Background
Chronic myeloid leukemia (CML) is a relatively common malignant hematopoietic disorder (~1-2 cases /100,000/year). CML accounts for approximately 15 % of leukaemia case in adults [
1,
2], and it is consistently associated with a reciprocal translocation of 9q34 with 22q11, which generates the Breakpoint cluster region/ Abelson oncogene (
BCR/ABL) fusion gene that is translated into an oncoprotein (P210
BCR/ABL) [
2‐
4]. The P210
BCR/ABL oncoprotein is a constitutively active tyrosine kinase that leads to uncontrolled cell growth and the malignant expansion of myeloid cells in the bone marrow and peripheral blood [
2,
5,
6]. Small molecule tyrosine kinase inhibitors (TKIs) that directly suppress
BCR-ABL activity are currently used to treat of CML [
7,
8]; however, resistance and intolerance to TKIs prevent a full therapeutic benefit in 20 -30 % of patients [
1,
9]. In addition, side-effects, such as diarrohea, skin toxicity and allergic reaction remain serious clinical problems [
10]. Therefore, a better understanding of the tumour biology of CML and alternative therapeutic avenues are urgently needed.
MicroRNAs (miRNAs, miR) are endogenous and highly conserved RNAs that normally base-pair with the 3’-untranslated region (UTR) of protein-encoding messenger RNA (mRNA), and suppress protein expression by inhibiting the translation and/or cleavage of target mRNAs [
11,
12]. miRNAs play key roles in numerous biological processes, including cell growth, cell cycle progression, apoptosis, migration and invasion [
13]. Dysregulated miRNAs may act as oncogenes or tumour suppressors, depending on the biological function of their targets [
14,
15]. For example, miR-370 reduces leukaemogenesis in acute lymphoblatic leukaemia (ALL) and CML by targeting the oncogene
FoxM1 [
16,
17], whereas miR-451 is known to targets
TSC1 and
GRSF1 in CML [
18], and miR-196b targets
HOXA in paediatric acute ALL [
19]. miR-362-5p was first reported by Bentwich and colleagues in primate testes [
20]. Our prior work has shown that miR-362-5p promotes hepatocellular carcinoma growth and metastasis by targeting
CYLD [
21]. However, the biological role and underlying mechanisms of miR-362-5p in CML have not been investigated.
Growth arrest and DNA damage-inducible (
GADD)
45α was originally identified as a tumour suppressor of multiple types of solid tumors and hematopoietic malignancies, and it was also implicated in stress signalling [
22,
23].
GADD45α is involved in proliferation, apoptosis, cell cycle control, and nucleotide excision repair [
24,
25]. Recent studies have shown that
GADD45α expression is frequently down-regulated in CML, and down-regulation of
GADD45α induces tumour cell proliferation, leukaemogenesis and CML progression [
26‐
29]. Nevertheless, the molecular mechanism underlying dsyregulated
GADD45α expression remains unknown.
GADD45α has been shown to play a predominant role in the regulation of c-Jun N-terminal kinase (JNK) and P38 mitogen-activated protein kinase (MAPK) signalling. Specifically, JNK and P38 MAPK are implicated in CML development and progression [
30,
31]. These two pathways are frequently found to be inactivated in CML. Conversely, the activation of P38 MAPK and JNK are generally implicated in the suppression of leukaemogenesis [
31‐
33].
In this study, we investigated whether miR-362-5p is an oncogene in CML and aimed to further understand the potential underlying mechanisms of its action in vitro and in vivo. This study reveals a novel role of miR-362-5p in CML tumourigenesis and progression, and we partially delineated the underlying molecular mechanism, providing novel insights into the tumour biology of CML.
Discussion
In this study, we analysed miR-362-5p expression during CML, and our data show that miR-362-5p expression is higher in both CML patient’ samples and cell lines compare to controls. Based on this novel finding, we further explored the role of this miRNA in CML. Interestingly, our observations indicate that the down-regulation of miR-362-5p significantly suppresses CML cell proliferation, enhances cell apoptosis, induces cell cycle arrest, and decreases migration and invasion in vitro, whereas a miR-362-5p inhibitor reduced tumour volume and tumour growth in vivo in a xenograft model. Furthermore, we found that miR-362-5p inhibitor increases the sensitivity of CML cell lines to the chemotherapeutic agent Ara-c. Taken together, these results indicate that miR-362-5p acts as a novel oncogenic miRNA (oncomiR) that exerts a important effects on CML progression.
Further mechanistic studies indicated that
GADD45α may be a key downstream target gene of miR-362-5p.
GADD45α, a P53 target gene, has been identified as a tumour suppressor [
38], because it promotes cell apoptosis and inhibits angiogenesis [
29,
38,
39]. Indeed, GADD45α-/- mice display increased mutation frequencies, and increased susceptibility to ionizing radiation and carcinogens [
24,
40]. Recently, GADD45α protein was shown to act as a sensor of oncogenic stress during the development of hematopoietic cells. Furthermore, altered
GADD45α expression may play a role in leukaemogenesis [
25,
29], because GADD45α expression is significantly down-regulated in AML and CML [
29,
41]. Previous studies have revealed that miR-148 suppresses the expression of
GADD45α in lung cancer and that
GADD45α is regulated by miR-130b in benign thyroid nodule tumorigenesis [
42,
43]. Thus, multiple miRNAs likely participate in the regulation of
GADD45α expression in different contexts; however, the ability of miR-362-5p to directly regulate
GADD45α remains unknown.
In this study, we found that GADD45α could be a direct target of miR-362-5p by the luciferase reporter assay and quantitative PCR, western blotting with samples from CML cell lines or patients’ samples further supports the idea. Knockdown of miR-362-5p inhibited the proliferation and promoted apoptotic of CML cells, which were attenuated by the siRNA mediated suppression of GADD45α. These data suggest that the miR-362-5p/GADD45α axis could be a key growth regulator of CML and that miR-362-5p is an oncomiR.
In addition, our data also suggest that miR-362-5p regulates activation of the P38 and JNK signalling pathways, likely via GADD45α. Although the mechanisms underlying this activation remains poorly understood, previously obtained data suggested that GADD45α can bind to the MEKK4 N-terminus. This binding activates the P38 and JNK signalling pathways via a conformational change that results in its autophosphorylation, activation, to strongly induce cell senescence and apoptosis [
36,
37,
44,
45].
Because aberrant miRNA expression appears to be a characteristic phenotype of many cancers, miRNA expression profiling likely has potential diagnostic value [
46], and the reintroduction or inhibition miRNA or inhibiting miRNAs may be a promising therapeutic approach [
46‐
48]. Indeed, several reports recently confirmed the feasibility of using microRNAs as a new therapeutic tool [
49‐
51]. Here, we propose that down-regulation of miR-362-5p expression with an miR-362-5p inhibitor can inhibit the proliferation and enhance apoptosis of cancer cells. Therefore, therapies targeting miR-362-5p in combination with existing conventional therapies may be a novel strategy against CML.
Methods
Cell lines and imatinib treatment
Ball-1 and Jurkat cells (human acute leukaemia cell lines), BV173 and K562 cells (human chronic myeloid leukaemia cell lines), and 293 T cells (human embryonic kidney cell line) were purchased from the Shanghai Institute of Cell Biology, Chinese Academy of Sciences (Shanghai, China). These five cell lines were cultured in RPMI-1640 media (Gibco, Carlsbad, CA, USA) containing 10 % heated-inactivated foetal bovine serum (FBS, Gibco) and 10 U/ml penicillin-streptomycin (Sigma, USA) in a humidified incubator at 37 °C in 5 % CO
2 and 95 % air. Primary CD34
+ cells (human normal bone marrow CD34
+ stem/progenitor cells) were kindly provided by Doctor Qing-sheng Li (Department of Haematology, First Affiliated Hospital, Anhui Medical University, Hefei, China). Imatinib-resistant K562 cells (K562IR) were kindly provided by Prof. Qiu-ying Huang (Department of Hematology, Affiliated Hospital, Suzhou university, Suzhou, China). The K562IR cells were cultured in the same medium containing 1 μM imatinib (STI571,Gleevec; Novartis) [
52].
BCR-ABL activity in K562 cells was inhibited by treatment with imatinib at a final concentration of 1 μM for 48 h [
52]. The cells were then harvested for real-time PCR.
Patients and normal controls
Forty newly diagnosed CML patients and 26 healthy controls were enrolled in this study (Additional file
8: Table S1). Approval was obtained from the Medical Ethics and Human Clinical Trial Committee at Anhui Medical University. All patients and healthy volunteers gave informed consent. Peripheral blood specimens were collected between April 2012 and September 2014 at the Department of Haematology, First Affiliated Hospital, Anhui Medical University, Hefei, China. The samples were immediately snap-frozen or stored at -80 °C. The samples were prepared with erythrocyte lysis buffer (Qiagen, Hilden, Germany) according to the manufacturer’s protocol prior to RNA extraction and protein analysis.
Quantitative real time PCR (qRT-PCR)
Total RNA was extracted from cells using TRIzol reagent (Invitrogen) following the manufacturer’s protocol. RNA purity and concentration were determined using a BioPhotometer (Eppendorf, Germany). Levels of mature miRNAs were measured using a Hairpin-it
TM miRNA qPCR Quantitation Kit (GenePharma, Shanghai, China) according to manufacturer’s instructions. The U6 small nuclear RNA gene (U6 snRNA) served as an internal control. Relative mRNA levels of
GADD45α were quantified using cDNA synthesized from total RNA, and (glyceraldehyde-3-phosphate dehydrogenase (GAPDH) served as an internal control). RNA was reverse-transcribed using RevertAid Moloneymurine leukaemia virus Reverse transcriptase (Thermo, USA) and random primers (Thermo). cDNA was then amplified with specific primers and Power SYBR Green PCR Master Mix (Applied Biosystems). The sequences of the primer used are listed in Additional file
8: Table S2.
Transient transfection of miRNA mimic, inhibitor, siRNA and Ara-c treament
293 T, BV173, and K562 cells were seeded in 6-well or 10-cm dishes. Transient transfections of miR-362-5p mimic and/or inhibitor, and negative control oligonucleotides (mimic-ctrl, or inhibitor-ctrl) (Genepharma, Shanghai, China) (Additional file
8: Table S3) at a final concentration of 50 nM were accomplished with Lipofectamine 2000 (Invitrogen, Carlsbad, CA, USA), following the manufacturer’s protocol. Similarly, cells were transiently transfected with siGADD45α and negative control scramble (si-ctrl) (Genepharma) at a final concentration of 25 nM. Protein assays and qRT-PCR analyses were conducted 48 h after transfection. Cell apoptosis/cell cycle were analyzed at 48/72 h after transfection. Soft-agar colony formation, migration and invasion assays were performed at 12 h after transfection.
BV173 and K562 cells were either left untransfected or were transfected with sythetic RNA (inhibitor-ctrl, inhibitor, inhibitor-ctrl and si-ctrl, inhibitor-ctrl and siGADD45α, inhibitor-ctrl and si-ctrl, or inhibitor and siGADD45α) similar above described. After 24 h, cells were treated with 5 μg/ml Ara-c (Hisun pharma, Taizhou, China). Cell apoptosis were analyzed at 24 h after Ara-c treatment.
Cell proliferation, cell cycle, and cell apoptosis analyses
Cell proliferation was evaluated at the indicated time points using the Cell Counting Kit-8 (CCK-8) Assay kit (Dojindo Molecular Technologies Inc, Kumamoto, Japan) according to the manufacturer’s protocol. Briefly, cells were incubated in 10 % CCK-8 for 2 h at 37 °C and the optical density value was then measured at 450 nm with a microplate reader (Sunrise, Tecan, Austria). The data are presented as the means ± SD of three independent experiments. The cell cycle distribution of BV173 and K562 cells was analysed 72 h after transfection with either a miR-362-5p inhibitor or a negative control. Cells were collected after washing twice with PBS, fixing in 75 % cold ethanol for 12 h, staining with propidium iodide (PI), and they were then evaluated by flow cytometry (BD Biosciences, Bedford, MD, USA). Similarly, cell apoptosis was analysed in BV173 and K562 cells by flow cytometry using the Annexin V-FITC/PI Apoptosis Detection Kit (BD Biosciences) 48 h after transfection with either an miR-362-5p inhibitor or a negative control.
Migration and invasion, soft-agar colony formation assay
Migration and invasion assays were performed with using Transwell Boyden Chamber (Corning, Cambridge, MA, USA). For the migration assay, cells were seeded into the upper compartment. For the invasion assay, cells were seeded into the upper chamber with an insert that was precoated with Matrigel (BD Biosciences). The chambers were then inserted into a 24-well culture plate and filled with RPMI-1640 medium containing 10 % FBS. After 12 h, the cells remaining on the upper surface of the membranes were scraped off, whereas, the cells located on the lower surface were fixed, stained with 0.1 % crystal violet, imaged, and counted under a microscope (Olympus, Tokyo, Japan). The same experiments were independently repeated three times.
To perform the soft agar colony-forming assay, a 1.4-ml base layer of agar (0.6 % agar in PRI1640 with 10 % FBS) was solidified in a six-well flat-bottomed plate before the addition of 1 ml of cell suspension in PRI1640 containing 3000 cells, 0.3 % agar and 10 % FBS, 24 h after transfection. The colonies were counted and imaged 12 days later.
The lentivirus was obtained from Genechem (Shanghai, China). For the control or miR-362-5p inhibition group, a sequence encoding a miR-362-5p negative control or its specific inhibitor was cloned into the lentiviral vector pCDH- CMV-MCS-EF1-coGFP. K562 cells(1 × 106) were infected with 1 × 107 lentivirus transducing units in the presence of 10 μg/ml polybrene (Sigma-Aldrich).
Animal experiments
All animal experiments were conducted with approval from the Animal Care and Use Committee of Anhui Medical University, China. Six-week-old female BALB/c nude mice (HFK, Beijing, China) were used to analyse tumourigenicity. 1 × 107 K562 cells infected with either a miR-362-5p-inhibitor or the control lentivirus were subcutaneously inoculated into the left subaxillary region of nude mice. The tumours were measured every 3 days, and tumour volumes were estimated using the following formula: 1/2(length × width2). The mice were sacrificed 21 days after cell injection, and then the final tumour weights and volumes were determined.
Western blotting and immunohistochemical analyses
For western blotting, cultured cells and peripheral blood specimens were lysed in RIPA buffer supplemented with complete protease inhibitor (Roche, Mannheim, Germany). Aliquots (100 μg) of total protein extracts were resolved on 12 % SDS-PAGE gels and transferred to PVDF membranes. The membranes were then incubated with antibodies against GADD45α (1:1000; Cell Signaling Technology, Boston, USA), JNK1/2 (1:1000; Cell Signaling Technology), phosphor-JNK1/2 (1:500; Cell Signaling Technology), p38 (1:1000; Cell Signaling Technology), phosphor-p38 (1:500; Cell Signaling Technology), or β-actin (1:10000; Santa Cruz Biotechnology, Santa Cruz, CA). Subsequently, the membranes were incubated with specific HRP-conjugated secondary antibodies (1:1000; Sigma-Aldrich, St Louis, MO, USA). Signals were detected using the enhanced chemiluminescence western blotting system (ComWin Biotech, Beijing, China).
For immunohistochemical analyses, tissues were harvested from xenograft model, fixed in 3 % formaldehyde at room temperature for 12 h, followed by fixation in methanol at -20 °C for 10 min. The samples were embedded and sectioned according to standard procedure. The sections were incubated with anti-GADD45α (Cell Signaling Technology) at 4 °C overnight, followed by incubation with an HRP-conjugated secondary antibody (Sigma-Aldrich). Image acquisition was performed on a DM100 Leica Photosystem (Leica, Milan, Italy).
Luciferase reporter assay
The 3’ untranslated region (3’-UTR) of human GADD45α was PCR-amplified from human genomic DNA and cloned into the XhoI/NotI sites of psi-CHECK2 (Promega, Madison, WI, USA) to generate the 3’-UTR wild-type reporter plasmid. A mutation of the GADD45α 3’-UTR sequence was performed using a Quick-change Site-Directed Mutagenesis kit (Stratagene, LaJolla, CA). 293 T, BV173, and K562 cells (1 × 105 cells/well) were transfected with 250 pg/μl psi-CHECK2 vector 50 nM miR-362-5p mimic and/or inhibitor, and control oligonucleotides. The cells were harvested 48 h after transfection and analysed for luciferase signals using Dual Luciferase Assays Kits (Promega) on a glomax-20/20 Luminometer (Promega).
Statistical analysis
The data are expressed as the mean ± SD of at least three independent experiments. Differences were analysed using Student’s t-test (two-tailed). A P value of 0.05 or less was considered statistically significant.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (#81272258, #81572749, #31300715), the Anhui Provincial Natural Science Foundation (#1308085QH136), Anhui Medical University Science Foundation (2015xkj095), and Anhui Medical University Training Program of National Outstanding Youth Foundation (GJYQ-1401).
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
PY carried out the qPCR assays and cell cycle analysis, drafted the manuscript. FN participated in the design of the study. QRD performed cells culture, participated in null mice feeding. QG carried out the Western blot assays, participated in qPCR assays. HZ carried out the apoptosis assays. MZY carried out CML samples collection. PY, XYW participated in the animal experiments. YZX performed the statistical analysis. LC carried out animal experiments. PY, DLC carried out the cell proliferation, migration and invasion assay. ZJC carried out the luciferase reporter assays, participated in the immunohistochemical analysis. LXK helped to draft the manuscript. PY, SYW conceived of the study, and participated in its design and coordination. All authors read and approved the final manuscript.