Impact of the Circadian Clock on UV‐Induced DNA Damage Response and Photocarcinogenesis

The skin is in constant exposure to various external environmental stressors, including solar ultraviolet (UV) radiation. Various wavelengths of UV light are absorbed by the DNA and other molecules in the skin to cause DNA damage and induce oxidative stress. The exposure to excessive ultraviolet (UV) radiation and/or accumulation of damage over time can lead to photocarcinogenesis and photoaging. The nucleotide excision repair (NER) system is the sole mechanism for removing UV photoproduct damage from DNA, and genetic disruption of this repair pathway leads to the photosensitive disorder xeroderma pigmentosum (XP). Interestingly, recent work has shown that NER is controlled by the circadian clock, the body's natural time‐keeping mechanism, through regulation of the rate‐limiting repair factor xeroderma pigmentosum group A (XPA). Studies have shown reduced UV‐induced skin cancer after UV exposure in the evening compared to the morning, which corresponds with times of high and low repair capacities, respectively. However, most studies of the circadian clock–NER connection have utilized murine models, and it is therefore important to translate these findings to humans to improve skin cancer prevention and chronotherapy.     

Effects of solar radiation on cutaneous detoxification pathways.

Because of its accessibility the human skin is constantly exposed to solar ultraviolet (UV) radiation. It is increasingly appreciated that exposure of the mammalian skin to UV plays a causal and decisive role in acute and chronic skin damage including the development of skin cancer. UV exposure causes sunburn, pigmentation, hyperplasia, immunosuppression, DNA damage, photoaging and photocarcinogenesis. To cope with constant environmental damage the skin possesses elaborate enzymatic detoxification systems. This paper briefly focuses on the effect of solar radiation, particularly UV spectrum, on detoxification pathways in the skin. Specifically the effect of solar radiation on cytochrome P450, glutathione, superoxide dismutase, glutathione peroxidase, catalase, glutathione-S-transferase and ceruloplasmin has been discussed.     

Deciphering UV-induced DNA Damage Responses to Prevent and Treat Skin Cancer

Ultraviolet (UV) radiation is among the most prevalent environmental factors that influence human health and disease. Even 1 h of UV irradiation extensively damages the genome. To cope with resulting deleterious DNA lesions, cells activate a multitude of DNA damage response pathways, including DNA repair. Strikingly, UV-induced DNA damage formation and repair are affected by chromatin state. When cells enter S phase with these lesions, a distinct mutation signature is created via error-prone translesion synthesis. Chronic UV exposure leads to high mutation burden in skin and consequently the development of skin cancer, the most common cancer in the United States. Intriguingly, UV-induced oxidative stress has opposing effects on carcinogenesis. Elucidating the molecular mechanisms of UV-induced DNA damage responses will be useful for preventing and treating skin cancer with greater precision. Excitingly, recent studies have uncovered substantial depth of novel findings regarding the molecular and cellular consequences of UV irradiation. In this review, we will discuss updated mechanisms of UV-induced DNA damage responses including the ATR pathway, which maintains genome integrity following UV irradiation. We will also present current strategies for preventing and treating nonmelanoma skin cancer, including ATR pathway inhibition for prevention and photodynamic therapy for treatment.     

Protective Effect of Curcumin Against Acute Ultraviolet B Irradiation‐induced Photo‐damage

Ultraviolet B (UVB) irradiation is one of the most dangerous insults for skin and causes sunburn, erythema, photoaging and photocarcinogenesis. Curcumin (diferuloylmethane), a yellow spice derived from dried rhizomes of Curcuma longa, has been shown to possess significant anti‐inflammatory, antioxidant, anticarcinogenic, antimutagenic, anticoagulant and anti‐infective effects. However, the protective effects of curcumin against acute photo‐damage are poorly understood. In this study, we investigated the photoprotective effects of curcumin against UVB‐induced acute photo‐damage in hairless mice and immortalized human keratinocytes (HaCaT). Topical application of curcumin significantly inhibited acute UVB (540 mJ cm^?2, for 3 successive days)‐induced inflammatory cells, collagen accrementition derangement and lipid peroxidation, and effectively induced NF‐E2‐related factor 2 (Nrf2) nuclear accumulation in uncovered (Uncv) hairless mice skin. Treatment of HaCaT cells with curcumin significantly attenuated acute UVB (300 mJ cm^?2)‐induced lactate dehydrogenase release, intracellular reactive oxygen species production and DNA damage, activated the expression of the phase II detoxifying enzymes and promoted DNA repair activity. The photoprotective effect provided by curcumin was potential associated with modulation of Nrf2‐dependent antioxidant response. Our study suggested that curcumin is a potential agent for preventing and/or treating UV radiation‐induced acute inflammation and photoaging.     

Crosstalk Among UV‐Induced Inflammatory Mediators,DNA Damage and Epigenetic Regulators Facilitates Suppression of the Immune System

The suppression of the immune system by overexposure to ultraviolet (UV) radiation has been implicated in the initiation and progression of photocarcinogenesis. Numerous changes occur in the skin on UVB exposure, including the generation of inflammatory mediators, DNA damage, epigenetic modifications, and migration and functional alterations in the antigen‐presenting dendritic cells. Although each of these alterations can elicit a cascade of events that have the potential to modulate immune sensitivity alone, there is emerging evidence that there is considerable crosstalk between these cascades. The development of an understanding of UV‐induced changes in the skin that culminate in UV‐induced immunosuppression, which has been implicated in the risk of nonmelanoma skin cancer, as a network of events has implications for the development of more effective chemopreventive strategies. In the current review article, we discuss the evidence of interactions between the various molecular targets and signaling mechanisms associated with UV‐induced immunosuppression.     

Kinetics of UV light-induced cyclobutane pyrimidine dimers in human skin in vivo: an immunohistochemical analysis of both epidermis and dermis

It is well known that UV exposure of human skin induces DNA damage, and the cumulative effect of such repeated damage is an important contributor to the development of skin cancer. Here, we demonstrate UV dose- and time-dependent induction of DNA damage in the form of cyclobutane pyrimidine dimers (CPD) in skin cells following a single exposure of human skin to UV radiation. CPD+ cells were identified by an immunohistochemical technique using monoclonal antibodies to thymine dimers. The percentage of CPD+ cells was UV dose-dependent, even a suberythemal (0.5 minimal erythemal dose [MED]) dose resulted in detectable level of cells that contained pyrimidine dimers. Forty-eight hours after irradiation the percent of total epidermal cells positive for CPD ranged from 19 +/- 8, 36 +/- 10, 57 +/- 12 and 80 +/- 10, and total percent dermal cells positive for CPD ranged from 1 +/- 1, 7 +/- 3, 16 +/- 3 and 20 +/- 5, respectively, following 0.5, 1.0, 2.0 and 4.0 MED. CPD were also observed in deeper reticular dermis, which suggest the penetrating ability of UV radiation into the skin. The change in CPD+ cells from 0.5 to 240 h post-UV exposure in both epidermal and dermal compartments of the skin was also quantitated. CPD+ cells were observed in skin biopsies at early time points after UV exposure which remained elevated for 48 h, then declined significantly by 3 days post-UV. A close examination of the skin at and after 3 days following UV exposure indicates the significant removal of DNA damaged cells from the epidermis. Ten days after UV exposure the levels of CPD+ cells in both epidermis and dermis were not significantly different from that in unirradiated skin.     

Effects and Mechanism of Nicotinamide Against UVA‐ and/or UVB‐mediated DNA Damages in Normal Melanocytes

Melanoma incidences are increasing rapidly, and ultraviolet (UV) radiation from the sun is believed to be its major contributing factor. UV exposure causes DNA damage in skin which may initiate cutaneous skin cancers including melanoma. Melanoma arises from melanocytes, the melanin‐producing skin cells, following genetic dysregulations resulting into hyperproliferative phenotype and neoplastic transformation. Both UVA and UVB exposures to the skin are believed to trigger melanocytic hyperplasia and melanomagenesis. Melanocytes by themselves are deficient in repair of oxidative DNA damage and UV‐induced photoproducts. Nicotinamide, an active form of vitamin B3 and a critical component of the human body's defense system has been shown to prevent certain cancers including nonmelanoma skin cancers. However, the mechanism of nicotinamide's protective effects is not well understood. Here, we investigated potential protective effects and mechanism of nicotinamide against UVA‐ and/or UVB‐ induced damage in normal human epidermal melanocytes. Our data demonstrated an appreciable protective effect of nicotinamide against UVA‐ and/or UVB‐ induced DNA damage in melanocytes by decreasing both cyclobutane pyrimidine dimers and 8‐hydroxy‐2′‐deoxyguanosine levels. We found that the photoprotective response of nicotinamide was associated with the activation of nucleotide excision repair genes and NRF2 signaling. Further studies are needed to validate our findings in in vivo models.     

Molecular Regulation of UV‐Induced DNA Repair

Ultraviolet (UV) radiation from sunlight is a major etiologic factor for skin cancer, the most prevalent cancer in the United States, as well as premature skin aging. In particular, UVB radiation causes formation of specific DNA damage photoproducts between pyrimidine bases. These DNA damage photoproducts are repaired by a process called nucleotide excision repair, also known as UV‐induced DNA repair. When left unrepaired, UVB‐induced DNA damage leads to accumulation of mutations, predisposing people to carcinogenesis as well as to premature aging. Genetic loss of nucleotide excision repair leads to severe disorders, namely, xeroderma pigmentosum (XP), trichothiodystrophy (TTD) and Cockayne syndrome (CS), which are associated with predisposition to skin carcinogenesis at a young age as well as developmental and neurological conditions. Regulation of nucleotide excision repair is an attractive avenue to preventing or reversing these detrimental consequences of impaired nucleotide excision repair. Here, we review recent studies on molecular mechanisms regulating nucleotide excision repair by extracellular cues and intracellular signaling pathways, with a special focus on the molecular regulation of individual repair factors.     

Inflammation Due to Voriconazole‐induced Photosensitivity Enhanced Skin Phototumorigenesis in Xpa‐knockout Mice

Voriconazole is an antifungal agent and used as a prophylactic measure, especially in immunocompromised patients. However, there have been several reports of its adverse reactions, namely photosensitivity with intense inflammatory rashes and subsequent skin cancer development. To assess the effects of photosensitizing drugs voriconazole and hydrochlorothiazide (HCTZ ) on the enhancement of UV ‐induced inflammatory responses and UV ‐induced tumorigenesis, we utilized Xpa ‐knockout mice, which is DNA repair‐deficient and more susceptible to UV ‐induced inflammation and tumor development than wild‐type mice. Administration of voriconazole prior to broadband UVB exposure significantly upregulated multiple inflammatory cytokines compared with the vehicle‐ or HCTZ ‐administered groups. Voriconazole administration along with chronic UVB exposure produced significantly higher number of skin tumors than HCTZ or vehicle in Xpa ‐knockout mice. Furthermore, the investigation of UVB ‐induced DNA damage using embryonic fibroblasts of Xpa ‐knockout mice revealed a significantly higher 8‐oxo‐7,8‐dihydroguanine level in cells treated with voriconazole N‐oxide, a voriconazole‐metabolite during UV exposure. The data suggest that voriconazole plus UVB ‐induced inflammatory response may be related to voriconazole‐induced skin phototumorigenesis.     

Attenuation of DNA damage in the dermis and epidermis of the albino hairless mouse by chronic exposure to ultraviolet-A and -B radiation

Mammalian skin is vulnerable to the photocarcinogenic and photoaging effects of solar UV radiation and defends itself using a variety of photoprotective responses including epidermal thickening, tanning and the induction of repair and antiradical systems. We treated Skh-1 albino hairless mice for 60 days with ultraviolet-A (UVA) or ultraviolet-B (UVB) radiation and measured the frequency of cyclobutane pyrimidine dimers and pyrimidine(6-4)pyrimidone photoproducts induced by a single acute sunburn dose of UVB at different stages of the chronic treatment. We found that both UVA and UVB exposure produced a photoprotective response in the dermis and epidermis and that the degree of photoproduct attenuation was dependent on dose, wavelength and the type of damage induced. Although epidermal thickening was important, our data suggest that UV protective compounds other than melanin may be involved in mitigating the damaging effects of sunlight in the skin.     

MOLECULAR MECHANISMS OF ULTRAVIOLET RADIATION CARCINOGENESIS

UV radiation is a potent DNA damaging agent and a known inducer of skin cancer in experimental animals. There is excellent scientific evidence to indicate that most non-melanoma human skin cancers are induced by repeated exposure to sunlight. UV radiation is unique in that it induces DNA damage that differs from the lesions induced by any other carcinogen. The prevalence of skin cancer on sun-exposed body sites in individuals with the inherited disorder XP suggests that defective repair of UV-induced DNA damage can lead to cancer induction. Carcinogenesis in the skin, as elsewhere, is a multistep process in which a series of genetic and epigenetic events leads to the emergence of a clone of cells that have escaped normal growth control mechanisms. The principal candidates that are involved in these events are oncogenes and tumor suppressor genes. Oncogenes display a positive effect on transformation, whereas tumor suppressor genes have an essentially negative effect, blocking transformation. Activated ras oncogenes have been identified in human skin cancers. In most cases, the mutations in the ras oncogenes have been localized to pyrimidine-rich sequences, which indicates that these sites are probably the targets for UV-induced DNA damage and subsequent mutation and transformation. The finding that activation of ras oncogenes in benign and self-regressing keratoacanthomas in both humans and in animals indicates that they play a role in the early stages of carcinogenesis (Corominas et al., 1989; Kumar et al., 1990). Since cancers do not arise immediately after exposure to physical or chemical carcinogens, ras oncogenes must remain latent for long periods of time. Tumor growth and progression into the more malignant stages may require additional events involving activation of other oncogenes or deletion of growth suppressor genes. In addition, amplification of proto-oncogenes or other genes may also be involved in tumor induction or progression. In contrast to the few studies that implicate the involvement of oncogenes in UV carcinogenesis, the role of tumor suppressor genes in UV carcinogenesis is unknown. Since cancer-prone individuals, particularly XP patients, lack one or more repair pathways, one can speculate that DNA repair enzymes would confer susceptibility to both spontaneous and environmentally induced cancers. Another potential candidate that can function as a tumor suppressor gene is the normal c-Ha-ras gene. Spandidos and Wilkie (1988) have shown that the normal c-Ha-ras gene can suppress transformation induced by the mutated ras gene.(ABSTRACT TRUNCATED AT 400 WORDS)     

Long‐term Genoprotection Effect of Sechium edule Fruit Extract Against UVA Irradiation in Keratinocytes

Photoprotection is essential to prevent the long‐term deleterious effects of ultraviolet (UV ), including skin cancer and photoaging. So far, there has been an increase in the use of natural bioactive phytochemicals for the development of more effective skin photoprotective agents. However, the molecular mechanisms underlying the photochemoprotection activity of such compounds remain largely unknown. The objective of this study was to investigate the effects of a Sechium edule fruit extract (SEE ) in terms of photoprotection against UVA in primary human keratinocytes. We found that SEE protected keratinocytes against UVA ‐induced cytotoxicity, decreased the intracellular amounts of reactive oxygen species, and reduced oxidatively induced DNA lesions after UVA exposure. Furthermore, SEE decreased the induction of CPD lesions in UVA ‐irradiated keratinocytes and exhibited increased DNA repair of such photoproducts at 24 h postexposure. Finally, using DNA repair biochips, we demonstrated that SEE ‐treated keratinocytes had DNA enzymatic repair activities more efficient for abasic sites, CPD and thymine glycols. Therefore, the benefits of SEE against UVA could be explained by a combination of antioxidant activity, the reduction in DNA damage, and the enhancement of DNA repair capacities.     

DNA Damage Induced by Late Spring Sunlight in Antarctica

Sunlight ultraviolet (UV) radiation constitutes an important environmental genotoxic agent that organisms are exposed to, as it can damage DNA directly, generating pyrimidine dimers, and indirectly, generating oxidized bases and single-strand breaks (SSBs). These lesions can lead to mutations, triggering skin and eye disorders, including carcinogenesis and photoaging. Stratospheric ozone layer depletion, particularly in the Antarctic continent, predicts an uncertain scenario of UV incidence on the Earth in the next decades. This research evaluates the DNA damage caused by environmental exposure to late spring sunlight in the Antarctic Peninsula, where the ozone layer hole is more pronounced. These experiments were performed at the Brazilian Comandante Ferraz Antarctic Station, at King’s George Island, South Shetlands Islands. For comparison, tropical regions were also analyzed. Samples of plasmid DNA were exposed to sunlight. Cyclobutane pyrimidine dimers (CPDs), oxidized base damage and SSBs were detected using specific enzymes. In addition, an immunological approach was used to detect CPDs. The results reveal high levels of DNA damage induced by exposure under the Antarctic sunlight, inversely correlated with ozone layer thickness, confirming the high impact of ozone layer depletion on the DNA damaging action of sunlight in Antarctica.     

Distinct Role of Sesn2 in Response to UVB‐Induced DNA Damage and UVA‐Induced Oxidative Stress in Melanocytes

Ultraviolet (UV) radiation, including both UVB and UVA irradiation, is the major risk factor for causing skin cancer including melanoma. Recently, we have shown that Sesn2, a member of the evolutionarily conserved stress‐inducible protein family Sestrins (Sesn), is upregulated in human melanomas as compared to melanocytes in normal human skin, suggesting an oncogenic role of Sesn2. However, the role of Sesn2 in UVB and UVA response is unknown. Here, we demonstrated that both UVB and UVA induce Sesn2 upregulation in melanocytes and melanoma cells. UVB induces Sesn2 expression through the p53 and AKT3 pathways. Sesn2 negatively regulates UVB‐induced DNA damage repair. In comparison, UVA induces Sesn2 upregulation through mitochondria but not Nrf2. Sesn2 ablation increased UVA‐induced Nrf2 induction and inhibits UVA‐induced ROS production, indicating that Sesn2 acts as an upstream regulator of Nrf2. These findings suggest previously unrecognized mechanisms in melanocyte response to UVB and UVA irradiation and potentially in melanoma formation.     

Proanthocyanidins from Grape Seeds Inhibit UV–Radiation‐Induced Immune Suppression in Mice: Detection and Analysis of Molecular and Cellular Targets

Ultraviolet (UV)–radiation‐induced immunosuppression has been linked with the risk of skin carcinogenesis. Approximately, 2 million new cases of skin cancers, including melanoma and nonmelanoma, diagnosed each year in the USA and therefore have a tremendous bad impact on public health. Dietary phytochemicals are promising options for the development of effective strategy for the prevention of photodamaging effects of UV radiation including the risk of skin cancer. Grape seed proanthocyanidins (GSPs) are such phytochemicals. Dietary administration of GSPs with AIN76A control diet significantly inhibits UV‐induced skin tumor development as well as suppression of immune system. UV‐induced suppression of immune system is commonly determined using contact hypersensitivity (CHS) model which is a prototype of T–cell‐mediated immune response. We present evidence that inhibition of UV‐induced suppression of immune system by GSPs is mediated through: (i) the alterations in immunoregulatory cytokines, interleukin (IL)‐10 and IL‐12, (ii) DNA repair, (iii) stimulation of effector T cells and (iv) DNA repair‐dependent functional activation of dendritic cells in mouse model. These information have important implications for the use of GSPs as a dietary supplement in chemoprevention of UV‐induced immunosuppression as well as photocarcinogenesis.     

PostExcision Events in Human Nucleotide Excision Repair

The nucleotide excision repair system removes a wide variety of DNA lesions from the human genome, including photoproducts induced by ultraviolet (UV) wavelengths of sunlight. A defining feature of nucleotide excision repair is its dual incision mechanism, in which two nucleolytic incision events on the damaged strand of DNA at sites bracketing the lesion generate a damage‐containing DNA oligonucleotide and a single‐stranded DNA gap approximately 30 nucleotides in length. Although the early events of nucleotide excision repair, which include lesion recognition and the dual incisions, have been explored in detail and are reasonably well understood, the fate of the single‐stranded DNA gaps and excised oligonucleotide products of repair have not been as extensively examined. In this review, recent findings that address these less‐explored aspects of nucleotide excision repair are discussed and support the concept that postincision gap and excised oligonucleotide processing are critical steps in the cellular response to DNA damage induced by UV light and other environmental carcinogens. Defects in these latter stages of repair lead to cell death and other DNA damage signaling responses and may therefore contribute to a number of human disease states associated with exposure to UV wavelengths of sunlight, including skin cancer, aging and autoimmunity.     

UV doses worldwide

UV radiation affects human health. Human exposure to UV radiation causes a few beneficial health effects like vitamin D3 formation but it causes many detrimental health effects: sunburn, ocular damage, photoaging, immune suppression, DNA damage and skin cancer. In countries with fair-skinned populations, skin cancer is the most diagnosed of all cancers. In the United States in 2002, there were over one million new skin cancer cases. That means one out of every 285 people got skin cancer. Skin cancer of fair-skinned individuals is increasing at an alarming rate (4-6% per year) around the world and has now reached so-called "pandemic" proportions. Thus, it is important to know what UV doses people around the world get throughout their lives. This review covers how the outdoor UV doses are weighted for different biological effects, the most commonly used measuring devices for terrestrial and personal UV doses, the natural and other effects on terrestrial and personal UV doses, the time people spend outside, their ambient exposures and the terrestrial and personal UV doses of adult outdoor and indoor workers as well as children and adolescents around the world. Overall, outdoor-working adults get about 10%, while indoor-working adults and children get about 3% (2-4%) of the total available annual UV (on a horizontal plane). People's UV doses increase with increasing altitude and decreasing latitude; most indoor-working adult Europeans get 10,000-20,000 J/m2 per year, Americans get 20,000-30,000 J/m2 per year and Australians are estimated to get 20,000-50,000 J/m2 per year (excluding vacation, which can increase the dose by 30% or more).