Effect of Total Flavonoids of Rhizoma drynariae on Tibial Dyschondroplasia by Regulating BMP-2 and Runx2 Expression in Chickens

Osterix/Sp7, a member of the Sp1 transcription factor family, plays an essential role in bone formation and osteoblastogenesis. Although Osterix has been shown to be induced by BMP2 in a mesenchymal cell line, the molecular basis of the regulation, expression and function of Osterix during osteoblast differentiation, is not fully understood. Thus we examined the role of BMP2 signaling in the regulation of Osterix using the mesenchymal cell lines C3H10T1/2 and C2C12. Osterix overexpression induced alkaline phosphatase activity and osteocalcin expression in C2C12 cells and stimulated calcification of murine primary osteoblasts. Considering that Runx2 overexpression induces Osterix, these results suggest that Osterix functions as downstream of Runx2. Surprisingly, BMP2 treatment induced Osterix expression and alkaline phosphatase activity in mesenchymal cells derived from Runx2-deficient mice. Furthermore, overexpression of Smad1 and Smad4 up-regulated Osterix expression, and an inhibitory Smad, Smad6, markedly suppressed BMP2-induced Osterix expression in the Runx2-deficient cells. Moreover, overexpression of a homeobox gene, Msx2, which is up-regulated by BMP2 and promotes osteoblastic differentiation, induced Osterix expression in the Runx2-deficient cells. Knockdown of Msx2 clearly inhibited induction of Osterix by BMP2 in the Runx2-deficient mesenchymal cells. Interestingly, microarray analyses using the Runx2-deficient cells revealed that the role of Osterix was distinct from that of Runx2. These findings suggest that Osterix is regulated via both Runx2-dependent and -independent mechanisms, and that Osterix controls osteoblast differentiation, at least in part, by regulating the expression of genes not controlled by Runx2.

Runx2 protein is first detected in preosteoblasts, and the expression is upregulated in immature osteoblasts, but downregulated in mature osteoblasts. Runx2 is the first transcription factor required for determination of the osteoblast lineage, while Sp7 and canonical Wnt-signaling further direct the fate of mesenchymal cells to osteoblasts, blocking their differentiation into chondrocytes. Runx2 induces the differentiation of multipotent mesenchymal cells into immature osteoblasts, directing the formation of immature bone, but Runx2 inhibits osteoblast maturation and mature bone formation. Normally, the protein level of Runx2 in osteoblasts reduces during bone development, and osteoblasts acquire mature phenotypes, which are required for mature bone formation. Furthermore, Runx2 triggers the expression of major bone matrix genes during the early stages of osteoblast differentiation, but Runx2 is not essential for the maintenance of these gene expressions in mature osteoblasts.

We present an overview of Runx involvement in regulatory mechanisms that are requisite for fidelity of bone cell growth and differentiation, as well as for skeletal homeostasis and the structural and functional integrity of skeletal tissue. Runx-mediated control is addressed from the perspective of support for biological parameters of skeletal gene expression. We review recent findings that are consistent with an active role for Runx proteins as scaffolds for integration, organization and combinatorial assembly of nucleic acids and regulatory factors within the three-dimensional context of nuclear architecture.