Tumorigenic and Immunosuppressive Effects of Endoplasmic Reticulum Stress in Cancer

Malignant cells utilize diverse strategies that enable them to thrive under adverse conditions while simultaneously inhibiting the development of anti-tumor immune responses. Hostile microenvironmental conditions within tumor masses, such as nutrient deprivation, oxygen limitation, high metabolic demand, and oxidative stress, disturb the protein-folding capacity of the endoplasmic reticulum (ER), thereby provoking a cellular state of “ER stress.” Sustained activation of ER stress sensors endows malignant cells with greater tumorigenic, metastatic, and drug-resistant capacity. Additionally, recent studies have uncovered that ER stress responses further impede the development of protective anti-cancer immunity by manipulating the function of myeloid cells in the tumor microenvironment. Here, we discuss the tumorigenic and immunoregulatory effects of ER stress in cancer, and we explore the concept of targeting ER stress responses to enhance the efficacy of standard chemotherapies and evolving cancer immunotherapies in the clinic. Malignant cells utilize diverse strategies that enable them to thrive under adverse conditions while simultaneously inhibiting the development of anti-tumor immune responses. Hostile microenvironmental conditions within tumor masses, such as nutrient deprivation, oxygen limitation, high metabolic demand, and oxidative stress, disturb the protein-folding capacity of the endoplasmic reticulum (ER), thereby provoking a cellular state of “ER stress.” Sustained activation of ER stress sensors endows malignant cells with greater tumorigenic, metastatic, and drug-resistant capacity. Additionally, recent studies have uncovered that ER stress responses further impede the development of protective anti-cancer immunity by manipulating the function of myeloid cells in the tumor microenvironment. Here, we discuss the tumorigenic and immunoregulatory effects of ER stress in cancer, and we explore the concept of targeting ER stress responses to enhance the efficacy of standard chemotherapies and evolving cancer immunotherapies in the clinic. Tumor growth persists despite many cell-intrinsic and cell-extrinsic stresses, including dysregulated proliferation, oxidative stress, nutrient and lipid deprivation, hypoxia, and acidic extracellular pH. Tumor progression despite these challenges requires frequent adaptation. The endoplasmic reticulum (ER) regulates this adaptive capacity by coordinating a wide array of fundamental cellular processes, including transmembrane and secretory protein folding, lipid biosynthesis, drug detoxification, and calcium storage and signaling. At steady state, the ER protein-folding machinery readily handles secretory pathway requirements. However, if misfolded proteins accumulate beyond a tolerable threshold, ER-resident sensors trigger an unfolded protein response (UPR) to transcriptionally and translationally improve ER protein-folding capacity. If these corrective efforts are insufficient, the cell will undergo apoptosis (Wang and Kaufman, 2014Wang M. Kaufman R.J. The impact of the endoplasmic reticulum protein-folding environment on cancer development.Nat. Rev. Cancer. 2014; 14: 581-597Crossref PubMed Scopus (221) Google Scholar). Despite these potentially fatal outcomes, robust ER stress responses have been documented in most major types of human cancer, including breast, pancreatic, lung, skin, prostate, brain, and even liquid malignancies (Wang and Kaufman, 2014Wang M. Kaufman R.J. The impact of the endoplasmic reticulum protein-folding environment on cancer development.Nat. Rev. Cancer. 2014; 14: 581-597Crossref PubMed Scopus (221) Google Scholar). Furthermore, ER stress in situ frequently correlates with advanced-stage disease and chemoresistance. The ability to tolerate persistent ER stress enhances cancer cell survival, angiogenesis, metastatic capacity, drug resistance, and immunosuppression. Yet this risky balancing act also endows cancer cells with selective vulnerabilities that could be harnessed to therapeutic advantage. In this review, we explore the causes and consequences of ER stress in malignancy within individual tumor cells and across their larger microenvironments. Detecting and resolving ER stress requires three major ER-spanning transmembrane proteins, inositol-requiring enzyme 1α (IRE1α) (encoded by ERN1), PKR-like ER kinase (PERK) (encoded by EIF2AK3), and activating transcription factor 6α (ATF6α) (encoded by ATF6). These sensors exhibit a broadly similar activation mechanism and regulate many unique and overlapping facets of the ER stress response. Each is bound intraluminally by the chaperone protein BiP (encoded by HSPA5), which locks them in monomeric, inactive states. If the level of intraluminal misfolded proteins exceeds the folding capacity of ER-resident chaperones, glycosylases, and oxido-reductases, BiP dissociates from IRE1α, PERK, and ATF6α (Bertolotti et al., 2000Bertolotti A. Zhang Y. Hendershot L.M. Harding H.P. Ron D. Dynamic interaction of BiP and ER stress transducers in the unfolded-protein response.Nat. Cell Biol. 2000; 2: 326-332Crossref PubMed Scopus (1380) Google Scholar, Shen et al., 2002Shen J. Chen X. Hendershot L. Prywes R. ER stress regulation of ATF6 localization by dissociation of BiP/GRP78 binding and unmasking of Golgi localization signals.Dev. Cell. 2002; 3: 99-111Abstract Full Text Full Text PDF PubMed Scopus (702) Google Scholar). These sensors subsequently drive mutually reinforcing signaling pathways to correct the protein-misfolding stress. If the burden can be reduced quickly, the cells successfully adapt to the insult, whereas insufficient clearance results in apoptotic cell death. IRE1α is a highly conserved dual enzyme possessing both kinase and endoribonuclease activity. After BiP dissociation, IRE1α dimerizes and autophosphorylates, triggering a conformational shift that allosterically activates its endoribonuclease domain. This nuclease then catalyzes a unique cytoplasmic mRNA-splicing reaction, specifically cleaving out 26 nucleotides from the XBP1 mRNA, which is subsequently re-ligated by the tRNA ligase RCTB (Lu et al., 2014Lu Y. Liang F.X. Wang X. A synthetic biology approach identifies the mammalian UPR RNA ligase RtcB.Mol. Cell. 2014; 55: 758-770Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar, Yoshida et al., 2001Yoshida H. Matsui T. Yamamoto A. Okada T. Mori K. XBP1 mRNA is induced by ATF6 and spliced by IRE1 in response to ER stress to produce a highly active transcription factor.Cell. 2001; 107: 881-891Abstract Full Text Full Text PDF PubMed Scopus (1963) Google Scholar). Re-ligation causes a reading frameshift and translation of the highly active transcription factor XBP1, which upregulates multiple foldases, oxido-reductases, intracellular trafficking components, ER-associated degradation machinery, and glycosylases to correct ER homeostasis (Shoulders et al., 2013Shoulders M.D. Ryno L.M. Genereux J.C. Moresco J.J. Tu P.G. Wu C. Yates 3rd, J.R. Su A.I. Kelly J.W. Wiseman R.L. Stress-independent activation of XBP1s and/or ATF6 reveals three functionally diverse ER proteostasis environments.Cell Rep. 2013; 3: 1279-1292Abstract Full Text Full Text PDF PubMed Scopus (122) Google Scholar). XBP1 also upregulates UPR-independent pathways, including pro-inflammatory cytokine production, lipid and hexosamine biosynthesis, and the hypoxia response (Bettigole and Glimcher, 2015Bettigole S.E. Glimcher L.H. Endoplasmic reticulum stress in immunity.Annu. Rev. Immunol. 2015; 33: 107-138Crossref PubMed Scopus (46) Google Scholar). XBP1 induction favors cell survival, as enforced overexpression rescues cell viability in vitro and in a transgenic rat model of retinitis pigmentosa (Lin et al., 2007Lin J.H. Li H. Yasumura D. Cohen H.R. Zhang C. Panning B. Shokat K.M. Lavail M.M. Walter P. IRE1 signaling affects cell fate during the unfolded protein response.Science. 2007; 318: 944-949Crossref PubMed Scopus (731) Google Scholar). However, under severe ER stress, IRE1α can also oligomerize and sequence-specifically degrade multiple ER-localized mRNAs and microRNAs in a pro-apoptotic process known as regulated IRE1α-dependent decay (RIDD) (Hollien et al., 2009Hollien J. Lin J.H. Li H. Stevens N. Walter P. Weissman J.S. Regulated Ire1-dependent decay of messenger RNAs in mammalian cells.J. Cell Biol. 2009; 186: 323-331Crossref PubMed Scopus (393) Google Scholar, Lerner et al., 2012Lerner A.G. Upton J.P. Praveen P.V. Ghosh R. Nakagawa Y. Igbaria A. Shen S. Nguyen V. Backes B.J. Heiman M. et al.IRE1α induces thioredoxin-interacting protein to activate the NLRP3 inflammasome and promote programmed cell death under irremediable ER stress.Cell Metab. 2012; 16: 250-264Abstract Full Text Full Text PDF PubMed Scopus (246) Google Scholar). Independently of its endoribonuclease function, phosphorylated IRE1α recruits TRAF2 to facilitate JNK and NFκB activation upon pharmacological ER stress (Tam et al., 2012Tam A.B. Mercado E.L. Hoffmann A. Niwa M. ER stress activates NF-κB by integrating functions of basal IKK activity, IRE1 and PERK.PLoS ONE. 2012; 7: e45078Crossref PubMed Scopus (0) Google Scholar, Urano et al., 2000Urano F. Wang X. Bertolotti A. Zhang Y. Chung P. Harding H.P. Ron D. Coupling of stress in the ER to activation of JNK protein kinases by transmembrane protein kinase IRE1.Science. 2000; 287: 664-666Crossref PubMed Scopus (1594) Google Scholar). Similarly, IRE1α constitutively associates with the transcription factor STAT3 in mouse primary hepatocytes, and this interaction is crucial for enhancing STAT3 phosphorylation both in vitro and in vivo (Liu et al., 2015Liu Y. Shao M. Wu Y. Yan C. Jiang S. Liu J. Dai J. Yang L. Li J. Jia W. et al.Role for the endoplasmic reticulum stress sensor IRE1α in liver regenerative responses.J. Hepatol. 2015; 62: 590-598Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar). IRE1α is thus well positioned to influence several key regulators of tumorigenesis independently of XBP1. Interestingly, decreased ER membrane fluidity resulting from increased ER phospholipid saturation induces IRE1α activation by forcing transmembrane domains of neighboring IRE1α monomers into contact. Exogenous saturated lipids, such as palmitate, or loss of lipid desaturases like SCD1 perturbs ER membrane composition and can induce this activation mode (Volmer et al., 2013Volmer R. van der Ploeg K. Ron D. Membrane lipid saturation activates endoplasmic reticulum unfolded protein response transducers through their transmembrane domains.Proc. Natl. Acad. Sci. USA. 2013; 110: 4628-4633Crossref PubMed Scopus (144) Google Scholar). Elegant studies using truncated IRE1α variants unable to bind BiP revealed that this lipid-mediated activation mode proceeds in the absence of misfolded proteins. Interestingly, IRE1α cannot oligomerize when activated by membrane lipid saturation, potentially favoring cell survival outcomes (Kitai et al., 2013Kitai Y. Ariyama H. Kono N. Oikawa D. Iwawaki T. Arai H. Membrane lipid saturation activates IRE1α without inducing clustering.Genes Cells. 2013; 18: 798-809Crossref PubMed Scopus (26) Google Scholar). However, given the immense difficulty in quantifying intracellular protein misfolding, the relative contribution of membrane rigidity versus unfolded protein accumulation to IRE1α activation in vivo remains unknown. Like IRE1α, PERK homodimerizes and autophosphorylates upon BiP dissociation or reduced ER membrane fluidity (Volmer et al., 2013Volmer R. van der Ploeg K. Ron D. Membrane lipid saturation activates endoplasmic reticulum unfolded protein response transducers through their transmembrane domains.Proc. Natl. Acad. Sci. USA. 2013; 110: 4628-4633Crossref PubMed Scopus (144) Google Scholar). Activated PERK phosphorylates the translation initiation factor eIF2α (encoded by EIF2S1), which reduces the influx of nascent proteins into the ER by restricting 5′ cap-dependent mRNA translation (Harding et al., 1999Harding H.P. Zhang Y. Ron D. Protein translation and folding are coupled by an endoplasmic-reticulum-resident kinase.Nature. 1999; 397: 271-274Crossref PubMed Scopus (1776) Google Scholar). Reduced translation rates facilitate focused refolding efforts by ER-localized chaperones. Paradoxically, global translational inhibition increases selective translation of the transcription factor ATF4, which directly upregulates the transcription factor C/EBP-homologous protein (CHOP) (encoded by DDIT3). Subsequently, ATF4 and CHOP cooperatively induce multiple genes involved in amino acid biosynthesis, amino acid transport, and the intracellular recycling system autophagy (B’chir et al., 2013B’chir W. Maurin A.C. Carraro V. Averous J. Jousse C. Muranishi Y. Parry L. Stepien G. Fafournoux P. Bruhat A. The eIF2α/ATF4 pathway is essential for stress-induced autophagy gene expression.Nucleic Acids Res. 2013; 41: 7683-7699Crossref PubMed Scopus (183) Google Scholar, Han et al., 2013Han J. Back S.H. Hur J. Lin Y.H. Gildersleeve R. Shan J. Yuan C.L. Krokowski D. Wang S. Hatzoglou M. et al.ER-stress-induced transcriptional regulation increases protein synthesis leading to cell death.Nat. Cell Biol. 2013; 15: 481-490Crossref PubMed Scopus (398) Google Scholar). Accelerated amino acid biosynthesis generates significant amounts of reactive oxygen species (ROS), which induces apoptosis if left unabated. However, PERK limits ROS accumulation by phosphorylating and stabilizing NRF2 (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 (576) Google Scholar), enhancing glutathione synthesis (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 (0) Google Scholar), and upregulating heme oxygenase-1 (HO-1) (Dey et al., 2015Dey S. Sayers C.M. Verginadis I.I. Lehman S.L. Cheng Y. Cerniglia G.J. Tuttle S.W. Feldman M.D. Zhang P.J. Fuchs S.Y. et al.ATF4-dependent induction of heme oxygenase 1 prevents anoikis and promotes metastasis.J. Clin. Invest. 2015; 125: 2592-2608Crossref PubMed Scopus (50) Google Scholar). PERK also activates NFκB by repressing translation of the NFκB inhibitor IκBα (Tam et al., 2012Tam A.B. Mercado E.L. Hoffmann A. Niwa M. ER stress activates NF-κB by integrating functions of basal IKK activity, IRE1 and PERK.PLoS ONE. 2012; 7: e45078Crossref PubMed Scopus (0) Google Scholar). After BiP dissociation, ATF6α translocates to the Golgi apparatus, where it is cleaved intramembranously by site 1 and site 2 proteases to liberate an active, soluble ATF6α transcription factor (Shen et al., 2002Shen J. Chen X. Hendershot L. Prywes R. ER stress regulation of ATF6 localization by dissociation of BiP/GRP78 binding and unmasking of Golgi localization signals.Dev. Cell. 2002; 3: 99-111Abstract Full Text Full Text PDF PubMed Scopus (702) Google Scholar). Disulfide bonding additionally regulates this ER-to-Golgi trafficking, and only monomeric, reduced ATF6α can properly access COPII endosomes (Schindler and Schekman, 2009Schindler A.J. Schekman R. In vitro reconstitution of ER-stress induced ATF6 transport in COPII vesicles.Proc. Natl. Acad. Sci. USA. 2009; 106: 17775-17780Crossref PubMed Scopus (0) Google Scholar). Unlike PERK and IRE1α, reduced ER membrane fluidity does not activate ATF6α, perhaps because dimerization is unfavorable for ATF6α activation. ATF6α fine-tunes the UPR by upregulating BiP and a subset of XBP1-dependent chaperones, oxidoreductases, and quality control and degradation machinery (Shoulders et al., 2013Shoulders M.D. Ryno L.M. Genereux J.C. Moresco J.J. Tu P.G. Wu C. Yates 3rd, J.R. Su A.I. Kelly J.W. Wiseman R.L. Stress-independent activation of XBP1s and/or ATF6 reveals three functionally diverse ER proteostasis environments.Cell Rep. 2013; 3: 1279-1292Abstract Full Text Full Text PDF PubMed Scopus (122) Google Scholar). Whereas IRE1α and PERK conditional or germline knockout mice often exhibit pronounced phenotypes, ATF6α germline knockout mice only yield clear phenotypes under pharmacological or pathological stresses (Yamamoto et al., 2010Yamamoto K. Takahara K. Oyadomari S. Okada T. Sato T. Harada A. Mori K. Induction of liver steatosis and lipid droplet formation in ATF6alpha-knockout mice burdened with pharmacological endoplasmic reticulum stress.Mol. Biol. Cell. 2010; 21: 2975-2986Crossref PubMed Scopus (116) Google Scholar), suggesting that ATF6α fine-tunes the UPR, which is largely controlled by the more dominant PERK/IRE1α responses. Multiple cell-intrinsic and cell-extrinsic mechanisms initiate and amplify ER stress within the cancer cell and the larger tumor microenvironment (Figure 1). Spatiotemporal differences in ER stress burden, driven by genetic, epigenetic, and microenvironmental heterogeneity, likely result in a range of pro-survival and pro-apoptotic responses. Anticancer interventions, such as chemotherapy, can also modulate UPR signaling, though the clinical implications are only beginning to be understood. Cancer initiation and development require both inactivation of tumor suppressors and/or the acquisition of oncogenic mutations that uncouple proliferation from extracellular, growth factor-mediated regulation. Transformation-associated increases in protein synthesis often overwhelm ER protein-folding capacity. In particular, highly secretory cancers, such as the B cell malignancy multiple myeloma, which produces extremely high levels of immunoglobulins, often undergo persistent ER stress (Obeng et al., 2006Obeng E.A. Carlson L.M. Gutman D.M. Harrington Jr., W.J. Lee K.P. Boise L.H. Proteasome inhibitors induce a terminal unfolded protein response in multiple myeloma cells.Blood. 2006; 107: 4907-4916Crossref PubMed Scopus (532) Google Scholar). Specific cellular behaviors can also influence protein secretory rates, as evidenced by PERK activation during epithelial-mesenchymal transition (EMT) (Feng et al., 2014Feng Y.X. Sokol E.S. Del Vecchio C.A. Sanduja S. Claessen J.H. Proia T.A. Jin D.X. Reinhardt F. Ploegh H.L. Wang Q. Gupta P.B. Epithelial-to-mesenchymal transition activates PERK-eIF2α and sensitizes cells to endoplasmic reticulum stress.Cancer Discov. 2014; 4: 702-715Crossref PubMed Scopus (56) Google Scholar). Similarly, oncogenic transformation driven by loss of the tumor suppressors p53, PTEN, TSC1, or TSC2 dramatically enhances protein synthesis rates, leading to ER stress (Hart et al., 2012Hart L.S. Cunningham J.T. Datta T. Dey S. Tameire F. Lehman S.L. Qiu B. Zhang H. Cerniglia G. Bi M. et al.ER stress-mediated autophagy promotes Myc-dependent transformation and tumor growth.J. Clin. Invest. 2012; 122: 4621-4634Crossref PubMed Scopus (133) Google Scholar, Namba et al., 2015Namba T. Chu K. Kodama R. Byun S. Yoon K.W. Hiraki M. Mandinova A. Lee S.W. Loss of p53 enhances the function of the endoplasmic reticulum through activation of the IRE1α/XBP1 pathway.Oncotarget. 2015; 6: 19990-20001Crossref PubMed Google Scholar, Signer et al., 2014Signer R.A. Magee J.A. Salic A. Morrison S.J. Haematopoietic stem cells require a highly regulated protein synthesis rate.Nature. 2014; 509: 49-54Crossref PubMed Scopus (99) Google Scholar). Enhanced protein synthesis and concomitant ER stress are also observed upon overexpression of oncogenic HRAS (G12E), BRAF (V600E), c-Myc, or Src (Chen et al., 2014Chen X. Iliopoulos D. Zhang Q. Tang Q. Greenblatt M.B. Hatziapostolou M. Lim E. Tam W.L. Ni M. Chen Y. et al.XBP1 promotes triple-negative breast cancer by controlling the HIF1α pathway.Nature. 2014; 508: 103-107Crossref PubMed Scopus (189) Google Scholar, Corazzari et al., 2015Corazzari M. Rapino F. Ciccosanti F. Giglio P. Antonioli M. Conti B. Fimia G.M. Lovat P.E. Piacentini M. Oncogenic BRAF induces chronic ER stress condition resulting in increased basal autophagy and apoptotic resistance of cutaneous melanoma.Cell Death Differ. 2015; 22: 946-958Crossref PubMed Scopus (17) Google Scholar, Denoyelle et al., 2006Denoyelle C. Abou-Rjaily G. Bezrookove V. Verhaegen M. Johnson T.M. Fullen D.R. Pointer J.N. Gruber S.B. Su L.D. Nikiforov M.A. et al.Anti-oncogenic role of the endoplasmic reticulum differentially activated by mutations in the MAPK pathway.Nat. Cell Biol. 2006; 8: 1053-1063Crossref PubMed Scopus (178) Google Scholar). Importantly, both the UPR signaling and leukemia development induced by conditional PTEN deletion (Signer et al., 2014Signer R.A. Magee J.A. Salic A. Morrison S.J. Haematopoietic stem cells require a highly regulated protein synthesis rate.Nature. 2014; 509: 49-54Crossref PubMed Scopus (99) Google Scholar) or c-Myc overexpression (Hart et al., 2012Hart L.S. Cunningham J.T. Datta T. Dey S. Tameire F. Lehman S.L. Qiu B. Zhang H. Cerniglia G. Bi M. et al.ER stress-mediated autophagy promotes Myc-dependent transformation and tumor growth.J. Clin. Invest. 2012; 122: 4621-4634Crossref PubMed Scopus (133) Google Scholar) were dramatically reduced or entirely abrogated upon heterozygous deletion of the key translation rate regulator ribosomal protein RPL24. This strongly implicates protein synthesis rate as a key driver of ER stress and tumorigenicity in vivo. However, oncogene expression does not always induce ER stress. In contrast to c-Myc transgene-driven B cell lymphoma (Hart et al., 2012Hart L.S. Cunningham J.T. Datta T. Dey S. Tameire F. Lehman S.L. Qiu B. Zhang H. Cerniglia G. Bi M. et al.ER stress-mediated autophagy promotes Myc-dependent transformation and tumor growth.J. Clin. Invest. 2012; 122: 4621-4634Crossref PubMed Scopus (133) Google Scholar), high MYC expression insulated a large panel of human cancer cell lines from ER stress upon exogenous proline depletion (Sahu et al., 2016Sahu N. Dela Cruz D. Gao M. Sandoval W. Haverty P.M. Liu J. Stephan J.P. Haley B. Classon M. Hatzivassiliou G. et al.Proline starvation induces unresolved ER stress and hinders mTORC1-dependent tumorigenesis.Cell Metab. 2016; 24: 753-761Abstract Full Text Full Text PDF PubMed Google Scholar). Similarly, Ras-transformed, Mychigh cells exhibited low basal ER stress but activated the UPR upon Ras inhibition, suggesting that additional layers of regulation coordinate MYC expression with ER homeostasis (Yaari-Stark et al., 2010Yaari-Stark S. Shaked M. Nevo-Caspi Y. Jacob-Hircsh J. Shamir R. Rechavi G. Kloog Y. Ras inhibits endoplasmic reticulum stress in human cancer cells with amplified Myc.Int. J. Cancer. 2010; 126: 2268-2281PubMed Google Scholar). Furthermore, exogenous desaturated lipids protected TSC2−/− mouse embryonic fibroblasts (MEFs) from ER stress (Young et al., 2013Young R.M. Ackerman D. Quinn Z.L. Mancuso A. Gruber M. Liu L. Giannoukos D.N. Bobrovnikova-Marjon E. Diehl J.A. Keith B. Simon M.C. Dysregulated mTORC1 renders cells critically dependent on desaturated lipids for survival under tumor-like stress.Genes Dev. 2013; 27: 1115-1131Crossref PubMed Scopus (57) Google Scholar). Recently transformed cells may initially undergo ER stress in response to the higher replicative and metabolic demands but can adapt by enhancing steady-state ER protein-folding capacity (Huber et al., 2013Huber A.L. Lebeau J. Guillaumot P. Pétrilli V. Malek M. Chilloux J. Fauvet F. Payen L. Kfoury A. Renno T. et al.p58(IPK)-mediated attenuation of the proapoptotic PERK-CHOP pathway allows malignant progression upon low glucose.Mol. Cell. 2013; 49: 1049-1059Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar). However, de novo genetic mutations and other cell-intrinsic and cell-extrinsic stresses likely contribute to the active UPR observed in most major cancer types (Wang and Kaufman, 2014Wang M. Kaufman R.J. The impact of the endoplasmic reticulum protein-folding environment on cancer development.Nat. Rev. Cancer. 2014; 14: 581-597Crossref PubMed Scopus (221) Google Scholar). Fundamental differences in the experimental approaches used, such as overexpression versus endogenous expression, primary cells versus cell lines, and in vivo versus in vitro models, may have also contributed to these discrepant findings. Future work should address how protein-translation rates and related processes, such as copy number alterations, epigenetic modifications, and microRNA-mediated regulatory mechanisms, influence the prevalence and intensity of ER stress responses in human tumors. Furthermore, identifying genetic defects and cell biological changes that influence the saturated:unsaturated ER phospholipid ratio will help distinguish protein misfolding from lipotoxic sources of ER stress. Nonsynonymous mutations can also directly destabilize intrinsic protein folding, triggering the UPR by overwhelming ER-resident chaperone capacity. Consistent with this, overexpressing certain destabilized smoothened (SMO) mutants induces robust ER stress in Drosophila in vivo (Marada et al., 2013Marada S. Stewart D.P. Bodeen W.J. Han Y.G. Ogden S.K. The unfolded protein response selectively targets active smoothened mutants.Mol. Cell. Biol. 2013; 33: 2375-2387Crossref PubMed Scopus (0) Google Scholar). Solid tumors possess dozens of nonsynonymous mutations, with certain cancers, such as melanoma and lung cancers, harboring upward of 200 mutations (Vogelstein et al., 2013Vogelstein B. Papadopoulos N. Velculescu V.E. Zhou S. Diaz Jr., L.A. Kinzler K.W. Cancer genome landscapes.Science. 2013; 339: 1546-1558Crossref PubMed Scopus (2281) Google Scholar). Identifying the spectrum of protein-destabilizing mutations that can trigger ER stress will help clarify the physiological relevance of this mechanism. The tumor microenvironment (TME) predominantly fuels ER stress via oxygen and nutrient deprivation and acidic waste accumulation, though hypernutrition can also contribute during obesity (Nakagawa et al., 2014Nakagawa H. Umemura A. Taniguchi K. Font-Burgada J. Dhar D. Ogata H. Zhong Z. Valasek M.A. Seki E. Hidalgo J. et al.ER stress cooperates with hypernutrition to trigger TNF-dependent spontaneous HCC development.Cancer Cell. 2014; 26: 331-343Abstract Full Text Full Text PDF PubMed Scopus (90) Google Scholar). Whereas normal cells primarily rely on oxidative phosphorylation or anaerobic glycolysis to generate ATP, cancer cells often favor aerobic glycolysis in a phenomenon known as the Warburg effect. Consequently, rapidly dividing cancer cells aggressively consume glucose and release large quantities of lactic acid waste regardless of local oxygen concentration, which lowers local extracellular pH. Tumors initially rely on resident tissue microvasculature to supply key nutrients and oxygen but eventually must generate their own local neovasculature to sustain growth. Though normal tissues possess highly ordered and efficient vasculature, tumor-generated neovasculature is generally leaky and torturous with slow, inconsistent blood flow. Such intermittent circulation limits nutrient accessibility, oxygen delivery, and waste drainage, thereby driving sporadic, acute hypoxia and lactic acidosis (Vaupel et al., 1989Vaupel P. Kallinowski F. Okunieff P. Blood flow, oxygen and nutrient supply, and metabolic microenvironment of human tumors: a review.Cancer Res. 1989; 49: 6449-6465PubMed Google Scholar). Each of these extracellular conditions can induce ER stress, though responsiveness varies depending on cell type. Low oxygen tension activates complex III of the mitochondrial electron transport chain to increase cytosolic ROS production, required for stabilizing the key hypoxia response transcription factor HIF1α (Guzy et al., 2005Guzy R.D. Hoyos B. Robin E. Chen H. Liu L. Mansfield K.D. Simon M.C. Hammerling U. Schumacker P.T. Mitochondrial complex III is required for hypoxia-induced ROS production and cellular oxygen sensing.Cell Metab. 2005; 1: 401-408Abstract Full Text Full Text PDF PubMed Scopus (776) Google Scholar). ROS can also generate highly reactive peroxidized lipid byproducts, which form destructive covalent adducts with various ER chaperones (Cubillos-Ruiz et al., 2015Cubillos-Ruiz J.R. Silberman P.C. Rutkowski M.R. Chopra S. Perales-Puchalt A. Song M. Zhang S. Bettigole S.E. Gupta D. Holcomb K. et al.ER stress sensor XBP1 controls anti-tumor immunity by disrupting dendritic cell homeostasis.Cell. 2015; 161: 1527-1538Abstract Full Text Full Text PDF PubMed Scopus (83) Google Scholar, Vladykovskaya et al., 2012Vladykovskaya E. Sithu S.D. Haberzettl P. Wickramasinghe N.S. Merchant M.L. Hill B.G. McCracken J. Agarwal A. Dougherty S. Gordon S.A. et al.Lipid peroxidation product 4-hydroxy-trans-2-nonenal causes endothelial activation by inducing endoplasmic reticulum stress.J. Biol. Chem. 2012; 287: 11398-11409Crossref PubMed Scopus (0) Google Scholar). Furthermore, both ER disulphide bond formation and lipid desaturation require molecular oxygen. Nutrient deprivation, particularly of glucose and glutamine, limits metabolic intermediates required for the hexosamine biosynthetic pathway (HBP). The HBP generates substrates for N-linked protein glycosylation, which is required for successful ER protein folding (Huber et al., 2013Huber A.L. Lebeau J. Guillaumot P. Pétrilli V. Malek M. Chilloux J. Fauvet F. Payen L. Kfoury A. Renno T. et al.p58(IPK)-mediated attenuation of the proapoptotic PERK-CHOP pathway allows malignant progression upon low glucose.Mol. Cell. 2013; 49: 1049-1059Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar). Proline starvation can also induce ER stress, potentially by inducing excessive ROS accumulation (Sahu et al., 2016Sahu N. Dela Cruz D. Gao M. Sandoval W. Haverty P.M. Liu J. Stephan J.P. Haley B. Classon M. Hatzivassiliou G. et al.Proline starvation induc