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

Cis-urocanic acid improves cell viability and suppresses inflammasome activation in human retinal pigment epithelial cells

Age-related macular degeneration (AMD) is a common eye disease among the elderly, which can result in impaired vision and irreversible loss of vision. The majority of patients suffer from the dry (also known as the atrophic) form of the disease, which is completely lacking an effective treatment. In the present study, we evaluated the potential of cis-urocanic acid (cis-UCA) to protect human ARPE-19 cells from cell damage and inflammasome activation induced by UVB light. Urocanic acid is a molecule normally present in human epidermis. Its cis-form has recently been found to alleviate UVB-induced inflammasome activation in human corneal epithelial cells. Here, we observed that cis-UCA is well-tolerated also by human retinal pigment epithelial (RPE) cells at a concentration of 100 μg/ml. Moreover, cis-UCA was cytoprotective and efficiently diminished the levels of mature IL-1β, IL-18, and cleaved caspase-1 in UVB-irradiated ARPE-19 cells. Interestingly, cis-UCA also reduced DNA damage, whereas its effect against ROS production was negligible. Collectively, cis-UCA protected ARPE-19 cells from UVB-induced phototoxicity and inflammasome activation. This study indicates that due to its beneficial properties of preserving cell viability and preventing inflammation, cis-UCA has potential in drug development of chronic ocular diseases, such as AMD.

The Prophylactic Effect of Vitamin C and Vitamin B12 against Ultraviolet-C-Induced Hepatotoxicity in Male Rats

Ultraviolet C (UVC) devices are an effective means of disinfecting surfaces and protecting medical tools against various microbes, including coronavirus. Overexposure to UVC can induce oxidative stress, damage the genetic material, and harm biological systems. This study investigated the prophylactic efficacy of vitamin C and B12 against hepatotoxicity in UVC-intoxicated rats. Rats were irradiated with UVC (725.76, 967.68, and 1048.36 J/cm²) for 2 weeks. The rats were pretreated with the aforementioned antioxidants for two months before UVC irradiation. The prophylactic effect of vitamins against UVC hepatotoxicity was evaluated by monitoring the alteration of liver enzyme activities, antioxidant status, apoptotic and inflammatory markers, DNA fragmentation, and histological and ultrastructural alterations. Rats exposed to UVC showed a significant increase in liver enzymes, oxidant-antioxidant balance disruption, and increased hepatic inflammatory markers (TNF-α, IL-1β, iNOS, and IDO-1). Additionally, obvious over-expression of activated caspase-3 protein and DNA fragmentation were detected. Histological and ultrastructural examinations verified the biochemical findings. Co-treatment with vitamins ameliorated the deviated parameters to variable degrees. In conclusion, vitamin C could alleviate UVC-induced hepatotoxicity more than vitamin B12 by diminishing oxidative stress, inflammation, and DNA damage. This study could provide a reference for the clinical practice of vitamin C and B12 as radioprotective for workers in UVC disinfectant areas.

Effect of acute ultraviolet radiation on Galleria mellonella health and immunity

For humans, acute and chronic overexposure to ultraviolet (UV) radiation can cause tissue damage in the form of sunburn and promote cancer(s). The immune-modulating properties of UV radiation and health-related consequences are not well known. Herein, we used the larvae of the wax moth Galleria mellonella, to determine UV-driven changes in cellular components of innate immunity. From immune cell (haemocyte) reactivity and the production of antimicrobial factors, these insects share many functional similarities with mammalian cellular innate immunity. After exposing insects to UVA or UVB for up to two hours, we monitored larval viability, susceptibility to infection, haemolymph (blood) physiology and faecal discharge. Prolonged exposure of larvae to UVB coincided with decreased survival, enhanced susceptibility to bacterial challenge, melanin synthesis in the haemolymph, compromised haemocyte functionality and changes in faecal (bacterial) content. We contend G. mellonella is a reliable in vivo model for assessing the impact of UV exposure at the whole organism and cellular levels.

UVB-Induced Microvesicle Particle Release and Its Effects on the Cutaneous Microenvironment

Ultraviolet B radiation (UVB) has profound effects on human skin that results in a broad spectrum of immunological local and systemic responses and is the major cause of skin carcinogenesis. One important area of study in photobiology is how UVB is translated into effector signals. As the skin is exposed to UVB light, subcellular microvesicle particles (MVP), a subtype of bioactive extracellular vesicles, are released causing a variety of local and systemic immunological effects. In this review, we highlight keratinocyte MVP release in keratinocytes in response to UVB. Specifically, Platelet-activating factor receptor agonists generated by UVB result in MVP released from keratinocytes. The downstream effects of MVP release include the ability of these subcellular particles to transport agents including the glycerophosphocholine-derived lipid mediator Platelet-activating factor (PAF). Moreover, even though UVB is only absorbed in the epidermis, it appears that PAF release from MVPs also mediates systemic immunosuppression and enhances tumor growth and metastasis. Tumor cells expressing PAF receptors can use this mechanism to evade chemotherapy responses, leading to treatment resistance for advanced cancers such as melanoma. Furthermore, novel pharmacological agents provide greater insight into the UVB-induced immune response pathway and a potential target for pharmacological intervention. This review outlines the need to more clearly elucidate the mechanism linking UVB-irradiation with the cutaneous immune response and its pathological manifestations. An improved understanding of this process can result in new insights and treatment strategies for UVB-related disorders from carcinogenesis to photosensitivity.

Investigation of Metabolome Underlying the Biological Mechanisms of Acute Heat Stressed Granulosa Cells

Heat stress affects granulosa cells and the ovarian follicular microenvironment, ultimately resulting in poor oocyte developmental competence. This study aims to investigate the metabo-lomics response of bovine granulosa cells (bGCs) to in vitro acute heat stress of 43 °C. Heat stress triggers oxidative stress-mediated apoptosis in cultured bGCs. Heat-stressed bGCs exhibited a time-dependent recovery of proliferation potential by 48 h. A total of 119 metabolites were identified through LC-MS/MS-based metabolomics of the spent culture media, out of which, 37 metabolites were determined as differentially involved in metabolic pathways related to bioenergetics support mechanisms and the physical adaptations of bGCs. Multiple analyses of metabolome data identified choline, citric acid, 3-hydroxy-3-methylglutaric acid, glutamine, and glycocyamine as being upregulated, while galactosamine, AICAR, ciliatine, 16-hydroxyhexadecanoic acid, lysine, succinic acid, uridine, xanthine, and uraconic acid were the important downregulated metabolites in acute heat stress. These differential metabolites were implicated in various important metabolic pathways directed towards bioenergetics support mechanisms including glycerophospholipid metabolism, the citrate cycle (TCA cycle), glyoxylate and dicarboxylate metabolism, and serine, threonine, and tyrosine metabolism. Our study presents important metabolites and metabolic pathways involved in the adaptation of bGCs to acute heat stress in vitro.

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.

Ethanol consumption synergistically increases ultraviolet radiation induced skin damage and immune dysfunction

BACKGROUND

Excessive UV radiation disrupts skin homeostasis by multiple mechanisms that extend beyond the simple erythema associated with sunburns including reduction of antioxidants, increased DNA damage, and impairment of skin immune responses. Recreational UV exposure frequently occurs concurrently with excessive ethanol (EtOH). Epidemiological studies suggest a harmful, dose-dependent impact of EtOH in the setting of high UV exposure, leading to increased severity of sunburns relative to those generated in the absence of EtOH. Furthermore, EtOH consumption and UV radiation have multiple overlapping effects on the skin that could account for the epidemiological association.

OBJECTIVE

To elucidate the relationship between excessive EtOH ingestion and UV exposures on early skin damage and downstream immune dysfunction.

METHODS

We examined the impact of UVB on local skin damage, including inflammation, sunburned cells, apoptotic cells, melanin and antioxidant levels, DNA damage and immune dysfunction in the presence or absence of EtOH ingestion by combining standard mouse models of EtOH consumption and UVB exposure models. To confirm that the observed changes in mouse skin were relevant to human skin, we investigated the effects of EtOH on UV-induced skin damage with human skin explants.

RESULTS

We demonstrated that EtOH consumption and UV exposure act synergistically to increase the severity of local skin damage resulting in impaired melanin responses, reduced antioxidants, greater DNA damage, and immune dysfunction as measured by reduced contact hypersensitivity.

CONCLUSIONS

The results support incorporation of the risks of combined UV exposure and excessive alcohol consumption into public health campaigns.

Protection from Ultraviolet Damage and Photocarcinogenesis by Vitamin D Compounds

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

Skin photo-protection with phytochemicals against photo-oxidative stress, photo-carcinogenesis, signal transduction pathways and extracellular matrix remodeling-An overview

Excessive exposure of skin to UV radiation triggers the generation of oxidative stress, inflammation, immunosuppression, apoptosis, matrix-metalloproteases production, and DNA mutations leading to the onset of photo ageing and photo-carcinogenesis. At the molecular level, these changes occur via activation of several protein kinases as well as transcription pathways, formation of reactive oxygen species, and release of cytokines, interleukins and prostaglandins together. Current therapies available on the market only provide limited solutions and exhibit several side effects. The present paper provides insight into scientific studies that have elucidated the positive role of phytochemicals in counteracting the UV-induced depletion of antioxidant enzymes, increased lipid peroxidation, inflammation, DNA mutations, increased senescence, dysfunctional apoptosis and immune suppression. The contribution of phytochemicals to the downregulation of expression of oxidative-stress sensitive transcription factors (Nrf2, NF-Kb, AP-1 and p53) and protein kinases (MSK, ERK, JNK, p38 MAPK, p90RSK2 and CaMKs) involved in inflammation, apoptosis, immune suppression, extracellular matrix remodelling, senescence, photo ageing and photo-carcinogenesis, is also discussed. Conclusively, several phytochemicals hold potential for the development of a viable solution against UV irradiation-mediated photo ageing, photo-carcinogenesis and related manifestations.

The impact of irradiance on UVB-induced cutaneous immunosuppression: Implications on administering most efficient phototherapy

BACKGROUND

Ultraviolet B (UVB) is commonly used for treating dermatologic conditions. Recently, high irradiance UVB (HIUVB) has been suggested to be more effective for treating skin conditions as compared to its low irradiance (LI) counterpart. The biological impact of UVB radiation emitted at different irradiance on cutaneous immunity remains obscure.

OBJECTIVE

This study aimed to explore the impacts of UVB radiation administered at equivalent fluence (mJ/cm²) but different irradiance (mW/cm²) on cutaneous immune response.

METHODS

Cultured bone marrow derived dendritic cell (BMDC) were treated with equivalent fluence of UVB radiation with HIUVB or LIUVB. The phenotypic and functional alterations of BMDCs were documented. Animal models were used to validate the in vitro results in vivo and explore the mechanisms involved.

RESULTS

After equivalent fluence of UVB radiation, the HIUVB treated BMDC showed significantly lower MHCII and CD86 expressions, reduced capacity to stimulate T cell proliferation, and enhanced activation of aryl hydrocarbon receptor (AhR)-activated genes as compared to control while their LIUVB treated counterpart showed no significant change. Using animal model, the HIUVB induced significantly higher immune suppressive effect in mice as compared to their LIUVB counterpart after equivalent fluence of UVB treatment. The superior immune suppressive effect of HIUVB over LIUVB radiation was not observed when similar experiments were performed using AhR-deficient mice.

CONCLUSION

We propose irradiance played an important role modulating UVB-induced cutaneous immune suppression. Future works on UVB phototherapy, both clinical and research, should incorporate this important parameter into consideration.

The Potential Role of Dietary Platelet-Activating Factor Inhibitors in Cancer Prevention and Treatment

Cancer is the second leading cause of mortality worldwide. The role of unresolved inflammation in cancer progression and metastasis is well established. Platelet-activating factor (PAF) is a key proinflammatory mediator in the initiation and progression of cancer. Evidence suggests that PAF is integral to suppression of the immune system and promotion of metastasis and tumor growth by altering local angiogenic and cytokine networks. Interactions between PAF and its receptor may have a role in various digestive, skin, and hormone-dependent cancers. Diet plays a critical role in the prevention of cancer and its treatment. Research indicates that the Mediterranean diet may reduce the incidence of several cancers in which dietary PAF inhibitors have a role. Dietary PAF inhibitors such as polar lipids have demonstrated inhibitory effects against the physiological actions of PAF in cancer and other chronic inflammatory conditions in vitro and in vivo. In addition, experimental models of radiotherapy and chemotherapy demonstrate that inhibition of PAF as adjuvant therapy may lead to more favorable outcomes. Although promising, there is limited evidence on the potential benefits of dietary PAF inhibitors on cancer prevention or treatment. Therefore, further extensive research is required to assess the effects of various dietary factors and PAF inhibitors and to elucidate the mechanisms in prevention of cancer progression and metastasis at a molecular level.

Ultraviolet radiation-induced immunosuppression and its relevance for skin carcinogenesis

The realisation that UV radiation (UVR) exposure could induce a suppressed immune environment for the initiation of carcinogenesis in the skin was first described more than 40 years ago. Van der Leun and his colleagues contributed to this area in the 1980s and 90s by experiments in mice involving UV wavelength and dose-dependency in the formation of such tumours, in addition to illustrating both the local and systemic effect of the UVR on the immune system. Since these early days, many aspects of the complex pathways of UV-induced immunosuppression have been studied and are outlined in this review. Although most experimental work has involved mice, it is clear that UVR also causes reduced immune responses in humans. Evidence showing the importance of the immune system in determining the risk of human skin cancers is explained, and details of how UVR exposure can down-regulate immunity in the formation and progression of such tumours reviewed. With increasing knowledge of these links and the mechanisms of UVR-induced immunosuppression, novel approaches to enhance immunity to skin tumour antigens in humans are becoming apparent which, hopefully, will reduce the burden of UVR-induced skin cancers in the future.

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

BACKGROUND

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

METHODS

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

OBJECTIVE

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

RESULTS

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

CONCLUSION

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

From Early Immunomodulatory Triggers to Immunosuppressive Outcome: Therapeutic Implications of the Complex Interplay Between the Wavebands of Sunlight and the Skin

Phototherapy is an efficient treatment for many cutaneous diseases that involve the activation of inflammatory pathways or the overgrowth of cells with aberrant phenotype. In this review, we discuss recent advances in photoimmunology, focusing on the effects of UV-based therapies currently used in dermatology. We describe the molecular responses to the main forms of photo(chemo)therapy such as UVB, UVA-1, and PUVA that include the triggering of apoptotic or immunosuppressive pathways and help to clear diseased skin. The early molecular response to UV involves DNA photoproducts, the isomerization of urocanic acid, the secretion of biophospholipids such as platelet activating factor (PAF), the activation of aryl hydrocarbon receptor and inflammasome, and vitamin D synthesis. The simultaneous and complex interaction of these events regulates the activity of the immune system both locally and systemically, resulting in apoptosis of neoplastic and/or benign cells, reduction of cellular infiltrate, and regulation of cytokines and chemokines. Regulatory T-cells and Langerhans cells, among other skin-resident cellular populations, are deeply affected by UV exposure and are therefore important players in the mechanisms of immunomodulation and the therapeutic value of UV in all its forms. We weigh the contribution of these cells to the therapeutic application of UV and how they may participate in transferring the direct impact of UV on the skin into local and systemic immunomodulation. Moreover, we review the therapeutic mechanisms revealed by clinical and laboratory animal investigations in the most common cutaneous diseases treated with phototherapy such as psoriasis, atopic dermatitis, vitiligo, and cutaneous T-cell lymphoma. Better understanding of phototherapeutic mechanisms in these diseases will help advance treatment in general and make future therapeutic strategies more precise, targeted, personalized, safe, and efficient.

Targeting Mitochondrial Oxidative Stress to Mitigate UV-Induced Skin Damage

Unmitigated UV radiation (UVR) induces skin photoaging and multiple forms of cutaneous carcinoma by complex pathways that include those mediated by UV-induced reactive oxygen species (ROS). Upon UVR exposure, a cascade of events is induced that overwhelms the skin’s natural antioxidant defenses and results in DNA damage, intracellular lipid and protein peroxidation, and the dysregulation of pathways that modulate inflammatory and apoptotic responses. To this end, natural products with potent antioxidant properties have been developed to prevent, mitigate, or reverse this damage with varying degrees of success. Mitochondria are particularly susceptible to ROS and subsequent DNA damage as they are a major intracellular source of oxidants. Therefore, the development of mitochondrially targeted agents to mitigate mitochondrial oxidative stress and resulting DNA damage is a logical approach to prevent and treat UV-induced skin damage. We summarize evidence that some existing natural products may reduce mitochondrial oxidative stress and support for synthetically generated mitochondrial targeted cyclic nitroxides as potential alternatives for the prevention and mitigation of UVR-induced skin damage.

Characterization of Retinal Pigment Epithelial Melanin and Degraded Synthetic Melanin Using Mass Spectrometry and In Vitro Biochemical Diagnostics

With increasing age, there is an observable loss of melanin in retinal pigment epithelial (RPE) cells. It is possible that degradation of the pigment contributes to the pathogenesis of retinal disease, as the cellular antioxidant material is depleted. Functionally, intact melanin maintains protective qualities, while oxidative degradation of melanin promotes reactive oxygen species (ROS) generation and formation of metabolic byproducts, such as melanolipofuscin. Understanding the structural and functional changes to RPE melanin with increasing age may contribute to a better understanding of disease progression and risk factors for conditions such as age-related macular degeneration (AMD). In this study, human donor RPE melanin is characterized using MALDI mass spectrometry to follow melanin degradation trends. In vitro models using ARPE-19 cells are used to assess photo-reactivity in repigmented cells. Significant protection against intracellular ROS produced by blue light is observed in calf melanin-pigmented cells versus unpigmented and black latex bead controls (P < 0.0001). UV-B exposure to aged human melanin-pigmented cells results in a significant increase in nitric oxide production versus control cells (P < 0.001). Peroxide-treated synthetic melanin is characterized to elucidate degradation products that may contribute to RPE cell damage.

Cytotoxic effect of the serotonergic drug 1-(1-Naphthyl)piperazine against melanoma cells

1-(1-Naphthyl)piperazine (1-NPZ) is a serotonergic derivative of quipazine acting both as antagonist and agonist of different serotonin receptors, with promising results for the management of skin cancer. In this work, we studied the effect of 1-NPZ on human MNT-1 melanoma cells by evaluating its effects on cell viability, ability to form colonies, cell cycle dynamics, reactive oxygen species (ROS) production and apoptosis. Treatment of MNT-1 cells with 1-NPZ for 24h decreased cell viability and induced apoptosis in a dose-dependent manner. Activity against melanoma was confirmed with a different melanoma cell line, SK-MEL-28. Simultaneously, 1-NPZ affected cell cycle progression by mediating a S-phase delay. Higher levels of ROS were also detected in MNT-1 cells after treatment with 1-NPZ. Furthermore, 1-NPZ significantly increased the expression of cyclooxygenase-2 in MNT-1 cells. These findings suggest that 1-NPZ pretreatment is able to induce oxidative stress, and consequently apoptotic cell death in melanoma cells. In conclusion, this study demonstrates the cytotoxic and genotoxic potential of 1-NPZ against melanoma cells.

Anti-Inflammatory Role of Langerhans Cells and Apoptotic Keratinocytes in Ultraviolet-B-Induced Cutaneous Inflammation

UV radiation, particularly UVB, is the major risk factor for the induction of skin cancer, and it induces skin inflammation and immunosuppression. Although reports documented that Langerhans cells (LCs) play various roles in photobiology, little is known about whether they contribute to UVB-induced cutaneous inflammation. Recently, the anti-inflammatory effect of apoptotic cells was noted. This study focuses on the roles of LCs and apoptotic cells in UVB-induced cutaneous inflammation. We show that LCs are essential for resolution of UVB-induced cutaneous inflammation. Administration of quinolyl-valyl-O-methylaspartyl-[2,6-difluophenoxy]-methyl ketone, a broad-spectrum caspase inhibitor with potent antiapoptotic properties, inhibited the formation of UVB-induced apoptotic cells and aggravated UVB-induced cutaneous inflammation in wild-type mice. In contrast, exacerbation of UVB-induced cutaneous inflammation following quinolyl-valyl-O-methylaspartyl-[2,6-difluophenoxy]-methyl ketone administration was not observed in LC-depleted mice. These results suggest that the interaction between LCs and apoptotic cells is critical for resolution of UVB-induced cutaneous inflammation. Interestingly, UVB-induced apoptotic keratinocytes were increased in LC-depleted mice. In addition, we revealed that UVB-induced apoptotic keratinocytes were phagocytosed by LCs ex vivo and that prolongation of UVB-induced cutaneous inflammation following treatment with Cytochalasin D, an inhibitor of phagocytosis, was partially attenuated in LC-depleted mice. Collectively, our findings demonstrate that the interaction between LCs and apoptotic cells, possibly via LC-mediated phagocytosis of apoptotic keratinocytes, has an essential anti-inflammatory role in the resolution of UVB-induced cutaneous inflammation.

Platelet activating factor-induced expression of p21 is correlated with histone acetylation

Ultraviolet (UV)-irradiated keratinocytes secrete the lipid mediator of inflammation, platelet-activating factor (PAF). PAF plays an essential role in UV-induced immune suppression and skin cancer induction. Dermal mast cell migration from the skin to the draining lymph nodes plays a prominent role in activating systemic immune suppression. UV-induced PAF activates mast cell migration by up-regulating mast cell CXCR4 surface expression. Recent findings indicate that PAF up-regulates CXCR4 expression via histone acetylation. UV-induced PAF also activates cell cycle arrest and disrupts DNA repair, in part by increasing p21 expression. Do epigenetic alterations play a role in p21 up-regulation? Here we show that PAF increases Acetyl-CREB-binding protein (CBP/p300) histone acetyltransferase expression in a time and dose-dependent fashion. Partial deletion of the HAT domain in the CBP gene, blocked these effects. Chromatin immunoprecipitation assays indicated that PAF-treatment activated the acetylation of the p21 promoter. PAF-treatment had no effect on other acetylating enzymes (GCN5L2, PCAF) indicating it is not a global activator of histone acetylation. This study provides further evidence that PAF activates epigenetic mechanisms to affect important cellular processes, and we suggest this bioactive lipid can serve as a link between the environment and the epigenome.

Serotonin signalling is crucial in the induction of PUVA-induced systemic suppression of delayed-type hypersensitivity but not local apoptosis or inflammation of the skin

Psoralen and UVA (PUVA) has immunosuppressive and proapoptotic effects, which are thought to be responsible alone or in combination for its therapeutic efficacy. However, the molecular mechanism by which PUVA mediates its effects is not well understood. Activation of the serotonin (5-hydroxytryptamine, 5-HT) pathway has been suggested to be involved in the modulation of T-cell responses and found to mediate UVB-induced immune suppression. In particular, the activation of the 5-HT2A receptor has been proposed as one mechanism responsible for UV-induced immune suppression. We therefore hypothesized that 5-HT may play a role in PUVA-induced effects. The model of systemic suppression of delayed-type hypersensitivity (DTH) to Candida albicans was used to study immune function after exposure of C3H and KIT(W) (-Sh/W-Sh) mice to a minimal inflammatory dose of topical PUVA. The intra-peritoneal injection of the 5-HT2 receptor antagonist ketanserin or cyproheptadine or an anti-5-HT antibody immediately before PUVA exposure entirely abrogated suppression of DTH but had no significant effect on inflammation, as measured by swelling and cellular infiltration of the skin, and apoptosis as determined by the number of sunburn cells in C3H mice. Importantly, the systemic injection of 5-HT recapitulated PUVA immune suppression of DTH but did not induce inflammation or apoptosis in the skin. KIT(W) (-Sh/W-Sh) mice (exhibiting myelopoietic abnormalities, including lack of 5-HT-containing mast cells) were resistant to PUVA-induced suppression of DTH but not local skin swelling. Thus, this points towards a crucial role of 5-HT signalling in PUVA-induced immune suppression but not inflammation or apoptosis in situ in the skin.

Understanding the connection between platelet-activating factor, a UV-induced lipid mediator of inflammation, immune suppression and skin cancer

Lipid mediators of inflammation play important roles in several diseases including skin cancer, the most prevalent type of cancer found in the industrialized world. Ultraviolet (UV) radiation is a complete carcinogen and is the primary cause of skin cancer. UV radiation is also a potent immunosuppressive agent, and UV-induced immunosuppression is a well-known risk factor for skin cancer induction. An essential mediator in this process is the glyercophosphocholine 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine commonly referred to as platelet-activating factor (PAF). PAF is produced by keratinocytes in response to diverse stimuli and exerts its biological effects by binding to a single specific G-protein-coupled receptor (PAF-R) expressed on a variety of cells. This review will attempt to describe how this lipid mediator is involved in transmitting the immunosuppressive signal from the skin to the immune system, starting from its production by keratinocytes, to its role in activating mast cell migration in vivo, and to the mechanisms involved that ultimately lead to immune suppression. Recent findings related to its role in regulating DNA repair and activating epigenetic mechanisms, further pinpoint the importance of this bioactive lipid, which may serve as a critical molecular mediator that links the environment (UVB radiation) to the immune system and the epigenome.

Diverse Functions of Plasma PAF-AH in Tumorigenesis

This chapter is focused on the role of the plasma form of platelet-activating factor-acetylhydrolase (PAF-AH), heretofore referred to as PAF-AH, in tumorigenic responses. Biochemical and other properties of this enzyme were discussed in detail in chapter "Plasma PAF-AH (PLA2G7): Biochemical Properties, Association with LDLs and HDLs, and Regulation of Expression" by Stafforini and in other chapters. Although phospholipases tend not to be drivers of tumorigenesis themselves, these enzymes and the lipid mediators whose levels they regulate interact with a variety of oncogenes and tumor suppressors [1]. Like other phospholipases, the functions of PAF-AH in cancer likely are related to its ability to regulate the levels of lipid mediators that participate in cellular processes related to initial tumorigenic events (e.g., proliferation, growth, inflammation) and/or spreading of the disease (e.g., matrix metalloproteinase secretion, actin cytoskeleton reorganization, migration, and angiogenesis) [1]. The importance of substrates and products of PAF-AH on key cellular functions has been evaluated in cell-based analyses which revealed that these metabolites can have pro- and antitumorigenic functions. Studies in genetically engineered mice lacking PAF-AH expression and genetic manipulation of PAF-AH levels in cancer cells demonstrated diverse functions of the protein in models of melanoma, prostate cancer, colon cancer, and others. The following sections highlight lessons learned from studies in cell lines and in mouse models regarding the diversity of functions of PAF-AH in cancer, and the potential of PAFAH transcripts, protein, and/or activity levels to become cancer biomarkers and therapeutic targets.

Platelet-activating factor induces cell cycle arrest and disrupts the DNA damage response in mast cells

Platelet-activating factor (PAF) is a potent phospholipid modulator of inflammation that has diverse physiological and pathological functions. Previously, we demonstrated that PAF has an essential role in ultraviolet (UV)-induced immunosuppression and reduces the repair of damaged DNA, suggesting that UV-induced PAF is contributing to skin cancer initiation by inducing immune suppression and also affecting a proper DNA damage response. The exact role of PAF in modulating cell proliferation, differentiation or transformation is unclear. Here, we investigated the mechanism(s) by which PAF affects the cell cycle and impairs early DNA damage response. PAF arrests proliferation in transformed and nontransformed human mast cells by reducing the expression of cyclin-B1 and promoting the expression of p21. PAF-treated cells show a dose-dependent cell cycle arrest mainly at G2–M, and a decrease in the DNA damage response elements MCPH1/BRIT-1 and ataxia telangiectasia and rad related (ATR). In addition, PAF disrupts the localization of p-ataxia telangiectasia mutated (p-ATM), and phosphorylated-ataxia telangiectasia and rad related (p-ATR) at the site of DNA damage. Whereas the potent effect on cell cycle arrest may imply a tumor suppressor activity for PAF, the impairment of proper DNA damage response might implicate PAF as a tumor promoter. The outcome of these diverse effects may be dependent on specific cues in the microenvironment.

Research on the immunosuppressive activity of ingredients contained in sunscreens

The immunosuppressive properties of Benzophenone-4, an UV-filter and three ingredients, Allantoin, Bisabolol and Enoxolon used in sunscreen formulation, previously characterized as anti-inflammatory compounds, are studied. The results of this study demonstrate that four tested molecules have effects on DCs and T cells which are the most important cells of the immune system. The impact is also visible on keratinocyte cells which are in the direct contact with skin sunscreens. Each ingredient should be used with caution at reduced doses or even removed from some cosmetic preparations, such as sunscreens.

Photoimmunology

The discipline that investigates the biologic effects of ultraviolet radiation on the immune system is called photoimmunology. Photoimmunology evolved from an interest in understanding the role of the immune system in skin cancer development and why immunosuppressed organ transplant recipients are at a greatly increased risk for cutaneous neoplasms. In addition to contributing to an understanding of the pathogenesis of nonmelanoma skin cancer, the knowledge acquired about the immunologic effects of ultraviolet radiation exposure has provided an understanding of its role in the pathogenesis of other photodermatologic diseases.

Nutritional abrogation of photoimmunosuppression: in vivo investigations

Skin cancer is a major public health concern, and the primary aetiological factor in the majority of skin cancers is ultraviolet radiation (UVR) exposure. UVR not only induces potentially mutagenic DNA damage but also suppresses cell-mediated immunity (CMI), allowing cancerous cells to escape destruction and progress to tumours. A considerable proportion of an individual's annual sun exposure is obtained outside the vacation period when topical and physical measures for photoprotection are irregularly used. Certain nutrients could provide an adjunctive protective role, and evidence is accruing from experimental studies to support their use in abrogation of photoimmunosuppression. Moreover, developments in clinical research methods to evaluate impact of solar-simulated radiation on cutaneous CMI allow the immune protective potential of nutritional agents to be examined in humans in vivo. This article summarises the mediation of CMI and its suppression by UVR, evaluates the methodology for quantitative assessment in vivo, reviews the human studies reported on nutritional abrogation of photoimmunosuppression including recent randomized controlled trials and discusses the mechanisms of photoprotection by the nutrients. This includes, in addition to antioxidants, novel studies of omega-3 polyunsaturated fatty acids and nicotinamide.

Understanding engineered nanomaterial skin interactions and the modulatory effects of ultraviolet radiation skin exposure

The study of engineered nanomaterials for the development of technological applications, nanomedicine, and nano-enabled consumer products is an ever-expanding discipline as is the concern over the impact of nanotechnology on human environmental health and safety. In this review, we discuss the current state of understanding of nanomaterial skin interactions with a specific emphasis on the effects of ultraviolet radiation (UVR) skin exposure. Skin is the largest organ of the body and is typically exposed to UVR on a daily basis. This necessitates the need to understand how UVR skin exposure can influence nanomaterial skin penetration, alter nanomaterial systemic trafficking, toxicity, and skin immune function. We explore the unique dichotomy that UVR has on inducing both deleterious and therapeutic effects in skin. The subject matter covered in this review is broadly informative and will raise awareness of potential increased risks from nanomaterial skin exposure associated with specific occupational and life style choices. The UVR-induced immunosuppressive response in skin raises intriguing questions that motivate future research directions in the nanotoxicology and nanomedicine fields.

Suppression of experimental autoimmune encephalomyelitis by 300-315nm ultraviolet light

Multiple sclerosis (MS) is a chronic debilitating disease, with lowest incidence in equatorial regions and highest incidence in temperate regions. This relationship is believed to be related to sunlight or UV light exposure. Recent evidence with experimental autoimmune encephalomyelitis (EAE), an animal model of MS, established that this suppression is not mediated by vitamin D production. UV is comprised of three general wave bands: UVC (100-280nm), UVB (280-320nm) and UVA (320-400nm). In the present study we used four lamps that emit different wavelengths of UV: (1) broad band UVB (BB-UVB: 280-320nm); (2) narrow band UVB (NB-UVB: 300-315nm); (3) broad band UVA (BB-UVA: 300-400nm); and (4) long wavelength UVA (UVA-1: 340-400nm). The effect of these light sources was studied in vitamin D-sufficient C57BL/6 mice. The NB-UVB largely accounted for the suppression and delay of onset of EAE by BB-UVB. In contrast, UVA-1 failed to suppress EAE severity at low (∼2.5KJ/m(2)), medium (∼5.0KJ/m(2)) and high (∼10.0KJ/m(2)) doses. Serum calcium and 25-(OH)D3 levels were unchanged after both NB-UVB and UVA-1 treatments. The results demonstrate that NB-UVB (300-315nm) is largely responsible for light-induced suppression of EAE and its effect is not via production of vitamin D.

The current state of play of rodent models to study the role of vitamin D in UV-induced immunomodulation

Ultraviolet radiation (UVR) from sunlight is immunomodulatory and the main source of vitamin D for humans. Vitamin D can also regulate adaptive immunity, through mechanisms that involve the induction or activation of regulatory T cells. Similar mechanisms have also been proposed for the induction of regulatory T cells after skin exposure to UVR. Here we discuss the converging and diverging immunoregulatory pathways of UVR and vitamin D, including the molecular pathways for regulatory T cell induction, non-genomic pathways regulated by vitamin D, antimicrobial peptides, skin integrity and potential interactions between vitamin D and other UVR-induced mediators. We then discuss possible in vivo approaches that could be used to demonstrate a direct (or otherwise) role for vitamin D in mediating the immunosuppressive effects of UVR such as the use of dietary vitamin D restriction to induce vitamin D deficiency, gene knockout mice or drugs to block enzymes of vitamin D metabolism. We end with discussion of the epigenetic effects of vitamin D and UVR for immunosuppression.

Molecular Mechanisms of UV-Induced Apoptosis and Its Effects on Skin Residential Cells: The Implication in UV-Based Phototherapy

The human skin is an integral system that acts as a physical and immunological barrier to outside pathogens, toxicants, and harmful irradiations. Environmental ultraviolet rays (UV) from the sun might potentially play a more active role in regulating several important biological responses in the context of global warming. UV rays first encounter the uppermost epidermal keratinocytes causing apoptosis. The molecular mechanisms of UV-induced apoptosis of keratinocytes include direct DNA damage (intrinsic), clustering of death receptors on the cell surface (extrinsic), and generation of ROS. When apoptotic keratinocytes are processed by adjacent immature Langerhans cells (LCs), the inappropriately activated Langerhans cells could result in immunosuppression. Furthermore, UV can deplete LCs in the epidermis and impair their migratory capacity, leading to their accumulation in the dermis. Intriguingly, receptor activator of NF-κB (RANK) activation of LCs by UV can induce the pro-survival and anti-apoptotic signals due to the upregulation of Bcl-xL, leading to the generation of regulatory T cells. Meanwhile, a physiological dosage of UV can also enhance melanocyte survival and melanogenesis. Analogous to its effect in keratinocytes, a therapeutic dosage of UV can induce cell cycle arrest, activate antioxidant and DNA repair enzymes, and induce apoptosis through translocation of the Bcl-2 family proteins in melanocytes to ensure genomic integrity and survival of melanocytes. Furthermore, UV can elicit the synthesis of vitamin D, an important molecule in calcium homeostasis of various types of skin cells contributing to DNA repair and immunomodulation. Taken together, the above-mentioned effects of UV on apoptosis and its related biological effects such as proliferation inhibition, melanin synthesis, and immunomodulations on skin residential cells have provided an integrated biochemical and molecular biological basis for phototherapy that has been widely used in the treatment of many dermatological diseases.

Pathobiology of actinic keratosis: ultraviolet-dependent keratinocyte proliferation

Actinic keratoses are proliferations of transformed neoplastic keratinocytes in the epidermis that are the result of cumulative ultraviolet (UV) radiation from sun exposure. They are commonly found on sites of sun-exposed skin such as the face, balding scalp, and back of the hand. Although UV exposure does exert certain beneficial effects on the skin, excessive exposure to UV radiation induces multiple cascades of molecular signaling events at the cellular level that produce inflammation, immunosuppression, failure of apoptosis, and aberrant differentiation. Cumulatively, these actions result in mutagenesis and, ultimately, carcinogenesis. This article provides a brief overview of the key mediators that are implicated in the pathobiology of actinic keratosis. Three evolutionary possibilities exist for these keratoses in the absence of treatment: (1) spontaneous remission, which can be common; (2) remaining stable, without further progression; or (3) transformation to invasive squamous cell carcinoma, which may metastasize. Because the effects of UV radiation on the skin are complex, it is not yet fully clear how all of the mediators of actinic keratosis progression are interrelated. Nonetheless, some represent potential therapeutic targets, because it is clear that directing therapy to the effects of UV radiation at a number of different levels could interrupt and possibly reverse the mechanisms leading to malignant transformation.

Exposure to UV Wavelengths in Sunlight Suppresses Immunity. To What Extent is UV-induced Vitamin D3 the Mediator Responsible?

Reduced immunity following exposure of skin to UV radiation (UVR) may explain the positive latitude gradient measured for a number of autoimmune diseases (greater incidence of disease with residence at higher latitudes), including multiple sclerosis, allergic asthma and diabetes. Humans obtain >80% of their vitamin D3 by exposure of skin to UVR in sunlight. In experimental models, both vitamin D3-dependent and vitamin D3-independent pathways have been implicated in the mechanisms of UVR-induced systemic suppression of immunity. However, where does the balance of control lie? How important is vitamin D3 other than providing a biomarker of sun exposure? Are other molecules/pathways activated by UVR more important? Murine and human studies suggest many molecules may play a role and their participation may vary with different diseases and the time of UVR exposure or vitamin D3 sufficiency/deficiency. Although low vitamin D3 levels have been associated with increased prevalence and progression of human autoimmune diseases, the benefits of supplementation with vitamin D3 have not been definitive. Vitamin D3 levels are a measure of past sun exposure but vitamin D3-dependent and vitamin D3-independent immunosuppressive effects of UVR may play a role in control of autoimmune diseases.

UVB immunosuppression: vitamin D or not vitamin D? That is the question

UVB radiation stimulates the production of vitamin D and also has immunosuppressive effects. Vitamin D is also known to alter immunological function. Thus, a relevant question is whether vitamin D is a mediator of the immunological effects of UVB. In this issue, Schwarz et al. have addressed this issue and have concluded that although vitamin D has similar effects to UVB on the immune system, UVB-induced immunosuppression can be achieved without the requirement for vitamin D action.

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

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

Effects of solar radiation on the skin

No one would underestimate the importance of sunlight to the evolution of life on the earth and its role in human development. However, all humans - especially individuals who are lightly pigmented or whose occupation or lifestyle exposes them to excessive amounts of sunlight - are potentially susceptible to its deleterious effects. These effects can range from acute biological responses, such as sunburn and skin tanning, to conditions resulting from chronic exposure such as photoaged skin and potentially life-threatening conditions such as skin cancer. The objective of this review is to present a concise and up-to-date perspective on the effects of UVB, UVA, visible, and infrared radiation on cutaneous biochemistry and physiology.

Guinea pig skin, a model for epidermal cellular and molecular changes induced by UVR in vivo and in vitro: effects on Mycobacterium bovis Bacillus Calmette-Guérin vaccination

Previously, we reported that ultraviolet B-radiation (UVR) suppressed Bacillus Calmette-Guérin (BCG) vaccine-induced resistance to Mycobacterium tuberculosis in guinea pigs (GP). Herein, we investigated the cellular and molecular changes within the irradiated GP epidermis and the in vivo effect of supernatants from UV-irradiated (200 J m(-2)) epidermal cells (UV-sup) on M. bovis BCG vaccination. UVR increased the number of nucleated keratinocytes in the skin, but caused a decrease in the proportions of CD25(+)T cells. In the spleen, UVR resulted in a decrease in the proportions of T-cell subsets including CD25(+)T cells, and major histocompatibility complex (MHC) class II(+) and CD14(+) cells. Similarly, significant up-regulation of several cytokine mRNAs including IL-10 was also observed. Furthermore, UV-sup significantly reduced the MHC class II expression in peritoneal cells and reduced T-cell proliferation to ConA. The proliferation to purified protein derivative (PPD) was restored to normal levels by anti-IL-10 antibody. The UV-sup when injected into BCG-vaccinated GP significantly diminished the skin test response and T-cell proliferation to PPD and up-regulated the expression of IL-10, IL-4, IL-1β and Foxp3 mRNAs in the lymph node or spleen. Thus, whole body UVR induces profound cellular and molecular changes and injection of UV-sup from epidermal cells mimics the effect of whole body UVR in BCG-vaccinated GP.

Loss of the platelet activating factor receptor in mice augments PMA-induced inflammation and cutaneous chemical carcinogenesis

Although platelet-activating factor (PAF) is a well-known acute inflammatory mediator, little is known regarding the role of PAF in chronic inflammation. Phorbol esters are known to stimulate PAF production. Moreover, the ability of repeated applications of phorbol esters to induce a sustained inflammatory response is crucial to their tumorigenic activity. We therefore examined whether PAF acts as a mediator of phorbol ester-induced inflammation and tumorigenesis. While PAF receptor knockout mice (PAFR(-/-)) showed an expected but modest reduction in the acute inflammatory response to phorbol 12-myristate 13-acetate (PMA), these mice exhibited a surprising increase in inflammation following chronic PMA application. This increased inflammation was documented by a number of findings that included: increased skin thickness, increased myeloperoxidase activity and expression and increased expression of known inflammatory mediators. Interestingly, vehicle-treated PAFR(-/-) mice also exhibited modest increases in levels of inflammatory markers. This suggests that the platelet activating factor receptor (PAFR) acts to suppress chronic inflammation in response to other stimuli, such as barrier disruption. The idea that chronic PAFR activation is anti-inflammatory was documented by repetitive topical PAFR agonist administration that resulted in reduced myeloperoxidase activity in skin. We next utilized a 7,12-dimethylbenz(a)anthracene/PMA carcinogenesis protocol to demonstrate that PAFR(-/-) mice exhibit significantly increased tumor formation and malignant progression compared with wild-type control mice. These studies provide evidence for two important, unexpected and possibly interrelated pathological roles for the PAFR: first, the PAFR acts to suppress PMA-induced chronic inflammation; secondly, the PAFR acts to suppress neoplastic development in response to chemical carcinogens.

The immunologic revolution: photoimmunology

UV radiation targets the skin and is a primary cause of skin cancer (both melanoma and nonmelanoma skin cancer). Exposure to UV radiation also suppresses the immune response, and UV-induced immune suppression is a major risk factor for skin cancer induction. The efforts of dermatologists and cancer biologists to understand how UV radiation exposure suppresses the immune response and contributes to skin cancer induction led to the development of the subdiscipline we call photoimmunology. Advances in photoimmunology have generally paralleled advances in immunology. However, there are a number of examples in which investigations into the mechanisms underlying UV-induced immune suppression reshaped our understanding of basic immunological concepts. Unconventional immune regulatory roles for Langerhans cells, mast cells, and natural killer T (NKT) cells, as well as the immune-suppressive function of lipid mediators of inflammation and alarmins, are just some examples of how advances in immunodermatology have altered our understanding of basic immunology. In this anniversary issue celebrating 75 years of cutaneous science, we provide examples of how concepts that grew out of efforts by immunologists and dermatologists to understand immune regulation by UV radiation affected immunology in general.

Hypoxia inducible factor-1α contributes to UV radiation-induced inflammation, epidermal hyperplasia and immunosuppression in mice

Hypoxia inducible factor-1α (HIF-1α), a ubiquitous inducible oxygen-sensing transcription factor, promotes cell survival under hypoxic conditions, including the early pre-angiogenic period of tumorigenesis, and is known to contribute to many malignancies. However HIF-1α can also be activated by inflammatory mediators, and can activate inflammation-modulating proteins itself, including heme oxygenase-1 (HO-1) and the cytokine IL-6. Recently HIF-1α was reported to be induced by UVB (290-320 nm) radiation in cultured human keratinocytes, acting as a stress protein associated with the release of reactive oxygen species. In this in vivo murine study we demonstrate that HIF-1α protein is an early responder to UV radiation in the skin, and its activation can be attenuated by treating mice with its post-translational inhibitor, YC-1. Treatment with YC-1 following UV-irradiation of mice has revealed the involvement of HIF-1α in UV-induced inflammation, IL-6 production, and epidermal hyperplasia. In addition, upregulated cutaneous HIF-1α was found to be an important factor in the UV-suppression of T cell-mediated immunity, measured by contact hypersensitivity (CHS). The mechanism remains unclear, however it did not appear to involve the immunosuppressive cutaneous photoproduct cis-urocanic acid, but HIF-1α induction was inhibited by irradiation with photoimmune protective UVA (320-400 nm), implicating a negative correlation between the two stress proteins, HIF-1α and the photoimmune protective UVA responder HO-1.

Roles of the immune system in skin cancer

Over the past several decades, there has been increasing interest in understanding the roles of the immune system in the development and progression of cancer. The importance of the immune system in human skin cancer has been long recognized based primarily upon the increased incidence of skin cancers in organ transplant recipients and mechanisms of ultraviolet (UV) radiation-mediated immunomodulation. In this review, we integrate multiple lines of evidence highlighting the roles of the immune system in skin cancer. First, we discuss the concepts of cancer immunosurveillance and immunoediting as they might relate to human skin cancers. We then describe the clinical and molecular mechanisms of skin cancer development and progression in the contexts of therapeutic immunosuppression in organ transplant recipients, viral oncogenesis, and UV radiation-induced immunomodulation with a primary focus on basal cell carcinoma and squamous cell carcinoma. The clinical evidence supporting expanding roles for immunotherapy is also described. Finally, we discuss recent research examining the functions of particular immune cell subsets in skin cancer and how they might contribute to both antitumour and protumour effects. A better understanding of the biological mechanisms of cancer immunosurveillance holds the promise of enabling better therapies.