Background
MicroRNAs (miRNAs) are small (~22 nt) non-coding RNAs of importance for protein level regulation. They act by interacting with the 3’UTR of the target mRNA which may cause mRNA degradation or translational inhibition [
1,
2]. Several miRNAs have been associated with processes involved in cancer progression,
e.g. proliferation, differentiation, apoptosis and tumorigenesis [
3] and miRNAs have been classified as both oncogenic and tumor suppressive [
4].
The miR-34 family consists of three homologous miRNAs located at chromosome 1 (miR-34a) and chromosome 11 (miR-34b/c) at positions frequently deleted in solid tumors,
e.g. neuroblastoma, breast, prostate and lung cancer [
5‐
9]. Several reports have also pointed out a decreased expression of miR-34 in numerous malignancies, such as miR-34c in prostate cancer [
10], miR-34a and -34c in colon [
11] and lung cancer [
12], miR-34a in neuroblastoma [
13], and miR-34a and -34b in breast cancer [
14,
15]. Many studies report tumor suppressor-like effects of miR-34, for instance in ovarian cancer [
16], prostate cancer [
10], and neuroblastoma cells [
5], putatively by regulating the expression of common miR-34 targets such as
CCND1[
17],
CCNE2[
18],
CDK4[
18,
19],
CDK6[
17,
20],
MET[
18,
19,
21,
22] and
E2F3[
5,
20].
A recent study with prostate cancer PC3 cells revealed that miR-34c expression also resulted in downregulation of protein kinase Cα (PKCα) mRNA [
21]. In addition, five target prediction tools (MiRanda [
23], DIANAmT [
24], miRWALK [
25], PICTAR5 [
26] and Targetscan [
27] predict
PRKCA as a putative miR-34c target. From a breast cancer perspective this could be of relevance since PKCα expression has been reported to be important for optimal breast cancer cell proliferation [
28,
29], support a cancer stem cell-like breast cancer cell population [
30] and to predict poorer survival [
28].
Taken together, these facts led us to investigate putative suppressive effects of miR-34c on growth properties of breast cancer cells. We found that miR-34c overexpression both blocks the proliferation of cultured basal-like breast cancer cells and induces cell death, although this was not mediated by PKCα downregulation.
Methods
Cell Culture
All cell lines were obtained from American Type Culture Collection. MDA-MB-231, MDA-MB-468, BT-549 and T47D breast cancer cells were maintained in RPMI 1640 medium (HyClone, Thermo Scientific) supplemented with 10% fetal bovine serum (Saveen & Werner AB), 1 mM sodium pyruvate (HyClone, Thermo Scientific) and 100 IU/ml penicillin-streptomycin solution (HyClone, Thermo Scientific). The media for BT-549 cells were additionally supplemented with 0.01 mg/ml insulin (Novo Nordisk A/S) and for T47D with 1% glucose.
Transfections
For miRNA transfections, cells were seeded at 50–60% confluency and grown in complete medium without antibiotics for 24 h. Cells were thereafter transfected for 5 hours with miRIDIAN microRNA Mimic (80 nM probe, Dharmacon, Lafayette, CO, USA) using 2 μl/ml Lipofectamine 2000 (Invitrogen) in Opti-MEM I (Invitrogen) followed by 96 hour incubation in complete medium, roughly according to the manufacturer’s recommendations. Control experiments were performed in parallel, transfecting cells with miRIDIAN microRNA Mimic Negative Control (Dharmacon). Transfection with 40 nM siRNA (Stealth RNAi, Invitrogen) was performed for 72 hours (sequences are listed in Table
1) according to the manufacturer’s protocol.
Table 1
siRNA nucleotides
Control 48% GC | UUACGGAUCGACUUAAGCCGUUGCA |
CDC23 I | GCUGCCCAGUGUUACAUCAAAUAUA |
CDC23 II | UAUAUUUGAUGUAACACUGGGCAGC |
CDC23 III | CCAAGCUCGAGAACUUGAUGGAUUU |
Thymidine incorporation
Cells were seeded in triplicates at a density of 5 × 104 cells per well in 12-well plates and transiently transfected for 5 hours. Cells were incubated with 1 μCi/ml [3H]-thymidine for 6 hours before harvesting the cells with 10 mM EDTA. The amount of radioactivity was measured with a Tri-carb 2810TR liquid scintillation analyzer (Perkin Elmer).
Cell cycle analysis
MDA-MB-231, MDA-MB-468 and BT-549 cells were seeded at a density of 150,000 cells per 35-mm cell culture dish and transiently transfected for 5 hours. Subsequently, cells were trypsinized and fixed in 70% ethanol for 20 minutes at −20°C, washed in PBS, and incubated with a solution containing 3.5 μM Tris- HCl pH 7.6, 10 mM NaCl, 50 μg/ml propidium iodide (PI), 20 μg/ml RNase, and 0.1% igepal CA-630 for 20 minutes on ice to label DNA. 10,000 events were acquired on the FL-2 channel for the PI signal. Sample acquisition and analyses were performed with CellQuest or FACSuite software (BD Biosciences).
Annexin V analysis
MDA-MB-231 and BT-549 cells were seeded at a density of 150,000 cells per 35-mm cell culture dish, and MDA-MB-468 cells were seeded at 200,000 cells per 35-mm cell culture dish and transfected for 5 hours. After 96 hour incubation in complete medium, floating cells, pooled with trypsinized adherent cells, were stained with Annexin V-allophycocyanin (APC; BD Pharmingen) according to the supplier’s protocol, and the amount of bound Annexin V-APC was quantified with a FACSCalibur cytometer (BD Biosciences). 10,000 events were acquired on the FL-4 channel for the Annexin V-APC signal.
Real-time qPCR
Total RNA was extracted from MDA-MB-231, MDA-MB-468 and BT549 cells with the RNeasy kit (Qiagen), and potential DNA contamination was eliminated with the RQ1 RNAse-Free DNase (Promega). Two micrograms of total RNA was used for cDNA synthesis with MultiScribe Reverse Transcriptase (Applied Biosystems). The cDNA was thereafter amplified by qPCR for evaluation of relative mRNA expression levels in an Applied Biosystems 7300 real-time quantitative PCR system using the SYBR Green PCR Master Mix (Applied Biosystems). The mRNA expression data were normalized to three reference genes (
SDHA,
UBC and
YWHAZ). For relative quantification of gene expression, the comparative Ct method was applied. The sequences of primers are listed in Table
2.
SDHA forward | TGGGAACAAGAGGGCATCTG |
SDHA reverse | CCACCACTGCATCAAATTCATG |
YWHAZ forward | ACTTTTGGTACATTGTGGCTTCAA |
YWHAZ reverse | CCGCCAGGACAAACCAGTAT |
UBC forward | ATTTGGGTCGCGGTTCTTG |
UBC reverse | TGCCTTGACATTCTCGATGGT |
PRKCA forward | AAACATCTCCACCCAAGACG |
PRKCA reverse | AATCCCTCCCTGCTCACTCT |
CCND1 forward | CCCTCGGTGTCCTACTTCAA |
CCND1 reverse | CTCCTCGCACTTCTGTTCCT |
CDCK4 forward | TGTGGAGTGTTGGCTGTATCTT |
CDCK4 reverse | GGTCGGCTTCAGAGTTTCC |
CDCK6 forward | TGGTGCCTCCTCTTGTCTG |
CDCK6 reverse | CTGCCTGTTCCCACTACTCC |
CDC23 forward | CGGAGTTGGCTTTCTCTCTC |
CDC23 reverse | CCTGGGCATCTTCCTCTGTA |
For analysis of miR-34b/c expression levels, total RNA was extracted from MDA-MB-231, MDA-MB-468 and T47D cells with Trizol according to manufacturer’s instructions (Invitrogen). Small RNAs were reversely transcribed with miRNA specific primers, quantified by the TaqMan MicroRNA assays (Applied Biosystems) and normalized to two reference genes (RNU44 and U47).
Western blot analysis
Cells were lysed in radioimmune precipitation assay buffer (10 Mm Tris–HCl (pH 7.2), 160 mM NaCl, 1% Triton X-100, 1% sodium deoxycholate, 0.1% SDS, 1 mM EDTA, and 1 mM EGTA) containing 40 μl/ml Complete protease inhibitor (Roche Applied Science) and incubated on ice for 30 min. Lysates were cleared by centrifugation at 14,000 × g for 10 min at 4°C, diluted in sample buffer containing β-mercaptoethanol, and boiled for 5 min. Protein concentration was determined by Bradford assay, equal amount of proteins were electrophoretically separated on either 10% or 12% NuPAGE Novex BisTris gels (Invitrogen) and transferred to polyvinylidene difluoride membranes (Millipore). Membranes were blocked with phosphate-buffered saline containing 5% nonfat milk and probed with antibodies to Cyclin D1 and PKCα (1:500; Santa Cruz Biotechnology), CDK4 (1:1000; Millipore), CDK6 (1:1000; Cell Signaling Technology), CDC23 (1:1000, Abcam) and actin (1:1000; MP Biomedicals). Proteins were visualized with horseradish peroxidase-labeled secondary antibody (Amersham Biosciences) using the SuperSignal system (Pierce) as substrate. Chemiluminescence was detected using a CCD camera (Fujifilm).
Data analysis
HiSeq miRNA expression data of 658 breast tumors and 86 normal breast tissue samples and mRNA data from corresponding samples were downloaded from the TCGA database (
http://cancergenome.nih.gov/). The data used were downloaded in December 2013. The tumors were clustered based on mRNA expression data using the hclust function in R. Survival analyses were performed on the 310 breast tumors that had follow up data using the Survival package. The TCGA “New tumor event” variable (recurrence) defined as new tumor event after initial treatment was used as end point for survival analyses. Pairwise comparisons were evaluated with a t-test.
Discussion
In cancers, dysregulation of miRNA is a common feature that can affect downstream targets and further influence tumorigenic events such as proliferation, metastasis and apoptosis [
34]. Family members of miR-34 have been reported to be downregulated in several different cancers, including prostate [
10], neuroblastoma [
13], colon [
11], lung [
12] and breast [
14,
15]. In addition, epigenetic silencing through CpG methylation [
35,
36] and homozygous deletions affecting the miR-34a and miR-34b/c loci (1p36 and 11q23, respectively) has been identified in neuroblastoma and other tumors [
5,
7,
37‐
39].
Our analyses of TCGA data indicate that low levels of miR-34b and/or miR-34c may predict a worse outcome of breast cancer. However, the data are not in line with previous reports indicating that miR-34a and miR-34b are downregulated in breast cancer [
40‐
42]. It was only for miR-34c in basal-like breast cancers that lower expression levels could be seen. This indicates that miR-34c may be the most relevant miR-34 family member to overexpress in basal-like breast cancer cells.
In this study, we have identified an anti-proliferative and pro-apoptotic effect by miR-34c in basal-like breast cancer cells, in concordance with reports from studies in other cancers [
16,
21]. Previous studies have pointed out a role for miR-34a [
13,
18,
35,
43‐
45], and in some cases for miR-34c [
18,
31], in suppression of the cell cycle, mainly by induction of G1 cell cycle arrest. Our data rather indicate that miR-34c induced a G2/M arrest in breast cancer cells. This is more in line with the miR-34a-promoted mitotic catastrophe and G2/M arrest in irradiated glioblastoma cells [
46]. One member of the anaphase-promoting complex (APC),
CDC23, has been reported to be a target of miR-34a [
33] and show a decreased mRNA expression in response to miR-34c in prostate cancer cells [
21]. In our analysis we detect a significant decrease of CDC23 both at mRNA and protein levels in response to miR-34c expression.
CDC23 may be a mediator of miR-34c effects, but more specific experiments are needed to settle
CDC23 as a direct miR-34c target. The decrease in G1 and increase in G2/M could be replicated by down regulation of CDC23 supporting the hypothesis that downregulation of CDC23 may mediate some of the observed miR-34c effects. However, there was no effect on cells in the sub-G1 phase suggesting that miR-34c-induced cell death may be mediated by other mechanisms.
PKCα protein levels were not influenced by miR-34c and a downregulation of PKCα is therefore conceivably not involved in the observed effects. However, the PRKCA mRNA levels were affected, albeit in different directions depending on cell line. The diverging effects on PRKCA mRNA levels suggest that it is less likely a direct target of miR-34c.
We also observed that miR-34c induces death in breast cancer cells. This could be a consequence of a G2/M arrest or involve other mechanisms, such as suppression of the pro-survival factors BCL2 [
13,
32] or SIRT1 [
47]. The fact that siCDC23 induces a G2/M arrest, but no increasing in sub-G1 phase, indicates that the effects may be separate. Induction of cell death actually seems to be a more general miR-34 effect since they have been shown to lead to increased cell death in several cell types [
5,
10,
48]. Along with the growth-suppressing and cell death-inducing effects shown in this study, miR-34c has been shown to reduce the migratory and self-renewing capacity of breast tumor-initiating cells [
49] and to inhibit metastatic invasion in vivo [
15]. Our study further indicates that miR-34c has tumor-suppressive effects in breast cancer and, together with other reports, this implies miR-34c to be a potential mediator for novel miRNA replacement therapies [
50].
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Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
CA and SW contributed to the experimental design, performed the experiments and assembled the drafts of the manuscript. YC participated in interpretative discussions and helped draft the manuscript. CL conceived the study, participated in the design of the experimental work, performed the statistical analyses and helped draft the manuscript. All authors read and approved the final manuscript.