Abstract 5431: Identification of serine/threonine kinase 39 (STK39) as a driver for multidrug resistance in hepatocellular carcinoma
Authors: Leung Carmen Oi Ning , Lee Terence Kin-Wah 
Published: March 2024

Abstract
Hepatocellular carcinoma (HCC) is a common type of cancer globally and is associated with a poor prognosis due to its high recurrence rate and resistance to treatment. Solid evidence showed the existence of cancer stem cells (CSCs) that are crucial for therapeutic resistance. However, there are only a few kinase inhibitors that have been found to specifically target liver T-ICs for the HCC treatment. To understand the regulatory mechanism of liver CSCs, we employed RNA-Seq analysis to compare the gene expression profiles, with a particular focus on protein kinases, between (1) sorted CD133+ liver CSCs and CD133- differentiated cells extracted from two HCC cell lines (Huh7 and PLC8024), as well as cultured hepatospheres and adherent differentiated cells of HCC cell line PLC/PRF/5. Our analysis identified Serine/Threonine kinase 39 (STK39) as a commonly up-regulated protein kinase in both CD133+ liver CSCs and CSC-enriched hepatospheres. Based on our analysis of the TCGA data, a positive correlation has consistently been observed between STK39 mRNA expression and the expression of liver CSC markers, including CD24, CD133, and CD47. Utilizing publicly available transcriptome datasets from the NCBI Gene Expression Omnibus (GEO), including GSE14520 and GSE25097, we found that overexpression of STK39 is detected in HCC tumor tissues (p&lt;0.001) and is significantly associated with poorer overall survival and disease-free survival. Through the use of lentiviral based overexpression and knockdown approaches, we have demonstrated the role of STK39 in regulating liver CSC properties, including self-renewal, tumorigenicity, cell invasiveness, and the expression of liver CSC markers. STK39 expression was found to be significantly elevated in in HCC cells that were resistant to both sorafenib and lenvatinib, and its inhibition resulted in increased sensitivity of HCC cells to these treatments. To identify the downstream target of STK39 for regulation of cancer stemness, tandem affinity purification coupled with mass spectrometry was employed. The analysis led to the identification of PARP1 as a novel protein binding partner of STK39. Upon suppression of STK39 in HCC cells, it was observed that the phosphorylation of PARP1 at Threonine 368 (T368) was significantly inhibited, indicating its role as a downstream effector of STK39-mediated liver CSC functions. Interestingly, STK39-mediated PARP1 phosphorylation led to development of resistance to PARP1 inhibitor, rucaparib. Additionally, STK39 plays a critical role in regulating PARP1-mediated chromatin decondensation, which enables it to open chromatin structure at the promoter regions of SOX2/OCT4. In summary, treatment of molecular targeted drugs including rucaparib and lenvatinib with STK39 inhibitor may be a potential therapeutic strategy for treatment of this deadly disease.
Citation Format: Carmen Oi Ning Leung, Terence Kin-Wah Lee. Identification of serine/threonine kinase 39 (STK39) as a driver for multidrug resistance in hepatocellular carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 5431.