Introduction
Colorectal carcinoma (CRC) is a commonly diagnosed cancer in males and females. It is estimated that over 1.2 million new cancer diagnoses and 608,700 deaths from colorectal cancer occurred in 2008 [
1]. Tumour metastasis is the most common cause of death in patients with CRC, but there are no effective therapies that target the development and progression of metastasis [
2]. One therapeutic strategy against tumour invasion and migration of recent interest is immunotherapy using agonists of Toll-like receptors (TLRs) as adjuvants [
3‐
5].
The TLRs are a family of receptors consisting of highly conserved molecules, that recognise the pathogen-associated molecular patterns (PAMP) of microorganisms and viruses and play critical roles in the innate immune response and subsequent induction of the adaptive immune response [
6‐
8]. Moreover, TLRs induce chronic inflammation after being activated by damage-associated molecular patterns (DAMPs) released from injured tissue or tumour tissue [
9,
10]. Activation of TLR signalling in the steady state maintains tissue architecture. However, in the presence of the deregulated inflammation that occurs during tumourigenesis, the TLR-driven tissue response may promote neoangiogenesis and tumour growth by poorly defined mechanisms [
11]. Recent studies have demonstrated that engagement of TLRs increases tumour growth and tumour immune evasion and induces resistance to apoptosis and chemoresistance in some tumour cells [
12,
13], implying the possibility of TLR-targeted cancer therapy. In lung cancer, it has been shown that engagement of TLR4 and TLR9 induces apoptosis in cancer cells [
14]. In ovarian cancer cells, it has been shown that activation of TLR4 signalling results in increased growth and chemoresistance [
13]. These findings demonstrate that different tumours express different functional TLRs, and the effects of TLR signalling in cancer cells may vary according to cancer cell type.
A growing body of evidence suggests that the Toll-like receptor 2 (TLR2) in the intestinal epithelium of patients with CRC or inflammatory bowel disease is up-regulated in comparison with healthy individuals [
15‐
17]. In general, stimulation of TLR2 induces a TH2, Treg, or TH17 type of immune response [
18,
19]. It is clear that TLR2 has a unique position in the regulation of tumour tolerance, cancer progression and metastasis [
20].
In this study, we identified a putative tumour suppressor miRNA, miR-143, that is frequently down-regulated in CRC. We demonstrated that miR-143 can directly regulate TLR2 expression in CRC cells. High levels of miR-143 can inhibit tumour invasion and migration in vitro and in vivo by reducing TLR2 activity. Our study provides strong evidence that TLR2 may be targeted by miR-143 to overcome chronic inflammation and down-regulate tumour invasion and migration.
Materials and methods
Ethics statement and human colorectal carcinoma tissues
The specimens were collected from patients who underwent surgery at the Shanghai Renji Hospital between January 2004 and January 2010. The mean follow-up time was 92 months (from 41 to 122 months). The protocol was approved by the Shanghai Jiao Tong University School of Medicine Clinical Research Ethics Committee, and the research was performed according to the 1975 Helsinki Declaration provisions. Written informed consent was obtained from all participants involved in the study.
Cell lines
The colorectal carcinoma cell lines, including LS174T (ATCC number: CL-188™, Dukes’ type B cells), SW480 (ATCC Number: CCL-228™, Dukes’ type B cells), SW620 (ATCC Number: CCL-227™, Dukes’ type C cells) and HCT116 (ATCC number: CCL-247, Dukes’ type D cells), were purchased from the Cell Bank of Type Culture Collection of the Shanghai Institute of Cell Biology, Chinese Academy of Sciences. These colorectal carcinoma cell lines were maintained in RPMI 1640 containing 10% foetal calf serum. Cultures were incubated at 37°C in standard tissue culture incubators.
Real-time PCR
Total RNA was extracted from cells using TRIzol Reagent (Life Technologies, Gaithersburg, MD, USA). Real-time PCR analyses were performed to detect TLR2 mRNA expression using SYBR Premix Ex Taq (TaKaRa, Dalian, China), and GAPDH was used as an internal control. Real-time PCR was performed under the following conditions: 95°C 10 m, 1 cycle; 95°C 10 s, 55°C 34 s, 40 cycles. The PCR primers were TLR2 forward 5′ GATGC CTACT GGGTG GAG 3′, TLR2 reverse 5′ AAAGA CGGAA ATGGG AGA 3′; GAPDH forward: 5′ TGGGG AAGGT GAAGG TCGG 3′ and GAPDH reverse: 5′ CTGGA AGATG GTGAT GGGA 3′.
The MiRcute miRNA qPCR detection kit (TIANGEN, Beijing, China) was used to quantify the expression levels of mature miR-143 according to the protocol provided, and GAPDH was used as an internal control. The primer sequence used was the miR-143 specific primer 5′ TGAGA TGAAG CACTG TAGCT C 3′. Real-time PCR was performed under the following conditions: 95°C 1 m, 1 cycle; 95°C 10 s, 65°C 40 s, 40 cycles.
For all results obtained by real-time PCR methods, we used the delta delta CT method to calculate the fold change in gene expression between different groups. We used a housekeeping gene (GAPDH) for endogenous normalisation. The amount of target (TLR2/miR-143), normalised to the endogenous housekeeping gene GAPDH and relative to a reference sample, is given by the following equation: amount of target =2-△△CT.
Vector constructs
The full length wild type 3′UTR (UTR-WT) of the TLR2 mRNA containing the putative miR-143 binding sites was amplified by PCR and cloned into the Xba1 site of the pGL3 control vector (Promega, Madison, WI, USA). The amplified PCR primers used were TLR2 UTR-WT forward: 5′ GATGC CTACT GGGTG GAG 3′, and TLR2 UTR-WT reverse 5′ AATAC TTTGC CTTGT TGC 3′. The mutant 3′UTR of TLR2 (UTR-MUT), which carried a mutation in the complementary site for the seed region of miR-143, was generated from the UTR-WT plasmid by overlap-extension PCR. The TLR2 transcript with a deletion mutation for 3′UTR (TLR2) was amplified and cloned into the pcDNA3.0 vector. The amplified PCR primers used were TLR2 CDS forward: 5′ CTGGA CAATG CCACA TAC 3′, and TLR2 CDS reverse: 5′ AAGAT CCCAA CTAGA CAAA 3′.
Oligonucleotide transfection
S transfectants over-expressing miR-143 were generated by lentiviral transduction using a pMIRNA1 plasmid carrying miR-143 (System Biosciences, Mountain View, CA). Mimics and inhibitors of miR-143 were purchased from Dharmacon (Lafayette, CO, US). Transient transfections of miR-143 mimics/inhibitors in cancer cells were performed using Lipofectamine™ 2000 (Invitrogen, Carlsbad, CA, USA) following the manufacturer’s protocol. The final concentration of miR-143 mimics/inhibitors in the transfection system was 100 nM. As a positive control, the mature sequence of has-miR-19a is 5′-UGUGC AAAUC UAUGC AAAAC UGA-3′. The final concentration of miR-19a mimics in the transfection system was 100 nM.
For the TLR2 knockdown, the sequences of the siRNA were: 5′-GGGCA GUCUU GAACA UUUAU U-3′ and 5′-UAAAU GUUCA AGACU GCCCU U-3′. All of the RNAi oligoribonucleotides were purchased from Genepharma (Shanghai, China). Transfection was performed with Lipofectamine™ 2000 following the manufacturer’s protocol. A final concentration of 100 nM of TLR2 siRNA and the negative controls was used for each transfection. Twenty-four hours after transfection, the biological behaviour of the cancer cells was observed.
Luciferase assay
SW620 cells were seeded in 24-well plates and co-transfected with 100 nM of miR-143 mimics, 100 ng/ml UTR-WT or UTR-MUT luciferase reporter construct, and 10 ng/ml pRL-CMV Renilla luciferase reporter using lipofectamine 2000. Cells were collected 24 h after transfection, and luciferase activity was measured with a dual-luciferase reporter assay (Promega, Madison, WI, USA). The luciferase activity was normalised to the Renilla luciferase activity.
Northern blotting
We extracted RNA with the mirVana™ miRNA Isolation kit (Ambion, Austin, TX) using the microRNA enrichment protocol. Mature miR-143 was measured by northern blotting using a Northern Max-Gly Kit (Ambion, Austin, TX). Briefly, after RNA electrophoresis, the transferred membrane was prehybridised with ULTRAhyb and detected with a miR-143-specific oligonucleotide probe (5′ GAGCT ACTGT GCTTC ATCTC A 3′) labelled with digoxigenin-ddUTP using a DIG Oligonucleotide 3′-End Labeling Kit (Roche Diagnostics, Indianapolis, IN). U2snRNA was used as an internal control. The U2snRNA-specific oligonucleotide probe used was 5′ TCGGA TAGAG GACGT ATCAG ATATT AAA 3′.
Western blot
Proteins were separated on a 10% SDS-PAGE gel and then transferred to a PVDF membrane. The membrane was incubated with a rabbit TLR2 polyclonal antibody (1:1000, Proteintech, Chicago, IL, USA). The secondary antibodies were labelled with IRDyes. The signals were observed using an Odyssey Infrared Imaging System (LI-COR Biosciences, Lincoln, NE, USA).
Growth assay
The proliferation assay was performed with the Cell Counting Kit-8 (Dojindo, Kumamoto, Japan) according to the manufacturer’s instructions. Before the addition of CCK-8, the cells were washed with warm culture media by spinning the plate at 500 rpm for 3 m and then discarding the supernatant.
Cell migration and invasion assays
Transwell migration and invasion assays were performed as described previously [
21]. The cell invasion assay was performed with matrigel (BD Biosciences, Sparks, MD) coated on the upper surface of a Transwell chamber (Corning, Lowell, MA). The cell migration assay was performed in a Transwell chamber untreated with matrigel. The cells that had migrated or invaded through the membrane were fixed with 4% paraformaldehyde and stained with Coomassie brilliant blue methanol. Photographs of 3 randomly selected fields of the fixed cells were taken, and the cells were counted.
Mouse experiments
Aliquots of 1 ×106 stable miR-143 or miR-NC over-expressing SW620 cells were injected into immunodeficient mice and evaluated for lung colonisation capacity in tail-vein assays. Lung colonisation was measured using bioluminescence at 1, 2, 3, 4, 5 and 6 weeks after injection.
Statistical analyses
The data are shown as the mean ± SD. Multiple group comparisons were performed by one-way ANOVA, and survival curver was generated by Log-Rank test followed by the SPSS procedure for comparison of means. A p < 0.05 was considered to be significant.
Discussion
Recently, TLR signalling pathways have been shown to be involved in tumour growth and immune evasion. Various microbial products have been used as adjuvants due to their ability to enhance tumour immunotherapy by stimulating TLR signalling and activation of both innate and adaptive immune responses [
31]. However, the results of studies using TLR agonists as adjuvants in cancer treatments are contradictory and require further investigation, as the use of TLR ligands in cancer immunotherapy could turn out to be a double-edged sword [
20]. Because our studies of elevated TLR2 expression in CRC do not support the use of agonists for treatment, it is important to study TLR expression in patients to better predict possible chemotherapeutic benefits.
TLR2, a unique member of the TLR family localised to the plasma membrane, recognises lipid and protein ligands and mediates both Th1 and Th2 immune re-sponses. TLR2 has been implicated in the activation of pro-inflammatory cytokines that enhance tumour invasion and migration [
32]. Elevated expression of TLR2 in our cancer patient group could be associated with invasive or migratory activity. In a series of in vitro experiments, TLR-2 was identified as a critical receptor for mediating inflammation via activation of the intracellular signalling pathway that results in the translocation of NF-κB and the secretion of TNF-α and IL-1β [
33]. In both tumour and host cells, TLR2 plays a crucial role in the establishment of a tumour-induced immunosuppressive environment. Targeting TLR2 leads, directly or indirectly, to the induction of strong anti-tumour immunity, which places TLR2 in a unique position as a promising target for cancer immunotherapy. However, the cause of TLR2 overexpression in human cancers has not been elucidated. The results presented here are the first line of evidence in support of the hypothesis that over-expression of TLR2 in CRC may result from under-expression of a specific miRNA molecule, miR-143; thus, our results demonstrate a new regulatory mechanism for CRC cell invasion and migration.
Aberrant miRNA expression profiles in cancer have been reported by several groups [
34,
35]. However, the pathological roles and molecular mechanisms of the aberrantly expressed miRNAs in CRC are poorly understood. miR-143 is particularly interesting because it possesses tumour suppressive activity and its expression is substantially reduced in several cancer types, especially in CRC [
36,
37]. Consistent with previous reports, our results reveal that miR-143 is under-expressed in most CRC samples. These results imply that miR-143 might serve as a novel diagnostic and therapeutic target for CRC. Many studies have identified several miR-143 targets, including MDM2 [
38], KRAS [
39], and HK2 [
40]. Ricci-Vitiani et al. found that restoring miR-143 and miR-145 in colon cancer cells decreases proliferation, migration and chemoresistance. They also identified CD44, KLF5, and BRAF as proteins targeted by miR-143 and miR-145 [
40]. As with many other miRNAs, the biological information available for miR-143 is largely limited to expression analysis. One significant obstacle that has limited the interpretation of many miRNA-profiling studies is that it is relatively difficult to identify specific miRNA targets.
In this study, overexpression of TLR2 was identified in a large-scale study of human CRC tissues. We also detected the pro-invasive and pro-migratory effects of TLR2 in CRC cell lines. The results indicate that TLR2 may function as an oncogene. To further investigate the causes of high TLR2 expression, we found a significant inverse correlation between TLR2 expression and miR-143 expression in CRC tissues using bioinformatics methods and nonparametric tests. These results indicate that the expression of TLR2 may be regulated by miR-143. For further study of the mechanisms, we used a dual luciferase report assay system to identify the functional binding site for miR-143 in TLR2 mRNA. We identified TLR2 as a novel target of miR-143 in CRC cells.
Next, we demonstrated that restoration of high miR-143 expression levels in CRC cells (SW620 and HCT116) is associated with decreased TLR2 expression. Considering the function of TLR2 in a pro-invasive and pro-migratory phenotype, we hypothesised that miR-143 may have the opposite function. As expected, miR-143 was able to reduce cancer cell invasion and migration. In animal experiments, miR-143 over-expression in SW620 cells significantly decreased lung metastatic colonisation. We demonstrated both in vivo and in vitro that miR-143 causes TLR2 down-regulation and subsequently inhibits cancer cell invasion and migration.
It was also important to note that miR-143 was more effective than siRNA (siTLR2) in inhibiting SW620 and HCT116 cell invasion and migration (Figures
2C,
2D, and
4E), despite the finding that the down-regulation of TLR2 translation by miR-143 was not more effective than siRNA (siTLR2) (Figures
2A and
4A). These findings suggest that other miR-143 targets may also play an important role in inhibiting cellular invasion and migration in CRC cells, such as DNMT3A [
41], CD44, KLF5, KRAS and BRAF [
40]. In addition, restoring miR-143 in colon cancer cells could inhibit proliferation [
40]. However, TLR2 knockdown did not affect the growth of colorectal carcinoma cells (Figure
2B). These findings suggest that TLR2 is a novel member of numerous targets for miR-143 and that different targets can mediate the specific effects of miR-143.
In summary, our results show that miR-143 and TLR2 form a novel regulatory pathway that controls CRC cell invasion and migration. Deciphering this mechanism is an important step towards unravelling the regulatory network that underlies tumourigenesis, thereby helping us realise the potential of miRNA in cancer treatment. Additionally, using TLR agonists as adjuvants in cancer treatment appears contradictory and requires further investigation.
Acknowledgments
This study was supported by the National Natural Science Foundation of China (Grant No. 81101515, 81201912, 81172327), the Program of Shanghai Municipal Education Commission (12YZ052, shjdy027), and the Shanghai Municipal Health Bureau (20114Y189). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
Haiyan Guo carried out the molecular biology studies and drafted the manuscript. Ying Chen carried out the bioinformatic analysis. Xiaobo Hu participated in the cell migration and invasion assays. Guanxiang Qian carried out the immunohistochemistry analysis. Shengfang Ge participated in the design of the study and performed the statistical analysis. Jianjun Zhang participated in the design of the study and coordination and helped to draft the manuscript. All authors read and approved the final manuscript.