Phosphorylation of initiation factor elF-2 and the control of reticulocyte protein synthesis

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

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

Endogenous inhibitors of the dsRNA-dependent eIF-2 alpha protein kinase PKR in normal and ras-transformed cells

The serine/threonine kinase PKR is activated by autophosphorylation in response to nanomolar concentrations of double-stranded RNA and other polyanions. We have previously shown that expression of an oncogenic ras gene induces an endogenous protein inhibitor of PKR activation in murine fibroblasts, as measured by a greatly reduced ability of PKR to autophosphorylate in response to double-stranded RNA in lysates from these ras-expressing cells. Immunoprecipitation of PKR away from the ras-transformed cell lysate restored the ability of PKR to become autophosphorylated. However, the autophosphorylation was no longer dsRNA-dependent. In the present work, PKR immobilized either by immunoprecipitation or by affinity precipitation on Agpoly(I) poly(C) was found to autophosphorylate in a dsRNA-independent manner when incubated in the presence of a detergent lysis buffer and ATP. When lysis buffer was replaced by cytoplasmic extract from normal or ras-transformed cells, autophosphorylation of the immobilized PKR was inhibited in the presence or absence of dsRNA, even though it could be shown that PKR remained bound and intact in the precipitate, and able to autophosphorylate if rewashed with lysis buffer. These findings suggest that PKR activation is regulated by an endogenous inhibitor in murine fibroblasts as well as by dsRNA or other polyanions.

2-Aminopurine inhibits the double-stranded RNA-dependent protein kinase both in vitro and in vivo

The autophosphorylation of interferon (IFN)-induced double-stranded RNA-dependent p68 protein kinase (PKR) and phosphorylation of the alpha-subunit of the translation initiation factor eIF-2 were inhibited by 10 mM 2-aminopurine in vitro. High concentrations of ATP overcame the inhibition. Kinetic studies indicated that 2-aminopurine is a competitive inhibitor with respect to ATP, suggesting that these two molecules bind the same site on the kinase. Treatment of HeLa cells with poly(I):poly(C) stimulated PKR autophosphorylation in vivo. The stimulated activity was inhibited by 10 mM 2-aminopurine to approximately the same extent as the in vitro inhibition.

Viral protein kinases and protein phosphatases

Certain large DNA viruses (e.g. herpesviruses and poxviruses) encode proteins related to cellular protein-serine/threonine kinases, and Hepatitis B virus and vesicular stomatitis virus may encode structurally different protein kinases. Other viruses activate cellular protein kinases, e.g. interferon-induced eukaryotic initiation factor-2 kinase, growth factor-induced kinases and protein kinases that regulate mitosis. Protein phosphatases are encoded by vaccinia virus and bacteriophage lambda and must also play a role in viral infection--as do cellular protein phosphatases. The functions of many of these viral enzymes remain to be determined, but they represent possible new targets for anti-viral therapy.

Reversal of an interferon-gamma-resistant phenotype by poly(I:C): possible role of double-stranded RNA-activated kinase in interferon-gamma signaling

Indoleamine 2,3-dioxygenase (IDO) is induced in neoplastic cell lines by interferon-gamma (IFN-gamma) treatment. In ME180 cervical carcinoma cells, there is a rapid increase in IDO mRNA accumulation beginning at 4 h after IFN-gamma treatment and continuing for at least 24 h. The IFN-gamma-resistant mutant of ME180, IR3B6B, expresses very low levels of IDO message after IFN-gamma treatment. However, pretreatment of this mutant with poly(I:C) restores normal levels of IDO mRNAs and IDO enzyme activity. Poly(I:C) mediated reversal of the IFN-gamma-resistant phenotype and induction of IDO mRNA are inhibited by 2-aminopurine. In vitro phosphorylation of calf thymus histone using the immunoprecipitated p68 kinase prepared from IFN-gamma-treated ME180 and IR3B6B cells revealed the deficiency of activation of this kinase in IR3B6B cells after IFN-gamma treatment, and treatment of this mutant cells with poly(I:C) restores p68 kinase activity. From these results, we conclude that a double-stranded RNA-dependent kinase is activated by IFN-gamma treatment and its activation correlates with IFN-gamma-mediated induction of the IDO gene.

Ribosomes terminated in vitro are in a tight association with non-phosphorylated elongation factor 2 (eEF-2) and GDP

A proportion of the ribosome population in the eukaryotic cell is present in the form of single 80-S ribosomes. These are not involved in translation and are tightly associated with eukaryotic elongation factor 2 (eEF-2). The factor dissociates from ribosomes when it is ADP-ribosylated. Attempts at reconstitution of such complexes from ribosomal subunits and eEF-2 were not successful. We have shown that monomeric ribosomes in a tight complex with eEF-2 can be obtained in vitro as terminated ribosomes in a reconstituted translation system containing isolated polyribosomes, elongation factors and pH5 enzymes (all from rabbit reticulocytes). Incubation of the system with radioactive GTP demonstrated that terminated ribosomes contain GDP. ADP-ribosylation of eEF-2 bound to terminated ribosomes by diphtheria toxin leads to dissociation of both eEF-2 and GDP to the same extent. Thus the presence of GDP in terminated ribosomes is eEF-2 dependent. Ribosomes terminated in vitro as well as native single ribosomes contain the non-phosphorylated form of eEF-2. We assume that tight association of terminated ribosomes with the non-phosphorylated form of eEF-2 excludes both the ribosome and active eEF-2 from the translational cycle and thus, maintains the optimal proportion of translating ribosomes and free eEF-2 in the cell.

Translational regulation of human beta interferon mRNA: association of the 3' AU-rich sequence with the poly(A) tail reduces translation efficiency in vitro

The 3' AU-rich region of human beta-1 interferon (hu-IFN beta) mRNA was found to act as a translational inhibitory element. The translational regulation of this 3' AU-rich sequence and the effect of its association with the poly(A) tail were studied in cell-free rabbit reticulocyte lysate. A poly(A)-rich hu-IFN beta mRNA (110 A residues) served as an inefficient template for protein synthesis. However, translational efficiency was considerably improved when the poly(A) tract was shortened (11 A residues) or when the 3' AU-rich sequence was deleted, indicating that interaction between these two regions was responsible for the reduced translation of the poly(A)-rich hu-IFN beta mRNA. Differences in translational efficiency of the various hu-IFN beta mRNAs correlated well with their polysomal distribution. The poly(A)-rich hu-IFN beta mRNA failed to form large polysomes, while its counterpart bearing a short poly(A) tail was recruited more efficiently into large polysomes. The AU-rich sequence-binding activity was reduced when the RNA probe contained both the 3' AU-rich sequence and long poly(A) tail, supporting a physical association between these two regions. Further evidence for this interaction was achieved by RNase H protection assay. We suggest that the 3' AU-rich sequence may regulate the translation of hu-IFN beta mRNA by interacting with the poly(A) tail.

MAP kinase activation during heat shock in quiescent and exponentially growing mammalian cells

In numerous cases of signal transduction, the mitogen-activated protein kinases (MAP kinases) or extracellular regulated kinases (ERKs) are found to be activated by phosphorylations which result in electrophoretic mobility changes. Activities of MAP kinases in cytosolic extracts can also be monitored by the capacity of such extracts to phosphorylate myelin basic protein. These two assays were used to demonstrate that MAP kinases were rapidly activated during heat shock of both quiescent and exponentially growing mammalian (hamster, rat, mouse and human) cells. Thus, the MAP kinase cascade is likely to also ensure heat-shock signal transduction and contribute to the regulation of the complex array of metabolic changes designated as the heat-shock response.

Translational regulation of human beta interferon mRNA: association of the 3' AU-rich sequence with the poly(A) tail reduces translation efficiency in vitro

The 3' AU-rich region of human beta-1 interferon (hu-IFN beta) mRNA was found to act as a translational inhibitory element. The translational regulation of this 3' AU-rich sequence and the effect of its association with the poly(A) tail were studied in cell-free rabbit reticulocyte lysate. A poly(A)-rich hu-IFN beta mRNA (110 A residues) served as an inefficient template for protein synthesis. However, translational efficiency was considerably improved when the poly(A) tract was shortened (11 A residues) or when the 3' AU-rich sequence was deleted, indicating that interaction between these two regions was responsible for the reduced translation of the poly(A)-rich hu-IFN beta mRNA. Differences in translational efficiency of the various hu-IFN beta mRNAs correlated well with their polysomal distribution. The poly(A)-rich hu-IFN beta mRNA failed to form large polysomes, while its counterpart bearing a short poly(A) tail was recruited more efficiently into large polysomes. The AU-rich sequence-binding activity was reduced when the RNA probe contained both the 3' AU-rich sequence and long poly(A) tail, supporting a physical association between these two regions. Further evidence for this interaction was achieved by RNase H protection assay. We suggest that the 3' AU-rich sequence may regulate the translation of hu-IFN beta mRNA by interacting with the poly(A) tail.

Inhibition of the dsRNA-dependent protein kinase by a peptide derived from the human immunodeficiency virus type 1 Tat protein

The human immunodeficiency virus (HIV) is the etiologic agent leading to the development of acquired immunodeficiency syndrome (AIDS). Interferons (IFNs) are known for eliciting antiviral responses from cells, and studies have indicated that infection with HIV induces the production of IFN. Previous studies have shown that the trans-acting response element (TAR) sequence of HIV-1 mRNA can activate the IFN-induced double-stranded (ds) RNA-dependent protein kinase (DAI). DAI, when activated, is a potent inhibitor of protein synthesis and has been implicated in mediating part of IFN's antiviral activity. Here, we report that a synthetic peptide containing the basic region of HIV Tat protein is effective in preventing the activation of DAI. Evidence is presented that indicates that the Tat peptide exerts its effect by binding to the TAR RNA sequence and thus preventing this RNA from binding to and activating DAI. It appears that in addition to its role in trans-activation, the tat protein may also function to overcome the antiviral activity of IFN by regulating DAI activity. Thus, inhibition of DAI by the Tat protein early in the life cycle of HIV may provide a mechanism by which the virus can escape a translational block imposed by the kinase.

Translational regulation by mRNA/protein interactions in eukaryotic cells: ferritin and beyond

The expression of certain eukaryotic genes is--at least in part--controlled at the level of mRNA translation. The step of translational initiation represents the primary target for regulation. The regulation of the intracellular iron storage protein ferritin in response to iron levels provides a good example of translational control by a reversible RNA/protein interaction in the 5' untranslated region of an mRNA. We consider mechanisms by which mRNA/protein interactions may impede translation initiation and discuss recent data suggesting that the ferritin example may represent the 'tip of the iceberg' of a more general theme for translational control.

Recycling and phosphorylation of eukaryotic initiation factor 2 on 60S subunits of 80S initiation complexes and polysomes

Phosphorylation of the alpha-subunit (38 kDa) of eukaryotic initiation factor 2 (eIF-2 alpha) regulates initiation of protein synthesis in eukaryotic cells. This phosphorylation is enhanced in cycloheximide-treated heme-deficient reticulocyte lysates in which polysomes are maintained. In early heme deficiency prior to polysome disaggregation, eIF-2(alpha P) accumulates primarily on the 60S subunits of polysomes. Further, isolated polysomes contain eIF-2 alpha that is efficiently phosphorylated in vitro by heme-regulated inhibitor (HRI). Immunoblot analysis of eIF-2 distribution in sucrose gradients of actively protein-synthesizing lysates indicates that eIF-2 is distributed at low levels throughout the polysome profiles. These findings suggest that polysome-bound eIF-2 alpha is a target of HRI under physiological conditions. The presence of eIF-2 on the 60S subunits of polysomes is incompatible with the conventional model in which eIF-2 is recycled during the joining of the 48S preinitiation complex and the 60S subunit to form the 80S initiation complex. A modified model is presented with emphasis on the translocation of eIF-2 from the 40S ribosomal subunit of the 48S preinitiation complex (eIF-2.GTP.Met-tRNA(f).40S.mRNA) to the 60S subunit of the 80S initiation complex.

Tumor suppressor function of the interferon-induced double-stranded RNA-activated protein kinase

RNA-dependent protein kinase is a M(r) 68,000 protein in human cells (p68 kinase) or a M(r) 65,000 protein in murine cells (p65 kinase). p65/p68 is a serine/threonine kinase induced by interferon treatment and generally activated by double-stranded RNAs. Once activated, the known function of this kinase is inhibition of protein synthesis through phosphorylation of the eukaryotic initiation factor 2. Here we have investigated the potential for tumorigenicity in mice of murine NIH 3T3 clones expressing human p68 kinase, either the wild-type or a mutant inactive kinase with a single amino acid substitution in the invariant lysine-296 in the catalytic domain II. Expression of the mutant p68 kinase was correlated with a malignant transformation phenotype, giving rise to the production of large tumors of at least 1 cm in diameter within 7-12 days in all inoculated mice. In contrast, no tumor growth was observed for several weeks in mice inoculated with NIH 3T3 cell clones expressing either the wild-type recombinant p68 kinase or only the endogenous p65 kinase, the murine analogue of the p68 kinase. These results suggest that functional p65/p68 kinase (recently called PKR), by a still undefined mechanism, may also act as a tumor suppressor. Consequently, one of the pathways by which interferon inhibits tumor growth might be through its capacity to induce the enhanced expression of this kinase.

Interactions between double-stranded RNA regulators and the protein kinase DAI

The interferon-induced protein kinase DAI, the double-stranded RNA (dsRNA)-activated inhibitor of translation, plays a key role in regulating protein synthesis in higher cells. Once activated, in a process that involves autophosphorylation, it phosphorylates the initiation factor eIF-2, leading to inhibition of polypeptide chain initiation. The activity of DAI is controlled by RNA regulators, including dsRNA activators and highly structured single-stranded RNAs which block activation by dsRNA. To elucidate the mechanism of activation, we studied the interaction of DAI with RNA duplexes of discrete sizes. Molecules shorter than 30 bp fail to bind stably and do not activate the enzyme, but at high concentrations they prevent activation by long dsRNA. Molecules longer than 30 bp bind and activate the enzyme, with an efficiency that increases with increasing chain length, reaching a maximum at about 85 bp. These dsRNAs fail to activate at high concentrations and also prevent activation by long dsRNA. Analysis of complexes between dsRNA and DAI suggests that at maximal packing the enzyme interacts with as little as a single helical turn of dsRNA (11 bp) but under conditions that allow activation the binding site protects about 80 bp of duplex. When the RNA-binding site is fully occupied with an RNA activator, the complex appears to undergo a conformational change.

Interactions between double-stranded RNA regulators and the protein kinase DAI

The interferon-induced protein kinase DAI, the double-stranded RNA (dsRNA)-activated inhibitor of translation, plays a key role in regulating protein synthesis in higher cells. Once activated, in a process that involves autophosphorylation, it phosphorylates the initiation factor eIF-2, leading to inhibition of polypeptide chain initiation. The activity of DAI is controlled by RNA regulators, including dsRNA activators and highly structured single-stranded RNAs which block activation by dsRNA. To elucidate the mechanism of activation, we studied the interaction of DAI with RNA duplexes of discrete sizes. Molecules shorter than 30 bp fail to bind stably and do not activate the enzyme, but at high concentrations they prevent activation by long dsRNA. Molecules longer than 30 bp bind and activate the enzyme, with an efficiency that increases with increasing chain length, reaching a maximum at about 85 bp. These dsRNAs fail to activate at high concentrations and also prevent activation by long dsRNA. Analysis of complexes between dsRNA and DAI suggests that at maximal packing the enzyme interacts with as little as a single helical turn of dsRNA (11 bp) but under conditions that allow activation the binding site protects about 80 bp of duplex. When the RNA-binding site is fully occupied with an RNA activator, the complex appears to undergo a conformational change.

Localization of Epstein-Barr virus-encoded RNAs EBER-1 and EBER-2 in interphase and mitotic Burkitt lymphoma cells

The subcellular distribution of the small Epstein-Barr virus-encoded RNAs EBER-1 and EBER-2 has been investigated by using a high-resolution in situ hybridization technique. The distribution patterns in Raji cells of fluorescent oligodeoxynucleotides complementary to each RNA were detected by confocal laser scanning microscopy. Both RNAs were found in the cytoplasm as well as in the nuclei of interphase cells. In contrast, use of the same technique indicated an exclusively nuclear location for cellular U2 RNA. In the cytoplasm distribution of the EBERs was similar to that of the double-stranded RNA-dependent protein kinase, to which these RNAs can bind, and was coincident with the rough endoplasmic reticulum. In cells undergoing mitosis the EBERs became localized around the chromosomes, whereas the protein kinase remained uniformly distributed in the cytoplasm. A cytoplasmic location for EBER-1 and EBER-2 in interphase cells is consistent with the evidence for a role for these small RNAs in translational control.

Plasmid cDNA-directed protein synthesis in a coupled eukaryotic in vitro transcription-translation system

A system is described in which transcription of cDNA clones by bacteriophage T7 RNA polymerase is coupled to translation in the micrococcal nuclease treated rabbit reticulocyte lysate in a single reaction of coupled transcription-translation. The monovalent and divalent cation requirements for translation are dominant for optimum expression in this coupled system, so that transcription is relatively inefficient. Nevertheless, the use of appropriate DNA concentrations leads to the synthesis of sufficient RNA to saturate the protein synthesis capacity of the system. The fidelity and efficiency of expression in this coupled system are high, and the degree of purification of the plasmid DNA is relatively uncritical. The system therefore offers very considerable advantages for rapid screening of 'mini-preparations' of cDNA plasmid constructs for retention of the correct reading frame and expression of the desired protein product.

Stable and specific association between the yeast recombination and DNA repair proteins RAD1 and RAD10 in vitro

The RAD1 and RAD10 genes of Saccharomyces cerevisiae are two of at least seven genes which are known to be required for damage-specific recognition and/or damage-specific incision of DNA during nucleotide excision repair. RAD1 and RAD10 are also involved in a specialized mitotic recombination pathway. We have previously reported the purification of the RAD10 protein to homogeneity (L. Bardwell, H. Burtscher, W. A. Weiss, C. M. Nicolet, and E. C. Friedberg, Biochemistry 29:3119-3126, 1990). In the present studies we show that the RAD1 protein, produced by in vitro transcription and translation of the cloned gene, specifically coimmunoprecipitates with the RAD10 protein translated in vitro or purified from yeast. Conversely, in vitro-translated RAD10 protein specifically coimmunoprecipitates with the RAD1 protein. The sites of this stable and specific interaction have been mapped to the C-terminal regions of both polypeptides. This portion of RAD10 protein is evolutionarily conserved. These results are the first biochemical evidence of a specific association between any eukaryotic proteins genetically identified as belonging to a recombination or DNA repair pathway and suggest that the RAD1 and RAD10 proteins act at the same or consecutive biochemical steps in both nucleotide excision repair and mitotic recombination.

Identification of double-stranded RNA-binding domains in the interferon-induced double-stranded RNA-activated p68 kinase

The double-stranded RNA (dsRNA)-binding domain of the human p68 kinase has been localized to the N-terminal half of the enzyme by using progressive deletion analysis and in vitro binding assays. To further define the domains responsible for binding to dsRNA, we cloned the mouse dsRNA-activated p65 kinase and used sequence alignment to identify conserved domains in the N-terminal region. Deletions in either of two 12-amino-acid-long and arginine- or lysine-rich regions abrogated binding to dsRNA. Moreover, in an in vivo growth inhibition assay in the yeast Saccharomyces cerevisiae, these mutants failed to exhibit a slow-growth phenotype.

Stable and specific association between the yeast recombination and DNA repair proteins RAD1 and RAD10 in vitro

The RAD1 and RAD10 genes of Saccharomyces cerevisiae are two of at least seven genes which are known to be required for damage-specific recognition and/or damage-specific incision of DNA during nucleotide excision repair. RAD1 and RAD10 are also involved in a specialized mitotic recombination pathway. We have previously reported the purification of the RAD10 protein to homogeneity (L. Bardwell, H. Burtscher, W. A. Weiss, C. M. Nicolet, and E. C. Friedberg, Biochemistry 29:3119-3126, 1990). In the present studies we show that the RAD1 protein, produced by in vitro transcription and translation of the cloned gene, specifically coimmunoprecipitates with the RAD10 protein translated in vitro or purified from yeast. Conversely, in vitro-translated RAD10 protein specifically coimmunoprecipitates with the RAD1 protein. The sites of this stable and specific interaction have been mapped to the C-terminal regions of both polypeptides. This portion of RAD10 protein is evolutionarily conserved. These results are the first biochemical evidence of a specific association between any eukaryotic proteins genetically identified as belonging to a recombination or DNA repair pathway and suggest that the RAD1 and RAD10 proteins act at the same or consecutive biochemical steps in both nucleotide excision repair and mitotic recombination.

Protein kinase-dependent effects of okadaic acid on hepatocytic autophagy and cytoskeletal integrity

The protein phosphatase inhibitor okadaic acid suppressed autophagy completely in isolated rat hepatocytes, as measured by the sequestration of electroinjected [3H]raffinose into sedimentable autophagic vacuoles. Okadaic acid was effectively antagonized by the general protein kinase inhibitors K-252a and KT-5926, the calmodulin antagonist W-7, and by KN-62, a specific inhibitor of Ca2+/calmodulin-dependent protein kinase II (CaMK-II). These inhibitors also antagonized a cytoskeleton-disruptive effect of okadaic acid, manifested as the disintegration of cell corpses after breakage of the plasma membrane. CaMK-II, or a closely related enzyme, would thus seem to play a role in the control of autophagy as well as in the control of cytoskeletal organization.

Mechanism and regulation of eukaryotic protein synthesis

This review presents a description of the numerous eukaryotic protein synthesis factors and their apparent sequential utilization in the processes of initiation, elongation, and termination. Additionally, the rare use of reinitiation and internal initiation is discussed, although little is known biochemically about these processes. Subsequently, control of translation is addressed in two different settings. The first is the global control of translation, which is effected by protein phosphorylation. The second is a series of specific mRNAs for which there is a direct and unique regulation of the synthesis of the gene product under study. Other examples of translational control are cited but not discussed, because the general mechanism for the regulation is unknown. Finally, as is often seen in an active area of investigation, there are several observations that cannot be readily accommodated by the general model presented in the first part of the review. Alternate explanations and various lines of experimentation are proposed to resolve these apparent contradictions.

Dioxin-dependent activation of murine Cyp1a-1 gene transcription requires protein kinase C-dependent phosphorylation

Transcriptional activation of the murine Cyp1a-1 (cytochrome P(1)450) gene by inducers such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (dioxin) requires the aromatic hydrocarbon (Ah) receptor and the interaction of an inducer-receptor complex with one or more of the Ah-responsive elements (AhREs) located about 1 kb upstream from the transcriptional initiation site. We find that treatment of mouse hepatoma Hepa-1 cells with 2-aminopurine, an inhibitor of protein kinase activity, inhibits CYP1A1 mRNA induction by TCDD as well as the concomitant increase in CYP1A1 enzyme activity. Formation of DNA-protein complexes between the Ah receptor and its AhRE target is also inhibited by 2-aminopurine, as determined by gel mobility shift assays. Phosphorylation is required for the formation of Ah receptor-specific complexes, since in vitro dephosphorylation of nuclear extracts from TCDD-treated Hepa-1 cells abolishes the capacity of the Ah receptor to form specific complexes with its cognate AhRE sequences. To determine whether any one of several known protein kinases was involved in the transcriptional regulation of the Cyp1a-1 gene, we treated Hepa-1 cells with nine other protein kinase inhibitors prior to induction with TCDD; nuclear extracts from these cells were analyzed for their capacity to form specific DNA-protein complexes. Only extracts from cells treated with staurosporine, a protein kinase C inhibitor, were unable to form these complexes. In addition, staurosporine completely inhibited CYP1A1 mRNA induction by TCDD. Depletion of protein kinase C by prolonged treatment with phorbol ester led to the complete suppression of CYP1A1 mRNA induction by TCDD. We conclude that (i) phosphorylation is necessary for the formation of a transcriptional complex and for transcriptional activation of the Cyp1a-1 gene; (ii) the phosphorylation site(s) exists on at least one of the proteins constituting the transcriptional complex, possibly the Ah receptor itself; and (iii) the enzyme responsible for the phosphorylation is likely to be protein kinase C.

Dioxin-dependent activation of murine Cyp1a-1 gene transcription requires protein kinase C-dependent phosphorylation

Transcriptional activation of the murine Cyp1a-1 (cytochrome P(1)450) gene by inducers such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (dioxin) requires the aromatic hydrocarbon (Ah) receptor and the interaction of an inducer-receptor complex with one or more of the Ah-responsive elements (AhREs) located about 1 kb upstream from the transcriptional initiation site. We find that treatment of mouse hepatoma Hepa-1 cells with 2-aminopurine, an inhibitor of protein kinase activity, inhibits CYP1A1 mRNA induction by TCDD as well as the concomitant increase in CYP1A1 enzyme activity. Formation of DNA-protein complexes between the Ah receptor and its AhRE target is also inhibited by 2-aminopurine, as determined by gel mobility shift assays. Phosphorylation is required for the formation of Ah receptor-specific complexes, since in vitro dephosphorylation of nuclear extracts from TCDD-treated Hepa-1 cells abolishes the capacity of the Ah receptor to form specific complexes with its cognate AhRE sequences. To determine whether any one of several known protein kinases was involved in the transcriptional regulation of the Cyp1a-1 gene, we treated Hepa-1 cells with nine other protein kinase inhibitors prior to induction with TCDD; nuclear extracts from these cells were analyzed for their capacity to form specific DNA-protein complexes. Only extracts from cells treated with staurosporine, a protein kinase C inhibitor, were unable to form these complexes. In addition, staurosporine completely inhibited CYP1A1 mRNA induction by TCDD. Depletion of protein kinase C by prolonged treatment with phorbol ester led to the complete suppression of CYP1A1 mRNA induction by TCDD. We conclude that (i) phosphorylation is necessary for the formation of a transcriptional complex and for transcriptional activation of the Cyp1a-1 gene; (ii) the phosphorylation site(s) exists on at least one of the proteins constituting the transcriptional complex, possibly the Ah receptor itself; and (iii) the enzyme responsible for the phosphorylation is likely to be protein kinase C.

2-Aminopurine overrides multiple cell cycle checkpoints in BHK cells

BHK cells blocked at any of several points in the cell cycle override their drug-induced arrest and proceed in the cycle when exposed concurrently to the protein kinase inhibitor 2-aminopurine (2-AP). For cells arrested at various points in interphase, 2-AP-induced cell cycle progression is made evident by arrival of the drug-treated cell population in mitosis. Cells that have escaped from mimosine G1 arrest, from hydroxyurea or aphidicolin S-phase arrest, or from VM-26-induced G2 arrest subsequently have all the hallmarks of mitosis--such as a mitotic microtubule array, nuclear envelope breakdown, and chromatin condensation. In a synchronous population, the time course of arrival in mitosis and its duration in 2-AP-treated cells that have escaped drug-induced cell cycle blocks is indistinguishable from control cells. Cells arrested in mitosis by nocodazole or taxol quickly escape mitotic arrest and enter interphase when exposed to 2-AP. 2-AP by itself does not influence the timing of cell cycle progression. We conclude that 2-AP acts to override checkpoints in every phase of the cell cycle, perhaps by inhibiting a protein kinase responsible for control of multiple cell cycle checkpoints.

Nerve growth factor employs multiple pathways to induce primary response genes in PC12 cells

Nerve growth factor (NGF) leads to neuronal differentiation of PC12 cells and promotes their survival in serum-free medium. Past studies have shown that purine analogues block some of the effects of NGF but not others and thus that they can be used to dissect the mechanistic pathways of its action. In the present work we used 2-aminopurine (2-AP) and 6-thioguanine (6-TG) to examine whether NGF causes activation of primary response genes through a single signaling pathway or via multiple pathways. Northern blot analysis and nuclear run-off transcription assays were used to assess the activation of c-fos, c-jun, TIS1, TIS8, and TIS11 after exposure of PC12 cells to NGF in the presence or absence of 2-AP and 6-TG. Our findings indicate that NGF appears to employ at least three distinct pathways to induce early genes in PC12 cells. This suggests that the NGF signaling mechanism diverges at an early point after interaction of NGF with its receptor.

Phosphorylation of initiation factor 2 alpha by protein kinase GCN2 mediates gene-specific translational control of GCN4 in yeast

We show that phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF-2) by the protein kinase GCN2 mediates translational control of the yeast transcriptional activator GCN4. In vitro, GCN2 specifically phosphorylates the alpha subunit of rabbit or yeast eIF-2. In vivo, phosphorylation of eIF-2 alpha increases in response to amino acid starvation, which is dependent on GCN2. Substitution of Ser-51 with alanine eliminates phosphorylation of eIF-2 alpha by GCN2 in vivo and in vitro and abolishes increased expression of GCN4 and amino acid biosynthetic genes under its control in amino acid-starved cells. The Asp-51 substitution mimics the phosphorylated state and derepresses GCN4 in the absence of GCN2. Thus, an established mechanism for regulating total protein synthesis in mammalian cells mediates gene-specific translational control in yeast.

Molecular cloning and characterization of an interferon induced human cDNA with sequence homology to a mammalian peptide chain release factor

Here we report the molecular cloning of several related human cDNAs from which a full-length sequence can be determined. The cDNAs encode a 2.8 kb mRNA that is strongly induced by interferon (IFN) gamma and the expression of which is not cell-restricted but observed in fibroblasts, macrophages and epithelial cells. The deduced amino acid sequence predicts a protein of 471 amino acids with high sequence similarity to a previously identified rabbit peptide chain release factor. Functional studies to demonstrate release factor activity showed that the protein encoded by this cDNA inhibited the readthrough activity of a yeast UGA suppressor tRNA in an in vitro translation system. The identification of this novel cDNA implies that translational control by IFN induced proteins may not be restricted to the initial steps of protein synthesis but may also act by regulation of peptide chain termination.

6-Methylmercaptopurine riboside is a potent and selective inhibitor of nerve growth factor-activated protein kinase N

Protein kinase N (PKN) is a soluble, apparently novel serine protein kinase that is activated by nerve growth factor (NGF) and other agents in PC12 pheochromocytoma cells as well as in several nonneuronal cell lines. Purine analogs, such as 6-thioguanine and 2-aminopurine, have been found to inhibit PKN in vitro. When applied to intact cells, these compounds suppress certain biological responses to NGF, but not others, a findings suggesting the presence of multiple pathways in the NGF mechanism. We report here that 6-methylmercaptopurine riboside (6-MMPR) inhibits NGF-stimulated PKN activity in vitro with an apparent Ki of approximately 5 nM. This is approximately 1,000-fold lower than the Ki of the most potent purine inhibitor of PKN. Compounds similar to 6-MMPR, but lacking the methyl or riboside groups, were much less potent as PKN inhibitors. A survey of six additional purified protein kinases shows no inhibitory effect of 6-MMPR, thus indicating a good degree of specificity of this compound for PKN. In contrast to NGF-stimulated PKN, a PKN-like activity stimulated in PC12 cells in response to activation of cyclic AMP-dependent protein kinase was nearly insensitive to 6-MMPR. Application of 6-MMPR to intact PC12 cells resulted in blockade of several responses to NGF (neurite regeneration and ornithine decarboxylase induction) but not of several others (rapid enhancement of tyrosine hydroxylase phosphorylation and PKN activation). These findings suggest that 6-MMPR is a potent and selective agent for characterizing PKN in vitro and for assessing its potential role in the multiple pathways of the NGF mechanism of action.

2-Aminopurine inhibits RNA and protein synthesis and reduces catecholamine desensitization in C6-2B rat glioma cells

We previously proposed that intracellular cyclic AMP accumulation induces a putative, rapidly turning over protein inhibitory to further hormone activation of adenylate cyclase. In the present study, 2-aminopurine, which has been reported to selectively block c-fos gene expression, was used to test the hypothesis that c-fos protein might be involved in the desensitization to catecholamines was observed in 2-aminopurine-treated C6-2B rat glioma cells. However, we found 2-aminopurine to inhibit, in a concentration-dependent manner, total cellular RNA and protein synthesis in C6-2B, HeLa, Swiss 3T3 and BALB/c cells. mRNA synthesis was also markedly reduced in 2-aminopurine-treated cells. These unexpected findings, while supporting our hypothesis of a protein synthesis-sensitive step in the development of refractoriness, raise concern about the specificity of action of 2-aminopurine to inhibit c-fos induction and thus any cellular process, including desensitization, which might be regulated by c-fos gene expression.

Role of the RNA-dependent protein kinase in the regulated expression of genes in transfected cells

The RNA-dependent P1/eIF-2 alpha protein kinase is a highly specific protein-serine/threonine kinase that catalyzes the phosphorylation of the alpha subunit of protein synthesis initiation factor eIF-2. The kinase plays a central role in translational control. The activity of the kinase is regulated by a variety of naturally occurring effector RNAs which bind to the regulatory domain of the enzyme. Certain RNAs are able to activate the eIF-2 alpha kinase activity inherent within protein P1, a process which involves an autophosphorylation of protein P1, whereas other RNAs are able to antagonize the activation process. Translational repression mediated by the kinase may also be disrupted by RNA binding proteins that sequester activator double-stranded RNAs and by site-directed mutants and homologs of the eIF-2 alpha translation factor substrate. The P1/eIF-2 alpha protein kinase is an important regulator of the translation of plasmid-derived mRNAs in transfected eukaryotic cells.

(2'-5')Oligoadenylate and intracellular immunity against retrovirus infection

1. The double-stranded RNA-dependent 2',5'-oligoadenylate (2-5A) synthetase/ribonuclease L (RNase L) system plays an essential role in the establishment of the antiviral state of a cell exposed to virus infection. 2. Until recently, the application of 2-5A derivatives to reinforce this system seemed to be limited mainly due to the low specificity of RNase L for viral RNA. 3. Two new strategies have been developed which yield a selective antiviral effect of 2-5As at least against human immunodeficiency virus-1 (HIV-1) infection: (i) an "intracellular immunization" approach using 2-5A synthetase cDNA linked to HIV trans-acting response element (TAR) and (ii) inhibition of retroviral reverse transcriptase activity by 2-5A analogues.

The conversion of eIF-2.GDP to eIF-2.GTP by eIF-2B requires Met-tRNA(fMet)

We have investigated why the recycling of eIF-2.GDP to eIF-2.GTP, mediated by the guanine nucleotide exchange factor eIF-2B, is rapid in rabbit reticulocyte lysate, reconstituted for optimal protein synthesis, but slow in an isolated reaction with purified eIF-2B. We have found that purified eIF-2B dissociates eIF-2.[3H]GDP as efficiently in the presence of GTP as it does in the presence of GDP provided Met-tRNA(fMet) is added. tRNA(fMet) is ineffective, and there is no Met-tRNA(fMet) requirement for exchange with GDP. Exchange of eIF-2 bound GDP for GTP is completely dependent upon Met-tRNA(fMet) in the presence of ATP, suggesting that under physiological conditions efficient recycling of eIF-2.GDP to eIF-2.GTP requires conversion of the latter, a relatively unstable complex, to a more stable Met-tRNA(fMet).eIF-2.GTP complex.

Functional expression and RNA binding analysis of the interferon-induced, double-stranded RNA-activated, 68,000-Mr protein kinase in a cell-free system

Eukaryotic viruses have devised numerous strategies to downregulate activity of the interferon-induced, double-stranded (dsRNA)-activated protein kinase (referred to as p68 on the basis of its Mr of 68,000 in human cells). Viruses must exert this control to avoid extensive phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF-2) by p68 and the resultant negative effects on protein synthesis initiation. To begin to define the molecular mechanisms underlying this regulation, we optimized expression of p68 in an in vitro transcription-translation system utilizing the full-length cDNA clone. The in vitro-expressed kinase was autophosphorylated in response to dsRNAs and heparin in a manner similar to that for the native p68 provided that the kinase inhibitor, 2-aminopurine, was present during the in vitro translation reaction. Further, the activated kinase efficiently phosphorylated its natural substrate, the alpha subunit of eIF-2. Binding experiments revealed that the expressed kinase complexed with the dsRNA activator, reovirus dsRNA, as well as the adenovirus-encoded inhibitor, VAI RNA. Interestingly, both the reovirus RNAs and VAI RNA also complexed with protein kinase molecules that lacked the carboxyl terminus and all catalytic domains. Deletion analysis confirmed that the p68 amino terminus contained critical determinants for reovirus dsRNA and VAI RNA binding. Further, reovirus dsRNA efficiently bound to, but failed to activate, p68 kinase molecules containing a single amino acid substitution in the invariant lysine 295 present in catalytic domain II. Taken together, these data demonstrate that this expression system permits a detailed mutagenic analysis of the regions of p68 required for interaction with virus-encoded activators and repressors.

Functional expression and RNA binding analysis of the interferon-induced, double-stranded RNA-activated, 68,000-Mr protein kinase in a cell-free system

Eukaryotic viruses have devised numerous strategies to downregulate activity of the interferon-induced, double-stranded (dsRNA)-activated protein kinase (referred to as p68 on the basis of its Mr of 68,000 in human cells). Viruses must exert this control to avoid extensive phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF-2) by p68 and the resultant negative effects on protein synthesis initiation. To begin to define the molecular mechanisms underlying this regulation, we optimized expression of p68 in an in vitro transcription-translation system utilizing the full-length cDNA clone. The in vitro-expressed kinase was autophosphorylated in response to dsRNAs and heparin in a manner similar to that for the native p68 provided that the kinase inhibitor, 2-aminopurine, was present during the in vitro translation reaction. Further, the activated kinase efficiently phosphorylated its natural substrate, the alpha subunit of eIF-2. Binding experiments revealed that the expressed kinase complexed with the dsRNA activator, reovirus dsRNA, as well as the adenovirus-encoded inhibitor, VAI RNA. Interestingly, both the reovirus RNAs and VAI RNA also complexed with protein kinase molecules that lacked the carboxyl terminus and all catalytic domains. Deletion analysis confirmed that the p68 amino terminus contained critical determinants for reovirus dsRNA and VAI RNA binding. Further, reovirus dsRNA efficiently bound to, but failed to activate, p68 kinase molecules containing a single amino acid substitution in the invariant lysine 295 present in catalytic domain II. Taken together, these data demonstrate that this expression system permits a detailed mutagenic analysis of the regions of p68 required for interaction with virus-encoded activators and repressors.

Two distinct alpha-interferon-dependent signal transduction pathways may contribute to activation of transcription of the guanylate-binding protein gene

The promoter of the gene encoding a cytoplasmic guanylate-binding protein (GBP) contains two overlapping elements: the interferon stimulation response element (ISRE), which mediates alpha interferon (IFN-alpha)-dependent transcription, and the IFN-gamma activation site (GAS), which is required for IFN-gamma-mediated stimulation. The ISRE binds a factor called ISGF-3 that is activated by IFN-alpha but not by IFN-gamma. The GAS binds a protein that is activated by IFN-gamma, which we have termed GAF (IFN-gamma activation factor; T. Decker, D. J. Lew, J. Mirkovitch, and J. E. Darnell, Jr., EMBO J., in press; D. J. Lew, T. Decker, I. Strehlow, and J. E. Darnell, Jr., Mol. Cell. Biol. 11:182-191, 1991). We now find that the GAS is also an IFN-alpha-responsive element in vivo and that IFN-alpha (in addition to activating ISGF-3) rapidly activates a GAS-binding factor, the IFN-alpha activation factor (AAF). The AAF has characteristics very similar to those of the previously described GAF. Through the use of inhibitors of protein synthesis and inhibitors of protein kinases, the activation conditions of AAF, GAF, and ISGF-3 could be distinguished. Therefore, not only do IFN-alpha and IFN-gamma stimulate transcription of GBP through different receptors linked to different signaling molecules, but occupation of the IFN-alpha receptor apparently leads to the rapid activation of two different DNA-binding proteins through the use of different intracellular pathways.