Cell differentiation and tumor-promoting action in skin carcinogenesis.

The early ultrastructural changes induced by the tumor promoter 12- O -tetradecanoyl-phorbol-13-acetate in mouse epidermis previously transformed by treatment with an initiating dose of 7,12-dimethylbenz(α)anthracene are described and compared with those in the basal cells of papillomas and carcinoma that appear later after repeated promoter treatment. Groups of cells with fine structural characteristics distinct from those of the other epidermal cells are observed 48 hr after a single application of the promoter to skin previously treated with an initiating dose of 7,12-dimethylbenz(α)anthracene. With repeated weekly treatment of the promoter, progressively larger groups of the atypical cells are observed, forming small papillomas by the fifth week. The striking similarity between the fine structural features of the atypical basal cells in the focal groups and those of the basal cells in papillomas and carcinomas, which develop later, suggests that these groups of phenotypically altered cells observed early after promotion are precursor neoplastic cells from which papilloma and carcinoma induced by the two-stage technique of skin carcinogenesis are derived. The absence of these phenotypically altered basal cells after treatment with a single, initiating dose of 7,12-dimethylbenz(α)anthracene, indicates that these phenotypic characteristics are acquired very early after treatment with the promoter and suggests that the phenotypic expression of the neoplastically transformed cells is the essential event of tumor-promoting action. On this basis, it is proposed that the events occurring during promotion can be dissociated into two main components. First, there are the changes induced by the promoter in the epidermal stem cells or in their microenvironment that enable the expression of the neoplastic phenotype. Second, there are the specific phenotypic alterations that occur in the transformed cells in response to promoting stimuli and the subsequent changes in these phenotypic properties during progression of the neoplastic process. The key to understanding tumor-promoting action is the ability to determine which of the many changes induced by promoters are essential for the phenotypic expression of neoplastic transformation. Two main possibilities are suggested to account for the phenotypic alterations observed in the basal cells of the atypical focal groups, papilloma and carcinoma. First, they are mutant-like cells induced to express their new phenotypic properties by the tumor promoter. Second, these phenotypic alterations in the neoplastic stem cells are due to loss of differentiated traits in the course of carcinogenesis and the acquisition of features found in embryonic cells. Whatever the nature of these phenotypic changes, they occur mainly in the basal cells, and their progeny differentiate normally, indicating that the neoplastic phenotype is reversible and that the relevant and essential changes to epidermal neoplasia are those of the stem cell fraction of the tumor population. It is proposed that the ability of the promoter to modulate gene activity, perhaps derepressing genes and inducing reversal of the epidermal cell differentiation, may be the alteration that is essential for the activation of the gene loci that code for the neoplastic phenotype.