Human subjects
We obtained 12 pairs of PTC and adjacent normal thyroid tissue from thyroidectomy conducted at the Luhe Hospital Capital Medical University. All these tissues were embedded in paraffin wax. In addition, 10 fine needle aspiration thyroid samples (6 cases of benign and 4 cases of malignancy) were collected for RNA-Seq [17]. The basic and pathological characteristics of PTC patients were extracted from medical records. Tumor staging was determined using the 7th edition of the American Joint Committee on Cancer Tumor-Node-Metastasis (AJCC-TNM) staging system. Thyroid tissue and cancer samples were from Center for Endocrine Metabolism and Immune Diseases, Beijing Luhe Hospital, Capital Medical University. All patients included in the protocol signed a declaration of informed consent. The research was approved by the Research Ethics Board of Luhe Hospital Capital Medical University and was carried out according to the World Medical Association Declaration of Helsinki.
In addition, mRNA expression data (RNA Seq v2) and clinical information for patients in The Cancer Genome Atlas thyroid cancer data set were downloaded from https://www.synapse.org and cBioPortal database (http://www.cbioportal.org), respectively, and used for analysis of differential mRNA expression and clinical prognosis. Moreover, the GEO dataset GSE3467, which consisted of 8 paired thyroid cancer and adjacent thyroid tissues, was used for analysis of differential mRNA expression.
Cell culture and transfection
Papillary thyroid cancer cell lines TPC-1 and BCPAP cells were purchased from the National Infrastructure of Cell Line Resource (Beijing, China). Nthy-ori-3-1 cells was kindly provided by Professor. Yang Yan. TPC-1, BCPAP, and Nthy-ori-3-1 cells were cultured in Roswell Park Memorial Institute (RPMI) 1640 medium (Gibco, Cleveland, TN, USA), with 10% fetal bovine serum (FBS) (Gibco, Cleveland, TN) and 1% penicillin/streptomycin in a 37 °C/5% CO2 incubator. Lipofectamine 2000 (Invitrogen, USA) was used to transfect small interfering RNA into TPC-1 cells. Forty-eight hours after transfection, the cells were collected and analyzed by western blot. The small interfering RNA was synthetized in Sangon Biotech (Shanghai, China). The small interfering RNA sequences were: siNC:UUC UCC GAA CGU GUC ACG UTT; siSGLT2 1#:CGACAAAUACCUGGGAGCAAUTT; si SGLT2 2#:ACCAUGAUUUACACGGUGACATT.
Proliferation assay
A Cell Counting Kit-8 (CCK8, Dojindo, Kumamoto, Japan) assay was used to assess cell proliferation rate. Cells were seeded at a density of 2000 cells/well into 96-well plates. The cells attached to the plates after 4 h incubation and were considered as 0 time point. The viable cells assessed by CCK8 assay using an Enspire microplate reader (Perkin Elmer, Waltham, MA, USA) at 450 nm.
Colony formation assay
Cells were digested into a single cell suspension and seeded in 6-well plates (800 cells per well). After incubation for 14 days, cells were stained with crystal viole and photographed.
Cell cycle
Cell cycles were examined by flow cytometry (FACScanto II, BD Biosciences, San Jose, CA, USA). Cells were fixed for overnight in 70% ethanol at 4 °C, and then incubated with propidium iodide and RNAase for 30 min at 37 °C before flow cytometry. ModFit software was used to analyze the data.
Glucose uptake assay
The glucose uptake rate was evaluated using the Glucose Uptake Assay Kit (ab136955). The cells were seeded into 96-well plates at a density of 3000 cells/well. After 12 h, the cells were cultured with 10 μM canagliflozin or DMSO in completed 1640 medium for 24 h. Cells were washed with PBS and starved in 1640 medium for 12 h. Cells were starved for glucose by pre-incubating them with 100 μL KRPH buffer containing 2% BSA for 40 min, and then 2- Deoxyglucose (2-DG; 10 mmol/L) was added and cultured for 20 min. 2-DG was omitted in respective negative controls. The rest of the protocol was performed according to the instructions from the manufacturer and subjected to the measurement of the 2-DG uptake using a microplate reader at 412 nm.
Seahorse XF Extracellular Flux analysis
The Seahorse Extracellular Flux Analyzer XF96 (Seahorse Bioscience, North Billerica, MA, USA) was used to measure the in vitro cells extracellular acidification rate (EACR) based on the manufacturer’s instructions. Briefly, 1.5 × 104cells were seeded per well in the XF96-well cell culture plate and incubated at 37℃ overnight. Next day, medium was changed to bio-carbonate free DMEM with 1 mM glutamine and then cells were incubated at 37℃ for 60 min in the CO2 free incubator to balance the media pH and temperature. The ECAR were monitored in baseline conditions and treated with 10 mM glucose, 1 µM oligomycin, 50 mM 2-deoxy glucose (2-DG). Data were normalized by the protein quantification.
Cell apoptosis
The apoptosis rate was evaluated by using the AnnexinV-FITC/PI Apoptosis Detection kit according to the instructions from the manufacturer. The cells were seeded into 6-well plates. Following starvation for 24 h (serum-free medium), the cells were collected, washed with PBS, and resuspended in 500μL Binding buffer. Then, 5μL Annexin V-FITC and 5μL PI were added to the buffer and incubated at room temperature for 15 min in the dark. Cells were analyzed by flow cytometry within 1 h. Annexin V positive cells were considered to be apoptotic cells.
RNA sequencing
Briefly, BCPAP cells were treated with 10 μM of canagliflozin or DMSO as negative control with three biological replicates for each group. After incubation for 36 h, cells were collected and total RNA was extracted using the TRIzol Reagent according to the manufacturer's instructions (Invitrogen). RNA quality was determined by 2100 Bioanalyser (Agilent) and quantified using the ND-2000 (NanoDrop Technologies). Then RNA prepared for library preparation and sequencing using the Illumina Hiseq2000 platform of Majorbio Biotech (Shanghai, China). The data were analyzed on the free online Majorbio Cloud Platform (www.i-sanger.com) according to the instructions.
Detection of reactive oxygen species (ROS)
Cells (5 × 105/well) were seeded in 6-well plates. After culturing overnight, cells were cultured with 10 μM canagliflozin or DMSO in medium for 24 h. Cells were then washed and re-suspended in PBS containing 10 μM of DCFH-DA and kept at 37 °C for 30 min in the dark. Next, cells were washed and analyzed by flow cytometry. Data processing was performed using FlowJo software version 10.5.0 for Windows (FlowJo LCC, Ashland, OR, USA).
In vivo xenograft formation assay
This study was performed following the Guide for the Care and Use of Laboratory Animals by National Institutes of Health, and all procedures were approved by the Animal Care and Use Committee of Capital Medical University.
TPC-1 cells were subcutaneously implanted in each of 5-week-old male Balb/c nude mice (1 × 105 cells in 0.1 ml PBS). Mice were then randomly divided into two groups when tumor volume grew to 80–100 mm3: vehicle control (0.5% CMC + 0.25% Tween 80) and canagliflozin group. The mice were monitored every two days for the growth of tumors, and they were sacrificed after 4 weeks. For euthanization, the mice were intraperitoneally injected with 100 mg/kg of sodium pentobarbital. The tumor xenografts were dissected and weighted after the deaths of the mice. Tumor volumes were estimated according to the equation: volume = width (mm) × width (mm) × length (mm)/2.
Immunohistochemistry (IHC)
The samples used for immunohistochemistry analysis include human tissues and mice tumor xenograft. Immunohistochemistry was performed as described previously [18]. Primary antibodies were incubated at the optimal conditions (SGLT2, 1:100, Abcam; Ki67, 1:100, Santa Cruz). Histochemistry score (H-SCORE) for thyroid cancer tissue and adjacent tissue were recorded separately to measure the expression levels of SGLT2. The staining intensity was transformed into corresponding values (0, negative; 1 + , weak; 2 + , moderate; and 3 + , strong). Based on positive cell number and staining intensity value, H-score was calculated as the following formula: H-SCORE = ∑(PI × I) = (percentage of cells with weak intensity × 1) + (percentage of cells with moderate intensity × 2) + (percentage of cells with strong intensity × 3).
Gene set enrichment analysis
The gene sets were obtained from the Molecular Signatures Database of the Broad Institute (http://software.broadinstitute.org/gsea/msigdb). Tests were performed by using default settings, with permutation number set at 1000. A false discovery rate (FDR) of < 0.25 was considered to indicate a statistically significant difference.
Cell migration and invasion assay
See Additional file 1: Wound-healing and transwell invasion assay.
Statistical analysis
Statistical analysis was performed using SPSS 18.0 (SPSS Inc., Chicago, IL, USA). Results are expressed as mean ± SD. Two-tailed unpaired Student’s t-test and repeated-measures analysis of variance were used to determine statistical significance. Statistical significance was accepted for p < 0.05.