Introduction
Colorectal cancer (CRC) is one of the most common digestive malignancies and the leading cause of cancer death in the world. In China, the incidence of CRC still continues to increase. Despite improvement in the treatment of CRC in the past decade, the overall survival of patients with CRC has not changed obviously. Metastasis is the main cause of mortalities and poor outcome [
1],[
2]. The underlying molecular mechanisms in CRC metastasis are still unclear. Hence, it is urgent to explore key molecules in tumor progression, which may be used to design new diagnostic strategies and specific targeted drugs.
MicroRNAs (miRNAs) are a class of diverse, small, noncoding RNAs that are processed from precursors with a characteristic hairpin secondary structure [
3]. They commonly function as critical gene regulators. In recent years, a large number of studies have confirmed that miRNAs are involved in tumorigenesis and metastasis by targeting various types of mRNAs [
4]. To date, dysregulated expression of several miRNAs, such as miR-21 [
5], miR-124 [
6], miR-625 [
7], miR-339-5p [
8] and miR-27b [
9], has been demonstrated to contribute to development and progression of CRC. In our recent study, miR-133a was identified as a tumor-suppressive factor in human CRC that acts by repressing tumor metastasis-associated protein LIM and SH3 protein 1 (LASP1) [
10], which provides additional evidence of a pivotal role for miRNAs in CRC progression [
11].
It has been demonstrated that that miR-1 were dysregulated and repress tumor progression in hepatocellular [
12], prostate [
13],[
14], thyroid [
15], bladder [
16] and renal [
17]cancer. In colorectal cancer, an experimental approach, called miRNA serial analysis of gene expression (miRAGE), was used to perform the largest experimental analysis of human miRNAs. The data showed that miR-1 was down-regulation in CRC tissues with up to 11.8-fold decrease, compared with control samples [
18]. Meanwhile, genome-wide profiling of chromatin signatures reveals epigenetic regulation of microRNA genes, and showed that miR-1 was methylated frequently in early and advanced colorectal cancer in which it may act as a tumor suppressor [
19]. A recent study
in vitro identified that concomitant downregulation of miR-1 and increase of metastasis-associated in colon cancer 1 (MACC1) can contribute to MET overexpression and to the metastatic behavior of colon cancer cells [
20]. However, the
in vivo function and underlying mechanism of miR-1 in CRC still have not been clarified clearly.
In this study, we detected miR-1 expression in CRC cells and tissue samples. Gain- or loss-of-function assays were performed to analyze the effect of miR-1 on tumor cell phenotypes. We established xenograft mice models to investigate its therapeutic role in vivo. Finally, we also explored the molecular mechanisms underlying the suppressive function of miR-1 and its potential targets.
Materials and methods
Cell culture and miRNA transfection
CRC cell lines HT29, HCT116, SW480, and SW620 were purchased from the American Type Culture Collection (ATCC; Manassas, Va) and maintained as previously described [
10]. Additionally, a human CRC cell subline with unique liver metastatic potential, designated SW480/M5, was established in our laboratory [
21] and used in the analysis. The cells were cultured in RPMI 1640 (Hyclone; Logan, Utah, USA) supplemented with 10% fetal bovine serum (FBS) (Gibco-BRL, Invitrogen; Paisley, UK) at a humidity of 5% CO
2 at 37°C.
miRNAs were transfected at a working concentration of 100 nmol/L using Lipofectamine 2000 reagent (Invitrogen; Carlsbad, Calif, USA). The miR-1 mimic, a nonspecific miR control, anti-miR-1 (miR-1 inhibitor), and a nonspecific anti-miR control were all purchased from GenePharma (Shanghai, China). Protein and RNA samples were extracted from subconfluent cells during the exponential phase of growth.
Tumor tissue sample
Fresh primary CRC specimens and paired noncancerous colorectal tissue were provided by the Tumor Tissue Bank of Nanfang Hospital. In each case, a diagnosis of primary CRC had been made, and the patient had undergone elective surgery for CRC in Nanfang Hospital between 2007 and 2010. The pathological diagnosis was made in the Department of Pathology of Nanfang Hospital of Southern Medical University. The study was approved by the Ethics Committee of Southern Medical University and all aspects of the study comply with the Declaration of Helsinki.
RNA isolation, reverse transcription, and quantitative real-time PCR
See Additional file
1 (available online only) for details.
Western blot analysis
Protein expression was assessed by immunoblot analysis of cell lysates (20–60 μg) in RIPA buffer in the presence of mouse antibodies to LIM and SH3 protein 1 (LASP1) (1:2000; Chemicon, Temecula, CA); E-cadherin, fibronectin (FN), β-actin (1:500; Santa Cruz, California, USA); rabbit antibodies to p-Akt (Ser473), p-Akt (Thr308), AKT, p44/42 MAPK (ERK1/2), p-p44/42 MAPK (ERK1/2), Rho GDP-dissociation inhibitor 1 (ARHGDIA) (1:1000; CST, Danvers, MA) and transgelin (TAGLN) (1:500; Abcam, Cambridge, UK).
Cell proliferation assays
See Additional file
1 (available online only) for details.
Cell migration analysis
See Additional file
1 (available online only) for details.
Preparation of lentiviral vectors
A DNA fragment corresponding to pre–miR-1 and the flanking sequence was amplified from human genomic DNA and then cloned into pGLV3/H1/GFP + puro lentiviral vector (
http://www.genepharma.com). The production, purification, and titration of lentivirus were performed as described by
Liu and colleagues [
22]. The packaged lentiviruses were named LV-miR-1. The empty lentiviral vector LV-con was used as a control.
Tumor growth assay
See Additional file
1 (available online only) for details.
See Additional file
1 (available online only) for details.
Proteomic analysis
See Additional file
1 (available online only) for details.
Potential miRNA targets were predicted and analyzed using 3 publicly available algorithms: PicTar, TargetScan, and miRanda [
23]. The number of false-positive results was decreased by accepting only putative target genes that were predicted by at least 2 programs.
miRNA target validation
A 2992-bp fragment of the LASP1 3’ untranslated region (3’UTR) was amplified by PCR and cloned downstream of the firefly luciferase gene in the psiCHECK-2 vector (Promega; Madison, Wis, USA). This vector was named wild-type (wt) 3’UTR. Site-directed mutagenesis of the miR-1 binding site in the LASP1 3’UTR was carried out using the GeneTailor Site-Directed Mutagenesis System (Invitrogen) and named mutant (mt) 3’UTR. For reporter assays, the wt or mt 3’UTR vector and miR-1 mimic or inhibitor were cotransfected. Luciferase activity was measured 48 h after transfection using the Dual-Luciferase Reporter Assay System (Promega, Madison, Wis, USA).
Statistical analysis
Data were analyzed using SPSS version 13.0 software (SPSS; Chicago, Ill, USA). The Student t-test and the one-way ANOVA test were carried out for qRT-PCR and CCK-8 analyses and to calculate the tumor growth curve. The correlation between miR-1 and LASP1 was determined using the Spearman rank correlation test. Statistical significance was established at P < 0.05.
Discussion
The expression of miRNAs was abnomal in various kinds of human cancer [
25],[
26]. More and more researches has documented that miRNAs play essential roles in multiple biological processes, including cell differentiation, proliferation, angiogenesis, invasion and migration [
27]-[
30]. Recently, several studies have showed the deregulation of miR-1 in many types of tumor, such as hepatocellular [
12], prostate [
13],[
14], thyroid [
15], bladder [
16] and renal [
17] cancer. In colorectal cancer, genome-wide profiling of chromatin signatures reveals epigenetic regulation of microRNA genes, and showed that miR-1 was methylated frequently in early and advanced colorectal cancer in which it may act as a tumor suppressor [
19]. Our data demonstrated that down-regulation of miR-1 was frequently exist in CRC tissue and cell lines, suggesting a tumor suppressive role of miR-1 in CRC development.
Until now, no functional evidence
in vivo of miR-1 has been documented in CRC. In prostate cancer, miR-1 inhibited cell proliferation, migration and invasion by suppressing the expression of purine nucleoside phosphorylase (PNP) [
14]. A follow-up study showed that miR-1 was reduced in patients with distant metastasis and recurrence, and may further serve as an independent prognostic factor [
13]. A recent study
in vitro identified that concomitant downregulation of miR-1 and increase of metastasis-associated in colon cancer 1 (MACC1) can contribute to MET overexpression and to the metastatic behavior of colon cancer cells [
20]. Similarly, our gain- and loss-of-function assay showed that miR-1 suppressed cell proliferation and migration
in vitro. Importantly, our findings showed that miR-1 reduced tumor growth and metastasis
in vivo, suggesting its suppressive role in CRC progression.
The molecular mechanisms underlying miR-1-mediated biological behaviors are still unclear. To comprehensively understand the effect of miR-1 on cancer cells, we performed proteomic analysis to screen the alteration of protein profiling in CRC cells. Using 2-D DIGE, we found a set of proteins that might be directly or indirectly modulated by miR-1. The candidate proteins have been reported involved in tumor development and progression. Interestingly, our previous study has demonstrated that a candidate protein, ARHGDIA, was upregulated in metastatic CRC and promoted cell migration of CRC cells [
31]. Another candidate protein, identified as TAGLN, has been demonstrated as a potential target of miR-1 using TargetScan software. Similarly, TAGLN2 has been reported as a target of miR-1 in renal cell carcinoma [
17], bladder cancer [
16] and head and neck squamous cell carcinoma [
32]. These findings are consistent with our proteomic results and a suppressive role of miR-1 in CRC.
Epithelial–mesenchymal transition (EMT) plays a pivotal role in the initiation of metastasis, a process in which epithelial cells lose adhesion and cytoskeletal components concomitant with a gain of mesenchymal components and the initiation of a migratory phenotype [
33],[
34]. Because the effect of miR-1 on cell migration and gene expression regulation, we detected the change of EMT markers in SW480 cells transfected with miR-1. The results supported that miR-1 may reverse EMT process to inhibit cell migration. Similarly, miRNA-1 suppresses prostate cancer metastasis by regulating epithelial-mesenchymal transition [
35]. Further study showed that Slug is a major regulator of mesenchymal differentiation and directly represses miR-1 transcription. Slug and miR-1/-200 act in a self-reinforcing regulatory loop, leading to amplification of EMT.
Our proteomic analysis revealed that miR-1 might be associated with cell signaling pathway. The findings of immunoblot assays demonstrated that miR-1 inactivated MAPK/ERK and PI3K/AKT pathway by dephosphorylation of ERK1/2 and AKT, which is classical signal transduction pathway and plays an essential role in tumor progression. Recent researches revealed that the activation of cap-dependent translation by cooperative ERK and AKT signaling is critical for promotion of CRC motility and metastasis. Inhibition of either ERK or AKT alone showed limited activity in inhibiting cell migration and invasion, but combined inhibition resulted in significant impact [
36]. To the best of its knowledge, we firstly demonstrated miR-1 exerted its biological functions by regulation of MAPK/ERK and PI3K/AKT pathway, which may also explain miR-1-mediated reversion of EMT process.
miRNAs generally exert their biological function by suppressing their specific target genes at a posttranscriptional level. A number of mRNAs were reported as direct targets of miR-1, such as transgelin 2 (TAGLN2) [
16],[
32], purine nucleoside phosphorylase (PNP) [
14],[
32], coding for the cyclin D2 (CCND2), CXC chemokine receptor 4 (CXCR4), stromal cell derived factor-1 (SDF-1) [
15], endothelin-1 [
12] and fibronectin1 [
37]
et al. In the research, we validated the targeting of LASP1 (a tumor metastasis-associated protein in CRC), showing that miR-1 may suppress tumors by binding directly in the LASP1 3’UTR. LASP1 is a specific focal adhesion protein involved in numerous biological and pathological processes [
38],[
39]. Overexpression of LASP1 has been described in several types of cancers [
40]-[
42]. In a previous study, we demonstrated overexpression of LASP1 in metastatic CRC tissue and found that the expression of this protein correlated closely with the overall survival of patients with CRC. RNA interference-mediated silencing of LASP1 in SW620 CRC cells inhibited cell proliferation and migration significantly. However, gene transfection-mediated overexpression of LASP1 in SW480 CRC cells resulted in aggressive cancer cell phenotypes and promoted cancer growth and metastasis (in contrast with the phenotypes induced by miR-1 restoration). These results show that LASP1 might be a promising target in developing treatments for patients with CRC [
10]. LASP1 overexpression can rescue miR-1-mediated biological activity. These results suggest that the inhibitory effect of miR-1 on the biological activity and MAPK or AKT signal pathway is mediated in part through the repression of LASP1 expression.
Taken together, the identification of miR-1 as a tumor-suppressive miRNA in human CRC that acts by repressing LASP1 provides x evidence of a pivotal role for miRNAs in CRC tumorigenesis and progression. Given that miR-1 is down-regulated in CRC, the re-introduction of this mature miRNA into tumor tissue could serve as a therapeutic strategy by reducing the expression of target genes. miRNA-based therapeutics are still in their infancy; however, our findings are encouraging and suggest that this miRNA could be targeted for the development of a treatment for patients with CRC, especially metastatic CRC, in the future.
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Competing interests
The authors have declared that no competing interest exists.