Increased phosphorylation of eukaryotic initiation factor 4 alpha during early activation of T lymphocytes correlates with increased initiation factor 4F complex formation

PABP1 Drives the Selective Translation of Influenza A Virus mRNA

Influenza A virus (IAV) is a human-infecting pathogen with a history of causing seasonal epidemics and on several occasions worldwide pandemics. Infection by IAV causes a dramatic decrease in host mRNA translation, whereas viral mRNAs are efficiently translated. The IAV mRNAs have a highly conserved 5'-untranslated region (5'UTR) that is rich in adenosine residues. We show that the human polyadenylate binding protein 1 (PABP1) binds to the 5'UTR of the viral mRNAs. The interaction of PABP1 with the viral 5'UTR makes the translation of viral mRNAs more resistant to canonical cap-dependent translation inhibition than model mRNAs. Additionally, PABP1 bound to the viral 5'UTR can recruit eIF4G in an eIF4E-independent manner. These results indicate that PABP1 bound to the viral 5'UTR may promote eIF4E-independent translation initiation.

Electromyostimulation with blood flow restriction enhances activation of mTOR and MAPK signaling pathways in rat gastrocnemius muscles

Neuromuscular electrical stimulation (NMES) combined with blood flow restriction (BFR) induces muscle hypertrophy. However, cellular mechanisms underlying the muscle hypertrophy induced by NMES combined with BFR remain unclear. We tested the hypothesis that NMES combined with BFR would enhance the mechanistic target of rapamycin (mTOR) and mitogen-activated protein kinase (MAPK) signaling pathways. Age-matched male Wistar rats (6 months old, n = 7 per group) were assigned randomly to control, BFR alone (BFR), NMES alone (NMES), and NMES combined with BFR (NMES/BFR) groups. NMES induced 25 isometric contractions lasting 8 s with 4-s resting periods between contractions in the gastrocnemius muscle. Four sets in total were performed, with 1-min intervals between sets. A latex cuff was placed on the proximal portion of the hind limb and BFR at 200 mm Hg was conducted in 4 sets (each set 5 min) with 1-min rest intervals between sets. Venous blood was collected from the lateral tail vein to determine pH, H⁺ concentration, and lactate concentration before and immediately after the treatments. Expression levels of proteins related to muscle hypertrophy were determined by Western blot analysis. The application of NMES/BFR promoted muscle fatigue more than NMES alone. NMES/BFR induced greater changes in accumulation of metabolites and increase in gastrocnemius muscle weight. The phosphorylation of mTOR and MAPK signaling-related proteins was also enhanced following NMES/BFR, compared with other conditions. Thus, NMES enhanced the activation of mTOR and MAPK signaling pathways when combined with BFR.

Calcium mobilization is both required and sufficient for initiating chromatin decondensation during activation of peripheral T-cells

Antigen engagement of the T-cell receptor (TCR) induces a rapid and dramatic decondensation of chromatin that is necessary for T-cell activation. This decondensation makes T-cells competent to respond to interleukin-2 providing a mechanism to ensure clonotypic proliferation during an immune response. Using murine T-cells, we investigated the mechanism by which TCR signaling can initiate chromatin decondensation, focusing on the role of calcium mobilization. During T-cell activation, calcium is first released from intracellular stores, followed by influx of extracellular calcium via store operated calcium entry. We show that mobilization of intracellular calcium is required for TCR-induced chromatin decondensation. However, the decondensation is not dependent on the activity of the downstream transcription factor NFAT. Furthermore, we show that the influx of extracellular calcium is dispensable for initiating chromatin decondensation. Finally, we show that mobilization of calcium from intracellular stores is sufficient to induce decondensation, independent of TCR engagement. Collectively, our data suggest that chromatin decondensation in peripheral T-cells is controlled by modulating intracellular calcium levels.

Chromatin condensation via the condensin II complex is required for peripheral T-cell quiescence

Naive T cells encountering their cognate antigen become activated and acquire the ability to proliferate in response to cytokines. Stat5 is an essential component in this response. We demonstrate that Stat5 cannot access DNA in naive T cells and acquires this ability only after T-cell receptor (TCR) engagement. The transition is not associated with changes in DNA methylation or global histone modification but rather chromatin decondensation. Condensation occurs during thymocyte development and proper condensation is dependent on kleisin-β of the condensin II complex. Our findings suggest that this unique chromatin condensation, which can affect interpretations of chromatin accessibility assays, is required for proper T-cell development and maintenance of the quiescent state. This mechanism ensures that cytokine driven proliferation can only occur in the context of TCR stimulation.

TCR-dependent translational control of GATA-3 enhances Th2 differentiation

The differentiation of CD4(+) T cells into the Th2 subset is controlled by the transcription factor GATA-3. GATA-3 is both necessary and sufficient for Th2 differentiation and works through the induction of chromatin remodeling at the Th2 effector cytokine loci. We show in this study that IL-4 stimulation induces GATA-3 mRNA upregulation, but the level of GATA-3 protein induced is insufficient for Th2 differentiation. The levels of GATA-3 protein and Th2 differentiation are enhanced by concomitant TCR signaling through the PI3K/mammalian target of rapamycin pathway. The PI3K-mediated increase in GATA-3 protein occurs without increasing the GATA-3 mRNA level. Rather, TCR signaling through PI3K specifically enhances the translation rate of GATA-3 without affecting the protein stability. Importantly, this role of TCR signaling is independent of the effects of TCR signaling in T cell survival and expansion. Thus, TCR signaling through PI3K may play a critical role in Th2 differentiation by the specific enhancement of GATA-3 translation.

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.

Activation of host translational control pathways by a viral developmental switch

In response to numerous signals, latent herpesvirus genomes abruptly switch their developmental program, aborting stable host–cell colonization in favor of productive viral replication that ultimately destroys the cell. To achieve a rapid gene expression transition, newly minted capped, polyadenylated viral mRNAs must engage and reprogram the cellular translational apparatus. While transcriptional responses of viral genomes undergoing lytic reactivation have been amply documented, roles for cellular translational control pathways in enabling the latent-lytic switch have not been described. Using PEL-derived B-cells naturally infected with KSHV as a model, we define efficient reactivation conditions and demonstrate that reactivation substantially changes the protein synthesis profile. New polypeptide synthesis correlates with 4E-BP1 translational repressor inactivation, nuclear PABP accumulation, eIF4F assembly, and phosphorylation of the cap-binding protein eIF4E by Mnk1. Significantly, inhibiting Mnk1 reduces accumulation of the critical viral transactivator RTA through a post-transcriptional mechanism, limiting downstream lytic protein production, and impairs reactivation efficiency. Thus, herpesvirus reactivation from latency activates the host cap-dependent translation machinery, illustrating the importance of translational regulation in implementing new developmental instructions that drastically alter cell fate.

Kaposi's sarcoma-associated herpesvirus (KSHV) is an important human pathogen and, like all herpesviruses, establishes a state of permanent residency in the infected host called latency. Major sites of KSHV latency are cells of the immune system and cells lining blood vessels. In individuals with weakened immunity, inappropriate growth of these cells driven by the resident virus can give rise to primary effusion lymphoma and Kaposi's sarcoma, respectively. These life-threatening cancers are most common in patients with HIV/AIDS and have become a major source of mortality in parts of sub-Saharan Africa. Under appropriate stimuli, herpesviruses change their relationship with the host cell and begin to manufacture proteins required to assemble new infectious virus particles that can be released and spread. To achieve this, the virus hijacks key processes within the cell and conscripts them into producing viral proteins. In this study, we describe for the first time how KSHV carefully manipulates the host protein synthesis machinery during the switch from latency to this specialized infectious virus production mode. Our results show that although overall protein synthesis is diminished, key components of the host's protein manufacturing machinery are actually stimulated, presumably to accelerate viral protein production.

Stimulation of muscle protein synthesis by somatotropin in pigs is independent of the somatotropin-induced increase in circulating insulin

Chronic treatment of growing pigs with porcine somatotropin (pST) promotes protein synthesis and doubles postprandial levels of insulin, a hormone that stimulates translation initiation. This study aimed to determine whether the pST-induced increase in skeletal muscle protein synthesis was mediated through an insulin-induced stimulation of translation initiation. After 7-10 days of pST (150 microg x kg(-1) x day(-1)) or control saline treatment, pancreatic glucose-amino acid clamps were performed in overnight-fasted pigs to reproduce 1) fasted (5 microU/ml), 2) fed control (25 microU/ml), and 3) fed pST-treated (50 microU/ml) insulin levels while glucose and amino acids were maintained at baseline fasting levels. Fractional protein synthesis rates and indexes of translation initiation were examined in skeletal muscle. Effectiveness of pST treatment was confirmed by reduced urea nitrogen and elevated insulin-like growth factor I levels in plasma. Skeletal muscle protein synthesis was independently increased by both insulin and pST. Insulin increased the phosphorylation of protein kinase B and the downstream effectors of the mammalian target of rapamycin, ribosomal protein S6 kinase, and eukaryotic initiation factor (eIF)4E-binding protein-1 (4E-BP1). Furthermore, insulin reduced inactive 4E-BP1.eIF4E complex association and increased active eIF4E.eIF4G complex formation, indicating enhanced eIF4F complex assembly. However, pST treatment did not alter translation initiation factor activation. We conclude that the pST-induced stimulation of skeletal muscle protein synthesis in growing pigs is independent of the insulin-associated activation of translation initiation.

Eukaryotic translation initiation factor 4E (eIF4E) expression in the brain tissue is induced by infusion of nerve growth factor into the mouse cisterna magnum: an in vivo study

In many cell types translation can be regulated by an expression of the translation initiation factor. Eukaryotic translation initiation factor eIF4E, which binds to the 5' cap structure of mRNA, plays an important role in translation regulation and it has been suggested that it is implicated in increased protein synthesis promoted by growth factors. In this study the effects of nerve growth factor (NGF) infusion into the cerebrospinal fluid (CSF) on eIF4E expression and phosphorylation in mouse brain tissue have been investigated. We investigated NGF as it is one of the most important growth factors and it is an important factor in cerebral cortical development, stimulating neuronal precursor proliferation. eIF4E level is also increased in response to infusion of NGF into the CSF. The present study shows that eIF4E is phosphorylated in the brain tissues treated with NGF. It is concluded that NGF regulates protein synthesis in the nervous tissue by enhancing expression and phosphorylation of eIF4E.

Sex-dependent differences in the regulation of myocardial protein synthesis following long-term ethanol consumption

Chronic heavy alcohol consumption alters cardiac structure and function. Controversies remain as to whether hearts from females respond to the chronic ethanol intake in a manner analogous to males. In particular, sex differences in the myocardial response to chronic alcohol consumption remain unresolved at the molecular level. The purpose of the present set of experiments was to determine whether alterations in cardiac structure and protein metabolism show sexual dimorphism following chronic alcohol consumption for 26 wk. In control animals, hearts from female rats showed lowered heart weights and had thinner ventricular walls compared with males. The smaller heart size was associated with a lower protein content that occurred in part from a reduced rate of protein synthesis. Chronic alcohol consumption in males, but not in females, caused a thinning of the ventricular wall and intraventricular septum, as assessed by echocardiography, correlating with the loss of heart mass. The alterations in cardiac size occurred, in part, through a lowering of the protein content secondary to a diminished rate of protein synthesis. The decreased rate of protein synthesis appeared related to a reduced assembly of active eukaryotic initiation factor (eIF)4G.eIF4E complex secondary to both a diminished phosphorylation of eIF4G and increased formation of inactive 4Ebinding protein (4EBP1).eIF4E complex. The latter effects occurred as a result of decreased phosphorylation of 4EBP1. None of these ethanol-induced alterations in hearts from males were observed in hearts from females. These data suggest that chronic alcohol-induced impairments in myocardial protein synthesis results, in part, from marked decreases in eIF4E.eIF4G complex formation in males. The failure of female rats consuming ethanol to show structural changes appears related to the inability of ethanol to affect the regulation protein synthesis to the same extent as their male counterparts.

Regulation of neonatal liver protein synthesis by insulin and amino acids in pigs

The high efficiency of protein deposition during the neonatal period is driven by high rates of protein synthesis, which are maximally stimulated after feeding. Infusion of amino acids, but not insulin, reproduces the feeding-induced stimulation of liver protein synthesis. To determine whether amino acid-stimulated liver protein synthesis is independent of insulin in neonates, and to examine the role of amino acids and insulin in the regulation of translation initiation in neonatal liver, we performed pancreatic glucose-amino acid clamps in overnight-fasted 7-day-old pigs. Pigs (n = 9-12/group) were infused with insulin at 0, 10, 22, and 110 ng.kg(-0.66).min(-1) to achieve 0, 2, 6, and 30 microU/ml insulin, respectively. At each insulin dose, amino acids were maintained at fasting or fed levels or, in conjunction with the highest insulin dose, allowed to fall to below fasting levels. Insulin had no effect on the fractional rate of protein synthesis in liver. Amino acids increased fractional protein synthesis rates in liver at each dose of insulin, including the 0 microU/ml dose. There was a dose-response effect of amino acids on liver protein synthesis. Amino acids and insulin increased protein S6 kinase and 4E-binding protein 1 (4E-BP1) phosphorylation; however, only amino acids decreased formation of the inactive 4E-BPI.eukaryotic initiation factor-4E (eIF4E) complex. The results suggest that amino acids regulate liver protein synthesis in the neonate by modulating the availability of eIF4E for 48S ribosomal complex formation and that this response does not require insulin.

Phosphorylation of eIF4E by Mnk-1 enhances HSV-1 translation and replication in quiescent cells

Although the activity of the translation initiation factor eIF4F is regulated in part by translational repressors (4E-BPs) that prevent incorporation of eIF4E, the cap-binding protein, into the initiation complex, the contribution of eIF4E phosphorylation to translational control remains controversial. Here, we demonstrate that the herpes simplex virus-1 (HSV-1) ICP0 gene product, a multifunctional transactivator of viral gene expression with ubiquitin E3 ligase activity that is important for vegetative replication and reactivation of latent infections, is required to stimulate phosphorylation of eIF4E as well as 4E-BP1, and promote assembly of eIF4F complexes in infected cells. Furthermore, 4E-BP1 is degraded by the proteasome in an ICP0-dependent manner, establishing that the proteasome can control 4E-BP1 steady-state levels. Preventing eIF4E phosphorylation by inhibiting the eIF4E kinase mnk-1 dramatically reduced viral replication and the translation of viral polypeptides in quiescent cells, providing the first evidence that phosphorylation of eIF4E by mnk-1 is critical for viral protein synthesis and replication. Thus, in marked contrast to many viruses that inactivate eIF4F, HSV-1 stimulates eIF4F complex assembly in quiescent, differentiated cells; moreover, this is important for viral replication, and may be crucial for HSV-1 to initiate its productive growth cycle in resting cells, such as latently infected neurons.

Regulation of translation initiation by insulin and amino acids in skeletal muscle of neonatal pigs

Previous studies have shown that intravenous infusion of insulin and/or amino acids reproduces the feeding-induced stimulation of muscle protein synthesis in neonates and that insulin and amino acids act independently to produce this effect. The goal of the present study was to delineate the regulatory roles of insulin and amino acids on muscle protein synthesis in neonates by examining translational control mechanisms, specifically the eukaryotic translation initiation factors (eIFs), which enable coupling of initiator methionyl-tRNAi and mRNA to the 40S ribosomal subunit. Insulin secretion was blocked by somatostatin in fasted 7-day-old pigs (n = 8-12/group), insulin was infused to achieve plasma levels of approximately 0, 2, 6, and 30 microU/ml, and amino acids were clamped at fasting or fed levels or, at the high insulin dose, below fasting. Both insulin and amino acids increased the phosphorylation of ribosomal protein S6 kinase (S6K1) and the eIF4E-binding protein (4E-BP1), decreased the binding of 4E-BP1 to eIF4E, increased eIF4E binding to eIF4G, and increased fractional protein synthesis rates but did not affect eIF2B activity. In the absence of insulin, amino acids had no effect on these translation initiation factors but increased the protein synthesis rates. Raising insulin from below fasting to fasting levels generally did not alter translation initiation factor activity but raised protein synthesis rates. The phosphorylation of S6K1 and 4E-BP1 and the amount of 4E-BP1 bound to eIF4E and eIF4E bound to eIF4G were correlated with insulin level, amino acid level, and protein synthesis rate. Thus insulin and amino acids regulate muscle protein synthesis in skeletal muscle of neonates by modulating the availability of eIF4E for 48S ribosomal complex assembly, although other processes also must be involved.

Translational control of protein synthesis in muscle and liver of growth hormone-treated pigs

GH treatment increases protein deposition and the efficiency of dietary protein used for growth. To identify the mechanisms that regulate tissue protein synthesis in response to exogenous GH treatment, fully fed, growing swine were treated with GH for 7 d. Fasted and fed pigs were infused with [1-(13)C]leucine to determine protein synthesis rates, and translation initiation factor activity levels were measured in skeletal muscle and liver. Feeding increased protein synthesis and translational efficiency in both muscle and liver of control and GH-treated pigs, and this was associated with increased 4E-BP1 and S6 kinase 1 phosphorylation, decreased association of eukaryotic initiation factor (eIF) 4E with 4E-BP1, and increased association of eIF4E with eIF4G. GH increased muscle protein synthesis and translational efficiency in fed pigs. GH increased liver protein synthesis of fasted and fed pigs in association with increased ribosome number. In muscle, but not liver, GH increased eIF2B activity and 4E-BP1 phosphorylation in both the fasted and fed state and increased the association of eIF4E with eIF4G in the fed state. We conclude that GH increases muscle protein synthesis in the fed state, in part, via mechanisms that enhance the binding of mRNA and methionyl-tRNA to the 40S ribosomal subunit, whereas GH increases liver protein synthesis in the fasted and fed states by increasing ribosome number. The results further indicate that the GH-induced protein synthetic response is dependent upon nutritional state and is tissue specific.

Oxidative stress increases eukaryotic initiation factor 4E phosphorylation in vascular cells

Dysregulated cell growth can be caused by increased activity of protein synthesis eukaryotic initiation factor (eIF) 4E. Dysregulated cell growth is also characteristic of atherosclerosis. It is postulated that exposure of vascular cells, such as endothelial cells, smooth muscle cells and monocytes/macrophages, to oxidants, such as oxidized low-density lipoprotein (oxLDL), leads to the elaboration of growth factors and cytokines, which in turn results in smooth muscle cell hyperproliferation. To investigate whether activation of eIF4E might play a role in this hyperproliferative response, vascular cells were treated with oxLDL, oxidized lipid components of oxLDL and several model oxidants, including H(2)O(2) and dimethyl naphthoquinone. Exposure to each of these compounds led to a dose- and time-dependent increase in eIF4E phosphorylation in all three types of vascular cells, correlated with a modest increase in overall translation rate. No changes in eIF4EBP, eIF2 or eIF4B modification state were observed. Increased eIF4E phosphorylation was paralleled by increased presence of eIF4E in high-molecular-mass protein complexes characteristic of its most active form. Anti-oxidants at concentrations typically employed to block oxidant-induced cell signalling likewise promoted eIF4E phosphorylation. The results of this study indicate that increased eIF4E activity may contribute to the pathophysiological events in early atherogenesis by increasing the expression of translationally inefficient mRNAs encoding growth-promoting proteins.

Translational initiation regulators are hypophosphorylated in rat liver during ethionine-mediated ATP depletion

Administration of ethionine to female rats is known to inhibit hepatic protein synthesis by reducing the level of hepatic ATP. Administration of methionine and/or adenine rapidly restores the ATP levels and protein synthesis. The ethionine administration causes a progressive disaggregation of hepatic polysomes, suggesting that the initiation step of protein synthesis is inhibited. Recent studies indicate that changes in initiation are associated with alterations in the phosphorylation states of translational initiation regulators such as eukaryotic initiation factor (eIF) 4E, eIF4E-binding protein 1 (4E-BP1), and the 70-kDa ribosomal protein S6 kinase (S6K1). We found that these initiation regulators are hypophosphorylated in rat liver during ethionine-mediated ATP depletion (60% of the control value). Furthermore, the restoration of the ATP levels by the administration of methionine and adenine brought about a complete recovery of the phosphorylation states of all these regulators. The present data suggest that hypophosphorylation of various initiation regulators represents the primary event in the ethionine-induced breakdown of polysomes and inhibition of protein synthesis in the liver. Possible involvement of mammalian target of rapamycin (mTOR), as a sensor of intracellular ATP level, was also discussed.

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.

Regulation of translation during in vitro maturation of bovine oocytes: the role of MAP kinase, eIF4E (cap binding protein) phosphorylation, and eIF4E-BP1

Meiotic maturation of mammalian oocytes (transition from prophase I to metaphase II) is accompanied by complex changes in the protein phosphorylation pattern. At least two major protein kinases are involved in these events; namely, cdc2 kinase and mitogen-activated protein (MAP) kinase, because the inhibition of these kinases arrest mammalian oocytes in the germinal vesicle (GV) stage. We show that during meiotic maturation of bovine oocytes, the translation initiation factor, eIF4E (the cap binding protein), gradually becomes phosphorylated. This substantial phosphorylation begins at the time of germinal vesicle breakdown (GVBD) and continues to the metaphase II stage. The onset of eIF4E phosphorylation occurs in parallel with a significant increase in overall protein synthesis. However, although eIF4E is nearly fully phosphorylated in metaphase II oocytes, protein synthesis reaches only basal levels at this stage, similar to that of prophase I oocytes, in which the factor remains unphosphorylated. We present evidence that a specific repressor of eIF4E, the binding protein 4E-BP1, is present and could be involved in preventing eIF4E function in metaphase II stage oocytes. Recently, two protein kinases, called Mnk1 and Mnk2, have been identified in somatic cells as eIF4E kinases, both of which are substrates of MAP kinase in vivo. In bovine oocytes, a specific inhibitor of cdk kinases, butyrolactone I, arrests oocytes in GV stage and prevents activation of both cdc2 and MAP kinase. Under these conditions, the phosphorylation of eIF4E is also blocked, and its function in initiation of translation is impaired. In contrast, PD 098059, a specific inhibitor of the MAP kinase activation pathway, which inhibits the MAP kinase kinase, called MEK function, leads only to a postponed GVBD, and a delay in MAP kinase and eIF4E phosphorylation. These results indicate that in bovine oocytes, 1) MAP kinase activation is only partially dependent on MEK kinase, 2) MAP kinase is involved in eIF4E phosphorylation, and 3) the abundance of fully phosphorylated eIF4E does not necessarily directly stimulate protein synthesis. A possible MEK kinase-independent pathway of MAP kinase phosphorylation and the role of 4E-BP1 in repressing translation in metaphase II oocytes are discussed.

Modifications of eukaryotic initiation factor 4F (eIF4F) in adult cardiocytes by adenoviral gene transfer: differential effects on eIF4F activity and total protein synthesis rates

In adult feline cardiocytes, increases in eukaryotic initiation factor 4F (eIF4F) activity are correlated with accelerated rates of total protein synthesis produced in response to increased load. Adenoviral gene transfer was employed to increase either eIF4F complex formation or the phosphorylation of eIF4E on Ser-209. To simulate load,cardiocytes were electrically stimulated to contract (2 Hz,5 ms pulses). Non-stimulated cardiocytes were used as controls.Adenovirus-mediated overexpression of wild-type eIF4E increased the total eIF4E pool by 120-140% above endogenous levels after 24 h and produced a corresponding increase in eIF4F content.However, it did not accelerate total protein synthesis rates inquiescent cardiocytes; neither did it potentiate the increase produced by contraction. To modify the affinity of eIF4F, cardiocytes were infected with a mutant (eIF4E/W56F) with a decreased binding affinity for the mRNA cap. Overexpression of eIF4E/W56F increased the quantity of eIF4F but the rate of total protein synthesis was decreased inquiescent and contracting cardiocytes. Overexpression of a mutant that blocked eIF4E phosphorylation (eIF4E/S209A) increased the quantity ofeIF4F without any significant effect on total protein synthesis rates in quiescent or contracting cardiocytes. Overexpression of the eIF4Ekinase Mnk-1 increased eIF4E phosphorylation without a corresponding increase in eIF4F complex formation or in the rate of total protein synthesis. We conclude the following: (1) eIF4F assembly is increased by raising eIF4E levels via adenoviral gene transfer; (2) the capbinding affinity of eIF4F is a rate-limiting determinant for total protein synthesis rates; and (3) increases in the quantity of eIF4Falone or in eIF4E phosphorylation are not sufficient to accelerate total protein synthesis rates.

Rapid dephosphorylation of eIF4E by dietary protein in the skeletal muscle and liver of food-deprived rats

The effect of dietary protein on eIF4E phosphorylation was examined in rats starved for 18 h and then fed on either a 20% casein diet (20C) or a protein-free diet (0C). Refeeding with the 20C diet, but not the 0C diet, resulted in partial dephosphorylation of eIF4E in both the skeletal muscle and liver. The results suggest that the dephosphorylation of eIF4E in response to food intake was regulated by the increase in plasma amino acid concentration that occurred after feeding with the 20C diet.

Developmental changes in the feeding-induced stimulation of translation initiation in muscle of neonatal pigs

The rapid gain in skeletal muscle mass in the neonate is associated with a marked elevation in skeletal muscle protein synthesis in response to feeding. The feeding-induced response decreases with development. To determine whether the response to feeding is regulated at the level of translation initiation, the expression, phosphorylation, and function of a number of eukaryotic initiation factors (eIF) were examined. Pigs at 7 and 26 days of age were either fasted overnight or fed porcine milk after an overnight fast. In muscle of 7-day-old pigs, the hyperphosphorylated form of the eIF4E repressor protein, 4E-binding protein 1 (4E-BP1), was undetectable in the fasting state but rose to 60% of total 4E-BP1 after feeding; eIF4E phosphorylation was unaffected by feeding status. The amount of eIF4E in the inactive 4E-BP1. eIF4E complex was reduced by 80%, and the amount of eIF4E in the active eIF4E. eIF4G complex was increased 14-fold in muscle of 7-day-old pigs after feeding. The amount of 70-kDa ribosomal protein S6 (p70(S6)) kinase in the hyperphosphorylated form rose 2.5-fold in muscle of 7-day-old pigs after feeding. Each of these feeding-induced responses was blunted in muscle of 26-day-old pigs. eIF2B activity in muscle was unaffected by feeding status but decreased with development. Feeding produced similar changes in eIF characteristics in liver and muscle; however, the developmental changes in liver were not as apparent as in skeletal muscle. Thus the results demonstrate that the developmental change in the acute stimulation of skeletal muscle protein synthesis by feeding is regulated by the availability of eIF4E for 48S ribosomal complex formation. The results further suggest that the overall developmental decline in skeletal muscle protein synthesis involves regulation by eIF2B.

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.

Effect of sepsis on eIE4E availability in skeletal muscle

Chronic septic abscess formation causes an inhibition of protein synthesis in gastrocnemius that is not observed in rats with a sterile abscess. The inhibition is associated with an impaired translation initiation. The present study was designed to investigate the effects of sepsis on phosphorylation and availability of eukaryotic initiation factor (eIF)4E in gastrocnemius 5 days after induction of a sterile or septic abscess. Neither sepsis nor sterile inflammation altered the extent of eIF4E phosphorylation. Moreover, no changes in the amount of the binding protein 4E-BP1 associated with eIF4E or in the phosphorylation of 4E-BP1 were observed during sepsis or sterile inflammation. In contrast, sepsis and sterile inflammation caused a reduction in the relative amount of eIF4G bound to eIF4E compared with controls. The diminished amount of eIF4G bound to eIF4E was not the result of a reduced abundance of eIF4E. Sepsis, but not sterile inflammation, caused an increase in the cellular abundance of eIF4E. The results provide evidence that alterations in the eIF4E system are probably not rate controlling for the synthesis of total, mixed proteins in gastrocnemius during sepsis. Instead, on the basis of our previous studies, changes in eIF2B appear to be responsible for limiting protein synthesis in skeletal muscle during sepsis.

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.

Orally administered leucine stimulates protein synthesis in skeletal muscle of postabsorptive rats in association with increased eIF4F formation

We investigated the protein synthetic response of skeletal muscle to an orally administered dose of leucine given alone or in combination with carbohydrate. Male rats were freely fed (F) or food deprived for 18 h; food-deprived rats were then administered saline (S), carbohydrate (CHO), leucine (L) or a combination of carbohydrate plus leucine (CL). CHO and CL meals were isocaloric and provided 15% of daily energy requirements. L and CL meals each delivered 270 mg leucine. Muscle protein synthesis in S was 65% of F (P<0.01) 1 h after meal administration. Concomitant with lower rates of protein synthesis, phosphorylation of the translational repressor, eukaryotic initiation factor (eIF)4E-binding protein 1 (4E-BP1), was less in S, leading to greater association of 4E-BP1.eIF4E, and reduced formation of the active eIF4G.eIF4E complex compared with F (P<0.01). Oral administration of leucine (L or CL), but not CHO, restored protein synthesis equal to that in F and resulted in 4E-BP1 phosphorylation that was threefold greater than that of S (P<0.01). Consequently, formation of 4E-BP1.eIF4E was inhibited and eIF4G.eIF4E was not different from F. The amount of eIF4E in the phosphorylated form was greater in S and CHO (P<0.01) than in all other groups. In contrast, no differences in the phosphorylation state of eIF2alpha or the activity of eIF2B were noted among treatment groups. Serum insulin was elevated 2.6- and 3.7-fold in CHO and CL, respectively, but was not different in L, compared with S (P<0.05). These results suggest that leucine stimulates protein synthesis in skeletal muscle by enhancing eIF4F formation independently of increases in serum insulin.

Role of eIF4E in stimulation of protein synthesis by IGF-I in perfused rat skeletal muscle

Insulin-like growth factor I (IGF-I) promotes anabolism by stimulating protein synthesis in skeletal muscle. In the present study, we have examined mechanisms by which IGF-I stimulates protein synthesis in skeletal muscle with a perfused rat hindlimb preparation. IGF-I (10 nM) stimulated protein synthesis over 2.7-fold. Total RNA content was unaffected, but translational efficiency was increased by IGF-I. We next examined the effect of IGF-I on eukaryotic initiation factor (eIF) 4E as a mechanism regulating translation initiation. IGF-I did not alter either the amount of eIF4E associated with the eIF4E binding protein 4E-BP1 or the phosphorylation state of 4E-BP1. Likewise, the phosphorylation state of eIF4E was unaltered by IGF-I. In contrast, the amount of eIF4E bound to eIF4G was increased threefold by IGF-I. We conclude that IGF-I regulates protein synthesis in skeletal muscle by enhancing formation of the active eIF4E x eIF4G complex.

Amino acid-induced stimulation of translation initiation in rat skeletal muscle

Amino acids stimulate protein synthesis in skeletal muscle by accelerating translation initiation. In the two studies described herein, we examined mechanisms by which amino acids regulate translation initiation in perfused skeletal muscle hindlimb preparation of rats. In the first study, the effects of supraphysiological amino acid concentrations on eukaryotic initiation factors (eIF) 2B and 4E were compared with physiological concentrations of amino acids. Amino acid supplementation stimulated protein synthesis twofold. No changes were observed in eIF2B activity, in the amount of eIF4E associated with the eIF4E-binding protein (4E-BP1), or in the phosphorylation of 4E-BP1. The abundance of eIF4E bound to eIF4G and the extent of phosphorylation of eIF4E were increased by 800 and 20%, respectively. In the second study, we examined the effect of removing leucine on translation initiation when all other amino acids were maintained at supraphysiological concentrations. Removal of leucine from the perfusate decreased the rate of protein synthesis by 40%. The inhibition of protein synthesis was associated with a 40% decrease in eIF2B activity and an 80% fall in the abundance of eIF4E. eIF4G complex. The fall in eIF4G binding to eIF4E was associated with increased 4E-BP1 bound to eIF4E and a reduced phosphorylation of 4E-BP1. In contrast, the extent of phosphorylation of eIF4E was unaffected. We conclude that formation of the active eIF4E. eIF4G complex controls protein synthesis in skeletal muscle when the amino acid concentration is above the physiological range, whereas removal of leucine reduces protein synthesis through changes in both eIF2B and eIF4E.

c-mos and cdc2 cooperate in the translational activation of fibroblast growth factor receptor-1 during Xenopus oocyte maturation

During oocyte maturation in Xenopus, previously quiescent maternal mRNAs are translationally activated at specific times. We hypothesized that the translational recruitment of individual messages is triggered by particular cellular events and investigated the potential for known effectors of the meiotic cell cycle to activate the translation of the FGF receptor-1 (XFGFR) maternal mRNA. We found that both c-mos and cdc2 activate the translation of XFGFR. However, although oocytes matured by injection of recombinant cdc2/cyclin B translate normal levels of XFGFR protein, c-mos depletion reduces the level of XFGFR protein induced by cdc2/cyclin B injection. In oocytes blocked for cdc2 activity, injection of mos RNA induced low levels of XFGFR protein, independent of MAPK activity. Through the use of injected reporter RNAs, we show that the XFGFR 3' untranslated region inhibitory element is completely derepressed by cdc2 alone. In addition, we identified a new inhibitory element through which both mos and cdc2 activate translation. We found that cdc2 derepresses translation in the absence of polyadenylation, whereas mos requires poly(A) extension to activate XFGFR translation. Our results demonstrate that mos and cdc2, in addition to functioning as key regulators of the meiotic cell cycle, cooperate in the translational activation of a specific maternal mRNA during oocyte maturation.

Control of translation initiation in animals

Regulation of translation initiation is a central control point in animal cells. We review our current understanding of the mechanisms of regulation, drawing particularly on examples in which the biological consequences of the regulation are clear. Specific mRNAs can be controlled via sequences in their 5' and 3' untranslated regions (UTRs) and by alterations in the translation machinery. The 5'UTR sequence can determine which initiation pathway is used to bring the ribosome to the initiation codon, how efficiently initiation occurs, and which initiation site is selected. 5'UTR-mediated control can also be accomplished via sequence-specific mRNA-binding proteins. Sequences in the 3' untranslated region and the poly(A) tail can have dramatic effects on initiation frequency, with particularly profound effects in oogenesis and early development. The mechanism by which 3'UTRs and poly(A) regulate initiation may involve contacts between proteins bound to these regions and the basal translation apparatus. mRNA localization signals in the 3'UTR can also dramatically influence translational activation and repression. Modulations of the initiation machinery, including phosphorylation of initiation factors and their regulated association with other proteins, can regulate both specific mRNAs and overall translation rates and thereby affect cell growth and phenotype.

Translational regulation of ornithine decarboxylase and other enzymes of the polyamine pathway

It has long been known that polyamines play an essential role in the proliferation of mammalian cells, and the polyamine biosynthetic pathway may provide an important target for the development of agents that inhibit carcinogenesis and tumor growth. The rate-limiting enzymes of the polyamine pathway, ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase (AdoMetDC), are highly regulated in the cell, and much of this regulation occurs at the level of translation. Although the 5' leader sequences of ODC and AdoMetDC are both highly structured and contain small internal open reading frames (ORFs), the regulation of their translation appears to be quite different. The translational regulation of ODC is more dependent on secondary structure, and therefore responds to the intracellular availability of active eIF-4E, the cap-binding subunit of the eIF-4F complex, which mediates translation initiations. Cell-specific translation of AdoMetDC appears to be regulated exclusively through the internal ORF, which causes ribosome stalling that is independent of eIF-4E levels and decreases the efficiency with which the downstream ORF encoding AdoMetDC protein is translated. The translation of both ODC and AdoMetDC is negatively regulated by intracellular changes in the polyamines spermidine and spermine. Thus, when polyamine levels are low, the synthesis of both ODC and AdoMetDC is increased, and an increase in polyamine content causes a corresponding decrease in protein synthesis. However, an increase in active eIF-4E may allow for the synthesis of ODC even in the presence of polyamine levels that repress ODC translation in cells with lower levels of the initiation factor. In contrast, the amino acid sequence that is encoded by the upstream ORF is critical for polyamine regulation of AdoMetDC synthesis and polyamines may affect synthesis by interaction with the putative peptide, MAGDIS.

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.

Insulin-stimulated kinase from rat fat cells that phosphorylates initiation factor 4E-binding protein 1 on the rapamycin-insensitive site (serine-111)

The effects of insulin and rapamycin on the phosphorylation of the translation regulator, initiation factor 4E-binding protein 1 (4E-BP1) have been studied in rat fat cells by following changes in the incorporation of 32P from [32P]Pi under steady-state conditions. Both unbound 4E-BP1 and 4E-BP1 bound to eukaryotic initiation factor 4E (eIF4E) were isolated from the cells and then digested with trypsin and other proteases; the radiolabelled phosphopeptides were then separated by two-dimensional thin- layer analysis and HPLC. The results provide confirmation of the conclusion of Fadden, Haystead and Lawrence [J. Biol. Chem. (1997) 272, 10240-10247] that insulin increases the phosphorylation of four sites that fit a Ser/Thr-Pro motif (Thr-36, Thr-45, Ser-64 and Thr-69) and that taken together these phosphorylations result in the dissociation of 4E-BP1 from eIF4E. The effects of insulin on the phosphorylation of these sites, and hence dissociation from eIF4E, are blocked by rapamycin. However, the present study also provides evidence that insulin increases the phosphorylation of 4E-BP1 bound to eIF4E on a further site (Ser-111) and that this is by a rapamycin-insensitive mechanism. Extraction of rat epididymal fat cells followed by chromatography on Mono-S and Superose 12 columns resulted in the separation of both an insulin-stimulated eIF4E kinase and an apparently novel kinase that is highly specific for Ser-111 of 4E-BP1. The 4E-BP1 kinase was activated more than 10-fold by incubation of the cells with insulin and was markedly more active towards 4E-BP1 bound to eIF4E than towards unbound 4E-BP1. The effects of insulin were blocked by wortmannin, but not by rapamycin. A 14-mer peptide based on the sequence surrounding Ser-111 of 4E-BP1 was also a substrate for the kinase, but peptide substrates for other known protein kinases were not. The kinase is quite distinct from casein kinase 2, which also phosphorylates Ser-111 of 4E-BP1. The possible importance of these kinases in the phosphorylation of 4E-BP1 in fat cells is discussed. It is suggested that the phosphorylation of Ser-111 might be a priming event that facilitates the subsequent phosphorylation of Thr-36, Thr-45, Ser-64 and Thr69 by a rapamycin-sensitive process that initiates the dissociation of 4E-BP1 from eIF4E and hence the formation of the eIF4F complex.

Effect of dietary protein on translation initiation in rat skeletal muscle and liver

The effect of dietary protein on the initiation of mRNA translation was examined in rats starved for 18 h and then fed isocaloric diets containing either 20% protein (20P) or no added protein (0P). Feeding the 20P diet, but not the 0P diet, stimulated protein synthesis in skeletal muscle and liver by 38 and 41%, respectively. The stimulation was associated with reduced binding of eukaryotic initiation factor (eIF) 4E to the translational repressor 4E-BP1, increased formation of the active eIF4E-eIF4G complex, and increased phosphorylation of 4E-BP1. In contrast, feeding a 0P diet had no effect on any of these parameters. Feeding a 20P diet resulted in partial dephosphorylation of eIF4E in both tissues. In liver, refeeding a 0P diet also resulted in partial eIF4E dephosphorylation, suggesting that the phosphorylation state of eIF4E is not important in the stimulation of protein synthesis under these conditions. Finally, plasma insulin concentrations were the same in rats fed either diet (14.8 +/- 4.9 vs. 15.5 +/- 4.5 microU/ml for 20P and 0P groups, respectively), suggesting that feeding-induced changes in plasma insulin are not sufficient to stimulate protein synthesis. Instead, a combination of dietary protein and insulin may be required to stimulate translation initiation.

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.

Availability of eIF4E regulates skeletal muscle protein synthesis during recovery from exercise

We examined the association of the mRNA cap binding protein eIF4E with the translational inhibitor 4E-BP1 in the acute modulation of skeletal muscle protein synthesis during recovery from exercise. Fasting male rats were run on a treadmill for 2 h at 26 m/min and were realimented immediately after exercise with either saline, a carbohydrate-only meal, or a nutritionally complete meal (54.5% carbohydrate, 14% protein, and 31.5% fat). Exercised animals and nonexercised controls were studied 1 h postexercise. Muscle protein synthesis decreased 26% after exercise and was associated with a fourfold increase in the amount of eIF4E present in the inactive eIF4E.4E-BP1 complex and a concomitant 71% decrease in the association of eIF4E with eIF4G. Refeeding the complete meal, but not the carbohydrate meal, increased muscle protein synthesis equal to controls, despite similar plasma concentrations of insulin. Additionally, eIF4E.4E-BP1 association was inversely related and eIF4E.eIF4G association was positively correlated to muscle protein synthesis. This study demonstrates that recovery of muscle protein synthesis after exercise is related to the availability of eIF4E for 48S ribosomal complex formation, and postexercise meal composition influences recovery via modulation of translation initiation.

Molecular mechanisms for the control of translation by insulin

Insulin acutely stimulates protein synthesis in mammalian cells, and this involves activation of the process of mRNA translation. mRNA translation is a complex multi-step process mediated by proteins termed translation factors. Several translation factors are regulated in response to insulin, often as a consequence of changes in their states of phosphorylation. The initiation factor eIF4E binds to the cap structure at the 5'-end of the mRNA and mediates assembly of an initiation-factor complex termed eIF4F. Assembly of this complex can be regulated by eIF4E-binding proteins (4E-BPs), which inhibit eIF4F complex assembly. Insulin induces phosphorylation of the 4E-BPs, resulting in alleviation of the inhibition. This regulatory mechanism is likely to be especially important for the control of the translation of specific mRNAs whose 5'-untranslated regions (5'-UTRs) are rich in secondary structure. Translation of another class of mRNAs, those with 5'-UTRs containing polypyrimidine tracts is also activated by insulin and this, like phosphorylation of the 4E-BPs, appears to involve the rapamycin-sensitive signalling pathway which leads to activation of the 70 kDa ribosomal protein S6 kinase (p70 S6 kinase) and the phosphorylation of the ribosomal protein S6. Overall stimulation of translation may involve activation of initiation factor eIF2B, which is required for all initiation events. This effect is dependent upon phosphatidylinositol 3-kinase and may involve the inactivation of glycogen synthase kinase-3 and consequent dephosphorylation of eIF2B, leading to its activation. Peptide-chain elongation can also be activated by insulin, and this is associated with the dephosphorylation and activation of elongation factor eEF2, probably as a consequence of the insulin-induced reduction in eEF2 kinase activity. Thus multiple signalling pathways acting on different steps in translation are involved in the activation of this process by insulin and lead both to general activation of translation and to the selective regulation of specific mRNAs.

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

Initiation factor (elF) 4E plays a key role in the regulation of translation. Its activity is modulated both by phosphorylation and by its association with an inhibitory protein, 4E-BP1, which precludes its interaction with eIF4G. Although increased eIF4E phosphorylation has been correlated with the activation of protein synthesis in T cells, the kinase(s) and/or phosphatase(s) involved have not been characterised. There is evidence for phosphorylation of eIF4E mediated by both protein kinase C-dependent and -independent signalling pathways. In these studies, I show that activation of protein kinase C with phorbol ester, stimulation via the T cell receptor complex with the monoclonal antibody OKT3 and cellular stresses increase the phosphorylation of eIF4E in Jurkat T cells. In contrast to published data, inhibition of either the ERK MAP kinase or p38 MAP kinase signalling pathways does not affect the PMA- or OKT3-stimulated increase in eIF4E phosphorylation. However, simultaneous inhibition of both of these pathways with selective inhibitors is required to completely abrogate the enhanced phosphorylation of eIF4E. These data show that in Jurkat cells, protein kinase C modulates the phosphorylation status of eIF4E indirectly via the ERK and/or p38 MAP kinase signalling pathways.

Involvement of stress-activated protein kinase and p38/RK mitogen-activated protein kinase signaling pathways in the enhanced phosphorylation of initiation factor 4E in NIH 3T3 cells

The initiation factor (eIF) 4E is regulated by modulating both the phosphorylation and the availability of the protein to participate in the initiation process. Here we show that either serum treatment or activation of the stress-activated protein kinase (JNK/SAPK) led to enhanced phosphorylation of eIF4E in quiescent NIH 3T3 cells. Although the immunosuppressant, rapamycin, was found to stabilize the association of eIF4E with its negative regulator, 4E-BP1, this drug did not prevent the early effects of serum stimulation on the overall rate of translation, polysome formation, the phosphorylation status of eIF4E, or the recruitment of eIF4E into the eIF4F complex. However, the rapid enhancement of eIF4E phosphorylation in response to serum was largely prevented by the inhibitor of mitogen-activated protein (MAP) kinase activation, PD98059. Activation of the JNK/SAPK signaling pathway with anisomycin resulted in enhanced phosphorylation of eIF4E, which was prevented by either rapamycin or the highly specific p38 MAP kinase inhibitor, SB203580. These data illustrate that multiple signaling pathways, including those of distinct members of the MAP kinase family, mediate the phosphorylation of eIF4E and that the association of eIF4E with 4E-BP1 does not necessarily prevent phosphorylation of eIF4E in vivo.

Ribosomal S6 kinase p90rsk and mRNA cap-binding protein eIF4E phosphorylations correlate with MAP kinase activation during meiotic reinitiation of mouse oocytes

During meiotic reinitiation of the mouse oocyte, entry into M-phase is regulated by changes of protein phosphorylation and by the stimulation of selective mRNA translation following the nuclear membrane dissolution. Our results reveal that M-phase kinases (MAP kinase and histone H1 kinase) are being activated together with S6 kinase and with the phosphorylation of eIF4E, the cap-binding subunit of the initiation factor eIF-4F. In order to test which signaling pathway(s) is(are) involved, okadaic acid and cycloheximide have been used as tools for differentially modulating MAP and histone H1 kinase activities. A role for MAP kinases in the phosphorylation of eIF4E and the activation of S6 kinase is suggested. The possible implication of p90rsk and/or of p70s6k in the overall increase in S6 kinase activity has been examined. p70s6k does not appear to be involved since phosphorylated forms are found in prophase and maturing oocytes. In contrast, p90rsk is phosphorylated and activated in maturing oocytes. p90rSk phosphorylation correlates with the activation of S6 kinase. These results suggest that the overall increase of S6 kinase activity is mostly due to p90rsk activation. The roles of eIF4E phosphorylation and S6 kinase activation in the physiological induction of M-phase and in the okadaic acid-induced premature mitotic events are discussed.

Insulin stimulates protein synthesis in skeletal muscle by enhancing the association of eIF-4E and eIF-4G

Insulin stimulated protein synthesis in gastrocnemius muscle of perfused rat hindlimb preparations by approximately twofold. The stimulation of protein synthesis was associated with a 12-fold increase in the amount of eukaryotic initiation factor eIF-4G bound to the mRNA cap-binding protein eIF-4E. In part, the increased binding of eIF-4G to eIF-4E was a result of release of eIF-4E bound to the translational regulator, PHAS-I, through a mechanism involving enhanced phosphorylation of PHAS-I. However, the insulin-induced association of eIF-4E and eIF-4G was not due to increased net phosphorylation of eIF-4E because insulin decreased the amount present in the phosphorylated form from 86 to 59% of total eIF-4E. Overall, the results suggest that insulin stimulates protein synthesis in gastrocnemius muscle through a mechanism involving increased binding of eIF-4G to eIF-4E, which is in part due to phosphorylation of PHAS-I, resulting in a release of eIF-4E from the inactive PHAS-I x eIF-4E complex.

Identification of the cap binding domain of human recombinant eukaryotic protein synthesis initiation factor 4E using a photoaffinity analogue

Binding of eIF-4E to the 5' m7G cap structure of eukaryotic mRNA signals the initiation of protein synthesis. In order to investigate the molecular basis for this recognition, photoaffinity labeling with [gamma-32P]8-N3GTP was used in binding site studies of human recombinant cap binding protein eIF-4E. Competitive inhibition of this cap analogue by m7GTP and capped mRNA indicated probe specificity for interaction at the protein binding site. Saturation of the binding site with [gamma-32P]8-N3GTP further demonstrated the selectivity of photoinsertion. Aluminum (III)-chelate chromatography and reverse-phase HPLC were used to isolate the binding site peptide resulting from digestion of photolabeled eIF-4E with modified trypsin. Amino acid sequencing identified the binding domain as the region containing the sequence Trp 113-Arg 122.Lys 119 was not identified in sequencing analysis nor was it cleaved by trypsin. These results indicate that Lys 119 is the residue directly modified by photoinsertion of [gamma-32P]8-N3GTP. A detailed understanding of eIF-4E.m7G mRNA cap interactions may lead the way to regulating this essential protein-RNA interaction for specific mRNA in vivo.

The regulated expression of cell cycle-related proteins as B-lymphocytes enter and progress through the G1 cell cycle stage following delivery of complete versus partial activation stimuli

Resting B-cells lack both cyclins D and E while constitutively expressing low levels of cdk4 and cdk2. B-cells receiving a complete growth stimulus express cyclin D2 by 10hr and cyclin E by 10-24hr poststimulation while increasing their protein levels of cdk4 and cdk2. B-cells receiving partial growth stimuli move into G1 without passing the G1 restriction point and transiently increase cyclin D2 mRNA levels without accumulating cyclin D2 protein. In the absence of cyclin D2 accumulation, cdk4 is not activated, and cyclin E is not expressed. These results suggest that signals responsible for moving B-cells through the G1 restriction point impact at the level of cyclin D2 protein accumulation. The possible implications of these results are discussed.

A reevaluation of the cap-binding protein, eIF4E, as a rate-limiting factor for initiation of translation in reticulocyte lysate

The cap-binding eukaryotic initiation factor, eIF4E, is a key target for the regulation of translation in mammalian cells and is widely thought to be present at very low molar concentrations. Here we present observations with the reticulocyte lysate that challenge this view. When reticulocyte ribosomes are harvested by centrifugation, most (approximately 75%) of the eIF4E remains in the postribosomal supernatant (PRS). In a reconstituted translation system we find that the ribosome-associated eIF4E alone can sustain much of the overall activity, suggesting that much of the factor in the PRS is functionally redundant. Consistent with this, our estimates of eIF4E in the reticulocyte lysate reveal much higher concentrations than previously reported. The association of a small proportion of eIF4E with the ribosome fraction appears to be functional and dependent on interaction with the factor eIF4G. This fraction of eIF4E is, as expected, more highly phosphorylated than that in the PRS; however, at least half the total phosphorylated eIF4E in reticulocyte lysate translation systems resides in the PRS fraction, suggesting that, while phosphorylation may enhance activity, it is not in itself sufficient to promote utilization of the factor. We also show that the eIF4E-binding factor, eIF4E-BP1 or PHAS-I, which regulates eIF4E activity in insulin-responsive cells, is present in the reticulocyte PRS at an approximately 1:1 molar ratio relative to eIF4E and demonstrate by co-immunoprecipitation studies that the binding of PHAS-I and eIF4G to eIF4E is mutually exclusive. These data are consistent with a potential regulatory role for PHAS-I in the reticulocyte lysate.

Initiation of protein synthesis in eukaryotic cells

It is becoming increasingly apparent that translational control plays an important role in the regulation of gene expression in eukaryotic cells. Most of the known physiological effects on translation are exerted at the level of polypeptide chain initiation. Research on initiation of translation over the past five years has yielded much new information, which can be divided into three main areas: (a) structure and function of initiation factors (including identification by sequencing studies of consensus domains and motifs) and investigation of protein-protein and protein-RNA interactions during initiation; (b) physiological regulation of initiation factor activities and (c) identification of features in the 5' and 3' untranslated regions of messenger RNA molecules that regulate the selection of these mRNAs for translation. This review aims to assess recent progress in these three areas and to explore their interrelationships.

Translational regulation during activation of porcine peripheral blood lymphocytes: association and phosphorylation of the alpha and gamma subunits of the initiation factor complex eIF-4F

Mature peripheral blood lymphocytes exist in a resting state both in vivo and when maintained in culture, exhibiting low translation rates consistent with their non-proliferative state. Previously we have shown that activation of these quiescent cells with either phorbol ester or concanavalin A leads to a rapid increase in the rate of protein synthesis and phosphate-labelling of initiation factor eIF-4 alpha [Morley, Rau, Kay and Pain (1993) Eur. J. Biochem. 218, 39-48]. We now show that neither the early enhanced translation rate nor the early increased phosphate-labelling of eIF-4 alpha requires the activity of the 70 kDa form of ribosomal protein S6 kinase. In addition, we demonstrate that eIF-4 gamma is phosphorylated in response to cell activation, an event which is correlated with phosphorylation of eIF-4 alpha and enhanced eIF-4F complex formation. In these studies, isoelectric focusing and immunoblot analysis of eIF-4 alpha indicate that phosphate-labelling of eIF-4 alpha following cell activation reflects a modest increase in steady-state phosphorylation, mediated by the enhanced activity of eIF-4 alpha kinase(s) and inhibition of eIF-4 alpha phosphatase activity. In the resting cell, eIF-4 alpha is associated with heat- and acid-stable insulin-responsive protein (PHAS-I; 4E-BP1); following acute stimulation with phorbol ester, there is a 40% decrease in the amount of PHAS-I associated with eIF-4 alpha. Incubation of anti-PHAS-I immunoprecipitates with extracts containing activated or immunprecipitated mitogen-activated protein kinase resulted in a small increase in phosphorylation of recovered PHAS-I and a modest release of eIF-4 alpha from the PHAS-I-eIF-4 alpha complex. These data suggest a possible role for PHAS-I in the regulation of eIF-4F complex formation and the rate of translation in primary cells.

Characterization of the in vivo phosphorylation sites of the mRNA.cap-binding complex proteins eukaryotic initiation factor-4E and p20 in Saccharomyces cerevisiae

Eukaryotic translation is believed to be regulated via the phosphorylation of specific eukaryotic initiation factors (eIFs), including one of the cap-binding complex proteins, eIF-4E. We show that in the yeast Saccharomyces cerevisiae, both eIF-4E and another cap-binding complex protein, p20, are phosphoproteins. The major sites of phosphorylation of yeast eIF-4E are found to be located in the N-terminal region of its sequence (Ser2 and Ser15) and are thus in a different part of the protein from the main phosphorylation sites (Ser53 and Ser209) proposed previously for mammalian eIF-4E. The most likely sites of p20 phosphorylation are at Ser91 and/or Ser154. All of the major sites in the two yeast proteins are phosphorylated by casein kinase II in vitro. Casein kinase II phosphorylation of cap-complex proteins should therefore be considered as potentially involved in the control of yeast protein synthesis. Mutagenesis experiments revealed that yeast eIF-4E activity is not dependent on the presence of Ser2 or Ser15. On the other hand, we observed variations in the amount of (phosphorylated) p20 associated with the cap-binding complex as a function of cell growth conditions. Our results suggest that interactions of yeast eIF-4E with other phosphorylatable proteins, such as p20, could play a pivotal role in translational control.

The phosphodiesterase inhibitor SQ 20006 selectively blocks mitogen activation of p70S6k and transition to S phase of the cell division cycle without affecting the steady state phosphorylation of eIF-4E

In quiescent cells high levels of protein synthesis are required in order to re-enter the cell cycle upon stimulation. Initiation of polypeptide synthesis is the step most often subject to regulation, controlled in part by phosphorylation of 40 S ribosomal protein S6 and a number of initiation factors. The kinase responsible for S6 phosphorylation is p70S6k. We now show that the p70S6k pathway can be selectively blocked by the aminopurine analogue, SQ 20006. This agent is known to raise cAMP levels, resulting in activation of protein kinase A. We present evidence that the increase in cAMP is not responsible for the inhibitory effect observed. We also show that SQ 20006 can prevent the activation of p70S6k in a rapid and reversible manner. The compound does not exert its inhibitory activity on p70S6k but can inhibit in vitro two protein kinase C isozymes (alpha and gamma). In a B lymphoblastoid cell line, treatment with SQ 20006 results in inhibition of protein synthesis at the initiation stage. In contrast, when tested directly upon the translational machinery in the reticulocyte lysate, inhibition is manifest at both the level of initiation and elongation. The role of protein kinase A in the modulation of p70S6k and the rate of translation is discussed.