GDF15 is an exercise-induced hepatokine regulated by glucagon and insulin in humans.

Objective: Growth differentiation factor (GDF)-15 is implicated in regulation of metabolism and circulating GDF15 increases in response to exercise. The source and regulation of the exercise-induced increase in GDF15 is, however not known. Method: Plasma GDF15 was measured by ELISA under the following conditions: 1) Arterial-to-hepatic venous differences sampled before, during, and after exercise in healthy male subjects (n=10); 2) exogenous glucagon infusion compared to saline infusion in resting healthy subjects (n=10); 3) an acute exercise bout with and without a pancreatic clamp (n=6); 4) healthy subjects for 36 hours (n=17), and 5) patients with anorexia nervosa (n=25) were compared to healthy age-matched subjects (n=25). Tissue GDF15 mRNA content was determined in mice in response to exhaustive exercise (n=16). Results: The splanchnic bed released GDF15 to the circulation during exercise and increasing the glucagon-to-insulin ratio in resting humans led to a 2.7-fold (P<0.05) increase in circulating GDF15. Conversely, inhibiting the exercise-induced increase in the glucagon-to-insulin ratio blunted the exercise-induced increase in circulating GDF15. Fasting for 36 hours did not affect circulating GDF15, whereas resting patients with anorexia nervosa displayed elevated plasma concentrations (1.4-fold, P<0.05) compared to controls. In mice, exercise increased GDF15 mRNA contents in liver, muscle, and adipose tissue. Conclusion: In humans, GDF15 is a "hepatokine" which increases during exercise and is at least in part regulated by the glucagon-to-insulin ratio. Moreover, chronic energy deprivation is associated with elevated plasma GDF15, which supports that GDF15 is implicated in metabolic signalling in humans.

Endurance Training Improves GLP-1 Sensitivity and Glucose Tolerance in Overweight Women.

Context and objective: Obesity and inactivity are risk factors for developing impaired glucose tolerance characterized by insulin resistance and reduced beta-cell function. The stimulatory effect of glucagon-like peptide 1 (GLP-1) on insulin secretion is also impaired in obese, inactive individuals. The aim of this study was to investigate whether endurance training influences beta-cell sensitivity to GLP-1. Participants and intervention: Twenty-four female participants, age 46 ±â€…2 years, body mass index 32.4 ±â€…0.9 kg/m2, and maximal oxygen consumption 24.7 ±â€…0.8 mL/kg/min participated in a 10-week exercise training study. Methods: Beta-cell sensitivity to GLP-1 was assessed in a subset of participants (n = 6) during a 120-minute hyperglycemic glucose clamp (8.5 mM) including a 1-hour GLP-1 (7-36 amide) infusion (0.4 pmol/kg/min). Changes in glucose tolerance, body composition, and cardiorespiratory fitness were assessed by oral glucose tolerance tests (OGTTs), dual-energy X-ray absorptiometry scans, magnetic resonance scans, and maximal oxygen consumption (VO2max) tests, respectively. Results: The c-peptide response to infusion of GLP-1 increased 28 ±â€…3% (P < 0.05) toward the end of the hyperglycemic clamp. The insulin response remained unchanged. Training improved glucose tolerance and reduced GLP-1, insulin, and glucagon levels during the OGTTs. Training increased VO2max (from 24.7 ±â€…0.8 to 27.0 ±â€…0.7 mL/kg/min; P < 0.05) and reduced visceral fat volume (from 4176 ±â€…265 to 3888 ±â€…266 cm3; P < 0.01). Conclusion: Along with improved glycemic control, endurance training improved beta-cell sensitivity to GLP-1 in overweight women. The study was deemed not to constitute a clinical trial and was not registered as such.

Identification and regulation of the xenometabolite derivatives cis- and trans-3,4-methylene-heptanoylcarnitine in plasma and skeletal muscle of exercising humans.

Little is known about xenometabolites in human metabolism, particularly under exercising conditions. Previously, an exercise-modifiable, likely xenometabolite derivative, cis-3,4-methylene-heptanoylcarnitine, was reported in human plasma. Here, we identified trans-3,4-methylene-heptanoylcarnitine, and its cis-isomer, in plasma and skeletal muscle by liquid chromatography-mass spectrometry. We analyzed the regulation by exercise and the arterial-to-venous differences of these cyclopropane ring-containing carnitine esters over the hepatosplanchnic bed and the exercising leg in plasma samples obtained in three separate studies from young, lean and healthy males. Compared with other medium-chain acylcarnitines, the plasma concentrations of the 3,4-methylene-heptanoylcarnitine isomers only marginally increased with exercise. Both isomers showed a more than twofold increase in the skeletal muscle tissue of the exercising leg; this may have been due to the net effect of fatty acid oxidation in the exercising muscle and uptake from blood. The latter idea is supported by a more than twofold increased net uptake in the exercising leg only. Both isomers showed a constant release from the hepatosplanchnic bed, with an increased release of the trans-isomer after exercise. The isomers differ in their plasma concentration, with a four times higher concentration of the cis-isomer regardless of the exercise state. This is the first approach studying kinetics and fluxes of xenolipid isomers from tissues under exercise conditions, supporting the hypothesis that hepatic metabolism of cyclopropane ring-containing fatty acids is one source of these acylcarnitines in plasma. The data also provide clear evidence for an exercise-dependent regulation of xenometabolites, opening perspectives for future studies about the physiological role of this largely unknown class of metabolites.

Muscle-Liver Substrate Fluxes in Exercising Humans and Potential Effects on Hepatic Metabolism.

CONTEXT: The liver is crucial to maintain energy homeostasis during exercise. Skeletal muscle-derived metabolites can contribute to the regulation of hepatic metabolism. OBJECTIVE: We aim to elucidate which metabolites are released from the working muscles and taken up by the liver in exercising humans and their potential influence on hepatic function. METHODS: In two separate studies, young healthy men fasted overnight and then performed an acute bout of exercise. Arterial-to-venous differences of metabolites over the hepato-splanchnic bed and over the exercising and resting leg were investigated by capillary electrophoresis- and liquid chromatography-mass spectrometry metabolomics platforms. Liver transcriptome data of exercising mice were analyzed by pathway analysis to find a potential overlap between exercise-regulated metabolites and activators of hepatic transcription. RESULTS: During exercise, hepatic O2 uptake and CO2 delivery were increased two-fold. In contrast to all other free fatty acids (FFA), those FFA with 18 or more carbon atoms and a high degree of saturation showed a constant release in the liver vein and only minor changes by exercise. FFA 6:0 and 8:0 were released from the working leg and taken up by the hepato-splanchnic bed. Succinate and malate showed a pronounced hepatic uptake during exercise and were also released from the exercising leg. The transcriptional response in the liver of exercising mice indicates the activation of HIF-, NRF2-, and cAMP-dependent gene transcription. These pathways can also be activated by succinate. CONCLUSION: Metabolites circulate between working muscles and the liver and may support the metabolic adaption to exercise by acting both as substrates and as signaling molecules.

Nasal insulin administration does not affect hepatic glucose production at systemic fasting insulin levels.

AIMS: To evaluate the effects of brain insulin on endogenous glucose production in fasting humans, with a focus on hepatic glucose release by performing a randomized, placebo-controlled, blinded, crossover experiment. MATERIALS AND METHODS: On two separate days, 2 H2 -glucose was infused to nine healthy lean men, and blood was sampled from the hepatic vein and a radial artery. On day 1, participants received 160 U human insulin through nasal spray, and on day 2 they received placebo spray, together with an intravenous insulin bolus to mimic spillover of nasal insulin to the circulation. Hepatic glucose fluxes and endogenous glucose production were calculated. RESULTS: Plasma insulin concentrations were similar on the two study days, and no differences in whole-body endogenous glucose production or hepato-splanchnic glucose turnover were detected. CONCLUSIONS: Nasal administration of insulin does not influence whole-body or hepatic glucose production in fasting humans. By contrast, pharmacological delivery of insulin to the brain might modulate insulin effectiveness in glucose-producing tissue when circulating insulin levels are elevated; therefore, the metabolic consequences of brain insulin action appear to be dependent on metabolic prandial status.

Metformin-induced glucagon-like peptide-1 secretion contributes to the actions of metformin in type 2 diabetes.

BACKGROUND: Metformin reduces plasma glucose and has been shown to increase glucagon-like peptide 1 (GLP-1) secretion. Whether this is a direct action of metformin on GLP-1 release, and whether some of the glucose-lowering effect of metformin occurs due to GLP-1 release, is unknown. The current study investigated metformin-induced GLP-1 secretion and its contribution to the overall glucose-lowering effect of metformin and underlying mechanisms in patients with type 2 diabetes. METHODS: Twelve patients with type 2 diabetes were included in this placebo-controlled, double-blinded study. On 4 separate days, the patients received metformin (1,500 mg) or placebo suspended in a liquid meal, with subsequent i.v. infusion of the GLP-1 receptor antagonist exendin9-39 (Ex9-39) or saline. During 240 minutes, blood was sampled. The direct effect of metformin on GLP-1 secretion was tested ex vivo in human ileal and colonic tissue with and without dorsomorphin-induced inhibiting of the AMPK activity. RESULTS: Metformin increased postprandial GLP-1 secretion compared with placebo (P = 0.014), and the postprandial glucose excursions were significantly smaller after metformin + saline compared with metformin + Ex9-39 (P = 0.004). Ex vivo metformin acutely increased GLP-1 secretion (colonic tissue, P < 0.01; ileal tissue, P < 0.05), but the effect was abolished by inhibition of AMPK activity. CONCLUSIONS: Metformin has a direct and AMPK-dependent effect on GLP-1-secreting L cells and increases postprandial GLP-1 secretion, which seems to contribute to metformin's glucose-lowering effect and mode of action. TRIAL REGISTRATION: NCT02050074 (https://clinicaltrials.gov/ct2/show/NCT02050074). FUNDING: This study received grants from the A.P. Møller Foundation, the Novo Nordisk Foundation, the Danish Medical Association research grant, the Australian Research Council, the National Health and Medical Research Council, and Pfizer Inc.

Angiogenin and Osteoprotegerin are type II muscle specific myokines protecting pancreatic beta-cells against proinflammatory cytokines.

Tissue cross-talk is emerging as a determinant way to coordinate the different organs implicated in glucose homeostasis. Among the inter-organ communication factors, muscle-secreted myokines can modulate the function and survival of pancreatic beta-cells. Using primary human myotubes from soleus, vastus lateralis and triceps brachii muscles, we report here that the impact of myokines on beta-cells depends on fiber types and their metabolic status. We show that Type I and type II primary myotubes present specific mRNA and myokine signatures as well as a different sensitivity to TNF-alpha induced insulin resistance. Finally, we show that angiogenin and osteoprotegerin are triceps specific myokines with beta-cell protective actions against proinflammatory cytokines. These results suggest that type I and type II muscles could impact insulin secretion and beta-cell mass differentially in type 2 diabetes through specific myokines secretion.

Quality Control of Serum and Plasma by Quantification of (4E,14Z)-Sphingadienine-C18-1-Phosphate Uncovers Common Preanalytical Errors During Handling of Whole Blood.

BACKGROUND: Nonadherence to standard operating procedures (SOPs) during handling and processing of whole blood is one of the most frequent causes affecting the quality of serum and plasma. Yet, the quality of blood samples is of the utmost importance for reliable, conclusive research findings, valid diagnostics, and appropriate therapeutic decisions. METHODS: UHPLC-MS-driven nontargeted metabolomics was applied to identify biomarkers that reflected time to processing of blood samples, and a targeted UHPLC-MS analysis was used to quantify and validate these biomarkers. RESULTS: We found that (4E,14Z)-sphingadienine-C18-1-phosphate (S1P-d18:2) was suitable for the reliable assessment of the pronounced changes in the quality of serum and plasma caused by errors in the phase between collection and centrifugation of whole blood samples. We rigorously validated S1P-d18:2, which included the use of practicality tests on >1400 randomly selected serum and plasma samples that were originally collected during single- and multicenter trials and then stored in 11 biobanks in 3 countries. Neither life-threatening disease states nor strenuous metabolic challenges (i.e., high-intensity exercise) affected the concentration of S1P-d18:2. Cutoff values for sample assessment were defined (plasma, ≤0.085 µg/mL; serum, ≤0.154 µg/mL). CONCLUSIONS: Unbiased valid monitoring to check for adherence to SOP-dictated time for processing to plasma or serum and/or time to storage of whole blood at 4 °C is now feasible. This novel quality assessment step could enable scientists to uncover common preanalytical errors, allowing for identification of serum and plasma samples that should be excluded from certain investigations. It should also allow control of samples before long-term storage in biobanks.

Angiopoietin-like protein 4 is an exercise-induced hepatokine in humans, regulated by glucagon and cAMP.

OBJECTIVE: Angiopoietin-like protein-4 (ANGPTL4) is a circulating protein that is highly expressed in liver and implicated in regulation of plasma triglyceride levels. Systemic ANGPTL4 increases during prolonged fasting and is suggested to be secreted from skeletal muscle following exercise. METHODS: We investigated the origin of exercise-induced ANGPTL4 in humans by measuring the arterial-to-venous difference over the leg and the hepato-splanchnic bed during an acute bout of exercise. Furthermore, the impact of the glucagon-to-insulin ratio on plasma ANGPTL4 was studied in healthy individuals. The regulation of ANGPTL4 was investigated in both hepatic and muscle cells. RESULTS: The hepato-splanchnic bed, but not the leg, contributed to exercise-induced plasma ANGPTL4. Further studies using hormone infusions revealed that the glucagon-to-insulin ratio is an important regulator of plasma ANGPTL4 as elevated glucagon in the absence of elevated insulin increased plasma ANGPTL4 in resting subjects, whereas infusion of somatostatin during exercise blunted the increase of both glucagon and ANGPTL4. Moreover, activation of the cAMP/PKA signaling cascade let to an increase in ANGPTL4 mRNA levels in hepatic cells, which was prevented by inhibition of PKA. In humans, muscle ANGPTL4 mRNA increased during fasting, with only a marginal further induction by exercise. In human muscle cells, no inhibitory effect of AMPK activation could be demonstrated on ANGPTL4 expression. CONCLUSIONS: The data suggest that exercise-induced ANGPTL4 is secreted from the liver and driven by a glucagon-cAMP-PKA pathway in humans. These findings link the liver, insulin/glucagon, and lipid metabolism together, which could implicate a role of ANGPTL4 in metabolic diseases.

Effect of Pancreatic Hormones on pro-Atrial Natriuretic Peptide in Humans.

Plasma concentrations of pro-Atrial natriuretic peptide, proANP, are decreased in obesity and diabetes. Decreased proANP concentrations have also been noted after meal intake, and recently, a glucose-mediated regulation of ANP gene expression was reported. Hence, we evaluated the effects of insulin, glucagon and glucose on plasma proANP in a series of observational and experimental studies. Six healthy men underwent seven days of bed rest. Before and after the bed rest, hyperinsulinemic euglycemic clamps with serial plasma measurements of proANP were performed. Moreover, plasma proANP was quantified in 65 individuals with normal or impaired glucose regulation. Finally, the effects of infusion-induced hyperglucagonemia were examined in ten healthy men. Bed rest decreased insulin sensitivity and plasma proANP. The decrease in proANP was not associated with insulin sensitivity and the peptide concentrations remained constant during euglycemic hyperinsulinemia and hyperglycemic hyperglucagonemia. Impaired glucose regulation was not associated with decreased proANP concentrations. Bed rest per se induces a marked decrease in plasma proANP concentrations whereas insulin resistance and impaired glucose regulation was not associated with lower proANP concentrations. Neither acute hyperinsulinemia nor hyperglucagonemia seems to affect plasma proANP. Our findings thus suggest that decreased plasma proANP concentrations occur late in the development of insulin resistance.

Circulating Follistatin Is Liver-Derived and Regulated by the Glucagon-to-Insulin Ratio.

CONTEXT: Follistatin is a plasma protein recently reported to increase under conditions with negative energy balance, such as exercise and fasting in humans. Currently, the perception is that circulating follistatin is a result of para/autocrine actions from various tissues. The large and acute increase in circulating follistatin in response to exercise suggests that it may function as an endocrine signal. OBJECTIVE: We assessed origin and regulation of circulating follistatin in humans. DESIGN/INTERVENTIONS: First, we assessed arterial-to-venous difference of follistatin over the splanchnic bed at rest and during exercise in healthy humans. To evaluate the regulation of plasma follistatin we manipulated glucagon-to-insulin ratio in humans at rest as well as in cultured hepatocytes. Finally, the impact of follistatin on human islets of Langerhans was assessed. RESULTS: We demonstrate that in humans the liver is a major contributor to circulating follistatin both at rest and during exercise. Glucagon increases and insulin inhibits follistatin secretion both in vivo and in vitro, mediated via the secondary messenger cAMP in the hepatocyte. Short-term follistatin treatment reduced glucagon secretion from islets of Langerhans, whereas long-term follistatin treatment prevented apoptosis and induced proliferation of rat ß cells. CONCLUSIONS: In conclusion, in humans, the liver secretes follistatin at rest and during exercise, and the glucagon-to-insulin ratio is a key determinant of circulating follistatin levels. Circulating follistatin may be a marker of the glucagon-to-insulin tone on the liver.

Type 2 diabetes alters metabolic and transcriptional signatures of glucose and amino acid metabolism during exercise and recovery.

AIMS/HYPOTHESIS: The therapeutic benefit of physical activity to prevent and treat type 2 diabetes is commonly accepted. However, the impact of the disease on the acute metabolic response is less clear. To this end, we investigated the effect of type 2 diabetes on exercise-induced plasma metabolite changes and the muscular transcriptional response using a complementary metabolomics/transcriptomics approach. METHODS: We analysed 139 plasma metabolites and hormones at nine time points, and whole genome expression in skeletal muscle at three time points, during a 60 min bicycle ergometer exercise and a 180 min recovery phase in type 2 diabetic patients and healthy controls matched for age, percentage body fat and maximal oxygen consumption (VO2). RESULTS: Pathway analysis of differentially regulated genes upon exercise revealed upregulation of regulators of GLUT4 (SLC2A4RG, FLOT1, EXOC7, RAB13, RABGAP1 and CBLB), glycolysis (HK2, PFKFB1, PFKFB3, PFKM, FBP2 and LDHA) and insulin signal mediators in diabetic participants compared with controls. Notably, diabetic participants had normalised rates of lactate and insulin levels, and of glucose appearance and disappearance, after exercise. They also showed an exercise-induced compensatory regulation of genes involved in biosynthesis and metabolism of amino acids (PSPH, GATM, NOS1 and GLDC), which responded to differences in the amino acid profile (consistently lower plasma levels of glycine, cysteine and arginine). Markers of fat oxidation (acylcarnitines) and lipolysis (glycerol) did not indicate impaired metabolic flexibility during exercise in diabetic participants. CONCLUSIONS/INTERPRETATION: Type 2 diabetic individuals showed specific exercise-regulated gene expression. These data provide novel insight into potential mechanisms to ameliorate the disturbed glucose and amino acid metabolism associated with type 2 diabetes.