ABSTRACT
Resistance to imatinib (IM) and other tyrosine kinase inhibitors (TKI)s is an increasing problem in leukemias caused by expression of BCR-ABL1. As chronic myeloid leukemia (CML) cell lines expressing BCR-ABL1 utilize an alternative non-homologous end-joining pathway (ALT NHEJ) to repair DNA double-strand breaks (DSB)s, we asked whether this repair pathway is a novel therapeutic target in TKI-resistant disease. Notably, the steady state levels of two ALT NHEJ proteins, poly-(ADP-ribose) polymerase 1 (PARP1) and DNA ligase IIIα, were increased in the BCR-ABL1-positive CML cell line K562 and, to a greater extent, in its imatinib-resistant (IMR) derivative. Incubation of these cell lines with a combination of DNA ligase and PARP inhibitors inhibited ALT NHEJ and selectively decreased survival with the effect being greater in the IMR derivative. Similar results were obtained with TKI-resistant derivatives of two hematopoietic cell lines that had been engineered to stably express BCR-ABL1. Together our results show that the sensitivity of cell lines expressing BCR-ABL1 to the combination of DNA ligase and PARP inhibitors correlates with the steady state levels of PARP1 and DNA ligase IIIα, and ALT NHEJ activity. Importantly, analysis of clinical samples from CML patients confirmed that the expression levels of PARP1 and DNA ligase IIIα correlated with the sensitivity to the DNA repair inhibitor combination. Thus, the expression levels of PARP1 and DNA ligase IIIα serve as biomarkers to identify a subgroup of CML patients who may be candidates for therapies that target the ALT NHEJ pathway when treatment with TKIs has failed.
Subject(s)
Benzamides/pharmacology , DNA Breaks, Double-Stranded/drug effects , DNA End-Joining Repair/drug effects , DNA Ligases/antagonists & inhibitors , Drug Resistance, Neoplasm/drug effects , Leukemia, Myelogenous, Chronic, BCR-ABL Positive/drug therapy , Piperazines/pharmacology , Poly(ADP-ribose) Polymerase Inhibitors , Pyrimidines/pharmacology , Antineoplastic Combined Chemotherapy Protocols , Apoptosis/drug effects , Blotting, Western , Cell Proliferation/drug effects , Comparative Genomic Hybridization , DNA Ligase ATP , DNA Ligases/genetics , DNA Ligases/metabolism , Enzyme Inhibitors/pharmacology , Fluorescent Antibody Technique , Fusion Proteins, bcr-abl/metabolism , Humans , Imatinib Mesylate , Immunoenzyme Techniques , Leukemia, Myelogenous, Chronic, BCR-ABL Positive/genetics , Leukemia, Myelogenous, Chronic, BCR-ABL Positive/pathology , Poly (ADP-Ribose) Polymerase-1 , Poly(ADP-ribose) Polymerases/genetics , Poly(ADP-ribose) Polymerases/metabolism , Poly-ADP-Ribose Binding Proteins , Protein Kinase Inhibitors/pharmacology , RNA, Messenger/genetics , RNA, Small Interfering/genetics , Real-Time Polymerase Chain Reaction , Reverse Transcriptase Polymerase Chain Reaction , Tumor Cells, Cultured , Xenopus ProteinsABSTRACT
MicroRNAs have been implicated as important mediators of cancer cell homeostasis, and accumulating data suggest compelling roles for them in the apoptosis pathway. X-linked inhibitor of apoptosis protein (XIAP) is a potent caspase inhibitor and an important barrier to apoptotic cell death, but the mechanisms that determine the diverse range of XIAP expression seen in cancer remains unclear. In this study, we present evidence that miR-24 directly targets the 3'UTR of the XIAP messenger RNA (mRNA) to exert translational repression. Using a heuristic algorithm of bioinformatics analysis and in vitro screening, we identified miR-24 as a candidate regulator of XIAP expression. Array comparative genomic hybridization and spectral karyotype analysis reveal that genomic copy number loss at the miR-24 locus is concordant with the loss of endogenous miR-24 in cancer cells. Using a luciferase construct of the XIAP 3'UTR, we showed that miR-24 specifically coordinates to the XIAP mRNA. Interference with miR-24's binding of the critical seed region, resulting from site-directed mutagenesis of the 3'UTR, significantly abrogated miR-24's effects on XIAP expression. Moreover, miR-24 overexpression can overcome apoptosis resistance in cancer cells via downregulation of XIAP expression, and the resulting cancer cell death induced by tumor necrosis factor-related apoptosis-inducing ligand is executed by the canonical caspase-mediated apoptosis pathway. In summary, our data suggest a novel mechanism by which miR-24 directly modulates XIAP expression level and consequently the apoptosis threshold in cancer cells.
