Effect of Subcellular Translocation of Protein Disulfide Isomerase on Tetrachlorobenzoquinone-Induced Signaling Shift from Endoplasmic Reticulum Stress to Apoptosis.

Our previous studies illustrated tetrachlorobenzoquinone (TCBQ)-caused toxicities in neuron-like cells which imply its association with neurodegenerative disorders. Although it is known that TCBQ induces oxidative damage that in turn results in endoplasmic reticulum (ER) stress and apoptosis, it is unclear how TCBQ triggers the signaling switch from pro-survival (to restore cellular homeostasis) to pro-death (trigger apoptosis). Protein disulfide isomerase family proteins (PDIs) regulate the progress of various neurodegenerative disorders, including Parkinson's disease and Alzheimer's disease. We tested the hypothesis that subcellular translocation of PDIs implicates the survival/death signaling switch by inducing mitochondrial outer membrane permeabilization (MOMP). The rat pheochromocytoma PC12 cells were exposed to TCBQ, and the concentration-dependent ER stress was observed upon TCBQ treatment, as indicated by the increase in inositol-requiring kinase/endonuclease 1α (IRE1α) phosphorylation, C/EBP homologous protein (CHOP) expression, X-box-binding protein 1 (XBP1) splicing, and caspase 12 activation. Interestingly, pharmacological (or siRNA) abrogation of PDIA1/PDIA3 aggravated the loss of cell viability induced by the relatively low concentration (10 μM) of TCBQ. However, inhibition of PDIA1/PDIA3 rescued the high concentration (20 μM) of TCBQ-induced cell death. Further mechanistic study illustrated that PDIs initially acted to restore cellular homeostasis to pro-survival but that the constant ER stress promoted the signaling switch to pro-apoptotis by the release of PDIA1/PDIA3 from the ER lumen to induce Bak-dependent MOMP. Our findings suggested that subcellular translocation of PDIs determined the "life or death" fate of PC12 cells to TCBQ-induced oxidative insult.