Homoharringtonine contributes to imatinib sensitivity by blocking the EphB4/RhoA pathway in chronic myeloid leukemia cell lines.

The purpose was to investigate the role of EphB4 in imatinib (IM) resistance and the mechanism responsible for homoharringtonine (HHT) contributing to imatinib sensitivity for a chronic myeloid leukemia (CML) cell lines. We established cell lines from a patient with CML at the time of first diagnosis and relapsed phase and designated them as NPhA1 and NPhA2, respectively. Stable underexpressing EphB4 cells (NPhA2-sh) were obtained. The activated signal proteins in cells were tested by Western blot. The EphB4 was overexpressed in IM-resistant NPhA2 in comparison with the NPhA1 cell line, but the expression of EphB4 mRNA and protein significantly decreased in knockdown NPhA2-EphB4-sh cells compared with NPhA2 and NPhA1 (P < 0.001) cell lines. NPhA2-EphB4-sh cells were sensitive to IM (IC50 0.93 mg/L), and NPhA2 showed IM resistance (IC50 5.45 mg/L) (P < 0.001). Meanwhile, phospho-Rac1/cdc42 was significantly increased in NPhA2 cells compared to NPhA2-EphB4-sh (P < 0.001). The apoptosis rate reached 58.71 ± 2.39 % with NPhA2 cells incubated with HHT + IM, which was higher than NPhA2 cells incubated with IM alone (P = 0.002). IC50 of NPhA2 cells incubated with IM was 5.45 mg/L. However, co-stimulation with HHT + IM decreased the IC50 of NPhA2 cells from 5.45 to 1.17 mg/L (P < 0.001). Furthermore, HHT blocked the expressions of EphB4/RhoA, but did not down-regulate the phospho-MEK/ERK in NPhA2 cells. The overexpression of EphB4 contributed to IM resistance in CML line cells. EphB4/RhoA may be a new marker of IM resistance. HHT + IM gained more treatment advantages than IM alone by blocking EphB4/RhoA pathways in CML cell lines.