Signalling through either the p38 or ERK mitogen-activated protein (MAP) kinase pathway is obligatory for phorbol ester and T cell receptor complex (TCR-CD3)-stimulated phosphorylation of initiation factor (eIF) 4E in Jurkat T cells

Implication of 4E-BP1 protein dephosphorylation and accumulation in pancreatic cancer cell death induced by combined gemcitabine and TRAIL

Pancreatic cancer cells show varying sensitivity to the anticancer effects of gemcitabine. However, as a chemotherapeutic agent, gemcitabine can cause intolerably high levels of toxicity and patients often develop resistance to the beneficial effects of this drug. Combination studies show that use of gemcitabine with the pro-apoptotic cytokine TRAIL can enhance the inhibition of survival and induction of apoptosis of pancreatic cancer cells. Additionally, following combination treatment there is a dramatic increase in the level of the hypophosphorylated form of the tumour suppressor protein 4E-BP1. This is associated with inhibition of mTOR activity, resulting from caspase-mediated cleavage of the Raptor and Rictor components of mTOR. Use of the pan-caspase inhibitor Z-VAD-FMK indicates that the increase in level of 4E-BP1 is also caspase-mediated. ShRNA-silencing of 4E-BP1 expression renders cells more resistant to cell death induced by the combination treatment. Since the levels of 4E-BP1 are relatively low in untreated pancreatic cancer cells these results suggest that combined therapy with gemcitabine and TRAIL could improve the responsiveness of tumours to treatment by elevating the expression of 4E-BP1.

Multi-Compartmentalisation in the MAPK Signalling Pathway Contributes to the Emergence of Oscillatory Behaviour and to Ultrasensitivity

Signal transduction through the Mitogen Activated Protein Kinase (MAPK) pathways is evolutionarily highly conserved. Many cells use these pathways to interpret changes to their environment and respond accordingly. The pathways are central to triggering diverse cellular responses such as survival, apoptosis, differentiation and proliferation. Though the interactions between the different MAPK pathways are complex, nevertheless, they maintain a high level of fidelity and specificity to the original signal. There are numerous theories explaining how fidelity and specificity arise within this complex context; spatio-temporal regulation of the pathways and feedback loops are thought to be very important. This paper presents an agent based computational model addressing multi-compartmentalisation and how this influences the dynamics of MAPK cascade activation. The model suggests that multi-compartmentalisation coupled with periodic MAPK kinase (MAPKK) activation may be critical factors for the emergence of oscillation and ultrasensitivity in the system. Finally, the model also establishes a link between the spatial arrangements of the cascade components and temporal activation mechanisms, and how both contribute to fidelity and specificity of MAPK mediated signalling.

Translational Control during Calicivirus Infection

In this review, we provide an overview of the strategies developed by caliciviruses to subvert or regulate the host protein synthesis machinery to their advantage. As intracellular obligate parasites, viruses strictly depend on the host cell resources to produce viral proteins. Thus, many viruses have developed strategies that regulate the function of the host protein synthesis machinery, often leading to preferential translation of viral mRNAs. Caliciviruses lack a 5′ cap structure but instead have a virus-encoded VPg protein covalently linked to the 5′ end of their mRNAs. Furthermore, they encode 2–4 open reading frames within their genomic and subgenomic RNAs. Therefore, they use alternative mechanisms for translation whereby VPg interacts with eukaryotic initiation factors (eIFs) to act as a proteinaceous cap-substitute, and some structural proteins are produced by reinitiation of translation events. This review discusses our understanding of these key mechanisms during caliciviruses infection as well as recent insights into the global regulation of eIF4E activity.

Murine norovirus 1 (MNV1) replication induces translational control of the host by regulating eIF4E activity during infection

Protein synthesis is a tightly controlled process responding to several stimuli, including viral infection. As obligate intracellular parasites, viruses depend on the translation machinery of the host and can manipulate it by affecting the availability and function of specific eukaryotic initiation factors (eIFs). Human norovirus is a member of the Caliciviridae family and is responsible for gastroenteritis outbreaks. Previous studies on feline calicivirus and murine norovirus 1 (MNV1) demonstrated that the viral protein, genome-linked (VPg), acts to direct translation by hijacking the host protein synthesis machinery. Here we report that MNV1 infection modulates the MAPK pathway to activate eIF4E phosphorylation. Our results show that the activation of p38 and Mnk during MNV1 infection is important for MNV1 replication. Furthermore, phosphorylated eIF4E relocates to the polysomes, and this contributes to changes in the translational state of specific host mRNAs. We propose that global translational control of the host by eIF4E phosphorylation is a key component of the host-pathogen interaction.

Morphoproteomics: exposing protein circuitries in tumors to identify potential therapeutic targets in cancer patients

Morphoproteomics combines the disciplines of histopathology, molecular biology and protein chemistry to paint a portrait of the protein circuitry in diseased cells for the purpose of uncovering molecular targets amenable to specific intervention, thereby customizing therapy for individual patients. This review considers the clinical application of morphoproteomics in malignant cells in the context of currently available pharmaceutical agents and discusses opportunities for combinatorial approaches that involve one or more small molecule inhibitors and single-agent chemotherapy with relatively low toxicity profiles. Future directions that involve focusing on points of convergence in signal transduction pathways and which integrate morphoproteomic with genomic and pharmacoproteomic and protein-function microarray data are offered.

Role of the eIF4E binding protein 4E-BP1 in regulation of the sensitivity of human pancreatic cancer cells to TRAIL and celastrol-induced apoptosis

BACKGROUND INFORMATION

Tumour cells can be induced to undergo apoptosis after treatment with the tumour necrosis factor α-related death-inducing ligand (TRAIL). Although human pancreatic cancer cells show varying degrees of response they can be sensitised to the pro-apoptotic effects of TRAIL in the presence of celastrol, a natural compound extracted from the plant Tripterygium wilfordii Hook F. One important aspect of the cellular response to TRAIL is the control of protein synthesis, a key regulator of which is the eukaryotic initiation factor 4E-binding protein, 4E-BP1.

RESULTS

We examined the effects of celastrol and TRAIL in several pancreatic cancer cell lines. In cells that are normally resistant to TRAIL, synergistic effects of TRAIL plus celastrol on commitment to apoptosis and inhibition of protein synthesis were observed. These were associated with a strong up-regulation and dephosphorylation of 4E-BP1. The enhancement of 4E-BP1 expression, which correlated with a threefold increase in the level of the 4E-BP1 transcript, was blocked by inhibitors of reactive oxygen species and the JNK protein kinase. When the expression of 4E-BP1 was reduced by an inducible micro-RNA, TRAIL-mediated apoptosis was inhibited.

CONCLUSION

These results suggest that 4E-BP1 plays a critical role in the mechanism by which TRAIL and celastrol together cause apoptotic cell death in human pancreatic tumour cells.

MNK1 kinase activity is required for abscission

MNK1 is a serine/threonine kinase identified as a target for MAP kinase pathways. Using chemical drug, kinase-dead expression or knockdown by RNA interference, we show that inhibition of MNK1 induces the formation of multinucleated cells, which can be rescued by expressing a form of MNK1 that is resistant to RNA interference. We found that the active human form of MNK1 localises to centrosomes, spindle microtubules and the midbody. Time-lapse recording of MNK1-depleted cells displays cytokinesis defects, as daughter cells fuse back together. When MNK1 activity was inhibited, no microtubule defect at the midbody was detected, however, anchorage of the membrane vesicle at the midbody was impaired as lumenal GFP-positive vesicles did not accumulate at the midbody. At the molecular level, we found that centriolin localisation was impaired at the midbody in MNK1-depleted cells. As a consequence, endobrevin - a v-SNARE protein implicated in the abscission step - was not properly localised to the midbody. Altogether, our data show that MNK1 activity is required for abscission.

Aldose reductase inhibitor fidarestat attenuates leukocyte-endothelial interactions in experimental diabetic rat retina in vivo

PURPOSE

Dysregulation of the polyol pathway has been implicated as a major cause of diabetic retinopathy. The aldose reductase inhibitor fidarestat was recently reported to prevent retinal oxidative stress and overexpression of vascular endothelial growth factor (VEGF) protein in diabetic rats. In this study, we investigated the effect of fidarestat on leukocyte-endothelial cell interactions in an in vivo experimental model for diabetic retina.

MATERIALS AND METHODS

Diabetes was induced in six-week-old male Long-Evans rats by intraperitoneal injection of streptozotocin (STZ) (75 mg/kg). The rats were divided into four experimental groups: non-diabetic control rats, untreated diabetic rats, and diabetic rats treated with a low (4 mg/kg/day) or high (16 mg/kg/day) oral dose of fidarestat. After four weeks of treatment, accumulated leukocytes in the retina were counted in vivo by acridine orange digital fluorography. Intercellular adhesion molecule-1 (ICAM-1) and VEGF-164 mRNA levels in the retina were analyzed using the quantitative reverse transcription-polymerase chain reaction. ICAM-1 protein expression in the retina was investigated by immunohistochemistry.

RESULTS

Fidarestat treatment significantly decreased concentrations of sorbitol and fructose in the retinas of STZ-induced diabetic rats. Leukocyte accumulation in the retinas of fidarestat-treated rats was significantly less than in the untreated diabetic group (P < 0.01). Fidarestat treatment significantly reduced the expression ICAM-1 mRNA, but not VEGF-164 mRNA, in the retina of diabetic rats. Immunohistochemical study also revealed the suppressive effect of fidarestat on expression of ICAM-1.

CONCLUSIONS

Oral administration of fidarestat attenuated leukocyte accumulation in the retina of STZ induced-diabetic rats, suggesting that fidarestat may have a therapeutic role in preventing the progression of diabetic retinopathy.

Analysis of mRNA associated factors during bovine oocyte maturation and early embryonic development

Regulation of gene expression at the translational level is particularly essential during developmental periods, when transcription is impaired. According to the closed-loop model of translational initiation, we have analyzed components of the 5 -mRNA cap-binding complex eIF4F (eIF4E, eIF4G, eIF4A), the eIF4E repressor 4E-BP1, and 3 -mRNA poly-(A) tail-associated proteins (PABP1 and 3, PAIP1 and 2, CPEB1, Maskin) during in vitro maturation of bovine oocytes and early embryonic development up to the 16-cell stage. Furthermore, we have elucidated the activity of distinct kinases which are potentially involved in their phosphorylation. Major phosphorylation of specific target sequences of PKA, PKB, PKC, CDKs, ATM/ATR, and MAPK were observed in M II stage oocytes. Furthermore, main changes in the abundance and/or phosphorylation of distinct mRNA-binding factors occur at the transition from M II stage oocytes to 2-cell embryos. In conclusion, the results indicate that, at the transition from oocyte to embryonic development, translational initiation is regulated by striking differences in the abundance and/or phosphorylation of 5 -end and 3 -end mRNA associated factors, mainly the poly-(A) bindings proteins PABP1 and 3, their repressor PAIP2 and a Maskin-like protein with distinct eIF4E-binding properties which prevents eIF4E/cap binding and eIF4F formation in vitro. Nevertheless, from the M II stage to 16-cell embryos a substantial amount of eIF4E and, to a lesser extent, of eIF4G was precipitated by (7)m-GTP-Separose indicating eIF4F complex formation. Therefore, it is likely that in general the reduction in PABP1 and 3 abundance represses overall translation during early embryonic development.

TRANSLATIONAL CONTROL IN T LYMPHOCYTES

Translational control is an important but relatively unappreciated mechanism that regulates levels of protein products. In addition to a global translational control that regulates the cell's response to external stimuli such as growth factors, cytokines, stress, and viral infections, selective translational control has recently been demonstrated to affect many genes related to growth and apoptotic processes. Translational infidelity has recently been suggested as a new mechanism of T cell dysregulation in SLE. This review discusses current data on translational control of T cell biology and the central aspect of translational control in the signalling pathway leading to T cell proliferation, apoptotic response, and cytokine production. The utility for global analysis by genomics to study translational control of T cell gene expression is also discussed.

Inhibition of mammalian target of rapamycin (mTOR) signalling in C2C12 myoblasts prevents myogenic differentiation without affecting the hyperphosphorylation of 4E-BP1

Current accepted models suggest that hypophosphorylated 4E-binding protein (4E-BP1) binds to initiation factor 4E (eIF4E) to inhibit cap-dependent translation, a process readily reversed by its phosphorylation following activation of mammalian target of rapamycin (mTORC1) signalling. Myogenic differentiation in the C2C12 myoblast model system reflects a concerted and controlled activation of transcription and translation following the exit of cells from the cell cycle. Here we show that myogenic differentiation is associated with increased rates of translation, the up-regulation of both 4E-BP1 mRNA and protein levels and enhanced levels of eIF4E/4E-BP1 complex. Paradoxically, treatment of C2C12 myoblasts with an inhibitor of mTOR signalling (RAD001) which inhibits translation, promotes the hyperphosphorylation of 4E-BP1 on novel sites and prevents the increase in 4E-BP1 levels. In contrast, eIF4E appears to be under translational control with a significant delay between induction of mRNA and subsequent protein expression.

Molecular mechanisms of cell proliferation induced by low power laser irradiation

Low power laser irradiation (LPLI) promotes proliferation of multiple cells, which (especially red and near infrared light) is mainly through the activation of mitochondrial respiratory chain and the initiation of cellular signaling. Recently, the signaling proteins involved in LPLI-induced proliferation merit special attention, some of which are regulated by mitochondrial signaling. Hepatocyte growth factor receptor (c-Met), a member of tyrosine protein kinase receptors (TPKR), is phosphorylated during LPLI-induced proliferation, but tumor necrosis factor alpha (TNF-alpha) receptor has not been affected. Activated TPKR could activate its downstream signaling elements, like Ras/Raf/MEK/ERK, PI3K/Akt/eIF4E, PI3K/Akt/eNOS and PLC-gamma/PKC pathways. Other two pathways, DeltaPsim/ATP/cAMP/JNK/AP-1 and ROS/Src, are also involved in LPLI-induced proliferation. LPLI-induced cell cycle progression can be regulated by the activation or elevated expressions of cell cycle-specific proteins. Furthermore, LPLI induces the synthesis or release of many molecules, like growth factors, interleukins, inflammatory cytokines and others, which are related to promotive effects of LPLI.

The proteasome inhibitor, MG132, promotes the reprogramming of translation in C2C12 myoblasts and facilitates the association of hsp25 with the eIF4F complex

The eukaryotic translation initiation factor (eIF) 4E, is regulated by modulating both its phosphorylation and its availability to interact with the scaffold protein, eIF4G, to form the mature eIF4F complex. Here we show that treatment of C2C12 myoblasts with the proteasomal inhibitor, MG132 (N-carbobenzoxyl-Leu-Leu-leucinal), resulted in an early decrease in protein synthesis rates followed by a partial recovery, reflecting the reprogramming of translation. The early inhibition of protein synthesis was preceded by a transient increase in eIF2alpha phosphorylation, followed by a sustained increase in eIF4E phosphorylation. Inhibition of eIF4E phosphorylation with CGP57380 failed to prevent translational reprogramming or the moderate decrease in eIF4F complexes at later times. Prolonged incubation with MG132 resulted in the increased expression of heat shock protein (hsp)25, alphaB-crystallin and hsp70, with a population of hsp25 associating with the eIF4F complex in a p38 mitogen-activated protein kinase-dependent manner. Under these conditions, eIF4GI, and to a lesser extent eIF4E, re-localized from a predominantly cytoplasmic distribution to a more perinuclear and granular staining. Although MG132 had little effect on the colocalization of eIF4E and eIF4GI, it promoted the SB203580-sensitive association of eIF4GI and hsp25, an effect not observed with alphaB-crystallin. Addition of recombinant hsp25 to an in vitro translation assay resulted in stimulation of on-going translation and a moderate decrease in de novo translation, indicating that this modified eIF4F complex containing hsp25 has a role to play in recovery of mRNA translation following cellular stress.

A role for PYK2 in ANG II-dependent regulation of the PHAS-1-eIF4E complex by multiple signaling cascades in vascular smooth muscle

Regulation of the PHAS-1-eukaryotic initiation factor-4E (eIF4E) complex is the rate-limiting step in the initiation of protein synthesis. This study characterized the upstream signaling pathways that mediate ANG II-dependent phosphorylation of PHAS-1 and eIF4E in vascular smooth muscle. ANG II-dependent PHAS-1 phosphorylation was maximal at 10 min (2.47 +/- 0.3 fold vs. control). This effect was completely blocked by the specific inhibitors of phosphatidylinositol 3-kinase (PI3-kinase, LY-294002), mammalian target of rapamycin, and extracellular signal-regulated kinase 1/2 (ERK1/2, U-0126) or by a recombinant adenovirus encoding dominant-negative Akt. PHAS-1 phosphorylation was followed by dissociation of eIF4E. Increased ANG II-induced eIF4E phosphorylation was observed at 45 min (2.63 +/- 0.5 fold vs. control), was maximal at 90 min (3.38 +/- 0.3 fold vs. control), and was sustained at 2 h. This effect was blocked by inhibitors of the ERK1/2 and p38 mitogen-activated protein (MAP) kinase pathways, but not by PI3-kinase inhibition, and was dependent on PKC, intracellular Ca2+, and tyrosine kinases. Downregulation of proline-rich tyrosine kinase 2 (PYK2) by antisense oligonucleotides led to a near-complete inhibition of PHAS-1 and eIF4E phosphorylation in response to ANG II. Therefore, PYK2 represents a proximal signaling intermediate that regulates ANG II-induced vascular smooth muscle cell protein synthesis via regulation of the PHAS-1-eIF4E complex.

Reactive oxygen species sensitivity of angiotensin II-dependent translation initiation in vascular smooth muscle cells

Translation initiation, the rate-limiting step in protein synthesis, is a key event in vascular smooth muscle cell growth, a major component of vascular disease. Translation initiation is regulated by interaction between PHAS-I and the eukaryotic initiation factor 4E (eIF4E). Although angiotensin II (Ang II)-induced vascular smooth muscle cell hypertrophy requires the generation of reactive oxygen species (ROS), the ROS sensitivity of these events and their upstream activators remain unclear. Here, we investigated the role of ROS in the regulation of PHAS-I phosphorylation on Thr-70 and Ser-65, an event required for the release of eIF4E from PHAS-I. Ang II-induced Ser-65 phosphorylation was ROS-dependent as assessed by pretreatment with ebselen (3.6 +/- 0.2 versus 1.1 +/- 0.2), diphenylene iodonium (3.6 +/- 0.2 versus 1.0 +/- 0.1), and N-acetyl cysteine (3.6 +/- 0.2 versus 1.2 +/- 0.1), but Ang II-stimulated phosphorylation of Thr-70 was ROS-insensitive. Although phosphatidylinositol 3-kinase pathway inhibition by LY294004 blocked both Ser-65 and Thr-70 phosphorylation (3.8 +/- 0.1 versus 0.8 +/- 0.1 and 3.2 +/- 0.2 versus 1.0 +/- 0.01, respectively), protein phosphatase 2A inhibition by okadaic acid selectively increased (3.3 +/- 0.1 versus 5.2 +/- 0.1) and p38 mitogen-activated protein kinase inhibition by SB203580 selectively decreased (3.8 +/- 0.1 versus 1.4 +/- 0.3) Ser-65 phosphorylation. Dominant negative Akt adenovirus also inhibited only Ser-65 phosphorylation (3.7 +/- 0.1 versus 1.0 +/- 0.03). These results demonstrate a unique differential ROS sensitivity of two separate residues on PHAS-I, which seems to be explained by the selective involvement of distinct signaling pathways in the regulation of these phosphorylation events.

Phosphorylation of initiation factor 4E is resistant to SB203580 in cells expressing a drug-resistant mutant of stress-activated protein kinase 2a/p38

Previous work has shown that increased phosphorylation of eukaryotic initiation factor (eIF) 4E at Ser209 in the C-terminal loop of the protein is observed in response to cellular stress. SB203580, a cell permeable inhibitor of stress-activated protein kinase 2a (SAPK2a/p38), suppresses this response in a number of cell types. To validate the in vivo specificity of this inhibitor for the investigation of signalling pathways, which modulate the phosphorylation of eIF4E, we have used 293 cells which inducibly express either a wild-type form (WT-SAPK2a) or a drug-resistant mutant of SAPK2a (DR-SAPK2a). These data show that while the arsenite-induced increase in the phosphorylation of eIF4E and hsp25 was sensitive to SB203580 in cells expressing WT-SAPK2a, these responses to SB203580 were abrogated in cells expressing DR-SAPK2a. In addition, the phosphorylation of the eIF4E kinase, MAP kinase integrating kinase-1 (Mnk1), which is activated in response to growth factors or stress, was insensitive to SB203580 in DR-SAPK2a-expressing cells. However, a cell-permeable, specific inhibitor of Mnk1, CGP57380 and the phosphatidylinositol-3-kinase (PI3-K) inhibitor, LY294002, prevented eIF4E phosphorylation in 293 cells irrespective of SAPK2a expression. Therefore, this study validates the use of SB203580 for investigating signalling pathways modulating the phosphorylation of eIF4E in cultured cells.

Aldose reductase inhibitor fidarestat prevents retinal oxidative stress and vascular endothelial growth factor overexpression in streptozotocin-diabetic rats

The study addressed the role for aldose reductase (AR) in 1) retinal oxidative stress and vascular endothelial growth factor (VEGF) overexpression in early diabetes, and 2) high glucose-induced oxidative stress in retinal endothelial cells. In vivo experiments were performed on control rats and diabetic rats treated with or without low or high dose of the AR inhibitor (ARI) fidarestat (2 or 16 mg. kg(-1). day(-1)). In vitro studies were performed on bovine retinal endothelial cells (BREC) cultured in either 5 or 30 mmol/l glucose with or without 1 micro mol/l fidarestat. Intracellular reactive oxygen species were assessed using the 5-(and-6)-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate (H(2)DCFDA) probe and flow cytometry. Both low and high doses of fidarestat (i.e., the doses that partially and completely inhibited sorbitol pathway hyperactivity) arrested diabetes-induced retinal lipid peroxidation. This was achieved due to upregulation of the key antioxidative defense enzyme activities rather than changes in reduced glutathione, oxidized glutathione, ascorbate and dehydroascorbate concentrations, and the glutathione and ascorbate redox states. Diabetes-associated 2.1-fold VEGF protein overexpression (enzyme-linked immunosorbent assay; ELISA) was dose-dependently prevented by fidarestat, whereas total VEGF mRNA and VEGF-164 mRNA (RT-PCR) abundance were not affected by either diabetes or the ARI. In BREC, fidarestat corrected hyperglycemia-induced increase in H(2)DCFDA fluorescence but not oxidative stress caused by three different pro-oxidants in normoglycemic conditions. In conclusion, increased AR activity contributes to retinal oxidative stress and VEGF protein overexpression in early diabetes. The findings justify the rationale for evaluation of fidarestat on diabetic retinopathy.

Low-energy laser irradiation enhances de novo protein synthesis via its effects on translation-regulatory proteins in skeletal muscle myoblasts

Low-energy laser irradiation (LELI) drives quiescent skeletal muscle satellite cells into the cell cycle and enhances their proliferation, thereby promoting skeletal muscle regeneration. Ongoing protein synthesis is a prerequisite for these processes. Here, we studied the signaling pathways involved in the LELI regulation of protein synthesis. High levels of labeled [35S]methionine incorporation were detected in LELI cells as early as 20 min after irradiation, suggesting translation of pre-existing mRNAs. Induced levels of protein synthesis were detected up until 8 h after LELI implying a role for LELI in de novo protein synthesis. Elevated levels of cyclin D1, associated with augmented phosphorylation of the eukaryotic initiation factor 4E (eIF4E) and its inhibitory binding protein PHAS-I, suggested the involvement of LELI in the initiation steps of protein translation. In the presence of the MEK inhibitor, PD98059, eIF4E phosphorylation was abolished and levels of cyclin D1 were dramatically reduced. The LELI-induced PHAS-I phosphorylation was abolished after preincubation with the PI3K inhibitor, Wortmannin. Concomitantly, LELI enhanced Akt phosphorylation, which was attenuated in the presence of Wortmannin. Taken together, these results suggest that LELI induces protein translation via the PI3K/Akt and Ras/Raf/ERK pathways.

Phosphorylation of eukaryotic initiation factor (eIF) 4E is not required for de novo protein synthesis following recovery from hypertonic stress in human kidney cells

Previous work has suggested that increased phosphorylation of eukaryotic initiation factor (eIF) 4E at Ser-209 in the C-terminal loop of the protein often correlates with increased translation rates. However, the functional consequences of phosphorylation have remained contentious with our understanding of the role of eIF4E phosphorylation in translational control far from complete. To investigate the role for eIF4E phosphorylation in de novo translation, we studied the recovery of human kidney cells from hypertonic stress. Results show that hypertonic shock caused a rapid inhibition of protein synthesis and the disaggregation of polysomes. These changes were associated with the dephosphorylation of eIF4G, eIF4E, 4E-binding protein 1 (4E-BP1), and ribosomal protein S6. In addition, decreased levels of the eIF4F complex and increased association of 4E-BP1 with eIF4E were observed over a similar time course. The return of cells to isotonic medium rapidly promoted the phosphorylation of these initiation factors, increased levels of eIF4F complexes, promoted polysome assembly, and increased rates of translation. However, by using a cell-permeable, specific inhibitor of eIF4E kinase, Mnk1 (CGP57380), we show that de novo initiation of translation and eIF4F complex assembly during this recovery phase did not require eIF4E phosphorylation.

The regulation of protein synthesis and translation factors by CD3 and CD28 in human primary T lymphocytes

BACKGROUND

Activation of human resting T lymphocytes results in an immediate increase in protein synthesis. The increase in protein synthesis after 16-24 h has been linked to the increased protein levels of translation initiation factors. However, the regulation of protein synthesis during the early onset of T cell activation has not been studied in great detail. We studied the regulation of protein synthesis after 1 h of activation using alphaCD3 antibody to stimulate the T cell receptor and alphaCD28 antibody to provide the co-stimulus.

RESULTS

Activation of the T cells with both antibodies led to a sustained increase in the rate of protein synthesis. The activities and/or phosphorylation states of several translation factors were studied during the first hour of stimulation with alphaCD3 and alphaCD28 to explore the mechanism underlying the activation of protein synthesis. The initial increase in protein synthesis was accompanied by activation of the guanine nucleotide exchange factor, eukaryotic initiation factor (eIF) 2B, and of p70 S6 kinase and by dephosphorylation of eukaryotic elongation factor (eEF) 2. Similar signal transduction pathways, as assessed using signal transduction inhibitors, are involved in the regulation of protein synthesis, eIF2B activity and p70 S6 kinase activity. A new finding was that the p38 MAPK alpha/beta pathway was involved in the regulation of overall protein synthesis in primary T cells. Unexpectedly, no changes were detected in the phosphorylation state of the cap-binding protein eIF4E and the eIF4E-binding protein 4E-BP1, or the formation of the cap-binding complex eIF4F.

CONCLUSIONS

Both eIF2B and p70 S6 kinase play important roles in the regulation of protein synthesis during the early onset of T cell activation.

Proteasome inhibitors and immunosuppressive drugs promote the cleavage of eIF4GI and eIF4GII by caspase-8-independent mechanisms in Jurkat T cell lines

Previously, we have shown that translation eukaryotic initiation factor (eIF) 4GI is cleaved during anti-Fas-mediated apoptosis. Here, we have investigated the effects of the proteasome inhibitors, MG132 and lactacystin, and the immunosuppressants, 2-amino-2[2-(4-octylphenyl)ethyl]-1,3,propane diol (FTY720) and cyclosporin A, on the integrity of eIF4GI and eIF4GII in T cells. Using wild-type Jurkat T cells, we show that the proteasome inhibitors MG132 and lactacystin promote the cleavage of eIF4G, activate caspase-8 and caspase-3-like activities and decrease cell viability. Furthermore, MG132 also promotes the cleavage of eIF4G and the activation of caspase-3-like activity in a caspase-8-deficient Jurkat cell line which is resistant to anti-Fas-mediated apoptosis. Using specific anti-peptide antisera, we show that both eIF4GI and eIF4GII are cleaved in either cell line in response to MG132 and lactacystin. In response to such treatments, we demonstrate that the fragments of eIF4GI generated include those previously observed with anti-Fas antiserum together with a novel product which lacks the ability to interact with eIF4E. In contrast, cells treated with the immunosuppressants FTY720 and cyclosporin A appear to contain only the novel cleavage fragment of eIF4GI and to lack those characteristic of cells treated with anti-Fas antiserum. These data suggest that caspase-8 activation is not required for apoptosis and eIF4G cleavage mediated by proteasome inhibitors and immunosuppressants in human T cells.

The mitogen-activated protein kinase signal-integrating kinase Mnk2 is a eukaryotic initiation factor 4E kinase with high levels of basal activity in mammalian cells

The cap-binding translation initiation factor eukaryotic initiation factor 4E (eIF4E) is phosphorylated in vivo at Ser209 in response to a variety of stimuli. In this paper, we show that the mitogen-activated protein kinase (MAPK) signal-integrating kinase Mnk2 phosphorylates eIF4E at this residue. Mnk2 binds to the scaffolding protein eIF4G, and overexpression of Mnk2 results in increased phosphorylation of endogenous eIF4E, showing that it can act as an eIF4E kinase in vivo. We have identified eight phosphorylation sites in Mnk2, of which at least three potential MAPK sites are likely to be essential for Mnk2 activity. In contrast to that of Mnk1, the activity of overexpressed Mnk2 is high under control conditions and could only be reduced substantially by a combination of PD98059 and SB203580, while the activity of endogenous Mnk2 in Swiss 3T3 cells was hardly affected upon treatment with these inhibitors. These compounds did not abolish phosphorylation of eIF4E, implying that Mnk2 may mediate phosphorylation of eIF4E in Swiss 3T3 cells. In vitro phosphorylation studies show that Mnk2 is a significantly better substrate than Mnk1 for extracellular signal-regulated kinase 2 (ERK2), p38MAPKalpha, and p38MAPKbeta. Therefore, the high levels of activity of Mnk2 under several conditions may be explained by efficient activation of Mnk2 by low levels of activity of the upstream kinases. Interestingly, we found that the association of both Mnk1 and Mnk2 with eIF4G increased upon inhibition of the MAPK pathways while activation of ERK resulted in decreased binding to eIF4G. This might reflect a mechanism to ensure rapid, but transient, phosphorylation of eIF4E upon stimulation of the MAPK pathways.

Signal transduction pathways that contribute to increased protein synthesis during T-cell activation

Protein synthesis rates were maximally stimulated in human lymphocytes by ionomycin and the phorbol ester PMA (I+P), which promotes proliferation, whereas PMA alone, which does not promote proliferation, stimulated protein synthesis to a lesser degree. Three translation-associated activities, eIF4E phosphorylation, eIF2B activity and 4E-BP1 phosphorylation also increased with stimulation by I+P and PMA, but only 4E-BP1 phosphorylation was differentially stimulated by these conditions. Correspondingly, signaling pathways activated in T cells were probed for their connection to these activities. Immunosuppressants FK506 and rapamycin partially blocked the protein synthesis rate increases by I+P stimulation. FK506 had less of an inhibitory effect with PMA stimulation suggesting that its mechanism mostly affected ionomycin-activated signals. I+P and PMA equally stimulated phosphorylation of ERK1/2, but I+P more strongly stimulated Akt, and p70(S6K) phosphorylation. An inhibitor that blocks ERK1/2 phosphorylation only slightly reduced protein synthesis rates stimulated by I+P or PMA, but greatly reduced eIF4E phosphorylation and eIF2B activity. In contrast, inhibitors of the PI-3 kinase and mTOR pathways strongly blocked early protein synthesis rate stimulated by I+P and PMA and also blocked 4E-BP1 phosphorylation and release of eIF4E suggesting that these pathways regulate protein synthesis activities, which are important for proliferation in T cells.

Regulation of eukaryotic initiation factor 4E phosphorylation in the nervous system of Aplysia californica

We have used an antibody that specifically recognizes eukaryotic initiation factor 4E (eIF4E) when it is phosphorylated at Ser(207) to characterize eIF4E phosphorylation in the nervous system of APLYSIA: The level of phosphorylated eIF4E, but not the level of total eIF4E, was significantly correlated with the basal rate of translation measured from different animals. Serotonin (5-HT), a transmitter that regulates the rate of translation in APLYSIA: neurons, had mixed effects on eIF4E phosphorylation. 5-HT decreased eIF4E phosphorylation in sensory cell clusters through activation of protein kinase C. 5-HT increased eIF4E phosphorylation in the whole pleural ganglia. In the APLYSIA: nervous system, eIF4E phosphorylation correlated with phosphorylation of the p38 MAP kinase, but not the p42 MAP kinase (ERK). Furthermore, an inhibitor of the p38 MAP kinase significantly decreased basal eIF4E phosphorylation, but an inhibitor of the MAP or ERK kinase (MEK) did not. Despite the correlation of eIF4E phosphorylation with the basal rate of translation, inhibition of eIF4E phosphorylation by an inhibitor of the p38 MAP kinase did not significantly decrease the rate of translation.

Differential requirements for caspase-8 activity in the mechanism of phosphorylation of eIF2alpha, cleavage of eIF4GI and signaling events associated with the inhibition of protein synthesis in apoptotic Jurkat T cells

Previously we have reported that induction of apoptosis in Jurkat cells results in an inhibition of overall protein synthesis with the selective and rapid cleavage of eukaryotic initiation factor (eIF) 4GI. For the cleavage of eIF4GI, caspase-3 activity is both necessary and sufficient in vivo, in a process which does not require signaling through the p38 MAP kinase pathway. We now show that activation of the Fas/CD95 receptor promotes an early, transient increase in the level of eIF2alpha phosphorylation, which is temporally correlated with the onset of the inhibition of translation. This is associated with a modest increase in the autophosphorylation of the protein kinase activated by double-stranded RNA. Using a Jurkat cell line that is deficient in caspase-8 and resistant to anti-Fas-induced apoptosis, we show that whilst the cleavage of eIF4GI is caspase-8-dependent, the enhancement of eIF2alpha phosphorylation does not require caspase-8 activity and occurs prior to the cleavage of eIF4GI. In addition, activation of the Fas/CD95 receptor results in the caspase-8-dependent dephosphorylation and degradation of p70(S6K), the enhanced binding of 4E-BP1 to eIF4E, and, at later times, the cleavage of eIF2alpha. These data suggest that apoptosis impinges upon the activity of several polypeptides which are central to the regulation of protein synthesis and that multiple signaling pathways are involved in vivo.

eIF4 initiation factors: effectors of mRNA recruitment to ribosomes and regulators of translation

Eukaryotic translation initiation factor 4F (eIF4F) is a protein complex that mediates recruitment of ribosomes to mRNA. This event is the rate-limiting step for translation under most circumstances and a primary target for translational control. Functions of the constituent proteins of eIF4F include recognition of the mRNA 5' cap structure (eIF4E), delivery of an RNA helicase to the 5' region (eIF4A), bridging of the mRNA and the ribosome (eIF4G), and circularization of the mRNA via interaction with poly(A)-binding protein (eIF4G). eIF4 activity is regulated by transcription, phosphorylation, inhibitory proteins, and proteolytic cleavage. Extracellular stimuli evoke changes in phosphorylation that influence eIF4F activity, especially through the phosphoinositide 3-kinase (PI3K) and Ras signaling pathways. Viral infection and cellular stresses also affect eIF4F function. The recent determination of the structure of eIF4E at atomic resolution has provided insight about how translation is initiated and regulated. Evidence suggests that eIF4F is also implicated in malignancy and apoptosis.

Translational control of viral gene expression in eukaryotes

As obligate intracellular parasites, viruses rely exclusively on the translational machinery of the host cell for the synthesis of viral proteins. This relationship has imposed numerous challenges on both the infecting virus and the host cell. Importantly, viruses must compete with the endogenous transcripts of the host cell for the translation of viral mRNA. Eukaryotic viruses have thus evolved diverse mechanisms to ensure translational efficiency of viral mRNA above and beyond that of cellular mRNA. Mechanisms that facilitate the efficient and selective translation of viral mRNA may be inherent in the structure of the viral nucleic acid itself and can involve the recruitment and/or modification of specific host factors. These processes serve to redirect the translation apparatus to favor viral transcripts, and they often come at the expense of the host cell. Accordingly, eukaryotic cells have developed antiviral countermeasures to target the translational machinery and disrupt protein synthesis during the course of virus infection. Not to be outdone, many viruses have answered these countermeasures with their own mechanisms to disrupt cellular antiviral pathways, thereby ensuring the uncompromised translation of virion proteins. Here we review the varied and complex translational programs employed by eukaryotic viruses. We discuss how these translational strategies have been incorporated into the virus life cycle and examine how such programming contributes to the pathogenesis of the host cell.

A role for the p38 MAP kinase pathway in the nuclear shuttling of NFATp

Calcium signals lead to the translocation of nuclear factor of activated T cells (NFAT) from the cytoplasm to the nucleus. This process is regulated by the calcium-activated phosphatase calcineurin, which can be cotransported with NFAT to the nucleus to maintain it transcriptionally active for the duration of calcium signaling. When the calcium signal ceases, NFAT is exported to the cytoplasm, and different NFAT kinases have been reported to oppose calcineurin activities and regulate the nuclear export of NFAT. Here we show that p38 MAPK phosphorylates in vitro and interacts in vivo with NFATp. Furthermore, the activation of this pathway in HeLa cells by cotransfection with activated MKK6 and p38 counteracts the calcium-induced nuclear accumulation of NFATp but not that of NFATc. By contrast, activation of JNK or ERK pathways failed to modify the nuclear shuttling of NFATp. Consistently, activation of p38, but not the JNK MAPK pathway, results in the inhibition of NFATp-driven transcription. In addition, the inhibition of the nuclear accumulation of NFATp by p38 appears to be mediated through the activation of NFATp nuclear export and takes place in a Leptomycin B-sensitive fashion, suggesting the involvement of the exportin CRM1 in this process. Thus, the p38 signal transduction pathway appears to play an important role in the regulation of the nuclear shuttling of NFATp and in cellular homeostasis.

Inhibition of p38 MAP kinase as a therapeutic strategy

Since the discovery of p38 MAP kinase in 1994, our understanding of its biology has progressed dramatically. The key advances include (1) identification of p38 MAP kinase homologs and protein kinases that act upstream and downstream from p38 MAP kinase, (2) identification of interesting and potentially important substrates, (3) elucidation of the role of p38 MAP kinase in cellular processes and (4) the establishment of the mechanism by which the pyridinylimidazole p38 MAP kinase inhibitors inhibit enzyme activity. It is now known that there are four members of the p38 MAP kinase family. They differ in their tissue distribution, regulation of kinase activation and subsequent phosphorylation of downstream substrates. They also differ in terms of their sensitivities toward the p38 MAP kinase inhibitors. The best-studied isoform is p38 alpha, whose activation has been observed in many hematopoietic and non-hematopoietic cell types upon treatment with appropriate stimuli. The pyridinylimidazole compounds, exemplified by SB 203580, were originally prepared as inflammatory cytokine synthesis inhibitors that subsequently were found to be selective inhibitors of p38 MAP kinase. SB 203580 inhibits the catalytic activity of p38 MAP kinase by competitive binding in the ATP pocket. X-ray crystallographic studies of the target enzyme complexed with inhibitor reinforce the observations made from site-directed mutagenesis studies, thereby providing a molecular basis for understanding the kinase selectivity of these inhibitors. The p38 MAP kinase inhibitors are efficacious in several disease models, including inflammation, arthritis and other joint diseases, septic shock, and myocardial injury. In all cases, p38 activation in key cell types correlated with disease initiation and progression. Treatment with p38 MAP kinase inhibitors attenuated both p38 activation and disease severity. Structurally diverse p38 MAP kinase inhibitors have been tested extensively in preclinical studies.

Phosphorylation of eIF-4E on Ser 209 in response to mitogenic and inflammatory stimuli is faithfully detected by specific antibodies

Phosphorylation of Ser 209 is thought to modulate the activity of the cap-binding factor eIF-4E which is a crucial component in the initiation complex for cap-dependent translation of mRNA. We report here the full reconstitution of the p38 Map kinase cascade leading to phosphorylation of eIF-4E in vitro and the generation of antibodies specific for phospho-serine 209 in eIF-4E. These antibodies were used to probe the phosphorylation of eIF-4E in mammalian cells stimulated with mitogens and pro-inflammatory cytokines. Treatment of human dermal fibroblasts with FCS led to a transient hyperphosphorylation, followed by hypophosphorylation and return to normal state phosphorylation at 16 h after the initial stimulation. By using a potent small molecular weight inhibitor of Mnk1, the upstream kinase for eIF-4E, we observed a rapid dephosphorylation of eIF-4E within 45 min after addition of the inhibitor, suggesting a high turnover of phosphate on eIF-4E mediated by Mnk1 and a yet unidentified phosphatase.

Gene expressions of lipopolysaccharide receptors, toll-like receptors 2 and 4, are differently regulated in mouse T lymphocytes

Toll-like receptors (TLRs) are a family of mammalian proteins homologous to Drosophila Toll. Human TLR2 was shown to mediate the responsiveness to lipopolysaccharide (LPS). On the other hand, gene mutations of mouse TLR4 (mTLR4) in LPS-hyporesponsive strains have suggested that mTLR4 is essential for LPS-signaling in mice, but the role of mTLR2 has not been explored. This report describes molecular cloning of the mTLR2 cDNA. Overexpression of mTLR2 and mouse CD14 conferred LPS-inducibility of c-Jun N-terminal kinase phosphorylation and nuclear factor-κB activation to COS7 cells, suggesting that mTLR2 is a signaling receptor for LPS. BothmTLR2 and mTLR4 genes were expressed in T cells. Treatment with anti-CD3ɛ, PMA plus ionomycin, or interleukin-2 (IL-2)/IL-15 increased mTLR2 but not mTLR4 messenger RNA (mRNA) in some T cell lines. Specific inhibitors of mitogen-activated extracellular signal-regulated kinase and fusion protein 38 (p38) kinase inhibited mTLR2 mRNA up-regulation by PMA plus ionomycin. This suggests that extracellular signal-regulated kinase and p38 kinase pathways were involved. Additionally, LPS treatment of EL-4 cell line decreasedIL-4 gene expression. Our results indicate that both mTLR2 and mTLR4 are involved in LPS signaling, but their expressions are regulated differently in T cells, and that LPS may directly affect T-cell functions by binding to TLRs.

Phosphorylation of the cap-binding protein eukaryotic translation initiation factor 4E by protein kinase Mnk1 in vivo

Eukaryotic translation initiation factor 4E (eIF4E) binds to the mRNA 5' cap and brings the mRNA into a complex with other protein synthesis initiation factors and ribosomes. The activity of mammalian eIF4E is important for the translation of capped mRNAs and is thought to be regulated by two mechanisms. First, eIF4E is sequestered by binding proteins, such as 4EBP1, in quiescent cells. Mitogens induce the release of eIF4E by stimulating the phosphorylation of 4EBP1. Second, mitogens and stresses induce the phosphorylation of eIF4E at Ser 209, increasing the affinity of eIF4E for capped mRNA and for an associated scaffolding protein, eIF4G. We previously showed that a mitogen- and stress-activated kinase, Mnk1, phosphorylates eIF4E in vitro at the physiological site. Here we show that Mnk1 regulates eIF4E phosphorylation in vivo. Mnk1 binds directly to eIF4G and copurifies with eIF4G and eIF4E. We identified activating phosphorylation sites in Mnk1 and developed dominant-negative and activated mutants. Expression of dominant-negative Mnk1 reduces mitogen-induced eIF4E phosphorylation, while expression of activated Mnk1 increases basal eIF4E phosphorylation. Activated mutant Mnk1 also induces extensive phosphorylation of eIF4E in cells overexpressing 4EBP1. This suggests that phosphorylation of eIF4E is catalyzed by Mnk1 or a very similar kinase in cells and is independent of other mitogenic signals that release eIF4E from 4EBP1.

The association of initiation factor 4F with poly(A)-binding protein is enhanced in serum-stimulated Xenopus kidney cells

Serum stimulation of cultured Xenopus kidney cells results in enhanced phosphorylation of the translational initiation factor (eIF) 4E and promotes a 2.8-fold increase in the binding of the adapter protein eIF4G to eIF4E, to form the functional initiation factor complex eIF4F. Here we demonstrate the serum-stimulated co-isolation of the poly(A)-binding protein (PABP) with the eIF4F complex. This apparent interaction of PABP with eIF4F suggests that a mechanism shown to be important in the control of translation in the yeast Saccharomyces cerevisiae also operates in vertebrate cells. We also present evidence that the signaling pathways modulating eIF4E phosphorylation and function in Xenopus kidney cells differ from those in several mammalian cell types studied previously. Experiments with the immunosuppressant rapamycin suggest that the mTOR signaling pathway is involved in serum-promoted eIF4E phosphorylation and association with eIF4G. Moreover, we could find little evidence for regulation of eIF4E function via interaction with the specific binding proteins 4E-BP1 or 4E-BP2 in these cells. Although rapamycin abrogated serum-enhanced rates of protein synthesis and the interaction of eIF4G with eIF4E, it did not prevent the increase in association of eIF4G with PABP. This suggests that serum stimulates the interaction between eIF4G and PABP by a distinct mechanism that is independent of both the mTOR pathway and the enhanced association of eIF4G with eIF4E.

Cleavage of translation initiation factor 4G (eIF4G) during anti-Fas IgM-induced apoptosis does not require signalling through the p38 mitogen-activated protein (MAP) kinase

Initiation factor (eIF) 4G plays a key role in the regulation of translation, acting as a bridge between eIF4E and eIF3, to allow an mRNA molecule to associate with the 40S ribosomal subunit. In this study, we show that activation of the Fas/CD95 receptor complex in Jurkat cells induces the degradation of eIF4G, the inhibition of total protein synthesis and cell death. These responses were prevented by the caspase inhibitors, zVAD.FMK and zDEVD.FMK. We also show that, in contrast to Saccharomyces cerevisiae, although rapamycin caused a modest inhibition of protein synthesis it did not induce apoptosis or the cleavage of eIF4G. Studies with the specific inhibitor, SB203580, have shown that signalling through the p38 MAP kinase pathway is not required for either the Fas/CD95-induced cleavage of eIF4G or cell death. These data suggest that the cleavage of eIF4G and the inhibition of translation play an integral role in Fas/CD95-induced cell death in Jurkat cells.