Studies on chemical transformation in vitro have been based primarily on fibroblastic cells. While these results have led to insights into the processes of neoplastic transformation, the majority of human cancers are derived from cells of epithelial origin. Many investigators have attempted to develop a transformation assay based on epithelial cells. Although some systems have proved to be relatively successful for assaying chemical carcinogens, a reproducible assay based on a well-characterized epithelial cell line does not yet exist. Recently, an epidermal keratinocyte cell line of murine origin (Balb/MK) has been developed which possesses stable genetic properties useful for transformation studies. Balb/MK cells have an absolute requirement for epidermal growth factor (EGF) for their continued proliferation. They also will undergo terminal differentiation when exposed to high levels of extracellular calcium. Previous studies have shown that infection of these cells by a variety of mammalian retroviruses leads to an abrogation of the EGF requirement as well as a block in the calcium-induced terminal differentiation pathway. Thus, these cells are potentially useful as a general model for epithelial cell transformation. Initial studies have shown that the treatment of Balb/MK cells with the chemical carcinogen 20'-methylcholanthrene (MCA) leads to the isolation of EGF-independent cell lines in a dose-dependent manner. In addition, these chemically treated cells are also blocked in their ability to respond to calcium-induced terminal differentiation. Therefore, it is proposed that the Balb/MK cell line can be used as the basis for a chemical transformation assay for epithelial cells. The validity of the system will be examined by comparing the effects of known chemical carcinogens in vivo with their effects on the Balb/MK cells. The growth properties of the chemically transformed Balb/MK cells will be characterized by known in vitro parameters as well as assays for tumorigenic potential. The effects of carcinogen treatment on the differentiation phenotype of these cells will be examined by the use of known markers of epidermal differentiation. Finally, the interaction between viral and chemical transformation of these cells will be assessed.
