OL Oncology Letters 1792-1074 1792-1082 D.A. Spandidos 10.3892/ol.2021.13059 OL-0-0-13059 Articles Knockdown of lncRNA NUTM2A-AS1 inhibits lung adenocarcinoma cell viability by regulating the miR-590-5p/METTL3 axis WangJie 1 ZhaJingyun 2 WangXiaolin 1 Department of Thoracic Surgery, Yangzhou University, Yangzhou, Jiangsu 225000, P.R. China Department of Respiratory Medicine, Hefei First People's Hospital, Hefei, Anhui 230001, P.R. China Correspondence to: Dr Xiaolin Wang, Department of Thoracic Surgery, Yangzhou University, 88 South University Road, Yangzhou, Jiangsu 225000, P.R. China, E-mail: wxl1908526@163.com 11 2021 20 09 2021 22 5 798 27052021 06082021 Copyright: © Wang et al. 2021 This is an open access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

Lung adenocarcinoma (LUAD) is the leading cause of cancer-related mortality worldwide. Long non-coding RNA (lncRNA) NUT family member 2A antisense RNA 1 (NUTM2A-AS1) is dysregulated in LUAD; however, its role in this disease remains unclear. The present study aimed to identify the underlying molecular mechanism of the effect of lncRNA NUTM2A-AS1 in LUAD by exploring whether lncRNA NUTM2A-AS1 could affect LUAD cell proliferation and apoptosis through the microRNA (miR)-590-5p/methyltransferase 3, N6-adenosine-methyltransferase complex catalytic subunit (METTL3) axis. miR-590-5p was predicted and verified as the direct target of NUTM2A-AS1 using bioinformatics analysis and a dual luciferase reporter assay. The expression levels of NUTM2A-AS1 and miR-590-5p in lung cancer cells, and the effects of NUTM2A-AS1 on cell viability and apoptosis were determined using MTT assays and flow cytometry, respectively. Reverse transcription-quantitative PCR analysis revealed that the expression levels of NUTM2A-AS1 were significantly upregulated, while those of miR-590-5p were significantly downregulated, in lung cancer cells compared with the control epithelial cells. NUTM2A-AS1 knockdown inhibited NCI-H23 cell viability and induced apoptosis by upregulating miR-590-5p expression. Moreover, the function and regulatory mechanism of miR-590-5p in LUAD were also investigated. It was determined that miR-590-5p could interact with METTL3, and further analysis of the expression levels of METTL3 in lung cancer cells demonstrated that METTL3 was significantly upregulated in NCI-H23 and A549 cells compared with the control cells. In addition, miR-590-5p inhibited NCI-H23 cell viability and induced apoptosis by downregulating METTL3 expression. In conclusion, the findings of the present study suggested that NUTM2A-AS1 knockdown may inhibit LUAD progression by regulating the miR-590-5p/METTL3 axis. These results may provide insight into the mechanisms underlying the tumorigenesis of LUAD and offer a new treatment strategy for the disease.

 lung adenocarcinoma long non-coding RNA NUT family member 2A antisense RNA 1 microRNA-590-5p methyltransferase 3 N6-adenosine-methyltransferase complex catalytic subunit Introduction

Lung cancer is the leading cause of cancer-related mortality worldwide (1). The most commonly diagnosed lung cancer subtypes are malignant epithelial tumors, small cell lung carcinoma and non-small cell lung carcinoma (NSCLC) (1). NSCLC accounts for 85–90% of lung cancer cases and can be further categorized into three subtypes: Lung adenocarcinoma (LUAD), lung squamous cell carcinoma (LUSC) and large cell carcinoma (2). LUAD represents 38.5% of all diagnosed lung cancer cases and is the only type of NSCLC associated with non-smokers (3). Generally, LUAD grows more slowly and exhibits smaller tumor masses compared with LUSC of the same stage; however, it tends to metastasize early (4). In addition, LUAD is characterized by high rates of somatic mutations and genomic rearrangements, which presents significant challenges when attempting to identify driver gene alterations; thus, only frequent mutations have been identified to date (5). The prognosis for patients with LUAD remains low, with an average 5-year survival rate of <20% (6). Therefore, further research is urgently required to identify novel biomarkers and effective targeted molecular therapies.

Long non-coding RNA (lncRNA) is a type of RNA ~200 nucleotides in length lacking protein-coding capacity (7). lncRNA regulates a wide range of biological functions, such as cell differentiation and development (8). Recent studies have reported that lncRNA molecules are abnormally expressed in tumors, where they can act either as oncogenes or as tumor suppressors, depending on the cancer type (9,10). Acha-Sagredo et al (11) investigated a number of abnormally expressed lncRNA molecules and determined that NUT family member 2A antisense RNA 1 (NUTM2A-AS1) was upregulated in NSCLC, which suggested that it may serve as an oncogene. Although NUTM2A-AS1 was among the first lncRNA molecules to be found to be aberrantly expressed in cancer, the underlying molecular mechanisms driving its dysregulated expression remain unclear, to the best of our knowledge. Thus, it may be important to investigate the role of this lncRNA in LUAD.

MicroRNA (miRNA/miR) is known to dysregulate target mRNA expression, which can induce tumorigenesis and drug resistance (12). Previous studies have reported that certain miRNA molecules could be used in early lung cancer detection (13,14). A previous study reported that the downregulation of members of the miR-34 family was positively associated with poor prognosis in patients with NSCLC (13). In addition, miR-223, miR-20a, miR-448 and miR-145, were shown to have high sensitivity (>80%) as serum or plasma miRNA signatures in stage I–II NSCLC samples (14). miR-590-5p expression is also dysregulated in various types of cancer; for example, miR-590-5p expression was downregulated ~7.5-fold in patients with NSCLC. Another study reported that miR-590-5p could suppress tumor growth in NSCLC and may therefore represent a promising biomarker for the diagnosis or prognosis of NSCLC (15). However, the role and molecular mechanism of miR-590-5p in LUAD remain to be explored.

Methyltransferase 3, N6-adenosine-methyltransferase complex catalytic subunit (METTL3) is a catalytic subunit that forms the N6-methyladenosine (m6A) methyltransferase complex with METTL14 and WT1 associated protein and RNA binding motif protein 15 (16). m6A modifications exert multiple functions on mRNA and lncRNA, including regulating mRNA biogenesis, decay and translation control (17). A previous study indicated that METTL3 acted as a functional and clinical oncogene in colorectal cancer by stabilizing hexokinase 2 and solute carrier family 2 member 1 expression via an m6A- and insulin-like growth factor 2 mRNA binding protein 2/3-dependent mechanism (18). In addition, the proliferation, survival and invasion of lung cancer cells were found to be increased following the methylation of target mRNA transcripts (19). Du et al (20) investigated the effect of miR-33a binding to the 3′-untranslated region (UTR) of METTL3 on NSCLC cell proliferation. Moreover, m6A mRNA methylation by METTL3 stabilizes Yes1-associated transcriptional regulator (YAP) mRNA, and metastasis-associated lung adenocarcinoma transcript 1 (MALAT1) can sponge miR-1914-3p to promote YAP expression, which promotes NSCLC drug resistance and metastasis (21). These aforementioned studies indicated that m6A demethylation may regulate mRNA expression. However, to the best of our knowledge, few lncRNA molecules have been reported to be susceptible to such post-translation modifications (22).

The aim of the present study was to determine whether the effects of lncRNA NUTM2A-AS1 in LUAD cell viability and apoptosis were regulated by the miR-590-5p/METTL3 axis. The relationship between METTL3 and miR-590-5p was examined. In addition, the expression levels of NUTM2A-AS1 were analyzed in LUAD cell lines.

 Materials and methods <sec> <title>Cell lines and culture

The human A549 and NCI-H23 LUAD cells and the human BEAS2B lung epithelial cells (all from American Type Culture Collection) were cultured in DMEM (Gibco; Thermo Fisher Scientific, Inc.) supplemented with 10% FBS (Gibco; Thermo Fisher Scientific, Inc.) and 1% penicillin-streptomycin at 37°C with 5% CO2.

 Cell transfection

Following 24 h of incubation at 37°C with 5% CO2, NCI-H23 cells in the logarithmic growth phase were transfected with 500 pmol control small interfering RNA (siRNA) (cat. no. siN0000001-1-5; Ribobio), 500 pmol NUTM2A-AS1-siRNA (cat. no. siB180131045110-1-5; Ribobio), 50 nM inhibitor control (5′-GUCCAGUGAAUUCCCAG-3′; Shanghai GenePharma Co., Ltd.), 50 nM miR-590-5p inhibitor (5′-AAAUAUGCUGUAUGUCAUGUGUU-3′; Shanghai GenePharma Co., Ltd.), 100 nM mimic control (5′-CGCCAAUAUCAUUAUACCUC-3′; Shanghai GenePharma Co., Ltd.), 100 nM miR-590-5p mimic (5′-GAGCUUAUUCAUAAAAUGCAG−3′; Shanghai GenePharma Co., Ltd.), 1 µg control-plasmid (sc-437275; Santa Cruz), 1 µg METTL3-plasmid (sc-404029-ACT; Santa Cruz Biotechnology, Inc.), 500 pmol NUTM2A-AS1-siRNA + 50 nM inhibitor control, 500 pmol NUTM2A-AS1-siRNA + 50 nM miR-590-5p inhibitor, 100 nM miR-590-5p mimic + 1 µg control-plasmid or 100 nM miR-590-5p mimic + 1 µg METTL3-plasmid using Lipofectamine® 2000 (Invitrogen; Thermo Fisher Scientific, Inc.) at 37°C for 48 h. 48 h after transfection, the transfection efficiencies were assessed using reverse transcription-quantitative PCR (RT-qPCR), and subsequent experiments were performed.

 RT-qPCR

Total RNA was extracted from cells using TRIzol® reagent (Invitrogen; Thermo Fisher Scientific, Inc.). Total RNA (1 µg) was reverse transcribed into cDNA using a PrimeScript RT Reagent kit (Takara Biotechnology Co., Ltd.). The reaction conditions were as follows: 70°C for 5 min, 37°C for 5 min and 42°C for 60 min. qPCR was subsequently performed using SYBR Premix Ex Taq (Takara Biotechnology Co., Ltd.) according to the manufacturer's instructions. Primers and probes used for the qPCR were designed using primer design software Oligo version 7 (Molecular Biology Insights, Inc.). qPCR assays were performed on a 7900 Real-Time PCR detection system (Applied Biosystems; Thermo Fisher Scientific, Inc.) using the following thermocycling conditions: Initial denaturation at 94°C for 15 min; followed by 40 cycles at 94°C for 15 sec (denaturation), 60°C for 15 sec (annealing) and 72°C for 15 sec (extension). The relative expression levels were calculated using the 2−ΔΔCq method (23). U6 for miRNA and GAPDH for mRNA were used as the internal controls. The following primer sequences were used for qPCR: GAPDH forward, 5′-TTTGGTATCGTGGAAGGACTC−3′ and reverse, 5′-GTAGAGGCAGGGATGATGTTCT-3′; U6 forward, 5′-CTCGCTTCGGCAGCAGCACATATA−3′ and reverse, 5′-AAATATGGAACGCTTCACGA-3′; NUTM2A-AS1 forward, 5′-TACCTCTAGTTCTTCCCGGC-3′ and reverse, 5′-TTTTGCTTTTCTCCTGGCCC-3′; miR-590-5p forward, 5′-GAGCTTATTCATAAAAGT−3′ and reverse, 5′-TCCACGACACGCACTGGATACGAC−3′; METTL3 forward, 5′-AAGCTGCACTTCAGACGAAT-3′ and reverse, 5′-GGAATCACCTCCGACACTC-3′; and PCNA forward, 5′-ATCTAGACGTCGCAACTCCG-3′ and reverse, 5′-GCTGCACTAAGGAGACGTGA-3′.

 Western blotting

Total protein was extracted from adherent cells in cell lysis buffer supplemented with 1 mM PMSF (Beyotime Institute of Biotechnology). Total protein concentration was quantified using a BCA Protein assay kit (Beyotime Institute of Biotechnology) and 20 µg protein/lane was separated by SDS-PAGE on 12 or 15% gels. The proteins were subsequently transferred to PVDF membranes (MilliporeSigma) at room temperature, then blocked with 5% skimmed milk in TBS-0.1% Tween 20 at room temperature for 1 h. The membranes were then incubated with the following primary antibodies overnight at 4°C: Anti-cleaved caspase-3 (1:1,000; cat. no. ab2302; Abcam), anti-caspase-3 (1:5,000; cat. no. ab32351; Abcam), anti-METTL3 (1:1,000; cat. no. ab195352; Abcam), anti-proliferating cell nuclear antigen (PCNA; 1:1,000; cat. no. 10205-2-AP; ProteinTech Group, Ltd.) and anti-GAPDH (1:1,000; cat. no. 5174; Cell Signaling Technology, Inc.). Following the primary antibody incubation, the membranes were incubated with an anti-mouse IgG antibody (1:3,000; cat. no. ab6728; Abcam) at 37°C for 45 min. Protein bands were visualized using an ECL detection kit (Beyotime Institute of Biotechnology). Densitometry was analyzed using ImageJ software (version 1.46; National Institutes of Health).

 Flow cytometry assay

An Annexin V-FITC Apoptosis Detection kit (Beyotime Institute of Biotechnology) was used to analyze cell apoptosis using flow cytometry. Briefly, the transfected cells were cultured for 48 h and resuspended in 1X binding buffer at a density of 3–5×105 cells/ml. The cells were incubated with 5 µl Annexin V-FITC and 10 µl propidium iodide at 4°C for 15 min in the dark. Apoptotic cells were analyzed using a FACSCalibur flow cytometer (BD Biosciences), and the apoptotic rate was calculated using Kaluza analysis software (version 2.1.1.20653; Beckman Coulter, Inc.).

 MTT assay

Cell viability was analyzed using a MTT assay (Sigma-Aldrich; Merck KGaA). Briefly, cells were seeded into 96-well plates (4×103 cells/well) and transfected as aforementioned. Following 48 h of transfection, 100 µl DMEM medium containing 0.5 mg/ml MTT was added to each well and incubated for a further 4 h in a humidified incubator with 5% CO2 at 37°C. The medium was subsequently removed and 150 µl DMSO (Nanjing Keygen Biotech Co., Ltd) was added to terminate the reaction. The absorbance was measured at a wavelength of 570 nm using a microplate reader (BioTek Instruments, Inc.).

 Dual luciferase reporter assay

The StarBase database (http://starbase.sysu.edu.cn/) was used to predict the binding site of lncRNA NUTM2A-AS1 and miR-590-5p. The binding sites between miR-590-5p and METTL3 were identified using TargetScan 7.2 (http://www.targetscan.org/vert_72/). Wild-type (WT) or mutant type (MUT) 3′-UTR sequences of NUTM2A-AS1 containing the putative target sites for miR-590-5p were synthesized and cloned into the pMirTarget vector (cat. no. PS100062; OriGene Technologies, Inc.) to generate the WT-3′-UTR NUTM2A-AS1 and MUT-3′-UTR NUTM2A-AS1 constructs, respectively. The reporter plasmids were co-transfected with the miR-590-5p mimic (200 mol/l) or mimic control into 293T cells (ATCC) using Lipofectamine® 2000 (Invitrogen; Thermo Fisher Scientific, Inc.). Using the same method, the WT and MUT 3′-UTRs of METTL3 were synthesized and cloned in the pMirTarget vector, then co-transfected into 293T cells with the miR-590-5p mimic or mimic control. Following 48 h of transfection, the relative luciferase activity was measured using a Dual Luciferase Reporter assay system (Promega Corporation), according to the manufacturer's protocol. The firefly luciferase activity (experimental) in the 293T cells was normalized to Renilla luciferase activity (control).

 Statistical analysis

Statistical analyses were performed using GraphPad Prism version 6.0 software (GraphPad Software, Inc.). Statistical comparisons among groups were analyzed using an unpaired Student's t-test or one-way ANOVA followed by Tukey's post hoc test. Data are presented as the mean ± SD from at least three independent experiments. P<0.05 was considered to indicate a statistically significant difference.

 Results <sec> <title>miR-590-5p directly targets NUTM2A-AS1

To understand the underlying molecular mechanisms of the effects of NUTM2A-AS1 in the progression of LUAD, the StarBase database was used to predict possible targets of NUTM2A-AS1. The results identified miR-590-5p as one of its potential targets (Fig. 1A). Furthermore, the interaction between lncRNA NUTM2A-AS1 and miR-590-5p was verified using a dual luciferase reporter assay. First, the transfection efficiency of the miR-590-5p mimic was evaluated. Compared with the mimic control group, the expression levels of miR-590-5p were significantly upregulated in 293T cells transfected with the miR-590-5p mimic (Fig. 1B). The results of the dual luciferase reporter assay revealed that the relative luciferase activity of WT-3′-UTR NUTM2A-AS1 significantly decreased following co-transfection with the miR-590-5p mimic compared with the mimic control-transfected cells (Fig. 1C), indicating that miR-590-5p may directly interact with NUTM2A-AS1.

 Expression levels of NUTM2A-AS1 and miR-590-5p in LUAD cells

The expression levels of NUTM2A-AS1 and miR-590-5p in LUAD cells (A549 and NCI-H23) and normal lung epithelial cells (BEAS-2B) were examined using RT-qPCR. Compared with BEAS-2B cells, the expression levels of NUTM2A-AS1 were significantly upregulated (Fig. 2A), while those of miR-590-5p were significantly downregulated (Fig. 2B) in A549 and NCI-H23 cells. These results suggest that the expression levels of NUTM2A-AS1 and miR-590-5p in LUAD cells are different from normal lung epithelial cells.

 Knockdown of NUTM2A-AS1 reduces NCI-H23 cell viability and apoptosis by upregulating miR-590-5p expression

The effects of NUTM2A-AS1 and miR-590-5p on NCI-H23 cell viability and apoptosis were investigated. NCI-H23 cells were transfected with control-siRNA, NUTM2A-AS1-siRNA, inhibitor control, miR-590-5p inhibitor, NUTM2A-AS1-siRNA + inhibitor control or NUTM2A-AS1-siRNA + miR-590-5p inhibitor for 48 h. RT-qPCR analysis demonstrated that, in NCI-H23 cells, NUTM2A-AS1-siRNA significantly downregulated NUTM2A-AS1 expression levels compared with control-siRNA (Fig. 3A). Transfection with the miR-590-5p inhibitor significantly downregulated miR-590-5p expression levels in NCI-H23 cells compared with the inhibitor control (Fig. 3B). Furthermore, transfection with NUTM2A-AS1-siRNA significantly upregulated miR-590-5p expression levels in NCI-H23 cells; however, this effect was reversed following co-transfection with the miR-590-5p inhibitor (Fig. 3C).

Moreover, MTT assays, western blot analysis and flow cytometry were performed. The results of the MTT assay demonstrated that NUTM2A-AS1 knockdown significantly decreased the viability of NCI-H23 cells (Fig. 4A). Furthermore, the mRNA and protein expression levels of PCNA were downregulated in NUTM2A-AS1-siRNA-transfected NCI-H23 cells (Fig. 4B and C). The apoptosis of NCI-H23 cells was detected by flow cytometry. The results suggested that the apoptotic rate of NCI-H23 cells transfected with NUTM2A-AS1-siRNA was increased compared with that in NCI-H23 cells transfected with control-siRNA (Fig. 4D and E). Compared with the control-siRNA group, transfection with NUTM2A-AS1-siRNA upregulated the expression levels of cleaved caspase-3 and the cleaved caspase-3/caspase-3 ratio in NCI-H23 cells (Fig. 4F and G). All these aforementioned effects of NUTM2A-AS1-siRNA transfection were significantly reversed following co-transfection with the miR-590-5p inhibitor, indicating that the upregulation expression of miR-590-5p and the downregulation of NUTM2A-AS1 regulate viability and apoptosis in LUAD cells in vitro.

 METTL3 is a direct target of miR-590-5p

The potential targets of miR-590-5p were predicted to further determine the underlying molecular regulatory mechanism of NUTM2A-AS1. StarBase database predicted that METTL3 was a potential downstream target gene of miR-590-5p (Fig. 5A). Thus, a dual luciferase reporter assay was conducted to validate the relationship between METTL3 and miR-590-5p. The relative luciferase activity of WT-3′-UTR METTL3 was significantly reduced following the co-transfection with the miR-590-5p mimic compared with the mimic control-transfected cells (Fig. 5B), which indicated that METTL3 may directly interact with miR-590-5p. In addition, the expression levels of METTL3 in A549 and NCI-H23 LUAD cell and BEAS-2B normal lung epithelial cells were analyzed using western blotting and RT-qPCR. The results revealed that the expression levels of METTL3 were significantly upregulated in LUAD cells compared with BEAS-2B cells (Fig. 5C and D). These results suggest that the expression levels of METTL3 may exert crucial roles in LUAD cell viability and apoptosis.

 miR-590-5p inhibits NCI-H23 cell viability and induces apoptosis by downregulating METTL3 expression

The effects of miR-590-5p and METTL3 on NCI-H23 cell viability and apoptosis were then examined. NCI-H23 cells were transfected with mimic control, miR-590-5p mimic, control-plasmid, METTL3-plasmid, miR-590-5p mimic + control-plasmid or miR-590-5p mimic + METTL3-plasmid for 48 h. RT-qPCR analysis showed that transfection with the miR-590-5p mimic significantly upregulated miR-590-5p expression levels in NCI-H23 cells compared with the mimic control-transfected cells (Fig. 6A). Transfection with the METTL3 plasmid also significantly upregulated the mRNA expression levels of METTL3 in NCI-H23 cells compared with the control-plasmid-transfected cells (Fig. 6B). Transfection with the miR-590-5p mimic significantly downregulated METTL3 expression levels in NCI-H23 cells compared with the mimic control-transfected cells, and this effect was reversed by transfection with the METTL3-plasmid (Fig. 6C and D). Further analysis revealed that the miR-590-5p mimic significantly impeded the viability of NCI-H23 cells (Fig. 7A), decreased the mRNA and protein expression levels of PCNA (Fig. 7B and C), induced cell apoptosis (Fig. 7D and E), upregulated the protein expression levels of cleaved caspase-3 (Fig. 7F) and increased the cleaved caspase-3/caspase-3 ratio (Fig. 7G) in NCI-H23 cells. All these functions induced by the miR-590-5p mimic were significantly inhibited following co-transfection with the METTL3-plasmid, which further indicated that miR-590-5p may downregulate the expression of METTL3 in LUAD.

 Discussion

lncRNA molecules have been associated with the pathogenesis of numerous types of human disease, including cancer (24). As the number of aberrantly expressed lncRNA molecules in lung cancer continues to increase with further research, the underlying regulatory mechanisms of lncRNA requires further validation (25). For example, several types of lncRNA, including MALAT1, HOX transcript antisense RNA, HNF1 homeobox A antisense RNA 1 and breast cancer anti-estrogen resistance 4, have been identified to be upregulated in LUAD (26–29). In addition, NUTM2A-AS1 is upregulated and its promoter region hypomethylated in LUAD (11). The hypomethylation of NUTM2A-AS1 and its upregulation may facilitate its role as an oncogene. However, the underlying molecular mechanism of the effects of NUTM2A-AS1 in lung cancer has not been reported, to the best of our knowledge. A previous study demonstrated that programmed death-ligand 1 partially rescued NUTM2A-AS1- and miR-376a-regulated gastric cancer (GC) cell tumorigenesis and drug resistance (30). In addition, miR-376a was recently identified to be associated with NUTM2A-AS1 and found to be critical for NUTM2A-AS1-induced tumorigenesis (30). Moreover, lncRNA realizes its function by regulating miRNA/mRNA expression (31). In addition to the bioinformatics prediction results, the data of the dual luciferase assay in the present study indicated that miR-590-5p may directly bind to NUTM2A-AS1. Moreover, NUTM2A-AS1 knockdown inhibited cell viability and lung cancer progression by upregulating miR-590-5p.

miRNA inhibits the protein expression of target mRNA transcripts by incomplete base pairing (32). Due to this function, miRNA molecules have been found to serve a role in carcinogenesis (33). Notably, the abnormal expression of mature miRNA can facilitate the early progression of human cancer (34). Research into the role of miR-590-5p in GC showed that the upregulated expression of miR-590-5p may promote the migration and invasion of GC cells (35). However, the mechanism through which serum exosomal miR-590-5p functions in patients remains to be elucidated, to the best of our knowledge. Shen et al (35) reported that the knockdown of reversion inducing cysteine rich protein with kazal motifs, which was identified as a direct target of miR-590-5p, promoted GC development. It was also found that miR-590-5p overexpression activated the AKT/ERK and STAT3 signaling pathways (35). In hepatocellular carcinoma, downregulation of miR-590-5p inhibits HepG2 cell proliferation and invasion by increasing TGF-β receptor II expression (36). However, the function of miR-590-5p in tumor remains controversial. For example, in osteosarcoma, overexpression of miR-590-5p significantly reduces the proliferation, migration and invasion of osteosarcoma cells (37). Furthermore, miR-590-5p has been demonstrated to inhibit the cell proliferation and tumor growth of malignant melanoma cells in vivo and in vitro by suppressing YAP1 expression (38). These reports (35–38) suggest that miR-590-5p plays a tumor-promoting role in gastric cancer and hepatocellular carcinoma, while it acts as a tumor suppressor in osteosarcoma and malignant melanoma. These inconsistencies may be due to the differences between sample origins and the tumor clinicopathological characteristics (33); therefore, further research should be conducted in various human cancer types to determine the effect of miR-590-5p. The role of miRNA molecules in lung cancer, such as miR-148b, miR-590-3p and miR-455-5p, has been investigated in previous studies (39–41). miR-590 has been proposed to act as an oncogene or as a tumor suppressor gene depending on the target (42). Based on these findings, the role of miR-590-5p in LUAD was explored in the present study, and its target, METTL3, was identified and shown to regulate the proliferation and apoptosis of LUAD cells.

m6A RNA methylation participates in the pathogenesis of numerous human cancer types by affecting RNA metabolism, and the formation of m6A is catalyzed by a methyltransferase complex including METTL3 (43). Zheng et al (44) suggested that the downregulation of the lncRNA family with sequence similarity 225 member A (FAM225A) inhibited nasopharyngeal tumorigenesis. Furthermore, FAM225A downregulation was shown to be due to the METTL3-induced activation of focal adhesion kinase/PI3K/AKT signaling and the enhanced competitive binding between miR-590-3p and miR-1275. Similar studies have also found that the downregulation of METTL3 suppressed the activation of signaling pathways (such as PI3K/AKT pathway and β-catenin pathway) in lung cancer cells, leading to the inhibition of tumorigenesis (45–47). METTL3 exerts a potential biological role in human cancer cells. For example, in lung cancer cell lines, the protein expression levels of METTL3 are upregulated, and METTL3 knockdown reduces cell proliferation and invasion, whilst increasing apoptosis (48,49). The results of the present study indicated that the expression levels of METTL3 were negatively regulated by miR-590-5p and that miR-590-5p may inhibit the viability and induce the apoptosis of lung cancer cells by downregulating METTL3 expression. However, the function and regulatory mechanism of NUTM2A-AS1 in LUAD was only examined in one LUAD cell line and should be explored in additional cell lines.

In conclusion, the findings of the current study indicated that NUTM2A-AS1 knockdown may suppress the viability and induce the apoptosis of LUAD cells by upregulating miR-590-5p expression. Moreover, METTL3 was identified as a direct target of miR-590-5p. Therefore, the lncRNA NUTM2A-AS1/miR-590-5p/METTL3 axis may represent a novel molecular mechanism involved in LUAD progression and potential therapeutic target for LUAD treatment. However, this study was only a preliminary in vitro study of the effect of lncRNA NUTM2A-AS1 in LUAD. To make the role of lncRNA NUTM2A-AS1 in LUAD more convincing, a lot of in-depth research is needed. For example, the function of lncRNA NUTM2A-AS1 in other LUAD cell lines, such as A549, should be determined. The role of lncRNA NUTM2A-AS1 in LUAD in an animal model should be explored. Furthermore, the expression of lncRNA NUTM2A-AS1 and miR-590-5p in LUAD tissue, and whether there is a correlation between lncRNA NUTM2A-AS1 and miR-590-5p in LUAD tissue should also be determined. Moreover, the relationship between the expression of lncRNA NUTM2A-AS1 and miR-590-5p in LUAD patients and the clinicopathological parameters of patients should clarified.

 Acknowledgements

Not applicable.

 Funding

No funding was received.

 Availability of data and materials

The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.

 Authors' contributions

JW contributed to the study design, data collection, statistical analysis, data interpretation and manuscript preparation. JZ contributed to data collection and statistical analysis. XW contributed to data collection, statistical analysis and manuscript preparation. JW and XW confirmed the authenticity of all the raw data. All authors read and approved the final manuscript.

 Ethics approval and consent to participate

Not applicable.

 Patient consent for publication

Not applicable.

 Competing interests

The authors declare that they have no competing interests.

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miR-590-5p is a direct target of the long non-coding RNA NUTM2A-AS1. (A) miR-590-5p was predicted as a potential target of NUTM2A-AS1 using StarBase. (B) miR-590-5p expression levels in 293T cells transfected with the mimic control or miR-590-5p mimic. (C) Dual luciferase reporter assay was performed to confirm the interaction between miR-590-5p and NUTM2A-AS1. **P<0.01 vs. mimic control group. miR/miRNA, microRNA; NUTM2A-AS1, NUT family member 2A antisense RNA 1; WT, wild-type; MUT, mutant; 3′-UTR.

Expression levels of NUTM2A-AS1 and miR-590-5p in lung adenocarcinoma cell lines. (A) RT-qPCR was used to determine that the expression levels of NUTM2A-AS1 were upregulated in A549 and NCI-H23 cells. (B) RT-qPCR was used to determine that the expression levels of miR-590-5p were downregulated in A549 and NCI-H23 cells. **P<0.01 vs. BEAS2B cells. lncRNA, long non-coding RNA; NUTM2A-AS1, NUT family member 2A antisense RNA 1; miR, microRNA; RT-qPCR, reverse transcription-quantitative PCR.

Long non-coding RNA NUTM2A-AS1 negatively regulates miR-590-5p expression in NCI-H23 cells. (A) NUTM2A-AS1 expression levels in NCI-H23 cells transfected with control-siRNA, or NUTM2A-AS1-siRNA. (B) miR-590-5p expression levels in NCI-H23 cells transfected with inhibitor control or miR-590-5p inhibitor. (C) miR-590-5p expression levels in NCI-H23 cells transfected with control-siRNA, NUTM2A-AS1-siRNA, NUTM2A-AS1-siRNA + inhibitor control or NUTM2A-AS1-siRNA + miR-590-5p inhibitor. **P<0.01 vs. control-siRNA; ##P<0.01 vs. inhibitor control; &&P<0.01 vs. NUTM2A-AS1-siRNA + inhibitor control. miR, microRNA; siRNA, small interfering RNA; NUTM2A-AS1, NUT family member 2A antisense RNA 1.

Long non-coding RNA NUTM2A-AS1 knockdown inhibits NCI-H23 cell viability and induces apoptosis by upregulating miR-590-5p. (A) Viability of NCI-H23 cells was assessed using an MTT assay after 24, 48 and 72 h of incubation. (B) mRNA and (C) protein expression levels of PCNA were assessed using reverse transcription-quantitative PCR and western blotting, respectively. (D and E) Apoptosis was detected using flow cytometry. (F) Cleaved caspase-3 and caspase-3 protein expression levels were measured using western blotting. (G) Cleaved caspase-3/caspase-3 ratio. **P<0.01 vs. control-siRNA; ##P<0.01 vs. NUTM2A-AS1-siRNA + inhibitor control. NUTM2A-AS1, NUT family member 2A antisense RNA 1; miR, microRNA; siRNA, small interfering RNA; PCNA, proliferating cell nuclear antigen.

METTL3 is a direct target of miR-590-5p. (A) METTL3 was predicted as a potential target of miR-590-5p using StarBase. (B) Dual luciferase reporter assay was performed to confirm the interaction between miR-590-5p and METTL3. (C and D) METTL3 expression levels were measured in A549 and NCI-H23 cells using western blotting and reverse transcription-quantitative PCR. **P<0.01 vs. mimic control; ##P<0.01 vs. BEAS2B cells. METTL3, methyltransferase 3, N6-adenosine-methyltransferase complex catalytic subunit; miR, microRNA; WT, wild-type; MUT, mutant.

miR-590-5p negatively regulates METTL3 in NCI-H23 cells. (A) miR-590-5p and (B) METTL3 expression levels were measured using RT-qPCR. METTL3 (C) mRNA and (D) protein expression levels were measured using RT-qPCR and western blotting, respectively. **P<0.01 vs. mimic control; ##P<0.01 vs. control-plasmid; &&P<0.01 vs. miR-590-5p mimic + control-plasmid. miR, microRNA; METTL3, methyltransferase 3, N6-adenosine-methyltransferase complex catalytic subunit; RT-qPCR, reverse transcription-quantitative PCR.

miR-590-5p inhibits NCI-H23 cell viability and induces apoptosis by downregulating METTL3. (A) Viability of NCI-H23 cells was assessed using an MTT assay after 24, 48 or 72 h of incubation. (B) mRNA and (C) protein expression levels of PCNA were assessed using reverse transcription-quantitative PCR and western blotting, respectively. (D and E) Apoptosis was detected using flow cytometry. (F) Cleaved caspase-3 and caspase-3 protein expression levels were measured using western blotting. (G) Cleaved caspase-3/caspase-3 ratio. **P<0.01 vs. mimic control; ##P<0.01 vs. miR-590-5p mimic + control-plasmid. miR, microRNA; METTL3, methyltransferase 3, N6-adenosine-methyltransferase complex catalytic subunit; PCNA, proliferating cell nuclear antigen.