LXR-dependent enhancer activation regulates the temporal organization of the liver's response to refeeding leading to lipogenic gene overshoot
Transitions between the fed and fasted state are common in mammals. The liver orchestrates adaptive responses to feeding/fasting by transcriptionally regulating metabolic pathways of energy usage and storage. Transcriptional and enhancer dynamics following cessation of fasting (refeeding) have not been explored.  We examined the transcriptional and chromatin events occurring upon refeeding in mice, including kinetic behavior and molecular drivers. We found that the refeeding response is temporally-organized with the early response focused on ramping up protein translation while the later stages of refeeding drive a bifurcated lipid synthesis program. While both the cholesterol biosynthesis and lipogenesis pathways were inhibited during fasting, most cholesterol biosynthesis genes returned to their basal levels upon refeeding while most lipogenesis genes markedly overshoot above pre-fasting levels. Gene knockout, enhancer dynamics and ChIP-seq analyses revealed that lipogenic gene overshoot is dictated by LXRa. These findings from unbiased analyses unravel the mechanism behind the long-known phenomenon of refeeding fat overshoot. Overall design: Males, 8 weeks-old mice (C57BL/6J) were randomly assigned to either group (6 mice per group). The experiment started after 1 week of acclimation. The Adlib group had ad libitum access to food (Teklad TD2018) and water throughout the experiment. For the rest of the groups, food was removed at the beginning of the inactive phase (shortly after lights on, zeitgeber time 1). For the Refed groups, food was put back in the cage 24 h later while the fasted group was anesthetized and euthanized. The Refed groups were euthanized 3 h, 10 h and 24 h after food reintroduction. At each group endpoint, mice were anesthetized and euthanized (ketamine:xylazine 30:6 mg/ml) and the liver was excised.