Cleavage of translation initiation factor 4G (eIF4G) during anti-Fas IgM-induced apoptosis does not require signalling through the p38 mitogen-activated protein (MAP) kinase

Mammalian CBX7 isoforms p36 and p22 exhibit differential responses to serum, varying functions for proliferation, and distinct subcellular localization

CBX7 is a polycomb group protein, and despite being implicated in many diseases, its role in cell proliferation has been controversial: some groups described its pro-proliferative properties, but others illustrated its inhibitory effects on cell growth. To date, the reason for the divergent observations remains unknown. While several isoforms for CBX7 were reported, no studies investigated whether the divergent roles of CBX7 could be due to distinct functions of CBX7 isoforms. In this study, we newly identified mouse CBX7 transcript variant 1 (mCbx7v1), which is homologous to the human CBX7 gene (hCBX7v1) and is expressed in various mouse organs. We revealed that mCbx7v1 and hCBX7v1 encode a 36 kDa protein (p36^CBX7) whereas mCbx7 and hCBX7v3 encode a 22 kDa protein (p22^CBX7). This study further demonstrated that p36^CBX7 was localized to the nucleus and endogenously expressed in proliferating cells whereas p22^CBX7 was localized to the cytoplasm, induced by serum starvation in both human and mouse cells, and inhibited cell proliferation. Together, these data indicate that CBX7 isoforms are localized in different locations in a cell and play differing roles in cell proliferation. This varying function of CBX7 isoforms may help us understand the distinct function of CBX7 in various studies.

Cap-Independent Translational Control of Carcinogenesis

Translational regulation has been shown to play an important role in cancer and tumor progression. Despite this fact, the role of translational control in cancer is an understudied and under appreciated field, most likely due to the technological hurdles and paucity of methods available to establish that changes in protein levels are due to translational regulation. Tumors are subjected to many adverse stress conditions such as hypoxia or starvation. Under stress conditions, translation is globally downregulated through several different pathways in order to conserve energy and nutrients. Many of the proteins that are synthesized during stress in order to cope with the stress use a non-canonical or cap-independent mechanism of initiation. Tumor cells have utilized these alternative mechanisms of translation initiation to promote survival during tumor progression. This review will specifically discuss the role of cap-independent translation initiation, which relies on an internal ribosome entry site (IRES) to recruit the ribosomal subunits internally to the messenger RNA. We will provide an overview of the role of IRES-mediated translation in cancer by discussing the types of genes that use IRESs and the conditions under which these mechanisms of initiation are used. We will specifically focus on three well-studied examples: Apaf-1, p53, and c-Jun, where IRES-mediated translation has been demonstrated to play an important role in tumorigenesis or tumor progression.

Surfactant protein SP-D modulates activity of immune cells: proteomic profiling of its interaction with eosinophilic cells

Surfactant protein D (SP-D), a C-type lectin, is known to protect against lung infection, allergy and inflammation. Its recombinant truncated form comprising homotrimeric neck and CRD region (rhSP-D) has been shown to bring down specific IgE levels, eosinophilia and restore Th2-Th1 homeostasis in murine models of lung hypersensitivity. SP-D knockout mice show intrinsic hypereosinophilia and airway hyper-responsiveness that can be alleviated by rhSP-D. The rhSP-D can bind activated eosinophils, inhibit chemotaxis and degranulation, and selectively induce oxidative burst and apoptosis in sensitized eosinophils. A global proteomics study of rhSP-D-treated eosinophilic cell line AML14.3D10 identified large-scale molecular changes associated with oxidative burst, cell stress and survival-related proteins potentially responsible for apoptosis induction. The data also suggested an involvement of RNA binding- and RNA splicing-related proteins. Thus, the proteomics approach yielded a catalog of differentially expressed proteins that may be protein signatures defining mechanisms of SP-D-mediated maintenance of homeostasis during allergy.

At the intersection of non-coding transcription, DNA repair, chromatin structure, and cellular senescence

It is well accepted that non-coding RNAs play a critical role in regulating gene expression. Recent paradigm-setting studies are now revealing that non-coding RNAs, other than microRNAs, also play intriguing roles in the maintenance of chromatin structure, in the DNA damage response, and in adult human stem cell aging. In this review, we will discuss the complex inter-dependent relationships among non-coding RNA transcription, maintenance of genomic stability, chromatin structure, and adult stem cell senescence. DNA damage-induced non-coding RNAs transcribed in the vicinity of the DNA break regulate recruitment of the DNA damage machinery and DNA repair efficiency. We will discuss the correlation between non-coding RNAs and DNA damage repair efficiency and the potential role of changing chromatin structures around double-strand break sites. On the other hand, induction of non-coding RNA transcription from the repetitive Alu elements occurs during human stem cell aging and hinders efficient DNA repair causing entry into senescence. We will discuss how this fine balance between transcription and genomic instability may be regulated by the dramatic changes to chromatin structure that accompany cellular senescence.

The tumor suppressive role of eIF3f and its function in translation inhibition and rRNA degradation

Deregulated translation plays an important role in human cancer. We previously reported decreased eukaryotic initiation factor 3 subunit f (eIF3f) expression in pancreatic cancer. Whether decreased eIF3f expression can transform normal epithelial cells is not known. In our current study, we found evidence that stable knockdown of eIF3f in normal human pancreatic ductal epithelial cells increased cell size, nuclear pleomorphism, cytokinesis defects, cell proliferation, clonogenicity, apoptotic resistance, migration, and formation of 3-dimensional irregular masses. Our findings support the tumor suppressive role of eIF3f in pancreatic cancer. Mechanistically, we found that eIF3f inhibited both cap-dependent and cap-independent translation. An increase in the ribosomal RNA (rRNA) level was suggested to promote the generation of cancer. The regulatory mechanism of rRNA degradation in mammals is not well understood. We demonstrated here that eIF3f promotes rRNA degradation through direct interaction with heterogeneous nuclear ribonucleoprotein (hnRNP) K. We showed that hnRNP K is required for maintaining rRNA stability: under stress conditions, eIF3f dissociates hnRNP K from rRNA, thereby preventing it from protecting rRNA from degradation. We also demonstrated that rRNA degradation occurred in non-P body, non-stress granule cytoplasmic foci that contain eIF3f. Our findings established a new mechanism of rRNA decay regulation mediated by hnRNP K/eIF3f and suggest that the tumor suppressive function of eIF3f may link to impaired rRNA degradation and translation.

Post-transcriptional and post-translational regulation of Bcl2

Bcl2 is an important pro-survival protein that has an essential function in normal immunity and whose constitutive expression leads to the development of lymphomas. Although transcriptional control of Bcl2 has been reported, increasing evidence suggests an important component of Bcl2 regulation is post-transcriptional. Phosphorylation of Bcl2 has been shown to enhance activity to allow response to extracellular growth-factor-mediated signals. Bcl2 mRNA contains regulatory elements in both its 5'- and 3'-UTRs (untranslated regions). An IRES (internal ribosome entry sequence) in the 5'-UTR permits continued translation in the presence of cellular stresses that reduce cap-dependent translation. The 3'-UTR of Bcl2 mRNA is 5.2 kb in length and contains multiple predicted miRNA (microRNA) and RNA-BP (RNA-binding protein)-binding sites. miR-15a and miR-16-1 have been found to inhibit Bcl2 expression in B-cells, whereas the RNA-BP nucleolin has been shown to increase Bcl2 expression by binding to the 3'-UTR and enhancing mRNA stability. Both decreased expression of miR-15a and miR-16-1 and increased nucleolin have been shown to be associated with increased Bcl2 expression and resistance to apoptosis in the common human disease, chronic lymphocytic leukaemia. miRNA-based therapeutic approaches to treat cancer are emerging. Bcl2 is highly regulated by miRNAs and is therefore an excellent candidate for such approaches.

Proteomic analysis of lanthanum citrate-induced apoptosis in human cervical carcinoma SiHa cells

Lanthanides possess diverse biological effect and have been shown to promote cell proliferation and induce apoptosis. Our previous studies showing that lanthanide citrate complex has significant antitumor activity in human cervical cancer HeLa cells. This study aims at determining if [LaCit(2)](3-) have the activity against another type of human cervical cancer cell line SiHa and the changes in protein expression that contribute to the mechanism(s) of [LaCit(2)](3-)-mediated apoptosis in SiHa cells. Cell growth inhibition was measured by MTT method, and apoptosis was detected by means of Hoechst 33258 staining and flow cytometry analysis. After [LaCit(2)](3-)-treatment the results show that the growth of SiHa cells was inhibited, the cells displayed typical apoptosis morphological changes, and increase in the rates of apoptosis. Using proteomics approaches, a variety of differentially expressed proteins were identified in SiHa cells before and after treatment with [LaCit(2)](3-). There were profound changes in 10 proteins relating to mitochondrial function and oxidative stress, suggesting that mitochondrial dysfunction plays a key role in [LaCit(2)](3-)-induced apoptosis. This was confirmed by a decrease in the mitochondrial transmembrane potential (Δψ(m)), and increases in H(2)O(2) generation in [LaCit(2)](3-)-treated cells. Among them the alerted proteins, Prx I, ANXA1 and TRAF5 were validated by western blotting analyses. These results suggest that there is an intrinsic molecular pathway of cell apoptosis in [LaCit(2)](3-)-treated SiHa cells. This observation is in accordance with our previous reports about the effects of [LaCit(2)](3-) and [YbCit(2)](3-) on HeLa cells and it provide a molecular mechanism underlying lanthanide citrate complex-mediated cell apoptosis.

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.

TRANSLATIONAL CONTROL IN T LYMPHOCYTES

Translational control is an important but relatively unappreciated mechanism that regulates levels of protein products. In addition to a global translational control that regulates the cell's response to external stimuli such as growth factors, cytokines, stress, and viral infections, selective translational control has recently been demonstrated to affect many genes related to growth and apoptotic processes. Translational infidelity has recently been suggested as a new mechanism of T cell dysregulation in SLE. This review discusses current data on translational control of T cell biology and the central aspect of translational control in the signalling pathway leading to T cell proliferation, apoptotic response, and cytokine production. The utility for global analysis by genomics to study translational control of T cell gene expression is also discussed.

Translational reprogramming following UVB irradiation is mediated by DNA-PKcs and allows selective recruitment to the polysomes of mRNAs encoding DNA repair enzymes

UVB-induced lesions in mammalian cellular DNA can, through the process of mutagenesis, lead to carcinogenesis. However, eukaryotic cells have evolved complex mechanisms of genomic surveillance and DNA damage repair to counteract the effects of UVB radiation. We show that following UVB DNA damage, there is an overall inhibition of protein synthesis and translational reprogramming. This reprogramming allows selective synthesis of DDR proteins, such as ERCC1, ERCC5, DDB1, XPA, XPD, and OGG1 and relies on upstream ORFs in the 5' untranslated region of these mRNAs. Experiments with DNA-PKcs-deficient cell lines and a specific DNA-PKcs inhibitor demonstrate that both the general repression of mRNA translation and the preferential translation of specific mRNAs depend on DNA-PKcs activity, and therefore our data establish a link between a key DNA damage signaling component and protein synthesis.

Phosphorylation of the eukaryotic initiation factor 3f by cyclin-dependent kinase 11 during apoptosis

eIF3f is a subunit of eukaryotic initiation factor 3 (eIF3). We previously showed that eIF3f is phosphorylated by cyclin dependent kinase 11 (CDK11(p46)) which is an important effector in apoptosis. Here, we identified a second eIF3f phosphorylation site (Thr119) by CDK11(p46) during apoptosis. We demonstrated that eIF3f is directly phosphorylated by CDK11(p46) in vivo. Phosphorylation of eIF3f plays an important role in regulating its function in translation and apoptosis. Phosphorylation of eIF3f enhances the association of eIF3f with the core eIF3 subunits during apoptosis. Our data suggested that eIF3f may inhibit translation by increasing the binding to the eIF3 complex during apoptosis.

Canonical initiation factor requirements of the Myc family of internal ribosome entry segments

Initiation of protein synthesis in eukaryotes requires recruitment of the ribosome to the mRNA and its translocation to the start codon. There are at least two distinct mechanisms by which this process can be achieved; the ribosome can be recruited either to the cap structure at the 5' end of the message or to an internal ribosome entry segment (IRES), a complex RNA structural element located in the 5' untranslated region (5'-UTR) of the mRNA. However, it is not well understood how cellular IRESs function to recruit the ribosome or how the 40S ribosomal subunits translocate from the initial recruitment site on the mRNA to the AUG initiation codon. We have investigated the canonical factors that are required by the IRESs found in the 5'-UTRs of c-, L-, and N-myc, using specific inhibitors and a tissue culture-based assay system, and have shown that they differ considerably in their requirements. The L-myc IRES requires the eIF4F complex and the association of PABP and eIF3 with eIF4G for activity. The minimum requirements of the N- and c-myc IRESs are the C-terminal domain of eIF4G to which eIF4A is bound and eIF3, although interestingly this protein does not appear to be recruited to the IRES RNA via eIF4G. Finally, our data show that all three IRESs require a ternary complex, although in contrast to c- and L-myc IRESs, the N-myc IRES has a lesser requirement for a ternary complex.

Translation regulation after taxol treatment in NIH3T3 cells involves the elongation factor (eEF)2

Changes to the translational machinery that occur during apoptosis have been described in the last few years. The two principal ways in which translational factors are modified during apoptosis are: (i) changes in protein phosphorylation and (ii) specific proteolytic cleavages. Taxol, a member of a new class of anti-tubulin drugs, is currently used in chemotherapeutic treatments of different types of cancers. We have previously demonstrated that taxol induces calpain-mediated apoptosis in NIH3T3 cells [Piñeiro et al., Exp. Cell Res., 2007, 313:369-379]. In this study we found that translation was significantly inhibited during taxol-induced apoptosis in these cells. We have studied the phosphorylation status and expression levels of eIF2a, eIF4E, eIF4G and the regulatory protein 4E-BP1, all of which are implicated in translation regulation. We found that taxol treatment did not induce changes in eIF2alpha phosphorylation, but strongly decreased eIF4G, eIF4E and 4E-BP1 expression levels. MDL28170, a specific inhibitor of calpain, prevented reduction of eIF4G, but not of eIF4E or 4E-BP1 levels. Moreover, the calpain inhibitor did not block taxol-induced translation inhibition. All together these findings demonstrated that none of these factors are responsible for the taxol-induced protein synthesis inhibition. On the contrary, taxol treatment increased elongation factor eEF2 phosphorylation in a calpain-independent manner, supporting a role for eEF2 in taxol-induced translation inhibition.

Polypyrimidine tract binding protein regulates IRES-mediated gene expression during apoptosis

During apoptosis there is a substantial reduction in the rate of protein synthesis, and yet some mRNAs avoid this translational inhibition. To determine the impact that receptor-mediated cell death has on the translational efficiency of a large number of mRNAs, translational profiling was performed on MCF7 cells treated with the apoptosis-inducing ligand TRAIL. Our data indicate that approximately 3% of mRNAs remain associated with the polysomes in apoptotic cells, and genes that are involved in transcription, chromatin modification/remodeling, and the Notch signaling pathway are particularly prevalent among the mRNAs that evade translational inhibition. Internal ribosome entry segments (IRESs) were identified in several of the mRNAs that remained associated with the polysomes during apoptosis, and, importantly, these IRESs functioned efficiently in apoptotic cells. Finally, the data showed that polypyrimidine tract binding protein (PTB, a known IRES trans-acting factor or ITAF) is pivotal in regulating the apoptotic process by controlling IRES function.

Specific cleavage of ribosomal RNA and mRNA during victorin-induced apoptotic cell death in oat

Here we report that rRNA and mRNA are specifically degraded in oat (Avena sativa L.) cells during apoptotic cell death induced by victorin, a host-selective toxin produced by Cochliobolus victoriae. Northern analysis indicated that rRNA species from the cytosol, mitochondria and chloroplasts were all degraded via specific degradation intermediates during victorin-induced apoptotic cell death but, in contrast, they were randomly digested in necrotic cell death induced by 30 mM CuSO(4) and heat shock. This indicates that specific rRNA cleavage could be controlled by an intrinsic program. We also observed specific cleavage of mRNA of housekeeping genes such as actin and ubiquitin during victorin-induced cell death. Interestingly, no victorin-induced mRNA degradation was detected with stress-responding genes such as PR-1, PR-10 and GPx throughout the experimental period. The RNA degradation mostly, but not always, occurred in parallel with DNA laddering, but pharmacological studies indicated that these processes are regulated by different signaling pathways with some overlapping upstream signals.

Decreased expression of eukaryotic initiation factor 3f deregulates translation and apoptosis in tumor cells

The eukaryotic initiation factor 3f (eIF3f) is the p47 subunit of the multi-subunit eIF3 complex. eIF3 plays an important role in translation initiation. In the present study, we investigate the biological function of eIF3f in translation and apoptosis in tumor cells. We demonstrated for the first time that eIF3f is downregulated in most human tumors using a cancer profiling array and confirmed by real-time reverse transcription PCR in melanoma and pancreatic cancer. Overexpression of eIF3f inhibits cell proliferation and induces apoptosis in melanoma and pancreatic cancer cells. Silencing of eIF3f protects melanoma cells from apoptosis. We further investigated the biological function of eIF3f. In vitro translation studies indicate that eIF3f is a negative regulator of translation and that the region between amino acids 170 and 248 of eIF3f is required for its translation regulatory function. Ectopic expression of eIF3f inhibits translation and overall cellular protein synthesis. Ribosome profile and ribosomal RNA (rRNA) fragmentation assays revealed that eIF3f reduces ribosomes, which may be associated with rRNA degradation. We propose that eIF3f may play a role in ribosome degradation during apoptosis. These data provide critical insights into the cellular function of eIF3f and in linking translation initiation and apoptosis.

Initiation factor modifications in the preapoptotic phase

Recent studies have identified several mechanistic links between the regulation of translation and the process of apoptosis. Rates of protein synthesis are controlled by a wide range of agents that induce cell death, and in many instances, the changes that occur to the translational machinery precede overt apoptosis and loss of cell viability. The two principal ways in which factors required for translational activity are modified prior to and during apoptosis involve (i) changes in protein phosphorylation and (ii) specific proteolytic cleavages. In this review, we summarise the principal targets for such regulation, with particular emphasis on polypeptide chain initiation factors eIF2 and eIF4G and the eIF4E-binding proteins. We indicate how the functions of these factors and of other proteins with which they interact may be altered as a result of activation of apoptosis and we discuss the potential significance of such changes for translational control and cell growth regulation.

Identification and characterization of the human ARD1-NATH protein acetyltransferase complex

Protein acetyltransferases and deacetylases have been implicated in oncogenesis, apoptosis and cell cycle regulation. Most of the protein acetyltransferases described acetylate epsilon-amino groups of lysine residues within proteins. Mouse ARD1 (homologue of yeast Ard1p, where Ard1p stands for arrest defective 1 protein) is the only known protein acetyltransferase catalysing acetylation of proteins at both alpha-(N-terminus) and epsilon-amino groups. Yeast Ard1p interacts with Nat1p (N-acetyltransferase 1 protein) to form a functional NAT (N-acetyltransferase). We now describe the human homologue of Nat1p, NATH (NAT human), as the partner of the hARD1 (human ARD1) protein. Included in the characterization of the NATH and hARD1 proteins is the following: (i) endogenous NATH and hARD1 proteins are expressed in human epithelial, glioma and promyelocytic cell lines; (ii) NATH and hARD1 form a stable complex, as investigated by reciprocal immunoprecipitations followed by MS analysis; (iii) NATH-hARD1 complex expresses N-terminal acetylation activity; (iv) NATH and hARD1 interact with ribosomal subunits, indicating a co-translational acetyltransferase function; (v) NATH is localized in the cytoplasm, whereas hARD1 localizes both to the cytoplasm and nucleus; (vi) hARD1 partially co-localizes in nuclear spots with the transcription factor HIF-1alpha (hypoxia-inducible factor 1alpha), a known epsilon-amino substrate of ARD1; (vii) NATH and hARD1 are cleaved during apoptosis, resulting in a decreased NAT activity. This study identifies the human homologues of the yeast Ard1p and Nat1p proteins and presents new aspects of the NATH and hARD1 proteins relative to their yeast homologues.

Translation inhibition during the induction of apoptosis: RNA or protein degradation?

The induction of apoptosis leads to a substantial inhibition of protein synthesis. During this process changes to the translation-initiation factors, the ribosome and the cellular level of mRNA have been documented. However, it is by no means clear which of these events are necessary to achieve translational shutdown. In this article, we discuss modifications to the translational apparatus that occur during apoptosis and examine the potential contributions that they make to the inhibition of protein synthesis. Moreover, we present evidence that suggests that a global increase in the rate of mRNA degradation occurs before the caspase-dependent cleavage of initiation factors. Increased mRNA decay is temporally correlated with the shutdown of translation and therefore plays a major role in the inhibition of protein synthesis in apoptotic cells.

The proteasome inhibitor, MG132, promotes the reprogramming of translation in C2C12 myoblasts and facilitates the association of hsp25 with the eIF4F complex

The eukaryotic translation initiation factor (eIF) 4E, is regulated by modulating both its phosphorylation and its availability to interact with the scaffold protein, eIF4G, to form the mature eIF4F complex. Here we show that treatment of C2C12 myoblasts with the proteasomal inhibitor, MG132 (N-carbobenzoxyl-Leu-Leu-leucinal), resulted in an early decrease in protein synthesis rates followed by a partial recovery, reflecting the reprogramming of translation. The early inhibition of protein synthesis was preceded by a transient increase in eIF2alpha phosphorylation, followed by a sustained increase in eIF4E phosphorylation. Inhibition of eIF4E phosphorylation with CGP57380 failed to prevent translational reprogramming or the moderate decrease in eIF4F complexes at later times. Prolonged incubation with MG132 resulted in the increased expression of heat shock protein (hsp)25, alphaB-crystallin and hsp70, with a population of hsp25 associating with the eIF4F complex in a p38 mitogen-activated protein kinase-dependent manner. Under these conditions, eIF4GI, and to a lesser extent eIF4E, re-localized from a predominantly cytoplasmic distribution to a more perinuclear and granular staining. Although MG132 had little effect on the colocalization of eIF4E and eIF4GI, it promoted the SB203580-sensitive association of eIF4GI and hsp25, an effect not observed with alphaB-crystallin. Addition of recombinant hsp25 to an in vitro translation assay resulted in stimulation of on-going translation and a moderate decrease in de novo translation, indicating that this modified eIF4F complex containing hsp25 has a role to play in recovery of mRNA translation following cellular stress.

Individual expression of poliovirus 2Apro and 3Cpro induces activation of caspase-3 and PARP cleavage in HeLa cells

The expression of individual viral genes enables the study of their effects on cellular functions. Our group previously generated stable HeLa cell lines that efficiently express poliovirus proteases 2A (clone 2A7d) and 3C (clone 3C7) under the control of tetracycline [Virology 266 (2000a) 352; J. Virol. 74 (2000b) 2383]. Upon induction of these proteases, the cells undergo drastic morphological alterations and eventually die. The present paper characterizes, in detail, the cellular and molecular events that lead to cell death in these lines. Several signs of apoptosis were observed in both 2A7d- and 3C7-induced cells, such as nuclear fragmentation, DNA breakdown (as determined by TUNEL), and phosphatidylserine translocation. Protease 2A induces the cleavage of poly-ADP-ribose-polymerase (PARP). This is blocked by the caspase-3 inhibitor DEVD in both 2A7d-On and 3C7-On cells suggesting that this enzyme might account for PARP cleavage in both cell lines. The results indicate that both poliovirus proteases induce apoptosis by mechanisms involving caspase activation, although the kinetics of apoptosis differs.

Cleavage of eukaryotic initiation factor eIF4G and inhibition of host-cell protein synthesis during feline calicivirus infection

Caliciviruses are small, non-enveloped, positive-stranded RNA viruses that are pathogenic for both animals and man. Although their capsid structure and genomic organization are distinct from picornaviruses, they have similarities to these viruses in their non-structural proteins. Picornaviruses induce a rapid inhibition of host-cell cap-dependent protein synthesis and this is mainly achieved through cleavage of eIF4G and/or dephosphorylation of 4E-BP1. In this study, the effect of calicivirus infection was examined on host-cell protein synthesis in order to determine whether they also induce host shut-off. We report that infection of cells with feline calicivirus (FCV) leads to the inhibition of cellular protein synthesis. This is accompanied by the cleavage of the eukaryotic translation initiation factors eIF4GI and eIF4GII in a manner reminiscent of that induced by picornaviruses. However, the cleavages occur at different sites. The potential mechanisms of these cleavage events and the implications for the translation of calicivirus mRNA are discussed.

Targets and mechanisms for the regulation of translation in malignant transformation

There is increasing evidence that deregulation of gene expression at the level of mRNA translation can contribute to cell transformation and the malignant phenotype. Two steps in the pathway of polypeptide chain initiation, viz. the assembly of the 43S initiation complex catalysed by polypeptide chain initiation factor eIF2 and the binding of eIF4E to eIF4G during the recruitment of mRNA to the ribosome, have been shown to be likely targets for changes associated with tumorigenesis. The activity of eIF2 is controlled by changes in phosphorylation of the alpha subunit of this factor. The availability of eIF4E for binding to eIF4G is regulated by the phosphorylation of a small family of eIF4E-binding proteins (the 4E-BPs). The activities of the protein kinases and/or phosphatases responsible for the (de)phosphorylation of these substrates may in turn be controlled by cellular and viral oncogenes and tumour-suppressor genes. This review will describe recent aspects of the mechanisms involved, with particular emphasis on the regulation of the eIF2 alpha kinase PKR and the control of 4E-BP phosphorylation by viral gene products, growth-inhibitory cytokines and the tumour-suppressor protein p53.

Activated eIF4E-binding Protein Slows G1 Progression and Blocks Transformation by c-myc without Inhibiting Cell Growth

Translation initiation is poised between global regulation of cell growth and specific regulation of cell division. The mRNA cap-binding protein (eIF4E) is a critical integrator of cell growth and division because it is rate-limiting for translation initiation and is also rate-limiting for G(1) progression. Translation initiation factor eIF4E is also oncogenic and a candidate target of c-myc. Recently, an activated inhibitory 4E-binding protein (4EBP) that blocks eIF4E was used to study its regulation of Drosophila growth. We adopted this approach in mammalian cells after identifying an autosensing mechanism that protects against increased levels of 4EBP1. Increased 4EBP1 induced a quantitative increase in the inactivated phosphorylated form of 4EBP1 in vitro and in vivo. To overcome this protective mechanism, we introduced alanine substitutions at four phosphorylation/inactivation sites in 4EBP1 to constitutively activate a 4EBP mu to block eIF4E. Overexpression of activated 4EBP mu inhibited cell proliferation and completely blocked transformation by both eIF4E and c-myc, although it did not block all tested oncogenes. Surprisingly, expression of the activated 4EBP mu increased cell size and protein content. Activated 4EBP mu blocked both cell proliferation and c-myc transformation by inhibiting G(1) progression and increasing apoptosis, without decreasing protein synthesis. Our results identify mammalian eIF4E as rate-limiting for cell cycle progression before it regulates cell growth. It further identifies G(1) control by translation initiation factors as an essential genetic target of c-myc that is necessary for its ability to transform cells.

p53-induced inhibition of protein synthesis is independent of apoptosis

Activation of a temperature-sensitive form of p53 in murine erythroleukaemia cells results in a rapid impairment of protein synthesis that precedes inhibition of cell proliferation and loss of cell viability by several hours. The inhibition of translation is associated with specific cleavages of polypeptide chain initiation factors eIF4GI and eIF4B, a phenomenon previously observed in cells induced to undergo apoptosis in response to other stimuli. Although caspase activity is enhanced in the cells in which p53 is activated, both the effects on translation and the cleavages of the initiation factors are completely resistant to inhibition of caspase activity. Moreover, exposure of the cells to a combination of the caspase inhibitor z-VAD.FMK and the survival factor erythropoietin prevents p53-induced cell death but does not reverse the inhibition of protein synthesis. We conclude that the p53-regulated cleavages of eIF4GI and eIF4B, as well as the overall inhibition of protein synthesis, are caspase-independent events that can be dissociated from the induction of apoptosis per se.

The p34cdc2-related cyclin-dependent kinase 11 interacts with the p47 subunit of eukaryotic initiation factor 3 during apoptosis

Cyclin-dependent kinase 11 (CDK11; also named PITSLRE) is part of the large family of p34(cdc2)-related kinases whose functions appear to be linked with cell cycle progression, tumorigenesis, and apoptotic signaling. However, substrates of CDK11 during apoptosis have not been identified. We used a yeast two-hybrid screening strategy and identified eukaryotic initiation factor 3 p47 protein (eIF3 p47) as an interacting partner of caspase-processed C-terminal kinase domain of CDK11 (CDK11(p46)). We demonstrate that the eIF3 p47 can interact with CDK11 in vitro and in vivo, and the interaction can be strengthened by stimulation of apoptosis. EIF3 p47 contains a Mov34/JAB domain and appears to interact with CDK11(p46) through this motif. We show in vitro that the caspase-processed CDK11(p46) can phosphorylate eIF3 p47 at a specific serine residue (Ser(46)) and that eIF3 p47 is phosphorylated in vivo during apoptosis. Purified recombinant CDK11(p46) inhibited translation of a reporter gene in vitro in a dose-dependent manner. In contrast, a kinase-defective mutant CDK11(p46M) did not inhibit translation of the reporter gene. Stable expression of CDK11(p46) in vivo inhibited the synthesis of a transfected luciferase reporter protein and overall cellular protein synthesis. These data provide insight into the cellular function of CDK11 during apoptosis.

Many cuts to ruin: a comprehensive update of caspase substrates

Apoptotic cell death is executed by the caspase-mediated cleavage of various vital proteins. Elucidating the consequences of this endoproteolytic cleavage is crucial for our understanding of cell death and other biological processes. Many caspase substrates are just cleaved as bystanders, because they happen to contain a caspase cleavage site in their sequence. Several targets, however, have a discrete function in propagation of the cell death process. Many structural and regulatory proteins are inactivated by caspases, while other substrates can be activated. In most cases, the consequences of this gain-of-function are poorly understood. Caspase substrates can regulate the key morphological changes in apoptosis. Several caspase substrates also act as transducers and amplifiers that determine the apoptotic threshold and cell fate. This review summarizes the known caspase substrates comprising a bewildering list of more than 280 different proteins. We highlight some recent aspects inferred by the cleavage of certain proteins in apoptosis. We also discuss emerging themes of caspase cleavage in other forms of cell death and, in particular, in apparently unrelated processes, such as cell cycle regulation and cellular differentiation.

Polypyrimidine tract-binding proteins are cleaved by caspase-3 during apoptosis

The polypyrimidine tract-binding protein (PTB), an RNA-binding protein, is required for efficient translation of some mRNAs containing internal ribosomal entry sites (IRESs). Here we provide evidence that the addition of apoptosis-inducing agents to cells results in the cleavage of PTB isoforms 1, 2, and 4 by caspase-3. This cleavage of PTB separated the N-terminal region, containing NLS-RRM1, from the C-terminal region, containing RRM2-3-4. Our data indicate that there are three noncanonical caspase-3 target sites in PTBs, namely Ile-Val-Pro-Asp(7)Ile, Leu-Tyr-Thr-Asp(139)Ser, and Ala-Ala-Val-Asp(172)Ala. The C-terminal PTB fragments localized to the cytoplasm, as opposed to the nucleus where most intact PTBs are found. Moreover, these C-terminal PTB fragments inhibited translation of polioviral mRNA, which contains an IRES element requiring PTB for its activation. This suggests that translation of some IRES-containing mRNAs is regulated by proteolytic cleavage of PTB during apoptosis.

Endoplasmic reticulum-bound ribosomes reside in stable association with the translocon following termination of protein synthesis

In current views, translation-coupled ribosome binding to the endoplasmic reticulum (ER) membrane is transient, with association occurring via the signal recognition particle pathway and dissociation occurring upon the termination of protein synthesis. Recent studies indicate, however, that ribosomal subunits remain membrane-bound following the termination of protein synthesis. To define the mechanism of post-termination ribosome association with the ER membrane, membrane-bound ribosomes were detergent-solubilized from tissue culture cells at different stages of the protein synthesis cycle, and the composition of the ribosome-associated membrane protein fraction was determined. We report that ribosomes reside in stable association with the Sec61alpha-translocon following the termination stage of protein synthesis. Additionally, in vitro experiments revealed that solubilized, gradient-purified ribosome-translocon complexes were able to initiate the translation of secretory and cytosolic proteins and were functional in assays of signal sequence recognition. Using this experimental system, synthesis of signal sequence-bearing polypeptides yielded a tight ribosome-translocon junction; synthesis of nascent polypeptides lacking a signal sequence resulted in a disruption of this junction. On the basis of these data, we propose that in situ, ribosomes reside in association with the translocon throughout the cycle of protein synthesis, with membrane release occurring upon translation of proteins lacking topogenic signals.

Caspase cleavage of initiation factor 4E-binding protein 1 yields a dominant inhibitor of cap-dependent translation and reveals a novel regulatory motif

Eukaryotic initiation factor 4E (eIF4E) binding proteins (4E-BPs) regulate the assembly of initiation complexes required for cap-dependent mRNA translation. 4E-BP1 undergoes insulin-stimulated phosphorylation, resulting in its release from eIF4E, allowing initiation complex assembly. 4E-BP1 undergoes caspase-dependent cleavage in cells undergoing apoptosis. Here we show that cleavage occurs after Asp24, giving rise to the N-terminally truncated polypeptide Delta4E-BP1, which possesses the eIF4E-binding site and all the known phosphorylation sites. Delta4E-BP1 binds to eIF4E and fails to become sufficiently phosphorylated upon insulin stimulation to bring about its release from eIF4E. Therefore, Delta4E-BP1 acts as a potent inhibitor of cap-dependent translation. Using a mutagenesis approach, we identify a novel regulatory motif of four amino acids (RAIP) which lies within the first 24 residues of 4E-BP1 and which is necessary for efficient phosphorylation of 4E-BP1. This motif is conserved among sequences of 4E-BP1 and 4E-BP2 but is absent from 4E-BP3. Insulin increased the phosphorylation of 4E-BP3 but not sufficiently to cause its release from eIF4E. However, a chimeric protein that was generated by replacing the N terminus of 4E-BP3 with the N-terminal sequence of 4E-BP1 (containing this RAIP motif) underwent a higher degree of phosphorylation and was released from eIF4E. This suggests that the N-terminal sequence of 4E-BP1 is required for optimal regulation of 4E-BPs by insulin.

Initiation factor eIF2 alpha phosphorylation in stress responses and apoptosis

The alpha subunit of polypeptide chain initiation factor eIF2 can be phosphorylated by a number of related protein kinases which are activated in response to cellular stresses. Physiological conditions which result in eIF2 alpha phosphorylation include virus infection, heat shock, iron deficiency, nutrient deprivation, changes in intracellular calcium, accumulation of unfolded or denatured proteins and the induction of apoptosis. Phosphorylated eIF2 acts as a dominant inhibitor of the guanine nucleotide exchange factor eIF2B and prevents the recycling of eIF2 between successive rounds of protein synthesis. Extensive phosphorylation of eIF2 alpha and strong inhibition of eIF2B activity can result in the downregulation of the overall rate of protein synthesis; less marked changes may lead to alterations in the selective translation of alternative open reading frames in polycistronic mRNAs, as demonstrated in yeast. These mechanisms can provide a signal transduction pathway linking eukaryotic cellular stress responses to alterations in the control of gene expression at the translational level.

Translation inhibition in apoptosis: caspase-dependent PKR activation and eIF2-alpha phosphorylation

The protein kinase PKR is a major player in the cellular antiviral response, acting mainly by phosphorylation of the alpha-subunit of the eukaryotic translation initiation factor 2 (eIF2-alpha) to block de novo protein synthesis. PKR activation requires binding of double-stranded RNA or PACT/RAX proteins to its regulatory domain. Since several reports have demonstrated that translation is inhibited in apoptosis, we investigated whether PKR and eIF2-alpha phosphorylation contribute to this process. We show that PKR is proteolysed and that eIF2-alpha is phosphorylated at the early stages of apoptosis induced by various stimuli. Both events coincide with the onset of caspase activity and are prevented by caspase inhibitors. Using site-directed mutagenesis we show that PKR is specifically proteolysed at Asp(251) during cellular apoptosis. This site is cleaved in vitro by recombinant caspase-3, caspase-7, and caspase-8 and not by the proinflammatory caspase-1 and caspase-11. The released kinase domain efficiently phosphorylates eIF2-alpha at the cognate Ser(51) residue, and its overexpression in mammalian cells impairs the translation of its own mRNA and of reporter mRNAs. Our results demonstrate a new and caspase-dependent activation mode for PKR, leading to eIF2-alpha phosphorylation and translation inhibition in apoptosis.

Insulin fails to stimulate muscle protein synthesis in sepsis despite unimpaired signaling to 4E-BP1 and S6K1

Induction of sepsis in rats causes an inhibition of protein synthesis in skeletal muscle that is resistant to the stimulatory actions of insulin. To gain a better understanding of the underlying reason for this lack of response, the present study was undertaken to investigate sepsis-induced alterations in insulin signaling to regulatory components of mRNA translation. Experiments were performed in perfused hindlimb preparations from rats 5 days after induction of a septic abscess. Sepsis resulted in a 50% reduction in protein synthesis in the gastrocnemius. Protein synthesis in muscles from septic rats, but not controls, was unresponsive to stimulation by insulin. The insulin-induced hyperphosphorylation response of the translation repressor protein 4E-binding protein 1 (4E-BP1) and of the 70-kDa S6 kinase (S6K1) (1), two targets of insulin action on mRNA translation, was unimpaired in gastrocnemius of septic rats. Hyperphosphorylation of 4E-BP1 in response to insulin resulted in its dissociation from the inactive eukaryotic initiation factor (eIF)4E. 4E-BP1 complex in both control and septic rats. However, assembly of the active eIF4F complex as assessed by the association of eIF4E with eIF4G did not follow the pattern predicted by the increased availability of eIF4E resulting from changes in the phosphorylation of 4E-BP1. Indeed, sepsis caused a dramatic reduction in the amount of eIF4G associated with eIF4E in the presence or absence of insulin. Thus the inability of insulin to stimulate protein synthesis during sepsis may be related to a defect in signaling to a step in translation initiation involved in assembly of an active eIF4F complex.

Proteasome inhibitors and immunosuppressive drugs promote the cleavage of eIF4GI and eIF4GII by caspase-8-independent mechanisms in Jurkat T cell lines

Previously, we have shown that translation eukaryotic initiation factor (eIF) 4GI is cleaved during anti-Fas-mediated apoptosis. Here, we have investigated the effects of the proteasome inhibitors, MG132 and lactacystin, and the immunosuppressants, 2-amino-2[2-(4-octylphenyl)ethyl]-1,3,propane diol (FTY720) and cyclosporin A, on the integrity of eIF4GI and eIF4GII in T cells. Using wild-type Jurkat T cells, we show that the proteasome inhibitors MG132 and lactacystin promote the cleavage of eIF4G, activate caspase-8 and caspase-3-like activities and decrease cell viability. Furthermore, MG132 also promotes the cleavage of eIF4G and the activation of caspase-3-like activity in a caspase-8-deficient Jurkat cell line which is resistant to anti-Fas-mediated apoptosis. Using specific anti-peptide antisera, we show that both eIF4GI and eIF4GII are cleaved in either cell line in response to MG132 and lactacystin. In response to such treatments, we demonstrate that the fragments of eIF4GI generated include those previously observed with anti-Fas antiserum together with a novel product which lacks the ability to interact with eIF4E. In contrast, cells treated with the immunosuppressants FTY720 and cyclosporin A appear to contain only the novel cleavage fragment of eIF4GI and to lack those characteristic of cells treated with anti-Fas antiserum. These data suggest that caspase-8 activation is not required for apoptosis and eIF4G cleavage mediated by proteasome inhibitors and immunosuppressants in human T cells.

Staurosporine inhibits phosphorylation of translational regulators linked to mTOR

Treatment of Swiss 3T3 cells with staurosporine resulted in dephosphorylation of two proteins which play key roles in regulating mRNA translation. This occurred before the execution of apoptosis, assessed by caspase-3 activity. These translation regulators are p70 S6 kinase, which phosphorylates ribosomal protein S6, and eukaryotic initiation factor (eIF) 4E binding protein 1 (4E-BP1), which both lie downstream of the mammalian target of rapamycin (mTOR). This resulted in decreased p70 S6 kinase activity, dephosphorylation of ribosomal protein S6, increased binding of 4E-BP1 to eIF4E and a concomitant decrease in eIF4F complexes. Our data show that staurosporine impairs mTOR signalling in vivo but that this not due to direct inhibition of mTOR or to inhibition of protein kinase C. It is becoming clear that agents which cause apoptosis inactivate mTOR signalling as a common early response prior to the execution of apoptosis, i.e., before caspase activation.

Disruption of the interaction of mammalian protein synthesis eukaryotic initiation factor 4B with the poly(A)-binding protein by caspase- and viral protease-mediated cleavages

Eukaryotic initiation factor (eIF) 4B interacts with several components of the initiation pathway and is targeted for cleavage during apoptosis. In a cell-free system, cleavage of eIF4B by caspase-3 coincides with a general inhibition of protein synthetic activity. Affinity chromatography demonstrates that mammalian eIF4B interacts with the poly(A)-binding protein and that a region consisting of the N-terminal 80 amino acids of eIF4B is both necessary and sufficient for such binding. This interaction is lost when eIF4B is cleaved by caspase-3, which removes the N-terminal 45 amino acids. Similarly, the association of eIF4B with the poly(A)-binding protein in vivo is reduced when cells are induced to undergo apoptosis. Cleavage of the poly(A)-binding protein itself, using human rhinovirus 3C protease, also eliminates the interaction with eIF4B. Thus, disruption of the association between mammalian eIF4B and the poly(A)-binding protein can occur during both apoptosis and picornaviral infection and is likely to contribute to the inhibition of translation observed under these conditions.

Protein factor requirements of the Apaf-1 internal ribosome entry segment: roles of polypyrimidine tract binding protein and upstream of N-ras

It has been reported previously that the 5' untranslated region of the mRNA encoding Apaf-1 (apoptotic protease-activating factor 1) has an internal ribosome entry site (IRES), whose activity varies widely among different cell types. Here it is shown that the Apaf-1 IRES is active in rabbit reticulocyte lysates, provided that the system is supplemented with polypyrimidine tract binding protein (PTB) and upstream of N-ras (unr), two cellular RNA binding proteins previously identified to be required for rhinovirus IRES activity. In UV cross-linking assays and electrophoretic mobility shift assays with individual recombinant proteins, the Apaf-1 IRES binds unr but not PTB; however, PTB binding occurs if unr is present. Over a range of different cell types there is a broad correlation between the activity of the Apaf-1 IRES and their content of PTB and unr. In cell lines deficient in these proteins, overexpression of PTB and unr stimulated Apaf-1 IRES function. This is the first example where an IRES in a cellular mRNA has been shown to be functionally dependent, both in vitro and in vivo, on specific cellular RNA binding proteins. Given the critical role of Apaf-1 in apoptosis, these results have important implications for the control of the apoptotic cascade.

Eukaryotic initiation factor 3 p110 mRNA is overexpressed in testicular seminomas

Testicular germ cell tumors are important neoplasms and seminoma accounts for 40 to 50% of these tumors. Little is known concerning the molecular events underlying the development of these malignancies. In the present study we used a modified differential display approach to compare gene expression between seminoma and normal testicular parenchyma, both of which are mixed tissues. We first analyzed mRNA (cDNA) expression by differential display and then directly used differentially expressed cDNAs for the synthesis of radiolabeled riboprobes to attribute differential expression to specific cell types in tissue sections by in situ hybridization. Using this approach along with real-time quantitative reverse transcriptase-polymerase chain reaction analysis we found an overexpression of eukaryotic initiation factor 3 p110 mRNA (EIF3S8) in seminoma cells compared to normal germ cells of testicular tubules. The elF3-p110 subunit may promote seminoma development by generally increasing translation leading to enhanced cellular growth and division rates.

Translational control of the antiapoptotic function of Ras

Activated Ras has been shown to provide powerful antiapoptotic signals to cells through well defined transcriptional and post- translational pathways, whereas translational control as a mechanism of Ras survival signaling remains unexplored. Here we show a direct relationship between assembly of the cap-dependent translation initiation apparatus and suppression of apoptosis by oncogenic Ras in vitro and in vivo. Decreasing protein synthesis with rapamycin, which is known to inhibit cap-dependent translation, increases the susceptibility of Ras-transformed fibroblasts to cytostatic drug-induced apoptosis. In contrast, suppressing global protein synthesis with equipotent concentrations of cycloheximide actually prevents apoptosis. Enforced expression of the cap-dependent translational repressor, the eukaryotic translation initiation factor (eIF) 4E-binding protein (4E-BPI), sensitizes fibroblasts to apoptosis in a manner strictly dependent on its ability to sequester eIF4E from a translationally active complex with eIF4GI and the co-expression of oncogenic Ras. Ectopic expression of 4E-BP1 also promotes apoptosis of Ras-transformed cells injected into immunodeficient mice and markedly diminishes their tumorigenicity. These results establish that eIF4E-dependent protein synthesis is essential for survival of fibroblasts bearing oncogenic Ras and support the concept that activation of cap-dependent translation by extracellular ligands or intrinsic survival signaling molecules suppresses apoptosis, whereas synthesis of proteins mediating apoptosis can occur independently of the cap.

Differential requirements for caspase-8 activity in the mechanism of phosphorylation of eIF2alpha, cleavage of eIF4GI and signaling events associated with the inhibition of protein synthesis in apoptotic Jurkat T cells

Previously we have reported that induction of apoptosis in Jurkat cells results in an inhibition of overall protein synthesis with the selective and rapid cleavage of eukaryotic initiation factor (eIF) 4GI. For the cleavage of eIF4GI, caspase-3 activity is both necessary and sufficient in vivo, in a process which does not require signaling through the p38 MAP kinase pathway. We now show that activation of the Fas/CD95 receptor promotes an early, transient increase in the level of eIF2alpha phosphorylation, which is temporally correlated with the onset of the inhibition of translation. This is associated with a modest increase in the autophosphorylation of the protein kinase activated by double-stranded RNA. Using a Jurkat cell line that is deficient in caspase-8 and resistant to anti-Fas-induced apoptosis, we show that whilst the cleavage of eIF4GI is caspase-8-dependent, the enhancement of eIF2alpha phosphorylation does not require caspase-8 activity and occurs prior to the cleavage of eIF4GI. In addition, activation of the Fas/CD95 receptor results in the caspase-8-dependent dephosphorylation and degradation of p70(S6K), the enhanced binding of 4E-BP1 to eIF4E, and, at later times, the cleavage of eIF2alpha. These data suggest that apoptosis impinges upon the activity of several polypeptides which are central to the regulation of protein synthesis and that multiple signaling pathways are involved in vivo.

Caspases are not involved in the cleavage of translation initiation factor eIF4GI during picornavirus infection

Infection of cells by many picornaviruses results in the rapid inhibition of cellular protein synthesis due to cleavage of the translation initiation factor eIF4G. The poliovirus (PV) 2A and foot-and-mouth disease virus (FMDV) L proteases are each sufficient to mediate this cleavage, but the cleavage mechanism may be indirect, involving an unidentified cellular protease(s). eIF4G is also targetted for cleavage by caspase-3 during apoptosis. Here, it is shown that caspase inhibitors do not inhibit the cleavage of eIF4GI during PV or FMDV infection. Similarly, in transient-expression studies, the cleavage of eIF4GI induced by PV 2A or FMDV L was unaffected by these inhibitors. Furthermore, the cleavage of eIF4GI was observed in PV-infected MCF-7 cells lacking caspase-3. These data, and the fact that induction of apoptosis yields different eIF4GI cleavage fragments, indicate that caspases do not have a major role in the cleavage of eIF4GI during PV or FMDV infection.

Translation initiation factor modifications and the regulation of protein synthesis in apoptotic cells

The rate of protein synthesis is rapidly down-regulated in mammalian cells following the induction of apoptosis. Inhibition occurs at the level of polypeptide chain initiation and is accompanied by the phosphorylation of the alpha subunit of initiation factor eIF2 and the caspase-dependent cleavage of initiation factors eIF4G, eIF4B, eIF2alpha and the p35 subunit of eIF3. Proteolytic cleavage of these proteins yields characteristic products which may exert regulatory effects on the translational machinery. Inhibition of caspase activity protects protein synthesis from long-term inhibition in cells treated with some, but not all, inducers of apoptosis. This review describes the initiation factor modifications and the possible signalling pathways by which translation may be regulated during apoptosis. We discuss the significance of the initiation factor cleavages and other changes for protein synthesis, and the implications of these events for our understanding of the cellular changes associated with apoptosis.

Cleavage of polypeptide chain initiation factor eIF4GI during apoptosis in lymphoma cells: characterisation of an internal fragment generated by caspase-3-mediated cleavage

Polypeptide chain initiation factor eIF4GI undergoes caspase-mediated degradation during apoptosis to give characteristic fragments. The most prominent of these has an estimated mass of approximately 76 kDa (Middle-Fragment of Apoptotic cleavage of eIF4G; M-FAG). Subcellular fractionation of the BJAB lymphoma cell line after induction of apoptosis indicates that M-FAG occurs in both ribosome-bound and soluble forms. Affinity chromatography on m7GTP-Sepharose shows that M-FAG retains the ability of eIF4GI to associate with both the mRNA cap-binding protein eIF4E and initiation factor eIF4A and that the ribosome-bound form of M-FAG is also present as a complex with eIF4E and eIF4A. These data suggest that the binding sites for eIF4E, eIF4A and eIF3 on eIF4GI are retained in the caspase-generated fragment. M-FAG is also a substrate for cleavage by the Foot-and-Mouth-Disease Virus-encoded L protease. These properties, together with the pattern of recognition by a panel of antibodies, define the origin of the apoptotic cleavage fragment. N-terminal sequencing of the products of caspase-3-mediated eIF4GI cleavage has identified the major cleavage sites. The pattern of eIF4GI degradation and the possible roles of the individual cleavage products in cells undergoing apoptosis are discussed.

p27KIP1 is down-regulated by two different mechanisms in human lymphoid cells undergoing apoptosis

The cyclin-dependent kinase inhibitor p27KIP1 is a crucial component of the mammalian restriction point, and as such is subject to multiple regulatory mechanisms. It has recently been shown that the abundance of p27KIP1 is also regulated during apoptosis; p27KIP1 is cleaved by a Z-VAD-fmk-sensitive caspase during apoptosis induced by growth factor deprivation in endothelial cells, and also following exposure of myeloid leukaemia cells to etoposide. Here, we investigate p27KIP1 regulation in B- and T-lymphoid cells undergoing apoptosis. We observe that p27KIP1 is down-regulated following exposure to a variety of apoptotic stimuli including an agonistic anti-Fas antibody, cycloheximide and etoposide. Further investigation revealed the existence of two different routes of p27KIP1 regulation in lymphoid cells undergoing apoptosis. The first pathway is utilized by lymphoid cells stimulated through Fas, is abrogated in a caspase-8-deficient T-cell line, and is blocked by the caspase inhibitors Z-VAD-fmk and Boc-D-fmk. In contrast, the loss of p27KIP1 in cells exposed to cycloheximide and etoposide occurs in the absence of caspase-8 or any Z-VAD-fmk- or Boc-D-fmk-sensitive caspase activities. Thus the down-regulation of p27KIP1 is a common occurrence in lymphoid cells undergoing apoptosis but, depending on the apoptotic trigger, this can be affected by two different mechanisms.

DNA-damaging agents cause inactivation of translational regulators linked to mTOR signalling

Treatment of cells with DNA-damaging agents, such as etoposide, can cause growth arrest or apoptosis. Treatment of Swiss 3T3 or RAT-1 cells with etoposide led to the dephosphorylation of both p70 S6 kinase and eukaryotic initiation factor (eIF) 4E-binding protein 1 (4E-BP1), resulting in decreased p70 S6 kinase activity and an increase in 4E-BP1 binding to eIF4E. These effects were not prevented by the general caspase inhibitor, Z-VAD.FMK. These findings indicate caspase-independent inhibition of signalling pathways that involve the mammalian target of rapamycin (mTOR). Similar effects were observed in response to two other DNA-damaging agents, cisplatin and mitomycin-C. These events preceded apoptosis, which was assessed by caspase-3 activity assays and FACS analysis. This shows that inhibition of mTOR signalling is not a consequence of apoptosis, although it may play a role in the events that precede cell death. 4E-BP1 was cleaved during apoptosis yielding a fragment that retained the ability to bind eIF4E. Cleavage of 4E-BP1 was inhibited by treatment of the cells with Z-VAD.FMK, indicating it is caspase-dependent. Insulin elicited full activation of p70 S6 kinase and phosphorylation of 4E-PB1 in etoposide-treated cells prior to the onset of apoptosis, but not during cell death. This suggests that mTOR signalling becomes irreversibly inhibited only after entry into apoptosis. Oncogene (2000).