Loss of mitochondrial membrane potential and caspase activation enhance apoptosis in irradiated K562 cells treated with herbimycin A.
OBJECTIVE
We previously reported that herbimycin A (HMA) alters the mode of cell death of K562 cells induced by radiation and enhanced their radiosensitivity. In the present study, we explored the apoptosis-inducing activity of HMA and the fundamental mechanism via which it regulates radiation-induced cell death.


METHODS
Chronic myelogenous leukemia (CML) cell line K562 was used. For X-irradiation and drug treatment, cells were plated at approximately 2x10(5) cells/ml. Exponentially growing cells were treated with 10 Gy of X-ray using a 6-MeV X-ray machine at a dose rate of 200-300 cGy/min. The cells were treated with 0.25 microM HMA immediately after irradiation and HMA remained for the entire culture period. The modes of cell death were discriminated by morphological changes, analysis of cell cycle, analysis of the mitochondrial events, and the expression of apoptosis-related proteins.


RESULTS
Our data demonstrates that radiation induced a significant time-dependent increase of cell death and failed to sustain a prolonged G2 arrest in K562 cells. Radiation-induced cell death caused the accumulation of cyclinB1 and weak nuclear fragmentation, suggesting a mitotic catastrophe. This mitotic catastrophe was dependent upon the mitochondrial permeability transition pore (PTP) opening and was independent of caspase-3. In contrast, K562 cells treated with radiation and HMA had an accelerated cell death and induced a p53-independent apoptosis. This apoptotic pathway was dependent upon an initial hyperpolarization of the mitochondrial inner membrane, following the release of cytochrome c and subsequent caspase-3 activation.


CONCLUSIONS
Two mechanisms of radiation-induced cell death in K562 cells, mitotic catastrophe and apoptosis, are regulated through distinct pathways, mitochondria and caspase-independent and -dependent, respectively. The findings of this study may provide new insights into improving the efficiency of radiotherapy in CML patients.
Jo
Wol-Soon
WS
Research Supporting Center for Medical Science (BK21 Program), College of Medicine, Dong-A University, Busan, Korea.
Jeong
Min-Ho
MH
Jin
Young-Hee
YH
Jang
Ji-Yeon
JY
Nam
Byung-Hyouk
BH
Son
Seok-Hyun
SH
Choi
Sun-Seob
SS
Yoo
Young-Hyun
YH
Kang
Chi-Dug
CD
Lee
Jae-Dong
JD
Jeong
Soo-Jin
SJ
D017209:Apoptosis / Q000528:radiation effects*; D016227:Benzoquinones; D020169:Caspases / Q000378:metabolism*; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D016195:G2 Phase; D006801:Humans; D047029:Lactams, Macrocyclic; D015464:Leukemia, Myelogenous, Chronic, BCR-ABL Positive / Q000473:pathology*; D008564:Membrane Potentials; D051336:Mitochondrial Membranes / Q000502:physiology*; D008938:Mitosis / Q000502:physiology / Q000528:radiation effects; D011809:Quinones / Q000494:pharmacology*; D017828:Rifabutin / Q000031:analogs & derivatives; D014407:Tumor Cells, Cultured; D014965:X-Rays
D016227:Benzoquinones; D004791:Enzyme Inhibitors; D047029:Lactams, Macrocyclic; D011809:Quinones; D017828:Rifabutin; C020754:herbimycin; D020169:Caspases