Nicotinamide Phosphoribosyltransferase Knockdown Leads to Lipid Accumulation in HepG2 Cells through The SIRT1-AMPK Pathway

Nicotinamide phosphoribosyltransferase (NAMPT), which is responsible for biosynthesis of nicotinamide adenine dinucleotide (NAD), has a regulatory role in cellular metabolism and thus, might be implicated in non-alcoholic fatty liver disease (NAFLD). This study aimed to show how NAMPT down-regulation in liver cells influences lipid metabolism and sirtiun 1 (SIRT1), as the main NAD-dependent deacetylase enzyme. In this experimental study, HepG2 cells were transfected with NAMPT siRNA and hepatic triglyceride (TG) content and SIRT1 deacetylase activity were measured by colorimetric and fluorometric methods, respectively. Gene expression of fatty acid synthase (FAS) and sterol regulatory element-binding protein-1c (SREBP- 1c) was evaluated by real-time polymerase chain reaction (PCR). Total protein level and the phosphorylated form of acetyl-CoA carboxylase (ACC) and AMP-activated protein kinase (AMPK) were also investigated by western blotting. Knockdown of NAMPT significantly promoted the accumulation of TG in HepG2 cells, accompanied by a remarkable decline in SIRT1 deacetylase activity. A significant rise in the gene expression of two key lipogenic factors, FAS and SREBP-1c was also observed. These effects were also accompanied by decreased phosphorylation of ACC and AMPK. On the other hand, treatment of transfected cells with either NAD, as the SIRT1 substrate or resveratrol, as the SIRT1 activator reversed the outcomes. These results demonstrated a protective role for NAMPT against NAFLD and its involvement in the regulation of de novo lipogenesis through the SIRT1/AMPK pathway.