Folic acid (FA), also named vitamin B9, is an essential cofactor for the synthesis of DNA bases and other biomolecules after bioactivation by dihydrofolate reductase (DHFR). FA is photoreactive and has been shown to generate DNA modifications when irradiated with UVA (360 nm) in the presence of DNA under cell-free conditions. To investigate the relevance of this reaction for cells and tissues, we irradiated three different cell lines (KB nasopharyngeal carcinoma cells, HaCaT keratinocytes, and a melanoma cell line) in the presence of FA and quantified cytotoxicity and DNA damage generation. The results indicate that FA is phototoxic and photogenotoxic by two different mechanisms. First, extracellular photodecomposition of FA gives rise to the generation of H2O2, which causes mostly DNA strand breaks. If this is prevented, e.g., by the presence of catalase, DNA damage generated by intracellular FA becomes evident. The damage spectrum in this case consists predominantly of oxidatively generated purine modifications sensitive to the repair glycosylase Fpg, as characteristic for type I photoreactions, and is associated with the formation of micronuclei. In KB cells, the DNA damage is strongly enhanced after pretreatment with the DHFR inhibitor methotrexate, which prevents the loss of the chromophore associated with the intracellular reduction of FA by DHFR. The results indicate that FA is photoreactive in cells and gives rise to nuclear DNA damage under irradiation.
Keywords: Folate; Free radicals; Methotrexate; Micronuclei; Oxidatively generated DNA damage; Photosensitization; Phototoxicity; UVA.
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