Cell lines and reagents
All cell lines were purchased from the American Type Culture Collection (Manassas, Virginia, USA). SGC-7901 cells were maintained in DMEM (Hyclone, Thermo Fisher Scientific, Florence, KY, USA) with 10% FBS and 100 U/ml penicillin-streptomycin. Dihydroartemisinin (DHA) was obtained from Chengdu MUST Bio-Technology., LTD.
Cell viability assays
A Cell Counting Kit 8 (CCK-8, Dalian Meliun Biotech Co., Ltd., Dalian, China) assay was performed to measure cell viability and proliferation. Briefly, cells were seeded in 96-well plates at a concentration of 5000 cells/well and incubated in a final volume of 200 ml culture medium. Cells were treated with different concentrations of Dihydroartemisinin (0-160 mg/ml) or other compounds for 48 h to measure the IC50. Cell viability was counted via the addition of CCK-8 reagent.
Colony formation assays
Cells were seeded into a 6-well plate in DMEM medium at a density of 1000 cells and cultured in a final volume of 2 ml culture medium containing DHA (0, 2, or 4 mg/ml) for 9-14 days until clones were visible to the naked eye. Colonies that contained > 30 stained cells were classified as clones. After washing, cells were fixed with 100% methanol at 4℃ for 10 min and stained with 0.05% crystal violet for 15 minutes at room temperature, and then washed twice with ddH2O and air-dried.
Cell apoptosis assay
Cells were seeded into a 6-well plate with 7×105 cells/well in DMEM medium for 24 h, followed by treatment with DHA (0, 2.5, 5, 10 mg/ml) for 48h. After treatment, the numbers of cells were adjusted approximately equal in each group. The cells were washed with PBS and suspended in a solution containing 5 mL Annexin-V FITC and 5 mL PI dye (BD, San Jose, CA, USA). Following incubation in the dark for 15 min, cell apoptosis was analyzed by using Accuri C6 flow cytometry (BD, San Jose, CA, USA).
Hoechst 33258 staining assay
An apoptotic Hoechst Staining Kit (Beyotime, Shanghai, China) was employed to detect cell apoptosis. Cells were seeded in 12-well plates with a coverslip at a density of 1.0 × 105 cells per well and incubated for 48 hours. Briefly, after fixation, the cells were rinsed with PBS and stained with Hoechst 33258. Cell apoptosis was detected at 350 nm by a confocal microscope, and the tumor cell apoptotic rate was calculated.
Total RNA isolation
Total RNA was extracted from the cultured cells (7×105 cells) and lysed in 1 mL Trizol reagent. 200 mL chloroform was added to separate the RNA at the aqueous phase, and 500 mL isopropanol was then applied. Finally, the RNA was centrifuged at 14,000 rpm (4℃) for 15 min and the RNA was dissolved in DEPC-MQ H2O. The concentration was measured with a Nanodrop 144 spectrometer N1000 (Thermo Fisher Scientific, Inc.).
Real-time qPCR
The extracted RNA was reverse-transcribed to cDNA using the Revert Aid First Strand cDNA Synthesis kit (Thermo Fisher Scientific, Inc.). The cDNA obtained via reverse transcription was diluted 10-fold and prepared according to the SYBR Green Reagent system for real-time fluorescence quantitative assay (Vazyme Biotech co. Ltd., Nanjing, China). Each qPCR reaction was repeated three times, and the expression of the target genes was normalized to the internal reference GAPDH. The thermal cycling of qPCR was as follows: 95°C for 2 min; 40 cycles of 95°C for 15 s and 60°C for 30 s. The mRNA expression of the target genes was analyzed using the 2-ΔΔCT calculation method. The primers are listed in the Supplementary Information.
Protein extraction and Western blot
SGC-7901 cells were collected with RIAP lysis buffer containing protease inhibitors. The cell lysates were centrifuged and the supernatants were collected. The protein concentration was determined using a BCA Protein Assay kit (Beyotime, China). The protein samples were mixed with sample loading buffer followed by boiling for 10 min, and subjected to SDS-PAGE before transferred onto polyvinylidene fluoride (PVDF) membranes. The membranes were incubated with primary antibodies (Supplementary Table 1) and HRP-conjugated secondary antibodies. Western Lighting Chemiluminescence Reagent Plus (Thermo Fisher Scientific, Inc.) and an Image Quant LAS 4000 biomolecular imager were used to visualize the expression levels.
Human Apoptosis Array
To evaluate the relative expressions of 35 apoptosis-related proteins, we used the Human Apoptosis Array Kit (R&D Systems, Minneapolis, MN, USA). Briefly, nitrocellulose membranes were blocked with an array buffer for 1 h at RT. Protein lysates were diluted and incubated overnight. After washed with 1× wash buffer to remove unbound proteins, membranes were exposed to a cocktail of biotinylated detection antibodies for 1 h at RT. Membranes were washed and incubated with streptavidin-HRP for 30 min at RT. Each capture spot corresponding to the amount of apoptotic bound protein was detected with enhanced chemiluminescence western blotting luminol reagent and visualized by X-ray film.
Statistical analysis
All experiments were performed at least three times. All data are presented as the mean ± standard deviation. Statistical analyses were conducted with Microsoft Excel 2010 Professional Plus (Version 14.0.7237.5000) and GraphPad Prism version 6.00 for Windows (GraphPad Software, San Diego, CA). Differences between two groups with similar variance were compared using Student’s t-tests. For all tests, a p<0.05 was considered to indicate a statistically significant difference.