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Research Articles: Therapeutics, Targets, and Development

A cell-active inhibitor of mitogen-activated protein kinase phosphatases restores paclitaxel-induced apoptosis in dexamethasone-protected cancer cells

Departments of ¹ Pharmacology and ² Chemistry, ³ University of Pittsburgh Drug Discovery Institute, and ⁴ Pittsburgh Molecular Libraries Screening Center, University of Pittsburgh, Pittsburgh, Pennsylvania

Requests for reprints: John S. Lazo, Department of Pharmacology, University of Pittsburgh, Biomedical Science Tower 3, 3501 Fifth Avenue, Pittsburgh, PA 15260. Phone: 412-648-9200; Fax: 412-648-9009. E-mail: lazo{at}pitt.edu

Abstract

Mitogen-activated protein kinase phosphatase (MKP)-1 is a dual-specificity phosphatase that negatively regulates the activity of mitogen-activated kinases and that is overexpressed in human tumors. Contemporary studies suggest that induction of MKP-1 during chemotherapy may limit the efficacy of clinically used antineoplastic agents. Thus, MKP-1 is a rational target to enhance anticancer drug activity, but suitable small-molecule inhibitors of MKP-1 are currently unavailable. Here, we have used a high-content, multiparameter fluorescence-based chemical complementation assay for MKP activity in intact mammalian cells to evaluate the cellular MKP-1 and MKP-3 inhibitory activities of four previously described, quinone-based, dual-specificity phosphatase inhibitors, that is, NSC 672121, NSC 95397, DA-3003-1 (NSC 663284), and JUN-1111. All compounds induced formation of reactive oxygen species in mammalian cells, but only one (NSC 95397) inhibited cellular MKP-1 and MKP-3 with an IC₅₀ of 13 µmol/L. Chemical induction of MKP-1 by dexamethasone protected cells from paclitaxel-induced apoptosis but had no effect on NSC 95397. NSC 95397 phenocopied the effects of MKP-1 small inhibitory RNA by reversing the cytoprotective effects of dexamethasone in paclitaxel-treated cells. Isobologram analysis revealed synergism between paclitaxel and NSC 95397 only in the presence of dexamethasone. The data show the power of a well-defined cellular assay for identifying cell-active inhibitors of MKPs and support the hypothesis that small-molecule inhibitors of MKP-1 may be useful as antineoplastic agents under conditions of high MKP-1 expression. [Mol Cancer Ther 2008;7(2):330–40]

Grant support: NIH grants CA52995 and CA78039 and Fiske Drug Discovery Fund.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 10/ 4/07; accepted 11/12/07.