A role for Src-family kinases in imatinib resistant chronic myelogenous leukemia

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Chronic myelogenous leukemia (CML) is characterized by reciprocal chromosomal translocation resulting in expression of the BCR-ABL chimeric gene product with tyrosine kinase transforming activity. Transformation is mediated by tyrosine phosphorylation of key signaling molecules that play a role in normal hematopoesis and cytokine signaling. The importance of BCR-ABL tyrosine kinase activity in CML is illustrated by the effectiveness of its targeted inhibition by drugs such as imatinib mesylate which provides effective therapy for CML patients, especially those in early phases of the disease. However, imatinib resistance occurs in CML patients in all phases of the disease through mechanisms that are only partially understood. Point mutations and amplification of the BCR-ABL gene are detected in resistant patients but these mechanisms alone do not account for resistance in all patients. Imatinib resistant CML cell models and analysis of clinical specimens suggested that activation of hematopoetic cell-type specific src-family kinases (SFK) that are not inhibited by imatinib (Lyn, Hck) reduce imatinib-mediated anti-leukemic activity and play a role in resistance. Animal models of CML demonstrated that both BCR-ABL and SFK inhibition are necessary to overcome imatinib resistance in vivo. In imatinib sensitive CML cells imatinib reduced BCR-ABL and specific substrate phosphorylation (CrkL) and co-ordinately reducing the activation of SFKs, suggesting upstream regulation of the SFKs by BCR-ABL in imatinib responsive cells. However in imatinib resistant CML patients that do not lose sensitivity to imatinib through BCR-ABL mutations, imatinib failed to suppress SFK, suggesting SFK regulation by BCR-ABL independent mechanisms. BCR-ABL/SFK complexes were detected in imatinib sensitive but not resistant cells. Autonomous regulation of SFKs in imatinib resistant CML specimens and cell survival could be overcome with a novel src/abl dual kinase inhibitor, BMS-354825, or targeted interference of SFK expression by siRNA. Together, these results suggest that SFKs play a role in imatinib resistance and inhibition of both BCR-ABL and SFKs may circumvent the development of imatinib resistance and increase the clinical activity of CML kinase-targeting therapies.