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Am J Physiol Endocrinol Metab. 2016 Apr 1;310(7):E484-94. doi: 10.1152/ajpendo.00492.2015. Epub 2016 Jan 26.

Lipotoxicity in steatohepatitis occurs despite an increase in tricarboxylic acid cycle activity.

Author information

1
Department of Chemistry, University of Florida, Gainesville, Florida;
2
Division of Endocrinology, Diabetes, and Metabolism, Department of Medicine, University of Florida, Gainesville, Florida;
3
Department of Integrative Biology, University of California, Berkeley, California;
4
Animal Care Services, University of Florida, Gainesville, Florida;
5
Advanced Magnetic Resonance Imaging and Spectroscopy Facility, McKnight Brain Institute, University of Florida, Gainesville, Florida;
6
Department of Chemistry, University of Florida, Gainesville, Florida; Department of Pathology, University of Florida, Gainesville, Florida.
7
Division of Endocrinology, Diabetes, and Metabolism, Department of Medicine, University of Florida, Gainesville, Florida; Division of Endocrinology, Diabetes, and Metabolism, Malcom Randall Veterans Administration Medical Center, Gainesville, Florida; Division of Diabetes, the University of Texas Health Science Center at San Antonio, San Antonio, Texas; Division of Diabetes, Audie L. Murphy Veterans Administration Medical Center, San Antonio, Texas; and.
8
Department of Pathology, University of Florida, Gainesville, Florida.
9
Division of Endocrinology, Diabetes, and Metabolism, Department of Medicine, University of Florida, Gainesville, Florida; nishanth.sunny@medicine.ufl.edu.

Abstract

The hepatic tricarboxylic acid (TCA) cycle is central to integrating macronutrient metabolism and is closely coupled to cellular respiration, free radical generation, and inflammation. Oxidative flux through the TCA cycle is induced during hepatic insulin resistance, in mice and humans with simple steatosis, reflecting early compensatory remodeling of mitochondrial energetics. We hypothesized that progressive severity of hepatic insulin resistance and the onset of nonalcoholic steatohepatitis (NASH) would impair oxidative flux through the hepatic TCA cycle. Mice (C57/BL6) were fed a high-trans-fat high-fructose diet (TFD) for 8 wk to induce simple steatosis and NASH by 24 wk. In vivo fasting hepatic mitochondrial fluxes were determined by(13)C-nuclear magnetic resonance (NMR)-based isotopomer analysis. Hepatic metabolic intermediates were quantified using mass spectrometry-based targeted metabolomics. Hepatic triglyceride accumulation and insulin resistance preceded alterations in mitochondrial metabolism, since TCA cycle fluxes remained normal during simple steatosis. However, mice with NASH had a twofold induction (P< 0.05) of mitochondrial fluxes (μmol/min) through the TCA cycle (2.6 ± 0.5 vs. 5.4 ± 0.6), anaplerosis (9.1 ± 1.2 vs. 16.9 ± 2.2), and pyruvate cycling (4.9 ± 1.0 vs. 11.1 ± 1.9) compared with their age-matched controls. Induction of the TCA cycle activity during NASH was concurrent with blunted ketogenesis and accumulation of hepatic diacylglycerols (DAGs), ceramides (Cer), and long-chain acylcarnitines, suggesting inefficient oxidation and disposal of excess free fatty acids (FFA). Sustained induction of mitochondrial TCA cycle failed to prevent accretion of "lipotoxic" metabolites in the liver and could hasten inflammation and the metabolic transition to NASH.

KEYWORDS:

hepatic insulin resistance; mitochondria; nonalcoholic steatohepatitis; steatosis

PMID:
26814015
PMCID:
PMC4824140
DOI:
10.1152/ajpendo.00492.2015
[Indexed for MEDLINE]
Free PMC Article

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