Over one million new cases of UVB-induced non-melanoma skin cancer are diagnosed yearly in the US. Cellular response to UVB results in induction of cyclooxygenase-2 (COX-2), a key enzyme in the conversion of arachidonic acid to prostaglandins, and overexpression of COX-2 is implicated in many forms of cancer, including skin cancer. Our laboratory and others have reported that COX-2 expression is induced in keratinocytes and skin following UVB exposure. COX-2 and prostaglandin production have been shown by our Co-investigator Susan Fischer to be critical for murine skin carcinogenesis. The [unreadable]-catenin signaling cascade is another important signaling pathway involved in tumorigenesis in a variety of tissues. Recently, Castellone et al. reported that COX-2 and its metabolite PGE2 promoted the growth of colon cancer cells by signaling through [unreadable]-catenin. To the best of our knowledge, this link between COX-2 and [unreadable]-catenin signaling, where COX-2 is induced by UVB exposure, has yet to be identified in normal epidermis. Our laboratory has obtained preliminary evidence that treatment of normal human epidermal keratinocytes (NHEKs) with UVB radiation resulted in increased levels of the signaling form of [unreadable]-catenin (active [unreadable]-catenin), concurrent with elevated COX- 2 expression. Given that UVB radiation induces COX-2 expression and PGE2 production, and our preliminary evidence that PGE2 treatment of NHEKs results in increased expression of active [unreadable]-catenin, we propose to test the novel hypothesis that exposure to UVB radiation results in increased [unreadable]-catenin signaling in the skin, which is dependent on COX-2 expression, and contributes to inappropriate proliferation of keratinocytes, leading to skin carcinogenesis. We propose to test this hypothesis in cell-based studies using primary mouse and human keratinocytes, as well as in mouse epidermis in vivo using COX-2+/- heterozygote mice and EP2-/- knockout mice and in human skin tumor samples. Furthermore, we will employ a novel strategy for transdermal delivery of the [unreadable]-catenin inhibitor, ICAT, to test whether inhibition of [unreadable]-catenin signaling attenuates UVB-induced hyperplasia. Aim #1 will investigate the effect of UVB on [unreadable]-catenin signaling in cultured mouse and human keratinocytes to determine if UVB-induction of active [unreadable]-catenin is transcriptionally functional, is dependent on COX-2 expression, and is part of a positive feedback loop. Aim #2 will investigate the effect of UVB radiation on [unreadable]-catenin signaling and subsequent effects on epidermal proliferation in vivo. This Aim will also establish whether UVB-induced [unreadable]-catenin signaling in mouse skin is dependent on COX-2 expression in vivo and whether a positive feedback loop between [unreadable]-catenin and COX-2 exists in vivo. Aim #3 will investigate the involvement of [unreadable]-catenin signaling in UVB-induced skin cancer and determine if inhibition of [unreadable]-catenin signaling can prevent UVB-induced tumors. Demonstrating the existence of a "UVB/COX-2/[unreadable]-catenin axis" in epidermis and identifying its contribution to skin carcinogenesis would have significant impact in the fields of normal skin biology and skin cancer.