Introduction
Colorectal cancer (CRC) is one of the most frequently diagnosed tumors with poor prognosis and the most common CRC is colon adenocarcinoma (COAD) [
1]. The development and progress of COAD is a multistep process in which accumulating genetic changes can play an important role. Although great progression has been made in surgery, chemotherapy, radiotherapy, and targeted drugs, there is no actual achievements in the overall survival rate of COAD patients [
2]. Hence, the investigation of promising therapeutic targets and molecular mechanism involved in the carcinogenesis of COAD remains especially crucial for the early diagnosis, timely treatment, and prognosis.
It is known that long non-coding RNAs (lncRNAs) are RNA molecules with more than 200 nucleotides [
3,
4]. Generally, lncRNAs modulate the expression level of targeted genes at the post-transcriptional period [
5]. More and more lncRNAs have been found to involve in many aspects of cellular homeostasis, such as angiogenesis, metastasis, cell proliferation, immunity adjustment, genomic stability, and so on [
6,
7]. Previous studies also suggest that lncRNAs are involved in the tumorigenesis through multiple mechanisms, such as transcriptional regulation, protein post-translational regulation miRNA regulation and so on [
8]. Previous studies also proved that a growing number of lncRNAs play crucial roles in COAD tumorigenesis and development, such as ZDHHC8P1, FOXD3-AS1, and ZEB1-AS [
9‐
11]. Therefore, lncRNAs could be regarded as potential diagnostic and prognostic biomarkers for human cancers.
LncRNA ACTA2-AS1 (ACTA2 Antisense RNA 1) is located at 10q23.31 with five exons [
12]. Recent studies revealed that dysregulation of ACTA2-AS1 has been found to be closely related to poor prognosis of several cancers, such as cervical cancer, hepatocellular carcinoma, liver cancer, breast cancer and lung adenocarcinoma [
13‐
15]. According to previous studies, ACTA2-AS1 may play an important role involved in the development of human cancers. However, the role of ACTA2-AS1 in COAD and its underlying molecular mechanisms remains unclear. Our previous studies found an obvious decrease of ACTA2-AS1 expression in both COAD cell lines and COAD tissues, and we presumed that ACTA2-AS1 may act as a crucial regulator in COAD. Therefore, this study is aimed to explore the specific function of ACTA2-AS1 in COAD and the molecular mechanisms involved.
Materials and methods
Cell lines and COAD tissues
Normal human colon mucosal epithelial cell line (CCD-18Co) and six COAD cell lines (SW480, HT29, LS174T, HCT116 and DLD-1) were procured from ATCC. The above cells were cultured in RPMI 1640 medium or DMEM (Invitrogen, USA) with 10% fetal bovine serum (Invitrogen, USA) and 1% Penicillin/Streptomycin (Sigma-Aldrich, USA) at 37 °C incubator containing 5% CO2. 82 newly diagnosed patients with COAD in The Fifth Hospital of Wuhan were included in the present study. All the experiments were carried out according to the principles of The Fifth Hospital of Wuhan.
Cell transfection
The design and construction of shRNAs and si-RNA for ACTA2-AS1 and BCL2L11, the synthesis of pcDNA 3.1, miR-4428 mimics vector, and the construction of a lentiviral vector overexpressing ACTA2-AS1 and BCL2L11 were separately conducted by Genechem (Shanghai, China). The transfection was carried out using Lipofectamine 2000 reagent (Invitrogen, USA) according to the guideline of manufacturer. The sequences of genes were as followed:
si-ACTA2-AS1#1: 5′-UAGAUUAUUAUGUCUUCCCAG-3′
si-ACTA2-AS1#2: 5′-UAGUAAAGCAACAUUCUUGGA-3′
si-BCL2L11: 5′-UUAAAUAACGUGAACAUGCUG-3′
miR-4428 mimics: 5′-GUUCCUCUGCCCUUGUACCUCG-3′
RNA extract and quantitative real-time PCR (qRT-PCR) assay
Trizol reagent (TaKaRa, China) was used to extract total RNA according to the manufacturer’s instructions. The PrimeScript RT Master Mix (TaKaRa, China) was used to reverse-transcribe lncRNA and mRNA. The SYBR Premix Ex Taq II Kit (TaKaRa, China) was employed to carry out Real-time PCR. The relative RNA expression was normalized to the expression levels of U6 and GAPDH. The primers used for quantitative PCR were as follows:
ACTA2-AS1:
5′-GTGGTTCTGGTTTGCCTGAT-3′ (forward),
5′-CTGGCCCTGTAACACCAGAT-3′ (reverse);
miR-4428:
5′-GTTCCTCTGCCCTTGTACCTCG-3′ (forward),
5′-GCGCGCGTA ACAGTCTACAGC-3′ (reverse);
BCL2L11:
5′-TAAGTTCTGAGTGTGACCGAGA-3′ (forward),
5′-GCTCTGTCTGTAGGGAGGTAGG-3′ (reverse);
U6:
5′-CTCGCTTCGGCAGCACA-3′ (forward),
5′-AACGCTTCACGAATTTGCGT-3′ (reverse);
GAPDH:
5′-TGCACCACCAACTGCTTAGC-3′ (forward),
5′-GGCATGCACTGTGGTCATGAG-3′ (reverse).
Cytoplasm and nuclear localization
The NE-PER™ Cytoplasmic and Nuclear Extraction Reagents Kit (Thermo Fisher Scientific) was done to confirm the cytoplasmic localization of ACTA2-AS1 in COAD cells. Following the manufacturer’s instructions, the COAD cells nuclear and cytoplasmic constituents were sorted and collected. Afterward, qRT-PCR was used to evaluate the ACTA2-AS1 expression in the nucleus and cell cytoplasm, respectively. GAPDH was used as the cytoplasm localization control while U6 was for the nucleus localization control.
Cell viability and colony assay
The Cell Counting Kit-8 (CCK-8) (Dojindo, Japan) experiment was used to measure cell viability. COAD cells were seeded into 96-well plates at 37 °C incubator with 5% CO2. After transfection, the OD 450 values were measured at 0, 24, 48, and 72 h (h) via CCK-8 assay after incubation for 2 h at 37 °C. Then, the absorbance values were measured on a microplate reader at 450 nm. For colony formation assay, 2000 SW480 and HT29 cells were seeded in 6-well plates and cultured for seven to ten days at 37 °C incubator with 5% CO2. Then, the cells were washed twice with PBS (phosphate-buffered saline), fixed with 4% paraformaldehyde (Sinopharm Chemical, China) and stained with crystal violet (Sigma, USA) for 15 min respectively. The clone spots were counted observed under a microscope (Olympus, Japan) with 5 random view fields.
Apoptotic assay
Apoptotic assay was performed using V-FITC Annexin and PI Apoptosis Detection Kit (Beyotime, China). SW480 and HT29 were fixed in 70% cooled ethanol and stained with Annexin V-FITC and PI for 20 min at room temperature according to the protocol, and then cell apoptosis was detected by flow cytometer.
Dual luciferase reporter assay
Through starbase 2.0, we found the miR-4428 binding site in ACTA2-AS1 and the downstream target gene of miR-4428 is BCL2L11. The 3ʹ-UTR of BCL2L11 and ACTA2-AS1 containing wild type (wt) and mutant type (mut) reporter vectors were purchased from Beijing TransGen Biotech Co., (Beijing, China). miR-4428 mimics binding sequence was inserted downstream of the firefly luciferase gene in psi-CHECK2 vector to synthesis the BCL2L11-wt or ACTA2-AS1-wt and psi-CHECK2-BCL2L11-mut or ACTA2-AS1-mut plasmids, respectively. The wt and mut plasmids subsequently were co-transfected into in SW480 and HT29 cells cells with negative control and miR-4428 mimics. After transfection for 48 h, the cells were lysed and the relative luciferase activity was measured via the Dual-luciferase reporter Assay System (Promega, Madison, WI, USA).
RNA pull-down assay
For RNA pull-down assay, the streptavidin-coated magnetic beads (Life Technologies, CA, USA) were covered by biotinylated ACTA2-AS1 (Bio-ACTA2-AS1) and Bio-Oligo according to its instruction and transfected into l × 106 SW480 and HT29 cells at 50 nM as a final concentration for 48 h. Subsequently, 0.7 mL lysis buffer (5 mM MgClz, 100 mM KCl, 20 mM Tris (pH 7.5), 0.3% NP-40) and complete protease inhibitor cocktail (Roche Applied Science, IN) were added into the cell pellets, then the cell lysates were incubated together with the RNA-tagged beads for the co-immunoprecipitation (Invitrogen, Carlsbad, CA, USA). The RNA–RNA complexes were subsequently collected by centrifugation at 10,000 r for 10 min and then the miR-4428 enrichment level was detected with qRT-PCR analysis.
Western blotting
Protein was extracted using Radioimmunoprecipitation assay (RIPA, Beyotime, China). Protein samples were separated through 10% sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and then transferred onto PVDF (polyvinylidene fluoride) membranes (Thermo Fisher Scientific, USA). Membranes were blocked in 5% bovine serum albumin (BSA) for 2 h and then incubated with primary antibodies, including anti-Bcl-2, anti- Bax and GAPDH (Cell Signaling Technology, USA) overnight at 4 °C. Next day, after washed with PBST for three times, the membranes were incubated with the HRP-conjugated secondary antibodies (Cell Signaling Technology, USA) at 37 °C for 1 h. Finally, the intensity of the bands was visualized by using enhanced chemiluminescence (ECL).
Tumor xenografts in nude mice
BALB/c nude mice (16–20 g, 5 weeks of age) were purchased from Shanghai Animal Laboratory Center (Shanghai, China). Cells transfected with sh-NC or sh-ACTA2-AS1 were digested with 0.25% trypsin, diluted in PBS, counted by trypan blue staining, adjusted to a concentration of 1.0 × 10^7 cells/mL, and 0.1 mL (1.0 × 10^6 cells) of this solution was injected hypodermically into the back flank of each mice. Tumor size was calculated every 7 days. Additionally, the present investigation was approved by the ethics committee of The Fifth Hospital of Wuhan.
Statistical analysis
The data in in study were presented as mean ± standard deviation (SD). The data were visualized through the GraphPad Prism 7.0 software. All statistical analysis was conducted via SPSS 20.0. p < 0.05 presented statistically significant.
Discussion
It is widely acknowledged that lncRNA could competitively sponge and regulate the expressions of miRNA to regulate tumorigenesis [
16‐
18]. For instance, lncRNA-ROR promotes tumor growth and metastasis of colon cancer cell by targeting miR-145 [
19]. In addition, MNX1-AS1 and ELFN1-AS1 were found to facilitate cell proliferation through regulating miR-218-5p/SEC61A1 axis in colon adenocarcinoma [
20,
21]. Recent evidence indicated that high-expression of ACTA2-AS1 was positively associated with poor prognosis in COAD patients, accelerating the pathological activities via regulating miR-4644/TRIM44 [
22]. LINC00342 and LINC00491 could accelerate progression of COAD by regulating miR-545-5p/MDM2 axis and sponging miR-145 respectively [
23,
24].
Recently, it was reported that ACTA2-AS1 is significantly over-expressed in cervical cancer, upregulating SMAD3 expression by competitively sponging miR-143-3p [
15]. However, ACTA2-AS1 may act as a tumor suppressor in lung adenocarcinoma and liver cancer cell via sequestering miR-378a-3p and miR-4428 to upregulate the expression of SOX7. However, the role of ACTA2-AS1 in COAD is not completely researched yet. This study suggested that low-expression of ACTA2-AS1 contributing to poor prognosis of COAD through KM-plot analysis. Inhibition or overexpression of ACTA2-AS1 promoted or inhibited cell proliferation, colony formation and induced apoptosis, demonstrating that ACTA2-AS1 might suppress the progress of COAD. Previously, Ying et al. showed that miR-4428 could bind with ACTA2-AS1 and posed positive effects on growth, migration and epithelial-mesenchymal transition process in non-small cell lung cancer [
14]. Evidences from our study indicated that miR-4428 could sponge ACTA2-AS1 and regulated the expression of
BCL2L11 negatively. Rescue assays suggested that the impaired COAD cells growth and facilitated apoptosis triggered by over-expression of ACTA2-AS1 could be recovered by knockdown of
BCL2L11 or over-expression of miR-4428.
BCL2L11 (also known as BIM) is a member of BCL-2 family, inducing apop-tosis and inhibiting autophagy by inactivating BCL2 or by activating BAX-BAK1 and by bridging BECN1 or DYNLL1, respectively [
25‐
27]. According to previous reports,
BCL2L11 is involved in biological pro-cesses in a variety of solid tumors such as ovarian cancer, endometrial adenocarcinoma, prostate tumor and gastric cancer [
28‐
30]. Cumulating evidence has demonstrated that the dysregulation of miRNAs plays crucial roles in the pathology tumorigenesis by directly tar-geting the 3′-UTRs of mRNA of target genes. For example, it is reported that the expression of
BCL2L11 is a direct target of miR-24 in gastric cancer, regulating cell growth and apoptosis. In human endometrial adenocarcinoma, miR-106a mimics co-transfected with wild-type
BCL2L11 3′-UTR markedly inhibited the relative luciferase activities of RL95-2 and HEC-1B cells, suggesting that
BCL2L11 is the direct target of miR-106a [
28]. In this study, we found the 3′-UTRs of
BCL2L11 could sponge miR-4428. Additionally,
BCL2L11 was low-expressed in COAD tissues and negatively correlated with miR-4428, and restoration of
BCL2L11 expression completely rescued the inhibitory effect of up-regulation of ACTA2-AS1 in COAD cells. These results revealed that
BCL2L11 was directly regulated by miR-4428 and might play a crucial role in COAD Additional file
1.
Taken together, ACTA2-AS1 plays a suppressive role in COAD via decoying miR-4428 to augment the expression of BCL2L11, inhibiting cell proliferation and promoting apoptosis. Our study demonstrated that ACTA2-AS1 may be a novel prognostic marker and therapeutic target biomarker in COAD.
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