UV-induced Cell Research Articles

Nse1 and Nse4, subunits of the Smc5-Smc6 complex, are involved in Dictyostelium development upon starvation.

The Smc5-Smc6 complex contains a heterodimeric core of two SMC proteins and non-Smc elements (Nse1-6), and plays an important role in DNA repair. We investigated the functional roles of Nse4 and Nse1 in Dictyostelium discoideum. Nse4 and Nse3 expressed as Flag-tagged fusion proteins were highly enriched in nuclei, while Nse1 was localized in whole cells. Using yeast two-hybrid assays, only the interaction between Nse3 and Nse1 was detected among the combinations. However, all of the interactions among these three proteins were recognized by co-immunoprecipitation assay using cell lysates prepared from the cells expressing green fluorescent protein (GFP)- or Flag-tagged fusion proteins. GFP-tagged Nse1, which localized in whole cells, was translocated to nuclei when co-expressed with Flag-tagged Nse3 or Nse4. RNAi-mediated Nse1 and Nse4 knockdown cells (Nse1 KD and Nse4 KD cells) were generated and found to be more sensitive to UV-induced cell death than control cells. Upon starvation, Nse1 and Nse4 KD cells had increases in the number of smaller fruiting bodies that formed on non-nutrient agar plates or aggregates that formed under submerged culture. We found a reduction in the mRNA level of pdsA, in vegetative and 8 h-starved Nse4 KD cells, and pdsA knockdown cells displayed effects similar to Nse4 KD cells. Our results suggest that Nse4 and Nse1 are involved in not only the cellular DNA damage response but also cellular development in D. discoideum.

The silkworm GSTe4 is sensitive to phoxim and protects HEK293 cells against UV-induced cell apoptosis.

Glutathione S-transferases (GSTs, EC 2.5.1.18) are a family of super enzymes with multiple functions that play a major role in the detoxification of endogenous and xenobiotic compounds. In our previous study, we have predicted 23 putative cytosolic GSTs in the silkworm genome using bioinformatic methods. In this study, we cloned and studied the insect-specific epsilon-class GST gene GSTe4 from the silkworm, Bombyx mori. The recombinant BmGSTe4 (Bac-BmGSTe4) was overexpressed in SF-9 cell lines, and it was found to have effective GST activity. We also found that the expression of BmGSTe4 was especially down-regulated after the silkworms were fumigated with or ingested phoxim. Moreover, BmGSTe4 protected HEK293 cells against UV-induced cell apoptosis. These results demonstrated that BmGSTe4 has GST activity, is sensitive to phoxim, and plays a role in inhibition of UV-induced cell apoptosis.

Novel Antioxidant Tripeptide "ACQ" Can Prevent UV-Induced Cell Death and Preserve the Number of Epidermal Stem Cells.

We found that tripeptide “ACQ: alanine-cysteine-glutamine” has significant DPPH scavenging activity compared to that of glutathione. Antioxidant effects of ACQ were tested in in vitro and in vivo models. When treated with H2O2, mock treated fibroblasts and keratinocytes showed strong staining by H2DCFA. But, ACQ showed good protective effects against hydrogen peroxide treatment. When mice were fed for 2 or 4 weeks, similar protective effects were observed. In the control group, epidermis was severely damaged by UV irradiation and apoptotic keratinocytes were observed. There were also numerous TUNEL positive cells. But in the ACQ group, epidermis became thicker and there was no sign of severe damage. Interestingly, the number of p63 cells was also higher in ACQ fed mice. To confirm the stem cell rescuing effects of ACQ, three-dimensional skin samples were constructed. Results showed that ACQ increased the expression of integrin α6 and the number of p63 positive cells. These findings showed that ACQ has good antioxidant activity and may increase stem cell activities by the regulation of integrin α6.

Cerium oxide nanoparticles, combining antioxidant and UV shielding properties, prevent UV-induced cell damage and mutagenesis.

Efficient inorganic UV shields, mostly based on refracting TiO2 particles, have dramatically changed the sun exposure habits. Unfortunately, health concerns have emerged from the pro-oxidant photocatalytic effect of UV-irradiated TiO2, which mediates toxic effects on cells. Therefore, improvements in cosmetic solar shield technology are a strong priority. CeO2 nanoparticles are not only UV refractors but also potent biological antioxidants due to the surface 3+/4+ valency switch, which confers anti-inflammatory, anti-ageing and therapeutic properties. Herein, UV irradiation protocols were set up, allowing selective study of the extra-shielding effects of CeO2vs. TiO2 nanoparticles on reporter cells. TiO2 irradiated with UV (especially UVA) exerted strong photocatalytic effects, superimposing their pro-oxidant, cell-damaging and mutagenic action when induced by UV, thereby worsening the UV toxicity. On the contrary, irradiated CeO2 nanoparticles, via their Ce(3+)/Ce(4+) redox couple, exerted impressive protection on UV-treated cells, by buffering oxidation, preserving viability and proliferation, reducing DNA damage and accelerating repair; strikingly, they almost eliminated mutagenesis, thus acting as an important tool to prevent skin cancer. Interestingly, CeO2 nanoparticles also protect cells from the damage induced by irradiated TiO2, suggesting that these two particles may also complement their effects in solar lotions. CeO2 nanoparticles, which intrinsically couple UV shielding with biological and genetic protection, appear to be ideal candidates for next-generation sun shields.

Anti-tyrosinase, antioxidant and UV-protective agents obtained from natural sources: A high-throughput bioguided approach

Tyrosinase is regarded as a target enzyme in skin-whitening and in therapeutic interventions for dermatological disorders associated with melanin hyperpigmentation. Abnormal melanin pigments are an aesthetic problem prevalent in middle-aged and elderly individuals [1], but may also occur upon UV radiation [2]. Skin ageing on the other hand, is associated with free radical formation and UV light exposure through activation of the reactive oxygen species (ROS) system [3]. Since many known agents have been proven toxic [4], tyrosinase inhibitors and antioxidant compounds from natural sources are in the center of research interest for the development of cosmeceutical products.This project with the acronym AGROCOS (FP7) aimed to carry to the stage of development candidates, plant derived small molecules with skin-whitening, antioxidant and UV-protective properties. 460 natural compounds with structurally diverse scaffolds and decoration patterns were evaluated for the above mentioned properties. The screens addressed radical scavenging properties, protection against reactive oxygen species (ROS) and UV-induced cell death in cell-based assays, and anti-tyrosinase properties. Based on this screening, a collection of 3600 extracts from biodiversity hotspots plants was emerged and screened on the same targets. 31 most antioxidant extracts, including many representatives from Myrtaceae and Fabaceae families, and 13 extracts with anti-tyrosinase properties, such as Glycyrrhiza glabra and Caesalpinia schlechteri were prioritized for an HPLC-based bioguided microfractionation. Active fractions were submitted to a final purification of bioactive compounds.This unique extract library combined with a high-throughput approach proved to be an efficient tool for the discovery of natural agents with antioxidant, UV-protective and anti-tyrosinase properties.

Improvement of polyphenol properties upon glucosylation in a UV-induced skin cell ageing model.

Polyphenols are strong antioxidant molecules allowing prevention of skin photo-ageing damages, but their use is limited due to low solubility and toxicity towards skin cells. We postulated that enzymatic glucosylation could improve their solubility, stability and, consequently, their efficacy. The aim of this work was to study changes induced by addition of a glucose moiety on two polyphenols displaying very different chemical structures [caffeic acid (CA), epigallocatechin-3-gallate (EGCG) and there glucosylated form, Glc-CA and Glc-EGCG] by assessing their cytotoxic properties and their antioxidant and anti-inflammatory activities. Their antioxidant effect was assessed first by the classical DPPH radical-scavenging method. Then, a panel of human skin cells (keratinocytes, melanocytes, fibroblasts and endothelial cells) was used to evaluate their effect on cell toxicity and their antioxidant activities. With this aim, a photo-ageing model based on UV irradiation of skin cells was established. Molecule activity was assessed on reactive oxygen species (ROS) production, on superoxide dismutase (SOD) and catalase activities and, finally, on inflammatory factor production IL-6, IL-8 and IL-1β. In an acellular model, antioxidant activity assessed by DPPH method was strongly reduced for Glc-CA compared to CA, whereas it remained the same for Glc-EGCG compared to EGCG. Glucosylated derivatives did not display more toxic effect on various skin cells. Moreover, toxicity was even strongly reduced for caffeic acid upon glucosylation. The efficacy of glucosyl-compounds against UV-induced ROS production was preserved, both with pre- and post-UV treatments. Particularly, a better antioxidant efficacy was shown by Glc-EGCG, vs. EGCG, on keratinocytes. In addition, an induction of SOD and catalase activity was clearly observed for Glc-CA. Both glucosyl-polyphenols display the same activity as their parent molecule in decreasing inflammatory factor production. Our results demonstrated that enzymatic glucosylation of CA and EGCG led to an improved or preserved antioxidant activity in a cellular model of UV-induced skin ageing, despite the decrease in instantaneous antioxidant properties observed for Glc-CA. Glc-EGCG is specifically more active on keratinocytes, suggesting a specific targeting. Such glucosylated polyphenols displaying improved physicochemical and biological properties should be better candidates than natural ones for use in food additives and cosmetics.

UV조사에 의해 유도된 피부섬유아세포의 세포사에 미치는 Sea Buckthorn (Hippophae rhamnoides L.) 열매추출물의 영향

Sea buckthorn (Hippophae rhamnoides L.)은 flavonoid, carotenoids, steroids, vitamin, tannins, oleic acid 등의 생물학적으로 활성이 높은 성분을 풍부하게 함유하고 있어 오랫동안 피부질환(skin diseases), 위궤양(gastric ulcers), 천식(asthma), 폐질환 등에 사용되어 왔다. 본 연구에서는 아시아와 유럽의 고지대에서 야생으로 자라는 Sea buckthorn의 열매추출물(SBFE)이 UV조사 후 유도되는 세포사와 연관성을 규명하기 위하여, 세포사와 관련된 다양한 인자의 변화를 관찰하였다. 그 결과, 세포사를 유발할 수 있는 UV조사농도는 400 mJ로 설정하였고, 세포생존율은 UV + LoC, UV + MeC, UV + HiC 처리군에서는 유의적으로 증가하였다. 또한 FACS와 DAPI염색의 결과에서 PI로 염색된 세포수는 UV + vehicle 처리군에서 급격히 증가한 이후 UV + SBFE 처리군에서 유의적으로 감소하였고, Annexin V가 염색된 세포 수는 고농도의 SFBE을 처리한 UV + HiC 처리군에서 유의적으로 감소하였다. 그러나 PI와 Annexin V가 동시에 염색된 세포의 수는 UV + LoC 처리군에서 오히려 증가하였고 나머지 그룹에서는 유의적인 변화가 관찰되지 않았다. 한편, 세포사와 관련된 단백질의 변화에 대한 연구에서 caspase-3는 pro 형태와 active 형태 모두에서 유의적인 차이를 나타내지 않았으며, Bax와 Bcl-2 모두 UV 무처리군에 비하여 UV + Vehicle 처리군에서 감소하였으나 Bax의 발현양은 UV + SBFE 처리군에서 농도의존적으로 증가한 반면, Bcl-2의 발현양은 UV + SBFE 처리군에서 추가적으로 더욱 감소하였다. 따라서 이상의 결과들은 SBFE가 UV조사에 의해 유도된 세포사를 억제하거나 회복시키는 기능을 가지고 있음을 제시하고 있어 향후 UV조사에 의해 유발되는 다양한 상해에 대한 치료제로서의 가능성을 제시하고 있다. Sea buckthorn (Hippophae rhamnoides L.) is a well-known and rich source of biologically active compounds, such as flavonoids, carotenoids, steroids, vitamins, tannins, and oleic acid. The effects of sea buckthorn fruit extract (SBFE) on ultraviolet (UV)-induced cell death was investigated in SK-MEL-2 cells cotreated with UV and a low concentration (LoC), medium concentration (MeC), or high concentration (HiC) of SBFE. Cell viability gradually decreased in accordance with an increase in the UV dose. The cell viability of the UV+SBFE cotreated cells increased significantly compared to that of UV+vehicle-treated cells during the application of an appropriate UV radiation dose (400 mJ). In addition, the number of 4',6-diamidino-2-phenylindole (DAPI), propidium iodine (PI)-, and annexin V-stained apoptotic cells was higher in the UV+vehicle-treated cells than in the UV untreated cells. The decrease of apoptotic cell numbers varied in each treated group, but it was most significant in the SBFE-treated group. The number of PI-stained cells dramatically decreased in accordance with the concentration of SBFE, and the maximum decrease was detected in the UV+HiC-treated group. In addition, Bax expression increased and Bcl-2 expression decreased in the SBFE-treated group compared with the UV-only treated group. The level of caspase-3 remained constant in all the groups. These results suggest that SBFE may contribute to a recovery from UV-induced cell death through the regulation of apoptotic protein expression and that it may have potential therapeutic utility in ameliorating UV-induced skin ageing.

Histone H3 K79 methylation states play distinct roles in UV-induced sister chromatid exchange and cell cycle checkpoint arrest in Saccharomyces cerevisiae.

Histone post-translational modifications have been shown to contribute to DNA damage repair. Prior studies have suggested that specific H3K79 methylation states play distinct roles in the response to UV-induced DNA damage. To evaluate these observations, we examined the effect of altered H3K79 methylation patterns on UV-induced G1/S checkpoint response and sister chromatid exchange (SCE). We found that the di- and trimethylated states both contribute to activation of the G1/S checkpoint to varying degrees, depending on the synchronization method, although methylation is not required for checkpoint in response to high levels of UV damage. In contrast, UV-induced SCE is largely a product of the trimethylated state, which influences the usage of gene conversion versus popout mechanisms. Regulation of H3K79 methylation by H2BK123 ubiquitylation is important for both checkpoint function and SCE. H3K79 methylation is not required for the repair of double-stranded breaks caused by transient HO endonuclease expression, but does play a modest role in survival from continuous exposure. The overall results provide evidence for the participation of H3K79 methylation in UV-induced recombination repair and checkpoint activation, and further indicate that the di- and trimethylation states play distinct roles in these DNA damage response pathways.

Small Kinetochore Associated Protein (SKAP) Promotes UV-Induced Cell Apoptosis through Negatively Regulating Pre-mRNA Processing Factor 19 (Prp19)

Apoptosis is a regulated cellular suicide program that is critical for the development and maintenance of healthy tissues. Previous studies have shown that small kinetochore associated protein (SKAP) cooperates with kinetochore and mitotic spindle proteins to regulate mitosis. However, the role of SKAP in apoptosis has not been investigated. We have identified a new interaction involving SKAP, and we propose a mechanism through which SKAP regulates cell apoptosis. Our experiments demonstrate that both overexpression and knockdown of SKAP sensitize cells to UV-induced apoptosis. Further study has revealed that SKAP interacts with Pre-mRNA processing Factor 19 (Prp19). We find that UV-induced apoptosis can be inhibited by ectopic expression of Prp19, whereas silencing Prp19 has the opposite effect. Additionally, SKAP negatively regulates the protein levels of Prp19, whereas Prp19 does not alter SKAP expression. Finally, rescue experiments demonstrate that the pro-apoptotic role of SKAP is executed through Prp19. Taken together, these findings suggest that SKAP promotes UV-induced cell apoptosis by negatively regulating the anti-apoptotic protein Prp19.

A comparative study of UV-induced cell signalling pathways in human keratinocyte-derived cell lines

Ultraviolet (UV) radiation can activate the p38 mitogen-activated protein kinase (MAPK), Jun N-terminal kinase (JNK) and nuclear factor-κB (NFκB) pathways in skin cells. HaCaT cells are widely used as a primary keratinocyte substitute to study these pathways. However, like most squamous cell carcinomas (SCCs), it contains a dysfunctional p53. It is unclear if HaCaT cells activate these signalling pathways similarly to SCC cells (Colo16) or to primary human epidermal keratinocytes (HEK). In this study, the UV activation (UVA, UVB, UVA+B, UVB+A) of p38 MAPK, JNK and NFκB pathways, and TNFα secretion by HEK, HaCaT and Colo16 cells were investigated. The signalling pathway activation was UV-type and dose-dependent with UVB+A radiation inducing a high p38 and JNK activation. HaCaT cells exhibited 2- to 4-fold higher activity of the p38 (771% at 60 min) and JNK (794% at 30 min) pathways following UVB+A radiation than did HEK cells (p38: 367% at 15 min and JNK: 184% at 30 min). While both HaCaT and Colo16 cells did not activate the NFκB pathway, Colo16 cells had a lower p38 and higher JNK activity than HaCaT cells. Irradiated HaCaT cells produced less TNFα (UVB: 3.5 pg/ml), while HEK cells produced the most (UVB: 1,296 pg/ml). When co-exposed to IL1α, irradiated HaCaT had the greatest fold of TNFα release (UVB: 16.2-fold, UVA+B: 8.9-fold and UVB+A: 6.1-fold). The pattern of activation and TNFα secretion of HaCaT cells mirrored that of Colo16 cells. It is likely that the presence of molecular alterations in HaCaT cells may be responsible for its different responses to that seen for HEK cells. The results of this study suggest caution in using HaCaT cells as a substitute for normal keratinocytes in investigating UV-induced cells signalling pathways.

An essential role for platelet-activating factor in activating mast cell migration following ultraviolet irradiation

The UVB (290-320 nm) radiation in sunlight is responsible for inducing skin cancer. Exposure to UV radiation is also immunosuppressive, and the systemic immune suppression induced by UV is a well-recognized risk factor for cancer induction. As UVB radiation is absorbed within the upper layers of the skin, indirect mechanisms must play a role in activating systemic immune suppression. One prominent example is mast cell migration, which from the skin to the draining LN is an essential step in the cascade of events leading to immune suppression. What triggers mast cell migration is not entirely clear. Here, we tested the hypothesis that PAF, a lipid mediator of inflammation produced by the skin in response to UV exposure, is involved. Mast cell-deficient mice (Kit(W-sh/W-sh)) are resistant to the suppressive effect of UV radiation, and reconstituting mast cell-deficient mice with normal bone marrow-derived mast cells restores susceptibility to immunosuppression. However, when mast cells from PAFR-/- mice were used, the reconstituted mice were not susceptible to the suppressive effects of UV. Furthermore, PAFR-/- mice showed impaired UV-induced mast cell migration when compared with WT mice. Finally, injecting PAF into WT mice mimicked the effect of UV irradiation and induced mast cell migration but not in PAFR-/- mice. Our findings indicate that PAFR binding induces mast cells to migrate from the skin to the LNs, where they mediate immune suppression.

Cell-laden microengineered and mechanically tunable hybrid hydrogels of gelatin and graphene oxide.

Incorporating graphene oxide inside GelMA hydrogels enhances their mechanical properties and reduces UV-induced cell damage while preserving their favorable characteristics for 3D cell encapsulation. NIH-3T3 fibroblasts encapsulated in GO-GelMA microgels demonstrate excellent cellular viability, proliferation, spreading, and alignment. GO reinforcement combined with a multi-stacking approach offers a facile engineering strategy for the construction of complex artificial tissues.

Abstract 1077: Dominant negative PI3-Kinase inhibits UV-induced squamous cell carcinoma in vivo.

Abstract Phosphoinositide-3 Kinase (PI3-Kinase) is an upstream regulator of cellular proliferation and anti-apoptotic signaling that has been implicated in cancer induction in many tissues, including skin. Pharmacological inhibition of PI3-Kinase is effective at inhibiting both chemical and UV-driven skin carcinogenesis. However, the specificity of these pharmacological inhibitors is often questioned. Therefore, specific genetic inhibition of PI3-Kinase in the epidermis needs to be carried out in mice. We report here on the generation of a new transgenic mouse which constitutively expresses the dominant negative mutant form of the regulatory subunit of the PI3-Kinase dimer, p85, in the epidermis under control of the keratin-5 promoter. These p85DN transgenic mice were derived on the SKH-1 hairless genetic background. A chronic UVB exposure experiment showed significant inhibition of both tumor size (p=0.0012) and tumor burden (p=0.0003) for mice carrying the transgene compared to wildtype littermates. The time to first tumor was also delayed in transgenic mice, but with only marginal significance (p=0.07). p85DN mice also showed significant inhibition of epidermal Activator Protein-1 (AP-1) transcription factor activity after acute UVB treatment (p=0.046). UVB induced activation of AP-1 in the epidermis has been shown to play a functional role in UVB mouse skin carcinogenesis. Further analysis of protein expression after acute treatment with UVB suggests that Akt signaling and COX-2 activation are both downregulated in p85DN transgenic mice. To our knowledge, this is the first report of genetic inhibition of PI3-Kinase in mouse skin carcinogenesis studies. Together, these data suggest a functional role of PI3-Kinase in UVB-induced squamous cell carcinoma and support the assertion that inhibition of this pathway is beneficial in chemoprevention of non-melanoma skin cancer. This work was supported in part by the following NIH grants: K07CA132956, K07CA132956-02S1, P01CA27502 and P30CA23074. Citation Format: Sally E. Dickinson, Erik R. Olson, Jadrian J. Rusche, P. Jane Criswell, Jack Zhang, Zagong Dong, Chengcheng Hu, Kathylynn Saboda, David S. Alberts, G. Timothy Bowden. Dominant negative PI3-Kinase inhibits UV-induced squamous cell carcinoma in vivo. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1077. doi:10.1158/1538-7445.AM2013-1077

Suppression of UV-Induced Wrinkle Formation by Induction of HSP70 Expression in Mice

UV-induced wrinkle formation owing to the degeneration of the extracellular matrix (ECM) is a major dermatological problem in which abnormal activation of matrix metalloproteinases (MMPs) and elastases have important roles. Heat shock protein 70 (HSP70) has cytoprotective and anti-inflammatory activities. In this study, we examined the effect of HSP70 expression on UV-induced wrinkle formation. Mild heat treatment (exposure to heated water at 42 °C) of the dorsal skin of hairless mice induced the expression of HSP70. The long-term repeated exposure to UV induced epidermal hyperplasia, decreased skin elasticity, degeneration of ECM, and wrinkle formation, which could be suppressed in mice concomitantly subjected to this heat treatment. The UV-induced epidermal hyperplasia, decreased skin elasticity, and degeneration of ECM were less apparent in transgenic mice expressing HSP70 than in wild-type mice. UV-induced fibroblast cell death, infiltration of inflammatory cells, and activation of MMPs and elastase in the skin were also suppressed in the transgenic mice. This study provides evidence for an inhibitory effect of HSP70 on UV-induced wrinkle formation. The results suggest that this effect is mediated by various properties of HSP70, including its cytoprotective and anti-inflammatory activities. We propose that HSP70 inducers used in a clinical context could prove beneficial for the prevention of UV-induced wrinkle formation.

The balneotherapy effect of hydrogen reduced water on UVB-mediated skin injury in hairless mice

Exposure to UVB radiation induced skin damage that results to increase risk of skin cancer. Despite the clinical importance of skin-induced damage, antioxidants imposed limited therapeutic success. Hydrogen molecule (H2) has been known as a safe antioxidant in the prevention and therapeutic approach towards several diseases. Drinking hydrogen reduced water (HRW), inhalation of hydrogen gas, and injecting H2-dissolved saline are widely accepted to incorporate H2 in the body. However, there is no document about the beneficial effect of hydrogen water bath. Here, we investigated the effect of hydrogen bathing on the UVB-induced skin damage in hairless mice. For this, mice of the bathing group are allowed to freely swim on HRW, and let the HRW penetrate for 60 mins. Scoring of skin injury, reactive oxygen species (ROS) enzyme activity quantification, cytokine analysis, and ultrastructural change of corneocytes were measured after exposure to UVB radiation of 360–540 mJ/cm2. In summary, the bathing with HRW significantly reduced the levels of skin damage, as well as increased activity of glutathione peroxidase. Further, the effect of HRW on cytokine network in the skin after UVB exposure revealed that HRW significantly decreased the level of inflammatory cytokines such as IL-1β, IL-6, TNF-α and IFN-γ. Finally, scanning electron microscopy data revealed low number of defected corneocytes and ultrastructural changes, suggesting that HRW bathing would protect UV-induced cell damage.

Vitamin D and Death by Sunshine

Exposure to sunlight is the major cause of skin cancer. Ultraviolet radiation (UV) from the sun causes damage to DNA by direct absorption and can cause skin cell death. UV also causes production of reactive oxygen species that may interact with DNA to indirectly cause oxidative DNA damage. UV increases accumulation of p53 in skin cells, which upregulates repair genes but promotes death of irreparably damaged cells. A benefit of sunlight is vitamin D, which is formed following exposure of 7-dehydrocholesterol in skin cells to UV. The relatively inert vitamin D is metabolized to various biologically active compounds, including 1,25-dihydroxyvitamin D3. Therapeutic use of vitamin D compounds has proven beneficial in several cancer types, but more recently these compounds have been shown to prevent UV-induced cell death and DNA damage in human skin cells. Here, we discuss the effects of vitamin D compounds in skin cells that have been exposed to UV. Specifically, we examine the various signaling pathways involved in the vitamin D-induced protection of skin cells from UV.

UV-Induced cell death in plants.

Plants are photosynthetic organisms that depend on sunlight for energy. Plants respond to light through different photoreceptors and show photomorphogenic development. Apart from Photosynthetically Active Radiation (PAR; 400–700 nm), plants are exposed to UV light, which is comprised of UV-C (below 280 nm), UV-B (280–320 nm) and UV-A (320–390 nm). The atmospheric ozone layer protects UV-C radiation from reaching earth while the UVR8 protein acts as a receptor for UV-B radiation. Low levels of UV-B exposure initiate signaling through UVR8 and induce secondary metabolite genes involved in protection against UV while higher dosages are very detrimental to plants. It has also been reported that genes involved in MAPK cascade help the plant in providing tolerance against UV radiation. The important targets of UV radiation in plant cells are DNA, lipids and proteins and also vital processes such as photosynthesis. Recent studies showed that, in response to UV radiation, mitochondria and chloroplasts produce a reactive oxygen species (ROS). Arabidopsis metacaspase-8 (AtMC8) is induced in response to oxidative stress caused by ROS, which acts downstream of the radical induced cell death (AtRCD1) gene making plants vulnerable to cell death. The studies on salicylic and jasmonic acid signaling mutants revealed that SA and JA regulate the ROS level and antagonize ROS mediated cell death. Recently, molecular studies have revealed genes involved in response to UV exposure, with respect to programmed cell death (PCD).

Differential Effect of Schisandrin B Stereoisomers on ATR-Mediated DNA Damage Checkpoint Signaling

We have previously reported that schisandrin B (SchB) is a specific inhibitor of ATR (ataxia telangiectasia and Rad-3-related) protein kinase. Since SchB consists of a mixture of its diastereomers gomisin N (GN) and γ-schisandrin (γ-Sch), the inhibitory action of SchB might result from a stereospecific interaction between one of the stereoisomers of SchB and ATR. Therefore, we investigated the effect of GN and γ-Sch on UV (UVC at 254 nm)-induced activation of DNA damage checkpoint signaling in A549 cells. UV-induced cell death (25 – 75 J/m2) was amplified by the presence of the diastereomers, especially GN. At the same time, GN, but not γ-Sch, inhibited the phosphorylation of checkpoint proteins such as p53, structural maintenance of chromosomes 1, and checkpoint kinase 1 in UV-irradiated cells. Moreover, GN inhibited the G2/M checkpoint during UV-induced DNA damage. The in vitro kinase activity of immunoaffinity-purified ATR was dose-dependently inhibited by GN (IC50: 7.28 μM) but not by γ-Sch. These results indicate that GN is the active component of SchB and suggest that GN inhibits the DNA damage checkpoint signaling by stereospecifically interacting with ATR.

3-Aminobenzamide protects primary human keratinocytes from UV-induced cell death by a poly(ADP-ribosyl)ation independent mechanism

Poly(ADP-ribosyl)ation (PARylation) is a NAD+-dependent protein modification carried out by PARP [poly(ADP-ribose) polymerase] enzymes. Here we set out to investigate whether PARylation regulates UVB-induced cell death in primary human keratinocytes. We used the benchmark PARP inhibitor 3-aminobenzamide (3AB) and a more potent and specific inhibitor PJ34 and found that UVB (0.05–0.2J/cm2) induced a dose dependent loss of viability that was prevented by 3AB but not by PJ34. Similarly to PJ34, two other new generation PARP inhibitors also failed to protect keratinocytes from UVB-induced loss of viability. Moreover, silencing PARP-1 in HaCaT human keratinocytes sensitized cells to UVB toxicity but 3AB provided protection to both control HaCaT cells and to PARP-1 silenced cells indicating that the photoprotective effect of 3AB is independent of PARP inhibition. Lower UVB doses (0.0125–0.05J/cm2) caused inhibition of proliferation of keratinocytes which was prevented by 3AB but augmented by PJ34. UVB-induced keratinocyte death displayed the characteristics of both apoptosis (morphology, caspase activity, DNA fragmentation) and necrosis (morphology, LDH release) with all of these parameters being inhibited by 3AB and apoptotic parameters slightly enhanced by PJ34. UVA also caused apoptotic and necrotic cell death in keratinocytes with 3AB protecting and PJ34 sensitizing cells to UVA-induced toxicity. 3AB prevented UVB-induced mitochondrial membrane depolarization and generation of hydrogen peroxide. In summary, PARylation is a survival mechanism in UV-treated keratinocytes. Moreover, 3-aminobenzamide is photoprotective and acts by a PARP-independent mechanism at a premitochondrial step of phototoxicity.

Sensing and Responding to UV-A in Cyanobacteria

Ultraviolet (UV) radiation can cause stresses or act as a photoregulatory signal depending on its wavelengths and fluence rates. Although the most harmful effects of UV on living cells are generally attributed to UV-B radiation, UV-A radiation can also affect many aspects of cellular processes. In cyanobacteria, most studies have concentrated on the damaging effect of UV and defense mechanisms to withstand UV stress. However, little is known about the activation mechanism of signaling components or their pathways which are implicated in the process following UV irradiation. Motile cyanobacteria use a very precise negative phototaxis signaling system to move away from high levels of solar radiation, which is an effective escape mechanism to avoid the detrimental effects of UV radiation. Recently, two different UV-A-induced signaling systems for regulating cyanobacterial phototaxis were characterized at the photophysiological and molecular levels. Here, we review the current understanding of the UV-A mediated signaling pathways in the context of the UV-A perception mechanism, early signaling components, and negative phototactic responses. In addition, increasing evidences supporting a role of pterins in response to UV radiation are discussed. We outline the effect of UV-induced cell damage, associated signaling molecules, and programmed cell death under UV-mediated oxidative stress.