Introduction
Gastric cancer (GC) is one of the malignant tumors with high mortality [
1]. The 5-year survival rate of GC is less than 20% due to the high incidence of metastasis [
2‐
4]. Chemotherapeutic resistance after surgery is therefore a huge challenge in patients with middle and late stages of GC [
5]. As one of the most common chemotherapeutic agents, oxaliplatin inhibits DNA synthesis and delays the progression of GC [
6]. Overcoming oxaliplatin resistance is one of the key approaches in the adjuvant therapy of GC.
Deviations in the mechanisms of chromosome maintenance and segregation caused by inappropriate activation of meiotic chromosome regulatory genes are critical in chemotherapy resistance [
6,
7]. We recently used bioinformatics to screen optimal genes in platinum-resistant tissues and found that meiotic nuclear divisions 1 (MND1) was significantly upregulated. As a meiotic regulatory protein, MND1 can form a complex with HOP2 to promote homologous chromosome pairing and meiotic double-strand break repair [
8]. Meanwhile, the MND1-HOP2 complex can drive meiotic recombination into the interhomologous pathway, which replaces the telomere elongation mechanism in cancer cells [
9]. MND1, Kruppel-like factor 6 (KLF6), and E2F transcription factor 1 (E2F1) can form a positive feedback loop to confer cisplatin resistance [
10]. However, the role of MND1 in GC and oxaliplatin resistance remains unknown.
Glucose metabolism and chemotherapy resistance are inseparable in tumor progression [
11]. Reprogramming of glucose metabolism after chemotherapy is a hallmark of cancer progression, especially with respect to an increase in aerobic glycolysis [
12]. Transketolase (TKT) is involved in the pentose phosphate pathway and is upregulated in various cancers [
13]. TKT regulates NADPH and EGFR pathway to promote liver cancer progression [
14]. AKT phosphorylates and activates TKT activity [
12], and in turn, the increase of TKT promotes the expression of AKT [
12]. TKT promotes the resistance of colorectal cancer and breast cancer cells to oxaliplatin [
15]. However, whether MND1 could regulate TKT and participate in the progression of GC remain unclear.
Herein, we demonstrated that, MND1 could regulate cell cycle, apoptosis, proliferation, metastasis and oxaliplatin resistance, and play an oncogene role in GC. Forkhead box protein A1 (FOXA1) could directly binds to the promoter of MND1 to inhibit its expression. Moreover, MND1 was observed to be coexpressed with TKT, which further promoted GC progression via activating PI3K/AKT signaling pathway. These results suggest that MND1 may be a potential therapeutic target in GC.
Materials and methods
Tissue collection
Six pairs of GC cancer tissues and adjacent normal tissues were collected at the First Affiliated Hospital of Anhui Medical University. All patients provided their signed informed consent. Our study received approval from the Ethics Committee of Anhui Medical University.
Cell culture
Normal gastric epithelial cell GES-1, and GC cell lines MNK-45, SGC-7901, SGC-803, HGC27, and AGS were all purchased from the Cell Bank of the Chinese Academy of Sciences. Cells were cultured in the RPMI-1640 medium (Gibco, USA) supplemented with 10% fetal bovine serum (FBS; Gibco).
RNA interference and plasmid construction
The sequences of MND1, FOXA1, and TKT were amplified by PCR, and the target sequences were subcloned into a pEGFP-N1 vector according to the restriction sites. The recombinant plasmids were amplified into monoclonal colonies (Solarbio, China).
The double-stranded DNA was digested with multiple enzymes and was inserted into the RNAi lentiviral pLENR-GPH vector. The LV-shMND1 sequence:5ʹ-CCGGCGCAAGTTGTGGAAGAAATTTCTCGAGAAATTTCTTCCACAACTTGCGTTTTTGAATTC-3ʹ and the Lv-shTKT sequence: 5ʹ-TCACCGTGGAGGACCATTATTCTCGAGAATAATGGTCCTCCACGGTGATTTTT-3ʹ. The ligated products were used to transform the competent cells of E. coli, and the recombinants were detected by PCR. Then, the recombinant lentivirus (100 μL, 1 × 107 units/mL) containing green fluorescent protein (GFP) were cultured at the density of 2 × 105 cells/well. At 24 h after transfection with the lipofectamine 3000 transfection reagent, the fluorescence intensity was determined.
RNA extraction and qRT-PCR analysis
QRT-PCR was performed as previously described [
16]. The Trizol lysis buffer (Tiangen, China) was used to extract the total cell RNA, which was further processed with the reverse transcription kit (Transgen, China) with the addition of the corresponding reagents mentioned by the manufacturer. The primers were shown in Additional file
4: Table S3. According to the requirements of the amplification kit, the necessary reagents were added to the 8-connected tubes for subsequent PCR analyses.
Western blotting
WB was performed as previously described [
16]. The samples were separated by gel electrophoresis SDS-PAGE, then transferred to the membrane. The membrane was washed thrice with TPBS for 30 min, blocked with blocking solution for 30 min and incubated with primary antibody (Additional file
5: Table S4). After overnight, the membrane was exposed to the ECL chemiluminescence reagent. Finally, the bands were analyzed with the Image J software.
CCK-8 and cell cloning experiments
CCK-8 and cell cloning experiments were performed as previously described [
16]. The cell density of 5000 cells/well was seeded in 96-well plate. CCK8 reagent was added to each well and the absorbance was measured at 450 nm. The cells in each group were seeded in a 6-well plate at the density of 1000 cells/well and cultured for 12 days. The pictures were taken after dyeing.
EDU assay
The treated cells from each group were planted in a 12-well plate and incubated with EDU (Beyotime) solution. Then, the cells were washed thrice, treated with 4% paraformaldehyde, fixed, and re-washed thrice with a permeabilization solution. According to the instructions, the reaction solution was prepared and was added to the cells. The cells were then configured with the DAPI staining solution, and the images were taken under a fluorescence microscope.
Cell cycle and apoptosis analysis
The cells in each group were digested, centrifuged, and fixed with 75% ethanol, and was further processed according to the cell cycle kit manufacturer’s instruction (Beyotime). Cell apoptosis was determined by using the Annexin V-APC/PI apoptosis kit (Biolite, China) and analyzed by using flow cytometry.
Dual-luciferase reporter assay
The MND1 target sequence was subcloned into the pGL3-basic vector, and the FOXA1 target sequence was transferred into the pEGFP-N1 vector. The recombinant plasmid was used for monoclonal cloning and amplification. 293 T cells were co-transfected with the MND1 or FOXA overexpression vector, empty pGL3, pGL3 promoter plasmid, and pRL vector luciferase (internal control). Luciferase activity was detected after 24 h.
Invasion, migration, and wound healing assays
The migration and invasion experiments were conducted as described previously [
17]. Transwell chambers was used to detect migration and invasion. The confluent cells were scratched horizontally in the 6-well plate and pictures were taken at 0 and 48 h, respectively.
Liquid chromatography-tandem mass spectrometry (LC–MS/MS) and co-immunoprecipitation (CO-IP)
The MND1-binding proteins were identified using HGC27 cells transfected with the Flag-MND1 vector. The cells were lysed by using the CO-IP kit (Absin, China) to obtain the input, IP, and IgG groups. The expressions of MND1 in the input, IP, and IgG groups were detected by WB. Subsequent LC–MS/MS analysis was performed by using a high-resolution mass spectrometer. CO-IP analysis was performed according to the obtained results.
Lactate production and glucose uptake assays
The cells in each group were treated with tissue lysate, and were centrifuged at 12,000 ×g for 5 min. Then, the supernatant was collected. The operation was further performed according to the manufacturer instructions of the glucose detection kit (Beyotime), and the absorbance was measured at 630 nm. The glucose content in the culture medium was measured according to the standard curve. Lactic acid production was determined and normalized by using the L-lactic acid detection kit (Solarbio).
Immunofluorescence (IF) staining
The medium was removed after the cells being treated in each group. Then, the cells were fixed with 4% paraformaldehyde, permeabilized with 0.1% Triton X 100, blocked with goat serum, and incubated with the primary antibodies (Additional file
5: Table S4). After overnight, the cells were washed thrice with PBS, incubated with a secondary antibody (Beyotime), and stained with DAPI for nuclei staining.
Tissue microarray (TMA) and immunohistochemistry (IHC)
A total of 159 pairs of GC cancer tissues (gastric adenocarcinoma) and adjacent tissues were collected from the First Affiliated Hospital of Anhui Medical University to perform the microarray tissue chips. Clinical characteristics of the patients were shown in Additional file
6: Table S5. IHC was performed as previously described [
16]. Antigen retrieval was performed and 3% hydrogen peroxide was used to block endogenous peroxidase. The slices were treated with the immunoblocking solution and were incubated with the primary antibody (Additional file
5: Table S4). Finally, the coverslips were air-dried and the slices were photographed under a fluorescence microscope.
Animal study
Four-week-old BALB/c female nude mice were purchased from Shanghai Lingchang Biotechnology Co., Ltd., and the animal experiments were approved by the ethics of the Anhui Medical University. The HGC27 cells were stably transfected, digested, and centrifuged in each group at the logarithmic growth phase. The cells were re-spun with a mixture of 100 μL/tube and were inoculated into the axilla of mice (1 cm). The tumor volume and growth curve were observed and measured. Mice were anesthetized with 1% chloral hydrate. Then, the removed tumors were sorted in a descending order and were photographed. Finally, the mice were killed by de-necking, and the skin on the tumor body was peeled off with ophthalmic scissors.
Statistical analyses
Biological analysis Use Rstudio V4.0.2 version was used to analyze the downloaded FPKM data. Statistical analysis was performed using Prism 8.0 (Graphpad software). Chi-square test was applied to compare the count data. The differences between the groups were analyzed by independent t-test or one-way analysis of variance (ANOVA). Overall survival (OS) was plotted using the Kaplan–Meier method and was analyzed using the COX regression model. P < 0.05 was considered to be statistically significant. All experiments were repeated thrice.
Discussion
Cancer can regulate the activity of genes by activating meiotic chromosomes, leading to deviations in the mechanisms that control chromosome segregation and maintenance. MND1 is directly related to poor prognosis in breast cancer and lung adenocarcinoma patients [
18,
19]. In the present study, the high expression of MND1 was found to be associated with an advanced pathological grade and TNM stage, and was an adverse prognosis factor in patients with GC. Previous literature has documented that cell cycle-related proteins are associated with platinum resistance [
20]. Moreover, CDK4/6 inhibitors promote the efficacy of oxaliplatin [
21]. In this study, silencing MND1 increased the sensitivity of the GC cells to oxaliplatin and supplyment with oxaliplatin after silencing MND1 promoted the GC cells apoptosis. This finding is consistent with the report by Zhang et al., who demonstrated that MND1 competitively bound to KLF6 and conferred cisplatin resistance in lung cancer [
10]. MND1 may be involved in cyclin regulation, thereby controlling the process of oxaliplatin resistance.
FOXA1 is a member of the FOX family. As a transcription factor, FOXA1 is involved in the biological processes of various cancers [
22]. FOXA1 can be degraded via ubiquitination of ZFP91 in GC to promote the cell progression [
23]. Meanwhile, FOXA1 can inhibit the proliferation, EMT, and apoptotic escape of GC cells [
24]. FOXA1 has a highly-conserved winged-helix DNA-binding domain that can replace histone H1 and maintain the open conformation of the enhancer region, thereby activating the target gene expression [
25,
26]. This study proposes for the first time that FOXA1 binds to the promoter region of MND1 and represses the MND1 expression. After the combination of FOXA1 overexpression and MND1 silencing, the inhibitory effect on the proliferation of GC cells is more evident, further indicating that FOXA1 inhibits the expression of MND1 and thereby induces a biological effect.
Cancer cells induce metastasis and chemoresistance via increased glucose uptake and the creation of a more acidic tumor microenvironment [
27]. By using MS and CO-IP, this study revealed, for the first time, that MND1 can be coexpressed with TKT. TKT is mainly involved in de novo nucleotide synthesis and pentose phosphate pathway toward the promotion of rapid cancer cell proliferation [
28]. Furthermore, TKT can enhance oxidative stress as well as the proliferative and metastatic capacities of hepatoma cells in vitro and promote tumor progression by binding to the EGFR pathway [
29]. Our results showed that silencing TKT could promote the proliferation of GC cells, which is consistent with past report [
30]. Tumor metastasis is inseparable from the enhancement of glycolytic capacity [
31]. We also found that silence of TKT can inhibit metastasis and reduce lactate generation and glucose uptake in GC cells. We speculate that TKT promotes the process of glycolysis and maintains the acidic tumor microenvironment, which facilitates metastasis of GC cells.
To date, the mechanism of TKT involved in oxaliplatin resistance is not clear. It has been reported that TKT activates the AKT expression via GRP78 phosphorylation [
30]. Moreover, AKT can phosphorylate TKT at thr382 to promote its expression and purine synthesis [
32]. Interestingly, TKT was significantly overexpressed in samples of oxaliplatin resistant colorectal cancer patients [
32]. It may be that TKT affects the aerobic glycolysis process of the tumor,,which is involved in oxaliplatin resistance. Park SY et al. [
33] recently found that inhibition of AMPK in oxaliplatin-resistant colorectal cancer cells induced autophagy by silencing AKT/mTOR pathway and reducing glycolytic enzymes. Therefore, targeting AMPK may increase the sensitivity of colorectal cancer cells to oxaliplatin. Overexpression of MiR-138 suppressed the PDK1 expression to decrease the oxaliplatin resistance of colorectal cancerr [
34]. Cheng et al. demonstrated that the down-regulation of
PTBP1 gene can overcome oxaliplatin resistance of drug-resistant colon cancer cells by regulating glycolysis [
35]. In addition, the PI3K/AKT signaling pathway has also been reported to be involved in mediating glycolysis and promoting oxaliplatin resistance. Fang et al. has recently shows that AKT mediated phosphorylation of
TOPBP1 at Ser1159 is involved in the activation of oxaliplatin resistant GC cells [
36]. In colon cancer cells,
SHP2 activates AKT to promote oxaliplatin resistance [
37].Our study found that TKT activated the phosphorylated expression of AKT and oxaliplatin resistance process of GC cells, which are consistent with the above reports [
33‐
37]. As a whole, this study indicating that TKT activates AKT and exerts functional effects via the PI3K/AKT signaling axis (Fig.
8O).
Tumor cells can obtain energy via glycolysis and pentose phosphate pathways, thereby enhancing their resistance to chemotherapeutic drugs [
11,
38]. Our study found that MND1 attenuated the toxic effects of oxaliplatin on the GC cells. Furthermore, our findings were validated in a nude mouse model. However, past studies have demonstrated that TKT is overexpressed in patients receiving oxaliplatin chemotherapy, which may be related to the activation of the glucose metabolism pathways after the tumor cells are damaged [
39]. Unfortunately, the correlation between TKT and oxaliplatin resistance is out of the scope of this study, and further investigation is needed.
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