Inhibition of S-phase progression triggered by UVA-induced ROS does not require a functional DNA damage checkpoint response in mammalian cells

Multi-endpoint assays reveal more severe toxicity induced by chloraminated effluent organic matter than chloraminated natural organic matter

Disinfection by chloramination produces toxic byproducts and the difference in toxicity of reclaimed and drinking water treated by chloramination remains unclear. This study investigated cytotoxic effects at the same concentrations of dissolved organic matter and showed that chloraminated effluent organic matter (EfOM) induced 1.7 times higher cytotoxicity than chloraminated natural organic matter (NOM) applied to simulate drinking water. Chloraminated EfOM induced more reactive nitrogen species than chloraminated NOM, and chloraminated EfOM and NOM induced similar and higher levels of reactive oxygen species than the negative control, respectively. Consequently, intracellular macromolecule damage indicated by DNA/RNA damage marker 8‑hydroxy-(deoxy)guanosine and the intracellular protein carbonyl concentration induced by chloraminated EfOM was higher and slightly more than that induced by chloraminated NOM, respectively. These data were consistent with the effects on cell physiological processes. Cell cycle arrest mainly occurred in G2 phase by chloraminated EfOM and NOM. Early apoptotic cells, which could return to normal, increased upon exposure to high concentrations of chloraminated EfOM and NOM. Moreover, necrotic cells were significantly increased from 0.5% to 2.5% when the concentration increased from 20- to 60-fold chloraminated EfOM, but were not obviously changed by chloraminated NOM. These results indicated that the comprehensive intracellular changes induced by toxic substances in chloraminated EfOM were more irreversible and induced more cell death than chloraminated NOM.

Distinct stem-like cell populations facilitate functional regeneration of the Cladonema medusa tentacle

Blastema formation is a crucial process that provides a cellular source for regenerating tissues and organs. While bilaterians have diversified blastema formation methods, its mechanisms in non-bilaterians remain poorly understood. Cnidarian jellyfish, or medusae, represent early-branching metazoans that exhibit complex morphology and possess defined appendage structures highlighted by tentacles with stinging cells (nematocytes). Here, we investigate the mechanisms of tentacle regeneration, using the hydrozoan jellyfish Cladonema pacificum. We show that proliferative cells accumulate at the tentacle amputation site and form a blastema composed of cells with stem cell morphology. Nucleoside pulse-chase experiments indicate that most repair-specific proliferative cells (RSPCs) in the blastema are distinct from resident stem cells. We further demonstrate that resident stem cells control nematogenesis and tentacle elongation during both homeostasis and regeneration as homeostatic stem cells, while RSPCs preferentially differentiate into epithelial cells in the newly formed tentacle, analogous to lineage-restricted stem/progenitor cells observed in salamander limbs. Taken together, our findings propose a regeneration mechanism that utilizes both resident homeostatic stem cells (RHSCs) and RSPCs, which in conjunction efficiently enable functional appendage regeneration, and provide novel insight into the diversification of blastema formation across animal evolution.

Changes in toxicity and adsorbable organic bromine concentrations in ozonated reclaimed water irradiated with sunlight

Disinfecting reclaimed water for safe reuse can produce toxic disinfection by-products such as adsorbable organic bromine (AOBr). Irradiating stored reclaimed water with sunlight is a "green" and free method for eliminating some toxic disinfection by-products, but the effects of irradiation with sunlight on ozonated reclaimed water containing bromide are not well understood. In this study, AOBr was found at concentrations of 171-180 (µg Br)/L in ozonated reclaimed water containing bromide at a concentration of 2 (mg Br)/L and dissolved organic carbon at a concentration of ∼5 (mg C)/L. Irradiation with sunlight degraded 53-74% of the AOBr in two reclaimed water samples in 8 h, and the pseudo-first-order rate constants (k) were 0.09-0.17 h^-1. The concentration of tribromomethane, a typical Br-containing disinfection by-product, was decreased by >96% by irradiation for 8 h (k = 0.42-0.47 h^-1). Irradiation with sunlight decreased the toxicity of ozonated reclaimed water to Chinese hamster ovary cells. Irradiation with sunlight decreased the degree of intracellular oxidative stress and oxidative damage caused by ozonated reclaimed water. Irradiation with sunlight for 8 h decreased cytotoxicity of the ozonated reclaimed water samples by 79% and 65%. The change in AOBr concentration correlated with the change in toxicity (R²=0.69, p<0.05). The relationships between sunlight wavelength and decreases in the AOBr concentration and toxicity were assessed, and it was found that UV in sunlight was predominantly responsible for decreasing the AOBr concentration and toxicity by reclaimed water. During irradiation for 8 h, UV was responsible for 65%-66% of the decrease in the AOBr concentration and 65-79% of the decrease in reclaimed water induced cytotoxicity. Irradiation with sunlight is a promising method for degrading AOBr and detoxifying ozonated reclaimed water during storage to allow the water to be reused.

Catechin-Albumin Conjugates: Enhanced Antioxidant Capacity and Anticancer Effects

(+)-Catechin conjugated with human serum albumin (CT–HSA) was prepared and evaluated as a drug carrier bearing anticancer effects. It was found that 2.4 mol of CT was conjugate to 1 mol HSA. The CT–HSA has an antioxidant capacity of about 3.3 times the amount of CT in the conjugate. Intracellular incorporation of the CT–HSA was analyzed by fluorescence-activated cell sorting (FACS) and confocal laser scanning microscopy (CLSM) measurements using fluorescein isothiocyanate (FITC)-labelled CT–HSA. The results indicated that the FITC-labelled CT–HSA was incorporated into HeLa cells in a concentration-dependent manner. The CT–HSA enhanced the binding of anticancer drugs (5-fluorouracil (5-Fu) and mitomycin C (MMC)) comparing with HSA, and the CT–HSA mixed with 5-Fu or MMC decreased significantly the HeLa cell viability as compared with the same concentration of each drug. In addition, intracellular reactive oxygen species (ROS) scavenging by the CT–HSA is likely to affect the anticancer effects. Thus, the CT–HSA enhanced anticancer drug efficacy in relation to controlling ROS-scavenging ability.

Inhibition of DNA Repair by Inappropriate Activation of ATM, PARP, and DNA-PK with the Drug Agonist AsiDNA

AsiDNA is a DNA repair inhibitor mimicking DNA double-strand breaks (DSB) that was designed to disorganize DSB repair pathways to sensitize tumors to DNA damaging therapies such as radiotherapy and chemotherapy. We used the property of AsiDNA of triggering artificial DNA damage signaling to examine the activation of DSB repair pathways and to study the main steps of inhibition of DNA repair foci after irradiation. We show that, upon AsiDNA cellular uptake, cytoplasmic ATM and PARP are rapidly activated (within one hour) even in the absence of irradiation. ATM activation by AsiDNA leads to its transient autophosphorylation and sequestration in the cytoplasm, preventing the formation of ATM nuclear foci on irradiation-induced damage. In contrast, the activation of PARP did not seem to alter its ability to form DNA repair foci, but prevented 53BP1 and XRCC4 recruitment at the damage sites. In the nucleus, AsiDNA is essentially associated with DNA-PK, which triggers its activation leading to phosphorylation of H2AX all over chromatin. This pan-nuclear phosphorylation of H2AX correlates with the massive inhibition, at damage sites induced by irradiation, of the recruitment of repair enzymes involved in DSB repair by homologous recombination and nonhomologous end joining. These results highlight the interest in a new generation of DNA repair inhibitors targeting DNA damage signaling.

Voriconazole enhances UV-induced DNA damage by inhibiting catalase and promoting oxidative stress

Cutaneous squamous cell carcinoma (cSCC) is the second most common form of skin cancer and is associated with cumulative UV exposure. Studies have shown that prolonged voriconazole use promotes cSCC formation; however, the biological mechanisms responsible for the increased incidence remain unclear. Here, we show that voriconazole directly increases oxidative stress in human keratinocytes and promotes UV-induced DNA damage as determined by comet assay, 8-oxoguanine immunofluorescence and mass spectrometry. Voriconazole treatment of human keratinocytes potentiates UV-induced apoptosis and activation of the p38 MAP kinase and 53BP1 UV stress response pathways. The p38 MAP kinase activation promoted by voriconazole exposure can be mitigated by pretreating keratinocytes with N-acetylcysteine. Voriconazole increases oxidative stress in keratinocytes by directly inhibiting catalase leading to lower intracellular NADPH levels and the triazole moieties in voriconazole are critical for inhibiting catalase. Furthermore, voriconazole is shown to promote UV-induced dysplasia in an in vivo model. Together, these data demonstrate that voriconazole potentiates oxidative stress in UV-irradiated keratinocytes through catalase inhibition. Use of antioxidants may mitigate the pro-oncogenic effects of voriconazole.

The key role of UVA-light induced oxidative stress in human Xeroderma Pigmentosum Variant cells

The UVA component of sunlight induces DNA damage, which are basically responsible for skin cancer formation. Xeroderma Pigmentosum Variant (XP-V) patients are defective in the DNA polymerase pol eta that promotes translesion synthesis after sunlight-induced DNA damage, implying in a clinical phenotype of increased frequency of skin cancer. However, the role of UVA-light in the carcinogenesis of these patients is not completely understood. The goal of this work was to characterize UVA-induced DNA damage and the consequences to XP-V cells, compared to complemented cells. DNA damage were induced in both cells by UVA, but lesion removal was particularly affected in XP-V cells, possibly due to the oxidation of DNA repair proteins, as indicated by the increase of carbonylated proteins. Moreover, UVA irradiation promoted replication fork stalling and cell cycle arrest in the S-phase for XP-V cells. Interestingly, when cells were treated with the antioxidant N-acetylcysteine, all these deleterious effects were consistently reverted, revealing the role of oxidative stress in these processes. Together, these results strongly indicate the crucial role of oxidative stress in UVA-induced cytotoxicity and are of interest for the protection of XP-V patients.

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.

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.

Protein oxidation, UVA and human DNA repair

Solar UVB is carcinogenic. Nucleotide excision repair (NER) counteracts the carcinogenicity of UVB by excising potentially mutagenic UVB-induced DNA lesions. Despite this capacity for DNA repair, non-melanoma skin cancers and apparently normal sun-exposed skin contain huge numbers of mutations that are mostly attributable to unrepaired UVB-induced DNA lesions. UVA is about 20-times more abundant than UVB in incident sunlight. It does cause some DNA damage but this does not fully account for its biological impact. The effects of solar UVA are mediated by its interactions with cellular photosensitizers that generate reactive oxygen species (ROS) and induce oxidative stress. The proteome is a significant target for damage by UVA-induced ROS. In cultured human cells, UVA-induced oxidation of DNA repair proteins inhibits DNA repair. This article addresses the possible role of oxidative stress and protein oxidation in determining DNA repair efficiency - with particular reference to NER and skin cancer risk.

Slow Replication Fork Velocity of Homologous Recombination-Defective Cells Results from Endogenous Oxidative Stress

Replications forks are routinely hindered by different endogenous stresses. Because homologous recombination plays a pivotal role in the reactivation of arrested replication forks, defects in homologous recombination reveal the initial endogenous stress(es). Homologous recombination-defective cells consistently exhibit a spontaneously reduced replication speed, leading to mitotic extra centrosomes. Here, we identify oxidative stress as a major endogenous source of replication speed deceleration in homologous recombination-defective cells. The treatment of homologous recombination-defective cells with the antioxidant N-acetyl-cysteine or the maintenance of the cells at low O₂ levels (3%) rescues both the replication fork speed, as monitored by single-molecule analysis (molecular combing), and the associated mitotic extra centrosome frequency. Reciprocally, the exposure of wild-type cells to H₂O₂ reduces the replication fork speed and generates mitotic extra centrosomes. Supplying deoxynucleotide precursors to H₂O₂-exposed cells rescued the replication speed. Remarkably, treatment with N-acetyl-cysteine strongly expanded the nucleotide pool, accounting for the replication speed rescue. Remarkably, homologous recombination-defective cells exhibit a high level of endogenous reactive oxygen species. Consistently, homologous recombination-defective cells accumulate spontaneous γH2AX or XRCC1 foci that are abolished by treatment with N-acetyl-cysteine or maintenance at 3% O₂. Finally, oxidative stress stimulated homologous recombination, which is suppressed by supplying deoxynucleotide precursors. Therefore, the cellular redox status strongly impacts genome duplication and transmission. Oxidative stress should generate replication stress through different mechanisms, including DNA damage and nucleotide pool imbalance. These data highlight the intricacy of endogenous replication and oxidative stresses, which are both evoked during tumorigenesis and senescence initiation, and emphasize the importance of homologous recombination as a barrier against spontaneous genetic instability triggered by the endogenous oxidative/replication stress axis.

Endogenous stress is an important stress because it challenges cells daily. However, endogenous stress is difficult to apprehend. Replication forks are routinely hindered by different endogenous stresses. Because homologous recombination plays a pivotal role in the reactivation of arrested replication forks, defects in homologous recombination reveal the initial endogenous stress(es). Here we identify endogenous oxidative stress among the different potential endogenous stresses as being responsible for spontaneous replication defects in homologous recombination-defective cells. Therefore, oxidative and replication stresses, which are both evoked during tumorigenesis and senescence initiation, are linked, and homologous recombination acts as a barrier against spontaneous genetic instability triggered by endogenous oxidative/replication stress.

Singlet Oxygen-Mediated Oxidation during UVA Radiation Alters the Dynamic of Genomic DNA Replication

UVA radiation (320–400 nm) is a major environmental agent that can exert its deleterious action on living organisms through absorption of the UVA photons by endogenous or exogenous photosensitizers. This leads to the production of reactive oxygen species (ROS), such as singlet oxygen (¹O₂) and hydrogen peroxide (H₂O₂), which in turn can modify reversibly or irreversibly biomolecules, such as lipids, proteins and nucleic acids. We have previously reported that UVA-induced ROS strongly inhibit DNA replication in a dose-dependent manner, but independently of the cell cycle checkpoints activation. Here, we report that the production of ¹O₂ by UVA radiation leads to a transient inhibition of replication fork velocity, a transient decrease in the dNTP pool, a quickly reversible GSH-dependent oxidation of the RRM1 subunit of ribonucleotide reductase and sustained inhibition of origin firing. The time of recovery post irradiation for each of these events can last from few minutes (reduction of oxidized RRM1) to several hours (replication fork velocity and origin firing). The quenching of ¹O₂ by sodium azide prevents the delay of DNA replication, the decrease in the dNTP pool and the oxidation of RRM1, while inhibition of Chk1 does not prevent the inhibition of origin firing. Although the molecular mechanism remains elusive, our data demonstrate that the dynamic of replication is altered by UVA photosensitization of vitamins via the production of singlet oxygen.

Identification of influential events concerning the Antarctic ozone hole over southern Brazil and the biological effects induced by UVB and UVA radiation in an endemic treefrog species

The increased incidence of solar ultraviolet radiation (UV) due to ozone depletion has been affecting both terrestrial and aquatic ecosystems and it may help to explain the enigmatic decline of amphibian populations in specific localities. In this work, influential events concerning the Antarctic ozone hole were identified in a dataset containing 35 years of ozone measurements over southern Brazil. The effects of environmental doses of UVB and UVA radiation were addressed on the morphology and development of Hypsiboas pulchellus tadpole (Anura: Hylidae), as well as on the induction of malformation after the conclusion of metamorphosis. These analyzes were complemented by the detection of micronucleus formation in blood cells. 72 ozone depletion events were identified from 1979 to 2013. Surprisingly, their yearly frequency increased three-fold during the last 17 years. The results clearly show that H. pulchellus tadpole are much more sensitive to UVB than UVA light, which reduces their survival and developmental rates. Additionally, the rates of micronucleus formation by UVB were considerably higher compared to UVA even after the activation of photolyases enzymes by a further photoreactivation treatment. Consequently, a higher occurrence of malformation was observed in UVB-irradiated individuals. These results demonstrate the severe genotoxic impact of UVB radiation on this treefrog species and its importance for further studies aimed to assess the impact of the increased levels of solar UVB radiation on declining species of the Hylidae family.

Replication stress in Mammalian cells and its consequences for mitosis

The faithful transmission of genetic information to daughter cells is central to maintaining genomic stability and relies on the accurate and complete duplication of genetic material during each cell cycle. However, the genome is routinely exposed to endogenous and exogenous stresses that can impede the progression of replication. Such replication stress can be an early cause of cancer or initiate senescence. Replication stress, which primarily occurs during S phase, results in consequences during mitosis, jeopardizing chromosome segregation and, in turn, genomic stability. The traces of replication stress can be detected in the daughter cells during G1 phase. Alterations in mitosis occur in two types: 1) local alterations that correspond to breaks, rearrangements, intertwined DNA molecules or non-separated sister chromatids that are confined to the region of the replication dysfunction; 2) genome-wide chromosome segregation resulting from centrosome amplification (although centrosomes do not contain DNA), which amplifies the local replication stress to the entire genome. Here, we discuss the endogenous causes of replication perturbations, the mechanisms of replication fork restart and the consequences for mitosis, chromosome segregation and genomic stability.

Red light interferes in UVA-induced photoaging of human skin fibroblast cells

The possible regulation mechanism of red light was determined to discover how to retard UVA-induced skin photoaging. Human skin fibroblasts were cultured and irradiated with different doses of UVA, thus creating a photoaging model. Fibroblasts were also exposed to a subtoxic dose of UVA combined with a red light-emitting diode (LED) for five continuous days. Three groups were examined: control, UVA and UVA plus red light. Cumulative exposure doses of UVA were 25 J cm(-2), and the total doses of red light were 0.18 J cm(-2). Various indicators were measured before and after irradiation, including cell morphology, viability, β-galactosidase staining, apoptosis, cycle phase, the length of telomeres and the protein levels of photoaging-related genes. Red light irradiation retarded the cumulative low-dose UVA irradiation-induced skin photoaging, decreased the expression of senescence-associated β-galactosidase, upregulated SIRT1 expression, decreased matrix metalloproteinase MMP-1 and the acetylation of p53 expression, reduced the horizon of cell apoptosis and enhanced cell viability. Furthermore, the telomeres in UVA-treated cells were shortened compared to those of cells in the red light groups. These results suggest that red light plays a key role in the antiphotoaging of human skin fibroblasts by acting on different signaling transduction pathways.

Protective Role of Comfrey Leave Extracts on UV-induced Zebrafish Fin Damage

In zebrafish, UV exposure leads to fin malformation phenotypes including fin reduction or absence. The present study evaluated UV-protective activities of comfrey leaves extracts in a zebrafish model by recording fin morphological changes. Chemopreventive effects of comfrey leave extracts were evaluated using Kaplan-Meier analysis and Cox proportional hazards regression. The results showed that (1) the mean times of return to normal fin in the UV+comfrey (50 and 100 ppm) groups were 3.43 and 2.86 days and were quicker compared with that in the UV only group (4.21 days); (2) zebrafish fins in the UV+comfrey (50 and 100 ppm) groups were 2.05 and 3.25 times more likely to return to normal than those in the UV only group; and (3) comfrey leave extracts had UV-absorbance abilities and significantly reduced ROS production in UV-exposed zebrafish embryos, which may attenuate UV-mediated apoptosis. In conclusion, comfrey leaves extracts may have the potential to be developed as UV-protective agents to protect zebrafish embryos from UV-induced damage.

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.

Cyclobutane Pyrimidine Dimer Density as a Predictive Biomarker of the Biological Effects of Ultraviolet Radiation in Normal Human Fibroblast

This study compared biological responses of normal human fibroblasts (NHF1) to three sources of ultraviolet radiation (UVR), emitting UVC wavelengths, UVB wavelengths, or a combination of UVA and UVB (solar simulator; emission spectrum, 94.3% UVA and 5.7% UVB). The endpoints measured were cytotoxicity, intra-S checkpoint activation, inhibition of DNA replication and mutagenicity. Results show that the magnitude of each response to the indicated radiation sources was best predicted by the density of DNA cyclobutane pyrimidine dimers (CPD). The density of 6-4 pyrimidine-pyrimidone photoproducts was highest in DNA from UVC-irradiated cells (14% of CPD) as compared to those exposed to UVB (11%) or UVA-UVB (7%). The solar simulator source, under the experimental conditions described here, did not induce the formation of 8-oxo-7,8-dihydroguanine in NHF1 above background levels. Taken together, these results suggest that CPD play a dominant role in DNA damage responses and highlight the importance of using endogenous biomarkers to compare and report biological effects induced by different sources of UVR.

Investigation of the phototoxicity and cytotoxicity of naproxen, a non-steroidal anti-inflammatory drug, in human fibroblasts

Non-steroidal anti-inflammatory drugs (NSAID) are widely used in the treatment of pain and inflammation associated with several diseases. Naproxen, 2-(6-methoxy-2-naphthyl) propionic acid (NAP), belongs to this pharmacological class and appears to be associated with a high incidence of both photoallergic and phototoxic reactions. In this study, using human fibroblasts, we examined the biological effects of NAP photosensitization induced by UVA, the predominant UV component of sunlight reaching the Earth's surface. We showed that NAP or UVA alone have no cytotoxic effects at the concentrations and doses used in this study. The same result was observed when cells were pre-incubated with NAP but irradiated without NAP. In marked contrast, exposure of cells in the presence of NAP led to a drastic reduction of cell viability. These results suggest that the phototoxicity is mainly due to irradiation of extracellular NAP that damages cell membranes. Moreover, we showed that NAP itself led to a low but reproducible production of reactive oxygen species (ROS), to protein modifications by lipid peroxidation-derived aldehydes, to p38 phosphorylation and to the slowing-down of DNA replication, while UVA treatment alone showed no effects. NAP photosensitization with UVA led to protein S-glutathionylation, oxidation of the proliferating cell nuclear antigen (PCNA), oxidation of cellular tryptophan, phosphorylation of Chk1 and inhibition of DNA replication. However, using small interfering RNA to down regulate Chk1 expression in cells, we showed that Chk1 is not required to slow the S-phase down. Nevertheless, inhibition of Chk1, but not of p38, sensitized the cells to the phototoxic effects of NAP. Collectively, our data suggest that the interaction of NAP with the cells triggers oxidative damage and a replication stress, which are exacerbated by UVA radiation. As oxidative and replication stress-induced genome instability are important factors in aging and tumor predisposition, it is of interest to evaluate the consequence of a non-steroidal anti-inflammatory drug, like naproxen, on genomic instability.

Oxidative stress in mammalian cells impinges on the cysteines redox state of human XRCC3 protein and on its cellular localization

In vertebrates, XRCC3 is one of the five Rad51 paralogs that plays a central role in homologous recombination (HR), a key pathway for maintaining genomic stability. While investigating the potential role of human XRCC3 (hXRCC3) in the inhibition of DNA replication induced by UVA radiation, we discovered that hXRCC3 cysteine residues are oxidized following photosensitization by UVA. Our in silico prediction of the hXRCC3 structure suggests that 6 out of 8 cysteines are potentially accessible to the solvent and therefore potentially exposed to ROS attack. By non-reducing SDS-PAGE we show that many different oxidants induce hXRCC3 oxidation that is monitored in Chinese hamster ovarian (CHO) cells by increased electrophoretic mobility of the protein and in human cells by a slight decrease of its immunodetection. In both cell types, hXRCC3 oxidation was reversed in few minutes by cellular reducing systems. Depletion of intracellular glutathione prevents hXRCC3 oxidation only after UVA exposure though depending on the type of photosensitizer. In addition, we show that hXRCC3 expressed in CHO cells localizes both in the cytoplasm and in the nucleus. Mutating all hXRCC3 cysteines to serines (XR3/S protein) does not affect the subcellular localization of the protein even after exposure to camptothecin (CPT), which typically induces DNA damages that require HR to be repaired. However, cells expressing mutated XR3/S protein are sensitive to CPT, thus highlighting a defect of the mutant protein in HR. In marked contrast to CPT treatment, oxidative stress induces relocalization at the chromatin fraction of both wild-type and mutated protein, even though survival is not affected. Collectively, our results demonstrate that the DNA repair protein hXRCC3 is a target of ROS induced by environmental factors and raise the possibility that the redox environment might participate in regulating the HR pathway.

Telomere shortening in cultured human dermal fibroblasts is associated with acute photodamage induced by UVA irradiation

Introduction

Photoaging is the superposition of chronic ultraviolet (UV)-induced damage on intrinsic aging. Telomere length is a molecular marker of cell aging, and genomic instability due to telomere shortening has been linked to several aging-related diseases.

Aim

To explore the effects of different doses of ultraviolet A (UVA) on the length of telomeres in human skin fibroblasts and partly reveal the mechanism of skin photoaging initiated by UVA irradiation.

Material and methods

Primary cultured human skin fibroblasts were irradiated with different doses of UVA light. Cell viability, cell cycle phase, β-galactosidase, and the length of telomeres were assessed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, flow cytometry, cytochemical staining, and real-time polymerase chain reactions, respectively.

Results

After UVA irradiation, inhibited proliferation, S phase accumulation and increased expression of senescence-associated β-galactosidase were observed in cultured fibroblasts. Moreover, the length of telomeres in UVA-treated cells was shortened in a dose-dependent manner as compared to controls (p < 0.05).

Conclusions

These results suggest that telomere length in human dermal fibroblasts can be shortened by a single high dosage of UVA radiation, and that acute photodamage might contribute to early photoaging in human skin via rapid telomere shortening. This study potentially provides the basis for better understanding of the molecular mechanism of photoaging.

Stress activated protein kinase pathway modulates homologous recombination in fission yeast

Rad52 is a key player in homologous recombination (HR), a DNA repair pathway that is dedicated to double strand breaks repair and recovery of perturbed replication forks. Here we show that fission yeast Rad52 homologue is phosphorylated when S phase cells are exposed to ROS inducers such as ultraviolet A radiation or hydrogen peroxide, but not to ultraviolet C or camptothecin. Phosphorylation does not depend on kinases Chk1, Rad3, Tel1 or Cdc2, but depends on a functional stress activated protein kinase (SAPK) pathway and can be partially prevented by anti-oxidant treatment. Indeed, cells lacking Sty1, the major fission yeast MAP kinase of the SAPK pathway, do not display Rad52 phosphorylation and have UVA induced Rad52 foci that persist longer if compared to wild type cells. In addition, spontaneous intrachromosomal HR is diminished in cells lacking Sty1 and, more precisely, gene conversion is affected. Moreover, HR induced by site-specific arrest of replication forks is twice less efficient in cells that do not express Sty1. Importantly, impairing HR by deletion of the gene encoding the recombinase Rhp51 leads to Sty1 dependent Rad52 phosphorylation. Thus, SAPK pathway impinges on early step of HR through phosphorylation of Rad52 in cells challenged by oxidative stress or lacking Rhp51 and is required to promote spontaneous gene conversion and recovery from blocked replication forks.

The effects of UV emission from compact fluorescent light exposure on human dermal fibroblasts and keratinocytes in vitro

Compact fluorescent light (CFL) bulbs can provide the same amount of lumens as incandescent light bulbs, using one quarter of the energy. Recently, CFL exposure was found to exacerbate existing skin conditions; however, the effects of CFL exposure on healthy skin tissue have not been thoroughly investigated. In this study, we studied the effects of exposure to CFL illumination on healthy human skin tissue cells (fibroblasts and keratinocytes). Cells exposed to CFLs exhibited a decrease in the proliferation rate, a significant increase in the production of reactive oxygen species, and a decrease in their ability to contract collagen. Measurements of UV emissions from these bulbs found significant levels of UVC and UVA (mercury [Hg] emission lines), which appeared to originate from cracks in the phosphor coatings, present in all bulbs studied. The response of the cells to the CFLs was consistent with damage from UV radiation, which was further enhanced when low dosages of TiO(2) nanoparticles (NPs), normally used for UV absorption, were added prior to exposure. No effect on cells, with or without TiO(2) NPs, was observed when they were exposed to incandescent light of the same intensity.

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.

Nonautonomous apoptosis is triggered by local cell cycle progression during epithelial replacement in Drosophila

Tissue remodeling involves collective cell movement, and cell proliferation and apoptosis are observed in both development and disease. Apoptosis and proliferation are considered to be closely correlated, but little is known about their coordinated regulation in physiological tissue remodeling in vivo. The replacement of larval abdominal epidermis with adult epithelium in Drosophila pupae is a simple model of tissue remodeling. During this process, larval epidermal cells (LECs) undergo apoptosis and are replaced by histoblasts, which are adult precursor cells. By analyzing caspase activation at the single-cell level in living pupae, we found that caspase activation in LECs is induced at the LEC/histoblast boundary, which expands as the LECs die. Manipulating histoblast proliferation at the LEC/histoblast boundary, either genetically or by UV illumination, indicated that local interactions with proliferating histoblasts triggered caspase activation in the boundary LECs. Finally, by monitoring the spatiotemporal dynamics of the S/G₂/M phase in histoblasts in vivo, we found that the transition from S/G₂ phases is necessary to induce nonautonomous LEC apoptosis at the LEC/histoblast boundary. The replacement boundary, formed as caspase activation is regulated locally by cell-cell communication, may drive the dynamic orchestration of cell replacement during tissue remodeling.

TAp73 induction by nitric oxide: regulation by checkpoint kinase 1 (CHK1) and protection against apoptosis

Nitric oxide (NO) is a potent activator of the p53 tumor suppressor protein, thereby inducing cell cycle arrest and apoptosis. However, little is known about the regulation of the two other p53-family members, p63 and p73, by nitrogen oxides. We report here an up-regulation of p73 by NO in p53-null K-562 leukemia cells. Chemical NO prodrugs or macrophage iNOS activity induced an accumulation of the TAp73α isoform in these cells, whereas macrophages from iNOS(-/-) mice did not. NO also up-regulated TAp73 mRNA expression, suggesting a transcriptional regulation. The checkpoint kinases Chk1 and Chk2 can regulate TAp73 induction after DNA damage. We show that these kinases were rapidly phosphorylated upon NO treatment. Genetic silencing or pharmacological inhibition of Chk1 impaired NO-mediated accumulation of TAp73α. Because NO is known to block DNA synthesis through ribonucleotide reductase inhibition, the up-regulation of TAp73α might be caused by DNA damage induced by an arrest of DNA replication forks. In support of this hypothesis, DNA replication inhibitors such as hydroxyurea and aphidicolin similarly enhanced TAp73α expression and Chk1 phosphorylation. Moreover, inhibition of Chk1 also prevented TAp73α accumulation in response to replication inhibitors. The knockdown of TAp73 with siRNA sensitized K-562 cells to apoptosis induced by a nitrosative (NO) or oxidative (H(2)O(2)) injury. Therefore, TAp73α has an unusual cytoprotective role in K-562 cells, contrasting with its pro-apoptotic functions in many other cell models. In conclusion, NO up-regulates several p53 family members displaying pro- and anti-apoptotic effects, suggesting a complex network of interactions and cross-regulations between NO production and p53-related proteins.

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

Longwave UVA is an independent class I carcinogen. A complete understanding of UVA-induced DNA damage and how this damage is processed in skin cells is therefore of utmost importance. A particular question that has remained contentious is whether UVA induces DNA double-strand breaks (DSBs), either directly or through processing of other types of DNA damage, such as recombination repair of replication forks stalled at DNA photoproducts. We therefore studied activation of the recombination repair pathway by solar available doses of UVA and assessed formation of DNA DSBs in primary skin fibroblasts. We found that, unlike ionizing radiation or UVB, UVA does not activate the Fanconi anemia/BRCA DNA damage response pathway or the "recombinase" RAD51 in primary skin fibroblasts. The fact that this pathway mediates recombination repair of DNA DSBs suggests that DNA DSBs are not formed by UVA. This is further supported by findings that UVA did not induce DNA DSBs, as assayed by neutral single-cell electrophoresis or by formation of γ-H2AX nuclear foci, considered the most sensitive assay for DNA DSBs. The lack of sufficient evidence for formation of DNA DSBs underlines the pivotal role of UVA-induced DNA photoproducts in UVA mutagenesis and carcinogenesis.

The human intra-S checkpoint response to UVC-induced DNA damage

The intra-S checkpoint response to 254 nm light (UVC)-induced DNA damage appears to have dual functions to slow the rate of DNA synthesis and stabilize replication forks that become stalled at sites of UVC-induced photoproducts in DNA. These functions should provide more time for repair of damaged DNA before its replication and thereby reduce the frequencies of mutations and chromosomal aberrations in surviving cells. This review tries to summarize the history of discovery of the checkpoint, the current state of understanding of the biological features of intra-S checkpoint signaling and its mechanisms of action with a focus primarily on intra-S checkpoint responses in human cells. The differences in the intra-S checkpoint responses to UVC and ionizing radiation-induced DNA damage are emphasized. Evidence that [6-4]pyrimidine-pyrimidone photoproducts in DNA trigger the response is discussed and the relationships between cellular responses to UVC and the molecular dose of UVC-induced DNA damage are briefly summarized. The role of the intra-S checkpoint response in protecting against solar radiation carcinogenesis remains to be determined.

Fbh1 limits Rad51-dependent recombination at blocked replication forks

Controlling the loading of Rad51 onto DNA is important for governing when and how homologous recombination is used. Here we use a combination of genetic assays and indirect immunofluorescence to show that the F-box DNA helicase (Fbh1) functions in direct opposition to the Rad52 orthologue Rad22 to curb Rad51 loading onto DNA in fission yeast. Surprisingly, this activity is unnecessary for limiting spontaneous direct-repeat recombination. Instead it appears to play an important role in preventing recombination when replication forks are blocked and/or broken. When overexpressed, Fbh1 specifically reduces replication fork block-induced recombination, as well as the number of Rad51 nuclear foci that are induced by replicative stress. These abilities are dependent on its DNA helicase/translocase activity, suggesting that Fbh1 exerts its control on recombination by acting as a Rad51 disruptase. In accord with this, overexpression of Fbh1 also suppresses the high levels of recombinant formation and Rad51 accumulation at a site-specific replication fork barrier in a strain lacking the Rad51 disruptase Srs2. Similarly overexpression of Srs2 suppresses replication fork block-induced gene conversion events in an fbh1Delta mutant, although an inability to suppress deletion events suggests that Fbh1 has a distinct functionality, which is not readily substituted by Srs2.