BMP‐2 promotes differentiation of osteoblasts and chondroblasts in <i>Runx2</i>‐deficient cell lines

<jats:title>Abstract</jats:title><jats:p>To investigate the molecular mechanism underlying the differentiation of osteoblasts and chondroblasts, we established a clonal cell lines, RD‐C6, from <jats:italic>Runx2</jats:italic>‐deficient mouse embryos. RD‐C6 cells expressed almost undetectable levels of phenotypes related to osteoblast and chondroblast differentiation at basal culture condition, whereas treatment with recombinant human bone morphogenetic protein‐2 (rhBMP‐2) or transduction of <jats:italic>BMP‐2</jats:italic> by adenovirus effectively induced this cell line to express mRNA related to the differentiation of osteoblasts and chondroblasts including <jats:italic>alkaline phosphatase</jats:italic>, <jats:italic>osteocalcin</jats:italic>, and <jats:italic>osterix</jats:italic>. Transduction of <jats:italic>Runx2</jats:italic> also induced the expression of these mRNA in RD‐C6 cells. <jats:italic>BMP‐2</jats:italic> transduction increased expression levels of mRNA for <jats:italic>Msx2</jats:italic> and <jats:italic>Dlx5</jats:italic>, but Runx2 transduction induced no significant increases in expression levels of these mRNA. Microarray analysis using RD‐C6 cells with or without rhBMP‐2 treatment demonstrated that BMP‐2 upregulated 66 genes including 13 transcription‐related molecules such as <jats:italic>Id1</jats:italic>, <jats:italic>Id2</jats:italic>, <jats:italic>Id4</jats:italic>, <jats:italic>Hey1</jats:italic>, <jats:italic>Smad6</jats:italic>, <jats:italic>Smad7</jats:italic>, and <jats:italic>Msx2</jats:italic>. To confirm bone and cartilage formation ability of RD‐C6 cells, we transplanted RD‐C6 cells into the peritoneal cavity of athymic mice using diffusion chambers with rhBMP‐2. RD‐C6 cells generated unmineralized cartilage but not bone. These results indicate that BMP‐2 induces <jats:italic>Runx2</jats:italic>‐deficient cells to express markers related to osteoblast and chondroblast differentiation using a <jats:italic>Runx2</jats:italic>‐independent pathway, but it failed to induce these cells to differentiate into bone‐forming osteoblasts and mature chondrocytes. J. Cell. Physiol. 211: 728–735, 2007. © 2007 Wiley‐Liss, Inc.</jats:p>