Investigating the content and function of stress-induced extracellular vesicles in colorectal cancer
Keywords: exosome
microenvironmental stress
colorectal cancer
Rab11a
amphiregulin

Despite advances in detection and treatment, the incidence of colorectal cancer (CRC), particularly in those under 40 years of age is increasing, and survival from metastatic disease remains very low. Adaptation of tumour cells to microenvironmental stresses and anti-cancer therapies is a key mechanism driving tumour progression and resistance. Extracellular vesicles (EVs) are nanosized membrane-bound vesicles, which carry rich bioactive cargoes that are able to influence and reprogramme recipient cell behaviour. In CRC, EVs are known to be potent drivers of primary tumour growth and metastasis, and have also been shown to modulate response to treatment by promoting drug resistance. The Goberdhan Group has recently shown that glutamine depletion can lead to change in EV biogenesis, including an increase in Rab11a-exosomes formed in Rab11a-positive recycling endosomes through inhibition of the nutrient-sensing kinase complex, mechanistic Target of Rapamycin Complex 1 (mTORC1). These EVs have enhanced tumorigenic function in CRC cells. Therefore, my DPhil focussed on investigating the potential pro-tumorigenic properties of these “switched” EVs in CRC cell lines in vitro and also assessing the composition of EVs in CRC patient plasma.
Having validated size exclusion chromatography as a method to isolate and detect purified preparations of switched EVs from HCT116 CRC cells, I showed that hypoxia, which reduces mTORC1 signalling, also induces the secretion of Rab11a-exosomes. I also demonstrated that mTORC1 inhibition induces the EV switch in a panel of other CRC cell lines (Caco-2, SW480 and SW620) that are representative of the genetic heterogeneity observed in CRC. My work provided evidence that switched EVs from HCT116 cells preferentially increase tumour cell growth in vitro and within an in vivo chick-embryo model. This growth-promoting property is ERK-MAPK-dependent and can be blocked by selective neutralisation of the EGFR ligand, Amphiregulin (AREG), which is upregulated in switched EVs isolated from HCT116 and Caco-2 cells. Furthermore, switched EVs isolated from HCT116 cells (KRAS mutant) promote AREG-dependent resistance to EGFR monoclonal antibody therapy in Caco-2 (KRAS wild-type) cells. I also showed that switched EVs acted in an AREG-independent fashion to reduce tumour cell migration and invasion, potentially reflecting the mesenchymal-to-epithelial transition at distant sites during CRC metastasis.
My final results chapter focussed on EV-associated microRNA cargo within HCT116 and SW480 cells, as well as within a cohort of patients with rectal cancer treated with neoadjuvant chemoradiotherapy (nCRT). A comparative analysis of the miRNA cargo of non-switched and switched EVs showed that HCT116 and SW480 cells secrete EVs with distinct miRNA expression patterns, although both are enriched in miRNAs known to participate in pro-tumorigenic processes and signalling pathways. After validating a SEC-based method to isolate EVs from patient plasma, I showed that nCRT for rectal cancer is associated with altered EV-miRNA cargo. Furthermore, individuals who show a clinical response to nCRT appear to exhibit a different EV-miRNA profile when compared to non-responders, which may provide a novel method to predict response to treatment.
In summary, this thesis reveals mechanisms connecting microenvironmental stress to the release of pro-tumorigenic EVs, alongside insights into how treatment for CRC modulates EV cargo. It therefore highlights potential novel EV-associated biomarkers and therapeutic targets for future research.