Hepatic steatosis (fatty liver) is associated with hepatitis virus infection, various drugs, and multiple genetic defects in energy metabolism. Fatty liver is an important factor in liver damage, and results in a further progression to cirrhosis and HCC [
32]. HBx has been reported to be associated with HBV-related pathogenesis [
33] and related to the development of hepatic steatosis in patients with HBV infections [
4,
24]. As demonstrated in the present study, Nar exhibits therapeutic effects on the early stages of HBx-induced hepatic steatosis, attenuating fatty liver as assessed by histopathology, serum chemistry, and gene expression.
The accumulation of free fatty acids (FFAs) and cholesterol in the liver results in the production of reactive oxygen species (ROS) and tumour necrosis factor (TNFα) -mediated inflammation [
34]. In the process of hepatic synthesis of endogenous fatty acids (FAs), ACC1 and FAS produce TGs, which are stored or rapidly metabolized. CD36 is the major FA transporter and enhances FFA uptake in hepatocytes [
35]. LXRα, along with SREBP1c (the major isoform in the liver), affects the transcription of the genes encoding ACC1 and FAS, and that of its own gene, consequently stimulating hepatic lipogenesis [
9,
36]. LXRα regulates the induction by HBx of SREBP1-mediated hepatic lipid synthesis and accumulation [
9]. Separately, the transcriptional regulator PPARγ controls adipogenesis and plays a major role in the process of lipid storage. HBx also reinforces the transcriptional activity of PPARγ1 and γ2, resulting in increased expression of the
CD36 gene and adipogenic genes, including adiposin, aP2, and adiponectin [
6]. HBx affects various lipid metabolic pathways and lipid deposition in liver. A previous study demonstrated that Nar significantly lowers serum triacylglycerol and cholesterol levels and decreases the expression of various lipogenic genes and SREBP-1c in HFD [
23]. Separate work showed that Nar modulates the activity of PPARγ and LXRα, thereby downregulating the expression of FAS [
22]. In the present study, we showed that Nar attenuates the HBx-induced accumulation of the transcripts encoding FAS, ACC1, CD36, adiponectin, aP2, PPAR1γ, PPARγ2, and LXRα in the in vivo model of HBx-transgenic mice. We further demonstrated, using the in vitro model of HepG2-HBx cells, that Nar exposure counteracts HBx-induced nuclear translocation and DNA-binding activity of SREBP.
Moreover, a previous study suggested that the activation of Akt is involved in the HBx-regulated process of survival and the activation of Srebp1c in the liver [
37]. In hepatocytes, PPARγ may play a predominant role in the metabolic adaptation downstream of PI3K/Akt2 pathway, leading to steatosis [
31] HBx also increases the expression of PPARγ, an effect thought to result from C/EBPα activation [
6]. Our results demonstrated that Nar exposure results in a significant attenuation of the HBx-induced accumulation of p-Akt, p-PI3K, and C/EBPα in HepG2-HBx cells, suggesting molecular mechanisms whereby Nar counteracts lipid accumulation in hepatocytes both in vivo and in vitro. Specifically, Nar appears to suppress lipogenesis by decreasing LXRα-Srebp1c signaling and, thus, suppresses the expression of downstream genes (such as
ACC and
FAS) regulated by this pathway. This decrease in LXRa-Srebp1c signaling apparently attenuates hepatic PPARγ overexpression, thereby providing hepatic protection from FFA-mediated damage in HBx-transgenic mice dosed with Nar.