Mechanisms by Which Glucagon Acutely Stimulates Hepatic Mitochondrial Oxidation and Gluconeogenesis

Abstract

Glucagon has been suggested to be both pathogenic and protective against type 2 diabetes (T2D) reflecting our poor understanding of the molecular mechanisms by which glucagon alters hepatic metabolism. Using Positional Isotopomer NMR Tracer Analysis (PINTA) we found that acute glucagon infusion stimulates rates of hepatic gluconeogenesisin glycogen-depleted mice by promoting increases in intrahepatic lipolysis and hepatic acetyl-CoA content (75±6 vs. 101±6 nmol/g, p=0.01), pyruvate carboxylase flux (VPC, 70.9±3.9 vs. 99.9±7.1 µmol/[kg-min], p=0.008) and mitochondrial oxidation (90.0±12.9 vs. 452.1±78.3 µmol/[kg-min], p=0.001) in vivo. Each of these effects was abrogated in liver-specific Inositol Trisphosphate Receptor-I (InsP3R-I) knockout mice demonstrating that intracellular calcium signaling is necessary for these processes. Liver specific knockdown of adipocyte triglyceride lipase abolished acute glucagon stimulation of gluconeogenesis (control 70.5±2.9 vs. 103.3±3.3 µmol/[kg-min], p<0.0001 and ATGL knockdown 69.7±1.7 vs. 69.6±2.0 µmol/[kg-min], p=0.9) and VPC (control 63.3±5.0 vs. 100.0±4.8 µmol/[kg-min], p=0.0003, ATGL knockdown 58.9±4.9 vs. 59.2±3.2 µmol/[kg-min], p=0.9) in WT mice demonstrating the necessity of hepatocellular lipolysis in mediating this process. Chronic increases in plasma glucagon concentrations reversed high fat diet-induced hepatic steatosis (liver triglyceride 50.1±8.5 vs. 28.3±5.1 mg/g, p=0.04; liver membrane diacylglycerol 801±142 vs. 448±47 nmol/g, p=0.03, PKCε membrane/cytosol 1.76±0.40 vs. 0.23±0.04, p=0.0009) and glucose intolerance (GTT glucose AUC 1.44±0.vs. 1.13±0.mM-min, p=0.003, insulin AUC 226.8±18.7 vs. 90.2±11.2 nM-min, p<0.0001). These results provide new insights into glucagon biology and suggest that glucagon and InsP3R-I may be therapeutic targets to reverse nonalcoholic fatty liver disease and T2D. Disclosure R.J. Perry: None. Y. Wang: None. A.L. Brill: None. L. Peng: None. D. Zhang: None. S. Dufour: None. Y. Zhang: None. X. Zhang: None. Y. Nozaki: None. G. Cline: None. B.E. Ehrlich: None. K. Petersen: Research Support; Spouse/Partner; Gilead Sciences, Inc.. Advisory Panel; Spouse/Partner; AstraZeneca, Merck & Co., Inc., Novo Nordisk A/S. G.I. Shulman: Advisory Panel; Self; AstraZeneca, Janssen Research & Development, Merck & Co., Inc., Novo Nordisk Inc.. Research Support; Self; Gilead Sciences, Inc..