Leptomycin B Overcomes Imatinib Resistance Mediated by Stromal Cells and Mutant BCR-ABL in Philadelphia Chromosome-Positive Acute Lymphoblastic Leukemia Cells.

Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph-ALL) is one of the most intractable hematological malignancies, and readily acquires resistance to chemotherapeutic drugs including imatinib mesylate. We hypothesized that the adhesive interaction of Ph-ALL cells with bone marrow stromal cells might cause their escape from drug-induced apoptosis and subsequent minimal residual disease, resulting in the generation of a chemoresistant clone such as a clone harboring mutant BCR-ABL. To gain insight into this possibility and a novel strategy against imatinib resistance, we used two Ph-ALL cell lines designated as IMS-PhL1 (L1) and IMS-PhL2 (L2). L1 cells had wild type BCR-ABL, whereas L2 cells had Y253H mutant and revealed 10-fold or more resistance to imatinib, compared with L1 cells. The growth of L1 cells was autonomous and their spontaneous apoptosis was suppressed by co-culture with a murine bone marrow stromal cell line, HESS-5. In contrast, the sustained growth and survival of L2 cells was absolutely dependent on direct contact with HESS-5. Both cell lines adhered to and migrated beneath the HESS-5 cell layer, resulting in the formation of cobblestone areas (CA). While floating L1 cells were eradicated by 1 mM imatinib, a portion of adherent L1 cells could survive even at 10 μM imatinib. Similarly, L2 cells forming CA beneath the HESS-5 cell layer considerably resisted prolonged exposure to 10 μM imatinib. Leptomycin B (LMB), a potent inhibitor of CRM1/exportin-1, can trap BCR-ABL in the nucleus and can aggressively eliminate BCR-ABL⁺ cells in combination with imatinib (Wang et al., 2001). We tested LMB for its ability to eliminate CA or adherent Ph-ALL cells in combination with imatinb. The result for L2 cells was shown in Figure. Dramatically, combined use of 10 μM imatinib and 1 nM LMB for 7 days exerted a synergistic effect on reduction in the number of CA. L1 cells were also susceptible to the combination of imatinib and LMB. Our results suggest that nuclear entrapment of BCR-ABL may be a promising strategy for overcoming imatinib resistance mediated by stromal cells as well as a certain BCR-ABL mutant.

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