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Introduction

Worldwide, ovarian cancer is one of the most common types of human gynecological tumor, and the morbidity and mortality rate is increased compared with other gynecological malignancies (1,2). A number of factors may lead to tumorigenesis in ovarian cancer, demonstrating the complex pathology of the disease (3). A previous clinical study indicated that the incidence of ovarian cancer is increasing worldwide (4). Currently, tumorectomy, radiotherapy and chemotherapy are the primary therapies for patients with ovarian cancer in the clinic. However, ineffective treatments and treatment noncompliance frequently contribute to a worsening of symptoms due to the apoptosis resistance of ovarian malignant cells in patients with cancer (5). Although novel treatments to improve clinical medicine have been investigated, the improvements have had little effect on the survival rate of patients with ovarian cancer (6).

Apoptosis resistance is important for tumor growth and aggressiveness, and is caused by regulatory disorders of the apoptotic signaling pathway (7). Numerous molecules and proteins have been identified to be associated with the biochemical processes underlying apoptosis resistance, and may regulate the apoptotic signaling pathway in various tumor cells (8–10). Cellular studies and clinical data have suggested a direct association between the expression of the anti-apoptotic gene survivin and the apoptotic susceptibility of human ovarian cancer cells (11–13). Resistance to radiotherapy and chemotherapy is the principal challenge for the treatment of recurrent ovarian cancer, and frequently causes varying degrees of immune system damage, treatment failure and tumor metastasis. Therefore, understanding the underlying mechanisms of radioresistance and chemoresistance in ovarian cancer may contribute to the inhibition of apoptosis resistance and the benefits of cancer therapy, in addition to the development of innovative systemic therapies.

Apoptosis resistance, particularly increased epithelial-mesenchymal transition (EMT), remains an intractable clinical problem in the treatment of ovarian cancer (14). A previous study revealed that EMT may affect the cell cycle, differentiation, survival and apoptosis, regulated by transforming growth factor (TGF)-β in tumor cells (15). In addition, TGF-β upregulation may suppress mothers against decapentaplegic homolog (Smad) and non-Smad signaling in mammary epithelial cells, leading to EMT and the inhibition of growth arrest and apoptosis (16). Additionally, Chorna et al (17) demonstrated that TGF-β production may act as a natural immunosupressor, which is regarded as an anti-apoptotic protein for doxorubicin. Research has indicated that the liver kinase B1 (LKB1)-salt-inducible kinase 1 (SIK1) signaling pathway is associated with lung cancer cell growth, and previous results have suggested that attenuating LKB1-SIK1 may promote tumor invasion via upregulation of TGF-β production in non-small cell lung cancer cells (18). However, the signaling pathway and molecular mechanisms of LKB1-SIK1 has not been investigated in ovarian cancer.

In the present study, to determine the role of LKB1-SIK1 in ovarian cancer, the activity and expression levels of LKB1-SIK1 were analyzed in ovarian cancer tissues with normal adjacent tissues as the control. Migratory and invasive capacities were evaluated using Transwell. The association between LKB1-SIK1, EMP, apoptosis resistance and ovarian cancer cell growth was investigated using western blotting, small interfering RNA (siRNA), protein overexpression and immunofluorescence.

Materials and methods

Ethics statement

The present clinical investigation (no. HMCH2010072508) was performed in strict accordance with the recommendations in the Guide for Haidian Maternal and Child Healthcare Center (Beijing, China) between May 2010 and October 2015. A total of 12 female patients with ovarian fibroma were required to review trial protocols and amendments, and to provide written informed consent. The present study was approved by the ethics committee of Haidian Maternal and Child Healthcare Center. The demographic and clinical pathological characteristics of the patients are summarized in Table I.

Table I. Characteristics of patients with ovarian cancer.

Table I.

Characteristics of patients with ovarian cancer.

Characteristic	Value
Patient no.	12
Age range, years	32.4–53.8
Cancer type	Ovarian fibroma
Tumor stage, n
I	8
II	3
III	1
IV	0
History of cancer	None
History of allergy	None
Prior treatment	None

Cell culture and reagents

Ovarian cancer cells (1×107) were isolated from patients with ovarian cancer using tumor cell separation methods (19). Ovarian cancer tissues and adjacent normal tissues were washed with PBS. Tumor tissue was cut into pieces and separated into individual cells using minimum essential medium (MEM) containing 5% pancreatin and penicillin/streptomycin (2 mM) (both from Gibco; Thermo Fisher Scientific, Inc., Waltham, MA, USA). Cells were maintained for 12 h at 37°C in the presence of 5% CO2. Subsequently, cells were filtered, collected and identified by microscopic investigation (20). Ovarian cancer cells were cultured in MEM (Gibco; Thermo Fisher Scientific, Inc.) supplemented with 10% fetal bovine serum (Invitrogen; Thermo Fisher Scientific, Inc.). Cells were cultured in a humidified atmosphere containing 5% CO2 at 37°C in a cell culture incubator (Gibco; Thermo Fisher Scientific, Inc.).

Western blotting

Ovarian cancer cells transfected with siRNA or the eukaryotic expression vector for LKB1 were homogenized in lysate buffer containing protease-inhibitor (Sigma-Aldrich; Merck KGaA, Darmstadt, Germany) and were centrifuged at 8,000 × g at 4°C for 10 min. The supernatant was used for analysis of the total protein using a bicinchoninic protein assay kit (Gibco; Thermo Fisher Scientific, Inc.). Protein samples (20 µg) were separated on 12% sodium dodecyl sulfate polyacrylamide gels and transferred onto polyvinylidene fluoride membranes (EMD Millipore, Billerica, MA, USA) as previously described (21). Protein was blocked with 5% bovine serum albumin reagent (Roche Diagnostics, Basel, Switzerland) for 1 h at 37°C. For western blotting, primary goat anti-human antibodies against TGF-β (cat. no. ab31013), zinc-finger protein SNAI1 (cat. no. ab53519), Snai2 (cat. no. ab187109), Twist-related protein 1 (cat. no. ab50887), zinc finger E-box-binding homeobox 1 (ZEB1; cat. no. ab71286), E-cadherin (cat. no. ab76319), vimentin (VIM; cat. no. ab137321), VEGF (cat. no. ab27278), angiotensin-1 (Ang-1; cat. no. ab53951) and β-actin (cat. no. ab8226) (all 1:500 dilution; Abcam, Cambridge, UK), were incubated overnight at 4°C, followed by incubation with horseradish peroxidase-conjugated polyclonal anti-rabbit immunoglobulin G antibody (1:10,000, cat. no. HAF008; R&D Systems, Inc., Minneapolis, MN, USA) for 1 h at room temperature. A Ventana Benchmark automated staining system was used for analyzing protein expression (Olympus BX51; Olympus, Tokyo, Japan).

siRNA transfection

Ovarian cancer cells (1×106) were transfected with HiPerFect reagent (Qiagen GmbH, Hilden, Germany), according to the manufacturer's instructions (22). siRNA-TGF-β (100 pmol), siRNA-LKB1 (100 pmol) and siRNA-vector (100 pmol) were transfected into ovarian cancer cells for 24 h at 37°C. The sequences of siRNA-TGF-β, siRNA-LKB1 and siRNA-vector were designed and are listed in Table II. siRNA oligonucleotide pools containing three sequences targeting TGF-β or LKB1 were purchased from Eurogentec, Ltd. (Liège, Belgium).

Table II. Sequences of siRNAs.

Table II.

Sequences of siRNAs.

Name	Sense	Antisense
TGF-β	5′-GGATACCAACTATTGCTTCAGCTCC-3′	5′-AGGCTCCAAATATAGGGGCAGGGTC-3′
LKB1	5′-CTAGCTCAGACCGTTAGACGCCAGGACGGGCTGTCAGGCTGGCGCCTTTT-3′	5′-AAAAGGCGCCAGCCTGACAGCCCGTCCTGGCGTCTAACGGTCTGAGCTAG-3′
Vector	5′-AGAGGGAAATCGTGCGTGAC-3′	5′-CAATAGTGATGACCTGGCCGT-3′

[i] siRNA, small interfering RNA; TGF-β, transforming growth factor-β; LKB1, liver kinase B1.

Endogenous expression of LKB1

In order to establish stable ovarian cancer cells with endogenous LKB1 expression, the eukaryotic expression vector pCMVp-NEO-BAN (Takara Biotechnology Co., Ltd., Dalian, China) was used to construct recombinant plasmids. LKB1 was cloned, sequenced and recombined into pCMVp-NEO-BAN to construct pCMVp-NEO-LKB1 (pLKB1). The recombinant plasmid pCMVp-NEO-LKB1 was subsequently transfected into ovarian cancer cells. pLKB1 (1.0 µg) or pvector (1.0 µg) was transfected into cultured ovarian cancer cells (5×106) using Lipofectamine 2000 (Sigma-Aldrich; Merck KGaA), according to the manufacturer's instructions. Stable LKB1-overexpressing ovarian cancer cells were selected by G418 screening (23). After 48 h transfection, LKB1-overexpressing ovarian cancer cells were used to subsequent experimentation.

Apoptosis analysis

Apoptosis analysis of ovarian tumor cells was performed using flow cytometry. Human ovarian tumor cells (5×106) were cultured in 6-well plates with paclitaxel (2.0 mg/ml; Sigma-Aldrich; Merck KGaA) for 48 h to achieve the maximal apoptosis rate. Ovarian tumor cells were harvested at 48 h post-treatment by trypsinization. Ovarian tumor cells were subsequently washed in cold PBS and adjusted to 1×106 cells/ml with PBS. Following double staining with fluorescein isothiocyanate (FITC)-Annexin V and propidium iodide using the FITC Annexin V Apoptosis Detection kit I (BestBio Biotechnology, Shanghai, China), cells were analyzed using a FACScan® flow cytometer equipped with Cell Quest software (version 3.3; BD Biosciences, San Jose, CA, USA), according to manufacturer's instructions, to detect apoptosis in ovarian tumor cells. All experiments were performed in triplicate.

MTT cytotoxicity assays

Ovarian cancer cells were incubated (1×103) with paclitaxel or PBS in 96-well plates for 48 h in triplicate. Subsequently, 20 µl MTT (5 mg/ml) in PBS solution was added to each well, the plate was further incubated for 4 h. Most of the medium was removed and 100 µl dimethyl sulfoxide was added into the wells to solubilize the crystals. The OD was measured using an ELISA reader (Bio-Rad Laboratories, Inc., Hercules, CA, USA) at wavelength of 450 nm.

Histological, immunohistochemical and immunofluorescence staining analyses

Ovarian tumor tissues were fixed in situ overnight in 10% buffered formalin. The fixed tissues were cut mid-sagittal and being embedded in paraffin (4 µm thickness) using standard protocols. Hematoxylin and eosin staining was used to visualize the area of myocardial infarction after treatment with matrine. Immunohistochemical staining was performed using an avidin-biotin-peroxidase technique. Tumor sections (4 µm) were deparaffinized in xylene, dehydrated through graded ethanol and treated with 0.3% hydrogen peroxide in methanol for 30 min at 37°C. Paraffin-embedded ovarian normal tissue and tumor sections were prepared and epitope retrieval was performed at 95°C for 15 min for further analysis. The paraffin sections were treated with hydrogen peroxide (3%) for 10–15 min, which subsequently was blocked with a blocking solution (5% skim milk powder) for 10–15 min at 37°C. Subsequently, the sections were incubated in goat anti-human anti-Snai2 (1:1,000, cat. no. ab187109), Twist (1:1,000, cat. no. ab50887), ZEB1 (1:1,000, cat. no. ab71286), LKB1 (1:1,000, cat. no. ab15095), SIK1 (1:1,000, cat. no. ab64428), phosphorylated (p-)LKB1 (1:1,000, cat. no. ab63473), p-SIK1 (1:1,000, cat. no. ab217809), TGF-β (1:1,000, cat. no. ab31013) and VIM (1:1,000, cat. no. ab137321) at 4°C for 12 h. All sections were washed three times and incubated with secondary rabbit anti-goat antibodies (1:2,000, cat. no. ab150117; Abcam) for 1 h at 37°C. For immunofluorescence, ovarian tumor cells were stained with goat anti-human vascular endothelial growth factor and angiopoietin-1 antibodies. Additionally, a terminal deoxynucleotidyl transferase dUTP nick end labeling assay was performed using a Peroxidase Apoptosis Detection kit (Chemicon; EMD Millipore). All sections were observed in six random fields in the confocal microscope at magnification, ×40 (Nikon E400, Nikon Corporation, Tokyo, Japan).

Analysis of the cell cycle

To analyze the effects of LKB1 overexpression on the cell cycle stage of ovarian cancer cells, flow cytometry was performed. Exponentially, culturing ovarian cancer cells (1×106) or LKB1 overexpression were cultured for 24 h at 37°C. Cells were washed and trypsinized and rinsed with phosphate-buffered saline (PBS). All cells were fixed in 75% ice-cold ethanol for 5 min and then washed with PBS three times. The fixed cells were washed with RNase A (20 µg ml/l, Fermentas; Thermo Fisher Scientific, Inc.) and stained with propidium iodide (20 µg ml/l, Sigma-Aldrich; Merck KGaA) for 10 min at 37°C. The percentages of cells in G1 phase were analyzed using BD FACSCalibur (Becton Dickinson; BD Biosciences, San Jose, CA, USA).

Cell invasion and migration assays

Ovarian tumor cells subjected to different treatments (LKB1 overexpression or TGF-β inhibition) were used to analyze invasion and migration. Migration and invasion assays in ovarian tumor cells were conducted in a 24-well MEM culture plate with chamber inserts (BD Biosciences) with 10% fetal bovine serum (FBS; Gibco; Thermo Fisher Scientific, Inc.) for 12 h at 37°C. For migration assays, 1×103 cells/well ovarian tumor cells were placed into the upper chamber with a non-coated membrane. For the invasion assays, cells (1×103 cells/well) were placed into the upper chamber with a Matrigel-coated membrane. Matrigel were fixed with 4% formaldehyde and stained with 4′,6-diamidino-2-phenylindole as well as counted in 6 random fields under a microscope. Invasion and migration were calculated in at least three randomly stained fields under a light microscope (Nikon E400; Nikon Corporation).

Statistical analysis

All data are presented as the mean ± standard error of triplicate samples, and analyses were performed using Prism 6.0 software (GraphPad Software, Inc., La Jolla, CA, USA). Statistical differences between experimental groups were analyzed using Student's t-test. P<0.05 was considered to indicate a statistically significant difference.

Results

Analysis of the expression levels of EMT proteins, LKB1 and SIK1 in ovarian tumor tissues

The expression levels of EMT molecules, LKB1 and SIK1 were measured in ovarian tumor tissues. As presented in Fig. 1A, expression of the EMT pathway components Snai2, Twist and ZEB1 was upregulated in ovarian tumor tissues compared with normal ovarian tissues. The results in Fig. 1B and C demonstrated that the expression and phosphorylation levels of LKB1 and SIK1 were downregulated in ovarian tumor tissues compared with normal ovarian tissues. It was additionally observed that the expression levels of TGF-β and VIM were markedly increased in ovarian tumor tissues compared with normal ovarian tissues (Fig. 1D). The results of the present study demonstrated that the knockdown of LKB1 expression promoted TGF-β and Snai1 expression in ovarian tumor cells (Fig. 1E). Additionally, it was observed that the knockdown of LKB1 promoted the apoptosis resistance of ovarian tumor cells treated with paclitaxel (Fig. 1F). The present results suggested that the expression levels of EMT, LKB1 and SIK1 were increased in ovarian tumor tissues, which may be associated with the aberrant growth and aggressiveness of ovarian tumor cells.

Figure 1. Expression levels of TGF-β, EMT, LKB1 and SIK1 in ovarian tumor tissues. (A) Expression levels of EMT components including Snai2, Twist and ZEB1 between ovarian tumor tissues and normal ovarian tissues. (B) Expression levels of LKB1 and SIK1 between ovarian tumor tissues and normal ovarian tissues. (C) Phosphorylation levels of LKB1 and SIK1 in ovarian tumor tissues compared with normal ovarian tissues. (D) Expression levels of TGF-β and vimentin in ovarian tumor tissues. (E) LKB1 knockdown suppressed Snai1 expression in ovarian tumor cells. (F) LKB1 knockdown decreased the apoptotic resistance of ovarian tumor cells treated with paclitaxel. TGF-β, transforming growth factor-β; Snai2, zinc-finger protein SNAI2; EMT, epithelial-mesenchymal transition; LKB1, liver kinase B1; SIK1, salt-inducible kinase 1; Twist, Twist-related protein 1; ZEB1, zinc finger E-box-binding homeobox 1; DLKB1, downregulated LKB1. Magnification, ×40.

Analysis of growth and apoptotic resistance of ovarian tumor cells following inhibition of TGF-β expression

As presented in Fig. 2A, the growth rate of ovarian tumor cells isolated from clinical patients was increased compared with normal ovarian cells. Proliferation and migration assays demonstrated that growth and aggressiveness was inhibited by the inhibition of TGF-β expression in ovarian tumor cells (Fig. 2B and C). Apoptosis experiments demonstrated that the inhibition TGF-β expression by siRNA promoted apoptotic sensitivity in cells treated with paclitaxel for 48 h (Fig. 2D). It was observed that the inhibition of TGF-β expression by siRNA inhibited the proliferation of ovarian tumor cells compared to si-vector-transfected cells (Fig. 2E). Additionally, it was observed that the inhibition of TGF-β expression suppressed the expression levels of Snai2, Twist and ZEB1, as determined by western blotting (Fig. 2F). The results of the present study suggested that TGF-β expression may be associated with the growth and apoptotic resistance of ovarian tumor cells.

Figure 2. Effects of TGF-β expression on the growth and apoptosis resistance of ovarian tumor cells. (A) The growth rate of ovarian tumor cells isolated from clinical patients was increased compared with normal ovarian cells. (B) Inhibition of TGF-β expression suppressed ovarian tumor cell growth. (C) Inhibition of TGF-β expression inhibited the aggressiveness of ovarian tumor cells. (D) Inhibition of TGF-β expression promoted apoptosis in ovarian tumor cells. (E) Inhibition of TGF-β expression inhibited the proliferation of ovarian tumor cells. (F) Inhibition of TGF-β expression decreased the expression of Snai2, Twist and ZEB1 in ovarian tumor cells, as determined by western blotting. **P<0.01. TGF-β, transforming growth factor-β; Snai2, zinc-finger protein SNAI2; Twist, Twist-related protein 1; ZEB1, zinc finger E-box-binding homeobox 1; ITGF-β, inhibited TGF-β.

LKB1 upregulation stimulates SIK1 expression and inhibits the EMT signaling pathway in ovarian tumor cells

The present study further analyzed the influences of LKB1 on TGF-β expression and the EMT signaling pathway in ovarian tumor cells. The results in Fig. 3A demonstrated that SIK1 expression was promoted by LKB1 upregulation in ovarian tumor cells, as determined by immunofluorescence analysis. Western blotting demonstrated that TGF-β expression was decreased by LKB1 (Fig. 3B). The apoptosis assay indicated that the apoptosis sensitivity of ovarian tumor cells was increased following upregulation of LKB1 (Fig. 3C). In addition, the results demonstrated that the expression of important regulatory factors in the EMT pathway, E-cad, VIM, Snail2 and γH2AX, was downregulated by LKB1 upregulation in ovarian tumor cells (Fig. 3D). Growth, migration and invasion assays demonstrated that growth and aggressiveness was inhibited following overexpression of LKB1 expression in ovarian tumor cells (Fig. 3E and F). The present findings suggested that activation of the LKB1 signaling pathway may inhibit the TGF-β-mediated EMT pathway and decrease growth, aggressiveness and apoptosis resistance in ovarian carcinoma cells.

Figure 3. Effects of LKB1 on SIK1 expression and the EMT signaling pathway in ovarian tumor cells. (A) SIK1 expression was promoted by LKB1 upregulation in ovarian tumor cells, as determined by immunofluorescence. (B) LKB1 overexpression suppressed TGF-β expression in ovarian tumor cells. (C) LKB1 upregulation promoted apoptotic sensitivity in ovarian tumor cells treated with paclitaxel. (D) LKB1 overexpression decreased E-cad, VIM, Snai2 and γH2AX expression in ovarian tumor cells. (E) Upregulated SIK1 suppressed the growth of ovarian tumor cells compared with the control. (F) LKB1 overexpression inhibited the aggressiveness of ovarian tumor cells. **P<0.01. LKB1, liver kinase B1; SIK1, salt-inducible kinase 1; Snai2, zinc-finger protein SNAI2; VIM, vimentin; E-cad, E-cadherin; TGF-β, transforming growth factor-β; γH2AX, γ-histone H2AX; LKB1OR, LKB1 overexpression; SIK1UE, SIK1 upregulation.

LKB1 overexpression inhibits cell cycle and apoptosis resistance-associated protein expression in ovarian tumor cells

The present study investigated the effects of LKB1 on cells survival- and apoptosis resistance-associated protein expression in ovarian tumor cells. The results demonstrated that LKB1 overexpression inhibited the survival and cell cycle progression of tumor cells treated with paclitaxel, compared with control cells (Fig. 4A and B). Western blotting demonstrated that the expression of Bcl-2, Bax and p53 was inhibited by LKB1 overexpression in ovarian tumor cells compared with control cells (Fig. 4C). However, caspase-3, caspase-8 and Apaf-1 expression was increased by LKB1 overexpression in ovarian tumor cells compared with control cells (Fig. 4D). The expression of aggressiveness-associated proteins FN and RNF-8 was decreased by LKB1 overexpression in ovarian tumor cells (Fig. 4E). Additionally, the results demonstrated that the expression levels of vascular endothelial growth factor (VEGF) and angiotensin-1 (Ang-1) were downregulated by LKB1 overexpression, as determined by immunofluorescence analysis (Fig. 4F). The present results suggested that LKB1 overexpression may inhibit aggressiveness- and apoptosis resistance-associated protein expression in ovarian tumor cells.

Figure 4. Effects of LKB1 overexpression on cell cycle- and apoptosis resistance-associated protein expression in ovarian tumor cells. (A) LKB1 overexpression decreased the survival rate of ovarian tumor cells treated with paclitaxel. (B) LKB1 overexpression promoted the cell cycle arrest in ovarian tumor cells treated with paclitaxel. (C) LKB1 overexpression decreased the expression of Bcl-2, Bax and p53 in ovarian tumor cells, as determined by western blotting. (D) LKB1 overexpression promoted the expression of caspase-3, caspase-8 and Apaf-1 in ovarian tumor cells, as determined by western blotting. (E) LKB1 overexpression decreased the expression of FN and RNF-8 in ovarian tumor cells. (F) LKB1 upregulation downregulated the expression levels of VEGF and Ang-1 in ovarian tumor cells. **P<0.01. LKB1, liver kinase B1; LKB1OR, LKB1 overexpression; Bcl-2, apoptosis regulator Bcl-2; Bax, apoptosis regulator BAX; p53, cellular tumor antigen p53; Apaf-1, apoptotic protease-activating factor 1; FN, fibronectin; RNF-8, E3 ubiquitin-protein ligase RNF-8; VEGF, vascular endothelial growth factor; Ang-1, angiotensin-1.

Discussion

Ovarian cancer is associated with poor prevention, intractable malignancy and a poor prognosis (24). Traditional treatments, including radiotherapy, chemotherapy and surgery, are limited to palliative approaches for patients with advanced ovarian cancer (25,26). Although novel therapeutic agents and protocols for patients with ovarian cancer have been proposed in previous reports, the mortality and survival rates remain poor due to an increased rate of recurrence and metastasis following surgical resection (27). It has been reported that intrinsic and acquired resistance to anticancer treatments has been recognized to be a notable impediment to favorable outcomes in the clinic (28,29). The present study investigated the growth and aggressiveness of clinical ovarian cancer cells and analyzed the potential molecular mechanisms of apoptotic resistance in cells treated with a chemotherapeutic drug. Previous studies have suggested that ovarian cells may survive exposure to chemotherapeutic drug treatment and may display cancer stem cell and EMT-positive phenotypes (30,31). Consistent with the results in the published literature, an additional previous study demonstrated that the apoptotic resistance of ovarian cancer was induced by the EMT phenotype and expression of EMT-associated proteins (32). Notably, the present findings suggested that the inhibition of LKB1-SIK1 may reverse apoptotic resistance in ovarian cancer cells through the TGF-β-mediated EMT signaling pathway.

Previous studies have indicated that the development of anti-angiogenic drugs for the treatment of patients with ovarian cancer is an emerging field of oncology hoping to enter the preclinical stage of clinical trials (33–35). Additionally, many patients with advanced ovarian cancer respond poorly to traditional treatments or experience limited benefit from these treatments (36). The present study demonstrated an association between the EMT and SIK1 signaling pathways in ovarian carcinoma cells. EMT leads to cellular heterogeneity and supports tumor engraftment, which has been associated with underlying tumor heterogeneity and growth, metastasis and progression of ovarian cancer (37). Tang et al (38) demonstrated that inhibiting vasculogenic mimicry formation by reducing EMT may contribute to tumor apoptosis in ovarian cancer. In the present study, the results indicated that key regulatory factors in the EMT signaling pathway were upregulated in clinical ovarian tumor tissues, resulting in rapid growth and aggressiveness of tumor cells. Inhibition of TGF-β expression led to an inhibition of growth and aggressiveness, in addition to a promotion of apoptosis, in ovarian carcinoma cells treated with paclitaxel. The present results demonstrated that VEGF and Ang-1 were downregulated by LKB1 overexpression, suggesting that LKB1 may exert regulatory effects on ovarian cancer cell growth.

A previous study demonstrated that the LKB1-SIK1 signaling pathway was suppressed in ovarian carcinoma cells compared with normal ovarian cells, which led to activation of the EMT signaling pathway (18). In the present study, it was observed that inhibition of TGF-β expression promoted paclitaxel-induced apoptosis and suppressed tumor metastasis-associated protein expression (Snai2, Twist and ZEB1) in ovarian tumor cells. Cha et al (39) suggested that the binding of transcription elongation factor A protein 3 to TGF-β receptor I may induce apoptosis regulated by the Smad- and mitogen-activated protein kinase-dependent pathways in ovarian cancer cells. The results of the present study indicated that the expression of LKB1-SIK1 was suppressed in ovarian carcinoma cells, and that increasing LKB1 expression was able to downregulate the expression of TGF-β, and EMT and anti-apoptosis proteins, and to decrease apoptotic resistance in ovarian carcinoma cells. A previous study demonstrated that the knockdown of the LKB1 tumor suppressor gene may potentially induce papillary serous ovarian cancer in the ovarian surface epithelium (40). An additional study indicated that suppression of the LKB1-p53-p21/WAF1 pathway may promote the conversion of normal ovarian cancer cells to cancer stem cells via regulation of miR-17 expression (41). Additionally, a study reported that the restoration of SIK1 expression led to an inhibition of proliferation, which further increased the understanding of the pathogenesis and progression of ovarian cancer (42). These previous reports suggested that the expression levels of LKB1 may be decreased in ovarian cancer cells. The results of the present study demonstrated that the upregulation of LKB1 promoted SIK1 expression and markedly suppressed the growth and aggressiveness of ovarian cancer cells. Upregulation of LKB1 additionally promoted apoptosis in ovarian carcinoma cells. The results also demonstrated that knockdown of LKB1 further promoted the expression of TGF-β and EMT proteins, which downregulated the chemosensitivity of ovarian carcinoma cells. In addition, overexpression of LKB1 in ovarian carcinoma cells increased the chemosensitivity of the cells, resulting in a marked inhibition of migration and invasion. The results of the present study demonstrated that LKB1 overexpression may serve an inhibitory role in SIK1 and apoptosis resistance-associated protein expression in ovarian tumor cells, indicating that LKB1 may be a potential anticancer molecule.

In conclusion, the present study demonstrated that the expression of LKB1 and SIK1 was downregulated, while TGF-β and EMT protein expression levels were upregulated in clinical ovarian tumor tissues and cells. The results of the present study demonstrated that targeting the LKB1-SIK1 signal pathway may suppress the TGF-β-mediated EMT pathway, which may provide a therapeutic target for ovarian cancer. The apoptosis resistance of ovarian cancer cells treated with paclitaxel was improved by increasing LKB1 expression, resulting in the suppression of growth and aggressiveness. The results of the present study additionally indicated that inhibition of TGF-β markedly suppressed EMT- and metastasis-associated protein expression in ovarian cancer cells. Notably, it was suggested that enhanced LKB1-SIK1 signaling may inhibit the TGF-β-mediated EMT signaling pathway and the chemoresistance of ovarian cancer cells, which may subsequently contribute to limited metastatic potential, suggesting that targeting the LKB1-SIK1-TGF-β-EMT signaling pathways may be a promising therapeutic option for promoting the chemosensitivity and inhibiting the growth and metastasis of ovarian cancer cells.

References