Transcription factor TGIF promotes osteoblast differentiation and bone formation by activating canonical Wnt signaling
The canonical Wnt signaling pathway has been shown to play a major role in bone formation. Wnt signaling promotes osteoblast differentiation through its ability to repress the differentiation of mesenchymal cells into chondrocytes and adipocytyes and to foster osteoblast proliferation. Here we report that a transcriptional corepressor TG interacting factor (TGIF) regulates canonical Wnt signaling and osteoblast differentiation which leads to less bone mass phonotype in TGIF-/- mice. We found TGIF interacted with several compenents of Wnt signaling by two-hybride system screen. b-Catenin is the central modulator of canonical Wnt signaling. We surmised that a physiological role of TGIF is to ensure stabilization of b-Catenin. First, induction of TGIF expression induced a marked increase b-Catenin accumulation. Second, depletion of TGIF in cells induced a significant decrease in the abundance of b-Catenin. Third, comparative studies using wild-type and TGIF-/- MEFs indicated that TGIF deficiency caused decreased accumulation of b-Catenin. Finally, deletion of TGIF induced a marked decrease in b-Catenin abundance in vivo. TGIF may function as a crucial component of the Wnt signaling pathway. Wnt luciferase reporter assay(TOP-FLASH) showed TGIF activated Wnt transcriptional responses. TGIF induced accumulation of Wnt target genes.TGIF was also required for those Wnt signaling target genes expression. Stable expressing of TGIF increased ALP staining in MC3T3-E1 cells, while the osteoblast differentiation was impaired in depleted TGIF of ST2 cells and primary calvarial cells derived from TGIF -/- mice compared to controls. Further, TGIF was required for Wnt3a-induced osteoblast differentiation in ST2 cells. On the other hand, Wnt induced the accumulation of TGIF in MC3T3-E1 cells and C3H10T1/2 cells, thus the positive forward loop facilitates TGIF regulating canonical Wnt signaling. In vivo, TGIF-/- mice showed low bone mass analyzed by microCT and histomorphometry. The mice lacking TGIF failed to accumulate trabecular and cortical bone . Trabecular bone volume (BV/TV) was decreased by ＞45% in TGIF -/- mice compared to controls. This defect in bone was due to significant decreases in trabecular numbers (Tb.N), thickness (Tb.Th)and cortical thickness(Cort.Th), and significant increases in trabecular spacing (Tb.Sp). The bone fomation rate (BFR) was decreased markly in TGIF -/- mice compared to the countermates. Taken together, these studies point to a key role for TGIF in canonical Wnt signaling.TGIF favors canonical Wnt signaling and fostered osteoblast differentiation in Wnt3A dependant manner. In vivo, TGIF knockout mice had low bone mass phenotype which could be caused by reducing canonical Wnt signaling.