S-Glutathionylation of cysteine residues within target proteins is a posttranslational modification that alters structure and function. We have shown that S-glutathionylation of protein disulfide isomerase (PDI) disrupts protein folding and leads to the activation of the unfolded protein response (UPR). PDI is a molecular chaperone for estrogen receptor alpha . Our present data show in breast cancer cells that S-glutathionylation of PDI interferes with its chaperone activity and abolishes its capacity to form a complex with . Such drug treatment also reverses estradiol-induced upregulation of c-Myc, cyclinD1, and , gene products involved in cell proliferation. Expression of an S-glutathionylation refractory PDI mutant diminishes the toxic effects of PABA/NO. Thus, redox regulation of PDI causes its S-glutathionylation, thereby mediating cell death through activation of the UPR and abrogation of stability and signaling. 1. Introduction Glutathione S-transferase pi (GSTP) is a biomarker protein in drug-resistant solid epithelial tumors, including ovarian, breast, liver, pancreatic, lung, and lymphoma . In some cases, GSTP can be the most abundant protein in the tumor and, consequently, has the potential to serve as an important drug target [2–4]. One therapeutic approach has been to develop prodrugs that are substrates for GSTP and become cytotoxic when liberated in cancer cells, yet exhibit diminished activation/toxicity in normal tissue. PABA/NO (O2-[2,4-dinitro-5-[4-(N-methylamino) benzoyloxy]phenyl] 1-(N,N-dimethylamino) diazen-1-ium-1,2-diolate)  is a GSTP-activated prodrug that releases high levels of nitric oxide (NO) at physiological pH. This reaction results in the formation of a Meisenheimer-complex intermediate and subsequently the leaving group of the reaction generates two molecules of NO. Elevated NO levels lead to cytotoxic effects by forming RNS/ROS intermediates that can alter protein function directly through posttranslational modifications on redox sensitive cysteine residues (S-nitrosylation, P-SNO or S-glutathionylation, P-SSG) [3, 6]. Prior studies have shown that protein disulfide isomerase (PDI) is a molecular target of PABA/NO treatment in cancer cells [2, 5, 7–9]. PDI is the most abundant chaperone/isomerase in the endoplasmic reticulum and plays a pivotal role in protein folding through isomerase and chaperone activity. The active site cysteine residues are S-glutathionylated (PDI-SSG) following PABA/NO treatment. The functional consequences are reduced isomerase activity, accumulation of unfolded/misfolded proteins, and
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