Enhanced Sensitivity to Neoplastic Transformation by137Cs γ-rays of Cells in the G2-/M-phase Age Interval

Two major factors involved in the reverse dose-rate effect for somatic mutation induction are the cell cycle position and LET value

To study mechanisms which could be involved in the reverse dose rate effect observed during mutation induction after exposure to high LET radiation, synchronized mouse L5178Y cells were exposed to carbon 290 MeV/n beams with different LET values at the G2/M, G1, G1/S or S phases in the cell cycle. The frequency of Hprt-deficient (6-thioguanine-resistant) mutant induction was subsequently determined. The results showed that after exposure to high LET value radiation (50.8 and 76.5 keV/microm), maximum mutation frequencies were seen at the G2/M phase, but after exposure to lower LET radiation (13.3 keV/microm), the highest mutation frequencies were observed at the G1 phase. The higher LET beam always produced higher mutation frequencies in the G2/M phase than in the G1 phase, regardless of radiation dose. These results suggest that cells in the G2/M phase is hyper-sensitive for mutation induction from high LET radiation, but not to mutation induction from low LET radiation. Molecular analysis of mutation spectra showed that large deletions (which could include almost entire exons) of the mouse Hprt gene were most efficiently induced in G2/M cells irradiated with high LET radiation. These entire exon deletions were not as frequent in cells exposed to lower LET radiation. This suggests that inappropriate recombination repair might have occurred in response to condensed damage in condensed chromatin in the G2/M phase. In addition, by using a hyper-sensitive mutation detection system (GM06318-10 cells), a reverse dose-rate effect was clearly observed after exposure to carbon beams with higher LET values (66 keV/microm), but not after exposure to beams with lower LET values (13.3 keV/microm). Thus, G2/M sensitivity towards mutation induction, and the dependence on radiation LET values could both be major factors involved in the reverse dose rate effect produced by high LET radiation.

The state of the art in the 1990's: NCRP Report No. 136 on the scientific bases for linearity in the dose-response relationship for ionizing radiation

To reassess the use of the linear-nonthreshold dose-response model in the light of advancing knowledge, the National Council on Radiation Protection and Measurements formed Scientific Committee 1-6 and charged it to evaluate the evidence for and against the linear-nonthreshold dose-response hypothesis without reference to any associated policy ramifications. To accomplish this task, the Committee reviewed the relevant theoretical, experimental, and epidemiological data on those effects of ionizing radiation that are generally postulated to be stochastic in nature (i.e., genetic and carcinogenic effects). From its review of the data, the Committee concluded that the weight of evidence suggests that lesions that are precursors to cancer (i.e., mutations and chromosome aberrations), and certain types of cancer as well, may increase in frequency linearly with the dose in the low-dose domain. On this basis, the Committee concluded that no alternative dose-response model is more plausible than the linear-nonthreshold model although other dose-response relationships cannot be excluded, especially in view of growing evidence that the dose-response relationship may be modified by adaptive responses, bystander effects, and other variables.

Neutron carcinogenesis: past, present, and future

An interest in the possible cancer causing ability of neutrons began soon after their discovery. Early use of neutrons from radioactive sources and from cyclotrons led to a need to define risk for such exposures. This need was soon followed by a more tangible need to define risk to the general population of high LET radiation from nuclear fall out and use of the Atomic bomb and possible use of the H-bomb. Neutrons were soon found to be very effective cell killing agents compared to conventional ionizing radiation. However High LET radiation sources and neutrons in particular, come in many different energies and from many types of sources. I will survey the differences between different energy neutrons and conventional types of radiation, particularly with respect to the dose rate of exposures and the influence of repair or lack thereof and more recently the effect of cell cycle distribution on the carcinogenic outcome. I will illustrate these ideas with examples of carcinogenicity studies and mutation studies from my own laboratory and in some cases from the work of others. Lastly I will introduce some possible avenues for molecular studies of neutron effects that might answer such vexing questions as the real risk at very low doses, is repair error free or error prone, do neutrons cause genetic instability for many cell generations after exposure, and others? There remain many questions about the biology of neutron action that require answers if we are to protect the ever increasing number of people exposed to them because of their growing use in medicine, in the military and in commercial industry.

p53 gene mutations in neoplastic transformation of C3H 10T1/2 and severe combined immunodeficiency fibroblasts

The relevance of p53 mutations to the neoplastic malignant transformation of rodent fibroblasts by genotoxic physical and chemical agents is not clear. In the present study, we investigated p53 mutations (in exons 5-8) in non-transformed and neoplastically transformed C3H 10T1/2 and severe combined immunodeficiency (SCID) cells. No p53 mutations were detected in 15 neoplastically transformed (two spontaneous, one 3-methylcholanthrene-induced, seven gamma-ray-induced and five 'hot particle'-induced) and two non-transformed 10T1/2 cells. Wild-type p53 gene was also detected in all non-transformed (immortalized) SCID cell lines analyzed (four lines) whereas all three neoplastically transformed (two spontaneous, one gamma-ray-induced) cell lines displayed missense mutations in the p53 gene. These mutations were all transitions: A > G in codon 123, G > A in codon 152, and C > T in codon 238. We conclude that mutation in the p53 gene appears to be an infrequent event in 10T1/2 cells regardless of the transforming agent, but a frequent event in the neoplastic transformation of immortalized SCID cells. Non-transformed SCID cells are deficient in repair of DNA double-strand breaks, and neoplastically transformed cells are assumed to be deficient as well.

Cell cycle dependence of radiation-induced homologous recombination in cultured monkey cells

A lacZ transgene recombination system that reports homologous recombination events involving duplicated lacZ segments was used to study recombination in monkey cells exposed to ionizing radiation at different points in the cell cycle. With this system, recombination events can be detected in single cells by histochemical staining soon after exposure of cells to DNA-damaging treatment. Ionizing radiation rapidly induced recombination 5-10-fold in cells that were at the mitosis stage of the cell cycle. Irradiation either of cells at other points in the cell cycle or of nonsynchronized cells had less of an effect on recombination between lacZ segments.

A diversity of responses displayed by a stochastic model of radiation carcinogenesis allowing for cell death

A stochastic model is presented of carcinogenesis induced by irradiation with arbitrary time-dependent dose rate. The key feature of the model is that it allows for radiation-induced cell killing to compete with the process of tumor promotion. Two versions of the model arise when considering target tissues with slow and rapid replacement of damaged cells. These versions show dissimilar shapes of the dose-response curves in the case of short-term exposure. The model provides a natural explanation of the basic experimental findings documented in the radiobiological literature.

Sensitivity of C3H 10T1/2 cells to radiation-induced killing and neoplastic transformation as a function of cell cycle

Cell-age sensitivity to both cell killing and neoplastic transformation induced by radiation was investigated using synchronized populations of C3H10T1/2 cells. Mitotic-cell suspensions, collected using a mitotic shake-off procedure, were irradiated with 4Gy 250 kVp X-rays or 0.5 Gy fission neutrons from the RSV-TAPIRO reactor at CR-Casaccia. For study of cell killing the mitotic-cell suspensions were either irradiated immediately after collection, or plated for subsequent irradiation, which was performed every hour, covering an interval of 17 h. The response pattern observed was similar after X-rays and neutron irradiation, but the magnitude of the variation through the cell cycle was smaller in the case of neutrons (1.3- compared with 5-fold). For study of neoplastic transformation induction the irradiation was performed immediately after collection, i.e. in M phase, or at later times corresponding to mid-G1, G1/S and G2 phases. The sensitivity of the G2/M phase was examined by irradiating the cells with 4Gy X-rays while still attached to the flask bottom, and dislodging them after 25 min. SimilarLy to cell survival, the transformation frequency showed a small variation after neutron irradiation (1.4- compared with 3.1-fold) for the phases examined.

Radon-induced cancer: a cell-based model of tumorigenesis due to protracted exposures

In 1982, results with C3H mouse embryo cells showed that the frequency of neoplastic transformation was enhanced when exposures to fission-spectrum neutrons were protracted in time. This finding was unexpected because the opposite was found with low-LET radiations. Similar neutron enhancements were reported with normal life-span Syrian hamster embryo cells, and with human hybrid cells. Because other studies did not confirm the preceding, in 1990--at a conference convened by the US Armed Forces Radiobiological Research Institute--a biophysical model was proposed to explain the basis for the enhancement observed in some experiments but not in others. The model attributed special sensitivities, related to killing and neoplastic transformation, to cells in and around mitosis. Subsequently, it was shown that late G2/M phase cells constituted this window of sensitivity. In the instance of tumorigenesis, the model predicted that protracted exposures to a high-LET radiation would result in enhanced frequencies of transformation providing that susceptible cells were cycling or could be induced to cycle. The model explained data on lung tumour induction in rats breathing radon at different concentrations, and uranium miners working in atmospheres containing different concentrations of radon. The model also explains the anomalous finding that lung cancer deaths are often sublinearly correlated with indoor radon concentration.

Application of the constant exposure time technique to transformation experiments with fission neutrons: failure to demonstrate dose-rate dependence

A direct comparison of the effectiveness of fission neutrons at high (11.0-31.3 cGy/min) or several low dose-rates (0.14-3.2 cGy/min) was carried out under identical conditions. Monolayers of exponentially growing C3H/10T1/2 cells were exposed at 37 degrees C to reactor-produced neutrons (fluence-mean energy En = 0.68 MeV, < or = 5% gamma component, frequency mean linear energy yF = 21 keV/micron, dose mean lineal energy yD = 42 keV/micron in an 8-micron spherical cavity). Survival or transformation induction were studied at five doses from 10.5 to 94 cGy. In low dose-rate irradiations, these doses were protracted over 0.5, 1, 3 or 4.5 h, resulting in 17 different dose-rates. Up to six experiments were performed at each of five exposure times. Concurrently with transformation we studied cell proliferation in control versus cells irradiated at 40 cGy (acute and a 4.5-h protraction) and found no evidence of a shift in the cell cycle distribution among these cells. At a given dose and dose-rate, the effect of dose protraction on survival or transformation was assessed by the dose-rate modifying factor (DRMF), defined as the low:high dose-rate effect ratio at the same dose. Survival or transformation induction curves were nearly linear with initial slopes, respectively, of about 6.5 x 10(-3) or 6.2 x 10(-6) cGy-1. Consistent with dose-response curves, DRMFs were independent of the dose and dose-rate. The mean values of the DRMF with their uncertainties and 99% confidence intervals, based on measurements in individual doses and dose-rates for survival or transformation were, respectively: 1.01 +/- 0.03 (0.92, 1.09) or 0.98 +/- 0.04 (0.83, 1.08) indicating a similar precision in determining DRMF for survival or transformation, and no dose or dose-rate influence on these end points.

Oncogenic potential of hyperthermia in combination with radiation

The C3H 10T1/2 mouse embryo cell line was used to determine the effect of hyperthermia on the in vitro oncogenic transforming potential of radiation. Heat exposures at 45 degrees C/15 min or at 43 degrees C/60 min administered alone yielded no significant transformation as previously reported. However, our recent results repeat our earlier findings that there is an increase in the in vitro transformation frequency after the combined treatment of hyperthermia and radiation, if foci/flask or foci/surviving cell are used to calculate transformation frequency, if high temperature exposures are used (e.g. 43 degrees C/60 min or 45 degrees C/15 min) and if the time between the combined treatments of hyperthermia and 200 cGy of 60Co radiation is < or = 5 min at ambient temperature. As can be seen in this and past reports whether the combination of hyperthermia and radiation show an increase, a decrease, or no change in in vitro oncogenic transformation, a number of factors are critical. These critical factors are (1) temperature/exposure time and radiation dose as expected; (2) stage of the cell cycle and growth conditions at each exposure; (3) time between treatments; and (4) method of data analysis, i.e. whether the transformation frequency was based on the foci/viable cells, foci/flask or the foci/total cells at risk (total cells plated x plating efficiency of the untreated cells). Recent publications have shown that the position of cells in the cell cycle determine the frequency of cell transformation (Cao et al. 1992, Miller et al. 1992). Factors 1-3 affect the cells position in the cell cycle.(ABSTRACT TRUNCATED AT 250 WORDS)

Radiation-induced chromosomal aberrations in mouse 10T1/2 cells: dependence on the cell-cycle stage at the time of irradiation

Cell-cycle stage radiosensitivity for the induction of chromosome aberrations has been investigated in C3H 10T1/2 cells. Exponentially growing cells were irradiated with 3 Gy X-rays (80 kVp) or 0.6 Gy alpha-particles (LET = 101 keV/micron). The two doses produce the same survival level (37%) in the asynchronous population. Cells were harvested at four different times following irradiation and cell-cycle phase at the time of irradiation was assessed by using the differential replication staining technique. The frequency of chromosome aberrations produced in a given stage of the cell cycle was not constant as a function of the sampling time, but this could not be simply related to the existence of subphases exhibiting different radiosensitivity, because of cell-cycle perturbation introduced by radiation. X-radiation induced more exchanges than deletions, whereas a predominance of isochromatid deletions was observed after alpha-irradiation. This can be interpreted on the basis of the different patterns of energy deposition of densely- and sparsely-ionizing radiation. Both X- and alpha-rays produced a significant increase in the frequency of Robertsonian translocations when cells were exposed in G1 or S phase, but not in G2 phase.

Transformation-sensitive cells in G2/M-phase are not promoted by TPA following 137Cs gamma-rays

Mouse C3H 10T1/2 cells are most sensitive to radiation-induced neoplastic transformation in the G2/M-phase of the cell cycle. When synchronized 10T1/2 cells were exposed to phorbol 12-myristate 13-acetate (TPA) after irradiation, transformation of cells not in the transformation-sensitive window was enhanced, but transformation of cells already in the transformation-sensitive window was not. Earlier work showed that (a) TPA enhances the frequency of transformation of both high and low dose-rate gamma-irradiated cells by about the same factor, but that (b) TPA enhances the transformation of cells exposed to low dose-rate fission spectrum neutrons appreciably less than cells exposed to high dose-rate fission spectrum neutrons. The latter observation is consistent with the inability of TPA to promote cells in the transformation-sensitive window and with the role of such cells in enhancing transformation by protracted doses of neutrons. The data provide a cellular basis for studying the biochemical/molecular aspects of TPA promotion in vitro.

Neoplastic transformation of C3H mouse embryo cells, 10T1/2: cell-cycle dependence for 50 kV X-rays and UV-B light

The variation of neoplastic transformation induced by 50 kV X-rays, and by solar-simulating UV-B light, was studied through the cell cycle of C3H mouse embryo cells designated 10T1/2. A mitotic shake-off method was used to harvest mitotic cells. The progression through the cell cycle of initially mitotic cells was followed as a function of time by flow cytometry, DNA labelling for passage through S-phase, and growth curves for cell number. At 2-3 h after shake-off, about 90% of the cells were in early G1-phase and by 15 h 60-70% of cells had reached S-phase. For 2.5 Gy, the transformation frequency per viable cell in M-phase was some five times higher than in S-phase. In contrast, at similar survival levels, UV-B light is less efficient in transforming mitotic cells. For both types of radiation, the frequency of neoplastic transformation per viable cell was roughly inversely proportional to survival.

Dose-rate effects of neutrons and gamma-rays on the induction of mutation and oncogenic transformation in plateau-phase mouse m5S cells

The dose-rate effect of 252-californium neutrons was investigated using confluent cultures of mouse m5S cells. The relative biological effectiveness (RBE) of neutrons for oncogenic transformation was increased from 3.3 to 5.1 when the dose-rate was reduced from 1.8 to 0.12 cGy/min. Similarly, neutron RBE values for HPRT- mutation were 4.9 and 7.4 at dose-rates of 1.8 and 0.12 cGy/min, respectively. The increases in RBE as dose-rate was reduced were due mainly to diminished transformation- and mutation-induction by gamma-rays (the standard radiation). The yields of neutron-induced oncogenic transformation as well as neutron-induced mutation were constant for both dose rates. Our observation contrasts with reports by others using proliferating cells where both oncogenic transformation and mutation were enhanced with neutron exposure at a reduced dose-rate, the so-called inverse dose-rate effect. Since m5S cells are sensitive to postconfluent inhibition of cell division, this observation could be ascribed to cell growth conditions used in these experiments. The mechanism of the inverse dose-rate effect of neutrons suggests that the enhancement of neutron-induced mutation and oncogenic transformation at a reduced dose-rate is strongly associated with cell proliferation during exposure.

Cell cycle dependence for the induction of 6-thioguanine-resistant mutations: G2/M stage is distinctively sensitive to 252Cf neutrons but not to 60Co gamma-rays

Cell cycle dependence for the induction of 6-thioguanine-resistant mutation was investigated using synchronized mouse L5178Y cells after exposure to 4 Gy to 60Co gamma-rays or 1 Gy of 252Cf radiation (fission neutrons). Maximal mutation frequency was observed immediately after release from colcemid block (G2/M phase) after 252Cf radiation, whereas it was 1 h after the release (G1 phase) after 60Co gamma-rays. When the mutation frequency was plotted against the surviving fraction, a general correlation was observed between the two parameters except for G2/M and G1 phases which were more mutable per lethal event for 252Cf and 60Co radiations, respectively.