage to the HSC niche can contribute to both the acute and long-term effects of IR on the hematopoietic system.

FUTURE DIRECTIONS

In this review, we have summarized a number of recent findings that provide new insights into the mechanisms whereby IR damages HSCs. These findings will provide new opportunities for developing a mechanism-based strategy to prevent and/or mitigate IR-induced BM suppression. Antioxid.

Total body irradiation causes long-term mouse BM injury via induction of HSC premature senescence in an Ink4a- and Arf-independent manner

Exposure to total body irradiation (TBI) induces not only acute hematopoietic radiation syndrome but also long-term or residual bone marrow (BM) injury. This residual BM injury is mainly attributed to permanent damage to hematopoietic stem cells (HSCs), including impaired self-renewal, decreased long-term repopulating capacity, and myeloid skewing. These HSC defects were associated with significant increases in production of reactive oxygen species (ROS), expression of p16(Ink4a) (p16) and Arf mRNA, and senescence-associated β-galacotosidase (SA-β-gal) activity, but not with telomere shortening or increased apoptosis, suggesting that TBI induces residual BM injury via induction of HSC premature senescence. This suggestion is supported by the finding that SA-β-gal(+) HSC-enriched LSK cells showed more pronounced defects in clonogenic activity in vitro and long-term engraftment after transplantation than SA-β-gal(-) LSK cells isolated from irradiated mice. However, genetic deletion of p16 and/or Arf had no effect on TBI-induced residual BM suppression and HSC senescence, because HSCs from irradiated p16 and/or Arf knockout (KO) mice exhibited changes similar to those seen in HSCs from wild-type mice after exposure to TBI. These findings provide important new insights into the mechanism by which TBI causes long-term BM suppression (eg, via induction of premature senescence of HSCs in a p16-Arf-independent manner).

Mitigation of radiation-induced hematopoietic injury via regulation of cellular MAPK/phosphatase levels and increasing hematopoietic stem cells

Here we describe a novel strategy for mitigation of ionizing radiation-induced hematopoietic syndrome by suppressing the activity of MKP3, resulting in ERK activation and enhanced abundance of hematopoietic stem cells, using the antioxidant flavonoid baicalein (5,6,7-trihydroxyflavone). It offered complete protection to mouse splenic lymphocytes against radiation-induced cell death. Inhibitors of ERK and Nrf-2 could significantly abrogate baicalein-mediated radioprotection in lymphocytes. Baicalein inhibited phosphatase MKP3 and thereby enhanced phosphorylation of ERK and its downstream proteins such as Elk and Nrf-2. It also increased the nuclear levels of Nrf-2 and the mRNA levels of its dependent genes. Importantly, baicalein administration to mice before radiation exposure led to significant recovery of loss of bone marrow cellularity and also inhibited cell death. Administration of baicalein increased the hematopoietic stem cell frequency as measured by side-population assay and also by antibody staining. Further, baicalein offered significant protection against whole-body irradiation (WBI; 7.5Gy)-induced mortality in mice. Interestingly, we found that baicalein works by activating the same target molecules ERK and Nrf-2 both in vitro and in vivo. Finally, administration of all-trans-retinoic acid (inhibitor of Nrf-2) significantly abrogated baicalein-mediated protection against WBI-induced mortality in mice. Thus, in contrast to the generalized conception of antioxidants acting as radioprotectors, we provide a rationale that antioxidants exhibit pleiotropic effects through the activation of multiple cellular signaling pathways.

NRF2-mediated Notch pathway activation enhances hematopoietic reconstitution following myelosuppressive radiation

A nuclear disaster may result in exposure to potentially lethal doses of ionizing radiation (IR). Hematopoietic acute radiation syndrome (H-ARS) is characterized by severe myelosuppression, which increases the risk of infection, bleeding, and mortality. Here, we determined that activation of nuclear factor erythroid-2-related factor 2 (NRF2) signaling enhances hematopoietic stem progenitor cell (HSPC) function and mitigates IR-induced myelosuppression and mortality. Augmenting NRF2 signaling in mice, either by genetic deletion of the NRF2 inhibitor Keap1 or by pharmacological NRF2 activation with 2-trifluoromethyl-2'-methoxychalone (TMC), enhanced hematopoietic reconstitution following bone marrow transplantation (BMT). Strikingly, even 24 hours after lethal IR exposure, oral administration of TMC mitigated myelosuppression and mortality in mice. Furthermore, TMC administration to irradiated transgenic Notch reporter mice revealed activation of Notch signaling in HSPCs and enhanced HSPC expansion by increasing Jagged1 expression in BM stromal cells. Administration of a Notch inhibitor ablated the effects of TMC on hematopoietic reconstitution. Taken together, we identified a mechanism by which NRF2-mediated Notch signaling improves HSPC function and myelosuppression following IR exposure. Our data indicate that targeting this pathway may provide a countermeasure against the damaging effects of IR exposure.

Ginsenoside Rg1 enhances the resistance of hematopoietic stem/progenitor cells to radiation-induced aging in mice

AIM

To investigate the effects of ginsenoside Rg1 on the radiation-induced aging of hematopoietic stem/progenitor cells (HSC/HPCs) in mice and the underlying mechanisms.

METHODS

Male C57BL/6 mice were treated with ginsenoside Rg1 (20 mg·kg(-1)·d(-1), ip) or normal saline (NS) for 7 d, followed by exposure to 6.5 Gy X-ray total body irradiation. A sham-irradiated group was treated with NS but without irradiation. Sca-1(+) HSC/HPCs were isolated and purified from their bone marrow using MACS. DNA damage was detected on d 1. The changes of anti-oxidative activities, senescence-related markers senescence-associated β-galactosidase (SA-β-gal) and mixed colony-forming unit (CFU-mix), P16(INK4a) and P21(Cip1/Waf1) expression on d 7, and cell cycle were examined on d 1, d 3, and d 7.

RESULTS

The irradiation caused dramatic reduction in the number of Sca-1(+) HSC/HPCs on d 1 and the number barely recovered until d 7 compared to the sham-irradiated group. The irradiation significantly decreased SOD activity, increased MDA contents and caused DNA damage in Sca-1(+) HSC/HPCs. Moreover, the irradiation significantly increased SA-β-gal staining, reduced CFU-mix forming, increased the expression of P16(INK4a) and P21(Cip1/Waf1) in the core positions of the cellular senescence signaling pathways and caused G1 phase arrest of Sca-1(+) HSC/HPCs. Administration of ginsenoside Rg1 caused small, but significant recovery in the number of Sca-1(+) HSC/HPCs on d 3 and d 7. Furthermore, ginsenoside Rg1 significantly attenuated all the irradiation-induced changes in Sca-1(+) HSC/HPCs, including oxidative stress reaction, DNA damage, senescence-related markers and cellular senescence signaling pathways and cell cycle, etc.

CONCLUSION

Administration of ginsenoside Rg1 enhances the resistance of HSC/HPCs to ionizing radiation-induced senescence in mice by inhibiting the oxidative stress reaction, reducing DNA damage, and regulating the cell cycle.

Reactive oxygen species in normal and tumor stem cells

Reactive oxygen species (ROS) play an important role in determining the fate of normal stem cells. Low levels of ROS are required for stem cells to maintain quiescence and self-renewal. Increases in ROS production cause stem cell proliferation/differentiation, senescence, and apoptosis in a dose-dependent manner, leading to their exhaustion. Therefore, the production of ROS in stem cells is tightly regulated to ensure that they have the ability to maintain tissue homeostasis and repair damaged tissues for the life span of an organism. In this chapter, we discuss how the production of ROS in normal stem cells is regulated by various intrinsic and extrinsic factors and how the fate of these cells is altered by the dysregulation of ROS production under various pathological conditions. In addition, the implications of the aberrant production of ROS by tumor stem cells for tumor progression and treatment are also discussed.

Survival efficacy of the PEGylated G-CSFs Maxy-G34 and neulasta in a mouse model of lethal H-ARS, and residual bone marrow damage in treated survivors

In an effort to expand the worldwide pool of available medical countermeasures (MCM) against radiation, the PEGylated G-CSF (PEG-G-CSF) molecules Neulasta and Maxy-G34, a novel PEG-G-CSF designed for increased half-life and enhanced activity compared to Neulasta, were examined in a murine model of the Hematopoietic Syndrome of the Acute Radiation Syndrome (H-ARS), along with the lead MCM for licensure and stockpiling, G-CSF. Both PEG-G-CSFs were shown to retain significant survival efficacy when administered as a single dose 24 h post-exposure, compared to the 16 daily doses of G-CSF required for survival efficacy. Furthermore, 0.1 mg kg of either PEG-G-CSF affected survival of lethally-irradiated mice that was similar to a 10-fold higher dose. The one dose/low dose administration schedules are attractive attributes of radiation MCM given the logistical challenges of medical care in a mass casualty event. Maxy-G34-treated mice that survived H-ARS were examined for residual bone marrow damage (RBMD) up to 9 mo post-exposure. Despite differences in Sca-1 expression and cell cycle position in some hematopoietic progenitor phenotypes, Maxy-G34-treated mice exhibited the same degree of hematopoietic stem cell (HSC) insufficiency as vehicle-treated H-ARS survivors in competitive transplantation assays of 150 purified Sca-1+cKit+lin-CD150+cells. These data suggest that Maxy-G34, at the dose, schedule, and time frame examined, did not mitigate RBMD but significantly increased survival from H-ARS at one-tenth the dose previously tested, providing strong support for advanced development of Maxy-G34, as well as Neulasta, as MCM against radiation.

Expression of the senescence marker p16INK4a in skin biopsies of acute lymphoblastic leukemia survivors: a pilot study

Background

Most childhood cancer survivors will develop ionizing radiation treatment-related health conditions that, in many instances, resemble age-associated pathologies. Treatment-induced premature senescence could be an underlying mechanism.

Findings

Here we wanted to know whether the expression of p16^INK4a, a senescence/aging biomarker, is increased in skin biopsies of acute lymphoblastic leukemia survivors (ALL), previously exposed to chemotherapy and radiation therapy. Several years post-treatments, we found p16^INK4a mRNA levels are 5.8 times higher in scalp skin biopsies (targeted by cranial irradiation therapy) compared to buttocks skin biopsies (n = 10, p = 0.01).

Conclusions

These results demonstrate for the first time that premature senescence is induced in pediatric cancer survivors and that p16^INK4a expression could be used as a potential biomarker in this population.

Role of p53 in regulating tissue response to radiation by mechanisms independent of apoptosis

Radiation exposure leads to diverse outcomes in vivo across different tissues and even within the same cell lineage. The diversity of radiation response in vivo is at least partially attributable to the status of the tumor suppressor p53, a master regulator of cellular response to stress, and activation of its transcriptional targets. In certain cells, such as hematopoietic progenitors and transit amplifying cells in the gastrointestinal epithelium, activation of p53 by radiation triggers the intrinsic pathway of apoptosis. However, in many other cells, activation of p53 by radiation does not result in apoptosis, which underscores the importance of understanding the role of p53 in regulating radiation response through alternative mechanisms. In this review, we summarize recent studies using genetically engineered mice to dissect the role of p53 in 1) cells where its activation is dissociated from the intrinsic pathway of apoptosis, such as hematopoietic stem cells and vascular endothelial cells and 2) tissues where activation of the intrinsic pathway of apoptosis does not promote the acute radiation syndrome, such as the gastrointestinal epithelium. We highlight findings showing that the apoptosis-independent response of p53 to radiation in vivo can contribute to death or survival in a cell-type dependent manner, which underscores the complexity by which p53 regulates the cellular and tissue response to radiation.

Deletion of Irf5 protects hematopoietic stem cells from DNA damage-induced apoptosis and suppresses γ-irradiation-induced thymic lymphomagenesis

Repeated low-dose γ-irradiation (IR) induces thymic lymphoma in mice because of oncogenic mutations propagating from a primitive hematopoietic stem/progenitor cell (HSC) in the bone marrow. It is well known that IR-induced thymic lymphomagenesis is markedly enhanced by p53 deficiency, yet data also indicate that p53-dependent apoptosis can actively drive tumor formation in this model. The latter was recently expounded on by findings from Puma-deficient mice, indicating that loss of this proapoptotic p53 target gene results in protection from IR-induced lymphomagenesis rather than enhanced susceptibility to. Similar to Puma, the transcription factor interferon regulatory factor 5 (Irf5) has been reported as a p53 target gene and is required for DNA damage-induced apoptosis. To date, no studies have been performed to elucidate the in vivo role of IRF5 in tumorigenesis. Given its essential role in DNA damage-induced apoptosis, we explored the tumor suppressor function of IRF5 in IR-induced thymic lymphomagenesis. Somewhat surprisingly, we found that thymic lymphoma development was significantly suppressed in Irf5(-/-) mice as compared with wild-type littermates. Suppression was due, in part, to reduced thymocyte and HSC apoptosis, resulting in reduced compensatory proliferation, and reduced replication stress-associated DNA damage. The observed effects were independent of p53 or Puma as these proteins were upregulated in Irf5(-/-) mice in response to IR. This study demonstrates an important new role for IRF5 in maintaining HSC homeostasis after IR and supports the non-redundant functions of IRF5, p53 and PUMA in DNA damage-induced lymphomagenesis. We propose that IRF5 may be an attractive target for developing therapeutic agents to ameliorate radiation-induced bone marrow injury.

Activation of p53 with Nutlin-3a radiosensitizes lung cancer cells via enhancing radiation-induced premature senescence

Radiotherapy is routinely used for the treatment of lung cancer. However, the mechanisms underlying ionizing radiation (IR)-induced senescence and its role in lung cancer treatment are poorly understood. Here, we show that IR suppresses the proliferation of human non-small cell lung cancer (NSCLC) cells via an apoptosis-independent mechanism. Further investigations reveal that the anticancer effect of irradiation correlates well with IR-induced premature senescence, as evidenced by increased senescence-associated β-glactosidase (SA-β-gal) staining, decreased BrdU incorporation and elevated expression of p16(INK4a) (p16) in irradiated NSCLC cells. Mechanistic studies indicate that the induction of senescence is associated with activation of the p53-p21 pathway, and that inhibition of p53 transcriptional activity by PFT-α attenuates IR-induced tumor cell killing and senescence. Gain-of-function assays demonstrate that restoration of p53 expression sensitizes H1299 cells to irradiation, whereas knockdown of p53 expression by siRNA inhibits IR-induced senescence in H460 cells. Furthermore, treatment with Nutlin-3a, a small molecule inhibitor of MDM2, enhances IR-induced tumor cell killing and senescence by stabilizing the activation of the p53-p21 signaling pathway. Taken together, these findings demonstrate for the first time that pharmacological activation of p53 by Nutlin-3a can sensitize lung cancer cells to radiation therapy via promoting IR-induced premature senescence.

Mechanisms of radiation toxicity in transformed and non-transformed cells

Radiation damage to biological systems is determined by the type of radiation, the total dosage of exposure, the dose rate, and the region of the body exposed. Three modes of cell death—necrosis, apoptosis, and autophagy—as well as accelerated senescence have been demonstrated to occur in vitro and in vivo in response to radiation in cancer cells as well as in normal cells. The basis for cellular selection for each mode depends on various factors including the specific cell type involved, the dose of radiation absorbed by the cell, and whether it is proliferating and/or transformed. Here we review the signaling mechanisms activated by radiation for the induction of toxicity in transformed and normal cells. Understanding the molecular mechanisms of radiation toxicity is critical for the development of radiation countermeasures as well as for the improvement of clinical radiation in cancer treatment.

Inhibiting TGFβ1 has a protective effect on mouse bone marrow suppression following ionizing radiation exposure in vitro

Ionizing radiation (IR) causes not only acute tissue damage but also residual bone marrow (BM) suppression. The induction of residual BM injury is primarily attributable to the induction of reactive oxygen species (ROS) pressure in hematopoietic cells. In this study, we examined if SB431542, a transforming growth factor β1 (TGFβ1) inhibitor, can mitigate IR-induced BM suppression in vitro. Our results showed that treatment with SB431542 protected mice bone marrow mononuclear cells (BMMNCs), hematopoietic progenitor cells (HPCs) and hematopoietic stem cells (HSCs) from IR-induced suppression using cell viability assays, clonogenic assays and competitive repopulation assays. Moreover, expression of gene-related ROS production in hematopoietic cells was analyzed. The expression of NOX1, NOX2 and NOX4 was increased in irradiated BMMNCs, and that of NOX2 and NOX4 was reduced by SB431542 treatment. Therefore, the results from this study suggest that SB431542, a TGFβ1 inhibitor, alleviates IR-induced BM suppression at least in part via inhibiting IR-induced NOX2 and NOX4 expression.

Prolonged expression of senescence markers in mice exposed to gamma-irradiation

Although ionizing radiation is known to induce cellular senescence in vitro and in vivo, its long-term in vivo effects are not well defined. In this study, we examined the prolonged expression of senescence markers in mice irradiated with single or fractionated doses. C57BL/6 female mice were exposed to 5 Gy of γ-rays in single or 5, 10, 25 fractions. At 2, 4, and 6 months after irradiation, senescence markers including mitochondrial DNA (mtDNA) common deletion, p21, and senescence-associated β-galactosidase (SA β-gal) were monitored in the lung, liver, and kidney. Increases of mtDNA deletion were detected in the lung, liver, and kidney of irradiated groups. p21 expression and SA β-gal staining were also increased in the irradiated groups compared to the non-irradiated control group. Increases of senescence markers persisted up to 6 months after irradiation. Additionally, the extent of mtDNA deletion and the numbers of SA β-gal positive cells were greater as the number of radiation fractions increased. In conclusion, our results showed that ionizing radiation, especially that delivered in fractions, can cause the persistent upregulation of senescence marker expression in vivo. This should be considered when dealing with chronic normal tissue injuries caused by radiation therapy or radiation accidents.

[Children who received a radiotherapy treatment of the cranio-cervico-facial region: appraisal of the orthodontic treatment need]

INTRODUCTION

Currently, 1 out of 900 adults from the ages of 16 to 44 is a survivor of childhood cancer. The global rate of survival for all types of childhood cancer is close to 80% and progress in treatment and care continues to offer promising results. In the years following treatment, these children, just like other healthy children, should have access to orthodontic care in spite of their chronic health problems.

OBJECTIVES

A transverse epidemiological study was conducted at the Leon Bernard Cancer Treatment Center in Lyon in order to respond to these requests and to assist orthodontists with the therapeutic difficulties presented by these patients. Our objective was to identify the orthodontic treatment required for children 7 years old or older who received antineoplastic therapy with radiation of the cranio-cervico-facial region.

RESULTS

The results show that children who received radiation therapy in the cranio-facial region presented delayed growth, facial asymmetry in 74% of the cases, overbite and overjet increased respectively in 70% and 61% of the cases. Dental discrepancies in development were present in 83% of the patients who presented with stunted teeth, premature closing of the apices and hypoplasias. We found sialopathy such as reduced saliva flow rate. Finally, considerable treatment was necessary in 61% of the cases and moderate treatment for 17% of the cases, because of missing teeth, retained deciduous teeth or an increased overjet.

The effects of p38 MAPK inhibition combined with G-CSF administration on the hematoimmune system in mice with irradiation injury

The acute and residual (or long-term) bone marrow (BM) injury induced by ionizing radiation (IR) is a major clinic concern for patients receiving conventional radiotherapy and victims accidentally exposed to a moderate-to-high dose of IR. In this study, we investigated the effects of the treatment with the p38 inhibitor SB203580 (SB) and/or granulocyte colony-stimulating factor (G-CSF) on the hematoimmune damage induced by IR in a mouse model. Specifically, C57BL/6 mice were exposed to a sublethal dose (6 Gy) of total body irradiation (TBI) and then treated with vehicle, G-CSF, SB, and G-CSF plus SB. G-CSF (1 µg/mouse) was administrated to mice by intraperitoneal (ip) injection twice a day for six successive days; SB (15 mg/kg) by ip injection every other day for 10 days. It was found that the treatment with SB and/or G-CSF significantly enhanced the recovery of various peripheral blood cell counts and the number of BM mononuclear cells 10 and 30 days after the mice were exposed to TBI compared with vehicle treatment. Moreover, SB and/or G-CSF treatment also increased the clonogenic function of BM hematopoietic progenitor cells (HPCs) and the frequency of BM lineage⁻Sca1⁺c-kit⁺ cells (LSK cells) and short-term and long term hematopoietic stem cells (HSCs) 30 days after TBI, in comparison with vehicle treated controls. However, the recovery of peripheral blood B cells and CD4⁺ and CD8⁺ T cells was not significantly affected by SB and/or G-CSF treatment. These results suggest that the treatment with SB and/or G-CSF can reduce IR-induced BM injury probably in part via promoting HSC and HPC regeneration.

Exacerbation of lung radiation injury by viral infection: the role of Clara cells and Clara cell secretory protein

Viral infections have been associated with exacerbation of disease in human cases of idiopathic pulmonary fibrosis. Since pulmonary fibrosis is a common outcome after irradiation to the lung, we hypothesized that viral infection after radiation exposure would exacerbate radiation-induced lung injury. Epithelial injury, a frequent outcome after infection, has been hypothesized to contribute to the pathogenesis of pulmonary fibrosis and bronchiolar epithelial Clara cells participate in epithelial repair. Therefore, it was further hypothesized that altered responses after irradiation involve the bronchiolar epithelial Clara cells. C57BL/6J or CCSP(-/-) mice were irradiated with 0 (sham), 5, 10 or 15 Gy to the whole thorax. At ten weeks post-irradiation, animals were mock infected or infected with influenza A virus and body weight and survival were monitored. Pulmonary function was assessed by whole-body plethysmography. The Clara cell markers, CCSP and Cyp2f2, were measured in the lung by qRT-PCR, and protein expression was visualized in the lung by immunofluorescence. Following pulmonary function tests, mice were sacrificed and tissues were collected for pathological analysis. In 15 Gy irradiated animals infected with influenza A virus, accelerated respiratory rates, reduced pulmonary function, and exacerbated lung pathology occurred earlier post-irradiation than previously observed after irradiation alone, suggesting infection accelerates the development of radiation injury. After irradiation alone, CCSP and Cyp2f2 mRNA levels were reduced, correlating with reductions in the number of Clara cells lining the airways. When combined with infection, these markers further declined and an apparent delay in recovery of mRNA expression was observed, suggesting that radiation injury leads to a chronic reduction in the number of Clara cells that may potentiate the epithelial injury observed after influenza A virus infection. This novel finding may have considerable therapeutic implications with respect to both thoracic tumor patients and recipients of bone marrow transplants.

Resveratrol induces premature senescence in lung cancer cells via ROS-mediated DNA damage

Resveratrol (RV) is a natural component of red wine and grapes that has been shown to be a potential chemopreventive and anticancer agent. However, the molecular mechanisms underlying RV's anticancer and chemopreventive effects are incompletely understood. Here we show that RV treatment inhibits the clonogenic growth of non-small cell lung cancer (NSCLC) cells in a dose-dependent manner. Interestingly, the tumor-suppressive effect of low dose RV was not associated with any significant changes in the expression of cleaved PARP and activated caspase-3, suggesting that low dose RV treatment may suppress tumor cell growth via an apoptosis-independent mechanism. Subsequent studies reveal that low dose RV treatment induces a significant increase in senescence-associated β–galactosidase (SA-β-gal) staining and elevated expression of p53 and p21 in NSCLC cells. Furthermore, we show that RV-induced suppression of lung cancer cell growth is associated with a decrease in the expression of EF1A. These results suggest that RV may exert its anticancer and chemopreventive effects through the induction of premature senescence. Mechanistically, RV-induced premature senescence correlates with increased DNA double strand breaks (DSBs) and reactive oxygen species (ROS) production in lung cancer cells. Inhibition of ROS production by N-acetylcysteine (NAC) attenuates RV-induced DNA DSBs and premature senescence. Furthermore, we show that RV treatment markedly induces NAPDH oxidase-5 (Nox5) expression in both A549 and H460 cells, suggesting that RV may increase ROS generation in lung cancer cells through upregulating Nox5 expression. Together, these findings demonstrate that low dose RV treatment inhibits lung cancer cell growth via a previously unappreciated mechanism, namely the induction of premature senescence through ROS-mediated DNA damage.

Resveratrol ameliorates ionizing irradiation-induced long-term hematopoietic stem cell injury in mice

Our recent studies showed that total body irradiation (TBI) induces long-term bone marrow (BM) suppression in part by induction of hematopoietic stem cell (HSC) senescence through NADPH oxidase 4 (NOX4)-derived reactive oxygen species (ROS). Therefore, in this study we examined whether resveratrol (3,5,4'-trihydroxy-trans-stilbene), a potent antioxidant and a putative activator of Sirtuin 1 (Sirt1), can ameliorate TBI-induced long-term BM injury by inhibiting radiation-induced chronic oxidative stress and senescence in HSCs. Our results showed that pretreatment with resveratrol not only protected mice from TBI-induced acute BM syndrome and lethality but also ameliorated TBI-induced long-term BM injury. The latter effect is probably attributable to resveratrol-mediated reduction of chronic oxidative stress in HSCs, because resveratrol treatment significantly inhibited TBI-induced increase in ROS production in HSCs and prevented mouse BM HSCs from TBI-induced senescence, leading to a significant improvement in HSC clonogenic function and long-term engraftment after transplantation. The inhibition of TBI-induced ROS production in HSCs is probably attributable to resveratrol-mediated downregulation of NOX4 expression and upregulation of Sirt1, superoxide dismutase 2 (SOD2), and glutathione peroxidase 1 expression. Furthermore, we showed that resveratrol increased Sirt1 deacetylase activity in BM hematopoietic cells; and Ex527, a potent Sirt1 inhibitor, can attenuate resveratrol-induced SOD2 expression and the radioprotective effect of resveratrol on HSCs. These findings demonstrate that resveratrol can protect HSCs from radiation at least in part via activation of Sirt1. Therefore, resveratrol has the potential to be used as an effective therapeutic agent to ameliorate TBI-induced long-term BM injury.

CD34+ cells from patients with myelodysplastic syndrome present different p21 dependent premature senescence

The mechanisms by which hematopoietic stem and progenitor cells (HSC and HPC) from myelodysplastic syndromes (MDS) undergo ineffective production of blood cells and disease transformation into acute myeloid leukemia remain to be investigated. It has been confirmed that increased production of reactive oxygen species (ROS) under various pathological conditions impairs HSC self-renewal and causes HSC premature exhaustion and BM suppression primarily via induction of HSC senescence, and oncogene induces accumulation of ROS and DNA damage and subsequently cellular senescence, which functions as an important barrier to prevent the growth of transformed cells to form a neoplasia. Here we investigated whether MDS CD34(+) cells enriched with HSC and HPC undergo senescence through accumulation of ROS and DNA damage and their action mechanisms. In this study, the percentages of SA-β-gal positive senescent CD34(+) cells increased in lower-risk MDS patients, but not in higher-risk MDS and AML patients, compared to that of healthy controls. The increases were associated with an elevated expression of p21 but not the activation of p38. Further study found that there were increased ROS and DNA damage in CD34(+)CD38(-) cells enriched with HSC progression from lower-risk MDS, higher-risk MDS to AML. Therefore, these data suggest that CD34(+) cells from patients with lower-risk MDS present p21 dependent premature senescence, increased accumulation of ROS and DNA damage in CD34(+)CD38(-) cells could contribute to this process; however, CD34(+) cells from patients with higher-risk MDS could develop some mechanisms to uncouple ROS and DNA damage induced senescence.

Compromised hematopoiesis and increased DNA damage following non-lethal ionizing radiation of a human hematopoietic system reconstituted in immunodeficient mice

PURPOSE

Precise understanding of radiation effects is critical to development of new modalities for the prevention and treatment of radiation-induced damage. In this study, we evaluated the effects of non-lethal doses of X-ray irradiation on human hematopoietic stem and progenitor cells (HSPC) reconstituted in NOD/Shi-scid, IL2Rγ(null) (NOG) immunodeficient mice.

MATERIALS AND METHODS

We transplanted cord blood CD34(+) HSPC into NOG mice irradiated with 2.0 Gy via tail veins. At the 12th week after transplantation, the NOG mice were irradiated with 0, 0.5, 1.0, 2.0, or 4.0 Gy, and the radiation effects on human HSPC in vivo were evaluated.

RESULTS

Although a majority of the mice irradiated with 2.0 Gy or more died in 12 weeks after irradiation, the mice that were exposed to 0.5 or 1.0 Gy of irradiation survived and were subjected to analysis. The chimerism of human CD45(+) hematopoietic cells in peripheral blood and bone marrow (BM) of the recipient mice was reduced in an X-ray dose-dependent manner after irradiation. Percentages of human CD34(+) HSPC as well as human (CD34+CD38-) HSC in BM similarly declined. (CD34+CD38-) HSC purified from the humanized mice at the 12th week after irradiation showed significantly increased numbers of phosphorylated H2AX (γH2AX) foci, a marker of DNA breaks, in an X-ray dose- dependent manner. Expression of p16INK4A, a hallmark of aging of HSC, was also detected only in HSPC from irradiated mice.

CONCLUSIONS

With further refinement, the humanized mouse model might be effectively used to study the biological effects of non-lethal radiation in vivo.

Long-term hematopoietic stem cell damage in a murine model of the hematopoietic syndrome of the acute radiation syndrome

Residual bone marrow damage (RBMD) persists for years following exposure to radiation and is believed to be due to decreased self-renewal potential of radiation-damaged hematopoietic stem cells (HSC). Current literature has examined primarily sublethal doses of radiation and time points within a few months of exposure. In this study, the authors examined RBMD in mice surviving lethal doses of total body ionizing irradiation (TBI) in a murine model of the Hematopoietic Syndrome of the Acute Radiation Syndrome (H-ARS). Survivors were analyzed at various time points up to 19 mo post-TBI for hematopoietic function. The competitive bone marrow (BM) repopulating potential of 150 purified c-Kit+ Sca-1+ lineage- CD150+ cells (KSLCD150+) remained severely deficient throughout the study compared to KSLCD150+ cells from non-TBI age-matched controls. The minimal engraftment from these TBI HSCs is predominantly myeloid, with minimal production of lymphocytes both in vitro and in vivo. All classes of blood cells as well as BM cellularity were significantly decreased in TBI mice, especially at later time points as mice aged. Primitive BM hematopoietic cells (KSLCD150+) displayed significantly increased cell cycling in TBI mice at all time points, which may be a physiological attempt to maintain HSC numbers in the post-irradiation state. Taken together, these data suggest that the increased cycling among primitive hematopoietic cells in survivors of lethal radiation may contribute to long-term HSC exhaustion and subsequent RBMD, exacerbated by the added insult of aging at later time points.

Bilirubin augments radiation injury and leads to increased infection and mortality in mice: molecular mechanisms

Our earlier results demonstrated that clinically relevant concentrations of unconjugated bilirubin (UCB) possessed immunotoxic effects. Whole-body irradiation (WBI) with 1 to 6 Gy leads to acute radiation syndrome, immunosuppression, and makes the host susceptible to infection. Since hyperbilirubinemia has been shown to be associated with several types of cancer, the present studies were undertaken to evaluate the radiomodifying effects of UCB in radiation-exposed mice having elevated levels of UCB. Pretreatment of splenic lymphocytes with UCB (1-50 μM at UCB/BSA ratio <1) augmented radiation-induced DNA strand breaks, MMP loss, calcium release, and apoptosis. Combination treatment of mice with UCB (50mg/kg bw) followed by WBI (2 Gy) 0.5h later, resulted in significantly increased splenic atrophy, bone marrow aplasia, decreased counts of peritoneal exudate cells, and different splenocyte subsets such as CD3+ T, CD4+ T, CD8+ T, CD19+ B, and CD14+ macrophages as compared to either UCB or WBI treatment. Hematological studies showed that WBI-induced lymphopenia, thrombocytopenia, and neutropenia were further aggravated in the combination treatment group. UCB pretreatment of mice potentiated WBI-induced apoptosis and decreased WBI-induced loss of functional response of various immune cells leading to augmentation of immunosuppression and infection susceptibility caused by WBI. In an acute bacterial peritonitis model, UCB pretreatment of mice significantly increased WBI-induced proinflammatory cytokines, nitric oxide, and peritoneal bacterial load resulting in increased infection and death. Studies using the pharmacological inhibitor of p38MAPK demonstrated the involvement of p38MAPK activation in the inflammatory cascade of peritonitis. These findings should prove useful in understanding the potential risk to hyperbilirubinemic patients during radiotherapy and victims of acute radiation exposure in the course of radiation accidents.

Reactive oxygen species mediate microRNA-302 regulation of AT-rich interacting domain 4a and C-C motif ligand 5 expression during transitions between quiescence and proliferation

Normal cell growth consists of two distinct phases, quiescence and proliferation. Quiescence, or G(0), is a reversible growth arrest in which cells retain the ability to reenter the proliferative cycle (G(1), S, G(2), and M). Although not actively dividing, quiescent cells are metabolically active and quiescence is actively maintained. Our results from microRNA PCR arrays and Taqman PCR assays showed a significant decrease (4-fold) in miR-302 levels during quiescence compared to proliferating normal human fibroblasts, suggesting that miR-302 could regulate cellular proliferation. Results from a Q-RT-PCR and dual-luciferase-3'-UTR reporter assays identified ARID4a (AT-rich interacting domain 4a, also known as RBP1) and CCL5 (C-C motif ligand 5) as targets for miR-302. Ionizing radiation decreased miR-302 levels, which was associated with an increase in its target mRNA levels, ARID4a and CCL5. Such an inverse correlation was also observed in cells treated with hydrogen peroxide as well as SOD2-overexpressing cells. Overexpression of miR-302 suppresses ARID4a and CCL5 mRNA levels, and increased the percentage of S-phase cells. These results identified miR-302 as an ROS-sensitive regulator of ARID4a and CCL5 mRNAs as well as demonstrate a regulatory role of miR-302 during quiescence and proliferation.

Lower phosphorylation of p38 MAPK blocks the oxidative stress-induced senescence in myeloid leukemic CD34(+)CD38 (-) cells

Leukemia seems to depend on a small population of "leukemia stem cells (LSCs)" for its growth and metastasis. However, the precise surviving mechanisms of LSCs remain obscure. Cellular senescence is an important obstacle for production and surviving of tumor cells. In this study we investigated the activated state of a pathway, in which reactive oxygen species (ROS) induces cellular senescence through DNA damage and phophorylation of p38 MAPK (p38), in myeloid leukemic CD34(+)CD38(-) cells. Bone marrow samples were obtained from patients with acute myeloid leukemia (AML, n=11) and chronic myeloid leukemia (CML, n=9). CD34(+)CD38(-) cells were isolated from mononuclear cells from these bone marrow samples, and K562 and KG1a cells (two kinds of myeloid leukemia cell lines) by mini-magnetic activated cell sorting. Hematopoietic stem cells (HSCs) from human cord blood served as controls. Intracellular ROS level was detected by flow cytometry. DNA damage defined as the γH2AX level was measured by immunofluorescence staining. Real-time RT-PCR was used to detect the expression of p21, a senescence-associated gene. Western blotting and immunofluorescence staining were employed to determine the p38 expression and activation. The proliferation and apoptosis of CD34(+)CD38(-) cells were detected by MTT assay and flow cytometry. Our results showed that ROS and DNA damage were substantially accumulated and p38 was less phosphorated in myeloid leukemic CD34(+)CD38(-) cells as compared with HSCs and H(2)O(2)-induced senescent HSCs. Furthermore, over-phosphorylation of p38 by anisomycin, a selective activator of p38, induced both the senescence-like growth arrest and apoptosis of CD34(+)CD38(-) cells from K562 and KG1a cell lines. These findings suggested that, although excessive accumulation of oxidative DNA damage was present in LSCs, the relatively decreased phosphorylation of p38 might help leukemic cells escape senescence and apoptosis.

Irradiated stem cells and ageing of the haematopoietic system

In the work presented here, changes in haematopoiesis of mice (B6129SF2/J) were studied 1 year after their whole-body exposure to a dose of 7 Gy (72% of mice survived). The irradiated mice were compared with non-irradiated younger (4 months of age) and older (16 months of age) mice. There was a significant increase in the relative abundance of primitive stem cells with long-term capability of the haematopoiesis recovery lin(-)/Sca-1(+)/CD117(+)/CD34(-) in the bone marrow of mice aged 16 months (irradiated and non-irradiated) compared with those aged 4 months. In terms of the ability to respond to further whole-body irradiation at a dose of 1 Gy, the presence of γH2A.X foci was studied in lin(-) bone marrow cells. There was a considerable number of persisting foci in lin(-) stem cells isolated from the bone marrow of the older irradiated mice. In the blood count from the peripheral blood of the older mice (both non-irradiated and irradiated at 7 Gy), there was a significant increase in granulocytes. In the group exposed to 7 Gy, the numbers of thrombocytes significantly increased, and on the contrary, the numbers of erythrocytes, the amount of haemoglobin, and haematocrit significantly decreased.

A sensitive and quantitative polymerase chain reaction-based cell free in vitro non-homologous end joining assay for hematopoietic stem cells

Hematopoietic stem cells (HSCs) are responsible for sustaining hematopoietic homeostasis and regeneration after injury for the entire lifespan of an organism. Maintenance of genomic stability is crucial for the preservation of HSCs, which depends on their efficient repair of DNA damage, particularly DNA double strand breaks (DSBs). Because of the paucity of HSCs and lack of sensitive assays, directly measuring the ability of HSCs to repair DSBs has been difficult. Therefore, we developed a sensitive and quantitative cell free in vitro non-homologous end joining (NHEJ) assay using linearized plasmids as the substrates and quantitative polymerase chain reaction (qPCR) technique. This assay can sensitively detect DSB repair via NHEJ in less than 1 µg 293T cell nuclear proteins or nuclear extracts from about 5,000 to 10,000 human BM CD34⁺ hematopoietic cells. Using this assay, we confirmed that human bone marrow HSCs (CD34⁺CD38⁻ cells) are less proficient in the repair of DSBs by NHEJ than HPCs (CD34⁺CD38⁺ cells). In contrast, mouse quiescent HSCs (Pyronin-Y^low LKS⁺ cells) and cycling HSCs (Pyronin-Y^hi LKS⁺ cells) repaired the damage more efficiently than HPCs (LKS⁻ cells). The difference in the abilities of human and mouse HSCs and HPCs to repair DSBs through NHEJ is likely attributed to their differential expression of key NHEJ DNA damage repair genes such as LIG4. These findings suggest that the qPCR-based cell free in vitro NHEJ assay can be used to sensitively measure the ability of human and mouse HSCs to repair DSBs.

An acute negative bystander effect of γ-irradiated recipients on transplanted hematopoietic stem cells

Ultimate success of hematopoietic stem cell transplantation (HSCT) depends not only on donor HSCs themselves but also on the host environment. Total body irradiation is a component in various host conditioning regimens for HSCT. It is known that ionizing radiation exerts "bystander effects" on nontargeted cells and that HSCs transplanted into irradiated recipients undergo proliferative exhaustion. However, whether irradiated recipients pose a proliferation-independent bystander effect on transplanted HSCs is unclear. In this study, we found that irradiated mouse recipients significantly impaired the long-term repopulating ability of transplanted mouse HSCs shortly (∼ 17 hours) after exposure to irradiated hosts and before the cells began to divide. There was an increase of acute cell death associated with accelerated proliferation of the bystander hematopoietic cells. This effect was marked by dramatic down-regulation of c-Kit, apparently because of elevated reactive oxygen species. Administration of an antioxidant chemical, N-acetylcysteine, or ectopically overexpressing a reactive oxygen species scavenging enzyme, catalase, improved the function of transplanted HSCs in irradiated hosts. Together, this study provides evidence for an acute negative, yet proliferation-independent, bystander effect of irradiated recipients on transplanted HSCs, thereby having implications for HSCT in both experimental and clinical scenarios in which total body irradiation is involved.

Effect of Ionizing Radiation Induced Damage of Endothelial Progenitor Cells in Vascular Regeneration

Objective—

A number of studies have revealed that stress signaling and subsequent stress responses in stem/progenitor cells are responsible for attenuated regeneration or degenerative disease. Because ionizing radiation (IR), which sensitizes diverse types of stem cells, reportedly induces cardio-circulatory diseases, we hypothesized that IR-induced vascular abnormalities are associated with defects in endothelial progenitor cells (EPCs) that are responsible for vascular homeostasis.

Methods and Results—

We used an irradiated mouse model to mimic the IR effect on vasculogenesis. Mouse EPCs isolated from irradiated mice and human EPCs exposed to IR were used for functional analysis and gene expression study. Under IR exposure, EPCs were depleted, and their function for vasculogenesis in vitro and in vivo was significantly reduced. In such IR-mediated stress responses, upregulating p21Cip1 and downregulating vascular endothelial growth factor (VEGF) were mediated by p53 transcriptional activity.

Conclusion—

The results of the present study suggest that suppression of p53 would be clinically applicable to (1) minimize the functional defects in EPCs in order to prevent the onset of vascular diseases caused by radiation therapy or radiation exposure and also to (2) provide novel insight into the mechanisms of IR-induced vascular damage and a possible strategy to minimize vascular damage by IR.