Hepatocyte-specific knockout of Nicotinamide phosphoribosyltransferase increases susceptibility for liver fibrosis in mice

Nicotinamide adenine dinucleotide (NAD+) supplementation has been suggested as a therapy against non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH). We investigated whether hepatocyte-specific knockout of nicotinamide phosphoribosyltransferase (Nampt) caused increased susceptibility towards liver damage in mice fed a low-methionine, choline-free 60% high-fat (MCD) diet. Knockout mice (HNKO) accumulated less hepatic triglyceride than WT littermates after 3 weeks of MCD, but had increased scores for liver inflammation, necrosis, and fibrosis. We found that fibrosis, necrosis, and portal inflammation was also present in HNKO mice on a purified control diet (PD) but not in chow-fed HNKO mice due to a higher content of NAD+ precursors in the diet. The PD induced fibrosis within 3 days of exposure could be prevented by supplementation with the NAD+ precursor nicotinamide riboside (NR). When NR was supplied after fibrosis induction, hepatic NAD+ levels markedly increased, and portal inflammation was attenuated. The fibrosis phenotype was associated with a decreased abundance of proteins involved in oxidation and reduction processes, particularly for proteins in oxidative phosphorylation (OXPHOS). NR supplementation increased abundance of these proteins. High-resolution respirometry revealed that PD-fed HNKO mice had decreased oxygen consumption when stimulated with succinate, and a decrease in maximal uncoupled respiratory capacity. Conclusions: We show that HNKO mice have increased susceptibility towards liver fibrosis when dietary NAD+ content is restricted, and that low NAD+ levels can affect hepatic mitochondrial function. Furthermore, our data suggest NR treatment has potential for preventing liver damage and NASH progression. Total RNA-seq of mus musculus liver in wild-type (WT) and hepatocyte specific NAMPT knockout (KO) mice on a purified diet. Samples were taken at 3, 6, 12 and 21 days of age.