Molecular cloning and tissue distribution of PHAS-I, an intracellular target for insulin and growth factors

Functional interaction between RAFT1/FRAP/mTOR and protein kinase cdelta in the regulation of cap-dependent initiation of translation

Hormones and growth factors induce protein translation in part by phosphorylation of the eukaryotic initiation factor 4E (eIF4E) binding protein 1 (4E-BP1). The rapamycin and FK506-binding protein (FKBP)-target 1 (RAFT1, also known as FRAP) is a mammalian homolog of the Saccharomyces cerevisiae target of rapamycin proteins (mTOR) that regulates 4E-BP1. However, the molecular mechanisms involved in growth factor-initiated phosphorylation of 4E-BP1 are not well understood. Here we demonstrate that protein kinase Cdelta (PKCdelta) associates with RAFT1 and that PKCdelta is required for the phosphorylation and inactivation of 4E-BP1. PKCdelta-mediated phosphorylation of 4E-BP1 is wortmannin resistant but rapamycin sensitive. As shown for serum, phosphorylation of 4E-BP1 by PKCdelta inhibits the interaction between 4E-BP1 and eIF4E and stimulates cap-dependent translation. Moreover, a dominant-negative mutant of PKCdelta inhibits serum-induced phosphorylation of 4E-BP1. These findings demonstrate that PKCdelta associates with RAFT1 and thereby regulates phosphorylation of 4E-BP1 and cap-dependent initiation of protein translation.

Role of load in regulating eIF-4F complex formation in adult feline cardiocytes

This study examined whether cardiocyte load increases eIF-4F complex formation. To increase load in vitro, adult feline cardiocytes were electrically stimulated to contract (1 Hz, 5-ms pulses). eIF-4F complex formation, measured by eIF-4G association with eIF-4E, increased 57 +/- 16% after 4 h of contraction compared with controls. eIF-4F complex formation did not increase on electrical stimulation with 2,3-butanedione monoxime (BDM), an inhibitor of active tension. Both insulin and phorbol ester increased eIF-4F complex formation, but these increases were unaffected by BDM. Insulin caused a shift of eIF-4E binding proteins (4E-BPs) into their hyperphosphorylated gamma-isoforms and dissociation of 4E-BPs from eIF-4E. Rapamycin inhibited 4E-BP phosphorylation in response to insulin but had no effect on eIF-4F complex formation. Electrically stimulated contraction caused a partial shift of 4E-BP1 and 4E-BP2 into the gamma-isoforms, but it had no effect on 4E-BP association with eIF-4E. Rapamycin blocked the increase in eIF-4F complex formation in electrically stimulated cardiocytes and depressed contractility. These data indicate that cardiocyte load causes a tension-dependent increase in eIF-4F complex formation that does not require dissociation of 4E-BPs from eIF-4E.

Striking multiplicity of eIF4E-BP1 phosphorylated isoforms identified by 2D gel electrophoresis regulation by heat shock

Eukaryotic initiation factor eIF4E-binding protein 1 (eIF4E-BP1), or PHAS-I, is multiply phosphorylated by insulin-stimulated protein kinase(s). Estimates for the number of phosphorylation sites range from two to greater than eight. IEF/SDS/PAGE can precisely differentiate protein isoforms based on their differences in charge (phosphorylation) and molecular mass. In this study, the diversity of eIF4E-BP1 isoforms was determined using IEF/SDS/PAGE/immunoblotting of unfractionated cell lysates. To investigate the molecular regulation of phosphorylation, alterations in eIF4E-BP1 in response to heat shock in HeLa cells were determined. In exponentially growing cells, 8-10 prominent eIF4E-BP1 isoforms were detected. Following heat shock, a rapid, temperature-dependent dephosphorylation of eIF4E-BP1 occurs roughly concurrent with protein synthesis inhibition; during recovery from heat shock rephosphorylation of eIF4E-BP1 parallels restoration of protein synthesis. However, eIF4E-BP1 and eIF4E kinases remain highly active during heat shock, as okadaic acid treatment restores phosphorylation of both factors in heat shocked cells. eIF4E-BP1 dephosphorylation is associated with eIF4E dissociation from large molecular mass complexes and increased binding to eIF4E-BP1. The amount of eIF4E-BP1 converted to the dephosphorylated state is sufficient to titrate all the eIF4E present. eIF4E-BP1 phosphorylation changes regulated by heat shock also occur in Drosophila. Of the 10 isoforms of eIF4E-BP1 resolved by IEF/SDS/PAGE, at least seven are labelled with [32P] and all 10 are recognized by (eIF4E-BP1)-specific antibodies. These results identify a complex set of eIF4E-BP1 phosphorylation isoforms; changes in the expression of these isoforms in response to stresses such as heat shock may contribute to translation repression.

A 45-kDa cAMP-dependent phosphoprotein which is related to the product of Mst57Dc in Drosophila melanogaster

In Drosophila, the catalytic subunit of cAMP-dependent protein kinase (PKA) is preferentially expressed in the brain and the male reproductive organs. Although the cAMP response element binding protein (CREB) is a major target of PKA in the brain, the target of PKA in the male reproductive organs has been unknown. In the present study, three cAMP-dependent phosphoproteins (referred to as pp45, pp20, and pp10) were detected in the lumen fluid of male accessory glands. They were tissue-specific secretory proteins that accumulated only after eclosion, and were transferred to females during mating as other secretory proteins of the accessory glands. Among them, the 45-kDa phosphoprotein was partially purified and characterized. The purified protein was phosphorylated in vitro by the catalytic subunit of PKA. The partial amino acid sequence of this 45-kDa phosphoprotein was identical to the predicted amino acid sequence of the Mst57Dc cDNA, which is a male accessory gland transcript.

Mutational analysis of sites in the translational regulator, PHAS-I, that are selectively phosphorylated by mTOR

Results obtained with PHAS-I proteins having Ser to Ala mutations in the five known phosphorylation sites indicate that mTOR preferentially phosphorylates Thr36 and Thr45. The effects of phosphorylating these sites on eIF4E binding were assessed in a far-Western analysis with a labeled eIF4E probe. Phosphorylation of Thr36 only slightly attenuated binding of PHAS-I to eIF4E, while phosphorylation of Thr45 markedly inhibited binding. Phosphorylation of neither site affected the electrophoretic mobility of the protein, indicating that results of studies that rely solely on a gel-shift assay to assess changes in PHAS-I phosphorylation must be interpreted with caution.

Regulation of 4E-BP1 phosphorylation: a novel two-step mechanism

The multisubunit eukaryotic translation initiation factor (eIF) 4F recruits 40S ribosomal subunits to the 5' end of mRNA. The eIF4F subunit eIF4E interacts directly with the mRNA 5' cap structure. Assembly of the eIF4F complex is inhibited by a family of repressor polypeptides, the eIF4E-binding proteins (4E-BPs). Binding of the 4E-BPs to eIF4E is regulated by phosphorylation: Hypophosphorylated 4E-BP isoforms interact strongly with eIF4E, whereas hyperphosphorylated isoforms do not. 4E-BP1 is hypophosphorylated in quiescent cells, but is hyperphosphorylated on multiple sites following exposure to a variety of extracellular stimuli. The PI3-kinase/Akt pathway and the kinase FRAP/mTOR signal to 4E-BP1. FRAP/mTOR has been reported to phosphorylate 4E-BP1 directly in vitro. However, it is not known if FRAP/mTOR is responsible for the phosphorylation of all 4E-BP1 sites, nor which sites must be phosphorylated to release 4E-BP1 from eIF4E. To address these questions, a recombinant FRAP/mTOR protein and a FRAP/mTOR immunoprecipitate were utilized in in vitro kinase assays to phosphorylate 4E-BP1. Phosphopeptide mapping of the in vitro-labeled protein yielded two 4E-BP1 phosphopeptides that comigrated with phosphopeptides produced in vivo. Mass spectrometry analysis indicated that these peptides contain phosphorylated Thr-37 and Thr-46. Thr-37 and Thr-46 are efficiently phosphorylated in vitro by FRAP/mTOR when 4E-BP1 is bound to eIF4E. However, phosphorylation at these sites was not associated with a loss of eIF4E binding. Phosphorylated Thr-37 and Thr-46 are detected in all phosphorylated in vivo 4E-BP1 isoforms, including those that interact with eIF4E. Finally, mutational analysis demonstrated that phosphorylation of Thr-37/Thr-46 is required for subsequent phosphorylation of several carboxy-terminal serum-sensitive sites. Taken together, our results suggest that 4E-BP1 phosphorylation by FRAP/mTOR on Thr-37 and Thr-46 is a priming event for subsequent phosphorylation of the carboxy-terminal serum-sensitive sites.

A potential role for extracellular signal-regulated kinases in prostaglandin F2alpha-induced protein synthesis in smooth muscle cells

To understand the mechanisms of prostaglandin F2alpha (PGF2alpha)-induced protein synthesis in vascular smooth muscle cells (VSMC), we have studied its effect on two major signal transduction pathways: mitogen-activated protein kinases and phosphatidylinositol 3-kinase (PI3-kinase) and their downstream targets ribosomal protein S6 kinase (p70(S6k)) and eukaryotic initiation factor eIF4E and its regulator 4E-BP1. PGF2alpha induced the activities of extracellular signal-regulated kinase 2 (ERK2) and Jun N-terminal kinase 1 (JNK1) groups of mitogen-activated protein kinases, PI3-kinase, and p70(S6k) in a time-dependent manner in growth-arrested VSMC. PGF2alpha also induced eIF4E and 4E-BP1 phosphorylation, global protein synthesis, and basic fibroblast growth factor-2 (bFGF-2) expression in VSMC. Whereas inhibition of PI3-kinase by wortmannin completely blocked the p70(S6k) activation, it only partially decreased the ERK2 activity, and had no significant effect on global protein synthesis and bFGF-2 expression induced by PGF2alpha. Rapamycin, a potent inhibitor of p70(S6k), also failed to prevent PGF2alpha-induced global protein synthesis and bFGF-2 expression, although it partially decreased ERK2 activity. In contrast, inhibition of ERK2 activity by PD 098059 led to a significant loss of PGF2alpha-induced eIF4E and 4E-BP1 phosphorylation, global protein synthesis, and bFGF-2 expression. PGF2alpha-induced phosphorylation of eIF4E and 4E-BP1 was also found to be sensitive to inhibition by both wortmannin and rapamycin. These findings demonstrate that 1) PI3-kinase-dependent and independent mechanisms appear to be involved in PGF2alpha-induced activation of ERK2; 2) PGF2alpha-induced eIF4E and 4E-BP1 phosphorylation appear to be mediated by both ERK-dependent and PI3-kinase-dependent rapamycin-sensitive mechanisms; and 3) ERK-dependent eIF4E phosphorylation but not PI3-kinase-dependent p70(S6k) activation correlates with PGF2alpha-induced global protein synthesis and bFGF-2 expression in VSMC.

Differential Regulation of 4E-BP1 and 4E-BP2, Two Repressors of Translation Initiation, During Human Myeloid Cell Differentiation

Human myeloid differentiation is accompanied by a decrease in cell proliferation. Because the translation rate is an important determinant of cell proliferation, we have investigated translation initiation during human myeloid cell differentiation using the HL-60 promyelocytic leukemia cell line and the U-937 monoblastic cell line. A decrease in the translation rate is observed when the cells are induced to differentiate along the monocytic/macrophage pathway or along the granulocytic pathway. The inhibition in protein synthesis correlates with specific regulation of two repressors of translation initiation, 4E-BP1 and 4E-BP2. Induction of HL-60 and U-937 cell differentiation into monocytes/macrophages by IFN-γ or PMA results in a dephosphorylation and consequent activation of 4E-BP1. Dephosphorylation of 4E-BP1 was also observed when U-937 cells were induced to differentiate into monocytes/macrophages following treatment with retinoic acid or DMSO. In contrast, treatment of HL-60 cells with retinoic acid or DMSO, which results in a granulocytic differentiation of these cells, decreases 4E-BP1 amount without affecting its phosphorylation and strongly increases 4E-BP2 amount. Taken together, these data provide evidence for differential regulation of the translational machinery during human myeloid differentiation, specific to the monocytic/macrophage pathway or to the granulocytic pathway.

The FKBP12-rapamycin-binding domain is required for FKBP12-rapamycin-associated protein kinase activity and G1 progression

The immunosuppressant rapamycin, in complex with its cellular receptor FKBP12, targets the cellular protein FKBP12-rapamycin-associated protein/mammalian target of rapamycin/rapamycin and FKBP12 target 1 (FRAP/mTOR/RAFT1) and inhibits/delays G1 cell cycle progression in mammalian cells. As a member of the novel phosphatidylinositol kinase-related kinase family, FRAP's kinase activity is essential for its signaling function. The FKBP12-rapamycin binding (FRB) domain in FRAP is also speculated to play an important role in FRAP function and signaling. However, the biochemical and physiological functions of FRB, as well as the mechanism for rapamycin inhibition, have been unclear. The present study focuses on investigation of FRB's role and the functional relationship between FRB domain and kinase domain in FRAP. Microinjection of purified FRB protein into human osteosarcoma MG63 cells results in a drastic blockage of the G1 to S cell cycle progression; such a dominant negative effect is reversed by a point mutation (Trp2027 --> Phe). The same mutation also abolishes kinase activity of FRAP without affecting ATP binding, and truncation studies suggest that upstream sequences including FRB are required for kinase activity in vitro. Given these data, we propose a model for FRAP function, in which the FRB domain is required for activation of the kinase domain, possibly through the interaction with an upstream activator. In addition, our observations provide direct evidence linking FRAP function to G1 cell cycle progression.

Translational repression by human 4E-BP1 in yeast specifically requires human eIF4E as target

4E-binding proteins (4E-BPs) are believed to have important regulatory functions in controlling the rate of translation initiation in mammalian cells. They do so by binding to the mRNA cap-binding protein, eIF4E, thereby inhibiting formation of the cap-binding complex, a process essential for cap-dependent translation initiation. We have reproduced the translation-repressive function of human 4E-BP1 in yeast and find its activity to be dependent on substitution of human eIF4E for its yeast counterpart. Translation initiation and growth are inhibited when human 4E-BP1 is expressed in a strain with the human eIF4E substitution, but not in an unmodified strain. We have compared the relative affinities of human 4E-BP1 for human and yeast eIF4E, both in vitro using an m7GTP cap-binding assay and in vivo using a yeast two-hybrid assay, and find that the affinity of human 4E-BP1 for human eIF4E is markedly greater than for yeast eIF4E. Thus yeast eIF4E lacks structural features required for binding to human 4E-BP1. These results therefore demonstrate that the features of eIF4E required for binding to 4E-BP1 are distinct from those required for cap-complex assembly.

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.

Attenuation of mammalian target of rapamycin activity by increased cAMP in 3T3-L1 adipocytes

Incubating 3T3-L1 adipocytes with forskolin, which increases intracellular cAMP by activating adenylate cyclase, mimicked rapamycin by attenuating the effect of insulin on stimulating the phosphorylation of four (S/T)P sites in PHAS-I, a downstream target of the mammalian target of rapamycin (mTOR) signaling pathway. To investigate the hypothesis that increasing cAMP inhibits mTOR, the protein kinase activity of mTOR was measured in an immune complex assay with recombinant PHAS-I as substrate. Both forskolin and 8-(4-chlorophenylthio)adenosine 3'-5'-monophosphate (CPT-cAMP) prevented the activation of mTOR by insulin in adipocytes, but neither agent affected mTOR activity when added directly to the immunopurified protein. In contrast, the cAMP phosphodiesterase inhibitor, theophylline, inhibited mTOR activity not only when added to intact adipocytes but also when added to immunopurified mTOR in vitro, demonstrating that certain methylxanthines are able to inhibit mTOR independently of increasing cAMP. Forskolin and CPT-cAMP blocked the effect of insulin on increasing mTOR phosphorylation, which was assessed using mTAb1, an antibody whose binding is inhibited by phosphorylation of mTOR. Although the mTAb1 epitope contains a consensus site for protein kinase B, neither agent inhibited the activation of protein kinase B produced by insulin. These findings support the interpretation that increasing cAMP attenuates the effects of insulin on PHAS-I, p70(S6K), and other downstream targets of the mTOR signaling pathway by inhibiting the phosphorylation and activation of mTOR.

Posttranscriptional control of gene expression in yeast

Studies of the budding yeast Saccharomyces cerevisiae have greatly advanced our understanding of the posttranscriptional steps of eukaryotic gene expression. Given the wide range of experimental tools applicable to S. cerevisiae and the recent determination of its complete genomic sequence, many of the key challenges of the posttranscriptional control field can be tackled particularly effectively by using this organism. This article reviews the current knowledge of the cellular components and mechanisms related to translation and mRNA decay, with the emphasis on the molecular basis for rate control and gene regulation. Recent progress in characterizing translation factors and their protein-protein and RNA-protein interactions has been rapid. Against the background of a growing body of structural information, the review discusses the thermodynamic and kinetic principles that govern the translation process. As in prokaryotic systems, translational initiation is a key point of control. Modulation of the activities of translational initiation factors imposes global regulation in the cell, while structural features of particular 5' untranslated regions, such as upstream open reading frames and effector binding sites, allow for gene-specific regulation. Recent data have revealed many new details of the molecular mechanisms involved while providing insight into the functional overlaps and molecular networking that are apparently a key feature of evolving cellular systems. An overall picture of the mechanisms governing mRNA decay has only very recently begun to develop. The latest work has revealed new information about the mRNA decay pathways, the components of the mRNA degradation machinery, and the way in which these might relate to the translation apparatus. Overall, major challenges still to be addressed include the task of relating principles of posttranscriptional control to cellular compartmentalization and polysome structure and the role of molecular channelling in these highly complex expression systems.

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.

Phosphorylation of the translational regulator, PHAS-I, by protein kinase CK2

The primary site in PHAS-I for phosphorylation by protein kinase CK2 in vitro was identified as Ser111. A relatively small amount of phosphorylation of Ser99 was also detected, and mutating Ser99 to Ala in PHAS-I slightly decreased phosphorylation by CK2 in vitro. In contrast, mutating Ser111 to Ala almost abolished phosphorylation, confirming Ser111 as the preferred site for CK2. Phosphorylation of Ser111 did not decrease binding of PHAS-I to eIF4E, and results of peptide mapping experiments with PHAS-I immunoprecipitated from 32P-labeled adipocytes indicated that Ser111 was not phosphorylated in cells. These results support the conclusion that CK2 is not involved in the control of PHAS-I.

Regulation of protein synthesis by cholecystokinin in rat pancreatic acini involves PHAS-I and the p70 S6 kinase pathway

BACKGROUND & AIMS

Cholecystokinin (CCK) stimulates protein synthesis in pancreatic acini at the translational level, although the signaling mechanisms involved remain uncharacterized. Two intermediates controlling translation are p70 S6 kinase and PHAS-I. We previously showed that CCK activates p70 S6K in pancreatic acini through phosphatidylinositol 3-kinase (PI 3K). In the present study we investigated the role of PI 3K, p70 S6K, and PHAS-I in mediating CCK-stimulated protein synthesis.

METHODS

Protein synthesis was measured by [35S]methionine incorporation into pancreatic protein using acini from rats with streptozotocin-induced diabetes. p70 S6 K activity was measured. PHAS-I was identified by Western analysis. PHAS-I/eIF-4E association was measured as the amount of PHAS-I recovered after purification of translation factor eIF-4E by 7-methyl guanosine triphosphate-Sepharose.

RESULTS

Rapamycin and PI 3K inhibitors, wortmannin and LY294002, blocked CCK-stimulated p70 S6K activity. Rapamycin inhibited basal protein synthesis and blocked the increase to all CCK concentrations. Wortmannin and LY294002 dose-dependently inhibited basal and CCK-stimulated protein synthesis and also blocked insulin-stimulated protein synthesis. CCK dose-dependently increased PHAS-I phosphorylation via a rapamycin- and LY294002-sensitive pathway and decreased the amount of PHAS-I associated with eIF-4E. Rapamycin and LY294002 eliminated this effect of CCK.

CONCLUSIONS

CCK stimulation of protein synthesis in pancreatic acini is sensitive to rapamycin and PI 3K inhibitors and involves PHAS-I phosphorylation and its association with eIF-4E.

Purification and characterization of a novel physiological substrate for calcineurin in mammalian cells

Although the calcium/calmodulin-regulated protein phosphatase calcineurin has been shown to play a role in a number of intracellular processes, relatively few of the downstream phosphoproteins that are dephosphorylated by this enzyme in cells have been described. Calcineurin was previously shown to play a role in amylase secretion by rat pancreatic acinar cells and to specifically dephosphorylate a 24-kDa cytosolic protein. The present study describes the purification and characterization of this novel phosphoprotein, termed CRHSP-24 (calcium-regulated heat-stable protein with a molecular mass of 24 kDa). Microgram quantities of CRHSP-24 were purified from a large-scale rat pancreas preparation in a procedure involving heat and acid precipitation, anion-exchange chromatography, preparative electrophoresis, electroelution, and two-dimensional electrophoresis. Internal amino acid sequence was obtained from two peptides following trypsin digestion and high pressure liquid chromatography. Both sequences matched with 100% identity nucleotide sequences of expressed sequence tags from human placenta and rat PC-12 cells. Two CRHSP-24 transcripts of 0.7 and 2. 9 kilobases were detected in multiple rat tissues by Northern analysis, whereas a single 24-kDa protein was observed by Western blotting. The CRHSP-24 protein is 147 amino acids in length, is composed of nearly 14% proline, and is phosphorylated entirely on serine residues. Western analysis and 32P metabolic labeling of acini revealed CRHSP-24 to be maximally phosphorylated in control cells and to undergo a rapid sustained dephosphorylation on at least 3 serine residues in response to calcium-mobilizing stimuli. Dephosphorylation of CRHSP-24 was completely inhibited by pretreatment of acini with cyclosporin A or FK506. Furthermore, the inhibitory effects of FK506 were blocked by excess rapamycin. The ubiquitous expression of CRHSP-24 in rat tissues suggests that this novel calcineurin substrate plays a common role in calcium-mediated signal transduction.

Amino acid sufficiency and mTOR regulate p70 S6 kinase and eIF-4E BP1 through a common effector mechanism

The present study identifies the operation of a signal tranduction pathway in mammalian cells that provides a checkpoint control, linking amino acid sufficiency to the control of peptide chain initiation. Withdrawal of amino acids from the nutrient medium of CHO-IR cells results in a rapid deactivation of p70 S6 kinase and dephosphorylation of eIF-4E BP1, which become unresponsive to all agonists. Readdition of the amino acid mixture quickly restores the phosphorylation and responsiveness of p70 and eIF-4E BP1 to insulin. Increasing the ambient amino acids to twice that usually employed increases basal p70 activity to the maximal level otherwise attained in the presence of insulin and abrogates further stimulation by insulin. Withdrawal of most individual amino acids also inhibits p70, although with differing potency. Amino acid withdrawal from CHO-IR cells does not significantly alter insulin stimulation of tyrosine phosphorylation, phosphotyrosine-associated phosphatidylinositol 3-kinase activity, c-Akt/protein kinase B activity, or mitogen-activated protein kinase activity. The selective inhibition of p70 and eIF-4E BP1 phosphorylation by amino acid withdrawal resembles the response to rapamycin, which prevents p70 reactivation by amino acids, indicating that mTOR is required for the response to amino acids. A p70 deletion mutant, p70Delta2-46/DeltaCT104, that is resistant to inhibition by rapamycin (but sensitive to wortmannin) is also resistant to inhibition by amino acid withdrawal, indicating that amino acid sufficiency and mTOR signal to p70 through a common effector, which could be mTOR itself, or an mTOR-controlled downstream element, such as a protein phosphatase.

4E-BP3, a new member of the eukaryotic initiation factor 4E-binding protein family

Translation initiation in eukaryotes is mediated by the cap structure (m7GpppN, where N is any nucleotide) present at the 5' end of all cellular mRNAs, except organellar. The cap is recognized by eukaryotic initiation factor 4F (eIF4F), which consists of three polypeptides, including eIF4E, the cap-binding protein subunit. The interaction of the cap with eIF4E facilitates the binding of the ribosome to the mRNA. eIF4E activity is regulated in part by two translational repressors, 4E-BP1 and 4E-BP2, which bind to it and prevent its assembly into eIF4F. We report here the isolation of 4E-BP3, a new member of the 4E-BP family. 4E-BP3 is homologous to 4E-BP1 and 4E-BP2, exhibiting 57 and 59% identity, respectively. The homology is most striking in the middle region of the protein, which contains the eIF4E binding motif and residues that are phosphorylated in 4E-BP1. 4E-BP3 is a heat stable protein that binds to eIF4E in vitro as well as in vivo. Further, 4E-BP3 overexpression specifically reduces eIF4E-dependent translation. The overlapping function and expression of the different 4E-BP family members imply that there is redundancy in this translational control mechanism, underscoring its importance.

The mRNA 5' cap-binding protein eIF4E and control of cell growth

Control of gene expression at the translational level is important in cell growth and proliferation. Recent work has identified pathways that transmit signals from extracellular stimuli to several translation components. A key participant in regulation of translation is eIF4E, the mRNA 5' cap-binding protein. Several signalling pathways impact on the activity of eIF4E. This review will summarise recent findings on the MAP kinase signalling pathway that leads to phosphorylation of eIF4E and on pathways that regulate repression of eIF4E function. A major unresolved question is how the changes in translation modulate cell growth rate, and a working model will be discussed.

Differential Regulation of Translation and eIF4E Phosphorylation During Human Thymocyte Maturation

Activation of peripheral blood T cells by cross-linking of CD3 results in a rapid and substantial rise in translation rates and proliferation, which coincides with an increase in the cap-binding protein, eIF4E activity. In contrast, immature CD4+CD8+ double-positive (DP) thymocytes undergo apoptosis in response to anti-CD3 mAb. We have investigated translation initiation in the response of immature thymocytes to activating signals. Activation by anti-CD3 + anti-CD4 of immature CD4+CD8+ DP thymocytes results in a rapid decrease in protein synthesis. In contrast, similar treatment of CD4+ or CD8+ single-positive (SP) thymocytes results in an increase in protein synthesis. The rate of protein synthesis is linked to the phosphorylation status of eIF4E. Following anti-CD3 + anti-CD4 stimulation, eIF4E phosphorylation strongly decreases in immature DP thymocytes, whereas it increases in mature SP thymocytes. The expression of 4E-BP2, a specific repressor of eIF4E function, is high in DP cells but decreases during maturation, raising the possibility of a role for 4E-BP2 in repressing eIF4E phosphorylation. These data provide evidence for differential regulation of the translational machinery during T cell development.

Nerve and epidermal growth factor induce protein synthesis and eIF2B activation in PC12 cells

The regulation of protein synthesis and of eukaryotic initiation factor eIF2B was studied in PC12 cells. An increase in protein synthesis was observed after nerve growth factor (NGF) and epidermal growth factor (EGF) treatment of PC12 cells, and this increase coincided with activation of eIF2B. Growth factor addition in the presence of the phosphatidylinositol-3'-OH kinase inhibitor wortmannin showed that both NGF- and EGF-induced protein synthesis and eIF2B activation were phosphatidylinositol-3'-OH kinase dependent. The EGF-induced stimulation of protein synthesis and activation of eIF2B was dependent upon FK506-binding protein-rapamycin-associated protein, as shown with the immunosuppressant rapamycin, whereas NGF induction was partially dependent upon FK506-binding protein-rapamycin-associated protein. The activities of two kinases that act on eIF2B, glycogen synthase kinase-3 and casein kinase II, were measured to assess their potential roles in the activation of eIF2B in PC12 cells. Inactivation of glycogen synthase kinase-3 was seen in response to both NGF and EGF and this coincided with activation of eIF2B. However, inactivation of glycogen synthase kinase-3 was not rapamycin sensitive, in contrast to the activation of eIF2B. This indicates the involvement of another protein kinase or regulatory mechanism in the eIF2B activation. Both growth factors activated casein kinase II. However, the time course of its activation and its insensitivity to wortmannin and rapamycin suggest that casein kinase II does not play a major regulatory role in eIF2B activation under these conditions.

RAFT1 phosphorylation of the translational regulators p70 S6 kinase and 4E-BP1

The complex of rapamycin with its intracellular receptor, FKBP12, interacts with RAFT1/FRAP/mTOR, the in vivo rapamycin-sensitive target and a member of the ataxia telangiectasia mutated (ATM)-related family of kinases that share homology with the catalytic domain of phosphatidylinositol 3-kinase. The function of RAFT1 in the rapamycin-sensitive pathway and its connection to downstream components of the pathway, such as p70 S6 kinase and 4E-BP1, are poorly understood. Here, we show that RAFT1 directly phosphorylates p70(S6k), 4E-BP1, and 4E-BP2 and that serum stimulates RAFT1 kinase activity with kinetics similar to those of p70(S6k) and 4E-BP1 phosphorylation. RAFT1 phosphorylates p70(S6k) on Thr-389, a residue whose phosphorylation is rapamycin-sensitive in vivo and necessary for S6 kinase activity. RAFT1 phosphorylation of 4E-BP1 on Thr-36 and Thr-45 blocks its association with the cap-binding protein, eIF-4E, in vitro, and phosphorylation of Thr-45 seems to be the major regulator of the 4E-BP1-eIF-4E interaction in vivo. RAFT1 phosphorylates p70(S6k) much more effectively than 4E-BP1, and the phosphorylation sites on the two proteins show little homology. This raises the possibility that, in vivo, an unidentified kinase analogous to p70(S6k) is activated by RAFT1 phosphorylation and acts at the rapamycin-sensitive phosphorylation sites of 4E-BP1.

4E binding proteins inhibit the translation factor eIF4E without folded structure

The 4E binding proteins (4E-BP1 and 4E-BP2) inhibit translation by binding to the limiting, proto-oncogenic initiation factor eIF4E. 4E-BPs produced in Escherichia coli had little or no folded structure, measured by NMR and CD. However, these proteins inhibited translation in reticulocyte lysate. Furthermore, they bound to isolated mouse eIF4E, showing a few broader, dispersed new NMR signals but no general increase in chemical shift dispersion. A peptide with the sequence of 4E-BP1 residues 49-68 was sufficient to bind eIF4E and to inhibit translation in reticulocyte lysate. These results suggest that a short central region of the 4E-BPs is responsible for eIF4E binding and translation inhibition while the remainder is unfolded and flexible.

The mammalian target of rapamycin phosphorylates sites having a (Ser/Thr)-Pro motif and is activated by antibodies to a region near its COOH terminus

The eukaryotic initiation factor 4E (eIF4E)-binding protein, PHAS-I, was phosphorylated rapidly and stoichiometrically when incubated with [gamma-32P]ATP and the mammalian target of rapamycin (mTOR) that had been immunoprecipitated with an antibody, mTAb1, directed against a region near the COOH terminus of mTOR. PHAS-I was phosphorylated more slowly by mTOR obtained either by immunoprecipitation with other antibodies or by affinity purification using a rapamycin/FKBP12 resin. Adding mTAb1 to either of these preparations of mTOR increased PHAS-I phosphorylation severalfold, indicating that mTAb1 activates the mTOR protein kinase. mTAb1-activated mTOR phosphorylated Thr36, Thr45, Ser64, Thr69, and Ser82 in PHAS-I. All five of these sites fit a (Ser/Thr)-Pro motif and are dephosphorylated in response to rapamycin in rat adipocytes. Thus, our findings indicate that Pro is a determinant of the mTOR protein kinase specificity and that mTOR contributes to the phosphorylation of PHAS-I in cells.

Disruption of the gene encoding the mitogen-regulated translational modulator PHAS-I in mice

PHAS-I is the prototype of a group of eIF4E-binding proteins that can regulate mRNA translation in response to hormones and growth factors. To investigate the importance of PHAS-I in the physiology of the intact animal, we disrupted the PHAS-I gene in mice. Tissues and cells derived from the knockout mice contained no detectable PHAS-I protein. A related protein, PHAS-II, and eIF4E were readily detectable in tissues from these animals, but neither appeared to be changed in a compensatory manner. Mice lacking PHAS-I appeared normal at birth. However, male knockout mice weighed approximately 10% less than controls at all ages, whereas female weights were similar to those of controls. Both males and females were fertile. Tissues from adult animals appeared to be normal by routine histological staining techniques, as were routine blood cell counts and chemistries. Fibroblasts derived from PHAS-I-deficient mouse embryos exhibited normal rates of growth and overall protein synthesis, responded normally to serum stimulation of ornithine decarboxylase activity and cell growth, and rapamycin inhibition of cell growth. Under these experimental conditions, PHAS-I is apparently not required for the normal development and reproductive behavior of female mice, but is required for normal body weight in male mice; the mechanisms responsible for this phenotype remain to be determined.

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.

Inactivation of eIF2B and phosphorylation of PHAS-I in heat-shocked rat hepatoma cells

Various factors are involved in the heat shock-induced inhibition of protein synthesis. Changes upon heat shock in phosphorylation, leading to inactivation, of eukaryotic initiation factors (eIFs) eIF2 and eIF4E have been shown for several cell types. However, in mammalian cells these changes occur at temperatures of 43 degrees C or higher while protein synthesis is already affected at milder heat shock temperatures. In searching for the cause for the inhibition of protein synthesis, the regulation of eIF2 and eIF4E by additional factors was analyzed. In this respect, the activity of eIF2B was measured during and after heat shock. A very clear correlation was found between the activity of this guanine exchange factor and the levels of protein synthesis, also at mild heat shock conditions. Changes in the phosphorylation of eIF4E and of the eIF4E-binding protein PHAS-I were also analyzed. Surprisingly, in H35 cells as well as in some other cell lines, PHAS-I phosphorylation was increased by heat shock, whereas in others it was decreased. Therefore, decreasing the eIF4E availability under stressful conditions does not seem to be a general mechanism to inhibit protein synthesis by heat shock. Regulation of eIF2B activity appears to be the main mechanism to control translation initiation after heat shock at mild temperatures.

Regulation of the ERK subgroup of MAP kinase cascades through G protein-coupled receptors

The extracellularly-responsive kinase (ERK) subfamily of mitogen-activated protein kinases (MAPKs) has been implicated in the regulation of cell growth and differentiation. Activation of ERKs involves a two-step protein kinase cascade lying upstream from ERK, in which the Raf family are the MAPK kinase kinases and the MEK1/MEK2 isoforms are the MAPK kinases. The linear sequence of Raf --> MEK --> ERK constitutes the ERK cascade. Although the ERK cascade is activated through growth factor-regulated receptor protein tyrosine kinases, they are also modulated through G protein-coupled receptors (GPCRs). All four G protein subfamilies (Gq/11 Gi/o, Gs and G12/13) influence the activation state of ERKs. In this review, we describe the ERK cascade and characteristics of its activation through GPCRs. We also discuss the identity of the intervening steps that may couple agonist binding at GPCRs to activation of the ERK cascade.

Rapamycin inhibits alpha 1-adrenergic receptor-stimulated cardiac myocyte hypertrophy but not activation of hypertrophy-associated genes. Evidence for involvement of p70 S6 kinase

The 70-kD S6 kinase (p70S6K) has been implicated in the regulation of protein synthesis in many cell types and in the angiotensin II-stimulated hypertrophy of cardiac myocytes. Our purpose was to determine whether p70S6K plays a role in cardiomyocyte hypertrophy induced by the alpha 1-adrenergic receptor (alpha 1-AR) agonist phenylephrine (PE). PE stimulated the activity of p70S6K > 3-fold, and this increase was blocked by rapamycin, an immunosuppressant macrolide that selectively inhibits p70S6K. When administered for 3 days, PE stimulated a 30% increase in total protein content, a 2-fold increase in the incorporation of [14C]phenylalanine (14C-Phe) into protein, and a 50% increase in two-dimensional myocyte area. Rapamycin pretreatment (> or = 500 pg/mL) significantly inhibited each of these PE-stimulated changes. Two days of PE treatment resulted in a 1.6-fold increase in total RNA yield per dish, a 2-fold increase in incorporation of [14C]uridine into myocyte RNA, and increases in relative mRNA levels of the hypertrophy-associated atrial natriuretic factor (ANF, 2.1-fold) and skeletal alpha-actin (SK, 2.2-fold) genes. Although rapamycin abolished the PE-stimulated increases in total RNA and incorporation of [14C]uridine, it had no effect on the induction of the ANF and SK genes. LY294002, a specific inhibitor of phosphatidylinositol 3-kinase (PI3-K) activity, inhibited PE-stimulated increases in p70S6K activity and the incorporation of labeled precursors into myocyte protein and RNA. These results demonstrate that p70S6K is activated by the hypertrophic agent PE and that a PI3-K or PI3-K-like activity is required for p70S6K activation and myocyte hypertrophy. The data suggest that p70S6K activation may be required for PE-stimulated hypertrophy of cardiac myocytes. Our results demonstrate that intracellular signaling pathways responsible for transcriptional and translational responses diverge early after alpha 1-AR stimulation in cardiac myocytes.

Phosphorylation of the translational repressor PHAS-I by the mammalian target of rapamycin

The immunosuppressant rapamycin interferes with G1-phase progression in lymphoid and other cell types by inhibiting the function of the mammalian target of rapamycin (mTOR). mTOR was determined to be a terminal kinase in a signaling pathway that couples mitogenic stimulation to the phosphorylation of the eukaryotic initiation factor (eIF)-4E-binding protein, PHAS-I. The rapamycin-sensitive protein kinase activity of mTOR was required for phosphorylation of PHAS-I in insulin-stimulated human embryonic kidney cells. mTOR phosphorylated PHAS-I on serine and threonine residues in vitro, and these modifications inhibited the binding of PHAS-I to eIF-4E. These studies define a role for mTOR in translational control and offer further insights into the mechanism whereby rapamycin inhibits G1-phase progression in mammalian cells.

Insulin activates a PD 098059-sensitive kinase that is involved in the regulation of p70S6K and PHAS-I

Incubating either Chinese hamster ovary (CHO) cells or 3T3-L1 adipocytes with insulin increased the phosphorylation of the eIF-4E-binding protein, PHAS-I. Insulin also activated p70S6K and the Erk-1 and Erk-2 isoforms of mitogen-activated protein kinase (MAP kinase). However, the concentrations of the hormone needed to activate MAP kinase were 10-100 times higher than those needed to increase PHAS-I phosphorylation and p70S6K activity. Incubating cells with the inhibitor of MAP kinase kinase (MEK) activation, PD 098059, blocked the effects of low concentrations of insulin on PHAS-I and p70S6K. The effects of the inhibitor were overcome by increasing concentrations of insulin. The results indicate that insulin activates a PD 098059-sensitive kinase that is involved in the regulation of both p70S6K and PHAS-I.

Identification of phosphorylation sites in the translational regulator, PHAS-I, that are controlled by insulin and rapamycin in rat adipocytes

Phosphorylation of PHAS-I by mitogen-activated protein (MAP) kinase in vitro decreased PHAS-I binding to eukaryotic initiation factor (eIF)-4E. The decrease in binding lagged behind the phosphorylation of PHAS-I in Ser64, the preferred site of MAP kinase. Binding of the Ala64 mutant of PHAS-I to eIF-4E was abolished by MAP kinase, indicating that phosphorylation of sites other than Ser64 control binding. To identify such sites, PHAS-I was phosphorylated with MAP kinase and [gamma-32P]ATP and then cleaved proteolytically before the resulting phosphopeptides were isolated by reverse phase chromatography and directly identified by amino acid sequencing. Phosphorylated residues were located by determining the cycles in which 32P was released when phosphopeptides were subjected to sequential Edman degradation. With an extended incubation in vitro, MAP kinase phosphorylated Thr36, Thr45, Ser64, Thr69, and Ser82. In rat adipocytes, the phosphorylation of all five sites was increased by insulin and decreased by rapamycin although there were differences in the magnitude of the effects. A form of PHAS-I phosphorylated exclusively in Thr36 remained bound to eIF-4E, indicating that phosphorylation of Thr36 is insufficient for dissociation of the PHAS-I.eIF-4E complex. In summary, our results indicate that multiple phosphorylation sites are involved in the control of PHAS-I. All five sites identified fit a (Ser/Thr)-Pro motif, suggesting that the phosphorylation of PHAS-I in cells is mediated by a proline-directed protein kinase.

PHAS-I phosphorylation in response to foetal bovine serum (FBS) is regulated by an ERK1/ERK2-independent and rapamycin-sensitive pathway in 3T3-L1 adipocytes

The phosphorylation state of PHAS-I is thought to be important in the regulation of protein synthesis initiation. PHAS-I phosphorylation significantly increases in response to growth factors and insulin. ERK1/ERK2 have previously been implicated as PHAS-I kinases. Present work utilised a specific phosphorothioate oligonucleotide antisense strategy against ERK1/ERK2 to determine whether ERK1/ERK2 mediate FBS-stimulated PHAS-I phosphorylation in vivo. Depleting > 90% of cellular ERK1/ERK2 had no effect on FBS-stimulated PHAS-I phosphorylation. However, treatment of cells with a specific p70S6k pathway inhibitor, rapamycin, markedly attenuated FBS-stimulated PHAS-I phosphorylation. These results indicate that PHAS-I phosphorylation in response to FBS occurs through an ERK1/ERK2-independent and rapamycin-sensitive pathway in 3T3-L1 adipocytes.

A new expression vector for high level protein production, one step purification and direct isotopic labeling of calmodulin-binding peptide fusion proteins

Calmodulin-binding peptide (CBP), a peptide of 26 amino acids derived from muscle myosin light chain kinase (MLCK), binds to calmodulin with nanomolar affinity. Proteins fused in frame with CBP can be purified from crude E. coli lysates in a single step using calmodulin affinity chromatography (Stofko-Hahn et al., 1992). Because the binding between CBP and calmodulin is calcium-dependent, the fusion protein can be eluted from the resin with virtually any buffer containing EGTA (2 mM) and used directly for many applications. To take full advantage of this affinity purification system, we constructed the versatile CBP fusion protein expression vector pCAL-n. The CBP coding sequence was positioned for fusion at the N-terminus, an advantage that ensures consistent high level synthesis of fusion proteins due to the efficient translation of the CBP in E. coli. The production of fusion proteins from pCAL-n is controlled by the tightly regulated T7(lac)O promoter. A versatile multiple cloning site (MCS) was included to facilitate the cloning of genes of interest. The protein coding sequence for the enzyme c-Jun N-terminal kinase (JNK) was inserted into the MCS of pCAL-n, and the resulting fusion protein CBP-JNK synthesized in E. coli cells at 15-20 mg/1 culture. CBP-JNK was purified to near homogeneity in one step with calmodulin affinity resin. Purified CBP-JNK is fully active, and the CBP peptide tag can be removed by cleavage with thrombin. We also show that CBP can be efficiently phosphorylated by cAMP-dependent protein kinase. Hence, the purified fusion proteins can be labeled directly with [gamma-32P]ATP and used to probe protein-protein or protein-nucleic acid interactions.

A novel translational repressor mRNA is edited extensively in livers containing tumors caused by the transgene expression of the apoB mRNA-editing enzyme

Transgene expression of the apolipoprotein B mRNA-editing enzyme (APOBEC-1) causes dysplasia and carcinoma in mouse and rabbit livers. Using a modified differential display technique, we identified a novel mRNA (NAT1 for novel APOBEC-1 target no. 1) that is extensively edited at multiple sites in these livers. The aberrant editing alters encoded amino acids, creates stop codons, and results in markedly reduced levels of the NAT1 protein in transgenic mouse livers. NAT1 is expressed ubiquitously and is extraordinarily conserved among species. It has homology to the carboxy-terminal portion of the eukaryotic translation initiation factor (eIF) 4G that binds eIF4A and eIF4E to form eIF4F. NAT1 binds eIF4A but not eIF4E and inhibits both cap-dependent and cap-independent translation. NAT1 is likely to be a fundamental translational repressor, and its aberrant editing could contribute to the potent oncogenesis induced by overexpression of APOBEC-1.

Insulin internalization and other signaling pathways in the pleiotropic effects of insulin

Insulin is the major anabolic hormone in humans and affects multiple cellular processes. Insulin rapidly regulates short-term effects on carbohydrate, lipid, and protein metabolism and is also a potent growth factor controlling cell proliferation and differentiation. The metabolic and growth-related effects require insulin binding to its receptor and receptor phosphorylation. Evidence suggests these events result in subsequent substrate phosphorylation and activation of multiple signaling pathways involving Src homology domain-containing proteins and the internalization of the insulin:receptor complex. The role of insulin internalization in insulin action is largely speculative. For more than two decades, extensive investigation has been carried out by numerous laboratories of the mechanisms by which insulin causes its pleiotropic responses and the cellular processing of insulin receptors. This chapter reviews our current knowledge of the phosphorylation signaling pathways activated by insulin and presents evidence that substrates other than insulin receptor substrate-1 are involved in insulin's regulation of immediate-early gene expression. We also review the mechanisms involved in insulin internalization and present evidence that internalization may play a key role in insulin action through both signal transduction processes and translocation of insulin to the cell cytoplasm and nucleus.

Purification and identification of a 28-kDa calcium-regulated heat-stable protein. A novel secretagogue-regulated phosphoprotein in exocrine pancreas

This study reports the purification and identification of a novel 28 kDa phosphoprotein from rat pancreatic acini, previously described as being highly regulated by calcium mobilizing secretagogues, which we have designated calcium-regulated heat-stable protein 28 (CRHSP-28). Internal amino acid sequences of purified CRHSP-28 were obtained following trypsin digestion and found to match with >95% identity the predicted amino acid sequence of a novel cDNA recently identified as being highly expressed in human breast carcinomas. Verification that this cDNA codes for human CRHSP-28 was demonstrated by the ability of antiserum raised against purified rat CRHSP-28 to recognize the recombinant human protein when expressed in bacteria. Furthermore, this antibody was found to specifically react with CRHSP-28 in rat acini following one- and two-dimensional electrophoresis and underwent a marked acidic shift in mobility after cholecystokinin stimulation, a phenomenon indicative of an increase in its phosphorylation. CRHSP-28 is predicted to be extremely hydrophilic, is phosphorylated entirely on serine residues, and bears little homology to any known proteins. Finally, the distribution of the CRHSP-28 protein in various rat tissues revealed that although it was present at low levels in almost all tissues, it was most highly expressed in pancreas, followed by the gastric, intestinal, and colonic mucosa. In view of its relative abundance throughout the digestive system and its apparent regulation by calcium-mobilizing agents, this protein may provide valuable insight into the mechanism(s) of calcium signaling in these tissues.

Isolation and characterization of a 14.5-kDa trichloroacetic-acid-soluble translational inhibitor protein from human monocytes that is upregulated upon cellular differentiation

A trichloroacetic-acid-soluble 14.5-kDa protein (p14.5) has been isolated from human mononuclear phagocytes (MNP) by a combination of trichloroacetic acid extraction, preparative electrophoresis and hydrophobic affinity chromatography; five tryptic peptides were subjected to protein sequencing. The full-length cDNA of the protein was cloned and sequenced from a lambda gt11 human liver library. The cDNA showed a remarkable similarity to a rat protein preferentially expressed in hepatocytes and renal tubular epithelial cells. The encoded protein is 137 amino acids long and similar to members of a new hypothetical family of small proteins with presently unknown function, named YER057c/YJGF. Human recombinant p14.5 inhibits in vitro protein synthesis in a rabbit reticulocyte lysate system. Unlike other inhibitors of protein synthesis, p14.5 is not phosphorylated despite the presence of putative phosphorylation sites. The p14.5 mRNA is weakly expressed in freshly isolated monocytes but is significantly upregulated when these monocytes are subjected to differentiation. This is also reflected by a differentiation-dependent increase in the protein concentration as demonstrated by immunoblots from cytosolic fractions and fluorescence-activated flow cytometry of permeabilized cells. A differentiation-dependent mRNA and protein expression of p14.5 is further suggested by the observation of a low expression in a variety of liver and kidney tumor cells and a high expression in fully differentiated cells as assessed by immunohistochemistry and northern blots. The highest mRNA expression was found in hepatocytes and renal distal tubular epithelial cells and only weak expression was found in other human tissues as evaluated by northern blot analysis. The preferential localization of the immunoreaction product seemed to be cytoplasmatic but, in less differentiated cells, nuclear labeling was occasionally visible. Immunoblotting of subcellular fractions confirmed these data. The high degree of evolutionary conservation of p14.5, the considerable upregulation during cellular differentiation and its potential role as a translational inhibitor may reflect an involvement in basic cellular mechanisms, e.g. a differentiation-dependent regulation of protein synthesis in hepatocytes, renal tubular epithelial cells, smooth muscle cells and MNP.

Rapamycin stimulates viral protein synthesis and augments the shutoff of host protein synthesis upon picornavirus infection

The immunosuppressant drug rapamycin blocks progression of the cell cycle at G1 in mammalian cells and yeast. We recently showed that rapamycin inhibits both in vitro and in vivo cap-dependent, but not cap-independent, translation. This inhibition is causally related to reduced phosphorylation and consequent activation of 4E-BP1, a repressor of the function of the cap-binding protein, eIF4E. Two members of the picornavirus family, encephalomyocarditis virus and poliovirus, inhibit phosphorylation of 4E-BP1. Since translation of picornavirus mRNAs is cap independent, inhibition of phosphorylation of 4E-BP1 could contribute to the shutoff of host protein synthesis. Here, we show that rapamycin augments both the shutoff of host protein synthesis and the initial rate of synthesis of viral proteins in cells infected with encephalomyocarditis virus and poliovirus.

Control of the translational regulators PHAS-I and PHAS-II by insulin and cAMP in 3T3-L1 adipocytes

The eukaryotic initiation factor 4E (eIF-4E)-binding proteins PHAS-I and PHAS-II were found to have overlapping but different patterns of expression in tissues. Both PHAS proteins were expressed in 3T3-L1 adipocytes, in which insulin stimulated their phosphorylation, promoted dissociation of PHAS.eIF-4E complexes, and decreased the ability of both to bind exogenous eIF-4E. The effects of insulin were attenuated by rapamycin and wortmannin, two agents that block activation of p70(S6K). Unlike PHAS-I, PHAS-II was readily phosphorylated by cAMP-dependent protein kinase in vitro; however, the effects of insulin on both PHAS proteins were attenuated by agents that increase intracellular cAMP, by cAMP derivatives, and by phosphodiesterase inhibitors. These agents also markedly inhibited the activation of p70(S6K). In summary, our results indicate that PHAS-I and -II are controlled by the mammalian target of rapamycin and p70(S6K) signaling pathway and that in 3T3-L1 adipocytes this pathway is inhibited by increased cAMP.

Phosphorylation of the eIF4E-binding protein PHAS-I after exposure of PC12 cells to EGF and NGF

PHAS-I or the eIF4E-binding protein 1 regulates the cap-binding activity of eIF4E by sequestering eIF4E. Binding of elF4E to PHAS-I is regulated by phosphorylation of PHAS-I. PC12 cells were used to study the signal transduction pathway leading to phosphorylation of PHAS-I. Both EGF and NGF induced phosphorylation of PHAS-I. Wortmannin, a PI-3 kinase inhibitor, staurosporine, a PKC inhibitor, and rapamycin, a FRAP inhibitor all blocked the phosphorylation of PHAS-I. Of the three inhibitors, only wortmannin was able to inhibit MAPK phosphorylation. This excludes a role for MAPK in NGF- and EGF-induced PHAS-I phosphorylation in PC12 cells. Apparently, PHAS-I was phosphorylated in a PI-3 kinase-, PKC-, and FRAP-dependent manner after EGF or NGF stimulation. Only PI-3 kinase and FRAP are involved in the regulation of the basal level of PHAS-I phosphorylation.

Cap-binding protein (eukaryotic initiation factor 4E) and 4E-inactivating protein BP-1 independently regulate cap-dependent translation

Cap-dependent protein synthesis in animal cells is inhibited by heat shock, serum deprivation, metaphase arrest, and infection with certain viruses such as adenovirus (Ad). At a mechanistic level, translation of capped mRNAs is inhibited by dephosphorylation of eukaryotic initiation factor 4E (eIF-4E) (cap-binding protein) and its physical sequestration with the translation repressor protein BP-1 (PHAS-I). Dephosphorylation of BP-I blocks cap-dependent translation by promoting sequestration of eIF-4E. Here we show that heat shock inhibits translation of capped mRNAs by simultaneously inducing dephosphorylation of eIF-4E and BP-1, suggesting that cells might coordinately regulate translation of capped mRNAs by impairing both the activity and the availability of eIF-4E. Like heat shock, late Ad infection is shown to induce dephosphorylation of eIF-4E. However, in contrast to heat shock, Ad also induces phosphorylation of BP-1 and release of eIF-4E. BP-1 and eIF-4E can therefore act on cap-dependent translation in either a mutually antagonistic or cooperative manner. Three sets of experiments further underscore this point: (i) rapamycin is shown to block phosphorylation of BP-1 without inhibiting dephosphorylation of eIF-4E induced by heat shock or Ad infection, (ii) eIF-4E is efficiently dephosphorylated during heat shock or Ad infection regardless of whether it is in a complex with BP-1, and (iii) BP-1 is associated with eIF-4E in vivo regardless of the state of eIF-4E phosphorylation. These and other studies establish that inhibition of cap-dependent translation does not obligatorily involve sequestration of eIF-4E by BP-1. Rather, translation is independently regulated by the phosphorylation states of eIF-4E and the 4E-binding protein, BP-1. In addition, these results demonstrate that BP-1 and eIF-4E can act either in concert or in opposition to independently regulate cap-dependent translation. We suggest that independent regulation of eIF-4E and BP-1 might finely regulate the efficiency of translation initiation or possibly control cap-dependent translation for fundamentally different purposes.

p21ras in carcinogenesis

The activation of p21ras proteins is required in signal transduction pathways that lead to cell proliferation. More recently, a role for p21ras proteins has also been suggested in pathways to apoptosis and in the regulation of the cell cycle. Pointmutated p21ras oncogenes code for constitutively activated p21ras proteins, which disturb the balance between cell growth and cell death in favour of cell growth. In this way, p21ras oncoproteins may contribute to carcinogenesis. The binding of growth factors to their receptors triggers a cascade of protein interactions, including activation of the p21ras proteins. In turn, p21ras proteins set the machinery for cell division in motion by stimulating different effector proteins which regulate the morphological alterations, the nutritional requirements, and the changes in gene expression necessary for cell division. The presence of p21ras oncoproteins constitutively stimulate proliferation, whilst the apoptotic pathway is suppressed along with the loss of cell cycle regulation. This review describes the function of the p21ras proteins in signal transduction pathways that control proliferation and apoptosis, and regulate the cell cycle. The dysregulation of these signal transduction pathways due to the presence of p21ras oncoproteins is discussed in the context of early carcinogenesis.

Both rapamycin-sensitive and -insensitive pathways are involved in the phosphorylation of the initiation factor-4E-binding protein (4E-BP1) in response to insulin in rat epididymal fat-cells

There is mounting evidence that in fat and other insulin-sensitive cells activation of protein synthesis may involve the dissociation of a protein (4E-BP1) from eukaryotic initiation factor (eIF)-4E thus allowing formation of the eIF-4F complex. This study compares the effects of insulin and epidermal growth factor (EGF) on the phosphorylation of 4E-BP1 in fat-cells (followed by gel-shift assays and incorporation of 32P) and on its association with eIF-4E. Several lines of evidence suggest that mitogenactivated protein kinase (MAP kinase) is not involved in these effects of insulin. Insulin causes much more extensive phosphorylation and dissociation of 4E-BP1 from eIF-4E than EGF, although EGF activates MAP kinase to a much greater extent than insulin. Moreover, MAP kinase does not phosphorylate 4E-BP1 when it is complexed with eIF-4E. In contrast, insulin activates the 40S ribosomal protein S6 kinase (p70S6K) 18-fold compared with a 2-fold activation by EGF, and the time course of this activation is similar to the phosphorylation and dissociation of 4E-BP1. Rapamycin, a specific inhibitor of the activation of this latter kinase, inhibits dissociation of 4E-BP1 from eIF-4E in cells incubated with insulin but reveals a phosphorylated from of 4E-BP1 which remains bound to eIF-4E. It is concluded that in rat epididymal fat-cells, the effects of insulin on 4E-BP1 involves multiple phosphorylation events. One phosphorylation event is rapamycin-insensitive, occurs only on bound 4E-BP1 and does not initiate dissociation. The second event does result in dissociation and is blocked by rapamycin, suggesting that the p70S6K signalling pathway is involved: p70S6K itself is probably not involved directly as this kinase does not phosphorylate 4E-BP1 in vitro.

Activation of the translational suppressor 4E-BP1 following infection with encephalomyocarditis virus and poliovirus

Infection of cells with picornaviruses, such as poliovirus and encephalomyocarditis virus (EMCV), causes a shutoff of host protein synthesis. The molecular mechanism of the shutoff has been partly elucidated for poliovirus but not for EMCV. Translation initiation in eukaryotes is facilitated by the mRNA 5' cap structure to which the multisubunit translation initiation factor eIF4F binds to promote ribosome binding. Picornaviruses use a mechanism for the translation of their RNA that is independent of the cap structure. Poliovirus infection engenders the cleavage of the eIF4G (formerly p220) component of eIF4F and renders this complex inactive for cap-dependent translation. In contrast, EMCV infection does not result in eIF4G cleavage. Here, we report that both EMCV and poliovirus activate a translational repressor, 4E-BP1, that inhibits cap-dependent translation by binding to the cap-binding subunit eIF4E. Binding of eIF4E occurs only to the underphosphorylated form of 4E-BP1, and this interaction is highly regulated in cells. We show that 4E-BP1 becomes dephosphorylated upon infection with both EMCV and poliovirus. Dephosphorylation of 4E-BP1 temporally coincides with the shutoff of protein synthesis by EMCV but lags behind the shutoff and eIF4G cleavage in poliovirus-infected cells. Dephosphorylation of 4E-BP1 by specifically inhibiting cap-dependent translation may be the major cause of the shutoff phenomenon in EMCV-infected cells.

Phosphorylation of eIF-4E on serine 209 by protein kinase C is inhibited by the translational repressors, 4E-binding proteins

Translation initiation in eukaryotes is facilitated by the mRNA 5' cap structure (m7GpppX, where X is any nucleotide) that binds the multisubunit initiation factor eIF4F through one of its subunits, eIF4E. eIF4E is a phosphoprotein whose phosphorylation state positively correlates with cell growth. Protein kinase C phosphorylates eIF4E in vitro, and possibly in vivo. Using recombinant eIF4E incubated in vitro with purified protein kinase C and analyzed by solid-phase phosphopeptide sequencing in combination with high performance liquid chromatography coupled to mass spectrometry, we demonstrated that the third amino acid of the peptide SGSTTK (Ser209) is the major site of phosphorylation. This finding is consistent with the newly assigned in vivo phosphorylation site of eIF4E (Joshi, B., Cai, A. L., Keiper, B. D., Minich, W. B., Mendez, R., Beach, C. M., Stepinski, J., Stolarski, R., Darzynkiewicz, E., and Rhoads, R. E. (1995) J. Biol. Chem. 270, 14597-14603). A S209A mutation resulted in dramatically reduced phosphorylation, both in vitro and in vivo. Furthermore, the mutant protein was phosphorylated on threonine (most probably threonine 210) in vivo. Here we show that in the presence of the recently characterized translational repressors 4E-BP1 or 4E-BP2, phosphorylation of eIF4E by protein kinase C is strongly reduced. This suggests a two-step model for the phosphorylation (and activation) of eIF4E by growth factors and hormones: first, dissociation of eIF4E from 4E-BPs, followed by eIF4E phosphorylation.

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.

Regulation of both glycogen synthase and PHAS-I by insulin in rat skeletal muscle involves mitogen-activated protein kinase-independent and rapamycin-sensitive pathways

Incubating rat diaphragm muscles with insulin increased the glycogen synthase activity ratio (minus glucose 6-phosphate/plus glucose 6-phosphate) by approximately 2-fold. Insulin increased the activities of mitogen-activated protein (MAP) kinase and the Mr = 90,000 isoform of ribosomal protein S6 kinase (Rsk) by approximately 1.5-2.0-fold. Epidermal growth factor (EGF) was more effective than insulin in increasing MAP kinase and Rsk activity, but in contrast to insulin, EGF did not affect glycogen synthase activity. The activation of both MAP kinase and Rsk by insulin was abolished by incubating muscles with the MAP kinase kinase (MEK) inhibitor, PD 098059; however, the MEK inhibitor did not significantly reduce the effect of insulin on activating glycogen synthase. Incubating muscles with concentrations of rapamycin that inhibited activation of p70S6K abolished the activation of glycogen synthase. Insulin also increased the phosphorylation of PHAS-I (phosphorylated heat- and acid-stable protein) and promoted the dissociation of the PHAS-I*eIF-4E complex. Increasing MAP kinase activity with EGF did not mimic the effect of insulin on PHAS-I phosphorylation, and the effect of insulin on increasing MAP kinase could be abolished with the MEK inhibitor without decreasing the effect of insulin on PHAS-I. The effects of insulin on PHAS-I were attenuated by rapamycin. Thus, activation of the MAP kinase/Rsk signaling pathway appears to be neither necessary nor sufficient for insulin action on glycogen synthase and PHAS-I in rat skeletal muscle. The results indicate that the effects of insulin on increasing the synthesis of glycogen and protein in skeletal muscle, two of the most important actions of the hormone, involve a rapamycin-sensitive mechanism that may include elements of the p70S6K signaling pathway.

Rapamycin blocks the phosphorylation of 4E-BP1 and inhibits cap-dependent initiation of translation

The immunosuppressant drug rapamycin blocks progression of the cell cycle at the G1 phase in mammalian cells and yeast. Here we show that rapamycin inhibits cap-dependent, but not cap-independent, translation in NIH 3T3 cells. Cap-dependent translation is also specifically reduced in extracts from rapamycin-treated cells, as determined by in vitro translation experiments. This inhibition is causally related to the dephosphorylation and consequent activation of 4E-BP1, a protein recently identified as a repressor of the cap-binding protein, eIF-4E, function. These effects of rapamycin are specific as FK506, a structural analogue of rapamycin, had no effect on either cap-dependent translation or 4E-BP1 phosphorylation. The rapamycin-FK506 binding protein complex is the effector of the inhibition of 4E-BP1 phosphorylation as excess of FK506 over rapamycin reversed the rapamycin-mediated inhibition of 4E-BP1 phosphorylation. Thus, inactivation of eIF-4E is, at least in part, responsible for inhibition of cap-dependent translation in rapamycin-treated cells. Furthermore, these results suggest that 4E-BP1 phosphorylation is mediated by the FRAP/TOR signalling pathway.