Apoptosis Induction And The Mechanisms Of Action Of Homoharringtonine In Human Leukemia Cells

Homoharringtonine (HHT) has been reported to be effective in a portion of patients with acute myeloid leukemia (AML) or chronic myeloid leukemia (CML) patients and is being evaluated for its efficacies in combination with other agents for AML treatment. To investigate reasons of different responses to HHT treatment, the cell growth inhibition and cytotoxicity of HHT were investigated in three AML cell lines, HL-60, NB4 and U937, and three CML cell lines, K562, KU812 and KCL22 as well as primary cells isolated from AML and CML patients. Although there was no evident difference in the cell growth inhibition among these cells, the AML cells were more sensitive than CML cells to HHT-induced cytoxicity. HHT at 20-40 nM induced apoptosis in AML cell lines but the apoptosis of HHT in CML cells required higher concentrations. Using HL-60 cells it was revealed that HHT decreased in the levels of c-FLIP, Mcl-1, survivin and XIAP protein and decreased the mitochondria membrane potential without production of ROS. Antioxidants N-acetylcysteine and catalase did not block HHT-induced apoptosis and that a H2O2-resistant subclone HL-60 were as sensitive as HL-60 to HHT-induced apoptosis. Jurkat cells with defect of FADD or caspse-8 were responsive to HHT apoptosis induction as parental cells, indicating that the extrinsic apoptotic pathway would not play an important role in HHT-induced apoptosis. U937, K562, KU812, and KCL22 cells expressed Bcl-XL and less responsive to HHT-induced apoptosis than HL-60cells. Silencing Mcl-1 or Bcl-XL, but not XIAP or surviving enhanced HHT-induced apoptosis in U937 cells. Unlike AML cells, CML cell lines express higher levels of Bcl-XL and were less responsive to HHT apoptosis induction. The apoptosis induction ability of HHT among K562, KCL22 and KU812 was inversely correlated with the levels of Bcl-XL independent of Mcl-1 protein inhibition. K562 cells expressing the higher level of Bcl-XL without Bcl-2 expression were less responsive to HHT apoptosis induction than KCL22 cells which express lower level of Bcl-XL even with high levels of Bcl-2 expression. Knock-down of Bcl-XL in K562 cells enhanced HHT-induced apoptosis and overexpression of Bcl-XL in KCL22 cells attenuated HHT-induced apoptosis. Our data suggest inhibition of Mcl-1 is not sufficient for HHT to induce apoptosis in CML cells and that Bcl-XL plays a more important role than Bcl-2 in protecting against HHT-induced apoptosis.