Ultraviolet radiation-mediated damage to cellular DNA.
ABSTRACT Emphasis is placed in this review article on recent aspects of the photochemistry of cellular DNA in which both the UVB and UVA components of solar radiation are implicated individually or synergistically. Interestingly, further mechanistic insights into the UV-induced formation of DNA photoproducts were gained from the application of new accurate and sensitive chromatographic and enzymic assays aimed at measuring base damage. Thus, each of the twelve possible dimeric photoproducts that are produced at the four main bipyrimidine sites can now be singled out as dinucleoside monophosphates that are enzymatically released from UV-irradiated DNA. This was achieved using a recently developed high-performance liquid chromatography-tandem mass spectrometry assay (HPLC-MS/MS) assay after DNA extraction and appropriate enzymic digestion. Interestingly, a similar photoproduct distribution pattern is observed in both isolated and cellular DNA upon exposure to low doses of either UVC or UVB radiation. This applies more specifically to the DNA of rodent and human cells, the cis-syn cyclobutadithymine being predominant over the two other main photolesions, namely thymine-cytosine pyrimidine (6-4) pyrimidone adduct and the related cyclobutyl dimer. UVA-irradiation was found to generate cyclobutane dimers at TT and to a lower extent at TC sites as a likely result of energy transfer mechanism involving still unknown photoexcited chromophore(s). Oxidative damage to DNA is also induced although less efficiently by UVA-mediated photosensitization processes that mostly involved 1O2 together with a smaller contribution of hydroxyl radical-mediated reactions through initially generated superoxide radicals.
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ABSTRACT: The role of UVA-radiation-the major fraction in sunlight-in human skin carcinogenesis is still elusive. We here report that different UVA exposure regime (4 x 5 J/cm(2) per week or 1 x 20 J/cm(2) per week) caused tumorigenic conversion (tumors in nude mice) of the HaCaT skin keratinocytes. While tumorigenicity was not associated with general telomere shortening, we found new chromosomal changes characteristic for each recultivated tumor. Since this suggested a nontelomere-dependent relationship between UVA irradiation and chromosomal aberrations, we investigated for alternate mechanisms of UVA-dependent genomic instability. Using the alkaline and neutral comet assay as well as gamma-H2AX foci formation on irradiated HaCaT cells (20-60 J/cm(2)), we show a dose-dependent and long lasting induction of DNA single and double (ds) strand breaks. Extending this to normal human skin keratinocytes, we demonstrate a comparable damage response and, additionally, a significant induction and maintenance of micronuclei (MN) with more acentric fragments (indicative of ds breaks) than entire chromosomes particularly 5 days post irradiation. Thus, physiologically relevant UVA doses cause long-lasting DNA strand breaks, a prerequisite for chromosomal aberration that most likely contribute to tumorigenic conversion of the HaCaT cells. Since normal keratinocytes responded similarly, UVA may likewise contribute to the complex karyotype characteristic for human skin carcinomas.Oncogene 08/2008; 27(31):4269-80. DOI:10.1038/onc.2008.70 · 8.56 Impact Factor
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ABSTRACT: Renal transplant patients are at a greatly increased risk of skin malignancy, particularly squamous cell carcinoma (SCC), a tumor closely associated with UV exposure. There is also significant interindividual skin cancer risk among transplant patients, with evidence suggesting that this derives from variation in response to oxidative stress. Our aim was to assess urinary 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG), by liquid chromatography-tandem mass spectrometry, in renal transplant patients with and without SCC. The relationships between SCC and urinary 8-oxodG were analyzed by conditional logistic regression and those between 8-oxodG and other candidate variables by linear regression, correcting for the effect of SCC. In SCC patients, urinary 8-oxodG was significantly elevated (p=0.03), both pre- and post-tumor development, compared to non-SCC transplant patients. Secondary analyses indicated that 8-oxodG was related to current heavy smoking (p=0.02) and darker skin type (p=0.02), but not measures of previous chronic sun exposure or current age and gender. Although subject numbers were limited, immunosuppression with azathioprine was positively associated with 8-oxodG in all patients combined (p=0.02). These results demonstrate, for the first time, that a subpopulation of renal transplant patients is under greater oxidative burden, and it is this population that is particularly predisposed to skin cancer.Free Radical Biology and Medicine 12/2007; 43(9):1328-34. DOI:10.1016/j.freeradbiomed.2007.07.024 · 5.71 Impact Factor
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ABSTRACT: Sunlight ultraviolet A (UVA) irradiation has been implicated in the etiology of human skin cancer. A genotoxic mode of action for UVA radiation has been suggested that involves photosensitization reactions giving rise to promutagenic DNA lesions. We investigated the mutagenicity of UVA in the lacI transgene in Big Blue mouse embryonic fibroblasts. UVA irradiation of these cells at a physiologically relevant dose of 18J/cm(2) caused a 2.8-fold increase in the lacI mutant frequency relative to control, i.e., 12.12+/-1.84 versus 4.39+/-1.99 x 10(-5) (mean+/-S.D.). DNA sequencing analysis showed that of 100 UVA-induced mutant plaques and 54 spontaneously arisen control plaques, 97 and 51, respectively, contained a minimum of one mutation along the lacI transgene. The vast majority of both induced- and spontaneous mutations were single base substitutions, although less frequently, there were also single and multiple base deletions and insertions, and tandem base substitutions. Detailed mutation spectrometry analysis revealed that G:C-->T:A transversions, the signature mutations of oxidative DNA damage, were significantly induced by UVA irradiation (P<0.003). The absolute frequency of this type of mutations was 7.4-fold increased consequent to UVA irradiation as compared to control (3.38 versus 0.454 x 10(-5); P<0.00001). These findings are in complete agreement with those previously observed in the cII transgene of the same model system, and reaffirm the notion that intracellular photosensitization reactions causing promutagenic oxidative DNA damage are involved in UVA genotoxicity.Mutation Research/Fundamental and Molecular Mechanisms of Mutagenesis 04/2007; 617(1-2):71-8. DOI:10.1016/j.mrfmmm.2006.12.003 · 4.44 Impact Factor