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Molecular & Cellular Proteomics 7:1077-1088, 2008.
© 2008 by The American Society for Biochemistry and Molecular Biology, Inc.

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Research

An Unbiased Evaluation of CK2 Inhibitors by Chemoproteomics

Characterization of Inhibitor Effects on CK2 and Identification of Novel Inhibitor Targets^*,S

James S. Duncan^,, Laszlo Gyenis, John Lenehan, Maria Bretner^¶,||, Lee M. Graves^**, Timothy A. Haystead and David W. Litchfield^,

From the Department of Biochemistry, The University of Western Ontario, London, Ontario N6A 5C1, Canada, ^¶ Institute of Biochemistry and Biophysics, Polish Academy of Sciences, 5a Pawiskiego St., 02-106 Warszawa, Poland, ^** Department of Pharmacology, University of North Carolina, Chapel Hill, North Carolina 27599-7365, and Department of Pharmacology, Duke University Medical Center, Durham, North Carolina 27710

Recently protein kinases have emerged as some of the most promising drug targets; and therefore, pharmaceutical strategies have been developed to inhibit kinases in the treatment of a variety of diseases. CK2 is a serine/threonine-protein kinase that has been implicated in a number of cellular processes, including maintenance of cell viability, protection of cells from apoptosis, and tumorigenesis. Elevated CK2 activity has been established in a number of cancers where it was shown to promote tumorigenesis via the regulation of the activity of various oncogenes and tumor suppressor proteins. Consequently the development of CK2 inhibitors has been ongoing in preclinical studies, resulting in the generation of a number of CK2-directed compounds. In the present study, an unbiased evaluation of CK2 inhibitors 4,5,6,7-tetrabromo-1H-benzotriazole (TBB), 4,5,6,7-tetrabromo-1H-benzimidazole (TBBz), and 2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole (DMAT) was carried out to elucidate the mechanism of action as well as inhibitor specificity of these compounds. Utilizing a chemoproteomics approach in conjunction with inhibitor-resistant mutant studies, CK2 and CK2' were identified as bona fide targets of TBB, TBBz, and DMAT in cells. However, inhibitor-specific cellular effects were observed indicating that the structurally related compounds had unique biological properties, suggesting differences in inhibitor specificity. Rescue experiments utilizing inhibitor-resistant CK2 mutants were unable to rescue the apoptosis associated with TBBz and DMAT treatment, suggesting the inhibitors had off-target effects. Exploitation of an unbiased chemoproteomics approach revealed a number of putative off-target inhibitor interactions, including the discovery of a novel TBBz and DMAT (but not TBB) target, the detoxification enzyme quinone reductase 2 (QR2). The results described in the present study provide insight into the molecular mechanism of action of the inhibitors as well as drug specificity that will assist in the development of more specific next generation CK2 inhibitors.

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