Age-dependent skin tumorigenesis and transgene expression in the Tg.AC (v-Ha-ras) transgenic mouse

Transgenic Tg,AC (v-Ha-ras)mice develop skin tumors in response to specific carcinogens and tumor promoters. The Tg,AC mouse carries the coding sequen ce of v-Ha ras, linked to a zeta -globin promoter and an SV40 polyadenylati on signal sequence, The transgene confers on these mice the property of gen etically initiated skin. This study examines the age-dependent sensitivity of the incidence of skin papillomas in Tg,AC mice exposed to topically appl ied 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment, full thickness sk in wounding or UV radiation. Skin tumor incidence and multiplicity were str ongly age-dependent, increasing with increasing age of the animal when firs t treated at 5, 10, 21 or 32 weeks of age. Furthermore, the temporal induct ion of transgene expression in keratinocytes isolated from TPA-treated mous e skin was also influenced by the age of the mice. Transgene expression was seen as early as 14 days after the start of TPA treatment in mice that wer e 10-32 weeks of age, but was not detected in similarly treated 5-week old mice. When isolated keratinocytes were fractionated by density gradient cen trifugation the highest transgene expression was found in the denser basal keratinocytes, Transgene expression could be detected in the denser keratin ocyte fraction as early as 9 days from start of TPA treatment in 32-week ol d mice. Using flow cytometry, a positive correlation was observed between e xpression of the v-sa-ras transgene and enriched expression of the cell sur face protein pl-integrin, a putative marker of epidermal stem cells, This r esult suggests that, in the Tg,AC mouse, an age-dependent sensitivity to tu mor promotion and the correlated induction of transgene expression are rela ted to changes in cellular development in the follicular compartment of the skin.