Identification of a Novel Link between the Protein Kinase NDR1 and TGFβ Signaling in Epithelial Cells

Transforming growth factor-beta (TGF beta) is a secreted polypeptide that plays essential roles in cellular development and homeostasis. Although mechanisms of TGF beta-induced responses have been characterized, our understanding of TGFb signaling remains incomplete. Here, we uncover a novel function for the protein kinase NDR1 (nuclear Dbf2-related 1) in TGF beta responses. Using an immunopurification approach, we find that NDR1 associates with SnoN, a key component of TGF beta signaling. Knockdown of NDR1 by RNA interference promotes the ability of TGF beta to induce transcription and cell cycle arrest in NMuMG mammary epithelial cells. Conversely, expression of NDR1 represses TGFb-induced transcription and inhibits the ability of TGF beta to induce cell cycle arrest in NMuMG cells. Mechanistically, we find that NDR1 acts in a kinase-dependent manner to suppress the ability of TGF beta to induce the phosphorylation and consequent nuclear accumulation of Smad2, which is critical for TGF beta-induced transcription and responses. Strikingly, we also find that TGF beta reciprocally regulates NDR1, whereby TGF beta triggers the degradation of NDR1 protein. Collectively, our findings define a novel and intimate link between the protein kinase NDR1 and TGF beta signaling. NDR1 suppresses TGFb-induced transcription and cell cycle arrest, and counteracting NDR1' s negative regulation, TGF beta signaling induces the downregulation of NDR1 protein. These findings advance our understanding of TGF beta signaling, with important implications in development and tumorigenesis.