The endoplasmic reticulum (ER) senses both extracellular and intracellular stresses that can disrupt its ability to facilitate the maturation of proteins destined for secretory pathways. The accumulation of misfolded proteins within the ER triggers an adaptive signaling pathway coined the Unfolded Protein Response (UPR). UPR activation contributes to cell adaptation by reducing the rate of protein translation, while increasing the synthesis of chaperones. Although we have gained considerable insight into the mechanisms that regulate gene expression and certain aspects of protein translation, the contribution of micro-RNAs (miRNAs) to UPR-dependent activities has only recently been investigated. Here, we highlight recent insights into the contribution of miRNAs to UPR-dependent cellular adaptive responses.
PERK; IRE1α; ATF6; ER stress; UPR; microRNA; apoptosis
Diacylglycerol (DAG) is a critical second messenger that mediates T cell receptor (TCR)–stimulated signaling. The abundance of DAG is reduced by the diacylglycerol kinases (DGKs), which catalyze the conversion of DAG to phosphatidic acid (PA) and thus inhibit DAG-mediated signaling. In T cells, the predominant DGK isoforms are DGKα and DGKζ, and deletion of the genes encoding either isoform enhances DAG-mediated signaling. We found that DGKζ, but not DGKα, suppressed the development of natural regulatory T (Treg) cells and predominantly mediated Ras and Akt signaling downstream of the TCR. The differential functions of DGKα and DGKζ were not attributable to differences in protein abundance in T cells or in their localization to the contact sites between T cells and antigen-presenting cells. RasGRP1, a key DAG-mediated activator of Ras signaling, associated to a greater extent with DGKζ than with DGKα; however, in silico modeling of TCR-stimulated Ras activation suggested that a difference in RasGRP1 binding affinity was not sufficient to cause differences in the functions of each DGK isoform. Rather, the model suggested that a greater catalytic rate for DGKζ than for DGKα might lead to DGKζ exhibiting increased suppression of Ras-mediated signals compared to DGKα. Consistent with this notion, experimental studies demonstrated that DGKζ was more effective than DGKα at catalyzing the metabolism of DAG to PA after TCR stimulation. The enhanced effective enzymatic production of PA by DGKζ is therefore one possible mechanism underlying the dominant functions of DGKζ in modulating Treg cell development.
Cyclin D1–cyclin-dependent kinase 4/6 (CDK4/6) dysregulation is a major contributor to melanomagenesis. Clinical evidence has revealed that p16INK4A, an allosteric inhibitor of CDK4/6, is inactivated in over half of human melanomas, and numerous animal models have demonstrated that p16INK4A deletion promotes melanoma. FBXO4, a specificity factor for the E3 ligase that directs timely cyclin D1 proteolysis, has not been studied in melanoma. We demonstrate that Fbxo4 deficiency induces Braf-driven melanoma and that this phenotype depends on cyclin D1 accumulation in mice, underscoring the importance of this ubiquitin ligase in tumor suppression. Furthermore, we have identified a substrate-binding mutation, FBXO4 I377M, that selectively disrupts cyclin D1 degradation while preserving proteolysis of the other known FBXO4 substrate, TRF1. The I377M mutation and Fbxo4 deficiency result in nuclear accumulation of cyclin D1, a key transforming neoplastic event. Collectively, these data provide evidence that FBXO4 dysfunction, as a mechanism for cyclin D1 overexpression, is a contributor to human malignancy.
Squamous cell carcinomas (SCCs) with an infiltrative invasion pattern carry a higher risk of treatment failure. Such infiltrative invasion may be mediated by a mesenchymal-like subpopulation of malignant cells that we have previously shown to arise from epithelial to mesenchymal transition (EMT) and resist epidermal growth factor receptor (EGFR) targeting. Here we demonstrate that SCCs with infiltrative, high risk invasion patterns contain abundant mesenchymal-like cells, which are rare in tumors with low risk patterns. This cellular heterogeneity was modeled accurately in three dimensional culture using collagen-embedded SCC spheroids, which revealed distinct invasive fronts created by collective migration of E-cadherin-positive cells versus infiltrative migration of individual mesenchymal-like cells. Because EGFR expression by mesenchymal-like cells was diminished in the spheroid model and in human SCCs, we hypothesized that SCCs shift toward infiltrative invasion mediated by this subpopulation during anti-EGFR therapy. Anti-EGFR treatment of spheroids using erlotinib or cetuximab enhanced infiltrative invasion by targeting collective migration by E-cadherin-positive cells while sparing mesenchymal-like cells; by contrast, spheroid invasion in absence of mesenchymal-like cells was abrogated by erlotinib. Similarly, cetuximab treatment of xenografts containing mesenchymal-like cells created an infiltrative invasive front comprised of this subpopulation, whereas no such shift was observed upon treating xenografts lacking these cells. These results implicate mesenchymal-like SCC cells as key mediators of the infiltrative invasion seen in tumors with locally aggressive behavior. They further demonstrate that EGFR inhibition can promote an infiltrative invasion front comprised of mesenchymal-like cells preferentially in tumors where they are abundant prior to therapy.
pattern of invasion; EGFR inhibition; squamous cell carcinoma; EMT; tumor heterogeneity
Viral and pharmacologic inducers of PKR-like ER kinase (PERK) were shown to accelerate the phosphorylation-dependent degradation of the IFNAR1 chain of the type 1 interferon (IFN) receptor and to limit cell sensitivity to IFN. Here we report that hypoxia can elicit these effects in a PERK-dependent manner. The altered fate of IFNAR1 affected by signaling downstream of PERK depends on phosphorylation of eIF2α and ensuing activation of p38α kinase. Activators of other eIF2α kinases such as PKR or GCN2 are also capable of eliminating IFNAR1 and blunting IFN responses. Modulation of constitutive PKR activity in human breast cancer cells stabilizes IFNAR1 and sensitizes these cells to IFNAR1-dependent anti-tumorigenic effects. Whereas downregulation of IFNAR1 and impaired IFNAR1 signaling can be elicited in response to amino acid deficit, the knockdown of GCN2 in melanoma cells reverses these phenotypes. We propose that, in cancer cells and the tumor microenvironment, activation of diverse eIF2α kinases followed by IFNAR1 downregulation enables multiple cellular components of tumor tissue to evade the direct and indirect anti-tumorigenic effects of Type 1 IFN.
interferon; tumor microenvironment; integrated stress response; PERK; PKR; GCN2; IFNAR1
Esophageal squamous cell carcinoma (ESCC) is one of the most aggressive forms of human cancer with poor prognosis due to late diagnosis and metastasis. Common genomic alterations in ESCC include p53 mutation, p120ctn inactivation, and overexpression of oncogenes such as cyclin D1, EGFR, and c-Met. Using esophageal epithelial cells transformed by the overexpression of EGFR and p53R175H, we find novel evidence of a functional link between p53R175H and the c-Met receptor tyrosine kinase to mediate tumor cell invasion. Increased c-Met receptor activation was observed upon p53R175H expression and enhanced further upon subsequent EGFR overexpression. We inhibited c-Met phosphorylation, resulting in diminished invasion of the genetically transformed primary esophageal epithelial cells (EPC-hTERT-EGFR-p53R175H), suggesting that the mechanism of increased invasiveness upon EGFR and p53R175H expression may be the result of increased c-Met activation. These results suggest that the use of therapeutics directed at c-Met in ESCC and other squamous cell cancers.
p53 mutation; c-Met; esophageal cancer; tumor invasion
Micro-RNAs typically function at the level of post-transcriptional gene silencing within the cytoplasm; however increasing evidence suggests that they may also function in nuclear, Argonaut containing complexes, to directly repress target gene transcription. We have investigated the role of micro-RNAs in mediating endoplasmic reticulum (ER) stress responses. ER stress triggers the activation of three signaling molecules: Ire-1α/β, PERK and ATF6 whose function is to facilitate adaption to the ensuing stress. We demonstrate that PERK induces miR-211, which in turn attenuates stress-dependent expression of the pro-apoptotic transcription factor chop/gadd153. MiR-211 directly targets the proximal chop/gadd153 promoter where it increases histone methylation and represses chop expression. Maximal chop accumulation ultimately correlates with miR-211 down regulation. Our data suggests a model where PERK-dependent miR-211 induction prevents premature chop accumulation and thereby provides a window of opportunity for the cell to re-establish homeostasis prior to apoptotic commitment.
miR-211; histone methylation; PERK; CHOP
Germ line PERK mutations are associated with diabetes mellitus and growth retardation in both rodents and humans. In contrast, late embryonic excision of PERK permits islet development and was found to prevent onset of diabetes, suggesting that PERK may be dispensable in the adult pancreas. To definitively establish the functional role of PERK in adult pancreata, we generated mice harboring a conditional PERK allele in which excision is regulated by tamoxifen administration. Deletion of PERK in either young adult or mature adult mice resulted in hyperglycemia associated with loss of islet and β cell architecture. PERK excision triggered intracellular accumulation of proinsulin and Glut2, massive endoplasmic reticulum (ER) expansion, and compensatory activation of the remaining unfolded-protein response (UPR) signaling pathways specifically in pancreatic tissue. Although PERK excision increased β cell death, this was not a result of decreased proliferation as previously reported. In contrast, a significant and specific increase in β cell proliferation was observed, a result reflecting increased cyclin D1 accumulation. This work demonstrates that contrary to expectations, PERK is required for secretory homeostasis and β cell survival in adult mice.
Cell division depends upon the coordinated action of positive and negative regulatory factors that ensure high fidelity replication of the genome and its equivalent separation into daughter cells following cytokinesis. The role of positive factors such as the cyclin-dependent kinases in promoting cell division is firmly established, as is the function of CDK inhibitors and phosphatases that antagonize CDKs. In addition to these, regulated protein destruction is now appreciated as essential for temporal regulation of cell cycle transitions. Protein degradation serves as an irreversible switch that ensures temporally regulated cell cycle transitions. Signal-dependent regulation of protein degradation is best understood with regard to the 26S proteasome. Proteins are directed to this machine subsequent to enzymatic transfer of a highly conserved small polypeptide, ubiquitin. The focus of this review is the regulatory molecules that direct the regulated attachment of ubiquitin, polyubiquitylation, to proteins destined for degradation as cells transition through the G1 phase into S phase. During the past decade, it has become increasingly apparent that these molecules are critical mediators of normal cell proliferation, and as such, they are frequently deregulated in human cancers.
F-box; cullin; cyclin; CDK
The endoplasmic reticulum (ER) resident PKR-like kinase (PERK) is necessary for Akt activation in response to ER stress. We demonstrate that PERK harbors intrinsic lipid kinase, favoring diacylglycerol (DAG) as a substrate and generating phosphatidic acid (PA). This activity of PERK correlates with activation of mTOR and phosphorylation of Akt on Ser473. PERK lipid kinase activity is regulated in a phosphatidylinositol 3-kinase (PI3K) p85α-dependent manner. Moreover, PERK activity is essential during adipocyte differentiation. Because PA and Akt regulate many cellular functions, including cellular survival, proliferation, migratory responses, and metabolic adaptation, our findings suggest that PERK has a more extensive role in insulin signaling, insulin resistance, obesity, and tumorigenesis than previously thought.
Insulin-like growth factor binding protein 3 (IGFBP3), a hypoxia-inducible gene, regulates a variety of cellular processes including cell proliferation, senescence, apoptosis and epithelial-mesenchymal transition (EMT). IGFBP3 has been linked to the pathogenesis of cancers. Most previous studies focus upon proapoptotic tumor suppressor activities of IGFBP3. Nevertheless, IGFBP3 is overexpressed in certain cancers including esophageal squamous cell carcinoma (ESCC), one of the most aggressive forms of squamous cell carcinomas (SCCs). The tumor-promoting activities of IGFBP3 remain poorly understood in part due to a lack of understanding as to how the tumor microenvironment may influence IGFBP3 expression and how IGFBP3 may in turn influence heterogeneous intratumoral cell populations. Here, we show that IGFBP3 overexpression is associated with poor postsurgical prognosis in ESCC patients. In xenograft transplantation models with genetically engineered ESCC cells, IGFBP3 contributes to tumor progression with a concurrent induction of a subset of tumor cells showing high expression of CD44 (CD44H), a major cell surface receptor for hyaluronic acid, implicated in invasion, metastasis and drug resistance. Our gain-of-function and loss-of-function experiments reveal that IGFBP3 mediates the induction of intratumoral CD44H cells. IGFBP3 cooperates with hypoxia to mediate the induction of CD44H cells by suppressing reactive oxygen species (ROS) in an insulin-like growth factor-independent fashion. Thus, our study sheds light on the growth stimulatory functions of IGFPB3 in cancer, gaining a novel mechanistic insight into the functional interplay between the tumor microenvironment and IGFBP3.
CD44; esophageal; squamous cell carcinoma; hypoxia; IGFBP3 and reactive oxygen species
Background & Aims
The Notch receptor family regulates cell fate through cell-cell communication. CSL (CBF-1/RBP-jκ, Su(H), Lag-1) drives canonical Notch-mediated gene transcription during cell lineage specification, differentiation and proliferation in the hematopoietic system, the intestine, the pancreas and the skin. However, the functional roles of Notch in esophageal squamous epithelial biology remain unknown.
Normal esophageal keratinocytes were stimulated with calcium chloride to induce terminal differentiation. The squamous epithelia were reconstituted in organotypic three-dimensional culture, a form of human tissue engineering. Notch was inhibited in culture with a γ-secretase inhibitor or dominant negative mastermind-like1 (DNMAML1). The roles of Notch receptors were evaluated by in vitro gain-of-function and loss-of-function experiments. Additionally, DNMAML1 was targeted to the mouse esophagus by cytokeratin K14 promoter-driven Cre (K14Cre) recombination of Lox-STOP-Lox-DNMAML1. Notch-regulated gene expression was determined by reporter transfection, chromatin immunoprecipitation (ChIP) assays, quantitative reverse-transcription polymerase chain reactions (RT-PCR), Western blotting, immunofluorescence and immunohistochemistry.
NOTCH1 (N1) was activated at the onset of squamous differentiation in the esophagus. Intracellular domain of N1 (ICN1) directly activated NOTCH3 (N3) transcription, inducing HES5 and early differentiation markers such as involucrin (IVL) and cytokeratin CK13 in a CSL-dependent fashion. N3 enhanced ICN1 activity and was required for squamous differentiation. Loss of Notch signaling in K14Cre;DNMAML1 mice perturbed esophageal squamous differentiation and resulted in N3 loss and basal cell hyperplasia.
Notch signaling is important for esophageal epithelial homeostasis. In particular, the crosstalk of N3 with N1 during differentiation provides novel, mechanistic insights into Notch signaling and squamous epithelial biology.
NOTCH1; NOTCH3; esophageal epithelium; squamous differentiation
Adhesion to the extracellular matrix (ECM) is critical for epithelial tissue homeostasis and function. ECM detachment induces metabolic stress and programmed cell death via anoikis. ECM-detached mammary epithelial cells are able to rapidly activate autophagy allowing for survival and an opportunity for re-attachment. However, the mechanisms controlling detachment-induced autophagy remain unclear. Here we uncover that the kinase PERK rapidly promotes autophagy in ECM-detached cells by activating AMP-activated protein kinase (AMPK), resulting in downstream inhibition of mTORC1-p70S6K signaling. LKB1 and TSC2, but not TSC1, are required for PERK-mediated inhibition of mammalian target of rapamycinin MCF10A cells and mouse embryo fibroblast cells. Importantly, this pathway shows fast kinetics, is transcription-independent and is exclusively activated during ECM detachment, but not by canonical endoplasmic reticulum stressors. Moreover, enforced PERK or AMPK activation upregulates autophagy and causes luminal filling during acinar morphogenesis by perpetuating a population of surviving autophagic luminal cells that resist anoikis. Hence, we identify a novel pathway in which suspension-activated PERK promotes the activation of LKB1, AMPK and TSC2, leading to the rapid induction of detachment-induced autophagy. We propose that increased autophagy, secondary to persistent PERK and LKB1-AMPK signaling, can robustly protect cells from anoikis and promote luminal filling during early carcinoma progression.
anoikis; breast cancer; unfolded protein response
In order to proliferate and expand in an environment with limited nutrients, cancer cells co-opt cellular regulatory pathways that facilitate adaptation and thereby maintain tumor growth and survival potential. The endoplasmic reticulum (ER) is uniquely positioned to sense nutrient deprivation stress and subsequently engage signaling pathways that promote adaptive strategies. As such, components of the ER stress-signaling pathway represent potential anti-neoplastic targets. However, recent investigations into the role of the ER resident protein kinase PERK have paradoxically suggested both pro- and anti-tumorigenic properties. We have utilized animal models of mammary carcinoma to interrogate PERK contribution in the neoplastic process. The ablation of PERK in tumor cells resulted in impaired regeneration of intracellular antioxidants and accumulation of reactive oxygen species triggering oxidative DNA damage. Ultimately, PERK deficiency impeded progression through the cell cycle due to the activation of the DNA damage checkpoint. Our data reveal that PERK-dependent signaling is utilized during both tumor initiation and expansion to maintain redox homeostasis and thereby facilitates tumor growth.
PERK; Nrf2; ROS; DNA damage; cell cycle checkpoints
Perturbations in the regulation of the core cell cycle machinery are frequently observed in human cancers. Cyclin D1 which functions as a mitogenic sensor and allosteric activator of CDK4/6, is one of the more frequently altered cell cycle regulators in cancers. Cyclin D1 is frequently overexpressed in cancers and its overexpression can be attributed to many factors including increased transcription, translation, and protein stability. Although cyclin D1 overexpression is clearly implicated in the affected cancers, overexpression of cyclin D1 is not sufficient to drive oncogenic transformation. Rather, emerging evidence suggests that nuclear retention of cyclin D1 resulting from altered nuclear trafficking and proteolysis is critical for the manifestation of its oncogenicity. This review provides a brief overview of current data documenting various mechanisms underlying aberrant cyclin D1 regulation in human cancers and their impact on neoplastic transformation.
Cyclin D1 levels are maintained at steady state by phosphorylation-dependent nuclear export and polyubiquitination by the SCFFBX4-αB crystallin E3 ligase. Inhibition of cyclin D1 proteolysis has been implicated as a causative factor leading to its overexpression in carcinomas of the breast and esophagus; however evaluation of the contribution of stable cyclin D1 to the genesis of such carcinomas has not been performed. We therefore generated transgenic mice wherein expression of either wild-type or a stable cyclin D1 allele (D1T286A) is regulated by MMTV-LTR. MMTV-D1T286A mice developed mammary adenocarcinomas at an increased rate relative to MMTV-D1 mice. Similar to human cancers that overexpress cyclin D1, D1T286A tumors were estrogen receptor positive and exhibited estrogen-dependent growth. MMTV-D1T286A tumors specifically overexpressed genes involved in DNA replication and DNA damage checkpoints suggesting that stabilization and nuclear accumulation of cyclin D1-dependent kinase contributes to genomic instability through perturbations in DNA replication. Collectively, these results suggest that temporal control of cyclin D1 subcellular localization and proteolysis is critical for the maintenance of homeostasis within the mammary epithelium.
αB crystallin; CDK4; cyclin D1; FBX4; mammary gland
Protein ubiquitylation is a complex enzymatic process that results in the covalent attachment of ubiquitin, via Gly-76 of ubiquitin, to an ε-NH2-group of an internal lysine residue in a given substrate. While E3 ligases frequently utilize lysines adjacent to the degron within the substrate, many substrates can be targeted to the proteasome via polyubiquitylation of any lysine. We have assessed the role of lysine residues proximal to the cyclin D1 phosphodegron for ubiquitylation by the SCFFbx4/αB-crystallin ubiquitin ligase and subsequent proteasome-dependent degradation of cyclin D1. The work described herein reveals a requisite role for Lys-269 (K269) for the rapid, poly-ubiquitin mediated degradation of cyclin D1. Mutation of lysine 269, which is proximal to the phosphodegron sequence surrounding Thr-286 in cyclin D1, not only stabilizes cyclin D1, but also triggers cyclin D1 accumulation within the nucleus thereby promoting cell transformation. In addition, D1-K269R is resistant to genotoxic stress induced degradation, similar to non-phosphorylatable D1-T286A, supporting the critical role for the post-translational regulation of cyclin D1 in the response to DNA damaging agents. Strikingly, while mutation of lysine 269 to arginine inhibits cyclin D1 degradation, it does not inhibit cyclin D1 ubiquitylation in vivo demonstrating that ubiquitylation of a specific lysine can influence substrate targeting to the 26S proteasome.
Ubiquitin mediated degradation of cyclin D1 following the G1/S transition counters its mitogen-dependent accumulation during G1 phase of the cell cycle. Although the cellular machinery responsible for this process has been identified, how this regulatory pathway interfaces to cellular stress responses, often referred to as checkpoints, remains to be established. One intensely investigated checkpoint is the cellular response to DNA damage. When DNA damage is sensed, the corresponding DNA damage checkpoint triggers the inhibition of CDK-dependent cell cycle progression, with arrest coordinated by induction of CDK inhibitors and rapid degradation of specific cyclins, such as cyclin D1. In recent work, we identified a phosphorylation- and Fbx4-dependent cyclin D1 degradation mechanism in response to genotoxic stress.18 This work revealed that loss of cyclin D1 regulation compromises the intra-Sphase response to DNA damage, promoting genomic instability and sensitization of cells to S-phase chemotherapy, highlighting a potential therapeutic strategy for cancers exhibiting cyclin D1 accumulation.
Cyclin D1; phosphorylation; ATM; CDK4; DNA damage; intra-S-phase checkpoint; SCFFbx4-αBCrystallin; GSK3β
Mantle cell lymphoma (MCL) is a B-cell lymphoma characterized by overexpression of cyclin D1 due to the t(11;14) chromosomal translocation. While expression of cyclin D1 is correlates with MCL development, expression of wild type cyclin D1 transgene in murine lymphocytes is unable to drive B-cell lymphoma. Because cyclin D1 mutants that are refractory to nuclear export display heighten oncogenicity in vitro compared with wild type D1, we generated mice expressing FLAG-D1/T286A, a constitutively nuclear mutant, under the control of the immunoglobulin enhancer, Eµ. D1/T286A transgenic mice universally develop a mature B-cell lymphoma. Expression of D1/T286A in B lymphocytes results in promiscuous S-phase entry and increased apoptosis in spleens of young pre-malignant mice. Lymphoma onset correlates with loss of p53 suggesting that inactivation of the p53 signaling axis precedes lymphoma development. Our results describe a cyclin D1-driven model of B-cell lymphomagenesis and provide evidence that nuclear-retention of cyclin D1 is oncogenic in vivo.
CDK4; cyclin D1; mantle cell lymphoma; apoptosis
Exposure of cells to Endoplasmic Reticulum (ER) stress leads to activation of phosphatidylinositol 3-kinase (PI3K)–Akt signaling pathway and transcriptional induction of the inhibitor of apoptosis family of proteins. One of the proximal effectors of the ER stress response, the PKR-like ER kinase (PERK), leads to cellular adaptation to stress by multiple mechanisms, including attenuation of protein synthesis, and transcriptional induction of pro-survival genes. While PERK activity leads to cellular adaptation to ER stress, we now demonstrate that PERK activity also inhibits the ER stress-induced apoptotic program through induction of cellular inhibitor of apoptosis (cIAP1 and cIAP2) proteins. This induction of IAPs occurs through both transcriptional and translational responses that are PERK-dependent. Reintroduction of cIAP1 or cIAP2 expression into PERK−/− MEFs during ER stress delays the early onset of ER stress-induced caspase activation and apoptosis observed in these cells. Furthermore, we demonstrate that activation of the PI3K-Akt pathway by ER stress is dependent on PERK, suggesting additional ways in which PERK activity protects cells from ER stress-induced apoptosis.
PERK; ER Stress; Apoptosis; IAP
Exposure of cells to endoplasmic reticulum (ER) stress leads to activation of PKR-like ER kinase (PERK), eukaryotic translation initiation factor 2α (eIF2α) phosphorylation, repression of cyclin D1 translation, and subsequent cell cycle arrest in G1 phase. However, whether PERK is solely responsible for regulating cyclin D1 accumulation after unfolded protein response pathway (UPR) activation has not been assessed. Herein, we demonstrate that repression of cyclin D1 translation after UPR activation occurs independently of PERK, but it remains dependent on eIF2α phosphorylation. Although phosphorylation of eIF2α in PERK–/– fibroblasts is attenuated in comparison with wild-type fibroblasts, it is not eliminated. The residual eIF2α phosphorylation correlates with the kinetics of cyclin D1 loss, suggesting that another eIF2α kinase functions in the absence of PERK. In cells harboring targeted deletion of both PERK and GCN2, cyclin D1 loss is attenuated, suggesting GCN2 functions as the redundant kinase. Consistent with these results, cyclin D1 translation is also stabilized in cells expressing a nonphosphorylatable allele of eIF2α; in contrast, repression of global protein translation still occurs in these cells, highlighting a high degree of specificity in transcripts targeted for translation inhibition by phosphorylated eIF2α. Our results demonstrate that PERK and GCN2 function to cooperatively regulate eIF2α phosphorylation and cyclin D1 translation after UPR activation.
The proto-oncogene c-Myc paradoxically activates both proliferation and apoptosis. In the pathogenic state, c-Myc–induced apoptosis is bypassed via a critical, yet poorly understood escape mechanism that promotes cellular transformation and tumorigenesis. The accumulation of unfolded proteins in the ER initiates a cellular stress program termed the unfolded protein response (UPR) to support cell survival. Analysis of spontaneous mouse and human lymphomas demonstrated significantly higher levels of UPR activation compared with normal tissues. Using multiple genetic models, we demonstrated that c-Myc and N-Myc activated the PERK/eIF2α/ATF4 arm of the UPR, leading to increased cell survival via the induction of cytoprotective autophagy. Inhibition of PERK significantly reduced Myc-induced autophagy, colony formation, and tumor formation. Moreover, pharmacologic or genetic inhibition of autophagy resulted in increased Myc-dependent apoptosis. Mechanistically, we demonstrated an important link between Myc-dependent increases in protein synthesis and UPR activation. Specifically, by employing a mouse minute (L24+/–) mutant, which resulted in wild-type levels of protein synthesis and attenuation of Myc-induced lymphomagenesis, we showed that Myc-induced UPR activation was reversed. Our findings establish a role for UPR as an enhancer of c-Myc–induced transformation and suggest that UPR inhibition may be particularly effective against malignancies characterized by c-Myc overexpression.
Zinc finger E-box binding (ZEB) proteins ZEB1 and ZEB2 are transcription factors essential in transforming growth factor (TGF)-β-mediated senescence, epithelial to mesenchymal transition (EMT) and cancer stem cell function. ZEBs are negatively regulated by members of the miR-200 microRNA family, but precisely how tumor cells expressing ZEBs emerge during invasive growth remains unknown. Here we report that NOTCH3-mediated signaling prevents expansion of a unique subset of ZEB-expressing cells. ZEB expression was associated with the lack of cellular capability of undergoing NOTCH3-mediated squamous differentiation in human esophageal cells. Genetic inhibition of the Notch-mediated transcriptional activity by dominant-negative Mastermind-like1 (DNMAML1) prevented squamous differentiation and induction of Notch target genes including NOTCH3. Moreover, DNMAML1 enriched EMT competent cells exhibited robust upregulation of ZEBs, downregulation of the miR-200 family, and enhanced anchorage independent growth and tumor formation in nude mice. RNA interference (RNAi) experiments suggested the involvement of ZEBs in anchorage independent colony formation, invasion and TGF-β-mediated EMT. Invasive growth and impaired squamous differentiation was recapitulated upon Notch inhibition by DNMAML1 in organotypic 3D culture, a form of human tissue engineering. Together, our findings indicate that NOTCH3 is a key factor limiting the expansion of ZEB-expressing cells, providing novel mechanistic insights into the role of Notch signaling in the cell fate regulation and disease progression of squamous esophageal cancers.
Notch; EMT; squamous cell differentiation; ZEB1; miR-200
The Unfolded Protein Response (UPR) is an ensemble of signal transduction pathways that respond to perturbations in the oxidative, pro-folding environment of the endoplasmic reticulum. During the past decade, ongoing research implicated these pathways in maintaining homeostasis of cells and organisms exposed to various stresses. Herein, we highlight recent findings regarding the functional role of the UPR in both normal and pathophysiological processes.