Introduction

Lung cancer is the leading cause of cancer-related deaths in men worldwide, accounting for 1.8 million deaths annually. The most common type, lung adenocarcinoma (LAD), constitutes almost 40% of lung cancers. Approximately 90% of patients with LAD develop distant metastasis at the advanced stage (Sequist et al. 2013; Dey et al. 2015; Wu et al. 2015). Therefore, to design effective therapeutic strategies for patients with LAD, it is very important to understand the molecular mechanisms underlying distant metastasis. Epithelial–mesenchymal transition (EMT) is an evolutionarily conserved developmental process during which epithelial cells lose polarity and then acquire a mesenchymal phenotype, and this transition has been involved in the initiation of metastasis (Brabletz 2012; Tang et al. 2016).

Fig. 1
figure 1

MiR-197 enhances LAD cell migration and invasion in vitro. The migration and invasion abilities of A549 (a) and PC9 (b) cells when transfected with the miR-197 overexpression vector or an empty vector as assessed by Transwell migration and Matrigel invasion assay for 24 h. All experiments were performed at least thrice, and the data are expressed as means \(\pm \hbox { SD}\). The statistical significance of differences was measured by unpaired student’s t-test. *\(P<0.05\), **\(P<0.01\).

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MicroRNAs (miRNAs) are small noncoding RNAs that upregulate or downregulate gene expression posttranscriptionally. It is well-known that miRNAs play a central role in tumour metastasis. For example, the miR-200 family members attenuate the EMT through targeting the repressors of E-cadherin that induce epithelial differentiation, or by targeting the EMT activators known as ZEB1/2 genes (Burk et al. 2008; Iliopoulos et al. 2010; Chang et al. 2011). Previously, miR-197 has reportedly been aberrantly expressed in different types of tumours. It plays a dual role by regulating tumour growth according to the tumour types, as oncogenes or tumour suppressors. On the one hand, miR-197 induces apoptosis, suppresses tumourigenicity, and inhibits cell proliferation and migration (Dai et al. 2014; Wang et al. 2015; Xin et al. 2015; Yang et al. 2015; Tian et al. 2016). Besides, miR-197 can suppress the expression of the tumour suppressor gene FUS1, and its expression is correlated with poor clinical outcomes in patients with LAD (Du et al. 2009; Mavridis et al. 2015). In these patients, miR-197 expression was significantly higher in tumours than in normal tissue and was associated with advanced tumour stage, overall poor survival and higher recurrence rates (Du et al. 2009; Mavridis et al. 2015), which suggest that miR-197 is important for LAD development. However, no studies have systematically determined the role of miR-197 in the development of metastatic disease in LAD.

In this study, we demonstrated the function of miR-197 in LAD and found that miR-197 strongly activates EMT and ultimately promotes LAD metastasis by targeting HIPK2. These observations suggest that miR-197 could be a therapeutic target for preventing LAD metastasis.

Materials and methods

Cell culture

The human LAD cell lines A549 and PC9 were obtained from the American Type Culture Collection (ATCC). The cell lines were maintained in Dulbecco’s modified Eagle’s medium (DMEM) with 10% foetal bovine serum (FBS), and they were incubated at 5% \(\hbox {CO}_{2}\) at \(37^{\circ }\hbox {C}\).

Real-time PCR

Total RNA was extracted from the LAD cells by using TRIzol (Invitrogen, Carlsbad, USA) following the manufacturer’s protocol. Quantification was determined by using NanoDrop 1000 spectrophotometer (Thermo Scientific, Pittsburgh, USA). Reverse transcription of RNA was performed using the ImProm-II reverse transcription system (Promega, Madison, USA) following the manufacturer’s instructions. Real-time PCR was performed in an ABI7500 Prism Sequence Detection System (Applied Biosystems, Foster City, USA) by using a SYBR Green kit (TaKaRa, Tokyo, Japan), and the relative changes were quantified. The \(2^{-\Delta \Delta \mathrm{CT}}\) method was used to measure gene expression. Each experiment was repeated at least thrice.

Fig. 2
figure 2

MiR-197 promoted LAD metastasis is mediated by the EMT. (a) The expression of EMT markers as analysed in miR-197 overexpressed LAD cell lines by real-time PCR. (b) EMT markers in two miR-197 overexpressed LAD cell lines as analysed by Western blots. All experiments were performed at least thrice and the data are expressed as means \(\pm \hbox { SD}\). The statistical significance of differences was measured by unpaired student’s t-test. *\(P<0.05\), **\(P<0.01\).

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RNA interference

LAD cells were stably infected with the premicroRNA expression construct known as the lenti-miR expression plasmid, which contained the full-length miR-197 in the H1-MCS-CMV-EGFP vector (GeneChem, Shanghai, China). The sh-miR-197 was cloned into the H1-MCS-CMV-EGFP vector (GeneChem) to generate H1-MCS-CMV-EGFP-sh-miR-197 (GeneChem). A nontargeting sequence was used as a lentivirus negative control and was purchased from GeneChem.

Migration and invasion assays

A migration assay was performed using 24-well culture inserts with porous polycarbonate membranes (\(8.0~\mu \hbox {m}\), Millipore, USA). For the Matrigel invasion assay, the filters were precoated with \(30~\mu \hbox {L}\) of Matrigel (BD Biosciences, San Jose, USA) for 4 h. In brief, \(5\times 10^{4}\) cells in \(200~\mu \hbox {L}\) of serum-free medium were added to the upper chamber, and \(800~\mu \hbox {L}\) of medium with 10% serum was placed in the lower chamber. The plates were incubated for 24 h at \(37^{\circ }\hbox {C}\) in 5% \(\hbox {CO}_{2}\). Cells that did not migrate or invade through the pores were removed with the cotton swab. Cells on the lower surface of the membrane were examined and counted under a microscope. Each experiment was repeated at least thrice.

Western blot assay

LAD cells were washed with PBS, collected, and lysed with RIPA lysis buffer (Cell Signaling Technology, Danvers, USA). Protein concentrations were measured using Qubit Protein Assay Kit and Qubit 2.0 Fluorometer (Invitrogen). Approximately \(50~\mu \hbox {g}\) of total protein was loaded on a 10% or 12% SDS-PAGE gel, and transferred to polyvinylidene difluoride membranes for immunoblotting. Membranes were blocked in 5% milk in TBST and incubated with primary antibody at \(4^{\circ }\hbox {C}\) overnight, followed with horseradish peroxidase (HRP)-linked secondary antibody, and then detected with Pierce ECL western blotting substrate (Thermo Scientific). The following antibodies were used: E-cadherin (1:1000, Abcam, Cambridge), snail (1:1000, Abcam, Cambridge), vimentin (1:1000, Abcam, Cambridge), HIPK2 (1:1000, Cell Signaling Technology, USA), and \(\upbeta \)-actin (1:5000, Boster, China).

Fig. 3
figure 3

The downregulation of miR-197 suppresses the EMT and migration ability in LAD cells. The migration and invasion abilities of A549 (a) and PC9 cells (b) as transfected with the sh-miR-197 vector or an empty vector and assessed by Transwell migration and Matrigel invasion assay for 24 h. The expression of EMT markers analysed in sh-miR-197 transfected A549 cells (c) and PC9 cells (d) by real-time PCR. All experiments were performed at least thrice and the data are expressed as the means \(\pm \hbox { SD}\). The statistical significance of differences was measured by unpaired student’s t-test. *\(P<0.05\), **\(P<0.01\).

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Fig. 4
figure 4

HIPK2 is a direct target of miR-197 in LAD cells. (a) A schematic representation of putative miR-197 binding in the \(3^{\prime }\)-UTR of HIPK2. (b) Dual-luciferase assays showing the repression of wild-type UTR (HIPK2-\(3^{\prime }\)-UTR) or mutant UTR (HIPK2-\(3^{\prime }\)-UTR mut) following the transfection of the vector or miR-197 overexpression vector. All experiments were performed at least thrice and the data are expressed as the means \(\pm \hbox { SD}\). The statistical significance of differences was measured by unpaired student’s t-test. **\(P<0.01\).

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Fig. 5
figure 5

HIPK2 was a direct functional target of miR-197 in LAD metastasis. (a) The migration capacities of A549 cells after the indicated treatment as assessed by Transwell migration assay. *\(P<0.05\), **\(P<0.01\). (b) The invasive capacities of A549 cells after the indicated treatment as assessed by Matrigel invasion assay. All experiments were performed at least thrice and the data are expressed as the means \(\pm \hbox { SD}\). The statistical significance of differences was measured by unpaired student’s t-test. *\(P<0.05\), **\(P<0.01\).

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3\(^{\prime }\)-UTR luciferase reporter assay

Lipofectamine-2000 (Invitrogen) was used to transfect A549 cells with luciferase vectors (an empty luciferase vector, a luciferase vector containing the \(3^{\prime }\)-UTR of the wild-type target gene, and a luciferase vector containing the \(3^{\prime }\)-UTR of the mutant-type target gene) for HIPK2 together with miR-197 or the negative control. After 48 h, transfected A549 cells in each well of 24-well plates were harvested with \(100~\mu \hbox {L}\) of lysis buffer. Ten \(\mu \hbox {L}\) of the cell extract was used to measure luciferase activity by using the Dual-Luciferase Reporter Assay System (Promega, Madison, USA). All assays were performed in triplicate and the data were presented as the ratios between firefly and Renilla fluorescence activities.

Statistical analyses

All data were presented as the means ± SD. The statistical analyses were performed using Student’s t-test. A difference was considered statistically significant when \(P<0.05\). All the statistical analyses were performed with SPSS 13.0 software.

Results

MiR-197 enhances LAD cell migration and invasion

First, we analysed the role of miR-197 in metastasis by migration and invasion studies. We generated LAD cell lines with stable miR-197 overexpression in A549 and PC9 cells using lentivirus transfection. Next, we performed a Boyden chamber migration/invasion assay to study metastatic function in vitro. The ectopic expression of miR-197 significantly increased the migratory and invasive abilities of both miR-197 overexpressing A549 and PC9 cells (figure 1, a–b). Collectively, our data suggested that miR-197 overexpression significantly increased the migratory and invasive abilities of LAD cells in vitro.

MiR-197 promotes LAD metastasis through EMT

EMT plays a critical role in the metastasis process, which provides the motility, invasion and migration properties of cancer cells (Brabletz 2012; Tang et al. 2016). Therefore, we further explored whether miR-197 promoted LAD metastasis is mediated through the EMT. To confirm this hypothesis, we first used miR-197 overexpressing A549 and PC9 cells to analyse the hallmark EMT changes at both the mRNA and protein levels. As shown in figure 2, a–b, the epithelial marker E-cadherin was dramatically decreased after miR-197 overexpression. However, the mesenchymal markers vimentin and snail were increased in miR-197 overexpressing cells compared to the vector cells. Taken together, these results demonstrate that miR-197 enhances the EMT process in LAD cells.

The downregulation of miR-197 suppresses the EMT and migration ability

Having shown that miR-197 overexpression could enhance the EMT process in LAD cells, we next employed a loss-of-function approach by using shRNA to investigate the role of miR-197 in the EMT process. As anticipated, the migratory and invasive capabilities of both A549 and PC9 cells were significantly reduced by miR-197 inhibition (figure 3, a–b). In addition, as shown in figure 3, c–d, the epithelial marker E-cadherin was increased, and the mesenchymal marker vimentin was decreased in sh-miR-197 transfected A549 and PC9 cells compared with the vector groups. Collectively, our findings suggest that miR-197-promoted LAD metastasis is mediated by EMT.

HIPK2 is a direct target of miR-197 in LAD cells

To investigate the roles for the target gene of miR-197 in the EMT process and metastasis in LAD, we first performed a miRNA target gene prediction with Targetscan database. We found that HIPK2 exhibited miR-197-binding sequences in its \(3^{\prime }\)-UTR regions (figure 4a). Next, the luciferase activity was decreased by miR-197 overexpression when the Luc-HIPK2-wt was present, compared with the luciferase activity in the Luc-HIPK2-mu, suggesting that miR-197 reduced the luciferase activity of Luc-HIPK2-wt but had no effect on Luc-HIPK2-mu (figure 4b). Taken together, our results suggest that miR-197 promotes LAD metastasis by directly targeting HIPK2.

HIPK2 is a direct functional target of miR-197 in LAD metastasis

We further tested whether HIPK2 is functionally regulated by miR-197 in LAD metastasis. Interestingly, the miR-197 overexpression-associated migration and invasion changes could be partially rescued by HIPK2 overexpression (figure 5, a–b). Collectively, these results demonstrated that miR-197 promotes LAD metastasis by targeting HIPK2.

Discussion

Many reports have demonstrated that the dysregulation of miRNAs contributes to cancer metastasis (Rabinowits et al. 2009; Martello et al. 2010; Song et al. 2013). However, it is not clear whether miRNAs play a key role in the regulation of LAD metastasis. The present study identified that miR-197 is involved in the promotion of LAD metastasis. For the first time, we found that miR-197 promoted the migratory and invasive abilities of LAD cells with two different cell lines and with overexpression and knockdown experiments. In this study, we provide strong evidence of the role of miR-197 through experiments performed in two different LAD cell lines. Moreover, we demonstrated that miR-197 enhanced EMT of LAD cells, an important process for metastasis, supporting the role of miR-197 as a factor that promotes LAD metastasis. This finding supports a previous report showing that miR-197 induces EMT by targeting p120 catenin in pancreatic cancer cells (Hamada et al. 2013).

The EMT process is a crucial step in initiating the metastatic spread of many tumour cells into distal organs in a variety of cancers (Tsai and Yang 2013; Puisieux et al. 2014). Importantly, miRNAs have been shown to regulate EMT (Song et al. 2014; Liang et al. 2015; Listing et al. 2015). Here, we found that miR-197 expression is involved in inducing the EMT process, as demonstrated by the gain of mesenchymal markers and loss of epithelial markers. Moreover, overexpression of miR-197 greatly promoted EMT in LAD cells, while knock down of miR-197 by shRNA inhibited EMT. Importantly, a high level of miR-197 expression was significantly associated with low E-cadherin expression and high vimentin and snail expression, which supports the loss-of-function and gain-of-function studies we performed in LAD tumour cell lines. To our knowledge, this study is the first to demonstrate that miR-197 enhances the EMT process in LAD.

It is well established that miRNA regulates a target gene expression to perform its function (Chen et al. 2015; Cekaite et al. 2016). Therefore, in this study, we investigated the functional target gene for miR-197 that was relative to LAD metastasis regulation. HIPK2 was predicted to be a functional target gene of miR-197 by a TargetScan database analysis. Moreover, we found that miR-197 directly bound to the \(3^{\prime }\)-UTR region of HIPK2 and suppressed HIPK2 expression. Previously, there was no direct evidence of a relationship between miR-197 and HIPK2 in cancer cells. This study provides the first evidence demonstrating that miR-197 directly regulates HIPK2 in LAD cells.

In conclusion, collectively our study has significant implications for understanding the underlying mechanisms of how miR-197 contributes to tumour progress in LAD. MiR-197 controls EMT and metastasis by directly silencing HIPK2. The findings from this study suggest that interfering with the miR-197 dependent regulation of HIPK2 could be a useful approach for the treatment of patients with late stage metastatic LAD.