Abstract PO-085: Therapeutic targeting of RPS6KB1/SQSTM1 axis to prevent biochemical recurrence
Authors: Kumar Addanki P. , Clark Alison , Villarreal Michelle R. , Huang Shih-Bo , Hussain Suleman S. , Yang Xiaoyu , Bedolla Roble G. , Rivas Paul , Jayamohan Sridharan , Wang Li-Ju , Chen Yidong , Ezhilan Caroline Xavier Paul , Natarajan Mohan , Michalek Joel E. , Reddick Robert , Miyamoto Hiroshi , Ghosh Rita 
Published: April 2021

Radiation therapy (RT) is a standard treatment for organ confined prostate cancer (PCa). Given that both intrinsic and acquired resistance to radiation causes progression to advanced metastatic PCa, there is an unmet need for adjuvant therapies that synergize with radiation to improve therapeutic efficacy. Previously, we demonstrated strong synergistic tumor growth inhibitory activities for Phellodendron amurense bark extract in combination with radiation in part through downregulation of ribosomal protein S6K (encoded by RPS6KB1). Subsequently we identified berberine (Ber), one of the active constituents of Nexrutine as a potential pharmacological inhibitor of RPS6KB1. Given these findings, we hypothesized that targeting RPS6KB1 could be an approach to enhance sensitivity to radiation. We tested the efficacy of Ber alone and in combination with radiation using an orthotopic implantation model and the mechanism associated with radio-sensitization using prostate cancer cells stably silenced and deleted for RPS6KB1. We observed that (i) RPS6KB1 deletion in androgen responsive early stage LNCaP cells and (ii) stable silencing in castrate resistant C4-2B cells increased sensitization to radiation relative to respective parental or non-targeted cells. Furthermore, pharmacological inhibition of RPS6KB1 with Ber sensitized cells to radiation. Mechanistically, RPS6KB1 silencing increased autophagic activity as revealed by changes in proteins identified through reverse phase protein array (RPPA) and decreased expression and levels of SQSTM1 via downregulation of NFkB. Pretreatment with chloroquine (CQ) rescued the observed radio-sensitization effects. Treatment with Ber alone or Ber plus radiation decreased levels of PSA that was sustained during the course of the experiment in mice orthotopically implanted with C4-2B cells. On the other hand animals treated with radiation alone developed recurrent cancer as evidenced by a resurgence of PSA. Notably, RPS6KB1 mRNA levels increased in tumor samples in patients experiencing biochemical recurrence (BCR). Taken together, these findings demonstrate that RPS6KB1 is a clinically relevant target and that Ber sensitizes prostate cancer cells to radiation in vitro and in vivo. Therefore, targeting RPS6KB1 signaling is an attractive strategy to prevent progression to BCR; given that rising PSA following conventional therapeutic approaches such as radiation remain a major clinical challenge. This work is supported in part by CPRIT RP190012 (APK).

Citation Format: Addanki P. Kumar, Alison Clark, Michelle R. Villarreal, Shih-Bo Huang, Suleman S. Hussain, Xiaoyu Yang, Roble G. Bedolla, Paul Rivas, Sridharan Jayamohan, Li-Ju Wang, Yidong Chen, Caroline Xavier Paul Ezhilan, Mohan Natarajan, Joel E. Michalek, Robert Reddick, Hiroshi Miyamoto, Rita Ghosh. Therapeutic targeting of RPS6KB1/SQSTM1 axis to prevent biochemical recurrence [abstract]. In: Proceedings of the AACR Virtual Special Conference on Radiation Science and Medicine; 2021 Mar 2-3. Philadelphia (PA): AACR; Clin Cancer Res 2021;27(8_Suppl):Abstract nr PO-085.