Repair of the three main types of bipyrimidine DNA photoproducts in human keratinocytes exposed to UVB and UVA radiations

Infliximab inhibits DNA repair in ultraviolet B-irradiated premalignant keratinocytes

Anti-tumor necrosis factor-alpha (TNFalpha) approaches are increasingly used in the therapy of autoimmune diseases. One of the safety concerns is the potential enhancement of skin carcinogenesis. The aim of this study was to investigate if the TNFalpha neutralizing antibody, infliximab, directly affects the cell cycle and DNA repair in premalignant human keratinocytes after ultraviolet-B (UVB) irradiation. We found that infliximab-treated cells exhibited an enhanced G2/M cell cycle arrest and increased apoptosis after 10-20 mJ/cm(2) UVB. In spite of this, the level of cyclobutane pyrimidine dimers (CPD) in infliximab-treated cells was significantly increased at both 24 and 48 h after irradiation with 10 mJ/cm(2) UVB. As we have recently shown that protein kinase B/Akt is involved in the TNFalpha signalling pathway and promotes cell survival and skin carcinogenesis, we measured activatory phosphorylations of Akt (Ser-473 and Thr-308) and the signalling via related pathways Erk 1/2, p38 and p70-S6K. Infliximab inhibited Akt and its downstream targets p70-S6K and Erk 1/2, and stimulated p38 both in sham-irradiated and UVB-irradiated cells. In conclusion, despite the fact that infliximab blocks Akt and stimulates the G2/M checkpoint and apoptosis in UVB-irradiated keratinocytes, the repair of CPD is impaired. It is conceivable that anti-TNFalpha treatments may contribute to the accumulation of mutagenic lesions in the epidermis and enhance the early stages of skin carcinogenesis.

Differential repair of UVB-induced cyclobutane pyrimidine dimers in cultured human skin cells and whole human skin

Cyclobutane pyrimidine dimers (CPDs) and pyrimidine (6-4) pyrimidone photoproducts (6-4PPs) are the two main classes of mutagenic DNA damages induced by UVB radiation. Numerous studies have been devoted so far to their formation and repair in human cells and skin. However, the biochemical methods used often lack the specificity that would allow the individual study of each of the four CPDs and 6-4PPs produced at TT, TC, CT and CC dinucleotides. In the present work, we applied an HPLC-mass spectrometry assay to study the formation and repair of CPDs and 6-4PPs photoproducts in primary cultures of human keratinocytes and fibroblasts as well as in whole human skin. We first observed that the yield of dimeric lesions was slightly higher in fibroblasts than in keratinocytes. In contrast, the rate of global repair was higher in the last cell type. Moreover, removal of DNA photoproducts in skin biopsies was found to be slower than in both cultured skin cells. In agreement with previous works, the repair of 6-4PPs was found to be more efficient than that of CPDs in the three types of samples, with no observed difference between the removal of the TT and TC derivatives. In contrast, a significant influence of the nature of the two modified pyrimidines was observed on the repair rate of CPDs. The decreasing order of removal efficiency was the following: C<>T>C<>C>T<>C>T<>T. These data, together with the known intrinsic mutational properties of the lesions, would support the reported UV mutation spectra. A noticeable exception concerns CC dinucleotides that are mutational hotspots with an UV-specific CC to TT tandem mutation, although related bipyrimidine photoproducts are produced in low yields and efficiently repaired.

Quantification of Inducible SOS-Like Photoprotective Responses in Human Skin

To document and quantify inducible photoprotective effects in human skin, explant cultures were treated once with thymidine dinucleotide (pTT) or diluent alone or UV-irradiated. Both pTT and UV increased the melanogenic protein levels on days 1-5 and comparably increased melanocyte dendricity and epidermal melanin content. Explants treated with pTT or UV but not with diluent alone showed initial inhibition of epidermal proliferation followed by mild reactive hyperplasia; melanocyte proliferation was minimal. To determine whether pTT and UV provide comparable protection against subsequent UV-induced DNA damage, explants were pTT- or diluent-treated or UV-irradiated. All explants were then irradiated with the same UV dose 72 hours later. Compared to diluent alone, pTT or UV pretreatment decreased the number of epidermal cells positive for cyclobutane pyrimidine dimers (CPDs) 50% immediately post-irradiation. In pTT- and UV- versus diluent-pretreated explants, the rate of CPD removal was also more rapid, approximately 80 vs 45% of the initial burden within 72 hours. These data confirm and quantify comparable SOS-like responses in human skin after pTT or UV irradiation, attributable to both increased epidermal melanin and increased DNA repair rate, in the case of pTT in the absence of initial damage.

Reactive oxygen-mediated damage to a human DNA replication and repair protein

Ultraviolet A (UVA) makes up more than 90% of incident terrestrial ultraviolet radiation. Unlike shorter wavelength UVB, which damages DNA directly, UVA is absorbed poorly by DNA and is therefore considered to be less hazardous. Organ transplant patients treated with the immunosuppressant azathioprine frequently develop skin cancer. Their DNA contains 6-thioguanine-a base analogue that generates DNA-damaging singlet oxygen ((1)O(2)) when exposed to UVA. Here, we show that this (1)O(2) damages proliferating cell nuclear antigen (PCNA), the homotrimeric DNA polymerase sliding clamp. It causes covalent oxidative crosslinking between the PCNA subunits through a histidine residue in the intersubunit domain. Crosslinking also occurs after treatment with higher-although still moderate-doses of UVA alone or with chemical oxidants. Chronic accumulation of oxidized proteins is linked to neurodegenerative disorders and ageing. Our findings identify oxidative damage to an important DNA replication and repair protein as a previously unrecognized hazard of acute oxidative stress.

Effects of intranasal phototherapy on nasal mucosa in patients with allergic rhinitis

RATIONALE

Rhinophototherapy has been shown to be effective in the treatment of allergic rhinitis. Considering that phototherapy with ultraviolet light (UV) induces DNA damage, it is of outstanding importance to evaluate the damage and repair process in human nasal mucosa.

METHODS

We have investigated eight patients undergoing intranasal phototherapy using a modified Comet assay technique and by staining nasal cytology samples for cyclobutane pyrimidine dimers (CPDs), which are UV specific photoproducts.

RESULTS

Immediately after last treatment Comet assay of nasal cytology samples showed a significant increase in DNA damage compared to baseline. Ten days after the last irradiation a significant decrease in DNA damage was observed compared to data obtained immediately after finishing the treatment protocol. Difference between baseline and 10 days after last treatment was not statistically significant. Two months after ending therapy, DNA damage detected by Comet assay in patients treated with intranasal phototherapy was similar with that of healthy individuals. None of the samples collected before starting intranasal phototherapy stained positive for CPDs. In all samples collected immediately after last treatment strong positive staining for CPDs was detected. The number of positive cells significantly decreased 10 days after last treatment, but residual positive staining was present in all the examined samples. This finding is consistent with data reported in skin samples after UV irradiation. Cytology samples examined two months after ending therapy contained no CPD positive cells.

CONCLUSION

Our results suggest that UV damage induced by intranasal phototherapy is efficiently repaired in nasal mucosa.

Oxidation of the sugar moiety of DNA by ionizing radiation or bleomycin could induce the formation of a cluster DNA lesion

Bleomycin, a radiomimetic drug currently used in human cancer therapy, is a well known carcinogen. Its toxicity is mostly attributed to its potentiality to induce DNA double strand breaks likely arising from the formation of two vicinal DNA strand breaks, initiated by C4-hydrogen abstraction on the 2-deoxyribose moiety. In this work we demonstrate that such a hydrogen abstraction reaction is able to induce the formation of a clustered DNA lesion, involving a 3' strand break together with a modified sugar residue exhibiting a reactive alpha,beta-unsaturated aldehyde that further reacts with a proximate cytosine base. The lesion thus produced was detected as a mixture of four isomers by HPLC coupled to tandem mass spectrometry subsequent to DNA extraction and enzymatic digestion. The modified nucleosides that constitute new types of cytosine adducts were identified as the likely two pairs of diastereomers of 6-(2-deoxy-beta-D-erythro-pentofuranosyl)-2-hydroxy-3(3-hydroxy-2-oxopropyl)-2,6-dihydroimidazo[1,2-c]-pyrimidin-5(3H)-one as inferred from mass spectrometry and NMR analyses of the chemically synthesized nucleosides. We demonstrate that bleomycin, and to a minor extent ionizing radiation, are able to induce significant amounts of the cytosine damage in cellular DNA. In addition, the repair kinetic of the lesion in a human lymphocyte cell line is rather slow, with a half-life of 10 h. The 2'-deoxycytidine adducts thus characterized that represent the first example of complex DNA lesions isolated and identified in cellular DNA upon one radical hit are likely to play an important role in the toxicity of bleomycin.

Effects of mRg2, a mixture of ginsenosides containing 60% Rg2, on the ultraviolet B-induced DNA repair synthesis and apoptosis in NIH3T3 cells

Ginseng has been used worldwidely as a traditional medicine of Asian countries for treatment of various diseases including cancer. The purpose of this study was to determine the effect of ginseng saponin mRg2, a mixture of ginsenosides containing 60% Rg2, on the repair and apoptosis of ultraviolet B (UVB)-exposed NIH3T3 cells. When cells were exposed to UVB and then incubated with normal growth medium for 48 h, cell viability, as determined by trypan blue exclusion assay decreased to about 25%. However, when mRg2 was included in the postincubation medium, the UVB-induced loss of cell viability was significantly reduced as compared with that postincubated in normal growth medium. 4,6-diamidino-2-phenylindole (DAPI) staining showed that postincubation of the UVB-exposed cells in medium containing mRg2 significantly reduced the apoptotic nuclear fragmentation. Interestingly, when cells were preincubated with mRg2 for 24 h and then exposed to various doses of UV, the amount of repair synthesis significantly increased as compared with those in cells exposed to UVB alone. Western blot analysis indicated that the mRg2 postincubation after UVB exposure potentiated the level of p53 and p21. The level of Triton nonextractable proliferating cell nuclear antigen (PCNA) also remained elevated by mRg2 postincubation. All these results suggest that mRg2 protects cells against UVB-induced genotoxicity by increasing DNA repair and decreasing apoptosis, in possible association with the modulation of protein levels involved in cell cycle arrest or progression.

Cell surface expression of melanocortin-1 receptor on HaCaT keratinocytes and alpha-melanocortin stimulation do not affect the formation and repair of UVB-induced DNA photoproducts

Ultraviolet (UV) exposure induces an up-regulation of melanocortin-1 receptor (MC1R) expression in human skin and the alpha-melanocyte-stimulating hormone (alpha-MSH) may reduce UVB-induced DNA damage in normal human melanocytes. Using high-performance liquid chromatography coupled to tandem mass spectrometry, we investigated the formation and repair of DNA lesions in UVB-irradiated HaCaT cells stably transfected with the wild type MC1R gene (HaCaT-MC1R). Similar levels of 8 bipyrimidine photoproducts including cyclobutane pyrimidine dimers (CPDs) (T<>T, T<>C, C<>T), (6-4) photoproducts ((6-4)PPs) (TT-(6-4)PPs, TC-(6-4)PPs) and their Dewar valence isomers together with 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo) were found to be generated in both non-transfected and HaCaT-MC1R cells after UVB exposure. Time-course studies of DNA photoproduct yields indicated that the DNA repair ability depended upon radiation doses. It was shown that (6-4)PPs were removed from the DNA of UVB-irradiated cells much more efficiently than CPDs. The repair efficiency of 8-oxodGuo, CPDs and (6-4)PPs was relatively similar in both cell lines and was not modified by stimulation with alpha-MSH before UVB-exposure. In conclusion, cell surface-enforced expression of MC1Rs on HaCaT keratinocytes and alpha-MSH stimulation do not affect the formation of UVB-induced DNA photoproducts and their subsequent repair.

Mechanism of UV-related carcinogenesis and its contribution to nevi/melanoma

Melanoma consists 4-5 % of all skin cancers, but it contributes to 71-80 % of skin cancers deaths. UV light affects cell and tissue homeostasis due to its damaging effects on DNA integrity and modification of expression of a plethora of genes. DNA repair systems protect cells from UV-induced lesions. Several animal models of melanoma have been developed (Xiphophorus, Opossum Monodelphis domestica, mouse models and human skin engrafts into other animals). This review discusses possible links between UV and genes significantly related to melanoma but does not discuss melanoma genetics. These include oncogenes, tumor suppressor genes, genes related to melanocyte-keratinocyte and melanocyte-matrix interaction, growth factors and their receptors, CRH, ACTH, α-MSH, glucocorticoids, ID1, NF-kappaB and vitamin D3.

Cyclobutane pyrimidine dimers are predominant DNA lesions in whole human skin exposed to UVA radiation

Solar UV radiation is the most important environmental factor involved in the pathogenesis of skin cancers. The well known genotoxic properties of UVB radiation (290-320 nm) mostly involve bipyrimidine DNA photoproducts. In contrast, the contribution of more-abundant UVA radiation (320-400 nm) that are not directly absorbed by DNA remains poorly understood in skin. Using a highly accurate and quantitative assay based on HPLC coupled with tandem mass spectrometry, we determined the type and the yield of formation of DNA damage in whole human skin exposed to UVB or UVA. Cyclobutane pyrimidine dimers, a typical UVB-induced DNA damage, were found to be produced in significant yield also in whole human skin exposed to UVA through a mechanism different from that triggered by UVB. Moreover, the latter class of photoproducts is produced in a larger amount than 8-oxo-7,8-dihydro-2'-deoxyguanosine, the most common oxidatively generated lesion, in human skin. Strikingly, the rate of removal of UVA-generated cyclobutane pyrimidine dimers was lower than those produced by UVB irradiation of skin. Finally, we compared the formation yields of DNA damage in whole skin with those determined in primary cultures of keratinocytes isolated from the same donors. We thus showed that human skin efficiently protects against UVB-induced DNA lesions, whereas very weak protection is afforded against UVA. These observations emphasize the likely role played by the UVA-induced DNA damage in skin carcinogenesis and should have consequences for photoprotection strategies.

Chemical synthesis of oligodeoxyribonucleotides containing the Dewar valence isomer of the (6-4) photoproduct and their use in (6-4) photolyase studies

The pyrimidine(6–4)pyrimidone photoproduct, a major UV lesion formed between adjacent pyrimidine bases, is transformed to its Dewar valence isomer upon exposure to UVA/UVB light. We have synthesized a phosphoramidite building block of the Dewar photoproduct formed at the thymidylyl(3′–5′)thymidine site and incorporated it into oligodeoxyribonucleotides. The diastereoisomers of the partially protected dinucleoside monophosphate bearing the (6–4) photoproduct, which were caused by the chirality of the phosphorus atom, were separated by reversed-phase chromatography, and the (6–4) photoproduct was converted to the Dewar photoproduct by irradiation of each isomer with Pyrex-filtered light from a high-pressure mercury lamp. The Dewar photoproduct was stable under both acidic and alkaline conditions at room temperature. After characterization of the isomerized base moiety by NMR spectroscopy, a phosphoramidite building block was synthesized in three steps. Although the ordinary method could be used for the oligonucleotide synthesis, benzimidazolium triflate as an alternative activator yielded better results. The oligonucleotides were used for the analysis of the reaction and the binding of Xenopus (6–4) photolyase. Although the affinity of this enzyme for the Dewar photoproduct-containing duplex was reportedly similar to that for the (6–4) photoproduct-containing substrate, the results suggested a difference in the binding mode.

UV-B radiation induces epithelial tumors in mice lacking DNA polymerase eta and mesenchymal tumors in mice deficient for DNA polymerase iota

DNA polymerase eta (Pol eta) is the product of the Polh gene, which is responsible for the group variant of xeroderma pigmentosum, a rare inherited recessive disease which is characterized by susceptibility to sunlight-induced skin cancer. We recently reported in a study of Polh mutant mice that Pol eta is involved in the somatic hypermutation of immunoglobulin genes, but the cancer predisposition of Polh-/- mice has not been examined until very recently. Another translesion synthesis polymerase, Pol iota, a Pol eta paralog encoded by the Poli gene, is naturally deficient in the 129 mouse strain, and the function of Pol iota is enigmatic. Here, we generated Polh Poli double-deficient mice and compared the tumor susceptibility of them with Polh- or Poli-deficient animals under the same genetic background. While Pol iota deficiency does not influence the UV sensitivity of mouse fibroblasts irrespective of Polh genotype, Polh Poli double-deficient mice show slightly earlier onset of skin tumor formation. Intriguingly, histological diagnosis after chronic treatment with UV light reveals that Pol iota deficiency leads to the formation of mesenchymal tumors, such as sarcomas, that are not observed in Polh(-/-) mice. These results suggest the involvement of the Pol eta and Pol iota proteins in UV-induced skin carcinogenesis.