Inhibition of PLK4 overcomes radioresistance in locally advanced rectal cancer through genomic instability and apoptotic cell death

Background: Polo-like kinase 4 (PLK4), a key regulator of centrosome duplication, has been implicated in cancer progression and therapeutic resistance. Although PLK4 overexpression is frequently observed in various malignancies, its functional contribution to radiotherapy (RT) response in colorectal cancer (CRC) remains poorly understood. Methods: To investigate whether PLK4 inhibition enhances radiosensitivity, CRC cell lines HCT116 (p53 wild-type) and HT29 (p53 mutant) were treated with the selective PLK4 inhibitor CFI-400945, RT or their combination. Cell viability and clonogenic assays were performed to evaluate cytotoxicity. DNA damage responses were assessed by gammaH2AX immunofluorescence and Western blot analysis of p-ATM, p-CHK2, and DNA-PKcs. Cell-cycle profiles and centrosomal abnormalities were examined by flow cytometry and gamma-tubulin staining, respectively. Apoptosis was determined through cleaved PARP-1 and caspase-3 expression levels. Results: CFI-400945 treatment reduced cell viability in a dose- and time-dependent manner and significantly enhanced the cytotoxic effects of RT in both CRC cell lines. Combined treatment markedly increased gammaH2AX foci formation and upregulated DNA damage response proteins, including p-ATM, p-CHK2, and DNA-PKcs. PLK4 inhibition also induced centrosome amplification, micronuclei formation, and multinucleation, indicating mitotic defects leading to genomic instability. Furthermore, the combination therapy enhanced G2/M arrest and apoptosis, as evidenced by elevated levels of cleaved PARP-1 and caspase-3. Conclusions: PLK4 inhibition sensitizes colorectal cancer cells to radiation by promoting DNA damage accumulation, mitotic catastrophe, and apoptotic cell death. These findings suggest that targeting PLK4 may serve as a potential therapeutic strategy to overcome radioresistance in CRC.