Abstract
Etoposide (VP-16) is extensively used to treat cancer, yet its efficacy is calamitously associated with an increased risk of secondary acute myelogenous leukemia. The mechanisms for the extremely high susceptibility of myeloid stem cells to the leukemogenic effects of etoposide have not been elucidated. We propose a mechanism to account for the etoposide-induced secondary acute myelogenous leukemia and nutritional strategies to prevent this complication of etoposide therapy. We hypothesize that etoposide phenoxyl radicals (etoposide-O⋅) formed from etoposide by myeloperoxidase are responsible for its genotoxic effects in bone marrow progenitor cells, which contain constitutively high myeloperoxidase activity. Here, we used purified human myeloperoxidase, as well as human leukemia HL60 cells with high myeloperoxidase activity and provide evidence of the following. 1) Etoposide undergoes one-electron oxidation to etoposide-O⋅ catalyzed by both purified myeloperoxidase and myeloperoxidase activity in HL60 cells; formation of etoposide-O⋅radicals is completely blocked by myeloperoxidase inhibitors, cyanide and azide. 2) Intracellular reductants, GSH and protein sulfhydryls (but not phospholipids), are involved in myeloperoxidase-catalyzed etoposide redox-cycling that oxidizes endogenous thiols; pretreatment of HL60 cells with a maleimide thiol reagent, ThioGlo1, prevents redox-cycling of etoposide-O⋅radicals and permits their direct electron paramagnetic resonance detection in cell homogenates. VP-16 redox-cycling by purified myeloperoxidase (in the presence of GSH) or by myeloperoxidase activity in HL60 cells is accompanied by generation of thiyl radicals, GS⋅, determined by HPLC assay of 5,5-dimethyl-1-pyrroline glytathionyl N-oxide glytathionyl nitrone adducts. 3) Ascorbate directly reduces etoposide-O⋅, thus competitively inhibiting etoposide-O⋅-induced thiol oxidation. Ascorbate also diminishes etoposide-induced topo II-DNA complex formation in myeloperoxidase-rich HL60 cells (but not in HL60 cells with myeloperoxidase activity depleted by pretreatment with succinyl acetone). 4) A vitamin E homolog, 2,2,5,7,8-pentamethyl-6-hydroxychromane, a hindered phenolic compound whose phenoxyl radicals do not oxidize endogenous thiols, effectively competes with etoposide as a substrate for myeloperoxidase, thus preventing etoposide-O⋅-induced redox-cycling. We conclude that nutritional antioxidant strategies can be targeted at minimizing etoposide conversion to etoposide-O⋅, thus minimizing the genotoxic effects of the radicals in bone marrow myelogenous progenitor cells, i.e., chemoprevention of etoposide-induced acute myelogenous leukemia.
Footnotes
- Received November 24, 1998.
- Accepted June 16, 1999.
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Send reprint requests to: Dr. Valerian E. Kagan, Department of Environmental and Occupational Health, University of Pittsburgh, 260 Kappa Dr., RIDC Park, Pittsburgh, PA 15238. E-mail:Kagan{at}vms.cis.pitt.edu
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↵1 On leave from the Institute of Evolutionary Physiology and Biochemistry, Russian Academy of Science, St. Petersburg, Russia.
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Supported by grants from the American Institute for Cancer Research 97-B128, American Cancer Society DHP-125, National Institutes of Health CA 74972, National Institutes of Health ES 09387, the Johns Hopkins Center for Alternatives to Animal Testing 9829 R3, the National Cancer Institute Oncology Research Faculty Development Program (to V.A.T.), and Leukemia Research Foundation fellowship (to Y.Y.T.).
- The American Society for Pharmacology and Experimental Therapeutics
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