Notch signaling is a direct determinant of keratinocyte growth arrest and entry into differentiation

The role of Notch signaling in growth/differentiation control of mammalian epithelial cells is still poorly defined. We show that keratinocyte-specifi c deletion of the Notch1 gene results in marked epidermal hyperplasia and d eregulated expression of multiple differentiation markers. In differentiati ng primary keratinocytes in vitro endogenous Notch1 is required for inducti on of p21(WAF1/Cip1) expression, and activated Notch1 causes growth suppres sion by inducing p21(WAF1/Cip1) expression. Activated Notch1 also induces e xpression of 'early' differentiation markers, while suppressing the late ma rkers. Induction of p21(WAF1/Cip1) expression and early differentiation mar kers occur through two different mechanisms, The RBP-JK protein binds direc tly to the endogenous p21 promoter and p21 expression is induced specifical ly by activated Notch1 through RBP-J kappa -dependent transcription. Expres sion of early differentiation markers is RBP-J kappa -independent and can b e induced by both activated Notch1 and Notch2, as well as the highly conser ved ankyrin repeat domain of the Notch1 cytoplasmic region. Thus, Notch sig naling triggers two distinct pathways leading to keratinocyte growth arrest and differentiation.