Abstract 2443: Serine/threonine kinase 39 (STK39) regulates cancer stemness through regulating PARP-mediated SOX2/OCT4 expression in hepatocellular carcinoma
Authors: Leung Carmen Oi Ning , Lee Terence Kin-Wah 
Published: April 2023

Abstract
Hepatocellular carcinoma (HCC) is one of the common malignancies worldwide. Poor prognosis of HCC patients is attributed to high frequency of tumor relapse and therapeutic resistance. Increasing evidence showed the existence of cancer stem cells (CSCs) which is crucial for tumor initiation and therapeutic resistance. However, there are limited kinase inhibitors available that show specific targeting against liver T-ICs for HCC treatment. To understand the mechanism for regulation of liver CSCs, we have employed RNA-Seq profiling to compare the gene expression profiles with focuses on protein kinases between (1) sorted CD133+ liver CSCs and CD133- differentiated counterparts isolated from two HCC cell lines (Huh7 and PLC8024) and cultured hepatospheres and adherent differentiated cells of HCC cell line PLC/PRF/5. This analysis revealed Serine/Threonine kinase 39 (STK39), also known as SPAK, to be the commonly up-regulated protein kinase common in both CD133+ liver CSCs and CSC enriched hepatospheres. Consistently, based on the TCGA data analysis, STK39 mRNA expression is positively correlated with the expression of liver CSC markers including CD24, CD133, and CD47. Using publicly available transcriptome datasets deposited in the NCBI Gene Expression Omnibus (GEO) including GSE14520 and GSE25097, we found that overexpression of STK39 was also observed in HCC tumor tissues (p&lt;0.001) and correlated with poorer overall survival and disease-free survival. By lentiviral based overexpression and knockdown approaches, we demonstrated the role of STK39 in regulation of liver CSC properties, including self-renewal, tumorigenicity, cell invasiveness and expression of liver CSC markers. Specially, STK39 expression was highly elevated in sorafenib- and lenvatinib-resistant HCC cells, and its inhibition led to sensitization of HCC cells towards these treatments. In order to identify downstream target of STK39 for regulation of cancer stemness, tandem affinity purification coupled with mass spectrometry was employed. Upon analysis, PARP1, was identified to be the novel protein binding partner of STK39. Upon suppression of STK39 in HCC cells, phosphorylation of PARP1 at Threonine 368 (T368) was found to be highly inhibited, suggesting its role as downstream effector of STK39-mediated liver CSC functions. Furthermore, STK39 regulates PARP1-mediated chromatin decondensation, and thus opens chromatin structure at the promoter regions of SOX2/OCT4. In summary, STK39 regulates cancer stemness and drug resistance by directly modulating PARP1-mediated SOX2/OCT4 expression via chromatin remodelling. Targeting STK39/PARP1 signaling cascade alone or in combination with molecular targeted drugs including rucaparib and lenvatnib can be the potential therapeutic strategy for treatment of this deadly disease.
Citation Format: Carmen Oi Ning Leung, Terence Kin-Wah Lee. Serine/threonine kinase 39 (STK39) regulates cancer stemness through regulating PARP-mediated SOX2/OCT4 expression in hepatocellular carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2443.