Background
Transforming growth factor-β (TGF-β) plays a critical role in biological processes, including development, homeostasis, fibrosis, and carcinogenesis [
1‐
4]. TGF-β initiates signaling across the plasma membrane to the nucleus by binding to TGF-β type II receptor (TβRII) which, in turn, recruits TGF-β type I receptor [
5,
6]. Following TGF-β receptor activation, receptor-associated Smad proteins (R-Smads), such as Smad2 and Smad3, are phosphorylated. Phosphorylated R-Smads recruit Smad4, and then, translocate to the nucleus to activate transcription of downstream target genes [
4,
7,
8]. In addition to this canonical TGF-β signaling pathway, TGF-β also activates Smad-independent non-canonical pathways, including PI3K/AKT, mitogen-activated protein kinases (MAPKs), NF-κB, Rho/Rac1, Cdc42, FAK, Src, and Abl [
4,
9]. TGF-β signaling is a double-edged sword in cancer development and progression. In the early stages of tumorigenesis, TGF-β functions as a tumor suppressor by inducing cell cycle arrest and apoptosis. Paradoxically, TGF-β signaling can exacerbate malignant phenotypes at later stages of tumorigenesis, by inducing epithelial-mesenchymal transition (EMT), tumor angiogenesis, and anti-immune reactions [
4,
9‐
11]. Overexpression of TGF-β1 correlates with progression of carcinomas, including breast, colon, esophageal, gastric, lung, ovarian, and pancreatic cancers. In contrast, weak or no TGF-β signaling due to mutations of TβRII, have been found in gliomas, biliary, breast, colon, esophageal, gastric, lung, ovarian, esophageal, pancreatic, prostate, and head and neck cancers [
7,
11,
12]. Oral squamous cell carcinoma (OSCC) represents one of the most common types of malignant tumors in head and neck. OSCC develops via stereotypical multistep processes. Abnormal TGF-β signaling has been proposed as one of the pathways leading to the carcinogenesis of OSCC [
13]. Yet, it is largely unclear how abnormal TGF-β signaling contributes to carcinogenesis of OSCC.
We have previously reported various mutations of TβRII detected in 18 human OSCC samples [
12]. In the present study, we investigated the I227T/N236D missense mutation located in the intracellular domain of TβRII, which harbors ACC (Thr) instead of ATC (Ile) at codon 227, and GAC (Asp) instead of AAC (Asn) at codon 236 (I227T/N236D). I227T/N236D mutation of TβRII, a novel mutation which has not been previously reported, was detected in the metastatic lymph node of one OSCC patient. We analyzed the effect of I227T/N236D TβRII mutation on TGF-β signaling and elucidated its role in the functional alterations leading to OSCC development.
Methods
Antibodies, plasmids, and other reagents
Matrigel (BD Matrigel Matrix, #356234) and type I collagen (#637–00653, Cellmatrix type I-A) were purchased from BD Biosciences (San Diego, CA, USA) and Nitta Gelatin (Osaka, Japan), respectively. Recombinant human TGF-β1 (#616455) was purchased from Calbiochem (Merck KGaA, Darmstadt, Germany). Curcumin (#C1386) and 3-(4,5-methylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) (#M5655) were obtained from Sigma-Aldrich (Merck KGaA, Darmstadt, Germany). Antibodies against TβRII (#sc-220, 1:1000) were purchased from Santa Cruz Biotechnology (CA, USA). Antibodies against phospho-Smad2 (Ser465/467, #3101, 1:1000), Smad2 (#3122, 1:1000), cleaved Caspase-3 (#9661, 1:1000), cleaved PARP (#9541, 1:1000), epidermal growth factor receptor (EGFR) (#2232, 1:1000), phospho-EGFR (Tyr1068, #2234, 1:1000), phospho-Akt (Ser473, #9271, 1:1000), Akt (#9272, 1:1000) were purchased from Cell Signaling Technology, Inc. (Danvers, MA, USA). Antibodies against β-actin (#BS6007M, 1:5000) were obtained from Bioworld Technology, Inc. (St. Louis, MN, USA). Tyrphostin AG 1478 (#9842), a selective EGFR inhibitor, was purchased from Cell Signaling Technology, Inc. The cDNA for human TβRII (#RC519855) was purchased from Origene (Rockville, MD, USA). p3TP-lux (#11767) and pRL-TK (Renilla reniformis luciferase under thymidine kinase promoter) (#E2241) were obtained from Addgene (Cambridge, MA, USA) and Promega (Madison, WI, USA), respectively. Restriction enzymes were purchased from New England Biolabs (Beverly, MA, USA). Primers for cloning and mutagenesis were synthesized by Bioneer (Daejeon, South Korea). Phusion High-Fidelity DNA Polymerase (#F530S) for TβRII cloning and mutagenesis was supplied by Thermo Fisher Scientific, Inc. (Carlsbad, CA, USA).
Cells and transient transfection
DR26 cells, mutant derivatives of Mv1Lu mink lung epithelial cells, which lack functional TβRII, were generously provided by Dr. J. Massague (Memorial Sloan-Kettering Cancer Center, New York, NY, USA). DR26 cells were cultured in Dulbecco’s modified Eagle’s medium (DMEM, #12-604F, Biowhittaker, Inc., Walkersville, MD, USA) supplemented with 10% fetal bovine serum (FBS, #26140079, Gibco, Thermo Fisher Scientific, Inc., Waltham, MA, USA) and 1% penicillin/streptomycin (#15140–163, Gibco, Thermo Fisher Scientific, Inc.) at 37 °C in the presence of 5% CO2. HSC-2 human OSCC cells were kindly provided by Prof. Takashi Muramatsu (Tokyo Dental College, Tokyo, Japan). HSC-2 cells were cultured in P medium (DMEM:Ham’s F-12; 3:1) supplemented with 10% FBS and 1% penicillin/streptomycin. Ham’s F-12 (#21700–075) was purchased from Thermo Fisher Scientific, Inc. Cells were transiently transfected using Lipofectamine 2000 (#11668–019, Invitrogen, Thermo Fisher Scientific, Inc.), following the manufacturer’s instructions.
Mutagenesis
The I227T/N236D double mutant TβRII was obtained by sequential site-directed mutagenesis. First, a TβRII mutant with a threonine residue instead of isoleucine at amino acid 227 (I227T) was constructed by site-directed mutagenesis using PCR as described in our earlier report [
10]. cDNA encoding full-length human TβRII was previously subcloned into pcDNA3 and pIRES2-EGFP vectors [
10]. These plasmids were used as templates for PCR. Primers 5′-TTGGATCCGGGGTCTGCCATGGGTC-3′ (F-BamHI) and 5′-AATCTAGACTATTTGGTAGTGTTTAGGGAGC-3′ (R-XbaI) were used to clone TβRII in pcDNA3; 5′-TTCTCGAGGGGGTCTGCCATGGGTC-3′ (F-XhoI) and 5′-AAACCGCGGCTATTTGGTAGTGTTTAGGGAGCC-3′ (R-SacII) were used to clone TβRII in pIRES2-EGFP. Primers used for site-directed mutagenesis are as follows: 5′-GCCATCATCCTGGTAGATGACCGCTC-3′ (sense) and 5′-GAGCGGTCATCTACCAGGATGATGGC-3′ (antisense). The PCR was performed using primers, F-BamHI (F-XhoI) with antisense and sense with R-XbaI (R-SacII). Subsequently, using the products of first PCR, a second round of PCR was carried out using the primers, F-BamHI (F-XhoI) and R-XbaI (R-SacII). The mutant PCR product was ligated to the corresponding restriction enzyme sites of the vector to generate the I227T mutant TβRII in pcDNA3 or pIRES2-EGFP. The N236D mutant TβRII was constructed in a similar fashion, using primers, 5′-CAACATCAACCACATCACAGAGCTGCTG-3′ (sense) and 5′-CAGCAGCTCTGTGATGTGGTTGATGTTG-3′ (antisense). I227T mutant TβRII plasmids were used as templates for the second PCR mutagenesis. The integrity of the products was confirmed by sequencing.
Construction of stable transfectant cells expressing TβRII
HSC-2 cells stably expressing wild-type or I227T/N236D mutant TβRII were constructed, as previously described [
10]. After transfection with pIRES2-EGFP vector, wild-type TβRII, and I227T/N236D TβRII in pIRES2-EGFP, the cells were selected in a P medium containing 10% FBS and 400 ng/ml of G418 (#G8168, Sigma-Aldrich).
3TP-lux promoter-reporter was used to test the transcriptional activities induced by TβRII mutation. DR26 cells seeded in 24-well plates were transfected with 0.2 μ g of 3TP-lux promoter-reporter constructs, 0.2 μg of wild type or mutant TβRII in pcDNA3, and 0.5 μg of pRL-TK. At 24 h after the transfection, cells were stimulated with 1 ng/ml of TGF-β in DMEM containing 0.2% FBS. After incubation for 24 h, the firefly luciferase and Renilla luciferase activities were detected using Dual Luciferase Reporter Assay System (#E1910, Promega, Madison, WI, USA), according to the manufacturer’s instructions. Data were normalized to Renilla luciferase activity for evaluation of transfection efficiency.
Cell viability assay
Cell viability was measured by MTT assay as previously described [
10]. For cytotoxicity analysis, cells (1 × 10
4) were seeded into the individual wells of a 96-well plate and treated with curcumin, AG1478 in the P medium containing 0.2% FBS. The optical density was measured at 570 nm using a microplate reader. All experiments were performed in triplicate.
Western blot analysis
The cells (1 × 106) were washed with cold PBS and lysed with Cell Lysis Buffer (#9803S, Cell Signaling Technology, Inc.) supplemented with phenylmethylsulfonyl fluoride (PMSF, # 78830, Cell Signaling Technology, Inc.). Protein concentration was determined using the Bicinchoninic Acid Protein Assay Kit (Pierce, Thermo Fisher Scientific, Inc.). Proteins (50 μg) were separated on 10% SDS-PAGE and transferred to PVDF membranes (#162–0177, Millipore, Billerica, MA, USA). The membranes were blocked in TBST (Tris-buffered saline with 0.5% of Triton X-100) containing 5% non-fat milk or BSA for 1 h at room temperature and incubated with appropriate primary antibodies at 4 °C overnight. After three washes with TBST, the membranes were incubated with horseradish peroxidase-conjugated anti-mouse (#7076S) or anti-rabbit (#7074S) secondary antibodies (Cell Signaling Technology, Inc.) for 1 h. Protein bands were visualized using chemiluminescence reagent (#W3651–024, GenDEPOT, Barker, TX, USA). The ImageJ software was used to quantify the signal intensity of protein bands.
RNA interference
Two different small interfering RNA (siRNA) sequences were used to target EGFR. The siRNAs were synthesized by Bioneer and the sequences were as follows: siEGFR#1 sense, 5′-GCAAAGUGUGUAACGGAAUAGGUAU-3′ and antisense, 5′-AUACCUAUUCCGUUACACACUUUGC-3′; siEGFR#2 sense, 5′-GAGGAAAUAUGUACUACGA-3′ and antisense, 5′-UCGUAGUACAUAUUUCCUC-3′ (antisense). A negative control siRNA (#SN-1003) was also purchased from Bioneer. Cells (4 × 105) were cultured in 60 mm dishes for 24 h. siRNA (100 nM) transfections were carried out using Lipofectamine RNAiMAX (#13778–075, Invitrogen, Thermo Fisher Scientific, Inc.), according to the manufacturers’ instructions. After 6 h of incubation, medium was replaced to fresh P medium containing 10% FBS and cells were cultured for an additional 42 h at 37 °C. For invasion assay, siRNA-transfected cells were stabilized for 18 h after the addition of fresh medium, and then, seeded into the upper chamber of transwell inserts.
Transwell migration assay and scratch wound healing assay
Two methods, transwell migration assay and scratch wound healing assay, were used to evaluate cell motility. In transwell migration assay, cells (2 × 104) were serum-starved for 16 h in P medium and seeded into the upper chamber of transwell inserts (8 μm pore size; #3422, Corning Costar, Lowell, MA, USA) with or without TGF-β1 (10 ng/ml). The lower chamber contained P medium supplemented with 10% FBS. After 24 h of incubation, cells that have migrated to the lower compartment were fixed in 10% formalin and stained with 0.025% crystal violet. The membrane filters were mounted on slides, and cells were counted under a microscope (five images per group; magnification, × 100). In scratch wound healing assay, cells (1 × 105) were seeded into 6-well plates and transfected with siRNAs. At 48 h post-transfection, the cell monolayer was wounded with a 200 μl sterile pipette tip. Cells were washed once with PBS and fresh serum-free medium was added with or without TGF-β1 (10 ng/ml). Following 18 h of incubation, migrated cells were counted under a light microscope (magnification, × 100).
Invasion assay
Transwell invasion assays were performed in a fashion similar to transwell migration assay except that the membrane inserts were precoated with 50 μl of 100 μg/ml Matrigel or 30 μl of 1.5 mg/ml Cellmatrix Type I-A. Briefly, after serum starvation for 16 h in P medium, cells (2 × 104) were seeded into the transwell chamber inserts in the presence or absence of TGF-β1 (10 ng/ml). The P medium containing 1% FBS was added to the lower chamber. After 48 h of incubation, cells that have penetrated the filter were fixed and stained with 0.025% crystal violet, and counting under a light microscope (five images per group; magnification, × 100).
Gelatin zymography
Gelatinase activities of matrix metalloproteinase-2 (MMP-2) and matrix metalloproteinase-9 (MMP-9) were analyzed by gelatin zymography as described previously [
10]. Cells were seeded in 6-well plates and incubated in P medium containing 0.2% FBS in the presence of vehicle or TGF-β1 (10 ng/ml) for 24 h. Cell conditioned medium (15 μg protein) was subjected to 10% SDS-PAGE containing 0.1% gelatin under non-reducing conditions. After rinsing four times with 2.5% triton X-100 to remove SDS at room temperature, the gels were incubated overnight at 37 °C in a developing buffer (50 mM Tris-HCl, pH 7.5, 0.2 M NaCl, 5 mM CaCl
2·2H
2O, and 0.02% Brij-35, pH 7.6). The gels were stained with 0.5% Coomassie Brilliant Blue R-250 in 10% acetic acid and 50% methanol, and then destained with 50% methanol and 10% acetic acid solution.
Annexin V/propidium iodide (PI) apoptosis assay
Cells (1 × 106) were seeded in a 100 mm dish and incubated at 37 °C overnight. The cells were treated with 20 μM curcumin for 24 h, and then, stained with an FITC annexin V apoptosis detection kit (#556547, BD Pharmingen™, BD Biosciences, San Jose, CA, USA), according to the manufacturers’ instructions. Briefly, the cells were harvested and resuspended in binding buffer at a density of 1 × 106 cells/ml. After the addition of annexin V-FITC (5 μl) and PI (20 μg/ml, 5 μl), the cells were incubated for 15 min at room temperature. Stained cells were analyzed by flow cytometry (BD FACSCanto II flow cytometer, BD Biosciences, Franklin Lakes, NJ, USA). Cytomics FC 500 with CXP software (Beckman Coulter, Fullerton, CA, USA) was used for data analysis.
In vivo xenograft tumor growth assay
All the animal procedures were performed in accordance with a protocol approved by the Institutional Animal Care and Use Committee of Yonsei University. Five-week-old male BALV/c nude mice were purchased from Central Laboratory Animal Inc. (Seoul, Korea). All the mice were housed in the Yonsei Laboratory Animal Research Center and maintained in a pathogen-free environment (12 h light/dark cycle, 25 ± 2 °C; humidity, 50 ± 10%). Food and water were freely available. HSC-2 stable transfectant cells (2 × 10
5 cells in 100 μl P medium) were orthotopically injected into the anterior tongue of the mice. Development of tongue tumors and weight loss was regularly monitored for 4 weeks. Tongue tissues containing tumor were fixed in 10% formalin neutral solution for at least 24 h at room temperature and embedded in paraffin. The tissues were sectioned into 4-μm-thick slices and stained with hematoxylin and eosin (H&E; #H-3502, Vector Laboratories, Inc., Burlingame, CA, USA). Tumor volume (TV) was calculated according to the formula: TV = (AxB
2)/2, by measuring the diameters of the major axis (A) and the minor axis (B) of tumor [
14].
Immunohistochemistry
4-μm thick sections were cut from the paraffin blocks and immunostained as described [
10]. Sections were deparaffinized in xylene, and hydrated in descending grades of ethanol. Sections were incubated with 3% hydrogen peroxide to inactivate endogenous peroxidase activities. After blocking in 1% normal goat serum (#S-1000, Vector Laboratories, Inc.) for 1 h, the sections were incubated with primary antibodies against phospho-EGFR (1:100) overnight at 4 °C, followed by incubation with biotinylated anti-rabbit IgG (#BA-1000, Vector Laboratories, Inc.) for 30 min. The sections were then incubated with horseradish peroxidase streptavidin (#SA-5004, Vector Laboratories, Inc.) for 30 min. The signal was developed using the 3,3′ diaminobenzidine tetrahydrochloride (DAB) kit (#SK-4100, Vector Laboratories, Inc.) and counterstained with Meyer’s hematoxylin (#H-3404, Vector Laboratories, Inc.). The staining intensity of the phosphorylated EGFR on the cell membrane was manually scored. An intensity scale ranging from 0 for no staining to 3+ for the most intense staining is used. H-Score is calculated using the following formula: H − Score = (%at 0) × 0 + (%at 1+) × 1 + (%at 2+) × 2 + (%at 3+) × 3.
Statistical analysis
Mann-Whitney U test was performed for statistical analysis. All variables were analyzed from three independent experiments, and each experiment was performed at least in triplicate. The results are shown as the mean ± standard deviation. A P-value < 0.05 was considered as statistically significant.
Discussion
Various mutations of TβRII have been found in a variety of tumors [
7,
11,
12,
22]. However, functional studies investigating specific mutations of TβRII from OSCC are rather scarce. The aim of the present study was to investigate the relationship between I227T/N236D mutation of TβRII and OSCC progression. The I227T/N236D mutation of TβRII resulted in enhanced TGF-β signaling, as revealed by higher transcriptional activities and Smad2 phosphorylation, as compared to wild-type. The transcriptional activity of TβRII was enhanced by I227T/N236D mutation regardless of the presence of TGF-β, suggesting that this mutation could elicit conformational changes to a higher affinity form for either TGF-β or TβRI. Another possibility for the higher transcriptional activity is that I227T/N236D mutation could modulate the rate of receptor internalization, leading to a prolonged and stronger TGF-β signaling as was previously observed under other TβRII mutations [
10]. Underlying mechanism for the higher transcriptional activity by I227T/N236D mutation of TβRII remains to be clarified.
To analyze the effect of I227T/N236D mutation on TGF-β signaling in human OSCC cells, HSC-2 cells were used. We have previously shown that HSC-2 cells exhibited TGF-β responsiveness such as TGF-β-mediated Smad2/3 phosphorylation and EMT induction [
10], suggesting that HSC-2 cells retain an intact TGF-β signaling machinery. Stable HSC-2 cells transfected with empty vector, wild-type TβRII, or I227T/N236D mutant TβRII did not exhibit significant differences in growth capabilities, which indicates that enhanced TGF-β signaling does not affect HSC-2 cell proliferation in vitro. However, in xenograft tumor growth assay, the I227T/N236D mutant TβRII exhibited around 3-fold higher tumor volume as compared to that of wild-type, implying that I227T/N236D mutation of TβRII promotes cell proliferation in vivo. To understand the discrepancy between in vivo and in vitro proliferative capacities, we focused on the differences in cell proliferation and death signaling among two-dimensional (2D) cultures, in vitro three-dimensional (3D) cultures, and in vivo growth conditions. It has previously been reported that cell proliferation rate is lower in 3D cell culture than in 2D culture as evidenced by the increased number of apoptotic cells and reduced S-phase cell population [
23,
24]. Apoptosis also occurs during tumor progression in vivo, which plays a critical role by imposing a highly selective pressure enabling clonal expansion of aggressive sub-clones [
25,
26]. The evasion of apoptosis in response to stress stimuli is thus an acquired hallmark of cancer. Differences in local availability of oxygen, nutrients, and signaling molecules within 3D culture or under in vivo conditions, are known factors that drive the variable cellular responses depending on their location [
24]. We wondered, in this context, whether differential in vivo growth capacities of HSC-2 stable cells expressing wild-type and I227T/N236D mutant TβRII are due to varying susceptibilities to apoptotic stimuli of these cells. We thus analyzed apoptosis in 2D culture conditions after treatment with apoptotic inducers, such as curcumin. The I227T/N236D mutant stable cells were more resistant to apoptosis as compared to wild-type cells following treatment with apoptotic inducers. These results suggest that the enhanced TGF-β signaling of I227T/N236D mutant TβRII cells contribute to enhanced in vivo proliferative capacities of these cells via suppression of apoptosis.
In response to stress stimuli, cancer cells not only acquire resistance to apoptosis but also induce survival signaling that is tightly linked. The balance between survival and death signaling controls the tumor growth [
26,
27]. Survival signaling involves abnormal activation of growth stimulating molecules, such as epidermal growth factor (EGF), platelet-derived growth factor (PDGF), nuclear factor κB (NF-κB), interleukins 2 and 3, mitogen-activated protein kinase (MAPK), and PI3K/AKT [
25,
28,
29]. Our data showed that EGFR signaling was enhanced by curcumin in HSC-2 stable cells, and upregulation of TGF-β signaling further promoted curcumin-mediated EGFR signaling. These results indicate that EGFR signaling acts as an important survival mechanism in I227T/N236D TβRII stable cells. A number of reports have revealed that enhanced EGFR signaling disrupts the balance between cell growth and apoptosis during development of a variety of solid tumors [
30,
31]. Abnormal activation of EGFR is known to occur mainly via overexpression, mutation, or autocrine stimulation of EGFR in cancers [
30‐
32]. Transactivation of EGFR by TGF-β signaling has also been documented in breast, gastric cancer cells, and hepatocytes [
33‐
36]. Taken together, our results indicate that EGFR activation via enhanced TGF-β signaling might play a critical role in tumor progression of OSCC by promoting cell survival.
Curcumin is a polyphenol found in the rhizome of
Curcuma longa and exhibits nontoxic chemopreventive and chemotherapeutic activities [
20]. Curcumin has been shown to induce apoptosis in a variety of cancer cells [
37‐
40]. We used curcumin as an apoptotic inducer since these seemingly opposite effects could be gradually modulated by controlling the dose. Since curcumin has various molecular targets, diverse apoptotic and growth inhibitory signaling pathways are modulated by curcumin treatment. Several reports have shown that curcumin inhibits EGFR signaling via suppression of EGFR expression, induction of EGFR degradation, inhibition of kinase activity of EGFR, and modulation of EGFR dimerization in lung adenocarcinoma, colon cancer, and epidermoid carcinoma cell lines [
19,
21,
41]. Curcumin induced apoptosis of HSC-2 cells at a concentration of 20 μM in 24 h. However, curcumin concentrations lower than 20 μM did not induce significant apoptosis in 24 h, but activated EGFR in all stable cells. We speculated that the exposure of HSC-2 cells to low concentrations of curcumin can activate pro-survival EGFR signaling as well as death signaling. EGFR signaling was higher in HSC-2 cells expressing I227T/N236D mutant TβRII than in wild-type counterpart. Concomitant with higher EGFR activation, the level of cleaved caspase-3 was lower in mutant cells than in wild-type cells upon curcumin treatment. AG1478, a pharmacological inhibitor of EGFR, efficiently suppressed EGFR activation, which led to the abrogation of apoptotic resistance of I227T/N236D mutant cells. Taken together, these results suggest that EGFR activation via enhanced TGF-β signaling is critical for the evasion of HSC-2 stable cells from apoptosis, which further supports the notion that I227T/N236D mutation of TβRII plays an important role in cell proliferation in vivo.
TGF-β treatment enhanced the migratory and invasive activities of HSC-2 cells. The I227T/N236D mutant TβRII cells exhibited higher migratory and invasive capabilities compared to wild-type cells regardless of the presence of TGF-β. To validate the role of EGFR in regulating migratory and invasive capabilities of I227T/N236D TβRII stable cells, EGFR was knockdowned in mutant cells. EGFR silencing dramatically reduced migratory and invasive capabilities of I227T/N236D TβRII cells, indicating that EGFR signaling plays a pivotal role in cell migration and invasion as well as in proliferation of OSCC.
In summary, our findings suggest that enhanced TGF-β signaling via I227T/N236D mutation of TβRII promotes EGFR activation, leading to tumorigenesis of OSCC by enhancing the hallmark features of cancer, such as apoptotic resistance and more invasive phenotypic changes.
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