Sunscreen protection against ultraviolet radiation-induced pyrimidine dimers in mouse epidermal DNA

Exploring Mycosporine-like Amino Acid UV-Absorbing Natural Products for a New Generation of Environmentally Friendly Sunscreens

Human skin needs additional protection from damaging ultraviolet radiation (UVR: 280-400 nm). Harmful UVR exposure leads to DNA damage and the development of skin cancer. Available sunscreens offer chemical protection from detrimental sun radiation to a certain extent. However, many synthetic sunscreens do not provide sufficient UVR protection due to the lack of photostability of their UV-absorbing active ingredients and/or the lack of ability to prevent the formation of free radicals, inevitably leading to skin damage. In addition, synthetic sunscreens may negatively affect human skin, causing irritation, accelerating skin aging and even resulting in allergic reactions. Beyond the potential negative effect on human health, some synthetic sunscreens have been shown to have a harmful impact on the environment. Consequently, identifying photostable, biodegradable, non-toxic, and renewable natural UV filters is imperative to address human health needs and provide a sustainable environmental solution. In nature, marine, freshwater, and terrestrial organisms are protected from harmful UVR through several important photoprotective mechanisms, including the synthesis of UV-absorbing compounds such as mycosporine-like amino acids (MAAs). Beyond MAAs, several other promising, natural UV-absorbing products could be considered for the future development of natural sunscreens. This review investigates the damaging impact of UVR on human health and the necessity of using sunscreens for UV protection, specifically UV-absorbing natural products that are more environmentally friendly than synthetic UV filters. Critical challenges and limitations related to using MAAs in sunscreen formulations are also evaluated. Furthermore, we explain how the genetic diversity of MAA biosynthetic pathways may be linked to their bioactivities and assess MAAs' potential for applications in human health.

The Role of Acetyl Zingerone and Its Derivatives in Inhibiting UV-Induced, Incident, and Delayed Cyclobutane Pyrimidine Dimers

Cyclobutane pyrimidine dimers (CPDs) are ultraviolet radiation (UV)-induced carcinogenic DNA photoproducts that lead to UV signature mutations in melanoma. Previously, we discovered that, in addition to their incident formation (iCPDs), UV exposure induces melanin chemiexcitation (MeCh), where UV generates peroxynitrite (ONOO^-), which oxidizes melanin into melanin-carbonyls (MCs) in their excited triplet state. Chronic MeCh and energy transfer by MCs to DNA generates CPDs for several hours after UV exposure ends (dark CPD, dCPDs). We hypothesized that MeCh and the resulting dCPDs can be inhibited using MeCh inhibitors, and MC and ONOO^- scavengers. Here, we investigated the efficacy of Acetyl Zingerone (AZ), a plant-based phenolic alkanone, and its chemical analogs in inhibiting iCPDs and dCPDs in skin fibroblasts, keratinocytes, and isogenic pigmented and albino melanocytes. While AZ and its methoxy analog, 3-(4-Methoxy-benzyl)-Pentane-2,4-dione (MBPD) completely inhibited the dCPDs, MBPD also inhibited ~50% of iCPDs. This suggests the inhibition of ~80% of total CPDs at any time point post UV exposure by MBPD, which is markedly significant. MBPD downregulated melanin synthesis, which is indispensable for dCPD generation, but this did not occur with AZ. Meanwhile, AZ and MBPD both upregulated the expression of nucleotide excision repair (NER) pathways genes including Xpa, Xpc, and Mitf. AZ and its analogs were non-toxic to the skin cells and did not act as photosensitizers. We propose that AZ and MBPD represent "next-generation skin care additives" that are safe and effective for use not only in sunscreens but also in other specialized clinical applications owing to their extremely high efficacy in blocking both iCPDs and dCPDs.

Purification and structural characterization of a novel natural pigment: cordycepene from edible and medicinal mushroom Cordyceps militaris

In the present work, a novel cordycepic pigment was successfully isolated and identified from Cordyceps militaris, as well as named as cordycepene (C₁₄H₁₇N₁O₄), according to the long unsaturated conjugated polyene structural characteristic. Cordycepene is sensitive to light, high temperature (≥ 60 °C), and acidic condition (pH ≤ 3), but possesses high stability against metal ions, and under alkaline and neutral conditions. Cordycepene shows a comparable DPPH (1,1-diphenyl-2-picrylhydrazyl) radical-scavenging activity at higher concentration (≥ 2 mg/mL) to vitamin C. Cordycepene promotes the growth of HSF (human skin fibroblast cell) after incubation for 72 h, and has an ability to repair the UV light-treated HSF cells. In addition, cordycepene increases the antioxidant activity (SOD, superoxide dismutase; GSH-Px, glutathione peroxidase; CAT, catalase) and decreases MDA (malondialdehyde) level, indicating that cordycepene inhibits the photochemical senescence of HSF by enhancing the antioxidant defense system. The discovery of cordycepene can provide a basis for research on light incubation and the accumulation of yellow pigment (carotenoids) from C. militaris.

Mexoryl: Broad-Spectrum Ultraviolet a Photoprotection

The deleterious effects of long-wave (320–400 nm) ultraviolet (UV) radiation on human skin have been recognized for decades. Human exposure to UV radiation may induce skin pigmentation and sunburn, cutaneous connective tissue alterations (photoaging), immunosuppression, and the development of skin cancers. Public awareness campaigns on the need for photoprotection advocate the regular use of sunscreens. Consumer demand and an expanding knowledge of the adverse effects of UV exposure have fueled the continual development of novel sunscreen formulations. Two organic UV filters, terephthlylidene dicamphor sulphonic acid (Mexoryl SX, L'Oréal, Paris, France) and drometrizole trisiloxane (Mexoryl XL, L'Oréal), provide effective protection from UV irradiation and offer improved safety profiles in terms of protection from UVA radiation. This article details the photoprotective benefits of Mexoryl SX and Mexoryl XL.

Physiological aspects of UV-excitation of DNA

Solar ultraviolet (UV) radiation, mainly UV-B (280-315 nm), is one of the most potent genotoxic agents that adversely affects living organisms by altering their genomic stability. DNA through its nucleobases has absorption maxima in the UV region and is therefore the main target of the deleterious radiation. The main biological relevance of UV radiation lies in the formation of several cytotoxic and mutagenic DNA lesions such as cyclobutane pyrimidine dimers (CPDs), 6-4 photoproducts (6-4PPs), and their Dewar valence isomers (DEWs), as well as DNA strand breaks. However, to counteract these DNA lesions, organisms have developed a number of highly conserved repair mechanisms such as photoreactivation, excision repair, and mismatch repair (MMR). Photoreactivation involving the enzyme photolyase is the most frequently used repair mechanism in a number of organisms. Excision repair can be classified as base excision repair (BER) and nucleotide excision repair (NER) involving a number of glycosylases and polymerases, respectively. In addition to this, double-strand break repair, SOS response, cell-cycle checkpoints, and programmed cell death (apoptosis) are also operative in various organisms to ensure genomic stability. This review concentrates on the UV-induced DNA damage and the associated repair mechanisms as well as various damage detection methods.

Skin DNA photodamage and its biological consequences

It is well established that ultraviolet (UV) radiation from sunlight damages skin cells' DNA. Wavelengths in the UVB range are absorbed by DNA and can induce mutagenic lesions such as pyrimidine dimers. On the other hand, genotoxic effects of solar UVA are mainly mediated by the activation of endogenous photosensitizers resulting in the generation of a local oxidative stress. Exogenous chemicals, such as drugs like psoralens or fluoroquinolones, sometimes amplify UV-induced harmful effects. DNA damage can lead to mutations and genetic instability. This is one of the reasons why sunlight overexposure increases the risk of skin cancer. But DNA photolesions can also be involved in other skin-specific responses to UV radiation: erythema, immunosuppression, and melanogenesis are examples reported in the literature. The aim of this short review is to summarize the general knowledge in the field of UV-induced DNA damage. Besides the biological consequences of DNA photolesions, this article also deals with technologies used for their detection and shows how helpful such approaches can be to assess photoprotection provided by sunscreens.

p53 tumor suppressor gene: a critical molecular target for UV induction and prevention of skin cancer

The relationship between exposure to UV radiation and development of skin cancer has been well established. Several studies have shown that UVB induces unique mutations (C-->T and CC-->TT transitions) in the p53 tumor suppressor gene that are not commonly induced by other carcinogens. Our studies have demonstrated that UV-induced mouse skin cancers contain p53 mutations at a high frequency and that these mutations can be detected in UV-irradiated mouse skin well before the appearance of skin tumors. This observation suggested that it might be possible to use p53 mutations as a biologic endpoint for testing the efficacy of sunscreens in photoprotection studies. Indeed, application of SPF 15 sunscreens to mouse skin before each UVB irradiation resulted in reduction in the number of p53 mutations. Because p53 mutations represent an early essential step in photocarcinogenesis, these results imply that inhibition of this event may protect against skin cancer development. This hypothesis was confirmed by our finding that sunscreens used in p53 mutation inhibition experiments also protected mice against UVB-induced skin cancer.

Photoprotection

Sun exposure is the main cause of photocarcinogenesis, photoageing, and photosensitivity; thus, photoprotection is an important issue. In a skin cancer prevention strategy, behavioural measures--eg, wearing sun protective clothes and a hat and reducing sun exposure to a minimum--should be preferred to sunscreens. Often this solution is deemed to be unacceptable in our global, outdoor society, and sunscreens could become the predominant mode of sun protection for various societal reasons (eg, healthiness of a tan, relaxation in the sun). The application of a liberal quantity of sunscreen has been shown to be by far the most important factor for effectiveness of the sunscreen, followed by the uniformity of application and the specific absorption spectrum of the agent used. The sunscreen market--crowded by numerous products--shows various differences worldwide. Nevertheless, sunscreens should not be abused in an attempt to increase time in the sun to a maximum. Controversies about safety of sunscreens and clinical recommendations are discussed.

Protective effect of the isoflavone equol against DNA damage induced by ultraviolet radiation to hairless mouse skin

Equol, an isoflavonoid metabolite produced from the dietary isoflavone daidzein by the gut microflora in mammals, has been found to protect not only against ultraviolet (UV) radiation-induced cutaneous inflammation and photoimmune suppression, but also have anti-photocarcinogenic properties in mice. Because the state of DNA damage has been correlated with suppression of the immune system and photocarcinogenesis, we have therefore examined the potential of equol to offer protection from solar-simulated UV (SSUV) radiation-induced DNA damage in hairless mice by the immunohistochemical approach using monoclonal antibody specific for cyclobutane pyrimidine dimers (CPDs; H3 antibody). Topical application of 20 µM equol lotion, which was applied both before and after SSUV significantly reduced the number of CPDs. This reduction was evident immediately after SSUV exposure, at 1 h after exposure, and at 24 h after exposure, revealing 54%, 50%, and 26% reduction in CPDs, respectively. When the same concentration was applied for 5 consecutive days after SSUV exposure, there was no significant difference in the reduction of CPDs immediately after SSUV irradiation or at 1 hour afterwards, but there were significant reductions of 23% and 42% at 24 and 48 h after SSUV exposure, respectively. Despite apparently reducing the number of CPDs post-SSUV, topically applied equol did not appear to increase the rate of dimer removal. To conclude, equol applied topically prior to SSUV irradiation offers protection against CPD formation in hairless mice, possibly by acting as a suncreen and thus inhibiting DNA photodamage.

Sunscreens Inadequately Protect Against Ultraviolet-A-Induced Free Radicals in Skin: Implications for Skin Aging and Melanoma?

Sunscreens are employed to mitigate the adverse effects of sunlight on skin but are primarily designed to prevent ultraviolet-B-associated burning and damage. The increasingly recognized role of ultraviolet A in aging, and possibly melanoma, highlights the need to include ultraviolet A screens; however, validation remains difficult. We have used a novel method to establish the efficacy of sunscreens, by measuring ultraviolet-A-induced free-radical production (thought to contribute towards ultraviolet-A-related aging and malignant change). Electron spin resonance spectroscopy was used to detect free radicals directly in human Caucasian skin during irradiation with levels of ultraviolet comparable to solar intensities. Using this system the protection afforded by three high factor sunscreens (sun protection factor 20+) that claim ultraviolet A protection was examined. Each sunscreen behaved similarly: at recommended application levels (> or = 2 mg per cm2) the ultraviolet-induced free radicals were reduced by only about 55%, and by about 45% at 0.5-1.5 mg per cm (0.5 mg per cm2 reported for common usage). A "free-radical protection factor" calculated on the basis of these results was only 2 at the recommended application level, which contrasts strongly with the erythema-based sun protection factors (mainly indicative of ultraviolet B protection) quoted by the manufacturers (20+). The disparity between these protection factors suggests that prolonged sunbathing (encouraged by use of these creams) would disproportionately increase exposure to ultraviolet A and consequently the risk of ultraviolet-A-related skin damage.

Non-invasive in vivo determination of UVA efficacy of sunscreens using diffuse reflectance spectroscopy

BACKGROUND

Evaluation of sunscreen efficacy is most relevant when measured on the surface it is meant to protect, namely on human skin in vivo. Application of any material to the surface of the skin alters its optical properties. Diffuse reflectance spectroscopy (DRS) is a non-invasive technique to measure changes in the optical properties of the skin decoupled from its biological responses following sunscreen application.

METHODS

This study compared measurements of UVA efficacy of oxybenzone and avobenzone at different concentrations (0-5%) using DRS, human phototest and an in vitro technique. Twenty subjects were enrolled for each product measured by DRS and 10 different subjects were enrolled for each product measured by human phototest. Six areas 5 cm x 10 cm were outlined on each subject's back. DRS measurements were performed on four subsites within each area before and 20 min after sunscreen application. UVA efficacy for each concentration of product was calculated from the measured transmission spectrum of a given product convoluted with the spectrum of a Xenon light source adequately filtered to obtain the UVA spectrum from 320 to 400 nm and the erythema action spectrum. Phototesting was performed using the same light source and persistent pigment darkening as the biological endpoint. Measurements were made with sunscreen coverage of 2 mg/cm2. In vitro measurements were performed using an Optometrics instrument.

RESULTS

All three techniques showed a linear response between calculated UVA efficacy and product concentration.

CONCLUSIONS

This study showed that DRS is a rapid and reproducible method to calculate UVA efficacy of sunscreen materials and that its results correlate closely with those obtained by human phototesting.

Mechanisms underlying UV-induced immune suppression: implications for sunscreen design

The ultraviolet (UV) radiation present in sunlight is immune-suppressive. Recently we showed that solar-simulated UV radiation (UVA + UVB; 295-400 nm), applied after immunization, suppressed immunological memory and the elicitation of delayed-type hypersensitivity to the common opportunistic pathogen, Candida albicans. Further, we found that wavelengths in the UVA region of the solar spectrum (320-400 nm), devoid of UVB, were equally effective in activating immune suppression as UVA + UVB radiation. Here we report on the mechanisms involved. No immune suppression was found in UV-irradiated mice injected with monoclonal anti-interleukin (IL)-10 antibody, or mice exposed to solar-simulated UV radiation and injected with recombinant IL-12. Antigen-specific suppressor T cells were found in the spleens of mice exposed to UVA + UVB radiation. Applying liposomes containing bacteriophage T4N5 to the skin of mice exposed to solar-simulated UVA + UVB radiation or mice exposed to UVA radiation blocked immune suppression, demonstrating an essential role for UV-induced DNA damage in the suppression of established immune reactions. These findings indicate that UV radiation activates similar immunological pathways to suppress the induction, or the elicitation, of the immune response.

Mechanisms underlying the suppression of established immune responses by ultraviolet radiation

The ultraviolet radiation present in sunlight is immune suppressive. Recently we showed that solar-simulated ultraviolet radiation (ultraviolet A + B; 295-400 nm), applied after immunization, suppressed immunologic memory and the elicitation of delayed-type hypersensitivity to the common opportunistic pathogen, Candida albicans. Further, we found that wavelengths in the ultraviolet A region of the solar spectrum (320-400 nm), devoid of ultraviolet B, were equally effective in activating immune suppression as ultraviolet A + B radiation. Here we report on the mechanisms involved. Maximal immune suppression was found when mice were exposed to solar-simulated ultraviolet radiation 7-9 d post immunization. No immune suppression was found in ultraviolet-irradiated mice injected with monoclonal anti-interleukin-10 antibody, or mice exposed to solar-simulated ultraviolet radiation and injected with recombinant interleukin-12. Suppressor lymphocytes were found in the spleens of mice exposed to ultraviolet A + B radiation. In addition, antigen-specific suppressor T cells (CD3+, CD4+, DX5+) were found in the spleens of mice exposed to ultraviolet A radiation. Applying liposomes containing bacteriophage T4N5 to the skin of mice exposed to solar-simulated ultraviolet A + B radiation, or mice exposed to ultraviolet A radiation, blocked immune suppression, demonstrating an essential role for ultraviolet-induced DNA damage in the suppression of established immune reactions. These findings indicate that overlapping immune suppressive mechanisms are activated by ultraviolet A and ultraviolet A + B radiation. Moreover, our findings demonstrate that ultraviolet radiation activates similar immunologic pathways to suppress the induction of, or the elicitation of, the immune response.

A computational study of physical and biological characterization of common UV sources and filters, and their relevance for substituting sunlight

Sunlight is the most important environmental UV source, affecting not only human health but also the whole terrestrial ecosystem. The use of artificial sources is advantageous since it is independent of geographical location and seasonal variations, however, in some photobiological/photochemical studies the choice of a specific UV source in relation to the biological end-point studied is sometimes questionable. Furthermore, it is often difficult to compare the results obtained in different laboratories due to 'slight' differences in the physical characteristics of the UV sources used. In an attempt to address these issues we calculated and compared the physical characteristics and the biological efficiency in UV-B and UV-A regions for two biological end-points (CPD and Fpg-sensitive sites formation) for frequently used UV-B, UV-A sources and solar light simulators (SLS). Our calculation shows that FS20 lamp is appropriate for studying the biological effects of UV-B radiation although differences in spectral characteristics of the associated filters may lead to at least 2-fold yields in CPD production. Furthermore, the use of a SLS with a Kodacel filter alone is inadequate for studying environmental UV effects. A metal-halide source with a Schott WG345 filter is appropriate for studies on biological effects due to UV-A region. Relative exposure duration was calculated to achieve equal amount of CPD or Fpg-sensitive sites, provided equal, total UV-(A+B) irradiance for the different UV sources.

Benefit and risk of organic ultraviolet filters

Modern sunscreen products provide broad-spectrum UV protection and may contain one or several UV filters. A modern UV filter should be heat and photostable, water resistant, nontoxic, and easy to formulate. Identification of a substance that meets these criteria is as difficult as discovering a new drug; hundreds of new molecules are synthesized and screened before a lead candidate is identified. The most important aspect in the development of a new UV filter is its safety. In our laboratories, the safety of new ultraviolet filters is assessed by an initial in vitro screen including photostability, cytotoxicity, photocytotoxicity, genotoxicity, and photogenotoxicity tests. These tests are performed in mammalian, yeast, and bacterial cell systems. Skin penetration potential is measured in vitro using human skin or, when required by regulations, in vivo. Because modern sunscreens are selected on the basis of their retention on and in the stratum corneum and are formulated as poorly penetrating emulsions, they generally have very low to negligible penetration rates. The safety and efficacy of UV filters are regulated and approved by national and international health authorities. Safety standards in the European Union, United States, or Japan stipulate that new filters pass a stringent toxicological safety evaluation prior to approval. The safety dossier of a new UV filter resembles that of a new drug and includes acute toxicity, irritation, sensitization, phototoxicity, photosensitization, subchronic and chronic toxicity, reproductive toxicity, genotoxicity, photogenotoxicity, carcinogenicity, and, in the United States, photocarcinogenicity testing. The margin of safety of new UV filters for application to humans is estimated by comparing the potential human systemic exposure with the no-effect level from in vivo toxicity studies. Only substances with a safe toxicological profile and a margin of safety of at least 100-fold are approved for human use. Finally, prior to marketing, new UV filters undergo stringent human testing to confirm their efficacy as well as the absence of irritation, sensitization, photoirritation, and photosensitization potential in man. UV filters not only protect against acute skin injury, such as sunburn, but also against long-term and chronic skin damage, including cellular DNA damage, photoinduced immune suppression, and, by extension, skin cancer. The protection provided by modern sunscreens against UV-induced skin cancer was shown in animal photocarcinogenicity studies and confirmed by numerous in vitro, animal, and human investigations: UV filters protect the p53 tumor suppressor gene from damage and prevent UV-induced immune suppression. Recent studies suggest that sunscreens protect against precursor lesions of skin cancer, such as actinic keratoses. Additional benefits of ultraviolet filters include prevention of photodermatoses, such as polymorphic light eruption, and, possibly, photoaging. Modern sunscreens are safe for children and adults. Percutaneous penetration and irritation rates of topically applied substances in children and adults are similar. The principal protective measure is to keep children out of the sun and/or to cover them with protective clothes; however, sunscreens are a safe and effective and often the only feasible defense of children against UV radiation. In conclusion, sunscreens are safe protective devices that undergo stringent safety and efficacy evaluation.

Modern approaches to photoprotection

UV light reacts with skin to produce undesirable changes, including photoaging and skin cancer. Sunscreen strategies are useful for protection against UV-B and short-wave UV-A, but complete protection against long-wave UV-A has not been achieved. Because UV-A is especially efficient at generating reactive oxygen species, it is being recognized increasingly as an important cause of photoaging and skin cancer.

UVAI-induced edema and pyrimidine dimers in murine skin

The induction of edema and pyrimidine dimers in epidermal DNA was determined in the skin of SKH:HR1 mice exposed to graded doses of ultraviolet radiation AI (UVAI; 340-400 nm). Exposure to UVAI induced 1.6 +/- 0.08 x 10(-6) (mean +/- standard error of mean) pyrimidine dimers per 10(8) Da of DNA per J/m2. Edema in irradiated animals was determined as an increase in skinfold thickness. A dose of 1.8 x 10(6) J/m2 of UVAI that resulted in a 50% increase in skinfold thickness (SFT50%) would have induced 1.0 x 10(5) dimers per basal cell genome. A similar increase in SFT induced by full spectrum solar ultraviolet radiation (290-400 nm) would accompany the induction of 11.0 x 10(5) pyrimidine dimers per basal cell genome. These results support a hypothesis that UVAI-induced pathological changes of the skin are mediated through the formation of nondimer photoproducts.

Protection by ultraviolet A and B sunscreens against in situ dipyrimidine photolesions in human epidermis is comparable to protection against sunburn

Sunscreens prevent sunburn and may also prevent skin cancer by protecting from ultraviolet-induced DNA damage. We assessed the ability of two sunscreens, with different spectral profiles, to inhibit DNA photodamage in human epidermis in situ. One formulation contained the established ultraviolet B filter octyl methoxycinnamate, whereas the other contained terephthalylidene dicamphor sulfonic acid, a new ultraviolet A filter. Both formulations had sun protection factors of 4 when assessed with solar simulating radiation in volunteers of skin type I/II. We tested the hypothesis that sun protection factors would indicate the level of protection against DNA photodamage. Thus, we exposed sunscreen-treated sites to four times the minimal erythema dose of solar simulating radiation, whereas vehicle and control sites were exposed to one minimal erythema dose. We used monoclonal antibodies against thymine dimers and 6-4 photoproducts and image analysis to quantify DNA damage in skin sections. A dose of four times the minimal erythema dose, with either sunscreen, resulted in comparable levels of thymine dimers and 6-4 photoproducts to one minimal erythema dose +/- vehicle, providing evidence that the DNA protection factor is comparable to the sun protection factor. The lack of difference between the sunscreens indicates similar action spectra for erythema and DNA photodamage and that erythema is a clinical surrogate for DNA photodamage that may lead to skin cancer.

Evaluation of the protective effect of sunscreens on in vitro reconstructed human skin exposed to UVB or UVA irradiation

We have previously shown that skin reconstructed in vitro is a useful model to study the effects of UVB and UVA exposure. Wavelength-specific biological damage has been identified such as the formation of sunburn cells (SBC) and pyrimidine dimers after UVB irradiation and alterations of dermal fibroblasts after UVA exposure. These specific effects were selected to evaluate the protection afforded by two sunscreens after topical application on the skin surface. Simplified formulations having different absorption spectra but similar sun protection factors were used. One contained a classical UVB absorber, 2-ethylhexyl-p-methoxycinnamate. The other contained a broad-spectrum absorber called Mexoryl SX, characterized by its strong absorbing potency in the UVA range. Both filters were used at 5% in a simple water/oil vehicle. The evaluation of photoprotection on in vitro reconstructed skin revealed good efficiency for both preparations in preventing UVB-induced damage, as shown by SBC counting and pyrimidine dimer immunostaining. By contrast, only the Mexoryl SX-containing preparation was able to efficiently prevent UVA-specific damage such as dermal fibroblast disappearance. Our data further support the fact that skin reconstructed in vitro is a reliable system to evaluate the photoprotection provided by different sunscreens against specific UVB and UVA biological damage.

Inhibition of solar simulator-induced p53 mutations and protection against skin cancer development in mice by sunscreens

We demonstrated previously that p53 mutations can be detected in ultraviolet B-irradiated mouse skin months before the gross appearance of skin tumors and that applying sun protection factor 15 sunscreens to mouse skin before each Kodacel-filtered FS40 sunlamp irradiation resulted in the reduction of such mutations. To determine whether there is an association between reduction of ultraviolet-induced p53 mutations by sunscreens and protection against skin cancer using an environmentally relevant light source, we applied sunscreens (sun protection factors 15-22) on to the shaved dorsal skin of C3H mice 30 min before each exposure to 4.54 kJ ultraviolet B (290-400 nm) radiation per m2 from a solar simulator. Control mice were treated 5 d per wk with ultraviolet only or vehicle plus ultraviolet. p53 mutation analysis indicated that mice exposed to ultraviolet only or vehicle plus ultraviolet for 16 wk (cumulative exposure to 359 kJ ultraviolet B per m2) developed p53 mutations at a frequency of 56%-69%, respectively, but less than 5% of mice treated with sunscreens plus ultraviolet showed evidence of p53 mutations. More importantly, 100% of mice that received a cumulative dose of 1000 kJ ultraviolet B per m2 only, or vehicle plus ultraviolet B developed skin tumors, whereas, the probability of tumor development in all the mice treated with the sunscreens plus 1000 kJ ultraviolet B per m2 was 2% and mice treated with sunscreens plus 1500 kJ ultraviolet B per m2 was 15%. These results demonstrate that the sunscreens used in this study not only protect mice against ultraviolet-induced p53 mutations, but also against skin cancers induced with solar-simulated ultraviolet. Because of this association, we conclude that inhibition of p53 mutations is a useful early biologic endpoint of photoprotection against an important initiating event in ultraviolet carcinogenesis.

The similarity of action spectra for thymine dimers in human epidermis and erythema suggests that DNA is the chromophore for erythema

The location of DNA photodamage within the epidermis is crucial as basal layer cells are the most likely to have carcinogenic potential. We have determined the action spectra for DNA photodamage in different human epidermal layers in situ. Previously unexposed buttock skin was irradiated with 0.5, 1, 2, and 3 minimal erythema doses of monochromatic UVR at 280, 290, 300, 310, 320, 340, and 360 nm. Punch biopsies were taken immediately after exposure and paraffin sections were prepared for immunoperoxidase staining with a monoclonal antibody against thymine dimers that were quantitated by image analysis. Dimers were measured at two basal layer regions, the mid and the upper living epidermis. The slopes of dose-response curves were used to generate four action spectra, all of which had maxima at 300 nm. Dimer action spectra between 300 and 360 nm were independent of epidermal layer, indicating comparable epidermal transmission at these wavelengths. Furthermore, we observed 300 nm-induced dimers in dermal nuclei; however, there was a marked effect of epidermal layer between 280 and 300 nm, showing relatively poor transmission of 280 and 290 nm to the basal layer. These data indicate that solar UVB (approximately 295-320 nm) is more damaging to basal cells than predicted from transmission data obtained from human epidermis ex vivo. The epidermal dimer action spectra were compared with erythema action spectra determined from the same volunteers and ultraviolet radiation sources. Overall, these spectral comparisons suggest that DNA is a major chromophore for erythema in the 280-340 nm region.