Crosstalk between Nuclear Factor I-C and Transforming Growth Factor-b1 Signaling Regulates Odontoblast Differentiation and Homeostasis

Abstract

Transforming growth factor-b1 (TGF-b1) signaling plays a key role in vertebrate development, homeostasis, and disease.Nuclear factor I-C (NFI-C) has been implicated in TGF-b1 signaling, extracellular matrix gene transcription, and tooth rootdevelopment. However, the functional relationship between NFI-C and TGF-b1 signaling remains uncharacterized. Thepurpose of this study was to identify the molecular interactions between NFI-C and TGF-b1 signaling in mouseodontoblasts. Real-time polymerase chain reaction and western analysis demonstrated that NFI-C expression levels wereinversely proportional to levels of TGF-b1 signaling molecules during in vitro odontoblast differentiation. Western blot andimmunofluorescence results showed that NFI-C was significantly degraded after TGF-b1 addition in odontoblasts, and theformation of the Smad3 complex was essential for NFI-C degradation. Additionally, ubiquitination assay results showed thatSmurf1 and Smurf2 induced NFI-C degradation and polyubiquitination in a TGF-b1-dependent manner. Both kinase and invitro binding assays revealed that the interaction between NFI-C and Smurf1/Smurf2 requires the activation of the mitogenactivatedprotein kinase pathway by TGF-b1. Moreover, degradation of NFI-C induced by TGF-b1 occurred generally in celltypes other than odontoblasts in normal human breast epithelial cells. In contrast, NFI-C induced dephosphorylation of p-Smad2/3. These results show that crosstalk between NFI-C and TGF-b1 signaling regulates cell differentiation andhomeostatic processes in odontoblasts, which might constitute a common cellular mechanism.