BCR‐ABL‐independent and RAS / MAPK pathway‐dependent form of imatinib resistance in Ph‐positive acute lymphoblastic leukemia cell line with activation of EphB4

Objective: We investigated the mechanism responsible for imatinib (IM) resistance in Philadelphia chromosome‐positive acute lymphoblastic leukemia (Ph⁺ ALL) cell lines. Methods: We established cell lines from a patient with Ph⁺ ALL at the time of first diagnosis and relapsed phase and designated as NPhA1 and NPhA2, respectively. We also derived IM‐resistant cells, NPhA2/STIR, from NPhA2 under gradually increasing IM concentrations. Results: NPhA1 was sensitive to IM (IC₅₀ 0.05 μm ) and NPhA2 showed mild IM resistance (IC₅₀ 0.3 μm ). NPhA2/STIR could be maintained in the presence of 10 μm IM. Phosphorylation of MEK and ERK was slightly elevated in NPhA2 and significantly elevated in NPhA2/STIR compared to NPhA1 cells. After treatment with IM, phosphorylation of MEK and ERK was not suppressed but rather increased in NPhA2 and NPhA2/STIR. Active RAS was also increased markedly in NPhA2/STIR after IM treatment. The expression of BCL‐2 was increased in NPhA2 compared to NPhA1, but no further increase in NPhA2/STIR. Proliferation of NPhA2/STIR was significantly inhibited by a combination of MEK inhibitor and IM. Analysis of tyrosine phosphorylation status with a protein tyrosine kinase array showed increased phosphorylation of EphB4 in NPhA2/STIR after IM treatment. Although transcription of EphB4 was suppressed in NPhA1 and NPhA2 after IM treatment, it was not suppressed and its ligand, ephrinB2, was increased in NPhA2/STIR. Suppression of EphB4 transcripts by introducing short hairpin RNA into NPhA2/STIR partially restored their sensitivity to IM. Conclusions: These results suggest a new mechanism of IM resistance mediated by the activation of RAS/MAPK pathway and EphB4.