MiR-122 Induces Radiosensitization in Non-Small Cell Lung Cancer Cell Line.

MiR-122 is a novel tumor suppresser and its expression induces cell cycle arrest, or apoptosis, and inhibits cell proliferation in multiple cancer cells, including non-small cell lung cancer (NSCLC) cells. Radioresistance of cancer cell leads to the major drawback of radiotherapy for NSCLC and the induction of radiosensitization could be a useful strategy to fix this problem. The present work investigates the function of miR-122 in inducing radiosensitization in A549 cell, a type of NSCLC cells. MiR-122 induces the radiosensitization of A549 cells. MiR-122 also boosts the inhibitory activity of ionizing radiation (IR) on cancer cell anchor-independent growth and invasion. Moreover, miR-122 reduced the expression of its targeted genes related to tumor-survival or cellular stress response. These results indicate that miR-122 would be a novel strategy for NSCLC radiation-therapy.

1. Introduction

Lung cancer is one of the most fatal human malignancies. It usually falls into two morphologic groups: the small cell lung cancer (SCLC) and the non-small cell lung cancer (NSCLC) [1,2]. NSCLC makes up to nearly 80% of all lung cancer clinical cases [3]. Among its three major subtypes, i.e., adenocarcinoma (AC), squamous cell carcinoma (SCC), and large cell carcinoma (LCC) [3], adenocarcinoma is the most common subtype of NSCLC [4]. At the time of first diagnosis with advanced stage NSCLC, the majority of patients are surgically unrespectable [5]. Thereby, for those patients, radiotherapy would be the first or unique option in medical treatment [6]. However, the clinical benefit of radiotherapy is somehow not fully satisfactory due to radioresistance [5,6]. Although there is progress in the study of molecular mechanisms relating to lung cancer biology, our understanding of the genetic and epigenetic alteration in radioresistance remains limited.

The microRNAs (miRs) are a series of small non-coding RNA containing 18–25 nucleotides and are involved in proliferation, metabolism, and apoptosis regulation of human cancer cells [7]. Recently, some miRNAs, for instance let-7 and miR-122 [8,9], have been identified to participate in the regulation of tumor progress as tumor-suppressors. Among them, miR-122 suppresses tumor proliferation via targeting to oncogenes cyclin G1 (CCNG1) and Bcl-w [10,11]. Over-expression of miR-122 could induce apoptosis and cell cycle arrest of H460 cells [11], and enhance the cytotoxic effect of gemcitabine on A549 cells [12]. In spite of the discovery of miR-122 in previous work, to develop its potential application in NSCLC treatment, more studies should be carried out on its physiological function. It is valuable to further assess the activity of miR-122 in NSCLC treatment. In this study, we show that expression of miR-122 in NSCLC cells induce radiosensitization. In order to increase radiotherapy efficiency, miR-122 could be a novel therapeutic strategy for MSCLC treatment.

2. Results and Discussion

2.1. MiR-122 Is Detectable in Hepatocyte Cell Lines but Not NSCLC Cells Lines

MiR-122 in different cell lines was detected by real-time RT-PCR assays. As shown in Figure 1, a high level of miR-122 was detected in L-02, a non-tumor hepatocyte cell line. HepG2 and MHCC-97L cells lines express miR-122. No endogenous miR-122 was observed in MHCC-97H, NSCLC A549 (adenocarcinoma, the most common subtype of NSCLC), and H460 (large cell carcinoma) cell lines. Therefore, to study the roles of miR-122 in NSCLC, we chose the A549 cell.

Figure 1

MiR-122 expression is detected in hepatocyte cells but not NSCLC cells and the expression of miR-122 via adenoviral vector inhibits the proliferation of A549 cells. (A) Total RNA extracted from the indicated cell lines were analyzed by Real time RT-PCR, and β-actin was chosen as loading control; (B) Relative RNA level was shown as mean ± SD from triplicate measurements with similar results; (C) A549 cells, which were infected with control or Ad-miR-122, were harvested at the indicated time points. Relative cell number was determined by MTT assay; (D) The level of miR-122 in A549 cell was shown. * p < 0.05 versus Ad-control or Ad-miR-122.

2.2. MiR-122 Suppresses Proliferation of NSCLC A549 Cells

Next, the effect of miR-122 on proliferation of A549 was investigated. Firstly, cell growth rate has no significant difference between A549 cells infected with control adenoviral vector (Ad-control) or not (the parental cells) (Figure 1C). Then, A549 cells that expressed miR-122 (Figure 1C) grew significantly slower than cells infected with Ad-control or not (Figure 1C,D). These results indicate that miR-122 attenuates the proliferation of A549 cells.

2.3. MiR-122 Enhances the Radiosensitization of A549 Cells

To determine whether miR-122 induces the radiosensitization of A549 cell, the colony formation ability of A549 cells was examined. Our results show that after a mild dose of IR (ionizing radiation) treatment (6 Gy), the colony formation of A549 cells was significantly attenuated (Figure 2A,B), and expression of miR-122 further enhanced the activity of IR (Figure 2A,B). These results reveal the radiosensitization activity of miR-122 in A549C cells.

Figure 2

MiR-122 enhances the radiosensitization of A549 cells. (A) A549 cells, which were infected with Ad-control or Ad-miR-122, then treated with indicated dose (0, 2, 4, 6, 8 or 10 Gy) of 60Co-γ IR (ionizing radiation). Then, cells (1 × 103 cells per well) were seeded in 6-well plates (Corning, Lowell, MA, USA) and cultured for 2–4 weeks, and the colonies were detected by staining with crystal violet (0.5% in 20% ethanol). Next, cells were harvested and measured by a multifunctional micro-plate reader at 546 nm. The results were shown as a typical photograph (A) or relative survival cell number (mean ± SD) from triplicate experiment with similar results (B). * p < 0.05 versus Ad-control or Ad-miR-122.

2.4. MiR-122 Enhances the DNA Double Strand Break (DSB) and Apoptosis Induced by IR

To further confirm that miR-122 increases radiosensitization of cancer cell, the DNA double strand break (DSB) and cell apoptosis were examined. To identify the DNA DSB, the foci formation of γ-H2AX was detected, which is a marker of DNA double strand break. As shown in Figure 3, miR-122 could not induce γ-H2AX foci formation in nuclear (Figure 3A,B), but the indicated dose (6 Gy) of 60Co-γ IR could (Figure 3A,C). In the presence of IR treatment, miR-122 significantly enhanced γ-H2AX foci formation induction (Figure 3A,C,D).

Figure 3

MiR-122 enhances the DNA double strands break (DSB) and apoptosis induced by IR in A549 cells. (A–D) A549 cells, infected with Ad-control or Ad-miR-122, were then treated without or with 6Gy IR. (A–D) Then, the Immunocytochemistry assays were performed. The γ-H2AX foci formation in the nucleus was shown as typical photograph (A–D); (E–I) A549 cells, which were infected with Ad-control (E,G) or Ad-miR-122 (F,H), were treated with IR (G,H) or not (E,F). Next, cells were harvested and examined by apoptosis analysis. The apoptotic cells were shown as typical photograph (E–H) or percentage of apoptosis cells (mean ± SD) from triplicate experiment with similar results (I). Arrows “↑” indicate the γ-H2AX foci formation (granule formation) in cellular nuclear (A–D), * p < 0.05 versus Ad-control or Ad-miR-122, * p < 0.05 versus without or with 6Gy IR.

Next, the apoptosis of A549 cells were examined. The percentage of apoptotic cells was significantly increased after Ad-miR-122 infection (from 4.85% to 14.49%) (Figure 3E,F). Moreover, treatment of indicated dose (6 Gy) of 60Co-γ ionizing radiation induced the apoptosis of A549 cells, the percentage of apoptotic cells were increased from 4.85% to 36.54% (Figure 3E,G). MiR-122 further enhanced the sensitivity of A549 cells to IR, the apoptosis rate increased from 36.54% to 62.48% (Figure 3G,H). Taken together, these findings support the fact that miR-122 enhances the radiosensitization of A549 cells.

2.5. MiR-122 Enhances the A549 Cells Anchorage-Independent Growth Inhibition Induced by IR

To examine the tumor-suppressing activity of miR-122, soft-agar assays were performed. There was no significant difference in anchorage-independent growth between cells infected with Ad-control and the parental cells (Figure 4A–F). After treating the indicated dose of IR (6 Gy), the anchorage-independent growth of A549 cells was significantly attenuated (Figure 4A–F). Further, miR-122 up-regulates this IR treatment effect (Figure 3A–F). These results further confirm the radio-sensitization enhancement of miR-122 in A549 cells.

Figure 4

Expression of miR-122 promotes the inhibitory activity of irradiation on A549 cells anchorage-independent growth and invasion. (A–E) A549 cells, infected with Ad-control or Ad-miR-122, were then treated with or without 6 Gy irradiation. Then, the soft-agar assay was performed. The results were shown as typical photograph (A–E) or colony number (mean ± SD) from triplicate experiment with similar results (F); (G–K) A549 cells, infected with Ad-control or Ad-miR-122 were then treated with or without 6 Gy irradiation. Then, the transwell assay was performed. The results were shown as typical photograph (G–K) or relative invasion cell number (mean ± SD) from triplicate experiment with similar results (L). * p < 0.05 versus control or miR-122, * p < 0.05 versus without or with 6 Gy irradiation.

2.6. miR-122 Promotes the Inhibition of A549 Cells Invasion Induced by Irradiation

Cell invasion is the main feature of metastatic malignancies. Transwell analysis was used to assess the effect of miR-122 on NSCLC cells invasion. A549 cells, infected with Ad-control or Ad-miR-122, were treated with a middle dose of IR (6 Gy). IR attenuated the invasion of A549 cells and miR-122 enhanced this effect (Figure 4G–L). Taken together with the soft agar results, our observations further support that miR-122 increases the radiosensitization of A549 cells.

2.7. MiR-122 Reduces the Expression of Its Pro-Survival or Anti-Apoptosis Targeted Genes in A549 Cells

Next, the mechanism of miR-122 function was studied. As shown in Figure 5, compared with Ad-control, Ad-miR-122 infection reduced the expression of pro-survival or anti-apoptosis regulators, BCL-W and IGF1R, in A549 cells (Figure 5A–C). These results suggest that miR-122 would play its proliferation inhibitory and radiosensitization roles via reducing the expression of its pro-survival or anti-apoptosis targeted genes.

Figure 5

MiR-122 represses the expression of IGF1R and BCL-W in A549 cells. (A) A549 cells, which were infected with Ad-control or Ad-miR-122, were then harvested for Western blot. The expression of IGF1R and BCL-W were detected by Anti-IGF1R or Anti-BCL-W antibodies. GAPDH was chosen as the loading control; (B,C) Relative protein level was shown as mean ± SD from triple experiments with similar results. * p < 0.05 versus Ad-control or Ad-miR-122.

2.8. MiR-122 Reduces the Expression of Stress-Response Regulators Survivin, c-IAP-1 and c-IAP-2 in A549 Cells

To further discover the mechanism of miR-122 in A549 radiosensitization enhancement, the effect of miR-122 on stress-response regulators was examined. As shown in Figure 6, compared with Ad-control, in A549 cells miR-122 reduced the expression of cell survival and stress-response regulators Survivin (Figure 6A,B), c-IAP-1 (Figure 6A,C) and c-IAP-2 (Figure 6A,D). These results indicate that miR-122 would enhance the sensitivity of A549 cells to radiosensitization via down-regulating cell stress-response regulators Survivin, c-IAP-1 and c-IAP-2.

Figure 6

MiR-122 represses the expression of Survivin, c-IAP-1 and c-IAP-2 in A549 cells. (A) A549 cells, which were infected with Ad-control or Ad-miR-122, were then harvested for Western blot. The expression of Survivin, c-IAP-1 and c-IAP-2 were detected by anti-Survivin, anti-cIAP1 or anti-cIAP2 antibodies. GAPDH was chosen as the loading control; (B–D) Relative protein level was shown as mean ± SD from triple experiments with similar results. * p < 0.05 versus Ad-control or Ad-miR-122.

Radiation therapy plays an important role in NSCLC treatment but the radioresistance of cancer cells would be a critical obstacle [6]. Therefore, it is valuable to identify novel strategies to overcome radioresistance and enhance radio-therapeutic efficacy [6]. Recently, several regulators related to irradiation have been identified, such as miR-126 or miR-34a [6,7,8]. In this work, we found miR-122 significantly enhanced the effect of IR on DNA DSB and apoptosis. Moreover, anchorage-independent growth and invasion of NSCLC A549 cells were also significantly attenuated after a combinational treatment of miR-122 and irradiation. In addition to previous finding that expression of miR-122 mediated by adenoviral vector could induce proliferation inhibition and cell cycle arrest of multiple cancer cells [11], our results demonstrate that miR-122 would be used for increasing susceptibility to radiation in NSCLC cells.

Recently, a lot of miRs have been implicated as tumor suppressors. Among these miRs, endogenous expression of miR-122 reduces in HCC (hepatocellular carcinoma) compared with non-tumor tissues [13]. Low level miR-122 correlates with cancer poor prognosis [14]. Overexpression of miR-122 also sensitizes cancer cells to anti-tumor agents, such as gemcitabine, sorafenib and doxorubicin [12,15,16]. Also, some genes including Bcl-w, ADAM10, Igf1R, SRF, cyclin G1, and ADAM17 have been identified as miR-122 targets [10]. However, the detailed function and mechanisms of miR-122 are still unclear. In this work, we report miR-122 can enhance the susceptibility of NSCLC cells to radiation therapy via targeting to BCL-W and IGF1R. Similar to BCL-2, BCL-W is a member of BCL-2 family and exerts anti-apoptotic activity [17].

Moreover, our results indicate that miR-122 reduces the expression of cell stress-response regulators Survivn, c-IAP-1, and c-IAP-2. These proteins protect cancer cells suffering from stress, including anti-tumor agents or irradiation [18,19,20]. Kang et al., also shows that down-regulation of these proteins can induce radiosensitization in NSCLC [6]. Since AKT signaling pathway participates in the anti-apoptotic property of cancer cell via up-regulation of Survivin, it is valuable to discover whether miR-122 suppresses the signaling cascade of PI3K-AKT pathway which can be activated by IGF1R. Based on the evidence that IGF1R, which is a target of miR-122, could induce the activation of MAPK or PI3S/AKT signaling, these results also support applying miR-122 as a specific sensitizer of MSCLC radio-therapy via enhancement of the sensitivity of cells to stress or injury in the future. Furthermore, activation of PI3K/AKT is also responsible for EMT (epithelial-mesenchymal transition) regulators E-cadherin and N-cadherin, which participate in the cancer metastasis [21]. It is valuable to further investigate the potential roles of miR-122 in EMT regulation.

3. Experimental Section

3.1. Cell Culture and Proliferation Analysis

A549 (adenocarcinoma) and H460 (large cell carcinoma) cell lines were obtained from cell resources center of Chinese Academy of Medical Sciences and Peking Union Medical College in China. HCC cell lines HepG2, MHCC-97H, MHCC-97L, and non-tumourigenic normal human hepatocyte L-02 cells were described in reference [22]. Cells were cultured in complete Dulbecco’s modified Eagle’s medium (DMEM) (Invitrogen, Carlsbad, CA, USA) in a sterile incubator maintained at 37 °C with 5% CO2. For the analysis of proliferation, cells were seeded in 96-well plates (Corning, Corning, NY, USA). Following incubation for one, two, three or four days, cells were harvested for MTT.

3.2. Adenovirus Vector Preparation

The adenovirus vector Ad-miR122 was constructed following the methods descripted by Niu et al. [2]. The sequence of miR-122 cloned into pShuttle-CMV vector was generated through annealing of two oligo DNA fragments with sticky end/cohesive terminus at the site of BamH I and Hind III: forward sequence, 5′-GATCCGCCTTAGCAGAGCTGTGGAGTGTGACAATGGTGTTTGTGTCTAAACTATCAAACG-3′; reverse sequence, 5′-AGCTTAAAAAAGGCCTAGCAGTAGCTATTTAGTGTGATAATGGCGTTTGATAGTTTAGACA-3′.

3.3. RNA Isolation and Real-Time Quantitative RT-PCR (Real Time qRT-PCR)

The miRNA examination kits of miR-122 were obtained from Invitrogen Company, USA. Total RNA was extracted using the PARISTM Kit (Applied Biosystems, Foster City, CA, USA) according to the manufacturer’s instructions. The real-time qRT-PCR was performed following the methods described by Kang et al., Tsai et al. and Kim et al. [6,23,24]. Briefly, all of the PCR reagents were from a SYBR green core reagent kit (Applied Biosystems) and the aliquots of a master mix containing all of the reaction components with the primers were dispensed into a real-time PCR plate (Applied Biosystems). Multiscribe™ Reverse Transcriptase (Applied Biosystems) was used to synthesize the complementary DNA templates. Real-time reverse transcription-polymerase chain reactions were performed in an Applied Biosystems 7500 Detection system using Maxima SYBR Green/ROX qPCR Master Mix Assays (Fermentas, Thermo Fisher, Waltham, MA, USA) following reference [25,26]. The housekeeping gene β-actin was chosen as the loading control, and relative expression levels were normalized to the expression of human β-actin mRNA. Real-time qRT-PCR was performed for 40 cycles of 95°C for 15s and 60°C for 1 min followed by thermal denaturation and the expression of miRNA-122 relative to β-actin RNA was calculated by using the 2−Δ method. The primers of β-actin were: forward primer: 5′-CTCCATCCTGGCC-TCGCTGT-3′; reverse primer: 5′-GCTGTCACCTTCACCGTTCC-3′.

3.4. Colony Formation

A549 cells, which were infected with Ad-control or Ad-miR-122, were treated with indicated dose (0, 2, 4, 6, 8 or 10 Gy) of 60Co-γ ionizing radiation. Then, cells (1 × 103 cells per well) were seeded in six-well plates (Corning, Lowell, MA, USA), cultured for two to four weeks, and the colonies were detected by staining with crystal violet (0.5% in 20% ethanol). Next, cells were harvested and measured by a multifunctional micro-plate reader at 546 nm. The relative colony number (relative survival cell number) = O.D. 546 administration group/O.D. 546 control group.

3.5. Immunocytochemistry (Cellular Immunofluorescence)

A549 cells, which were infected with Ad-control or Ad-miR-122, were seeded in Costar® 3603 96-well plate (3 × 103 cells per well) from corning company, USA and treated with 6 Gy dose of 60Co-γ ionizing radiation. After 4 h, the plates were fixed with 3% paraformaldehyde for 30 min and then permeablized by Triton X-100 (0.5%) treatment at 4 °C for 10 min. Next, plates were blocking by 10% goat serum diluted in PBS. After blocking, plates were incubated with primary antibody, anti-γ-H2AX (1:500) diluted in PBS at 37 °C for 1 h followed by washing three times with PBS and incubated with FITC-labeled secondary antibody (goat anti-rabbit IgG) for an additional 1 h at 37 °C while keeping from light. The fluorescence signals were visualized by a confocal microscope.

3.6. Apoptosis and Flow Cytometry Analysis

The apoptosis and flow cytometry analysis were performed following the methods descripted by Feng et al., 2015 [21]. Briefly, A549 cells, which were cultured in 60 mm dishes (Corning, Corning, NY, USA), were infected with Ad-control or Ad-miR-122. Next, cells were treated with 6 Gy dose of 60Co-γ ionizing radiation. After 48 h, cells were harvested and labeled with propidium iodide (PI) and Annexin V according to the manufacturer’s instructions (BD Biosciences, Franklin Lakes, NJ, USA). The apoptosis of A549 cells were examined by a FACScalibur Flow Cytometer (Becton Dickinson, BD Biosciences, Franklin Lakes, NJ, USA).

3.7. Anchorage-Independent Growth

A549 cells, which were treated without or with a single dose of irradiation (6 Gy), were plated on six-well plates (500 per well) (Corning, Lowell, MA, USA), with a bottom layer of 0.7% low-melting-temperature agar in DMEM and a top layer of 0.25% agar in DMEM. Colony number was the mean ± SD of three independent experiments scored after three to four weeks of growth [27,28].

3.8. Tranwell Invasion Assay

A549 cells, which were treated without or with a middle dose of 60Co-γ irradiation (6 Gy), were analyzed by transwell assays performed in 24-well plates chamber (Corning, Lowell, MA, USA) fitted with a polyethylene terephthalate filter membrane with 8-μm pores. The membrane undersurface was coated with 30 μL ECM (Extracellular matrix) gel (Sigma, St. Louis, MO, USA) mixed with RPMI-1640 serum-free medium in 1:5 dilution for 4 h at 37 °C. The top chambers of the transwells were filled with 0.2 mL of cells (5 × 105 cells/mL) in serum-free medium, and the bottom chambers were filled with 0.25 mL of RPMI 1640 medium containing 10% FBS. The cells were incubated in the transwells at 37 °C in 5% CO2 for 12 h. The relative invading cells were measured following the methods described in reference [29]. Values were corrected for protein concentration and presented as the mean ± SD of replicate experiments [19,29,30].

3.9. Antibodies and Western Blot

Antibodies against BCL-W, IGFR, Survivin, c-IAP-1, c-IAP-2 and GAPDH were obtained from Santa Cruz Biotechnology (Santa Cruz Biotech, CA, USA). A polyclonal anti-rabbit IgG antibody conjugated with the horseradish peroxidase (HRP) were from Sigma (St. Louis, MO, USA). Cells, which were seeded and cultured in six-well plates, were then harvested by RIPA buffer supplemented with protease inhibitors cocktails (Sigma). Total protein samples were performed by SDS-PAGE and trans-printed to poly-vinylidene fluoride (PVDF) membranes (Millipore, Billerica, MA, USA). Then, membranes were blocked with 10% BSA in TBST buffer and then incubated 2 h at 37 °C with primary antibody against BCL-W (1:1000), IGFR (1:1000), Survivin (1:2000), c-IAP-1 (1:500), c-IAP-2 (1:500) or GAPDH (1:5000) diluted in TBST containing 5% BSA and subsequently washed three times in TBST for 5 min each. Then membranes were incubated with the HRP-conjugated secondary antibodies (1:5000) after being washed three times in TBST for 5 min each. At last, the blot was developed with enhanced chemiluminescence reagents (Pierce, Rockford, IL, USA) by X-ray films. The blots were performed on three independent occasions with similar results.

3.10. Statistical Analysis

The results from real-time RT-PCR or Western blot were analyzed using ALPHA INNOTECH software. The relative expression level was calculated as follows: (indicated group expression level/loading control expression level)/(control group expression level/loading control expression level) [19]. Statistical significance was determined using SPSS statistical software Bonferroni’s correction with or without two-way ANOVA.

4. Conclusions

In summary, our results indicated that miR-122 induced the susceptibility of NSCLC cell line to radiation, and expression of miR-122 via adenoviral vector could be a novel strategy in NSCLC radiotherapy or gene-therapy.