Mugita N, Honda Y, Nakamura H, Fujiwara T, Tanaka K, Omura S, Shimbara N, Ogawa M, Saya H, Nakao M
Department of Tumor Genetics and Biology, Kumamoto University School of Medicine, 2-2-1 Honjo, Kumamoto 860-0811, Japan.
Int J Mol Med. 1999 Feb;3(2):127-37.
The murine C2C12 myocytes terminally differentiate to myotubes in the mitogen-depletion, and a portion of the cells undergo apoptosis. In this study, a specific proteasome inhibitor lactacystin induced cell cycle withdrawal and precocious expression of myosin in C2C12 cells in mitogen-enriched medium, but these cells did not fuse to form myotubes. Mitogen-starved myocytes could not differentiate to myotubes under the proteasome inhibition. The genes for p21, MyoD, Myogenin and RB were activated, and p27 gene was repressed under the proteasome inhibition, suggesting the transcriptional regulation of these genes linked to the proteasome activity. The induction of p21 prior to MyoD may contribute to the incomplete myogenesis in the presence of lactacystin. In addition, lactacystin-treated C2C12 cells did not undergo apoptosis, while proteasome accumulated in the nuclei of apoptotic cells but not in those of myotubes during mitogen-depleted differentiation. Further, lactacystin induced similarly incomplete differentiation in human RD embryonal rhabdomyosarcoma cells. Our findings demonstrated that proteasome has an essential role in myogenesis, especially in transcriptional control of myogenic and cell cycle regulators, cell fusion forming myotubes, and apoptosis.
在有丝分裂原耗竭的情况下，小鼠C2C12肌细胞会终末分化为肌管，并且一部分细胞会发生凋亡。在本研究中，一种特异性蛋白酶体抑制剂乳胞素在富含丝裂原的培养基中诱导C2C12细胞退出细胞周期并过早表达肌球蛋白，但这些细胞并未融合形成肌管。在蛋白酶体抑制作用下，缺乏有丝分裂原的肌细胞无法分化为肌管。在蛋白酶体抑制作用下，p21、MyoD、生肌调节因子和RB的基因被激活，而p27基因被抑制，这表明这些基因的转录调控与蛋白酶体活性相关。在乳胞素存在的情况下，p21在MyoD之前的诱导可能导致肌生成不完全。此外，经乳胞素处理的C2C12细胞未发生凋亡，而在有丝分裂原耗竭的分化过程中，蛋白酶体在凋亡细胞的细胞核中积累，而不在肌管的细胞核中积累。此外，乳胞素在人RD胚胎性横纹肌肉瘤细胞中诱导出类似的不完全分化。我们的研究结果表明，蛋白酶体在肌生成中具有重要作用，尤其是在对肌生成和细胞周期调节因子的转录控制、细胞融合形成肌管以及细胞凋亡方面。
