Phosphorylation of the proto-oncogene product eukaryotic initiation factor 4E is a common cellular response to tumor necrosis factor

TNF-alpha increases protein content in C2C12 and primary myotubes by enhancing protein translation via the TNF-R1, PI3K, and MEK

Recent evidence supports that TNF-alpha, long considered a catabolic factor, may also have a physiological function in skeletal muscle. The catabolic view, mainly based on correlative studies in human and in vivo animal models, was challenged by experiments with myoblasts, in which TNF-alpha induced differentiation. The biological effects of TNF-alpha in differentiated muscle, however, remain poorly understood. In the present study, we tested whether TNF-alpha has growth-promoting effects in myotubes, and we characterized the mechanisms leading to these effects. Treatment of C(2)C(12) myotubes with TNF-alpha for 24 h increased protein synthesis (PS) and enhanced cellular dehydrogenase activity by 22 and 26%, respectively, without changing cell numbers. These effects were confirmed in myotubes differentiated from primary rat myoblasts. TNF-alpha activated two signaling cascades: 1) ERK1/2 and its target eIF4E and 2) Akt and its downstream effectors GSK-3, p70(S6K), and 4E-BP1. TNF-alpha-induced phosphorylation of Akt, and ERK1/2 was inhibited by an antibody against TNF-alpha receptor 1 (TNF-R1). PD-98059 pretreatment abolished TNF-alpha-induced phosphorylation of ERK1/2 and eIF4E, whereas PS was only partially inhibited. LY-294002 completely abolished TNF-alpha-induced stimulation of PS as well as phosphorylation of Akt and its downstream targets GSK-3, p70(S6K), and 4E-BP1. Rapamycin inhibited TNF-alpha-induced phosphorylation of the mTOR C1 target p70(S6K) without altering TNF-alpha-induced PS and 4E-BP1 phosphorylation. In conclusion, our results provide evidence that TNF-alpha enhances PS in myotubes and that this is based on enhanced protein translation mediated by the TNF-R1 and PI3K-Akt and MEK-ERK signaling cascades.

Acute treatment with TNF-alpha attenuates insulin-stimulated protein synthesis in cultures of C2C12 myotubes through a MEK1-sensitive mechanism

Insulin and TNF-alpha exert opposing effects on skeletal muscle protein synthesis that are mediated in part by the rapamycin-sensitive mammalian target of rapamycin (mTOR) pathway and the PD-98059-sensitive, extracellular signal-regulated kinase (ERK)1/2 pathway. The present study examined the separate and combined effects of insulin (INS), TNF, PD-98059, or dnMEK1 adenovirus on the translational control of protein synthesis in C(2)C(12) myotubes. Cultures were treated with INS, TNF, PD-98059, dnMEK1, or a combination of INS + TNF with PD-98059 or dnMEK1. INS stimulated protein synthesis, enhanced eIF4E.eIF4G association, and eIF4G phosphorylation and repressed eIF4E.4E-BP1 association vs. control. INS also promoted phosphorylation of ERK1/2, S6K1, and 4E-BP1 and dephosphorylation of eIF4E. TNF alone did not have an effect on protein synthesis (vs. control), eIF4E.eIF4G association, or the phosphorylation of eIF4G, S6K1, or 4E-BP1, although it transiently increased ERK1/2 and eIF4E phosphorylation. When myotubes were treated with TNF + INS, the cytokine blocked the insulin-induced stimulation of protein synthesis. This appeared to be due to an attenuation of insulin-stimulated eIF4E.eIF4G association, because other stimulatory effects of INS, e.g., phosphorylation of ERK1/2, 4E-BP1, S6K1, eIF4G, and eIF4E and eIF4E.4E-BP1 association, were unaffected. Finally, treatment of myotubes with PD-98059 or dnMEK1 adenovirus before TNF + INS addition resulted in a derepression of protein synthesis and the association of eIF4G with eIF4E. These findings suggest that TNF abrogates insulin-induced stimulation of protein synthesis in myotubes through a decrease in eIF4F complex assembly independently of S6K1 and 4E-BP1 signaling and dependently on a MEK1-sensitive signaling pathway.

Alpha interferon inhibits human T-cell leukemia virus type 1 assembly by preventing Gag interaction with rafts

Alpha-2a interferon (IFN-alpha2a) has beneficial clinical effects on human T-cell leukemia virus type 1 (HTLV-1) infection, but its antiviral mechanism of action is unknown. Antiviral effects of IFN-alpha2a were studied in 293T cells expressing HTLV-1 proviral DNA and in HTLV-1-infected cells (HOS/PL, MT2, and HUT102). In 293T cells, an 50% inhibitory concentration of 10 U of IFN-alpha2a/ml was determined by p19 antigen ELISA. Analysis of IFN-treated cells demonstrated no defect in viral protein synthesis but did show a decrease in the level of released virus, as determined by immunoblot assays. Electron microscopy studies of IFN-treated cells revealed neither a defect in the site of virus budding nor tethering of virus particles at the plasma membrane, thus arguing against an effect on virus release. Cell fractionation studies and confocal microscopy showed no effect of IFN on Gag association with membranes. However, the level of Gag association with lipid rafts was decreased, suggesting a role of IFN in inhibiting HTLV-1 assembly.

eIF4 initiation factors: effectors of mRNA recruitment to ribosomes and regulators of translation

Eukaryotic translation initiation factor 4F (eIF4F) is a protein complex that mediates recruitment of ribosomes to mRNA. This event is the rate-limiting step for translation under most circumstances and a primary target for translational control. Functions of the constituent proteins of eIF4F include recognition of the mRNA 5' cap structure (eIF4E), delivery of an RNA helicase to the 5' region (eIF4A), bridging of the mRNA and the ribosome (eIF4G), and circularization of the mRNA via interaction with poly(A)-binding protein (eIF4G). eIF4 activity is regulated by transcription, phosphorylation, inhibitory proteins, and proteolytic cleavage. Extracellular stimuli evoke changes in phosphorylation that influence eIF4F activity, especially through the phosphoinositide 3-kinase (PI3K) and Ras signaling pathways. Viral infection and cellular stresses also affect eIF4F function. The recent determination of the structure of eIF4E at atomic resolution has provided insight about how translation is initiated and regulated. Evidence suggests that eIF4F is also implicated in malignancy and apoptosis.

ABC50, a novel human ATP-binding cassette protein found in tumor necrosis factor-alpha-stimulated synoviocytes

We have used the recently developed technique of differential display polymerase chain reaction to seek for new genes modulated by tumor necrosis factor-alpha (TNF-alpha) in cultured synoviocytes. One PCR fragment was shown to correspond to a new gene that was mapped by high-resolution fluorescence in situ hybridization to band 6p21.33. The cDNA of this gene was cloned, and the deduced amino acid sequence revealed consensus motifs for the nucleotide binding folds of the ATP-binding cassette (ABC) family of proteins. However, a hydropathy curve showed that the polypeptide does not contain the transmembrane domains that are typical of the subfamily of ABC transporters and are associated with transporter/channel functions. The new gene, called ABC50, is the first human and mammalian ABC protein found to lack transmembrane domains. Homology with some yeast ABC proteins suggests that ABC50 codes for a new human ribosomal protein involved in translation of mRNA. It could therefore play a role in the enhancement of protein synthesis that follows TNF-alpha treatment of synoviocytes and thus participate in the inflammatory processes mediated by this cytokine. Furthermore, since TNF-alpha also modulates the expression of MHC class I genes, and these genes are known to map to 6p21.33, it is hypothesized that ABC50 and MHC class I are part of the same chromatin expression domain.

Cloning and characterization of human eIF4E genes

Two human eukaryotic initiation factor 4E (eIF4E) genes were isolated and characterized from placental and chromosome 4-specific genomic libraries. One of the genes (EIF4E1) contained six introns, but the other gene (EIF4E2) was intronless, flanked by Alu sequences and 14-base pair (bp) direct repeats, and terminated by a short poly(A) stretch, all characteristics of retrotransposons. Numerous additional intronless eIF4E pseudogenes were found, but unlike EIF4E2, all contained premature in-frame stop codons. The entire EIF4E1 gene spanned >50 kilobase pairs. The coding regions of these two genes differed in four nucleotide residues, resulting in two amino acid differences in the predicted proteins. The promoter of EIF4E1 has been characterized previously. The putative promoter of EIF4E2 contained no TATA box but did contain a transcription initiator region (Inr) and numerous other sequence motifs characteristic of regulated promoters. EIF4E2 contained only two of the three polyadenylation signals present in EIF4E1. Evidence for transcription of both genes was obtained from primer extension, S1 mapping, ribonuclease protection, and reverse transcriptase-polymerase chain reaction experiments. Transcription was found to initiate 19 bp upstream of the translational initiation codon in the case of EIF4E1 and 80 bp in the case of EIF4E2. The two genes were differentially expressed in four human cell lines, Wish, Chang, K562, and HeLa.

Intracellular calcium mobilization suppresses the TNF-alpha-stimulated synthesis of PAI-1 in human endothelial cells. Indications that calcium acts at a translational level

We investigated in human umbilical vein endothelial cells (HUVECs) the interaction between the signaling pathways triggered by calcium mobilization and those affected by human recombinant tumor necrosis factor-alpha (TNF) on the expression of type-1 plasminogen activator inhibitor (PAI-1). Calcium ionophore A23187 alone exerted a modest increase (50%) on PAI-1 synthesis. TNF alone increased PAI-1 accumulation in the culture medium in a time- and dose-dependent fashion, but this increase was abolished when A23187 was added simultaneously with TNF. The downregulating effect of A23187 was not the result of impaired protein secretion, proteolysis, cytotoxicity, or an apoptotic process. A23187 did not decrease the TNF-enhanced PAI-1 mRNA level but did provoke a significant shift in the distribution pattern of PAI-1 transcripts by increasing the 2.3-kb relative to the 3.2-kb form. Comparable inhibitory effects on PAI-1 protein synthesis were observed when A23187 was added 7 hours after the onset of TNF stimulation, strongly suggesting a posttranscriptional inhibitory action of calcium signaling on TNF-stimulated PAI-1 synthesis. However, treatment with actinomycin D showed that PAI-1 mRNA stability was not altered by the various treatments. Chelation of extracellular calcium by EGTA did not prevent the A23187-induced inhibition of TNF-stimulated PAI-1 protein synthesis, emphasizing the role of internal calcium stores in the inhibition of PAI-1 synthesis. Sucrose gradient fractionation of cell lysates revealed that regardless of which treatment was used, both PAI-1 mRNA transcripts exhibited similar sedimentation profiles in the actively translating polysomal pool, suggesting that the A23187-induced shift had no functional consequence on translation. However, in TNF-stimulated cells, A23187 induced a higher proportion of PAI-1 mRNAs that sedimented in fractions corresponding to less dense polysomes, a phenomenon that usually reflects a slower initiation rate during mRNA translation. A23187 also abolished the increase in PAI-1 synthesis induced by recombinant human interleukin 1 beta, and thapsigargin exerted effects comparable to those of A23187 on PAI-1 synthesis in TNF-stimulated cells. It is proposed that in HUVECs, the A23187-induced release of calcium from endoplasmic stores suppresses at the translational level the increase in PAI-1 synthesis triggered by proinflammatory cytokines.

Inhibition of protein tyrosine phosphatases causes phosphorylation of tyrosine-331 in the p60 TNF receptor and inactivates the receptor-associated kinase

Inhibition of protein tyrosine phosphatases blocks tumor necrosis factor (TNF)-induced growth modulation and NF-kappaB activation, both mediated primarily through the p60 TNF receptor. How inhibition of the phosphatases affects the p60 TNF receptor or the recently described receptor-associated serine/threonine kinase (p60TRAK) is not known. In this report, we show that this inhibition, when induced by pervanadate, caused the tyrosine phosphorylation of the cytoplasmic domain (CD) of the p60 receptor, as revealed by phosphoamino acid analysis. Furthermore, site-directed mutagenesis indicated that pervanadate specifically induced the phosphorylation of tyrosine-331, which is located in the death domain of the TNF receptor, a domain to which p60TRAK binds. This tyrosine residue was also phosphorylated by purified, recombinant pp60Src in vitro. Inhibition of protein tyrosine phosphatases by pervanadate also led to the inactivation of p60TRAK. In contrast, okadaic acid, a specific inhibitor of protein serine/threonine phosphatase, increased p60TRAK activity. Taken together, these results suggest that protein tyrosine phosphatases play an essential role in phosphorylation of the cytoplasmic domain of the TNF receptor and in regulation of the receptor-associated kinase, and this in turn may play a role in TNF-mediated growth modulation and NF-kappaB activation.

TNF-alpha pretreatment induces protective effects against focal cerebral ischemia in mice

Cytokines are recognized to play an important role in acute stroke. Tumor necrosis factor-alpha (TNF) is one of the pro-inflammatory cytokines and is expressed in ischemic brain. We hypothesized that TNF might play a role in the regulation of tolerance to ischemia when administered prior to the ischemic episode. We studied the effects of pretreatment of TNF administered intravenously, intraperitoneally, or intracisternally in mice that were subjected to middle cerebral artery occlusion (MCAO) 48 h later. MCAO was performed in BALB/C mice by direct cauterization of distal MCA, which resulted in pure cortical infarction. A significant reduction in infarct size was noted in mice pretreated by TNF at the dose of 0.5 microgram/mouse (p < 0.01) intracisternally. At the doses used in this study, administration of TNF by intravenous or intraperitoneal routes was not effective. Immunohistochemical analysis of brains subjected to 24 h of MCAO revealed a significant decrease in CD11b immunoreactivity after TNF pretreatment compared with control MCAO. Preconditioning with TNF affects infarct size in a time- and dose-dependent manner. TNF induces significant protection against ischemic brain injury and is likely to be involved in the signaling pathways that regulate ischemic tolerance.

Inhibition of tumor necrosis factor signal transduction in endothelial cells by dimethylaminopurine

Tumor necrosis factor (TNF) promotes diverse responses in endothelial cells that are important to the host response to infections and malignancies; however, less is known of the postreceptor events important to TNF action in endothelial cells than in many other cell types. Since phosphorylation cascades are implicated in cytokine signaling, the effects of the protein kinase inhibitor dimethylaminopurine (DMAP) on TNF action in bovine aortic endothelial cells (BAEC) were investigated. In BAEC, TNF promotes phosphorylation of eukaryotic initiation factor 4E (eIF-4E), c-Jun N-terminal kinase (JNK) and ceramide-activated protein kinase activities, Jun-b expression, prostacyclin production, and, when protein synthesis is inhibited, cytotoxicity. DMAP abrogated or significantly attenuated each of these responses to TNF, without affecting the specific binding of TNF to its receptors. Histamine, another agent active in the endothelium, promotes phosphorylation of elongation factor-2 (EF-2) and prostacyclin production, but not phosphorylation of eIF-4E in BAEC. Histamine-stimulated EF-2 phosphorylation was not inhibited and prostacyclin production was unaffected by DMAP. These observations demonstrate that a distinct signal transduction cascade, which can be selectively inhibited by DMAP, promotes the response of BAEC to TNF. Thus, we have identified a reagent, DMAP, that may be useful for characterizing the TNF signal transduction pathway.

Role of protein kinase activity in apoptosis

The transmission of signals from the plasma membrane to the nucleus involves a number of different pathways all of which have in common protein modification. The modification is primarily in the form of phosphorylation which leads to the activation of a series of protein kinases. It is now evident that these pathways are common to stimuli that lead to mitogenic and apoptotic responses. Even the same stimuli under different physiological conditions can cause either cell proliferation or apoptosis. Activation of specific protein kinases can in some circumstances protect against cell death, while in others it protects the cell against apoptosis. Some of the pathways involved lead to activation of transcription factors and the subsequent induction of genes involved in the process of cell death or proliferation. In other cases, such as for the tumour suppressor gene product p53, activation may be initiated both at the level of gene expression or through pre-existing proteins. Yet in others, while the initial steps in the pathway are ill-defined, it is clear that downstream activation of a series of cystein proteases is instrumental in pushing the cell towards apoptosis. In this report we review the involvement of protein kinases at several different levels in the control of cell behaviour.

Phosphorylation of eIF-4E and initiation of protein synthesis in P19 embryonal carcinoma cells

Mitogenic stimulation of protein synthesis is accompanied by an increase in eIF-4E phosphorylation. The effect on protein synthesis by induction of differentiation is less well known. We treated P19 embryonal carcinoma cells with the differentiating agent retinoic acid and found that protein synthesis increased during the first hour of addition. However, the phosphorylation state, as well as the turnover of phosphate on eIF-4E, remained unchanged. Apparently, the change in protein synthesis after RA addition is regulated by another mechanism than eIF-4E phosphorylation. By using P19 cells overexpressing the EGF receptor, we show that the signal transduction pathway that leads to phosphorylation of eIF-4E is present in P19 cells; the EGF-induced change in phosphorylation of eIF-4E in these cells is likely to be regulated by a change in eIF-4E phosphatase activity. These results suggest that the onset of retinoic acid-induced differentiation is triggered by a signal transduction pathway which involves changes in protein synthesis, but not eIF-4E phosphorylation.

Casein kinase-1 phosphorylates the p75 tumor necrosis factor receptor and negatively regulates tumor necrosis factor signaling for apoptosis

Cellular responses initiated by tumor necrosis factor (TNF) are mediated by two different cell surface receptors with respective molecular masses of 55 kDa (p55) and 75 kDa (p75). p55 is functional in almost every cell type and can independently transmit most biological activities of TNF. In contrast, TNF signaling via p75 seems so far largely restricted to cells of lymphoid origin, where it can induce proliferation, cytokine production, and/or apoptosis. The mechanisms that regulate TNF receptor activity are largely unknown. Here we report that the p75 of unstimulated p75-responsive PC60 T cells is phosphorylated on serine by a kinase activity present in p75 immune complexes. Several lines of evidence indicate that the latter kinase is casein kinase-1 (CK-1). Previous results have shown that the p75 TNF receptor is constitutively phosphorylated in vivo. Our data show that the latter in vivo phosphorylation is also at least partially due to CK-1. Pretreatment of cells with TNF had no detectable effect on p75 phosphorylation in vitro or in vivo. However, a specific CK-1 inhibitor potentiated TNF-induced apoptosis mediated by p75, suggesting an inhibitory role for phosphorylation by CK-1. Although in vivo p75 phosphorylation could be seen in both p75-unresponsive and p75-responsive cell lines, in vitro p75 phosphorylation in p75 coimmunoprecipitates could not be observed in cell lines that were biologically unresponsive to p75 stimulation. The latter observation further indicates a regulatory role for p75 phosphorylation in p75-mediated signaling. Taken together, our data demonstrate that the p75 TNF receptor is phosphorylated and associated with CK-1, which negatively regulates p75-mediated TNF signaling.

Effects of tumor necrosis factor-alpha on antimitogenicity and cell cycle-related proteins in MCF-7 cells

Tumor necrosis factor-alpha (TNF-alpha) demonstrated antimitogenic activity in MCF-7 cells (estrogen receptor-positive human breast cancer cells) in a dose- and time-dependent manner (EC-50 of 2.5 ng/ml). This antimitogenic effect of TNF-alpha was accompanied by a decreased number of cells in S phase in a dose- and time-dependent manner. Based on growth arrest experiments using aphidicolin, it is apparent that TNF-alpha acted in early G1 phase. It did not show antimitogenic effects once cells reentered the S phase based on [3H]thymidine incorporation into DNA and cell cycle analysis. Specificity of TNF-alpha was established by using monoclonal anti-human TNF-alpha antibody. On the basis of Western immunoblot analysis of Rb, p53 and cell cycle inhibitory protein (Cip1) (p21) proteins, TNF-alpha decreased Rb protein expression in a dose- and time-dependent manner whereas it increased the expression level of tumor suppressor p53 protein. TNF-alpha also increased the expression level of Cip1 (p21) protein in a dose-dependent manner. This induction of Cip1 (p21) protein was preceded by the induction of p53 protein in MCF-7 cells. Cip1 (p21) protein associated with cyclin D was also increased. Tumor suppressor Rb protein expression was increased during G1 to S phase progression. Cyclin D protein expression levels were not changed in response to TNF-alpha treatment, although serine/threonine kinase inhibitors such as H7 and the protein kinase C inhibitor staurosporine decreased cyclin D expression levels in MCF-7 cells. Based on experiments with staurosporine, it appears that TNF-alpha does not utilize a protein kinase C pathway in MCF-7 cells. Other cell cycle-related proteins such as Cdk2, Cdc2, and Cdk4 did not show any change in response to TNF-alpha. TNF-alpha did not affect complexes between cyclin D and Cdk2, Cdk4, and Rb proteins in MCF-7 cells. Taken together these results suggest that Rb, p53, and Cip1 (p21) proteins mediate TNF-alpha antimitogenic activity, and TNF-alpha induces growth arrest in the G1 phase in MCF-7 cells.

Phosphorylation of eukaryotic protein synthesis initiation factor 4E by insulin-stimulated protamine kinase

Insulin-stimulated protamine kinase (cPK) and protein kinase C (PKC) phosphorylated eukaryotic protein synthesis initiation factor 4E (eIF-4E) on serine and threonine residues located on an identical tryptic fragment as judged by two-dimensional phosphopeptide mapping. With cPK and PKC, the apparent Km for eIF-4E was about 1.2 and 50 microM, respectively. Relative to recombinant human eIF-4E, cPK exhibited about 100% and < or = 5% activity with eIF-4ES209A and eIF-4ET210A, respectively, and eIF-4ES209A was phosphorylated exclusively on threonines. Bovine kidney eIF-4E enhanced up to 1.8-fold globin synthesis in m7GTP-Sepharose-treated reticulocyte lysates. In contrast, following incubation with cPK, these eIF-4E preparations stimulated globin synthesis up to 6-fold. Compared to the dephosphorylation of the cPK-modified serine on eIF-4E, reticulocyte lysates and highly purified protein phosphatase 2A exhibited marked preference for the cPK-modified threonine. The results indicate that cPK phosphorylates eIF-4E on Ser209 and Thr210, that the hydroxyl group or phosphorylation of Thr210 is necessary for cPK to act on Ser209, and that Ser209 phosphorylation activates reticulocyte globin synthesis. The results suggest that cPK could contribute to the insulin-stimulated phosphorylation of eIF-4E, but that protein phosphatase 2A may confer the site specificity of this response.

Glucose transporter gene expression: regulation of transcription and mRNA stability

The facilitated diffusion of D-glucose across the plasma membrane is carried out by a set of stereospecific transport proteins known as the glucose transporters. These integral membrane proteins are members of a gene family where tissue-specific expression of one or more members will determine in part the net rate of glucose entry into the cell. The regulation of glucose transporter gene expression is a critical feature of cellular homeostasis, as defects in specific transporter expression can lead to profound alterations in cellular physiology. In this review, we provide a brief descriptive background on the family of glucose transporters and examine in depth the regulation of the two transporters expressed in adipose tissue, GLUTI, a basal growth-related transporter and GLUT4, the insulin-responsive glucose transporter.

Signal transduction mechanisms in the regulation of protein synthesis

Control of polypeptide synthesis plays an important role in cell proliferation and translation rates generally reflect the growth state of the cultured eukaryotic cell. Physiological regulation of protein synthesis is almost always exerted at the level of polypeptide chain initiation, with the binding of mRNA to the small ribosomal subunit a rate-limiting step in many cell systems. Studies have indicated key roles in the regulation of protein synthesis for the structural features of mRNA molecules and phosphorylation of initiation factors which catalyse this process. This review focuses on translational regulation at the level of mRNA binding to the ribosome and the role of phosphorylation of initiation factors in mediating both quantitative and qualitative control. The identity of putative kinases which may mediate these processes is addressed and a possible model for the role of a transient activation of initiation factors in cell growth or differentiation is presented.

The presence of tat protein or tumor necrosis factor alpha is critical for herpes simplex virus type 1-induced expression of human immunodeficiency virus type 1

Tat-independent transcription of human immunodeficiency virus type 1 (HIV-1) plays an important role in virus life cycle before biologically significant levels of Tat protein have been accumulated. Using a latently infected T-cell line containing an integrated Tat-defective HIV-1 provirus, we examined whether factors known to up-regulate the HIV-1 expression in vitro can replace the requirement for a functional Tat protein and induce the expression of the Tat-defective HIV-1 provirus. Both tumor necrosis factor alpha (TNF-alpha) and herpes simplex virus type 1 (HSV-1) infection stimulated transcription of the Tat-defective HIV-1 provirus to comparable levels, but in HSV-1-infected cells, the cytoplasmic HIV-1 transcripts were not efficiently translated in the absence of Tat protein and were excluded from the large polysomes. However, HSV-1 infection did not affect the distribution of cellular gamma-actin RNA or 28S RNA in the polysomal fractions. The translational block of HIV-1 RNA was not mediated by the virion-associated host cell shutoff protein (vhs); dissociation of HIV-1 transcripts from the polysomes and inefficient translation was also observed in cells infected with the vhs-defective mutant of HSV-1 (vhs-1). Overexpression of Rev protein did not rescue the synthesis of HIV-1 proteins in these cells; however, the observed inhibition of HIV-1 RNA translation was efficiently overcome in the presence of Tat protein or TNF-alpha. These findings suggest that, in contrast to TNF-alpha, HSV-1 infection is not able to induce a full cycle of HIV-1 replication and that cytokines and Tat have a critical role in the activation of HIV-1 provirus by HSV-1.

Increased phosphorylation of eukaryotic initiation factor 4 alpha during early activation of T lymphocytes correlates with increased initiation factor 4F complex formation

Mature porcine peripheral blood mononuclear cells (PPBMCs) exist in a resting state both in vivo and when maintained in culture, with low translation rates consistent with their non-proliferative state. When cultured in the presence of the appropriate mitogen, there is a 2-4-fold increase in the rate of protein synthesis per ribosome within 4 h of stimulation [Kay, J. E., Ahern, T. and Atkins, M. (1971) Biochim. Biophys. Acta 247, 322-334]. Studies on extracts prepared from unstimulated cells have suggested lesions in initiation factor activity, primarily affecting the binding of mRNA to ribosomes [Ahern, T., Sampson, J. and Kay, J. E. (1974) Nature 248, 519-521]. In these studies, we have demonstrated that activation of quiescent PPBMCs with the phorbol ester phorbol 12-myristate 13-acetate or concanavalin A leads to a rapid 2-4-fold increase in the rate of protein synthesis within 1 h or 4 h, respectively, which is insensitive to the transcriptional inhibitor, 5,6-dichlorobenzimidazole riboside. Relative to control cells, both phorbol ester and concanavalin A induce a 2-4-fold increase in labelling of the eukaryotic initiation factor eIF-4 alpha with phosphate in vivo, which primarily reflects a small net increase in phosphorylation rather than phosphate turnover on eIF-4 alpha. Similarly, with the human leukaemic T cell line JURKAT, stimulation of the T cell receptor with the monoclonal antibody, OKT-3, or treatment with phorbol ester induces a 2-3-fold increase in eIF-4 alpha phosphorylation within 30 min. Analysis of phosphorylation by two-dimensional gel electrophoresis and measurement of kinase activity towards synthetic peptides, indicate that this increased labelling also reflects increased eIF-4 alpha kinase activity rather than phosphate turnover on eIF-4 alpha. Of central importance is the finding that, concomitant with increased rates of protein synthesis following stimulation of PPBMCs with either phorbol ester or concanavalin A, there is a significant increase in the level of eIF-4 alpha recovered in high-molecular-mass complexes. These data suggest that, in quiescent PPBMCs, eIF-4F may be limiting and that the association of eIF-4 alpha and eIF-4 gamma into high-molecular-mass complexes is regulated by phosphorylation and may play a pivotal role in translational control.

Role of protein phosphorylation in TNF-induced apoptosis: phosphatase inhibitors synergize with TNF to activate DNA fragmentation in normal as well as TNF-resistant U937 variants

This study examined the role of protein phosphorylation in TNF induction of apoptosis in several tumor cell lines by testing the effects of agents that either stimulate or inhibit protein phosphorylation. The serine-threonine phosphatase inhibitors, okadaic acid (OKA) and calyculin A (CLA), synergistically augmented TNF-induced apoptosis in several TNF-sensitive tumor cell lines including the U937 histiocytic lymphoma, the BT-20 mammary carcinoma, and the LNCap prostatic tumor cell line. Furthermore, the phosphatase inhibitors completely reversed the TNF resistance of a variant (U9-TR) derived from U937. CLA also inhibited phosphatase activity in cell-free extracts from both U937 and U9-TR at the same concentrations (0.4-2.0 nM) that it synergized with TNF. In contrast, TNF treatment of U937 cells did not result in inhibition of phosphatase activity mediated by protein phosphatase 1 (PP1) and PP2A in cell extracts. Since the phosphatase inhibitors are known to increase the overall levels of protein phosphorylation in cells, this suggested that TNF may act by stimulating protein kinase (PK) activity. This hypothesis was supported by the results of testing a panel of relatively specific protein kinase inhibitors. TNF activation of DNA fragmentation was blocked by a potent inhibitor of myosin light chain kinase (MLCK) but was unaffected by inhibitors of cAMP or cGMP-dependent PKs. We postulate that a defect in the activation of MLCK or possibly some other as yet unknown PK may be responsible for the TNF resistance of U9-TR. Furthermore, this resistance may be circumvented by promoting protein phosphorylation with the serine-threonine-dependent phosphatase inhibitors.

Mechanism of action of developmentally regulated sea urchin inhibitor of eIF-4

The developmentally regulated inhibitor of eIF-4 function found in unfertilized sea urchin eggs has been partially purified and its mechanism of action studied in vitro using purified recombinant eIF-4 alpha and cell-free translation systems. The results demonstrate that although the phosphorylation of eIF-4 alpha is necessary to promote protein synthesis, it is not sufficient to maintain all aspects of eIF-4 function. The egg inhibitor does not change eIF-4 alpha phosphorylation state. During the blockage of initiation caused by the egg inhibitor, eIF-4 alpha remains phosphorylated but accumulates in a 48S initiation intermediate. This suggests that the egg inhibitor functions by preventing the release of eIF-4 alpha from the small ribosomal subunit. The characteristics of the inhibitor in a reticulocyte translation system demonstrate that eIF-4 activity is inhibited within 3-6 min. However, the inhibitor's characteristics in a mRNA-dependent translation system contrast with this. Preincubation with the inhibitor for 5-25 min prior to the addition of mRNA does not prevent endogenous eIF-4 from participating in translation but diminishes its ability to be reutilized, consistent with the accumulation of eIF-4 alpha on the small ribosomal subunit. The ribosomal localization of the inhibitor suggests that it could prevent eIF-4 alpha release by direct binding. The gradual inactivation of the inhibitor following fertilization indicates that it represents a component of a novel regulatory cascade that modulates eIF-4 activity.