Homoharringtonine Contributes to Imatinib Sensitivity by Blocking EphB4/RhoA Pathway in Ph+ Myeloid Leukemia

Abstract Abstract 4420 Objective: The purpose was to investigate the role of the EphB4 in imatinib (IM) resistance and the mechanism why the homoharringtonine (HHT)+IM regimen gained more treatment profits than simple HHT or IM treating myeloid leukemia. Method: The stable under-expressing EphB4 cells (K562-R-EphB4-sh) were obtained. The cell viability and IC50 under the incubation of IM or HHT+IM was tested by MTT. PE Annexin V apoptosis detected the apoptosis rate of K562-R cells. Subcutaneous K562 xenograft models were established. The activated signal proteins in cells and tissues such as RhoA, MEK and ERK were tested by Western blot. Result: K562-R-EphB4-sh cell and xenograft were sensitivity to IM. Activated RhoA was not involved in K562-R-EphB4-sh cell and xenograft tissue. But phosphorylation of MEK/ERK was overexpression in K562-R-EphB4-sh cell and tissue. The apoptosis rate reached 58.71 ± 2.39% with K562-R cell incubated with HHT+IM, which was higher to K562-R cell incubated with IM (P=0.002). IC50 of K562-R cell incubated by IM was 5.45 mg/L. But under the stimulation of HHT+IM, IC50 of K562-R decreased from 5.45 to 1.17 mg/L (P<0.001). K562-R xenograft volumes significantly decreased with IM+HHT treatment comparing with before treatment (1692.82 ± 317.14 mm3 versus 975.56 ± 132.42 mm3, P<0.001). HHT blocked the expressions of EphB4/RhoA in K562-R cell and xenograft, but HHT cannot down-regulate the expression of P- MEK/ERK. Conclusions: A new marker of IM resistance mediated by the activation of EphB4/RhoA pathway. HHT+IM regimen gained more treatment profits than simple HHT or IM treating myeloid leukemia by blocking EphB4/RhoA pathway in Ph+ myeloid leukemia. Disclosures: No relevant conflicts of interest to declare.