Radiation resistance is a significant challenge in the treatment of breast cancer in humans. Human breast cancer is commonly treated with surgery and adjuvant chemotherapy/radiotherapy, but recurrence and metastasis upon the development of therapy resistance results in treatment failure. Exosomes are extracellular vesicles secreted by most cell types and contain biologically active cargo that, when transferred to recipient cells, can influence the cells’ genome and proteome. We propose that exosomes secreted by radioresistant (RR) cells may be able to disseminate the RR phenotype throughout the tumour. Here, we isolated exosomes from the human breast cancer cell line, MDA-MB-231, and the canine mammary carcinoma cell line, REM134, and their RR counterparts to investigate the effects of exosomes derived from RR cells on non-RR recipient cells. Canine mammary cancer cells lines have previously been shown to be excellent translational models of human breast cancer. This is consistent with our current data showing that exosomes derived from RR cells can increase cell viability and colony formation in naïve recipient cells and increase chemotherapy and radiotherapy resistance, in both species. These results are consistent in cancer stem cell and non-cancer stem cell populations. Significantly, exosomes derived from RR cells increased the tumoursphere-forming ability of recipient cells compared to exosomes derived from non-RR cells. Our results show that exosomes are potential mediators of radiation resistance that could be therapeutically targeted.

Adult T-cell leukemia/lymphoma (ATLL) is a type of cancer with aggressive and rapid progression and has been linked to infection with the human T-cell lymphotropic virus type 1 (HTLV-1). Standard treatment consists of chemotherapy, radiation therapy, and stem-cell transplant, but the development of chemoresistance is common. Despite major advances in understanding the pathogenesis of the disease, patients who do not respond or achieve only transient responses remain a challenge for clinicians. Exosomes are small extracellular vesicles (30-150 nm in diameter) involved in intercellular communication and biologic material transference (including mRNA, miRNA, lipids, and metabolites) between the cells. Exosomes are an emerging key to understanding cancer cell communication and signaling pathway activation during the chemoresistance process. Our hypothesis is that exosomes secreted by chemoresistant (CR) ATLL cells can drive phenotypic changes in nonresistant ATLL cells. For this purpose, we induced chemoresistance using the CHOP protocol (dexamethasone, doxorubicin, and vincristine cocktail) in Jurkat cells (human acute T-cell leukemia/lymphoma). We determined IC50 values as 1 nM of dexamethasone, 0.05 μM of doxorubicin, and 1 nM of vincristine. Acquisition of chemoresistance was demonstrated by increased cell viability after treatment using IC50 of drugs cocktail. We found that Jurkat-CR have slower doubling time compared to respective naïve cells, 26 hours and 21 hours to CR-Jurkat and naïve, respectively. Exosomes derived from the putative chemoresistant were isolated and used to treat their respective naïve cells. There was an increase in cell proliferation after the exosome's treatment comparing to control for Jurkat (P<0.0035). However, there was no difference in proliferation comparing the treatment using exosomes derived from naïve cells and those derived from CR cells (P=0.91). The cell proliferation was higher after 78 hours of exosomes' treatment (P<0.001). Those results are preliminary data. Despite the differences in proliferation related to exosomes treatment, further experiments are required to prove our initial hypothesis. Citation Format: Taismara Garnica, Jéssika Lesbon, Arina Rochetti, Chantell Payton, Rhona Muirhead, Lisa Pang, David Argyle, Juliano da Silveira, Heidge Fukumasu. Investigating the role of exosomes derived from chemotherapy-resistant leukemia cells as mediators of cellular plasticity [abstract]. In: Proceedings of the AACR Virtual Meeting: Advances in Malignant Lymphoma; 2020 Aug 17-19. Philadelphia (PA): AACR; Blood Cancer Discov 2020;1(3_Suppl):Abstract nr PO-21.