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Introduction

Asthma is the most common chronic childhood disease (1). Neutrophilic asthma (NA) is a subtype of asthma, which occurs in 15-25% of cases and responds poorly to glucocorticoid treatment (2). NA is characterized by neutrophil-mediated inflammation of the airway, and can result in reduced bronchodilator reversibility and fixed airflow obstruction, which are also the features of chronic obstructive pulmonary disease (3,4). Understanding the immunopathology of NA may result in the discovery of targeted treatments for patients (3).

MicroRNAs (miRNAs/miRs) are small non-coding RNAs that regulate the post-transcriptional expression of multiple genes by complementary binding to the 3′ untranslated region (3′UTR) of the target gene (5). miR-29a-3p has been demonstrated to participate in a variety of diseases and different pathological processes, including cell proliferation, apoptosis, fibrosis and immunomodulation (6,7). Epithelial-mesenchymal transition (EMT) is a process in which epithelial cells lose the epithelial phenotype and undergo the transition to typical mesenchymal characteristics. Recent evidence has suggested that EMT is involved in airway remodeling and the development of asthma (8,9). More importantly, miR-29a-3p has been implicated in the inhibition of gene expression in EMT and metastasis (6). However, the effect of miR-29a-3p on EMT in NA remains unknown.

Thus, the present study aimed to investigate the role of miR-29a-3p in NA. To study the role of miR-29a-3p in NA, a mouse model of NA was established and these animals were compared to normal controls. Both groups of mice were subjected to lung function tests and histopathological analysis to confirm the induction of the NA model. Human bronchial epithelial cells (16HBE) were grown in culture, and in vitro experiments were performed. The expression levels of miR-29a-3p, secreted protein acidic rich in cysteine (SPARC) and EMT-related markers were measured and luciferase reporter assays were performed to identify the direct regulatory relationship between miR-29a-3p and SPARC.

Materials and methods

Animals

Female C57BL/6 mice (n=12; certificate no. SCXK 2020-0003) aged 6-8 weeks and weighing 18-20 g were purchased from Guangxi Medical University Animal Center (license no. SYXK 2020-0004). Mice were housed under specific pathogen-free conditions with a relatively stable temperature (20-24°C) and humidity (55±10%) at 12-h light/dark cycles. All animal experiments were approved by The Ethics Committee of The First Affiliated Hospital of Guangxi Medical University [approval no. 2019 (KY-E-035); Nanning, China].

Animal groups

Mice were randomly divided into the normal control group (NC) and a NA group (NA) (n=6 in each group). The mouse model with NA was established using previously outlined protocols (10,11). Mice were sensitized by airway delivery of 100 µg ovalbumin (OVA; Grade II & V; Sigma-Aldrich; Merck KGaA) and 1 µg lipopolysaccharide (LPS; Sigma-Aldrich; Merck KGaA) in a total volume of 50 µl PBS on days 1, 7 and 14. The mixture was instilled along the posterior oropharyngeal wall and inhaled into the airway, followed by a challenge with 1% OVA aerosol for 1 h from day 21 for 7 consecutive days. The mice in the NC group received the equivalent amount of PBS treatment instead of OVA + LPS for sensitization and challenge.

Lung function measurements

Mice were anesthetized with 1% pentobarbital sodium (50 mg/kg body weight) by intraperitoneal injection. Measurements of dynamic resistance were assessed using whole-body plethysmography (Buxco® FinePointe Noninvasive Airway Mechanics; Data Sciences International) and induced with methacholine (Sigma-Aldrich; Merck KGaA) at doses of 12.5, 25 and 50 mg/ml 24 h after the final OVA challenge. Each mouse was exposed to aerosolized PBS (baseline) for 3 min followed by the administration of increasing concentrations of methacholine solutions. Airway resistance [enhanced pause (Penh)] values were then evaluated for 5 min. The results were expressed as the percentage of the baseline Penh value for each concentration of methacholine used (12).

Cell classification of bronchoalveolar fluid (BALF)

Mice were sacrificed 24 h after the final aerosolization. Cervical dislocation was used for euthanasia and death was confirmed by the onset of rigor mortis, according to The National Institutes of Health Guide for the Care and Use of Laboratory Animals. The lungs were subjected to bronchoalveolar lavage twice 24 h after the final aerosolization with 0.5 ml PBS (recovery rate ≥80%) and the total volume of BALF was 0.8 ml. Total and differential cell counts in BALF were determined by Diff-Quick staining (Beijing Solarbio Science & Technology Co., Ltd.) for 1 min at room temperature, according to previously outlined protocols (11).

Histopathological analysis

Lungs were fixed in 4% paraformaldehyde solution for 24 h at room temperature, and then subjected to gradient alcohol dehydration and paraffin-embedding. Then, the lungs were sliced into 5-7-µm thick sections and subsequently stained with hematoxylin and eosin (H&E) at room temperature for 2-3 min. An optical microscope (Olympus Corporation) was used to evaluate the general inflammation using previously outlined protocols (11).

Cell culture and transfection

Human bronchial epithelial cells (16HBE) were purchased from FuHeng Biology (cat. no. FH1013) and maintained in Keratinocyte medium (cat. no. 2101; ScienCell Research Laboratories, Inc.) in a humidified atmosphere of 5% CO2 at 37°C. Cells were authenticated using the STR genotype test and passed the mycoplasma testing. Cell passage four was used for this study. For lentiviral transfection, pLV-h-SPARC-CMV-MCS-3FLAG-EF1-ZsGreen-PURO [Lentivirus (LV)-SPARC] and pLV-CMV-MCS-3FLAG-EF1-Zs-Green-T2A-PURO (LV-control; both purchased from Sangon Biotech Co., Ltd.) were transfected into 293T cells (National Collection of Authenticated Cell Cultures; National Science & Technology Infrastructure), 2nd generation system was used [ratio of the lentiviral plasmid (10 µg):packaging vector (10 µg):envelope (5 µg)]. For virus collection, the supernatant was collected 48 and 72 h after transfection of the packaged cells, centrifuged at 2,000 × g for 10 min to remove cell debris, and ultracentrifuged at 82,700 × g for 120 min, then the pellet was resuspend in the culture medium to determine the titer. miR-29a-3p mimic and miR-29a-3p inhibitor were provided by Sangon Biotech Co., Ltd. 16HBE cells (1×106 cells/ml) at 60-80% confluence were treated with LV-SPARC [at 50 multiplicity of infection (MOI)] for 24 h, and then a miR-29a-3p mimic or miR-29a-3p inhibitor (100 nM) mixed with Lipofectamine® 6000 transfection reagent (Invitrogen; Thermo Fisher Scientific, Inc.) was transfected into cells at room temperature for 5 min, and incubated for 24 h. The negative control group (NeC; untreated cells) was set. miRNAs were labeled with FAM, and transfection efficacy was determined by the expression of immunofluorescence using fluorescence microscopy (Olympus Corporation). The cells were lysed for reverse transcription-quantitative (RT-q)PCR and western blotting. The sequences were as follows: miR-29a-3p mimic, 5′-UAG CAC CAU CUG AAA UCG GUU A-3′; miR-29a-3p inhibitor, 5′-UAA CCG AUU UCA GAU GGU GCU A-3′; miR-29a-3p mimic control, 5′-UUU GUA CUA CAC AAA AGU ACU G-3′; and miR-29a-3p inhibitor control, 5′-CAG UAC UUU UGU GUA GUA CAA A-3′.

Luciferase reporter gene assay

The target gene of miR-29a-3p was predicted using the online bioinformatics software, TargetScan3.1 (http://www.targetscan.org/vert_71/). Cells were co-transfected with miR-29a-3p mimic or miR mimic control, and psiCHECK-2-SPARC-3′-UTR-wild-type (WT) plasmid or a psiCHECK-2-SPARC-3′-UTR-mutant (MUT) plasmid (Promega Corporation) using Lipofectamine® 2000 (Invitrogen; Thermo Fisher Scientific, Inc.). Luciferase activity was measured 48 h after transfection using Dual-Luciferase system (Promega Corporation) in comparison with Renilla luciferase activity.

RT-qPCR

The lung samples and cultured cells were subjected to RNA extraction using TRIzol (cat. no. 15596-026; Invitrogen; Thermo Fisher Scientific, Inc.). cDNA for RNA and miRNA were synthesized using the PrimeScript™ RT Reagent kit with gDNA Eraser (cat. no. RR047A; Takara Bio, Inc.) and Mir-X™ miRNA First-Strand Synthesis kit (cat. no. 638313; Takara Bio, Inc.) according to the manufacturer's protocol, respectively. qPCR was performed using an ABI 7500 Real-Time PCR instrument (Applied Biosystems; Thermo Fisher Scientific, Inc.) with TB Green® Premix Ex Taq™ II (Tli RNase H Plus) (cat. no. RR820A; Takara Bio, Inc.) using the following primers: U6 forward, 5′-GCT TCG GCA GCA CAT ATA CTA AAA T-3′ and reverse, 5′-CGC TTC ACG AAT TTG CGT GTC AT-3′; miR-29a-3p, 5′-CGC TAG CAC CAT CTG AAA TCG GTT A-3′; glyceraldehyde-3-phosphate dehydrogenase (GAPDH) forward, 5′-CCT CTG CGC CCT TGA GCT AGG A-3′ and reverse, 5′-CAC AAG AAG ATG CGG CCG TCT C-3′; SPARC forward, 5′-GCT CCC ATT GGC GAG TTT G-3′ and reverse, 5′-GAT GTA GTC CAG GTG GAG CTT GTG-3′; E-cadherin forward, 5′-CAC CGA TGG TGA GGG TAC ACA G-3′ and reverse, 5′-GGC TTC AGG AAT ACA TGG ACA AAG A-3′; Vimentin forward, 5′-AAA GCG TGG CTG CCA AGA A-3′ and reverse, 5′-ACC TGT CTC CGG TAC TCG TTT GA-3′; U6 (human) forward, 5′-CTC GCT TCG GCA GCA CA-3′ and reverse, 5′-AAC GCT TCA CGA ATT TGC GT-3′; miR-29a-3p (human) forward, 5′-TAG CAC CAT CTG AAA TCG GTT A-3′ and reverse, 5′-TGG TGT CGT GGA GTC G-3′; GAPDH (human) forward, 5′-GCA CCG TCA AGG CTG AGA AC-3′ and reverse, 5′-TGG TGA AGA CGC CAG TGG A-3′; SPARC (human) forward, 5′-ACA TAA GCC CAG TTC ATC ACC A-3′ and reverse, 5′-ACA ACC GAT TCA CCA ACT CCA-3′; E-cadherin (human) forward, 5′-GGA TTG CAA ATT CCT GCC ATT C-3′ and reverse, 5′-AACGTTGTCCCGGGTGTCA-3′; and vimentin (human) forward, 5′-GGA AGG CGA GGA GAG CAG GAT T-3′ and reverse, 5′-TTC AAG GTC ATC GTG ATG CTG AGA AG-3′. qPCR thermocycling conditions were as follows: Denaturation at 95°C for 30 sec, annealing at 95°C for 5 sec, and extension at 60°C for 34 sec for 40 cycles. The miRNA and mRNA levels were normalized to U6 or GAPDH, respectively. The fold-change for each gene was calculated using the 2−ΔΔCq method (13).

Western blotting

The lung tissues and cells were lysed in a mixture of RIPA lysis buffer (Beyotime Institute of Biotechnology) and protease inhibitors (Thermo Fisher Scientific, Inc.). The protein concentration of samples was measured using a BCA protein assay kit (Applygen Technologies, Inc.). Equal amounts of total protein (20 µl) were loaded onto 10% sodium dodecyl sulfate-polyacrylamide gels and subjected to electrophoresis, and then separated proteins were electro-transferred onto PVDF membranes (Thermo Fisher Scientific, Inc.). Membranes were blocked with 5% skimmed milk or 5% BSA (cat. no. A8020; Beijing Solarbio Science & Technology Co., Ltd.) for 1 h and incubated at 4°C overnight with primary antibodies against the following proteins: SPARC (cat. no. 8725; 1:1,000; Cell Signaling Technology, Inc.), E-cadherin (cat. no. 3195; 1:1,000; Cell Signaling Technology, Inc.), vimentin (cat. no. 5741; 1:1,000; Cell Signaling Technology, Inc.), ERK (cat. no. 4695; 1:1,000; Cell Signaling Technology, Inc.), p-ERK (cat. no. 4370; 1:2,000; Cell Signaling Technology, Inc.) and GADPH (cat. no. 21612; 1:5,000; Signalway Antibody LLC). Following which, membranes were incubated at room temperature for 1 h with a secondary antibody (anti-rabbit IgG; cat. no. L3012; 1:5,000; Signalway Antibody LLC). An ECL kit (cat. no. BL520A; Biosharp Life Sciences) was used to view the protein bands, and ImageJ software version 1.8.0 (National Institutes of Health) was applied for the analysis of the relative intensities of protein bands.

Statistical analysis

Data were analyzed using GraphPad Prism 6 software (GraphPad Software, Inc.) and expressed as the mean ± SD of three independent experimental repeats. The statistical significance was determined by an unpaired Student's t-test or one-way ANOVA with Tukey's post hoc test for multiple comparisons. For non-normally distributed data, the significance was determined using a Kruskal-Wallis test with a post hoc Dunn's multiple comparison test. P<0.05 was considered to indicate a statistically significant difference.

Results

Establishment of the NA mouse model

The mouse model was established and presented with the following features of NA: Presence of bronchial hyperresponsiveness, the accumulation of inflammatory cells in the lung, particularly increased neutrophils, as well as a high number of neutrophils in the BALF. As shown in the H&E staining images, NA mice exhibited a thick basement membrane and increased inflammatory cell infiltration around the bronchus (Fig. 1A). Airway resistance was elevated in the NA group when compared with the NC group after methacholine challenges at doses of 25 and 50 mg/ml (Fig. 1B). Compared with the NC mice, the total number of cells and neutrophils were significantly increased, while no eosinophilic cells were observed in the BALF of NA mice (Fig. 1C).

Figure 1 Establishment of the NA mouse model. (A) Representative images of hematoxylin and eosin-stained lung tissue (magnification, ×100). (B) Airway resistance of the mouse model. (C) The differential cell counts in the BALF. n=6 mice per group. The results are presented as the mean ± SD. Data were analyzed using a two-way ANOVA. **P<0.01, ****P<0.0001 vs. NC group. NC, normal control; NA, neutrophilic asthma; Mch, methacholine; BALF, bronchoalveolar fluid.

miR-29a-3p and SPARC are involved in NA

miR-29a-3p and SPARC expression levels were determined via RT-qPCR. Compared with the NC group, miR-29a-3p expression was decreased (Fig. 2A), while SPARC expression was increased in NA mice (Fig. 2B and E). The levels of EMT-related markers in the lung of NA mice were then determined. The results showed that the mRNA and protein expression levels of the epithelial marker, E-cadherin, were decreased in NA mice compared with the NC group (Fig. 2C and E). On the contrary, the mRNA and protein expression levels of the mesenchymal marker, vimentin, were increased in NA mice compared with the NC group (Fig. 2D and E). The data revealed that the NA mouse model exhibited an EMT phenotype with decreased levels of miR-29a-3p and increased SPARC.

Figure 2 miR-29a-3p and SPARC are involved in NA. (A) miR-29a-3p, (B) SPARC, (C) E-cadherin and (D) Vimentin mRNA expression as measured via reverse transcription-quantitative PCR. (E) SPARC, E-cadherin and Vimentin protein expression levels were measured by western blotting. n=6 mice per group. The results are presented as the mean ± SD. Data were analyzed using an unpaired Student's t-test. *P<0.05, ***P<0.001, ****P<0.0001. NC, normal control; NA, neutrophilic asthma; miR, microRNA; SPARC, secreted protein acidic rich in cysteine.

SPARC regulates EMT in 16HBE cells

Next, the function of SPARC during EMT was investigated in 16HBE cells. SPARC was overexpressed in 16HBE cells following LV-SPARC transfection (Fig. 3A). The mRNA expression of E-cadherin was significantly reduced in the LV-SPARC group compared with the NeC group, whereas mRNA expression of vimentin was significantly increased by LV-SPARC administration (Fig. 3B). E-cadherin protein expression was reduced, while vimentin expression was elevated in SPARC-treated 16HBE cells compared with the LV-control group, as analyzed via western blotting (Fig. 3C), suggesting that SPARC participated in the EMT of 16HBE cells.

Figure 3 Effects of SPARC on the epithelial-mesenchymal transition of 16HBE cells. (A) SPARC mRNA was measured via RT-qPCR in 16HBE cells treated with LV-SPARC for 24 h. (B) mRNA expression levels of E-cadherin and Vimentin were measured via RT-qPCR. (C) Protein expression levels of E-cadherin and Vimentin were measured by western blotting. Three independent experiments were conducted for each treatment. The data are expressed as the mean ± SD. Data were analyzed using a one-way ANOVA. *P<0.05, **P<0.01, ***P<0.001. LV, lentivirus; SPARC, secreted protein acidic rich in cysteine; RT-qPCR, reverse transcription-quantitative PCR; NeC, negative control group.

miR-29a-3p regulates SPARC expression in vitro

miRNAs are hypothesized to be involved in various diseases where they inhibit gene expression through binding to the 3′UTR of the target gene (6). In the present study, the target gene of miR-29a-3p was predicted using the online bioinformatics software, TargetScan. The results showed that SPARC was a target gene for miR-29a-3p and the predicted binding site is shown in Fig. 4A. Dual-luciferase reporter gene assays demonstrated the direct regulation of miR-29a-3p by SPARC (Fig. 4B). Furthermore, the expression of SPARC in cultured 16HBE cells treated with a miR-29a-3p mimic was determined. miR-29a-3p expression following transfection is shown in Fig. 4C. It was found that SPARC mRNA expression was reduced after transfection of 16HBE cells with a miR-29a-3p mimic compared with the NeC group (Fig. 4D). These data supported the concept that reducing the expression of miR-29a-3p can regulate the mRNA synthesis of SPARC.

Figure 4 SPARC is the target gene for miR-29a-3p. (A) The predicted miR-29a-3p binding sites in the SPARC 3′UTR. (B) The direct regulation of miR-29a-3p to SPARC was determined using dual-luciferase reporter gene assays. (C) miR-29a-3p and (D) SPARC mRNA expression as measured via reverse transcription-quantitative PCR in miR-29a-3p mimic-treated 16HBE cells. Three independent experiments were conducted for each treatment. The data are expressed as the mean ± SD. Data were analyzed using one-way or two-way ANONA as applicable.**P<0.01, ****P<0.0001. UTR, untranslated region; WT, wild-type; MUT, mutant; SPARC, secreted protein acidic rich in cysteine; miR, microRNA; NeC, negative control group.

Effect of miR-29a-3p on EMT in 16HBE cells

Transfection efficacy was higher in 16HBE cells administrated with miR-29a-3p mimic at 100 nM, rather than at 20 or 50 nM, as determined using fluorescence microscopy (Fig. 5A). Transfection of 16HBE cells with a miR-29a-3p inhibitor was shown to successfully induce the knockdown of miR-29a-3p compared with the miR inhibitor control group (Fig. 5B). E-cadherin expression was increased, while vimentin expression was decreased in 16HBE cells transfected with the miR-29a-3p mimic at 100 nM compared with the NeC group, as measured via RT-qPCR (Fig. 5C). E-cadherin expression was decreased, while p-ERK was increased in the miR-29a-3p inhibitor group compared with the NeC group, which was verified via western blotting (Fig. 5D and E). Vimentin protein was not affected by administration of the miR-29a-3p inhibitor. These data indicated that miR-29a-3p regulated the mRNA expression of EMT-related markers, while the effect on protein expression was not significant. We speculated that a post-translational modification may be involved in the protein expression process. p-ERK expression increased after inhibitor administration, implying that ERK may participate in the process of EMT formation.

Figure 5 miR-29a-3p regulates the epithelial-mesenchymal transition of 16HBE cells. (A) Transfection efficiency was evaluated via fluorescence microscopy. (B) miR-29a-3p inhibitor induced the knockdown of miR-29a-3p. (C) E-cadherin and vimentin expression was measured via reverse transcription-quantitative PCR. (D and E) E-cadherin, Vimentin and p-ERK protein expression levels were measured via western blotting. Three independent experiments were conducted for each treatment. The data are expressed as the mean ± SD. Data were analyzed using one-way ANOVA. **P<0.01, ****P<0.0001. miR, microRNA; p-, phosphorylated; t-, total protein; NeC, negative control group.

miR-29a-3p reduces SPARC-induced EMT in 16HBE cells

Transfection with the miR-29a-3p mimic increased E-cadherin expression, and decreased vimentin expression in 16HBE cells pretreated with LV-SPARC compared with transfection with LV-SPARC alone, as determined via western blotting (Fig. 6). This suggested that miR-29a-3p administration reversed the SPARC-induced EMT observed in 16HBE cells. p-ERK expression was upregulated by transfection with LV-SPARC alone, but this was reversed by administration of the miR-29a-3p mimic (Fig. 6). These results implied that ERK may be involved in the EMT process and that it is regulated by miR-29a-3p.

Figure 6 miR-29a-3p suppresses the epithelial-mesenchymal transition of SPARC-induced 16HBE cells. E-cadherin, Vimentin and p-ERK protein expression levels were assessed by western blotting. Representative images and semi-quantification of protein expression are shown. Three independent experiments were conducted for each treatment. The data are expressed as the mean ± SD. Data were analyzed using one-way ANOVA. *P<0.05, ***P<0.001, ****P<0.0001. miR, microRNA; SPARC, secreted protein acidic rich in cysteine; p-, phosphorylated; t-, total protein; LV, lentivirus; NeC, negative control group.

Discussion

It is commonly known that patients with NA may exhibit irreversible airflow obstruction and poor response to treatment (3,4). Dysregulated extracellular matrix (ECM) remodeling has also been reported to result in EMT, which is considered to be involved in airway remodeling, which is the main cause of fixed airflow limitations that occur during asthma attacks (8). A previous study demonstrated that the loss of miR-29a-3p is crucial for ECM protein deposit and pulmonary fibrosis (14). However, findings concerning the underlying function of miR-29a-3p in asthma are still limited. The present study reported that miR-29a-3p was significantly decreased in the lung of the mouse model with NA and may mediate EMT via SPARC/ERK signaling pathway.

miR-29a-3p has been reported in a multitude of diseases, ranging from intrauterine inflammation in extremely preterm infants (15) and airway epithelial remodeling in chronic obstructive pulmonary disease (16) to proliferation and differentiation in retinal progenitors (17). Studies have reported that miR-29a-3p could be a therapeutic tool for certain diseases, such as pulmonary fibrosis (14) and Alzheimer's disease (18). The existing data revealed that miR-29a-3p inhibited Th17 cell differentiation and activation in a disease model with inflammatory bowel syndrome (19). Our previous study showed that NA was characterized by airway neutrophil inflammation and Th17 cell dominance (11).

In the present study, it was that miR-29a-3p was downregulated in NA when compared with the control mice. miR-29a-3p is well-characterized by its ability to regulate ECM proteins, including collagen, elastin and fibrillin, which play important roles in airway remodeling (20). For instance, downregulation of miR-29a-3p was found to result in the enhanced expression of the collagen proteins in pulmonary fibrosis (21). Moreover, it could attenuate TGF-β1-induced fibrosis in primary human endometrial stromal cells (22) and mediate the remodeling process of airway epithelial cells (16). Therefore, we suggest that miR-29a-3p has a possible role in airway remodeling during NA. EMT is one of the mechanisms of airway remodeling in asthma (23). In the present study, it was found that the NA mouse model exhibited the EMT phenotype with decreased miR-29a-3p levels, suggesting it may be involved in airway remodeling of NA. It was demonstrated that miR-29a-3p regulated EMT in cultured 16HBE cells, which was similar to the regulation of miR-29a-3p in fibroblast accumulation in the kidneys (24). However, the present study also demonstrated that while E-cadherin and vimentin mRNA expression changed significantly, protein expression did not show the same obvious alternations. We speculated that this phenomenon may be related to post-translational modification. This deduction needs further research and discussion, which is a limitation of this study.

miRNAs regulate gene expression by binding to the 3′UTR of the target genes (25). An increasing number of studies have demonstrated that miR-29a-3p exhibits negative regulation of mRNAs encoding ECM proteins that play essential roles in matrix deposition and EMT (21,22). In the present study, the online bioinformatics software, TargetScan, was used to predict the target genes for miR-29a-3p and SPARC. SPARC is a molecule that regulates cell proliferation, differentiation, ECM deposition and EMT, as well as participating in airway remodeling of chronic airways disease (26). Studies have shown that SPARC-null mice exhibited reduced collagen deposition and tissue fibrosis (27,28). SPARC inhibition may, therefore, represent a potential therapeutic approach in fibrotic diseases (29). The present data showed that SPARC was upregulated in the lung of the mouse model with NA, implying that it may participate in airway remodeling. SPARC was reported to participate in the EMT of cancer cells (30,31). In the current study, it was found that SPARC overexpression increased vimentin and decreased E-cadherin expression in vitro, suggesting that this molecule may contribute to EMT of 16HBE cells.

The direct link between miR-29a-3p and SPARC was verified using dual-luciferase reporter gene assays. Moreover, it was found that miR-29a-3p administration reversed the SPARC-induced EMT formation in 16HEB cells, supporting the concept that this miRNA inhibited EMT by suppressing SPARC synthesis directly. The data implied that the regulation of miR-29a-3p may participate in EMT of a mouse model with NA. The regulation of miR-29a-3p by SPARC was not verified in vivo, due to restricted access to the animal center due to COVID-19, and this is a limitation of the current study.

The potential signaling pathways involved in the miR-29a-3p-mediated effects on the EMT of 16HBE cells were also studied in the present study. The function of SPARC has been reported to be mediated by the phosphorylation of c-Jun N-terminal kinase and p38-MAPK signaling pathways on limbal epithelial stem cells (32). SPARC is upregulated during the EMT process in lung cancer cells and overexpression of SPARC can induce the increased expression of p-Akt and p-ERK (31). Consistent with the existing studies, the present study found that SPARC overexpression led to the elevated expression of p-ERK and miR-29a-3p inhibited this increase. This suggested that ERK may participate in the SPARC-induced EMT process mediated by miR-29a-3p and SPARC observed in NA. Presenting the ratio of p-protein/total protein may improve the evidence for the activation of this signaling pathway. The band for total protein expression of ERK was not present because of the experimental design and the restricted laboratory access due to COVID-19, which was a limitation of this study.

In conclusion, the current study found that miR-29a-3p expression was decreased, while SPARC was elevated in a mouse model of NA. SPARC was observed to induce EMT in cultured 16HBE cells in vitro and this was directly targeted by miR-29a-3p, and may be mediated by p-ERK. The data suggested that miR-29a-3p may participate in the airway remodeling of NA.

Availability of data and materials

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

Authors' contributions

XZ conceived the present study, performed the experiments, analyzed the data, and wrote the manuscript. JX and HS conducted the animal studies. QW analyzed and interpreted the data. GN designed and supervised the study, critically revised the manuscript for intellectual content, and was primarily responsible for the writing of the manuscript. XZ and JX confirm the authenticity of all the raw data. All authors have read and approved the final manuscript.

Ethics approval and consent to participate

The present study was approved by The Ethics Committee of The First Affiliated Hospital of Guangxi Medical University [approval no. 2019 (KY-E-035); Nanning, China].

Patient consent for publication

Not applicable.

Competing interests

The authors declare that they have no competing interests.

Acknowledgments

The authors would like to thank Dr Dev Sooranna of Imperial College London (London, UK) for editing the manuscript.

References