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: 4.73). 04/2012; 53(6):1080-92. DOI: 10.1194/jlr.M023382
Source: PubMed

ABSTRACT 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.

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Hepatic insulin resistance is associated with increased collagen. Integrin α1β1 is a collagen binding receptor expressed on hepatocytes. Here we show that expression of the α1 subunit is increased in hepatocytes isolated from high fat fed (HF-fed) mice. To determine whether the integrin α1 subunit protects against impairments in hepatic glucose metabolism, we analyzed glucose tolerance and insulin sensitivity in HF-fed integrin α1-null (itgα1-/-) and wild-type littermates (itgα1+/+). Using the insulin clamp, we found that insulin-stimulated hepatic glucose production was suppressed by ~50% in HF-fed itgα1+/+ mice. In contrast, it was not suppressed in HF-fed itgα1-/- mice indicating severe hepatic insulin resistance. This was associated with decreased hepatic insulin signaling in HF-fed itgα1-/- mice. Interestingly, hepatic triglyceride and diglyceride content were normalized to chow-fed levels in HF-fed itgα1-/- mice. This indicates that hepatic steatosis is dissociated from insulin resistance in HF-fed itgα1-/- mice. The decrease in hepatic lipid accumulation in HF-fed itgα1-/- mice was associated with altered free fatty acid metabolism. These studies establish a role for integrin signaling in facilitating hepatic insulin action, while promoting lipid accumulation in mice challenged with a HF diet. Copyright © 2015, The American Society for Biochemistry and Molecular Biology.
    The Journal of biological chemistry. 01/2015;
  • [Show abstract] [Hide abstract]
    ABSTRACT: High-protein (HP) diets are effective anti-steatotic treatment options for patients with non-alcoholic fatty liver disease, but whether these diets also decrease steatosis in hyperlipidaemic conditions is not known. The aim of the present study was to determine the effects of a HP diet on hepatic steatosis and inflammation in hyperlipidaemic mice. Hyperlipidaemic male and female APOE2 knock-in (APOE2ki) mice were fed a semi-synthetic low-protein (LP) or HP diet in combination with a low-fat diet or a high-fat diet for 3 weeks. The HP diets reduced hepatic fat and cholesterol concentrations to 40-55 % of those induced by the corresponding LP diets and attenuated hepatic inflammation mildly. The VLDL-associated plasma cholesterol concentrations decreased to 60-80 %, but those of TAG increased 3-4-fold. APOE2-mediated restriction of fat import into the liver did not modify the effects of a HP diet previously observed in wild-type mice. Female APOE2ki mice exhibited a higher expression of lipogenic, cholesterol-synthesising, inflammatory and cell-stress genes than wild-type female or male APOE2ki mice, but a similar response to HP diets. Low Apob expression and unchanged plasma APOB100 concentrations suggest that HP diets increase the plasma concentrations of TAG by slowing their clearance. The decrease in plasma leptin and hepatic fat and glycogen concentrations and the increase in fatty acid-oxidising gene and phosphoenolpyruvate carboxykinase 1 protein expression suggest a HP diet-mediated increase in mitochondrial metabolism. In conclusion, a HP diet reduces hepatic lipid content in dyslipidaemic mice and lowers the activation status of inflammatory cells in the liver.
    British Journal Of Nutrition 08/2014; · 3.34 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The hallmark of NAFLD is steatosis of unknown etiology. To test how dietary protein decreases steatosis, we fed female C57BL/6 J mice low-fat (8 en%) or high-fat (42 en%) combined with low-protein (11 en%), high-protein (HP; 35 en%), or extra-high-protein (HPX; 58 en%) diets for 3 weeks. The 35en% protein diets reduced hepatic triglyceride, free fatty-acid, cholesterol, and phospholipid contents to ~50% of that in 11en% protein diets. Every additional 10 en% protein reduced hepatic fat content ~1.5 g%. HP diets had no effect on lipogenic or fatty-acid oxidizing genes except Ppargc1α (+30%), increased hepatic PCK1 content 3-5-fold, left plasma glucose and hepatic glycogen concentration unchanged, and decreased inflammation and cell stress (decreased Fgf21 and increased Gsta expression). The HP-mediated decrease in steatosis correlated inversely with plasma branched-chain amino-acid (BCAA) concentrations and hepatic content of BCAA-derived monomethyl branched-chain fatty acids (mmBCFAs) 14-methylpentadecanoic (14-MPDA; valine-derived) and, to a lesser extent, 14-methylhexadecanoic acid (isoleucine-derived). Liver lipid content was 1.6-1.8-fold higher in females than in males, but the anti-steatotic effect of high-protein diets was equally strong. The strong upregulation of PCK1 and literature data showing an increase in phosphoenolpyruvate and a decline in tricarboxylic-acid cycle intermediates in liver reveal that an increased efflux of these intermediates from mitochondria represents an important effect of a high-protein diet. The high-protein diet-induced increase in 14-MPDA and the dietary response in gene expression were more pronounced in females than males. Our findings are compatible with a facilitating role of valine-derived mmBCFAs in the antisteatotic effect of high-protein diets.
    The Journal of Nutritional Biochemistry 09/2014; · 4.59 Impact Factor


Available from
May 30, 2014