Abstract 4: Intercepting ribosomal protein S6KB1 signaling: Prevention of prostate cancer recurrence
Authors: Kumar Addanki Pratap , Clark Alison , Villarreal Michelle , Jayamohan Sridharan , Huang Shih-Bo , Hussain Suleman S. , Yang Xiaoyu , Rivas Paul , Patel Darpan , Pierce Bethany L. , Tripathy Shreya , Osmulski Pawel , Gaczynska Maria , Zhao Lai , Wang Li-Ju , Chen Yidong , Ezhilan Caroline Xavier Paul , Natarajan Mohan , Michalek Joel E. , Reddick Robert L. , Ghosh Rita 
Published: June 2022

Abstract
There is an urgent need for innovative strategies such as the discovery of adjuvants that can prevent relapse and improve quality of life for patients treated with radiotherapy. Previously we demonstrated the utility of Nexrutine (Nex) as a neo-adjuvant with radiation. Nex was safe and well tolerated in PCa patients and potentiated radiation response in part through downregulation of ribosomal protein S6K (encoded byRPS6KB1). We now show that RPS6KB1 depleted prostate cancer cells with higher basal levels of γ-H2AX, a marker for DNA double strand breaks (i) are more sensitive to radiation and (ii) form smaller tumors with reduced levels of prostate specific antigen (PSA). Depletion of RPS6KB1 hindered DNA double-strand break repair predominantly through the alternate end-joining pathway, induction of G2/M checkpoint and NFκB pathway activation. Collectively these events led to improved radiation sensitivity. We further identified Berberine (Ber), one of the active constituents of Nex as a potential pharmacological inhibitor of RPS6KB1. In an orthotopic implantation model of C4-2B, treatment with Ber alone or Ber plus radiation decreased PSA levels that was sustained during the course of the experiment. On the other hand animals treated with radiation alone developed recurrent cancer as evidenced by a resurgence of PSA. Animals administered Ber followed by XRT intervention had increased levels of RANTES while there was no change in animals that received XRT followed by Ber. The observed reversal of the Bereffect with the sequence of intervention is statistically significant (p=0.0298). Among animals not subject to XRT, the mean PSA increased in those that did not receive Ber relative to those that did; mean difference=-1.93, 95% CI -3.75 to -0.105, p=0.04 with no significant changes in body weight. Notably,RPS6KB1 mRNA levels increased in tumor samples in patients experiencing biochemical recurrence(BCR). Given that rising PSA following conventional therapeutic approaches such as radiation remain a major clinical challenge, targeting RPS6KB1 signaling with radiation therapy is an attractive strategy to prevent BCR. Supported in part by CPRIT RP190012 (APK).
Citation Format: Addanki Pratap Kumar, Alison Clark, Michelle Villarreal, Sridharan Jayamohan, Shih-Bo Huang, Suleman S. Hussain, Xiaoyu Yang, Paul Rivas, Darpan Patel, Bethany L. Pierce, Shreya Tripathy, Pawel Osmulski, Maria Gaczynska, Lai Zhao, Li-Ju Wang, Yidong Chen, Caroline Xavier Paul Ezhilan, Mohan Natarajan, Joel E. Michalek, Robert L. Reddick, Rita Ghosh. Intercepting ribosomal protein S6KB1 signaling: Prevention of prostate cancer recurrence [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 4.