Rapid activation of calmodulin-dependent protein kinase III in mitogen-stimulated human fibroblasts. Correlation with intracellular Ca2+ transients

Revealing eEF-2 kinase: recent structural insights into function

The α-kinase eukaryotic elongation factor 2 kinase (eEF-2K) regulates translational elongation by phosphorylating its ribosome-associated substrate, the GTPase eEF-2. eEF-2K is activated by calmodulin (CaM) through a distinctive mechanism unlike that in other CaM-dependent kinases (CAMK). We describe recent structural insights into this unique activation process and examine the effects of specific regulatory signals on this mechanism. We also highlight key unanswered questions to guide future structure-function studies. These include structural mechanisms which enable eEF-2K to interact with upstream/downstream partners and facilitate its integration of diverse inputs, including Ca2+ transients, phosphorylation mediated by energy/nutrient-sensing pathways, pH changes, and metabolites. Answering these questions is key to establishing how eEF-2K harmonizes translation with cellular requirements within the boundaries of its molecular landscape.

Signal Integration at Elongation Factor 2 Kinase: THE ROLES OF CALCIUM, CALMODULIN, AND SER-500 PHOSPHORYLATION

Eukaryotic elongation factor 2 kinase (eEF-2K), the only calmodulin (CaM)-dependent member of the unique α-kinase family, impedes protein synthesis by phosphorylating eEF-2. We recently identified Thr-348 and Ser-500 as two key autophosphorylation sites within eEF-2K that regulate its activity. eEF-2K is regulated by Ca²⁺ ions and multiple upstream signaling pathways, but how it integrates these signals into a coherent output, i.e. phosphorylation of eEF-2, is unclear. This study focuses on understanding how the post-translational phosphorylation of Ser-500 integrates with Ca²⁺ and CaM to regulate eEF-2K. CaM is shown to be absolutely necessary for efficient activity of eEF-2K, and Ca²⁺ is shown to enhance the affinity of CaM toward eEF-2K. Ser-500 is found to undergo autophosphorylation in cells treated with ionomycin and is likely also targeted by PKA. In vitro, autophosphorylation of Ser-500 is found to require Ca²⁺ and CaM and is inhibited by mutations that compromise binding of phosphorylated Thr-348 to an allosteric binding pocket on the kinase domain. A phosphomimetic Ser-500 to aspartic acid mutation (eEF-2K S500D) enhances the rate of activation (Thr-348 autophosphorylation) by 6-fold and lowers the EC₅₀ for Ca²⁺/CaM binding to activated eEF-2K (Thr-348 phosphorylated) by 20-fold. This is predicted to result in an elevation of the cellular fraction of active eEF-2K. In support of this mechanism, eEF-2K knock-out MCF10A cells reconstituted with eEF-2K S500D display relatively high levels of phospho-eEF-2 under basal conditions. This study reports how phosphorylation of a regulatory site (Ser-500) integrates with Ca²⁺ and CaM to influence eEF-2K activity.

Calcium Homeostasis and Ionic Mechanisms in Pulmonary Fibroblasts

Fibroblasts are key cellular mediators of many chronic interstitial lung diseases, including idiopathic pulmonary fibrosis, scleroderma, sarcoidosis, drug-induced interstitial lung disease, and interstitial lung disease in connective tissue disease. A great deal of effort has been expended to understand the signaling mechanisms underlying the various cellular functions of fibroblasts. Recently, it has been shown that Ca(2+) oscillations play a central role in the regulation of gene expression in human pulmonary fibroblasts. However, the mechanisms whereby cytosolic [Ca(2+)] are regulated and [Ca(2+)] oscillations transduced are both poorly understood. In this review, we present the general concepts of [Ca(2+)] homeostasis, of ionic mechanisms responsible for various Ca(2+) fluxes, and of regulation of gene expression by [Ca(2+)]. In each case, we then also summarize the original findings that pertain specifically to pulmonary fibroblasts. From these data, we propose an overall signaling cascade by which excitation of the fibroblasts triggers pulsatile release of internally sequestered Ca(2+), which, in turn, activates membrane conductances, including voltage-dependent Ca(2+) influx pathways. Collectively, these events produce recurring Ca(2+) oscillations, the frequency of which is transduced by Ca(2+)-dependent transcription factors, which, in turn, orchestrate a variety of cellular events, including proliferation, synthesis/secretion of extracellular matrix proteins, autoactivation (production of transforming growth factor-β), and transformation into myofibroblasts. That unifying hypothesis, in turn, allows us to highlight several specific cellular targets and therapeutic intervention strategies aimed at controlling unwanted pulmonary fibrosis. The relationships between Ca(2+) signaling events and the unfolded protein response and apoptosis are also explored.

Proteasomal degradation of eukaryotic elongation factor-2 kinase (EF2K) is regulated by cAMP-PKA signaling and the SCFβTRCP ubiquitin E3 ligase

Protein translation and degradation are critical for proper protein homeostasis, yet it remains unclear how these processes are dynamically regulated, or how they may directly balance or synergize with each other. An important translational control mechanism is the Ca(2+)/calmodulin-dependent phosphorylation of eukaryotic elongation factor-2 (eEF-2) by eukaryotic elongation factor-2 kinase (EF2K), which inhibits elongation of nascent polypeptide chains during translation. We previously described a reduction of EF2K activity in PC12 cells treated with NGF or forskolin. Here, we show that both forskolin- and IGF-1-mediated reductions of EF2K activity in PC12 cells are due to decreased EF2K protein levels, and this is attenuated by application of the proteasome inhibitor, MG132. We further demonstrate that proteasome-mediated degradation of EF2K occurs in response to A2A-type adenosine receptor stimulation, and that activation of protein kinase A (PKA) or phospho-mimetic mutation of the previously characterized PKA site, Ser-499, were sufficient to induce EF2K turnover in PC12 cells. A similar EF2K degradation mechanism was observed in primary neurons and HEK cells. Expression of a dominant-negative form of Cul1 in HEK cells demonstrated that EF2K levels are regulated by an SCF-type ubiquitin E3 ligase. Specifically, EF2K binds to the F-box proteins, βTRCP1 and βTRCP2, and βTRCP regulates EF2K levels and polyubiquitylation. We propose that the proteasomal degradation of EF2K provides a mechanistic link between activity-dependent protein synthesis and degradation.

Phosphorylation of elongation factor-2 kinase differentially regulates the enzyme's stability under stress conditions

Eukaryotic elongation factor-2 kinase (eEF-2K) is a Ca(2+)/calmodulin-dependent enzyme that negatively regulates protein synthesis. eEF-2K has been shown to be up-regulated in cancer, and to play an important role in cell survival through inhibition of protein synthesis. Post-translational modification of protein synthesis machinery is important for its regulation and could be critical for survival of cancer cells encountering stress. The purpose of our study was to examine the regulation of eEF-2K during stress with a focus on the roles of phosphorylation in determining the stability of eEF-2K. We found that stress conditions (nutrient deprivation and hypoxia) increase eEF-2K protein. mRNA levels are only transiently increased and shortly return to normal, while eEF-2K protein levels continue to increase after further exposure to stress. A seemingly paradoxical decrease in eEF-2K stability was found when glioma cells were subjected to stress despite increased protein expression. We further demonstrated that phosphorylation of eEF-2K differentially affects the enzyme's turnover under both normal and stress conditions, as evidenced by the different half-lives of phosphorylation-defective mutants of eEF-2K. We further found that the eEF-2K site (Ser398) phosphorylated by AMPK is pivotal to the protein's stability, as the half-life of S398A mutant increases to greater than 24h under both normal and stress conditions. These data indicate that eEF-2K is regulated at multiple levels with phosphorylation playing a critical role in the enzyme's turnover under stressful conditions. The complexity of eEF-2K phosphorylation highlights the intricacies of protein synthesis control during cellular stress.

Amyloid-beta interrupts the PI3K-Akt-mTOR signaling pathway that could be involved in brain-derived neurotrophic factor-induced Arc expression in rat cortical neurons

The deposition of amyloid-beta (Abeta) contributes to the pathogenesis of Alzheimer's disease. Even at low levels, Abeta may interfere with various signaling cascades critical for the synaptic plasticity that underlies learning and memory. Brain-derived neurotrophic factor (BDNF) is well known to be capable of inducing the synthesis of activity-regulated cytoskeleton-associated protein (Arc), which plays a fundamental role in modulating synaptic plasticity. Our recent study has demonstrated that treatment of fibrillar Abeta at a nonlethal level was sufficient to impair BDNF-induced Arc expression in cultured rat cortical neurons. In this study, BDNF treatment alone induced the activation of the phosphatidylinositol 3-kinase-Akt-mammlian target of rapamycin (PI3K-Akt-mTOR) signaling pathway, the phosphorylation of eukaryotic initiation factor 4E binding protein (4EBP1) and p70 ribosomal S6 kinase (p70S6K), the dephosphorylation of eukaryotic elongation factor 2 (eEF2), and the expression of Arc. Interrupting the PI3K-Akt-mTOR signaling pathway by inhibitors prevented the effects of BDNF, indicating the involvement of this pathway in BDNF-induced 4EBP1 phosphorylation, p70S6K phosphorylation, eEF2 dephosphorylation, and Arc expression. Nonlethal Abeta pretreatment partially blocked these effects of BDNF. Double- immunofluorescent staining in rat cortical neurons further confirmed the coexistence of eEF2 dephosphorylation and Arc expression following BDNF treatment regardless of the presence of Abeta. These results reveal that, in cultured rat cortical neurons, Abeta interrupts the PI3K-Akt-mTOR signaling pathway that could be involved in BDNF-induced Arc expression. Moreover, this study also provides the first evidence that there is a close correlation between BDNF-induced eEF2 dephosphorylation and BDNF-induced Arc expression. (c) 2009 Wiley-Liss, Inc.

A pharmacoproteomic approach implicates eukaryotic elongation factor 2 kinase in ER stress-induced cell death

Apoptosis triggered by endoplasmic reticulum (ER) stress has been implicated in many diseases but its cellular regulation remains poorly understood. Previously, we identified salubrinal (sal), a small molecule that protects cells from ER stress-induced apoptosis by selectively activating a subset of endogenous ER stress-signaling events. Here, we use sal as a probe in a proteomic approach to discover new information about the endogenous cellular response to ER stress. We show that sal induces phosphorylation of the translation elongation factor eukaryotic translation elongation factor 2 (eEF-2), an event that depends on eEF-2 kinase (eEF-2K). ER stress itself also induces eEF-2K-dependent eEF-2 phosphorylation, and this pathway promotes translational arrest and cell death in this context, identifying eEF-2K as a hitherto unknown regulator of ER stress-induced apoptosis. Finally, we use both sal and ER stress models to show that eEF-2 phosphorylation can be activated by at least two signaling mechanisms. Our work identifies eEF-2K as a new component of the ER stress response and underlines the utility of novel small molecules in discovering new cell biology.

Elongation Factor-2 Kinase Regulates Autophagy in Human Glioblastoma Cells

Elongation factor-2 kinase (eEF-2 kinase), also known as Ca(2+)/calmodulin-dependent kinase III, regulates protein synthesis by controlling the rate of peptide chain elongation. The activity of eEF-2 kinase is increased in glioblastoma and other malignancies, yet its role in neoplasia is uncertain. Recent evidence suggests that autophagy plays an important role in oncogenesis and that this can be regulated by mammalian target of rapamycin (mTOR). Because eEF-2 kinase lies downstream of mTOR, we studied the role of eEF-2 kinase in autophagy using human glioblastoma cell lines. Knockdown of eEF-2 kinase by RNA interference inhibited autophagy in glioblastoma cell lines, as measured by light chain 3 (LC3)-II formation, acidic vesicular organelle staining, and electron microscopy. In contrast, overexpression of eEF-2 kinase increased autophagy. Furthermore, inhibition of autophagy markedly decreased the viability of glioblastoma cells grown under conditions of nutrient depletion. Nutrient deprivation increased eEF-2 kinase activity and decreased the activity of S6 kinase, suggesting an involvement of mTOR pathway in the eEF-2 kinase regulation of autophagy. These results suggest that eEF-2 kinase plays a regulatory role in the autophagic process in tumor cells; and eEF-2 kinase is a downstream member of the mTOR signaling; eEF-2 kinase may promote cancer cell survival under conditions of nutrient deprivation through regulating autophagy. Therefore, eEF-2 kinase may be a part of a survival mechanism in glioblastoma and targeting this kinase may represent a novel approach to cancer treatment.

Enhancement of translation elongation in neurons by brain-derived neurotrophic factor: implications for mammalian target of rapamycin signaling

The effects and signaling mechanisms of brain-derived neurotrophic factor (BDNF) on translation elongation were investigated in cortical neurons. BDNF increased the elongation rate approximately twofold, as determined by measuring the ribosomal transit time. BDNF-accelerated elongation was inhibited by rapamycin, implicating the mammalian target of rapamycin (mTOR). To explore the mechanisms underlying these effects, we examined the protein phosphorylation cascades that lead to the activation of translation elongation in neurons. BDNF increased eukaryote elongation factor 1A (eEF1A) phosphorylation and decreased eEF2 phosphorylation. Whereas eEF2 phosphorylation levels altered by BDNF were inhibited by rapamycin, eEF1A phosphorylation was not affected by rapamycin or PD98059, a mitogen-activated protein kinase kinase (MEK) inhibitor. BDNF induced phosphorylation of eEF2 kinase (Ser366), as well as decreased its kinase activity. All these events were inhibited by rapamycin. Furthermore, mTOR siRNA, which reduced mTOR levels up to 50%, inhibited the BDNF-induced enhancement in elongation rate and decrease in eEF2 phosphorylation. These results strongly suggest that BDNF enhances translation elongation through the activation of the mTOR-eEF2 pathway.

Identification of the ubiquitin-proteasome pathway in the regulation of the stability of eukaryotic elongation factor-2 kinase

Eukaryotic elongation factor-2 kinase (eEF-2 kinase) is a highly conserved calcium/calmodulin-dependent enzyme involved in the regulation of protein translation and cell proliferation. Rapid changes in the activity and abundance of eEF-2 kinase have been observed on growth stimulation, and increased enzyme activity is characteristic of malignant cell growth. Yet the mechanism for controlling the turnover of this kinase is unknown. The ubiquitin-proteasome pathway regulates the degradation of many cellular proteins, including transcription factors, cell cycle regulators, and signal transduction proteins. Therefore, we determined whether the ubiquitin-proteasome pathway regulates the turnover of eEF-2 kinase. We found that eEF-2 kinase was a relatively short-lived protein with a half-life of less than 6 hours. eEF-2 kinase was ubiquitinated in vivo as determined by coimmunoprecipitation and polyubiquitin affinity matrix. Incubation of purified eEF-2 kinase with a source of ubiquitination enzymes (rabbit reticulocyte lysate), purified ubiquitin, and ATP revealed the presence of increasing molecular weight species of ubiquitinated eEF-2 kinase. Treatment of cells with MG132, a proteasome inhibitor, inhibited eEF-2 kinase degradation and induced the accumulation of polyubiquitinated forms of the enzyme, resulting in an increase in its half-life. These results suggest involvement of the proteasome in the turnover of the ubiquitinated kinase. Because eEF-2 kinase is chaperoned by heat shock protein 90 (Hsp90), we next determined if disruption of the Hsp90-eEF-2 kinase complex promoted degradation of the kinase. Treatment of cells with geldanamycin, an Hsp90 inhibitor, enhanced ubiquitination of eEF-2 kinase and decreased the half-life of the kinase to less than 2 hours. These results indicate that cellular levels of eEF-2 kinase are maintained by a balance between association with Hsp90 and degradation by the ubiquitin-proteasome pathway. In conclusion, these data show that the turnover of eEF-2 kinase is regulated by the ubiquitin-proteasome pathway and, therefore, modulating the ubiquitination of eEF-2 kinase might control the abundance of this enzyme and have implications in the treatment of certain forms of cancer.

IGF-I activates the eIF4F system in cardiac muscle in vivo

IGF-I acutely stimulates protein synthesis in cardiac muscle through acceleration of mRNA translation. In the present study, we examined the regulatory signaling pathways and translation protein factors that potentially contribute to the myocardial responsiveness of protein synthesis to IGF-I in vivo. IGF-I was injected IV into rats and 20 min later the hearts were excised and homogenized for assay of regulatory proteins. IGF-I increased assembly of the translationally active eukaryotic initiation factor (eIF)4G.eIF4E complex. The increased assembly of eIF4G.eIF4E was associated with an enhanced eIF4G phosphorylation and increased availability of eIF4E. Increased availability of eIF4E occurred as a consequence of diminished abundance of the inactive 4E-BP1.eIF4E complex following IGF-I. The assembly of the 4E-BP1.eIF4E complex appeared to be decreased through an IGF-I-induced phosphorylation of 4E-BP1. IGF-I also caused an increase in the phosphorylation of S6K1. Activation of the potential upstream regulators of 4E-BP1 and S6K1 phosphorylation via PKB and mTOR was also observed. In contrast, there was no effect of IGF-I on phosphorylation of elongation factor (eFE)2. The results suggest the major impact of IGF-I in cardiac muscle occurred via stimulation of translation initiation rather than elongation. Furthermore, the results are consistent with a role for assembly of active eIF4G.eIF4E complex and activation of S6K1 in mediating the stimulation of mRNA translation initiation by IGF-I through a PKB/mTOR signaling pathway.

Expression and phosphorylation of delta-CaM kinase II in cultured Alzheimer fibroblasts

Dysregulation of calcium homeostasis is among the major cellular alterations in Alzheimer's disease (AD). We studied Ca(2+)/calmodulin-dependent protein kinase II (CaM kinase II), one of the major effectors regulating neuronal responses to changes in calcium fluxes, in cultured skin fibroblasts from subjects with sporadic AD. We found, by using PCR and Western analysis, that human fibroblasts express the delta-isoform of this kinase, and that CaM kinase II is the major Ca(2+)/calmodulin-dependent kinase in these cells. Protein expression level of the kinase was not significantly different in AD fibroblasts. However, the total activity of the kinase (stimulated by Ca(2+)/calmodulin) was significantly reduced in AD cell lines, whereas Ca(2+)-independent activity was significantly enhanced. The percent autonomy of the kinase (%Ca(2+)-independent/Ca(2+)-dependent activity) in AD cell lines was 62.8%, three-fold the corresponding percentage in control fibroblasts. The abnormal calcium-independent activity was not due to enhanced basal autophosphorylation of Thr(287). The observed abnormalities, if present in brain tissue, may be implicated either in dysfunction of neuroplasticity and cognitive functions or in dysregulation of cell cycle.

Regulation of the Multifunctional Ca2+/Calmodulin-dependent Protein Kinase II by the PP2C Phosphatase PPM1F in Fibroblasts

The regulation of the multifunctional calcium/calmodulin dependent protein kinase II (CaMKII) by serine/threonine protein phosphatases has been extensively studied in neuronal cells; however, this regulation has not been investigated previously in fibroblasts. We cloned a cDNA from SV40-transformed human fibroblasts that shares 80% homology to a rat calcium/calmodulin-dependent protein kinase phosphatase that encodes a PPM1F protein. By using extracts from transfected cells, PPM1F, but not a mutant (R326A) in the conserved catalytic domain, was found to dephosphorylate in vitro a peptide corresponding to the auto-inhibitory region of CaMKII. Further analyses demonstrated that PPM1F specifically dephosphorylates the phospho-Thr-286 in autophosphorylated CaMKII substrate and thus deactivates the CaMKII in vitro. Coimmunoprecipitation of CaMKII with PPM1F indicates that the two proteins can interact intracellularly. Binding of PPM1F to CaMKII involves multiple regions and is not dependent on intact phosphatase activity. Furthermore, overexpression of PPM1F in fibroblasts caused a reduction in the CaMKII-specific phosphorylation of the known substrate vimentin(Ser-82) following induction of the endogenous CaM kinase. These results identify PPM1F as a CaM kinase phosphatase within fibroblasts, although it may have additional functions intracellularly since it has been presented elsewhere as POPX2 and hFEM-2. We conclude that PPM1F, possibly together with the other previously described protein phosphatases PP1 and PP2A, can regulate the activity of CaMKII. Moreover, because PPM1F dephosphorylates the critical autophosphorylation site of CaMKII, we propose that this phosphatase plays a key role in the regulation of the kinase intracellularly.

Regulation of elongation factor-2 kinase by pH

Elongation factor-2 kinase (eEF-2K) is a Ca(2+)/calmodulin-dependent protein kinase that phosphorylates and inactivates eEF-2 and that can regulate the rate of protein synthesis at the elongation stage. Here we report that a slight decrease in pH, within the range observed in vivo, leads to a dramatic activation of eEF-2K. The activity of eEF-2K in mouse liver extracts, as well as the activity of purified recombinant human eEF-2K, is low at pH 7.2-7.4 and is increased by severalfold when the pH drops to 6.6-6.8. eEF-2K requires calmodulin for activity at neutral as well as acidic pH. Kinetic studies demonstrate that the pH does not affect the K(M) for ATP or eEF-2 and activation of eEF-2K at acidic pH is due to an increase in V(max). To analyze the potential role of eEF-2K in regulating protein synthesis by pH, we constructed a mouse fibroblast cell line that expresses eEF-2K in a tetracycline-regulated manner. Overexpression of eEF-2K led to a decreased rate of protein synthesis at acidic pH, but not at neutral pH. Our results suggest that pH-dependent activation of eEF-2K may play a role in the global inhibition of protein synthesis during tissue acidosis, which accompanies such processes as hypoxia and ischemia.

Detection of anti-elongation factor 2 kinase (calmodulin-dependent protein kinase III) antibodies in patients with systemic lupus erythematosus

Elongation factor 2 kinase (eEF-2K), also known as calmodulin-dependent protein kinase III, is a member of the calmodulin-mediated signaling pathway that links activation of cell surface receptors to cell division. The activity of eEF-2K is increased in many human cancers and may be a valid target for anti-cancer treatment. It is one of the unconventional eukaryotic protein kinases with respect to its structural domains in comparison to other members of the serine/threonine protein kinase superfamily. eEF-2K is highly conserved in nature. For example, the amino acid sequence of human eEF-2K is 90% identical to mouse and rat eEF-2Ks and 40% identical to that of the C. elegans enzyme. Therefore it has been difficult to generate high-titer and high-specificity antibodies to the human enzyme by traditional techniques. Patients with systemic lupus erythematosus (SLE) produce auto-antibodies to a variety of cellular proteins, including members of the protein translation apparatus. Hence, we developed an ELISA assay that could detect anti-eEF2K antibodies from sera of SLE patients using purified eEF-2K as an antigen. We screened 117 sera from SLE patients. High-titer anti-eEF-2K antibodies were detected in 72 subjects. One of the high-titer sera was used for further characterization. The auto-antibody recognized eEF-2K on immunoblots and immunoprecipitated the kinase with intact enzyme activity. In conclusion, anti-eEF-2K antibodies are found in sera of SLE patients and are useful tools to study the role of this highly conserved enzyme.

Involvement of protein kinase Cdelta in contact-dependent inhibition of growth in human and murine fibroblasts

There is evidence that protein kinase C delta (PKCdelta) is a tumor suppressor, although its physiological role has not been elucidated so far. Since important anti-proliferative signals are mediated by cell-cell contacts we studied whether PKCdelta is involved in contact-dependent inhibition of growth in human (FH109) and murine (NIH3T3) fibroblasts. Cell-cell contacts were imitated by the addition of glutardialdehyde-fixed cells to sparsely seeded fibroblasts. Downregulation of the PKC isoforms alpha, delta, epsilon, and mu after prolonged treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA, 0.1 microM) resulted in a significant release from contact-inhibition in FH109 cells. Bryostatin 1 selectively prevented TPA-induced PKCdelta-downregulation and reversed TPA-induced release from contact-inhibition arguing for a role of PKCdelta in contact-inhibition. In accordance, the PKCdelta specific inhibitor Rottlerin (1 microM) totally abolished contact-inhibition. Interestingly, immunofluorescence revealed a rapid translocation of PKCdelta to the nucleus when cultures reached confluence with a peak in early-mid G1 phase. Nuclear translocation of PKCdelta in response to cell-cell contacts could also be demonstrated after subcellular fractionation by Western blotting and by measuring PKCdelta-activity after immunoprecipitation. Transient transfection of NIH3T3 cells with a dominant negative mutant of PKCdelta induced a transformed phenotype. We conclude that PKCdelta is involved in contact-dependent inhibition of growth.

Inhibition of protein synthesis in cortical neurons during exposure to hydrogen peroxide

Transient cerebral ischemia, which is accompanied by a sustained release of glutamate and zinc, as well as H(2)O(2) formation during the reperfusion period, strongly depresses protein synthesis. We have previously demonstrated that the glutamate-induced increase in cytosolic Ca(2+) is likely responsible for blockade of the elongation step of protein synthesis, whereas Zn(2+) preferentially inhibits the initiation step. In this study, we provide evidence indicating that H(2)O(2) and thapsigargin mobilized a common intracellular Ca(2+) pool. H(2)O(2) treatment stimulated a slow increase in intracellular Ca(2+), and precluded the effect of thapsigargin on Ca(2+) mobilization. H(2)O(2) stimulated the phosphorylation of both eIF-2alpha and eEF-2, in a time- and dose-dependent manner, suggesting that both the blockade of the elongation and of the initiation step are responsible for the H(2)O(2)-induced inhibition of protein synthesis. However, kinetic data indicated that, at least during the first 15 min of H(2)O(2) treatment, the inhibition of protein synthesis resulted mainly from the phosphorylation of eEF-2. In conclusion, H(2)O(2) inhibits protein translation in cortical neurons by a process that involves the phosphorylation of both eIF-2alpha and eEF-2 and the relative contribution of these two events depends on the duration of H(2)O(2) treatment.

Novel compounds, '1,3-selenazine derivatives' as specific inhibitors of eukaryotic elongation factor-2 kinase

The inhibitory activities of 5,6-dihydro-4H-1,3-selenazine derivatives on protein kinases were investigated. In a multiple protein kinase assay using a postnuclear fraction of v-src-transformed NIH3T3 cells, 4-ethyl-4-hydroxy-2-p-tolyl-5, 6-dihydro-4H-1,3-selenazine (TS-2) and 4-hydroxy-6-isopropyl-4-methyl-2-p-tolyl-5,6-dihydro-4H-1, 3-selenazine (TS-4) exhibited selective inhibitory activity against eukaryotic elongation factor-2 kinase (eEF-2K) over protein kinase A (PKA), protein kinase C (PKC) and protein tyrosine kinase (PTK). In further experiments using purified kinases, TS-2 (IC(50)=0.36 microM) and TS-4 (IC(50)=0.31 microM) inhibited eEF-2K about 25-fold more effectively than calmodulin-dependent protein kinase-I (CaMK-I), and about 6-fold (TS-2) or 33-fold (TS-4) more effectively than calmodulin-dependent protein kinase-II (CaMK-II), respectively. TS-2 and TS-4 showed much weaker inhibitory activity toward PKA and PKC, while TS-4, but not TS-2, moderately inhibited immunoprecipitated v-src kinase. TS-2 (10.7-fold) and TS-4 (12.5-fold) demonstrated more potent and more specific eEF-2K inhibitory activity than rottlerin, a previously identified eEF-2K inhibitor. TS-2 inhibited ATP or eEF-2 binding to eEF-2K in a competitive or non-competitive manner, respectively. In cultured v-src-transformed NIH3T3 cells, TS-2 also decreased phospho-eEF-2 protein level (IC(50)=4.7 microM) without changing the total eEF-2 protein level. Taken together, these results suggest that TS-2 and TS-4 are the first identified selective eEF-2K inhibitors and should be useful tools for studying the function of eEF-2K.

Heterogeneity of cardiac rat and human elongation factor 2

Elongation factor 2 (EF-2) catalyses the last step of the elongation cycle, translocation, in the course of protein biosynthesis. A system for analyzing post-translational modifications of EF-2, which is a single polypeptide of 857 amino acids, is reported and its application to cytosolic extracts of cultured neonatal rat heart myocytes, neonatal and adult rat cardiac tissue, and extracts of human left ventricular myocardium is described. Comparing different pH ranges in immobilized pH gradient-isoelectric focusing (IPG-IEF), a range of pH 3 - 10 and 4 - 9 resulted in a highly defined and reproducible resolution of six different EF-2 variants of all extracts in the first dimension. These six variants were detected by the "imaging plate" (phosphor radiation image sensor) after specific labeling with Pseudomonas exotoxin A catalyzed [32P]ADP-ribosylation. This finding could be confirmed in Western blot analysis with a specific polyclonal rabbit antibody. Using two-dimensional polyacrylamide gel electrophoresis (2-D-PAGE), five to six EF-2 variants could be demonstrated in all extracts. By application of a second IPG indicator strip to the 2-D gel, they could be aligned with corresponding spots in a silver-stained 2-D separation of human myocardial tissue, revealing that the EF-2 variants belong to the group of low-abundance proteins.

Translation elongation factor 2 is encoded by a single essential gene in Candida albicans

Translation elongation factor 2 (eEF2) is a large protein of more than 800 amino acids which establishes complex interactions with the ribosome in order to catalyze the conformational changes needed for translation elongation. Unlike other yeasts, the pathogenic fungus Candida albicans was found to have a single gene encoding this factor per haploid genome, located on chromosome 2. Expression of this locus is essential for vegetative growth, as evidenced by placing it under the control of a repressible promoter. This C. albicans gene, named EFT2, was cloned and sequenced (EMBL accession number Y09664). Genomic and cDNA sequence analysis identified common transcription initiation and termination signals and an 842 amino acid open reading frame (ORF), which is interrupted by a single intron. Despite some genetic differences, CaEFT2 was capable of complementing a Saccharomyces cerevisiae Deltaeft1 Deltaeft2 null mutant, which lacks endogenous eEF2, indicating that CaEFT2 can be expressed from its own promoter and its intron can be correctly spliced in S. cerevisiae.

cAMP inhibits translation by inducing Ca2+/calmodulin-independent elongation factor 2 kinase activity in IPC-81 cells

Treatment of IPC-81 cells led to inhibition of protein synthesis, which was accompanied by an increase in the average size of polysomes and a decreased rate of elongation, indicating that it involved inhibition of peptide chain elongation. This inhibition was also associated with increased phosphorylation of elongation factor eEF2 (which inhibits its activity) and enhanced Ca2+/calmodulin-independent activity of eEF2 kinase. Previous work has shown that phosphorylation of eEF2 kinase by cAMP-dependent protein kinase (cAPK) in vitro induces such activator-independent activity, and the present data show that such a mechanism can occur in intact cells to link physiological levels of cAPK activation with inhibition of protein synthesis.

Activity and regulation by growth factors of calmodulin-dependent protein kinase III (elongation factor 2-kinase) in human breast cancer

Calmodulin-dependent protein kinase III (CaM kinase III, elongation factor-2 kinase) is a unique member of the Ca2+/CaM-dependent protein kinase family. Activation of CaM kinase III leads to the selective phosphorylation of elongation factor 2 (eEF-2) and transient inhibition of protein synthesis. Recent cloning and sequencing of CaM kinase III revealed that this enzyme represents a new superfamily of protein kinases. The activity of CaM kinase III is selectively activated in proliferating cells; inhibition of the kinase blocked cells in G0/G1-S and decreased viability. To determine the significance of CaM kinase III in breast cancer, we measured the activity of the kinase in human breast cancer cell lines as well as in fresh surgical specimens. The specific activity of CaM kinase III in human breast cancer cell lines was equal to or greater than that seen in a variety of cell lines with similar rates of proliferation. The specific activity of CaM kinase III was markedly increased in human breast tumour specimens compared with that of normal adjacent breast tissue. The activity of this enzyme was regulated by breast cancer mitogens. In serum-deprived MDA-MB-231 cells, the combination of insulin-like growth factor I (IGF-I) and epidermal growth factor (EGF) stimulated cell proliferation and activated CaM kinase III to activities observed in the presence of 10% serum. Inhibition of enzyme activity blocked cell proliferation induced by growth factors. In MCF-7 cells separated by fluorescence-activated cell sorting. CaM kinase III was increased in S-phase over that of other phases of the cell cycle. In summary, the activity of Ca2+/CaM-dependent protein kinase III is controlled by breast cancer mitogens and appears to be constitutively activated in human breast cancer. These results suggest that CaM kinase III may contribute an important link between growth factor/receptor interactions, protein synthesis and the induction of cellular proliferation in human breast cancer.

An examination of the role of increased cytosolic free Ca2+ concentrations in the inhibition of mRNA translation

Mobilization of Ca2+ sequestered by the endoplasmic reticulum (ER) produces the phosphorylation of initiation factor (eIF) 2, whereas an increase in cytosolic free Ca2+ ([Ca2+]i) due to plasmalemmal Ca2+ influx increases the phosphorylation of elongation factor (eEF) 2. In nucleated mammalian cells, depletion of ER Ca2+ stores has been demonstrated to inhibit translational initiation, but evidence that increased [Ca2+]i per se causes slowing of peptide chain elongation is lacking. L-type Ca2+ channel activity of GH3 pituitary cells, which are enriched in calmodulin-dependent eEF-2 kinase, was manipulated such that the impact of [Ca2+]i on eEF-2 phosphorylation and translational rate could be examined for up to 10 min without inhibiting initiation. At 1 mM extracellular Ca2+, resting [Ca2+]i values were high (154-255 nM) and eEF-2 was phosphorylated. The Ca2+ channel antagonist, nisoldipine, lowered [Ca2+]i and reduced eEF-2 phosphorylation by half but had no effect on amino acid incorporation. The Ca2+ channel agonist, Bay K 8644, produced sustained elevations of [Ca2+]i that were associated with 25-50% increases in eEF-2 phosphorylation, but no changes in protein synthetic rates occurred. Larger Ca2+ influxes were achievable with either 25 mM KCl or KCl plus Bay K 8644. These treatments further increased eEF-2 phosphorylation (50-100% above control) and inhibited leucine incorporation by 20-70% but ATP content was reduced by 25-50% and total cell-associated Ca2+ contents rose by 3- to 13-fold. eIF-2alpha was not phosphorylated during these treatments. Addition of low concentrations of ionomycin, which do not lower ATP content, was associated with complex changes in [Ca2+]i that resembled alterations in eEF-2 phosphorylation. The inhibition of leucine incorporation in response to ionomycin, however, coincided only with the phosphorylation of eIF-2alpha, not eEF-2. It is concluded that changes in [Ca2+]i occurring in the absence of ATP depletion alter the phosphorylation state of eEF-2 but are not regulatory for mRNA translation.

T-cadherin is a major glycophosphoinositol-anchored protein associated with noncaveolar detergent-insoluble domains of the cardiac sarcolemma

Sucrose-density flotation analysis of Triton-insoluble membrane domains isolated from highly purified sheep ventricular sarcolemma revealed the presence of two major 120- and 100-kDa proteins. Both species migrated in two-dimensional isoelectric focussing/SDS gels with an apparent pI of approximately 4.3, suggesting that they might be related. Microsequence analysis of peptides derived from the 100-kDa protein yielded amino acid sequences with high homology to T-cadherin, a truncated cadherin lacking a cytoplasmic domain. The similarity was confirmed using antibodies to chicken T-cadherin that reacted with both proteins on immunoblots. T-cadherin was released from the detergent-insoluble sarcolemmal fraction by phospholipase C treatment indicating that it is linked to the membrane by a glycophosphoinositol anchor. T-cadherin could be ADP-ribosylated by a transferase that was also present in the caveolin-enriched Triton-insoluble fraction. T-cadherin-containing membrane fragments cofractionated on sucrose gradients with caveolin-3, a marker protein for myocyte caveolae. However, immunopurified caveolin-3-containing membranes contained no associated T-cadherin. Immunocytochemical analysis of cultured rat atrial myocytes revealed that T-cadherin and caveolin have related but nonoverlapping staining patterns. These results suggest that T-cadherin is a major glycophosphoinositol-linked protein in cardiac myocytes and that it may be located in plasma membrane "rafts" distinct from but possibly adjacent to caveolae.

Regulation of translational initiation during cellular responses to stress

Chemicals and conditions that damage proteins, promote protein misfolding, or inhibit protein processing trigger the onset of protective homeostatic mechanisms resulting in "stress responses" in mammalian cells. Included in these responses are an acute inhibition of mRNA translation at the initiation step, a subsequent induction of various protein chaperones, and the recovery of mRNA translation. Separate, but closely related, stress response systems exist for the endoplasmic reticulum (ER), relating to the induction of specific "glucose-regulated proteins" (GRPs), and for the cytoplasm, pertaining to the induction of the "heat shock proteins" (HSPs). Activators of the ER stress response system, including Ca(2+)-mobilizing and thiol-reducing agents, are discussed and compared to activators of the cytoplasmic stress system, such as arsenite, heavy metal cations, and oxidants. An emerging integrative literature is reviewed that relates protein chaperones associated with cellular stress response systems to the coordinate regulation of translational initiation and protein processing. Background information is presented describing the roles of protein chaperones in the ER and cytoplasmic stress response systems and the relationships of chaperones and protein processing to the regulation of mRNA translation. The role of chaperones in regulating eIF-2 alpha kinase activities, eIF-2 cycling, and ribosomal loading on mRNA is emphasized. The putative role of GRP78 in coupling rates of translation to processing is modeled, and functional relationships between the HSP and GRP chaperone systems are discussed.

Identification of a new class of protein kinases represented by eukaryotic elongation factor-2 kinase

The several hundred members of the eukaryotic protein kinase superfamily characterized to date share a similar catalytic domain structure, consisting of 12 conserved subdomains. Here we report the existence and wide occurrence in eukaryotes of a protein kinase with a completely different structure. We cloned and sequenced the human, mouse, rat, and Caenorhabditis elegans eukaryotic elongation factor-2 kinase (eEF-2 kinase) and found that with the exception of the ATP-binding site, they do not contain any sequence motifs characteristic of the eukaryotic protein kinase superfamily. Comparison of different eEF-2 kinase sequences reveals a highly conserved region of approximately 200 amino acids which was found to be homologous to the catalytic domain of the recently described myosin heavy chain kinase A (MHCK A) from Dictyostelium. This suggests that eEF-2 kinase and MHCK A are members of a new class of protein kinases with a novel catalytic domain structure.

Phosphorylation of eukaryotic elongation factor 2 in differentiating and proliferating HL-60 cells

Ca(2+)-and calmodulin-dependent protein kinase III (CaM PKIII) phosphorylates eukaryotic elongation factor 2 (eEF-2) in HL-60 cells. Dephosphorylation of the factor in these cells is catalyzed by phosphoprotein phosphatase 2A alone. Differentiation of the HL-60 cells by all-trans retinoic acid resulted in a reduced growth rate and a marked decrease in the intracellular concentration of eEF-2. During differentiation the activity of the eEF-2 kinase is gradually reduced and reaches 10% of that found in undifferentiated cells 5 days after the onset of differentiation. The capacity to dephosphorylate phospho-eEF-2 remained unaltered in the growth-arrested cells. Differentiation without reduced proliferation was induced in the HL-60 cells by interferon-gamma. Under these conditions, differentiation had no effect on the cellular content of eEF-2 or the ability to dephosphorylate phospho-eEF-2. However, the differentiated cells showed a dramatic decrease in the specific activity of the eEF-2 kinase. The results show that the cellular content of eEF-2 varies with the rate of proliferation and that the activity of the eEF-2 kinase is high in undifferentiated proliferating cells and decreases upon differentiation even under conditions of an unaltered growth rate.

Effects of KS-501, KS-502 and their enantiomers on calmodulin-sensitive enzyme activity and cellular proliferation

Calmodulin plays an important role in cellular proliferation as part of a signal transduction pathway activated by phospholipase C. Drugs that block the ability of calmodulin to bind to and activate its target enzymes inhibit the growth of a wide variety of malignant cells. To identify more potent and selective inhibitors of this potential target for new drug development, we studied two recently synthesized compounds, KS-501 and KS-502, for their activity against calmodulin-sensitive enzymes and for their ability to block the growth of parental and multidrug-resistant leukemic cells. KS-501 and KS-502 inhibited the activation of a calmodulin-sensitive cyclic nucleotide phosphodiesterase. The mechanism of enzyme inhibition was through interfering with calmodulin activation rather than through a direct effect on the enzyme. KS-501 was more potent than KS-502 and was studied in greater detail. This compound inhibited the activation of calmodulin kinase I and II, but had less effect against cyclic adenosine 3',5'-monophosphate (cyclic AMP)-sensitive kinase. KS-501 was also more effective than KS-502 in inhibiting the growth of sensitive L1210 leukemic lymphocytes. Both compounds were less effective inhibitors of multidrug-resistant L1210 leukemia than of the parental line. These studies identify a new class of calmodulin inhibitor, with selectivity for calmodulin-dependent kinases over cyclic AMP-dependent protein kinase. Since the total synthesis of the KS-compounds has been accomplished, it should now be possible to develop derivatives with greater activity and selectivity.

Capsaicin-induced neurotoxicity in cultured dorsal root ganglion neurons: involvement of calcium-activated proteases

We examined the mechanism by which capsaicin produces its toxic effects on cultures of rat sensory neurons. Capsaicin caused a robust increase in [Ca2+]i in a subpopulation of cultured rat dorsal root ganglion neurons. Similarly, a brief exposure to capsaicin resulted in delayed degeneration of a subpopulation of the cells. This subpopulation (about 35% of the cells present) was characterized by a capsaicin-induced uptake of Co2+, which could be detected cytochemically. Both capsaicin-induced Co2+ uptake and capsaicin-induced cell death were blocked by the capsaicin antagonist Ruthenium Red. Cell death was also prevented by removal of external calcium or by inhibiting calcium-activated proteases such as calpain. Evidence that calpain activity was increased was provided by examining the amount of degradation of the preferred calpain substrate alpha-spectrin. Capsaicin treatment produced a significant increase in the levels of the 150,000 molecular weight spectrin breakdown product. Furthermore, applying the protease inhibitors E64 or MDL 28,170 reduced capsaicin-mediated cell death. It is concluded that capsaicin kills a subpopulation of sensory neurons by activating a receptor-operated channel. The consequent Ca2+ ion influx causes large increases in [Ca2+]i and subsequent activation of Ca(2+)-sensitive proteases. This model provides support for the role of [Ca2+]i as the orchestrator of delayed neuronal degeneration.

Effect of oxidizing agents and haemin on the phosphorylation of eukaryotic elongation factor 2 in rabbit reticulocyte lysates

Incubation of rabbit reticulocyte lysates in the absence of added haemin resulted in the phosphorylation of a 95 kDa protein. This protein was suggested to be elongation factor 2 (eEF-2) based on the following observations, (i) phosphorylation of the 95 kDa protein was Ca2+ and CaM-dependent. (ii) eEF-2 supplemented to the lysates became phosphorylated and co-migrated with the endogenous 95 kDa phosphoprotein upon electrophoresis in SDS gels. (iii) The tryptophane specific cleavage pattern obtained from the isolated 95 kDa phosphoprotein was identical to that of phosphorylated eEF-2. Phosphorylation of the 95 kDa protein was stimulated by oxidizing agents such as oxidized glutathione and NAD+ and inhibited by addition of haemin. The haemin concentration needed for 50% inhibition (IC50) was 2.5 microM. Haemin also had an inhibitory effect on eEF-2 phosphorylation in a system containing highly purified components (IC50 = 2 microM). In this system haemin inhibited phosphorylation of eEF-2 even in the presence of a 100-fold excess of beta-mercaptoethanol. Oxidizing agents had no effect on the kinase activity in the purified system.

Cellular ADP-ribosylation of elongation factor 2

A cellular ADP-ribosyltransferase activity has been found in a variety of animals and tissues. The enzyme transfers ADP-ribose from NAD to elongation factor 2, inactivating the factor and thus inhibiting in vitro protein synthesis. Although, the mechanism of action of the cellular enzyme appears similar to diphtheria toxin and Pseudomonas exotoxin A, it differs from the toxins in that only a fraction of the EF-2 pool is modified. The endogenously ADP-ribosylated EF-2 has been detected by a variety of methods including two-dimensional electrophoresis and immunoprecipitation with elongation factor 2 antibody. The nature of the cellular ADP-ribosyltransferase and its physiological significance are unknown.

Elongation factor-2 kinase: effective inhibition by the novel protein kinase inhibitor rottlerin and relative insensitivity towards staurosporine

The elongation factor-2 (eEF-2) is selectively phosphorylated by the eEF-2 kinase (calmodulin-dependent kinase III). This phosphorylation can be inhibited by calmodulin antagonists, such as CGS 9343B (IC50 = 4 microM). The novel protein kinase inhibitor rottlerin is shown to suppress eEF-2 phosphorylation with an IC50 of 5.3 microM. By contrast, the eEF-2 kinase is rather resistant towards the potent but non-selective protein kinase inhibitor staurosporine (IC50 > 50 microM) and thus can be differentiated from most other protein kinases that are suppressed by staurosporine in the nM range.

Regulation of elongation factor-2 by multisite phosphorylation

We have studied the phosphorylation of protein synthesis elongation factor eEF-2, the effects of phosphorylation on its activity and the dephosphorylation of phosphorylated eEF-2 by protein phosphatases-2A and -2C. Extensive analysis of phosphopeptides generated from eEF-2 phosphorylated in vitro by subsequent digestion with CNBr and trypsin indicated that Thr56 and Thr58 are the only residues significantly phosphorylated, consistent with our earlier report. They are also the only two residues to be significantly phosphorylated in reticulocyte lysates: in this system monophosphorylated eEF-2 corresponded only to phosphorylation of Thr56, no factor phosphorylated at only Thr58 being detected. Phosphorylation of Thr56 and Thr58 was found to be an ordered process, modification of Thr56 preceding, and apparently being required for, phosphorylation of Thr58. This presumably explains why the only species of mono-phosphorylated eEF-2 detected are phosphorylated at Thr56. The eEF-2 kinase could phosphorylate a synthetic peptide based on residues 49-60 of eEF-2 (RAGETRFTDTRK), albeit only at a very low rate, and with a very high Km, compared to eEF-2 itself. The kinase phosphorylated the residues corresponding to Thr56 and Thr58, apparently in a random manner, but not Thr53. In the light of the existence of two phosphorylation sites in eEF-2, the relationship between phosphorylation and activity was investigated. Activity was measured in the poly(U)-directed synthesis of polyphenylalanine, where both the bis- and mono-phosphorylated (mono at Thr56) forms of the factor were found to be completely inactive. Indeed, the phosphorylated species appeared to be able to impair the activity of non-phosphorylated eEF-2 in this system. Experiments using reticulocyte lysates also indicated that both phosphorylated forms of eEF-2 were inactive in the translation of physiological templates, but no evidence for dominant inhibition by these species was obtained. Protein phosphatases-2A and -2C (PP-2A and PP-2C) can each efficiently dephosphorylate phosphorylated eEF-2. While bis-phosphorylated eEF-2 was a better substrate for PP-2A than monophosphorylated factor (phosphorylated at Thr56), the converse was true for PP-2C. This seemed to be due, at least in part, to the inhibition of dephosphorylation of Thr56 by PP-2C by the presence of phosphate on Thr58. Nevertheless, PP-2C exhibited a preference for dephosphorylation of Thr56 in bis-phosphorylated eEF-2, while PP-2A showed no such preference. These findings are discussed in terms of current knowledge of the specificity of these two protein phosphatases.

Purification and phosphorylation of elongation factor-2 kinase from rabbit reticulocytes

Eukaryotic elongation-factor-2 kinase has been purified to homogeneity from rabbit reticulocytes through a seven-step procedure and has been identified as a protein with a molecular mass of approximately 103 kDa as judged by SDS/PAGE. A degradation product of about 95 kDa was also evident in some preparations. The activity of the purified kinase was completely dependent on calcium and calmodulin. The kinase rapidly underwent extensive autophosphorylation, incorporating 1 mol phosphate/mol within 1 min; 5 mol phosphate/mol were incorporated within 1 h. The autophosphorylation was Ca2+/calmodulin-dependent; phosphopeptide mapping revealed multiple phosphopeptides even after just 0.5 min of autophosphorylation, suggesting that a number of sites became rapidly phosphorylated. Autophosphorylation occurred on serine and threonine residues. Preincubation in the presence of Ca2+, Mg2+ and ATP produced a rapid 2-3-fold activation of the kinase and also induced partial Ca(2+)-independent activity. Preincubation in the absence of the ligands showed that all three were required for full activation and induction of Ca(2+)-independent activity.

Regulation of protein synthesis by modulation of intracellular calcium in rat liver

The rate of protein synthesis can be modulated in intact cells by varying the concentration and subcellular distribution of intracellular calcium. Because the biochemical reactions required for the pathway of protein synthesis occur in the cytosol of the cell, it might be expected that protein synthesis would be controlled by free cytosolic calcium rather than the sequestered cation. However, a recent report proposed that maintenance of optimal rates of protein synthesis depends on the amount of calcium sequestered in the endoplasmic reticulum rather than free cytosolic calcium (C.O. Brostrom and M. A. Brostrom, Annu. Rev. Physiol. 52: 577-590, 1990). In the present study, rat livers were perfused with buffer containing various compounds previously shown to alter intracellular calcium concentration and distribution in isolated cells. It was found that conditions designed to cause a rise in free cytosolic calcium had no effect on protein synthesis. In contrast, conditions designed to cause depletion of sequestered calcium resulted in an inhibition of protein synthesis characterized by a reduction in peptide-chain initiation relative to elongation. The inhibition of protein synthesis was further localized to a decrease in the activity of eukaryotic initiation factor (eIF) 2B as measured in extracts from perfused livers. The inhibition of eIF-2B activity was associated with a 2.4-fold increase in the proportion of the alpha-subunit of eIF-2 in the phosphorylated form. In summary, the results of the present study support a model whereby mobilization of calcium sequestered in the endoplasmic reticulum results in an inhibition of protein synthesis in rat liver.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential inhibition by staurosporine of phorbol ester, bryostatin and okadaic acid effects on mouse skin

The tumor promoters 12-O-tetradecanoyl-phorbol-13-acetate (TPA), a strong activator of protein kinase C (PKC) and okadaic acid, which is ineffective in this respect, induce a rapidly developing ('early') edema of the mouse ear. Bryostatin, another potent activator of PKC, is unable to induce an 'early' edema but causes a more delayed development of edema at a time when most of the PKC is down-regulated. The PKC inhibitor staurosporine neither inhibits the early TPA- nor the late bryostatin-induced edema, but suppresses the okadaic acid-induced edema very effectively. TPA as well as bryostatin, but not okadaic acid cause a down-regulation of PKC, which is not inhibited by staurosporine. The calmodulin antagonist cyclosporine A, which does not suppress PKC activity, very effectively inhibits the TPA-induced edema and down regulation of PKC. Hence we conclude that protein phosphorylation catalyzed by staurosporine-suppressable PKC is not involved in the induction of edema and PKC down-regulation by TPA but that a calmodulin dependent process may play a critical role in these and other TPA effects in mouse skin.