1 Background
Colorectal cancer (CRC) is the fourth most common cancer worldwide, and its incidence is on the rise [
1]. Despite advances in the diagnosis and treatment of CRC over the past few decades, its mortality remains high, and this is primarily due to recurrence and distant organ metastases [
2].
Recent studies have found that angiogenesis in the tumor microenvironment is critical to the recurrence and distant organ metastasis of CRC because blood is required to provide essential oxygen and nutrients during these processes [
3‐
5]. Currently, anti-angiogenesis therapy has been one of the most commonly used methods for the treatment of tumor recurrence and drug-resistant distant organ metastasis. However, the 5-year survival rate of patients after standardized anti-vascular therapy has not significantly improved [
6]. Studies regarding the mechanism of angiogenesis in CRC are mostly focused on the transcriptional level [
7‐
9]. Therefore, there is an urgent need to explore new mechanisms of CRC angiogenesis, especially at the post-transcriptional level, so as to provide a new theoretical basis for anti-vascular therapy.
Epigenetic regulatory mechanisms, such as DNA methylation, histone methylation, and acetylation or
N6-methyladenosine (m6A), are emerging research frontiers in tumor biology [
10‐
13]. As the most abundant post-transcriptional modification, m6A has become an important regulator of mRNA, affecting various basic biological processes [
14]. methyltransferase-like 3 (METTL3) is a key member of the m6A methyltransferase complex that has recently been reported to play an important role in influencing angiogenesis and promoting tumor progression [
15‐
19]. However, the mechanism by which METTL3 promotes angiogenesis in CRC remains unexplored.
LncRNAs, as non-coding RNAs longer than 200 nt, are involved in a variety of biological processes. Recent studies have found that they can affect tumor angiogenesis and play an important role in promoting tumor recurrence and distant organ metastasis [
4,
20,
21]. In this study, LINC00662, a lncRNA with a length of 2097 nt, was found to be located on the long arm of chromosome 19. In the TCGA (The Cancer Genome Atlas) CRC database, LINC00662 was highly expressed in CRC and was also significantly positively correlated with VEGFA.
In this study, we first demonstrated that METTL3, LINC00662, and vascular endothelial growth factor A(VEGFA) were significantly positively correlated not only in the TCGA CRC database, but also in CRC tissue specimens. It was also confirmed that both METTL3 and LINC00662 promoted angiogenesis in CRC. In addition, MeRIP experiments confirmed that METTL3 dually regulated the stability of the LINC00662 and VEGFA RNAs to maintain their expression, thereby promoting angiogenesis in CRC.
2 Methods
2.1 Clinical tissue specimens
Sixty-four CRC and 64 adjacent normal tissues were obtained during surgery from the Nanjing First Hospital, which is affiliated with the Nanjing Medical University. The operation to collect tissue samples from patients is from 2019 to 2021. The diagnosis of CRC and adjacent normal tissues was confirmed according to the pathological evidence. Tissues were snap-frozen in liquid nitrogen and stored at − 80 °C before detection was performed. This study was approved by the Ethics Committee of the Nanjing Medical University, and informed consent was obtained from all participants.
2.2 Cell culture and transfection
Normal colon cells (FHC), human umbilical vein endothelial cells (HUVEC), and CRC cells (HCT116, HT29, SW480, and DLD1; Shanghai Cell Bank, Shanghai, China) were cultured in Dulbecco’s Modified Eagle Medium supplemented with 10% fetal bovine serum (FBS) and penicillin/streptomycin (100 U/ml) in an incubator with 5% CO2 at 37 °C (the HUVEC cells were cultured in endothelial cell medium (ECM)). All of the cell lines used in this study were mycoplasma-free. Lipofectamine 2000 (Invitrogen, Carlsbad, California, USA) was used for cell siRNA transfection, and the X-Tremegene HP DNA transfection reagent (Roche, Mannheim, Germany) was used for cell plasmid transfection. During transfection, 50–70% of cells in the six-well plate should be transfected. Six hours after the siRNA transfection, the cells required a medium change, but not for the plasmid transfection. RNA was collected 24 h after cell transfection, and protein was collected 48 h after cell transfection.
2.3 Plasmid and interference sequence construction
The siRNA interference sequences of LINC00662, METTL3, and insulin-like growth factor 2 mRNA binding protein 1 (IGF2BP1) are listed in Supplementary Table 1. The full lengths of the human METTL3 (NM_019852.5), VEGFA (NM_001025366.3), IGF2BP1 (NM_001160423.2), and LINC00662 (NR_027301.1) were also cloned into the pcDNA3.1(+) vector (Invitrogen # V80020). The primers are listed in Sect.
2.4.
2.4 DNA, RNA extraction and qRT-PCR
The plasmids METTL3, LINC00662, and VEGFA were extracted from
E. coli using the Endo free Plasmids Mini Kit II (50) Kit (OMEGA, Norcross, GA, USA). The total RNA was extracted from the CRC cell lines or fresh frozen samples using the Trizol reagent (Invitrogen, Carlsbad, California, USA), and the cDNA was reversed transcribed using the PrimeScript RT Reagent Kit (Takara, Shiga, Japan) according to the manufacturer’s protocol and stored at − 20 °C. Quantitative real-time PCR was performed in triplicate in the Step One Plus TM real-time PCR Instrument (Applied Biosystems by Thermo Fisher Scientific, Singapore). GAPDH served as the internal reference gene. The primers used in this paper are listed in Table
1.
Table 1
The primers used in this study
GAPDH | Forward primer TGGTATGAGAGCTGGGGAATG Reverse primer CCTCCCCACCTTGAAAGGAA |
LINC00662 | Forward primer ACGCTGCTGCCACTGTAATAA Reverse primer GTCCGCCTTTCACAGAACTGA |
IGF2BP1 | Forward primer GCGGCCAGTTCTTGGTCAA Reverse primer TTGGGCACCGAATGTTCAATC |
VEGFA | Forward primer CGCTCGGTGCTGGAATTTGAT Reverse primer CCGTCGGCCCGATTCAAGT |
METTL3 | Forward primer TTGTCTCCAACCTTCCGTAGT Reverse primer CCAGATCAGAGAGGTGGTGTAG |
2.5 Angiogenesis assay in vitro
The pre-cooled 96-well plate was prepared in advance and placed on ice during the experiment. The normal concentration of the Corning gel thawed in advance was evenly separated in each well of the 96-well plate, and 50 μl was added to each well. The transfected cell supernatant was mixed with the HUVEC cells (3 × 104) and dropped into the coagulated matrix glue. These were cultured in the incubator for 6–8 h to observe angiogenesis. Photos were taken (4×) under an Olympus microscope IX53 (Olympus, Center Valley, PA, USA). Image Pro was used to calculate the number of angiogenesis. Each experiment was analyzed in triplicate.
2.6 Cell counting MTT assay and colony formation assay
After the transfection of the HCT116 and HT29 cells was completed, these cells were seeded on 96-well plates at a concentration of 3 × 104 cells and cultured for 0, 24, 48 and 72 h. Subsequently, 20 μl of the cell counting MTT was added to each well. After incubation at 37 °C for 4 h, the liquid was discarded and dimethyl sulfoxide (DMSO) was added. The absorbance value was then detected at 490 nm. After the transfection of the HCT116 and HT29 cells was completed, 3000–5000 cells were planted in each well of the six-well plate and cultured for 14 days. After methanol fixation, crystal violet staining was performed, and the number of clone cells was counted using Image J after photos were taken (4×) under an Olympus microscope IX53 (Olympus, Center Valley, USA). The concentration of crystal violet used in this experiment is 5‰. Each experiment was analyzed in triplicate.
2.7 Western blotting
Cells were collected and lysed, and the protein concentration was determined. 10% SDS-PAGE gel was added into the protein, and electrophoresis was performed first. After marker separation, 0.45 μm PVDF membrane was transferred between the gel after methanol activated membrane. The protein was separated and transferred to a polyvinylidene difluoride membrane followed by incubation with 5% milk at 20 ± 5 °C for 1 h. The membrane was incubated at 4 °C overnight with the following antibodies: METTL3 (1:1000, AB195352, Abcam, Cambridge, MA, USA), VEGFA (1:1000, AB185238, Abcam, Cambridge, MA, USA), and GAPDH (1:1000, Beijing Zhongshan Golden Bridge Biotechnology Co. Ltd., Beijing, China). The secondary antibody was then added. The second antibody was sheep anti-rat and sheep anti-rabbit, and the dilution ratio was 1:5000 and the membrane was incubated at room temperature for 1 h. The protein expression was observed using a chemiluminescence gel imaging system (Tanon 5200, Shanghai, China).
2.8 Immunohistochemistry
Tissue samples from the normal group and the CRC group were fixed and then cut into 4 μm sections for the IHC. In simple terms, the tissue samples were treated with ethylene diamine tetraacetic acid (EDTA) as an antigen extract and then treated with CD31 antibody (1:2000, AB76533, Abcam, Cambridge, MA, USA), CD34 antibody (1:400, AB81289, Abcam, Cambridge, MA, USA), VEGFA antibody (1:400, AB185238, Abcam, Cambridge, MA, USA), m6A antibody (1:200, AB151230, Abcam, Cambridge, MA,USA), METTL3 antibody (1:500, AB195352, Abcam, Cambridge, MA, USA), and IGF2BP1 antibody (1:4000, AB229700, Abcam, Cambridge, MA, USA), separately. These samples were then incubated overnight at 4 °C. Subsequently, the second antibody was incubated at 37 °C for 1 h. Finally, the samples were stained and imaged.
2.9 m6A-qRT-PCR
The total RNA was extracted from the CRC cell lines using the Trizol reagent (Invitrogen, Carlsbad, California, USA). Approximately 100 μg of the total RNA was digested by DNase (Takara, Shiga, Japan) in a 150 μl reaction system at 37 °C for 20 min. After digestion, the total RNA was extracted again using the Trizol reagent, followed by RNA fragmentation using fragmentation reagents (Invitrogen, Carlsbad, California, USA) at 71 °C for 5 min. Then the termination buffer was added immediately. The fragmented RNA was extracted using the Trizol reagent and dissolved in 200 μl of diethylpyrocarbonate (DEPC) water. Approximately 160 μl of fragmented RNA was diluted with the MeRIP buffer (150 mM KCl, 25 mM Tris, 5 mM EDTA, 0.5% Triton X-100, 0.5 mM DTT, protease inhibitor (1:100) (Invitrogen, Carlsbad, California, USA), and RNAase inhibitor (1:1000) (ABclonal, Wuhan, China)) and divided into two tubes that were incubated with the anti-m6A antibody (ABclonal, Wuhan, China) or the control IgG antibody with protein A/G conjugated magnetic beads (MCE, Monmouth Junction, NJ, USA) in 900 μl of the RNA binding protein immunoprecipitation (RIP) lysis buffer at 4 °C for 4 h. In total, 20 ul of fragmented RNA was collected. The bound RNAs were immunoprecipitated with beads. The beads were washed with the RIP buffer four times and treated with 10 μl of 10% sodium dodecyl sulfate (SDS), 10 μl of proteinase K (Takara, Shiga, Japan), and 130 μl of the MeRIP buffer for 30 min at 55 °C. Then the treated liquid was transferred to new tubes. In each tube, 1 ml of the Trizol reagent and chloroform was added in turn. After centrifugation, the upper water phase was collected. A 1/10 volume of 3 M sodium acetate and an equal volume of isopropyl alcohol land glycogen with a final concentration of 100 ug/ml were added. After that, the samples were kept at − 80 °C overnight and then centrifuged at 12,000×g at 4 °C for 15 min. They were then washed twice with 75% ethanol. Finally, the precipitation was dissolved with an equal volume of DEPC water and analyzed using two-step quantitative RT-PCR (Takara, Shiga, Japan).
2.10 RNA binding protein immunoprecipitation (RIP)
The CRC cells were washed twice with ice-cold PBS and lysed in 1 ml of the RIP Lysis buffer (150 mM KCl, 25 mM Tris, 5 mM EDTA, 0.5% Triton X-100, 0.5 mM DTT, protease inhibitor (1:100), and RNAase inhibitor (1:1000)) on ice for 30 min. The cell lysates were centrifuged at 12,000×g at 4 °C for 15 min. A total of 10% of the supernatant was collected, and the remaining supernatant was incubated with anti-METTL3 (Ab195352, Abcam, USA), anti-METTL16 (Ab252420, Abcam, USA), anti-ALKBH5 (ABE547, MERCK, USA), anti-FTO (ABE552, MERCK, USA), anti-YTHDC1 (Ab264375, Abcam, USA), anti-YTHDF1 (Ab220162, Abcam, USA), anti-YTHDF2 (Ab220163, Abcam, USA), anti-YTHDF3 (Ab220161, Abcam, USA), anti-IGF2BP1 (Ab184305, Abcam, USA), anti-IGF2BP2 (Ab128175, Abcam, USA), and anti-IGF2BP3 (Ab177477, Abcam, USA) antibody or the control IgG antibody with protein A/G conjugated magnetic beads (MCE, USA) in 900 μl of the RIP lysis buffer at 4 °C for 4 h. Bound RNAs were immunoprecipitated with beads. The beads were washed with RIP buffer four times and treated with 10 μl 10% SDS, 10 μl of proteinase K (Takara, Shiga Japan), and 130 μl of the MeRIP buffer for 30 min at 55 °C. RNA in the immunoprecipitation (IP) or input group was recovered with the Trizol reagent (Invitrogen, Carlsbad, California, USA) according to the manufacturer’s instruction and analyzed by quantitative RT-PCR. The enrichment ratio was calculated as a ratio of its amount in the IP to that in the input.
2.11 Matrigel plug assay in nude mice
All animal care and procedures were conducted in accordance with the standards of the Administrative Regulations on Laboratory Animals approved by the State Council of the People’s Republic of China. The animal experiments were approved by the Ethics Committee of Experimental Animal Welfare of the Nanjing Medical University (IACUC-1705037). In this experiment, female BALB/c nude mice aged 8 weeks were purchased from the Viton lievera experiment and maintained in the condition without pathogens. The cells were resuspended in serum-free medium. Then, 0.2 ml of cell suspension was mixed with 0.2 ml of high-concentration matrix gel (BD Biosciences, San Jose, CA, USA), and the mixture was immediately injected subcutaneously into the back of nude mice. Mice were sacrificed 14 days after injection, and the matrix thrombus was removed. Next, photos were taken (4×) under an Olympus stereoscopic microscope MVX10 (Olympus, Center Valley, PA, USA). A total of 200 ml of FITC-conjugated lectin (1 mg/ml) was analyzed for capillary perfusion. The mice were injected intravenously 30 min before they were killed. The Drabkin reagent kit (Sigma-Aldrich) was used to analyze the hemoglobin according to the manufacturer’s instructions. The final hemoglobin concentration was measured at 540 nm and calculated according to the standard calibration curve. The expression levels of HE, CD34 and VEGFA were analyzed by immunohistochemistry.
2.12 Zebrafish model animal
For the proliferation and migration studies, we chose the AB* strain zebrafish that were initially obtained from crosses between the AB strain (Zebrafish International Resource Center, ZIRC) and the local wild-type strain and subsequently inbred in the laboratory. A total of 48 h after cells were transfected in each well of a six-well plate, we used trypsin to digest and collect the cells and washed them twice with PBS. Then, we suspended the cells in a serum-free medium of 1 ml 1640 and counted the cells to make sure that the total number of cells was 1 × 106. Next, we added 5 μl of the cell-labeling solution (Invitrogen) to per ml of the cell suspension and mixed the wells by gentle pipetting and incubated for 20 min at 37 °C. Furthermore, we cleaned the cells three times by 1640 serum-free medium and resuspended cells in 30 μl of DMEM culture medium and then injected. Four days later, photos were taken (4×) under an Olympus microscope IX53 (Olympus, Center Valley, USA). Each experiment was analyzed in triplicate.
2.13 RNA stability assays
CRC cells were seeded in six-well plates overnight, and then treated with actinomycin D (5 μg/ml, HY-17559, MedChemExpress) at 0, 3, 6 and 9 h. The total RNA was then isolated by Trizol (Invitrogen, USA) and analyzed by qRT-PCR. The mRNA expression for each group at the indicated time was calculated and normalized by GAPDH. The mRNA half-lives time was estimated according to the linear regression analysis.
2.14 Statistics and reproducibility
Each experiment was performed at least three biological replicates. Results are presented as the mean ± SD, comparisons were made using two-tailed Student’s t-test, two-tailed paired t-test, or two-tailed nonparametric Mann–Whitney U-test, and P < 0.05 indicates statistical significance (*P < 0.5; **P < 0.1; ***P < 0.01;****P < 0.001). Statistical analyses were performed using the GraphPad Prism 7 software. Quantization of the fluorescence area for proliferation and migration of zebrafish was performed by Image J software. The western blot results were quantified using Image J 1.53 software and normalized to the loading control or GAPDH.
4 Discussion
CRC is a common gastrointestinal malignancy with increasing incidence. The 5-year survival rate of patients with CRC is 65% [
22], but the 5-year survival rate of patients with advanced CRC is extremely low. Therefore, technologies that help implement CRC treatment strategies are urgently required because they can improve patient survival [
23]. It has been reported that the tumor microenvironment plays an obvious role in promoting the recurrence, drug resistance, and distal organ metastasis of CRC, especially angiogenesis. When tumors are larger than 2 mm, tumor angiogenesis greatly promotes the progression of CRC because blood is required to provide the needed oxygen and nutrients during tumor recurrence and metastasis. In recent years, studies on CRC angiogenesis have primarily focused on the transcriptional level [
7‐
9]. Therefore, exploring new molecular mechanisms, especially regulation at the post-transcriptional level, will greatly enrich the study of CRC angiogenesis and provide a theoretical basis for colorectal diagnosis and treatment. Recent studies have found that m6A modification plays a key role in CRC [
24‐
27]. In addition, m6A has also been reported to influence tumor progression by affecting angiogenesis [
17,
28‐
32]. However, the effect of m6A on angiogenesis and progression remains unclear in CRC.
In this study, we first found a high expression of vascular-related indicators in CRC through the database, suggesting that angiogenesis may promote CRC progression. In the database analysis, we found that METTL3, the vascular-related m6A enzyme, was highly expressed in CRC tissues, indicating that METTL3 may promote the progression of CRC as well. We then confirmed that angiogenesis related indicators, such as VEGFA, are significantly overexpressed in CRC using qRT-PCR and IHC, confirming that angiogenesis could promote CRC progression. In addition, the IHC staining revealed a significantly high expression of m6A and METTL3 in CRC, suggesting that m6A modification, especially METTL3, may be indeed be involved in the progression of CRC. In addition, METTL3 was significantly positively correlated with VEGFA, and VEGFA mRNA and protein levels were significantly down-regulated after METTL3 interference, indicating that METTL3 may affect angiogenesis by regulating VEGFA in CRC.
With continuous studies on lncRNAs, it has become common to see that lncRNAs can promote the tumor microenvironment, especially angiogenesis, and thus can promote tumor progression [
4,
20,
21]. The recent literature has found that METTL3 can promote the malignant progression of glioma by regulating the stability of MALAT1 and activating NF-κB [
33]. In this study, we found LINC00662 through the CRC database. It was positively correlated with VEGFA. Further verification by histological qRT-PCR also showed that LINC00662 was highly expressed in CRC and was significantly positively correlated with VEGFA. This suggested that LINC00662 may promote CRC progression by influencing angiogenesis. It was worthy to note that qRT-PCR in CRC tissues also showed a significant positive correlation between METTL3 and LINC00662. After METTL3 interference, LINC00662 RNA levels decreased significantly, suggesting that METTL3 may regulate both LINC00662 and VEGFA RNAs to influence CRC angiogenesis.
Generally, m6A regulates the expression of genes by affecting the stability of its RNAs. In this study, the expression levels of the LINC00662 and VEGFA RNAs were observed separately after the interference and overexpression of METTL3. The results showed that after interference of METTL3 and the stabilities of LINC00662 and VEGFA RNAs were all decreased. Accordingly, the stabilities were increased after overexpression. Thus, the most important innovation of this study was that METTL3 could promote CRC angiogenesis by the dual regulation of LINC00662 and VEGFA RNAs stability.
M6A modification is primarily mediated by m6A methyltransferase, demethylase, and the reader protein that regulate pre-mRNA splicing, miRNA processing, translation and mRNA attenuation [
34]. In this study, the RIP experiments showed that in addition to METTL3 interacting with LINC00662, the reader protein, IGF2BP1, also interacted with LINC00662. However, the stabilities and expression levels of LINC00662 and VEGFA RNAs did not change after the interference and overexpression of IGF2BP1. These results suggested that IGF2BP1 had no direct regulatory effect on LINC00662 and VEGFA. In addition, the overall survival of CRC patients with high METTL3 levels was shorter, which is consistent with those reported in other studies [
26,
35]. Subsequently, we validated the role of METTL3 and LINC00662 in angiogenesis, proliferation, and migration in CRC. The knockdown of METTL3 could inhibit angiogenesis, proliferation, invasion, and migration in the HCT116 and HT29 cells. In addition, we established a multi-group rescue experiment in the studies of angiogenesis, proliferation, invasion, and the migration function. It showed that METTL3 played the most important role in promoting CRC angiogenesis through the METTL3/LINC00662/VEGFA axis, which greatly enriched the study of METTL3 on CRC angiogenesis. In addition, the theoretical basis of the m6A influence on the tumor microenvironment was strengthened. These results suggested that METTL3, as an oncogene, promoted the progression of CRC. The key role of METTL3 in various cancers has also been reported [
33,
36,
37]. Therefore, METTL3 may become a new potential therapeutic target for cancer. In conclusion, our study suggests that METTL3 dually regulates the LINC00662 and VEGFA RNAs stability to promote CRC angiogenesis, which is a novel mechanism for regulating CRC angiogenesis. Although we demonstrated the regulatory mechanism of METTL3 in CRC angiogenesis, there were still deficiencies. For example, the selective primary catalytic inhibitor METTL3 (STM2457) has been reported as a therapeutic strategy for acute myeloid leukemia [
38]. STM2457 combined with anti-PD-1 therapy also can achieve good anti-CRC tumor effect [
39], but no METTL3 inhibitor has been found to be used for CRC anti-angiogenesis. Therefore, further studies are required to determine the role of METTL3 inhibitors in CRC angiogenesis. The failure of this study to construct stable cell lines with METTL3 knockdown to further confirm its role in the stability of LINC00662 and VEGFA RNAs is another deficiency of this paper.
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