Targeting EZH2-mediated methylation of histone 3 inhibits proliferation of pediatric acute monocytic leukemia cells in vitro

Abstract

Background: Enhancer of zeste homolog 2 (EZH2) is a histone methyltransferase and a catalytic subunit of the polycomb repressive complex 2 (PRC2) that catalyzes the mono-, di-, and tri-methylation of histone H3 at Lys 27 (H3K27me3) to facilitate chromatin remodeling and gene silencing functions. The overexpression of EZH2 is associated with high levels of H3K27me3 in various types of cancers. Previous reports showed a significant association of EZH2 aberrations in pediatric cancers such as soft tissue sarcomas and glioblastoma. Recent reports in human subjects and animal models have also suggested a central role of EZH2 in the induction and progression of acute myeloid leukemia. Methods: In this study, we aimed to investigate the molecular status of EZH in cell lines derived from distinct pediatric leukemia to assess the efficacy of targeting EZH2 to suppress cancer cell survival and proliferation. Cell cytotoxicity and the half maximal inhibitory concentration (IC50) were measured by Alamar blue cytotoxicity assay. The molecular status of EZH2 was profiled by DNA sequencing and western blot analysis of EZH2 protein levels in THP-1, SEM, and MV4:11. In addition, Methylation of histone H3 was evaluated by immunoblotting and immunostaining analyses. Results: Our results showed that EZH2 protein is overexpressed in the pediatric monocytic cell line THP-1 , but not in other leukemia derived cell lines MV4;11 and SEM. Screening a panel of methyltransferase inhibitors revealed that three inhibitors; GSK126, UNC1999 and EPZ-5687 are the most potent inhibitors that suppressed EZH2 activity selectively on lysine 27 which resulted in increased apoptosis and inhibition of AKT and ERK protein phosphorylation in THP-1 cells. Our data demonstrated a significant increase in apoptosis in cells treated with drug combination (EZH2i and selinexor) compared to EZH2i inhibitors alone. Conclusions: Taken together, our data provide initial evidence that targeting EZH2 is a promising therapeutic strategy for the treatment of subtypes of pediatric AML. Also, combining EZH2 inhibitors with selinexor may increase the treatment efficacy in these patients. Findings from this study indicate further evaluation and consideration of these compounds for early phase clinical studies in selected pediatric malignancies in the future.