Immunohistochemistry in a tissue microarray
A commercial uterine cervical tissue microarray (Alenabio, China), consisting of 40 cases of cancer tissues and 40 of non-cancerous tissues, was used to evaluate CXCL1 protein expression. Of the cancer tissues, 23 patients aged from 30 to 50, and 17 aged from 51 to 70. The patients were classified into 3 clinical stages, comprising 4 cases of stage I, 17 of stage II and 19 of stage III. Moreover, tumor infiltration was distinguished with scores increasing from T1 to T3, including 34 with grade 1, 5 with grade 2 and 1 with grade 3. Immunohistochemical staining was conducted as described previously [16]. Protein expression was determined using a primary antibody targeting CXCL1 (1:100; Abcam, USA), which was further matched with a HRP-linked second antibody (Boster Biological Technology, China) at the dilution of 1:200. Immunoreactivity was graded according to the following two parameters, staining intensity and percentage of positive cells. Staining intensity was scored with a value of 1 (weak), 2 (moderate), 3 (strong) and 4 (super strong), and percentage of positive cells was scored with a value of 1 (< 25%), 2 (25-50%), 3 (50-75%) and 4 (>75%). The total score for CXCL1 staining was calculated as the sum of the score of both parameters.
Cell lines, cell culture and cell transfection
HeLa human cervical cancer cells and PHM1-41 human immortalized cervical stromal cells were obtained from American type culture collection (ATCC, USA). HeLa and PHM1-41 were, respectively, cultured in RPMI‑1640 and Dulbecco’s Modified Eagle’s Medium (DMEM; Gibco, USA), which were supplemented with 10% fetal bovine serum (FBS; Gibco, USA) containing antibiotics (100 units/ml penicillin and 100 mg/ml streptomycin) in a 37°C humidified incubator with 5% CO2. To overexpress CXCL1 gene in HeLa and PHM1-41 cells, full-length cDNA of human CXCL1 was subcloned into pcDNA3.1 vectors and the specific vectors targeting cells were ransfected using a lipofectamine3000 kit (Invitrogen, USA) according to the supplier's instructions. Finally, the transfection efficiency of CXCL1 in HeLa and PHM1-41 cells were tested using ELISA assays.
ELISA analysis
The culture supernatant derived from cell medium was collected and CXCL1 secretory protein was determined using a Human CXCL1 ELISA Kit (Boster Biological Technology, China) following the manufacturer's protocol. Briefly, a total of 100 μl of culture supernatant as well as different concentrations of recombinant CXCL1 standard protein was added to each well followed by incubating the plate at 37 °C for 90 min. Following triple rinsing with washing buffer, the plate was coated with 100 μl biotin-conjugated primary antibody against CXCL1 prior to incubation at 37 °C for 60 min. After rinsing three times, the plate was coated with 100 μl of working fluid and further incubated at 37 °C for 30 min. Upon the formation of immunocomplex, the plate was washed five times, and 100 μl of TMB developer solution was placed into each well for colorimetric determination. After reaction at 37 °C for 20 min, absorbance of each well was immediately measured at 450 nm using a spectrophotometer (BioTek Instruments, USA). CXCL1 protein level in culture supernatant was quantified and analyzed based on a standard curve constructed by the absorbance of CXCL1 standard protein.
Cell proliferation detection
Three separate experiments were employed to evaluate HeLa cell viability. (I) Exogenous effect: HeLa cells at a density of 2x103 per well were seed into each well of 96-well plates supplemented with 100 µl of medium containing various concentrations of human recombinant CXCL1 (PeproTech, USA). (II) Autocrine effect: The same number of HeLa cells overexpressing CXCL1 and their mock controls as described in experiment I were grown in 96-well plates, 100 µl of medium supplemented with or without the ERK1/2 inhibitor PD98059 (Sigma, USA) was added into each well. (III) Paracrine effect: Firstly, the conditioned medium (CM) was prepared as follows: CXCL1-overexpressing PHM1-41 cells and their mock controls were inoculated in 100-mm diameter dishes at a density of 5x106 cells per 1000 µl of medium. After incubation at 37˚C for 12 h, the culture supernatant was harvested by centrifugation, and CM from PHM1-41 cells was produced by addition of different proportions of the supernatant into complete medium. In the paracrine experiment, HeLa cells at a density of 2x103 per well were seeded into 96-well plates, and each well was incubated with 100 µl of various proportions of CM. The cells in the above three groups were incubated at 37˚C for 24 h, 48 h and 72 h, cell viability was thereafter addressed using the Cell Counting Kit-8 (CCK-8) assay.
Cell migration assay
Consistent with proliferation assay, three experiments for migration assay were as follows. (I) Exogenous effect: The same number of HeLa cells at a density of 2x103 were seeded in the top chamber in 100 μl serum-free medium, the low chamber was coated with 600 μl serum-supplemented medium supplemented with various concentrations of human recombinant CXCL1. (II) Autocrine effect: 2x104 of CXCL1-overexpressing HeLa cells and their mock controls in 100 μl serum-free medium containing with or without the ERK inhibitor PD98059 were seeded in top chamber, whereas 600 μl medium with 10% FBS was placed in the low chamber for chemoattraction. (III) Paracrine effect: HeLa cells were maintained in the upper chamber as described in experiment I, and 600 μl serum-supplemented medium containing various proportions of CM was placed in the low chamber. 24 h after migration, the cells that had attached on the bottom surface of the transwell filters were fixed and stained with methanol/crystal violet solution.
Cell apoptosis assay
Cell apoptosis was determined by using a commercial apoptotic-Hoechst Staining Kit (Beyotime, Shanghai, China) according to the manufacturer' instructions. Following detection with a confocal microscope at 350 nm, cancer cell apoptosis rate was estimated by the following formula: Apoptosis rate (%) = apoptosis cell numbers/ total cell numbers x 100%.
Western blotting analysis
Total cell lysates were prepared from cell pellets using RIPA buffer (Solarbio, China) following the manufacturer's protocol. After quantification of protein concentration, a total of 40 µg of protein extracts was subjected to western blotting analysis. Different primary antibodies, including anti-p-ERK (1:1,000 dilution; ImmunoWay, USA), anti- ERK (1:1,000 dilution; ImmunoWay, USA), anti-cyclin D1 (1:5,000 dilution; Abcam, USA), anti-Bax (1:1,000; OriGene, China) and anti-β‑actin (1:1,000 dilution; Boster, China) antibodies, were utilized in the analysis, followed by incubation with various HRP-conjugated second antibodies (Boster Biological Technology, China). Immune complexes were visualized using an Enhanced Chemiluminescence (ECL) kit (Thermo Fisher Scientific, USA), and the band intensities were calculated using a Gel imaging system (Tanon, China).
Statistical analysis
All assays were done at least three times, SPSS 20.0 software was applied for statistical analyses. All values were expressed as mean value with their standard deviation, statistical difference between two groups was determined by analysis of variance followed by the Student's t-test. Statistically significant was defined as a P-value < 0.05.