Bois Philippe R J, Grosveld Gerard C
St Jude Children's Research Hospital, Department of Genetics, 332 North Lauderdale, Memphis, TN 38105, USA.
EMBO J. 2003 Mar 3;22(5):1147-57. doi: 10.1093/emboj/cdg116.
Activation of the transcription factor FKHR (Forkhead in human rhabdomyosarcoma, FOXO1a) in various established cell lines induces cell cycle arrest followed by apoptosis. These effects are inhibited through activation of the phosphatidylinositol 3-kinase/Akt pathway, resulting in FKHR phosphorylation and its export from the nucleus, thus blocking its pro-apoptotic activity. Here we report that FKHR regulates fusion of differentiating primary myoblasts. We demonstrate that FKHR is localized in the cytoplasm of proliferating myoblasts, yet translocates to the nucleus by a phosphorylation-independent pathway following serum starvation, a condition that induces myoblast differentiation. FKHR phosphorylation during terminal differentiation appears to downregulate its fusion activity, as a dominant-active non-phosphorylatable FKHR mutant dramatically augments the rate and extent of myotube fusion. However, this FKHR mutant exerts its effects only after other events initiated the differentiation pro cess. Conversely, enforced expression of a dominant-negative FKHR mutant blocks myotube formation whereas wild-type FKHR has no effect. We conclude that in addition to the role of FoxO proteins in regulating cell cycle progress and apoptosis, FKHR controls the rate of myotube fusion during myogenic differentiation.
在多种已建细胞系中，转录因子FKHR（人横纹肌肉瘤中的叉头蛋白，FOXO1a）的激活会诱导细胞周期停滞，随后引发细胞凋亡。通过磷脂酰肌醇3激酶/Akt途径的激活，这些效应受到抑制，导致FKHR磷酸化并从细胞核输出，从而阻断其促凋亡活性。在此我们报告，FKHR调节分化中的原代成肌细胞的融合。我们证明，FKHR定位于增殖的成肌细胞的细胞质中，但在血清饥饿（一种诱导成肌细胞分化的条件）后，通过一条不依赖磷酸化的途径转运至细胞核。终末分化过程中的FKHR磷酸化似乎会下调其融合活性，因为一种显性激活的不可磷酸化的FKHR突变体显著增强了肌管融合的速率和程度。然而，这种FKHR突变体仅在其他事件启动分化过程后才发挥其作用。相反，显性负性FKHR突变体的强制表达会阻断肌管形成，而野生型FKHR则无此作用。我们得出结论，除了FoxO蛋白在调节细胞周期进程和细胞凋亡中的作用外，FKHR在成肌分化过程中还控制肌管融合的速率。
