Lower plasma NAMPT/visfatin levels are associated with impaired hepatic mitochondrial function in non-diabetic obese women: a potential link between obesity and non-alcoholic fatty liver disease

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

Objective

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.

Materials and Methods

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.

Results

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.

Conclusion

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.

Mitochondrial role in the neonatal predisposition to developing nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is a global epidemic in obese children and adults, and the onset might have fetal origins. A growing body of evidence supports the role of developmental programming, whereby the maternal environment affects fetal and infant development, altering the risk profile for disease later in life. Human and nonhuman primate studies of maternal obesity demonstrate that risk factors for pediatric obesity and NAFLD begin in utero. The pathologic mechanisms for NAFLD are multifactorial but have centered on altered mitochondrial function/dysfunction that might precede insulin resistance. Compared with the adult liver, the fetal liver has fewer mitochondria, low activity of the fatty acid metabolic enzyme carnitine palmitoyl-CoA transferase-1, and little or no gluconeogenesis. Exposure to excess maternal fuels during fetal life uniquely alters hepatic fatty acid oxidation, tricarboxylic acid cycle activity, de novo lipogenesis, and mitochondrial health. These events promote increased oxidative stress and excess triglyceride storage, and, together with altered immune function and epigenetic changes, they prime the fetal liver for NAFLD and might drive the risk for nonalcoholic steatohepatitis in the next generation.

Nicotinamide phosphoribosyltransferase in malignancy: a review

Nicotinamide phosphoribosyltransferase (Nampt) catalyzes the rate-limiting step of nicotinamide adenine dinucleotide (NAD) synthesis. Both intracellular and extracellular Nampt (iNampt and eNampt) levels are increased in several human malignancies and some studies demonstrate increased iNampt in more aggressive/invasive tumors and in tumor metastases. Several different molecular targets have been identified that promote carcinogenesis following iNampt overexpression, including SirT1, CtBP, and PARP-1. Additionally, eNampt is elevated in several human cancers and is often associated with a higher tumor stage and worse prognoses. Here we review the roles of Nampt in malignancy, some of the known mechanisms by which it promotes carcinogenesis, and discuss the possibility of employing Nampt inhibitors in cancer treatment.

Effects of dietary supplementation with epigallocatechin-3-gallate on weight loss, energy homeostasis, cardiometabolic risk factors and liver function in obese women: randomised, double-blind, placebo-controlled clinical trial

The aim of the present study was to examine the effects of green tea epigallocatechin-3-gallate (EGCG) on changes in body composition, energy and substrate metabolism, cardiometabolic risk factors and liver function enzymes after an energy-restricted diet intervention in obese women. In the present randomised, double-blind, placebo-controlled study, eighty-three obese (30 kg/m² > BMI < 40 kg/m²) pre-menopausal women consumed 300 mg/d of EGCG or placebo (lactose). We measured body weight and adiposity (dual-energy X-ray absorptiometry), energy expenditure and fat oxidation rates (indirect calorimetry), blood lipid levels (TAG, total cholesterol, LDL-cholesterol and HDL-cholesterol), insulin resistance, C-reactive protein and liver function markers (aspartate aminotransferase, alanine aminotransferase, alkaline phosphatase, γ-glutamyltransferase, urea, bilirubin and 2-keto[1-¹³C]isocaproate oxidation) before and after the intervention in the EGCG and control groups. We did not find any significant difference in the changes in body weight (-0.3 kg, 95% CI -5.0, 4.3), fat mass (-0.7 kg, 95% CI -3.5, 2.1), energy (0.3 kJ/kg per d, 95% CI -3.1, 2.7) and fat (-0.1 g/min, 95% CI -0.03, 0.01) metabolism, homeostasis assessment model for insulin resistance (0.2, 95% CI -0.2, 0.7), total cholesterol (-0.21 mmol/l, 95% CI -0.55, 0.13), LDL-cholesterol (-0.15 mmol/l, 95% CI -0.50, 0.20), TAG (-0.4 mmol/l, 95% CI -0.56, 0.29) and liver function markers between the EGCG and control groups. In conclusion, the present results suggest that dietary supplementation with 300 mg/d of EGCG for 12 weeks did not enhance energy-restricted diet-induced adiposity reductions, and did not improve weight-loss-induced changes in cardiometabolic risk factors in obese Caucasian women. The intake of 300 mg/d of EGCG for 12 weeks did not cause any adverse effect on liver function biomarkers.

What is the role of adiponectin in obesity related non-alcoholic fatty liver disease?

Non-alcoholic fatty liver disease (NAFLD) is recognized as the most common type of chronic liver disease in Western countries. Insulin resistance is a key factor in the pathogenesis of NAFLD, the latter being considered as the hepatic component of insulin resistance or obesity. Adiponectin is the most abundant adipose-specific adipokine. There is evidence that adiponectin decreases hepatic and systematic insulin resistance, and attenuates liver inflammation and fibrosis. Adiponectin generally predicts steatosis grade and the severity of NAFLD; however, to what extent this is a direct effect or related to the presence of more severe insulin resistance or obesity remains to be addressed. Although there is no proven pharmacotherapy for the treatment of NAFLD, recent therapeutic strategies have focused on the indirect upregulation of adiponectin through the administration of various therapeutic agents and/or lifestyle modifications. In this adiponectin-focused review, the pathogenetic role and the potential therapeutic benefits of adiponectin in NAFLD are analyzed systematically.