Background
Cancer is the second leading cause of death worldwide, among which gastric cancer (GC) is one of the most common malignancies of the digestive system, ranking second in cancer-related mortality [
1]. The main causes of GC death are rapid proliferation, invasion, metastasis, and anti-cancer drug resistance [
2,
3]. The prognosis of patients can be significantly improved by the combination of radiotherapy, neoadjuvant therapy, and other treatment measures [
4,
5]. However, symptoms of early GC are inconspicuous. An important factor in the gastric cancer-related mortality is the usual “advanced stage at the diagnosis” [
6]. Therefore, the occurrence and development mechanisms of GC remain to be investigated.
Abnormal cellular processes such as proliferation and apoptosis in cancer cells are significant causes of cancer progression. Recent studies have found that kinesin motor proteins could facilitate cancer progression via promoting cell proliferation [
7]. Increasing evidence demonstrate that kinesin proteins play carcinogenesis role in the initiation and progression of human cancers [
8]. Kinesin superfamily proteins (KIFs) belong to kinesin motor proteins, which can support intracellular transport such as RNA, intracellular vesicles, and other materials [
9,
10]. Up to date, over 600 members of the kinesin superfamily have been identified in addition to more than 45 members of mammals [
11]. Increasing studies [
8,
12‐
16] have shown that KIFs is related to the initiation and progression of breast cancer, lung cancer, and pancreatic cancer. Moreover, KIFs may be therapeutic targets for non-small cell lung cancer (NSCLC) [
17].
Kinesin family member 15 (KIF15), a member of KIFs, belongs to the kinesin-12 subfamily, and is the only motor protein which is structurally similarly to kinesin-5 [
18,
19]. KIF15 localization and motility can be regulated by the C-terminus of TPX2 and microtubule dynamics. The regulation of KIF15 makes centrosome separate during bipolar spindle assembly [
20,
21]. Recently, KIF15 was reported to be overexpressed in several types of human diseases such as glioma and breast cancer [
22,
23], and may facilitate these diseases. Moreover, KIF15 has been reported to promote development of digestive tumors such as pancreatic cancer [
16]. However, the effects of KIF15 in GC has not been reported by consulting literatures online.
In this study, we attempted to investigate the function of KIF15 in GC through in vitro and in vivo experiments. Data from The Cancer Genome Atlas (TCGA) database were downloaded to analyze the relationship between KIF15 expression level and clinicopathologic features of GC patients. MTT and colony formation assays were performed to detected cell proliferation. Annexin V-APC staining was used to assess the apoptosis rate change in GC cells. GC xenograft mouse models were established to verify the results of in vitro experiments. Mechanistically, Human Apoptosis Antibody Array kit including 43 proteins was used to screening potential apoptotic targets of GC in GC. In summary, KIF15 serves a promoting role in GC and might be an early diagnosis marker.
Methods
Human samples
Clinicopathological data of GC patients were downloaded from TCGA database. Informed consent was obtained from all 122 GC patients who underwent survival follow-up in our hospital. All GC tissues enrolled in this study were taken from the Bio-Bank of Department of General Surgery in the Fourth Affiliated Hospital of Harbin Medical University. The experiment has been approved by the Medical Ethics Committee of the Fourth Affiliated Hospital of Harbin Medical University. Ethical approval number is YXLLSC-2018-01. All of the patients underwent gastrectomy, of which 41 GC tissues with paired paracarcinoma tissues from patients were obtained for the immunohistochemical assay. The specimens were pathologically diagnosed as GC. All patients have signed informed consent. All experiments were performed with mycoplasma-free cells.
Cell culture
GC cell lines (AGS, SGC-7901, BGC-823 and MGC-803) and normal human gastric epithelial cell line GES-1 were bought from Shanghai Institutes for Biological Sciences, China. These cells were cultured with Dulbecco’s Modified Eagle medium (DMEM) medium supplemented with 10% fetal bovine serum (FPS), 100 U/ml of penicillin, and 100 μg/ml of streptomycin (HyClone).
Immunohistochemistry (IHC)
For IHC assay, 5-mm sections from formalin-fixed-paraffin embedded tissue were transferred to polylysine-coated slides and incubated with primary antibodies against KIF15 (1:50, fine test, Wuhan, Hubei, China). Subsequently, the GTVision Two-step Visualization System (Genetech, Shanghai, China) was used to visualize the immunostaining, and hematoxylin was used to counter-stain. Finally, the sections were examined under light microscopy and evaluated by the Immuno-Reactive-Score (IRS), which was calculated by multiplying staining proportion score (PS) and staining intensity score (IS). PS was defined as 0 (0%), 1 (1–25%), 2 (26–50%), 3 (51–75%), and 4 (76–100%). IS was classified as 0 (negative staining), 1 (weak), 2 (moderate) and 3 (strong).
Gene knockdown
BR-V-108 carrier was used in this experiment. Green fluorescent protein (GFP) was consulted in the lentivirus. After the design software was evaluated and determined, the following sequences were selected as interference targets: GCTGAAGTGAAGAGGCTCAAA. The lentivirus-containing short hairpin RNA (shRNA) targeting KIF15 was purchased from Biosci Res (Shanghai, China). The shRNAs were transfected into the GC cell lines using Lipofectamine 3000 (Invitrogen) according to the manufacturer s instructions. Each group infected the cells with 40 μl of lentivirus at a concentration of 1 × 108 TU/ml. At 48 h post-transfection, the cells were selected with puromycin (2 μg/ml) for 2 weeks to construct stable cell lines. The transfection efficiency was verified by light microscopy, fluorescence microscope, qRT-PCR and WB.
RNA extraction and qRT-PCR
To determine the knockdown efficiency, RNA samples were isolated from the collected cell lines. RNA extraction was performed using Trizol reagent (Invitrogen, Waltham, MA, USA) following the protocols. Then the RNA sample was reversely transcribed to cDNA using M-MLV RT (Promega Corporation, Madison, USA). Real-time PCR was carried out using the SYBR Premix Ex Taq reaction system, and cycling conditions were set according to the manufacturer’s instructions. GAPDH was employed as an internal control for different samples. qRT-PCR primers were all obtained from Sangon Biotech (Shanghai, China). Knockdown levels were calculated using the 2(−ΔΔCt) method. The primers for gene application were listed as follows: KIF15-F5-CTCTCACAGTTGAATGTCCTTG and KIF15-R5-CTCCTTGTCAGCAGAATGAAG; and GAPDH-F5-TGACTTCAACAGCGACACCCA and GAPDH-R5-CACCCTGTTGCTGTAGCCAAA.
Western blot (WB)
To detect the expression level of KIF15 protein, cells were lysed and total proteins were denatured after quantification. The same amount of protein was loaded on sodium dodecyl sulfate (SDS) polyacrylamide gels for electrophoresis, and then electrophoretically transferred onto polyvinylidene difluoride (PVDF) membranes. Non-fat milk powder with a concentration of 5% was used to block the non-specific sites. The blots were then incubated with various primary antibodies at 4 °C overnight. Fluorescent-based anti-rabbit IgG secondary antibody (Beyotime, Shanghai, China) was used to detect the captured primary antibodies on the membranes. Protein bands were visualized by ECL reagent (Bio-Rad, Hercules, Canada) and imaged with the ChemiDoc XRS System (Bio-Rad, Hercules, CA, USA). Finally, Image J Software was used to quantify the proteins.
Cell proliferation and colony formation assay
Cells were digested with trypsin and cultured into suspension and counted. The cells were placed into 96-well plates, and cultured for 5 days. To determine the cell proliferation, 20 μl of 5 mg/ml 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT, Sigma) solution was added into each well. After another 4 h incubation, supernatants were discarded, and 100 μl dimethyl sulfoxide (DMSO, Sigma) was supplemented to dissolve the formazan crystals. Finally, the absorbance was determined on a microplate reader at a wavelength of 490 nm (Bio-Rad).
For colony formation assays, cells were seeded in six-well plates at a density of 2 ml/well with each experimental group being plated in three wells. The cells were incubated over a period of 8 days. The cell colonies were fixed with 1 ml paraformaldehyde each well (Sinopharm Group, Beijing, China) for 30 min followed by Giemsa staining (500 μl/well; Shanghai Dingguo, Shanghai, China) for 20 min. Finally, the colonies were photographed and counted by fluorescence microscopy (Olympus, Japan). Each experiment was performed in triplicate.
Apoptosis assay and tunnel staining
sh-KIF15 cell and sh-Ctrl cell cultures were trypsinized in the logarithmic growth phase for 1 h. Both adherent and floating cells were collected and washed three times with ice-cold PBS (Phosphate Buffered Saline). Subsequently, Annexin V-APC staining was used to assess the apoptosis according to the manufacturer’s instructions. Flow Cytometer (eBioscience, California, USA) was used to analyzed the GFP and Annexin V-APC of the stained cells within 15 min. GFP showed green fluorescence, and Annexin V-APC showed red fluorescence. Each experiment was performed in triplicate.
Tumor sections from GC xenograft mouse model were taken and tunel staining was performed for 1 h according to the instructions. The ratio of red light to blue light was observed by fluorescence microscope, and the apoptosis rate was calculated.
Establishment of GC xenograft mouse model and in vivo imaging assay
Cells at concentration 1 × 107 cells/ml with or without KIF15 knockdown were injected subcutaneously into the back next to the right forelimb of the nude mouse, and permitted to grow until palpable. Each mouse was injected with 200 μl. Both sh-Ctrl and sh-KIF15 group were consisted of six mice. The tumor growth was observed on the fifth day. Then tumors were measured every 3 days with a vernier caliper until the eighteenth day. Tumor volume was calculated according to the following formula. Tumor volume (mm3) = π/6 × L×W × W (W, width at the widest point; L, perpendicular width). Before the mice were sacrificed at the eighteenth day, vivisection imager Lumina LT (Perkin Elmer, Massachusetts, USA) was used for in vivo imaging, and fluorescence quantitative analysis was performed. All subcutaneous tumor tissues were harvested, weighted, fixed with 4% paraformaldehyde, and embedded, resected, stained with hematoxylin-eosin (HE) as well as Ki-67 to evaluate proliferation.
Protein array analysis
shRNA was used to knockdown KIF15 mRNA expression in GC cell line SGC- 7901. Analysis of proteins by Human Apoptosis Antibody Array kit (ab134001, Abcam, Cambridge, UK) was conducted following the manufacturer’s instructions. The intensity score of each array spot was measured by the ImageJ software program. Apoptotic proteins expression levels were detected by comparing the sh-KIF15 group with the sh-Ctrl group.
Statistical analysis
All the experiments were conducted two or three times. The statistical analyses in the current study were carried out using the software of GraphPad Prism version 8.0.1 (GraphPad Inc, San Diego, CA, USA). The Student’s t test was used to analyse difference between two groups. High and low age groups were divided by the median age of all patients. ANOVA was used to compare the statistical differences in more than three groups. Kaplan–Meier survival analysis and the log-rank test were used for patient survival analysis. The correlation between KIF15 and seven apoptotic genes was calculated using Spearman’s correlation. Values of P less than 0.05 were considered to indicate a significant statistically difference.
Discussion
Previous studies have proposed that KIFs play important roles in different diseases even various of cancers. It can lead to congenital thrombocytopenia, breast cancer, oral cancer and bladder cancer [
8,
23‐
26]. Recently, KIF15 has been reported to be highly expressed in gastrointestinal (GI) track cancers such as pancreatic cancer and hepatocellular cancer [
16,
27]. Furthermore, abnormal high expression of KIF15 could predict a poor prognosis in patients with lung carcinoma [
15]. However, the function of KIF15 in GI track disease remains to be explored. And the mechanisms of KIF15 on apoptosis in GI track cancers have not been reported as well. Here, our research proves that abnormal high expression of KIF15 is a common phenomenon in patients with GC by consulting data in TCGA and conducting IHC. Our study demonstrated that KIF15 facilitate GC via promoting cell proliferation and inhibiting cell apoptosis. Particularly, our results showed that KIF15 knockdown could lead to significantly downregulation of several apoptotic proteins such as sTNF-R2 and HSP60. Furthermore, higher expression of KIF15 was observed in elder patients (≥ 67 years old) and early GC (G1/G2). High expression of KIF15 could predict poor prognosis in patients with GC. These results indicate that KIF15 is highly expressed in the early stage of GC, and may be used in the early diagnosis of GC.
The expression level of KIF15 was found up-regulated in GC. However, its effect on GC was still unknown. It was reported that KIF15 could promote pancreatic cancer cell proliferation as a tumor promoting factor [
16]. The uncontrolled proliferation of cells is an important factor in the progression of cancer. Our results of MTT assay and colony formation found that the cell viability in sh-KIF15 group is significantly decreased, and the cell proliferation ability is also remarkably reduced. This proved that KIF15 can promote proliferation of GC cells. Spindle formation is a critical step in the cell division cycle, and KIF15 is a kinase participating in spindle formation to power mitosis [
28]. Mitotic centromere-associated kinesin (MCAK) is also an important protein regulating mitosis, and it could promote GC cell proliferation by promoting mitosis [
29]. Therefore, we speculate that the mechanism of KIF15 to control proliferation may be similar to MCAK, which regulates mitosis to control GC cell proliferation. In addition, whether KIF15 is affected by other factors to affect cell proliferation is also worth studying.
In addition to abnormal proliferation, changes in the cell apoptosis are also key causes of cancer progression. Cell apoptosis is programmed cell death, and it plays an important role in animal tissue homeostasis [
30]. The normal survival of cells require active apoptosis inhibition, which is achieved by inhibiting the expression of pro-apoptotic factors and promoting the expression of anti-apoptotic factors [
31]. Abnormal regulation of apoptosis can contribute to a variety of human diseases including cancer. Recent study has reported that knockdown of KIF15 effectively promoted the apoptosis in breast cancer cells [
12]. However, the function and molecular mechanism of KIF15 in regulating GC cell apoptosis has not been demonstrated.
Apoptosis has been widely studied, in which several apoptotic protein families were generally known. These apoptotic protein families, such as tumor necrosis factor receptor (TNFR) and inhibitor of apoptosis proteins (IAPs), play important roles in apoptotis pathways. TNFR includes TNR-R1, TNF-R2, and their soluble forms. It has been confirmed that TNFR was significantly higher in GC tissues than paracarcinoma tissues [
32]. sTNF-R2 is the soluble form of TNF-R2, which is known as an anti-apoptotic factor. It has been reported that high expression of sTNF-R2 is associated with ovarian cancer [
33]. Whether sTNF-R2 is linked to GC is worth investigating. IAPs includes XIAP, Livin, and Survivin. IAPs has also been verified to promote GC cell apoptosis and suppressing GC cell growth [
34]. XIAP can inhibit Caspase-3, Caspase-7, and Caspase-9 to inhibit apoptosis [
35]. Caspase, a member of cysteinyl aspartate-directed proteases that breakdown intracellular proteins, is the central regulatory molecule of cell apoptosis [
36]. Once activated, Caspase hydrolyzes downstream substrates, triggering a series of events that lead to the desired biological response [
37]. In addition to Caspases, XIAP can regulate apoptosis through mitochondrial pathway [
38]. It have been also reported that XIAP was a kind of NF-κB-regulated gene products, and IAPs participate in many TNFR-induced pathways [
25,
39,
40]. Thus, XIAP expression may be regulated by NF-κB which is associated with TNFR, and this speculation remains to be proved in GC cancers.
Therefore, we hypothesized that KIF15 may regulate the apoptosis of GC cells through these well known apoptotic factors. The Human Apoptosis Antibody Array protein kit was used in this study to detect apoptotic proteins which may be regulated by KIF15. With the results of this kit, we can infer the apoptotic pathways possibly regulated by KIF15 in GC. In our experiments, expression levels of seven common apoptotic factors were decreased markedly after silencing KIF15. Interestingly, in addition to the superfamily members mentioned above, the expression levels of IGF-2 and HSP60 were also significantly reduced when KIF15 was knocked down. IGF-2 is another anti-apoptotic factor which can also inhibit cell apoptosis [
41]. HSP60 belongs to heat shock proteins (HSPs) which are well known as tumor promoting factors [
42]. Studies have shown that HSP60 could promote GC via inhibiting GC cell apoptosis [
43]. All the seven proteins which were detected by The Human Apoptosis Antibody Array protein kit could suppress cell apoptosis. Therefore, we believe that KIF15 may inhibit apoptosis through regulating one or more apoptosis signaling pathways. However, the specific pathway of its regulation of apoptosis remains to be verified by further experiments. Besides the changes of the expression of these seven proteins in the kit, the expression of their corresponding genes are also correlated with the expression of KIF15 in TCGA database. From the Human Apoptosis Antibody Array protein kit results, three of the seven genes were significantly correlated with KIF15. Unlike the other two genes, TNFRSF1A is negatively correlated with KIF15 expression level. This result may be because sTNF-R1 protein is not only regulated by KIF15, but also affected by other pathway proteins. In addition, although the correlation between XIAP and KIF15 is not significant, some studies have found that survivin and XIAP can regulate GC proliferation and apoptosis [
34]. These results prove that KIF15 may regulate these apoptosis proteins from another aspect.
Elevated KIF15 expression can predict a poor prognosis in patients with lung adenocarcinoma [
15]. Similarly, most of GC are adenocarcinomas, thus the expression level of KIF15 may also be used to assess postoperative survival in patients with GC. The clinical data of patients with GC in the TCGA database were analyzed to explore the relationship between the expression level of KIF15 and the clinical characteristics of patients with GC. It was found that KIF15 expression level was statistically higher in patients with high age (> 67 years old) or early histologic stages (G1/G2). Combined with the common phenomenon of high expression level of KIF15 in GC, we hypothesis that high expression of KIF15 may be used to predict the progression of GC in elderly people. Histological grading suggested that KIF15 might be related to the progression of GC, and this mechanism remains to be further investigated. In addition, similar to lung cancer, the follow-up data of all 122 GC tissues in our biobank showed that patients with high expression level of KIF15 had lower 5-year survival rate than patients with low expression level of KIF15. This result showed that KIF15 could serve not only as a biomarker for early diagnosis of GC, but also as a marker for prognosis of GC patients.
Furthermore, except for proliferation and apoptosis play vital roles in GC, invasion and metastasis are important causes of poor prognosis of GC. Kinesin family member 26B (KIF26B) is a novel oncogene upregulated in metastatic GC tissues, and it could promote metastasis via VEGF pathway in GC [
44]. However, no report has been published on whether KIF15 is related to the invasion and metastasis of GC so far. Moreover, it has been reported that KIF15 could assist cancer cells to develop chemotherapy resistance, thus affecting the efficiency of cancer treatment [
45]. KIF15 tends to be compensatorily up-regulated when Eg5 is inhibited by anticancer drugs because of its structural and functional similarities to Eg5 in mitosis [
46,
47]. Therefore, whether KIF15 plays a role in chemotherapy of GC also needs further investigation. The combined inhibitor of KIF15 and Eg5 may provide a novel strategy for figuring out chemotherapeutic resistance.
Hence, the present study verified the high expression of KIF15 in GC, and further explore the possible mechanism of promoting proliferation and inhibiting apoptosis of KIF15 in GC progression. This study also confirmed the important role of KIF15 in clinical practice, and showed KIF15 may be used as a diagnostic and prognostic factor of GC.
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