Background
Epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs), including gefitinib, erlotinib, afatinib, and dacomitinib, are effective as first-line treatment for advanced non-small-cell lung cancer (NSCLC) harbouring activating
EGFR mutations (e.g., deletions in exon 19 and the exon 21 L858R mutation) [
1‐
5].
EGFR T790M mutation emerges following EGFR-TKI therapy, which accounts for 55% of acquired resistance to first- and second-generation EGFR-TKIs [
6,
7]. In addition, the molecular alternations that lead to EGFR-TKI resistance include bypass pathway activation [e.g., MET amplification (MET-amp), HER2 amplification (HER2-amp)] and downstream signalling pathways activation (e.g., PI3K and BRAF mutations) [
6,
8,
9]. Histological transformations [e.g., small cell and epithelial-mesenchymal transition (EMT)] are also involved in TKI resistance [
10,
11]. However, the mechanism remains unknown in ~ 15% of patients with acquired resistance to EGFR-TKIs.
Osteopontin (OPN) is a secretory extracellular matrix glycosylated phosphoprotein that was first identified in bone tissue as a major sialoprotein in modulating bone formation and remodelling [
12]. It is a member of the small integrin-binding ligand N-linked glycoproteins, a family of five integrin binding glycophosphoproteins, including bone sialoprotein, dentin matrix protein 1, dentin sialophosphoprotein, and matrix extracellular phosphoglycoprotein [
13]. OPN is an extracellular matrix (ECM) ligand for integrins and a likely candidate to promote angiogenesis in the uterus and placenta. OPN is highly expressed in osteoblasts and osteoclasts. OPN also plays an important role in biomineralization [
14], and it also contributes to various metastasis-associated mechanisms, including proliferation, survival, adhesion, migration, invasion, and angiogenesis [
15‐
17]. Moreover, OPN has been demonstrated to play a role in the metastasis of NSCLC. OPN is up-regulated in NSCLC and even more in cells with strong potential and capacity of metastasis and invasion [
18,
19], which can be attenuated by its deletion [
20]. Up-regulation of OPN is proposed to be associated with stages, severities, lymph node metastasis, poor prognosis, and high recurrence [
21‐
23]. However, it is still unclear whether OPN is responsible for acquired resistance to EGFR-TKIs.
This study aims to identify whether expression of OPN correlates with acquired resistance as well as the exact signalling pathways involved in OPN-mediated acquired resistance to EGFR-TKIs.
Materials and methods
Cell culture and reagents
PC9, HCC827 and H1975 cell lines were purchased from the Cell Bank of Type Culture Collection of the Chinese Academy of Sciences (Shanghai, China). Caicun Zhou at Tongji University School of Medicine provided PC9GR cells. HCC827GR cells were derived from HCC827 cells by exposure to gefitinib, as previously described [
24]. PC9 and PC9GR cells were routinely cultured in RPMI-1640 medium supplemented with 10% foetal bovine serum (Gibco, Carlsbad, CA, USA). HCC827, HCC827GR, and H1975 cells were cultured in RPMI-1640 medium supplemented with 10% heat-inactivated FBS (Gibco). Cells were grown in a humidified incubator with 5% CO
2 at 37 °C.
Gefitinib was supplied by AstraZeneca (London, UK). Osimertinib and the FAK inhibitor VS-6063 were purchased from Selleck Chemicals (Houston, TX, USA).
Viability and proliferation assays
Cells were plated in each well of a 96-well plate at a density of 3000 cells per well, grown overnight and then treated with varying drug concentrations for 72 h. The Cell Counting Kit-8 (CCK-8) assay kit (Boster, Wuhan, China) was used according to the manufacturer’s instructions to assess cell viability. Fluorescence at 630 nm and 450 nm was measured using a microplate reader after 1–2 h (Thermo, Waltham, MA, USA).
Proteome profiler array analysis
The protein profile was analysed using a human soluble receptor array kit, non-haematopoietic panel-ARY012 (R&D Inc. Minneapolis, MN, USA) according to the manufacturer’s protocols. PC9, HCC827, PC9GR and HCC827R cells were lysed with lysis buffer mixed with proteinase cocktail inhibitor (Roche, Branford, CT, USA). Cell lysates were pipetted up and down for resuspension on ice for 30 min and then centrifuged at 14,000 × g at 4 °C for 5 min. The protein lysates were collected, and the concentrations were determined by the bicinchoninic acid assay (BCA assay). The protein lysates (100–300 μg per membrane) were incubated overnight with nitrocellulose membranes containing 62 soluble receptors. The membranes were subsequently incubated first with a specific cocktail of biotinylated detection antibodies and later with the streptavidin–horseradish peroxidase solution. Signals were detected by using a chemifluorescence detection system (Bio-Rad, Hercules, CA, USA) according to the manufacturer’s protocol. The relative density of specific protein expression was determined using Quantity One software.
Kinase and western blot assays
We used Human Phospho-Kinase Array Kit-ARY003B (R&D) for the human receptor tyrosine kinase (RTK) assay. Cells were seeded into 6-well plates at a concentration of 3 × 105 cells/well. After 24 h, the cells were harvested and lysed in RIPA buffer (Cell Signalling Technology, Danvers, MA, USA) containing a protease and phosphatase inhibitor cocktail (Sigma-Aldrich, Louis, MO, USA). The protein lysates were incubated with the array membrane, and the protein signal was visualized using chemifluorescent detection (Bio-Rad) according to the manufacturer’s protocol. The relative density of specific protein expression was determined using Quantity One software.
Antibodies against the following were obtained from Cell Signalling Technology: p-EGFR Y1068 (#2234), ERK (#9102), p-ERK T202/Y204 (#9101), AKT (#9272), p-AKT S473(#4060), FAK (#3285), p-FAK Y397 (#8556), SRC (#2108), p-SRC Y416 (#2101), and PARP (#9542). The β-actin antibody (CW0096M), GAPDH antibody (CW0100M), horseradish peroxidase (HRP)-conjugated anti-mouse antibody (CW0102), and HRP-conjugated anti-rabbit antibody (CW0103) were purchased from CoWin Biosciences (Beijing, China). Anti-EGFR (C-2), OPN (LFMb-14), -integrin αV (P2W7), and -integrin β3 (B-7) antibodies were purchased from Santa Cruz Biotechnology. The anti-integrin β1 (AF5379) antibody was purchased from Affinity Biotechnology. For immunoblotting, cells were harvested, washed in PBS, and lysed in RIPA buffer [50 mmol/L Tris–HCl (pH 8.0), 150 mmol/L sodium chloride, 5 mmol/L magnesium chloride, 1% Triton X-100; 0.5% sodium deoxycholate, 0.1% SDS, 40 mmol/L sodium fluoride, 1 mmol/L sodium orthovanadate, and complete protease inhibitors (Selleck Chemicals, Houston, TX, USA)]. Western Lightning ECL reagent (Thermo) was used for signal detection.
siRNA experiments
Transfection was carried out using Lipofectamine 2000 transfection reagent (Invitrogen, Carlsbad, NM, USA) according to the manufacturer's protocol. The target siRNA sequences used are listed in Additional file
1: Table S1. siRNAs were used at a concentration of 10 nmol/L (GenePharma, Shanghai, China). The efficacy of transfection was verified by qRT-PCR.
Flow cytometry analysis
Cells were seeded in 6-well plates at a density of 5 × 104 cells per well and treated with drugs at different concentrations or DMSO as a negative control. We analysed apoptosis and the cell cycle status of the cells by using Annexin V-FITC and propidium iodide (PI) (R&D) staining according to the manufacturer’s protocol.
ELISA
The secreted OPN (sOPN) was measured using ELISA (MultiSciences, Hangzhou, China). Briefly, 50 μL of diluent was added to each well of the microplate. Then, another 50 μL standard, control, or sample was added to each well and incubated for 2 h at room temperature. After washing four times, the conjugate reagent was added to each well and incubated for 2 h at room temperature. After washing four times, 100 μL substrate solution was added to each well, and the reaction was stopped after 30 min by adding the stop solution. Absorbance at 450 nm was measured using a spectrophotometer (Thermo). PC9GR cells were exposed to medium containing 1% FBS for 12 h and then treated with the following inhibitors separately: the autophagy inhibitor 3-methyladenine (1 mM, Selleck Chemicals), the protein transport inhibitor brefeldin A (10 ng/ml, APExBIO, Houston, TX, USA), and the exosome secretion inhibitor 5-(N, N-dimethyl)- amiloride DMA (50 nM, APExBIO). The supernatant was collected from the cultured medium 24 h later.
RNA extraction and quantitative real-time PCR analysis
Total RNA was extracted from cells using TRIzol reagent (Invitrogen) and reverse transcribed to cDNA using reverse transcription reagents (Takara Bio, Shiga, Japan) according to the manufacturer's protocol. The primer sequences used for reverse transcription quantitative polymerase chain reaction (qRT‐PCR) are listed in Additional file
1: Table S1. Quantitative RT-PCR was performed using SYBR Premix ExTaq™ (Takara) with an ABI StepOnePlus Real-Time PCR system (Applied Biosystems, Foster City, CA) according to the operator's manual. The expression values of genes were normalized to the internal control GAPDH.
Human tissue and IHC
Seven NSCLC tissue samples were collected from patients between 2017 and 2018 at the Respiratory Department of the First Affiliated Hospital of Soochow University. All participants provided written informed consent at the time of recruitment. All cases had clinically and pathologically confirmed diagnoses of NSCLC in accordance with the Revised International System for Staging Lung Cancer.
Immunohistochemical (IHC) analysis was conducted in our previous study. Briefly, sections were incubated with anti-ITGαV (EPR16800, 1:200 dilution; Abcam) overnight at 4 °C and then with biotinylated secondary antibodies. The reactions were developed using DAB Kit (BD Bioscience, San Jose, CA, USA), and the sections were counterstained with haematoxylin. The staining area was scored using the following scale: 0, 0–10% of tissue stained positive; 1, 10–20% stained positive; 2, 20–40% stained positive; 3, 40–70% stained positive; and 4, > 70% positive cells. The IHC score was generated from three different areas of the slides, and the average score was calculated for each sample.
Immunofluorescence staining
Cultured cells were fixed with 4% paraformaldehyde for 15 min at room temperature, permeabilized with Triton (0.1% in TBS) for 30 min and blocked with 5% BSA in PBS for 1 h at room temperature. The cells were then incubated overnight at 4 °C with anti-ITGαV antibodies (EPR16800, 1:200 dilution; Abcam) followed by Alexa Fluor 488-conjugated anti-rabbit IgG (Beyotime, Shanghai, China) for 90 min. Finally, the samples were incubated in DAPI for 10 min (Life Technologies) for nuclear counterstaining. Images were acquired using a Leica SP8 confocal microscope with optimal settings for the fluorescent markers used.
Mouse xenograft models and establishment of EGFR-TKI-resistant lung cancer tumours in vivo
Male athymic BALB/c nude mice were purchased from the Experimental Animal Center of Soochow University and bred under pathogen-free conditions. All experimental procedures were reviewed and approved in accordance with the guidelines for the care and use of laboratory animals, and informed written consent was obtained from Soochow University. To establish mouse xenograft models, the same amount of the indicated tumour cells was injected subcutaneously into both flanks of each mouse. Tumour volumes (mm3) were calculated as length × width2/2. When tumours reached ~ 200 mm3, gefitinib and VS-6063 were given by gavage at 12.5 mg/kg and 25 mg/kg daily, respectively, until the mice were sacrificed.
Statistical analysis
All results are presented as the mean standard deviation (SD). Two-way ANOVA was used to calculate the difference in IC50 for EGFR-TKIs among cells with various treatments. Statistical comparisons were determined with Student's t test, and P < 0.05 was regarded as significant. All statistical analyses were performed with GraphPad Prism 7.0 (GraphPad, San Diego, CA) and SPSS 17.0 software (SPSS, Chicago, IL).
Discussion
Acquired EGFR-TKI resistance limits the long-term clinical efficacy of these drugs. Although most of the mechanisms of acquired EGFR-TKI resistance have been revealed, the mechanism of ~ 15% of cases has not yet been elucidated. In this study, we report that OPN contributes to acquired EGFR-TKI resistance by up-regulating expression of integrins αv and β3, which activates the downstream FAK/AKT and ERK signalling pathways to promote cell proliferation in NSCLC. These results provide theoretical bases for novel alternatives and treatment strategies for patients with EGFR-TKI-acquired resistant NSCLC.
To identify novel mechanisms contributing to EGFR-TKI acquired resistance in NSCLC, we performed proteome profiler array analysis in gefitinib-sensitive parent cell lines (PC9 and HCC827) and gefitinib-resistant cell lines (PC9GR and HCC827GR). A total of 119 soluble receptors and related proteins were detected. Interestingly, OPN was most significantly increased in both resistant cell lines, indicating that OPN plays an important role in EGFR-TKI acquired resistance in NSCLC. However, secretory OPN was only increased in PC9GR cells and not in HCC827GR cells (Fig.
1b). This might be because OPN secretion in HCC827GR cells is regulated, leading to a different mechanism of EGFR-TKI resistance in HCC827GR cells from that in PC9GR cells. OPN is a secretory extracellular matrix glycosylated phosphoprotein [
12]. High expression of SPP1 was reported to be involved in tumour invasion, progression, and metastasis in multiple cancers, including breast, ovarian, and colon cancer [
33‐
35]. In addition, OPN is up-regulated in NSCLC and even more in cells with strong potential and capacity of metastasis and invasion [
18,
19]. Wang and colleagues found that OPN was involved in the acquired resistance of lung cancer to afatinib, and its mechanism needs to be further explored [
36]. In this study, we found that OPN contributes to acquired resistance by enhancing expression of integrins αV and β3. OPN acts as a ligand for integrins, and OPN/integrin forms a positive feedback pathway to jointly induce acquired resistance.
The ECM alone can induce tumour cell resistance to treatment [
37]. As a family of cell surface receptors, integrins play an important role in interaction with the ECM. Integrin biochemical and mechanical signalling regulates cell survival, proliferation, differentiation, migration, adhesion, apoptosis, anoikis, polarity and stemness [
38‐
40]. Integrin αVβ3 is a large family of integrins. Integrin αVβ3 expression and activation drive the intracellular signalling that promotes cancer cell survival, invasion, metastasis, angiogenesis, and self-renewal [
39,
41], as well as chemotherapy resistance [
42,
43] and radiotherapy resistance [
44,
45]. Several studies have reported that integrin β3 is up-regulated after EGFR-TKI treatment [
46,
47], consistent with our findings. More importantly, our findings add to current knowledge about how integrin β3 is upregulated in resistant tumours. According to a previous study, the Kras/RalB/NF-κB pathway and miR-483-3p are essential for integrin β3-mediated EGFR-TKI resistance. However, our results showed that up-regulated integrin αVβ3 in gefitinib-resistant cells resulting from OPN down-regulation activated the FAK/Akt/Erk pathway (Fig.
7). Similarly, Kanda and colleagues found that acquired erlotinib resistance was mediated by the integrin β1/Src/Akt signalling pathway in lung cancer [
48]. Overexpression of OPN/integrin αvβ3 results in activation of downstream FAK signalling, which is a key component of the signal transduction pathways activated by integrins and has an essential role in cancer cell survival, EMT, metastasis, and stemness [
49]. Several studies have shown that activation of FAK signalling is associated with EGFR-TKI resistance [
46,
47]. Based on our results, VS-6063 can enhance sensitivity to gefitinib in PC9GR cells. We reason that inhibitors targeting FAK might interact with EGFR TKIs to prevent or delay the occurrence of acquired resistance and progression of lung cancer. However, the interaction between cancer cells and the microenvironment and the mechanisms of EGFR-TKI acquired resistance in tumour cells are rather complicated. Consequently, additional investigations are needed to better understand the roles of the OPN/integrin αvβ3/FAK pathway in acquired resistance to EGFR-TKIs.
There are some limitations to our study. First, the acquired resistance mechanism induced by the OPN/integrin αVβ3 pathway was confirmed only in EGFR-TKI-resistant PC9 cells, and our findings should be validated in various EGFR-TKI-resistant lung cancers. Second, although we demonstrated that OPN/integrin αVβ3 can induce acquired resistance in vitro, we still need to verify this in vivo to fully support our conclusion. Finally, we should investigate how expression of OPN is regulated in the process of acquired resistance, which will be helpful for better understanding and overcoming EGFR-TKI acquired resistance in NSCLC.
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.