Multifractionation of 60Co gamma-rays reduces neoplastic transformation in vitro

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

C3H mouse 10T1/2 cells, exposed to low doses of fission-spectrum neutrons, have an enhanced frequency of neoplastic transformation if protracted exposures are used (Hill et al. 1982, 1984a, 1985). To explain this anomaly, a biophysical model was proposed (Elkind 1991a,b) having the following essential features: (1) a narrow age interval exists in the growth cycle of 10T1/2 cells in which cells have high sensitivities to transformation; (2) in the latter age interval, cells are also sensitive to killing; (3) with increasing dose, cells at ages earlier in the growth cycle are progressively delayed from entering the sensitive age window; and (4) with increasing dose, the transformation sensitivity of cells in the sensitive window is not expressed due to increased killing. Protracted low doses result in elevated frequencies because of less killing, and reductions in delays in cell progression. Therefore, transformation-sensitive cells can progress into the sensitive interval to replace those that have progressed out of it. The unique shape and radiobiological properties of cells in and around mitosis, led to the proposal that the sensitive window is mitosis and possible cells just preceding or just following M phase (Elkind 1991a,b). Because of the likelihood that the properties of the cells in a sensitive window would not be evident only when fission-spectrum neutrons are used, this study was undertaken using 137Cs gamma-rays. We have found that late G2- to M-phase 10T1/2 cells have a maximal sensitivity to neoplastic transformation as well as to killing by 137Cs gamma-rays.

Effect of Multiple Irradiation with Low Doses of Gamma-rays on Morphological Transformation and Growth Ability of Human Embryo Cellsin Vitro

We have measured expression of transformed phenotypes in human embryo (HE) cells repeatedly irradiated with a dose of 7.5 cGy per week throughout the life span of these cells in vitro. Irradiation was repeated until the cells had accumulated 195 cGy at which time the cells had reached the equivalent of their 26th passage and samples of cells at several passages were assayed for cell survival by colony formation, for mutation at hypoxanthine guanine phosphoribosyl transferase (HGPRT) locus and for transformation by focus formation. The lifespan (mean population doublings) of multiple irradiated cultures with a total dose of 97.5 cGy was slightly, but significantly, prolonged over that of controls. For example, if cells had accumulated 195 cGy, the maximum number of cell division of HE cells in vitro extended to 130-160% of non-irradiated control. Although transformed foci were not observed with cells until cells had accumulated 97.5 cGy, it increased with increasing accumulated dose. No cells, however, showed unlimited life span in vitro and also expressed tumorigenicity.

Radiation Research Society. 1952-2002. Historical and current highlights in radiation biology: has anything important been learned by irradiating cells?

Around 30 years ago, a very prominent molecular biologist confidently proclaimed that nothing of fundamental importance has ever been learned by irradiating cells! The poor man obviously did not know about discoveries such as DNA repair, mutagenesis, connections between mutagenesis and carcinogenesis, genomic instability, transposable genetic elements, cell cycle checkpoints, or lines of evidence historically linking the genetic material with nucleic acids, or origins of the subject of oxidative stress in organisms, to name a few things of fundamental importance learned by irradiating cells that were well known even at that time. Early radiation studies were, quite naturally, phenomenological. They led to the realization that radiations could cause pronounced biological effects. This was followed by an accelerating expansion of investigations of the nature of these radiobiological phenomena, the beginnings of studies aimed toward better understanding the underlying mechanisms, and a better appreciation of the far-reaching implications for biology, and for society in general. Areas of principal importance included acute tissue and tumor responses for applications in medicine, whole-body radiation effects in plants and animals, radiation genetics and cytogenetics, mutagenesis, carcinogenesis, cellular radiation responses including cell reproductive death, cell cycle effects and checkpoint responses, underlying molecular targets leading to biological effects, DNA repair, and the genetic control of radiosensitivity. This review summarizes some of the highlights in these areas, and points to numerous examples where indeed, many things of considerable fundamental importance have been learned by irradiating cells.

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.

Alteration of irradiated shuttle vector processing by exposure of human lymphoblast host cells to single or split gamma-ray doses

The repair of damaged DNA by mammalian cells exposed to single or split doses of radiation was probed with shuttle vector pZ189. Human lymphoblast hosts who received a single 120 cGy dose 2 h before transfection with 2500 cGy-damaged pZ189 yielded a two-fold higher frequency of progeny plasmids with mutations in their supF-tRNA target genes than did unirradiated host cells. Delaying transfection for 12 h, however, reduced the mutation frequency by half versus unirradiated controls. Plasmid survival was also affected by the time between host cell irradiation and transfection. Splitting doses of 50-500 cGy into two equal fractions separated by 4 h lowered mutation frequency and increased plasmid survival compared with equivalent acute doses; increasing the interval between dose fractions to 8 h, however, lowered plasmid survival compared with acute doses. Sequence analyses of the target gene in mutant plasmids revealed increased multiple-base substitution mutations among progenies recovered from irradiated hosts, indicating enhanced excision repair. These findings support modulation of mammalian cell DNA repair by ionizing radiation, disclose the transient nature of the effect of radiation on DNA repair, and demonstrate a quantitative difference in the effectiveness of single and split doses.

Lack of dose rate modification (0.0049 vs. 0.12 Gy/min) of fission-neutron-induced neoplastic transformation in C3H/10T1/2 cells

Clonogenic survival and neoplastic transformation of asynchronous cultures of C3H/10T1/2 cells were used to assay the effect of dose protraction of reactor-produced fission neutrons. Cells were exposed to eight neutron doses ranging from 0.05 to 0.9 Gy delivered at 11.7 or at 0.49 cGy/min. For each dose level, high and low dose rate irradiations were performed on the same day. At each dose a similar effectiveness of fission neutron irradiation at high or low dose rates was measured for both cell survival and transformation. The combined high and low dose-rate data were analysed by two- or three-parameter models. Depending on the model used, values of the effectiveness per unit dose derived as parameters of linear terms of the respective dose-response curves were 0.9-1.2 Gy-1 for clonogenic survival and 5-8 x 10(-4) Gy-1 for neoplastic transformation. It is concluded that the modification of fission neutron dose-response curves by dose rate is negligible or absent in the range of doses and dose rates examined, in contrast to results with other sources of fission or fast neutrons.

Interpretation of the increase in the frequency of neoplastic transformations observed for some ionising radiations at low dose rates

The anomalous increase of transformation frequency with decreasing dose rate observed by Hill et al. (1982, 1984b) for mouse fibroblast cells irradiated with fission neutrons cannot be satisfactorily explained by current models of radiation action. Recently a new model has been proposed which predicts the enhancement of damage with prolongation of irradiation, for equal doses. This is applied to the transformation studies in an attempt to interpret the enhancement observed for some radiations but not for others. Evidence is presented which suggests that repaired double-strand breaks in the DNA of cells which survive are the precursors of transformation. A critical physical factor is the total irradiation time rather than the dose rate. Approximately 1 per cent of repaired surviving cells go on to transform. From the results an explanation emerges of why transformation enhancement at low dose rates is not observed for natural alpha radiation and for photons or electrons, but is observed for fission neutrons and fast iron ions.

Lack of inverse dose-rate effect on fission neutron induced transformation of C3H/10T1/2 cells

Exponential and density-inhibited cultures of C3H/10T1/2 cells were exposed to a single dose of 0.3 Gy of fission neutrons delivered at rates ranging from 0.005 to 0.1 Gy/min. No discernible effect upon cell survival or transformation was observed by a lowering of the fission neutron dose rate in either exponential or plateau cultures. At the level of 2.3 x 10(-4) transformants per surviving cell, the RBE for neoplastic transformation was three at acute dose rates and ten at the lowest dose rate studied (0.005 Gy/min for neutrons and 0.01 Gy/min for X-rays).

Effect of X-ray dose protraction and a tumor promoter on transformation induction in vitro

We have investigated the effects of X-rays given in a brief exposure (1 min or less) or protracted over 5 h, on cell survival and the induction of neoplastic transformation in C3H/10T1/2 cells with an emphasis on latent transformation damage remaining after protracted irradiation. This latent damage and its expression were investigated at accumulated doses of 0.25 to 4 Gy by chronic treatment with TPA (12-O-tetradecanoyl-phorbol-13-acetate or phorbol myristate acetate) at 0.1 microgram/ml beginning after irradiation. Transformation incidence from protracted as well as brief X-irradiations was linearly related to X-ray dose in the presence of 0.1 microgram/ml TPA/ml. In the absence of TPA, the best fits were obtained with cubic rather than quadratic functions. The effect-modifying factors due to dose protraction were similar with or without TPA and averaged 4.6 at low doses (up to 2 Gy). Also within this dose range average transformation enhancement due to TPA was approximately 4. Our results indicate that dose protraction does not change the shape the dose-response curve for transformation, and that the shape change induced by TPA is also independent of dose protraction.

Effects of split-dose irradiation on survival and oncogenic transformation induced by 31 MeV protons in C3H10T1/2 cells

Survival and oncogenic transformation were studied in C3H10T1/2 cells exposed to 31 MeV protons. Total doses of 0.5, 1 and 7 Gy were delivered as single and two equal fractions with various time intervals up to 10 h between doses. With split doses as compared with single doses to a total dose of 7 Gy, survival increased by a factor of 2.5 +/- 0.2, whereas the frequency of transformation per surviving cell declined by a factor of 3.1 +/- 0.5. Maximal split-dose recovery occurred within the first 5 h for both endpoints. Further, the transformation frequency decreased by factors of 3.1 +/- 0.6 and 1.5 +/- 0.3 respectively for total doses of 0.5 and 1.0 Gy split into two equal fractions. The data for 1 and 7 Gy are compatible with data in the literature for other low LET radiations.

Repair time for oncogenic transformation in C3H/10T1/2 cells subjected to protracted X-irradiation

With exponential cultures of C3H/10T1/2 cells, we have investigated the effect of X-ray dose protraction on oncogenic cell transformation in the dose range 0.25-2 Gy. Within a particular experiment a constant exposure time was used. In different experiments exposure time varied between 1 and 5h. Cell transformation was analysed using the linear-quadratic relation, gamma (D) = alpha 1D + alpha 2D2, between transformation frequency per surviving cell and X-ray dose. Based on values of the linear coefficients, we developed an empirical formula for relating slopes of dose induction curves obtained at high or reduced dose rate condition. Our estimate of repair half-time for cell transformation with 95 per cent confidence limits is 2.4 (1.8, 3.0) h.

Dose protraction studies with low- and high-LET radiations on neoplastic cell transformation in vitro

A major objective of our heavy-ion research is to understand the potential carcinogenic effects of cosmic rays and the mechanisms of radiation-induced cell transformation. During the past several years, we have studied the relative biological effectiveness of heavy ions with various atomic numbers and linear energy transfer on neoplastic cell transformation and the repair of transformation lesions induced by heavy ions in mammalian cells. All of these studies, however, were done with a high dose rate. For risk assessment, it is extremely important to have data on the low-dose-rate effect of heavy ions. Recently, with confluent cultures of the C3H10T1/2 cell line, we have initiated some studies on the low-dose-rate effect of low- and high-LET radiation on cell transformation. For low-LET photons, there was a decrease in cell killing and cell transformation frequency when cells were irradiated with fractionated doses and at low dose rate. Cultured mammalian cells can repair both subtransformation and potential transformation lesions induced by X rays. The kinetics of potential transformation damage repair is a slow one. No sparing effect, however, was found for high-LET radiation. There was an enhancement of cell transformation for low-dose-rate argon (400 MeV/u; 120 keV/micrometer) and iron particles (600 MeV/u; 200 keV/micrometer). The molecular mechanisms for the enhancement effect is unknown at present.

Single, split and fractionated dose X-radiation-induced malignant transformation in A31-11 mouse BALB/3T3 cells

Studies were conducted to determine the effects of a single, split and fractionated doses (separated by 1-h intervals) of 100 rad of X-irradiation on the morphological transformation of BALB/c 3T3 clone A31-11 mouse fibroblasts grown in media containing calf serum. Both spontaneous and radiation-induced transformation levels were lower for these cells grown in the cell-serum containing media than previously reported for these cells grown in fetal calf serum containing media. In the studies reported here, cells were irradiated either as density-inhibited plateau phase cultures or as low density cultures at 10-14 h after being reseeded from confluent dishes. We observed that a 4-fraction 100-rad dose resulted in a reduced yield of transformants compared to a single dose of 100 rad when plateau phase cultures were utilized for the radiation exposures, but not in low density cultures in which the cells were allowed to proliferate during the radiation exposures, these results suggest that the growth phase of the cells can play a major role in determining the yield of transformants induced by fractionated doses of radiation. It is noteworthy that, for the other data obtained in these studies, in none of 12 different experimental points (involving 5 separate experiments) did a fractionated dose protocol result in a reduced yield of transformants when compared to a single dose protocol.

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.