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

Risk of basal cell carcinoma in a randomized clinical trial of aspirin and folic acid for the prevention of colorectal adenomas

BACKGROUND

Aspirin may reduce the risk of several types of cancer.

OBJECTIVES

To evaluate if folic acid is associated with risk of basal cell carcinoma (BCC).

METHODS

BCC incidence was evaluated in a randomized, double-blind, placebo-controlled clinical trial of aspirin (81 mg daily or 325 mg daily for ~3 years) and/or folic acid (1 mg daily for ~6 years) for the prevention of colorectal adenomas among 1121 participants with a previous adenoma. BCC was confirmed by blinded review of pathology reports.

RESULTS

One hundred and four of 958 non-Hispanic white participants were diagnosed with BCC over a median follow-up of 13·5 years. Cumulative incidence of BCC was 12% [95% confidence interval (CI) 7-17] for placebo, 16% (95% CI 11-21) for 81 mg aspirin daily and 15% (95% CI 10-20) for 325 mg aspirin daily [hazard ratio (HR) for any aspirin 1·45 (95% CI 0·93-2·26); HR for 81 mg daily 1·57 (95% CI 0·96-2·56); HR for 325 mg daily 1·33 (95% CI 0·80-2·20)]. BCC risk was higher with aspirin use in those without previous skin cancer but lower with aspirin use in those with previous skin cancer (P_interaction = 0·02 for 81 mg aspirin daily; P_interaction = 0·03 for 325 mg aspirin daily). Folic acid supplementation was unrelated to BCC incidence (HR 0·85; 95% CI 0·57-1·27).

CONCLUSIONS

Neither aspirin nor folic acid treatment had a statistically significant effect on risk of BCC. Subgroup analysis suggested that chemopreventive effects of nonsteroidal anti-inflammatory drugs may be specific to those at high risk for BCC.

Aryl hydrocarbon receptor (AhR) a possible target for the treatment of skin disease

Aryl hydrocarbon receptor (AhR) is a transcription factor expressed in all skin cells type. It responds to exogenous and endogenous chemicals by inducing/repressing the expression of several genes with toxic or protective effects in a wide range of species and tissues. In healthy skin, AhR signalling contributes to keratinocytes differentiation, skin barrier function, skin pigmentation, and mediates oxidative stress. In the last years, some studies have shown that AhR seems to be involved in the pathogenesis of some skin diseases, even if the currently available data are contradictory. Indeed, while the blocking the AhR signalling activity could prevent or treat skin cancer, the AhR activation seems to be advantageous for the treatment of inflammatory skin diseases. Therefore, for its multifaceted role in skin diseases, AhR seems to be an attractive therapeutic target. Indeed, recently some molecules have been identified for the prevention of skin cancer and the treatment of inflammatory skin diseases.

Molecular pathways regulating the formation of brown-like adipocytes in white adipose tissue

Adipose tissue is functionally composed of brown adipose tissue and white adipose tissue. The unique thermogenic capacity of brown adipose tissue results from expression of uncoupling protein 1 in the mitochondrial inner membrane. On the basis of recent findings that adult humans have functionally active brown adipose tissue, it is now recognized as playing a much more important role in human metabolism than was previously thought. More importantly, brown-like adipocytes can be recruited in white adipose tissue upon environmental stimulation and pharmacologic treatment, and this change is associated with increased energy expenditure, contributing to a lean and healthy phenotype. Thus, the promotion of brown-like adipocyte development in white adipose tissue offers novel possibilities for the development of therapeutic strategies to combat obesity and related metabolic diseases. In this review, we summarize recent advances in understanding the molecular mechanisms involved in the recruitment of brown-like adipocyte in white adipose tissue.

Cell-type-specific roles for COX-2 in UVB-induced skin cancer

In human tumors, and in mouse models, cyclooxygenase-2 (COX-2) levels are frequently correlated with tumor development/burden. In addition to intrinsic tumor cell expression, COX-2 is often present in fibroblasts, myofibroblasts and endothelial cells of the tumor microenvironment, and in infiltrating immune cells. Intrinsic cancer cell COX-2 expression is postulated as only one of many sources for prostanoids required for tumor promotion/progression. Although both COX-2 inhibition and global Cox-2 gene deletion ameliorate ultraviolet B (UVB)-induced SKH-1 mouse skin tumorigenesis, neither manipulation can elucidate the cell type(s) in which COX-2 expression is required for tumorigenesis; both eliminate COX-2 activity in all cells. To address this question, we created Cox-2(flox/flox) mice, in which the Cox-2 gene can be eliminated in a cell-type-specific fashion by targeted Cre recombinase expression. Cox-2 deletion in skin epithelial cells of SKH-1 Cox-2(flox/flox);K14Cre(+) mice resulted, following UVB irradiation, in reduced skin hyperplasia and increased apoptosis. Targeted epithelial cell Cox-2 deletion also resulted in reduced tumor incidence, frequency, size and proliferation rate, altered tumor cell differentiation and reduced tumor vascularization. Moreover, Cox-2(flox/flox);K14Cre(+) papillomas did not progress to squamous cell carcinomas. In contrast, Cox-2 deletion in SKH-1 Cox-2(flox/flox); LysMCre(+) myeloid cells had no effect on UVB tumor induction. We conclude that (i) intrinsic epithelial COX-2 activity plays a major role in UVB-induced skin cancer, (ii) macrophage/myeloid COX-2 plays no role in UVB-induced skin cancer and (iii) either there may be another COX-2-dependent prostanoid source(s) that drives UVB skin tumor induction or there may exist a COX-2-independent pathway(s) to UVB-induced skin cancer.

Molecular mechanisms of mouse skin tumor promotion

Multiple molecular mechanisms are involved in the promotion of skin carcinogenesis. Induction of sustained proliferation and epidermal hyperplasia by direct activation of mitotic signaling pathways or indirectly in response to chronic wounding and/or inflammation, or due to a block in terminal differentiation or resistance to apoptosis is necessary to allow clonal expansion of initiated cells with DNA mutations to form skin tumors. The mitotic pathways include activation of epidermal growth factor receptor and Ras/Raf/mitogen-activated protein kinase signaling. Chronic inflammation results in inflammatory cell secretion of growth factors and cytokines such as tumor necrosis factor-α and interleukins, as well as production of reactive oxygen species, all of which can stimulate proliferation. Persistent activation of these pathways leads to tumor promotion.

Progression of liver oncogenesis in the double transgenic mice c-myc/TGF α is not enhanced by cyclooxygenase-2 expression

Cyclooxygenase-2 (COX-2) has been associated with cell growth regulation, tissue remodeling and carcinogenesis. Overexpression of COX-2 in hepatocytes constitutes an ideal condition to evaluate the role of prostaglandins (PGs) in liver pathogenesis. The effect of COX-2-dependent PGs in genetic hepatocarcinogenesis has been investigated in triple c-myc/transforming growth factor α (TGF-α) transgenic mice that express human COX-2 in hepatocytes on a B6CBAxCD1xB6DBA2 background. Analysis of the contribution of COX-2-dependent PGs to the development of hepatocarcinogenesis, evaluated in this model, suggested a minor role of COX-2-dependent prostaglandins to liver oncogenesis as indicated by liver histopathology, morphometric analysis and specific markers of tumor progression. This allows concluding that COX-2 is insufficient for modifying the hepatocarcinogenesis course mediated by c-myc/TGF-α.

The role of the EP receptors for prostaglandin E2 in skin and skin cancer

One of the most common features of exposure of skin to ultraviolet (UV) light is the induction of inflammation, a contributor to tumorigenesis, which is characterized by the synthesis of cytokines, growth factors and arachidonic acid metabolites, including the prostaglandins (PGs). Studies on the role of the PGs in non-melanoma skin cancer (NMSC) have shown that the cyclooxygenase-2 (COX-2) isoform of the cyclooxygenases is responsible for the majority of the pathological effects of PGE₂. In mouse skin models, COX-2 deficiency significantly protects against chemical carcinogen- or UV-induced NMSC while overexpression confers endogenous tumor promoting activity. Current studies are focused on identifying which of the G protein-coupled EP receptors mediate the tumor promotion/progression activities of PGE₂ and the signaling pathways involved. As reviewed here, the EP1, EP2, and EP4 receptors, but not the EP3 receptor, contribute to NMSC development, albeit through different signaling pathways and with somewhat different outcomes. The signaling pathways activated by the specific EP receptors are context specific and likely depend on the level of PGE₂ synthesis, the differential levels of expression of the different EP receptors, as well as the levels of expression of other interacting receptors. Understanding the role and mechanisms of action of the EP receptors potentially offers new targets for the prevention or therapy of NMSCs.

Protein kinases and transcription factors activation in response to UV-radiation of skin: implications for carcinogenesis

Solar ultraviolet (UV) radiation is an important environmental factor that leads to immune suppression, inflammation, photoaging, and skin carcinogenesis. Here, we reviewed the specific signal transduction pathways and transcription factors involved in the cellular response to UV-irradiation. Increasing experimental data supporting a role for p38, MAPK, JNK, ERK1/2, and ATM kinases in the response network to UV exposure is discussed. We also reviewed the participation of NF-κB, AP-1, and NRF2 transcription factors in the control of gene expression after UV-irradiation. In addition, we discussed the promising chemotherapeutic intervention of transcription factors signaling by natural compounds. Finally, we focused on the review of data emerging from the use of DNA microarray technology to determine changes in global gene expression in keratinocytes and melanocytes in response to UV treatment. Efforts to obtain a comprehensive portrait of the transcriptional events regulating photodamage of intact human epidermis after UV exposure reveals the existence of novel factors participating in UV-induced cell death. Progress in understanding the multitude of mechanisms induced by UV-irradiation could lead to the potential use of protein kinases and novel proteins as specific targets for the prevention and control of skin cancer.

Cyanidin-3-glucoside suppresses B[a]PDE-induced cyclooxygenase-2 expression by directly inhibiting Fyn kinase activity

Benzo[a]pyrene-7,8-diol-9,10-epoxide (B[a]PDE) is a well-known carcinogen that is associated with skin cancer. Abnormal expression of cyclooxygenase-2 (COX-2) is an important mediator in inflammation and tumor promotion. We investigated the inhibitory effect of cyanidin-3-glucoside (C3G), an anthocyanin present in fruits, on B[a]PDE-induced COX-2 expression in mouse epidermal JB6 P+ cells. Pretreatment with C3G resulted in the reduction of B[a]PDE-induced expression of COX-2 and COX-2 promoter activity. The activation of activator protein-1 (AP-1) and nuclear factor-κB (NF-κB) induced by B[a]PDE was also attenuated by C3G. C3G attenuated the B[a]PDE-induced phosphorylation of MEK, MKK4, Akt, and mitogen-activated protein kinases (MAPKs), but no effect on the phosphorylation of the upstream MAPK regulator Fyn. However, kinase assays demonstrated that C3G suppressed Fyn kinase activity and C3G directly binds Fyn kinase noncompetitively with ATP. By using PP2, a pharmacological inhibitor for SFKs, we showed that Fyn kinase regulates B[a]PDE-induced COX-2 expression by activating MAPKs, AP-1 and NF-κB. These results suggest that C3G suppresses B[a]PDE-induced COX-2 expression mainly by blocking the activation of the Fyn signaling pathway, which may contribute to its chemopreventive potential.

The prostaglandin E2 receptor, EP2, regulates survivin expression via an EGFR/STAT3 pathway in UVB-exposed mouse skin

We previously reported that cycloogenase (COX)-2-generated prostaglandin E2 (PGE2) had anti-apoptotic effects in UVB-exposed mouse skin that involved EP2-mediated signaling (Chun et al., Cancer Res. 2007; 67: 2015). Because survivin is a regulator of cell survival, the possible involvement of COX-2 and EP2 in survivin expression following UVB exposure of mouse skin was investigated. In wild type mice, UVB exposure time-dependently increased the levels of survivin and phosphorylated-signal transducer and activator of transcription 3 (p-STAT3), a transcription factor that regulates survivin expression; and COX-2- or EP2-deficiency significantly reduced their induction. Topical application of the COX-2 inhibitor, celecoxib, also reduced UVB-induced survivin levels. To further investigate the roles of PGE2 and EP2 in the regulation of survivin, indomethacin was used to inhibit UVB-induced endogenous PG production. UVB-induced survivin levels were reduced by indomethacin, and PGE2 and the EP2 agonist, butaprost, partially restored survivin levels. The epidermal growth factor receptor (EGFR) is a downstream effector of EP2 and EGFR inhibition (AG1478) significantly reduced UVB activation of STAT3 and survivin levels. UVB-induced epidermal apoptosis in COX-2-/- mice was reduced by butaprost and EGFR inhibition blocked butaprost’s protective effects. Furthermore, butaprost in the absence of UVB exposure time-dependently increased p-EGFR, p-STAT3, and survivin levels in naïve mouse skin, whereas the EP4 agonist, PGE1 alcohol, did not significantly increase p-STAT3 or survivin levels. These data suggest that COX-2-generated PGE2 regulates survivin expression in mouse skin, in part, via an EP2-mediated EGFR/STAT3 pathway. Therefore, targeting the EP2/survivin pathway may provide a strategy for the chemoprevention/chemotherapy of skin cancer.

Docosahexaenoic acid reverts resistance to UV-induced apoptosis in human keratinocytes: involvement of COX-2 and HuR

The dramatic increase in the incidence of nonmelanoma skin cancer over the last decades has been related to the augmented exposure to ultraviolet (UV) radiation (UVR). It is known that apoptosis is induced as a protective mechanism after the acute irradiation of keratinocytes, whereas apoptotic resistance and carcinogenesis may follow the chronic exposure to UVR. We found that not all the human keratinocytes lines studied underwent apoptosis following acute exposure to UVR (10-60 mJ/cm(2)). Whereas UVR induced apoptosis in the HaCaT cells, NCTC 2544 and nr-HaCaT cells showed apoptosis resistance. The cytokeratin pattern of the apoptosis-resistant cells indicated that they possessed a degree of differentiation lower than that of HaCaT cells. They also showed an enhanced expression of cyclooxygenase-2 (COX-2), an early marker of carcinogenesis in various tissues, including skin. n-3 polyunsaturated fatty acids have drawn increasing interest as nutritional factors with the potential to reduce UVR carcinogenesis, and since they are apoptosis inducers and COX-2 inhibitors in cancer cells, we investigated the ability of n-3 polyunsaturated fatty acids to influence the resistance to UVR-induced apoptosis in keratinocytes. We observed that docosahexaenoic acid (DHA) reverted the resistance of nr-HaCaT cells to UVR-induced apoptosis, increasing the Bax/Bcl-2 ratio and caspase-3 activity, and reduced COX-2 levels by inhibiting the expression of the human antigen R (HuR), a known COX-2 mRNA stabilizer in keratinocytes. The transfection of nr-HaCaT cells with HuR siRNA mimicked the proapoptotic effect of DHA. Overall, our findings further support the role of DHA as a suitable anticarcinogenic factor against nonmelanoma skin cancers.

Targeting COX-2 expression by natural compounds: a promising alternative strategy to synthetic COX-2 inhibitors for cancer chemoprevention and therapy

Cyclooxygenase (COX)-2 is a pro-inflammatory immediate early response protein, chronically up-regulated in many pathological conditions. In autoimmune diseases, it is responsible for degenerative effects whereas in cancer, it correlates with poor prognosis. A constitutive expression of COX-2 is triggered since the earliest steps of carcinogenesis. Consequently, strategies aimed at inhibiting COX-2 enzymatic activity have been clinically applied for the treatment of autoimmune disorders; in addition, the same approaches are currently investigated for anti-cancer purposes. However, COX-2 protein inhibitors (i.e., NSAIDs and COXIBs) are not amenable to prolonged administration since they may cause severe side effects, and efforts are underway to identify alternative approaches for chemoprevention/therapy. COX-2 expression is a multi-step process, highly regulated at transcriptional and post-transcriptional levels. Defects in the modulation of one or both of these steps may be found in pathological conditions. Targeting COX-2 expression may therefore represent a promising strategy, by which the same preventive and therapeutic benefits may be gained while avoiding the severe side effects of COX-2 enzymatic inhibition. Naturally occurring compounds derived from plants/organisms represent a huge source of biologically active molecules, that remains largely unexplored. Derived from plants/organisms used in traditional forms of medicine or as dietary supplements, these compounds have been experimentally investigated for their anti-inflammatory and anti-cancer potential. In this review, we will analyze how natural compounds may modulate the multistep regulation of COX-2 gene expression and discuss their potential as a new generation of COX-2 targeting agents alternative to the synthetic COX-2 inhibitors.

Cyclooxygenase-2 controls energy homeostasis in mice by de novo recruitment of brown adipocytes

Obesity results from chronic energy surplus and excess lipid storage in white adipose tissue (WAT). In contrast, brown adipose tissue (BAT) efficiently burns lipids through adaptive thermogenesis. Studying mouse models, we show that cyclooxygenase (COX)-2, a rate-limiting enzyme in prostaglandin (PG) synthesis, is a downstream effector of beta-adrenergic signaling in WAT and is required for the induction of BAT in WAT depots. PG shifted the differentiation of defined mesenchymal progenitors toward a brown adipocyte phenotype. Overexpression of COX-2 in WAT induced de novo BAT recruitment in WAT, increased systemic energy expenditure, and protected mice against high-fat diet-induced obesity. Thus, COX-2 appears integral to de novo BAT recruitment, which suggests that the PG pathway regulates systemic energy homeostasis.

Cell death in the skin

The skin is the largest organ of the body and protects the organism against external physical, chemical and biological insults, such as wounding, ultraviolet radiation and micro-organisms. The epidermis is the upper part of the skin that is continuously renewed. The keratinocytes are the major cell type in the epidermis and undergo a specialized form of programmed cell death, called cornification, which is different from classical apoptosis. In keep with this view, several lines of evidence indicate that NF-kB is an important factor providing protection against keratinocyte apoptosis in homeostatic and inflammatory conditions. In contrast, the hair follicle is an epidermal appendage that shows cyclic apoptosis-driven involution, as part of the normal hair cycle. The different cell death programs need to be well orchestrated to maintain skin homeostasis. One of the major environmental insults to the skin is UVB radiation, causing the occurrence of apoptotic sunburn cells. Deregulation of cell death mechanisms in the skin can lead to diseases such as cancer, necrolysis and graft-versus-host disease. Here we review the apoptotic and the anti-apoptotic mechanisms in skin homeostasis and disease.

UVB irradiation regulates Cox-2 mRNA stability through AMPK and HuR in human keratinocytes

Considerable evidence has demonstrated that UVB irradiation is a strong carcinogen for nonmelanoma skin cancer. Up-regulation of cyclooxygenase-2 (Cox-2) has been shown to be a crucial event in human keratinocytes in their responses to UVB irradiation. To further understand the molecular mechanisms governing Cox-2 regulation, we found that UVB irradiation significantly increased Cox-2 mRNA stability by inducing cytoplasmic localization and protein abundance of human antigen R (HuR). We also found that AMP-activated kinase (AMPK) mediates these events and that UVB reduces AMPK activity by down-regulating LKB1 kinase. Finally, we propose a novel model in which UVB regulates Cox-2 mRNA stability through the LKB1/AMPK pathway.

Caffeic acid, a phenolic phytochemical in coffee, directly inhibits Fyn kinase activity and UVB-induced COX-2 expression

Caffeic acid (3,4-dihydroxycinnamic acid) is a well-known phenolic phytochemical present in many foods, including coffee. Recent studies suggested that caffeic acid exerts anticarcinogenic effects, but little is known about the underlying molecular mechanisms and specific target proteins. In this study, we found that Fyn, one of the members of the non-receptor protein tyrosine kinase family, was required for ultraviolet (UV) B-induced cyclooxygenase-2 (COX-2) expression, and caffeic acid suppressed UVB-induced skin carcinogenesis by directly inhibiting Fyn kinase activity. Caffeic acid more effectively suppressed UVB-induced COX-2 expression and subsequent prostaglandin E(2) production in JB6 P+ mouse skin epidermal (JB6 P+) cells compared with chlorogenic acid (5-O-caffeoylquinic acid), an ester of caffeic acid with quinic acid. Data also revealed that caffeic acid more effectively induced the downregulation of COX-2 expression at the transcriptional level mediated through the inhibition of activator protein-1 (AP-1) and nuclear factor-kappaB transcription activity compared with chlorogenic acid. Fyn kinase activity was suppressed more effectively by caffeic acid than by chlorogenic acid, and downstream mitogen-activated protein kinases (MAPKs) were subsequently blocked. Pharmacological Fyn kinase inhibitor (3-(4-chlorophenyl)1-(1,1-dimethylethyl)-1H-pyrazolo[3,4-d]pyrimidin-4-amine and leflunomide) data also revealed that Fyn is involved in UVB-induced COX-2 expression mediated through the phosphorylation of MAPKs in JB6 P+ cells. Pull-down assays revealed that caffeic acid directly bound with Fyn and non-competitively with adenosine triphosphate. In vivo data from mouse skin also supported the idea that caffeic acid suppressed UVB-induced COX-2 expression by blocking Fyn kinase activity. These results suggested that this compound could act as a potent chemopreventive agent against skin cancer.

Activation of the Nrf2-antioxidant system by a novel cyclooxygenase-2 inhibitor furan-2-yl-3-pyridin-2-yl-propenone: implication in anti-inflammatory function by Nrf2 activator

Furan-2-yl-3-pyridin-2-yl-propenone (FPP-3) is a novel synthetic compound and has demonstrated anti-inflammatory activity by inhibiting cyclooxygenase-2 (COX-2). It is widely accepted that reactive oxygen species (ROS) generated by activated inflammatory cells can exacerbate inflammation. In this study, the potential antioxidative efficacy of FPP-3 has been investigated in murine cells. FPP-3 increased the expression of multiple antioxidative enzymes, including NAD(P)H:quinone oxidoreductase 1 (Nqo1), gamma-glutamylcysteine ligase (GCL) and heme oxygenase-1 (HO-1), by facilitating the nuclear translocation of nuclear factor-erythroid 2-p45-related factor 2 (Nrf2). Inducibility of antioxidant proteins such as HO-1 were lost in nrf2-deficient murine fibroblasts. As a result of enhanced cellular antioxidative capacity, elevation of NF-kappaB-driven reporter gene expression by lipopolysaccharide was attenuated by FPP-3 treatment in murine fibroblasts. Furthermore, FPP-3 treatment inhibited UVA-mediated induction of COX-2 in murine keratinocytes. Our current study suggests that FPP-3, which has been developed as a novel COX-2 inhibitor, has antioxidative properties by activating the Nrf2-ARE pathway. The dual function of this compound may provide a better strategy to block/attenuate the inflammation process and to alleviate ROS-associated inflammatory complications.

NFAT regulates induction of COX-2 and apoptosis of keratinocytes in response to ultraviolet radiation exposure

The nuclear factor of activated T cells (NFAT) transcription factors are regulated by calcium/calcineurin signals and play important roles in T cells, muscle, bone, and neural tissue. NFAT is expressed in the epidermis, and although recent data suggest that NFAT is involved in the skin’s responses to ultraviolet radiation (UVR), the wavelengths of radiation that activate NFAT and the biological function of UV-activated NFAT remain undefined. We demonstrate that NFAT transcriptional activity is preferentially induced by UVB wavelengths in keratinocytes. The derived action spectrum for NFAT activation indicates that NFAT transcriptional activity is inversely associated with wavelength. UVR also evoked NFAT2 nuclear translocation in a parallel wavelength-dependent fashion and both transcriptional activation and nuclear translocation were inhibited by the calcineurin inhibitor cyclosporin A. UVR also evoked NFAT2 nuclear translocation in three-dimensional skin equivalents. Evidence suggests that COX-2 contributes to UV-induced carcinogenesis. Inhibiting UV-induced NFAT activation in keratinocytes, reduced COX-2 protein induction, and increased UV-induced apoptosis. COX-2 luciferase reporters lacking functional NFAT binding sites were less responsive to UVR, highlighting that NFAT is required for UV-induced COX-2 induction. Taken together, these data suggest that the proinflammatory, antiapoptotic, and procarcinogenic functions of UV-activated COX-2 may be mediated, in part, by upstream NFAT signaling. Flockhart, R. J., Diffey, B. L., Farr, P. M., Lloyd, J., Reynolds, N. J. NFAT regulates induction of COX-2 and apoptosis of keratinocytes in response to ultraviolet radiation exposure.

Cyclooxygenase-2 expression is critical for chronic UV-induced murine skin carcinogenesis

While it has been established that both the constitutive and inducible forms of cyclooxygenase (COX-1 and COX-2, respectively) play important roles in chemical initiation-promotion protocols with phorbol ester tumor promoters, the contribution of these two enzymes to ultraviolet (UV) light-induced skin tumors has not been fully assessed. To better understand the contribution of COX-1 and COX-2 to UV carcinogenesis, we transferred the null allele for each isoform onto the SKH-1 hairless strain of mouse. Due to low viability on this background with complete knockout of COX-2, heterozygous mice were used in UV carcinogenesis experiments. While the lack of one allele of COX-1 had no effect on tumor outcome, the lack of one allele of COX-2 resulted in a 50-65% reduction in tumor multiplicity and a marked decrease in tumor size. Additionally, transgenic SKH-1 mice that overexpress COX-2 under the control of a keratin 14 promoter developed 70% more tumors than wild-type SKH-1 mice. The lack of one allele of either COX-1 or COX-2 reduced prostaglandin (PG) E2 levels in response to a single UV treatment. The proliferative response to UV was significantly reduced in COX-2, but not COX-1, heterozygous mice. UV-induced apoptosis, however, was greater in COX-2 heterozygous mice. Collectively, these results clearly establish the requirement for COX-2 in the development of skin tumors.

The Aryl hydrocarbon receptor: an illuminating effector of the UVB response

The aryl hydrocarbon receptor (AhR) is a cytosolic ligand-activated transcription factor that mediates most of the toxic and carcinogenic effects of drugs and environmental toxins collectively known as xenobiotics. Ligand activation of the AhR stimulates the transcription of genes that encode several xenobiotic-metabolizing enzymes. The molecular mechanisms and signaling pathways evoked by the activation of the AhR are becoming increasingly understood and underscore the participation of the AhR in crucial processes, including cellular stress response, proliferation, differentiation, inflammation, and carcinogenesis. Studies now implicate the AhR as an integral part of the multifaceted signal transduction pathway initiated by the exposure of keratinocytes to ultraviolet B radiation (UVB), which is the most ubiquitous hazard to human skin and the principal risk factor for skin cancer. Ligand-dependent activation of the AhR in the cytosol provides a molecular bridge that links cytoplasmic events to nuclear signals, thus unmasking a previously unknown role for this transcription factor in the complex cellular response to UVB.

Cyclooxygenase-2 inhibits UVB-induced apoptosis in mouse skin by activating the prostaglandin E2 receptors, EP2 and EP4

Cyclooxygenase-2 (COX-2) is induced by UVB light and reduces UVB-induced epidermal apoptosis; however, the mechanism is unclear. Therefore, wild-type (WT) and COX-2-/- mice were acutely treated with UVB (5 kJ/m(2)), and apoptotic signaling pathways were compared. Following exposure, apoptosis was 2.5-fold higher in COX-2-/- compared with WT mice. Because prostaglandin E(2) (PGE(2)) is the major UV-induced prostaglandin and manifests its activity via four receptors, EP1 to EP4, possible differences in EP signaling were investigated in WT and COX-2-/- mice. Following UVB exposure, protein levels of EP1, EP2, and EP4 were elevated in WT mice, but EP2 and EP4 levels were 50% lower in COX-2-/- mice. Activated cyclic AMP-dependent protein kinase (PKA) and Akt are downstream in EP2 and EP4 signaling, and their levels were reduced in UVB-exposed COX-2-/- mice. Furthermore, p-Bad (Ser(136) and Ser(155)), antiapoptotic products of activated Akt and PKA, respectively, were significantly reduced in UVB-exposed COX-2-/- mice. To further study the roles of EP2 and EP4, UVB-exposed CD-1 mice were topically treated with indomethacin to block endogenous PGE(2) production, and PGE(2), the EP2 agonist (butaprost) or EP4 agonist (PGE(1) alcohol), was applied. Indomethacin reduced PKA and Akt activation by approximately 60%, but PGE(2) and the agonists restored their activities. Furthermore, both agonists decreased apoptosis in COX-2-/- mice by 50%. The data suggest that COX-2-generated PGE(2) has antiapoptotic roles in UVB-exposed mouse skin that involves EP2- and EP4-mediated signaling.

A role for cyclooxygenase-2 in ultraviolet light-induced skin carcinogenesis

Nonmelanoma skin cancer is the most prevalent cancer in the United States and its incidence is on the rise. These cancers generally arise on sun-exposed areas of the body and the ultraviolet (UV) B spectrum of sunlight has been clearly identified as the major carcinogen responsible for skin cancer development. Besides inducing DNA damage directly, UV exposure of the skin induces the expression of the enzyme cyclooxygenase-2 (COX-2), which catalyzes the first step in the conversion of arachidonic acid to prostaglandins, the primary product in skin being prostaglandin E(2) (PGE(2)). COX-2 has been shown to be overexpressed in premalignant lesions as well as in nonmelanoma skin cancers in both humans and mice chronically exposed to UV. Through the use of COX-2-selective inhibitors and COX-2 knockout mice, it has been shown that UV-induced COX-2 expression plays a major role in UV-induced PGE(2) production, inflammation, edema, keratinocyte proliferation, epidermal hyperplasia, and generation of a pro-oxidant state leading to oxidative DNA damage. Chronic exposure to UV leads to chronic up-regulation of COX-2 expression and chronic inflammation along with the accumulation of DNA damage and mutations, all of which combine to induce malignant changes in epidermal keratinocytes and skin cancers. Both inhibition of COX-2 activity and reduction in COX-2 expression by genetic manipulations significantly reduce, while overexpression of COX-2 in transgenic mice significantly increases UV-induced skin carcinogenesis. Together these studies demonstrate that COX-2 expression/activity is critical to the development of UV-related nonmelanoma skin cancers.

A proposed COX-2 and PGE2 receptor interaction in UV-exposed mouse skin

The cyclooxygenases (COX-1 and COX-2) and the prostaglandins (PGs) they generate play a major role in the skin's response to sunlight. Sunlight, especially the ultraviolet B (UVB) component, induces COX-2 and increases PG levels. However, PGs can have both beneficial and adverse cutaneous effects. To elucidate the roles of the COXs and the PGs they generate in response to UVB exposure, experiments with the COX-1- and COX-2-deficient mice have provided insight into the specific roles of each isoform. Furthermore, because PGE(2) is the major PG produced following UV exposure and PGE(2) manifests its biological activity via four membrane receptors (EP1, EP2, EP3, EP4), elucidating contributions of these receptors is essential for understanding the roles of PGs in UVB-induced effects. In this review, we summarize recent findings from the COX-deficient mice showing how COX-2 generated PGE(2) acting via the receptors EP2 and EP4 could contribute to short term beneficial, but also contribute to long-term carcinogenic effects in response to UVB exposure.