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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    • "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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