Carbohydrates and lipids are primary energy sources in mammals; thus, their oxidation and synthesis are carefully controlled. The tricarboxylic acid (TCA) cycle is necessary to oxidize these substrates, and in liver it also supports their synthesis. However, its role in regulating these processes, particularly de novo lipogenesis (DNL) is underappreciated. We report that activation of hepatic gluconeogenesis (GNG) is required to trigger the canonical lipid and carbohydrate response to fasting in mice. Phosphoenolpyruvate carboxykinase (PEPCK) catalyzes cataplerotic flux from the TCA cycle and is required for GNG from lactate, pyruvate and amino acids. Knockdown (KD) or knockout (KO) of PEPCK caused the massive accumulation of TCA cycle intermediates and KO mice developed fatty liver after an overnight fast. Lipidomic profiling of liver suggested increased lipid desaturation and elongation with loss of PEPCK. 2H NMR analysis of liver lipids following 2H2O administration demonstrated elevated fasting DNL in KO mice, which was similar to fed WT mice. In addition, fasting ketogenesis, estimated by infusions of [U-13C]BHB and [3,4-13C]AcAc, was decreased in KO mice. Despite an impaired fasting response, the expression of genes related to control of fat oxidation and DNL were not decreased, suggesting that loss of regulation was mediated by a metabolic mechanism. Inhibition of GNG produced a reduced mitochondrial redox state which limited oxidative pathways and decreased acetyl-CoA concentration. Inhibition of GNG also decreased fasting ATP requirements, resulting in a high energy charge which suppressed activation of AMPK, its canonical suppression of acetyl-CoA carboxylase and caused increased malonyl-CoA during fasting. Hence, PEPCK mediated cataplerosis and GNG during fasting is not only an important source of glucose, it is also necessary to shift cellular redox and energetics to states necessary to suppress lipogenesis and maintain ketogenesis during fasting.
S. Deja: None. J. Duarte: None. J.A. Fletcher: None. B. Kucejova: None. X. Fu: None. G. Vale: None. J. Young: Board Member; Self; Metalytics. Consultant; Self; Pfizer Inc. J. Browning: None. S.C. Burgess: None.
National Institutes of Health (R01DK078184, P41EB015908); Robert A. Welch Foundation (I-1804)