Development of phorbol ester responsiveness in neonatal mouse epidermis: correlation between hyperplastic response and sensitivity to first-stage tumor promotion.

As compared with the adult state, neonatal mouse skin (strain NMRI) has a hyperplastic appearance which gradually changes into the mature type between postnatal day 3 and 10. Concomitantly, the late fetal and neonatal keratin polypeptide pattern is replaced by the mature pattern. As long as the adult type of epidermal differentiation is not sufficiently developed (i.e., prior to postnatal day 5), topical application of the phorbol ester tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA) causes neither morphological alterations nor a measurable induction of cellular proliferation and ornithine decarboxylase activity. TPA application at day 7 evokes, however, (i) a hyperplastic reaction followed by a massive 'psoriasis-like' hyperkeratosis, (ii) an increase of ornithine decarboxylase activity and (iii) a restoration of the neonatal keratin polypeptide pattern. Multistage tumorigenesis experiments carried out with prenatally initiated mice show that during the early period of development (prior to postnatal day 5) mouse skin is also resistant to the effects of TPA as a stage I tumor promoter. Since both the hyperplastic response and the sensitivity to tumor promotion develops in a strictly parallel manner, reactions involved in the induction of epidermal hyperplasia are assumed to provide an important condition of stage I skin tumor promotion.