ERdj5 Sensitizes Neuroblastoma Cells to Endoplasmic Reticulum Stress-induced Apoptosis

Christophoros G. Thomas,Giannis Spyrou

doi:10.1074/jbc.m806189200

Christophoros G. Thomas, Giannis Spyrou

Open Access

https://doi.org/10.1074/jbc.m806189200

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Abstract

Down-regulation of the unfolded protein response (UPR) can be therapeutically valuable in cancer treatment, and endoplasmic reticulum (ER)-resident chaperone proteins may thus be targets for developing novel chemotherapeutic strategies. ERdj5 is a novel ER chaperone that regulates the ER-associated degradation of misfolded proteins through its associations with EDEM and the ER stress sensor BiP. To investigate whether ERdj5 can regulate ER stress signaling pathways, we exposed neuroblastoma cells overexpressing ERdj5 to ER stress inducers. ERdj5 promoted apoptosis in tunicamycin, thapsigargin, and bortezomib-treated cells. To provide further evidence that ERdj5 induces ER stress-regulated apoptosis, we targeted Bcl-2 to ER of ERdj5-overexpressing cells. Targeting the Bcl-2 to ER prevented the apoptosis induced by ER stress inducers but not by non-ER stress apoptotic stimuli, suggesting induction of ER stress-regulated apoptosis by ERdj5. ERdj5 enhanced apoptosis by abolishing the ER stress-induced phosphorylation of eukaryotic translation initiation factor 2alpha (eIF2alpha) and the subsequent translational repression. ERdj5 was found to inhibit the eIF2alpha phosphorylation under ER stress through inactivating the pancreatic endoplasmic reticulum kinase. The compromised integrated stress response observed in ERdj5-overexpressing ER-stressed cells due to repressed eIF2alpha phosphorylation correlated with impaired neuroblastoma cell resistance under ER stress. These results demonstrate that ERdj5 decreases neuroblastoma cell survival by down-regulating the UPR, raising the possibility that this protein could be a target for anti-tumor approaches.

Highlights

Accumulation of unfolded proteins in the lumen of the endoplasmic reticulum (ER),2 a condition that produces ER stress, induces an intracellular signaling cascade named the unfolded protein response (UPR) [1, 2]
ERdj5 Induces ER Stress-mediated Apoptosis—We previously demonstrated that ERdj5 exhibits ATP-dependent binding to the master regulator of the UPR BiP and that ERdj5 mRNA levels are induced upon ER stress [21]
Apoptosis was induced to a similar extent in both Bcl-2cb5-overexpressing cells and the cells carrying the empty vector after treatment with the non-ER stressors doxorubicin and etoposide (Fig. 4, A and B)

Summary

Introduction

Accumulation of unfolded proteins in the lumen of the endoplasmic reticulum (ER), a condition that produces ER stress, induces an intracellular signaling cascade named the unfolded protein response (UPR) [1, 2]. The ER stress chaperones GRP78/BiP and GRP94 are induced at different stages of tumor progression, and the X box-binding protein (XBP1), the major transcriptional regulator of the IRE1 arm of the UPR, is essential for tumor growth under hypoxic conditions [8, 12,13,14]. Both PERK and IRE1 are obvious candidates, because as kinases they are viable drug targets [17] Drugs such as bortezomib (Velcade) kill tumor cells in vitro by causing ER stress while inhibiting part of the UPR [18]. The ER contains several proteins endowed with oxidoreductase activity that catalyze formation, isomerization, and reduction of disulfide bonds in proteins that are undergoing folding and quality control [20] Among these proteins, ERdj is a recently identified, ER-resident molecule that features a unique combination of domains, including a dnaj, a protein-disulfide isomerase (PDI)-like, and a thioredoxin domain. We show that ERdj decreases neuroblastoma cell survival by inducing ER stress-regulated apoptosis

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