Targeted disruption of signal transducer and activator of transcription 3 (Stat3) in bulge region keratinocyte stem cells inhibits chemically-induced skin carcinogenesis

Abstract

2501

Keratinoctye stem cells (KSCs) located in the bulge region of hair follicles are self-renewing cells that have slowly cycling and quiescent characteristics. KSCs maintain hair growth and epidermal homeostasis by providing transit amplifying cells that differentiate into cutaneous lineage cells. Studies focusing on KSCs as the origin of skin tumors have suggested that KSCs, especially those in the bulge region of hair follicles, could be a potential target of skin carcinogenesis. Recently, we have shown that Stat3 is required for both the initiation and promotion stages of skin carcinogenesis. During initiation, Stat3 appears to be critical for survival of DNA-damaged keratinocytes, especially those found in the bulge region. During tumor promotion, Stat3 appears to be critical for proliferation of keratinocytes and clonal expansion of initiated cells. To determine functional roles of Stat3 in bulge region KSC survival and proliferation during skin carcinogenesis, we generated inducible bulge region KSC-specific Stat3 deficient mice (K15.CrePR1.Stat3^fl/fl mice) that allow targeted disruption of Stat3 in KSCs following RU486 treatment. Following treatment with DMBA, Ha-ras codon 61 A¹⁸²→T mutations were significantly decreased in bulge region KSCs isolated from K15.CrePR1.Stat3^fl/fl mice treated with RU486 compared with those of nontransgenic littermates. In addition, inducible deletion of Stat3 prior to DMBA treatment significantly increased the number of apoptotic keratinocytes in the bulge region compared with nontransgenic littermates. Furthermore, following treatment with TPA, α6- and CD34-positive KSCs isolated from hair follicles of K15.CrePR1.Stat3^fl/fl mice treated with RU486 showed reduced proliferative response compared with those of nontransgenic littermates, as analyzed by FACS. In two-stage skin carcinogenesis experiments, both tumor incidence and the average number of papillomas per mouse were significantly reduced (~80% reduction in the average number of papillomas per mouse) in K15.CrePR1.Stat3^fl/fl mice treated with RU486 prior to initiation compared with similarly treated control mice. Collectively, these results provide strong evidence that Stat3 is required for survival of bulge region KSCs during initiation with DMBA. These data also support the hypothesis that bulge region KSCs contribute significantly to skin tumor development in this model of epithelial carcinogenesis.