eIF-2 kinases: regulators of general and gene-specific translation initiation

Cryo-EM structure of Influenza A virus NS1 and antiviral protein kinase PKR complex

Influenza A virus is the cause of a widespread human disease with high morbidity and mortality rates. The influenza virus encodes non-structural protein 1 (NS1), an exceedingly multifunctional virulence component. NS1 plays essential roles in viral replication and evasion of the cellular innate immune system. Protein kinase RNA-activated also known as protein kinase R (PKR) phosphorylates translation initiation factor eIF-2α on serine 51 to inhibit protein synthesis in virus-infected mammalian cells. Consequently, PKR activation inhibits mRNA translation, which results in the assert of both viral protein synthesis and cellular and possibly apoptosis in response to virus infection. Host signaling pathways are important in the replication of influenza virus, but the mechanisms involved remain to be characterized. Herein, the structure of NS1 and PKR complex was determined using Cryo-EM. We found the N91, E94, and G95 residues of PKR bind directly with N188, D125, and K126, respectively, of NS1. Furthermore, the study shows that PKR peptide offers a potential treatment for Influenza A virus infections.

An emerging role for the endoplasmic reticulum in stress granule biogenesis

Stress granules (SGs), structurally dynamic, optically resolvable, macromolecular assemblies of mRNAs, RNA binding proteins (RBPs), translation factors, ribosomal subunits, as well as other interacting proteins, assemble in response to cell stress conditions that elicit phosphorylation of eukaryotic initiation factor 2α (eIF2α) and consequently, the inactivation of translation initiation. SG biology is conserved throughout eukaryotes and has recently been linked to the pathological sequelae of neurodegenerative disorders, cancer biology, and viral infection. Substantial insights into mechanisms of SG biogenesis, and more broadly the phenomenon of biological liquid-liquid phase separation (LLPS), have been aided by detailed proteomic and transcriptomic studies as well as in vitro reconstitution approaches. A particularly interesting and largely unexplored element of SG biology is the cell biological context of SG biogenesis, including its subcellular organization and more recently, evidence that the endoplasmic reticulum (ER) membrane may serve important functions in RNA granule biology generally and SG biogenesis specifically. A central role for the ER in SG biogenesis is discussed and a hypothesis linking SG formation on the ER to the trafficking, localization and de novo translation of newly exported mRNAs is presented.

EIF2AK2 selectively regulates the gene transcription in immune response and histones associated with systemic lupus erythematosus

PKR, also known as EIF2AK2, is an IFN-stimulated gene (ISG) and shows a higher expression in probands with systemic lupus erythematosus (SLE), which is likely responsible for the impaired translational and proliferative responses to mitogens in T cells from SLE patients. In this study, we overexpressed EIF2AK2 in HeLa cells to study EIF2AK2-regulated genes using RNA-seq technology, followed by bioinformatic analysis of target genes of EIF2AK2-regulated transcriptional factors (TFs). Overexpression of EIF2AK2 promotes HeLa cell apoptosis. EIF2AK2 selectively represses the transcription of histone protein genes associated with SLE, immune response genes and TF genes, which was validated by RT-qPCR experiments. Analysis of motifs overrepresented in the promoter regions of EIF2AK2-regulated genes revealed eighteen EIF2AK2-regulated TFs involved in establishing the EIF2AK2 network. Eight out of these predicted EIF2AK2-regulated TFs were further verified by RT-qPCR selectively in both HeLa and Jurkat cells, and most such as HEY2, TFEC, BATF2, GATA3 and ATF3 and FOXO6 are known to regulate immune response. Our results suggest that the dsRNA-dependent kinase EIF2AK2 selectively regulates the transcription of immune response and SLE-associated histone protein genes, and such a selectivity is likely to be operated by EIF2AK2-targeted TFs. The EIF2AK2-TFs axis potentially offers new therapeutic targets for counteracting immunological disease in the future.

Dystonia 16 (DYT16) mutations in PACT cause dysregulated PKR activation and eIF2α signaling leading to a compromised stress response

Dystonia 16 (DYT16) is caused by mutations in PACT, the protein activator of interferon-induced double-stranded RNA-activated protein kinase (PKR). PKR regulates the integrated stress response (ISR) via phosphorylation of the translation initiation factor eIF2α. This post-translational modification attenuates general protein synthesis while concomitantly triggering enhanced translation of a few specific transcripts leading either to recovery and homeostasis or cellular apoptosis depending on the intensity and duration of stress signals. PKR plays a regulatory role in determining the cellular response to viral infections, oxidative stress, endoplasmic reticulum (ER) stress, and growth factor deprivation. In the absence of stress, both PACT and PKR are bound by their inhibitor transactivation RNA-binding protein (TRBP) thereby keeping PKR inactive. Under conditions of cellular stress these inhibitory interactions dissociate facilitating PACT-PACT interactions critical for PKR activation. While both PACT-TRBP and PKR-TRBP interactions are pro-survival, PACT-PACT and PACT-PKR interactions are pro-apoptotic. In this study we evaluate if five DYT16 substitution mutations alter PKR activation and ISR. Our results indicate that the mutant DYT16 proteins show stronger PACT-PACT interactions and enhanced PKR activation. In DYT16 patient derived lymphoblasts the enhanced PACT-PKR interactions and heightened PKR activation leads to a dysregulation of ISR and increased apoptosis. More importantly, this enhanced sensitivity to ER stress can be rescued by luteolin, which disrupts PACT-PKR interactions. Our results not only demonstrate the impact of DYT16 mutations on regulation of ISR and DYT16 etiology but indicate that therapeutic interventions could be possible after a further evaluation of such strategies.

The Role of PKR as a Potential Target for Treating Cardiovascular Diseases

Cardiovascular diseases are the leading cause of death globally with limited treatment options. Despite improved pharmacological therapy, scientific understandings on the root mechanisms of cardiovascular diseases are still not fully understood. It is well known that inflammation plays a key role in the pathogenesis of cardiovascular diseases and controlling this inflammatory pathway may inhibit the progression of this chronic disease. Protein Kinase R (PKR), a serine threonine kinase is activated during various pathological conditions. Activation of PKR can induce apoptosis, inflammation and oxidative stress. Since PKR has multidimensional roles, thus PKR is an attractive target for treating cardiovascular and metabolic disorders. The goal of this review is to discuss potential role of PKR in cardiovascular diseases, pathways activated by it and association between pathways activated.

Model of the delayed translation of cyclin B maternal mRNA after sea urchin fertilization

Sea urchin eggs exhibit a cap-dependent increase in protein synthesis within minutes after fertilization. This rise in protein synthesis occurs at a constant rate for a great number of proteins translated from the different available mRNAs. Surprisingly, we found that cyclin B, a major cell-cycle regulator, follows a synthesis pattern that is distinct from the global protein population, so we developed a mathematical model to analyze this dissimilarity in biosynthesis kinetic patterns. The model includes two pathways for cyclin B mRNA entry into the translational machinery: one from immediately available mRNA (mRNAcyclinB) and one from mRNA activated solely after fertilization (XXmRNAcyclinB). Two coefficients, α and β, were added to fit the measured scales of global protein and cyclin B synthesis, respectively. The model was simplified to identify the synthesis parameters and to allow its simulation. The calculated parameters for activation of the specific cyclin B synthesis pathway after fertilization included a kinetic constant (k_a ) of 0.024 sec^-1 , for the activation of XXmRNAcyclinB, and a critical time interval (t₂ ) of 42 min. The proportion of XXmRNAcyclinB form was also calculated to be largely dominant over the mRNAcyclinB form. Regulation of cyclin B biosynthesis is an example of a select protein whose translation is controlled by pathways that are distinct from housekeeping proteins, even though both involve the same cap-dependent initiation pathway. Therefore, this model should help provide insight to the signaling utilized for the biosynthesis of cyclin B and other select proteins. Mol. Reprod. Dev. 83: 1070-1082, 2016. © 2016 Wiley Periodicals, Inc.

The unfolded protein response in retinal vascular diseases: implications and therapeutic potential beyond protein folding

Angiogenesis is a complex, step-wise process of new vessel formation that is involved in both normal embryonic development as well as postnatal pathological processes, such as cancer, cardiovascular disease, and diabetes. Aberrant blood vessel growth, also known as neovascularization, in the retina and the choroid is a major cause of vision loss in severe eye diseases, such as diabetic retinopathy, age-related macular degeneration, retinopathy of prematurity, and central and branch retinal vein occlusion. Yet, retinal neovascularization is causally and dynamically associated with vasodegeneration, ischemia, and vascular remodeling in retinal tissues. Understanding the mechanisms of retinal neovascularization is an urgent unmet need for developing new treatments for these devastating diseases. Accumulating evidence suggests a vital role for the unfolded protein response (UPR) in regulation of angiogenesis, in part through coordinating the secretion of pro-angiogenic growth factors, such as VEGF, and modulating endothelial cell survival and activity. Herein, we summarize current research in the context of endoplasmic reticulum (ER) stress and UPR signaling in retinal angiogenesis and vascular remodeling, highlighting potential implications of targeting these stress response pathways in the prevention and treatment of retinal vascular diseases that result in visual deficits and blindness.

CREB/TRH pathway in the central nervous system regulates energy expenditure in response to deprivation of an essential amino acid

BACKGROUND

In the central nervous system (CNS), thyrotropin-releasing hormone (TRH) has an important role in regulating energy balance. We previously showed that dietary deprivation of leucine in mice increases energy expenditure through CNS-dependent regulation. However, the involvement of central TRH in this regulation has not been reported.

METHODS

Male C57J/B6 mice were maintained on a control or leucine-deficient diet for 7 days. Leucine-deprived mice were either third intracerebroventricular (i.c.v.) injected with a TRH antibody followed by intraperitoneal (i.p.) injection of triiodothyronine (T3) or i.c.v. administrated with an adenovirus of shCREB (cAMP-response element binding protein) followed by i.c.v. injection of TRH. Food intake and body weight were monitored daily. Oxygen consumption, physical activity and rectal temperature were assessed after the treatment. After being killed, the hypothalamus and the brown adipose tissue were collected and the expression of related genes and proteins related was analyzed. In other experiments, control or leucine-deficient medium incubated primary cultured neurons were either infected with adenovirus-mediated short hairpin RNA targeting extracellular signal-regulated kinases 1 and 2 (Ad-shERK1/2) or transfected with plasmid-overexpressing protein phosphatase 1 regulatory subunit 3C (PPP1R3C).

RESULTS

I.c.v. administration of anti-TRH antibodies significantly reduced leucine deprivation-stimulated energy expenditure. Furthermore, the effects of i.c.v. TRH antibodies were reversed by i.p. injection of T3 during leucine deprivation. Moreover, i.c.v. injection of Ad-shCREB (adenovirus-mediated short hairpin RNA targeting CREB) significantly suppressed leucine deprivation-stimulated energy expenditure via modulation of TRH expression. Lastly, TRH expression was regulated by CREB, which was phosphorylated by ERK1/2 and dephosphorylated by PPP1R3C-containing protein Ser/Thr phosphatase type 1 (PP1) under leucine deprivation in vitro.

CONCLUSIONS

Our data indicate a novel role for TRH in regulating energy expenditure via T3 during leucine deprivation. Furthermore, our findings reveal that TRH expression is activated by CREB, which is phosphorylated by ERK1/2 and dephosphorylated by PPP1R3C-containing PP1. Collectively, our studies provide novel insights into the regulation of energy homeostasis by the CNS in response to an essential amino-acid deprivation.

Diversity of human tRNA genes from the 1000-genomes project

The sequence diversity of individual human genomes has been extensively analyzed for variations and phenotypic implications for mRNA, miRNA, and long non-coding RNA genes. TRNA (tRNA) also exhibits large sequence diversity in the human genome, but tRNA gene sequence variation and potential functional implications in individual human genomes have not been investigated. Here we capitalize on the sequencing data from the 1000-genomes project to examine the diversity of tRNA genes in the human population. Previous analysis of the reference human genome indicated an unexpected large number of diverse tRNA genes beyond the necessity of translation, suggesting that some tRNA transcripts may perform non-canonical functions. We found 24 new tRNA sequences in>1% and 76 new tRNA sequences in>0.2% of all individuals, indicating that tRNA genes are also subject to evolutionary changes in the human population. Unexpectedly, two abundant new tRNA genes contain base-pair mismatches in the anticodon stem. We experimentally determined that these two new tRNAs have altered structures in vitro; however, one new tRNA is not aminoacylated but extremely stable in HeLa cells, suggesting that this new tRNA can be used for non-canonical function. Our results show that at the scale of human population, tRNA genes are more diverse than conventionally understood, and some new tRNAs may perform non-canonical, extra-translational functions that may be linked to human health and disease.

Chemical genetics reveals a kinase-independent role for protein kinase R in pyroptosis

Formation of the inflammasome, a scaffolding complex that activates caspase-1, is important in numerous diseases. Pyroptotic cell death induced by anthrax lethal toxin (LT) is a model for inflammasome-mediated caspase-1 activation. We discovered 7-desacetoxy-6,7-dehydrogedunin (7DG) in a phenotypic screen as a small molecule that protects macrophages from LT-induced death. Using chemical proteomics, we identified protein kinase R (PKR) as the target of 7DG and show that RNAi knockdown of PKR phenocopies treatment with 7DG. Further, we show that PKR's role in ASC assembly and caspase-1 activation induced by several different inflammasome stimuli is independent of PKR's kinase activity, demonstrating that PKR has a previously uncharacterized role in caspase-1 activation and pyroptosis that is distinct from its reported kinase-dependent roles in apoptosis and inflammasome formation in lipopolysaccharide-primed cells. Remarkably, PKR has different roles in two distinct cell death pathways and has a broad role in inflammasome function relevant in other diseases.

A mathematical model of the unfolded protein stress response reveals the decision mechanism for recovery, adaptation and apoptosis

Background

The unfolded protein response (UPR) is a major signalling cascade acting in the quality control of protein folding in the endoplasmic reticulum (ER). The cascade is known to play an accessory role in a range of genetic and environmental disorders including neurodegenerative and cardiovascular diseases, diabetes and kidney diseases. The three major receptors of the ER stress involved with the UPR, i.e. IRE1 α, PERK and ATF6, signal through a complex web of pathways to convey an appropriate response. The emerging behaviour ranges from adaptive to maladaptive depending on the severity of unfolded protein accumulation in the ER; however, the decision mechanism for the switch and its timing have so far been poorly understood.

Results

Here, we propose a mechanism by which the UPR outcome switches between survival and death. We compose a mathematical model integrating the three signalling branches, and perform a comprehensive bifurcation analysis to investigate possible responses to stimuli. The analysis reveals three distinct states of behaviour, low, high and intermediate activity, associated with stress adaptation, tolerance, and the initiation of apoptosis. The decision to adapt or destruct can, therefore, be understood as a dynamic process where the balance between the stress and the folding capacity of the ER plays a pivotal role in managing the delivery of the most appropriate response. The model demonstrates for the first time that the UPR is capable of generating oscillations in translation attenuation and the apoptotic signals, and this is supplemented with a Bayesian sensitivity analysis identifying a set of parameters controlling this behaviour.

Conclusions

This work contributes largely to the understanding of one of the most ubiquitous signalling pathways involved in protein folding quality control in the metazoan ER. The insights gained have direct consequences on the management of many UPR-related diseases, revealing, in addition, an extended list of candidate disease modifiers. Demonstration of stress adaptation sheds light to how preconditioning might be beneficial in manifesting the UPR outcome to prevent untimely apoptosis, and paves the way to novel approaches for the treatment of many UPR-related conditions.

ER stress activates NF-κB by integrating functions of basal IKK activity, IRE1 and PERK

NF-κB, a transcription factor, becomes activated during the Unfolded Protein Response (UPR), an endoplasmic reticulum (ER) stress response pathway. NF-κB is normally held inactive by its inhibitor, IκBα. Multiple cellular pathways activate IKK (IκBα Kinase) which phosphorylate IκBα leading to its degradation and NF-κB activation. Here, we find that IKK is required for maximum activation of NF-κB in response to ER stress. However, unlike canonical NFκB activation, IKK activity does not increase during ER stress, but rather the level of basal IKK activity is critical for determining the extent of NF-κB activation. Furthermore, a key UPR initiator, IRE1, acts to maintain IKK basal activity through IRE1's kinase, but not RNase, activity. Inputs from IRE1 and IKK, in combination with translation repression by PERK, another UPR initiator, lead to maximal NF-κB activation during the UPR. These interdependencies have a significant impact in cancer cells with elevated IKK/NF-κB activity such as renal cell carcinoma cells (786-0). Inhibition of IKK by an IKK inhibitor, which significantly decreases NF-κB activity, is overridden by UPR induction, arguing for the importance of considering UPR activation in cancer treatment.

PKCδ Regulates Translation Initiation through PKR and eIF2α in Response to Retinoic Acid in Acute Myeloid Leukemia Cells

Translation initiation and activity of eukaryotic initiation factor-alpha (eIF2α), the rate-limiting step of translation initiation, is often overactivated in malignant cells. Here, we investigated the regulation and role of eIF2α in acute promyelocytic (APL) and acute myeloid leukemia (AML) cells in response to all-trans retinoic acid (ATRA) and arsenic trioxide (ATO), the front-line therapies in APL. ATRA and ATO induce Ser-51 phosphorylation (inactivation) of eIF2α, through the induction of protein kinase C delta (PKCδ) and PKR, but not other eIF2α kinases, such as GCN2 and PERK in APL (NB4) and AML cells (HL60, U937, and THP-1). Inhibition of eIF2α reduced the expression of cellular proteins that are involved in apoptosis (DAP5/p97), cell cycle (p21Waf1/Cip1), differentiation (TG2) and induced those regulating proliferation (c-myc) and survival (p70S6K). PI3K/Akt/mTOR pathway is involved in regulation of eIF2α through PKCδ/PKR axis. PKCδ and p-eIF2α protein expression levels revealed a significant association between the reduced levels of PKCδ (P = 0.0378) and peIF2 (P = 0.0041) and relapses in AML patients (n = 47). In conclusion, our study provides the first evidence that PKCδ regulates/inhibits eIF2α through induction of PKR in AML cells and reveals a novel signaling mechanism regulating translation initiation.

Post-transcriptional control of type I interferon induction by porcine reproductive and respiratory syndrome virus in its natural host cells

Porcine reproductive and respiratory syndrome virus (PRRSV) is not only a poor inducer of type I interferon but also inhibits the efficient induction of type I interferon by porcine transmissible gastroenteritis virus (TGEV) and synthetic dsRNA molecules, Poly I:C. However, the mechanistic basis by which PRRSV interferes with the induction of type I interferon in its natural host cells remains less well defined. The purposes of this review are to summarize the key findings in supporting the post-transcriptional control of type I interferon in its natural host cells and to propose the possible role of translational control in the regulation of type I interferon induction by PRRSV.

Sigma-1Rs are upregulated via PERK/eIF2α/ATF4 pathway and execute protective function in ER stress

Sigma-1 receptors (Sig-1Rs) are the ER resident proteins. Sig-1Rs in the brain have been reported to be significantly reduced in patients with schizophrenia. The impediment of regulating Sig-1Rs expression levels increases the risk for schizophrenia. Thus elucidating the mechanism regulating Sig-1Rs expression might provide the strategy to prevent mental disorders. In this study, we have demonstrated that Sig-1Rs were transcriptionally upregulated by ATF4 in ER stress. Moreover, ATF4 directly bounds to the 5' flanking region of Sig-1R gene. The reporter activities using this region were enhanced in ER stress, or by ATF4 alone. The reporter activities with the pathogenic polymorphisms (GC-241-240TT, T-485A) were reduced. In addition, the processing of Caspase-4 was inhibited by Sig-1Rs. These results indicate that Sig-1Rs are transcriptionally upregulated via the PERK/eIF2α/ATF4 pathway and ameliolate cell death signaling. This study is the first report identifying the transcription factor regulating Sig-1Rs expression.

Stress Granules and Virus Replication

Viruses are dependent on the cellular translation machinery for protein synthesis. Part of the innate immune response to infection is activation of the stress kinase PKR which phosphorylates the alpha subunit of the initiation factor eIF2. This results in inhibition of translation and is intended to block virus replication. A downstream effect of translational shutoff involves the formation of cytoplasmic granules, termed stress granules (SGs), that contain mRNAs, initiation factors, ribosomal subunits, and other mRNA regulatory proteins. SGs hold mRNAs in a translationally inactive state until cells recover from stress. Recent studies have begun to elucidate the impact of SGs on virus replication. Not surprisingly, viruses from diverse families have been found to modulate SG formation in infected cells by associating with important SG effecter proteins. This review describes the current knowledge on SGs and their interaction with and impact on virus replication.

Aquatic birnavirus-induced ER stress-mediated death signaling contribute to downregulation of Bcl-2 family proteins in salmon embryo cells

Aquatic birnavirus induces mitochondria-mediated cell death, but whether connects to endoplasmic reticulum (ER) stress is still unknown. In this present, we characterized that IPNV infection triggers ER stress-mediated cell death via PKR/eIF2α phosphorylation signaling for regulating the Bcl-2 family protein expression in fish cells. The IPNV infection can induce ER stress as follows: (1) ER stress sensor ATF6 cleavaged; (2) ER stress marker GRP78 upregulation, and (3) PERK/eIF2α phosphorylation. Then, the IPNV-induced ER stress signals can induce the CHOP expression at early (6 h p.i.) and middle replication (12 h p.i.) stages. Moreover, IPNV-induced CHOP upregulation dramatically correlates to apparently downregulate the Bcl-2 family proteins, Bcl-2, Mcl-1 and Bcl-xL at middle replication stage (12 h p.i.) and produces mitochondria membrane potential (MMP) loss and cell death. Furthermore, with GRP78 synthesis inhibitor momitoxin (VT) and PKR inhibitor 2-aminopurine (2-AP) treatment for blocking GRP78 expression and eIF2α phosphorylation, PKR/PERK may involve in eIF2α phosphorylation/CHOP upregulation pathway that enhances the downstream regulators Bcl-2 family proteins expression and increased cell survival. Taken together, our results suggest that IPNV infection activates PKR/PERK/eIF2α ER stress signals for regulating downstream molecules CHOP upregulation and Bcl-2 family downregulation that led to induce mitochondria-mediated cell death in fish cells, which may provide new insight into RNA virus pathogenesis and disease.

Hyperthermia induces the ER stress pathway

BACKGROUND

The ER chaperone GRP78/BiP is a homolog of the Hsp70 family of heat shock proteins, yet GRP78/BiP is not induced by heat shock but instead by ER stress. However, previous studies had not considered more physiologically relevant temperature elevation associated with febrile hyperthermia. In this report we examine the response of GRP78/BiP and other components of the ER stress pathway in cells exposed to 40°C.

METHODOLOGY

AD293 cells were exposed to 43°C heat shock to confirm inhibition of the ER stress response genes. Five mammalian cell types, including AD293 cells, were then exposed to 40°C hyperthermia for various time periods and induction of the ER stress pathway was assessed.

PRINCIPAL FINDINGS

The inhibition of the ER stress pathway by heat shock (43°C) was confirmed. In contrast cells subjected to more mild temperature elevation (40°C) showed either a partial or full ER stress pathway induction as determined by downstream targets of the three arms of the ER stress pathway as well as a heat shock response. Cells deficient for Perk or Gcn2 exhibit great sensitivity to ER stress induction by hyperthermia.

CONCLUSIONS

The ER stress pathway is induced partially or fully as a consequence of hyperthermia in parallel with induction of Hsp70. These findings suggest that the ER and cytoplasm of cells contain parallel pathways to coordinately regulate adaptation to febrile hyperthermia associated with disease or infection.

Endoplasmic reticulum stress inducers provide protection against 6-hydroxydopamine-induced cytotoxicity

6-Hydroxydopamine (6-OHDA) is a neurotoxin used to establish experimental models of Parkinson's disease. Exposure to 6-OHDA results in cell death associated with oxidative stress. Pretreatments with sublethal oxidative stress and some pharmacological drugs have been shown to exert preconditioning effects on cytotoxicity caused by 6-OHDA. In this study, we investigated whether endoplasmic reticulum (ER) stress exerts preconditioning effects on 6-OHDA-induced cytotoxicity in human neuroblastoma SH-SY5Y cells. Pretreatment with ER stress inducers, thapsigargin (Tg) and tunicamycin (Tm), promoted GRP78 mRNA induction and ATF4 translation, which are ER stress markers, under our experimental conditions and protected against the cytotoxicity. The protective effect of Tg was more potent than that of Tm. We also found that Tg induced the expression of the antioxidant gene heme oxygenase-1 (HO-1) in a dose-dependent manner, whereas Tm had a weak effect on HO-1 induction. Flow cytometric analysis revealed that reactive oxygen species generated by 6-OHDA were more effectively suppressed in cells pretreated with Tg than with Tm. Therefore, it is likely that Tg enhances antioxidative defenses in SH-SY5Y cells compared with Tm. Because actinomycin D inhibited HO-1 induction by Tg, the induction of HO-1 may be regulated at the transcriptional level. Moreover, the specific eIF2α phosphatase inhibitor salubrinal augmented Tg-induced HO-1 expression. Therefore, the downstream signaling pathway of eIF2α might be involved in Tg-induced HO-1 expression. On the other hand, the reporter assay revealed that Tg stimulated the antioxidant response element (ARE) that is located in regulatory regions of antioxidant genes such as HO-1. Taken together, our data suggest that preconditioning effects induced by Tg mediate an adaptive response to 6-OHDA-induced cytotoxicity via phosphorylation of eIF2α and activation of the ARE.

Cytochrome P450 regulation: the interplay between its heme and apoprotein moieties in synthesis, assembly, repair, and disposal

Heme is vital to our aerobic universe. Heme cellular content is finely tuned through an exquisite control of synthesis and degradation. Heme deficiency is deleterious to cells, whereas excess heme is toxic. Most of the cellular heme serves as the prosthetic moiety of functionally diverse hemoproteins, including cytochromes P450 (P450s). In the liver, P450s are its major consumers, with >50% of hepatic heme committed to their synthesis. Prosthetic heme is the sine qua non of P450 catalytic biotransformation of both endo- and xenobiotics. This well-recognized functional role notwithstanding, heme also regulates P450 protein synthesis, assembly, repair, and disposal. These less well-appreciated aspects are reviewed herein.

Novel small molecule drugs inhibit tumor cell metabolism and show potent anti-tumorigenic potential

BACKGROUND

Rapidly dividing tumor cells have an increased demand for nutrients to support their characteristic unabated growth; this demand is met by an increased availability of nutrients such as amino acids through vasculogenesis and by the enhanced cellular entry of nutrients through the upregulation of specific transporters. Deprivation of intracellular amino acids or block of amino acid uptake has been shown to be cytotoxic to many established human cancer cell lines in vitro and in human cancer xenograft models.

RESULTS

In this paper, we provide evidence that the two small molecule oxyphenisatine analogs TOP001 and TOP216 exert their anti-cancer effect by affecting tumor cell metabolism and inducing intracellular amino acid deprivation, leading to a block of cell proliferation. GCN2-mediated phosphorylation of eIF2α as well as mTOR pathway inhibition supports the above notion. In addition, these novel anti-cancer compounds inhibit DNA and protein synthesis and induce apoptosis in a broad spectrum of cancer cell lines. In vivo, the compounds induce tumor stasis and regression in mouse xenograft models of human breast, prostate, ovarian and pancreatic cancer, both when administered intravenously and orally.

CONCLUSION

In conclusion, these small molecules, built on a 1,3-dihydroindole-2-one scaffold, elicit strong anti-proliferative and cytotoxic activity, and importantly, a strong anti-tumorigenicity is observed in in vivo xenograft models of human breast, ovary, prostate and pancreatic cancers encouraging the translation of this class of compounds into the clinic.

Translational control of eukaryotic gene expression

Translational control mechanisms are, besides transcriptional control and mRNA stability, the most determining for final protein levels. A large number of accessory factors that assist the ribosome during initiation, elongation, and termination of translation are required for protein synthesis. Cap-dependent translational control occurs mainly during the initiation step, involving eukaryotic initiation factors (eIFs) and accessory proteins. Initiation is affected by various stimuli that influence the phosphorylation status of both eIF4E and eIF2 and through binding of 4E-binding proteins to eIF4E, which finally inhibits cap- dependent translation. Under conditions where cap-dependent translation is hampered, translation of transcripts containing an internal ribosome entry site can still be supported in a cap-independent manner. An interesting example of translational control is the switch between cap-independent and cap-dependent translation during the eukaryotic cell cycle. At the G1-to-S transition, translation occurs predominantly in a cap-dependent manner, while during the G2-to-M transition, cap-dependent translation is inhibited and transcripts are predominantly translated through a cap-independent mechanism.

Adenosine deaminase ADAR1 increases gene expression at the translational level by decreasing protein kinase PKR-dependent eIF-2alpha phosphorylation

ADAR1 (adenosine deaminase acting on RNA) catalyzes the deamination of adenosine to inosine on RNA substrates with double-stranded character. Here, we show that coexpression of ADAR1 in mammalian cells markedly increases plasmid-based gene expression in transfected cells. The enhanced expression was independent of the nature of the promoter (viral and cellular) used to drive gene expression, of the protein reporter (luciferase and RRP) tested, and of the human cell line examined (293T and HeLa). Exogenous protein levels were increased by approximately 20-fold to approximately 50-fold when ADAR1 was coexpressed, whereas RNA transcript levels changed by less than 2-fold. The activation of PKR (protein kinase regulated by RNA) protein kinase and the phosphorylation of translation initiation factor eIF-2alpha seen following plasmid DNA transfection were both greatly reduced in ADAR1-transfected cells. Stable knockdown of the PKR kinase increased reporter gene expression in the absence, but not in the presence, of ADAR1 coexpression. Both size forms of ADAR1-the p150-inducible form and the p110-like constitutive form-enhanced plasmid-based gene expression. Taken together, these results indicate that the ADAR1 deaminase increases exogenous gene expression at the translational level by decreasing PKR-dependent eIF-2alpha phosphorylation.

Old target new approach: an alternate NF-kappaB activation pathway via translation inhibition

Activation of the transcription factor NF-kappaB is a highly regulated multi-level process. The critical step during activation is the release from its inhibitor IkappaB, which as any other protein is under the direct influence of translation regulation. In this review, we summarize in detail the current understanding of the impact of translational regulation on NF-kappaB activation. We illustrate a newly developed mechanism of eIF2alpha kinase-mediated IkappaB depletion and subsequent NF-kappaB activation. We also show that the classical NF-kappaB activation pathways occur simultaneously with, and are complemented by, translational down regulation of the inhibitor molecule IkappaB, the importance of one or the other being shifted in accordance with the type and magnitude of the stressing agent or stimuli.

Expression of HBX, an oncoprotein of hepatitis B virus, blocks reoviral oncolysis of hepatocellular carcinoma cells

Although reovirus has been used in tests as a potential cancer therapeutic agent against a variety of cancer cells, its application to hepatocellular carcinoma cells, in which the hepatitis B virus (HBV) X (HBX) protein of HBV plays a primary role, has not yet been explored. Here, we describe experiments in which we use reovirus to treat Chang liver carcinoma cells expressing either a vector only (Chang-vec) or a vector encoding HBX protein (Chang-HBX). Although Chang-vec cells readily support reoviral proliferation and undergo apoptosis, Chang-HBX cells are highly resistant to reoviral infection and virus-induced apoptosis, even though HBX protein induces activation of Ras and inactivation of PKR, which are normally thought to enhance reoviral oncolysis. The resistance of Chang-HBX cells to reovirus may instead be explained by HBX-induced downregulation of death receptor 5 and activation of Stat1. Phosphorylated Stat1 activates interferon (IFN)-stimulated regulatory element (ISRE)- and IFN-gamma-activated sequence (GAS)-mediated transcription, leading to the production of IFN-beta, whereas the reduced expression of Stat1 with its siRNA results in a decrease in IFN-beta production, by which Chang-HBX cells eventually succumb to reovirus infection. This result further indicates that HBX induces the establishment of an antiviral state through Stat1 activation. Thus, it appears that active Ras does not override the antiviral effect mediated by the activation of Stat1. Accordingly, we report that HBX, an oncoprotein of HBV, can prevent reoviral oncolysis of hepatocellular carcinoma. This suggests there may be limits to the practical application of reovirus in the treatment of human cancers already expressing other oncoviral proteins.

Essential role of PACT-mediated PKR activation in tunicamycin-induced apoptosis

Cellular stresses such as disruption of calcium homeostasis, inhibition of protein glycosylation, and reduction of disulfide bonds result in accumulation of misfolded proteins in the endoplasmic reticulum (ER) and lead to cell death by apoptosis. Tunicamycin, which is an inhibitor of protein glycosylation, induces ER stress and apoptosis. In this study, we examined the involvement of double-stranded RNA (dsRNA)-activated protein kinase (PKR) and its protein activator PACT in tunicamycin-induced apoptosis. We demonstrate for the first time that PACT is phosphorylated in response to tunicamycin and is responsible for PKR activation by direct interaction. Furthermore, PACT-induced PKR activation is essential for tunicamycin-induced apoptosis, since PACT as well as PKR null cells are markedly resistant to tunicamycin and show defective eIF2alpha phosphorylation and C/EBP homologous protein (CHOP, also known as GADD153) induction especially at low concentrations of tunicamycin. Reconstitution of PKR and PACT expression in the null cells renders them sensitive to tunicamycin, thus demonstrating that PACT-induced PKR activation plays an essential function in induction of apoptosis.

Paramyxovirus-induced shutoff of host and viral protein synthesis: role of the P and V proteins in limiting PKR activation

The paramyxovirus simian virus 5 (SV5) establishes highly productive persistent infections of epithelial cells without inducing a global inhibition of translation. Here we show that an SV5 mutant (the P/V-CPI(-) mutant) with substitutions in the P subunit of the viral polymerase and the accessory V protein also establishes highly productive infections like wild-type (WT) SV5 but that cells infected with the P/V-CPI(-) mutant show an overall shutdown of both host and viral translation at late times postinfection. Reduced host and viral protein synthesis with the P/V-CPI(-) virus was not due to lower levels of mRNA or caspase-dependent apoptosis and correlated with phosphorylation of the translation initiation factor eIF-2alpha. WT SV5 was a poor activator of the eIF-2alpha kinase protein kinase R (PKR). By contrast, the P/V-CPI(-) mutant induced PKR phosphorylation, which correlated with the time course of translation inhibition but was independent of interferon signaling. In HeLa cells that expressed the PKR inhibitor influenza A virus NS1 or reovirus sigma3, the rate of host protein synthesis at late times after infection with the P/V-CPI(-) mutant was restored to approximately 50% that of control HeLa cells. By contrast, the rates of P/V-CPI(-) viral protein synthesis in HeLa cells expressing NS1 or sigma3 were dramatically enhanced, between 5- and 20-fold, while levels of viral mRNA were increased only slightly (NS1-expressing cells) or remained constant (sigma3-expressing cells). Similar results were found using HeLa cells where PKR levels were reduced due to knockdown by small interfering RNA. Expression of either the WT P or the WT V protein from the genome of the P/V-CPI(-) mutant resulted in lower levels of PKR activation and rates of host and viral protein synthesis that closely matched those seen with WT SV5. Despite higher rates of translation, cells infected with the V- or P-complemented virus accumulated viral mRNAs to lower levels than that seen with the parental P/V-CPI(-) mutant. We present a model in which the paramyxovirus P/V gene products limit induction of PKR by limiting the synthesis of aberrant viral mRNAs and double-stranded RNA and thus prevent the shutdown of translation by a mechanism that differs from that of other PKR inhibitors such as NS1 and sigma3.

Protein kinase PKR plays a stimulus- and virus-dependent role in apoptotic death and virus multiplication in human cells

The protein kinase regulated by double-stranded RNA (dsRNA), PKR, is implicated in a range of biologic processes, including apoptotic death and interferon antiviral responses, based in part on studies with mouse cells genetically deficient in Pkr. To test the role of the PKR protein in human cells, an RNA interference silencing strategy was used to generate stable HeLa cell lines with less than 2% of the PKR protein (PKR deficient) compared to either parental or control knockdown HeLa lines. Phosphorylation of the alpha subunit of eukaryotic initiation factor 2 on serine 51 was not detectably increased in response to dsRNA in PKR-deficient HeLa cells but was elevated severalfold in PKR-sufficient cells. PKR-deficient cells displayed reduced dsRNA-induced apoptosis compared to PKR-sufficient cell lines, whereas tumor necrosis factor alpha (TNF-alpha)-induced apoptosis was comparable between the HeLa lines. NF-kappaB was activated to a comparable extent in PKR-deficient and PKR-sufficient HeLa cells upon treatment with either dsRNA or TNF-alpha. The antiviral response against vesicular stomatitis virus was reduced in interferon-treated PKR-deficient compared to PKR-sufficient HeLa cells. However, the growth of two human viruses, adenovirus and reovirus, was unaffected by the PKR knockdown. Surprisingly, the yield of mutant adenovirus that fails to encode VAI RNA was not enhanced in PKR-deficient cells, indicating the importance of host factors in addition to PKR in conferring the VAI RNA phenotype.

Heme: a versatile signaling molecule controlling the activities of diverse regulators ranging from transcription factors to MAP kinases

Heme (iron protoporphyrin IX) is an essential molecule for numerous living organisms. Not only does it serve as a prosthetic group in enzymes, it also acts as a signaling molecule that controls diverse molecular and cellular processes ranging from signal transduction to protein complex assembly. Deficient heme synthesis or function impacts the hematopoietic, hepatic and nervous systems in humans. Recent studies have revealed a series of heme-regulated transcription factors and signal transducers including Hap1, a heme-activated transcription factor that mediates the effects of oxygen on gene transcription in the yeast Saccharomyces cerevisiae; Bach1, a transcriptional repressor that is negatively regulated by heme in mammalian cells; IRR, an iron regulatory protein that mediates the iron-dependant regulation of heme synthesis in the bacterium Bradyrhizobium japonicum; and heme-regulated inhibitor, an eucaryotic initiation factor 2alpha kinase that coordinates protein synthesis with heme availability in reticulocytes. In this review, we summarize the current knowledge about how heme controls the activity of these transcriptional regulators and signal transducers, and discuss diseases associated with defective heme synthesis, degradation and function.

Viruses, endoplasmic reticulum stress, and interferon responses

Viral infection induces endoplasmic reticulum (ER) stress and interferon responses. While viral double-stranded RNA intermediates trigger interferon responses, viral polypeptides synthesized during infection stimulate ER stress. Among the interferon-regulated gene products, the double-stranded RNA-dependent protein kinase (PKR) plays a key role in limiting viral replication. Thus, to establish productive infection, viruses have evolved mechanisms to overcome the deleterious effects of PKR. It has become clear that ER stress causes translational attenuation and transcriptional upregulation of genes encoding proteins that facilitate folding or degradation of proteins. Notably, prolonged ER stress triggers apoptosis. Therefore, viruses are confronted with the consequences of ER stress. Emerging evidence suggests that viruses not only interfere with the interferon system involving PKR but also manipulate the programs emanating from the ER in a complex way, which may facilitate viral replication or pathogenesis. This review highlights recent progress in these areas.

Unshackling the links between reovirus oncolysis, Ras signaling, translational control and cancer

Reovirus has an inherent preference for replicating in cells with dysregulated growth factor signaling cascades that comprise Ras activation. Precisely how reovirus exploits the host cell Ras pathway is unclear, but there is evidence suggesting that activated Ras signaling is important for efficient viral protein synthesis. Defining the molecular mechanism of reovirus oncolysis will shed light on reovirus replication and important aspects of cellular transformation, Ras signaling cascades and regulation of protein translation.

AMPK activation regulates apoptosis, adipogenesis, and lipolysis by eIF2alpha in adipocytes

AMP-activated protein kinase (AMPK) is a metabolic master switch regulating glucose and lipid metabolism. Recently, AMPK has been implicated in the control of adipose tissue content. Yet, the nature of this action is controversial. We examined the effect on F442a adipocytes of the AMPK activator-AICAR. Activation of AMPK induced dose-dependent apoptotic cell death, inhibition of lipolysis, and downregulatation key adipogenic genes, such as peroxisome proliferator-activated receptor (PPARgamma) and CCAAT/enhancer-binding protein alpha (C/EBPalpha). We have identified the alpha-subunit of the eukaryotic initiation factor-2 (eIF2alpha) as a target gene which is phosphorylated following AICAR treatment. Such phosphorylation is one of the best-characterized mechanisms for downregulating protein synthesis. 2-Aminopurine (2-AP), an inhibitor of eIF2alpha kinases, could overcome the apoptotic effect of AICAR, abolishing the reduction of PPARgamma and C/EBPalpha and the lipolytic properties of AMPK. Thus, AMPK may diminish adiposity via reduction of fat cell number through eIF2alpha-dependent translation shutdown.

Lead-induced upregulation of the heme-regulated eukaryotic initiation factor 2α kinase is compromised by hemin in human K562 cells

Expression and kinase activity of the heme-regulated-eIF-2alpha kinase or -inhibitor (HRI) are induced during cytoplasmic stresses leading to inhibition of protein synthesis. Using a reporter construct with HRI promoter, we have determined the promoter activity during heat-shock and lead toxicity in human K562 cells. These two conditions induced HRI promoter activity by 2- to 3-fold. Contrary to this, hemin, a suppressor of HRI kinase activity, downregulated HRI promoter activity and stimulated hemoglobin synthesis. Interestingly, when hemin-treated cells were transfected and exposed to lead, hemin compromised lead-effect substantially by downregulating HRI promoter activity, HRI transcription and HRI kinase activity. These results together suggest that heme signaling in relation to translation regulation is not only restricted to the cytoplasm (modulating HRI kinase activity) alone but it also spans to the nucleus modulating HRI expression. Hemin may thus be useful for alleviation of stress-induced inhibition of protein synthesis.

Coordination of ER and oxidative stress signaling: the PERK/Nrf2 signaling pathway

In the broadest sense, cellular stress describes conditions wherein cells encounter and react to a 'non-normal' state. Perturbations may originate through both extracellular and intracellular means. Whereas transient levels of stress are expected to occur on a regular basis, a series of checks and balances ensures that cells are well equipped to maintain a homeostatic state. In the case of supra-physiological stress signaling, cellular challenges are more severe, and programmed cell death may be the best option for the organism. The ability of a cell, and by extension, an organism, to adequately manage cellular stress is fundamental--a question of life or death. The endoplasmic reticulum (ER) is exquisitely poised to sense and respond to cellular stresses including those that result from metabolic and/or protein folding imbalances. In response to stress originating from within the ER, the PERK and Ire1 protein kinases, along with other proximal signaling molecules, initiate a program of transcriptional and translational regulation termed the unfolded protein response. A consequence of ER stress is the accumulation of reactive oxygen species that promotes a state of oxidative stress. PERK signaling, via activation of the Nrf2 and ATF4 transcription factors, coordinates the convergence of ER stress with oxidative stress signaling. Here we discuss progress regarding the signaling pathways involved in these cellular stresses and the implications of the intersection between the two signaling pathways.

The thioredoxin system protects ribosomes against stress-induced aggregation

We previously showed that thioredoxins are required for dithiothreitol (DTT) tolerance, suggesting they maintain redox homeostasis in response to both oxidative and reductive stress conditions. In this present study, we screened the complete set of viable deletion strains in Saccharomyces cerevisiae for sensitivity to DTT to identify cell functions involved in resistance to reductive stress. We identified 195 mutants, whose gene products are localized throughout the cell. DTT-sensitive mutants were distributed among most major biological processes, but they particularly affected gene expression, metabolism, and the secretory pathway. Strikingly, a mutant lacking TSA1, encoding a peroxiredoxin, showed a similar sensitivity to DTT as a thioredoxin mutant. Epistasis analysis indicated that thioredoxins function upstream of Tsa1 in providing tolerance to DTT. Our data show that the chaperone function of Tsa1, rather than its peroxidase function, is required for this activity. Cells lacking TSA1 were found to accumulate aggregated proteins, and this was exacerbated by exposure to DTT. Analysis of the protein aggregates revealed that they are predominantly composed of ribosomal proteins. Furthermore, aggregation was found to correlate with an inhibition of translation initiation. We propose that Tsa1 normally functions to chaperone misassembled ribosomal proteins, preventing the toxicity that arises from their aggregation.

GCN2 phosphorylation of eIF2alpha activates NF-kappaB in response to UV irradiation

In response to UV irradiation, mammalian cells elicit a gene expression programme designed to repair damage and control cell proliferation and apoptosis. Important members of this stress response include the NF-kappaB (nuclear factor-kappaB) family. However, the mechanisms by which UV irradiation activates NF-kappaB are not well understood. In eukaryotes, a variety of environmental stresses are recognized and remediated by a family of protein kinases that phosphorylate the alpha subunit of eIF2 (eukaryotic initiation factor-2). In the present study we show that NF-kappaB in MEF (murine embryo fibroblast) cells is activated by UV-C and UV-B irradiation through a mechanism requiring eIF2alpha phosphorylation. The primary eIF2alpha kinase in response to UV is GCN2 (general control non-derepressible-2), with PEK/PERK (pancreatic eIF2alpha kinase/RNA-dependent-protein-kinase-like endoplasmic-reticulum kinase) carrying out a secondary function. Our studies indicate that lowered protein synthesis accompanying eIF2alpha phosphorylation, combined with eIF2alpha kinase-independent turnover of IkappaBalpha (inhibitor of kappaBalpha), reduces the levels of IkappaBalpha in response to UV irradiation. Release of NF-kappaB from the inhibitory IkappaBalpha would facilitate NF-kappaB entry into the nucleus and targeted transcriptional control. We also find that loss of GCN2 in MEF cells significantly enhances apoptosis in response to UV exposure similar to that measured in cells deleted for the RelA/p65 subunit of NF-kappaB. These results demonstrate that GCN2 is central to recognition of UV stress, and that eIF2alpha phosphorylation provides resistance to apoptosis in response to this environmental insult.

New insights into erythropoiesis: the roles of folate, vitamin B12, and iron

Erythropoiesis is the process in which new erythrocytes are produced. These new erythrocytes replace the oldest erythrocytes (normally about one percent) that are phagocytosed and destroyed each day. Folate, vitamin B12, and iron have crucial roles in erythropoiesis. Erythroblasts require folate and vitamin B12 for proliferation during their differentiation. Deficiency of folate or vitamin B12 inhibits purine and thymidylate syntheses, impairs DNA synthesis, and causes erythroblast apoptosis, resulting in anemia from ineffective erythropoiesis. Erythroblasts require large amounts of iron for hemoglobin synthesis. Large amounts of iron are recycled daily with hemoglobin breakdown from destroyed old erythrocytes. Many recently identified proteins are involved in absorption, storage, and cellular export of nonheme iron and in erythroblast uptake and utilization of iron. Erythroblast heme levels regulate uptake of iron and globin synthesis such that iron deficiency causes anemia by retarded production rates with smaller, less hemoglobinized erythrocytes.

Reinitiation involving upstream ORFs regulates ATF4 mRNA translation in mammalian cells

During cellular stresses, phosphorylation of eukaryotic initiation factor-2 (eIF2) elicits gene expression designed to ameliorate the underlying cellular disturbance. Central to this stress response is the transcriptional regulator activating transcription factor, ATF4. Here we describe the mechanism regulating ATF4 expression involving the differential contribution of two upstream ORFs (uORFs) in the 5' leader of the mouse ATF4 mRNA. The 5' proximal uORF1 is a positive-acting element that facilitates ribosome scanning and reinitiation at downstream coding regions in the ATF4 mRNA. When eIF2-GTP is abundant in nonstressed cells, ribosomes scanning downstream of uORF1 reinitiate at the next coding region, uORF2, an inhibitory element that blocks ATF4 expression. During stress conditions, phosphorylation of eIF2 and the accompanying reduction in the levels of eIF2-GTP increase the time required for the scanning ribosomes to become competent to reinitiate translation. This delayed reinitiation allows for ribosomes to scan through the inhibitory uORF2 and instead reinitiate at the ATF4-coding region. Increased expression of ATF4 would contribute to the expression of genes involved in remediation of cellular stress damage. These results suggest that the mechanism of translation reinitiation involving uORFs is conserved from yeast to mammals.

The role of translation in neoplastic transformation from a pathologist's point of view

Increased cell proliferation, which is a hallmark of aggressive malignant neoplasms, requires a general increase in protein synthesis and a specific increase in the synthesis of replication-promoting proteins. Transient increase in the general protein synthesis rate, as well as preferential translation of specific mRNAs coding for growth promoting proteins (e.g. cyclin D1), takes place during normal mitogenic response. A number of extensively studied growth signal transduction pathways (Ras, PI3K, MAPK, mTOR-dependent pathways) activate the function and expression of various components of the translational machinery. In abnormal situations, constitutive activation of signal transduction pathways (e.g. oncogenic activation of Ras or Myc) leads to continuous upregulation of key elements of translational machinery. On the other hand, tumor suppressor genes (p53, pRb) downregulate ribosomal and tRNA synthesis, and their inactivation results in uncontrolled production of these translational components. During recent years, a significant effort has been dedicated to determining whether expression of translation factors is increased in human tumors using clinical biopsy specimens. The results of these studies indicate that expression of particular translation initiation factors is not always increased in human neoplasms. The pattern of expression is characteristic for a particular tumor type. For example, eIF-4E is usually increased in bronchioloalveolar carcinomas but not in squamous cell carcinomas of the lung. Interestingly, in certain highly proliferative and aggressive neoplasms (e.g. squamous cell carcinoma of the lung, melanoma), the expression of eIF-4E is barely detectable. These findings suggest that mechanisms for increasing general protein synthesis in various neoplasms differ significantly. Finally, the possibility of qualitative alterations in the translational machinery, rather than a simple increase in the activity of its components, is discussed along with the possibility of targeting those qualitative differences for tumor therapy.

Dimerization Is Required for Activation of eIF2 Kinase Gcn2 in Response to Diverse Environmental Stress Conditions

In the yeast Saccharomyces cerevisiae, starvation for amino acids induces phosphorylation of the alpha subunit of eukaryotic initiation factor 2alpha by Gcn2 protein kinase, leading to elevated translation of GCN4. Gcn4p is a transcriptional activator of hundreds of genes involved in remedying nutrient deprivation. In addition to a conserved kinase domain, Gcn2p has a regulatory region homologous to histidyl tRNA synthetase enzymes that binds uncharged tRNA that accumulates during amino acid starvation. Flanking the carboxyl terminus of the histidyl-tRNA synthetase-related domain is a region spanning 162 residues that participates in the activation of the protein kinase. Gel filtration and chemical cross-linking analysis of the recombinant carboxyl-terminal Gcn2 protein revealed that this region is a stable homodimer that is highly resistant to high concentrations of salt. Residue alterations in three hydrophobic segments and one segment with a proposed amphipathic alpha-helix in this Gcn2p carboxyl terminus blocked oligomerization, supporting the role of hydrophobic interactions in the dimerization interface of Gcn2p. Introduction of residue substitutions that impaired dimerization into the full-length protein prevented the ability of Gcn2p to phosphorylate its substrate eukaryotic initiation factor-2alpha and induce GCN4 translational expression in yeast cells subjected to a variety of stresses including amino acid limitation or exposure to rapamycin or high levels of NaCl. This latter stress can be overcome by addition of increasing amounts of K+ ions, indicating that the Na+/K+ ion balance is central to this stress induction. We conclude that dimerization involving hydrophobic segments in the carboxyl-terminal region is required for activation of Gcn2p in response to a multitude of stresses.