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Norwegian Radiation Protection Authority
# Photosensitizing effects of bilirubin Terje Christensen, Gunnar Kinn, Iwona Gradzka*, Alicja Jaworska and Ellen B. Roll Radiation medicine department, Norwegian Radiation Protection Authority and Institutes of Radiology and Pathology, Norwegian National Hospital, University of Oslo, Norway. *Institute of Chemistry and Nuclear Technology, Department of Radiology and Health Protection, Warsaw, Poland Post address: Norwegian Radiation Protection Authority. P.O.Box 55, N-1345 Østerås, Norway, e-mail: nrpa@nrpa.no, Internet: http://www.nrpa.no E-Mail: Terje.Christensen@nrpa.no, Gunnar.Kinn@nrpa.no, Iwonag@orange.ichtj.waw.pl, Alicja.Jaworska@nrpa.no, Ellen.Roll@nrpa.no Norwegian Radiation Protection Authority: nrpa@nrpa.no Telephone: xx 47 67162500 Telefax: xx 47 22461304 Key words: Bilirubin, photosensitization, cell death, apoptosis, DNA-breaks, phototherapy ## Abstract Cells from the established mouse epidermal cell line 308 were exposed to bilirubin and light in vitro. It was observed that bilirubin was toxic to the cells in the dark, and that the treatment of cells with blue light affected the bilirubin toxicity. At low light doses the toxicity was reduced, probably as a result of the formation of photoisomers. Higher doses of light induced the formation of reactive species, probably hydrogen peroxide, singlet oxygen and breakdown products of bilirubin that caused a photosensitised effect on the cell survival. The morphology of the cells did not change the first hours after treatment, but an effect of a combination of bilirubin and light that induced about 80% reduction of clonogenic cell survival could be observed after 22 hours. The nuclei showed no fragmentation characteristic for late stages of apoptosis, although high molecular weight fragments of DNA were observed 1h after treatment with the same dose of bilirubin and light. The fragments could either be a result of the induction of apoptosis in some cells or the fact that unrepaired DNA-damage was present after irradiation.