Bcl-xL is a dominant antiapoptotic protein that inhibits homoharringtonine-induced apoptosis in 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). To investigate its mechanism of action, cell growth inhibition and cytotoxicity of HHT were investigated in three AML cell lines, HL-60, NB4, and U937, and in three CML cell lines, K562, KU812, and KCL22. AML cells were more sensitive than CML cells to HHT-induced cytotoxicity. Using HL-60 cells, it was revealed that HHT decreased the levels of myeloid cell leukemia 1 (Mcl-1), X-linked inhibitor of apoptosis protein (XIAP), survivin, and B-cell lymphoma 2 (Bcl-2)-homology domain 3 (BH3)-only proteins as well as the mitochondrial membrane potential. The levels of Bcl-2, Bcl-2-associated X protein (Bax), and Bcl-2 homologous antagonist/killer (Bak) proteins in HL-60 cells were not changed after HHT treatment. U937, K562, KU812, and KCL22 cells expressed B-cell lymphoma-extra large (Bcl-xL) and were less responsive to HHT-induced apoptosis than HL-60 cells. Silencing Mcl-1 or Bcl-xL, but not XIAP or survivin, enhanced HHT-induced apoptosis in U937 cells. The levels of HHT-induced apoptosis in K562, KCL22, and KU812 cells were inversely correlated with the levels of Bcl-xL but not those of Bcl-2 or Mcl-1. K562 cells expressing high levels of Bcl-xL but no Bcl-2 were less responsive to HHT-induced apoptosis than KCL22 cells that expressed lower levels of Bcl-xL and higher levels of Bcl-2 protein. In K562 cells, knockdown of Bcl-xL, but not of Mcl-1, enhanced HHT-induced apoptosis. Transfection of Bcl-xL into KCL22 cells attenuated HHT-induced apoptosis. These data suggest that Bcl-xL plays a more important role than Bcl-2 and Mcl-1 in protecting against HHT-induced apoptosis.