Transcriptional regulation of lipid metabolism genes during fasting occurs via two distinct modules

During fasting, the liver produces glucose and ketones to supply fuels to the body. Triglycerides derived from adipose tissue serve as substrates for fuel production: the fatty acid component is oxidized and used for ketogenesis while the glycerol backbone serves as a gluconeogenic substrate. Our goal is to investigate the transcriptional regulation of lipid metabolism genes in response to fasting and to identify the possible cooperation between the transcription factors (TFs) that regulate it. RNA sequencing of fasted mice revealed 70 fasting-induced genes related to lipid metabolism. Pathway enrichment analysis of these genes revealed two main pathways: phospholipid metabolism and fatty acid oxidation. To understand how fasting-related signals affect gene expression, we measured gene levels in primary mouse hepatocytes following these treatments: corticosterone, glucagon, and WY-14634 (which activate the transcription factors GR, CREB, and PPARα respectively). We found two distinct modules of gene expression: a synergistic response in fatty acid oxidation- and ketogenesis-related genes following a dual treatment of corticosterone and WY-14634. In contrast, phospholipid-metabolism genes showed a synergistic response in the dual treatment of corticosterone and glucagon. The two expression patterns indicate two distinctly-operating modules, a GR-PPARα module responsible for ketogenesis and a CREB-GR module responsible for phospholipid metabolism. This, together with our previous study, suggest that phospholipid metabolism genes take part in supplying substrates for gluconeogenesis. Indeed, inhibition of a GR-CREB-induced gene that potentially provides gluconeogenic substrates, decreased hepatic glucose production. Our findings shed new light on transcriptional regulation of lipid metabolism during fasting.