Regulation and Targeting of Eg5, a Mitotic Motor Protein in Blast Crisis CML: Overcoming Imatinib Resistance

Patients with blast crisis (BC) CML frequently become resistant to Imatinib, a Bcr-Abltyrosine kinase-targeting agent. Eg5, a microtubule-associated motor protein has beendescribed to be highly expressed in BC CML by microarray analysis (Nowicki et al,Oncogene 22:3952-3963, 2003). We investigated the regulation of Eg5 by Bcr-Abltyrosine kinase and its potential as a therapeutic target in BC CML. Eg5 was highlyexpressed in all Philadelphia chromosome positive (Ph+) cell lines and BC CML patientsamples. Inhibition of Bcr-Abl by Imatinib downregulated Eg5 expression in ImatinibsensitiveKBM5 and HL-60p185 cells, but not in Imatinib-resistant KBM5-STI571,harboring a T315I mutation, and Bcr-Abl-negative HL-60 cells. Blocking Eg5 expressionwith antisense oligonucleotide (Eg5-ASO) or inhibiting its activity with the smallmoleculeEg5 inhibitor, S-trityl-L-cysteine induced G2/M cell cycle block and subsequentcell death in both Imatinib-sensitive and -resistant cells. Further, Eg5-ASO treatment ofSCID mice harboring KBM5 cell xenografts significantly prolonged the median survivalof the animals (p=0.03). Our findings suggest that Eg5 is downstream of and regulated byBcr-Abl tyrosine kinase in Philadelphia chromosome positive cells. Inhibition of Eg5expression or its activity blocks cell cycle progression and induces cell death independentof the cellular response to Imatinib. Therefore, Eg5 could be a potential therapeutic targetfor the treatment of BC CML, in particular Imatinib-resistant BC CML.