Demonstration of a radiation-induced bystander effect for low dose low LET β-particles

Radiation-induced bystander mutagenesis at a relatively low dose range was investigated using low LET β-particles in a three-dimensional cell culture model. CHO cells were labeled with 0, 0.5, 1.0 or 5.0 μCi tritiated thymidine (³HdTTP) for 12 h and subsequently incubated with A_L cells for 24 h at 11°C. The cell mixture was centrifuged to produce a spheroid of 4 × 10⁶ cells of which there was five times more A_L than CHO cells. The short-range β-particles emitted by ³HdTTP result in self-irradiation of labeled CHO cells, thus biological effects on neighboring A_L cells can be attributed to the bystander response. To evaluate such response, non-labeled bystander A_L cells were isolated from among labeled CHO cells and studied independently for survival and mutagenesis. Treatment of CHO cells with ³HdTTP resulted in a dose-dependent increase in bystander mutation incidence among neighboring A_L cells compared to controls. In addition, multiplex PCR analysis revealed the types of mutants to be significantly different from those of spontaneous origin. These data provide evidence that low dose low LET radiation can induce bystander mutagenesis in a three-dimensional model. The results of this study will address the relevant issues of actual target size and radiation quality, and are likely to have a significant impact on our current understanding of radiation risk assessment.     

Radiation induced bystander signals are independent of DNA damage and DNA repair capacity of the irradiated cells

Evidence is accumulating that irradiated cells produce signals, which interact with non-exposed cells in the same population. Here, we analysed the mechanism for bystander signal arising in wild-type CHO cells and repair deficient varients, focussing on the relationship between DNA repair capacity and bystander signal arising in irradiated cells. In order to investigate the bystander effect, we carried out medium transfer experiments after X-irradiation where micronuclei were scored in non-targeted DSB repair deficient xrs5 cells. When conditioned medium from irradiated cells was transferred to unirradiated xrs5 cells, the level of induction was independent of whether the medium came from irradiated wild-type, ssb or dsb repair deficient cells. This result suggests that the activation of a bystander signal is independent of the DNA repair capacity of the irradiated cells. Also, pre-treatment of the irradiated cells with 0.5% DMSO, which suppresses micronuclei induction in CHO but not in xrs5 cells, suppressed bystander effects completely in both conditioned media, suggesting that DMSO is effective for suppression of bystander signal arising independently of DNA damage in irradiated cells. Overall the work presented here adds to the understanding that it is the repair phenotype of the cells receiving bystander signals, which determines overall response rather than that of the cell producing the bystander signal.     

Role of homologous recombination in the alpha-particle-induced bystander effect for sister chromatid exchanges and chromosomal aberrations

The bystander effect for sister chromatid exchanges (SCEs) and chromosomal aberrations was examined in hamster cell lines deficient in either DNA-PKcs (V3 cells, deficient in nonhomologous end joining, NHEJ) or RAD51C (irs3 cells, deficient in homologous recombination, HR). Cells synchronized in G0/G1 phase were irradiated with very low fluences of alpha particles such that < 1% of the nuclei were traversed by an alpha particle. Wild-type cells showed a prominent bystander response for SCE induction; an even greater effect was observed in V3 cells. On the other hand, no significant induction of SCE was observed in the irs3 RAD51C-deficient bystander cells irradiated at various stages in the cell cycle. Whereas a marked bystander effect for chromosomal aberrations occurred in V3 cells, the induction of chromosomal aberrations in irs3 bystander cells was minimal and similar to that of wild-type cells. Based on these findings, we hypothesize that HR is essential for the induction of SCE in bystander cells; however, HR is unable to repair the DNA damage induced in NHEJ-deficient bystander cells that leads to either SCE or chromosomal aberrations.     

Investigation of the bystander effect in CHO-K1 cells

AimInvestigation of the bystander effect in Chinese Hamster Ovary cells (CHO-K1) co-cultured with cells irradiated in the dose range of 0.1–4 Gy of high LET ¹²C ions and X-rays.BackgroundThe radiobiological effects of charged heavy particles on a cellular or molecular level are of fundamental importance in the field of biomedical applications, especially in hadron therapy and space radiation biology.Materials and methodsA heavy ion ¹²C beam from the Heavy Ion Laboratory of the University of Warsaw (HIL) was used to irradiate CHO-K1 cells. Cells were seeded in Petri dishes specially designed for irradiation purposes. Immediately after irradiation, cells were transferred into transwell culture insert dishes to enable co-culture of irradiated and non-irradiated cells. Cells from the membrane and well shared the medium but could not touch each other. To study bystander effects, a clonogenic survival assay was performed.ResultsThe survival fraction of cells co-cultured with cells irradiated with ¹²C ions and X-rays was not reduced.ConclusionsThe bystander effect was not observed in these studies.     

Indirect influences of radiation on unirradiated cells through irradiated cells.

There has been a recent upsurge of interest in radiation-induced bystander effects. Previously we reported that the accumulation of inducible nitric oxide (NO) synthase (iNOS) was induced only in human glioblastoma mutant (m) p53 cells by acute irradiation with X-rays, suggesting a suppression of iNOS induction after acute irradiation with X-rays in wtp53 cells. NO secreted from the irradiated mp53 cells induced the accumulation of p53 in unirradiated wtp53 cells. The radiosensitivity of wtp53 cells was reduced by exposure to the conditioned medium from irradiated mp53 cells, suggesting that NO is an initiator of radiation-induced bystander effects. In the present study, we found that the accumulation of iNOS in wtp53 cells was induced by chronic irradiation with gamma-rays followed by acute irradiation with X-rays, but not by each one. It is suggested that the accumulation of iNOS may be due to the depression of acute irradiation-induced p53 functions by pre-chronic irradiation. We found that chronic irradiation with gamma-rays did not inhibit the accumulation of p53 after exposure to the conditioned medium from the irradiated mp53 cells. However, the decay of accumulated p53 was stimulated by chronic irradiation with gamma-rays. At the same time, the accumulation of Hdm2 was observed; suggesting that chronic irradiation with gamma-rays may stimulate the degradation of p53 accumulated by NO-mediated bystander effects.     

Induction of apoptosis in bone marrow cells after treatment of mice with WR-2721 and gamma-rays: relationship to the cell cycle

Apoptosis and cell proliferation are accepted to be responsible for the maintenance of homeostasis in the hematopoietic system. Understanding of the mechanisms of action of the aminothiols and ionizing radiation on normal hematopoietic cells requires determination of the correlation between apoptotic cell death and cell cycle distribution. The effects of WR-2721 ((S)-2-/3-aminopropylamino/ethylphosphorothioic acid; Amifostine) and ⁶⁰Co gamma-rays on apoptosis and cell cycle progression in the mouse bone marrow were determined. Adult male Swiss mice were exposed to 6 Gy gamma-rays only, or pretreated with WR-2721, at a dose of 400 mg/kg body weight, 30 min before gamma-irradiation. The laser scanning cytometry APO-BRDU^TM assay based on simultaneous analysis of cellular DNA content and the in situ detection of DNA strand breaks was used to identify apoptotic cells and to reveal the cell cycle position of apoptotic and nonapoptotic cells. Temporary changes in the frequency of apoptotic cells with fluorescein isothiocyanate (FITC) labeling of DNA strand breaks, and all bone marrow cells including apoptotic and nonapoptotic ones, whose DNA stained with propidium iodide, were observed in the particular phases of the cell cycle throughout the 96-h period after WR-2721 application and gamma-irradiation. The cell cycle phase specificity of WR-2721 and ⁶⁰Co gamma-irradiation was shown in terms of induction of apoptosis in bone marrow cells. The patterns of alterations in the frequency of apoptotic cells and all bone marrow cells with respect to their cell cycle position were dependent on the agent(s) applied and the time interval after treatment of mice with WR-2721 and/or gamma-rays. A modulatory, suppressive action of WR-2721 on apoptosis induction and the cell cycle perturbation caused in normal cells of the mouse bone marrow by gamma-rays was found.     

Low doses of alpha particles do not induce sister chromatid exchanges in bystander Chinese hamster cells defective in homologous recombination

We reported previously that the homologous recombinational repair (HRR)-deficient Chinese hamster mutant cell line irs3 (deficient in the Rad51 paralog Rad51C) showed only a 50% spontaneous frequency of sister chromatid exchange (SCE) as compared to parental wild-type V79 cells. Furthermore, when irradiated with very low doses of alpha particles, SCEs were not induced in irs3 cells, as compared to a prominent bystander effect observed in V79 cells [H. Nagasawa, Y. Peng, P.F. Wilson, Y.C. Lio, D.J. Chen, J.S. Bedford, J.B. Little, Role of homologous recombination in the alpha-particle-induced bystander effect for sister chromatid exchanges and chromosomal aberrations, Radiat. Res. 164 (2005) 141-147]. In the present study, we examined additional Chinese hamster cell lines deficient in the Rad51 paralogs Rad51C, Rad51D, Xrcc2, and Xrcc3 as well as another essential HRR protein, Brca2. Spontaneous SCE frequencies in non-irradiated wild-type cell lines CHO, AA8 and V79 were 0.33SCE/chromosome, whereas two Rad51C-deficient cell lines showed only 0.16SCE/chromosome. Spontaneous SCE frequencies in cell lines defective in Rad51D, Xrcc2, Xrcc3, and Brca2 ranged from 0.23 to 0.33SCE/chromosome, 0-30% lower than wild-type cells. SCEs were induced significantly 20-50% above spontaneous levels in wild-type cells exposed to a mean dose of 1.3mGy of alpha particles (<1% of nuclei traversed by an alpha particle). However, induction of SCEs above spontaneous levels was minimal or absent after alpha-particle irradiation in all of the HRR-deficient cell lines. These data suggest that Brca2 and the Rad51 paralogs contribute to DNA damage repair processes induced in bystander cells (presumably oxidative damage repair in S-phase cells) following irradiation with very low doses of alpha particles.     

Radio-adaptive response: characterization of a cytogenetic repair induced by low-level ionizing radiation in cultured Chinese hamster cells

Pretreatment with low doses of beta-rays from incorporated tritiated thymidine ([3H]dThd) or of Co-60 gamma-rays (1 or 5 cGy) rendered actively growing Chinese hamster V79 cells more resistant to the induction of micronuclei or sister-chromatid exchanges (SCEs) by a subsequent high dose of gamma-rays (1 Gy). This adaptive response to ionizing radiation (radio-adaptive response) can be induced by an optimal range of low doses of 3H beta-rays, but not by much lower or higher adapting doses. Full expression of the adaptive response induced by the exposure to low doses of 60Co gamma-rays occurred 4 h after the adapting dose. The cells pre-exposed to low doses of gamma-rays showed cross-resistance to challenge doses of gamma-rays themselves and also of mitomycin C (MMC) and near ultraviolet light (UV-B, 313 nm), but not to those of ethyl methanesulfonate (EMS) or cis-platinum (II) diammine dichloride (cisplatin) for SCE induction. These results suggest that the radio-adaptive response mechanistically couples to the repair network which copes with chromatin lesions induced by MMC and UV-B.     

Determination of residual dimethylsulfoxide in cryopreserved cardiovascular allografts

Dimethylsulfoxide (DMSO) is a solvent which protects the structure of allografts during the cryopreservation and thawing process. However, several toxic effects of DMSO in patients after transplantation of cryopreserved allografts have been described. The aim of this study is to determine the residual DMSO in the cardiovascular allografts after thawing and preparation of cryopreserved allografts for clinical application following guidelines of the European Pharmacopoeia for DMSO detection. Four types of EHB allografts (aortic valve-AV, pulmonary valve-PV, descending thoracic aorta-DA, and femoral artery-FA) are cryopreserved using as cryoprotecting solution a 10% of DMSO in medium 199. Sampling is carried out after thawing, after DMSO dilution and after delay of 30 min from final dilution (estimated delay until allograft implantation). After progressive thawing in sterile water bath at 37–42 °C (duration of about 20 min), DMSO dilution is carried out by adding consecutively 33, 66 and 200 mL of saline. Finally, tissues are transferred into 200 mL of a new physiologic solution. Allograft samples are analysed for determination of the residual DSMO concentration using a validated Gas Chromatography analysis. Femoral arteries showed the most important DMSO reduction after the estimated delay: 92.97% of decrease in the cryoprotectant final amount while a final reduction of 72.30, 72.04 and 76.29% in DMSO content for AV, PV and DA, was found, respectively. The residual DMSO in the allografts at the moment of implantation represents a final dose of 1.95, 1.06, 1.74 and 0.26 mg kg^?1 in AV, PV, DA and FA, respectively, for men, and 2.43, 1.33, 2.17 and 0.33 mg kg^?1 for same tissues for women (average weight of 75 kg in men, and 60 kg in women). These results are seriously below the maximum recommended dose of 1 g DMSO kg^?1 (Regan et al. in Transfusion 50:2670–2675, 2010) of weight of the patient guaranteeing the safety and quality of allografts.     

Chromosomal damage induced by maleic hydrazide in mammalian cells in vitro and in vivo

The induction of sister-chromatid exchanges (SCE) and chromosomal aberrations (Ch.Ab.) by the herbicide maleic hydrazide (MH) has been investigated in Chinese hamster ovary (CHO) cells grown in vitro and in bone marrow cells of mice treated in vivo. MH induces SCE and Ch.Ab. in CHO cells without metabolic activation; however, no induction of SCE was found in the in vivo experiments.     

Function and organization of the G region of bacteriophage Mu; Host specificity determined by gene splicing

Chinese hamster ovary (CHO) cells were exposed to [³H]ethyl nitrosourea (ENU) or [³H]ethyl methanesulfonate (EMS) and the following DNA ethylation products were quantitated: 3- and 7-ethyladenine, O²-ethylcytosine, 3-, 7- and O⁶-ethylguanine, O²- and O⁴-ethyldeoxythymidine and the representative ethylated phosphodiester, deoxythymidylyl (3′–5′)ethyl-deoxythymidine. When mutations at the hypoxanthine-guanine phosphoribosyl transferase (HGPRT) locus induced by these same treaments were compared with the observed ethylation products, mutations were found to correlate best with 3- and O⁶-ethylguanine. EMS induced approximately twice as many sister-chromatid exchanges (SCEs) as ENU at doses yielding equal mutation frequencies. When SCEs were indirectly compared with DNA ethylation products, 3-ethyladenine and ethylated phosphodiesters related best to SCE formation. Because mutation and SCE induction appear, at least in part, to be related to different DNA adducts, SCE induction by simple ethylating agents may not be a quantitative indicator of potentially mutagenic DNA damage.     

Delayed repair of DNA single-strand breaks does not increase cytogenetic damage

DNA damage and cytogenetic effects of ionizing radiation were investigated in Chinese hamster ovary (CHO) cells and unstimulated human peripheral blood lymphocytes. DNA damage and repair were analysed by alkaline elution under conditions that predominantly measured DNA single-strand breaks (ssb). X-radiation (2.5 Gy) induced ssb in both CHO cells and unstimulated lymphocytes, and the breaks were repaired within 30 and 90 min, respectively. This rapid repair was delayed by the poly(ADP-ribose) polymerase inhibitor, 3-aminobenzamide (3AB). The cytogenetic effects of the 3AB-induced delay in DNA repair were examined by analysing sister chromatid exchange (SCE) frequency in CHO cells and fragmentation of prematurely condensed chromosomes (PCC) in unstimulated human lymphocytes after 2.5 Gy of X-rays. Although 3AB delayed the rejoining of DNA ssb, this delay did not result in increased cytogenetic damage manifested as either SCE or fragmentation of PCC. These results indicate that the rapidly rejoining DNA ssb are not important in the production of chromosome damage.     

DNA strand break and rejoining in cultured human fibroblasts exposed to fast neutrons or gamma rays

The production and rejoining of DNA single-strand and double-strand breaks have been monitored in monolayer cultures of proliferating human skin fibroblasts by means of sensitive techniques. Cells were irradiated with low doses of either 60Co gamma-rays or 14.6 MeV neutrons at 0 degrees C (0-5 Gy for measurement of single-strand breaks by alkaline elution and 0-50 Gy for double-strand breaks measured by neutral elution). The yield of single-strand breaks induced by neutrons was 30 per cent of that produced by the same dose of gamma-rays; whilst in the induction of double-strand breaks neutrons were 1.6 times as effective as gamma-rays. Upon post-irradiation incubation of cells at 37 degrees C, neutron-induced single-strand and double-strand breaks were rejoined with a similar time-course to gamma-induced breaks. Rejoining followed biphasic kinetics; of the single-strand breaks, 50 per cent disappeared within 2 min after gamma-rays and 6-10 min after neutrons. Fifty per cent of the double-strand breaks disappeared within 10 min, after gamma-rays and neutrons. Cells derived from patients suffering from ataxia-telangiectasia showed the same capacity for repair of single- and double-strand breaks induced by 14.6 MeV neutrons, as cells established from normal donors. The comparison of neutrons and gamma-rays in the induction of DNA breaks did not explain the elevated r.b.e. on high LET radiation. However, a study of the variation in the spectrum of lesions induced by different radiation sources will probably contribute to the clarification of the relative importance of other radio products.     

Vitamin C is positive in the DNA synthesis inhibition and sister-chromatid exchange tests

Ascorbate caused a dose-dependent increase in sister-chromatid exchanges (SCEs) in Chinese hamster ovary (CHO) cells and in human lymphocytes. Moreover, in the DNA synthesis inhibition test with HeLa cells, ascorbate gave results typical of DNA-damaging chemicals. Catalase reduced SCE induction by ascorbate, prevented its cytotoxicity in CHO cells, and prevented its effect on HeLa DNA synthesis. Ascorbate reduced induction of SCE in CHO cells by N-methylN′-nitrosoguanidine (MNNG) by direct inactivation of MNNG.     

Stress proteins are not induced in mammalian cells exposed to radiofrequency or microwave radiation

The induction of stress proteins in HeLa and CHO cells was investigated following a 2 h exposure to radiofrequency (RF) or microwave radiation. Cells were exposed or sham exposed in vitro under isothermal (37 ± 0.2 °C) conditions. HeLa cells were exposed to 27- or 2450 MHz continuous wave (CW) radiation at a specific absorption rate (SAR) of 25 W/kg. CHO cells were exposed to CW 27 MHz radiation at a SAR of 100 W/kg. Parallel positive control studies included 2 h exposure of HeLa or CHO cells to 40 °C or to 45 μM cadmium sulfate. Stress protein induction was assayed 24 h after treatment by electrophoresis of whole-cell extracted protein labeled with [³⁵S]-methionine. Both cell types exhibited well-characterized responses to the positive control stresses. Under these exposure conditions, neither microwave nor RF radiation had a detectable effect on stress protein induction as determined by either comparison of RF-exposed cells with sham-exposed cells or comparison with heat-stressed or Cd⁺⁺ positive control cells. Bioelectromagnetics 18:499–505, 1997. © 1997 Wiley-Liss, Inc.     

Evidence for induction of DNA double strand breaks in the bystander response to targeted soft X-rays in CHO cells

This study investigated the role of DNA double strand breaks and DNA base damage in radiation-induced bystander responses in Chinese hamster ovary (CHO) cell lines. Two CHO repair-deficient clones, xrs5 (DNA double strand break repair-deficient) and EM9 (DNA base excision repair-deficient) were used in addition to the wild type (CHO). The Gray Cancer Institute ultrasoft X-ray microprobe is a powerful tool for investigating the bystander response, because it permits the irradiation of only a single nucleus of a cell, as reported previously. In order to investigate the bystander effect in each repair-deficient cell line, we irradiated a single cell within a population and scored the formation of micronuclei. When a single nucleus in the population was targeted with 1 Gy, elevated numbers of micronuclei were induced in the neighbouring unirradiated cells in the EM9 and xrs5 cell lines, whereas induction was not observed in CHO. The induction of micronuclei in xrs5 was significantly higher than that in EM9. Under these conditions, the surviving fraction in the neighbouring cells was significantly lower in xrs5 than in the other cell lines, showing a higher cell killing effect in xrs5. To confirm that bystander factors secreted from irradiated cells caused these effects, we carried out medium transfer experiments using conventional X-irradiation. Medium conditioned for 24 h with irradiated cells was transferred to unirradiated cells and elevated induction of micronuclei was observed in xrs5. These results suggest that DNA double strand breaks rather than base damage are caused by factors secreted in the medium from irradiated cells.     

Chromosomal instability in mutagen-sensitive mutants isolated from mouse lymphoma L5178Y cells. II. Abnormal induction of sister-chromatid exchanges and chromosomal aberrations by mutagens in an ionizing radiation-sensitive mutant (M10) and an alkylating agent-sensitive mutant (MS1)

To determine the mutual relationships between cell survival and induction of sister-chromatid exchanges (SCEs) as well as chromosomal aberrations (CAs), mutagen-induced SCEs and CAs were analyzed in an ionizing radiation-sensitive mutant (M10) and an alkylating agent-sensitive mutant (MS 1) isolated from mouse lymphoma L5178Y cells. The levels of CA induction in both mutants strictly corresponded to the sensitivity to lethal effects of mutagens, except that caffeine-induced CAs in M10 are considerably lower than those in L5178Y. The results clearly indicate that except for caffeine-induced CAs in M10, mutagen-induced lethal lesions are responsible for CA induction. In contrast, SCE induction in mutants was complicated. In M10, hypersensitive to killing by gamma-rays, methyl methanesulfonate (MMS), and 4-nitroquinoline 1-oxide (4NQO), but not sensitive to UV or caffeine, the frequency of SCEs induced by gamma-rays was barely higher than that in L5178Y, and the frequencies of MMS- and UV-induced SCEs were similar to those in L5178Y, but 4NQO- and caffeine-induced SCEs were markedly lower than those in L5178Y. MS 1, which is hypersensitive to MMS and caffeine, but not sensitive to UV or 4NQO, responded to caffeine with an enhanced frequency of SCEs and had a normal frequency of MMS-induced SCEs, but a reduced frequency of UV- and 4NQO-induced SCEs. Thus, susceptibility to SCE induction by mutagens is not necessarily correlated with sensitivity of mutants to cell killing and/or CA induction by mutagens. Furthermore, the spontaneous levels of SCEs are lower in M10 and higher in MS 1 than that in L5178Y (Tsuji et al., 1987). Based on these results, we speculate that M10 may be partially defective in the processes for the formation of SCEs caused by mutagens. On the other hand, MS 1 may modify SCE formation-related lesions induced by UV and 4NQO to some repair intermediates that do not cause SCE formation. In addition, MMS-induced lethal lesions in MS 1 may not be responsible for SCE induction whereas caffeine-induced lethal lesions are closely correlated with SCE induction. Thus, the lesions or mechanisms involved in SCE production are in part different from those responsible for cell lethality or CA production.     

The effects of alanine ingestion on metabolic responses to exercise in cyclists

The influence of alanine on plasma amino acid concentrations and fuel substrates as well as cycling performance was examined. Four solutions [6% alanine (ALA); 6% sucrose (CHO); 6% alanine and 6% sucrose (ALA–CHO); an artificially sweetened placebo (PLC)] were tested using a double-blind, randomised, cross-over design. During each trial, ten cyclists ingested 500 mL of test solution 30 min before exercise and 250 mL after 15, 30, and 45 min of exercise. Participants cycled for 45 min at 75% VO₂max followed by a 15-min performance trial. Blood was collected before beverage consumption and prior to the performance trial. Alanine concentration was increased (p < 0.05) by approximately tenfold for ALA and ALA–CHO and less than twofold for CHO and PLC. Alanine ingestion increased concentrations of most gluconeogenic amino acids. Overall, alanine supplementation tended to produce favourable metabolic effects, but did not influence performance.     

Biochemical and cytogenetical characterization of Chinese hamster ovary X-ray-sensitive mutant cells xrs 5 and xrs 6. V. The correlation of DNA strand breaks and base damage to chromosomal aberrations and sister-chromatid exchanges induced by X-irradiation

The X-ray-sensitive Chinese hamster ovary (CHO) mutant cell lines xrs 5 and xrs 6 were used to study the relation between X-ray-induced DNA lesions and biological effects. The frequencies of chromosomal aberrations and sister-chromatid exchanges (SCE) were determined in wild-type CHO-K1 as well as mutants xrs 5 and xrs 6 cells following X-irradiation under aerobic and anaerobic conditions. Furthermore, we used a newly developed immunochemical method (based on the binding of a monoclonal antibody to single-stranded DNA) to assay DNA single-strand breaks (SSBs) induced by gamma-rays in these CHO cells, after a repair time of up to 4 h. For all cell lines tested the frequency of X-ray-induced chromosomal aberrations was strongly increased after irradiation in air compared with hypoxic conditions. When compared to the wild-type line, the xrs mutants known to have a defect in repair of DNA double-strand breaks (DSBs) exhibited a markedly enhanced sensitivity to aerobic irradiation, and a high OER (oxygen enhancement ratio) of 2.8-3.5, compared with 1.8-2 in CHO-K1 cells. The induction of SCE by X-rays was relatively little affected in CHO-K1 irradiated in air compared with hypoxic conditions (OER = 0.8), and in xrs 5 (OER = 0.7). A dose-dependent increase in the frequency of SCEs was obtained in xrs 6 cells treated with X-rays in air, and a further increase by a factor of 2 was evident under hypoxic conditions (OER = 0.4). With the immunochemical assay of SSB following gamma-irradiation, no difference was found between wild-type and mutant strains in the number of SSBs induced. The observed rate of rejoining of SSBs was also the same for all cell lines studied.     

Response of X-ray-sensitive CHO mutant cells (xrs-6c) to radiation. II. Relationship between cell survival and the induction of chromosomal damage with low doses of alpha particles

The induction of cytotoxicity, chromosomal aberrations, and sister chromatid exchanges (SCEs) was measured in CHO K-1c cells and in isogenic X-ray-sensitive mutant xrs-6c cells that had been irradiated with X rays and alpha particles in isoleucine-deficient alpha-minimal essential medium in G1 phase of the cell cycle. There was a noticeable shoulder region on the survival curve for CHO K-1c cells irradiated with very low doses of alpha particles, whereas this feature was absent for xrs-6c cells with alpha-particle doses as low as 0.5 cGy. Higher frequencies of chromatid-type aberrations were induced in G1-phase xrs-6c cells than in G1-phase CHO K-1c cells by both gamma- and alpha-particle irradiation. Induction of nonlethal chromosomal aberrations was observed following exposure to 2-6 cGy of alpha particles, doses yielding 97-100% cell survival. Irradiation with 0.5 cGy of alpha particles induced SCE; nearly 60% of irradiated cells contained significantly increased levels of SCE. However, only 3% of the nuclei of cells exposed to 0.5 cGy of alpha-particle radiation were actually traversed by an alpha particle. The observation that a large fraction of cells apparently survive exposure to very low doses of alpha-particle radiation with persistent genetic damage manifested by both chromosomal aberrations and SCEs may have important implications for the carcinogenic hazards of high-LET radiation.