Bcl-xL Is a Dominant Antiapoptotic Protein that Inhibits Homoharringtonine-Induced Apoptosis in Leukemia Cells

Abstract

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.

Carbon-Ion Irradiation Suppresses Migration and Invasiveness of Human Pancreatic Carcinoma Cells MIAPaCa-2 via Rac1 and RhoA Degradation

“…SiRNA targeting X-linked inhibitor of apoptosis protein (XIAP) and negative control siRNA (Santa Cruz Biotechnology, Inc., Santa Cruz, CA, USA) were used as previously described (12). Cells grown to ~60% confluence in 6-well plates were transfected with 50 pmol siRNA mixed with LipoTrust Ex Oligo reagent (Hokkaido System Science Co., Ltd., Hokkaido, Japan) for 48 h in serum-free medium.…”

Ethacrynic Acid Oxadiazole Analogs Induce Apoptosis in Malignant Hematologic Cells through Downregulation of Mcl-1 and c-FLIP, Which Was Attenuated by GSTP1-1

“…The activation of caspase-9 is controlled by the antiapoptotic proteins Bcl-2, Bcl-xL, and Mcl-1 by maintaining MTP (19). Bcl-xL is not expressed in HL-60 cells (20). The levels of Bcl-2 and Mcl-1 proteins in both HL-60 and HP100-1 cells after treatments with either 6s or 6u were compared and it was found that Mcl-1 levels, but not those of Bcl-2, were decreased in both cell lines (Fig.…”

Homoharringtonine and SAHA synergistically enhance apoptosis in human acute myeloid leukemia cells through upregulation of TRAIL and death receptors

“…Previously, HHT has been revealed to induce apoptosis in various types of leukemic cells, which was characterized by cytochrome c release and activation of caspase (30). In AML cells, HHT leads to a decrease in mitochondrial membrane potential (31), which is regulated by activation of caspase-9 via cleaved Bid as a result of caspase-8 activation. In the present study, enhanced activation of caspase-9 and increased release of cytochrome c were observed, and may have been the result of enhanced activation of the extrinsic apoptosis pathway induced by upregulation of TRAIL and DR4/DR5.…”