OR33-05 Glucocorticoids Instigate Transcriptional Waves To Maintain Temporally Organized Fuel Production During Fasting

Abstract

Abstract Disclosure: D. Goldberg: None. I. Goldstein: None. Most animals are well equipped to endure prolonged fasting periods by converting energy stores into available biochemical fuel. This is achieved by the hepatic production of two fuel types supplying other tissues: glucose (originating from gluconeogenesis or glycogen breakdown) and ketone bodies (produced via ketogenesis). The adaptive response to fasting involves several hormones that together maintain homeostasis in the face of energy shortage. Glucocorticoids (GCs) are principal hormonal cues regulating the transcriptional response to fasting and were shown to augment both glucose and ketone production. We aimed to decipher how GCs control both the gluconeogenic and ketogenic programs. We profiled the transcriptional response and genome-wide enhancer dynamics of primary mouse hepatocytes to GC treatment. In addition to the induction of the expected fasting metabolism genes, we were surprised to find that GCs induced various genes encoding transcription factors (TFs). In fact, 13% of GC-induced genes were TFs, many of which were similarly induced in livers of fasted mice. Perturbation experiments and chromatin immunoprecipitation sequencing (ChIP-seq) showed that GCs induced these TFs via the glucocorticoid receptor (GR) which bound to DNA regulatory elements of the genes. We found that GCs induce two groups of TFs. The first group are TFs that maintain the early gluconeogenic fasting response. Indeed, knock-down of GC-induced TFs (C/EBPβ, MAFB or KLF15) reduced gluconeogenic gene expression. The second group are TFs which dictate a secondary transcriptional wave serving to produce fuel in prolonged fasting. Notably, GCs Initiate a TF cascade in which GR induces the expression of the Ppara gene, leading to increased PPARα protein. Upon subsequent ligand stimulation of PPARα, a ketogenic gene program is activated. We show that a dual treatment of GCs with a PPARα agonist leads to synergistic induction of ketogenic genes and that this induction is dependent on protein synthesis. We have analyzed the transcriptomic result of this TF cascade and found that dozens of ketogenic genes are synergistically induced. Genome-wide analysis of enhancer dynamics revealed numerous enhancers activated by the GR-PPARα cascade. These enhancers are proximal to ketogenic genes, are enriched for the PPARα binding motif and show increased PPARα binding as profiled by ChIP-seq. Taken together, our work reveals a GC-induced program whereby the expression of TFs increases during fasting, serving to rapidly support gluconeogenic capacity in short-term fasting and initiate a delayed ketogenic program brought about in prolonged fasting. These two regulatory modules may dictate the well-known temporal organization of the fasting response whereby gluconeogenesis predominates in early fasting but is gradually replaced by ketogenesis as fasting persists. Presentation: Sunday, June 18, 2023