miR-744-5p Inhibits Non-Small Cell Lung Cancer Proliferation and Invasion by Directly Targeting PAX2.

Non-small cell lung cancer is one of the leading causes of cancer-related death worldwide. MicroRNAs have been characterized as critical regulators for cancer progression including non-small cell lung cancer. This work explored microRNA-744-5p expression in non-small cell lung cancer cell lines and normal cell line using quantitative real-time polymerase chain reaction. Connection of microRNA-744-5p and paired box 2 was analyzed with bioinformatic analysis, luciferase activity reporter assay, and Western blot. Effects of microRNA-744-5p or paired box 2 expression on non-small cell lung cancer cell behaviors were analyzed using a series of in vitro experiments. MicroRNA-744-5p was found to have decreased expression in non-small cell lung cancer cell lines compared with normal cell line. Paired box 2 was identified as a direct target for microRNA-744-5p in non-small cell lung cancer. Overexpression of microRNA-744-5p inhibits non-small cell lung cancer cell proliferation, colony formation, and cell invasion in vitro through targeting paired box 2. The present study provided novel insights into the biological functions of microRNA-744-5p in non-small cell lung cancer.

Introduction

Non-small cell lung cancer (NSCLC) accounts for approximately 80% of all lung cancer cases.[1,2] Although significant improvements in treatment methods for NSCLC have been achieved, the reported 5-year overall survival rate remains undesirable.[3] Hence, a better understanding of the abnormally expressed molecules in NSCLC progression is urgently needed.

MicroRNAs (miRNAs) are reported to have crucial roles in human development via repressing downstream targets expression.[4] Emerging evidence indicated altered expression of miRNA is crucial for carcinogenesis and functioned as either tumor suppressive or oncogenic miRNA.[5] Importantly, some miRNAs could be used as biomarkers for tumor early diagnosis, treatment, or prognosis prediction in oral squamous cell carcinoma, acute myeloid leukemia, and NSCLC.[6-9]

Abnormal expression of microRNA (miR)-744 has been found in several human cancer types.[10-13] For instance, miR-744 was overexpressed in prostate cancer and activated Wnt/β-catenin signaling pathway.[10] Furthermore, miR-744 was found to regulate LKB1 expression to promote prostate cancer progression.[11] These results suggested the oncogenic role of miR-744.[10,11] On the contrary, miR-744 was found to reduce expression in glioblastoma and its overexpression suppresses cancer progression via targeting NOB1.[12] In addition, it was found that miR-744-5p could induce ovarian cancer cell death through regulating HNRNPC and NFIX.[13] However, the role of miR-744-5p in NSCLC remains unclear.

Paired box 2 (PAX2) belongs to PAX protein family and was regarded as a crucial driver for human cancer development.[14-16] For example, PAX2 was found to promote endometrial cancer cell proliferation and invasion via targeting cyclin-dependent kinase 1.[14] In esophageal cancer, high PAX2 expression was found closely correlated with advanced tumor stage, lymph node metastasis, and lymphatic invasion.[15] In addition, it was found that PAX2 could activate interleukin 5 expression to promote tumor metastasis.[15] Moreover, PAX2 expression was found to be regulated by miR-497 to influence ovarian cancer cell proliferation and apoptosis.[16]

In this study, we explored expression levels of miR-744-5p and PAX2 in NSCLC cell lines. Effects of miR-744-5p or PAX2 on NSCLC cell proliferation, colony formation, and invasion were investigated using a series of in vitro experiments. Relationships between miR-744-5p and PAX2 were analyzed with luciferase activity reporter assay and Western blot.

Materials and Methods

Cell Culture

Non-small cell lung cancer cell lines (A549 and PC-9) and lung epithelial cell line BEAS-2B were purchased from Cell Bank of Chinese Academy of Sciences (Shanghai, China). Non-small cell lung cancer cells were incubated in Dulbecco’s modified Eagle medium (Invitrogen, Thermo Fisher Scientific, Inc, Waltham, Massachusetts) supplemented with 10% fetal bovine serum (FBS, Invitrogen). BEAS-2B cell line was incubated in Bronchial Epithelial Cell Growth Medium (BEGM) (Invitrogen, Thermo Fisher Scientific, Inc) containing 10% FBS. All these cells were maintained at a 37 °C humidified incubator containing 5% of CO2.

Cell Transfection

Cells were seeded into a 6-well plate at the density of 3 × 105 cells/well and cultured until 70% to 80% confluence. For upregulation of miR-744-5p, 100 nmol/L miR-744-5p mimic (5′-UGCGGGGCUAGGGCUAACAGCA-3′) was transfected into NSCLC cell lines using Lipofectamine 2000 (Invitrogen). The negative control mimic (NC-mimic) sequence was 5′-GAGCUACGGUAGAGCCGGUAGC-3′; pcDNA3.1 containing the full-length complementary DNA (cDNA) of PAX2 (pPAX2, 5 μg) purchased from GenScript was used to upregulate PAX2 expression in NSCLC cells. After transfection for 48 hours, cells were collected for following analyses.

Quantitative Real-Time Polymerase Chain Reaction

Total RNA from cultured cells was extracted using TRIzol reagent (Invitrogen) according to manufacturer’s protocol. Complementary DNA was synthesized using SuperScript II (Invitrogen). Quantitative real-time polymerase chain reaction was performed at ABI 7500 system (Applied Biosystem, Foster City, California) using SYBR Green (Applied Biosystems). Primer sequences used were as follows: miR-744-5p forward, 5′-AATGCGGGGCTAGGGCTA-3′ and reverse, 5′-GTGCAGGGTCCGAGGT-3′; and U6 small nuclear RNA (snRNA) forward, 5′-CTCGCTTCGGCAGCACA-3′ and reverse, 5′-AACGCTTCACGAATTTGCGT-3′. The following protocol was used: denaturation at 95°C for 10 minutes, followed by 40 cycles of denaturation at 95°C for 15 seconds, annealing at 60°C for 30 seconds, and extension at 72°C for 1 minute. Relative expression level was analyzed using 2−ΔΔCt method with U6 snRNA as internal control. Experiments were performed in triplicates.

Western Blot

Total protein from cultured cells was isolated using radioimmunoprecipitation assay lysis buffer (Beyotime, Haimen, Jiangsu, China) and protease inhibitors (Beyotime). Then, protein sample (50 μg) was isolated using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and transferred to polyvinylidene difluoride membrane. Membranes were incubated with primary antibody (1:1,000, rabbit monoclonal anti-PAX2: ab79389 [rabbit], 1:1,000, rabbit monoclonal anti-GAPDH: ab181602 [rabbit]; Abcam, Cambridge, Massachusetts) at 4°C for overnight after washing with (Tris-HCl buffer solution tween) TBST and blocked by 5% fat-free milk. Membrane was incubated with horseradish peroxidase–conjugated goat anti-rabbit secondary antibody (1:5,000, ab6721; Abcam) at room temperature for 2 hours. Protein signals were developed using BeyoECL kit (Beyotime). All the experiments were conducted in triplicates.

Cell Proliferation Assay

Cell Counting Kit 8 (CCK-8) assay was used to analyze cell proliferation rate. Cells were seeded into 96-well plate (2 × 103 cells/well) and incubated for 0, 24, 48, and 72 hours. Then, 10-μL CCK-8 reagent (Beyotime) was added to each well and further incubated for 2 hours. Optical density at 490 nm was analyzed using microplate reader. Three independent experiments were performed.

Colony Formation Assay

Cells were incubated at 6-well plates for 14 days at the abovementioned conditions. Colonies were fixed with methanol, stained with crystal violet, and counted under microscope. Experiments were repeated in triplicates.

Invasion Assay

Transwell chambers (8-µm pore size membranes, Corning, New York, New York) precoated with Matrigel (BD Biosciences, Franklin Lakes, New Jersey) were used for cell invasion analysis; 5  ×  104 cells in serum-free medium were filled into the upper chamber. Dulbecco’s modified Eagle medium containing FBS was added to the lower chamber. After incubation for 48 hours, invaded cells were fixed with methanol, stained with crystal violet, and counted under microscope. Experiments were conducted in triplicates.

Luciferase Reporter Assay

According to the bioinformatic analyses, results were obtained from TargetScan (http://www.targetscan.org/vert_72/).[17] Among all these 48 predicted targets, we found PAX2 ranks as the first place, indicating PAX2 has a high potential to be validated as miR-744-5p target. Importantly, PAX2 was revealed to be overexpressed in several human cancers and thus promote cancer progression.[14-16] However, the connection of miR-744-5p and PAX2 was not reported till now. Hence, PAX2 was selected for following analyses. The 3′-untranslated region (3′-UTR) of PAX2 was cloned from genome and inserted into pmirGLO (Promega, Madison, Wisconsin) to generate wild-type (wt)-PAX2. Mutant luciferase vector mutant (mt)-PAX2 was generated using site-direct mutagenesis kit (Takara, Dalian, Liaoning, China). Cells were then cotransfected with luciferase reporter (10 ng) and synthetic miRNAs (50 nmol/L) using Lipofectamine 2000. After incubation for 48 hours, relative luciferase activity was analyzed with Dual-Luciferase Reporter Assay System (Promega). Experiments were performed in triplicates.

Statistical Analysis

Data were presented as mean ± standard deviation after analyzing at SPSS (SPSS, Inc, Chicago, Illinois). Differences in groups were analyzed with Student t test or analysis of variance and Tukey post hoc test. P <  .05 was set as indicator for statistical significance.

Results

miR-744-5p Was Downregulated in NSCLC Cell Lines

We found miR-744-5p expression level was significantly reduced in NSCLC cell lines (A549 and PC-9) compared with lung epithelial cell line BEAS-2B (Figure 1).

Figure 1.

MicroRNA-744-5p was downregulated in NSCLC cell lines as assessed by qRT-PCR. ***P < .001. miR-744-5p indicates microRNA-744-5p; NSCLC, non-small cell lung cancer; qRT-PCR, quantitative real-time polymerase chain reaction.

Overexpression of miR-744-5p Inhibits BEAS-2B Cell Proliferation

Then, we investigated the biological roles of miR-744-5p by introducing miR-744-5p mimic into BEAS-2B cell line. We found miR-744-5p mimic transfection increased the levels of miR-744-5p in BEAS-2B cell (Figure 2A). Cell Counting Kit-8 assay revealed miR-744-5p overexpression significantly inhibited the proliferation of BEAS-2B cell line (Figure 2B).

Figure 2.

MicroRNA-744-5p overexpression inhibits the proliferation of BEAS-2B cells. A, Expression of miR-744-5p and (B) cell proliferation in BEAS-2B cells transfected with miR-744-5p mimic and NC-mimic. ***P < .001, *P < .05. miR-744-5p indicates microRNA-744-5p; NC-mimic, negative control mimic; NSCLC, non-small cell lung cancer.

miR-744-5p Inhibits NSCLC Cell Proliferation, Colony Formation, and Cell Invasion

To validate the role of miR-744-5p in NSCLC, NSCLC cells were transfected with miR-744-5p mimic. We showed miR-744-5p level in NSCLC cell lines was significantly elevated by miR-744-5p mimic as compared with NC-mimic (Figure 3A). Cell Counting Kit 8 assay revealed that miR-744-5p mimic transfection significantly inhibited NSCLC cell proliferation rate compared with NC-mimic (Figure 3B). Colony formation assay confirmed the results of CCK-8 assay (Figure 3C). In addition, we found miR-744-5p overexpression inhibited NSCLC cell invasion (Figure 3D).

Figure 3.

MicroRNA-744-5p plays an inhibitory role in the growth and invasive abilities of NSCLC cells. A, Expression of miR-744-5p, (B) cell proliferation, (C) colony formation, and (D) cell invasion in NSCLC cells transfected with miR-744-5p mimic and NC-mimic. ***P < .001, **P < .01. miR-744-5p indicates microRNA-744-5p; NC-mimic, negative control mimic; NSCLC, non-small cell lung cancer.

PAX2 Was a Direct Target Gene of miR-744-5p

Paired box 2 was demonstrated to be a putative target of miR-744-5p using TargetScan (Figure 4A). Luciferase reporter assay was conducted to validate the connection of miR-744-5p and 3′-UTR of PAX2; miR-744-5p mimic transfection significantly inhibited relative luciferase activity in cells transfected with wt-PAX2 but had no effect on those transfected with mt-PAX2 (Figure 4B). Western blot showed that PAX2 expression was downregulated by miR-744-5p mimic in NSCLC cells (Figure 4C).

Figure 4.

Paired box 2 was a direct target gene of miR-744-5p. A, The PAX2 3′-UTR contains the predicted miR-744-5p binding site. B, The NSCLC cells were transfected with wt or mt PAX2 3′-UTR and miR-744-5p mimic or NC-mimic. C, Effects of miR-744-5p mimic on PAX2 expression in NSCLC cell lines. ***P < .001. miR-744-5p indicates microRNA-744-5p; mt, mutant; NC-mimic, negative control mimic; ns, not significant; NSCLC, non-small cell lung cancer; PAX2, paired box 2; UTR, untranslated region; wt, wild-type.

Overexpression of PAX2 Reversed the Tumor Suppressive Effects of miR-744-5p on NSCLC

To investigate the biological connection of miR-744-5p and PAX2, pPAX2 was cotransfected with miR-744-5p mimic into NSCLC cells. It was found PAX2 expression could be elevated by pPAX2 (Figure 4A). Meanwhile, overexpression of PAX2 could partially reversed the effects of miR-744-5p mimic on PAX2 expression (Figure 5A). As expected, PAX2 overexpression promotes NSCLC cell proliferation, colony formation, and cell invasion (Figure 5B-D). Moreover, PAX2 overexpression abolished the inhibitory effects of miR-744-5p on NSCLC cell proliferation, colony formation, and cell invasion (Figure 5B-D).

Figure 5.

Paired box 2 reintroduction reverses the effects of miR-744-5p on NSCLC cell behaviors. A, Expression of PAX2, (B) cell proliferation, (C) colony formation, and (D) cell invasion in NSCLC cells transfected with pPAX2, pcDNA3.1, or pPAX2, and miR-744-5p mimic. ***P < .001, **P < .01, *P < 0.05. miR-744-5p indicates microRNA-744-5p; NSCLC, non-small cell lung cancer; PAX2, paired box 2.

Discussion

Roles of miRNAs in tumor development have gained increasing attention in recent years.[5] Multiple miRNAs have been found to function in crucial roles in NSCLC.[18,19] For instance, force miR-34b expression was found to be able to inhibit NSCLC cell migration and invasion but promote cell apoptosis through targeting YY1-associated factor 2.[18] MicroRNA-382 was found to inhibit NSCLC metastasis through regulating LIM-only protein 3.[19]

In this study, we observed miR-744-5p expression was reduced in NSCLC cell lines compared with normal cell line, and miR-744-5p overexpression inhibits NSCLC cell proliferation, colony formation, and cell invasion in vitro. These results indicated miR-744-5p may function in a tumor suppressive role in NSCLC, which is inconsistent with its role in glioblastoma and ovarian cancer.[12,13]

Furthermore, we validated PAX2 as a functional target of miR-744-5p in NSCLC. Furthermore, PAX2 overexpression promoted NSCLC cell growth and invasion and partially reversed the suppressive effects of miR-744-5p. Previous work has indicated that PAX2 could promote the epithelial–mesenchymal transition process in NSCLC.[20] Hence, we believe our work provided novel evidence to support the oncogenic role of PAX2 in NSCLC. Although this study has achieved progress regarding the role of miR-744-5p/PAX2 axis in NSCLC, we have to admit several drawbacks in this current study: (1) This work investigated the role of miR-744-5p/PAX2 in NSCLC cells, however, studies using an animal model were not done; and (2) Patients should be enrolled to investigate the expression of miR-744-5p and PAX2 in human tissues.

In conclusion, we found miR-744-5p had reduced expression in NSCLC cell lines. Further functional experiments revealed miR-744-5p directly targets PAX2 to exert a tumor suppressive role in NSCLC. This study provided new clues to guide novel therapeutic target development for NSCLC.