Absence of radioadaptive responses in four cell-lines in vitro as determined by colony formation assay

DNA double-strand break repair and adaptive responses of low-dose radiation in normal and tumor lung cell lines

The adaptive response (AR), which can be induced by low-dose ionizing radiation (LD), may influence the therapeutic ratio of cancer treatment. We investigated the AR and the DNA double-strand break (DSB) repair pathway in human lung tumor cells and normal cells. We measured viability and proliferation of normal lung cells (MRC-5) and lung cancer cells (QU-DB) using the MTT and colony formation assays. Flow cytometric analysis of γ-H2AX was used to measure DNA-DSBs induction, repair, and residual damages. AR was seen in the normal cells but not in the cancer cells. Our findings suggest that LD stimulates DSB repair and that this may contribute to distinctive AR in normal vs. cancer cells.

Changes in the Proliferation Rate, Clonogenicity, and Radiosensitivity of Cultured Cells During and After Continuous Low-Dose-Rate Irradiation

We investigated the effects of continuous low-dose radiation on proliferation, clonogenicity, radiosensitivity, and repair of DNA double-strand breaks (DSBs) in human salivary gland (HSG) tumor cells. Human salivary gland cells were cultured on acrylic boards above very-low-dose (4.3 μSv/h) or low-dose (27 μSv/h) radiation-emitting sheets or without sheets. Total cell numbers and plating efficiencies were compared among the 3 groups every 1 or 2 weeks until 6 weeks after starting culture. At 2, 4, and 6 weeks, surviving fractions of HSG cells after irradiation at 2 to 8 Gy cultured on the very-low-dose or low-dose sheets were compared to those of the control. At 4 weeks, HSG cells irradiated at 2 Gy were assessed for phosphorylated histone (γH2AX) foci formation, and DSBs were evaluated. No significant differences were observed in total cell number or plating efficiencies with or without low-dose-emitting sheets. The surviving fractions after irradiation of the very-low-dose group at 2 to 6 weeks and those of the low-dose group at 2 to 4 weeks were higher than those of the control (P < .01). Thus, a radioadaptive response was clearly demonstrated. From the γH2AX foci quantification, the adaptive responses were considered to be associated with the efficient repair of DSB, especially slow repair, in this cell line.

Overview of Biological, Epidemiological, and Clinical Evidence of Radiation Hormesis

The effects of low-dose radiation are being increasingly investigated in biological, epidemiological, and clinical studies. Many recent studies have indicated the beneficial effects of low doses of radiation, whereas some studies have suggested harmful effects even at low doses. This review article introduces various studies reporting both the beneficial and harmful effects of low-dose radiation, with a critique on the extent to which respective studies are reliable. Epidemiological studies are inherently associated with large biases, and it should be evaluated whether the observed differences are due to radiation or other confounding factors. On the other hand, well-controlled laboratory studies may be more appropriate to evaluate the effects of low-dose radiation. Since the number of such laboratory studies is steadily increasing, it will be concluded in the near future whether low-dose radiation is harmful or beneficial and whether the linear-no-threshold (LNT) theory is appropriate. Many recent biological studies have suggested the induction of biopositive responses such as increases in immunity and antioxidants by low-dose radiation. Based on recent as well as classical studies, the LNT theory may be out of date, and low-dose radiation may have beneficial effects depending on the conditions; otherwise, it may have no effects.

Continuous Low-Dose-Rate Irradiation Promotes Growth of Silkworms

To investigate the influence of low-dose-rate irradiation on the growth of silkworms, Bombyx mori, eggs of silkworms were randomly divided into 2 groups and were grown on either low-dose-radiation-emitting sheets or control sheets. On the radiation-emitting sheets, the dose rate was measured as 66.0 (4.3) μSv/h (mean [standard deviation]) by a Geiger-Müller counter for α, β, and γ rays and 3.8 (0.3) μSv/h by a survey meter for γ rays. The silkworms became larger when bred on the radiation-emitting sheets, and their body weight was about 25% to 37% heavier on day 42 to 49 after starting the experiment. Continuous low-dose-rate irradiation promoted the growth of silkworms. It should be further investigated whether this phenomenon could be utilized by the silk industry.

Different responses of tumor and normal cells to low-dose radiation

AIM OF THE STUDY

We demonstrated stimulation of both erythrocyte immune function and superoxide dismutase activity in tumor-bearing mice in response to whole-body 75 mGy X-rays. In addition, we enhanced the chemotherapeutic effect by exposing tumor-bearing mice to low-dose radiation (LDR). This study aims to investigate the different responses of tumor cells and normal cells to LDR.

MATERIAL AND METHODS

Survival fraction, micronucleus frequency, and cell cycle of Lewis cells and primary human fibroblast AG01522 cells were measured. S180 sarcoma cells were implanted in mice, and tumor sizes were measured in vivo.

RESULTS

In response to LDR exposure in vitro, a stimulating effect was observed in AG01522 cells but not in Lewis cells. Low-dose radiation did not cause an adaptive response in the Lewis cell cycle. Lack of an LDR-induced radioadaptive response in tumor cells was observed in tumor-bearing mouse models. Furthermore, a higher apoptotic effect and lower expression of the anti-apoptosis gene Bcl-2 were found in tumor cells of tumor-bearing mice exposed to D1 + D2 than those in tumor cells of tumor-bearing mice exposed to D2 alone.

CONCLUSIONS

Different responses of tumor cells and normal cells to LDR were found. Low-dose radiation was found to stimulate the growth of normal cells but not of tumor cells in vitro and in vivo, which is a very important and clinically relevant phenomenon.

Low-dose radiation induces adaptive response in normal cells, but not in tumor cells: in vitro and in vivo studies

Biological effects of low-dose radiation (LDR) are distinguishable from those of high-dose radiation. Hormetic and adaptive responses are such two examples. However, whether adaptive response could be induced in tumor cells by LDR, especially under in vivo condition, remains elusive, and was systemically investigated in the present study. Four tumor cell lines: two human leukemia cell lines (erythroleukemia cell line K562, and acute promyelocytic leukemia cell line HL60), and two human solid tumor cell lines (lung carcinoma cell line NCI-H446 and glioma cell line U251), along with one normal cell line (human fibroblast cells, MRC-5), were irradiated with LDR at 75 mGy of X-rays as D1 and then 4 Gy of X-rays as D2 (i.e.: D1 + D2) or only 4 Gy of X-rays (D2 alone). Three tumor-bearing animal models were also used to further define whether LDR induces adaptive response in tumor cells in vivo. Adaptive response was observed only in normal cell line, but not in four tumor cell lines, in response to LDR, showing a resistance to subsequent D2-induced cell growth inhibition. Three tumor-bearing mouse models with U251, NCI-H446 or S180 tumor cells were used to confirm that pre-exposure of tumor-bearing mice to D1 did not induce the resistance of tumor cells in vivo to D2-induced tumor growth inhibition. Furthermore, a higher apoptotic effect, along with higher expression of apoptosis-related genes P53 and Bax and lower expression of anti-apoptosis gene Bcl-2, was found in tumor cells of the tumor-bearing mice exposed to D1 + D2 than those in the tumor cells of the tumor-bearing mice exposed to D2 alone. These results suggest that LDR does not induce adaptive response in the tumor cells under both in vitro and in vivo conditions, which is a very important, clinic-relevant phenomenon.

Low-dose whole-body irradiation induced radioadaptive response in C57BL/6 mice

Radioadaptive survival responses after relatively low doses of radiation were investigated in C57BL/6 mice. The 8-week-old mice received whole-body mid-lethal challenging irradiation (5.9 Gy) at various intervals after conditioning whole-body irradiation with 50-400 mGy. Thereafter, survival of the mice was observed for 30 days. The mice receiving 400 mGy at 6 h before the challenging dose had a lower survival rate than the control group, but it was not observed when the conditioning 400-mGy irradiation was given 24 h beforehand. The conditioning doses of 100 and 200 mGy did not influence the survival of mice after the challenging dose. The mice receiving 50 mGy at 1 day, 3 days or 1 week before the challenging dose had a higher survival rate than the control, although this adaptive response was not observed when 50 mGy was given 6 h, 12 h, 3.5 weeks, or 5 weeks beforehand. When 50 mGy was given 2 weeks before the challenging dose, the adaptive response was observed in an experiment in which the mice were caged in our laboratory at the age of 5 weeks, whereas it was not observed in another experiment in which the mice were caged at 3 weeks. This study confirmed the presence of radioadaptive survival responses at the dose of 50 mGy given relatively shortly before the challenging dose.