Immunoprotective UVA (320-400 nm) irradiation upregulates heme oxygenase-1 in the dermis and epidermis of hairless mouse skin

The induction of heme oxygenase-1 (HO-1) by ultraviolet A (UVA) (320-400 nm) radiation provides a protective cellular defence against oxidative stress, and has been well demonstrated in cultured human skin fibroblasts, although keratinocytes were unreactive. The UVA responsiveness of HO-1 however, has not been confirmed in intact skin. Previously, we reported that UVA-inducible HO enzyme activity in mouse skin is protective against UVB-induced immunosuppression. This study identifies the induced HO isoform and its localization in mouse skin irradiated in vivo with such an immunoprotective UVA dose. We found that HO-1 mRNA was expressed in UVA-irradiated skin, but not in normal or UVB-irradiated skin, whereas constitutive HO-2 was always present. UVA-irradiated skin had increased HO enzyme activity and bilirubin content, and decreased heme content, consistent with HO-1 induction. In situ hybridization and immunohistochemical staining localized HO-1 mRNA and protein to both epidermis and dermis, with strongest expression in basal keratinocytes and weaker expression in dermal fibroblast-like and other cells, in contrast with UVA-induced HO-1 in cultured human skin fibroblasts. This suggests that cultured skin cells may not fully represent skin functions in vivo, or that there may be inherent differences between human and hairless mouse skin HO-1 responses.     

Dermal contributions to UVA-induced oxidative stress in skin

Background: When the skin is exposed to solar irradiation, UVA photons interact with skin tissues and induce excessive reactive oxygen species, resulting in oxidative stress. We have shown in a previous study that in vivo chemiluminescence's measurement can be used to evaluate the overall level of UVA-induced oxidative stress in human skin. However, the origin of the observed chemiluminescence signals remains unclear.
Methods: UVA-induced chemiluminescence measurements were conducted: (a) in vitro on collagen solutions and solid collagen sheet preparations, (b) ex vivo on human and mouse skin biopsies, and (c) in vivo on human skin of various constitutive pigmentation levels. Fluorescence was measured on collagen in vitro as well as on skin for the in vivo experiments.
Results: We found in the in vitro experiments that UVA-induced chemiluminescence increases with the presence of collagen cross-links. When dermal sides were exposed to UVA irradiation, both mouse and human skin biopsies demonstrated significantly higher chemiluminescence levels than when epidermal sides were exposed to UVA. The amount of collagen cross-links decreases slightly following UVA exposure, as shown both by in vivo fluorescence and by UVA-induced chemiluminescence. Finally, there was less measurable UVA-induced chemiluminescence in dark skin compared with light pigmented skin in vivo .
Conclusions: The dermis is very sensitive to UVA photons. Dermal cross-links are potential UVA sensitizers. The oxidative stress induced by UVA and measured by chemiluminescence may largely be attributed to the breakdown of dermal collagen cross-links.     

Individual photosensitivity of human skin and UVA-induced pyrimidine dimers in DNA

Delineation of the DNA-damaging properties of UVA radiation is a major issue in understanding solar carcinogenesis. Emphasis was placed in this study on the formation of cyclobutane pyrimidine dimers (CPDs), which are now well established as the most frequent UVA-induced DNA lesions in human skin. The yield of CPDs was determined by a chromatographic assay following ex vivo UVA and UVB irradiation of biopsies taken from either phototype II or IV volunteers. A clear correlation was found between the frequency of UVB-induced CPDs and both the phototype and the minimum erythemal dose (MED). Similar results were obtained for the induction of CPDs upon exposure to UVA. Moreover, an excellent correlation was observed for each donor between the yield of DNA damage induced by either UVB or UVA. These observations show that the key parameters driving UVA-induced formation of CPDs are attenuation of radiation in the skin and the number of photons reaching skin cells rather than the cellular content in photosensitizers. In addition, the results show that both MED and phototype are good predictors of the vulnerability of DNA toward UVB and UVA in the skin. This result is of importance for the identification of individuals to be extensively protected.     

Ultraviolet a radiation-induced immediate iron release is a key modulator of the activation of NF-kappaB in human skin fibroblasts

Ultraviolet A (UVA, 320-400 nm) radiation, an oxidizing component of sunlight, leads to an immediate increase in the labile iron in human skin fibroblasts. Exposure of skin fibroblasts to UVA radiation is also known to induce nuclear factor-kappaB (NF-kappaB) DNA-binding activity, although the underlying mechanism is unclear. We report here that in skin fibroblasts, the extent of NF-kappaB activation by UVA tightly correlates with the level of "UVA-induced" labile iron release as shown by both iron chelation and iron loading treatments. Furthermore, our data indicate that the slow kinetics of induction of NF-kappaB by UVA relative to other oxidants previously studied is due to a transient increase in permeability of nuclear membrane to proteins and occurs as a result of labile iron-mediated damage to nuclear membrane. Since in addition to iron chelators, lipid peroxidation inhibitors also decrease the UVA-mediated induction of NF-kappaB, we propose that the rapid release of labile iron by UVA might act as a catalyst to exacerbate the generation of lipid secondary messengers in skin cell membranes that are responsible for induction of NF-kappaB. This novel role for iron in amplifying NF-kappaB mobilization in response to UVA-induced oxidative stress aids understanding of its involvement in UV-induced skin inflammation.     

S100A8 induction in keratinocytes by ultraviolet A irradiation is dependent on reactive oxygen intermediates

Cutaneous exposure to ultraviolet (UV) A (320-400 nm) results in the formation of damaging reactive oxygen intermediates, which are implicated as mediators of DNA damage, apoptosis, and photoaging. S100A8 is a low-molecular-weight calcium-binding protein, highly sensitive to oxidation. In this study, UVA-induced S100A8 expression by keratinocytes was investigated. UVA (50-100 kJ per m2) strongly induced S100A8 in differentiated keratinocytes in the epidermis of BALB/c mice. Similarly, S100A8 mRNA and monomeric protein were significantly upregulated in PAM212 cells (a murine keratinocyte cell line) in response to 10 kJ per m2 UVA 24 h after irradiation. Although S100A9 associates with S100A8 in neutrophils and abnormally differentiated keratinocytes (human psoriasis), in this study it was not coinduced with keratinocyte S100A8. Dorsal application of 4-hydroxy-tempo (a superoxide dismutase-mimicking agent) to mice concentration-dependently reduced UVA-induced S100A8 expression. Incubation of PAM212 cells with superoxide dismutase and catalase during UVA irradiation also abrogated S100A8 induction. These results suggest that UVA-induced S100A8 is expressed by keratinocytes in response to generation of reactive oxygen intermediates.     

Effect of ultraviolet A on ornithine decarboxylase and metallothionein gene expression in mouse skin

BACKGROUND: Ultraviolet A (UVA) is known to induce the expression of many stress responsive genes due to the generation of reactive oxygen species (ROS). However, UVA's role in inducing metallothionein (MT) gene expression has not been studied. Furthermore, our group demonstrated that UVA enhanced 12-O-tetradecanoylphorbol-13-acetate (TPA)-mediated induction of ornithine decarboxylase (ODC) activity in mouse skin (1). METHODS: We examined the interaction of UVA, TPA and antioxidants on the induction of MT and ODC mRNA in mouse skin. Female CD-1 mice were exposed to UVA (19 J/cm2) and total RNA was isolated from the skin. Northern blot analysis for MT and ODC mRNAs was performed. ODC activity in mouse epidermis was also determined in some experiments. RESULTS: UVA induced MT mRNA in mouse skin; however, it did not increase ODC mRNA. 1,4-Diazabicylo-[2,2,2]-octane (DABCO), a singlet oxygen scavenger, reduced UVA-mediated induction of MT mRNA by 40%. The data suggest that ROS produced by UVA exposure may contribute to its ability to induce MT mRNA. UVA slightly enhanced TPA-mediated ODC mRNA induction, while it enhanced ODC enzyme activity 70%. UVA additively intensified TPA-mediated MT mRNA induction. alpha-Tocopherol pretreatment inhibited the induction of ODC enzyme activity by TPA treatment combined with UVA exposure (TPA + UVA); however, alpha-tocopherol had less of an inhibitory effect on ODC mRNA induction by TPA + UVA. Curcumin, a plant pigment, dramatically inhibited both TPA- and TPA + UVA-induced expression of ODC and MT genes. CONCLUSIONS: These results demonstrate that UVA can induce MT gene expression and enhance TPA-induced ODC and MT gene expression. The data further suggest that these effects are partially mediated by ROS.     

Modulation of IL-10, IL-12, and IFN-gamma in the epidermis of hairless mice by UVA (320-400 nm) and UVB (280-320 nm) radiation

We have observed recently that the suppression of contact hypersensitivity (CHS) induced in mice by UVB irradiation may be prevented by suberythemal exposure to UVA radiation. Because the UVB-immunosuppressed state is associated with an upregulation of the Th2-associated cytokines IL-10 and IL-4, and a deficiency in Th1-associated IL-2, IL-12, and IFN-gamma, and because UVA photoimmunoprotection appeared to be IFN-gamma- dependent, we tested the hypothesis that UVA immunoprotection results from an ability to prevent the UVB-induced cytokine disarray. This study describes changes in epidermal IL-10, IL-12 and IFN-gamma for 5 d following irradiation of hairless mice with the CHS-modulating doses of UVB, UVA, or UVA + UVB, using immuno-histochemical detection in paraffin embedded skin sections, followed by image analysis quantitation. We found that UVB, but not UVA exposure, caused an increase in epidermal IL-10 expression, peaking at 3 d. UVA irradiation, but not UVB, resulted in increased epidermal IL-12 expression, peaking at 3 d, and increased epidermal IFN-gamma expression peaking earlier at 1 d. Irradiation with UVA + UVB abrogated the UVB-enhanced expression of IL-10, and caused small but significant increases in IL-12 and IFN-gamma at 3 d and 1 d, respectively. These findings suggest that UVA photoimmunoprotection is mediated via prevention of IL-10 release, and thus the maintenance of the Th1/Th2 balance, probably by upregulation of IL-12 and IFN-gamma, which are known to antagonize IL-10 in numerous models. The time course suggests that IFN-gamma responds initially to UVA radiation, and may stimulate the increased expression of IL-12.     

Protective role of AQP3 in UVA-induced NHSFs apoptosis via Bcl2 up-regulation

Aquaporin-3 (AQP3), a water/glycerol-transporting protein that facilitates water, urea, and glycerol transport, can inhibit arsenite-induced apoptosis by up-regulating Bcl-2. However, whether it has a protective role in ultraviolet A (UVA)-induced apoptosis in normal human skin fibroblasts is not known. In this study, we demonstrate that mild UVA treatment fails to induce oxidative cell stress and apoptosis in normal human skin fibroblasts (NHSFs) overexpressing AQP3. After severe UVA irradiation, there was an increase in oxidative cell stress and apoptosis when AQP3 levels decreased. We also found that silencing AQP3 sensitized NHSFs to low-dose UVA. Overexpressing AQP3 was protective against high-dose UVA-induced oxidative stress and apoptosis. Besides, we observed that Bcl-2 may be involved in UVA-induced apoptosis. Our findings suggested that the water/glycerol-transporting protein AQP3 plays a role in resistance to UVA-induced apoptosis.     

Influence of UVA and UVB irradiation on hepatic and cutaneous P450 isoenzymes

The influence of UVA and UVB irradiation of the skin for 1, 2 and 4 weeks on the activities of the hepatic and cutaneous P450 isoenzymes was investigated in female Wistar rats before and after systemic administration of hexachlorobenzene (HCB), a well-known porphyrogenic agent, which additionally induces P450 1A1 and P450 1A2 isoenzymes. UVA and UVB irradiation of the skin of the controls and HCB-treated animals did not influence porphyrin metabolism. In the nonporphyric rats hepatic EROD (P450 1A1) activity was induced by UVB, but the activity of ADM (P450 2B) and EMDM (P450 3A) was either minimally or not affected. In the HCB-treated (porphyric) rats UVA and UVB irradiation resulted in a significant depression of HCB-induced EROD in the liver and in the skin. In both the nonporphyric and the porphyric rats UVA and UVB irradiation had no effect on hepatic ADM activity. In the liver of the nonporphyric animals EMDM activity remained unchanged after UVA and UVB irradiation, whereas in the HCB-treated animals the activity of this enzyme was increased. Finally, after UVA and UVB irradiation cutaneous EMDM activity was increased in the controls, whereas the HCB-induced increase of this enzyme in porphyric animals was decreased. In addition long-term (28 days) UVB irradiation decreased hepatic GSH content significantly in normal and porphyric rats. These experimental findings cannot be directly extrapolated to humans; however, they suggest that exposure of human skin to UV radiation may result in alterations in the activity of cutaneous, hepatic and other extracutaneous P450 isoenzymes. Received: 27 June 1996     

Nox1-based NADPH oxidase is the major source of UVA-induced reactive oxygen species in human keratinocytes

UVA radiation is a major environmental stress on skin, causing acute and chronic photodamage. These responses are mediated by reactive oxygen species (ROS), although the cellular source of these ROS is unknown. We tested the hypotheses that UVA-induced activation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase is required for ROS generation in human keratinocytes (HK) and that these ROS initiate rapid prostaglandin E2 (PGE2) synthesis. Treatment of HK with a non-toxic dose of UVA rapidly increased NADPH oxidase activity and intracellular ROS, which were partially blocked by an inhibitor of NADPH oxidase and by a mitochondria-selective antioxidant. Depleting the Nox1 isoform of the catalytic subunit of NADPH oxidase using small interfering RNA (siRNA) blocked the UVA-induced ROS increase, indicating that ROS produced by mitochondria or other sources are downstream from Nox1. Nox1 siRNA also blocked UVA-initiated PGE2 synthesis. The mechanism for activation of Nox1 is mediated by an increase in intracellular calcium. Ceramide, which has been proposed to mediate responses to UVA in HK, also activated NADPH oxidase. These results indicate that UVA activates Nox1-based NADPH oxidase to produce ROS that stimulate PGE2 synthesis, and that Nox1 may be an appropriate target for agents designed to block UVA-induced skin injury.     

Photoprotection of bacterial-derived melanin against ultraviolet A–induced cell death and its potential application as an active sunscreen

Background  The increase in the incidence of non-melanoma skin tumours, photoaging, and immunosuppression demand for more effective sunscreen on ultraviolet A (UVA) irradiation.
Objectives  The aim of the study is to evaluate the photoprotective effects of a bacterial-derived melanin against UVA-induced damages in vitro and in vivo .
Methods  Human fibroblasts were used to assess the role of the bacterial-derived melanin on cell viability against UVA. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay and nuclear morphology were employed to evaluate the photoprotection at the cellular level. Fluorometric assays were performed to detect the formation of reactive oxygen species (ROS) in the cells. Evaluations of the bacterial-derived melanin as a sunscreen were measured by transmission test and persistent pigment darkening on human skin.
Results  Bacterial-derived melanin efficiently scavenged ROS in the fibroblasts after UVA irradiation. The cell viability of xeroderma pigmentosum (XP) fibroblast treated with varied doses of melanin increased dramatically in comparison with untreated control and the treated XP fibroblasts became more resistant to UVA-induced apoptosis than normal fibroblasts. Although the relative transmission didn't change too much with different concentration of bacterial-derived melanin, this melanin could keep UVA-irradiated skin from pigment darkening and act as an active sunscreen on skin.
Conclusions  The bacterial-derived melanin provided significant protection to fibroblast cell and human skin against the UVA radiation. It has the potential to be developed as an active sunscreen for the patients with photosensitivity skin to sun exposure.     

Chronic UVA (365-nm) irradiation induced scratching in hairless mice: dose-time dependency and the effect of ketanserin

Abstract In a study on the dose–response relationship for longwave UVA (UVAI; 340–400 nm) carcinogenesis in hairless mice scratch marks appeared after months of daily exposure as an unwanted side effect. Tumor induction in the highest of the 4 tested dose groups (receiving a daily dose of 430 kJ/m² of 365-nm radiation) could not be determined because extensive scarification occurred prior to the development of any tumors. The induction of scratch marks could be scored and quantified in all 4 dose groups tested. The UVA I dose-dependencies for the induction of tumors and scratch marks were compared. We found that the induction of scratch marks depended mainly on the cumulative UVA1 exposure, whereas tumor induction showed a lesser dose-dependency. An attempt was made to prevent the apparent pruritogenic effect of UVA1 irradiation and to understand its mechanism. The influence of ketanserin, a serotonin/histamine antagonist, on the UVA1 induction of scratch marks was tested in groups of 8 mice daily irradiated with 430 kJ/m². No difference was found between treated and untreated animals. Histological examination of skin biopsies from irradiated mice from the 430-kJ/m² dose group from the UVA1 careinogenic experiment, showed no changes in numbers of mast cells or other inflammatory features when compared to skin biopsies from unirradiated control mice. This indicated that UVA1-induced scratching is not mediated through mast cell release of scrotonin and/or histamine. An adequate therapeutic treatment which can prevent UVA1-induccd scratching would enable us to lest tumor induction with UVA1 over a larger close range, and may provide additional insight in how this radiation damages the skin. It remains conjectural whether there exists an analogous UVA-induccd pruritus in human skin.     

A chemiluminescence study of UVA-induced oxidative stress in human skin in vivo

Oxidative stress is defined as an imbalance between pro-oxidants and antioxidants in favor of pro-oxidants. Photon emission (also called chemiluminescence) has been widely used to study oxidative stress in biological systems in vitro. In vivo chemiluminescence has been proposed as a non-invasive method to assess oxidative stress in the skin. UVA (320-400 nm part of the ultraviolet radiation) exposure is generally accepted as a source of oxidative stress in the skin. In this study, UVA-induced oxidative stress was studied by using an in vivo chemiluminescence detection method. First, the dose response and the fluence rate response of the UVA-induced oxidative stress in human skin were investigated by examining the decay kinetics of the chemiluminescence signal following UVA exposure. A kinetic model was proposed to help differentiate these two responses. We found that the initial burst of the chemiluminescence signal depended on the UVA fluence rate, whereas the decay of the signal following exposure can be related to the UVA dose involved. Second, a significant reduction of UVA-induced chemiluminescence signal was observed after tape-stripping, indicating that stratum corneum is a major source of UVA-induced oxidative stress in the skin. Furthermore, the oxygen dependence of UVA-induced chemiluminescence signal was also confirmed by application of a pressure cuff, implying that some of the oxidative stress occurs in the deeper layers of the skin. Finally, topical application of vitamin C before exposure significantly reduced the UVA-induced chemiluminescence signal. We thus conclude that chemiluminescence is an effective method to assess the oxidative stress induced by UVA in human skin in vivo.     

Interleukin-1beta and macrophage migration inhibitory factor (MIF) in dermal fibroblasts mediate UVA-induced matrix metalloproteinase-1 expression

BACKGROUND: Exposure to solar UV radiation is the main environmental factor that causes premature aging of the skin. Matrix metalloproteinases (MMP)-1 is a member of the MMP family and degrades types I and III collagens, which are the major structural components of the dermis. OBJECTIVE: We evaluated the involvement IL-1beta and macrophage migration inhibitory factor (MIF) in MMP-1 expression under ultraviolet A (UVA) irradiation. METHODS: IL-1beta and MIF in MMP-1 expression in cultured human dermal fibroblasts and the UVA effects on MMPs production using IL-1alpha/beta-deficient mice were analyzed. Furthermore, fibroblasts derived from MIF-deficient mice were used to analyze the effect of IL-1beta-induced MMPs production. RESULTS: IL-1beta-enhanced MIF expression and induced MMP-1 in cultured human dermal fibroblasts. IL-1beta-induced MMP-1 expression is inhibited by neutralizing anti-MIF antibody. Dermal fibroblasts of IL-1alpha/beta-deficient mice produced significantly decreased levels of MMPs compared to wild-type mice after UVA irradiation. Furthermore, fibroblasts of MIF-deficient mice were much less sensitive to IL-1beta-induced MMPs production. On the contrary, IL-1beta produced significantly decreased levels of MMPs in MIF-deficient mice fibroblasts. The up-regulation of MMP-1 mRNA by IL-1beta stimulation was found to be inhibited by a p38 inhibitor and a JNK inhibitor. In contrast, the MEK inhibitor and inhibitor were found to have little effect on expression of MMP-1 mRNA. CONCLUSIONS: IL-1beta is involved in the up-regulation of UVA-induced MMP-1 in dermal fibroblasts, and IL-1beta and MIF cytokine network induce MMP-1 and contribute to the loss of interstitial collagen in skin photoaging.     

UVA-induced immune suppression in human skin: protective effect of vitamin E in human epidermal cells in vitro

Summary UVA (320–400nm) radiation damage to membranes, proteins, DNA and other cellular targets is predominantly related to oxidative processes. In the present study, we demonstrated that cutaneous UVA-induced immunosuppression can be related, at least in part, to the appearance of these oxidative processes. The UVA-induced oxidative processes in freshly isolated epidermal cells were monitored by measuring the thiobarbituric acid reactive substances (TBARS) as an index of peroxidation. The in vitro immunosuppressive effects of UVA were demonstrated by measuring the allogeneic lymphocyte proliferation induced by epidermal cells or purified Langerhans cells in the mixed epidermal cell–lymphocyte reaction (MECLR). In addition, the effects of a potent antioxidant (vitamin E) on these two UVA-induced processes were analysed. Our results showed that the antigen-presenting function of Langerhans cells measured in the MECLR is dose-dependently decreased by UVA radiation (up to 20 J/cm²). Overnight incubation of epidermal cells with vitamin E (400 μmol/1) before irradiation partially protected epidermal cells from the immunosuppressive effects of UVA radiation, and decreased TBARS release into the supernatant (a decrease of 35% compared with a control without vitamin E). Our results suggest that UVA radiation may alter cellpresenting antigen function partly via the generation of reactive oxygen species which trigger peroxidative processes, and these data contribute to the understanding of the role of oxidative mechanisms in immune suppression induced by UVA radiation. Our in vitro model can be used to quantify UV-mediated epidermal cell damage and the degree of immune photoprotection provided by various agents.     

Modulatory effects of an algal extract containing astaxanthin on UVA-irradiated cells in culture

UV radiation from sunlight is the most potent environmental risk factor in skin cancer pathogenesis. In the present study the ability of an algal extract to protect against UVA-induced DNA alterations was examined in human skin fibroblasts (1BR-3), human melanocytes (HEMAc) and human intestinal CaCo-2 cells. The protective effects of the proprietary algal extract, which contained a high level of the carotenoid astaxanthin, were compared with synthetic astaxanthin. DNA damage was assessed using the single cell gel electrophoresis or comet assay. In 1BR-3 cells, synthetic astaxanthin prevented UVA-induced DNA damage at all concentrations (10 nM, 100 nM, 10 microM) tested. In addition, the synthetic carotenoid also prevented DNA damage in both the HEMAc and CaCo-2 cells. The algal extract displayed protection against UVA-induced DNA damage when the equivalent of 10 microM astaxanthin was added to all three-cell types, however, at the lower concentrations (10 and 100 nM) no significant protection was evident. There was a 4.6-fold increase in astaxanthin content of CaCo-2 cells exposed to the synthetic compound and a 2.5-fold increase in cells exposed to algal extract. In 1BR-3 cells, exposure to UVA for 2 h resulted in a significant induction of cellular superoxide dismutase (SOD) activity, coupled with a marked decrease in cellular glutathione (GSH) content. However pre-incubation (18 h) with 10 microM of the either the synthetic astaxanthin or the algal extract prevented UVA-induced alterations in SOD activity and GSH content. Similarly, in CaCo-2 cells a significant depletion of GSH was observed following UVA-irradiation which was prevented by simultaneously incubating with 10 microM of either synthetic astaxanthin or the algal extract. SOD activity was unchanged following UVA exposure in the intestinal cell line. This work suggests a role for the algal extract as a potentially beneficial antioxidant.     

Ultraviolet A radiation: its role in immunosuppression and carcinogenesis

Ultraviolet A (UVA) radiation is immunosuppressive and mutagenic in humans and carcinogenic in animals. UVA suppresses immunity with a bell-shaped dose response. At doses equivalent to 15-20 minutes of sun exposure at noon, UVA contributes to approximately 75% of sunlight-induced immunosuppression. A recent action spectrum, indicating that 360-380 nm but not 320-350 nm UVA suppresses immunity in humans, suggests an important role for reactive oxygen species. UVA also causes an energy crisis in cells, and normalization of adenosine triphosphate with nicotinamide prevents UVA immunosuppression. UVA activation of the alternative complement pathway and defects in memory T-cell development are also involved. Human skin cancers contain mutations in the p53 and BRM genes that are consistent with being induced by UVA. UVA is also mutagenic in human skin equivalents. The basal layer of human skin is more susceptible to UVA-induced mutations than the upper layers. Because skin cancers arise from these basal proliferating cells, this finding is likely to be important and could be attributable to low levels of the DNA repair enzyme OGG1 in basal cells. UVA is therefore likely to make a larger contribution to UVA-induced skin carcinogenesis in humans than is predicted by small animal models as the result of being immunosuppressive and mutagenic for basal keratinocytes.     

Photocarcinogenesis by near-ultraviolet (UVA) radiation in Sencar mice

The carcinogenic effect of long-term exposure to UVA (315-400 nm) radiation was examined in Cr:ORL Sencar mice. Daily exposure to FR40T12 PUVA bulbs, filtered with Mylar to eliminate wavelengths below 315 nm, induced skin tumors with 50% probability of tumor development (T50) occurring after 68 weeks of irradiation. Tumors developed primarily on the dorsa of mice and included squamous cell carcinomas, poorly differentiated (spindle cell) tumors, and benign squamous papillomas. By comparison, thrice-weekly exposure of Sencar mice to unfiltered FS40 sunlamps containing both UVB (280-315 nm) and UVA radiation, induced skin tumors with T50 occurring after 23 weeks of irradiation. Tumors developed primarily on the ears and included squamous cell carcinomas and spindle-cell tumors. A small number of spontaneous mammary adenocarcinomas occurred in mice (both irradiated and unirradiated controls) that were older than 50 weeks. This study demonstrates that UVA radiation, which is the major UV waveband in solar radiation, is carcinogenic in a haired mouse strain, although far less carcinogenic than combined UVB/UVA radiation.     

Evaluating the UVA protection of sunscreens

Because the protection factor of sunscreens concerns only UVB protection, usually nothing is known about the protection offered in the UVA range. Using different methods, we compared six commercially available sunscreens to determine the UVA protection factor and, thus, to select the most appropriate sunscreen. Two clinical methods on human skin (inhibition of UVA-induced tanning with the use of a high-intensity UVA source and inhibition of methoxsalen plus UVA-induced phototoxicity) were compared with a method in animals (inhibition of UVA-induced sunburn cell production in mice treated with methoxsalen) and with two in vitro techniques (solution-dilution and sandwich spectrophotometry). We conclude that all five methods used give a quantitative estimate of UVA protection, but none can be accepted as a standard because the UVA protection factor varies according to the method used and the reading time.