Background
Nasopharyngeal carcinoma (NPC) has a high prevalence in southeast Asia, especially in southern China [
1,
2]. Although Epstein-Barr virus (EBV) infection is the most well-characterized risk factor [
3‐
5], other potential genetic and environmental factors have also been suggested to contribute to the pathogenesis of this malignancy [
6,
7]. Obesity has been suggested by some studies as a risk factor of NPC for decades, but the findings from different groups have been inconsistent [
8‐
10]. Recent published studies have proposed that altered levels of adipose-derived adipokines, such as adiponectin, leptin, and resistin, may have contributed to the development of various malignancies [
11,
12].
Adiponectin is an adipocytokine almost exclusively secreted by the adipose tissue [
13,
14]. Circulating levels of adiponectin are paradoxically reduced in obesity and diabetes [
15]. Mounting evidence has clearly shown its intimate involvement in the regulation of cardiovascular function, glucose/lipid metabolism, and chronic inflammation [
16‐
18]. A series of clinical studies have also revealed that circulating adiponectin is inversely associated with the risk of several malignancies, such as multiple myeloma, prostate, breast, colorectal, and pancreatic cancers [
19‐
22]. In fact, adiponectin elicits anti-proliferative effects in different tumor histocytes in vivo and in vitro, including breast, prostate, hepatocellular, and endometrial carcinomas [
23‐
25].
It is still unclear if adipose tissue, through the secreted adiponectin, plays a role in controlling the development of NPC. Herein, we set out to examine the relationship between blood concentrations of adiponectin and the risk of developing NPC in two cohorts from Guangdong province, including a hospital-based case–control study with 152 cases and 132 controls, and a nested case–control study with 71 cases and 142 controls within a community-based NPC screening cohort. Importantly, we also investigated whether, and by what mechanisms, adiponectin directly regulates the growth of NPC cells.
Materials and methods
Animal breeding and subcutaneous transplantation
All animal experimental procedures were approved by the Experimental Animal Academic Ethics Committee of Guangdong Pharmaceutical University (gdpulacspf2017064).
Adiponectin deficient mice were generously provided by Professor Philipp Scherer of the University of Texas Southwestern (Dallas, TX, USA). Male nude mice were purchased from the GemPharmatech (Nanjing, Jiangsu, China), and crossed with APN−/− female mice to generate three genotypes of nude mice: APN+/+, APN+/−, and APN−/−. Mice were kept in the Laboratory Animal Center of Guangdong Pharmaceutical University (Guangzhou, Guangdong, China), and maintained in specific pathogen-free conditions with stationary temperature of 23–25 °C and 12-h light/dark cycles.
1 × 106 CNE-2 or 5-8F cells were resuspended in 100 µL PBS and subcutaneously injected into the right armpit region of five- to six-week-old male nude mice. Tumors were measured using digital Vernier calipers every day, with tumor volume calculated using the formula [sagittal dimension (mm) × cross dimension (mm)]2/2 and expressed in cm3. All animals were sacrificed, tumor tissues were collected, imaged, and weighed.
For AdipoRon administration, four days after injection NPC cells, the mice were randomly allocated into two groups (Vehicle and AdipoRon groups) of 6 mice per group. In the AdipoRon group, mice were intragastrically administered 50 mg/kg AdipoRon suspended in corn oil every other day. In the Vehicle group, mice were administered solvent alone in corn oil.
Cell culture and regents
The CNE-2 and S18 cell lines were kindly gifted by Professor Chaonan Qian at SYSUCC. HNE2, 5-8F, C666-1 and 6-10B cells were from the Central South University Advanced Research Center (Changsha, Hunan, China). HNE2, 5-8F and 6-10B cells were cultured in RPMI-1640 medium, CNE-2 and S18 cells were cultured in Dulbecco's modified eagle medium containing 4.5 mg/mL glucose, all supplemented with 10% fetal bovine serum (Gibco, Carlsbad, CA, USA), 100 U/mL penicillin and 100 ug/mL streptomycin (Hyclone, Logan, UT, USA). Cells were maintained in a humidified atmosphere of 5% CO2 at 37 °C. The cell line was authenticated via deoxyribonucleic-acid profiling using short tandem repeat analysis.
Recombination human full-length adiponectin was dissolved in deionized water to prepare a working stock solution of approximately 0.5 mg/mL (BioVendor, Brno, Czech Republic). AdipoRon was dissolved in DMSO to prepare a working stock solution of approximately 50 mM (Selleck Chemicals, Houston, TX, USA). Compound C was purchased from MedChem Express (Monmouth Junction, NJ, USA), and was prepared as a stock concentration at 10 mM in DMSO and stored at − 80 °C.
Cell viability and proliferation assays
Cell viability was measured using cell counting kit-8 (CCK-8) (Sangon Biotech, Shanghai, China). Cells were cultured in 96-well plates, with six duplicate wells in each group, and pre-treated in 100 μL medium with or without different concentrations of inhibitors for 1 h, followed by solvent alone, AdipoRon or APN for the indicated period. After incubation, CCK-8 solution was added to each well followed by a further 2 h incubation under 5% CO2 at 37 °C. Absorbance was automatically measured at 450 nm with a microplate reader (Infinite F50, Tecan Group Ltd., Mannedorf, Switzerland). The relative cell viability was calculated as the percentage of untreated cells.
Cell proliferation was measured using plate clone formation and 5-ethynyl-2'-deoxyuridine (EdU) assays. CNE-2 cells were plated in 12-well plates and treated with human recombinant adiponectin or AdipoRon. Then, the culture medium was replaced with fresh medium containing adiponectin every 3 days. After 7 days’ treatment, the medium was removed, and cell colonies were fixed and stained with crystal violet (Sangon Biotech). Images were taken with a digital camera, colonies contained more than 50 cells in each well were counted. The EdU assay were preformed according to manufacturer’s instructions (RiboBio, Guangzhou, Guangdong, China). The EdU-positive rate was calculated as EdU-positive cells/Hoechst-stained cells × 100%. The assays were repeated in triplicate.
Transient transfection with small interfering RNA
The small interfering RNA (siRNA) oligos against AdipoR1, AdipoR2 and scrambled control siRNA were commercially synthesized by RiboBio (Guangzhou, Guangdong, China), and transfected with riboFECT CP transfection reagent (RiboBio, Guangzhou, Guangdong, China) according to the manufacturer’s protocol. The siRNA duplexes used for this study are listed in Additional file
1: Table S2. Two days after transfection, the cells were subjected to total RNA isolation and viability assays.
Cell cycle assay
CNE-2 cells were incubated in serum-free medium overnight, and then cells were treated with adiponectin or AdipoRon. Cells were collected, washed, and suspended in cold PBS. Cells were then fixed in 70% cold ethanol at 4 °C overnight. After fixation, the cells were washed with PBS twice, resuspended in 0.2 mL PI/RNase staining buffer (BD Biosciences, San Jose, CA, USA) for 30 min at room temperature. The cell cycle distribution was determined by the DxP Athena flow cytometry system (Cytek Biosciences, Fremont, CA, USA), and the percentages of different phases of cell cycle were determined using ModFit LT 5.0 (Verity Software house, Topsham, ME, USA).
Cell apoptosis assays
PE Annexin V apoptosis detection kit (BD Biosciences, #559763) was used to determine cell apoptosis. Cells treated with the indicated drug concentrations. After treatment, we harvested the cells, washed them twice with PBS, and stained them using Annexin V-PE and 7-AAD for 15 min in the dark, followed by analysis using the DxP Athena flow cytometry system (Cytek Biosciences). The upper right quadrant represents late apoptotic cells, and the lower right quadrant represents early apoptotic cells. The assessment of the apoptosis rate was the sum of early and late apoptosis.
RNA extraction and qRT-PCR
Total RNA was extracted from cell by using Trizol reagent (Sigma; T9424). The quantity and quality of RNA were determined using a ScanDrop2 nano-volume spectrophotometer (Analytik Jena), and reversely transcribed based on the HiScript II Q RT kit (Vazyme; R223) according to the manufacturer’s instructions. Amplification and real-time detection were performed on a qTOWER3 G real-time PCR system (Analytik Jena) by using ChamQ Universal SYBR qPCR Master Mix (Vazyme; Q711) in 20 μL reaction. The relative expression levels of each targeted gene were normalized by subtracting the corresponding mouse β-actin threshold cycle (CT) values by using the ΔΔCT comparative method. Three biological replicates per group were used for qPCR. Primers were synthesized by Sangon Biotech (Shanghai, China). Sequences of all primers used are provided in Additional file
1: Table S3.
Immunoblotting analysis
Cells were collected and homogenized in RIPA lysis buffer containing a protease inhibitor (Beyotime Biotechnology, Shanghai, China). The protein concentration was determined using bicinchoninic acid protein assay kit (Thermo Fisher Scientific, Waltham, MA, USA). Then the equivalent proteins were separated by SDS-PAGE, and transferred on Immobilon-p Transfer Membrane (Millipore, Billerica, MA, USA) with the wet electrical transfer method using Mini Trans-Blot (Bio-Rad Laboratories, Hercules, CA, USA). The membranes were blocked with 5% nonfat dried milk in TBS containing 0.1% Tween-20 for 1 h at room temperature; followed by the primary antibody incubation overnight at 4 °C and the secondary antibody for 1 h at room temperature. The bands were detected with ECL detection system according to the manufacturer’s protocol (Thermo Fisher Scientific) using ChemiDoc XRS + system (Bio-Rad). The gray intensities of bands were measured using ImageJ software (National Institutes of Health, Bethesda, MD, USA) and were normalized for β-actin. The antibodies used were as follows: mouse anti-β-actin (A5316) and mouse anti-GAPDH (G8795) (Sigma-Aldrich, St. Louis, MO, USA); rabbit anti-p21 (#2947), rabbit anti-p27 (#3686), rabbit anti-CDK2 (#2546), rabbit anti-CDK4 (#12790), rabbit anti-Cyclin B1 (#12231), rabbit anti-Cyclin D1 (#2978), rabbit anti-ERK1/2 (#4695), rabbit anti-p-ERK1/2 (#4370), rabbit anti-LKB1 (#3047), rabbit anti-p-LKB1 (#3482), rabbit anti-AMPKα (#5831), and rabbit anti-p-AMPKα (#2535) (Cell Signaling Technology, Danvers, MA, USA); mouse anti-AdipoR2 (sc-514045) (Santa Cruz Biotechnology, Santa Cruz, CA, USA); rabbit anti-AdipoR1 (ab126611) (Abcam, Cambridge, MA, USA); Goat anti-mouse-HRP and goat anti-rabbit-HRP (Jackson ImmunoResearch, West Grove, PA, USA). The densitometry of the bands was quantified using ImageJ software.
Immunohistochemistry staining
IHC was carried out as described previously [
26]. The sections were deparaffinized, rehydrated and performed antigen retrieval with microwave method in 10 mM citrate buffer. The sections blocked with 3% H
2O
2 for 15 min, incubated with 5% normal goat serum in PBST for 1 h at 37 °C. Then sections were incubated with primary antibodies mouse anti-AdipoR2 (Santa Cruz; 1:50), rabbit anti-AdipoR1 (Abcam; 1:100), rabbit anti-Ki-67 (#9027, CST) and rabbit anti-CD31 (#77699, CST) at 4 °C overnight. After washing, followed by horseradish peroxidase-conjugated secondary antibody incubation for 1 h. Sections were incubated with developing solution (diaminobenzidine, DAB) and counterstained with hematoxylin (ZSGB-Bio, Beijing, China). Goat anti-mouse-HRP and goat anti-rabbit-HRP (Jackson ImmunoResearch) were used as the secondary antibodies.
Messenger RNA (mRNA) expression data for 566 head and neck squamous cell carcinoma (HNSC) samples were downloaded from The Cancer Genome Atlas (TCGA) data portal (
https://xenabrowser.net/datapages/). According anatomic neoplasm subdivision, including 44 tonsil, 9 oropharynx, 143 oral tongue, 87 oral cavity, 3 lip, 128 larynx, 10 hypopharynx, 7 hard palate, 66 floor of mouth, 22 buccal mucosa, 29 base of tongue and 18 alveolar ridge tumors.
Microarray gene expression profiling data including GSE12452 (10 normal controls and 31 NPC samples) [
27], GSE53819 (21 normal controls and 18 NPC samples) [
28], GSE61218 (21 normal controls and 18 NPC samples) [
29], GSE64634 (4 normal controls and 12 NPC samples) [
30], GSE103611 (48 NPC samples) [
31], GSE132112 (95 NPC samples) [
32], and GSE13597 (3 normal controls and 25 NPC samples) [
33]. The RNA-seq data of NPC samples including GSE102349 (113 NPC samples) [
34] and GSE68799 (4 normal controls and 42 NPC samples). These data were downloaded from the Gene Expression Omnibus (GEO) database.
Statistical analysis
Data were expressed as mean ± SD. Statistical analyses were performed using GraphPad Prism 7.0 (GraphPad Software, La Jolla, CA, USA). The statistical significance between groups was assessed by Student’s t test or by analysis of variance (ANOVA) with Sidak's multiple comparisons test. A value of P < 0.05 was considered statistically significant.
Discussion
The findings derived from the retrospective and prospective case–control studies established for the first time the inverse relationship between adiponectin and the risk of NPC. This newly identified inverse relationship is completely independent of other well known risk factors, such as age, EBV infection status, family history, suggesting an independent regulation of NPC development by an adipocyte-derived metabolic hormone. The current study did not stratify the correlation against body weight owing to the lack of such data in both cohorts. However, previous studies have largely ruled out the association of body weight with risk of NPC [
8‐
10]. Thus, we do not believe that this issue will change the outcomes of this analysis. Interestingly, we did not find evidence of an association among women, which might be due to the relatively few female NPC cases. Extending investigation is required to confirm our findings, and to better elucidate sex hormones affect the relationship between adiponectin and NPC.
In corroborating the outcomes of such strong clinical associations, we further established the causative effects as well as the underlying mechanisms, of adiponectin on human NPC development. Co-incubation with adiponectin or adiponectin-receptor agonist suppressed the growth of human NPC cells, arrested cell cycle via AdipoR1- and AdipoR2-mediated AMPK activation. Importantly, adiponectin-deficiency significantly accelerated, while administration of adiponectin-receptor agonist inhibited, the growth of human NPC cell-derived xenografts in the nude mice. Taken together, these results unequivocally solidified that adiponectin is not just a correlative circulating factor but also a direct regulatory factor in the development of NPC.
Initially discovered as a crucial regulator of inflammation, energy balance, glucose/lipid metabolism [
15,
18], adiponectin has been reported to have direct anti-proliferative effects in several malignant cell lines [
36]. In this study, we demonstrated that adiponectin could directly suppress the growth of NPC cells by arresting cell cycles at the G0/G1 phase through regulating the expression of several cell cycle key regulators, with the activation of AMPKα as the most likely initiating signaling event. Tumor suppressor LKB1 as the critical upstream kinase, its phosphorylation leads to the activation of AMPK [
35]. We found adiponectin treatment did not affect the level of total LKB1, it significantly increased the level of p-LKB1, further demonstrating that adiponectin suppresses NPC growth through activating LKB1/AMPK signaling. Furthermore, adiponectin induced AMPK activation probably suppressed the phosphorylation of ERK1/2, further enhancing the anti-proliferative effect. Recent studies have shown that AMPK signaling can play a critical role in the regulation of cancer cell proliferation via induction of apoptosis and cell cycle arrest [
37‐
40]. Several tumor suppressor genes, such as p53, mTOR, and p27, are considered as the downstream signaling components of AMPK activation [
41,
42]. 5-Aminoimidazole-4-carboxamide ribonucleoside (AICAR), a pharmacological activator of AMPK, suppresses cell growth of head and neck squamous cell carcinoma [
43]. Moreover, metformin has recently received attention as an anti-tumor drug, since it induces inhibition of cancer cell proliferation via activation of AMPK signaling [
44]. Our previous study has already shown that adiponectin could arrests endometrial cancer cells at the G0/G1 stage, possibly by activating AMPK [
24]. Such result is well corroborated by the observation that the inhibitory effect of adiponectin on NPC cell proliferation was neutralized by inhibition of AMPK activity with a specific AMPK inhibitor, ComC, particularly adiponectin-controlled cell cycle progression in NPC cells. Besides AMPK, other molecular mechanisms could also play critical. For instance, chronic inflammation is an important even in propelling the development of NPC [
3]. Owing to its well-established anti-inflammatory function, adiponectin can also prevent NPC development by suppressing proinflammatory cytokines such as IL6, tumor necrosis factor-α, and interferon γ [
45], as well as inducing the expression of anti-inflammatory cytokines such as IL-10 and IL-1RA [
46].
From a translational perspective, we have tested if stimulation of adiponectin receptor activity would attenuate the growth of human nasopharyngeal carcinoma, and applied AdipoRon, the first oral adiponectin receptor agonist capable of binding and activating both AdipoR1 and AdipoR2 [
47], in the human NPC model. AdipoRon has emerged as a possible candidate for the treatment of different pathological conditions, including metabolic, cardiovascular, and cognitive dysfunction of Alzheimer’s disease, specifically comorbidity between depression and obesity [
47‐
50]. In this study, we have demonstrated that oral administration of AdipoRon exhibited a robust anti-cancer effect against human NPC derived xenograft tumors, and the dosing of AdipoRon applied in our study (at 50 mg/kg) closely matched those reported in other mouse models [
47,
49,
51,
52]. With multitudes of mechanisms, AdipoRon may represent a therapeutic agent that can be applied towards the treatment of human NPC.
Conclusions
In conclusion, our findings from this study shed some new light on the pathogenesis of NPC, highlighting the importance of an adipocyte-derived endocrine hormone, adiponectin, as a crucial inhibitor to NPC tumorigenesis via AMPK activation. Further investigations are needed to establish the linkages between other adipocyte-derived endocrine hormones, in addition to adiponectin, with the progression and pathological features of NPC, such as tumor grade, vascular invasion, and metastasis. Our findings herein may provide knowledge of adiponectin as a novel therapeutic target in NPC therapy.
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