A PLK1 Kinase Inhibitor Enhances the Chemosensitivity of Cisplatin by Inducing Pyroptosis in Esophageal Squamous Cell Carcinoma

Objective/Background: Esophageal squamous cell carcinoma (ESCC) urgently requires more effective chemotherapies because of its relatively low overall survival rate. Targeting PLK1 has recently been proven as a viable therapeutic strategy against ESCC. Therefore, this study aimed to explore whether the PLK1 inhibitor BI2536 is able to sensitize ESCC cells to cisplatin (DDP) and determine the underlying mechanisms.   Methods: Viability, clonogenicity, cell cycle distribution and apoptosis were assessed in KYSE150 and KYSE510 cell lines treated with BI2536 or DDP alone or in combination. DNA damage was assessed by phospho-histone H2AX (γH2AX) and RAD51 foci, and checkpoint activation was examined by Western blotting and immunohistochemistry. A mouse xenograft model was used to assess the efficacy of the co-treatment. GSDME expression and prognostic significance were assessed by a staining tissue microarray that included more than 100 samples of esophageal and esophageal cancer tissues.   Results: We found that the combination of BI2536 and DDP was synergistic in ESCC cells. BI2536 enhanced the anti-proliferative and pro-apoptotic effects of DDP and blocked the cell cycle at the G2/M phase. The combination treatment induced pyroptosis in ESCC cells at low doses. Moreover, mechanistic studies revealed that BI2536 significantly induced DNA damage and impaired the DNA damage repair pathway in DDP-treated cells both in vitro and in vivo. Interestingly, we found that co-treatment with BI2536 and DDP caused pyroptosis in a subset of cells via the BAX/caspase-3/GSDME pathway but not via the caspase-1/GSDMD pathway. Importantly, our study found that GSDME was more highly expressed in tumor tissue than that seen in normal esophageal mucosa tissue, and it may serve as a prognostic factor.   Conclusions: In conclusion, BI2536 is able to sensitize ESCC cells to DDP by inhibiting the DNA damage repair pathway and inducing pyroptosis, suggesting new information for understanding pyroptosis. Our study also reveals that the PLK1 inhibitor BI2536 might be an attractive candidate for ESCC targeted therapy, especially when combined with DDP for treating the GSDME overexpression subtype.   Funding Statement: This work is supported by the National 973 Program (2015CB553904), National Natural Fund of China (81490753, 81830086 and 81802780), China Postdoctoral Science Foundation (2017M620010), National Postdoctoral Program for Innovative Talent, Science Foundation of Peking University Cancer Hospital (2017-27). Declaration of Interests: No potential conflicts of interest were disclosed. Ethics Approval Statement: Tissue microarrays (TMAs) of ESCC specimens were obtained from Shanghai Outdo Biotech Co., Ltd. (SOBC), with the approval of the Institutional Review Board. Detailed clinicopathological characteristics for all specimens are summarized in Supplementary Table S1. Written informed consent was obtained from all patients prior to the study. The use of the clinical specimens for research purposes was approved by the Institutional Research Ethics Committee.