FgfR2 Activation Induces Cbfa1/Runx2 and BMP-2 Expression in Mouse Calvarial Osteoblasts

Human genetic studies have demonstrated that fibroblast growth factor receptor 2 (FgfR2) plays important roles in skeletal development. However, the mechanism by which FgfR2 regulates bone formation remains poorly understood. FgfR2 is expressed in osteogenic precursors and early osteoblasts, and has been suggested to regulate cell proliferation. Interestingly, analyses of transgenic mice expressing dominant negative (DN)-FgfR2 or activating mutation (P253R) have revealed that FgfR2 also regulates osteogenic differentiation. Objectives: To better understand the role of FgfR2 in osteogenic differentiation, we asked whether FgfR2 regulates expression of Runx2, an essential transcription factor for osteogenic commitment and early differentiation, and BMP-2, a potent osteoblast differentiation factor. Methods: Calvarial bone cell cultures prepared from postnatal day 10 mice were treated with FGF-9, a cognate ligand for FgfR2, for different periods (0,3,6,9,12,24 hrs). RNAs extracted from these cells were analyzed by Northern blot hybridization to determine the expression levels of Runx2 and BMP-2 genes. We have also analyzed calvarial bone cells isolated from transgenic mice expressing FgfR2 with DN- or P253R activating mutations. Results: The Runx2 gene is upregulated by FGF9 treatment, reaching a peak in 3 hours but decreasing thereafter. Also, the BMP-2 gene expression is rapidly increased within 3 hours by FGF9 treatment, reaching a peak at 12 hours. Expression of both Runx2 and BMP-2 genes are increased in calvarial bone cells isolated from transgenic mice expressing P253R-FgfR2, but decreased in the cells from mice expressing DN-FgfR2. Conclusions: Based on these data, we conclude that Runx2 and BMP2 are downstream target genes of FgfR2-mediated signaling. In addition, given the mitogenic functions of FgfR2, we propose that FgfR2 plays an important role in increasing the number of early bone cells that are competent for differentiation by inducing Runx2 and BMP2. (Supported by NICHD HD40282 and a grant from AAOF)