Abstract:

Background and purpose: Polo-like kinase 4 (PLK4) is a cell cycle regulatory protein, which is closely related to the occurrence and development of a variety of malignant tumors. The aim of this study was to investigate the role of PLK4 in the proliferation, invasion and apoptosis of oral squamous cell carcinoma (OSCC). Methods: Gene Expression Profiling Interactive Analysis 2 (GEPIA2) and the University of Alabama at Birmingham Cancer Data Analysis Portal (UALCAN) were used to analyze the expression of PLK4 in head and neck squamous cell carcinoma and surrounding normal tissues. Real-time fluorescence quantitative polymerase chain reaction (RTFQ-PCR) and Western blot were employed to evaluate the mRNA and protein expression of PLK4 in OSCC cells. We further analyzed PLK4, phosphoinositide 3-kinase (PI3K), phosphorylated phosphoinositide 3-kinase (p-PI3K), protein kinase B (AKT), phosphorylated protein kinase B (p-AKT), survivin, and cyclin D1 protein expression. The effects of PLK4 on cell proliferation, apoptosis and invasion were analyzed by cell counting kit-8 (CCK-8) assay, flow cytometry and transwell assay. In addition, 12 4-week-old SPF BALB/c female nude mice were divided into sh-NC group (n=6) and sh-PLK4 group (n=6) by random number table method. The sh-NC/sh-PLK4 cells were injected into the right anterior armpit of nude mice for subcutaneous tumor formation. The body weight, tumor volume and tumor weight of the two groups of nude mice were observed, and the stripped tumor tissues were analyzed by H-E staining. The animal experiments were approved by the Animal Experiment Center of Shandong Second Medical University (No: 2024SDL840). Results: The results of GEPIA2 and UALCAN online databases showed that PLK4 was highly expressed in OSCC compared with normal tissues. In addition, PLK4 was highly expressed in OSCC cell lines (HN6, Cal-27, SCC-4, SCC-9, SCC-25) compared with oral mucosal epithelial cells (P<0.05). Western blot results showed that the expression levels of PI3K/AKT signaling pathway proteins p-PI3K, p-AKT, cyclin D1 and survivin were decreased after PLK4 knockdown and increased after PLK4 overexpression in OSCC cells (P<0.05). The cell proliferation activity and the number of transmembrane cells were positively correlated with the decrease and increase of PLK4 expression (P<0.05), while the cell apoptotic rate was negatively correlated (P<0.05), indicating that cell proliferation and apoptosis were both affected. In addition, compared with the sh-NC group, the tumor volume and weight of the nude mice in the sh-PLK4 group were decreased (P<0.05), while there was no significant difference in the body weight of the nude mice between the two groups (P>0.05). Moreover, the nuclear atypia of tumor tissues in sh-PLK4 group was lower than that in sh-NC group (P<0.05). Conclusion: PLK4 can regulate the invasion, proliferation and apoptosis of OSCC cells, potentially through the activation of PI3K/AKT signaling pathway.

Key words: PLK4, Oral squamous cell carcinoma, PI3K/AKT signaling pathway, Proliferation, Apoptosis, Invasion