Departments of Animal Sciences and Applied Biological Chemistry, Graduate School of Agriculture and Life Sciences, The University of Tokyo, Tokyo, Japan.
Laboratory of Cell Regulation, Departments of Animal Sciences and Applied Biological Chemistry, Graduate School of Agriculture and Life Sciences, The University of Tokyo, Tokyo, Japan.
Front Endocrinol (Lausanne). 2020 Feb 28;11:96. doi: 10.3389/fendo.2020.00096. eCollection 2020.
Insulin receptor substrate (IRS)-1 is a major substrate of insulin-like growth factor (IGF)-I receptors. It is well-known that IGF-I and II play essential roles in myogenesis progression. Herein, we report an unexpected phenomenon that IRS-1-overexpressing L6 myoblasts are eliminated from normal cell layers at the beginning of differentiation. Initially, the IRS protein level and apoptosis were examined during myogenic differentiation in L6 myoblasts. We found that the IRS-1 protein level decreased, whereas active caspase 3 increased around 1 day after induction of differentiation. The addition of a pan-caspase inhibitor, Z-VAD-FMK, inhibited differentiation-induced suppression of the IRS-1 protein level. Apoptosis was not enhanced in L6 myoblasts stably expressing high levels of IRS-1 (L6-IRS-1). However, when L6-IRS-1 was cultured with control cells (L6-mock), we observed that L6-IRS-1 was eliminated from the cell layer. We have recently reported that, in L6-IRS-1, internalization of the IGF-I receptor was delayed and IGF signal activation was sustained for a longer period than in L6-mock. When cells stably expressing IRS-1 3YA mutant, which could not maintain the IGF signals, were cultured with normal cells, elimination from the cell layer was not detected. These data suggested that the high level of IRS-1 in myoblasts induces elimination from the cell layer due to abnormal sustainment of IGF-I receptor activation.
胰岛素受体底物 (IRS)-1 是胰岛素样生长因子 (IGF)-I 受体的主要底物。众所周知，IGF-I 和 IGF-II 在肌发生进展中发挥重要作用。在此，我们报告了一个意想不到的现象，即在分化开始时，IRS-1 过表达的 L6 成肌细胞从正常细胞层中消失。最初，在 L6 成肌细胞的肌发生分化过程中检查了 IRS 蛋白水平和细胞凋亡。我们发现 IRS-1 蛋白水平下降，而活性 caspase 3 在诱导分化后约 1 天增加。添加泛半胱天冬酶抑制剂 Z-VAD-FMK 抑制分化诱导的 IRS-1 蛋白水平下降。在稳定表达高水平 IRS-1（L6-IRS-1）的 L6 成肌细胞中，细胞凋亡没有增强。然而，当 L6-IRS-1 与对照细胞（L6-mock）共培养时，我们观察到 L6-IRS-1 从细胞层中消失。我们最近报道称，在 L6-IRS-1 中，IGF-I 受体的内化延迟，并且 IGF 信号激活持续时间更长。当稳定表达 IRS-1 3YA 突变体（不能维持 IGF 信号）的细胞与正常细胞共培养时，未检测到从细胞层中消失。这些数据表明，成肌细胞中 IRS-1 的高水平导致 IGF-I 受体激活的异常持续导致从细胞层中消除。
