Nakashima Aiko, Katagiri Takenobu, Tamura Masato
Department of Biochemistry and Molecular Biology, Graduate School of Dental Medicine, Hokkaido University, Sapporo, Japan.
J Biol Chem. 2005 Nov 11;280(45):37660-8. doi: 10.1074/jbc.M504612200. Epub 2005 Sep 2.
Loss of function of the Wnt co-receptor, lipoprotein receptor-related protein 5, decreases bone formation, and a point mutation in this gene results in high bone mass, indicating the importance of this signaling pathway in bone formation. However, the exact mechanism is currently unknown. We examined a potential role for Wnt signaling and functional cross-talk of bone morphogenetic protein 2 (BMP-2) in osteoblast differentiation. To assess the contribution of Wnt, we generated C2C12 cells over-expressing Wnt3a or Wnt5a and treated these with BMP-2. We showed that expression of matrix extracellular phosphoglycoprotein was induced by BMP-2 in Wnt3a over-expressing C2C12 cells but not in Wnt5a over-expressing C2C12 cells. Over-expression of Wnt3a blocked BMP-2-induced inhibition of myotube formation in C2C12 cells when switched to low mitogen medium. In these cultures, expression of inhibitor of DNA binding/differentiation (Id) 1, a helix-loop-helix protein induced by BMP-2, decreased in stable Wnt3a- but not in Wnt5a-expressing cells. This suppression is mediated by a GC-rich region of the BMP-2-responsive element of the Id1 gene promoter, and interaction between Smad1/4 and beta-catenin is crucial for Wnt-mediated suppression of the BMP-2 response in C2C12 cells. Over-expression of the inhibitor of canonical Wnt signaling, Dickkopf, inhibits this suppression. In contrast, BMP-2 or Smad1/4 up-regulated Wnt3a or activated beta-catenin-induced lymphoid-enhancing factor 1/T cell factor-dependent transcriptional activity. These findings identify functional cross-talk of Id1 expression between Wnt and BMP signaling and demonstrate a novel mechanism for Wnt regulation of the BMP-2 response, linking Id1 expression to Wnt/beta-catenin signaling.
Wnt 共受体脂蛋白受体相关蛋白 5 的功能丧失会减少骨形成，该基因的一个点突变会导致高骨量，这表明该信号通路在骨形成中具有重要性。然而，确切机制目前尚不清楚。我们研究了 Wnt 信号传导以及骨形态发生蛋白 2（BMP-2）在成骨细胞分化中的潜在作用及其功能相互作用。为了评估 Wnt 的作用，我们构建了过表达 Wnt3a 或 Wnt5a 的 C2C12 细胞，并使用 BMP-2 对其进行处理。我们发现，在过表达 Wnt3a 的 C2C12 细胞中，BMP-2 可诱导基质细胞外磷酸糖蛋白的表达，而过表达 Wnt5a 的 C2C12 细胞中则无此现象。当转换至低丝裂原培养基时，Wnt3a 的过表达可阻断 BMP-2 诱导的 C2C12 细胞中肌管形成的抑制作用。在这些培养物中，由 BMP-2 诱导的螺旋-环-螺旋蛋白 DNA 结合/分化抑制因子（Id）1 在稳定表达 Wnt3a 的细胞中表达降低，但在表达 Wnt5a 的细胞中未降低。这种抑制作用由 Id1 基因启动子的 BMP-2 反应元件富含 GC 的区域介导，并且 Smad1/4 与β-连环蛋白之间的相互作用对于 Wnt 介导的 C2C12 细胞中 BMP-2 反应的抑制至关重要。经典 Wnt 信号传导抑制剂 Dickkopf 的过表达可抑制这种抑制作用。相反，BMP-2 或 Smad1/4 上调 Wnt3a 或激活β-连环蛋白诱导的淋巴细胞增强因子 1/T 细胞因子依赖性转录活性。这些发现确定了 Wnt 和 BMP 信号传导之间 Id1 表达的功能性相互作用，并证明了 Wnt 调节 BMP-2 反应的新机制，将 Id1 表达与 Wnt/β-连环蛋白信号传导联系起来。
