EGF Inhibits Wnt/-Catenin-Induced Osteoblast Differentiation by Promoting -Catenin Degradation

Bone morphogenetic protein (BMP) and canonical Wnts are representative developmental signals that enhance osteoblast differentiation and bone formation. Previously, we demonstrated that epidermal growth factor (EGF) inhibits BMP2-induced osteoblast differentiation by inducing Smurf1 expression. However, the regulatory role of EGF in Wnt/-catenin-induced osteoblast differentiation has not been elucidated. In this study, we investigated the effect of EGF on Wnt/-catenin signaling-induced osteoblast differentiation using the C2C12 cell line. EGF significantly suppressed the expression of osteoblast marker genes, which were induced by Wnt3a and a GSK-3 inhibitor. EGF increased the expression levels of Smurf1 mRNA and protein. Smurf1 knockdown rescued Wnt/-catenin-induced osteogenic marker gene expression in the presence of EGF. EGF treatment or Smurf1 overexpression did not affect -catenin mRNA expression levels, but reduced -catenin protein levels and TOP-Flash activity. EGF and Smurf1 promoted -catenin ubiquitination. Co-immunoprecipitation and GST pull-down assays showed that Smurf1 associates with -catenin. These results suggest that EGF/Smurf1 inhibits Wnt/-catenin-induced osteogenic differentiation and that Smurf1 downregulates Wnt/-catenin signaling by enhancing proteasomal degradation of -catenin. J. Cell. Biochem. 116: 2849-2857, 2015. (c) 2015 Wiley Periodicals, Inc.