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Introduction

Liver cancer is one of the most common causes of cancer-associated mortality worldwide, particularly in China (1). Hepatocellular carcinoma accounts for 70–85% of all liver cancer cases (2). There are currently several treatment options for liver cancer; however, none have been shown to be a cure-all, with regards to recurrence, patient survival and longevity (3). Patients with early-stage liver cancer can be treated with surgery, which may involve removal of part of the liver or transplantation; the 5-year survival rate in these cases often exceeds 70%. However, the majority of cases are not suitable for surgical treatment; the overall 5-year survival rate in these cases is ~15% (4). Until 2017, there was no systemic treatment option for patients with advanced liver cancer. Therefore, it is important to investigate the molecular mechanisms underlying liver carcinogenesis, and to identify novel approaches that can effectively inhibit liver cancer cell growth and metastasis.

Taurolidine (TRD) is a substance derived from the amino sulfonic acid taurine, which was originally used to treat peritonitis and catheter-associated infections (5). Previous studies have revealed that TRD inhibits proliferation of cancer cells and induces the differentiation of malignant cells, including glioblastoma (6,7), melanoma (8,9), mesothelioma (10,11), colon carcinoma (12,13), squamous cell esophageal carcinoma (14) and sarcoma cell lines (15,16). However, the effect and mechanism of TRD in liver cancer remain to be elucidated.

Gene associated with retinoid-interferon-induced mortality-19 (GRIM-19) is located on human chromosome 19p-13.1 and encodes a 16-kD protein comprising 144 amino acids (17). GRIM-19 serves an important role in the mitochondrial respiratory chain (18). Our previous study (19) revealed that GRIM-19 is an important regulator of tumor cell survival. Notably, downregulation of GRIM-19 and hyperactivation of phosphorylated (p)-signal transducer and activator of transcription 3 (STAT3) expression in liver cancer lesions are closely correlated with increased histological grading in liver cancer (19,20). Favorable pharmacokinetic and safety data have been reported for TRD following systemic application in healthy volunteers (21), as well as in patients with locally advanced gastric carcinoma and glioblastoma (22–24). However, the role of TRD in the induction of cell apoptosis remains to be fully elucidated. The mitochondria-dependent pathway (7,15,25–27) and the death receptor-associated pathway have been reported to be associated with TRD-induced apoptosis (14,16,27,28). However, it is not clear whether TRD can induce liver cancer cell apoptosis; therefore, the potential mechanism requires further exploration.

In the present study, the effects of TRD treatment on GRIM-19, cyclin D1, STAT3, p-STAT3, B-cell lymphoma 2 (Bcl-2) and Bcl-2-associated X protein (Bax) expression, and on the apoptosis of liver cancer cells, were investigated in vitro. The results indicated that treatment with TRD induced liver cancer cell apoptosis in a dose-dependent manner, which was associated with an increase in GRIM-19 expression and deactivating the STAT3 signaling pathway.

Materials and methods

Cell lines and transfection

HepG2 and SNU-423 human liver cancer cell lines were obtained from the Shanghai Preservation Center Cell Bank (Shanghai, China). HepG2 cells were cultured in Dulbecco's modified Eagle's medium (Gibco; Thermo Fisher Scientific, Inc., Waltham, MA, USA), whereas SNU-423 cells were cultured in Roswell Park Memorial Institute 1640 medium (Gibco; Thermo Fisher Scientific, Inc.), both media were supplemented with 10% fetal bovine serum (FBS; Gibco; Thermo Fisher Scientific, Inc.), 100 U/ml penicillin and 100 µg/ml streptomycin (Thermo Fisher Scientific, Inc.) at 37°C in an atmosphere containing 5% CO2. Small interfering RNA (siRNA) duplex oligoribonucleotides targeting the coding region of GRIM-19 were obtained from Thermo Fisher Scientific, Inc. HepG2 and SNU-423 cells (105 cells/well) were transfected with control siRNA (5′-UUCUCCGAACGUGUCACGUTT-3′, 50 nM) or specific GRIM-19 siRNA (5′-GCUUCAUGUGGUACACGUATT-3′, 50 nM) using Lipofectamine® 2000 (Thermo Fisher Scientific, Inc., Waltham, MA, USA), according to the manufacturer's protocol. The culture medium was replaced after 6 h of incubation. Subsequently, 48 h post-transfection, the cells were collected. GRIM-19 knockdown cells were pretreated with niclosamide (p-STAT3 inhibitor, 10 µM; 50-65-7; Selleck Chemicals, Houston, TX, USA) for 30 min, after which, 100 µM TRD (Santa Cruz Biotechnology, Inc., Dallas, TX, USA) was added for 24 h.

MTT assay

The effects of treatment with TRD on HepG2 and SNU-423 cell viability were measured using the MTT assay. Cells (103 cells/well) were cultured in 96-well plates overnight and were treated with TRD at various concentrations (10, 50, 100 and 20 0 µM) for 0, 12, 24, 48 or 72 h at 37°C in an atmosphere containing 5% CO2. During the last 4 h of culture, the cells in each well were exposed to 20 µl MTT (5 mg/ml; Merck KGaA, Darmstadt, Germany). The generated formazan in each well was then dissolved in 150 µl dimethyl sulfoxide (DMSO) and the absorbance was measured at 490 nm using a microplate reader, as previously described (29).

Flow cytometry

HepG2 cells were treated with various concentrations of TRD (50, 100 and 200 µM) and SNU-423 cells were treated with 200 µM TRD for 24 or 48 h at 37°C. In addition, GRIM-19 knockdown cells were treated with 100 µM TRD for 24 h at 37°C. Cells (3×105 cells/well) were then transferred to a tube in which Annexin V (5 µl) and propidium iodide (5 µl) (APOAF; Sigma-Aldrich; Merck KGaA, Darmstadt, Germany) were added, and the cells were incubated for 30 min at 37°C. Subsequently, the percentages of apoptotic cells in the different groups were determined by flow cytometry, using the BD FACSCelesta™ flow cytometer (BD Biosciences, San Jose, CA, USA). Data were analysed with FCS Express 4 (De Novo Software, Los Angeles, CA, USA). In addition, some cells were stained with trypan blue (ab233465; Abcam, Hong Kong, China) and surviving cells were counted under a microscope (Olympus IX51; Leeds Precision Instruments, Inc., Minneapolis, MN, USA) in a blinded manner. Cell survival rate was determined using the following equation: Survival rate (%) = number of unstained cells/total number of cells observed × 100. Survival rate of cells before TRD treatment exceeded 95% in all groups.

Immunofluorescence

HepG2 cells (105 cells/slide) were cultured on glass cover slips in 24-well plates overnight, fixed with 4% paraformaldehyde (Merck KGaA) in PBS and washed with PBS. Following permeabilization, the glass cover slips were blocked with 1 mM CaCl2, 1 mM MgCl2, 5% normal goat serum (SL038; Beijing Solarbio Science & Technology Co., Ltd., Beijing, China), 10% FBS and 2% bovine serum albumin (pH 7.6, A8010; Beijing Solarbio Science & Technology Co., Ltd.) in PBS and were incubated with mouse anti-human GRIM-19 antibody (1:1,000 dilution, ab110240; Abcam) at 4°C overnight. After washing with PBS containing 0.1% Triton X-100, the glass cover slips were incubated with tetramethyl rhodamine isothiocyanate-conjugated secondary antibodies (1:100 dilution, BA1089; Boster Biological Technology, Pleasanton, CA, USA) for 30 min at 37°C, followed by counterstaining with DAPI and mounting with Gel Mount™ Aqueous Mounting Medium (Merck KGaA). Images were acquired using a Leica™ fluorescence microscope equipped with a Leica™ camera (LEICA-DFC320; Leica Microsystems, Inc., Buffalo Grove, IL, USA).

Western blot analysis

Cells (3×105 cells/well) were cultured in 24-well plates overnight, and were harvested and lysed with radioimmunoprecipitation assay buffer (R0020; Beijing Solarbio Science & Technology Co., Ltd.). Protein concentrations were determined using the Bicinchoninic Acid Reagent (Thermo Fisher Scientific, Inc., after which, the cell lysates (30 µg/lane) were separated by 8–15% SDS-PAGE and transferred onto nitrocellulose membranes (GE Healthcare, Chicago, IL, USA). After blocking with 5% non-fat dry milk (for GRIM-19, STAT3, Bcl-2, Bax and cyclin D1) or 5% bovine serum albumin (for p-STAT3) in 10 mM Tris-Cl (pH 8.0), 150 mM NaCl and 0.05% Tween 20 for 1 h, the membranes were incubated with various antibodies overnight at 4°C. The bound antibodies were detected using horseradish peroxidase-conjugated immunoglobulin G secondary antibodies (1:5,000 dilutions, NA931 or NA9340; GE Healthcare) for 1 h at room temperature and visualized using enhanced chemiluminescence (GE Healthcare). The primary antibodies included mouse polyclonal anti-human GRIM-19 (1:1,000 dilution, ab110240; Abcam), mouse monoclonal anti-human STAT3 (1:5,000 dilution, ab119352; Abcam), rabbit monoclonal anti-human cyclin D1 (1:10,000 dilution, ab134175; Abcam), rabbit monoclonal anti-human Bcl-2 (1:1,000 dilution, ab32124; Abcam), rabbit monoclonal anti-human Bax (1:2,000 dilution, ab32503; Abcam), mouse monoclonal anti-human p-STAT3 (Tyr705) (1:2,000 dilution, #4113; Cell Signaling Technology, Inc., Danvers, MA, USA), and anti-β-actin (1:1,000 dilution, sc-517582; Santa Cruz Biotechnology, Inc.). The levels of target protein expression relative to the control protein, β-actin, were analyzed by densitometric analysis using a detection system (Amersham Imager 600; GE Healthcare).

Reverse transcription-quantitative polymerase chain reaction (RT-qPCR)

Total RNA was isolated from individual groups of cells using TRIzol® reagent (Invitrogen; Thermo Fisher Scientific, Inc.), according to the manufacturer's protocol. RNA was reverse transcribed into cDNA using the RT2 First Strand kit (330404; Qiagen, Inc., Valencia, CA, USA), according to the manufacturer's protocol. The expression levels of GRIM-19 mRNA transcripts relative to the β-actin control were determined by RT-qPCR using SYBR® Premix Ex Taq™ (Takara Biotechnology Co., Ltd., Dalian, China) and specific primers on the LC480 Real-time PCR system (Roche Applied Science, Penzberg, Germany). The thermocycling conditions were as follows: 95°C for 30 sec, followed by 40 cycles at 95°C for 5 sec, 60°C for 30 sec and 72°C for 30 sec, with a final extension step at 72°C for 5 min. The primer sequences were as follows: GRIM-19 (116 bp), forward 5′-CATCGACTACAAACGGAACTTG-3′, reverse 5′-GCTCACGGTTCCACTTCATTA-3′; and β-actin (196 bp), forward 5′-TGACGTGGACATCCGCAAAG-3′ and reverse 5′-CTGGAAGGTGGACAGCGAGG-3′. The data were quantified using the 2−ΔΔCq method (30,31).

Statistical analysis

All statistical analyses were performed using SPSS 10.0 software (SPSS, Inc., Chicago, IL, USA). All data are expressed as the means ± standard deviation. Multiple comparisons of means were determined using one- or two-way analysis of variance, followed by the post hoc Newman-Keuls test. P<0.05 was considered to indicate a statistically significant difference.

Results

TRD significantly reduces HepG2 and SNU-423 cell viability and induces apoptosis

To determine the effects of TRD on liver cancer cell viability, HepG2 cells were treated with various concentrations of TRD (10, 50, 100 and 200 µM) for 0, 12, 24, 48 or 72 h. Cell viability was then analyzed using the MTT assay. As shown in Fig. 1A, treatment with any concentration of TRD treatment for 12 h, and with 10 and 50 µM TRD at any time-point, did not significantly inhibit cell viability. However, TRD (100 and 200 µM) significantly inhibited cell viability in the 24-h treatment group (0.688±0.062 vs. 0.997±0.055, P<0.01; 0.652±0.074 vs. 0.997±0.055, P<0.01), the 48-h treatment group (0.987±0.037 vs. 1.386±0.063, P<0.01; 0.817±0.045 vs. 1.386±0.063, P<0.01) and the 72-h treatment group (1.245±0.125 vs. 1.772±0.116, P<0.01; 1.034±0.108 vs. 1.772±0.116, P<0.01). These findings suggested that TRD inhibited HepG2 cell viability in a dose-dependent manner. Notably, the proliferation of HepG2 cells in the TRD group was significantly reduced, but not completely stopped; similar results were observed in the SNU-423 cells line.

Figure 1. TRD significantly reduces HepG2 and SNU-423 cell viability, and induces apoptosis. (A) Cells were treated with various concentrations of TRD (10, 50, 100 and 200 µM) for 0, 12, 24, 48 or 72 h. Cell viability of the different groups was determined by the MTT assay. (B) HepG2 cells were treated with various concentrations of TRD (50, 100 and 200 µM) for 24 or 48 h and cell apoptosis was analyzed by flow cytometry. SNU-423 cells were treated with 200 µM TRD for 24 or 48 h to observe cell apoptosis by flow cytometry. Data are representative images or expressed as the means ± standard deviation of three separate experiments. *P<0.05 compared with the control group. &P<0.05 compared with the 24 h group. OD, optical density; TRD, taurolidine.

Based on these results, HepG2 cells were then treated with various concentrations of TRD (50, 100 and 200 µM) for 24 or 48 h, in order to observe cell apoptosis by flow cytometry. The results revealed that TRD (100 and 200 µM) significantly increased cell apoptosis in the 24- and 48-h groups (P<0.01). In addition, TRD treatment (100 and 200 µM) for 48 h significantly accelerated cell apoptosis compared with at 24 h (P<0.05; P<0.01). TRD treatment (50 µM) for 24 or 48 h had no effect on cell apoptosis (P=0.35; P=0.28). These findings indicated that TRD induced apoptosis of HepG2 cells in a dose- and time-dependent manner (Fig. 1B). Subsequently, SNU-423 cells were treated with TRD (200 µM) for 24 and 48 h, and cell apoptosis was detected by flow cytometric analysis. The results further confirmed that TRD induced apoptosis of liver cancer cells.

TRD upregulates GRIM-19 expression in HepG2 cells

GRIM-19 is a regulator of cell death, which is particularly associated with the stabilization and function of mitochondrial complex 1 (32). To understand the importance of GRIM-19 in TRD-induced apoptosis of liver cancer cells, HepG2 cells were treated with TRD at various concentrations (50, 100 and 200 µM) for 24, 48 or 72 h. The expression levels of GRIM-19 were detected by western blotting. The results revealed that treatment with TRD (100 and 200 µM) significantly increased GRIM-19 protein expression in HepG2 cells in the 24-h group (1.57±0.12 vs. 0.52±0.11, P<0.01; 2.35±0.31 vs. 0.52±0.11, P<0.01), in the 48-h group (1.99±0.22 vs. 0.68±0.17, P<0.01; 2.49±0.22 vs. 0.68±0.17, P<0.01) and in the 72-h group (2.44±0.22 vs. 0.97±0.35, P<0.01; 3.62±0.37 vs. 0.97±0.35, P<0.01). The effects of TRD on the upregulation of GRIM-19 protein expression appeared to be dose-dependent (Fig. 2A-C).

Figure 2. TRD upregulates GRIM-19 expression in HepG2 cells. HepG2 cells were treated with TRD at various concentrations (50, 100 and 200 µM) for 24, 48 or 72 h. (A-C) Protein expression level of GRIM-19 were detected in TRD-treated HepG2 cells by western blot analysis. (D-F) TRD treatment enhanced GRIM-19 mRNA expression in HepG2 cells. (G) HepG2 cells were cultured on glass cover slips and stained with anti-GRIM-19 and tetramethyl rhodamine isothiocyanate-conjugated secondary antibodies, followed by counterstaining with DAPI. GRIM-19 expression was characterized under a fluorescence microscope. Data are representative images (magnification, ×400) or are expressed as the means ± standard deviation of three separate experiments. *P<0.05 compared with the control group. GRIM-19, retinoic-interferon-induced mortality 19; TRD, taurolidine.

The mRNA expression levels of GRIM-19 mRNA in the different groups were detected by RT-qPCR. TRD treatment (100 and 200 µM) significantly enhanced GRIM-19 mRNA expression in HepG2 cells in the 24-h group (0.93±0.07 vs. 0.36±0.05, P<0.01; 1.14±0.11 vs. 0.36±0.05, P<0.01), in the 48-h group (1.12±0.14 vs. 0.44±0.06, P<0.01; 1.39±0.21 vs. 0.44±0.06, P<0.01) and in the 72-h group (1.22±0.22 vs. 0.5±0.09, P<0.01; 1.68±0.25 vs. 0.5±0.09, P<0.01) (Fig. 2D-F).

Our previous study revealed that GRIM-19 is predominantly expressed in the cytoplasm of liver cancer tissue slices, whereas its expression in nuclei is limited (19). To confirm this observation in HepG2 cells, the cellular localization of endogenous GRIM-19 was analyzed by immunofluorescence. Consistent with the histological staining results of our previous study, GRIM-19 expression was detected in the cytoplasm, and its expression was markedly increased in the presence of TRD (Fig. 2G).

GRIM-19 serves a critical role in TRD-induced inhibition of viability and promotion of apoptosis in liver cancer cells

To determine the effects of GRIM-19 on the viability and apoptosis of liver cancer cells, GRIM-19 expression was knocked down by RNA interference in HepG2 and SNU-423 cells. Post-transfection with GRIM-19 siRNA or control siRNA for 24 h, the cells were treated with 100 µM TRD for 24 h. As shown in Fig. 3A and B, the protein and mRNA expression levels of GRIM-19 were significantly decreased in the TRD + GRIM-19 siRNA group compared with in the TRD treatment group (protein: 0.64±0.06 vs. 1.77±0.25, P<0.01; mRNA: 0.52±0.04 vs. 0.93±0.17, P<0.01). However, there was no significant difference between the GRIM-19 siRNA and control groups (protein: 0.43±0.11 vs. 0.52±0.09, P=0.23; mRNA: 0.27±0.08 vs. 0.36±0.03, P=0.25). This may be related to the weak expression of GRIM-19 in the control group. Furthermore, GRIM-19 protein and mRNA expression were significantly increased in the TRD treatment group compared with in the control group.

Figure 3. GRIM-19 serves a critical role in TRD-induced inhibition of viability and enhanced apoptosis in HepG2 cells and SNU-423 cells. Post-transfection with GRIM-19 or control siRNA for 24 h, cells were treated with TRD for 24 h. (A) GRIM-19 protein expression in the four groups of HepG2 and SNU-423 cells. (B) GRIM-19 mRNA expression in the four groups. (C and D) Effects of GRIM-19 siRNA transfection on cell viability and apoptosis of TRD-treated HepG2 cells and SNU-423 cells. GRIM-19, retinoic-interferon-induced mortality 19; OD, optical density; siRNA, small interfering RNA; TRD, taurolidine.

As shown in Fig. 3C, TRD, which is an inducer of GRIM-19, significantly inhibited HepG2 cell viability compared with in the control group (0.339±0.09 vs. 0.585±0.13, P<0.01). Conversely, the effects of TRD on cell viability were reversed following GRIM-19 knockdown (0.339±0.09 vs. 0.472±0.1, P<0.05); however, cell viability was not completely recovered. Similar findings were observed in SNU-423 cells. There may be other molecular mechanisms participating in the regulation of TRD in liver cancer cell viability. Taken together, these findings suggested that GRIM-19 was involved in the regulation of cell viability.

As shown in Fig. 3D, flow cytometry revealed that TRD treatment increased the percentage of apoptotic cells compared with control siRNA transfection (P<0.01). Cell apoptosis was significantly inhibited in the GRIM-19 siRNA + TRD group compared with in the TRD group (P<0.01) and the percentage of apoptotic cells was increased compared with the GRIM-19 siRNA group (P<0.05). Cell apoptosis in the TRD treatment group was the highest observed among the four groups. In addition, there was no significant difference in cell apoptosis between the GRIM-19 siRNA and control siRNA groups (P=0.13).

GRIM-19 regulates HepG2 cell proliferation and apoptosis via a STAT3-dependent pathway

To clarify the mechanism underlying the effects of TRD and GRIM-19 on cell viability and apoptosis, the expression of proliferation- and apoptosis-associated molecules, including STAT3, p-STAT3, cyclin D1, Bcl-2 and Bax, was detected by western blotting. As shown in Fig. 4A, the expression levels of p-STAT3, cyclin D1 and Bcl-2 were increased in the GRIM-19 siRNA group, whereas those of Bax were decreased. TRD treatment was revealed to have the opposite effect, inhibiting the expression of p-STAT3, cyclin D1 and Bcl-2, and increasing that of Bax, compared with the control group. These results suggested that GRIM-19 regulated HepG2 cell viability and apoptosis through the STAT3 pathway.

Figure 4. GRIM-19 regulates HepG2 cell proliferation and apoptosis via a STAT3-dependent pathway. The expression levels of proliferation- and apoptosis-associated molecules, STAT3, p-STAT3, cyclin D1, Bcl-2 and Bax, were detected by western blotting. (A) Expression of p-STAT3, cyclin D1 and Bcl-2 was increased, and that of Bax was decreased, in the GRIM-19 siRNA group, whereas TRD treatment had the opposite effect, inhibiting the expression of p-STAT3, cyclin D1 and Bcl-2, and increasing that of Bax, compared with the control group. (B) High expression levels of GRIM-19 and Bax were detected in the TRD and TRD + niclosamide groups, which were accompanied by low levels of cyclin D1, Bcl-2 and-STAT3. Conversely, low expression levels of GRIM-19 and Bax, and high expression levels of cyclin D1, Bcl-2 and p-STAT3 were detected in the TRD + GRIM-19 siRNA group. (C) Schematic diagram. Data are representative images or expressed as the means ± standard deviation of each group from three separate experiments. Bax, Bcl-2-associated X protein; Bcl-2, B-cell lymphoma 2; GRIM-19, retinoic-interferon-induced mortality 19; p-STAT3, phosphorylated-STAT3; siRNA, small interfering RNA; STAT3, signal transducer and activator of transcription 3; TRD, taurolidine.

To further confirm the role of the STAT3 signaling pathway in TRD treatment, GRIM-19 knockdown cells were pretreated with a p-STAT3 inhibitor, niclosamide, for 30 min, and 100 µM TRD was then added for 24 h. Western blot analysis detected high GRIM-19 and Bax expression in the TRD and TRD + niclosamide groups, accompanied by low expression levels of cyclin D1, Bcl-2 and p-STAT3. Conversely, low expression levels of GRIM-19 and Bax, and high expression levels of cyclin D1, Bcl-2 and p-STAT3 were detected in the TRD + GRIM-19 siRNA group. Niclosamide reduced the expression of p-STAT3, cyclin D1 and Bcl-2, and increased that of Bax, in the TRD-treated HepG2 cells, irrespective of GRIM-19 knockdown. However, there was no effect on the expression levels of GRIM-19, irrespective of the addition of niclosamide (Fig. 4B). This phenomenon indicated that GRIM-19 was an upstream molecule of the STAT3 signaling pathway. These findings suggested that TRD may enhance GRIM-19 expression and induce apoptosis of human liver cancer cells via a STAT3-dependent pathway.

Discussion

TRD is one of the most common types of human cancer, and a leading cause of cancer-associated mortality worldwide. Statistics have demonstrated that the majority of patients with liver cancer are not suitable for surgical resection, thus carrying a considerably poor prognosis. Particularly in patients with advanced liver cancer, the benefits of conventional therapy are limited; therefore, the need for more effective systemic treatments is urgent. TRD is an antiseptic agent derived from the amino sulfonic acid taurine, which has been used to treat peritonitis and catheter-associated infections (5). Recently, TRD has been used to treat malignant diseases (33–36), including gastrointestinal carcinoma, glioblastoma, fibrosarcoma, prostate cancer and melanoma (34,36–39). However, its precise mechanism of action remains unclear. In the present study, treatment with TRD significantly inhibited the viability and triggered the apoptosis of liver cancer cells.

In our previous study (19), it was revealed that GRIM-19 expression is reduced in liver cancer compared with in adjacent liver tissue. Furthermore, the decreased expression of GRIM-19 in liver cancer lesions is closely associated with an increased histological grading in liver cancer. In the present study, treatment with TRD significantly increased GRIM-19 expression in the cytoplasm of HepG2 cells. Simultaneously, cell viability was inhibited, whereas apoptosis was induced in TRD-treated HepG2 and SNU-423 cells, accompanied by an increase in GRIM-19 expression. In addition, the viability and apoptosis of HepG2 and SNU-423 cells, which had been affected by TRD treatment, was recovered when GRIM-19 was knocked down. These findings indicated that TRD inhibited cell viability and induced apoptosis via the GRIM-19 pathway in TRD. Wei et al (32) revealed that GRIM-19 is predominantly expressed in the nuclei of HeLa cells. Haura et al (40) reported that GRIM-19 is predominantly expressed in the inner mitochondrial membrane and is a component of the mitochondrial complex I. In the present study, it was revealed that GRIM-19 was predominantly expressed in the cytoplasm of liver cancer cells, which is consistent with our previous results (19).

The aim of the present study was to expand the investigation of the in vitro effects of TRD treatment on cell apoptosis. The apoptotic mechanism of cancer cells is composed of both upstream regulators and downstream effector components. The regulators can be divided into two major circuits. One receives and processes extracellular death-inducing signals, and the other senses and integrates various signals of intracellular origin. Bcl-2 is an upstream effector molecule in the apoptotic pathway, which has been identified as a potent suppressor of apoptosis. Bcl-2 has been reported to form a heterodimer complex with the proapoptotic Bax, thereby neutralizing its proapoptotic effects. Therefore, the ratio of Bax/Bcl-2 is a decisive factor and serves an important role in determining whether cells die or survive (41–44). The expression levels of these proteins were detected by western blot analysis in the present study, in order to determine their involvement in the molecular mechanisms underlying the role of TRD in liver cancer cell apoptosis.

Our previous study detected a significantly higher nuclear expression of p-STAT3 in liver cancer lesions. Notably, an inverse correlation was identified between GRIM-19 and p-STAT3 expression in liver cancer tissues (19). To verify this mechanism in vitro, a p-STAT3 inhibitor (niclosamide) was added and the expression of proliferation- and apoptosis-associated molecules, STAT3, p-STAT3, cyclin D1, Bcl-2 and Bax, was detected by western blotting. The result demonstrated that treatment with TRD not only enhanced the expression of GRIM-19, but also reduced the expression of cyclin D1, Bcl-2 and p-STAT3 in HepG2 cells. This result further supported the hypothesis that TRD may induce the apoptosis of human liver cancer cells through GRIM-19-induced regulation of a STAT3-dependent pathway. The present data demonstrated that niclosamide reduced the expression levels of p-STAT3, cyclin D1 and Bcl-2, and increased those of Bax in TRD-treated HepG2 cells, irrespective of GRIM-19 knockdown. This phenomenon indicated that GRIM-19 may be an upstream molecule of the STAT3 signaling pathway.

In conclusion, the present study indicated that GRIM-19 was predominantly expressed in the cytoplasm of liver cancer cells, and the knockdown of GRIM-19 was accompanied by an increase in the expression of cyclin D1, Bcl-2 and p-STAT3 in these cells. As shown in Fig. 4C, treatment with TRD significantly upregulated GRIM-19 expression, but downregulated cyclin D1, Bcl-2 and p-STAT3 expression in HepG2 cells and SNU-423 cells. However, these expression levels were recovered following GRIM-19 knockdown. In summary, TRD inhibited liver cancer cell viability and induced apoptosis via GRIM-19-induced regulation of a STAT3-dependent pathway; therefore, TRD may serve as a novel effective treatment for liver cancer.

Acknowledgements

Not applicable.

Funding

This study was supported in part by grants from the Shandong Province Medical and Health Science and Technology Development Project (grant no. 2017WS465), the National Natural Science Foundation of China (grant nos. 81600469 and 81472685), the Science and Technology Development Projects of Shandong Province (grant nos. 2017GSF218053 and 2016GSF201126), and the Major Special Plan of Science and Technology of Shandong Province (grant no. 2015ZDXX0802A01).

Availability of data and materials

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Authors' contributions

FL contributed to experimentation and manuscript writing. JQ was responsible for technical guidance and data interpretation. CQ was responsible for data analysis and statistical analysis. ZF and WR made substantial contributions to conception and design. All authors read and approved the final manuscript.

Ethics approval and consent to participate

Not applicable.

Patient consent for publication

Not applicable.

Competing interests

The authors declare that they have no competing interests.

Glossary

Abbreviations

Abbreviations:

References