UV-A induces persistent genomic instability in human keratinocytes through an oxidative stress mechanism

UVR Exposure and Prevention of Street Construction Workers in Colombia and Germany

(1) Solar ultraviolet radiation (UVR) poses a major risk factor for developing skin cancer after years of chronic exposure. The irradiation is strongly dependent upon the activity or occupation carried out, but also on the climate conditions at the workplace. Knowledge of both has been tested within the occupational group of road construction workers in Colombia and Germany. (2) The GENESIS-UV measurement system has been used at both locations for consistency. A number of workers in both countries wore an electronic data logging dosimeter for several months to deliver detailed information on UVR exposure. (3) It was found that in a tropical climate, UVR exposure remains constant throughout the year, while in a temperate climate seasonal effects are visible, superimposed by behavioural aspects e.g., in springtime. The daily distribution of the radiation shows a distinct dip, especially in the Colombian data. Derived data show the high fraction of working days exceeding a threshold set by the skin type. (4) Road construction work involves high UVR exposure. In both countries, preventive measures are required to reduce the personal exposure to a minimum. Exceedance of the minimal erythema dose (MED) suggests a possible enhancing effect, especially in fair skinned people. Intercomparison of UVR exposure at workplaces is possible between countries and climate zones, emphasizing efforts for global action against skin cancer.

Genome-Wide RNA Sequencing Analysis in Human Dermal Fibroblasts Exposed to Low-Dose Ultraviolet A Radiation

Ultraviolet A (UVA) radiation can pass through the epidermis and reach the dermal skin layer, contributing to photoaging, DNA damage, and photocarcinogenesis in dermal fibroblasts. High-dose UVA exposure induces erythema, whereas low-dose, long-term UVA exposure causes skin damage and cell senescence. Biomarkers for evaluating damage caused by low-dose UVA in fibroblasts are lacking, making it difficult to develop therapeutic agents for skin aging and aging-associated diseases. We performed RNA-sequencing to investigate gene and pathway alterations in low-dose UVA-irradiated human skin-derived NB1RGB primary fibroblasts. Differentially expressed genes were identified and subjected to Gene Ontology and reactome pathway analysis, which revealed enrichment in genes in the senescence-associated secretory phenotype, apoptosis, respiratory electron transport, and transcriptional regulation by tumor suppressor p53 pathways. Insulin-like growth factor binding protein 7 (IGFBP7) showed the lowest p-value in RNA-sequencing analysis and was associated with the senescence-associated secretory phenotype. Protein–protein interaction analysis revealed that Fos proto-oncogene had a high-confidence network with IGFBP7 as transcription factor of the IGFBP7 gene among SASP hit genes, which were validated using RT-qPCR. Because of their high sensitivity to low-dose UVA radiation, Fos and IGFBP7 show potential as biomarkers for evaluating the effect of low-dose UVA radiation on dermal fibroblasts.

Genomic instability induced by radiation-mimicking chemicals is not associated with persistent mitochondrial degeneration

Ionizing radiation has been shown to cause induced genomic instability (IGI), which is defined as a persistently increased rate of genomic damage in the progeny of the exposed cells. In this study, IGI was investigated by exposing human SH-SY5Y neuroblastoma cells to hydroxyurea and zeocin, two chemicals mimicking different DNA-damaging effects of ionizing radiation. The aim was to explore whether IGI was associated with persistent mitochondrial dysfunction. Changes to mitochondrial function were assessed by analyzing mitochondrial superoxide production, mitochondrial membrane potential, and mitochondrial activity. The formation of micronuclei was used to determine immediate genetic damage and IGI. Measurements were performed either immediately, 8 days, or 15 days following exposure. Both hydroxyurea and zeocin increased mitochondrial superoxide production and affected mitochondrial activity immediately after exposure, and mitochondrial membrane potential was affected by zeocin, but no persistent changes in mitochondrial function were observed. IGI became manifested 15 days after exposure in hydroxyurea-exposed cells. In conclusion, immediate responses in mitochondrial function did not cause persistent dysfunction of mitochondria, and this dysfunction was not required for IGI in human neuroblastoma cells.

Bystander effect of ultraviolet A radiation protects A375 melanoma cells by induction of antioxidant defense

Ultraviolet (UV) irradiated cells release factors that result in varied responses by non-irradiated cells via bystander effects (BE). The UV-BE is dependent on the cell types involved and on the wavelength of the radiation. Using conditioned medium from UVA-irradiated A375 human melanoma cells (UVA-CM), UVA-bystander response was evaluated on the viability of naïve A375 cells. UVA-CM treatment itself did not alter cell viability; however, UVA-CM treated bystander cells were more resistant to the lethal action of UVA, UVB, UVC or H₂O₂. Effects of UVA-CM on cell proliferation, mechanism of cell death, DNA damage, malondialdehyde formation, generation of reactive oxygen species (ROS) and antioxidant status were studied in A375 cells. We observed that UVA-CM triggered antioxidant defenses to elicit protective responses through elevation of antioxidant enzyme activities in cells, which persisted until 5 h after exposure to UVA-CM. This was possibly responsible for decreased generation of ROS and diminished DNA and membrane damage in cells. These bystander cells were resistant to killing when exposed to different genotoxic agents. Damaged nuclei, induction of apoptosis and autophagic death were also lowered in these cells. The influence of UVA-CM on cancer stem cells side population was assessed.Highlights:UVA radiation induced bystander effects in A375 cellsDamage by genotoxicants is suppressed due to lower ROS generation on UVA-CM treatmentUVA-CM exposure enhanced higher activities of CAT and GPxResistance to genotoxic agents in such cells was due to elevated antioxidant defenceUVA-bystander phenomenon was a protective response.

Wavelengths and temporal effects on the response of mammalian cells to UV radiation: Limitations of action spectra illustrated by genotoxicity

All photobiological events depend on the wavelength of the incident radiation. In real-life situations and in the vast majority of laboratory experiments, exposure always involves sources with various emission spectra spreading over a wide wavelength range. Action spectra are often used to describe the efficiency of a process at different wavelengths and to predict the effects of a given light source by summation of the individual effects at each wavelength. However, a full understanding of the biological effects of complex sources requires more than considering these concomitant events at each specific wavelength. Indeed, photons of different energies may not have additive but synergistic or inhibitory effects on photochemical processes and cellular responses. The evolution of a photobiological response with post-irradiation time must also be considered. These two aspects may represent some limitations to the use of action spectra. The present review, focused on mammalian cells, illustrates the concept of action spectrum and discusses its drawbacks using theoretical considerations and examples taken from the literature. Emphasis is placed on genotoxicity for which wavelength effects have been extensively studied. Other effects of UV exposure are also mentioned.

Evaluation of Inflammation by Cytokine Production Following Combined Exposure to Ultraviolet and Radiofrequency Radiation of Mobile Phones on 3D Reconstructed Human Skin In Vitro

The absorption of exposure to radiofrequency (RF) emitted by wireless devices leads to a high specific absorption rate in the skin. Ultraviolet (UV) radiation can induce several damages to the skin. The aim of this study was to examine whether combined, consecutive exposure to solar UV radiation and 1950 MHz RF exposure of third generation (3G) mobile system have any effect on inflammation processes in the skin. Under in vitro experiments, the inflammation process was examined by cytokines (IL-1α, IL-6, and IL-8) and MMP-1 enzyme secretion on 3D full thickness human skin model. The RF exposure was applied before or after UV irradiation, in order to study either the possible cooperative or protective effects of exposure to RF and UV. We did not find changes in cytokines due to exposure to RF alone. The RF exposure did not enhance the effects of UV radiation. There was a statistically not-significant decrease in cytokines when the skin tissues were pre-exposed to RF before being exposed to 4 standard erythemal dose (SED) UV compared to UV exposure alone. We found that RF exposure reduced the previously UV-treated MMP-1 enzyme concentration. This study might support the evaluation of the effects on the skin exposed to microwave radiation of 5G mobile technology.

Proteomic characterization of HaCaT keratinocytes provides new insights into changes associated with SDS exposure

Background

Using human keratinocyte HaCaT cell line model, we screened for proteins that changed their content due to SDS exposure in non-toxic dose (25 μg/ml, as determined by the MTT assay and microscopic examination) during 48 h.

Methods

The altered level of proteins from HaCaT keratinocytes exposed to SDS was analyzed by LC-MS/MS approach and quantified using Progenesis LC software.

Results

The Pathview map of 131 upregulated proteins was built, and enhancement of glycolysis/gluconeogenesis was found.

Conclusions

The results of our study admit the possibility of promotion of the cutaneous neoplasia and/or the peculiarity of the response of immortalized keratinocytes to the SDS treatment and provide new insights into possible role of SDS as integrator of diverse signaling that influence cell fate decisions.

Electromagnetic Fields, Genomic Instability and Cancer: A Systems Biological View

This review discusses the use of systems biology in understanding the biological effects of electromagnetic fields, with particular focus on induction of genomic instability and cancer. We introduce basic concepts of the dynamical systems theory such as the state space and attractors and the use of these concepts in understanding the behavior of complex biological systems. We then discuss genomic instability in the framework of the dynamical systems theory, and describe the hypothesis that environmentally induced genomic instability corresponds to abnormal attractor states; large enough environmental perturbations can force the biological system to leave normal evolutionarily optimized attractors (corresponding to normal cell phenotypes) and migrate to less stable variant attractors. We discuss experimental approaches that can be coupled with theoretical systems biology such as testable predictions, derived from the theory and experimental methods, that can be used for measuring the state of the complex biological system. We also review potentially informative studies and make recommendations for further studies.

An Evaluation of the Effects of Photoactivation of Bithionol, Amiodarone and Chlorpromazine on Human Keratinocytes In Vitro

Human skin is a continual target for chemical toxicity, due to its constant exposure to xenobiotics. The skin possesses a number of protective antioxidant systems, including glutathione and enzymic pathways, which are capable of neutralising reactive oxygen species (ROS). In combination with certain chemicals, the presence of ROS might augment the levels of toxicity, due to photoactivation of the chemical or, alternatively, due to an oxidatively-stressed state in the skin which exisited prior to exposure to the chemical. Bithionol is a phototoxic anti-parasitic compound. The mechanism of its toxicity and the possible methods of protection from its damaging effects have been explored. The capacity of keratinocytes to protect themselves from bithionol and other phototoxic chemicals has been investigated. In addition, the potential of endogenous antioxidants, such as vitamin C and E, to afford protection to the cells, has been evaluated. The intracellular glutathione stores of HaCaT keratinocytes were reduced following treatment with biothionol. Following photoactivation, both bithionol and chlorpromazine had similar effects, which suggests that glutathione is important in the detoxification pathway of these chemicals. This was confirmed by means of the visual identification of fluorescently-labelled glutathione. Endogenous antioxidants were unable to protect the HaCaT keratinocytes from bithionol toxicity or chlorpromazine phototoxicity. Amiodarone was shown to have no effect on cellular glutathione levels, which suggests that an alternative mechanism of detoxification was occurring in this case. This was supported by evidence of the protection of HaCaT cells from amiodarone phototoxicity via endogenous antioxidants. Thus, it appears that amiodarone toxicity is dependent on the levels of non-gluathione antioxidants present, whilst bithionol and chlorpromazine detoxification relies on the glutathione antioxidant system. This type of approach could indicate the likely mechanisms of phototoxicity of chemicals in vitro, with relevance to potential effects in vivo.

Oocyte atresia in WT (HdrR) and P53 (-/-) medaka (Oryzias latipes) exposed to UVA

The negative effects of ambient ultraviolet (UVA) on the water environment have been recently highlighted; UVA can create deleterious effects by stimulating stress on pelagic organisms. Little is known about UVA effects on oocyte characteristics of female fish. In the present study we explored the effects of exposure to ecologically relevant levels of simulated UVA radiation on ovaries of two major strains WT (HdrR) and P53 (-/-) of medaka (Oryzias latipes) mature female. Fish were assigned to control and three UVA-exposed groups as (15 min, 30 min, and 60 min/day) for three days and sample selection was 24 h and 14 days after exposure. Histological alterations and oocyte atresia percentage were analyzed in the UVA-exposed fish compared to control. Alteration comprised hyperthrophied follicular cells with increased thickness, breakdown of egg chorion (zona radiata), damage of cortical alveoli, and distorted nucleus and cytoplasm. The atresia percentages significantly increased with higher UVA exposure dose and time for both the wild type and the p53 deficient fish. The wild type displayed significantly higher oocyte atresia percentage than the p53 mutant. These results suggested that UVA exposure provoked histological alterations in both p53 and WT medaka oocytes leading to follicular atresia, which reduce female reproductive ability and larval production. UVA oocyte response showed p53 dependent and independent histological alteration, however, the p53 mutant was less sensitive to UVA than the wild type in medaka fish.

TCDD-induced mitochondrial superoxide production does not lead to mitochondrial degeneration or genomic instability in human SH-SY5Y neuroblastoma cells

Several genotoxic and non-genotoxic agents have been reported to cause delayed genetic damage in the progeny of the exposed cells. Such induced genomic instability (IGI) may be a driving force in carcinogenesis, and it is thus highly important to understand the cellular events accompanying it. The aim of this study was to investigate whether 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) affects mitochondrial integrity and can consequently induce genomic instability. Mitochondrial integrity was evaluated by measuring mitochondrial superoxide production, mitochondrial membrane potential, and mitochondrial activity. Micronucleus formation was used to assess immediate genetic damage and IGI. The assays were performed either immediately, 8 or 15d after the exposure. Mitochondrial superoxide production was increased by TCDD immediately after the exposure. No consistent effects on mitochondrial integrity were observed at later time points, although slightly decreased mitochondrial membrane potential at 8d and increased mitochondrial superoxide potential production at 15 after exposure were observed in the TCDD-exposed cells. TCDD did not cause immediate genetic damage, and significant IGI was not observed. In conclusion, the present results suggest that immediate TCDD-induced increase in mitochondrial superoxide level does not lead to persistent loss of mitochondrial integrity or IGI in human SH-SY5Y neuroblastoma cells.

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.

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.

Potential Effects of Corneal Cross-Linking upon the Limbus

Corneal cross-linking is nowadays the most used strategy for the treatment of keratoconus and recently it has been exploited for an increasing number of different corneal pathologies, from other ectatic disorders to keratitis. The safety of this technique has been widely assessed, but clinical complications still occur. The potential effects of cross-linking treatment upon the limbus are incompletely understood; it is important therefore to investigate the effect of UV exposure upon the limbal niche, particularly as UV is known to be mutagenic to cellular DNA and the limbus is where ocular surface tumors can develop. The risk of early induction of ocular surface cancer is undoubtedly rare and has to date not been published other than in one case after cross-linking. Nevertheless it is important to further assess, understand, and reduce where possible any potential risk. The aim of this review is to summarize all the reported cases of a pathological consequence for the limbal cells, possibly induced by cross-linking UV exposure, the studies done in vitro or ex vivo, the theoretical bases for the risks due to UV exposure, and which aspects of the clinical treatment may produce higher risk, along with what possible mechanisms could be utilized to protect the limbus and the delicate stem cells present within it.

An Evaluation of Root Phytochemicals Derived from Althea officinalis (Marshmallow) and Astragalus membranaceus as Potential Natural Components of UV Protecting Dermatological Formulations

As lifetime exposure to ultraviolet (UV) radiation has risen, the deleterious effects have also become more apparent. Numerous sunscreen and skincare products have therefore been developed to help reduce the occurrence of sunburn, photoageing, and skin carcinogenesis. This has stimulated research into identifying new natural sources of effective skin protecting compounds. Alkaline single-cell gel electrophoresis (comet assay) was employed to assess aqueous extracts derived from soil or hydroponically glasshouse-grown roots of Althea officinalis (Marshmallow) and Astragalus membranaceus, compared with commercial, field-grown roots. Hydroponically grown root extracts from both plant species were found to significantly reduce UVA-induced DNA damage in cultured human lung and skin fibroblasts, although initial Astragalus experimentation detected some genotoxic effects, indicating that Althea root extracts may be better suited as potential constituents of dermatological formulations. Glasshouse-grown soil and hydroponic Althea root extracts afforded lung fibroblasts with statistically significant protection against UVA irradiation for a greater period of time than the commercial field-grown roots. No significant reduction in DNA damage was observed when total ultraviolet irradiation (including UVB) was employed (data not shown), indicating that the extracted phytochemicals predominantly protected against indirect UVA-induced oxidative stress. Althea phytochemical root extracts may therefore be useful components in dermatological formulations.

Genomic instability induced by 50Hz magnetic fields is a dynamically evolving process not blocked by antioxidant treatment

Increased level of micronuclei was observed in SH-SY5Y cells in a previous study at 8 and 15 days after exposure to extremely low frequency (ELF) magnetic fields (MF), indicating possible induction of genomic instability in the progeny of the exposed cells. The aim of this study was to further explore the induction of genomic instability by ELF MFs by increasing the follow-up time up to 45 days after exposure. Human SH-SY5Y neuroblastoma cells were exposed to a 50Hz, 100μT MF for 24h with or without co-exposure to menadione (MQ), a chemical agent that increases cellular superoxide production. Micronuclei, reactive oxygen species (ROS) and lipid peroxidation (LPO) were measured at 15, 30 and 45 days after exposure. To study the possible causal role of ROS in the delayed effects of MF, the antioxidant N-acetylcysteine (NAC) was administered before MF exposure. Consistently with the previous study, the level of micronuclei was statistically significantly elevated 15 days after exposure. A similar effect was observed at 30 days, but not at 45 days after exposure. The level of LPO was statically significantly decreased 30 and 45 days after exposure. Consistently with our previous findings, the MF effect did not depend on co-exposure to MQ. Treatment with NAC effectively decreased cellular ROS level and suppressed the effect of MQ on ROS, but it did not block the MF effect, indicating that increase in ROS is not needed as a causal link between MF exposure and induction of delayed effects. The results presented here are consistent with genomic instability that persists in the progeny of MF-exposed cells up to at least 30 days after exposure. Changes in LPO observed at 30 and 45 days after exposure indicates that the MF-initiated process may continue up to at least 45 days after exposure.

Dermato-protective properties of ergothioneine through induction of Nrf2/ARE-mediated antioxidant genes in UVA-irradiated Human keratinocytes

UVA irradiation-induced skin damage and redox imbalance have been shown to be ameliorated by ergothioneine (EGT), a naturally occurring sulfur-containing amino acid. However, the responsible molecular mechanism with nanomolar concentrations of EGT remains unclear. We investigated the dermato protective efficacies of EGT (125-500nM) against UVA irradiation (15J/cm(2)), and elucidated the underlying molecular mechanism in human keratinocyte-derived HaCaT cells. We found that EGT treatment prior to UVA exposure significantly increased the cell viability and prevented lactate dehydrogenase release into the medium. UVA-induced ROS and comet-like DNA formation were remarkably suppressed by EGT with a parallel inhibition of apoptosis, as evidenced by reduced DNA fragmentation (TUNEL), caspase-9/-3 activation, and Bcl-2/Bax dysregulation. Furthermore, EGT alleviated UVA-induced mitochondrial dysfunction. Dose-dependent increases of antioxidant genes, HO-1, NQO-1, and γ-GCLC and glutathione by EGT were associated with upregulated Nrf2 and downregulated Keap-1 expressions. This was confirmed by increased nuclear accumulation of Nrf2 and inhibition of Nrf2 degradation. Notably, augmented luciferase activity of ARE may explain Nrf2/ARE-mediated signaling pathways behind EGT dermato-protective properties. We further demonstrated that Nrf2 translocation was mediated by PI3K/AKT, PKC, or ROS signaling cascades. This phenomenon was confirmed with suppressed nuclear Nrf2 activation, and consequently diminished antioxidant genes in cells treated with respective pharmacological inhibitors (LY294002, GF109203X, and N-acetylcysteine). Besides, increased basal ROS by EGT appears to be crucial for triggering the Nrf2/ARE signaling pathways. Silencing of Nrf2 or OCTN1 (EGT carrier protein) signaling with siRNA showed no such protective effects of EGT against UVA-induced cell death, ROS, and apoptosis, which is evidence of the vitality of Nrf2 translocation and protective efficacy of EGT in keratinocytes. Our findings conclude that EGT at nanomolar concentrations effectively ameliorated UVA-induced skin damage, and may be considered as a desirable food supplement for skin protection and/or preparation of skin care products.

UV exposure modulates hemidesmosome plasticity, contributing to long-term pigmentation in human skin

Human skin colour, ie pigmentation, differs widely among individuals, as do their responses to various types of ultraviolet radiation (UV) and their risks of skin cancer. In some individuals, UV-induced pigmentation persists for months to years in a phenomenon termed long-lasting pigmentation (LLP). It is unclear whether LLP is an indicator of potential risk for skin cancer. LLP seems to have similar features to other forms of hyperpigmentation, eg solar lentigines or age spots, which are clinical markers of photodamage and risk factors for precancerous lesions. To investigate what UV-induced molecular changes may persist in individuals with LLP, clinical specimens from non-sunburn-inducing repeated UV exposures (UVA, UVB or UVA + UVB) at 4 months post-exposure (short-term LLP) were evaluated by microarray analysis and dataset mining. Validated targets were further evaluated in clinical specimens from six healthy individuals (three LLP+ and three LLP-) followed for more than 9 months (long-term LLP) who initially received a single sunburn-inducing UVA + UVB exposure. The results support a UV-induced hyperpigmentation model in which basal keratinocytes have an impaired ability to remove melanin that leads to a compensatory mechanism by neighbouring keratinocytes with increased proliferative capacity to maintain skin homeostasis. The attenuated expression of SOX7 and other hemidesmosomal components (integrin α6β4 and plectin) leads to increased melanosome uptake by keratinocytes and points to a spatial regulation within the epidermis. The reduced density of hemidesmosomes provides supporting evidence for plasticity at the epidermal-dermal junction. Altered hemidesmosome plasticity, and the sustained nature of LLP, may be mediated by the role of SOX7 in basal keratinocytes. The long-term sustained subtle changes detected are modest, but sufficient to create dramatic visual differences in skin colour. These results suggest that the hyperpigmentation phenomenon leading to increased interdigitation develops in order to maintain normal skin homeostasis in individuals with LLP.

Role of microRNAs and DNA Methyltransferases in Transmitting Induced Genomic Instability between Cell Generations

There is limited understanding of how radiation or chemicals induce genomic instability, and how the instability is epigenetically transmitted to the progeny of exposed cells or organisms. Here, we measured the expression of microRNAs (miRNAs) and DNA methyltransferases (DNMTs) in murine embryonal fibroblasts exposed to ionizing radiation or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which were previously shown to induce genomic instability in this cell line. Cadmium was used as a reference agent that does not induce genomic instability in our experimental model. Measurements at 8 and 15 days after exposure did not identify any such persistent changes that could be considered as signals transmitting genomic instability to the progeny of exposed cells. However, measurements at 2 days after exposure revealed findings that may reflect initial stages of genomic instability. Changes that were common to TCDD and two doses of radiation (but not to cadmium) included five candidate signature miRNAs and general up-regulation of miRNA expression. Expression of DNMT3a, DNMT3b, and DNMT2 was suppressed by cadmium but not by TCDD or radiation, consistently with the hypothesis that sufficient expression of DNMTs is necessary in the initial phase of induced genomic instability.

Responses to ionizing radiation mediated by inflammatory mechanisms

Since the early years of the twentieth century, the biological consequences of exposure to ionizing radiation have been attributed solely to mutational DNA damage or cell death induced in irradiated cells at the time of exposure. However, numerous observations have been at variance with this dogma. In the 1950s, attention was drawn to abscopal effects in areas of the body not directly irradiated. In the 1960s reports began appearing that plasma factors induced by irradiation could affect unirradiated cells, and since 1990 a growing literature has documented an increased rate of DNA damage in the progeny of irradiated cells many cell generations after the initial exposure (radiation-induced genomic instability) and responses in non-irradiated cells neighbouring irradiated cells (radiation-induced bystander effects). All these studies have in common the induction of effects not in directly irradiated cells but in unirradiated cells as a consequence of intercellular signalling. Recently, it has become clear that all the various effects demonstrated in vivo may reflect an ongoing inflammatory response to the initial radiation-induced injury that, in a genotype-dependent manner, has the potential to contribute primary and/or ongoing damage displaced in time and/or space from the initial insult. Importantly, there is direct evidence that non-steroidal anti-inflammatory drug treatment reduces such damage in vivo. These new findings highlight the importance of tissue responses and indicate additional mechanisms of radiation action, including the likelihood that radiation effects are not restricted to the initiation stage of neoplastic diseases, but may also contribute to tumour promotion and progression. The various developments in understanding the responses to radiation exposures have implications not only for radiation pathology but also for therapeutic interventions.

Induction of bystander effects by UVA, UVB, and UVC radiation in human fibroblasts and the implication of reactive oxygen species

Radiation-induced bystander effects are various types of responses displayed by nonirradiated cells induced by signals transmitted from neighboring irradiated cells. This phenomenon has been well studied after ionizing radiation, but data on bystander effects after UV radiation are limited and so far have been reported mainly after UVA and UVB radiation. The studies described here were aimed at comparing the responses of human dermal fibroblasts exposed directly to UV (A, B, or C wavelength range) and searching for bystander effects induced in unexposed cells using a transwell co-incubation system. Cell survival and apoptosis were used as a measure of radiation effects. Additionally, induction of senescence in UV-exposed and bystander cells was evaluated. Reactive oxygen species (ROS), superoxide radical anions, and nitric oxide inside the cells and secretion of interleukins 6 and 8 (IL-6 and IL-8) into the medium were assayed and evaluated as potential mediators of bystander effects. All three regions of ultraviolet radiation induced bystander effects in unexposed cells, as shown by a diminution of survival and an increase in apoptosis, but the pattern of response to direct exposure and the bystander effects differed depending on the UV spectrum. Although UVA and UVB were more effective than UVC in generation of apoptosis in bystander cells, UVC induced senescence both in irradiated cells and in neighbors. The level of cellular ROS increased significantly shortly after UVA and UVB exposure, suggesting that the bystander effects may be mediated by ROS generated in cells by UV radiation. Interestingly, UVC was more effective at generation of ROS in bystanders than in directly exposed cells and induced a high yield of superoxide in exposed and bystander cells, which, however, was only weakly associated with impairment of mitochondrial membrane potential. Increasing concentration of IL-6 but not IL-8 after exposure to each of the three bands of UV points to its role as a mediator in the bystander effect. Nitric oxide appeared to play a minor role as a mediator of bystander effects in our experiments. The results demonstrating an increase in intracellular oxidation, not only in directly UV-exposed but also in neighboring cells, and generation of proinflammatory cytokines, processes entailing cell damage (decreased viability, apoptosis, senescence), suggest that all bands of UV radiation carry a potential hazard for human health, not only due to direct mechanisms, but also due to bystander effects.

Bystander effect induced by UVC radiation in Chinese hamster V79 cells

In past decades, researches on radiation-induced bystander effect mainly focused on ionizing radiation such as α-particle, β-particle, X-ray and γ-ray. But few researches have been conducted on the ability of ultraviolet (UV) radiation-induced bystander effect, and knowledge of UVC-induced bystander effect is far limited. Here, we adopted medium transfer experiment to detect whether UVC could cause bystander effect in Chinese hamster V79 cells. We determined the cell viability, apoptosis rate, chromosome aberration and ultrastructure changes, respectively. Our results showed that: (1) the viability of UVC-irradiated V79 cells declined significantly with the dosage of UVC; (2) similar to the irradiated cells, the main death type of bystander cells cultured in irradiation conditioned medium (ICMs) was also apoptosis; (3) soluble factors secreted by UVC-irradiated cells could induce bystander effect in V79 cells; (4) cells treated with 4 h ICM collected from 90 mJ cm(-2) UVC-irradiated cells displayed the strongest response. Our data revealed that UVC could cause bystander effect through the medium soluble factors excreted from irradiated cells and this bystander effect was a novel quantitative and kinetic response. These findings might provide a foundation to further explore the exact soluble bystander factors and detailed mechanism underlying UVC-induced bystander effect.

Induction of genomic instability, oxidative processes, and mitochondrial activity by 50Hz magnetic fields in human SH-SY5Y neuroblastoma cells

Epidemiological studies have suggested that exposure to 50Hz magnetic fields (MF) increases the risk of childhood leukemia, but there is no mechanistic explanation for carcinogenic effects. In two previous studies we have observed that a 24-h pre-exposure to MF alters cellular responses to menadione-induced DNA damage. The aim of this study was to investigate the cellular changes that must occur already during the first 24h of exposure to MF, and to explore whether the MF-induced changes in DNA damage response can lead to genomic instability in the progeny of the exposed cells. In order to answer these questions, human SH-SY5Y neuroblastoma cells were exposed to a 50-Hz, 100-μT MF for 24h, followed by 3-h exposure to menadione. The main finding was that MF exposure was associated with increased level of micronuclei, used as an indicator of induced genomic instability, at 8 and 15d after the exposures. Other delayed effects in MF-exposed cells included increased mitochondrial activity at 8d, and increased reactive oxygen species (ROS) production and lipid peroxidation at 15d after the exposures. Oxidative processes (ROS production, reduced glutathione level, and mitochondrial superoxide level) were affected by MF immediately after the exposure. In conclusion, the present results suggest that MF exposure disturbs oxidative balance immediately after the exposure, which might explain our previous findings on MF altered cellular responses to menadione-induced DNA damage. Persistently elevated levels of micronuclei were found in the progeny of MF-exposed cells, indicating induction of genomic instability.

Consistency of the proteome in primary human keratinocytes with respect to gender, age, and skin localization

Keratinocytes account for 95% of all cells of the epidermis, the stratified squamous epithelium forming the outer layer of the skin, in which a significant number of skin diseases takes root. Immortalized keratinocyte cell lines are often used as research model systems providing standardized, reproducible, and homogenous biological material. Apart from that, primary human keratinocytes are frequently used for medical studies because the skin provides an important route for drug administration and is readily accessible for biopsies. However, comparability of these cell systems is not known. Cell lines may undergo phenotypic shifts and may differ from the in vivo situation in important aspects. Primary cells, on the other hand, may vary in biological functions depending on gender and age of the donor and localization of the biopsy specimen. Here we employed metabolic labeling in combination with quantitative mass spectrometry-based proteomics to assess A431 and HaCaT cell lines for their suitability as model systems. Compared with cell lines, comprehensive profiling of the primary human keratinocyte proteome with respect to gender, age, and skin localization identified an unexpected high proteomic consistency. The data were analyzed by an improved ontology enrichment analysis workflow designed for the study of global proteomics experiments. It enables a quick, comprehensive and unbiased overview of altered biological phenomena and links experimental data to literature. We guide through our workflow, point out its advantages compared with other methods and apply it to visualize differences of cell lines compared with primary human keratinocytes.

Hyperbaric oxygen preconditioning protects skin from UV-A damage

Hyperbaric oxygen therapy (HBOT) is used for a number of applications, including the treatment of diabetic foot ulcers and CO poisoning. However, we and others have shown that HBOT can mobilize cellular antioxidant defenses, suggesting that it may also be useful under circumstances in which tissue protection from oxidative damage is desired. To test the protective properties of hyperbaric oxygen (HBO) on a tissue level, we evaluated the ability of a preconditioning treatment regimen to protect cutaneous tissue from UV-A-induced oxidative damage. Three groups of hairless SKH1-E mice were exposed to UV-A 3 days per week for 22 weeks, with two of these groups receiving an HBO pretreatment either two or four times per week. UV-A exposure increased apoptosis and proliferation of the skin tissue, indicating elevated levels of epithelial damage and repair. Pretreatment with HBO significantly reduced UV-A-induced apoptosis and proliferation. A morphometric analysis of microscopic tissue folds also showed a significant increase in skin creasing following UV-A exposure, which was prevented by HBO pretreatment. Likewise, skin elasticity was found to be greatest in the group treated with HBO four times per week. The effects of HBO were also apparent systemically as reductions in caspase-3 activity and expression were observed in the liver. Our findings support a protective function of HBO pretreatment from a direct oxidative challenge of UV-A to skin tissue. Similar protection of other tissues may likewise be achievable.

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.

Dioxin induces genomic instability in mouse embryonic fibroblasts

Ionizing radiation and certain other exposures have been shown to induce genomic instability (GI), i.e., delayed genetic damage observed many cell generations later in the progeny of the exposed cells. The aim of this study was to investigate induction of GI by a nongenotoxic carcinogen, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Mouse embryonic fibroblasts (C3H10T1/2) were exposed to 1, 10 or 100 nM TCDD for 2 days. Micronuclei (MN) and expression of selected cancer-related genes were assayed both immediately and at a delayed point in time (8 days). For comparison, similar experiments were done with cadmium, a known genotoxic agent. TCDD treatment induced an elevated frequency of MN at 8 days, but not directly after the exposure. TCDD-induced alterations in gene expression were also mostly delayed, with more changes observed at 8 days than at 2 days. Exposure to cadmium produced an opposite pattern of responses, with pronounced effects immediately after exposure but no increase in MN and few gene expression changes at 8 days. Although all responses to TCDD alone were delayed, menadione-induced DNA damage (measured by the Comet assay), was found to be increased directly after a 2-day TCDD exposure, indicating that the stability of the genome was compromised already at this time point. The results suggested a flat dose-response relationship consistent with dose-response data reported for radiation-induced GI. These findings indicate that TCDD, although not directly genotoxic, induces GI, which is associated with impaired DNA damage response.

Zebrafish have a competent p53-dependent nucleotide excision repair pathway to resolve ultraviolet B-induced DNA damage in the skin

Ultraviolet (UV) light is a primary environmental risk factor for melanoma, a deadly form of skin cancer derived from the pigmented cells called melanocytes. UVB irradiation causes DNA damage, mainly in the form of pyrimidine dimers (cis-syn cyclobutane pyrimidine dimers and pyrimidine (6-4) pyrimidone photoproducts), and organisms have developed complex multiprotein repair processes to cope with the DNA damage. Zebrafish is becoming an important model system to study the effects of UV light in animals, in part because the embryos are easily treated with UV irradiation, and the DNA damage repair pathways appear to be conserved in zebrafish and mammals. We are interested in exploring the effects of UV irradiation in young adult zebrafish, so that we can apply them to the study of gene-environment interactions in models of skin cancer. Using the Xiphophorus UV melanoma model as a starting point, we have developed a UV irradiation treatment chamber, and established UV treatment conditions at different ages of development. By translating the Xiphophorus UV treatment methodology to the zebrafish system, we show that the adult zebrafish skin is competent for nucleotide excision DNA damage repair, and that like in mammalian cells, UV treatment promotes phosphorylation of H2AX and a p53-dependent response. These studies provide the groundwork for exploring the role of UV light in melanoma development in zebrafish.

Synthesis and application of a novel sunscreen-antioxidant

Background information on the inefficacy of sunscreens to provide free radical protection in skin, despite their usefulness in preventing sunburn/erythema, prompted us to synthesize a compound which would display in the same molecule both UV-absorbing and antioxidant capacities. For this purpose, the UVB absorber, 2-ethylhexyl-4-methoxycinnamate (OMC) was combined with the piperidine nitroxide TEMPOL, which has antioxidant properties. The spectral properties of the new nitroxide-based sunscreen (MC-NO) as well as its efficacy to prevent photo-oxidative damage to lipids induced by UVA, natural sunlight and 4-tert-butyl-4-methoxydibenzoylmethane (BMDBM), a photo-unstable sunscreen which generates free radicals upon UV radiation, was studied. The results obtained demonstrate that MC-NO: (a) absorbs in the UVB region even after UVA irradiation; (b) acts as free radical scavenger as demonstrated by EPR experiments; (c) strongly reduces both UVA-, sunlight- and BMDBM-induced lipid peroxidation in liposomes, measured as reduced TBARS levels; and (d) has comparable antioxidant activity to that of commonly used vitamin E and BHT in skin care formulations. These results suggest that the use of the novel sunscreen-antioxidant or of other nitroxide-based sunscreens in formulations aimed at reducing photoinduced skin damage may be envisaged.

Induction of oxidative cell damage by photo-treatment with zincmetaN-methylpyridylporphyrin

We have previously reported that isomeric Zn(II) N-methylpyridylporphyrins (ZnTM-2(3,4)-PyP4 + ) can act as photosensitizers with efficacy comparable to that of hematoporphyrin derivative (HpD) in preventing cell proliferation and causing cell death in vitro. To better understand the biochemical basis of this activity, the effects of photo-activated ZnTM-3-PyP4 + on GSH/GSSG ratio, lipid peroxidation, membrane permeability, oxidative DNA damage, and the activities of SOD, catalase, glutathione reductase, and glutathione peroxidase were evaluated. Light exposure of ZnTM-3-PyP4 + -treated colon adenocarcinoma cells caused a wide spectrum of oxidative damage including depletion of GSH, inactivation of glutathione reductase and glutathione peroxidase, oxidative DNA damage and peroxidation of membrane lipids. Cell staining with Hoechst-33342 showed morphological changes consistent with both necrotic and apoptotic death sequences, depending upon the presence of oxygen.

Cellular and sub-cellular responses to UVA in relation to carcinogenesis

PURPOSE

UVA radiation (315-400 nm) contributes to skin aging and carcinogenesis. The aim of this review is to consider the mechanisms that underlie UVA-induced cellular damage, how this damage may be prevented or repaired and the signal transduction processes that are elicited in response to it.

RESULTS

Exposure to ultraviolet (UV) light is well-established as the causative factor in skin cancer. Until recently, most work on the mechanisms that underlie skin carcinogenesis focused on shorter wavelength UVB radiation (280-315 nm), however in recent years there has been increased interest in the contribution made by UVA. UVA is able to cause a range of damage to cellular biomolecules including lipid peroxidation, oxidized protein and DNA damage, such as 8-oxoguanine and cyclobutane pyrimidine dimers. Such damage is strongly implicated in both cell death and malignant transformation and cells have a number of mechanisms in place to mitigate the effects of UVA exposure, including antioxidants, DNA repair, and stress signalling pathways.

CONCLUSIONS

The past decade has seen a surge of interest in the biological effects of UVA exposure as its significance to the process of photo-carcinogenesis has become increasingly evident. However, unpicking the unique complexity of the cellular response to UVA, which is only now becoming apparent, will be a major challenge for the field of photobiology in the 21st century.

Watching the watcher: regulation of p53 by mitochondria

p53 has been referred to as the 'guardian of the genome' because of its role in protecting the cell from DNA damage. p53 performs its duties by regulating cell-cycle progression and DNA repair and, in cases of irreparable DNA damage, by executing programmed cell death. Mitochondria are an important target of transcription-dependent and -independent actions of p53 to carry out the apoptotic function. However, increasing evidence suggests that p53 activity is regulated by mitochondria. Cellular insults that alter mitochondrial function can have important consequences on p53 activity. In light of these new findings, the following review focuses on p53/mitochondria connections, in particular how reactive oxygen species generated at mitochondria regulate p53 activity. A better understanding of the mechanisms by which mitochondria regulate p53 may have an impact on our understanding of the development and progression of many diseases, especially cancer.

Cellular effects of long wavelength UV light (UVA) in mammalian cells

UVA should receive significant consideration as a human health risk as it is a large proportion of the solar spectrum that reaches the earth's surface and because of its ability to penetrate human skin. It is only relatively recently that this has been recognized and this previously under-researched part of the UV spectrum is becoming increasingly well characterized at doses that are quite low in relation to those experienced by humans. Absorption of UVA in a cell leads to the production of reactive oxygen and nitrogen species that can damage major biomolecules including DNA and membrane lipids. Various types of damage induced in these molecules lead to significant biological effects including cytotoxicity, mutations and alterations in cell signalling pathways. Longer-term effects such as persistent genomic instability and bystander effects have also been observed following UVA treatment of mammalian cells and, as with ionizing radiation, this changes some of the fundamental thinking around tissue effects of irradiation. Antioxidants have been assessed extensively for their ability to protect against the biological effects of UVA and a number have been shown to be successful at least in-vitro, for example vitamin E and epigallocatechin-3-gallate. Other potential targets for protection are suggested through the increased understanding of some of the signalling mechanisms activated following treatment, for example the inhibition of NADPH oxidase is seen to reduce a bystander effect. The search for appropriate and successful photoprotective agents remains an important area of research.

In vitro photostability and photoprotection studies of a novel 'multi-active' UV-absorber

This paper reports on the synthesis and properties of a new UV-absorber (OC-NO) based on the most popular UV filter worldwide, ethylhexyl methoxycinnamate (OMC) in which the methoxy group has been replaced with a pyrrolidine nitroxide bearing antioxidant activity. This sunscreen active has therefore both UV-absorbing and antioxidant properties which could ideally address both the UV-B and UV-A skin photo-damage. For broad-spectrum coverage, the combinations of OC-NO with two commonly used UV-A absorbers (BMDBM and DHHB) were also studied. The results obtained reveal that OC-NO: (a) is as photostable as OMC after UV-A exposure; (b) acts as free radical scavenger as demonstrated by EPR and chemical studies; (c) reduces UV-A and UV-A+BMDBM induced lipid peroxidation in liposomes and cells, measured as reduced TBARS levels and increased C11-BODIPY red fluorescence, respectively; (d) has comparable antioxidant activity to that of vitamin E and BHT commonly used in skin care formulations; (e) is non-cytotoxic to human skin fibroblasts as assessed with the MTT assay when exposed to increasing doses of UV-A; and (f) OC-NO+DHHB is a promising, photostable broad spectrum UV-filter combination that concomitantly reduces UV-induced free radical damage. These results suggest that nitroxide/antioxidant-based UV-absorbers may pave the way for the utilization of 'multi-active' ingredients in sunscreens thereby reducing the number of ingredients in these formulations.

Skin DNA photodamage and its biological consequences

It is well established that ultraviolet (UV) radiation from sunlight damages skin cells' DNA. Wavelengths in the UVB range are absorbed by DNA and can induce mutagenic lesions such as pyrimidine dimers. On the other hand, genotoxic effects of solar UVA are mainly mediated by the activation of endogenous photosensitizers resulting in the generation of a local oxidative stress. Exogenous chemicals, such as drugs like psoralens or fluoroquinolones, sometimes amplify UV-induced harmful effects. DNA damage can lead to mutations and genetic instability. This is one of the reasons why sunlight overexposure increases the risk of skin cancer. But DNA photolesions can also be involved in other skin-specific responses to UV radiation: erythema, immunosuppression, and melanogenesis are examples reported in the literature. The aim of this short review is to summarize the general knowledge in the field of UV-induced DNA damage. Besides the biological consequences of DNA photolesions, this article also deals with technologies used for their detection and shows how helpful such approaches can be to assess photoprotection provided by sunscreens.

UVA radiation causes DNA strand breaks, chromosomal aberrations and tumorigenic transformation in HaCaT skin keratinocytes

The role of UVA-radiation-the major fraction in sunlight-in human skin carcinogenesis is still elusive. We here report that different UVA exposure regime (4 x 5 J/cm(2) per week or 1 x 20 J/cm(2) per week) caused tumorigenic conversion (tumors in nude mice) of the HaCaT skin keratinocytes. While tumorigenicity was not associated with general telomere shortening, we found new chromosomal changes characteristic for each recultivated tumor. Since this suggested a nontelomere-dependent relationship between UVA irradiation and chromosomal aberrations, we investigated for alternate mechanisms of UVA-dependent genomic instability. Using the alkaline and neutral comet assay as well as gamma-H2AX foci formation on irradiated HaCaT cells (20-60 J/cm(2)), we show a dose-dependent and long lasting induction of DNA single and double (ds) strand breaks. Extending this to normal human skin keratinocytes, we demonstrate a comparable damage response and, additionally, a significant induction and maintenance of micronuclei (MN) with more acentric fragments (indicative of ds breaks) than entire chromosomes particularly 5 days post irradiation. Thus, physiologically relevant UVA doses cause long-lasting DNA strand breaks, a prerequisite for chromosomal aberration that most likely contribute to tumorigenic conversion of the HaCaT cells. Since normal keratinocytes responded similarly, UVA may likewise contribute to the complex karyotype characteristic for human skin carcinomas.

The Dose Rate of UVA Treatment Influences the Cellular Response of HaCaT Keratinocytes

The contribution of UV exposure to the etiology of skin cancer and photoaging is undisputed. However, the effect of altering the intensity or dose rate of UV, which varies considerably with geographical location, the time of day or year, and the use of sunscreens, is not understood. In this study, the effect of altering the dose rate of UVA was investigated in the immortalized human keratinocyte cell line, HaCaT. Lowering the dose rate of UVA resulted in increased cytotoxicity, which correlated with increases in both lipid peroxidation and DNA damage. Furthermore, exposure at low dose rate did not appear to reduce the ability of UVA to induce the phenomenon of persistent genomic instability. Pretreatment with the antioxidant vitamin E significantly protected against UVA dose-rate effects observed with respect to lipid peroxidation and survival. Additionally, cell populations irradiated at low dose rate exhibited a shift towards a more pro-oxidant state. Taken together, these observations suggest an oxidative stress mechanism is underlying the UVA dose-rate effect. This study demonstrates that dose rates must be included as a key factor when evaluating the biological effects of UVA, especially considering the concerns, which exist regarding the efficacy and photostability of sunscreens to UVA.

Naringenin protects HaCaT human keratinocytes against UVB-induced apoptosis and enhances the removal of cyclobutane pyrimidine dimers from the genome

Many naturally occurring agents are believed to protect against UV-induced skin damage. In this study, we have investigated the effects of naringenin (NG), a naturally occurring citrus flavonone, on the removal of UVB-induced cyclobutane pyrimidine dimers (CPD) from the genome and apoptosis in immortalized p53-mutant human keratinocyte HaCaT cells. The colony-forming assay shows that treatment with NG significantly increases long-term cell survival after UVB irradiation. NG treatment also protects the cells from UVB-induced apoptosis, as indicated by the absence of the 180 base pair DNA ladders and the appearance of sub-G1 peak using agarose gel electrophoresis and flow cytometric analysis, respectively. The UVB-induced poly (ADP-ribose) polymerase-1 (PARP-1) cleavage, caspase activation and Bax/Bcl2 ratio were modulated following NG treatment, indicating an antiapoptotic effect of NG in UVB-damaged cells that occurs at least in part via caspase cascade pathway. Moreover, treatment of UVB-irradiated HaCaT cells with NG enhances the removal of CPD from the genome, as observed by both direct quantitation of CPD in genomic DNA and immuno-localization of the damage within the nuclei. The study provides a molecular basis for the action of NG as a promising natural flavonoid in preventing skin aging and carcinogenesis.