PSMD14 is a subunit of the 19S regulatory particle of the proteasome with deubiquitinating enzyme activity and its high expression was reported to be associated with the poor prognosis of many cancers. In the present study, I showed that the silencing of PSMD14, as well as pharmacological inhibition of PSMD14 with capzimin (CZM), induces paraptosis, a cell death mode accompanied by dilation of the endoplasmic reticulum (ER) and mitochondria in breast cancer cells. I found that CZM markedly induced ER stress and cycloheximide, a protein synthesis inhibitor, almost completely blocked CZM-induced paraptosis. I also found that CZM induced the depletion and ER Ca2+ and subsequent increases in cytosolic and mitochondrial Ca2+ in a proteasome-independent manner. Interestingly, pretreatment with BAPTA-AM, an intracellular Ca2+ chelator, effectively attenuated CZM-induced increase in cytosolic Ca2+ levels and paraptotic cell death. Furthermore, CZM induced disruption of the Golgi apparatus and possibly protein transport system, whereas bortezomib, a proteasome inhibitor, did not. Unexpectedly, inhibition of IRE1α or JNK enhanced CZM-induced increase in cytosolic Ca2+ levels and accelerated vacuolization. Taken together, these results demonstrate that disruption of both proteostasis and Ca2+ homeostasis may contribute to the paraptosis induced by PSMD14 inhibition.
