Preferential Translation Research Articles

The CXCR4-LASP1-eIF4F Axis Promotes Translation of Oncogenic Proteins in Triple-Negative Breast Cancer Cells.

Triple-negative breast cancer (TNBC) remains clinically challenging as effective targeted therapies are lacking. In addition, patient mortality mainly results from the metastasized lesions. CXCR4 has been identified to be one of the major chemokine receptors involved in breast cancer metastasis. Previously, our lab had identified LIM and SH3 Protein 1 (LASP1) to be a key mediator in CXCR4-driven invasion. To further investigate the role of LASP1 in this process, a proteomic screen was employed and identified a novel protein-protein interaction between LASP1 and components of eukaryotic initiation 4F complex (eIF4F). We hypothesized that activation of the CXCR4-LASP1-eIF4F axis may contribute to the preferential translation of oncogenic mRNAs leading to breast cancer progression and metastasis. To test this hypothesis, we first confirmed that the gene expression of CXCR4, LASP1, and eIF4A are upregulated in invasive breast cancer. Moreover, we demonstrate that LASP1 associated with eIF4A in a CXCL12-dependent manner via a proximity ligation assay. We then confirmed this finding, and the association of LASP1 with eIF4B via co-immunoprecipitation assays. Furthermore, we show that LASP1 can interact with eIF4A and eIF4B through a GST-pulldown approach. Activation of CXCR4 signaling increased the translation of oncoproteins downstream of eIF4A. Interestingly, genetic silencing of LASP1 interrupted the ability of eIF4A to translate oncogenic mRNAs into oncoproteins. This impaired ability of eIF4A was confirmed by a previously established 5′UTR luciferase reporter assay. Finally, lack of LASP1 sensitizes 231S cells to pharmacological inhibition of eIF4A by Rocaglamide A as evident through BIRC5 expression. Overall, our work identified the CXCR4-LASP1 axis to be a novel mediator in oncogenic protein translation. Thus, our axis of study represents a potential target for future TNBC therapies.

Divergent rRNAs as regulators of gene expression at the ribosome level.

It is generally assumed that each organism has evolved to possess a unique ribosomal RNA (rRNA) species optimal for its physiological needs. However, some organisms express divergent rRNAs, the functional roles of which remain unknown. Here, we show that ribosomes containing the most variable rRNAs, encoded by the rrnI operon (herein designated as I-ribosomes), direct the preferential translation of a subset of mRNAs in Vibrio vulnificus, enabling the rapid adaptation of bacteria to temperature and nutrient shifts. In addition, genetic and functional analyses of I-ribosomes and target mRNAs suggest that both I-ribosomal subunits are required for the preferential translation of specific mRNAs, the Shine-Dalgarno sequences of which do not play a critical role in I-ribosome binding. This study identifies genome-encoded divergent rRNAs as regulators of gene expression at the ribosome level, providing an additional level of regulation of gene expression in bacteria in response to environmental changes.

Estrogen receptor α promotes protein synthesis by fine-tuning the expression of the eukaryotic translation initiation factor 3 subunit f (eIF3f)

Approximately two thirds of all breast cancer cases are estrogen receptor (ER)-positive. The treatment of this breast cancer subtype with endocrine therapies is effective in the adjuvant and recurrent settings. However, their effectiveness is compromised by the emergence of intrinsic or acquired resistance. Thus, identification of new molecular targets can significantly contribute to the development of novel therapeutic strategies. In recent years, many studies have implicated aberrant levels of translation initiation factors in cancer etiology and provided evidence that identifies these factors as promising therapeutic targets. Accordingly, we observed reduced levels of the eIF3 subunit eIF3f in ER-positive breast cancer cells compared with ER-negative cells, and determined that low eIF3f levels are required for proper proliferation and survival of ER-positive MCF7 cells. The expression of eIF3f is tightly controlled by ERα at the transcriptional (genomic pathway) and translational (nongenomic pathway) level. Specifically, estrogen-bound ERα represses transcription of the EIF3F gene, while promoting eIF3f mRNA translation. To regulate translation, estrogen activates the mTORC1 pathway, which enhances the binding of eIF3 to the eIF4F complex and, consequently, the assembly of the 48S preinitiation complexes and protein synthesis. We observed preferential translation of mRNAs with highly structured 5'-UTRs that usually encode factors involved in cell proliferation and survival (e.g. cyclin D1 and survivin). Our results underscore the importance of estrogen-ERα-mediated control of eIF3f expression for the proliferation and survival of ER-positive breast cancer cells. These findings may provide rationale for the development of new therapies to treat ER-positive breast cancer.

PIE-1 Translation in the Germline Lineage Contributes to PIE-1 Asymmetry in the Early Caenorhabditis elegans Embryo.

In the C. elegans embryo, the germline lineage is established through successive asymmetric cell divisions that each generate a somatic and a germline daughter cell. PIE-1 is an essential maternal factor that is enriched in embryonic germline cells and is required for germline specification. We estimated the absolute concentration of PIE-1::GFP in germline cells and find that PIE-1::GFP concentration increases by roughly 4.5 fold, from 92 nM to 424 nM, between the 1 and 4-cell stages. Previous studies have shown that the preferential inheritance of PIE-1 by germline daughter cells and the degradation of PIE-1 in somatic cells are important for PIE-1 enrichment in germline cells. In this study, we provide evidence that the preferential translation of maternal PIE-1::GFP transcripts in the germline also contributes to PIE-1::GFP enrichment. Through an RNAi screen, we identified Y14 and MAG-1 (Drosophila tsunagi and mago nashi) as regulators of embryonic PIE-1::GFP levels. We show that Y14 and MAG-1 do not regulate PIE-1 degradation, segregation or synthesis in the early embryo, but do regulate the concentration of maternally-deposited PIE-1::GFP. Taken together, or findings point to an important role for translational control in the regulation of PIE-1 levels in the germline lineage.

O-GlcNAc modification of eIF4GI acts as a translational switch in heat shock response.

Heat shock response (HSR) is an ancient signaling pathway leading to thermoprotection of nearly all living organisms. Emerging evidence suggests that intracellular O-linked β-N-acetylglucosamine (O-GlcNAc) serves as a molecular 'thermometer' by reporting ambient temperature fluctuations. Whether and how O-GlcNAc modification regulates HSR remains unclear. Here we report that, upon heat shock stress, the key translation initiation factor eIF4GI undergoes dynamic O-GlcNAcylation at the N-terminal region. Without O-GlcNAc modification, the preferential translation of stress mRNAs is impaired. Unexpectedly, stress mRNAs are entrapped within stress granules (SGs) that are no longer dissolved during stress recovery. Mechanistically, we show that stress-induced eIF4GI O-GlcNAcylation repels poly(A)-binding protein 1 and promotes SG disassembly, thereby licensing stress mRNAs for selective translation. Using various eIF4GI mutants created by CRISPR/Cas9, we demonstrate that eIF4GI acts as a translational switch via reversible O-GlcNAcylation. Our study reveals a central mechanism linking heat stress sensing, protein remodeling, SG dynamics and translational reprogramming.

P.042 Raloxifene sensitizes glioblastoma cells to hypoxia-induced death through inhibition of stress granule dissolution

Background: Glioblastoma (GBM) is the most common primary malignant brain tumour. Despite aggressive therapy, median survival is only 14 months. Death typically results from treatment failure and local recurrence. The GBM microenvironment is highly hypoxic, which correlates with treatment resistance. Cytoplasmic RNA stress granules (SGs) form in response to hypoxic stress and act as sights of mRNA triage, allowing preferential translation of pro-survival mRNA during stress. We hypothesize that SGs may play a role in hypoxia-induced resistance to therapy, and may be targetable by chemotherapeutics to improve outcomes. Methods: We screened 1280 approved compounds to identify drugs that inhibited formation or dissolution of SGs in U251 glioma cells. Raloxifene inhibited SG dissolution in a dose dependent manner. We treated cells with raloxifene and incubated them in hypoxia, and then measured rates of cell death using cell counting and Presto blue. Results: Cell death rates were synergistically higher in cells treated with the combination of raloxifene and hypoxia compared to either treatment alone. Conclusions: Raloxifene inhibits the dissolution of SGs in glioma cells, and combination treatment results in synergistic tumour cell death. Taken together, this provides evidence that inhibition of SG dissolution may be a viable target for future GBM chemotherapeutics.

Bovine adenovirus‐3 protein VIII associates with eukaryotic initiation factor‐6 during infection

Adenovirus protein VIII appears to connect core with the inner surface of the adenovirus capsid. Because protein-protein interactions are central to virus replication, identification of proteins interacting with protein VIII may help in understanding their role in adenovirus infection. Our yeast 2-hybrid assay indicated that protein VIII interacts with eukaryotic initiation factor 6 (eIF6). These findings were confirmed by Glutathione S-transferase-pull down assay, bimolecular fluorescent complementation assay, and coimmunoprecipitation assay in plasmid DNA transfected and bovine adenovirus-3 (BAdV-3) infected cells. The C-terminus amino acids 147 to 174 of protein VIII and N-terminus amino acids 44 to 97 of eIF6 are involved in these interactions. Polysome analysis demonstrated increased level of 60S ribosomal subunit and decreased level of 80S complex in protein VIII expressing cells or BAdV-3 infected cells. Our results suggest that formation of functional 80S ribosome appears impaired in the presence of protein VIII at late times post infection. We speculate that this impaired ribosome assembly may be responsible for the inhibition of cellular mRNA translation observed late in adenovirus infected cells. Moreover, analysis of recombinant BAdV-3 expressing mutant protein VIII (deletion of eIF6 interacting domain) suggests that interaction of protein VIII and eIF6 may help in preferential translation of adenovirus genes during late phase of adenovirus infection.

Chronic oxidative stress promotes GADD34-mediated phosphorylation of the TAR DNA-binding protein TDP-43, a modification linked to neurodegeneration

Oxidative and endoplasmic reticulum (ER) stresses are hallmarks of the pathophysiology of ALS and other neurodegenerative diseases. In these stresses, different kinases phosphorylate eukaryotic initiation factor eIF2α, enabling the translation of stress response genes; among these is GADD34, the protein product of which recruits the α-isoform of protein phosphatase 1 catalytic subunit (PP1α) and eIF2α to assemble a phosphatase complex catalyzing eIF2α dephosphorylation and resumption of protein synthesis. Aberrations in this pathway underlie the aforementioned disorders. Previous observations indicating that GADD34 is induced by arsenite, a thiol-directed oxidative stressor, in the absence of eIF2α phosphorylation suggest other roles for GADD34. Here, we report that arsenite-induced oxidative stress differs from thapsigargin- or tunicamycin-induced ER stress in promoting GADD34 transcription and the preferential translation of its mRNA in the absence of eIF2α phosphorylation. Arsenite also stabilized GADD34 protein, slowing its degradation. In response to oxidative stress, but not ER stress, GADD34 recruited TDP-43, and enhanced cytoplasmic distribution and cysteine modifications of TDP-43 promoted its binding to GADD34. Arsenite also recruited a TDP-43 kinase, casein kinase-1ϵ (CK1ϵ), to GADD34. Concomitant with TDP-43 aggregation and proteolysis after prolonged arsenite exposure, GADD34-bound CK1ϵ catalyzed TDP-43 phosphorylations at serines 409/410, which were diminished or absent in GADD34-/- cells. Our findings highlight that the phosphatase regulator, GADD34, also functions as a kinase scaffold in response to chronic oxidative stress and recruits CK1ϵ and oxidized TDP-43 to facilitate its phosphorylation, as seen in TDP-43 proteinopathies.

Reduced Expression of Eukaryotic Translation Initiation Factor 3 Subunit e and Its Possible Involvement in the Epithelial-Mesenchymal Transition in Endometriosis.

Epithelial-mesenchymal transition (EMT) is now well documented to be involved in the development of endometriosis through the promotion of invasion and fibrogenesis. To date, several factors have been reported to be involved in EMT in endometriosis. The eukaryotic translation initiation factor 3 subunit e (eIF3e) protein is a component of the multisubunit eIF3 complex essential for cap-dependent translation initiation. The aim of this study was to investigate whether eIF3e is involved in EMT in endometriosis. We recruited 40 premenopausal women (34.7 [6.8] years) with laparoscopically and histologically diagnosed ovarian endometriomas, and their ectopic endometrial tissue samples were collected after informed consent. As controls, endometrial tissue samples were obtained after informed consent from 40 premenopausal women, roughly age-matched (36.9 [6.4] years) and menstrual phase-matched with endometriosis group, who underwent surgery for benign gynecologic disorders or cervical intraepithelial neoplasia but without endometriosis, adenomyosis, or uterine fibroids. All tissue samples were subjected to immunohistochemistry analysis of eIF3e, transforming growth factor (TGF-β1), Snail, E-cadherin, vimentin, and proliferating cell nuclear antigen (PCNA). We found significantly reduced immunoreactivity against eIF3e and E-cadherin but elevated immunoreactivity against TGF-β1, Snail, vimentin, and PCNA in endometriotic epithelial cells when compared to that of control endometrium (all P values <.05). The eIF3e staining levels correlated negatively with that of TGF-β1 and Snail but positively with that of E-cadherin (all P values <.05). These data suggest that eIF3e downregulation may be involved in EMT in endometriosis, possibly through preferential translation of Snail. Future studies are warranted to confirm whether this is the mechanism.

Participation of dengue virus NS4B protein in the modulation of immune effectors dependent on ER stress in insect cells.

Organisms' reactions to adverse events result in the generation of immune effectors, which, in the case of insects, may be produced from the direct activation of pathways such as Toll, Jak-STAT, Imd, or RNAi or may be derived from the crosstalk of different intracellular pathways. One such pathway, the unfolded protein response (UPR), has the primary objective of restoring homeostasis in the endoplasmic reticulum. In addition, the UPR participates in signaling crosstalk with the immune pathways, generating protection against pathogenic organisms. Dengue virus is a plus-strand RNA virus belonging to the Flavivirus genus that uses the ER as a replication site; during the infection, there are indicators of the activation of the UPR, which in turn, induces the synthesis of internal membranes and preferential translation of viral proteins enhancing the replication. One of the dengue virus proteins, the NS4B can block the pathway of α/β interferon in mammals. However, what happen in insects is interesting because the lack of the main antiviral pathway, the interferon and the role of the NS4B protein in the UPR-immunity relationship can be better understood. Thus, in this study, we demonstrated that the DENV2/16681 NS4B protein is capable of modulating the immune effectors that result from the activation of the UPR in insect cells.

Gemtuzumab ozogamicin in acute myeloid leukemia.

CD33 is variably expressed on leukemia blasts in almost all patients with acute myeloid leukemia (AML) and possibly leukemia stem cells in some. Efforts to target CD33 therapeutically have focused on gemtuzumab ozogamicin (GO; Mylotarg), an antibody-drug conjugate delivering a DNA-damaging calicheamicin derivative. GO is most effective in acute promyelocytic leukemia but induces remissions in other AML types and received accelerated approval in the US in 2000. However, because a large follow-up study showed no survival improvement and increased early deaths the drug manufacturer voluntarily withdrew the US New Drug Application in 2010. More recently, a meta-analysis of data from several trials reported better survival in adults with favorable- and intermediate-risk cytogenetics but not adverse-risk AML randomized to receive GO along with intensive induction chemotherapy. As a result, GO is being re-evaluated by regulatory agencies. Responses to GO are diverse and predictive biological response markers are needed. Besides cytogenetic risk, ATP-binding cassette transporter activity and possibly CD33 display on AML blasts may predict response, but established clinical assays and prospective validation are lacking. Single-nucleotide polymorphisms in CD33 may also be predictive, most notably rs12459419 where the minor T-allele leads to decreased display of full-length CD33 and preferential translation of a splice variant not recognized by GO. Data from retrospective analyses suggest only patients with the rs12459419 CC genotype may benefit from GO therapy but confirmation is needed. Most important may be markers for AML cell sensitivity to calicheamicin, which varies over 100 000-fold, but useful assays are unavailable. Novel CD33-targeted drugs may overcome some of GO's limitations but it is currently unknown whether such drugs will be more effective in patients benefitting from GO and/or improve outcomes in patients not benefitting from GO, and what the supportive care requirements will be to enable their safe use.

Translation initiation factor eIF4G1 preferentially binds yeast transcript leaders containing conserved oligo-uridine motifs.

Translational control of gene expression plays essential roles in cellular stress responses and organismal development by enabling rapid, selective, and localized control of protein production. Translational regulation depends on context-dependent differences in the protein output of mRNAs, but the key mRNA features that distinguish efficiently translated mRNAs are largely unknown. Here, we comprehensively determined the RNA-binding preferences of the eukaryotic initiation factor 4G (eIF4G) to assess whether this core translation initiation factor has intrinsic sequence preferences that may contribute to preferential translation of specific mRNAs. We identified a simple RNA sequence motif—oligo-uridine—that mediates high-affinity binding to eIF4G in vitro. Oligo(U) motifs occur naturally in the transcript leader (TL) of hundreds of yeast genes, and mRNAs with unstructured oligo(U) motifs were enriched in immunoprecipitations against eIF4G. Ribosome profiling following depletion of eIF4G in vivo showed preferentially reduced translation of mRNAs with long TLs, including those that contain oligo(U). Finally, TL oligo(U) elements are enriched in genes with regulatory roles and are conserved between yeast species, consistent with an important cellular function. Taken together, our results demonstrate RNA sequence preferences for a general initiation factor, which cells potentially exploit for translational control of specific mRNAs.

Human Keratinocyte Differentiation Requires Translational Control by the eIF2α Kinase GCN2

Appropriate and sequential differentiation of keratinocytes is essential for all functions of the human epidermis. Although transcriptional regulation has proven to be important for keratinocyte differentiation, little is known about the role of translational control. A key mechanism for modulating translation is through phosphorylation of the α subunit of eukaryotic initiation factor 2 (eIF2). A family of different eIF2α kinases function in the integrative stress response to inhibit general protein synthesis coincident with preferential translation of select mRNAs that participate in stress alleviation. Here we demonstrate that translational control through eIF2α phosphorylation is required for normal keratinocyte differentiation. Analyses of polysome profiles revealed that key differentiation genes, including involucrin, are bound to heavy polysomes during differentiation, despite decreased general protein synthesis. Induced eIF2α phosphorylation by the general control nonderepressible 2 (GCN2) protein kinase facilitated translational control and differentiation-specific protein expression during keratinocyte differentiation. Furthermore, loss of GCN2 thwarted translational control, normal epidermal differentiation, and differentiation gene expression in organotypic skin culture. These findings underscore a previously unknown function for GCN2 phosphorylation of eIF2α and translational control in the formation of an intact human epidermis.

Differentiation of human keratinocytes requires translational control by the eIF2 kinase GCN2

In the human epidermis, differentiation of keratinocytes is essential to form a barrier to protect against a variety of environmental insults. This differentiation process can become dysregulated in many skin diseases, including psoriasis, eczema, and non‐melanoma skin cancer. Although changes in transcriptional networks during keratinocyte differentiation are well characterized, little is known about the role of translational control. A major mechanism of translational control involves phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF2‐P). In response to environmental stresses, induced eIF2‐P represses translation initiation, allowing cells to conserve resources and trigger cytoprotective gene expression. In addition to repression of global protein synthesis, eIF2‐P induces preferential translation of specific mRNAs that promote stress adaptation, including the transcription factors ATF4 and CHOP. Since keratinocytes undergo a massive change in morphology and gene expression during differentiation, we hypothesized that translational control plays a major role. To investigate this hypothesis we measured both genome‐wide and gene‐specific levels of translation. We observed a global reduction in translation initiation as measured by polysome profiling during differentiation in both monolayer and 3D organotypic keratinocyte cultures. Expression of eIF2‐P, ATF4, and CHOP was limited to suprabasal (differentiated) keratinocytes in vivo as visualized histologically in human skin tissue. Keratinocyte differentiation in vitro induced eIF2‐P to levels similar to that seen following endoplasmic reticulum stress, but eIF2‐P alone was not sufficient to induce differentiation. Of interest, the epidermal differentiation transcript involucrin (IVL) was preferentially bound to large polysomes following differentiation, indicative of preferential translation, which was similar to that seen for ATF4 and CHOP. Inhibition of eIF2‐P blocked both general and gene‐specific translational control during differentiation. Loss of either eIF2‐P or the upstream protein kinase GCN2 abrogated both IVL protein and mRNA expression. Strikingly, loss of GCN2 altered epidermal tissue formation in a 3D organotypic model as characterized by disorganized keratinocyte layers and a decreased number of cornified cells. These results provide novel evidence that a whole‐cell repression of protein synthesis alongside gene‐specific translational control occur during normal epidermal differentiation. These findings suggest that translational control through GCN2/eIF2‐P is important for proper epidermal stratification, likely though translational regulation of differentiation genes such as IVL. The GCN2/eIF2‐P pathway in keratinocyte biology provides for novel targets for intervention in dermatological diseases characterized by abnormal differentiation.Support or Funding InformationRuth L. Kirschstein National Research Service Award (1F31ES026517 to AEC)

Phosphorylation of eIF2 directs multiple mechanisms of preferential translation for cell adaptation to environmental stress

Phosphorylation of eIF2 (eIF2‐P) lowers protein synthesis to conserve resources and facilitate preferential translation of specific transcripts that promote adaptation to environmental stresses. Central to the mechanisms of preferential translation are upstream ORFs (uORFs) embedded within the 5′‐leaders of mRNAs, which serve as “bar codes” that help ribosomes determine the levels of translation of mRNA coding sequences (CDS) during eIF2‐P. We have been studying the functions of uORFs that demarcate translational control and the roles that these preferentially translated genes play in cell adaptation to stress. Preferentially translated GADD34 (PPP1R15A) and constitutively expressed CReP (PPP1R15B) each function to dephosphorylate eIF2‐P and restore protein synthesis. Both GADD34 and CReP mRNAs contain two upstream uORFs, and using biochemical and genetic approaches we show that distinct features of these uORFs are critical for differential CDS translation. In the absence of stress and low eIF2‐P, translation of an inhibitory uORF in GADD34 acts as a barrier by inefficient translation termination that prevents subsequent reinitiation at the GADD34 CDS. Induced eIF2‐P during stress directs ribosome bypass of the inhibitory uORF, leading to enhanced translation of the GADD34 CDS. By comparison, CReP expression occurs independent of eIF2‐P via an uORF that allows for translation reinitiation at the CReP CDS independent of stress conditions. Alterations in the GADD34 uORF not only affect the status of eIF2‐P and translational control, but also cell adaptation to stress. Translation of other preferentially translated genes also feature bypass of uORFs, although by mechanisms featuring different uORF properties. For example, bypass of a single uORF directs preferential translation of CHOP (DDIT3/GADD153), a transcription factor that triggers cell death during periods of extended stress. We discovered that the CHOP uORF serves to stall elongating ribosomes and prevent ribosome reinitiation at the downstream CHOP CDS. Deletion of the CHOP uORF increases the levels and alters the pattern of induced CHOP expression during eIF2‐P. Elevated CHOP expression leads to increased cell death in response to stress. Finally, we determined that bypass of uORFs in IBTKalpha enhances translation of the CDS in response to eIF2‐P. Elevated levels of IBTKalpha localize to endoplasmic reticulum exit sites and facilitate initiation of autophagy. These examples and others indicate that the proper mixing and matching of uORFs with distinct properties are central for directing gene‐specific translation during eIF2‐P. Preferentially translated genes are critical for the timing and extent of translation and transcription, and alter metabolism and reclamation processes such as autophagy, which together can determine cell survival during environmental stresses.Support or Funding InformationNIH GM049164

Abstract B12: Expression of Integrated Stress Response proteins during the progression of UVB-induced squamous cell carcinoma

Abstract The major environmental risk factor for developing non-melanoma skin cancers (NMSCs) is exposure to the ultraviolet-B (UVB) wavelengths found in sunlight. Unlike other types of cancer where the occurrence each year is precisely known, so many NMSCs occur each year in the United States that the incidence of NMSC is not accurately reported. Treatment of NMSC costs the U.S. healthcare system over 1 billion dollars annually. Therefore while NMSC is rarely lethal, the expense of NMSC treatment is the fifth highest among all cancer treatments. In response to a myriad of environmental stresses, including UVB irradiation, eukaryotic cells rapidly modulate protein synthesis. An important mechanism for translation regulation involves phosphorylation of the α subunit of eukaryotic initiation factor 2 (eIF2α), which results in a prompt decrease in global protein synthesis, concurrent with preferential translation of cytoprotective gene transcripts. Because a range of different stresses can activate one of four eIF2 kinases, this pathway is referred to as the Integrated Stress Response (ISR). Although the ISR pathway has been implicated in carcinogenesis in a variety of tissues, little is known about whether the ISR can modulate the development of NMSC in human skin. We previously defined the cytoprotective role of the ISR in response to UVB-irradiation of human keratinocytes in vitro. Following UVB exposure, keratinocytes lacking an ISR response have a shortened G1 arrest, a decreased ability to repair UVB-damaged DNA, a diminished senescence response, and an increased susceptibility to apoptosis. Collectively, these functions attributed to the ISR response in keratinocytes following UVB-irradiation are tumor suppressive. To address how the ISR is involved during photocarcinogenesis, we analyzed the expression of ISR proteins (eIF2α-phosphorylation, ATF4, CHOP, GADD34) in the progression of a subtype of NMSC, squamous cell carcinoma (SCC). The progression ranges from normal skin, to a precursor lesion of SCC (actinic keratosis, AK), and finally to SCC tumors. ISR marker proteins were detected using immunofluorescence on formalin-fixed, paraffin-embedded tissue from at least four distinct specimens in each category. In normal skin, expression of ISR proteins were found in differentiating layers of the epidermis. eIF2α phosphorylation was predominantly observed in both the nucleus and cytoplasm of keratinocytes in the squamous and granular layers of the skin. Similarly, GADD34, ATF4, and CHOP were expressed in normal differentiating keratinocytes. In contrast, using a human skin model of AK, ATF4 expression was greatly increased in all but the basal cell layer of the epidermis. CHOP and GADD34 expression, as well as eIF2α phosphorylation, in human AKs followed a similar pattern. Progression to SCC resulted in a striking decrease in ISR protein expression. ATF4 and CHOP expression were absent, no eIF2α phosphorylation was detected, and GADD34 expression was greatly diminished. Therefore, the standard ISR response in differentiating keratinocytes is enhanced in AK lesions but silenced as the tumor progresses to SCC. These data suggest that sustaining the ISR response in UVB-damaged epidermis could be a valuable therapeutic target for the prevention of SCC development. Citation Format: Dan F. Spandau, Ann Collier, Ron Wek. Expression of Integrated Stress Response proteins during the progression of UVB-induced squamous cell carcinoma. [abstract]. In: Proceedings of the AACR Special Conference on Translational Control of Cancer: A New Frontier in Cancer Biology and Therapy; 2016 Oct 27-30; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2017;77(6 Suppl):Abstract nr B12.

Regulation of heat shock gene expression in response to stress

Heat shock (HS) genes, or stress genes, code for a number of proteins that collectively form the most ancient and universal stress defense system. The system determines the cell capability of adaptation to various adverse factors and performs a variety of auxiliary functions in normal physiological conditions. Common stress factors, such as higher temperatures, hypoxia, heavy metals, and others, suppress transcription and translation for the majority of genes, while HS genes are upregulated. Transcription of HS genes is controlled by transcription factors of the HS factor (HSF) family. Certain HSFs are activated on exposure to higher temperatures or other adverse factors to ensure stress-induced HS gene expression, while other HSFs are specifically activated at particular developmental stages. The regulation of the main mammalian stress-inducible factor HSF1 and Drosophila melanogaster HSF includes many components, such as a variety of early warning signals indicative of abnormal cell activity (e.g., increases in intracellular ceramide, cytosolic calcium ions, or partly denatured proteins); protein kinases, which phosphorylate HSFs at various Ser residues; acetyltransferases; and regulatory proteins, such as SUMO and HSBP1. Transcription factors other than HSFs are also involved in activating HS gene transcription; the set includes D. melanogaster GAF, mammalian Sp1 and NF-Y, and other factors. Transcription of several stress genes coding for molecular chaperones of the glucose-regulated protein (GRP) family is predominantly regulated by another stress-detecting system, which is known as the unfolded protein response (UPR) system and is activated in response to massive protein misfolding in the endoplasmic reticulum and mitochondrial matrix. A translational fine tuning of HS protein expression occurs via changing the phosphorylation status of several proteins involved in translation initiation. In addition, specific signal sequences in the 5'-UTRs of some HS protein mRNAs ensure their preferential translation in stress.

One Gene and Two Proteins: a Leaderless mRNA Supports the Translation of a Shorter Form of the Shigella VirF Regulator.

ABSTRACTVirF, an AraC-like activator, is required to trigger a regulatory cascade that initiates the invasive program of Shigella spp., the etiological agents of bacillary dysentery in humans. VirF expression is activated upon entry into the host and depends on many environmental signals. Here, we show that the virF mRNA is translated into two proteins, the major form, VirF30 (30 kDa), and the shorter VirF21 (21 kDa), lacking the N-terminal segment. By site-specific mutagenesis and toeprint analysis, we identified the translation start sites of VirF30 and VirF21 and showed that the two different forms of VirF arise from differential translation. Interestingly, in vitro and in vivo translation experiments showed that VirF21 is also translated from a leaderless mRNA (llmRNA) whose 5′ end is at position +309/+310, only 1 or 2 nucleotides upstream of the ATG84 start codon of VirF21. The llmRNA is transcribed from a gene-internal promoter, which we identified here. Functional analysis revealed that while VirF30 is responsible for activation of the virulence system, VirF21 negatively autoregulates virF expression itself. Since VirF21 modulates the intracellular VirF levels, this suggests that transcription of the llmRNA might occur when the onset of the virulence program is not required. We speculate that environmental cues, like stress conditions, may promote changes in virF mRNA transcription and preferential translation of llmRNA.

Abstract 230: The role of integrated stress response (ISR) transcription factor ATF4 in metabolic adaptation of tumors to c-Myc activation

Abstract The ability of cancer cells to adapt to non-cell autonomous (extrinsic) and cell autonomous (intrinsic) stresses is critical for maintaining cell viability and growth. The Integrated Stress Response (ISR) pathway is essential in cancer cell adaptation to extrinsic stresses such as hypoxia and nutrient deprivation both in vitro and in vivo. The ISR kinases, PERK and GCN2, phosphorylate eIF2α, transiently inhibiting protein translation during times of stress. Phosphorylation of eIF2α promotes the preferential translation of the transcription factor ATF4, which activates a gene expression program that enhances uptake and synthesis of amino acids, antioxidant defense and chaperone expression to promote recovery from nutrient deprivation and ER stress. Here, we demonstrate the importance of ISR signaling in MYC oncogene induced intrinsic stress both in vitro and in vivo. We previously reported that activation of PERK, during c-Myc induction is vital for activating cytoprotective autophagy and suppressing c-Myc induced apoptosis. Here, we demonstrate GCN2 is also robustly activated during c-Myc induction. GCN2 activation after c-Myc induction regulates ATF4 transcription and translation. ATF4 was necessary for survival and suppression of oxidative stress during c-Myc activation. We have identified ATF4 targets that regulate amino acid uptake and involved in amelioration of oxidative stress. Furthermore, siRNA mediated knockdown or genetic knockout of GCN2 was synthetic lethal with c-Myc activation. Interestingly, PERK is still able to phosphorylate eIF2α in the absence of GCN2. However, eIF2α phosphorylation was completely inhibited in the absence of both kinases suggesting that these kinases compensate for each other and both are required for optimal phosphorylation of eIF2α after c-Myc activation. Consistent with our in vitro data, we observe higher levels of p-eIF2α and ATF4 in lymphoma cells compared to WT B cells in vivo in a mouse model of Myc driven lymphoma. These findings highlight a novel mechanism of stress response pathway activation to support c-Myc driven tumorigenesis involving an amino acid sensor kinase GCN2. Citation Format: Feven Tameire, Maria A. Monroy, Constantinos Koumenis. The role of integrated stress response (ISR) transcription factor ATF4 in metabolic adaptation of tumors to c-Myc activation. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 230.

Prioritized Expression of BDH2 under Bulk Translational Repression and Its Contribution to Tolerance to Severe Vanillin Stress in Saccharomyces cerevisiae.

Vanillin is a potent fermentation inhibitor derived from the lignocellulosic biomass in biofuel production, and high concentrations of vanillin result in the pronounced repression of bulk translation in Saccharomyces cerevisiae. Studies on genes that are efficiently translated even in the presence of high concentrations of vanillin will be useful for improving yeast vanillin tolerance and fermentation efficiency. The BDH1 and BDH2 genes encode putative medium-chain alcohol dehydrogenase/reductases and their amino acid sequences are very similar to each other. Although BDH2 was previously suggested to be involved in vanillin tolerance, it has yet to be clarified whether Bdh1/Bdh2 actually contribute to vanillin tolerance and reductions in vanillin. Therefore, we herein investigated the effects of Bdh1 and Bdh2 on vanillin tolerance. bdh2Δ cells exhibited hypersensitivity to vanillin and slower reductions in vanillin than wild-type cells and bdh1Δ cells. Additionally, the overexpression of the BDH2 gene improved yeast tolerance to vanillin more efficiently than that of BDH1. Only BDH2 mRNA was efficiently translated under severe vanillin stress, however, both BDH genes were transcriptionally up-regulated. These results reveal the importance of Bdh2 in vanillin detoxification and confirm the preferential translation of the BDH2 gene in the presence of high concentrations of vanillin. The BDH2 promoter also enabled the expression of non-native genes under severe vanillin stress and furfural stress, suggesting its availability to improve of the efficiency of bioethanol production through modifications in gene expression in the presence of fermentation inhibitors.