Photocarcinogenesis--models and mechanisms

Detection and Quantification of UV-irradiation-induced DNA Damages by Liquid Chromatography-Mass Spectrometry and Immunoassay†

Solar ultraviolet (UV)-induced DNA lesions are associated with skin carcinogenesis. The detection of these DNA lesions is important to understand their genotoxicity and health effects. However, sunlight exposure-relevant DNA damage measurement is still a challenge. Here, we summarize our recent progresses on the related analytical techniques, including synthesis of dimeric lesions, the optimization of procedures for ultrahigh performance liquid chromatography-coupled mass spectrometry (UHPLC-MS/MS), and the maturation of anti-T(6-4)T photoproduct antibodies and their potential applications for immunoassay.

Photo-co-carcinogenesis of Topically Applied Retinyl Palmitate in SKH-1 Hairless Mice

Cosmetic products that contain retinyl palmitate are popular as antiaging skin treatments; however, recent studies suggest a risk for enhanced skin tumor development with topical retinyl palmitate applications and exposure to solar ultraviolet radiation (UVR). In this study, we investigated the potential of retinyl palmitate to enhance UVR-induced photo-co-carcinogenesis. Groups of 36 male and 36 female SKH-1 hairless mice were exposed to simulated solar light (SSL) and treated with the control cream or creams containing retinyl palmitate, 5 days per week for 40 weeks. Other groups of mice were exposed to SSL and received no cream treatment or received cream treatments and were exposed to ultraviolet-A or ultraviolet-B. Mice were monitored for the development of skin tumors, and the incidences and multiplicities of squamous cell neoplasia were determined by histopathology. In both the absence and presence of SSL, mice administered the control cream developed skin tumors earlier and had higher incidences and multiplicities of skin squamous cell neoplasms than mice that received no cream treatment. Compared to the control cream groups, mice exposed to SSL and administered the retinyl palmitate creams demonstrated earlier onsets of skin tumors and had increased incidences and multiplicities of squamous cell skin neoplasms.

Repeated Treatments with Ingenol Mebutate Prevents Progression of UV-Induced Photodamage in Hairless Mice

Background and Aim

Ingenol mebutate (IngMeb) is an effective treatment for actinic keratosis. In this study, we hypothesized that repeated treatments with IngMeb may prevent progression of UV-induced photodamage, and that concurrent application of a corticosteroid may reduce IngMeb-induced local skin responses (LSR).

Methods

Hairless mice (n = 60; 3 groups of 20 mice) were irradiated with solar simulated ultraviolet radiation (UVR) throughout the study. Five single treatments with IngMeb were given at 4-week intervals (Days 21, 49, 77, 105, and 133). Clobetasol propionate (CP) was applied once daily for 5 days prior to each IngMeb application, as well as 6 h and 1 day post treatment. One week after IngMeb treatment No. 1, 3, and 5 (Days 28, 84, and 140), biopsies from four mice in each group were collected for histological evaluation of UV-damage on a standardized UV-damage scale (0–12). LSR (0–24) were assessed once daily (Days 1–7) after each IngMeb treatment.

Results

IngMeb prevented progression of photodamage in terms of keratosis grade, epidermal hypertrophy, dysplasia, and dermal actinic damage with a lower composite UV-damage score on day 140 (UVR 10.25 vs. UVR+IngMeb 6.00, p = 0.002) compared to UVR alone. IngMeb induced LSR, including erythema, flaking, crusting, bleeding, vesiculation, and ulceration. Concurrent CP increased LSR (max LSR Tx 1–5: UVR+IngMeb+CP 3.6–5.5 vs. UVR+IngMeb 2.6–4.3) and provided better prevention of photodamage compared to IngMeb alone (Day 140: UVR+IngMeb 6.00 vs. UVR+IngMeb+CP 3.00 p < 0.001).

Conclusion

Repeated field-directed treatments with IngMeb prevent progression of cutaneous photodamage in hairless mice, while CP cannot be used to alleviate IngMeb-induced LSR. The findings suggest that IngMeb may potentially serve as a prophylactic treatment for UV-induced tumors.

Development and validation of a new transgenic hairless albino mouse as a mutational model for potential assessment of photocarcinogenicity

Short-term phototoxicity testing is useful in selecting test agents for the longer and more expensive photocarcinogenesis safety tests; however, no validated short-term tests have been proven reliable in predicting the outcome of a photocarcinogenesis safety test. A transgenic, hairless, albino (THA) mouse model was developed that carries the gpt and red/gam [Spi(-)] genes from the gpt delta mouse background and the phenotypes from the SKH-1 mouse background to use as a short-term test in lieu of photocarcinogenesis safety tests. Validation of the THA mouse model was confirmed by exposing groups of male mice to sub-erythemal doses of ultraviolet B (UVB) irradiation for three consecutive days emitted from calibrated overhead, Kodacel-filtered fluorescent lamps and measuring the mutant frequencies (MFs) in the gpt and red/gam (Spi(-)) genes and types of mutations in the gpt gene. The doses or irradiation were monitored with broad-spectrum dosimeters that were calibrated to a NIST-traceable standard and cumulative CIE-weighted doses were 20.55 and 41.0mJ/cm(2) (effective). Mice were sacrificed 14 days after the final UVB exposure and MFs in both the gpt and red/gam genes were evaluated in the epidermis. The exposure of mice to UVB induced significant ten- to twelve-fold increases in the gpt MF and three- to five-fold increases in the Spi(-) MF over their respective background MF, 26±3×10(-6) and 9±1×10(-6). The gpt mutation spectra were significantly different between that of the UVB-irradiated and that of non-irradiated mice although the mutation spectra of both groups were dominated by C→T transitions (84% and 66%). In mice exposed to UVB, the C→T transitions occurred almost exclusively at dipyrimidine sites (92%), whereas in non-irradiated control mice, the C→T transitions occurred at CpG sites (86%). These results suggest that the newly developed THA mice are a useful and reliable model for testing UVB-induced mutagenicity in skin tissue. The application of this model for short-term prediction of solar-induced skin carcinogenicity is presently under investigation.

Investigating the role of melanin in UVA/UVB- and hydrogen peroxide-induced cellular and mitochondrial ROS production and mitochondrial DNA damage in human melanoma cells

Skin cancer incidence is dramatically increasing worldwide, with exposure to ultraviolet radiation (UVR) a predominant factor. The UVA component initiates oxidative stress in human skin, although its exact role in the initiation of skin cancer, particularly malignant melanoma, remains unclear and is controversial because there is evidence for a melanin-dependent mechanism in UVA-linked melanoma studies. Nonpigmented (CHL-1, A375), moderately pigmented (FM55, SKmel23), and highly pigmented (FM94, hyperpigmented FM55) human melanoma cell lines have been used to investigate UVA-induced production of reactive oxygen species using FACS analysis, at both the cellular (dihydrorhodamine-123) and the mitochondrial (MitoSOX) level, where most cellular stress is generated. For the first time, downstream mtDNA damage (utilizing a quantitative long-PCR assay) has been investigated. Using UVA, UVB, and H(2)O(2) as cellular stressors, we have explored the dual roles of melanin as a photoprotector and photosensitizer. The presence of melanin has no influence over cellular oxidative stress generation, whereas, in contrast, melanin protects against mitochondrial superoxide generation and mtDNA damage (one-way ANOVA with post hoc Tukey's analysis, P<0.001). We show that if melanin binds directly to DNA, it acts as a direct photosensitizer of mtDNA damage during UVA irradiation (P<0.001), providing evidence for the dual roles of melanin.

Neonatal mice do not have increased sensitivity to induction of squamous cell carcinomas

BACKGROUND

Squamous cell carcinoma (SCC) is linked with the lifelong cumulative effect of ultraviolet radiation (UVR). In contrast, epidemiological data have shown that sunburn in childhood is a stronger risk factor for cutaneous malignant melanoma than continuous UVR, indicating a higher carcinogenic sensitivity early in life.

METHODS

We investigated how a high neonatal dose of UVR affects the development of SCC in mice irradiated later in life. We used simulated solar radiation (sun) and solarium radiation (solarium). Ninety-nine C3.Cg/TifBomTac-immunocompetent hairless mice received 0, 25 or 35 standard erythema doses (SED) UVR when they were 4 days old followed by 4 SED sun or 4 SED solarium three times/weekly from 9 weeks of age.

RESULTS

Tumours developed faster in mice treated with 35 SED UVR + 4 SED sun compared with 4 SED sun, but no change was observed in the cumulative dose required to achieve tumours. Tumours also developed faster in mice treated with 35 SED UVR + 4 SED solarium compared with 4 SED solarium, and a difference was also observed in the cumulative dose required to achieve tumours. If the Skin Cancer Utrecht-Philadelphia-murine spectrum was used to weigh the delivered irradiance instead of the International Commission on Illumination erythema action spectrum, tumours developed after the same accumulated dose.

CONCLUSION

In conclusion, this study does not indicate increased sensitivity to induction of SCC early in life.

The grape antioxidant resveratrol for skin disorders: promise, prospects, and challenges

Resveratrol, a phytoalexin antioxidant found in red grapes, has been shown to have both chemopreventive and therapeutic effects against many diseases and disorders, including those of the skin. Studies have shown protective effects of resveratrol against ultraviolet radiation-mediated oxidative stress and cutaneous damages including skin cancer. Because many of the skin conditions stem from ultraviolet radiation and oxidative stress, this antioxidant appears to have promise and prospects against a wide range of cutaneous disorders including skin aging and skin cancers. However, there are a few roadblocks in the way of this promising agent regarding its translation from the bench to the bedside. This review discusses the promise and prospects of resveratrol in the management of skin disorders and the associated challenges.

Ultraviolet A within sunlight induces mutations in the epidermal basal layer of engineered human skin

The ultraviolet B (UVB) waveband within sunlight is an important carcinogen; however, UVA is also likely to be involved. By ascribing mutations to being either UVB or UVA induced, we have previously shown that human skin cancers contain similar numbers of UVB- and UVA-induced mutations, and, importantly, the UVA mutations were at the base of the epidermis of the tumors. To determine whether these mutations occurred in response to UV, we exposed engineered human skin (EHS) to UVA, UVB, or a mixture that resembled sunlight, and then detected mutations by both denaturing high-performance liquid chromatography and DNA sequencing. EHS resembles human skin, modeling differential waveband penetration to the basal, dividing keratinocytes. We administered only four low doses of UV exposure. Both UVA and UVB induced p53 mutations in irradiated EHS, suggesting that sunlight doses that are achievable during normal daily activities are mutagenic. UVA- but not UVB-induced mutations predominated in the basal epidermis that contains dividing keratinocytes and are thought to give rise to skin tumors. These studies indicate that both UVA and UVB at physiological doses are mutagenic to keratinocytes in EHS.

Protein kinase Cepsilon inhibits UVR-induced expression of FADD, an adaptor protein, linked to both Fas- and TNFR1-mediated apoptosis

Protein kinase C (PKC)epsilon overexpression in FVB/N transgenic mice sensitized skin to UVR-induced development of squamous cell carcinomas and suppressed formation of sunburn cells, which are DNA-damaged keratinocytes undergoing apoptosis. Here, we elucidated the mechanisms associated with the inhibition of UVR-induced appearance of sunburn cells in PKCepsilon transgenic mice. We found that the inhibition of UVR-induced sunburn cell formation in PKCepsilon transgenic mice may be the result of the inhibition of the expression of Fas, Fas ligand, and the mammalian death adaptor protein termed Fas-associated with death domain (FADD). The adaptor protein FADD is the key component of the death-inducing signaling complex of both Fas and tumor necrosis factor receptor 1. A decreased expression of epidermal FADD was observed after a single UVR exposure. However, a complete loss of FADD expression was found after four (Monday, Wednesday, Friday, and Monday) repeated UVR exposures. FADD transmits apoptotic signals from death receptors to the downstream initiator caspase-8 and connects to the mitochondrial intrinsic apoptotic signal transduction pathway by the cleavage of Bid, a Bcl-2 family member. PKCepsilon-mediated loss of FADD expression inhibited UVR signals to the activation of both extrinsic and intrinsic apoptotic pathways.

Cyclobutane pyrimidine dimers do not fully explain the mutagenicity induced by UVA in Chinese hamster cells

UVA generates low levels of cyclobutane pyrimidine dimers (CPDs). Here we asked the question whether CPDs could fully explain the level of mutations induced by UVA. Relative mutagenicities of UVA and UVC were calculated at equal levels of CPDs in cell lines, deficient in different aspects of repair. Survival and gene mutations in the hprt locus were analyzed in a set of Chinese hamster ovary (CHO) cell lines, i.e., wild-type, Cockayne syndrome B protein-deficient (CSB), XRCC3-deficient and XRCC1-deficient adjusted to the same level of CPDs which was analyzed as strand breaks as a result of DNA cleavage by T4 endonuclease V at CPD sites. Induced mutagenicity of UVA was approximately 2 times higher than the mutagenicity of UVC in both wild-type and XRCC1-deficient cells when calculated at equal level of CPDs. Since this discrepancy could be explained by the fact that the TT-dimers, induced by UVA, might be more mutagenic than C-containing CPDs induced by UVC, we applied acetophenone, a photosensitizer previously shown to generate enhanced levels of TT-CPDs upon UVB exposure. The results suggested that the TT-CPDs were actually less mutagenic than the C-containing CPDs. We also found that the mutagenic effect of UVA was not significantly enhanced in a cell line deficient in the repair of CPDs. Altogether this suggests that neither base excision- nor nucleotide excision-repair was involved. We further challenge the possibility that the lesion responsible for the mutations induced by UVA was of a more complex nature and which possibly is repaired by homologous recombination (HR). The results indicated that UVA was more recombinogenic than UVC at equal levels of CPDs. We therefore suggest that UVA induces a complex type of lesion, which might be an obstruction during replication fork progression that requires HR repair to be further processed.

Protein kinase Cepsilon interacts with Stat3 and regulates its activation that is essential for the development of skin cancer

Protein kinase C (PKC) represents a large family of phosphatidylserine (PS)-dependent serine/threonine protein kinases. At least six PKC isoforms (alpha, delta, epsilon, eta, micro, and zeta) are expressed in epidermis. PKC is a major intracellular receptor for 12-O-tetradecanoylphorbol-13-acetate (TPA) and is also activated by a variety of stress factors including ultraviolet radiation (UVR). PKC isozymes (alpha, delta, epsilon, and eta), exhibit specificities to the development of skin cancer. PKCepsilon, a calcium-insensitive PKC isoform, is linked to the development of squamous cell carcinoma (SCC) elicited either by the 7,12-Dimethylbenzanthracene (DMBA)-TPA protocol or by repeated exposures to UVR. PKCepsilon overexpressing transgenic mice, when treated either with TPA or exposed to UVR, elicit similar responses such as inhibition of apoptosis, promotion of cell survival, and development of SCC. PKCepsilon overexpression increases Stat3 activation after either TPA treatment or UVR exposure. Both PKCepsilon and signal transducers and activators of transcription-3 (Stat3) are implicated in the development of SCC. However, the link between PKCepsilon and Stat3 remains elusive. We found that PKCepsilon interacts with Stat3. PKCepsilon interaction with Stat3 was dependent upon UVR treatment. In reciprocal immunoprecipitation/blotting experiments, Stat3 coimmunoprecipitated with PKCepsilon. Colocalization of PKCepsilon with Stat3 was confirmed by double immunofluorescence staining. PKCepsilon interaction with Stat3 was PKCepsilon isoform specific and was not observed with other protein kinases. As observed in vitro with immunocomplex kinase assay with immunopurified PKCepsilon and Stat3, PKCepsilon phosphorylated Stat3 at the serine 727 residue. PKCepsilon depletion prevented Stat3Ser727 phosphorylation, Stat3 DNA binding, and transcriptional activity. The results presented indicate that PKCepsilon mediates Stat3 activation.

Protein kinase C epsilon, which sensitizes skin to sun's UV radiation-induced cutaneous damage and development of squamous cell carcinomas, associates with Stat3

Chronic exposure to UV radiation (UVR) is the major etiologic factor in the development of human skin cancers including squamous cell carcinoma (SCC). We have shown that protein kinase C(epsilon) (PKC(epsilon)), a Ca(2+)-independent, phospholipid-dependent serine/threonine kinase, is an endogenous photosensitizer. PKC(epsilon) is among the six isoforms (alpha, delta, epsilon, eta, mu, and zeta) expressed in both mouse and human skin. PKC(epsilon) transgenic mice, which overexpress PKC(epsilon) in the basal epidermal cells and cells of the hair follicle, are highly sensitive to UVR-induced cutaneous damage and development of SCC. We now present that PKC(epsilon)-overexpressing, but not PKC(delta)-overexpressing, transgenic mice, when exposed to a single (4 kJ/m(2)) or repeated (four doses, 2 kJ/m(2)/dose, thrice weekly) UVR, emitted by Kodacel-filtered FS-40 sun lamps, elicit constitutive phosphorylation of signal transducers and activators of transcription 3 (Stat3) at both Tyr705 and Ser727 residues. UVR-induced phosphorylation of Stat3 accompanied increased expression of Stat3-regulated genes (c-myc, cyclin D1, cdc25A, and COX-2). In reciprocal immunoprecipitation/blotting experiments, phosphorylated Stat3 co-immunoprecipitated with PKC(epsilon). As observed in vivo using PKC(epsilon) knockout mice and in vitro in an immunocomplex kinase assay, PKC(epsilon) phosphorylated Stat3 at Ser727 residue. These results indicate for the first time that (a) PKC(epsilon) is a Stat3Ser727 kinase; (b) PKC(epsilon)-mediated phosphorylation of StatSer727 may be essential for transcriptional activity of Stat3; and (c) UVR-induced phosphorylation of Ser727 may be a key component of the mechanism by which PKC(epsilon) imparts sensitivity to UVR-induced development of SCC.

Protein kinase C delta overexpressing transgenic mice are resistant to chemically but not to UV radiation-induced development of squamous cell carcinomas: a possible link to specific cytokines and cyclooxygenase-2

Protein kinase C delta (PKCdelta), a Ca(2+)-independent, phospholipid-dependent serine/threonine kinase, is among the novel PKCs (delta, epsilon, and eta) expressed in mouse epidermis. We reported that FVB/N transgenic mice that overexpress ( approximately 8-fold) PKCdelta protein in basal epidermal cells and cells of the hair follicle are resistant to the development of both skin papillomas and squamous cell carcinoma (SCC) elicited by 7,12-dimethylbenz(a)anthracene initiation and 12-O-tetradecanoylphorbol-13-acetate (TPA) promotion protocol. We now present that PKCdelta overexpression in transgenic mice failed to suppress the induction of SCC developed by repeated exposures to UV radiation (UVR), the environmental carcinogen linked to the development of human SCC. Both TPA and UVR treatment of wild-type mice (a) increased the expression of proliferating cell nuclear antigen (PCNA) and apoptosis; (b) stimulated the expression of cytokines tumor necrosis factor-alpha (TNF-alpha), granulocyte macrophage colony-stimulating factor (GM-CSF), and granulocyte CSF (G-CSF); and (c) increased cyclooxygenase-2 (COX-2) expression and expression of phosphorylated Akt (p-Akt), p38, extracellular signal-regulated kinase-1 (ERK1), and ERK2. PKCdelta overexpression in transgenic mice enhanced TPA-induced but not UVR-induced apoptosis and suppressed TPA-stimulated but not UVR-stimulated levels of cell PCNA, cytokines (TNF-alpha, G-CSF, and GM-CSF), and the expression of COX-2, p-Akt, and p38. The results indicate that UVR-mediated signal transduction pathway to the induction of SCC does not seem to be sensitive to PKCdelta overexpression. The proapoptotic activity of PKCdelta coupled with its ability to suppress TPA-induced expression of proinflammatory cytokines, COX-2 expression, and the phosphorylation of Akt and p38 may play roles in the suppression of TPA-promoted development of SCC.

Review of the potential photo-cocarcinogenicity of topical calcineurin inhibitors. Position statement of the European Dermatology Forum

Topical Calcineurin Inhibitors (TCIs) used for the treatment of atopic eczema modify the immune regulatory function of the skin and may have the potential to enhance immunosuppressive ultraviolet (UV) effects. Current recommendations on UV protection in eczema patients treated with PCIs are inconsistent and have given rise to uncertainty and anxiety in patients. Therefore, the European Dermatology Forum (EDF) developed a position statement which reviews critically the available data with regard to the problem, especially analysing and commenting the limitations of rodent models for the human situation. There is no conclusive evidence from rodent trials to indicate that long-term application of TCIs is photococarcinogenic. There is a need for further studies to investigate the validity of mouse models as well as long-term cohort studies in patients using TCIs. Available data suggest that long-term application of TCIs is safe, that there is no evidence of increased skin cancer risk and that it is ethical to treat patients with TCIs when indicated.

Skin cancer prevention: a possible role of 1,25dihydroxyvitamin D3 and its analogs

We previously reported that the natural hormone 1,25dihydroxyvitamin D3 (1,25(OH)(2)D(3)) protects human skin cells from ultraviolet radiation (UVR)-induced apoptosis. UVR-induced pre-mutagenic cyclobutane pyrimidine dimers are diminished in number from 0.5h after cessation of UVR in all skin cell types, by treatment with three different Vitamin D compounds: by 1,25(OH)(2)D(3), by the rapid acting, low calcemic analog, 1alpha,25(OH)(2)lumisterol(3) (JN) and by the low calcemic but transcriptionally active hybrid analog 1alpha-hydroxymethyl-16-ene-24,24-difluoro-25-hydroxy-26,27-bis-homovitamin D3 QW-1624F2-2 (QW), which may explain the enhanced cell survival. The rapid response antagonist analog 1beta,25(OH)(2)D(3) (HL) abolished the photoprotective effects of 1,25(OH)(2)D(3) whilst a genomic antagonist, (23S)-25-dehydro-1alpha-hydroxyvitamin D(3)-26,23-lactone (TEI-9647), had no effect. UVR increased p53 expression in human skin cells, whilst concurrent treatment with 1,25(OH)(2)D(3) further enhanced this effect several fold, at 3 and 6h after UVR. Combined with previously reported lower nitrite levels with 1,25(OH)(2)D(3), this increased p53 expression may favor DNA repair over apoptosis. We now report that topical application of 1,25(OH)(2)D(3) or QW also suppressed solar simulated UV (SSUVR-induced pyrimidine dimers in the epidermis of irradiated hairless Skh:HR1 mice, measured 24h after irradiation. Furthermore, UVR-induced immunosuppression in the mice was markedly reduced by topical application of either 1,25(OH)(2)D(3) or QW. These preliminary results show, for the first time, a protective effect of Vitamin D compounds against DNA photodamage in vivo.

Protein kinase C epsilon is an endogenous photosensitizer that enhances ultraviolet radiation-induced cutaneous damage and development of squamous cell carcinomas

Chronic exposure to UV radiation (UVR), especially in the UVA (315-400 nm) and UVB (280-315 nm) spectrum of sunlight, is the major risk factor for the development of nonmelanoma skin cancer. UVR is a complete carcinogen, which both initiates and promotes carcinogenesis. We found that protein kinase C epsilon (PKCepsilon), a member of the phospholipid-dependent threonine/serine kinase family, is an endogenous photosensitizer, the overexpression of which in the epidermis increases the susceptibility of mice to UVR-induced cutaneous damage and development of squamous cell carcinoma. The PKCepsilon transgenic mouse (FVB/N) lines 224 and 215 overexpressed 8- and 18-fold PKCepsilon protein, respectively, over endogenous levels in basal epidermal cells. UVR exposure (1 kJ/m(2) three times weekly) induced irreparable skin damage in high PKCepsilon-overexpressing mouse line 215. However, the PKCepsilon transgenic mouse line 224, when exposed to UVR (2 kJ/m(2) three times weekly), exhibited minimum cutaneous damage but increased squamous cell carcinoma multiplicity by 3-fold and decreased tumor latency by 12 weeks. UVR exposure of PKCepsilon transgenic mice compared with wild-type littermates (1) elevated the levels of neither cyclobutane pyrimidine dimer nor pyrimidine (6-4) pyrimidone dimer, (2) reduced the appearance of sunburn cells, (3) induced extensive hyperplasia and increased the levels of mouse skin tumor promoter marker ornithine decarboxylase, and (4) elevated the levels of tumor necrosis factor alpha (TNFalpha) and other growth stimulatory cytokines, granulocyte colony-stimulating factor, and granulocyte macrophage colony-stimulating factor. The role of TNFalpha in UVR-induced cutaneous damage was evaluated using PKCepsilon transgenic mice deficient in TNFalpha. UVR treatment three times weekly for 13 weeks at 2 kJ/m(2) induced severe cutaneous damage in PKCepsilon transgenic mice (line 215), which was partially prevented in PKCepsilon-transgenic TNFalpha-knockout mice. Taken together, the results indicate that PKCepsilon signals UVR-induced TNFalpha release that is linked, at least in part, to the photosensitivity of PKCepsilon transgenic mice.

The basal layer in human squamous tumors harbors more UVA than UVB fingerprint mutations: a role for UVA in human skin carcinogenesis

We hypothesized that a substantial portion of the mutagenic alterations produced in the basal layer of human skin by sunlight are induced by wavelengths in the UVA range. Using laser capture microdissection we examined separately basal and suprabasal keratinocytes from human skin squamous cell carcinomas and premalignant solar keratosis for both UVA- and UVB-induced adduct formation and signature mutations. We found that UVA fingerprint mutations were detectable in human skin squamous cell carcinomas and solar keratosis, mostly in the basal germinative layer, which contrasted with a predominantly suprabasal localization of UVB fingerprint mutations in these lesions. The epidermal layer bias was confirmed by immunohistochemical analyses with a superficial localization of cyclobutane thymine dimers contrasting with the localization of 8-hydroxy-2'-deoxyguanine adducts to the basal epithelial layers. If unrepaired, these adducts may lead to fixed genomic mutations. The basal location of UVA-rather than UVB-induced DNA damage suggests that longer-wavelength UVR is an important carcinogen in the stem cell compartment of the skin. Given the traditional emphasis on UVB, these results may have profound implications for future public health initiatives for skin cancer prevention.

Comparison of the expression of p53, p21, Bax and the induction of apoptosis between patients with basal cell carcinoma and normal controls in response to ultraviolet irradiation

AIM

Ultraviolet light (UV) is known to cause DNA damage in the epidermis. The damaged DNA is repaired or deleted by apoptosis to prevent the generation of cancer. It has been suggested that a deficient apoptotic mechanism may predispose individuals to skin cancer. Therefore, the response of normal controls and patients with basal cell carcinoma (BCC) to UV irradiation was investigated.

METHODS

The buttock skin from normal volunteers and patients with BCC was irradiated using solar simulated radiation (SSR). SSR mimics the effect of natural sunlight. Skin biopsies were excised and examined for p53, p21, and Bax protein expression and for the induction of apoptosis.

RESULTS

At 33 hours after UV irradiation, the induction of apoptosis was significantly higher (p = 0.04) in patients with BCC than in normal volunteers (Mann Whitney test). A trend towards higher p21 expression was found at 33 hours in patients with BCC (mean, 18.69 positive cells/field) than in normal volunteers (mean, 9.89), although this difference was not significant (p = 0.05 positive cells/field).

CONCLUSION

These results may imply that patients with BCC have enhanced sensitivity to UV irradiation or that there is some defect in the cell arrest or repair pathways, which results in damaged cells been pushed into apoptosis rather than repair.

The expression of p53, p21, Bax and induction of apoptosis in normal volunteers in response to different doses of ultraviolet radiation

BACKGROUND

Ultraviolet radiation (UVR) damages keratinocytes. Direct DNA damage may undergo enzymatic repair followed by resumption of the normal cell cycle. Cells may also be eliminated without inflammation by the error-free process of programmed cell death or apoptosis. Necrosis of cells can occur after overwhelming damage. Failure of apoptosis leads to retention of cells with persistent mutations.

OBJECTIVES

This study investigates p53-dependent apoptotic responses in normal skin following solar-simulated radiation (SSR).

METHODS

Sun-protected buttock skin from normal volunteers with no history or clinical evidence of skin cancer was exposed to graded doses of SSR, 0.5, 1, 2 and 3 times the minimal erythema dose (MED). Biopsies taken at a range of time points (4.5, 9, 24, 33, 48 and 72 h) after UVR, quantified the time course and dose-response of apoptosis and the expression of the relevant proteins, p53, p21waf1/Cip1 and Bax, by single and double labelling techniques.

RESULTS

Apoptosis was upregulated in a dose-dependent manner as was the expression of p53, p21waf1/Cip1 and Bax in response to SSR. Following exposure to 3 MEDs it was found that: (i) the maximum number of apoptotic cells occurred at 48 h; (ii) p53 protein expression was upregulated from 4 to 72 h preceding peak p21waf1/Cip1 protein expression (9-48 h) and peak Bax protein expression (33 h).

CONCLUSIONS

These results suggest that, following SSR, normal human skin induces apoptosis by the p53, p21waf1/Cip1, Bax pathway in vivo. In addition, induction of apoptosis and expression of p53, p21waf1/Cip1 and Bax occurs in a dose-dependent manner.

Tannins protect against skin tumor promotion induced by ultraviolet-B radiation in hairless mice

We recently showed that Tarapod tannic acid (TA), a hydrolyzable tannin extracted from the pods of the Tara tree (Caesalpinia spinosa), was more effective than other tannins tested at inhibiting ultraviolet-B (UV-B)-stimulated hydrogen peroxide activity (an indirect measure of free radicals) in the skin of hairless mice. We also found that Tarapod TA inhibited UV-B-induced ornithine decarboxylase activity and UV-B-stimulated DNA synthesis, two biochemical markers linked to the skin tumor-promoting ability of this physical carcinogen. For this reason, we examined the effect of topical application, force feeding (gavage), and intraperitoneal injections of Tarapod TA on mouse skin chronically treated with UV-B light. Mice were initiated by a single topical application of 7,12-dimethylbenz[a]anthracene (50 nmol) and promoted by two weekly treatments with UV-B light (250 mJ/cm2) for 25 weeks. Topical application of Tarapod TA, 20 minutes before irradiation, resulted in a dose-dependent inhibition of tumor incidence (number of mice with tumors) and tumor yield (number of tumors/mouse), with 8 mg of TA inhibiting tumor yield by 70% at Week 25. Intraperitoneal injections of low doses (10 mg/kg mouse body wt), but not of high doses (25 mg/kg body wt), of TA afforded protection against UV-B-induced papillomas. However, the protection by intraperitoneal injection was lower than that observed by topical application: 10 mg/kg body wt of TA reduced tumor yield by 55%. The force feeding of 10 mg of Tarapod TA before irradiation failed to significantly inhibit the yield of tumors at the end of the experiment but delayed tumor appearance by six weeks. These results suggest that plant tannins administered topically or injected intraperitoneally reduce the tumor-promoting effects of UV-B radiation and thus could be useful photoprotectants.

The effect of suntan parlor exposure on delayed and contact hypersensitivity

Cutaneous and systemic immune function are believed to play an important role in cutaneous carcinogenesis. We therefore sought to determine whether the suntan parlor radiation sources commonly used in the United States cause measurable qualitative suppression of immune function and quantitative alterations in circulating T cell subpopulations. Subjects (n = 22) were recruited and randomly assigned to receive suntan parlor exposures (10 full-body UV exposures over a 2 week period, shielding only the right flexural arm) or no exposure. Baseline circulating T lymphocyte subpopulations (T helper lymphocyte, CD4; T suppressor/cytotoxic lymphocyte, CD8) were measured. Two weeks later (upon completion of UV exposures for those in this group), circulating T cell subpopulations were measured and dinitrochlorobenzene (DNCB) sensitization (in the UV group, on the UV-exposed buttock) was performed. Subsequent DNCB elicitation was performed in a bilateral fashion (in the UV group, on the right UV-shielded and the left UV-exposed upper arm). We found that subjects in the UV group demonstrated localized suppression of contact hypersensitivity sensitization and elicitation and also an increase in circulating CD8 cells when compared to the control group (P < or = 0.05). We conclude that suntan parlor exposures, as typically received in this country, suppress contact hypersensitivity and increase the circulating T suppressor/cytotoxic cell number quantity.