Notch signaling is necessary for epithelial growth arrest by TGF-β

Transforming growth factor beta (TGF-beta) and Notch act as tumor suppressors by inhibiting epithelial cell proliferation. TGF-beta additionally promotes tumor invasiveness and metastasis, whereas Notch supports oncogenic growth. We demonstrate that TGF-beta and ectopic Notch1 receptor cooperatively arrest epithelial growth, whereas endogenous Notch signaling was found to be required for TGF-beta to elicit cytostasis. Transcriptomic analysis after blocking endogenous Notch signaling uncovered several genes, including Notch pathway components and cell cycle and apoptosis factors, whose regulation by TGF-beta requires an active Notch pathway. A prominent gene coregulated by the two pathways is the cell cycle inhibitor p21. Both transcriptional induction of the Notch ligand Jagged1 by TGF-beta and endogenous levels of the Notch effector CSL contribute to p21 induction and epithelial cytostasis. Cooperative inhibition of cell proliferation by TGF-beta and Notch is lost in human mammary cells in which the p21 gene has been knocked out. We establish an intimate involvement of Notch signaling in the epithelial cytostatic response to TGF-beta.