Identification and characterization of pancreatic eukaryotic initiation factor 2 alpha-subunit kinase, PEK, involved in translational control

The role of the unfolded protein response in tumour development: friend or foe?

Having accumulated mutations that overcome cell-cycle and apoptotic checkpoints, the main obstacle to survival faced by a cancer cell is the restricted supply of nutrients and oxygen. These conditions impinge on protein folding in the endoplasmic reticulum and activate a largely cytoprotective signalling pathway called the unfolded protein response. Prolonged activation of this response can, however, terminate in apoptosis. Recent delineation of the components of this response, coupled with several clinical studies, indicate that it is uniquely poised to have a role in regulating the balance between cancer cell death, dormancy and aggressive growth, as well as altering the sensitivity of solid tumours to chemotherapeutic agents.

Endoplasmic reticulum stress links obesity, insulin action, and type 2 diabetes

Obesity contributes to the development of type 2 diabetes, but the underlying mechanisms are poorly understood. Using cell culture and mouse models, we show that obesity causes endoplasmic reticulum (ER) stress. This stress in turn leads to suppression of insulin receptor signaling through hyperactivation of c-Jun N-terminal kinase (JNK) and subsequent serine phosphorylation of insulin receptor substrate-1 (IRS-1). Mice deficient in X-box-binding protein-1 (XBP-1), a transcription factor that modulates the ER stress response, develop insulin resistance. These findings demonstrate that ER stress is a central feature of peripheral insulin resistance and type 2 diabetes at the molecular, cellular, and organismal levels. Pharmacologic manipulation of this pathway may offer novel opportunities for treating these common diseases.

Parasite-specific eIF2 (eukaryotic initiation factor-2) kinase required for stress-induced translation control

The ubiquitous intracellular parasite Toxoplasma gondii (phylum Apicomplexa) differentiates into an encysted form (bradyzoite) that can repeatedly re-emerge as a life-threatening acute infection (tachyzoite) upon impairment of immunity. Since the switch from tachyzoite to bradyzoite is a stress-induced response, we sought to identify components related to the phosphorylation of the alpha subunit of eIF2 (eukaryotic initiation factor-2), a well-characterized event associated with stress remediation in other eukaryotic systems. In addition to characterizing Toxoplasma eIF2alpha (TgIF2alpha), we have discovered a novel eIF2 protein kinase, designated TgIF2K-A (Toxoplasma gondii initiation factor-2kinase). Although the catalytic domain of TgIF2K-A contains sequence and structural features that are conserved among members of the eIF2 kinase family, TgIF2K-A has an extended N-terminal region that is highly divergent from other eIF2 kinases. TgIF2K-A specifically phosphorylates the regulatory serine residue of yeast eIF2alpha in vitro and in vivo, and can modulate translation when expressed in the yeast model system. We also demonstrate that TgIF2K-A phosphorylates the analogous regulatory serine residue of recombinant TgIF2alpha in vitro. Finally, we demonstrate that TgIF2alpha phosphorylation in tachyzoites is enhanced in response to heat shock or alkaline stress, conditions known to induce parasite differentiation in vitro. Collectively, this study suggests that eIF2 kinase-mediated stress responses are conserved in Apicomplexa, and a novel family member exists that may control parasite-specific events, including the clinically relevant conversion into bradyzoite cysts.

Coupling endoplasmic reticulum stress to the cell death program

The endoplasmic reticulum (ER) regulates protein synthesis, protein folding and trafficking, cellular responses to stress and intracellular calcium (Ca(2+)) levels. Alterations in Ca(2+) homeostasis and accumulation of misfolded proteins in the ER cause ER stress that ultimately leads to apoptosis. Prolonged ER stress is linked to the pathogenesis of several different neurodegenerative disorders. Apoptosis is a form of cell death that involves the concerted action of a number of intracellular signaling pathways including members of the caspase family of cysteine proteases. The two main apoptotic pathways, the death receptor ('extrinsic') and mitochondrial ('intrinsic') pathways, are activated by caspase-8 and -9, respectively, both of which are found in the cytoplasm. Recent studies point to the ER as a third subcellular compartment implicated in apoptotic execution. Here, we review evidence for the contribution of various cellular molecules that contribute to ER stress and subsequent cellular death. It is hoped that dissection of the molecular components and pathways that alter ER structure and function and ultimately promote cellular death will provide a framework for understanding degenerative disorders that feature misfolded proteins.

Glucose-stimulated protein synthesis in pancreatic beta-cells parallels an increase in the availability of the translational ternary complex (eIF2-GTP.Met-tRNAi) and the dephosphorylation of eIF2 alpha

In pancreatic beta-cells, glucose causes a rapid increase in the rate of protein synthesis. However, the mechanism by which this occurs is poorly understood. In this report, we demonstrate, in the pancreatic beta-cell line MIN6, that glucose stimulates the recruitment of ribosomes onto the mRNA, indicative of an increase in the rate of the initiation step of protein synthesis. This increase in the rate of initiation is not mediated through an increase in the availability of the initiation complex eIF4F, because glucose is unable to stimulate eIF4F assembly or, in the absence of amino acids, modulate the phosphorylation status of 4E-BP1. Moreover, in MIN6 cells and isolated islets of Langerhans, rapamycin, an inhibitor of the mammalian target of rapamycin, only partially inhibited glucose-stimulated protein synthesis. However, we show that glucose stimulates the dephosphorylation of eIF2 alpha in MIN6 cells and the assembly of the translational ternary complex, eIF2-GTP.Met-tRNAi, in both MIN6 cells and islets of Langerhans. The changes in the phosphorylation of eIF2 alpha are not mediated by the PKR-like endoplasmic reticulum eIF2 alpha kinase (PERK), because PERK is not phosphorylated at low glucose concentrations and overexpression of a dominant negative form of PERK has no significant effect on either glucose-stimulated protein synthesis or the phosphorylation of eIF2 alpha. Taken together, these results indicate that glucose-stimulated protein synthesis in pancreatic beta-cells is regulated by a mechanism largely independent of the activity of mammalian target of rapamycin, but which is likely to be dependent on the availability of the translational ternary complex, regulated by the phosphorylation status of eIF2 alpha.

Dilated cardiomyopathy caused by aberrant endoplasmic reticulum quality control in mutant KDEL receptor transgenic mice

Aberrant protein folding beyond the capacity of endoplasmic reticulum (ER) quality control leads to stress response in the ER. The Lys-Asp-Glu-Leu (KDEL) receptor, a retrieval receptor for ER chaperones in the early secretory pathway, contributes to ER quality control. To elucidate the function of the KDEL receptor in vivo, we established transgenic mice expressing a mutant KDEL receptor. We found that the mutant KDEL receptor sensitized cells to ER stress and that the mutant mice developed dilated cardiomyopathy. Ultrastructural analyses revealed expanded sarcoplasmic reticulums and protein aggregates that obstructed the adjacent transverse tubules of the mutant cardiomyocytes. Cardiomyocytes from the mutant mice were sensitive to ER stress when treated with tunicamycin and showed a functional defect in the L-type Ca(2+) current. We observed ubiquitinated protein aggregates, enhanced expression of CHOP (a death-related transcriptional factor expressed upon ER stress), and apoptosis in the mutant hearts. These findings suggest that impairment of the KDEL receptor disturbs ER quality control, resulting in accumulation of misfolded proteins in the ER in an in vivo system, and that the dilated cardiomyopathy found in the mutant KDEL receptor transgenic mice is associated with ER stress.

Endoplasmic reticulum stress stimulates the expression of cyclooxygenase-2 through activation of NF-kappaB and pp38 mitogen-activated protein kinase

Expression of mutant proteins or viral infection may interfere with proper protein folding activity in the endoplasmic reticulum (ER). Several pathways that maintain cellular homeostasis were activated in response to these ER disturbances. Here we investigated which of these ER stress-activated pathways induce COX-2 and potentially oncogenesis. Tunicamycin and brefeldin A, two ER stress inducers, increased the expression of COX-2 in ML-1 or MCF-7 cells. Nuclear translocation of NF-kappaB and activation of pp38 MAPK were observed during ER stress. IkappaBalpha kinase inhibitor Bay 11-7082 or IkappaBalpha kinase dominant negative mutant significantly inhibited the induction of COX-2. pp38 MAPK inhibitor SB203580 or eIF2alpha phosphorylation inhibitor 2-aminopurine attenuated the nuclear NF-kappaB DNA binding activity and COX-2 induction. Expression of mutant hepatitis B virus (HBV) large surface proteins, inducers of ER stress, enhanced the expression of COX-2 in ML-1 and HuH-7 cells. Transgenic mice showed higher expression of COX-2 protein in liver and kidney tissue expressing mutant HBV large surface protein in vivo. Similarly, increased expression of COX-2 mRNA was observed in human hepatocellular carcinoma tissue expressing mutant HBV large surface proteins. In ML-1 cells expressing mutant HBV large surface protein, anchorage-independent growth was enhanced, and the enhancement was abolished by the addition of specific COX-2 inhibitors. Thus, ER stress due either to expression of viral surface proteins or drugs can stimulate the expression of COX-2 through the NF-kappaB and pp38 kinase pathways. Our results provide important insights into cellular carcinogenesis associated with latent endoplasmic reticulum stress.

Global mRNA stabilization preferentially linked to translational repression during the endoplasmic reticulum stress response

The stability of mRNAs undergoing translation has long been a controversial question. Here, we systematically investigate links between mRNA turnover and translation during the endoplasmic reticulum (ER) stress response, a process during which protein synthesis is potently regulated. cDNA array-based approaches to assess the stability and translational status of each mRNA were devised. First, ER stress-triggered changes in mRNA stability were studied by comparing differences in steady-state mRNA levels with differences in gene transcription. Second, changes in translational status were monitored by studying ER stress-induced shifts in the relative distribution of each mRNA along sucrose gradients. Together, the array-derived data reveal complex links between mRNA stability and translation, with all regulatory groups represented: both stabilized and destabilized mRNAs were found among translationally induced as well as translationally suppressed mRNA collections. Remarkably, however, the subset of stabilized mRNAs was prominently enriched in translationally suppressed transcripts, suggesting that ER stress was capable of causing the stabilization of mRNAs associated with a global reduction in protein synthesis. The cDNA array-based approach described here can be applied to global analyses of mRNA turnover and translation and can serve to investigate subsets of mRNAs subject to joint posttranscriptional control.

Synaptic plasticity and translation initiation

It is widely accepted that protein synthesis, including local protein synthesis at synapses, is required for several forms of synaptic plasticity. Local protein synthesis enables synapses to control synaptic strength independent of the cell body via rapid protein production from pre-existing mRNA. Therefore, regulation of translation initiation is likely to be intimately involved in modulating synaptic strength. Our understanding of the translation-initiation process has expanded greatly in recent years. In this review, we discuss various aspects of translation initiation, as well as signaling pathways that might be involved in coupling neurotransmitter and neurotrophin receptors to the translation machinery during various forms of synaptic plasticity.

Caspase-12 and ER-stress-mediated apoptosis: the story so far

The labyrinth of the endoplasmic reticulum (ER) interweaves the cytosol and connects to the nucleus, mitochondria, and the plasma membrane. In the lumen of the ER, the essential function of lipid synthesis, Ca(2+) storage, folding, and maturation of proteins take place. Therefore, the tight regulation and maintenance of ER homeostasis is vital. Disturbance of the Ca(2+) homeostasis during hypoxia, or imbalance between the demand and capacity of the protein-folding apparatus, initiates an adaptive response of the cell, termed the unfolded protein response (UPR, ER stress response). As a result, ER-localized chaperones are induced, protein synthesis is slowed down, and a protein degrading system is initiated. However, if the ER stress cannot be alleviated, it culminates in apoptosis. This paper reviews the newly outlined signaling pathways of the unfolded protein response and describes the central role of caspase-12 in the initiation of cell death. The complex role of the ER and its signaling pathways provides a novel angle on apoptosis research and may offer a key to apoptosis-associated diseases.

The unfolded protein response represses differentiation through the RPD3-SIN3 histone deacetylase

In Saccharomyces cerevisiae, splicing of HAC1 mRNA is initiated in response to the accumulation of unfolded proteins in the endoplasmic reticulum by the transmembrane kinase-endoribonuclease Ire1p. Spliced Hac1p (Hac1ip) is a negative regulator of differentiation responses to nitrogen starvation, pseudohyphal growth, and meiosis. Here we show that the RPD3-SIN3 histone deacetylase complex (HDAC), its catalytic activity, recruitment of the HDAC to the promoters of early meiotic genes (EMGs) by Ume6p, and the Ume6p DNA-binding site URS1 in the promoters of EMGs are required for nitrogen-mediated negative regulation of EMGs and meiosis by Hac1ip. Co-immunoprecipitation experiments demonstrated that Hac1ip can interact with the HDAC in vivo. Systematic analysis of double deletion strains revealed that HAC1 is a peripheral component of the HDAC. In summary, nitrogen-induced synthesis of Hac1ip and association of Hac1ip with the HDAC are physiological events in the regulation of EMGs by nutrients. These data also define for the first time a gene class that is under negative control by the UPR, and provide the framework for a novel mechanism through which bZIP proteins repress transcription.

Herp Is Dually Regulated by Both the Endoplasmic Reticulum Stress-specific Branch of the Unfolded Protein Response and a Branch That Is Shared with Other Cellular Stress Pathways

The mammalian unfolded protein response (UPR) includes two major branches: one(s) specific to ER stress (Ire1/XBP-1 and ATF6-dependent), and one(s) shared by other cellular stresses (PERK/eIF-2alpha phosphorylation-dependent). Here, we demonstrate that the ER-localized protein Herp represents a second target, in addition to CHOP, that is dually regulated by both the shared and the ER stress-specific branches during UPR activation. For the first time, we are able to assess the contribution of each branch of the UPR in the induction of these targets. We demonstrate that activation of the shared branch of the UPR alone was sufficient to induce Herp and CHOP. ATF4 was not required during ER stress when both branches were used but did contribute significantly to their induction. Conversely, stresses that activated only the shared branch of the UPR were completely dependent on ATF4 for CHOP and Herp induction. Thus, the shared and the ER stress-specific branches of the UPR diverge to regulate two groups of targets, one that is ATF6 and Ire1/XBP-1-dependent, which includes BiP and XBP-1, and another that is eIF-2alpha kinase-dependent, which includes ATF4 and GADD34. The two branches also converge to maximally up-regulate targets like Herp and CHOP. Finally, our studies reveal that a PERK-dependent target other than ATF4 is contributing to the cross-talk between the two branches of the UPR that has previously been demonstrated.

Phosphorylation of eIF2alpha is involved in the signaling of indispensable amino acid deficiency in the anterior piriform cortex of the brain in rats

Sensing of indispensable amino acid (IAA) deficiency, an acute challenge to protein homeostasis, is demonstrated by rats as rejection of IAA-deficient diets within 20 min. The anterior piriform cortex (APC) of the brain in rats and birds is essential for this nutrient sensing, and is activated by IAA deficiency. Yet the mechanisms that sense and transduce IAA reduction to signaling in the APC, or indeed in any animal cells, are unknown. Because rejection of a deficient diet within 20 min is too rapid to be explained by transcription-derived signals, brain tissue was taken from rats after 20 min access to either a threonine-basal, -devoid, or -corrected diet and examined for proteins associated with early signaling of IAA deficiency in the yeast model. Western blots and immunohistochemistry showed that the phosphorylation of eukaryotic initiation factor 2-alpha (p-eIF2alpha[Ser51]) and translation of its downstream product, c-Jun, were increased (47%, P < 0.005, and 55%, P < 0.025, respectively) in APC from rats offered devoid, but not corrected diets, compared with those offered basal diets. This was not seen in other brain areas. In cells intensely labeled for cytoplasmic p-eIF2alpha, there was intense fluorescence for c-Jun in the nucleus. Thus, p-eIF2alpha, which is pivotal in the initiation of global protein translation, and its downstream product, the leucine zipper protein, c-Jun, are increased in the mammalian APC within the time frame necessary for the behavioral response. We suggest that p-eIF2alpha and c-Jun participate in signaling nutrient deficiency in the IAA-sensitive neurons of the APC.

Defects in translational regulation mediated by the alpha subunit of eukaryotic initiation factor 2 inhibit antiviral activity and facilitate the malignant transformation of human fibroblasts

Suppression of protein synthesis through phosphorylation of the translation initiation factor alpha subunit of eukaryotic initiation factor 2 (eIF2alpha) is known to occur in response to many forms of cellular stress. To further study this, we have developed novel cell lines that inducibly express FLAG-tagged versions of either the phosphomimetic eIF2alpha variant, eIF2alpha-S51D, or the phosphorylation-insensitive eIF2alpha-S51A. These variants showed authentic subcellular localization, were incorporated into endogenous ternary complexes, and were able to modulate overall rates of protein synthesis as well as influence cell division. However, phosphorylation of eIF2alpha failed to induce cell death or sensitize cells to killing by proapoptotic stimuli, though it was able to inhibit viral replication, confirming the role of eIF2alpha in host defense. Further, although the eIF2alpha-S51A variant has been shown to transform NIH 3T3 cells, it was unable to transform the murine fibroblast 3T3 L1 cell line. To therefore clarify this issue, we explored the role of eIF2alpha in growth control and demonstrated that the eIF2alpha-S51A variant is capable of collaborating with hTERT and the simian virus 40 large T antigen in the transformation of primary human kidney cells. Thus, dysregulation of translation initiation is indeed sufficient to cooperate with defined oncogenic elements and participate in the tumorigenesis of human tissue.

There is more to life and death than mitochondria: Bcl-2 proteins at the endoplasmic reticulum

Proteins of the Bcl-2 family are important regulators of cell fate. The role of these proteins in controlling mitochondrial apoptotic processes has been extensively investigated, although exact molecular mechanisms are incompletely understood. However, mounting evidence indicates that these proteins also function at the endoplasmic reticulum and other locations within the cell. Both pro- and anti-apoptotic Bcl-2 family members can regulate endoplasmic reticulum calcium, cellular pH and endoplasmic reticulum resident proteins. In this review, we discuss the activities and potential targets of Bcl-2 family members at the endoplasmic reticulum and other cellular locations.

Activating transcription factor 3 is integral to the eukaryotic initiation factor 2 kinase stress response

In response to environmental stress, cells induce a program of gene expression designed to remedy cellular damage or, alternatively, induce apoptosis. In this report, we explore the role of a family of protein kinases that phosphorylate eukaryotic initiation factor 2 (eIF2) in coordinating stress gene responses. We find that expression of activating transcription factor 3 (ATF3), a member of the ATF/CREB subfamily of basic-region leucine zipper (bZIP) proteins, is induced in response to endoplasmic reticulum (ER) stress or amino acid starvation by a mechanism requiring eIF2 kinases PEK (Perk or EIF2AK3) and GCN2 (EIF2AK4), respectively. Increased expression of ATF3 protein occurs early in response to stress by a mechanism requiring the related bZIP transcriptional regulator ATF4. ATF3 contributes to induction of the CHOP transcriptional factor in response to amino acid starvation, and loss of ATF3 function significantly lowers stress-induced expression of GADD34, an eIF2 protein phosphatase regulatory subunit implicated in feedback control of the eIF2 kinase stress response. Overexpression of ATF3 in mouse embryo fibroblasts partially bypasses the requirement for PEK for induction of GADD34 in response to ER stress, further supporting the idea that ATF3 functions directly or indirectly as a transcriptional activator of genes targeted by the eIF2 kinase stress pathway. These results indicate that ATF3 has an integral role in the coordinate gene expression induced by eIF2 kinases. Given that ATF3 is induced by a very large number of environmental insults, this study supports involvement of eIF2 kinases in the coordination of gene expression in response to a more diverse set of stress conditions than previously proposed.

PERK-dependent activation of Nrf2 contributes to redox homeostasis and cell survival following endoplasmic reticulum stress

The accumulation of unfolded proteins elicits a cellular response that triggers both pro-survival and pro-apoptotic signaling events. PERK-dependent activation of NF-E2-related factor-2 (Nrf2) is critical for survival signaling during this response; however, the mechanism whereby Nrf2 confers a protective advantage to stressed cells remains to be defined. We now demonstrate that Nrf2 activation contributes to the maintenance of glutathione levels, which in turn functions as a buffer for the accumulation of reactive oxygen species during the unfolded protein response. The deleterious effects of Nrf2 or PERK deficiencies could be attenuated by the restoration of cellular glutathione levels or Nrf2 activity. In addition, the inhibition of reactive oxygen species production attenuated apoptotic induction following endoplasmic reticulum stress. Our data suggest that perturbations in cellular redox status sensitize cells to the harmful effects of endoplasmic reticulum stress, but that other factors are essential for apoptotic commitment.

The double-stranded RNA-activated protein kinase mediates viral-induced encephalitis

The double-stranded (ds) RNA-activated protein kinase (PKR) plays an important role in control of viral infections and cell growth. We have studied the role of PKR in viral infection in mice that are defective in the PKR signaling pathway. Transgenic mice were derived that constitutively express a trans-dominant-negative kinase-defective mutant PKR under control of the beta-actin promoter. The trans-dominant-negative PKR mutant expressing transgenic mice do not have a detectable phenotype, similar to observations with PKR knock-out mice. The requirement for PKR in viral pathogenesis was studied by intracerebral infection of mice with a mouse-adapted poliovirus. Histopathological analysis revealed diffuse encephalomyelitis with severe inflammatory lesions throughout the central nervous system (CNS) in infected wild-type mice. In contrast, histopathological evaluation of virus-injected trans-dominant-negative PKR transgenic mice as well as PKR knock-out mice yielded no signs of tissue damage associated with inflammatory host responses. However, the virus did replicate in both models of PKR-deficient mice at a level equal to that observed in wild-type infected mice. Although the results indicate a clear difference in susceptibility to poliovirus-induced encephalitis, this difference manifests clinically as a slight delay in fatal neuropathy in trans-dominant-negative PKR transgenic and PKR knock-out animals. Our observations support the finding that viral-induced PKR activation may play a significant role in pathogenesis by mediating the host response to viral CNS infection. They support PKR to be an effective target to control tissue damage due to deleterious host responses to viral infection.

XBP-1 regulates a subset of endoplasmic reticulum resident chaperone genes in the unfolded protein response

The mammalian unfolded protein response (UPR) protects the cell against the stress of misfolded proteins in the endoplasmic reticulum (ER). We have investigated here the contribution of the UPR transcription factors XBP-1, ATF6alpha, and ATF6beta to UPR target gene expression. Gene profiling of cell lines lacking these factors yielded several XBP-1-dependent UPR target genes, all of which appear to act in the ER. These included the DnaJ/Hsp40-like genes, p58(IPK), ERdj4, and HEDJ, as well as EDEM, protein disulfide isomerase-P5, and ribosome-associated membrane protein 4 (RAMP4), whereas expression of BiP was only modestly dependent on XBP-1. Surprisingly, given previous reports that enforced expression of ATF6alpha induced a subset of UPR target genes, cells deficient in ATF6alpha, ATF6beta, or both had minimal defects in upregulating UPR target genes by gene profiling analysis, suggesting the presence of compensatory mechanism(s) for ATF6 in the UPR. Since cells lacking both XBP-1 and ATF6alpha had significantly impaired induction of select UPR target genes and ERSE reporter activation, XBP-1 and ATF6alpha may serve partially redundant functions. No UPR target genes that required ATF6beta were identified, nor, in contrast to XBP-1 and ATF6alpha, did the activity of the UPRE or ERSE promoters require ATF6beta, suggesting a minor role for it during the UPR. Collectively, these results suggest that the IRE1/XBP-1 pathway is required for efficient protein folding, maturation, and degradation in the ER and imply the existence of subsets of UPR target genes as defined by their dependence on XBP-1. Further, our observations suggest the existence of additional, as-yet-unknown, key regulators of the UPR.

Nrf2 is a direct PERK substrate and effector of PERK-dependent cell survival

Activation of PERK following the accumulation of unfolded proteins in the endoplasmic reticulum (ER) promotes translation inhibition and cell cycle arrest. PERK function is essential for cell survival following exposure of cells to ER stress, but the mechanisms whereby PERK signaling promotes cell survival are not thoroughly understood. We have identified the Nrf2 transcription factor as a novel PERK substrate. In unstressed cells, Nrf2 is maintained in the cytoplasm via association with Keap1. PERK-dependent phosphorylation triggers dissociation of Nrf2/Keap1 complexes and inhibits reassociation of Nrf2/Keap1 complexes in vitro. Activation of PERK via agents that trigger the unfolded protein response is both necessary and sufficient for dissociation of cytoplasmic Nrf2/Keap1 and subsequent Nrf2 nuclear import. Finally, we demonstrate that cells harboring a targeted deletion of Nrf2 exhibit increased cell death relative to wild-type counterparts following exposure to ER stress. Our data demonstrate that Nrf2 is a critical effector of PERK-mediated cell survival.

Delineation of a negative feedback regulatory loop that controls protein translation during endoplasmic reticulum stress

Transient protein synthesis inhibition is an important protective mechanism used by cells during various stress conditions including endoplasmic reticulum (ER) stress. This response centers on the phosphorylation state of eukaryotic initiation factor (eIF)-2 alpha, which is induced by kinases like protein kinase R-like ER kinase (PERK) and GCN2 to suppress translation and is later reversed so translation resumes. GADD34 was recently identified as the factor that activates the type 1 protein serine/threonine phosphatase (PP1), which dephosphorylates eIF-2 alpha during cellular stresses. Our study delineates a negative feedback regulatory loop in which the eIF-2 alpha-controlled inhibition of protein translation leads to GADD34 induction, which promotes translational recovery. We show that activating transcription factor-4 (ATF4), which is paradoxically translated during the eIF-2 alpha-mediated translational block, is required for the transactivation of the GADD34 promoter in response to ER stress and amino acid deprivation. ATF4 directly binds to and trans-activates a conserved ATF site in the GADD34 promoter during ER stress. Examination of ATF4-/- MEFs revealed an absence of GADD34 induction, prolonged eIF-2 alpha phosphorylation, delayed protein synthesis recovery, and diminished translational up-regulation of BiP during ER stress. These studies demonstrate the essential role of GADD34 in the resumption of protein synthesis, define the pathway for its induction, and reveal that cytoprotective unfolded protein response targets like BiP are sensitive to the eIF-2 alpha-mediated block in translation.

Translational control of protein synthesis in pancreatic acinar cells

Translational control of protein synthesis in the pancreas is important in regulating growth and the synthesis of digestive enzymes. Regulation of translation is primarily directed at the steps in initiation and involves reversible phosphorylation of initiation factors (eIFs) and ribosomal proteins. Major sites include the assembly of the eIF4F mRNA cap binding complex, the activity of guanine nucleotide exchange factor eIF2B, and the activity of ribosomal S6 kinase. All of these involve phosphorylation by different regulatory pathways. Stimulation of protein synthesis in acinar cells is primarily mediated by the phosphatidylinositol 3-kinase-mTOR pathway and involves both release of eIF4E (the limiting component of eIF4F) from its binding protein and phosphorylation of ribosomal S6 protein by S6K. eIF4E is itself phosphorylated by a distinct pathway. Inhibition of acinar protein synthesis can be mediated by inhibition of eIF2B following phosphorylation of eIF2alpha.

Proteasome inhibitors disrupt the unfolded protein response in myeloma cells

Novel agents that target the proteasome, a proteolytic complex responsible for the degradation of ubiquitinated proteins, have demonstrated remarkable therapeutic efficacy in multiple myeloma, a plasma cell malignancy. However, the mechanism by which these compounds act remains unknown. A signaling pathway called the unfolded protein response (UPR) allows cells to handle the proper folding of proteins. The transcription factor XBP-1, a regulator of the UPR, is also required for plasma cell differentiation, suggesting a link between the UPR and plasma cell differentiation. Here we show that proteasome inhibitors target XBP-1 and the UPR in myeloma cells. Proteasome inhibitors suppress the activity of the translumenal endoplasmic reticulum endoribonuclease/kinase, IRE1 alpha, to impair the generation of the active, spliced XBP-1 species and simultaneously stabilize the unspliced species that acts as a dominant negative. Myeloma cells rendered functionally deficient in XBP-1 undergo increased apoptosis in response to endoplasmic reticulum stress. Identification of compounds that target the activity of IRE1 alpha/XBP-1 may yield novel therapies for the treatment of multiple myeloma and other malignancies that rely on an intact UPR.

Phosphorylation of the alpha subunit of eukaryotic initiation factor 2 is required for activation of NF-kappaB in response to diverse cellular stresses

Nuclear factor kappaB (NF-kappaB) serves to coordinate the transcription of genes in response to diverse environmental stresses. In this report we show that phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF2) is fundamental to the process by which many stress signals activate NF-kappaB. Phosphorylation of this translation factor is carried out by a family of protein kinases that each respond to distinct stress conditions. During impaired protein folding and assembly in the endoplasmic reticulum (ER), phosphorylation of eIF2alpha by PEK (Perk or EIF2AK3) is essential for induction of NF-kappaB transcriptional activity. The mechanism by which NF-kappaB is activated during ER stress entails the release, but not the degradation, of the inhibitory protein IkappaB. During amino acid deprivation, phosphorylation of eIF2alpha by GCN2 (EIF2AK4) signals the activation of NF-kappaB. Furthermore, inhibition of general translation or transcription by cycloheximide and actinomycin D, respectively, elicits the eIF2alpha phosphorylation required for induction of NF-kappaB. Together, these studies suggest that eIF2alpha kinases monitor and are activated by a range of stress conditions that affect transcription and protein synthesis and assembly, and the resulting eIFalpha phosphorylation is central to activation of the NF-kappaB. The absence of NF-kappaB-mediated transcription and its antiapoptotic function provides an explanation for why eIF2alpha kinase deficiency in diseases such as Wolcott-Rallison syndrome leads to cellular apoptosis and disease.

The function of GADD34 is a recovery from a shutoff of protein synthesis induced by ER stress: elucidation by GADD34-deficient mice

GADD34 is a protein that is induced by stresses such as DNA damage. The function of mammalian GADD34 has been proposed by in vitro transfection, but its function in vivo has not yet been elucidated. Here we generated and analyzed GADD34 knockout mice. Despite their embryonic stage- and tissue-specific expressions, GADD34 knockout mice showed no abnormalities at fetal development and in early adult life. However, in GADD34-/- mouse embryonic fibroblasts (MEFs), recovery from a shutoff of protein synthesis was delayed when MEFs were exposed to endoplasmic reticulum (ER) stress. The phosphorylation of eukaryotic translation initiation factor 2 alpha (eIF2alpha) at Ser51 induced by thapsigargin or DTT was prolonged in GADD34-/- MEF, although following treatment with tunicamycin, the eIF2alpha phosphorylation level did not change in either GADD34+/+ or GADD34-/- cells. ER stress stimuli induced expressions of Bip (binding Ig protein) and CHOP (C/EBP homologous protein) in MEF of wild-type mice. These expressions were strongly reduced in GADD34-/- MEF, which suggests that GADD34 up-regulates Bip and CHOP. These results indicate that GADD34 works as a sensor of ER stress stimuli and recovers cells from shutoff of protein synthesis.

Cloning and characterization of an eukaryotic initiation factor-2alpha kinase from the silkworm, Bombyx mori

Eukaryotic initiation factor 2alpha (eIF-2alpha) kinases are involved in the translational regulations that occur in response to various types of environmental stress, and play an important role in the cellular defense system operating under unfavorable conditions. The identification of additional eIF-2alpha kinases and the elucidation of their functions are necessary to understand how different eIF-2alpha kinases can specifically respond to distinct stimuli. Here, we report a novel eIF-2alpha kinase, termed BeK, from the silkworm, Bombyx mori. This gene encodes 579 amino acids and contains all 11 catalytic domains of protein-serine/threonine kinases. Most notably, it contains an "Ile-Gln-Met-Xaa-Xaa-Cys" motif, which is highly conserved from yeast to mammalian eIF-2alpha kinases. BeK does not show any significant homology in the NH(2) terminal regulatory domain, suggesting a distinct regulatory mechanism of this novel eIF-2alpha kinase. BeK is ubiquitously expressed in the various tissues throughout the final larval stage. Importantly, BeK is activated in Drosophila Schneider cells following heat shock and osmotic stress, and activated-BeK has been shown to phosphorylate an eIF-2alpha subunit at the Ser(50) site. However, other forms of stress, such as immune stress, endoplasmic reticulum stress and oxidative stress, cannot significantly elicit BeK activity. Interestingly, the baculovirus gene product, PK2, can inhibit BeK enzymatic activity, suggesting that BeK may be an endogenous target for a viral gene product. Taken together, these data indicate that BeK is a novel eIF-2alpha kinase involved in the stress response in B. mori.

Stress tolerance of misfolded carboxypeptidase Y requires maintenance of protein trafficking and degradative pathways

The accumulation of aberrantly folded proteins can lead to cell dysfunction and death. Currently, the mechanisms of toxicity and cellular defenses against their effects remain incompletely understood. In the endoplasmic reticulum (ER), stress caused by misfolded proteins activates the unfolded protein response (UPR). The UPR is an ER-to-nucleus signal transduction pathway that regulates a wide variety of target genes to maintain cellular homeostasis. We studied the effects of ER stress in budding yeast through expression of the well-characterized misfolded protein, CPY*. By challenging cells within their physiological limits to resist stress, we show that the UPR is required to maintain essential functions including protein translocation, glycosylation, degradation, and transport. Under stress, the ER-associated degradation (ERAD) pathway for misfolded proteins is saturable. To maintain homeostasis, an "overflow" pathway dependent on the UPR transports excess substrate to the vacuole for turnover. The importance of this pathway was revealed through mutant strains compromised in the vesicular trafficking of excess CPY*. Expression of CPY* at levels tolerated by wild-type cells was toxic to these strains despite retaining the ability to activate the UPR.

Amino acids as regulators of gene expression at the level of mRNA translation

Amino acids act through a number of signaling pathways and mechanisms to mediate control of gene expression at the level of mRNA translation. This report reviews recent findings that illustrate the manner through which amino acids act to regulate the initiation phase of mRNA translation. The report focuses on signaling pathways that involve the eukaryotic initiation factor-2 (eIF2) protein kinase, general control non-derepressing kinase-2 and the mammalian target of rapamycin (mTOR) protein kinase. It also describes the mechanisms through which amino acid-induced modulation of eIF2 phosphorylation and mTOR-mediated signaling cause derepression of translation of specific mRNAs and result in an overall change in the pattern of gene expression. Finally, it provides examples of mRNAs whose translation is modulated through these mechanisms.

Transcriptional and translational control in the Mammalian unfolded protein response

Cells monitor the physiological load placed on their endoplasmic reticulum (ER) and respond to perturbations in ER function by a process known as the unfolded protein response (UPR). In metazoans the UPR has a transcriptional component that up-regulates expression of genes that enhance the capacity of the organelle to deal with the load of client proteins and a translational component that insures tight coupling between protein biosynthesis on the cytoplasmic side and folding in the ER lumen. Together, these two components adapt the secretory apparatus to physiological load and protect cells from the consequences of protein malfolding.

Plasma cell differentiation and the unfolded protein response intersect at the transcription factor XBP-1

The transcription factor X-box binding protein 1 (XBP-1) is essential for the differentiation of plasma cells and the unfolded protein response (UPR). Here we show that UPR-induced splicing of XBP-1 by the transmembrane endonuclease IRE1 is required to restore production of immunoglobulin in XBP-1-/- mouse B cells, providing an integral link between XBP-1, the UPR and plasma cell differentiation. Signals involved in plasma cell differentiation, specifically interleukin-4, control the transcription of XBP-1, whereas its post-transcriptional processing is dependent on synthesis of immunoglobulins during B cell differentiation. We also show that XBP-1 is involved in controlling the production of interleukin-6, a cytokine that is essential for plasma cell survival. Thus, signals upstream and downstream of XBP-1 integrate plasma cell differentiation with the UPR.

Characterization of rainbow trout and zebrafish eukaryotic initiation factor 2alpha and its response to endoplasmic reticulum stress and IPNV infection

The cDNAs of rainbow trout and zebrafish eIF2alpha have been isolated and found to encode proteins of similar molecular weight and isoelectric point to the alpha-subunit of the human translational initiation factor, eIF2. The rainbow trout (36.0kDa) and zebrafish (36.2kDa) eIF2alphas share 93 and 91% identity to the human protein, respectively, and are recognized by antibodies raised to the human form. In mammals, the phosphorylation of the alpha-subunit of eIF2 plays a key role in the regulation of protein synthesis in response to a range of cellular stresses. Regions corresponding to the human phosphorylation and kinase-docking sites are identical in the proteins of both fish species, as are residues that interact with the eIF2 recycling factor, eIF2B. Moreover, both recombinant rainbow trout and zebrafish eIF2alphas can be phosphorylated in vitro by the mammalian heme-sensitive eIF2alpha-kinase, HRI/HCR, as well as the interferon-inducible, dsRNA sensitive kinase, PKR. Phosphorylation of rainbow trout and zebrafish eIF2alpha can also occur in vivo. RTG-2 and ZFL cells subjected to endoplasmic reticulum (ER) stress by treatment with the Ca(2+)-ionophore A23187 showed increased levels of eIF2alpha phosphorylation, suggesting similarity between the ER stress response in fish and other higher eukaryotes. Furthermore, RTG-2 cells responded to treatment with poly(I).poly(C) or to infection by infectious pancreatic necrosis virus, IPNV, by increasing eIF2alpha phosphorylation. These data imply that RTG-2 cells express the interferon-induced eIF2alpha-kinase, PKR and suggests that the interferon/eIF2alpha/PKR response to virus infection may be a conserved vertebrate characteristic. Overall these data are consistent with the premise that fish are able to regulate protein synthesis in response to cellular stresses through phosphorylation of eIF2alpha.

Protein synthesis and endoplasmic reticulum stress can be modulated by the hepatitis C virus envelope protein E2 through the eukaryotic initiation factor 2alpha kinase PERK

The hepatitis C virus envelope protein, E2, is an endoplasmic reticulum (ER)-bound protein that contains a region of sequence homology with the double-stranded RNA-activated protein kinase PKR and its substrate, the eukaryotic translation initiation factor 2 (eIF2). We previously reported that E2 modulates global translation through inhibition of the interferon-induced antiviral protein PKR through its PKR-eIF2alpha phosphorylation site homology domain (PePHD). Here we show that the PKR-like ER-resident kinase (PERK) binds to and is also inhibited by E2. At low expression levels, E2 induced ER stress, but at high expression levels, and in vitro, E2 inhibited PERK kinase activity. Mammalian cells that stably express E2 were refractory to the translation-inhibitory effects of ER stress inducers, and E2 relieved general translation inhibition induced by PERK. The PePHD of E2 was required for the rescue of translation that was inhibited by activated PERK, similar to our previous findings with PKR. Here we report the inhibition of a second eIF2alpha kinase by E2, and these results are consistent with a pseudosubstrate mechanism of inhibition of eIF2alpha kinases. These findings may also explain how the virus promotes persistent infection by overcoming the cellular ER stress response.

The mammalian endoplasmic reticulum as a sensor for cellular stress

The recent elucidation of the mammalian unfolded protein response pathway has revealed a unique and transcriptionally complex signal transduction pathway that protects cells from a variety of physical and biochemical stresses that can occur during normal development and in disease states. Although the stress conditions are monitored in the endoplasmic reticulum, the beneficial effects of this pathway are extended to other cellular organelles and to the organism itself.

Dominant negative pathogenesis by mutant proinsulin in the Akita diabetic mouse

Autosomal dominant diabetes in the Akita mouse is caused by mutation of the insulin 2 gene, whose product replaces a cysteine residue that is engaged in the formation of an intramolecular disulfide bond. These heterozygous mice exhibit severe insulin deficiency despite coexpression of normal insulin molecules derived from three other wild-type alleles of the insulin 1 and 2 genes. Although the results of our previous study suggested that the mutant proinsulin 2 is misfolded and blocked in the transport from the endoplasmic reticulum to the Golgi apparatus, its dominant negative nature has not been fully characterized. In the present study, we investigated the possible pathogenic mechanisms induced by the mutant proinsulin 2. There is no evidence that the mutant proinsulin 2 attenuates the overall protein synthesis rate or promotes the formation of aberrant disulfide bonds. The trafficking of constitutively secreted alkaline phosphatase, however, is significantly decreased in the islets of Akita mice, indicating that the function of early secretory pathways is nonspecifically impaired. Morphological analysis has revealed that secretory pathway organelle architecture is progressively devastated in the beta-cells of Akita mice. These findings suggest that the organelle dysfunction resulting from the intracellular accumulation of misfolded proinsulin 2 is primarily responsible for the defect of coexisting wild-type insulin secretion in Akita beta-cells.

Mammalian stress granules represent sites of accumulation of stalled translation initiation complexes

In eukaryotic cells subjected to environmental stress, untranslated mRNA accumulates in discrete cytoplasmic foci that have been termed stress granules. Recent studies have shown that in addition to mRNA, stress granules also contain 40S ribosomal subunits and various translation initiation factors, including the mRNA binding proteins eIF4E and eIF4G. However, eIF2, the protein that transfers initiator methionyl-tRNA(i) (Met-tRNA(i)) to the 40S ribosomal subunit, has not been detected in stress granules. This result is surprising because the eIF2. GTP. Met-tRNA(i) complex is thought to bind to the 40S ribosomal subunit before the eIF4G. eIF4E. mRNA complex. In the present study, we show in both NIH-3T3 cells and mouse embryo fibroblasts that stress granules contain not only eIF2 but also the guanine nucleotide exchange factor for eIF2, eIF2B. Moreover, we show that phosphorylation of the alpha-subunit of eIF2 is necessary and sufficient for stress granule formation during the unfolded protein response. Finally, we also show that stress granules contain many, if not all, of the components of the 48S preinitiation complex, but not 60S ribosomal subunits, suggesting that they represent stalled translation initiation complexes.

When translation meets metabolism: multiple links to diabetes

Type 2 diabetes is a polygenic disorder characterized by multiple biochemical defects including transcriptional, translational, and posttranslational abnormalities. Although major progress has been made in elucidation of factors at the transcriptional and posttranslational levels, defects at the translational level remain elusive. Mutation of a kinase that regulates translation initiation has been implicated in the etiology of a monogenic form of diabetes known as Wolcott-Rallison syndrome. Characterization of mice rendered deficient in eukaryotic initiation factors has provided model systems to study the involvement of translation in regulating insulin synthesis and secretion, hepatic function, peripheral insulin resistance, and diabetic complications. Recent progress in the understanding of endoplasmic reticulum overload by unfolded proteins has begun to uncover mechanisms leading to pancreatic beta-cell exhaustion. Future advances in this area may lead to identification of the missing links in the pathogenesis of beta-cell failures due to conditions such as hyperinsulinemia, hyperglycemia, and long-term treatment with sulfonylureas, and thus may identify novel therapeutic targets for diabetes.

Functional characterization of Drosophila melanogaster PERK eukaryotic initiation factor 2alpha (eIF2alpha) kinase

Four distinct eukaryotic initiation factor 2alpha (eIF2alpha) kinases phosphorylate eIF2alpha at S51 and regulate protein synthesis in response to various environmental stresses. These are the hemin-regulated inhibitor (HRI), the interferon-inducible dsRNA-dependent kinase (PKR), the endoplasmic reticulum (ER)-resident kinase (PERK) and the GCN2 protein kinase. Whereas HRI and PKR appear to be restricted to mammalian cells, GCN2 and PERK seem to be widely distributed in eukaryotes. In this study, we have characterized the second eIF2alpha kinase found in Drosophila, a PERK homologue (DPERK). Expression of DPERK is developmentally regulated. During embryogenesis, DPERK expression becomes concentrated in the endodermal cells of the gut and in the germ line precursor cells. Recombinant wild-type DPERK, but not the inactive DPERK-K671R mutant, exhibited an autokinase activity, specifically phosphorylated Drosophila eIF2alpha at S50, and functionally replaced the endogenous Saccharomyces cerevisiae GCN2. The full length protein, when expressed in 293T cells, located in the ER-enriched fraction, and its subcellular localization changed with deletion of different N-terminal fragments. Kinase activity assays with these DPERK deletion mutants suggested that DPERK localization facilitates its in vivo function. Similar to mammalian PERK, DPERK forms oligomers in vivo and DPERK activity appears to be regulated by ER stress. Furthermore, the stable complexes between wild-type DPERK and DPERK-K671R mutant were mediated through the N terminus of the proteins and exhibited an in vitro eIF2alpha kinase activity.

Activation of an unfolded protein response during differentiation of antibody-secreting B cells

The unfolded protein response pathway (UPR) is believed to detect and compensate for excessive protein accumulation in the endoplasmic reticulum (ER). The UPR can be induced by pharmacological agents that perturb ER functions, but may also occur during cellular developmental processes such as the transition of B-lymphocytes into antibody-secreting plasma cells. Here we show that major UPR components are activated in B cells stimulated to secrete antibody. Increased expression of UPR targets including the ER chaperones BiP and GRP94 and the transcription factor XBP-1 initiates early in the differentiation program prior to up-regulated synthesis of Ig chains. Furthermore, these same kinetics are observed during differentiation for cleavage of the ER-localized ATF6alpha protein and splicing of XBP-1 mRNA to generate p50ATF6alpha and p54XBP-1, the two known UPR transcriptional activators. All of these UPR events reach maximal levels once Ig synthesis and secretion are markedly induced. Interestingly, these events are not accompanied by expression of CHOP, a transcription factor induced by ER stress agents commonly used to investigate the UPR. These results suggest that a physiological UPR elicited during differentiation of B-lymphocytes into high-rate secretory cells may be distinct from the UPR defined by agents that disrupt protein maturation in the ER.

Control of PERK eIF2alpha kinase activity by the endoplasmic reticulum stress-induced molecular chaperone P58IPK

P58(IPK) is an Hsp40 family member known to inhibit the interferon (IFN)-induced, double-stranded RNA-activated, eukaryotic initiation factor 2alpha (eIF2alpha) protein kinase R (PKR) by binding to its kinase domain. We find that the stress of unfolded proteins in the endoplasmic reticulum (ER) activates P58(IPK) gene transcription through an ER stress-response element in its promoter region. P58(IPK) interacts with and inhibits the PKR-like ER-localized eIF2alpha kinase PERK, which is normally activated during the ER-stress response to protect cells from ER stress by attenuating protein synthesis and reducing ER client protein load. Levels of phosphorylated eIF2alpha were lower in ER-stressed P58(IPK)-overexpressing cells and were enhanced in P58(IPK) mutant cells. In the ER-stress response, PKR-like ER kinase (PERK)-mediated translational repression is transient and is followed by translational recovery and enhanced expression of genes that increase the capacity of the ER to process client proteins. The absence of P58(IPK) resulted in increased expression levels of two ER stress-inducible genes, BiP and Chop, consistent with the enhanced eIF2alpha phosphorylation in the P58(IPK) deletion cells. Our studies suggest that P58(IPK) induction during the ER-stress response represses PERK activity and plays a functional role in the expression of downstream markers of PERK activity in the later phase of the ER-stress response.

Endoplasmic reticulum stress and the development of diabetes: a review

The early steps of insulin biosynthesis occur in the endoplasmic reticulum (ER), and the beta-cell has a highly developed and active ER. All cells regulate the capacity of their ER to fold and process client proteins and they adapt to an imbalance between client protein load and folding capacity (so-called ER stress). Mutations affecting the ER stress-activated pancreatic ER kinase (PERK) and its downstream effector, the translation initiation complex eukaryotic initiation factor 2 (eIF2), have a profound impact on islet cell development, function, and survival. PERK mutations are associated with the Wolcott-Rallison syndrome of infantile diabetes and mutations that prevent the alpha-subunit of eIF2 from being phosphorylated by PERK, block beta-cell development, and impair gluconeogenesis. We will review this and other rare forms of clinical and experimental diabetes and consider the role of ER stress in the development of more common forms of the disease.

Heat shock protein 90 modulates the unfolded protein response by stabilizing IRE1alpha

The molecular chaperone HSP90 regulates stability and function of multiple protein kinases. The HSP90-binding drug geldanamycin interferes with this activity and promotes proteasome-dependent degradation of most HSP90 client proteins. Geldanamycin also binds to GRP94, the HSP90 paralog located in the endoplasmic reticulum (ER). Because two of three ER stress sensors are transmembrane kinases, namely IRE1alpha and PERK, we investigated whether HSP90 is necessary for the stability and function of these proteins. We found that HSP90 associates with the cytoplasmic domains of both kinases. Both geldanamycin and the HSP90-specific inhibitor, 514, led to the dissociation of HSP90 from the kinases and a concomitant turnover of newly synthesized and existing pools of these proteins, demonstrating that the continued association of HSP90 with the kinases was required to maintain their stability. Further, the previously reported ability of geldanamycin to stimulate ER stress-dependent transcription apparently depends on its interaction with GRP94, not HSP90, since geldanamycin but not 514 led to up-regulation of BiP. However, this effect is eventually superseded by HSP90-dependent destabilization of unfolded protein response signaling. These data establish a role for HSP90 in the cellular transcriptional response to ER stress and demonstrate that chaperone systems on both sides of the ER membrane serve to integrate this signal transduction cascade.

Regulation of protein synthesis by hypoxia via activation of the endoplasmic reticulum kinase PERK and phosphorylation of the translation initiation factor eIF2alpha

Hypoxia profoundly influences tumor development and response to therapy. While progress has been made in identifying individual gene products whose synthesis is altered under hypoxia, little is known about the mechanism by which hypoxia induces a global downregulation of protein synthesis. A critical step in the regulation of protein synthesis in response to stress is the phosphorylation of translation initiation factor eIF2alpha on Ser51, which leads to inhibition of new protein synthesis. Here we report that exposure of human diploid fibroblasts and transformed cells to hypoxia led to phosphorylation of eIF2alpha, a modification that was readily reversed upon reoxygenation. Expression of a transdominant, nonphosphorylatable mutant allele of eIF2alpha attenuated the repression of protein synthesis under hypoxia. The endoplasmic reticulum (ER)-resident eIF2alpha kinase PERK was hyperphosphorylated upon hypoxic stress, and overexpression of wild-type PERK increased the levels of hypoxia-induced phosphorylation of eIF2alpha. Cells stably expressing a dominant-negative PERK allele and mouse embryonic fibroblasts with a homozygous deletion of PERK exhibited attenuated phosphorylation of eIF2alpha and reduced inhibition of protein synthesis in response to hypoxia. PERK(-/-) mouse embryo fibroblasts failed to phosphorylate eIF2alpha and exhibited lower survival after prolonged exposure to hypoxia than did wild-type fibroblasts. These results indicate that adaptation of cells to hypoxic stress requires activation of PERK and phosphorylation of eIF2alpha and suggest that the mechanism of hypoxia-induced translational attenuation may be linked to ER stress and the unfolded-protein response.

The unfolded protein response is involved in the pathology of Alzheimer's disease

The endoplasmic reticulum (ER) performs the synthesis, posttranslational modification, and proper folding of proteins. A variety of conditions can be ER stress, causing the accumulation of unfolding or misfolding proteins in the ER. Eukaryotic cells have three different mechanisms for dealing with an accumulation of unfolded proteins in the ER known as the unfolded protein response (UPR): transcriptional induction, translational attenuation, and degradation. This paper focuses on the relationship between UPR and the pathogenesis of AD. Our results indicate a new mechanism by which PS1 mutations may affect the sensing of ER stress. Experimental manipulation of IRE1, PERK, or eIF2alpha phosphorylation or GRP78 expression might allow the development of therapeutic strategies for FAD.

Phosphorylation of eukaryotic initiation factor 2 by heme-regulated inhibitor kinase-related protein kinases in Schizosaccharomyces pombe is important for fesistance to environmental stresses

Protein synthesis is regulated by the phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF2alpha) in response to different environmental stresses. One member of the eIF2alpha kinase family, heme-regulated inhibitor kinase (HRI), is activated under heme-deficient conditions and blocks protein synthesis, principally globin, in mammalian erythroid cells. We identified two HRI-related kinases from Schizosaccharomyces pombe which have full-length homology with mammalian HRI. The two HRI-related kinases, named Hri1p and Hri2p, exhibit autokinase and kinase activity specific for Ser-51 of eIF2alpha, and both activities were inhibited in vitro by hemin, as previously described for mammalian HRI. Overexpression of Hri1p, Hri2p, or the human eIF2alpha kinase, double-stranded-RNA-dependent protein kinase (PKR), impeded growth of S. pombe due to elevated phosphorylation of eIF2alpha. Cells from strains with deletions of the hri1(+) and hri2(+) genes, individually or in combination, exhibited a reduced growth rate when exposed to heat shock or to arsenic compounds. Measurements of in vivo phosphorylation of eIF2alpha suggest that Hri1p and Hri2p differentially phosphorylate eIF2alpha in response to these stress conditions. These results demonstrate that HRI-related enzymes are not unique to vertebrates and suggest that these eIF2alpha kinases are important participants in diverse stress response pathways in some lower eukaryotes.

Distinct roles of activating transcription factor 6 (ATF6) and double-stranded RNA-activated protein kinase-like endoplasmic reticulum kinase (PERK) in transcription during the mammalian unfolded protein response

In response to accumulation of unfolded proteins in the endoplasmic reticulum (ER), a homoeostatic response, termed the unfolded protein response (UPR), is activated in all eukaryotic cells. The UPR involves only transcriptional regulation in yeast, and approx. 6% of all yeast genes, encoding not only proteins to augment the folding capacity in the ER, but also proteins working at various stages of secretion, are induced by ER stress [Travers, Patil, Wodicka, Lockhart, Weissman and Walter (2000) Cell (Cambridge, Mass.) 101, 249-258]. In the present study, we conducted microarray analysis of HeLa cells, although our analysis covered only a small fraction of the human genome. A great majority of human ER stress-inducible genes (approx. 1% of 1800 genes examined) were classified into two groups. One group consisted of genes encoding ER-resident molecular chaperones and folding enzymes, and these genes were directly regulated by the ER-membrane-bound transcription factor activating transcription factor (ATF) 6. The ER-membrane-bound protein kinase double-stranded RNA-activated protein kinase-like ER kinase (PERK)-mediated signalling pathway appeared to be responsible for induction of the remaining genes, which are not involved in secretion, but may be important after cellular recovery from ER stress. In higher eukaryotes, the PERK-mediated translational-attenuation system is known to operate in concert with the transcriptional-induction system. Thus we propose that mammalian cells have evolved a strategy to cope with ER stress different from that of yeast cells.