The study was approved by the Research Ethics Committee of Tangdu Hospital, the Fourth Military Medical University (Xi'an, China). All patients willingly consented to participate in the study by signing the written informed consent form. All specimens and data were handled according to the relevant ethical and legal standards.

A total of 54 glioma specimens (24 male and 30 female), resected between October 2008 and Octover 2010 and stored in liquid nitrogen, were retrieved from the archives of the Department of Pathology, Tangdu Hospital (Xi'an, China). The age of these patients ranged from 6 to 75 (mean age of 42.74 and median age of 42.5 years). These retrieved specimens were directly used for reverse transcription- quantitative PCR (RT-qPCR) analysis, and formalin-fixed and paraffin-embedded for immunohistochemical (IHC) analysis. Conferring to the World Health Organization (WHO) classification (11), the histopathological sections were evaluated by two pathologists, with differences resolved by careful discussion. Accordingly, a total of 29/54 gliomas were classified as low-grade gliomas [7 pilocytic astrocytomas (WHO I) and 22 diffuse astrocytomas (WHO II)] and 25/54 as high-grade gliomas [20 anaplasia astrocytomas (WHO III) and 5 primary glioblastomas (WHO IV)]. None of the patients had been treated with chemotherapy or radiotherapy prior to surgery. The clinicopathological features of all patients are summarized in Table I.

The patient's survival time was assessed by calculating the days from the date of the initial glioma-related surgery to mortality. Patients who succumbed to diseases not directly associated with the gliomas or those who succumbed for other reasons were excluded from this study. The association between the overall survival time and the TXNIP expression level was established using the Kaplan-Meier method.

Formalin-fixed, paraffin-embedded, sectioned tissues (4-µm thick) were stained using the Elivision Plus/horseradish peroxidase (HRP) detection system (Fuzhou Maixin Biotech Co., Ltd., Fuzhou, China) and counterstained with hematoxylin. In brief, following a peroxidase block with 3% H₂O₂/methanol (Fuzhou Maixin Biotech Co., Ltd.) for 30 min, specimens were blocked with 5% normal goat serum. The slides were then incubated overnight with mouse polyclonal anti-human TXNIP primary antibody (cat. no. K0205-3; Medical & Biological Laboratories Co., Ltd., Nagoya, Japan) at 1:50 dilution, at 4°C. An IHC antibody diluent (cat. no. ZLI-9029; ZSGB-BIO, Beijing, China) at 1:50 dilution replaced the primary antibody for negative control slides. The specimens were then briefly washed three times with phosphate-buffered saline (PBS) and incubated at room temperature with Solution A for 20-min, followed by a 30-min incubation with Solution B (EnVision Plus/HRP mouse/Rabbit IHC kit; cat. no. KIT-9902; Fuzhou Maixin Biotech Co., Ltd.) at 37°C. The signal visualization was performed by treatment with DAB chromogen for 2 to 3 min. After washing with water, specimens were counterstained with Meyer's hematoxylin (Maixin Biotech, Fuzhou, China). The WHO I neoplastic brain tissues of 54 glioma specimens, which were obtained from the archives of the Department of Pathology, Tangdu Hospital, were used as control tissues.

The evaluation standards of the pathologists were as follows: i) The intensity of the cell staining; non-staining as negative (−), light brown staining as weakly positive (+), brown staining as medium positive (++) and sepia staining as strong positive (+++/++++); ii) the number of positive cells; the numbers of positive cells <25% (+), the numbers of positive cells among 25–49% (++) and the numbers of positive cells >50% (+++). Qualitative and half-quantitative staining results were achieved by combined the analysis results of the two experiments. A total of 5–10 high power fields were randomly observed and the averages were taken. Negative (−) and weakly positive (+) was regarded as low expression and the others were regarded as high expression.

The glioma U251MG cell line was purchased from the Chinese Academy of Sciences Cell Bank (Shanghai, China) and was stored in liquid nitrogen until use. The cell line was authenticated by short tandem repeat profiling. Under culture, the cells were maintained at 37°C in high-glucose Dulbecco's Modified Eagle's Medium (DMEM; Invitrogen; Thermo Fisher Scientific, Inc., Waltham, MA, USA) supplemented with 10% fetal bovine serum (Gibco; Thermo Fisher Scientific, Inc.) and 1% penicillin/streptomycin (Sigma-Aldrich; Merck KGaA, Darmstadt, Germany) in a humidified 95% air and 5% CO₂ incubator.

A total of 3 sequences of human lentiviral TXNIP short hairpin RNA (shRNA) (Table II) and the non-specific control shRNA were purchased and packaged into the lentivirus by Shanghai GenePharma Co., Ltd. (Shanghai, China). U251-MG cells were infected with 0.5 ml shRNA lentiviruses (1×10⁶/ml) and cultured at 37°C in conventional DMEM supplemented with 2 µg/ml puro (Sigma-Aldrich; Merck KGaA) in a humidified 95% air and 5% CO₂ incubator for 1 week following transfection. These four cell lines were labeled as LV3-TXNIP shRNA1-U251, LV3-TXNIP shRNA2-U251, LV3-TXNIP shRNA3-U251 and LV3-NC-U251. The TXNIP expression level was evaluated by RT-qPCR and western blotting.

For western blot analysis, cells (LV3-TXNIP shRNA1-U251, LV3-TXNIP shRNA2-U251, LV3-TXNIP shRNA3-U251 and LV3-NC-U251) were cultured to cell densities of 2–3×10⁶ in 60 mm dishes. Subsequently, the cells were harvested and lysed with radio immunoprecipitation assay buffer (Beijing Solarbio Science & Technology Co., Ltd., Beijing, China). Total protein concentrations were determined using the bicinchoninic acid protein assay kit (Boster Systems, Inc., Pleasanton, CA, USA). Total proteins (20 µg) were divided by 12% SDS-PAGE (Genshare Biological, Shaanxi, China) and transferred onto polyvinylidene fluoride membranes (EMD Millipore, Billerica, MA, USA). Following a 2-h blocking period with TBST buffer (TBS plus 0.1% Tween-20) supplemented with 5% w/v non-fat milk, the membranes were incubated overnight in TXNIP antibody (cat. no. K0205-3, Medical & Biological Laboratories Co., Ltd.) at a 1:1,500 dilution at 4°C. Subsequently, the membranes were incubated with HRP-conjugated goat anti-mouse secondary antibody (cat. no. BS50350, 1:5,000; Bioworld Technology, Inc., St. Louis Park, MN, USA) at room temperature for 1 h. Following washing, proteins were visualized using an ECL detection system (Genshare Biological).

For RT-qPCR, RNA samples were extracted from the cells and 54 glioma tissues. Typically, 1 µg total RNA was used to generate cDNA using SuperScript II RT (Takara Bio, Inc., Otsu, Japan) with an oligo-dT primer. RT-qPCR was performed using the Power SYBR-Green PCR Master mix, as per the manufacturer's instruction (Applied Biosystems; Thermo Fisher Scientific, Inc.). Thermo cycling for all reactions was initiated with a denaturation step at 95°C for 10 min, followed by 40 cycles at 95°C for 5 sec and 60°C for 30 sec. Each assay was performed in triplicate. GAPDH was used as the internal control. SDS 2.2.1 software (Applied Biosystems; Thermo Fisher Scientific, Inc.) was used to perform relative quantification of target genes using the comparative cycle threshold (2^−∆∆Cq) method (12). The primer sequences are described in Table II (13).

Cell proliferation was analyzed in vitro using tetrazolium salt 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). A total of five identical 96-well plates were seeded simultaneously with the four types of cells aforementioned, at a concentration of 2×10³ cells/well in 200 µl DMEM. Each cell type was seeded into seven wells in each plate. After a designated culture time (12, 24, 48, 72 and 96 h), the medium was replaced with 200 µl medium supplemented with 0.5 mg/ml MTT (Sigma-Aldrich; KGaA) reagent and incubated at 37°C for 4 h. Next, the supernatant was extracted, and the cells were lysed in 200 µl dimethyl sulfoxide (Sigma-Aldrich; KGaA) for 10 min at 37°C. The optical density values were measured by recording the absorbance at 490 nm using a plate reader. The obtained values were presented as the fold increase with respect to the control group.

U251-MG cells were seeded in 6-well plates and cultured until they reached confluence. A wound was then created by manually scraping the cell layer with a 200-µl pipette tip. The floating cells were removed by washing with PBS three times. The cells were then incubated in DMEM supplemented with 1% FBS for 24 h. For each group, the number of cells that had migrated into the simulated wound was counted by imaging eight randomly selected microscopic fields at 0, 12 and 24 h after wound creation. Images were acquired with a Nikon DS-5 M Camera System (Nikon Corporation, Tokyo, Japan) mounted on a phase-contrast Leitz microscope (Leica Microsystems GmbH, Wetzlar, Germany) and were processed using Adobe Photoshop 7.0 (Adobe Systems, Inc., San Jose, CA, USA).

Migration and invasion experiments were performed using a QCM 24-Well Cell Invasion assay kit (Cell Biolabs, Inc., San Diego, CA, USA). The top chamber of the Transwell system received 2×10³ cells, plated directly on the polycarbonate Transwell filters (for the Transwell migration assay) or on the Matrigel-coated polycarbonate Transwell filter (for the Transwell matrix-penetration invasion assay). For the Transwell migration assay, the top (containing the cells) and bottom chambers received serum-free medium. However, for the invasion assay, cells in the top chamber were suspended in serum-free medium, whereas the bottom chamber received medium supplemented with 10% FBS, serving as a chemoattractant. The cells were incubated at 37°C for 8 h (for the migration assay) or 16 h (for the invasion assay). The non-migratory or non-invading cells on the upper surface of the polycarbonate membrane in the top chamber were wiped away using cotton swabs. The migrated and invaded cells on the lower surface of the membrane were fixed in 95% ethanol for 5 min, air-dried and stained with 4 g/l crystal violet prior to counting under a microscope. A total of three independent experiments were conducted and the data are presented as the mean ± standard error of the mean (SEM).

All computations were performed using SPSS version 13.0 software, for Windows (SPSS, Inc., Chicago, IL, USA). Data are expressed as the mean ± SEM throughout. Statistical differences between the levels of TXNIP expression in different pathological grades were evaluated with the non-parametric Kruskal-Wallis test, and those differences in the binominal clinical categories were evaluated with the non-parametric Mann-Whitney test. The survival time was calculated according to the Kaplan-Meier method. Differences between values obtained from in vitro experiments of the treatment groups were compared to the untreated control group by paired Student's t-test. P<0.05 was considered to indicate statistical significance.

In the 54 patients with gliomas, IHC analysis revealed high TXNIP expression, localized mainly in the nucleus in 51.9% (28/54) of samples (Fig. 1A), the majority of which were from low-grade glioma patients. The remaining 48.1% (26/54) of samples demonstrated lower TXNIP expression, which was found to be associated with a higher histopathological glioma grade (P<0.01).

To validate the specificity of the immunohistochemical results, RT-qPCR analysis was performed. The primers are listed in Table II. TXNIP was strongly expressed in low-grade glioma samples, but was not detected or was only weakly detected in high-grade gliomas, particularly in glioblastoma (grade IV) (Fig. 1B). These results were consistent with those of the IHC analysis.

To analyze the overall survival time, the patients were followed up for a total of 40 months from the initiation of the study, during which 52/54 patients succumbed to glioma. The Kaplan-Meier analysis showed a strong association of TXNIP expression with the overall survival of glioma patients (Fig. 1C); high expression was associated with increased overall survival time.

U251-MG cells were transfected with one of the TXNIP shRNA lentiviruses (shRNA1-3) or the scrambled shRNA lentivirus. The results of the western blot analysis (Fig. 2A) and RT-qPCR (Fig. 2B) revealed that transfection of shRNA2 and shRNA3 was more efficient than shRNA1, and that shRNA3 transfection was more efficient (P<0.01) than shRNA2 transfection (P<0.05).

Several studies have shown that TXNIP is strongly downregulated in several types of clinical tumor samples and tumor cell lines (10,14–16). It was hypothesized that the downregulation of TXNIP would induce a more aggressive behavior in cultured glioma cells. To test this hypothesis, the glioma U251-MG cell line was infected with a lentivirus encoding an shRNA sequence targeted at TXNIP. First, an MTT assay was used to examine the effect of TXNIP on cell proliferation. The cell proliferation was increased in the TXNIP shRNA group compared with the control groups at 24 h after transfection, indicating that downregulation of TXNIP could significantly promote the proliferation of glioma cells (Fig. 3A). A wound-healing assay was then used to examine the effect of TXNIP on cell migration. Compared with the LV-NC cells, the TXNIP shRNA cells exhibited rapid migration (Fig. 3B). Furthermore, Transwell assays revealed that the downregulation of TXNIP significantly enhanced the invasion (Fig. 3C) and migration (Fig. 3D) of glioma cells.