Imatinib triggers mesenchymal-like conversion of CML cells associated with increased aggressiveness

Chronic myelogenous leukemia (CML) is a cytogenetic disorder resulting from the expression of p210BCR-ABL. Imatinib, an inhibitor of BCR-ABL, has emerged as the leading compound to treat CML patients. Despite encouraging clinical results, resistance to imatinib represents a major drawback for therapy, as a substantial proportion of patients are refractory to this treatment. Recent publications have described the existence of a small cancer cell population with the potential to exhibit the phenotypic switch responsible for chemoresistance. To investigate the existence of such a chemoresistant cellular subpopulation in CML, we used a two-step approach of pulse and continuous selection by imatinib in different CML cell lines that allowed the emergence of a subpopulation of adherent cells (IM-R Adh) displaying an epithelialmesenchymal transition (EMT)-like phenotype. Overexpression of several EMT markers was observed in this CML subpopulation, as well as in CD34 CML primary cells from patients who responded poorly to imatinib treatment. In response to imatinib, this CD44 ^high/CD24^low IM-R Adh subpopulation exhibited increased adhesion, transmigration and invasion in vitro and in vivo through specific overexpression of the αVβ3 receptor. FAK/Akt pathway activation following integrin β3 (ITGβ3) engagement mediated the migration and invasion of IM-R Adh cells, whereas persistent activation of ERK counteracted BCR-ABL inhibition by imatinib, promoting cell adhesion-mediated resistance.