Introduction
Nonalcoholic fatty liver disease (NAFLD) is part of a spectrum of liver diseases called hepatic steatosis and one of the most common liver diseases. It is characterized by the accumulation of large blisters of triglycerides in hepatocytes and occurs in the absence of secondary causes [
1,
2]. Nonalcoholic steatohepatitis (NASH) is a progressive and inflammatory subtype of NAFLD characterized by hepatocellular injury and inflammation, with various stages of fibrosis [
3,
4]. A small percentage of patients with simple steatosis will progress to cirrhosis, but more than 20% of patients with NASH will.[
1,
5] Patients with NASH have a significantly higher risk of developing hepatocellular carcinoma than those with other diseases [
6]. According to a survey, from 2004 to 2016, there was a 114% and 80% increase in the number of men and women, respectively, in the liver transplant waitlist for NASH [
7]. At the same time, patients with NASH are more susceptible to developing cirrhosis, end-stage liver disease, and an increased risk of morbidity and mortality, even if the disease is often clinically silent [
8]. As a result, diagnosing and treating NASH has become a formidable challenge for clinicians.
Although several drugs for NASH have entered phase 3 clinical trials in the past few years, no FDA-approved medical treatment for NASH has been identified [
9‐
11]. At present, the therapeutic interventions for NASH focus on weight loss and lifestyle changes, such as diet and exercise. The demand for pharmacological therapy for NASH remains high because the degree of weight loss required for the histological improvement of the liver is difficult to achieve and maintain [
12]. Unfortunately, the only effective therapy for end-stage liver disease and liver failure is liver transplantation [
12,
13].
Previous studies indicate that multiple factors can predispose a person to the initial development of NASH, including metabolic alterations and genetic susceptibility [
14]. However, accumulating evidence has shown that inflammation and inflammatory signaling pathways play an important role in NASH progression [
15,
16]. The activation of liver resident macrophages or Kupffer cells induces NLRP3 inflammasome activation, triggering an inflammatory response. NLRP3 inflammasome activation leads to a broad immune response, including the production of pro-inflammatory cytokines and chemokines, the subsequent recruitment of neutrophils and other immune cells, and cell death [
17,
18]. Growing evidence has shown that inflammasome-driven inflammation is associated with tissue damage and liver fibrosis in NASH [
17‐
19].
Inflammasomes are cytoplasmic multiprotein complexes usually composed of NLRP3 (NOD-, LRR- and pyrin domain-containing protein (3), the adaptor protein ASC (apoptosis-associated speck-like protein containing CARD), and the effector molecule pro-caspase-1 [
20‐
22]. Upon stimulation, NLRP3 binds to ASC, which in turn interacts with the cysteine protease caspase-1 and forms a complex that leads to caspase-1 activation. Caspase-1 activation can promote cleavage of the pro-inflammatory cytokines interleukin-1β (IL-1β) and IL-18 into their active forms and the release of the cytosolic protein GSDMD [
23,
24]. It is important to note that the development of type 2 diabetes, mitochondrial dysfunction, and insulin resistance are strongly associated with NLRP3 inflammasome activation, which are major risk factors for developing NASH [
25,
26].
Several compounds, including MCC950, OLT1177, parthenolide, sulforaphane, and isoliquiritigenin, have been shown to strongly inhibit NLRP3 inflammasome activation [
27‐
32]. MCC950 is a well-studied and specific NLRP3 inhibitor, which can alleviate the symptoms of mouse models of NLRP3-dependent disease, including NASH, type 2 diabetes, and Alzheimer’s disease [
27]. However, MCC950 could induce potential hepatotoxicity in phase II clinical trials. In addition, OLT1177, a selective inhibitor of the NLRP3 inflammasome, also has been investigated in phase II clinical trials [
28,
33]. Therefore, it is important to develop effective, broadly applicable, and safe NLRP3 inflammasome inhibitors to treat inflammasome-mediated diseases.
Epalrestat, an aldose reductase inhibitor, is used to improve the function of the peripheral nerves in diabetes mellitus [
34]. Aldose reductase is a rate-controlling enzyme in the polyol pathway and is a potential drug target for preventing and treating diabetic neuropathy [
35,
36]. Several key inhibitors of aldose reductase have been developed to treat diabetes complications via the polyol pathway [
37,
38]. Nevertheless, epalrestat is the only aldose reductase inhibitor currently available for clinical use in many countries, including China, Japan, and India [
39,
40]. Therefore, epalrestat is a clinically effective and safe drug.
In this study, we report that epalrestat prevented the progression of diabetic retinopathy/nephropathy and could also be used as an anti-inflammatory agent. Here, we describe epalrestat as a highly potent and specific inhibitor of NLRP3, which is active in various NLRP3-dependent mouse models, particularly in NASH.
Materials and methods
Mice
C57BL/6 mice were obtained from SPF Biotechnology Co., Ltd (Beijing, China). All animals were reared at SPF conditions, with 40–70% humidity and 12 h of light/12 h of dark per day.
Cell culture
Bone marrow-derived macrophages (BMDMs) were obtained from C57BL/6 mice (10 weeks old) and cultured in Dulbecco’s modified Eagle’s medium (DMEM) supplemented with 10% FBS, 1% penicillin/streptomycin (P/S) and 50 ng/mL murine macrophage colony-stimulating factor (M-CSF). THP1 cells were cultured in RPMI-1640 medium supplemented with 10% FBS and 1% P/S. HEK-293 T cells were cultured in DMEM supplemented with 10% FBS and 1% P/S. All cells were incubated at 37 °C in a humidified atmosphere with 5% (v/v) CO2.
Antibodies and reagents
Phorbol-12-myristate-13-acetate (PMA), dimethyl sulfoxide (DMSO), adenosine triphosphate (ATP), Nigericin, LPS, TRIzol, and ultrapure LPS were purchased from Sigma-Aldrich (Munich, Germany). Silicon dioxide (SiO2), poly (I: C), poly (dA: dT), and Pam3CSK4 were obtained from InvivoGen (San Diego, USA). MCC950 was purchased from TopScience (Shanghai, China). Epalrestat, ponalrestat, ranirestat, and tolrestat were purchased from MedChemExpress (New Jersey, USA). MitoSOX was purchased from Invitrogen (Carlsbad, USA). Certified Fetal Bovine Serum (FBS) were obtained from VivaCell (Shanghai, China). Anti-mouse caspase-1 antibodies (AG-20B-0042) were purchased from Adipogen (San Diego, USA). Anti-human cleaved IL-1β (12,242), anti-human caspase-1 (4199S), anti-mouse IL-1β (12,507), and anti-NLRP3 (15101S) antibodies were purchased from CST (Boston, USA). Anti-ASC (sc-22514-R) antibodies were purchased from Santa Cruz Biotechnology (Dallas, USA). Anti-DDDK tag (20,543-1-AP) and anti-GAPDH (60,004-1-1G) antibodies were purchased from the Proteintech Group (Chicago, USA). Anti-β-actin (ab8226), anti-NEK7 (ab133514), and anti-aldose reductase (ab268058) antibodies were purchased from Abcam (Cambridge, UK). Color Prestained Protein marker (20AB01) was purchased from GenStar (Beijing, China). Salmonella was a gift from Dr. Tao Li of the National Center of Biomedical Analysis (Beijing, China).
Plasmids and transfection
The plasmids pCMV-Flag-Vector and pCMV-NLRP3-Flag were kindly provided by Dr. Tao Li from the National Center of Biomedical Analysis (Beijing, China).
Inflammasome activation
THP1 and BMDMs were seeded in 24-well culture dishes at a density 1 × 106 cells/mL or 1.5 × 106 cells/mL and cultured overnight. Afterward, the cells were primed for 4 h with 50 ng/mL LPS. Then, the medium was replaced with Opti-MEM supplemented with epalrestat. After 1 h, the activation of NLRP3 was typically achieved through the following treatments: 5 mM ATP for 45 min, 7.5 μM nigericin for 30 min, 200 μg/mL SiO2 for 6 h, or transfection with 1 μg/mL poly (I: C) using Lipofectamine 2000 for 6 h. NLRC4 or AIM2 inflammasome activation was accomplished using Salmonella or transfection with 1 μg/mL poly (dA: dT) using Lipofectamine 2000 for 6 h.
Western blot analysis
To detect caspase-1 p20 and IL-1β production, the cell supernatants were concentrated with TCA and then centrifuged at 12,000 ×g for 10 min at 4 °C. Next, the concentrated proteins were washed by turning up and down with acetone and centrifuged at 12,000 ×g for 5 min at 4 °C. The acetone-containing supernatant was discarded, the samples were placed in 1 × loading sample buffer, and the proteins were denatured at 105 ℃ for 15 min. The protein samples were then resolved on SDS–PAGE gels and transferred to a polyvinylidene difluoride (PVDF) membrane using a wet-transfer system. Next, the PVDF membranes were blocked in TBST (20 mM Tris–HCl [pH 7.6], 150 mM NaCl, and 0.1% [v/v] Tween-20) containing 5% (w/v) non-fat milk for 1 h at room temperature and then incubated with primary antibodies diluted in 5% (w/v) BSA in TBST overnight at 4 °C. Afterward, the membranes were washed thrice with TBST and incubated with the corresponding horseradish peroxidase (HRP)-conjugated secondary antibodies diluted in 5% (w/v) non-fat milk in TBST for 1 h. Following three washes with TBST, the bands generated on the membrane were visualized on X-ray film using a chemiluminescent western blotting detection system.
To detect the expression of NLRP3, pro-IL-1β, ASC, and caspase-1 p45, the cell lysates were collected via direct lysis in a 1 × loading sample buffer. Western blot analysis was then performed as described above.
Cell viability assay
BMDMs were seeded overnight at a density of 1 × 106 cells/mL in 96-well culture dishes. The cells were then incubated at 37 °C and treated with epalrestat for 24 h. After incubation, the spent medium was replaced with DMEM containing a with cell counting kit-8 (CCK-8) reagent and the cells were incubated for 30 min. The optical density (OD) values were then measured at 450 nm.
Lactate dehydrogenase (LDH) assay
The release of LDH in the cell supernatants was measured using an LDH cytotoxicity assay kit (Beyotime, Shanghai, China) according to the manufacturer’s instructions.
Enzyme-linked immunosorbent assay (ELISA)
Cell culture supernatants, peritoneal lavage fluid, and mouse serum were collected. ELISA kits to detect mouse IL-1β (R&D Systems, SMLB00C), TNF-α (Dakewe, 1217202), and IL-6 (Dakewe, 1210602), and human IL-1β (Dakewe, 1110122) and TNF-α (Dakewe, 1117202) were used to detect the levels of the indicated cytokines according to the respective manufacturer’s instructions.
Caspase-1 activity assay
The activity of caspase-1 in the cell supernatants was measured using a Caspase-Glo® 1 Inflammasome Assay (Promega, Beijing, China) according to the manufacturer’s instructions.
Real-time PCR analysis
Total RNA from mouse tissue was extracted using TRIzol reagent. cDNA was then synthesized from 2 μg RNA using an RT Master Mix for qPCR (MCE, HY-K0510) and analyzed with a SYBR Green qPCR Master Mix (MCE, HY-K0522) using an iQ6 Real-Time PCR Detection System (Bio-Rad) for real-time PCR analysis. The mRNA level of the target genes was normalized to that of the housekeeping gene GAPDH.
ASC oligomerization
BMDMs were seeded in 12-well culture dishes at density of 1 × 106 cells/mL overnight and then treated with 50 ng/mL LPS for 4 h. Next, the spent medium was replaced with Opti-MEM supplemented with epalrestat and incubated for 1 h. NLRP3, NLRC4, and AIM2 inflammasome activation was achieved using similar treatments as described above. Cells were lysed with a Triton buffer [50 mM Tris–HCl (pH 7.5), 150 mM NaCl, 0.5% Triton X-100, and EDTA-free protease inhibitor cocktail] and then centrifuged at 6000 × g for 10 min at 4 °C. Supernatants were then centrifuged at 6000 × g for 10 min. The pellet fractions were then washed with PBS and cross-linked with 2 mM disuccinimidyl suberate (DSS) in PBS for 30 min at 37 °C, followed by centrifugation at 6000 ×g for 15 min. Finally, the cross-linked pellets were dissolved in 1 × loading buffer and analyzed using a chemiluminescent western blotting detection system.
Immunoprecipitation and pull-down assays
HEK-293 T cells were transfected with Flag-tagged plasmids (Flag-Vector and Flag-NLRP3) for 24 h and then treated with epalrestat for 6 h. Afterward, the cells were lysed with a lysis buffer (50 mM NaCl, 50 mM Tris, pH 7.8, 0.1% [v/v] Nonidet-P40, 5 mM EDTA and 10% [v/v] glycerol) containing an EDTA-free protease inhibitor cocktail and the cell lysates were collected and centrifuged at 12,000 rpm for 15 min. The supernatants were then immunoprecipitated with anti-Flag M2 affinity beads according to the manufacturer’s instructions. Cell lysates or immunoprecipitates were separated using a chemiluminescent western blotting detection system.
Epalrestat was also conjugated with EAH-activated Sepharose 4B (GE Healthcare). RIPA buffer containing an EDTA-free protease inhibitor cocktail was used to lyse BMDMs, and the lysates were centrifuged at 12,000 ×g for 15 min at 4 °C. Next, the combination of epalrestat-conjugated Sepharose 4B beads and cell lysates were co-incubated at 4 °C overnight. Finally, the beads were washed six times with RIPA buffer, and the proteins were analyzed via immunoblotting.
Intracellular K+ and Ca2+ measurements
BMDMs were seeded in 12-well plates overnight and then treated with 50 ng/mL LPS for 4 h. The spent medium was then replaced with Opti-MEM supplemented with epalrestat. The cells were then incubated for 1 h and then treated with nigericin or ATP. To measure the intracellular K+ levels, the spent medium was aspirated before washing the cells three times with PBS. Ultrapure HNO3 was then added to lyse the cells, and the lysates were boiled at 100 °C for 30 min. Intracellular K+ measurements were then performed via inductively coupled plasma mass spectrometry. To measure the intracellular Ca2+ levels, a trace showing ATP-induced Ca2+ flux was analyzed using a FLIPRT Tetra system (Molecular Devices, USA).
Toxicity of epalrestat in vivo
8 week-old male or female C57BL/6 mice were gavaged with epalrestat (120 mg/kg/day) or vehicle daily for 14 days. The body weight of the mice was measured daily. On the 15th day, after anesthetization, the serum of the mice was collected and assessed for AST, ALT, creatinine, TBIL, and glucose levels using commercial kits according to their respective manufacturer’s instructions.
LPS-induced systemic inflammation
8 week-old female C57BL/6 mice were gavaged with epalrestat (20 or 40 mg/kg), MCC950 (40 mg/kg), or vehicle for 1 h. After intraperitoneal injection with LPS (20 mg/kg) for 6 h, the mice were anesthetized, and the serum and peritoneal lavage fluids were collected. Cytokine levels in the lavage fluids and serum were detected using ELISA. The stained cells were analyzed using flow cytometry.
Methionine- and choline-deficient (MCD) diet-induced steatohepatitis and fibrosis
C57BL/6 mice (8 week-old, male) were fed with an MCD diet (518,810, Dyets, USA) or an identical diet supplemented with methionine and choline (MCS; 518,811, Dyets) for 6 weeks according to the manufacturer’s instructions. Afterward, the mice were randomly separated into groups and gavaged with epalrestat (20 mg/kg), MCC950 (20 mg/kg), or vehicle daily for a total of 5 days, and then 40 mg/kg every other day, for up to 6 weeks. Finally, the mice were anesthetized, and their liver and serum were collected for analysis.
Statistical analyses
All statistical calculations were performed using GraphPad Prism 7 software (GraphPad Software) and Microsoft Excel. Data are expressed as the mean ± SD. Statistical analysis was carried out using a standard two-tailed unpaired Student’s t-test for single comparisons and one-way ANOVA for multiple comparisons. Data were considered statistically significant when P < 0.05.
Discussion
Our study revealed that epalrestat has a powerful suppressive function on NLRP3 inflammasome activation. As a powerful and specific NLRP3 inhibitor, epalrestat is effective in vitro in BMDMs and in vivo in mice. We also revealed that epalrestat has a potential therapeutic in NLRP3 inflammasome-driven diseases, such as NASH. Until now, there only a few small molecules have been reported to directly target the NLRP3 inflammasome, but OLT1177 is the only one that has undergone phase II clinical trials [
28]. In this research, we describe the effect of epalrestat, a clinically effective and safe drug that could also improve the function of peripheral nerves in diabetes mellitus. Therefore, epalrestat may be useful and important for treating NLRP3-driven diseases.
We also elucidated the mechanism by which epalrestat inhibits NLRP3 inflammasome in this study. The data indicated that treatment with epalrestat, either before or after LPS stimulation, failed to inhibit NF-κB-mediated pro-IL-1β expression and TNF-α production. Moreover, epalrestat has no effect on K+ efflux, Ca2+ flux, or ROS production but could dose-dependently block ASC oligomerization. We also characterized the interaction between epalrestat and inflammasome-associated proteins and indicated that epalrestat could not directly interact with the proteins essential for NLRP3-inflammasome activation, such as NLRP3, caspase-1, NEK7, and ASC.
Epalrestat, as an aldose reductase inhibitor, could improve the function of peripheral nerves in diabetes patients and has been successfully used in the clinic [
34,
39,
40]. Aldose reductase is a key rate-limiting enzyme of the polyol pathway, and controlling its activity can lower blood glucose and thus alleviate diabetic complications [
36,
58‐
60]. However, recent research evidence suggests that aldose reductase is an outstanding facilitator to regulate inflammatory signals [
37,
61]. Accordingly, preventing inflammatory complications may be a potential use of aldose reductase inhibition. Currently, aldose reductase inhibitors are used to treat endotoxemia, sepsis, or inflammatory diseases [
62‐
64]. Aldose reductase inhibitors have been clinically studied for diabetic complications over the past few years [
62,
65]. Therefore, aldose reductase inhibitors could be explored as a treatment for inflammatory diseases. Besides epalrestat, ponalrestat, ranirestat, and tolrestat were also selected in this study to verify whether aldose reductase inhibitors could block NLRP3 inflammasome activation. However, epalrestat is the only aldose reductase inhibitor that is effective and safe in clinical use [
34,
66]. We have demonstrated that epalrestat could suppress NLRP3 inflammasome activation by targeting aldose reductase.
NASH is characterized by hepatocyte ballooning, hepatic steatosis, inflammatory cell infiltration of the liver lobules, hepatocellular injury and fibrosis [
67,
68]. Several studies indicate that NLRP3 inflammasomes could induce tissue damage and liver fibrosis in NASH [
20,
69,
70]. In our study, epalrestat could target NLRP3 inflammasome to alleviate liver inflammation and fibrosis in MCD-fed mice and has comparable therapeutic potential to MCC950, which may be a plausible direction for NASH pharmacotherapy. Therefore, another important discovery is that epalrestat could decrease liver inflammation and fibrosis by suppressing NLRP3 inflammasome activation in NASH.
In this research, we identified epalrestat as a prospective novel and potential powerful NLRP3 inflammasome antagonist. Epalrestat inhibited NLRP3 inflammasome activation in vitro and in vivo through its function of inhibiting aldose reductase activity. Epalrestat also displayed significant therapeutic potential in NASH. Unfortunately, there are still no FDA-approved medical treatments available for NASH. Epalrestat is a clinically effective and safe drug and has already been used in many countries, such as China, Japan, and India; therefore, epalrestat may be used as a favorable candidate drug for treating NASH.
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