OBJECTIVETo investigate the role of the host-encoded silent information regulator 1 (SIRT1) on hepatocytes' lipid metabolism under conditions of hepatitis C virus (HCV) infection and assess its potential effects on virus replication in vitro.

METHODSThe Huh-7.5 human hepatocyte cell line was used as the control group and Huh-7.5 cells stably expressing the HCV replicon (Huh7.5-HCV) were used as the experimental group. Effects of interferon (IFN) treatment and activation of SIRT1 by resveratrol were also observed. The mRNA and protein expression levels of SIRT1 were detected by real time (q)PCR and western blotting. Effects on SIRT1 protein activity were tested by measuring the levels of reactive oxygen species (ROS) and the nicotinamide adenine dinucleotide (NAD+)/beta-nicotinamide adenine dinucleotide, reduced (NADH) by flow cytometry and chromatometry, and the levels of triacylglycerol (TG), total cholesterol (TC), and fatty acid beta oxidation rate by enzymatic analysis and liquid scintillation counting. Effects on mRNA expression of SIRT1 downstream lipid-metabolism genes were measured by qPCR.

RESULTSThe Huh7.5-HCV cells had a significantly higher level of ROS (3.8+/-0.5 vs. Huh-7.5: 1.0+/-0.2; t = 12.736, P less than 0.01) but significantly lower levels of NAD+/NADH (0.03+/-0.01 vs. 0.12+/-0.03; t = 6.971, P less than 0.01), SIRT1 activity (0.3+/-0.1 vs. 1.0+/-0.2, 0.9+/-0.2, F = 6.766, P less than 0.01), SIRT1 mRNA (0.4+/-0.1 vs. 1.0+/-0.3, 0.9+/-0.2, F = 5.864, P less than 0.01), and SIRT1 protein (0.3+/-0.1 vs. 0.8+/-0.2, 0.9+/-0.2, F = 5.419, P less than 0.01). The lower levels of SIRT1 in Huh7.5-HCV cells accompanied decreased phosphorylation of the forkhead box O1 (FoxO1), which not only up-regulated the downstream genes of SREBP-1c, FAS, ACC, SREBP-2, HMGR and HMGS (which increased fatty acid synthesis) but also down-regulated the downstream genes of PPAR and CPT1A genes (which decreased fatty acid beta oxidation). IFN treatment restored all of the aforementioned changes. Resveratrol-induced SIRT activation improved the perturbations in lipid metabolism pathways, as evidenced by an increase in fatty acid beta oxidation and a decrease in TG and TC synthesis, as well as inhibited HCV replication.

CONCLUSIONHCV may decrease the NAD+/NADH ratio in hepatocytes, leading to a down-regulation of SIRT1 activity and expression and perturbing the downstream expression profile of lipid metabolism-related factors, ultimately causing lipid metabolism disorders and establishing a permissive intracellular environment for HCV replication.