Mitochondrial function in liver cells is resistant to perturbations in NAD+ salvage capacity
Dall M, Trammell SAJ, Asping M, Hassing AS, Agerholm M, Vienberg SG, Gillum MP, Larsen S, Treebak JT
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.
Journal
Journal of Biological Chemistry
Model
Mouse
Impact Factor
4.010
Keywords
NAMPT, nicotinamide adenine dinucleotide (NAD), mitochondrial metabolism, hepatocyte, NAD biosynthesis, fatty acid metabolism, tryptophan metabolism, respiration