Elevated TCA cycle function in the pathology of diet-induced hepatic insulin resistance and fatty liver

Advanced Imaging Research Center, University of Texas Southwestern Medical Center, Dallas, TX, USA.
The Journal of Lipid Research (Impact Factor: 4.42). 04/2012; 53(6):1080-92. DOI: 10.1194/jlr.M023382
Source: PubMed


The manner in which insulin resistance impinges on hepatic mitochondrial function is complex. Although liver insulin resistance is associated with respiratory dysfunction, the effect on fat oxidation remains controversial, and biosynthetic pathways that traverse mitochondria are actually increased. The tricarboxylic acid (TCA) cycle is the site of terminal fat oxidation, chief source of electrons for respiration, and a metabolic progenitor of gluconeogenesis. Therefore, we tested whether insulin resistance promotes hepatic TCA cycle flux in mice progressing to insulin resistance and fatty liver on a high-fat diet (HFD) for 32 weeks using standard biomolecular and in vivo (2)H/(13)C tracer methods. Relative mitochondrial content increased, but respiratory efficiency declined by 32 weeks of HFD. Fasting ketogenesis became unresponsive to feeding or insulin clamp, indicating blunted but constitutively active mitochondrial β-oxidation. Impaired insulin signaling was marked by elevated in vivo gluconeogenesis and anaplerotic and oxidative TCA cycle flux. The induction of TCA cycle function corresponded to the development of mitochondrial respiratory dysfunction, hepatic oxidative stress, and inflammation. Thus, the hepatic TCA cycle appears to enable mitochondrial dysfunction during insulin resistance by increasing electron deposition into an inefficient respiratory chain prone to reactive oxygen species production and by providing mitochondria-derived substrate for elevated gluconeogenesis.

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Available from: Blanka Kucejova, Oct 10, 2015
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    • "Dysfunction in this complex inhibits the mitochondrial electron flow from the Fe-S complex I to the ubiquinone centers and therefore may block the entire process of oxidative phosphorylation. EGCG had the effect of enhancing the activity of this complex and consequently enhancing the electron transport chain [40] [55] [59] [60]. Complex II is the second entry point of reducing equivalents into the mitochondrial respiratory chain via FADH, and it is the only complex that pumps protons across the inner mitochondrial membrane. "
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    ABSTRACT: Nonalcoholic fatty liver disease has been considered the hepatic manifestation of obesity. It is unclear whether supplementation with green tea extract rich in epigallocatechin-3-gallate (EGCG) influences the activity of mitochondrial respiratory chain complexes and insulin resistance in the liver. EGCG regulated hepatic mitochondrial respiratory chain complexes and was capable of improving lipid metabolism, attenuating insulin resistance in obese mice. Mice were divided into four groups: control diet+water (CW) or EGCG (CE) and hyperlipidic diet+water (HFW) or EGCG (HFE). All animals received water and diets ad libitum for 16weeks. Placebo groups received water (0.1ml/day) and EGCG groups (0.1ml EGCG and 50mg/kg/day) by gavage. Cytokines concentrations were obtained by ELISA, protein expression through Western blotting and mitochondrial complex enzymatic activity by colorimetric assay of substrate degradation. HFW increased body weight gain, adiposity index, retroperitoneal and mesenteric adipose tissue relative weight, serum glucose, insulin and Homeostasis Model Assessment of Basal Insulin Resistance (HOMA-IR); glucose intolerance was observed in oral glucose tolerance test (OGTT) as well as ectopic fat liver deposition. HFE group decreased body weight gain, retroperitoneal and mesenteric adipose tissue relative weight, HOMA-IR, insulin levels and liver fat accumulation; increased complexes II-III and IV and malate dehydrogenase activities and improvement in glucose uptake in OGTT and insulin sensitivity by increased protein expression of total AKT, IRα and IRS1. We did not find alterations in inflammatory parameters analyzed. EGCG was able to prevent obesity stimulating the mitochondrial complex chain, increasing energy expenditure, particularly from the oxidation of lipid substrates, thereby contributing to the prevention of hepatic steatosis and improved insulin sensitivity. Copyright © 2015. Published by Elsevier Inc.
    The Journal of nutritional biochemistry 07/2015; DOI:10.1016/j.jnutbio.2015.07.002 · 3.79 Impact Factor
    • "In contrast to these findings in skeletal muscle, mitochondrial oxidative metabolism is 2-fold greater in hepatocytes of individuals of hepatic steatosis (Sunny et al. 2011). Animal studies suggest this hyperactivity of fattyacid induced mitochondrial oxidation (or TCA flux) is associated with mitochondrial inefficiency that leads, in the long term, to insulin resistance (Satapati et al. 2012). In the absence of properly designed longitudinal and experimental studies, however, the cause and effect nature of the association between mitochondrial efficiency/flux and insulin resistance remains equivocal (Holloszy 2009). "
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    ABSTRACT: The prevalence of type 2 diabetes (T2D) has increased dramatically over the past two decades, not only among adults but also among adolescents. T2D is a systemic disorder affecting every organ system and is especially damaging to the cardiovascular system, predisposing individuals to severe cardiac and vascular complications. The precise mechanisms that cause T2D are an area of active research. Most current theories suggest that the process begins with peripheral insulin resistance that precedes failure of the pancreatic β-cells to secrete sufficient insulin to maintain normoglycemia. A growing body of literature has highlighted multiple aspects of mitochondrial function, including oxidative phosphorylation, lipid homeostasis, and mitochondrial quality control in the regulation of peripheral insulin sensitivity. Whether the cellular mechanisms of insulin resistance in adults are comparable to that in adolescents remains unclear. This review will summarize both clinical and basic studies that shed light on how alterations in skeletal muscle mitochondrial function contribute to whole body insulin resistance and will discuss the evidence supporting high-intensity exercise training as a therapy to circumvent skeletal muscle mitochondrial dysfunction to restore insulin sensitivity in both adults and adolescents.
    Biochemistry and Cell Biology 05/2015; DOI:10.1139/bcb-2015-0012 · 2.15 Impact Factor
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    • "Oxidative stress and elevated CAC flux in the liver are characteristics of obesity, NAFLD/NASH, and hepatic lipotoxicity [3] [4] [11] [14] [32]. In the current study, we demonstrate that lipotoxic concentrations of the SFA palmitate are associated with the net redistribution of intracellular calcium from the ER to the mitochondria. "
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    ABSTRACT: Palmitate overload induces hepatic cell dysfunction characterized by enhanced apoptosis and altered citric acid cycle (CAC) metabolism; however, the mechanism of how this occurs is incompletely understood. We hypothesize that elevated doses of palmitate disrupt intracellular calcium homeostasis resulting in a net flux of calcium from the ER to mitochondria, activating aberrant oxidative metabolism. We treated primary hepatocytes and H4IIEC3 cells with palmitate and calcium chelators to identify the roles of intracellular calcium flux in lipotoxicity. We then applied 13C metabolic flux analysis (MFA) to determine the impact of calcium in promoting palmitate-stimulated mitochondrial alterations. Co-treatment with the calcium-specific chelator BAPTA resulted in a suppression of markers for apoptosis and oxygen consumption. Additionally, 13C MFA revealed that BAPTA co-treated cells had reduced CAC fluxes compared to cells treated with palmitate alone. Our results demonstrate that palmitate-induced lipoapoptosis is dependent on calcium-stimulated mitochondrial activation, which induces oxidative stress.
    Molecular Metabolism 08/2014; 3(5). DOI:10.1016/j.molmet.2014.05.004
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