Wang Ling-Fang, Wang Xiao-Nv, Huang Cong-Cong, Hu Long, Xiao Yun-Fei, Guan Xiao-Hui, Qian Yi-Song, Deng Ke-Yu, Xin Hong-Bo
Institute of Translational Medicine, Nanchang University, 999 Xuefu Load, Honggutan District, Nanchang, 330031, China.
School of Life Sciences, Nanchang University, Nanchang, 330031, China.
Lipids Health Dis. 2017 Apr 27;16(1):82. doi: 10.1186/s12944-017-0464-z.
Nonalcoholic 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.
Hepatic 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.
FK866 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.
Our 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.
非酒精性脂肪性肝病是世界上最常见的肝脏疾病之一，是代谢综合征的典型肝脏表现，其特征是肝脏中脂质蓄积。烟酰胺磷酸核糖转移酶（NAMPT）最近被鉴定为一种参与烟酰胺腺嘌呤二核苷酸（NAD）生物合成的酶，在哺乳动物多种器官的细胞代谢中起重要作用。本研究旨在探讨NAMPT对高脂饮食诱导的肝脂肪变性的影响。
在体内用高脂饮食（HFD）诱导C57BL/6小鼠发生肝脂肪变性模型。用NAMPT载体质粒转染HepG2和Hep1-6肝细胞，或用NAMPT抑制剂FK866处理，然后与油酸孵育。通过HE染色或油红染色检测脂质蓄积情况。采用定量RT-PCR和蛋白质印迹法检测参与脂肪生成合成的基因表达。
FK866显著促进高脂饮食喂养小鼠的肝脏脂肪变性和体外肝脏脂质蓄积，同时伴有脂肪生成基因如固醇调节元件结合蛋白1（SREBP1）和脂肪酸合酶（FASN）表达增加。烟酰胺单核苷酸（NMN）和NAD在体外显著挽救了FK866的作用。相反，在HepG2和Hep1-6肝细胞中过表达NAMPT可改善肝脏脂质蓄积。此外，FK866降低了高脂饮食喂养小鼠肝脏中Sirt1和磷酸化AMPKα的蛋白水平。此外，Sirt1激活剂白藜芦醇显著降低脂肪生成基因表达，而Sirt1特异性抑制剂EX-527则产生相反作用。
我们的结果表明，抑制NAMPT通过抑制Sirt1介导的信号通路加重高脂饮食或油酸诱导的肝脂肪变性。一方面，抑制NAMPT通过抑制NAD补救途径减少NAD生成，导致Sirt1活性降低，进而减弱SREBP1的去乙酰化，其中抑制SREBP1活性促进FASN和ACC的表达。另一方面，Sirt1活性降低减轻了AMPKα的激活，从而进一步增强SREBP1活性。
