Background
Nonalcoholic fatty liver disease (NAFLD) is a clinicopathological condition, which is defined as excessive fat accumulation and formation of lipid droplets in the cytoplasm of hepatocytes, accompanied by enlargement of liver and inflammation. It ranges from simple steatosis to nonalcoholic steatohepatitis (NASH) and could eventually lead to cirrhosis and hepato-cellular carcinoma [
1,
2]. Currently, it has become clear that NALFD is a multifactorial disease closely related to liver steatosis, insulin resistance, oxidative stress, inflammatory reaction, etc. Persistent endoplasmic reticulum (ER) stress and mitochondrial dysfunction participate in the regulation of the above physiological changes and both play an important role in the progression from NALFD to NASH [
3,
4].
Calcium ion (Ca
2+), a critical and versatile intracellular secondary messenger, is involved in various cellular processes. ER is known to be the most important intracellular Ca
2+ store [
5]. The abnormal release of ER Ca
2+ not only induces ER stress and mitochondrial dysfunction, but also exacerbates the hepatic cell lipotoxicity [
6]. Sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) pump, the main regulator of intracellular Ca
2+, actively re-accumulates released Ca
2+ back into the ER, and therefore maintains Ca
2+ homeostasis. SERCA activity is reduced in NALFD, while enhanced SERCA activity alleviates ER stress and apoptosis [
7,
8]. Recent studies have showed that the homeostasis of Ca
2+ is closely related to the development of NALFD to NASH [
9‐
11].
Natural monomers or extracts isolated from plants or herbs have been demonstrated to effectively treat various diseases with relatively low toxicity. Matrine (Mat), a tetracyclo-quinolizidine alkaloid, is mainly derived from leguminosae such as
Sophora flavescens and
Sophora subprostrata. Recent researches have shown that Mat can exert a wide range of pharmacological effects and has the potential to treat a variety of diseases, including cardiac fibrosis, parkinson’s disease and arthritis [
12‐
14]. In addition, the clinical drugs based on Mat have been applied for the treatment of hepatitis, and it has been reported that the intramuscular injection of Mat effectively improves the clinical symptoms of patients with chronic hepatitis B, recovers liver function and alters serum conversion from positive to negative hepatitis B virus DNA [
15]. In addition, some studies have shown that Mat and oxymatrine (oxy-Mat) can inhibit the endoplasmic reticulum stress in steatohepatitis and sodium arsenite induced hepatic injury, but other reported that Mat can induce the ER stress and promote tumor cell apoptosis [
16,
17]. Then what is the role of Mat in non-alcoholic fatty liver disease and in the relationship between ER stress response and calcium homeostasis? The present study investigated accordingly whether Mat has a therapeutic effect on NALFD, and made a preliminary inquiry into the molecular mechanisms involved.
Methods
Materials
Matrine (M5319), palmitic acid (V900121), 2-Aminoethoxydiphenyl borate (D9754) and thapsigargin (T9033) were purchased from Sigma-Aldrich (St. Louis, MO, USA). The antibodies included anti-Phospho-PERK (Thr980) (#3179), anti-PERK (#3192) anti-CHOP (#2895), anti-BiP (#3177) and anti-cleaved caspase3 (#9664) were purchased from Cell Signaling Technology (USA). GAPDH (AT0002, CMCTAG, USA). Other antibodies included anti-ATF6 (ab37149), anti-phospho-IRE1 (phospho S724) (ab48187), anti-IRE1 (ab37073), anti-SREBP1 (ab28481), Anti-Fatty Acid Synthase (ab22759), anti-Acetyl Coenzyme A Carboxylase (ab45174), anti-NF-κB p65, (ab16502), anti-c-jun (phospho S63) (ab32385) were purchased from Abcam (Cambridge, UK).
Animals and experimental designs
The vivo studies were performed in male C57BL/6J (3–4 weeks) mice, which were purchased from the animal center of the Air Force Medical University (Xi’an, China). The mice were kept under a light-controlled condition (12 h/12 h light/dark cycle) in a special room with constant temperature (23 ± 2 °C) and humidity (55 ± 15%), and were free to eat and drink. For the HFD-fed mice, mice were acclimated for 2 weeks and randomly divided into 5 groups (n = 10). The control group received regular chow, whereas the HFD group received the high-fat diet (TP-26301, Trophic, Nantong, China) for 12 weeks, and HFD + Mat groups (HFD + Mat L, M, H) received high-fat diet for 4 weeks and then combined with Mat (0.5 mg/kg, 2.5 mg/kg, 10 mg/kg, respectively) intervention from 5- to 12-week. For the MCD diet-fed mice, mice were acclimated for 2 weeks and divided into 5 groups (n = 10). The control group received regular chow, whereas the MCD group received the MCD diet (TP-3005G, Trophic, Nantong, China) for 6 weeks, and MCD + Mat groups (MCD + Mat L, M, H) received MCD diet for 2 weeks and then combined with Mat (0.5 mg/kg, 2.5 mg/kg, 10 mg/kg, respectively) intervention from 3- to 6-week. In the end of experiment, all the mice were euthanized after a 12-h fast, and the liver tissues and blood samples were collected for evaluation.
Biochemical analysis and histopathology
Serum triglyceride (TG), total cholesterol (TC), alanine aminotransferase (ALT), and aspartate aminotransferase (AST) were assessed by automatic biochemical analyzer (200FR, TOSIBA, Japan). Pro-inflammatory cytokines (TNF-α, IL-6, IL-10) in serum were assessed with commercial kits based on colorimetric method. Liver tissue in 4% paraformaldehyde was stained with hematoxylin and eosin (H&E) to observe the damage and was subjected to oil red O staining to visualize lipid droplets in the liver. The images were captured and analyzed by a fluorescence microscope (NI, Nikon, Tokyo, Japan).
Cell culture
L02 human liver cells (a kind gift from the professor Zhang of Air Force Medical University) were used for in vitro study. L02 cells were cultured in Dulbecco’s Modified Eagle medium (DMEM) supplemented with 10% (v/v) fetal bovine serum (FBS) and 1% penicillin/streptomycin at 37 °C in a humidified atmosphere of 5% CO2. Future experiments were conducted after the completion of cell processing.
Oil red O staining
The cultured L02 cells grown on cell plates were washed with phosphate-buffered saline (PBS) three times and fixed with 4% paraformaldehyde for 1 h. Then, the fixed cells were washed with 60% isopropanol for 30 s and PBS three times. In darkness, the cells were stained with freshly diluted Oil Red O working solution for 1 h at 37 °C, after which they were counterstained with hematoxylin for 1 min. The cells were washed with PBS three times and ultimately observed and photographed by an inverted fluorescence microscope (IX53, Olympus, Tokyo, Japan).
ROS detection
Intracellular ROS levels were detected as the manufacturer’s instructions described. L02 cells were cultured on cell plates with suitable density. After the corresponding treatment, cells were washed with serum-free medium for three times, and incubated with 10 μM 21,71-dichlorofluorescin diacetate (S0033, Beyotime Biotechnology, Jiangsu, China) in serum-free medium at 37 °C for 30 min. Then, cells were washed with the same serum-free medium for three times. Imaging was observed and photographed by the fluorescence microscope. DCFH fluorescence was measured by fluorescence spectrophotometer (excitation at 488 nm and emission at 525 nm, 180 manual).
Immunohistochemistry
ER histochemistry was detected using Streptavidin Rabbit HRP Kit (CW2035, CWBIO, Beijing, China), according to the manufacturer’s instructions. HRP can catalyze the color development of substrates, thus inferring the presence and distribution of antigens to be detected. Primary antibodies were CHOP, GRP78 and ATF6. Secondary antibodies for fluorescent imaging were tagged with the corresponding species to the primary antibody. Imaging was observed and photographed by the fluorescence microscope.
Apoptosis assay
Apoptosis was detected using Annexin V-FITC/PI cell apoptosis detection kit (C1063, Beyotime Biotechnology, Jiangsu, China). Cells from different groups were digested with trypsin without EDTA, resuspended in the binding buffer, stained with Annexin V-FITC for 15 min and PI for 5 min, and then analyzed by flow cytometry (Novocyte 2040R, ACEA, USA).
Measurement of cytosolic Ca2+
Cytosolic Ca2+ ([Ca2+]c) in L02 cells was measured using the Fura-3AM fluorescent indicator (S1056, Beyotime Biotechnology, Jiangsu, China) as described in the specifications. Briefly, L02 cells seeded in 96-well plates or 6-well plates and received the corresponding treatment respectively. For measuring the [Ca2+]c, cells were washed and incubated with 5 μM Fura-3/AM and 0.1% Pluronic F-127 (ST501, Beyotime) in Hanks’ balanced salt solution (HBSS) for 45 min at 37 °C. The cells were then washed with PBS three times. Fluorescence was subsequently measured by fluorescence spectrophotometer (Tecan infinite M200 Pro, Switzerland) (excitation at 488 nm and emission at 530 nm, 180 manual). Background fluorescence was subtracted from all signals. Changes in [Ca2+]c are represented by changes in fluorescence expressed as: F/F0, where F0 is control fluorescence.
Measurement of mitochondrial membrane potential
Mitochondrial membrane potential was detected using mitochondrial membrane potential assay kit with JC-1 (C2006, Beyotime Biotechnology, Jiangsu, China), according to the manufacturer’s instructions. JC-1 staining was observed and photographed by the fluorescence microscope. In addition, its fluorescence was measured by fluorescence spectrophotometer.
Western blotting
Total protein was separated by SDS-PAGE (Bio-Rad, CA, USA) and electro-transferred onto the polyvinylidene difluoride (PVDF) membranes. Further, the PVDF membranes were incubated with specific primary antibodies (1:1000) overnight at 4 °C. The appropriate secondary antibodies (1:4000) were used to tag primary antibody for 3 h and an enhanced chemiluminescence (ECL) detection system (Millpore immobilonTM HRP Substrate) was used to develop the immunoreactive bands. Finally, the protein band densities were quantified with Fusion software.
Molecular docking studies
The docking studies were carried out using the AutoDock Vina software. The input files were prepared in the graphic interface AutoDock Tools 1.5.2 [
18]. The protein crystal structure of rabbit SERCA1a (PDB code: 4BEW) and SERCA2 (PDB code: 5MPM) was downloaded from the protein data bank as a basis for modeling. All the ligands in the computational study were converted into 3-D format. The docking parameters were as follows: blind docking procedure with a grid box of 126 × 126 × 126 Å
3 and a grid spacing of 0.375 Å. For the scoring we selected the Lamarckian Genetic Algorithm with maximum number of energy evaluations of 1 × 10
7.
SERCA activity analysis
SERCA activity was analyzed using the Ca2+-ATP enzyme assay kit (Biological Technology, Beijing, China) according to the manufacturer’s instructions. Protein concentration was determined by the BCA protein assay kit, and the amount of inorganic phosphate reflects SERCA activity.
Statistical analysis
Dates in this study were expressed as the mean ± standard error of mean (SEM). Statistical analyses were performed with independent t-test, one-way ANOVA, followed by a least significant difference post hoc analysis. For all analysis, P values < 0.05 were considered statistically significant.
Discussion
Nonalcoholic fatty liver disease is a clinical pathological syndrome characterized by diffuse hepatocytic bullous steatosis. Levels of serum TG are statistically significantly higher in patients with NAFLD [
30]. We demonstrated that Mat could effectively reduce serum TC and TG content in HFD mice. Obesity is a major risk factor for many health complications, and NAFLD has increased in parallel with obesity in the United States [
31]. In the present study, Mat remarkably reduced body weight gain and lipid generation related protein levels in HFD-induced obese mice, which may prevent the progression of NAFLD to a certain extent. Since Mat treatment reduced body weight, all the resulting phenotypes such as reduction in hyperlipidemia, lipogenesis, steatosis might be secondary and may not be a direct effect mediated by Mat treatment. Therefore, in order to further investigate the effect of Mat on liver lipid metabolism, we examined the effect of Mat on liver lipid metabolism-related proteins in later experiments. Our results showed that Mat down-regulated the proteins levels of sterol regulatory element binding protein 1c (SREBP-1c), fatty acid synthase (FAS), and acetyl-coenzyme A-carboxylase (ACC) in HFD mice. NAFLD is a spectrum of pathologic changes in the liver that ranges from simple steatosis to NASH, early fibrosis, cirrhosis and can progress to hepatocellular carcinoma [
32]. In most cases, ALT and AST are elevated in NASH [
33]. Meanwhile, inflammatory response is an important feature of NASH and an important driving factor in its progression to liver fibrosis [
21]. The elevated levels of inflammatory factors and aminotransferase were observed in MCD mice, while administration of Mat could attenuate this effect. Furthermore, the results of H&E and oil red O staining showed that Mat administration markedly improved liver injury and lipid accumulation in HFD and MCD mice. In this study, Mat exerted a certain protective effect against the progression from steatosis to steatohepatitis in HFD and MCD diet-fed mice.
ER is notable for its central roles in Ca
2+ storage, lipid biosynthesis, protein sorting and processing. The ER stress signal is mainly mediated by the following three ER-transmembrane proteins: PERK, IRE1α, and ATF6α [
34]. When the ER homeostasis is unbalanced, GRP78 is separated from the transmembrane protein by binding with unfolded or misfolded proteins, and then PERK, IRE1α and ATF6α and their downstream signaling pathways are activated, resulting in increased lipid synthesis and apoptosis [
35]. Our work showed that ER stress was significantly inhibited by Mat in the livers of MCD diet-fed mice, as well as in PA-induced L02 cells. Moreover, the expression of p-IRE1, ATF6, GRP78, and p-PERK were up-regulated in the livers MCD diet-fed mice which were reduced by long-term administration of Mat. In addition, low dose and middle dose Mat treatment could inhibit the ER stress in PA-induced L02 cells as well. These results indicated that Mat could effectively attenuate excessive ER stress in NAFLD.
ER stress is the central link in the occurrence of NAFLD. Excessive ER stress, mitochondrial dysfunction and oxidative stress can form a vicious cycle, which accelerates the progression of NAFLD. Since PA induces mitochondrial depolarization and suppresses autophagic activity, the proportion of dysfunctional mitochondria may be increased [
36]. Our results showed that low and middle dose Mat reduced the damage of mitochondria induced by PA in L02 cells. Both ER stress and mitochondrial damage can generate a large amount of ROS [
37,
38]. In turn, excessive ROS aggravates ER stress and mitochondrial dysfunction [
27,
39,
40]. Massive generation of mitochondrial ROS opens the mitochondrial permeability transition pore (PTP), promotes apoptosis and necrosis, and leads to hepatic fibrosis [
10]. Severe or persistent ER stress can lead to cell apoptosis [
28,
41]. We found that low and middle dose Mat reduced ROS formation and apoptosis induced by PA in L02 cells. These results indicated that low and middle dose Mat which could effectively attenuate mitochondrial damage, oxidative stress and apoptosis induced by excessive ER stress in NAFLD, but not high dose Mat.
Ca
2+ is considered to play a key role in ER stress, mitochondrial dysfunction, oxidative stress and liver insulin resistance, all of which are important features of NAFLD [
7,
25,
42,
43]. The destruction of cytosolic Ca
2+ balance, especially the excessive release of ER Ca
2+, is the core factor to lead to ER stress, mitochondrial damage and a series of problems that followed. The SERCA pump is a key site to regulate the ER calcium balance, because it participates in the redistribution of Ca
2+ from cytoplasm to ER. Recent studies showed that PA induced ER stress, apoptosis and accompanied by a reduction in SERCA2b activity, mimicking nonalcoholic steatohepatitis [
7,
44]. We observed that cytosolic Ca
2+ levels were increased in PA-induced L02 cells after 12 h incubation, while Mat treatment attenuated this effect. This result indicated that Mat was involved in Ca
2+ regulation in NAFLD. In order to further clarify the role of Mat, we conducted a series of instantaneous stimulation experiments in normal L02 cells. Firstly, we found that Mat stimulation increased [Ca
2+]
c in a concentration-dependent manner and this effect was partially suppressed when combined with 2-APB (IP
3R inhibitor). However, the effect was mostly suppressed in cells after Tg (SERCA inhibitor) treated 12 h. Studies have pointed out that 2-APB reduces ER Ca
2+ when ER Ca
2+ store is replete [
29]. We also observed that there was a slight increase when 2-APB (20 μM) stimulated L02 cells and the phenomenon disappeared when cells were stimulated with Mat in advance for 10 min. This indicated that Mat treatment reduced the Ca
2+ concentration in ER, which is a similar effect with Tg. Moreover, we found that Tg stimulation was able to increase the cytosolic Ca
2+ level while Mat and Tg simultaneous stimulation showed a lower Ca
2+ wave than that in Tg stimulation. As is known to all, the calcium exchange between the ER and the cytoplasm depends mainly on three calcium channels, including IP
3R, RyR and SERCA. RyR has a poor effect on liver cells for it is mainly located in cardiac muscle and skeletal muscle. Our stimulation tests were carried out in a non-calcium environment, which excludes the influence of external calcium ions. Therefore, we suspected that Mat was likely involved in calcium regulation in the SERCA pathway. In order to verify this, we conducted molecular docking experiment. Among the entire cluster of complexes for SERCA1a and SERCA2 predicted by AutoDock, the most populated cluster conformations together with the lowest energy conformation show that Mat might also function as a SERCA pump inhibitor. Furthermore, SERCA activity assay showed that Mat up-regulated SERCA activity in PA-induced L02 cells, although this enzyme activity might be reduced when high dose Mat was used in control group. These results supported that Mat mitigated the ER stress of NAFLD through competitive inhibition of SERCA.
In the present study, the highest dose of Mat in mouse models was converted from clinical common dose in people and all doses of Mat showed a beneficial effect. However, in PA-induced L02 cells, high dose Mat has different pharmacological effects from middle and low dose Mat, which seems to be contradictory. In fact, the diverse pharmacological effects of Mat on ER stress have been reported in the previous literature. On the one hand, hepatitis B and atherosclerosis are considered to be associated with excessive ER stress, and Mat has a good therapeutic effect on these two diseases [
45‐
48]. On the other hand, some researchers have discovered that Mat also exerts anti-cancer effects by aggravating ER stress [
49]. The increase of SERCA2 activity was demonstrated in cancer cells and is considered to protect cancer cells from apoptosis [
50]. Our experiments show that the anti-cancer effects of Mat may be exerted by inhibiting SERCA2. In some other diseases, in spite of the stress, Mat as a inhibitor of SERCA, it can exert its competitive inhibitory effect to improve SERCA activity and maintains the ER function, thereby improving the state of the disease.
Authors’ contributions
LS and YZ designed this study. XBG, SG, SZ, LS and AL performed the experiments and all authors participated in data analysis. XBG, LS and YZ wrote the manuscript. All authors read and approved the final manuscript.