Evaluating Cks1-Skp2 Interface As An Effective Therapeutic Target For Rb1-Deficient Metastatic Castration-Resistant Prostate Cancer (Mcrpc).

Abstract Most prostate cancers are locally confined and can be cured by surgery, radiation therapy, and androgen deprivation therapy (ADT). Many prostate cancers initially respond to ADT but become resistant over time and ultimately relapse with a more aggressive, chemotherapy-resistant, and often metastatic disease, termed metastatic castration-resistant prostate cancer (mCRPC), with five-year survival rate <30%. Compared with regional and castration-sensitive prostate cancer, mCRPC harbor higher frequency of inactivating RB1 mutations. Unfortunately, no targeted therapy is available for RB1-deficient tumors, as it is not possible to restore this tumor suppressor in tumor cells. Therefore, we aim to identify downstream signaling targets of pRb which could serve as drug targets and thereby restore the critical functions of pRb. Our group previously identified the E3 ubiquitin ligase SCF-Skp2 as a direct repression target of pRb. Skp2 knockout completely blocks prostate tumorigenesis and cancer-related deaths in an Rb1 and Trp53 double deficient mouse model. As a substrate of Skp2, the CDK inhibitor p27 is upregulated upon Skp2 deletion, which slows cell cycle progression and thus cell proliferation. Our research findings indicate that Skp2 is a promising target for RB1 deficient mCRPC treatment. Skp2 post-translationally regulates a large number of proteins, while Cks1 serves as an essential adaptor protein only for Skp2-mediated p21/p27/p57 ubiquitination, and its Asn45 plays a critical role in binding with Skp2. A Cks1 N45R mutant has been reported to abolish the binding of Cks1 with Skp2. In order to specifically and effectively target Skp2's function in p27 degradation, we generated Cks1 N45R knockin mice and evaluated its anti-tumorigenesis effect. Here we show Cks1 N45R knockin completely inhibits tumorigenesis in the Rb1 and Trp53 double deficient prostate cancer mouse model, with p27 being elevated in the prostate to the comparable amount of that in Skp2 knockout. Surprisingly, Cks1 N45R leads to reduced Skp2 protein stability and consequently the total protein level of Skp2 decreases by more than 50%. We found that the CDK1/2 inhibitor Dinaciclib downregulates Skp2 protein levels by promoting Skp2's interaction and ubiquitination with the E3 ligase Cdh1. Our preliminary data suggests that increased p27 facilitates Cdh1-mediated Skp2 degradation by suppressing Cdk2/CyclinA-mediated Skp2 and Cdh1 phosphorylation. This study highlights the potential of developing small molecule inhibitors targeting the Cks1-Skp2 interface for RB1-mutant mCPRC, it also shows the regulation of Skp2 protein levels may be a viable strategy to phenocopy the antitumor effect of Skp2 knockout. Furthermore, we expect that the combination of Cdk1/2 inhibitors and Cks1-Skp2 interface inhibitors would increase the efficacy and selectivity of targeted therapy in mCRPC. Citation Format: Yingjiao Xue, Liang Zhu, Hongling Zhao, Edward L. Schwartz. Evaluating Cks1-Skp2 interface as an effective therapeutic target for RB1-deficient metastatic castration-resistant prostate cancer (mCRPC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2928.