RUNX2 expression in mesenchymal cells induces osteoblast differentiation and bone formation. BMP blocking agents were used to show that RUNX2-dependent osteoblast differentiation and transactivation activity both require BMP signaling and, further, that RUNX2 enhances the responsiveness of cells to BMPs.Introduction: BMPs and the RUNX2 transcription factor are both able to stimulate osteoblast differentiation and bone formation. BMPs function by activating SMAD proteins and other signal transduction pathways to stimulate expression of many target genes including RUNX2. In contrast, RUNX2 induces osteoblast-specific gene expression by directly binding to enhancer regions in target genes. In this study, we examine the interdependence of these two factors in controlling osteoblast differentiation in mesenchymal progenitor cells. Materials and Methods: C3H10T1/2 mesenchymal cells and primary cultures of marrow stromal cells were transduced with a RUNX2 adenovirus and treated with BMP blocking antibodies or the natural antagonist, NOGGIN. Osteoblast differentiation was determined by assaying alkaline phosphatase and measuring osteoblast-related mRNA using quantitative RT/PCR. Activation of BMP-responsive signal transduction pathways (SMAD, extracellular signal-regulated kinase [ERK], p38, and c-jun-N-terminal kinase [JNK]) was assessed on Western blots. Results and Conclusions: C3H10T1/2 cells constitutively synthesize BMP2 and 4 mRNA and protein, and this BMP activity is sufficient to activate basal levels of SMAD phosphorylation. Inhibition of BMP signaling was shown to disrupt the ability of RUNX2 to stimulate osteoblast differentiation and transactivate an osteocalcin gene promoter-luciferase reporter in C3H10T1/2 cells. BMP blocking antibodies also inhibited RUNX2-dependent osteoblast differentiation in primary cultures of murine marrow stromal cells. Conversely, RUNX2 expression synergistically stimulated BMP2 signaling in C3H10T1/2 cells. However, RUNX2 did not increase the ability of this BMP to activate SMAD, ERK, p38, and JNK pathways. This study shows that autocrine BMP production is necessary for the RUNX2 transcription factor to be active and that BMPs and RUNX2 cooperatively interact to stimulate osteoblast gene expression.

(2004) Cell 117, 387-398). However, the mechanisms of ATF4 in bone cells are still not clear. In this study, we determined the molecular mechanisms through which ATF4 activates the mouse osteocalcin (Ocn) gene 2 (mOG2) expression and mOG2 promoter activity. ATF4 increased the levels of Ocn mRNA and mOG2 promoter activity in Runx2-containing osteoblasts but not in non-osteoblastic cells that lack detectable Runx2 protein. However, ATF4 increased Ocn mRNA and mOG2 promoter activity in non-osteoblastic cells when Runx2 was co-expressed. Mutational analysis of the OSE1 (ATF4-binding site) and the two OSE2s (Runx2-binding sites) in the 657-bp mOG2 promoter demonstrated that ATF4 and Runx2 activate Ocn via cooperative interactions with these sites. Pull-down assays using nuclear extracts from osteoblasts or COS-7 cells overexpressing ATF4 and Runx2 showed that both factors are present in either anti-ATF4 and anti-Runx2 immunoprecipitates. In contrast, pull-down assays using purified glutathione S-transferase fusion proteins were unable to demonstrate a direct physical interaction between ATF4 and Runx2. Thus, accessory factors are likely involved in stabilizing interactions between these two molecules. Regions within Runx2 required for ATF4 complex formation and activation were identified. Deletion analysis showed that the leucine zipper domain of ATF4 is critical for Runx2 activation. This study is the first demonstration that cooperative interactions between ATF4 and Runx2/Cbfa1 stimulate osteoblast-specific Ocn expression and suggests that this regulation may represent a novel intramolecular mechanism regulating Runx2 activity and, thereby, osteoblast differentiation and bone formation.

Bone regeneration requires interactions between a number of factors including bone morphogenetic proteins (BMPs), growth factors, and transcriptional regulators such as Runx2/Cbfa1 (Runx2). Because each component may provide a unique contribution to the overall osteogenic response, we hypothesized that bone formation may be enhanced by using combinations of complimentary factors. As an initial test of this concept, interactions between BMP2 and Runx2 were examined using adenovirus-based expression vectors (AdCMVRunx2, AdCMV-BMP2) in the pluripotent C3H10T1/2 cell line. Cells transduced with AdCMV-Runx2 strongly expressed osteoblast markers, such as alkaline phosphatase and osteocalcin, but formed only a weakly mineralized extracellular matrix in vitro, whereas cells transduced with AdCMV-BMP2 exhibited higher levels of mineralization, but only expressed low levels of Runx2 and osteocalcin mRNA. Significantly, when cells were transduced with optimal titers of both viruses, osteoblast differentiation was stimulated to levels that were 10-fold greater than those seen with either AdCMV-Runx2 or AdCMV-BMP2 alone. To measure in vivo osteogenic activity, virally transduced cells were subcutaneously implanted into immunodeficient mice. Cells transduced with control virus produced only fibrous tissue while those with AdCMV-Runx2 produced limited amounts of both cartilage and bone. In contrast, cells transduced with either AdCMV-BMP2 alone or AdCMV-BMP2 plus AdCMV-Cbfa1 generated large ossicles containing cartilage, bone, and a marrow cavity. However, ossification in the AdCMV-BMP2 plus AdCMV-Cbfa1 group was more extensive in that both mineral content and fractional bone area were greater than that seen in the AdCMV-BMP2 group. Thus, the increased osteoblast differentiation observed with combined adenovirus treatment in vitro is also manifested by increased bone formation in vivo. These results suggest that Runx2 and BMP2 have distinct, but complementary, roles in osteogenesis and that their combined actions may be necessary for optimal bone formation. (J Bone Miner Res 2003;18:705-715)

The bone sialoprotein (Bsp) gene provides an excellent model for studying mechanisms controlling osteoblast-specific gene expression. Although the RUNX2 transcription factor directly regulates many osteoblastrelated genes, its function in Bsp expression remains uncertain. By using chromatin immunoprecipitation (ChIP) analysis in MC3T3-E1 (clone MC-4) preosteoblast cells, RUNX2 was shown to bind a chromatin fragment containing the proximal Bsp promoter. Two putative RUNX2-binding sites (R1 and R2) were identified within this region of the mouse, rat, and human genes and were shown to bind RUNX2 in vitro and in vivo (by ChIP assay). Site-specific mutagenesis established that both sites act as osteoblast-specific transcriptional enhancers and together account for nearly two-thirds of the total promoter activity. In addition, functional cooperativity was observed between the R2 site and an adjacent homeodomain protein-binding site previously characterized by this laboratory (the C site). All three sites (R1, R2, and C) are necessary for maximal promoter activity in osteoblasts. DLX5 in MC-4 cell nuclear extracts binds to the C site in vitro. Furthermore, ChIP assays revealed that DLX5 is selectively associated with chromatin in the vicinity of the C site only when Bsp is transcriptionally active. Finally, co-immunoprecipitation assays detected a physical complex containing DLX5 and RUNX2. Taken together, our data show that RUNX2 is a direct regulator of Bsp in osteoblasts and that it functions in cooperation with DLX5 or a related factor to activate osteoblast-specific gene expression.

Insulin and its downstream signaling pathway are indispensable for postnatal bone growth and turnover by having influence on both osteoblast and osteoclast development. Insulin signaling regulates both bone formation by osteoblasts and bone resorption by osteoclasts; however, the regulation occurs mainly through the insulin signaling pathway within osteoblasts. An impairment of osteoblastic insulin signaling leads to an impaired bone quality by affecting osteoblast proliferation, differentiation and survival. The insulin signaling pathway and MAPK and PI3K/Akt pathways play pivotal roles in the differentiation, function and survival of bone cells. Current evidence suggests that osteoblastic insulin signaling not only modulates bone growth and turnover but is also required for energy metabolism. Several mice models with impaired insulin signaling exhibited both bone and metabolic phenotypes, including symptoms of low bone mass, obesity, glucose intolerance and insulin resistance. In this review, we discuss the key findings that suggest a pivotal role of osteoblastic insulin signaling in both bone and energy metabolism.