Regulation of epidermal transglutaminase activity and terminal differentiation by retinoids and phorbol esters

A number of characteristics of phorbol ester-mediated mouse skin tumor promotion indicate that cell selection is the underlying biological process. Studies in vivo and in cultured mouse epidermal cells suggest that selection is based on heterogeneous responses of subpopulations of basal cells which can be induced to proliferate or differentiate in response to promoter exposure. Retinoids are effective inhibitors of tumor promotion in mouse skin but do not influence the proliferative response to phorbol esters. Since both retinoids and phorbol esters modify epidermal differentiation, the antipromoter action of retinoids could be related to differentiation responses. Retinoic acid induces transglutaminase activity in cultured mouse epidermal basal cells grown in less than 0.1 mM Ca2+. While this enzyme is associated with terminal differentiation in skin, retinoic acid paradoxically blocks the terminal differentiation of cultured cells. In contrast, phorbol esters and extracellular calcium greater than 0.1 mM induce both transglutaminase activity and terminal differentiation. Enzyme kinetic analyses indicate that the transglutaminases induced by all three inducers are the same enzyme. The increase in activity of transglutaminase by all three inducers requires RNA and protein synthesis. However, the time course of increase and decay of each activity differs for each inducer. A variety of biologically active retinoids induce transglutaminase activity, and their effectiveness correlates with their reported antipromoter activity. Exposures to both retinoic acid and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate are antagonistic resulting in less than additive induction. Induction kinetics with both inducers are more like those of retinoic acid than those of the phorbol ester. Simultaneous exposure to retinoic acid and 12-O-tetradecanoylphorbol-13-acetate protects the epidermal cell population from induced terminal differentiation and cell loss which is observed in response to the promoter alone. These results suggest that the antipromoting action of retinoids could be mediated by modification of phorbol ester-accelerated terminal differentiation through an effect on transglutaminase and cornification. This action of retinoids would block a critical aspect of cell selection involving loss of cells and subsequent regenerative hyperplasia, although simple hyperplasia may still occur.