Background
Non-alcoholic fatty liver disease (NAFLD) has become a public health issue because of the ongoing epidemics of obesity and type 2 diabetes. The spectrum of NAFLD ranges from simple steatosis to steatohepatitis, liver fibrosis, cirrhosis, and eventually hepatocellular carcinoma [
1]. According to the “two-hit-hypothesis” of non-alcoholic steatohepatitis (NASH), hepatic steatosis (“first hit”) is a prerequisite for the development of subsequent adverse events (“second hit”) when combined with unfavorable environmental and genetic factors, leading to inflammatory liver damage and fibrogenesis in NASH [
2]. In humans with NAFLD, circulating free fatty acids (FFAs) are commonly elevated and their plasma levels correlate with disease severity. Consistently, FFAs have been proposed to contribute to the development and progression of NAFLD and NASH [
3,
4].
NASH is known to be associated with an increased generation of reactive oxygen species [
5] through nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-mediated pathways that play a crucial role in the development of hepatic fibrogenesis [
6]. On the other hand, nuclear factor kappa (NFκB) is a dimeric transcription factor comprised of five family members RelA (p65), RelB, c-Rel, p50 and p52 and is a critical mediator of inflammatory responses [
7,
8]. Recent studies have shown that modulation of lipid homeostasis and suppression of NFκB activation counteract the progression of liver injury in experimental NASH model [
9].
Statins are specific inhibitors of 3-hydroxy-3methylglutaryl coenzyme A (HMG-CoA) reductase, the rate-limiting enzyme in cholesterol biosynthesis [
10]. In addition to their cholesterol-lowering effects, statins also possess pleiotropic properties that account for their anti-inflammatory, anti-proliferative, anti-thrombotic, anti-oxidative, anti-cancer, and immunomodulatory actions
in vitro and
in vivo. It has been shown that statins can reduce liver triglyceride [
11] and ameliorate severe hepatic steatosis [
12], thereby improving NASH-related fibrogenesis. Clinically, the therapeutic role of statins in the treatment of patients with NASH is still controversial [
13]. Although it has been reported that statins could improve liver steatosis and reduce the NAFLD activity score [
14], their efficacy against fibrosis remains unclear. This study, therefore, aimed at investigating the therapeutic potential of statins against NASH-related fibrogenesis through
in vitro and
in vivo studies.
Discussion
In the present study, we demonstrated
in vitro and
in vivo the anti-fibrotic effects of Flu in steatosis-induced hepatocyte damage models. The results of this study showed that not only could Flu ameliorate PA-induced ROS production, NFκB activity, and pro-inflammatory gene expressions (Figures
1 and
2) in PRHs and HepG2 cells, but it could also reduce α-SMA protein expression and profibrogenic genes expressions in PA- or TGF-β1-treated HSC-T6 cells (Figure
3C and D). Furthermore, the results of the present study revealed suppression of both mRNA and protein expressions of α-SMA in HSC-T6 incubated with the CM collected from PA-Flu-treated PRHs or Flu-treated PRHs compared to those incubated with CM collected from PA-treated PRHs or CM without Flu treatment (Figure
3E and F). Consistent
in vivo findings are the demonstration of the anti-fibrotic (Figures
4 and
5) and anti-inflammatory (Figure
6) effects of Flu in the CDAA rat model. In addition to supporting the proposal that Flu could alleviate steatosis-induced hepatic fibrogenesis through suppressing inflammation and oxidative stress, the most striking finding of the present study is the revelation of the need for paracrine effect of hepatocyte in the process of PA-elicited HSC activation.
Increased reactive oxygen species (ROS) generation has been implicated in the progression of chronic liver diseases. NOX, which is membrane-bound enzyme complex found in the plasma membrane and in the membranes of phagosomes, is present in a variety of hepatic cells including the Kupffer cells, hepatocytes, and HSCs, and participates in liver inflammation and fibrosis [
27,
28]. Structurally, NOX contains two membrane-integrated components, gp91
phox
and p22
phox
, and a number of cytosolic regulatory proteins (p47
phox
, p67
phox
, p40
phox
, and Rac). The pro-fibrotic and pro-inflammatory roles of gp91
phox
are illustrated by the result of a previous study that demonstrated amelioration of hepatic fibrosis through knocking out gp91
phox
in mice [
29,
30] and that of another study showing inflammation through TNF receptor-1 expression and NFκB activation elicited by gp91
phox
induction [
31]. Moreover, attenuating oxidative stress and decreasing the expression of NF-κB and subsequent pro-inflammatory cytokines production have been reported to be associated with the alleviation of CCl
4-induced hepatic fibrogenesis [
32]. Our
in vitro results demonstrated that PA induced both gp91
phox
expression and NFκB p65 nuclear translocation in HepG2 and PRHs. The activation of gp91
phox
and NFκB, therefore, are possible mechanisms by which lipid accumulation causes inflammation and fibrosis. Furthermore, Flu (1–20 μM) inhibited PA-induced ROS production, gp91
phox
expression (Figure
1), and NFκB p65 nuclear translocation (Figure
2A), suggesting anti-oxidative, anti-inflammatory, and anti-fibrotic effects of Flu in PRHs and HepG2 cells.
A number of recent studies have reported that not only does statin reduce the concentration of circulating low-density lipoprotein cholesterol, but it also decreases hepatic lipid deposition in patients with NASH. However, it is still controversial whether it could suppress hepatic inflammation and fibrosis. In terms of organs other than the liver, several studies have previously shown that statin exerts anti-fibrotic effects on the kidneys, the lungs, and the heart [
33-
36]. Regarding fibrogenesis in the liver, compared to the results of the present investigation, a previous study showed that simvastatin improves NASH-related fibrosis by increasing the expression of eNOS, decreasing the expression of iNOS, and inhibiting the activation of HSC
in vitro [
37]. However, that study did not investigate the role of hepatocyte in the setting of lipidemia. In this aspect, the present study demonstrated a critical role of hepatocyte in the process of free fatty acid-induced HSC activation.
In vivo, it has been shown that statin treatment could ameliorate the progression of hepatic steatosis, inflammation, and fibrosis induced by high-fat [
37] and CDAA [
38] diets, indicating that statin is protective against the pathogenesis of NASH. In this study, we consistently demonstrated the therapeutic effects of Flu on CDAA-induced liver fibrosis
in vivo (Table
2, Figures
4 and
5), and provided the
in vitro effects of Flu on PA-treated hepatocytes and CM-treated HSCs. Our
in vivo results revealed the down-regulation of the mRNA expression of pro-inflammatory and pro-fibrogenic genes as well as plasma TNFα level in CDAA rats (Table
2, Figures
5 and
6) following Flu treatment, suggesting the anti-inflammatory and anti-fibrotic properties of Flu
in vivo. On the other hand, our
in vitro data identified increased expressions of pro-inflammatory cytokines (i.e., ICAM-1, IL-6, and TNF-α) in PA-treated hepatocytes, while Flu treatment inhibited these pro-inflammatory cytokines expressions in PA-treated hepatocytes (Figures
2B, C). Furthermore, our results also showed that α-SMA protein expression was significantly suppressed in HSC-T6 cells incubated with CM collected from PA-Flu-treated PRHs or Flu-treated PRHs compared to those incubated with CM collected from PA-treated PRHs or CM without Flu treatment (Figure
3E and F), indicating that Flu may decrease the levels of inflammatory cytokines in CM, thereby attenuating the activation of HSCs. Therefore, our results demonstrated PA-induced HSC activation through paracrine effect of hepatocyte
in vitro that was significantly suppressed by the treatment of Flu. Accordingly, our results revealed that Flu could reduce PA-induced ROS production, gp91
phox
expression (Figure
1), NFκB p65 nuclear translocation (Figure
2A), and pro-inflammatory genes expressions (Figure
2B and C) in PRHs and HepG2 cells. Consistently, Flu was found to attenuate CM-induced or TGF-β1- induced α-SMA protein expression (marker of HSC activation) and the expressions of pro-fibrogenic genes in HSC-T6 cells (Figure
3C and D). Taken together, the
in vitro findings suggest dual therapeutic effects of Flu on both steatotic hepatocyte and HSCs.
The present study has two noteworthy limitations. Firstly, studies were not performed on animals with established diet-induced NASH to elucidate the therapeutic effects of Flu. Secondly, studies were not performed on animals treated only with Flu and regular diet to evaluate the direct effects of Flu on normal rats, its hepatotoxicity and adverse effects. Nevertheless, q-PCR on the expressions of three inflammation indicators (i.e., ICAM1, IL-6, and TNF-α) in HepG2 cells and PRHs using Flu alone were performed in the present study that showed no significant differences in the expressions of these markers between the control group and the group treated with Flu alone.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.
The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
L-WC.: Designed the study, performed experimental work, data analysis and interpretation, and wrote the manuscript; Y-C.H.: Participated in data analysis and interpretation, and wrote the manuscript; T-FL.: Performed experimental work, data analysis and interpretation; YL.: Performed experimental work, data analysis and interpretation; Y-TC.: Participated in the collection and/or assembly of immuno-histochemistry data, data analysis and interpretation; K-CY.: Conceived the study, and participated in its design; J-CW: Conceived the study, and participated in its design; Y-TH.: Conceived the study, participated in its design and coordination, and final approval of the version to be published. All authors read and approved the final manuscript to be published.