Three-dimensional visualization of ultraviolet-induced DNA damage and its repair in human cell nuclei

Fluorescence-microscopy-based assay assessing regulatory mechanisms of global genome nucleotide excision repair in cultured cells

It remains uncertain how global genome nucleotide excision repair (GG-NER) efficiently removes various helix distorting DNA lesions in the cell nucleus. Here, we present a protocol to assess the contribution of factors of interest to GG-NER using two types of fluorescence-microscopy-based techniques. First, we describe steps for analyzing the localization of the factors upon local ultraviolet (UV) irradiation. We then detail the second technique, which quantifies the removal of UV-induced photolesions combined with lesion-specific antibodies and program-based image analysis. For complete details on the use and execution of this protocol, please refer to Kusakabe et al.1.

The Over-Irradiation Metabolite Derivative, 24-Hydroxylumister-ol3, Reduces UV-Induced Damage in Skin

The hormonal form of vitamin D₃, 1,25(OH)₂D₃, reduces UV-induced DNA damage. UV exposure initiates pre-vitamin D₃ production in the skin, and continued UV exposure photoisomerizes pre-vitamin D₃ to produce "over-irradiation products" such as lumisterol₃ (L₃). Cytochrome P450 side-chain cleavage enzyme (CYP11A1) in skin catalyzes the conversion of L₃ to produce three main derivatives: 24-hydroxy-L₃ [24(OH)L₃], 22-hydroxy-L₃ [22(OH)L₃], and 20,22-dihydroxy-L₃ [20,22(OH)L₃]. The current study investigated the photoprotective properties of the major over-irradiation metabolite, 24(OH)L₃, in human primary keratinocytes and human skin explants. The results indicated that treatment immediately after UV with either 24(OH)L₃ or 1,25(OH)₂D₃ reduced UV-induced cyclobutane pyrimidine dimers and oxidative DNA damage, with similar concentration response curves in keratinocytes, although in skin explants, 1,25(OH)₂D₃ was more potent. The reductions in DNA damage by both compounds were, at least in part, the result of increased DNA repair through increased energy availability via increased glycolysis, as well as increased DNA damage recognition proteins in the nucleotide excision repair pathway. Reductions in UV-induced DNA photolesions by either compound occurred in the presence of lower reactive oxygen species. The results indicated that under in vitro and ex vivo conditions, 24(OH)L₃ provided photoprotection against UV damage similar to that of 1,25(OH)₂D₃.

UV-induced DNA Damage in Skin is Reduced by CaSR Inhibition

The epidermis maintains a cellular calcium gradient that supports keratinocyte differentiation from its basal layers (low) to outer layers (high) leading to the development of the stratum corneum, which resists penetration of UV radiation. The calcium‐sensing receptor (CaSR) expressed in keratinocytes responds to the calcium gradient with signals that promote differentiation. In this study, we investigated whether the CaSR is involved more directly in protection from UV damage in studies of human keratinocytes in primary culture and in mouse skin studied in vivo. siRNA‐directed reductions in CaSR protein levels in human keratinocytes significantly reduced UV‐induced direct cyclobutane pyrimidine dimers (CPD) by ~80% and oxidative DNA damage (8‐OHdG) by ~65% compared with control transfected cells. Similarly, in untransfected cells, the CaSR negative modulator, NPS‐2143 (500 nm), reduced UV‐induced CPD and 8‐OHdG by ~70%. NPS‐2143 also enhanced DNA repair and reduced reactive oxygen species (ROS) by ~35% in UV‐exposed keratinocytes, consistent with reduced DNA damage after UV exposure. Topical application of NPS‐2143 also protected hairless Skh:hr1 mice from UV‐induced CPD, oxidative DNA damage and inflammation, similar to the reductions observed in response to the well‐known photoprotection agent 1,25(OH)₂D₃ (calcitriol). Thus, negative modulators of the CaSR offer a new approach to reducing UV‐induced skin damage.

Transcriptome analysis of molecular response to UVC irradiation in zebrafish embryos

Ultraviolet (UV) rays can be both harmful and beneficial to humans. This study aimed to investigate the toxicity and safety of ultraviolet C (UVC) exposure in living organisms and the corresponding biodefense molecular mechanisms. Zebrafish embryos, at an early developmental stage (5-6 h post-fertilization), were irradiated with increasing UVC dosages using high-efficiency deep-ultraviolet light-emitting diodes (278 nm). Morphological phenotypes including survival rate, hatching rate, heart rate, and malformation rate were evaluated. Compared to un-irradiated controls, all zebrafish embryos exposed to 4.5 mJ/cm² UVC survived and showed no significant difference in hatching and heart rate. However, 7.5 mJ/cm² of UVC irradiation caused a significantly decreased survival rate (37.5%) and an increased malformation rate (81.8%). Therefore, 4.5 mJ/cm² was chosen as the limit dosage that the internal biodefense system of zebrafish embryos can protect against UVC radiation. Transcriptome analysis (RNA sequencing) performed on 3 min and 3 days post-irradiation embryos (4.5 mJ/cm²) revealed the molecular mechanisms underlying the response of zebrafish embryos to irradiation. The embryos quickly responded to UVC-induced stress by activating the p53 signaling pathway. In addition, after 3 days of recuperation, the embryos showed activation of signal transducer and activator of transcription (STAT) signaling pathway. To our knowledge, this is the first study to evaluate the toxicological effects and the molecular mechanism of biodefense in zebrafish embryos upon 278 nm UVC irradiation.

Safety of 222 nm UVC Irradiation to the Surgical Site in a Rabbit Model

For the prevention of surgical site infection (SSI), continuous disinfection could be helpful. Short wavelength ultraviolet radiation C (UVC) is highly bactericidal but shows cytotoxicity. Radiation of UVC with a wavelength of 222 nm to the skin is considered to be safe because it only reaches the stratum corneum. However, the safety of 222 nm irradiation to the surgical field not covered with skin is unknown. The purpose of this study was to examine the safety of 222 nm UVC irradiation on a surgical field in a rabbit model. Five types of tissue were surgically exposed and irradiated with 222 or 254 nm UVC. Immunohistological assessment against cyclobutane pyrimidine dimer (CPD), an index of DNA damage by UVC, was performed. The CPD-positive cell rate was significantly higher in the 254 nm group than in the other groups in all tissues. A 222 nm group showed significantly more CPD than control in fat tissue, but no significant difference in all other tissues. In fat tissue collected 24 h after irradiation, the 254 nm group showed higher CPD than the other groups, while the 222 nm group had reduced to the control level. These data suggest that 222 nm UVC irradiation could be a new method to safely prevent SSI.

Time kinetics of cyclobutane pyrimidine dimer formation by narrowband and broadband UVB irradiation

BACKGROUND

Maximum cyclobutane pyrimidine dimer (CPD) formation in the skin induced by ultraviolet B (UVB) irradiation is thought to occur within a few minutes and is immediately decreased by the DNA repair system.

OBJECTIVE

We evaluated the time course and differential effects of narrowband (NB-UVB) and broadband (BB-UVB) UVB on CPD formation.

METHODS

We investigated CPD formation at various time-points in vivo, from 3 min to 72 h, after UVB irradiation using 2 mouse strains, C57BL/6 J and BALB/c. The backs of the mice were shaved and irradiated with NB-UVB or BB-UVB. Skin specimens were obtained and stained with anti-CPD antibody. Positive signals in the epidermis were measured using ImageJ. DNA was extracted from the isolated epidermis and subjected to quantitative CPD analysis by enzyme-linked immunosorbent assay (ELISA).

RESULTS

CPDs induced by UVB irradiation (1 minimum erythemal dose) in epidermal skin were detected in the nucleus. Although the CPD levels increased immediately after irradiation (3 min), the highest level was detected at 1 h and the increase lasted 24-48 h after irradiation. BB-UVB tended to induce greater CPD levels than NB-UVB in both mouse strains. The ELISA showed similar results.

CONCLUSIONS

CPDs were induced immediately after UV irradiation, with the maximum level observed 1 h after irradiation. BB-UVB irradiation tended to induce greater levels of CPD formation. In addition to the direct effects of UVB, the presence of CPDs in hair follicles, which were not irradiated by UVB, suggests that reactive oxygen species are also involved in CPD formation in the skin.

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.

Protection from Ultraviolet Damage and Photocarcinogenesis by Vitamin D Compounds

Exposure of skin cells to UV radiation results in DNA damage, which if inadequately repaired, may cause mutations. UV-induced DNA damage and reactive oxygen and nitrogen species also cause local and systemic suppression of the adaptive immune system. Together, these changes underpin the development of skin tumours. The hormone derived from vitamin D, calcitriol (1,25-dihydroxyvitamin D₃) and other related compounds, working via the vitamin D receptor and at least in part through endoplasmic reticulum protein 57 (ERp57), reduce cyclobutane pyrimidine dimers and oxidative DNA damage in keratinocytes and other skin cell types after UV. Calcitriol and related compounds enhance DNA repair in keratinocytes, in part through decreased reactive oxygen species, increased p53 expression and/or activation, increased repair proteins and increased energy availability in the cell when calcitriol is present after UV exposure. There is mitochondrial damage in keratinocytes after UV. In the presence of calcitriol, but not vehicle, glycolysis is increased after UV, along with increased energy-conserving autophagy and changes consistent with enhanced mitophagy. Reduced DNA damage and reduced ROS/RNS should help reduce UV-induced immune suppression. Reduced UV immune suppression is observed after topical treatment with calcitriol and related compounds in hairless mice. These protective effects of calcitriol and related compounds presumably contribute to the observed reduction in skin tumour formation in mice after chronic exposure to UV followed by topical post-irradiation treatment with calcitriol and some, though not all, related compounds.

C1D is not directly involved in the repair of UV-damaged DNA but protects cells from oxidative stress by regulating gene expressions in human cell lines

A small nuclear protein, C1D, has roles in various cellular processes, transcription regulation, genome stability surveillance, DNA repair and RNA processing, all of which are required to maintain the host life cycles. In the previous report, C1D directly interacts with XPB, a component of the nucleotide excision repair complex, and C1D knockdown reduced cell survival of 27-1 cells, CHO derivative cells, after UV irradiation. To find out the role of C1D in UV-damaged cells, we used human cell lines with siRNA or shRNA to knockdown C1D. C1D knockdown reduced cell survival rates of LU99 and 786-O after UV irradiation, although C1D knockdown did not affect the efficiency of the nucleotide excision repair. Immunostaining data support that C1D is not directly involved in the DNA repair process in UV-damaged cells. However, H2O2 treatment reduced cell viability in LU99 and 786-O cells. We also found that C1D knockdown upregulated DDIT3 expression in LU99 cells and downregulated APEX1 in 786-O cells, suggesting that C1D functions as a co-repressor/activator. The data accounts for the reduction of cell survival rates upon UV irradiation.

UV scintillating particles as radiosensitizer enhance cell killing after X-ray excitation

BACKGROUND AND PURPOSE

Radiation therapy is the gold standard treatment for inoperable malignant tumors. However, due to the heterogeneity of the tumor, some regions are more radio resistant and can lead to metastasis and tumor recurrence. In this study, we propose combining traditional X-ray treatment with UVC-emitting LuPO4:Pr3+ nanoparticles (NPs) to increase the tumor control as well as to reduce tumor recurrence and metastasis. These NPs convert ionizing radiation into UVC-photons (UVC range: 200-280 nm) locally at the tumor site. Unlike X-ray, UVC-photons damage DNA directly via an oxygen-independent mechanism, which could improve treatment of radioresistant tumors such as hypoxic tumors.

MATERIALS AND METHODS

The effect of X-ray generated UVC-photons was tested on human fibroblasts incubated with NPs prior to radiation treatment. The surviving fraction of the cells was assessed by means of colony formation assay. Experiments were performed on normal and UVC sensitive cell lines to demonstrate the presence of UVC photons during treatment. In addition, UV-specific DNA damages were investigated using an immunofluorescence assay to measure cyclopyrimidine dimers (CPDs).

RESULTS

Combined treatment showed an increased cell death of over 50%, compared to radiation alone. This results in a dose equivalent of 4 Gy for the combined treatment with 2 Gy irradiation. The formation of CPDs and the increased effect on UV sensitive cells indicate the presence of UV photons. The generated amount of CPDs is comparable to an UVC exposure of about 15 J × m-2.

CONCLUSION

Combining NPs with ionizing radiation results in a localized dose surge, which could increase tumor control. It could also allow lowering the total applied dose to minimize unwanted side effects to the surrounding normal tissue while maintaining tumor control.

Brahma deficiency in keratinocytes promotes UV carcinogenesis by accelerating the escape from cell cycle arrest and the formation of DNA photolesions

BACKGROUND

Ultraviolet radiation (UVR) is the principal cause of keratinocyte skin cancers. Previous work found that levels of the chromatin remodelling protein, Brahma (Brm), are diminished during the progression from actinic keratoses to cutaneous squamous cell carcinomas in humans, and its loss in UV-irradiated mouse skin causes epidermal hyperplasia and increased tumour incidence.

METHODS

The skins of mice and mouse and human keratinocytes deficient in Brm were exposed to UVR and evaluated for cell cycle progression and DNA damage response.

OBJECTIVE

To identify the mechanisms by which loss of Brm contributes to UVR-induced skin carcinogenesis.

RESULTS

In both mouse keratinocytes and HaCaT cells, Brm deficiency led to an increased cell population growth following UVR exposure compared to cells with normal levels of Brm. Cell cycle analysis using a novel assay showed that Brm-deficient keratinocytes entered cell cycle arrest normally, but escaped from cell cycle arrest faster, enabling them to begin proliferating earlier. In mouse keratinocytes, Brm primarily affected accumulation in G₀/G₁-phase, whereas in HaCaT cells, which lack normal p53, accumulation in G₂/M-phase was affected. Brm-deficient keratinocytes in mouse skin and human cell cultures also had higher levels of UVR-induced cyclobutane pyrimidine dimer photolesions. These effects occurred without any compensatory increase in DNA repair or cell death to remove photolesions or the cells that harbor them from the keratinocyte population.

CONCLUSION

The loss of Brm in keratinocytes exposed to UVR enables them to resume proliferation while harboring DNA photolesions, leading to an increased fixation of mutations and, consequently, increased carcinogenesis.

The interplay of UV and cutaneous papillomavirus infection in skin cancer development

Cutaneous human papillomaviruses (HPVs) are considered as cofactors for non-melanoma skin cancer (NMSC) development, especially in association with UVB. Extensively studied transgenic mouse models failed to mimic all aspects of virus-host interactions starting from primary infection to the appearance of a tumor. Using the natural model Mastomys coucha, which reflects the human situation in many aspects, we provide the first evidence that only UVB and Mastomys natalensis papillomavirus (MnPV) infection strongly promote NMSC formation. Using UVB exposures that correspond to UV indices of different geographical regions, irradiated animals developed either well-differentiated keratinizing squamous cell carcinomas (SCCs), still supporting productive infections with high viral loads and transcriptional activity, or poorly differentiated non-keratinizing SCCs almost lacking MnPV DNA and in turn, early and late viral transcription. Intriguingly, animals with the latter phenotype, however, still showed strong seropositivity, clearly verifying a preceding MnPV infection. Of note, the mere presence of MnPV could induce γH2AX foci, indicating that viral infection without prior UVB exposure can already perturb genome stability of the host cell. Moreover, as shown both under in vitro and in vivo conditions, MnPV E6/E7 expression also attenuates the excision repair of cyclobutane pyrimidine dimers upon UVB irradiation, suggesting a viral impact on the DNA damage response. While mutations of Ras family members (e.g. Hras, Kras, and Nras) were absent, the majority of SCCs harbored-like in humans-Trp53 mutations especially at two hot-spots in the DNA-binding domain, resulting in a loss of function that favored tumor dedifferentiation, counter-selective for viral maintenance. Such a constellation provides a reasonable explanation for making continuous viral presence dispensable during skin carcinogenesis as observed in patients with NMSC.

The activation of PPARγ by 2,4,6-Octatrienoic acid protects human keratinocytes from UVR-induced damages

Increasing attention is addressed to identify products able to enhance skin photoprotection and to prevent skin carcinogenesis. Several studies have demonstrated that the α-melanocyte stimulating hormone (αMSH), acting on a functional MC1R, provides a photoprotective effect by inducing pigmentation, antioxidants and DNA repair. We discovered a link between αMSH and the nuclear receptor Peroxisome Proliferator-Activated Receptor-γ (PPARγ), suggesting that some of the αMSH protective effects may be dependent on PPARγ transcriptional activity. Moreover, we demonstrated that the activation of PPARγ by the parrodiene 2,4,6-octatrienoic acid (Octa) induces melanogenesis and antioxidant defence in human melanocytes and counteracts senescence-like phenotype in human fibroblasts. In this study, we demonstrate that the activation of PPARγ by Octa exerts a protective effect against UVA- and UVB-induced damage on normal human keratinocytes (NHKs), the major target cells of UV radiation. Octa promotes the antioxidant defence, augments DNA repair and reduces the induction of proteins involved in UV-induced DNA damage response. Our results contribute to deepen the analysis of the αMSH/PPARγ connection and suggest perspectives for the development of new molecules and formulations able to prevent cutaneous UV damage by acting on the different skin cell populations through PPARγ activation.

The pathogenic role of interleukin-22 and its receptor during UVB-induced skin inflammation

Recent studies show that IL-22, a cytokine produced by activated CD4+ T cells and NK cells, plays a pathogenic role in acute and chronic skin diseases. While IL-22 is produced by immune cells, the expression of IL-22Rα, the functional subunit of IL-22R, is mostly restricted to non-hematopoietic cells in organs such as the skin and pancreas. Although it is well known that ultraviolet B (UVB) radiation induces skin inflammation, there have been no reports regarding the effect of UVB on the expression of IL-22Rα. This study investigated IL-22Rα expression and IL-22-mediated proliferation and pro-inflammatory cytokine production by UVB-irradiated keratinocytes. IL-22Rα was increased in HaCaT and primary human keratinocytes after UVB irradiation through the translocation of IL-22Rα from the cytosol to the membrane. This increase in the expression of IL-22Rα was mediated by the PI3K/Akt pathway. Moreover, the suppression of keratinocyte proliferation by UVB irradiation was inhibited by treatment with IL-22. At the same time, IL-22 increased the production of IL-1α, IL-6, and IL-18 in UVB-irradiated HaCaT cells and primary human keratinocytes. Finally, IL-22Rα expression was increased in UVB-irradiated human and mouse skin by immunohistochemistry. The increased expression of IL-22Rα therefore promotes keratinocyte proliferation and pro-inflammatory cytokine production during UVB-induced skin inflammation, suggesting that UVB facilitates skin inflammation by increasing the responsiveness of keratinocytes to IL-22. This study provides a new insight into UVB-induced skin inflammation and the regulation of related inflammatory skin diseases.

Thymine DNA glycosylase modulates DNA damage response and gene expression by base excision repair-dependent and independent mechanisms

Thymine DNA glycosylase (TDG) is a base excision repair (BER) enzyme, which is implicated in correction of deamination-induced DNA mismatches, the DNA demethylation process and regulation of gene expression. Because of these pivotal roles associated, it is crucial to elucidate how the TDG functions are appropriately regulated in vivo. Here, we present evidence that the TDG protein undergoes degradation upon various types of DNA damage, including ultraviolet light (UV). The UV-induced degradation of TDG was dependent on proficiency in nucleotide excision repair and on CRL4^CDT2 -mediated ubiquitination that requires a physical interaction between TDG and DNA polymerase clamp PCNA. Using the Tdg-deficient mouse embryonic fibroblasts, we found that ectopic expression of TDG compromised cellular survival after UV irradiation and repair of UV-induced DNA lesions. These negative effects on cellular UV responses were alleviated by introducing mutations in TDG that impaired its BER function. The expression of TDG induced a large-scale alteration in the gene expression profile independently of its DNA glycosylase activity, whereas a subset of genes was affected by the catalytic activity of TDG. Our results indicate the presence of BER-dependent and BER-independent functions of TDG, which are involved in regulation of cellular DNA damage responses and gene expression patterns.

Epidermal Rac1 regulates the DNA damage response and protects from UV-light-induced keratinocyte apoptosis and skin carcinogenesis

Non-melanoma skin cancer (NMSC) is the most common type of cancer. Increased expression and activity of Rac1, a small Rho GTPase, has been shown previously in NMSC and other human cancers; suggesting that Rac1 may function as an oncogene in skin. DMBA/TPA skin carcinogenesis studies in mice have shown that Rac1 is required for chemically induced skin papilloma formation. However, UVB radiation by the sun, which causes DNA damage, is the most relevant cause for NMSC. A potential role of Rac1 in UV-light-induced skin carcinogenesis has not been investigated so far. To investigate this, we irradiated mice with epidermal Rac1 deficiency (Rac1-EKO) and their controls using a well-established protocol for long-term UV-irradiation. Most of the Rac1-EKO mice developed severe skin erosions upon long-term UV-irradiation, unlike their controls. These skin erosions in Rac1-EKO mice healed subsequently. Surprisingly, we observed development of squamous cell carcinomas (SCCs) within the UV-irradiation fields. This shows that the presence of Rac1 in the epidermis protects from UV-light-induced skin carcinogenesis. Short-term UV-irradiation experiments revealed increased UV-light-induced apoptosis of Rac1-deficient epidermal keratinocytes in vitro as well as in vivo. Further investigations using cyclobutane pyrimidine dimer photolyase transgenic mice revealed that the observed increase in UV-light-induced keratinocyte apoptosis in Rac1-EKO mice is DNA damage dependent and correlates with caspase-8 activation. Furthermore, Rac1-deficient keratinocytes showed reduced levels of p53, γ-H2AX and p-Chk1 suggesting an attenuated DNA damage response upon UV-irradiation. Taken together, our data provide direct evidence for a protective role of Rac1 in UV-light-induced skin carcinogenesis and keratinocyte apoptosis probably through regulating mechanisms of the DNA damage response and repair pathways.

Subnuclear localization, rates and effectiveness of UVC-induced unscheduled DNA synthesis visualized by fluorescence widefield, confocal and super-resolution microscopy

Unscheduled DNA synthesis (UDS) is the final stage of the process of repair of DNA lesions induced by UVC. We detected UDS using a DNA precursor, 5-ethynyl-2'-deoxyuridine (EdU). Using wide-field, confocal and super-resolution fluorescence microscopy and normal human fibroblasts, derived from healthy subjects, we demonstrate that the sub-nuclear pattern of UDS detected via incorporation of EdU is different from that when BrdU is used as DNA precursor. EdU incorporation occurs evenly throughout chromatin, as opposed to just a few small and large repair foci detected by BrdU. We attribute this difference to the fact that BrdU antibody is of much larger size than EdU, and its accessibility to the incorporated precursor requires the presence of denatured sections of DNA. It appears that under the standard conditions of immunocytochemical detection of BrdU only fragments of DNA of various length are being denatured. We argue that, compared with BrdU, the UDS pattern visualized by EdU constitutes a more faithful representation of sub-nuclear distribution of the final stage of nucleotide excision repair induced by UVC. Using the optimized integrated EdU detection procedure we also measured the relative amount of the DNA precursor incorporated by cells during UDS following exposure to various doses of UVC. Also described is the high degree of heterogeneity in terms of the UVC-induced EdU incorporation per cell, presumably reflecting various DNA repair efficiencies or differences in the level of endogenous dT competing with EdU within a population of normal human fibroblasts.

Modeling xeroderma pigmentosum associated neurological pathologies with patients-derived iPSCs

Xeroderma pigmentosum (XP) is a group of genetic disorders caused by mutations of XP-associated genes, resulting in impairment of DNA repair. XP patients frequently exhibit neurological degeneration, but the underlying mechanism is unknown, in part due to lack of proper disease models. Here, we generated patient-specific induced pluripotent stem cells (iPSCs) harboring mutations in five different XP genes including XPA, XPB, XPC, XPG, and XPV. These iPSCs were further differentiated to neural cells, and their susceptibility to DNA damage stress was investigated. Mutation of XPA in either neural stem cells (NSCs) or neurons resulted in severe DNA damage repair defects, and these neural cells with mutant XPA were hyper-sensitive to DNA damage-induced apoptosis. Thus, XP-mutant neural cells represent valuable tools to clarify the molecular mechanisms of neurological abnormalities in the XP patients.

Detection of UV-induced cyclobutane pyrimidine dimers by near-infrared spectroscopy and aquaphotomics

Ultraviolet (UV) radiation causes cellular DNA damage, among which cyclobutane pyrimidine dimers (CPDs) are responsible for a variety of genetic mutations. Although several approaches have been developed for detection of CPDs, conventional methods require time-consuming steps. Aquaphotomics, a new approach based on near-infrared spectroscopy (NIRS) and multivariate analysis that determines interactions between water and other components of the solution, has become an effective method for qualitative and quantitative parameters measurement in the solutions. NIR spectral patterns of UVC-irradiated and nonirradiated DNA solutions were evaluated using aquaphotomics for detection of UV-induced CPDs. Groups of UV-irradiated and nonirradiated DNA samples were classified (87.5% accuracy) by soft independent modeling of class analogy (SIMCA). A precise regression model calculated from NIR water spectral patterns based on UVC doses (r Val = 0.9457) and the concentration of cis-syn cyclobutane thymine dimers (cis-syn T<>Ts; r Val = 0.9993) was developed using partial least squares regression (PLSR), while taking advantage of water spectral patterns, particularly around 1400-1500 nm. Our results suggested that, in contrast to DNA, the formation of cis-syn T<>Ts increased the strongly hydrogen bonded water. Additionally, NIRS could qualitatively and quantitatively detect cis-syn T<>Ts in isolated DNA aqueous solutions upon UVC exposure.

The contribution of mitochondrial thymidylate synthesis in preventing the nuclear genome stress

In quiescent fibroblasts, the expression levels of cytosolic enzymes for thymidine triphosphate (dTTP) synthesis are down-regulated, causing a marked reduction in the dTTP pool. In this study, we provide evidence that mitochondrial thymidylate synthesis via thymidine kinase 2 (TK2) is a limiting factor for the repair of ultraviolet (UV) damage in the nuclear compartment in quiescent fibroblasts. We found that TK2 deficiency causes secondary DNA double-strand breaks formation in the nuclear genome of quiescent cells at the late stage of recovery from UV damage. Despite slower repair of quiescent fibroblast deficient in TK2, DNA damage signals eventually disappeared, and these cells were capable of re-entering the S phase after serum stimulation. However, these cells displayed severe genome stress as revealed by the dramatic increase in 53BP1 nuclear body in the G1 phase of the successive cell cycle. Here, we conclude that mitochondrial thymidylate synthesis via TK2 plays a role in facilitating the quality repair of UV damage for the maintenance of genome integrity in the cells that are temporarily arrested in the quiescent state.

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

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

Non-invasive diffuse reflectance measurements of cutaneous melanin content can predict human sensitivity to ultraviolet radiation

The diversity of human skin phenotypes and the ubiquitous exposure to ultraviolet radiation (UVR) underscore the need for a non-invasive tool to predict an individual's UVR sensitivity. We analysed correlations between UVR sensitivity, melanin content, diffuse reflectance spectroscopy (DR) and UVR-induced DNA damage in the skin of subjects from three racial/ethnic groups: Asian, black or African American and White. UVR sensitivity was determined by evaluating each subject's response to one minimal erythemal dose (MED) of UVR one day after the exposure. Melanin content was measured using DR and by densitometric analysis of Fontana-Masson staining (FM) in skin biopsies taken from unexposed areas. An individual's UVR sensitivity based on MED was highly correlated with melanin content measured by DR and by FM. Therefore, a predictive model for the non-invasive determination of UVR sensitivity using DR was developed. The MED precision was further improved when we took race/ethnicity into consideration. The use of DR serves as a tool for predicting UVR sensitivity in humans that should be invaluable for determining appropriate UVR doses for therapeutic, diagnostic and/or cosmetic devices.

Nicotinamide enhances repair of ultraviolet radiation-induced DNA damage in human keratinocytes and ex vivo skin

Nicotinamide (vitamin B3) protects from ultraviolet (UV) radiation-induced carcinogenesis in mice and from UV-induced immunosuppression in mice and humans. Recent double-blinded randomized controlled Phase 2 studies in heavily sun-damaged individuals have shown that oral nicotinamide significantly reduces premalignant actinic keratoses, and may reduce new non-melanoma skin cancers. Nicotinamide is a precursor of nicotinamide adenine dinucleotide (NAD(+)), an essential coenzyme in adenosine triphosphate (ATP) production. Previously, we showed that nicotinamide prevents UV-induced ATP decline in HaCaT keratinocytes. Energy-dependent DNA repair is a key determinant of cellular survival after exposure to DNA-damaging agents such as UV radiation. Hence, in this study we investigated whether nicotinamide protection from cellular energy loss influences DNA repair. We treated HaCaT keratinocytes with nicotinamide and exposed them to low-dose solar-simulated UV (ssUV). Excision repair was quantified using an assay of unscheduled DNA synthesis. Nicotinamide increased both the proportion of cells undergoing excision repair and the repair rate in each cell. We then investigated ssUV-induced cyclobutane pyrimidine dimers (CPDs) and 8-oxo-7,8-dihydro-2'-deoxyguanosine (8oxoG) formation and repair by comet assay in keratinocytes and with immunohistochemistry in human skin. Nicotinamide reduced CPDs and 8oxoG in both models and the reduction appeared to be due to enhancement of DNA repair. These results show that nicotinamide enhances two different pathways for repair of UV-induced photolesions, supporting nicotinamide's potential as an inexpensive, convenient and non-toxic agent for skin cancer chemoprevention.

HCMV-infected cells maintain efficient nucleotide excision repair of the viral genome while abrogating repair of the host genome

Many viruses subvert the host cell's ability to mount and complete various DNA damage responses (DDRs) after infection. HCMV infection of permissive fibroblasts activates host DDRs at the time of viral deposition and during replication, but the DDRs remain uncompleted without arrest or apoptosis. We believe this was in part due to partitioning of the damage response and double strand break repair components. After extraction of soluble proteins, the localization of these components fell into three groups: specifically associated with the viral replication centers (RCs), diffused throughout the nucleoplasm and excluded from the RCs. Others have shown that cells are incapable of processing exogenously introduced damage after infection. We hypothesized that the inability of the cells to process damage might be due to the differential association of repair components within the RCs and, in turn, potentially preferential repair of the viral genome and compromised repair of the host genome. To test this hypothesis we used multiple strategies to examine repair of UV-induced DNA damage in mock and virus-infected fibroblasts. Comet assays indicated that repair was initiated, but was not completed in infected cells. Quantitative analysis of immunofluorescent localization of cyclobutane pyrimidine dimers (CPDs) revealed that after 24 h of repair, CPDs were significantly reduced in viral DNA, but not significantly changed in the infected host DNA. To further quantitate CPD repair, we developed a novel dual-color Southern protocol allowing visualization of host and viral DNA simultaneously. Combining this Southern methodology with a CPD-specific T4 endonuclease V alkaline agarose assay to quantitate repair of adducts, we found efficient repair of CPDs from the viral DNA but not host cellular DNA. Our data confirm that NER functions in HCMV-infected cells and almost exclusively repairs the viral genome to the detriment of the host's genome.

Human cytomegalovirus (HCMV) is a leading cause of birth defects. This may be due in part to this virus' ability to inflict specific damage to its host's DNA, combined with the disruption of an infected cell's ability to repair damage. Earlier studies found that components of the cell's repair machinery were differentially associated with the HCMV viral replication centers in the nucleus. Experiments here extend this observation to include components of the machinery involved in UV lesion repair. We hypothesized that association of components of the DNA repair machinery within the viral replication centers could favor the repair of viral DNA, but more importantly, be detrimental to the repair of cellular DNA. Infected cells were irradiated and examined for repair by three different methods. In the course of this study, we developed a new technique allowing simultaneous evaluation of both the viral and host genomes in an infected cell. These experiments found rapid, selective removal of UV lesions from the viral and not the cellular DNA within infected cells. Our results indicate the differential association of certain cellular repair proteins with this virus may have far-reaching implications in the disease pathogenesis of HCMV infection.

1α,25(OH)₂-vitamin D and a nongenomic vitamin D analogue inhibit ultraviolet radiation-induced skin carcinogenesis

Exposure to ultraviolet radiation (UVR) can lead to a range of deleterious responses in the skin. An important form of damage is the DNA photolesion cyclobutane pyrimidine dimer (CPD). CPDs can be highly mutagenic if not repaired prior to cell division and can lead to UV-induced immunosuppression, making them potentially carcinogenic. UVR exposure also produces vitamin D, a prehormone. Different shapes of the steroid hormone 1α,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] can produce biological responses through binding either to its cognate nuclear receptor (VDR) to regulate gene transcription or to the VDR associated with plasma membrane caveolae to produce, via signal transduction, nongenomic physiologic responses. Here, we show that both 1,25(OH)₂D₃ and 1α,25(OH)₂-lumisterol (JN), a conformationally restricted analogue that can generate only nongenomic responses, are effective inhibitors of UV damage in an immunocompetent mouse (Skh:hr1) model susceptible to UV-induced tumors. Both 1,25(OH)₂D₃ and JN significantly reduced UVR-induced CPD, apoptotic sunburn cells, and immunosuppression. Furthermore, these compounds inhibited skin tumor development, both papillomas and squamous cell carcinomas, in these mice. The observed reduction of these UV-induced effects by 1,25(OH)₂D₃ and JN suggests a role for these compounds in prevention against skin carcinogenesis. To the best of our knowledge, this is the first comprehensive report of an in vivo long-term biological response generated by chronic dosing with a nongenomic-selective vitamin D steroid.

Caffeine promotes ultraviolet B-induced apoptosis in human keratinocytes without complete DNA repair

In response to ultraviolet B damage, keratinocytes undergo apoptosis to eliminate damaged cells, thereby preventing tumorigenic transformation. Caffeine, the most widely consumed psychoactive substance, produces complex pharmacological actions; it has been shown to be chemopreventive in non-melamona skin cancer in mice through increasing apoptosis. Here we have investigated the molecular and cellular mechanisms in the pro-apoptotic effect of caffeine on UVB-irradiated human HaCaT keratinocytes. Pretreatment with caffeine increased UVB-induced apoptosis in HaCaT cells. Caffeine blocked UVB-induced Chk1 phosphorylation. In addition, similar to the effect of the PI3K inhibitor LY294002, caffeine also inhibited phosphorylation of AKT and up-regulation of COX-2, two critical oncogenic pathways in skin tumorigenesis. However, phosphorylation of EGFR or ERK was unaffected. Inhibiting ATR pathways by siRNA targeting ATR had little effect on UVB-induced apoptosis or AKT activation, indicating that the inhibitory effect of caffeine on apoptosis and the AKT pathway does not require the ATR pathway. Inhibiting AKT by caffeine blocked UVB-induced COX-2 up-regulation. Expression of constitutively active AKT that was not inhibited by caffeine was found to protect cells from caffeine-promoted apoptosis post-UVB irradiation, indicating that AKT is an essential inhibitory target for caffeine to promote apoptosis. Caffeine specifically sensitized cells with unrepaired DNA damage to UVB-induced apoptosis. These findings indicate that in HaCaT keratinocytes, inhibiting the AKT/COX-2 pathways through an ATR-independent pathway is a critical molecular mechanism by which caffeine promotes UVB-induced apoptosis of unrepaired keratinocytes for elimination.

Regulation of global genome nucleotide excision repair by SIRT1 through xeroderma pigmentosum C

Disruption of the nucleotide excision repair (NER) pathway by mutations can cause xeroderma pigmentosum, a syndrome predisposing affected individuals to development of skin cancer. The xeroderma pigmentosum C (XPC) protein is essential for initiating global genome NER by recognizing the DNA lesion and recruiting downstream factors. Here we show that inhibition of the deacetylase and longevity factor SIRT1 impairs global genome NER through suppressing the transcription of XPC in a SIRT1 deacetylase-dependent manner. SIRT1 enhances XPC expression by reducing AKT-dependent nuclear localization of the transcription repressor of XPC. Finally, we show that SIRT1 levels are significantly reduced in human skin tumors from Caucasian patients, a population at highest risk. These findings suggest that SIRT1 acts as a tumor suppressor through its role in DNA repair.

XPC branch-point sequence mutations disrupt U2 snRNP binding, resulting in abnormal pre-mRNA splicing in xeroderma pigmentosum patients

Mutations in two branch-point sequences (BPS) in intron 3 of the XPC DNA repair gene affect pre-mRNA splicing in association with xeroderma pigmentosum (XP) with many skin cancers (XP101TMA) or no skin cancer (XP72TMA), respectively. To investigate the mechanism of these abnormalities we now report that transfection of minigenes with these mutations revealed abnormal XPC pre-mRNA splicing that mimicked pre-mRNA splicing in the patients' cells. DNA oligonucleotide-directed RNase H digestion demonstrated that mutations in these BPS disrupt U2 snRNP-BPS interaction. XP101TMA cells had no detectable XPC protein but XP72TMA had 29% of normal levels. A small amount of XPC protein was detected at sites of localized ultraviolet (UV)-damaged DNA in XP72TMA cells which then recruited other nucleotide excision repair (NER) proteins. In contrast, XP101TMA cells had no detectable recruitment of XPC or other NER proteins. Post-UV survival and photoproduct assays revealed greater reduction in DNA repair in XP101TMA cells than in XP72TMA. Thus mutations in XPC BPS resulted in disruption of U2 snRNP-BPS interaction leading to abnormal pre-mRNA splicing and reduced XPC protein. At the cellular level these changes were associated with features of reduced DNA repair including diminished NER protein recruitment, reduced post-UV survival and impaired photoproduct removal.

Agents that reverse UV-Induced immune suppression and photocarcinogenesis affect DNA repair

UV exposure induces skin cancer, in part, by inducing immune suppression. Repairing DNA damage, neutralizing the activity of cis-urocanic acid, and reversing oxidative stress abrogate UV-induced immune suppression and skin cancer induction, suggesting that DNA, UCA, and lipid photo-oxidation serve as UV photoreceptors. What is not clear is whether signaling through each of these different photoreceptors activates independent pathways to induce biological effects or whether there is a common checkpoint where these pathways converge. Here, we show that agents known to reverse photocarcinogenesis and photoimmune suppression, such as platelet-activating factor (PAF) and serotonin (5-HT) receptor antagonists, regulate DNA repair. Pyrimidine dimer repair was accelerated in UV-irradiated mice injected with PAF and 5-HT receptor antagonists. Nucleotide excision repair (NER), as measured by unscheduled DNA synthesis, was accelerated by PAF and 5-HT receptor antagonists. Injecting PAF and 5-HT receptor antagonists into UV-irradiated Xeroderma pigmentosum complementation group A-deficient mice, which lack the enzymes responsible for NER, did not accelerate photoproduct repair. Similarly, UV-induced formation of 8-oxo-deoxyguanosine was reduced by PAF and 5-HT receptor antagonists. We conclude that PAF and 5-HT receptor antagonists accelerate DNA repair caused by UV radiation, which prevents immune suppression and interferes with photocarcinogenesis.

XPC initiation codon mutation in xeroderma pigmentosum patients with and without neurological symptoms

Two unrelated xeroderma pigmentosum (XP) patients, with and without neurological abnormalities, respectively, had identical defects in the XPC DNA nucleotide excision repair (NER) gene. Patient XP21BE, a 27-year-old woman, had developmental delay and early onset of sensorineural hearing loss. In contrast, patient XP329BE, a 13-year-old boy, had a normal neurological examination. Both patients had marked lentiginous hyperpigmentation and multiple skin cancers at an early age. Their cultured fibroblasts showed similar hypersensitivity to killing by UV and reduced repair of DNA photoproducts. Cells from both patients had a homozygous c.2T>G mutation in the XPC gene which changed the ATG initiation codon to arginine (AGG). Both had low levels of XPC message and no detectable XPC protein on Western blotting. There was no functional XPC activity in both as revealed by the failure of localization of XPC and other NER proteins at the sites of UV-induced DNA damage in a sensitive in vivo immunofluorescence assay. XPC cDNA containing the initiation codon mutation was functionally inactive in a post-UV host cell reactivation (HCR) assay. Microsatellite markers flanking the XPC gene showed only a small region of identity ( approximately 30kBP), indicating that the patients were not closely related. Thus, the initiation codon mutation resulted in DNA repair deficiency in cells from both patients and greatly increased cancer susceptibility. The neurological abnormalities in patient XP21BE may be related to close consanguinity and simultaneous inheritance of other recessive genes or other gene modifying effects rather than the influence of XPC gene itself.

Multiphoton time-domain fluorescence lifetime imaging microscopy: practical application to protein–protein interactions using global analysis

Förster resonance energy transfer (FRET) detected via fluorescence lifetime imaging microscopy (FLIM) and global analysis provide a way in which protein–protein interactions may be spatially localized and quantified within biological cells. The FRET efficiency and proportion of interacting molecules have been determined using bi-exponential fitting to time-domain FLIM data from a multiphoton time-correlated single-photon counting microscope system. The analysis has been made more robust to noise and significantly faster using global fitting, allowing higher spatial resolutions and/or lower acquisition times. Data have been simulated, as well as acquired from cell experiments, and the accuracy of a modified Levenberg–Marquardt fitting technique has been explored. Multi-image global analysis has been used to follow the epidermal growth factor-induced activation of Cdc42 in a short-image-interval time-lapse FLIM/FRET experiment. Our implementation offers practical analysis and time-resolved-image manipulation, which have been targeted towards providing fast execution, robustness to low photon counts, quantitative results and amenability to automation and batch processing.

Comparative study of nucleotide excision repair defects between XPD-mutated fibroblasts derived from trichothiodystrophy and xeroderma pigmentosum patients

To get a clue to understand how mutations in the XPD gene result in different skin cancer susceptibilities in patients with xeroderma pigmentosum (XP) or trichothiodystrophy (TTD), a thorough understanding of their nucleotide excision repair (NER) defects is essential. Here, we extensively characterize the possible causes of NER defects in XP-D and in TTD fibroblasts. The 3 XP-D cell strains examined were similarly deficient in repairing UV-induced cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts (6-4PPs) from genomic DNA. The severity of NER defects correlated with their UV sensitivities. Possible alterations of TFIIH (which consists of 10 subunits including XPD) were then examined. All XP-D cell strains were normal in their concentrations of TFIIH, and displayed normal abilities to recruit TFIIH to sites of UV-induced DNA damage. However, replication protein A (RPA; single-stranded DNA binding protein) accumulation at DNA damage sites, which probably reflects the in vivo XPD helicase activity of TFIIH, is similarly impaired in all XP-D cell strains. Meanwhile, all 3 TTD cell strains had approximately 50% decreases in cellular TFIIH content. Importantly, 2 of the 3 TTD cell strains, which carry the major XPD mutations found in TTD patients, showed defective recruitment of TFIIH to DNA damage sites. Moreover, RPA accumulation at damage sites was impaired in all TTD cell strains to different degrees, which correlated with the severity of their NER defects. These results demonstrate that XP-D and TTD cells are both deficient in the repair of CPDs and 6-4PPs, but TTD cells have more multiple causes for their NER defects than do XP-D cells. Since TFIIH is a repair/transcription factor, TTD-specific alterations of TFIIH possibly result in transcriptional defects, which might be implication for the lack of increased incidence of skin cancers in TTD patients.

Persistence of repair proteins at unrepaired DNA damage distinguishes diseases with ERCC2 (XPD) mutations: cancer-prone xeroderma pigmentosum vs. non-cancer-prone trichothiodystrophy

Patients with xeroderma pigmentosum (XP) have a 1,000-fold increase in ultraviolet (UV)-induced skin cancers while trichothiodystrophy (TTD) patients, despite mutations in the same genes, ERCC2 (XPD) or ERCC3 (XPB), are cancer-free. Unlike XP cells, TTD cells have a nearly normal rate of removal of UV-induced 6-4 photoproducts (6-4PP) in their DNA and low levels of the basal transcription factor, TFIIH. We examined seven XP, TTD, and XP/TTD complex patients and identified mutations in the XPD gene. We discovered large differences in nucleotide excision repair (NER) protein recruitment to sites of localized UV damage in TTD cells compared to XP or normal cells. XPC protein was rapidly localized in all cells. XPC was redistributed in TTD, and normal cells by 3 hr postirradiation, but remained localized in XP cells at 24-hr postirradiation. In XP cells recruitment of other NER proteins (XPB, XPD, XPG, XPA, and XPF) was also delayed and persisted at 24 hr (p<0.001). In TTD cells with defects in the XPD, XPB, or GTF2H5 (TTDA) genes, in contrast, recruitment of these NER proteins was reduced compared to normals at early time points (p<0.001) and remained low at 24 hr postirradiation. These data indicate that in XP persistence of NER proteins at sites of unrepaired DNA damage is associated with greatly increased skin cancer risk possibly by blockage of translesion DNA synthesis. In contrast, in TTD, low levels of unstable TFIIH proteins do not accumulate at sites of unrepaired photoproducts and may permit normal translesion DNA synthesis without increased skin cancer.

Polydeoxyribonucleotide promotes cyclobutane pyrimidine dimer repair in UVB-exposed dermal fibroblasts

BACKGROUND

DNA is the main cellular chromophore for ultraviolet B (UVB). Its absorption leads to the generation of typical photoproducts. The most frequent types (about 80%) are cyclobutane pyrimidine dimers (CPDs). Several studies have suggested that treatment with deoxyribonucleosides can protect some cell types from DNA damage. The aim of this work was to evaluate the ability of the polydeoxyribonucleotide (PDRN) to protect human dermal fibroblasts from UVB-induced DNA damage.

METHODS

Human dermal fibroblasts were irradiated with 600 mJ/cm(2) of UVB radiation. Cells were analyzed at increasing time points from irradiation to study the recovery from UVB-induced DNA photodamage. Damage repair was subsequently assessed by immunocytochemical analysis of CPDs levels and by measurement of p53 protein expression.

RESULTS

The extracellular addition of 100 microg/ml PDRN immediately after irradiation caused a strong activation of p53 protein in the first 24 h. This signal was accompanied by an increase in CPDs repair rates at early time points of recovery.

CONCLUSIONS

The addition of PDRN to the culture medium supports CPDs repair probably providing a faster supply of precursors for the deoxyribonucleotide triphosphates pool necessary to UVB-damaged cells. This condition could promote the action of the salvage pathway, thereby accelerating DNA repair, but other inducible responses linked to increased p53 expression could be involved.

Influence of XPB helicase on recruitment and redistribution of nucleotide excision repair proteins at sites of UV-induced DNA damage

The XPB DNA helicase, a subunit of the basal transcription factor TFIIH, is also involved in nucleotide excision repair (NER). We examined recruitment of NER proteins in XP-B cells from patients with mild or severe xeroderma pigmentosum (XP) having different XPB mutations using local UV-irradiation through filters with 5 microm pores combined with fluorescent antibody labeling. XPC was rapidly recruited to UV damage sites containing DNA photoproducts (cyclobutane pyrimidine dimers, CPD) in all the XP-B and normal cells, thus reflecting its role in damage recognition prior to the function of XPB. Cells from the mild XP-B patients, with a missense mutation, showed delayed recruitment of all NER proteins except XPC to UV damage sites, demonstrating that this mutation impaired localization of these proteins. Surprisingly, in cells from severely affected patients, with a C-terminal XPB mutation, XPG and XPA proteins were normally recruited to UV damage sites demonstrating that this mutation permits recruitment of XPG and XPA. In marked contrast, in all the XP-B cells recruitment of XPF was absent immediately after UV and was delayed by 0.5 and 3 h in cells from the mild and severely affected XP patients, respectively. Redistribution of NER proteins was nearly complete in normal cells by 3 h but by 24 h redistribution was only partially present in cells from mild patients and virtually absent in cells from the severely affected patients. Ineffectual repair of UV-induced photoproducts resulting from delayed recruitment and impaired redistribution of NER proteins may contribute to the markedly increased frequency of skin cancer in XP patients.

Neurons and astrocytes exhibit lower activities of global genome nucleotide excision repair than do fibroblasts

Nucleotide excision repair (NER) is a DNA repair pathway, which eliminates various types of helix-distorting DNA damage including some forms of oxidative damage and UV-induced photoproducts. To understand why patients with NER-defective disorders develop progressive neurological abnormalities, we investigated NER capabilities in neural cells. Primary cultured neurons and astrocytes derived from rat embryonic brains were prepared in mixed-cell cultures, and fibroblasts from the same embryos were cultured for comparison. Neurons in culture were unable to proliferate, while cultured astrocytes maintained that capacity. Determination of (6-4) photoproducts in situ using antibodies against those DNA lesions was used to measure NER capabilities in individual neural cells, which were identified by staining with specific cell markers. The results demonstrate that both neurons and astrocytes have significantly lower NER capabilities than fibroblasts. That result was consistent with the finding that levels of an NER-related protein (proliferating cell nuclear antigen, PCNA) recruited at the localized UV-damage sites were lower in neurons and in astrocytes than in fibroblasts. Interestingly, the degrees of NER deterioration in those neural cells were almost equivalent to those found in NER-defective human fibroblasts (TTD2VI) that show an increased sensitivity to UV. Thus, the present study suggests that an attenuated NER capacity is not specific to post-mitotic neurons, but may be common to neural cells constituting the central nervous system regardless of their residual proliferative capacity. Although the reduced but substantial NER capability of neural cells is indispensable to preventing progressive neurological abnormalities, that low NER capability might have implications for brain ageing.

Low-molecular-weight vitellogenin 1-like proteins are components of a UV-damaged-DNA binding activity highly expressed in zebrafish (Danio rerio) embryos

A strong UV-damaged-DNA binding activity had been detected in the extracts of zebrafish embryos at 12 hr after fertilization by gel shift assay (Hsu et al. 2002. Fish Physiol Biochem 25:41-51). We attempted to study the components of this binding activity and their importance in DNA damage recognition. Among the proteins extracted from gel retardation complexes, a 30- and a 35-kDa polypeptide binding preferentially to 6-4photoproducts (6-4PPs) generated by UV irradiation were identified by peptide mass fingerprinting (PMF) as homologs of zebrafish vitellogenin I (zfVg1), a 150-kDa metalloprotein known as the precursor of yolk proteins in embryos. zfVg1-like polypeptides ranging from 25 to 105 kDa were detected in 12- and 96-hr-old zebrafish extracts by immunoblot analysis. Immunoblot analysis of affinity-captured proteins confirmed the preferential binding of the 30-35-kDa polypeptides to the 6-4PP probe, while 96-hr-old larval extracts containing very low levels of these two factors failed to recognize 6-4PPs. The presence of zfVg1-like factors was important in maintaining the embryonic UV-binding activity, as inclusion of a monoclonal anti-zfVg1 antibody in reaction mixtures caused a concentration-dependent reduction in 6-4PP-specific binding. In contrast, DNA damage recognition was not disturbed at all by an anti-HSP 70 antibody. The formation of 6-4PP-binding complexes was abolished after the addition of the metal chelating agent 1,10-phenanthroline (OP) to zebrafish extracts and the loss of UV-binding capacity correlated with the disappearance of the 35-kDa factor in OP-treated extracts. Our results demonstrated the ability of low-molecular-weight zfVg1-like proteins in zebrafish embryos to bind UV-damaged DNA and the expression of this embryonic UV-binding activity was metal dependent. Whether zfVg1-like UV-binding proteins are involved in repairing damaged DNA in embryos or in processing helical structures similar to UV-distorted DNA needs further investigation.

Spatiotemporal dynamics of p21CDKN1A protein recruitment to DNA-damage sites and interaction with proliferating cell nuclear antigen

The cyclin-dependent kinase inhibitor p21CDKN1A plays a fundamental role in the DNA-damage response by inducing cell-cycle arrest, and by inhibiting DNA replication through association with the proliferating cell nuclear antigen (PCNA). However, the role of such an interaction in DNA repair is poorly understood and controversial. Here, we provide evidence that a pool of p21 protein is rapidly recruited to UV-induced DNA-damage sites, where it colocalises with PCNA and PCNA-interacting proteins involved in nucleotide excision repair (NER), such as DNA polymerase δ, XPG and CAF-1. In vivo imaging and confocal fluorescence microscopy analysis of cells coexpressing p21 and PCNA fused to green or red fluorescent protein (p21-GFP, RFP-PCNA), showed a rapid relocation of both proteins at microirradiated nuclear spots, although dynamic measurements suggested that p21-GFP was recruited with slower kinetics. An exogenously expressed p21 mutant protein unable to bind PCNA neither colocalised, nor coimmunoprecipitated with PCNA after UV irradiation. In NER-deficient XP-A fibroblasts, p21 relocation was greatly delayed, concomitantly with that of PCNA. These results indicate that early recruitment of p21 protein to DNA-damage sites is a NER-related process dependent on interaction with PCNA, thus suggesting a direct involvement of p21 in DNA repair.

Colcemid inhibits the rejoining of the nucleotide excision repair of UVC-induced DNA damages in Chinese hamster ovary cells

In our previous study, we found that colcemid, an inhibitor of mitotic spindle, promotes UVC-induced apoptosis in Chinese hamster ovary cells (CHO.K1). In this study, a brief treatment of colcemid on cells after but not before UV irradiation could synergistically reduce the cell viability. Although colcemid did not affect the excision of UV-induced DNA damages such as [6-4] photoproducts or cyclobutane pyrimidine dimers, colcemid accumulated the DNA breaks when it was added to cells following UV-irradiation. This colcemid effect required nucleotide excision repair (NER) since the same accumulation of DNA breaks was barely or not detected in two NER defective strains of CHO cells, UV5 or UV24. Furthermore, the colcemid effect was not due to semi-conservative DNA replication or mitosis since the colcemid-caused accumulation of DNA breaks was also seen in non-replicating cells. Moreover, colcemid inhibited rejoining of DNA breaks accumulated by hydroxyurea/cytosine arabinoside following UV irradiation. Nevertheless, colcemid did not affect the unscheduled DNA synthesis as assayed by the incorporation of bromodeoxyuridine. Taken together, our results suggest that colcemid might inhibit the step of ligation of NER pathways.

In situ detection of acetylaminofluorene-DNA adducts in human cells using monoclonal antibodies

The present study was performed to generate monoclonal antibodies capable of detecting N-acetoxy-2-acetylaminofluorene (NA-AAF)-derived DNA adducts in human cells in situ. As an immunogen, we employed NA-AAF-modified single-stranded DNA coupled electrostatically to methylated protein and we produced five different monoclonal antibodies. All of them showed strong binding to NA-AAF-modified DNA, but had undetectable or minimal binding to undamaged DNA. Competitive inhibition experiments revealed that the epitope recognized by these antibodies is N-(deoxyguanosin-8-yl)-2-acetylaminofluorene (dG-C8-AAF) in DNA, although deacetylated N-(deoxyguanosin-8-yl)-2-aminofluorene in DNA is also recognized with slightly less efficiency. In contrast, these antibodies did not bind to 3-(deoxyguanosin-N(2)-yl)-2-acetylaminofluorene in DNA or to UV-induced lesions in DNA. Interestingly, they showed only minimal binding to small AAF-nucleoside adducts (dG-C8-AAF), indicating that DNA regions flanking a DNA-bound adduct, in addition to the adduct itself, are essential for the stable binding of the antibodies. Using an enzyme-linked immunosorbent assay with the most promising antibody (AAF-1), we detected the concentration-dependent induction of NA-AAF-modified adducts in DNA from repair deficient xeroderma pigmentosum (XP) cells treated with physiological concentrations of NA-AAF. Moreover, the assay enabled to confirm that normal human cells efficiently repaired NA-AAF-induced DNA adducts but not XP-A cells. Most importantly, the formation of NA-AAF-induced DNA adducts in individual nuclei of XP cells could be clearly visualized using indirect immunofluorescence. Thus, we succeeded in establishing novel monoclonal antibodies capable of the in situ detection of NA-AAF-induced DNA adducts in human cells.

Low concentrations of formaldehyde induce DNA damage and delay DNA repair after UV irradiation in human skin cells

Long-term occupational exposure to formaldehyde (FA) increases the risk for nasopharyngeal squamous cell carcinoma. As the skin is also in contact with FA by environmental exposure, we tested the genotoxic properties of appropriate low concentrations (<100 microM) of FA on cultured keratinocytes and fibroblasts of human skin. The initial DNA damage was assessed by comet assay. The induction of DNA protein crosslinks was measured by the ability of FA to reduce DNA migration induced by methyl-methane-sulfonate. Upon 4 h of exposure to FA, significant (P < 0.05) crosslink formations were observed in fibroblasts (50 microM FA) and in keratinocytes (25 microM FA). Upon 8 h of exposure to FA (25 microM FA), significant crosslink formations were observed in both the cell types. FA is known to inhibit different DNA repair pathways. Therefore, we studied the effect of FA on UV-induced repair. Human keratinocytes and fibroblasts exposed to 10 microM FA prior to UV irradiation showed disturbed repair kinetics after UVC and UVB, but not after UVA irradiation. Single-strand breaks (SSBs) derived from nucleotide excision repair disappeared 6 h after solely UVC (3 mJ/cm2) or 3 h solely UVB (30 mJ/cm2) exposure in both the cell types. In the presence of FA, SSBs were still present at these time points containing a reference to a delay in DNA resynthesis/ligation. FA at a concentration not inducing micronuclei (12.5 microM) caused significant increase of UVC-induced (4 mJ/cm2) chromosomal damage. Proliferation of keratinocytes and fibroblasts was in parallel to observed DNA damages. In conclusion, our data suggest that environmental exposure to FA may contribute to UV-induced skin carcinogenesis.

Trichothiodystrophy fibroblasts are deficient in the repair of ultraviolet-induced cyclobutane pyrimidine dimers and (6-4)photoproducts

A photosensitive form of trichothiodystrophy (TTD) results from mutations in the same XPD gene as the DNA-repair-deficient genetic disorder xeroderma pigmentosum group D (XP-D). Nevertheless, unlike XP, no increase in skin cancers appears in patients with TTD. Although the ability to repair ultraviolet (UV)-induced DNA damage has been examined to explain their cancer-free phenotype, the information accumulated to date is contradictory. In this study, we determined the repair kinetics of cyclobutane pyrimidine dimers (CPD) and (6-4)photoproducts (6-4PP) in three TTD cell strains using an enzyme-linked immunosorbent assay. We found that all three TTD cell strains are deficient in the repair of CPD and of 6-4PP. UV sensitivity correlated well with the severity of repair defects. Moreover, accumulation of repair proteins (XPB and proliferating cell nuclear antigen) at localized DNA damage sites, detected using micropore UV irradiation combined with fluorescent antibody labeling, reflected their DNA repair activity. Importantly, mutations of the XPD gene affected both the recruitment of the TFIIH complex to DNA damage sites and the TFIIH expression. Our results suggest that there is no major difference in the repair defect between TTD and XP-D and that the cancer-free phenotype in TTD is unrelated to a DNA repair defect.

UV-induced skin damage

Solar radiation induces acute and chronic reactions in human and animal skin. Chronic repeated exposures are the primary cause of benign and malignant skin tumors, including malignant melanoma. Among types of solar radiation, ultraviolet B (290-320 nm) radiation is highly mutagenic and carcinogenic in animal experiments compared to ultraviolet A (320-400 nm) radiation. Epidemiological studies suggest that solar UV radiation is responsible for skin tumor development via gene mutations and immunosuppression, and possibly for photoaging. In this review, recent understanding of DNA damage caused by direct UV radiation and by indirect stress via reactive oxygen species (ROS) and DNA repair mechanisms, particularly nucleotide excision repair of human cells, are discussed. In addition, mutations induced by solar UV radiation in p53, ras and patched genes of non-melanoma skin cancer cells, and the role of ROS as both a promoter in UV-carcinogenesis and an inducer of UV-apoptosis, are described based primarily on the findings reported during the last decade. Furthermore, the effect of UV on immunological reaction in the skin is discussed. Finally, possible prevention of UV-induced skin cancer by feeding or topical use of antioxidants, such as polyphenols, vitamin C, and vitamin E, is discussed.

The total amount of DNA damage determines ultraviolet-radiation-induced cytotoxicity after uniformor localized irradiation of human cells

We have recently developed a micropore ultraviolet irradiation technique. An isopore membrane filter with 3 microm diameter pores shields ultraviolet C radiation from cultured human fibroblasts, leading to partial irradiation within the cells with an average of about three exposed areas per nucleus. This study addressed the question of whether the spatial distribution of DNA damage within a cell nucleus is important in triggering ultraviolet-induced cytotoxicity. We have examined whether there are differences in cytotoxicity between partially ultraviolet-irradiated cells and uniformly irradiated cells after equal amounts of DNA damage were induced in the cell nuclei. We first determined DNA damage formation in normal human fibroblasts using an enzyme-linked immunosorbent assay. We found that 5 J per m2 ultraviolet irradiation produced an equivalent amount of cyclobutane pyrimidine dimers and (6-4) photoproducts per cell as 100 J per m2 with the membrane filter shield. At those doses, we found that both types of ultraviolet irradiation induced similar levels of cytotoxicity as assessed by a 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium assay. Both types of ultraviolet-irradiated cells also had similar cell-cycle distribution and apoptosis as measured by flow cytometry. Moreover, no significant differences in repair kinetics for either type of photolesion were observed between the two different ultraviolet treatments. Similar results were obtained in Cockayne syndrome cells that are defective in transcription-coupled nucleotide excision repair. Present results indicate that in the range of photoproducts studied, the spatial distribution of DNA damage within a cell is less important than the amount of damage in triggering ultraviolet-induced cytotoxicity.