AP-1-independent sensitization to oxidative stress-induced apoptosis by proteasome inhibitors

Hydrogen peroxide (H2O2) induces apoptosis of mesangial cells via c-Jun N-terminal kinase (JNK)-activator protein-1 (AP-1) and extracellular signal-regulated kinase (ERK)-AP-1 pathways. We recently found that subtoxic doses of proteasome inhibitors, MG132 and lactacystin, dramatically enhanced H2O2-induced apoptosis in mesangial cells. In this report, we examined molecular mechanisms involved in this phenomenon, especially focusing on AP-1 pathways. Reporter assays showed that MG132 induced activation of AP-1. However, pharmacological inhibitors of AP-1, retinoic acid, and curcumin, did not suppress the proapoptotic effect of MG132. Suppression of JNK-AP-1 by transfection with either a dominant-negative mutant of JNK or a dominant-negative mutant of c-Jun did not attenuate the apoptosis enhancement by MG132. Similarly, suppression of ERK-AP-1 by PD98059 or dominant-negative mutants of ERK did not affect the apoptosis-promoting effect of MG132. Interestingly, pretreatment with MG132 did not enhance activation of AP-1 by H2O2. These data suggested a novel, AP-1-independent promotion of apoptosis by proteasome inhibitors. (C) 2004 Elsevier Inc. All rights reserved.