Staphylococcal enterotoxin C2 promotes osteogenesis and suppresses osteoclastogenesis of human mesenchymal stem cells

As a super-antigen, staphylococcal enterotoxin C2 (SEC2) stimulates the release of massive inflammatory cytokines such as interferon-gamma (IFN-γ), interleukin-1 (IL-1) and interleukin-2 (IL-2) which are documented to implicate osteoblast differentiation. In the present study, SEC2 was found to significantly improve the osteoblast differentiation by up-regulating BMP2 and Runx2/Cbfa1 expression. Interferon (IFN)-inducible gene IFI16, a co-activator of Runx2/Cbfa1, was also activated by SEC2 in the osteoblast differentiation. In addition, exogenous introduction of SEC2 stimulated OPG expression and suppressed RANKL, suggesting suppression of osteoclastogenesis in hMSCs. Therefore, our results displayed that SEC2 plays an important role in the commitment of MSC to the osteoblast and it might be a potential new therapeutic candidate for bone regeneration.     

Role of Runx genes in chondrocyte differentiation

Runx2/Cbfa1 plays a central role in skeletal development as demonstrated by the absence of osteoblasts/bone in mice with inactivated Runx2/Cbfa1 alleles. To further investigate the role of Runx2 in cartilage differentiation and to assess the potential of Runx2 to induce bone formation, we cloned chicken Runx2 and overexpressed it in chick embryos using a retroviral system. Infected chick wings showed multiple phenotypes consisting of (1) joint fusions, (2) expansion of carpal elements, and (3) shortening of skeletal elements. In contrast, bone formation was not affected. To investigate the function of Runx2/Cbfa1 during cartilage development, we have generated transgenic mice that express a dominant negative form of Runx2 in cartilage. The selective inactivation of Runx2 in chondrocytes results in a severe shortening of the limbs due to a disturbance in chondrocyte differentiation, vascular invasion, osteoclast differentiation, and periosteal bone formation. Analysis of the growth plates in transgenic mice and in chick limbs shows that Runx2 is a positive regulator of chondrocyte differentiation and vascular invasion. The results further indicate that Runx2 promotes chondrogenesis either by maintaining or by initiating early chondrocyte differentiation. Furthermore, Runx2 is essential but not sufficient to induce osteoblast differentiation. To analyze the role of runx genes in skeletal development, we performed in situ hybridization with Runx2- and Runx3-specific probes. Both genes were coexpressed in cartilaginous condensations, indicating a cooperative role in the regulation of early chondrocyte differentiation and thus explaining the expansion/maintenance of cartilage in the carpus and joints of infected chick limbs.     

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.     

Bone morphogenetic protein-2 stimulates Runx2 acetylation

Runx2/Cbfa1/Pebp2aA is a global regulator of osteogenesis and is crucial for regulating the expression of bone-specific genes. Runx2 is a major target of the bone morphogenetic protein (BMP) pathway. Genetic analysis has revealed that Runx2 is degraded through a Smurf-mediated ubiquitination pathway, and its activity is inhibited by HDAC4. Here, we demonstrate the molecular link between Smurf, HDACs and Runx2, in BMP signaling. BMP-2 signaling stimulates p300-mediated Runx2 acetylation, increasing transactivation activity and inhibiting Smurf1-mediated degradation of Runx2. HDAC4 and HDAC5 dea-cetylate Runx2, allowing the protein to undergo Smurf-mediated degradation. Inhibition of HDAC increases Runx2 acetylation, and potentiates BMP-2-stimulated osteoblast differentiation and increases bone formation. These results demonstrate that the level of Runx2 is controlled by a dynamic equilibrium of acetylation, deacetylation, and ubiquitination. These findings have important medical implications because BMPs and Runx2 are of tremendous interest with regard to the development of therapeutic agents against bone diseases.     

Hey1 basic helix-loop-helix protein plays an important role in mediating BMP9-induced osteogenic differentiation of mesenchymal progenitor cells

Pluripotent mesenchymal stem cells (MSCs) are bone marrow stromal progenitor cells that can differentiate into osteogenic, chondrogenic, adipogenic, and myogenic lineages. We previously demonstrated that bone morphogenetic protein (BMP) 9 is one of the most potent and yet least characterized BMPs that are able to induce osteogenic differentiation of MSCs both in vitro and in vivo. Here, we conducted gene expression-profiling analysis and identified that Hey1 of the hairy/Enhancer of split-related repressor protein basic helix-loop-helix family was among the most significantly up-regulated early targets in BMP9-stimulated MSCs. We demonstrated that Hey1 expression was up-regulated at the immediate early stage of BMP9-induced osteogenic differentiation. Chromatin immunoprecipitation analysis indicated that Hey1 may be a direct target of the BMP9-induced Smad signaling pathway. Silencing Hey1 expression diminished BMP9-induced osteogenic differentiation both in vitro and in vivo and led to chondrogenic differentiation. Likewise, constitutive Hey1 expression augmented BMP9-mediated bone matrix mineralization. Hey1 and Runx2 were shown to act synergistically in BMP9-induced osteogenic differentiation, and Runx2 expression significantly decreased in the absence of Hey1, suggesting that Runx2 may function downstream of Hey1. Accordingly, the defective osteogenic differentiation caused by Hey1 knockdown was rescued by exogenous Runx2 expression. Thus, our findings suggest that Hey1, through its interplay with Runx2, may play an important role in regulating BMP9-induced osteoblast lineage differentiation of MSCs.     

Metformin attenuates trauma‐induced heterotopic ossification via inhibition of Bone Morphogenetic Protein signalling

AMP‐activated protein kinase (AMPK) is an intracellular sensor of energy homoeostasis that is activated under energy stress and suppressed in energy surplus. AMPK activation leads to inhibition of anabolic processes that consume ATP. Osteogenic differentiation is a process that highly demands ATP during which AMPK is inhibited. The bone morphogenetic proteins (BMPs) signalling pathway plays an essential role in osteogenic differentiation. The present study examines the inhibitory effect of metformin on BMP signalling, osteogenic differentiation and trauma‐induced heterotopic ossification. Our results showed that metformin inhibited Smad1/5 phosphorylation induced by BMP6 in osteoblast MC3T3‐E1 cells, concurrent with up‐regulation of Smad6, and this effect was attenuated by knockdown of Smad6. Furthermore, we found that metformin suppressed ALP activity and mineralization of the cells, an event that was attenuated by the dominant negative mutant of AMPK and mimicked by its constitutively active mutant. Finally, administration of metformin prevented the trauma‐induced heterotopic ossification in mice. In conjuncture, AMPK activity and Smad6 and Smurf1 expression were enhanced by metformin treatment in the muscle of injured area, concurrently with the reduction of ALK2. Collectively, our study suggests that metformin prevents heterotopic ossification via activation of AMPK and subsequent up‐regulation of Smad6. Therefore, metformin could be a potential therapeutic drug for heterotopic ossification induced by traumatic injury.     

Haploinsufficiency of Runx2 results in bone formation decrease and different BSP expression pattern changes in two transgenic mouse models

Runx2 has been identified as "a master gene" for the differentiation of osteoblasts and Runx2-deficient mice has demonstrated a complete absence of mature osteoblast and ossification. To further characterize the Runx2 responsive elements within the bone sialoprotein (BSP) promoter and further investigate into the role of Runx2 haploinsufficiency in osteoblast differentiation, mBSP9.0Luc mice and mBSP4.8Luc mice were crossed with Runx2-deficient mice respectively. Luciferase assay, micro CT scan, and histological analysis were performed using tissues isolated from mBSP9.0luc/Runx2+/- mice, mBSP4.8luc/Runx2+/- mice and their corresponding Runx2+/+ littermates. Alkaline phosphatase activity, mineralization assays and RT-PCR analysis using calvarial osteoblasts isolated from these transgenic mice were also performed. Luciferase assay demonstrated an early increase in luciferase expression in mBSP9.0luc/Runx2+/- mice before the expression level of luciferase dramatically decreased and turned lower than that in their control littermates in later stages. In contrast, luciferase expression in mBSP4.8luc/Runx2+/- failed to show such an early increase. Micro CT scan and histological analysis showed that BMD and trabecular bone volume were decreased and bone formation was delayed in Runx2+/- mice. Furthermore, mineralization assay and semi-quantitative RT-PCR assay demonstrated a gene-dose-dependent decrease in bone nodule formation and bone marker genes expression levels in cultured calvarial osteoblasts derived from Runx2 knockout mice. Reconstitution of Runx2-null cells with Runx2 vector partially rescued the osteoblast function defects. In conclusion, the 9.0 kb BSP promoter demonstrated a higher tissue-specific regulation of the BSP gene by Runx2 in vivo and full Runx2 gene dose is essential for osteoblast differentiation and normal bone formation.     

Malignant reversion of a human osteosarcoma cell line,Saos-2, by inhibition of NFkappaB

Beyond a pivotal role in neoplastic transformation and malignant progression, NFkappaB is intricately involved in bone biology, pointed up by the osteopetrotic phenotype of NFkappaB (p50-p52) double knock-out mice. Osteopetrosis results from intrinsic defects in osteoclastogenesis, loss of osteoclast bone resorptive activity and, questionably, increased osteoblast activity (bone matrix apposition and mineralization). We here report that inhibition of NFkappaB signaling activity in Saos-2 cells results in a marked decrease in cellular proliferation, assessed by the incorporation of radioactive thymidine into cellular DNA. Decreased cellular proliferation was accompanied by the induction of bone morphogenic proteins (BMP) 4, 7, and the osteoblast specific transciption factor, Cbfa1, heralding osteoblast differentiation, given the induction of alkaline phosphatase, osteopontin, and osteocalcin message levels and the attendant increase in matrix deposition and mineralization in vitro. These results point to the negative regulation of osteoblast differentiation by NFkappaB, with implications in the pathogenesis and progression of osteosarcomas.     

BMP7基因沉默抑制钙盐诱导猪主动脉瓣膜间质细胞成骨分化

目的:探究小干扰RNA(small interference RNA,siRNA)介导的骨形态发生蛋白7(bone morphogenetic protein7,BMP7)基因沉默对钙盐诱导猪主动脉瓣膜间质细胞成骨分化的影响及机制,为钙化性主动脉瓣膜病(calcific aortic valve disease,CAVD)的干预及治疗提供理论依据。方法:非CAVD瓣膜组织(non-CAVD组)取自手术治疗的主动脉夹层患者,CAVD瓣膜组织(CAVD组)取自因钙化性主动脉瓣狭窄而进行主动脉瓣膜置换术的患者,采用免疫组化和Western blot法检测non-CAVD组和CAVD组中BMP7、Runt相关转录因子2(Runx2)的蛋白质表达水平。选取健康家猪处死后即刻于无菌条件下取主动脉瓣叶,采用胶原酶连续消化法分离主动脉瓣膜间质细胞,观察其形态特征,并用免疫荧光染色行表型鉴定。采用脂质体转染法将BMP7-siRNA转染猪主动脉瓣膜间质细胞,采用qPCR和Western blot法验证BMP7表达的变化;利用钙盐培养基诱导细胞成骨分化,建立体外主动脉瓣膜间质细胞钙化模型后,采用ALP染色和茜素红S染色实验分别检测细胞早期及晚期成骨分化能力;采用qPCR和Western blot法分别检测细胞成骨相关基因及蛋白质Runx2、OCN和OPN的表达情况。并用Western blot法检测BMP7下游信号通路中Smad1/5/8的磷酸化水平。结果:BMP7和Runx2蛋白在CAVD组中表达明显高于non-CAVD组。成功分离出原代猪主动脉瓣膜间质细胞,α-平滑肌肌动蛋白(α-SMA)及波形蛋白(vimentin)染色阳性,血管性血友病因子(von willebrand factor,vWF)染色阴性。转染BMP7-siRNA后猪主动脉瓣膜间质细胞中BMP7的mRNA和蛋白质水平均明显下调,早期及晚期成骨分化能力均明显降低。沉默BMP7基因的表达,可下调Runx2、OCN和OPN的基因及蛋白质表达,且磷酸化的Smad1/5/8(p-Smad1/5/8)蛋白水平明显降低。结论:BMP7基因沉默抑制钙盐诱导的主动脉瓣膜间质细胞的成骨分化能力,BMP7/Smads信号通路可能在该过程中发挥重要作用。