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Introduction

The incidence of gastric cancer (GC) has declined significantly worldwide over the past half-century (1). Nevertheless, GC remains a global health concern, as it is the fifth leading cancer and third most common cause of cancer-related mortality worldwide (2). The development and progression of GC are thought to be multistep processes involving several genetic mutations in proto-oncogenes or tumor-suppressor genes (3). Unfortunately, the majority of patients with GC have a poor prognosis, as this type of cancer is typically diagnosed at a late stage of the disease (4). Therefore, further insight into the molecular mechanisms underlying GC progression may identify novel therapeutic targets and improve the prognosis of GC.

Currently, cytokine-based immunotherapy has been shown to influence the progression of GC (5). Several studies have suggested that interleukin-17 (IL-17) (6), tumor necrosis factor α (TNFα) (7) and IL-23 (8) are associated with the metastasis and prognosis of GC. Functional studies have indicated that cytokine immunotherapy can reduce cancer cell apoptosis, invasion and proliferation, suggesting that cytokine immunotherapy may be considered an efficient tool with which to intervene against cancer development (9,10). However, several studies have also indicated that immunotherapy results in treatment failure due to drug resistance (7,9,11). Increasing evidence suggests that resistance to cytokine immunotherapy contributes to the tumor escape from the apoptotic signal (12). Therefore, determining methods with which to enhance cytokine-induced apoptosis is the key to enhancing the treatment efficacy.

Mitochondria, which are organelles present in all cells of the human body apart from erythrocytes, play a pivotal role in energy production (13). In addition to this vital function, mitochondria are involved in other complex processes, such as metabolism, maintaining the homeostatic control of reactive oxygen species (ROS) and nitrogen species production, calcium regulation, cellular metabolism and proliferation, as well as cell division and programmed cell death (apoptosis) (14–16). Previous studies have indicated that cytokines, particularly TNFα (7,9), induce GC cell apoptosis via the activation of the caspase-9-dependent apoptotic pathway. These data illustrate that the mitochondria are a target of cytokine-based therapies. Therefore, we hypothesized that resistance to treatment may involve mitochondrial protection. Recent studies have confirmed that mitophagy functions as the housekeeper for damaged mitochondria (17,18). Mitophagy labels injured mitochondria via LC3II and facilitates the entry of the injured mitochondria to the lysosome, leading to the removal of deficient mitochondria (19,20). Accordingly, mitophagy is capable of blocking the mitochondrial apoptotic signal. Based on the protective role played by mitophagy in mitochondrial apoptosis, we therefore wished to determine whether mitophagy is involved in TNFα-based immunotherapeutic resistance, and if so, we wished to elucidate the molecular links between mitophagy and mitochondrial protection under TNFα treatment.

Nuclear receptor subfamily 4 group A member 1 (NR4A1), the subfamily of NR4A orphan receptors, plays an essential role in metabolic processes, inflammation and the central nervous system (21). Previous studies have demonstrated that NR4A1 is downregulated in multiple human tumors, including acute leukemia and breast cancer (22,23). It acts as an anti-oncogenic factor that reduces cancer cell and tumor growth and migration/metastasis (24). A recent study illustrated that NR4A1 regulates mitophagy activity (21). Based on this, we aimed to determine whether NR4A1 has the ability to increase the vulnerability of GC in response to TNFα treatment via the modification of mitophagy. Therefore, the aim of this study was to investigate the mechanisms underlying resistance to TNFα-based immunotherapy, particularly as regards the role of protective mitophagy in mitochondrial apoptosis. In addition, we examined whether NR4A1 affects the susceptibility of GC cells treated with TNFα via the regulation of mitophagy.

Materials and methods

Cell culture and treatment

The human GC cell line, AGS, was purchased from the American Type Culture Collection (ATCC; Manassas, VA, USA). The AGS cells were cultured in Dulbecco's modified Eagle's medium (DMEM; Gibco Life Technologies, Carlsbad, CA, USA) supplemented with 10% fetal bovine serum (FBS), 2 mM L-glutamine, 100 U/ml penicillin, and 100 lg/ml streptomycin at 37°C in a humidified atmosphere containing 5% CO2. To suppress and activate the c-Jun N-terminal kinase (JNK) pathway, SP600125 (SP, 10 μM) and Anisomycin (Ani, 10 μM) (both from Selleck Chemicals, Houston, TX, USA) were used, respectively to treat the cells for ~2 h at room temperature. To activate mitophagy, carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP, 3 μM; cat. no. C2920; Sigma-Aldrich, St. Louis, MO, USA) was used for ~1 h at room temperature.

Immunofluorescence staining and fluorescence microscopy imaging

The cells were washed with phosphate-buffered saline (PBS) and fixed with 4% paraformaldehyde for 30 min. The cells were incubated with the primary antibody at 4°C overnight. The cells were then washed with PBS 3 times and stained with fluorescent secondary antibody (Alexa-Fluor 488 donkey anti-rabbit secondary antibody (1:1,000, cat. no. A-21206; Invitrogen, Carlsbad, CA, USA) at 37°C for 30 min in the dark. DAPI (cat. no. 28718-90-3; Sigma-Aldrich) was used for nuclear staining, as previously described (25). After fluorescence quenching, the images were acquired with the same exposure settings using a fluorescence microscope with standard excitation filters (Olympus Corp., Tokyo, Japan). The primary antibodies used in the present study were as follows: translocase of outer mitochondrial membrane 20 (Tom20; #ab78547), lysosomal-associated membrane protein 1 (LAMP1; #ab24170), cytochrome c (Cyt-c; (#ab133504) (all from Abcam, Camridge, MA, USA), Parkin (#2132) and p-JNK (#2656) (both from Cell Signaling Technology, Danvers, MA, USA). PI staining was conducted using live cells as previously described (26).

Western blot analysis

The cells were trypsinized, washed with PBS and then lysed. The lysates were incubated at 4°C for 20 min and centrifuged at 12,000 × g for 15 min. Equal amounts of lysate (20 or 30 μg) were resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and transferred onto polyvinylidene difluoride membranes (Millipore, Billerica, MA, USA). The membranes were then blocked in 5% non-fat skim milk/TBST [20 mM Tris-HCl (pH 7.4), 150 mM NaCl, and 0.1% Tween-20] at room temperature for 2 h and detected with primary antibodies at room temperature for 2 h, as previously described (27). The membranes were then blotted for 1 h at room temperature with an appropriate horseradish peroxidase-linked horseradish peroxidase-conjugated secondary antibodies (Beyotime Institute of Biotechnology, Shanghai, China), followed by enhanced chemiluminescence western blot detection reagents (Amersham Pharmacia Biotech, Piscataway, NJ, USA). The primary antibodies used were as follows: pro-caspase-3 (1:1,000, #9662), cleaved caspase-3 (1:1,000, #9664) (both from Cell Signaling Technology), survivin (1:1,000, #ab469), cellular inhibitor of apoptosis protein-1 (c-IAP1; 1:1,000, #ab25939), Bad (1:2,000, #ab90435) (all from Abcam), Bax (1:2,000, #5023; Cell Signaling Technology), caspase-9 (1:1,000, #ab32539; Abcam), LC3II (1:1,000, #3868; Cell Signaling Technology), p62 (1:1,000, #ab56416; Abcam), Beclin1 (1:1,000, #3495), autophagy-related 5 (ATG5; 1:1,000, #12994), Parkin (1:1,000, #2132) and JNK (1:1,000, #2656) (all from Cell Signaling Technology), complex III subunit core (CIII-core2, 1:1,000; cat. no. 459220; Invitrogen), complex II (CII-30, 1:1,000, #ab110410), complex IV subunit II (CIV-II, 1:1,000, #ab110268), complex I subunit NDUFB8 (CI-20, 1:1,000, #ab110242) (all from Abcam). The blots were visualized using enhanced chemiluminescence reagents (BeyoECL Plus; Beyotime Institute of Biotechnology). The mean densities of the bands were represented as the optical density (OD) in units per square millimeter and normalized to those of β-actin (Quantity One, version 4.6.2; Bio-Rad Laboratories, Inc., Hercules, CA, USA).

Terminal deoxynucleotidyltransferase-mediated dUTP nick end labelling (TUNEL) staining

The apoptosis of the cells was detected using a TUNEL assay kit according to the manufacturer's instructions (Roche Applied Science, Indianapolis, IN, USA). In brief, the fixed cells were permeabilized with proteinase K for 15 min at room temperature, as previously described (28). The cells were then treated with 3% H2O2 to block endogenous peroxidase and then incubated with equilibration buffer and terminal deoxynucleotidyl transferase (TdT) enzyme. Finally, the cells were incubated with anti-digoxigenin-peroxidase conjugate. The cells were examined under a light microscope (magnification, ×100; BX51; Olympus Corp., Tokyo, Japan).

Measurement of intracellular ROS levels

Intracellular ROS levels were measured by flow cytometry using the peroxide-sensitive fluorescent probe 2′7′-dichlorofluorescein diacetate (DCF-DA). Briefly, the cells (3.5×106 cells/well) were plated in 6-well culture plates for 6 h. The cells were then incubated with DCF-DA (25 μM) in PBS at 37°C for 30 min, washed twice with PBS, and detached by treatment with trypsin-EDTA. The detached cells were collected and resuspended in PBS, and the fluorescence intensity of the cells was measured using a flow cytometer (BD FACSVerse; BD Biosciences, San Jose, CA, USA) (29).

Measurement of mitochondrial membrane potential (MMP)

The MitoProbe™ JC-1 assay kit (Thermo Fisher Scientific Inc., Waltham, MA, USA) was used to detect changes in MMP. The assay was performed according to the manufacturer's instructions, and the results of the assay were obtained using the BD FACSAria II flow cytometer (BD Biosciences). JC-1 forms J-aggregates emitting red fluorescence at 590 nm in healthy mitochondria and J-monomers emitting green fluorescence at 490 nm in depolarized mitochondria. An increased ratio of J-monomers indicates mitochondrial damage. Carbonyl cyanide m-chlorophenyl hydrazone (CCCP, 50 μM), a disruptor of MMP, was used as a positive control (30).

Detection of cell viability

Cell viability was detected by determining caspase-3 and -9 activity, as well as by MMT assay. Caspase-9 and -3 activities were determined using caspase-assay kits (Beyotime Institute of Biotechnology), which detect the production of the chromophore p-nitroanilide after its cleavage from the peptide substrate DEVD-p-nitroanilide and LEHD-p-nitroanilide (31). MMT assay was conducted as previously described (26). Briefly, the cells were seeded in a 96-well plate at a density of 1×106 cells in triplicate for 7 days at 37°C with 5% CO2. Subsequently, 20 μl 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) (5 mg/ml; pH 7.4; Sigma-Aldrich) were added to the cells for 4 h. The supernatants were then discarded and 100 μl dimethyl sulfoxide (Sigma-Aldrich) was added to each well for 10 min. The OD of the samples was measured at an absorbance of 490 nm using a spectrophotometer (Epoch 2; BioTek Instruments, Inc., Winooski, VT, USA). The assay was repeated 3 times.

Detection of oxidative stress

Malondialdehyde (MDA) is an end product of lipid peroxidation that results from oxidative damage and reflects the level of cellular damage during oxidative injury. Glutathione (GSH), glutathione peroxidase (GPx) and superoxide dismutase (SOD) are important antioxidants that scavenge free radicals. MDA activity was assessed using a Lipid Peroxidation (MDA) assay kit (Sigma-Aldrich) according to the manufacturer's instructions. The results are expressed as nmol/g tissue. GPx activity (a marker of the systemic antioxidant status) was measured using a Glutathione Peroxidase assay kit (Cayman Chemical Co., Ann Arbor, MI, USA), as per the manufacturer's instructions. The SOD and GSH concentrations were measured using commercial kits (Beyotime Institute of Biotechnology) following the manufacturer's instructions and as previously described (32).

Measurement of lactate production, glucose uptake, mitochondrial respiratory function and ATP production

Extracellular lactate levels were measured using the cell culture medium with a lactate assay kit (#K607-100; BioVision, Milpitas, CA, USA). Intracellular glucose levels were measured using cell lysates with a glucose assay kit (#K606-100; BioVision). Adenosine triphosphate (ATP) levels were measured using an ATP assay kit (Celltiter-Glo Luminescent Cell Viability assay; Promega, Madison, WI, USA). The uptake of glucose, the production of lactate and the levels of ATP were all measured according to the manufacturer's instructions and as previously described (33). Mitochondrial respiration was initiated by the addition of glutamate/malate to a final concentration of 5 and 2.5 mmol/l, respectively. Signal transducer and activator of transcription 3 (Stat3) respiration was initiated by the addition of ADP (150 nmol/l); stat4 was measured as the rate of oxygen consumption following ADP phosphorylation.

Construction of adenovirus for NR4A1 overexpression

To induce the overexpression of NR4A1, the pDC316-mCMV-NR4A1 plasmid was purchased from Vigene Bioscience (Rockville, MD, USA) and was transfected with the framework plasmid (1:1) into 293T cells (purchased from ATCC) using Lipofectamine 2000 (Thermo Fisher Scientific, Inc.). Transfection was carried out for 48 h and the viral supernatant was then collected and identified by PCR. Following amplification, the supernatant was acquired again and filtered through a 0.45-μm filter to obtain the adenovirus-NR4A1 (Ad-NR4A1). Subsequently, Ad-NR4A1 was used to transfect the cells and the cells stably expressing NR4A1 were examined by western blot analysis.

Statistical analysis

The data are expressed as the means ± SD of at least 3 independent experiments. Statistical analysis was performed using one-way analysis of variance (ANOVA) followed by Bonferroni's multiple comparison test. A P-value <0.05 was considered to indicate a statistically significant difference.

Results

Overexpression of NR4A1 enhances cell death induced by TNFα

First, we used various concentrations of TNFα to induce a cell model of GC. Compared to the control group, TNFα reduced the viability of the human GC cell line, AGS, in a dose-dependent manner (Fig. 1A). As there was no difference observed between the 10 and the 20 ng/ml treatment groups, 10 ng/ml was used as the treatment concentration in the following experiments. To explore the role of NR4A1 in TNFα-mediated damage in GC, we examined changes in the expression of NR4A1. We found that NR4A1 expression progressively decreased following treatment with TNFα (Fig. 1B and C). Subsequently, to obtain information as to the role of NR4A1 in cancer cell damage, we overexpressed NR4A1 by transfection with adenovirus (Ad-NR4A1). We demonstrated that transfection with Ad-NR4A1 increased the expression of NR4A1 which had been decreased by TNFα (Fig. 1B and C). Subsequently, we performed TUNEL assay to observe the effects of NR4A1 overexpression on TNFα-induced cell death. Unexpectedly, the overexpression of NR4A1 further increased cellular death when compared to TNFα treatment alone (Fig. 1D and E). However, the number of TUNEL-positive cells in the control group transfected with control adenovirus (Ad-ctrl) did not differ significantly from that of the cells treated with TNFα alone (Fig. 1D and E).

Figure 1 Effects of nuclear receptor subfamily 4 group A member 1 (NR4A1) on cell viability. (A) Cell viability was detected by MTT assay. (B and C) Alterations in NR4A1 in response to tumor necrosis factor α (TNFα). TNFα induced the donwnregulation of NR4A1. The overexpression of NR4A1 was carried out by adenoviral transfection. (D and E) TUNEL assay was used to evaluate the role of NR4A1 in cell death. NR4A1 has the ability to further reduced cellular activity. (F and G) Alterations in caspase-3 and caspase-9 activities. (H-L) Alterations in the levels of proteins related to mitochondrial apoptosis. *P<0.05 vs. Ctrl group; #P<0.05 vs. TNFα group.

To exclude the role of accidental factors, we also examined the activities of caspase-3 and -9 (Fig. 1F and G). The results revealed that caspase-3 and -9 activities were -in accordance with the above-mentioned finding that the re-introduction (overexpression) of NR4A1 enhanced the sensitivity of GC cells to TNFα-mediated damage. Apart from the changes in protein activity, we also detected alterations in protein expression. Compared to the control group, TNFα elevated the expression of caspase-3 and -9 (Fig. 1H–L). However, the overexpression of NR4A1 further increased the levels of pro-apoptotic proteins, but reduced the expression levels of anti-apoptotic factors (survivin and c-IAP1) (Fig. 1H–L). Taken tother, these data indicated that regaining NR4A1 expression further increased GC cell death induced by TNFα.

NR4A1 facilitates TNFα-evoked mitochondrial injury. It has been reported that NR4A1 induces mitochondrial damage (34). In this study, to determine whether mitochondrial damage is responsible for the promoting effects of NR4A1 on TNFα-induced cell death, we detected mitochondrial function. MMP is the source for the mitochondria to generate ATP, which fuels a cell's biological function. However, we observed that NR4A1 further reduced MMP, as revealed by less red fluorescence and more green fluorescence (Fig. 2A and B). Furthermore, the collapse of MMP may be a result of the mitochondrial permeability transition pore (mPTP) opening (35). As shown in Fig. 2C, compared to the control group, TNFα induced more mPTP opening. However, regaining NR4A1 expression further contributed to mPTP opening.

Figure 2 Nuclear receptor subfamily 4 group A member 1 (NR4A1) augments the mitochondrial damage. (A and B) Changes in mitochondrial membrane potential (MMP). The red fluorescence indicated the normal mitochondria with high membrane potential. The green fluorescence was the marker of damaged mitochondria with reduced membrane potential. (C) Changes in mitochondrial permeability transition pore (mPTP) opening. NR4A1 further enhanced the mPTP opening ratio. (D and E) Changes in the levels of cellular ROS. NR4A1 augmented cellular oxidative stress in the presence of tumor necrosis factor α (TNFα). (F–I) Changes in the levels of MDA, GSH, SOD and GPx. (J) The leakage of Cyt-c from the mitochondria into the cytoplasm. *P<0.05 vs. Ctrl group; #P<0.05 vs. TNFα group.

The consequence of mPTP opening and the reduction of MMP is evidence of mitochondrial metabolism disruption and the initiation of apoptotic signaling (36). In this study, we first focused on mitochondrial apoptosis. Mitochondrial apoptosis is characterized by excessive ROS production and subsequent cellular oxidative injury, which contributes to the leakage of pro-apoptotic factors from the mitochondria (29). In this study, using flow cytometric analysis, we found that TNFα treatment enhanced the levels of ROS (Fig. 2D and E). Furthermore, the overexpression of NR4A1 amplified ROS overproduction. In response to excessive ROS production, the levels of antioxidant factors, such as GSH, SOD and GPX, were reduced, suggesting an imbalance of the redox status (Fig. 2F–I). The enhancement of NR4A1 further consumed antioxidant factors, but generated greater levels of MDA, an end product of lipid peroxidation. Finally, we observed greater amounts of Cyt-c released into the cytoplasm in response to TNFα treatment, which was similar to the results in the NR4A1 overexpression group (Fig. 2J). Collectively, these data suggested that NR4A1 enhanced TNFα-induced mitochondrial damage.

NR4A1 further suppresses mitochondrial energy metabolism

In addition to cellular apoptosis, mitochondrial energy production is another determinant for cancer growth and cellular survival (37). Considering that NR4A1 promoted mitochondrial apoptosis, we therefore wished to determine whether NR4A1 hinders mitochondrial energy production in the presence of TNFα. First, compared to the control group, TNFα abated the content of ATP, which was further reduced once NR4A1 was expression was recovered (Fig. 3A), suggesting that NR4A1 has the ability to influence mitochondrial energy generation. Since ATP is produced via the mitochondrial respiratory complex, we therefore examined changes in the mitochondrial respiratory complex. Compared to the control group, TNFα suppressed the contents of the mitochondrial respiratory complex (Fig. 3B–F). However, NR4A1 further suppressed the expression of these factors of the mitochondrial respiratory complex (Fig. 3B–F). Correspondingly, mitochondrial Stat3 (Fig. 3G) and Stat4 (Fig. 3H) respiratory functions also declined when NR4A1 was overexpressed. These data indicated that NR4A1 impaired mitochondrial respiratory function. The reduced mitochondrial energy metabolism may be associated with a decrease in glucose intake and lactate production. Thus, we measured the content of glucose in medium. As shown in Fig. 3I and J, TNFα indeed reduced glucose consumption and lactate generation, suggesting the termination of glycometabolism in GC cells. By contrast, the overexpression of NR4A1 further limited glucose intake and lactate production. Taken together, these data indicated that NR4A1 enhanced the suppressive effects of TNFα on mitochondrial energy metabolism.

Figure 3 Nuclear receptor subfamily 4 group A member 1 (NR4A1) impaires mitochondrial metabolism. (A) Changes in cellular ATP production. (B) Changes in the levels of contents of the mitochondrial respiratory complex. NR4A1 had the ability to further reduce the contents of mitochondrial respiratory complex when compared to the tumor necrosis factor α (TNFα) alone group. (C–F) Quantitative analysis of the expression of contents of the mitochondrial respiratory complex. (G and H) The Stat3 and Stat4 respiratory rate, which was the reflection of mitochondrial energy metabolism efficiency. NR4A1 further abated mitochondria-related energy production efficiency. (I) Cellular glucose uptake was further suppressed in response to NR4A1 overexpression when compared to the TNFα alone group. (J) The extracellular lactate was measured using the cell culture medium with lactate assay kit. NR4A1 attenuated the production of lactate. *P<0.05 vs. Ctrl group; #P<0.05 vs. TNFα group.

NR4A1 inhibits mitophagy to enhance the sensitivity of GC cells to TNFα

To determine the mechanisms through which NR4A1 enhanced TNFα-induced mitochondrial damage, we focused on the mitochondria repairing system itself, mitophagy. Damaged mitochondria activate mitophagy, which facilitates the removal and clearing of malfunctioning mitochondria via the lysosome, leading to the preservation of mitochondrial quantity and quality (38). Thus, mitophagy counteracts TNFα-induced mitochondrial damage (39). Based on this, we first investigated mitophagy activity. In response to TNFα treatment, the levels of mito-LC3II, Beclin1, ATG5 and p62 were increased (Fig. 4A–E), suggestive of mitophagy activation. However, the overexpression of NR4A1 reversed this tendency, illustrating the inhibitory effect of NR4A1 on mitophagy. To provide further direct evidence of mitophagy, we used immunofluorescence to label the mitochondria and lysosomes at the same time. As shown in Fig. 4F and G, in the control group, few mitochondria were merged with the lysosomes. Following treatment with TNFα, most of the mitochondria were contained by lysosomes, indicative of mitophagy activation. However, the re-introduction of NR4A1 suppressed the lysosome-mitochondria interaction, displaying the inhibitory role of NR4A1 on mitophagy. Subsequently, to determine whether mitophagy inhibition contributes to the excess mitochondrial damage following NR4A1 overexpression, we used FCCP to activate mitophagy. After the activation of mitophagy, caspase-9 activity increased despite the overexpression of NR4A1 (Fig. 4H). These data indicated that mitophagy, the protective system of the mitochondria, was activated by TNFα and contributed to mitochondrial protection and resistance to apoptosis. The recovery of NR4A1 expression inhibited mitophagy and enhanced the sensitivity of GC cells to TNFα-induced cell death.

Figure 4 Nuclear receptor subfamily 4 group A member 1 (NR4A1) regulates mitophagy to enhance cellular apoptosis in response to tumor necrosis factor α (TNFα) treatment. (A) Changes in the levels of mitophagy markers. (B-E) Quantitative analysis of the expression of ATG5, p62, Beclin1 and mitochondrially-located LC3II (mito-LC3II). (F and G) Co-staining of mitochondria and lysosomes. Mitophagy was activated under TNFα treatment but significantly suppressed by NR4A1 application. (H) Changes in caspase-9 activity with the activator of mitophagy. FCCP (5 μM) was used to activate mitophagy 2 h prior to treatment. *P<0.05 vs. Ctrl group; #P<0.05 vs. TNFα group; @P<0.05 vs. TNFα+Ad-NR4A1 group.

Mitophagy is activated via Parkin through JNK

Finally, we wished to determine the underlying signal that modulates mitophagy activity under TNFα treatment conditions. Parkin is the primary receptor for mitophagy, and it mediates the damaged mitochondrial removal in several types of cells (40). Therefore, we first examined changes in Parkin expression. We found that Parkin expression was upregulated by TNFα treatment, but was decreased upon the overexpression of NR4A1 (Fig. 5A–C). This information established the regulatory effects of TNFα on Parkin. Furthermore, to explain the mechanisms through which TNFα upregulates Parkin expression, we focused on JNK. JNK is considered the upstream trigger of Parkin expression (41). To determine whether TNFα regulates Parkin-required mitophagy via JNK, an inhibitor (SP600125, SP) and activator (Ani) of JNK were used. As shown in Fig. 5A and B, compared to the control group, TNFα treatment increased JNK activity as evidenced by greater levels of phosphorylated JNK. However, NR4A1 overexpression suppressed TNFα-induced JNK activation. Next, upon the inhibition of JNK via SP600125 under TNFα treatment, JNK activity was blocked, and Parkin expression was also inhibited (Fig. 5A–C). By contrast, the activation of JNK in the cells overexpressing NR4A1 increased Parkin expression (Fig. 5A–C). In addition, the results of co-immunofluorescence of p-JNK and Parkin were also in agreement with the above-mentioned results (Fig. 5F). These data indicated that the TNFα-induced activation of the JNK pathway was responsible for Parkin upregulation.

Figure 5 Nuclear receptor subfamily 4 group A member 1 (NR4A1) inhibits the activation of the c-Jun N-terminal kinase (JNK)/Parkin pathway to inactivate mitophagy. (A) The influence of NR4A1 on JNK and Parkin. The activator of JNK partly abated the effects of NR4A1 on the mitophagy receptor, Parkin. (B and C) Quantitative analysis of the expression of JNK and Parkin. (D and E) Quantitative analysis of the expression of mito-LC3II and p62. Inhibition of JNK by SP reduced mitophagy activation under tumor necrosis factor α (TNFα) treatment conditions. However, the re-activation of JNK re-activated mitophagy despite of NR4A1 overexpression. (F) Co-staining of p-JNK and Parkin. (G) PI assay was used as a marker of cellular apoptosis. The activation of JNK attenuated the pro-apoptotic effects of NR4A1 as evidenced by less PI-positive cells. *P<0.05 vs. Ctrl group; #P<0.05 vs. TNFα group; @P<0.05 vs. TNFα+Ad-NR4A1 group.

To provide further direct evidence of the role of JNK in Parkin-mediated mitophagy, we examined markers of mitophagy. After the blockade of JNK under TNFα treatment, mitophagy was reduced, as evidenced by less mito-LC3II and p62 expression (Fig. 5A, D and E), which was similar to the results observed in the NR4A1 group. By contrast, the activation of JNK re-evoked the expression of mito-LC3II and p62 despite the overexpression of NR4A1 (Fig. 5A, D and E).

Finally, to determine whether JNK is also involved in the resistance of GC cells to TNFα-induced damage, we used PI to detect cellular damage in cells treated with or without JNK activator. As shown in Fig. 5G, the inhibition of JNK increased the number of TNFα-induced PI-positive cells. However, following NR4A1 overexpression, the activation of JNK reduced the number of PI-positive cells. Taken together, these data suggested that JNK/Parkin-dependent mitophagy was responsible for the therapeutic resistance observed with TNFα and that NR4A1 enhanced the fatal effects of TNFα via the inhibition of JNK/Parkin-dependent mitophagy.

Discussion

GC is one of the leading causes of cancer-related mortality worldwide (42). The number of patients with GC increases each year, and many of them are diagnosed at the advanced stages of the disease with overt metastasis, thus missing the best opportunity for curative surgery (43). Although patients during the early stages of GC can be cured by surgery, the majority of patients with GC are diagnosed at the advanced stages of the disease and present with extensive invasion, lymphatic metastasis and other organ metastases (44). Understanding tumorigenesis may aid in diagnosis the disease stage and even at earlier stages. GC tumorigenesis is a multistep and multifactorial process involving diverse genetic alterations, including the inactivation of tumor suppressor genes, the activation of oncogenes and the abnormal expression of cancer-related genes. Thus, it is crucial to investigate the novel mechanisms that govern the development of GC to elucidate the molecular mechanisms and develop effective therapeutic strategies.

Although immunotherapy with IL-2, TNFα and interferon-α (IFN-α) has been applied to atteunate or intervene with GC, as the response rate in patients with the disease to such treatment is only 10–20% (5,45). In this study, we demonstrated that NR4A1 enhanced the TNFα therapeutic efficacy by augmenting cellular apoptosis and reducing drug resistance through mitophagy. To the best of our knowledge, this is the first study to describe the molecular signal of NR4A1 as the adjuvant to enhance cytokine-based immunotherapy for GC. We found that the enhancement of NR4A1 further elevated GC cell apoptosis induced by TNFα. On the one hand, NR4A1 augmented cell death via strengthening caspase-9-dependent mitochondrial apoptosis. NR4A1 treatment induced excessive oxidative stress and evoked extensive mPTP opening. After mPTP opening, the pro-apoptotic factors in the mitochondria, such as Cyt-c, were released from the mitochondria into the cytoplasm. On the other hand, NR4A1 impaired mitochondrial energy production by suppressing the mitochondria respiratory complex, leading to the collapse of mitochondria-related energy metabolism. Through the two above-mentioned mechanisms, NR4A1 enhanced GC cellular damage in response to TNFα treatment. Considering that NR4A1 was downregulated following TNFα treatment, according to our findings, the enhancement of NR4A1 would be useful for tumor treatments in clinical practice.

In the present study, we also investigated the mechanisms through which NR4A1 sensitizes GC cells to TNFα. Apart from the energy production and death signal transmission, the mitochondria itself have a protection system to reduce excessive cellular damage, and this is mitophagy (46). Mitophagy neutralizes the damaged mitochondria via lysosomes, leading to the removal of bad mitochondria (47). It has been shown that mitophagy blocks mitochondrial apoptosis and promotes cellular survival (48). Moreover, mitochondria also employ mitophagy to provide the energy substrate to fuel mitochondria energy production (49). Therefore, through mitophagy, the mitochondria sweep out the defective mitochondria and offer nutrients to the cell. Given the important role of mitophagy in cellular protection, we aimed to determine whether mitophagy was involved in treatment failure or resistance. We found that TNFα treatment indeed increased the overlap of mitochondria and lysosomes and the markers of mitophagy, suggesting the activation of mitophagy. Furthermore, NR4A1 inhibited the mitophagy activity. By contrast, the activation of mitophagy influenced the pro-apoptotic effects of NR4A1. These findings indicate that mitophagy may be the molecular signal for the therapeutic insensitivity. However, further evidence is required to support this notion in vivo and in clinical practice.

In this study, we determined that the JNK/Parkin pathway was the upstream signal that was activated by TNFα to trigger mitophagy, and NR4A1 suppressed mitophagy via the inhibition of JNK. In the process of mitophagy, several factors a Bcl-2 interacting protein 3 (BNIP3), mitofusin 2 (Mfn2) and dynamin-1-like protein (Drp1) (50–53). In our study, Parkin was upregulated and led to mitophagy activation. The underlying mechanism was attributed to the JNK pathway. TNFα increased JNK activity, which activated Parkin. Therefore, these findings explain the molecular signal for mitophagy activation by TNFα treatment, which offers a potential target to intervene against mitophagy.

In the present study, we illustrated the important role of NR4A1 in GC cell apoptosis induced by TNFα (Fig. 6). NR4A1 amplified the therapeutic sensitivity of GC to TNFα via targeting to the mitochondria. In this process, mitophagy was the indispensable element in the protection of the mitochondria and contributed to therapeutic resistance. NR4A1 abated mitophagy to further augment mitochondrial apoptosis and energy disruption, finally evoking greater cellular damage. These findings, on the one hand, explained the mechanisms underling the drug resistance of GC cells, and on the other hand, we provided an easy and effective method with which to elevate TNFα-mediated GC cell death. Thus, the enhancement of NR4A1 may be a practical and efficient adjuvant for GC treatment. However, further insight into this combination should be obtained to provide ample evidence for clinical applications.

Figure 6 Tumor necrosis factor α (TNFα) induced gastric cancer (GC) mitochondrial dysfunction as evidenced by Cyt-c leakage, mitochondrial membrane potential (MMP) collapse and energy metabolism disruption, which finally contributed to the activation of cellular death. Although TNFα treatment caused GC cell death, it also triggered protective mitophagy. TNFα increased c-Jun N-terminal kinase (JNK) activity which upregulated Parkin which then activated mitophagy to remove bad mitochondrial and inhibit cellular apoptosis. The overexpression of Nuclear receptor subfamily 4 group A member 1 (NR4A1) inhibited mitophagy, increasing the sensitivity of GC cells to TNFα-induced cell death.

Acknowledgments

This study was supported by grants from the project of Songjiang District Science and Technology Commission (no. 15SJGG29). The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.

References