Pitx2 prevents osteoblastic transdifferentiation of myoblasts by bone morphogenetic proteins
Pitx2 prevents osteoblastic transdifferentiation of myoblasts by bone morphogenetic proteins
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DOI：
10.1074/jbc.m708154200
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发表时间：
2008-01-04
影响因子：
4.8
通讯作者：
Imamura, Takeshi
中科院分区：
文献类型：
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作者：
Hayashi, Makoto;Maeda, Shingo;Imamura, Takeshi
在病理性肌肉和/或骨骼条件下，肌肉细胞经常暴露于骨形态发生蛋白(BMPs)。由于BMP既是强骨诱导剂又是肌肉生成抑制因子，某些分子可能会阻止肌肉细胞转化为病理性骨；如果没有这些分子，创伤性骨化性肌炎就会形成新生骨。当C2C12成肌细胞暴露在BMPs中时，它们会分化为成骨细胞，但不能成熟为骨细胞。由于成骨细胞分化的转录因子Osterix基因仅在BMP刺激下在C2C12细胞中瞬时诱导，我们假设未知的转录抑制因子(S)抑制Osterix的表达，阻止成骨细胞的完全分化。基因芯片分析确定了可能的成骨细胞分化抑制因子，并鉴定了在左右不对称中发挥重要调节作用的配对样同源域转录因子Pitx2(也称为Rieg)。在BMP刺激C2C12细胞2天后，结合Osterix下调表达，可诱导Pitx2表达。Pitx2的过表达抑制了Osterix的表达和随后的成骨细胞分化，而最上游的成骨调节因子Runx2没有受到影响。相反，在C2C12细胞中诱导Pitx2的短发夹状RNA促进了Osterix的表达和成骨细胞在BMP刺激下的成熟。此外，含有来自Pitx2缺失胚胎的成肌细胞的小鼠胚胎成纤维细胞在BMP刺激下显示出增强的Osterix表达。这些发现表明，Pitx2抑制了成肌细胞中BMPs诱导的成骨信号，以防止其成骨转化。
Muscle cells are often exposed to bone morphogenetic proteins (BMPs) in pathological muscle and/or bone conditions. Because BMPs function as strong bone inducers as well as myogenesis inhibitors, certain molecules likely prevent muscle cells from converting into pathologic bone; without these molecules, de novo bone would form as observed in myositis ossificans traumatica. When C2C12 myoblasts are exposed to BMPs, they differentiate into osteoblastic cells but cannot mature into bone cells. As the Osterix gene, a transcription factor for osteoblast differentiation, is only transiently induced upon BMP stimulation in C2C12 cells, we hypothesized that unknown transcriptional repressor(s) inhibit Osterix expression and prevent complete osteoblastic differentiation. Gene microarray analyses were performed to identify putative inhibitors for osteoblastic differentiation, and the paired-like homeodomain transcription factor Pitx2 (also termed Rieg), which plays an important regulatory role in left-right asymmetry, was identified. Pitx2 was induced 2 days after BMP stimulation in C2C12 cells in concert with Osterix down-regulation. Overexpression of Pitx2 repressed Osterix expression and subsequent osteoblastic differentiation, whereas Runx2, the most upstream regulator of osteogenesis, was unaffected. Conversely, the induction of short hairpin RNA for Pitx2 in C2C12 cells enhanced Osterix expression and osteoblastic maturation upon BMP stimulation. Moreover, mouse embryonic fibroblasts containing myoblasts from Pitx2-null embryos showed enhanced Osterix expression upon BMP stimulation. These findings suggest that Pitx2 suppresses osteogenic signals induced by BMPs in myoblasts to prevent their osteoblastic conversion.