Suppression of keratinocyte growth and differentiation by transforming growth factor beta 1 involves multiple signaling pathways

Transforming growth factor beta1 treatment of keratinocytes results in a su ppression of differentiation, an induction of extracellular matrix producti on, and a suppression of growth. In this study we utilized markers specific for each of these functions to explore the signaling pathways involved in mediating these transforming-growth-factor-beta1-induced activities. In the first instance, we found that the induction of extracellular matrix produc tion (characterized by 3TP-Lux reporter activity) was induced in both kerat inocytes and a keratinocyte-derived carcinoma cell line, SCC25, in a dose-d ependent manner. Furthermore, transforming growth factor beta1 also suppres sed the differentiation-specific marker gene, transglutaminase type 1, in b oth keratinocytes and SCC25 cells. In contrast, transforming growth factor beta1 inhibited proliferation of keratinocytes but did not cause growth inh ibition in the SCC25 cells. Transforming-growth-factor-beta1-induced growth inhibition of keratinocytes was characterized by decreases in DNA synthesi s, accumulation of hypophosphorylated Rb, and the inhibition of the E2F:Rb- responsive promoter, cdc2, and an induction of the p21 promoter. When the n egative regulator of transforming growth factor beta1 signaling, SMAD7, was overexpressed in keratinocytes it could prevent transforming-growth-factor -beta1-induced activation of the 3TP-Lux and the p21 promoter. SMAD7 could also prevent the suppression of the transglutaminase type 1 by transforming growth factor beta1 but it could not inhibit the repression of the cdc2 pr omoter. These data indicate that the induction of 3TP-Lux and p21 and the s uppression of transglutaminase type 1 are mediated by a different proximate signaling pathway to that regulating the suppression of the cdc2 gene. Com bined, these data indicate that the regulation of transforming growth facto r beta1 actions are complex and involve multiple signaling pathways.