Background
Neuroblastoma is the most common solid tumor of infancy. It is thought to arise from the anomalous arrest of multi-potential embryonal cells of neural crest origin during differentiation. The disordered differentiation contributes to the pathogenesis of the disease [
1]. Prognosis of neuroblastoma is in part related to tumor stage, the presence or absence of N-myc amplification, nuclear ploidy and the age of onset [
2‐
4]. Advanced neuroblastoma in children over 1 year old has a very poor prognosis and is resistant to standard chemotherapy. Although complete or partial remissions are achieved in 74% of these children with multi-agent high-dose therapy, long-term survivors represent only 15–20% of relapsed patients [
5,
6]. Relapse and metastasis are the dominated negative factors for survival. New approaches to inhibit aggressiveness and increase chemo-sensitivity of neuroblastoma to anticancer agents are required.
Astrocyte elevated gene-1 (
AEG-1) was originally characterized as a human immunodeficiency virus (HIV)-1-inducible gene in primary human fetal astrocyte [
7,
8], which is a downstream target molecule of Ha-
ras and c-
myc mediating their tumor promoting effects [
9].
AEG-1 is ubiquitously expressed in numerous cell types, elevated levels have also been observed in some solid tumors including those of breast, brain and prostate [
9,
10]. Intriguingly,
AEG-1 expression is elevated in diverse neoplastic conditions, it cooperates with Ha-
ras to promote transformation, and its overexpression in Hela cells induces increased anchorage-independent growth and invasiveness and increase expression of adhesion molecules by activating the NF-κB pathway [
11]. However, such studies are lacking in neuroblastoma.
Recently, we found that AEG-1 is also frequently overexpressed in neuroblastoma (submitted). In patients with advanced neuroblastoma, poor clinical outcome were observed related to AEG-1 overexpression, highlight a potential role of AEG-1 in promoting tumor progression and metastasis of neuroblastoma.
In the present study, we hypothesize that overexpressed
AEG-1 enhances tumorogenic properties of neuroblastoma cells in the same manner as observed in cultured HeLa cells [
11]. The inhibition of
AEG-1 expression could be a new adjuvant therapy for neuroblastoma.
Methods
Cell lines and culture
Human neuroblastoma cell lines M17 and SK-N-SH (Chinese Type Culture Collection, Beijing, China) were maintained in Dulbecco's modified Eagle's medium (DMEM, Invitrogen, Carlsbad, CA, USA) supplemented with 10% heat-inactivated fetal bovine serum (FBS, Gibco, AUS) at 37°C in an atmosphere of 5% CO2 with humidity.
AEG-1-siRNA transfection
Knockdown of
AEG-1 expression was achieved using transfection of
AEG-1-siRNA.
AEG-1-siRNA1 and
AEG-1-siRNA2 targeting nucleotides 971–991 and 1355–1375 of human
AEG-1 mRNA sequence (GenBank Accession No. NM_178812.3) were synthesized by Genepharma (Shanghai, China) as shown in Table
1 and annealed to form siRNA duplexes according to manufacturer's instructions. Non-targeting siRNA was used to control for non-specific effects. Cells were transfected 24 hours under standard culture conditions with 100 nM siRNA duplexes using Lipofectamine™ 2000 (Invitrogen, Carlsbad, CA) following manufacturer's protocols.
Table 1
Targeted AEG-1 sequences and the control siRNA were chemically synthesized by Genepharma (Shanghai, China).
AEG-1 siRNA 1 | s: 5'-GACACUGGAGAUGCUAAUAUU-3' as: 5'-UAUUAGCAUCUCCAGUGUCUU-3' |
AEG-1 siRNA 2 | s: 5'-GGUGAAGAUAACUCUACUGUU-3' as: 5'-CAGUAGAGUUAUCUUCACCUU-3' |
Control siRNA | s: 5'-UUCUCCGAACGUGUCACGUTT-3' as: 5'-ACGUGACACGUUCGGAGAATT-3' |
Real-time RT-PCR
Fourty-eight hours after transfection, cells were harvested in TRIzol Reagent (Invitrogen) and total RNA was isolated following the manufacturer's instructions. The first strand cDNA was synthesized according to the manufacturer's instructions (TaKaRa RT kit, Dalian, China). Quantitative determination of AEG-1 transcript concentrations was performed by real-time RT-PCR with GAPDH as an internal control. Primers for AEG-1 (sense 5' GGC AAT TGG GTA GAC GAA GA 3'; antisense 5' CCT GTT TTG GAC GGG TTT TA 3') and GAPDH (sense 5' GAG TCA ACG GAT TTG GTC GT 3'; antisense 5' TTG ATT TTG GAG GGA TCT CG 3') synthesized by Sangon (Shanghai, China) and were used to measure gene expression. Amplification reaction assays were set up triplicate for each sample using the SYBR Green system (TaKaRa, Dalian, China). In order to quantify the gene expression changes, the ΔΔCt method was used to calculate the relative fold-changes normalized against GAPDH.
Western blot analysis
After 48 hours of transfection, cells and supernatant of each group would be collected. Proteins were extracted after break-down of cells by SDS boiling method. Proteins were quantified by Bradford method. 50 μg of protein underwent SDS-PAGE and was transferred to PVDF membrane afterward. It was then sealed at room temperature for 2 hours. The primary antibodies, rabbit anti-human AEG-1 antibody (Invitrogen, Carlsbad, CA), was added at a ratio of 1:1000, and incubated overnight at 4°C. The membrane was washed with PBS. Then, the secondary antibody, mouse anti-rabbit IgG/HRP antibodies (Amersham Biosciences), was added at a ratio of 1:5000, and incubated at room temperature for 2 hours. The membrane was washed three times and reacted with chemiluminescent agent for 5 minutes. It was then ECL tabletting, exposed, and displayed. The amount of each protein sample was controlled by β-actin.
Cell proliferation assay
M17 and SK-N-SH cells were transfected in 6-well plate. 24 hours late, the transfected cells were trypsinized and plated in 96-well plates with 1.0 × 103 cells in 100 μl of the medium and allowed to attach for 24 h, then 10 μl of MTT (5 mg/ml in PBS) was added for 4 h incubation at 37°C after 4, 24, 48, 72 h, respectively. Subsequently the formazan crystals were solubilized with 100 μl of 10% sodium dodecyl sulfate (SDS) in 0.01 M HCl for 24 h. The absorbance was measured using a Microplate Reader (Bio-rad 680, Bio-rad, USA) with a test wavelength of 570 nm and a reference wavelength of 630 nm and all experiments were performed in triplicate. The cell proliferation curve was plotted using the absorbance at each time point.
Colonogenic assay
The number of colonies was determined as described previously [
12]. Briefly, following transfection for 48 h, cells were trypsinized, counted, and seeded for the colony forming assay in 60 mm dishes at 200 cells per dish. After incubation for 14 days, colonies were stained with crystal violet and the numbers of positive cells counted. Colonies containing more than 50 cells were scored, and triplicates containing 10–150 colonies/dish were counted in each treatment.
Flow cytometry analysis
Detection of apoptosis and cell cycle by flow cytometry was performed using the Annexin V-FITC kit (Becton Dickinson, CA, USA). The transfected cells were harvested with trypsinization, fixed with cold 70% ethanol at 4°C for 24 hours. The staining was performed according to the producer's manual. Flow cytometry (Becton Dickinson, CA, USA) was performed immediately.
Cell viability assay
Cell viability assay was performed as described previously [
12]. Cells were seeded in 96-well plates (Corning, NY, USA). After overnight culture, they were exposed to various concentrations of cisplatin or doxorubicin for 48 h in a CO
2 incubator. MTT assay as described above was used to detect the chemo-sensitivity of cells. Absorbance values were expressed as percentages relative to controls, and the concentrations resulting in 50% inhibition of cell growth (IC
50 values) were calculated.
Statistical analysis
Results were presented as means of three independent experiments (± SD). Statistical analyses were performed using SPSS 13.0. Comparisons of optical density values, percentage of viable cells and number of apoptotic cells among groups were performed using the two-tailed Student's t test or ANOVA. P < 0.05 was considered statistically significant.
Discussion
Neuroblastoma is one of the most frequently occurring solid tumors in children, especially in the first year of life, when it accounts for 50% of all tumors. It is the second most common cause of death in children, only preceded by accidents [
5]. Despite many advances in the past three decades, neuroblastoma has remained an enigmatic challenge to clinical and basic scientists. Elucidation of the exact molecular pathways of neuroblastoma will enable researchers and clinicians to stratify the disease and adapt therapy to the risk of relapse or progress. A large body of basic research into genes and oncogenes has accumulated up till present.
Increased/decreased expression of the molecular factors, MYCN, H-ras, and trkA is well known in neuroblastoma [
1‐
4]. However, the poor prognosis for advanced neuroblastoma still reflects in part the lack of knowledge about the tumor's basic biology.
Aberrant
AEG-1 expression has been observed in some solid tumors including breast, brain and prostate [
13,
14]. Our earlier data have demonstrated that
AEG-1 expression was increased in human neuroblastoma tissues and cultured cells compared to normal brain tissues. The expression level of
AEG-1 was correlated with the clinical staging of neuroblastoma. Multivariate analysis suggested that
AEG-1 might be an independent biomarker for the prediction of prognosis of neuroblastoma (submitted). In our current study, we evaluated the possibility of
AEG-1 as a therapeutic target of neuroblastoma.
AEG-1 has been reported to be upregulated in several malignancies and play a critical role in Ha-
ras-mediated oncogenesis through the phosphatidylinositol 3-kinase/AKT signaling pathway [
15]. Emdad et al. documented that
AEG-1 is a significant positive regulator of NF-κB [
11]. Activation of NF-κB by
AEG-1 could represent a key molecular mechanism by which
AEG-1 promotes anchorage-independent growth and invasion, two central features of the neoplastic phenotype. Furthermore, Kikuno et al. revealed that aberrant
AEG-1 expression as a positive auto-feedback activator of AKT and as a suppressor of FOXO3a in prostatic cancer cells [
10]. In this study, we adopted a strategy of RNA interference to inhibit expression of
AEG-1 in two neuroblastoma cell lines, M17 and SK-N-SH. The results revealed that after transfection with
AEG-1 siRNA, mRNA level and protein level of the
AEG-1 gene decreased, and meanwhile cell growth inhibited and apoptosis increased. Therefore, our data also confirmed that
AEG-1 serves in regulating both cell proliferation and survival.
AEG-1 knockdown may not only effect the NF-κB signaling pathway, but also the PI3K/AKT signaling pathway, either directly or indirectly and also influences the function of several PI3K/AKT downstream substrates. Our reports also shown that knockdown of
AEG-1 resulted in accumulation in the G
0/G
1 phase and reduction of S and G
2/M phase cell. The eukaryotic cell cycle is regulated via the sequential activation and inactivation of CDKs that drive cell cycle progression through the phosphorylation and dephosphorylation of regulatory proteins. The underlying mechanisms are still unclear.
Since AEG-1 might play important role in neuroblastoma cell growth, we explored the therapeutic role of AEG-1 in combination with chemotherapeutic drug. We found that knockdown of AEG-1 synergistically enhanced the cytotoxicity of cisplatin and doxorubicin. Cisplatin forms inter- and intra-strand DNA cross-links. The cytotoxic effect was likely a result of inhibition of replication by cisplatin-DNA adducts and induction of apoptosis. Cisplatin is a widely used anticancer agent and frequently applied via transarterial chemo-embolization or systemically in neuroblastoma. Our results suggest that cisplatin chemotherapy could be more effective in combination with RNAi mediated knockdown of AEG-1. Clearly, for the development of such a therapeutic strategy for clinical use, a suitable vector system is necessary. These will be further explored in future work.
In summary, our present study suggests that overexpressed AEG-1 enhance the tumorogenic properties of neuroblastoma cells. Knockdown of AEG-1 could inhibit proliferation and enhance chemo-sensitivity to cisplatin or doxorubicin in neuroblastoma cells and therefore it could be a new adjuvant therapy for neuroblastoma.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
HL and LW carried out cell transfection, immunoblotting analysis; CL and LX contributed to cell transfection, cell treatments, RT-PCR and flow cytometry analysis. HL, XS and RS supervised experimental work and wrote the manuscript. All authors read and approved the final manuscript.