Transcriptional cascades during fasting amplify gluconeogenesis and instigate a secondary wave of ketogenic gene transcription

Abstract

Background & AimsDuring fasting, bodily homeostasis is maintained due to hepatic production of glucose (gluconeogenesis) and ketone bodies (ketogenesis). The main hormones governing hepatic fuel production are glucagon and glucocorticoids that initiate transcriptional programs aimed at supporting gluconeogenesis and ketogenesis. MethodsUsing primary mouse hepatocytes as an ex vivo model, we employed transcriptomic analysis (RNA-seq), genome-wide profiling of enhancer dynamics (ChIP-seq), perturbation experiments (inhibitors, shRNA), hepatic glucose production measurements and computational analyses. ResultsWe found that in addition to the known metabolic genes transcriptionally induced by glucagon and glucocorticoids, these hormones induce a set of genes encoding transcription factors (TFs) thereby initiating transcriptional cascades. Upon activation by glucocorticoids, the glucocorticoid receptor (GR) induced the genes encoding two TFs: CCAAT/enhancer-binding protein beta (C/EBP{beta}) and peroxisome proliferator-activated receptor alpha (PPAR). We found that C/EBP{beta} mainly serves as an amplifier of hormone-induced gene programs in hepatocytes. C/EBP{beta} augmented gluconeogenic gene expression and hepatic glucose production. Conversely, the GR-PPAR cascade initiated a secondary transcriptional wave of genes supporting ketogenesis. The cascade led to synergistic induction of ketogenic genes which is dependent on protein synthesis. Genome-wide analysis of enhancer dynamics revealed numerous enhancers activated by the GR-PPAR cascade. These enhancers were proximal to ketogenic genes, enriched for the PPAR response element and showed increased PPAR binding. ConclusionThis study reveals abundant transcriptional cascades occurring during fasting. These cascades serve two separated purposes: the amplification of the primary gluconeogenic transcriptional program and the induction of a secondary gene program aimed at enhancing ketogenesis. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=137 SRC="FIGDIR/small/588039v1_ufig1.gif" ALT="Figure 1"> View larger version (26K): org.highwire.dtl.DTLVardef@e02207org.highwire.dtl.DTLVardef@da0c7aorg.highwire.dtl.DTLVardef@1695a8borg.highwire.dtl.DTLVardef@1b78fb0_HPS_FORMAT_FIGEXP M_FIG C_FIG