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

Supplementation with NAD precursors such as nicotinamide riboside (NR) has been shown to enhance mitochondrial function in the liver and to 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 deficient hepatocytes in mice is unknown. Here, we assessed role in maintaining mitochondrial and metabolic functions in the mouse liver. Using the Cre-LoxP system, we generated hepatocyte-specific 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 were 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 [N]glutamine, HNKO hepatocytes had a higher [N]NAD enrichment than WT hepatocytes, indicating that HNKO mice compensate through NAD synthesis. We conclude that NAMPT-deficient hepatocytes can maintain substantial NAD levels and that the knockout has only minor consequences for mitochondrial function in the mouse liver.