Formation Of Pyrimidine Dimers Research Articles

UV and ionizing radiations induced DNA damage, differences and similarities

Both UV and ionizing radiations damage DNA. Two main mechanisms, so-called direct and indirect pathways, are involved in the degradation of DNA induced by ionizing radiations. The direct effect of radiation corresponds to direct ionization of DNA (one electron ejection) whereas indirect effects are produced by reactive oxygen species generated through water radiolysis, including the highly reactive hydroxyl radicals, which damage DNA. UV (and visible) light damages DNA by again two distinct mechanisms. UVC and to a lesser extend UVB photons are directly absorbed by DNA bases, generating their excited states that are at the origin of the formation of pyrimidine dimers. UVA (and visible) light by interaction with endogenous or exogenous photosensitizers induce the formation of DNA damage through photosensitization reactions. The excited photosensitizer is able to induce either a one-electron oxidation of DNA (type I) or to produce singlet oxygen (type II) that reacts with DNA. In addition, through an energy transfer from the excited photosensitizer to DNA bases (sometime called type III mechanism) formation of pyrimidine dimers could be produced. Interestingly it has been shown recently that pyrimidine dimers are also produced by direct absorption of UVA light by DNA, even if absorption of DNA bases at these wavelengths is very low. It should be stressed that some excited photosensitizers (such as psoralens) could add directly to DNA bases to generate adducts. The review will described the differences and similarities in terms of damage formation (structure and mechanisms) between these two physical genotoxic agents.

Photoexcitation Dynamics of Thymine in Acetonitrile and an Ionic Liquid Probed by Time‐resolved Infrared Spectroscopy

Femtosecond transient IR absorption spectroscopy was used to probe the decay mechanism of electronically excited thymine (a naturally occurring pyrimidine base in DNA) dissolved in an ionic liquid ([Bmim][ BF4 ]) or CD3CN after the absorption of UV light (267 nm). In both solvents, an absorption band grew on a picosecond timescale, along with decaying bleach and evolving red‐shifted absorption signals. A population analysis of the observed kinetic data suggested that most of the photoexcited thymine underwent a sub‐picosecond non‐radiative relaxation to the vibrationally hot ground electronic state. About 4% (16%) of the excited thymine in the ionic liquid (CD3CN) relaxed to an intermediate electronic state, which relaxed into a low‐lying triplet state by intersystem crossing (ISC) (ISC did not relax to the ground electronic state within the experimental period (1 ns)). The low ISC yield for thymine in an ionic liquid was correlated with molecular properties of the solvent. This observation is significant because the ISC to triplet state transition for excited thymine has been considered as a precursor to cyclobutane–pyrimidine dimer formation, which led to functional damage of the base after UV absorption. This finding may shed light on the photostability of DNA in ionic liquids.

Quantum Yield of Cyclobutane Pyrimidine Dimer Formation Via the Triplet Channel Determined by Photosensitization.

UV-induced formation of the cyclobutane pyrimidine dimer (CPD) lesion is investigated by stationary and time-resolved photosensitization experiments. The photosensitizer 2'-methoxyacetophenone with high intersystem crossing efficiency and large absorption cross-section in the UV-A range was used. A diffusion controlled reaction model is presented. Time-resolved experiments confirmed the validity of the reaction model and provided information on the dynamics of the triplet sensitization process. With a series of concentration dependent stationary illumination experiments, we determined the quantum efficiency for CPD formation from the triplet state of the thymine dinucleotide TpT to be 4 ± 0.2%.

The Protection Effect of Topical Cocoa Extract on Cyclobutane Pyrimidine Dimer (CPD) Formation and Tissue Neutrophil in Mice Induced by Ultraviolet B

Ultraviolet (UV) radiation has dangerous effects on the skin that may cause multiple clinical and cellular effects, which will produce DNA damage characterized by formation of photoproducts such as cyclobutane pyrimidine dimer (CPD), and acute inflammation characterized by neutrophil infiltration in the skin tissues. Cocoa flavanols are natural anti inflammatory molecules that has a role in preventing cutaneous UV damage. The aims of this study are to assess protection effects of topical cocoa extract against formation of CPD and tissue neutrophils caused by ultraviolet B radiation. This study is a true experimental study using mice as animal model, conducted at animal laboratory of Hasanuddin University and Sentra Diagnostic Patologia Laboratory from April to Mei 2016. Total of samples were 30. Protection effects of CPD formation occur with application of 200 ppm cocoa extract, with frequency of negative and weak CPD expression were 60% and 40%, respectively. There was an increased in protection with application of 400 ppm cocoa extract, with frequency of negative and weak CPD expression were 80% and 20%, respectively, and there was decrease of protection with application of 800 ppm cocoa extract, with frequency of negative and weak CPD expression were 40% and 60% respectively. The group which only exposed with UVB has higher mean of tissue neutrophils (mean=10,4), and groups with application of 400 ppm and 800 ppm cocoa extract have lowest mean of tissue neutrophils (mean=1,4). There was no significant differences between groups with application of 400 ppm and 800 ppm cocoa extract. The best protection effects on CPD formation and tissue neutrophil achieved with application 400 ppm of cocoa extract.

Blocking cyclobutane pyrimidine dimer formation by steric hindrance.

The efficiency of thymine (Thy) and uracil (Ura) to form cyclobutane pyrimidine dimers (CPDs) in solution, upon UV irradiation differs by one order of magnitude. This could to be partially related to the steric hindrance induced by the methyl at C5 in thymine. The aim of the present work is to establish the influence of a bulky moiety at this position on the photoreactivity of pyrimidines. With this purpose, photosensitization with benzophenone and acetone of a 5-tert-butyl uracil derivative () and the equivalent Thy () has been compared. Introduction of the tert-butyl group completely blocks CPD formation. Moreover, the mechanistic insight obtained by laser flash photolysis is in accordance with the observed photoreactivity.

Study on DNA damages induced by UV radiation

DNA damages in Escherichia coli (E. coli) exposed to UV radiation have been investigated. After 30 min of exposure to UV radiation of 5 mJ/cm2, the growth of E. coli in LB broth medium was about only 10% in compared with non-irradiated one. This results suggested that the UV radiation caused the damages for E. coli genome resulted in reduction in its growth and survival, and those lesions can be somewhat recovered. For both solutions of plasmid DNAs and E. coli cells containing plasmid DNA, this dose also caused the breakages on single and double strands of DNA, shifted the morphology of DNA plasmid from supercoiled to circular and linear forms. The formation of pyrimidine dimers upon UV radiation significantly reduced when the DNA was irradiated in the presence of Ganoderma lucidum extract. Thus, studies on UV-induced DNA damage at molecular level are very essential to determine the UV radiation doses corresponding to the DNA damages, especially for creation and selection of useful radiation-induced mutants, as well as elucidation the protective effects of the specific compounds against UV light.

In Vivo Spectrum of UVC-induced Mutation in Mouse Skin Epidermis May Reflect the Cytosine Deamination Propensity of Cyclobutane Pyrimidine Dimers.

Although ultraviolet radiation (UVR) has a genotoxicity for inducing skin cancers, the skin may tolerate UVC component because the epidermal layer prevents this short wavelength range from passing through. Here, UVC genotoxicity for mouse skin was evaluated in terms of DNA damage formation and mutagenicity. UVC induced UVR photolesions and mutations remarkably in the epidermis but poorly in the dermis, confirming the barrier ability of the epidermis against shorter UVR wavelengths. Moreover, the epidermis itself responded to UVC mutagenicity with mutation induction suppression, which suppressed the mutant frequencies to a remarkably low, constant level regardless of UVC dose. The mutation spectrum observed in UVC-exposed epidermis showed a predominance of UV-signature mutation, which occurred frequently in 5'-TCG-3', 5'-TCA-3' and 5'-CCA-3' contexts. Especially, for the former two contexts, the mutations recurred at several sites with more remarkable recurrences at the 5'-TCG-3' sites. Comparison of the UVC mutation spectrum with those observed in longer UVR wavelength ranges led us to a mechanism that explains why the sequence context preference of UV-signature mutation changes according to the wavelength, which is based on the difference in the mCpG preference of cyclobutane pyrimidine dimer (CPD) formation among UVR ranges and the sequence context-dependent cytosine deamination propensity of CPD.

Rapid Deamination of Cyclobutane Pyrimidine Dimer Photoproducts at TCG Sites in a Translationally and Rotationally Positioned Nucleosome in Vivo

Sunlight-induced C to T mutation hot spots in skin cancers occur primarily at methylated CpG sites that coincide with sites of UV-induced cyclobutane pyrimidine dimer (CPD) formation. The C and 5-methyl-C in CPDs are not stable and deaminate to U and T, respectively, which leads to the insertion of A by the DNA damage bypass polymerase η, thereby defining a probable mechanism for the origin of UV-induced C to T mutations. Deamination rates for T(m)CG CPDs have been found to vary 12-fold with rotational position in a nucleosome in vitro. To determine the influence of nucleosome structure on deamination rates in vivo, we determined the deamination rates of CPDs at TCG sites in a stably positioned nucleosome within the FOS promoter in HeLa cells. A procedure for in vivo hydroxyl radical footprinting with Fe-EDTA was developed, and, together with results from a cytosine methylation protection assay, we determined the translational and rotational positions of the TCG sites. Consistent with the in vitro observations, deamination was slower for one CPD located at an intermediate rotational position compared with two other sites located at outside positions, and all were much faster than for CPDs at non-TCG sites. Photoproduct formation was also highly suppressed at one site, possibly due to its interaction with a histone tail. Thus, it was shown that CPDs of TCG sites deaminate the fastest in vivo and that nucleosomes can modulate both their formation and deamination, which could contribute to the UV mutation hot spots and cold spots.

Defective removal of ribonucleotides from DNA promotes systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a multifactorial autoimmune disease in which environmental exposures like virus infection and UV-irradiation trigger activation of the innate and adaptive immune system in genetically predisposed individuals. Heterozygous mutations of the 3' repair exonuclease 1 (TREX1) are associated with SLE. Biallelic mutations in TREX1 and the three subunits of ribonuclease H2 (RNASEH2A-C) cause Aicardi-Goutieres syndrome, an inflammatory encephalopathy with clinical overlap with SLE. We therefore investigated the role of RNase H2 in SLE pathogenesis. RNase H2 is responsible for the removal of misincorporated ribonucleotides from DNA and is indispensable for genome integrity. We demonstrated a genetic association for rare RNase H2 sequence variants with SLE. RNase H2-deficient fibroblasts of AGS and SLE patients accumulated ribonucleotides in genomic DNA resulting in chronic low-level DNA damage, constitutive p53 phosphorylation and senescence. Patient fibroblasts proliferated slower than fibroblasts from healthy individuals and showed impairment of cell cycle progression. In addition, patient fibroblasts exhibited constitutive up-regulation of interferon-stimulated genes and an enhanced type I interferon response to the nucleic acid poly(I:C) and UV-irradiation. UV-irradiation induced enhanced cyclobutane pyrimidine dimer formation in ribonucleotide-containing DNA. This suggests that innate immune activation may be caused by immune recognition of DNA metabolites of DNA damage repair and may also explain photosensitivity in SLE patients with RNase H2 mutation. In summary, our findings implicate RNase H2 in the pathogenesis of SLE, and suggest a role of DNA damage-associated pathways in the initiation of autoimmunity.

Abstract 2811: Silver nanoparticles protect human keratinocytes from deleterious effects of ultraviolet radiation: Implications for skin cancer chemoprevention

Abstract Skin cancer is the most common form of cancer worldwide and its incidence has been increasing at an alarming rate in the United States. A solar ultraviolet (UV) radiation, particularly UVB, is the main etiological cause of skin carcinogenesis due to its DNA damaging ability. Therefore, multidisciplinary scientific research has placed specific emphasis on finding novel ways for skin cancer chemoprevention. Here, we provide first experimental evidence for chemopreventive efficacy of silver-nanoparticles (AgNPs) against UVB-induced DNA damage and apoptosis in immortalized human epidermal keratinocytes (HaCaT). AgNPs synthesized by chemical reduction method were characterized for their physicochemical properties. The nanoparticles were spherical (> 90%) in shape, in the size range of 20-50 nm, and had net negative surface charge with an average zeta potential of - 47.7 mV. AgNPs were largely non-toxic to HaCaT cells and their pretreatment protected them from UVB-induced apoptosis. Significant reduction in cyclobutane pyrimidine dimer (CPD) formation upon UVB exposure was also observed in AgNPs-pretreated HaCaT cells. In addition, AgNPs pre-treatment leads to G1 phase arrest of cell cycle in UVB-exposed HaCaT cells thus allowing sufficient time for CPDs repair. Moreover, enhanced AgNPs internalization and localization into cytoplasmic and nuclear compartments was observed by UVB irradiated HaCaT cells. Furthermore, AgNPs pretreatment altered the expression of various genes involved in cell-cycle, apoptosis and nucleotide-excision repair in UVB-irradiated HaCaT cells. Together, these findings clearly suggest the potential utility of AgNPs as novel chemopreventive agents against UVB-induced skin carcinogenesis. Citation Format: Sumit Arora, Nikhil Tyagi, Arun Bhardwaj, Lilia Rusu, Rohan Palanki, Ajay P. Singh, James E. Carter, Seema Singh. Silver nanoparticles protect human keratinocytes from deleterious effects of ultraviolet radiation: Implications for skin cancer chemoprevention. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2811. doi:10.1158/1538-7445.AM2015-2811

Two photolyases in Agrobacterium tumefaciens

Agrobacterium tumefaciens is a model organism which delivers genes into plants and induces plant tumors. We study the photobiology of A .tumefaciens, which contains two phytochromes, Agp1 and Agp2, and two photolyases, PhrA and PhrB. Photolyases are DNA repair enzymes that revert the UV-induced formation of cyclobutan pyrimidine dimers (CPD) or (6-4) photoproducts in a light dependent manner. PhrA is a CPD photolyase, whereas PhrB serves as (6-4) photolyase, as shown by HPLC based repair assays. We have obtained the crystal structures of PhrA and PhrB . Both structures revealed interesting insight into function and evolution of photolyases. PhrA has two Trp triades that serve as electron pathways for photoreduction of the chromophore, FAD. An antenna chromophore, methenyl tetrahydrofolate (MTHF) is bound to a site of the protein that is different from other photolyases. This finding could be a clue for the antenna chromophore of plant cryptochromes, which are closely related with PhrA. PhrB, which is so far the only prokaryotic protein with documented (6-4) photorepair activity, has a 6-7 dimethyl 8 ribityllumazin antenna chromophore and an FeS cluster. Phylogenetic studies reveal that PhrB is a representative of an ancient type of photolyases.

Defective removal of ribonucleotides from DNA promotes systemic autoimmunity.

Genome integrity is continuously challenged by the DNA damage that arises during normal cell metabolism. Biallelic mutations in the genes encoding the genome surveillance enzyme ribonuclease H2 (RNase H2) cause Aicardi-Goutières syndrome (AGS), a pediatric disorder that shares features with the autoimmune disease systemic lupus erythematosus (SLE). Here we determined that heterozygous parents of AGS patients exhibit an intermediate autoimmune phenotype and demonstrated a genetic association between rare RNASEH2 sequence variants and SLE. Evaluation of patient cells revealed that SLE- and AGS-associated mutations impair RNase H2 function and result in accumulation of ribonucleotides in genomic DNA. The ensuing chronic low level of DNA damage triggered a DNA damage response characterized by constitutive p53 phosphorylation and senescence. Patient fibroblasts exhibited constitutive upregulation of IFN-stimulated genes and an enhanced type I IFN response to the immunostimulatory nucleic acid polyinosinic:polycytidylic acid and UV light irradiation, linking RNase H2 deficiency to potentiation of innate immune signaling. Moreover, UV-induced cyclobutane pyrimidine dimer formation was markedly enhanced in ribonucleotide-containing DNA, providing a mechanism for photosensitivity in RNase H2-associated SLE. Collectively, our findings implicate RNase H2 in the pathogenesis of SLE and suggest a role of DNA damage-associated pathways in the initiation of autoimmunity.

Synergistic modulation of cyclobutane pyrimidine dimer photoproduct formation and deamination at a TmCG site over a full helical DNA turn in a nucleosome core particle.

Sunlight-induced C to T mutation hotspots in skin cancers occur primarily at methylated CpG sites that coincide with sites of UV-induced cyclobutane pyrimidine dimer (CPD) formation. The C or 5-methyl-C in CPDs are not stable and deaminate to U and T, respectively, which leads to the insertion of A by DNA polymerase η and defines a probable mechanism for the origin of UV-induced C to T mutations. We have now determined the photoproduct formation and deamination rates for 10 consecutive T=mCG CPDs over a full helical turn at the dyad axis of a nucleosome and find that whereas photoproduct formation and deamination is greatly inhibited for the CPDs closest to the histone surface, it is greatly enhanced for the outermost CPDs. Replacing the G in a T=mCG CPD with A greatly decreased the deamination rate. These results show that rotational position and flanking sequence in a nucleosome can significantly and synergistically modulate CPD formation and deamination that contribute to C to T mutations associated with skin cancer induction and may have influenced the evolution of the human genome.

Photoprotective and antioxidative constituents of an alpine Scabiosa species

UV-related damage is one of the main factors leading to skin cancer. The cosmetic industry puts intensive research efforts into the discovery of new ingredients with photoprotective and antioxidative properties, as it has been recognized that high sun protection factors alone are not sufficient to block UV-induced effects. Switzerland possesses a rich and diverse alpine flora which represents an attractive source of new ingredients with skin-protective effects, as alpine plants typically produce UV-protective secondary metabolites in response to high UV exposure. More than 30 ethanolic extracts from alpine plants were tested for protecting activity against cyclobutane pyrimidine dimer (CPD) formation in keratinocytes, and for DPPH radical scavenging properties. A Scabiosa species was identified as a promising candidate in both assays, and was selected for comprehensive investigation of its metabolite profile, and characterization of the bioactive constituents. The leaves were extracted successively by dichloromethane and methanol. Fractionation of the methanolic extract by a combination of different chromatographic methods, such as column chromatography on Sephadex LH-20, and preparative and semipreparative HPLC afforded 14 compounds including flavonoids (luteolin and isoorientin O- and C-glucosides), caffeic acid derivatives, and iridoid and secoiridoid glucosides. These metabolites account for most peaks detected in the HPLC-UV/ESIMS profile of the crude extract. Flavone glucosides with catechol groups and caffeoylquinic acid derivatives are responsible for the DPPH scavenging activity. Protection against CPD formation could be assigned to fractions containing caffeoylquinic acid derivatives. Compound 1, consisting of an ester of menthiafolic acid with secologanol, is a new secondary metabolite.

Protective effects of a new phloretin derivative against UVB-induced damage in skin cell model and human volunteers.

The phenolic compound phloretin is a prominent member of the chemical class of dihydrochalcones. Phloretin is specifically found in apple and apple juice and known for its biological properties. We were particularly interested in its potential dermo-cosmetic applications. However, practical limitations of phloretin do exist due to its poor water-solubility. Phloretin was sulfonated with sulfuric acid (98%, wt) and mixed with saturated salt water to produce phloretin 3',3-disulfonate in order to increase its water-solubility. Here we reported the photoprotective effect of phloretin 3',3-disulfonate (PS), a new semi-synthetic derivative of phloretin. Results showed that PS attenuated cyclobutane pyrimidine dimer (CPDs) formation, glutathione (GSH) depletion and apoptosis induced by ultraviolet B (UVB). The photoprotective effect of PS is tightly correlated to the enhancement of nucleotide excision repair (NER) gene expression. Furthemore, PS had inhibitory effects on UVB-induced release of the inflammatory mediators, such as IL-6 and prostaglandin-E2. We also confirmed the safety and clinical efficacy of PS on human skin. Overall, the results demonstrated significant benefits of PS on the protection of keratinocytes against UVB-induced injuries and suggested its potential use in skin photoprotection.

High resolution mapping of modified DNA nucleobases using excision repair enzymes.

The incorporation and creation of modified nucleobases in DNA have profound effects on genome function. We describe methods for mapping positions and local content of modified DNA nucleobases in genomic DNA. We combined in vitro nucleobase excision with massively parallel DNA sequencing (Excision-seq) to determine the locations of modified nucleobases in genomic DNA. We applied the Excision-seq method to map uracil in E. coli and budding yeast and discovered significant variation in uracil content, wherein uracil is excluded from the earliest and latest replicating regions of the genome, possibly driven by changes in nucleotide pool composition. We also used Excision-seq to identify sites of pyrimidine dimer formation induced by UV light exposure, where the method could distinguish between sites of cyclobutane and 6-4 photoproduct formation. These UV mapping data enabled analysis of local sequence bias around pyrimidine dimers and suggested a preference for an adenosine downstream from 6-4 photoproducts. The Excision-seq method is broadly applicable for high precision, genome-wide mapping of modified nucleobases with cognate repair enzymes.

Effect of Tinospora cordifolia on the reduction of ultraviolet radiation‐induced cytotoxicity and DNA damage in PC12 cells

The safety of Tinospora cordifolia and its potential to protect against ultraviolet radiation‐induced cytotoxicity and DNA damage in PC12 cells were investigated. To evaluate the safety of T. cordifolia, cell viability and agarose gel electrophoresis were carried out using PC12 cells treated with 0 to 100 μg mL−1 of methanol extract of T. cordifolia. T. cordifolia extracts did not show cytotoxicity ranging 0 to 100 μg mL−1. In addition, T. cordifolia extracts significantly increased cell viability at 1 ng, 10 ng and 1 μg mL−1 concentrations in serum‐deprived medium compared to control. To confirm the protective role against UV‐induced damage, PC12 cells alone or in the presence of 10 ng, 100 ng, or 1 μg mL−1 of T. cordifolia extract were exposed to 250, 270 and 290 nm of UV radiation, which corresponded to doses of 120, 150 and 300 mJ cm−2, respectively. Treatment with T. cordifolia extracts significantly increased the cell survival rate irradiated at 290 nm. In addition, T. cordifolia extracts significantly reduced cyclobutane pyrimidine dimer formation induced by UV irradiation at all wavelengths. In conclusion, T. cordifolia is not toxic and safe for cells. Our findings can support its application as phototherapy in the medical sector.

Effect of sequence and metal ions on UVB-induced anti cyclobutane pyrimidine dimer formation in human telomeric DNA sequences

Irradiation of G-quadruplex forming human telomeric DNA with ultraviolet B (UVB) light results in the formation of anti cyclobutane pyrimidine dimers (CPDs) between loop 1 and loop 3 in the presence of potassium ions but not sodium ions. This was unexpected because the sequences involved favor the nonphotoreactive hybrid conformations in K+ solution, whereas a potentially photoreactive basket conformation is favored in Na+ solution. To account for these contradictory results, it was proposed that the loops are too far apart in the basket conformation in Na+ solution but close enough in a two G-tetrad basket-like form 3 conformation that can form in K+ solution. In the current study, Na+ was still found to inhibit anti CPD formation in sequences designed to stabilize the form 3 conformation. Furthermore, anti CPD formation in K+ solution was slower for the sequence previously shown to exist primarily in the proposed photoreactive form 3 conformation than the sequence shown to exist primarily in a nonphotoreactive hybrid conformation. These results suggest that the form 3 conformation is not the principal photoreactive conformation, and that G-quadruplexes in K+ solution are dynamic and able to access photoreactive conformations more easily than in Na+ solution.

Modulation of the UVB-induced lethality by furocoumarins in Staphylococcus aureus

Furocumarins (FCs) are photoactive compounds capable of binding to DNA, and once excited by UVA light (∼365nm), they form photoadducts which can lead to mutagenicity and lethality. However, the biological effects of FCs combined with UVB light (312nm) is still little investigated. In the present study, the lethal effect of UVB light alone and combined with different concentrations of 8-methoxypsoralen (8-MOP), 4,5′,8-trimethylpsoralen (TMP) and 3-carbethoxypsoralen (3-CPs) was evaluated in a strain of Staphylococcus aureus. 8-MOP-UVB and TMP-UVB were more effective in inducing lethality compared to UVB alone, indicating that these FCs act as photosensitizing agents for UVB. The increase in concentration of 8-MOP resulted in a greater mortality. On the contrary, a decrease in mortality was found with an increase in TMP concentration. 3-CPs protected bacteria against damage induced by UVB, which can be attributed to the inhibition of cyclobutyl pyrimidine dimer formation. The different modulatory effects on lethality induced by UVB shown by the FCs tested could be related to differences in the specificity of each compound for particular nucleotide sequences, as well as other chemical characteristics of each molecule could influence the number and types of adducts formed, contributing to the photosensitizing or photoprotective effects observed.

ELF magnetic fields do not affect cell survival and DNA damage induced by ultraviolet B

We investigated whether extremely low frequency (ELF) magnetic field exposure has modification effects on cell survival after ultraviolet B (UV-B) irradiation and on repair process of DNA damage induced by UV-B irradiation in WI38VA13 subcloned 2RA and XP2OS(SV) cells. The ELF magnetic field exposure was conducted using a Helmholtz coil-based system that was designed to generate a sinusoidal magnetic field at 5 mT and 60 Hz. Cell survival was assessed by WST assay after UV-B irradiation at 20-80 J/m(2) , ELF magnetic field exposure for 24 h, followed by incubation for 48 h. DNA damage was assessed by quantification of cyclobutane pyrimidine dimer formation and 6-4 photoproduct formation using ELISA after UV-B irradiation at 20-80 J/m(2) followed by ELF magnetic field exposure for 24 h. No significant changes were observed in cell survival between ELF magnetic field and sham exposures. Similarly, DNA damage induced by UV-B irradiation did not change significantly following ELF magnetic field exposure. Our results suggest that ELF magnetic field exposure at 5 mT does not have modification effect on cell survival after UV-B irradiation and on repair process of DNA damage induced by UV-B irradiation.