Overexpression of NRK1 ameliorates diet- and age-induced hepatic steatosis and insulin resistance.

NAD is a co-enzyme in redox reactions and a substrate required for activity of various enzyme families, including sirtuins and poly(ADP-ribose) polymerases. Dietary supplementation of NAD precursors nicotinamide mononucleotide (NMN) or nicotinamide riboside (NR) protects against metabolic disease, neurodegenerative disorders and age-related physiological decline in mammals. Here we sought to identify the roles of nicotinamide riboside kinase 1 (NRK1) plays in regulating hepatic NAD biosynthesis and lipid metabolism. Using adenovirus mediated gene transduction to overexpress or knockdown NRK1 in mouse liver, we have demonstrated that NRK1 is critical for maintaining hepatic NAD levels and triglyceride content. We have further shown that the hepatic expression of Nmrk1 mRNA is significantly decreased either in mice treated with high-fat diet or in aged mice. However, adenoviral delivery of NRK1 in these diet- and age-induced mice elevates hepatic NAD levels, reduces hepatic steatosis, and improves glucose tolerance and insulin sensitivity. Our results provide important insights in targeting NRK1 for treating hepatic steatosis.