Initiation factor modifications in the preapoptotic phase

Effects of Progesterone and Selective Ligands of Membrane Progesterone Receptors in HepG2 Cells of Human Hepatocellular Carcinoma

Progesterone exerts multiple effects in different tissues through nuclear receptors (nPRs) and through membrane receptors (mPRs) of adiponectin and progestin receptor families. The effect of progesterone on the cells through different types of receptors can vary significantly. At the same time, it affects the processes of proliferation and apoptosis in normal and tumor tissues in a dual way, stimulating proliferation and carcinogenesis in some tissues, suppressing them and stimulating cell death in others. In this study, we have shown the presence of high level of mPRβ mRNA and protein in the HepG2 cells of human hepatocellular carcinoma. Expression of other membrane and classical nuclear receptors was not detected. It could imply that mPRβ has an important function in the HepG2 cells. The main goal of the work was to study functions of this protein and mechanisms of its action in human hepatocellular carcinoma cells. Previously, we have identified selective mPRs ligands, compounds LS-01 and LS-02, which do not interact with nuclear receptors. Their employment allows differentiating the effects of progestins mediated by different types of receptors. Effects of progesterone, LS-01, and LS-02 on proliferation and death of HepG2 cells were studied in this work, as well as activating phosphorylation of two kinases, p38 MAPK and JNK, under the action of three steroids. It was shown that all three progestins after 72 h of incubation with the cells suppressed their viability and stimulated appearance of phosphatidylserine on the outer surface of the membranes, which was detected by binding of annexin V, but they did not affect DNA fragmentation of the cell nuclei. Progesterone significantly reduced expression of the proliferation marker genes and stimulated expression of the p21 protein gene, but had a suppressive effect on the expression of some proapoptotic factor genes. All three steroids activated JNK in these cells, but had no effect on the p38 MAPK activity. The effects of progesterone and selective mPRs ligands in HepG2 cells were the same in terms of suppression of proliferation and stimulation of apoptotic changes in outer membranes, therefore, they were mediated through interaction with mPRβ. JNK is a member of the signaling cascade activated in these cells by the studied steroids.

Phosphorylation of ribosomal protein S6 differentially affects mRNA translation based on ORF length

Phosphorylation of Ribosomal Protein S6 (RPS6) was the first post-translational modification of the ribosome to be identified and is a commonly-used readout for mTORC1 activity. Although the cellular and organismal functions of RPS6 phosphorylation are known, the molecular consequences of RPS6 phosphorylation on translation are less well understood. Here we use selective ribosome footprinting to analyze the location of ribosomes containing phosphorylated RPS6 on endogenous mRNAs in cells. We find that RPS6 becomes progressively dephosphorylated on ribosomes as they translate an mRNA. As a consequence, average RPS6 phosphorylation is higher on mRNAs with short coding sequences (CDSs) compared to mRNAs with long CDSs. We test whether RPS6 phosphorylation differentially affects mRNA translation based on CDS length by genetic removal of RPS6 phosphorylation. We find that RPS6 phosphorylation promotes translation of mRNAs with short CDSs more strongly than mRNAs with long CDSs. Interestingly, RPS6 phosphorylation does not promote translation of mRNAs with 5′ TOP motifs despite their short CDS lengths, suggesting they are translated via a different mode. In sum this provides a dynamic view of RPS6 phosphorylation on ribosomes as they translate mRNAs and the functional consequence on translation.

Influence of Varying Dietary ω6 to ω3 Fatty Acid Ratios on the Hepatic Transcriptome, and Association with Phenotypic Traits (Growth, Somatic Indices, and Tissue Lipid Composition), in Atlantic Salmon (Salmo salar)

Simple Summary

Plant oils are routinely used in fish feeds as a fish oil replacement. However, these terrestrial alternatives typically contain high levels of ω6 fatty acids (FA) and, thus, high ω6 to ω3 (ω6:ω3) FA ratios, which influence farmed fish and their consumers. The ω6:ω3 ratio is known to affect many biological processes (e.g., inflammation, FA metabolism) and human diseases; however, its impacts on fish physiology and the underlying molecular mechanisms are less well understood. In this study, we used 44 K microarrays to examine which genes and molecular pathways are altered by variation in dietary ω6:ω3 in Atlantic salmon. Our microarray study showed that several genes related to immune response, lipid metabolism, cell proliferation, and translation were differentially expressed between the two extreme ω6:ω3 dietary treatments. We also revealed that the PPARα activation-related transcript helz2 is a potential novel molecular biomarker of tissue variation in ω6:ω3. Further, correlation analyses illustrated the relationships between liver transcript expression and tissue (liver, muscle) lipid composition, and other phenotypic traits in salmon fed low levels of fish oil. This nutrigenomic study enhanced the current understanding of Atlantic salmon gene expression response to varying dietary ω6:ω3.

Abstract

The importance of dietary omega-6 to omega-3 (ω6:ω3) fatty acid (FA) ratios for human health has been extensively examined. However, its impact on fish physiology, and the underlying molecular mechanisms, are less well understood. This study investigated the influence of plant-based diets (12-week exposure) with varying ω6:ω3 (0.4–2.7) on the hepatic transcriptome of Atlantic salmon. Using 44 K microarray analysis, genes involved in immune and inflammatory response (lect2a, itgb5, helz2a, p43), lipid metabolism (helz2a), cell proliferation (htra1b), control of muscle and neuronal development (mef2d) and translation (eif2a, eif4b1, p43) were identified; these were differentially expressed between the two extreme ω6:ω3 dietary treatments (high ω6 vs. high ω3) at week 12. Eight out of 10 microarray-identified transcripts showed an agreement in the direction of expression fold-change between the microarray and qPCR studies. The PPARα activation-related transcript helz2a was confirmed by qPCR to be down-regulated by high ω6 diet compared with high ω3 diet. The transcript expression of two helz2 paralogues was positively correlated with ω3, and negatively with ω6 FA in both liver and muscle, thus indicating their potential as biomarkers of tissue ω6:ω3 variation. Mef2d expression in liver was suppressed in the high ω6 compared to the balanced diet (ω6:ω3 of 2.7 and 0.9, respectively) fed fish, and showed negative correlations with ω6:ω3 in both tissues. The hepatic expression of two lect2 paralogues was negatively correlated with viscerosomatic index, while htra1b correlated negatively with salmon weight gain and condition factor. Finally, p43 and eif2a were positively correlated with liver Σω3, while these transcripts and eif4b2 showed negative correlations with 18:2ω6 in the liver. This suggested that some aspects of protein synthesis were influenced by dietary ω6:ω3. In summary, this nutrigenomic study identified hepatic transcripts responsive to dietary variation in ω6:ω3, and relationships of transcript expression with tissue (liver, muscle) lipid composition and other phenotypic traits.

JunB can enhance the transcription of IL-8 in oral squamous cell carcinoma

Proteasome inhibitor MG132 was shown to enhance the secretion of interleukin 8 (IL-8) by various cells. The enhancement is regulated by the transcription factor activator protein-1 (AP-1) at the transcriptional level. AP-1 is a dimer formed by AP-1 family proteins. The purpose of the present study was to explore the combinations of the AP-1 family proteins that contribute to MG132-driven IL-8 secretion. Oral squamous cell carcinoma-derived cell lines, Ca9-22 and HSC3, were used to demonstrate their response to MG132. IL-8 secretion was augmented by MG132 in both cell lines. c-Jun expression was detected in both the cell lines, whereas c-Fos expression was detected only in the HSC3. The influence of MG132 stimulation on c-Jun and c-Fos expression was further examined by western blot analysis. c-Jun expression was increased by MG132 stimulation, whereas c-Fos expression was not detected even after MG132 stimulation. As JunB is reported to inhibit the transcriptional activity of the AP-1 complex, we speculated that the c-Jun homodimer should contribute to IL-8 enhancement. Expression vectors encoding wild type and c-Jun mutants, M17 and M22-23, respectively, were constructed and transfected into the Ca9-22 cells. In contrast to our expectations, MG132-induced IL-8 secretion was significantly reduced in all the transfectants suggesting that other c-Jun members might form homodimers with c-Jun and contribute to IL-8 enhancement. Transfection of the cells with c-Jun or JunB small hairpin RNA (shRNA) reduced IL-8 secretion up to 50% and 65% of the control shRNA transfectant. Furthermore, cotransfection of both shRNA almost completely inhibited the IL-8 secretion. These results indicate that JunB not only inhibits but also enhances the transcription of c-Jun targets in combination with c-Jun.

Trans-acting translational regulatory RNA binding proteins

The canonical molecular machinery required for global mRNA translation and its control has been well defined, with distinct sets of proteins involved in the processes of translation initiation, elongation and termination. Additionally, noncanonical, trans-acting regulatory RNA-binding proteins (RBPs) are necessary to provide mRNA-specific translation, and these interact with 5' and 3' untranslated regions and coding regions of mRNA to regulate ribosome recruitment and transit. Recently it has also been demonstrated that trans-acting ribosomal proteins direct the translation of specific mRNAs. Importantly, it has been shown that subsets of RBPs often work in concert, forming distinct regulatory complexes upon different cellular perturbation, creating an RBP combinatorial code, which through the translation of specific subsets of mRNAs, dictate cell fate. With the development of new methodologies, a plethora of novel RNA binding proteins have recently been identified, although the function of many of these proteins within mRNA translation is unknown. In this review we will discuss these methodologies and their shortcomings when applied to the study of translation, which need to be addressed to enable a better understanding of trans-acting translational regulatory proteins. Moreover, we discuss the protein domains that are responsible for RNA binding as well as the RNA motifs to which they bind, and the role of trans-acting ribosomal proteins in directing the translation of specific mRNAs. This article is categorized under: RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes Translation > Translation Regulation Translation > Translation Mechanisms.

Cord Blood CD8+ T Cells Have a Natural Propensity to Express IL-4 in a Fatty Acid Metabolism and Caspase Activation-Dependent Manner

How T cells differentiate in the neonate may critically determine the ability of the infant to cope with infections, respond to vaccines and avert allergies. Previously, we found that naïve cord blood CD4⁺ T cells differentiated toward an IL-4-expressing phenotype when activated in the presence of TGF-β and monocyte-derived inflammatory cytokines, the latter are more highly secreted by infants who developed food allergy. Here, we show that in the absence of IL-2 or IL-12, naïve cord blood CD8⁺ T cells have a natural propensity to differentiate into IL-4-producing non-classic T_C2 cells when they are activated alone, or in the presence of TGF-β and/or inflammatory cytokines. Mechanistically, non-classic T_C2 development is associated with decreased expression of IL-2 receptor alpha (CD25) and glycolysis, and increased fatty acid metabolism and caspase-dependent cell death. Consequently, the short chain fatty acid, sodium propionate (NaPo), enhanced IL-4 expression, but exogenous IL-2 or pan-caspase inhibition prevented IL-4 expression. In children with endoscopically and histologically confirmed non-inflammatory bowel disease and non-infectious pediatric idiopathic colitis, the presence of TGF-β, NaPo, and IL-1β or TNF-α promoted T_C2 differentiation in vitro. In vivo, colonic mucosa of children with colitis had significantly increased expression of IL-4 in CD8⁺ T cells compared with controls. In addition, activated caspase-3 and IL-4 were co-expressed in CD8⁺ T cells in the colonic mucosa of children with colitis. Thus, in the context of colonic inflammation and limited IL-2 signaling, CD8⁺ T cells differentiate into non-classic T_C2 that may contribute to the pathology of inflammatory/allergic diseases in children.

Dual abrogation of MNK and mTOR: a novel therapeutic approach for the treatment of aggressive cancers

Targeting the translational machinery has emerged as a promising therapeutic option for cancer treatment. Cancer cells require elevated protein synthesis and exhibit augmented activity to meet the increased metabolic demand. Eukaryotic translation initiation factor 4E is necessary for mRNA translation, its availability and phosphorylation are regulated by the PI3K/AKT/mTOR and MNK1/2 pathways. The phosphorylated form of eIF4E drives the expression of oncogenic proteins including those involved in metastasis. In this article, we will review the role of eIF4E in cancer, its regulation and discuss the benefit of dual inhibition of upstream pathways. The discernible interplay between the MNK and mTOR signaling pathways provides a novel therapeutic opportunity to target aggressive migratory cancers through the development of hybrid molecules.

Proteasome inhibitors bortezomib and carfilzomib used for the treatment of multiple myeloma do not inhibit the serine protease HtrA2/Omi

The proteasome inhibitor bortezomib is associated with the development of peripheral neuropathy in patients, but the mechanism is not fully understood.

The proteasome inhibitor bortezomib is associated with the development of peripheral neuropathy in patients, but the mechanism by which bortezomib can induce peripheral neuropathy is not fully understood. One study suggested that off-target inhibition of proteases other than the proteasome, particularly HtraA2/Omi, may be the underlying mechanism of the neuropathy. The same study also concluded that carfilzomib, a second proteasome inhibitor that is associated with less peripheral neuropathy in patients than bortezomib, showed no inhibition of HtrA2/Omi. The goal of the work described here was to determine whether either proteasome inhibitors truly affected HtrA2/Omi activity. A variety of methods were used to test the effects of both bortezomib and carfilzomib on HtrA2/Omi activity that included in vitro recombinant enzyme assays, and studies with the human neuroblastoma SH-SY5Y cell line and HtrA2/Omi-knockout mouse embryonic fibroblasts. The compound ucf-101 was used to assess the effects of specific HtrA2/Omi inhibition. In contrast to previously published data, our results clearly demonstrated that neither bortezomib nor carfilzomib inhibited HtrA2/Omi activity in recombinant enzyme assays at concentrations up to 100 μM, while the specific inhibitor ucf-101 did inhibit the enzyme. The proteasome inhibitors did not inhibit HtrA2/Omi activity in either SH-SY5Y cells or mouse embryonic fibroblasts, as determined by expression of the HtrA2/Omi substrates eIF4G1 and UCH-L1. Based on our biochemical and cell-based assays, we conclude that neither bortezomib nor carfilzomib inhibited HtrA2/Omi activity. Therefore, it is unlikely that bortezomib associated peripheral neuropathy is a direct result of off-target inhibition of HtrA2/Omi.

The role of mTOR signalling in the regulation of skeletal muscle mass in a rodent model of resistance exercise

Resistance exercise (RE) activates signalling by the mammalian target of rapamycin (mTOR), and it has been suggested that rapamycin-sensitive mTOR signalling controls RE-induced changes in protein synthesis, ribosome biogenesis, autophagy, and the expression of peroxisome proliferator gamma coactivator 1 alpha (PGC-1α). However, direct evidence to support the aforementioned relationships is lacking. Therefore, in this study, we investigated the role of rapamycin-sensitive mTOR in the RE-induced activation of muscle protein synthesis, ribosome biogenesis, PGC-1α expression and hypertrophy. The results indicated that the inhibition of rapamycin-sensitive mTOR could prevent the induction of ribosome biogenesis by RE, but it only partially inhibited the activation of muscle protein synthesis. Likewise, the inhibition of rapamycin-sensitive mTOR only partially blocked the hypertrophic effects of chronic RE. Furthermore, both acute and chronic RE promoted an increase in PGC-1α expression and these alterations were not affected by the inhibition of rapamycin-sensitive mTOR. Combined, the results from this study not only establish that rapamycin-sensitive mTOR plays an important role in the RE-induced activation of protein synthesis and the induction of hypertrophy, but they also demonstrate that additional (rapamycin-sensitive mTOR-independent) mechanisms contribute to these fundamentally important events.

Sumoylation of eIF4A2 affects stress granule formation

Regulation of protein synthesis is crucial for cells to maintain viability and to prevent unscheduled proliferation that could lead to tumorigenesis. Exposure to stress results in stalling of translation, with many translation initiation factors, ribosomal subunits and mRNAs being sequestered into stress granules or P bodies. This allows the re-programming of the translation machinery. Many aspects of translation are regulated by post-translational modification. Several proteomic screens have identified translation initiation factors as targets for sumoylation, although in many cases the role of this modification has not been determined. We show here that eIF4A2 is modified by SUMO, with sumoylation occurring on a single residue (K226). We demonstrate that sumoylation of eIF4A2 is modestly increased in response to arsenite and ionising radiation, but decreases in response to heat shock or hippuristanol. In arsenite-treated cells, but not in hippuristanol-treated cells, eIF4A2 is recruited to stress granules, suggesting sumoylation of eIF4A2 correlates with its recruitment to stress granules. Furthermore, we demonstrate that the inability to sumoylate eIF4A2 results in impaired stress granule formation, indicating a new role for sumoylation in the stress response.

Summary: In response to stress, proteins required to initiate protein synthesis are modified; we demonstrate that sumoylation of eIF4A2 correlates with its recruitment to stress granules.

Biological insights into the expression of translation initiation factors from recombinant CHOK1SV cell lines and their relationship to enhanced productivity

We show for translation initiation factors involved in formation of the closed loop mRNA, their expression is associated with recombinant antibody productivity in Chinese hamster ovary cells and maintaining these is important in determining the cells capacity for antibody productivity.

mTOR kinase-dependent, but raptor-independent regulation of downstream signaling is important for cell cycle exit and myogenic differentiation

Myogenic differentiation in the C2C12 myoblast model system reflects a concerted and controlled activation of transcription and translation following the exit of cells from the cell cycle. Previously we have shown that the mTORC1 signaling inhibitor, RAD001, decreased protein synthesis rates, delayed C2C12 myoblast differentiation, decreased p70S6K activity but did not affect the hypermodification of 4E-BP1. Here we have further investigated the modification of 4E-BP1 during the early phase of differentiation as cells exit the cell cycle, using inhibitors to target mTOR kinase and siRNAs to ablate the expression of raptor and rictor. As predicted, inhibition of mTOR kinase activity prevented p70S6K, 4E-BP1 phosphorylation and was associated with an inhibition of myogenic differentiation. Surprisingly, extensive depletion of raptor did not affect p70S6K or 4E-BP1 phosphorylation, but promoted an increase in mTORC2 activity (as evidenced by increased Akt Ser473 phosphorylation). These data suggest that an mTOR kinase-dependent, but raptor-independent regulation of downstream signaling is important for myogenic differentiation.

Apoptosis Triggers Specific, Rapid, and Global mRNA Decay with 3' Uridylated Intermediates Degraded by DIS3L2

Apoptosis is a tightly coordinated cell death program that damages mitochondria, DNA, proteins, and membrane lipids. Little is known about the fate of RNA as cells die. Here, we show that mRNAs, but not noncoding RNAs, are rapidly and globally degraded during apoptosis. mRNA decay is triggered early in apoptosis, preceding membrane lipid scrambling, genomic DNA fragmentation, and apoptotic changes to translation initiation factors. mRNA decay depends on mitochondrial outer membrane permeabilization and is amplified by caspase activation. 3′ truncated mRNA decay intermediates with nontemplated uridylate-rich tails are generated during apoptosis. These tails are added by the terminal uridylyl transferases (TUTases) ZCCHC6 and ZCCHC11, and the uridylated transcript intermediates are degraded by the 3′ to 5′ exonuclease DIS3L2. Knockdown of DIS3L2 or the TUTases inhibits apoptotic mRNA decay, translation arrest, and cell death, whereas DIS3L2 overexpression enhances cell death. Our results suggest that global mRNA decay is an overlooked hallmark of apoptosis.

Weighing up the possibilities: Controlling translation by ubiquitylation and sumoylation

Regulation of protein synthesis is of fundamental importance to cells. It has a critical role in the control of gene expression, and consequently cell growth and proliferation. The importance of this control is supported by the fact that protein synthesis is frequently upregulated in tumor cells. The major point at which regulation occurs is the initiation stage. Initiation of translation involves the interaction of several proteins to form the eIF4F complex, the recognition of the mRNA by this complex, and the subsequent recruitment of the 40S ribosomal subunit to the mRNA. This results in the formation of the 48S complex that then scans the mRNA for the start codon, engages the methionyl-tRNA and eventually forms the mature 80S ribosome which is elongation-competent. Formation of the 48S complex is regulated by the availability of individual initiation factors and through specific protein-protein interactions. Both of these events can be regulated by post-translational modification by ubiquitin or Ubls (ubiquitin-like modifiers) such as SUMO or ISG15. We provide here a summary of translation initiation factors that are modified by ubiquitin or Ubls and, where they have been studied in detail, describe the role of these modifications and their effects on regulating protein synthesis.

The role of IRES trans-acting factors in carcinogenesis

Regulation of protein expression through RNA metabolism is a key aspect of cellular homeostasis. Upon specific cellular stresses, distinct transcripts are selectively controlled to modify protein output in order to quickly and appropriately respond to stress. Reprogramming of the translation machinery is one node of this strict control that typically consists of an attenuation of the global, cap-dependent translation and accompanying switch to alternative mechanisms of translation initiation, such as internal ribosome entry site (IRES)-mediated initiation. In cancer, many aspects of the RNA metabolism are frequently misregulated to provide cancer cells with a growth and survival advantage. This includes changes in the expression and function of RNA binding proteins termed IRES trans-acting factors (ITAFs) that are central to IRES translation. In this review, we will examine select emerging, as well as established, ITAFs with important roles in cancer initiation and progression, and in particular their role in IRES-mediated translation. This article is part of a Special Issue entitled: Translation and Cancer.

The S. pombe translation initiation factor eIF4G is Sumoylated and associates with the SUMO protease Ulp2

SUMO is a small post-translational modifier, that is attached to lysine residues in target proteins. It acts by altering protein-protein interactions, protein localisation and protein activity. SUMO chains can also act as substrates for ubiquitination, resulting in proteasome-mediated degradation of the target protein. SUMO is removed from target proteins by one of a number of specific proteases. The processes of sumoylation and desumoylation have well documented roles in DNA metabolism and in the maintenance of chromatin structure. To further analyse the role of this modification, we have purified protein complexes containing the S. pombe SUMO protease, Ulp2. These complexes contain proteins required for ribosome biogenesis, RNA stability and protein synthesis. Here we have focussed on two translation initiation factors that we identified as co-purifying with Ulp2, eIF4G and eIF3h. We demonstrate that eIF4G, but not eIF3h, is sumoylated. This modification is increased under conditions that produce cytoplasmic stress granules. Consistent with this we observe partial co-localisation of eIF4G and SUMO in stressed cells. Using HeLa cells, we demonstrate that human eIF4GI is also sumoylated; in vitro studies indicate that human eIF4GI is modified on K1368 and K1588, that are located in the C-terminal eIF4A- and Mnk-binding sites respectively.

Modulation of the sensitivity of Jurkat T-cells to inhibition of protein synthesis by tumor necrosis factor α-related apoptosis-inducing ligand

Tumor necrosis factor α-related apoptosis-inducing ligand (TRAIL) is a potent inducer of apoptosis in Jurkat T lymphoma cells. One of the characteristics of the phase preceding overt apoptosis is the marked downregulation of protein synthesis. We have investigated factors that can influence this response and have explored some of the signaling pathways involved. We show that interferon-α (IFNα) pretreatment desensitizes Jurkat cells to TRAIL-induced inhibition of protein synthesis, such that the concentration of TRAIL required for 50% inhibition is increased by 10-fold. The inhibition of translation is characterized by dephosphorylation of the eIF4E-binding protein 4E-BP1 and IFNα desensitizes Jurkat cells to this effect. IFNα also inhibits TRAIL-mediated dephosphorylation of the growth-promoting protein kinase B (Akt). Since Jurkat cells are defective for phosphatase and tensin homolog deleted on chromosome 10 (PTEN) and therefore have constitutive phosphoinositide 3-kinase (PI3K) activity, we investigated the consequences for protein synthesis of inhibiting PI3K using LY294002. Inhibition of PI3K partially inhibits translation, but also enhances the effect of a suboptimal concentration of TRAIL. However, LY294002 does not block the ability of IFNα to protect protein synthesis from TRAIL-induced inhibition. Data are presented suggesting that IFNα impairs the process of activation of caspase-8 within the TRAIL death-inducing signaling complex.

The novel arylindolylmaleimide PDA-66 displays pronounced antiproliferative effects in acute lymphoblastic leukemia cells

Background

Prognosis of adult patients suffering from acute lymphoblastic leukemia (ALL) is still unsatisfactory. Targeted therapy via inhibition of deregulated signaling pathways appears to be a promising therapeutic option for the treatment of ALL. Herein, we evaluated the influence of a novel arylindolylmaleimide (PDA-66), a potential GSK3β inhibitor, on several ALL cell lines.

Methods

ALL cell lines (SEM, RS4;11, Jurkat and MOLT4) were exposed to different concentrations of PDA-66. Subsequently, proliferation, metabolic activity, apoptosis and necrosis, cell cycle distribution and protein expression of Wnt and PI3K/Akt signaling pathways were analyzed at different time points.

Results

PDA-66 inhibited the proliferation of ALL cells significantly by reduction of metabolic activity. The 72 h IC50 values ranged between 0.41 to 1.28 μM PDA-66. Additionally, caspase activated induction of apoptosis could be detected in the analyzed cell lines. PDA-66 influenced the cell cycle distribution of ALL cell lines differently. While RS4;11 and MOLT4 cells were found to be arrested in G2 phase, SEM cells showed an increased cell cycle in G0/1 phase.

Conclusion

PDA-66 displays significant antileukemic activity in ALL cells and classifies as candidate for further evaluation as a potential drug in targeted therapy of ALL.

Phosphorylation of eIF4GII and 4E-BP1 in response to nocodazole treatment: a reappraisal of translation initiation during mitosis

Translation mechanisms at different stages of the cell cycle have been studied for many years, resulting in the dogma that translation rates are slowed during mitosis, with cap-independent translation mechanisms favored to give expression of key regulatory proteins. However, such cell culture studies involve synchronization using harsh methods, which may in themselves stress cells and affect protein synthesis rates. One such commonly used chemical is the microtubule de-polymerization agent, nocodazole, which arrests cells in mitosis and has been used to demonstrate that translation rates are strongly reduced (down to 30% of that of asynchronous cells). Using synchronized HeLa cells released from a double thymidine block (G 1/S boundary) or the Cdk1 inhibitor, RO3306 (G 2/M boundary), we have systematically re-addressed this dogma. Using FACS analysis and pulse labeling of proteins with labeled methionine, we now show that translation rates do not slow as cells enter mitosis. This study is complemented by studies employing confocal microscopy, which show enrichment of translation initiation factors at the microtubule organizing centers, mitotic spindle, and midbody structure during the final steps of cytokinesis, suggesting that translation is maintained during mitosis. Furthermore, we show that inhibition of translation in response to extended times of exposure to nocodazole reflects increased eIF2α phosphorylation, disaggregation of polysomes, and hyperphosphorylation of selected initiation factors, including novel Cdk1-dependent N-terminal phosphorylation of eIF4GII. Our work suggests that effects on translation in nocodazole-arrested cells might be related to those of the treatment used to synchronize cells rather than cell cycle status.

Cap-independent translation initiation of apaf-1 mRNA based on a scanning mechanism is determined by some features of the secondary structure of its 5' untranslated region

We have earlier shown that the 5'-untranslated region (5' UTR) of the mRNA coding for activation factor of apoptotic peptidase 1 (Apaf-1) can direct translation in vivo by strictly 5' end-dependent way even in the absence of m(7)G-cap. Dependence of translational efficiency on the cap availability for this mRNA turned out to be relatively low. In this study we demonstrate that this surprising phenomenon is determined the 5'-proximal part (domains I and II) of highly structured Apaf-1 5' UTR. Remarkably, domain II by itself was able to reduce dependence of the mRNA on the cap on its transferring to a short 5' UTR derived from a standard vector. We suggest that the low cap-dependence inherent to some cellular mRNAs may have an important physiological significance under those stress conditions when the function of cap-binding factor eIF4E is impaired.

Live or let die: posttranscriptional gene regulation in cell stress and cell death

Studies of the regulation of gene expression historically focused on transcription. However, during stress and apoptosis, profound gene expression changes occur more rapidly and globally than is possible by regulating transcription. Posttranscriptional changes in mRNA processing and translation in response to diverse stresses shut down most protein translation to conserve energy and lead to rapid remodeling of the proteome to promote repair. Pre-mRNA splicing and mRNA stability are fundamentally altered under some stress conditions. Stress pathways coordinate a cytoprotective repair response, while simultaneously initiating signaling that can ultimately trigger cell death. How the cell mediates the decision between repair and apoptosis is largely not understood. In some stresses, microRNAs may tip the balance. Here, we review what is known about posttranscriptional gene regulation during stress, focusing on what is still unknown and how new technologies might be used to understand what changes are most physiologically important in different forms of stress and death.

Combined inhibition of p97 and the proteasome causes lethal disruption of the secretory apparatus in multiple myeloma cells

Inhibition of the proteasome is a widely used strategy for treating multiple myeloma that takes advantage of the heavy secretory load that multiple myeloma cells (MMCs) have to deal with. Resistance of MMCs to proteasome inhibition has been linked to incomplete disruption of proteasomal endoplasmic-reticulum (ER)-associated degradation (ERAD) and activation of non-proteasomal protein degradation pathways. The ATPase p97 (VCP/Cdc48) has key roles in mediating both ERAD and non-proteasomal protein degradation and can be targeted pharmacologically by small molecule inhibition. In this study, we compared the effects of p97 inhibition with Eeyarestatin 1 and DBeQ on the secretory apparatus of MMCs with the effects induced by the proteasome inhibitor bortezomib, and the effects caused by combined inhibition of p97 and the proteasome. We found that p97 inhibition elicits cellular responses that are different from those induced by proteasome inhibition, and that the responses differ considerably between MMC lines. Moreover, we found that dual inhibition of both p97 and the proteasome terminally disrupts ER configuration and intracellular protein metabolism in MMCs. Dual inhibition of p97 and the proteasome induced high levels of apoptosis in all of the MMC lines that we analysed, including bortezomib-adapted AMO-1 cells, and was also effective in killing primary MMCs. Only minor toxicity was observed in untransformed and non-secretory cells. Our observations highlight non-redundant roles of p97 and the proteasome in maintaining secretory homeostasis in MMCs and provide a preclinical conceptual framework for dual targeting of p97 and the proteasome as a potential new therapeutic strategy in multiple myeloma.

mRNA encoding WAVE-Arp2/3-associated proteins is co-localized with foci of active protein synthesis at the leading edge of MRC5 fibroblasts during cell migration

During cell spreading, mammalian cells migrate using lamellipodia formed from a large dense branched actin network which produces the protrusive force required for leading edge advancement. The formation of lamellipodia is a dynamic process and is dependent on a variety of protein cofactors that mediate their local regulation, structural characteristics and dynamics. In the present study, we show that mRNAs encoding some structural and regulatory components of the WAVE [WASP (Wiskott-Aldrich syndrome protein) verprolin homologous] complex are localized to the leading edge of the cell and associated with sites of active translation. Furthermore, we demonstrate that steady-state levels of ArpC2 and Rac1 proteins increase at the leading edge during cell spreading, suggesting that localized protein synthesis has a pivotal role in controlling cell spreading and migration.

The 5' untranslated region of Apaf-1 mRNA directs translation under apoptosis conditions via a 5' end-dependent scanning mechanism

We have previously shown that translation driven by the 5' UTR of Apaf-1 mRNA is relatively efficient in the absence of m7G-cap, but no IRES is involved. Nevertheless, it may be speculated that a "silent" IRES is activated under apoptosis conditions. Here, we show that translation of the mRNA with the Apaf-1 5' UTR is relatively resistant to apoptosis induced by etoposide when eIF4E is sequestered by 4E-BP and eIF4G is partially cleaved. However, translation under these conditions remains governed by 5' end-dependent scanning. We hypothesize that the observed phenomenon is based on the intrinsic low cap-dependence of the Apaf-1 5' UTR.

Multiple components of eIF4F are required for protein synthesis-dependent hippocampal long-term potentiation

Persistent forms of synaptic plasticity are widely thought to require the synthesis of new proteins. This feature of long-lasting forms of plasticity largely has been demonstrated using inhibitors of general protein synthesis, such as either anisomycin or emetine. However, these drugs, which inhibit elongation, cannot address detailed questions about the regulation of translation initiation, where the majority of translational control occurs. Moreover, general protein synthesis inhibitors cannot distinguish between cap-dependent and cap-independent modes of translation initiation. In the present study, we took advantage of two novel compounds, 4EGI-1 and hippuristanol, each of which targets a different component of the eukaryotic initiation factor (eIF)4F initiation complex, and investigated their effects on long-term potentiation (LTP) at CA3-CA1 synapses in the hippocampus. We found that 4EGI-1 and hippuristanol both attenuated long-lasting late-phase LTP induced by two different stimulation paradigms. We also found that 4EGI-1 and hippuristanol each were capable of blocking the expression of newly synthesized proteins immediately after the induction of late-phase LTP. These new pharmacological tools allow for a more precise dissection of the role played by translational control pathways in synaptic plasticity and demonstrate the importance of multiple aspects of eIF4F in processes underlying hippocampal LTP, laying the foundation for future studies investigating the role of eIF4F in hippocampus-dependent memory processes.

Programmed cell death 4 (PDCD4): a novel player in ethanol-mediated suppression of protein translation in primary cortical neurons and developing cerebral cortex

BACKGROUND

Prenatal exposure to ethanol (EtOH) elicits a range of neuro-developmental abnormalities, microcephaly to behavioral deficits. Impaired protein synthesis has been connected to pathogenesis of EtOH-induced brain damage and abnormal neuron development. However, mechanisms underlying these impairments of protein synthesis are not known. In this study, we illustrate the effects of EtOH on programmed cell death protein 4 (PDCD4), a tumor and translation repressor.

METHODS

Primary cortical neurons (PCNs) were treated with 2.5 and 4 mg/ml EtOH for different time points (4 to 24 hours), and PDCD4 expression was detected by Western blotting. Protein synthesis was determined using [(35) S] methionine incorporation assay. Methyl cap pull-down assay was performed to establish the effect of EtOH on association of eukaryotic initiation factor 4A (eIF4A) with capped mRNA. Luciferase assay was performed to determine the in vivo translation. A 2-day acute 5-dose binge model with EtOH (4 g/kg body wt, 25% v/v) was performed in Sprague-Dawley rats at 12-hour intervals and analyzed for PDCD4, eIF4A, and eIF4A-methyl cap association.

RESULTS

EtOH increased PDCD4 expression in a time- and dose-dependent manner in PCNs, which inhibited the association of eIF4A with methyl cap. EtOH and ectopic PDCD4 expression suppressed in vivo translation in PCNs and RNAi targeting of PDCD4 blocked the inhibitory effect of EtOH on protein synthesis. In utero exposure of pregnant rats to EtOH resulted in a significant increase in PDCD4 in fetal cerebral cortex along with the inhibition of methyl cap-associated eIF4A, compared with isocaloric controls. Increased PDCD4 also occurred in pooled fractions of remaining brain regions.

CONCLUSIONS

Our data, for the first time, illustrate that PDCD4 mediates inhibitory effects of EtOH on protein synthesis in PCNs and developing brain.

IRES-Dependent Translational Control during Virus-Induced Endoplasmic Reticulum Stress and Apoptosis

Many virus infections and stresses can induce endoplasmic reticulum (ER) stress response, a host self-defense mechanism against viral invasion and stress. During this event, viral and cellular gene expression is actively regulated and often encounters a switching of the translation initiation from cap-dependent to internal ribosome-entry sites (IRES)-dependent. This switching is largely dependent on the mRNA structure of the 5′ untranslated region (5′ UTR) and on the particular stress stimuli. Picornaviruses and some other viruses contain IRESs within their 5′ UTR of viral genome and employ an IRES-driven mechanism for translation initiation. Recently, a growing number of cellular genes involved in growth control, cell cycle progression and apoptosis were also found to contain one or more IRES within their long highly structured 5′ UTRs. These genes initiate translation usually by a cap-dependent mechanism under normal physiological conditions; however, in certain environments, such as infection, starvation, and heat shock they shift translation initiation to an IRES-dependent modality. Although the molecular mechanism is not entirely understood, a number of studies have revealed that several cellular biochemical processes are responsible for the switching of translation initiation to IRES-dependent. These include the cleavage of translation initiation factors by viral and/or host proteases, phosphorylation (inactivation) of host factors for translation initiation, overproduction of homologous proteins of cap-binding protein eukaryotic initiation factors (eIF)4E, suppression of cap-binding protein eIF4E expression by specific microRNA, activation of enzymes for mRNA decapping, as well as others. Here, we summarize the recent advances in our understanding of the molecular mechanisms for the switching of translation initiation, particularly for the proteins involved in cell survival and apoptosis in the ER stress pathways during viral infections.

Protein phosphatase 2A promotes endothelial survival via stabilization of translational inhibitor 4E-BP1 following exposure to tumor necrosis factor-α

OBJECTIVE

Tumor necrosis factor-α (TNFα) may change from a stimulator of reversible activation of endothelial cells (ECs) to a killer when combined with cycloheximide (CHX). The means by which endothelial cells are destined to either the survival pathway or the apoptotic pathway are not fully understood. We investigated the role of p38 mitogen-activated protein kinase (MAPK) and protein phosphatase 2A (PP2A) activation and their regulation of 4E-BP1 stability in ECs to determine whether this pathway contributes to apoptosis induced by TNFα and CHX.

METHODS AND RESULTS

Apoptosis was induced in human umbilical vein ECs (HUVECs) by treating them with a combination of TNFα and CHX (TNFα/CHX). Activation of p38 MAPK was increased in HUVECs undergoing apoptosis, which was associated with degradation of eukaryotic initiation factor 4A regulator 4E-BP1 in a p38 MAPK-dependent manner. CHX attenuated a TNFα-stimulated increase in the expression and activity of PP2A. Silencing PP2A expression with small interfering RNA transfection mimicked CHX sensitization, increasing HUVEC apoptosis with TNFα stimulation and suggesting a protective role for PP2A in the apoptotic process.

CONCLUSION

Our data suggest that (1) TNFα stimulates PP2A and HUVECs elude apoptosis by PP2A-dependent dephosphorylation of p38 MAPK, and (2) CHX-induced inhibition of PP2A leads to maintenance of p38 activity and degradation of 4E-BP1, resulting in enhanced TNFα-induced apoptosis.

Ischemia-induced calpain activation causes eukaryotic (translation) initiation factor 4G1 (eIF4GI) degradation, protein synthesis inhibition, and neuronal death

Persistent protein synthesis inhibition (PSI) is a robust predictor of eventual neuronal death following cerebral ischemia. We thus tested the hypothesis that persistent PSI inhibition and neuronal death are causally linked. Neuronal viability strongly correlated with both protein synthesis and levels of eukaryotic (translation) initiation factor 4G1 (eIF4G1). We determined that in vitro ischemia activated calpain, which degraded eIF4G1. Overexpression of the calpain inhibitor calpastatin or eIF4G1 resulted in increased protein synthesis and increased neuronal viability compared with controls. The neuroprotective effect of eIF4G1 overexpression was due to restoration of cap-dependent protein synthesis, as well as protein synthesis-independent mechanisms, as inhibition of protein synthesis with cycloheximide did not completely prevent the protective effect of eIF4G1 overexpression. In contrast, shRNA-mediated silencing of eIF4G1 exacerbated ischemia-induced neuronal injury, suggesting eIF4G1 is necessary for maintenance of neuronal viability. Finally, calpain inhibition following global ischemia in vivo blocked decreases in eIF4G1, facilitated protein synthesis, and increased neuronal viability in ischemia-vulnerable hippocampal CA1 neurons. Collectively, these data demonstrate that calpain-mediated degradation of a translation initiation factor, eIF4G1, is a cause of both persistent PSI and neuronal death.

Translation initiation factors and active sites of protein synthesis co-localize at the leading edge of migrating fibroblasts

Cell migration is a highly controlled essential cellular process, often dysregulated in tumour cells, dynamically controlled by the architecture of the cell. Studies involving cellular fractionation and microarray profiling have previously identified functionally distinct mRNA populations specific to cellular organelles and architectural compartments. However, the interaction between the translational machinery itself and cellular structures is relatively unexplored. To help understand the role for the compartmentalization and localized protein synthesis in cell migration, we have used scanning confocal microscopy, immunofluorescence and a novel ribopuromycylation method to visualize translating ribosomes. In the present study we show that eIFs (eukaryotic initiation factors) localize to the leading edge of migrating MRC5 fibroblasts in a process dependent on TGN (trans-Golgi network) to plasma membrane vesicle transport. We show that eIF4E and eIF4GI are associated with the Golgi apparatus and membrane microdomains, and that a proportion of these proteins co-localize to sites of active translation at the leading edge of migrating cells.

Multimodality molecular imaging of apoptosis in oncology

OBJECTIVE

The purposes of this review are to describe the signaling pathways of and the cellular changes that occur with apoptosis and other forms of cell death, summarize tracers and modalities used for imaging of apoptosis, delineate the relation between apoptosis and inhibition of protein translation, and describe spectroscopic technologies that entail high-frequency ultrasound and infrared and midinfrared light in characterizing the intracellular events of apoptosis.

CONCLUSION

Apoptosis is a highly orchestrated set of biochemical and morphologic cellular events. These events present many potential targets for the imaging of apoptosis in vivo. Imaging of apoptosis can facilitate early assessment of anticancer treatment before tumor shrinkage, which may increase the effectiveness of delivery of chemotherapy and radiation therapy and speed drug development.

Rat eggs cannot wait: Spontaneous exit from meiotic metaphase-II arrest

Mammalian eggs await fertilisation while arrested at the second metaphase stage of meiotic division. A network of signalling pathways enables the establishment and maintenance of this metaphase-II arrest. In the absence of fertilisation, mammalian eggs can spontaneously exit metaphase II when parthenogenetically stimulated, or sometimes without any obvious stimulation. Ovulated rat eggs abortively release from metaphase-II arrest once removed from egg donors. Spontaneously activated rat eggs extrude the second polar body and proceed to the so-called metaphase III-'like' stage, with clumps of condensed chromatin scattered in the egg cytoplasm. It is still unclear what makes rat eggs susceptible to spontaneous activation; however, a vague picture of the signalling pathways involved in the process of spontaneous activation is beginning to emerge. Such cell cycle instability is one of the major reasons why it is more difficult to establish nuclear transfer in the rat. This review examines the known predisposing factors and biochemical mechanisms involved in spontaneous activation. The strategies used to prevent spontaneous metaphase-II release in rat eggs will also be discussed.

The human insulin mRNA is partly translated via a cap- and eIF4A-independent mechanism

The aim of this study was to investigate whether cap-independent insulin mRNA translation occurs in human pancreatic islets at basal conditions, during stimulation at a high glucose concentration and at conditions of nitrosative stress. We also aimed at correlating cap-independent insulin mRNA translation with binding of the IRES trans-acting factor polypyrimidine tract binding protein (PTB) to the 5'-UTR of insulin mRNA. For this purpose, human islets were incubated for 2h in the presence of low (1.67 mM) or high glucose (16.7 mM). Nitrosative stress was induced by addition of 1 mM DETA/NO and cap-dependent mRNA translation was inhibited with hippuristanol. Insulin biosynthesis rates were determined by radioactive labeling and immunoprecipitation. PTB affinity to insulin mRNA 5'-UTR was assessed by a magnetic micro bead pull-down procedure. We observed that in the presence of 1.67 mM glucose, approximately 70% of the insulin mRNA translation was inhibited by hippuristanol. Corresponding value from islets incubated at 16.7 mM glucose was 93%. DETA/NO treatment significantly decreased the translation of insulin by 85% in high glucose incubated islets, and by 50% at a low glucose concentration. The lowered insulin biosynthesis rates of DETA/NO-exposed islets were further suppressed by hippuristanol with 55% at 16.7 mM glucose but not at 1.67 mM glucose. Thus, hippuristanol-induced inhibition of insulin biosynthesis was less pronounced in DETA/NO-treated islets as compared to control islets. We observed also that PTB bound specifically to the insulin mRNA 5'-UTR in vitro, and that this binding corresponded well with rates of cap-independent insulin biosynthesis at the different conditions. In conclusion, our studies show that insulin biosynthesis is mainly cap-dependent at a high glucose concentration, but that the cap-independent biosynthesis of insulin can constitute as much as 40-100% of all insulin biosynthesis during conditions of nitrosative stress. These data suggest that the pancreatic β-cell is able to uphold basal insulin synthesis at conditions of starvation and stress via a cap- and eIF4A-independent mechanism, possibly mediated by the binding of PTB to the 5'-UTR of the human insulin mRNA.

Inhibition of the interactions between eukaryotic initiation factors 4E and 4G impairs long-term associative memory consolidation but not reconsolidation

Considerable evidence indicates that the general blockade of protein synthesis prevents both the initial consolidation and the postretrieval reconsolidation of long-term memories. These findings come largely from studies of drugs that block ribosomal function, so as to globally interfere with both cap-dependent and -independent forms of translation. Here we show that intra-amygdala microinfusions of 4EGI-1, a small molecule inhibitor of cap-dependent translation that selectively disrupts the interaction between eukaryotic initiation factors (eIF) 4E and 4G, attenuates fear memory consolidation but not reconsolidation. Using a combination of behavioral and biochemical techniques, we provide both in vitro and in vivo evidence that the eIF4E-eIF4G complex is more stringently required for plasticity induced by initial learning than for that triggered by reactivation of an existing memory.

Analysis of the protein expression changes during taxol-induced apoptosis under translation inhibition conditions

Taxol is currently used in chemotherapeutic treatments of different types of cancers. In this article, we demonstrate that taxol induces apoptosis and translation down-regulation in human embryonic kidney (HEK293T) cells. Antibody arrays are a promising new tool for the analysis of protein levels changes in cells responding to different stimuli. Using this approach, we have identified changes in the expression of 38 proteins (20 down-regulated and 18 up-regulated), implicated in several cellular processes mainly in apoptosis, cell cycle and signal transduction pathways, and also cytoskeleton proteins. Among them, we have confirmed a considerable decrease in the expression of p14(ARF) and a significant increase in the levels of dystrophin and c-Myc. It is known that c-Myc mRNA has an internal ribosome entry segment (IRES) element in its 5'UTR that could regulate its expression under global protein synthesis inhibition conditions. We demonstrate that after taxol treatment, the c-Myc IRES activity is maintained meanwhile cap-dependent activity is inhibited. In addition, an increase in c-Myc mRNA was also observed after taxol treatment. We conclude that taxol-induced c-Myc expression is regulated at both transcriptional and translational levels, the last of them by a mechanism mediated by IRES.

Natural occurrence and physiological role of a truncated eIF4E in the porcine endometrium during implantation

The present study is the first report providing evidence for a physiological role of a truncated form of the mRNA cap-binding protein eIF4E1 (eukaryotic initiation factor 4E1). Our initial observation was that eIF4E, which mediates the mRNA cap function by recruiting the eIF4F complex (composed of eIF4E, 4G and 4A), occurs in two forms in porcine endometrial tissue in a strictly temporally restricted fashion. The ubiquitous prototypical 25 kDa form of eIF4E was found in ovariectomized and cyclic animals. A new stable 23 kDa variant, however, is predominant during early pregnancy at the time of implantation. Northern blotting, cDNA sequence analysis, in vitro protease assays and MS showed that the 23 kDa form does not belong to a new class of eIF4E proteins. It represents a proteolytically processed variant of eIF4E1, lacking not more than 21 amino acids at the N-terminus. Steroid replacements indicated that progesterone in combination with 17β-oestradiol induced the formation of the 23 kDa eIF4E. Modified cell-free translation systems mimicking the situation in the endometrium revealed that, besides eIF4E, eIF4G was also truncated, but not eIF4A or PABP [poly(A)-binding protein]. The 23 kDa form of eIF4E reduced the repressive function of 4E-BP1 (eIF4E-binding protein 1) and the truncated eIF4G lacked the PABP-binding site. Thus we suggest that the truncated eIF4E provides an alternative regulation mechanism by an altered dynamic of eIF4E/4E-BP1 binding under conditions where 4E-BP1 is hypophosphorylated. Together with the impaired eIF4G–PABP interaction, the modified translational initiation might particularly regulate protein synthesis during conceptus attachment at the time of implantation.

Localization of ribosomes and translation initiation factors to talin/beta3-integrin-enriched adhesion complexes in spreading and migrating mammalian cells

BACKGROUND INFORMATION

The spatial localization of translation can facilitate the enrichment of proteins at their sites of function while also ensuring that proteins are expressed in the proximity of their cognate binding partners.

RESULTS

Using human embryonic lung fibroblasts and employing confocal imaging and biochemical fractionation techniques, we show that ribosomes, translation initiation factors and specific RNA-binding proteins localize to nascent focal complexes along the distal edge of migrating lamellipodia. 40S ribosomal subunits appear to associate preferentially with beta3 integrin in focal adhesions at the leading edges of spreading cells, with this association strongly augmented by a synergistic effect of cell engagement with a mixture of extracellular matrix proteins. However, both ribosome and initiation factor localizations do not require de novo protein synthesis.

CONCLUSIONS

Taken together, these findings demonstrate that repression, complex post-transcriptional regulation and modulation of mRNA stability could potentially be taking place along the distal edge of migrating lamellipodia.

Visualizing and quantifying the differential cleavages of the eukaryotic translation initiation factors eIF4GI and eIF4GII in the enterovirus-infected cell

Enterovirus (EV) infection has been shown to cause a marked shutoff of host protein synthesis, an event mainly achieved through the cleavages of eukaryotic translation initiation factors eIF4GI and eIF4GII that are mediated by viral 2A protease (2A(pro)). Using fluorescence resonance energy transfer (FRET), we developed genetically encoded and FRET-based biosensors to visualize and quantify the specific proteolytic process in intact cells. This was accomplished by stable expression of a fusion substrate construct composed of the green fluorescent protein 2 (GFP(2)) and red fluorescent protein 2 (DsRed2), with a cleavage motif on eIF4GI or eIF4GII connected in between. The FRET biosensor showed a real-time and quantifiable impairment of FRET upon EV infection. Levels of the reduced FRET closely correlated with the cleavage kinetics of the endogenous eIF4Gs isoforms. The FRET impairments were solely attributed to 2A(pro) catalytic activity, irrespective of other viral-encoded protease, the activated caspases or general inhibition of protein synthesis in the EV-infected cells. The FRET biosensors appeared to be a universal platform for several related EVs. The spatiotemporal and quantitative imaging enabled by FRET can shed light on the protease-substrate behaviors in their normal milieu, permitting investigation into the molecular mechanism underlying virus-induced host translation inhibition.

Inhibition of mammalian target of rapamycin (mTOR) signalling in C2C12 myoblasts prevents myogenic differentiation without affecting the hyperphosphorylation of 4E-BP1

Current accepted models suggest that hypophosphorylated 4E-binding protein (4E-BP1) binds to initiation factor 4E (eIF4E) to inhibit cap-dependent translation, a process readily reversed by its phosphorylation following activation of mammalian target of rapamycin (mTORC1) signalling. Myogenic differentiation in the C2C12 myoblast model system reflects a concerted and controlled activation of transcription and translation following the exit of cells from the cell cycle. Here we show that myogenic differentiation is associated with increased rates of translation, the up-regulation of both 4E-BP1 mRNA and protein levels and enhanced levels of eIF4E/4E-BP1 complex. Paradoxically, treatment of C2C12 myoblasts with an inhibitor of mTOR signalling (RAD001) which inhibits translation, promotes the hyperphosphorylation of 4E-BP1 on novel sites and prevents the increase in 4E-BP1 levels. In contrast, eIF4E appears to be under translational control with a significant delay between induction of mRNA and subsequent protein expression.

Immunity and the regulation of protein synthesis: surprising connections

The plasticity that is needed by the cell to respond to rapid changes in its environment cannot only be provided by means of transcriptional regulation, which generally confers on cells a set of stable properties. Alternatively, the control of mRNA translation allows the cell to modulate rapidly and over short period of time its gene expression program, without invoking the slower nuclear pathways for mRNA synthesis and transport. Several recent findings indicate that regulation of translation affects directly antigen presentation, cytokine production, as well as the survival of dendritic cells. I describe here some of the regulatory mechanisms that control translation in response to microbial products or cytokine exposure and their contribution to the overall immune response.

PI3K signaling regulates rapamycin-insensitive translation initiation complex formation in vaccinia virus-infected cells

How vaccinia virus (VV) regulates assembly of the host translation initiation complex eIF4F remains unclear. Here, we show that VV activated host PI3K to stimulate downstream mammalian target of rapamycin (mTOR), a kinase that inactivates the translational repressor 4E-BP1. However, although the mTOR inhibitor rapamycin suppressed VV-induced inactivation of 4E-BP1, it failed to inhibit eIF4F assembly. In contrast, PI3K inhibition in VV-infected cells increased the abundance of hypophosphorylated 4E-BP1 and disrupted eIF4F complex formation. PI3K signaling, therefore, plays a critical role in regulating protein production during VV infection, at least in part by controlling the abundance and activity of 4E-BP1.

Lovastatin effect in rat neuroblasts of the CNS: inhibition of cap-dependent translation

Mevalonate biosynthesis pathway is important in cell growth and survival and its blockade by 3-hydroxy-3-methylglutaryl CoA reductase inhibitors, statins, arrest brain neuroblasts growth and induce apoptosis. Translation is among the main biochemical mechanisms that controls gene expression and therefore cell growth or apoptosis. In the CNS, translation regulates synaptic plasticity. Thus, our aim was to investigate the effect of lovastatin in protein translation in rat neuroblasts of the CNS and the biochemical pathways involved. Lovastatin treatment in rat brain neuroblasts causes a significant time- and concentration-inhibition of protein synthesis, which is partially mediated by phosphatydilinositol 3-kinase/mammalian target of rapamycin (mTOR) pathway inhibition. Lovastatin treatment decreases the phosphorylation state of mTOR substrates, p70S6K and eukaryotic translation initiation factor (eIF) 4E-binding protein 1 and simultaneously increases eIF4E-binding protein 1 in a time-dependent manner. Concomitantly, lovastatin causes a decrease in eIF4G cellular amount, which is partially mediated by caspase(s) activity excluding caspase 3. These biochemical pathways affected by lovastatin might explain the protein translation inhibition observed in neuroblasts. Cycloheximide treatment, which blocked protein synthesis, does not induce neuroblasts apoptosis. Therefore, we suggest that lovastatin-induced protein synthesis inhibition might not contribute to the concomitant neuroblasts apoptosis previously observed.

Nitric oxide mediates NMDA-induced persistent inhibition of protein synthesis through dephosphorylation of eukaryotic initiation factor 4E-binding protein 1 and eukaryotic initiation factor 4G proteolysis

Cerebral ischaemia causes long-lasting protein synthesis inhibition that is believed to contribute to brain damage. Energy depletion promotes translation inhibition during ischaemia, and the phosphorylation of eIF (eukaryotic initiation factor) 2α is involved in the translation inhibition induced by early ischaemia/reperfusion. However, the molecular mechanisms underlying prolonged translation down-regulation remain elusive. NMDA (N-methyl-D-aspartate) excitotoxicity is also involved in ischaemic damage, as exposure to NMDA impairs translation and promotes the synthesis of NO (nitric oxide), which can also inhibit translation. In the present study, we investigated whether NO was involved in NMDA-induced protein synthesis inhibition in neurons and studied the underlying molecular mechanisms. NMDA and the NO donor DEA/NO (diethylamine–nitric oxide sodium complex) both inhibited protein synthesis and this effect persisted after a 30 min exposure. Treatments with NMDA or NO promoted calpain-dependent eIF4G cleavage and 4E-BP1 (eIF4E-binding protein 1) dephosphorylation and also abolished the formation of eIF4E–eIF4G complexes; however, they did not induce eIF2α phosphorylation. Although NOS (NO synthase) inhibitors did not prevent protein synthesis inhibition during 30 min of NMDA exposure, they did abrogate the persistent inhibition of translation observed after NMDA removal. NOS inhibitors also prevented NMDA-induced eIF4G degradation, 4E-BP1 dephosphorylation, decreased eIF4E–eIF4G-binding and cell death. Although the calpain inhibitor calpeptin blocked NMDA-induced eIF4G degradation, it did not prevent 4E-BP1 dephosphorylation, which precludes eIF4E availability, and thus translation inhibition was maintained. The present study suggests that eIF4G integrity and hyperphosphorylated 4E-BP1 are needed to ensure appropriate translation in neurons. In conclusion, our data show that NO mediates NMDA-induced persistent translation inhibition and suggest that deficient eIF4F activity contributes to this process.

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 translation is required for dendritic cell function and survival during activation

In response to inflammatory stimulation, dendritic cells (DCs) have a remarkable pattern of differentiation (maturation) that exhibits specific mechanisms to control antigen processing and presentation. Here, we show that in response to lipopolysaccharides, protein synthesis is rapidly enhanced in DCs. This enhancement occurs via a PI3K-dependent signaling pathway and is key for DC activation. In addition, we show that later on, in a manner similar to viral or apoptotic stress, DC activation leads to the phosphorylation and proteolysis of important translation initiation factors, thus inhibiting cap-dependent translation. This inhibition correlates with major changes in the origin of the peptides presented by MHC class I and the ability of mature DCs to prevent cell death. Our observations have important implications in linking translation regulation with DC function and survival during the immune response.