Riboflavin activated by ultraviolet A1 irradiation induces oxidative DNA damage-mediated mutations inhibited by vitamin C

Abstract

An increasingly popular theory ascribes UVA (>320-400 nm) carcinogenicity to the ability of this wavelength to trigger intracellular photosensitization reactions, thereby giving rise to promutagenic oxidative DNA damage. We have tested this theory both at the genomic and nucleotide resolution level in mouse embryonic fibroblasts carrying the lambda phage cII transgene. We have also tested the hypothesis that inclusion of a cellular photosensitizer (riboflavin) can intensify UVA-induced DNA damage and mutagenesis, whereas addition of an antioxidant (vitamin C) can counteract the induced effects. Cleavage assays with formamidopyrimidine DNA glycosylase (Fpg) coupled to alkaline gel electrophoresis and ligation-mediated PCR (LM-PCR) showed that riboflavin treatment (1 microM) combined with UVA1 (340-400 nm) irradiation (7.68 J/cm(2)) or higher dose UVA1 irradiation alone induced Fpg-sensitive sites (indicative of oxidized and/or ring-opened purines) in the overall genome and in the cII transgene, respectively. Also, the combined treatment with riboflavin and UVA1 irradiation gave rise to single-strand DNA breaks in the genome and in the cII transgene determined by terminal transferase-dependent PCR (TD-PCR). A cotreatment with vitamin C (1 mM) efficiently inhibited the formation of the induced lesions. Mutagenicity analysis showed that riboflavin treatment combined with UVA1 irradiation or high-dose UVA1 irradiation alone significantly increased the relative frequency of cII mutants, both mutation spectra exhibiting significant increases in the relative frequency of G:C --> T:A transversions, the signature mutations of oxidative DNA damage. The induction of cII mutant frequency was effectively reduced consequent to a cotreatment with vitamin C. Our findings support the notion that UVA-induced photosensitization reactions are responsible for oxidative DNA damage leading to mutagenesis.

Fig. 1.

Cytotoxicity of riboflavin and/or UVA1 irradiation in the presence and absence of vitamin C to Big Blue mouse embryonic fibroblasts. Cell cultures were treated with increasing concentrations of riboflavin or solvent in the presence or absence of vitamin C at different concentrations for 20 min at 37°C in the dark. Subsequently, the treated cultures were irradiated with various doses of UVA1, and afterward cell viability was determined by the trypan blue dye exclusion assay. Viability is expressed as a percentage of total cell number. Error bars = SD.

Fig. 2.

Quantification of induced DNA lesions. (a) DNA damage in the genome. Cleavage assay with Fpg digestion and alkaline gel electrophoresis of the genomic DNA of mouse embryonic fibroblasts treated with riboflavin or irradiated with UVA1, individually or combined, in the presence and absence of vitamin C. (b and c) LM-PCR mapping of DNA damage in the cII transgene. DNA footprinting in mouse embryonic fibroblasts treated with riboflavin or irradiated with UVA1, individually or combined: in the presence and absence of vitamin C with (+) and without (−) predigestion with Fpg enzyme (b) and with (+) and without (−) pretreatment with T4 Endo V plus CPD photolyase reactivation (c). bp, base pair; M, molecular size marker; nt, nucleotide position.

Fig. 3.

Comparative mutation spectra of the cII transgene in Big Blue mouse embryonic fibroblasts treated with riboflavin (1 μM) combined with UVA1 irradiation (7.68 J/cm²), UVA1 irradiation alone (18.0 J/cm²), or control. Ins, insertion; Del, deletion. ∗, As compared with “nontreated control”; P < 0.001. †, as compared with “nontreated control”; P < 0.05.