miR‐9 as Post‐Transcriptional Modulator of DNA Topoisomerase IIα (TOP2α) in Human Leukemia K562 Cells with Acquired Resistance to the Anticancer Drug Etoposide
Authors: Kania Evan E , Hernandez Victor A , English Anthony E , Shkolnikov Nick J , Yalowich Jack C , Elton Terry S 
Published: June 2021

microRNAs (miRNAs) are short, noncoding RNAs that inhibit translation by binding primarily to the 3′‐untranslated region (3′‐UTR) of mRNA. The enzyme TOP2α induces covalent complexes with DNA and produces transient double‐strand DNA breaks crucial for processes such as replication and normal chromosomal dysjunction at mitosis. TOP2α is an important target for clinically effective anticancer agents, such as etoposide (VP‐16) since these drugs stabilize the otherwise short‐lived enzyme‐DNA covalent complexes, thereby inducing cytotoxic DNA damage. However, the efficacy of these agents is limited by chemoresistance. Our lab has characterized acquired resistance to VP‐16 in human leukemia K562 cells. This cloned resistant cell line, K/VP.5, contains reduced levels of TOP2α compared to parental K562 cells. The goal of this project is to test the hypothesis that TOP2α levels are decreased in K/VP.5 cells, in part, through miRNA‐mediated mechanisms.Pooled miRNA qPCR profiling experiments were performed to investigate the expression levels of ~500 miRNAs in K562 and K/VP.5 cells. hsa‐miR‐9‐3p and ‐5p (miR‐9‐3p and ‐5p) were overexpressed in K/VP.5 cells compared to K562 cells. The TOP2α 3′‐UTR harbors putative miRNA recognition elements (MRE) for these miRNAs. Therefore, these miRNAs were chosen for further study. To assess post‐transcriptional regulation of TOP2α by miRNAs, a dual luciferase reporter plasmid harboring the entire 3′‐UTR of TOP2α mRNA (998 bp) was constructed (psiTOP2α/UTR). Transfection with psiTOP2α/UTR demonstrated decreased luciferase expression in K/VP.5 compared with K562 cells (p<0.001), suggesting altered post‐transcriptional regulation in resistant cells. K562 cells that were co‐transfected with psiTOP2α/UTR and miR‐9‐5p or ‐3p mimic resulted in a decrease in luciferase expression only for miR‐9‐5p (p<0.001). Mutating the putative miR‐9‐5p seed sequence prevented the decrease in luciferase activity, demonstrating a direct interaction of this miRNA with the MRE of TOP2α. Immunoblotting for TOP2α in K562 cells transfected with miR‐9‐3p or ‐5p mimic resulted in decreased TOP2α protein compared to mock transfected K562 cells (miR‐9‐3p; p<0.05, miR‐9‐5p; p=0.01). In contrast, immunoblotting for TOP2α in K/VP.5 cells transfected with miR‐9‐3p or ‐5p inhibitor resulted in an increase of TOP2α protein (p<0.05), strongly suggesting a role for both miRNAs in acquired resistance to VP‐16.Our findings indicate that miR‐9‐3p and ‐5p reduce TOP2α expression levels. In addition, results presented here contribute to the elucidation of chemoresistance mechanisms and have the translational potential for circumvention of drug resistance by modulation of miRNA concentrations.Support or Funding InformationPatrick and Jane O'Neill Endowed Scholarship, Honors and Scholars Enrichment Grant, Research Scholars AwardThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.