Various cancer cells are resistant to chemotherapeutic drugs-induced apoptosis. Thus, cancer cells that have acquired resistance to apoptosis need novel strategies for inducing non-apoptotic cell death. Paraptosis is a kind of non-apoptotic cell death and characterized by extensive vacuolization due to dilation of mitochondria and the endoplasmic reticulum (ER). However, the regulatory mechanisms that control paraptotic events are not yet fully understood. Recently, we found that celastrol, a triterpene extracted from the Chinese “Thunder of God Vine”, induced paraptosis accompanied by dilation of mitochondria and the ER in MDA-MB 435S and MCF-7 breast cancer cells. Inhibition of protein synthesis by cycloheximide blocked celastrol-induced vacuolation and subsequent cell death indicating that protein synthesis is required for this process. Generation of reactive oxygen species and mitochondrial Ca²⁺ overload was shown to act as a critical early signal in celastrol-induced paraptosis-like cell death contributing to the dilation of mitochondria/ER and subsequent paraptotic cell death. Inhibition of mitochondrial Ca2+ uniporter employing ruthenium red and IP₃ receptor employing 2-APB very effectively blocked celastrol-induced paraptosis-like cell death. Taken together, our results suggest that Ca²⁺ overload into the mitochondria via mitochondrial Ca2+ uniporter and IP3 receptor critically contribute to celastrol-induced paraptosis-like cell death.
