Chemical carcinogenesis studies in mouse epidermal cell cultures

Studies of tumor induction on mouse skin have provided insight into the basis biology of chemical carcinogenesis, but molecular mechanisms have been more difficult to elucidate. Mouse epidermal cell cultures have proven to be a valuable model for performing mechanistic studies. Previous data have indicated that such cultures proliferate and differentiate in a manner highly analogous to epidermis in vivo. In addition, carcinogen metabolism, DNA repair, and responses to tumor promoters are quite similar in mouse skin in vivo and in vitro. Recent data have extended these observations toward defining the biological characteristics of initiated cells and elucidating the mechanism of action of promoters and antipromoters. When mouse epidermis is cultured under conditions of low extracellular Ca++, proliferation is enhanced and terminal differentiation is inhibited. Addition of Ca++ induces terminal differentiation. If cells are treated with carcinogens under low Ca++ conditions and subsequently switched to standard Ca++, cell colonies which do not terminally differentiate evolve. Such colonies continue to synthesize keratin, are subculturable, and may represent preneoplastic cells. In other experiments, epidermal cells derived from mouse skin treated with carcinogens in vivo also demonstrate prolonged in vitro survival and subculturability while controls have a limited lifespan. Such studies suggest that biological alterations can be detected in epidermal cells exposed to carcinogens well before and the phenotypic expression of neoplasia. Exposure of epidermal cells to phorbol-ester tumor promoters induces ornithine decarboxylase (ODC). This induction is enhanced by corticosteroids and markedly inhibited by retinoids. Ultraviolet light also induces ODC in epidermal cells, but kinetic studies suggest that the early pathway of induction (afferent to the nucleus) is different from that of phorbol esters. The later pathways (efferent from the nucleus-i.e., transcription and translation) appear to be similar. Retinoids have only a minor suppressive effect on ODC induction by UV while corticosteroids enhance UV induction to the same extent as seen with phorbol esters These results suggest that the site of retinoids is in the afferent pathway while steroids act on the efferent pathway.