CircPTK2 Suppresses the Progression of Gastric Cancer by Targeting miR-196a-3p/AATK Axis


 BackgroundGastric cancer is a type of malignant tumor with high morbidity and mortality. It has been shown that circular RNAs (circRNAs) exert critical functions in gastric cancer progression via working as microRNA (miRNA) sponges to regulate gene expression. However, the role and potential molecular mechanism of circRNAs in gastric cancer remain largely unknown.MethodsCircPTK2 (hsa_circ_0005273) was identified by bioinformatics analysis and validated by RT-qPCR assay. Bioinformatics prediction, dual-luciferase reporter and RNA pull down assays were used to determine the interaction between circPTK2, miR-196a-3p, AATK.ResultsThe level of circPTK2 was markedly downregulated in gastric cancer tissues. Upregulation of circPTK2 suppressed the proliferation, migration and invasion of gastric cancer cells, while downregulation of circPTK2 exhibited opposite effects. Mechanically, circPTK2 could competitively bind to miR-196a-3p and prevent miR-196a-3p to reduce the expression of AATK. In addition, circPTK2 overexpression inhibited tumorigenesis in a xenograft mouse model of gastric cancer.ConclusionCollectively, circPTK2 functions as a tumor suppressor to suppress gastric cancer cell proliferation through regulating miR-196a-3p/AATK axis, suggesting that circPTK2 may serve as novel therapeutic target for gastric cancer.


Introduction
Gastric cancer is the fth most common malignancy worldwide and is the third leading cause of cancerrelated deaths [1][2][3]. Although the current clinical diagnosis and treatment for gastric cancer are continuously improving, the 5-year survival rate of patients with gastric cancer is less than 30% [4,5]. Gastric cancer is a complex cellular network, currently, the etiology and pathogenesis are not yet clear, which brings corresponding di culties to its early diagnosis and treatment [6,7]. Circular RNAs (circRNAs) are a class of non-coding RNAs derived from back-spliced exons [8,9]. Unlike linear RNA, circRNAs are covalently closed continuous loops that lack 5' (cap) and 3' (polyadenylation) ends [10]. Meanwhile, circRNAs are found to be relatively stable and evolutionally conserved in the cytoplasm [10,11]. CircRNAs have been found to be implicated in the progression of cancers [12,13].
Signi cantly, circRNAs are aberrantly expressed in multiform types of cancer, including gastric cancer [14,15]. However, the biological function of circRNAs in gastric cancer remains largely unclear.
MicroRNAs (miRNAs) are a kind of non-coding RNAs with 19-25 nucleotides in length [16,17]. It has been shown that miRNA can regulate gene expression at the post-transcriptional level [18,19]. Considerable studies reported that circRNAs could exhibit their biological roles through acting as "miRNA sponges" to regulate gene expressions [20,21]. For instance, zhang et al showed that circNRIP1 could promote gastric cancer progression through sponging miR-149-5p [22]. Luo et al found that circCCDC9 inhibited gastric cancer cell proliferation via targeting miR-6792-3p/CAV1 axis [23].
In this study, we screened differentially expressed circRNAs (DEcircRNAs) in gastric cancer and found that circPTK2 was signi cantly downregulated in gastric tissues. In addition, circPTK2 could inhibit proliferation, migration and invasion of gastric cancer cells through functioning as a miRNA sponge to upregulate the expression of tumor suppressor gene AATK. These data indicated that circPTK2 may be used as a potential target in gastric cancer therapy.

Materials And Methods
Identi cation ofdifferentially expressed circRNAs (DEcircRNAs) Gastric cancer-related datasets (GSE93541, GSE89143 and GSE78092) were downloaded from the GEO database. For GSE93541 dataset, R language was utilized to analyze the DEcircRNAs in plasma samples from gastric cancer patients and healthy controls. For GSE89143 and GSE78092 datasets, R language was utilized to analyze the DEcircRNAs between gastric cancer tissues and adjacent normal tissues. The threshold value of differentially expressed genes was set at 2 times of different multiple and p<0.05. The intersection of DEcircRNAs from three datasets was performed using the Venn Diagram package.

Specimen collection
Gastric cancer tissues and matched adjacent normal tissues were obtained from the First A liated Hospital of Soochow University. The written consent was obtained from each patient with gastric cancer.
All samples were frozen in liquid nitrogen and stored at -80°C. This study was approved by the ethics committee of The First A liated Hospital of Soochow University.

Cell culture
Human AGS and MKN45 cells were obtained from ATCC (Manassas, VA, USA). Cells were maintained in DMEM medium (Thermo Fisher Scienti c, Waltham, MA, USA) containing 10% fetal bovine serum (FBS) in a 5% CO 2 incubator at 37°C.
Lentivirus-containing short hairpin RNA (shRNA) targeting circPTK2 or AATK plasmids were purchased from Hanbio (Shanghai, China). After that, 293T cells were transfected with above-mentioned lentiviral plasmids were transduced into 293 T cells to package lentivirus to infect AGS and MKN45 cells.
Subsequently, the infected cells were selected by 2 µg/mL of puromycin.

RT-PCR and RT-qPCR
TRIzol Reagent (Thermo Fisher Scienti c) was used to extract total RNA. Meanwhile, genomic DNA (gDNA) was isolated using the Genomic DNA Isolation Kit (Sangon Biotech, Shanghai, China). After that, cDNA was synthesized using EntiLink™ 1st Strand cDNA Synthesis Kit (ELK Biotechnology). Later on, the DreamTaq DNA Polymerase (Thermo Fisher Scienti c) was used for PCR. Then, the cDNA and gDNA PCR products were analyzed 2% agarose gel electrophoresis. In addition, qPCR was performed using the EnTurbo™ SYBR Green PCR SuperMix on a Verse ow cytometry system (BD Biosciences). β-actin and U6 were used as internal controls. The primers were listed in Table 1.
Actinomycin D and RNase R treatment AGS and MKN45 cells were incubated with Actinomycin D (2 μg/ml; Sigma) for 0, 6, 12, 18 and 24 h to assess the stability of circPTK2 and its linear isoform. In addition, total RNA (10 μg) was treated with RNase R (5 U/μg; Epicenter Technologies) for 30 min at 37°C, then, the level of circPTK2 was detected using RT-qPCR assay.

Cell viability assay
Transfected AGS and MKN45 cells were seeded onto 96 well plates and cultured for the indicated times.
Later on, cell viability was determined using the Cell Counting Kit-8 (CCK-8) reagent (Dojindo Laboratories, Kumamoto, Japan). Subsequently, the absorbance was measured at a wavelength of 450 nm.

Colony formation assay
Transfected AGS and MKN45 cells were plated onto 6 well plates. After 2 weeks of incubation, cells were xed with 4% paraformaldehyde for 20 min and then stained with 0.1% crystal violet at room temperature. After that, cell colonies were imaged and counted using a light microscope.

Transwell assay
Transfected AGS and MKN45 cells were suspended in 200 μL serum-free medium and placed into the upper chambers (Corning, NY, USA). Later on, the lower chambers were loaded with DMEM medium (600 μL) containing 10% FBS. After 24 h of incubation, the cells on the lower surface were xed with 4% formaldehyde, and then stained with 0.1% crystal violet solution. After that, the stained cells were imaged using a light microscope. Transwell chambers that coated with Matrigel were used for cell invasion assay.
Animal study BALB/c nude mice (5-6 weeks old) were purchased from the Jingda experimental animal Co., Ltd. (Changsha, China). This study was approved by the First A liated Hospital of Soochow University and conducted according to the institutional guidelines. Animals were divided into 4 groups (shRNA NC; circPTK2 shRNA2; OE NC; circPTK2 OE). AGS or MKN45 cells (5 x 10 6 cells/mouse) were subcutaneously injected into the right ank of each mouse. The size of the tumor was measured every 5 days. The tumor volume was calculated by the formula: (length x width 2 )/2. In the end of the experiment, the tumor was removed, and tumor weight was measured.

Luciferase reporter assay
The sequences including miR-196a-3p binding sites in the circPTK2 3' UTRs and AATK 3' UTR were cloned into the luciferase reporter vector pGL6-miR (Beyotime). After that, AGS or MKN45 cells were cotransfected with the luciferase plasmids and miR-196a-3p mimics for 48 h. Later on, the re y and Renilla luciferase activities were measured by a dual-luciferase reporter assay system (Promega, USA).

RNA-pull down assay
The biotinylated circPTK2 or biotinylated miR-196a-3p probe were incubated with Streptavidin magnetic beads (Thermo Fisher Scienti c) for 2 h at room temperature. Later on, AGS or MKN45 cells were incubated with the magnetic beads at 4°C overnight. After that, the complex was pulled down and analyzed by RT-qPCR assay.

Immunohistochemistry (IHC)
The tumor tissues were xed in 4% paraformaldehyde and then embedded in para n. Later on, tissues were sectioned (5-μm thick) and then stained with primary antibody speci c for AATK (Abcam) overnight at 4°C. Images were captured by a uorescence microscope. Coimmunoprecipitation (co-IP) Cells were transfected with pcDNA3.1-AATK or pcDNA3.1-STK39 plasmids After that, the transfected cells were lysed using RIPA buffer, and then cell lysates were treated with anti-AATK, anti-STK39 or anti-IgG antibodies. Later on, the samples were incubated with protein A and G Sepharose beads for 4 h at 4°C. Then, the protein binding complex was isolated and subjected to western blot assay.

Statistical analysis
Data were presented as mean ± standard deviation (S.D.). Differences between three or more groups were analyzed by One-way analysis of variance (ANOVA) and Tukey's tests. P < 0.05 was considered statistically signi cant.

Differential expression of circRNAs in gastric cancer
To identify DEcircRNAs in gastric cancer, R language was performed to analyze the circRNA expression pro les from three gastric cancer-related datasets (GSE93541, GSE89143 and GSE78092). As shown in Figure 1A, heat map showed that 538, 268 and 211 DEcircRNAs were identi ed in the GSE93541, GSE89143 and GSE78092 datasets respectively. Using a Venn diagram, 12 overlapping DEcircRNAs (4 were upregulated, while 8 were downregulated) were identi ed in these three datasets ( Figure 1B). To verify these results, all of these circRNAs were chosen for further con rmation in gastric cancer tissues and normal tissues using RT-qPCR ( Figure 1C). The results showed that hsa_circ_0005273 (circPTK2) exhibited the most signi cant difference in gastric cancer tissues ( Figure 1C). Furthermore, we found that circPTK2 is derived from exons 27, 28, 29 of the PTK2 gene ( Figure 2A). In addition, sanger sequencing veri ed the head-to-tail splicing in the RT-qPCR product of circPTK2 ( Figure  2A). Next, RNase R digestion assay showed that linear form of PTK2 was markedly decreased under the RNase R treatment, while the circular isoform was resistant to RNase R digestion, suggesting that circPTK2 harbors a loop structure ( Figure 2B and 2C). Meanwhile, the stability of circPTK2 in AGS and MKN45 cells was detect using the actinomycin D assay. The data showed that the linear PTK2 mRNA transcript was less stable than circPTK2 transcript in AGS and MKN45 cells under treatment with Actinomycin D (Figure 2D and 2E). Furthermore, to con rm the existence of circPTK2, we designed convergent primers to amplify PTK2 mRNA and divergent primers to amplify circPTK2. The results of PCR showed that circPTK2 was only ampli ed by cDNA templates from AGS and MKN45 cells using divergent primers ( Figure 2F). To sum up, circPTK2 is decreased in gastric cancer tissues and is a stable circRNA from PTK2.

Overexpression of circPTK2 inhibits gastric cancer cell proliferation and tumor growth
To explore the biological role of circPTK2 in gastric cancer cells, we used shRNAs to downregulate the level of circPTK2 in gastric cancer cells. (Figure 3A). Meanwhile, we established circPTK2 stably overexpressing gastric cancer cells via transfecting with circPTK2 OE plasimds ( Figure 3A). Additionally, downregulation of circPTK2 notably promoted the viability, proliferation, migration and invasion of AGS and MKN45 cells, while circPTK2 overexpression exhibited opposite effects. (Figure 3B, 3C, 3D and 3E). We further investigated the effect of circPTK2 on tumor growth in vivo. Silencing of circPTK2 markedly increased the tumor volume weight in mouse xenografts. However, overexpression of circPTK2 obviously inhibited the tumor growth of AGS and MKN45 cells ( Figure 4A, 4B and 4C). Collectively, circPTK2 may play a tumor-suppressive role in gastric cancer in vitro and in vivo.

CircPTK2 acts as the sponge of miR-196a-3p
It has been shown that circRNAs can regulate gene expression via acting as miRNA sponges [24]. Thus, we predicted the potential miRNAs binding to circPTK2 using the CircInteractome dataset. The data showed that miR-196a-3p functioned as the target of circPTK2 with complementary binding sites ( Figure   5A). In addition, miR-196a-3p mimics notably reduced the luciferase activity in AGS and MKN45 cells cotransfected with circPTK2-WT; however, miR-196a-3p mimics had no effect on luciferase activity in AGS and MKN45 cells co-transfected with circPTK2-MT, suggesting that miR-196a-3p is a direct binding target of circPTK2 ( Figure 5B). The RNA pull-down assay results showed that miR-196a-3p was pulled down by biotin-labeled circPTK2 probe in both AGS and MKN45 cells and circPTK2 was pulled down by biotinlabeled miR-196a-3p probe, indicating that miR-196a-3p directly interacted with circPTK2 ( Figure 5C and 5D). Meanwhile, RT-qPCR results showed that miR-196a-3p was highly expressed in gastric cancer tissues ( Figure 5E). To sum up, circPTK2 could act as a miRNA sponge for miR-196a-3p in gastric cancer.

AATK is a direct binding target of miR-196a-3p
Two datasets miRDB and TargetScan were used to predict the potential binding targets of miR-196a-3p and found that AATK might be a potential target of miR-196a-3p ( Figure 6A and 6B). Additionally, miR-196a-3p mimics decreased the luciferase activity in cells co-transfected with AATK-WT ( Figure 6C). In addition, miR-196a-3p mimics signi cantly downregulated the expression of AATK in AGS and MKN45 cells, while miR-196a-3p inhibitor displayed the opposite results ( Figure 6D and 6E). Moreover, RNA pulldown assay showed that AATK was pulled down by biotin-labeled miR-196a-3p probe, indicating that miR-196a-3p directly interacted with AATK ( Figure 6F). Moreover, AATK expression was negatively correlated with the expression of miR-196a-3p (r=-0.674, p<0.05) and its expression was positively correlated with circPTK2 expression (r=-0.793, p<0.05) ( Figure 6G and 6H). Furthermore, the expression of AATK was downregulated in gastric cancer tissues ( Figure 6I and 6J). Regarding prognosis, Kaplan-Meier curves showed that low AATK expression correlated with poor survival rate of patients of gastric cancer ( Figure 6K). Collectively, AATK is a direct target gene of miR-196a-3p and is downregulated in gastric cancer tissues.

Knockdown of AATK reverses the tumor-suppressing effect of circPTK2
Next, to further con rm the interaction among circPTK2, miR-196a-3p and AATK, rescue experiments were performed. We found that upregulation of circPTK2 notably increased the expression of AATK; however, these phenomena were reversed by miR-196a-3p mimics or by AATK knockdown (Figure 7A). In addition, the data of CCK8 and colony formation showed that overexpression of circPTK2 signi cantly inhibited the viability and proliferation of AGS and MKN45 cells; however, these changes were reversed by miR-196a-3p mimics or by AATK knockdown (Figure 7B and 7C). Meanwhile, upregulation of circPTK2 markedly suppressed the migration and invasion, triggered the apoptosis of AGS and MKN45 cells; however, these changes were reversed by miR-196a-3p mimics or by AATK knockdown (Figure 7D, 7E and  7F). These data indicated that circPTK2 inhibited gastric cancer tumorigenesis by sponging miR-196a-3p, thus increasing AATK expression.

Interaction of AATK with STK39 in gastric cancer
We further elucidated the anti-tumor mechanism of circPTK2 in gastric cancer. Protein-Protein Interaction network showed that AATK might interact with STK39, CDK5, and CDK5R1, respectively ( Figure 8A).
Serine/threonine kinase 39 (STK39) has been found to function as a tumor oncogene in human cancers [25,26]. In addition, zhao et al found that downregulation of STK39 could induce the apoptosis of renal cell carcinoma cells via inactivating p38 signaling pathway [27]. Thus, among the candidates for AATKinteracting proteins, STK39 was selected for further study. To explore the protein interactions between AATK and STK39, co-IP experiments were performed on gastric cancer cells and showed that AATK could bind with STK39 in gastric cancer cell contexts ( Figure 8B). Moreover, AATK overexpression reduced the expressions of p-STK39, p-p38 and Bcl-2 and increased the expressions of Bax and cleaved caspase 3 in AGS and MKN45 cells ( Figure 8C and 8D). Collectively, circPTK2 could induce the apoptosis of gastric cancer cells by regulating miR-196a-3p/AATK/STK39/p39 pathways ( Figure 8E).

Discussion
CircRNA has identi ed as a novel non-coding RNA, which plays key roles in tumor progression and affect the hallmarks of cancer [22,28]. However, the functions of circRNAs in gastric cancer remain largely unclear. In this study, we identi ed a cancer-associated circRNA, circPTK2, originating from exons 27, 28, 29 of its host gene PTK2, and found that circPTK2 was signi cantly downregulated in gastric cancer tissues. In addition, upregulation of circPTK2 could inhibit gastric cancer cell proliferation, migration and invasion by targeting miR-196a-3p/AATK axis.
Recently, circRNAs have been proved to exert various biological functions via acting as miRNA sponges [29]. To determine whether circPTK2 could regulate gastric cancer progression via sponging miRNAs, CircInteractome dataset was used to predict the potential miRNAs. We found that miR-196a-3p might be sponged by circPTK2, which was veri ed by Luciferase reporter assay and RNA-pull down assay. Rescue experiments revealed that overexpression of miR-196a-3p attenuated the anti-cancer role of circPTK2 in gastric cancer, suggesting that circPTK2 regulated the progression of gastric cancer via sponging miR-196a-3p. Besides, circRNAs could sponge miRNAs and prevent them from interacting with target mRNA, which in turn upregulate target gene expression [30]. In this study, we found that apoptosis-associated tyrosine kinase 1 (AATK) is a downstream target gene of miR-196a-3p. Ma et al showed that AATK was downregulated in metastatic melanoma cells, and overexpression of AATK could suppress the migration and trigger the apoptosis of melanoma cells [31]. In agreement with previous study, we found that AATK was downregulated in gastric cancer cells. In addition, overexpression of circPTK2 markedly upregulated the expression of AATK in gastric cancer cells, suggesting that circPTK2 could interact with miR-196a-3p and function as a miRNA sponge to regulate AATK expression.
Importantly, the association of AATK with the development of gastric cancer has not been described. Bioinformatics analysis indicated that AATK might interact with STK39 (also known as SPAK), which was veri ed by co-IP assay. Evidence has shown that STK39 could promote cervical cancer progression via NF-κB/p38 MAPK/MMP2 pathway [32]. Our data indicated that overexpression of AATK notably downregulated the expressions of p-STK39 and p-p38 in gastric cancer cells, suggesting that AATK might inhibit gastric cancer progression via inactivating STK39/p38 signaling pathway. These data indicated that circPTK2 can sponge miR-196a-3p and up-regulate miR-196a-3p targeting gene AATK, thereby inactivating STK39/p38 signaling pathway.

Conclusion
In summary, we found that circPTK2 could serve as a tumor suppressive circRNA. Mechanistically, circPTK2 could suppress the proliferation, migration and invasion of gastric cancer cells through directly binding to miR-196a-3p and subsequently decrease the inhibiting ability of miR-196a-3p on AATK. These data indicated that exosomal circPTK2 may be a therapeutic target in gastric cancer.

Declarations
Ethics approval and consent to participate All animal procedures were approved by the First A liated Hospital of Soochow University and conducted according to the institutional guidelines.

Consent for publication
Not applicable.
Availability of data and materials The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Con icts of interest
The authors declare no competing nancial interests.

Funding
National Natural Science Foundation of China (Grant No. 81974375).

Authors' contributions
Ling Gao and Tingting Xia made major contributions to the conception, design and manuscript drafting of this study. Mingde Qin, Xiaofeng Xue and Linhua Jiang were responsible for data acquisition, data analysis, data interpretation and manuscript revision. Xinguo Zhu made substantial contributions to conception and design of the study and revised the manuscript. All authors agreed to be accountable for all aspects of the work. All authors read and approved the nal manuscript.