Background
Oesophageal cancer is a highly aggressive cancer with a poor 5-year survival rate (<20 %) [
1]. Current treatment regimens fail to eradicate disseminated tumour cells which can re-establish as a drug resistant secondary cancer [
2]. Our research group previously analysed a panel of oesophageal cancer cell lines and their response to treatment with standard chemotherapeutics (5-fluorouracil (5-FU) and cisplatin). Cells that were sensitive to these drugs were both apoptosis and autophagy competent and did not recover following withdrawal of the drug. In contrast, cells that were drug resistant exhibited only autophagy with limited Type II cell death and recovered following removal of the chemotherapeutics. Silencing of key autophagy regulators Beclin 1 and ATG7 verified the crucial role autophagy plays in this recovery process [
3].
Autophagy is an evolutionary conserved catabolic process in which cells self-digest their own organelles, protein aggregates and other macromolecules to maintain cellular homeostasis. However, when a tumour has become established, cancer cells utilise autophagy as a mechanism to protect themselves from adverse stressful conditions including nutrient starvation, hypoxia and anticancer treatments [
4]. Inducing excessive autophagy (Type II cell death) has also been identified as an important cell death pathway [
5]. Autophagy with Type II cell death has been observed following the treatment of several different types of cancer with chemotherapeutics [
6]. It is crucial that we have a clearer understanding of the signalling pathways required to enhance cell death associated autophagy and prevent autophagy that promotes survival of cancer cells.
The importance of microRNAs (miRNAs) in the regulation of diverse cellular processes implicated in tumourigenesis including proliferation, migration, angiogenesis, metastasis, apoptosis and autophagy has only begun to emerge. MiRNAs are small, non-coding RNA molecules of 22–24 nucleotides in length. They act as powerful negative regulators of gene expression (acting at the posttranscriptional and translational levels) and are believed to regulate at least 30 % of human genes. A single miRNA may target several mRNAs at once and consequently can act on complex regulatory networks. Depending on the cellular context and the availability of direct mRNA targets, the same miRNA may have quite diverse functions. MiRNAs have now emerged as potential targets to be exploited for anticancer drug development [
7‐
10]. MiRNAs have been identified to have crucial roles in various stages of autophagy including induction (e.g. miR-101), vesicle nucleation (e.g. miR-30a), elongation and completion (e.g. miR-181a), docking and fusion (e.g. miR-34a) and degradation and recycling (e.g. miR-17/20/93/106 complex) [
11‐
14]. Several miRNAs have also been implicated in the development of drug resistance [
15]. However, as this remains an emerging field, there are likely to be numerous miRNAs, which have yet to be identified, that regulate cell survival/death mechanisms and thereby impact on chemosensitivity.
In this study, we present evidence that miR-193b has an important role in determining the chemosensitivity of oesophageal cancer cells. Overexpression of miR-193b significantly reduces the ability of cancer cells to recover following treatment with 5-FU. MiR-193b may mediate its effects, at least in part, through decreasing the expression of stathmin 1. Stathmin 1 is a ubiquitous cytosolic phosphoprotein that regulates microtubule dynamics. Its expression levels have previously been reported to influence the responsiveness of cancer cells to chemotherapeutics [
16‐
18]. We found that increased expression of miR-193b does not activate apoptosis but elevates autophagy and non-apoptotic cell death.
Methods
Cell culture/reagents
Human oesophageal cancer cell lines OE19, OE21 and OE33 were obtained from the European Collection of Cell Cultures (96071721, 96062201 and 96070808). KYSE450 cells were from DSMZ (Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH, ACC-387). OE21, OE19 and OE33 cells were grown in RPMI 1640 medium (Sigma, R8758). KYSE450 cells were maintained in 50: 50 RPMI: F12 HAMS medium (Sigma, N6658), all supplemented with 10 % foetal calf serum (Sigma, F7524) and 1 % Penicillin/Streptomycin (Gibco-BRL, 15070-063) and grown at 37 °C, 5 % CO2. Chloroquine (C6628) and 5-FU (F6627) were purchased from Sigma.
Microarray and data analysis
MiRNA expression profiling and data analysis was performed as previously described [
19,
20]. The probe set contained 1344 probes capable of detecting 725 human miRNAs in addition to mouse, rat and viral probes. These experiments were carried out in triplicate. The microarray expression data have been deposited to the Gene Expression Omnibus (GEO) data repository (accession number GSE77132).
Real-time PCR
RNA was extracted from all cell lines, cDNA synthesised and PCR performed as per manufacturer’s instructions (Qiagen, 217004, 218161 and 218073) using the Lightcycler system (Roche). MiR-193b (MS00031549) and RNU6B (used for normalisation; MS00033740) primers were designed and synthesised by Qiagen. Results represent at least three independent experiments. Expression levels were quantified using the ‘Delta-Delta’ formula [
21].
Transfections
MiR-193b mimics (C-300764-05-0010), scrambled miRNAs (negative control; CN-001000-01-05), stathmin 1 siRNA (L-005152-00-0005) and scrambled siRNAs (negative control; D-001810-01-20) were designed and synthesised by Dharmacon. Cells were transiently transfected using Lipofectamine 2000 (Invitrogen, 11668-027) according to the protocol provided with the reagent. SiGlo green transfection indicator (Dharmacon, D-001630-01-05) was used to assess transfection efficiency (which was always greater than 70 %).
Western blotting
Protein extracts were lysed in modified RIPA buffer (50 mM Tris–HCl pH 7.4, 150 mM NaCl, 0.25 % sodium deoxycholate, 1 % Igepal, 1 mM EDTA, 1× PefaBloc, 1× Protease Inhibitor Cocktail, 1 mM Na3VO4, 1 mM NaF). Protein samples were separated on pre-cast NuPAGE 4–12 % Bis-Tris gels (Invitrogen, NP0322) and electrophoretically transferred onto PVDF membranes (Invitrogen, IB401001). Primary antibodies were: anti-stathmin 1 (Abcam, ab52630), anti-LC3 (MBL, PD014) and anti-actin (Sigma, A5441). Proteins were visualised using relevant IR-Dye secondary antibodies and quantified using the Odyssey Infrared imaging system (Li-Cor).
Following treatment, all adherent cells were trypsinised and counted. Fifteen hundred viable cells were reseeded in fresh media (without drug) into a well of a six well plate (in triplicate) and allowed to grow for 10–14 days. Colonies were then fixed with 96 % ethanol and stained with ProDiff Solution C (Braidwood Laboratories, 22009). Plates were scanned using the Odyssey Infrared Imaging system (Li-Cor) and colonies quantified. Results are presented as integrated intensity ± standard error of the mean (SEM) from at least three independent experiments. When colony numbers were very low, colonies were manually counted.
Flow cytometry assay
BD-LSRII flow cytometer (BD BioSciences) was used to detect fluorescence. Data analysis and histogram overlays were done using the FlowJo software.
Morphological examination of cell death
Following cytospinning onto glass slides, cells were stained with Pro-Diff (Braidwood Laboratories, 22007, 22008 and 22009). Morphology images were captured using a DP70 digital microscope camera and Olympus DP-Soft823 version 3.2 software. All images are representative of at least three independent experiments. Apoptosis is identified by shrinking of the cell, chromatin condensation and DNA fragmentation into ‘apoptotic bodies’ within an intact plasma membrane. Type II cell death was characterised by loss of cytoplasmic material, an increase in the number of cytoplasmic vesicles, pyknosis (condensed regions) of the nuclear material and an intact nuclear membrane [
22].
HMGB1 ELISA
Following treatment, supernatant was removed from cells and subsequently diluted 1:1 with dilution buffer. Measurement of High Mobility Group Box 1 (HMGB1) concentration was determined using the HMGB1 ELISA kit (Chondrex, 6010) according to manufacturer’s instructions.
Statistical analysis
All statistical analysis was performed using GraphPad Prism version 5. Comparisons between groups were assessed using Student’s t test.
Discussion
In this study, we have shown that expression of miR-193b increases the chemosensitivity of oesophageal cancer cells. Our results suggest that miR-193b may affect chemosensitivity, in part, by decreasing the expression of stathmin 1. Overexpression of miR-193b results in the induction of autophagy. Heightened levels of autophagy may be important for some of the functional activities of miR-193b or it may be an indirect consequence of its activity.
To date, several miRNAs including miR-141, miR-200c, miR-148a, miR-296, miR-27a, miR-483 and miR-214 have been identified to play a role in oesophageal cancer development and or in determining the responsiveness of these cancer cells to chemotherapeutics [
29‐
31]. Our data suggests that miR-193b can now be added to this list of miRNAs whose expression affects the sensitivity of oesophageal cancer cells to chemotherapy.
MiR-193b has been postulated to be a tumour suppressor in several cancers including gastric cancer due to its lower expression in malignant tissue compared to the corresponding normal tissue [
24,
32‐
35]. Putative functions identified to date for miR-193b include the negative regulation of proliferation, cell cycle control, migration and invasion of hepatocellular carcinoma cells and non-small cell lung cancer cells [
34,
36]. MiR-193b has previously been linked to enhancing chemosensitivity. In melanoma cells, overexpression of miR-193b reduced Mcl-1 expression and sensitised these cells to ABT-737 (BH3 mimetic) induced apoptosis [
33]. In hepatitis B virus associated hepatocellular carcinoma cells, enhancing miR-193b expression lowered the IC50 of these cells to sorafenib treatment and apoptosis was triggered in these cells [
37]. Our research is in agreement with these studies showing the importance of miR-193b for chemosensitivity. Recently, a study of two oesophageal cancer cell lines (OE19 and KYSE410) identified several miRNAs associated with chemoresistance. In that study, they found that OE19 cells which were engineered to be resistant to 5-FU had higher expression levels of miR-193b compared to their parental cells [
38]. This is in contradiction to our research which showed that chemosensitive cells had higher expression levels of miR-193b. The mechanism of chemoresistance was not investigated in that analysis. However, our work is the first reported study showing that miR-193b overexpression in combination with 5-FU treatment results in the induction of autophagy and non-apoptotic cell death. MiR-193b’s functional role has never been previously investigated in oesophageal cancer cells and our work indicated that overexpression of this miRNA in combination with 5-FU treatment is effective in sensitising these drug resistant cells.
Several reports have shown that miR-193b directly targets Mcl-1 [
33,
37]. The authors report that suppressing the anti-apoptotic protein Mcl-1 induced apoptosis in these cells. However, in our study, overexpression of miR-193b did not trigger apoptosis in our cells and consequently alternative targets of miR-193b were investigated. We identified that miR-193b may influence chemosensitivity through the negative regulation of stathmin 1. Silencing of stathmin 1 could at least partially re-capitulate the enhanced sensitivity to 5-FU that was observed with the overexpression of miR-193b. MiR-193b has been shown to regulate the expression of stathmin 1 and consequently affected tumour growth and metastasis in pancreatic cells [
24]. Stathmin 1 has also been reported to affect cell proliferation and migration capabilities of melanoma cells [
23]. Knockdown or low levels of stathmin 1 could sensitise breast cancer cells to paclitaxel, vinblastine or docetaxel respectively demonstrating its importance in determining the responsiveness of cancer cells to chemotherapeutics [
16,
18]. High expression levels of stathmin 1 have been associated with lymph node metastasis and increased malignancy in oesophageal adeno and squamous cell carcinoma [
39,
40]. Of note, stathmin 1 has also been putatively identified as an autophagy regulator, potentially through its importance for microtubule dynamics [
12,
41]. As miR-193b potentially influences the expression of greater than 200 genes, it is likely that there are additional targets of this miRNA which may, at least in part, affect chemosensitivity.
There have been no previously reported studies linking miR-193b and autophagy. In our study, we showed that overexpression of miR-193b results in an increase of LC3 II and autophagic vesicles (as determined by Cyto-ID) and also affected autophagy flux. It is unclear from this study whether increased expression of miR-193b directly or indirectly affected autophagy. To date, there have been far in excess of twenty miRNAs implicated directly in the regulation of autophagy and this list is increasing rapidly [
11‐
14]. The impact of de-regulation of these miRNAs on autophagy and on cellular processes including cell death is only beginning to emerge. Further analysis could add miR-193b to this list. For example, miR-23b has been shown to directly target ATG12b. Overexpression of this miRNA sensitised pancreatic cells to radiotherapy by inhibiting radiation induced autophagy. The mechanism which resulted in the reduced survivability of these cells was not delineated [
42]. In melanoma cells, miR-290-295 cluster suppressed autophagic cell death in response to glucose starvation through the down-regulation of ATG7 and ULK1 [
43].
In our study, it is possible that miR-193b’s relationship with stathmin 1 (potential autophagy regulator) may be important for the autophagy induction observed following miR-193b overexpression. However, it remains a possibility that miR-193b increases the autophagy level in these cells through an indirect mechanism. For example, we could postulate that enhanced miR-193b expression could elevate accumulation of macro-molecular structures or organelles (e.g. through effects on microtubules) and consequently induces autophagy (indirect mechanism). The exact relationship between miR-193b and autophagy remains to be fully elucidated.
Acknowledgements
The authors would like to thank the staff of the Cork Cancer Research Centre for review of data and critical feedback. This work was funded by the Irish Cancer Society fellowship programme (CRF12NYH) and Breakthrough Cancer Research, Cork, Ireland. The funding sources had no involvement in study design, in the collection, analysis and interpretation of data, in the writing of the report and in the decision to submit the article for publication.
Competing interests
All authors declare no conflict of interest.
Authors’ contributions
MN was involved in the design of the study, carried out the majority of the experiments, critically reviewed data and drafted the manuscript; TO’D was involved in the design of the study, performed FACS analysis studies, critically reviewed data and helped to draft the manuscript; AB carried out and analysed the miRNA expression profiling studies; EW was involved in the design of the study, analysis of miRNA expression profiling studies, critical review of all data and of the resultant manuscript; SMcK conceived the study, participated in its design, critically reviewed all data and helped to draft the manuscript. All authors have read and approved the final version of the manuscript.