Fibroblast Growth Factor 2 Stimulation of Osteoblast Differentiation and Bone Formation Is Mediated by Modulation of the Wnt Signaling Pathway

Abstract

Fibroblast growth factor 2 (FGF2) positively modulates osteoblast differentiation and bone formation. However, the mechanism(s) is not fully understood. Because the Wnt canonical pathway is important for bone homeostasis, this study focuses on modulation of Wnt/␤-catenin signaling using Fgf2 Both embryonic and postnatal skeletogenesis involve mesenchymal precursor cells progressively differentiating into boneforming cells, osteoblasts (1). Osteoblast differentiation is controlled by extracellular signals, including both the FGF (2) and Wnt pathways (3).FGF2 is one member of the FGF family (2, 4). It is expressed by osteoblasts (5-7) and stored in the extracellular matrix (7). Our previous studies show that FGF2 positively regulates bone formation and osteoblast differentiation. Disruption of the Fgf2 gene resulted in significantly decreased bone mass and bone formation as revealed by histomorphometry and micro-CT (8). At the cellular level, the effects of FGF2 ablation on osteoblast phenotypes included decreased osteoblast proliferation, reduced colony-forming efficiency in Fgf2 Ϫ/Ϫ bone marrow stromal cell (BMSC) 2 cultures, and decreased alkaline phosphatase and von Kossa staining (8 -10). The osteoblast function was also confirmed in vivo: bone formation rates were significantly reduced in Fgf2 Ϫ/Ϫ mice compared with wild-type littermates (8) The decreased osteoblast differentiation in Fgf2 Ϫ/Ϫ BMSC cultures can be partially rescued by adding exogenous FGF2 in vitro (8, 9). FGF2 also stimulates in vivo bone formation (11)(12)(13)(14). Differentiation of BMSCs into the osteoblast lineage is governed by transcription factors, including runt-related transcription factor 2 (Runx2), Osterix, and activating transcription factor 4 (ATF4) (1). Decreased osteoblast differentiation might be due to reduced expression of transcription factors Runx2 (9) and ATF4 (15), which were associated with reduced bone formation in Fgf2 Ϫ/Ϫ mice. Furthermore, exogenous FGF2 was able to induce Runx2 (16) and ATF4 (15) expression. These studies suggest that FGF2 is a positive regulator of bone formation. In addition, FGF2 is required for osteoclast formation and bone resorption because FGF2 deletion results in reduced osteoclast formation and resorption both in vitro (17) and in vivo (8). The present study focused on potential mechanisms by which FGF2 regulates osteoblast differentiation and bone formation.Wnts are secreted glycoproteins with a family of 19 proteins in mammals, including Wnt3a and Wnt10b (18). Wnt10b stimulates osteogenic transcription factor Runx2, thereby promoting osteoblastogenesis and bone formation (19,20). Wnt ligands can signal through either the ␤-catenin-dependent or -independent pathways (21). The ␤-catenin-dependent Wnt pathway is well studied in osteoblasts (3). In the absence of Wnt ligands, ␤-catenin is degraded by a destruction complex which includes glycogen synthase kinase 3␤ (GSK3␤) (3,21,22). Thus, GSK3␤ is a negative regulator of the ␤-catenin/Wnt pathway. Binding of Wnt ligands to the Frizzled rec...

MEKK2 mediates an alternative β-catenin pathway that promotes bone formation

“…Thus, the profound effect on bone seen in mice with a conditional deletion of β-catenin exon 3, which prevents this basal ubiquination of β-catenin, would be expected to reflect a decoupling from both the WNT and FGF2/ MEKK2 pathways (1,24). Notably, these findings provide a mechanistic basis for prior observations that β-catenin protein levels and activity are reduced in the absence of FGF2 (25). The phenotype of Mekk2 −/− mice also is consistent with the reduced bone mass and decreased BFR seen in adult Fgf2 −/− mice, indicating that a FGF2/MEKK2 pathway contributes to the maintenance of adult bone mass (26).…”

Fibroblast growth factor signaling in skeletal development and disease

“…In osteoprogenitor cells, FGF2 was found to antagonize WNT/β-catenin signaling Ambrosetti et al 2008). In vivo, however, Wnt genes are decreased significantly in osteoblasts from Fgf2 −/− mice, which display reduced osteoblast differentiation (Fei et al 2011). Consistent with this, exogenous FGF2 can promote β-catenin nuclear accumulation and rescue osteoblast differentiation in Fgf2 −/− mice, indicating that endogenous FGF2 stimulates cell differentiation in more mature osteoblasts in part by activating WNT/β-catenin signaling (Fei et al 2011).…”

Effect of FGF/FGFR Signal in Fluid Shear Stress and Estrogen Regulating Bone Metabolism

“…FGF/FGFR signal and canonical Wnt/β-catenin pathway had complicated interaction. The loss of efficiency of Wnt inhibitor Axin2 could lead to craniosynostosis of mice [95,96]. Experiments of Liu et al [95] showed that lack of Axin2 could promote β-catenin signal and ultimately cause ossification of periosteum.…”

“…Wnt-dependent BMP signals could guide intracellular β-catenin distribution, featuring an important positive feedback regulation effect upon β-catenin [96]. FGF signal and BMP signal, apart from the Wnt/ β-catenin mediated indirect relations, also shared certain direct relations.…”

“…And Wnt signals could also hinder Runx2 activity through P38 and ERK signals [23]. In addition, FGF genes are also the direct target genes of Wnt signals [96,[99][100][101].…”