Reduction of IKKα Expression Promotes Chronic Ultraviolet B Exposure-Induced Skin Inflammation and Carcinogenesis

Ultraviolet B light (UVB) is a common cause of human skin cancer. UVB irradiation induces mutations in the tumor suppressor p53 gene as well as chronic inflammation, which are both essential for UVB carcinogenesis. Inhibitor of nuclear factor ®B kinase-α (IKKα) plays an important role in maintaining skin homeostasis, and expression of IKKα was found to be down-regulated in human and murine skin squamous cell carcinomas. However, the role of IKKα in UVB skin carcinogenesis has not been investigated. Thus, here we performed UVB carcinogenesis experiments on Ikkα+/+ and Ikkα+/− mice. Ikkα+/− mice were found to develop a twofold greater number of skin tumors than Ikkα+/+ mice after chronic UVB irradiation. In addition, tumor latency was significantly shorter and tumors were bigger in Ikkα+/− than in Ikkα+/+ mice. At an early stage of carcinogenesis, an increase in UVB-induced p53 mutations as well as macrophage recruitment and mitogenic activity, and a decrease in UVB-induced apoptosis, were detected in Ikkα+/− compared with those in Ikkα+/+ skin. Also, reduction of IKKα levels in keratinocytes up-regulated the expression of monocyte chemoattractant protein-1 (MCP-1/CCL2), TNFα, IL-1, and IL-6, and elevated macrophage migration, which might promote macrophage recruitment and inflammation. Therefore, these findings suggest that reduction of IKKα expression orchestrates UVB carcinogen, accelerating tumorigenesis.