The Intracellular ABC Transporter A3 Contributes to Imatinib Resistance of CML Cells by Subcellular Sequestration.

Inhibition of bcr-abl tyrosine kinase activity has evolved as a mainstay of therapy for patients with chronic myeloid leukaemia (CML). However, a fraction of leukaemia cells persists under targeted therapy and can lead to disease progression on cessation of treatment. Analysing leukemic progenitor cells with the side population phenotype we found that the bcr-abl positive leukemic clones contributed to this compartment in CML patients, and that the intracellular ABC Transporter A3 is strongly expressed in such leukemic SP cells. Subsequently we found ABCA3 expressed in all of 39 whole bone marrow and peripheral blood samples from 39 CML patients, with transcripts numbers substantially exceeding those of patients with AML or normal volunteers. Similarly, the CML cell lines K562, Lama-84 and BV17.3 showed high endogenous ABCA3 expression levels, allowing specific downregulation by siRNA. As measured in viability and colony forming assays, the functional knock down of ABCA3 induced a significantly increased susceptibility to imatinib. Furthermore, we applied subcellular fractionation techniques to investigate the subcelluar distribution of imatinib, and found the majority of [14C]-labelled imatinib to accumulate in the lyosomal fraction, in K562 and Lama84 leukemic cells as well as the HEK293 model system. In conclusion, the intracellular ABC transporter A3 is expressed consistently and at high levels expressed in CML cells, and may contribute to imatinib resistance via lysosomal sequestration.