Transcriptome-wide analysis of PGC-1 alpha-binding RNAs identifies genes linked to glucagon metabolic action
. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA 2020
The peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1 alpha) is a transcriptional coactivator that controls expression of metabolic/energetic genes, programming cellular responses to nutrient and environmental adaptations such as fasting, cold, or exercise. Unlike other coactivators, PGC-1 alpha contains protein domains involved in RNA regulation such as serine/arginine (SR) and RNA recognition motifs (RRM5). However, the RNA targets of PGC-1 alpha and how they pertain to metabolism are unknown. To address this, we performed enhanced ultraviolet (UV) cross-linking and immunoprecipitation followed by sequencing (eCLIP-seq) in primary hepatocytes induced with glucagon. A large fraction of RNAs bound to PGC-1 alpha were intronic sequences of genes involved in transcriptional, signaling, or metabolic function linked to glucagon and fasting responses, but were not the canonical direct transcriptional PGC-1 alpha targets such as OXPHOS or gluconeogenic genes. Among the top-scoring RNA sequences bound to PGC-1 alpha were Foxo1, Camk1 delta, Pert, Klf15, Pln4, Cluh, Trpc5, Gfra1, and Slc25a25. PGC-1 alpha depletion decreased a fraction of these glucagon-induced messenger RNA (mRNA) transcript levels. Importantly, knockdown of several of these genes affected glucagon-dependent glucose production, a PGC-1 alpha-regulated metabolic pathway. These studies show that PGC-1 alpha binds to intronic RNA sequences, some of them controlling transcript levels associated with glucagon action.
Autophagy mediates hepatic GRK2 degradation to facilitate glucagon-induced metabolic adaptation to fasting
. FASEB J 2020
The liver plays a key role during fasting to maintain energy homeostasis and euglycemia via metabolic processes mainly orchestrated by the insulin/glucagon ratio. We report here that fasting or calorie restriction protocols in C57BL6 mice promote a marked decrease in the hepatic protein levels of G protein-coupled receptor kinase 2 (GRK2), an important negative modulator of both G protein-coupled receptors (GPCRs) and insulin signaling. Such downregulation of GRK2 levels is liver-specific and can be rapidly reversed by refeeding. We find that autophagy, and not the proteasome, represents the main mechanism implicated in fasting-induced GRK2 degradation in the liver in vivo. Reducing GRK2 levels in murine primary hepatocytes facilitates glucagon-induced glucose production and enhances the expression of the key gluconeogenic enzyme Pck1. Conversely, preventing full downregulation of hepatic GRK2 during fasting using adenovirus-driven overexpression of this kinase in the liver leads to glycogen accumulation, decreased glycemia, and hampered glucagon-induced gluconeogenesis, thus preventing a proper and complete adaptation to nutrient deprivation. Overall, our data indicate that physiological fasting-induced downregulation of GRK2 in the liver is key for allowing complete glucagon-mediated responses and efficient metabolic adaptation to fasting in vivo.