"NR drives NAD+ biosynthesis through a highly conserved path from yeast to mammals. It is initiated with the phosphorylation of NR by NR Kinases (NRK1 or NRK2), generating NMN which is subsequently converted in NAD+ via NMN adenylyl transferase enzymes (NMNATs)"
Supplementation with an alternative NAD+ precursor (nicotinamide) did not prevent the defects of the NRK1 deficient mice and failed to restore NAD+. NRK1 is essential and rate-limiting for NAD+ synthesis from exogenous NR and NMN in hepatocytes.
This shows that in mice NAD+ precursors are not fully exchangeable and NR, compared to nicotinamide, has a unique ability to sustain high hepatic NAD-levels and this is important for DNA-repair.
Endogenous nicotinamide riboside metabolism protects against diet-induced liver damage
Published in NATURE: 20 September 2019
https://www.nature.com/articles/s41467-019-12262-xSupplementation with the NAD+ precursor nicotinamide riboside (NR) ameliorates and prevents a broad array of metabolic and aging disorders in mice. However, little is known about the physiological role of endogenous NR metabolism.
We have previously shown that NR kinase 1 (NRK1) is rate-limiting and essential for NR-induced NAD+ synthesis in hepatic cells. To understand the relevance of hepatic NR metabolism, we generated whole body and liver-specific NRK1 knockout mice.
Here, we show that NRK1 deficiency leads to decreased gluconeogenic potential and impaired mitochondrial function. Upon high-fat feeding, NRK1 deficient mice develop glucose intolerance, insulin resistance and hepatosteatosis. Furthermore, they are more susceptible to diet-induced liver DNA damage, due to compromised PARP1 activity.
Our results demonstrate that endogenous NR metabolism is critical to sustain hepatic NAD+ levels and hinder diet-induced metabolic damage, highlighting the relevance of NRK1 as a therapeutic target for metabolic disorders.