Skin cancer is currently the most common type of human cancer in Americans. Myricetin, a naturally occurring phytochemical, has potent anticancer-promoting activity and contributes to the chemopreventive potential of several foods, including red wine. Here, we show that myricetin suppresses UVB-induced cyclooxygenase-2 (COX-2) expression in mouse skin epidermal JB6 P+ cells. The activation of activator protein-1 and nuclear factor-KB induced by UVB was dose-dependently inhibited by myricetin treatment. Western blot and kinase assay data revealed that myricetin inhibited Fyn kinase activity and subsequently attenuated UVB-induced phosphorylation of mitogen-activated protein kinases. Pull-down assays revealed that myricetin competitively bound with ATP to suppress Fyn kinase activity. Importantly, myricetin exerted similar inhibitory effects compared with 4-amino-5-(4-chloro-phenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimiidine, a well-known pharmacologic inhibitor of Fyn. In vivo mouse skin data also revealed that myricetin inhibited Fyn kinase activity directly and subsequently attenuated UVB-induced COX-2 expression. Mouse skin tumorigenesis data clearly showed that pretreatment with myricetin significantly suppressed UVB-induced skin tumor incidence in a dose-dependent manner. Docking data suggest that myricetin is easily docked to the ATP-binding site of Fyn, which is located between the N and C lobes of the kinase domain. Overall, these results indicated that myricetin exerts potent chemopreventive activity mainly by targeting Fyn in skin carcinogenesis. [Cancer Res 2008;68(14):6021-30]

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 P1 mouse skin epidermal (JB6 P1) 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-kB 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 UVBinduced COX-2 expression mediated through the phosphorylation of MAPKs in JB6 P1 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.

Luteolin, a flavonoid present in various vegetables including onion and broccoli, has been reported to possess anticarcinogenic effects. However, its chemopreventive effect on UV-induced skin cancer and its mechanism are not fully understood. Herein, we examined the chemopreventive effect and associated mechanisms of luteolin in the JB6 P+ cell line and the SKH-1 hairless mouse model. Luteolin suppressed UVB-induced cyclooxygenase-2 expression and activator protein-1 and nuclear factor-κB activity in JB6 P+ cells. Immunoblot and kinase assay data showed that luteolin attenuated protein kinase Cε (PKCε) and Src kinase activities and subsequently inhibited UVB-induced phosphorylation of mitogen-activated protein kinases and the Akt signaling pathway. In addition, pull-down assays revealed that luteolin binds directly to PKCε and Src in an ATP-competitive manner. Importantly, luteolin suppressed tumor incidence, multiplicity, and overall size in SKH-1 hairless mice. Analysis of the skin by immunohistochemistry and immunoblotting showed that luteolintreated groups had a substantial reduction in the levels of cyclooxygenase-2, tumor necrosis factor-α, and proliferating cell nuclear antigen compared with groups treated with only UVB. Further analysis using skin lysates showed that luteolin inhibited PKCε and Src kinase activity. Together, these data suggest that luteolin exerts potent chemopreventive activity against UVB-induced skin cancer mainly by targeting PKCε and Src.