Cells with the ability to grow in 0.5 μg/mL of cisplatin were obtained 4 months after the initial drug exposure, named as U251R. Cell viability Cell lines were seeded into 96-well plates at a density of 5 × 103 cells/100 μL medium per well. After check details adherence, cells
were treated with various concentrations of cisplatin for 48 h, with DMSO as negative controls. At the end of treatment, the tetrazolium compound, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT, Sigma) was added and then incubated for additional 4 h at 37°C in the dark. The formazan crystals were dissolved by DMSO, and the absorbance was recorded using an ELISA plate reader. Plasmid construction Cyclin D1 shRNA (cyclin-sh) and negative scramble shRNA (SCR) were inserted into pGPHI vector. The primers were as follows: For cyclin-sh, forward primer 5-CACCGATCGTCGCCACCTGGATGTTCAAGAGACATCCAGGTGGCGACGATCTTTTTTG-3, and reverse primer 5-GATCCAAAAAAGATCGTCGCCACCTGGATGTCTCTTGAACATCCAGGTGGCGACGATC-3; for SCR, forward primer 5-CACCGTTCTCCGAACGTGTCACGTCAAGAGATTACGTGACACGTTCGGAGAATTTTTTG-3, and reverse primer 5-GATCCAAAAAA TTCTCCGAACGTGTCACGTAATCTCTTGACGTGACACGTTCGGAGAAC-3. Cyclin D1 3’-UTR sequence was cloned into pGL3-Luc vector. The primers were as follows: forward primer 5-GCTCTAGAGCTGACTCCAAATCTCAATGAAGCCA-3, and reverse primer 5-GCTCTAGAGCTAACCAGAAATGCACAGACCCAG-3. click here MiRNA microarray analysis
Total RNA was extracted from each cell line using TRIzol reagent (Invitrogen) according to the manufacturer’s instructions. The RNA samples were submitted to KangChen Bio-tech (Shanghai, China), then labeled with Hy3™ fluorescent dye for hybridization on a miRCURY™ LNA microRNA array (Exiqon, Vedbaek, Denmark). Expression levels of selected miRNAs differed by at least 2-fold between cisplatin-resistant U251R cell line and parental U251 cell line. Immunoblot analysis Cell almost lysates were loaded onto 10% SDS–polyacrylamide gels, electrophoresed and transferred to PVDF membranes (Millipore, Billerica, MA,
USA). Membranes were blocked in TBS-Tween-20 containing 5% non-fat milk at room temperature for 1 h and then incubated with primary antibodies at 4°C overnight. On the second day, the blots were incubated with HRP-linked secondary antibodies at room temperature for 1 h. After three times’ wash in TBST buffer, the blots were visualized by ECL Reagent (Cell Signaling Technology) as previously described . Luciferase reporter assay This assay was performed as previously described . Briefly, cells were seeded in a 24-well plate and transfected with miRNA mimics expression vectors, additional pGL3-Luc/cyclin D1-3’-UTR plasmid, and pRL-TK plasmid. Twenty-four hours after transfection, cells were lysed and then luciferase activities were measured according to the manufacturer’s protocol (Promega, Madison, WI, USA). Each sample’s luciferase activity was normalized to that of renilla.