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ORIGINAL ARTICLE

Biochemical and Proteomics Approaches to Characterize Topoisomerase II Cysteines and DNA as Targets Responsible for Cisplatin-Induced Inhibition of Topoisomerase II

Faculty of Pharmacy, University of Manitoba, Winnipeg, Manitoba, Canada (B.B.H., X.W.); Department of Physics and Astronomy, University of Manitoba and Manitoba Centre for Proteomics, Winnipeg, Manitoba, Canada (O.V.K., W.E., K.G.S.); Department of Molecular Pharmacology, St. Jude Children's Research Hospital, Memphis, Tennessee (J.L.N.); and Department of Pharmacology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania (T.S., A.G., S.Y., Y.J., J.C.Y.)

Abstract

Cisplatin was shown to strongly inhibit the decatenation and relaxation activity of isolated human DNA topoisomerase II. This inhibition was not accompanied by stabilization of a covalent topoisomerase II-DNA intermediate. Pretreatment of kinetoplast plasmid DNA (kDNA) or pBR322 DNA with submicromolar concentrations of cisplatin quickly rendered these substrates incompetent in the topoisomerase II catalytic assay. Cisplatin nearly equally inhibited growth of a parental K562 and an etoposide-resistant K/VP.5 cell line that contained decreased topoisomerase II levels, a result consistent with isolated enzyme experiments demonstrating that cisplatin was not a topoisomerase II poison. Because cisplatin is known to react with protein sulfhydryl groups, the 13 cysteine groups in the topoisomerase II monomer were evaluated by mass spectrometry to determine which cysteines were free and disulfide-bonded to identify possible sites of cisplatin adduction. High-pressure liquid chromatography-matrix-assisted laser desorption ionization mass spectrometry showed that topoisomerase II contained at least five free cysteines (170, 216, 300, 392, and 405) and two disulfide-bonded cysteine pairs (427–455 and 997-1008). Cysteine 733 was also disulfide-bonded, but its partner cysteine could not be identified. Cisplatin antagonized the formation of a fluorescence adduct between topoisomerase II and the sulfhydryl-reactive maleimide reagent 10-(2,5-dihydro-2,5-dioxo-1H-pyrrol-1-yl)-9-methoxy-3-oxo-3H-naphtho[2,1-b]pyran-2-carboxylic acid methyl ester (ThioGlo-1). Dithiothreitol, which was shown by spectrophotometry to react rapidly with cisplatin (6-min half-time), diminished the capacity of cisplatin to interfere with ThioGlo-1 binding to topoisomerase II. The results of this study suggest that cisplatin may exert some of its cell growth inhibitory and antitumor activity by inhibition of topoisomerase II through reaction with critical enzyme sulfhydryl groups and/or by forming DNA adducts that render the DNA substrate refractory to topoisomerase II.

Address correspondence to: Dr. Brian Hasinoff, Faculty of Pharmacy, University of Manitoba, Winnipeg, MB R3T 2N2, Canada. E-mail: b_hasinoff{at}umanitoba.ca

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