The SP/NK1R System-Mediated ROS Generation in GBM Cells through Inhibiting Glutaredoxin Protein.

Altered redox balance is among the main contributing factors developing glioblastoma multiforme (GBM), a highly aggressive grade IV brain tumor. Neuropeptide substance P (SP) plays a key role in modifying the cellular redox environment by activating the neurokinin-1 receptor (NK1R). In this study, we aimed to investigate the redox-modulating properties of both SP and a commercially available NK1R antagonist, aprepitant in GBM cells. To detect the effect of aprepitant on the viability of U87 glioblastoma cells, resazurin assay was applied. The level of intracellular ROS was assessed using 2',7'-dichlorodihydrofluorescein diacetate (H2DCFDA) assay. The expression of glutaredoxin, a well-known redox-active protein, was measured by quantitative real-time polymerase chain reaction (qRT-PCR). Concurrently, the activity of glutaredoxin was also analyzed by a commercial kit (ZellBio GmbH). We found that SP increased the intracellular levels of reactive oxygen species (ROS) in U87 GBM cells, and aprepitant remarkably decreased this effect. We also explored the effects of SP/NK1R signaling on the glutaredoxin system as a major cellular redox buffer in GBM cells. SP reduced both expression and enzymatic activity of glutaredoxin, and these effects were significantly decreased by aprepitant. In conclusion, our results suggest a possible involvement of SP/NK1R signaling in GBM pathogenesis through oxidative stress and offering new insight for the application of aprepitant as a redox-modulating strategy in GBM patients.

1. Introduction

Glioblastoma multiforme (GBM) is a highly aggressive grade IV brain tumor originating from a type of supporting cell in the brain called astrocytic glial cell [1, 2]. Surgical resection, chemotherapy, and radiation therapy are the currently used therapeutic strategies in GBM; however, treatment with these strategies has not led to improved survival rates, and patients have a poor prognosis with a median survival of 14 months after diagnosis [2-4]. Therefore, there is a need to elucidate the molecular mechanisms involved in GBM pathogenesis to improve therapeutic strategies and patient's survival. The exact etiology of GBM is not yet fully understood; but, inherited genetic abnormalities and high dose therapeutic ionizing radiation may increase the risk of GBM [2]. There is also a strong link between GBM and oxidative stress [5].

Oxidative stress is a consequence of oxidant-antioxidant imbalance, leading to excessive production of reactive oxygen species (ROS) [6]. ROS including superoxide, hydroxyl radical, and hydrogen peroxide are highly reactive molecules causing deleterious effects to DNA, proteins, and lipids, consequently contributing to genetic instability and GBM tumor initiation and progression [5]. Remarkably, the brain is more vulnerable to ROS-mediated oxidative damage, mainly due to its high oxygen demand accompanied by low antioxidant enzymes activity, presence of highly peroxidizable lipids, and a low cellular regenerative capacity [5, 7, 8]. To counteract oxidative stress, the body is equipped with enzymatic antioxidant defense mechanisms including superoxide dismutase (SOD), catalase, and glutaredoxin and thioredoxin system [9]. Accordingly, altered intracellular antioxidant enzymes levels have been found in patients with cancers, including brain tumors [10, 11]. Given the importance of an altered redox balance in GBM pathogenesis, identifying the redox regulatory mechanism is essential to get a clearer view and better management of GBM.

Substance P (SP), a member of the tachykinin neuropeptides family, plays critical roles in GBM tumor growth and development through the activation of neurokinin-1 receptor (NK1R), a class of neurokinin G protein-coupled receptors [12-14]. The results of previous studies showed that NK1R is highly expressed on glioblastoma cells and is associated with worse prognosis and advanced tumor stages [15]. Its interaction with SP could support the proliferation and development of GBM [16]. Accordingly, Muñoz et al. also indicated that in the presence of SP, the overexpressed NK1R isoforms in GBM-derived GAMG cells enhance the proliferative and growth capacity of malignant cells [17]. SP has been shown to affect the redox balance of the body and further exacerbate the pathological condition of various clinical disorders [18, 19]. Given the importance of the SP/NK1R axis in the pathogenesis of GBM, it is not surprising that blockage of this pathway could be an effective approach in GBM therapy [12, 14]. L-733,060 is one of the antagonists of NK1R that its antiproliferative effects have been reported in glioblastoma cell lines. Muñoz et al. reported that when GBM-derived GAMG cells were treated with micromolar concentrations of L-733,060, the proliferation of the cells was inhibited even in the presence of SP [17]. Aprepitant is also a selective inhibitor of NK1R that was first administrated as an anxiolytic, antidepressant, and antiemetic agent. However, it became evident that this agent might have anticancer effects [15, 20]. In GBM-derived cell lines, it has been reported that aprepitant could inhibit the growth of malignant cells through inducing apoptotic cell death [21]. Importantly, NK1R antagonists have been found as potent redox-modulating agents in various stress-related diseases [22-24]. However, the redox-modulating potential of NK1R antagonists in tumor cells has not yet been fully investigated. The importance of both SP/NK1R signaling and the altered redox balance in GBM pathogenesis prompted us to investigate the relationship between SP/NK1R signaling and the redox status of GBM in the hope of supporting future studies on the therapeutic potential of aprepitant in GBM. In this study, we investigated the effects of exogenous SP and NK1R antagonist, aprepitant, on ROS levels, and the glutaredoxin system as one of the main intracellular redox buffers in the body.

2. Materials and Methods

2.1. Cell Culture and Reagents

Experiments were performed using the U87 cell line, a human primary glioblastoma cell line. The cells were bought from the National Cell Bank of Institute Pasteur of Iran (Tehran, Iran). Cells were cultured in RPMI 1640 and Ham's F12 (RPMI/F12) media (Gibco-BRL, Life technology, Paisley, Scotland), supplemented with 10% fetal bovine serum (Gibco-BRL, Life technology, Paisley, Scotland) and 1% penicillin-streptomycin (Gibco-BRL, Life technology, Paisley, Scotland). SP and aprepitant were purchased from Sigma-Aldrich Company (St. Louis, MO, USA).

2.2. Resazurin Cell Viability Assay

Resazurin cell viability assay relies on the cellular reduction of nonfluorescent dye resazurin to the strongly fluorescent dye resorufin in metabolically active cells [25]. The amount of fluorescence output correlates with the number of viable cells in a sample. In brief, U87 glioblastoma cells were seeded into 96-well plates at a density of 2.5 × 104 cells per well and cultured for 24 h. Then, cells were treated with various concentrations of aprepitant 0 (control), 5, 10, 25, 35, and 50 μM for 24 h. Following treatment with the indicated concentrations, the medium was removed, and 10 μL resazurin solution (0.01 mg/mL dissolved in phosphate-buffered saline; Sigma-Aldrich) was added to each well, and the wells were incubated for 3 h at 37°C. The fluorescence intensity of the sample was quantified at certain wavelengths of 570 nm and 600 nm using a fluorescence spectrometer. The obtained values were transformed to percentage survival rates by comparing the absorbance values of treated cells to the values of untreated control cells, and the 50% inhibitory concentration (IC50) value was determined using the GraphPad Prism® 6 software.

2.3. Measurement of Reactive Oxygen Species Activity

The level of intracellular ROS was assessed using 2′,7′-dichlorodihydrofluorescein diacetate (H2DCFDA, Sigma, USA) assay. Following diffusion of DCFDA into the cell, DCFDA is deacetylated by cellular esterases to generate the nonfluorescent compound H2DCF. H2DCF in presence of ROS is then quickly oxidized to highly fluorescent dichlorofluorescein (DCF). In brief, U87 glioblastoma cells were seeded into 6-well plates at a density of 75 × 104 cells per well and cultured for 24 h. Afterward, the cells were incubated for 30 min at 37°C with 10 μM DCFH-DA in the dark. Subsequently, U87 glioblastoma cells were treated with SP (100 and 400 nM) alone or in combination with aprepitant (15 μM) for another 24 h. Tertbutyl hydrogen peroxide (TBHP) (Abcam, UK) at 50 mM concentration was used as a positive control. Fluorescent signals are then measured at 495/529 nm (Excitation/Emission) using a Perkin-Elmer atomic absorption spectrophotometer.

2.4. RNA Extraction and Quantitative Real-Time PCR (qRT-PCR)

Total RNA was isolated using FavorPrep blood/cultured cell total RNA mini kit (Yekta Tajhiz, Iran) according to the manufacturer's protocol. The concentration and purity of the extracted RNA were evaluated by a nanodrop spectrophotometer (NanoDrop 1000™, USA) and agarose gel electrophoresis. Total RNA was reverse transcribed to complementary DNA (cDNA) using the cDNA synthesis kit (Pars Tous Biotechnology, Iran) as instructed. qRT-PCR amplifications were carried out in a Roche real-time thermal cycler (Mannheim, Germany) using SYBR Green qPCR Master Mix (No ROX) (Amplicon, Denmark). The housekeeping GAPDH (glyceraldehyde-3-phosphate dehydrogenase) gene was also used as an internal reference gene, and the relative levels were analyzed using the 2−DDCT method.

2.5. Assessment of the Glutaredoxin Activity

For analyzing the activity of glutaredoxin, a commercial kit (ZellBio GmbH, Germany) was used. ZellBio GmbH kit is based on a quantitative sandwich enzyme-linked immunosorbent assay (ELISA) to measure glutaredoxin. The experimental procedure was carried out according to the instructions of the kit. The activity of glutaredoxin was evaluated in ng/mL according to the manufacturer's instructions with a sensitivity of 0.1 ng/mL.

2.6. Statistical Analysis

All experiments were carried out in triplicates, and the results are indicated as mean ± standard deviation (SD) (n = 3). The GraphPad Prism® 6.0 software (San Diego, CA, USA) for Windows was used for all statistical analyses. Bonferroni's t-test was applied to analyze multigroup comparisons following ANOVA. The p value below 0.05 was considered statistically significant.

3. Results

3.1. The Results of the Cell Viability

The results of the resazurin-based cell viability assay at indicated concentrations (5–100 μM) of aprepitant are shown in Figure 1. Aprepitant dose-dependently decreased cell viability and metabolic activity of U87 glioblastoma cells. Following the exposure of cells to aprepitant, a marked reduction in metabolic activity of U87 cells was observed with an estimated IC50 value of around 36.14 μM. Regarding the dose-dependent changes in cell viability, 15 μM was preferred as the experimental concentration.

Figure 1

The results of resazurin-based cell viability assay in U87 glioblastoma cells at increasing concentrations (5–100 μM) of aprepitant for 24 h. The IC50 value of about 36.14 μM was observed for aprepitant in this cell line. Data show the mean ± standard deviation (SD) of three distinct experiments.

3.2. Aprepitant Significantly Reduced the Intracellular ROS Levels in U87 Glioblastoma Cells

The SP/NK1R system has been shown to affect the redox balance of the body by increasing the generation of ROS in different cell types and further exacerbating the pathological condition of various clinical disorders [22-24]. To determine the redox-modulating properties of SP and aprepitant in U87 glioblastoma cells, we evaluated the intracellular ROS levels in response to SP (100 and 400 nM) alone or in combination with aprepitant (15 μM) using DCFH-DA probe. The levels of DCF-positive cells indicate the elevation of ROS production. As shown in Figure 2, SP increased ROS levels in U87 glioblastoma cells; however, significant effects were observed when cells were exposed to SP (400 nM). Moreover, ROS production was significantly reduced in cells treated with aprepitant (15 μM) with or without pretreatment with SP (100 and 400 nM) for 24 h. These results demonstrated the redox modulatory effect of aprepitant through reducing the intracellular ROS levels which might have clinical significance in ROS-associated cancer including GBM.

Figure 2

The effects of SP and aprepitant on intracellular ROS levels. U87 glioblastoma cells are exposed to the preferred concentration of SP (100 and 400 nM) alone and in combination with aprepitant (15 μM) for 24 h, and ROS formation was assessed by the DCFH-DA assay. The results demonstrate that intracellular ROS production is significantly reduced in cells treated with aprepitant with or without the pretreatment with SP.

3.3. Aprepitant Significantly Reduced the mRNA Expression of Glutaredoxin in U87 Glioblastoma Cells

To counteract oxidative stress, the body is equipped with several enzymatic antioxidant defense mechanisms. Among antioxidant systems in the cell, the glutaredoxin system is the major cellular redox buffer. To further investigate the redox-modulating properties of SP and aprepitant in U87 glioblastoma cells, we evaluated the mRNA expression levels of glutaredoxin enzyme in response to SP (100 and 400 nM) alone or in combination with aprepitant (15 μM) using quantitative RT-PCR. As shown in Figure 3, SP reduced glutaredoxin expression in U87 glioblastoma cells; however, significant effects were observed when cells were exposed to SP (400 nM). Moreover, glutaredoxin expression was significantly increased in cells treated with aprepitant (15 μM) with or without pretreatment with SP (100 and 400 nM) for 24 h. These results suggested that the redox regulatory functions of aprepitant might be mediated through alteration of glutaredoxin enzyme.

Figure 3

The effects of SP and aprepitant on the mRNA expression level of glutaredoxin in U87 glioblastoma cells. The results demonstrate that mRNA expression of glutaredoxin is significantly increased in cells treated with aprepitant (15 μM) with or without pretreatment with SP (100 and 400 nM) as compared to the untreated control cells. The level of expression of glutaredoxin was normalized by GAPDH mRNA levels and indicated as mean ± SD (p < 0.05).

3.4. Aprepitant Significantly Reduced the Glutaredoxin Activity in U87 Glioblastoma Cells

To further support the observed effects of SP and aprepitant on the glutaredoxin system, we also assessed the glutaredoxin activity. We evaluated the activity of glutaredoxin enzyme in response to SP (100 and 400 nM) alone or in combination with aprepitant (15 μM) using the ZellBio GmbH kit. As shown in Figure 4, SP reduced the glutaredoxin activity in U87 glioblastoma cells; however, significant effects were observed when cells were exposed to SP (400 nM). Moreover, the glutaredoxin activity was significantly increased in cells treated with aprepitant (15 μM) with or without pretreatment with SP (100 and 400 nM) for 24 h. Accordingly, these findings were consistent with the results obtained from the qRT-PCR analysis of glutaredoxin mRNA expression (Figure 3).

Figure 4

The effects of SP and aprepitant on glutaredoxin activity in U87 glioblastoma cells. The results demonstrate that glutaredoxin activity is significantly increased in cells treated with aprepitant (15 μM) with or without pretreatment with SP (100 and 400 nM) as compared to the untreated control cells. The activity of glutaredoxin is indicated as ng/mL.

4. Discussion

This study investigated the redox regulatory mechanism mediated by SP/NK1R signaling in GBM cells. Our results demonstrated that the exogenous SP increased ROS generation and reduced both expression and enzymatic activity of the glutaredoxin, and these effects were remarkably decreased by aprepitant.

The overproduction of ROS is deleterious to cell homeostasis, structures, and functions. Oxidative damages of cellular structures (e.g., proteins, nucleic acids, and lipids) adversely affect multiple processes associated with GBM pathogenesis including cell proliferation, apoptosis, migration, and resistance to therapy. Several endogenous antioxidant enzymes inhibit the production of cytotoxic ROS. Importantly, a significant reduction in antioxidants enzymes activity and subsequent increased oxidative damage has been observed in brain gliomas associated with aggressive tumors [10, 26–28]. Among antioxidant systems in the cell, the glutaredoxin system is deemed to be the major cellular redox buffer owing to its ability to provide an abundance of reducing equivalents in response to oxidative stress [29]. The glutaredoxin system is composed of tripeptide glutathione (GSH), glutaredoxin (GRX), glutathione reductase (GR), and an electron donor, NADPH. GRX is a small thiol/disulfide oxidoreductase enzyme that reduces mixed disulfides between oxidized protein thiol groups and glutathione (S-glutathionylated proteins), thereby, maintaining glutathione homeostasis. Oxidized GRX is reduced by the oxidation of GSH. GR in turn returns oxidized GSH (GSSG) to its reduced form by NADPH. The GRX system has been also shown to regulate the activity of the antioxidant enzyme glutathione peroxidases (GPx) [30, 31]. An important point, GRX has a key role in healthy neural development, and dysregulation of GRX is associated with the development of human neurological disorders and brain tumors [32, 33]. The neurotoxicity of methylmercury (MeHg) in human astrocytoma cells is associated with inhibition of GRX [32]. Given these points, our finding that SP increased ROS generation and reduced GRX expression and activity highlights the importance of GRX in controlling ROS levels in GBM cells. Furthermore, SP-mediated alteration of ROS and GRX and further inhibition of these effects by aprepitant might reflect the involvement of SP/NK1R signaling in GBM pathogenesis through oxidative stress. However, further validations in future studies are required to elucidate more precisely the redox regulatory mechanism mediated by SP/NK1R signaling in GBM and verify the therapeutic potential of targeting this system. In this line, several studies have also suggested that NK1R antagonists would provide benefits in various clinical disorders via regulating oxidative stress. In animal models of traumatic brain injury (TBI), NK1R antagonist, L-733,060, exerts favorable effects on the neurological outcome through inhibiting SP-mediated oxidative stress and neuroinflammation [24]. Intraarterial administration of NK1R antagonists, CP-96345, reduces ROS generation and thus further attenuates SP-mediated hyperactivity of rat bladder [34]. Liu et al. found that aprepitant significantly inhibits NOX4-mediated ROS production, suggesting the therapeutic potential of aprepitant in rheumatoid arthritis through regulating oxidative stress [22]. Consistently, our findings also verified the redox-modulating properties of aprepitant; however, its clinical significance in GBM requires further validation studies.

When it comes to oxidative stress and particularly ROS production in cancer cells, there is always a matter of concern. It is well-established that excessive production of ROS within a malignant cell could induce mitochondrial damage-mediated apoptotic cell death [35] and suppress the proliferative capacity of the cells through stimulating DNA damage responses [36]. In fact, many chemotherapeutic drugs such as doxorubicin recruit this mechanism to eliminate the population of cancer cells [37]. There is a wealth of evidence suggesting that NK1R antagonists also reduce the viability of malignant cells by increasing ROS's intracellular levels. In acute myeloid leukemia, Chentao et al. indicated that inhibiting NK1R increases the intracellular levels of ROS and in turn induces mitochondria-mediated apoptotic cell death [38]. SR140333, an antagonist of NK1R, was reported to induce its anticancer effects by increasing ROS's intracellular levels. The importance of ROS in the anticancer property of SR140333 was to the degree that Trolox, an ROS scavenger, could prevent SR140333-mediated induction of cell death [38]. Despite these findings, it should not be forgotten that the excessive production of ROS within cancer cells is not always beneficial, as the constant presence of this reactive oxygen species could increase the risk of both chemoresistance and cancer metastasis [39]. The harmful effect of ROS and its derivatives on the activity of thiol-containing proteins could also lead to the accumulation of harmful mutations in DNA, suggesting that these free radicals might act in favor of tumorigenesis [40].

Among the wide range of oncogenic signaling pathways that could guarantee ROS production in cancer cells, constant activation of NK1R is one of the most important ones [41]. In esophageal squamous cell carcinoma, it has been reported that SP/NK1R interaction activates the PI3K/Akt and NF-κB signaling axes and thereby reinforces the survival of malignant cells through the production of ROS [15]. In this regard, it has also been declared that blockage of NK1R could reduce the survival of cancer cells by reducing ROS. Aprepitant was shown to decrease the viability of U87 cells by elevating the expression of catalase superoxide dismutase, two important scavenging enzymes of ROS [42]. Or, in triple-negative breast cancer cells, aprepitant attenuated doxorubicin-induced ROS and, thereby, prevented cardiomyopathy [43]. In colon cancer, Ghahremanloo et al. proposed that through suppressing the PI3K/Akt signaling axis and diminishing intracellular levels of ROS, aprepitant decreased the survival of SW480 cells [44]. It is hard to conclude, but perhaps, NK1R-induced ROS production in some tissues will contribute to tumorigenesis. The possible explanations concerning this discordance are the different oxidative capacities of cancer cells, different experimental conditions, different treatment doses of SP and aprepitant and duration of exposure, and finally the activation of or inactivation of other redox regulatory pathways in the different tumor microenvironment.

It is believed that high levels of ROS may play a dual role in cancer development by eliciting both proapoptotic and prosurvival effects according to intensity and duration of exposure. With this in view, these results support the concept that aprepitant can efficiently target the dual actions of ROS concerning the applied dose. However, the discrepancy related to the redox-modulating properties of aprepitant in different tumor cells remains to be further elucidated. Interestingly, hypomagnesemia occurred following treatment with some of the common chemotherapeutic drugs, which can increase the neuronal release of SP via activation of N-methyl-D-aspartate (NMDA) receptor, promote the induction of oxidative and inflammatory responses [45-47]. Accordingly, blocking NK1R using aprepitant improves cardiac functions in erlotinib-treated rats by reducing erlotinib-induced hypomagnesemia and subsequent SP elevation, thereby, inhibiting SP-induced oxidative/inflammation stress [48].

Given that aprepitant approved by the United States Food and Drug Administration and currently used as antiemetic drugs, these results offer new insight for applying aprepitant as anticancer agents [16, 49].

In summary, we report that SP activation of NK1R significantly affected the redox status of GBM cells by inhibiting the GRX antioxidant system and further increasing ROS generation. The present findings could also open new avenues for therapeutic modulation of redox status by aprepitant in GBM. Further in vitro and in vivo experiments should be performed to verify the redox regulatory mechanism mediated by SP/NK1R signaling and the clinical significance of aprepitant as a redox-modulating strategy in GBM patients.