p38 MAP kinase plays a functional role in UVB-induced mouse skin carcinogenesis

UVB irradiation of epidermal keratinocytes results in the activation of the p38 mitogen-activated protein kinase (MAPK) pathway and subsequently activator protein-1 (AP-1) transcription factor activation and cyclooxygenase-2 (COX-2) expression. AP-1 and COX-2 have been shown to play functional roles in UVB-induced mouse skin carcinogenesis. In this study, the experimental approach was to express a dominant negative p38α MAPK (p38DN) in the epidermis of SKH-1 hairless mice and assess UVB-induced AP-1 activation, COX-2 expression, and the skin carcinogenesis response in these mice compared to wild-type littermates. We observed a significant inhibition of UVB-induced AP-1 activation and COX-2 expression in p38DN transgenic mice, leading to a significant reduction of UVB-induced tumor number and growth compared to wild-type littermates in a chronic UVB skin carcinogenesis model. A potential mechanism for this reduction in tumor number and growth rate is an inhibition of chronic epidermal proliferation, observed as reduced Ki-67 staining in p38DN mice compared to wild-type. Although we detected no difference in chronic apoptotic rates between transgenic and nontransgenic mice, analysis of acutely irradiated mice demonstrated that expression of the p38DN transgene significantly inhibited UVB-induced apoptosis of keratinocytes. These results counter the concerns that inhibition of p38 MAPK in a chronic situation could compromise the ability of the skin to eliminate potentially tumorigenic cells. Our data indicate that p38 MAPK is a good target for pharmacological intervention for UV-induced skin cancer in patients with sun damaged skin, and suggest that inhibition of p38 signaling reduces skin carcinogenesis by inhibiting COX-2 expression and proliferation of UVB-irradiated cells.