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Introduction

Apoptosis, or programmed cell death, occurs through both extrinsic and intrinsic pathways, which are modulated by apoptotic proteins including cytochrome c, caspase-8 and −3 (1). Morphological and biochemical hallmarks of apoptosis include cell shrinkage, chromatin condensation, DNA fragmentation and formation of apoptotic bodies (2). Anticancer agents and radiation are able to induce apoptotic protein-mediated signal transduction pathways and consequent inhibition of tumor growth (1,3). Anticancer agents and radiation-induced apoptosis can be blocked by overexpression of anti-apoptotic proteins in cancer cells leading to treatment failure (4). Human hepatocellular carcinoma (HCC) is endemic in Asia and among the deadliest types of cancers (5). Overexpression of anti-apoptotic proteins such as cellular FLICE-like inhibitory protein (c-FLIP), myeloid cell leukemia-1 (Mcl-1), and X-linked inhibitor of apoptosis protein (XIAP) has been identified in HCC and is associated with the poor prognosis of HCC patients (6–8).

Nuclear factor-κB (NF-κB) is a transcription factor of a number of oncogenes which modulate tumorigenesis (9). Cancer hallmarks that include self-sufficiency in proliferative growth signals, insensitivity to anti-growth signals, evasion of apoptosis, limitless replicative potential, tissue invasion and metastasis, and sustained angiogenesis have been related to NF-κB-modulated expression of downstream effector proteins (10). Various anticancer agents and radiation not only trigger apoptosis, but also activate expression of NF-κB-induced anti-apoptotic proteins resulting in the reduction of therapeutic efficacy in HCC both in vitro and in vivo (11,12). Constitutive NF-κB activation is observed in patients with advanced HCC and may be used as a negative prognostic biomarker (13). Therefore, development of NF-κB signal inhibitors may facilitate the treatment of HCC patients.

Regorafenib (Stivarga®) is a multi-kinase inhibitor with a similar chemical structure to sorafenib (Nexavar®), but has an additional functional group, which produces more potent activity to inhibit oncogenic receptor tyrosine and cytoplasmic signaling kinases (14). Regorafenib has been approved to treat colorectal cancer and gastrointestinal stromal tumors. A recent update of an ongoing phase III clinical trial reported that regorafenib was effective in patients with sorafenib-resistant HCC (15). In our previous study, sorafenib, as an inhibitor of NF-κB signaling, was found to reduce the expression of NF-κB-modulated anti-apoptotic proteins in HCC both in vitro and in vivo (12). However, whether regorafenib, an analogue of sorafenib, can induce apoptosis through blockage of NF-κB activation in HCC cells remains obscure. The aim of the present study was to investigate the role of NF-κB inactivation on regorafenib-induced apoptosis in SK-HEP-1 cells using MTT assay, flow cytometry, DNA gel electrophoresis, western blotting and NF-κB reporter gene assay. ERK, AKT, JNK and P38 inhibitors were used to determine the mechanism of regorafenib-induced NF-κB inactivation in HCC.

Materials and methods

Agents and antibodies

Regorafenib was provided by Bayer Health Care Pharmaceuticals (Whippany, NJ, USA). Dulbecco's modified Eagles medium (DMEM), fetal bovine serum (FBS), L-glutamine and penicillin-streptomycin (PS) were purchased from Gibco/Life Technologies (Carlsbad, CA, USA). Propidium iodide (PI) and DiOC6 were purchased from BioVision (Mountain View, CA, USA) and Enzo Life Sciences (Farmingdale, NY, USA), respectively. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and RNase were purchased from Sigma-Aldrich (St. Louis, MO, USA) and Fermentas (St. Leon-Rot, Baden-Württemberg, Germany), respectively. Primary antibodies of cleaved-caspase-3, cellular FADD-like IL-1β-converting enzyme (FLICE)-inhibitory protein (c-FLIP) and pAKT (Ser473) were purchased from Cell Signaling Technology (Beverly, MA, USA). Primary antibodies of caspase-8 and X-linked inhibitor of apoptosis protein (XIAP) were obtained from Thermo Fisher Scientific (Fremont, CA, USA). Primary antibodies of ERK, AKT, NF-κB p65, β-actin and cytochrome c were purchased from Santa Cruz Biotechnology (Santa Cruz, CA, USA). Primary antibodies of MCL-1 and pERK were purchased from BioVision (Milpitas, CA, USA) and Merck Millipore (Billerica, MA, USA), respectively. Secondary antibodies were purchased from Jackson ImmunoResearch (West Grove, PA, USA). Nuclear and Cytoplasmic Extraction and Genomic DNA Miniprep kits were obtained from Chemicon (Temecula, CA, USA) and Axygen Biosciences (Union City, CA, USA), respectively. NF-κB inhibitor 4-N-[2-(4-phenoxyphenyl)ethyl]quinazoline-4,6-diamine (QNZ), AKT inhibitor LY294002, c-Jun N-terminal kinase (JNK) inhibitor SP600125, P38 inhibitor SB203580 and extracellular signal-regulated kinase (ERK) inhibitor PD98059 were purchased from Selleckchem (Houston, TX, USA). NF-κB-luciferrase2 vector (pNF-κB/luc2) and D-luciferin were obtained from Promega (Madison, WI, USA) and Caliper Life Science (Hopkinton, MA, USA), respectively. Hygromycin B was purchased from Santa Cruz Biotechnology.

Cell culture

SK-HEP-1 cells were gifted by Professor Jing-Gung Chung of the Department of Biological Science and Technology, China Medical University, (Taichung, Taiwan). Cells were cultured in DMEM and supplemented with 10% FBS, 2 mM L-glutamine and PS (100 U/ml and 100 µg/ml) at 37°C in an atmosphere of 5% CO2 (16).

Plasmid transfection and stable clone selection

SK-HEP-1 cells were transfected with pNF-κB/luc2 using JetPEI™. Cells (1×106) were seeded into 10-cm dish and incubated overnight. DNA solution (10 µg NF-κB/luc2 plasmid dissolved in 250 µl of 150 mM NaCl) was mixed with 250 µl JetPEI solution (20 µl of JetPEI reagent diluted in 230 µl of 150 mM NaCl), and then incubated for 30 min at room temperature to make 500 µl DNA/JetPEI mixture. The DNA/JetPEI mixture was added to the SK-HEP-1 cells in a 10-cm diameter dish and incubated for 24 h. After transfection, the cells were cultured in medium containing 200 µg/ml of hygromycin B for two weeks. The surviving clones were subsequently subcultured into 96-well plates. Function of the NF-κB reporter gene in each clone was assayed using the IVIS 200 Imaging System (Xenogen, Alameda, CA, USA). Cells with functional NF-κB reporter gene product were renamed as SK-HEP-1/NF-κB-luc2 cells (12).

MTT assay

SK-HEP-1 cells were seeded into 96-well plates at a density of 3×104 cells/well and incubated overnight. Cells were treated with different concentrations of QNZ (0–0.4 µM in 0.1% DMSO) or regorafenib (0–50 µM in 0.1% DMSO) for different periods, and then the change in cell viability was determined with the MTT assay as previously described (17).

Detection of mitochondrial membrane potential (MMP)

SK-HEP-1 cells were seeded into 12-well plates at a density of 2×105 cells/well and incubated overnight. Cells were treated with 0.4 µM QNZ or 20 µM regorafenib for different periods. Cells were harvested by centrifugation and washed twice with phosphate-buffered saline (PBS), and then stained with DiOC6 solution (4 µM DiOC6 in 500 µl PBS) for 30 min at 37°C. Detection of MMP was performed using flow cytometry (FACS101; FACScan; Becton-Dickinson, Franklin Lakes, NJ, USA) as described by Wang et al (18).

Analysis of the sub-G1 population

SK-HEP-1 cells were seeded into 12-well plates at a density of 2×105 cells/well and incubated overnight. Cells were treated with 0.4 µM QNZ or 20 µM regorafenib for different periods. The cells were harvested by centrifugation and fixed with 70% ethanol and incubated overnight at −20°C. Cells were washed twice with PBS and then stained with 500 µl of PI buffer (40 µg/ml PI, 100 µg/ml RNase and 1% Triton X-100 in PBS) for 1 h in darkness at room temperature. Detection of the sub-G1 cell population was performed using flow cytometry (FACS101; FACScan) as described by Huang et al (19).

Western blot assay

SK-HEP-1 cells (3×106) were seeded into 10-cm diameter dishes and incubated overnight. Then, the cells were treated with 0.4 µM QNZ or 20 µM regorafenib for different periods. Lysis buffer (50 mM Tris-HCl pH 8.0, 120 mM NaCl, 0.5% NP-40 and 1 mM phenylmethanesulfonyl fluoride) was used for total protein extraction from the cells in the different treatment groups. Cytosolic proteins from cells in each group were extracted using a cytosol extraction kit following the instructions provided by the manufacturer. Protein expression of NF-κB p65, NF-κB p65 (Ser536), XIAP, Mcl-1, c-FLIP, cleaved-caspase-3, caspase-8, cytochorme c, ERK, pERK, AKT and pAKT were evaluated with western blot assay as described by Ting et al (20). Quantification of protein bands was performed using ImageJ software (National Institutes of Health, Bethesda, MD, USA).

Detection of DNA fragmentation

SK-HEP-1 cells were seeded into 6-well plates at a density of 1×106 cells/well and incubated overnight, and then treated with 0.4 µM QNZ or 20 µM regorafenib for different periods. Genomic DNA from the cells was purified using the GenElute™ Mammalian Genomic DNA Miniprep kit (Sigma-Aldrich) following the instructions provided by the manufacturer. Analysis of DNA fragmentation was performed using 1.5% agarose gel electrophoresis (12).

NF-κB reporter gene assay

SK-HEP-1 cells were seeded into 96-well plates at a density of 3×104 cells/well and incubated overnight. The detailed conditions for the different treatment groups are provided in detail in the figure legends. D-luciferin solution (500 µM D-luciferin in 100 µl PBS) was added to each well, and photon signal was acquired for 1 min using the IVIS 200 Imaging System. Relative NF-κB activity was corrected by cell viability which was evaluated by the MTT assay as previously described (17).

Statistical analysis

Results are all representative of at least three independent experiments. Statistical significance was determined using the Student's t-test. p-values of <0.05 were considered statistically significant.

Results

NF-κB inhibitor diminishes the expression of anti-apoptotic proteins and induces both extrinsic and intrinsic apoptosis in the SK-HEP-1 cells

In order to verify the effects of NF-κB inactivation on pro-apoptotic and anti-apoptotic signal transduction, SK-HEP-1 cells were initially treated with different concentrations of QNZ for different periods. Subsequently, cell viability, expression of NF-κB p65 (Ser536), anti-apoptotic and pro-apoptotic proteins, and the effects of apoptosis were evaluated with MTT assay, western blotting, DNA gel electrophoresis and flow cytometry. Fig. 1A indicates that QNZ significantly reduced cell viability in a dose- and time-dependent manner as compared to that noted in the control cells (vehicle treatment with 0.1% DMSO). Fig. 1B shows that QNZ not only inhibited expression of NF-κB p65 (Ser536) and anti-apoptotic proteins (XIAP, MCL-1 and c-FLIP), but also increased levels of pro-apoptotic proteins (cleaved-caspase-3 and −8, and cytochrome c). DNA fragmentation is one of the apoptotic hallmarks and QNZ-induced DNA fragmentation is demonstrated in Fig. 1C. Apoptosis also can be measured by flow cytometry to detect the sub-G1 cell population and MMP. The sub-G1 cell population and loss of MMP were significantly enhanced by regorafenib treatment in a time-dependent manner as compared to the control (Fig. 1D and E).

Figure 1. Effects of the NF-κB inhibitor QNZ on cell viability, expression of NF-κB-modulated anti-apoptotic proteins and apoptosis pathways in SK-HEP-1 cells. SK-HEP-1 cells were treated with different concentrations (0, 0.1, 0.2, 0.3 and 0.4 µM in 0.1% DMSO) of QNZ for 24 and 48 h. (A) Change in cell viability was determined with the MTT assay. * *p<0.01. (B) Protein levels of NF-κB p65 (Ser536), XIAP, c-FLIP, MCL-1, cleaved-caspase-3 and −8, and cytochrome c were evaluated using western blot assay. ap<0.05 and bp<0.01 as compared with the control. (C) Detection of DNA fragmentation was performed using gel electrophoresis. (D) Determination of the sub-G1 cell population was carried out using flow cytometry. **p<0.01 as compared with the control. (E) The change in MMP was investigated using flow cytometry. *p<0.05 and **p<0.01 as compared with the control.

Regorafenib inhibits expression of NF-κB-modulated anti-apoptotic proteins and induces both extrinsic and intrinsic apoptosis in the SK-HEP-1 cells

The SK-HEP-1 cells were treated with different concentrations of regorafenib for different periods. Cell viability, expression of NF-κB p65 (Ser536), expression of anti-apoptotic and pro-apoptotic proteins, and regorafenib effects on apoptosis were evaluated with MTT assay, western blotting, DNA gel electrophoresis, and flow cytometry. Regorafenib significantly decreased cell viability in a dose- and time-dependent manner as compared to that noted in the control cells (Fig. 2A). Regorafenib also significantly inhibited expression of NF-κB p65 (Ser536) and anti-apoptotic proteins (XIAP, MCL-1 and c-FLIP) while increased the levels of pro-apoptotic proteins (cleaved-caspase-3 and −8, and cytochrome c) in a time-dependent manner as compared to the control group (Fig. 2B). Fig. 2C shows that regorafenib induced DNA fragmentation and significantly induced the sub-G1 cell population and loss of MMP in a time-dependent manner as compared to the control (Fig. 2D and E).

Figure 2. Effects of regorafenib on cell viability, expression of NF-κB-modulated anti-apoptotic proteins and apoptosis pathways in SK-Hep1 cells. Cells were treated with different concentration (0, 10, 20, 30, 40 and 50 µM in 0.1% DMSO) of regorafenib for 24 and 48 h. (A) Change in cell viability was determined with MTT assay. **p<0.01. (B) Protein levels of NF-κB p65 (Ser536), XIAP, c-FLIP, MCL-1, cleaved-caspase-3 and −8, and cytochrome c were evaluated by western blot assay. ap<0.05 and bp<0.01 as compared with the control. (C) Detection of DNA fragmentation was performed using gel electrophoresis. Determination of Sub G1 population was performed using flow cytometry. **p<0.01 as compared with the control. (E) Change of MMP was investigated using flow cytometry. *p<0.05 and **p<0.01 as compared with the control.

Regorafenib inhibits NF-κB activation through ERK dephosphorylation in the SK-HEP-1 cells

We found that regorafenib reduced NF-κB activation and this turns regorafenib into an inhibitor of NF-κB signaling. In the next step, we used different kinase (AKT, JNK, P38 and ERK) inhibitors to investigate the mechanism of regorafenib-induced NF-κB inactivation in the SK-HEP-1 cells. Fig. 3A and B shows that regorafenib, QNZ (NF-κB inhibitor) and the ERK inhibitor (PD98059) significantly reduced NF-κB activation. Fig. 3C indicates that regorafenib also inhibited ERK and AKT phosphorylation in a time-dependent manner in the SK-HEP-1 cells.

Figure 3. Effects of different kinase inhibitors on NF-κB activation and the effects of regorafenib on ERK and AKT phosphorylation in SK-Hep1 cells. SK-Hep1 cells were treated with 20 µM regorafenib, 0.4 µM QNZ and 10 µM various kinase (AKT, JNK, P38 and ERK) inhibitors for 24 and 48 h. (A) NF-κB activation was evaluated with NF-κB reporter gene assay after treatment for 24 h. Images were acquired by IVIS 200. (B) NF-κB activation was evaluated with NF-κB reporter gene assay after treatments for 48 h. Images were acquired by IVIS 200. (C) Protein expression of phosphorylated ERK and AKT (Ser473) was determined with western blot assay. *p<0.05 and **p<0.01 as compared with the control.

Discussion

Regorafenib, a sorafenib analogue, has been approved for the treatment of metastatic colorectal cancer and advanced gastrointestinal stromal tumors (15). Sorafenib, as an inhibitor of NF-κB signaling, was indicated in our previous study to reduce the expression of NF-κB-modulated anti-apoptotic proteins and trigger the apoptotic pathway in HCC both in vitro and in vivo (12). However, whether regorafenib induces apoptosis through inhibition of NF-κB activation in HCC cells requires elucidation. Therefore, we evaluated the effects of regorafenib on NF-κB inhibition-related apoptosis and the mechanism in HCC SK-HEP-1 cells in vitro. First, we found that the NF-κB inhibitor QNZ reduced NF-κB activation and anti-apoptotic protein levels (XIAP, c-FLIP and MCL-1) while triggered extrinsic and intrinsic apoptotic pathways (Fig. 1A-E). Secondly, regorafenib as inhibitor of NF-κB signaling also suppressed NF-κB activation and anti-apoptotic protein levels, while induced extrinsic and intrinsic apoptotic pathways (Fig. 2A-E). Finally, we found that the ERK inhibitor reduced NF-κB activation and regorafenib diminished ERK phosphorylation (Fig. 3A and B).

Expression of anti-apoptotic proteins such as XIAP, c-FLIP and MCL-1 is linked to constitutive NF-κB activation in cancer cells (12,21). XIAP can interact with the active site of caspase-3 resulting in inhibition of caspase-3-mediated apoptosis (22). c-FLIP, a caspase-8 inhibitor, disrupts caspase-8 and prevents initiation of the extrinsic apoptotic pathway (23). MCL-1 suppresses loss of MMP and cytochrome c release from mitochondria that subsequently leads to inhibition of the intrinsic apoptotic pathway (24,25). The present study results demonstrated that both the NF-κB inhibitor and regorafenib inhibited NF-κB activation, reduced anti-apoptotic protein (XIAP, c-FLIP and MCL-1) expression, and activated extrinsic and intrinsic apoptotic pathways. Chen et al suggested that regorafenib activates NF-κB-regulated expression of p53-upregulated modulator of apoptosis (PUMA) and inhibits colorectal tumor growth (26). RAF/mitogen-activated protein kinase kinase (MEK)/ERK and phosphoinositide 3-kinase (PI3K)/AKT signaling transduction are the most critical pathways in the development and progression of HCC. Activation of ERK and AKT can be used as biomarkers to predict poor prognosis in HCC (27). Sorafenib induces apoptosis and inhibits angiogenesis in HCC via blockage of the RAF/MEK/ERK pathway. However, AKT activation is not inhibited by sorafenib (28). Fig. 3C shows that regorafenib significantly reduced both ERK and AKT phosphorylation. NF-κB can be activated through different kinases, such as AKT, JNK, P38 or ERK in different types of cancer cells (12,29–30). We used inhibitors of AKT, JNK, P38 and ERK to verify the mechanism of regorafenib-induced NF-κB inactivation in the SK-HEP-1 cells. We found that the ERK inhibitor revealed similar effects in the inhibition of NF-κB activation as regorafenib or QNZ (Fig. 3A and B). Therefore, we suggest that regorafenib inhibits NF-κB activation via dephosphorylation of ERK. In previous studies, we also found that sorafenib inhibited NF-κB-modulated tumor progression through suppression of ERK activation in HCC Huh7 cells (12,17).

In conclusion, the present study demonstrated that regorafenib triggered extrinsic and intrinsic apoptotic pathways through blockage of ERK/NF-κB activation in SK-HEP-1 cells in vitro. We propose that regorafenib may be a potential anticancer agent for the treatment of advanced HCC.

Acknowledgements

The present study was supported by a grant to J.-J.T. (RD2016-020) from the National Yang-Ming University Hospital (Yilan, Taiwan). We acknowledge the technical services provided by the Clinical Medicine Research Laboratory of National Yang-Ming University Hospital.

Glossary

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References