Oleanolic acid-derived synthetic triterpenoids are a promising new class of compounds with antitumorigenic activity against various types of cancer cells. In this study, we show that methyl-2-cyano-3, 12-dioxooleana-1, 9(11)-dien-28-oate (CDDO-ME) triggered paraptosis-like cellular vacuolation, which was mainly derived from the dilation of the endoplasmic reticulum, but finally killed breast cancer cells via apoptosis. We found that the increase in intracellular Ca2+ levels preceded CDDO-ME-induced vacuolation and cell death. In addition, this increase was accompanied by the increase in reactive oxygen species (ROS) levels. Pretreatment with Ca2+ chelator, BAPTA, BAPTA-AM or antioxidant, N-acetylcysteine effectively blocked CDDO-ME-induced vacuolation and cell death, suggesting that Ca2+ influx and ROS generation are initial critical signals for CDDO-ME-induced anti-cancer effect. Moreover, CDDO-ME markedly reduced the protein levels of c-FLIPL. Overexpression of c-FLIPL did not affect CDDO-ME-induced vacuolation, but significantly attenuated CDDO-ME-induced cell death. Interestingly, CDDO-ME increased intracellular Ca2+ and ROS levels at much lower doses, compared to CDDO and CDDO rather increased the protein levels of c-FLIPL via its proteasome inhibitory activity. Taken together, our results clearly show that both intracellular Ca2+ influx, ROS generation and c-FLIP downregulation critically contribute to the potent anti-cancer effect of CDDO-ME in breast cancer cells.
