Regulation of eukaryotic protein synthesis by protein kinases that phosphorylate initiation factor eIF-2

Wnt5a promotes hippocampal postsynaptic development and GluN2B-induced expression via the eIF2α HRI kinase

Wnt signaling plays a key role in neurodevelopment and neuronal maturation. Specifically, Wnt5a stimulates postsynaptic assemblies, increases glutamatergic neurotransmission and, through calcium signaling, generates nitric oxide (NO). Trying to unveil the molecular pathway triggering these postsynaptic effects, we found that Wnt5a treatment induces a time-dependent increases in the length of the postsynaptic density (PSD), elicits novel synaptic contacts and facilitates F-actin flow both in in vitro and ex vivo models. These effects were partially abolished by the inhibition of the Heme-regulated eukaryotic initiation factor 2α (HRI) kinase, a kinase which phosphorylates the initiation translational factor eIF2α. When phosphorylated, eIF2α normally avoids the translation of proteins not needed during stress conditions, in order to avoid unnecessary energetic expenses. However, phosphorylated eIF2α promotes the translation of some proteins with more than one open reading frame in its 5′ untranslated region. One of these proteins targeted by Wnt-HRI-eIF2α mediated translation is the GluN2B subunit of the NMDA receptor. The identified increase in GluN2B expression correlated with increased NMDA receptor function. Considering that NMDA receptors are crucial for excitatory synaptic transmission, the molecular pathway described here contributes to the understanding of the fast and plastic translational mechanisms activated during learning and memory processes.

EIF2AK2 selectively regulates the gene transcription in immune response and histones associated with systemic lupus erythematosus

PKR, also known as EIF2AK2, is an IFN-stimulated gene (ISG) and shows a higher expression in probands with systemic lupus erythematosus (SLE), which is likely responsible for the impaired translational and proliferative responses to mitogens in T cells from SLE patients. In this study, we overexpressed EIF2AK2 in HeLa cells to study EIF2AK2-regulated genes using RNA-seq technology, followed by bioinformatic analysis of target genes of EIF2AK2-regulated transcriptional factors (TFs). Overexpression of EIF2AK2 promotes HeLa cell apoptosis. EIF2AK2 selectively represses the transcription of histone protein genes associated with SLE, immune response genes and TF genes, which was validated by RT-qPCR experiments. Analysis of motifs overrepresented in the promoter regions of EIF2AK2-regulated genes revealed eighteen EIF2AK2-regulated TFs involved in establishing the EIF2AK2 network. Eight out of these predicted EIF2AK2-regulated TFs were further verified by RT-qPCR selectively in both HeLa and Jurkat cells, and most such as HEY2, TFEC, BATF2, GATA3 and ATF3 and FOXO6 are known to regulate immune response. Our results suggest that the dsRNA-dependent kinase EIF2AK2 selectively regulates the transcription of immune response and SLE-associated histone protein genes, and such a selectivity is likely to be operated by EIF2AK2-targeted TFs. The EIF2AK2-TFs axis potentially offers new therapeutic targets for counteracting immunological disease in the future.

Translational control mechanisms in cutaneous malignant melanoma: the role of eIF2α

BACKGROUND

Melanoma cells develop adaptive responses in order to cope with particular conditions of tumor microenvironment, characterized by stress conditions and deregulated proliferation. Recently, the interplay between the stress response and the gene expression programs leading to metastatic spread has been reported.

METHODS

We evaluated levels and localization of eIF2α/peIF2α in V600BRAF and wtBRAF metastatic melanoma cell lines by means of western blot and confocal microscopy analyses. Furthermore, we performed a sequence analyses and structure and dynamics studies of eIF2α protein to reveal the role of eIF2α and its correlations in different pathways involved in the invasive phase of melanoma.

RESULTS

We found peIF2α both in cytoplasm and nucleus. Nuclear localization was more represented in V600BRAF melanoma cell lines. Our studies on eIF2α protein sequence indicated the presence of a predicted bipartite NLS as well as a nuclear export signal NES and an S1 domain, typical of RNA interacting proteins. Furthermore, we found high levels of transcription factor EB (TFEB), a component of the MiT/TFE family, and low β-catenin levels in V600BRAF cells.

CONCLUSIONS

Based on our results, we suggest that peIF2α nuclear localization can be crucial in ER stress response and in driving the metastatic spread of melanoma, through lysosomal signaling and Wnt/β-catenin pathway. In conclusion, this is the first evidence of nuclear localization of peIF2α, representing a possible target for future therapeutic approaches for metastatic melanoma.

Small molecule modulators of eukaryotic initiation factor 2α kinases, the key regulators of protein synthesis

Eukaryotic initiation factor 2 alpha kinases (eIF-2α kinases) are key mediators of stress response in cells. In mammalian cells, there are four eIF-2α kinases, namely HRI (Heme-Regulated Inhibitor), PKR (RNA-dependent Protein Kinase), PERK (PKR-like ER Kinase) and GCN2 (General Control Non-derepressible 2). These kinases get activated during diverse cytoplasmic stress conditions and phosphorylate the alpha-subunit of eIF2, leading to global protein synthesis inhibition. Therefore, eIF-2α kinases play a vital role in various cellular processes such as proliferation, differentiation, apoptosis and cell signaling. Deregulation of eIF-2α kinases and protein synthesis has been linked to numerous pathological conditions such as certain cancers, anemia and neurodegenerative disorders. Thus, modulation of these kinases by small molecules holds a great therapeutic promise. In this review we have compiled the available information on inhibitors and activators of these four eIF-2α kinases. The review concludes with a note on the selectivity issue of currently available modulators and future perspectives for the design of specific small molecule probes.

P58(IPK): a novel "CIHD" member of the host innate defense response against pathogenic virus infection

To support their replication, viruses take advantage of numerous cellular factors and processes. Recent large-scale screens have identified hundreds of such factors, yet little is known about how viruses exploit any of these. Influenza virus infection post-translationally activates P58(IPK), a cellular inhibitor of the interferon-induced, dsRNA-activated eIF2alpha kinase, PKR. Here, we report that infection of P58(IPK) knockout mice with influenza virus resulted in increased lung pathology, immune cell apoptosis, PKR activation, and mortality. Analysis of lung transcriptional profiles, including those induced by the reconstructed 1918 pandemic virus, revealed increased expression of genes associated with the cell death, immune, and inflammatory responses. These experiments represent the first use of a mammalian infection model to demonstrate the role of P58(IPK) in the antiviral response. Our results suggest that P58(IPK) represents a new class of molecule, a cellular inhibitor of the host defense (CIHD), as P58(IPK) is activated during virus infection to inhibit virus-induced apoptosis and inflammation to prolong host survival, even while prolonging viral replication.

Essential role of PACT-mediated PKR activation in tunicamycin-induced apoptosis

Cellular stresses such as disruption of calcium homeostasis, inhibition of protein glycosylation, and reduction of disulfide bonds result in accumulation of misfolded proteins in the endoplasmic reticulum (ER) and lead to cell death by apoptosis. Tunicamycin, which is an inhibitor of protein glycosylation, induces ER stress and apoptosis. In this study, we examined the involvement of double-stranded RNA (dsRNA)-activated protein kinase (PKR) and its protein activator PACT in tunicamycin-induced apoptosis. We demonstrate for the first time that PACT is phosphorylated in response to tunicamycin and is responsible for PKR activation by direct interaction. Furthermore, PACT-induced PKR activation is essential for tunicamycin-induced apoptosis, since PACT as well as PKR null cells are markedly resistant to tunicamycin and show defective eIF2alpha phosphorylation and C/EBP homologous protein (CHOP, also known as GADD153) induction especially at low concentrations of tunicamycin. Reconstitution of PKR and PACT expression in the null cells renders them sensitive to tunicamycin, thus demonstrating that PACT-induced PKR activation plays an essential function in induction of apoptosis.

Re-programming of translation following cell stress allows IRES-mediated translation to predominate

There is now an overwhelming body of evidence to suggest that internal ribosome entry is required to maintain the expression of specific proteins during patho-physiological situations when cap-dependent translation is compromised, for example, following heat shock or during mitosis, hypoxia, differentiation and apoptosis. Translational profiling has been used by several groups to assess the extent to which alternative mechanisms of translation initiation selectively recruit mRNAs to polysomes during cell stress. The data from these studies have shown that under each condition 3-5% of coding mRNAs remain associated with the polysomes. Importantly, the genes identified in each of these studies do not show a significant amount of overlap, suggesting that 10-15% of all mRNAs have the capability for their initiation to occur via alternative mechanism(s).

Regulation of initiation factors controlling protein synthesis on cultured astrocytes in lactic acid-induced stress

The goals of this work were first to assess whether the lactic acidosis observed in vivo in ischemia may by itself explain the inhibition of protein synthesis described in the literature and second to study the factors controlling the initiation of protein synthesis under lactic acid stress. Primary rat astrocyte cultures exposed to pH 5.25 underwent cell death and a strong inhibition of protein synthesis assessed by [3H]methionine incorporation, which was solely due to acidity of the extracellular medium and was not related to lactate concentrations. This result was associated with a weak phosphorylation of eukaryotic initiation factor (eIF)4E and a rapid phosphorylation of eIF2alpha via the kinases PKR and PKR-like endoplasmic reticulum kinase. The inhibition of PKR by PRI led first to a significant but not complete dephosphorylation of eIF2alpha that probably contributed to maintain the inhibition of the protein synthesis and second to surprising phosphorylations of extracellular signal-regulated protein kinase, p70S6K and eIF4E, suggesting a possible cross-link between the two pathways. Conversely, cell death was weak at pH 5.5. Protein synthesis was decreased to a lesser extent, the phosphorylation of eIF2alpha was limited, extracellular signal-regulated protein kinase 1/2 was activated and its downstream targets, p70S6K and eIF4E, were phosphorylated. However, the strong phosphorylation of eIF4E was not associated with an activation of the eIF4F complex. This last result may explain why protein synthesis was not stimulated at pH 5.5. However, when astrocytes were exposed at pH 6.2, corresponding to the lower pH observed in hyperglycemic ischemia, no modification in protein synthesis was observed. Consequently, lactic acidosis cannot, by itself, provide an explanation for the decrease in protein synthesis previously reported in vivo in ischemia.

Neuroprotective effects of pituitary adenylate cyclase–activating polypeptide (PACAP) in MPP+-induced alteration of translational control in Neuro-2a neuroblastoma cells

Parkinson's disease (PD) and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxicity are both associated with dopaminergic neuron death in the substantia nigra. Although a variety of evidence has shown that degenerative cells have apoptotic features, the role of apoptosis in disease pathology remains controversial. The 1-methyl-4-phenylpyridinium ion (MPP(+)), a metabolite of MPTP, was recently shown to alter the expression of proteins involved in translational control. The initiation step of translational control is regulated by a cascade of phosphorylation affecting proteins of the antiapoptotic way controlled by mammalian target of rapamycin (mTOR) and of the proapoptotic way controlled by double-stranded RNA protein-dependent kinase (PKR). A study showed that MPP(+) induced an increase in eIF2alpha phosphorylation, leading to inhibition of protein synthesis.

THE AIMS OF OUR STUDY WERE

(1) to assess the effects of MPP(+) toxicity on molecular factors of PKR and mTOR signaling pathways in murine neuroblastoma cells, and (2) to examine the ability of VIP and PACAP peptides to counteract the MPP(+) toxicity. Our findings showed that MPP(+) induced phosphorylation of eIF2alpha and significantly reduced the expression of phosphorylated mTOR, p70S6K, eIF4E, and 4E-BP1, suggesting its toxicity in controlling protein synthesis. Furthermore, the VIP peptide had no effect on either the PKR or the mTOR signaling pathway. On the contrary, the PACAP 27 neuropeptide prevented MPP(+)-induced eIF2alpha phosphorylation and blocked MPP(+) toxicity in molecular factors of the mTOR pathway. And last, PACAP 27 seemed to protect Neuro-2a cells from the apoptotic process as assessed by the decreased nuclear condensation after DAPI staining. These results could open new paths of research of PACAP in PD.

AMPK activation regulates apoptosis, adipogenesis, and lipolysis by eIF2alpha in adipocytes

AMP-activated protein kinase (AMPK) is a metabolic master switch regulating glucose and lipid metabolism. Recently, AMPK has been implicated in the control of adipose tissue content. Yet, the nature of this action is controversial. We examined the effect on F442a adipocytes of the AMPK activator-AICAR. Activation of AMPK induced dose-dependent apoptotic cell death, inhibition of lipolysis, and downregulatation key adipogenic genes, such as peroxisome proliferator-activated receptor (PPARgamma) and CCAAT/enhancer-binding protein alpha (C/EBPalpha). We have identified the alpha-subunit of the eukaryotic initiation factor-2 (eIF2alpha) as a target gene which is phosphorylated following AICAR treatment. Such phosphorylation is one of the best-characterized mechanisms for downregulating protein synthesis. 2-Aminopurine (2-AP), an inhibitor of eIF2alpha kinases, could overcome the apoptotic effect of AICAR, abolishing the reduction of PPARgamma and C/EBPalpha and the lipolytic properties of AMPK. Thus, AMPK may diminish adiposity via reduction of fat cell number through eIF2alpha-dependent translation shutdown.

Two mechanisms of caspase 9 processing in double-stranded RNA- and virus-triggered apoptosis

Viral double-stranded RNA (dsRNA) is a ubiquitous intracellular "alert signal" used by cells to detect viral infection and to mount anti-viral responses. DsRNA triggers a rapid (complete within 2-4 h) apoptosis in the highly-susceptible HeLa cell line. Here, we demonstrate that the apical event in this apoptotic cascade is the activation of procaspase 8. Downstream of caspase 8, the apoptotic signaling cascade bifurcates into a mitochondria-independent caspase 8/caspase 3 arm and a mitochondria-dependent, caspase 8/Bid/Bax/Bak/cytochrome c arm. Both arms impinge upon, and activate, procaspase 9 via two different cleavage sites within the procaspase 9 molecule (D330 and D315, respectively). This is the first in vivo demonstration that the "effector" caspase 3 plays an "initiator" role in the regulation of caspase 9. The dsRNA-induced apoptosis is potentiated by the inhibition of protein synthesis, whose role is to accelerate the execution of all apoptosis steps downstream of, and including, the activation of caspase 8. Thus, efficient apoptosis in response to viral dsRNA results from the co-operation of the two major apical caspases (8 and 9) and the dsRNA-activated protein kinase R (PKR)/ribonuclease L (RNase L) system that is essential for the inhibition of protein synthesis in response to viral infection.

Tumor specific activation of PKR as a non-toxic modality of cancer treatment

Over the past decade progress has been made in the development of therapies against cancer. Small molecules, mainly tyrosine kinase inhibitors (tyrphostins) like Gleevec, Iressa targeting CML and EGFR overexpressing tumors have entered the clinic, where a large number of other tyrphostins are at various stages of clinical development. In parallel a few antibodies like Herceptin targeting breast cancer overexpressing Her-2 and Rituxan targeting B cell malignancies are utilized in the clinic. In all these cases success is moderate and restricted to a narrow population of patients, except for Gleevec which is effective for a long duration for chronic CML. The cancer community agrees that this is actually a unique exception that proves the rule. Over the past few years a few modalities of cancer gene therapies have emerged. In this short review we shall summarize our efforts to develop methods to activate PKR selectively in cancer cells.

The human SNF5/INI1 protein facilitates the function of the growth arrest and DNA damage-inducible protein (GADD34) and modulates GADD34-bound protein phosphatase-1 activity

The growth arrest and DNA damage-inducible protein (GADD34) mediates growth arrest and apoptosis in response to DNA damage, negative growth signals, and protein malfolding. GADD34 binds to protein phosphatase-1 (PP1) and can attenuate translational elongation of key transcriptional factors through dephosphorylation of eukaryotic initiation factor-2alpha. We reported previously that the human trithorax leukemia fusion protein (HRX) can bind to GADD34 and abrogate GADD34-mediated apoptosis in response to UV irradiation. We found that hSNF5/INI1, a component of the hSWI/SNF chromatin remodeling complex, also binds to GADD34 and can coexist with GADD34 and HRX fusion proteins as a trimolecular complexes in vivo. In the present report, we demonstrate that hSNF5/INI1 binds to GADD34 in part through the PP1 docking site within a domain homologous to herpes simplex virus-1 ICP34.5. We found that hSNF5/INI1 can bind PP1 independently and weakly stimulate its phosphatase activity in solution and in complex with GADD34. hSNF5/INI1 and PP1 do not compete for binding to GADD34 but rather form a stable heterotrimeric complex with GADD34. We also show that Epstein-Barr nuclear protein 2, which binds hSNF5/INI1, can disrupt hSNF5/INI1 binding to GADD34 and partially reverse the GADD34-mediated growth suppression function in Ha-ras expressing HIH-3T3 (3T3-ras) cells. These results implicate hSNF5/INI1 in the function of GADD34 and suggest that hSNF5/INI1 may regulate PP1 activity in vivo.

Translational control by the ER transmembrane kinase/ribonuclease IRE1 under ER stress

Under conditions of endoplasmic reticulum (ER) stress, mammalian cells induce both translational repression and the unfolded protein response that transcriptionally activates genes encoding ER-resident molecular chaperones. To date, the only known pathway for translational repression in response to ER stress has been the phosphorylation of eIF-2alpha by the double-stranded RNA-activated protein kinase (PKR) or the transmembrane PKR-like ER kinase (PERK). Here we report another pathway in which the ER transmembrane kinase/ribonuclease IRE1beta induces translational repression through 28S ribosomal RNA cleavage in response to ER stress. The evidence suggests that both pathways are important for efficient translational repression during the ER stress response.

Translational control of viral gene expression in eukaryotes

As obligate intracellular parasites, viruses rely exclusively on the translational machinery of the host cell for the synthesis of viral proteins. This relationship has imposed numerous challenges on both the infecting virus and the host cell. Importantly, viruses must compete with the endogenous transcripts of the host cell for the translation of viral mRNA. Eukaryotic viruses have thus evolved diverse mechanisms to ensure translational efficiency of viral mRNA above and beyond that of cellular mRNA. Mechanisms that facilitate the efficient and selective translation of viral mRNA may be inherent in the structure of the viral nucleic acid itself and can involve the recruitment and/or modification of specific host factors. These processes serve to redirect the translation apparatus to favor viral transcripts, and they often come at the expense of the host cell. Accordingly, eukaryotic cells have developed antiviral countermeasures to target the translational machinery and disrupt protein synthesis during the course of virus infection. Not to be outdone, many viruses have answered these countermeasures with their own mechanisms to disrupt cellular antiviral pathways, thereby ensuring the uncompromised translation of virion proteins. Here we review the varied and complex translational programs employed by eukaryotic viruses. We discuss how these translational strategies have been incorporated into the virus life cycle and examine how such programming contributes to the pathogenesis of the host cell.

Activation of p38 mitogen-activated protein kinase and c-Jun NH(2)-terminal kinase by double-stranded RNA and encephalomyocarditis virus: involvement of RNase L, protein kinase R, and alternative pathways

Double-stranded RNA (dsRNA) accumulates in virus-infected mammalian cells and signals the activation of host defense pathways of the interferon system. We describe here a novel form of dsRNA-triggered signaling that leads to the stimulation of the p38 mitogen-activated protein kinase (p38 MAPK) and the c-Jun NH(2)-terminal kinase (JNK) and of their respective activators MKK3/6 and SEK1/MKK4. The dsRNA-dependent signaling to p38 MAPK was largely intact in cells lacking both RNase L and the dsRNA-activated protein kinase (PKR), i. e., the two best-characterized mediators of dsRNA-triggered antiviral responses. In contrast, activation of both MKK4 and JNK by dsRNA was greatly reduced in cells lacking RNase L (or lacking both RNase L and PKR) but was restored in these cells when introduction of dsRNA was followed by inhibition of ongoing protein synthesis or transcription. These results are consistent with the notion that the role of RNase L and PKR in the activation of MKK4 and JNK is the elimination, via inhibition of protein synthesis, of a labile negative regulator(s) of the signaling to JNK acting upstream of SEK1/MKK4. In the course of these studies, we identified a long-sought site of RNase L-mediated cleavage in the 28S rRNA, which could cause inhibition of translation, thus allowing the activation of JNK by dsRNA. We propose that p38 MAPK is a general participant in dsRNA-triggered cellular responses, whereas the activation of JNK might be restricted to cells with reduced rates of protein synthesis. Our studies demonstrate the existence of alternative (RNase L- and PKR-independent) dsRNA-triggered signaling pathways that lead to the stimulation of stress-activated MAPKs. Activation of p38 MAPK (but not of JNK) was demonstrated in mouse fibroblasts in response to infection with encephalomyocarditis virus (ECMV), a picornavirus that replicates through a dsRNA intermediate. Fibroblasts infected with EMCV (or treated with dsRNA) produced interleukin-6, an inflammatory and pyrogenic cytokine, in a p38 MAPK-dependent fashion. These findings suggest that stress-activated MAPKs participate in mediating inflammatory and febrile responses to viral infections.

Characterization of the hemin-sensitive eukaryotic initiation factor 2alpha kinase from mouse nonerythroid cells

The heme-regulated eukaryotic initiation factor 2alpha (eIF2alpha) kinase (heme-regulated inhibitor (HRI)) is activated by heme deficiency in reticulocytes and plays an important role in translational control in these cells. Previously, HRI was cloned from rabbit reticulocytes and rat brain, but a heme-regulated eIF2alpha kinase activity has only been purified from erythroid cells. In this study, we report the purification of a heme-sensitive eIF2alpha kinase activity from both mouse liver and NIH 3T3 cell extracts. Furthermore, we have cloned and characterized this mouse liver eIF2alpha kinase (mHRI), which exhibits 83 and 94% identities to rabbit and rat HRIs, respectively. Both the purified enzyme and recombinant mHRI exhibited an autokinase and an eIF2alpha kinase activity, and both activities were inhibited in vitro by hemin. In addition, wild-type mHRI, but not the inactive mHRI-K196R mutant, was autophosphorylated in vivo when it was expressed in 293 cells. Quantitation of mHRI mRNA expression in various mouse tissues by reverse transcription-polymerase chain reaction revealed relatively high levels in liver, kidney, and testis. These results provide strong evidence that mHRI is a ubiquitous eIF2alpha kinase of mammalian cells, suggesting that it could play important roles in the translational regulation of nonerythroid tissues.

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.

The solution structure of the S1 RNA binding domain: a member of an ancient nucleic acid-binding fold

The S1 domain, originally identified in ribosomal protein S1, is found in a large number of RNA-associated proteins. The structure of the S1 RNA-binding domain from the E. coli polynucleotide phosphorylase has been determined using NMR methods and consists of a five-stranded antiparallel beta barrel. Conserved residues on one face of the barrel and adjacent loops form the putative RNA-binding site. The structure of the S1 domain is very similar to that of cold shock protein, suggesting that they are both derived from an ancient nucleic acid-binding protein. Enhanced sequence searches reveal hitherto unidentified S1 domains in RNase E, RNase II, NusA, EMB-5, and other proteins.

The plant translational apparatus

Protein synthesis in both eukaryotic and prokaryotic cells is a complex process requiring a large number of macromolecules: initiation factors, elongation factors, termination factors, ribosomes, mRNA, amino-acylsynthetases and tRNAs. This review focuses on our current knowledge of protein synthesis in higher plants.

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.

Initiation of protein synthesis in eukaryotic cells

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

Phosphorylation of initiation factor 2 alpha subunit and apoptosis in Ca2+ ionophore-treated cultured neuronal cells

The initial step of protein synthesis is regulated by the eukaryotic initiation factor 2 (eIF-2) whose phosphorylation in the alpha subunit by specific kinases, as double-stranded RNA-dependent protein kinase (PKR), produces an inhibition of the translational rates. Besides, intracellular Ca2+ mobilization has been associated with an increase in the PKR activity. Our results show, in primary neuronal cultures, that the treatment with the Ca2+ ionophore A23187 induces an increase in the phosphorylation of the alpha subunit of eIF-2 (eIF-2 alpha) and inhibition of protein synthesis. Those biochemical changes run parallel to the appearance of specific apoptosis characteristics such as cell shrinkage, segmentation of chromatin into small round bodies, and cleavage of DNA into 180 bp multimers. These results indicate that one of the targets during the process of apoptosis induced by the rise in the intracellular Ca2+, could be eIF-2 factor.

Regulation of the interferon-inducible protein kinase PKR and (2'-5')oligo(adenylate) synthetase by a catalytically inactive PKR mutant through competition for double-stranded RNA binding

The interferon-inducible double-stranded RNA-dependent protein kinase PKR has been suggested to function as a tumour suppressor gene product. Catalytically inactive mutants of PKR give rise to a tumorigenic phenotype when overexpressed in NIH-3T3 fibroblasts and this has been attributed to a dominant negative effect on the activity of the wild-type enzyme. Here we show that the mutant with Lys296 replaced by Arg, [K296R]PKR, not only inhibits the protein kinase activity of wild-type PKR but is also inhibitory towards another double-stranded RNA-dependent enzyme, the 40-kDa form of (2'-5')oligo(adenylate) synthetase. Inhibition of both wild-type PKR and (2'-5')oligo(adenylate) synthetase is reversed by adding higher concentrations of double-stranded RNA. These results suggest competition between [K296R]PKR and wild-type PKR or (2'-5')oligo(adenylate) synthetase for limiting amounts of double-stranded RNA. Moreover, the data imply that the tumorigenic effect of this PKR mutant could be due to inhibition of additional pathways requiring low levels of double-stranded RNA for activation and cannot be unambiguously attributed to inhibition of endogenous PKR itself.