Introduction
Lung cancer is one of the most common human malignancies and is a leading cause of cancer-related death among both men and women around the world (1). In 2016, it was estimated that there were 224,390 new cases and 158,080 deaths due to lung cancer in the United States of America (2). Lung cancer can be divided into two groups: non-small cell lung cancer (NSCLC) and small lung cancer (SLC) (3). NSCLC, an aggressive type of lung cancer, accounts for ~85% of all lung cancer cases (4). In total, 32-40% of NSCLCs are adenocarcinomas, with squamous (25-30%) and large cell (8-16%) carcinomas making up the bulk of the other types of NSCLCs (5). Despite the considerable progress made in the development of NSCLC therapies, the prognosis of patients with this disease remains poor, with a 5-year survival rate of >16% (6). In addition, tumor recurrence and metastasis are frequent challenges in the treatments of NSCLC (7,8). Therefore, development of novel effective therapeutic strategies for patients with NSCLC is essential.
MicroRNAs (miRNAs) are a series of endogenous, short-length, single-stranded, and non-coding RNA molecules of ~21-25 nucleotides in length (9). miRNAs suppress the expression of genes by directly binding to the 3′-untranslated region (3′-UTR) of their target genes and promoting RNA degradation or inhibiting translation (10,11). miRNAs are involved in various biological processes, including cell proliferation, cell cycle, apoptosis, differentiation, generation, migration, invasion and metastasis (12-14). Increasingly, studies are indicting that miRNAs may be key regulators in the initiation and progression of human cancers (15-17). For NSCLC, various miRNAs has been demonstrated to be aberrantly expressed, including miR-98 (18), miR-373 (19), miR-484 (20) and miR-1258 (21). The dysregulation of miRNAs may have tumor-suppressing or oncogenic roles in NSCLC, and thus serve crucial roles in the genesis and development of NSCLC by regulating multiple cellular biological processes (22-24). Further investigation into the specific roles of deregulated miRNAs in NSCLC could facilitate the identification of novel therapeutic targets for patients with this disease.
miR-802 is dysregulated in multiple types of human cancer, and exerts tumor-suppressive (25-28) or promoting (29) roles. However, the expression levels and function roles of miR-802 in NSCLC remain largely unknown. In the present study, miR-802 expression was examined in NSCLC tissues and cell lines. The clinical role of miR-802 in NSCLC patients was also examined. Then, miR-802 expression was restored in cell lines to explore its effects in regulating the aggressive behavior of NSCLC cells. Finally, the regulatory mechanisms by which miR-802 affects the malignant development of NSCLC were investigated.
Materials and methods
Human tissues
NSCLC tissues and paired normal adjacent tissues (NATs) were obtained from 52 patients undergoing surgical resection at The First Affiliated Hospital of Guangzhou Medical University (Guangzhou, China) between August 2015 and July 2017. None of these patients received adjuvant therapy prior to surgery. All tissues were immediately frozen in liquid nitrogen and then stored at −80°C prior to RNA isolation. Written informed consent was obtained from all patients and this study was approved by the Ethics Committee of The First Affiliated Hospital of Guangzhou Medical University (Guangzhou, China).
Cell lines and transfection
A total of five human NSCLC cell lines (A549, H522, H1299, H460, and SK-MES-1) and a non tumorigenic bronchial epithelium cell line, BEAS-2B, were purchased from Shanghai Institute of Biochemistry and Cell Biology (Shanghai, China). All cells were cultured in Dulbecco’s modified Eagle’s medium (DMEM; Gibco; Thermo Fisher Scientific, Inc., Waltham, MA, USA) supplemented with 10% heat inactivated fetal bovine serum (FBS; Gibco; Thermo Fisher Scientific, Inc.) and 1% v/v penicillin-streptomycin (Sigma-Aldrich; Merck KGaA, Darmstadt, Germany) at 37°C in a humidified atmosphere containing 5% CO2.
miR-802 mimics, miRNA mimic negative control (miR-NC), small interfering RNA (siRNA) against fibroblast growth factor receptor 1 (FGFR1; termed here si-FGFR1) and negative control siRNA (si-NC) were generated by GenePharma Co., Ltd. (Shanghai, China). The sequences were: miR-802 mimics, 5′-CAGUAACAAAGAUUCAUCCUUGU-3′; miR-NC, 5′-UUCUCCGAACGUGUCACGUTT-3′; si-FGFR1, 5′-GGAGGUGCUUCACUUAAGATT-3′; and si-NC, 5′-UUC UCCGAACGUGUCACGUTT-3′. FGFR1 overexpression vector pCMV-FGFR1 and empty pCMV plasmid were chemically synthesized by OriGene Technologies, Inc., (Beijing, China). Lipofectamine 2000 (Invitrogen; Thermo Fisher Scientific, Inc.) was used for cell transfections according to the manufacturer’s protocol. After incubation for 6-8 h, the culture medium was discarded, and fresh DMEM containing 10% FBS was added into each well.
Reverse transcription-quantitative polymerase chain reaction (RT-qPCR)
Total RNA was extracted from tissues and cells using the TRIzol reagent (Thermo Fisher Scientific, Inc.), and quantified using an ND-2000 spectrophotometer (NanoDrop Technologies; Thermo Fisher Scientific, Inc., Wilmington, DE, USA), following the manufacturer’s protocol. The All-in-One miRNA RT-qPCR Detection kit (GeneCopoeia, Inc., Rockville, MD, USA) was used to determine miR-802 expression. To analyze FGFR1 mRNA expression, reverse transcription was performed using a PrimeScript RT reagent kit (Takara Biotechnology Co., Ltd., Dalian, China). Subsequently, the relative expression of FGFR1 mRNA was detected with SYBR Premix Ex Taq (Takara Biotechnology Co., Ltd.) using an Applied Biosystems 7900HT Real-Time PCR system (Thermo Fisher Scientific, Inc.). U6 snRNA and GAPDH were used as internal references for miR-802 and FGFR1, respectively. Relative expression levels were determined according to the 2−∆∆Cq method (30). The primers were designed as follows: miR-802, 5′-GGACCACCGCTCGCT CATCGCTAA-3′ (forward) and 5′-TCGCGTTACTATAT GCCAAGCCCTAG-3′ (reverse); U6, 5′-ACCCATGGCGCCC AAATTGATTCCC-3′ (forward) and 5′-ATCGCTAGTCTG TTATTCCAAGTTG-3′ (reverse); FGFR1, 5′-CTGGTGACA GAGGACAATG-3′ (forward) and 5′-AGATCCGGTCAA ATAATGCC-3′ (reverse); and GAPDH, 5′-CGGAGTCAAC GGATTTGGTCGTAT-3′ (forward) and 5′-AGCCTTCTC CATGGTGGTGAAGAC-3′ (reverse).
MTT assay
Transfected cells were harvested 24 h post-transfection and re-seeded in 96-well plates at a density of 3,000cells/well. Cells were then incubated at 37°C for 0, 24, 48 and 72 h. At each time point, 20 µl MTT solution (5 mg/ml; Sigma-Aldrich; Merck KGaA) was added in each well and incubated for another 4 h at 37°C. The culture medium was removed and 150 µl of dimethyl sulfoxide (DMSO; Sigma-Aldrich; Merck KGaA) was added into each well. Finally, the optical density (OD) at 490 nm was detected using an enzyme linked immunosorbent assay reader (Bio Rad Laboratories, Inc., Hercules, CA, USA).
Colony formation assay
Transfected cells were collected at 24 h post-transfection and seeded into 6-well plates at a density of 1×103 cells/well in 2 ml of culture medium. After incubation at 37°C with 5% CO2, the colonies were washed with PBS (Gibco; Thermo Fisher Scientific, Inc.), fixed with 4% paraformaldehyde, and then stained with methyl violet (Beyotime Institute of Biotechnology, Inc., Shanghai, China). The number of colonies was counted under an inverted light microscope (Olympus Corporation, Tokyo, Japan).
Cell apoptosis analysis
Transfected cells were harvested at 48 h post-tansfection, washed with ice-cold PBS, and then stained with the Annexin V-fluorescein isothiocyanate (FITC) Apoptosis Detection kit (Biolegend, Inc., San Diego, CA, USA), which is used to detect cells undergoing apoptosis. In specific, transfected cells were resuspended in 100 µl of binding buffer followed by staining with 5 µl Annexin V-FITC and 5 µl propidium iodide. Following a 15 min incubation at room temperature in the dark, a flow cytometer (EPICS Xl-4; Beckman Coulter, Inc., Brea, CA, USA) was used to determine the % of apoptotic cells. FlowJo software (version 7.6.1; FlowJo LLC, Ashland, OR, USA) was used to analyze the data.
Cell migration and invasion assays
Cell migration and invasion was determined using uncoated and matrigel-coated transwell chambers (8 µm pore size; BD Biosciences, San Jose, CA, USA), respectively. Transfected cells were harvested 48 h post-transfection, washed with PBS, and suspended in FBS-free DMEM. Fifty thousand cells were added into the upper chamber and 500 µl DMEM containing 20% FBS was placed into the lower chamber as a chemoattractant. After incubation at 37°C for 24 h, non-migrated and non-invaded cells were wiped away carefully with cotton wool. The migrated and invaded cells on the lower surface were fixed in 4% paraformaldehyde, stained with 0.5% crystal violet, photographed, and counted in five randomly selected areas under an inverted light microscope (Olympus Corporation).
In vivo tumor growth assay
A total of eight female BALB/c nude mice (20 g; aged 4-6 weeks) were obtained from the Shanghai Laboratory Animal Center (Shanghai, China). All nude mice were maintained under specific pathogen-free conditions (25°C; 50% humidity; 10-h light/14-h dark cycle). Cells were transfected with miR-802 mimics or miR-NC. Following incubation at 37°C with 5% CO2 for 24 h, the transfected cells were harvested and injected subcutaneously into the flank region of nude mice (n=4 for each group). The volume of tumor xenografts was calculated every four days using the following formula: length × width2 × 0.5. One month later, all nude mice were sacrificed, and the xenografts were removed and weighted. The animal experiments were approved by the Scientific Investigation Board of The First Affiliated Hospital of Guangzhou Medical University (Guangzhou, China) and performed in accordance with the guidelines of the National Institutes of Health Guidance for the Care and Use of Laboratory Animals.
Bioinformatics prediction and luciferase reporter assay
TargetScan (http://www.targetscan.org/) and miRDB (http://www.mirdb.org/) were used to predict the potential targets of miR-802.
For the luciferase reporter assay, the fragments of FGFR1 3′-UTR containing the wild-type (wt) and mutant (mut) miR-802 binding site were constructed by GenePharma Co., Ltd., and were then inserted into the pMIR-REPORT miRNA Expression Reporter vector (Ambion; Thermo Fisher Scientific, Inc.). The chemically synthesized luciferase reporter plasmids were termed pMIR-FGFR1-3′-UTR-wt and pMIR-FGFR1-3′-UTR-mut, respectively. Cells were plated into 24-well plates one night prior to transfection. miR-802 mimics or miR-NC in combination with pMIR-FGFR1-3′-UTR-wt or pMIR-FGFR1-3′-UTR-mut were cotransfected into cells using Lipofectamine 2000, according to the manufacturer’s instructions. Luciferase activity was detected 48 h post-transfection using a dual-luciferase reporter assay system (Promega Corporation, Madison, WI, USA). Renilla luciferase activity was normalized against firefly luciferase activity.
Western blot analysis
Total protein was extracted from tissue samples and cells using radioimmunoprecipitation assay buffer (Beyotime Institute of Biotechnology, Haimen, China) supplemented with protease and phosphatase inhibitors (Roche Applied Science, Mannheim, Germany). The protein concentration was measured using the BCA assay (KeyGen Biotech. Co., Ltd., Nanjing, China). Equal amounts of protein were separated on 10% SDS-polyacrylamide gels and then transferred onto polyvinylidene difluoride (PVDF; Millipore, Billerica, MA, USA) membranes. After blocking in TBST containing 5% non fat dry milk, the membranes were incubated with primary antibodies at 4°C overnight. Afterwards, the membranes were washed with TBST followed by incubation with goat anti-rabbit (cat no. ab6721; 1:5,000 dilution) or goat anti-mouse (cat. no. ab6789; 1:5,000 dilution) horseradish peroxidase (HRP)-conjugated secondary antibodies (Abcam, Cambridge, UK) at room temperature for 2 h. Finally, enhanced chemiluminescence reagents (ECL; Pierce; Thermo Fisher Scientific, Inc.) were used for protein band detection. Quantity One software version 4.62 (Bio Rad Laboratories, Inc.) was used for densitometry. The primary antibodies used in this study were as follows: rabbit anti-human monoclonal FGFR1 antibody (cat. no. ab76464; 1:1,000 dilution; Abcam), mouse anti-human monoclonal phosphoinositide 3-kinase (PI3K; cat. no. ab189403; 1:1,000 dilution; Abcam), rabbit anti-human polyclonal phosphorylated (p-) PI3K antibody (cat. no. ab182651; 1:500 dilution; Abcam), mouse anti-human monoclonal p-AKT serine/threonine kinase (Akt) antibody (cat. no. sc-81433; 1:1,000 dilution; Santa Cruz Biotechnology, Inc., Callas, TX, USA), mouse anti-human monoclonal Akt antibody (cat. no. sc-56878; 1:1,000 dilution; Santa Cruz Biotechnology, Inc.), rabbit anti-human monoclonal mammalian target of rapamycin (mTOR) antibody (cat. no. ab2732; 1:1,000 dilution; Abcam), rabbit anti-human monoclonal p-mTOR antibody (cat. no. ab109268; 1:1,000 dilution; Abcam), and rabbit anti-human monoclonal GAPDH antibody (cat. no. ab181603; 1:1,000 dilution; Abcam).
Statistical analysis
All data were presented as the mean ± standard deviation. SPSS 15.0 (SPSS Inc., Chicago, IL, USA) was used for statistical analysis. The relationship between miR-802 and the clinicopathological variables of NSCLC patients was determined using the χ2 test. Student’s t-test was utilized to examine the differences between two groups. Significant differences between multiple groups were investigated using one-way analysis of variance, followed by a Bonferroni’s post hoc test. The association between miR-802 and FGFR1 mRNA levels in NSCLC tissues was analyzed using Spearman’s correlation analysis. P<0.05 was considered to indicate a statistically significant difference.
Results
miR-802 expression is downregulated in NSCLC tissues and cell lines
To uncover the expression pattern of miR-802 in NSCLC, RT-qPCR analysis was utilized to detect miR-802 expression in 52 pairs of NSCLC tissues and paired NATs. miR-802 was downregulated in NSCLC tissues as compared with NATs (Fig. 1A; P<0.05). To clarify the clinical significance of miR-802 in NSCLC, all NSCLC patients were divided into two groups: miR-802-low expression group or miR-802-high expression group. The median value of miR-802 in NSCLC patients was used as a cut-off point. The low miR-802 expression group was correlated with several indicators of NSCLC progression, including tumor stage (P=0.026), lymph node metastasis (P=0.005) and brain metastasis (P=0.004; Table I). Next, the levels of miR-802 were determined in five human NSCLC cell lines (A549, H522, H1299, H460, and SK-MES-1) and in a non tumorigenic bronchial epithelial cell line, BEAS-2B. Consistent with the patient tissue results, miR-802 displayed decreased expression in all five NSCLC cell lines compared with the normal BEAS-2B cell line (Fig. 1B; P<0.05). Thus, downregulation of miR-802 may play crucial roles in NSCLC progression.
miR-802 inhibits the growth, migration and invasion of NSCLC cells in vitro
To determine the functional role of miR-802 in NSCLC cells, H522 and H460 cells were selected for transfection based on their lowest levels of miR-802 expression among the five NSCLC cell lines tested. miR-802 mimics or miR-NC were transfected into H522 and H460 cells and then RT-qPCR was performed to determine the miR-802 expression levels. RT-qPCR revealed that miR-802 was significantly increased in miR-802 mimics-transfected H522 and H460 cells, compared with the cells transfected with miR-NC (Fig. 2A; P<0.05). MTT and colony formation assays were then used to investigate the effect of miR-802 in the proliferation of NSCLC cells. As illustrated in Fig. 2B and C, ectopic miR-802 expression resulted in a significant reduction in the proliferative (P<0.05) and colony formation (P<0.05) abilities of H522 and H460 cells. Additionally, miR-802 upregulation promoted apoptosis in H522 and H460 cells as detected by cell apoptosis flow cytometry analysis (Fig. 2D; P<0.05).
Since miR-802 expression was strongly correlated with brain metastasis, cell migration and invasion assays were performed to examine whether miR-802 could affect NSCLC metastasis-related functions. The migration (Fig. 2E; P<0.05) and invasion (Fig. 2F; P<0.05) of H522 and H460 cells transfected with miR-802 mimics decreased significantly compared with the cells transfected with miR-NC. Taken together, these results implied that miR-802 may have a tumor-suppressive role in NSCLC growth and invasion in vitro.
FGFR1 is a direct target of miR-802 in NSCLC cells
To investigate the underlying mechanism for the tumor-suppressing function of miR-802 in NSCLC cells, bioinformatics analysis was performed to search for the putative targets of miR-802. The analysis indicated that the 3′-UTR of FGFR1 contained a highly conserved binding site for miR-802 (Fig. 3A). FGFR1 was selected for further investigation because it has been previously demonstrated to be implicated in the formation and progression of NSCLC (31-37). To confirm this prediction, first a luciferase reporter assay was conducted determine whether miR-802 could directly target the 3′-UTR of FGFR1 in NSCLC cells. The results demonstrated that the luciferase activity was suppressed in H522 and H460 cells that were co-transfected with miR-802 mimics and the reporter plasmid carrying the wild-type miR-802 binding site (Fig. 3B; P<0.05); however, the luciferase activity was unaffected when the binding site in the 3′-UTR of FGFR1 for miR-802 was mutated (Fig. 3B). Additionally, the FGFR1 expression was detected in NSCLC tissues and its potential association with the miR-802 expression was explored. The data obtained from RT-qPCR analysis revealed that the expression levels of FGFR1 mRNA were significantly higher in NSCLC tissues compared with NATs (Fig. 3C; P<0.05). In addition, western blot analysis revealed that FGFR1 protein was highly expressed in NSCLC tissues in comparison with NATs (Fig. 3D; P<0.05). Furthermore, an inverse correlation was identified between miR-802 and FGFR1 mRNA levels in NSCLC tissues by Spearman correlation analysis (Fig. 3E; r=-0.5109, P=0.0001). Finally, the regulatory effects of miR-802 upregulation on endogenous FGFR1 expression in NSCLC cells were examined through RT-qPCR and western blot analysis. FGFR1 mRNA (Fig. 3F; P<0.05) and protein (Fig. 3G; P<0.05) levels were significantly suppressed by miR-802 overexpression in H522 and H460 cells. These results provide strong evidence that FGFR1 is a direct target gene of miR-802 in NSCLC cells.
FGFR1 inhibition can simulate the inhibitory effects of miR-802 in NSCLC cells
siRNA against FGFR1 (si-FGFR1) was introduced into H522 and H460 cells to knock down endogenous FGFR1 expression and examine the functions of FGFR1 in NSCLC cells. Western blot analysis verified that FGFR1 expression was efficiently suppressed in H522 and H460 cells following si-FGFR1 transfection (Fig. 4A; P<0.05). Next, cell proliferation (by MTT assay), colony formation, apoptosis, migration and invasion were examined in the FGFR1-silenced cells. These experiments revealed that the silencing of FGFR1 notably restricted cell proliferation (Fig. 4B; P<0.05) and colony formation (Fig. 4C; P<0.05), induced cell apoptosis (Fig. 4D; P<0.05), and reduced cell migration (Fig. 4E; P<0.05) and invasion (Fig. 4F; P<0.05) in H522 and H460 cells. The effects of FGFR1 inhibition in NSCLC cells were similar to those caused by miR-802 mimics, which further suggests that FGFR1 is a downstream target of miR-802 in NSCLC cells.
FGFR1 rescues the suppressive effects of miR-802 on the malignant phenotypes of NSCLC cells
The aforementioned results demonstrated that miR-802 decreased the growth and invasion of NSCLC cells in vitro and that FGFR1 was a direct target gene of miR-802. Rescue experiments were next performed to further confirm that the inhibitory effects of miR-802 on the malignant phenotypes of NSCLC cells were mediated though FGFR1. miR-802-overexpressing H522 and H460 cells were transfected with a FGFR1-overexpression vector, pCMV-FGFR1, to recover FGFR1 expression levels (Fig. 5A; P<0.05). Functional experiments revealed that FGFR1 restoration effectively rescued the tumor-suppressing effects of miR-802 overexpression on the proliferation (Fig. 5B; P<0.05), colony formation (Fig. 5C; P<0.05), apoptosis (Fig. 5D; P<0.05), migration (Fig. 5E; P<0.05), and invasion (Fig. 5F; P<0.05) abilities of H522 and H460 cells. These observations indicated that FGFR1 served as a downstream effector of miR-802 in NSCLC cells.
miR-802 deactivates the PI3K/Akt/mTOR pathway in NSCLC cells through downregulating FGFR1 expression
FGFR1 activates the PI3K/Akt/mTOR signaling pathway and contributes to the aggressive behaviors of NSCLC cells (38). Thus, next it was attempted to explore whether miR-802 can deactivate the PI3K/Akt/mTOR pathway in NSCLC cells. miR-802 mimics in combination with pCMV-FGFR1 or empty pCMV plasmid were co-transfected into H522 and H460 cells. Post-transfection, western blot analysis revealed that miR-802 upregulation decreased the p-PI3K, p-Akt, and p-mTOR levels in H522 and H460 cells, whereas the total PI3K, Akt, and mTOR protein levels were unaffected (Fig. 6). Notably, the downregulation of p-PI3K, p-Akt, and p-mTOR caused by miR-802 mimics was restored in H522 and H460 cells after co-transfection with pCMV-FGFR1 (Fig. 6; P<0.05). These results suggest that miR-802 deactivated the PI3K/Akt/mTOR pathway in NSCLC cells by interfering with FGFR1 expression.
miR-802 impairs the growth of NSCLC cells in vivo
Finally, an in vivo tumor growth assay was performed to assess whether miR-802 influenced the tumor growth of NSCLC cells in vivo. The volumes (Fig. 7A and B; P<0.05) and weights (Fig. 7C; P<0.05) of the xenograft tumors in the miR-802 mimics group were significantly decreased compared with the control miR-NC group. Upregulation of miR-802 was confirmed in the xenograft tumors derived from miR-802 mimics-transfected H460 cells, as detected by RT-qPCR analysis (Fig. 7D; P<0.05). Western blot analysis of the xenograft tumor tissues revealed a significant downregulation of FGFR1, p-PI3K, p-Akt, and p-mTOR in the miR-802 mimics group compared with the control miR-NC group (Fig. 7E; P<0.05). These results suggest that miR-802 overexpression decreased the growth of NSCLC cells in vivo. The downregulation of FGFR1 and deactivation of the PI3K/Akt/mTOR pathway may be responsible for the tumor growth inhibition induced by miR-802 overexpression in NSCLC cells.
Discussion
Emerging reports have revealed that several miRNAs are abnormally expressed in NSCLC (39-41). miRNAs have been identified as oncogenes or tumor suppressors, which regulate various biological processes including oncogenesis and development (42). Therefore, insights into the functions of miRNAs in NSCLC might facilitate the identification of potential diagnostic markers and therapeutic targets for patients with NSCLC. In the current study, for the first time, the expression of miR-802 was detected in NSCLC patient tissues and its clinical value was evaluated. The biological roles and mechanisms underlying the function of miR-802 in NSCLC were also explored. The present findings suggest that miR-802 may be a notable tumor-suppressive miRNA in NSCLC, and it may be important for the maintenance of the properties of NSCLC cells.
miR-802 is downregulated in prostate cancer, and its downregulation is correlated with Gleason score, distant metastasis, and the pathological stage of the cancer (25). miR-802 is also weakly expressed in tongue squamous cell carcinoma (26), breast cancer (27), and gastric cancer (28). By contrast, miR-802 is upregulated in osteosarcoma tissues and cell lines (29). These inconsistent findings triggered us to examine the expression status of miR-802 in NSCLC and evaluate its clinical significance in patients with this disease. The present results revealed that miR-802 expression was decreased in NSCLC tissues and cell lines. Low miR-802 expression was significantly correlated with tumor stage, lymph node metastasis and brain metastasis in NSCLC patients. These findings suggest that miR-802 may be a potential diagnostic and prognostic marker of patients with this specific type of cancer.
miR-802 has been demonstrated to be a tumor suppressor in several human cancer types. For instance, miR-802 overexpression inhibits prostate cancer cell proliferation, promotes cell apoptosis in vitro, and decreases in vivo tumor growth. In addition, upregulation of miR-802 attenuates cell metastasis and epithelial-mesenchymal transition in prostate cancer (25). In tongue squamous cell carcinoma, ectopic miR-802 expression restricts cell proliferation, invasion, and epithelial-mesenchymal transition in vitro (26). In breast cancer, enforced miR-802 expression inhibits cancer cell growth both in vitro and in vivo (27). In gastric cancer, resumption of miR-802 expression decreases cell growth, promotes cell apoptosis, impairs cell metastasis in vitro, and hinders tumor growth in vivo (28). By contrast, miR-802 exerts oncogenic roles in the growth of osteosarcoma cells (29). Nevertheless, the functional roles of miR-802 in NSCLC had remained largely unknown. In the present study, miR-802 was demonstrated to have a tumor-suppressing role in the development of NSCLC, and was implicated in the regulation of NSCLC cell proliferation, colony formation, apoptosis, migration, invasion in vitro, and tumor growth in vivo. These findings suggest that miR-802 might be a promising therapeutic target in the treatment of patients with NSCLC.
Multiple human genes, including flotillin-2 in human prostate cancer (25), mitogen-activated protein kinase 4 in tongue squamous cell carcinoma (26), forkhead box protein M1 in breast cancer (27), RAB23 in gastric cancer (28), and p27 in osteosarcoma (29), have been validated as direct target genes of miR-802. In the present study, FGFR1, a member of the fibroblast growth factor family (43), was identified as a novel downstream target of miR-802 in NSCLC cells. It is highly expressed in a variety of human cancers, such as prostate cancer (44), gastric cancer (45), colorectal cancer (46) and breast cancer (47). FGFR1 is also upregulated in NSCLC, and its upregulation is associated with gender, smoking status, and disease stage (48). Patients with NSCLC with high FGFR1 expression exhibit shorter survival periods compared with patients with low FGFR1 expression (31). In addition, FGFR1 is demonstrated to be an independent favorable prognostic factor for patients with NSCLC (32). Functionally, FGFR1 is strongly correlated with the oncogenicity of NSCLC cells through its regulation of tumor aggression (33-37). The present study confirmed that miR-802 functioned as a tumor suppressor that retarded NSCLC progression, and this suppressive effect was mediated by FGFR1 silencing. These findings suggest that increased miR-802 expression, which results in FGFR1 knockdown, might be an effective therapeutic technique for the management of patients with NSCLC.
In summary, the present study demonstrated that miR-802 was downregulated in NSCLC and might contribute to the development of NSCLC. Mechanistically, miR-802 exerted its tumor-suppressive role, at least partially, by directly targeting the FGFR1-mediated PI3K/Akt/mTOR pathway in NSCLC. These observations help to improve our understanding of the mechanisms underlying the genesis and development of NSCLC, and may potentially promote the development of novel targeted therapies against this disease. However, a limitation of the present study is that miR-802 inhibitors were not used to knockdown endogenous miR-802 expression, and to examine the effects of miR-802 knockdown on the oncogenicity of NSCLC cells. Further studies will be required to fully elucidate the functions of miR-802 in NSCLC and its potential as a diagnostic or therapeutic target.