Background
Epithelial ovarian cancer (EOC) is a female malignant disease. The mechanism of the occurrence and development of EOC is complex. Cytokines and growth factors may play important roles in ovarian tumorigenesis. The signal transducer and activator of transcription 1 (STAT1) is one of the members of STAT family and functions as a signal messenger, transcription factor, and immune modulator, participating in cellular processes including cell proliferation, differentiation, apoptosis, and immunosurveillance [
1‐
3]. STAT1 has two isoforms, a full-length STAT1α and a truncated STAT1β. STAT1α carries two phosphorylation sites, tyrosine 701 (Y701) and serine 727 (S727). The latter is located at a C-terminal trans-activation domain (TAD) [
4]. STAT1β is expressed at a low level and lacks the TAD but is efficiently phosphorylated on Y701 [
5,
6]. The canonical signaling pathway of STAT1 is triggered by Janus kinase (JAK) upon ligands, such as interferon-γ (IFN-γ), stimulation [
7]. The phosphorylated and activated STAT1 then translocate into the nucleus and regulates the expression of target genes. For example, IFN-γ can enhance the expression of Smad7 through the JAK1/STAT1 signaling pathway [
8]. Smad7 is an inhibitory Smad which prevents the interaction of Smad3 with transforming growth factor-β (TGF-β) receptor [
9].
It has been shown that TGF-β plays an important role in ovarian cancer [
10,
11]. The canonical TGF-β signaling pathway acts through the intracellular transducer proteins such as receptor-activated Smad (R-Smad). Upon TGF-β binding, the constitutively activated type II receptor (TβRII) recruits and phosphorylates the type I receptor (TβRI) on the cell surface [
12,
13]. TβRI, also known as activin receptor-like kinase (ALK), has seven members [
14]. ALK5 (TGFBRI, expressed in most types of cells) and ALK1 (ACVRL1, expressed mainly in endothelial cells) are two subtypes of TβRI for human TGF-β [
15]. TGF-β1-activated ALK5 and/or ALK1 further recruit and phosphorylate R-Smads, such as Smad2/3 by ALK5 and Smad1/5/8 by ALK1. The phosphorylated and activated R-Smads then form a complex with common Smad (Co-Smad) Smad4 and the resulting complex then translocates into the nucleus where it acts as a transcription factor by binding to the promoter of a target gene to regulate its expression [
16]. Smad6 and Smad7 are two inhibitory Smads (I-Smads) which prevent or inhibit Smad2/3 phosphorylation and nuclear translocation, hence suppressing their downstream function [
17].
The interference between STAT1 and TGF-β signaling has been reported previously [
8,
18,
19]. For instance, a bipyridyl compound CaeA can enhance TGF-β/Smad3 signaling by suppressing IFN-γ/STAT1 signaling in regulatory T cells [
18]. An inhibitory action of STAT1 on TGF-β signaling is via the induction of inhibitory Smad7 [
8]. On the other hand, TGF-β1 suppresses IFN-γ-induced STAT1 signaling through the promotion of STAT1 and its inhibitor protein interaction [
19]. All these suggest that the crosstalk between two pathways is the downstream event of receptor activation and no exact mechanism of interference from each other is explored at the receptor level. The physical interaction of STAT1 with the signaling components of TGF-β is never speculated.
Our recent study using high-throughput luminescence-based mammalian interactome mapping technology showed that STAT1 as a potential binding protein is one of the novel interactors of the TGF-β1 receptor [
20]. The current study validated for the first time that STAT1 isoforms directly interacts with TGF-β receptors and determined the consequence of this interaction particularly on the downstream signaling of TGF-β1. Meanwhile, we determined whether TGF-β1 activates STAT1 and STAT1/TGF-β receptor complex. The expression of STAT1 in EOC and the function of STAT1 on EOC cell behaviours were also examined.
Methods
Patients and ovarian tissue preparation
Human fresh ovarian tissue samples were obtained with informed consent from patients. The study on human subjects was approved by the Ethics Committee of Jinshan Hospital, Fudan University. A total of 20 ovarian samples were collected from patients who underwent cytoreductive surgery (5 normal samples from patients with non-ovarian tumor and 15 ovarian tumor samples, including 6 benign, 3 borderline, and 6 malignant tumors) with median age 50 years (range 25–70 years) at Jinshan Hospital from January, 2013 to January, 2016. None of the patients had received chemotherapy or radiotherapy before surgery.
Tissue microarray and immunohistochemistry
A human ovarian tissue microarray was obtained from Alena Biotechnology Co., Ltd. (Cat# OV1005a, Xi’an, Shanxi, China). All tissues were 10% formalin-fixed and paraffin-embedded. A total of 100 ovarian tissue specimens (20 normal controls and 80 ovarian tumors) were examined by immunohistochemistry (IHC). Among 100 specimens in a slide, seven came off during the IHC staining process. In the end, 20 normal controls (3 from normal ovaries and 17 from adjacent normal ovary tissues) with median age 48.5 years (range 19–63 years) and 73 ovarian tumors (12 benign, 7 borderline, 44 malignant, 10 metastatic) with median age 49.0 years (range 17–75 years) were statistically analyzed.
After blocking with 10% normal goat serum (Fuzhou Maixin Biotech Co., Ltd., Maixin Bio, Fuzhou, Fujian, China), the sections were incubated with rabbit monoclonal antibodies against STAT1 (Cat# 9175), pSTAT1-Y701 (Cat# 9167) and pSTAT1-S727 (Cat# 8826) (Cell Signaling Technology, Inc., Danvers, MA, USA), respectively, overnight, followed by incubation with biotinylated anti-rabbit secondary antibody (Cell Signaling Technology) at room temperature for 1 h. Scoring of STAT1 immunoreactive staining was performed by two independent examiners without any prior view of patient’s clinical data and classified as described previously using staining index (SI) system [
21].
Cell culture
Human epithelial ovarian cancer cell lines (OVCAR-3 and SK-OV-3) and human embryonic kidney cell line (HEK-293 T) were obtained from American Type Culture Collection (ATCC, Manassas, VA, USA). Non-tumorous human ovarian surface epithelial cells (HOSEpiC) were obtained from ScienCell Research Laboratories (Carlsbad, CA, USA). HOSEpiC and OVCAR-3 were cultured in RPMI-1640 medium (HyClone, Thermo Fisher Scientific Inc., Beijing, China), whereas SK-OV-3 and 293 T cells were cultured in Dulbecco’s Modified Eagle’s Medium (DMEM) (HyClone), supplemented with 10% fetal bovine serum (FBS) (Invitrogen, Carlsbad, CA, USA).
Treatment with TGF-β1 and inhibitor of TGF-β type I receptor kinase
Cells were seeded into 6-well plate at 5 × 105 cells/well for 24 h and then treated with TGF-β1 (0, 0.1, 1 or 10 ng/ml, R&D Systems, Minneapolis, MN, USA) for 24 h or 10 ng/ml of TGF-β1 for a time period as indicated. In order to block the TGF-β signaling, cells were pre-treated with an inhibitor of TGF-β type I receptor kinase (10 μM SB-431542, Sigma, Saint Louis, MO, USA) for 30 min, followed by 10 ng/ml of TGF-β1 treatment for 24 h.
Quantitative real-time polymerase chain reaction (qRT-PCR)
Total RNA was extracted from cells and tissues using the RNeasy Mini Kit (Qiagen, Gaithersburg, MD, USA) according to the manufacturer’s instruction. Five-hundred nanogram of total RNA was reverse-transcribed using a Transcriptor First Strand cDNA Synthesis Kit (Roche Diagnostics, Indianapolis, IN, USA). The primers for total STAT1, STAT1α, STAT1β, and β-actin (shown in Additional file
1: Table S1) were synthesized (GenePharma Co. Ltd., Shanghai, China). PCR amplification was performed using a Power SYBR Green PCR Master Mix Kit by 7300 Real-Time PCR System (Applied Biosystems, Foster City, CA, USA) according to the manufacturer’s recommendations.
Transfection of small interfering RNA
Cells were seeded into 6-well plate at 2.5 × 10
5 cells/well and transfected with 1 μg of human STAT1-small interfering RNA (STAT1-siRNA) or scramble non-specific control siRNA (NC-siRNA) (GenePharma Co. Ltd.; Additional file
1: Table S1) using X-tremeGENE siRNA transfection reagent (Roche Diagnostics) according to the manufacturer’s instruction, followed by incubation for the indicated time.
Generation of constructs
TGF-β receptor constructs were used as described previously [
20]. STAT1α and STAT1β constructs were generated by inserting the PCR products into a mammalian expression vector. Briefly, the cDNA encoding STAT1α or STAT1β was amplified by PCR using the Pfu Ultra II Fusion HS DNA Polymerase (Stratagene, Agilent Technologies, CA, USA) with specific primers (shown in Additional file
1: Table S1). After purification of PCR, the product was inserted into the
Kpn I and
Sac II sites of pcDNA4/TO/myc-His (B) vector (Invitrogen). Two plasmids named as pStat1α-myc and pStat1β-myc were generated and the presence of insert was confirmed by restriction enzyme digestion as well as by sequencing.
Transient transfection and co-immunoprecipitation (co-IP)
HEK-293 T cells were seeded into 6-well plate at 2.5 × 105 cells/well and were transfected or co-transfected with 4 μg receptor plasmid (ALK1-HA, ALK5-HA or TβRII-HA) and/or STAT1 plasmid (Stat1α-myc or Stat1β-myc) using DNA Transfection Reagent (GBC lifetech, Miami, FL, USA). After incubation for 48 h, cells were lysed with Pierce RIPA Buffer (Thermo Scientific, Rockford, IL, USA) supplemented with phosphatase inhibitor (KeyGEN BioTECH, Nanjing, Jiangsu, China) and PMSF (Beyotime, Haimen, Jiangsu, China) on ice for 20 min. Cell lysates (500 μg of total proteins) were then incubated with 5 μl anti-Myc IP Affinity gel or anti-HA IP Affinity gel (GBC lifetech) overnight at 4 °C according to the manufacturer’s instruction. After extensive washing, bound proteins were eluted with 4X sample buffer. Eluate and input proteins were then subjected to immunoblotting.
Immunoblotting (IB)
Cells were lysed using Pierce RIPA buffer supplemented with 1% PMSF and phosphatase inhibitors (KeyGEN BioTECH). Protein concentration was measured using the BCA Protein Assay (Thermo Scientific). Equal amounts of protein were separated on 10% SDS-PAGE and transferred to a PVDF membrane (Millipore, Billerica, MA, USA). After blocking, the membrane was incubated with a primary antibody at 4 °C overnight and subsequently incubated with horseradish peroxidase-conjugated goat anti-rabbit or anti-mouse IgG (Cell Signaling Technology, Inc., Danvers, MA, USA) for 1 h at room temperature. The following primary antibodies were used: rabbit monoclonal anti-STAT1, anti-pSTAT1-Y701, anti-pSTAT1-S727, mouse monoclonal anti-Smad2, rabbit polyclonal anti-pSmad2, anti-β-actin (Cell Signaling Technology), rabbit polyclonal anti-HA and anti-c-Myc (Santa Cruz Biotechnology, Inc., CA, USA). Signals were detected using Immobilon™ Western Chemiluminescent HRP Substrate (Millipore) and quantified using Tanon-4500 Gel Imaging System with GIS ID Analysis Software v4.1.5 (Tanon Science and Technology Co., Ltd., Shanghai, China).
Cell proliferation, migration, and invasion assays
For the cell proliferation assay, cells were seeded into 96-well culture plate at a density of 3 × 103 cells/well and incubated for 24 h, followed by transfection with STAT1 plasmids or STAT1-siRNA or their counterpart controls in the absence or presence of 10 ng/ml of TGF-β1 for 48 h. Cell proliferation was measured by the Cell Proliferation Reagent (WST-1 kit, Roche) according to the manufacturer’s instruction. The signal was read by a microplate reader (BioTek Epoch, Winooski, VT, USA) at 450 nm.
For the migration assay, SK-OV-3 cells were seeded into 6-well plate at a density of 3 × 105 cells/well and cultured for 24 h. The cell monolayer was then scraped using a pipette tip to make a scratch wound. After washing, cells were transfected with STAT1 plasmids or STAT1-siRNA as well as their counterpart controls and incubated for 24, 48, and 72 h. Cell migration was determined by wound healing. Images of the wound were obtained by photography and the gap widths were measured and analyzed.
Cell invasion was performed in a plate with a Transwell containing a porous membrane (pore size 8 μm, Costar, Corning Incorporated, New York, NY, USA) coated with Matrigel (final concentration of 250 μg/ml/well, BD Biosciences, Bedford, MA, USA). After transfection of SK-OV-3 cells with STAT1 plasmids or STAT1-siRNA or their counterpart controls for 24 h, the transfected cells were seeded on the top chamber of Transwells without serum at a density of 1 × 104 cells/well. The bottom chamber was supplemented with 10% FBS as a chemoattractant. After incubation at 37 °C for 48 h, the non-invaded cells were removed by wiping the upper layer of the chamber. The invaded cells on the bottom surface were fixed with 4% paraformaldehyde and stained with 5% Crystal Violet Staining Solution (Beyotime). The cell number was counted in three random fields under a light microscope (BX43, Olympus, Tokyo, Japan). All experiments were repeated at least three times.
Statistical analysis
Data were analyzed using GraphPad Prism 5 (GraphPad Software Inc., San Diego, CA, USA) and SPSS Statistics 21 for Windows (SPSS, Chicago, IL, USA). For comparison between two groups or multiple comparisons in treatment experiments, a Student’s t-test or one-way ANOVA was applied. Results are presented as the mean ± the standard error of the mean (SEM). The difference at P < 0.05 was considered statistically significant.
Discussion
The current study demonstrated for the first time that STAT1α and STAT1β directly interact with TGF-β receptors (ALK1/ALK5/TβRII) and that the phosphorylation of STAT1 on Y701 and S727 is mediated by TGF-β1. Furthermore, the overexpression or knockdown of STAT1 influences TGF-β1-induced Smad2 phosphorylation and affects EOC cell proliferation, migration, and invasion. We convince that the crosstalk between two pathways initiates at the receptor level.
STAT1 is a transcriptional factor which mediates responses to all types of IFNs and regulates a variety of cellular activities [
22], whereas the impairment of TGF-β signaling has been found in various diseases, including cancer [
23]. Although the interference between STAT1 and TGF-β signaling has been reported previously, all data indicate that the crosstalk between these two pathways is the downstream event of receptor activation. Furthermore, no exact mechanism of interference from each other is explored and the physical interaction of STAT1 with the signaling components of TGF-β is never hypothesized. Using a high-throughput luminescence-based mammalian interactome mapping technology we have recently reported that STAT1 might interact with the TGF-β type I and type II receptors [
20]. The present study verified that STAT1α/β indeed bind to TβRII/TβRI (ALK1 and ALK5) and negatively regulates TGF-β signaling.
We consistently observed that TGF-β1 induced STAT1 phosphorylation on Y701 and S727 in non-tumorous cells. In cancerous cells, however, TGF-β1 induced STAT1 phosphorylation on S727 only (specific to STAT1α form) and inhibits STAT1 phosphorylation on Y701 (specific to both STAT1α and STAT1β forms). These data suggest that, besides IFNs, TGF-β1 also modulates the STAT1 signaling pathway. Interestingly, the binding status of STAT1α and STAT1β with ALK5, the main type I receptor of TGF-β, was different between non-cancerous and cancerous cells upon the administration of a ligand TGF-β1. It might depend on the certain circumstances. We hypothesize that there is a homeostasis function of STAT1α and STAT1β in the normal situation, which TGF-β1 activates both sites (Y701 and S727); whereas there is abnormal higher level of STAT1 in a tumor cell, which TGF-β1 increases the phosphorylation of STAT1 at S727 site (STAT1α) and decreases the phosphorylation of STAT1 at Y701 site (STAT1α/β). TGF-β-mediated STAT1 phosphorylation and activation may be cell-type specific and may reflect a molecular shift in tumorigenesis. Similar to our finding, it has been shown that TGF-β inhibits the activation and phosphorylation of STAT1 on Y701 induced by insulin-like growth factor binding protein-3 (IGFBP-3) in mesenchymal chondroprogenitor cells [
24]. TGF-β1 also inhibits IFN-γ-induced phosphorylation of STAT1 on Y701 and S727 in glial cells from rat brain [
25]. However, none of these studies show that the phosphorylation of STAT1 is a consequence of its binding to the TGF-β receptor. The current study provides conceivable evidence that an increase of STAT1 phosphorylation on S727, similar to Smad2 phosphorylation, can be detected within a short time (30 min) upon TGF-β1 stimulation, indicating that this process is associated with TGF-β receptor binding after the cytokine treatment.
Our study has shown that the status of the association or dissociation of STAT1/TβRI/TβRII complex is altered after TGF-β1 treatment. Classically, the activated STAT1 should dissociate from the TβRII/ALK5 complex, dimerize and translocate to the nucleus. However based on our co-immunoprecipitation data we speculate that the interaction between STAT1α and TGF-β receptor is not transient and that STAT1α constitutively binds to the TGF-β receptor, and blocks Smad phosphorylation and hence the downstream TGF-β signaling pathway. The phosphorylation of STAT1α by TGF-β1 leads to its activation and the dissociation of STAT1α from TβRII/ALK5 receptor complex that releases the blockage and, in turn, increases the phosphorylation of Smad2, executing TGF-β signal transduction in ovarian cancer cells. Furthermore, the balance between STAT1α and STAT1β is important in ovarian tumorigenesis. It has been showed the overexpression of STAT1β can inhibit the phosphorylation of STAT1α as well as the DNA-binding and transcriptional activities in B lymphocytes [
26], indicating that the altered levels of the STAT1 isoforms may affect the pathophysiological processes. In support of our data that STAT
−/− mice had high activation of the TGF-β signaling pathway during liver fibrosis [
27], knockdown of STAT1 enhances TGF-β1-induced phospho-Smad2, whereas overexpression of STAT1 suppresses TGF-β1-induced phospho-Smad2, strongly pointing toward the influence of STAT1 on TGF-β signaling pathway.
With respect to tumorigenesis both STAT1 and TGF-β1 present controversial roles. STAT1 has been reported for its tumor suppressive as well as tumor promoting functions [
28], whereas TGF-β inhibits cell proliferation at an early stage and promotes invasion and metastasis at the later stage of cancer [
29]. TGF-β-mediated STAT1 activation via STAT1α phosphorylation may result in the promotion of tumorigenesis. The current study showed that the expression level of STAT1 was higher in ovarian cancer cells (OVCAR-3 and SK-OV-3) than non-cancerous ovarian cells (HOSEpiC). High level of STAT1 was also observed in patients with high-grade serous EOC. The overexpression of STAT1 in ovarian cancer may result in the tumorigenic effect of TGF-β signaling and therefore partially explains the controversial behavior of TGF-β in tumorigenesis. Similar to the results reported previously in endometrial cancer cells [
30], our study demonstrated that the elevation of STAT1 expression promotes while its knockdown inhibits EOC cell proliferation, migration, and invasion.
In the present study, we found overexpression of STAT1 at both mRNA and protein levels in human epithelial-type ovarian borderline and malignant tumor tissues. High level of STAT1 was found in ovarian serous malignant tumors rather than in mucinous tumors, indicating that it is a tissue biomarker at least and is tumor-type specific. STAT1 has been recently identified as a drug resistance biomarker in ovarian cancer [
31]. It has been reported that STAT1 is a potential indicator predicting chemoresistance in EOC [
32,
33]. Activating the FAK/STAT1 signaling pathway induces a malignant potential in ovarian epithelium [
34] and targeting this signaling pathway is a good therapeutic strategy for ovarian cancer [
35].