Phosphorylation of eukaryotic initiation factor 2 during physiological stresses which affect protein synthesis

Expression of the eukaryotic translation initiation factors 4E and 2alpha in non-Hodgkin's lymphomas

Transition of cells from quiescence to proliferation requires an increase in the rate of protein synthesis, which is regulated in part by two key translation initiation factors, 4E and 2alpha. The expression and activity of both factors are increased transiently when normal resting cells are stimulated to proliferate. They are constitutively elevated in oncogene transformed cultured cells, and overexpression of either initiation factor in rodent cells makes them tumorigenic. In this study we investigate an association between the expression of translation initiation factors and lymphomagenesis. We have analyzed the expression of the protein synthesis initiation factors 4E and 2alpha by immunohistochemistry in reactive lymph nodes and several types of non-Hodgkin's lymphoma representing a wide range of clinical behaviors based on the Revised European-American Lymphoma behavioral classification. The study included 7 benign lymph nodes with follicular hyperplasia, 26 indolent lymphomas (6 marginal zone lymphomas, 7 small lymphocytic lymphomas, and 13 follicular lymphomas, grades 1 and 2), 16 moderately aggressive lymphomas (8 mantle cell lymphomas and 8 follicular lymphomas, grade 3), 24 aggressive lymphomas (14 large-B-cell lymphomas and 10 anaplastic large-cell lymphomas), and 15 highly aggressive lymphomas (7 lymphoblastic lymphomas and 8 Burkitt's lymphomas). Strong expression of initiation factors 4E and 2alpha was demonstrated in the germinal centers of reactive follicles. Minimal or no expression was seen in the mantle zones and surrounding paracortices, indicating that high expression of initiation factors 4E and 2alpha is associated with the active proliferation of lymphocytes. Most cases of aggressive and highly aggressive lymphomas showed strong expression of initiation factors 4E and 2alpha, in contrast to the cases of indolent and moderately aggressive lymphoma, in which their expression was intermediate between the germinal centers and the mantles of reactive follicles. A positive correlation was found between the expression of both initiation factors 4E and 2alpha and the Revised European-American Lymphoma behavior classification (P < 0.05). Thus, constitutively increased expression of initiation factors 4E and 2alpha may play an important role in the development of lymphomas and is correlated with their biological aggressiveness.

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.

Abnormal levels and minimal activity of the dsRNA-activated protein kinase, PKR, in breast carcinoma cells

The interferon induced, dsRNA-activated, protein kinase, PKR, is a key regulator of translational initiation, playing an important role in the regulation of cell proliferation, apoptosis and transformation. PKR levels correlate inversely with proliferative activity in several human tumor systems. This inverse relationship breaks down in human invasive ductal breast carcinomas which exhibit high levels of PKR (Haines et al., Tumor Biol. 17 (1996) 5-12). Consistent with the data from human tumors, the levels of PKR in several breast carcinoma cell lines, MCF7, T47D, BT20, MDAMB231 and MDAMB468, are paradoxically high compared to those found in the normal breast cell lines MCF10A and Hs578Bst. The activity of affinity- or immuno-purified PKR from MCF7, T47D, and BT20 cells appears to be severely attenuated, as judged by its ability to autophosphorylate, or phosphorylate eIF2 alpha. Furthermore, the activity of the kinase from breast carcinoma cells is refractory to stimulation by dsRNA or heparin. However, PKR from breast carcinoma cells remains functional with respect to its ability to bind dsRNA. The activity of PKR from MCF10A cells is reduced by prior incubation with extracts from MCF7 cells, suggesting that MCF7 extracts contain a transdominant inhibitor of PKR. Deregulation of PKR may therefore provide a mechanism for the development or maintenance of a transformed phenotype of human breast carcinomas, mimicking the effects of manipulation of PKR or eIF2 activity observed in experimental systems. Thus, breast carcinomas may provide the first indication of a role for PKR in the pathogenesis of a naturally occurring human cancer.

Suppression of protein synthesis in brain during hibernation involves inhibition of protein initiation and elongation

Protein synthesis (PS) has been considered essential to sustain mammalian life, yet was found to be virtually arrested for weeks in brain and other organs of the hibernating ground squirrel, Spermophilus tridecemlineatus. PS, in vivo, was below the limit of autoradiographic detection in brain sections and, in brain extracts, was determined to be 0.04% of the average rate from active squirrels. Further, it was reduced 3-fold in cell-free extracts from hibernating brain at 37 degreesC, eliminating hypothermia as the only cause for protein synthesis inhibition (active, 0.47 +/- 0.08 pmol/mg protein per min; hibernator, 0.16 +/- 0.05 pmol/mg protein per min, P < 0.001). PS suppression involved blocks of initiation and elongation, and its onset coincided with the early transition phase into hibernation. An increased monosome peak with moderate ribosomal disaggregation in polysome profiles and the greatly increased phosphorylation of eIF2alpha are both consistent with an initiation block in hibernators. The elongation block was demonstrated by a 3-fold increase in ribosomal mean transit times in cell-free extracts from hibernators (active, 2.4 +/- 0.7 min; hibernator, 7.1 +/- 1.4 min, P < 0.001). No abnormalities of ribosomal function or mRNA levels were detected. These findings implicate suppression of PS as a component of the regulated shutdown of cellular function that permits hibernating ground squirrels to tolerate "trickle" blood flow and reduced substrate and oxygen availability. Further study of the factors that control these phenomena may lead to identification of the molecular mechanisms that regulate this state.

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.

Thioredoxin reductase mediates cell death effects of the combination of beta interferon and retinoic acid

Interferons (IFNs) and retinoids are potent biological response modifiers. By using JAK-STAT pathways, IFNs regulate the expression of genes involved in antiviral, antitumor, and immunomodulatory actions. Retinoids exert their cell growth-regulatory effects via nuclear receptors, which also function as transcription factors. Although these ligands act through distinct mechanisms, several studies have shown that the combination of IFNs and retinoids synergistically inhibits cell growth. We have previously reported that IFN-beta-all-trans-retinoic acid (RA) combination is a more potent growth suppressor of human tumor xenografts in vivo than either agent alone. Furthermore, the IFN-RA combination causes cell death in several tumor cell lines in vitro. However, the molecular basis for these growth-suppressive actions is unknown. It has been suggested that certain gene products, which mediate the antiviral actions of IFNs, are also responsible for the antitumor actions of the IFN-RA combination. However, we did not find a correlation between their activities and cell death. Therefore, we have used an antisense knockout approach to directly identify the gene products that mediate cell death and have isolated several genes associated with retinoid-IFN-induced mortality (GRIM). In this investigation, we characterized one of the GRIM cDNAs, GRIM-12. Sequence analysis suggests that the GRIM-12 product is identical to human thioredoxin reductase (TR). TR is posttranscriptionally induced by the IFN-RA combination in human breast carcinoma cells. Overexpression of GRIM-12 causes a small amount of cell death and further enhances the susceptibility of cells to IFN-RA-induced death. Dominant negative inhibitors directed against TR inhibit its cell death-inducing functions. Interference with TR enzymatic activity led to growth promotion in the presence of the IFN-RA combination. Thus, these studies identify a novel function for TR in cell growth regulation.

Stress (heat shock) proteins: molecular chaperones in cardiovascular biology and disease

How a cell responds to stress is a central problem in cardiovascular biology. Diverse physiological stresses (eg, heat, hemodynamics, mutant proteins, and oxidative injury) produce multiple changes in a cell that ultimately affect protein structures and function. Cells from different phyla initiate a cascade of events that engage essential proteins, the molecular chaperones, in decisions to repair or degrade damaged proteins as a defense strategy to ensure survival. Accumulative evidence indicates that molecular chaperones such as the heat shock family of stress proteins (HSPs) actively participate in an array of cellular processes, including cytoprotection. The versatility of the ubiquitous HSP family is further enhanced by stress-inducible regulatory networks, both at the transcriptional and posttranscriptional levels. In the present review, we discuss the regulation and function of HSP chaperones and their clinical significance in conditions such as cardiac hypertrophy, vascular wall injury, cardiac surgery, ischemic preconditioning, aging, and, conceivably, mutations in genes encoding contractile proteins and ion channels.

An examination of the role of increased cytosolic free Ca2+ concentrations in the inhibition of mRNA translation

Mobilization of Ca2+ sequestered by the endoplasmic reticulum (ER) produces the phosphorylation of initiation factor (eIF) 2, whereas an increase in cytosolic free Ca2+ ([Ca2+]i) due to plasmalemmal Ca2+ influx increases the phosphorylation of elongation factor (eEF) 2. In nucleated mammalian cells, depletion of ER Ca2+ stores has been demonstrated to inhibit translational initiation, but evidence that increased [Ca2+]i per se causes slowing of peptide chain elongation is lacking. L-type Ca2+ channel activity of GH3 pituitary cells, which are enriched in calmodulin-dependent eEF-2 kinase, was manipulated such that the impact of [Ca2+]i on eEF-2 phosphorylation and translational rate could be examined for up to 10 min without inhibiting initiation. At 1 mM extracellular Ca2+, resting [Ca2+]i values were high (154-255 nM) and eEF-2 was phosphorylated. The Ca2+ channel antagonist, nisoldipine, lowered [Ca2+]i and reduced eEF-2 phosphorylation by half but had no effect on amino acid incorporation. The Ca2+ channel agonist, Bay K 8644, produced sustained elevations of [Ca2+]i that were associated with 25-50% increases in eEF-2 phosphorylation, but no changes in protein synthetic rates occurred. Larger Ca2+ influxes were achievable with either 25 mM KCl or KCl plus Bay K 8644. These treatments further increased eEF-2 phosphorylation (50-100% above control) and inhibited leucine incorporation by 20-70% but ATP content was reduced by 25-50% and total cell-associated Ca2+ contents rose by 3- to 13-fold. eIF-2alpha was not phosphorylated during these treatments. Addition of low concentrations of ionomycin, which do not lower ATP content, was associated with complex changes in [Ca2+]i that resembled alterations in eEF-2 phosphorylation. The inhibition of leucine incorporation in response to ionomycin, however, coincided only with the phosphorylation of eIF-2alpha, not eEF-2. It is concluded that changes in [Ca2+]i occurring in the absence of ATP depletion alter the phosphorylation state of eEF-2 but are not regulatory for mRNA translation.

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.

Regulation of guanine nucleotide exchange through phosphorylation of eukaryotic initiation factor eIF2alpha. Role of the alpha- and delta-subunits of eiF2b

The guanine nucleotide exchange activity of eIF2B plays a key regulatory role in the translation initiation phase of protein synthesis. The activity is markedly inhibited when the substrate, i. e. eIF2, is phosphorylated on Ser51 of its alpha-subunit. Genetic studies in yeast implicate the alpha-, beta-, and delta-subunits of eIF2B in mediating the inhibition by substrate phosphorylation. However, the mechanism involved in the inhibition has not been defined biochemically. In the present study, we have coexpressed the five subunits of rat eIF2B in Sf9 cells using the baculovirus system and have purified the recombinant holoprotein to >90% homogeneity. We have also expressed and purified a four-subunit eIF2B complex lacking the alpha-subunit. Both the five- and four-subunit forms of eIF2B exhibit similar rates of guanine nucleotide exchange activity using unphosphorylated eIF2 as substrate. The five-subunit form is inhibited by preincubation with phosphorylated eIF2 (eIF2(alphaP)) and exhibits little exchange activity when eIF2(alphaP) is used as substrate. In contrast, eIF2B lacking the alpha-subunit is insensitive to inhibition by eIF2(alphaP) and is able to exchange guanine nucleotide using eIF2(alphaP) as substrate at a faster rate compared with five-subunit eIF2B. Finally, a double point mutation in the delta-subunit of eIF2B has been identified that results in insensitivity to inhibition by eIF2(alphaP) and exhibits little exchange activity when eIF2(alphaP) is used as substrate. The results provide the first direct biochemical evidence that the alpha- and delta-subunits of eIF2B are involved in mediating the effect of substrate phosphorylation.

Disruption of cellular translational control by a viral truncated eukaryotic translation initiation factor 2alpha kinase homolog

Phosphorylation of eukaryotic translation initiation factor 2alpha (eIF2alpha) is a common cellular mechanism to limit protein synthesis in stress conditions. Baculovirus PK2, which resembles the C-terminal half of a protein kinase domain, was found to inhibit both human and yeast eIF2alpha kinases. Insect cells infected with wild-type, but not pk2-deleted, baculovirus exhibited reduced eIF2alpha phosphorylation and increased translational activity. The negative regulatory effect of human protein kinase RNA-regulated (PKR), an eIF2alpha kinase, on virus production was counteracted by PK2, indicating that baculoviruses have evolved a unique strategy for disrupting a host stress response. PK2 was found in complex with PKR and blocked kinase autophosphorylation in vivo, suggesting a mechanism of kinase inhibition mediated by interaction between truncated and intact kinase domains.

Regulation of protein synthesis in ventricular myocytes by vasopressin. The role of sarcoplasmic/endoplasmic reticulum Ca2+ stores

Protein synthesis in H9c2 ventricular myocytes was subject to rapid inhibition by agents that release Ca2+ from the sarcoplasmic/endoplasmic reticulum, including thapsigargin, ionomycin, caffeine, and arginine vasopressin. Inhibitions were attributable to the suppression of translational initiation and were coupled to the mobilization of cell-associated Ca2+ and the phosphorylation of eIF2alpha. Ionomycin and thapsigargin produced relatively stringent degrees of Ca2+ mobilization that produced an endoplasmic reticulum (ER) stress response. Translational recovery was associated with the induction of ER chaperones and resistance to translational inhibition by Ca2+-mobilizing agents. Vasopressin at physiologic concentrations mobilized 60% of cell-associated Ca2+ and decreased protein synthesis by 50% within 20-30 min. The inhibition of protein synthesis was exerted through an interaction at the V1 vascular receptor, was imposed at physiologic extracellular Ca2+ concentrations, and became refractory to hormonal washout within 10 min of treatment. Inhibition was found to attenuate after 30 min, with full recovery occurring in 2 h. Translational recovery did not involve an ER stress response but rather was derived from the partial repletion of intracellular Ca2+ stores. Longer exposures to vasopressin were invariably accompanied by increased rates of protein synthesis. Translational inhibition by vasopressin, but not by Ca2+-mobilizing drugs, was both preventable and reversible by treatment with phorbol ester, which reduced the extent of Ca2+ mobilization occurring in response to the hormone. Larger and more prolonged translational inhibitions occurred after down-regulation of protein kinase C. This report provides the first compelling evidence that hormonally induced mobilization of sarcoplasmic/endoplasmic reticulum Ca2+ stores is regulatory upon mRNA translation.

Potential Alu function: regulation of the activity of double-stranded RNA-activated kinase PKR

Cell stress, viral infection, and translational inhibition increase the abundance of human Alu RNA, suggesting that the level of these transcripts is sensitive to the translational state of the cell. To determine whether Alu RNA functions in translational homeostasis, we investigated its role in the regulation of double-stranded RNA-activated kinase PKR. We found that overexpression of Alu RNA by cotransient transfection increased the expression of a reporter construct, which is consistent with an inhibitory effect on PKR. Alu RNA formed stable, discrete complexes with PKR in vitro, bound PKR in vivo, and antagonized PKR activation both in vitro and in vivo. Alu RNAs produced by either overexpression or exposure of cells to heat shock bound PKR, whereas transiently overexpressed Alu RNA antagonized virus-induced activation of PKR in vivo. Cycloheximide treatment of cells decreased PKR activity, coincident with an increase in Alu RNA. These observations suggest that the increased levels of Alu RNAs caused by cellular exposure to different stresses regulate protein synthesis by antagonizing PKR activation. This provides a functional role for mammalian short interspersed elements, prototypical junk DNA.

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.

Relation of neuronal endoplasmic reticulum calcium homeostasis to ribosomal aggregation and protein synthesis: implications for stress-induced suppression of protein synthesis

Results from experiments performed with permanent non-neuronal cell lines suggest that endoplasmic reticulum (ER) calcium homeostasis plays a key role in the control of protein synthesis (PS). It has been concluded that disturbances in ER calcium homeostasis may contribute to the suppression of PS triggered by a severe metabolic stress (W. Paschen, Med. Hypoth., 47 (1996) 283-288). To elucidate how an emptying of ER calcium stores of these cells would effect PS and ribosomal aggregation of non-transformed fully differentiated cells, experiments were run on primary neuronal cell cultures. ER calcium stores were depleted by treating cells with thapsigargin (TG, a selective, irreversible inhibitor of ER Ca(2+)-ATPase), cyclopiazonic acid (CPA, a reversible inhibitor of ER Ca(2+)-ATPase), or caffeine (an agonist of ER ryanodine receptor). Changes in intracellular calcium activity were evaluated by fluorescence microscopy using fura-2-loaded cells. Protein synthesis was determined by measuring the incorporation of [3H]leucine into proteins. The degree of aggregation of ribosomes was evaluated by electron microscopy. TG induced a permanent inhibition of PS to about 10% of control which was only partially reversed within 2 h of recovery. CPA caused about 70% inhibition of PS, and PS recovered completely 60 min after treatment. Caffeine produced an inhibition of PS to about 50% of control. Loading cells with the calcium chelator BAPTA-AM (33.3 microM) alone suppressed PS without reversing TG- or caffeine-induced inhibition of PS, indicating that the suppression of PS was caused by a depletion of ER calcium stores and not by an increase in cytosolic calcium activity. TG-treatment of cells induced a complete disaggregation of polysomes which was not reversed within the 4 h recovery period following TG-treatment. After caffeine treatment of cells, we observed a heterogenous pattern of ribosomal aggregation: in some neurons ribosomes were almost completely aggregated while in other cells a significant portion of polyribosomes were disaggregated. The results indicate that a depletion of neuronal ER calcium stores disturbs protein synthesis in a similar way to the effects of transient forms of metabolic stress (ischemia, hypoglycemia or status epilepticus), thus implying that a disturbance in ER calcium homeostasis may contribute to the pathological process of stress-induced cell injury.

The double-stranded RNA-dependent protein kinase PKR: structure and function

This review describes the structure and function of the interferon (IFN)-inducible, double-stranded RNA-activated protein kinase PKR. This protein kinase has been studied extensively in recent years, and a large body of evidence has accumulated concerning its expression, interaction with regulatory RNA and protein molecules, and modes of activation and inhibition. PKR has been shown to play a variety of important roles in the regulation of translation, transcription, and signal transduction pathways through its ability to phosphorylate protein synthesis initiation factor eIF2, I-kappaB (the inhibitor of NF-kappaB), and other substrates. Expression studies involving both the wild-type protein and dominant negative mutants of PKR have established roles for the enzyme in the antiviral effects of IFNs, in the responses of uninfected cells to physiologic stresses, and in cell growth regulation. The possibility that PKR may function as a tumor suppressor and inducer of apoptosis suggests that this IFN-regulated protein kinase may be of central importance to the control of cell proliferation and transformation.

p67 transcription regulates translation in serum-starved and mitogen-activated KRC-7 cells

The regulation of protein synthesis was studied in KRC-7 cells (rat hepatoma) grown in complete medium, during serum starvation, and mitogen activation. Upon serum starvation, the cells lost almost completely p67 mRNA, p67 protein, and protein synthesis activity. After phorbol 12-myristate 13-acetate addition, the same serum-starved cells regained p67 mRNA, p67 protein, and protein synthesis activity. Also, the extracts from the serum-starved cells phosphorylated the eukaryotic initiation factor-2 (eIF-2) alpha-subunit. This eIF-2 alpha-subunit phosphorylation was not observed when the extracts from either the cells grown in complete medium or mitogen-activated cells were used (Gupta, S., Wu, S., Chatterjee, N., Ilan, J., Ilan, J., Osterman, J. C., and Gupta, N. K. (1995) Gene Expr. 5, 113-122). We now report the following. 1) The eIF-2 kinase activity was the same in the cells grown in complete medium, after serum starvation, and subsequent mitogen stimulation. However, the eIF-2 kinase in the cells grown in complete medium and also after mitogen activation of the serum-starved cells cannot phosphorylate eIF-2 alpha-subunit as these cells contain p67. After removal of endogenous p67 by p67 antibodies, the extracts from all these cells similarly phosphorylated exogenously added eIF-2. 2) None of the cell extracts showed p67 deglycosylase activity. 3) The p67 mRNA was synthesized in serum-starved cells by expression of a p67 cDNA. The appearance of p67 mRNA in the serum-starved cells was accompanied by the appearance of p67 protein. Also, the rates of protein synthesis in the serum-starved cells were restored nearly to the level observed in the confluent cells. The expression of p67 cDNA also significantly increased protein synthesis rates in the cells grown in complete medium and in mitogen-activated cells. These results show that the loss of protein synthesis activity in serum-starved cells was due to loss of p67 mRNA. The expressed p67 mRNA was stable in serum-starved cells. These results, therefore, suggest that the loss of p67 mRNA in serum-starved cells is due to loss of p67 transcription. The p67 transcription regulates translation.

Use of vertical slab isoelectric focusing and immunoblotting to evaluate steady-state phosphorylation of eIF2 alpha in cultured cells

The combination of vertical, one-dimensional isoelectric focusing and immunoblotting works very well for the evaluation of the phosphorylation state of the alpha-subunit of eIF2 using reticulocyte lysate or purified eIF2. However, the method is more difficult to apply to the analysis of eIF2 alpha phosphorylation in cultured cells. In part this reflects the fact that the protein content of cultured cell extracts is rarely as high as that found in extracts produced from reticulocytes, and in part this reflects the fact that some component(s) of cell extracts interferes with the entry of eIF2 alpha into the isoelectric focusing gel. To overcome these difficulties, we have modified the earlier method to include immunoprecipitation of eIF2 from cell extracts prior to isoelectric focusing, as well as a low sodium dodecyl sulfate concentration in the isoelectric focusing sample buffer. Since the PKR activation state and therefore the eIF2 alpha phosphorylation state change with cell density and nutritional status, we routinely set up consistent feeding schedules and recommend the collection of data over a range of cell densities.

Inhibition of translational initiation by activators of the glucose-regulated stress protein and heat shock protein stress response systems. Role of the interferon-inducible double-stranded RNA-activated eukaryotic initiation factor 2alpha kinase

Depletion of endoplasmic reticulum (ER) Ca2+ perturbs protein folding and processing within the organelle while inhibiting translational initiation through activation of the double-stranded RNA-activated eukaryotic initiation factor (eIF)-2alpha kinase (PKR) (Prostko, C. R., Dholakia, J. N., Brostrom, M. A., and Brostrom, C. O. (1995) J. Biol. Chem. 270, 6211-6215). The glucose-regulated stress protein (GRP) chaperones are subsequently induced. We now report that sodium arsenite, a prototype for stressors fostering cytoplasmic protein misfolding, also inhibits translational initiation through activation of PKR while subsequently inducing the heat shock protein (HSP) chaperones. Arsenite neither mobilized ER-associated Ca2+ nor slowed peptide chain elongation. Various HSP-inducing chemicals caused rapid phosphorylation of eIF-2alpha. When incubated with double-stranded RNA, extracts derived from arsenite-treated cells displayed greater degrees of phosphorylation of PKR and eIF-2alpha than did control extracts. Cells overexpressing a dominant negative PKR mutation resisted translational inhibition and eIF-2alpha phosphorylation in response to ER or cytoplasmic stressors. Induction of either the HSP or GRP chaperones was accompanied by development of translational tolerance to either Ca2+-mobilizing agents or arsenite. Following induction of the HSPs by arsenite, cells remained susceptible to induction of the GRPs by Ca2+-mobilizing agents. Conversely, cells possessing induced GRP contents in response to Ca2+-mobilizing agents readily induced the HSPs in response to arsenite. It is concluded that the two chaperone systems function independently except for their mutual suppression of PKR.

Changes in the phosphorylation of eukaryotic initiation factor 2 alpha, initiation factor 2B activity and translational rates in primary neuronal cultures under different physiological growing conditions

Phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 (eIF-2) is one of the best known mechanisms regulating protein synthesis in a wide range of eukaryotic cells, from yeast to human. To determine whether this mechanism operates in primary neuronal cells, we have cultured primary neuronal cells for 7 days under two optimal growing conditions, complete medium (containing 15% serum) and serum-free medium, and determined the protein synthesis rate, eukaryotic initiation 2 and 2B (eIF-2B) activities, as well as the level of phosphorylation of eIF-2. Cells cultured in serum-free medium exhibited a lower rate of protein synthesis (75%), concomitant to a decreased eIF-2 activity (71%), and slightly higher eIF-2(alpha P) levels (from 10 to 16% of total eIF-2) with respect to cells cultured in complete media. eIF-2B activity, as measured at saturating eIF-2. GDP concentrations (assay independent on the presence of eIF-2(alpha P)) was similar under the two culture conditions. When neurons cultured in serum-free medium are exposed to complete medium for only 24 h, there is a clear decrease in the phosphorylation of eIF-2 alpha (16-3%). This decrease correlates in time with an increase in the protein synthesis rate (154%), as well as eIF-2 activity (236%). The increased levels of eIF-2(alpha P), a competitive inhibitor of eIF-2B in the guanine-exchange reaction, are responsible for the decreased eIF-2B activity found in the neurons cultured in serum-free medium. Additionally, eIF-2(alpha P) is accountable for the lower effect of exogenous eIF-2B in ternary complex formation from preformed eIF-2. GDP in the serum-free media. These changes in phosphorylation of eIF-2 alpha in normal mammalian cells in response to changes in the extracellular medium are reported here for the first time.

Phosphorylation of plant eukaryotic initiation factor-2 by the plant-encoded double-stranded RNA-dependent protein kinase, pPKR, and inhibition of protein synthesis in vitro

Regulation of protein synthesis by eukaryotic initiation factor-2alpha (eIF-2alpha) phosphorylation is a highly conserved phenomenon in eukaryotes that occurs in response to various stress conditions. Protein kinases capable of phosphorylating eIF-2alpha have been characterized from mammals and yeast. However, the phenomenon of eIF2-alpha-mediated regulation of protein synthesis and the presence of an eIF-2alpha kinase has not been demonstrated in higher plants. We show that plant eIF-2alpha (peIF-2alpha) and mammalian eIF-2alpha (meIF-2alpha) are phosphorylated similarly by both the double-stranded RNA-binding kinase, pPKR, present in plant ribosome salt wash fractions and the meIF-2alpha kinase, PKR. By several criteria, phosphorylation of peIF-2alpha is directly correlated with pPKR protein and autophosphorylation levels. Significantly, pPKR is capable of specifically phosphorylating Ser51 in a synthetic eIF-2alpha peptide, a key characteristic of the eIF-2alpha kinase family. Taken together, these data support the concept that pPKR is a member of the eIF-2alpha kinase family. In addition, the inhibition of brome mosaic virus RNA in vitro translation in wheat germ lysates by the addition of double-stranded RNA, phosphorylated peIF-2alpha, meIF-2alpha, or activated human PKR suggests that plant protein synthesis may be regulated via phosphorylation of eIF-2alpha.

Activation of the double-stranded-RNA-activated protein kinase and induction of vascular cell adhesion molecule-1 by poly (I).poly (C) in endothelial cells

Double-stranded RNA (dsRNA) induces the vascular cell adhesion molecule VCAM-1 to high levels of expression in human umbilical vein endothelial (HUVE) cells. Although VCAM-1 is also induced by the cytokine interleukin 1 beta (IL-1 beta), activation of the dsRNA-activated protein kinase (PKR) occurs only in response to incubation with dsRNA but not with IL-1 beta. Incubation of HUVE cells with the synthetic dsRNA, poly (I).poly (C), activates PKR with increased autophosphorylation, increased phosphorylation of the translation factor eIF2 alpha, and increased activation of the transcription factor NF-kappa B. Promoter analysis in HUVE cells using a VCAM-1 promoter linked to CAT reporter gene demonstrates that poly (I).poly (C) responsiveness resides in the minimal VCAM-1 promoter that contains two NF-kappa B sites, and deletion of the NF-kappa B sites eliminates basal and poly (I).poly (C)-induced CAT activity, supporting the importance of NF-kappa B in the poly (I).poly (C)-mediated induction of VCAM-1. In vitro studies using purified reagents demonstrate that PKR is capable of phosphorylating I kappa B alpha (the inhibitory subunit of NF-kappa B) in a dsRNA-dependent manner. This suggests that phosphorylation of I kappa B alpha by PKR could be an initial step in the activation of NF-kappa B by dsRNA. NF-kappa B is also activated by IL-1 beta in HUVE cells, but this activation occurs without increased PKR autophosphorylation or eIF2 alpha phosphorylation. Poly (I).poly (C) induces VCAM-1 mRNA levels that are dramatically higher and sustained longer than levels induced by IL-1 beta. Although phosphorylation of eIF2 alpha interferes with protein translation, sufficient VCAM-1 mRNA translation occurs in response to poly (I).poly (C) to yield VCAM-1 protein levels that are similar to levels that are induced by IL-1 beta. This suggests that the higher, sustained VCAM-1 mRNA levels that occur in response to incubation with poly (I).poly (C) compensate for the partial translational block resulting from increased eIF2 alpha phosphorylation. These studies indicate that transcriptional and translational regulatory events that occur in response to activation of PKR by dsRNA are important in the regulation of VCAM-1 gene expression in HUVE cells.

Nuclear-envelope nucleoside triphosphatase kinetics and mRNA transport following brain ischemia and reperfusion

STUDY HYPOTHESIS

We attempted to determine whether the reduced egress of mRNA from brain nuclei following in vivo ischemia and reperfusion is caused by direct damage to the nuclear pore-associated NTPase that impairs the system for nuclear export of polyadenylated, or poly(A)+, mRNA.

DESIGN

Prospective animal study.

INTERVENTIONS

NTPase activity and poly(A)+ mRNA transport were studied in nuclear envelope vesicles (NEVs) prepared from canine parietal cortex isolated after 20 minutes of ischemia or 20 minutes of ischemia and 2 or 6 hours of reperfusion.

RESULTS

Brain NEV NTPase Michaelis-Menten constant (Km) and maximum uptake velocity (Vmax) and the ATP-stimulated poly(A)+ mRNA egress rates were not significantly affected by ischemia and reperfusion. In vitro exposure of the NEVs to the OH. radical-generating system completely abolished NTPase activity.

CONCLUSION

We conclude that brain ischemia and reperfusion do not induce direct inhibition of nucleocytoplasmic transport of poly(A)+ mRNA. This suggests that the nuclear membrane is not exposed to significant concentrations of OH. radical during reperfusion.

Independent signaling of grp78 gene transcription and phosphorylation of eukaryotic initiator factor 2 alpha by the stressed endoplasmic reticulum

Perturbation of endoplasmic reticular (ER) function signals increased expression of the gene encoding the ER resident chaperone Grp78/BiP and rapid suppression of translational initiation accompanied by phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 (eIF-2). eIF-2 alpha phosphorylation and grp78 mRNA induction were measured in GH3 pituitary cells subjected to varied degrees of ER stress to ascertain whether activation of an eIF-2 alpha kinase is involved in both events. grp78 mRNA was induced at low concentrations of ionomycin and dithiothreitol that did not provoke eIF-2 alpha phosphorylation or inhibition of amino acid incorporation. Mobilization of the bulk of cell-associated Ca2+ and the induction of grp78 mRNA occurred at comparable low concentrations of ionomycin, whereas phosphorylation of eIF-2 alpha and inhibition of protein synthesis required higher ionophore concentrations. Pretreatment for 1 h with cycloheximide suppressed grp78 mRNA induction and eIF-2 alpha phosphorylation in response to either stressor. Prolonged (17 h) cycloheximide blockade increased eIF-2 alpha phosphorylation without inducing grp78 mRNA. Upon release from the blockade, grp78 mRNA was induced and eIF-2 alpha was dephosphorylated. Translational tolerance to ionomycin or dithiothreitol, accompanied by dephosphorylation of eIF-2 alpha, was observed whenever grp78 mRNA was induced. Induction of grp78 mRNA preceded significant eIF-2 alpha phosphorylation during treatment with brefeldin A. It is concluded that signaling of grp78 gene transcription can occur independently of eIF-2 alpha phosphorylation or translational repression and that greater degrees of ER stress are required for eIF-2 alpha phosphorylation than for grp78 mRNA induction.

Molecular cloning of a human complementary DNA encoding an initiation factor 2-associated protein (p67)

Rat p67 is an initiation factor-2 associated protein, which plays important roles in translational regulation. A cDNA that encodes a homologue of rat p67 was isolated from a human liver cDNA library. The encoded protein contains 478 amino acids with a calculated molecular mass of 52,891 and an isoelectric point of 5.64. The amino acid sequence is 92% identical to that of rat p67. The corresponding mRNA has a size of approximately 2.1 kb and is present in all tested human tissues.

Purification and characterization of eukaryotic translational initiation factor eIF-2B from liver

Eukaryotic initiation factor (eIF)-2B was purified to greater than 95% homogeneity from both rat and bovine liver. The purified protein consisted of five nonidentical subunits with apparent molecular weights ranging from 30.9 to 89.1 kDa. The holoprotein was characterized in terms of its Stokes radius and frictional coefficient. The isoelectric points for the beta-, gamma-, and epsilon-subunits were found to be 6.4, 6.9, and approximately 6.0, respectively; the alpha- and delta-subunits did not focus well because their isoelectric points as predicted by the nucleotide sequences of cDNAs for the two proteins are greater than 8.5. The purified protein was used as antigen to generate monoclonal antibodies to the epsilon-subunit. The eIF-2B epsilon monoclonal antibodies and monoclonal antibodies to the alpha-subunit of eIF-2 were then used to directly quantitate the amounts of eIF-2B and eIF-2 in rat liver and rat reticulocytes. The ratio of eIF-2B to eIF-2 was found to be approx. 0.6 and 0.3 in liver and reticulocytes, respectively, supporting the proposition that phosphorylation of only part of the total cellular eIF-2 could potentially sequester all of the eIF-2B into an inactive eIF-2.eIF-2B complex. The purified protein was also used as substrate in protein kinase assays. Extracts of rat liver were shown to contain protein kinase activity directed toward the epsilon-subunit, but no other subunit of eIF-2B. Overall, the studies presented here are the first to show a direct quantitation of eIF-2 and eIF-2B in different tissues. They also provide evidence that the epsilon-subunit of eIF-2B is the only subunit of eIF-2B that is phosphorylated by protein kinase(s) present in extracts of rat liver.

The RNA-binding properties of protein synthesis initiation factor eIF-2

Protein synthesis initiation factor eIF-2 bound ATP in the presence or absence of Mg2+ ions. ATP impaired the binding of GTP or GDP to eIF-2. However, excess GTP did not significantly decrease the binding of ATP to eIF-2, suggesting eIF-2 has distinct ATP and GTP binding sites. Highly purified eIF-2 can bind mRNA, and this did not require the mRNA to be capped. mRNA binding was saturable, and maximal binding corresponded to about 0.4 mol mRNA bound per mol eIF-2. GTP, and, at lower concentrations, GDP, inhibited the binding of mRNA to eIF-2. In addition, ATP and other nucleoside triphosphates decreased mRNA binding. The implications of these findings for the structure and function of eIF-2 are discussed. Preparations of eIF-2 deficient in the beta-subunit showed reduced ability to bind mRNA, suggesting that while it is not essential for mRNA binding, this subunit is involved in the interaction. Consistent with this is the observation that ultraviolet crosslinking of mRNA to eIF-2 resulted primarily in labelling of the beta-subunit. Subsequent analysis revealed that mRNA was cross-linked to the C-terminal region of eIF-2b which contains a putative Zn-finger structure.

Translational regulation of the heat shock response

All organisms from bacteria to man respond to an exposure to higher than physiological temperatures by reprogramming their gene expression, leading to the increased synthesis of a unique set of proteins termed heat shock proteins (hsps). The hsps function as molecular chaperones in both normal and stressed cells. The rapid and efficient synthesis of hsps is achieved as a result of changes occurring at gene transcription, RNA processing and degradation, and mRNA translation. With regard to the translational regulation, the emerging picture is that the two key steps of polypeptide chain initiation, namely mRNA binding and Met-tRNA(i) binding to ribosomes, are regulated in heat-shocked mammalian cells. In Drosophila, mRNA binding is regulated by a structural feature of the leader of heat shock mRNAs and by the inactivation of eukaryotic initiation factor- (eIF-) 4F. No clear evidence for changes in Met-tRNA(i) binding has been obtained yet.

Rat fibroblasts transfected with the human 70-kDa heat shock gene exhibit altered translation and eukaryotic initiation factor 2 alpha phosphorylation following heat shock

Heat shock inhibits translation in a wide variety of cells. After heating, eukaryotic initiation factor 2-alpha (eIF-2 alpha) becomes phosphorylated which prevents the binding of Met-tRNA to the 40s ribosomal subunit inhibiting initiation of translation. Thermotolerant cells demonstrate resistance to inhibition of translation by additional heating suggesting that heat shock proteins may help to maintain translational integrity following thermal stress. Here we have examined the effects of increased intracellular levels of hsp70 protein on translation and eIF-2 alpha phosphorylation using rat fibroblasts stably transfected with a cloned human hsp70 gene. We observed a decrease in the rate of translational inhibition following heat shock in both hsp70-transfected and thermotolerant cells. Upon recovery at 37 degrees C, both hsp70-transfected and thermotolerant cells exhibit a faster rate of translational recovery. Utilizing slab gel isoelectric focusing coupled with immunoblotting we demonstrate that 45 degrees C heat shock leads to a rapid 4-5-fold increase in eIF-2 alpha phosphorylation, with little difference seen between control cells and hsp70-transfected cells. However, dephosphorylation of eIF-2 alpha occurs faster in the hsp70-transfected cells. These results suggest that hsp70 may play a role in facilitating the dephosphorylation of eIF-2 alpha as well as reversing the inhibition of translation following heat shock.

Regulation of eukaryotic initiation factor-2 expression during sepsis

Protein synthesis is stimulated at the level of peptide chain initiation in livers from rats with a sterile or septic abscess. In contrast, peptide chain initiation is inhibited in fast-twitch skeletal muscles from septic rats. We investigated the possible mechanisms responsible for these differential changes in peptide chain initiation between liver and skeletal muscle during sepsis by measuring the cellular content of eukaryotic initiation factor-2 (eIF-2), the extent of phosphorylation of the alpha-subunit of eIF-2, and the activity of eIF-2B. In skeletal muscle, neither the eIF-2 content nor the extent of phosphorylation of eIF-2 alpha was altered during sepsis. However, a significant decrease (P < 0.001) in eIF-2B activity was observed in fast-twitch muscles. In liver, neither the extent of phosphorylation of eIF-2 alpha nor the activity of eIF-2B was different in rats with a sterile or septic abscess compared with control. However, the amount of eIF-2 in liver was increased in both sterile inflammation and sepsis. The relative abundance of eIF-2 alpha mRNA was not increased in either condition compared with control. Analysis of the distribution of eIF-2 alpha mRNA from control rats revealed that only approximately 40% of the message was associated with polysomes. Sterile inflammation or sepsis caused a 50% increase in the proportion of eIF-2 alpha mRNA associated with the polysomes compared with control.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of heme-regulated eIF-2 alpha kinase and its expression in erythroid cells

In this article we focus first on the molecular mechanisms controlling the activity of the heme-regulated translational inhibitor, HRI, in erythroid cells. Then we discuss the tissue-specific expression of HRI. The experimental evidence obtained to date indicates that the major physiological role of HRI is in adjusting the synthesis of globin to the availability of heme.

Translational control during amino acid starvation

Amino acid starvation of mammalian cells results in a pronounced fall in the overall rate of protein synthesis. This is associated with increased phosphorylation of the alpha-subunit of the initiation factor eIF-2, which in turn impairs the activity of the guanine nucleotide exchange factor, eIF-2B. Similar mechanisms have now been found to operate in the yeast, Saccharomyces cerevisiae, where the major physiological result is to circumvent the lack of external amino acids by promoting the translation of a transcription factor, GCN4, that facilitates the expression of a number of enzymes required for amino acid biosynthesis. This article reviews current knowledge of these mechanisms in both mammalian and yeast cells and identifies questions still requiring elucidation.

Translational control during heat shock

Regulation of translation during heat shock of Drosophila and mammalian cells is reviewed. Protein synthesis is severely inhibited by elevated temperatures but synthesis of heat shock proteins (HSPs) is resistant to this inhibition. The primary site of regulation is polypeptide chain initiation. The activities of two initiation factors, eIF-2 and eIF-4F, are modulated during heat shock. A protein kinase which modulates eIF-2 activity appears to be associated with heat shock proteins (HSPs). Evidence is emerging that HSP70 acts as a heat sensor by detecting the presence of accumulating denatured proteins. In the rabbit reticulocyte lysate denatured proteins bind HSP70 releasing an eIF-2 kinase to shut down protein synthesis. It appears highly likely that a similar mechanism is acting in heat shocked cells. Cell-free protein synthesizing systems prepared from heat shocked cells are deficient in eIF-4F. Modulation of eIF-4F can explain in part the apparent preferential translation of HSP mRNAs.

Reversible phosphorylation of eukaryotic initiation factor 2 alpha in response to endoplasmic reticular signaling

Agents, such as EGTA, thapsigargin, and ionophore A23187, that mobilize sequestered Ca2+ from the endoplasmic reticulum (ER) or dithiothreitol (DTT) that compromises the oxidizing environment of the organelle, disrupt early protein processing and inhibit translational initiation. Increased phosphorylation of eIF-2 alpha (5-fold) and inhibition of eIF-2B activity (50%) occur in intact GH3 cells exposed to these agents for 15 min (Prostko et al. J. Biol. Chem. 267:16751-16754, 1992). Alterations in eIF-2 alpha phosphorylation and translational activity in response to EGTA were reversed by addition of Ca2+ in excess of chelator while responses to DTT were reversible by washing. Exposure for 3 h to either A23187 or DTT, previously shown to induce transcription-dependent translational recovery, resulted in dephosphorylation of eIF-2 alpha in a manner blocked by actinomycin D. Phosphorylation of eIF-2 alpha in response to A23187 or DTT was not prevented by conventional inhibitors of translation including cycloheximide, pactamycin, puromycin, or verrucarin. Prolonged inhibition of protein synthesis to deplete the ER of substrates for protein processing resulted in increased eIF-2 alpha phosphorylation, decreased eIF-2B activity, and reduced monosome content that were indicative of time-dependent blockade; these inhibitors did not abolish polysomal content. Superphosphorylation of eIF-2 alpha occurred upon exposure of these preparations to either A23187 or DTT. Tunicamycin, an inhibitor of co-translational transfer of core oligosaccharide, provoked rapid phosphorylation of eIF-2 alpha and inhibition of translational initiation whereas sugar analog inhibitors of glycoprotein processing did neither. A flow of processible protein to the ER does not appear to be required for the phosphorylation of eIF-2 alpha in response to ER perturbants. We hypothesize that perturbation of the translocon, rather than suppression of protein processing, initiates the signal emanating from the ER culminating in eIF-2 alpha phosphorylation and translational repression.

Depression of neuronal protein synthesis initiation by protein tyrosine kinase inhibitors

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

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

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

The role of the beta-subunit of initiation factor eIF-2 in initiation complex formation

The functional properties of preparations of protein synthesis initiation factor eIF-2 which lack the beta-subunit (as confirmed immunologically) were compared with those of the heterotrimeric factor. The former can bind guanine nucleotides but not initiator tRNA, and also exhibits a substantially reduced rate of initiation factor eIF-2B-mediated GDP/GTP-exchange.

Translational initiation factor eIF-2 subcellular levels and phosphorylation status in the developing rat brain

We have quantified the levels of the alpha subunit of initiation factor 2 (eIF-2) in the postmicrosomal supernatant and the ribosomal salt wash fractions from suckling and adult rat brain. The levels of eIF-2 in the ribosomal salt wash decrease in adult with respect to that present in suckling rat brain, but the total amount remains fairly constant, and a very close parallelism exists between the eIF-2 associated with ribosomes and RNA levels in the microsomal fraction in the two age groups. The phosphorylation state of eIF-2 alpha, as determined by isoelectric focusing followed by protein immunoblotting, in the same subcellular fractions, did not reveal the presence of the phosphorylated form in any of the fractions studied. These results suggest that phosphorylation of the alpha subunit is not implied in the regulation of protein synthesis initiation during brain development, and some other component regulates both the number of active ribosomes and eIF-2 levels in microsomes.

Characteristics of the eukaryotic initiation factor 2 associated 67-kDa polypeptide

A eukaryotic initiation factor 2 (eIF-2) associated 67-kDa polypeptide (p67) protects the eIF-2 alpha-subunit from eIF-2 kinase(s) catalyzed phosphorylation, and this promotes protein synthesis in the presence of active eIF-2 kinase(s), [Datta, B., et al. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 3324-3328]. This report presents the results of studies related to characteristics of p67 action and the mechanism of p67:eIF-2 interaction: (1) p67 antibodies inhibited protein synthesis in hemin-supplemented rabbit reticulocyte lysates, and such inhibition was reversed by preincubation of the antibodies, specifically with p67. (2) p67 inhibited HRI- and dsI-catalyzed phosphorylations of the eIF-2 alpha-subunit and histones, but it did not inhibit casein kinase catalyzed phosphorylation of the eIF-2 beta-subunit. (3) p67 bound specifically to the eIF-2 gamma-subunit. p67 co-immunoprecipitated with the eIF-2 subunits when a p67/eIF-2 mixture was treated with p67 or eIF-2 subunit antibodies and protein A agarose. However, when eIF-2 was preincubated specifically with the eIF-2 gamma-subunit antibodies, subsequent co-immunoprecipitation of p67 with the eIF-2 subunits was completely inhibited. Similarly, preincubation of p67 and p67 antibodies prevented subsequent p67 binding to eIF-2. Preincubation of eIF-2, with either eIF-2 alpha- or beta-subunit antibodies, had no effect on p67 co-immunoprecipitation with the eIF-2 subunits. (4) p67:eIF-2 interaction is necessary for p67 activity to protect the eIF-2 alpha-subunit from eIF-2 kinase(s) catalyzed phosphorylation.(ABSTRACT TRUNCATED AT 250 WORDS)

Aminoacylation of initiator methionyl-tRNA(i) under conditions inhibitory to initiation of protein synthesis

Inhibition of protein synthesis in perfused rat liver deprived of either methionine or tryptophan results from a defect in peptide-chain initiation. Similarly, the decreased rate of protein synthesis in liver from rats deprived of food for 24 h and in skeletal muscle after 2 days of diabetes results from a defect in initiation. In the present study, the tissue content of tRNA(iMet) and its level of aminoacylation were measured in these conditions to determine whether methionyl-tRNA(iMet) formation is a mechanism involved in the regulation of initiation. The extent of aminoacylation of tRNA(iMet) in livers perfused with supplemented medium or medium deficient in either methionine or tryptophan was 64 +/- 2, 61 +/- 3, and 66 +/- 2% of the total accepting activity, respectively. The total tissue content of tRNA(iMet), expressed as a percentage of total RNA, was 1.7 +/- 0.1, 1.6 +/- 0.1, and 1.6 +/- 0.1 for the three conditions, respectively. In livers from starved rats, the extent of aminoacylation of tRNA(iMet) was 80 +/- 7% and the total tissue content of tRNA(iMet) was 1.9 +/- 0.1% compared with control values of 82 +/- 6 and 2.0 +/- 0.1%, respectively. In skeletal muscle from diabetic rats, the extent of aminoacylation of tRNA(iMet) was 79 +/- 4% and the total tissue content of tRNA(iMet) was 2.0 +/- 0.3% compared with values of 79 +/- 5 and 2.0 +/- 0.2% for control animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Translational stimulation by reovirus polypeptide sigma 3: substitution for VAI RNA and inhibition of phosphorylation of the alpha subunit of eukaryotic initiation factor 2

COS cells transfected with plasmids that activate DAI depend on expression of virus-associated I (VAI) RNA to prevent the inhibitory effects of the alpha subunit of eukaryotic initiation factor 2 (eIF-2 alpha) kinase (DAI) and restore the translation of vector-derived dihydrofolate reductase mRNA. This VAI RNA requirement could be completely replaced by reovirus polypeptide sigma 3, consistent with its double-stranded RNA (dsRNA)-binding activity. S4 gene transfection of 293 cells also partially restored adenovirus protein synthesis after infection with the VAI-negative dl331 mutant. In dl331-infected 293 cells, eIF-2 alpha was present mainly in the acidic, phosphorylated form, and trans complementation with polypeptide sigma 3 or VAI RNA decreased the proportion of eIF-2 alpha (P) from approximately 85 to approximately 30%. Activation of DAI by addition of dsRNA to extracts of S4 DNA-transfected COS cells required 10-fold-higher levels of dsRNA than extracts made from cells that were not producing polypeptide sigma 3. In extracts of reovirus-infected mouse L cells, the concentration of dsRNA needed to activate DAI was dependent on the viral serotype used for the infection. Although the proportion of eIF-2 alpha (P) was greater than that in uninfected cells, most of the factor remained in the unphosphorylated form, even at 16 h after infection, consistent with the partial inhibition of host protein synthesis observed with all three viral serotypes. The results indicate that reovirus polypeptide sigma 3 participates in the regulation of protein synthesis by modulating DAI and eIF-2 alpha phosphorylation.