Alterations of Central Liver Metabolism of Pediatric Patients with Non-Alcoholic Fatty Liver Disease

Abstract

Non-alcoholic fatty liver disease (NAFLD) is the most common chronic liver disease in children and is associated with overweight and insulin resistance (IR). Almost nothing is known about in vivo alterations of liver metabolism in NAFLD, especially in the early stages of non-alcoholic steatohepatitis (NASH). Here, we used a complex mathematical model of liver metabolism to quantify the central hepatic metabolic functions of 71 children with biopsy-proven NAFLD. For each patient, a personalized model variant was generated based on enzyme abundances determined by mass spectroscopy. Our analysis revealed statistically significant alterations in the hepatic carbohydrate, lipid, and ammonia metabolism, which increased with the degree of obesity and severity of NAFLD. Histologic features of NASH and IR displayed opposing associations with changes in carbohydrate and lipid metabolism but synergistically decreased urea synthesis in favor of the increased release of glutamine, a driver of liver fibrosis. Taken together, our study reveals already significant alterations in the NASH liver of pediatric patients, which, however, are differently modulated by the simultaneous presence of IR.

Keywords: histology; liver tissue; mathematical modeling; plasma profile; proteomics.

Figure 1

Model validation. (a) Relationship between hepatic triacylglycerol (TAG) content and histologically assessed grade of steatosis; (b) Relationship between homeostatic model assessment for insulin resistance (HOMAR-IR) and hepatic glucose uptake.

Figure 2

Relationship between the body mass index (BMI) and hepatic metabolic functions. (a) Glycolysis; (b) Fatty acid uptake; (c) TAG synthesis; (d) Lipoprotein synthesis.

Figure 3

Relationship between histologically assessed liver fibrosis and various hepatic capacities. (a) Glycolysis; (b) TAG synthesis; (c) Lipoprotein production. A high degree of fibrosis is associated with decreased TAG synthesis, reduced glycolysis, and decreased lipoprotein synthesis.

Figure 4

Significant associations between fatty acid uptake and other metabolic functions. (a) Lipoprotein synthesis; (b) Ketone Body production; (c) β-oxidation; (d) Cholesterol synthesis; (e) Ammonia uptake; (f) Urea production. For full information, see Supplemental Figure S1.

Figure 5

Clinical parameters of pediatric patients in four different disease classes (variant A). (a) Age; (b) BMI; (c) BMI Standard Deviation Score (SDS); (d) NAFLD Activity Score (NAS); (e) Inflammation score; (f) Degree of fibrosis. Crossbars indicate differences between groups based on two-sided t-testing (orange: p < 0.1; black: p < 0.05; red: p < 0.01). Corresponding p-values are given above bars.

Figure 6

Diurnal differences in metabolic functions between the four disease classes (variant A) defined by the degree of fibrosis and HOMA-IR. (a) Rate of glycolysis; (b) Fatty acid uptake; (c) β-oxidation; (d) Ketone body production; (e) Urea production; (f) Glutamate release. Crossbars indicate differences between groups based on two-sided t-testing (orange: p < 0.1; black: p < 0.05; red: p < 0.01). Corresponding p-values are given above bars.

Figure 7

Association strength of NASH and IR with metabolic changes of the liver. The association strength was quantified by the normalized regression coefficients α and β in Equation (1). (a) Association strength if the severity of NASH and IR is measured by inflammation grading and HOMA-IR; (b) Association strength if the severity of NASH and IR is measured by fibrosis grading and HOMA-IR.

Figure 8

Schematic model representation. Reactions and transport processes between compartments are symbolized by arrows. Single pathways as defined in biochemical textbooks are numbered and highlighted by different coloring: (1) glycogen metabolism, (2) fructose metabolism, (3) galactose metabolism, (4) glycolysis, (5) gluconeogenesis, (6) oxidative pentose phosphate pathway, (7) non-oxidative pentose phosphate pathway, (8) fatty acid synthesis, (9) triglyceride synthesis, (10) synthesis and degradation of lipid droplets and synthesis of VLDL lipoprotein, (11) cholesterol synthesis, (12) tricarbonic acid cycle, (13) respiratory chain and oxidative phosphorylation, (14) β-oxidation of fatty acids, (15) urea cycle, (16) ethanol metabolism, (17) ketone body synthesis, (18) glutamine metabolism, (19) serine utilization, and (20) alanine utilization. Lipid droplet synthesis and degradation pathways include de novo synthesis of lipid droplets, lipid droplet filling, lipid droplet growth and fusion as well as lipid droplet degradation in dependence on regulatory surface proteins [15]. Small cylinders and cubes symbolize ion channels and ion transporters. Double arrows indicate reversible reactions, which may proceed in both directions according to the value of the thermodynamic equilibrium constant and cellular concentrations of their reactants. Reactions are labeled by the short names of the catalyzing enzyme or membrane transporter given in the small boxes attached to the reactions arrow. Red boxes indicate enzymes that are regulated by hormone-dependent reversible phosphorylation. Metabolites are denoted by their short names. Full names of metabolites and kinetic rate laws of reaction rates are outlined in Berndt et al. [14] and Wallstab et al. [15]. The figure was adapted from Figure 1 of reference [14] by updating and rearranging pathways (9)–(11) (http://creativecommons.org/licenses/by/4.0/).