Dent, P., Yacoub, A., Contessa, J., Caron, R., Amorino, G., Valerie, K., Hagan, M. P., Grant, S. and Schmidt-Ullrich, R. Stress and Radiation-Induced Activation of Multiple Intracellular Signaling Pathways. Radiat. Res. 159, 283–300 (2003).

Exposure of cells to a variety of stresses induces compensatory activations of multiple intracellular signaling pathways. These activations can play critical roles in controlling cell survival and repopulation effects in a stress-specific and cell type-dependent manner. Some stress-induced signaling pathways are those normally activated by mitogens such as the EGFR/RAS/PI3K-MAPK pathway. Other pathways activated by stresses such as ionizing radiation include those downstream of death receptors, including pro-caspases and the transcription factor NFKB. This review will attempt to describe some of the complex network of signals induced by ionizing radiation and other cellular stresses in animal cells, with particular attention to signaling by growth factor and death receptors. This includes radiation-induced signaling via the EGFR and IGFI-R to the PI3K, MAPK, JNK, and p38 pathways as well as FAS-R and TNF-R signaling to pro-caspases and NFKB. The roles of autocrine ligands in the responses of cells and bystander cells to radiation and cellular stresses will also be discussed. Based on the data currently available, it appears that radiation can simultaneously activate multiple signaling pathways in cells. Reactive oxygen and nitrogen species may play an important role in this process by inhibiting protein tyrosine phosphatase activity. The ability of radiation to activate signaling pathways may depend on the expression of growth factor receptors, autocrine factors, RAS mutation, and PTEN expression. In other words, just because pathway X is activated by radiation in one cell type does not mean that pathway X will be activated in a different cell type. Radiation-induced signaling through growth factor receptors such as the EGFR may provide radioprotective signals through multiple downstream pathways. In some cell types, enhanced basal signaling by proto-oncogenes such as RAS may provide a radioprotective signal. In many cell types, this may be through PI3K, in others potentially by NFKB or MAPK. Receptor signaling is often dependent on autocrine factors, and synthesis of autocrine factors will have an impact on the amount of radiation-induced pathway activity. For example, cells expressing TGFα and HB-EGF will generate protection primarily through EGFR. Heregulin and neuregulins will generate protective signals through ERBB4/ERBB3. The impact on radiation-induced signaling of other autocrine and paracrine ligands such as TGFβ and interleukin 6 is likely to be as complicated as described above for the ERBB receptors.

Dugan, L. C. and Bedford, J. S. Are Chromosomal Instabilities Induced by Exposure of Cultured Normal Human Cells to Low- or High-LET Radiation? Radiat. Res. 159, 301–311 (2003).

Radiation-induced genomic instability has been proposed as a very early, if not an initiating, step in radiation carcinogenesis. Numerous studies have established the occurrence of radiation-induced chromosomal instability in various cells of both human and rodent origin. In many of these studies, however, the cells were not “normal” initially, and in many cases they involved tumor-derived cell lines. The phenomenon clearly would be of even greater interest if it were shown to occur generally in cells that are normal at the outset, rather than cells that may have been “selected” because of a pre-existing susceptibility to induced instability. As a test of the generality of the phenomenon, we studied low-passage normal diploid human fibroblasts (AG1521A) to determine whether they are susceptible to the induction of chromosomal instability in the progeny of surviving cells after exposure in G₀ to low- and high-LET radiation. Cytogenetic assays for instability were performed on both mixed populations of cells and clones of cells surviving exposure. We found no evidence for the induction of such instability as a result of radiation exposure, though we observed a senescence-related chromosomal instability in the progeny of both irradiated and unirradiated cell populations.

Kang, C-M., Park, K-P., Song, J-E., Jeoung, D-I., Cho, C-K., Kim, T-H., Bae, S., Lee, S-J. and Lee, Y-S. Possible Biomarkers for Ionizing Radiation Exposure in Human Peripheral Blood Lymphocytes. Radiat. Res. 159, 312–319 (2003).

Biomarkers to indicate past exposure to radiation have not been entirely satisfactory. Using cDNA microarray hybridization to find new potential biomarkers, we identified highly expressed genes in human peripheral blood lymphocytes (PBLs) after irradiation 1 Gy ex vivo. The present set of radiation markers in PBLs was identified 12 h after radiation. A total of 44 genes were identified. However, when RT-PCR was performed with mRNA from the PBLs of five individuals, only four genes, including TRAIL receptor 2, DRAL (now known as FHL2), cyclin G, and cyclin protein gene, showed greater than 50% agreement between gene induction as detected by microarray analysis and by RT-PCR. When more than 32 donors were tested for the above four genes, greater than 85% agreement was obtained between gene induction measured by microarray analysis and by RT-PCR. There was a linear dose–response relationship between 0.5 and 4 Gy 12 h after irradiation; however, there was less linearity at later times. These results suggested that the relative expression levels of genes such as TRAIL receptor 2, FHL2, cyclin G, and cyclin protein gene in PBLs may provide estimates of radiation exposures.

Mitchel, R. E. J., Jackson, J. S., Morrison, D. P. and Carlisle, S. M. Low Doses of Radiation Increase the Latency of Spontaneous Lymphomas and Spinal Osteosarcomas in Cancer-Prone, Radiation-Sensitive Trp53 Heterozygous Mice. Radiat. Res. 159, 320–327 (2003).

Mice heterozygous for Trp53 are radiation-sensitive and cancer-prone, spontaneously developing a variety of cancer types. Osteosarcomas in the spine lead to paralysis, while lymphomas lead rapidly to death, distinct events that provide objective measures of latency. The effects of a single low-dose (10 or 100 mGy), low-dose-rate (0.5 mGy/min) ⁶⁰Co γ irradiation on lymphoma or spinal osteosarcoma frequency and latency, defined as time of death or of onset of paralysis, respectively, were examined. Compared to spontaneous lymphomas or to spinal osteosarcomas leading to paralysis in unexposed mice, an exposure of 7–8-week-old Trp53 ^/− mice to 10 or 100 mGy had no significant effect on tumor frequency, indicating no effect on tumor initiation. All tumors are therefore assumed to be of spontaneous origin. However, a 10-mGy exposure reduced the risk of both lymphomas and spinal osteosarcomas by significantly increasing tumor latency, indicating that the main in vivo effect of a low-dose exposure is a reduction in the rate at which spontaneously initiated cells progress to malignancy. The effect of this adaptive response persisted for the entire life span of all the animals that developed these tumors. Exposure to 100 mGy delayed lymphoma latency longer than the 10-mGy exposure. However, the 100-mGy dose increased spinal osteosarcoma risk by decreasing overall latency compared to unexposed control mice. That result suggested that this higher dose was in a transition zone between reduced and increased risk, but that the dose at which the transition occurs varies with the tumor type.

Burd, R., Lavorgna, S. N., Daskalakis, C., Wachsberger, P. R., Wahl, M. L., Biaglow, J. E., Stevens, C. W. and Leeper, D. B. Tumor Oxygenation and Acidification are Increased in Melanoma Xenografts after Exposure to Hyperglycemia and meta-Iodo-benzylguanidine. Radiat. Res. 159, 328–335 (2003).

Tumor oxygen tension and extracellular pH (pH_e) are physiological parameters that can be manipulated to improve current cancer therapies. Many human tumors consist of cells that are chronically exposed to low pH_e. Exposure of tumor cells in culture to glucose decreases oxygen consumption (oxygen sparing or Crabtree effect), and while this effect is absent in low pH-adapted tumor cells, it can be restored by combining the respiratory inhibitor meta-iodo-benzylguanidine (MIBG) with glucose (Burd et al., Cancer Res. 61, 5630–5635, 2001). The effects of hyperglycemia and MIBG on tumor oxygen tension and on pH_e were investigated in human melanoma xenografts in SCID mice. An oral gavage of 1 M glucose (2 g/kg) increased the average blood glucose concentration from <140 mg/dl to ∼400 mg/dl. Although tumor pH_e decreased from pH 6.7 to pH 6.5 (P < 0.01) after about 60 min, no change in tumor oxygen tension was observed. However, when oral glucose and MIBG (15 mg/kg) were administered together, oxygen tension increased from 2.8 mmHg to ∼17 mmHg, and tumor pH_e decreased from pH 6.7 to pH 6.3 (P < 0.01) after about 115 min. In conclusion, administration of glucose together with MIBG increases tumor oxygen tension and also increases the magnitude and duration of acidification. Hyperglycemia plus MIBG has the potential to improve response to radiation therapy as well as to hyperthermia and some chemotherapies.

Kirkpatrick, J. P., Brizel, D. M. and Dewhirst, M. W. A Mathematical Model of Tumor Oxygen and Glucose Mass Transport and Metabolism with Complex Reaction Kinetics. Radiat. Res. 159, 336–344 (2003).

Hypoxia imparts radioresistance to tumors, and various approaches have been developed to enhance oxygenation, thereby improving radiosensitivity. This study explores the influence of kinetic and physical factors on substrate metabolism in a tumor model, based on a Krogh cylinder. In tissue, aerobic metabolism is assumed to depend on glucose and oxygen, represented by the product of Michaelis-Menten expressions. For the base case, an inlet pO₂ of 40 mmHg, a hypoxic limit of 5 mmHg, and a tissue/capillary radius ratio of 10 are used. For purely aerobic metabolism, a hypoxic fraction of 0.16 and volume-average pO₂ of 8 mmHg are calculated. Reducing the maximum oxygen rate constant by 9%, decreasing the tissue cylinder radius by 5%, or increasing the capillary radius by 8% abolishes the hypoxic fraction. When a glycolytic term is added, concentration profiles are similar to the base case. Using a distribution of tissue/capillary radius ratios increases the hypoxic fraction and reduces sensitivity to the oxygen consumption rate, compared to the case with a single tissue/capillary radius ratio. This model demonstrates that hypoxia is quite sensitive to metabolic rate and geometric factors. It also predicts quantitatively the effects of inhibited oxygen metabolism and enhanced mass transfer on tumor oxygenation.

Pohl, F., Hassel, S., Nohe, A., Flentje, M., Knaus, P., Sebald, W. and Koelbl, O. Radiation-Induced Suppression of the Bmp2 Signal Transduction Pathway in the Pluripotent Mesenchymal Cell Line C2C12: An In Vitro Model for Prevention of Heterotopic Ossification by Radiotherapy. Radiat. Res. 159, 345–350 (2003).

Heterotopic ossification is a common complication after total hip replacement. Clinical studies showed the effectiveness of radiation for prevention of heterotopic ossification. The mechanism of radiotherapy responsible for the reduction of heterotopic ossification is unclear. The purpose of this study was to study an analogue model showing a time- and dose-dependent effect of radiation. Using cells of the defined embryonic mouse cell line C2C12, the influence of ionizing radiation on the Bmp-induced signal cascade leading to osteogenic differentiation was analyzed. Binding of iodinated Bmp2 to the receptors, Smad1 activation, and alkaline phosphatase (ALP) activity were determined in cells with or without irradiation. The cytotoxic effect of radiotherapy was evaluated using viability tests. Radiotherapy reduced formation of the Bmp2/Bmp receptor complex. This effect was dependent on dose. The phosphorylation (activation) of Smad1 decreased after irradiation in a time-dependent manner, whereas the level of total Smads was not influenced by radiotherapy. The ALP activity decreased after radiotherapy. A dose of 7 Gy delivered 6 h before or after incubation with Bmp resulted in about a 30% decrease in ALP activity. No signs of cytotoxic effects were observed within the time window studied using doses of 0 to 20 Gy. The time- and dose-dependent effect of radiotherapy for prevention of heterotopic ossification known from the results of clinical studies has an analogue in the C2C12 cell model. The primary mechanism of radiotherapy seems to be an influence on cellular responsiveness to the Bmp2-induced osteoblastic differentiation. The results suggest a down-regulation of the Bmp2/receptor complex.

Takagi, K., Yamaguchi, K., Sakurai, T., Asari, T., Hashimoto, K. and Terakawa, S. Secretion of Saliva in X-Irradiated Rat Submandibular Glands. Radiat. Res. 159, 351–360 (2003).

The mechanism of radiation-induced dysfunction in rat submandibular glands was investigated at the cellular level. After X irradiation (single dose, 15 Gy), a vacuolation in the acinar cells or an enlargement of the acinar lumen was observed as a typical morphological change for 2 weeks. As observed using a video-enhanced contrast differential interference contrast (VEC-DIC) microscope, exocytosis and shrinkage of the acinar cells induced by application of pilocarpine (100 μM) were markedly suppressed for 5 days and then recovered to 80% of the control levels. Using an immunohistochemical method, no significant change was observed in amylase distribution, but a marked loss of aquaporin 5 was found in the acinar cells after the irradiation. The extent and time course of pilocarpine-induced mobilization of intracellular Ca² did not change after the irradiation. We conclude that radiation-induced dysfunction in the salivary glands is due to an impairment of exocytosis and a reduction of water secretion. The loss of aquaporin 5 and possibly other membrane-fusion proteins in acinar cells may be the major mechanism underlying such a dysfunction.

Guo, H., Seixas-Silva, Jr., J. A., Epperly, M. W., Gretton, J. E., Shin, D. M., Bar-Sagi, D., Archer, H. and Greenberger, J. S. Prevention of Radiation-Induced Oral Cavity Mucositis by Plasmid/Liposome Delivery of the Human Manganese Superoxide Dismutase (SOD2) Transgene. Radiat. Res. 159, 361–370 (2003).

Oral cavity mucositis is a major toxicity of radiation therapy for head and neck cancer. In the present mouse model studies, we evaluated intraoral administration of SOD2-PL complexes 24 h before single-fraction 30-Gy irradiation for the prevention of oral cavity mucositis. Expression of the human SOD2 transgene in the oral cavity of C3H/HeNsd mice was demonstrated by nested reverse transcriptase polymerase chain reaction (RT-PCR). Mice treated intraorally with bacterial β-galactosidase gene-plasmid/liposome (LacZ-PL) or hemagglutinin (HA)-manganese superoxide dismutase-plasmid/liposome (HA-SOD2-PL) demonstrated LacZ or HA-SOD2 expression, respectively, 24 h after injection. In a second strain of mouse, SOD2-PL-treated female athymic nude mice demonstrated significantly decreased ulceration at day 5 after 30 Gy, compared to LacZ-PL-injected, irradiated mice or irradiated controls. No further reduction in radiation-induced ulceration was detected in mice treated with both SOD2-PL and 10 mg/kg of amifostine (WR-2721) 30 min before 30 Gy compared to SOD2-PL alone. No significant protection of orthotopically transplanted murine squamous cell carcinoma (SCC-VII) tumors was detected in mice that received SOD2-PL treatment before 18 Gy. Thus overexpression of human SOD2 in the oral cavity mucosa can prevent radiation-induced mucositis with no detectable compromise in the therapeutic response of orthotopically transplanted tumors.

Jagetia, G. C., Rajanikant, G. K. and Rao, S. K. Evaluation of the Effect of Ascorbic Acid Treatment on Wound Healing in Mice Exposed to Different Doses of Fractionated Gamma Radiation. Radiat. Res. 159, 371–380 (2003).

Alteration of the radiation-induced changes in wound contraction, collagen synthesis and wound histology by ascorbic acid was studied in mice exposed to 10, 16 and 20 Gy of fractionated (2 Gy/fraction) γ radiation. The animals were given double-distilled water or ascorbic acid daily before exposure to 2 Gy/day of fractionated irradiation. A full-thickness skin wound was created on the dorsum of the irradiated mice, and the progression of wound contraction and collagen synthesis were examined and histological evaluations were carried out at various times after wounding. Irradiation caused a dose-dependent delay in wound contraction, and pretreatment with ascorbic acid resulted in a significant increase in wound contraction. The greatest increase in wound contraction was observed 6 and 9 days after wounding in both groups. Pretreatment with ascorbic acid augmented the synthesis of collagen significantly as revealed by an increase in hydroxyproline content. The collagen deposition and fibroblast and vasculature densities declined in a dose-dependent manner in groups receiving radiation alone as indicated by histological evaluation. Pretreatment with ascorbic acid ameliorated the observed effect significantly. These studies demonstrate that pretreatment with ascorbic acid resulted in a significant reduction of radiation-induced delay in wound healing as shown by earlier wound closure and increased collagen content and fibroblast and vascular densities.

Miller, J., Zeitlin, C., Cucinotta, F. A., Heilbronn, L., Stephens, D. and Wilson, J. W. Benchmark Studies of the Effectiveness of Structural and Internal Materials as Radiation Shielding for the International Space Station. Radiat. Res. 159, 381–390 (2003).

Accelerator-based measurements and model calculations have been used to study the heavy-ion radiation transport properties of materials in use on the International Space Station (ISS). Samples of the ISS aluminum outer hull were augmented with various configurations of internal wall material and polyethylene. The materials were bombarded with high-energy iron ions characteristic of a significant part of the galactic cosmic-ray (GCR) heavy-ion spectrum. Transmitted primary ions and charged fragments produced in nuclear collisions in the materials were measured near the beam axis, and a model was used to extrapolate from the data to lower beam energies and to a lighter ion. For the materials and ions studied, at incident particle energies from 1037 MeV/nucleon down to at least 600 MeV/nucleon, nuclear fragmentation reduces the average dose and dose equivalent per incident ion. At energies below 400 MeV/nucleon, the calculation predicts that as material is added, increased ionization energy loss produces increases in some dosimetric quantities. These limited results suggest that the addition of modest amounts of polyethylene or similar material to the interior of the ISS will reduce the dose to ISS crews from space radiation; however, the radiation transport properties of ISS materials should be evaluated with a realistic space radiation field.

Chiu, S-J., Lee, M-Y., Chou, W-G. and Lin, L-Y. Germanium Oxide Enhances the Radiosensitivity of Cells. Radiat. Res. 159, 391–400 (2003).

We investigated here the combined effect of GeO₂ and radiation on cell viability. Cells were treated with 0 to 22 mM GeO₂ for 12 h followed by 1 Gy X irradiation. A synergistic cytotoxic effect was observed for the combined treatment with a dose-dependent reduction of cell viability. Complete survival curves showed a 2.3- and 2.75-fold increase in radiosensitivity for 50% cell death in the presence of 5 and 15 mM GeO₂, respectively. The increased radiosensitivity also occurred when GeO₂ was given either 4 h prior to irradiation or immediately after radiation exposure. GeO₂ did not affect the total soluble thiol content or the activities of catalase and glutathione S-transferase. Analysis of the production of reactive oxygen species (ROS) revealed that the combined treatment dramatically increased the synthesis of ROS. Addition of N-acetyl cysteine (NAC, 20 mM) decreased the production of ROS in cells. NAC, however, increased cell viability only slightly after treatment with GeO₂ and radiation. Thus increased production of ROS makes little or no contribution to the observed death. The combination of GeO₂ and X radiation, however, significantly increased the frequency of DNA double-strand breaks (DSBs). Notably, the presence of GeO₂ also reduced the efficiency of DNA repair. We conclude that treatment with GeO₂ followed by X irradiation increases DNA DSBs and cell death.

Friedland, W., Jacob, P., Bernhardt, P., Paretzke, H. G. and Dingfelder, M. Simulation of DNA Damage after Proton Irradiation. Radiat. Res. 159, 401–410 (2003).

The biophysical radiation track simulation model PARTRAC was improved by implementing new interaction cross sections for protons in water. Computer-simulated tracks of energy deposition events from protons and their secondary electrons were superimposed on a higher-order DNA target model describing the spatial coordinates of the whole genome inside a human cell. Induction of DNA double-strand breaks was simulated for proton irradiation with LET values between 1.6 and 70 keV/μm and various reference radiation qualities. The yield of DSBs after proton irradiation was found to rise continuously with increasing LET up to about 20 DSBs per Gbp and Gy, corresponding to an RBE up to 2.2. About half of this increase resulted from a higher yield of DSB clusters associated with small fragments below 10 kbp. Exclusion of experimentally unresolved multiple DSBs reduced the maximum DSB yield by 30% and shifted it to an LET of about 40 keV/μm. Simulated fragment size distributions deviated significantly from random breakage distributions over the whole size range after irradiation with protons with an LET above 10 keV/μm. Determination of DSB yields using equations derived for random breakage resulted in an underestimation by up to 20%. The inclusion of background fragments had only a minor influence on the distribution of the DNA fragments induced by radiation. Despite limited numerical agreement, the simulations reproduced the trends in proton-induced DNA DSBs and fragment induction found in recent experiments.

Cai, Z. and Sevilla, M. D. Electron and Hole Transfer from DNA Base Radicals to Oxidized Products of Guanine in DNA. Radiat. Res. 159, 411–419 (2003).

An investigation of electron and hole transfer to oxidized guanine bases in DNA is reported. Guanine in DNA was preferentially oxidized by Br₂^·− at 298 K to 8-oxo-7,8-dihydro-guanine (8-oxo-G) and higher oxidation products. HPLC-EC analysis of irradiated DNA shows that the formation of 8-oxo-G could not be increased above the ratio of one 8-oxo-G to 127 ± 6 bp regardless of dose. 8-oxo-G can be produced only at low levels because it is further oxidized to other species. These oxidation products of guanine have been extensively investigated and identified by others. Our electron spin resonance studies suggest that at 77 K 8-oxo-G is a trap for radiation-produced holes, but certain further oxidation products of 8-oxo-G (G_ox) are found to be efficient electron traps. Gamma irradiation of oxidized DNA samples in frozen (D₂O) aqueous ices and glassy 7 M LiBr solutions resulted in radicals formed by electron attachment to the G_ox sites that were monitored by electron spin resonance spectroscopy (ESR) at 77 K. These ESR spectra suggest that those oxidation products of 8-oxo-G containing α-diketo groups account for the electron traps (G_ox) in oxidized DNA with oxaluric acid a likely major trap. Electron transfer from DNA anion radicals to G_ox was followed by monitoring of their ESR signals with time at 77 K. Using typical values for the tunneling constant β estimates of the relative amount of G_ox to base pairs were obtained. Radicals formed by UV photolysis of oxidized DNA in 8 M NaClO₄ glassy aqueous solutions were also investigated. The 8-oxo-G cation accounts for less than 10% of all the radicals observed after either γ irradiation of oxidized DNA in frozen (D₂O) aqueous solution or UV photolysis of oxidized DNA in 8 M NaClO₄ glassy aqueous solutions. We estimate hole transfer distances of about 7 ± 1 bp at 1 min from G^· to 8-oxo-G.

Wang, H., Wang, X., Iliakis, G. and Wang, Y. Caffeine Could Not Efficiently Sensitize Homologous Recombination Repair-Deficient Cells to Ionizing Radiation-Induced Killing. Radiat. Res. 159, 420–425 (2003).

Caffeine inhibits ATM and ATR, two important checkpoint regulators, abolishes ionizing radiation-induced checkpoint response, and radiosensitizes cells. Radiation-induced DNA double-strand breaks (DSBs) are repaired by two major processes, homologous recombination repair (HRR) and nonhomologous end joining (NHEJ). It remains unclear which repair process, HRR or NHEJ, is affected when the checkpoint responses are abolished by caffeine. In this study we observed the effect of caffeine on gene-targeted DT40 chicken lymphoblast cells. We show that caffeine efficiently abolishes S- and G₂-phase checkpoint responses after irradiation in all cell lines tested and greatly radiosensitizes wild-type and ATM^–/– cells, the partially checkpoint-deficient cells. However, caffeine has a much smaller radiosensitizing effect on RAD54^–/– cells and has no effect on RAD51-deficient cells. RAD51 and RAD54 are the important factors for HRR. Our results indicate that the checkpoint responses abolished by caffeine (S and G₂) mainly affect HRR, which results in cell radiosensitization.

Wang, X., Wang, H., Iliakis, G. and Wang, Y. Caffeine-Induced Radiosensitization is Independent of Nonhomologous End Joining of DNA Double-Strand Breaks. Radiat. Res. 159, 426–432 (2003).

After exposure to ionizing radiation, proliferating cells actively slow down progression through the cell cycle through the activation of checkpoints to provide time for repair. Two major complementary DNA double-strand break (DSB) repair pathways exist in mammalian cells, homologous recombination repair (HRR) and nonhomologous end joining (NHEJ). The relationship between checkpoint activation and these two types of DNA DSB repair pathways is not clear. Caffeine, as a nonspecific inhibitor of ATM and ATR, abolishes multi-checkpoint responses and sensitizes cells to radiation-induced killing. However, it remains unknown which DNA repair process, NHEJ or HRR, or both, is affected by caffeine-abolished checkpoint responses. We report here that caffeine abolishes the radiation-induced G₂-phase checkpoint and efficiently sensitizes both NHEJ-proficient and NHEJ-deficient mammalian cells to radiation-induced killing without affecting NHEJ. Our results indicate that caffeine-induced radiosensitization occurs by affecting an NHEJ-independent process, possibly HRR.

Redpath, J. L., Short, S. C., Woodcock, M. and Johnston, P. J. Low-Dose Reduction in Transformation Frequency Compared to Unirradiated Controls: The Role of Hyper-radiosensitivity to Cell Death. Radiat. Res. 159, 433–436 (2003).

Calculations based on plausible parameters taken from the existing experimental database, and new measurements on the cell cycle dependence of low-dose hyper-radiosensitivity (HRS) of non-tumorigenic HeLa × skin fibroblast human hybrid cells, provide the first experimental evidence that the selective killing of a transformation-sensitive G₂/M-phase subpopulation as a consequence of low-dose HRS could account in part for the observed reduction of induced transformation frequencies at low doses to values below that observed spontaneously. However, it is clear that other mechanisms associated with classical adaptive response, such as induced DNA repair, are also likely to be involved.