PLK1 overexpression suppresses homologous recombination and confers cellular sensitivity to PARP inhibition

The overexpression of Polo-like kinase 1 (PLK1) is associated with poor clinical outcomes in various malignancies, making it an attractive target for anticancer therapies. Although recent studies suggest PLK1's involvement in homologous recombination (HR), the impact of its overexpression on HR remains unclear. In this study, we investigated the effect of PLK1 overexpression on HR using bioinformatics and experimental approaches. Analyzing The Cancer Genome Atlas (TCGA) and Cancer Cell Line Encyclopedia (CCLE) datasets with the Homologous Recombination Deficiency (HRD) score, we found a positive correlation between PLK1 expression and HRD score, indicating that increased PLK1 expression suppresses HR. To validate these findings, we performed cell line-based experiments, demonstrating that PLK1 overexpression attenuates RAD51 focus formation and HR, as measured by ASHRA in T47D cells. Since HR-deficient cells are hypersensitive to PARP inhibitors, we further confirmed that PLK1 overexpression increases sensitivity to PARP inhibitors, both in CCLE dataset analysis and experiments using T47D cells. Additionally, we found that the effects of PLK1 overexpression on HR suppression and increased PARP inhibitor sensitivity were mitigated by either a PLK1 kinase inhibitor or the kinase-dead mutant [T210A]. This suggests that PLK1's impact on HR and PARP inhibitor sensitivity is mediated through its kinase activity. Moreover, analysis of clinical ovarian cancer samples revealed that higher PLK1 expression correlates with increased sensitivity to PARP inhibitors. Our results suggest that PLK1 overexpression suppresses homologous recombination, leading to enhanced sensitivity to PARP inhibition, presenting a potential therapeutic strategy for targeting cancers with overexpression of PLK1.