Effect of RhoC on the epithelial-mesenchymal transition process induced by TGF-β1 in lung adenocarcinoma cells.

According to recent research, Ras homolog gene family member C (RhoC) is confirmed to have a powerful regulatory effect on cell motility mediated by the cytoskeleton, and this process is closely associated with tumor invasion and metastasis. In addition, the epithelial-mesenchymal transition (EMT) process which causes cytoskeleton rearrangement, also plays a pivotal role in tumor invasion and metastasis.Consequently, in the present study, we aimed to ascertain whether RhoC has an effect on the EMT process induced by TGF-β1 in lung adenocarcinoma cells and whether RhoC promotes tumor invasion by mediating the occurrence of EMT. Based on the findings, we demonstrated that RhoC was an essential mediator of the EMT process in lung adenocarcinoma cell line A549 which was evaluated by observing the morphological characteristics of the cells and by assessing the expression levels of two EMT marker proteins: E-cadherin and vimentin. During the process of EMT in the A549 cells induced by TGF-β1 (5 ng/ml), upregulated RhoC protein and RhoC activity were detected, which was associated with the enhanced invasive capability of the cells in vitro. Conversely, downregulation of the expression of RhoC by shRNA markedly impeded EMT progression as well as the invasion of A549 cells. Our results may provide a novel target towards the prevention of metastasis in advanced lung adenocarcinoma.

Introduction

Lung cancer is one of the most common malignant tumors and the leading cause of cancer-related morbidity and mortality. As the cancer statistics in 2016 collected by the American National Cancer Institute show, lung cancer is still the leading cause of cancer-related deaths among both men and women with a mortality rate of 27 and 26%, respectively (1). Early metastasis is the key factor for the poor prognosis of lung cancer; therefore, elucidating the mechanism of carcinoma invasion and metastasis is critical for prognostic assessment and therapeutic guidance.

Invasion and metastasis are quite complicated processes driven by multiple genes which experience multistep developmental changes on the basis of weakened adhesion and enhanced motile ability between tumor cells (2). Several studies have shown that epithelial-mesenchymal transition (EMT) plays an important role in the process of neoplasm metastasis (3–5). EMT is a physiological process in which epithelial cells go through a series of biochemical changes, and it is characterized by obtaining a mesenchymal phenotype and losing cell apico-basal polarity (6). At present, the recognized molecular mechanisms of EMT include decreased expression of epithelial markers such as E-cadherin and cytokeratins which reduce cell adhesion and lead to loosened attachments to adjacent cells. Thus, the functional loss of E-cadherin is regarded as a symbolic event in EMT (7); the cytoskeleton is rearranged through a change in its essential components from keratin to vimentin, forming cell-matrix adhesion, causing cell phenotype transition. Together, these conversions result in enhanced capability of motility and eventually mediate cell invasion and migration (8). Under normal physiological conditions, EMT participates in a variety of processes such as animal embryo implantation and development, organ formation, tissue damage repair, tissue regeneration and organ fibrosis. In tumors, EMT affects the invasive capability of cells. A recent study indicated that malignant tumor metastasis and local infiltration imitate the occurrence of EMT during embryonic development (9). However, researchers have reported that the process of EMT can be observed in the cell invasion and metastasis of colon (10), breast (11), prostate (12), liver (13), lung (14) and cervical tumors (15). Moreover, the number of tumor cells appearing during EMT is directly related to the degree of invasion and metastasis.

Emerging evidence suggests that Rho proteins contribute to the process of EMT (16,17). Rho proteins are a subset of the oncogenic Ras superfamily, which is also named RhoGTPase since it possesses GTP enzyme activity. The primary biological effects of Rho proteins include cytoskeleton remodeling, involvement in cell adhesion and regulation of cell movement and migration. The Rho family can be classified into three subtypes including RhoA, RhoB and Ras homolog gene family member C (RhoC). Compared with other members of the Rho protein family, RhoC has the strongest binding force with its downstream effector ROCK (Rho-associated coiled-coil kinase). Thus, it possesses remarkable adjustment ability. Similar to other members of the Ras superfamily, RhoC cycles between its activated state (combined with GTP) and inactivated state (combined with GDP) and this cycling can be regulated by different proteins. However, unlike other homologues, RhoC gene mutations in tumors are rare (18), but often manifests at abnormal expression levels. Bellovin et al found that RhoC expression was negatively correlated with the expression level of E-cadherin which regulated cell junction when cells were induced by EMT (19). Mukai et al showed that RhoC was essential in rat ascites hepatoma metastasis (20). In addition, Hakem et al demonstrated that downregulation of the RhoC gene was able to significantly inhibit breast cancer metastasis, even though the suppression of tumor growth was not observed. Notably, the researchers also found that deficiency of RhoC did not affect normal physiological functions, such as embryonic development and immune response (21). Consequently, we aimed to ascertain the effects of RhoC on the EMT process induced by TGF-β1 in lung adenocarcinoma cells and whether RhoC promotes tumor invasion by mediating the occurrence of EMT.

Materials and methods

Cell line, cell culture and treatment

Human lung adenocarcinoma cell line (A549) was purchased from the Cell Center of Central South University Xiangya Medical College (Changsha, Hunan, China). Cells were cultured in high-glucose Dulbecco's modified Eagle's medium (DMEM; HyClone-GE Life Sciences, Logan, UT, USA) supplemented with 10% fetal bovine serum (Gibco, Carlsbad, CA, USA), 100 U/ml penicillin, 100 µg/ml streptomycin sulphate, 2 mM glutamate in a humidified atmosphere of 5% CO2 at 37°C. A549 cells were exposed to 5 ng/ml TGF-β1 from PeproTech, Inc. (Rocky Hill, NJ, USA) at different time-points (0, 24, 48 and 72 h).

shRNA for RhoC knockdown

RhoC small hairpin RNAs (shRNAs) expressing green fluorescence protein (GFP) were synthesized by GeneChem Co., Ltd. (Shanghai, China). The target sequences of RhoC shRNA were: RhoC-1, 5′-AAGCCTTGACTTCATCTCAGC-3′ (sense) and 3′-GCTGAGATGAAGTCAAGGCTT-5′ (antisense); RhoC-2, 5′-CACCATGGCTGCAATCCGAAA-3′ (sense) and 3′-TTTCGGATTGCAGCCATGGTG-5′ (antisense); RhoC-3, 5′-GAGGTGTTTGAGATGGCCACT-3′ (sense) and 3′-AGTGGCCATCTCAAACACCTC-5′ (antisense); RhoC-NC, 5′-AGCTTAAGTTTAAACCGCTG-3′ (sense) and 3′-TCAATAATGACGTATGTTCCC-5′ (antisense). Each RhoC shRNA was extracted and purified, and was identified through genetic sequencing. According to the graphic analysis of the sequencing, it was observed that the proposed plasmid DNA was consistent with the plasmid information provided by GeneChem (data not shown), which produced a successful plasmid amplification extraction. Each shRNA was mixed with Lipofectamine 2000 reagent (Invitrogen, Carlsbad, CA, USA) following the manufacturer's instructions which was added to each plate when cells grew to a confluence of 30–50%. After 8 h of incubation, the transfection medium was removed, the plates were washed with phosphate-buffered saline (PBS) 3 times, and replaced with complete medium. Twenty-four hours later, the knockdown efficiency of the RhoC gene was estimated by flow cytometry and western blotting.

Western blotting

Cells were trypsinized and washed 3 times with PBS before being lysed on ice for 30 min with RIPA lysis buffer and a protease inhibitor (Sigma-Aldrich, St. Louis, MO, USA). The lysates were centrifuged at 13,000 × g at 4°C for 20 min. Protein concentrations were measured using the Bradford method as described in the manufacturers protocol. Total protein (40 µg) from each sample was separated by 10% sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and transferred onto polyvinylidene difluoride (PVDF) membranes (Millipore Inc., Billerica, MA, USA), and then blocked for 1 h in Tris-buffered saline and Tween-20 (TBST) containing 5% skimmed milk. The proteins of interest were incubated with the corresponding antibodies at 4°C overnight. The primary antibodies were used as follows. Goat anti-human RhoC (1:500 dilution) was purchased from Santa Cruz Biotechnology (Santa Cruz, CA, USA), rabbit anti-human vimentin (1:600) and rabbit anti-human E-cadherin (1:400) were purchased from Assay Biotech (Sunnyvale, CA, USA). After incubation with the corresponding secondary antibodies for 2 h at room temperature (RT) the desired proteins were detected using ECL chemiluminescence. Image Lab 4.1 software was used to analyze the gray area value of the protein bands and the relative protein expression level was normalized to the reference protein β-actin.

Analysis of RhoC activity by pull-down assay

Pull-down assay was used to assess RhoC activity as previously described (22). For these experiments, A549 cells were harvested with RIPA lysis buffer coupled with a protease inhibitor, centrifuged at 13,000 × g at 4°C for 20 min to obtain a supernatant. The extracts were incubated for 40 min at 4°C with 20 µg glutathione beads bound to GST-Rhotekin RBD fusion protein (Cytoskeleton, Inc., Denver, CO, USA), then washed 3 times with washing buffer. The Rho content of the beads was eluted and boiled to denaturation, and then separated using 15% SDS-PAGE, transferred to a PVDF membrane and immunoblotted with a RhoC antibody.

Transwell assay

Invasion assays were carried out in 24-well Transwell chambers (BD Biosciences, Bedford, MD, USA). The upper compartment of the chambers was filled with 100 µl pre-chilled Matrigel (1:6; BD Biosciences). The Matrigel remained at 37°C for 4 h for solidification. Subsequently, cells were harvested, resuspended in DMEM containing 1% BSA, and then seeded in the upper chamber. An additional 400 µl 10% DMEM was added to the lower chamber as a chemoattractant. The chambers were incubated in 5% CO2 at 37°C for 24 h. Cells were stained with crystal violet after the cells and the Matrigel in the upper chamber had been wiped off using a cotton swab, carefully. The number of invasive cells was counted using an inverted microscope. Five random fields in each chamber were measured. Finally, the staining dye was dissolved with 10% acetic acid. The absorbance wavelength at 570 nm was measured using an enzyme-linked immunometric meter (Bio-Rad Laboratories, Inc., Hercules, CA, USA), so as to indirectly evaluate the number of invasive cells. Assays were performed in triplicate.

F-actin staining

Phalloidin conjugated to FITC was used to stain filamentous actin (F-actin) (23). When cell confluence on the coverslips reached 60%, 4% paraformaldehyde was added for 20 min to the fixed cells. The fixed cells were then permeabilized with 0.5% Triton X-100 TBS for 10 min and blocked with 10% goat serum for 10 min. In the dark, the slips were stained with 5 µg/ml FITC-phalloidin (Sigma-Aldrich), for 30 min at RT, then washed with 0.1% Triton X-100 TBS three times and 50% glycerol was used to seal the slides. Finally, the slides were observed under a fluorescence microscope.

Statistical analysis

SPSS 18.0 software (SPSS, Inc., Chicago, IL, USA) was used for statistical analysis. Data in each group are presented as the mean ± standard deviation (mean ± SD), and the difference between the groups was calculated using the two-tailed Student's t-test. p<0.05 was assumed to indicate a statistical difference.

Results

EMT is induced in A549 cells by TGF-β1

TGF-β1 is commonly used as an inducer in models of EMT in cancer cells (24). The A549 cell line originates from alveolar type II epithelial cells and possesses typical epithelial cell characteristics which often present cobblestone-like morphology. In the present study, we found that the cell morphology of the A549 cells was transformed into fusiform and spindle-shape cells with increasing pseudopodia after TGF-β1 stimulation. In other words, the A549 cells displayed features of mesenchymal cells. Furthermore, TGF-β1 exerted effects on the A549 cells in a time-dependent manner. The 24 h group did not exhibit an obvious effect on the cell morphology, while the cell shape began to change after 48 h and the difference became more noticeable at the 72 h time-point (Fig. 1A). E-cadherin and vimentin protein expression documented by western blotting assay revealed that E-cadherin expression was decreased (p<0.05) and vimentin expression was increased (p<0.05) in response to stimulation with 5 ng/ml of TGF-β1 for 48 h (Fig. 1B and C). All the aforementioned results provided evidence that EMT was induced in the A549 cells by TGF-β1 at 48 h, and this phenomenon was demonstrated by the fact that the A549 cells exhibited altered morphology and consequent alterations on a molecular expression level. We selected 48 h as the detection time of change of EMT characteristics mediated by TGF-β1.

Figure 1.

EMT is induced by TGF-β1 in A549 cells. (A) The cell morphology of the A549 cell line treated with 5 ng/ml of TGF-β1. Observation were carried out at 4 time-points including 0, 24, 48 and 72 h under a light microscope (magnification, ×100). (B and C) A549 cells were treated with 5 ng/ml of TGF-β1 for 0, 24, 48 and 72 h and the protein expression of (B) E-cadherin and (C) vimentin was detected by western blotting and normalized to β-actin.

Expression and activity of RhoC are enhanced during the process of EMT in the A549 cells induced by TGF-β1

As aforementioned, RhoC exhibits some similarities with EMT and it is necessary to investigate the origin of this similarity, which could be interrelated or independent from each other. To clarify whether RhoC takes part in the regulation of EMT in the A549 cells, we performed a western blot assay to detect the RhoC protein expression and a pull-down assay to detect the RhoC activity. After incubation with TGF-β1 (5 ng/ml) for >48 h, we observed that both the expression and activity of RhoC were markedly increased (Fig. 2A and B; p<0.05). This tendency coincides with loss of E-cadherin and the acquisition of vimentin (Fig. 1B and C). These results suggest that RhoC may act as a positive regulator of EMT in the A549 cells induced by TGF-β1.

Figure 2.

Expression and activity of RhoC are enhanced during the process of EMT in the A549 cells induced by TGF-β1. (A) A549 cells were treated with TGF-β1 (5 ng/ml) and RhoC protein expression levels were assessed by western blotting at 0, 24, 48 and 72 h. (B) A549 cells were treated with TGF-β1 (5 ng/ml) and RhoC activity was assessed by a pull-down assay at 0, 24, 48 and 72 h.

The influence of TGF-β1-mediated EMT on A549 cell invasion and cytoskeleton rearrangement

Further experiments were carried out to determine whether TGF-β1 affects A549 cell invasion and the cytoskeleton during EMT. Using a Transwell chamber assay and an enzyme-linked test, we found that the number of A549 cells which invaded into the lower chamber containing Matrigel increased in a time-dependent manner (absorbance of the 0 h group vs. the 24 h group, the 0 h group vs. the 48 h group, and the 0 h group vs. the 72 h group were 0.045±0.0023 vs. 0.054±0.0034, 0.045±0.0023 vs. 0.078±0.0056 and 0.045±0.0023 vs. 0.104±0.0063, respectively; p<0.05; Fig. 3A). Additionally, in order to assess the role of TGF-β1 on the cytoskeleton of the A549 cells, an F-actin assay was performed. The cytoskeleton of the A549 cells showed evident change. As shown in Fig. 3B, the actin stress fibers were regularly arranged in the 0 h control group, while a change occurred in the cellular morphology after stimulation with TGF-β1. A radial pattern arrangement emerged and a great quantity of actin stress fibers formed around the cells following TGF-β1 treatment of the A549 cells at >48 h. The results of the present study further support that TGF-β1 induced EMT in the A549 cells.

Figure 3.

A549 cell invasion and cytoskeleton rearrangement are influenced by TGF-β1-mediated EMT. (A) A549 cells were treated with TGF-β1 (5 ng/ml). A Transwell chamber assay and crystal violet staining using an enzyme-linked test were used to evaluate the number of invasive cells (magnification, ×100; *p<0.05, **p<0.05 and ***p<0.05, compared with the 0 h group). (B) F-actin staining using phalloidin-FITC was observed under a fluorescence microscope at different time-points (0, 24, 48 and 72 h) after A549 cells were treated with TGF-β1 (5 ng/ml).

Regulation of RhoC mediates EMT by TGF-β1 in A549 cells

Based upon the aforementioned results, RhoC expression is associated with EMT in A549 cells. In order to confirm whether RhoC regulates EMT in the A549 cells, the RhoC gene was silenced with shRNA prior to exposure to TGF-β1 or following it. Three shRNAs were designed by GeneChem. Fluorescence microscopy and flow cytometry were used to evaluate the efficiency of transfection (Fig. 4A), and western blotting was used to measure the silencing effect (Fig. 4B). Through a series of verification tests, RhoC-3 shRNA proved to be the most valuable and was selected to engage in the follow-up experiment.

Figure 4.

Regulation of RhoC on TGF-β1-mediated EMT in A549 cells. (A) A549 cells displaying green fluorescence under fluorescence microscope showed that the cells were successfully transfected. Flow cytometry was used to evaluate the efficiency of transfection. (B) The RhoC silencing effect was assessed by western blotting after transfection with shRNA for 48 h (p<0.05). (C) A549 cells were transfected with RhoC shRNA before or after incubation with TGF-β1 (5 ng/ml), and the morphology of the cells was observed using a light inverted microscope. a, RhoC shRNA; b, RhoC shRNA + TGF-β1; c, TGF-β1; d, TGF-β1 + RhoC shRNA (magnification, ×100). (D and E) A549 cells were transfected with the RhoC shRNA or the control plasmid before or after incubation with TGF-β1 (5 ng/ml) and the expression of E-cadherin and vimentin were detected by western blotting.

The RhoC gene-knockdown group prior to TGF-β1 exposure did not show typical mesenchymal morphology. In contrast, the A549 cells exhibited mesenchymal morphology when RhoC was downregulated after TGF-β1 treatment (Fig. 4C). With respect to molecular expression, our results demonstrated that not only the cell group with RhoC silencing prior to TGF-β1 treatment but also the group with RhoC silencing after TGF-β1 treatment markedly abrogated the downregulation of E-cadherin (Fig. 4D) and the upregulation of vimentin (Fig. 4E). The present data demonstrated that RhoC was involved in the process of EMT in the A549 cells induced by TGF-β1 treatment and that the inhibition of RhoC expression impeded EMT progression.

Change in invasion of A549 cells after the RhoC gene is silenced

EMT is known to play an important role in tumor cell invasion. Considering that downregulation of the RhoC gene impedes the development of EMT in the A549 cells, we performed an assay to determine whether downregulation of the RhoC gene affects the invasive capacity of the A549 cells during the course of EMT. Only the group with RhoC silencing prior to TGF-β1 treatment exhibited suppressed invasive ability induced by TGF-β1 (the absorbances of the con group vs. the NC group vs. the shRNA group vs. the TGF-β1 group vs. the NC + TGF-β1 group vs. the shRNA + TGF-β1 group vs. the TGF-β1 + NC group vs. the TGF-β1 + shRNA group were 0.053±0.0026 vs. 0.054±0.0025 vs. 0.032±0.0032 vs. 0.110±0.0043 vs. 0.103±0.0085 vs. 0.077±0.0034 vs. 0.100±0.0042 vs. 0.102±0.0049, respectively; Fig. 5). Therefore, these results further revealed that downregulation of RhoC retarded the progression of EMT in the A549 cells while it did not reverse EMT.

Figure 5.

Change in the invasive ability of A549 cells after RhoC gene silencing. A549 cells were transfected with RhoC shRNA before or after incubation with TGF-β1 (5 ng/ml), Transwell chamber assay and crystal violet staining using an enzyme-linked test were used to evaluate the number of invasive cells; a, Con; b, NC; c, RhoC shRNA; d, TGF-β1; e, NC + TGF-β1; f, RhoC shRNA + TGF-β1; g, TGF-β1 + NC; h, TGF-β1 + RhoC shRNA (magnification ×100, *p<0.05; #p<0.05; △p<0.05; ✩p>0.05).

Discussion

EMT is a significant initial event in the process of tumor metastasis. Alterations in cell morphology, EMT-related classic marker protein expression and cell invasion capability are various characteristics of EMT (25). TGF-β1, a cytokine, is known to induce the occurrence of EMT. During the invasion and metastasis of a tumor, TGF-β1 plays an important role in tumor progression. It not only blocks the cell cycle and apoptosis of normal epithelial cells, but also induces and maintains the EMT of cells (26). Various cell models, such as the human mammary gland epithelial cell line MCF-10A underwent a typical EMT change after TGF-β1 treatment (27), therefore, these cell lines have been frequently used to research EMT induced by TGF-β1. In the present study, we observed the cell morphology of the A549 cell line and the protein expression of EMT-related genes at different time-points by adopting the concentration of 5 ng/ml of TGF-β1 (28,29), so as to demonstrate the successful establishment of EMT in the A549 cells.

With respect to cell morphology, the morphological transition of the cells was obvious as treatment time with TGF-β1 gradually progressed. We observed A549 cell morphological changes at multiple time-points in our experiment and found that the cell morphology began to change after treatment at 24 h, and that the cell morphological changes were the most marked in the 48 h group. By comparing the differences between the 0 h control and 48 h groups, we found that A549 cells underwent shape change from cobblestone-like into spindle and fusiform, with the addition of pseudopodia after being exposed to TGF-β1 for >48 h. In other words, A549 displayed obvious mesenchymal cell morphological changes. Similarly, Kasai et al (30) pointed out that human alveolar epithelial type II cells derived from the A549 cell line transitioned from a pebble-shaped epithelial phenotype into a narrow mesenchymal phenotype after TGF-β1 (5 ng/ml) treatment for 48 h. Meanwhile the connection between adjacent cells also became loose. These findings were in accordance with our experimental results.

The alteration of EMT-related gene expression is another characteristic of EMT. During the generation of the process of EMT, epithelial phenotype marker protein E-cadherin whose expression is downregulated displays a mesenchymal phenotype similar to the protein fibronectin or to vimentin overexpression (31). E-cadherin is a transmembrane protein, whose effect on maintaining epithelial structural integrity and polarity cannot be ignored, as its downregulated expression leads to separation of adjacent and epithelial cells which lose their polarity. For this reason E-cadherin is deemed as a crucial marker protein of EMT, and downregulation of its expression always prompts the onset of EMT. Mesenchymal phenotypic marker protein vimentin is an important component of intermediate filaments of the cytoskeleton, which participate in the regulation of the cell polarity shift, the cytoplasm distribution and the cytoskeleton structure (32). Lee et al (33) reported that epidermal growth factor (EGF) acted on cervical squamous epithelial cells, inhibited E-cadherin expression and enhanced vimentin expression, which can induce EMT and took part in the elongation of cells and in compact cell junctions, thereby increasing the invasive ability of cells. In a study on prostate cancer, researchers found that IGF-1 elevated ZEBl expression which inhibited E-cadherin, thus promoting EMT occurrence (34). Our results also revealed that the E-cadherin protein expression decreased and the vimentin protein expression increased significantly after the A549 cells were treated with TGF-β1 for 48 h. The EMT-related gene expression indicated that the A549 cells underwent EMT, in addition, the expression level of the proteins was consistent with the morphological transformation of the A549 cells.

EMT is an important biological process in which malignant tumors derived from epithelial cells obtain the ability of migration and invasion. The tight junction among tumor cells appears to sever upon the onset of EMT, owing to the reduced expression of E-cadherin. Thus, it is possible for tumor cells to detach from the primary carcinoma nest (35), which contributes to the spread and invasion of the extracellular matrix. Using Transwell assay, we observed the invasive and metastatic ability of the A549 cells enhanced with prolongation of exposure time to TGF-β1 in a time-dependent manner. This phenomenon was demonstrated in multiple tumors as well. The study by Gjerdrum et al suggested that metastasis of breast cancer depends on EMT. Cell invasion and distant metastasis can be accelerated by Axl in breast cancer, and Axl expression increased when breast cancer cells were stimulated with Twist which is an EMT transcription factor (36). A similar phenomenon was confirmed in both hepatic carcinoma cells and in an animal model (37). Nevertheless, the underlying mechanism remains controversial and is believed to involve several signaling pathways including TGF-β (Smad/Rho-GTP) (38), the tyrosine kinase receptor, integrin, the Wnts and the NF-κB pathways (39).

As it is already known, the RhoC protein is able to regulate the reorganization of cytoskeleton actin in all eukaryotic cells. On the one hand, the RhoC protein gathers stress fibers to maintain cell morphology, toughness and strength and it also assembles integrin and related proteins such as fibronectin into adhesion complexes (40). The RhoC protein combines with the cytoskeleton proteins such as focal adhesion kinase, myosin light chain and pixillin which directly take part in regulating cell movement and migration. On the other hand, overexpression of RhoC can increase the protein level of vascular endothelial growth factor (VEGF), activate the mitogen-activated protein kinase (MAPK), phospholipase C (PLC), phosphatidyl inositol 3 kinase (PI3K) signal transduction pathways, and promote tumor invasion and metastasis (41,42). Symons and Segall (43) considered that the balance between Rho and Rac activity determines cell morphology and biological behavior. When the Rho activity occupies a leading position, cells tend to undergo mesenchymal phenotype conversion, with a decrease in E-cadherin expression, onset of EMT and an increase in invasive ability. Whereas, the opposite results can be seen when Rac activity is dominant. As aforementioned, we hypothesized that EMT induced by TGF-β1 may be associated with RhoC in lung cancer. In the present study, we demonstrated that both the RhoC protein expression and the RhoC-GTP enzyme activity were markedly increased following TGF-β1 treatment in the A549 cells. Furthermore, using F-actin staining, we showed cellular tonofilament aggregation and separation and a disordered arrangement, which suggested cytoskeleton change. These data indicated that RhoC exerts significant effects on TGF-β1-induced EMT in the A549 cells. Notably, Rho-GTP enzyme activity was also demonstrated to be a key step in the EMT model in renal tubular epithelial HK2 cells (44).

Several studies have raised the possibility that RhoC can be a vital target in blocking EMT which motivated us to explore this hypothesis. Recently, one study that was performed using RNA interference technology, indicated that RhoC knockdown inhibited the onset of EMT in cervical cancer cells (45). In addition, Cho and Yoo (46), using Y-27632 which is a selective inhibitor of Rho downstream effector ROCKs, found that EMT induced by TGF-β1 was completely interrupted in lens epithelial cells. In order to determine whether RhoC influences the process of EMT in lung adenocarcinoma A549 cells, we transfected RhoC shRNA into the A549 cells before TGF-β1 treatment. Likewise, we found that downregulation of RhoC prevented the EMT of A549 cells in a similar pattern and further suppressed the invasive capability of lung adenocarcinoma cells. Consequently, we have reason to speculate that targeted inhibition of the activity of RhoC could become an important approach for the therapy of lung adenocarcinoma. However, in the present study, the silencing of the RhoC gene did not reverse EMT when RhoC shRNA was transfected after TGF-β1 treatment in the A549 cells. Thus, a concrete reason for this and the molecular mechanism involved have not yet been elucidated and need to be explored in future research.