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Glycine N -methyltransferase (GNMT) is the most abundant liver methyltransferase regulating the availability of the biological methyl donor, S -adenosylmethionine (SAM). Moreover, GNMT has been identified to be down-regulated in hepatocellular carcinoma (HCC). Despite its role in regulating SAM levels and association of its down-regulation with liver tumorigenesis, the impact of reduced GNMT on metabolic reprogramming before the manifestation of HCC has not been investigated in detail. Herein, we used 2 H/ 13 C metabolic flux analysis in conscious, unrestrained mice to test the hypothesis that the absence of GNMT causes metabolic reprogramming. GNMT-null (KO) mice displayed a reduction in blood glucose that was associated with a decline in both hepatic glycogenolysis and gluconeogenesis. The reduced gluconeogenesis was due to a decrease in liver gluconeogenic precursors, citric acid cycle fluxes, and anaplerosis and cataplerosis. A concurrent elevation in both hepatic SAM and metabolites of SAM utilization pathways was observed in the KO mice. Specifically, the increase in metabolites of SAM utilization pathways indicated that hepatic polyamine synthesis and catabolism, transsulfuration, and de novo lipogenesis pathways were increased in the KO mice. Of note, these pathways utilize substrates that could otherwise be used for gluconeogenesis. Also, this metabolic reprogramming occurs before the well-documented appearance of HCC in GNMT-null mice. Together, these results indicate that GNMT deletion promotes a metabolic shift whereby nutrients are channeled away from glucose formation toward pathways that utilize the elevated SAM., (© 2018 Hughey et al.)

Pathologies including diabetes and conditions such as exercise place an unusual demand on liver energy metabolism, and this demand induces a state of energy discharge. Hepatic AMP-activated protein kinase (AMPK) has been proposed to inhibit anabolic processes such as gluconeogenesis in response to cellular energy stress. However, both AMPK activation and glucose release from the liver are increased during exercise. Here, we sought to test the role of hepatic AMPK in the regulation of in vivo glucose-producing and citric acid cycle-related fluxes during an acute bout of muscular work. We used 2 H/ 13 C metabolic flux analysis to quantify intermediary metabolism fluxes in both sedentary and treadmill-running mice. Additionally, liver-specific AMPK α1 and α2 subunit KO and WT mice were utilized. Exercise caused an increase in endogenous glucose production, glycogenolysis, and gluconeogenesis from phosphoenolpyruvate. Citric acid cycle fluxes, pyruvate cycling, anaplerosis, and cataplerosis were also elevated during this exercise. Sedentary nutrient fluxes in the postabsorptive state were comparable for the WT and KO mice. However, the increment in the endogenous rate of glucose appearance during exercise was blunted in the KO mice because of a diminished glycogenolytic flux. This lower rate of glycogenolysis was associated with lower hepatic glycogen content before the onset of exercise and prompted a reduction in arterial glucose during exercise. These results indicate that liver AMPKα1α2 is required for maintaining glucose homeostasis during an acute bout of exercise., (© 2017 by The American Society for Biochemistry and Molecular Biology, Inc.)

Metabolic stress, as well as several antidiabetic agents, increases hepatic nucleotide monophosphate (NMP) levels, activates AMP-activated protein kinase (AMPK), and suppresses glucose production. We tested the necessity of hepatic AMPK for the in vivo effects of an acute elevation in NMP on metabolism. 5-Aminoimidazole-4-carboxamide 1-β-D-ribofuranoside (AICAR; 8 mg·kg(-1)·min(-1))-euglycemic clamps were performed to elicit an increase in NMP in wild type (α1α2(lox/lox)) and liver-specific AMPK knock-out mice (α1α2(lox/lox) + Albcre) in the presence of fixed glucose. Glucose kinetics were equivalent in 5-h fasted α1α2(lox/lox) and α1α2(lox/lox) + Albcre mice. AMPK was not required for AICAR-mediated suppression of glucose production and increased glucose disappearance. These results demonstrate that AMPK is unnecessary for normal 5-h fasting glucose kinetics and AICAR-mediated inhibition of glucose production. Moreover, plasma fatty acids and triglycerides also decreased independently of hepatic AMPK during AICAR administration. Although the glucoregulatory effects of AICAR were shown to be independent of AMPK, these studies provide in vivo support for the AMPK energy sensor paradigm. AICAR reduced hepatic energy charge by ∼20% in α1α2(lox/lox), which was exacerbated by ∼2-fold in α1α2(lox/lox) + Albcre. This corresponded to a ∼6-fold rise in AMP/ATP in α1α2(lox/lox) + Albcre. Consistent with the effects on adenine nucleotides, maximal mitochondrial respiration was ∼30% lower in α1α2(lox/lox) + Albcre than α1α2(lox/lox) livers. Mitochondrial oxidative phosphorylation efficiency was reduced by 25%. In summary, these results demonstrate that the NMP capacity to inhibit glucose production in vivo is independent of liver AMPK. In contrast, AMPK promotes mitochondrial function and protects against a more precipitous fall in ATP during AICAR administration.