Hormone-controlled cooperative binding of transcription factors drives synergistic gene induction during fasting

Episodes of fasting are an inherent aspect of physiology with most animals experiencing frequent bouts of fasting. The mammalian fasting response includes a diverse set of endocrine and metabolic cues that together affect metabolism to maintain homeostasis in the face of energy shortage. A central aspect of the fasting response is the hepatic production of glucose via gluconeogenesis. Gluconeogenesis is heavily regulated at the transcriptional level with several hormones controlling rates of gluconeogenic gene transcription. However, how the different hormone signals are integrated within cells to produce a robust and coherent transcriptional output is unknown. Glucagon and glucocorticoids are principal fasting hormones that cooperate in regulating gluconeogenesis. We used genome-wide profiling of gene expression, enhancer dynamics and transcription factor (TF) binding in primary mouse hepatocytes to uncover the mode of cooperation between glucagon and glucocorticoids. We found that compared to a single treatment with each hormone, a dual treatment directs hepatocytes to a pro-gluconeogenic gene program by synergistically inducing gluconeogenic genes. The cooperative mechanism driving synergistic gene expression is based on ‘assisted loading’ whereby a glucagon-activated TF – CREB, leads to enhancer activation which facilitates binding of the glucocorticoid receptor (GR) upon glucocorticoid stimulation. Glucagon does not only activate single enhancers but also activates enhancer clusters, thereby assisting the loading of GR also across enhancer units within the cluster. In summary, we showed that two hormonal signals are integrated to boost a biological response by coordinating enhancer activation, TF binding and synergistic gene expression. This work unraveled critical regulatory mechanisms at play in the liver during fasting.