Differential recovery from potentially lethal damage in normal human lung fibroblasts after irradiation with 60Co gamma-rays and accelerated N-ion beam

[Steps to Regulatory Science]

The research achievements in the field of regulatory science from the beginning of my research are described in an overview. First, I was interested in the complexity of development and pursued my studies on the mechanisms of DNA replication and repair, the mutagenicity of air pollutants, and the oncogene. After researching new phenomena based on the discovery of basic research in molecular/biochemistry, my research interests turned to the field of regulatory science which applies scientific evidence to social systems. I was able to successfully contribute to the field of drinking water quality in Japan through the establishment of drinking water quality standards and standard values, primarily for organic and agricultural chemicals, the development of analysis techniques, and the creation of an organization for ensuring safety. Research on the water quality in public water areas, which are also the sources of drinking water, was another subject in which I was involved. I took part in developing the concept and evaluation methodology for the environmental impact assessment of active pharmaceutical ingredients as well as conducting environmental monitoring on urban rivers in Japan. I have also been engaged in studies on the security and safety of human health with an ecosystem conservation background. It has been a great pleasure to collaborate on research projects with so many people toward a common aim.

Increased effectiveness of carbon ions in the production of reactive oxygen species in normal human fibroblasts

The production of reactive oxygen species (ROS), especially superoxide anions (O2 (·-)), is enhanced in many normal and tumor cell types in response to ionizing radiation. The influence of ionizing radiation on the regulation of ROS production is considered as an important factor in the long-term effects of irradiation (such as genomic instability) that might contribute to the development of secondary cancers. In view of the increasing application of carbon ions in radiation therapy, we aimed to study the potential impact of ionizing density on the intracellular production of ROS, comparing photons (X-rays) with carbon ions. For this purpose, we used normal human cells as a model for irradiated tissue surrounding a tumor. By quantifying the oxidization of Dihydroethidium (DHE), a fluorescent probe sensitive to superoxide anions, we assessed the intracellular ROS status after radiation exposure in normal human fibroblasts, which do not show radiation-induced chromosomal instability. After 3-5 days post exposure to X-rays and carbon ions, the level of ROS increased to a maximum that was dose dependent. The maximum ROS level reached after irradiation was specific for the fibroblast type. However, carbon ions induced this maximum level at a lower dose compared with X-rays. Within ∼1 week, ROS decreased to control levels. The time-course of decreasing ROS coincides with an increase in cell number and decreasing p21 protein levels, indicating a release from radiation-induced growth arrest. Interestingly, radiation did not act as a trigger for chronically enhanced levels of ROS months after radiation exposure.

Cell killing, nuclear damage and apoptosis in Chinese hamster V79 cells after irradiation with heavy-ion beams of (16)O, (12)C and (7)Li

Chinese hamster V79 cells were exposed to high LET (linear energy transfer) (16)O-beam (625keV/mum) radiation in the dose range of 0-9.83Gy. Cell survival, micronuclei (MN), chromosomal aberrations (CA) and induction of apoptosis were studied as a follow up of our earlier study on high LET radiations ((7)Li-beam of 60keV/mum and (12)C-beam of 295keV/mum) as well as (60)Co gamma-rays. Dose dependent decline in surviving fraction was noticed along with the increase of MN frequency, CA frequency as well as percentage of apoptosis as detected by nuclear fragmentation assay. The relative intensity of DNA ladder, which is a useful marker for the determination of the extent of apoptosis induction, was also increased in a dose dependent manner. Additionally, expression of tyrosine kinase lck-1 gene, which plays an important role in response to ionizing radiation induced apoptosis, was increased with the increase of radiation doses and also with incubation time. The present study showed that all the high LET radiations were generally more effective in cell killing and inflicting other cytogenetic damages than that of low LET gamma-rays. The dose response curves revealed that (7)Li-beam was most effective in cell killing as well as inducing other nuclear damages followed by (12)C, (16)O and (60)Co gamma-rays, in that order. The result of this study may have some application in biological dosimetry for assessment of genotoxicity in heavy ion exposed subjects and in determining suitable doses for radiotherapy in cancer patients where various species of heavy ions are now being generally used.

DNA-lesion and cell death by alpha-particles and nitrogen ions

When the natural logarithm of the surviving fraction is plotted against the dose of radiation, curves with shoulders at relatively high survival levels are obtained after gamma-rays. The curves were practically linear in case of HMV-I and HA-1 cells irradiated by charged particle beams. These cells were derived from human malignant melanoma and Chinese hamster cells, respectively. The amount of DNA single strand breaks (ssb) by gamma-rays or nitrogen-ions (LET=530KeV/micrometers) in HMV-I cells increases linearly with increment in dose, when the ssb is detected using the alkaline elution technique. There is no close relationship between the dose-response curve of the ssb and the dose-survival curves after gamma-rays or N-ions. The amount of DNA double strand breaks (dsb) by gamma-rays increases quadratically with increment of dose, in both HMV-I cells and HA-1 cells, when the dsb is detected using the neutral elution technique. The survival fraction for HA-1 cells is slightly higher than that for HMV-I cells, at the same dose, and the amount of dsb for HA-1 cells is considerably greater than that for HMV-I cells. These results suggest that the radiosensitivities to gamma-rays in different cell lines do not correspond to the number of DNA strand breaks. The amount of both non-repairable ssb and dsb also increases quadratically with increment of dose for gamma-rays and almost linearly with increment of dose for N-ions and alpha-particles (LET=36keV/micrometers for HA-1 cells and LET=77keV/micrometers for HMV-I cells). The dose-response curves for non-repairable dsb in case of these radiations seemed to mirror image the dose-survival curves for these radiations, in both cell lines. The number of non-repairable DNA strand breaks in the two cell lines, at the same level of survival was much the same. These results show the close relationship between the induction of non-repairable DNA strand breaks and cell killing.