Feedback inhibition of the unfolded protein response by GADD34-mediated dephosphorylation of eIF2alpha

Gene expression during acute and prolonged hypoxia is regulated by distinct mechanisms of translational control

Hypoxia has recently been shown to activate the endoplasmic reticulum kinase PERK, leading to phosphorylation of eIF2alpha and inhibition of mRNA translation initiation. Using a quantitative assay, we show that this inhibition exhibits a biphasic response mediated through two distinct pathways. The first occurs rapidly, reaching a maximum at 1-2 h and is due to phosphorylation of eIF2alpha. Continued hypoxic exposure activates a second, eIF2alpha-independent pathway that maintains repression of translation. This phase is characterized by disruption of eIF4F and sequestration of eIF4E by its inhibitor 4E-BP1 and transporter 4E-T. Quantitative RT-PCR analysis of polysomal RNA indicates that the translation efficiency of individual genes varies widely during hypoxia. Furthermore, the translation efficiency of individual genes is dynamic, changing dramatically during hypoxic exposure due to the initial phosphorylation and subsequent dephosphorylation of eIF2alpha. Together, our data indicate that acute and prolonged hypoxia regulates mRNA translation through distinct mechanisms, each with important contributions to hypoxic gene expression.

Functional coupling of p38-induced up-regulation of BiP and activation of RNA-dependent protein kinase-like endoplasmic reticulum kinase to drug resistance of dormant carcinoma cells

It has been proposed that occult, disseminated metastatic cells are refractory to chemotherapy due to lack of proliferation. We have shown that p38 activation induces dormancy of squamous carcinoma cells. We now show that p38 signaling in these cells activates a prosurvival mechanism via the up-regulation of the endoplasmic reticulum (ER) chaperone BiP and increased activation of the ER stress-activated eukaryotic translation initiator factor 2alpha kinase RNA-dependent protein kinase-like ER kinase (PERK) allowing dormant tumor cells to resist drug toxicity. RNA interference and dominant-negative expression studies revealed that both BiP and PERK signaling promote survival and drug resistance of dormant cells, and that BiP up-regulation prevents Bax activation. We propose that stress-dependent activation of p38 via BiP up-regulation and PERK activation protects dormant tumor cells from stress insults, such as chemotherapy.

Coping with stress: eIF2 kinases and translational control

In response to environmental stresses, a family of protein kinases phosphorylate eIF2 (eukaryotic initiation factor 2) to alleviate cellular injury or alternatively induce apoptosis. Phosphorylation of eIF2 reduces global translation, allowing cells to conserve resources and to initiate a reconfiguration of gene expression to effectively manage stress conditions. Accompanying this general protein synthesis control, eIF2 phosphorylation induces translation of specific mRNAs, such as that encoding the bZIP (basic leucine zipper) transcriptional regulator ATF4 (activating transcription factor 4). ATF4 also enhances the expression of additional transcription factors, ATF3 and CHOP (CCAAT/enhancer-binding protein homologous protein)/GADD153 (growth arrest and DNA-damage-inducible protein), that assist in the regulation of genes involved in metabolism, the redox status of the cells and apoptosis. Reduced translation by eIF2 phosphorylation can also lead to activation of stress-related transcription factors, such as NF-κB (nuclear factor κB), by lowering the steady-state levels of short-lived regulatory proteins such as IκB (inhibitor of NF-κB). While many of the genes induced by eIF2 phosphorylation are shared between different environmental stresses, eIF2 kinases function in conjunction with other stress-response pathways, such as those regulated by mitogen-activated protein kinases, to elicit gene expression programmes that are tailored for the specific stress condition. Loss of eIF2 kinase pathways can have important health consequences. Mice devoid of the eIF2 kinase GCN2 [general control non-derepressible-2 or EIF2AK4 (eIF2α kinase 4)] show sensitivity to nutritional deficiencies and aberrant eating behaviours, and deletion of PEK [pancreatic eIF2α kinase or PERK (RNA-dependent protein kinase-like endoplasmic reticulum kinase) or EIF2AK3] leads to neonatal insulin-dependent diabetes, epiphyseal dysplasia and hepatic and renal complications.

Doxorubicin prevents endoplasmic reticulum stress-induced apoptosis

Several cellular stress signaling pathways initiate apoptosis in eukaryotic cells, but the interactions and coordination between the pathways have not been elucidated. In this study, apoptosis was triggered in MCF7 human breast carcinoma cells using doxorubicin, a topoisomerase inhibitor, and an endoplasmic reticulum (ER) stress inducer, thapsigargin, the latter causing the unfolded protein response (UPR). Interestingly, compared to treatment with doxorubicin or thapsigargin alone, cell death was reduced by treatment with both stress inducers. In contrast to another topoisomerase inhibitor, etoposide, doxorubicin markedly decreased apoptosis induced by thapsigargin; this doxorubicin effect was accompanied by reduced expression of the UPR-specific proapoptotic protein, C/EBP-homologous protein, and its upstream transcription factor, ATF4. We further found that doxorubicin downregulates the expression of ATF4 mRNA, indicating that doxorubicin interferes with the UPR at the level of ATF4 transcription. Taken together, the data suggest that ER stress-initiated cell death might be regulated by doxorubicin.

To replicate or not to replicate: achieving selective oncolytic virus replication in cancer cells through translational control

To ensure that their mRNAs are translated and that the viral proteins necessary for assembling the next generation of infectious progeny are produced, viruses must effectively seize control of the translational machinery within their host cells. In many cases, the ability to productively engage host translational components can determine if a given cell type can support viral replication, illustrating the critical importance of this task in the viral life cycle. Failure to interface properly with the host translational apparatus can compromise the productive growth cycle, resulting in an abortive infection and radically restricting viral replication. Not only have viruses become facile at commandeering this machinery, they are also particularly adept at manipulating cellular translation control pathways for their own ends. In this review, the mechanisms by which numerous viruses manipulate host translational control circuits are discussed. Furthermore, particular attention is devoted to understanding how interfering with the ability of a virus to properly regulate translation in its host can be exploited to generate oncolytic strains that selectively replicate in cancer cells.

Cellular abnormalities linked to endoplasmic reticulum dysfunction in cerebrovascular disease—therapeutic potential

Unfolded proteins accumulate in the lumen of the endoplasmic reticulum (ER) as part of the cellular response to cerebral hypoxia/ischemia and also to the overexpression of the mutant genes responsible for familial forms of degenerative diseases such as Alzheimer's disease, Parkinson's disease, Huntington's disease, amyothrophic lateral sclerosis, and Huntington's disease, as well as other disorders that are caused by an expanded CAG repeat. This accumulation arises from an imbalance between the load of proteins that need to be folded and processed in the ER lumen and the ER folding/processing capacity. To withstand such potentially lethal conditions, stress responses are activated that includes the shutdown of translation to reduce the ER work load and the activation of the expression of genes coding for proteins involved in the folding and processing reactions, to increase folding/processing capacity. In transient cerebral ischemia, ER stress-induced suppression of protein synthesis is believed to be too severe to permit sufficient activation of the genetic arm of the ER stress response. Mutations associated with Alzheimer's disease down-regulate the ER stress response and make cells more vulnerable to conditions associated with ER stress. When the functioning of the ER is severely impaired and affected cells can no longer withstand these stressful conditions, programmed cell death is induced, including a mitochondria-driven apoptotic pathway. Raising the resistance of cells to conditions that interfere with ER functions and activating the degradation and refolding of unfolded proteins accumulated in the ER lumen are possible strategies for blocking the pathological process leading to cell death at an early stage.

Bioactive small molecules reveal antagonism between the integrated stress response and sterol-regulated gene expression

Phosphorylation of translation initiation factor 2alpha (eIF2alpha) coordinates a translational and transcriptional program known as the integrated stress response (ISR), which adapts cells to endoplasmic reticulum (ER) stress. A screen for small molecule activators of the ISR identified two related compounds that also activated sterol-regulated genes by blocking cholesterol biosynthesis at the level of CYP51. Ketoconazole, a known CYP51 inhibitor, had similar effects, establishing that perturbed flux of precursors to cholesterol activates the ISR. Surprisingly, compound-mediated activation of sterol-regulated genes was enhanced in cells with an ISR-blocking mutation in the regulatory phosphorylation site of eIF2alpha. Furthermore, induction of the ISR by an artificial drug-activated eIF2alpha kinase reduced the level of active sterol regulatory element binding protein (SREBP) and sterol-regulated mRNAs. These findings suggest a mechanism by which interactions between sterol metabolism, the ISR, and the SREBP pathway affect lipid metabolism during ER stress.

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.

Distinct endoplasmic reticulum stress responses are triggered during human liver transplantation

Injury due to cold ischaemia-reperfusion (IR) represents a major cause of primary graft non-function following human liver transplantation. This major cellular response translates into a dramatic decrease in intracellular ATP concentration during the ischaemic phase, thus sensitizing cells to reperfusion shock. We postulated that IR-induced cellular damage might cause alterations of the secretory pathway, particularly at the level of endoplasmic reticulum (ER) function. Under these circumstances, the ER triggers an adaptive response named the 'unfolded protein response' (UPR). In this study, we show that the expression of BiP, CHOP/GADD153 and GADD34, known to be induced specifically upon ER stress, are differentially affected upon IR, thus suggesting that distinct ER stress responses are activated during each phase of transplantation. With an approach combining semi-quantitative RT-PCR and immunoblotting using phospho-specific antibodies, we show that the IRE-1 pathway is activated upon early ischaemia and, in a second phase, upon early reperfusion. This occurs through the atypical splicing of XBP-1 mRNA, its translation into a transcriptionally active XBP-1 protein and the subsequent increase in EDEM mRNA expression, and may also contribute to the observed reperfusion-induced activation of MAPK/SAPK. In contrast, we demonstrate that the PERK pathway, leading to inhibition of cap-dependent translation, is mainly activated upon reperfusion, as shown by PERK and eIF2alpha phosphorylation. PERK activation is detected restrictively in sinusoidal endothelial cells and could contribute to the exaggerated sensivity of this liver cell type to IR injury. These results correlate well with the observed defect in protein secretion and suggest that the biphasic ER stress response may influence liver secretory functions and, as a consequence, condition liver transplantation outcomes.

Stable ribosome binding to the endoplasmic reticulum enables compartment-specific regulation of mRNA translation

In eukaryotic cells, protein synthesis is compartmentalized; mRNAs encoding secretory/membrane proteins are translated on endoplasmic reticulum (ER)-bound ribosomes, whereas mRNAs encoding cytosolic proteins are translated on free ribosomes. mRNA partitioning between the two compartments occurs via positive selection: free ribosomes engaged in the translation of signal sequence-encoding mRNAs are trafficked from the cytosol to the ER. After translation termination, ER-bound ribosomes are thought to dissociate, thereby completing a cycle of mRNA partitioning. At present, the physiological basis for termination-coupled ribosome release is unknown. To gain insight into this process, we examined ribosome and mRNA partitioning during the unfolded protein response, key elements of which include suppression of the initiation stage of protein synthesis and polyribosome breakdown. We report that unfolded protein response (UPR)-elicited polyribosome breakdown resulted in the continued association, rather than release, of ER-bound ribosomes. Under these conditions, mRNA translation in the cytosol was suppressed, whereas mRNA translation on the ER was sustained. Furthermore, mRNAs encoding key soluble stress proteins (XBP-1 and ATF-4) were translated primarily on ER-bound ribosomes. These studies demonstrate that ribosome release from the ER is termination independent and identify new and unexpected roles for the ER compartment in the translational response to induction of the unfolded protein response.

Evolution of GADD34 expression after focal cerebral ischaemia

GADD34, a stress response protein associated with cell rescue, DNA repair and apoptosis, is expressed in the ischaemic brain. The C-terminal region of GADD34 has homology with the Herpes Simplex Virus protein, ICP34.5, which overcomes the protein synthesis block after viral infection by actively dephosphorylating eukaryotic translation initiation factor 2alpha (eIF2alpha). The carboxy terminus of GADD34 is also capable of dephosphorylating eIF2alpha and therefore has the capacity to restore the protein synthesis shutoff associated with ischaemia. This study examines the distribution and time course of GADD34 expression after focal cerebral ischaemia. Focal ischaemia or sham procedure was carried out on Sprague-Dawley rats with survival times of 4, 12, 24 h, 7 and 30 days. Brains were processed for histology and immunohistochemistry. Ischaemic damage was mapped onto line diagrams and GADD34 positive cells counted in selected regions of cortex and caudate. GADD34 immunopositive cells (mainly neurones), expressed as cells/mm2, were present in ischaemic brains at 4 h (e.g., peri-infarct cortex 20 +/- 5; contralateral cortex 3 +/- 1, P < 0.05). Of the time points examined, numbers of GADD34 positive cells were highest 24 h after ischaemia (peri-infarct cortex 31 +/- 7.3, contralateral cortex 0.1 +/- 0.1, P < 0.05). Immunopositive cells, following a similar time course, were identified within the peri-infarct zone in the caudate nucleus and in ipsilateral cingulate cortex (possibly as a consequence of cortical spreading depression). GADD34 positive cells did not co-localise with a marker of irreversible cell death (TUNEL). Taken together, GADD34 positive cells in key neuroanatomical locations pertinent to the evolving ischaemic lesion, the lack of co-localisation with TUNEL and the protein's known effects on restoring protein synthesis, repairing DNA and involvement in ischaemic pre-conditioning suggests that it has the potential to influence cell survival in ischaemically compromised tissue.

Potential molecular mechanism for rodent tumorigenesis: mutational generation of Progression Elevated Gene-3 (PEG-3)

Progression Elevated Gene-3 (PEG-3) was cloned using subtraction hybridization as an upregulated transcript associated with transformation and tumor progression of rat embryo fibroblast cells. PEG-3 is a unique gene facilitating tumor progression by modulating multiple pathways in transformed cells, including genomic stability, angiogenesis and invasion. PEG-3 originates from mutation in the growth arrest and DNA damage inducible gene GADD34. A one base deletion in rat GADD34 results in a frame-shift and premature appearance of a stop-codon resulting in a C-terminally truncated molecule that is PEG-3. We now document that mutation in the GADD34 gene is a frequent event during transformation and/or immortalization of rodent cells. Sequencing of the GADD34 gene in a number of independent rat tumor cell lines revealed that in a majority of these the GADD34 gene is mutated to either PEG-3 or a PEG-3-like gene with similar C-terminal truncations. An important function of GADD34 is to inhibit cell growth, predominantly by apoptosis, and we demonstrate that PEG-3 or C-terminal truncations of human GADD34 resembling PEG-3 prevent growth inhibition by both human and rat GADD34. Phosphorylation of p53 by GADD34 is one mechanism by which it inhibits growth and PEG-3 could prevent GADD34-induced p53 phosphorylation. In contrast, PEG-3 was unable to block other GADD34-induced changes, including eIF2 alpha dephosphorylation, indicating that its effects on GADD34 may be related more to its effect on cell growth rather than a global inhibitor of all GADD34 functions. We hypothesize that mutational generation of PEG-3 or a similar molecule is a critical event during rodent carcinogenesis. The inherent property of PEG-3 to function as a dominant negative of the growth inhibitory property of GADD34 might rescue cells from DNA damage-induced apoptosis leading to growth independence and tumorigenesis.

Regulation of SNARK activity in response to cellular stresses

SNARK is a member of the AMPK subfamily of serine/threonine protein kinases. In this study, we examined the regulation of SNARK activity in kidney (BHK, HEK293), pancreatic beta-cell insulinoma (INS-1), hepatocarcinoma (H4IIE) and keratinocyte (NRKC)-derived cell lines in response to diverse cellular stresses. We show that SNARK activity is regulated by glucose- or glutamine-deprivation, induction of endoplasmic reticulum stress by homocysteine or DTT, elevation of cellular AMP and/or depletion of ATP, hyperosmotic stress, salt stress, ultraviolet B radiation and oxidative stress caused by hydrogen peroxide. Moreover, the regulation of SNARK activity in response to cellular stresses depends greatly upon cell type. Furthermore, SNARK activity is downregulated by metformin in a dose- and time-dependent manner in H4IIE cells. These observations support a role for SNARK as a molecular component of the cellular stress response.

CHOP induces death by promoting protein synthesis and oxidation in the stressed endoplasmic reticulum

Unfolded and malfolded client proteins impose a stress on the endoplasmic reticulum (ER), which contributes to cell death in pathophysiological conditions. The transcription factor C/EBP homologous protein (CHOP) is activated by ER stress, and CHOP deletion protects against its lethal consequences. We find that CHOP directly activates GADD34, which promotes ER client protein biosynthesis by dephosphorylating phospho-Ser 51 of the alpha-subunit of translation initiation factor 2 (eIF2alpha) in stressed cells. Thus, impaired GADD34 expression reduces client protein load and ER stress in CHOP(-/-) cells exposed to perturbations that impair ER function. CHOP(-/-) and GADD34 mutant cells accumulate less high molecular weight protein complexes in their stressed ER than wild-type cells. Furthermore, mice lacking GADD34-directed eIF2alpha dephosphorylation, like CHOP(-/-) mice, are resistant to renal toxicity of the ER stress-inducing drug tunicamycin. CHOP also activates ERO1alpha, which encodes an ER oxidase. Consequently, the ER of stressed CHOP(-/-) cells is relatively hypo-oxidizing. Pharmacological and genetic manipulations that promote a hypo-oxidizing ER reduce abnormal high molecular weight protein complexes in the stressed ER and protect from the lethal consequences of ER stress. CHOP deletion thus protects cells from ER stress by decreasing ER client protein load and changing redox conditions within the organelle.

A Selective Inhibitor of eIF2 Dephosphorylation Protects Cells from ER Stress

Most protein phosphatases have little intrinsic substrate specificity, making selective pharmacological inhibition of specific dephosphorylation reactions a challenging problem. In a screen for small molecules that protect cells from endoplasmic reticulum (ER) stress, we identified salubrinal, a selective inhibitor of cellular complexes that dephosphorylate eukaryotic translation initiation factor 2 subunit alpha (eIF2alpha). Salubrinal also blocks eIF2alpha dephosphorylation mediated by a herpes simplex virus protein and inhibits viral replication. These results suggest that selective chemical inhibitors of eIF2alpha dephosphorylation may be useful in diseases involving ER stress or viral infection. More broadly, salubrinal demonstrates the feasibility of selective pharmacological targeting of cellular dephosphorylation events.

Herpes simplex virus 1 infection activates the endoplasmic reticulum resident kinase PERK and mediates eIF-2alpha dephosphorylation by the gamma(1)34.5 protein

The gamma(1)34.5 protein of herpes simplex virus (HSV) plays a crucial role in virus infection. Although the double-stranded RNA-dependent protein kinase (PKR) is activated during HSV infection, the gamma(1)34.5 protein inhibits the activity of PKR by mediating dephosphorylation of the translation initiation factor eIF-2alpha. Here we show that HSV infection also induces phosphorylation of an endoplasmic reticulum (ER) resident kinase PERK, a hallmark of ER stress response. The virus-induced phosphorylation of PERK is blocked by cycloheximide but not by phosphonoacetic acid, suggesting that the accumulation of viral proteins in the ER is essential. Notably, the maximal phosphorylation of PERK is delayed in PKR+/+ cells compared to that seen in PKR-/- cells. Further analysis indicates that hyperphosphorylation of eIF-2alpha caused by HSV is greater in PKR+/+ cells than in PKR-/- cells. However, expression of the gamma(1)34.5 protein suppresses the ER stress response caused by virus, dithiothreitol, and thapsigargin as measured by global protein synthesis. Interestingly, the expression of GADD34 stimulated by HSV infection parallels the status of eIF-2alpha phosphorylation. Together, these observations suggest that regulation of eIF-2alpha phosphorylation by the gamma(1)34.5 protein is an efficient way to antagonize the inhibitory activity of PKR as well as PERK during productive infection.

Phosphorylation of the alpha-subunit of the eukaryotic initiation factor-2 (eIF2alpha) reduces protein synthesis and enhances apoptosis in response to proteasome inhibition

Protein ubiquitination and subsequent degradation by the proteasome are important mechanisms regulating cell cycle, growth and differentiation, and apoptosis. Recent studies in cancer therapy suggest that drugs that disrupt the ubiquitin/proteasome pathway induce apoptosis and sensitize malignant cells and tumors to conventional chemotherapy. In this study we addressed the role of phosphorylation of the alpha-subunit eukaryotic initiation factor-2 (eIF2), and its attendant regulation of gene expression, in the cellular stress response to proteasome inhibition. Phosphorylation of eIF2alpha in mouse embryo fibroblast (MEF) cells subjected to proteasome inhibition leads to a significant reduction in protein synthesis, concomitant with induced expression of the bZIP transcription regulator, ATF4, and its target gene CHOP/GADD153. The primary eIF2alpha kinase activated by exposure of these fibroblast cells to proteasome inhibition is GCN2 (EIF2AK4), which has a central role in the recognition of cytoplasmic stress signals. Endoplasmic reticulum (ER) stress is not effectively induced in MEF cells subjected to proteasome inhibition, with minimal activation of the ER stress sensory proteins, eIF2alpha kinase PEK (PERK/EIF2AK3), IRE1 protein kinase and the transcription regulator ATF6 following up to 6 h of proteasome inhibitor treatment. Loss of eIF2alpha phosphorylation thwarts caspase activation and delays apoptosis. Central to this pro-apoptotic function of eIF2alpha kinases during proteasome inhibition is the transcriptional regulator CHOP, as deletion of CHOP in MEF cells impedes apoptosis. We conclude that eIF2alpha kinases are integral to cellular stress pathways induced by proteasome inhibitors, and may be central to the efficacy of anticancer drugs that target the ubiquitin/proteasome pathway.

Endoplasmic reticulum stress as a correlate of cytotoxicity in human tumor cells exposed to diindolylmethane in vitro

The dietary phytochemical indole-3-carbinol (I3C) protects against cervical cancer in animal model studies and in human clinical trials. I3C and its physiologic condensation product diindolylmethane (DIM) also induce apoptosis of tumor cells in vitro and in vivo, suggesting that these phytochemicals might be useful as therapeutic agents as well as for cancer prevention. Deoxyribonucleic acid microarray studies on transformed keratinocytes and tumor cell lines exposed to pharmacologic concentrations of DIM in vitro are consistent with a cellular response to nutritional deprivation or disruptions in protein homeostasis such as endoplasmic reticulum (ER) stress. In this report we investigate whether specific stress response pathways are activated in tumor cells exposed to DIM and whether the ER stress response might contribute to DIM's cytotoxicity. Induction of the stress response genes GADD153, GADD34 and GADD45A, XBP-1, GRP78, GRP94, and asparagine synthase was documented by Western blot and real-time reverse transcriptase-polymerase chain reaction in C33A cervical cancer cells, and induction of a subset of these was also observed in cancer cell lines from breast (MCF-7) and prostate (DU145). The results are consistent with activation of more than 1 stress response pathway in C33A cells exposed to 75 microM DIM. Phosphorylation elF2alpha was rapidly and transiently increased, followed by elevated levels of ATF4 protein. Activation of IRE1alpha was indicated by a rapid increase in the stress-specific spliced form of XBP-1 messenger ribonucleic acid and a rapid and persistent phosphorylation of JNK1 and JNK2. Transcriptional activation dependent on an ATF6-XBP-1 binding site was detected by transient expression in MCF-7, C33A, and a transformed epithelial cell line (HaCaT); induction of the GADD153 (CHOP) promoter was also confirmed by transient expression. Cleavage of caspase 12 was observed in both DIM-treated and untreated C33A cells but did not correlate with cytotoxicity, whereas caspase 7 was cleaved at later times, coinciding with the onset of apoptosis. The results support the hypothesis that cytotoxic concentrations of DIM can activate cellular stress response pathways in vitro, including the ER stress response. Conversely, DIM was especially cytotoxic to stressed cells. Thapsigargin and tunicamycin, agents that induce ER stress, sensitized cells to the cytotoxic effects of DIM to differing degrees; nutrient limitation had a similar, but even more pronounced, effect. Because DIM toxicity in vitro is enhanced in cells undergoing nutritional deprivation and ER stress, it is possible that stressed cells in vivo, such as those within developing solid tumors, also have increased sensitivity to killing by DIM.

Translational repression mediates activation of nuclear factor kappa B by phosphorylated translation initiation factor 2

Numerous stressful conditions activate kinases that phosphorylate the alpha subunit of translation initiation factor 2 (eIF2alpha), thus attenuating mRNA translation and activating a gene expression program known as the integrated stress response. It has been noted that conditions associated with eIF2alpha phosphorylation, notably accumulation of unfolded proteins in the endoplasmic reticulum (ER), or ER stress, are also associated with activation of nuclear factor kappa B (NF-kappaB) and that eIF2alpha phosphorylation is required for NF-kappaB activation by ER stress. We have used a pharmacologically activable version of pancreatic ER kinase (PERK, an ER stress-responsive eIF2alpha kinase) to uncouple eIF2alpha phosphorylation from stress and found that phosphorylation of eIF2alpha is both necessary and sufficient to activate both NF-kappaB DNA binding and an NF-kappaB reporter gene. eIF2alpha phosphorylation-dependent NF-kappaB activation correlated with decreased levels of the inhibitor IkappaBalpha protein. Unlike canonical signaling pathways that promote IkappaBalpha phosphorylation and degradation, eIF2alpha phosphorylation did not increase phosphorylated IkappaBalpha levels or affect the stability of the protein. Pulse-chase labeling experiments indicate instead that repression of IkappaBalpha translation plays an important role in NF-kappaB activation in cells experiencing high levels of eIF2alpha phosphorylation. These studies suggest a direct role for eIF2alpha phosphorylation-dependent translational control in activating NF-kappaB during ER stress.

The unfolded protein response--a stress signaling pathway of the endoplasmic reticulum

The endoplasmic reticulum (ER) is a factory for folding and maturation of newly synthesized transmembrane and secretory proteins. The ER provides stringent quality control systems to ensure that only correctly folded proteins exit the ER and unfolded or misfolded proteins are retained and ultimately degraded. A number of biochemical and physiological stimuli can change ER homeostasis, impose stress to the ER, and subsequently lead to accumulation of unfolded or misfolded proteins in the ER lumen. The ER has evolved stress response signaling pathways collectively called the unfolded protein response (UPR) to cope with the accumulation of unfolded or misfolded proteins. This review summarizes our understanding of the UPR signaling developed in the recent years.

Survival and apoptosis signals in ER stress: the role of protein kinases

The endoplasmic reticulum (ER) is the organelle in which newly synthesized secretory and transmembrane proteins form their proper tertiary structure by post-translational modification, folding, and oligomerization. However, many of these proteins are unfolded or misfolded by extracellular or intracellular stimuli. The accumulation of misfolded proteins constitutes a risk for living cells. Eukaryotic cells possess at least three different mechanisms to adapt to ER stress and thereby survive: (1) translational attenuation to limit further accumulation of misfolded proteins; (2) transcriptional activation of genes encoding ER-resident chaperones; and (3) the ER-associated degradation (ERAD) pathway to restore the folding capacity. If the cells are exposed to prolonged or strong ER stress, the cells are destroyed by apoptosis. Recent evidence indicates that ER stress signaling pathways are mediated in part by several protein kinases and play an important role in the pathogenesis of neurodegenerative disorders. The main purpose of this review is to summarize current knowledge about the protein kinases involved in ER stress, and their involvement in the pathogenesis of neurodegenerative disorders.

Building an antibody factory: a job for the unfolded protein response

Plasma cells are highly specialized, terminally differentiated secretory cells that produce tremendous quantities of a single product, the antibody molecule. In differentiating from a quiescent B cell, the plasma cell must undergo a dramatic architectural metamorphosis. This process entails augmenting the secretory organelles and the proteins that populate them, upregulating their energy and translation potential, and increasing the quality control system to do the job. This transformation is accomplished by an interplay between B lineage-specific transcriptional programs that control plasma cell differentiation and an unfolded protein response.

eIF2 and the control of cell physiology

Eukaryotic initiation factor eIF2 and its 'exchange factor' eIF2B play a key role in the regulation of protein synthesis in eukaryotes from yeast to mammals. Phosphorylation of eIF2 inhibits eIF2B and thus translation initiation. Four eIF2 kinases are now known in mammalian cells and these are activated in response to specific stress conditions. While phosphorylation of eIF2 serves to impair general protein synthesis, it causes upregulation of the translation of certain specific mRNAs that encode transcription factors. It can, therefore, exert effects on gene expression at multiple levels. The importance of correct control of eIF2 and eIF2B for normal physiology is exemplified by data from transgenic mice carrying knock-in or knock-out mutations and by the fact that mutations in the genes for the eIF2 kinase PERK or for eIF2B give rise to serious human diseases.

Cerebral ischemia and the unfolded protein response

We review studies of endoplasmic reticulum (ER) stress and the unfolded protein response (UPR) following cerebral ischemia and reperfusion (I/R). The UPR is a cell stress program activated when misfolded proteins accumulate in the ER lumen. UPR activation causes: (i) a PERK-mediated phosphorylation of eIF2alpha, inhibiting protein synthesis to prevent further accumulation of unfolded proteins in the ER and (ii) upregulation of genes coding for ER-resident enzymes and chaperones and others, via eIF2alpha(p), and ATF6 and IRE1 activation. UPR-induced transcription increases capacity of the ER to process misfolded proteins. If ER stress and the UPR are prolonged, apoptosis ensues. Multiple forms of ER stress have been observed following brain I/R. The UPR following brain I/R is not isomorphic between in vivo I/R models and in vitro cell culture systems with pharmacological UPR induction. Although PERK and IRE1 are activated in the initial hours of reperfusion, total PERK decreases, ATF6 is not activated, and there is delayed appearance of UPR-induced mRNAs. Thus, multiple damage mechanisms associated with brain I/R alter UPR expression and contribute to a pro-apoptotic phenotype in neurons. Insights resulting from these studies will be important for the development of therapies to halt neuronal death following brain I/R.

Translation reinitiation at alternative open reading frames regulates gene expression in an integrated stress response

Stress-induced eukaryotic translation initiation factor 2 (eIF2) alpha phosphorylation paradoxically increases translation of the metazoan activating transcription factor 4 (ATF4), activating the integrated stress response (ISR), a pro-survival gene expression program. Previous studies implicated the 5' end of the ATF4 mRNA, with its two conserved upstream ORFs (uORFs), in this translational regulation. Here, we report on mutation analysis of the ATF4 mRNA which revealed that scanning ribosomes initiate translation efficiently at both uORFs and ribosomes that had translated uORF1 efficiently reinitiate translation at downstream AUGs. In unstressed cells, low levels of eIF2alpha phosphorylation favor early capacitation of such reinitiating ribosomes directing them to the inhibitory uORF2, which precludes subsequent translation of ATF4 and represses the ISR. In stressed cells high levels of eIF2alpha phosphorylation delays ribosome capacitation and favors reinitiation at ATF4 over the inhibitory uORF2. These features are common to regulated translation of GCN4 in yeast. The metazoan ISR thus resembles the yeast general control response both in its target genes and its mechanistic details.

Regulation of mouse GADD34 gene transcription after DNA damaging agent methylmethane sulfonate

The GADD34 gene is transcriptionally induced by growth arrest and DNA damage. However, the mechanisms underlying the transcriptional regulation are still unclear. We analyzed the promoter of mouse GADD34 gene and the methylmethane sulfonate (MMS)-induced transcriptional regulation of this gene. By introducing genome mutants, which were linked to the luciferase reporter, into NIH3T3 cells, we defined a 100-bp fragment upstream of the transcriptional initiating site as the minimal promoter of the GADD34 gene. Subsequent study revealed that CRE-binding site located in this minimal promoter was critical for MMS-induced transcription of the GADD34 gene. In vitro binding experiments showed that phosphorylated c-Jun was contained in the CRE/DNA complex. Overexpression of the dominant negative form of c-Jun led to a decrease of MMS-responsive promoter activity. From these results, we conclude that the CRE site of the GADD34 promoter is indispensable to the MMS-responsive cis-element that c-Jun is the essential transcription factor for MMS-stimulated regulation of GADD34 gene expression and that the upstream signaling is dependent on JNK.

Activating transcription factor 4 is translationally regulated by hypoxic stress

Hypoxic stress results in a rapid and sustained inhibition of protein synthesis that is at least partially mediated by eukaryotic initiation factor 2alpha (eIF2alpha) phosphorylation by the endoplasmic reticulum (ER) kinase PERK. Here we show through microarray analysis of polysome-bound RNA in aerobic and hypoxic HeLa cells that a subset of transcripts are preferentially translated during hypoxia, including activating transcription factor 4 (ATF4), an important mediator of the unfolded protein response. Changes in mRNA translation during the unfolded protein response are mediated by PERK phosphorylation of the translation initiation factor eIF2alpha at Ser-51. Similarly, PERK is activated and is responsible for translational regulation under hypoxic conditions, while inducing the translation of ATF4. The overexpression of a C-terminal fragment of GADD34 that constitutively dephosphorylates eIF2alpha was able to attenuate the phosphorylation of eIF2alpha and severely inhibit the induction of ATF4 in response to hypoxic stress. These studies demonstrate the essential role of ATF4 in the response to hypoxic stress, define the pathway for its induction, and reveal that GADD34, a target of ATF4 activation, negatively regulates the eIF2alpha-mediated inhibition of translation. Taken with the concomitant induction of additional ER-resident proteins identified by our microarray analysis, this study suggests an important integrated response between ER signaling and the cellular adaptation to hypoxic stress.

GADD34 induces p53 phosphorylation and p21/WAF1 transcription

Recently, others and we have shown that one of the functions of GADD34 is a recovery from a shutoff of protein synthesis induced by endoplasmic reticulum stress. GADD34 has been shown to induce growth arrest and apoptosis. Main protein of apoptosis is p53, especially phosphorylation of p53. And one of the main proteins of growth arrest is p21/WAF1. Here we analyzed the effects of GADD34 on p53 phosphorylation and p21/WAF1 transcription. Transfected Myc-tagged p53 was dose-dependently phosphorylated at Ser15 by increasing the amount of GADD34. Transfection of GADD34 also induced the endogenous phosphorylation of p53 and enhanced p21 protein expression. Transfection of GADD34 induced p21/WAF1 promoter activity. This activity was dependent on p53, because GADD34 transfection to p53-deficient cells produced only a slight increase of p21/WAF1 promoter activity. The p21/WAF1 promoter activity was greatly enhanced by the transfection of p53. Both GADD34 and p53 transfection induced much higher p21/WAF1 promoter activity. The promoter activity of p21/WAF1 was very low in GADD34 deficient MEF. The transfection of GADD34 increased the p21/WAF1 promoter activity in GADD34 deficient MEF.

A trip to the ER: coping with stress

The accumulation of unfolded proteins in the lumen of the endoplasmic reticulum (ER) induces a coordinated adaptive program called the unfolded protein response (UPR). The UPR alleviates stress by upregulating protein folding and degradation pathways in the ER and inhibiting protein synthesis. With a basic conceptual framework for the UPR, including the identification of key mediators of the response, now in place, recent work has turned towards investigating how the response is regulated and how its effects radiate beyond the immediate realm of protein secretion. This review highlights advances in these areas and attempts to forecast important issues that must be addressed soon.

Gene expression profiling reveals novel targets of estramustine phosphate in prostate cancer cells

Estramustine phosphate (EMP) is a compound widely used for the treatment of hormone-refractory prostate cancer. In order to better understand the precise molecular mechanism(s) by which EMP exerts its effects on hormone-resistant PC3 prostate cancer cells, we have utilized microarray to interrogate 22,215 known genes to determine the gene expression profiles altered by EMP treatment. The purpose of this investigation was to identify gene expression profile first and then in future studies determine the specific role of these genes in EMP-induced apoptosis in prostate cancer cells. We found a total of 726 genes which showed >2 fold change after EMP treatment. Clustering analysis showed 12 different types of expression alteration. These genes were also subjected to cluster analysis according to their biological functions. We found that EMP regulated the expression of genes, which are critically involved in the regulation of cell growth, cell cycle, apoptosis, iron homeostasis, cytoskeleton and cell signaling transduction. Real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis was used to confirm the results of microarray, and the results of real-time quantitative RT-PCR were consistent with the microarray data. From these results, we conclude that EMP caused changes in the expression of a large number of genes that are related to the control of cell survival and physiological behaviors. The gene expression profiles may provide comprehensive molecular mechanism(s) by which EMP exerts its pleiotropic effects on prostate cancer cells. EMP-induced regulation of these genes may be further exploited for devising therapeutic strategies for prostate cancer.

Discordance of UPR signaling by ATF6 and Ire1p-XBP1 with levels of target transcripts

Accumulation of misfolded proteins within the lumen of the mammalian endoplasmic reticulum (ER) activates the unfolded protein response (UPR). ATF6 and Ire1p are ER-associated proteins that control UPR-specific transcription systems in mammals; UPR signaling involves cleavage of ATF6 and splicing of XBP1 mRNA initiated by Ire1p. We tested the hypothesis that activation of ATF6 and/or Ire1p determines the levels of mRNAs derived from target genes encoding GRP78/BiP and EDEM. By subjecting dermal fibroblasts to multiple stresses, strong correlations were found between ATF6 activation and XBP1 splicing, and between GRP78/BiP mRNA and EDEM mRNA accumulation. Surprisingly, there was no reasonable correlation between activation of either signal transducer with accumulation of either target transcript. Thus, ATF6 and Ire1p signaling do not define the magnitude of UPR-dependent mRNA increases, even though they may be necessary for gene activation, suggesting the existence of additional stress-sensitive factors acting as "coincidence detectors" for transcript accumulation.

The making of a professional secretory cell: architectural and functional changes in the ER during B lymphocyte plasma cell differentiation

B lymphocytes are small cells that express antigen receptors and secrete little if any IgM. Upon encounter with antigen, they differentiate into short-lived plasma cells, which secrete large amounts of polymeric IgM. Plasma cell differentiation entails a massive development of the endoplasmic reticulum to sustain high levels of Ig production. Recent findings suggest a role for the unfolded protein response in orchestrating the architectural and functional changes during terminal plasma cell differentiation.

Control of alpha subunit of eukaryotic translation initiation factor 2 (eIF2 alpha) phosphorylation by the human papillomavirus type 18 E6 oncoprotein: implications for eIF2 alpha-dependent gene expression and cell death

Phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF2alpha) at serine 51 inhibits protein synthesis in cells subjected to various forms of stress including virus infection. The human papillomavirus (HPV) E6 oncoprotein contributes to virus-induced pathogenicity through multiple mechanisms including the inhibition of apoptosis and the blockade of interferon (IFN) action. We have investigated a possible functional relationship between the E6 oncoprotein and eIF2alpha phosphorylation by an inducible-dimerization form of the IFN-inducible protein kinase PKR. Herein, we demonstrate that HPV type 18 E6 protein synthesis is rapidly repressed upon eIF2alpha phosphorylation caused by the conditional activation of the kinase. The remainder of E6, however, can rescue cells from PKR-mediated inhibition of protein synthesis and induction of apoptosis. E6 physically associates with GADD34/PP1 holophosphatase complex, which mediates translational recovery, and facilitates eIF2alpha dephosphorylation. Inhibition of eIF2alpha phosphorylation by E6 mitigates eIF2alpha-dependent responses to transcription and translation of proapoptotic genes. These findings demonstrate, for the first time, a role of the oncogenic E6 in apoptotic signaling induced by PKR and eIF2alpha phosphorylation. The functional interaction between E6 and the eIF2alpha phosphorylation pathway may have important implications for HPV infection and associated pathogenesis.

Identification of cell surface targets for HIV-1 therapeutics using genetic screens

Human immunodeficiency virus (HIV) drugs designed to interfere with obligatory utilization of certain host cell factors by virus are less likely to encounter development of resistant strains than drugs directed against viral components. Several cellular genes required for productive infection by HIV were identified by the use of genetic suppressor element (GSE) technology as potential targets for anti-HIV drug development. Fragmented cDNA libraries from various pools of human peripheral blood mononuclear cells (PBMC) were expressed in vitro in human immunodeficiency virus type 1 (HIV-1)-susceptible cell lines and subjected to genetic screens to identify GSEs that interfered with viral replication. After three rounds of selection, more than 15000 GSEs were sequenced, and the cognate genes were identified. The GSEs that inhibited the virus were derived from a diverse set of genes including cell surface receptors, cytokines, signaling proteins, transcription factors, as well as genes with unknown function. Approximately 2.5% of the identified genes were previously shown to play a role in the HIV-1 life cycle; this finding supports the biological relevance of the assay. GSEs were derived from the following 12 cell surface proteins: CXCR4, CCR4, CCR7, CD11C, CD44, CD47, CD68, CD69, CD74, CSF3R, GABBR1, and TNFR2. Requirement of some of these genes for viral infection was also investigated by using RNA interference (RNAi) technology; accordingly, 10 genes were implicated in early events of the viral life cycle, before viral DNA synthesis. Thus, these cell surface proteins represent novel targets for the development of therapeutics against HIV-1 infection and AIDS.

Protein phosphatase 1, but not protein phosphatase 2A, dephosphorylates DNA-damaging stress-induced phospho-serine 15 of p53

Okadaic acid (OA) is a protein phosphatase (PP) inhibitor and induces hyperphosphorylation of p53. We investigated whether the inhibition of PP1 by OA promotes the phosphorylation of the serine 15 of p53. In vitro dephosphorylation assay showed that PP1 dephosphorylated ultraviolet C (UVC)-induced phospho-ser15 of p53, and that OA treatment inhibited it. One of the PP1 regulators, growth arrest and DNA damage 34 (GADD34), disturbed PP1 binding with p53, interfered with the dephosphorylation of p53 and increased the amount of phospho-p53 after UVC-treatment. This report provides the first evidence that PP1, but not PP2A, dephosphorylates phospho-serine 15 of p53.

Elevated gadd153/chop expression and enhanced c-Jun N-terminal protein kinase activation sensitizes aged cells to ER stress

The endoplasmic reticulum (ER), as a processing plant for the folding and posttranslational modification of proteins, is exquisitely sensitive to changes in its internal environment. Various conditions, collectively termed 'ER stress', can perturb ER functions, leading to the activation of a complex response known as the unfolded protein response. Here, we investigated the response of hepatocytes derived from young (4-5 months) and aged (24-26 months) rats to two agents, thapsigargin (TG) and tunicamycin (TM), which act via different mechanisms to induce ER stress. Old hepatocytes displayed greater cell death than young cells following treatment with TG or TM, associated with higher expression of the pro-apoptotic gene gadd153 (also known as chop) and enhanced c-Jun N-terminal protein kinase (JNK) activation. Pharmacologic inhibition of JNK decreased the expression of TG-stimulated gadd153 in old cells and reduced their sensitivity to TG-induced cell death. Inhibition of p38, on the other hand, enhanced TG-induced gadd153 expression and JNK activation, and augmented TG-induced cell death. Additional experiments implicated the PERK/eIF-2 alpha signaling pathway as a contributor to the higher Gadd153 expression and JNK activation, and greater sensitivity of old cells to ER stress.

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.

Functional diversity of protein phosphatase-1, a cellular economizer and reset button

The protein serine/threonine phosphatase protein phosphatase-1 (PP1) is a ubiquitous eukaryotic enzyme that regulates a variety of cellular processes through the dephosphorylation of dozens of substrates. This multifunctionality of PP1 relies on its association with a host of function-specific targetting and substrate-specifying proteins. In this review we discuss how PP1 affects the biochemistry and physiology of eukaryotic cells. The picture of PP1 that emerges from this analysis is that of a "green" enzyme that promotes the rational use of energy, the recycling of protein factors, and a reversal of the cell to a basal and/or energy-conserving state. Thus PP1 promotes a shift to the more energy-efficient fuels when nutrients are abundant and stimulates the storage of energy in the form of glycogen. PP1 also enables the relaxation of actomyosin fibers, the return to basal patterns of protein synthesis, and the recycling of transcription and splicing factors. In addition, PP1 plays a key role in the recovery from stress but promotes apoptosis when cells are damaged beyond repair. Furthermore, PP1 downregulates ion pumps and transporters in various tissues and ion channels that are involved in the excitation of neurons. Finally, PP1 promotes the exit from mitosis and maintains cells in the G1 or G2 phases of the cell cycle.

Nck-1 Antagonizes the Endoplasmic Reticulum Stress-induced Inhibition of Translation

Eukaryotic cells have developed specific mechanisms to overcome environmental stress. Here we show that the Src homology 2/3 (SH2/SH3) domain-containing protein Nck-1 prevents the unfolded protein response normally induced by pharmacological endoplasmic reticulum (ER) stress agents. Overexpression of Nck-1 enhances protein translation, whereas it abrogates eukaryotic initiation factor 2alpha (eIF2alpha) phosphorylation and inhibition of translation in response to tunicamycin or thapsigargin treatment. Nck-1 overexpression also attenuates induction of the ER chaperone, the immunoglobulin heavy chain-binding protein (BiP), and impairs cell survival in response to thapsigargin. We provided evidence that in these conditions, the effects of Nck on the unfolded protein response (UPR) involve its second SH3 domain and a calyculin A-sensitive phosphatase activity. In addition, we demonstrated that protein translation is reduced in mouse embryonic fibroblasts lacking both Nck isoforms and is enhanced in similar cells expressing high levels of Nck-1. In these various mouse embryonic fibroblasts, we also provided evidence that Nck modulates the activation of the ER resident eIF2alpha kinase PERK and consequently the phosphorylation of eIF2alpha on Ser-51 in response to stress. Our study establishes that Nck is required for optimal protein translation and demonstrates that, in addition to its adaptor function in mediating signaling from the plasma membrane, Nck also mediates signaling from the ER membrane compartment.

The Tumor Suppressor Activity of MDA-7/IL-24 Is Mediated by Intracellular Protein Expression in NSCLC Cells

mda-7/IL-24 (HGMW-approved symbol IL24) is a tumor suppressor gene whose expression is lost during tumor progression. Gene transfer using adenoviral mda-7/IL-24 (Ad-mda7) exhibits minimal toxicity on normal cells while inducing potent apoptosis in a variety of cancer cell lines. Ad-mda7-transduced cells express high levels of MDA-7 protein intracellularly and also secrete a soluble form of MDA-7 protein. In this study, we sought to determine whether the intracellular or secreted MDA-7 protein was responsible for anti-tumor activity in H1299 lung tumor cells. Ad-mda7 transduction of lung tumor cells increased expression of stress-related proteins, including BiP, GADD34, PP2A, caspases 7 and 12, and XBP-1, consistent with activation of the UPR pathway, a key sensor of endoplasmic reticulum (ER)-mediated stress. Blocking secretion of MDA-7 did not inhibit apoptosis, demonstrating that intracellular MDA-7 was responsible for cytotoxicity. Consistent with this result, when applied directly to lung cancer cells, soluble MDA-7 protein exhibited minimal cytotoxic effect. We then generated mda-7 expression constructs using vectors that target the expressed protein to various subcellular compartments, including cytoplasm, nucleus, and ER. Only full-length and ER-targeted MDA-7 elicited cell death in tumor cells. Thus in lung cancer cells, Ad-mda7 activates the UPR stress pathway and induces apoptosis via intracellular MDA-7 expression in the secretory pathway.

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.

The protein kinase PKR: a molecular clock that sequentially activates survival and death programs

Cell death and survival play a key role in the immune system as well as during development. The control mechanisms that balance cell survival against cell death are not well understood. Here we report a novel strategy used by a single protein to regulate chronologically cell survival and death. The interferon-induced protein kinase PKR acts as a molecular clock by using catalysis-dependent and -independent activities to temporally induce cell survival prior to cell death. We show that the proapoptotic protein PKR surprisingly activates a survival pathway, which is mediated by NF-kappaB to delay apoptosis. Cell death is then induced by PKR through the phosphorylation of eIF-2alpha. This unique temporal control might serve as a paradigm for other kinases whose catalytic activity is not required for all of their functions.

Inhibition of a constitutive translation initiation factor 2alpha phosphatase, CReP, promotes survival of stressed cells

Phosphorylation of eukaryotic translation initiation factor 2α (eIF2α) on serine 51 is effected by specific stress-activated protein kinases. eIF2α phosphorylation inhibits translation initiation promoting a cytoprotective gene expression program known as the integrated stress response (ISR). Stress-induced activation of GADD34 feeds back negatively on this pathway by promoting eIF2α dephosphorylation, however, GADD34 mutant cells retain significant eIF2α-directed phosphatase activity. We used a somatic cell genetic approach to identify a gene encoding a novel regulatory subunit of a constitutively active holophosphatase complex that dephosphorylates eIF2α. RNAi of this gene, which we named constitutive repressor of eIF2α phosphorylation (CReP, or PPP1R15B), repressed the constitutive eIF2α-directed phosphatase activity and activated the ISR. CReP RNAi strongly protected mammalian cells against oxidative stress, peroxynitrite stress, and more modestly against accumulation of malfolded proteins in the endoplasmic reticulum. These findings suggest that therapeutic inhibition of eIF2α dephosphorylation by targeting the CReP-protein–phosphatase-1 complex may be used to access the salubrious qualities of the ISR.

GADD34-PP1c recruited by Smad7 dephosphorylates TGFbeta type I receptor

The cascade of phosphorylation is a pivotal event in transforming growth factor β (TGFβ) signaling. Reversible phosphorylation regulates fundamental aspects of cell activity. TGFβ-induced Smad7 binds to type I receptor (TGFβ type I receptor; TβRI) functioning as a receptor kinase antagonist. We found Smad7 interacts with growth arrest and DNA damage protein, GADD34, a regulatory subunit of the protein phosphatase 1 (PP1) holoenzyme, which subsequently recruits catalytic subunit of PP1 (PP1c) to dephosphorylate TβRI. Blocking Smad7 expression by RNA interference inhibits association of GADD34–PP1c complex with TβRI, indicating Smad7 acts as an adaptor protein in the formation of the PP1 holoenzyme that targets TβRI for dephosphorylation. SARA (Smad anchor for receptor activation) enhances the recruitment PP1c to the Smad7–GADD34 complex by controlling the specific subcellular localization of PP1c. Importantly, GADD34–PP1c recruited by Smad7 inhibits TGFβ-induced cell cycle arrest and mediates TGFβ resistance in responding to UV light irradiation. The dephosphorylation of TβRI mediated by Smad7 is an effective mechanism for governing negative feedback in TGFβ signaling.

Cytoprotection by pre-emptive conditional phosphorylation of translation initiation factor 2

Transient phosphorylation of the alpha-subunit of translation initiation factor 2 (eIF2alpha) represses translation and activates select gene expression under diverse stressful conditions. Defects in the eIF2alpha phosphorylation-dependent integrated stress response impair resistance to accumulation of malfolded proteins in the endoplasmic reticulum (ER stress), to oxidative stress and to nutrient deprivations. To study the hypothesized protective role of eIF2alpha phosphorylation in isolation of parallel stress signaling pathways, we fused the kinase domain of pancreatic endoplasmic reticulum kinase (PERK), an ER stress-inducible eIF2alpha kinase that is normally activated by dimerization, to a protein module that binds a small dimerizer molecule. The activity of this artificial eIF2alpha kinase, Fv2E-PERK, is subordinate to the dimerizer and is uncoupled from upstream stress signaling. Fv2E-PERK activation enhanced the expression of numerous stress-induced genes and protected cells from the lethal effects of oxidants, peroxynitrite donors and ER stress. Our findings indicate that eIF2alpha phosphorylation can initiate signaling in a cytoprotective gene expression pathway independently of other parallel stress-induced signals and that activation of this pathway can single-handedly promote a stress-resistant preconditioned state.

Ischaemic preconditioning in the rat brain: effect on the activity of several initiation factors, Akt and extracellular signal-regulated protein kinase phosphorylation, and GRP78 and GADD34 expression

Translational repression induced during reperfusion of the ischaemic brain is significantly attenuated by ischaemic preconditioning. The present work was undertaken to identify the components of the translational machinery involved and to determine whether translational attenuation selectively modifies protein expression patterns during reperfusion. Wistar rats were preconditioned by 5-min sublethal ischaemia and 2 days later, 30-min lethal ischaemia was induced. Several parameters were studied after lethal ischaemia and reperfusion in rats with and without acquired ischaemic tolerance (IT). The phosphorylation pattern of the alpha subunit of eukaryotic initiation factor 2 (eIF2) in rats with IT was exactly the same as in rats without IT, reaching a peak after 30 min reperfusion and returning to control values within 4 h in both the cortex and hippocampus. The levels of phosphorylated eIF4E-binding protein after lethal ischaemia and eIF4E at 30 min reperfusion were higher in rats with IT, notably in the hippocampus. eIF4G levels diminished slightly after ischaemia and reperfusion, paralleling calpain-mediated alpha-spectrin proteolysis in rats with and without IT, but they did not show any further decrease after 30 min reperfusion in rats with IT. The phosphorylated levels of eIF4G, phosphatidylinositol 3-kinase-protein B (Akt) and extracellular signal-regulated kinases (ERKs) were very low after lethal ischaemia and increased following reperfusion. Ischaemic preconditioning did not modify the observed changes in eIF4G phosphorylation. All these results support that translation attenuation may occur through multiple targets. The levels of the glucose-regulated protein (78 kDa) remained unchanged in rats with and without IT. Conversely, our data establish a novel finding that ischaemia induces strong translation of growth arrest and DNA damage protein 34 (GADD34) after 4 h of reperfusion. GADD34 protein was slightly up-regulated after preconditioning, besides, as in rats without IT, GADD34 levels underwent a further clear-cut increase during reperfusion, this time as earlier as 30 min and coincident with translation attenuation.

Stress-Induced Apoptosis in Spodoptera frugiperda (Sf9) Cells: Baculovirus p35 Mitigates eIF2α Phosphorylation

Spodoptera frugiperda (Sf9) ovarian cells, natural hosts for baculovirus, are good model systems to study apoptosis and also heterologous gene expression. We report that uninfected Sf9 cells readily undergo apoptosis and show increased phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF2alpha) in the presence of agents such as UVB light, etoposide, high concentrations of cycloheximide, and EGTA. In contrast, tunicamycin, A23187, and low concentrations of cycloheximide promoted eIF2alpha phosphorylation in Sf9 cells but without apoptosis. These findings therefore suggest that increased eIF2alpha phosphorylation does not always necessarily lead to apoptosis, but it is a characteristic hallmark of stressed cells and also of cells undergoing apoptosis. Cell death induced by the above agents was abrogated by infection of Sf9 cells with wild-type (wt) AcNPV. In contrast, Sf9 cells when infected with vAcdelta35, a virus carrying deletion of the antiapoptotic p35 gene, showed increased apoptosis and enhanced eIF2alpha phosphorylation. Further, a recombinant wt virus vAcS51D expressing human S51D, a phosphomimetic form of eIF2alpha, induced apoptosis in UVB pretreated Sf9 cells. However, infection with vAcS51A expressing a nonphosphorylatable form (S51A) of human eIF2alpha partially reduced apoptosis. Consistent with these findings, it has been observed here that caspase activation has led to increased eIF2alpha phosphorylation, while caspase inhibition by z-VAD-fmk reduced eIF2alpha phosphorylation selectively in cells exposed to proapoptotic agents. These findings therefore suggest that the stress signaling pathway determines apoptosis, and caspase activation is a prerequisite for increased eIF2alpha phosphorylation in Sf9 cells undergoing apoptosis. The findings also reinforce the conclusion for the first time that the "pancaspase inhibitor" baculovirus p35 mitigates eIF2alpha phosphorylation.

Calcium dynamics and endoplasmic reticular function in the regulation of protein synthesis: implications for cell growth and adaptability

The endoplasmic reticulum (ER) possesses the structural and functional features expected of an organelle that supports the integration and coordination of major cellular processes. Ca(2+) sequestered within the ER sustains lumenal protein processing while providing a reservoir of the cation to support stimulus-response coupling in the cytosol. Release of ER Ca(2+) sufficient to impair protein processing promotes ER stress and signals the "unfolded protein response" (UPR). The association of the UPR with an acute suppression of mRNA translational initiation and a longer term up-regulation of ER chaperones and partial translational recovery is discussed. Regulatory sites in mRNA translation and the mechanisms responsible for the early and later phases of the UPR are reviewed. The regulatory significance of GRP78/BiP, a multifunctional, broad-specificity ER chaperone, in the coordination of ER protein processing with mRNA translation during acute and chronic ER stress is addressed. The relationship of ER stress to protein misfolding in the cytoplasm is examined. Translational alterations in embryonic cardiomyocytes during treatments with various Ca(2+)-mobilizing, growth-promoting stimuli are described. The importance of ER Ca(2+) stores, ER chaperones, and cytosolic-free Ca(2+) in translational control and growth promotion by these stimuli is assessed. Some perspectives are provided regarding Ca(2+) as an integrating factor in the generation or diversion of metabolic energy. Circumstances impacting upon cellular adaptability during exposure to growth stimuli or during stressful conditions that require rapid adjustments in ATP for continued viability are considered.