Background
Gastric cancer is one of the most commonly diagnosed cancers and a cause of cancer-related death worldwide [
1]. Although great progress has been made in the treatment of gastric cancer in recent years, the prognosis is still poor, especially for advanced-stage patients. Tumor progression and distant metastasis are the main causes of death. For metastatic gastric cancer patients, chemotherapy is still the main therapeutic strategy. In recent years, increasing evidence has shown that immune evasion is essential for tumor survival and development [
2,
3]. It has been reported that in the tumor microenvironment, tumor cells can recruit immunosuppressive cells, such as CD4
+ T cells, to disrupt the cytotoxic functions of CD8
+ T cells [
4,
5]. In addition, programmed death ligand 1 (PD-L1), a B7 family ligand, can bind to its receptor programmed death-1 (PD-1) to influence tumor-specific T cells, induce apoptosis and inhibit the activity of CD8
+ T cells, leading to immune evasion in tumors. Clinically, an increasing number of studies have shown that blocking the PD-1 checkpoint with an anti-PD-1 antibody is an efficient immunotherapy approach in different cancers [
6,
7]. Our group confirmed the clinical benefit of PD-1 antibody therapy in gastric cancer patients [
8]. However, most gastric cancer patients are resistant to anti-PD-1 therapy, and only some patients benefit from this treatment [
9]. PD-L1 has been proven to be a predictive biomarker of anti-PD-1 efficacy in several tumor types. However, the predictive role of PD-L1 expression in gastric cancer is controversial [
10]. The KENOTE-061 and KENOTE-062 trials showed better survival in patients with PD-L1-positive tumors after pembrolizumab treatment [
11,
12]. On the other hand, data from the Checkmate032, JAVELIN Gastric 300, and ATTRCTION-2 trials did not support the concept of PD-L1 positivity as a predictive biomarker of anti-PD-1 efficacy [
13‐
15]. Therefore, a better understanding of the molecular mechanisms that contribute to gastric cancer progression and immune evasion is critical for developing effective therapeutic strategies for this disease.
Circular RNAs (circRNAs) are a class of recently identified noncoding RNAs or protein-coding RNAs that are characterized by covalently closed loops without 5′ to 3′ polarity or polyadenylation (polyA) tails [
16]. Previous reports have found that circRNAs are generally derived from the back-splicing of pre-mRNA transcripts and are mainly located in the cytoplasm [
17,
18]. Studies have demonstrated that circRNAs are conserved, stable and abundant in different tumor cells and tissues [
19]. Increasing studies have indicated that circRNAs participate in multiple physiological and pathological processes by modulating gene expression, apoptosis, the cell cycle, cell migration and invasion [
20]. Our previous study identified a circRNA signature that can predict postoperative recurrence in stage II-III colon cancer [
21]. Mechanistically, circRNAs can exert their functions in different ways, such as sponging miRNAs, interacting with RNA-binding proteins, and translating proteins [
22‐
24]. For instance, our previous study found that circUBXN7 expression was downregulated in bladder cancer, and forced circUBXN7 expression could suppress cell growth and invasion by sponging miR-1247 to enhance B4GALT3 expression in bladder cancer cells [
25]. Rong et al. [
26] demonstrated that circPSMC3 could inhibit the proliferation and metastasis of gastric cancer. Han et al. [
27] reported that circMTO1 modulated the progression of hepatocellular carcinoma through the regulation of p21 expression by sponging miR-9. Hsiao et al. [
28] verified that the circRNA CCDC66 promoted the progression and metastasis of colon cancer. Although the molecular mechanisms of dysregulated circRNA-associated pathways have been extensively explored, the role of circRNAs in gastric cancer progression and immune evasion remains poorly understood.
In this study, to identify circRNAs involved in gastric cancer progression and immune evasion, we performed a circRNA array using primary and distant metastatic tumor tissues as well as tissues sensitive or resistant to anti-PD-1 therapy. We found that the circRNA circDLG1 (hsa_circ_0008583), which is derived from the DLG1 gene, was significantly upregulated in distant metastatic tissues and primary gastric cancer tissues resistant to anti-PD-1 therapy. More importantly, circDLG1 expression was significantly associated with an aggressive tumor phenotype and adverse prognosis in gastric cancer patients who received anti-PD-1 therapy. In addition, ectopic expression of circDLG1 promoted in vitro cell proliferation, invasion, immune evasion, and in vivo tumorigenesis and metastasis in immunocompetent mice. Mechanistically, circDLG1 could directly interact with miR-141-3p, acting as a miRNA sponge to increase the expression of the miR-141-3p target gene chemokine 12 (CXCL12), thereby promoting the progression of gastric cancer. Thus, circDLG1 might be a promising therapeutic target and biomarker for gastric cancer.
Methods
Human tissue samples and cell lines
Fresh-frozen or paraffin-embedded tissues were obtained from advanced-stage gastric cancer patients who underwent gastroscopy and biopsy, fine needle biopsy, or palliative surgery and received anti-PD-1 therapy at Sun Yat-sen University Cancer Center from August 2018 to October 2019. These patients were all included in a real-world study of PD-1 antibody therapy in gastric cancer (No: NCT04086888). A total of 126 gastric cancer patients were enrolled. Eighty-two of these patients had primary tumor tissues available. Seventy-three patients could be evaluated for the efficacy of PD-1 antibody therapy, among which 30 patients had tissues available (including primary tissues, adjacent normal tissues, and distant metastatic tissues). These patients were evaluated every 3-4 cycles of therapy and followed up regularly. The response rate was evaluated based on RECIST 1.1 guidelines. The tumor response included complete response (CR), partial response (PR), stable disease (SD), and progressive disease (PD). The objective response rate (ORR) was defined as the percentage of patients who achieved CR and PR. The clinical and pathological parameters, including age, sex, tumor size, tumor cell differentiation, peritoneal metastasis, and Lauren’s classification, were collected from patient records. This study was approved by the ethics committee of the Sun Yat-sen University Cancer Center, and informed consent was obtained from all patients. Overall survival (OS) was calculated from the start of PD-1 antibody treatment to the date of death or last contact, and progression-free survival (PFS) was calculated from the start of PD-1 antibody treatment to the date of progression or death.
Human gastric cancer cell lines (HGC27, BGC823, MKN45, MKN28, SGC7901, AGS), the normal gastric epithelial cell line GES-1, the murine gastric cancer cell line MFC and human embryonic kidney (HEK) 293 T cells were purchased from the Shanghai Institute of Cell Biology, Chinese Academy of Science (Shanghai, China). All cell lines were cultured according to the provider’s instructions.
CircRNA microarray assays
To identify the potential circRNAs involved in gastric cancer progression and anti-PD-1 resistance, 3 patients who responded to anti-PD-1 (experienced PR) therapy with PFS of more than 10 months and 6 patients who were nonresponsive to anti-PD-1 (2 SD and 4 PD) therapy with PFS of less than 5 months were selected for circRNA microarray analysis.
The circRNA microarray was performed by Kangchen Biotech (Shanghai, China). All sample preparation and microarray hybridization steps were conducted according to the instructions of Arraystar (Rockville, MD, USA). In brief, circRNAs were enriched by removing the linear RNAs with RNase R treatment, and the enriched circRNAs were then amplified and transcribed into fluorescent cRNA with a random priming method (Arraystar Super RNA Labeling Kit). Then, the labeled cRNAs were hybridized onto Human circRNA Array V2 (8 × 15 k, Arraystar). Finally, the arrays were scanned using the Agilent Scanner G2505C and analyzed with Agilent Feature Extraction software (version 11.0.1.1). The random variance model was used to identify the differentially expressed genes. The paired t-test was used to calculate the P value. The threshold for differentially expressed genes was fold change ≥2.0 and a P value ≤0.05.
Quantitative real-time polymerase chain reaction (qRT–PCR)
Total RNA was isolated from human gastric cancer tissues and cell lines using TRIzol reagent (Sigma–Aldrich, St. Louis, MO, USA) according the manufacturer’s instructions. Reverse transcription of mRNA and miRNA was performed using random primers and stem–loop primers, respectively. qRT–PCR was conducted using a TaqMan Universal Master Mix II kit on a Bio–Rad CFX96 qPCR system, and fold changes were determined by using the relative quantification 2
-ΔΔCT method. The nuclear and cytoplasmic fractions of cells were separated using the PARIS Kit (Life Technologies) according to the manufacturer’s instructions. RNA was extracted from both fractions. Then, qRT–PCR was performed to determine the expression ratios of specific RNA molecules between the nuclear and cytoplasmic fractions. GAPDH and U6 served as cytoplasmic and nuclear markers, respectively. The primers used are presented in Additional file
1: Table S1.
Western blotting analysis
Western blotting analysis was performed according to a previously described method [
29]. Briefly, proteins were extracted from gastric cancer cell lines, and the protein concentration was calculated using the Pierce BCA protein assay kit (Thermo Scientific, Rockford, IL, USA). The following antibodies were used in this study: anti-GAPDH (CST, #2118 L) and anti-CXCL12 (CST, #3740S).
Immunohistochemistry (IHC) analysis
The IHC analysis was performed according to the method we described previously [
30]. Briefly, the paraffin-embedded tissue blocks were cut into 4-μm slides. Immunostaining images were captured using a microscope (Leica, Germany). The immunoreactivity in each tissue section was assessed by two pathologists, and the degree of positivity was evaluated according to the percentage of positive tumor cells. The following antibodies were used: anti-CXCL12 (CST, 97958S; 1:200 dilution), anti-PD-L1 (22C3 pharmDX; Dako, Carpinteria, CA, USA), and anti-CD33 (clone PWS44; 1:200 dilution; Leica Biosystems, Nussloch, Germany). PD-L1 positivity was defined as a membrane staining intensity ≥1% in the tumor cells or tumor infiltrating immune cells. The median value of PD-L1 expression was used as the cutoff value to stratify high and low PD-L1 expression.
Flow cytometry
The detailed flow cytometry procedure was performed as previously described [
31]. The following antibodies were used: anti-mouse IFNγ (BD Biosciences), anti-mouse CD45 (BD Biosciences), anti-mouse Gr-1 (TONBO Biosciences), anti-mouse Ly6G (TONBO Biosciences), anti-mouse ly6C (BD Biosciences), anti-mouse CD11b (TONBO Biosciences), anti-mouse F4/80 (BioLegend), anti-human CD11b (TONBO Biosciences), anti-human CD33 (BD Biosciences), and anti-human HLA-DR (BD Biosciences). Detailed information on the antibodies is provided in Additional file
2: Table S2. Briefly, the cells were digested and suspended as single cells, washed with PBS, and then resuspended in cell stabilizing buffer (BioLegend cat. No. 420201); the supernatant was discarded, and the sample was centrifuged at 350 g for 5 min. Then, 5 μl of Human TruStain FcX™ (Fc Receptor Blocking Solution, BioLegend Cat. No. 422301) and the antibodies were added and cultured for 15-20 min at room temperature. Finally, the cells were assessed by flow cytometry. For intracellular staining of IFNγ, fixation buffer (BioLegend Cat. No. 420801) was added and incubated for 20 min at room temperature. Then, the cells were washed with Intracellular Staining Perm Wash Buffer (BioLegend Cat. No. 421002), resuspended and centrifuged. IFNγ antibody was added, and the cells were cultured for 20 min and finally assessed by flow cytometry.
Fluorescence in situ hybridization (FISH) assays
The FISH assay was performed according to previously described methods [
25]. Briefly, cells were hybridized with Cy3-labeled circDLG1 probe and Cy5-labeled miR-141-3p probe (GenePharma, China) for 12 h at 37 °C. Then, the nuclei were counterstained using 4′,6-diamidino-2-phenylindole (DAPI) (Yeasen, Shanghai, China). Finally, the cells were viewed and captured under a ZEISS LSM800 confocal microscope (Carl Zeiss AG, Germany).
Cell proliferation, colony formation, migration and invasion assays
Cell counting kit-8 (CCK-8) and colony formation assays were performed to assess the cell proliferation ability. Transwell assays were conducted to evaluate cell migration and invasion abilities. The details of these assays were described in our previous study [
32].
RNA immunoprecipitation (RIP) and luciferase activity assays
The RIP assay was conducted using the Magna RIP RNA-bing Protein Immunoprecipitation kit (Millipore, USA) according to the provider’s protocol. In brief, cell lysates were cultured with Dynabeads-coated IgG antibody (Millipore, USA) or AGO2 antibody (Cell Signaling Technology, USA) for 12 h at 4 °C. The purified RNA was subjected to qRT–PCR to detect the enriched circDLG1 and miRNA.
For the luciferase activity assay, potential binding sites were predicted using StarBase v3.0 and TargetScanHuman 7.2. Gastric cells were cotransfected with pGL-luc-circDLG1, pGL-luc-CXCL12 3′-UTR, and miR-141-3p mimics or negative control mimics for 48 h, and luciferase activity was detected using the dual-luciferase reporter assay system (Promega, USA) according to the manufacturer’s instructions.
circDLG1 knockdown or overexpression transfection experiment
Small hairpin RNAs (shRNAs) targeting the junction region of the circDLG1 sequence and the circDLG1-overexpressing lentiviral vector were synthesized by Geneseed Biotech Co., Ltd. (Guangzhou, China). Gastric cancer cell lines were transfected with circDLG1 shRNA or the circDLG1-overexpressing lentiviral vector following the manufacturer’s instructions according to a previously described method [
25].
The in vivo tumorigenesis and metastasis experiments in BALB/c nude mice and C57BL/6 mice were approved by the Animal Experiment Ethics Committee of Sun Yat-sen University Cancer Center. The mice were purchased from the Shanghai Institute of Material Medicine (Shanghai, China) and were fed according to the provider’s instructions. The experiments were conducted as described in our previous study [
33]. Briefly, to evaluate the in vivo tumorigenesis effect of circDLG1, MFC-sh-circDLG1 and MFC-sh-NC cells (1 × 10
6 cells/mouse) were inoculated subcutaneously into the flanks of two groups of BALB/c nude mice and C57BL/6 mice (ten for each cell group). Tumor size was measured every 4 days, and tumor volume was estimated. After 4 weeks, the mice were sacrificed, and the tumors were excised. To investigate the effect of circDLG1 on tumor metastasis, MFC-sh-circDLG1 and MFC-sh-NC cells (2 × 10
6 cells/mouse) were inoculated into the tail vein of two groups of BALB/c nude mice and C57BL/6 mice (ten for each cell group). Four weeks later, the mice were sacrificed, and the lungs were excised and paraffin embedded. Consecutive sections (4 μm) were made and stained with hematoxylin-eosin. The micrometastases in the lungs were examined and counted under a dissecting microscope.
Mouse xenograft anti-PD-1 therapy experiment
To evaluate the effect of CXCL12 on immune evasion and the efficacy of anti-PD-1 therapy in vivo, an in vivo xenograft experiment was conducted in C57BL/6 mice. In brief, four groups of mice were implanted subcutaneously in the left flanks with MFC-sh-NC, and two groups of mice were implanted subcutaneously in the left flanks with MFC-sh-CXCL12 cells (ten mice for each group, 1 × 10
6 cells/mouse). The MFC-sh-NC implanted mice were treated with 0.9% normal saline (NS), anti-PD-1 antibody, AMD3100 (a CXCR4 inhibitor), and the combination of AMD3100 and anti-PD-1 antibody. MFC-sh-CXCL12-implanted mice were treated with 0.9% normal saline (NS) and an anti-PD-1 antibody. A mouse anti-PD-1 antibody was purchased from Bio X Cell (West Lebanon, NH, USA). AMD3100 was purchased from Selleck (Shanghai, China) and dissolved in phosphate-buffered saline (PBS). The mice were intraperitoneally injected with 0.9% NS, AMD3100, or anti-PD-1 antibody according to a previously described method [
34]. The mice were sacrificed at 4 weeks or before becoming moribund. The survival time was defined as the date of first therapy to the date of sacrifice.
RNA in situ hybridization (ISH), sphere formation assay, and RNA sequencing (RNA-seq) analysis
The methods for ISH, sphere formation assay and RNA-seq are described in Additional file
3: Supplementary materials and methods.
Statistical analysis
All data are presented as the mean ± standard deviation unless otherwise noted. Statistical analyses were performed with SPSS 17.0 software (SPSS Inc., Chicago, IL, USA) or GraphPad Prism 7 (GraphPad Software, Inc., La Jolla, CA, USA). For comparisons between groups, Student’s t-test, chi-squared test, one-way ANOVA, and Pearson correlation analysis were used, as appropriate. PFS was analyzed using the Kaplan–Meier method with the log-rank test, and Cox’s proportional hazard regression model was performed to evaluate the independent prognostic indicators. A P-value of < 0.05 was considered significant.
Discussion
Increasing evidence has indicated that circRNAs are involved in different diseases [
37,
38]. Moreover, many circRNAs are expressed in a cell type-specific or tissue-specific manner [
39], indicating that they might play important biological functions. The dysregulation of circRNAs has been found in several pathological processes, including cardiac hypertrophy, neurological disorders and tumor development [
38,
40]. By analyzing the circRNA expression profiles in primary and paired distant metastatic gastric cancer tissues as well as tissues that respond to or are resistant to anti-PD-1 therapy, we found that circDLG1 (circRNA ID: hsa_circ_0008583) was upregulated in distant metastatic lesions and tissues resistant to anti-PD-1 therapy, and high circDLG1 expression was associated with worse PFS in gastric cancer patients treated with anti-PD-1 therapy. This finding indicated that circDLG1 might play an important role in gastric cancer progression and anti-PD-1 resistance.
Further biological experiments demonstrated that circDLG1 could promote cell proliferation, migration, EMT, and stem cell formation. Moreover, an in vivo assay showed that circDLG1 induced gastric cancer progression and metastasis in immunocompetent mice but not in immunodeficient mice. This finding implies that circDLG1 might be involved in the regulation of immunity. Indeed, further analysis showed that knockdown of circDLG1 increased CD8
+ T cells while decreasing MDSCs. Previous reports have indicated that EMT characteristics are associated with immune evasion and anti-PD-1 resistance in different tumors [
41‐
43]. Our results showed that circDLG1 is critical for the two hallmark processes of tumors, EMT and immune evasion.
Previous studies have shown that several biomarkers, including MSI, PD-L1 expression, TMB, EBV status and Pold/Pole, might be associated with the response to anti-PD-1 therapy [
9,
44]. Our previous study showed that the circulating tumor DNA landscape could predict the efficacy of immune checkpoint inhibitors in some gastric cancer patients [
45]. However, more biomarkers are needed to identify gastric cancer patients who can benefit from anti-PD-1 therapy. In this study, we found that circDLG1 upregulated the expression of CXCL12. Indeed, previous studies have indicated that CXCL12 is involved in the progression and metastasis of gastric cancer [
46]. Moreover, increasing evidence indicates that CXCL12 can induce immune evasion by recruiting infiltrating MDSCs into the tumor microenvironment [
36]. In this study, we showed that knockdown of CXCL12 could inhibit the invasion ability and improve the sensitivity of gastric cancer to anti-PD-1 therapy. CXCL12 expression is associated with the PFS of patients who receive anti-PD-1 therapy. This finding is in accordance with previous studies of other tumor types [
34,
47]. Taken together, these data demonstrated that CXCL12 might be used as a biomarker to predict gastric cancer resistance to anti-PD-1 therapy. Furthermore, the CXCL12/CXCR4 axis has been found to play an important role in anti-PD-1 resistance in other cancer types [
34]. Blocking CXCL12/CXCR4 using AMD3100 (a CXCR4 inhibitor) could improve the efficacy of anti-PD-1 in melanoma [
47]. Interestingly, we found that AMD3100 treatment could significantly improve the efficacy of anti-PD-1 therapy in gastric cancer in vivo. This finding indicates that targeting CXCL12 might be a promising strategy to improve anti-PD-1 efficacy in gastric cancer.
Increasing evidence indicates that circRNAs can act as miRNA sponges to regulate the expression of protein-coding genes. For example, the circRNA ACVR2A suppresses bladder cancer cell proliferation and metastasis through the miR-626/EYA4 axis [
48]; the circRNA circ-RanGAP1 regulates VEGFA expression by targeting miR-877-3p to facilitate gastric cancer invasion and metastasis [
49], and the circRNA circCCDC9 acts as a sponge for miR-6792-3p to inhibit gastric cancer progression [
50]. In the present study, we found that circDLG1 was mainly located in the cytoplasm of gastric cancer cells, and RIP and RNA pulldown assays showed that circDLG1 could interact with miR-141-3p. Luciferase activity assays confirmed the direct relationship between miR-141-3p and circDLG1. In addition, RNA FISH assays indicated that miR-141-3p and circDLG1 could colocalize in the cytoplasm. These data demonstrate that circDLG1 could sponge miR-141-3p to regulate the expression of CXCL12. Our study has several limitations. First, the sample size was relatively small, and further validation is needed using a larger cohort of patients. Second, we did not detect circDLG1 in the plasma, which might be due to the degradation of RNA after a long storage time of the samples. Further validation is needed to confirm the existence of circDLG1 in plasma or exosomes. Third, the mouse model could not fully reflect the tumor immune microenvironment of the patients, and further study using a patient-derived xenograft model is needed to confirm our results.
Conclusion
In conclusion, we identified a novel circRNA, circDLG1, that is upregulated in gastric cancer distant metastatic lesions and tissues resistant to anti-PD-1 therapy. Further functional studies showed that circDLG1 could promote gastric cancer proliferation, invasion, immune evasion and anti-PD-1 therapy resistance. Clinically, circDLG1 expression was associated with adverse prognosis in gastric cancer patients treated with anti-PD-1 therapy. Additional experiments indicated that circDLG1 acts as a miR-141-3p sponge to regulate the expression of CXCL12, which could induce MDSC infiltration to impair the function of CD8+ T cells. Our study revealed the significance of the circDLG1/CXCL12 axis in the immune evasion and anti-PD-1 resistance of gastric cancer.
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