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Disulfiram Chelated With Copper Inhibits the Growth of Gastric Cancer Cells by Modulating Stress Response and Wnt/β-catenin Signaling

Disulfiram (DSF) is a well-known drug for alcohol abuse. In recent decades, DSF has been demonstrated to exhibit anti-tumor activity; DSF chelated with copper shows enhanced anti-tumor effect. Our goal was to explore the effect of DSF/Cu complex on the growth and metastasis of gastric cancer (GC) in vitro and in vivo. DSF/Cu complex suppressed the proliferation, migration of MKN-45 and BGC-823 GC cells. Furthermore, DSF/Cu treatment reduced the tumor volume in GC mouse models with a tumor suppression rate of 48.24%. Additionally, DSF/Cu induced apoptosis in vitro in MKN-45 and BGC-823 GC cells in a dose- and time-dependent manner as well as in vivo in the xenograft tumor mouse model. Furthermore, DSF/Cu induced autophagy and autophagic flux in MKN-45 and BGC-823 cells, increased the expression of autophagy-related Beclin-1 and LC3 proteins in vivo. Additionally, DSF/Cu suppressed aerobic glycolysis and oxidative phosphorylation by reducing oxygen consumption rate and extracellular acidification rate, respectively, in MKN-45 and BGC-823 cells. Treatment with DSF/Cu induced oxidative stress and DNA damage response by elevating the reactive oxygen species levels; increasing the expression of P53, P21, and γ-H2AX proteins; and inhibiting Wnt/β-catenin signaling in vitro and in vivo. Thus, DSF/Cu suppressed the growth and metastasis of GC cells via modulating the stress response and Wnt/β-catenin signaling. Hence, DSF may be used as a potential therapeutic agent for the treatment of GC.


INTRODUCTION
In 2018, gastric cancer (GC) ranked fifth in among all cancers regarding incidence and third regarding cancer-related deaths globally (1). In China, GC occupied the second and third rank in terms of cancer incidences and deaths, respectively, in 2015 (2); however, it was associated with the highest recorded morbidity and mortality in 2013 in Gansu province (3). Although different management techniques and comprehensive therapies have been used for treating GC, their clinical benefits remain unsatisfactory (4). Furthermore, only a few patients with GC are diagnosed at the early stages of the disease. Moreover, metastasis remains the dominating cause of deaths in majority of the cases. In GC patients with metastasis, a 5-year survival rate has been reported to be only 5% (5). Therefore, drugs with high efficacy and low toxicity are required for GC treatment.
Currently, there has been increasing attention in anti-tumor role of the anti-alcoholism drug disulfiram (DSF). Multiple studies have explored the anti-tumor role of DSF in various cancers, such as breast cancer, liver cancer, pancreatic cancer, and so on; moreover, the anti-cancer activity of DSF has been shown to increase when it is chelated with copper (Cu) (6,7). The underlying mechanisms associated with the anti-cancer activity of DSF may involve inhibition of the proteasome activity, suppression of metastasis, proliferation of cancer stem cell, DNA methylation, and elevation of reactive oxygen species (ROS) as well as the modulation of different signaling pathways (8).
Stress response and genetic aberrations are the universal phenomena associated with tumorigenesis (9). Stress response, including responses to metabolic and oxidative stress, unfolded proteins, heat shock proteins, and DNA damage, plays a dual role in the process of tumor development and progression (10)(11)(12)(13). The canonical Wnt/b-catenin signaling pathway is involved in embryonic and organ development and is reportedly activated in GC and plays a pivotal role in the carcinogenesis, invasion, metastasis, progression, and drug resistance of GC cells (14).
The aim of the current study was to explore if DSF chelated with Cu could suppress the growth and metastasis of GC cells using MKN45 GC cell line and tumor xenograft mouse model.

Cell Culture and Transfection
Human gastric carcinoma cell lines of MKN-45, BGC-823 and human embryonic kidney cell line HEK293T were purchased from the Type Culture Collection of the Chinese Academy of Sciences (Shanghai, China). The cells were cultured in RPMI 1640 medium supplemented with 10% FBS at 37°C and 5% CO 2 . Green fluorescent protein-light chain 3 (GFP-LC3) was donated Yunsheng Zhang (Hunan University), and monomeric red fluorescent protein-GFP-LC3 (mRFP-GFP-LC3) plasmid was purchased from Hedgehog Bio (China). MKN-45 and BGC-823 cells were grown overnight in six-well plates (3 × 10 5 cells per well), and then incubated with a mixture of GFP-LC3 plasmid and Lip2000, which was prepared by using the OptiMEM medium at a ratio of 4mg plasmid/4ml of Lip2000 per well. After 6 h of incubation, the cells were again cultured in RPMI 1640 medium. Following 72 h incubation, the cells were transfected with the lentivirus containing the insert and screened by puromycin. HEK293T cells were cultured overnight in six-well plates (1 × 10 6 cells per well) and incubated with a mixture of pSPAX2, pMD2G, and mRFP-GFP-LC3 OptiMEM medium.
where As, Ac, and Ab represent the absorbance of treatment, control, and blank groups, respectively.

Western Blotting
MKN-45 and BGC-823 cells were cultured overnight in sixwell plates (3 × 10 5 cells per well) followed by treatment with DMSO or DSF/Cu (100 or 300 mM) for 24 or 48 h. The cells were then lysed using a mixture of RIPA buffer and PMSF (1,000:1) followed by protein purification. The protein content in the cell lysate was quantified using BCA assay. The protein samples (20 mg) were separated by 10% or 12% sodium dodecyl sulfate-polyacrylamide gel electrophoresis, transferred to nitrocellulose membrane, and blocked with 5% BSA for 2 h at 37°C. The membranes were subsequently incubated with primary antibodies overnight at 4°C followed by incubation with secondary antibodies for 1 h at room temperature. Finally, the membranes were incubated with ECL and the images were captured with VersaDoc imaging system (Bio-Rad, USA).

Transmission Electron Microscopy
MKN-45 and BGC-823 cells were obtained after DSF/Cu (100 or 300 mM) treatment for 48 h, fixed in 2.5% glutaraldehyde, dehydrated through a graded series of ethanol solution (50, 70, 80, 90, and 95%), and embedded in resin. The specimens were then cut into ultra-thin sections, stained, and examined under HT7700 transmission electron microscope (HITACHI, Japan).   slides were deparaffinized, hydrated, stained, and sealed, whereas for IHC staining, the slides were deparaffinized, and rehydrated followed by incubation with the primary and subsequently secondary antibodies. The slides were then stained with 3,3'diaminobenzidine, counterstained with hematoxylin and analyzed under light microscope (AMG, USA). For TUNEL assay, the slides were deparaffinized, digested with Proteinase K, incubated with DeadEnd ™ Fluorometric TUNEL System, mounted in antifade mounting medium with DAPI, and imaged by confocal microscopy.

Statistical Analysis
Data were analyzed by the Image J and GraphPad Prism 7 software. The differences between two groups were calculated by one-way analysis of variance, and results with P value of <0.05 were considered statistically significant.

Disulfiram/Cu Induced Apoptosis in Gastric Cancer Cells
As shown in Figure 5, the percentage of apoptotic cells in the control, 100 mM DSF/Cu-treated, and 300 mM DSF/Cu-treated groups was 2.7 ± 0.23%, 8.69 ± 0.38%, and 23.83 ± 2.32%, respectively, after 24 h of treatment. The apoptosis rate further increased to 15.47 ± 0.61% and 42.11 ± 1.29% at concentrations of 100 and 300 mM, respectively, after 48 h of treatment. Furthermore, the expression of B-cell lymphoma-2 (Bcl-2) decreased and that of Bax protein increased in the DSF/Cu-treated cells compared with that in the DMSO (control)-treated cells ( Figure 5).

Disulfiram/Cu Induces Autophagy and Autophagic Flux in Gastric Cancer Cells
The effect of DSF/Cu treatment on autophagy was demonstrated by various methods in MKN-45 and BGC-823 cells (Figures 6-8). The formation of double-membrane autolysosomes and autophagosomes was observed in DSF/Cutreated MKN45 cells by transmission electron microscopy ( Figures 6A, B). Furthermore, western blotting results showed an increase in Beclin1 levels and LC3 II/I ratio in DSF/Cu-treated MKN-45 and BGC-823 cells in a dose and time-dependent manner ( Figures 6C, D). Additionally, we investigated the distribution of GFP-LC3 in MKN-45 and BGC-823 cells transiently expressing GFP-LC3 (Figure 7). The results showed an increase in the distribution of GFP-LC3 in DSF/Cu-treated MKN-45 and BGC-823 cells in a dose and time-dependent manner ( Figure 7A). This was further confirmed quantitatively by flow cytometry assays ( Figures 7B, C).

Disulfiram/Cu Modulates the Stress Response in Gastric Cancer Cells
The role of DSF/Cu on modulating the stress responses was demonstrated by various experimental strategies in MKN-45 and BGC-823 cells. As shown in Figure 9, various parameters of OCR, including glycolysis, glycolytic capacity, and glycolytic reserve, were found to be decreased by DSF/Cu treatment, indicating a decrease in the aerobic glycolysis in DSF/Cutreated MKN-45 and BGC-823 cells. Furthermore, various parameters of ECAR, including basal respiration, maximal respiration, and ATP production, were found to be reduced in DSF/Cu-treated MKN-45 and BGC-823 cells, indicating a reduction in oxidative phosphorylation (OXPHOS). The fluorescence microscopy and flow cytometry results showed that DSF/Cu elevated ROS levels in MKN-45 and BGC-823 cells in a concentration-and time-dependent manner ( Figure  10). Furthermore, the expression of P53, P21, and g-H2AX proteins increased in DSF/Cu-treated MKN-45 and BGC-823 cells compared with that in the control cells, with expression being higher at higher concentration (300 vs 100 mM) of DSF/Cu ( Figure 11).

Disulfiram/Cu Inhibits Wnt/b-catenin Signaling in Gastric Cancer Cells
As shown in Figure 12, DSF/Cu treatment reduced the expression of Wnt/b-catenin signaling pathway related proteins, such as frizzled-7 (Fzd7), b-catenin, C-myc, and Cyclin D1, compared with control treatment. A slight decrease in the expression of these proteins was observed at a concentration of 100 mM of DSF/Cu; however, the decrease  was found to be significantly higher at a higher dose of DSF/Cu (300 mM).

Anti-Cancer Activities of DSF/Cu In Vivo
The in vivo activity of DSF/Cu was demonstrated using a subcutaneous tumor xenograft mouse model of MKN-45 cells.
The DSF/Cu (5 mg/kg) group showed a reduction in the tumor volume, as indicated by a TSR of 48.24% (Figures 13A, B). As shown in Figure 13C, there was no significant difference observed in the body weight between the control and DSF/Cutreated mice (P = 0.66). Furthermore, H&E staining demonstrated the induction of necrosis was increased by DSF/ Cu treatment ( Figure 14A). The expression of Ki-67 protein, a marker for cellular proliferation, significantly decreased ( Figure  14B) by DSF/Cu treatment. TUNEL assay demonstrated the induction of apoptosis in the tumor tissues of DSF/Cu-treated mice compared to that in the tissues of DMSO-treated mice ( Figure 15A). Furthermore, the expression of Beclin1 and LC3 ( Figures 15B, C). In addition, the oxidative stress associated proteins (P53 and g-H2AX), and the Wnt/b-catenin signaling associated proteins, including Fzd7, b-catenin, C-myc, and Cyclin D1 were shown to be reduced by DSF/Cu treatment ( Figure 16).

DISCUSSION
Since 1800s, DSF has been used in rubber production (15). Since the year 1940, it has been used for treating alcoholism and currently has been approved by the U.S. Food and Drug Administration (16). Furthermore, the anti-cancer activity of DSF has been explored since 1977 (17). DSF is rapidly converted to diethyldithiocarbamate in the system, which forms a stable complex with Cu; the complex exhibits a stronger anti-tumor effect than DSF alone (18). DSF/Cu complex has been reported to suppress the growth and metastasis of non-small-cell lung cancer (NSCLC), hepatocellular carcinoma, and oral squamous cell carcinoma (6,19,20). Zhang  PCD, the cell-intrinsic suicidal pathway, is associated with the development, homeostasis, and pathogenesis of cells (22). Apoptosis is a form of type I PCD (22). Bcl-2 family proteins are important in cell apoptosis and regulate the mitochondrial outer membrane permeabilization (MOMP) (23). The accumulation of BAX-BAK oligomers promote MOMP and results in the release of intermembrane proteins to the cytosol, which leads to apoptosis body formation (24). Our study revealed that DSF/Cu treatment increased the rate of apoptosis in GC cells as well as induced apoptosis in vivo, which is agreement with the findings from a previous cancer study (7).
Autophagy (also known as macroautophagy), a form of type II PCD, results in the sequestration of cytoplasmic components, protein aggregates, and organelles and formation of autophagosomes (25,26). Because of the various cancer types and cancer stages, the dual role of autophagy in cancer development has been reported (27). Beclin1, a homolog of yeast autophagy associated gene-6 (ATG6), regulates the formation of autophagosomes by regulating Bcl-2 family of proteins, ATG14L, ultraviolet radiation resistance-associated gene, vacuolar protein sorting 34 (VPS34), and kinase activity (27)(28)(29). Existing studies on GC report varying levels of Beclin1. For instance, increased levels of Beclin1 has been reported to increase autophagy, which may be related to tumorigenesis in GC (30). However, in the poorly differentiated GC cells (MKN-45 and BGC-823 cells) and human GC tissues, the expression of Beclin1 was found to be reduced (31). Our results found that the level of Beclin1 increased after the treatment of MKN-45 and BGC-823 cells with DSF/Cu, which is in accordance with the finding from a study that showed BECN1 to be a haploid-insufficient tumor suppressor in carcinoma (32). LC3, a homolog of yeast ATG8, contains LC3-I and LC3-II chains. Cytosolic LC3-I is activated by ATG7, which is then transferred to ATG3 and results in the formation of the membrane-bound protein form, LC3-II (33). LC3-II is involved in the formation and expansion of autophagosomes, and thus autophagy (34). In our study, expression of Beclin1 and LC3-II/I ratio were found to be  (35). In the current study, we successfully constructed MKN-45 and BGC-823 cells transiently expressing GFP-LC3, and confirmed that DSF/Cu treatment increases the accumulation of punctate GFP-LC3. In addition, we showed that LC3-II specifically localizes to autophagosomes and is degraded in the autolysosome. The autophagic flux may be analyzed by mRFP-GFP-tagged LC3 (35). Similar to that in the lysosomal lumen, GFP is degraded under acidic conditions, whereas mRFP is stable under this condition. Thus, an mRFP signal indicates the formation of autolysosomes, whereas yellow puncta (coexistence of GFP and RFP fluorescence) represents a phagophore or an autophagosome (35). In the current study, we constructed MKN-45 and BGC-823 cells stably expressing mRFP-GFP-LC3 and showed increased accumulation of yellow LC3-II puncta in DSF/Cu-treated cells, indicating increased autophagic flux, which is consistent with the findings reported for non-small cell lung cancer (36). Thus, our findings suggest that DSF/Cu treatment induces apoptosis and autophagy in MKN-45 and BGC-823 GC cells, leading to cell death. Redox homeostasis is important in maintaining the cellular functions and cell survival. ROS triggers oxidative stress, which plays a pivotal role in the development and treatment of tumors (37). However, the role of ROS in cancer progression is controversial. ROS at low and moderate levels is associated   Representative images of immunohistochemical analysis are presented. The column chart shows mean ± SD, n ≥ 3. *P < 0.05, **P < 0.01, and ***P < 0.001. DSF/Cu treatment (49). Furthermore, g-H2AX, a marker for DSB, is associated with the mediator of DNA damage checkpoint 1 (MDC1) (50,51). We showed an increase in g-H2AX in MKN-45 and BGC-823 cells treated with DSF/Cu, similar to the findings from studies on prostate cancer and atypical teratoid/ rhabdoid tumors (52,53). Thus, our findings suggested that DSF/ Cu triggered oxidative stress and DDR by increasing ROS, P53, and P21 levels. The Warburg effect, according to which cancer cells undergo aerobic glycolysis rather than OXPHOS even in the presence of oxygen, is a well-known phenomenon in cancer growth and proliferation (54). Further, metabolic reprogramming is important during cancer metastasis (55). Wnt/b-catenin signaling has been known to increase the glucose uptake and repress mitochondrial respiration (52). For instance, Wnt signaling reduces aerobic glycolysis in colon cancer (56), whereas mitochondrial function is suppressed via C-myc, the downstream gene of Wnt/b-catenin signaling, in triple-negative breast cancer (57). In prostatic cancer, DSF/Cu regulates cell metabolism (52), whereas in glioblastoma, DSF suppresses the Wnt signaling pathway (58). Our study showed that DSF/Cu not only inhibited glycolysis and OXPHOS by repressing OCR and ECAR in MKN-45 and BGC-823 cells but also decreased the levels of Fzd7, b-catenin, C-myc, and Cyclin D1 in MKN-45 and BGC-823 cells, indicating the deregulation of Wnt/ b-catenin signaling pathway in GC.
In the tumor xenograft mouse model, DSF/Cu inhibited tumor volume, as indicated by a TSR of 48.47%. Further, no change in the body weight between DSF/Cu-treated and control mice groups indicated the safety and low toxicity of DSF/Cu. Additionally, DSF/Cu inhibited cancer cell proliferation, induced necrosis, apoptosis, and autophagy in vivo. Moreover, DSF/Cu treatment triggered oxidative stress and modulated the expression of proteins associated Wnt/b-catenin signaling in the tumor tissues.
As this study was originally designed to explore the role of DSF/Cu in inhibiting the growth of GC cells, a positive control was not used; this is a limitation of the current study. Multiple studies have demonstrated that DSF-nanoparticles have enhanced anti-cancer effect (18). In the future, we plan to perform nano-modification of DSF and use docetaxel as a positive control to further explore the role of DSF in cancer. Furthermore, we are hopeful that more preclinical trials will be conducted before DSF is used in clinical practice.
In conclusion, our study demonstrated that DSF/Cu exhibits anti-cancer effects by modulating stress responses and the Wnt/b-catenin signaling pathway in MKN-45 and BGC-823 GC cells and xenograft tumor mouse model. Therefore, DSF may be a beneficial chemotherapeutic agent for the treatment of GC.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/supplementary materials; further inquiries can be directed to the corresponding author.

ETHICS STATEMENT
The animal study was reviewed and approved by the Ethics Committee of Lanzhou University Second Hospital Animal Center.

AUTHOR CONTRIBUTIONS
LW designed and performed the experiments as well as prepared and edited the original manuscript draft. XC designed the experiments. RW, HZ, CZ, and LX conducted the experiments. MP edited and reviewed the manuscript. YL supervised the complete project and edited the manuscript. All authors contributed to the article and approved the submitted version.

FUNDING
Our work was financially supported by the grants from National Natural Science Foundation of China (grant no. 31770537) and International Science and Technology Cooperation Project of Gansu Province (grant no. 18YF1WA113).