Coupling of endoplasmic reticulum stress to CDDO-Me-induced up-regulation of death receptor 5 via a CHOP-dependent mechanism involving JNK activation

The synthetic triterpenoid methyl-2-cyano-3,12-dioxoolean-1,9-dien-28-oate (CDDO-Me) is in phase I clinical trials as a novel cancer therapeutic agent. We previously showed that CDDO-Me induces c-jun N(2)-terminal kinase (JNK)dependent death receptor 5 (DR5) expression and augments death receptor-induced apoptosis. The current study focused on addressing how CDDO-Me induces JNK-dependent DR5 expression. Analysis of DR5 promoter regions defines that the CCAAT/enhancer binding protein homologous protein (CHOP) binding site is responsible for CDDO-Me-induced transactivation of the DR5 gene. Consistently, CDDO-Me induced DR5 expression and parallel CHOP up-regulation. Blockade of CHOP up-regulation also abrogated CDDO-Me-induced DR5 expression. These results indicate that CDDO-Me induces CHOP-dependent DR5 up-regulation. Moreover, the JNK inhibitor SP600125 abrogated CHOP induction by CDDO-Me, suggesting a JNK-dependent CHOP up-regulation by CDDO-Me as well. Importantly, knockdown of CHOP attenuated CDDO-Me-induced apoptosis, showing that CHOP induction is involved in CDDO-Me-induced apoptosis. Additionally, CDDO-Me increased the levels of Bip, phosphorylated eukaryotic translation initiation factor 2(1, inositol requiring kinase 1 alpha, and activating transcription factor 4, all of which are featured changes during endoplasmic reticulum (ER) stress. Furthermore, salubrinal, an inhibitor of ER stress-induced apoptosis, inhibited JNK activation and up-regulation of CHOP and DR5 by CDDO-Me and protected cells from CDDO-Me-induced apoptosis. Thus, ER stress seems to be important for CDDO-Me-induced JNK activation, CHOP and DR5 up-regulation, and apoptosis. Collectively, we conclude that CDDO-Me triggers ER stress, leading to JNK-dependent, CHOP-mediated DR5 up-regulation and apoptosis.