The protective effect of indomethacin on photocarcinogenesis in hairless mice

Keratinocyte Carcinoma and Photoprevention: The Protective Actions of Repurposed Pharmaceuticals, Phytochemicals and Vitamins

Simple Summary

Keratinocyte carcinoma is the most common type of cancer. Sun exposure and ultraviolet radiation are significant contributors to the development of carcinogenesis, mediated by DNA damage, increased oxidative stress, inflammation, immunosuppression and dysregulated signal transduction. Photoprevention involves using different compounds to delay or prevent ultraviolet radiation-induced skin cancer. In this review, we look at new avenues for systemic photoprevention that are based on pharmaceuticals, plant-derived phytochemicals and vitamins. We also investigate the mechanisms underlying these strategies for preventing the onset of carcinogenesis.

Abstract

Ultraviolet radiation (UVR) arising from sun exposure represents a major risk factor in the development of keratinocyte carcinomas (KCs). UVR exposure induces dysregulated signal transduction, oxidative stress, inflammation, immunosuppression and DNA damage, all of which promote the induction and development of photocarcinogenesis. Because the incidence of KCs is increasing, better prevention strategies are necessary. In the concept of photoprevention, protective compounds are administered either topically or systemically to prevent the effects of UVR and the development of skin cancer. In this review, we provide descriptions of the pathways underlying photocarcinogenesis and an overview of selected photoprotective compounds, such as repurposed pharmaceuticals, plant-derived phytochemicals and vitamins. We discuss the protective potential of these compounds and their effects in pre-clinical and human trials, summarising the mechanisms of action involved in preventing photocarcinogenesis.

Photoimmunology: how ultraviolet radiation affects the immune system

Ultraviolet (UV) radiation is a ubiquitous component of the environment that has important effects on a wide range of cell functions. Short-wavelength UVB radiation induces sunburn and is a potent immunomodulator, yet longer-wavelength, lower-energy UVA radiation also has effects on mammalian immunity. This Review discusses current knowledge regarding the mechanisms by which UV radiation can modify innate and adaptive immune responses and how this immunomodulatory capacity can be both beneficial in the case of inflammatory and autoimmune diseases, and detrimental in the case of skin cancer and the response to several infectious agents.

Reassessment of a Free Radical Theory of Cancer With Emphasis on Ultraviolet Carcinogenesis

Pro-oxidants, reactive species and free radicals, are toxic substances that can cause oxidative damage to major constituents of biological systems. In contradistinction, antioxidants are defined as any substance that significantly prevents the pro-oxidant-initiated oxidation of a substrate. Consequently, it was suggested that it might be possible to reduce free radical damage and thus cancer risk through 3 dietary changes: (1) caloric reduction, that is, lowering the level of free radical reactions arising in the course of normal metabolism; (2) minimize dietary components that increase the level of free radical reactions (eg, polyunsaturated fats); and (3) supplement the diet with one or more free radical reaction inhibitors (antioxidants). Lipid peroxidation exemplifies the type of chain reaction initiated by free radicals in (2) and (3). Both the phenolic antioxidant butylated hydroxytoluene (BHT) and the carotenoid β-carotene can terminate such reactions and have been shown to influence ultraviolet (UV) carcinogenesis. However, there is a lack of correlation between physicochemical and patho-physiological responses in both instances. Whereas the influence on UV carcinogenesis of both antioxidants has been reported to diminish as the level of dietary fat decreases, pointing to the involvement of lipid peroxidative reactions, the mode of BHT’s action in inhibiting UV carcinogenesis appears to be related to UV dose diminution through increased spectral absorbance of the stratum corneum. β-carotene has no such effect and may actually exacerbate UV carcinogenesis under certain dietary conditions. This paradox points to the complex relationship between chemical mechanisms and biological mode of action of antioxidants. Recent clinical and experimental data suggest that antioxidant supplementation of the complex and intricately balanced natural antioxidant defense system as a cancer prevention strategy will demand extreme caution.

Does Photosensitivity Predict Photococarcinogenicity?

Assessment of short-term and long-term effects of light (pho—totesting) is part of the safety evaluation of drugs. Results are incorporated into drug package inserts to advise patients and health care providers about the use of drug products on sun—exposed skin. We undertook an exhaustive literature search and a search of archived studies at the Center for Drug Evaluation and Research (CDER), Food and Drug Administration (FDA), in order to evaluate the potential of short—term photoassays to predict long—term effects of drugs used in sunlight (280–700 nm). The correlation between the findings from the photococarcinogenicity assays in mice that used exposure to simulated sunlight and those from photogenotoxicity and photosensitivity studies was examined. Results indicated that photosensitivity and photogenotoxicity assays did not necessarily predict effects in photococarcinogenicity studies in mice. Effects of drugs on skin that are not due to photoactivation of drug can be important factors in enhancement of UV—induced skin carcinogenesis.

The relevance of piroxicam for the prevention and treatment of nonmelanoma skin cancer and its precursors

Piroxicam (PXM), a nonsteroidal anti-inflammatory drug, is an enolic benzothiazine and a potent member of the oxicam series. The drug suppresses the synthesis of proinflammatory enzymes, such as cyclo-oxygenases-1 and -2 (COX-1 and 2), downregulates the production of prostaglandins (PGs) and tromboxanes, and inhibits polyamines production by blocking ornithine decarboxylase induction involved in nonmelanoma skin carcinogenesis. In addition, PXM is able to induce tumor cell apoptosis and suppresses metalloproteinase 2 activities. Skin carcinogenesis is a multistep process in which the accumulation of genetic events leads to a gradually dysplastic cellular expression, deregulation of cell growth, and carcinomatous progression. COX-1 upregulation plays a significant role in PG and vascular epidermal growth factor production supporting tumor growth. Increased level of PGs in premalignant and/or malignant cutaneous tumors is also favored by upregulation of COX-2 and downregulation of the tumor suppressor gene 15-hydroxy-prostaglandin dehydrogenase. Chemoprevention can be a hopeful approach to inhibit carcinoma occurrence before an invasive tumor develops. The chemopreventive effect of nonsteroidal anti-inflammatory drugs on nonmelanoma skin cancers has been established. In this study, we highlighted the different modalities of action of PXM on the pathogenesis of nonmelanoma skin cancer, analyzing and evaluating binding modes and energies between COX-1 or COX-2 and PXM by protein–ligand molecular docking. Our clinical experience about the local use of PXM on actinic keratoses and field cancerization is also reported, confirming its efficacy as target therapy.

Oral and systemic photoprotection

Photoprotection can be provided not only by ultraviolet (UV) blockers but also by oral substances. Epidemiologically identified associations between foods and skin cancer and interventional experiments have discovered mechanisms of UV skin damage. These approaches have identified oral substances that are photoprotective in humans. UV inhibits adenosine triphosphate (ATP) production causing an energy crisis, which prevents optimal skin immunity and DNA repair. Enhancing ATP production with oral nicotinamide protects from UV immunosuppression, enhances DNA repair and reduces skin cancer in humans. Reactive oxygen species also contribute to photodamage. Nontoxic substances consumed in the diet, or available as oral supplements, can protect the skin by multiple potential mechanisms. These substances include polyphenols in fruit, vegetables, wine, tea and caffeine-containing foods. UV-induced prostaglandin E2 (PGE2 ) contributes to photodamage. Nonsteroidal anti-inflammatory drugs and food substances reduce production of this lipid mediator. Fish oils are photoprotective, at least partially by reducing PGE2 . Orally consumed substances, either in the diet or as supplements, can influence cutaneous responses to UV. A current research goal is to develop an oral supplement that could be used in conjunction with other sun protective strategies in order to provide improved protection from sunlight.

Use of nonsteroidal anti-inflammatory drugs and risk of basal cell carcinoma in the United States Radiologic Technologists study

Nonsteroidal anti-inflammatory drugs (NSAIDs) have been associated with reduced risk of colorectal and other cancers, but the association with basal cell carcinoma (BCC) is unclear. Previous epidemiological studies have been small in size, conducted in especially vulnerable populations, or have not accounted for solar ultraviolet exposure, a major risk factor for BCC. In the United States Radiologic Technologists cohort, we followed subjects to assess NSAID use on risk of first incident BCC. We included Caucasian participants who responded to both second and third questionnaires (administered from 1994 to 1998 and 2003 to 2005, respectively), and who reported no cancer at the time of the second questionnaire, N = 58,213. BCC, constituent risk factors (e.g., eye color, complexion, hair color) and sun exposure history were assessed through self-administered survey. Hazard ratios (HRs) and 95% confidence intervals (CIs) were calculated using Cox proportional hazards models. Of the 58,213 people in the study population, 2,291 went on to develop BCC. Any NSAID use was not associated with subsequent incidence of BCC (HR = 1.04, 95% CI: 0.92-1.16) after adjusting for age, sex and estimated lifetime summer sun exposure. Neither association was observed when stratified by NSAID type (aspirin and other NSAIDs), nor did dose-response patterns emerge by frequency of use (average days per month). Further analyses did not reveal interaction with sex, birth cohort, smoking, alcohol consumption, sun exposure, occupational radiation exposure or personal risk factors for BCC. In this large nationwide study, we observed no association between NSAID use and subsequent BCC risk.

High death rate in mice treated topically with diclofenac

Recently, 3% diclofenacnatrium gel (diclofenac) was introduced for the treatment of actinic keratoses. Data on photocarcinogenesis of topical diclofenac are limited, and we wished to investigate whether topical diclofenac can accelerate photocarcinogenesis using simulated solar radiation (SSR). Diclofenac was applied topically on the backs of hairless, female, C3.Cg/TifBomTac immunocompetent mice three times weekly followed by ultraviolet radiation (2, 3, or 4 Standard Erythema Dose) until death. There was a significant difference in survival between diclofenac-treated groups and control groups (P<0.0001). Physical examination of the diclofenac-treated mice showed peptic ulcers, oesophageal ulcers and gastrointestinal bleeding. To be sure that these side effects were not caused by topical absorption without oral ingestion, one group of mice was wearing Elizabethan collars and was single housed. Nevertheless, these mice also had gastrointestinal side effects. We terminated the experiment after 151 days when only a few mice remained in the diclofenac-treated groups and most had symptoms of discomfort and weight loss. No tumors developed as a result of the early termination.

Association between nonsteroidal anti-inflammatory drug use and cutaneous squamous cell carcinoma

OBJECTIVE

To examine the association between nonsteroidal anti-inflammatory drug (NSAID) use and cutaneous squamous cell carcinoma (SCC).

DESIGN

Retrospective case-control study.

SETTING

Kaiser Permanente Northern California (KPNC), a large population based-health maintenance organization.

PATIENTS

Random sample of 415 KPNC members diagnosed as having a pathologically verified SCC in 2004 and 415 age-, sex-, and race-matched controls with no history of skin cancer.

MAIN EXPOSURE MEASURE

Self-reported NSAID use in the 10 years prior to baseline. Use of NSAIDs was categorized based on type (any NSAIDs, aspirin, ibuprofen, and nonaspirin NSAIDs). Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated using conditional logistic regression to estimate the association of SCC with regular use, dose, and duration of exposure to the different NSAID types. Information on pharmacy-dispensed NSAIDs was also examined to assess its association with SCC risk. Models were adjusted for all ascertained SCC risk factors (fully adjusted model) and only those variables associated with both SCC risk and NSAID use (parsimonious model).

RESULTS

Fully adjusted analyses showed no statistically significant reduction in SCC risk with self-reported regular use of any NSAID (OR, 1.32; 95% CI, 0.92-1.89), aspirin (OR, 1.38; 95% CI, 0.96-1.97), ibuprofen (OR, 0.74; 95% CI, 0.46-1.19), or nonaspirin NSAIDs (OR, 0.84; 95% CI, 0.56-1.26). Analyses examining duration, dose, and variables combining duration and dose of NSAID exposure did not appreciably change results. An analysis using the parsimonious model showed similar results. The data on pharmacy-dispensed NSAIDs also showed no association with SCC risk.

CONCLUSION

Neither self-reported nor pharmacy-dispensed NSAID exposure was associated with cutaneous SCC risk.

Polymorphisms in COX-2, NSAID use and risk of basal cell carcinoma in a prospective study of Danes

We investigated the risk of basal cell carcinoma (BCC) in relation to a number of single nucleotide polymorphisms in genes involved in the inflammatory response. A case-control study including 322 BCC cases and a similar number of controls was nested in a population-based prospective study of 57,053 individuals (aged 50-64 at inclusion) in Denmark. NSAID use was associated with a slightly decreased risk of BCC (IRR=0.85, 95% CI=0.66-1.10). We found that two polymorphisms in COX-2, COX-2 A-1195G and T8473C were associated with risk of BCC. Carriers of the variant allele of COX-2 A-1195G had lower risk of BCC than homozygous wild type carriers (IRR=0.54, 95% CI=0.47-0.89). Homozygous carriers of the variant allele of COX-2 T8473C were at 2.27-fold higher risk of BCC (95% CI=1.31-3.92) than homozygous wild type allele carriers. The polymorphisms IL6 G-174C, IL8 T-251A, PPARgamma2 Pro(12)Ala, IL1beta T-31C, and IL10 C-592A were not associated with risk of BCC. We found no statistically significant interaction between polymorphisms and NSAID use in relation to risk of BCC. While it cannot be ruled out that the present findings are due to chance, the results indicate that high COX-2 expression may increase risk of BCC while NSAID use may be protective.

Cyclooxygenase-2 deficiency increases epidermal apoptosis and impairs recovery following acute UVB exposure

The cyclooxygenases, COX-1 and COX-2, are involved in cutaneous responses to both acute and chronic UV exposure. In the present study, wild-type (WT), COX-1-/- and COX-2-/- mice were used to determine the influence of the individual isoform on mouse skin responses to acute UVB treatment. Immunohistochemistry and Western analysis indicated that COX-2, and not COX-1, was induced by UVB (2.5 or 5.0 kJ/m2), but that COX-1 remained the major source of prostaglandin E2 production. UVB exposure significantly increased epidermal apoptosis in all genotypes compared to untreated mice. However, while the number of apoptotic cells in WT and COX-1-/- mice were about equal, the number of apoptotic cells was 2.5-fold greater in COX-2-/- mice. Apoptosis in WT and COX-2-/- mice peaked at 24 h post-exposure. The increased apoptosis and reduced proliferation in COX-2-/- mice resulted in about a 50% decrease in epidermal thickness at 24-48 h post-exposure compared to about a 50% increase in epidermal thickness in WT mice. UVB-induced cell replication, as measured by BrdU labeling, was reduced in COX-2-/- compared to WT mice at 24-96 h. However, by 96 h post-exposure, both WT and COX-2-/- mice showed epidermal hyperplasia. The data indicate that COX-2 induction initially protects against the acute sunburn effects of UVB, but that continuous induction of COX-2 may contribute to skin cancer in chronic UVB exposure.

Effect of NSAIDs on the recurrence of nonmelanoma skin cancer

Experimental studies have consistently shown a protective effect of nonsteroidal antiinflammatory drugs (NSAIDs) against nonmelanoma skin cancers (NMSC). However, little human epidemiological research has been done in this regard. We used data from the Skin Cancer Chemoprevention Study to explore the association of NSAID use and with the risk of basal-cell carcinoma (BCC) and squamous-cell carcinoma (SCC). 1,805 subjects with a recent history of NMSC were randomized to placebo or 50 mg of daily beta-carotene. Participants were asked about their use of over-the-counter and prescription medications at baseline and every 4 months during the trial. Skin follow-up examinations were scheduled annually with a study dermatologist; confirmed lesions were the endpoints in the study. We used a risk set approach to the analysis of grouped times survival data and unconditional logistic regression to compute odds ratios [ORs] for various exposures to NSAIDs. The use of NSAIDs was reported in over 50% of questionnaires. For BCC, NSAIDs exhibited a weak protective effect in crude analyses, which attenuated markedly after adjustment. For SCC, the use of NSAIDs in the year previous to diagnosis reduced the odds by almost 30% (adjusted OR=0.71, 95% CI 0.48-1.04). When we accounted for frequency of use, results for BCC were not striking, and there were inconsistent suggestions of an inverse association with SCC. There were some indications of a modest, nonsignificant reduction on the number of BCCs and SCCs with NSAID use. Our data suggest a weak and inconsistent chemopreventive effect of NSAIDs on BCC and SCC.

Fatty acid composition of red blood cell membranes and risk of squamous cell carcinoma of the skin

Differential effects of fatty acids on carcinogenesis suggest that fatty acid composition is important in tumor development. Arachidonic acid and its metabolites elicit inflammation and promote tumor formation in mouse skin. Inhibitors of the arachidonic cascade inhibit tumor incidence. A population-based case control study in Southeastern Arizona tested the hypothesis that lower levels of arachidonic acid in RBC membranes were associated with decreased risk of skin squamous cell carcinoma (SCC; n = 335 SCC cases and 321 controls). Extracted and esterified RBC fatty acids were analyzed using capillary gas chromatography. Individual peaks for 14 fatty acids were measured as a percentage of total fatty acids. Logistic regression was used to estimate odds ratios (OR), adjusting for SCC risk factors (age, gender, actinic keratosis history, freckling, and tanning ability). Increased levels of arachidonic acid in RBC membranes were associated with increased risk of SCC [odds ratio (OR), 1.08 per mg/100 mL change; 95% confidence interval (95% CI), 1.02-1.15] and this association remained when controls with actinic keratosis precursor lesions were excluded. SCC risk was highest among the upper quartile of arachidonic acid (OR, 2.38; 95% CI, 1.37-4.12). In contrast, increasing proportions of palmitic acid (OR, 0.94; 95% CI, 0.89-1.00) and palmitoleicacid (OR, 0.49; 95% CI, 0.30-0.81) were associated with reduced SCC risk. More studies are needed to elucidate the function of RBC fatty acids so that recommendations can be made to alter the human diet for cancer prevention.

Mouse skin tumor promotion by sodium arsenate is associated with enhanced PCNA expression

Drinking water contamination by arsenicals remains a major public health problem in many parts of the world more particularly in India and Bangladesh. Despite arsenic being a health hazard and implicated in human carcinogenesis, the experimental evidence available is much limited even now and the mechanisms involved during carcinogenesis and tumor promotions are not clear. Accordingly, in this study, we have studied the tumor promoter effects of sodium arsenate on mouse skin tumor promoter model system using 9,10-dimethyl-1,2-benzanthracene (DMBA) as a initiating carcinogen. Our studies showed development of papillomas on mice skin treated with only DMBA. However, mice treated with DMBA on skin and administered arsenate (As) in drinking water showed development of well differentiated squamous cell carcinomas. Further, both by immunohistochemistry and western blotting analysis studies higher levels of proliferating cell nuclear antigen (PCNA) was observed in mice treated with DMBA plus arsenate compared to only DMBA treated group. PCNA is known to be associated with S phase and DNA replication of the cell cycle. The plain controls and arsenate controls did not show significant difference either in tumor development or in PCNA levels. The present study demonstrates mouse skin tumor promoting effect of arsenate which seems to be associated with abnormal cell proliferation as indicated by higher levels of PCNA expression.

UVA-mediated activation of signaling pathways involved in skin tumor promotion and progression

Each year more than 1,000,000 cases of non-melanoma skin cancer (NMSC) are diagnosed in the Unites States. Solar radiation has been described as an important etiological factor in the development of NMSC. UVA comprises the largest portion of solar radiation reaching the surface of the earth (90-99%) and has been described to lead to benign tumor formation as well as malignant cancers, squamous cell carcinomas (SCCs). While much research has focused upon the effects of UVB radiation, little is known about UVA-induced signaling pathways and their role in tumor promotion. Here we focus on UVA-mediated activation of mitogen-activated protein kinase (MAPK) pathways and their role in activator protein-1 (AP-1) mediated transcription and cyclooxygenase-2 (COX-2) expression. AP-1 and COX-2 have been found to play a role in angiogenesis in other tissues. We propose UVA-mediated increases in AP-1 and COX-2 may play a role in tumor promotion through increases in interleukin-8 (IL-8) and vascular endothelial growth factor (VEGF). Since MAPKs, specifically p38 and JNK, appear to play a major role in the expression of UVA-induced AP-1 and COX-2, pharmacological inhibitors may be of benefit in the chemoprevention of non-melanoma skin cancer.

Inhibition of cutaneous ultraviolet light B-mediated inflammation and tumor formation with topical celecoxib treatment

Inflammation, which includes the release of growth factors, proinflammatory cytokines and prostaglandins, the infiltration and activation of inflammatory cells, and the induction of oxidative DNA damage, is known to play a role in cancer development. The combination of damage to the skin resulting from chronic ultraviolet light B (UVB) exposure itself and the inflammatory response it induces is a major source of skin cancer development. Cyclooxygenase-2 (COX-2), an inflammatory enzyme responsible for the production of prostaglandins, is now implicated in the development of epithelial cancers, including squamous cell carcinoma in the skin. Previous work conducted in our laboratory has shown that topical treatment with celecoxib following UVB irradiation inhibits several parameters of acute inflammation, including vascular permeability, the infiltration and activation of neutrophils, and the production of prostaglandin E(2) (PGE(2)). The present studies expanded these observations, demonstrating the ability of topical celecoxib to inhibit acute oxidative damage. In addition, long-term studies illustrate the effectiveness of topical treatment with this drug in reducing chronic inflammation and UVB-induced papilloma/carcinoma formation. This data provides compelling evidence to explore the clinical efficacy of topically applied COX-2 inhibitors for the prevention of human skin cancers.

Omega 3 but not omega 6 fatty acids inhibit AP-1 activity and cell transformation in JB6 cells

Epidemiological and animal-based investigations have indicated that the development of skin cancer is in part associated with poor dietary practices. Lipid content and subsequently the derived fatty acid composition of the diet are believed to play a major role in the development of tumorigenesis. Omega 3 (omega3) fatty acids, including docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), can effectively reduce the risk of skin cancer whereas omega 6 (omega6) fatty acids such as arachidonic acid (AA) reportedly promote risk. To investigate the effects of fatty acids on tumorigenesis, we performed experiments to examine the effects of the omega3 fatty acids EPA and DHA and of the omega6 fatty acid AA on phorbol 12-tetradecanoate 13-acetate (TPA)-induced or epidermal growth factor (EGF)-induced transcription activator protein 1 (AP-1) transactivation and on the subsequent cellular transformation in a mouse epidermal JB6 cell model. DHA treatment resulted in marked inhibition of TPA- and EGF-induced cell transformation by inhibiting AP-1 transactivation. EPA treatment also inhibited TPA-induced AP-1 transactivation and cell transformation but had no effect on EGF-induced transformation. AA treatment had no effect on either TPA- or EGF-induced AP-1 transactivation or transformation, but did abrogate the inhibitory effects of DHA on TPA- or EGF-induced AP-1 transactivation and cell transformation in a dose-dependent manner. The results of this study demonstrate that the inhibitory effects of omega3 fatty acids on tumorigenesis are more significant for DHA than for EPA and are related to an inhibition of AP-1. Similarly, because AA abrogates the beneficial effects of DHA, the dietary ratio of omega6 to omega3 fatty acids may be a significant factor in mediating tumor development.

The role of cyclooxygenase and lipoxygenase in cancer chemoprevention

The involvement of prostaglandins (PGs) and other eicosanoids in the development of human cancer has been known for over two decades. Importantly, an increase in PG synthesis may influence tumor growth in human beings and experimental animals, and numerous studies have illustrated the effect of PG synthesis on carcinogen metabolism, tumor cell proliferation and metastatic potential. PGs produced by cyclooxygenases (COXs) are represented by a large series of compounds that mainly enhance cancer development and progression, acting as carcinogens or tumor promoters, with profound effects on carcinogenesis. Further investigations suggest that arachidonic acid (AA) metabolites derived from lipoxygenase (LOX) pathways play an important role in growth-related signal transduction, implying that intervention through these pathways should be useful for arresting cancer progression. We discuss here the implications of COX and LOX in colon, pancreatic, breast, prostate, lung, skin, urinary bladder and liver cancers. Select inhibitors of COX and LOX are described, including nonsteroidal antiinflammatory drugs (NSAIDs), selective COX-2 inhibitors, curcumin, tea, silymarin and resveratrol, as well as a method useful for evaluating inhibitors of COX. Although a substantial amount of additional work is required to yield a better understanding of the role of COX and LOX in cancer chemoprevention, it is clear that beneficial therapeutic effects can be realized through drug-mediated modulation of these metabolic pathways.

Protective effect of isoflavone derivative against photocarcinogenesis in a mouse model

Plant-derived isoflavones are currently receiving much attention because of their phyto-estrogenic and antioxidant activities. In this study, we describe novel photoprotective effects of one isoflavone derivative from red clover (NV07), following its application topically in Skh:HR-1 hairless mice. We found that in mice irradiated in the short-term (3 days) with minimally erythemal solar simulated UV radiation, topical lotions containing NV07 dose-responsively reduced the erythema-associated oedema, the induction of ornithine decarboxylase, and the suppression of contact hypersensitivity. In mice irradiated chronically (50 days), daily application of topical NV07-lotion reduced photocarcinogenesis significantly, and appeared to be actively protective during both the initiation phase and the later promotion phase of tumour induction.

Sunlight, immunosuppression and skin cancer: role of histamine and mast cells

1. The development into tumours of skin cells transformed by ultraviolet (UV) B radiation of wavelengths 290-320 nm is enhanced by the ability of UVB to suppress an immune response that would otherwise destroy them. Ultraviolet B-induced immunomodulation may be by multiple mechanisms, but generally manifests in an antigen-presenting cell defect and an altered cytokine environment in the draining lymph nodes. 2. Immune responses to microbial or self-antigens may be dysfunctional by similar mechanisms following UVB exposure. 3. Earliest-acting intermediates in the initiation of UVB-induced immunosuppression are the UVB absorbers (photoreceptors) of the skin, notably DNA resulting in immunoregulatory cytokine production, and trans-urocanic acid (UCA), which, upon isomerization to its cis isomer, signals downstream immunosuppressive events. 4. In mice, dermal mast cells are critical to UVB-induced systemic immunomodulation. In mice, there is a functional link as well as a linear relationship between the prevalence of histamine-staining dermal mast cells and the log of the dose of UVB required for 50% immunosuppression. Studies with histamine receptor antagonists support histamine as the main' product of mast cells involved. Histamine acts in large part via a prostanoid-dependent pathway. 5. Approximately 50% of humans and greater than 90% of patients with non-melanoma skin cancer are UVB susceptible for suppression of a contact hypersensitivity response. Neither cytokine polymorphisms nor UVB-induced levels of cis-UCA in irradiated skin have been linked to UVB susceptibility. Patients with basal cell carcinomas (BCC) have an increased dermal mast cell prevalence in non-sun-exposed buttock skin. We propose that mast cells function in humans, as in mice, by initiating immunosuppression and, thereby, allowing a permissive environment for BCC development.

The role of orally administered dimethylarsinic acid, a main metabolite of inorganic arsenics, in the promotion and progression of UVB-induced skin tumorigenesis in hairless mice

The effect of dimethylarsinic acid (DMA) on skin tumorigenesis by UVB irradiation was examined. Hairless mice (Hos: HR-1) irradiated with UVB at a dose of 2 kJ/m(2) twice weekly, were fed with drinking water containing 1000 ppm DMA, a main metabolite of inorganic arsenics, produced more skin tumors than DMA-untreated mice. Histopathological examination revealed that the mouse malignant tumors with severe atypism appeared only in the treatment group of UVB plus 1000 ppm DMA. These positive results point out the importance of dimethylated metabolites of inorganic arsenic in the process of skin carcinogenesis.

Reduction of UV-induced skin tumors in hairless mice by selective COX-2 inhibition

UV light is a complete carcinogen, inducing both basal and squamous cell skin cancers. The work described uses the selective COX-2 inhibitor celecoxib to examine the efficacy of COX-2 inhibition in the reduction of UV light-induced skin tumor formation in hairless mice. UVA-340 sun lamps were chosen as a light source that effectively mimics the solar UVA and UVB spectrum. Hairless mice were irradiated for 5 days a week for a total dose of 2.62 J/cm(2). When 90% of the animals had at least one tumor, the mice were divided into two groups so that the tumor number and multiplicity were the same (P < 0.31). Half of the mice were then fed a diet containing 1500 p.p.m. celecoxib. Tumor number, multiplicity and size were then observed for the next 10 weeks. Ninety-five percent of the tumors formed were histopathologically evaluated as squamous cell carcinoma. COX-2 expression and activity were increased in tumors. After 10 weeks, the difference in tumor number and multiplicity in the drug-treated group was 56% of UV controls (P < 0.001). The results show that the orally administered selective COX-2 inhibitor celecoxib prevents new tumor formation after the onset of photocarcinogenesis and suggest that treatment with celecoxib may be very useful in preventing UV-induced skin tumors in humans.

Modulation of the UVA activation of haem oxygenase, collagenase and cyclooxygenase gene expression by epigallocatechin in human skin cells

We have investigated the modifying effects of epigallocatechin, a major polyphenolic constituent of green tea, on ultraviolet-A-activated gene expression in human fibroblasts and keratinocytes using the stress responsive enzymes: haem oxygenase-1, interstitial collagenase and cyclooxygenase-2. Although epigallocatechin strongly reduced ultraviolet-A-induced haem oxygenase-1 activation in skin-derived 'fibroblasts, the same compound activated collagenase and cyclooxygenase expression. In a keratinocyte cell line, ultraviolet-A-mediated haem oxygenase-1 over-expression was low and epigallocatechin failed to modulate it further. In contrast to the results with fibroblasts, ultraviolet-A activation of cyclooxygenase in keratinocytes was reduced by epigallocatechin. The results indicate that the effect of this green tea polyphenol on cellular stress responses is complex and may involve direct effects on signal transduction as well as changes that may be associated with its antioxidant activity.

Growth regulation of primary human keratinocytes by prostaglandin E receptor EP2 and EP3 subtypes

We examined the contribution of specific EP receptors in regulating cell growth. By RT-PCR and northern hybridization, adult human keratinocytes express mRNA for three PGE2 receptor subtypes associated with cAMP signaling (EP2, EP3, and small amounts of EP4). In actively growing, non-confluent primary keratinocyte cultures, the EP2 and EP4 selective agonists, 11-deoxy PGE1 and 1-OH PGE1, caused complete reversal of indomethacin-induced growth inhibition. The EP3/EP2 agonist (misoprostol), and the EP1/EP2 agonist (17-phenyl trinor PGE2), showed less activity. Similar results were obtained with agonist-induced cAMP formation. The ability of exogenous dibutyryl cAMP to completely reverse indomethacin-induced growth inhibition support the conclusion that growth stimulation occurs via an EP2 and/or EP4 receptor-adenylyl cyclase coupled response. In contrast, activation of EP3 receptors by sulprostone, which is virtually devoid of agonist activity at EP2 or EP4 receptors, inhibited bromodeoxyuridine uptake in indomethacin-treated cells up to 30%. Although human EP3 receptor variants have been shown in other cell types to markedly inhibit cAMP formation via a pertussis toxin sensitive mechanisms, EP3 receptor activation and presumably growth inhibition was independent of adenylyl cyclase, suggesting activation of other signaling pathways.

Dependence of photocarcinogenesis and photoimmunosuppression in the hairless mouse on dietary polyunsaturated fat

A series of semi-purified diets containing 20% fat by weight, of increasing proportions (0, 5%, 10%, 15% or 20%) of polyunsaturated sunflower oil mixed with hydrogenated saturated cottonseed oil, was fed to groups of Skh:HR-1 hairless mice during induction and promotion of photocarcinogenesis. The photocarcinogenic response was of increasing severity as the polyunsaturated content of the mixed dietary fat was increased, whether measured as tumour incidence, tumour multiplicity, progression of benign tumours to squamous cell carcinoma, or reduced survival. At the termination of the study approximately 6 months following the completion of the 10-week chronic UV irradiation treatment, when most mice bore tumours, the contact hypersensitivity (CHS) reactions in those groups supporting the highest tumour leads (fed 15% or 20% polyunsaturated fat), were significantly suppressed in comparison with the mice bearing smaller tumour loads (fed 0, 5% or 10% polyunsaturated fat). When mice were exposed acutely to UV radiation (UVR), a diet of 20% saturated fat provided almost complete protection from the suppression of CHS, whereas feeding 20% polyunsaturated fat resulted in 57% suppression; the CHS of unirradiated mice was unaffected by the nature of the dietary fat. These results suggest that the enhancement of photocarcinogenesis by the dietary polyunsaturated fat component is mediated by an induced predisposition to persistent immunosuppression caused by the chronic UV irradiation, and supports the evidence for an immunological role in dietary fat modulation of photocarcinogenesis in mice.

8-hydroxy-2'-deoxyguanosine is increased in epidermal cells of hairless mice after chronic ultraviolet B exposure

8-Hydroxy-2'-deoxyguanosine (8-OHdG) is a mutation-prone (G:C to T:A transversion) DNA base-modified product generated by reactive oxygen species or photodynamic action. G:C to T:A transversions are observed in the p53 and ras genes of UVB-induced skin cancers of mice and in squamous and basal cell carcinomas of human skin exposed to sunlight. In the current study, 8-OHdG formation was evaluated in the epidermis of hairless mice after repeated exposure to UVB, and possible mechanisms involved were studied. Exposure of hairless mice to either 3.4 [2 minimal erythema dose (MED)] or 16.8 (10 MED) kJ/m2 of UVB three times a week for 2 wk induced a 2.5- or 6.1-fold increase, respectively, in the levels of 8-OHdG in DNA, compared to the unexposed controls. An immunohistochemical method using a monoclonal antibody specific for 8-OHdG showed stronger and more extensive staining in the nuclei of UV-irradiated epidermal cells than in those of nonirradiated cells. Western blots probed with antibodies against 4-hydroxy-2-nonenal-modified proteins confirmed the involvement of reactive oxygen species in the epidermal damage induced by chronic UVB exposure. 3-Nitro-L-tyrosine was detected in western blots in a concentration-dependent manner, suggesting that peroxynitrite derived from the reaction of nitric oxide and superoxide, both of which were probably released from inflammatory cells, was involved in modifying the DNA bases. Therefore, the formation of 8-OHdG after UVB exposure appears to be regulated by at least three pathways: photodynamic action, lipid peroxidation, and inflammation and may play a role in sunlight-induced skin carcinogenesis.

Topical tretinoin enhances corticosteroid-induced inhibition of tumorigenesis in hairless mice previously exposed to solar simulating radiation

The role of retinoids in hairless mouse photocarcinogenesis has been studied extensively but remains controversial. By contrast, the role of corticosteroids has hardly been investigated. Many findings indicate an antagonism between the action of these two drugs. To examine these issues, we irradiated hairless mice thrice weekly for 10 weeks with 1.5 minimal erythema doses of solar simulating radiation (UVB + UVA). In the post-UV period, groups of mice were treated topically for 30 weeks with tretinoin, corticosteroid or emollient vehicle alone or with a sequential combination of corticosteroid and tretinoin. A control group remained topically untreated. All three agents, when used alone, significantly inhibited tumorigenesis. The sequential combination produced the greatest inhibition.