Endoplasmic Reticulum Stress in Malignancy

The combination of relative nutrient deprivation and dysregulation of protein synthesis make malignant cells especially prone to protein misfolding. Endoplasmic reticulum stress, which results from protein misfolding within the secretory pathway, has a profound effect on cancer cell proliferation and survival. In this review, we examine the evidence implicating endoplasmic reticulum dysfunction in the pathology of cancer and discuss how recent findings may help to identify novel therapeutic targets. The combination of relative nutrient deprivation and dysregulation of protein synthesis make malignant cells especially prone to protein misfolding. Endoplasmic reticulum stress, which results from protein misfolding within the secretory pathway, has a profound effect on cancer cell proliferation and survival. In this review, we examine the evidence implicating endoplasmic reticulum dysfunction in the pathology of cancer and discuss how recent findings may help to identify novel therapeutic targets. In the crowded molecular environment of the endoplasmic reticulum (ER), protein maturation requires the coordinated activity of many chaperones and folding enzymes. BiP is an abundant ER HSP70 chaperone that binds to exposed stretches of hydrophobic residues of immature polypeptide chains, while GRP94 is an HSP90 chaperone involved in subsequent folding steps for a subset of ER client proteins. When the efficiency of secretory protein folding is threatened, the cell is said to experience “ER stress” and elicits a homeostatic “unfolded protein response” (UPR) (Figure 1) (Walter and Ron, 2011Walter P. Ron D. The unfolded protein response: from stress pathway to homeostatic regulation.Science. 2011; 334: 1081-1086Crossref PubMed Scopus (3881) Google Scholar). The diverse substrate repertoire of BiP enables it to function as a master regulator of the UPR by binding to and inactivating the three ER stress sensors, PERK, IRE1, and ATF6. During ER stress, increased levels of unfolded substrates lead to the sequestration of BiP, freeing the sensors to initiate UPR signaling. PERK ameliorates ER stress through phosphorylation of the translation initiation factor eIF2α. This causes generalized attenuation of protein synthesis while also promoting the translation of a subset of UPR target proteins, including the transcription factor ATF4. ATF4 induces expression of the transcription factor CHOP and, subsequently, the phosphatase subunit GADD34, which specifically dephosphorylates eIF2α, enabling the recovery of protein translation (Marciniak et al., 2004Marciniak S.J. Yun C.Y. Oyadomari S. Novoa I. Zhang Y. Jungreis R. Nagata K. Harding H.P. Ron D. CHOP induces death by promoting protein synthesis and oxidation in the stressed endoplasmic reticulum.Genes Dev. 2004; 18: 3066-3077Crossref PubMed Scopus (1490) Google Scholar). The induction of ER oxidase 1α (ERO1α) by CHOP promotes oxidative protein folding in the ER, but this increased formation of disulphide bonds can contribute to worsening cellular stress through the generation of reactive oxygen species (ROS). However, additional targets of ATF4 include enzymes necessary to withstand oxidative stress, which tend to limit this toxicity. Additional targets of ATF4 promote amino acid import and synthesis, thus playing a cytoprotective role during a variety of stressful insults. Because other eIF2α kinases responding to different stresses can trigger this pathway—for example, GCN2 responds to amino acid deprivation—it has been named the integrated stress response (ISR; Figure 2) (Harding et al., 2003Harding H.P. Zhang Y. Zeng H. Novoa I. Lu P.D. Calfon M. Sadri N. Yun C. Popko B. Paules R. et al.An integrated stress response regulates amino acid metabolism and resistance to oxidative stress.Mol. Cell. 2003; 11: 619-633Abstract Full Text Full Text PDF PubMed Scopus (2364) Google Scholar).Figure 2The ISRShow full captionPhosphorylation of eIF2α serves as a hub for integration of signals mediated by a family of kinases: PERK responds to ER stress, HRI responds to iron deficiency and to oxidative stress, PKR is activated by dsRNA during some viral infections, and GCN2 is activated during amino acid starvation (Harding et al., 2003Harding H.P. Zhang Y. Zeng H. Novoa I. Lu P.D. Calfon M. Sadri N. Yun C. Popko B. Paules R. et al.An integrated stress response regulates amino acid metabolism and resistance to oxidative stress.Mol. Cell. 2003; 11: 619-633Abstract Full Text Full Text PDF PubMed Scopus (2364) Google Scholar). In unstressed conditions, eIF2α supports new protein synthesis as a component of the eIF2 complex that recruits initiator methionyl-tRNA to the ribosome. During its catalytic cycle, the eIF2 complex hydrolyzes bound GTP and must interact with the guanine nucleotide exchange factor eIF2B to be recharged with GTP. Once eIF2α is phosphorylated, it becomes a potent antagonist of eIF2B and thus attenuates the rate of protein translation. Low basal levels of eIF2α phosphorylation are antagonized by the constitutively expressed eIF2α phosphatase CReP, but during stress, this is overwhelmed and phospho-eIF2α accumulates. While translation of most mRNAs is reduced by phosphorylation of eIF2α, a subset is translated more efficiently, most notably, the transcript factor ATF4. This transactivates most genes of the ISR, including amino acid transporters and synthetases, which help counter amino acid limitation while providing the thiol moieties necessary for synthesis of the antioxidant glutathione. Subsequently, ATF4 induces another transcription factor CHOP, which induces the eIF2α phosphatase GADD34 leading to dephosphorylation of eIF2α and the resumption of normal rates of cap-dependent translation. CHOP also induces the ER oxidase ERO1α, thus promoting oxidative protein folding. While the induction of GADD34 and ERO1α can be seen as adaptive during the response to transient ER stress, their induction during chronic stressful circumstances can contributes to worsening stress and result in exaggerated toxicity.View Large Image Figure ViewerDownload Hi-res image Download (PPT) Phosphorylation of eIF2α serves as a hub for integration of signals mediated by a family of kinases: PERK responds to ER stress, HRI responds to iron deficiency and to oxidative stress, PKR is activated by dsRNA during some viral infections, and GCN2 is activated during amino acid starvation (Harding et al., 2003Harding H.P. Zhang Y. Zeng H. Novoa I. Lu P.D. Calfon M. Sadri N. Yun C. Popko B. Paules R. et al.An integrated stress response regulates amino acid metabolism and resistance to oxidative stress.Mol. Cell. 2003; 11: 619-633Abstract Full Text Full Text PDF PubMed Scopus (2364) Google Scholar). In unstressed conditions, eIF2α supports new protein synthesis as a component of the eIF2 complex that recruits initiator methionyl-tRNA to the ribosome. During its catalytic cycle, the eIF2 complex hydrolyzes bound GTP and must interact with the guanine nucleotide exchange factor eIF2B to be recharged with GTP. Once eIF2α is phosphorylated, it becomes a potent antagonist of eIF2B and thus attenuates the rate of protein translation. Low basal levels of eIF2α phosphorylation are antagonized by the constitutively expressed eIF2α phosphatase CReP, but during stress, this is overwhelmed and phospho-eIF2α accumulates. While translation of most mRNAs is reduced by phosphorylation of eIF2α, a subset is translated more efficiently, most notably, the transcript factor ATF4. This transactivates most genes of the ISR, including amino acid transporters and synthetases, which help counter amino acid limitation while providing the thiol moieties necessary for synthesis of the antioxidant glutathione. Subsequently, ATF4 induces another transcription factor CHOP, which induces the eIF2α phosphatase GADD34 leading to dephosphorylation of eIF2α and the resumption of normal rates of cap-dependent translation. CHOP also induces the ER oxidase ERO1α, thus promoting oxidative protein folding. While the induction of GADD34 and ERO1α can be seen as adaptive during the response to transient ER stress, their induction during chronic stressful circumstances can contributes to worsening stress and result in exaggerated toxicity. As solid cancers grow, their nutrient requirements eventually exceed the capacity of the existing vascular bed. Although many cancers adapt by triggering angiogenesis, inevitably the cores of most tumors become hypoxic and nutrient depleted. Impaired generation of ATP compromises ER protein folding, thus leading to activation of the UPR and ISR, while amino acid starvation further contributes to ISR activation. Indeed, phosphorylation of eIF2α by PERK has been shown to be necessary for the growth of larger solid tumors (Bi et al., 2005Bi M. Naczki C. Koritzinsky M. Fels D. Blais J. Hu N. Harding H. Novoa I. Varia M. Raleigh J. et al.ER stress-regulated translation increases tolerance to extreme hypoxia and promotes tumor growth.EMBO J. 2005; 24: 3470-3481Crossref PubMed Scopus (586) Google Scholar). During hypoxia, generation of ROS increases both in mitochondria (Brunelle et al., 2005Brunelle J.K. Bell E.L. Quesada N.M. Vercauteren K. Tiranti V. Zeviani M. Scarpulla R.C. Chandel N.S. Oxygen sensing requires mitochondrial ROS but not oxidative phosphorylation.Cell Metab. 2005; 1: 409-414Abstract Full Text Full Text PDF PubMed Scopus (599) Google Scholar) and the ER, partly through UPR-mediated induction of ERO1α (Marciniak et al., 2004Marciniak S.J. Yun C.Y. Oyadomari S. Novoa I. Zhang Y. Jungreis R. Nagata K. Harding H.P. Ron D. CHOP induces death by promoting protein synthesis and oxidation in the stressed endoplasmic reticulum.Genes Dev. 2004; 18: 3066-3077Crossref PubMed Scopus (1490) Google Scholar, Song et al., 2008Song B. Scheuner D. Ron D. Pennathur S. Kaufman R.J. Chop deletion reduces oxidative stress, improves beta cell function, and promotes cell survival in multiple mouse models of diabetes.J. Clin. Invest. 2008; 118: 3378-3389Crossref PubMed Scopus (541) Google Scholar). Accordingly, a key function of the ISR is to defend against oxidative stress, primarily by increasing biosynthesis of the antioxidant glutathione (Harding et al., 2003Harding H.P. Zhang Y. Zeng H. Novoa I. Lu P.D. Calfon M. Sadri N. Yun C. Popko B. Paules R. et al.An integrated stress response regulates amino acid metabolism and resistance to oxidative stress.Mol. Cell. 2003; 11: 619-633Abstract Full Text Full Text PDF PubMed Scopus (2364) Google Scholar). The resulting increased capacity for oxidative protein folding is beneficial for tumor growth. Levels of ERO1α correlate with a worse prognosis in breast cancer (Kutomi et al., 2013Kutomi G. Tamura Y. Tanaka T. Kajiwara T. Kukita K. Ohmura T. Shima H. Takamaru T. Satomi F. Suzuki Y. et al.Human endoplasmic reticulum oxidoreductin 1-alpha is a novel predictor for poor prognosis of breast cancer.Cancer Sci. 2013; 104: 1091-1096Crossref PubMed Scopus (60) Google Scholar), and depleting breast carcinoma cells of PERK increases ROS production and impairs cell growth (Bobrovnikova-Marjon et al., 2010Bobrovnikova-Marjon E. Grigoriadou C. Pytel D. Zhang F. Ye J. Koumenis C. Cavener D. Diehl J.A. PERK promotes cancer cell proliferation and tumor growth by limiting oxidative DNA damage.Oncogene. 2010; 29: 3881-3895Crossref PubMed Scopus (211) Google Scholar). Moreover, the loss of PERK promotes G2/M cell cycle delay due to oxidative damage of DNA (Bobrovnikova-Marjon et al., 2010Bobrovnikova-Marjon E. Grigoriadou C. Pytel D. Zhang F. Ye J. Koumenis C. Cavener D. Diehl J.A. PERK promotes cancer cell proliferation and tumor growth by limiting oxidative DNA damage.Oncogene. 2010; 29: 3881-3895Crossref PubMed Scopus (211) Google Scholar). This PERK-mediated resistance to oxidative stress is also implicated in resistance to radiotherapy (Rouschop et al., 2013Rouschop K.M. Dubois L.J. Keulers T.G. van den Beucken T. Lambin P. Bussink J. van der Kogel A.J. Koritzinsky M. Wouters B.G. PERK/eIF2α signaling protects therapy resistant hypoxic cells through induction of glutathione synthesis and protection against ROS.Proc. Natl. Acad. Sci. USA. 2013; 110: 4622-4627Crossref PubMed Scopus (164) Google Scholar, Rouschop et al., 2010Rouschop K.M. van den Beucken T. Dubois L. Niessen H. Bussink J. Savelkouls K. Keulers T. Mujcic H. Landuyt W. Voncken J.W. et al.The unfolded protein response protects human tumor cells during hypoxia through regulation of the autophagy genes MAP1LC3B and ATG5.J. Clin. Invest. 2010; 120: 127-141Crossref PubMed Scopus (612) Google Scholar), an effect of tumor adaptation to preconditioned ER stress. In addition to the ATF4-dependent antioxidant response, cells can induce antioxidant pathways via Nrf2. This transcription factor is normally held inactive within the cytosol through binding to Keap1, which promotes its ubiquitination by Cul3 and subsequent proteasomal degradation. Upon oxidative stress, Keap1 releases Nrf2 to transactivate target genes within the nucleus. It has been suggested that this interaction is modulated by PERK. Two early reports suggested that Nrf2 could be phosphorylated by PERK during ER stress, triggering dissociation from Keap1 and induction of antioxidant genes (Cullinan and Diehl, 2004Cullinan S.B. Diehl J.A. PERK-dependent activation of Nrf2 contributes to redox homeostasis and cell survival following endoplasmic reticulum stress.J. Biol. Chem. 2004; 279: 20108-20117Crossref PubMed Scopus (564) Google Scholar, Cullinan et al., 2003Cullinan S.B. Zhang D. Hannink M. Arvisais E. Kaufman R.J. Diehl J.A. Nrf2 is a direct PERK substrate and effector of PERK-dependent cell survival.Mol. Cell. Biol. 2003; 23: 7198-7209Crossref PubMed Scopus (934) Google Scholar). Indeed, Nrf2 appears to be beneficial during ER stress-induced oxidative stress. However, activation of PERK’s kinase domain in the absence of ER stress leads to induction of ISR target genes in a manner that is entirely dependent on phosphorylation of eIF2α (Lu et al., 2004Lu P.D. Jousse C. Marciniak S.J. Zhang Y. Novoa I. Scheuner D. Kaufman R.J. Ron D. Harding H.P. Cytoprotection by pre-emptive conditional phosphorylation of translation initiation factor 2.EMBO J. 2004; 23: 169-179Crossref PubMed Scopus (305) Google Scholar). This suggests either that phosphorylation of Nrf2 plays a minor role in the transcriptional response to ER stress or that it is important only when additional arms of the UPR are active. Recent observations suggest that activation of the UPR in hypoxic tumors leads to increased autophagy (Rouschop et al., 2010Rouschop K.M. van den Beucken T. Dubois L. Niessen H. Bussink J. Savelkouls K. Keulers T. Mujcic H. Landuyt W. Voncken J.W. et al.The unfolded protein response protects human tumor cells during hypoxia through regulation of the autophagy genes MAP1LC3B and ATG5.J. Clin. Invest. 2010; 120: 127-141Crossref PubMed Scopus (612) Google Scholar). Autophagy is cytoprotective during stress by liberating amino acids from long-lived proteins and the removal of damaged organelles. Accordingly, hypoxic regions of human tumor xenografts demonstrate increased expression of autophagy factors, such as LC3, and increased autophagy. In multiple cell lines, PERK mediates the upregulation of LC3 and autophagy-related gene 5 via ATF4 and CHOP, respectively, promoting phagophore formation (Rouschop et al., 2010Rouschop K.M. van den Beucken T. Dubois L. Niessen H. Bussink J. Savelkouls K. Keulers T. Mujcic H. Landuyt W. Voncken J.W. et al.The unfolded protein response protects human tumor cells during hypoxia through regulation of the autophagy genes MAP1LC3B and ATG5.J. Clin. Invest. 2010; 120: 127-141Crossref PubMed Scopus (612) Google Scholar). The inability of PERK-deficient cells to replenish LC3 correlates with impaired survival when subjected to hypoxia. Although PERK clearly plays an important role in the survival of hypoxic tumor cells, the IRE1 arm of the UPR is also important. During hypoxia-induced ER stress, IRE1-driven XBP1 splicing (to generate XBP1s) increases tumor cell tolerance to hypoxia, whereas loss of XBP1 impairs hypoxic tumor growth (Romero-Ramirez et al., 2004Romero-Ramirez L. Cao H. Nelson D. Hammond E. Lee A.H. Yoshida H. Mori K. Glimcher L.H. Denko N.C. Giaccia A.J. et al.XBP1 is essential for survival under hypoxic conditions and is required for tumor growth.Cancer Res. 2004; 64: 5943-5947Crossref PubMed Scopus (449) Google Scholar). Indeed, in breast cancer, increased splicing of XBP1 is associated with a worse prognosis, perhaps reflecting an increased tolerance of tumor cells to hypoxia (Davies et al., 2008Davies M.P. Barraclough D.L. Stewart C. Joyce K.A. Eccles R.M. Barraclough R. Rudland P.S. Sibson D.R. Expression and splicing of the unfolded protein response gene XBP-1 are significantly associated with clinical outcome of endocrine-treated breast cancer.Int. J. Cancer. 2008; 123: 85-88Crossref PubMed Scopus (137) Google Scholar). It is well established that tumor hypoxia and glucose deprivation induce angiogenesis. Hypoxia achieves this via HIF, but the mechanism of nutrient limitation has remained obscure until recently. Evidence suggests that the PERK-ATF4 arm of the UPR directly upregulates vascular endothelial growth factor (VEGF) while downregulating inhibitors of angiogenesis (Blais et al., 2006Blais J.D. Addison C.L. Edge R. Falls T. Zhao H. Wary K. Koumenis C. Harding H.P. Ron D. Holcik M. Bell J.C. Perk-dependent translational regulation promotes tumor cell adaptation and angiogenesis in response to hypoxic stress.Mol. Cell. Biol. 2006; 26: 9517-9532Crossref PubMed Scopus (247) Google Scholar, Wang et al., 2012Wang Y. Alam G.N. Ning Y. Visioli F. Dong Z. Nör J.E. Polverini P.J. The unfolded protein response induces the angiogenic switch in human tumor cells through the PERK/ATF4 pathway.Cancer Res. 2012; 72: 5396-5406Crossref PubMed Scopus (130) Google Scholar). Depleting cells of PERK prevents upregulation of VEGF by glucose deprivation, whereas antagonism of HIF1α does not (Wang et al., 2012Wang Y. Alam G.N. Ning Y. Visioli F. Dong Z. Nör J.E. Polverini P.J. The unfolded protein response induces the angiogenic switch in human tumor cells through the PERK/ATF4 pathway.Cancer Res. 2012; 72: 5396-5406Crossref PubMed Scopus (130) Google Scholar). Similarly, inhibition of PERK, which reduces the growth of xenograft tumors in mice, decreases tumor vascularity and perfusion (Wang et al., 2012Wang Y. Alam G.N. Ning Y. Visioli F. Dong Z. Nör J.E. Polverini P.J. The unfolded protein response induces the angiogenic switch in human tumor cells through the PERK/ATF4 pathway.Cancer Res. 2012; 72: 5396-5406Crossref PubMed Scopus (130) Google Scholar). In addition, hypoxia-induced vascularization is modulated by IRE1α (Drogat et al., 2007Drogat B. Auguste P. Nguyen D.T. Bouchecareilh M. Pineau R. Nalbantoglu J. Kaufman R.J. Chevet E. Bikfalvi A. Moenner M. IRE1 signaling is essential for ischemia-induced vascular endothelial growth factor-A expression and contributes to angiogenesis and tumor growth in vivo.Cancer Res. 2007; 67: 6700-6707Crossref PubMed Scopus (177) Google Scholar). Ire1α−/− mouse embryonic fibroblasts and glioblastoma cells expressing dominant-negative IRE1α induce less VEGFA in ischemic conditions, limiting growth and angiogenesis of xenografts. In a human glioma model, it has been demonstrated that IRE1α is involved in the expression of angiogenic factors, including VEGFA and interleukin-6 (IL6), while suppressing the expression of antiangiogenic factors (Auf et al., 2010Auf G. Jabouille A. Guérit S. Pineau R. Delugin M. Bouchecareilh M. Magnin N. Favereaux A. Maitre M. Gaiser T. et al.Inositol-requiring enzyme 1alpha is a key regulator of angiogenesis and invasion in malignant glioma.Proc. Natl. Acad. Sci. USA. 2010; 107: 15553-15558Crossref PubMed Scopus (213) Google Scholar). Consequently, loss of IRE1α impairs glioma growth with increased overall survival of glioma-implanted animals. However, the relationship between IRE1α signaling and angiogenesis appears to be complex since, in nonmalignant models of ischemia, IRE1 has been shown to impair vascular regeneration by degrading mRNA encoding the neurovascular guidance cue netrin-1 via the process of regulated IRE1 dependent decay (RIDD) (Binet et al., 2013Binet F. Mawambo G. Sitaras N. Tetreault N. Lapalme E. Favret S. Cerani A. Leboeuf D. Tremblay S. Rezende F. et al.Neuronal ER stress impedes myeloid-cell-induced vascular regeneration through IRE1α degradation of netrin-1.Cell Metab. 2013; 17: 353-371Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar). It is surprising that, although antagonism of IRE1 can impair tumor vascularity and improve survival, it may promote tumor invasion (Auf et al., 2010Auf G. Jabouille A. Guérit S. Pineau R. Delugin M. Bouchecareilh M. Magnin N. Favereaux A. Maitre M. Gaiser T. et al.Inositol-requiring enzyme 1alpha is a key regulator of angiogenesis and invasion in malignant glioma.Proc. Natl. Acad. Sci. USA. 2010; 107: 15553-15558Crossref PubMed Scopus (213) Google Scholar). Hypovascularity may contribute to invasiveness, but a more complex picture is likely, as the induction of angiogensis does not fully suppress the infiltrative properties of IRE1-deficient glioma cells. Recent analysis has revealed that the increased migratory phenotype likely reflects changes in the secretome via a reduction in RIDD (Dejeans et al., 2012bDejeans N. Pluquet O. Lhomond S. Grise F. Bouchecareilh M. Juin A. Meynard-Cadars M. Bidaud-Meynard A. Gentil C. Moreau V. et al.Autocrine control of glioma cells adhesion and migration through IRE1α-mediated cleavage of SPARC mRNA.J. Cell Sci. 2012; 125: 4278-4287Crossref PubMed Scopus (91) Google Scholar). For example, antagonism of IRE1 increases levels of the RIDD target BM-40, promoting cell adhesion and migration (Dejeans et al., 2012bDejeans N. Pluquet O. Lhomond S. Grise F. Bouchecareilh M. Juin A. Meynard-Cadars M. Bidaud-Meynard A. Gentil C. Moreau V. et al.Autocrine control of glioma cells adhesion and migration through IRE1α-mediated cleavage of SPARC mRNA.J. Cell Sci. 2012; 125: 4278-4287Crossref PubMed Scopus (91) Google Scholar). This suggests that, while suppression of IRE1 signaling may offer a novel approach to target tumor vascularization, it also risks promoting tumor invasion and so deserves further study. Tumor invasion is also influenced by epithelial-to-mesenchymal transition (EMT), a known characteristic of ER-stressed cells. During embryonic development and in malignancy, HIF1α and Notch signaling link hypoxia with EMT, causing loss of epithelial integrity through downregulation of adhesion molecules such as E-cadherin (Lester et al., 2007Lester R.D. Jo M. Montel V. Takimoto S. Gonias S.L. uPAR induces epithelial-mesenchymal transition in hypoxic breast cancer cells.J. Cell Biol. 2007; 178: 425-436Crossref PubMed Scopus (220) Google Scholar, Sahlgren et al., 2008Sahlgren C. Gustafsson M.V. Jin S. Poellinger L. Lendahl U. Notch signaling mediates hypoxia-induced tumor cell migration and invasion.Proc. Natl. Acad. Sci. USA. 2008; 105: 6392-6397Crossref PubMed Scopus (648) Google Scholar). Simultaneously, increased chemotaxis accompanies the induction of the chemokine receptor CXCR4 (Azab et al., 2012Azab A.K. Hu J. Quang P. Azab F. Pitsillides C. Awwad R. Thompson B. Maiso P. Sun J.D. Hart C.P. et al.Hypoxia promotes dissemination of multiple myeloma through acquisition of epithelial to mesenchymal transition-like features.Blood. 2012; 119: 5782-5794Crossref PubMed Scopus (232) Google Scholar, Barriga et al., 2013Barriga E.H. Maxwell P.H. Reyes A.E. Mayor R. The hypoxia factor Hif-1α controls neural crest chemotaxis and epithelial to mesenchymal transition.J. Cell Biol. 2013; 201: 759-776Crossref PubMed Scopus (95) Google Scholar). Thus, EMT promotes metastasis by removing impediments to the egress of cells from their original tumor while also honing them to new niches (reviewed in Hanahan and Weinberg, 2011Hanahan D. Weinberg R.A. Hallmarks of cancer: the next generation.Cell. 2011; 144: 646-674Abstract Full Text Full Text PDF PubMed Scopus (42748) Google Scholar). ER stress has been shown to drive EMT in vitro and in animal models of fibrosis through src-mediated signaling (Tanjore et al., 2011Tanjore H. Cheng D.S. Degryse A.L. Zoz D.F. Abdolrasulnia R. Lawson W.E. Blackwell T.S. Alveolar epithelial cells undergo epithelial-to-mesenchymal transition in response to endoplasmic reticulum stress.J. Biol. Chem. 2011; 286: 30972-30980Crossref PubMed Scopus (180) Google Scholar, Ulianich et al., 2008Ulianich L. Garbi C. Treglia A.S. Punzi D. Miele C. Raciti G.A. Beguinot F. Consiglio E. Di Jeso B. ER stress is associated with dedifferentiation and an epithelial-to-mesenchymal transition-like phenotype in PC Cl3 thyroid cells.J. Cell Sci. 2008; 121: 477-486Crossref PubMed Scopus (102) Google Scholar). It is therefore plausible that ER stress may contribute to EMT in cancer invasion, although more formal examinations of this are needed. A further consideration is that phenotypic change from epithelium to mesenchyme will affect the secretory capacity of a cell, thus altering its vulnerability to ER stress. Consistent with this, evidence suggests that expression of the ER stress markers CHOP and GADD34 change during dedifferentiation of mesothelioma cells (Dalton et al., 2013Dalton L.E. Clarke H.J. Knight J. Lawson M.H. Wason J. Lomas D.A. Howat W.J. Rintoul R.C. Rassl D.M. Marciniak S.J. The endoplasmic reticulum stress marker CHOP predicts survival in malignant mesothelioma.Br. J. Cancer. 2013; 108: 1340-1347Crossref PubMed Scopus (47) Google Scholar). Through a proteostatic network, impaired protein folding in one cellular location leads to the propagation of cell-wide responses. The interplay between the mitochondrial HSP90 chaperone networks and the protein-folding environment of the ER exemplifies such a mechanism. HSP90 and its related chaperone, TRAP-1, are abundant in the mitochondria of tumor cells but not in those of most normal tissues, and they appear to antagonize mitochondrial death pathways (Chae et al., 2012Chae Y.C. Caino M.C. Lisanti S. Ghosh J.C. Dohi T. Danial N.N. Villanueva J. Ferrero S. Vaira V. Santambrogio L. et al.Control of tumor bioenergetics and survival stress signaling by mitochondrial HSP90s.Cancer Cell. 2012; 22: 331-344Abstract Full Text Full Text PDF PubMed Scopus (93) Google Scholar). It is not surprising that impaired function of mitochondrial HSP90 leads to a mitochondrial UPR and the induction of autophagy (Siegelin et al., 2011Siegelin M.D. Dohi T. Raskett C.M. Orlowski G.M. Powers C.M. Gilbert C.A. Ross A.H. Plescia J. Altieri D.C. Exploiting the mitochondrial unfolded protein response for cancer therapy in mice and human cells.J. Clin. Invest. 2011; 121: 1349-1360Crossref PubMed Scopus (120) Google Scholar). More recently, it has been shown that inhibition of mitochondrial HSP90 using the small molecule gamitrinib disrupts tumor bioenergetics to such an extent that ER stress pathways are activated (Chae et al., 2012Chae Y.C. Caino M.C. Lisanti S. Ghosh J.C. Dohi T. Danial N.N. Villanueva J. Ferrero S. Vaira V. Santambrogio L. et al.Control of tumor bioenergetics and survival stress signaling by mitochondrial HSP90s.Cancer Cell. 2012; 22: 331-344Abstract Full Text Full Text PDF PubMed Scopus (93) Google Scholar). Notably, activation of the classical UPR of the ER was necessary for survival of mitochondrial proteotoxicity. Direct communication between the mitochondrion and ER during stress serves to modulate the function of both organelles. PERK is enriched at mitochondrial-ER contact sites and appears to tether mitochondria to the ER membrane (Figure 3) (Verfaillie et al., 2012Verfaillie T. Rubio N. Garg A.D. Bultynck G. Rizzuto R. Decuypere J.P. Piette J. Linehan C. Gupta S. Samali A. Agostinis P. PERK is required at the ER-mitochondrial contact sites to convey apoptosis after ROS-based ER stress.Cell Death Differ. 2012; 19: 1880-1891Crossref PubMed Scopus (500) Google Scholar). Mitofusin 2 (Mfn2), a GTPase of the mitochondrial outer membrane that mediates mitochondrial fusion, has recently been shown to bind directly to PERK (Muñoz et al., 2013Muñoz J.P. Ivanova S. Sánchez-Wandelmer J. Martínez-Cristóbal P. Noguera E. Sancho A. Díaz-Ramos A. Hernández-Alvarez M.I. Sebastián D. Mauvezin C. et al.Mfn2 modulates the UPR and mitochondrial function via repression of PERK.EMBO J. 2013; 32: 2348-2361Crossref PubMed Scopus (252) Google Scholar). Because cells lacking Mfn2 experience basal activation of PERK, it has been suggested that Mfn2 may normally inhibit PERK signaling. However, enhanced signaling in all three branches of the UPR in Mfn2−/− cells is difficult to explain by this dysinhibition of PERK alone, since exaggerated phosphorylation of eIF2α would reduce ER stress by attenuating protein translation. It therefore seems likely that ER-mitochondrial signaling is affected more extensively. Indeed, PERK modulates mitochondrial morphology and function, with overexpression of PERK causing mitochondrial fragmentation and reduced respiration, while depletion of PERK reduces mitochondrial calcium overload and ROS production in Mfn2-deficient cells (Muñoz et al., 2013Muñoz J.P. Ivanova S. Sánchez-Wandelmer J. Martínez-Cristóbal P. Noguera E. Sancho A. Díaz-Ramos A. Hernández-Alvarez M.I. Sebastián D. Mauvezin C. et al.Mfn2 modulates the UPR and mitochondrial function via repression of PERK.EMBO J. 2013; 32: 2348-2361Crossref PubMed Scopus (252) Google Scholar). It is interesting that the tethering function of PERK appears independent of its kinase activity and facilitates ROS-mediated proapoptotic signaling be