FOXO1 cooperates with C/EBPδ and ATF4 to regulate skeletal muscle atrophy transcriptional program during fasting

Catabolic conditions, such as starvation, inactivity, and cancer cachexia, induce Forkhead box O (FOXO) transcription factor(s) expression and severe muscle atrophy via the induction of ubiquitin-proteasome system-mediated muscle proteolysis, resulting in frailty and poor quality of life. Although FOXOs are clearly essential for the induction of muscle atrophy, it is unclear whether there are other factors involved in the FOXO-mediated transcriptional regulation. As such, we identified FOXO-CCAAT/enhancer-binding protein delta (C/EBP delta) signaling pathway as a novel proteolytic pathway. By comparing the gene expression profiles of FOXO1-transgenic (gain-of-function model) and FOXO1,3a,4(-/-) (lossof-function model) mice, we identified several novel FOXO1-target genes in skeletal muscle including Reddl, Sestrin1, Castor2, Chad, Depp1, Lat3, as well as C/EBP delta. During starvation, C/EBP delta abundance was increased in a FOXOs-dependent manner. Notably, knockdown of C/EBP delta prevented the induction of the ubiquitin-proteasome system and decrease of myofibers in FOXO1-activated myotubes. Conversely, C/EBP delta overexpression in primary myotubes induced myotube atrophy. Furthermore, we demonstrated that FOXOI enhances the promoter activity of target genes in cooperation with C/EBP delta and ATF4. This research comprehensively identifies novel FOXO1 target genes in skeletal muscle and clarifies the pathophysiological role of FOXO1, a master regulator of skeletal muscle atrophy.

分解代谢状况，如饥饿、不活动和癌症恶病质，通过诱导泛素 - 蛋白酶体系统介导的肌肉蛋白水解，诱导叉头框O（FOXO）转录因子表达和严重的肌肉萎缩，导致身体虚弱和生活质量差。尽管FOXO对于诱导肌肉萎缩显然是必不可少的，但尚不清楚在FOXO介导的转录调控中是否涉及其他因素。因此，我们确定了FOXO - CCAAT/增强子结合蛋白δ（C/EBPδ）信号通路是一种新的蛋白水解通路。通过比较FOXO1转基因（功能获得模型）和FOXO1、3a、4（ - / - ）（功能缺失模型）小鼠的基因表达谱，我们在骨骼肌中鉴定了几个新的FOXO1靶基因，包括Redd1、Sestrin1、Castor2、Chad、Depp1、Lat3以及C/EBPδ。在饥饿期间，C/EBPδ的丰度以依赖FOXO的方式增加。值得注意的是，敲低C/EBPδ可阻止FOXO1激活的肌管中泛素 - 蛋白酶体系统的诱导和肌纤维的减少。相反，在原代肌管中过表达C/EBPδ会导致肌管萎缩。此外，我们证明了FOXO1与C/EBPδ和ATF4协同增强靶基因的启动子活性。这项研究全面鉴定了骨骼肌中新的FOXO1靶基因，并阐明了FOXO1（骨骼肌萎缩的主要调节因子）的病理生理作用。