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Thapsigargin, a weak skin tumor promoter, alters the growth and differentiation of mouse keratinocytes in culture
Laboratory of Cellular Carcinogenesis and Tumor Promotion, National Cancer Institute Bethesda, MD 20892, USA
1To whom correspondence should be addressed
Thapsigargin (Tg), applied twice weekly to the backs of CD-1 mice initiated with 7, 12-dimethylbenz(a)anthracene, promoted papillomas on the skin of
50% of the mice. Tg alone induced papillomas in 10% of the mice. Although Tg and 12-O-tetradecanoylphorbol-13-acetate (TPA) are synergistic in a keratinocyte co-culture assay for promotion, skin tumor promotion by TPA was inhibited by co-treatment with Tg. Treatment of cultured keratinocytes with non-toxic doses of Tg increased the level of intracellular free Ca2+ and delayed Ca2+-induced stratification. Tg blocked expression of the spinous layer differentiation marker keratin 1 (K1), while inducing cornification, a marker of differentiation in the granular layer/stratum corneum. In medium with 1.4 mM Ca2+, Tg prolonged keratinocyte proliferation and selected foci of monolayer cells. A Tg-independent cell line, TK-1, was developed from a single dish in which foci continued to expand after Tg removal. Grafting TK-1 cells on to the backs of nude mice as part of a reconstituted skin resulted in the development of dysplastic papillomas with a high rate of progression to squamous cell carcinomas.
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