Auranofin induces paraptosis by dual inhibition of thioredoxin reductase and proteasome in breast cancer cells

Auranofin (AF), a gold (I)-containing phosphine compound, is being investigated for oncological application as a repurposed drug. We show here that AF induces paraptosis, a non-apoptotic cell death mode characterized by the dilation of the endoplasmic reticulum (ER) and mitochondria, in breast cancer cells. Although the covalent inhibition of thioredoxin reductase (TrxR), an enzyme that critically controls intracellular redox homeostasis, is considered the primary mechanism of AF’s anticancer activity, knockdown of TrxR1 did not induce paraptosis. Instead, TrxR1 knockdown plus the proteasome inhibitor (PI), bortezomib (Bz), or low doses of AF plus Bz induced paraptosis, mimicking the effect of high-dose AF. These results suggest that the paraptosis induced by high-dose AF requires the inhibition of both TrxR1 and proteasome. We found that TrxR1 knockdown/Bz or subtoxic doses of AF and Bz induced paraptosis selectively in breast cancer cells, sparing non-transformed MCF10A cells, whereas high-dose AF killed both cancer and MCF10A cells. GSH depletion was found to be critically involved in the paraptosis induced by dual TrxR1/proteasome inhibition, independent of ROS generation. In this process, the ATF4/CHAC1 (glutathione-specific gamma-glutamylcyclotransferase 1) axis plays a crucial role in GSH degradation, contributing to proteotoxic stress possibly due to accumulation of the misfolded thiol-containing proteins. These results suggest that the paraptosis-inducing strategy of AF plus a PI may provide an effective therapeutic strategy against pro-apoptotic therapy-resistant cancers and reduce the potential side effects by high-dose AF.