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

SIRT6 pharmacological inhibition delays skin cancer progression in the squamous cell carcinoma

Sirtuin 6 (SIRT6) has a critical role in cutaneous Squamous Cell Carcinoma (cSCC): SIRT6 silencing in skin SCC cells has pro-differentiating effects and SIRT6 deletion abrogated DMBA-TPA-induced skin tumorigenesis in mice. On the other hand, SIRT6 acts as tumor suppressor in SCC by enhancing glycolysis in tumor propagating cells. Herein, pharmacological modulation of SIRT6 deacetylase activity was investigated in cSCC, with S6 (inhibitor) or MDL-800 (activator). In cSCC cells, S6 recreated the pro-differentiating effects of SIRT6 silencing, as the levels of Keratin 1, Keratin 10 and Loricrin were upregulated compared to controls. Next, the effects of SIRT6 pharmacological modulation were evaluated in a DMBA-TPA-induced skin cancer mouse model. Mice treated with the inhibitor S6 in a preventive approach, i.e. at the beginning of the promotion stage, presented reduced number and size of papillomas, compared to the controls. The epidermal hyperproliferation marker Keratin 6 and the cSCC marker Keratin 8 were less abundant when SIRT6 was inhibited. In S6-treated lesions, the Epithelial-Mesenchymal Transition (EMT) markers Zeb1 and Vimentin were less expressed compared to untreated lesions. In a therapeutic approach, i.e. treatment starting after papilloma appearance, the S6 group presented reduced papillomas (number and size), whereas MDL-800-treated mice displayed an opposite trend. In S6-treated lesions, Keratin 6 and Keratin 8 were less expressed, EMT was less advanced, with a higher E-cadherin/Vimentin ratio, indicating a delayed carcinogenesis when SIRT6 was inhibited. Our results confirm that SIRT6 plays a role in skin carcinogenesis and suggest SIRT6 pharmacological inhibition as a promising strategy in cSCC.

Dynamic stem cell selection safeguards the genomic integrity of the epidermis

Maintaining genomic integrity and stability is crucial for life; yet, no tissue-driven mechanism that robustly safeguards the epithelial genome has been discovered. Epidermal stem cells (EpiSCs) continuously replenish the stratified layers of keratinocytes that protect organisms against various environmental stresses. To study the dynamics of DNA-damaged cells in tissues, we devised an in vivo fate tracing system for EpiSCs with DNA double-strand breaks (DSBs) and demonstrated that those cells exit from their niches. The clearance of EpiSCs with DSBs is caused by selective differentiation and delamination through the DNA damage response (DDR)-p53-Notch/p21 axis, with the downregulation of ITGB1. Moreover, concomitant enhancement of symmetric cell divisions of surrounding stem cells indicates that the selective elimination of cells with DSBs is coupled with the augmented clonal expansion of intact stem cells. These data collectively demonstrate that tissue autonomy through the dynamic coupling of cell-autonomous and non-cell-autonomous mechanisms coordinately maintains the genomic quality of the epidermis.

Toll-Like Receptor-4 Antagonist Enhances the Repair of Ultraviolet Radiation-Induced DNA Damage and Augments Anti-Tumor Immune Responses in Mice

Simple Summary

Ultraviolet B (UVB) radiation is largely responsible for the development of skin cancer. When UVB-induced DNA damage in cells is not repaired, it can lead to the initiation of non-melanoma skin cancers. Xeroderma pigmentosum (XP) disease is caused by a defect in the repair of damaged DNA. Toll-like receptor-4 (TLR4) and NLR family pyrin domain containing 3 (NLRP3) belong to the family of innate immune receptors and are highly expressed in skin tumors. In this study, we determined the mechanism through which TLR4 inhibitor TAK-242 regulates inflammation and prevents skin cancer.

Abstract

Ultraviolet (UV) irradiation of the skin is related to the development of skin cancer. UVB also causes DNA damage in the form of cyclobutane pyrimidine dimers (CPDs), which can result in stable mutations. Toll-like receptor 4 (TLR4), a component of innate immunity, plays a key role in cancer. Previous studies from our laboratory have observed that TLR4 deficiency resulted in the repair of UVB-induced DNA damage, inhibition of UVB-induced immune suppression, and carcinogenesis. In this study, we determined the efficacy of TLR4 antagonist TAK-242 in regulation of UVB-induced DNA damage, inflammation, and tumor development. Our results indicate that TAK-242 treatment increased the expression of xeroderma pigmentosum group A (XPA) mRNA, resulting in the repair of UVB-induced CPDs in skin of SKH-1 mice. Treatment with TAK-242 also inhibited the activation of NLR family pyrin domain containing 3 (NLRP3) in UVB-exposed skin of SKH-1 mice. Cutaneous carcinogenesis was significantly reduced in mice treated with TAK-242 in comparison to vehicle-treated mice. The proinflammatory cytokines IL-1β, IL-6, and TNF-α were also found to be significantly greater in vehicle-treated mice than TAK-242-treated mice. Finally, treatment with TAK-242 augmented anti-tumor immune responses in mice. Our data provide further evidence that activation of the TLR4 pathway promotes the development of UV-induced non-melanoma skin cancer mediated at least in part on its negative effects on DNA damage. Moreover, treatment with the TLR4 inhibitor TAK-242 may be effective for prevention of skin cancer.

Toll-like receptor-4 deficiency inhibits ultraviolet radiation-induced tumor development by modulation of immune and inflammatory responses

Ultraviolet (UV) B irradiation of the skin induces acute inflammation, as characterized by erythema, edema, and immunosuppression, and is subsequently linked to the progression of skin cancer. Toll-like receptor 4 (TLR4), a component of innate immunity, has been shown to play an important role in cancer. To elucidate the role of TLR4 in UVB-induced tumor development, TLR4-proficient (C3H/HeN) and TLR4-deficient (C3H/HeJ) mice were exposed to multiple doses of UVB radiation (200 mJ/cm² ) for 40 weeks. Photocarcinogenesis was retarded in terms of tumor incidence, and tumor latency, in mice deficient in TLR4 compared with TLR4-proficient mice, whereas significantly greater numbers of tumors occurred in TLR4-proficient mice. There was significant upregulation of inflammatory markers like COX-2, PGE₂ , S100A8, and S100A9 in the skin of TLR4-proficient mice than the skin of TLR4-deficient mice. Furthermore, we found that TLR4-proficient mice had a significantly higher number of Gr1+CD11b+ myeloid cells CD4+CD25+ regulatory T-cells than TLR4-deficient mice. Furthermore, the levels of interferon (IFN)-γ cytokine was increased and the levels of interleukin (IL)-4, IL-10, and IL-17 cytokines were decreased in serum, skin, and tumor lysates of TLR4-deficient mice in comparison with samples from TLR4-proficient mice. Together, our data indicate that TLR4-mediated inflammation may cause suppression of antitumor responses and trigger the development of UVB-induced skin cancers. Thus, strategies to inhibit TLR4-mediated immune suppression may allow us to develop preventive and therapeutic approaches for the management of UVB-induced cutaneous tumors.

Molecular Insights into the Interaction of RONS and Thieno[3,2-c]pyran Analogs with SIRT6/COX-2: A Molecular Dynamics Study

SIRT6 and COX-2 are oncogenes target that promote the expression of proinflammatory and pro-survival proteins through a signaling pathway, which leads to increased survival and proliferation of tumor cells. However, COX-2 also suppresses skin tumorigenesis and their relationship with SIRT6, making it an interesting target for the discovery of drugs with anti-inflammatory and anti-cancer properties. Herein, we studied the interaction of thieno[3,2-c]pyran analogs and RONS species with SIRT6 and COX-2 through the use of molecular docking and molecular dynamic simulations. Molecular docking studies revealed the importance of hydrophobic and hydrophilic amino acid residues for the stability. The molecular dynamics study examined conformational changes in the enzymes caused by the binding of the substrates and how those changes affected the stability of the protein-drug complex. The average RMSD values of the backbone atoms in compounds 6 and 10 were calculated from 1000 ps to 10000 ps and were found to be 0.13 nm for both compounds. Similarly, the radius of gyration values for compounds 6 and 10 were found to be 1.87 ± 0.03 nm and 1.86 ± 0.02 nm, respectively. The work presented here, will be of great help in lead identification and optimization for early drug discovery.

Phosphodiesterase 4B plays a role in benzophenone-3-induced phototoxicity in normal human keratinocytes

Benzophenone-3 (BP-3), which is extensively used in organic sunscreen, has phototoxic potential in human skin. Phosphodiesterase 4B (PDE4B) has a well-established role in inflammatory responses in immune cells. Currently, it is unknown if PDE4B is associated with BP-3-induced phototoxicity in normal human keratinocytes (NHKs). We found that BP-3 significantly increased PDE4B expression in ultraviolet B (UVB)-irradiated NHKs. Notably, BP-8, a sunscreen agent that shares the 2-hydroxy-4-methoxyphenyl methanone moiety with BP-3, also upregulated PDE4B expression in NHKs. Upon UVB irradiation, BP-3 upregulated the expression of pro-inflammatory factors, such as prostaglandin endoperoxide synthase 2, tumor necrosis factor α, interleukin 8, and S100A7, and downregulated the level of cornified envelope associated proteins, which are important in the development of the epidermal permeability barrier. The additive effects of UVB-activated BP-3 on the expression of both pro-inflammatory mediators and cornified envelope associated proteins were antagonized by treatment with the PDE4 inhibitor rolipram. The BP-3 and UVB co-stimulation-induced PDE4B upregulation and its association with the upregulation of pro-inflammatory mediators and the downregulation of epidermal differentiation markers were confirmed in a reconstituted three dimensional human epidermis model. Therefore, PDE4B has a role in the mechanism of BP-3-induced phototoxicity.

Loss of INK4a/Arf gene enhances ultraviolet radiation-induced cutaneous tumor development

The CDKN2A locus encodes for tumor suppressor genes p16^INK4a and p14^Arf which are frequently inactivated in human skin tumors. The purpose of this study was to determine the relationship between loss of INK4a/Arf activity and inflammation in the development of ultraviolet (UV) radiation-induced skin tumors. Panels of INK4a/Arf^-/- mice and wild-type (WT) mice were treated with a single dose of UVB (200 mJ/cm² ). For long-term studies, these mice were irradiated with UVB (200 mJ/cm² ) three times weekly for 30 weeks. At the end of the experiment, tissues were harvested from mice and assayed for inflammatory biomarkers and cytokines. A single dose of UVB resulted in a significant increase in reactive oxygen species (ROS) and 8-dihydroxyguanosine (8-oxo-dG) lesions in INK4a/Arf^-/- mice compared to WT mice. When subjected to chronic UVB, we found that 100% of INK4a/Arf^-/- mice had tumors, whereas there were no tumors in WT controls after 24 weeks of UVB exposure. The increase in tumor development correlated with a significant increase in nuclear factor (NF)-κB, cyclooxygenase-2 (COX-2), prostaglandin E₂ (PGE₂ ) and its receptors both in UVB-exposed skin and in the tumors. A significant increase was seen in inflammatory cytokines in skin samples of INK4a/Arf^-/- mice following treatment with chronic UVB radiation. Furthermore, significantly more CD11b⁺ Gr1⁺ myeloid cells were present in UVB-exposed INK4a/Arf^-/- mice compared to WT mice. Our data indicate that by targeting UVB-induced inflammation, it may be possible to prevent UVB-induced skin tumors in individuals that carry CDKN2A mutation.

Autophagy in UV Damage Response

UV radiation exposure from sunlight and artificial tanning beds is the major risk factor for the development of skin cancer and skin photoaging. UV-induced skin damage can trigger a cascade of DNA damage response signaling pathways, including cell cycle arrest, DNA repair and, if damage is irreparable, apoptosis. Compensatory proliferation replaces the apoptotic cells to maintain skin barrier integrity. Disruption of these processes can be exploited to promote carcinogenesis by allowing the survival and proliferation of damaged cells. UV radiation also induces autophagy, a catabolic process that clears unwanted or damaged proteins, lipids and organelles. The mechanisms by which autophagy is activated following UV exposure, and the functions of autophagy in UV response, are only now being clarified. Here, we summarize the current understanding of the mechanisms governing autophagy regulation by UV, the roles of autophagy in regulating cellular response to UV-induced photodamage and the implications of autophagy modulation in the treatment and prevention of photoaging and skin cancer.

Honokiol inhibits ultraviolet radiation-induced immunosuppression through inhibition of ultraviolet-induced inflammation and DNA hypermethylation in mouse skin

Ultraviolet (UV) radiation exposure induces immunosuppression, which contributes to the development of cutaneous malignancies. We investigated the effects of honokiol, a phytochemical found in plants of the genus Magnolia, on UVB-induced immunosuppression using contact hypersensitivity (CHS) as a model in C3H/HeN mice. Topical application of honokiol (0.5 and 1.0 mg/cm2 skin area) had a significant preventive effect on UVB-induced suppression of the CHS response. The inflammatory mediators, COX-2 and PGE2, played a key role in this effect, as indicated by honokiol inhibition of cyclooxygenase-2 (COX-2) expression and PGE2 production in the UVB-exposed skin. Honokiol application also inhibited UVB-induced DNA hypermethylation and its elevation of the levels of TET enzyme, which is responsible for DNA demethylation in UVB-exposed skin. This was consistent with the restoration of the CHS response in mice treated with the DNA demethylating agent, 5-aza-2'-deoxycytidine, after UVB exposure. There was no significant difference in the levels of inhibition of UVB-induced immunosuppression amongst mice that were treated topically with available anti-cancer drugs (imiquimod and 5-fluorouracil). This study is the first to show that honokiol has the ability to inhibit UVB-induced immunosuppression in preclinical model and, thus, has potential for use as a chemopreventive strategy for UVB radiation-induced malignancies.

Naproxen Inhibits UVB-induced Basal Cell and Squamous Cell Carcinoma Development in Ptch1+/- /SKH-1 Hairless Mice

Naproxen possesses anti-proliferative and pro-apoptotic effects besides its known anti-inflammatory functions. Here, we demonstrate the anticancer effects of naproxen against UVB-induced basal cell carcinoma (BCCs) and squamous cell carcinoma (SCCs) in a highly susceptible murine model of UVB carcinogenesis. Naproxen significantly inhibited UVB-induced BCCs and SCCs in this model. Tumor number and volume were significantly decreased (P < 0.005 and P < 0.05, respectively). Inhibition in UVB-induced SCCs and BCCs was 77% and 86%, respectively, which was associated with reduced PCNA and cyclin D1 and increased apoptosis. As expected, inflammation-related iNOS, COX-2 and nuclear NFκBp65 were also diminished by naproxen treatment. Residual tumors excised from naproxen-treated animal were less invasive and showed reduced expression of epithelial-mesenchymal transition (EMT) markers N-cadherin, Vimentin, Snail and Twist with increased expression of E-cadherin. In BCC and SCC cells, naproxen-induced apoptosis and activated unfolded protein response (UPR) signaling with increased expression of ATF4, p-eIF2α and CHOP. Employing iRNA-based approaches, we found that naproxen-induced apoptosis was regulated by CHOP as sensitivity of these cutaneous neoplastic cells for apoptosis was significantly diminished by ablating CHOP. In summary, these data show that naproxen is a potent inhibitor of UVB-induced skin carcinogenesis. ER stress pathway protein CHOP may play an important role in inducing apoptosis in cancer cells.

Resistance to ursolic acid-induced apoptosis through involvement of melanogenesis and COX-2/PGE2 pathways in human M4Beu melanoma cancer cells

Melanoma is one of the most aggressive forms of cancer with a continuously growing incidence worldwide and is usually resistant to chemotherapy agents, which is due in part to a strong resistance to apoptosis. Previously, we had showed that B16-F0 murine melanoma cells undergoing apoptosis are able to delay their own death induced by ursolic acid (UA), a natural pentacyclic triterpenoid compound. We had demonstrated that tyrosinase and TRP-1 up-regulation in apoptotic cells and the subsequent production of melanin were implicated in an apoptosis resistance mechanism. Several resistance mechanisms to apoptosis have been characterized in melanoma such as hyperactivation of DNA repair mechanisms, drug efflux systems, and reinforcement of survival signals (PI3K/Akt, NF-κB and Raf/MAPK pathways). Otherwise, other mechanisms of apoptosis resistance involving different proteins, such as cyclooxygenase-2 (COX-2), have been described in many cancer types. By using a strategy of specific inhibition of each ways, we suggested that there was an interaction between melanogenesis and COX-2/PGE2 pathway. This was characterized by analyzing the COX-2 expression and activity, the expression of tyrosinase and melanin production. Furthermore, we showed that anti-proliferative and proapoptotic effects of UA were mediated through modulation of multiple signaling pathways including Akt and ERK-1/2 proteins. Our study not only uncovers underlying molecular mechanisms of UA action in human melanoma cancer cells but also suggest its great potential as an adjuvant in treatment and cancer prevention.

Vitamin D Induces Cyclooxygenase 2 Dependent Prostaglandin E2 Synthesis in HaCaT Keratinocytes

The active metabolite of vitamin D calcitriol and its analogs are well-known for their anti-inflammatory action in the skin, while their main side effect associated with topical treatment of inflammatory disorders is irritant contact dermatitis. Prostaglandin E2 (PGE2 ) is pro-inflammatory at the onset of inflammation and anti-inflammatory at its resolution. We hypothesized that induction of PGE2 synthesis by calcitriol in epidermal keratinocytes may contribute both to its pro-inflammatory and anti-inflammatory effects on the skin. Treatment of human immortalized HaCaT keratinocytes with calcitriol (3-100 nM, 2-24 h) increased PGE2 production due to increased mRNA and protein expression of COX-2, but not to increase of COX-1 or release of arachidonic acid. The effect of calcitriol on COX-2 mRNA was observed also in primary human keratinocytes. The increase in COX-2 mRNA is associated with COX-2 transcript stabilization. Calcitriol exerts this effect by a rapid (2 h) and protein synthesis independent mode of action that is dependent on PKC and Src kinase activities. Treatment with a COX-2 inhibitor partially prevented the attenuation of the keratinocyte inflammatory response by calcitriol. We conclude that upregulation of COX-2 expression with the consequent increase in PGE2 synthesis may be one of the mechanisms explaining the Janus face of calcitriol as both a promoter and attenuator of cutaneous inflammation. J. Cell. Physiol. 231: 837-843, 2016. © 2015 Wiley Periodicals, Inc.

SIRT6 promotes COX-2 expression and acts as an oncogene in skin cancer

SIRT6 is a SIR2 family member that regulates multiple molecular pathways involved in metabolism, genomic stability, and aging. It has been proposed previously that SIRT6 is a tumor suppressor in cancer. Here, we challenge this concept by presenting evidence that skin-specific deletion of SIRT6 in the mouse inhibits skin tumorigenesis. SIRT6 promoted expression of COX-2 by repressing AMPK signaling, thereby increasing cell proliferation and survival in the skin epidermis. SIRT6 expression in skin keratinocytes was increased by exposure to UVB light through activation of the AKT pathway. Clinically, we found that SIRT6 was upregulated in human skin squamous cell carcinoma. Taken together, our results provide evidence that SIRT6 functions as an oncogene in the epidermis and suggest greater complexity to its role in epithelial carcinogenesis.

A higher dietary ratio of long-chain omega-3 to total omega-6 fatty acids for prevention of COX-2-dependent adenocarcinomas

Compelling evidence that daily low-dose aspirin decreases risk for a number of adenocarcinomas likely reflects the fact that a modest but consistent inhibition of cyclooxygenase-2 (COX-2) activity can have a meaningful protective impact on risk for such cancers. The cancer-promoting effects of COX-2 are thought to be mediated primarily by prostaglandin E2 (PGE2), synthesized from arachidonic acid. The long-chain omega-3s eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), abundant in many fatty fish, can interfere with the availability of arachidonate to COX-2 by multiple complementary mechanisms; moreover, the PGE3 produced by COX-2 from EPA is a competitive inhibitor of the receptors activated by PGE2. These considerations have given rise to the hypothesis that a high dietary intake of EPA/DHA, relative to omega-6 (from which arachidonate is generated), should lessen risk for a number of adenocarcinomas by impeding PGE2 production and activity-while not posing the risk to vascular health associated with COX-2-specific nonsteroidal antiinflammatory agents. Analyses that focus on studies in which the upper category of fish consumption (not fried or salt-preserved) is 2 or more servings weekly, and on studies that evaluate the association of long-term fish oil supplementation with cancer risk yields a number of findings that are consistent with the hypothesis. Further studies of this nature may help to clarify the impact of adequate regular intakes of long-chain omega-3 on cancer risk, and perhaps provide insight into the dose-dependency of this effect.

Fisetin inhibits UVB-induced cutaneous inflammation and activation of PI3K/AKT/NFκB signaling pathways in SKH-1 hairless mice

Solar ultraviolet B (UVB) radiation has been shown to induce inflammation, DNA damage, p53 mutations and alterations in signaling pathways eventually leading to skin cancer. In this study, we investigated whether fisetin reduces inflammatory responses and modulates PI3K/AKT/NFκB cell survival signaling pathways in UVB-exposed SKH-1 hairless mouse skin. Mice were exposed to 180 mJ cm(-2) of UVB radiation on alternate days for a total of seven exposures, and fisetin (250 and 500 nmol) was applied topically after 15 min of each UVB exposure. Fisetin treatment to UVB-exposed mice resulted in decreased hyperplasia and reduced infiltration of inflammatory cells. Fisetin treatment also reduced inflammatory mediators such as COX-2, PGE2 as well as its receptors (EP1-EP4) and MPO activity. Furthermore, fisetin reduced the level of inflammatory cytokines TNFα, IL-1β and IL-6 in UVB-exposed skin. Fisetin treatment also reduced cell proliferation markers as well as DNA damage as evidenced by increased expression of p53 and p21 proteins. Further studies revealed that fisetin inhibited UVB-induced expression of PI3K, phosphorylation of AKT and activation of the NFκB signaling pathway in mouse skin. Overall, these data suggest that fisetin may be useful against UVB-induced cutaneous inflammation and DNA damage.

COX-2 inhibitors arrest prostate cancer cell cycle progression by down-regulation of kinetochore/centromere proteins

BACKGROUND

Previous studies have shown that COX-2 inhibitors inhibit cancer cell proliferation. However, the molecular mechanism remains elusive.

METHODS

Prostate cancer LNCaP, 22Rv1, and PC3 cells were cultured and treated with the COX-2 inhibitors celecoxib and CAY10404. Knockdown of COX-2 in LNCaP cells was carried out using lentiviral vector-loaded COX-2 shRNA. Cell cycle progression and cell proliferation were analyzed by flow cytometry, microscopy, cell counting, and the MTT assay. The antagonists of EP1, EP2, EP3, and EP4 were used to examine the effects of the PGE2 signaling. The effect of COX-2 inhibitors and COX-2 knockdown on expression of the kinetochore/centromere genes and proteins was determined by RT-PCR and immunoblotting.

RESULTS

Treatment with the COX-2 inhibitors celecoxib and CAY10404 or knockdown of COX-2 significantly inhibited prostate cancer cell proliferation. Flow-cytometric analysis and immunofluorescent staining confirmed the cell cycle arrested at the G2/M phase. Biochemical analysis showed that inhibition of COX-2 or suppression of COX-2 expression induced a dramatic down-regulation of key proteins in the kinetochore/centromere assembly, such as ZWINT, Cdc20, Ndc80, CENP-A, Bub1, and Plk1. Furthermore, the EP1 receptor antagonist SC51322, but not the EP2, EP3, and EP4 receptor antagonists, produced similar effects to the COX-2 inhibitors on cell proliferation and down-regulation of kinetochore/centromere proteins, suggesting that the effect of the COX-2 inhibition is through inactivation of the EP1 receptor signaling.

CONCLUSIONS

Our studies indicate that inhibition of COX-2 can arrest prostate cancer cell cycle progression through inactivation of the EP1 receptor signaling and down-regulation of kinetochore/centromere proteins.

Prostaglandin E2 Promotes UV radiation-induced immune suppression through DNA hypermethylation

Exposure of mice to UV radiation results in suppression of the contact hypersensitivity (CHS) response. Here, we report that the UV-induced suppression of CHS is associated with increases in the levels of cyclooxygenase-2 (COX-2), prostaglandin E2 (PGE2), and PGE2 receptors in the exposed skin. UV radiation-induced suppression of CHS was inhibited by topical treatment of the skin with celecoxib or indomethacin (inhibitors of COX-2) or AH6809 (an EP2 antagonist). Moreover, mice deficient in COX-2 were found to be resistant to UV-induced suppression of CHS. The exposure of wild-typemice to UVB radiation resulted in DNA hypermethylation, increased DNA methyltransferase (Dnmt) activity, and elevated levels of Dnmt1, Dnmt3a, and Dnmt3b proteins in the skin, and these responses were downregulated on topical treatment of the site of exposure after irradiation with indomethacin or EP2 antagonist. Topical treatment of UVB-exposed COX-2-deficient mice with PGE2 enhanced the UVB-induced suppression of CHS as well as global DNA methylation and elevated the levels of Dnmt activity and Dnmt proteins in the skin. Intraperitoneal injection of 5-aza-2'-deoxycytidine (5-Aza-dc), a DNA demethylating agent, restored the CHS response to 2,4-dinitrofluorobenzene in UVB-exposed skin and this was associated with the reduction in global DNA methylation and Dnmt activity and reduced levels of Dnmt proteins. Furthermore, treatment with 5-Aza-dc reversed the effect of PGE2 on UV-induced suppression of CHS in COX-2-deficient mice. These findings reveal a previously unrecognized role for PGE2 in the promotion of UVB-induced immunosuppression and indicate that it is mediated through PGE2 regulation of DNA methylation.

Impact of EPA ingestion on COX- and LOX-mediated eicosanoid synthesis in skin with and without a pro-inflammatory UVR challenge--report of a randomised controlled study in humans

Scope

Eicosapentaenoic acid (EPA), abundant in oily fish, is reported to reduce skin inflammation and provide photoprotection, potential mechanisms include competition with arachidonic acid (AA) for metabolism by cyclooxygenases/lipoxygenases to less pro-inflammatory mediators. We thus examine impact of EPA intake on levels of AA, EPA and their resulting eicosanoids in human skin with or without ultraviolet radiation (UVR) challenge.

Methods and results

In a double-blind randomised controlled study, 79 females took 5 g EPA-rich or control lipid for 12 wk. Pre- and post-supplementation, red blood cell and skin polyunsaturated fatty acids were assessed by GC, and eicosanoids from unexposed and UVR-exposed skin by LC-MS/MS. Active supplementation increased red blood cell and dermal EPA versus control (both p < 0.001), lowering relative AA:EPA content (4:1 versus 15:1 and 5:1 versus 11:1, respectively; both p < 0.001). Pre-supplementation, UVR increased PGE₂, 12-hydroxyeicosatetraenoic acids, 12-HEPE (all p < 0.001) and PGE₃ (p < 0.05). Post-EPA, PGE₂ was reduced in unchallenged skin (p < 0.05) while EPA-derived PGE₃ (non-sign) and 12-HEPE (p < 0.01) were elevated post-UVR. Thus, post-EPA, PGE₂:PGE₃ was lower in unchallenged (12:1 versus 28:1; p < 0.05) and UVR exposed (12:1 versus 54:1; p < 0.01) skin; 12-hydroxyeicosatetraenoic acids:12-HEPE was lower in UVR-exposed skin (3:1 versus 11:1; p < 0.001).

Conclusion

Dietary EPA augments skin EPA:AA content, shifting eicosanoid synthesis towards less pro-inflammatory species, and promoting a regulatory milieu under basal conditions and in response to inflammatory insult.

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.

Nitric oxide-releasing sulindac is a novel skin cancer chemopreventive agent for UVB-induced photocarcinogenesis

Nitric oxide (NO)-releasing non-steroidal anti-inflammatory drugs (NO-NSAIDs) which have been synthesized to reduce gastro-intestinal and cardiovascular toxicities of NSAIDs, possess anti-proliferative, pro-apoptotic and anti-cancer activities. Here, we show that NO-sulindac inhibited UVB-induced skin tumorigenesis in SKH-1 hairless mice. Topical application of NO-sulindac reduced tumor incidence, number (p<0.05) and volume (p<0.005) as compared to UVB (alone)-irradiated vehicle-treated mice. An increase in TUNEL-positive cells in skin lesions was accompanied by the enhanced Bax:Bcl-2 ratio. The expression of pro-apoptotic Bax was increased whereas anti-apoptotic Bcl-2 reduced. However, proliferation was identified as the major target of NO-sulindac in this study. A reduced expression of PCNA and cyclin D1 associated with the dampening of cell cycle progression was observed. The mechanism of this inhibition was related to the reduction in UVB-induced Notch signaling pathway. UVB-induced inflammatory responses were diminished by NO-sulindac as observed by a remarkable reduction in the levels of phosphorylated MAP Kinases Erk1/2, p38 and JNK1/2. In this regard, NO-sulindac also inhibited NFκB by enhancing IκBα as evidenced by the reduced expression of iNOS and COX-2, the direct NFκB transcription target proteins. NO-sulindac significantly diminished the progression of benign lesions to invasive carcinomas by suppressing the tumor aggressiveness and retarding epithelial-mesenchymal transition. A marked decrease in the expression of mesenchymal markers such as Fibronectin, N-cadherin, SNAI, Slug and Twist and an increase in epithelial cell polarity marker E-cadherin were noted in NO-sulindac-treated tumors. Our data suggest that NO-sulindac is a potent inhibitor of UVB-induced skin carcinogenesis and acts by targeting proliferation-regulatory pathways.

Soybean isoflavone genistein regulates apoptosis through NF-κB dependent and independent pathways

Cyclooxygenase-2 (COX-2), the key enzyme of the conversion of arachidonic acid to prostaglandins is an important regulator of inflammation and perhaps apoptosis. Genistein is an active component of legumes and other related food associated with prevention of degenerative diseases possibly through modulating certain signaling pathways. It was investigated whether the induction of apoptosis with genistein was carried out via COX-2 suppression through the regulation of NF-κB. The cox-2 positive and negative cells were used to compare the effect of genistein on the modulation of NF-κB in COX-2 expressed or non-expressed genotypic systems. Suppression of COX-2 as well as decreasing NF-κB DNA binding activity was accompanied with the induction of apoptosis in genistein-treated COX-2 expressed cells. However, in cox-2 negative cells, apoptosis occurred without any involvement of NF-κB with genistein treatement. Genistein induced apoptosis through the generation of reactive oxygen species (ROS) both of cox-2 positive and negative cells. These results suggested that genistein is capable of exihibiting NF-κB-dependent and NF-κB-independent apoptotic control via ROS generation depending on genetic cell types.

siRNA-mediated knock-down of COX-2 in melanocytes suppresses melanogenesis

Cyclooxygenase-2 (COX-2) is an enzyme induced in response to multiple mitogenic and inflammatory stimuli, including UV light. UV-induced COX-2 expression induces production of prostaglandin E2 (PGE2) in keratinocytes, which mediates inflammation and cell proliferation. Until recently, studies regarding COX-2 and PGE2 in the skin have focused on keratinocytes and skin cancer and the effect of PGs produced by keratinocytes on melanocytes. However, the effects of COX-2 itself or COX-2 inhibitors on melanogenesis are not well known. Therefore, to establish the role of COX-2 in melanogenesis, we investigated the effects of knock-down of COX-2 in melanocytes on melanin production and the expression of melanogenic molecules through silencing of COX-2 expression with COX-2 short interfering RNA (siRNA). COX-2 knock-down in melanocytes decreased the expressions of tyrosinase, TRP-1, TRP-2, gp100 and MITF and also reduced tyrosinase enzyme activity. Furthermore, COX-2 siRNA-transfected melanocytes showed markedly reduced alpha-melanocyte stimulating hormone (α-MSH)-induced melanin production. In addition, α-MSH-induced COX-2 expression in both scrambled siRNA-transfected and COX-2 siRNA-transfected melanocytes was greater than α-MSH-untreated cells. Our results suggest that COX-2 might be a candidate target for the development of anti-melanogenic agents and α-MSH-induced pigmentation could be closely associated with COX-2 expression. COX-2 inhibitors might therefore be of particular use in whitening cosmetics for hyperpigmentation disorders such as melasma, postinflammatory hyperpigmentation and solar lentigo.

Ultraviolet C irradiation induces different expression of cyclooxygenase 2 in NIH 3T3 cells and A431 cells: the roles of COX-2 are different in various cell lines

Ultraviolet C (UVC) is a DNA damage inducer, and 20 J/m² of UVC irradiation caused cell growth inhibition and induced cell death after exposure for 24–36 h. The growth of NIH 3T3 cells was significantly suppressed at 24 h after UVC irradiation whereas the proliferation of A431 cells was inhibited until 36 h after UVC irradiation. UVC irradiation increased COX-2 expression and such up-regulation reached a maximum during 3–6 h in NIH 3T3 cells. In contrast, UVC-induced COX-2 reached a maximum after 24–36 h in A431 cells. Measuring prostaglandin E2 (PGE2) level showed a biphasic profile that PGE2 release was rapidly elevated in 1–12 h after UVC irradiation and increased again at 24 h in both cell lines. Treatment with the selective COX-2 inhibitor, SC-791, during maximum expression of COX-2 induction, attenuated the UVC induced-growth inhibition in NIH 3T3 cells. In contrast, SC-791 treatment after UVC irradiation enhanced death of A431 cells. These data showed that the patterns of UVC-induced PGE2 secretion from NIH 3T3 cells and A431 cells were similar despite the differential profile in UVC-induced COX-2 up-regulation. Besides, COX-2 might play different roles in cellular response to UVC irradiation in various cell lines.

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.

Role of PGE-type receptor 4 in auditory function and noise-induced hearing loss in mice

This study explored the physiological roles of PGE-type receptor 4 (EP4) in auditory function. EP4-deficient mice exhibited slight hearing loss and a reduction of distortion-product otoacoustic emissions (DPOAEs) with loss of outer hair cells (OHCs) in cochleae. After exposure to intense noise, these mice showed significantly larger threshold shifts of auditory brain-stem responses (ABRs) and greater reductions of DPOAEs than wild-type mice. A significant increase of OHC loss was confirmed morphologically in the cochleae of EP4-deficient mice. Pharmacological inhibition of EP4 had a similar effect to genetic deletion, causing loss of both hearing and OHCs in C57BL/6 mice, indicating a critical role for EP4 signaling in the maintenance of auditory function. Pharmacological activation of EP4 significantly protected OHCs against noise trauma, and attenuated noise-induced hearing loss in C57BL/6 mice. These findings suggest that EP4 signaling is necessary for the maintenance of cochlear physiological function and for cochlear protection against noise-induced damage, in particular OHCs. EP4 might therefore be an effective target for cochlear disease therapeutics.

The eicosanoid response to high dose UVR exposure of individuals prone and resistant to sunburn

High personal UVR doses can be gained during leisure activities, causing intense self-resolving inflammation (sunburn) of unprotected skin. UVR activates release of membrane fatty acids and upregulates their metabolism by cyclooxygenases (COX) and lipoxygenases (LOX) to different eicosanoids. While COX-derived prostaglandin (PG)E(2) is a potent mediator of sunburn vasodilatation, LOX-derived 15-hydroxyeicosatetraenoic acid (HETE) and its lipoxin metabolites may contribute to sunburn limitation. We explored the relationships between expression of these lipid mediators and the clinical and histological outcomes, comparing responses of individuals prone and more resistant to sunburn. An acute UVR exposure of 12 SED (standard erythema dose) was applied to buttock skin of 32 white Caucasians (n = 16 phototype I/II, n = 16 phototype III/IV), and over the subsequent 72 h assessments were made of skin erythema, immunohistochemical expression of leukocyte markers, COX-2, 12-LOX, 15-LOX and nitric oxide synthase (NOS), and eicosanoid levels by LC/ESI-MS/MS. Evidence of a significant inflammatory response was seen earlier in phototype I/II with regard to expression of erythema (4 h, p < 0.001), neutrophil infiltration (24 h, p = 0.01), epidermal COX-2 (24 h, p < 0.05) and 12-LOX (24 h, p < 0.01), and dermal eNOS (24 h, p < 0.05) proteins, although CD3+ lymphocyte infiltration showed an earlier increase in phototype III/IV (24 h, p < 0.05). Although erythema was equivalent at 72 h in both groups, phototype I/II showed higher PGE(2) accompanied by elevated 15-HETE, and a strong positive correlation was seen between these mediators (n = 18, r = 0.805, p = 0.0001). Hence anti-inflammatory eicosanoid 15-HETE may temper the pro-inflammatory milieu in sunburn, having greater influence in those prone to sunburn than those more resistant, given the same high UVR exposure conditions.

UVB radiation-induced β-catenin signaling is enhanced by COX-2 expression in keratinocytes

UVB radiation is the major carcinogen responsible for skin carcinogenesis, thus elucidation of the molecular pathways altered in skin in response to UVB would reveal novel targets for therapeutic intervention. It is well established that UVB leads to upregulation of cyclooxygenase 2 (COX-2) in the skin which contributes to skin carcinogenesis. Overexpression of COX-2 has been shown to promote colon cancer cell growth through β-catenin signaling, however, little is known about the connection between UVB, COX-2, and β-catenin in the skin. In the present study, we have identified a novel pathway in which UVB induces β-catenin signaling in keratinocytes, which is modulated by COX-2 expression. Exposure of the mouse 308 keratinocyte cell line (308 cells) and primary normal human epidermal keratinocytes (NHEKs) to UVB resulted in increased protein levels of both N-terminally unphosphorylated and total β-catenin. In addition, we found that UVB-enhanced β-catenin-dependent TOPflash reporter activity and expression of a downstream β-catenin target gene. We demonstrated that UVB-induced β-catenin signaling is modulated by COX-2, as treatment of keratinocytes with the specific COX-2 inhibitor NS398 blocked UVB induction of β-catenin. Additionally, β-catenin target gene expression was reduced in UVB-treated COX-2 knockout (KO) MEFs compared to wild-type (WT) MEFs. Furthermore, epidermis from UVB-exposed SKH-1 mice exhibited increased N-terminally unphosphorylated and total β-catenin protein levels and increased staining for total β-catenin, and both responses were reduced in COX-2 heterozygous mice. Taken together, these results suggest a novel pathway in which UVB induces β-catenin signaling in keratinocytes which is enhanced by COX-2 expression.

Cyclooxygenase-2-dependent phosphorylation of the pro-apoptotic protein Bad inhibits tonicity-induced apoptosis in renal medullary cells

During antidiuresis, cell survival in the renal medulla requires cyclooxygenase-2 (COX-2) activity. We have recently found that prostaglandin E2 (PGE2) promotes cell survival by phosphorylation and, hence, inactivation of the pro-apoptotic protein Bad during hypertonic stress in Madin-Darby canine kidney (MDCK) cells in vitro. Here we determine the role of COX-2-derived PGE(2) on phosphorylation of Bad and medullary apoptosis in vivo using COX-2-deficient mice. Both wild-type and COX-2-knockout mice constitutively expressed Bad in tubular epithelial cells of the renal medulla. Dehydration caused a robust increase in papillary COX-2 expression, PGE2 excretion, and Bad phosphorylation in wild-type, but not in the knockout mice. The abundance of cleaved caspase-3, a marker of apoptosis, was significantly higher in papillary homogenates, especially in tubular epithelial cells of the knockout mice. Knockdown of Bad in MDCK cells decreased tonicity-induced caspase-3 activation. Furthermore, the addition of PGE2 to cells with knockdown of Bad had no effect on caspase-3 activation; however, PGE2 caused phosphorylation of Bad and substantially improved cell survival in mock-transfected cells. Thus, tonicity-induced COX-2 expression and PGE2 synthesis in the renal medulla entails phosphorylation and inactivation of the pro-apoptotic protein Bad, thereby counteracting apoptosis in renal medullary epithelial cells.

Caffeine promotes ultraviolet B-induced apoptosis in human keratinocytes without complete DNA repair

In response to ultraviolet B damage, keratinocytes undergo apoptosis to eliminate damaged cells, thereby preventing tumorigenic transformation. Caffeine, the most widely consumed psychoactive substance, produces complex pharmacological actions; it has been shown to be chemopreventive in non-melamona skin cancer in mice through increasing apoptosis. Here we have investigated the molecular and cellular mechanisms in the pro-apoptotic effect of caffeine on UVB-irradiated human HaCaT keratinocytes. Pretreatment with caffeine increased UVB-induced apoptosis in HaCaT cells. Caffeine blocked UVB-induced Chk1 phosphorylation. In addition, similar to the effect of the PI3K inhibitor LY294002, caffeine also inhibited phosphorylation of AKT and up-regulation of COX-2, two critical oncogenic pathways in skin tumorigenesis. However, phosphorylation of EGFR or ERK was unaffected. Inhibiting ATR pathways by siRNA targeting ATR had little effect on UVB-induced apoptosis or AKT activation, indicating that the inhibitory effect of caffeine on apoptosis and the AKT pathway does not require the ATR pathway. Inhibiting AKT by caffeine blocked UVB-induced COX-2 up-regulation. Expression of constitutively active AKT that was not inhibited by caffeine was found to protect cells from caffeine-promoted apoptosis post-UVB irradiation, indicating that AKT is an essential inhibitory target for caffeine to promote apoptosis. Caffeine specifically sensitized cells with unrepaired DNA damage to UVB-induced apoptosis. These findings indicate that in HaCaT keratinocytes, inhibiting the AKT/COX-2 pathways through an ATR-independent pathway is a critical molecular mechanism by which caffeine promotes UVB-induced apoptosis of unrepaired keratinocytes for elimination.

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.

Cyclooxygenase-2 enhances antimicrobial peptide expression and killing of Staphylococcus aureus

Antimicrobial peptides such as human β-defensins (hBDs) and cathelicidins are critical for protection against infection and can be induced by activation of TLRs, a pathway that also activates cyclooxygenase(Cox)-2 expression. We hypothesized that Cox-2 is induced by TLR activation and is necessary for optimal AMP production, and that inhibitors of Cox-2 may therefore inhibit antimicrobial action. Normal human keratinocytes (NHEKs) stimulated with a TLR2/6 ligand, macrophage-activating lipopeptide-2, or a TLR3 ligand, polyinosinic-polycytidylic acid, increased Cox-2 mRNA and protein and increased PGE(2), a product of Cox-2. Treatment with a Cox-2 selective inhibitor (SC-58125) or Cox-2 small interfering RNA attenuated hBD2 and hBD3 production in NHEKs when stimulated with macrophage-activating lipopeptide-2, polyinosinic-polycytidylic acid, or UVB (15 mJ/cm(2)), but it did not attenuate vitamin D3-induced cathelicidin. SC-58125 also inhibited TLR-dependent NF-κB activation. Conversely, treatment with Cox-derived prostanoids PGD(2) or 15-deoxy-Δ(12,14)-PGJ(2) induced hBD3 or hBD2 and hBD3, respectively. The functional significance of these observations was seen in NHEKs that showed reduced anti-staphylococcal activity when treated with a Cox-2 inhibitor. These findings demonstrate a critical role for Cox-2 in hBD production and suggest that the use of Cox-2 inhibitors may adversely influence the risk for bacterial infection.

The prostaglandin E2 receptor, EP2, stimulates keratinocyte proliferation in mouse skin by G protein-dependent and {beta}-arrestin1-dependent signaling pathways

The prostaglandin E(2) (PGE(2)) G protein-coupled receptor (GPCR), EP2, plays important roles in mouse skin tumor development (Chun, K. S., Lao, H. C., Trempus, C. S., Okada, M., and Langenbach, R. (2009) Carcinogenesis 30, 1620-1627). Because keratinocyte proliferation is essential for skin tumor development, EP2-mediated signaling pathways that contribute to keratinocyte proliferation were investigated. A single topical application of the EP2 agonist, butaprost, dose-dependently increased keratinocyte replication via activation of epidermal growth factor receptor (EGFR) and PKA signaling. Because GPCR-mediated activation of EGFR can involve the formation of a GPCR-β-arrestin-Src signaling complex, the possibility of a β-arrestin1-Src complex contributing to EP2-mediated signaling in keratinocytes was investigated. Butaprost induced β-arrestin1-Src complex formation and increased both Src and EGFR activation. A role for β-arrestin1 in EP2-mediated Src and EGFR activation was demonstrated by the observation that β-arrestin1 deficiency significantly reduced Src and EGFR activation. In agreement with a β-arrestin1-Src complex contributing to EGFR activation, Src and EGFR inhibition (PP2 and AG1478, respectively) indicated that Src was upstream of EGFR. Butaprost also induced the activation of Akt, ERK1/2, and STAT3, and both β-arrestin1 deficiency and EGFR inhibition (AG1478 or gefitinib) decreased their activation. In addition to β-arrestin1-dependent EGFR activation, butaprost increased PKA activation, as measured by phospho-GSK3β (p-GSK3β) and p-cAMP-response element-binding protein formation. PKA inhibition (H89 or R(P)-adenosine-3',5'-cyclic monophosphorothioate (R(P)-cAMPS)) decreased butaprost-induced cAMP-response element-binding protein and ERK activation but did not affect EGFR activation, whereas β-arrestin1 deficiency decreased EGFR activation but did not affect butaprost-induced PKA activation, thus indicating that they were independent EP2-mediated pathways. Therefore, the results indicate that EP2 contributed to mouse keratinocyte proliferation by G protein-independent, β-arrestin1-dependent activation of EGFR and G protein-dependent activation of PKA.

Honokiol, a phytochemical from the Magnolia plant, inhibits photocarcinogenesis by targeting UVB-induced inflammatory mediators and cell cycle regulators: development of topical formulation

To develop newer and more effective chemopreventive agents for skin cancer, we assessed the effect of honokiol, a phytochemical from the Magnolia plant, on ultraviolet (UV) radiation-induced skin tumorigenesis using the SKH-1 hairless mouse model. Topical treatment of mice with honokiol in a hydrophilic cream-based topical formulation before or after UVB (180 mJ/cm(2)) irradiation resulted in a significant protection against photocarcinogenesis in terms of tumor multiplicity (28-60%, P < 0.05 to <0.001) and tumor volume per tumor-bearing mouse (33-80%, P < 0.05 to 0.001, n = 20). Honokiol also inhibited and delayed the malignant progression of papillomas to carcinomas. To investigate the in vivo molecular targets of honokiol efficacy, tumors and tumor-uninvolved skin samples from the tumor-bearing mice were analyzed for inflammatory mediators, cell cycle regulators and survival signals using immunostaining, western blotting and enzyme-linked immunosorbent assay. Treatment with honokiol significantly inhibited UVB-induced expression of cyclooxygenase-2, prostaglandin E(2) (P < 0.001), proliferating cell nuclear antigen and proinflammatory cytokines, such as tumor necrosis factor-α (P < 0.001), interleukin (IL)-1β (P < 0.01) and IL-6 (P < 0.001) in the skin as well as in skin tumors. Western blot analysis revealed that honokiol: (i) inhibited the levels of cyclins D1, D2 and E and associated cyclin-dependent kinases (CDKs)2, CDK4 and CDK6, (ii) upregulated Cip/p21 and Kip/p27 and (iii) inhibited the levels of phosphatidylinositol 3-kinase and the phosphorylation of Akt at Ser(473) in UVB-induced skin tumors. Together, our results indicate that honokiol holds promise for the prevention of UVB-induced skin cancer by targeting inflammatory mediators, cell cycle regulators and cell survival signals in UVB-exposed skin.

UV radiation inhibits 15-hydroxyprostaglandin dehydrogenase levels in human skin: evidence of transcriptional suppression

Elevated levels of prostaglandins (PG) have been detected in the skin following UV radiation (UVR). PGs play an important role in mediating both the acute and the chronic consequences of UVR exposure. UVR-mediated induction of cyclooxygenase-2 (COX-2) contributes to increased PG synthesis. In theory, reduced catabolism might also contribute to increased PG levels. 15-Hydroxyprostaglandin deyhdrogenase (15-PGDH), a tumor suppressor gene, plays a major role in PG catabolism. In this study, we investigated whether UVR exposure suppressed 15-PGDH while inducing COX-2 in keratinocytes and in human skin. UVR exposure caused dose-dependent induction of COX-2, suppression of 15-PGDH, and increased prostaglandin E(2) (PGE(2)) production in HaCaT cells. Exposure to UVR suppressed the transcription of 15-PGDH, resulting in reduced 15-PGDH mRNA, protein, and enzyme activities. UVR exposure induced Slug, a repressive transcription factor that bound to the 15-PGDH promoter. Silencing Slug blocked UVR-mediated downregulation of 15-PGDH. The effects of UVR were also evaluated in the EpiDerm skin model, a three-dimensional model of human epidermis. Here too, COX-2 levels were induced and 15-PGDH levels suppressed following UVR exposure. Next, the effects of UVR were evaluated in human subjects. UVR treatment induced COX-2 while suppressing 15-PGDH mRNA in the skin of 9 of 10 subjects. Collectively, these data suggest that reduced expression of 15-PGDH contributes to the elevated levels of PGs found in the skin following UVR exposure. Possibly, agents that prevent UVR-mediated downregulation of 15-PGDH will affect the acute or the long-term consequences of UVR exposure, including nonmelanoma skin cancer.

UV-induced squamous cell carcinoma--a role for antiapoptotic signalling pathways

The incidence of nonmelanoma skin cancer including squamous cell carcinoma (SCC) and basal cell carcinoma (BCC) has dramatically increased in the last decades, and chronic sun exposure was identified as a main etiologic agent. UV radiation may produce DNA damage either directly or through reactive oxygen species (ROS). As mutations caused by UV may lead to skin cancer due to oncogene activation and tumor suppressor gene inactivation, efficient safeguard mechanisms have been developed during evolution. These enclose induction of apoptosis and formation sunburn cells aiming at the removal of premalignant cells. The keratinocyte apoptotic machinery in response to UV consists of both intrinsic/mitochondrial and extrinsic/death receptor-mediated cell-death pathways, which are particularly regulated by mitogen-activated protein kinases (MAPKs, JNK and p38) and the tumor-suppressor protein p53. For development of skin cancer, it appears that critical steps in apoptosis control are dysregulated leading to resistance both to death ligand-mediated and intrinsic proapoptotic pathways. These particularly include inactivation of p53, as well as activation of EGFR, COX-2 and MAPKs, which result in specific regulation of Bcl-2 proteins, death ligands and death receptors. The final unravelling of apoptosis regulation in epithelial skin cancer may allow the development of new targeted therapeutic strategies.

Leptin deficiency-induced obesity exacerbates ultraviolet B radiation-induced cyclooxygenase-2 expression and cell survival signals in ultraviolet B-irradiated mouse skin

Obesity has been implicated in several inflammatory diseases and in different types of cancer. Chronic inflammation induced by exposure to ultraviolet (UV) radiation has been implicated in various skin diseases, including melanoma and nonmelanoma skin cancers. As the relationship between obesity and susceptibility to UV radiation-caused inflammation is not clearly understood, we assessed the role of obesity on UVB-induced inflammation, and mediators of this inflammatory response, using the genetically obese (leptin-deficient) mouse model. Leptin-deficient obese (ob/ob) mice and wild-type counterparts (C57/BL6 mice) were exposed to UVB radiation (120 mJ/cm(2)) on alternate days for 1 month. The mice were then euthanized and skin samples collected for analysis of biomarkers of inflammatory responses using immunohistochemistry, western blotting, ELISA and real-time PCR. Here, we report that the levels of inflammatory responses were higher in the UVB-exposed skin of the ob/ob obese mice than those in the UVB-exposed skin of the wild-type non-obese mice. The levels of UVB-induced cyclooxygenase-2 expression, prostaglandin-E(2) production, proinflammatory cytokines (i.e., tumor necrosis factor-alpha, interleukin-1beta, interleukin-6), and proliferating cell nuclear antigen and cell survival signals (phosphatidylinositol-3-kinase and p-Akt-Ser(473)) were higher in the skin of the ob/ob obese mice than the those in skin of their wild-type non-obese counterparts. Compared with the wild-type non-obese mice, the leptin-deficient obese mice also exhibited greater activation of NF-kappaB/p65 and fewer apoptotic cells in the UVB-irradiated skin. Our study suggests for the first time that obesity in mice is associated with greater susceptibility to UVB-induced inflammatory responses and, therefore, obesity may increase susceptibility to UVB-induced inflammation-associated skin diseases, including the risk of skin cancer.

Gq protein mediates UVB-induced cyclooxygenase-2 expression by stimulating HB-EGF secretion from HaCaT human keratinocytes

Ultraviolet (UV) radiation induces cyclooxygenase-2 expression to produce cellular responses including aging and carcinogenesis in skin. We hypothesised that heterotrimeric G proteins mediate UV-induced COX-2 expression by stimulating secretion of soluble HB-EGF (sHB-EGF). In this study, we aimed to elucidate the role and underlying mechanism of the alpha subunit of Gq protein (Galphaq) in UVB-induced HB-EGF secretion and COX-2 induction. We found that expression of constitutively active Galphaq (GalphaqQL) augmented UVB-induced HB-EGF secretion, which was abolished by knockdown of Galphaq with shRNA in HaCaT human keratinocytes. Galphaq was found to mediate the UVB-induced HB-EGF secretion by sequential activation of phospholipase C (PLC), protein kinase Cdelta (PKCdelta), and matrix metaloprotease-2 (MMP-2). Moreover, GalphaqQL mediated UVB-induced COX-2 expression in an HB-EGF-, EGFR-, and p38-dependent manner. From these results, we concluded that Galphaq mediates UV-induced COX-2 expression through activation of EGFR by HB-EGF, of which ectodomain shedding was stimulated through sequential activation of PLC, PKCdelta and MMP-2 in HaCaT cells.

Mechano-transduction in osteoblastic cells involves strain-regulated estrogen receptor alpha-mediated control of insulin-like growth factor (IGF) I receptor sensitivity to Ambient IGF, leading to phosphatidylinositol 3-kinase/AKT-dependent Wnt/LRP5 receptor-independent activation of beta-catenin signaling

The capacity of bones to adjust their mass and architecture to withstand the loads of everyday activity derives from the ability of their resident cells to respond appropriately to the strains engendered. To elucidate the mechanisms of strain responsiveness in bone cells, we investigated in vitro the responses of primary mouse osteoblasts and UMR-106 osteoblast-like cells to a single period of dynamic strain. This stimulates a cascade of events, including activation of insulin-like growth factor I receptor (IGF-IR), phosphatidylinositol 3-kinase-mediated phosphorylation of AKT, inhibition of GSK-3β, increased activation of β-catenin, and associated lymphoid-enhancing factor/T cell factor-mediated transcription. Initiation of this pathway does not involve the Wnt/LRP5/Frizzled receptor and does not culminate in increased IGF transcription. The effect of strain on IGF-IR is mimicked by exogenous des-(1–3)IGF-I and is blocked by the IGF-IR inhibitor H1356. Inhibition of strain-related prostanoid and nitric oxide production inhibits strain-related (and basal) AKT activity, but their separate ectopic administration does not mimic it. Strain-related IGF-IR activation of AKT requires estrogen receptor α (ERα) with which IGF-1R physically associates. The ER blocker ICI 182,780 increases the concentration of des-(1–3)IGF-I necessary to activate this cascade, whereas estrogen inhibits both basal AKT activity and its activation by des-(1–3)IGF-I. These data suggest an initial cascade of strain-related events in osteoblasts in which strain activates IGF-IR, in association with ERα, so initiating phosphatidylinositol 3-kinase/AKT-dependent activation of β-catenin and altered lymphoid-enhancing factor/T cell factor transcription. This cascade requires prostanoid/nitric oxide production and is independent of Wnt/LRP5.

PGE2 receptor EP2 mediates the antagonistic effect of COX-2 on TGF-beta signaling during mammary tumorigenesis

The molecular mechanisms that enable cyclooxygenase-2 (COX-2) and its mediator prostaglandin E2 (PGE2) to inhibit transforming growth factor-beta (TGF-beta) signaling during mammary tumorigenesis remain unknown. We show here that TGF-beta selectively stimulated the expression of the PGE2 receptor EP2, which increased normal and malignant mammary epithelial cell (MEC) invasion, anchorage-independent growth, and resistance to TGF-beta-induced cytostasis. Mechanistically, elevated EP2 expression in normal MECs inhibited the coupling of TGF-beta to Smad2/3 activation and plasminogen activator inhibitor-1 (PAI1) expression, while EP2 deficiency in these same MECs augmented Smad2/3 activation and PAI expression stimulated by TGF-beta. Along these lines, engineering malignant MECs to lack EP2 expression prevented their growth in soft agar, restored their cytostatic response to TGF-beta, decreased their invasiveness in response to TGF-beta, and potentiated their activation of Smad2/3 and expression of PAI stimulated by TGF-beta. More important, we show that COX-2 or EP2 deficiency both significantly decreased the growth, angiogenesis, and pulmonary metastasis of mammary tumors produced in mice. Collectively, this investigation establishes EP2 as a potent mediator of the anti-TGF-beta activities elicited by COX-2/PGE2 in normal and malignant MECs. Our findings also suggest that pharmacological targeting of EP2 receptors may provide new inroads to antagonize the oncogenic activities of TGF-beta during mammary tumorigenesis.-Tian, M., Schiemann, W. P. PGE2 receptor EP2 mediates the antagonistic effect of COX-2 on TGF-beta signaling during mammary tumorigenesis.