Glucagon plays an important role in hepatic glucose metabolism as well as protein/amino acid metabolism but detailed knowledge about glucagon’s role in hepatic autophagy is limited. We hypothesized that glucagon regulate hepatic protein metabolism partly through activation of AMP-kinase (AMPK) that in turn lead to inhibition of mammalian target of rapamycin complex1 (mTORC1) and activation of Uncoordinated-51 like autophagy activating kinase-1 (ULK1) resulting in increased autophagy. In order to test this hypothesis, we infused chronically catheterized awake mice with [13C5]-glutamine [0.225 mg/(kg-min)] for 120 min after which a second line was connected infusing somatostatin [4 µg/(kg-min)] and insulin [0.1 mU/(kg-min)], with (n=7) or without glucagon [10 ng/(kg-min)] (n=6) for 90 min. This raised plasma glucagon concentration ~18-fold compared to control. Glucagon infusion increased phosphorylation of hepatic: AMPKThr172, RaptorSer792, ULK1Ser555 (P=0.03, P=0.03, and P=0.06, respectively, compared to control), and resulted in hepatic Microtubule-associated protein 1A/1B-light chain 3 (LC3)-II accumulation (P=0.02), a marker of autophagy, assessed by immunoblotting. Glucagon treatment also increased the 13C enrichment (m+3) in liver glucose (P<0.0001), indicating increased conversion of [13C5]-glutamine to glucose as well as increased plasma urea concentrations (P=0.04). Consistent with these in vivo results, parallel studies performed in liver slices obtained from overnight fasted male rats (n=6) incubated in Krebs-Henseleit bicarbonate buffer supplemented with [3-13C]-alanine (1 mM), glucagon (250 µM) treatment increased: glucose production (P=0.01), urea production (P=0.0005), and intracellular levels of leucine (P=0.045).
In conclusion, our data suggest that glucagon promotes hepatic proteolysis and ureagenesis by AMPK-mediated suppression of mTOC1 activity, leading to increased hepatic autophagy.
K.D. Galsgaard: None. N.J. Wewer Albrechtsen: Research Support; Self; Mercodia, Novo Nordisk A/S, Novo Nordisk Foundation. Speaker’s Bureau; Self; Merck Sharp & Dohme Corp. J.J. Holst: Advisory Panel; Self; AstraZeneca, Merck Sharp & Dohme Corp., Novo Nordisk A/S, Zealand Pharma A/S. Other Relationship; Spouse/Partner; Antag Therapeutics. G.I. Shulman: Advisory Panel; Self; AstraZeneca, Janssen Research & Development, LLC, Merck & Co., Inc. Advisory Panel; Spouse/Partner; Merck & Co., Inc. Consultant; Self; Novo Nordisk A/S. Consultant; Spouse/Partner; Novo Nordisk A/S. Other Relationship; Self; Gilead Sciences, Inc., iMetabolic Biopharma Corporation. Other Relationship; Spouse/Partner; iMetabolic Biopharma Corporation. Other Relationship; Self; Maze Therapeutics. K. Petersen: Advisory Panel; Spouse/Partner; Merck Sharp & Dohme Corp. Research Support; Self; Merck Sharp & Dohme Corp., National Institute of Diabetes and Digestive and Kidney Diseases. Research Support; Spouse/Partner; National Institute of Diabetes and Digestive and Kidney Diseases. A. Nasiri: Employee; Spouse/Partner; Medtronic. G. Cline: None. X. Zhang: None. J. Lee: None. B.T. Hubbard: None.