CPDs and 6-4PPs play different roles in UV-induced cell death in normal and NER-deficient human cells. DNA Repair (Amst). 2008;7:303-312. [PMID: 18096446 DOI: 10.1016/j.dnarep.2007.11.003]

Efficacy of Combinational Treatment versus Nicotinamide Monotherapy in the Prevention of Ultraviolet Radiation-Induced Skin Cancer

INTRODUCTION

Ultraviolet radiation (UVR) is the primary risk factor for keratinocyte carcinomas. Oral supplementation with nicotinamide (NAM) is reported to reduce the formation of new keratinocyte carcinomas. NAM's photoprotection is mediated by enhanced DNA repair. We wanted to explore whether NAM in combination with antiproliferative (metformin [Met]) or antioxidant (phloroglucinol [PG]) compounds could potentially enhance its photoprotective effects.

METHODS

Hairless mice (C3.Cg-Hrhr/TifBomTac) were treated orally with either a standard dose of NAM monotherapy (NAM-mono; 600 mg/kg) or NAM (400 mg/kg) combined with Met (200 mg/kg) (NAM-Met) or PG (75 mg/kg) (NAM-PG). Mice were irradiated with 3.5 standard erythema doses of UVR three times per week to induce tumour development. Photoprotective effects were based on (i) tumour onset of the first three tumours, (ii) skin photodamage, and (iii) DNA damage (cyclobutane pyrimidine dimers [CPDs] and pyrimidine-pyrimidone (6-4) photoproducts [6-4PPs]).

RESULTS

All mice treated with NAM demonstrated a delay in tumour onset and reduced tumour burden compared to the UV control group (NAM, NAM-Met, NAM-PG vs. UV control: p ≤ 0.015). NAM-mono and NAM-PG increased time until all three tumours with no difference between them, indicating a similar degree of photoprotection. NAM-mono had no effect on DNA damage compared to the UV control group (p > 0.05), whereas NAM-PG reduced 6-4PP lesions (p < 0.01) but not CPDs (p > 0.05) compared to NAM-mono. NAM-Met delayed the onset of the third tumour compared to the UV control but demonstrated a quicker onset compared to NAM-mono, suggesting inferior photoprotection compared to nicotinamide monotherapy.

CONCLUSION

NAM-PG was as effective in delaying UVR-induced tumour onset as NAM-mono. The reduction in 6-4PP lesions may indicate that the mechanism of NAM-PG is better suited for photoprotection than NAM-mono. NAM-mono was superior to NAM-Met, indicating a dose dependency of NAM's photoprotection. These results highlight the potential for combining photoprotective compounds to enhance photoprotection.

α-Tocopherol application as a countermeasure to UV-B stress in bread wheat (Triticum aestivum L.)

The source of energy for all photoautotrophic organisms is light, which is absorbed by photosynthetic processes and used to transform carbon dioxide and H2O into organic molecules. The majority of UV-B light (280 to 320 nm) is absorbed by stratospheric ozone layer, although some of it does reach at the Earth's surface. Because of the sedentary lifestyle of plants, this form of abiotic stress is unavoidable and can induce growth and even cell death. Ten-day-old calli generated from mature Kirik wheat embryos were subjected to UV-B radiation for 0, 2, 4, and 6 h to examine the function of exogenous α-tocopherol, a lipophilic antioxidant, in wheat tolerance to UV-B radiation stress. The calli were then moved to a callus medium containing α-tocopherol (0, 50, and 100 mg/l) and cultivated there for 20 days after being subjected to UV-B stress. For plant regeneration, embryogenic calli were put on a medium for plant regeneration after 30 days. The findings of this investigation demonstrated that an increase in UV-B exposure period resulted in a substantial drop in the relative growth rate of callus, the rate of embryogenic callus, the rate of responding embryogenic callus, and the number of plants in each explant. On the other hand, with the application of α-tocopherol, all these parameters improved, and the best result was observed in the application of 100 mg/l of α-tocopherol in terms of plant regeneration under UV-B stress.

Surface plasmon resonance detection of UV irradiation-induced DNA damage and photoenzymatic repair processes through specific interaction between consensus double-stranded DNA and p53 protein

DNA damage, such as DNA lesions and strand breaks, impairs normal cell functions and failure in the DNA repair process could lead to gene mutation, cell apoptosis and disease occurrence. p53 is a tumor suppressor and DNA-binding protein, and DNA damage might affect their interaction and the subsequent p53 function. Herein, real-time monitoring of DNA damage and repair processes through DNA-p53 protein interaction was performed by surface plasmon resonance (SPR). The target DNA with consecutive pyrimidine nucleobases was first damaged upon UVC (254 nm) irradiation and then photoenzymatically repaired under UVA (365 nm) irradiation. The as-formed double-stranded (ds) DNA between probe DNA and normal, damaged or repaired target DNA was immobilized on the sensor chips, followed by the injection of p53 protein. By measuring the SPR signals under different cases, the DNA damage and repair processes could be conveniently monitored. The SPR signals were inversely proportional to the UVC doses ranging from 0.021 to 1.26 kJ m^-2, providing a viable means for the quantification of the DNA damage level. The binding affinity between p53 and the dsDNA formed upon the hybridization of probe DNA and normal, damaged, or photoenzymatically repaired target DNA was estimated. This is the first report on measuring the equilibrium dissociation constant (K_D) between the p53 protein and the dsDNA with photodamaged or repaired target sequences. The sensing strategy by SPR thus opens a new avenue for real-time measurement of the DNA damage and the repair processes.

Preparation of CPD Photolyase Nanoliposomes Derived from Antarctic Microalgae and Their Effect on UVB-Induced Skin Damage in Mice

UVB radiation is known to trigger the block of DNA replication and transcription by forming cyclobutane pyrimidine dimer (CPD), which results in severe skin damage. CPD photolyase, a kind of DNA repair enzyme, can efficiently repair CPDs that are absent in humans and mice. Although exogenous CPD photolyases have beneficial effects on skin diseases, the mechanisms of CPD photolyases on the skin remain unknown. Here, this study prepared CPD photolyase nanoliposomes (CPDNL) from Antarctic Chlamydomonas sp. ICE-L, which thrives in harsh, high-UVB conditions, and evaluated their protective mechanisms against UVB-induced damage in mice. CPDNL were optimized using response surface methodology, characterized by a mean particle size of 105.5 nm, with an encapsulation efficiency of 63.3%. Topical application of CPDNL prevented UVB-induced erythema, epidermal thickness, and wrinkles in mice. CPDNL mitigated UVB-induced DNA damage by significantly decreasing the CPD concentration. CPDNL exhibited antioxidant properties as they reduced the production of reactive oxygen species (ROS) and malondialdehyde. Through activation of the NF-κB pathway, CPDNL reduced the expression of pro-inflammatory cytokines including IL-6, TNF-α, and COX-2. Furthermore, CPDNL suppressed the MAPK signaling activation by downregulating the mRNA and protein expression of ERK, JNK, and p38 as well as AP-1. The MMP-1 and MMP-2 expressions were also remarkably decreased, which inhibited the collagen degradation. Therefore, we concluded that CPDNL exerted DNA repair, antioxidant, anti-inflammation, and anti-wrinkle properties as well as collagen protection via regulation of the NF-κB/MAPK/MMP signaling pathways in UVB-induced mice, demonstrating that Antarctic CPD photolyases have the potential for skincare products against UVB and photoaging.

XPG in the Nucleotide Excision Repair and Beyond: a study on the different functional aspects of XPG and its associated diseases

Several proteins are involved in DNA repair mechanisms attempting to repair damages to the DNA continuously. One such protein is Xeroderma Pigmentosum Complementation Group G (XPG), a significant component in the Nucleotide Excision Repair (NER) pathway. XPG is accountable for making the 3' incision in the NER, while XPF-ERCC4 joins ERCC1 to form the XPF-ERCC1 complex. This complex makes a 5' incision to eliminate bulky DNA lesions. XPG is also known to function as a cofactor in the Base Excision Repair (BER) pathway by increasing hNth1 activity, apart from its crucial involvement in the NER. Reports suggest that XPG also plays a non-catalytic role in the Homologous Recombination Repair (HRR) pathway by forming higher-order complexes with BRCA1, BRCA2, Rad51, and PALB2, further influencing the activity of these molecules. Studies show that, apart from its vital role in repairing DNA damages, XPG is also responsible for R-loop formation, which facilitates exhibiting phenotypes of Werner Syndrome. Though XPG has a role in several DNA repair pathways and molecular mechanisms, it is primarily a NER protein. Unrepaired and prolonged DNA damage leads to genomic instability and facilitates neurological disorders, aging, pigmentation, and cancer susceptibility. This review explores the vital role of XPG in different DNA repair mechanisms which are continuously involved in repairing these damaged sites and its failure leading to XP-G, XP-G/CS complex phenotypes, and cancer progression.

Photorepair of Either CPD or 6-4PP DNA Lesions in Basal Keratinocytes Attenuates Ultraviolet-Induced Skin Effects in Nucleotide Excision Repair Deficient Mice

Ultraviolet (UV) radiation is one of the most genotoxic, universal agents present in the environment. UVB (280-315 nm) radiation directly damages DNA, producing cyclobutane pyrimidine dimers (CPDs) and pyrimidine 6-4 pyrimidone photoproducts (6-4PPs). These photolesions interfere with essential cellular processes by blocking transcription and replication polymerases, and may induce skin inflammation, hyperplasia and cell death eventually contributing to skin aging, effects mediated mainly by keratinocytes. Additionally, these lesions may also induce mutations and thereby cause skin cancer. Photolesions are repaired by the Nucleotide Excision Repair (NER) pathway, responsible for repairing bulky DNA lesions. Both types of photolesions can also be repaired by distinct (CPD- or 6-4PP-) photolyases, enzymes that specifically repair their respective photolesion by directly splitting each dimer through a light-dependent process termed photoreactivation. However, as photolyases are absent in placental mammals, these organisms depend solely on NER for the repair of DNA UV lesions. However, the individual contribution of each UV dimer in the skin effects, as well as the role of keratinocytes has remained elusive. In this study, we show that in NER-deficient mice, the transgenic expression and photorepair of CPD-photolyase in basal keratinocytes completely inhibited UVB-induced epidermal thickness and cell proliferation. On the other hand, photorepair by 6-4PP-photolyase in keratinocytes reduced but did not abrogate these UV-induced effects. The photolyase mediated removal of either CPDs or 6-4PPs from basal keratinocytes in the skin also reduced UVB-induced apoptosis, ICAM-1 expression, and myeloperoxidase activation. These findings indicate that, in NER-deficient rodents, both types of photolesions have causal roles in UVB-induced epidermal cell proliferation, hyperplasia, cell death and inflammation. Furthermore, these findings also support the notion that basal keratinocytes, instead of other skin cells, are the major cellular mediators of these UVB-induced effects.

Simultaneously monitoring UVC-induced DNA damage and photoenzymatic repair of cyclobutane pyrimidine dimers by electrochemical impedance spectroscopy

Cyclobutane pyrimidine dimers (CPDs) are the major DNA photoproducts of thymine-thymine dinucleotides upon ultraviolet (UV) irradiation. Failure in the repair of damaged DNA may lead to DNA replication errors, DNA mutations, and even cell death. Photoreactivation can mediate the repair of UV-induced DNA lesions by photolyases upon UVA (315-400 nm) or blue light (400-500 nm) irradiation. Herein, the UVC (254 nm)-induced DNA damage and photoenzymatic repair of the CPD products were simultaneously monitored by electrochemical impedance spectroscopy (EIS). The UVC-damaged dT₂₀ was first immobilized on the gold electrode, and the specific recognition by the anti-CPD antibody leads to significantly increased EIS signals. The electron transfer resistance (R_et) values were linearly proportional to the concentrations of damaged dT₂₀ ranging from 0.005 to 0.1 μM, and a detection limit of 3.06 nM was achieved. Using surface plasmon resonance, the equilibrium dissociation constant (K_D) between the CPDs in dT₂₀ and anti-CPD antibody was estimated to be (3.32 ± 0.31) × 10^-12 M, indicating the strong binding affinity. Evidenced by EIS, the CPDs in the damaged dT₂₀ could be repaired by the attached DNA photolyase under UVA (365 nm) photoexcitation, and the detachment of the photolyase from the DNA strand was accomplished after completion of the repair process. The repair efficiency was calculated to be 70.0% by EIS, being consistent with that of 71.4% by UV spectroscopy. The electrochemical method is simple, sensitive and straightforward, holding great potential for assaying other types of DNA lesions and their repair processes.

On Broken Ne(c)ks and Broken DNA: The Role of Human NEKs in the DNA Damage Response

NIMA-related kinases, or NEKs, are a family of Ser/Thr protein kinases involved in cell cycle and mitosis, centrosome disjunction, primary cilia functions, and DNA damage responses among other biological functional contexts in vertebrate cells. In human cells, there are 11 members, termed NEK1 to 11, and the research has mainly focused on exploring the more predominant roles of NEKs in mitosis regulation and cell cycle. A possible important role of NEKs in DNA damage response (DDR) first emerged for NEK1, but recent studies for most NEKs showed participation in DDR. A detailed analysis of the protein interactions, phosphorylation events, and studies of functional aspects of NEKs from the literature led us to propose a more general role of NEKs in DDR. In this review, we express that NEK1 is an activator of ataxia telangiectasia and Rad3-related (ATR), and its activation results in cell cycle arrest, guaranteeing DNA repair while activating specific repair pathways such as homology repair (HR) and DNA double-strand break (DSB) repair. For NEK2, 6, 8, 9, and 11, we found a role downstream of ATR and ataxia telangiectasia mutated (ATM) that results in cell cycle arrest, but details of possible activated repair pathways are still being investigated. NEK4 shows a connection to the regulation of the nonhomologous end-joining (NHEJ) repair of DNA DSBs, through recruitment of DNA-PK to DNA damage foci. NEK5 interacts with topoisomerase IIβ, and its knockdown results in the accumulation of damaged DNA. NEK7 has a regulatory role in the detection of oxidative damage to telomeric DNA. Finally, NEK10 has recently been shown to phosphorylate p53 at Y327, promoting cell cycle arrest after exposure to DNA damaging agents. In summary, this review highlights important discoveries of the ever-growing involvement of NEK kinases in the DDR pathways. A better understanding of these roles may open new diagnostic possibilities or pharmaceutical interventions regarding the chemo-sensitizing inhibition of NEKs in various forms of cancer and other diseases.

Inhibitors of Nucleotide Excision Repair Decrease UVB-Induced Mutagenesis-An In Vitro Study

The high incidence of skin cancers in the Caucasian population is primarily due to the accumulation of DNA damage in epidermal cells induced by chronic ultraviolet B (UVB) exposure. UVB-induced DNA photolesions, including cyclobutane–pyrimidine dimers (CPDs), promote mutations in skin cancer driver genes. In humans, CPDs are repaired by nucleotide excision repair (NER). Several commonly used and investigational medications negatively influence NER in experimental systems. Despite these molecules’ ability to decrease NER activity in vitro, the role of these drugs in enhancing skin cancer risk is unclear. In this study, we investigated four molecules (veliparib, resveratrol, spironolactone, and arsenic trioxide) with well-known NER-inhibitory potential in vitro, using UVB-irradiated CHO epithelial and HaCaT immortalized keratinocyte cell lines. Relative CPD levels, hypoxanthine phosphoribosyltransferase gene mutation frequency, cell viability, cell cycle progression, and protein expression were assessed. All four molecules significantly elevated CPD levels in the genome 24 h after UVB irradiation. However, veliparib, spironolactone, and arsenic trioxide reduced the mutagenic potential of UVB, while resveratrol did not alter UVB-induced mutation formation. UVB-induced apoptosis was enhanced by spironolactone and arsenic-trioxide treatment, while veliparib caused significantly prolonged cell cycle arrest and increased autophagy. Spironolactone also enhanced the phosphorylation level of mammalian target of rapamycin (mTOR), while arsenic trioxide modified UVB-driven mitochondrial fission. Resveratrol induced only mild changes in the cellular UVB response. Our results show that chemically inhibited NER does not result in increased mutagenic effects. Furthermore, the UVB-induced mutagenic potential can be paradoxically mitigated by NER-inhibitor molecules. We identified molecular changes in the cellular UVB response after NER-inhibitor treatment, which may compensate for the mitigated DNA repair. Our findings show that metabolic cellular response pathways are essential to consider in evaluating the skin cancer risk–modifying effects of pharmacological compounds.

Dynamics of caspase activation upon UV induced genotoxic injury

PURPOSE

Caspases are common mediators of cell death. Evasion of cell death including apoptosis are considered to be hallmarks of cancer. A deeper understanding of the apoptotic cascade may aid improving cancer therapies. Our aim was to characterize the progression of cell death following UV-induced genotoxic injury in a defined cell culture model.

MATERIALS AND METHODS

Hela cells were UV-irradiated with doses ranging from 0.1 to 60 mJ/cm². Cells were counted and colony forming assays were performed with caspase inhibitors.

RESULTS

In our model of HeLa cells, cells remain >90% viable until 6 hrs after UV radiation (UVR), but more than half of the cells are dead after 12 - 72 hrs after UVR. Within a dose range between 0.1 and 50 mJ/cm², viability ranges roughly between 20 and 30%. The difference between the lowest dose applied (0.1 mJ/cm²) and the other doses applied is significant, with the exception of the next higher dose of 1 mJ/cm². The activation of caspases precedes the cell death induction by several hrs. Caspase-9 starts to be activated at 1 hr after UVR followed by caspases 3, 6 and 7 which are fully active at 2 hrs after UVR while caspase-8 is fully active only 3 hrs after UVR. Most caspases are only weakly or not active at 0.1 mJ/cm² after 3 hrs, but fully active at the same time point with increased radiation doses. PARP-1, a caspase substrate, is cleaved immediately after activation of the caspases. Colony formation activity of the tumor cells decreases exponentially after UVR dropping down to < 0.01% plating efficiency at a dose of 60 mJ/cm². Interestingly, this drop in plating efficiency cannot be rescued by any of the two caspase inhibitors tested.

CONCLUSIONS

UV-induced cell death in this model involves the activation of apoptosis-related caspases, but this activation seems to be dispensable for the execution of cell death. Further experiments should clarify which mechanisms of cell death are really necessary for the execution of this type of cell death.

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

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

RHOAming Through the Nucleotide Excision Repair Pathway as a Mechanism of Cellular Response Against the Effects of UV Radiation

Typical Rho GTPases include the enzymes RhoA, Rac1, and Cdc42 that act as molecular switches to regulate essential cellular processes in eukaryotic cells such as actomyosin dynamics, cell cycle, adhesion, death and differentiation. Recently, it has been shown that different conditions modulate the activity of these enzymes, but their functions still need to be better understood. Here we examine the interplay between RhoA and the NER (Nucleotide Excision Repair) pathway in human cells exposed to UVA, UVB or UVC radiation. The results show high levels and accumulation of UV-induced DNA lesions (strand breaks and cyclobutane pyrimidine dimers, CPDs) in different cells with RhoA loss of function (LoF), either by stable overexpression of negative dominant RhoA (RhoA-N19 mutant), by inhibition with C3 toxin or by transient silencing with siRNA. Cells under RhoA LoF showed reduced levels of γH2AX, p-Chk1 (Ser345) and p-p53 (Ser15) that reflected causally in their accumulation in G1/S phases, in low survival rates and in reduced cell proliferation, also in accordance with the energy of applied UV light. Even NER-deficient cells (XPA, XPC) or DNA translesion synthesis (TLS)-deficient cells (XPV) showed substantial hypersensitivity to UV effects when previously submitted to RhoA LoF. In contrast, analyses of apoptosis, necrosis, autophagy and senescence revealed that all cells displaying normal levels of active RhoA (RhoA-GTP) are more resistant to UV-promoted cell death. This work reaffirms the role of RhoA protein signaling in protecting cells from damage caused by UV radiation and demonstrates relevant communicating mechanisms between actin cytoskeleton and genomic stability.

The 6-4 photoproduct is the trigger of UV-induced replication blockage and ATR activation

The most prevalent human carcinogen is sunlight-associated ultraviolet (UV), a physiologic dose of which generates thousands of DNA lesions per cell, mostly of two types: cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts (6-4PPs). It has not been possible, in living cells, to precisely characterize the respective contributions of these two lesion types to the signals that regulate cell cycle progression, DNA replication, and cell survival. Here we coupled multiparameter flow cytometry with lesion-specific photolyases that eliminate either CPDs or 6-4PPs and determined their respective contributions to DNA damage responses. Strikingly, only 6-4PP lesions activated the ATR-Chk1 DNA damage response pathway. Mechanistically, 6-4PPs, but not CPDs, impeded DNA replication across the genome as revealed by microfluidic-assisted replication track analysis. Furthermore, single-stranded DNA accumulated preferentially at 6-4PPs during DNA replication, indicating selective and prolonged replication blockage at 6-4PPs. These findings suggest that 6-4PPs, although eightfold fewer in number than CPDs, are the trigger for UV-induced DNA damage responses.

Investigating the efficacy of UVSE protein at repairing CPD and 6-4 pp DNA damages in human cells

UV exposure could induce carcinogenic mutation in human cells, including CPD (Cyclobutane pyrimidine dimer), and 6-4 pp (6-4 photoproduct) DNA damages. Spiting the active BER (Base Excision Repair) system of human cells, it lacks initiator glycosylase, rendering these damages to be only repaired through NER (Nucleotide Excision Repair) system. Some microorganisms such as Deinococcus radiodurans bacteria have a BER system for repairing these damages with an enzyme coded by the uvsE gene. This study evaluated the efficacy of the recombinant UVSE protein for repairing the CPD and 6-4 pp DNA damages in human cells. At the current study, the optimized sequence of the uvsE gene was synthesized and expressed in Hek293T cell line. The identity of protein was ascertained through ELISA assay and the stability of expression was measured via qPCR. The human Hek293T cells with the recombinant protein and without it were exposed to the UV light, and the repair of DNA damages was analyzed in both conditions using CPD and 6-4PP ELISA Combo Kit. The results indicated that uvsE gene was successfully colonized and expressed and expression showed to be stable. Hek293T cells with recombinant uvsE gene showed efficacy at repairing 80% of CPD and 85% of 6-4 photoproducts during one hour, and more than 95% of damages over 4 h' repair time. Considering the outcome of this study, it could be concluded that the uvsE recombinant product is highly effective at repairing both CPD and 6-4 pp damages and could be considered as a preventive agent for UV-induced skin cancers.

Fluorescently-labelled CPD and 6-4PP photolyases: new tools for live-cell DNA damage quantification and laser-assisted repair

UV light induces cyclobutane pyrimidine dimers (CPDs) and pyrimidine-pyrimidone (6-4) photoproducts (6-4PPs), which can result in carcinogenesis and aging, if not properly repaired by nucleotide excision repair (NER). Assays to determine DNA damage load and repair rates are invaluable tools for fundamental and clinical NER research. However, most current assays to quantify DNA damage and repair cannot be performed in real time. To overcome this limitation, we made use of the damage recognition characteristics of CPD and 6-4PP photolyases (PLs). Fluorescently-tagged PLs efficiently recognize UV-induced DNA damage without blocking NER activity, and therefore can be used as sensitive live-cell damage sensors. Importantly, FRAP-based assays showed that PLs bind to damaged DNA in a highly sensitive and dose-dependent manner, and can be used to quantify DNA damage load and to determine repair kinetics in real time. Additionally, PLs can instantly reverse DNA damage by 405 nm laser-assisted photo-reactivation during live-cell imaging, opening new possibilities to study lesion-specific NER dynamics and cellular responses to damage removal. Our results show that fluorescently-tagged PLs can be used as a versatile tool to sense, quantify and repair DNA damage, and to study NER kinetics and UV-induced DNA damage response in living cells.

Assessing the Roles of Rho GTPases in Cell DNA Repair by the Nucleotide Excision Repair Pathway

Ultraviolet light crossing the ozone layer in the atmospheric barrier affects all forms of living beings on earth. In eukaryotic cells, the nucleotide excision repair (NER) pathway protects the DNA by removing cyclobutane pyrimidine dimers (CPDs) and 6-4-photoproduct (6-4-PP) lesions caused by ultraviolet (UV) light, allowing cells to proliferate. On the other hand, adhesion and invasion processes, primarily regulated by the typical Rho GTPases Rho, Rac, and Cdc42, are also affected by UV radiation effects. Studies focused on determining whether or not these GTPases might affect the NER pathway in different cell models are enlightening and should start with classical experimental methodologies. In this chapter we describe two methods (host cell reactivation assay, or HCR, and slot-blots for CPDs and 6-4-PPs) to assess the direct or indirect involvement of these three GTPases on the NER pathway.

Major Roles for Pyrimidine Dimers, Nucleotide Excision Repair, and ATR in the Alternative Splicing Response to UV Irradiation

We have previously found that UV irradiation promotes RNA polymerase II (RNAPII) hyperphosphorylation and subsequent changes in alternative splicing (AS). We show now that UV-induced DNA damage is not only necessary but sufficient to trigger the AS response and that photolyase-mediated removal of the most abundant class of pyrimidine dimers (PDs) abrogates the global response to UV. We demonstrate that, in keratinocytes, RNAPII is the target, but not a sensor, of the signaling cascade initiated by PDs. The UV effect is enhanced by inhibition of gap-filling DNA synthesis, the last step in the nucleotide excision repair pathway (NER), and reduced by the absence of XPE, the main NER sensor of PDs. The mechanism involves activation of the protein kinase ATR that mediates the UV-induced RNAPII hyperphosphorylation. Our results define the sequence UV-PDs-NER-ATR-RNAPII-AS as a pathway linking DNA damage repair to the control of both RNAPII phosphorylation and AS regulation.

Xeroderma Pigmentosa Group A (XPA), Nucleotide Excision Repair and Regulation by ATR in Response to Ultraviolet Irradiation

The sensitivity of Xeroderma pigmentosa (XP) patients to sunlight has spurred the discovery and genetic and biochemical analysis of the eight XP gene products (XPA-XPG plus XPV) responsible for this disorder. These studies also have served to elucidate the nucleotide excision repair (NER) process, especially the critical role played by the XPA protein. More recent studies have shown that NER also involves numerous other proteins normally employed in DNA metabolism and cell cycle regulation. Central among these is ataxia telangiectasia and Rad3-related (ATR), a protein kinase involved in intracellular signaling in response to DNA damage, especially DNA damage-induced replicative stresses. This review summarizes recent findings on the interplay between ATR as a DNA damage signaling kinase and as a novel ligand for intrinsic cell death proteins to delay damage-induced apoptosis, and on ATR's regulation of XPA and the NER process for repair of UV-induced DNA adducts. ATR's regulatory role in the cytosolic-to-nuclear translocation of XPA will be discussed. In addition, recent findings elucidating a non-NER role for XPA in DNA metabolism and genome stabilization at ds-ssDNA junctions, as exemplified in prematurely aging progeroid cells, also will be reviewed.

Translesion synthesis mechanisms depend on the nature of DNA damage in UV-irradiated human cells

Ultraviolet-induced 6-4 photoproducts (6-4PP) and cyclobutane pyrimidine dimers (CPD) can be tolerated by translesion DNA polymerases (TLS Pols) at stalled replication forks or by gap-filling. Here, we investigated the involvement of Polη, Rev1 and Rev3L (Polζ catalytic subunit) in the specific bypass of 6-4PP and CPD in repair-deficient XP-C human cells. We combined DNA fiber assay and novel methodologies for detection and quantification of single-stranded DNA (ssDNA) gaps on ongoing replication forks and postreplication repair (PRR) tracts in the human genome. We demonstrated that Rev3L, but not Rev1, is required for postreplicative gap-filling, while Polη and Rev1 are responsible for TLS at stalled replication forks. Moreover, specific photolyases were employed to show that in XP-C cells, CPD arrest replication forks, while 6-4PP are responsible for the generation of ssDNA gaps and PRR tracts. On the other hand, in the absence of Polη or Rev1, both types of lesion block replication forks progression. Altogether, the data directly show that, in the human genome, Polη and Rev1 bypass CPD and 6-4PP at replication forks, while only 6-4PP are also tolerated by a Polζ-dependent gap-filling mechanism, independent of S phase.

Diagnosis of eight groups of xeroderma pigmentosum by genetic complementation using recombinant adenovirus vectors

Because patients with xeroderma pigmentosum (XP) must avoid ultraviolet (UV) light from an early age, an early diagnosis of this disorder is essential. XP is composed of seven genetic complementation groups, XP-A to -G, and a variant type (XP-V). To establish an easy and accurate diagnosis of the eight disease groups, we constructed recombinant adenoviruses that expressed one of the XP cDNA. When fibroblasts derived from patients with XP-A, -B, -C, -D, -F or -G were infected with the adenovirus expressing XPA, XPB, XPC, XPD, XPF or XPG, respectively, and UV-C at 5-20 J/m² was irradiated, cell viability was clearly recovered by the corresponding recombinant adenoviruses. In contrast, XP-E and XP-V cells were not significantly sensitive to UV irradiation and were barely complemented by the matched recombinant adenoviruses. However, co-infection of Ad-XPA with Ad-XPE increased survival rate of XP-E cells after UV-C exposure. When XP-V cell strains, including one derived from a Japanese patient, were infected with Ad-XPV, exposed to UV-B and cultured with 1 mmol/L of caffeine, flow cytometry detected a characteristic decrease in the S phase in all the XP-V cell strains. From these results, the eight groups of XP could be differentiated by utilizing a set of recombinant adenoviruses, indicating that our procedure provides a convenient and correct diagnostic method for all the XP groups including XP-E and XP-V.

Integration and scaling of UV-B radiation effects on plants: from DNA to leaf

A process-based model integrating the effects of UV-B radiation through epidermis, cellular DNA, and its consequences to the leaf expansion was developed from key parameters in the published literature. Enhanced UV-B radiation-induced DNA damage significantly delayed cell division, resulting in significant reductions in leaf growth and development. Ambient UV-B radiation-induced DNA damage significantly reduced the leaf growth of species with high relative epidermal absorbance at longer wavelengths and average/low pyrimidine cyclobutane dimers (CPD) photorepair rates. Leaf expansion was highly dependent on the number of CPD present in the DNA, as a result of UV-B radiation dose, quantitative and qualitative absorptive properties of epidermal pigments, and repair mechanisms. Formation of pyrimidine-pyrimidone (6-4) photoproducts (6-4PP) has no effect on the leaf expansion. Repair mechanisms could not solely prevent the UV-B radiation interference with the cell division. Avoidance or effective shielding by increased or modified qualitative epidermal absorptance was required. Sustained increased UV-B radiation levels are more detrimental than short, high doses of UV-B radiation. The combination of low temperature and increased UV-B radiation was more significant in the level of UV-B radiation-induced damage than UV-B radiation alone. Slow-growing leaves were more affected by increased UV-B radiation than fast-growing leaves.

Differential responses to high- and low-dose ultraviolet-B stress in tobacco Bright Yellow-2 cells

Ultraviolet (UV)-B irradiation leads to DNA damage, cell cycle arrest, growth inhibition, and cell death. To evaluate the UV-B stress-induced changes in plant cells, we developed a model system based on tobacco Bright Yellow-2 (BY-2) cells. Both low-dose UV-B (low UV-B: 740 J m(-2)) and high-dose UV-B (high UV-B: 2960 J m(-2)) inhibited cell proliferation and induced cell death; these effects were more pronounced at high UV-B. Flow cytometry showed cell cycle arrest within 1 day after UV-B irradiation; neither low- nor high-UV-B-irradiated cells entered mitosis within 12 h. Cell cycle progression was gradually restored in low-UV-B-irradiated cells but not in high-UV-B-irradiated cells. UV-A irradiation, which activates cyclobutane pyrimidine dimer (CPD) photolyase, reduced inhibition of cell proliferation by low but not high UV-B and suppressed high-UV-B-induced cell death. UV-B induced CPD formation in a dose-dependent manner. The amounts of CPDs decreased gradually within 3 days in low-UV-B-irradiated cells, but remained elevated after 3 days in high-UV-B-irradiated cells. Low UV-B slightly increased the number of DNA single-strand breaks detected by the comet assay at 1 day after irradiation, and then decreased at 2 and 3 days after irradiation. High UV-B increased DNA fragmentation detected by the terminal deoxynucleotidyl transferase dUTP nick end labeling assay 1 and 3 days after irradiation. Caffeine, an inhibitor of ataxia telangiectasia mutated (ATM) and ataxia telangiectasia and Rad3-related (ATR) checkpoint kinases, reduced the rate of cell death in high-UV-B-irradiated cells. Our data suggest that low-UV-B-induced CPDs and/or DNA strand-breaks inhibit DNA replication and proliferation of BY-2 cells, whereas larger contents of high-UV-B-induced CPDs and/or DNA strand-breaks lead to cell death.

Site-directed Mutagenesis (Y52E) of POLH Affects Its Ability to Bypass Ultraviolet-induced DNA Lesions in HaCaT Cells

DNA polymerase eta (Pol η) is one of several Y family trans-lesion synthesis (TLS) polymerases in humans and plays an important role in maintaining genome stability after ultraviolet (UV) irradiation, as it carries out error-free TLS at sites of UV-induced lesions. We performed site-directed mutagenesis of human polymerase η gene (Y52E), confirmed by sequencing, then cloned wild-type mutant and POLH genes into the eukaryotic vector pEGFP-N1. After transfecting wild-type and mutant plasmids into HaCaT keratinocytes, we tested for UV induced cis-syn cyclobutane pyrimidine dimer (CPDs) DNA lesions, and analysed cellular viability by MTT cell proliferation assay. The results showed that CPD levels decreased both with empty vector control (EVC), wild-type POLH, and Y52E-POLH over 48 hours post UV irradiation with 0.1 mW/cm2 UVB for 15 minutes (p = 0.025). The rate in CPD reduction of mutant POLH was less than in wild-type POLH. Cell viabilities of all three groups increased over 48 hours after UV irradiation, with the increased rate in the wild-type being higher than for mutant protein (p = 0.046). We conclude that Y52E POLH has reduced capacity to bypass UV induced DNA lesions in HaCaT cells.

Extreme tolerance and developmental buffering of UV-C induced DNA damage in embryos of the annual killifish Austrofundulus limnaeus

Free-living aquatic embryos are often at risk of exposure to ultraviolet radiation (UV-R). Successful completion of embryonic development depends on efficient removal of DNA lesions, and thus many aquatic embryos have mechanisms to reverse DNA lesions induced by UV-R. However, little is known of how embryos that are able to enter embryonic dormancy may respond to UV-R exposure and subsequent DNA damage. Embryos of the annual killifish Austrofundulus limnaeus are unique among vertebrates because their normal embryonic development includes (1) a complete dispersion of embryonic blastomeres prior to formation of the definitive embryonic axis, and (2) entry into a state of metabolic depression and developmental arrest termed diapause. Here, we show that developing and diapausing embryos of A. limnaeus have exceptional tolerance of UV-C radiation and can successfully complete embryonic development after receiving substantial doses of UV-C, especially if allowed to recover in full-spectrum light. Recovery in full-spectrum light permits efficient removal of the most common type of DNA lesion induced by UV-R: cyclobutane pyrimidine dimers. Interestingly, whole-mount embryo TUNEL assays suggest that apoptosis may not be a major contributor to cell death in embryos UV-C irradiated during dispersion/reaggregation or diapause. We also observed embryo mortality to be significantly delayed by several weeks in diapausing embryos irradiated and allowed to recover in the dark. These atypical responses to UV-R induced DNA damage may be due to the unique annual killifish life history and provide insight into DNA damage repair and recognition mechanisms during embryonic dormancy.

An unexpected deamination reaction after hydrolysis of the pyrimidine (6-4) pyrimidone photoproduct

Pyrimidine (6-4) pyrimidone photoproduct (6-4PP), a common DNA photolesion formed under solar irradiation, was indicated to hydrolyze under strong basic conditions, breaking the N3-C4 bond at the 5'-thymine. The reanalysis of this reaction revealed that the resulting water adduct may not be stable as previously proposed; it readily undergoes an esterification reaction induced by the 5-OH group at 6-4PP to form a five-membered ring, eliminating a molecule of ammonia.

d-limonene prevents ultraviolet irradiation: Induced cyclobutane pyrimidine dimers in Skh1 mouse skin

AIM: To establish whether d-limonene can protect against induction of cyclobutane pyrimidine dimers (CPDs) and sunburn in ultraviolet irradiation (UVR) irradiated mouse skin.

METHODS: The d-limonene was given in 4 daily oral 20 μL aliquots at different concentrations as follows: 100%, 10% or 1% in liponate and 100% liponate as control. One day after the final d-limonene treatment, the mice were anesthetized with i.p. sodium pentobarbital and placed in boxes to allow a rectangular (2 cm × 4 cm) region of dorsal skin to be irradiated with a single, ultraviolet radiation dose of 1.5 kJ/m². Skin samples from UVR irradiated area were obtained at 5 min after UVR exposure for CPD detection, at 6 d after UVR exposure, skin samples were obtained for in situ analysis for N-myc downstream regulating gene 1 (NDRG1) (a stress response gene), proliferating cell nuclear antigen (PCNA) (an S-phase marker) and filaggrin (a barrier integrity gene). Based on immunohistochemistry staining, the number of CPD, NDRG1 and PCNA positive cells, as well as unstained cells was counted in 3 different individually selected areas and percentage of positive cells was established.

RESULTS: CPD reduction occurred as follows: liponate only-none; 1% d-limonene-54.3% reduction of CPDs; 10% d-limonene-73.4% reduction of CPDs; 100% d-limonene-86.1% reduction of CPDs, the latter equivalent to a UV dose of only 0.21 kJ/m². Sunburn was also dose-dependently reduced by d-limonene. The NDRG1 protein was strongly induced by UVR (70.0% ± 10.4% positive cells), but 1% d-limonene reduced the response to 64.6% ± 9.2%, 10% d-limonene reduced the response to 16.2% ± 3.4% and 100% d-limonene reduced the response to 6.3% ± 1.7%. Similarly, PCNA was 52.4% ± 9.9% positive in UVR exposed skin, and 1% d-limonene reduced it to 42.9% ± 8.1%, 10% d-limonene reduced it to 36.2% ± 6.7% and 100% d-limonene reduce it to 13.8% ± 3.4%. NDRG1 and PCNA were increased by d-limonene or UVR separately, but combined they produced less than either agent separately owing to the protective effect of pre-exposure to d-limonene.

CONCLUSION: Overall d-limonene acted to protect against ultraviolet B-induced DNA photodamage and sunburn in UVR exposed skin.

Detrimental effects of UV-B radiation in a xeroderma pigmentosum-variant cell line

DNA polymerase η (pol η), of the Y-family, is well known for its in vitro DNA lesion bypass ability. The most well-characterized lesion bypassed by this polymerase is the cyclobutane pyrimidine dimer (CPD) caused by ultraviolet (UV) light. Historically, cellular and whole-animal models for this area of research have been conducted using UV-C (λ=100-280 nm) owing to its ability to generate large quantities of CPDs and also the more structurally distorting 6-4 photoproduct. Although UV-C is useful as a laboratory tool, exposure to these wavelengths is generally very low owing to being filtered by stratospheric ozone. We are interested in the more environmentally relevant wavelength range of UV-B (λ=280-315 nm) for its role in causing cytotoxicity and mutagenesis. We evaluated these endpoints in both a normal human fibroblast control line and a Xeroderma pigmentosum variant cell line in which the POLH gene contains a truncating point mutation, leading to a nonfunctional polymerase. We demonstrate that UV-B has similar but less striking effects compared to UV-C in both its cytotoxic and its mutagenic effects. Analysis of the mutation spectra after a single dose of UV-B shows that a majority of mutations can be attributed to mutagenic bypass of dipyrimidine sequences. However, we do note additional types of mutations with UV-B that are not previously reported after UV-C exposure. We speculate that these differences are attributed to a change in the spectra of photoproduct lesions rather than other lesions caused by oxidative stress.

Transfection of pseudouridine-modified mRNA encoding CPD-photolyase leads to repair of DNA damage in human keratinocytes: a new approach with future therapeutic potential

UVB irradiation induces harmful photochemical reactions, including formation of Cyclobutane Pyrimidine Dimers (CPDs) in DNA. Accumulation of unrepaired CPD lesions causes inflammation, premature ageing and skin cancer. Photolyases are DNA repair enzymes that can rapidly restore DNA integrity in a light-dependent process called photoreactivation, but these enzymes are absent in humans. Here, we present a novel mRNA-based gene therapy method that directs synthesis of a marsupial, Potorous tridactylus, CPD-photolyase in cultured human keratinocytes. Pseudouridine was incorporated during in vitro transcription to make the mRNA non-immunogenic and highly translatable. Keratinocytes transfected with lipofectamine-complexed mRNA expressed photolyase in the nuclei for at least 2days. Exposing photolyase mRNA-transfected cells to UVB irradiation resulted in significantly less CPD in those cells that were also treated with photoreactivating light, which is required for photolyase activity. The functional photolyase also diminished other UVB-mediated effects, including induction of IL-6 and inhibition of cell proliferation. These results demonstrate that pseudouridine-containing photolyase mRNA is a powerful tool to repair UVB-induced DNA lesions. The pseudouridine-modified mRNA approach has a strong potential to discern cellular effects of CPD in UV-related cell biological studies. The mRNA-based transient expression of proteins offers a number of opportunities for future application in medicine.

Protective effect of a Phyllanthus orbicularis aqueous extract against UVB light in human cells

CONTEXT

One approach to protect human skin against the dangerous effects of solar ultraviolet (UV) irradiation is the use of natural products, such as photoprotectors. Phyllanthus orbicularis Kunth (Euphorbiaceae) is a Cuban endemic plant used in popular medicine. Its antigenotoxicity effect against some harmful agents has been investigated. However, the effect in ultraviolet B (UVB)-irradiated human cells has not been previously assessed.

OBJECTIVE

The protective effect of a P. orbicularis extract against UVB light-induced damage in human cells was evaluated.

MATERIALS AND METHODS

DNA repair proficient (MRC5-SV) and deficient (XP4PA, complementation group XPC) cell-lines were used. Damaging effects of UVB light were evaluated by clonogenic assay and apoptosis induction by flow cytometry techniques. The extent of DNA repair itself was determined by the removal of cyclobutane pyrimidine dimers (CPDs). The CPDs were detected and quantified by slot-blot assay.

RESULTS

Treatment of UVB-irradiated MRC5-SV cells with P. orbicularis extract increased the percentage of colony-forming cells from 36.03 ± 3.59 and 4.42 ± 1.45 to 53.14 ± 8.8 and 14.52 ± 1.97, for 400 and 600 J/m(2), respectively. A decrease in apoptotic cell population was observed in cells maintained within the extract. The P. orbicularis extract enhanced the removal of CPD from genomic DNA. The CPDs remaining were found to be about 27.7 and 1.1%, while with plant extract, treatment these values decreased to 16.1 and 0.2%, for 3 and 24 h, respectively.

DISCUSSION AND CONCLUSION

P. orbicularis aqueous extract protects human cells against UVB damage. This protective effect is through the modulation of DNA repair effectiveness.

XPA-mediated regulation of global nucleotide excision repair by ATR Is p53-dependent and occurs primarily in S-phase

Cell cycle checkpoints play an important role in regulation of DNA repair pathways. However, how the regulation occurs throughout the cell cycle remains largely unknown. Here we demonstrate that nucleotide excision repair (NER) is regulated by the ATR/p53 checkpoint via modulation of XPA nuclear import and that this regulation occurs in a cell cycle-dependent manner. We show that depletion of p53 abrogated the UV-induced nuclear translocation of XPA, while silencing of Chk1 or MAPKAP Kinase-2 (MK2) had no effect. Inhibition of p53 transcriptional activities and silencing of p53-Ser15 phosphorylation also reduced the damage-induced XPA nuclear import. Furthermore, in G1-phase cells the majority of XPA remained in the cytoplasm even after UV treatment. By contrast, while most of the XPA in S-phase cells was initially located in the cytoplasm before DNA damage, UV irradiation stimulated bulk import of XPA into the nucleus. Interestingly, the majority of XPA molecules always were located in the nucleus in G2-phase cells no matter whether the DNA was damaged or not. Consistently, the UV-induced Ser15 phosphorylation of p53 occurred mainly in S-phase cells, and removal of cyclobutane pyrimidine dimers (CPDs) was much more efficient in S-phase cells than in G1-phase cells. Our results suggest that upon DNA damage in S phase, NER could be regulated by the ATR/p53-dependent checkpoint via modulation of the XPA nuclear import process. In contrast, the nuclear import of XPA in G(1) or G(2) phase appears to be largely independent of DNA damage and p53.

Biological sensors for solar ultraviolet radiation

Solar ultraviolet (UV) radiation is widely known as a genotoxic environmental agent that affects Earth ecosystems and the human population. As a primary consequence of the stratospheric ozone layer depletion observed over the last decades, the increasing UV incidence levels have heightened the concern regarding deleterious consequences affecting both the biosphere and humans, thereby leading to an increase in scientific efforts to understand the role of sunlight in the induction of DNA damage, mutagenesis, and cell death. In fact, the various UV-wavelengths evoke characteristic biological impacts that greatly depend on light absorption of biomolecules, especially DNA, in living organisms, thereby justifying the increasing importance of developing biological sensors for monitoring the harmful impact of solar UV radiation under various environmental conditions. In this review, several types of biosensors proposed for laboratory and field application, that measure the biological effects of the UV component of sunlight, are described. Basically, the applicability of sensors based on DNA, bacteria or even mammalian cells are presented and compared. Data are also presented showing that on using DNA-based sensors, the various types of damage produced differ when this molecule is exposed in either an aqueous buffer or a dry solution. Apart from the data thus generated, the development of novel biosensors could help in evaluating the biological effects of sunlight on the environment. They also emerge as alternative tools for using live animals in the search for protective sunscreen products.

Photoprotective potential of Cordyceps polysaccharides against ultraviolet B radiation-induced DNA damage to human skin cells

BACKGROUND

Ultraviolet (UV) radiation causes DNA damage resulting in photoageing and skin cancer. UVB (290-320 nm) interacts directly with DNA, inducing two major photoproducts: cyclobutane-pyrimidine dimers (CPDs) and (6-4) pyrimidine-pyrimidone photoproducts. Cordyceps sinensis (Berk.) Sacc. is a medicinal fungus with reported anticancer and cytoprotective effects.

OBJECTIVES

To investigate genoprotective effects of polysaccharide-rich Cordyceps mycelial components against UVB-induced damage in normal human fibroblast cells.

METHODS

Cultured human fibroblasts (BJ cells) were treated for 30 min and, separately, for 24 h with hot water extract of Cordyceps fungal mycelia or exopolysaccharides. Cells were washed, irradiated with UVB (302 nm), and immediately lysed, after which DNA damage, as strand breaks, was measured using an enzyme-assisted comet assay that detects CPDs.

RESULTS

DNA damage in UVB-irradiated cells was significantly lowered (P < 0·01) with Cordyceps pretreatment. Similar results were seen with 30 min and 24 h pretreatment. Specifically, and in comparison with irradiated cells with no Cordyceps pretreatment, there was a 27% reduction in CPDs in irradiated cells with 24 h pretreatment with 200 μg mL(-1) of the hot water Cordyceps extract, and a 34% reduction with 24 h pretreatment with 200 μg mL(-1) of the exopolysaccharide extract.

CONCLUSIONS

Clear evidence of protection against UVB-induced CPDs was seen with Cordyceps mycelial extracts. Results indicate that Cordyceps may offer photoprotection and lower the risk of basal cell carcinoma, the main skin cancer caused by CPDs. Further study is needed to identify protective mechanisms.

Theoretical studies on photoisomerizations of (6-4) and Dewar photolesions in DNA

The (6-4) photoproduct ((6-4) PP) is one of the main lesions in UV-induced DNA damage. The (6-4) PP and its valence isomer Dewar photoproduct (Dewar PP) can have a great threat of mutation and cancer but gained much less attention to date. In this study, with density functional theory (DFT) and the complete active space self-consistent field (CASSCF) methods, the photoisomerization processes between the (6-4) PP and the Dewar PP in the gas phase, the aqueous solution, and the photolyase have been carefully examined. Noticeably, the solvent effect is treated with the CASPT2//CASSCF/Amber (QM/MM) method. Our calculations show that the conical intersection (CI) points play a crucial role in the photoisomerization reaction between the (6-4) PP and the Dewar PP in the gas and the aqueous solution. The ultrafast internal conversion between the S(2) ((1)ππ*) and the S(0) states via a distorted intersection point is found to be responsible for the formation of the Dewar PP lesion at 313 nm, as observed experimentally. For the reversed isomeric process, two channels involving the "dark" excited states have been identified. In addition to the above passages, in the photolyase, a new electron-injection isomerization process as an efficient way for the photorepair of the Dewar PP is revealed.

UBE2W interacts with FANCL and regulates the monoubiquitination of Fanconi anemia protein FANCD2

Fanconi anemia (FA) is a rare cancer-predisposing genetic disease mostly caused by improper regulation of the monoubiquitination of Fanconi anemia complementation group D2 (FANCD2). Genetic studies have indicated that ubiquitin conjugating enzyme UBE2T and HHR6 could regulate FANCD2 monoubiquitination through distinct mechanisms. However, the exact regulation mechanisms of FANCD2 monoubiquitination in response to different DNA damages remain unclear. Here we report that UBE2W, a new ubiquitin conjugating enzyme, could regulate FANCD2 monoubiquitination by mechanisms different from UBE2T or HHR6. Indeed, UBE2W exhibits ubiquitin conjugating enzyme activity and catalyzes the monoubiquitination of PHD domain of Fanconi anemia complementation group L (FANCL) in vitro. UBE2W binds to FANCL, and the PHD domain is both necessary and sufficient for this interaction in mammalian cells. In addition, over-expression of UBE2W in cells promotes the monoubiquitination of FANCD2 and down-regulated UBE2W markedly reduces the UV irradiation-induced but not MMC-induced FANCD2 monoubiquitination. These results indicate that UBE2W regulates FANCD2 monoubiquitination by mechanisms different from UBE2T and HRR6. It may provide an additional regulatory step in the activation of the FA pathway.

Molecular mechanisms of ultraviolet radiation-induced DNA damage and repair

DNA is one of the prime molecules, and its stability is of utmost importance for proper functioning and existence of all living systems. Genotoxic chemicals and radiations exert adverse effects on genome stability. Ultraviolet radiation (UVR) (mainly UV-B: 280-315 nm) is one of the powerful agents that can alter the normal state of life by inducing a variety of mutagenic and cytotoxic DNA lesions such as cyclobutane-pyrimidine dimers (CPDs), 6-4 photoproducts (6-4PPs), and their Dewar valence isomers as well as DNA strand breaks by interfering the genome integrity. To counteract these lesions, organisms have developed a number of highly conserved repair mechanisms such as photoreactivation, base excision repair (BER), nucleotide excision repair (NER), and mismatch repair (MMR). Additionally, double-strand break repair (by homologous recombination and nonhomologous end joining), SOS response, cell-cycle checkpoints, and programmed cell death (apoptosis) are also operative in various organisms with the expense of specific gene products. This review deals with UV-induced alterations in DNA and its maintenance by various repair mechanisms.

Site-specific analysis of UV-induced cyclobutane pyrimidine dimers in nucleotide excision repair-proficient and -deficient hamster cells: Lack of correlation with mutational spectra

Irradiation of cells with UVC light induces two types of mutagenic DNA photoproducts, i.e. cyclobutane pyrimidine dimers (CPD) and pyrimidine (6-4) pyrimidone photoproducts (6-4 PP). To investigate the relationship between the frequency of UV-induced photolesions at specific sites and their ability to induce mutations, we quantified CPD formation at the nucleotide level along exons 3 and 8 of the hprt gene using ligation-mediated PCR, and determined the mutational spectrum of 132 UV-induced hprt mutants in the AA8 hamster cell line and of 165 mutants in its nucleotide excision repair-defective derivative UV5. In AA8 cells, transversions predominated with a strong strand bias towards thymine-containing photolesions in the non-transcribed strand. As hamster AA8 cells are proficient in global genome repair of 6-4 PP but selectively repair CPD from the transcribed strand of active genes, most mutations probably resulted from erroneous bypass of CPD in the non-transcribed strand. However, the relative incidence of CPD and the positions where mutations most frequently arose do not correlate. In fact some major damage sites hardly gave rise to the formation of mutations. In the repair-defective UV5 cells, mutations were almost exclusively C>T transitions caused by photoproducts at PyC sites in the transcribed strand. Even though CPD were formed at high frequencies at some TT sites in UV5, these photoproducts did not contribute to mutation induction at all. We conclude that, even in the absence of repair, large variations in the level of induction of CPD at different sites throughout the two exons do not correspond to frequencies of mutation induction.

T4 endonuclease V: review and application to dermatology

BACKGROUND

T4 endonuclease V was originally isolated from Escherichia coli infected with T4 bacteriophage. It has been shown to repair ultraviolet (UV)-induced cyclobutane pyrimidine dimers in DNA, which, when unrepaired, contribute to mutations that result in actinic keratoses and non-melanoma skin cancers (NMSC). This is a particular concern in patients with genetic defects in their DNA repair systems, especially those with xeroderma pigmentosum (XP). When packaged in liposomes and applied topically, T4 endonuclease V can traverse the stratum corneum and become incorporated within the cytoplasm and nucleus of epidermal keratinocytes and Langerhans cells.

OBJECTIVE

To review all major studies evaluating the efficacy of T4 endonuclease V in animals and humans, the toxicity and safety profile of the topical medication and its potential clinical uses.

METHODS

A literature search was performed through PubMed/Medline, using the keywords 'T4N5', 'T4 endonuclease V' and 'dimericine'. Papers found in the bibliographies of those identified in the initial search and deemed relevant were also included.

CONCLUSION

This enzyme increases the repair of UV-damaged DNA and produces other beneficial effects on UV-damaged cells. In clinical trials in XP patients, topical application of liposome-encapsulated T4 endonuclease V reduced the incidence of basal cell carcinomas by 30% and of actinic keratoses by > 68%. Adverse effects were minimal, and there was no evidence of allergic or irritant contact dermatitis. Although the photoprotective effect of T4N5 has been investigated only in XP patients, the possibility exists that it may benefit others likely to develop premalignant keratoses and NMSC, such as organ transplant recipients receiving immunosuppressive therapy and individuals who have had numerous psoralen plus UVA photochemotherapy treatments. It may be also be effective for normal individuals.

How DNA lesions are turned into powerful killing structures: insights from UV-induced apoptosis

Mammalian cells treated with ultraviolet (UV) light provide one of the best-known experimental systems for depicting the biological consequences of DNA damage. UV irradiation induces the formation of DNA photoproducts, mainly cyclobutane pyrimidine dimers (CPDs) and (6-4) pyrimidine-pyrimidone photoproducts [(6-4)PPs], that drastically impairs DNA metabolism, culminating in the induction of cell death by apoptosis. While CPDs are the most important apoptosis-inducing lesions in DNA repair proficient cells, recent data indicates that (6-4)PPs also signals for apoptosis in DNA repair deficient cells. The toxic effects of these unrepaired DNA lesions are commonly associated with transcription blockage, but there is increasing evidence supporting a role for replication blockage as an apoptosis-inducing signal. This is supported by the observations that DNA double-strand breaks (DSBs) arise at the sites of stalled replication forks, that these DSBs are potent inducers of apoptosis and that inhibition of S phase progression diminishes the apoptotic response. Reactive oxygen species, generated after exposure of mammalian cells to longer UV wavelengths, may also induce apoptotic responses. In this regard, emphasis is given to the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-OxoG), but indirect induced lesions such as lipoperoxide DNA adducts also deserve attention. ATR is the main established sensor molecule for UV-induced DNA damage. However, there is evidence that ATM as well as the MAPK pathway also play a role in the UV response by activating either the death receptor or the mitochondrial damage pathway. Adding more complexity to the subject, cells under stress suffer other types of processes that may result in cell death. Autophagy is one of these processes, with extensive cross-talks with apoptosis. No matter the mechanisms, cell death avoids cells to perpetuate mutations induced by genotoxic lesions. The understanding of such death responses may provide the means for the development of strategies for the prevention and treatment of cancer.

Defective transcription/repair factor IIH recruitment to specific UV lesions in trichothiodystrophy syndrome

Most trichothiodystrophy (TTD) patients present mutations in the xeroderma pigmentosum D (XPD) gene, coding for a subunit of the transcription/repair factor IIH (TFIIH) complex involved in nucleotide excision repair (NER) and transcription. After UV irradiation, most TTD/XPD patients are more severely affected in the NER of cyclobutane pyrimidine dimers (CPD) than of 6-4-photoproducts (6-4PP). The reasons for this differential DNA repair defect are unknown. Here we report the first study of NER in response to CPDs or 6-4PPs separately analyzed in primary fibroblasts. This was done by using heterologous photorepair; recombinant adenovirus vectors carrying photolyases enzymes that repair CPD or 6-4PP specifically by using the energy of light were introduced in different cell lines. The data presented here reveal that some TTD/XPD mutations affect the recruitment of TFIIH specifically to CPDs, but not to 6-4PPs. This deficiency is further confirmed by the inability of TTD/XPD cells to recruit, specifically for CPDs, NER factors that arrive in a TFIIH-dependent manner later in the NER pathway. For 6-4PPs, we show that TFIIH complexes carrying an NH(2)-terminal XPD mutated protein are also deficient in recruitment of NER proteins downstream of TFIIH. Treatment with the histone deacetylase inhibitor trichostatin A allows the recovery of TFIIH recruitment to CPDs in the studied TTD cells and, for COOH-terminal XPD mutations, increases the repair synthesis and survival after UV, suggesting that this defect can be partially related with accessibility of DNA damage in closed chromatin regions.