Background
Renal cell carcinoma (RCC) comprises approximately 3% of malignant tumors [
1], and its incidence rate has been rising over the past decade. Around 90% of RCCs are the clear cell carcinoma (ccRCC) subtype [
2]. Primary RCC is commonly treated with radical nephrectomy; however, despite surgical resection, approximately 30% of patients with RCC eventually develop metastasis, which is associated with high levels of mortality [
3,
4]. During recent decades, a great variety of novel biomarkers and their underlying mechanisms have been discovered in RCC, and demonstrated significant relevance in clinical practice [
5]. Nevertheless, the development and progression of RCC remain incompletely understood, and more efforts are required to identify the molecular mechanisms that promote RCC development and progression, to facilitate better treatment of this disease.
Circular RNAs (circRNAs) are a subclass of small RNAs characterized by being covalently closed loops, without a 5′ cap or 3′ poly-A tail [
6,
7]. CircRNAs were initially considered to represent ‘noise’ generated during transcription, and to have no significant cellular functions [
8]. Recently, due to the wide application of high throughput sequencing, numerous novel circRNAs have been discovered in mammalian cells [
9]. Among these circRNAs, a large proportion have important roles in both physiological and pathological processes, including cancer development and progression [
10]. Emerging evidence shows that there is an interactive relationship between micro RNAs (miRNAs) and circRNAs, referred to as the “miRNA sponge” effect [
11]. During this process, circRNAs trap miRNAs on specific binding sites, thus preventing miRNA from interfering with mRNA expression. This process can mediate cancer progression [
12]; for example, circMAPK4 acts as a sponge for miR-125a-3p, which participates glioma progression via the MAPK signaling pathway [
13]. Further, circZNF609 can interact with and downregulate miR-138-5p, promoting RCC progression [
14]; however, the biological functions and clinical significance of circRNAs in RCC remain largely unknown, and require elucidation.
Here, we conducted bioinformatics analysis of published circRNA microarray data from the Gene expression omnibus (GEO,
https://www.ncbi.nlm.nih.gov/gds/) database and determined that circSDHC (hsa_circRNA_100372) may have an oncogenic role in RCC development and progression. By performing in vitro and in vivo experiments, we demonstrate that circSDHC serves as a sponge for miRNA-127-3p, thereby regulating the CDKN3/E2F1 axis. Therefore, circSDHC is a promising potential prognostic biomarker and therapeutic target in patients with RCC.
Materials and methods
Cell lines and cell culture
Human RCC cell lines (A498, 786-O, 769P and Caki-1), a human renal proximal tubular epithelial cell line (HK2), and a human embryonic kidney cell line (HEK-293 T), were purchased from the Chinese Academy of Sciences. 786-O, and 769P were cultured in RPMI 1640 (Gibco, China) supplemented with 10% FBS (PAN-Seratech, Germany). A498, Caki-1, HK2, and HEK-293 T were cultured in DMEM (Gibco, China) supplemented with 10% FBS (PAN-Seratech, Germany). The incubation environment was at 37 °C with 5% CO2. Cells were routinely checked for mycoplasma infection during cell culture (Beyotime, China).
ccRCC patient samples and follow-up data
A total of 140 patients with ccRCC, who underwent radical nephrectomy without neoadjuvant chemotherapy or radiotherapy between 2002 and 2012, at the First Affiliated Hospital of Sun Yat-sen University (Guangzhou, China), were recruited into the study cohort. Patients were followed up regularly, with a median follow-up time of 99.0 months. Overall survival (OS) was defined as the duration from the date of surgery to the date of patient’s death for any reason. Formalin-fixed, paraffin-embedded (FFPE) samples of both tumor and adjacent normal tissue, were collected from these patients for analysis of RNA expression. Total RNA was extracted from tissue specimens using a nucleic acid isolation kit for FFPE (ThermoFisher, USA). Samples used in this study were approved by the Medical Ethics Committee of the First Affiliated Hospital of Sun Yat-sen University.
Microarray datasets were obtained from GEO by searching using the keywords “circRNA’” and “renal cell carcinoma”. Two datasets were retrieved: GSE100186, consisting of 4 tumors and matched adjacent normal tissue; and GSE137836, consisting of 3 primary tumors and 3 metastatic tumors. Arraystar Human circRNA microarray V2 chip info was downloaded for further analysis reference (
https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GPl21825). The Cancer Genome Atlas (TCGA) clear cell renal cell carcinoma (ccRCC) sequencing and clinical data were downloaded from Firebrowse (
http://firebrowse.org/). R (version 3.4.3) (
https://www.r-project.org/) was used for subsequent data analysis.
RNA and gDNA extraction
Total RNA samples were extracted from cells using Trizol (Invitrogen, USA) according to the manufacturer’s instructions. Genomic DNA (gDNA) was extracted using a genomic DNA isolation kit (Sangon Biotech, China).
RNase R treatment, cDNA synthesis, and PCR
Aliquots of total RNA (2 μg) were incubated with or without 3 U/μg RNase R (Epicenter Technologies, USA) for 30 min at 37 °C and the product RNAs purified using an RNeasy MinElute cleaning Kit (Qiagen, Germany). Isolated RNA was first reverse-transcribed to cDNA using the PrimeScript RT Master Mix (Takara, China) containing random and oligo (dT) primers. Then, PCR was performed using GoTaq Green Master Mix (Promega, USA), according to the manufacturer’s instructions. Primer sequences are provided in Additional file
1: Table S1. PCR products were subjected to electrophoresis on a 2% agarose gels and visualized using Safe Green (Biosharp, China).
Quantitative RT-PCR (qRT-PCR)
For qRT-PCR assays, 2X SYBR Green Pro Taq HS Premix II (AGbio, China) was used and reactions were conducted on QuantStudio 5 real-time-PCR instruments (ThermoFisher, USA). Primer sequences are provided in Additional file
1: Table S1. CircRNA and mRNA levels were normalized to those of
GAPDH, while miRNA levels were normalized to those of small nuclear
U6. The 2
−ΔΔCt method was used to calculate relative expression levels.
Actinomycin D assay
Cells were cultured with or without 2 μg/ml actinomycin D (Sigma, USA) in medium. Then, the cells were harvested at different time points, followed by RNA extraction and qRT-PCR detection of RNA stability, as described above.
Fluorescence in situ hybridization (FISH)
Cy3-labeled circSDHC and FAM-labeled miRNA-127-3p probes were synthesized by RiboBio (China). A Fluorescent in Situ Hybridization Kit (RiboBio, China) was used to hybridize the probes to cells. Images were captured on a confocal laser scanning microscope (FV1000; Olympus, Japan).
Western blot
Cells were harvested and lysed on ice in RIPA buffer (ThermoFisher, USA) containing proteinase inhibitor (Beyotime, China). Then, lysates were incubated on ice for 15 min before being centrifuged for 15 min (13,000 RPM, 4 °C). Supernatants were collected and protein concentration measured using a BCA protein assay kit (ThermoFisher, USA). Protein samples (20 μg) were loaded in each lane of SDS-PAGE gels. After electrophoresis, proteins were transferred onto PVDF membranes, which were blocked in non-fat milk. Then, membranes were incubated overnight at 4 °C with primary antibody, and subsequently with secondary antibody for 1 h at room temperature. Hybridizations were detected using a western blot substrate kit (Tanon, China) on a FluorChem E System (ProteinSimple, USA). Antibodies used in western blots were as follows: CKDN3 (1:1000 dilution, Abcam, USA), E2F1 (1:1000 dilution, Cell signaling, USA), GAPDH (1:1000 dilution, Cell signaling, USA), CDK1 (1:1000 dilution, ThermoFisher, USA), CDK2 (1:1000 dilution, ThermoFisher, USA), HRP-conjugated goat anti-mouse (1:5000 dilution, Proteintech, China), and HRP-conjugated goat anti-rabbit antibody (15,000 dilution, Proteintech, China).
Plasmid construction and siRNA interference assay
For circSDHC over-expression plasmids, human circSDHC cDNA was synthesized and cloned into a pLVX-cir vector (Genomeditech, China); empty vector was used as the negative control. For siRNA assays, two targeting siRNAs and one scrambled siRNA (negative control) were synthesized by RiboBio (China) (Additional file
1: Table S1). Both overexpression plasmid and siRNAs were transfected using Lipofectamine 3000 (Invitrogen, USA), according to the manufacturer’s instructions. Functional assays were carried out 48 h after transfection. Protein and RNA were harvested 48 h after transfection.
Pull-down assay with biotinylated circSDHC
A biotinylated probe targeting the junction area of circSDHC was synthesized (RiboBio, China); an oligo probe served as the negative control. Briefly, to generate probe-coated beads, probes were incubated with streptavidin magnetic beads (Invitrogen, USA) at room temperature for 2 h. Then, the probe-coated beads were mixed with cell lysates overnight at 4 °C. After centrifuging to wash the beads, pulled down miRNAs were extracted using Trizol (Invitrogen, USA) and subjected to qRT-PCR.
Pull-down assay with biotinylated miRNA
Biotinylated miR-127-3p and scrambled negative control miRNA were synthesized (RiboBio, China). Biotinylated miRNAs were transfected into cells using Lipofectamine 3000 (Invitrogen, USA), then cell lysates collected 48 h after transfection and incubated with streptavidin magnetic beads (Invitrogen, USA) at room temperature for 2 h. After centrifuging to wash the beads, pulled down circRNAs were extracted using Trizol (Invitrogen, USA) and subjected to qRT-PCR.
Luciferase reporter assay
Twenty-four hours before transfection, 3 × 103 HEK-293 T cells per well were seeded in 96-well plates. A mixture of 50 ng luciferase reporter vectors, 5 ng Renilla luciferase reporter vectors (pRL-TK), and different miRNA mimics were co-transfected into the cells. After 48 h of incubation, luciferase activity was measured using a dual luciferase reporter assay kit (Promega, USA) on a Varioskan LUX machine (Thermo, USA). Luciferase values were normalized to those of corresponding Renilla luciferase, and fold-changes in luciferase values calculated.
Cell migration and invasion assays
Transwell assays were used to evaluate the invasion and migration ability of cells in vitro. Prior to assays, cells were starved by culture in serum-free medium for 8 h. Then, cells were collected, adjusted to a concentration of 1 × 105 in 100 μl of serum-free medium, and added to transwell inserts (Corning, USA), which were coated with (in the invasion assay) and without (in the migration assay) 2% Matrigel (Corning, USA). Medium supplemented with 10% FBS was added to the lower chamber as a nutritional attractant. After incubation (8 h for migration assay and 16 h for invasion assay), transwell inserts were collected, fixed with 4% polyformaldehyde (Beyotime, China), and stained with 0.4% crystal violet (Beyotime, China) for 20 min. Cells on the upper surface were wiped out with a cotton swab, and invaded/migrated cells calculated by capturing five random fields under an Olympus IX83 inverted microscope (Japan).
MTT assay
For MTT assays, 1 × 103 cells were seeded in each well of a 96-well plate and incubated for a specific period of time. At the harvesting time point, 10 μl of 5 mg/ml MTT (Beyotime, China) was added to each well and incubated for 2 h. Then, the medium was aspirated and 100 μl of DMSO (MP Biomedicals, USA) added to each well, followed by brief shaking to dissolve the crystals, and measurement of absorbance at a wavelength of 490 nm.
Hematoxylin and eosin (HE) and immunohistochemistry (IHC) staining
These procedures were performed as described previously [
15,
16]. Antibodies used for IHC were as follows: CDKN3 (1:100 dilution, ThermoFisher, USA) and E2F1 (1:100 dilution, ThermoFisher, USA). Paraffin sections (thickness, 5 μm) were used for staining. Images were captured with an Olympus IX83 inverted microscope (Japan). Pathological samples were evaluated and scored separately by two qualified pathologists. The IHC scoring is as follows: 0 for no staining, 1+, 2+, 3+ and 4+ for 1–24, 25–49%, 50–74% and over 75% staining intensity, respectively.
Animal experiments
All animal care and experimental procedures were conducted according to the guidelines of the National Institutes of Health, and were approved by the Institutional Animal Care and Use Committee of Sun Yat-sen University. BALB/c nude mice (3–4 weeks old) were purchased from Vital River Laboratory Animal Technology (China). Stable transfection of control vector and circSDHC shRNA was performed in the 786-O cell line, which was subsequently used for animal studies. For the metastasis experiment, there were eight mice per group. Cells (1 × 106) transfected with control vector or shRNA were injected via tail veins. Mouse body weights were measured weekly. All mice were euthanized after 8 weeks and lung tissues collected and subjected to HE staining. Images were captured using an Olympus IX83 inverted microscope (Japan) and lung metastatic foci were counted in each sample. For the tumor growth study, eight mice were used in each group. Cells (5 × 106) transfected with control vector or shRNA were inoculated subcutaneously into the left side of the body. Tumor size was measured weekly. All mice were euthanized after 4 weeks and subcutaneous tumors were dissected and collected. Final tumor weights were measured and tumor samples were subjected to HE and IHC staining.
Statistical analysis
All statistical analyses were conducted using GraphPad Prism version 7.0 and R (version 3.4.3) (
https://www.r-project.org/). The student’s t-test was used for comparisons between two experimental groups. Pearson coefficients were calculated to assess correlations. Survival analysis was performed using Kaplan–Meier curves, with application of the logrank test to calculate statistical significance. Univariate and multivariate hazard ratios (HRs) were calculated using Cox regression. All quantitative experimental data are from at least three repeated experiments and are presented as mean ± SD. All
p values < 0.05 were considered significant (*
P < 0.05; **
P < 0.001; ***
P < 0.0001).
Discussion
circRNAs were once considered noise generated by transcription, with no significant biological function [
8]; however, the development of high-throughput sequencing has revealed that there are actually a great variety of functional circRNAs in mammalian cells [
9]. CircRNAs have unique functions in regulation of gene expression [
9], and can influence the development and progression of many different types of cancer [
13,
27‐
32]. Increasing numbers of circRNAs are being identified as potentially promising biomarkers [
5]; however, the functions of circRNAs in RCC remain largely unknown, and warrant further exploration.
In our study, we first acquired circRNA microarray data from the GEO database: one dataset comparing tumors and adjacent normal tissue, and the other one compared primary tumors with matched metastatic lesions. Using filtering steps, we identified a number of circRNAs located on chromosomes that are frequently amplified in ccRCC, as having substantial oncogenic potential. After testing in our own patient cohort, circSDHC emerged as the most consistently associated circRNA, exhibiting higher expression in ccRCC tissue and correlation with unfavorable outcomes. In mechanistic experiments, circSDHC was found to downregulate mir-127-3p expression via a sponge effect, thereby activating the CDKN3/E2F1 pathway, and promoting RCC cell proliferation and metastasis. Moreover, in vivo animal studies confirmed the oncogenic characteristics of circSDHC. To our best knowledge, this is the first report on the expression and regulatory function of circSDHC in RCC.
There is accumulating evidence supporting the role of circRNAs as sponges for miRNAs in mediating various biological functions [
17]. Notably, previous studies have demonstrated that circRNA cytoplasmic localization is closely associated with miRNA sponge effects. In the present study, we confirmed that circSDHC was predominantly distributed in the cytoplasm by FISH analysis and used two databases (CSCD and ENCORI) to predict miRNAs potentially bound by circSDHC. Subsequently, we used a biotin-labeled probe targeting circSDHC and luciferase reporter assays to confirm that miR-127-3p is a target of circSDHC. Further, analysis of TCGA dataset and data from our own patient cohort showed lower expression of miR-127-3p in tumor, compared with normal tissues. Our findings are consistent with previous studies, which proved that miR-127-3p functions as a tumor suppressor in several different cancers, including osteosarcoma [
33], oral squamous cell carcinoma [
34], and prostate cancer [
35].
We also identified CDKN3 and its downstream E2F1 pathway as a target of miR-127-3p. CDKN3, as a CDK1 and CDK2 inhibitor protein, is traditionally considered a negative regulator of cell cycle progression [
36]. Despite the negative regulatory effects of CDKN3 on CDK1 and CDK2, an oncogenic role for aberrant overexpression of CDKN3 has been implicated in numerous types of human cancer, including prostate cancer [
37], gastric cancer [
38], nasopharyngeal carcinoma [
39], and esophageal cancer [
40]. In esophageal cancer, CDKN3 influences cancer progression by promoting the cell cycle and chemo-resistance [
40]. Moreover, CDKN3 can act as a positive regulator of CDK and Cyclin in certain cancer type, including gastric cancer, ovarian cancer and esophageal cancer [
38,
41,
42]. Our data showed similar result, in which CDKN3 can upregulate the expression of CDK1 and CDK2. Further, the downstream E2F1 pathway is well-established as oncogenic, as it promotes the growth and metastasis of multiple cancer types [
22‐
25]. Specifically, E2F1 promotes tumor malignancy and correlates with TNM stage in ccRCC [
26]. Therefore, we predicted that the CDKN3/E2F1 pathway may be a target of circSDHC and miR-127-3p. According to the results of bioinformatics analysis, dual luciferase reporter assays, and rescue experiments, we verified a novel regulatory axis comprising circSDHC/miR-127-3p/CDKN3/E2F1 in RCC.
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