Glucagon and lipid interactions in the regulation of hepatic AMPK signaling and expression of PPARalpha and FGF21 transcripts in vivo.

Hepatic glucagon action increases in response to accelerated metabolic demands and is associated with increased whole body substrate availability, including circulating lipids. The hypothesis that increases in hepatic glucagon action stimulate AMP-activated protein kinase (AMPK) signaling and peroxisome proliferator-activated receptor-α (PPARα) and fibroblast growth factor 21 (FGF21) expression in a manner modulated by fatty acids was tested in vivo. Wild-type (gcgr(+/+)) and glucagon receptor-null (gcgr(-/-)) littermate mice were studied using an 18-h fast, exercise, and hyperglucagonemic-euglycemic clamps plus or minus increased circulating lipids. Fasting and exercise in gcgr(+/+), but not gcgr(-/-) mice, increased hepatic phosphorylated AMPKα at threonine 172 (p-AMPK(Thr(172))) and PPARα and FGF21 mRNA. Clamp results in gcgr(+/+) mice demonstrate that hyperlipidemia does not independently impact or modify glucagon-stimulated increases in hepatic AMP/ATP, p-AMPK(Thr(172)), or PPARα and FGF21 mRNA. It blunted glucagon-stimulated acetyl-CoA carboxylase phosphorylation, a downstream target of AMPK, and accentuated PPARα and FGF21 expression. All effects were absent in gcgr(-/-) mice. These findings demonstrate that glucagon exerts a critical regulatory role in liver to stimulate pathways linked to lipid metabolism in vivo and shows for the first time that effects of glucagon on PPARα and FGF21 expression are amplified by a physiological increase in circulating lipids.

Hepatic glucagon action is essential for exercise-induced reversal of mouse fatty liver.

OBJECTIVE: Exercise is an effective intervention to treat fatty liver. However, the mechanism(s) that underlie exercise-induced reductions in fatty liver are unclear. Here we tested the hypothesis that exercise requires hepatic glucagon action to reduce fatty liver. RESEARCH DESIGN AND METHODS: C57BL/6 mice were fed high-fat diet (HFD) and assessed using magnetic resonance, biochemical, and histological techniques to establish a timeline for fatty liver development over 20 weeks. Glucagon receptor null (gcgr(-/-)) and wild-type (gcgr(+/+)) littermate mice were subsequently fed HFD to provoke moderate fatty liver and then performed either 10 or 6 weeks of running wheel or treadmill exercise, respectively. RESULTS: Exercise reverses progression of HFD-induced fatty liver in gcgr(+/+) mice. Remarkably, such changes are absent in gcgr(-/-) mice, thus confirming the hypothesis that exercise-stimulated hepatic glucagon receptor activation is critical to reduce HFD-induced fatty liver. CONCLUSIONS: These findings suggest that therapies that use antagonism of hepatic glucagon action to reduce blood glucose may interfere with the ability of exercise and perhaps other interventions to positively affect fatty liver.

Hepatic energy state is regulated by glucagon receptor signaling in mice.

The hepatic energy state, defined by adenine nucleotide levels, couples metabolic pathways with energy requirements. This coupling is fundamental in the adaptive response to many conditions and is impaired in metabolic disease. We have found that the hepatic energy state is substantially reduced following exercise, fasting, and exposure to other metabolic stressors in C57BL/6 mice. Glucagon receptor signaling was hypothesized to mediate this reduction because increased plasma levels of glucagon are characteristic of metabolic stress and because this hormone stimulates energy consumption linked to increased gluconeogenic flux through cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C) and associated pathways. We developed what we believe to be a novel hyperglucagonemic-euglycemic clamp to isolate an increment in glucagon levels while maintaining fasting glucose and insulin. Metabolic stress and a physiological rise in glucagon lowered the hepatic energy state and amplified AMP-activated protein kinase signaling in control mice, but these changes were abolished in glucagon receptor- null mice and mice with liver-specific PEPCK-C deletion. 129X1/Sv mice, which do not mount a glucagon response to hypoglycemia, displayed an increased hepatic energy state compared with C57BL/6 mice in which glucagon was elevated. Taken together, these data demonstrate in vivo that the hepatic energy state is sensitive to glucagon receptor activation and requires PEPCK-C, thus providing new insights into liver metabolism.