RETRACTED ARTICLE: STMN-1 is a potential marker of lymph node metastasis in distal esophageal adenocarcinomas and silencing its expression can reverse malignant phenotype of tumor cells

Patient characteristics

STMN-1 expression in tumor samples

STMN-1 protein expression was analyzed in the distal esophageal adenocarcinomas tissue in 63 patients. Western blot analysis of samples showed that STMN-1 was up regulated in 31 (49.21%) of the 63 cases. Twenty one patients (33.33%) with pN1 and pN2 were found positive for STMN-1 and ten patients (15.87%) with pN0 also showed overexpression of STMN-1 (Table 1).

STMN-1 status and its correlation with patient clinicopathological characteristics

Next, we investigated the association between STMN-1 expression and the clinicopathological characteristics of distal esophageal adenocarcinoma cases and found that STMN-1 overexpression was significantly associated with pN classification (P = 0.001). The expression of the STMN-1 protein in distal esophageal adenocarcinoma in the pN1-pN2 stage was significantly higher than in the pN0 stage and in the T3-T4a stage, it was significantly higher than in the T1-T2 stage. However, no significant correlation was found between STMN-1 expression and gender, age, or degree of differentiation (Table 1).

STMN-1 expression positively correlates with pN stage in adenocarcinoma of the distal esophagus

STMN-1 expression in esophageal adenocarcinoma cell lines

In the current study, we evaluated the state of STMN-1 protein expression in tissue samples by Western blot analysis, (Figure 1A).

We further assessed the level of STMN-1 protein expression in esophageal adenocarcinoma derived cell lines by immunocytochemistry (Figure 1B, C) and immunofluorescence analysis (Figure 1D, E). Strong immunoreactivity of STMN-1 protein was detected in the cytoplasm of esophageal adenocarcinoma cells.

Transfection & transduction efficiency

Successful packing of lentivirus was done in HEK 293 T. STMN-1 shRNA transfected cells and control shown in (Figure 2A, B). To determines the efficacies of viral vectors, viral supernatants prepared from either non-silencing-shRNA (control) or STMN-1 shRNA were added to esophageal adenocarcinoma cells. GFP expression in photomicrographs of STMN-1-shRNA cells and control shown in (Figure 2C, D, E, and F) confirmed > 90% transduction efficiency. To evaluate the silencing efficiency qRT-PCR and western blotting analysis were performed.

Lentivirus-mediated RNAi efficiently suppressed STMN-1 expression in esophageal adenocarcinoma cells

To investigate the role of STMN-1 in esophageal adenocarcinoma cell’s growth and metastasis, we constructed lentivirus vector with STMN-1 shRNA and infected esophageal adenocarcinoma cells. Stable expression of STMN-1 shRNAs specifically knocks down STMN-1 expression and activity in esophageal adenocarcinoma cell. After viral infection, more than 80% of the cells were GFP-positive, indicating a high efficiency of shRNA delivery (Figure 2C, D). The results of the western blot showed that the average band intensities of STMN-1 normalized to actin (Figure 3A).

Stable knockdown of oncogenic STMN-1 by lenti-shRNA significantly inhibited esophageal adenocarcinoma cell’s growth in vitro

To investigate the possible anti-proliferative effects of STMN-1 knockdown in vitro, a CCK-8 assay was performed and a cell growth curve was generated. STMN-1 shRNA transduced esophageal adenocarcinoma cells showed significantly reduced viability relative to Non-silencing control-transduced cells. STMN-1 knockdown inhibited the proliferation of esophageal adenocarcinoma cells in vitro, indicating that the differential STMN-1 gene expression affects the growth of esophageal adenocarcinoma cells (Figure 4A).

Stable knockdown of STMN-1 expression inhibits migration, invasion and metastasis of esophageal adenocarcinoma cells

Cell migration was evaluated in the Boyden migration assay two days after esophageal adenocarcinoma cells were transfected with Non-silencing shRNA infected cells or STMN1shRNA. STMN-1 shRNA transfected inhibited migration (Figure 5A, B, C, D). To evaluate the function of STMN-1 on Esophageal adenocarcinoma cell invasion, Matrigel invasion chambers were utilized. Inhibited STMN-1 expression led to a significantly decreased invasive ability of esophageal adenocarcinoma cells (Figure 5E).

Knockdown of STMN-1 expression induced cell cycle arrest at the G1/S phase

The Flow cytometery results demonstrated that, STMN-1 shRNA treated cells induce an accumulation of esophageal adenocarcinoma cells in the G1/S phase of the cell cycle (Figure 4C, D). In the control group 48.6% of cells were in the G1 phase and 46% of the cells were in S of the cell cycle, and 24 post transfection, cells accumulated in G1 54.4% but reduced to 40.5% in S phase. These results demonstrate that knockdown of STMN-1 induces G1/S phase arrest in esophageal adenocarcinoma cells.

Knockdown of STMN-1 expression induced esophageal adenocarcinoma cells apoptosis

As shown in Figure 4B, the percentage of apoptotic cells infected with STMN-1 shRNA was much higher than that in control shRNA group (P < 0.01). No significant difference was found between control shRNA infected cells and non-infected cells. These data indicated that knockdown of STMN-1 expression induced apoptosis in the esophageal adenocarcinoma cells.

In vivo studies of esophageal adenocarcinoma xenograft tumor models in nude mice

To further evaluate the effects of reduced STMN-1 expression on the tumorigenic phenotype and in particular its contribution to in vivo tumor growth. Esophageal adenocarcinoma cells infected with non-silencing shRNA and STMN1 shRNA or untreated were injected into mice, (Figure 6C, D). STMN1 shRNA transfected grew rapidly as compared to negative control. These results demonstrate that in vivo tumor growth was inhibited by shRNA-mediated knockdown of STMN-1 expression in esophageal adenocarcinoma cells.

Ethics statement

The study protocol was approved by the Ethics Committee of Provincial Hospital Affiliated to Shandong University. All the participants provided their written informed consent for inclusion in the data analysis and manuscript publication. Data were analyzed anonymously.

Patient selection and distal esophageal adenocarcinoma specimen

From June 2010 to June 2012, sixty-three eligible patients with previously untreated, potentially resectable, clinical stage I-III distal esophageal adenocarcinomas were admitted to the Department of Thoracic Surgery, Provincial Hospital Affiliated to Shandong University, China. These patients were all diagnosed with adenocarcinoma of the distal esophagus. Complete clinical data, along with their medical histories was kept in separate files for each patient enrolled in the study. None of the patients had received chemotherapy or radiotherapy prior to surgery. Informed consent to obtain tissue samples for the study was taken from each patient. Tissue samples were obtained at the time of surgical resection. There were forty-five men and eighteen women, and the median age was fifty-seven. The age range was thirty-four to seventy-nine years old. To define the stage of adenocarcinoma of the distal esophagus, we used TNM classification according to the 2009 UICC standard [4].

Mice

BALB/C-nu/nu male mice (weight 18–22 g, 6 weeks old) were obtained from the Shanghai Tumor Institution.

All experimental procedures using animals in the present study had received prior approval by the Institutional Animal Care and Use Committee of Shandong University under Contract 2011–0015.

Cell lines, culture medium and reagents

Human Embryonic Kidney 293 cells (HEK93T) were obtained from the Shanghai Tumor Institution. Human esophageal adenocarcinoma cells were kindly provided by the pathology department of provincial hospital. Classical Liquid Media Dulbecco’s Modified Eagles Medium (DMEM), High Glucose RPMI Media 1640 was purchased from HyClone (Thermo Scientific). Fetal bovine serum (FBS) was purchased from Gibco (Invitrogen Co., USA). Dimethyl sulfoxide (DMSO) was purchased from Sigma-Aldrich (St Louis, Missouri, USA). Rabbit Anti-STMN1 Polyclonal antibody and HRP-conjugated secondary antibodies were purchased from Abcam (Cambridge, MA).

Operation method

Patients were operated through the 7th intercostal space with left posterolateral thoracotomy. The lower esophagus was mobilized and periesophageal lymph nodes were cleared. Thoracoabdominal two-field lymph node dissection was undertaken. An incision was made through the diaphragm to enter the abdominal cavity. The greater and lesser curvatures were mobilized. We routinely removed perigastric lymph nodes, including left gastric artery node (Figure 6A). For the nodal stations around the celiac trifurcation, we performed selective dissection when the swollen lymph nodes were found or touched, and sometimes we only performed lymph node sampling when no obviously swollen one was found (Figure 6B). From the upper and lower margin of the tumor more than 5 cm esophageal and gastric tissue were resected and then Gastro-esophageal anastomosis was made mechanically at the level of the inferior pulmonary vein. The average age in this group was fifty-seven. This operation was performed on forty-five males and eighteen females. The average lymph node clearance was 21.5% per patient.

Cell culture

Esophageal adenocarcinoma cells were grown in RPMI 1640 medium supplemented with 10% fetal bovine serum (FBS) and 100 units/ml penicillin, and 100 mg/ml streptomycin. The cells were grown at 37°C in a humidified atmosphere containing 5% CO2. Stock cultures of each cell line were routinely sub-cultured at least once a week and the medium was changed every 2–3 days.

Lentivirus-mediated short hairpin RNA (shRNA) knockdown of gene expression

To knockdown STMN-1 expression, pGIPZ-lentiviral shRNAmir vectors targeting human STMN1 and Non-silencing pGIPZ control vector were purchased from Open Biosystems (Thermo Fisher Scientific, Inc.). The sequences of STMN-1 shRNA are as following: 5′-TTATTAGCTTCCATTTTGT-3′; 5′-TCTCTTCTATTGCCTTCTG-3′ and 5′-TTATTAACCATTCAAGTCC-3′. Lentiviral shRNA was produced by Co-transfection of the Trans-Lentiviral packaging mix with a shRNA transfer vector into HEK293T packaging cells (OpenBiosystems). For cell infection, viral supernatants were supplemented with 6 μg/mL polybrene and incubated with cells for 24 hours. Esophageal adenocarcinoma cell lines were transduced by the lentiviral particles followed by puromycin selection (1 μg/mL) for 10 days. The cells stably expressing shRNA were maintained in puromycin (0.2 μg/mL).

RNA extraction and qRT-PCR

Total RNA extraction was performed using Trizol reagent (Invitrogen) according to the manufacturer’s instruction. RNA concentration was measured by Nano Drop 1000 (Thermo Fisher Scientific). One microgram of total RNA extracted from the cells was subjected to reverse Transcription (RT). Verso cDNA Ki (Thermo Scientific) was used for cDNA synthesis. Real-time RT-PCR was used to quantify the expression level of STMN-1 gene in Esophageal adenocarcinoma cell line using ABI 7300 real-time PCR thermal cycle instrument (ABI, USA), according to the supplied protocol. Amplification conditions were as follows: Reverse-transcription reaction: 42°C, 30 minutes per cycle. PCR cycling conditions were as follows: Enzyme activation 95°C 15 minutes per cycle, denaturation 95°C at 15 seconds per 40 cycles and Annealing/Extension at 60°C for 60 seconds.

A Real-time PCR reaction was performed using the Solaris qPCR Gene Expression Master Mix with LOW ROX premixed and 1 μL of total cDNA in each well, Stathmin specific primers were as follows:

(F, AGAATACACTGCCTGTCGCTTG; R, AGGCACGCTTCTCCAGTT). The relative expression levels were normalized to expression of endogenous Beta-Actin. (Primers: F, TGGAGAAAATCTGGCACCAC; R, GGTCTCAAACATGATCTGG).

Protein extraction and western blotting

For whole-cell protein extraction, esophageal adenocarcinoma cell line were washed with cold PBS and subsequently lysed in cold RIPA lysis buffer (50 mM Tris–HCl, pH 7.4, 150 mM NaCl, 1 mM dithiothreitol [DTT], 0.25% sodium deoxycholate, 0.1% NP-40) containing 1 mM phenylmethysulfonyl fluoride (PMSF), 50 mM sodiumpyrophosphate, 1 mM Na3VO4, 1 mM NaF, 5 mM EDTA, 5 mM EGTA, and protease inhibitors cocktail (Roche). Cell lysis was performed on ice for 30 minutes. Clear protein extracts were obtained by centrifugation for 30 minutes at 4°C. Protein concentrations were determined by the method of Bradford using the Bio-Rad protein assay reagent (Bio-Rad) and 20–40 mg of protein mixed with loading buffer was loaded per lane, separated by 12% SDS-polyacrylamide gel electrophoresis (SDS-PAGE). Proteins were transferred to PVDF membrane filters (Millipore, USA). Nonspecific binding was blocked by incubation in phosphate-buffered saline (PBS) containing 0.1% Tween 20 (PBS-T) and 5% skim milk. PVDF membranes were blocked with 5% dry milk for one hour at 4°C Membranes were incubated in STMN-1 primary antibody (1:1000) overnight at 4°C. The membranes were then incubated with the corresponding secondary antibody (1:2000, horseradish peroxidase-conjugated anti-rabbit) in TBST-5% nonfat milk for 1 hour at room temperature and the immunoreactive bands were visualized using EZ ECL Chemiluminescence Detection Kit for HRP (Biological Industries Ltd, Israel). Images were acquired using the LAS3000 Imager (Fujifilm). Membranes were re-probed for Beta-Actin as a loading control.

Cell proliferation assay

Cell Counting Kit-8 (CCK-8; Dojindo) was used in cell proliferation assay. 3000 viable cells per well into 96-well tissue culture plates in a final volume of 100 μl. Every 24 hours, a plate was subjected to assay by adding 10 μl of CCK-8 solution to each well, and the plate was further incubated for 4 hours at 37°C. The absorbance at 450 nm was measured with a micro plate reader. The experiment was performed in eight replicates.

Adhesion assay

Cell adhesion was analyzed using a Vybrant Cell Adhesion Assay kit (Molecular Probes, USA) was used according to the manufacter’s instructions. Briefly, cells in suspension were labeled with Calcein AM dye, and equal numbers of cells were plated in 96-well plates. At timed intervals, non-adherent cells were removed and adherent cells were quantified with the use of a SAFIRE fluorescence plate reader.

Migration and invasion assays

For trans-well migration assay, 50,000 cells were added to upper chamber in serum free media and migration at 37°C towards 10% FBS containing growth media was determined either after 24 hours or 48 hrs. Cells migrated through the membrane were fixed, stained with H&E (Sigma) and counted under light microscope. For invasion assay, lower chambers of matrigel coated invasion plates were coated with 10 mg/ml fibronectin overnight at 4°C and cells invading through matrigel were fixed and stained after 48 hours.

TUNEL assay

Terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (In Situ Cell Death Detection Kit POD) was used for proving apoptosis in individual cells. The TUNEL working procedure was carried out following the producer’s directions (Roche). Endogenous peroxidase was blocked by incubation in 1.3% H2O2 in PBS for 10 min at room temperature before enzymatic labeling. During the TUNEL procedure samples were washed in PBS. The fluorescent signal conversion using anti-fluorescence antibody conjugated with peroxidase and substrate color reaction applying chromogen DAB (3,3′-diaminobenzidine tetrahydrochloride; Sigma-Aldrich, Germany) were proceeded after enzymatic labeling.

Flow cytometric cell cycle analysis

For cell cycle analysis esophageal adenocarcinoma cells were collected in twenty-four post transfection, 6 cm plates. Cells were harvested using cold PBS and fixed in 70% cold ethanol overnight in 4°C and treated with 1 ng/ml RNase A for 10 minutes at 37°C. Cellular DNA was stained with 15 ng/ml propidium iodide (PI) for 30 minutes at 37°C in the dark. The cells then were sorted by FACS Calibur Flow Cytometer (Becton Dickinson, CA) and cell-cycle profiles were determined using the ModFitLT software (Becton Dickinson, SanDiego, CA). The experiments were repeated on two separate occasions.

Immunocytochemistry

Stably transfected esophageal adenocarcinoma cells were seeded into 4-chambered glass slides (Nunc Lab-Tek Chamber Slide System). Cells were then incubated overnight. After 24 hours, cells were rinsed with PBS, fixed with 3.7% w/v paraformaldehyde (Sigma), rinsed with PBS, and permeablized in 0.5% Triton X-100 (Sigma). Nonspecific immunoglobulin binding was blocked with 5% normal goat serum and 0.5% NP-40 (Sigma). Primary antibodies recognizing with STMN-1 (abicam) were diluted 1:100 in blocking solution. After incubation with primary antibody, cells were rinsed with 0.05% Tween-20 (Bio-Rad) in PBS, and then incubated with secondary antibody for 1 h at RT. Stain with 3,3′-Diaminobenzidine (DAB) and observed under light microscope.

Immunofluorescence

To examine the protein expression of STMN-1 gene, we performed immunofluorescence analysis. Briefly, the cells were washed with PBS and fixed in 4% paraformaldehyde for 10 minutes at 37°C followed by absolute methanol for 10 minutes at 4°C and blocked in PBS containing 1% skimmed milk for 10 minutes. The samples were then incubated with affinity-purified Rabbit Anti-Human STMN-1 Polyclonal Antibody (Abcam, USA) at a dilution of 1: 100 for 2 hours, rinsed twice with PBS, and incubated with goat anti-rabbit IgG Alexa Fluor 488 (Molecular Probes) for 1 hour.

In vivo studies of esophageal adenocarcinoma cell line xenograft tumor models in nude mice

Six-week-old male BALB/c nude mice were housed in a temperature-controlled, pathogen-free animal facility with 12-hour light and dark cycles. Mice were injected subcutaneously into bilateral flanks with untransfected cells, or transfected with Non-silencing shRNA and STMNEshRNA (2 × 10⁶ cells in 200 μl PBS) to establish tumors. Tumor mass (xenograft) volume was measured every week from week 3 to week 7. After 7 week mice were sacrificed, and tumors were harvested.

Statistical analysis

For comparison of more than three groups, we used one-way analysis of variance, followed by Tukey’s multiple comparison-values < 0.05 were considered statistically significant. One-way analysis of variance (ANOVA), followed by the LSD post hoc test was used to compare mean differences in 2 or more groups. Associations between categorical variables were evaluated by Pearson’s chi-square test. P values represent 2- sided tests and are of statistical significance when P < 0.05. These statistically significant variables were found in the univariate analysis (P, 0.05), and the Logistics Regression Model. Similar ratio statistics were employed to further evaluate them for multivariate. All statistical analysis was performed by using IBM SPSS version 20.0.