Smad3 differently affects osteoblast differentiation depending upon its differentiation stage.

Smad3, a critical component of the TGF-beta signaling pathways, plays an important role in the regulation of bone formation. However, how Smad3 affects osteoblast at the different differentiation stage remains still unknown. In the present study, we examined the effects of Smad3 on osteoblast phenotype by employing mouse bone marrow ST-2 cells and mouse osteoblastic MC3T3-E1 cells at the different differentiation stage. Smad3 overexpression significantly inhibited bone morphogenetic protein-2 (BMP-2)-induced ALP activity in ST-2 cells, indicating that Smad3 suppresses the commitment of pluripotent mesenchymal cells into osteoblastic cells. Smad3 increased the levels of COLI and ALP mRNA at 7 day cultures in MC3T3-E1 cells, and its effects on COL1 were decreased as the culture periods progress, although its effects on ALP were sustained during 21 day cultures. Smad3 overexpression enhanced the level of Runx2 and OCN mRNA at 14 day and 21 day cultures. Smad3 increased the levels of MGP and NPP-1 mRNA, although the extent of increase in MGP and NPP-1 was reduced and enhanced during the progression of culture period, respectively. Smad3 did not affect the level of ANK mRNA. On the other hand, Smad3 enhanced the level of MEPE mRNA at 14 and 21 day cultures, although Smad3 decreased it at 7 day cultures. In conclusion, Smad3 inhibits the osteoblastic commitment of ST-2 cells, while promotes the early stage of differentiation and maturation of osteoblastic committed MC3T3-E1 cells. Also, Smad3 enhanced the expression of mineralization-related genes at the maturation phase of MC3T3-E1 cells.     

Parathyroid hormone-responsive Smad3-related factor, Tmem119, promotes osteoblast differentiation and interacts with the bone morphogenetic protein-Runx2 pathway

The mechanisms whereby the parathyroid hormone (PTH) exerts its anabolic action on bone are incompletely understood. We previously showed that inhibition of ERK1/2 enhanced Smad3-induced bone anabolic action in osteoblasts. These findings suggested the hypothesis that changes in gene expression associated with the altered Smad3-induced signaling brought about by an ERK1/2 inhibitor would identify novel bone anabolic factors in osteoblasts. We therefore performed a comparative DNA microarray analysis between empty vector-transfected mouse osteoblastic MC3T3-E1 cells and PD98059-treated stable Smad3-overexpressing MC3T3-E1 cells. Among the novel factors, Tmem119 was selected on the basis of its rapid induction by PTH independent of later increases in endogenous TGF-β. The levels of Tmem119 increased with time in cultures of MC3T3-E1 cells and mouse mesenchymal ST-2 cells committed to the osteoblast lineage by BMP-2. PTH stimulated Tmem119 levels within 1 h as determined by Western blot analysis and immunocytochemistry in MC3T3-E1 cells. MC3T3-E1 cells stably overexpressing Tmem119 exhibited elevated levels of Runx2, osteocalcin, alkaline phosphatase, and β-catenin, whereas Tmem119 augmented BMP-2-induced Runx2 levels in mesenchymal cells. Tmem119 interacted with Runx2, Smad1, and Smad5 in C2C12 cells. In conclusion, we identified a Smad3-related factor, Tmem119, that is induced by PTH and promotes differentiation in mouse osteoblastic cells. Tmem119 is an important molecule in the pathway downstream of PTH and Smad3 signaling in osteoblasts.     

AMP-activated protein kinase (AMPK) positively regulates osteoblast differentiation via induction of Dlx5-dependent Runx2 expression in MC3T3E1 cells

This study examined the role of AMPK activation in osteoblast differentiation and the underlining mechanism. An AMPK activator (AICAR or metformin) stimulated osteoblast differentiation with increases in ALP and OC protein production as well as the induction of AMPK phosphorylation in MC3T3E1 cells. In addition, metformin induced the phosphorylation of Smad1/5/8 and expression of Dlx5 and Runx2, whereas compound C or dominant negative AMPK inhibited these effects. Transient transfection studies also showed that metformin increased the BRE-Luc and Runx2-Luc activities, which were inhibited by DN-AMPK or compound C. Down-regulation of Dlx5 expression by siRNA suppressed metformin-induced Runx2 expression. These results suggest that the activation of AMPK stimulates osteoblast differentiation via the regulation of Smad1/5/8-Dlx5-Runx2 signaling pathway.