title: Inducible nitric oxide synthase mediates MG132 lethality in leukemic cells through mitochondrial depolarization.
abstract: Proteasomes are highly expressed in rapidly growing neoplastic cells and essential for controlling the cell cycle process and mitochondrial homeostasis. Pharmacological inhibition of the proteasome shows a significant anticancer effect on hematopoietic malignancies that is usually associated with the generation of reactive oxygen species. In this study, we comprehensively investigated the role of endogenous oxidants in various cellular events of K562 leukemic cells in response to treatment with MG132, a proteasome inhibitor. MG132 at 1.4 µM potently triggered G2/M arrest, mitochondrial depolarization, and apoptosis. By such treatment, the protein level of inducible nitric oxide synthase (iNOS) was doubled and cellular oxidants, including nitric oxide, superoxide, and their derivatives, were increasingly produced. In MG132-treated cells, the increase in iNOS-derived oxidants was responsible for mitochondrial depolarization and caspase-dependent apoptosis, but was insignificant in G2/M arrest. The amount of iNOS was negatively correlated with that of manganese superoxide dismutase (MnSOD). Whereas iNOS activity was inhibited by aminoguanidine, cellular MnSOD levels as well as mitochondrial membrane potentials were upregulated, and consequentially G2/M arrest and apoptosis were thoroughly reversed. It is suggested that cells rich in functional mitochondria possess improved proteasome activity, which antagonizes the cytotoxic and cytostatic effects of MG132. In contrast to iNOS, endothelial NOS-driven cGMP-dependent signaling promoted mitochondrial function and survival of MG132-stressed cells. In conclusion, the functional interplay of proteasomes and mitochondria is crucial for leukemic cell growth, wherein iNOS plays a key role.
authors.affiliation: Institute of Medical Sciences, College of Medicine, Tzu-Chi University, Hualien 97004, Taiwan.
authors.forename: Tung Hui
authors.initials: TH
authors.lastname: Chao
authors.affiliation: Institute of Medical Sciences, College of Medicine, Tzu-Chi University, Hualien 97004, Taiwan; Department of Medical Research, Buddhist Tzu-Chi General Hospital, Hualien, Taiwan.
authors.forename: Meng-Ya
authors.initials: MY
authors.lastname: Chang
authors.affiliation: Department of Medical Laboratory Science and Biotechnology, School of Medicine and Health Sciences, FooYin University, Kaohsiung, Taiwan.
authors.forename: Shu-Jem
authors.initials: SJ
authors.lastname: Su
authors.affiliation: Institute of Medical Sciences, College of Medicine, Tzu-Chi University, Hualien 97004, Taiwan; Department of Molecular Biology and Human Genetics, College of Life Sciences, Tzu-Chi University, Hualien 97004, Taiwan; Department of Laboratory Medicine and Biotechnology, College of Medicine, Tzu-Chi University, Hualien 97004, Taiwan. Electronic address: shsuapril@mail.tcu.edu.tw.
authors.forename: Shu-Hui
authors.initials: SH
authors.lastname: Su
mesh_terms: D017209:Apoptosis / Q000187:drug effects*; D020169:Caspases / Q000378:metabolism; D006152:Cyclic GMP / Q000378:metabolism; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology*; D059565:G2 Phase Cell Cycle Checkpoints / Q000187:drug effects; D006801:Humans; D020014:K562 Cells; D007938:Leukemia / Q000201:enzymology* / Q000473:pathology; D007976:Leupeptins / Q000494:pharmacology*; D053078:Membrane Potential, Mitochondrial / Q000187:drug effects; D008928:Mitochondria / Q000187:drug effects* / Q000502:physiology; D009569:Nitric Oxide / Q000378:metabolism; D052247:Nitric Oxide Synthase Type II / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects; D013482:Superoxide Dismutase / Q000378:metabolism
chemical_list: D015853:Cysteine Proteinase Inhibitors; D007976:Leupeptins; D009569:Nitric Oxide; D052247:Nitric Oxide Synthase Type II; D013482:Superoxide Dismutase; D020169:Caspases; D046988:Proteasome Endopeptidase Complex; D006152:Cyclic GMP; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde
keywords: Apoptosis; Free radicals; Mitochondrial depolarization; Nitric oxide synthase; Proteasome inhibitor; Reactive oxygen nitrogen species