Molecular and cellular basis for tumor promotion in mouse skin

Mouse skin painting studies have indicated that initiation-promotion protocols primarily yield benign tumors of monoclonal origin. The conversion of benign papillomas to carcinomas is not accelerated by promoters but is enhanced by mutagens. In contrast to initiation-promotion protocols, repeated application of complete carcinogens to mouse skin results in numerous carcinomas, and many of these cancers are multiclonal. These biological differences in tumor formation resulting from a promotion stage, as well as the reversibility of this stage, suggest an epigenetic mechanism of action for promoting agents. The monoclonal nature of the benign tumor indicates that cell selection may play an important role in tumor promotion. The ability to culture mouse epidermal cells has provided a model for delineating the mechanism of action of initiators and promoters. Exposure of cultured mouse keratinocytes to initiating agents results in cellular foci with altered behavior in which the cells resist the inhibitory growth signal associated with the induction of terminal differentiation. Such cells are also resistant to the induction of terminal differentiation in response to phorbol ester tumor promoters whereas subpopulations of normal keratinocytes are accelerated to differentiate by these agents. These differential responses to phorbol esters may be mediated by qualitative differences in phorbol ester receptors among keratinocyte subpopulations. Initiated cells appear to have unique phorbol ester receptor characteristics. Using oncogenic retroviruses as vectors, studies have been performed to assess the role of ras oncogene expression in initiation and promotion. An activated ras gene induces dramatic increases in epidermal basal cell proliferation, but such cells differ from initiated cells in that growth ceases in response to differentiation signals. While promoters can also enhance basal cell proliferation, they do not seem to induce a major increase in ras gene expression. We propose that the genetic change in the differentiation program induced by initiators provides a basis for selective clonal expansion of the initiated population by promoters to form a benign tumor. A subsequent genetic change, perhaps in the growth control program, may lead to malignant conversion. Altered expression of a ras gene may be relevant in this progression.