Edmondson D G, Olson E N
Department of Biochemistry and Molecular Biology, University of Texas M.D. Anderson Cancer Center, Houston 77030.
Genes Dev. 1989 May;3(5):628-40. doi: 10.1101/gad.3.5.628.
MyoD1 is a nuclear phosphoprotein that is expressed in skeletal muscle in vivo and in certain muscle cell lines in vitro; it has been shown to convert fibroblasts to myoblasts through a mechanism requiring a domain with homology to the myc family of proteins. The BC3H1 muscle cell line expresses skeletal muscle-specific genes upon exposure to mitogen-deficient medium, but does not express MyoD1 at detectable levels. To determine whether BC3H1 cells may express regulatory genes functionally related to MyoD1, a cDNA library prepared from differentiated BC3H1 myocytes, was screened at reduced stringency with the region of the MyoD1 cDNA that shares homology with c-myc. From this screen, a cDNA was identified that encodes a major open reading frame with 72% homology to the myc domain and basic region of MyoD1. The mRNA encoded by this MyoD1-related gene is expressed in skeletal muscle in vivo and in differentiated skeletal myocytes in vitro and is undetectable in cardiac or smooth muscle, nonmuscle tissues, or nonmyogenic cell types. During myogenesis, the MyoD1-related mRNA accumulates several hours prior to other muscle-specific mRNAs and therefore represents an early molecular marker for entry of myoblasts into the differentiation pathway. Transient transfection of 10T1/2 or 3T3 cells with the MyoD1-related cDNA is sufficient to induce myosin heavy-chain expression and to activate a reporter gene under transcriptional control of the muscle creatine kinase 5' enhancer, which functions only in differentiated myocytes. Expression of this cDNA in stably transfected 10T1/2 cells also leads to fusion and muscle-specific gene expression upon exposure to mitogen-deficient medium. Thus, the product of this MyoD1-related gene is sufficient to activate the muscle differentiation program and may substitute for MyoD1 in certain developmental situations. Together, these results suggest the existence of a family of myogenic regulatory genes that share a conserved motif with c-myc.
肌分化蛋白1（MyoD1）是一种核磷蛋白，在体内的骨骼肌以及体外某些肌肉细胞系中表达；研究表明，它可通过一种机制将成纤维细胞转化为成肌细胞，该机制需要一个与myc蛋白家族具有同源性的结构域。BC3H1肌肉细胞系在暴露于缺乏促有丝分裂原的培养基时会表达骨骼肌特异性基因，但检测不到MyoD1的表达水平。为了确定BC3H1细胞是否可能表达与MyoD1功能相关的调控基因，用与c-myc具有同源性的MyoD1 cDNA区域，以较低的严谨性筛选了从分化的BC3H1肌细胞制备的cDNA文库。通过该筛选，鉴定出一个cDNA，其编码的主要开放阅读框与MyoD1的myc结构域和碱性区域具有72%的同源性。这个与MyoD1相关的基因编码的mRNA在体内的骨骼肌和体外分化的骨骼肌细胞中表达，在心脏或平滑肌、非肌肉组织或非成肌细胞类型中检测不到。在肌生成过程中，与MyoD1相关的mRNA在其他肌肉特异性mRNA之前数小时积累，因此代表了成肌细胞进入分化途径的早期分子标志物。用与MyoD1相关的cDNA瞬时转染10T1/2或3T3细胞足以诱导肌球蛋白重链表达，并激活在肌肉肌酸激酶5'增强子转录控制下的报告基因，该增强子仅在分化的肌细胞中起作用。在稳定转染的10T1/2细胞中表达该cDNA，在暴露于缺乏促有丝分裂原的培养基时也会导致细胞融合和肌肉特异性基因表达。因此，这个与MyoD1相关的基因产物足以激活肌肉分化程序，并且在某些发育情况下可能替代MyoD1。总之，这些结果表明存在一个与c-myc共享保守基序的成肌调节基因家族。
