NBPF4 mitigates progression in colorectal cancer through the regulation of EZH2-associated ETFA.

Colorectal cancer (CRC) is one of the leading causes of death worldwide, and hence, there is a need to elucidate the molecular mechanisms contributing to the progression of CRC. In this study, we aimed at assessing the role of long non-coding RNA NBPF4 on the tumorigenesis of CRC. Silencing or overexpression experiments were performed on HCT116 and SW260 in vitro models. BALB/c athymic female nude mice aged 5-6 weeks were used as in vivo models. To assess the relationship between NBPF4 and its regulatory RNA pull-down assay, RNA immunoprecipitation, luciferase activity, Western blotting and qRT-PCR were employed. Initially, we identified that NBPF4 was downregulated in CRC tissues and cell lines. Furthermore, we observed that NBPF4 decreased tumorigenesis in both in vitro and in vivo models. Additionally, we identified that ETFA was highly expressed in CRCs and was negatively associated with NBPF4. Subsequently, we identified that EZH2, a transcriptional factor, activated ETFA by enhancing the methylation of its promoter, and EZH2 was also highly regulated in CRCs. Using COAD and READ databases, we confirmed that EZH2 and ETFA were positively correlated. Furthermore, we identified NBPF4 and EZH2 were targets for ZFP36, which bound and positively regulated NBPF4. This prevented NBPF4 from binding to its negative regulator miR-17-3p. Our results demonstrated that NBPF4 downregulated EZH2 and stabilized itself by binding to ZFP36, thus escaping from inhibition by miR-17-3p, which allowed mitigation of CRC through inhibition of ETFA.

INTRODUCTION

Colorectal cancer (CRC) stands as the third leading cause of cancer‐related deaths among men and women worldwide. Risk factors such as alcohol consumption, smoking, unhealthy diet, lack of exercise and excessive body weight contribute to more than half of the deaths. Originating from the glandular, epithelial cells of the large intestine, CRCs are a heterogeneous group of diseases due to the wide array of contributing mutations. Till date, relatively less is known about the molecular cause of this disease, making the development of therapeutic strategies more difficult. Therefore, surgery remains one of the critical treatment options for early diagnosed cases, while chemotherapy and other cytotoxic methods are used for later stages. However, due to increase in drug resistance, there is a need to identify new targets and treatment strategies for CRC., ,

Non‐coding RNAs (ncRNAs) have little or no protein‐coding capacity, but participate in important cellular processes and are categorized into groups based on their length or function. MicroRNAs (miRs) are short, 20 nt in length, and are well studied among ncRNAs, whereas long non‐coding RNAs (lncRNAs) are ~200 nt long and have recently garnered immense attention in the cancer research. NcRNAs have varied functions, ranging from regulation of transcription, protein stability and translation. Many lncRNAs such as MALAT1, HOTAIR, NEAT1 and KCNQ1OT1  have been reported to play important roles in cancer. MALAT1 and HOTAIR have been associated with increased metastasis in the lungs, liver, and CRCs.,

In our study, for the first time we identified lncRNA neuroblastoma breakpoint family 4 (NBPF4) to play an important role in the regulation of tumorigenesis in CRCs. Even though NBP family members have been identified to play crucial roles in brain evolution and cognitive function,, relatively less is known about NBPF4 in cancer.

Enhancer of zeste homolog 2 (EZH2) is a histone methyl transferase subunit of the polycomb repressor complex and has been known to be mutated in many forms of cancer. EZH2 has been reported to be correlated with poor prognosis in CRC patients, with high EZH2 expression associated with tumour size, stage and metastasis and hence considered as a prognostic marker for CRCs. In this study, we identified that miR‐17‐3p and ZFP36 competitively bound to NBPF4 or EZH2 and regulated tumorigenesis through the ETFA methylation. Using in vitro and in vivo models, we confirmed that NBPF4 decreased metastasis, proliferation and thus tumorigenesis in CRCs. However, NBPF4 regulated promoter methylation by interacting with ETFA to increase tumorigenesis. Understanding of the detailed molecular mechanisms contributing to metastasis and tumorigenesis would aid in the identification of prognostic markers and the development of potential therapeutic strategies for CRCs.

MATERIALS AND METHODS

Patient's samples

We collected paired tumours and adjacent normal tissues from patients with colon cancer, who received treatment at Zhongshan Hospital, Fudan University. The tissues were immediately cut and stored in RNAlater (Ambion), and they were subjected to quantitative real‐time PCR (qRT‐PCR) analysis. Importantly, no patients received any neoadjuvant therapy. This research was approved by the Ethical Committee of Zhongshan Hospital, Fudan University, in compliance with the ethical standards set out in the Declaration of Helsinki. All participants were provided with written informed consent.

Cell lines and culture

The cell lines used in this study, including normal colonic epithelial cells (NCM460) and colon cancer cell lines (SW620, HT29, SW480, LoVo and HCT116), were purchased from American Type Culture Collection (Manassas, USA). All cell lines were cultured, passaged and maintained for less than six months based on the supplier's instructions. Furthermore, the cell lines were confirmed to be mycoplasma free and DNA fingerprinting was used to assess the authenticity of these cells before use.

Plasmid construction and cell transfection

To overexpress NBPF4, ETFA or EZH2, the respective full‐length cDNA sequences in a pcDNA3.1 vector (Invitrogen, USA) were used. Short hairpin RNA (shRNA) targeting NBPF4 or EZH2 (shNBPF4 or shEZH2) were obtained from GenePharma, whereas small interfering RNA (siRNA) against ETFA or ZFP36 (si‐ETFA or si‐ZFP36) were obtained from Abcam. Empty pcDNA3.1 vector and scrambled shRNA or siRNA were used as negative controls. Furthermore, using Lipofectamine™ 2000 (Invitrogen, USA), SW620 and HCT116 cells were transfected with the above‐mentioned plasmids as needed. After 48 h of transfection, cells were collected for different subsequent uses.

Quantitative real‐time PCR (qRT‐PCR)

From cultured cells, total RNA was isolated with the aid of TRIzol reagent (Thermo Fisher Scientific, USA). Furthermore, reverse transcription using GoScript reverse transcription system (Qiagen GmbH, Germany) was performed. qRT‐PCR analysis was performed using the SYBR‐Green PCR Master Mix kit (Takara, Dalian, China) and the ABI 7900 detection system (Applied Biosystems, USA). The relative expression of genes was calculated and normalized to GAPDH using the 2−ΔΔCt method.

CCK‐8 cell viability assay

Using Cell Counting Kit‐8 (CCK‐8) (Dojindo Laboratories, Kumamoto, Japan), cell viability was measured as instructed by the manufacturer. In 96‐well plates, cells were seeded at a density of 1 × 104 cells/well. After the indicated treatments, CCK‐8 solution was added to the medium at a dilution of 1:10 and cells were incubated at 37℃ for 4 h. Absorbance was measured using a microplate reader at 450 nm (Bio‐Rad Labs, Sunnyvale, CA).

Colony formation test

Cells (800 cells) were seeded into each well in a 12‐well plate in triplicate, and the cells were cultured for 2 weeks with a change in medium for every 3 days. Furthermore, colonies containing more than 50 cells were methanol fixed and stained with crystal violet (Sigma, USA). The colonies formed were counted manually, and the rate of colony formation was calculated using the formula: colony formation rate = (number of colonies / number of inoculated cells) × 100%.

Transwell assay

To assess the levels of cell migration and invasion, transwell assays were performed using Transwell chambers (BD Biosciences, San Jose, CA, USA). To measure cell migration, a cell suspension containing 1 × 105 cells was added to the upper chamber, and to the lower chamber, 600 μl of DMEM containing 10% FBS was added. After 24 h of incubation, cells were fixed with methanol and stained with crystal violet. Furthermore, cells were counted from images obtained under a microscope from three random fields. For assessment of cell invasion assay, the upper chamber was precoated with Matrigel at 37℃ for 1 h. The other steps were similar to the transwell migration assay.

Immunofluorescence (IF)

Immunofluorescence staining kit (Elabscience, China) was used to detect the expression of E‐cadherin and N‐cadherin in SW620 and HCT116. The cells were collected and fixed with 0.5 ml of fixation solution for 10 min. After the final centrifugation, the slides were sealed with mounting fluid for 60 min. The cells were then incubated with the diluted primary antibody (Abcam, UK) overnight at 4℃ and then incubated with the fluorescently labelled secondary antibody (Abcam, UK) for 60 minutes in the dark. Finally, fluorescence was observed with a fluorescence microscope (Leica, Germany).

In vivo mouse assays

BALB/c athymic female nude mice aged 5–6 weeks were obtained from Animal centres. In vivo tumorigenic effects were assessed using methods previously described. Briefly, 1 × 106 SW620 cells (per mouse) were injected subcutaneously into the flanks of nude mice. Size of the tumour was measured once every 4 days, and the tumour volume was also calculated simultaneously. Post five weeks, mice were sacrificed and the tumour samples were collected. To assess liver metastasis, mice were first anesthetized by isoflurane inhalation (0.5%–1.0%). With an incision of 1 cm in the left abdomen lateral region, 106 cells were injected into the spleen. The incision was revealed and after 5 or 6 weeks, mice were sacrificed, and spleen and liver were removed and paraffin embedded.

Immunohistochemistry staining

Initially, tissue slides were blocked using goat serum for 15 min at room temperature. Furthermore, the slides were incubated with anti‐F4/80 antibody (Santa Cruz Biotechnology) overnight at 4℃. PBS washings of the slides were performed, and the slides were incubated with secondary antibodies conjugated with horseradish peroxidase at 37℃ for 30 min. Visualizations of the sections were performed using diaminobenzidine tetrahydrochloride. Counterstaining of the sections was performed using haematoxylin. The sections which were stained were observed under a light microscope (DP73; Olympus).

Chromatin immunoprecipitation (ChIP)

Using Magna ChIP kit, ChIP analysis was performed according to the manufacturer's instructions (Millipore, Billerica, Mass.). Initially, the cells were fixed with formaldehyde for 30 min to specifically fix the DNA and protein cross‐link. Using sonication, DNA was isolated from the cells and fragmented into 200–1000 bp. The fragmented DNA was subsequently incubated overnight with protein A/G beads having anti‐EZH2 or IgG antibodies (negative control). The DNA fragments were further quantified using qRT‐PCR.

RNA pull‐down assay

RNA pull‐down assay was performed as previously described. Briefly, antisense RNA was incubated with 1 mg of protein extract. Furthermore, the extract was passed through streptavidin beads and subsequently washed. Using gel electrophoresis, proteins were resolved and the bands of interest were excised and analysed using mass spectrometry. Furthermore, using the Biotin RNA labelling kit (Genisphere, Hatfield, PA), the RNA pull‐down assay was performed.

RNA immunoprecipitation (RIP)

Using Millipore EZ‐Magna RIP RNA‐Binding Protein Immunoprecipitation kit, RIP assays were performed (Millipore, Bedford, MA, USA) based on the manufacturer's recommendations. RNAs which were precipitated were further tested using qRT‐PCR. For the RIP assay, the antibodies used were as follows: anti‐ZFP36 (Abcam, ab230507), anti‐Ago2 (Abcam, ab186733) and anti‐IgG (Millipore, PP64). IgG was used as the negative control and input as the positive control. All experiments were repeated thrice.

Luciferase reporter assay

To examine the effect of EZH2 on ETFA transcription, as well as that of NBPF4 on EZH2 transcription, pGL3 plasmids containing a firefly reporter were used to construct recombinant plasmids with ETFA or EZH2 promoter. Then, the recombinant plasmids were transfected into appropriate HEK293T cells (with altered EZH2 or NBPF4 expression) by using Lipofectamine™ 2000 (Invitrogen, USA). To detect the binding of miR‐186‐5p to NBPF4 or EZH2, the psiCHECK2 vector (Promega, Madison, WI) was applied to obtain NBPF4‐WT, NBPF4‐Mut, EZH2‐WT and EZH2‐Mut, and then, HEK293T cells were co‐transfected. Plasmids and miR‐NC, miR‐17‐3p mimics or miR‐17‐3p mimics together with pcDNA3.1/ NBPF4. 48 h post‐transfection, the luciferase activities were evaluated with dual‐luciferase reporter assay system (Promega, Madison, WI, USA).

Western blotting

Initially, the protein was isolated from cells and tissues using RIPA lysis buffer (Beyotime, Guangzhou, China). Protein concentration was assessed using BCA protein assay kit (Pierce, Rockford, USA) based on the manufacturer's instructions. Western blotting was performed as previously described.

Statistical Analysis

Data collected from at least three independent experiments were analysed using SPSS 17.0 (SPSS Inc, USA), and the mean of the results is expressed as the mean ± SD. Then, statistical significances between two groups were assessed using Student's t test or for three groups or more, one‐way ANOVA was used. Differences with p values below 0.05 were defined as statistically significant.

RESULTS

NBPF4 negatively regulates tumorigenesis by modulating members of the EMT pathway

In this study, we assessed the role of long non‐coding RNA NBPF4 in CRC. Initially, we checked the levels of NBPF4 in CRC lines (SW620, HT29, SW480, LoVo and HCT116) (Figure 1A). Compared with the control cell line NCM460, the expression level of NBPF4 was significantly lower in the CRC cell lines. Specifically, HCT116 had the least expression and SW620 had higher expression of NBPF4. Hence, we chose HCT116 and SW620 lines for subsequent experiments. Using pcDNA 3.1 vector and short hairpin RNA (shRNA), we either overexpressed or silenced NBPF4 in HCT116 and SW620, respectively. We confirmed the efficacy of overexpression and silencing using qRT‐PCR analysis (Figure 1B, C). Furthermore, we assessed their proliferation capacity using CCK‐8 assay and it was evident that overexpression of NBPF4 decreased proliferation in HCT116 cells, whereas its silencing increased SW620 cell's proliferation (Figure 1D, E). We also performed colony formation assay and observed that the overexpression of NBPF4 decreased colony‐forming units, when compared to cells transfected with an empty vector in HCT116 cells. However, silencing of NBPF4 in SW620 increased the colony‐forming units (Figure 1F–I). Next, we performed migration and invasion assays and observed that overexpression of NBPF4 decreased migration and invasion, whereas silencing did the opposite (Figure 1J–M). To further explore to the role of NBPF4 in tumorigenesis, we assessed the influence levels of EMT markers in the presence or absence of NBPF4. Observations from our Western blot analysis indicated that overexpression of NBPF4 increased E‐cadherin levels and decreased N‐cadherin and vimentin levels. Alternatively, silencing of NBPF4 decreased E‐cadherin and increased N‐cadherin and vimentin levels (Figure 1N). These results were further confirmed using immunofluorescence staining in both HCT116 and SW260 cells (Figure 1O).

FIGURE 1

NBPF4 negatively regulates tumorigenesis by modulating members of the EMT pathway. (A) mRNA expression profiles of various CRC cell lines SW620, HT29, SW480, LoVo, HCT116 and normal colonic epithelial cells NCM460. (B) Cells were transfected with either NBPF4 or empty vector, and relative NBPF4 mRNA expression was assessed in HCT116. (C) Cells transfected with either shNBPF4 or shCtrl, and relative NBPF4 mRNA expression was assessed in SW620. (D) CCK‐8 proliferation assay of HCT116 cells after transfection with NBPF4 or empty vector. (E) CCK‐8 proliferation assay of SW620 after cells was transfected with shNBPF4 or shCtrl. (F, G) Colony formation assay of HCT116 after transfection with NBPF4 or empty vector. (H, I) Colony formation assay of SW620 after transfection with shNBPF4 or shCtrl. (J, K) Assessment of migration and invasion of HCT116 cells after cells were transfected with NBPF4 or vector by Transwell assay. (L, M) Assessment of migration and invasion of SW620 cells by Transwell assay after cells were transfected with shNBPF4 or shCtrl. (N) Detection of EMT‐related protein expression levels using Western blotting. (O) Immunofluorescence detection of EMT‐related proteins. *p < 0.05; **p < 0.01, ***p < 0.001 versus NCM460 or empty vector or shCtrl

Expression of NBPF4 is associated with decreased tumorigenesis in mice models

We further developed an in vivo mice model using BALB/c athymic female nude mice to assess the effects of NBPF4 on tumorigenesis. Initially, 1 × 106 cells SW620 cells (per mouse; overexpressed for NBPF4 or silenced for NBPF4) were injected subcutaneously into the flanks of nude mice. Volume of the tumour was measured every 4 days until day 20 post‐injection. It was evident that mice injected with cells overexpressing NBPF4 had significantly smaller tumour volume and weight at day 20, compared to the mice injected with either shNBPF4 or control cells. Furthermore, mice injected with cells silenced for NBPF4 had a higher tumour volume and weight when compared to mice injected with either overexpressed NBPF4 or control cells (Figure 2A–C). We also assessed the levels of EMT markers in the tumour samples at day 20 post‐injection. Similar to the in vitro results, NBPF4 overexpression increased E‐cadherin levels and decreased N‐cadherin and vimentin levels. Alternatively, silencing of NBPF4 decreased E‐cadherin and increased N‐cadherin and vimentin levels, when compared to controls (Figure 2D, E). Additionally, we also performed immunohistochemical staining of the tissue samples and confirmed the results using Western blotting (Figure 2F). To assess liver metastasis, mice were first anesthetized by isoflurane inhalation (0.5%–1.0%). With an incision of 1 cm in the left abdomen lateral region, 1 × 106 cells were injected into the spleen. The incision was revealed and after 6 weeks mice were sacrificed and the organs were separated for further analysis. Figure 2G indicates representative images of liver tissues from mice injected with cells containing shNBPF4, NBPF4 overexpression or control vector. Evidently, we observed fewer metastatic nodules in mice containing cells overexpressing NBPF4. Alternatively, mice containing cells silenced for NBPF4 had more metastatic nodules in the liver tissues, when compared to control mice (Figure 2G, H).

FIGURE 2

Expression of NBPF4 is associated with decreased tumorigenesis in mouse models. (A) Representative images of tumour size, (B) tumour volume and (C) tumour weight at 20 days post‐injection of cells. Western blotting (D) and qRT‐PCR (E) detection of EMT‐related proteins. (F) Protein levels of E‐cadherin, N‐cadherin and vimentin in tumours from indicated cells were estimated by IHC. Representative tissue images (G) and HE staining (H) of metastatic nodules. **p < 0.01; ***p < 0.001 versus control

NBPF4 decreased the progression of CRC through the inhibition of ETFA

Using the Human Protein Atlas (https://www.proteinatlas.org/, we identified that ETFA is highly expressed in CRC (Figure 3A), and we further validated its high expression levels in clinical samples and CRC cell lines (Figure 3B, C). Interestingly, HCT116 cells had high ETFA expression compared to the control. Furthermore, we overexpressed or silenced ETFA in HCT116 or SW620 cells, respectively (Figure 3D–G). Additionally, we checked the levels of ETFA in cells that were silenced or overexpressed for NBPF4. It was evident that overexpression of NBPF4 significantly decreased ETFA expression in HCT116 cells, whereas overexpression of NBPF4 and ETFA did rescue its expression (Figure 3H–K). Furthermore, silencing of NBPF4 increased ETFA expression in SW260, but simultaneously silencing NBPF4 and ETFA decreased ETFA expression (Figure 3L, M). These results indicated the expression levels of ETFA and NBPF4 were inversely proportional in CRC cells. To assess the effects of NBPF4 and ETFA on cell proliferation, we performed CCK‐8 assay on HCT116 and SW260 cells. Initially, overexpression of NBPF4 decreased the proliferation of HCT116 cells, but simultaneous overexpression of NBPF4 and ETFA significantly increased proliferation. Additionally, silencing of NBPF4 increased the proliferation of SW260 cells, but simultaneously silencing NBPF4 and ETFA significantly decreased its proliferation (Figure 3N, O). From these results, it was further clear that NBPF4 decreased proliferation through regulation of ETFA. Interestingly, we could observe similar results from our colony formation, migration and invasion assay, wherein it was clear that overexpression of NBPF4 decreased migration and invasion and simultaneous overexpression of ETFA increased the migration and invasion in HCT116 cells (Figure 3P, Q). Furthermore, we observed that overexpression of NBPF4 increased E‐cadherin expression, decreased N‐cadherin and vimentin expression levels, when compared to control. However, overexpression of both NBPF4 and ETFA simultaneously decreased NBPF4’s effect on EMT markers in HCT116 cells. In SW260 cells, silencing of NBPF4 decreased the expression of NBPF4, decreased E‐cadherin and increased N‐cadherin and vimentin. Alternatively, silencing of NBPF4 and ETFA returned the expression of E‐cadherin, N‐cadherin and vimentin to levels similar to that of the control (Figure 3R).

FIGURE 3

NBPF4 decreases the progression of CRC through the inhibition of ETFA. (A) Using Human Protein Atlas, we identified high expression of ETFA in CRC patients. (B) Cancer tissues or adjacent tissues (n = 20) were assessed for qRT‐PCR to detect ETFA expression. (C) mRNA expression profiles of various CRC cell lines SW620, HT29, SW480, LoVo, HCT116 and normal colonic epithelial cells NCM460. (D, E) Cells were transfected with ETFA or empty vector, and relative ETFA mRNA expression was assessed in HCT116. (F, G) Cells were transfected with shETFA or shCtrl, and relative ETFA mRNA expression was assessed in SW620. (H, I) Cells transfected with NBPF4 or empty vector and relative ETFA mRNA expression was assessed in HCT116. (J, K) Cells were transfected with shNBPF4 or shCtrl, and relative ETFA mRNA expression was assessed in SW620. (L, M) Alterations in ETFA mRNA levels in the presence (overexpression) or absence (silencing) of NBPF4 and/or ETFA. (N, O). Proliferation assay in the presence (overexpression) or absence (silencing) of NBPF4 or ETFA. (P) Colony formation assay in the presence (overexpression) or absence (silencing) of NBPF4 and/or ETFA. (Q) Migration and invasion assay in the presence (overexpression) or absence (silencing) of NBPF4 and/or ETFA. (R) EMT‐associated protein expression levels using Western blotting. **p < 0.01; ***p < 0.001 versus NCM460 or Non‐cancer or empty vector or shCtrl. #p < 0.05; ##p < 0.01

EZH2 plays a key role in CRC by regulating ETFA expression

From COAD and READ databases, we observed that EZH2 is highly expressed in CRC and based on gene expression profiling interactive analysis (GEPIA), we observed that there is a positive correlation between ETFA and EZH2 expression (Figure 4A–C). We also assessed EZH2 expression levels in different CRC cell lines and observed that EZH2 was highly expressed in HCT116 cells (Figure 4D). We further silenced EZH2 and observed a lowered expression of ETFA in SW620 cells, compared to the control. We also overexpressed EZH2 and observed an increase in the expression of ETFA in HCT116 cells (Figure 4E–L). Using ChiP‐seq analysis, we precipitated the EZH2 protein and assessed the ETFA promoter methylation levels (Figure 4M). We also assessed the levels of trimethylation at the 27th lysine position of the histone residue (H3K27me3) of ETFA promoter after silencing of EZH2, and clearly, we observed an increase in H3K27me3 of ETFA promoter, when compared to the control (Figure 4N, O). These results indicated that EZH2 potentially regulates ETFA through methylation of the ETFA promoter. However, to further confirm if EZH2 regulated the expression of ETFA at the mRNA level, we performed luciferase activity. Initially, the SW620 cells were initially silenced for EZH2 and with the aid of pGL3 ‐ETFA promotor luciferase plasmid, we identified that lack of EZH2 significantly decreased the luciferase activity associated with ETFA transcriptional activation. Additionally, in HCT116 cells, which overexpressed EZH2, luciferase activity was significantly increased, when compared to the empty vector control (Figure 4P, Q). These results suggest that EZH2 potentially binds and regulates the expression of ETFA.

FIGURE 4

EZH2 plays a key role in CRC by regulating ETFA expression. Correlation between the expression of EZH2 and CRC based on COAD (A) and READ (B) from TCGA database. (C) The expression of ETFA and EZH2 in COAD as predicted by TCGA database. (D) The relative expression of EZH2 in CRC cell lines and NCM460 cells was detected by qRT‐PCR. (E–H) The expressions of EZH2 and ETFA in EZH2 silenced SW620 cells or EZH2 overexpressed HCT116 cells were determined by qRT‐PCR. (I) ChIP analysis indicated the interaction between EZH2 and ETFA promoter. (J‐M) ChIP analysis and luciferase assay assessing the interaction between EZH2 and ETFA in the presence or absence of EZH2. *p < 0.05; **p < 0.01; ***p < 0.001 versus NCM460 or empty vector or shCtrl or IgG. #p < 0.05; ##p < 0.01

NBPF4 collectively regulates the stability of EZH2 mRNA in a ZFP36‐dependent manner

Furthermore, to identify the association between EZH2 and NBPF4, we silenced NBPF4 and observed that the EZH2 mRNA and protein levels were highly expressed. Subsequently, overexpression of NBPF4 significantly decreased EZH2 mRNA and protein levels (Figure 5A–D). In addition, we performed a luciferase assay with the pGL3‐EZH2 promoter and observed that in the presence of shNBPF4 or NBPF4, the luciferase activity remained unchanged. We therefore hypothesized that NBPF4 regulated EZH2 at the post‐transcriptional stage in the CRC (Figure 5E). A recent study demonstrated that RNA binding proteins (RBPs) are gene expression regulators and are modulated by lncRNA. Using StarBase 2.0 (http://starbase.sysu.edu.cn/starbase2/index.php, we predicted that ZFP36 is a shared RBP that interacts with both NBPF4 and EZH2 mRNA. Initially, using RIP analysis, we confirmed that ZFP36 significantly harvested NBPF4 and EZH2 in SW620 and HCT116 cells. Similarly, after silencing of NBPF4, ZFP36 bound EZH2 mRNA levels were significantly increased, but overexpression of NBPF4 decreased the EZH2 mRNA levels (Figure 5H, I). NBPF4’s effect on the inhibition or activation of EZH2 depends on the up‐or down‐regulation of ZFP36 (Figure 5J, K). In summary, NBPF4 collectively regulates the stability of EZH2 mRNA in a ZFP36‐dependent manner.

FIGURE 5

NBPF4 collectively regulates the stability of EZH2 mRNA in a ZFP36‐dependent manner. The effect of NBPF4 on EZH2 expression in SW620 (A, B) and HCT116 (C, D) CRC cells were assessed using qRT‐PCR and Western blot analysis. (E) Luciferase reporter gene assay was performed to assess the effect of NBPF4 on EZH2 transcription. (F, G) RIP analysis verified the common interaction of ZFP36 with NBPF4 and EZH2 mRNA. (H, I) Evaluate the effect of NBPF4 on ZFP36 interacting EZH2 mRNA by pre‐performed RIP analysis. qRT‐PCR results of EZH2 levels in the SW620 (J) and HCT116 (K) CRC cells. **p < 0.01, ***p < 0.001 versus empty vector or shCtrl or IgG

miR‐17‐3p regulates the expression of NBPF4 and EZH2

Finally, through Starbase, we predicted that NBPF4 and EZH2 are both targets of miR‐17‐3p (Figure 6A). With the aid of RNA pull‐down, we observed that in the presence of overexpressed WT miR‐17‐3p, both EZH2 and NBPF4 levels were increased in both SW620 and HCT116 cells (Figure 6B, C). We further developed miR‐17‐3p mutants and observed that in its presence, the expression levels of both EZH2 and NBPF4 were similar to that of the control (Figure 6D, E). This indicated the potential of miR‐17‐3p to bind and regulate EZH2 and NBPF4. Furthermore, RIP analysis confirmed that miR‐17‐3p, EZH2, and NBPF4 are bound to the ETFA promoter. Additionally, we developed luciferase experiments with mutations at the 3’‐UTR of EZH2 and NBPF4, and observed that in cells with WT‐NBPF4, miR‐17‐3p mimics can significantly decrease the luciferase activity potentially due to the binding and suppression of NBPF4. Furthermore, this decrease in function could be recovered slightly when NBPF4 was overexpressed. However, for the cells with the NBPF4‐mutation, there was no significant difference in the luciferase activity when compared to the control (Figure 6F, G). These results indicate that miR‐17‐3p binds to the 3’UTR of NBPF4 and regulates its expression. Furthermore, we silenced NBPF4 and observed increased expression of ETFA, whereas simultaneous silencing of EZH2 and NBPF4 significantly decreased the expression of ETFA. These results further confirmed that NBPF4 negatively regulates ETFA expression, whereas EZH2 is essential for appropriate expression of ETFA (Figure 6H, I). We have shown that the above three molecules exist together in RNA‐induced silencing complexes. In combination with miR‐17‐3p, a competitive relationship between NBPF4 and EZH2 mRNA could be observed. It is evident that EZH2 mediates the regulation of ETFA through ZFP36 stabilized‐NBPF4. Additionally, by regulating EZH2 mediated methylation on the ETFA promoter, NBPF4 decreased the expression of ETFA (Figure 6J, K).

FIGURE 6

miR‐17‐3p regulates the expression of NBPF4 and EZH2. (A) StarBase 2.0 based prediction of the binding sequence of wild‐type and mutant EZH2 or NBPF4 and miR‐17‐3p. (B, C) RNA pull‐down analysis confirmed the interaction of NBPF4 and EZH2 mRNA with miR‐17‐3p. (D, E) RIP analysis confirmed the interaction of NBPF4, miR‐17‐3p and EZH2 mRNA. (F, G) The luciferase reporter gene assay explained that NBPF4 and EZH2 mRNA bind miR‐117‐3p in a competitive manner. (H‐K) The effect of NBPF4 on ETFA expression was verified by performing qRT‐PCR and ChIP. **p < 0.01; ***p < 0.001 versus NC or IgG. #p < 0.05; ##p < 0.01

DISCUSSION

Competitive endogenous RNA (ceRNA) are a group of ncRNA that have recently gained immense interest due to their role in regulation of transcriptional and post‐transcriptional activities., Many studies have observed that certain transcripts can competitively bind and inhibit the ability of non‐coding RNA to bind to other targets. This lack of binding could lead to transcriptional activation or inactivation and thus lead to the suppression or activation of certain pathways. Interactions between miRNA and miRNA response elements (MREs) are key steps in the regulation of certain RNA. MREs are either located in the 3′ untranslated region (3′ UTR), 5′ UTR, or coding sequence of the RNA and interestingly they are also present in lncRNA sequences., Adding to the complexity, similar MREs occur in multiple regions of different RNAs indicating that the same ncRNA can bind and regulate multiple RNAs at the same time., , Due to the presence of varied ncRNA pool that have multiple targets, there exists a competition between multiple RNAs sharing the same MREs to bind to the ncRNA, thus leading to a competition between them. Recent research has indicated that such MREs exist in lncRNA as well and thus in turn can be controlled by other ncRNAs., Interestingly, such activity can determine the progression of tumorigenesis in many different cancers., , In this study, we identified a lncRNA, NBPF4 to be highly downregulated in CRCs, and its overexpression led to decreased tumorigenesis and metastasis in our in vitro and in vivo models, respectively. In vitro, overexpression of NBPF4 can significantly reduce tumour cell viability, tumour cell proliferation and invasion, while affecting EMT. In vivo, NBPF4 significantly reduces the size of animal tumours, while also affecting EMT, and significantly reducing tumour metastasis (Figures 1 and 2). Furthermore, to identify the mechanism regulating its expression in CRC, we explored other key players that were regulated upstream or downstream of NBPF4 in CRC. Interestingly, using the human protein atlas, we observed that electron transfer flavoprotein subunit alpha (ETFA) was highly regulated in CRCs, and down‐regulation of ETFA significantly decreased the progression of CRC. Studies have shown that overexpression of NBPF4 will reduce the expression of ETFA. Further, ETFA can reverse the tumour cell viability, proliferation, and invasion which was decreased by NBPF4. Finally, ETFA also has an impact on EMT (Figure 3). However, relatively less information is known about ETFA in cancer. Previously, studies indicated that ETFA could be a prognostic marker for glioblastoma in European and Chinese populations., Furthermore, to assess the link between NBPF4 and ETFA, we explored possible upstream mechanisms and identified that EZH2 is also upregulated in CRCs. Interestingly, we further identified that EZH2 is required for the methylation and activation of ETFA promoter (Figure 4). Overexpression of EZH2 has been associated with many cancers such as hepatocellular carcinoma, breast cancer, bladder cancer and CRC., , A study by Chen et al. identified that EZH2 overexpression was associated with increased tumour size, stage and metastasis and thus could act as a potential prognostic marker for CRC. EZH2 functions through di‐or trimethylation of H3K27, and studies indicate it is essential for the proliferation of cancer cells. Studies have also identified mutations and loss of function in genes that usually antagonize EZH2. UTX (ubiquitously transcribed tetratricopeptide repeat gene on X chromosome), a histone demethylase usually functions by removing the EZH2’s methylation of the di‐or trimethylated H3K27, and mutations in UTX has been identified in many cancers such as bladder, pancreatic, renal, medulloblastoma and multiple myeloma., , , In our study, we identified that EZH2 overexpression increased tumorigenesis in CRC cell lines, and its expression is negatively correlated with NBPF4 (Figure 5). However, NBPF4 and EZH2 did not interact at an RNA level, as indicated by the luciferase activity assay. Further, RBPs are also considered to be important regulators in gene expression. Recently, the involvement of RBPs in gene expression and its regulation by lncRNAs has been revealed., For example, overexpression of ZFP36 can reverse the upregulation of EZH2 caused by shNBPF4 and vice versa. Furthermore, when we explored the commonalities between them from RNA immunoprecipitation and qRT‐PCR results, we identified that both NBPF4 and EZH2 are bound and regulated by RNA binding protein ZFP36. ZFP36 is an RBP associated with tumorigenesis and is involved in transcription regulation and RNA processing., Evidently, previous studies have shown that loss of ZFP36 enhances EMT transition in CRC. On the other hand, the ceRNA network has been widely regarded as one of the main mechanisms by which lncRNA functions in cancer., At present, we have further explored that NBPF4 participates in the ceRNA regulatory network by competitively binding miR‐17‐3p and EZH2. Alternatively, we also established that miR‐17‐3p bound and regulated the expression of NBPF4 and EZH2 (Figure 6). Specifically, miR‐17‐3p inhibits NBPF4, and however, NBPF4 competitively binds to ZFP36 and escapes the negative regulation by miR‐17‐3p.

CONCLUSION

Hence, in this study, we identified that NBPF4 modulates tumorigenesis in CRC through regulation of ETFA via miR‐17‐3p/ZSP36/EZH2 axis. This identification could aid in the development of prognostic markers and potential treatment strategies for CRC.

CONFLICT OF INTEREST

The authors confirm that there are no conflicts of interest.

AUTHOR CONTRIBUTIONS

Wankun Chen: Conceptualization (lead); Formal analysis (equal); Funding acquisition (equal); Investigation (lead); Writing‐review & editing (equal). Di Zhou: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Writing‐review & editing (equal). Zhaoyuan Chen: Investigation (equal); Methodology (equal); Validation (equal). Ke Nan: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Writing‐review & editing (equal). Jiahui Gu: Data curation (equal); Investigation (equal); Writing‐original draft (equal). Feng Ge: Funding acquisition (equal); Investigation (equal); Methodology (equal); Validation (equal); Writing‐original draft (equal). Jin Long Liu: Supervision (equal); Writing‐review & editing (equal). Hao Zhang: Writing‐original draft (lead). Changhong Miao: Conceptualization (equal); Funding acquisition (equal); Project administration (equal); Supervision (equal); Writing‐review & editing (equal).

ETHICAL APPROVAL

Upon receiving written informed consent from all patients, human colon samples were obtained. The research was approved by the Ethics Committee of Zhongshan Hospital, Fudan University, China (B2020‐062R2).

CONSENT FOR PUBLICATION

All authors have agreed to publish this manuscript.