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J Biol Chem. 2019 Jul 18. pii: jbc.RA118.006756. doi: 10.1074/jbc.RA118.006756. [Epub ahead of print]

Mitochondrial function in liver cells is resistant to perturbations in NAD+ salvage capacity.

Author information

1
University of Copenhagen, Denmark.
2
NNF Center for Basic Metabolic Research, University of Copenhagen, Denmark.

Abstract

Supplementation with nicotinamide adenine dinucleotide (NAD+) precursors such as nicotinamide riboside (NR) has been shown to enhance mitochondrial function in the liver and prevent hepatic lipid accumulation in high-fat diet (HFD)-fed rodents. Hepatocyte-specific knockout of the NAD+-synthesizing enzyme nicotinamide phosphoribosyltransferase (NAMPT) reduces liver NAD+ levels, but the metabolic phenotype of Nampt-deficient hepatocytes in mice is unknown. Here, we assessed Nampt's role in maintaining mitochondrial and metabolic functions in the mouse liver. Using the Cre-LoxP system, we generated hepatocyte-specific Nampt knockout (HNKO) mice, having a 50% reduction of liver NAD+ levels. We screened the HNKO mice for signs of metabolic dysfunction following 60% HFD feeding for 20 weeks +/- NR supplementation and found that NR increases hepatic NAD+ levels without affecting fat mass or glucose tolerance in HNKO or WT animals. High-resolution respirometry revealed that NR supplementation of the HNKO mice did not increase state III respiration, which was observed in WT mice following NR supplementation.  Mitochondrial oxygen consumption and fatty-acid oxidation was unaltered in primary HNKO hepatocytes. Mitochondria isolated from whole HNKO livers had only a 20% reduction in NAD+, suggesting that the mitochondrial NAD+ pool is less affected by HNKO than the whole-tissue pool. When stimulated with tryptophan in the presence of 15N-glutamine, HNKO hepatocytes had a higher 15N-NAD+ enrichment than WT hepatocytes, indicating that HNKO mice compensate through de novo NAD+ synthesis. We conclude that NAMPT-deficient hepatocytes can maintain substantial NAD+ levels and that the Nampt knockout has only minor consequences for mitochondrial function in the mouse liver.

KEYWORDS:

NAD biosynthesis; NAMPT; fatty acid metabolism; hepatocyte; mitochondrial metabolism; nicotinamide adenine dinucleotide (NAD); respiration; tryptophan metabolism

PMID:
31320478
DOI:
10.1074/jbc.RA118.006756
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