Clegg C H, Hauschka S D
J Cell Biol. 1987 Aug;105(2):937-47. doi: 10.1083/jcb.105.2.937.
MM14 mouse myoblasts withdraw irreversibly from the cell cycle and become postmitotic within a few hours of being deprived of fibroblast growth factor (Clegg, C. H., T. A. Linkhart, B. B. Olwin, and S. D. Hauschka, 1987, J. Cell Biol., 105:949-956). To examine the mechanisms that may regulate this developmental state of skeletal muscle, we tested the mitogen responsiveness of various cell types after their polyethylene glycol-mediated fusion with post-mitotic myocytes. Heterokaryons containing myocytes and quiescent nonmyogenic cells such as 3T3, L cell, and a differentiation-defective myoblast line (DD-1) responded to mitogen-rich medium by initiating DNA synthesis. Myonuclei replicated DNA and reexpressed thymidine kinase. In contrast, (myocyte x G1 myoblast) heterokaryons failed to replicate DNA in mitogen-rich medium and became postmitotic. This included cells with a nuclear ratio of three myoblasts to one myocyte. Proliferation dominance in (myocyte x 3T3 cell) and (myocyte x DD-1) heterokaryons was conditionally regulated by the timing of mitogen treatment; such cells became postmitotic when mitogen exposure was delayed for as little as 6 h after cell fusion. In addition, (myocyte x DD-1) heterokaryons expressed a muscle-specific trait and lost epidermal growth factor receptors when they became postmitotic. These results demonstrate that DNA synthesis is not irreversibly blocked in skeletal muscle; myonuclei readily express proliferation-related functions when provided with a mitogenic signal. Rather, myocyte-specific repression of DNA synthesis in heterokaryons argues that the postmitotic state of skeletal muscle is regulated by diffusible factors that inhibit processes of cellular mitogenesis.
MM14小鼠成肌细胞在被剥夺成纤维细胞生长因子后的数小时内，会不可逆地退出细胞周期并进入有丝分裂后状态（克莱格，C.H.，T.A.林克哈特，B.B.奥尔文，和S.D.豪施卡，1987年，《细胞生物学杂志》，105:949 - 956）。为了研究可能调节骨骼肌这种发育状态的机制，我们检测了各种细胞类型在通过聚乙二醇介导与有丝分裂后肌细胞融合后的促有丝分裂原反应性。含有肌细胞和静止的非肌源性细胞（如3T3、L细胞和一个分化缺陷的成肌细胞系（DD - 1））的异核体通过启动DNA合成对富含促有丝分裂原的培养基作出反应。肌细胞核复制DNA并重新表达胸苷激酶。相比之下，（肌细胞×G1期成肌细胞）异核体在富含促有丝分裂原的培养基中未能复制DNA并进入有丝分裂后状态。这包括成肌细胞与肌细胞核比例为三比一的细胞。（肌细胞×3T3细胞）和（肌细胞×DD - 1）异核体中的增殖优势受到促有丝分裂原处理时间的条件性调节；当细胞融合后促有丝分裂原暴露延迟至仅6小时时，此类细胞进入有丝分裂后状态。此外，（肌细胞×DD - 1）异核体在进入有丝分裂后状态时表达一种肌肉特异性特征并失去表皮生长因子受体。这些结果表明，骨骼肌中的DNA合成并非不可逆地被阻断；当提供促有丝分裂信号时，肌细胞核很容易表达与增殖相关的功能。相反，异核体中肌细胞特异性的DNA合成抑制表明，骨骼肌的有丝分裂后状态受抑制细胞有丝分裂过程的可扩散因子调节。
