Molecular mechanisms in the control of translation by hormones and growth factors

Mechanism underlying the regulation of sortilin expression and its trafficking function

This review summarizes and analyzes the updated information on the regulation of sortilin expression and its trafficking function. Evidence indicates that the expression and function of sortilin are closely regulated at four levels: DNA, messenger RNA (mRNA), protein, and trafficking function. DNA methylation, several mutations, and minor single-nucleotide polymorphisms within DNA fragments affect the expression of SORT1 gene. A few transcription factors and microRNAs modulate its transcription as well as the splicing or stability of the mRNA. Moreover, several translation factors control the synthesis of sortilin protein, and posttranslational modifications affect its degradation processes. Multiple adaptor molecules modulate the sortilin trafficking function in the anterograde or retrograde pathway. Recent advances in the regulation of sortilin expression and function, and its related mechanisms will help the ongoing research related to sortilin and promote future clinical application via sortilin intervention.

Reduced adiposity and improved insulin sensitivity in obese mice with antisense suppression of 4E-BP2 expression

To investigate the possible role of eukaryotic initiation factor 4E-binding protein-2 (4E-BP2) in metabolism and energy homeostasis, high-fat diet-induced obese mice were treated with a 4E-BP2-specific antisense oligonucleotide (ASO) or a control 4E-BP2 ASO at a dose of 25 mg/kg body wt or with saline twice a week for 6 wk. 4E-BP2 ASO treatment reduced 4E-BP2 levels by >75% in liver and white (WAT) and brown adipose (BAT) tissues. Treatment did not change food intake but lowered body weight by approximately 7% and body fat content by approximately 18%. Treatment decreased liver triglyceride (TG) content by >50%, normalized plasma glucose and insulin levels, and reduced glucose excursion during glucose tolerance test. 4E-BP2 ASO-treated mice showed >8.5% increase in metabolic rate, >40% increase in UCP1 levels in BAT, >45% increase in beta(3)-adrenoceptor mRNA, and 40-55% decrease in mitochondrial dicarboxylate carrier, fatty acid synthase, and diacylglycerol acyltransferase 2 mRNA levels in WAT. 4E-BP2 ASO-transfected mouse hepatocytes showed an increased fatty acid oxidation rate and a decreased TG synthesis rate. In addition, 4E-BP2 ASO-treated mice demonstrated approximately 60 and 29% decreases in hepatic glucose-6-phosphatase and phosphoenolpyruvate carboxykinase mRNA, respectively, implying decreased hepatic glucose output. Furthermore, increased phosphorylation of Akt(Ser473) in both liver and fat of 4E-BP2 ASO-treated mice and increased GLUT4 levels in plasma membrane in WAT of the ASO-treated mice were observed, indicating enhanced insulin signaling and increased glucose uptake as a consequence of reduced 4E-BP2 expression. These data demonstrate for the first time that peripheral 4E-BP2 plays an important role in metabolism and energy homeostasis.

Characterization of the eIF2-associated protein p67 during brain ischemia and reperfusion

OBJECTIVES

Within the first few minutes of reperfusion after global brain ischemia, there is a severe depression of protein translation owing to phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 (eIF2). There is a 67 kDa peptide (p67) that, in its glycosylated form, binds to eIF2 and protects eIF2alpha from phosphorylation. Moreover, cells with high p67 content exhibit enhanced resistance to eIF2alpha phosphorylation. To examine the possibilities that deglycosylation of brain p67 occurs during ischemia and/or early reperfusion or that p67 deglycosylation may be more extensive in the vulnerable neurons, these experiments were undertaken to characterize the localization and activation state of p67 during early brain reperfusion

METHODS

Western blots using antibodies that recognize total p67, glycosylated p67 and phosphorylated eIF2alpha were used to characterize total p67 and glycosylated p67 during reperfusion-induced phosphorylation of eIF2alpha. We also characterized the immunohistochemical distribution of glycosylated p67 before and after brain ischemia and reperfusion.

RESULTS

There was a large increase in phosphorylated eIF2alpha, but there was no decrease in the levels of total or glycosylated p67 from those observed in controls following 10 minutes complete brain ischemia and 10 or 60 minutes subsequent reperfusion. Furthermore, there was no reduction in localized immunostaining for glycosylated p67 in vulnerable neurons during ischemia and reperfusion.

DISCUSSION

It does not appear that p67 plays a significant role in regulating the phosphorylation of eIF2alpha following transient brain ischemia.

The binding between p67 and eukaryotic initiation factor 2 plays important roles in the protection of eIF2α from phosphorylation by kinases

Phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 is the major regulatory step in the initiation of protein synthesis in mammals. P67, a cellular glycoprotein, protects phosphorylation of eIF2alpha from kinases. Previously, we reported that the D6/2 mutant of p67 has higher levels of protection of eIF2alpha phosphorylation (POEP) activity. In this study, we report that the D6/2 mutant and its double mutants containing second-site alanine substitutions at the five conserved amino acid residues (D251, D262, H331, E364, and E459) show increased POEP activity in serum-starved rat tumor hepatoma cells. Serum-restoration to those cells did not abolish their increased POEP activity except the D6/2+H331A double mutant. The latter mutant shows slight inhibition of POEP activity during serum starvation and this inhibition increased significantly during serum restoration. KRC-7 cells constitutively expressing the D6/2 mutant showed slightly decreased levels of PKR phosphorylation and significantly low level of phosphorylation of ERKs 1 and 2. The D6/2 mutant also showed increased binding with eIF2alpha and eIF2gamma and almost similar binding with ERKs 1 and 2 as compared to wild type p67. Altogether, our data demonstrate that the increased binding of the D6/2 mutant with the subunits of eIF2 may be in part the cause for its high POEP activity.

Eukaryotic initiation factor 2-associated glycoprotein, p67, shows differential effects on the activity of certain kinases during serum-starved conditions

Phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 is the major regulatory step in the initiation of protein synthesis in mammals. P67, a cellular glycoprotein, protects phosphorylation of eIF2alpha from kinases. P67 has five conserved amino acid residues at the D251, D262, H331, E364, and E459 positions. To determine the roles of these conserved amino acid residues in eIF2alpha phosphorylation during serum-starved conditions, we constitutively expressed D251A, D262A, H331A, E364A, and E459A mutants in rat tumor hepatoma cells. We find that the point mutants D251A, H331A, and E364A lower the levels of eIF2alpha phosphorylation. These low levels of phosphorylation decrease when serum-starved cells are grown in medium containing serum. To understand the mechanism of action of the p67 mutants in eIF2alpha phosphorylation during serum-starvation, we performed detailed biochemical analyses with the D251A mutant. We find that neither the O-GlcNAc modification on the D251A mutant nor the binding of D251A mutant with eIF2gamma has significant effects on eIF2alpha phosphorylation during serum-starved conditions. However, the D251A mutant inhibits p67's activity to suppress the activity of ERK1/2. Our data suggest that both p67 and the D251A mutant bind to ERK1, thus strengthening the idea that p67 regulates the activity of ERK1. During serum-starvation conditions, both PKR and PERK are phosphorylated and the D251A mutant shows increased stability of PERK as well as a slight decrease in its activity. Altogether, our data provide evidence to suggest that p67 modulates the expression and activity of certain eIF2alpha-specific kinases.

IGF-I/IGFBP-3 ameliorates alterations in protein synthesis, eIF4E availability, and myostatin in alcohol-fed rats

Chronic alcohol consumption decreases the concentration of the anabolic hormone IGF-I, and this change is associated with impaired muscle protein synthesis. The present study evaluated the ability of IGF-I complexed with IGF-binding protein (IGFBP)-3 to modulate the alcohol-induced inhibition of muscle protein synthesis in gastrocnemius. After 16 wk on an alcohol-containing diet, either the IGF-I/IGFBP-3 binary complex (BC) or saline was injected two times daily for three consecutive days. After the final injection of BC (3 h), plasma IGF-I concentrations were elevated in alcohol-fed rats to values not different from those of similarly treated control animals. Alcohol feeding decreased the basal rate of muscle protein synthesis by limiting translational efficiency. BC treatment of alcohol-fed rats increased protein synthesis back to basal control values, but the rate remained lower than that of BC-injected control rats. The BC partially reversed the alcohol-induced decrease in the binding of eukaryotic initiation factor (eIF)4E with eIF4G. This change was associated with reversal of the alcohol-induced dephosphorylation of eIF4G but was independent of changes in the phosphorylation of either 4E-BP1 or eIF4E. However, BC reversed the alcohol-induced increase in IGFBP-1 and muscle myostatin, known negative regulators of IGF-I action and muscle mass. Hence, exogenous IGF-I, administered as part of a BC to increase its circulating half-life, can in part reverse the decreased protein synthesis observed in muscle from chronic alcohol-fed rats by stimulating selected components of translation initiation. The data support the role of IGF-I as a mediator of chronic alcohol myopathy in rats.

Cyclic AMP inhibits translation of cyclin D3 in T lymphocytes at the level of elongation by inducing eEF2-phosphorylation

The purpose of the present study was to understand the mechanism by which activated protein kinase A (PKA) leads to down-regulation of cyclin D3 in lymphocytes. By using Jurkat cells as a model system, we have been able to demonstrate that cyclin D3 is reduced at the level of translation by inhibition of elongation. One of the important factors involved in translational elongation is the eukaryotic elongation factor 2 (eEF2). eEF2 promotes translation in its unphosphorylated form, and we observed a rapid phosphorylation of the eEF2-protein upon forskolin treatment. When using specific inhibitors of the eEF2-kinase prior to forskolin treatment, we were able to inhibit the increased phosphorylation of eEF2. Furthermore, inhibition of eEF2-kinase prevented the forskolin-mediated down-regulation of cyclin D3. Taken together, it appears that activation of PKA in Jurkat cells reduces the expression of cyclin D3 at the level of translational elongation by increasing the phosphorylation of eEF2 and thereby inhibiting its activity.

Negative regulation of the protection of eIF2alpha phosphorylation activity by a unique acidic domain present at the N-terminus of p67

Eukaryotic initiation factor 2 (eIF2)-associated glycoprotein, p67, has protection of eIF2alpha phosphorylation (POEP) activity, and this activity requires lysine-rich domains I and II of p67. Another unique acidic residue-rich domain is also present at the N-terminus of p67. In this study we analyzed the role of this acidic residue-rich domain in POEP activity. Our data revealed that constitutive expression of a mutant form of p67 (D6/2) in mammalian cells resulted in increased POEP activity, and this activity was partially inhibited when second-site alanine substitutions at the conserved amino acids D251, D262, E364, and E459 were introduced in the D6/2 mutant. In contrast, a similar mutation at the conserved H331 position did not show any effect on POEP activity. Individual alanine substitutions at the above conserved amino acids in wild-type p67 did not show any significant effect on POEP activity except the E459 position where alanine substitution caused approximately 50% increase in POEP activity as compared to the wild type. Although, the levels of endogenous p67 and p67-deglycosylase did not correlate with the POEP activity, we found that the D6/2 mutant of p67 was glycosylated at a higher level in mammalian cells as compared to wild-type p67. The increased POEP activity of the D6/2 mutant also correlated with the higher rate of overall protein synthesis in mammalian cells constitutively expressing this mutant form of p67. Taken together, these data suggest that the acidic residue-rich domain present at the N-terminus of p67 may have a negative role in POEP activity.

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

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

Upregulation of iNOS expression and phosphorylation of eIF-2alpha are paralleled by suppression of protein synthesis in rat hypothalamus in a closed head trauma model

When the inducible form of nitric oxide synthase (iNOS) is expressed after challenge to the nervous system, it results in abnormally high concentrations of nitric oxide (NO). Under such conditions, NO could phosphorylate the eukaryotic translation initiation factor (eIF)-2alpha, thus suppressing protein synthesis in neurons that play a role in endocrine and autonomic functions. Using the Marmarou model of traumatic brain injury (TBI), we observed a rapid increase (at 4 h after TBI) of iNOS mRNA in magno- and parvocellular supraoptic and paraventricular neurons, declining gradually by approximately 30% at 24 h and by approximately 80% at 48 h. Western analysis indicated a trend towards increased iNOS protein synthesis at 4 h, which peaked at 8 h, and tended to decrease at the later time points. At the same time points, we detected immunocytochemically the phosphorylated form of eIF-2alpha (eIF-2alpha[P]) as cytoplasmic and more often as nuclear labeling. The incidence of double-labeled [iNOS and eIF-2alpha(P)] neuronal profiles, particularly at 24 h and 48 h after TBI, was high. De novo protein synthesis assessed quantitatively after infusion of 35S methionine/cysteine was reduced by approximately 20% at 4 h, remained depressed at 24 h, and did not return to control levels up to 48 h following the trauma. The results suggest that iNOS may trigger phosphorylation of eIF-2alpha, which in turn interferes with protein synthesis at the translational (ribosomal complex) and transcriptional (chromatin) levels. The depression in protein synthesis may include downregulation of iNOS itself, which could be an autoregulatory inhibitory feedback mechanism for NO synthesis. Excessive amounts of NO may also participate in dysfunction of hypothalamic circuits that underlie endocrine and autonomic alterations following TBI.

Phosphorylation of elongation factor-2 kinase on serine 499 by cAMP-dependent protein kinase induces Ca2+/calmodulin-independent activity

Elongation factor-2 kinase (eEF-2K) negatively regulates mRNA translation via the phosphorylation and inactivation of elongation factor-2 (eEF-2). We have shown previously that purified eEF-2K can be phosphorylated in vitro by cAMP-dependent protein kinase (PKA) and that this induces significant Ca(2+)/calmodulin (CaM)-independent eEF-2K activity [Redpath and Proud (1993) Biochem. J. 293, 31-34]. Furthermore, elevation of cAMP levels in adipocytes also increases the level of Ca(2+)/CaM-independent eEF-2K activity to a similar extent, providing a mechanistic link between elevated cAMP and the inhibition of protein synthesis [Diggle, Redpath, Heesom and Denton (1998) Biochem. J. 336, 525-529]. Here we describe the expression of glutathione S-transferase (GST)-eEF-2K fusion protein and the identification of two serine residues that are phosphorylated by PKA in vitro. Endoproteinase Arg-C digestion of GST-eEF-2K produced two phosphopeptides that were separated by HPLC and sequenced. (32)P Radioactivity release from these peptides indicated that the sites of phosphorylation were Ser-365 and Ser-499, both of which lie C-terminal to the catalytic domain. Mutation of these sites to non-phosphorylatable residues indicated that both sites need to be phosphorylated to induce Ca(2+)/CaM-independent eEF-2K activity in vitro. However, expression of Myc-tagged eEF-2K in HEK 293 cells, followed by treatment with chlorophenylthio-cAMP (CPT-cAMP), showed that Ser-499 phosphorylation alone induced Ca(2+)/CaM-independent eEF-2K activity in cells. Co-expression of wild-type eEF-2K with luciferase resulted in a 2-3-fold reduction in luciferase expression. Expression of eEF-2K S499D resulted in a 10-fold reduction in luciferase expression despite the fact that this mutant was expressed at very low levels. This indicates that eEF-2K S499D is constitutively active when expressed in cells, thus leading to the suppression of its own expression. Our data demonstrate an important role for the phosphorylation of Ser-499 in the activation of eEF-2K by PKA and the inhibition of protein synthesis.

Regulation of RNA polymerase III transcription during cell cycle entry

Increased rates of RNA polymerase (pol) III transcription constitute a central feature of the mitogenic response, but little is known about the mechanism(s) responsible. We demonstrate that the retinoblastoma protein RB plays a major role in suppressing pol III transcription in growth-arrested fibroblasts. RB knockout cells are compromised in their ability to down-regulate pol III following serum withdrawal. RB binds and represses the pol III-specific transcription factor TFIIIB during G(0) and early G(1), but this interaction decreases as cells approach S phase. Full induction of pol III coincides with mid- to late G(1) phase, when RB becomes phosphorylated by cyclin D- and E-dependent kinases. TFIIIB only associates with the underphosphorylated form of RB, and overexpression of cyclins D and E stimulates pol III transcription in vivo. The RB-related protein p130 also contributes to the repression of TFIIIB in growth-arrested fibroblasts. These observations provide insight into the mechanisms responsible for controlling pol III transcription during the switch between growth and quiescence.

Amino acid regulation of gene expression

The impact of nutrients on gene expression in mammals has become an important area of research. Nevertheless, the current understanding of the amino acid-dependent control of gene expression is limited. Because amino acids have multiple and important functions, their homoeostasis has to be finely maintained. However, amino-acidaemia can be affected by certain nutritional conditions or various forms of stress. It follows that mammals have to adjust several of their physiological functions involved in the adaptation to amino acid availability by regulating the expression of numerous genes. The aim of the present review is to examine the role of amino acids in regulating mammalian gene expression and protein turnover. It has been reported that some genes involved in the control of growth or amino acid metabolism are regulated by amino acid availability. For instance, limitation of several amino acids greatly increases the expression of the genes encoding insulin-like growth factor binding protein-1, CHOP (C/EBP homologous protein, where C/EBP is CCAAT/enhancer binding protein) and asparagine synthetase. Elevated mRNA levels result from both an increase in the rate of transcription and an increase in mRNA stability. Several observations suggest that the amino acid regulation of gene expression observed in mammalian cells and the general control process described in yeast share common features. Moreover, amino acid response elements have been characterized in the promoters of the CHOP and asparagine synthetase genes. Taken together, the results discussed in the present review demonstrate that amino acids, by themselves, can, in concert with hormones, play an important role in the control of gene expression.

RNA polymerase III transcription: its control by tumor suppressors and its deregulation by transforming agents

The level of RNA polymerase (pol) III transcription is tightly linked to the rate of growth; it is low in resting cells and increases following mitogenic stimulation. When mammalian cells begin to proliferate, maximal pol III activity is reached shortly before the G1/S transition; it then remains high throughout S and G2 phases. Recent data suggest that the retinoblastoma protein RB and its relatives p107 and p130 may be largely responsible for this pattern of expression. During G0 and early G1 phase, RB and p130 bind and repress the pol III-specific factor TFIIIB; shortly before S phase they dissociate from TFIIIB, allowing transcription to increase. At the end of interphase, when cells enter mitosis, pol III transcription is again suppressed; this mitotic repression is achieved through direct phosphorylation of TFIIIB. Thus, pol III transcription levels fluctuate as mammalian cells cycle, being high in S and G2 phases and low during mitosis and early G1. In addition to this cyclic regulation, TFIIIB can be bound and repressed by the tumor suppressor p53. Conversely, it is a target for activation by several viruses, including SV40, HBV, and HTLV-1. Some viruses also increase the activity of a second pol III-specific factor called TFIIIC. A large proportion of transformed and tumor cell types express abnormally high levels of pol III products. This may be explained, at least in part, by the very high frequency with which RB and p53 become inactivated during neoplastic transformation; loss of function of these cardinal tumor suppressors may release TFIIIB from key restraints that operate in normal cells.

Chronic alcohol feeding impairs hepatic translation initiation by modulating eIF2 and eIF4E

The present study examined potential cellular mechanisms responsible for the inhibition of protein synthesis in liver after chronic alcohol consumption. Rats were maintained on an alcohol-containing diet for 14 wk; control animals were fed isocalorically. Hepatic ATP content was not different in alcohol-fed and control animals. No alcohol-induced reduction in total hepatic RNA content (an estimate of ribosomal RNA) was detected, suggesting that alcohol decreased translational efficiency. Alcohol feeding increased the proportion of 40S and 60S ribosomal subunits in the nonpolysome-associated fraction by 30%. To identify mechanisms responsible for the impairment in initiation, several eukaryotic initiation factors (eIF) were analyzed. Alcohol feeding decreased hepatic eIF2B activity by 36%. This reduction was associated with a 20% decrease in eIF2Bepsilon content and a 90% increase in eIF2alpha phosphorylation. Alcohol also dramatically influenced the distribution of eIF4E. Compared with pair-fed control values, alcohol feeding increased the amount of eIF4E present in the inactive 4E-binding protein 1 (4E-BP1). eIF4E complex by 80% and decreased binding of eIF4G to eIF4E by 70%. However, the phosphorylation status of 4E-BP1 and eIF4E was not altered by alcohol. Although the plasma concentrations of threonine, proline, and citrulline were mildly decreased, the circulating amount of total amino acids was not altered by alcohol feeding. In summary, these data suggest that chronic alcohol consumption impairs translation initiation in liver by altering eIF2B activity as well as eIF4F function via changes in eIF4E availability.

Insulin induces dephosphorylation of eukaryotic initiation factor 2alpha and restores protein synthesis in vulnerable hippocampal neurons after transient brain ischemia

Brain reperfusion causes prompt, severe, and prolonged protein synthesis suppression and increased phosphorylation of eukaryotic initiation factor 2alpha [eIF2alpha(P)] in hippocampal CA1 and hilar neurons. The authors hypothesized that eIF2alpha(P) dephosphorylation would lead to recovery of protein synthesis. Here the effects of insulin, which activates phosphatases, were examined by immunostaining for eIF2alpha(P) and autoradiography of in vivo 35S amino acid incorporation. Rats resuscitated from a 10-minute cardiac arrest were given 0, 2, 10 or 20 U/kg of intravenous insulin, underwent reperfusion for 90 minutes, and were perfusion fixed. Thirty minutes before perfusion fixation, control and resuscitated animals received 500 microCi/kg of 35S methionine/cysteine. Alternate 30-microm brain sections were autoradiographed or immunostained for eIF2alpha(P). Controls had abundant protein synthesis and no eIF2alpha(P) in hippocampal neurons. Untreated reperfused neurons in the CA1, hilus, and dentate gyrus had intense staining for eIF2alpha(P) and reduced protein synthesis; there was little improvement with treatment with 2 or 10 U/kg of insulin. However, with 20 U/kg of insulin, these neurons recovered protein synthesis and were free of eIF2alpha(P). These results show that the suppression of protein synthesis in the reperfused brain is reversible; they support a causal association between eIF2alpha(P) and inhibition of protein synthesis, and suggest a mechanism for the neuroprotective effects of insulin.

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.

cAMP inhibits translation by inducing Ca2+/calmodulin-independent elongation factor 2 kinase activity in IPC-81 cells

Treatment of IPC-81 cells led to inhibition of protein synthesis, which was accompanied by an increase in the average size of polysomes and a decreased rate of elongation, indicating that it involved inhibition of peptide chain elongation. This inhibition was also associated with increased phosphorylation of elongation factor eEF2 (which inhibits its activity) and enhanced Ca2+/calmodulin-independent activity of eEF2 kinase. Previous work has shown that phosphorylation of eEF2 kinase by cAMP-dependent protein kinase (cAPK) in vitro induces such activator-independent activity, and the present data show that such a mechanism can occur in intact cells to link physiological levels of cAPK activation with inhibition of protein synthesis.

Eukaryotic initiation factor 2alpha kinase and phosphatase activity during postischemic brain reperfusion

When ischemic brain is reperfused, there is in vulnerable neurons immediate inhibition of protein synthesis associated with a large increase in phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 [eIF2alpha, phosphorylated form eIF2alpha(P)]. We examined eIF2alpha kinase and eIF2alpha(P) phosphatase activity in brain homogenate postmitochondrial supernatants obtained from rats after 3 to 30 min of global brain ischemia (cardiac arrest), after 5 min of ischemia and 5 min of reperfusion (5R), and after 10 min of ischemia and 90 min reperfusion (90R). Because it has been suggested that PKR might be specifically responsible for producing eIF2alpha(P) during reperfusion, we also examined in brain homogenates from wild-type and PKR0/0 C57BL/6J x 129/SV mice the effect of 5 min of ischemia and 5 min of reperfusion on eIF2alpha(P). Cytosolic brain eIF2alpha(P) in the 5R and 90R rats was 18- and 23-fold that of nonischemic controls without any change in the rate of eIF2alpha(P) dephosphorylation. There was no change in eIF2alpha kinase activity between 3 and 30 min of ischemia but an 85% decrease in the 5R group; the 90R group was similar to controls. In wild-type and PKR0/0 mice total eIF2alpha was identical, and there was an identical 16-fold increase in eIF2alpha(P) at 5 min of reperfusion. Our observations contradict hypotheses that PKR activation, loss of eIF2alpha(P) phosphatase activity, or any general increase in eIF2alpha kinase activity are responsible for reperfusion-induced phosphorylation of eIF2alpha, and we suggest that the mechanism may involve regulation of the availability of eIF2alpha to a kinase.

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 protein-synthesis elongation-factor-2 kinase by cAMP in adipocytes

Treatment of primary rat epididymal adipocytes or 3T3-L1 adipocytes with various agents which increase cAMP led to the phosphorylation of eukaryotic translation elongation factor-2 (eEF-2). The increase in eEF-2 phosphorylation was a consequence of the activation of eEF-2 kinase (eEF-2K), which is a Ca2+/calmodulin-dependent kinase. eEF-2K was shown to be essentially inactive at less than 0.1 microM free Ca2+ when measured in cell-free extracts. Treatment of adipocytes with isoproterenol induced Ca2+-independent eEF-2K activity, and an 8-10-fold activation of eEF-2K was observed at Ca2+ concentrations of less than 0.1 microM. Increased cAMP in 3T3-L1 adipocytes led to the inhibition of total protein synthesis and decreased the rate of polypeptide-chain elongation. We also show that the phosphorylation of eEF-2 and the activity of eEF-2K are insulin-regulated in adipocytes. These results demonstrate a novel mechanism for the control of protein synthesis by hormones which act by increasing cytoplasmic cAMP.

Implication of eIF2B rather than eIF4E in the regulation of global protein synthesis by amino acids in L6 myoblasts

The present study was designed to investigate the mechanism through which leucine and histidine regulate translation initiation in L6 myoblasts. The results show that both amino acids stimulate initiation and coordinately regulate the activity of eukaryotic initiation factor eIF2B. The changes in eIF2B activity could be explained in part by modulation of the phosphorylation state of the alpha-subunit of eIF2. The activity changes might also be a result of modulation of the phosphorylation state of the eIF2B epsilon-subunit, because deprivation of either amino acid caused a decrease in eIF2Bepsilon kinase activity. Leucine, but not histidine, additionally caused a redistribution of eIF4E from the inactive eIF4E.4E-BP1 complex to the active eIF4E.eIF4G complex. The redistribution was a result of increased phosphorylation of 4E-BP1. The changes in 4E-BP1 phosphorylation and eIF4E redistribution associated with leucine deprivation were not observed in the presence of insulin. However, the leucine- and histidine-induced alterations in global protein synthesis and eIF2B activity were maintained in the presence of the hormone. Overall, the results suggest that both leucine and histidine regulate global protein synthesis through modulation of eIF2B activity. Furthermore, under the conditions employed herein, alterations in eIF4E availability are not rate-controlling for global protein synthesis but might be necessary for regulation of translation of specific mRNAs.

Amino acid availability regulates p70 S6 kinase and multiple translation factors

Incubation of Chinese hamster ovary cells without amino acids for up to 60 min caused a rapid marked decrease in p70 S6 kinase activity and increased binding of initiation factor eIF4E to its inhibitory regulator protein 4E-BP1. This was associated with dephosphorylation of 4E-BP1 and eIF4E and dissociation of eIF4E from eIF4G. All these effects were rapidly reversed by resupplying a mixture of amino acids and this was blocked by rapamycin and by inhibitors of phosphatidylinositol 3-kinase, implying a role for phosphatidylinositol 3-kinase in the signalling pathway linking amino acids with the control of p70 S6 kinase activity and the phosphorylation of these translation factors. Amino acid withdrawal also led to changes in the phosphorylation of other translation factors; phosphorylation of eIF4E decreased whereas elongation factor eEF2 became more heavily phosphorylated, each of these changes being associated with decreased activity of the factor in question. Earlier studies have suggested that protein kinase B (PKB) may act upstream of p70 S6 kinase. However, amino acids did not affect the activity of PKB, indicating that amino acids activate p70 S6 kinase through a pathway independent of this enzyme. Studies with individual amino acids suggested that the effects on p70 S6 kinase activity and translation-factor phosphorylation were independent of cell swelling. The data show that amino acid supply regulates multiple translation factors in mammalian cells.

Requirement for activation of the serine-threonine kinase Akt (protein kinase B) in insulin stimulation of protein synthesis but not of glucose transport

A wide variety of biological activities including the major metabolic actions of insulin is regulated by phosphatidylinositol (PI) 3-kinase. However, the downstream effectors of the various signaling pathways that emanate from PI 3-kinase remain unclear. Akt (protein kinase B), a serine-threonine kinase with a pleckstrin homology domain, is thought to be one such downstream effector. A mutant Akt (Akt-AA) in which the phosphorylation sites (Thr308 and Ser473) targeted by growth factors are replaced by alanine has now been shown to lack protein kinase activity and, when overexpressed in CHO cells or 3T3-L1 adipocytes with the use of an adenovirus vector, to inhibit insulin-induced activation of endogenous Akt. Akt-AA thus acts in a dominant negative manner in intact cells. Insulin-stimulated protein synthesis, which is sensitive to wortmannin, a pharmacological inhibitor of PI 3-kinase, was abolished by overexpression of Akt-AA without an effect on amino acid transport into the cells, suggesting that Akt is required for insulin-stimulated protein synthesis. Insulin activation of p70 S6 kinase was inhibited by approximately 75% in CHO cells and approximately 30% in 3T3-L1 adipocytes, whereas insulin-induced activation of endogenous Akt was inhibited by 80 to 95%, by expression of Akt-AA. Thus, Akt activity appears to be required, at least in part, for insulin stimulation of p70 S6 kinase. However, insulin-stimulated glucose uptake in both CHO cells and 3T3-L1 adipocytes was not affected by overexpression of Akt-AA, suggesting that Akt is not required for this effect of insulin. These data indicate that Akt acts as a downstream effector in some, but not all, of the signaling pathways downstream of PI 3-kinase.

The molecular basis of viral oncolysis: usurpation of the Ras signaling pathway by reovirus

NIH-3T3 cells, which are resistant to reovirus infection, became susceptible when transformed with activated Sos or Ras. Restriction of reovirus proliferation in untransformed NIH-3T3 cells was not at the level of viral gene transcription, but rather at the level of viral protein synthesis. An analysis of cell lysates revealed that a 65 kDa protein was phosphorylated in untransformed NIH-3T3 cells, but only after infection with reovirus. This protein was not phosphorylated in infected or uninfected transformed cells. The 65 kDa protein was determined to be the double-stranded RNA-activated protein kinase (PKR), whose phosphorylation leads to translation inhibition. Inhibition of PKR phosphorylation by 2-aminopurine, or deletion of the Pkr gene, led to drastic enhancement of reovirus protein synthesis in untransformed cells. The emerging picture is one in which early viral transcripts trigger PKR phosphorylation in untransformed cells, which in turn leads to inhibition of translation of viral genes; this phosphorylation event is blocked by an element(s) in the Ras pathway in the transformed cells, allowing viral protein synthesis to ensue. The usurpation of the Ras signaling pathway therefore constitutes the basis of reovirus oncolysis.

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.

Synthesis of 3-substituted benzamides and 5-substituted isoquinolin-1(2H)-ones and preliminary evaluation as inhibitors of poly(ADP-ribose)polymerase (PARP)

Inhibitors of poly(ADP-ribose)polymerase (PARP) inhibit repair of damaged DNA and thus potentiate radiotherapy and chemotherapy of cancer. 3-Substituted benzamides and 5-substituted isoquinolin-1-ones have been synthesised and evaluated for inhibition of PARP. Reduction of 3-(bromoacetyl)benzamide, followed by treatment with base, gave RS-3-oxiranylbenzamide. Reduction of 3-(hydroxyacetyl)benzonitrile with bakers' yeast gave the R-diol which was converted to R-3-(1,2-dihydroxyethyl)benzamide. Similar reduction of 3-(acetoxyacetyl)benzonitrile led towards the S-diol which was converted to its cyclic acetonide. E-2-(2,6-Dicyanophenyl)-N,N-dimethylethenamine was formed by condensation of 2,6-dicyanotoluene with dimethylformamide dimethyl acetal (DMFDMA); cyclisation under acidic conditions afforded 5-cyanoisoquinolin-1-one. Heck coupling of 5-iodoisoquinolin-1-one with propenoic acid formed E-3-(1-oxoisoquinolin-5-yl)propenoic acid. 3-Oxiranylbenzamide, 5-bromoisoquinolin-1-one and 5-iodoisoquinolin-1-one were among the most potent inhibitors of PARP activity in a preliminary screen in vitro.

A distinct difference in the metabolic stimulus-response coupling pathways for regulating proinsulin biosynthesis and insulin secretion that lies at the level of a requirement for fatty acyl moieties

The regulation of proinsulin biosynthesis in pancreatic beta-cells is vital for maintaining optimal insulin stores for glucose-induced insulin release. The majority of nutrient fuels that induce insulin release also stimulate proinsulin biosynthesis, but since insulin exocytosis and proinsulin synthesis involve different cellular mechanisms, a point of divergence in the respective metabolic stimulus-response coupling pathways must exist. A parallel examination of the metabolic regulation of proinsulin biosynthesis and insulin secretion was undertaken in the same beta-cells. In MIN6 cells, glucose-induced proinsulin biosynthesis and insulin release shared a requirement for glycolysis to generate stimulus-coupling signals. Pyruvate stimulated both proinsulin synthesis (threshold 0.13-0.2 mM) and insulin release (threshold 0.2-0.3 mM) in MIN6 cells, which was eliminated by an inhibitor of pyruvate transport (1 mM alpha-cyano-4-hydroxycinnamate). A combination of alpha-oxoisohexanoate and glutamine also stimulated proinsulin biosynthesis and insulin release in MIN6 cells, which, together with the effect of pyruvate, indicated that anaplerosis was necessary for instigating secondary metabolic stimulus-coupling signals in the beta-cell. A consequence of increased anaplerosis in beta-cells is a marked increase in malonyl-CoA, which in turn inhibits beta-oxidation and elevates cytosolic fatty acyl-CoA levels. In the beta-cell, long-chain fatty acyl moieties have been strongly implicated as metabolic stimulus-coupling signals for regulating insulin exocytosis. Indeed, it was found in MIN6 cells and isolated rat pancreatic islets that exogenous oleate, palmitate and 2-bromopalmitate all markedly potentiated glucose-induced insulin release. However, in the very same beta-cells, these fatty acids in contrast inhibited glucose-induced proinsulin biosynthesis. This implies that neither fatty acyl moieties nor beta-oxidation are required for the metabolic stimulus-response coupling pathway specific for proinsulin biosynthesis, and represent an early point of divergence of the two signalling pathways for metabolic regulation of proinsulin biosynthesis and insulin release. Therefore alternative metabolic stimulus-coupling factors for the specific control of proinsulin biosynthesis at the translational level were considered. One possibility examined was an increase in glycerophosphate shuttle activity and change in cytosolic redox state of the beta-cell, as reflected by changes in the ratio of alpha-glycerophosphate to dihydroxyacetone phosphate. Although 16.7 mM glucose produced a significant rise in the alpha-glycerophosphate/dihydroxyacetone phosphate ratio, 1 mM pyruvate did not. It follows that the cytosolic redox state and fatty acyl moieties are not necessarily involved as secondary metabolic stimulus-coupling factors for regulation of proinsulin biosynthesis. However, the results indicate that glycolysis and the subsequent increase in anaplerosis are indeed necessary for this signalling pathway, and therefore an extramitochondrial product of beta-cell pyruvate metabolism (that is upstream of the increased cytosolic fatty acyl-CoA) acts as a key intracellular secondary signal for specific control of proinsulin biosynthesis by glucose at the level of translation.

Expression and purification of the alpha-subunit of eukaryotic initiation factor eIF2: use as a kinase substrate

The alpha-subunit of eukaryotic initiation factor eIF2 (eIF2alpha) plays an important role in the regulation of mRNA translation through modulation of the interaction of eIF2 and a second initiation factor, eIF2B. The interaction of the two proteins is regulated in vivo by phosphorylation of eIF2alpha at Ser51. In the present study, rat eIF2alpha was expressed in Sf21 cells using the baculovirus expression system. The recombinant protein was purified to >90% homogeneity in a single immunoaffinity chromatographic step. The protein was free of endogenous eIF2alpha kinase activity and was rapidly phosphorylated by the eIF2alpha kinases HCR and PKR. A variant of eIF2alpha in which the phosphorylation site was changed to Ala was also expressed and purified. The variant eIF2alpha was not phosphorylated by either HCR or PKR, demonstrating that the kinases specifically phosphorylate the correct site in the recombinant protein even in the absence of the other two subunits of the protein. In summary, a rapid and inexpensive method for obtaining eIF2alpha has been developed. Use of the wildtype and variant forms of eIF2alpha to measure eIF2alpha kinase activity in cell and tissue extracts should greatly facilitate examination of the regulation of mRNA translation under a variety of conditions.

Translational control: a general mechanism for gene regulation during T cell activation

Distributional changes of individual mRNAs between free ribonucleoprotein particles (mRNP) and ribosome-bound transcripts are used to assess translational control. Simultaneous analysis of many mRNA species is required to estimate the overall contribution of translation to the regulation of gene expression. To this purpose, total cytoplasmic RNA was fractionated in sucrose step gradients and poly(A)+ RNA was prepared from mRNP and ribosome-bound fractions. Since direct, simultaneous analysis of a profusion of mRNAs is not feasible, distribution of their in vitro translation products was examined after separation in 2-dimensional gels, followed by computer-based analysis of autoradiographs. When this analysis was applied to antigenically stimulated T cells, 36% of in vitro translation products showed a greater than 10-fold increase in intensity, suggesting transcriptional activation of the corresponding mRNAs. In comparison, 7.9% of individual mRNAs (54 of 685 species) were translationally activated. They were redistributed from free mRNP to ribosome-associated fractions; 4.7% (32 species) were translationally repressed, as indicated by the opposite pattern. The differential recruitment of 12.6% of mRNA species demonstrates specificity and the general significance of translational control during T cell activation, which implies that translation may play a similar role in regulating gene expression in a variety of physiological processes.

Regulation of eukaryotic initiation factor eIF2B: glycogen synthase kinase-3 phosphorylates a conserved serine which undergoes dephosphorylation in response to insulin

Eukaryotic initiation factor eIF2B catalyses a key regulatory step in mRNA translation. eIF2B and total protein synthesis are acutely activated by insulin, and this requires phosphatidylinositol 3-kinase (PI 3-kinase). The epsilon-subunit of eIF2B is phosphorylated by glycogen synthase kinase-3 (GSK-3), which is inactivated by insulin in a PI 3-kinase-dependent manner. Here we identify the phosphorylation site in eIF2Bepsilon as Ser540 and show that treatment of eIF2B with GSK-3 inhibits its activity. Ser540 is phosphorylated in intact cells and undergoes dephosphorylation in response to insulin. This is blocked by PI 3-kinase inhibitors. Insulin-induced dephosphorylation of this inhibitory site in eIF2B seems likely to be important in the overall activation of translation by this hormone.

Signalling pathways regulating protein turnover in skeletal muscle

The protein content of skeletal muscle is determined by the relative rates of synthesis and degradation which must be regulated coordinately to maintain equilibrium. However, in conditions such as fasting where amino acids are required for gluconeogenesis, or in cancer cachexia, this equilibrium is disrupted and a net loss of protein ensues. This review, utilising studies performed in several situations, summarizes the current state of knowledge on the possible signalling pathways regulating protein turnover in skeletal muscle and highlights areas for future work.

Signal transduction pathways involved in the regulation of protein synthesis by insulin in L6 myoblasts

The phosphorylation states of three proteins implicated in the action of insulin on translation were investigated, i.e., 70-kDa ribosomal protein S6 kinase (p70S6k), eukaryotic initiation factor (eIF) 4E, and the eIF-4E binding protein 4E-BP1. Addition of insulin caused a stimulation of protein synthesis in L6 myoblasts in culture, an effect that was blocked by inhibitors of phosphatidylinositide-3-OH kinase (wortmannin), p70S6k (rapamycin), and mitogen-activated protein kinase (MAP kinase) kinase (PD-98059). The stimulation of protein synthesis was accompanied by increased phosphorylation of p70S6k, an effect that was blocked by rapamycin and wortmannin but not PD-98059. Insulin caused dephosphorylation of eIF-4E, an effect that appeared to be mediated by the p70S6k pathway. Insulin also stimulated phosphorylation of 4E-BP1 as well as dissociation of the 4E-BP1.eIF-4E complex. Both rapamycin and wortmannin completely blocked the insulin-induced changes in 4E-BP1 phosphorylation and association of 4E-BP1 and eIF-4E; PD-98059 had no effect on either parameter. Finally, insulin stimulated formation of the active eIF-4G.eIF-4E complex, an effect that was not prevented by any of the inhibitors. Overall, the results suggest that insulin stimulates protein synthesis in L6 myoblasts in part through utilization of both the p70S6k and MAP kinase signal transduction pathways.

Reduced 40S initiation complex formation in skeletal muscle during sepsis

Severe muscle wasting is a characteristic feature of sepsis. We have previously established that the rate of protein synthesis in muscles composed of fast-twitch fibers is severely diminished in response to sepsis. The present studies investigate the biochemical reactions responsible for the decreased rate of protein synthesis using gastrocnemius from control and septic rats perfused in situ. Analysis of free ribosomal subunits indicated peptide-chain initiation was impaired by infection. To characterize biochemical reactions in the pathway of peptide-chain initiation affected, the effect of sepsis on the incorporation of initiator [35S]methionyl-tRNA (met-tRNA(imet)) into the 40S initiation complex was examined. Sepsis caused a 65% decrease in the binding of radiolabelled met-tRNA(imet) to the 40S initiation complex compared with controls. The binding of met-tRNA(met) to the 40S ribosome is regulated by eukaryotic initiation factor eIF-2B, whose activity can be modulated in part by the redox state of pyridine dinucleotides. The mean cytoplasmic NADH/NAD+ ratio was increased 2 fold in sepsis, while the NADPH/NADP+ ratio was unchanged. These findings identify the formation of the 40S initiation complex as a defect in the protein synthesis machinery during sepsis. The decreased formation of the 40S initiation complex in muscle could not be explained by changes in the cytoplasmic redox state.

Heat shock increases the association of binding protein-1 with initiation factor 4E

The effects of heat shock on the regulation of the cap-binding initiation factor 4E (eIF4E) and its inhibitory binding protein, 4E-BP1, have been examined in Chinese hamster ovary cells and in cardiac myocytes. Heat shock increased the association between eIF4E and 4E-BP1, and this was associated with a dephosphorylation of 4E-BP1. These effects did not appear to be due wholly to decreased activity of the p70 S6 kinase pathway, which is implicated in the control of 4E-BP1, and they were not mediated by the stress-activated p38 microtubule-associated protein kinase pathway. Increased binding of 4E-BP1 to eIF4E correlated with a decrease in the amount of eIF4G which co-purified with the latter. This could account for the previously observed impairment of eIF4F function during heat shock, and, since heat shock protein mRNAs are believed to be relatively cap-independent, could provide a mechanism for the selective up-regulation of the synthesis of heat shock proteins and other stress proteins during heat shock.

Effect of brain ischemia and reperfusion on the localization of phosphorylated eukaryotic initiation factor 2 alpha

Postischemic brain reperfusion is associated with a substantial and long-lasting reduction of protein synthesis in selectively vulnerable neurons. Because the overall translation initiation rate is typically regulated by altering the phosphorylation of serine 51 on the alpha-subunit of eukaryotic initiation factor 2 (eIF-2 alpha), we used an antibody specific to phosphorylated eIF-2 alpha [eIF-2(alpha P)] to study the regional and cellular distribution of eIF-2(alpha P) in normal, ischemic, and reperfused rat brains. Western blots of brain postmitochondrial supernatants revealed that approximately 1% of all eIF-2 alpha is phosphorylated in controls, eIF-2(alpha P) is not reduced by up to 30 minutes of ischemia, and eIF-2(alpha P) is increased approximately 20-fold after 10 and 90 minutes of reperfusion. Immunohistochemistry shows localization of eIF-2(alpha P) to astrocytes in normal brains, a massive increase in eIF-2(alpha P) in the cytoplasm of neurons within the first 10 minutes of reperfusion, accumulation of eIF-2(alpha P) in the nuclei of selectively vulnerable neurons after 1 hour of reperfusion, and morphology suggesting pyknosis or apoptosis in neuronal nuclei that continue to display eIF-2(alpha P) after 4 hours of reperfusion. These observations, together with the fact that eIF-2(alpha P) inhibits translation initiation, make a compelling case that eIF-2(alpha P) is responsible for reperfusion-induced inhibition of protein synthesis in vulnerable neurons.

Modulation of translation initiation in rat skeletal muscle and liver in response to food intake

Protein synthesis is altered in both skeletal muscle and liver in response to nutritional status with food deprivation being associated with an inhibition of mRNA translation. In the present study, the effect of food-intake on the initiation of mRNA translation was examined in rats fasted for 18-h and then refed a complete diet. Fasting and refeeding caused alterations in translation initiation in both skeletal muscle and liver that were not associated with any detectable changes in the activity of eIF2B or in the phosphorylation state of eIF2 alpha. Instead, alterations in initiation were associated with changes in the phosphorylation state of eIF4E and/or the association of eIF4E with eIF4G as well as the eIF4E binding protein, 4E-BP1. In muscle from fasted rats, the amount of eIF4E present in an inactive complex with 4E-BP1 was increased 5-fold compared to freely fed control animals. One hour after refeeding a complete diet, the amount of 4E-BP1 bound to eIF4E was reduced to freely fed control values. Reduced association of the two proteins was the result of increased phosphorylation of 4E-BP1. Refeeding a complete diet also stimulated the binding of eIF4E to eIF4G to form the active eIF4F complex. In liver, the amount of eIF4E associated with eIF4G, but not the amount of eIF4E associated with 4E-BP1, was altered by fasting and refeeding. Furthermore, in liver, but not in skeletal muscle, fasting and refeeding resulted in modulation of the phosphorylation state of eIF4E. Overall, the results suggest that protein synthesis may be differentially regulated in muscle and liver in response to fasting and refeeding. In muscle, protein synthesis is regulated through modulation of the binding of eIF4E to eIF4G and in liver through modulation of both phosphorylation of eIF4E as well as binding of eIF4E to eIF4G.

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.

Wheat germ initiation factor 2 (WG x eIF2) decreases the inhibition in protein synthesis and eIF2B activity of reticulocyte lysates mediated by eIF2alpha phosphorylation

Phosphorylation of serine 51 residue in the alpha-subunit of eukaryotic initiation factor 2 (eIF2alpha) impairs the guanine nucleotide exchange (GNE) activity of eIF2B protein and thereby inhibits protein synthesis in mammalian systems, insects and yeast. It is not known if phosphorylation of plant eIF2 can inhibit an eIF2B-like activity. Interestingly purified wheat germ eIF2 (WG x eIF2) can exchange guanine nucleotides in vitro without the addition of any protein factor like eIF2B. It is not clear if this is due to a contaminant eIF2B-like activity associated with WG x eIF2 or because the affinity of WG x eIF2 for GDP and GTP is not markedly different. Our observations here indicate that the GNE activity of WG x eIF2 is not inhibited upon phosphorylation of the p41-42 doublet subunit in WG x eIF2 by reticulocyte eIF2alpha kinases, or in the presence of reticulocyte eIF2(alphaP) in which serine 51 residue is phosphorylated. Further, addition of WG x eIF2 reduces the inhibition in eIF2B activity, protein synthesis, and also the formation of 15S complex that occurs between reticulocyte eIF2(alphaP) and eIF2B protein in heme-deficient or poly(IC)-treated reticulocyte lysates, presumably by a mechanism of competition between wheat germ and reticulocyte eIF2 for phosphorylation. Unlike reticulocyte eIF2(alphaP), phosphorylated WG x eIF2 is unable to interact with reticulocyte eIF2B to form a 15S complex. The ability of WG x eIF2 to exchange guanine nucleotides independent of an eIF2B like protein and the inability of phosphorylated WG x eIF2 to interact with reticulocyte eIF2B suggests that WG x eIF2 is different from mammalian eIF2 and these differences may have occurred in evolution probably due to some changes in the amino acid sequences around the phosphorylation site in eIF2alpha.

The effects of pyrroloquinoline quinone on heme-regulated eIF-2alpha kinase and eIF-2B activities in eukaryotic protein synthesis

Pyrroloquinoline quinone (PQQ), a novel cofactor of biological redox processes, is ubiquitous in animal cells. We have examined the effects of PQQ on protein synthesis. PQQ inhibits protein synthesis in hemin-supplemented rabbit reticulocyte lysates. This inhibition is characterized by increased phosphorylation of eIF-2alpha and by diminished guanine nucleotide exchange activity of eIF-2B. The increased eIF-2alpha phosphorylation is the result of activation by PQQ of the heme-regulated eIF-2alpha kinase (HRI). The addition of 10 microM PQQ completely inhibits the increase in protein synthesis that occurs on the addition of hemin (20 microM) to heme-deficient lysates, whereas a lower concentration of PQQ (100 nM) causes a very slight stimulation of protein synthesis. The increased eIF-2alpha phosphorylation that occurs at high concentrations of PQQ inhibits eIF-2B activity, presumably due to formation of a 15S complex [eIF-2(alphaP).eIF-2B] in which eIF-2B becomes non-functional. Low concentrations of PQQ (0.1-1 microM) do not affect eIF-2alpha phosphorylation, but rather enhance the guanine nucleotide exchange activity of eIF-2B in reticulocyte lysates. In Chinese hamster ovary cell extract which is devoid of significant eIF-2alpha kinase activity, addition of both low and high concentrations of PQQ results in an increase in eIF-2B activity. The addition of PQQ to reticulocyte lysates activates HRI whereas addition of PQQ to purified HRI in vitro inhibits the autokinase and eIF-2alpha kinase activity of the HRI; the inhibition of purified HRI by PQQ is observed both in the presence and absence of hemin. These findings suggest that PQQ inhibits purified HRI by acting as an oxidant whereas in lysates in which PQQ is readily reduced, the PQQ acts as a reductant and increases the activities of both HRI and eIF-2B.

Activation of translation initiation factor eIF2B by insulin requires phosphatidyl inositol 3-kinase

Eukaryotic initiation factor eIF2B mediates a key regulatory step in peptide-chain initiation and is acutely activated by insulin, although, it is not clear how. Inhibitors of phosphatidylinositide 3-kinase blocked activation of eIF2B, although rapamycin, which inhibits the p70 S6 kinase pathway, did not. Furthermore, a dominant negative mutant of PI 3-kinase also prevented activation of eIF2B, while a Sos-mutant, which blocks MAP kinase activation, did not. The data demonstrate that a pathway distinct from MAP and p70 S6 kinases regulates eIF2B. Glycogen synthase kinase-3 (GSK-3) phosphorylates and inactivates eIF2B. In all cases, eIF2B and GSK-3 were regulated reciprocally. Dominant negative PI 3-kinase abolished the insulin-induced inhibition of GSK-3. These data strongly support the hypothesis that insulin activates eIF2B through a signalling pathway involving PI 3-kinase and inhibition of GSK-3.

Subunit assembly and guanine nucleotide exchange activity of eukaryotic initiation factor-2B expressed in Sf9 cells

Eukaryotic initiation factor-2B (eIF-2B) is a guanine nucleotide exchange factor (GEF) that plays a key role in the regulation of protein synthesis. In this study, we have used the baculovirus-infected Sf9 insect cell system to express and characterize the five dissimilar subunits of rat eIF-2B. GEF activity was detected in extracts of Sf9 cells expressing the epsilon-subunit alone and was greatly increased when all five subunits were coexpressed. In addition, high GEF activity was observed in extracts containing a four-subunit complex lacking the alpha-subunit. Assembly of an eIF-2B holoprotein was confirmed by coimmunoprecipitation of all five subunits. Gel filtration chromatography revealed that recombinant eIF-2B had the same molecular mass as eIF-2B purified from rat liver and that it did indeed possess GEF activity. Phosphorylation of the substrate eIF-2 inhibited the GEF activity of the five-subunit eIF-2B; this inhibition required the eIF-2B alpha-subunit. The results demonstrate that eIF-2Balpha functions as a regulatory subunit that is not required for GEF activity, but instead mediates the regulation of eIF-2B by substrate phosphorylation. Furthermore, eIF-2Bepsilon is necessary and is perhaps sufficient for GEF activity in vitro.

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.

Angiotensin II stimulates phosphorylation of the translational repressor 4E-binding protein 1 by a mitogen-activated protein kinase-independent mechanism

To investigate the molecular basis of the hypertrophic action of angiotensin II (AII) in vascular smooth muscle cells (SMC), we have examined the ability of the hormone to regulate the function of the translational repressor 4E-binding protein 1 (4E-BP1). Addition of AII to quiescent aortic SMC potently increased the phosphorylation of 4E-BP1 as revealed by a decreased electrophoretic mobility and an increased phosphate content of the protein. The stimulation of 4E-BP1 phosphorylation was maximal at 15 min and persisted up to 120 min. Results from affinity chromatography on m7GTP-agarose demonstrated that AII-induced phosphorylation of 4E-BP1 promotes its dissociation from eIF4E in target cells. Further characterization of 4E-BP1 phosphorylation by phosphoamino acid analysis and phosphopeptide mapping revealed that 4E-BP1 is phosphorylated on eight distinct peptides containing serine and threonine residues in AII-treated cells. The combination of results obtained from kinetics experiments, phosphopeptide analysis of in vitro and in vivo phosphorylated 4E-BP1, and pharmacological studies with the MAP kinase kinase inhibitor PD 98059 provided strong evidence that the MAP kinases ERK1/ERK2 are not involved in the regulation of 4E-BP1 phosphorylation in aortic SMC. Together, our results demonstrate that AII treatment of vascular SMC leads to hyperphosphorylation of the translational regulator 4E-BP1 and to its dissociation from eIF4E by a MAP kinase-independent mechanism.

The measurement of tissue protein turnover

Tissue protein turnover can be assessed by a number of semi-, quantitative and qualitative methods. There are a number of static indices of the state of turnover of protein, for example amount of RNA per DNA or protein, the state of aggregation of ribosomes (i.e. the polyribosome index), the abundance of mRNA for particular proteins, and the enzymatic activity of proteins such as proteases, ribonuclease, etc. In addition, the concentration of particular amino acids such as glutamine or non-re-utilizable amino acids, formed post-translationally, such as 3-methylhistidine or hydroxyproline, are able to provide snapshot indices. However, since turnover is a dynamic process it should, ideally, be probed using methods such as the incorporation of tracer amino acids into protein or the dilution of tracer amino acids in the free pool by protein breakdown. The combination of tracer and tissue or limb balance methods is especially powerful since all the dynamic processes can potentially be quantified. The use of stable isotopes to label metabolic tracers has dramatically increased the feasibility of carrying out measurements of protein synthesis and breakdown and there has been a substantial growth in the application of the methods to a wide variety of tissues sampled by biopsy or at operation. Summaries of a number of currently feasible methods are provided, together with commentary on the relative efficacy of the methods and of the instrumental techniques required. There is also a discussion of suitable tracer labels and amino acids, plus a summary of the most reliable current values for protein turnover in a variety of tissues. The review also contains descriptions of potential methods which have not yet been applied in human beings but which are feasible, given the current recent increases in the accuracy and sensitivity of instrumentation for measurement of stable isotope labelling.