Oxidatively-generated damage to DNA and proteins mediated by photosensitized UVA

Abstract

UVA accounts for about 95% of the solar ultraviolet (UV) radiation that reaches Earth and most likely contributes to human skin cancer risk. In contrast to UVB, which comprises the remaining 5% and is absorbed by DNA nucleobases to cause direct photodamage, UVA damages DNA indirectly. It does this largely through its interactions with cellular chromophores that act as photosensitisers to generate reactive oxygen species. Exogenously supplied chemicals, including some widely-prescribed medicines, may also act as photosensitisers and these drugs are associated with an increased risk of sun-related cancer. Because they amplify the effects of UVA on cells, they provide a means to investigate the mechanisms and effects of UVA-induced photodamage. Here, we describe some of the major lesions induced by two groups of UVA photosensitisers, the DNA thionucleotides and the fluoroquinolone antibiotics. In thionucleotides, replacement of the oxygen atoms of canonical nucleobases by sulfur converts them into strong UVA chromophores that can be incorporated into DNA. The fluoroquinolones are also UVA chromophores. They are not incorporated into DNA and induce a different range of DNA damages. We also draw attention to the potentially important contribution of photochemical protein damage to the cellular effects of photosensitised UVA. Proteins targeted for oxidation damage include DNA repair factors and we suggest that UVA-mediated protein damage may contribute to sunlight-induced cancer risk.

Keywords: DNA lesions; Photosensitizer; Protein oxidation; Thiopurines; Thiopyrimidines; UVA.

Graphical abstract

Fig. 1

Structures of UVA photosensitisers. Azathioprine, mercaptopurine and 6-thioguanine are all converted to 6-TG deoxyribonucleotides, which are in turn incorporated into DNA. This is a prerequisite for the clinical effectiveness of thiopurines. Thiopyrimidine deoxynucleosides are incorporated into DNA of cells via the TK-dependent pyrimidine nucleoside salvage pathway. The fluoroquinolone class of antibiotics acts as inhibitors of DNA topoisomerases and intercalate rather than incorporate into DNA.

Fig. 2

UVA photoproducts of DNA 6-thioguanine (6-TG) and 4-thiothymidine A. 6-TG. Reactive oxygen species generated from the interaction between 6-TG and UVA oxidise 6-TG or DNA 6-TG to guanine sulfinate and sulfonate as predominant products. These oxidised forms are also produced by MMPP treatment of 6-TG-substituted DNA. B.Potential intrastrand crosslink between DNA 6-TG and an adjacent imidazole ring-opened deoxyadenosine. UVA-mediated generation of covalent adducts of this type between 6-thioinosine and deoxyadenosine has been demonstrated in solution. C.4-thiothymidine. The thietane photoproduct generated by interstrand crosslinking between DNA 4-thiothymidine and a 5’-DNA thymidine.

Fig. 3

UVA photoproducts of 4-thio-5-iododeoxyuridine (SIdU). UVA irradiation of free SIdU in solution or of DNA containing incorporated SIdU causes deiodination and the generation of SdU via a reactive a 5-thiyl radical. Further reactions induced by UVA or MMPP generate (unidentified) thiol-oxidised intermediates – possibly the sulfinate [bracketed]. The intermediate(s) can undergo loss of the thiol group to generate dU as a final product. The thiyl radical and the oxidised thiol DNA intermediates can also react with nucleobases on the complementary DNA strand to generate DNA interstrand crosslinks (ICLs) or potentially with protein functional groups to form DNA-protein crosslinks (DPCs).

Fig. 4

UVA photoproducts of 4-thio-5-bromodeoxyuridine (SBrdU). In free solution, UVA irradiation or MMPP treatment of the deoxynucleoside generates 5-bromodeoxyuridine (BrdU) via an (unidentified) oxidised intermediate (shown here as the sulfinate). The photochemistry of DNA SBrdU is different. UVA-mediated degradation of SBrdU is not followed by the formation of detectable BrdU. This suggests that reaction with the complementary DNA strand or proteins in preferred. A potentially lethal DNA lesion that is a substrate for the Escherichia coli uracil-DNA N-glycosylase (UNG) has not been identified. UVA does not induce debromination of SBrdU either in free solution or in DNA.