Background
Bladder cancer is a most common urological cancer in China, and the rest of the world [
1]. Radical cystectomy with pelvic lymphadenectomy is the standard treatment for bladder cancer. However, bladder cancer may recur due to the incomplete elimination of cancer stem cells (CSCs). Therefore, new strategies targeting bladder CSCs are urgently needed.
S100A4, also known as metastasin/FSP1/pEL98/mts-1, is a gene encoding a small calcium binding protein that interacts with other proteins to enhance apoptosis, cell motility, and tumorigenesis [
2].
S100A4 is overexpressed in most cancers, including breast cancer, gastric cancer, and non-small cell lung cancer (NSCLC) [
3]. In addition,
S100A4 expression is correlated with patients’ outcome and cancer metastasis [
4]. It has been recently reported that S100A4 is a novel marker and a critical regulator of glioma stem cells, with the enhanced S100A4 expression contributing to the presentation of a metastatic phenotype [
5]. These findings indicate that S100A4 may be a promising therapeutic target for bladder CSCs. Through bioinformatics analysis in preliminary experiments, we found that there was a close relationship between S100A4 protein and the nuclear factor kappa B (NF-κB) signaling pathway.
In the present study, we demonstrate that S100A4 is up-regulated in MB49 bladder cancer stem cells (MCSCs). Additionally, overexpression of S100A4 enhances the proliferation capacity of MCSCs in vitro, and also upregulates inhibitor of nuclear factor kappa B NF-κB kinase (IKK) and activates the NF-κBsignaling pathway, whereas knockdown of S100A4 resulted in the opposite effects. The findings of this study suggest that S100A4 may promote the proliferation capacity and upregulate IKK in MCSCs by activating the NF-κB signaling pathway. Therefore, S100A4 may have the potential to be a therapeutic target in MCSCs.
Methods
Establishment and characterizations of MCSCs
MCSCs were obtained from MB49 bladder cancer cells, which was a mouse cell line, using limited dilution and serum-free culture medium method described previously [
6]. The serum-free culture medium was consisted of RPMI1640 supplemented with leukemia inhibitory factor (20 ng/ml, eBioscience, San Diego, CA), basic fibroblast growth factor (20 ng/ml, Peprotech, Rocky Hill, NJ), epidermal growth factor (20 ng/ml, Peprotech), bovine serum albumin (4 μg/ml, Thermo Scientific HyClone, Logan, UT), and B-27 serum-free supplement (20 μl/ml, Invitrogen, Grand Island, NY).
The validation of MCSCs was performed as previously reported [
6]. Cancer stem cell markers CD133 and CD44 was detected by flow cytometry analysis, Western blotting, and quantitative polymerase chain reaction (QPCR). The proliferative ability and susceptibility to chemotherapy were examined by Cell Counting Kit-8 reagent assay. Cell migration ability was examined with the transwell assay. The tumorigenic ability was verified using nude mice.
Western blot analysis
The MB49 cells and MCSCs were respectively harvested. Equal amount proteins were extracted from cells, and separated by 10% sodium dodecyl sulfate -polyacrylamide gel electrophoresis followed by transferring to polyvinylidene difluoride membranes (Millipore, Billerica, MA). The membranes were blocked with 5% skim milk in PBS, and incubated overnight at 4 °C with the primary antibodies including anti-S100A4 (Abcam, Cambridge, MA), anti- IKK (Abcam) and anti-β-actin antibody (Abcam) followed by secondary antibodies (Abcam). Bands were visualized using Fluor Chem FC2 (Alpha Innotech, San Leandro, CA).
Quantitative polymerase chain reaction
Total RNA was extracted using the Arcturus PicoPure RNA isolation kit (Applied Biosciences, Carlsbad, NM). RNA quality was tested using the Bioanalyzer RNA Pico Chip (Agilent Technologies, Santa Clara, CA). Total RNA were transcribed reversely with Superscript III (Invitrogen), followed by synthesizing the first-strand cDNA which was amplified using a SYBR green PCR master mix (Bio-Rad, Hercules, CA) performed on a 7500 real-time PCR system (AB Applied Biosystems, Singapore). The cycling systems were denaturation (95 °C for 10 s), annealing and extension (60 °C for 60 s). The primers were designed using Primer Express version 2.0 (Applied Biosystems, Foster City, CA), and are shown in Table
1. The relative expression level were analyzed using the ∆∆C
t method.
GAPDH was used as the internal control.
Table 1
Primers of selected genes
S100A4 | F: 5’- CCCTGGATGTGATGGTGT-3’ | 615 bp |
R: 5’- GTTGTCCCTGTTGCTGTC-3’ |
Interleukin (IL)-2 | F: 5’- GAATGGAATTAATAATTACAAGAA-3’ | 401 bp |
R; 5’-TGTTTCAGATCCCTTTAGTTCCAG-3’ |
Tumor necrosis factor (TNF) | F: 5’- CCAGGCAGTCAGATCATCTTCTC-3’ | 179 bp |
R: 5’- AGCTGGTTATCTCTCAGCTCCAC-3’ |
GAPDH | F: 5’-CCATGGAGAAGGCTGGGG-3’ | 198 bp |
R: 5’-CAAAGTTGTCATCCATGAC-3’ |
Plasmid construction, and RNA interference and transfection
Full length of S100A4 gene were inserted into a vector plasmid pCMV6-XL5 (OriGene Technologies). MCSC cells were transfected with pCMV6-XL5-S100A4 plasmid (pCMV-S100A4) performing in Lipofectamine™ 2000 (Invitrogen) following the manufacturer’s instructions, which were referred to as MCSCs/S100A4-vector. The expression of S100A4 was detected at 48 and 72 h after transfection on transcription and translation level respectively.
S100A4 was knockdown by S100A4-siRNA transfection. Double-stranded siRNAs specific to S100A4 were bought from Shanghai GenePharma Co., Ltd. (Shanghai, China). The S100A4 siRNA sequences were 5’-TGTAACGAATTCTTTGAAG-3′, and 5’-ACGAATTCTTTGAAGGCTT-3′. The non-coding (NC) siRNAs sequence was 5’-UUCUCCGAACGUGUCACGUTT-3′. MCSCs were transfected with a final concentration of 20 nM of siRNA performing in Lipofectamine™ 2000, which were referred to as MCSCs/S100A4-siRNA or MCSCs/NC-siRNA cells, respectively.
Cell counting Kit-8 (CCK-8) assay
Cells were plated at a density of 1 × 103 cells per well in a 96-well plate, followed by incubating for 72 h. After incubation, cell counting kit-8 reagent (CCK-8, Dojindo Molecular Technologies, Kumamoto, Japan) was added to each well with 10 μl at a time periods of 24, 48, and 72 h. Incubated for 4 h, the absorbance value was read at 450 nm performing in EnSpire 2300 multilabel reader (PerkinElmer, Singapore).
In vivo tumorigenicity study
All animal experiments were obeyed the Chinese animal protection laws and guidelines, and approved by the Ethics Committee of Southern Medical University (Contract 1,116,904).
Four-week-old immune deficient nude mice were purchased from Experimental Animals Center (Southern Medical University, Guangzhou, China), and fed under specific, pathogen-free conditions. Cells (1 × 104) were injected into mice subcutaneously. Tumor xenograft formation was recorded at 10, 20, 30, and 45 days, calculated the tumor volume according to the formula d2 × D/2, where D and d were the longest and the shortest diameters, respectively. Then mice were sacrificed after CO2 anesthesia.
Luciferase reporter assay
NF-κB transcriptional activity was examined using the pNF-κB-luciferase reporter and control plasmids (Clontech, Mountain View, CA). The cells were plated at a sub confluent density, followed by co-transfecting with 0.5 μg of NF-κB luciferase reporter plasmid or negative plasmid, and 0.02 μg of Renilla luciferase pRL-TK plasmid (Promega, Madison, WI) performing in Lipofectamine 2000 reagent (Invitrogen). Cell lysates were prepared 24 h after transfection, and the reporter activity was measured using the Dual-luciferase reporter assay system (Promega).
Immunoprecipitation
Cells were washed with ice-cold PBS, followed by lysing in Tris-buffered saline (pH 7.4), containing 150 mM NaCl, 50 mM Tris, 0.1% Nonidet P-40, 1 mM EDTA, 1 mM Na3VO4, 10 mM NaF, 2.5 mg/ml aprotinin and leupeptin, 1 mM β-glycerophosphate and 4-(2-aminoethyl) benzenesulfonyl fluoride hydrochloride, and 10 mM iodoacetate. After incubation, cellular debris and nuclei were removed by centrifugation. Cell lysates were incubated with specific antibody overnight, and then with Protein A-Sepharose (Amersham Biosciences, Piscataway, NJ) beads for another 4 h. The immunoprecipitates were washed in Tris-buffered saline four times and boiled in Laemmli buffer included of 0.02% blue bromophenol and 2% bmercaptoethanol.
Statistical analysis
SPSS19.0 software was used for all statistical analyses. Numeric data were described as the mean value ± standard deviation. Comparisons were performed by Students t-test. A value of P < 0.05 was considered to indicate statistical significance.
Discussion
S100A4 has been reported to be an important regulator for modulating the cell cycle, proliferation, apoptosis, and migration in different kinds of cells through various mechanisms [
7]. In adult animals, S100A4 expression is restricted to very few kinds of normal tissue or cells, whereas it is usually overexpressed in cancerous tissues [
2]. However, little is known about the function of S100A4 on bladder CSCs. In this research, we demonstrate firstly that S100A4 is able to enhance the proliferation capacity of mouse bladder MCSCs. Another study has indicated that S100A4 could be a novel marker and regulator of glioma stem cells in human and murine malignant gliomas [
5]. Different expression levels of S100A4 lead to different stem cell characteristics: it promotes self-renewal at a lower level, while promotes quiescence through asymmetric stem progenitor divisions at a higher level [
5]. Several works have also demonstrated that
S100A4 may have oncogenic effects in a multitude of tumor types [
8]. However, further researches are needed to manifest the exact function of S100A4 in various cancer stem cells.
In the present research, we found that cell proliferation was significantly prompted by S100A4 overexpression, while was inhibited by S100A4 suppression. Luciferase reporter assays manifested that the transcriptional activity of NF-κB was enhanced significantly by overexpression of S100A4, implying NF-κB may play an crucial part in the S100A4-induced proliferation capacity of MCSCs.
NF-κB is the collective name of a family of transcription factors consisting of seven proteins, encoded by five genes: c-Rel, RelA, RelB, p100/p52 and p105/p50 [
9]. NF-κB has been widely known for its regulatory effects on immunological and inflammatory processes, like a serious of other pathological and physiological responses containing of the development and progression of cancer. NF-κB activation is mediated frequently by plenty of chemotherapeutic agents, which generally means inducing a strong anti-apoptotic response which limits the efficacy of treatments [
10]. As shown in this research, activation of NF-κB signaling is regulated by IKK in a negative way. The IκB family of inhibitory proteins generally holds the NF-κB pathway in an inactive status by sequestering NF-κB in the cytoplasm. Many extracellular stimuli could result in the activation of IKK [
11]. Following stimulation, IKK is recruited to the combined signaling complex of late around membrane receptors, which affording a platform in phosphorylation and activation subsequently [
12]. As many works have reported, the accurate regulation of IKK activity is an important procedure in activating NF-κB pathway [
13]. Accordingly, the regulation of IKK recruitment is crucial in activating NF-κB induced by an extracellular stimulation.
Consistent with those previous works, our research also demonstrated that overexpression of S100A4 upregulated the level of IKK, followed by raising the activation of NF-κB ultimately. In addition, overexpression of S100A4 upregulated some genes, IL-2 and TNF, which downstream of the NF-κB signaling pathway. IL-2 has been demonstrate to play an crucial role in tumor proliferation [
14]. TNF-α takes a paramount role in proliferation during the development and progression in different kinds of cancer [
15]. Accordingly, it would be attractive to detect whether IL-2 or TNF act a part in proliferation and disease progression in MCSCs.
There are some limitations to this study that needed to be taken into account. The IKK complex is formed by three subunits: IKKα, IKKβ, and IKKγ [
16], and it would be better to test the individual subunits separately rather than IKK alone. In addition, our study did not test whether S100A4 enhanced the stemness of MB49 cells or not, and it would be better to overexpressed or knocked-down S100A4 not only in MCSCs but also in MB49 cells.