Radioresistance Mechanisms in Prostate Cancer Cell Lines Surviving Ultra-Hypo-Fractionated EBRT: Implications and Possible Clinical Applications

Simple Summary Radiotherapy is an option for curing localized and locally advanced prostate cancer. However, radioresistance can occur, determining treatment failure and poor prognosis. Herein, we developed a model of radio-resistant prostate cancer cells by irradiating the bone metastasis-derived PC3 highly metastatic prostate cancer cell line and the brain-derived moderately metastatic DU-145 prostate cancer cell line, both castration-resistant. Ultra-hypo-fractionated radiotherapy was used, with doses and intervals similar to the ones used in clinical practice. These in vitro models were tested to gain information on the molecular mechanisms used by prostate cancer cells to survive radiation-induced death. Results from bioassays and molecular assays show that in the highly metastatic cells (PC3), the acquired radioresistance-though enhancing clonogenic efficiency, enrichment of cancer stem cells, proliferation rate and migration ability-interestingly results in significantly higher sensitivity to Docetaxel. This behaviour was not observed using the moderately metastatic DU-145 prostate cancer cells. It can be hypothesised that subgroups of patients with highly metastatic prostate cancer could benefit from chemotherapy immediately after the failure of radiotherapy, before a re-challenge with hormonal treatment or other strategies. The use of a higher dose per fraction to overcome the high radioresistance of prostate cancer cells has been unsuccessfully proposed. Herein, we present PC3 and DU-145, castration-resistant prostate cancer cell lines that survived a clinically used ultra-higher dose per fraction, namely, radioresistant PC3 and DU-145 cells (PC3RR and DU-145RR). Compared to PC3, PC3RR showed a higher level of aggressive behaviour, with enhanced clonogenic potential, DNA damage repair, migration ability and cancer stem cell features. Furthermore, compared to PC3, PC3RR more efficiently survived further radiation by increasing proliferation and down-regulating pro-apoptotic proteins. No significant changes of the above parameters were described in DU-145RR, suggesting that different prostate cancer cell lines that survive ultra-higher dose per fraction do not display the same grade of aggressive phenotype. Furthermore, both PC3RR and DU-145RR increased antioxidant enzymes and mesenchymal markers. Our data suggest that different molecular mechanisms could be potential targets for future treatments plans based on sequential strategies and synergistic effects of different modalities, possibly in a patient-tailored fashion. Moreover, PC3RR cells displayed an increase in specific markers involved in bone remodeling, indicating that radiotherapy selects a PC3 population capable of migrating to secondary metastatic sites. Finally, PC3RR cells showed a better sensitivity to Docetaxel as compared to native PC3 cells. This suggests that a subset of patients with castration-resistant metastatic disease could benefit from upfront Docetaxel treatment after the failure of radiotherapy.