microRNAs (miRNAs) are highly conserved, non-protein-coding RNAs that function to regulate gene expression. In mammals this regulation is primarily carried out by repression of translation. miRNAs play important roles in homeostatic processes such as development, cell proliferation and cell death. Recently the dysregulation of miRNAs has been linked to cancer initiation and progression, indicating that miRNAs may play roles as tumour suppressor genes or oncogenes. The role of miRNAs in apoptosis is not fully understood, however, evidence is mounting that miRNAs are important in this process. The dysregulation of miRNAs involved in apoptosis may provide a mechanism for cancer development and resistance to cancer therapy. This review examines the biosynthesis of miRNA, the mechanisms of miRNA target regulation and the involvement of miRNAs in the initiation and progression of human cancer. It will include miRNAs involved in apoptosis, specifically those miRNAs involved in the regulation of apoptotic pathways and tumour suppressor/oncogene networks. It will also consider emerging evidence supporting a role for miRNAs in modulating sensitivity to anti-cancer therapy.

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Chemoradiation therapy (CRT) prior to surgery is increasingly the standard of care for locally advanced oesophageal cancer. Radiation therapy is important for local tumour control; however, tumour resistance to radiation is a substantial clinical problem. The mechanism(s) of radioresistance are still poorly understood, however, mounting evidence supports a role for microRNA (miRNA) in modulating key cellular pathways mediating response to radiation. Global miRNA profiling of an established isogenic model of radioresistance in oesophageal adenocarcinoma demonstrated a significant downregulation of miR-31 in radioresistant cells, both basally and in response to radiation. Ectopic re-expression of miR-31 significantly re-sensitised radioresistant cells to radiation. miR-31 was demonstrated to alter the expression of 13 genes involved in DNA repair, which is a critical cellular defence against radiation-induced DNA damage. In oesophageal tumours, miR-31 expression was significantly reduced in patients demonstrating poor histomorphologic response to neoadjuvant CRT, whilst expression of the miR-31-regulated DNA repair genes was significantly increased. Our data suggest a possible mechanism for resistance to CRT, potentially via enhanced DNA repair. This study demonstrates, for the first time, a role for miR-31 in modulating radioresistance and highlights the need for further study investigating the potential role of miR-31 as both a predictive marker of response and a novel therapeutic agent with which to enhance the efficacy of radiation therapy.

To study radioresistance in esophageal adenocarcinoma, we generated an isogenic cell line model by exposing OE33 esophageal adenocarcinoma cells to clinically relevant fractionated doses of radiation (cumulative dose 50 Gy). A clonogenic assay confirmed enhanced survival of the radioresistant OE33 subline (OE33 R). To our knowledge, we are the first to generate an isogenic model of radioresistance in esophageal adenocarcinoma. This model system was characterized in terms of growth, cell cycle distribution and checkpoint operation, apoptosis, reactive oxygen species generation and scavenging, and DNA damage. While similar properties were found for both the parental OE33 (OE33 P) cells and radioresistant OE33 R cells, OE33 R cells demonstrated greater repair of radiation-induced DNA damage. Our results suggest that the radioresistance of OE33 R cells is due at least in part to increased DNA repair.

Neoadjuvant chemoradiation therapy (CRT) is increasingly the standard of care for locally advanced oesophageal cancer. A complete pathological response to CRT is associated with a favourable outcome. Radiation therapy is important for local tumour control, however, radioresistance remains a substantial clinical problem. We hypothesise that alterations in mitochondrial function and energy metabolism are involved in the radioresistance of oesophageal adenocarcinoma (OAC). To investigate this, we used an established isogenic cell line model of radioresistant OAC. Radioresistant cells (OE33 R) demonstrated significantly increased levels of random mitochondrial mutations, which were coupled with alterations in mitochondrial function, size, morphology and gene expression, supporting a role for mitochondrial dysfunction in the radioresistance of this model. OE33 R cells also demonstrated altered bioenergetics, demonstrating significantly increased intracellular ATP levels, which was attributed to enhanced mitochondrial respiration. Radioresistant cells also demonstrated metabolic plasticity, efficiently switching between the glycolysis and oxidative phosphorylation energy metabolism pathways, which were accompanied by enhanced clonogenic survival. This data was supported in vivo, in pre-treatment OAC tumour tissue. Tumour ATP5B expression, a marker of oxidative phosphorylation, was significantly increased in patients who subsequently had a poor pathological response to neoadjuvant CRT. This suggests for the first time, a role for specific mitochondrial alterations and metabolic remodelling in the radioresistance of OAC.

AbbreviationsSNP, single nucleotide polymorphism; PDB, protein data-bank; siRNA, small-interfering ribonucleic acid; LARS, gene encoding the cytoplasmic leucyl-tRNA synthetase; MMP, mitochondrial membrane potential; ROS, reactive oxygen species; LeuRS, leucyl-tRNA synthetase enzyme; aaRS, aminoacyl-tRNA synthetase; MELAS, mitochondrial encephalomyopathy lactic acidosis and stroke-like symptoms; LARS2, gene encoding the mitochondrial leucyl-tRNA synthetase; BCAAs, branched chain amino acids.

The risk of recurrence following radiation therapy remains high for a significant number of prostate cancer patients. The development of in vitro isogenic models of radioresistance through exposure to fractionated radiation is an increasingly used approach to investigate the mechanisms of radioresistance in cancer cells and help guide improvements in radiotherapy standards. We treated 22Rv1 prostate cancer cells with fractionated 2 Gy radiation to a cumulative total dose of 60 Gy. This process selected for 22Rv1-cells with increased clonogenic survival following subsequent radiation exposure but increased sensitivity to Docetaxel. This RR-22Rv1 cell line was enriched in S-phase cells, less susceptible to DNA damage, radiation-induced apoptosis and acquired enhanced migration potential, when compared to wild type and aged matched control 22Rv1 cells. The selection of radioresistant cancer cells during fractionated radiation therapy may have implications in the development and administration of future targeted therapy in conjunction with radiation therapy.

Resistance to neoadjuvant chemoradiation therapy (CRT) remains a critical barrier to the effective treatment of esophageal adenocarcinoma (EAC). Cancer stem-like cells (CSCs) are a distinct subpopulation of cells implicated in the resistance of tumors to anti-cancer therapy. However, their role in the resistance of EAC to CRT is largely unknown. In this study, using a novel in vitro isogenic model of radioresistant EAC, we demonstrate that radioresistant EAC cells have enhanced tumorigenicity in vivo, increased expression of CSC-associated markers and enhanced holoclone forming ability. Further investigation identified a subpopulation of cells that are characterised by high aldehyde dehydrogenase (ALDH) activity, enhanced radioresistance and decreased expression of miR-17-5p. In vitro, miR-17-5p was demonstrated to significantly sensitise radioresistant cells to X-ray radiation and promoted the repression of genes with miR-17-5p binding sites, such as C6orf120. In vivo, miR-17-5p was significantly decreased, whilst C6orf120 was significantly increased, in pre-treatment EAC tumour samples from patients who demonstrated a poor response to neoadjuvant CRT. This study sheds novel insights into the role of CSCs in the resistance of EAC to CRT and highlights miR-17-5p as a potential biomarker of CRT sensitivity and novel therapeutic target in treatment resistant EAC.