Background
Colorectal cancer (CRC) is one of the most common cancer in the world, ranking as the fourth diagnosed cancer [
1]. Since 5-fluorouracil (5-FU) was discovered in 1957 [
2], it has been used as first-line therapy in colorectal cancer [
3]. Despite this, the efficacy of 5-FU-based chemotherapy is disappointing [
4]. Most patients acquire resistance during treatment with a median survival of approximately 20 months [
5,
6]. Thus, resistance to 5-FU-based chemotherapy has become the major hurdle to improve treatment efficacy.
The mechanisms of 5-FU resistance are complex. Some studies have demonstrated that cancer cells are resistant to chemotherapy intrinsically. Mutant p53 tumor suppressor gene is found in many human tumors and its activity is associated with 5-FU resistance [
7]. Some members of Bcl-2 family, such as Bcl-xL and Bax, are related to 5-FU resistance in colorectal cancer [
8]. Overexpression of Astrocyte elevated gene-1 (AEG-1) increases 5-FU resistance in human hepatocellular carcinoma (HCC) [
9]. While other studies have indicated that drug resistance could be acquired. Tumor microenvironment includes many stromal cells and plays a critical role in chemoresistance. Cancer-associated fibroblasts (CAFs) are capable of decreasing drug uptake in tumors and causing resistance during chemotherapy [
10]. Tumor associated macrophages (TAMs) could protect colorectal cancer cells from 5-FU-based chemotherapy via putrescine [
11]. In triple-negative breast cancer, TGF-β increases the stem-like properties of cancer cells and causes drug-resistance [
12].
Exosomes secreted by different kinds of cells are a kind of vesicles consisted of lipid bilayer membrane and play important roles in cell to cell communication [
13]. These vesicles contain various proteins and nucleic acids including mRNAs and microRNAs [
14,
15]. Recent studies have indicated that exosomes display multiple roles in tumor progression. Exosomes from mesenchymal stem cells could transfer angiogenesis-related microRNAs [
16]. Exosomal microRNA-9 released by nasopharyngeal carcinoma cells inhibits angiogenesis by targeting MDK and is associated with good survival in patients [
17]. MicroRNA-103 secreted by hepatoma cells is capable of promoting tumor cell metastasis [
18]. Metastatic organotropism is associated with exosomes and the integrins of exosomes could be used to predict tumor metastasis [
19]. Until now, most studies have focused on exosomal microRNAs transfer in various cancers. However, the mechanisms by which proteins in exosomes affect the phenotype of recipient cells have not been fully interpreted, especially in chemotherapy resistance.
Taken together, these findings promote us to investigate whether exosomes derived from 5-FU resistant cells could mediate 5-FU resistance in colorectal cancer cells. We verified the function of exosomes from 5-FU resistant cells RKO/R (Exo/R), analyzed the proteomics results of Exo/R and RNA-sequencing of 5-FU sensitive cells (RKO/P) and resistant cells (RKO/R). By doing this, we found that exosomal p-STAT3 enhanced 5-FU resistance in sensitive cells (RKO/P) via caspase cascade. Our results may open new avenues for discovering diagnostic markers and therapeutic targets for acquired 5-FU resistance.
Methods
Cell culture
Human colorectal cancer cell lines RKO and HCT116 were purchased from the Chinese Academy of Science (Shanghai, China). A 5-FU resistant cell line (RKO/R) was established from RKO parental cell line (RKO/P) as previously described [
20]. RKO/P and RKO/R cells were cultured in DMEM (Gibco, USA), and HCT116 cells were cultured in RPMI-1640 medium (Gibco, USA). RKO/R cells were cultured without 5-FU for this experiment. All medium contained 10% FBS (Gibco USA) and a penicillin-streptomycin solution (Gibco, USA). Cells were incubated in a humidified incubator with 5% CO
2 at 37 °C. For cell apoptosis assays, cells were seeded in 48-well plates. After 24 h, 200 μl of culture medium containing 5-FU, DMSO, inhibitors (Table
1) and exosomes was added to each well. Cells were then harvested for FACS analysis at different time points.
5-fluorouracil | F8423-5G | Sigma-Aldrich (USA) |
DMSO | D2650 | Sigma-Aldrich (USA) |
ezatiostat | A8225 | APExBIO Technology (USA) |
stattic | A2224 | APExBIO Technology (USA) |
Exosome isolation and normalization
Exosomes were extracted from the same volume of culture medium without FBS. The supernatant was centrifuged at 300 g for 10 min at 4 °C and then at 1,000 g for 10 min at 4 °C to remove apoptotic bodies, followed by 10,000 g for 30 min at 4 °C. Finally, the supernatant from the former step was centrifuged by 100,000 g for 70 min at 4 °C using an ultracentrifuge (Beckman Coulter, USA). After centrifugation, the exosomes were resuspended in complete culture medium or phosphate-buffered saline (PBS). Exo/P or Exo/R came from 106 RKO/P or RKO/R cells was added to each well of 48-well plates.
Flow cytometry
Apoptosis assays were performed after 72 h, and the cells were harvested according to the manufacturer’s instructions using an Annexin V-APC/7-AAD staining kit (MultiSciences, China). The apoptotic rate of the cells was determined by flow cytometry (BD FACSCanto, USA). All data were exported as FCS 3.0 documents and analyzed with FlowJo software 10.0.7. Cell cycle assays were performed after 24 h, and cells were harvested according to the manufacturer’s instructions for the cell cycle staining buffer (MultiSciences, China).
BrdU proliferation assay
Cells were plated in 96-well plates and incubated for 24 h. Then, BrdU, 5-FU and exosomes were added to each well and incubated for 24 h. Cell proliferation was measured using a BrdU cell proliferation ELISA kit (Abcam, UK). The absorbance of the samples was measured according to the manufacturer’s instructions.
Transmission electron microscopy (TEM)
Exosomes were resuspended in PBS and placed onto copper grids at room temperature. Extra solution was removed with filter paper, and phosphotungstic acid solution was added. Then, filter paper was used again to remove the excess solution. The copper grids were dried at room temperature for 2 min, and the exosomes were observed with transmission electron microscopy (JEM-1200EX, Japan).
High sensitivity flow cytometry (HSFCM)
To determine size distribution, exosomes were resuspended with PBS after ultracentrifugation and detected with a Flow NanoAnalyzer (NanoFCM, China).
Silver staining and shotgun proteomics
Exosomes were lysed with SDT buffer (4% SDS, 100 mM DTT, 150 mM Tris-HCl) and boiled for 5 min. The same volume of each sample was loaded. Exo/P and Exo/R proteins were visualized using a silver staining kit (Beyotime, China). Exo/R were lysed with SDT buffer and boiled for 15 min for mass spectrometry. Then, detergent, DTT and other low-molecular-weight components were removed, and the protein suspensions were digested at 37 °C overnight. The resulting peptides were desalted on C18 cartridges (Sigma, USA). Mass spectrometry was performed on a Q Exactive mass spectrometer (Thermo Scientific, USA). Finally, the data were analyzed with MaxQuant software version 1.5.3.17 (Max Planck Institute of Biochemistry, Germany) and searched against the UniProt database.
RNA-sequencing
Total RNA was extracted from RKO/P and RKO/R. RNA molecules were purified using oligo (dT) attached magnetic beads. Then, mRNA was fragmented into small pieces using fragmentation reagent. For cDNA synthesis, first-strand cDNA was generated using random N6 primers, followed by second strand synthesis. PCR was used to amplify the cDNA fragments with adaptors from the previous step. The double stranded PCR products were heat separated, and the single strand circle RNA was formatted as the final library. RNA-seq libraries were sequenced using a BGISEQ-500 sequencer. Raw reads that contained the sequences of adaptor and low-quality reads were filtered before downstream analysis. Clean reads were mapped to reference genes using Bowtie2. Gene expression levels were quantified by the RSEM software package, and NOISeq software was used to screen differentially expressed genes.
WB analysis
Cells and exosomes extracts were lysed with RIPA buffer (Beyotime, China) containing protease inhibitor (KeyGEN Biotech, China). Equal amounts of protein samples were loaded and run on SDS-PAGE gels equally and transferred to Hybridization Nitrocellulose Filter (Merck Millipore, Germany). Then, the membranes were blocked with 5% skim milk (BD Biosciences, USA) dissolved in 0.1% Tris-buffered saline with Tween-20 (0.1% TBST) at room temperature for 1 h. Next, the membranes were incubated with primary antibodies (Table
2) diluted in primary antibody solution (Toyobo, Japan) overnight at 4 °C. On the following day, 0.1% TBST was used to wash the membranes. Then, the membranes were incubated with secondary antibodies diluted at 1:5000 in 0.1% TBST at room temperature for 2 h. HRP-conjugated goat anti-rabbit IgG (H + L) (Thermo Scientific, USA) and goat anti-mouse IgG (H + L) (Thermo Scientific, USA) were used for the secondary antibodies. Signals were developed with ECL (Santa Cruz, USA) and protein bands were visualized on X-ray film.
Table 2
Primary antibody list
ALIX | GTX42812 | Genetex (USA) | 1:200 | |
Caspase-9 | 9502 | Cell Signaling Technology (USA) | 1:1000 | |
Caspase-3 | 9662 | Cell Signaling Technology (USA) | 1:1000 | |
CD63 | ab59479 | Abcam (UK) | 1:200 | |
CD9 | ab92726 | Abcam (UK) | 1:200 | |
GAPDH | 10494–1-AP | Proteintech (USA) | 1:1000 | |
GSTP1 | GTX112695 | Genetex (USA) | 1:1000 | 1:200 |
LaminB1 | 12987–1-AP | Proteintech (USA) | 1:1000 | |
p-STAT3 (Tyr705) | 9145 | Cell Signaling Technology (USA) | 1:200 | 1:200 |
STAT3 | 9139 | Cell Signaling Technology (USA) | 1:1000 | |
TBP | 44059 | Cell Signaling Technology (USA) | 1:1000 | |
TSG101 | ab125011 | Abcam (UK) | 1:200 | |
β-actin | 60008–1-Ig | Proteintech (USA) | 1:1000 | |
Confocal microscopy analysis
RKO/P cells were cultured in confocal dishes (NEST, China). Exo/P and Exo/R were labeled with the fluorescent dye PKH-26 (Sigma, USA) following the manufacturer’s instructions. Then, exosomes were ultracentrifuged again. RKO/P cells were incubated with the PKH-26 labeled Exo/P and Exo/R. Cells were fixed with 4% paraformaldehyde for 20 min, and washed with PBS containing 0.1% Tween (0.1% PBST) for confocal microscopy. Cell membranes were permeabilized with 0.2% Triton X-100 or methanol for 10 min and washed with 0.1% PBST. For immunofluorescence, cells were incubated with QuickBlock™ Blocking Buffer for Immunol Staining (Beyotime, China) for 1 h at room temperature and were then incubated with primary antibodies (Table
2) diluted in QuickBlock™ Primary Antibody Dilution Buffer for Immunol Staining (Beyotime, China) at 4 °C in a humidified chamber overnight. Subsequently, the dishes were washed and incubated with anti-rabbit or anti-mouse Alexa Fluor® Plus 488 secondary Antibody (1:500, Thermo Scientific, USA) at room temperature for 2 h. Cell nuclei were labeled with DAPI (Thermo Scientific, USA) for 2 min. Finally, the dishes were observed under a laser scanning confocal microscope (Leica TCS-SP8, Germany).
Real-time cellular analysis (RTCA)
RTCA (ACEA Biosciences, USA) was used to monitor cell viability, and was performed according to the manufacturer’s instructions. Cell index is used to monitor cell status including cell numbers and cell attachment. When cells adhere to the surface of E-plate and influence the electrical impedance across the array, the xCELLigence software records electrical values and converts it into cell index [
21]. First, 50 μl of culture medium was added to measure the background value. Then, the cells were mixed with 50 μl of culture medium and seeded into E-plates. Culture medium containing exosomes, 5-FU, DMSO and inhibitors was added when the cell index reached 1.0. The data were documented and exported with ACEA Biosciences RTCA software 2.0 and analyzed by Microsoft Excel and GraphPad Prism 7.0.
siRNA transfection
Chemically synthesized GSTP1 and STAT3 siRNAs (Table
3) and control siRNAs were purchased from RiboBio (China). The sequences of the siRNAs are shown in Table
3. The siRNAs were transiently transfected into RKO/P and RKO/R cells using Lipofectamine RNAiMAX (Invitrogen, USA) according to the manufacturer’s instructions. After 48 h of transfection, the cells were analyzed by WB, and the conditional medium was collected.
GSTP1 | si-1 | CCTACACCGTGGTCTATTT |
si-2 | TACATCTCCCTCATCTACA |
STAT3 | si-1 | CCGTGGAACCATACACAAA |
si-2 | CATCTGCCTAGATCGGCTA |
Caspase-3 activity assay
Cells were seeded in 96-well plates, and culture medium containing exosomes, 5-FU, DMSO and inhibitors were added after 24 h. Caspase-3 activity was determined per well according to the manufacturer’s instructions for the GreenNuc™ Caspase-3 activity assay kit (Beyotime, China). Fluorescence images were obtained using an Operetta CLS™ high-content cell imaging analysis system (PerkinElmer, USA), and a 20× objective lens was used in our experiment. The percentage of caspase-3 was calculated by the software Harmony 4.5 and GraphPad Prism 7.0.
Animal experiments
Animal experiments were approved by the Committee on the Ethics of Animal Experiments of The Sixth Affiliated Hospital, Sun Yat-sen University. Male BALB/c nude mice (4–5 weeks) were purchased from Charles river (China). RKO/P cells were injected to the right flank of each mouse subcutaneously (3 × 106 cells in 200 μl PBS per mouse). All mice were divided into four groups (Exo/P + DMSO, Exo/P + stattic, Exo/R + DMSO, Exo/R + stattic) when tumors reached volume of 50–100 mm3. 5-FU (50 mg/kg) and stattic (25 mg/kg) were administered intraperitoneally (i.p.) every 2 days. Exosomes (10 μg) were injected into the xenograft tumors every 4 days. Tumor volumes and body weights were measured every 2 days. The mice were sacrificed after 17 days of treatment, and the size and weight of tumors from each group were measured.
Statistical analysis
Statistical analysis was performed with the GraphPad Prism 7 software. All data were presented for two replicates or three times. Statistical significance was determined by Student’s t-test for two groups. We also used One-way ANOVA multiple comparison analysis with Tukey’s posttest. Two-way ANOVA with Fisher’s LSD test was used to compare more groups. P < 0.05 was regarded as statistically significant.
Discussion
5-FU-based chemotherapy significantly prolonged the life expectancy of patients, however, resistance to 5-FU is a major limitation to treatment success [
22]. Due to complicated and variable biological processes, the mechanisms of chemoresistance are still elusive [
23]. It is known that the functions of exosomes are not restricted to maintaining normal biological processes but also encompass drug resistance. Previous studies have shown that exosomes secreted by bone marrow stromal cells (BMSCs), CAFs and tumor cells promote chemotherapy resistance in human tumors [
24‐
26]. In this study, we isolated exosomes from 5-FU-resistant cells, identified exosomal p-STAT3 with shotgun proteomics analysis and verified its function in 5-FU resistance. These findings demonstrated an unconventional mechanism of acquired drug resistance in colorectal cancer.
Exosomes play multiple roles in intercellular communication by transmitting RNAs and proteins [
27,
28]. In previous studies, exosomes were described as important vesicles disseminating drug resistance. MicroRNAs in exosomes, which could change various pathways related to chemotherapy resistance have been reported in different cancers. For instance, cisplatin resistance in lung cancer is associated with exosomal miR-100-5p [
29], and the PI3K/Akt pathway in hepatocellular carcinoma (HCC) is activated by miR-32-5p delivered by exosomes from resistant cells [
30]. Efflux of the tumor-suppressors miR-145 and miR-34a via microvesicles is responsible for 5-FU resistance in colon cancer cells [
31]. Notably, the transmission of proteins by exosomes is significant in regulating chemotherapy resistance. For instance, some researchers have shown that TrpC-5-containing extracellular vesicles in breast cancer and P-glycoprotein (P-gp)-containing microvesicles in ovarian cancer are responsible for chemotherapeutic resistance [
32,
33]. GSTP1, which is associated with detoxification and glutathione conjugation, has been reported in adriamycin-resistant breast cancer cells [
34,
35]. However, in the above studies, the functional proteins were selected by subjective conjecture instead of screening objectively. Thus, only some well-known proteins were identified and novel and pivotal components in the exosomes were not explored. In contrast, we performed proteome profiling analysis and subsequent validation studies. Our results demonstrated for the first time that p-STAT3 accumulation in exosomes is relevant to 5-FU resistance. More importantly, the results suggested that more large-scale mass spectrum-based analyses should be performed to screen potential chemoresistant proteins in exosomes.
Previous study has found that IL-6 could induce STAT3 phosphorylation and subsequently regulate transcription [
36]. STAT3 is activated in many cancers and associates with patient’s survival [
37]. In addition, increased p-STAT3 levels have been reported in CRC and correlated with chemoradiotherapy [
38]. Recent studies revealed that the inhibition of STAT3 sensitized colorectal cancer cells to 5-FU treatment through down-regulating cyclinD1 [
39]. Nevertheless, few studies have focused on exosomal p-STAT3 in colorectal cancer and the selectivity of proteins transported by exosomes has not been fully understood. A previous study indicated that cells treated with 5-FU were larger than control cells, and their nuclear staining was pale [
40]. Our results showed that the nuclei of RKO/R cells appeared irregular shape and had pale DAPI staining. In brief, our findings not only illustrated the enrichment of p-STAT3 in exosomes but also implied the possible mechanisms that regulate this enrichment in exosomes.
Stattic, which selectively inhibits the activation, dimerization and nuclear translocation of STAT3, was used to prevent p-STAT3 from translocating to the nucleus [
41]. In our study, p-STAT3 was enriched in Exo/R and participated in exosome-mediated 5-FU resistance. These results suggested that developing inhibitors that selectively impair the function of p-STAT3 would be an effective way to reduce exosome-mediated chemotherapy resistance. Additionally, more evidence suggests that exosomes could be used as promising biomarkers to diagnose cancer [
42]. However, their application in diagnosing chemotherapy resistance is still lacking. Thus, the application of p-STAT3 as a potential biomarker would help to monitor the 5-FU resistance of patients during treatment. More investigations are needed to validate the clinical use of p-STAT3-containing exosomes as a therapeutic target and biomarker in colorectal cancer.
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