BMP2 Differentially Regulates the Expression of Gremlin1 and Gremlin2, the Negative Regulators of BMP Function, During Osteoblast Differentiation

Bone morphogenetic proteins (BMPs) control the expressions of many genes involved in bone formation. On the basis of our hypothesis that BMP2 stimulation-regulated gene expression plays a critical role in osteoblast differentiation, we performed genome-wide screening of messenger RNA from BMP2-treated and -untreated C2C12 cells using a DNA microarray technique. We found that the expressions of Gremlin1 and Gremlin2, which are known BMP antagonists, were bidirectionally regulated by BMP2. Gremlin1 was down-regulated by BMP2, while Gremlin2 was up-regulated in both time- and dose-dependent manners. Ablation of Gremlin1 or Gremlin2 enhanced osteoblast differentiation induced by BMP2. On the other hand, treatment with recombinant Gremlin1 inhibited BMP2-induced osteoblast differentiation. Furthermore, treatment with Smad4 siRNA and the p38 MAPK inhibitor SB203580 suppressed BMP2-induced Gremlin2 gene expression. The differential regulation of Gremlin1 and Gremlin2 gene expressions by BMP2 may explain the critical function of these genes during osteoblast differentiation.     

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。     

Microarray analysis reveals expression regulation of Wnt antagonists in differentiating osteoblasts

Wnt signaling has been implicated in regulating bone formation by controlling osteoblast proliferation and function. Although stabilization of beta-catenin by Wnt has been shown to increase alkaline phosphatase expression and osteoblast differentiation, the precise role of Wnt signaling during the process of osteoblast differentiation is largely unknown. In this study, we used microarray technology to investigate expression regulation of Wnt signaling components during in vitro osteoblast differentiation. Expression was analyzed during bone morphogenetic protein 2 (BMP2)-induced osteoblast differentiation of murine C2C12 and MC3T3 cells and data were compared with expression in BMP2-treated NIH3T3 fibroblasts. During osteoblast differentiation, particularly strong expression regulation of the Wnt antagonists Sfrp2 (secreted frizzled related protein 2) and Wif1 (Wnt inhibitory factor 1) was observed in the late phase of differentiation. In situ expression analysis in murine tail vertebrae supported Wif1 expression during late phase bone cell differentiation, since Wif1 was found to be expressed in vivo in trabecular, but not in cortical bone. We further analyzed the effects of continuous activation of Wnt signaling by lithium chloride and observed that osteoblast differentiation was reduced, as measured by expression of osteoblast marker genes encoding alkaline phosphatase, osteocalcin, and osterix, as well as by the amount of calcium release. Taken together, our data indicate that endogenous expression of Wnt antagonists by osteoblasts provides a negative Wnt feedback loop which is essential in controlling osteoblast maturation.