Abstract 4992: Taxol-elevated PLK1 overcomes BETi-resistant in prostate cancer via triggering phosphorylation-dependent degradation of BRD4
Authors: Zhang Yanquan , Fong Ka Will , Mao Fengyi , Wang Ruixin , Kong Yifan , Li Chaohao , Liu Jinghui , Li Zhiguo , Allison Derek B. , Napier Dana , Li Daheng , Liu Jinpeng , Wang Chi , Zhang Yeqing , Li Guangbing , Liu Xiaoqi 
Published: April 2023

Abstract
Background: Bromodomain-containing protein 4 (BRD4) has been widely studied as an attractive therapeutic target of prostate cancer (PCa). Currently, a number of BRD4 inhibitors or degraders have been discovered. However, recent studies revealed that SPOP mutant of prostate cancer patients leads to BRD4 stabilization and resulted in JQ1-resistant. Therefore, to reveal the mechanism underlying BRD4 stability may improve the clinical response rate and efficacy of BRD4-targeted therapy in PCa patients.
Methods: To evaluate the level of BRD4 during the mitosis phase, we applied a Confocal microscope and WB. In vitro kinase assay and Mass spectrometry were applied to test the phosphorylation of BRD4 by PLK1. A homemade phosphorylation-specific antibody was used to verify p-BRD4 in PCa cells and a tissue micro-array. A PDX-tumor model was used to evaluate the combination efficacy of Taxol to overcome JQ1-resistant.
Result: We found that the level of BRD4 obviously decreased during the mitosis phase of PCa cells. Upon releasing from M-phase arrest, BRD4 level increased along with PLK1 decreasing in PCa cell lines, suggesting PLK1 might be involved in the degradation of BRD4 in the M-phase. We found that PBD domain of PLK1 was responsible for interacting with CTD of BRD4. Enforced overexpression of PLK1 promoted BRD4 dramatically reduced. Reversely, tet-induced depletion or inhibition of PLK1 led to BRD4 stabilization. Furthermore, applying gain or loss of function assay, we found that PLK1 directly phosphorylated BRD4 and triggered its phosphorylation-dependent degradation in M-phase, which was confirmed in clinical samples that high level of PLK1 was negatively correlated with BRD4. Accordingly, we found that overexpression of PLK1 lowered the stabilized BRD4 caused by SPOP mutant in PCa cells and consequently made cells sensitive to JQ1. Intriguingly, upon treated with Taxol, a commonly used medicine for PCa patients, PLK1 level was dramatically elevated as well as p-BRD4 status but total BRD4 downregulated in PCa cells. Moreover, sequential treatment of Taxol and JQ1 resulted in significant inhibition of proliferation, colony formation, and PDX tumor carrying SPOP mutant, indicating application of Taxol overcame JQ-1 resistant.
Conclusion: Collectively, our results suggested that PLK1 phosphorylates BRD4 and consequently triggers its phosphorylation-dependent degradation in PCa cells. Sequential treatment of Taxol and JQ1 overcomes SPOP mutant-related BETi-resistant in prostate cancer.
Citation Format: Yanquan Zhang, Ka Will Fong, Fengyi Mao, Ruixin Wang, Yifan Kong, Chaohao Li, Jinghui Liu, Zhiguo Li, Derek B. Allison, Dana Napier, Daheng Li, Jinpeng Liu, Chi Wang, Yeqing Zhang, Guangbing Li, Xiaoqi Liu. Taxol-elevated PLK1 overcomes BETi-resistant in prostate cancer via triggering phosphorylation-dependent degradation of BRD4. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4992.