Hepatic energy state is regulated by glucagon receptor signaling in mice

Abstract

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. IntroductionThe energy state in the cell is defined by adenine nucleotide levels and is critically coupled to nearly all metabolic processes (1). In the cell, the adenine nucleotides ATP, ADP, and AMP are tied directly or indirectly to all energetic pathways and allosterically control numerous regulatory enzymes (2-6). Changes in adenine nucleotides typically occur such that ATP and AMP deviate in reciprocal directions, while ADP remains constant (1). Such changes are the basis for using the ratio AMP/ATP (1, 7) or the equation for cellular energy charge ([ATP + (0.5 × ADP)] / [ATP + ADP + AMP]) (8, 9) as indices of the metabolic environment. Metabolic stress is thus characterized by a rise in AMP paired with a fall in ATP levels, reflecting a decrease in energy state. A fall in energy state is of considerable importance, in part due to the regulatory role of AMP/ATP ratios on AMPK activity (10). AMPK is a metabolic switch sensitive to high AMP/ATP ratios and functions to protect the energy state by inhibiting ATP-consuming processes while stimulating ATP-producing processes. (10).In most tissues, the environment is controlled to maintain a high energy state (low AMP/ATP ratio). In skeletal muscle, for example, creatine kinase limits reductions in ATP during conditions such as exercise, when energy utilization is accelerated (11,12). The liver, in contrast, lacks creatine kinase, and exercise has been shown to markedly decrease the hepatic energy state and increase the phosphorylation of AMPK (11, 13). The regulatory...

Augmenting energy expenditure by mitochondrial uncoupling: a role of AMP-activated protein kinase

“…AMPK is believed to control liver glucose homeostasis, due to downregulation of key gluconeogenic genes. However, recent studies suggest that AMPK rather supports than inhibits hepatic gluconeogenesis by improving hepatic energy status during fasting (Berglund et al 2009), also in accordance with the notion that metformin, a glucose-lowering drug, inhibits hepatic gluconeogenesis independently of LKB1/AMPK pathway and independently of the transcriptional repression of gluconeogenesis, via a decrease in hepatic energy state . Thus, the role of AMPK in the control of hepatic glucose metabolism by thiazolidinediones (TZDs), and also by adiponectin, an insulin-sensitizing adipokine (Zhang et al 2009;Yamauchi et al 2002), should be reconsidered.…”

“…Hepatic AMPK has a central role in the metabolic adaptation to acute and chronic nutritional stress (Berglund et al 2009). AMPK influences hepatic lipid metabolism, where its activation leads to a reduction in lipogenesis and cholesterol synthesis and a simultaneous increase in fatty acid oxidation leading to a decreased hepatic lipid deposition (Hardie 2008;Zhang et al 2009).…”

5-Aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR) Effect on Glucose Production, but Not Energy Metabolism, Is Independent of Hepatic AMPK in Vivo

“…Applying physiological stressors that reduce energy state (i.e., acute, high intensity exercise) may uncover a role for hepatic AMPK in glucoregulation that is not evident when energy state is reduced using AICAR. These and other recent studies (6,8,10) emphasize the need to demarcate the in vivo effects of AMPK in physiology from pharmacology. Recent literature on glycemic regulation by glucagon (10) and biguanides (6,8) has reinvigorated the conversation surrounding AMP and energy charge as regulatory variables (46,47).…”

“…Recent literature on glycemic regulation by glucagon (10) and biguanides (6,8) has reinvigorated the conversation surrounding AMP and energy charge as regulatory variables (46,47). Glucagon-mediated flux through phosphoenolpyruvate carboxykinase appears to generate an autoregulatory feedback loop limiting the amount of ATP disposed to gluconeogenesis (10). These studies demonstrate that the unique plasticity of liver EC has an important function in response to a normal elevation in endocrine action.…”

“…The supernatant was neutralized with 0.5 M K 2 CO 3 . Samples sat on ice for another 3 min and were centrifuged at 3200 ϫ g at 4°C for 5 min, and the supernatants were saved for HPLC nucleotide analysis as previously described (10). Energy charge (EC) was calculated from the results as fol-…”