Background
Glioblastoma multiforme (GBM) is the most aggressive malignancy in adults and thus persists as a major unsolved clinical challenge [
1]. Despite impressive advances in surgical techniques, radiotherapy and chemotherapy, the median survival time of patients with GBM remains dismally at 14.6 months [
2].
Diffuse infiltrative invasion of GBM cells into the adjacent normal brain areas is a major cause of invariable recurrence and relapse after resection of primary tumors [
3].
A number of pathological features in GBM provide the basis for understanding the functional consequences of changes in gene expression. For example, hypoxia is a pathological hallmark of GBM. Hypoxia-inducible factor 1 (HIF1), a dimeric transcription factor, is one of the primary regulators that coordinate cellular responses to hypoxia. HIF1 is composed of α and β subunits (HIF1α; HIF1β). HIF1α is rapidly degraded under normoxic conditions but is often stable under hypoxic conditions. However, when HIF1α binds to hypoxia-responsive elements (HREs), it activates transcription of downstream genes, which are involved in tumor angiogenesis, invasion, cell survival, and glucose metabolism [
4]. Therefore, identifying HIF1α-targeted molecules will provide further understanding in the development and treatment of human glioma.
Coiled coils are among the most ubiquitous folding motifs identified in proteins and have not only been found in structural proteins but also play a necessary role in various intracellular regulation processes [
5]. Coiled coils are involved in signal-transducing events and act as a molecular recognition system. Furthermore, they provide mechanical stability to cells and are involved in movement processes [
6]. Increasing evidence suggests that aberrant expression of coiled-coil domain containing proteins influences the migration, invasion and proliferation of various human cancers, including bladder cancer [
7], pancreatic cancer [
8], gastric cancer [
9], papillary thyroid carcinoma [
10], leukemia [
11], prostate cancer [
12], breast cancer [
13].
CCDC109B, also known as mitochondrial calcium uniporter b (MCUb), is an MCU isogene [
14]. CCDC109B is an evolutionarily conserved protein, which possesses two coiled-coil domains and two transmembrane domains [
15]. Functionally, MCUb acts as a negative subunit of the MCU channel, and the MCU/MCUb ratio seems to vary in different tissues, providing a molecular mechanism to mediate the efficiency of mitochondrial calcium (Ca
2+) intake [
16]. The failure of mitochondria to intake calcium leads to the abnormal activation of cytosolic Ca
2+-dependent enzymes, including calpain proteases [
17] and calmodulin-dependent kinases [
18] and ultimately leads to changes in cellular signaling cascades which directly regulate cell growth [
19], tumor cell invasion [
20]. However, the biological significance of CCDC109B in human glioma remains unclear.
Here, we investigated expression of CCDC109B in human glioma tissues and cell lines by analyzing our own cohort and publicly available molecular databases. Then, functional experiments were performed with model systems in vitro and in vivo. We uncovered a potential oncogenic role for CCDC109B in glioma progression and identified HIF1α as a possible transcriptional regulator. These results, support CCDC109B as a new therapeutic target for the treatment of human glioma.
Methods
Ethics statement
Human brain tumor (n = 68; WHO grade II–IV) and non-neoplastic tissue (n = 4) samples were obtained from surgeries performed at the Department of Neurosurgery at Qilu Hospital (Shandong, China). Written informed consent was obtained from all patients, and approval for experiments was obtained from Ethics Committee of the Qilu Hospital. All surgeries and post-operative animal care were approved by the Institutional Animal Care and Use Committee (IACUC) of Shandong University (Shandong, China). Our research complies with the commonly-accepted ‘3Rs’: replacement of animals by alternatives wherever possible, reduction in the number of animals used, and refinement of experimental conditions and procedures to minimize harm to animals.
Cell culture and hypoxic treatment
Human glioma cell lines, U87MG, U251 and T98 were obtained from the Culture Collection of the Chinese Academy of Sciences (Shanghai, China). The normal human astrocytes (NHA) cell line was a kind gift from the Department of Biomedicine at the University of Bergen (Bergen, Norway). Cells were cultured in Dulbecco’s modified Eagle’s medium (DMEM; Thermo Fisher Scientific, Waltham, MA, USA) supplemented with 10% fetal bovine serum (FBS; Thermo Fisher Scientific) and maintained at 37 °C in a humidified chamber containing 5% CO2. For hypoxic treatment, cells were placed in a modulator incubator (HERAcell 150i, Thermo Fisher Scientific) in 94% N2, 5% CO2, and 1% O2. For stable CCDC109B-knockdown, U87MG and U251 cells were infected with lentivirus expressing short hairpin RNA (shRNA) (sh-CCDC109B-1). After 48 h, U87MG or U251 cells were exposed to 0.5 or 2 µg/mL puromycin (A1113802, Thermo Fisher Scientific), respectively, in complete DMEM for an additional 2 weeks. Cells were subsequently treated with PX478 (S7612, Selleck Chemicals; Shanghai, China) and HIF1α siRNA to inhibit HIF1α expression and harvested after 48 h. Sequences of synthesized shRNAs (Genepharma; Shanghai, China) were the following: sh-Negative Control (sh-NC) 5′-TTCTCCGAACGTGTCACGTtt-3′; sh-CCDC109B-1 5′-CAGTCACACCATTATAGTAtt-3′; sh-CCDC109B-2 5′-CTCGACAGGATTATACTTAtt-3′; sh-CCDC109B-3 5′-GCAAGTAGAAGAACTCAATtt-3′. Sequences of synthesized siRNAs (Genepharma) were the following: si-NC 5′-TTCTCCGAAGGTGTCACGG-3′; si-HIF1α-1 5′TACGTTGTGAGTGGTATTATT-3′; si-HIF1α-2 5′-CTGATGACCAGCAACTTGA-3′.
IHC
Samples were fixed in 4% formalin, paraffin-embedded, and sectioned (4 µm). After de-waxing and rehydration, the sections were incubated with 0.01 M citrate buffer for 20 min at 95 °C for antigen retrieval. Endogenous peroxidase activity and non-specific antigens were blocked with 3% hydrogen peroxide (ZSGB-Bio; Beijing, China) and 10% normal goat serum (ZSGB-Bio) respectively, followed by incubation with primary antibody at 4 °C overnight. Sections were rinsed with phosphate buffered saline (PBS), treated with goat anti-rabbit secondary antibody (ZSGB-Bio), visualized using 3, 3′-diaminobenzidine (DAB, ZSGB-Bio) as substrate, and counterstained with hematoxylin (Beyotime; Haimen, China). Normal mouse serum was used as the negative control. Staining of cancer cells was scored as follows: 0, no staining; 1, weak staining in <50% cells; 2, weak staining in ≥50% cells; 3, strong staining in <50% cells; and 4, strong staining in ≥50% cells. The following primary antibodies (Abcam, Cambridge, UK) were used at the dilutions indicated: CCDC109B (1:200), HIF1α (1:200), Ki-67 (1:500), MMP2 (1:100) and MMP9 (1:200).
Western blot analysis
Cells and tissues were incubated 30 min in RIPA buffer containing protein inhibitor cocktail for lysis (Thermo Fisher Scientific). After centrifugation and denaturation, protein (20 μg) was separated by 10% polyacrylamide gel electrophoresis and electrophoretically transferred to polyvinylidene difluoride (PVDF) membranes (Merck Millipore; Shanghai, China). Membranes were blocked with Tris Buffered Saline with Tween 20 (TBST, 10 mM Tris, 150 mM NaCl, 0.1% Tween 20) containing 5% bovine serum albumin (BSA, Thermo Fisher Scientific),and incubated overnight at 4 °C with the following primary antibodies against CCDC109B (1:500), HIF1α (1:1000), MMP2 (1:1000), MMP9 (1:1000) and β-Tubulin (1:1000; Cell Signaling Technology; Danvers, MA, USA). Membranes were incubated the next day with secondary antibody (1:5000; Santa Cruz; Dallas, TX, USA) conjugated to horseradish peroxidase (HRP) for 1 h at room temperature. Proteins were quantified using a system for detecting chemiluminescence (Bio-Rad; Irvine, CA, USA), according to the manufacturer’s protocol. Representative images and data were obtained from at least three independent biological replicate experiments.
Cell migration and invasion assay
Cell migration and invasion assays were performed in uncoated and matrigel-coated (BD Biosciences; San Jose, CA, USA) Transwell chambers (8 μm pores; Corning Costar; Corning, NY, USA). Cells (2 × 104) in medium (200 µL) with 1% FBS were seeded in the top chamber. The lower chamber was filled with medium (600 µL) containing 30% FBS. Chambers were incubated for 24 h under normoxic or hypoxic conditions. Cells that migrated to or invaded into the lower surface were fixed with 4% paraformaldehyde (Solarbio; Beijing, China), stained with crystal violet (Solarbio) for 15 min and counted under bright field microscopy. Images were acquired from 5 random fields in each well, and cell numbers were determined using Kodak MI software. Each experiment was repeated three times in triplicate.
Immunofluorescence
To assess the distribution and expression levels of CCDC109B, NHA and glioma cells were seeded onto glass slides. The cells were then washed twice with PBS and fixed with 4% paraformaldehyde for 20 min at room temperature. Cells were rinsed with PBS, permeabilized with 0.5% Triton X-100 (Solarbio) for 15 min, and blocked with 10% normal goat serum for 60 min at room temperature. Cells were stained with primary antibody against CCDC109B (1:100) at 4 °C overnight, followed by incubation with Alexa Fluor 594 goat anti-rabbit IgG (Abcam, UK; 1:800) for 1 h at room temperature. Cell nuclei were stained with DAPI (Sigma-Aldrich, Germany) at 37 °C for 10 min, and images were obtained with confocal microscopy (LSM780, Zeiss).
Proliferation assay
Cell proliferation was measured using the EdU Apollo 567 Cell Tracking Kit (Ribo-bio; Guangzhou, China). Cells (2 × 104) under different treatments were seeded onto 24-well plates, exposed to 200 μM of 5-ethynyl-20-deoxyuridine for 2 h at 37 °C, fixed with 4% paraformaldehyde for 20 min, and treated with 0.5% Triton X-100 for 10 min. Cells were rinsed with PBS three times, and incubated with 100 μL of Apollo reagent for 30 min. Nuclie were stained with Hochest33342. The percentages of EdU-positive cells were determined from 500 cells and three independent experiments were performed.
NC and sh-CCDC109B-1 glioma cells were seeded onto six-well plates (120 cells per well) and cultured for 2 weeks in medium that was changed twice each week. Colonies of more than 50 cells were counted after fixation and staining with 100% methanol and 5% crystal violet. Data reported represent the average of three independent experiments.
Quantitative real-time PCR
Total RNA was isolated from cells using Trizol reagent (Takara; Tokyo, Japan) according to the manufacturer’s protocol. Total RNA was reverse-transcribed, and the resulting cDNA was used as template in real-time quantitative PCR performed with the standard SYBR premix Ex Taq (Takara) on the Real Time PCR Detection System (480II, Roche; Pleasanton, CA, USA). GAPDH served as an internal control, and independent experiments were conducted in triplicate. The following primers were used: GAPDH, forward, 5′-AATGAAGGGGTCATTGATGG-3′, reverse, 5′-AAGGTGAAGGTCGGAGTCAA-3′; HIF1α, forward, 5′-TGGCAGCAACGACACAGAAA-3′, reverse, 5′-TGCAGGGTCAGCACTACTTC-3′; CCDC109B, forward, 5′-ACACTGCTGAGATGGAACACAT-3′, reverse, 5′- TTGGCTTCCGAATGAGCTTCTA-3′.
Animal studies
For generation of the subcutaneous GBM model, female 4-week-old nude mice (SLAC laboratory animal Center; Shanghai, China) were maintained in a barrier facility on high-efficiency particulate air (HEPA)-filtered racks. Digoxin and saline were purchased from Qilu Hospital, Shandong University. Nude mice (n = 16) were divided into two groups (U87MG + saline, U87MG + digoxin, 8 mice per group). Cells were harvested by trypsinization, resuspended at 107 cells/mL in a 1:1 solution of PBS/Matrigel (BD Biosciences, USA), and injected subcutaneously into the right shoulder of the mouse. The tumor tissues were isolated 37 days after injection, and then used for protein extraction.
For orthotopic xenografts, 4-week-old female nude mice (n = 16) were divided into two groups (sh-CCDC109B-1 and NC group), and U87MG or U87MG modified cells (1 × 106) were implanted into the brain using a stereotactic apparatus (KDS310, KD Scientific; Holliston, MA, USA). Animals which displayed symptoms such as severe hunchback posture, apathy, decreased motion or activity, dragging legs, or drastic loss of body weight were euthanized by cervical dislocation. Excised tumor tissues were formalin-fixed, paraffin-embedded, and sectioned for Hematoxylin–Eosin (HE) staining and IHC.
Statistical analysis
All data are presented as a mean ± the standard error of the mean (S.E.M). The Student’s t test was used when only two groups were being compared. Analysis of variance (ANOVA) was used in cases where there were more than two groups being compared. Survival curves were estimated by the Kaplan–Meier method and compared using the log-rank test. For multivariate analysis, independent prognostic factors were determined using the Cox’ proportional hazards model. Variables that might be dependent on other variables were excluded from the model. A two-tailed χ2 test was used to determine the association between CCDC109B and HIF1α. GraphPad Prism version 7.00 software program (GraphPad; La Jolla, CA, USA) was used to analyze in vitro and in vivo experiments. Differences were considered to be statistically significant when P < 0.05.
Discussion
Over the past decades, rapid advancement in technologies has enabled us to describe human gliomas with greater molecular detail. However, the value of established biomarkers is limited. In this regard, identification of new molecular targets and a better understanding of underlying pathways might improve the prognosis and the efficiency of treatment for glioma patients. In the present study, we found that CCDC109B was highly expressed in HGG relative to LGG and normal brain tissues. Silencing of CCDC109B inhibited glioma proliferation, migration and invasion of glioma cells in vitro and led to decreased tumor volume and prolonged OS in vivo. Unexpectedly, we found CCDC109B expression to be drastically upregulated under hypoxia and that subsequent knockdown inhibited hypoxia-induced migration and invasion of glioma cells. Finally, functional disruption with siRNAs revealed HIF1α as a potential transcriptional regulator of CCDC109B expression both in vitro and in vivo. Our study for the first time identifies CCDC109B as a potential tumor promotor in glioma progression and provides rational for targeting CCDC109B as novel treatment or prognostic marker in human glioma.
CCDC109B was first identified as a paralogue of MCU, with two predicted transmembrane domains. In Hela cells, CCDC109B acts as a dominant negative mediator of MCU, attenuating mitochondria calcium increases evoked by agonist stimulation [
16]. In this study, we found that CCDC109B expression was elevated in HGG tissues and observed high expression level of CCDC109B in human glioma cell lines. Then, analysis of publicly available data revealed that increased expression of
CCDC109B mRNA level was highly associated with the mesenchymal molecular subtype in human glioma. Next, we confirmed this finding in a cohort of glioma and non-neoplastic brain tissue samples. Consistent with our results, higher expression of CCDC109B in GBM was reported in a meta-analysis performed with a large cohort [
29]. In addition, results from gene profiling analysis conducted by another group revealed increased CCDC109B as a possible factor contributing to/associated with temozolomide (TMZ) resistance in malignant gliomas [
30]. Finally, CCDC109B overexpression has also been reported in leukemia [
31]. All together, these results indicate that CCDC109B might function as an oncogene in human gliomas and possibly other cancers as well.
Importantly, we took our molecular analysis a step further and examined the functional consequences of inactivating CCDC109B with shRNAs in human glioma cell lines. Our data demonstrated that knockdown of CCDC109B significantly attenuated proliferation, migration and invasion of glioma cells in vitro and led to decreased tumor volume and prolonged OS of tumor-bearing mouse in orthotopic models. Moreover, we demonstrated that decreased expression of MMP2 and MMP9, proteins linked to invasion/migration accompanied CCDC109B knockdown. Mounting evidence suggests that a critical role of coiled-coil motif proteins in human tumorigenesis is in their mediation of cellular processes, mainly proliferation and invasion [
6,
29,
30]. As one member of the family of coiled-coil motif proteins, CCDC109B plays an important role in facilitating Ca
2+ flux across the inner mitochondrial membrane (IMM) [
14]. Aberrant expression of CCDC109B has been shown to lead to mitochondrial Ca
2+ remodeling and the subsequent activation of signaling cascades associated with cancer formation and maintenance [
32]. Our results parallel a study conducted by Flotho et al. [
31] where investigators demonstrated that CCDC109B regulates cell proliferation and predicts treatment outcome in childhood acute lymphoblastic leukemia. Collectively, we and others have demonstrated that CCDC109B contributes to glioma and possibly more generally to cancer development by promoting cellular processes such as proliferation and invasion/migration.
An unexpected finding in our study was that CCDC109B expression was induced by hypoxia. Intratumoral hypoxia, which plays a key role in tumor angiogenesis, growth and invasion, has been directly associated with an aggressive phenotype of GBM [
33,
34]. HIF1α, is a critical mediator of cellular response to hypoxia and therefore has been found to be involved in cancer progression and metastasis [
35,
36]. Inhibition of HIF1α blocked hypoxia-induced CCDC109B both in vitro and in vivo, indicating that HIF1α could regulate CCDC109B expression. Silencing of CCDC109B decreased hypoxia-induced migration and invasion. However, the underlying mechanisms in CCDC109B-mediated glioma invasion/migration under hypoxic conditions remains not fully clear. Further examination of regulation of HIF1α under normoxia and hypoxia may provide additional insight into its in GBM pathophysiology [
37] and interacting factors may provide alternative therapeutic targets for the treatment of GBM.
Authors’ contributions
RX, XL and JW conceived and designed the experiments; RX performed the experiments; MH and JJ analyzed the data; BH, AC and DZ contributed reagents/materials/analysis tools; JW and RX wrote the paper. All authors read and approved the final manuscript.