Role of Cbfa1 in osteoblast differentiation and function

Among the multiple cell lineages whose differentiation is affected by a runt-related gene the osteoblast is a relative newcomer. Molecular biology, developmental biology and mouse and human genetic studies have demonstrated that Cbfa1 is a critical regulator of osteoblast differentiation in vertebrates. Cbfa1 is not only a differentiation factor but also a regulator of bone formation by differentiated osteoblasts beyond development. Thus, Cbfa1 controls osteogenesis at multiple stages.     

Runx2 is a common target of transforming growth factor beta1 and bone morphogenetic protein 2, and cooperation between Runx2 and Smad5 induces osteoblast-specific gene expression in the pluripotent mesenchymal precursor cell line C2C12

When C2C12 pluripotent mesenchymal precursor cells are treated with transforming growth factor beta1 (TGF-beta1), terminal differentiation into myotubes is blocked. Treatment with bone morphogenetic protein 2 (BMP-2) not only blocks myogenic differentiation of C2C12 cells but also induces osteoblast differentiation. The molecular mechanisms governing the ability of TGF-beta1 and BMP-2 to both induce ligand-specific responses and inhibit myogenic differentiation are not known. We identified Runx2/PEBP2alphaA/Cbfa1, a global regulator of osteogenesis, as a major TGF-beta1-responsive element binding protein induced by TGF-beta1 and BMP-2 in C2C12 cells. Consistent with the observation that Runx2 can be induced by either TGF-beta1 or BMP-2, the exogenous expression of Runx2 mediated some of the effects of TGF-beta1 and BMP-2 but not osteoblast-specific gene expression. Runx2 mimicked common effects of TGF-beta1 and BMP-2 by inducing expression of matrix gene products (for example, collagen and fibronectin), suppressing MyoD expression, and inhibiting myotube formation of C2C12 cells. For osteoblast differentiation, an additional effector, BMP-specific Smad protein, was required. Our results indicate that Runx2 is a major target gene shared by TGF-beta and BMP signaling pathways and that the coordinated action of Runx2 and BMP-activated Smads leads to the induction of osteoblast-specific gene expression in C2C12 cells.     

Activation of p38 and Smads mediates BMP-2 effects on human trabecular bone-derived osteoblasts

The bone morphogenetic proteins (BMPs) are potent osteoinductive factors that accelerate osteoblast maturation, accompanied by increased cell-substrate adhesion. BMP-2 treatment of osteoblastic cells increases phosphorylation of the cytoplasmic BMP-2 signaling molecules, Smad1 and Smad5. We have previously reported that BMP-2 treatment increase cytoskeletal organization of human trabecular bone-derived osteoblast-like cells (osteoblasts), which is also accompanied by an activation of the focal adhesion kinase p125(FAK). We report here that activation of p125(FAK) occurs with the same kinetics as the phosphorylation of Smad1, suggesting that BMP-2 initiates cross-talk between Smad signaling and the adhesion-mediated signaling pathway. As an adjunct to these effects, we examined activation of mitogen-activated protein (MAP) kinase family members in response to focal adhesion contact formation. Although phosphorylated forms of all three kinases were apparent, only SAPK2alpha/p38 (p38) was activated in response to BMP-2 treatment. Inhibition of p38 kinase activity suppressed BMP-2 induced Smad1 phosphorylation, as well as its translocation to the nucleus, suggesting the integration of p38 activation with Smad1 signaling. Finally, inhibition of p38 in osteoblasts also led to the complete abrogation of BMP-2 induced osteocalcin gene expression and matrix mineralization. These findings suggest that BMP-2 must activate p38 in order to mediate osteogenic differentiation and maturation.     

Cbfa1/Runx2与成骨细胞分化调控

成骨细胞是由间充质干细胞经骨原细胞和前成骨细胞分化而来的。近年来已鉴定转录因子Cbfal(core binding factor α1)是成骨细胞分化和骨形成的关键调控因子。在成骨细胞分化的过程中,Cbfal通过调控成骨细胞特异性细胞外基质蛋白基因的表达和成骨细胞周期参与成骨细胞的分化过程。新近发现Cbfal能通过自身的PST序列区域与Smads结合形成复合物共同参与成骨细胞的分化调控。