The thiosemicarbazone Me(2)NNMe(2) induces paraptosis by disrupting the ER thiol redox homeostasis based on protein disulfide isomerase inhibition

Due to their high biological activity, thiosemicarbazones have been developed for treatment of diverse diseases, including cancer, resulting in multiple clinical trials especially of the lead compound Triapine. During the last years, a novel subclass of anticancer thiosemicarbazones has attracted substantial interest based on their enhanced cytotoxic activity. Increasing evidence suggests that the double-dimethylated Triapine derivative Me(2)NNMe(2) differs from Triapine not only in its efficacy but also in its mode of action. Here we show that Me(2)NNMe(2)- (but not Triapine)-treated cancer cells exhibit all hallmarks of paraptotic cell death including, besides the appearance of endoplasmic reticulum (ER)-derived vesicles, also mitochondrial swelling and caspase-independent cell death via the MAPK signaling pathway. Subsequently, we uncover that the copper complex of Me(2)NNMe(2) (a supposed intracellular metabolite) inhibits the ER-resident protein disulfide isomerase, resulting in a specific form of ER stress based on disruption of the Ca(2+) and ER thiol redox homeostasis. Our findings indicate that compounds like Me(2)NNMe(2) are of interest especially for the treatment of apoptosis-resistant cancer and provide new insights into mechanisms underlying drug-induced paraptosis.