No formation of DNA double-strand breaks and no activation of recombination repair with UVA

Overexpression of SIRT6 regulates NRF2/HO-1 and NF-κB signaling pathways to alleviate UVA-induced photoaging in skin fibroblasts

Skin photoaging, resulting from prolonged exposure to sunlight, especially UVA rays, has been identified as a key contributor to age-related skin degeneration. However, the mechanism by which UVA radiation induces skin cell senescence has not been fully elucidated. In this investigation, bioinformatics technology was employed to identify SIRT6 as the core hub gene involved in the progression of skin photoaging. The study evinced that prolonged exposure of cutaneous fibroblasts to UVA radiation results in a marked reduction in the expression of SIRT6, both in vivo and in vitro. Knockdown of SIRT6 in skin fibroblasts resulted in the upregulation of genes associated with cellular aging, thereby exacerbating the effects of UVA radiation-induced photoaging. Conversely, overexpression of SIRT6 decreased the expression of cell aging-related genes, indicating that SIRT6 plays a role in the regulation of senescence in skin fibroblasts induced by UVA radiation. We proffer substantiation that overexpression of SIRT6 protects skin fibroblasts from UVA-induced oxidative stress by activating the NRF2/HO-1 signaling cascade. Moreover, SIRT6 overexpression also reduced UVA-induced type I collagen degradation by inhibiting NF-κB signaling cascade. In summary, our findings showed that overexpression of SIRT6 inhibits UVA-induced senescence phenotype and type I collagen degradation in skin fibroblasts by modulating the NRF2/HO-1 and NF-κB signaling pathways. And the regulation of these signaling pathways by SIRT6 may be achieved through its deacetylase activity. Therefore, SIRT6 is a novel and promising therapeutic target for skin aging related to age and UV.

Ataxia Telangiectasia Mutated Signaling Delays Skin Pigmentation upon UV Exposure by Mediating MITF Function toward DNA Repair Mode

Skin pigmentation is paused after sun exposure; however, the mechanism behind this pausing is unknown. In this study, we found that the UVB-induced DNA repair system, led by the ataxia telangiectasia mutated (ATM) protein kinase, represses MITF transcriptional activity of pigmentation genes while placing MITF in DNA repair mode, thus directly inhibiting pigment production. Phosphoproteomics analysis revealed ATM to be the most significantly enriched pathway among all UVB-induced DNA repair systems. ATM inhibition in mouse or human skin, either genetically or chemically, induces pigmentation. Upon UVB exposure, MITF transcriptional activation is blocked owing to ATM-dependent phosphorylation of MITF on S414, which modifies MITF activity and interactome toward DNA repair, including binding to TRIM28 and RBBP4. Accordingly, MITF genome occupancy is enriched in sites of high DNA damage that are likely repaired. This suggests that ATM harnesses the pigmentation key activator for the necessary rapid, efficient DNA repair, thus optimizing the chances of the cell surviving. Data are available from ProteomeXchange with the identifier PXD041121.

Unravelling the molecular mechanism of mutagenic factors impacting human health

Environmental mutagens are chemical and physical substances in the environment that has a potential to induce a wide range of mutations and generate multiple physiological, biochemical, and genetic modifications in humans. Most mutagens are having genotoxic effects on the following generation through germ cells. The influence of germinal mutations on health will be determined by their frequency, nature, and the mechanisms that keep a specific mutation in the population. Early prenatal lethal mutations have less public health consequences than genetic illnesses linked with long-term medical and social difficulties. Physical and chemical mutagens are common mutagens found in the environment. These two environmental mutagens have been associated with multiple neurological disorders and carcinogenesis in humans. Thus in this study, we aim to unravel the molecular mechanism of physical mutagens (UV rays, X-rays, gamma rays), chemical mutagens (dimethyl sulfate (DMS), bisphenol A (BPA), polycyclic aromatic hydrocarbons (PAHs), 5-chlorocytosine (5ClC)), and several heavy metals (Ar, Pb, Al, Hg, Cd, Cr) implicated in DNA damage, carcinogenesis, chromosomal abnormalities, and oxidative stress which leads to multiple disorders and impacting human health. Biological tests for mutagen detection are crucial; therefore, we also discuss several approaches (Ames test and Mutatox test) to estimate mutagenic factors in the environment. The potential risks of environmental mutagens impacting humans require a deeper basic knowledge of human genetics as well as ongoing research on humans, animals, and their tissues and fluids.

Urolithin A protects human dermal fibroblasts from UVA-induced photoaging through NRF2 activation and mitophagy

Photoaging, caused by exposure to sunlight and especially UVA, has been identified as one of the culprits for age-related skin deterioration. Here, we initially demonstrated that urolithin A (UroA), a metabolite derived from intestine microflora, possessed sufficient photoprotective capacity and attenuated UVA-induced senescent phenotypes in human fibroblasts, such as growth inhibition, senescence-associated β-galactosidase activity, breakdown of extracellular matrix, synthesis of senescence-associated secretory phenotypes and cell cycle arrest. Furthermore, UroA lessened the accumulation of intracellular reactive oxygen species, which promoted the phosphorylation and afterwards nuclear translocation of NRF2, subsequently driving the activation of downstream antioxidative enzymes. In parallel, we proved that UroA restored mitochondrial function by induction of mitophagy, which was regulated by the SIRT3-FOXO3-PINK1-PARKIN network. Taken together, our results showed that UroA protected dermal fibroblast from UVA damage through NRF2/ARE activation and mitophagy process, thus supporting UroA as a potential therapeutic agent for photoaging.

All You Need Is Light. Photorepair of UV-Induced Pyrimidine Dimers

Although solar light is indispensable for the functioning of plants, this environmental factor may also cause damage to living cells. Apart from the visible range, including wavelengths used in photosynthesis, the ultraviolet (UV) light present in solar irradiation reaches the Earth's surface. The high energy of UV causes damage to many cellular components, with DNA as one of the targets. Putting together the puzzle-like elements responsible for the repair of UV-induced DNA damage is of special importance in understanding how plants ensure the stability of their genomes between generations. In this review, we have presented the information on DNA damage produced under UV with a special focus on the pyrimidine dimers formed between the neighboring pyrimidines in a DNA strand. These dimers are highly mutagenic and cytotoxic, thus their repair is essential for the maintenance of suitable genetic information. In prokaryotic and eukaryotic cells, with the exception of placental mammals, this is achieved by means of highly efficient photorepair, dependent on blue/UVA light, which is performed by specialized enzymes known as photolyases. Photolyase properties, as well as their structure, specificity and action mechanism, have been briefly discussed in this paper. Additionally, the main gaps in our knowledge on the functioning of light repair in plant organelles, its regulation and its interaction between different DNA repair systems in plants have been highlighted.

Focus on UV-Induced DNA Damage and Repair-Disease Relevance and Protective Strategies

The protective ozone layer is continually depleting due to the release of deteriorating environmental pollutants. The diminished ozone layer contributes to excessive exposure of cells to ultraviolet (UV) radiation. This leads to various cellular responses utilized to restore the homeostasis of exposed cells. DNA is the primary chromophore of the cells that absorbs sunlight energy. Exposure of genomic DNA to UV light leads to the formation of multitude of types of damage (depending on wavelength and exposure time) that are removed by effectively working repair pathways. The aim of this review is to summarize current knowledge considering cellular response to UV radiation with special focus on DNA damage and repair and to give a comprehensive insight for new researchers in this field. We also highlight most important future prospects considering application of the progressing knowledge of UV response for the clinical control of diverse pathologies.

Genetic Heterogeneity of BRAF Fusion Kinases in Melanoma Affects Drug Responses

BRAF fusions are detected in numerous neoplasms, but their clinical management remains unresolved. We identified six melanoma lines harboring BRAF fusions representative of the clinical cases reported in the literature. Their unexpected heterogeneous responses to RAF and MEK inhibitors could be categorized upon specific features of the fusion kinases. Higher expression level correlated with resistance, and fusion partners containing a dimerization domain promoted paradoxical activation of the mitogen-activated protein kinase (MAPK) pathway and hyperproliferation in response to first- and second-generation RAF inhibitors. By contrast, next-generation αC-IN/DFG-OUT RAF inhibitors blunted paradoxical activation across all lines and had their therapeutic efficacy further increased in vitro and in vivo by combination with MEK inhibitors, opening perspectives in the clinical management of tumors harboring BRAF fusions.

Whole-exome sequencing reveals the impact of UVA light mutagenesis in xeroderma pigmentosum variant human cells

UVA-induced mutagenesis was investigated in human pol eta-deficient (XP-V) cells through whole-exome sequencing. In UVA-irradiated cells, the increase in the mutation frequency in deficient cells included a remarkable contribution of C>T transitions, mainly at potential pyrimidine dimer sites. A strong contribution of C>A transversions, potentially due to oxidized bases, was also observed in non-irradiated XP-V cells, indicating that basal mutagenesis caused by oxidative stress may be related to internal tumours in XP-V patients. The low levels of mutations involving T induced by UVA indicate that pol eta is not responsible for correctly replicating T-containing pyrimidine dimers, a phenomenon known as the ‘A-rule’. Moreover, the mutation signature profile of UVA-irradiated XP-V cells is highly similar to the human skin cancer profile, revealing how studies involving cells deficient in DNA damage processing may be useful to understand the mechanisms of environmentally induced carcinogenesis.

RNA-Seq analysis revealed genes associated with UV-induced cell necrosis through MAPK/TNF-α pathways in human dermal fibroblast cells as an inducer of premature photoaging

Exposing the skin to solar UV radiation induces cascades of signaling pathways and biological alterations such as redox imbalance, suppression of antioxidant genes and programmed cell death. Therefore, the aim of this study was to use RNA-Seq to unravel the effects of UV radiation on Normal Human Adult Fibroblast cells (NHDF). Cells were exposed to UV (20 mJ/cm2 for 3 mins) and incubated for 24 h. Total mRNA from the cells generated libraries of 72,080,648 and 40,750,939 raw reads from UV-treated and control cells respectively. Of the differentially expressed genes (DEGs) produced 2,007 were up-regulated and 2,791 were down-regulated (fold change ≥2, p < 0.05). The expression of 4 genes was validated with RT-qPCR. Chemokine signaling pathways in cancer were significantly activated and antioxidant genes were down-regulated. This study applied Next Generation Sequencing technology to reveal the genes and pathways involved in UV-induced human dermal fibroblast cells necrosis.

Formation of UV-induced DNA damage contributing to skin cancer development

UV-induced DNA damage plays a key role in the initiation phase of skin cancer. When left unrepaired or when damaged cells are not eliminated by apoptosis, DNA lesions express their mutagneic properties, leading to the activation of proto-oncogene or the inactivation of tumor suppression genes. The chemical nature and the amount of DNA damage strongly depend on the wavelength of the incident photons. The most energetic part of the solar spectrum at the Earth's surface (UVB, 280-320 nm) leads to the formation of cyclobutane pyrimidine dimers (CPDs) and pyrimidine (6-4) pyrimidone photoproducts (64PPs). Less energetic but 20-times more intense UVA (320-400 nm) also induces the formation of CPDs together with a wide variety of oxidatively generated lesions such as single strand breaks and oxidized bases. Among those, 8-oxo-7,8-dihydroguanine (8-oxoGua) is the most frequent since it can be produced by several mechanisms. Data available on the respective yield of DNA photoproducts in cells and skin show that exposure to sunlight mostly induces pyrimidine dimers, which explains the mutational signature found in skin tumors, with lower amounts of 8-oxoGua and strand breaks. The present review aims at describing the basic photochemistry of DNA and discussing the quantitative formation of the different UV-induced DNA lesions reported in the literature. Additional information on mutagenesis, repair and photoprotection is briefly provided.

ATR/Chk1 Pathway is Activated by Oxidative Stress in Response to UVA Light in Human Xeroderma Pigmentosum Variant Cells

The crucial role of DNA polymerase eta in protecting against sunlight-induced tumors is evidenced in Xeroderma Pigmentosum Variant (XP-V) patients, who carry mutations in this protein and present increased frequency of skin cancer. XP-V cellular phenotypes may be aggravated if proteins of DNA damage response (DDR) pathway are blocked, as widely demonstrated by experiments with UVC light and caffeine. However, little is known about the participation of DDR in XP-V cells exposed to UVA light, the wavelengths patients are mostly exposed. Here, we demonstrate the participation of ATR kinase in protecting XP-V cells after receiving low UVA doses using a specific inhibitor, with a remarkable increase in sensitivity and γH2AX signaling. Corroborating ATR participation in UVA-DDR, a significant increase in Chk1 protein phosphorylation, as well as S-phase cell cycle arrest, is also observed. Moreover, the participation of oxidative stress is supported by the antioxidant action of N-acetylcysteine (NAC), which significantly protects XP-V cells from UVA light, even in the presence of the ATR inhibitor. These findings indicate that the ATR/Chk1 pathway is activated to control UVA-induced oxidatively generated DNA damage and emphasizes the role of ATR kinase as a mediator of genomic stability in pol eta defective cells.

NADPH Oxidases and Their Roles in Skin Homeostasis and Carcinogenesis

SIGNIFICANCE

Skin protects the body from dehydration, pathogens, and external mutagens. NADPH oxidases are central components for regulating the cellular redox balance. There is increasing evidence indicating that reactive oxygen species (ROS) generated by members of this enzyme family play important roles in the physiology and pathophysiology of the skin. Recent Advances: NADPH oxidases are active producers of ROS such as superoxide and hydrogen peroxide. Different isoforms are found in virtually all tissues. They play pivotal roles in normal cell homeostasis and in the cellular responses to various stressors. In particular, these enzymes are integral parts of redox-sensitive prosurvival and proapoptotic signaling pathways, in which they act both as effectors and as modulators. However, continuous (re)activation of NADPH oxidases can disturb the redox balance of cells, in the worst-case scenario in a permanent manner. Abnormal NADPH oxidase activity has been associated with a wide spectrum of diseases, as well as with aging and carcinogenesis.

CRITICAL ISSUES

Sunlight with its beneficial and deleterious effects induces the activation of NADPH oxidases in the skin. Evidence for the important roles of this enzyme family in skin cancer and skin aging, as well as in many chronic skin diseases, is now emerging.

FUTURE DIRECTIONS

Understanding the precise roles of NADPH oxidases in normal skin homeostasis, in the cellular responses to solar radiation, and during carcinogenesis will pave the way for their validation as therapeutic targets not only for the prevention and treatment of skin cancers but also for many other skin-related disorders. Antioxid. Redox Signal. 28, 1238-1261.

Genome and Epigenome Surveillance Processes Underlying UV Exposure in Plants

Land plants and other photosynthetic organisms (algae, bacteria) use the beneficial effect of sunlight as a source of energy for the photosynthesis and as a major source of information from the environment. However, the ultraviolet component of sunlight also produces several types of damage, which can affect cellular and integrity, interfering with growth and development. In order to reduce the deleterious effects of UV, photosynthetic organisms combine physiological adaptation and several types of DNA repair pathways to avoid dramatic changes in the structure. Therefore, plants may have obtained an evolutionary benefit from combining genome and surveillance processes, to efficiently deal with the deleterious effects of UV radiation. This review will present the different mechanisms activated upon UV exposure that contribute to maintain genome and integrity.

Immunostaining of UVA-induced DNA damage in erythrocytes of medaka (Oryzias latipes)

Some authors have recently reported that UVA induces double-strand breaks (DSBs) in DNA. Only a few researchers have reported on the induction of DSBs upon UVA exposure, as measured using the Comet assay and γ-H2AX as markers of DSB formation. In the present study, we have investigated for the first time the dose-dependent induction of DSBs by UVA in medaka (Oryzias latipes) erythrocytes. Adult female medaka fish were exposed to UVA for 15, 30, and 60min/day for three continuous days; an unirradiated control group was kept in the same laboratory conditions. At 0h and 24h after UVA exposure, blood was collected to detect DNA damage and repair. The number of γ-H2AX foci was higher than the control value at 0h after UVA exposure and decreased within a 24h. the comet assay showed that DNA repair began during the recovery period. These findings confirm our pervious findings of genotoxic effects after UVA exposure in medaka erythrocytes and suggest that the replication-independent formation of UVA-induced DSBs is mediated through the generation of reactive oxygen species. In conclusion, these results suggest that DNA damage and repair occur after UVA exposure in medaka fish. UVA is the main component of solar UV radiation and is used for artificial UV exposure. Our results may have implications for skin cancer research.

Sunlight damage to cellular DNA: Focus on oxidatively generated lesions

The routine and often unavoidable exposure to solar ultraviolet (UV) radiation makes it one of the most significant environmental DNA-damaging agents to which humans are exposed. Sunlight, specifically UVB and UVA, triggers various types of DNA damage. Although sunlight, mainly UVB, is necessary for the production of vitamin D, which is necessary for human health, DNA damage may have several deleterious consequences, such as cell death, mutagenesis, photoaging and cancer. UVA and UVB photons can be directly absorbed not only by DNA, which results in lesions, but also by the chromophores that are present in skin cells. This process leads to the formation of reactive oxygen species, which may indirectly cause DNA damage. Despite many decades of investigation, the discrimination among the consequences of these different types of lesions is not clear. However, human cells have complex systems to avoid the deleterious effects of the reactive species produced by sunlight. These systems include antioxidants, that protect DNA, and mechanisms of DNA damage repair and tolerance. Genetic defects in these mechanisms that have clear harmful effects in the exposed skin are found in several human syndromes. The best known of these is xeroderma pigmentosum (XP), whose patients are defective in the nucleotide excision repair (NER) and translesion synthesis (TLS) pathways. These patients are mainly affected due to UV-induced pyrimidine dimers, but there is growing evidence that XP cells are also defective in the protection against other types of lesions, including oxidized DNA bases. This raises a question regarding the relative roles of the various forms of sunlight-induced DNA damage on skin carcinogenesis and photoaging. Therefore, knowledge of what occurs in XP patients may still bring important contributions to the understanding of the biological impact of sunlight-induced deleterious effects on the skin cells.

Micronucleated erythrocytes in newborns rats exposed to three different types of ultraviolet-A (UVA) lamps from commonly uses devices

Exposure to ultraviolet-A (UVA) light can accidentally cause adverse effects in the skin and eyes. UVA induces DNA damage directly by creating pyrimidine dimers or by the formation of reactive oxygen species that can indirectly affect DNA integrity. UVA radiation is emitted by lamps from everyday devices. In adult rats, micronucleated erythrocytes (MNE) are removed from the circulation by the spleen. However, in newborn rats, MNE have been observed in peripheral blood erythrocytes. The objective of this study was to use micronucleus tests to evaluate the DNA damage caused in newborn rats exposed to UVA light from three different types of UVA lamps obtained from commonly used devices: counterfeit detectors, insecticide devices, and equipment used to harden resins for artificial nails. Rat neonates were exposed to UVA lamps for 20min daily for 6days. The neonates were sampled every third day, and the numbers of MNE and micronucleated polychromatic erythrocytes (MNPCE) in the peripheral blood were determined. The rat neonates exposed to the three types of UVA lamps showed increased numbers of MNE and MNPCE from 48h to 144h (P<0.05 and P<0.001 respectively). However, no relationship was observed between the number of MNE and the wattage of the lamps. In conclusion, under these conditions, UVA light exposure induced an increase in MNE without causing any apparent damage to the skin.

Small-molecule inhibitors of Ataxia Telangiectasia and Rad3 related kinase (ATR) sensitize lymphoma cells to UVA radiation

BACKGROUND

Psoralen plus ultraviolet A (PUVA) photochemotherapy is a combination treatment used for inflammatory and neoplastic skin diseases such as mycosis fungoides (MF), the most common type of cutaneous T-cell lymphoma (CTCL). However, 30% of MF patients do not respond sufficiently to PUVA and require more aggressive therapies.

OBJECTIVE

The aim of this project was to investigate whether inhibition of Ataxia Telangiectasia and Rad3 related kinase (ATR) may enhance efficacy of phototherapy.

METHODS

CTCL cell lines (MyLa2000, SeAx and Mac2a) served as in vitro cell models. ATR and Chk1 were inhibited by small molecule antagonists VE-821, VE-822 or Chir-124, or by small interfering RNAs (siRNAs). Cell cycle and viability were assessed by flow cytometry.

RESULTS

Small molecule inhibitors of ATR and Chk1 potently sensitized all cell lines to PUVA and, importantly, also to UVA, which by itself did not cause apoptotic response. VE-821/2 blocked ATR pathway activation and released the cells from the G2/M block caused by UVA and PUVA, but did not affect apoptosis caused by other chemotherapeutics (etoposide, gemcitabine, doxorubicine) or by hydrogen peroxide. Knockdown of ATR and Chk1 with siRNA also blocked the ATR pathway and released the cells from G2/M block but did not sensitize the cells to UVA as observed with the small molecule inhibitors. The latter suggested that the synergism between VE-821/2 or Chir-124 and UVA was not solely caused by specific blocking of ATR kinase but also ATR-independent photosensitization. This hypothesis was further verified by administrating VE-821/2 or Chir-124 before and after UVA irradiation, as well as comparing their activity with other ATR and Chk1 inhibitors (AZD6738 and MK8776). We found that only VE-821/2 and Chir-124 kinase inhibitors had synergistic effect with UVA, and only if applied before treatment with UVA.

CONCLUSION

Small molecule ATR and Chk1 inhibitors potently sensitize lymphoma cells to UVA radiation and induce a prominent apoptotic response. Interestingly, this effect is due to the dual (kinase inhibiting and photosensitizing) mode of action of these compounds.

Oxidatively generated damage to cellular DNA by UVB and UVA radiation

This review article focuses on a critical survey of the main available information on the UVB and UVA oxidative reactions to cellular DNA as the result of direct interactions of UV photons, photosensitized pathways and biochemical responses including inflammation and bystander effects. UVA radiation appears to be much more efficient than UVB in inducing oxidatively generated damage to the bases and 2-deoxyribose moieties of DNA in isolated cells and skin. The UVA-induced generation of 8-oxo-7,8-dihydroguanine is mostly rationalized in terms of selective guanine oxidation by singlet oxygen generated through type II photosensitization mechanism. In addition, hydroxyl radical whose formation may be accounted for by metal-catalyzed Haber-Weiss reactions subsequent to the initial generation of superoxide anion radical contributes in a minor way to the DNA degradation. This leads to the formation of both oxidized purine and pyrimidine bases together with DNA single-strand breaks at the exclusion, however, of direct double-strand breaks. No evidence has been provided so far for the implication of delayed oxidative degradation pathways of cellular DNA. In that respect putative characteristic UVA-induced DNA damage could include single and more complex lesions arising from one-electron oxidation of the guanine base together with aldehyde adducts to amino-substituted nucleobases.

Skin-resident T cells sense ultraviolet radiation-induced injury and contribute to DNA repair

Skin-resident T cells have been shown to play important roles in tissue homeostasis and wound repair, but their role in UV radiation (UVR)-mediated skin injury and subsequent tissue regeneration is less clear. In this study, we demonstrate that acute UVR rapidly activates skin-resident T cells in humans and dendritic epidermal γδ T cells (DETCs) in mice through mechanisms involving the release of ATP from keratinocytes. Following UVR, extracellular ATP leads to an increase in CD69 expression, proliferation, and IL-17 production, and to changes in DETC morphology. Furthermore, we find that the purinergic receptor P2X7 and caspase-1 are necessary for UVR-induced IL-1 production in keratinocytes, which increases IL-17 secretion by DETCs. IL-17, in turn, induces epidermal TNF-related weak inducer of apoptosis and growth arrest and DNA damage-associated gene 45, two molecules linked to the DNA repair response. Finally, we demonstrate that DETCs and human skin-resident T cells limit DNA damage in keratinocytes. Taken together, our findings establish a novel role for skin-resident T cells in the UVR-associated DNA repair response and underscore the importance of skin-resident T cells to overall skin regeneration.

DNA damage as a biological sensor for environmental sunlight

Solar ultraviolet (UV) radiation is widely known as an environmental genotoxic agent that affects ecosystems and the human population, generating concerns and motivating worldwide scientific efforts to better understand the role of sunlight in the induction of DNA damage, cell death, mutagenesis, and ultimately, carcinogenesis. In this review, general aspects of UV radiation at the Earth's surface are reported, considering measurements by physical and biological sensors that monitor solar UV radiation under different environmental conditions. The formation of DNA photoproducts and other types of DNA damage by different UV wavelengths are compared with the present information on their roles in inducing biological effects. Moreover, the use of DNA-based biological dosimeters is presented as a feasible molecular and cellular tool that is focused on the evaluation of DNA lesions induced by natural sunlight. Clearly, direct environmental measurements demonstrate the biological impact of sunlight in different locations worldwide and reveal how this affects the DNA damage profile at different latitudes. These tools are also valuable for the quantification of photoprotection provided by commercial sunscreens against the induction of DNA damage and cell death, employing DNA repair-deficient cells that are hypersensitive to sunlight. Collectively, the data demonstrate the applicability of DNA-based biosensors as alternative, complementary, and reliable methods for registering variations in the genotoxic impact of solar UV radiation and for determining the level of photoprotection sunscreens provided at the level of DNA damage and cell death.

The variety of UV-induced pyrimidine dimeric photoproducts in DNA as shown by chromatographic quantification methods

Induction of DNA damage is one of the major consequences of exposure to solar UV radiation in living organisms. UV-induced DNA photoproducts are mostly pyrimidine dimers, including cyclobutane pyrimidine dimers, pyrimidine (6-4) pyrimidone photoproducts and Dewar valence isomers. In the last few decades, a large number of methods have been developed for the quantification of these pyrimidine dimers. The present review emphasizes the contribution of chromatographic techniques to our better understanding of the basic DNA photochemistry and the better description of damage in cells.

Physiological aspects of UV-excitation of DNA

Solar ultraviolet (UV) radiation, mainly UV-B (280-315 nm), is one of the most potent genotoxic agents that adversely affects living organisms by altering their genomic stability. DNA through its nucleobases has absorption maxima in the UV region and is therefore the main target of the deleterious radiation. The main biological relevance of UV radiation lies in the formation of several cytotoxic and mutagenic DNA lesions such as cyclobutane pyrimidine dimers (CPDs), 6-4 photoproducts (6-4PPs), and their Dewar valence isomers (DEWs), as well as DNA strand breaks. However, to counteract these DNA lesions, organisms have developed a number of highly conserved repair mechanisms such as photoreactivation, excision repair, and mismatch repair (MMR). Photoreactivation involving the enzyme photolyase is the most frequently used repair mechanism in a number of organisms. Excision repair can be classified as base excision repair (BER) and nucleotide excision repair (NER) involving a number of glycosylases and polymerases, respectively. In addition to this, double-strand break repair, SOS response, cell-cycle checkpoints, and programmed cell death (apoptosis) are also operative in various organisms to ensure genomic stability. This review concentrates on the UV-induced DNA damage and the associated repair mechanisms as well as various damage detection methods.

DNA base damage by reactive oxygen species, oxidizing agents, and UV radiation

Emphasis has been placed in this article dedicated to DNA damage on recent aspects of the formation and measurement of oxidatively generated damage in cellular DNA in order to provide a comprehensive and updated survey. This includes single pyrimidine and purine base lesions, intrastrand cross-links, purine 5',8-cyclonucleosides, DNA-protein adducts and interstrand cross-links formed by the reactions of either the nucleobases or the 2-deoxyribose moiety with the hydroxyl radical, one-electron oxidants, singlet oxygen, and hypochlorous acid. In addition, recent information concerning the mechanisms of formation, individual measurement, and repair-rate assessment of bipyrimidine photoproducts in isolated cells and human skin upon exposure to UVB radiation, UVA photons, or solar simulated light is critically reviewed.

UVA-induced DNA double-strand breaks result from the repair of clustered oxidative DNA damages

UVA (320-400 nm) represents the main spectral component of solar UV radiation, induces pre-mutagenic DNA lesions and is classified as Class I carcinogen. Recently, discussion arose whether UVA induces DNA double-strand breaks (dsbs). Only few reports link the induction of dsbs to UVA exposure and the underlying mechanisms are poorly understood. Using the Comet-assay and γH2AX as markers for dsb formation, we demonstrate the dose-dependent dsb induction by UVA in G(1)-synchronized human keratinocytes (HaCaT) and primary human skin fibroblasts. The number of γH2AX foci increases when a UVA dose is applied in fractions (split dose), with a 2-h recovery period between fractions. The presence of the anti-oxidant Naringin reduces dsb formation significantly. Using an FPG-modified Comet-assay as well as warm and cold repair incubation, we show that dsbs arise partially during repair of bi-stranded, oxidative, clustered DNA lesions. We also demonstrate that on stretched chromatin fibres, 8-oxo-G and abasic sites occur in clusters. This suggests a replication-independent formation of UVA-induced dsbs through clustered single-strand breaks via locally generated reactive oxygen species. Since UVA is the main component of solar UV exposure and is used for artificial UV exposure, our results shine new light on the aetiology of skin cancer.

Photoinduced damage to cellular DNA: direct and photosensitized reactions

The survey focuses on recent aspects of photochemical reactions to cellular DNA that are implicated through the predominant formation of mostly bipyrimidine photoproducts in deleterious effects of human exposure to sunlight. Recent developments in analytical methods have allowed accurate and quantitative measurements of the main DNA photoproducts in cells and human skin. Highly mutagenic CC and CT bipyrimidine photoproducts, including cyclobutane pyrimidine dimers and pyrimidine (6-4) pyrimidone photoproducts (6-4PPs) are generated in low yields with respect to TT and TC photoproducts. Another striking finding deals with the formation of Dewar valence isomers, the third class of bipyrimidine photoproducts that is accounted for by UVA-mediated isomerization of initially UVB generated 6-4PPs. Cyclobutadithymine (T<>T) has been unambiguously shown to be involved in the genotoxicity of UVA radiation. Thus, T<>T is formed in UVA-irradiated cellular DNA according to a direct excitation mechanism with a higher efficiency than oxidatively generated DNA damage that arises mostly through the Type II photosensitization mechanism. C<>C and C<>T are repaired at rates intermediate between those of T<>T and 6-4TT. Evidence has been also provided for the occurrence of photosensitized reactions mediated by exogenous agents that act either in an independent way or through photodynamic effects.

Oxidatively generated complex DNA damage: tandem and clustered lesions

There is an increasing interest for oxidatively generated complex lesions that are potentially more detrimental than single oxidized nucleobases. In this survey, the recently available information on the formation and processing of several classes of complex DNA damage formed upon one radical hit including mostly hydroxyl radical and one-electron oxidants is critically reviewed. The modifications include tandem base lesions, DNA-protein cross-links and intrastrand (purine 5',8-cyclonucleosides, adjacent base cross-links) and interstrand cross-links. Information is also provided on clustered lesions produced essentially by exposure of cells to ionizing radiation and high energetic heavy ions through the involvement of multiple radical events that induce several lesions DNA in a close spatial vicinity. These consist mainly of double strand breaks (DSBs) and non-DSB clustered lesions that are referred as to oxidatively generated clustered DNA lesions (OCDLs).

DNA damage after acute exposure of mice skin to physiological doses of UVB and UVA light

Solar ultraviolet (UV) radiation is an important risk factor in skin carcinogenesis. This has been attributed mainly to the UVB waveband because the high-energetic photons are capable of interacting with DNA and inducing DNA damage. Recently, UVA light has also gained increasing interest in relation to DNA alteration. Although UVA photons are less energetic than UVB, they comprise a major fraction of sunlight UV radiation and penetrate deep into the skin. The study was carried out to compare the acute effects of UVA and UVB light on SKH-1 mice in relation to DNA damage and associated parameters. Mice were exposed to UVA (10 and 20 J/cm(2)) or UVB (200 and 800 mJ/cm(2)) radiation. The number of DNA single-strand breaks (SSB) in lymphocytes, amount of phosphorylated histone H2AX (gamma-H2AX) and apoptosis or DNA fragmentation (TUNEL-positive cells) in skin sections and level of gamma-H2AX, activated caspase-3 and phosphorylated p53 in skin were evaluated after 4 and 24 h. SSB analyzed by alkaline comet assay were found to be 4 and 24 h following UVB and UVA treatment, respectively. TUNEL and gamma-H2AX-positive cell were observed only in UVB exposed animals at both time intervals. The level of activated caspase-3 and phospho-p53 was increased 24 h after UVA and UVB radiation and was more apparent in UVB treated mice. The results indicate that the mechanism of DNA damage caused by acute UVA exposure includes formation of SSB (oxidative damage), but not double-strand breaks.

DNA damage by singlet oxygen and cellular protective mechanisms

Reactive oxygen species, as singlet oxygen ((1)O(2)) and hydrogen peroxide, are continuously generated by aerobic organisms, and react actively with biomolecules. At excessive amounts, (1)O(2) induces oxidative stress and shows carcinogenic and toxic effects due to oxidation of lipids, proteins and nucleic acids. Singlet oxygen is able to react with DNA molecule and may induce G to T transversions due to 8-oxodG generation. The nucleotide excision repair, base excision repair and mismatch repair have been implicated in the correction of DNA lesions induced by (1)O(2) both in prokaryotic and in eukaryotic cells. (1)O(2) is also able to induce the expression of genes involved with the cellular responses to oxidative stress, such as NF-κB, c-fos and c-jun, and genes involved with tissue damage and inflammation, as ICAM-1, interleukins 1 and 6. The studies outlined in this review reinforce the idea that (1)O(2) is one of the more dangerous reactive oxygen species to the cells, and deserves our attention.

Comparison of DNA damage responses following equimutagenic doses of UVA and UVB: a less effective cell cycle arrest with UVA may render UVA-induced pyrimidine dimers more mutagenic than UVB-induced ones

Mechanisms of UVA-mutagenesis remain a matter of debate. Earlier described higher rates of mutation formation per pyrimidine dimer with UVA than with UVB and other evidence suggested that a non-pyrimidine dimer-type of DNA damage contributes more to UVA- than to UVB-mutagenesis. However, more recently published data on the spectra of UVA-induced mutations in primary human skin cells and in mice suggest that pyrimidine dimers are the most common type of DNA damage-inducing mutations not only with UVB, but also with UVA. As this rebuts a prominent role of non-dimer type of DNA damage in UVA-mutagenesis, we hypothesized that the higher mutation rate at UVA-induced pyrimidine dimers, as compared to UVB-induced ones, is caused by differences in the way UVA- and UVB-exposed cells process DNA damage. Therefore, we here compared cell cycle regulation, DNA repair, and apoptosis in primary human fibroblasts following UVB- and UVA-irradiation, using the same physiologic and roughly equimutagenic doses (100-300 J m(-2) UVB, 100-300 kJ m(-2) UVA) we have used previously for mutagenesis experiments with the same type of cells. ELISAs for the detection of pyrimidine dimers confirmed that much fewer dimers were formed with these doses of UVA, as compared to UVB. We found that cell cycle arrests (intra-S, G1/S, G2/M), mediated at least in part by activation of p53 and p95, are much more prominent and long-lasting with UVB than with UVA. In contrast, no prominent differences were found between UVA and UVB for other anti-mutagenic cellular responses (DNA repair, apoptosis). Our data suggest that less effective anti-mutagenic cellular responses, in particular different and shorter-lived cell cycle arrests, render pyrimidine dimers induced by UVA more mutagenic than pyrimidine dimers induced by UVB.

γH2Ax: biomarker of damage or functional participant in DNA repair "all that glitters is not gold!"

The phosphorylation of H2Ax on its S139 site, γH2Ax, is important for the assembly of repair complexes at DNA double strand breaks (DSBs). The formation and functional role of γH2Ax after other kinds of DNA damage, especially UV light, where DSBs are rare, is less clear. Following UV light in the UVB and UVC ranges, complex distributions of γH2Ax can be identified, quite unlike the discrete enumerable foci seen after ionizing radiation. Several distinct distributions of γH2Ax occur: a low level nuclear-wide distribution of γH2Ax occurs during nucleotide excision repair; irregular focal distributions occur at arrested replication forks; high intensity nuclear-wide γH2Ax occurs in association with S-phase apoptosis. The intensity and distributions of γH2Ax vary according to the activity of excision repair, bypass polymerase and apoptotic caspases. The frequency of DSBs at arrested replication forks is low but highly variable in different cell types, and probably caused by enzymatic action. Despite the prominence of S139 phosphorylation following UV damage, mutation of this site has no influence on the UV damage response indicating that γH2Ax is a biomarker but not a participant in the UV-DNA damage response.