Background
Cervical cancer is the second most frequent malignancy in women and is responsible for substantial number of morbidities and mortalities throughout the word [
1]. Molecular and epidemiological studies have shown that infection with a high-risk human papillomavirus (HPV) type is a major factor in the development of cervical cancer and is responsible for nearly all cases of cervical cancer [
2]. Of the over 100 HPV types known thus far, HPV-16 is the most common high-risk virus and causes more than 50% of all cases of cervical carcinoma [
3]. During carcinogenesis, HPV-16 DNA integrates into the host cell genome, resulting in a loss of expression of the regulatory gene E2, which is relatively conserved among papillomaviruses [
4]. In addition to being a transcriptional regulator, the E2 protein negatively regulates the HPV viral oncogenes E6 and E7 in benign lesions [
5]. E2 proteins from high-risk HPV have been proven to affect cellular processes such as anti-proliferation, apoptosis, regulation of the viral life cycle, and gene expression [
6],[
7]. Previous studies have shown a direct relationship between HPV E2 gene expression and mitochondrial dysfunction in apoptosis in cervical cancer cells [
8].
The main function of mitochondria is to produce energy by synthesising ATP. Reactive oxygen species (ROS) production contributes to mitochondrial damage in the pathological level and also plays an important role in redox signalling from the organelle to the rest of the cell [
9]. Studies have demonstrated that the high-risk HPV-16 E2 protein can induce mitochondrial dysfunction by regulating protein expression and the localisation of proteins to mitochondrial membranes [
10]. The receptor for the globular head of C1q, gC1qR, was initially identified as a protein within the mitochondrial matrix [
11] that could mediate many biological responses, including growth perturbations, morphological abnormalities and the initiation of apoptosis [
12]. This study aimed to comprehensively identify the effect of the gC1qR gene on HPV-16 E2-induced apoptosis of cells and to investigate whether the gC1qR-induced biological changes acted through a mitochondria-dependent pathway in HPV-16 E2-transfected cervical squamous carcinoma cells.
Materials and methods
Chemicals and reagents
The human cervical squamous carcinoma cell lines C33a (HPV-16 negative) and SiHa (HPV-16 positive) were purchased from Hangzhou Hibio Bio-tech Co., Ltd (Hangzhou, Zhejiang, China). Dulbecco's Modified Eagle's Medium (DMEM) powder, penicillin and streptomycin were purchased from Invitrogen/Gibco (Grand Island, NY, USA). The Phototope-HRP Western Blot Detection System, including an anti-mouse IgG, an HRP-linked antibody, a biotinylated protein ladder, 20X LumiGLO Reagent and 20X peroxide, was purchased from Cell Signalling Technology (Beverly, MA, USA). Lipofectamine 2000, was purchased from Invitrogen (Carlsbad, CA, USA). 2',7'-dichlorodihydrofluorescein diacetate (H2DCFDA) was obtained from Molecular Probes. The Annexin V-FITC/Propidium Iodide (PI) Flow Cytometry Assay Kit was purchased from Invitrogen (Carlsbad, CA, USA). Antibodies targeting HPV-16 E2, gC1qR, and actin were purchased from Santa Cruz (Santa Cruz, CA, USA) and Cell Signalling Technology. The pcDNA-HPV-16 E2 and pcDNA-HPV-16 E2 mutant (mut) plasmids were kindly supplied by Hangzhou Hibio Bio-tech Co., Ltd. gC1qR small-interfering RNA (siRNA) and negative siRNA (siRNA directed toward an unrelated gene as a negative control) were synthesised by Wuhan Genesil Biotechnology Co., Ltd (Wuhan, China). Cell culture supplies were purchased from Life Technologies (Gaithersburg, MD, USA). Unless otherwise specified, all of the other reagents were of analytical grade.
Tissue procurement and preparation
Between October 2009 and January 2012, we recruited women who underwent radical hysterectomies due to cervical carcinoma at Nanjing Maternity and Child Health Care Hospital. This study was approved by the Ethical Committee of the Chinese Academy of Sciences and the Nanjing Maternity and Child Health Care Hospital in Nanjing. All of the study participants gave informed consent for the collection of the tissues and blood samples. Human cervical cancer specimens were obtained from 30 HPV-16-positive patients (median age of 45 years, age range between 22-59 years), which were studied along with a control group (median age of 43 years, age range between 21-54 years). Some cervix tissues from non-cervical cancer patients who have had a hysterectomy for hysteromyoma or adenomyosis etc were collected, the other from patients who have had a tissue biopsy for non cancer diagnoses. From those tissues, thirty cases from the patients (HPV-16 is positive, and the HPV typing was detected using gene chip technique in this study) were chosen as the control group, which pathological diagnosis was mild cervicitis or have no obvious pathological changes. The human cervical squamous cell carcinoma tissues and non-cancerous cervix tissues were all reviewed by a pathologist and histologically. The infection of other sexually transmitted pathogens including CT, NG, GV, MG, TV, MH, and HSV-2 was detected by routine clinical microbiology methods before the HPV analysis.
Cell culture
The human cervical carcinoma cell lines C33a and SiHa were cultured in DMEM medium containing 10% foetal bovine serum, 1% nonessential amino acids, 2 mM glutamine, and antibiotics (100 units/ml penicillin and streptomycin). The cells were maintained in a humidified 5% CO2 incubator at 37°C.
Cloning and transfection of the HPV-16 E2 plasmids
The full-length HPV-16 E2 open reading frame (ORF) was constructed in-frame into the pcDNA 3.1 expression plasmid (Invitrogen, Carlsbad, CA) by PCR amplification using the BamHI and EcoRI restriction sites according to the pBR322 reference clone. Primer-F (5'-GAT GGA GAC TCT TTG CCA ACG-3') and Primer-R (5'-TCA TAT AGA CAT AAA TCC AGT AGA C-3') were used to clone the HPV-16 E2. The mutant HPV-16 E2 plasmid was created by PCR mutagenesis using the Primer-F (5'-GAT GGA GAC TCT TTG CCA ACG-3') and mutant Primer-R (5'-TCC CAT TCT CTG GCC TTG TAA ATA GCA CA
TGC TAG-3'), where the mutated codons are denoted in bold and italic. The HPV-16 E2 mutant reduced DNA replication activity and transactivation regulation [
13]. The resulting pcDNA-HPV-16 E2 vector and mutant HPV-16 E2 vector were then transfected into C33a and SiHa cells, respectively. Twenty-four hours after plating, the cells were serum starved in RPMI-1640 medium containing 0.5% FBS for an additional 24 h until the cells became quiescent. Following serum starvation, pcDNA-HPV-16 E2 was transfected into the cells (90% confluent) at passage numbers 6, 9 and 12 using Lipofectamine™ reagent (Life Technologies, Inc.) according to the manufacturer's protocol. Reporter gene levels were normalised to the amount of total protein, and each experiment was independently performed three to five times.
gC1qR siRNA-expressing plasmid construction
To silence the objective genes, the siRNA target gene sequence was designed to be homologous to nucleotides 408-426 of the human gC1qR mRNA. The forward siRNA sequence was 5'-AAC AAC AGC AUC CCA CCA ACA UU-3'. The 5' end oligonucleotides contained BamHI and HindIII restriction site overhangs. The gC1qR siRNA-expressing plasmid was constructed using pGenesil-1 as the vector backbone. The siRNA was synthesised, annealed and ligated into the BamHI and HindIII restriction sites in the linearised pGenesil-1 expression vector. At the same time, a vector containing the siRNA for an unrelated gene was used as a negative control.
Scanning and transmission electron microscopy
Biopsies were taken immediately after surgery. Tumour specimens were obtained by cutting longitudinal sections of 3-5-mm maximum thickness, which were immersed in phosphate-buffered 2.5% glutaraldehyde for 2 h. Following an overnight washing with 0.1 M sodium phosphate buffer, the tissue blocks were post-fixed in 1% OsO4 in a 0.1 M phosphate buffer (pH 7.4) for 1 h, stained with 1% uranyl acetate, and then dehydrated in an acetone gradient. For transmission electron microscopy, ultrathin (60-70 nm) sections were stained with uranyl acetate and lead citrate. The cell morphology was examined at 3700X and 12500X magnification and photographed using a JEOL JEM-2000EX transmission electron microscope (Tokyo, Japan).
Western blot analysis
Following various treatments for 48 h, cells were harvested in ice-cold PBS, pelleted at 15,000 rpm for 5 min, and then incubated in lysis buffer containing 50 mM Tris-HCl (pH 7.4), 0.5% NP-40, 150 mM NaCl, 50 mM NaF, 1 mM Na3VO4, 1% Triton X-100, 1 mM EDTA, 1 mM PMSF, 10% glycerol, and protease inhibitor cocktail on ice for 30 min. The supernatants were centrifuged for 20 min at 13,000 rpm at 4°C. The protein was estimated using the Bradford reagent. Equal amounts of protein were loaded and separated on a 10-15% SDS-polyacrylamide gel and then transferred onto a PVDF membrane. The membranes were blocked for 1 h in 5% non-fat milk in PBST (PBS containing 0.05% Tween 20) and then incubated with the appropriate primary antibodies against HPV-16 E2 or actin at a 1:500 dilution. The membrane was washed in PBST and incubated with the secondary IgG HRP-conjugated antibody at a 1:500 dilution. The protein bands were visualised using the enhanced chemiluminescence (ECL) Western Detection System, and the densitometry analysis was performed on the scanned immunoblot images using the Image J gel analysis tool.
Assay of intracellular ROS
ROS production was measured using the cell-permeable probe H
2DCFDA, which preferentially measures peroxides. Briefly, C33a and SiHa cells were grown on cover slips and incubated with 10 μM H
2DCFDA under various conditions for 15 min in the dark. The cells were then lysed with RIPA buffer in ice-cold conditions [
14]. H
2DCFDA fluorescence was detected using fluorescence microscopy at an excitation wavelength of 488 nm and an emission wavelength of 530 nm. A spectrofluorometer with a slit width of 5 nm was used to quantify the fluorescence levels of the supernatants. The experiments were repeated at least 10 times. The results were determined according to the increase in fluorescence intensity with respect to normoxic untreated controls by subtracting the basal fluorescence levels.
Measurement of the intracellular Ca2+ concentration ([Ca2+]i)
Fluo-4 AM fluorescence was used to quantify the intracellular Ca2+ levels. C33a and SiHa cells were treated under various conditions at the indicated times and then washed with ice-cold PBS. The cells were resuspended in 1 mL of PBS and incubated with 5 mL of 1 mM Fluo-4 AM for 1 h. The fluorescence intensity was detected using a Beckman Coulter Paradigm™ Detection Platform at an excitation wavelength of 485 nm and an emission wavelength of 530 nm to determine the intracellular Ca2+ concentrations. Fluorometric measurements were performed in 10 different sets and expressed as the fold increase in fluorescence per microgram of protein compared with the control group.
Measurement of the mitochondrial membrane potential (Δψm)
The loss of mitochondrial membrane potential (Δψm) was measured in C33a and SiHa cells after treatment under varying conditions at different time intervals using the fluorescent cationic dye JC-1, which is a mitochondria-specific fluorescent dye [
15]. The dye accumulates in mitochondria with increasing Δψm under monomeric conditions and can be detected at an excitation wavelength of 485 nm and an emission wavelength of 530 nm. C33a and SiHa cells that had undergone the various treatments were washed with serum-free medium after 60 h of growth and incubated with 10 μM JC-1 at 37°C. Then, the C33a and SiHa cells were resuspended in medium containing 10% serum, and the fluorescence levels were measured at the two different wavelengths. The data are representative of 10 individual experiments.
Detection of apoptotic cells
Apoptosis measurements were performed using Annexin V-FITC/propidium iodide staining with flow cytometry analysis. After different treatments at the indicated times, C33a and SiHa cells were washed and resuspended in binding buffer (2.5 mM CaCl2, 10 mM HEPES, pH 7.4, and 140 mM NaCl) before being transferred to a 5-mL tube. The cells were incubated in the dark with 5 μL each of Annexin V-FITC and propidium iodide for 15 min. The binding buffer was then added to each tube, and the samples were analysed using a Beckman Coulter Epics XL flow cytometer. The normal cells were found in the Q1_LL region, and the early and late apoptotic cells were distributed in the Q1_LR and Q1_UR regions, respectively. The necrotic cells were located in the Q1_UL region.
Statistical analysis
Unless otherwise indicated, the results represent the mean ± standard deviation (SD). Differences between the various datasets were tested for significance using Student's t-test, and p-values less than 0.05 were considered significant (*p < 0.05; **p < 0.01; #
p > 0.05).
Discussion
The results of this study show that over-expression of HPV-16 E2 is associated with apoptosis of human cervical squamous carcinoma cells. Our study has further confirmed that over-expression of HPV-16 E2 could regulate the expression of the gC1qR gene, which induces mitochondrial dysfunction. These findings constitute the first evidence that gC1qR is a target during HPV-16 E2-induced apoptosis in human cervical squamous carcinoma cells.
Experimental evidence has demonstrated that the primary host defence mechanism against viral infection is to target apoptotic proteins, which, when released from the mitochondria, regulate cellular responses including the promotion of cell proliferation or induction of cell death [
14]. For example, high-risk HPV-16 E6 was reported to maintain mitochondrial morphology and integrity while inhibiting the release of the pro-apoptotic factor cytochrome c, a potent catalyst of apoptosis [
15]. Others studies also demonstrated that apoptosis is triggered by over-expression of HPV-16 E2 by enhancing the expression of pro-apoptotic Bax, inhibiting the expression of anti-apoptotic Bcl xl, releasing cytochrome c from the mitochondria, and activating caspases-9 and -3 [
16]. Interestingly, only high-risk HPV E2 proteins, such as the six regulatory molecules (E1, E2, E4, E5, E6, and E7) encoded by the HPV-16 genome, seem to be effective in modulating mitochondrial metabolism [
17]. E2 could not only influence the activity of the cell cycle (benign lesions) but may also regulate viral gene expression (including the E6/E7 oncogenes) [
18]. Our results demonstrate that E2 transfection into HPV-negative C33A cells or HPV-16-positive SiHa cells increased gC1qR expression, induced mitochondrial dysfunction, and enhanced apoptosis.
The gC1qR protein is primarily localised at the outer mitochondrial membrane [
19], enhances ROS production and increases Ca
2+ uptake by reducing the electron flow from complex I [
20]. In recent years, it has become increasingly evident that gC1qR-induced mitochondrial dysfunction is linked to apoptosis and the release of cytotoxic factors such as ROS, which are generated in excess in defective mitochondria. ROS induction, as a by-product, can regulate signalling pathways leading to the inhibition of cell proliferation or cell death [
21]. Our previous study demonstrated that gC1qR vector-treated C33a and SiHa cells expressing gC1qR generated increased levels of ROS. Oxidant generation correlated with intracellular Ca
2+ accumulation and a decrease in the relative Δψm values, which in turn induced cell apoptosis. However, treatment with metformin may reverse gC1qR-induced C33a and SiHa cell apoptosis. This observation was also supported by the results derived from the treatment that silenced the gC1qR gene in HPV-16 E2-induced apoptosis.
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Competing interests
The authors declare that they have no competing interests.
Authors' contributions
LJG and PQG conceived the study and drafted the manuscript. ZLC participated in designing the study and helped draft the manuscript. YJS performed the molecular biology studies and the statistical analysis. HLZ collected the patient information. All of the authors read and approved the final manuscript.