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 δ (C/EBPδ) signaling pathway as a novel proteolytic pathway. By comparing the gene expression profiles of FOXO1-transgenic (gain-of-function model) and FOXO1,3a,4-/- (loss-of-function model) mice, we identified several novel FOXO1-target genes in skeletal muscle including Redd1, Sestrin1, Castor2, Chac1, Depp1, Lat3, as well as C/EBPδ. During starvation, C/EBPδ abundance was increased in a FOXOs-dependent manner. Notably, knockdown of C/EBPδ prevented the induction of the ubiquitin-proteasome system and decrease of myofibers in FOXO1-activated myotubes. Conversely, C/EBPδ overexpression in primary myotubes induced myotube atrophy. Furthermore, we demonstrated that FOXO1 enhances the promoter activity of target genes in cooperation with C/EBPδ 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.

Vitamin D Activates Various Gene Expressions, Including Lipid Metabolism, in C2C12 Cells.

Vitamin D is a fat-soluble molecule, well known for its role in regulating calcium homeostasis in bone. It has become increasingly clear that it also has important effects in many other organs, including the skeletal muscle. In order to gain insight into the role of vitamin D in the skeletal muscle, we performed microarray analysis using C2C12 myoblasts treated with 1,25-dihydroxyvitamin D (1,25(OH)2D), active form of vitamin D. We found multiple genes upregulated by 1,25(OH)2D. Some of them, i.e., vitamin D receptor (Vdr), diacylglycerol O-acyltransferase (Dgat1 and Dgat2, the rate limiting steps of triacylglycerol acylation), and vascular endothelial growth factor A (Vegfa), were previously reported to be upregulated by 1,25(OH)2D in C2C12 cells. RT-qPCR analysis confirmed increased mRNA levels of Rarres2, Dio2, Tgm2, Lpl, Mdfi, Igfbp3, Dgat1, Crabp2, Gadd45a, Vagfa, Dgat2, C3, Ldhb, Cebpa, Igfbp5, Mrc2, Vdr. Thus, many genes, including lipid metabolism genes as well as genes related to muscle functions, appear to be upregulated by 1,25(OH)2D in muscle cells.