Intracellular messengers and the control of protein synthesis

Structural dynamics of the complex of calmodulin with a minimal functional construct of eukaryotic elongation factor 2 kinase and the role of Thr348 autophosphorylation

The calmodulin (CaM) activated α-kinase, eukaryotic elongation factor 2 kinase (eEF-2K), plays a central role in regulating translational elongation by phosphorylating eukaryotic elongation factor 2 (eEF-2), thereby reducing its ability to associate with the ribosome and suppressing global protein synthesis. Using TR (for truncated), a minimal functional construct of eEF-2K, and utilizing hydrogen/deuterium exchange mass spectrometry (HXMS), solution-state nuclear magnetic resonance (NMR) and biochemical approaches, we investigate the conformational changes accompanying complex formation between Ca²⁺ -CaM and TR and the effects of autophosphorylation of the latter at Thr348, its primary regulatory site. Our results suggest that a CaM C-lobe surface, complementary to the one involved in recognizing the calmodulin-binding domain (CBD) of TR, provides a secondary TR-interaction platform. CaM helix F, which is part of this secondary surface, responds to both Thr348 phosphorylation and pH changes, indicating its integration into an allosteric network that encompasses both components of the Ca²⁺ -CaM•TR complex. Solution NMR data suggest that CaM_H107K , which carries a helix F mutation, is compromised in its ability to drive the conformational changes in TR necessary to enable efficient Thr348 phosphorylation. Biochemical studies confirm the diminished capacity of CaM_H107K to induce TR autophosphorylation compared to wild-type CaM.

Structural Dynamics of the Activation of Elongation Factor 2 Kinase by Ca2+-Calmodulin

Eukaryotic elongation factor 2 kinase (eEF-2K), the only known calmodulin (CaM)-activated α-kinase, phosphorylates eukaryotic elongation factor 2 (eEF-2) on a specific threonine (Thr-56) diminishing its affinity for the ribosome and reducing the rate of nascent chain elongation during translation. Despite its critical cellular role, the precise mechanisms underlying the CaM-mediated activation of eEF-2K remain poorly defined. Here, employing a minimal eEF-2K construct (TR) that exhibits activity comparable to the wild-type enzyme and is fully activated by CaM in vitro and in cells, and using a variety of complimentary biophysical techniques in combination with computational modeling, we provide a structural mechanism by which CaM activates eEF-2K. Native mass analysis reveals that CaM, with two bound Ca²⁺ ions, forms a stoichiometric 1:1 complex with TR. Chemical crosslinking mass spectrometry and small-angle X-ray scattering measurements localize CaM near the N-lobe of the TR kinase domain and the spatially proximal C-terminal helical repeat. Hydrogen/deuterium exchange mass spectrometry and methyl NMR indicate that the conformational changes induced on TR by the engagement of CaM are not localized but are transmitted to remote regions that include the catalytic site and the functionally important phosphate binding pocket. The structural insights obtained from the present analyses, together with our previously published kinetics data, suggest that TR, and by inference, wild-type eEF-2K, upon engaging CaM undergoes a conformational transition resulting in a state that is primed to efficiently auto-phosphorylate on the primary activating T348 en route to full activation.

Translation in the mammalian oocyte in space and time

A hallmark of oocyte development in mammals is the dependence on the translation and utilization of stored RNA and proteins rather than the de novo transcription of genes in order to sustain meiotic progression and early embryo development. In the absence of transcription, the completion of meiosis and early embryo development in mammals relies significantly on maternally synthesized RNAs. Post-transcriptional control of gene expression at the translational level has emerged as an important cellular function in normal development. Therefore, the regulation of gene expression in oocytes is controlled almost exclusively at the level of mRNA and protein stabilization and protein synthesis. This current review is focused on the recently emerged findings on RNA distribution related to the temporal and spatial translational control of the meiotic progression of the mammalian oocyte.

Calcium/calmodulin stimulates the autophosphorylation of elongation factor 2 kinase on Thr-348 and Ser-500 to regulate its activity and calcium dependence

Eukaryotic elongation factor 2 kinase (eEF-2K) is an atypical protein kinase regulated by Ca(2+) and calmodulin (CaM). Its only known substrate is eukaryotic elongation factor 2 (eEF-2), whose phosphorylation by eEF-2K impedes global protein synthesis. To date, the mechanism of eEF-2K autophosphorylation has not been fully elucidated. To investigate the mechanism of autophosphorylation, human eEF-2K was coexpressed with λ-phosphatase and purified from bacteria in a three-step protocol using a CaM affinity column. Purified eEF-2K was induced to autophosphorylate by incubation with Ca(2+)/CaM in the presence of MgATP. Analyzing tryptic or chymotryptic peptides by mass spectrometry monitored the autophosphorylation over 0-180 min. The following five major autophosphorylation sites were identified: Thr-348, Thr-353, Ser-445, Ser-474, and Ser-500. In the presence of Ca(2+)/CaM, robust phosphorylation of Thr-348 occurs within seconds of addition of MgATP. Mutagenesis studies suggest that phosphorylation of Thr-348 is required for substrate (eEF-2 or a peptide substrate) phosphorylation, but not self-phosphorylation. Phosphorylation of Ser-500 lags behind the phosphorylation of Thr-348 and is associated with the Ca(2+)-independent activity of eEF-2K. Mutation of Ser-500 to Asp, but not Ala, renders eEF-2K Ca(2+)-independent. Surprisingly, this Ca(2+)-independent activity requires the presence of CaM.

Purification and characterization of tagless recombinant human elongation factor 2 kinase (eEF-2K) expressed in Escherichia coli

The eukaryotic elongation factor 2 kinase (eEF-2K) modulates the rate of protein synthesis by impeding the elongation phase of translation by inactivating the eukaryotic elongation factor 2 (eEF-2) via phosphorylation. eEF-2K is known to be activated by calcium and calmodulin, whereas the mTOR and MAPK pathways are suggested to negatively regulate kinase activity. Despite its pivotal role in translation regulation and potential role in tumor survival, the structure, function, and regulation of eEF-2K have not been described in detail. This deficiency may result from the difficulty of obtaining the recombinant kinase in a form suitable for biochemical analysis. Here we report the purification and characterization of recombinant human eEF-2K expressed in the Escherichia coli strain Rosetta-gami 2(DE3). Successive chromatography steps utilizing Ni-NTA affinity, anion-exchange, and gel filtration columns accomplished purification. Cleavage of the thioredoxin-His(6)-tag from the N-terminus of the expressed kinase with TEV protease yielded 9 mg of recombinant (G-D-I)-eEF-2K per liter of culture. Light scattering shows that eEF-2K is a monomer of ∼85 kDa. In vitro kinetic analysis confirmed that recombinant human eEF-2K is able to phosphorylate wheat germ eEF-2 with kinetic parameters comparable to the mammalian enzyme.

ERK1/2 map kinase metabolic pathway is responsible for phosphorylation of translation initiation factor eIF4E during in vitro maturation of pig oocytes

Eukaryotic initiation factor 4E (eIF4E) plays an important role in mRNA translation by binding the 5'-cap structure of the mRNA and facilitating the recruitment to the mRNA of other translation factors and the 40S ribosomal subunit. eIF4E undergoes regulated phosphorylation on Ser-209 and this phosphorylation is believed to be important for its binding to mRNA and to other initiation factors. The findings showing that the translation initiation factor eIF4E becomes gradually phosphorylated during in vitro maturation (IVM) of pig oocytes with a maximum in metaphase II (M II) stage oocytes have been documented by us recently (Ellederova et al., 2006). The aim of this work was to study in details the metabolic pathways involved in this process. Using inhibitors of cyclin-dependent kinases, Butyrolactone I (BL I) and protein phosphatases, okadaic acid (OA) we show that ERK1/2 MAP kinase pathway is involved in this phosphorylation. We also demonstrate that activation and phosphorylation of ERK1/2 MAP kinase and eIF4E is associated with the activating phosphorylation of Mnk1 kinase, one of the two main kinases phosphorylating eIF4E in somatic cells.

Hydrogen peroxide mediates arsenite activation of p70(s6k) and extracellular signal-regulated kinase

To define the mechanism of arsenite-induced tumor promotion, we examined the role of reactive oxygen species (ROS) in the signaling pathways of cells exposed to arsenite. Arsenite treatment resulted in the persistent activation of p70(s6k) and extracellular signal-regulated kinase 1/2 (ERK1/2) which was accompanied by an increase in intracellular ROS production. The predominant produced appeared to be H(2)O(2), because the arsenite-induced increase in dichlorofluorescein (DCF) fluorescence was completely abolished by pretreatment with catalase but not with heat-inactivated catalase. Elimination of H(2)O(2) by catalase or N-acetyl-L-cysteine inhibited the arsenite-induced activation of p70(s6k) and ERK1/2, indicating the possible role of H(2)O(2) in the arsenite activation of the p70(s6k) and the ERK1/2 signaling pathways. A specific inhibitor of p70(s6k), rapamycin, and calcium chelators significantly blocked the activation of p70(s6k) induced by arsenite. While the phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin and LY294002 completely abrogated arsenite activation of p70(s6k), ERK1/2 activation by arsenite was not affected by these inhibitors, indicating that H(2)O(2) might act as an upstream molecule of PI3K as well as ERK1/2. Consistent with these results, none of the inhibitors impaired H(2)O(2) production by arsenite. DNA binding activity of AP-1, downstream of ERK1/2, was also inhibited by catalase, N-acetyl-L-cysteine, and the MEK inhibitor PD98059, which significantly blocked arsenite activation of ERK1/2. Taken together, these studies provide insight into mechanisms of arsenite-induced tumor promotion and suggest that H(2)O(2) plays a critical role in tumor promotion by arsenite through activation of the ERK1/2 and p70(s6k) signaling pathways.

Constitutive activation of p70S6k in cancer cells

The mitogen-stimulated serine/threonine kinase p70S6k plays an important role in the progression of cells from G0/G1 to S phase of the cell cycle by translational up-regulation of a family of mRNA transcripts family of mRNA transcripts which contain polypyrimidine tract at their 5 transcriptional start site. Here, we report that p70S6k was constitutively phosphorylated and activated to various degrees in serum-deprived AGS, A2058, HT-1376, MG63, MCF7, MDA-MB-435S, MDA-MB-231 and MB-157. Rapamycin treatment induced a significant dephosphorylation and inactivation of p70S6k in all cancer cell lines, while wortmannin, a specific inhibitor of PI3-K, caused a mild dephosphorylation of p70S6k in AGS, MDA-MB-435S and MB-157. In addition, SQ20006, methylxanthine phosphodiesterase inhibitor, reduced the phosphorylation of p70S6k in all cancer cells tested. Consistent with inhibitory effect of rapamycin on p70S6k activity, rapamycin inhibited [3H]-thymidine incorporation and increased the number of cells at G0/G1 phase. Furthermore, these inhibitory effects were accompanied by the decrease in growth of cancer cells. Taken together, the results indicate that the antiproliferative activity of rapamycin might be attributed to cell cycle arrest at G0/G1 phase in human cancer cells through the inhibition of constitutively activated p70S6k of cancer cells and suggest p70S6k as a potential target for therapeutic strategies aimed at preventing or inhibiting tumor growth.

Modulation of protein translation by Nck-1

In mammals, Nck represented by two genes, is a 47-kDa SH2/SH3 domain-containing protein lacking intrinsic enzymatic function. Here, we reported that the first and the third SH3 domains of Nck-1 interact with the C-terminal region of the beta subunit of the eukaryotic initiation factor 2 (eIF2 beta). Binding of eIF2 beta was specific to the SH3 domains of Nck-1, and in vivo, the interaction Nck/eIF2 beta was demonstrated by reciprocal coimmunoprecipitations. In addition, Nck was detected in a molecular complex with eIF2 beta in an enriched ribosomal fraction, whereas no other SH2/SH3 domain-containing adapters were found. Cell fractionation studies demonstrated that the presence of Nck in purified ribosomal fractions was enhanced after insulin stimulation, suggesting that growth factors dynamically regulate translocation of Nck to ribosomes. In HEK293 cells, we observed that transient overexpression of Nck-1 significantly enhanced Cap-dependent and -independent protein translation. This effect of Nck-1 required the integrity of its first and third SH3 domains originally found to interact with eIF2 beta. Finally, in vitro, Nck-1 also increased protein translation, revealing a direct role for Nck-1 in this process. Our study demonstrates that in addition to mediate receptor tyrosine kinase signaling, Nck-1 modulates protein translation potentially through its direct interaction with an intrinsic component of the protein translation machinery.

Comparative analysis of protein synthesis, transcription and cytoplasmic polyadenylation of mRNA during maturation of bovine oocytes in vitro

In many species, large numbers of macromolecules are accumulated during oocyte growth. The messenger and ribosomal RNAs produced in these cells are far in excess of those necessary to support protein synthesis. Thus, the aim of this study was to elucidate the processes of translational regulation during meiotic maturation. The relationship between transcription, translation and polyadenylation of mRNA during in-vitro maturation (IVM) of bovine oocytes was investigated. The results presented here show that overall protein synthesis is stimulated during meiotic maturation (approximately three times) concomitantly with the onset of germinal vesicle breakdown after 6 to 10 h of IVM. However, in metaphase II, the incorporation of [35S]methionine into proteins showed only basal levels, as in the germinal vesicle (GV) stage. Furthermore, in the course of IVM, de-novo transcription strongly declines as determined by measuring the incorporation of [3H]uridine into RNA. In contrast to this finding, the incorporation of [3H]adenosine increased and showed a peak during the time interval from 6 to 10 h of IVM, parallel with the onset of germinal vesicle breakdown (GVBD) and translation. In the further course of maturation, only a moderate decrease of [3H]adenosine incorporation was observed. These results indicate that (i) translation increased at the time of GVBD; (ii) these processes were accompanied by polyadenylation of mRNA; and (iii) although transcription declines, polyadenylated mRNA is accumulated until metaphase II (as shown by poly(U)-hybridization), in which protein synthesis is low. The correlation of these processes is discussed here. A detailed knowledge of the biochemical and molecular processes which occur during oocyte maturation can be useful for the improvement of IVM conditions.

Involvement of phosphatidylinositol 3 kinase in the progesterone-induced oocyte maturation in Rana dybowskii

Previously, we observed that 70-kDa ribosomal protein S6 kinase (p70(s6k)) plays an essential role during the early phase of oocyte maturation in Rana dybowskii. To investigate further the early signal transduction components involved in this process, the possible role of phosphatidylinositol-3 kinase (PI3 kinase) during oocyte maturation was examined. Progesterone-induced oocyte maturation was significantly inhibited by wortmannin and LY294002, specific inhibitors of PI3 kinase. In contrast, protein kinase C activator 12-0-tetradecanoylphorbol-13-acetate-induced oocyte maturation was not inhibited by wortmannin. Protein synthesis was also significantly suppressed by wortmannin treatment during oocyte maturation. Moreover, PI3 kinase inhibitor suppressed progesterone-induced phosphorylation of S6 kinase in a dose-dependent manner. Likewise, PI3 kinase inhibitors significantly inhibited the phosphorylation of mitogen-activated protein (MAP) kinase which was increased during oocyte maturation. Finally, progesterone-induced H1 kinase activity was also inhibited by PI3 kinase inhibitors in a dose-dependent manner. Taken together, these results suggest that PI3 kinase is an initial component of the signal transduction pathway which precedes p70(s6k), MAP kinase, and MPF production during progesterone-induced maturation of amphibian oocyte.

Mild hypothermia increases Bcl-2 protein expression following global cerebral ischemia

Mild hypothermia protects the brain against experimental ischemia, but the reasons are not well known. We examined whether the protective effects of mild hypothermia could be correlated with alterations in expression of Bcl-2, an anti-apoptotic protein in a rat model of transient global ischemia. Following 10 min of forebrain ischemia, hippocampal neurons were examined 72 h later for survival, expression of Bcl-2 family proteins and apoptosis. Intraischemic mild hypothermia was applied for 3 h (33 degrees C, isch-33) or normal body temperature was maintained (37 degrees C, isch-37). Survival of CA1 neurons was significantly improved in the isch-33 group compared to the isch-37 group (90 vs. 53% survival; P<0.01). The proportion of Bcl-2-positive cells among surviving CA1 neurons in the isch-33 group was increased compared to that of sham and isch-37 groups (P<0.01). Bax expression in CA1 was no different between sham and isch-33 groups, but was significantly decreased in isch-37 (P<0.05). TUNEL staining was positive in many isch-37 CA1 neurons, but absent in isch-33. Utilizing electron microscopy, more cells meeting criteria for apoptosis were observed in the isch-37 than isch-33. These data suggest that mild hypothermia attenuates apoptotic death, and that this protection may be related to increases in Bcl-2.

Inhibition of protein synthesis by didemnins: cell potency and SAR

Synthetic and naturally occurring didemnins are potent and specific inhibitors of protein synthesis in vitro. Structure-activity analysis indicates a requirement for the intact macrocycle; however, the smaller ring size represented by the didemnin analogue, tamandarin A, is equipotent to didemnin B. Replacement of the N,O-dimethyltyrosine by a N-methylphenylalanine or N-methylleucine residue is also well-tolerated. The rank order for inhibition of protein synthesis in vitro appears to be retained in MCF-7 cells, albeit at much higher potency. This increase in potency is explained for the first time by data indicating that MCF-7 cells can accumulate didemnin B up to 2-3 orders of magnitude compared to the growth medium.

Role of S6 phosphorylation and S6 kinase in cell growth

This article reviews our current knowledge of the role of ribosomal protein S6 phosphorylation and the S6 kinase (S6K) signaling pathway in the regulation of cell growth and proliferation. Although 40S ribosomal protein S6 phosphorylation was first described 25 years ago, it only recently has been implicated in the translational up-regulation of mRNAs coding for the components of protein synthetic apparatus. These mRNAs contain an oligopyrimidine tract at their 5' transcriptional start site, termed a 5'TOP, which has been shown to be essential for their regulation at the translational level. In parallel, a great deal of information has accumulated concerning the identification of the signaling pathway and the regulatory phosphorylation sites involved in controlling S6K activation. Despite this knowledge we are only beginning to identify the direct upstream elements involved in growth factor-induced kinase activation. Use of the immunosupressant rapamycin, a bacterial macrolide, in conjunction with dominant interfering and activated forms of S6K1 has helped to establish the role of this signaling cascade in the regulation of growth and proliferation. In addition, current studies employing the mouse as well as Drosophila melanogaster have provided new insights into physiological function of S6K in the animal. Deletion of the S6K1 gene in mouse cells led to an animal of reduced size and the identification of the S6K1 homolog, S6K2, whereas loss of dS6K function in Drosophila demonstrated its paramount importance in development and growth control.

p68 Sam is a substrate of the insulin receptor and associates with the SH2 domains of p85 PI3K

The 68 kDa Src substrate associated during mitosis is an RNA binding protein with Src homology 2 and 3 domain binding sites. A role for Src associated in mitosis 68 as an adaptor protein in signaling transduction has been proposed in different systems such as T-cell receptors. In the present work, we have sought to assess the possible role of Src associated in mitosis 68 in insulin receptor signaling. We performed in vivo studies in HTC-IR cells and in vitro studies using recombinant Src associated in mitosis 68, purified insulin receptor and fusion proteins containing either the N-terminal or the C-terminal Src homology 2 domain of p85 phosphatidylinositol-3-kinase. We have found that Src associated in mitosis 68 is a substrate of the insulin receptor both in vivo and in vitro. Moreover, tyrosine-phosphorylated Src associated in mitosis 68 was found to associate with p85 phosphatidylinositol-3-kinase in response to insulin, as assessed by co-immunoprecipitation studies. Therefore, Src associated in mitosis 68 may be part of the signaling complexes of insulin receptor along with p85. In vitro studies demonstrate that Src associated in mitosis 68 associates with the Src homology 2 domains of p85 after tyrosine phosphorylation by the activated insulin receptor. Moreover, tyr-phosphorylated Src associated in mitosis 68 binds with a higher affinity to the N-terminal Src homology 2 domain of p85 compared to the C-terminal Src homology 2 domain of p85, suggesting a preferential association of Src associated in mitosis 68 with the N-terminal Src homology 2 domain of p85. This association may be important for the link of the signaling with RNA metabolism.

Inhibition of S6 kinase by rapamycin blocks maturation of Rana dybowskii oocytes

Studies were carried out to define the hormone-induced signal transduction pathway during maturation of Rana dybowskii oocytes. Rapamycin, a specific inhibitor of S6 kinase, blocked progesterone-induced oocyte germinal vesicle breakdown (GVBD) in a dose-dependent manner indicating that S6 kinase is required for meiotic maturation of Rana oocytes. Addition of rapamycin within 3 h, but not 6 h, of progesterone treatment inhibited GVBD. In contrast, cycloheximide, a general protein synthesis inhibitor, blocked GVBD even when added 9 h after progesterone addition. A twofold increase in S6 kinase activity occurred within 1 h of progesterone stimulation and rapamycin inhibited this activity. Rapamycin also suppressed, in a dose-dependent manner, progesterone-induced protein synthesis during the first 12 h of culture but less effectively later. Histone H1 kinase activity (maturation-promoting factor, MPF) was observed in oocyte extracts at two different times (between 6 and 9 h and at 24 h) following progesterone stimulation. Rapamycin blocked H1 kinase activity between 6 and 9 h of culture but not that observed at 24 h. In contrast, cycloheximide suppressed progesterone-induced H1 kinase activity as well as protein synthesis throughout the course of incubation. Such results indicate that rapamycin and cycloheximide have common and unique effects on oocyte maturation and suggest that progesterone-induced S6 kinase activity is closely associated with induction of protein synthesis and activation of MPF during oocyte maturation. Results in Rana contrast with those obtained in Xenopus where rapamycin inhibited S6 kinase but failed to inhibit GVBD or protein synthesis. Differences in the response of Rana and Xenopus oocytes to rapamycin are discussed in relation to seasonal, biochemical, and species variations.

Regulation of p70s6k/p85s6k and its role in the cell cycle

Two to three-fold increases in the rate of protein synthesis are required both to enter the G1 phase of the cell cycle from G0 and to proceed to S phase in response to growth factors and mitogens. This increase is in part regulated via multiple phosphorylation of the 40S ribosomal protein S6 by the mitogen-stimulated p70s6k/p85s6k. At the protein synthesis level this event appears to be involved in specifically increasing the efficiency of translation of a family of essential mRNAs containing a polypyrimidine tract at their 5' transcriptional start site. The activation of p70s6k/p85s6k and maintenance of its activity throughout G1 is controlled via multiple phosphorylation events mediated by a complex signalling network acting on distinct sets of phosphorylation sites.

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.

Influence of mitochondrial protein synthesis inhibition on deafferentation-induced ultrastructural changes in nucleus magnocellularis of developing chicks

Following cochlea removal in developing chicks, about 30% of the neurons in the ipsilateral second-order auditory nucleus, nucleus magnocellularis, undergo cell death. Administration of chloramphenicol, a mitochondrial protein synthesis inhibitor, results in a pronounced increase in deafferentation-induced cell death. In this study, we examined whether the chloramphenicol enhancement of deafferentation-induced cell death reveals the same ultrastructural characteristics that are seen in degenerating nucleus magnocellularis neurons after cochlea removal alone. Unilateral cochlea removal was performed on anaesthetized posthatch chicks. One group of animals was simultaneously treated with chloramphenicol. Six, twelve, or twenty-four hours following cochlea removal, n. magnocellularis neurons were studied by routine transmission electron microscopy. Particular attention was paid to the integrity of the polyribosomes and rough endoplasmic reticulum. Two ultrastructurally different types of neuronal degeneration were observed in the deafferented nucleus magnocellularis neurons: an early onset electron-lucent type that always involved ribosomal dissociation and a late-onset electron-dense type displaying nuclear pyknosis and severely damaged mitochondria. The percentage of nucleus magnocellularis neurons displaying ribosomal disintegration following cochlea removal was found to be markedly increased after chloramphenicol treatment. This finding suggests that mitochondrial function is important for the maintenance of a functional protein synthesis apparatus following deafferentation.

Xenopus poly (A) binding protein maternal RNA is localized during oogenesis and associated with large complexes in blastula

Maternal mRNAs are synthesized during oogenesis and often stored for use during early embryogenesis, before the onset of zygotic transcription. The temporal and spatial regulation of maternal RNAs is likely to be crucial mechanism for the establishment of the body pattern. In the course of a study that identified a Xenopus maternal mRNA that is translationally regulated along the dorsoventral axis, several RNAs were found to behave anomalously in polysomal analysis and are further characterized here. As controls for polysome analysis, elF4E RNA and D7.1 RNA were equally translated in both dorsal and ventral cells, whereas the cell-cell signaling factor noggin RNA was not translated in either cell type. Maternal RNAs encoding poly (A) binding protein (PABP), Vg1 and Xcat-2 were associated with large complexes that, in contrast to polysomes, were not dissociated in magnesium-free buffer. Vg1 and Xcat-2 maternal mRNAs have been shown to be localized during oogenesis to the vegetal hemisphere of the oocyte [Rebagliati et al., 1985; Mosquera et al., 1993]. In situ hybridization analysis indicated that PABP RNA was also localized during oogenesis, to the animal hemisphere in stage VI oocytes. This suggests that association of maternal mRNAs with large EDTA-insensitive mRNP complexes is correlated with intracellular localization, but the specific localization within the oocyte is dependent upon the RNA species.

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.

Phosphorylation of eukaryotic elongation factor 2 in differentiating and proliferating HL-60 cells

Ca(2+)-and calmodulin-dependent protein kinase III (CaM PKIII) phosphorylates eukaryotic elongation factor 2 (eEF-2) in HL-60 cells. Dephosphorylation of the factor in these cells is catalyzed by phosphoprotein phosphatase 2A alone. Differentiation of the HL-60 cells by all-trans retinoic acid resulted in a reduced growth rate and a marked decrease in the intracellular concentration of eEF-2. During differentiation the activity of the eEF-2 kinase is gradually reduced and reaches 10% of that found in undifferentiated cells 5 days after the onset of differentiation. The capacity to dephosphorylate phospho-eEF-2 remained unaltered in the growth-arrested cells. Differentiation without reduced proliferation was induced in the HL-60 cells by interferon-gamma. Under these conditions, differentiation had no effect on the cellular content of eEF-2 or the ability to dephosphorylate phospho-eEF-2. However, the differentiated cells showed a dramatic decrease in the specific activity of the eEF-2 kinase. The results show that the cellular content of eEF-2 varies with the rate of proliferation and that the activity of the eEF-2 kinase is high in undifferentiated proliferating cells and decreases upon differentiation even under conditions of an unaltered growth rate.

Phosphorylation of eukaryotic protein synthesis initiation factor 4E by insulin-stimulated protamine kinase

Insulin-stimulated protamine kinase (cPK) and protein kinase C (PKC) phosphorylated eukaryotic protein synthesis initiation factor 4E (eIF-4E) on serine and threonine residues located on an identical tryptic fragment as judged by two-dimensional phosphopeptide mapping. With cPK and PKC, the apparent Km for eIF-4E was about 1.2 and 50 microM, respectively. Relative to recombinant human eIF-4E, cPK exhibited about 100% and < or = 5% activity with eIF-4ES209A and eIF-4ET210A, respectively, and eIF-4ES209A was phosphorylated exclusively on threonines. Bovine kidney eIF-4E enhanced up to 1.8-fold globin synthesis in m7GTP-Sepharose-treated reticulocyte lysates. In contrast, following incubation with cPK, these eIF-4E preparations stimulated globin synthesis up to 6-fold. Compared to the dephosphorylation of the cPK-modified serine on eIF-4E, reticulocyte lysates and highly purified protein phosphatase 2A exhibited marked preference for the cPK-modified threonine. The results indicate that cPK phosphorylates eIF-4E on Ser209 and Thr210, that the hydroxyl group or phosphorylation of Thr210 is necessary for cPK to act on Ser209, and that Ser209 phosphorylation activates reticulocyte globin synthesis. The results suggest that cPK could contribute to the insulin-stimulated phosphorylation of eIF-4E, but that protein phosphatase 2A may confer the site specificity of this response.

Hormone-induced meiotic maturation in Xenopus oocytes occurs independently of p70s6k activation and is associated with enhanced initiation factor (eIF)-4F phosphorylation and complex formation

Hormone-induced meiotic maturation of the Xenopus oocyte is regulated by complex changes in protein phosphorylation. It is accompanied by a stimulation in the rate of translation, manifest at the level of polypeptide chain initiation. At laser times in the maturation process, this reflects an increased ability for mRNA to interact with the 40 S ribosomal subunit. In mammalian cells there is growing evidence for the regulation of translation by phosphorylation of ribosomal protein S6 and of initiation factors responsible for the binding of mRNA to ribosomes. In this report, we show that although the 70 kDa form of S6 kinase is activated within 1.5 hours in response to progesterone or insulin, a time critical for protein synthesis, its activation is not required for hormone-induced stimulation of translation rates or maturation. In response to progesterone, activation of translation occurs in parallel with enhanced phosphate labelling of eIF-4 alpha and eIF-4 gamma and eIF-4F complex formation, events which are thought to facilitate the interaction of eIF-4F with the mRNA cap structure. However, with insulin, activation of translation occurs prior to detectable de novo phosphorylation of eIF-4F, although a small enhancement of turnover of phosphate on eIF-4 alpha may occur at this early time. With either hormone, enhanced phosphate labelling of eIF-4 alpha is shown to reflect activation of eIF-4 alpha kinase(s), which coincides temporally with activation of p42 MAP and p90rsk kinases. The possible role of initiation factor modification on increased translation rates during meiotic maturation is discussed.

P62 association with RNA is regulated by tyrosine phosphorylation

The ras-GAP associated protein, p62, is a major tyrosine phosphoprotein in transformed and growth factor treated cells. Although its exact function is not known, it can bind directly to src-family tyrosine kinases and has been implicated as a linker protein bridging activated src family tyrosine kinases with downstream effectors. One novel feature of p62, revealed by its predicted amino acid sequence, is the presence of an RNA-binding region, the KH domain. As p62 becomes tyrosine phosphorylated when src-kinases become activated, we compared the RNA binding ability of p62 in both its phosphorylated and unphosphorylated state. The ability of p62 to bind RNA was severely impaired when p62 was tyrosine phosphorylated. This suggests that the ability of p62 to bind RNA is regulated by tyrosine phosphorylation and implicates the regulation of RNA as a component of tyrosine kinase signaling pathways.

Mitogenesis and protein synthesis: a role for ribosomal protein S6 phosphorylation?

It has been known for 20 years that the ribosomal protein S6 is rapidly phosphorylated when cells are stimulated to grow or divide. Furthermore, numerous studies have documented that there is a strong correlation between increases in S6 phosphorylation and protein synthesis, leading to the idea that S6 phosphorylation is involved in up-regulating translation. In an attempt to define a mechanism by which S6 phosphorylation exerts translational control, other studies have focused on characterizing the sites of phosphorylation of this protein and its location within the ribosome. Recent data show that S6 is a protein which may have diverse cellular functions and is essential for normal development, and that it may be involved in the translational regulation of a specific class of messages.

Signal transduction mechanisms in the regulation of protein synthesis

Control of polypeptide synthesis plays an important role in cell proliferation and translation rates generally reflect the growth state of the cultured eukaryotic cell. Physiological regulation of protein synthesis is almost always exerted at the level of polypeptide chain initiation, with the binding of mRNA to the small ribosomal subunit a rate-limiting step in many cell systems. Studies have indicated key roles in the regulation of protein synthesis for the structural features of mRNA molecules and phosphorylation of initiation factors which catalyse this process. This review focuses on translational regulation at the level of mRNA binding to the ribosome and the role of phosphorylation of initiation factors in mediating both quantitative and qualitative control. The identity of putative kinases which may mediate these processes is addressed and a possible model for the role of a transient activation of initiation factors in cell growth or differentiation is presented.

Tyrosine phosphorylation and activation of mitogen-activated protein kinase in the rat brain following transient cerebral ischemia

Activation of trophic factor receptors stimulates tyrosine phosphorylation on proteins and supports neuronal survival. We report that in the recovery phase following reversible cerebral ischemia, tyrosine phosphorylation increases in the membrane fraction of the resistant hippocampal CA3/dentate gyrus (DG) region, whereas in the sensitive CA1 region or striatum, tyrosine phosphorylation is less marked or decreases. In the cytosolic fractions, a 42-kDa protein, identified as mitogen-activated protein (MAP) kinase, is markedly phosphorylated and activated immediately following ischemia, in particular in CA3/DG, but not in striatum. In the CA1 region, phosphorylation of MAP kinase is less intense and decreases later during reperfusion, which could explain the delay of neuronal degeneration in this structure. The data suggest that in ischemia-resistant neurons the growth factor receptor-coupled signaling cascade is stimulated and, through its effects on DNA transcription and mRNA translation, supports neuronal survival.

S6 phosphorylation and the p70s6k/p85s6k

Activation of cell growth leads to the multiple phosphorylation of 40S ribosomal protein S6. The kinase responsible for controling this event is termed p70s6k/p85s6k. Both isoforms of the kinase are derived from a common gene activated by a complex set of phosphorylation events; each resides in a unique cellular compartment: the p70s6k in the cytoplasm and the p85s6k in the nucleus. Although p70s6k/p85s6k represent the first mitogen-activated serine/threonine kinase described, the signaling pathway leading to activation of both isoforms remains obscure. Recent studies have shown that this pathway is distinct from that of p21ras and the p42mapk/p44mapk, and that bifurcation of these pathways takes place at the level of the receptor. Experiments with point mutants of the PDGF receptor and inhibitors of phosphatidyl-inositol-3-OH kinase have implicated the latter molecule in this signaling event, but more recent findings suggest an alternative route may be employed. The p70s6k signaling pathway can also be ablated by the immunosuppressant rapamycin, which blocks p70s6k activation and S6 phosphorylation without affecting the other kinases whose activation is triggered by mitogen treatment. In parallel, rapamycin suppresses the translation of a family of mRNAs that contain a polypyrimidine tract at their 5' transcriptional start site. The implication is that this event is mediated by the phosphorylated form of S6 that may either (1) directly interact with the polypyrimidine tract or (2) alter the affinity of the 40S ribosome mRNA binding site for polypyrimidine tract mRNAs, or (3) recognize proteins that directly bind to the polypyrimidine tract.

Regulation of translation and cell growth by eIF-4E

This review discusses the regulation of a key controlling step in the initiation of protein synthesis, the binding of mRNA to ribosomes. Particular focus is given to the phosphorylation of the cap-binding factor, eIF-4E, and the role of this factor in the regulation of cell growth.

The effect of global ischemia and recirculation of rat brain on protein synthesis in vitro

Transient cerebral ischemia causes long-lasting inhibition of protein synthesis despite recovery of energy metabolism. We investigated the question if this inhibition is due to the formation of a suppression factor which interferes with the function of the protein synthesizing machinery. For this purpose rats were submitted to 20 minutes four vessel-occlusion followed by recirculation times from 30 minutes to 7 days. Post-mitochondrial supernatant (PMS) from various brain regions was added to a self-contained, cell-free rabbit reticulocyte translational system, and the effect on in vitro protein synthesis was assessed by measuring 14C-leucine incorporation over a duration of 45 minutes. PMS prepared at the end of ischemia from hippocampus, striatum and cerebellum inhibited in vitro protein synthesis by 40%-60% but there was only a minor inhibition by PMS from cerebral cortex. During post-ischemic recirculation cortical PMS transiently induced inhibition of in vitro protein synthesis by 30% but this effect gradually disappeared within one week. The inhibition caused by PMS from hippocampus, striatum and cerebellum was not reversed during recirculation and still amounted to about 40% after 7 days. Inhibition of in vitro protein synthesis could be blocked by heating PMS to 100 degrees C, indicating that the suppressor factor is a protein. The comparison of the in vitro effect of postischemic PMS with previously described in vivo inhibition of protein synthesis demonstrates that the here observed suppressor factor is not able to explain the overall disturbance of protein synthesis in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

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

Mature porcine peripheral blood mononuclear cells (PPBMCs) exist in a resting state both in vivo and when maintained in culture, with low translation rates consistent with their non-proliferative state. When cultured in the presence of the appropriate mitogen, there is a 2-4-fold increase in the rate of protein synthesis per ribosome within 4 h of stimulation [Kay, J. E., Ahern, T. and Atkins, M. (1971) Biochim. Biophys. Acta 247, 322-334]. Studies on extracts prepared from unstimulated cells have suggested lesions in initiation factor activity, primarily affecting the binding of mRNA to ribosomes [Ahern, T., Sampson, J. and Kay, J. E. (1974) Nature 248, 519-521]. In these studies, we have demonstrated that activation of quiescent PPBMCs with the phorbol ester phorbol 12-myristate 13-acetate or concanavalin A leads to a rapid 2-4-fold increase in the rate of protein synthesis within 1 h or 4 h, respectively, which is insensitive to the transcriptional inhibitor, 5,6-dichlorobenzimidazole riboside. Relative to control cells, both phorbol ester and concanavalin A induce a 2-4-fold increase in labelling of the eukaryotic initiation factor eIF-4 alpha with phosphate in vivo, which primarily reflects a small net increase in phosphorylation rather than phosphate turnover on eIF-4 alpha. Similarly, with the human leukaemic T cell line JURKAT, stimulation of the T cell receptor with the monoclonal antibody, OKT-3, or treatment with phorbol ester induces a 2-3-fold increase in eIF-4 alpha phosphorylation within 30 min. Analysis of phosphorylation by two-dimensional gel electrophoresis and measurement of kinase activity towards synthetic peptides, indicate that this increased labelling also reflects increased eIF-4 alpha kinase activity rather than phosphate turnover on eIF-4 alpha. Of central importance is the finding that, concomitant with increased rates of protein synthesis following stimulation of PPBMCs with either phorbol ester or concanavalin A, there is a significant increase in the level of eIF-4 alpha recovered in high-molecular-mass complexes. These data suggest that, in quiescent PPBMCs, eIF-4F may be limiting and that the association of eIF-4 alpha and eIF-4 gamma into high-molecular-mass complexes is regulated by phosphorylation and may play a pivotal role in translational control.

Depression of neuronal protein synthesis initiation by protein tyrosine kinase inhibitors

Growth factors stimulate cellular protein synthesis, but the intracellular signaling mechanisms that regulate initiation of mRNA translation in neurons have not been clarified. A rate-limiting step in the initiation of protein synthesis is the formation of the ternary complex among GTP, eukaryotic initiation factor 2 (eIF-2), and the initiator tRNA. Here we report that genistein, a specific tyrosine kinase inhibitor, decreases tyrosine kinase activity and the content of phosphotyrosine proteins in cultured primary cortical neurons. Genistein inhibits protein synthesis by > 80% in a dose-dependent manner (10-80 micrograms/ml) and concurrently decreases ternary complex formation by 60%. At the doses investigated, genistein depresses tyrosine kinase activity and concomitantly stimulates PKC activity. We propose that a protein tyrosine kinase participates in the initiation of protein synthesis in neurons, by affecting the activity of eIF-2 directly or through a protein kinase cascade.

Heat-shock inhibits protein synthesis and eIF-2 activity in cultured cortical neurons

Stress, such as heat-shock, hypoxia and hypoglycemia, inhibits the initiation of protein synthesis. The effects of heat-shock on protein synthesis, eucaryotic initiation factor 2 (eIF-2) activity, protein kinase C (PKC), and casein kinase II (CKII) activities were studied in primary cortical neuronal cultures. In neurons exposed to heat-shock at 44 degrees C for 20 min, protein synthesis is inhibited by more than 80%, and is accompanied by a 60% decrease in eIF-2 activity. Steady state PKC and CK II activities were not affected by heat-shock. Vanadate (200 microM), a protein phosphotyrosine phosphatase inhibitor, partially prevented the depression of eIF-2 activity during heat-shock, and increased CKII activity by 90%. In contrast, staurosporine (62nM), a protein kinase C inhibitor, did not affect eIF-2 activity. We conclude that heat-shock causes a change in the phosphorylation/dephosphorylation of regulatory proteins leading to a depressed eIF-2 activity and protein synthesis in neurons.

Initiation of protein synthesis and heat-shock protein-72 expression in the rat brain following severe insulin-induced hypoglycemia

Following stress such as heat shock or transient cerebral ischemia, global brain protein synthesis initiation is depressed through modulation of eucaryotic initiation factor (eIF) activities, and modification of ribosomal subunits. Concomitantly, expression of a certain class of mRNA, heat-shock protein (HSP) mRNA, is induced. Here we report that the activity of eucaryotic initiation factor-2 (eIF-2), a protein that participates in the regulation of a rate-limiting initiation step of protein synthesis, transiently decreases following insulin-induced severe hypoglycemia in the rat brain neocortex. Expression of HSP 72, a 72-kDa HSP, in surviving neurons was seen at 1-7 days of recovery following 30 min of hypoglycemic coma, but not at 1 h and 6 h of recovery. In the neocortex, HSP 72 was first seen in layer IV, and later also in surviving neurons in layer II. In the CA1 region and in the crest of dentate gyrus, HSP 72 expression was evident in cells adjacent to irreversibly damaged neurons. In the CA3 region and the hilus of dentate gyrus, HSP 72 was expressed in a few scattered neurons. In septal nucleus, HSP 72 was expressed in a lateral to medial fashion over a period of 1-3 days of recovery. We conclude that severe insulin-induced hypoglycemia induces a stress response in neurons in the recovery phase, including inhibition of protein synthesis initiation, depression of eIF-2 activity, and a delayed and prolonged expression of HSP 72 in surviving neurons. The HSP 72 expression may be a protective response to injurious stress.

Inhibition of protein phosphatases activates glucose-6-phosphatase in isolated rat hepatocytes

Incubation of hepatocytes in the presence of microcystin-LR, okadaic acid, calyculin A (inhibitors of protein phosphatases PP1 and PP2A) or microcystin-RR (a specific inhibitor of PP2A) activated glucose-6-phosphatase both in the supernatant and in intact or disrupted microsomes. Puromycin, an inhibitor of protein synthesis, totally suppressed this activating effect, suggesting the involvement of protein phosphatases in the regulation of glucose-6-phosphatase synthesis.

Stimulation of translation in 3T3-L1 cells in response to insulin and phorbol ester is directly correlated with increased phosphate labelling of initiation factor (eIF-) 4F and ribosomal protein S6

One of the earliest responses to growth-promoting compounds in quiescent or serum-starved cells is stimulation of phosphorylation of ribosomal protein S6. Exposure of 3T3-L1 cells to insulin or phorbol ester (phorbol 12-myristate 13-acetate, PMA) also promotes phosphorylation of specific initiation factors. In this study, stimulation of phosphate labelling of S6, eIF-4F and eIF-4B in response to insulin and PMA has been examined in 3T3-L1 cells and compared with changes in protein synthesis. The rate of phosphate incorporation into eIF-4F and S6 is rapid and parallels the transient stimulation of protein synthesis observed with insulin. Whilst a similar correlation exists with PMA, the response is not as great as with insulin, but is more sustained. A role for the co-ordinate phosphorylation of initiation factors and ribosomal protein S6 in the stimulation of protein synthesis is discussed.