| Summary |
The most common type of cancer in the United States is non-melanoma skin cancer (NMSC), with an estimated two million new cases diagnosed each year. Numerous malignancies including NMSC, colon, breast, and prostate cancer overexpress cyclooxygenase-2 (COX-2). COX-2 catalyzes the synthesis of prostaglandins including prostaglandin E2 (PGE₂) which activates E-type prostaglandin (EP) receptors and promotes tumor development when overexpressed. Pharmacological inhibition or genetic deletion of COX-2 significantly decreases skin carcinogenesis mediated by the two-stage chemical carcinogenesis model using 7, 12-dimethylbenz[a]anthracene (DMBA) and 12-O-tetradecanoylphorbol-13-acetate (TPA) or by UVB-light. Apigenin is a non-mutagenic bioflavonoid present in various foods. Previous work in our lab has shown that apigenin blocks TPA- and UV-light induced COX-2 expression in vitro and apigenin also inhibits skin carcinogenesis; however, the mechanism by which apigenin blocks tumor formation is unclear. These observations suggest that apigenin prevents chemically-induced tumor formation through the inhibition of the COX-2/PGE₂ pathway. In specific aim 1, our goal was to determine the acute effect of apigenin on TPA-induced COX-2 expression, PGE2 production, and EP receptor expression in vivo to gain a better understanding of the molecular mechanisms that may be responsible for its antitumor activity. Female SKH-1 hairless mice were pretreated with apigenin or vehicle one hour before exposure to TPA. Animals were euthanized 8 hours later and the epidermis isolated, pooled, and analyzed for COX-2 expression, PGE₂ production, and EP receptor expression. Apigenin significantly inhibited COX-2 expression, PGE₂ production, and EP1 receptor expression in the epidermis. In specific aim 2, our goal was to determine if apigenin-mediated blockade of chemically-induced skin Tumorigenesis occurs as a result of the inhibition of COX-2 expression, PGE₂ production, and EP receptor expression. Skin carcinogenesis was induced in animals using the two-stage chemical carcinogenesis model. Female SKH-1 mice were exposed to a single topical dose of DMBA (tumor initiating agent) followed by biweekly applications of TPA (tumor promoting agent) for 25 weeks. Animals were treated one hour prior to TPA exposure with apigenin or vehicle. At the end of the study, the number of tumors and the size of each tumor were determined. Tumors and surrounding epidermal tissue were then harvested, classified, and subjected to immunohistochemical analysis. Apigenin caused a dose-dependent decrease in tumor multiplicity and incidence in animals exposed to DMBA/TPA. Apigenin significantly inhibited the chemically-mediated increase in COX-2 expression, PGE₂ production, and EP receptor expression in skin surrounding the tumors whereas all tumors overexpressed COX-2. In addition, apigenin increased terminal differentiation and decreased proliferation in the surrounding epidermis of tumor bearing animals. Collectively, our findings demonstrate that apigenin suppresses chemically-induced tumor formation and also inhibits COX-2 expression, PGE₂ production, and EP receptor expression in the surrounding epidermis. Inhibition of the COX-2/PGE₂ pathway by apigenin may be responsible for its antitumor activity thus supporting the development of apigenin or apigenin-derivatives as chemopreventive agents for NMSC. |
