1 Introduction
Nasopharyngeal carcinoma (NPC) is a squamous cell carcinoma of the head and neck with high aggressiveness [
1]. Diagnosis of advanced NPC happens in more than 70% of tumor cases due to the insidious symptoms of NPC, tumor aggressiveness, and poor patient awareness [
2]. Under the comprehensive treatment of radiotherapy, the 5-year survival rate of NPC has reached about 85–90% [
3,
4]. However, about 8–10% of patients still experience disease recurrence or distant metastasis, which is the main reason for treatment failure in some patients [
4]. Therefore, the identification of novel and sensitive biomarkers is needed to develop new therapeutic strategies.
Unlike normal differentiated cells, most cancer cells rely on aerobic glycolysis to obtain high energy to rapidly adapt to growth, a phenomenon known as the "Warburg effect" [
5‐
7]. Therefore, a proposal that targeted blockade of Warburg effect has therapeutic prospects to control cancers [
8,
9]. Aberrant glycolysis is closely associated with NPC proliferation and metastasis [
10‐
12]. However, the mechanism for regulating metabolism at the post-transcriptional level is still insufficient, such as the role of circular RNAs (circRNAs) and microRNAs (miRNAs).
The study of circRNAs has become the frontier of biomedical research [
13,
14]. CircRNAs with covalently closed loop structures are characterized by high stability and abundance [
15,
16]. The competing endogenous RNA mechanism is recognized as the main regulatory mechanism of circRNAs in regulating biological processes [
17]. Furthermore, many studies have revealed that circRNAs participate in malignant biological processes, including Warburg effect [
18,
19]. Many circRNAs have been identified to be differentially expressed in NPC, and have the potential to regulate the malignant behavior of NPC [
20], such as circular RNA Ran-binding protein 17 [
21] and circ-itchy E3 ubiquitin protein ligase [
22]. Our research group noticed that hsa_circ_0044569 (circCOL1A1) is a novel oncogenic circRNA in gastric cancer that is involved in the malignant behavior of cancer cells [
23]. In the preliminary experiment, circCOL1A1 was found to be significantly up-regulated in NPC tissues by PCR analysis. This finding suggests that circCOL1A1 may play a role in the biological process of NPC. Given the limited understanding of the role of circCOL1A1 in NPC, our study aims to fill this gap.
The purpose of this study was to investigate the effect of circCOL1A1 on the malignant behavior of NPC cells and its mechanism. These results may provide a new perspective for Warburg effect in NPC progression, and more importantly, may also provide a new theoretical basis for targeted therapy of NPC.
2 Materials and methods
2.1 Clinical samples
NPC tissues (n = 42) and normal nasopharyngeal epithelial tissues (n = 16) were collected from The First Affiliated Hospital of Jinan University. All tissue samples were confirmed by pathological examination. Neither radiation, chemotherapy nor other treatments were received by patients. The resected samples were stored at − 80 °C until RNA or protein extraction. The ethics committee of The First Affiliated Hospital of Jinan University provided approval for the study and all patients provided written informed consent (Ethical approval number: 202108GZ2161). Table
1 shows clinical information on the subjects.
Table 1
Clinical characteristics of healthy subjects and NPC patients
Average age (years) | 45.94 | 43.02 | 0.127 |
Age standard deviation (years) | 14.56 | 15.27 |
Average weight (kg) | 80.75 | 75.79 | 0.262 |
Weight standard deviation (kg) | 16.02 | 14.50 |
BMI average | 24.94 | 26.22 | 0.324 |
BMI standard deviation | 5.54 | 5.02 |
Smoking history (yes) (cases) | 6 | 21 | 0.576 |
Drinking history (yes) (cases) | 6 | 24 | 0.296 |
Hypertension history (yes) (cases) | 3 | 17 | 0.212 |
2.2 Cell culture
Human NPC cell lines SNU46, SUNE1, CNE-1, HONE-1 and 6-10B, and human immortalized nasopharyngeal epithelial cell line (NP-69) were obtained from Cell Bank of the Chinese Academy of Sciences (Shanghai, China). Cells were grown in Roswell Park Memorial Institute-1640 medium (Gibco, USA), 10% fetal bovine serum (FBS, Gibco), and 1% streptomycin/penicillin (Invitrogen, Carlsbad, USA) at 37 °C under 5% CO2. All cells were validated by short tandem repeat assays and detected negative for mycoplasma contamination.
2.3 Real-time reverse transcriptase-polymerase chain reaction (RT-qPCR)
Total RNA extracts were obtained using Trizol (Invitrogen). The complementary DNA (cDNA) synthesis for circRNA and mRNA was performed using the HiScript II first-strand cDNA synthesis kit (Vazyme, Nanjing, China). The cDNA synthesis for miRNA was carried out using the All-in-One miRNA first-strand cDNA synthesis kit (GeneCopoeia, Rockville, MD, USA). Gene expression was detected on a 7500 Real-Time PCR System (Applied Biosystems). GAPDH and U6 were internal controls. Gene expression was normalized by 2
−ΔΔCt method. All primers are listed in Table
2.
Table 2
RT-qPCR primer sequence
Has_circ_0044569 | Forward: 5'-ACCCACCGACCAAGAAACC-3' |
Reverse: 5'-TTGTCGCAGACGCAGATC-3' |
MiR-370-5p | Forward: 5'-ACACTCCAGCTGGGCAGGTCACGTCTCTGC-3' |
Reverse: 5'-TGGTGTCGTGGAGTCG-3' |
PTMA | Forward: 5'-GAGGTAGACGAAGAAGAG-3' |
Reverse: 5'- GAAGTGGAGGGTGAATAG-3' |
GAPDH | Forward: 5'-CACCCACTCCTCCACCTTTG-3' |
Reverse: 5'-CCACCACCCTGTTGCTGTAG-3' |
U6 | Forward: 5'-CTCGCTTCGGCAGCACA-3' |
Reverse: 5'-AACGCTTCACGAATTTGCGT-3' |
2.4 Subcellular isolation
circCOL1A1 localization was assessed using PARIS™ kit (Thermo Fisher Scientific). Cells were subjected to incubation in a lysis buffer followed by centrifugation at 12,000 × g. The resulting supernatant and nuclear precipitation were utilized for the extraction of RNA. Subsequently, RT-qPCR was conducted to evaluate the levels of circCOL1A1 in both the cytoplasmic and nuclear compartments of cells. 18S rRNA was chosen as the control for the cytoplasmic portion, while U6 served as the control for the nuclear portion.
2.5 Actinomycin D and RNase R assays
To examine the resilience of circCOL1A1 within the cellular milieu of SUNE1 cells, we conducted a series of assays utilizing actinomycin D and RNase R treatments. For the actinomycin D assay, SUNE1 cells were cultivated in a nutrient-rich medium fortified with 2 μg/mL of actinomycin D (Sigma, MO, USA) to inhibit transcriptional activity. A parallel culture was maintained in a medium containing dimethyl sulfoxide (Sigma) to serve as the vehicle control. In parallel, for the assessment of RNA stability against enzymatic degradation, total RNA from SUNE1 cells was incubated with RNase R (3 U/μg, obtained from Geneseed, Guangzhou, China), an enzyme that selectively digests linear RNA, or treated with water that had been rendered inert with diethyl pyrocarbonate (Sigma) to ensure an RNase-free environment. The quantification of circCOL1A1 and the linear COL1A1 isoform was meticulously performed via RT-qPCR to evaluate the differential stability and expression post-treatment.
2.6 Plasmid engineering
Small interfering RNA targeting circCOL1A1 and prothymosin alpha (PTMA) and negative control (NC) (si-circCOL1A1, si-PTMA, si-NC), overexpression plasmids targeting circCOL1A1 and PTMA, and NC (oe-circCOL1A1, oe-PTMA and oe-NC), miR-370-5p mimic/inhibitor, mimic/inhibitor NC were purchased from Gene Pharma (Shanghai, China). Vector and oligonucleotide were transfected into cells with Lipofectamine 3000 (Invitrogen). After 48 h, the transfection efficiency was assessed by RT-qPCR.
2.7 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) method
A total of 5 × 103 cells were introduced into 96-well plates and incubated for 24, 48, or 72 h. Following the removal of the supernatant, 100 μL MTT solution (0.5 mg/mL, Sigma) was added to each well and incubated for a duration of 4 h. Subsequently, the purple crystal was dissolved using dimethyl sulfoxide (150 μL, Sigma). Finally, the absorbance at 570 nm was measured using a microplate reader (Bio-Rad, Hercules, CA, USA).
2.8 Colonies detection
A total of 5 × 103 cells per pore were introduced onto a 6-well plate. Following a two-week incubation period, the cells were immobilized using 4% paraformaldehyde (Sigma) and subsequently subjected to staining with 0.5% crystal violet (Sigma). Enumeration of colonies was performed utilizing an inverted microscope (Nikon, Tokyo, Japan).
2.9 Transwell assays
Transwell chambers with 8-μm pores (Corning, NY, USA) were utilized to determine cell invasion and migration. The chambers were coated or not coated with Matrigel (50 μL, BD Biosciences, NJ, USA) [
24]. A total of 5 × 10
4 cells suspended in serum-free medium were introduced into the upper chamber (upper chamber volume 200 μL). The lower chamber (lower chamber volume 600 μL) contained medium supplemented with 10% FBS. After 24 h, the cells were fixed with paraformaldehyde and stained with 0.1% crystal violet. The resulting samples were observed under a microscope at a magnification of 100 times (Olympus). Cell counts were determined using image analysis software.
2.10 Glycolysis analysis
Cellular glucose consumption, lactic acid production and adenosine triphosphate (ATP) levels were measured as previously mentioned [
25]. Glucose assay kits (Sigma), lactate colorimetric/fluorometric kits, and ATP assay kit (Thermo Fisher Scientific) were purchased to determine glucose consumption, lactate production, and ATP level, respectively.
2.11 ATP/adenosine diphosphate (ADP) detection
ATP/ADP ratio was measured using the ApoSENSOR ADP/ATP Ratio Assay Kit (#K255-200, BioVision) [
26]. The luminescence was quantified using a spectroscopic technique (Molecular Devices, Cal, US). A total of 1 × 10
4 cells were introduced into the photometer and subsequently incubated with a nucleotide release buffer. Following this, 1 μL ATP-monitoring enzyme was added. The luminescence was recorded for a duration of 1 min (Data A) and 10 min (Data B). Subsequently, ADP convertase was introduced and the corresponding sample values were obtained (Data C). The ATP/ADP ratio was calculated as Data A divided by (Data C minus Data B).
2.12 Nicotinamide adenine dinucleotide (NAD)+/Nicotinamide adenine dinucleotide (NADH) determination
NAD
+/NADH ratio was tested using the EnzyChrom™ NAD
+/NADH Ratio Assay Kit (E2ND-100, Bioassay Systems, CA, USA) [
27]. Cells were suspended in NAD
+ and NADH extraction buffers to conduct NAD
+ and NADH assays. A total of 1 × 10
5 cells were collected. The resulting homogenate was placed in a 1.5 ml Eppendorf tube, with 100 ml NAD extraction buffer added for NAD assay or 100 ml NADH extraction buffer added for NADH assay. Subsequently, 20 μl assay buffer was introduced to the extract, followed by the addition of 100 ml back extraction buffer to neutralize the extract. The mixture was then centrifuged, and the resulting supernatant was transferred to the working reagent. Optical density values at 565 nm were measured at both 0 and 15 min.
2.13 Immunoblotting
Proteins present in cells and tissues were extracted utilizing the radioimmunoprecipitation lysis buffer (Beyotime). The quantification of protein concentration was accomplished using the bicinchoninic acid Protein Assay Kit (Beyotime). Subsequently, all cell lysates, each containing 40 μg protein, were subjected to SDS-PAGE and subsequently transferred onto polyvinylidene fluoride membranes through electrophoretic imprinting. The membrane was sealed with Tween-Tris buffered saline containing 5% skim milk at room temperature for a duration of 2 h. Subsequently, it was incubated with primary antibodies, namely GAPDH (2118, Cell Signaling Technology), PTMA (YN2871, Immunoway), hexokinase 2 (HK2; 22029-1-AP, Proteintech), and pyruvate kinases type M2 (PKM2; 4053, Cell Signaling Technology). Following this, the membrane containing protein bands was incubated with horseradish peroxidase-polymerized secondary antibody at room temperature for another 2 h. The imprints were visualized using the enhanced chemiluminescence detection system. The software FluorChem2.0 was employed to calculate the integral density values.
2.14 Luciferase activity
The putative miR-370-5p target binding sequences and mutant sequences in circCOL1A1 and PTMA 3'untranslated region (UTR) were synthesized and cloned into the pmirGLO-promoter vector (Promega). Wild-type pmirGLO-circCOL1A1/PTMA (or mutant pmirGLO-circCOL1A1/PTMA) reporter plasmid was transfected with miR-370-5p mimic/inhibitor or mimic/inhibitor NC into cells via Lipofectamine 3000 (Invitrogen). The dual luciferase reporter gene assay system (Promega) measured firefly luciferase and renilla luciferase 48 h after transfection. The ratio of firefly to renilla luciferase activity is considered to be relative to luciferase activity.
2.15 RNA immunoprecipitation (RIP) experiment
RIP experiments were performed according to the manufacturer (Magna RIP™, Millipore, MA, USA) (Supplementary File 1). Cells were lysed using RIP buffer to release RNA and proteins. RNase inhibitors and DNase I were added to avoid RNA degradation and DNA contamination. The specific antibody Ago2 antibody was added to the lysate and incubated overnight to form antibody-protein complexes. Subsequently, mouse IgG-coupled A/G magnetic beads were added to allow binding of the beads to the antibody complex. The complexes were harvested with proteinase K and the immunoprecipitated RNA was then isolated. RNA concentration measurements and RNA quality assessment were performed by spectrophotometer (NanoDrop, Thermo Scientific, MA, USA). To demonstrate the presence of binding targets, purified RNA was collected and tested by RT-qPCR.
2.16 Data analysis
Data were obtained from at least three biological replicates and assessed by GraphPad Prism 9.0 software. Shown as mean ± standard deviation, the data were compared by one-way analysis of variance and Tukey's multiple comparison test or unpaired Student's t-test. Clinical association between NPC patients’ features and circCOL1A1 expression was analyzed by chi-square test. P < 0.05 was taken as a hallmark of significant difference.
4 Discussion
Local recurrence and distant metastasis of nasopharyngeal carcinoma hinder the clinical therapeutic effect of nasopharyngeal carcinoma, so it is crucial to explore its mechanism and find the active molecules in the mechanism. Recent studies have shown that miRNAs [
30] and circRNAs [
20] are in essence in NPC, such as miR-506 [
31] and circ_0000215 [
32]. Here, circCOL1A1 was determined to regulate the miR-370-5p/PTMA axis to produce oncogenic effects in NPC. Therefore, it can serve as a new model for studying circRNA regulatory mechanisms and developing tumor drug targets.
circCOL1A1 was only selected because it is highly abundant and stable in NPC cells. Furthermore, it was more rich in NPC tumor tissues and was associated with lymph node metastasis and TNM staging. Follow-up analysis of cases is requested in future studies to clarify whether circCOL1A1 is able to be an effective marker for liquid biopsy. In NPC, some circRNAs have been reported to be oncogenes that promote NPC development [
21]. A previous paper analyzes that circCOL1A1 is involved in the malignant behavior of cancer cells [
23]. Similarly, the current work revealed that circCOL1A1 downregulation blocked NPC cell malignancy, suggesting that circCOL1A1 may have an oncogenic function during NPC progression.
It is now recognized that even with adequate oxygen, cancer cells tend to meet their energy needs through aerobic glycolysis rather than oxidative phosphorylation, thereby promoting cancer cell proliferation and growth and ultimately promoting the malignant development of cancer [
33‐
35]. A large amount of lactate produced by glycolysis changes the microenvironment of tumor cells, inducing tumor cell invasion and metastasis [
36] whereas inhibiting aerobic glycolysis reduces the proliferation and invasive capacity of NPC cells [
37]. The work revealed a critical role for circCOL1A1 in NPC progression, but previous studies have provided little evidence for the function of circCOL1A1 in cancer metabolism. Interestingly, data analysis in this study showed that silencing circCOL1A1 decreased cellular glucose consumption, lactate production, ATP levels and ATP/ADP and increased NAD
+/NADH, as well as decreased glycolysis-related proteins HK2 and PKM2. Intriguingly, circCOL1A1 is posited to modulate the metabolic landscape of cancer cells through a miRNA sponging mechanism, particularly by sequestering a cohort of miRNAs including miR-370-5p, thereby attenuating their suppressive impact on multiple downstream target genes. These targets likely encompass key glycolytic enzymes such as HK2 and PKM2, whose hyperactivation is directly implicated in the Warburg effect [
38,
39]. Knockdown of circCOL1A1 resulted in the release of miR-370-5p's repressive hold, consequently reinforcing the inhibition of these enzymes and curtailing the upregulation of glycolysis. Moreover, a reduction in intracellular ATP levels may reflect a metabolic shift from glycolysis to oxidative phosphorylation [
40], a transition that potentially involves the activation of AMP-activated protein kinase (AMPK) [
41], a sentinel in cellular energy homeostasis under conditions of energy deprivation. Activation of AMPK is known to curb the growth and proliferation of tumor cells and propel a reprogramming of cellular energy metabolism [
41]. The observed depletion of ATP levels intimates that cells may be transitioning away from glycolysis as the principal energetic pathway, instead favoring aerobic metabolism [
42]. A direct repercussion of dampened glycolytic throughput could be an elevated NAD
+/NADH ratio, given that NAD
+ is more efficiently recycled during aerobic respiration [
43].
To explore the regulatory mechanism of circCOL1A1 in NPC, the subcellular localization of circCOL1A1 was analyzed and it was mainly localized in the cytoplasm of NPC cells, suggesting that circCOL1A1 mainly acts as a ceRNA of downstream miRNAs (miR-370-5p was selected) to play a post-transcriptional regulation role on downstream gene expression. Interestingly, miR-370-5p expression was lower in NPC tissues and cells. Previously, miRNAs have been shown to be important regulators of cancer progression and metabolic reprogramming [
44,
45]. MiR-370-5p confers tumor-suppressing actions in some human cancers and has the potential to suppress the malignant phenotype of cancer cells, such as breast, lung [
28], and colorectal cancer [
29]. Consistent with previous studies, our data proved that miR-370-5p had the potential to suppress the malignant phenotypes of NPC cells.
It is well known that miRNAs usually bind to the 3'UTR end of downstream target genes [
46]. As a target gene of miR-370-5p in this work, PTMA, a nuclear protein, has been reported to be activated in esophageal cancer [
47], colorectal cancer [
48], and glioma [
49]. Importantly, a recent study shows that PTMA is downregulated in NPC and involved in miR-1-induced apoptosis [
50]. Consistent with these findings, PTMA was expressed lowly in NPC.
Although this study found that circCOL1A1 promotes NPC progression by miR-370-5p/PTMA axis, it has not been further validated in vivo. In addition, circCOL1A1 expression in the serum of NPC patients should be detected in the follow-up analysis to verify whether it is a biomarker for blood biopsies of NPC patients. Finally, functional and signaling pathways should be expanded to elucidate how aberrant PTMA expression promotes malignant tumor progression through metabolic reprogramming.
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