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.

Nucleotide excision repair (NER) protects against sunlight-induced skin cancer. Defective NER is associated with photosensitivity and a high skin cancer incidence. Some clinical treatments that cause photosensitivity can also increase skin cancer risk. Among these, the immunosuppressant azathioprine and the fluoroquinolone antibiotics ciprofloxacin and ofloxacin interact with UVA radiation to generate reactive oxygen species that diminish NER capacity by causing protein damage. The replication protein A (RPA) DNA-binding protein has a pivotal role in DNA metabolism and is an essential component of NER. The relationship between protein oxidation and NER inhibition was investigated in cultured human cells expressing different levels of RPA. We show here that RPA is limiting for NER and that oxidative damage to RPA compromises NER capability. Our findings reveal that cellular RPA is surprisingly vulnerable to oxidation, and we identify oxidized forms of RPA that are associated with impaired NER. The vulnerability of NER to inhibition by oxidation provides a connection between cutaneous photosensitivity, protein damage, and increased skin cancer risk. Our findings emphasize that damage to DNA repair proteins, as well as to DNA itself, is likely to be an important contributor to skin cancer risk.

Potentially mutagenic DNA lesions induced by UVB (wavelengths 280–320 nm) are important risk factors for solar ultraviolet (UV) radiation-induced skin cancer. The carcinogenicity of the more abundant UVA (320–400 nm) is less well understood but is generally regarded to reflect its interaction with cellular chromophores that act as photosensitisers. The arylhydrocarbon receptor agonist 6-formylindolo[3,2-b] carbazole (FICZ), is a UVB photoproduct of tryptophan and a powerful UVA chromophore. Combined with UVA, FICZ generates reactive oxygen species (ROS) and induces oxidative DNA damage. Here we demonstrate that ROS generated by FICZ/UVA combinations also cause extensive protein damage in HaCaT human keratinocytes. We show that FICZ/UVA-induced oxidation significantly inhibits the removal of potentially mutagenic UVB-induced DNA photolesions by nucleotide excision repair (NER). DNA repair inhibition is due to FICZ/UVA-induced oxidation damage to the NER proteome and DNA excision repair is impaired in extracts prepared from FICZ/UVA-treated cells. NER protects against skin cancer. As a likely UVB photoproduct of intracellular tryptophan, FICZ represents a de facto endogenous UVA photosensitiser in sun-exposed skin. FICZ formation may increase the risk of solar UV-induced skin cancer by promoting photochemical damage to the NER proteome and thereby preventing the removal of UVB-induced DNA lesions.

Long wavelength ultraviolet radiation (UVA, 320–400 nm) interacts with chromophores present in human cells to induce reactive oxygen species (ROS) that damage both DNA and proteins. ROS levels are amplified, and the damaging effects of UVA are exacerbated if the cells are irradiated in the presence of UVA photosensitizers such as 6-thioguanine (6-TG), a strong UVA chromophore that is extensively incorporated into the DNA of dividing cells, or the fluoroquinolone antibiotic ciprofloxacin. Both DNA-embedded 6-TG and ciprofloxacin combine synergistically with UVA to generate high levels of ROS. Importantly, the extensive protein damage induced by these photosensitizer+UVA combinations inhibits DNA repair. DNA is maintained in intimate contact with the proteins that effect its replication, transcription, and repair, and DNA–protein cross-links (DPCs) are a recognized reaction product of ROS. Cross-linking of DNA metabolizing proteins would compromise these processes by introducing physical blocks and by depleting active proteins. We describe a sensitive and statistically rigorous method to analyze DPCs in cultured human cells. Application of this proteomics-based analysis to cells treated with 6-TG+UVA and ciprofloxacin+UVA identified proteins involved in DNA repair, replication, and gene expression among those most vulnerable to cross-linking under oxidative conditions.