Objective

The objective of our study was to examine the direct action of insulin-like growth factor-1(IGF-1) signaling on energy homeostasis in myocytes.

Design

We studied the IGF-1 stimulation of mitochondrial uncoupling protein 3 (UCP3) expression in the HEK 293 derived cell line TSA201, murine C2C12 skeletal muscle myoblasts, and rat L6 skeletal myoblasts. We also investigated the direct effect of IGF-1 on the Insulin/IGF-1 receptor (IGF-1R)/phosphatidylinositol 3 (PI3)-Akt/forkhead box O4 (FOXO4) pathway using a combination of a reporter assay, semi-quantitative polymerase chain reaction, western blotting, and animal experiments.

Results

We demonstrated that IGF-1 regulates UCP3 expression via phosphorylation of FOXO4, which is a downstream signal transducer of IGF-1. UCP3 expression increased with activated FOXO4 in a dose-dependent manner. We also examined the functional FOXO4 binding site consensus sequences and identified it as the −1922 bp site in the UCP3 promoter region. UCP3 was also found to be concomitantly expressed with IGF-1 during differentiation of C2C12 myoblasts. Our animal experiments showed that high fat diet induced IGF-1 levels which likely influenced UCP3 expression in the skeletal muscle.

Conclusion

Our findings demonstrate that that IGF-1 directly stimulates UCP3 expression via the IGF-1/IGF-1R/PI3-Akt/FOXO4 pathway.

中文翻译：

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胰岛素样生长因子1通过叉头盒O4直接介导线粒体解偶联蛋白3的表达。

目的

我们研究的目的是研究胰岛素样生长因子-1（IGF-1）信号传导对肌细胞能量稳态的直接作用。

设计

我们研究了IGF-1刺激HEK 293衍生的细胞系TSA201，鼠C2C12骨骼肌成肌细胞和大鼠L6骨骼成肌细胞中的线粒体解偶联蛋白3（UCP3）表达。我们还研究了IGF-1对胰岛素/ IGF-1受体（IGF-1R）/磷脂酰肌醇3（PI3）-Akt /叉头盒O4（FOXO4）途径的直接作用，使用了一种报告剂检测，半定量检测聚合酶链反应，蛋白质印迹和动物实验。

结果

我们证明了IGF-1通过FOXO4的磷酸化调节UCP3表达，FOXO4是IGF-1的下游信号转导子。激活的FOXO4以剂量依赖性方式增加UCP3表达。我们还检查了功能性FOXO4结合位点共有序列，并将其鉴定为UCP3启动子区域中的-1922 bp位点。还发现在C2C12成肌细胞分化过程中，UCP3与IGF-1同时表达。我们的动物实验表明，高脂饮食会诱导IGF-1水平，这很可能会影响骨骼肌中UCP3的表达。

结论

我们的发现表明，IGF-1通过IGF-1 / IGF-1R / PI3-Akt / FOXO4途径直接刺激UCP3表达。