Promoter-enhanced neoplastic transformation after gamma-ray exposure at 10 cGy/day

We have measured gamma-ray-induced neoplastic transformation in C3H10T1/2 mouse embryo cells irradiated at an average 10 cGy/day throughout the useful life span of these cells for transformation studies. At cumulative total doses of 50, 150, 300, and 450 cGy, samples of cells were assayed for cell survival and neoplastic transformation with or without the administration of 0.1 micrograms/ml of 12-O-tetradecanoylphorbol-13-acetate (TPA) starting 24 h after the irradiation. The results indicate that, at a dose rate of 10 cGy/day, the rate of induction of neoplastic transformation is reduced by a factor of thirteen compared to that at 100 cGy/min. Still, frequencies above the background level are observed. These results are consistent with previous data which, at 144 cGy/day (0.1 cGy/min), showed that radiation-induced initiation events could be repaired during exposure, thus reducing the frequency of transformation from that observed at 100 cGy/min [A. Han et al., Cancer Res. 40, 3328-3332 (1980)]. Although the addition of TPA after the delivery of a particular dose at 10 cGy/day produced a significant increase in the frequency of neoplastic transformation, the degree of enhancement was less than after higher-dose-rate exposures [C.K. Hill et al., Radiat. Res. 109, 347-351 (1987)]. These results indicate that during 7 weeks of exposure, the repair of radiation-induced initiation was extensive but not complete, and suggest that a significant part of the damage persists which can be promoted by TPA. These observations support the inference that initiation and promotion are not tightly coupled and are probably independent processes.     

Adaptation of the dichlorofluorescein assay for detection of radiation-induced oxidative stress in cultured cells

The oxidation of 2'7'-dichlorofluorescin (DCFH) to 2'7'-dichlorofluorescein (DCF), a fluorescent DCFH oxidation product, is a highly sensitive indicator that is used to measure oxidative stress in cells. In the present study, a DCF assay has been adapted to quantify oxidative stress in human breast epithelial cell cultures after exposure to gamma rays. The results demonstrate that the sensitivity and specificity of the DCF assay is strongly influenced by the timing of DCFH diacetate (DCFH-DA) substrate loading in relation to radiation exposure and by the matrix in which the cells were loaded with DCFH-DA substrate. Under the conditions optimized in this study, the DCF assay is capable of detecting increased DCFH oxidation in cell cultures irradiated with gamma rays at a dose as low as 1.5 cGy. The increase in fluorescence was directly proportional to the radiation dose, which ranged from 0 to 2 Gy, and a minimal level of fluorescence was observed in sham-irradiated cells. These results indicate that the DCF assay optimized in this study is highly sensitive, linear and specific for measuring oxidative stress in irradiated cells.     

Split-dose exposures versus dual ion exposure in human cell neoplastic transformation

Since radiation fields of space contain many-fold more protons than high atomic number, high energy (HZE) particles, cells in astronaut crews will experience on average several proton hits before an HZE hit. Thus radiation regimes of proton exposure before HZE particle exposure simulate space radiation exposure, and measurement of the frequency of neoplastic transformation of human primary cells to anchorage-independent growth simulates an initial step in cancer induction. Although previous investigations indicated a synergistic increase in transformation yields in the cells exposed to protons followed by HZE particles, these experiments did not differentiate between the effect of splitting of the dose into two fractions and that of changing the ion beams. To test this, we irradiated cells with split doses of either protons or HZE particles, then measured clonogenic survival and neoplastic transformation, as measured by colony formation in semi-solid soft agar medium. The data show that the split dose of 20 cGy plus 20 cGy of either H or HZE ions gave about the same effect as the 40 cGy uninterrupted dose, quite different from the effect of the mixed ion beam H + HZE irradiation. We also asked if lower proton doses than 20 cGy followed 15 min later by 20 cGy of HZE ions gave greater than additive transformation frequencies. Substantial increases in transformation levels were observed for all proton doses tested, including 1 cGy. These results point to the signal importance of protons in affecting the effect of space radiation on human cells.     

Production of delayed death and neoplastic transformation in CGL1 cells by radiation-induced bystander effects

Other investigators have demonstrated by transfer of medium from irradiated cells and by irradiation with low-fluence alpha particles or microbeams that cells do not have to be directly exposed to ionizing radiation to be detrimentally affected, i.e. bystander effects. In this study, we demonstrate by transfer of medium from X-irradiated human CGL1 hybrid cells that the killing of bystander cells reduces the plating efficiency of the nonirradiated CGL1 cells by 33 +/- 6%. In addition, we show that the amount of cell death induced by bystander effects is not dependent on X-ray dose, and that the induction of apoptosis does not appear to be responsible for the cell death. Furthermore, we found that the reduction in plating efficiency in bystander cells is evident for over 18 days, or 22 cell population doublings, after medium transfer, despite repeated refeeding of the cell cultures. Finally, we report the novel observation that bystander effects induced by the transfer of medium from irradiated cells can induce neoplastic transformation. Exposing unirradiated CGL1 cells to medium from cells irradiated with 5 or 7 Gy increased the frequency of neoplastic transformation significantly from 6.3 x 10(-6) in unirradiated controls to 2.3 x 10(-5) (a factor of nearly four). We conclude that the bystander effect induces persistent, long-term, transmissible changes in the progeny of CGL1 cells that result in delayed death and neoplastic transformation. The data suggest that neoplastic transformation in bystander cells may play a significant role in radiation-induced neoplastic transformation at lower doses of X rays.     

Low doses of very low-dose-rate low-LET radiation suppress radiation-induced neoplastic transformation in vitro and induce an adaptive response

The purpose of this study was to determine whether adaptation against neoplastic transformation could be induced by exposure to very low-dose-rate low-LET radiation. HeLa x skin fibroblast human hybrid cells were irradiated with approximately 30 kVp photons from an array of (125)I seeds. The initial dose rate was 4 mGy/day. Cell samples were taken at four intervals at various times over a period of 88 days and assayed for neoplastic transformation and the presence of reactive oxygen species (ROS). The dose rate at the end of this treatment period was 1.4 mGy/day. Transformation frequencies and ROS levels were compared to those of parallel unirradiated controls. At the end of 3 months and an accumulated dose of 216 mGy, cells treated with very low-dose-rate radiation were exposed to a high-dose-rate 3-Gy challenge dose of (137)Cs gamma rays, and the effects compared with the effect of 3 Gy on a parallel culture of previously unirradiated cells. Cells exposed to very low-dose-rate radiation exhibited a trend toward a reduction in neoplastic transformation frequency compared to the unirradiated controls. This reduction seemed to diminish with time, indicating that the dose rate, rather than accumulated dose, may be the more important factor in eliciting an adaptive response. This pattern was in general paralleled by a reduction of ROS present in the irradiated cultures compared to controls. The very low-dose-rate-treated cells were less sensitive to the high challenge dose than unirradiated controls, suggesting the induction of an adaptive response. Since there was a suggestion of a dose-rate threshold for induction suppression, a second experiment was run with a fresh batch of cells at an initial dose rate of 1 mGy/day. These cells were allowed to accumulate 40 mGy over 46 days (average dose rate=0.87 mGy/day), and there was no evidence for suppression of transformation frequency compared to parallel unirradiated controls. It is concluded that doses of less than 100 mGy delivered at very low dose rates in the range 1 to 4 mGy/day can induce an adaptive response against neoplastic transformation in vitro. When the dose rate drops below approximately 1 mGy/day, this suppression is apparently lost, suggesting a possible dose-rate-dependent threshold for this process.     

Neoplastic transformation in vitro by mixed beams of high-energy iron ions and protons

The radiation environment in space is complex in terms of both the variety of charged particles and their dose rates. Simulation of such an environment for experimental studies is technically very difficult. However, with the variety of beams available at the National Space Research Laboratory (NSRL) at Brookhaven National Laboratory (BNL) it is possible to ask questions about potential interactions of these radiations. In this study, the end point examined was transformation in vitro from a preneoplastic to a neoplastic phenotype. The effects of 1?GeV/n iron ions and 1?GeV/n protons alone provided strong evidence for suppression of transformation at doses ≤5?cGy. These ions were also studied in combination in so-called mixed-beam experiments. The specific protocols were a low dose (10?cGy) of protons followed after either 5-15?min (immediate) or 16-24?h (delayed) by 1?Gy of iron ions and a low dose (10?cGy) of iron ions followed after either 5-15?min or 16-24?h by 1?Gy of protons. Within experimental error the results indicated an additive interaction under all conditions with no evidence of an adaptive response, with the one possible exception of 10?cGy iron ions followed immediately by 1?Gy protons. A similar challenge dose protocol was also used in single-beam studies to test for adaptive responses induced by 232?MeV/n protons and (137)Cs γ radiation and, contrary to expectations, none were observed. However, subsequent tests of 10?cGy of (137)Cs γ radiation followed after either 5-15?min or 8?h by 1?Gy of (137)Cs γ radiation did demonstrate an adaptive response at 8?h, pointing out the importance of the interval between adapting and challenge dose. Furthermore, the dose-response data for each ion alone indicate that the initial adapting dose of 10?cGy used in the mixed-beam setting may have been too high to see any potential adaptive response.     

Gene expression comparisons performed for biodosimetry purposes on in vitro peripheral blood cellular subsets and irradiated individuals

We examined the benefit of gene expression analysis on peripheral blood cellular subsets of different radiosensitivity to elucidate their utility as biodosimeters for estimation of dose in irradiated individuals. Peripheral mononucleated cells were isolated from 18 healthy volunteers employing density separation in a CPT-NH tube. Peripheral mononucleated cells were cultured in RPMI 1640 medium containing 10% autologous serum and were irradiated with 0.1-1 Gy (240 kV, 13 mA, X rays at 1 Gy/min). A low-dose study was performed with isolated peripheral mononucleated cells from one healthy donor in three independent experiments. Peripheral mononucleated cells were irradiated at 0 (sham), 1, 2.5 and 5 cGy (70 kV, 13 mA X rays at 1 cGy/min) and gene expression was measured 24 and 48 h after irradiation. After irradiation, CD4(+) or CD8(+) cells were isolated by magnetic beads in independent experiments. RNA from lymphocyte subsets and peripheral mononucleated cells was isolated after 24 and 48 h and converted into cDNA. Gene expression of GADD45, CDKN1A, DDB2, PCNA, BAX and ATF3 were determined using RTQ-PCR. Data were analyzed employing linear and logistic regression analysis. The same examinations were performed in 5 individuals either diagnosed using CT scans (up to 4.3 cGy) or by administering (F-18)-fluoro-2-deoxy-d-glucose (F-18 FDG, 0.6 cGy). Methodological, intra- and inter-individual variability in 90-95% of measurements did not exceed the introduced twofold change over sham-irradiated control values in peripheral mononucleated cells and CD4(+) cells, and therefore no false positive results were observed. Dose reconstruction in peripheral mononucleated cells in opposite to CD4(+) lymphocytes required fewer genes and appeared more efficient (R-square = 84.8% compared to 51.8%). In vitro samples exposed to 10 cGy could be completely discriminated from sham-irradiated samples without individual pre-exposure controls, which coincided with our preliminary in vivo results. However, in vitro differential gene expression was measured relative to control values and did not differ significantly at 24 and 48 h after irradiation in contrast to our preliminary in vivo data. In addition, below 5 cGy in vitro data did not show reproducible significant changes in gene expression, which was opposite to our preliminary in vivo data. Therefore a twofold change in gene expression over control sufficiently controls for different sources of variance, and measuring gene expression in peripheral mononucleated cell for biological dosimetry purposes appears superior over measurements in lymphocyte subsets. The increased gene expression measured after low absorbed doses in vivo and in vitro might indicate a particular applicability of this method for a low-level radiation scenario in the absence of individual pre-exposure controls. However, the constant gene expression values measured up to 48 h in our in vitro model at doses >10 cGy, and the absence of reproducible and statistically significant gene expression changes below 5 cGy contrast to the preliminary in vivo results performed at similar doses. Therefore, measurements with our in vitro models should be interpreted cautiously.     

Fission-spectrum neutrons at a low dose rate enhance neoplastic transformation in the linear, low dose region (0-10 cGy)

The neoplastic transformation of C3H 10T1/2 cells induced by fission-spectrum neutrons delivered at a high dose rate is linear up to 40 cGy. Reducing the dose rate increases the frequency of transformation in the low dose region. At a dose rate of 0.086 cGy min-1, the initial part of the induction curve remains linear but it has a slope 9-fold greater than the initial part of the curve at a high dose rate.     

Absence of a dose-fractionation effect on neoplastic transformation induced by fission-spectrum neutrons in C3H 10T1/2 cells

We have investigated the effect of fission-spectrum neutron dose fractionation on neoplastic transformation of exponentially growing C3H 10T1/2 cells. Total doses of 10.8, 27, 54, and 108 cGy were given in single doses or in five equal fractions delivered at 24-h intervals in the biological channel of the RSV-TAPIRO reactor at CRE-Casaccia. Both cell inactivation and neoplastic transformation were more effectively induced by fission neutrons than by 250-kVp X rays. No significant effect on cell survival or neoplastic transformation was observed with split doses compared to single doses of fission-spectrum neutrons. Neutron RBE values relative to X rays determined from data for survival and neoplastic transformation were comparable.     

Increased frequency of spontaneous neoplastic transformation in progeny of bystander cells from cultures exposed to densely ionizing radiation

An increased risk of carcinogenesis caused by exposure to space radiation during prolonged space travel is a limiting factor for human space exploration. Typically, astronauts are exposed to low fluences of ionizing particles that target only a few cells in a tissue at any one time. The propagation of stressful effects from irradiated to neighboring bystander cells and their transmission to progeny cells would be of importance in estimates of the health risks of exposure to space radiation. With relevance to the risk of carcinogenesis, we investigated, in model C3H 10T½ mouse embryo fibroblasts (MEFs), modulation of the spontaneous frequency of neoplastic transformation in the progeny of bystander MEFs that had been in co-culture 10 population doublings earlier with MEFs exposed to moderate doses of densely ionizing iron ions (1 GeV/nucleon) or sparsely ionizing protons (1 GeV). An increase (P<0.05) in neoplastic transformation frequency, likely mediated by intercellular communication through gap junctions, was observed in the progeny of bystander cells that had been in co-culture with cells irradiated with iron ions, but not with protons.     

Radiosensitivity parameters for neoplastic transformations in C3H10T1/2 cells

We have evaluated radiosensitivity parameters for cellular transformation from published experimental data on neoplastic transformations induced in C3H10T1/2 cells by BEVALAC ions. The measured RBE values are well reproduced by a track theory calculation using sets of m-target parameters with either m = 2 or m = 3, suggesting a quadratic or cubic extrapolation to low doses of gamma rays. Using track theory one is thus able to predict transformation frequencies in those cells after an arbitrary radiation field, under known or assumed conditions of exposure, in a manner shown earlier for cellular survival. Extension of these calculations to interpret cancer incidence in vivo is also discussed.     

The neoplastic transformation of SCID cells by radiation.

Severe combined immunodeficiency (SCID) cells are hypersensitive to killing by ionizing radiation because of deregulation of DNA-dependent protein kinase (DNA-PK) and a concomitant deficiency in the repair of DNA double-strand breaks. The effect of this condition on the neoplastic transformation of SCID fibroblasts, designated SCID 3T1, has been investigated. The spontaneous transformation rate was approximately 2 x 10(-5) at early passages and increased up to approximately 7 x l0(-3) at later passages. The radiation survival curves of transformed cells had thresholds and therefore appeared to be qualitatively similar to the survival curves of C3H 10T(1/2) mouse fibroblast cells, but the initial slopes were steeper. In contrast, per unit dose, SCID cells were more sensitive to transformation than 10T(1/2) cells. Eight transformed clones were tested for tumorigenicity, and all produced fibrosarcomas in athymic nude mice. Properties associated with the tumor suppressor Trp53 (formerly known as p53) were examined in three of the clones. In these clones, although Trp53 protein was overexpressed, a lower expression of Cdkn1a (formerly known as p21, Cip1) protein was observed compared to parental cells. The expression of Trp53 and Cdkn1a and the G(1)-phase arrest (one set of data on G(1)-phase delay is included as an example) was not induced by ionizing radiation in these transformed clones; each clone carried a point mutation in Trp53. This suggests that the deficiency in the repair of DNA double-strand breaks increased the tumorigenicity and the genomic instability of transformed SCID cells.     

Ionizing radiation alters neuronal excitability in hippocampal slices of the guinea pig

To investigate the effects of ionizing radiation on an isolated neuronal network without complicating systemic factors, slices of hippocampus from the guinea pig were isolated and studied in vitro. Slices were irradiated with a 60Co source and compared to paired, sham-irradiated controls. Electrophysiological activity in the CA 1 population of pyramidal cells was evoked by stimulation of the stratum radiatum. Analysis of the somatic and dendritic responses suggested sites of radiation damage. Orthodromically evoked activity was significantly decreased in slices receiving greater than 75 Gy gamma radiation. The effects were dose and dose-rate dependent. At 20 Gy/min, doses of 50 Gy and greater produced synaptic impairment while doses of 75 Gy and greater also produced postsynaptic damage (i.e., the ability of the synaptic response to generate an action potential). A lower dose rate, 5 Gy/min, reduced the sensitivity of synaptic damage to radiation exposure; synaptic impairment required a dose of 100 Gy or greater at the lower dose rate. In contrast, postsynaptic damage was not sensitive to dose rate. This study demonstrates that ionizing radiation can directly affect the integrated functional activity of neurons.     

Possible involvement of L-type voltage-gated calcium channels in release of dopamine in the striatum of irradiated rats

The object of this study was to determine the effect of exposure to gamma radiation on potassium chloride (KCl)-stimulated release of dopamine (DA) in the striatum of the rat. In addition, the effect of some calcium channel blockers [nicardipine, a blocker of the L-type voltage-gated N-type VGCC; Omega-agatoxin TK, a selective blocker of P-type VGCC; and nickel chloride (NiCl(2)), which preferentially blocks the T-type VGCC] on KCl-stimulated release of DA in the striatum in sham-irradiated and irradiated rats was determined. Exposure of rats to 1-10 Gy (60)Co gamma rays had no significant effect on KCl-stimulated release of DA in the striatum in comparison to sham-irradiated animals. Administering 100, 300 and 500 nM of Omega-agatoxin TK or 50, 100 and 200 nM of Omega-conotoxin GVIA significantly decreased the release of DA stimulated by KCl in both irradiated and sham-irradiated animals in a dose-dependent manner. However, 10, 30 and 50 microM of nicardipine decreased the release of DA in irradiated animals but not in sham-irradiated animals. It is unknown why doses of 5-20 microM NiCl(2) had no effect on the release of DA in sham-irradiated and irradiated animals. The results demonstrate that the doses of radiation used in this study had no effect on release of DA in the striatum. Multiple calcium channel types coexist to regulate release of DA. P- and N-type VGCCs are involved in release of DA in sham-irradiated and irradiated animals, whereas only L-type VGCCs are involved in release of DA in irradiated animals.     

Stimulation of radiation-impaired plasminogen activator release by phorbol ester in aortic endothelial cells

Ionizing radiation has been reported to affect the fibrinolytic activity of exposed tissue. With cultured bovine aortic endothelial cells, radiation suppresses the release of plasminogen activator to the conditioned media, with a concomitant increase in intracellular plasminogen activator. Thus study was undertaken to determine whether radiation-impaired plasminogen activator release can be modified by phorbol ester. We exposed cultured bovine aortic endothelial cells to a sterilizing dose of 10 Gy of gamma-rays and found the treatment led to cell injury, as evidenced by an increased release of prelabeled chromium, and to a reduction of plasminogen activator in the conditioned media with elevated intracellular plasminogen activator in irradiated cells. Phorbol ester enhanced plasminogen activator activity in both sham-irradiated and irradiated endothelial cells. It was interesting to note that the increased plasminogen activator in phorbol ester-stimulated sham-irradiated cells was largely retained inside the cell, while it was released to the conditioned media in irradiated cells. Apparently, altered plasminogen activator activity of radiation-sterilized endothelial cells can be modified by exogenous stimuli.     

Acute radiation effects on the content and release of plasminogen activator activity in cultured aortic endothelial cells

Confluent monolayers from three lines of bovine aortic endothelial cells were exposed to a single dose of 10 Gy of 60Co gamma rays. Seventy-two hours later, the morphology of the irradiated and sham-irradiated monolayers was examined, and cellular DNA and protein contents were determined. In addition, the release of plasminogen activator (PA) activity into the culture media and PA activity in the cell lysates were assayed. Irradiated monolayers maintained their cobblestone appearance, but individual endothelial cells were enlarged considerably compared to sham-irradiated cells. DNA and protein contents in the irradiated monolayers were reduced to 43-50% and 72-95% of the control levels, respectively. These data indicate that radiation induced cell loss (detachment and/or lysis) from the monolayer, with hypertrophy of surviving (attached) cells to preserve the continuity of the monolayer surface. Total PA activity (lysate plus medium) in the irradiated dishes was reduced to 50-75% of the control level. However, when endothelial PA activity was expressed on the basis of DNA content, the irradiated monolayers from two of the three cell lines contained significantly more PA activity than did sham-irradiated monolayers. Most importantly, the percentage of the total PA activity released into the culture medium by irradiated cells (5-22%) was significantly (P less than 0.001) lower than that released by sham-irradiated cells (23-68%). These data suggest that fibrinolytic defects observed in irradiated tissues in situ may be attributable at least in part to a radiation-induced inhibition of PA release by vascular endothelial cells.     

The effect of dose rate on radiation-induced neoplastic transformation in vitro by low doses of low-LET radiation

The dependence of the incidence of radiation-induced cancer on the dose rate of the radiation exposure is a question of considerable importance to the estimation of risk of cancer induction by low-dose-rate radiation. Currently a dose and dose-rate effectiveness factor (DDREF) is used to convert high-dose-rate risk estimates to low dose rates. In this study, the end point of neoplastic transformation in vitro has been used to explore this question. It has been shown previously that for low doses of low-LET radiation delivered at high dose rates, there is a suppression of neoplastic transformation frequency at doses less than around 100 mGy. In the present study, dose-response curves up to a total dose of 1000 mGy have been generated for photons from (125)I decay (approximately 30 keV) delivered at doses rates of 0.19, 0.47, 0.91 and 1.9 mGy/min. The results indicate that at dose rates of 1.9 and 0.91 mGy/min the slope of the induction curve is about 1.5 times less than that measured at high dose rate in previous studies with a similar quality of radiation (28 kVp mammographic energy X rays). In the dose region of 0 to 100 mGy, the data were equally well fitted by a threshold or linear no-threshold model. At dose rates of 0.19 and 0.47 mGy/min there was no induction of transformation even at doses up to 1000 mGy, and there was evidence for a possible suppressive effect. These results show that for this in vitro end point the DDREF is very dependent on dose rate and at very low doses and dose rates approaches infinity. The relative risks for the in vitro data compare well with those from epidemiological studies of breast cancer induction by low- and high-dose-rate radiation.     

Pathology effects at radiation doses below those causing increased mortality

Mortality data from experiments conducted at the Argonne National Laboratory (ANL) on the long-term effects of external whole-body irradiation on B6CF(1) mice were used to investigate radiation-induced effects at intermediate doses of (60)Co gamma rays or fission-spectrum neutrons either delivered as a single exposure or protracted over 60 once-weekly exposures. Kaplan-Meier analyses were used to identify the lowest dose in the ANL data (within radiation quality, pattern of exposure, and sex) at which radiation-induced mortality caused by primary tumors could be detected (approximately 1-2 Gy for gamma rays and 10-15 cGy for neutrons). Doses at and below these levels were then examined for radiation-induced shifts in the spectrum of pathology detected at death. To do this, specific pathology events were pooled into larger assemblages based on whether they were cancer, cardiovascular disease or non-neoplastic diseases detected within the lungs and pleura, liver and biliary tract, reproductive organs, or urinary tract. Cancer and cardiovascular disease were further subdivided into categories based on whether they caused death, contributed to death, or were simply observed at death. Counts of how often events falling within each of these combined pathology categories occurred within a mouse were then used as predictor variables in logistic regression to determine whether irradiated mice could be distinguished from control mice. Increased pathology burdens were detected in irradiated mice at doses lower than those causing detectable shifts in mortality-22 cGy for gamma rays and 2 cGy for neutrons. These findings suggest that (1) models based on mortality data alone may underestimate radiation effects, (2) radiation may have adverse health consequences (i.e. elevated health risks) even when mortality risks are not detected, and (3) radiation-induced pathologies other than cancer do occur, and they involve multiple organ systems.     

Response of primary human fibroblasts exposed to solar particle event protons

Solar particle events (SPEs) present a major radiation-related risk for manned exploratory missions in deep space. Within a short period the astronauts may absorb doses that engender acute effects, in addition to the risk of late effects, such as the induction of cancer. Using primary human cells, we studied clonogenic survival and the induction of neoplastic transformation after exposure to a worst case scenario SPE. We simulated such an SPE with monoenergetic protons (50, 100, 1000 MeV) delivered at a dose rate of 1.65 cGy min?1 in a dose range from 0 to 3 Gy. For comparison, we exposed the cells to a high dose rate of 33.3 cGy min?1. X rays (100 kVp, 8 mA, 1.7 mm Al filter) were used as a reference radiation. Overall, we observed a significant sparing effect of the SPE dose rate on cell survival. High-dose-rate protons were also more efficient in induction of transformation in the dose range below 30 cGy. However, as dose accumulated at high dose rate, the transformation levels declined, while at the SPE dose rate, the number of transformants continued to increase up to about 1 Gy. These findings suggest that considering dose-rate effects may be important in evaluating the biological effects of exposure to space radiation. Our analyses of the data based on particle fluence showed that lethality and transforming potential per particle clearly increased with increasing linear energy transfer (LET) and thus with the decreasing energy of protons. Further, we found that the biological response was determined not only by LET but also type of radiation, e.g. particles and photons. This suggests that using γ or X rays may not be ideal for assessing risk associated with SPE exposures.