Type 1 phosphatase inhibitors reduce the restoration of guanine nucleotide exchange activity of eukaryotic initiation factor 2B inhibited reticulocyte lysates rescued by hemin

CCCP-induced mitochondrial dysfunction - characterization and analysis of integrated stress response to cellular signaling and homeostasis

Mitochondrial dysfunction mediated by CCCP (carbonyl cyanide m-chlorophenyl hydrazone), an inhibitor of mitochondrial oxidative phosphorylation, evokes the integrated stress response (ISR), which is analyzed here by eIF2α phosphorylation and expression profiles of ATF4 and CHOP proteins. Our findings suggest that the CCCP-induced ISR pathway is mediated by activation of HRI kinase, but not by GCN2, PERK, or PKR. Also, CCCP activates AMPK, a cellular energy sensor, and AKT, a regulator implicated in cell survival, and suppresses phosphorylation of mTORC1 substrates eIF4E-BP1 and S6K. CCCP also downregulates translation and promotes autophagy, leading to noncaspase-mediated cell death in HepG2 cells. All these events are neutralized by NAC, an anti-ROS, suggesting that CCCP-induced mitochondrial dysfunction promotes oxidative stress. ISRIB, an inhibitor of the ISR pathway, mitigates CCCP-induced expression of ATF4 and CHOP, activation of AKT, and autophagy, similar to NAC. However, it fails to reverse CCCP-induced AMPK activation, suggesting that CCCP-induced autophagy is dependent on ISR and independent of AMPK activation. ISRIB restores partly, inhibition in eIF4E-BP1 phosphorylation, promotes eIF2α phosphorylation, albeit slowly, and mitigates suppression of translation accordingly, in CCCP-treated cells. These findings are consistent with the idea that CCCP-induced oxidative stress leading to eIF2α phosphorylation and ATF4 expression, which is known to stimulate genes involved in autophagy, play a pro-survival role together with AKT activation and regulate mTOR-mediated eIF4E-BP1 phosphorylation.

Insulin treatment promotes tyrosine phosphorylation of PKR and inhibits polyIC induced PKR threonine phosphorylation

Tyrosine phosphorylation of insulin receptor beta (IRβ) in insulin treated HepG2 cells is inversely correlated to ser(51) phosphorylation in the alpha-subunit of eukaryotic initiation factor 2 (eIF2α) that regulates protein synthesis. Insulin stimulates interaction between IRβ and PKR, double stranded RNA-dependent protein kinase, also known as EIF2AK2, and phosphorylation of tyrosine residues in PKR, as analyzed by immunoprecipitation and pull down assays using anti-IRβ and anti-phosphotyrosine antibodies, recombinant IRβ and immunopurified PKR. Further polyIC or synthetic double stranded RNA-induced threonine phosphorylation or activation of immunopurified and cellular PKR is suppressed in the presence of insulin treated purified IRβ and cell extracts. Acute, but not chronic, insulin treatment enhances tyrosine phosphorylation of IRβ, its interaction with PKR and tyrosine phosphorylation of PKR. In contrast, lipopolysaccharide that stimulates threonine phosphorylation of PKR and eIF2α phosphorylation and AG 1024, an inhibitor of the tyrosine kinase activity of IRβ, reduces PKR association with the receptor, IRβ in HepG2 cells. These findings therefore may suggest that tyrosine phosphorylated PKR plays a role in the regulation of insulin induced protein synthesis and in maintaining insulin sensitivity, whereas, suppression of polyIC-mediated threonine phosphorylation of PKR by insulin compromises its ability to fight against virus infection in host cells.

Phosphorylation of eIF2alpha in response to 26S proteasome inhibition is mediated by the haem-regulated inhibitor (HRI) kinase

The present study demonstrates that even brief inhibition of degradation by the 26S proteasome inhibits global protein synthesis, mediated through increased phosphorylation of eIF2alpha (eukaryotic translational initiation factor 2alpha) by the HRI (haem-regulated inhibitor) kinase. Exposure of COS-7 cells to the proteasome inhibitor MG-132 (the proteasome inhibitor carbobenzoxy-L-leucyl-L-leucyl-leucinal) for 4 h resulted in a 55-60% decrease in protein synthesis rate compared with control cells. This repression of protein synthesis after treatment with MG-132 is not due to induction of apoptosis, which is known to occur after longer periods of 26S inhibition. Instead, we observed a significantly increased phosphorylation of eIF2alpha, which is known to repress global protein synthesis. In three MEF (mouse embryonic fibroblast) knockout cell lines lacking one of the four kinases known to phosphorylate eIF2alpha, increased phosphorylation of eIF2alpha still occurred after inhibition of the 26S proteasome. These three cell lines included a deletion of the PKR (double-stranded-RNA-dependent protein kinase); a deletion of the PERK (PKR-like endoplasmic reticulum resident kinase); or a deletion of the GCN2 (positive general control of transcription-2) kinase, indicating that none of these kinases was primarily responsible for the observed phosphorylation of eIF2alpha. In contrast, in a fourth MEF knockout cell line, HRI(-/-) cells lacking the HRI kinase failed to increase eIF2alpha phosphorylation upon proteasome inhibitor treatment (MG-132 or various doses of Bortezomib), indicating that the HRI kinase is the primary kinase activated by brief treatment of MEFs with 26S proteasome inhibitors.

Reversible inhibition of the protein phosphatase 1 by hydrogen peroxide. Potential regulation of eIF2 alpha phosphorylation in differentiated PC12 cells

Oxidative inactivation of protein tyrosine phosphatases and calcineurin is a well established mechanism; however, little information with regard to the effect of oxidants on PP1 and PP2A activity is available. Herein, we show that PP1 activity is inhibited by H(2)O(2) treatment in differentiated PC12 cells both in vitro and in vivo experiments. Thiol-antioxidant N-acetyl-cysteine (NAC) and reduced glutathione (GSH), when added in vitro to lysates from H(2)O(2)-treated cells, reversed PP1 inhibition. H(2)O(2) treatment increased eIF2 alpha phosphorylated levels (eIF2 alpha P) in a time- and dose-dependent fashion and promoted protein synthesis inhibition. Interestingly, NAC pretreatment protected cells from H(2)O(2)-induced PP1 inactivation and, consequently, it abolished increased H(2)O(2)-induced eIF2 alpha phosphorylation and protein synthesis inhibition. In addition, PP1 inhibitor tautomycin prevented both NAC-induced PP1 reactivation and eIF2 alpha P dephosphorylation in H(2)O(2)-treated cells. Taken together, our findings support a role for PP1 in eIF2 alpha phosphorylation and oxidative stress-triggered translation down regulation.

Growth arrest and DNA damage-inducible protein GADD34 targets protein phosphatase 1 alpha to the endoplasmic reticulum and promotes dephosphorylation of the alpha subunit of eukaryotic translation initiation factor 2

The growth arrest and DNA damage-inducible protein, GADD34, associates with protein phosphatase 1 (PP1) and promotes in vitro dephosphorylation of the alpha subunit of eukaryotic translation initiation factor 2, (eIF-2 alpha). In this report, we show that the expression of human GADD34 in cultured cells reversed eIF-2 alpha phosphorylation induced by thapsigargin and tunicamycin, agents that promote protein unfolding in the endoplasmic reticulum (ER). GADD34 expression also reversed eIF-2 alpha phosphorylation induced by okadaic acid but not that induced by another phosphatase inhibitor, calyculin A (CA), which is a result consistent with PP1 being a component of the GADD34-assembled eIF-2 alpha phosphatase. Structure-function studies identified a bipartite C-terminal domain in GADD34 that encompassed a canonical PP1-binding motif, KVRF, and a novel RARA sequence, both of which were required for PP1 binding. N-terminal deletions of GADD34 established that while PP1 binding was necessary, it was not sufficient to promote eIF-2 alpha dephosphorylation in cells. Imaging of green fluorescent protein (GFP)-GADD34 proteins showed that the N-terminal 180 residues directed the localization of GADD34 at the ER and that GADD34 targeted the alpha isoform of PP1 to the ER. These data provide new insights into the mode of action of GADD34 in assembling an ER-associated eIF-2 alpha phosphatase that regulates protein translation in mammalian cells.

Translation and phosphorylation of wheat germ lysate: phosphorylation of wheat germ initiation factor 2 by casein kinase II and in N-ethylmaleimide-treated lysates

Previously, we observed that N-ethylmaleimide (NEM), a thiol-alkylating agent, was found to stimulate the phosphorylation of several proteins in translating wheat germ (WG) lysates, including the phosphorylation of alpha, the p41-42 doublet subunit, and beta, the p36 subunit, of the WG initiation factor 2 (eIF2). We find now that NEM increases phosphorylation of several proteins significantly in lysates which are moderate or low in their translation compared to optimally active lysates. Heat treatment, which stimulates oxidation of protein sulfhydryls, decreases the translation and phosphorylation ability of WG lysates. The decrease in phosphorylation, but not translation, that occurs in heat-treated lysates is prevented very efficiently by NEM and partially by reducing agents such as dithiothreitol (DTT) and GSH. DTT prevents, however, completely the loss of sulfhydryl content of heat-treated WG lysates and does not at all prevent heat-induced inhibition of translation. In contrast, DTT prevents completely the diamide-induced translational inhibition and also the loss of sulfhydryl content. These findings therefore suggest that in addition to the maintenance of sulfhydryl groups, heat-labile proteins and their interactions with other proteins play an important role in overall translation and phosphorylation. It is also observed here that heat treatment stimulates the phosphorylation of rabbit reticulocyte eIF2 alpha but not the alpha subunit (p41-42 doublet) of WG eIF2. A phosphospecific anti-eIF2 alpha antibody recognizes the WG eIF2 alpha(P) that is phosphorylated by an authentic eIF2 alpha kinase such as double-stranded RNA-dependent protein kinase, but it is unable to recognize the eIF2 alpha that is phosphorylated in NEM-treated lysates. These findings therefore suggest that phosphorylation of WG eIF2 alpha in NEM-treated lysates occurs on a site different from the serine 51 residue that is phosphorylated by authentic eIF2 alpha kinases. In addition, it also suggests that WG eIF2 alpha, unlike reticulocyte eIF2 alpha, is phosphorylated by eIF2 alpha kinases and also by other kinases. Consistent with this idea, it has been observed here that casein kinase II (CKII) phosphorylates WG eIF2 alpha and the phosphorylation is enhanced by NEM in vitro and in lysates. The phosphopeptide analysis suggests that WG eIF2 alpha has separate phosphorylation sites for CKII and heme-regulated eIF2 alpha kinase (a well-characterized mammalian eIF2 alpha kinase), and NEM-induced phosphorylation in WG lysates resembles CKII-mediated phosphorylation.

Ischemia-induced inhibition of the initiation factor 2alpha phosphatase activity in the rat brain

Rats were subjected to the standard four-vessel occlusion model of brain transient ischemia for 30 min. Following different recirculation periods, the level of phosphorylation of the initiation factor 2 subunit alpha (eIF2alpha) and the eIF2alpha kinase/s and phosphatase/s activity were determined. eIF2alpha phosphorylation significantly increased very early during reperfusion (10-30 min), recovering at 4 h of reperfusion. Activation of any eIF2alpha kinases studied during ischemia or reperfusion was not noted. Conversely, eIF2alpha phosphatase activity significantly decreased at 10-15 min of reperfusion, reaching values even higher than in controls at 2-4 h of reperfusion. Our results support the hypothesis that the reperfusion-induced phosphorylated eIF2alpha changes are at least a result of the transiently eIF2alpha phosphatase inhibition.

Ischemia-induced phosphorylation of initiation factor 2 in differentiated PC12 cells: role for initiation factor 2 phosphatase

An in vitro model of ischemia was obtained by subjecting PC12 cells differentiated with nerve growth factor to a combination of glucose deprivation plus anoxia. Immediately after the ischemic period, the protein synthesis rate was significantly inhibited (80%) and western blots of cell extracts revealed a significant accumulation of phosphorylated eukaryotic initiation factor 2, alpha subunit, eIF2(alphaP) (42%). Upon recovery, eIF2(alphaP) levels returned to control values after 30 min, whereas protein synthesis was still partially inhibited (33%) and reached almost control values within 2 h. The activities of the mammalian eIF2alpha kinases, double-stranded RNA-activated protein kinase, mammalian GCN2 homologue, and endoplasmic reticulum-resident kinase, were determined. None of the eIF2alpha kinases studied showed increased activity in ischemic cells as compared with controls. Exposure of cells to cell-permeable inhibitors of protein phosphatases 1 and 2A, calyculin A or tautomycin, induced dose- and time-dependent accumulation of eIF2(alphaP), mimicking an ischemic effect. Protein phosphatase activity, as measured with [(32)P]phosphorylase a as a substrate, diminished during ischemia and returned to control levels upon 30-min recovery. In addition, the rate of eIF2(alphaP) dephosphorylation was significantly lower in ischemic cells, paralleling both the greatest translational inhibition and the highest eIF2(alphaP) levels. The endogenous phosphatase activity from control and ischemic extracts showed different sensitivity to inhibitor 2 and fostriecin in in vitro assays, inhibitor-2 effect in ischemic cells being lower than in control cells. Together these results indicate that an eIF2alpha phosphatase, probably protein phosphatase 1, is implicated in the ischemia-induced eIF2(alphaP) accumulation in PC12 cells.

Cell death, calcium mobilization, and immunostaining for phosphorylated eukaryotic initiation factor 2-alpha (eIF2alpha) in neuronally differentiated NB-104 cells: arachidonate and radical-mediated injury mechanisms

These experiments examine the effects of arachidonate with respect to cell death, radical-mediated injury, Ca2+ mobilization, and formation of ser-51-phosphorylated eukaryotic initiation factor 2alpha [eIF2alpha(P)]. It is known that during brain ischemia the concentration of free arachidonate can reach 180 microM, and during reperfusion oxidative metabolism of arachidonate leads to generation of superoxide that can reduce stored ferric iron and promote lipid peroxidation. During early brain reperfusion, we have shown an approximately 20-fold increase in eIF2alpha(P) which maps to vulnerable neurons that display inhibition of protein synthesis. Here in neuronally differentiated NB-104 cells, equivalent cell death (assessed by LDH release) was induced by 40 microM arachidonate and 20 microM cumene hydroperoxide (CumOOH, a known alkoxyl radical generator). In these injury models (1) radical inhibitors (BHA, BHT, and the lipophilic iron chelator EMHP) block CumOOH-induced cell death but do not block arachidonate-induced death; (2) 40 microM arachidonate (but not up to 40 microM CumOOH) rapidly induces Ca2+ release from intracellular stores; (3) both 40 microM arachidonate and 20 microM CumOOH induce intense immunostaining for eIF2alpha(P); and (4) the elF2alpha(P) immunostaining induced by CumOOH but not that induced by arachidonate is completely blocked by anti-radical intervention with EMHP. Arachidonate-induced formation of eIF2alpha(P) and cell death do not require iron-mediated radical mechanisms and are associated with Ca2+ release from intracellular stores; however, radical-mediated injury also induces both eIF2alpha(P) and cell death without release of intracellular Ca2+. Our data link eIF2alpha(P) formation during brain reperfusion to two established injury mechanisms that may operate concurrently.

Reducing agents mitigate protein synthesis inhibition mediated by vanadate and vanadyl compounds in reticulocyte lysates

Recently, we synthesized and characterized vanadyl saccharides to evaluate the effects of various vanadate and vanadyl complexes, which differ in their oxidation states on various biomacromolecules and cellular activities (1, 2). Here, we report that both vanadate (+V oxidation state) and different vanadyl species (+IV oxidation state) such as vanadyl D-glucose, vanadyl diascorbate, and vanadyl sulfate, impair the formation of polysomes and inhibit the initiation of protein synthesis in hemin-supplemented rabbit reticulocyte lysates. Vanadate inhibits protein synthesis more severely than vanadyl species and is consistent with the idea that vanadate is reduced to vanadyl state intracellularly. The inhibition of protein synthesis caused by low concentrations (10-20 microM) of vanadate and vanadyl species is effectively mitigated by reducing agents such as dithiothreitol, reduced glutathione (GSH), or reduced pyridine dinucleotide. A significant decrease in the protein synthesis inhibition in vanadate-treated lysates by GSH suggests that the mechanism of protein synthesis inhibition by vanadate is different than the action of other oxidants such as heavy metal ions and oxidized glutathione. This suggestion is also consistent with the findings that vanadium compounds do not stimulate phosphorylation of the alpha (alpha) subunit of initiation factor 2 (eIF2) or decrease the guanine nucleotide exchange activity of eIF2B, which is required to exchange GDP for GTP in eIF2.GDP binary complex. The reduction of vanadate to vanadyl state and the subsequent complex formation of vanadyl species with the endogenous reducing compounds or with the -SH groups of certain proteins may be the cause for protein synthesis inhibition in lysates.

Effect of brain ischemia and reperfusion on the localization of phosphorylated eukaryotic initiation factor 2 alpha

Postischemic brain reperfusion is associated with a substantial and long-lasting reduction of protein synthesis in selectively vulnerable neurons. Because the overall translation initiation rate is typically regulated by altering the phosphorylation of serine 51 on the alpha-subunit of eukaryotic initiation factor 2 (eIF-2 alpha), we used an antibody specific to phosphorylated eIF-2 alpha [eIF-2(alpha P)] to study the regional and cellular distribution of eIF-2(alpha P) in normal, ischemic, and reperfused rat brains. Western blots of brain postmitochondrial supernatants revealed that approximately 1% of all eIF-2 alpha is phosphorylated in controls, eIF-2(alpha P) is not reduced by up to 30 minutes of ischemia, and eIF-2(alpha P) is increased approximately 20-fold after 10 and 90 minutes of reperfusion. Immunohistochemistry shows localization of eIF-2(alpha P) to astrocytes in normal brains, a massive increase in eIF-2(alpha P) in the cytoplasm of neurons within the first 10 minutes of reperfusion, accumulation of eIF-2(alpha P) in the nuclei of selectively vulnerable neurons after 1 hour of reperfusion, and morphology suggesting pyknosis or apoptosis in neuronal nuclei that continue to display eIF-2(alpha P) after 4 hours of reperfusion. These observations, together with the fact that eIF-2(alpha P) inhibits translation initiation, make a compelling case that eIF-2(alpha P) is responsible for reperfusion-induced inhibition of protein synthesis in vulnerable neurons.

Wheat germ initiation factor 2 (WG x eIF2) decreases the inhibition in protein synthesis and eIF2B activity of reticulocyte lysates mediated by eIF2alpha phosphorylation

Phosphorylation of serine 51 residue in the alpha-subunit of eukaryotic initiation factor 2 (eIF2alpha) impairs the guanine nucleotide exchange (GNE) activity of eIF2B protein and thereby inhibits protein synthesis in mammalian systems, insects and yeast. It is not known if phosphorylation of plant eIF2 can inhibit an eIF2B-like activity. Interestingly purified wheat germ eIF2 (WG x eIF2) can exchange guanine nucleotides in vitro without the addition of any protein factor like eIF2B. It is not clear if this is due to a contaminant eIF2B-like activity associated with WG x eIF2 or because the affinity of WG x eIF2 for GDP and GTP is not markedly different. Our observations here indicate that the GNE activity of WG x eIF2 is not inhibited upon phosphorylation of the p41-42 doublet subunit in WG x eIF2 by reticulocyte eIF2alpha kinases, or in the presence of reticulocyte eIF2(alphaP) in which serine 51 residue is phosphorylated. Further, addition of WG x eIF2 reduces the inhibition in eIF2B activity, protein synthesis, and also the formation of 15S complex that occurs between reticulocyte eIF2(alphaP) and eIF2B protein in heme-deficient or poly(IC)-treated reticulocyte lysates, presumably by a mechanism of competition between wheat germ and reticulocyte eIF2 for phosphorylation. Unlike reticulocyte eIF2(alphaP), phosphorylated WG x eIF2 is unable to interact with reticulocyte eIF2B to form a 15S complex. The ability of WG x eIF2 to exchange guanine nucleotides independent of an eIF2B like protein and the inability of phosphorylated WG x eIF2 to interact with reticulocyte eIF2B suggests that WG x eIF2 is different from mammalian eIF2 and these differences may have occurred in evolution probably due to some changes in the amino acid sequences around the phosphorylation site in eIF2alpha.