Glucagon acutely regulates hepatic amino acid catabolism and the effect may be disturbed by steatosis

Abstract

Objective Glucagon is well known to regulate blood glucose but may be equally important for amino acid metabolism. Plasma levels of amino acids are regulated by glucagon-dependent mechanism(s), while amino acids stimulate glucagon secretion from alpha cells, completing the recently described liver-alpha cell axis. The mechanisms underlying the cycle and the possible impact of hepatic steatosis are unclear. Methods We assessed amino acid clearance in vivo in mice treated with a glucagon receptor antagonist (GRA), transgenic mice with 95% reduction in alpha cells, and mice with hepatic steatosis. In addition, we evaluated urea formation in primary hepatocytes from ob/ob mice and humans, and we studied acute metabolic effects of glucagon in perfused rat livers. We also performed RNA sequencing on livers from glucagon receptor knock-out mice and mice with hepatic steatosis. Finally, we measured individual plasma amino acids and glucagon in healthy controls and in two independent cohorts of patients with biopsy-verified non-alcoholic fatty liver disease (NAFLD). Results Amino acid clearance was reduced in mice treated with GRA and mice lacking endogenous glucagon (loss of alpha cells) concomitantly with reduced production of urea. Glucagon administration markedly changed the secretion of rat liver metabolites and within minutes increased urea formation in mice, in perfused rat liver, and in primary human hepatocytes. Transcriptomic analyses revealed that three genes responsible for amino acid catabolism ( Cps1 , Slc7a2 , and Slc38a2 ) were downregulated both in mice with hepatic steatosis and in mice with deletion of the glucagon receptor. Cultured ob/ob hepatocytes produced less urea upon stimulation with mixed amino acids, and amino acid clearance was lower in mice with hepatic steatosis. Glucagon-induced ureagenesis was impaired in perfused rat livers with hepatic steatosis. Patients with NAFLD had hyperglucagonemia and increased levels of glucagonotropic amino acids, including alanine in particular. Both glucagon and alanine levels were reduced after diet-induced reduction in Homeostatic Model Assessment for Insulin Resistance (HOMA-IR, a marker of hepatic steatosis). Conclusions Glucagon regulates amino acid metabolism both non-transcriptionally and transcriptionally. Hepatic steatosis may impair glucagon-dependent enhancement of amino acid catabolism.

Integrated Metabolomics and Lipidomics Analysis Reveal Remodeling of Lipid Metabolism and Amino Acid Metabolism in Glucagon Receptor–Deficient Zebrafish

“…Previous studies have revealed that GCGR blockade decreased hepatic amino acid catabolism and increased the serum amino acid level, revealing the important role of GCGR in regulating amino acid metabolism (Solloway et al, 2015 ; Dean et al, 2017 ; Galsgaard et al, 2018 ; Winther-Sorensen et al, 2020 ). In our study, the KEGG pathway analysis also indicated that 11 pathways were enriched in amino acid metabolism, which was in the second place of metabolism-related pathways ( Figure 2C ).…”

“…GCGR blockade–induced hyperaminoacidemia has also been documented in mammals, from mice, monkey, to human patients (Okamoto et al, 2015 ; Larger et al, 2016 ; Galsgaard et al, 2018 ; Li et al, 2018 ). Studies further revealed that plasma hyperaminoacidemia was due to the decreased liver amino acid catabolism after GCGR inhibition (Solloway et al, 2015 ; Kim et al, 2017 ; Winther-Sorensen et al, 2020 ). High levels of plasma amino acids in turn stimulated the pancreatic α-cell hyperplasia, which has been defined as the liver-α cell axis (Dean et al, 2017 ; Galsgaard et al, 2018 ; Wewer Albrechtsen et al, 2018a ).…”

“…Knockout or inhibition of GCGR in mice displayed pancreatic α-cell hyperplasia and increased the plasma concentrations of glucagon, glucagon-like peptide-1, low-density lipoprotein, and amino acids. Hyperaminoacidemia was observed in Gcgr −/− mice in many studies (Yang et al, 2011 ; Solloway et al, 2015 ; Dean et al, 2017 ; Kim et al, 2017 ; Galsgaard et al, 2018 ; Winther-Sorensen et al, 2020 ). Moreover, a lot of genes involved in amino acid catabolism were found to be down-regulated in Gcgr −/− mice, especially those related to glutamine, serine, and arginine metabolism (Yang et al, 2011 ; Dean et al, 2017 ; Kim et al, 2017 ; Winther-Sorensen et al, 2020 ).…”

“…Hyperaminoacidemia was observed in Gcgr −/− mice in many studies (Yang et al, 2011 ; Solloway et al, 2015 ; Dean et al, 2017 ; Kim et al, 2017 ; Galsgaard et al, 2018 ; Winther-Sorensen et al, 2020 ). Moreover, a lot of genes involved in amino acid catabolism were found to be down-regulated in Gcgr −/− mice, especially those related to glutamine, serine, and arginine metabolism (Yang et al, 2011 ; Dean et al, 2017 ; Kim et al, 2017 ; Winther-Sorensen et al, 2020 ). Further research revealed that deficiency of GCGR caused disturbed amino acid catabolism and reduced amino acid clearance in the liver and led to elevated plasma amino acid levels, which in turn stimulated the pancreatic α-cell hyperplasia and glucagon secretion (Dean et al, 2017 ; Galsgaard et al, 2018 ; Winther-Sorensen et al, 2020 ).…”

“…Moreover, a lot of genes involved in amino acid catabolism were found to be down-regulated in Gcgr −/− mice, especially those related to glutamine, serine, and arginine metabolism (Yang et al, 2011 ; Dean et al, 2017 ; Kim et al, 2017 ; Winther-Sorensen et al, 2020 ). Further research revealed that deficiency of GCGR caused disturbed amino acid catabolism and reduced amino acid clearance in the liver and led to elevated plasma amino acid levels, which in turn stimulated the pancreatic α-cell hyperplasia and glucagon secretion (Dean et al, 2017 ; Galsgaard et al, 2018 ; Winther-Sorensen et al, 2020 ). These results suggested an endocrine loop of liver-α cell axis through glucagon signaling (Holst et al, 2017 ; Wewer Albrechtsen et al, 2018a , b ; Dean, 2020 ).…”