Loss of hepatic phosphoenolpyruvate carboxykinase 1 dysregulates metabolic responses to acute exercise but enhances adaptations to exercise training in mice

Abstract

Acute exercise increases liver gluconeogenesis to supply glucose to working muscle. Concurrently, elevated liver lipid breakdown fuels the high energetic cost of gluconeogenesis. This functional coupling between liver gluconeogenesis and lipid oxidation has been proposed to underlie the ability of regular exercise to enhance liver mitochondrial oxidative metabolism and decrease liver steatosis in individuals with non-alcoholic fatty liver disease. Herein we tested whether repeated bouts of increased hepatic gluconeogenesis are necessary for exercise training to lower liver lipids. Experiments used diet-induced obese mice lacking hepatic phosphoenolpyruvate carboxykinase 1 (KO) to inhibit gluconeogenesis and wild type (WT) littermates. 2H/13C metabolic flux analysis quantified glucose and mitochondrial oxidative fluxes in untrained mice at rest and during acute exercise. Circulating and tissue metabolite levels were determined during sedentary conditions, acute exercise, and refeeding post-exercise. Mice also underwent six weeks of treadmill running protocols to define hepatic and extrahepatic adaptations to exercise training. Untrained KO mice were unable to maintain euglycemia during acute exercise resulting from an inability to increase gluconeogenesis. Liver triacylglycerides were elevated following acute exercise and circulating β-hydroxybutyrate was higher during post-exercise refeeding in untrained KO mice. In contrast, exercise training prevented liver triacylglyceride accumulation in KO mice. This was accompanied by pronounced increases in indices of skeletal muscle mitochondrial oxidative metabolism in KO mice. Together, these results show that hepatic gluconeogenesis is dispensable for exercise training to reduce liver lipids. This may be due to responses in ketone body metabolism and/or metabolic adaptations in skeletal muscle to exercise.

Exercise training adaptations in liver glycogen and glycerolipids require hepatic AMP-activated protein kinase in mice

“…Following a one-hour acclimation period, an 80 μl arterial blood sample was obtained to determine natural isotopic enrichment of plasma glucose. Immediately after this sample acquisition, a stable isotope infusion protocol was initiated to allow for the quantification of endogenous glucose production and associated oxidative fluxes as previously performed (5,6,(26)(27)(28)(29)(30). Briefly, a 2 H 2 O (99.9%)-saline bolus containing [6,6-2 H 2 ]glucose (99%) was infused over 25 minutes to concurrently label the H 2 O pool and provide a [6,6-2 H 2 ]glucose prime (440 μmol•kg -1 ).…”

“…Briefly, a 2 H 2 O (99.9%)-saline bolus containing [6,6-2 H 2 ]glucose (99%) was infused over 25 minutes to concurrently label the H 2 O pool and provide a [6,6-2 H 2 ]glucose prime (440 μmol•kg -1 ). An independent, continuous infusion of [6, were obtained while mice were completing an acute treadmill running bout at 35% of their posttraining maximal running speed. Plasma samples were stored at -20°C for subsequent isotopic analysis.…”

“…Exercise accelerates liver glucose production to match the higher rate of muscle glucose uptake (1,2). The increased liver glucose production is accomplished by a rise in both liver glycogenolysis and gluconeogenesis (1)(2)(3)(4)(5)(6). De novo synthesis of glucose via gluconeogenesis is energetically costly and contributes to the elevated hydrolysis of ATP to ADP and AMP during exercise (7).…”

“…De novo synthesis of glucose via gluconeogenesis is energetically costly and contributes to the elevated hydrolysis of ATP to ADP and AMP during exercise (7). To mitigate this energy discharge and support the increased gluconeogenic flux, fatty acid oxidation within the liver is stimulated (5,6,8). Thus, acute exercise requires that the liver transform the potential energy in fatty acids to the common energy currency of ATP (9).…”

Exercise training adaptations in liver glycogen and glycerolipids require hepatic AMP-activated protein kinase in mice

“…Following a one-hour acclimation period, an 80 μl arterial blood sample was obtained to determine natural isotopic enrichment of plasma glucose. Immediately after this sample acquisition, a stable isotope infusion protocol was initiated to allow for the quantification of endogenous glucose production and associated oxidative fluxes as previously performed (5,6,(26)(27)(28)(29)(30) ]glucose prime. Four arterial blood samples (100 μl) were obtained 90-120 minutes following the [U-13 C]propionate bolus (time = 0-30 minutes of treadmill running) to determine arterial blood glucose using an Accu-Chek® glucometer (Roche Diagnostics, Indianapolis, IN, USA) and to perform 2 H/ 13 C metabolic flux analysis.…”