Cab45S inhibits the ER stress-induced IRE1-JNK pathway and apoptosis via GRP78/BiP

Disturbance of endoplasmic reticulum (ER) homeostasis causes ER stress and leads to activation of the unfolded protein response, which reduces the stress and promotes cell survival at the early stage of stress, or triggers cell death and apoptosis when homeostasis is not restored under prolonged ER stress. Here, we report that Cab45S, a member of the CREC family, inhibits ER stress-induced apoptosis. Depletion of Cab45S increases inositol-requiring kinase 1 (IRE1) activity, thus producing more spliced forms of X-box-binding protein 1 mRNA at the early stage of stress and leads to phosphorylation of c-Jun N-terminal kinase, which finally induces apoptosis. Furthermore, we find that Cab45S specifically interacts with 78-kDa glucose-regulated protein/immunoglobulin heavy chain binding protein (GRP78/BiP) on its nucleotide-binding domain. Cab45S enhances GRP78/BiP protein level and stabilizes the interaction of GRP78/BiP with IRE1 to inhibit ER stress-induced IRE1 activation and apoptosis. Together, Cab45S, a novel regulator of GRP78/BiP, suppresses ER stress-induced IRE1 activation and apoptosis by binding to and elevating GRP78/BiP, and has a role in the inhibition of ER stress-induced apoptosis.     

The therapeutic mavericks: Potent immunomodulating chaperones capable of treating human diseases

Two major chaperones, calreticulin (CRT) and binding immunoglobulin protein (GRP78/BiP) dependent on their location, have immunoregulatory or anti-inflammatory functions respectively. CRT induces pro-inflammatory cytokines, dendritic cell (DC) maturation and activates cytotoxic T cells against tumours. By contrast, GRP78/BiP induces anti-inflammatory cytokines, inhibits DC maturation and heightens T-regulatory cell responses. These latter functions rebalance immune homeostasis in inflammatory diseases, such as rheumatoid arthritis. Both chaperones are therapeutically relevant agents acting primarily on monocytes/DCs. Endogenous exposure of CRT on cancer cell surfaces acts as an ‘eat-me’ signal and facilitates improved elimination of stressed and dying tumour cells by DCs. Therefore, therapeutics that promote endogenous CRT translocation to the cell surface can improve the removal of cancer cells. However, infused recombinant CRT dampens this cancer cell eradication by binding directly to the DCs. Low levels of endogenous BiP appear as a surface biomarker of endoplasmic reticulum (ER) stress in some types of tumour cells, a reflection of cells undergoing proliferation, in which resulting hypoxia and nutrient deprivation perturb ER homeostasis triggering the unfolded protein response, leading to increased expression of GRP78/BiP and altered cellular location. Conversely, infusion of an analogue of GRP78/BiP (IRL201805) can lead to long-term immune resetting and restoration of immune homeostasis. The therapeutic potential of both chaperones relies on them being relocated from their intracellular ER environment. Ongoing clinical trials are employing therapeutic interventions to either enhance endogenous cell surface CRT or infuse IRL201805, thereby triggering several disease-relevant immune responses leading to a beneficial clinical outcome.     

Overexpression of the 78-kDa glucose-regulated protein/immunoglobulin-binding protein (GRP78/BiP) inhibits tissue factor procoagulant activity

Previous studies have demonstrated that overexpression of GRP78/BiP, an endoplasmic reticulum (ER)-resident molecular chaperone, in mammalian cells inhibits the secretion of specific coagulation factors. However, the effects of GRP78/BiP on activation of the coagulation cascade leading to thrombin generation are not known. In this study, we examined whether GRP78/BiP overexpression mediates cell surface thrombin generation in a human bladder cancer cell line T24/83 having prothrombotic characteristics. We report here that cells overexpressing GRP78/BiP exhibited significant decreases in cell surface-mediated thrombin generation, prothrombin consumption and the formation of thrombin-inhibitor complexes, compared with wild-type or vector-transfected cells. This effect was attributed to the ability of GRP78/BiP to inhibit cell surface tissue factor (TF) procoagulant activity (PCA) because conversion of factor X to Xa and factor VII to VIIa were significantly lower on the surface of GRP78/BiP-overexpressing cells. The additional findings that (i) cell surface factor Xa generation was inhibited in the absence of factor VIIa and (ii) TF PCA was inhibited by a neutralizing antibody to human TF suggests that thrombin generation is mediated exclusively by TF. GRP78/BiP overexpression did not decrease cell surface levels of TF, suggesting that the inhibition in TF PCA does not result from retention of TF in the ER by GRP78/BiP. The additional observations that both adenovirus-mediated and stable GRP78/BiP overexpression attenuated TF PCA stimulated by ionomycin or hydrogen peroxide suggest that GRP78/BiP indirectly alters TF PCA through a mechanism involving cellular Ca(2+) and/or oxidative stress. Similar results were also observed in human aortic smooth muscle cells transfected with the GRP78/BiP adenovirus. Taken together, these findings demonstrate that overexpression of GRP78/BiP decreases thrombin generation by inhibiting cell surface TF PCA, thereby suppressing the prothrombotic potential of cells.     

Thapsigargin down-regulates protein levels of GRP78/BiP in INS-1E cells

Pancreatic β-cells have a well-developed endoplasmic reticulum (ER) and express large amounts of chaperones and protein disulfide isomerases (PDI) to meet the high demand for synthesis of proteins. We have observed an unexpected decrease in chaperone protein level in the β-cell model INS-1E after exposure to the ER stress inducing agent thapsigargin. As these cells are a commonly used model for primary β-cells and has been shown to be vulnerable to ER stress, we hypothesize these cells are incapable of mounting a chaperone defense upon activation of ER stress. To investigate the chaperone expression during an ER stress response, induced by thapsigargin in INS-1E cells, we used quantitative mass spectrometry based proteomics. The results displayed a decrease of GRP78/BiP, PDIA3 and PDIA6. Decrease of GRP78/BiP was verified by Western blot and occurred in parallel with enhanced levels of p-eIF2α and CHOP. In contrast to INS-1E cells, GRP78/BiP was not decreased in MIN6 cell or rat and mouse islets after thapsigargin exposure. Investigation of the decreased protein levels of GRP78/BiP indicates that this is not a consequence of reduced mRNA expression. Rather the reduction results from the combined effect of reduced protein synthesis and enhanced proteosomal degradation and possibly also degradation via autophagy. Induction of ER stress with thapsigargin leads to lower protein levels of GRP78/BiP, PDIA3 and PDIA6 in INS-1E cells which may contribute to the susceptibility of ER stress in this β-cell model.     

The role of endoplasmic reticulum‐related BiP/GRP78 in interferon gamma‐induced persistent Chlamydia pneumoniae infection

Direct interaction of Chlamydiae with the endoplasmic reticulum (ER) is essential in intracellular productive infection. However, little is known about the interplay between Chlamydiae and the ER under cellular stress conditions that are observed in interferon gamma (IFN‐γ) induced chlamydial persistent infection. ER stress responses are centrally regulated by the unfolded protein response (UPR) under the control of the ER chaperone BiP/GRP78 to maintain cellular homeostasis. In this study, we could show that the ER directly contacted with productive and IFN‐γ‐induced persistent inclusions of Chlamydia pneumoniae (Cpn). BiP/GRP78 induction was observed in the early phase but not in the late phase of IFN‐γ‐induced persistent infection. Enhanced BiP/GRP78 expression in the early phase of IFN‐γ‐induced persistent Cpn infection was accompanied by phosphorylation of the eukaryotic initiation factor‐2α (eIF2α) and down‐regulation of the vesicle‐associated membrane protein‐associated protein B. Loss of BiP/GRP78 function resulted in enhanced phosphorylation of eIF2α and increased host cell apoptosis. In contrast, enhanced BiP/GRP78 expression in IFN‐γ‐induced persistent Cpn infection attenuated phosphorylation of eIF2α upon an exogenous ER stress inducer. In conclusion, ER‐related BiP/GRP78 plays a key role to restore cells from stress conditions that are observed in the early phase of IFN‐γ‐induced persistent infection.     

Human cytomegalovirus specifically controls the levels of the endoplasmic reticulum chaperone BiP/GRP78, which is required for virion assembly

The endoplasmic reticulum (ER) chaperone BiP/GRP78 regulates ER function and the unfolded protein response (UPR). Human cytomegalovirus infection of human fibroblasts induces the UPR but modifies it to benefit viral replication. BiP/GRP78 protein levels are tightly regulated during infection, rising after 36 h postinfection (hpi), peaking at 60 hpi, and decreasing thereafter. To determine the effects of this regulation on viral replication, BiP/GRP78 was depleted using the SubAB subtilase cytotoxin, which rapidly and specifically cleaves BiP/GRP78. Toxin treatment of infected cells for 12-h periods beginning at 36, 48, 60, and 84 hpi caused complete loss of BiP but had little effect on viral protein synthesis. However, progeny virion formation was significantly inhibited, suggesting that BiP/GRP78 is important for virion formation. Electron microscopic analysis showed that infected cells were resistant to the toxin and showed none of the cytotoxic effects seen in uninfected cells. However, all viral activity in the cytoplasm ceased, with nucleocapsids remaining in the nucleus or concentrated in the cytoplasmic space just outside of the outer nuclear membrane. These data suggest that one effect of the controlled expression of BiP/GRP78 in infected cells is to aid in cytoplasmic virion assembly and egress.     

Lysozyme Mutants Accumulate in Cells while Associated at their N-terminal Alpha-domain with the Endoplasmic Reticulum Chaperone GRP78/BiP

Amyloidogenic human lysozyme variants deposit in cells and cause systemic amyloidosis. We recently observed that such lysozymes accumulate in the endoplasmic reticulum (ER) with the ER chaperone GRP78/BiP, accompanying the ER stress response. Here we investigated the region of lysozyme that is critical to its association with GRP78/BiP. In addition to the above-mentioned variants of lysozyme, we constructed lysozyme truncation or substitution mutants. These were co-expressed with GRP78/BiP (tagged with FLAG) in cultured human embryonic kidney cells, which were analyzed by western blotting and immunocytochemistry using anti-lysozyme and anti-FLAG antibodies. The amyloidogenic variants were confirmed to be strongly associated with GRP78/BiP as revealed by the co-immunoprecipitation assay, whereas N-terminal mutants pruned of 1-41 or 1-51 residues were found not to be associated with the chaperone. Single amino acid substitutions for the leucine array along the α-helices in the N-terminal region resulted in wild-type lysozyme remaining attached to GRP78/BiP. These mutations also tended to show lowered secretion ability. We conclude that the N-terminal α-helices region of the lysozyme is pivotal for its strong adhesion to GRP78/BiP. We suspect that wild-type lysozyme interacts with the GRP at this region as a step in the proper folding monitored by the ER chaperone.     

Involvement of androgen receptor and glucose-regulated protein 78 kDa in human hepatocarcinogenesis

Previous studies demonstrated that androgen receptor (AR) is expressed in human hepatocellular carcinoma (HCC), one of the male-dominant diseases. Glucose-regulated protein 78 kDa (GRP78/Bip), which has a role in cancer development, is one of the androgen response genes in prostate cell lines. The aim of this study was to investigate the impact of AR on endoplasmic reticulum (ER)-stress signaling in human hepatoma. AR and GRP78 expressions were examined in human liver tissue panels. Human hepatoma cells stably expressing short hairpin RNA targeting AR and cells over-expressing AR were generated. The expressions of ER-stress molecules and AR were measured by real-time RT-PCR and Western blotting. The effect of AR on ER-stress responsive gene expression was examined by reporter assay. Strong positive correlation between AR mRNA and GRP78 mRNA was observed in stage I/II-HCCs. AR enhanced ER-stress responsive element activities and GRP78 expression, and regulated ER-stress response in hepatocytes. Sorafenib strongly induced significant apoptosis in HepG2 cells by the inhibition of AR and inhibition of the downstream GRP78. AR seems a co-regulator of GRP78 especially in earlier-stage HCC. AR plays a critical role in controlling ER-stress, providing new therapeutic options against HCC.     

Loss of BiP/GRP78 function blocks translocation of secretory proteins in yeast

BiP/GRP78 is an essential member of the HSP70 family that resides in the lumen of the endoplasmic reticulum. In yeast, BiP/GRP78 is encoded by the KAR2 gene. A temperature sensitive mutation was isolated in KAR2 and found to cause a rapid block in protein secretion. Secretory precursors of a number of proteins (invertase, carboxypeptidase Y, alpha-factor, and BiP) accumulated that were characteristic of a block in translocation into the lumen of the ER. Protease protection experiments confirmed that the precursors accumulated on the cytoplasmic side of the ER membrane. Moreover, depletion of wild-type KAR2 protein also resulted in a block in translocation of secretory proteins. These results implicate BiP/GRP78 function in the continued translocation of proteins into the lumen of the ER.     

Bip/GRP78-induced production of cytokines and uptake of amyloid-beta(1-42) peptide in microglia

In the brains of Alzheimer's disease (AD) patients, fibrillar amyloid-beta peptides (Abeta) are markedly accumulated and the microglia associate with the amyloid plaques. However, the regulation of Abeta clearance is still unclear. In the present study, we examined the effect of a chaperone protein BiP/GRP78 on the microglial function. Exogenous addition of recombinant BiP/GRP78 induced the production of cytokines such as interleukin-6 and tumor necrosis factor-alpha, but heat treatment of this protein abolished the activity. Although Abeta(1-42) did not induce cytokine production, it was taken up by the microglia. In addition, the amount of Abeta(1-42) uptake and the number of microglia that phagocytosed Abeta(1-42) were markedly increased by BiP/GRP78. Exogenous BiP/GRP78 also translocated to the endoplasmic reticulum (ER). These results suggest that BiP/GRP78 stimulates Abeta clearance in the microglia, and that dysfunction in the ER may cause the accumulation of extracellular Abeta(1-42).     

La3+ binds to BiP/GRP78 and induces unfolded protein response in HepG2 cells

The effects of La3+ on the unfolded protein response signaling pathways were investigated in human hepatoblastoma HepG2 cells. Our data showed that La3+ could induce unfolded protein response in HepG2 cells, including a significant increase of BiP/GRP78 level, which is an important ER residential chaperone and an ER stress hallmark, in a concentration and time-dependent manner, UPR transducer IRE1 phosphorylation and splicing activation IRE1 downstream substrate XBP1 mRNA. By using La3+-affinity chromatography, the possible cellular target of La3+ leading to UPR events was shown to be the ER residential chaperone BiP/GRP78. BiP/GRP78 was shown to be a La3+ binding protein and the interaction of La3+ with BiP/GRP78 resulted in dissociation of BiP-IRE1 complexes. La3+ induced dissociation of the BiP/GRP78-IRE1 complex was in a time and concentration manner. The apparent dissociation constant was estimated to be 4 nM. In addition, La3+ was observed to slightly stimulate the production of cellular ROS and cause alteration of intracellular Ca2+, indicating the possible involvement of ROS and Ca2+ alteration in La3+ induced UPR. The present work provides a new perspective for understanding the biological and toxicological effects of La3+.     

Altered localization of amyloid precursor protein under endoplasmic reticulum stress

Recent reports have shown that the endoplasmic reticulum (ER) stress is relevant to the pathogenesis of Alzheimer disease. Following the amyloid cascade hypothesis, we therefore attempted to investigate the effects of ER stress on amyloid-beta peptide (Abeta) generation. In this study, we found that ER stress altered the localization of amyloid precursor protein (APP) from late compartments to early compartments of the secretory pathway, and decreased the level of Abeta 40 and Abeta 42 release by beta- and gamma-cutting. Transient transfection with BiP/GRP78 also caused a shift of APP and a reduction in Abeta secretion. It was revealed that the ER stress response facilitated binding of BiP/GRP78 to APP, thereby causing it to be retained in the early compartments apart from a location suitable for the cleavages of Abeta. These findings suggest that induction of BiP/GRP78 during ER stress may be one of the regulatory mechanisms of Abeta generation.     

Overexpressing GRP78 influences Ca2+ handling and function of mitochondria in astrocytes after ischemia-like stress

Ca(2+) transfer from endoplasmic reticulum (ER) to mitochondria at contact sites between the organelles can induce mitochondrial dysfunction and programmed cell death after stress. The ER-localized chaperone glucose-regulated protein 78kDa (GRP78/BiP) protects neurons against excitotoxicity and apoptosis. Here we show that overexpressing GRP78 protects astrocytes against ischemic injury, reduces net flux of Ca(2+) from ER to mitochondria, increases Ca(2+) uptake capacity in isolated mitochondria, reduces free radical production, and preserves respiratory activity and mitochondrial membrane potential after stress. We conclude that GRP78 influences ER-mitochondrial Ca(2+) crosstalk to maintain mitochondrial function and protect astrocytes from ischemic injury.     

ATM blocks tunicamycin-induced endoplasmic reticulum stress

Endoplasmic reticulum stress (ER-stress) is associated with ataxia telangiectasia mutated (ATM) gene. We present here conclusive data showing that ATM blocks ER-stress induced by tunicamycin or ionizing radiation (IR). X-box protein-1 (XBP-1) splicing, GRP78 expression and caspase-12 activation were increased by tunicamycin or IR in Atm-deficient AT5BIVA fibroblasts. Activation of caspase-12 and caspase-3 by tunicamycin was significantly reduced in cells transfected with wild-type Atm (AT5BIVA/wtATM). Atm knockdown by siRNA, however, noticeably elevated ER-stress and chemosensitivity to tunicamycin. In summary, we present substantial data demonstrating that ATM blocks the ER stress signaling associated with cancer cell proliferation.     

Hyperthermia induces the ER stress pathway

Background

The ER chaperone GRP78/BiP is a homolog of the Hsp70 family of heat shock proteins, yet GRP78/BiP is not induced by heat shock but instead by ER stress. However, previous studies had not considered more physiologically relevant temperature elevation associated with febrile hyperthermia. In this report we examine the response of GRP78/BiP and other components of the ER stress pathway in cells exposed to 40°C.

Methodology

AD293 cells were exposed to 43°C heat shock to confirm inhibition of the ER stress response genes. Five mammalian cell types, including AD293 cells, were then exposed to 40°C hyperthermia for various time periods and induction of the ER stress pathway was assessed.

Principal Findings

The inhibition of the ER stress pathway by heat shock (43°C) was confirmed. In contrast cells subjected to more mild temperature elevation (40°C) showed either a partial or full ER stress pathway induction as determined by downstream targets of the three arms of the ER stress pathway as well as a heat shock response. Cells deficient for Perk or Gcn2 exhibit great sensitivity to ER stress induction by hyperthermia.

Conclusions

The ER stress pathway is induced partially or fully as a consequence of hyperthermia in parallel with induction of Hsp70. These findings suggest that the ER and cytoplasm of cells contain parallel pathways to coordinately regulate adaptation to febrile hyperthermia associated with disease or infection.