Tumor Promotion: A Problem of Differential Responses of Normal and Neoplastic Cells to Trophic Stimuli

Increased understanding of the tumor promotion phase of chemical carcinogenesis has resulted from the discovery of pure tumor promoting-agents and the development of animal models in which tumor promoters modify experimental carcinogenesis (1). Simultaneously, interest in this area of cancer research has been stimulated by epidemiological studies indicating that a promotion phase is important in the development of human cancer (2). Tumor promoters cause or allow the expression of the latent tumor phenotype induced in some cells by limited doses of carcinogens. Commonly, the model systems where tumor promotion plays an important role in neoplastic development are those involving epithelial tissues, in particular those composed of more than one cell type or cells in different states of maturation such as skin, breast and bronchus. As a classical model for carcinogenesis in a lining epithelium, mouse skin has provided most of the conceptual framework regarding the biology of tumor promotion (1,3).