THAPSIGARGIN, A WEAK SKIN TUMOR PROMOTER, ALTERS THE GROWTH AND DIFFERENTIATION OF MOUSE KERATINOCYTES IN CULTURE

Thapsigargin (Tg), applied twice weekly to the backs of CD-1 mice init iated with 7,12-dimethylbenz[a]anthracene, promoted papillomas on the skin of similar to 50% of the mice, Tg alone induced papillomas in 10% of the mice, Although Tg and 12-O-tetradecanoylphorbol-13-acetate (TP A) are synergistic in a keratinocyte co-culture assay for promotion, s kin tumor promotion by TPA was inhibited by co-treatment with Tg, Trea tment of cultured keratinocytes with non-toxic doses of Tg increased t he level of intracellular free Ca2+ and delayed Ca2+-induced stratific ation, Tg blocked expression of the spinous layer differentiation mark er keratin 1 (K1), while inducing cornification, a marker of different iation in the granular layer/stratum corneum, In medium with 1.4 mM Ca 2+, Tg prolonged keratinocyte proliferation and selected foci of monol ayer cells. A Tg-independent cell line, TK-1, was developed from a sin gle 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 ski n resulted in the development of dysplastic papillomas with a high rat e of progression to squamous cell carcinomas.