Abstract 845: Silibinin radiosensitizes prostate cancer cells by enhancing radiation-induced cell death and inhibiting nuclear EGFR-mediated DNA repair

Abstract

Abstract Nearly 60% of all cancer patients undergo radiotherapy at some juncture of their treatment schedule. However, the problem of therapeutic resistance and side effects to normal dividing cells wanes the positive facets of radiotherapy. Radiosensitizing agents have met with limited success in clinical settings. Hence, development of novel radiosensitizers with a selective response is greatly desired. Herein, we analyzed the radiosensitizing effects of silibinin, a natural plant flavonoid on prostate cancer (PCa) cells in vitro. Clonogenic assay and soft agar colony formation assay revealed that silibinin (25-100µM) could significantly enhance radiation (2.5-10 Gy) induced inhibition in colony formation selectively in PCa cells. G2/M phase is the most sensitive phase for radiation-induced damage. Silibinin enhanced and prolonged radiation-induced G2/M arrest in both DU145 and PC-3 cells. Even though low doses of silibinin (25µM) alone did not induce significant cell death, it could substantially enhance radiation-induced apoptosis. This could also be due to increased and prolonged ROS production observed in combination treatment. Foremost among the pro-survival pathways activated by radiation which contributes to therapeutic resistance is the EGFR pathway. Radiation-induced activation of EGFR subdues the response, both by induction of pro-survival pathway and by activating DNA repair after its nuclear translocation. We observed that silibinin could reduce radiation-induced EGFR signaling and consequently reverse the resistance mediating mechanisms within the cell. Silibinin down regulated levels of anti-apoptotic BCl-2 and survivin, enhanced by radiation. Inhibiting DNA repair would essentially enhance therapeutic index of radiation. Hence, we examined whether silibinin can down-regulate the repair pathways activated in response to radiation. Silibinin reversed the radiation-induced activation of CHK1 and CHK2 at both mRNA and protein levels. We further analyzed its effect on radiation-induced nuclear translocation of EGFR. Our study revealed that silibinin inhibited the nuclear translocation of EGFR and subsequently also affected the nuclear localization of DNA-PK, which is known to be one of the most important mediators of DSB repair in human cells. This was further evident by the increase in the number of pH2AX (ser139) foci suggesting lesser DNA repair in these cells. Thus, we found that silibinin not only enhanced the cell death by reversing the pro-survival signaling activated by radiation; but also suppressed the repair of DSBs by inhibiting nuclear-EGFR mediated DNA-PK activation. Overall, since silibinin is already in phase II clinical trial for prostate cancer patients, the present findings indicate that a combinatorial approach involving silibinin with radiation could prove to be more effective against radioresistant PCa. Citation Format: Dhanya K. Nambiar, Paulraj Rajamani, Rana P. Singh. Silibinin radiosensitizes prostate cancer cells by enhancing radiation-induced cell death and inhibiting nuclear EGFR-mediated DNA repair. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 845. doi:10.1158/1538-7445.AM2014-845