Abstract 4651: Polo-like kinase 1 phosphorylates and stabilizes KLF4 to promote tumorigenesis in nasopharyngeal carcinoma
Authors: Mai Jia , Zhong Zhuo-Yan , Chen Xiu-Xing , Xiang Yan-Qun , Li Xuan , Zhang Hai-Liang , Deng Rong , Zhu Xiao-Feng 
Published: August 2019

Abstract
INTRODUCTION Advanced nasopharyngeal carcinoma (NPC) is an aggressive disease with no targeted therapies and poor outcomes. New innovative targets are urgently needed. Krüppel-like factor 4 (klf4, gklf) is a zinc-finger transcription factor involved in a wide variety of cellular processes, including apoptosis, cell cycle progression and stem cell renewal. Emerging data indicate that klf4 dysregulation either facilitates or impedes tumor progression. The function of klf4 in NPC remains elusive.
METHODS we examined the expression of klf4 in human NPC (n=152) with a tissue immunohistochemistry (IHC) assay, and performed klf4 knockdown stable cell line with two different small hairpin RNAs (shRNAs) to evaluate the impact of klf4 on NPC. Global gene expression experiments were performed to explore the molecular mechanisms underlying klf4-dependent tumorigenesis. Small-molecule kinase inhibitor screening was performed to identify potential upstream kinases of klf4. Co-immunoprecipitation assay used to test the interaction between plk1 and klf4. Mass spectrometry analysis was performed to determine the specific site at which klf4 is phosphorylated by plk1. Statistical analyses were conducted using GraphPad Prism, SPSS software and JavaGSEA Desktop Application. A p value of &lt;0.05 was considered to indicate statistical significance.
RESULTS Our investigation showed that high expression of klf4 was correlated with poor prognosis in NPC. Moreover, genome-wide profiling revealed that klf4 directly activated oncogenic programmes, including gene sets associated with KRAS, VEGF, and MYC signalling. We further found that inhibition of polo-like kinase 1 could downregulate the expression of klf4 and that plk1 directly phosphorylated klf4 at Ser234. Notably, phosphorylation of klf4 by plk1 caused the recruitment and binding of the E3 ligase traf6, which resulted in klf4 K32 K63-linked ubiquitination and klf4 stabilization. Moreover, klf4 could enhance traf6 expression at the transcriptional level, thus initiating a klf4-traf6 feed-forward loop. Treatment with the plk1 inhibitor volasertib (BI6727) significantly inhibited tumor growth in nude mice.
CONCLUSION In this study, we revealed that klf4 is upregulated in NPC and is correlated with poor prognoses. Our study demonstrated for the first time that plk1 can directly interact with klf4 and phosphorylate Ser234, alter the affinity of klf4 for the E3 ligase traf6, and promote the K63-linked ubiquitination of klf4 K32. In addition, klf4 can enhance traf6 expression at the transcriptional level and further promote klf4 expression. The resulting increase in klf4 ubiquitination leads to stabilization and upregulation of klf4, which leads to tumorigenesis in NPC. These results expand our understanding of the role of KLF4 in NPC and validate PLK1 inhibitors as potential therapeutic agents for NPC.
Citation Format: Jia Mai, Zhuo-Yan Zhong, Xiu-Xing Chen, Yan-Qun Xiang, Xuan Li, Hai-Liang Zhang, Rong Deng, Xiao-Feng Zhu. Polo-like kinase 1 phosphorylates and stabilizes KLF4 to promote tumorigenesis in nasopharyngeal carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4651.