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

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

Production and release of acylcarnitines by primary myotubes reflect the differences in fasting fat oxidation of the donors.

Context:Acylcarnitines are biomarkers of incomplete β-oxidation and mitochondrial lipid overload but indicate also high rates of mitochondrial fatty acid oxidation. It is unknown whether the production of acylcarnitines in primary human myotubes obtained from lean, metabolically healthy subjects reflects the fat oxidation in vivo.Objective:Our objective was to quantify the acylcarnitine production in myotubes obtained from subjects with low and high fasting respiratory quotient (RQ).Methods:Fasting RQ was determined by indirect calorimetry. Muscle biopsies from the vastus lateralis muscle were taken from 6 subjects with low fasting RQ (mean 0.79 ± 0.03) and 6 with high fasting RQ (0.90 ± 0.03), and satellite cells were isolated, cultured, and differentiated to myotubes. Myotubes were cultivated with 125μM 13C-labeled palmitate for 30 minutes and 4 and 24 hours. Quantitative profiling of 42 intracellular and 31 extracellular acylcarnitines was performed by stable isotope dilution-based metabolomics analysis by liquid chromatography coupled to mass spectrometry.Results:Myotubes from donors with high fasting RQ produced and released significant higher amounts of medium-chain acylcarnitines. High 13C8 and 13C10 acylcarnitine levels in the extracellular compartment correlated with high fasting RQ. The decreased expression of medium-chain acyl-coenzyme A dehydrogenase (MCAD) in these myotubes can explain the higher rate of incomplete fatty acid oxidation. A lower intracellular [13C]acetylcarnitine to carnitine and lower intracellular 13C16/13C18 acylcarnitine to carnitine ratio indicate reduced fatty acid oxidation capacity in these myotubes. Mitochondrial DNA content was not different.Conclusion:Acylcarnitine production and release from primary human myotubes of donors with high fasting RQ indicate a reduced fatty acid oxidation capacity and a higher rate of incomplete fatty acid oxidation. Thus, quantitative profiling of acylcarnitine production in human myotubes can be a suitable tool to identify muscular determinants of fat oxidation in vivo

Common Genetic Variation in the Human <i>FNDC5</i> Locus, Encoding the Novel Muscle-Derived ‘Browning’ Factor Irisin, Determines Insulin Sensitivity

<div><p>Aims/hypothesis</p><p>Recently, the novel myokine irisin was described to drive adipose tissue ‘browning’, to increase energy expenditure, and to improve obesity and insulin resistance in high fat-fed mice. Here, we assessed whether common single nucleotide polymorphisms (SNPs) in the <i>FNDC5</i> locus, encoding the irisin precursor, contribute to human prediabetic phenotypes (overweight, glucose intolerance, insulin resistance, impaired insulin release).</p><p>Methods</p><p>A population of 1,976 individuals was characterized by oral glucose tolerance tests and genotyped for <i>FNDC5</i> tagging SNPs. Subgroups underwent hyperinsulinaemic-euglycaemic clamps, magnetic resonance imaging/spectroscopy, and intravenous glucose tolerance tests. From 37 young and 14 elderly participants recruited in two different centres, muscle biopsies were obtained for the preparation of human myotube cultures.</p><p>Results</p><p>After appropriate adjustment and Bonferroni correction for the number of tested variants, SNPs rs16835198 and rs726344 were associated with <i>in vivo</i> measures of insulin sensitivity. Via interrogation of publicly available data from the Meta-Analyses of Glucose and Insulin-related traits Consortium, rs726344’s effect on insulin sensitivity was replicated. Moreover, novel data from human myotubes revealed a negative association between <i>FNDC5</i> expression and appropriately adjusted <i>in vivo</i> measures of insulin sensitivity in young donors. This finding was replicated in myotubes from elderly men.</p><p>Conclusions/interpretation</p><p>This study provides evidence that the <i>FNDC5</i> gene, encoding the novel myokine irisin, determines insulin sensitivity in humans. Our gene expression data point to an unexpected insulin-desensitizing effect of irisin.</p></div

Association of <i>FNDC5</i> SNPs rs16835198, rs3480, rs726344, and rs1746661 with glycaemia and insulin sensitivity (statistics).

<p>Prior to statistical analysis, all parameters were adjusted for gender, age, and bioelectrical impedance-derived percentage of body fat. Nominal associations are marked by bold fonts;</p>#<p>significant after Bonferroni correction (p<0.0127). HOMA-IR – homeostasis model assessment of insulin resistance; HWE – Hardy-Weinberg equilibrium; ISI – insulin sensitivity index; OGTT – oral glucose tolerance test; SNP – single nucleotide polymorphism.</p

Association of <i>FNDC5</i> SNPs rs16835198 and rs726344 with insulin sensitivity.

<p>HOMA-IR (A and C) and ISI OGTT (B and D) data were adjusted for gender, age, and bioelectrical impedance-derived percentage of body fat. Diamonds represent means ±SE. HOMA-IR – homeostasis model assessment of insulin resistance; ISI OGTT – oral glucose tolerance test-derived insulin sensitivity index; SNP – single nucleotide polymorphism.</p

Association of human myotube <i>FNDC5</i> mRNA expression with <i>PPARGC1A</i> mRNA expression <i>in vitro</i> and donors’ insulin sensitivity <i>in vivo</i>.

<p>The association between human myotube <i>FNDC5</i> and <i>PPARGC1A</i> mRNA contents (A) was assessed using simple linear regression analysis. The association between human myotube <i>FNDC5</i> mRNA expression and fasting insulin levels (B), HOMA-IR (C), ISI OGTT (D), and 2-h plasma glucose levels (E) of 37 young healthy donors recruited in Tübingen and with fasting insulin levels (F) of 14 elderly men recruited in Stockholm was tested by multiple linear regression analysis with gender, age, and bioelectrical impedance-derived percentage of body fat (Tübingen volunteers) or with BMI (Stockholm volunteers) as confounding variables (leverage plots shown). Dotted lines indicate the 95% confidence interval of the regression. HOMA-IR – homeostasis model assessment of insulin resistance.</p