Ebert Scott M, Monteys Alex Mas, Fox Daniel K, Bongers Kale S, Shields Bridget E, Malmberg Sharon E, Davidson Beverly L, Suneja Manish, Adams Christopher M
Department of Internal Medicine, Roy J. and Lucille A. Carver College of Medicine, The University of Iowa, Iowa City, Iowa 52242, USA.
Mol Endocrinol. 2010 Apr;24(4):790-9. doi: 10.1210/me.2009-0345. Epub 2010 Mar 2.
Prolonged fasting alters skeletal muscle gene expression in a manner that promotes myofiber atrophy, but the underlying mechanisms are not fully understood. Here, we examined the potential role of activating transcription factor 4 (ATF4), a transcription factor with an evolutionarily ancient role in the cellular response to starvation. In mouse skeletal muscle, fasting increases the level of ATF4 mRNA. To determine whether increased ATF4 expression was required for myofiber atrophy, we reduced ATF4 expression with an inhibitory RNA targeting ATF4 and found that it reduced myofiber atrophy during fasting. Likewise, reducing the fasting level of ATF4 mRNA with a phosphorylation-resistant form of eukaryotic initiation factor 2alpha decreased myofiber atrophy. To determine whether ATF4 was sufficient to reduce myofiber size, we overexpressed ATF4 and found that it reduced myofiber size in the absence of fasting. In contrast, a transcriptionally inactive ATF4 construct did not reduce myofiber size, suggesting a requirement for ATF4-mediated transcriptional regulation. To begin to determine the mechanism of ATF4-mediated myofiber atrophy, we compared the effects of fasting and ATF4 overexpression on global skeletal muscle mRNA expression. Interestingly, expression of ATF4 increased a small subset of five fasting-responsive mRNAs, including four of the 15 mRNAs most highly induced by fasting. These five mRNAs encode proteins previously implicated in growth suppression (p21(Cip1/Waf1), GADD45alpha, and PW1/Peg3) or titin-based stress signaling [muscle LIM protein (MLP) and cardiac ankyrin repeat protein (CARP)]. Taken together, these data identify ATF4 as a novel mediator of skeletal myofiber atrophy during starvation.
长期禁食会改变骨骼肌基因表达，从而促进肌纤维萎缩，但其潜在机制尚未完全明确。在此，我们研究了激活转录因子4（ATF4）的潜在作用，ATF4是一种在细胞对饥饿的反应中具有古老进化作用的转录因子。在小鼠骨骼肌中，禁食会增加ATF4 mRNA的水平。为了确定肌纤维萎缩是否需要增加ATF4表达，我们用靶向ATF4的抑制性RNA降低了ATF4表达，发现这减少了禁食期间的肌纤维萎缩。同样，用真核起始因子2α的磷酸化抗性形式降低ATF4 mRNA的禁食水平也减少了肌纤维萎缩。为了确定ATF4是否足以减小肌纤维大小，我们过表达了ATF4，发现它在没有禁食的情况下减小了肌纤维大小。相比之下，转录无活性的ATF4构建体并没有减小肌纤维大小，这表明需要ATF4介导的转录调控。为了开始确定ATF4介导的肌纤维萎缩机制，我们比较了禁食和ATF4过表达对整体骨骼肌mRNA表达的影响。有趣的是，ATF4的表达增加了一小部分由五个对禁食有反应的mRNA组成的子集，其中包括禁食诱导程度最高的15个mRNA中的4个。这五个mRNA编码的蛋白质先前被认为与生长抑制（p21（Cip1/Waf1）、GADD45α和PW1/Peg3）或基于肌联蛋白的应激信号传导[肌肉LIM蛋白（MLP）和心脏锚蛋白重复蛋白（CARP）]有关。综上所述，这些数据确定ATF4是饥饿期间骨骼肌纤维萎缩的一种新型介质。
