Transcription-coupled and global genome repair differentially influence UV-B-induced acute skin effects and systemic immunosuppression

A deep dive into UV-based phototherapy: Mechanisms of action and emerging molecular targets in inflammation and cancer

UV-based phototherapy (including psoralen plus UVA (PUVA), UVB and UVA1) has a long, successful history in the management of numerous cutaneous disorders. Photoresponsive diseases are etiologically diverse, but most involve disturbances in local (and occasionally systemic) inflammatory cells and/or abnormalities in keratinocytes that trigger inflammation. UV-based phototherapy works by regulating the inflammatory component and inducing apoptosis of pathogenic cells. This results in a fascinating and complex network of simultaneous events-immediate transcriptional changes in keratinocytes, immune cells, and pigment cells; the emergence of apoptotic bodies; and the trafficking of antigen-presenting cells in skin-that quickly transform the microenvironment of UV-exposed skin. Molecular elements in this system of UV recognition and response include chromophores, metabolic byproducts, innate immune receptors, neurotransmitters and mediators such as chemokines and cytokines, antimicrobial peptides, and platelet activating factor (PAF) and PAF-like molecules that simultaneously shape the immunomodulatory effects of UV and their interplay with the microbiota of the skin and beyond. Phototherapy's key effects-proapoptotic, immunomodulatory, antipruritic, antifibrotic, propigmentary, and pro-prebiotic-promote clinical improvement in various skin diseases such as psoriasis, atopic dermatitis (AD), graft-versus-host disease (GvHD), vitiligo, scleroderma, and cutaneous T-cell lymphoma (CTCL) as well as prevention of polymorphic light eruption (PLE). As understanding of phototherapy improves, new therapies (UV- and non-UV-based) are being developed that will modify regulatory T-cells (Treg), interact with (resident) memory T-cells and /or utilize agonists and antagonists as well as antibodies targeting soluble molecules such as cytokines and chemokines, transcription factors, and a variety of membrane-associated receptors.

Long non-coding RNAs in cutaneous biology and keratinocyte carcinomas

Long non-coding RNAs (lncRNAs) are a largely uncharacterized group of non-coding RNAs with diverse regulatory roles in various biological processes. Recent observations have elucidated the functional roles of lncRNAs in cutaneous biology, e.g. in proliferation and differentiation of epidermal keratinocytes and in cutaneous wound repair. Furthermore, the role of lncRNAs in keratinocyte-derived skin cancers is emerging, especially in cutaneous squamous cell carcinoma (cSCC), which presents a significant burden to health care services worldwide and causes high mortality as metastatic disease. Elucidation of the functions of keratinocyte-specific lncRNAs will improve understanding of the molecular pathogenesis of epidermal disorders and skin cancers and can be exploited in development of new diagnostic and therapeutic applications for keratinocyte carcinomas. In this review, we summarize the current evidence of functionally important lncRNAs in cutaneous biology and in keratinocyte carcinomas.

Roles of UVA radiation and DNA damage responses in melanoma pathogenesis

The growing incidence of melanoma is a serious public health issue that merits a thorough understanding of potential causative risk factors, which includes exposure to ultraviolet radiation (UVR). Though UVR has been classified as a complete carcinogen and has long been recognized for its ability to damage genomic DNA through both direct and indirect means, the precise mechanisms by which the UVA and UVB components of UVR contribute to the pathogenesis of melanoma have not been clearly defined. In this review, we therefore highlight recent studies that have addressed roles for UVA radiation in the generation of DNA damage and in modulating the subsequent cellular responses to DNA damage in melanocytes, which are the cell type that gives rise to melanoma. Recent research suggests that UVA not only contributes to the direct formation of DNA lesions but also impairs the removal of UV photoproducts from genomic DNA through oxidation and damage to DNA repair proteins. Moreover, the melanocyte microenvironment within the epidermis of the skin is also expected to impact melanomagenesis, and we therefore discuss several paracrine signaling pathways that have been shown to impact the DNA damage response in UV-irradiated melanocytes. Lastly, we examine how alterations to the immune microenvironment by UVA-associated DNA damage responses may contribute to melanoma development. Thus, there appear to be multiple avenues by which UVA may elevate the risk of melanoma. Protective strategies against excess exposure to UVA wavelengths of light therefore have the potential to decrease the incidence of melanoma. Environ. Mol. Mutagen. 59:438-460, 2018. © 2018 Wiley Periodicals, Inc.

The protective effect of some Thai plants and their bioactive compounds in UV light-induced skin carcinogenesis

Skin cancer, represents a major public health concern. While the vast majority is non-melanoma skin cancers, melanomas are mostly responsible for mortality. Solar UVB radiation is mutagenic and carcinogenic. It is primarily responsible for both non-melanoma and melanoma skin cancers via excessive production of reactive oxygen species (ROS), which mediate changes in inflammation and immunity, and have been implicated in all three stages of skin cancer development. Due to their regulatory role in numerous functions of cells, signaling pathways are targets for chemoprevention. The current standards in melanoma therapy are targeted and combination therapies, which, albeit prolong survival responses, are still prone to development of drug resistance. To this extent, drugs of natural origin continue to spark great interest. Thailand has a rich biodiversity of indigenous flora, which have traditionally been used to treat a variety of pathologies. The active components in plant extracts that have medicinal properties, termed 'bioactive compounds,' are efficient chemopreventive agents due to their antioxidant, antimutagenic, anticarcinogenic, and carcinogen detoxification properties. Thai plants and their bioactive compounds have shown protective effects on UV light-induced skin cancer in different experimental models. This warrants further in vivo investigations and translation to clinical studies to determine efficacy and safety, for use as lead compounds in targeted/combination therapy or adjuvant therapy with existing regimes. Coupled with a strategy for prevention, this offers a promising outlook for protection against photocarcinogenesis.

Propolis Inhibits UVA-Induced Apoptosis of Human Keratinocyte HaCaT Cells by Scavenging ROS

Propolis is a resinous material collected by honeybees from several plant sources. This research aimed at showing its protective effect against UVA-induced apoptosis of human keratinocyte HaCaT cells. Using Hoechst staining, it was demonstrated that propolis (5 and 10 μg/mL) significantly inhibited the apoptosis of HaCaT cells induced by UVA-irradiation. Propolis also showed the protective effect against loss of mitochondrial membrane potential induced by UVA-irradiaiton in HaCaT cells. Propolis also inhibited the expression of activated caspase-3 induced by UVA-irradiation. To investigate the role of ROS in UVA-induced apoptosis and protection by propolis, the generation of ROS was determined in cells. The results showed that the generation of ROS was markedly reduced in cells pretreated with propolis. Consequently, propolis protected human keratinocyte HaCaT cells against UVA-induced apoptosis, which might be related to the reduction of ROS generation by UVA-irradiation.

Patients with xeroderma pigmentosum complementation groups C, E and V do not have abnormal sunburn reactions

BACKGROUND

Xeroderma pigmentosum (XP) is a rare autosomal recessive disorder of DNA repair. It is divided into eight complementation groups: XP-A to XP-G (classical XP) and XP variant (XP-V). Severe and prolonged sunburn reactions on minimal sun exposure have been considered a cardinal feature of classical XP. However, it has recently become clear that not all patients have abnormal sunburn reactions.

OBJECTIVES

To examine sunburn reactions in a cohort of patients with XP and correlate this to the complementation group.

METHODS

Sixty patients with XP attending the U.K. National XP Service from 2010 to 2012 were studied. Their history of burning after minimal sun exposure was assessed using a newly developed sunburn severity score. The age at which the first skin cancer was histologically diagnosed in each patient, and the presence of any neurological abnormality, was also recorded.

RESULTS

Sunburn severity scores were abnormally high in patients with XP-A, XP-D, XP-F and XP-G compared with non-XP controls. There was no significant difference in sunburn score of patients with XP-C, XP-E and XP-V compared with controls (P > 0·05). Patients with XP-C, XP-E and XP-V were more likely to have skin cancer diagnosed at an earlier age than those with severe sunburn on minimal sun exposure. In addition, patients with XP with severe sunburn had an increased frequency of neurological abnormalities.

CONCLUSIONS

Not all patients with XP have a history of severe and prolonged sunburn on minimal sun exposure. The normal sunburn response of patients with XP-C, XP-E and XP-V may relate to the preservation of transcription-coupled DNA repair in these groups. Those with a history of severe sunburn on minimal sun exposure developed their first skin cancer at an older age compared with patients with XP-C, XP-E and XP-V, but they had an increased frequency of neurological abnormalities. Physicians need to be aware that about half of all patients with XP will present without a history of abnormal sunburn.

DNA double helix unwinding triggers transcription block-dependent apoptosis: a semiquantitative probe of the response of ATM, RNAPII, and p53 to two DNA intercalators

We have previously shown the binding modes of two DNA interacting analogues (1)a {3-(4-methyl-piperazin)-8-oxo-8H-acenaphtho[1,2-b]pyrrole-9-carbonitrile} and (3)a {3-(3-dimethylamino-propylamino)-8-oxo-8H-acenaphtho[1,2-b]pyrrole-9-carbonitrile} with the DNA double helix. In this study, we have determined the notably different DNA damage signal pathway elicited by (1)a and (3)a due to the different extents to which they unwind the DNA double helix. First, we have identified that ataxia-telangiectasia-mutated (ATM) protein kinase can respond to DNA double helix unwinding caused by both (1)a and (3)a. In addition, the amount of ATM activation is consistent with the degree to which the DNA double helix was unwound. Consequently, we used (1)a and (3)a to semiquantitatively probe the response of RNA polymerase II (RNAPII) and p53 toward DNA double helix unwinding in vivo. By means of flow cytometry, immunocytochemistry, ChIP, quantitative real-time polymerase chain reaction, and Western blot analyses, we measured the level of p53 and RNAPII phosphorylation, in addition to the dynamics of the RNAPII distribution along the c-Myc gene. These results provided novel evidence for the impact of subtle DNA structural changes on the activity of RNAPII and p53. Moreover, DNA double helix conformational damage-dependent apoptosis was studied for the first time. These results indicated that (1)a can induce transcriptional blockage following a shift of the unphosphorylated IIa form of RNAPII to the phosphorylated IIo form, while (3)a is unable to induce the same effect. Subsequently, p53 accumulation and phosphorylation events occur that lead to apoptosis in the case of (1)a exposure. This suggests that the transcriptional blockage is also correlated to the degree of double helix unwinding. Furthermore, we found that the degree of DNA conformational damage determines whether or not apoptosis occurs through transcriptional blockage. Under our experimental conditions, ATM does not participate in the downstream events even when it has been activated. Thus, p53-mediated apoptosis may be independently triggered by transcriptional blockage.

Decreased transcription-coupled nucleotide excision repair capacity is associated with increased p53- and MLH1-independent apoptosis in response to cisplatin

Background

One of the most commonly used classes of anti-cancer drugs presently in clinical practice is the platinum-based drugs, including cisplatin. The efficacy of cisplatin therapy is often limited by the emergence of resistant tumours following treatment. Cisplatin resistance is multi-factorial but can be associated with increased DNA repair capacity, mutations in p53 or loss of DNA mismatch repair capacity.

Methods

RNA interference (RNAi) was used to reduce the transcription-coupled nucleotide excision repair (TC-NER) capacity of several prostate and colorectal carcinoma cell lines with specific defects in p53 and/or DNA mismatch repair. The effect of small inhibitory RNAs designed to target the CSB (Cockayne syndrome group B) transcript on TC-NER and the sensitivity of cells to cisplatin-induced apoptosis was determined.

Results

These prostate and colon cancer cell lines were initially TC-NER proficient and RNAi against CSB significantly reduced their DNA repair capacity. Decreased TC-NER capacity was associated with an increase in the sensitivity of tumour cells to cisplatin-induced apoptosis, even in p53 null and DNA mismatch repair-deficient cell lines.

Conclusion

The present work indicates that CSB and TC-NER play a prominent role in determining the sensitivity of tumour cells to cisplatin even in the absence of p53 and DNA mismatch repair. These results further suggest that CSB represents a potential target for cancer therapy that may be important to overcome resistance to cisplatin in the clinic.

Guanine sulphinate is a major stable product of photochemical oxidation of DNA 6-thioguanine by UVA irradiation

The DNA of patients taking the immunosuppressant and anticancer drugs azathioprine or 6-mercaptopurine contains 6-thioguanine (6-TG). The skin of these patients is selectively sensitive to ultraviolet A radiation (UVA) and they suffer an extremely high incidence of sunlight-induced skin cancer with long-term treatment. DNA 6-TG interacts with UVA to generate reactive oxygen species, which oxidize 6-TG to guanine sulphonate (G(SO3)). We suggested that G(SO3) is formed via the reactive electrophilic intermediates, guanine sulphenate (G(SO)) and guanine sulphinate (G(SO2)). Here, G(SO2) is identified as a significant and stable UVA photoproduct of free 6-TG, its 2'-deoxyribonucleoside, and DNA 6-TG. Mild chemical oxidation converts 6-TG into G(SO2), which can be further oxidized to G(SO3)-a stable product that resists further reaction. In contrast, G(SO2) is converted back to 6-TG under mild conditions. This suggests that cellular antioxidant defences might counteract the UVA-mediated photooxidation of DNA 6-TG at this intermediate step and ameliorate its biological effects. In agreement with this possibility, the antioxidant ascorbate protected DNA 6-TG against UVA oxidation and prevented the formation of G(SO3).

Reactive oxygen species generated by thiopurine/UVA cause irreparable transcription-blocking DNA lesions

Long-term treatment with the anticancer and immunosuppressant thiopurines, azathioprine or 6-mercaptopurine, is associated with acute skin sensitivity to ultraviolet A (UVA) radiation and a high risk of skin cancer. 6-thioguanine (6-TG) that accumulates in the DNA of thiopurine-treated patients interacts with UVA to generate reactive oxygen species. These cause lethal and mutagenic DNA damage. Here we show that the UVA/DNA 6-TG interaction rapidly, and essentially irreversibly, inhibits transcription in cultured human cells and provokes polyubiquitylation of the major subunit of RNA polymerase II (RNAPII). In vitro, 6-TG photoproducts, including the previously characterized guanine-6-sulfonate, in the transcribed DNA strand, are potent blocks to RNAPII transcription whereas 6-TG is only slightly inhibitory. In vivo, guanine-6-sulfonate is removed poorly from DNA and persists to a similar extent in the DNA of nucleotide excision repair-proficient and defective cells. Furthermore, transcription coupled repair-deficient Cockayne syndrome cells are not hypersensitive to UVA/6-TG, indicating that potentially lethal photoproducts are not selectively excised from transcribed DNA. Since persistent transcription-blocking DNA lesions are associated with acute skin responses to sunlight and the development of skin cancer, our findings have implications for skin cancer in patients undergoing thiopurine therapy.

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

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

UV-induced immunosuppression in the balance

Around 1980, experiments with hairless mice showed us that UV-induced actinic keratoses (AK) and ensuing skin carcinomas did not arise independently: the rate of occurrence in one skin area was increased considerably if AKs had already been induced separately in another distant skin area, i.e. a systemic effect. The ground laying work of Margaret Kripke in the 1970s provided a fitting explanation: UV-induced immunosuppression and tolerance toward the UV-induced tumors. From Kripke's work a new discipline arose: "Photoimmunology." Enormous strides were made in exploring and expanding the effects from UV carcinogenesis to infectious diseases, and in elucidating the mechanisms involved. Stemming from concerns about a depletion of the ozone layer and the general impact of ambient UV radiation, the groups I worked in and closely collaborated with explored the anticipated adverse effects of UV-induced immunosuppression on healthy individuals. An important turning point was brought about in 1992 when the group of Kevin Cooper reported that immunosuppression could be induced by UV exposure in virtually all human subjects tested, suggesting that this is a normal and sound physiological reaction to UV exposure. This reaction could actually protect us from illicit immune responses against our UV-exposed skin, such as observed in idiopathic polymorphic light eruption. This premise has fruitfully rekindled the research on this common "sun allergy," affecting to widely varying degrees about one in five Europeans with indoor professions.

Subchronic inhalation of mixtures of cigarette smoke constituents in Xpa-/-p53+/- knock-out mice: a comparison of intermittent with semi-continuous exposure to acetaldehyde, formaldehyde, and acrolein

We investigated whether inhaling peak concentrations of aldehydes several times daily is more damaging than semi-continuously inhaling low-dose aldehydes. We exposed Xpa-/-p53+/- knock-out mice either intermittently or semi-continuously to mixed acetaldehyde, formaldehyde, and acrolein. The intermittent regimen entailed exposure to the aldehydes 7 min every 45 min, 12 times/day, 5 days/week, corresponding to concentrations inhaled by smokers. Semi-continuously exposed animals received half the dose of aldehydes in 8h/day, 5 days/week. Some mice in each group were sacrificed after 13 weeks of exposure; the rest breathed clean air until the end of 1 year. Mice injected intratracheally with benzo[a]pyrene formed a positive control group. The nasal cavity, lungs, and any macroscopically abnormal organs of all mice were analysed histopathologically. After 13 weeks of exposure, the subacute, overall, histopathological changes induced by the inhalation differed noticeably between the intermittently and semi-continuously treated Xpa-/-p53+/- knock-out mice. After 13 weeks of mixed aldehyde exposure, atrophy of the olfactory epithelium generally appeared, but disappeared after 1 year (adaptation and/or recovery). Respiratory epithelial metaplasia of the olfactory epithelium occurred at a higher incidence at 1 year. Except for a significantly greater number of tumours observed in knock-out mice compared to wild mice (semi-continuous aldehyde exposure and controls), no differences between the semi-continuous and intermittent exposure groups were observed.

Tissue specific mutagenic and carcinogenic responses in NER defective mouse models

Several mouse models with defects in genes encoding components of the nucleotide excision repair (NER) pathway have been developed. In NER two different sub-pathways are known, i.e. transcription-coupled repair (TC-NER) and global-genome repair (GG-NER). A defect in one particular NER protein can lead to a (partial) defect in GG-NER, TC-NER or both. GG-NER defects in mice predispose to cancer, both spontaneous as well as UV-induced. As such these models (Xpa, Xpc and Xpe) recapitulate the human xeroderma pigmentosum (XP) syndrome. Defects in TC-NER in humans are associated with Cockayne syndrome (CS), a disease not linked to tumor development. Mice with TC-NER defects (Csa and Csb) are - except for the skin - not susceptible to develop (carcinogen-induced) tumors. Some NER factors, i.e. XPB, XPD, XPF, XPG and ERCC1 have functions outside NER, like transcription initiation and inter-strand crosslink repair. Deficiencies in these processes in mice lead to very severe phenotypes, like trichothiodystrophy (TTD) or a combination of XP and CS. In most cases these animals have a (very) short life span, display segmental progeria, but do not develop tumors. Here we will overview the available NER-related mouse models and will discuss their phenotypes in terms of (chemical-induced) tissue-specific tumor development, mutagenesis and premature aging features.

Differential role of basal keratinocytes in UV-induced immunosuppression and skin cancer

Cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts (6-4PPs) comprise major UV-induced photolesions. If left unrepaired, these lesions can induce mutations and skin cancer, which is facilitated by UV-induced immunosuppression. Yet the contribution of lesion and cell type specificity to the harmful biological effects of UV exposure remains currently unclear. Using a series of photolyase-transgenic mice to ubiquitously remove either CPDs or 6-4PPs from all cells in the mouse skin or selectively from basal keratinocytes, we show that the majority of UV-induced acute effects to require the presence of CPDs in basal keratinocytes in the mouse skin. At the fundamental level of gene expression, CPDs induce the expression of genes associated with repair and recombinational processing of DNA damage, as well as apoptosis and a response to stress. At the organismal level, photolyase-mediated removal of CPDs, but not 6-4PPs, from the genome of only basal keratinocytes substantially diminishes the incidence of skin tumors; however, it does not affect the UVB-mediated immunosuppression. Taken together, these findings reveal a differential role of basal keratinocytes in these processes, providing novel insights into the skin's acute and chronic responses to UV in a lesion- and cell-type-specific manner.

The mechanisms and consequences of ultraviolet-induced immunosuppression

Exposure to ultraviolet radiation (UVR) can result in immune suppression to antigens encountered within a few days of the irradiation. The process leading to the down-regulation in immune responses is complex. It is initiated by several photoreceptors located in the skin surface, namely DNA, trans-urocanic acid and membrane components. The absorption of UVR by these chromophores then leads to the release of a wide range of mediators that can affect antigen presenting cells locally or systemically. The final steps include the generation of antigen-specific T cells capable of regulating immunity. The consequences of the UV-induced changes in the skin immune system for the control of skin cancers, infectious diseases including vaccination, and autoimmune diseases are considered. Finally, the effects of active vitamin D, synthesised in the epidermis following UVR, are discussed in the context of the skin immune response.

Ultraviolet radiation-induced non-melanoma skin cancer in the Crl:SKH1:hr-BR hairless mouse: augmentation of tumor multiplicity by chlorophyllin and protection by indole-3-carbinol

Over 1 million new cases of ultraviolet radiation-induced non-melanoma skin cancers (NMSC) per year now occur in the USA and the incidence of these diseases continues to increase. New preventative strategies are required. The hypothesis tested was that dietary administration of the putative cancer chemopreventatives sodium-copper-chlorophyllin (Chlor) or indole-3-carbinol (I3C) would inhibit UV-induced skin carcinogenesis in the Crl:SKH1:hr-BR hairless mouse. Groups of 20 mice were pre-fed isocaloric/isonutritive 20% corn-oil AIN-76a based diets that contained either Chlor (1.52 g%), I3C (5.08 g%) or no chemopreventative (control) for 2 weeks followed by exposure of their dorsal skin to a 10 week incremental, sub-erythemal, carcinogenic simulated solar UV exposure regime. Feeding was continued for the duration of the experiment. Matched non-UV exposed dietary groups were also included in the experimental design. The diets had no significant (p > 0.05) effect on body weight, feed consumption, cutaneous methanol-extractable UV photoprotective substances or on cutaneous UV-reflective characteristics. By day 180, UV-irradiated mice fed the Chlor had a significantly (p < 0.05) higher tumor multiplicity (33.6 +/- 4.72; mean +/- SEM) than UV-irradiated control animals (22.8 +/- 4.25). UV-irradiated mice fed I3C had a significantly (p < 0.001) lower tumor multiplicity (13.0 +/- 2.42) than that of both the UV-irradiated control and UV-irradiated Chlor-fed mice. The Chlor or I3C diets did not significantly (p > 0.05) affect UV-induced systemic suppression of contact hypersensitivity responses. These results demonstrate augmentation of the UV-induced cutaneous carcinogenic process by dietary chlorophyllin and protection from this carcinogenic process by indole-3-carbinol via mechanisms that do not involve changes in skin optical properties, modulation of photoimmunosuppression or caloric/nutrient effects.

Transcription - guarding the genome by sensing DNA damage

Cells induce the expression of DNA-repair enzymes, activate cell-cycle checkpoints and, under some circumstances, undergo apoptosis in response to DNA-damaging agents. The mechanisms by which these cellular responses are triggered are not well understood, but there is recent evidence that the transcription machinery might be used in DNA-damage surveillance and in triggering DNA-damage responses to suppress mutagenesis. Transcription might also act as a DNA-damage dosimeter where the severity of blockage determines whether or not to induce cell death. Could transcription therefore be a potential therapeutic target for anticancer strategies?

Transgenic and knockout mice for DNA repair functions in carcinogenesis and mutagenesis

Genetically modified mouse models with defects in DNA repair pathways, especially in nucleotide excision repair (NER) and mismatch repair (MMR), are powerful tools to study processes like carcinogenesis and mutagenesis. The use of mutant mice in these studies has many advantages over using normal wild type mice with respect to costs, number of animals, predictive value towards carcinogenic compounds and the duration of study. Short-term carcinogenicity assays still require considerable number of animals and extensive pathological analyses. Therefore, alternatives demanding less animals and shorter exposure times would be desirable. In this respect, one approach could be the use of transgenic mice harbouring marker genes, that can easily detect mutagenic features of carcinogenic compounds, especially when such models are in a DNA repair deficient background. Here, we review the progress made in the development and use of DNA repair deficient mouse models as replacements for long-term cancer assays and discuss the applicability of enhanced gene mutant frequencies as early indicators of tumourigenesis. Although promising models exist, there is still a need for more universally responding and highly sensitive mouse models, since it is likely that non-genotoxic carcinogens will go undetected in a DNA repair deficient mouse. One attractive candidate mouse model, having a presumptive broad detective range, is the Xpa/p53 mutant mouse model, which will be discussed in more detail.

Association of transcription-coupled repair but not global genome repair with ultraviolet-B-induced Langerhans cell depletion and local immunosuppression

Exposure to ultraviolet-B radiation impairs cellular immune responses. This immunosuppression seems to be associated with Langerhans cell migration. DNA damage appears to play a key role because enhanced nucleotide excision repair, a pathway essential for elimination of ultraviolet-B-induced DNA lesions, strongly counteracts immunosuppression. To determine the effect of DNA repair on ultraviolet-B-induced local immunosuppression and Langerhans cell disappearance, three mouse strains carrying different defects in nucleotide excision repair were compared. XPC mice, which were defective in global genome repair, were as sensitive to ultraviolet-B-induced local suppression of contact hypersensitivity to picryl chloride as their wild-type littermates. CSB mice, defective in transcription-coupled repair, were far more sensitive for immunosuppression as were XPA mice, defective in both transcription-coupled repair and global genome repair. Only a moderate depletion of Langerhans cells was observed in XPC mice and wild-type littermates. Ultraviolet-B-induced Langerhans cell depletion was enhanced in CSB and XPA mice. Hence, the major conclusion is that local immunosuppression is only affected when transcription-coupled DNA repair is impaired. Furthermore, a defect in transcription-coupled repair was linked to enhanced ultraviolet-B-induced Langerhans cell depletion. In combination with earlier experiments, it can be concluded that Langerhans cell disappearance is related to ultraviolet-B-induced local but not to systemic immunosuppression.

Skin infections in renal transplant recipients and the relation with solar ultraviolet radiation

BACKGROUND

Ultraviolet radiation (UVR) is an important risk factor for skin cancer in transplant recipients. In view of the potential suppressive effect of UVR on host resistance it was examined whether exposure to UVR was also associated with the occurrence of various skin infections.

METHODS

In a cohort of renal transplant recipients (n = 137), lifetime exposure was assessed by means of a retrospective questionnaire on cumulative sunlight exposure. Diagnosed skin infections since renal transplantation were extracted from the patient's medical charts. Season of diagnosis was regarded as indicative of short-term exposure.

RESULTS

In comparison with winter a high rate of herpes simplex infections was found in spring [rate ratio (RR) = 4.09, 95% confidence interval (CI) 1.2-14.5], and high rates of herpes zoster infections (RR = 1.6, 95% CI: 0.8-3.5) and fungal/yeast infections in summer (RR = 2.1, 95% CI: 1.3-3.4). A higher lifetime exposure (RR = 2.31, 95% CI: 1.04-5.1) and a greater cumulative number of reported sunburns (RR = 2.3, 95% CI: 1.1-5.1) were independently associated with a higher risk of bacterial infections.

CONCLUSIONS

The seasonal association with the occurrence of clinical herpes infections indicates an effect of short-term UVR. Our data suggest that the number of sunburn episodes in the past is also relevant for the susceptibility to certain skin infections.

Cytokine polymorphisms play a role in susceptibility to ultraviolet B-induced modulation of immune responses after hepatitis B vaccination

UVB exposure can alter immune responses in experimental animals and humans. In an earlier human volunteer study, we demonstrated that hepatitis B-specific humoral and cellular immunity after vaccination on average were not significantly affected by UVB exposure. However, it is known that individuals differ in their susceptibility to UVB-induced immunomodulation, and it was hypothesized that polymorphisms in specific cytokines may play a role in this susceptibility. In this respect, we previously demonstrated that immune responses after hepatitis B vaccination are influenced by the minor allelic variant of IL-1 beta in the general population. For all volunteers, single nucleotide polymorphisms were determined for the following UV response-related cytokines: IL-1 receptor antagonist (+2018), IL-1 alpha (+4845), IL-1 beta (+3953), TNF-alpha (-308), and TNF-alpha (-238). Exposure to UVB significantly suppressed Ab responses to hepatitis B in individuals with the minor variant for the IL-1 beta polymorphism. Increased minimal erythema dose values (just perceptible), which resulted in higher absolute UVB exposures, were observed in the same individuals. There were no associations observed between UVB-induced immunomodulation and the other cytokine polymorphisms examined. This study indicates that individual susceptibility to UVB radiation needs to be considered when studying the effects of UVB in humans.

Immunological disorders and DNA repair

Cellular DNA continuously incurs damage and a range of damage response mechanisms function to maintain genomic integrity in the face of this onslaught. During the development of the immune response, the cell utilises three defined processes, V(D)J recombination, class switch recombination and somatic hypermutation, to create genetic diversity in developing T and B cells. Curiously, the damage response mechanisms employed to maintain genomic stability in somatic cells have been exploited and adapted to help generate diversity during immune development. As a consequence of this overlap, there is mounting evidence that disorders attributable to impaired damage response mechanisms display associated immunodeficiency. Since double strand breaks (DSB) are created during at least two of the mechanisms used to create immunoglobulin diversity, namely V(D)J recombination and class switch recombination, it is not surprising that disorders associated with defects in the response to double strand breaks are those most associated with immunodeficiency. Here, we review the steps involved in the generation of genetic diversity during immune development with a focus on the damage response mechanisms employed and then consider human immunodeficiency disorders associated with impaired damage response mechanisms.

Ultraviolet light-induced apoptosis is associated with S-phase in primary human fibroblasts

Transcription-coupled nucleotide excision repair (tcNER)-deficient human fibroblasts are extremely sensitive to the induction of apoptosis in response to low doses of ultraviolet light (UV light), but are less sensitive to the induction of apoptosis following exposure to high doses [J. Invest. Dermatol. 117 (2001) 1162]. This seemingly paradoxical observation led us to re-evaluate the relationship between UV dose and the induction of apoptosis. Here we report that the reduction in the extent of UV-induced apoptosis in tcNER-deficient strains following exposure to elevated doses of UV light does not result from impaired gene expression alone because neither inhibitors of transcription nor inhibitors of translation blocked UV-induced apoptosis. Furthermore, UV-induced apoptosis was greatly reduced by inhibiting S-phase progression with either mimosine or serum withdrawal. Importantly, DNA synthesis following UV-irradiation occurred only at doses that induced apoptosis in these cell lines and the apoptotic cells contained nascent DNA. Moreover, deregulation of G(1)- to S-phase transition by expression of human papillomavirus E7 sensitized cells to UV-induced apoptosis. Taken together these results suggest that the induction of apoptosis requires S-phase progression following UV-irradiation.

Exposure to solar ultraviolet radiation in young Dutch children: assessment by means of a 6-week retrospective questionnaire

We designed a 6-week retrospective questionnaire on sunlight exposure. Estimation of the short-term exposure to sunlight is important for observational human studies concerning the effects of ultraviolet radiation (UVR) on the human immune system and related resistance to infections. This questionnaire was given to the parents of 1672 1-year-old children in the Netherlands who participated in a birth cohort study. We evaluated the questionnaire and estimated the personal 6-week cumulative exposure to solar UVR. Only 910 questionnaires (54.4%) were filled out completely and consistently. This suggests that reporting data on children's outdoor exposure, even for the recent past, is often difficult. The data from these questionnaires indicated that the crude number of reported outdoor hours was enough to obtain a relative estimate of the individual exposure to ambient UVR, but that weighting for the effect of clothing was essential for the classification of the systemic UVR dosage received. Sunny weeks in the Netherlands in 1998, as were established by independent measurements of the levels of ambient UVR, vacations abroad, and sunburn, were associated with a comparatively high mean estimated exposure. These results support the suitability of the questionnaire for classifying the participants with respect to their short-term exposure to solar UVR.

UVB radiation-induced cancer predisposition in Cockayne syndrome group A (Csa) mutant mice

Cockayne syndrome (CS) is an inherited photosensitive neurodevelopmental disorder caused by a specific defect in the transcription-coupled repair (TCR) sub-pathway of NER. Remarkably, despite their DNA repair deficiency, CS patients do not develop skin cancer. Here, we present a mouse model for CS complementation group A. Like cells from CS-A patients, Csa-/- mouse embryonic fibroblasts (MEFs): (i) are ultraviolet (UV)-sensitive; (ii) show normal unscheduled DNA synthesis (indicating that the global genome repair sub-pathway is unaffected); (iii) fail to resume RNA synthesis after UV-exposure and (iv) are unable to remove cyclobutane pyrimidine dimers (CPD) photolesions from the transcribed strand of active genes. CS-A mice exhibit UV-sensitivity and pronounced age-dependent loss of retinal photoreceptor cells but otherwise fail to show the severe developmental and neurological abnormalities of the human syndrome. In contrast to human CS, Csa-/- animals develop skin tumors after chronic exposure to UV light, indicating that TCR in mice protects from UV-induced skin cancer development. Strikingly, inactivation of one Xpc allele (encoding a component of the damage recognition complex involved in the global genome repair sub-pathway) in Csa-/- mice resulted in a strongly enhanced UV-mediated skin cancer sensitivity, indicating that in a TC repair defective background, the Xpc gene product may be a rate-limiting factor in the removal of UV-induced DNA lesions.

Antagonistic activity of Lactobacillus casei strain shirota against gastrointestinal Listeria monocytogenes infection in rats

In the present study, the effect of ingested viable Lactobacillus casei Shirota strain YIT9029 on oral infection with the enteric pathogen Listeria monocytogenes in Wistar rats was investigated. Rats were orally infected with 10(9) viable L. monocytogenes. Starting 3 days before the infection, rats received a daily dosage of 10(9) viable L. casei. It was shown that supplementation of L. casei significantly reduced the numbers of L. monocytogenes in stomach, caecum, faeces, spleen and liver, 2 days after L. monocytogenes infection. The number of L. monocytogenes in the mesenteric lymph nodes was not affected by the ingestion of L. casei. In comparison with control animals, the levels of the liver-specific alanine aminotransferase were lower in L. casei-fed rats. Histological analysis of spleen and liver revealed no differences between the experimental and control animals. In a parallel study with orally L. monocytogenes infected rats, it was shown that L. casei was able to increase cellular immunity significantly as determined with the delayed-type hypersensitivity response against heat-killed L. monocytogenes. In conclusion, in the present study it was shown that orally administered L. casei is able to enhance host resistance against oral L. monocytogenes infection. In the gastrointestinal tract, as well as in the spleen and liver, L. monocytogenes numbers were reduced. Furthermore, it is concluded that the enhancement of this anti-Listeria activity might be, at least partly, due to increased cell-mediated immunity.

A review of studies on the effects of ultraviolet irradiation on the resistance to infections: evidence from rodent infection models and verification by experimental and observational human studies

Recent studies on the immunosuppressive effects of ultraviolet radiation (UVR) and the related resistance to infections in rodents and humans are presented. The waveband dependency of trans-to-cis isomerisation of urocanic acid in the stratum corneum and the role of DNA damage in UVR-induced erythema and immunosuppression were investigated to further elucidate the underlying mechanisms. Furthermore, human experimental studies on UVR-induced immunomodulation were performed. It appeared that the doses needed to suppress various immune parameters in humans (e.g. NK activity, contact hypersensitivity) were higher than those needed in experiments in rodents. Still, extrapolation of experimental animal data to the human situation showed that UVR may impair the resistance to different systemic infections at relevant outdoor doses. In observational human studies we aimed to substantiate the relevance of UVR for infections in humans. It was shown that sunny season was associated with a slightly retarded but clinically non-relevant antibody response to hepatitis B vaccination. Furthermore, sunny season appeared to be associated with a small decline in the number of CD4+ T-helper cells in a cohort of HIV-infected persons and a higher recurrence of herpes simplex and herpes zoster in a cohort of renal transplant recipients. However, in a study among young children a higher exposure to solar UVR was associated with a lower occurrence of upper respiratory tract symptoms. As disentangling the effects of UVR from other relevant factors is often impossible in observational studies, concise quantitative risk estimations for the human situation cannot be given at present.

Photoimmunology and nucleotide excision repair: impact of transcription coupled and global genome excision repair

Ultraviolet (UV) light generates damage to DNA which is removed by a versatile mechanism called nucleotide excision repair (NER). There are two subpathways for NER: the transcription coupled repair (TCR) pathway which removes DNA damage from actively transcribed genes and the global genome repair pathway which removes damage throughout the genome. Most types of DNA lesions are processed more rapidly by TCR than by GGR. It is widely accepted that immunological processes play a pivotal role in the generation of skin tumours induced by exposure to ultraviolet light and first evidence is emerging that GGR and TCR play different roles in skin reactions such as erythema and delayed type hypersensitivity. The relationship between UV-induced responses of the skin and the two NER subpathways is discussed.

Ultraviolet light induced injury: immunological and inflammatory effects

This article reviews many of the complex events that occur after cutaneous ultraviolet (UV) exposure. The inflammatory changes of acute exposure of the skin include erythema (sunburn), the production of inflammatory mediators, alteration of vascular responses and an inflammatory cell infiltrate. Damage to proteins and DNA accumulates within skin cells and characteristic morphological changes occur in keratinocytes and other skin cells. When a cell becomes damaged irreparably by UV exposure, cell death follows via apoptotic mechanisms. Alterations in cutaneous and systemic immunity occur as a result of the UV-induced inflammation and damage, including changes in the production of cytokines by keratinocytes and other skin-associated cells, alteration of adhesion molecule expression and the loss of APC function within the skin. These changes lead to the generation of suppressor T cells, the induction of antigen-specific immunosuppression and a lowering of cell-mediated immunity. These events impair the immune system's capacity to reject highly antigenic skin cancers. This review gives an overview of the acute inflammatory and immunological events associated with cutaneous UV exposure, which are important to consider before dealing with the complex interactions that occur with chronic UV exposure, leading to photocarcinogenesis.

Effects of solar radiation on the human immune system

On UV irradiation of the skin, a complex cascade of immunological changes results, initiated by cutaneous chromophores and ending in suppression of some local and systemic immune responses. In this review, the stages in this process are outlined first, concentrating on the roles of DNA and urocanic acid as photoreceptors. Evidence indicating UV-induced immunomodulation of delayed hypersensitivity and resistance to infectious diseases in human subjects follows. Aspects of genetic susceptibility to the immunosuppressive effects of UV exposure and extrapolation of the data obtained in animal models to the human situation are included. Finally uncertain and unknown factors relating to the impact of UV on the human immune system are discussed.

Differential ultraviolet-B-induced immunomodulation in XPA, XPC, and CSB DNA repair-deficient mice

Ultraviolet B irradiation has serious consequences for cellular immunity and can suppress the rejection of skin tumors and the resistance to infectious diseases. DNA damage plays a crucial role in these immunomodulatory effects of ultraviolet B, as impaired repair of ultraviolet-B-induced DNA damage has been shown to cause suppression of cellular immunity. Ultraviolet-B-induced DNA damage is repaired by the nucleotide excision repair mechanism very efficiently. Nucleotide excision repair comprises two subpathways: transcription-coupled and global genome repair. In this study the immunologic consequences of specific nucleotide excision repair defects in three mouse models, XPA, XPC, and CSB mutant mice, were investigated. XPA mice carry a total nucleotide excision repair defect, whereas XPC and CSB mice only lack global genome and transcription-coupled nucleotide excision repair, respectively. Our data demonstrate that cellular immune parameters in XPA, XPC, and CSB mice are normal compared with their wild-type (control) littermates. This may indicate that the reported altered cellular responses in xeroderma pigmentosum patients are not constitutive but could be due to external factors, such as ultraviolet B. Upon exposure to ultraviolet B, only XPA mice are very sensitive to ultraviolet-B-induced inhibition of Th1-mediated contact hypersensitivity responses and interferon-gamma production in skin draining lymph nodes. Lipopolysaccharide-stimulated tumor necrosis factor alpha and interleukin-10 production are significantly augmented in both XPA and CSB mice after ultraviolet B exposure. Lymph node cell numbers were increased very significantly in XPA, mildly increased in CSB, and not in XPC mice. In general XPC mice do not exhibit any indication of enhanced ultraviolet B susceptibility with regard to the immune parameters analyzed. These data suggest that both global genome repair and transcription-coupled repair are needed to prevent immunomodulation by ultraviolet B, whereas transcription-coupled repair is the major DNA repair subpathway of nucleotide excision repair that prevents the acute ultraviolet-B-induced effects such as erythema.

Dose response for UV-induced immune suppression in people of color: differences based on erythemal reactivity rather than skin pigmentation

Ultraviolet radiation (UVR) is known to suppress immune responses in human subjects. The purpose of this study was to develop dose responses across a broad range of skin pigmentation in order to facilitate risk assessment. UVR was administered using FS 20 bulbs. Skin pigmentation and UVR sensitivity were evaluated using Fitzpatrick classifications, minimal erythemal dose (MED), slope of the erythemal dose response curve (sED), baseline pigmentation and tanning response. To assess immune responses dinitrochlorobenzene (DNCB) was applied to irradiated buttock skin 72 h after irradiation. Two weeks later DNCB was applied to the inside upper arm. Skin thickness was measured before and after challenge. Dose response was modeled (to obtain a regression line) for the entire group of 185 subjects. With the exception of sED none of the above-mentioned pigmentation indicators contributed significantly to variability around the regression line. Thus, differences in sensitivity for multiple skin types based on Fitzpatrick classification or MED were not observed. However, differences in immune sensitivity to UVR were detected between subjects with steep erythemal dose response curves and those with moderate or flat responses. For subjects with steep erythemal responses the dose calculated to suppress the immune response by 50% was 114 mJ/cm2. This group included individuals with Fitzpatrick skin types I-V, MED for these subjects ranged from 30 to 80 mJ/cm2. The 50% suppression dose for subjects with weak or no erythemal response could not be computed (the dose response was flat). This resistant group included subjects with skin types IV-VI and MED for these subjects ranged from 41 to > 105 mJ/cm2. This study provides a human dose response for UVR suppression of contact sensitivity that will be useful in risk assessment. It is the first study to provide this information using the FS sun lamp and is the first study to include people of color. The sED appears to be a new variable for identifying sensitive subjects at risk of UVR-induced immune suppression.

UVA exposure affects UVB and cis-urocanic acid-induced systemic suppression of immune responses in Listeria monocytogenes-infected Balb/c mice

Ultraviolet radiation can inhibit immune responses locally as well as systemically. Such effects have been measured in animals and humans exposed to ultraviolet B (wavelength 280-315 nm) (UVB) and ultraviolet A (315-400 nm) (UVA). The precise wavelength dependence is important for the identification of possible molecular targets and for assessments of risk of different artificial UV sources and solar UV. In such analyses, it is commonly assumed that radiation energy from each wavelength contributes to the effect independent of the other wavelengths. Here we show that this assumption does not hold good. In the present study, it was investigated whether exposure to broadband UVA or longwave ultraviolet A 1 (340-400 nm) (UVA 1) prior to the standard immunosuppressive UVB protocol might modulate the immunosuppressive effects induced by UVB. Preexposure to broadband UVA or longwave UVA 1, 1 day prior to the standard immunosuppressive UVB protocol, inhibited the UVB-induced suppression of delayed type hypersensitivity (DTH) to Listeria monocytogenes significantly. This effect was not associated with restoring the number of interleukin (IL-12)-positive cells in the spleen. Since isomerization of trans-urocanic acid (UCA) into the immunosuppressive cis-UCA isomer plays a crucial role in UVB-induced immunomodulation, in a second set of experiments it was investigated whether immunosuppression induced by cis-UCA might also be downregulated by preexposure to UVA. Animals were exposed to broad-band UVA or longwave UVA 1 prior to application of an immunosuppressive dose of cis- or trans-UCA as a control. Both UVA and UVA 1 appear to inhibit the cis-UCA-induced systemic immunosuppression (DTH and IL-12) to L. monocytogenes. These studies clearly show that UVA radiation modulates both UVB and cis-UCA-induced immunomodulation. In general, our studies indicate that both broadband UVA and longwave UVA 1 could induce modulation of UVB and cis-UCA-induced immunomodulation. As sunlight contains both UVA and UVB radiation the balance between these two radiations apparently determines the net immunomodulatory effect.