Peptide-bond modified glutathione conjugate analogs modulate GSTπ function in GSH-conjugation, drug sensitivity and JNK signaling

Glutathione S-transferase pi (CST, E.C.2.5.1.18) overexpression contributes to resistance of cancer cells towards cytostatic drugs. Furthermore, GST pi is involved in the cellular stress response through inhibition of Jun N-terminal-kinase (JNK), a process that can be modulated by CST inhibitors. GSH conjugates are potent CST inhibitors, but are sensitive towards gamma-glutamyltranspeptidase (gamma GT)-mediated breakdown. In search for new peptidase stable CST inhibitors we employed the following strategy: (1) selection of a suitable (GST inhibiting) peptide-bond isostere from a series of previously synthesized gamma GT stabilized GSH-analogs. (2) The use of this peptidomimetic strategy to prepare a GSTT, selective inhibitor. Two gamma GT stable GSH conjugate analogs inhibited human GSTs, although non-selectively. One of these, a urethane-type peptide-bond is well accepted by GSTs and we selected this modification for the development of a gamma GT stable, GST pi selective inhibitor, UrPhg-Et-2. This compound displayed selectivity for GST pi compared to alpha and mu class enzymes. Furthermore, the inhibitor reversed GST pi-mediated drug resistance (MDR) in breast tumor cells. In addition, short-term exposure of cells to UrPhg-Et-2 led to GST pi oligornerization and JNK activation, suggesting that it activates the JNK-cJun signaling module through GST pi dissociation. Altogether, we show the successful use of peptidomimetic glutathione conjugate analogs as CST inhibitors and MDR-modifiers. As many MDR related enzymes, such as MRP1, glyoxalase 1 and DNA-pk are also inhibited by GSH conjugates, these peptidomimetic compounds can be used as scaffolds for the development of multi-target MDR drugs. (c) 2005 Elsevier Inc. All rights reserved.