GRP78 (glucose-regulated protein of 78 kDa) is traditionally regarded as a major ER (endoplasmic reticulum) chaperone facilitating protein folding and assembly, protein quality control, Ca2+ binding and regulating ER stress signalling. It is a potent anti-apoptotic protein and plays a critical role in tumour cell survival, tumour progression and angiogenesis, metastasis and resistance to therapy. Recent evidence shows that GRP78 can also exist outside the ER. The finding that GRP78 is present on the surface of cancer but not normal cells in vivo represents a paradigm shift on how GRP78 controls cell homoeostasis and provides an opportunity for cancer-specific targeting. Cell-surface GRP78 has emerged as an important regulator of tumour cell signalling and viability as it forms complexes with a rapidly expanding repertoire of cell-surface protein partners, regulating proliferation, PI3K (phosphoinositide 3-kinase)/Akt signalling and cell viability. Evidence is also emerging that GRP78 serves as a receptor for viral entry into host cells. Additionally, a novel cytosolic form of GRP78 has been discovered prominently in leukaemia cells. These, coupled with reports of nucleus- and mitochondria-localized forms of GRP78, point to the previously unanticipated role of GRP78 beyond the ER that may be critical for cell viability and therapeutic targeting.
Unlike other members of the Hsp70 (heat-shock protein 70) gene family, GRP78 (glucose-regulated protein of 78 kDa) is encoded by a single copy gene in the eukaryotic genome and its induction is primarily regulated at the level of transcription [1,2]. Since its discovery in the 1980s, GRP78 has been well established as an ER (endoplasmic reticulum) chaperone and is widely used as a marker for ER stress. It is highly induced in a wide range of tumours through intrinsic factors, such as altered glucose metabolism of cancer cells, compounded by extrinsic factors including glucose deprivation, hypoxia and acidosis in the microenvironment of poorly perfused solid tumours . The induction of GRP78 by ER stress leads to an increase in GRP78 in the ER compartment, as well as promotion of GRP78 relocalization from the ER to the cell surface . There, GRP78 assumes a new function as a co-receptor for cell-surface signalling [5,6]. ER stress also enhances retention of intron 1 of the Grp78 transcript, leading to translation of a novel isoform of GRP78 (Grp78va) that lacks the ER signal peptide and is localized in the cytosol . Although GRP78va is in low abundance compared with the ER form of GRP78, it exhibits cytoprotective properties and has the potential to regulate UPR (unfolded protein response) signalling from the cytosol. Additionally, GRP78 is reported to associate with the mitochondria and, in mouse models , perturbation of GRP78 levels in the ER elicits compensatory responses in the ER, as well as in the mitochondria . Although least understood, GRP78 has been observed in the nucleus following overexpression . Furthermore, a secreted form of GRP78 is linked to endothelial cell drug resistance . In the present review, we summarize the occurrence and putative function of GRP78 beyond the ER (Figure 1). These findings support the emerging notion of functional importance of minor subpopulations of various proteins generated by different means to influence fundamental biological processes.
Summary of GRP78 functions in different subcellular compartments
With the first report of the cell-surface localization of GRP78 in 1997 , evidence has rapidly accumulated that GRP78 exists on the cell surface of select cell types. However, there have been inconsistencies in the literature on the expression of cell-surface GRP78 in cultured cell lines, probably due to inherent diversity of cells maintained in culture or technical issues. Nonetheless, whole-organism targeting of cell-surface GRP78 has shown reactivity primarily observed with pathological tissues, such as cancer, and cell types under stress conditions, including hypoxia or glucose starvation. This suggests potential applications in targeted therapy. The key known functions of cell-surface GRP78 are summarized in Figures 1 and 2.
Examples of GRP78 on the cell-surface serving as a receptor and regulator of cell signalling
GRP78 on the surface of tumour cells and the control of oncogenic signalling
GRP78 was identified on the cell surface of malignant lymphocytes of lymphoma and leukaemia from patients with acquired immunodeficiency syndrome . KDEL-motif-containing proteins, including GRP78, were expressed on the surface of NG108-15 cells, a neuroblastoma glioma hybrid cell line . Established human cancer cell lines of neuroblastoma (SH-SY5Y), lung adenocarcinoma (A549), colon adenocarcinoma (LoVo), acute lymphoblastic leukaemia B-cells (Sup-B15 ALL-B cell) and ovarian tumour cells freshly isolated from patient ascites fluid showed cell-surface GRP78 expression, as determined by global profiling of the cell-surface proteome of tumour cells . Cell-surface GRP78 was also found on the melanoma cell line Me6652/4, osteosarcoma SJSA-1, hepatoma cell line HepG2 , breast cancer cells (Hs578T, MDA-MB-231, BrCa-MZ-01 and MCF-7) , gastric cancer 23132/87 and pancreatic cancer BXPC-3 cells .
Through the formation of complexes with other proteins on the cell surface, GRP78 is reported to mediate tumour cell signal transduction (Figure 2). For example, in highly metastatic and invasive 1-LN prostate cancers, cell-surface GRP78 acts as a receptor for activated α2-macroglobulin, leading to activation of PAK-2 (p21-activated kinase-2) and, together with LIMK1 (LIM domain kinase 1) and cofilin phosphorylation, increases motility for metastasis [18,19]. The interaction of α2-macroglobulin with cell-surface GRP78 is also reported to promote cell proliferation by activating ERK1/2 (extracellular-signal-regulated kinase 1/2), p38 MAPK (mitogen-activated protein kinase) and PI3K (phosphoinositide 3-kinase), and cell survival by the Akt and NF-κB (nuclear factor κB) signalling cascade . Autoantibodies from serum of prostate cancer patients against an amino acid segment of GRP78 (Leu98–Leu115) induces cell proliferation, suggesting that it serves as an agonist of activated α2-macroglobulin, which recognizes the same site of GRP78 . Additionally, treatment of a human bladder carcinoma cell line with the same autoantibody against GRP78 increases tissue factor procoagulant activity . Interestingly, the autoantibody caused the release of Ca2+ from the ER store, which may account for the increase in risk of venous thromboembolism. In another example, Cripto, a multifunctional cell-surface protein which is key to vertebrate embryogenesis and human tumour progression, was bound to cell-surface GRP78 . The complex of Cripto and GRP78 can enhance tumour growth via inhibition of TGF-β (transforming growth factor-β) signalling. Furthermore, it was determined that blockade of Cripto binding to cell-surface GRP78 by an antibody against the N-terminus of GRP78 inhibited oncogenic Cripto signalling and this involved the MAPK/PI3K and Smad2/3 pathway .
Anti-cancer therapy by targeting cell-surface GRP78 on tumour cells
By screening the circulating pool of antibodies from patients' serum, GRP78 was identified as a tumour antigen highly expressed in bone marrow metastases of prostate cancer patients, whereas weakly expressed in normal prostate tissue . This opens up the opportunity for cell-surface GRP78 ligand-directed targeting cancer therapy, such that synthetic chimaeric ligand peptides containing a programmed-cell-death-inducing sequence when administered to mice suppressed tumour growth in xenograft and isogenic mouse models of prostate and breast cancer . In other applications, Pep42, a cyclic oligopeptide that specifically bound to cell-surface GRP78 and internalized into cells, enabled taxol-conjugated Pep42 to target and kill melanoma cells by recognizing GRP78 on their surface . Further studies have shown that the fusion of Pep42 conjugated to apoptosis-inducing peptide D-(KLAKLAK)2 selectively killed human cancer cell lines in vitro by binding to cell-surface GRP78, but with minimal toxicity to normal cells where no GRP78 was detected on the cell surface .
GRP78-modification variants may represent novel targets for cancer therapy. Phage-display-derived human monoclonal antibodies isolated by binding to primary breast cancer cells recognize a modified form of cell-surface GRP78, involving a putative glycosylation site at the C-terminus of GRP78 . Another report indicates the existence of an 82 kDa tumour-specific variant of GRP78 . The epitope is an O-linked carbohydrate moiety and is specific for malignant cells, which may account for the escape of GRP78 from immune surveillance and immune response. Serum autoantibodies against this form of GRP78 from cancer patients when added to malignant cells leads to lipid accumulation and cell death .
A commercial polyclonal antibody directed against the C-terminus of GRP78 was reported to induce apoptosis in melanoma cells (A375) and prostate cancer cells (1-LN and DU145), but not in another prostate cancer cell line, PC-3, where GRP78 expression was undetectable on the surface . The proposed mechanism is that this antibody leads to the up-regulation of p53, inhibition of NF-κB1 and NF-κB2 activation, and suppression of Ras/MAPK and PI3K/Akt signalling [29–32]. In another study, using prostate cancer PC-3 cells, apoptosis induced by extracellular Par-4 and TRAIL (tumour-necrosis-factor-related apoptosis-inducing ligand) was observed to be dependent on the binding of Par-4 to cell-surface GRP78 and resulted in activation of the extrinsic apoptosis pathways, and this was enhanced by ER stress or TRAIL . Par-4 was regarded previously as a cytosolic and nuclear protein that promotes cell death; however, it was found that Par-4 can spontaneously be secreted in normal and cancer cell culture, and it was proposed that ER stress or TRAIL caused translocation of Par-4–GRP78 complexes from the ER to plasma membrane [34,35]. Nonetheless, how Par-4 enters the ER and the conflicting reports of whether GRP78 is expressed at significant levels on the surface of PC-3 cells remain to be resolved, since other studies have shown no cell-surface GRP78 expression in PC-3 cells compared with high levels in more malignant and invasive 1-LN cells [36,37].
GRP78 on the surface of proliferating endothelial cells
GRP78 is expressed on the cell surface of proliferating endothelial cells and monocytic cells [38,39] (Figure 2). GRP78 associates with MHC class I on the surface of these cells and is required for MHC class I expression . GPI (glycosylphosphatidylinositol)-anchored T-cadherin is reported to associate with GRP78 on the surface of vascular endothelial cells and, in this capacity, GRP78 influences endothelial cell survival as a cell-surface signalling receptor .
As tumour progression typically requires angiogenesis for nutrient and oxygen supply, anti-angiogenic therapy exploits this requirement to block tumour growth. Kringle 5 of human plasminogen has been shown to be a binding partner of GRP78 on the surface of proliferating endothelial cells and stimulated tumour cells . rK5 (recombinant Kringle 5) induces apoptosis of proliferating endothelial cells and tumour cells through binding of surface-expressed GRP78 and enhancing caspase 7 activity by disruption of the GRP78–procaspase 7 complex . Further studies have shown that prior irradiation significantly sensitizes glioma microvessel endothelial cells to rK5-induced apoptosis, which required LRP1 (low-density lipoprotein receptor-related protein 1) and GRP78 . In addition, the expression of cell-surface GRP78 is elevated in VEGF (vascular endothelial growth factor)-activated HUVECs (human umbilical vein endothelial cells) and is required for endothelial cell proliferation . The same study showed that cell-surface GRP78 is a promising target for effective liposome drug delivery in anti-neovascular cancer therapy .
GRP78 was recently identified as the endothelial cell receptor required for Mucorales to penetrate and damage endothelial cells . Moreover, serum from mice vaccinated with recombinant GRP78 protected diabetic ketoacidosis mice from infection with mucormycosis, providing a novel approach for therapeutic intervention against lethal mucormycosis . GRP78 also exists on the atherosclerotic plaque endothelial surface and negatively regulates tissue factor-mediated initiation of the coagulation cascade . In another study, a novel peptide, RoY, was demonstrated to alleviate mouse hindlimb ischaemia through binding surface-expressed GRP78 on hypoxic endothelial cells . Furthermore, another peptide derived from ADAM15 (a disintegrin and metalloproteinase 15) has also been shown to activate GRP78 on endothelial cell membranes under hypoxic conditions, inducing VEGF-independent angiogenesis, implying that cell-surface GRP78 can serve as an angiogenic receptor for ischaemic disease therapy .
Cell-surface GRP78 as a co-receptor for virus internalization
Evidence is emerging that GRP78 serves as a critical portal for viral entry into host cells (Figure 2). Previous studies on viral entry of Coxsackie virus A9 into host cells determined that it required MHC class I molecules. GRP78 was later found to act as a co-receptor for virus internalization by associating with MHC class I molecules on the cell surface . GRP78 expressed on the surface of liver cancer cells acts as a receptor for dengue virus serotype 2 entry, and antibodies directed against both the N- and C-terminus of GRP78 had a major effect on the binding of the virus to the cell surface as well as virus infectivity . Recently, during a study of the Borna disease virus, which is characterized by highly neutropic and non-cytopathic infection, GRP78 was also found on the surface of Borna disease virus-targeted cells. Borna disease virus entry was mediated by the association of cell-surface GRP78 with the N-terminus-cleaved product of the envelope glycoprotein of Borna disease virus, GP1 . An antibody against the N-terminus of GRP78 (N20) was shown to inhibit GP1 binding to cells expressing GRP78 on the cell surface and to reduce virus infection.
Potential mechanism for GRP78 translocating from the ER to the cell surface
The mechanisms for GRP78 trafficking from the ER to the plasma membrane are just emerging. The C-terminal tetra-peptide KDEL has been shown to prevent GRP78 secretion and maintain it within the ER lumen . Since KDEL receptor expression was not co-ordinately up-regulated with ER stress in HeLa cells , increases in the intracellular level of GRP78 triggered by ER stress may exceed the retention capacity of the KDEL retrieval system, resulting in an escape from the ER to the cell surface. It is also possible that the activity of the various components of the KDEL system is altered under ER stress or pathological conditions. Another possible mechanism for GRP78 transport to the cell surface may involve the masking of the KDEL motif by glycosylation or other modification to the protein sequence adjacent to KDEL. A glycosylated form of GRP78 has been detected, and potential glycosylation sites exist at the C-terminus in close proximity to the KDEL motif [4,16,17]. Additionally, specific GRP78-interacting protein partners may facilitate its transport from the ER to the cell surface, and this can be cell-type-specific and/or acting in combination. For example, a DnaJ-like transmembrane protein, MTJ-1, binds GRP78 and silencing MTJ-1 expression apparently suppress cell-surface GRP78 expression in macrophages . However, in PC3 cells, Par-4 was reported to be required for translocation of GRP78 from the ER to the plasma membrane .
In cell cultures, the ER stress agent thapsigargin actively promotes the cell-surface expression of GRP78, as the increase in cell-surface GRP78 is severalfold higher than the increase in intracellular GRP78 induced by thapsigargin . Nonetheless, ER stress is not required for cell-surface localization of GRP78. Thus ectopic expression of GRP78 can induce the translocation in the absence of ER stress as indicated by the lack of CHOP (CCAAT/enhancer-binding protein-homologous protein) induction . Moreover, deletion of the ER-retention motif KDEL alters GRP78 relocation in a dose-dependent manner, such that GRP78 devoid of the KDEL motif had more surface localization at lower dosages. This suggests that the KDEL-retrieval system plays a significant role in regulating the extent of GRP78 leaving the ER. Mutation analysis of the putative O-linked glycosylation site Thr648, which is two amino acids away from KDEL, has no effect on modification and translocation of GRP78 to the cell surface . It is still possible that the modification only plays a minor role in the translocation progress thus it is below the detection limit or the experiments were not performed in the correct cell types.
The extracellular domains of GRP78 exposed on the cell surface have been identified by flow cytometry assays using different epitopes at the N-terminus, C-terminus and a middle segment of GRP78. These studies show that the N-terminus, C-terminus and middle portion of GRP78 are all exposed on the cell surface [4,15,33,45]. Furthermore, the blockade of function of Cripto, GPI-anchored T-cadherin and Par-4 by using an antibody against the N-terminus of GRP78 is in agreement with the existence of an extracellular N-terminal region of cell-surface GRP78 [24,33,41]. Similarly, an antibody against the C-terminus of GRP78 also affects cellular signalling, consistent with the exposed C-terminus of GRP78 on the cell surface .
Since addition of extracellular recombinant GRP78 did not appear to bind to the surface of recipient cells, cell-surface GRP78 is unlikely to be due to the adhesion of secreted GRP78 to the cell surface . This raises the issue of whether GRP78 exists as a transmembrane protein. Although GRP78 is a hydrophilic protein, the TMpred program showed several potential transmembrane segments, with a predicted structural model of cell-surface GRP78 presented as a transmembrane protein with the C-terminus exposed extracellularly [4,10]. This is supported by biochemical analysis of microsomal fractions showing that a subpopulation of GRP78 is an ER-transmembrane protein with the N-terminus protruding into the cell cytosol, as demonstrated by limited trypsin digestion and cell-membrane separation by sodium carbonate extraction . Future studies are required to fully dissect the domains of GRP78 that are extracellular.
GRP78 IN THE CYTOPLASM
Although unexpected, GRP78 has also been detected in the cytoplasm with functions including regulation of UPR signalling, assembly of viral proteins and amelioration of Pb (lead) neurotoxicity (Figure 1). A cytosolic GRP78 isoform (GRP78va), which is generated by alternative splicing, has recently been discovered  (Figure 3). GRP78va mRNA contains the intron 1 of Grp78 and encodes a truncated form of GRP78 due to alternative translational initiation. The abundance of Grp78va is likely to be affected by NMD (nonsense-mediated RNA decay), given that the retention of intron 1 introduces a premature stop codon. However, ER-stress-induced eIF2α (eukaryotic initiation factor 2α) phosphorylation leads to repression of NMD , which consequently stabilizes the Grp78va mRNAs. Therefore GRP78va is up-regulated by ER stress, due to elevated pre-mRNA levels of Grp78 and increased stability of the Grp78va transcript. Notably, GRP78va is overexpressed in leukaemia cells and samples from leukaemia patients. As this isoform is devoid of the signalling peptide and retains the major functional domains of GRP78, it is localized in the cytosol, where it can potentially interact with many client proteins. The first GRP78va-associated protein reported is P58IPK, the inhibitor of PERK (double-stranded-RNA-dependent protein kinase-like ER kinase) during UPR (Figure 3). Although P58IPK primarily resides in the ER, inefficient translocation can lead to cytosolic localization . When overexpressed, GRP78va interacts with P58IPK and antagonizes its inhibitory effect on PERK, resulting in activated PERK signalling and increased cell survival under ER stress. That study suggested that GRP78va has the potential to influence survival of cancer cells in adaptation to ER stress through modulating UPR signalling and other yet unknown processes in the cytosol.
Role of GRP78 and its isoform GRP78va in regulating the UPR signalling pathway and cell survival
GRP78 plays an important role in HCMV (human cytomegalovirus) assembly and egress . During HCMV infection, the vigorous synthesis of viral proteins requires an increased capacity of the protein maturation machinery and therefore GRP78 expression is induced . GRP78 actively binds to the viral proteins US2 and US11, which facilitates the virus-mediated degradation of MHC classes I and II. A recent study indicated that GRP78 is localized in the cytosolic-assembly compartment possibly through masking of its C-terminal KDEL signal by the protein complex during assembly .
GRP78 also plays a protective role in Pb-induced oxidative stress in astroglia cells . It has been shown that Pb directly targets GRP78 through direct binding and the GRP78 expression is elevated . Further studies revealed the compartmentalized cytosolic distribution of GRP78 in the astrocytoma cells exposed to Pb. The induction and redistribution of GRP78 protects astroglia against Pb neurotoxicity. However, the detailed molecular mechanism needs further investigation to interpret why and how GRP78 acts in the cytoplasm in astroglia cells under Pb insults.
GRP78 IN THE MITOCHONDRIA
The ER and mitochondria are physically and functionally linked, and there is increasing evidence of the GRPs influencing ER and mitochondrial cross-talk to maintain mitochondrial function (Figure 1). For example, the ER chaperone GRP94 interacts with the essential mitochondrial chaperone GRP75 to facilitate GRP75-mediated import of mitochondrial proteins . Interestingly, GRP78 is also implicated in the regulation of mitochondria energy balance. Grp78 heterozygosity ameliorates high-fat-diet-induced obesity and Type 2 diabetes, as a consequence of an activated adaptive UPR and increased insulin sensitivity in WAT (white adipose tissue) . The partial reduction in GRP78 in WAT leads to the elevated expression of GRP75, suggesting increased energy expenditure in the mitochondria probably as a compensatory measure. It is possible that GRP78 might physically interact with GRP75 in the mitochondria, since it has been reported that GRP78 is also localized in the mitochondria under ER stress . Submitochondrial fractionation studies identified the mitochondrial location of GRP78 mainly in the intermembrane space, inner membrane and mitochondria matrix, where GRP75 also locates. ER stress and UPR signalling induce the overexpression of GRP78, which results in the mitochondrial localization of GRP78. Furthermore, Ca2+ transfer from ER to mitochondria at contact sites between the organelles can induce mitochondrial dysfunction and programmed cell death after stress. Recently, it was discovered that overexpressing GRP78 protects astrocytes against ischaemic injury, reduces the net flux of Ca2+ from the ER to mitochondria, increases Ca2+ uptake capacity in isolated mitochondria, reduces free radical production, and preserves respiratory activity and mitochondrial membrane potential after stress . Collectively, these findings imply that GRP78 can potentially regulate mitochondrial function, such as balancing energy expenditure and maintaining mitochondrial homoeostasis, especially under ER stress.
GRP78 IN THE NUCLEUS
GRP78 has also been observed in the nucleus when it is ectopically overexpressed or induced by ER stress [10,62]. GV (gilvocarcin V) is an antitumour antibiotic with a coumarin-based aromatic structure that promotes protein–DNA cross-linking when photoactivated by near-UV light. Interestingly, the mature form of GRP78 lacking the hydrophobic leader sequence was selectively cross-linked to DNA in human fibroblasts by photoactivated GV . A proteomics study to isolate proteins involved in irradiation-induced DNA protein cross-linking in mammalian cells using γ-rays under either aerated and/or hypoxic conditions identified GRP78 as being cross-linked to DNA .
Recently, it was reported that capsaicin, a pungent ingredient of red pepper, induces apoptosis and also promotes cytoplasmic CHOP expression and nuclear translocation of GRP78 in human hepatoma HepG2 cells . It was found that capsaicin-induced apoptosis is mediated through elevation of intracellular production of ROS (reactive oxygen species), and regulation of the mitochondrial Bcl2 family and caspase 3. In another example, knockdown of GRP78 sensitizes cells to UVC-induced cell death, primarily due to an impaired DNA repair capacity . Taken together, it is tempting to speculate that the nuclear form of GRP78 might play a role against DNA-damage-induced apoptosis through a distinct regulatory mechanism in the nucleus (Figure 1).
In the early 1990s, immunogold electron microscopy studies showed that, in exocrine pancreatic cells, the ER luminal proteins GRP78, PDI (protein disulfide-isomerase) and GRP94 are exported from the ER to other intracellular organelles, such as nuclear-envelope cisternae, the trans-Golgi cisternae, secretory granules and plasma membranes, and even secreted into the extracellular space . Although all of these chaperone proteins have the KDEL ER-retention signal, it was speculated that the saturation of KDEL receptors or defects in the protein-sorting system might cause the inability to retrieve these KDEL-bearing proteins to the ER lumen, especially in cells with intensive requirements for protein synthesis and maturation. Following these observations, secretion of GRP78 was also detected in tumours. In the study of the inhibitory effects of bortezomib on tumour angiogenesis, it was discovered that a few tumour cell lines secreted a high amount of GRP78 into the tumour microenvironment . The inhibition of the ubiquitin–proteasome pathway by bortezomib, a proteasome inhibitor, causes accumulation of misfolded proteins, leading to ER stress and UPR activation. In that study, it was proposed that induction of GRP78 under ER stress in tumour cells leads to the increased secretion of GRP78 and, by binding to cell-surface receptors of endothelial cells, extracellular GRP78 activates the ERK and Akt pathways, and protects endothelial cells from the anti-angiogenic effect of bortezomib  (Figure 1). In a proteomic study of gastric cancer, GRP78 was identified in the sera of 28% of patients, but not in healthy individuals . Importantly, circulating GRP78 autoantibodies have been detected in patients with gastric and prostate cancer and have been implicated in tumour cell proliferation [21,68]. Further studies would help to clarify whether soluble GRP78 in sera from patients is from tumour cells or only dead cells in peripheral blood. Interestingly, GRP78 was also detected in oviductal fluids from women in the periovulatory period . It was found to be secreted from human oviduct epithelial cells and has the ability to modulate sperm–zona pellucida binding during fertilization . Therefore secreted GRP78 can potentially regulate a multitude of biological processes in both pathological and physiological conditions.
CONCLUSIONS AND PERSPECTIVES
GRP78 localized in multiple intracellular organelles is correlated with facilitating cell adaptation to stress for survival (Figure 1). This also raises the following issues. How does ER stress promote GRP78 export from the ER, especially to the cell surface, cytosol and mitochondria? Is it possible that the protein transportation machinery is altered in cancer cells, or cells with high protein-processing demands, such as pancreatic acinar cells, and those experiencing ER stress or oxidative stress? Since cell-surface GRP78 plays a critical role for cell signalling and survival, blocking its expression on the cell surface may sensitize cancer cells and proliferating endothelial cells within the tumour microenvironment to chemotoxic agents. For the cytosolic isoform of GRP78, what are its interactive partners and what pathways are affected? Finally, GRP78 is only a paradigm of ER proteins. There might be similar regulation of redistribution on other ER-chaperone proteins, such as GRP94 and PDI. It will be important to explore atypical forms of these proteins outside the ER as they may also have unexpected functions important for controlling cell viability and signalling.
CCAAT/enhancer-binding protein-homologous protein
mitogen-activated protein kinase
nuclear factor κB
nonsense-mediated RNA decay
double-stranded-RNA-dependent protein kinase-like ER kinase
recombinant Kringle 5
tumour-necrosis-factor-related apoptosis-inducing ligand
unfolded protein response
vascular endothelial growth factor
white adipose tissue
We thank members of the Lee laboratory for helpful discussions, and apologize to all of the colleagues whose work could not be cited due to space constraints.
The authors' work was supported in part by the National Institutes of Health [grant numbers CA027607, CA111700 and DK079999 to (A.S.L.)].
These authors contributed equally to the paper.
Present address: Dana-Farber Cancer Institute, Boston, MA 02115, U.S.A.