1. ^*Radioisotope Research Center, Nara Medical University, Kashihara, Nara 634-8521, Japan
2. ^†Departments of Dermatology, Nara Medical University, Kashihara, Nara 634-8521, Japan
3. ^‡Public Health, Nara Medical University, Kashihara, Nara 634-8521, Japan
4. ^§Medical Genetics Research Center, Nara Medical University, Kashihara, Nara 634-8521, Japan

Correspondence: Toshio Mori, PhD, Radioisotope Research Center, Nara Medical University, Kashihara, Nara 634-8521, Japan. Email: tmori@naramed-u.ac.jp

Received 11 April 2002; Revised 19 July 2002; Accepted 8 August 2002.

Abstract

We have recently developed a micropore ultraviolet irradiation technique. An isopore membrane filter with 3 m diameter pores shields ultraviolet C radiation from cultured human fibroblasts, leading to partial irradiation within the cells with an average of about three exposed areas per nucleus. This study addressed the question of whether the spatial distribution of DNA damage within a cell nucleus is important in triggering ultraviolet-induced cytotoxicity. We have examined whether there are differences in cytotoxicity between partially ultraviolet-irradiated cells and uniformly irradiated cells after equal amounts of DNA damage were induced in the cell nuclei. We first determined DNA damage formation in normal human fibroblasts using an enzyme-linked immunosorbent assay. We found that 5 J per m² ultraviolet irradiation produced an equivalent amount of cyclobutane pyrimidine dimers and (6–4) photoproducts per cell as 100 J per m² with the membrane filter shield. At those doses, we found that both types of ultraviolet irradiation induced similar levels of cytotoxicity as assessed by a 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium assay. Both types of ultraviolet-irradiated cells also had similar cell-cycle distribution and apoptosis as measured by flow cytometry. Moreover, no significant differences in repair kinetics for either type of photolesion were observed between the two different ultraviolet treatments. Similar results were obtained in Cockayne syndrome cells that are defective in transcription-coupled nucleotide excision repair. Present results indicate that in the range of photoproducts studied, the spatial distribution of DNA damage within a cell is less important than the amount of damage in triggering ultraviolet-induced cytotoxicity.