FOXO1 cooperates with C/EBP delta 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.