Activation of mitogen-activated protein kinase cascades is involved in regulation of bone morphogenetic protein-2-induced osteoblast differentiation in pluripotent C2C12 cells

Bone morphogenetic protein (BMP)-2, a member of the transforming growth factor-beta (TGF-beta) superfamily, is able to induce osteoblastic differentiation of C2C12 cells. Both Smad and mitogen-activated protein kinase (MAPK) pathways are essential components of the TGF-beta superfamily signaling machinery. Although Smads have been demonstrated to participate in the BMP-2-induced osteoblastic differentiation of C2C12 cells, the role of MAPK has not been addressed. This report shows that BMP-2 activates ERK and p38, but not JNK, in C2C12 cells. Pretreatment of cells with the p38 inhibitor, SB203580, dramatically reduced BMP-2-induced expression of the osteoblast markers alkaline phosphatase (ALP) and osteocalcin (OC). Nevertheless, overexpression of MKK3, a protein kinase that phosphorylates and activates p38, failed to induce ALP or OC expression in the absence of BMP-2, indicating that p38 activation is necessary but not sufficient for the acquisition of the osteoblast phenotype by these cells. Although ALP induction was increased slightly in the presence of PD-98059, a selective inhibitor of the ERK cascade, this compound significantly inhibited both steady-state and BMP-2-induced OC RNA levels. Our results indicate that p38 and ERK cascades play a crucial role in the osteoblast differentiation of C2C12 cells mediated by BMP-2.     

Bone morphogenetic protein activities are enhanced by 3',5'-cyclic adenosine monophosphate through suppression of Smad6 expression in osteoprogenitor cells

Bone morphogenetic proteins (BMPs) belong to the transforming growth factor (TGF)-beta superfamily, and some display potent osteogenic activity both in vivo and in vitro. The BMP signaling cascade involving BMP receptors at the cell membrane and intracellular messengers (Smads) has been elucidated, but the regulatory mechanisms of BMP signaling have not been clarified. We previously found that pentoxifyline (PeTx), a nonspecific inhibitor of phosphodiesterase (PDE), and rolipram, a PDE-4-specific inhibitor, enhance BMP-4-induced osteogenic differentiation of mesenchymal cells, probably through the elevation of intracellular cyclic adenosine monophosphate (cAMP) accumulation and modulation of BMP signaling pathways as enhanced BMP-4 action was reproduced by addition of dibutylyl-cAMP (dbcAMP). However, the precise mechanisms underlying the enhancing effects of those agents on BMP signaling were not completely revealed. As already reported, BMPs utilize a specific intracellular signaling cascade to target genes via R-Smads (Smad1,5,8), Co-Smad (Smad4) and I-Smads (Smad6,7). One possibility for cAMP-mediated effects on BMP signaling might be suppression of I-Smads expression since these proteins form a negative feedback loop in BMP signaling. To examine this possibility, changes in I-Smad (Smad6) expression on addition of dbcAMP or PeTx were examined in a bone-marrow-derived osteogenic cell line (ST2). Alkaline phosphatase activity in ST2 cells was consistently induced by BMP-4 treatment (300 ng/ml), and Smad6 mRNA expression was also induced by BMP-4 treatment. Although concurrent treatment of ST2 cells with BMP-4 and dbcAMP elicited further activation of alkaline phosphatase, addition of dbcAMP reduced BMP-4-induced Smad6 expression in a dose-dependent manner. Furthermore, detection of phosphorylated Smad1/5/8 on Western blotting analysis was prolonged, suggesting prolonged kinase activity of BMP receptors through suppressed expression of Smad6. Elevated intracellular cAMP might thus enhance BMP signaling by suppressing Smad6 induction and prolonging intracellular BMP signaling.     

Osteogenic activity of the fourteen types of human bone morphogenetic proteins (BMPs)

BACKGROUND: Bone morphogenic proteins (BMPs) are known to promote osteogenesis, and clinical trials are currently underway to evaluate the ability of certain BMPs to promote fracture-healing and spinal fusion. The optimal BMPs to be used in different clinical applications have not been elucidated, and a comprehensive evaluation of the relative osteogenic activity of different BMPs is lacking. METHODS: To identify the BMPs that may possess the most osteoinductive activity, we analyzed the osteogenic activity of BMPs in mesenchymal progenitor and osteoblastic cells. Recombinant adenoviruses expressing fourteen human BMPs (BMP-2 to BMP-15) were constructed to infect pluripotent mesenchymal progenitor C3H10T1/2 cells, preosteoblastic C2C12 cells, and osteoblastic TE-85 cells. Osteogenic activity was determined by measuring the induction of alkaline phosphatase, osteocalcin, and matrix mineralization upon BMP stimulation. RESULTS: BMP-2, 6, and 9 significantly induced alkaline phosphatase activity in pluripotential C3H10T1/2 cells, while BMP-2, 4, 6, 7, and 9 significantly induced alkaline phosphatase activity in preosteoblastic C2C12 cells. In TE-85 osteoblastic cells, most BMPs (except BMP-3 and 12) were able to induce alkaline phosphatase activity. The results of alkaline phosphatase histochemical staining assays were consistent with those of alkaline phosphatase colorimetric assays. Furthermore, BMP-2, 6, and 9 (as well as BMP-4 and, to a lesser extent, BMP-7) significantly induced osteocalcin expression in C3H10T1/2 cells. In C2C12 cells, osteocalcin expression was strongly induced by BMP-2, 4, 6, 7, and 9. Mineralized nodules were readily detected in C3H10T1/2 cells infected with BMP-2, 6, and 9 (and, to a lesser extent, those infected with BMP-4 and 7). CONCLUSIONS: A comprehensive analysis of the osteogenic activity of fourteen types of BMPs in osteoblastic progenitor cells was conducted. Our results suggest an osteogenic hierarchical model in which BMP-2, 6, and 9 may play an important role in inducing osteoblast differentiation of mesenchymal stem cells. In contrast, most BMPs are able to stimulate osteogenesis in mature osteoblasts.     

Chondrocytes provide morphogenic signals that selectively induce osteogenic differentiation of mesenchymal stem cells.

During endochondral bone development cartilage formation always precedes that of bone, leading to the hypothesis that chondrocytes provide inductive signals for osteogenesis. To test this hypothesis, C3H10T1/2 mesenchymal stem cells were cocultured in membrane separated trans-well culture chambers with nonhypertrophic chondrocytes, hypertrophic chondrocytes, calvaria osteoblasts, or tendon fibroblasts derived from embryonic chickens to assess if individual cell types would selectively promote osteogenic differentiation. Then, differentiation of C3H10T1/2 mesenchymal stem cells in coculture were compared with that induced by bone morphogenetic protein 7 or osteogenic protein-1 (BMP-7; OP-1) treatment. Osteogenesis, as determined by the expression of Cbfa1 and osteocalcin (OC) messenger RNAs (mRNAs), was induced strongly in C3H10T1/2 cells cocultured with both chondrocyte cell populations but was not induced by coculture with either osteoblasts or skin fibroblasts. Interestingly, treatment of C3H10T1/2 cells with BMP-7 induced both chondrogenesis and osteogenesis, and only osteogenic differentiation was observed in the C3H10T1/2 cells cocultured with chondrocytes. No alterations in the expression of mRNAs for BMP-1 to -8 were observed in the C3H10T1/2 cells under any of the coculture conditions. This shows that the induction of endogenous BMPs by coculture does not regulate osteogenesis in an autocrine manner. These results show that chondrocytes express soluble morphogenetic factors that selectively promote osteogenesis, and this selective effect is not mimicked by an exogenously added BMP.     

核结合因子a1的调控机制

Cbfa1是骨发育过程中调节骨髓基质干细胞向成骨细胞分化和成熟的重要转录因子。Cbfa1的表达水平异常与骨骼系统疾病有关。体内体外实验证实多种通路(如Wnt/LRP5/-catenin,BMP/Smads,1,25-(OH)2-vitaminD3/VDR/VDRE途径)和调节蛋白(Msx2,Dlx5,Twists)在Cbfa1基因表达、活性和随后的骨形成过程中起关键作用。这些发现对调控成骨细胞分化和治疗骨质疏松以及其他伴有骨量改变的疾病治疗提供了新的思路,这些疾病有可能用控制Cbfa1表达来进行治疗。     

Protein phosphatase magnesium‐dependent 1A–mediated inhibition of BMP signaling is independent of Smad dephosphorylation

Phosphorylation of Smad1/5/8 at carboxyl‐terminal serine residues by type I receptors activates downstream bone morphogenetic protein (BMP) signaling. Protein phosphatase magnesium‐dependent 1A (PPM1A) has been shown to suppress BMP activity by dephosphorylating phospho‐Smads. We report here that PPM1A suppresses BMP signaling via a novel mechanism. PPM1A inhibited a constitutively activated Smad1 mutant lacking BMP receptor phosphorylation sites. PPM1A reduced the protein levels not only of Smad1 but also of Smad5 and Smad8. A proteasome inhibitor blocked the inhibitory effects of PPM1A on Smad1, but the Smurf‐binding motif in the Smad1 linker region was not involved in this inhibition. The phosphatase activity of PPM1A is essential for inhibition. Taken together, these findings suggest that through the dephosphorylation of unidentified substrate(s), PPM1A inhibits BMP signaling by decreasing Smad protein levels via the proteasome pathway. Moreover, knockdown of endogenous PPM1A stimulated osteoblastic differentiation, suggesting that PPM1A may physiologically suppress BMP signaling via Smads. © 2010 American Society for Bone and Mineral Research     

血管内皮生长因子对脂肪干细胞成骨分化基因转录水平的调节

目的探讨内源性血管内皮生长因子（VEGF）体外诱导脂肪干细胞（ADSCs）分化成骨的转录活化机制。方法脂肪干细胞来自大白鼠腹股沟处脂肪,脂肪内干细胞的分离根据JMGimblel所描述的方法获取,培养鉴定后以BMP-2修饰组作为阳性对照组,提取细胞总RNA,采用RT-PCR法及琼脂糖凝胶电泳方法分析在不同诱导条件下Cbfal和Osterix表达的差异。结果BMP-2重组蛋白组的Cbfal基因转录水平明显高于VEGF诱导组（P〈0．01）,两者联合应用未见Cbfal基因转录增强;在以VEGF为诱导因子的ADSCs组,Osterix的mRNA水平明显低于BMP-2组（P〈0．01）,与Cbfal基因转录模式不同,VEGF联合BMP-2共同诱导ADSCs成骨分化组,Osterix基因的mRNA水平有增高的趋势。结论通过Cbfal和Osterix在ADSC成骨分化的不同时间的表达差异,本研究认为在ADSCs诱导成骨分化过程中,Cbfal启动和激活Osterix转录,仍是主要的信号传导途径。VEGF具有一定的诱导ADSCs成骨分化能力,但其作用主要是非依赖性Cbfal通路调控的Osterix表达。     

Autogenous regulation of a network of bone morphogenetic proteins (BMPs) mediates the osteogenic differentiation in murine marrow stromal cells

The expression patterns of (bone morphogenetic proteins) BMPs during fracture repair and pre-natal bone development suggest that these processes are regulated through the coordinated actions of multiple BMPs. Murine bone marrow stromal cells (MSCs) in culture provide a well recognized ex vivo system of mesenchymal stem cell differentiation in which the effects of BMPs can be examined. Studies were performed to determine if MSC differentiation is dependent on the endogenous expression of multiple BMPs and to characterize their interactions. MSCs were harvested from the bone marrow of tibiae and femora of 8 to 10-week-old male C57/B6 mice and prepared by standard methods. Osteogenic differentiation was assessed by histological assays, alkaline phosphatase enzyme activity and assays for the expression of multiple mRNAs for BMPs and osteogenic development. The role of autogenously expressed BMPs in controlling the osteogenic differentiation of marrow stromal cells in vitro was assessed in both gain-of-function and loss-of-function experiments. Gain of function experiments were carried out in the presence of exogenously added BMP-2 or -7 and loss-of-function experiments were carried out by BMP antagonism with noggin and BMP-2 antibody blockade. Osteogenic differentiation was concurrent with and proportional to increases in the expression of BMPs-2, -3, -4, -5, -6 and -8A. BMP antagonism with either noggin or BMP-2 antibody blockade inhibited osteogenic differentiation by 50% to 80%, respectively, and reduced the expression of endogenous levels of BMPs-2, -3, -5 and -8A. In contrast, antagonism induced the expression of BMP-4 and -6. The addition of rhBMP-2 or -7 enhanced osteogenic differentiation and produced a reciprocal expression profile in the endogenous BMPs expression as compared to BMP antagonism. BMP antagonism could be rescued through the competitive addition of rhBMP-2. These studies demonstrated that osteogenic differentiation was regulated by a complex network of multiple BMPs that showed selective increased and decreased expression during differentiation. They further demonstrated that BMP-2 was a central regulator in this network.     

Phospho‐Smad1 modulation by nedd4 e3 ligase in BMP/TGF‐β signaling

A considerable number of studies have focused on the regulation of mothers against decapentaplegic homologue (Smad)–dependent or –independent pathways in the signaling by each transforming growth factor β (TGF‐β) superfamily member in diverse biologic contexts. The sophisticated regulation of the actions of these molecules and the underlying molecular mechanisms still remain elusive. Here we show new mechanisms of ambilateral R (receptor‐regulated)–Smad regulation of bone morphogenetic protein 2 (BMP‐2)/TGF‐β1 signals. In a specific context, both signals regulate the nonclassic Smads pathway reciprocally, BMP‐2 to Smad2/3 and TGF‐β1 to Smad1/5/8, as well as their own classic linear Smad pathway. Interestingly, in this study, we found that C‐terminal phosphorylated forms of each pathway Smad degraded rapidly 3 hours after stimulation of nonclassic signals but are dramatically restored by treatment with via proteasomal inhibition. Furthermore, an E3 ligase, neural precursor cell expressed, developmentally down‐regulated 4 (Nedd4), also was found as one of the important modulators of the p‐Smad1 in both BMP‐2 and TGF‐β1 action. Overexpressed Nedd4 suppressed the BMP‐induced osteoblast transdifferentiation process of premyoblast C2C12 cells or alkaline phosphatase (ALP) level of human osteosarcoma cells and promoted TGF‐β1‐induced degradation of p‐Smad1 via physical interaction and polyubiquitination. Conversely, siNedd4 potentiated BMP signals through upregulation of p‐Smad1 and ALP activity, the effect of which led to an increased the rate of P_i‐induced calcification of human vascular smooth muscle cells. These new insights about proteasomal degradation–mediated phosphorylated nonclassic Smad regulation of BMP‐2/TGF‐β1 could, in part, help to unravel the complex mechanisms of abnormal nonosseous calcification by the aberrant activity of BMP/TGF‐β/Smads. © 2011 American Society for Bone and Mineral Research.     

Osteogenic Protein-1 Stimulates mRNA Levels of BMP-6 and Decreases mRNA Levels of BMP-2 and -4 in Human Osteosarcoma Cells

Bone morphogenetic proteins (BMPs) are novel growth and differentiation factors that act on mesenchymal stem cells to initiate new bone formation in vivo and promote the growth and differentiation of cells in the osteoblastic lineage. In the present study, we examined the effects of recombinant human osteogenic protein-1 (also known as BMP-7) on the expression of related members of the BMP family using SaOS-2 and U2-OS, two human osteosarcoma cell strains. Evaluation of BMP-2, -4, and -6 mRNA expression indicates that OP-1 stimulated the mRNA levels of BMP-6 in both SaOS-2 cells (threefold) and U2-OS cells (fivefold) after 24 hours of treatment, while decreasing the mRNA levels of BMP-4 in SaOS-2 cells (80%) and BMP-2 and BMP-4 in U2-OS cells by 50% and 72%, respectively. BMP-2 mRNA expression, as examined by Northern blot analysis, was below detectable limits in SaOS-2 cultures. These results demonstrate that OP-1 modulates the mRNA expression of related members of the BMP family, suggesting a possible mode of action of OP-1 on the growth and differentiation of cells in the osteoblastic lineage in vitro. Received: 7 May 1996 / Accepted: 24 September 1996     

Sustained Delivery of BMP-2 and Platelet-rich Plasma-released Growth Factors Contributes to Osteogenesis of Human Adipose-derived Stem Cells

Platelet-rich plasma (PRP) has a pool of multiple growth factors efficient at inducing the proliferation and osteogenic differentiation of human adipose-derived stem cells (hADSCs). Bone morphogenetic protein (BMP)-2 is a strong stimulator for the osteogenic differentiation of hADSCs. The purpose of this study was to verify the effect of PRP-released growth factors and microsphere-encapsulated BMP-2 on the proliferation and osteoblastic differentiation of hADSCs and to construct a novel tissue-engineered bone. The BMP-2-loaded microspheres and hADSCs were embedded in activated PRP gel. Another 5 composites (hADSCs/platelet-poor plasma [PPP]; hADSCs/PRP; hADSCs/BMP-2/PPP; hADSCs/BMP-2/PRP; and hADSCs/BMP-2+microspheres/PPP) were also constructed. The DNA content, alkaline phosphatase activity, mRNA expression of alkaline phosphatase, osteopontin, osteocalcin, and mineralization of hADSCs in each composite were compared. The DNA content was higher in all PRP-containing composites, meaning that PRP-released growth factors stimulated proliferation of hADSCs. Alkaline phosphatase increased in BMP-2/PRP and BMP-2+microspheres/PRP composites in the first 7 days, meaning that BMP-2 had a synergistic effect with PRP in the early differentiation of hADSCs. Osteopontin, osteocalcin, and mineralization assays were higher in BMP-2+microspheres/PRP composite than in the BMP-2/PRP composite up to 21 days, meaning that a continuous delivery of BMP-2 stimulates osteoblastic differentiation of hADSCs at the early stage and the final maturation stage. These results suggest that sustained delivery of BMP-2 in combination with PRP is better than a single administration of PRP or BMP-2 in the osteogenic differentiation of hADSCs.     

木通皂苷D促进糖皮质激素环境下小鼠骨髓间充质干细胞成骨分化

目的观察木通皂苷D(ASD)对糖皮质激素(GC)环境下小鼠骨髓间充质干细胞(BMSCs)成骨分化的影响及其机制。方法体外培养小鼠BMSCs,实验分为三组,分别用成骨诱导培养基(OBI)、OBI+地塞米松(DEXA,0.1μmol/L)、OBI+DEXA(0.1μmol/L)+ASD(10μmol/L)干预。ALP试剂盒检测ALP活性,茜素红染色检测矿化情况,qRT-PCR检测成骨因子Runx2、骨钙素(OCN)及Smad1、Smad5 mRNA表达,Western blot检测Smad1/5磷酸化水平及Smad1/5/8总蛋白表达。结果OBI+DEXA组ALP活性明显较OBI组低(P<0.001),OBI+DEXA+ASD组ALP活性明显较OBI+DEXA升高(P<0.01)。OBI+DEXA干预组成骨相关因子Runx2、OCN、Smad1和Smad5 mRNA表达较OBI组均明显下调(P<0.01),OBI+DEXA+ASD组Runx2、OCN、Smad1和Smad5 mRNA表达较OBI+DEXA干预组明显上调(P<0.01和P<0.05)。三组Smad1/5/8总蛋白表达差异无统计学意义,OBI+DEXA干预组pSmad1/5较OBI干预组表达降低,OBI+DEXA+ASD干预组pSmad1/5表达较OBI+DEXA干预组增加。结论ASD可促进GC环境下小鼠BMSCs成骨分化,其机制可能与激活BMP/Smad信号通路有关。