Inhibition of NAMPT aggravates high fat diet-induced hepatic steatosis in mice through regulating Sirt1/AMPKα/SREBP1 signaling pathway

Abstract

BackgroundNonalcoholic fatty liver disease is one of the most common liver diseases in the world and is a typical hepatic manifestation of metabolic syndrome which is characterized with lipid accumulation in liver. Nicotinamide phosphoribosyltransferase (NAMPT) has been recently identified as an enzyme involved in nicotinamide adenine dinucleotide (NAD+) biosynthesis and plays an important role in cellular metabolism in variety of organs in mammals. The aim of this study was to investigate the effects of NAMPT on high fat diet-induced hepatic steatosis.MethodsHepatic steatosis model was induced by high fat diet (HFD) in C57BL/6 mice in vivo. HepG2 and Hep1-6 hepatocytes were transfected with NAMPT vector plasmid or treated with NAMPT inhibitor FK866 and then incubated with oleic acid. Lipids accumulation was examined by HE staining or oil red staining. Quantitative RT-PCR and Western blot were used to measure expressions of the genes involved in lipogenic synthesis.ResultsFK866 significantly promoted liver steatosis in the mice fed with HFD and hepatic lipid accumulation in vitro, accompanied by the increases of the expressions of lipogenic genes such as sterol regulatory element-binding protein 1 (SREBP1) and fatty acid synthase (FASN). Nicotinamide mononucleotide (NMN) and NAD+ significantly rescued the actions of FK866 in vitro. In contrast, overexpression of NAMPT in HepG2 and Hep1-6 hepatocytes ameliorated hepatic lipid accumulation. In addition, FK866 decreased the protein levels of Sirt1 and phospho-AMPKα in liver of the HFD fed mice. Furthermore, Resveratrol, a Sirt1 activator, significantly reduced lipogenic gene expressions, while EX-527, a Sirt1 specific inhibitor, had the opposite effects.ConclusionOur results demonstrated that inhibition of NAMPT aggravated the HFD- or oleic acid-induced hepatic steatosis through suppressing Sirt1-mediated signaling pathway. On the one hand, the inhibition of NAMPT reduced the production of NAD+ through inhibiting the NAD+ salvage pathway, resulting in the decrease of Sirt1 activity, and then attenuated the deacetylation of SREBP1 in which the inhibition of SREBP1 activity promoted the expressions of FASN and ACC. On the other hand, the reduced Sirt1 activity alleviated the activation of AMPKα to further enhance SREBP1 activities.Electronic supplementary materialThe online version of this article (doi:10.1186/s12944-017-0464-z) contains supplementary material, which is available to authorized users.

Reply to Moon and Minhas: Teasing apart NAD+ metabolism in inflammation: commentary on Zhou et al. (2016). Br J Pharmacol 173: 2352–2368

“…Nevertheless, we disagree with their claim that our conclusion is misleading. In fact, our results have been confirmed by a recent study reporting that inhibition of NAMPT aggravated the hepatic steatosis induced by high‐fat diet or oleic acid, which was rescued by replenishment of the NAMPT enzymic product nicotinamide mononucleotide (Wang et al, ). For a better understanding of the actions of the de novo and salvage pathways of NAD + biosynthesis in liver health and disease, further investigations are warranted.…”

Healthy Effects of Plant Polyphenols: Molecular Mechanisms

“…Sirtuins, NAD + -dependent enzymes, are considered amongst the most promising anti-aging targets, leading to define these deacetylases as "the enzymes of youth". Sirtuins depend on AMPK via stimulation of nicotinamide phosphoribosyltransferase (NAMPT), an enzyme involved in NAD + synthesis that regulates cellular metabolism in several mammalian organs and tissues [265] and whose inhibition worsens diet-induced hepatic steatosis in mice [266].…”

Adipokines and Non-Alcoholic Fatty Liver Disease: Multiple Interactions

“…Although NAMPT also has a crucial role in cancer cell metabolism, a specific role in HCC associated with NAFLD has never been demonstrated. Despite several studies the role of NAMPT in human NAFLD remains inconclusive and recently it has been demonstrated that inhibition of NAMPT aggravated the HFD- or oleic acid-induced hepatic steatosis through suppression of Sirt1-mediated signaling pathways [ 79 ]. The interaction with Sirt1 might be relevant, as we have shown earlier that weight loss after bariatric surgery is able to induce expression of Sirt1 and -3, in both adipose and liver tissue [ 80 ].…”