Scropolioside B Inhibits IL-1β and Cytokines Expression through NF-κB and Inflammasome NLRP3 Pathways

Chronic inflammation is associated with various chronic illnesses including immunity disorders, cancer, neurodegeneration, and vascular diseases. Iridoids are compounds with anti-inflammatory properties. However their anti-inflammatory mechanism remains unclear. Here, we report that scropolioside B, isolated from a Tibetan medicine (Scrophularia dentata Royle ex Benth.), blocked expressions of TNF, IL-1, and IL-32 through NF-κB pathway. Scropolioside B inhibited NF-κB activity in a dose-dependent manner with IC50 values of 1.02 μmol/L. However, catalpol, similar to scropolioside B, was not effective in inhibiting NF-κB activity. Interestingly, scropolioside B and catalpol decreased the expression of NLRP3 and cardiolipin synthetase at both the mRNA and protein level. Our results showed that scropolioside B is superior in inhibiting the expression, maturation, and secretion of IL-1β compared to catalpol. These observations provide further understanding of the anti-inflammatory effects of iridoids and highlight scropolioside B as a potential drug for the treatment of rheumatoid arthritis and atherosclerosis.


Introduction
Acute inflammatory responses are essential for pathogen control and tissue repair and can also cause severe tissue damage. During chronic infections and age-associated immune dysregulation, inflammatory processes may induce a variety of harmful effects on an organism [1]. Chronic inflammation is associated with chronic illnesses including cancer, neurodegeneration, and vascular diseases [2][3][4]. Infection or cell damage triggers the release of proinflammatory cytokines such as interleukin-(IL-) 1 and tumor necrosis factor-(TNF-) , which are key mediators of the host immune response. Signal transduction of inflammatory cytokines includes ligands, receptors, coreceptors, and cytosolic and nuclear signaling mechanisms. These mechanisms can activate the NF-B, JNK, p38 MAPK, STAT, and PI3K signaling pathways [5]. On the other hand, inflammasomes play a key role in the regulation of inflammation and immune responses by producing proinflammatory cytokines [6]. Studies have shown that inflammasomes are involved in atherosclerosis [7], metabolic syndrome [8], type 2 diabetes [9], alcoholic steatohepatitis [10], mucosal immune response [11], rheumatoid arthritis [12], and gout [13]. The nucleotide-binding domain-(NOD-) like receptor protein 3 (NLRP3) inflammasome is a multiprotein complex that regulates the maturation of proinflammatory cytokines IL-1 and IL-18. It consists of NOD-like receptor, NLRP3, the adaptor protein ASC (apoptosis-associated speck-like protein containing caspase-1 activator domain, CARD), and caspase-1. Upon exogenous and endogenous stimuli, the NLRP3 inflammasome forms through activation of NLRP3 and recruitment of ASC and pro-caspase-1, resulting in caspase-1 activation and subsequently processing of pro-IL-1 and pro-IL-18 into their active forms [14].
Iridoid is derived from Scrophulariaceae, Rubiaceae, Labiatae, Gentianaceae, Oleaceae, and so on; it is mainly 2 Mediators of Inflammation derived from Scrophularia L. [15]. The iridoids are comprised of a large family of distinctive bicyclic monoterpenes that possess a wide range of pharmacological properties, including anticancer, anti-inflammatory, antifungal, and antibacterial activities [16,17]. Scropolioside A exhibited anti-inflammatory properties against different experimental models of delayed-type hypersensitivity. Scropolioside A also inhibited the production of prostaglandin E2, leukotriene B4, nitric oxide, and some interleukin but had no effect on the production of IL-10. Moreover, it modified the expression of both nitric oxide synthase-2 and cyclooxygenase-2, as well as the activation of NF-B in RAW 264.7 macrophages [18]. Scropolioside D also possessed significant antidiabetic and anti-inflammatory activity [19]. However, although scropolioside B exhibited moderate antibacterial activity against strains of multidrug and methicillin-resistant Staphylococcus aureus (MRSA) and a panel of rapidly growing mycobacteria with minimum inhibitory concentration (MIC) values ranging from 32 to 128 g/mL [20], it had no significant effect on TXB2-release [21]. We previously reported different anti-inflammatory effects of other iridoid components [22,23]. Scrophularia dentata Royle ex Benth. in Tibet is used for antiviral and anti-inflammatory treatment. Therefore, in this study, we examined scropolioside B isolated from S. dentata Royle ex Benth. We determine whether scropolioside B exhibits anti-inflammatory effect and further analyze its underlying mechanism in human monocytes.

Cell Cultures and Reagents. Human Embryonic Kidney 293 cells (HEK293 cells) were purchased from Yongzheng
Grubber Products Corporation (Nanjing, China) and THP-1 cells were from the Chinese Academy of Sciences (Shanghai, China). Cells were cultured on 100 mm tissue culture dishes or 100 mL flasks in Dulbecco's modified Eagle's medium (DMEM) containing 10% fetal bovine serum (Gibco, Invitrogen, USA) at 37 ∘ C in a humidified incubator under 5% CO 2 and 95% air. During experiments, the cells were plated in 24well plates or 30 mm tissue culture dishes for 16 or 24 h.

Luciferase Assay.
To assay NF-B promoter activity, THP-1 cells were transiently transfected with a luciferase reporter gene. pNF-B-TA-Luc was purchased from Stratagene (USA). Cells were plated one day prior to transfection so that cells will be approximately 80% confluent on the day of transfection. On the day of transfection, DNA was diluted to 2 g per 100 L of serum-free medium, and an appropriate amount of FUGENE HD Transfection Reagent (Promega, USA) was added to achieve the proper ratio of reagent to DNA. The mixture was incubated for 0-15 minutes, and 100 L was added to each well to be transfected. Cells were transfected for 5 hours before changing to fresh media. One hour after transfection, TNF-was added to the cells for 16-20 hours. Luciferase activity was measured in the cell lysates using the Promega Luciferase Assay System according to the manufacturer's instructions (Promega, USA).

Quantitative Real-Time PCR (qRT-PCR).
Total RNA was extracted using TRIzol (Life Technology, Carlsbad, CA, USA) according to the manufacturer's instructions. Real-time PCR amplification and detection were performed using the SYBR Green qPCR SuperMix-UDG with ROX (Life Technologies) in a fluorescence thermal cycler (StepOne Real-Time PCR system, Life Technologies) according to the manufacturer's protocol. Gene expression was normalized using GAPDH as a reference gene. Relative mRNA expression levels were calculated following the ΔΔCt method with the following primers: GAPDH, IL-1 , TNF-, IL-32 , IL-32 , CLS1, and NLRP3 in Table 1. All amplifications were conducted within the linear range of the assay and normalized to respective GAPDH levels using SPSS Version 18.0 (SPSS Institute, Inc., Chicago, IL, USA).

Date Analysis.
Each experiment was performed at least 3 times. The results were presented as means ± standard error of mean (SD). All data was analyzed using SPSS software, and one-way ANOVA was used to determine the statistical significance of differences between the means. Differences were statistically significant when < 0.05.

Blocking IL-1 and TNF-Expression by Scropolioside B.
Since scropolioside B contains structures of catalpol and two phenylpropanoids (Figure 1(a)), we compared and tested the anti-inflammatory capabilities of both scropolioside B and catalpol in THP-1 cells. The expression of IL-1 and TNFwas significantly induced by lipopolysaccharide (LPS) or palmitic acid (PA), a free fatty acid with potential proinflammatory mediators, compared to control-treated THP-1 cells (Figure 1). This indicated that cellular exposure to LPS or PA induced the secretion of various cytokines that lead to the initiation and amplification of inflammation. To investigate the anti-inflammatory effect of scropolioside B, we preincubated THP-1 cells with the compound for 1 h and subsequently stimulated the cells with LPS or PA. We found that scropolioside B significantly blocked the increase in IL-1 and TNF-levels induced by LPS or PA (Figure 1). However, at the concentration of 50 mol/L, catalpol did not effectively block expression of IL-1 and TNF-, although these antiinflammatory effects had been reported [24,25]. These observations suggested that scropolioside B has stronger antiinflammatory activity compared to catalpol.

Inhibition of Nuclear Factor B Activation by Scropolioside
B. NF-B is an essential transcription factor involved in the production of several cytokines that mediate the inflammatory response. To investigate overall anti-inflammatory activity of scropolioside B, we used a luciferase reporter assay to determine nuclear factor kappa B (NF-B) activity. After HEK293 cells were transferred with either the NF-B or the control plasmid, cells were treated with or without scropolioside B for 1 h and then stimulated with 100 ng/mL of TNF-. An increase in luciferase activity was observed after stimulation with TNF-, suggesting that NF-B was activated by TNF- (Figure 2(a)). Pretreatment with scropolioside B (0.08-50 mol/L) inhibited TNF--induced NF-B activation in a concentration-dependent manner. Furthermore, scropolioside B exhibited an IC 50 value of 1.02 mol/L (Figure 2(b)). These results showed that scropolioside Bmediated inhibition of inflammatory cytokine induction was due to the suppression of NF-B.

Scropolioside B Reduced the IL-32
Expression. IL-32 is a proinflammatory cytokine involved in several diseases, including infections, chronic inflammation, and cancer. TNF-or LPS are known inducers of IL-32, IL-32-dependent effects of IL-1 on endothelial cell functions [26]. We next determined the inhibitory effects of scropolioside B on IL-32 expression. Pretreatment with scropolioside B significantly diminished the increase in mRNA expression levels of IL-32 and IL-32 induced by LPS stimulation (Figures 3(a) and   3(b)), similar to IL-1 and TNF-expression pattern, in LPSinduced THP-1 cells.

Scropolioside B Decreases Expression of NLRP3.
Inflammasomes regulate maturation of IL-1 and IL-18 and pyroptosis. NLRP3 is a member of inflammasomes which constitute the compound with ASC and caspase-1 to catalyze the maturation of IL-1 [27]. To observe whether inflammatory * * factors induce NLRP3 expression, we stimulated THP-1 cells with LPS for 24 h. We observed that LPS upregulated NLRP3 mRNA and protein (Figures 4(a), 4(c), and 4(d)). Similarly, LPS also significantly enhanced mRNA expression of cardiolipin synthetase 1 (CLS1) (Figure 4(b)), a mitochondrial enzyme catalysing cardiolipin synthesis necessary for inflammasome NLRP3 activity [28]. As shown in Figures  4(a)-4(d), pretreatment with scropolioside B inhibited the expressions of NLRP3 mRNA and protein, as well as CLS1 mRNA. We also found that catalpol was as equally effective as scropolioside B (Figures 4(a)-4(d)), suggesting that this inhibitory effect may be from the same catalpol structure ( Figure 6).

Discussion
Scropolioside B which is from S. dentata Royle ex Benth. has antipyretic detoxicating effect. It is used in Tibetan medicine, such as smallpox, measles, other infectious fevers, and inflammatory diseases. In this study, we demonstrated that scropolioside B, an iridoid glycoside containing a catalpol and two phenylpropanoids, was more effective than catalpol in inhibiting the expressions of IL-1 and TNF-in THP-1 cells activated by LPS or palmitic acid ( Figure 1). Our results suggested that scropolioside B has higher anti-inflammatory activity than catalpol, although some studies have reported that catalpol does demonstrate anti-inflammatory effects at high dose by inhibiting COX-2 activity, TNF-formation, monocyte chemotactic protein-1 (MCP-1), and nitric oxide production [24,25]. As shown in Figure 2, the anti-inflammatory effects of scropolioside B were mediated by blocking NF-B activity ( Figure 2). IL-1 is involved in the onset of acute local or systemic inflammation and contributes to a variety of chronic noninfectious diseases, including ischemic injury, atherosclerosis, type 2 diabetes, and osteoarthritis. Endogenous metabolites, such as oxidized fatty acids, high glucose, uric acid crystals, activated complement, necrotic cells, and cytokines, can stimulate the synthesis of the inactive IL-1 precursor, which awaits processing by the inflammasome complex to be activated [29]. Inflammasomes regulate maturity of IL-1 , IL-18, and pyroptosis and recognize microbial products or endogenous molecules released from damaged cells [27]. Inflammasomes have several member proteins, including NLRP1, NLRP3, NLRC4, AIM2, and NRP6 [6]. Our results showed that scropolioside B can inhibit mRNA and protein expression of inflammasome NLRP3 and prevents the secretion of IL-1 ( Figure 4). Interestingly, catalpol has a similar inhibitory effect on NLRP3 expression compared to scropolioside B, suggesting that scropolioside B and catalpol inhibit IL-1 and NLRP3 expression by different mechanisms. Scropolioside B and catalpol also block the expression of CLS1, which is an enzyme in the final step of mitochondrial cardiolipin synthesis by catalysing the transfer of a phosphatidyl residue from diacylglycerol to phosphatidylglycerol. Iyer et al. [28] reported that mitochondrial cardiolipin and reactive oxygen species are needed for inflammasome NLRP3 activity. Cardiolipin can bind directly to NLRP3 and silencing of cardiolipin synthesis specifically inhibits inflammasome NLRP3 activation [28]. Based on these observations, we believe that scropolioside B not only blocks NF-B pathway but also inhibits NLRP3, CLS1, and IL-1 expressions. However, catalpol only prevents the expression of NLRP3 and CLS1 ( Figure 5).
In conclusion, scropolioside B significantly diminished expression and secretion of IL-1 , IL-32, and TNF-. We show that this is mediated by modulating NF-B, NLRP3, and CLS1 levels. Additional studies are needed to further elucidate other targets by which scropolioside and catalpol regulate inflammation. The results of this study strengthen previous understanding of the anti-inflammatory effects of iridoids and highlight scropolioside B as a potential drug for the treatment of rheumatoid arthritis and atherosclerotic disease.