Inhibition of rabbit reticulocyte lysate protein synthesis by heavy metal ions involves the phosphorylation of the alpha-subunit of the eukaryotic initiation factor 2

Inhibition of Protein Synthesis after Exposure of Caco2 Cells to Heavy Metals

The toxicity of two essential trace elements, the heavy metals zinc and copper, has been studied in Caco2 cells, a human colon carcinoma cell line able to differentiate in culture into mature enterocytes.

The ID50 for zinc and copper in Caco2 cells was determined from dose-effect curves. The ID50 is the same (500μM) for zinc and copper in undifferentiated cells, but in differentiated cells the ID50 for zinc (1mM) is higher than the ID50 for copper (600μM). Total protein synthesis is more severely affected by both metals in undifferentiated Caco2 cells than in differentiated cells. This difference could be explained by the different intracellular accumulation of the two metals at the two stages of differentiation. Copper inhibits total protein synthesis more than zinc, at both stages of differentiation. A protective role of zinc-induced metallothionein is discussed in relation to these results.

KSHV activates unfolded protein response sensors but suppresses downstream transcriptional responses to support lytic replication

Herpesviruses usurp host cell protein synthesis machinery to convert viral mRNAs into proteins, and the endoplasmic reticulum (ER) to ensure proper folding, post-translational modification and trafficking of secreted and transmembrane viral proteins. Overloading ER folding capacity activates the unfolded protein response (UPR), whereby sensor proteins ATF6, PERK and IRE1 initiate a stress-mitigating transcription program that accelerates catabolism of misfolded proteins while increasing ER folding capacity. Kaposi’s sarcoma-associated herpesvirus (KSHV) can be reactivated from latency by chemical induction of ER stress, which causes accumulation of the XBP1s transcription factor that transactivates the viral RTA lytic switch gene. The presence of XBP1s-responsive elements in the RTA promoter suggests that KSHV evolved a mechanism to respond to ER stress. Here, we report that ATF6, PERK and IRE1 were activated upon reactivation from latency and required for efficient KSHV lytic replication; genetic or pharmacologic inhibition of each UPR sensor diminished virion production. Despite UPR sensor activation during KSHV lytic replication, downstream UPR transcriptional responses were restricted; 1) ATF6 was cleaved to activate the ATF6(N) transcription factor but ATF6(N)-responsive genes were not transcribed; 2) PERK phosphorylated eIF2α but ATF4 did not accumulate; 3) IRE1 caused XBP1 mRNA splicing, but XBP1s protein did not accumulate and XBP1s-responsive genes were not transcribed. Ectopic expression of the KSHV host shutoff protein SOX did not affect UPR gene expression, suggesting that alternative viral mechanisms likely mediate UPR suppression during lytic replication. Complementation of XBP1s deficiency during KSHV lytic replication inhibited virion production in a dose-dependent manner in iSLK.219 cells but not in TREx-BCBL1-RTA cells. However, genetically distinct KSHV virions harvested from these two cell lines were equally susceptible to XBP1s restriction following infection of naïve iSLK cells. This suggests that cell-intrinsic properties of BCBL1 cells may circumvent the antiviral effect of ectopic XBP1s expression. Taken together, these findings indicate that while XBP1s plays an important role in reactivation from latency, it can inhibit virus replication at a later step, which the virus overcomes by preventing its synthesis. These findings suggest that KSHV hijacks UPR sensors to promote efficient viral replication while sustaining ER stress.

Like all viruses, Kaposi’s sarcoma-associated herpesvirus (KSHV) uses cellular machinery to create viral proteins. Some of these proteins are folded and modified in the endoplasmic reticulum (ER) and traverse the cellular secretory apparatus. Exceeding ER protein folding capacity activates the unfolded protein response (UPR), which resolves ER stress by putting the brakes on protein synthesis and turning on stress-mitigating genes. We show that KSHV replication activates the three cellular proteins that sense ER stress, which are each required to support efficient viral replication. By contrast, KSHV blocks the UPR gene expression program downstream from each of these activated sensor proteins. The failure to resolve ER stress might normally be expected to put the virus at a disadvantage, but we demonstrate that reversal of this scenario is worse; when we supplement infected epithelial cells with the UPR transcription factor XBP1s to artificially stimulate the production of UPR-responsive gene products, virus replication is blocked at a late stage and very few viruses are released from infected cells. Taken together, these observations suggest that KSHV requires UPR sensor protein activation to replicate but has dramatically altered the outcome to prevent the synthesis of new UPR proteins and sustain stress in the ER compartment.

Eukaryotic initiation factor 2α kinase is a nitric oxide-responsive mercury sensor enzyme: Potent inhibition of catalysis by the mercury cation and reversal by nitric oxide

The activity of one of the eukaryotic initiation factor 2alpha kinases, heme-regulated inhibitor (HRI), is modulated by heme binding. Here, we demonstrate for the first time that Hg2+ strongly inhibits the function of HRI (IC50=0.6 microM), and nitric oxide fully reverses this inhibition. Other divalent metal cations, such as Fe2+, Cu2+, Cd2+, Zn2+ and Pb2+, also significantly inhibit kinase activity with IC50 values of 1.9-8.5 microM. Notably, inhibition by cations other than Hg2+ is not reversed by nitric oxide. Our present data support dual roles of Hg2+ and nitric oxide in the regulation of protein synthesis during cell emergency states.

Atypical, bidirectional regulation of cadmium-induced apoptosis via distinct signaling of unfolded protein response

Cadmium is a widely distributed nephrotoxic metal that causes renal tubular injury. In this report, we investigated involvement of endoplasmic reticulum (ER) stress and individual unfolded protein responses in cadmium-initiated apoptosis of tubular epithelial cells. Cadmium chloride (CdCl(2)) induced expression of endogenous ER stress markers, GRP78, GRP94 and CHOP in vitro and in vivo, and subsequently caused cytological changes typical of apoptosis. Attenuation of ER stress by transfection with ER chaperone GRP78 or ORP150 suppressed CdCl(2)-triggered apoptosis. In response to CdCl(2), phosphorylation of RNA-dependent protein kinase-like ER kinase (PERK) and eukaryotic translation initiation factor 2alpha (eIF2alpha) was observed. Enhanced phosphorylation of eIF2alpha attenuated, whereas inhibition of eIF2alpha exacerbated CdCl(2)-induced apoptosis. Activating transcription factor 6 (ATF6) was also activated by CdCl(2) and blockade of this process suppressed induction of CHOP and thereby improved cell survival. CdCl(2) also triggered activation of the inositol-requiring ER-to-nucleus signal kinase 1 (IRE1)-X-box-binding protein 1 (XBP1) pathway and inhibition of XBP1 attenuated apoptosis independent of GRP78 and CHOP. c-Jun N-terminal kinase (JNK), another molecule downstream of IRE1, was also phosphorylated by CdCl(2) and its inhibition attenuated apoptosis. These results evidenced bidirectional regulation of apoptosis in cadmium-exposed cells. The ATF6 and IRE1 pathways cooperatively caused apoptosis via induction of CHOP, activation of XBP1 and phosphorylation of JNK, and the PERK-eIF2alpha pathway counteracted the proapoptotic processes.

Lead-induced upregulation of the heme-regulated eukaryotic initiation factor 2α kinase is compromised by hemin in human K562 cells

Expression and kinase activity of the heme-regulated-eIF-2alpha kinase or -inhibitor (HRI) are induced during cytoplasmic stresses leading to inhibition of protein synthesis. Using a reporter construct with HRI promoter, we have determined the promoter activity during heat-shock and lead toxicity in human K562 cells. These two conditions induced HRI promoter activity by 2- to 3-fold. Contrary to this, hemin, a suppressor of HRI kinase activity, downregulated HRI promoter activity and stimulated hemoglobin synthesis. Interestingly, when hemin-treated cells were transfected and exposed to lead, hemin compromised lead-effect substantially by downregulating HRI promoter activity, HRI transcription and HRI kinase activity. These results together suggest that heme signaling in relation to translation regulation is not only restricted to the cytoplasm (modulating HRI kinase activity) alone but it also spans to the nucleus modulating HRI expression. Hemin may thus be useful for alleviation of stress-induced inhibition of protein synthesis.

c-Jun N-terminal kinase is involved in mercuric ions-mediated interleukin-4 secretion in mast cells

BACKGROUND

Interleukin (IL)-4 plays a prominent role in immune response. Mercuric compounds upregulate IL-4 expression in animal tissues, and this upregulation plays a role in mercuric-mediated immunomodulation. Mercuric ions-mediated IL-4 expression was observed in vitro in T lymphocytes and mast cells. In the present study, we investigated molecular mechanisms responsible for this effect of mercuric ions in mast cells.

METHODS

C1.MC/C57.1 mouse mast cells were exposed in vitro to increasing concentrations of Hg(2+) in the absence or presence of the specific c-Jun N-terminal kinase (JNK) inhibitor SP600125. The level of phosphorylated c-Jun in mast cells was determined by Western blotting, JNK activity assessed with in vitro kinase assay and the amount of secreted IL-4 determined by ELISA.

RESULTS

We observed that Hg(2+) upregulated c-Jun phosphorylation on Ser 73 at concentrations which overlapped concentrations mediating IL-4 secretion. Phosphorylation of c-Jun in mast cells was associated with an increase in JNK activity. The specific JNK inhibitor SP600125 abolished both mercuric-induced c-Jun phosphorylation and IL-4 secretion in mast cells.

CONCLUSIONS

These observations are consistent with the hypothesis that JNK is one of the signaling proteins mediating the effect of Hg(2+) on IL-4 expression in mast cells and is engaged in environmentally mediated immunomodulation.

Quantifying immunohistochemical staining of phospho-eIF2alpha, heme oxygenase-2 and NADPH cytochrome P450 reductase in oligodendrocytes during experimental autoimmune encephalomyelitis

As a consequence of inflammation associated with multiple sclerosis and its animal model, experimental autoimmune encephalomyelitis (EAE), stress responses are induced in many cells within the CNS, however, those that occur within the primary pathological target, the oligodendrocyte, are not fully established. Recently, we found that phosphorylated eukaryotic initiation factor-2alpha (eIF2alpha), an inhibitor of protein translation associated with the stress response, is expressed in a greater number of oligodendrocytes in EAE animals compared to controls. However, since numerous oligodendrocytes in control animals also expressed phospho-eIF2alpha, a method was developed to detect expression levels within oligodendrocytes that did not rely on the number of oligodendrocytes that were stained. This method utilized a high dilution of the primary antibody so that the staining density was kept below a maximum plateau which could eliminate expression differences. Furthermore, the staining density within oligodendrocytes, as determined by image analysis, was corrected by the background density or that within neurons. In either case, the density of staining was greater in oligodendrocytes from EAE animals versus controls. The expression of heme oxygenase-2 and NADPH cytochrome P450 reductase also were examined, but unlike phospho-eIF2alpha, neither was increased in oligodendrocytes from EAE animals compared to controls. In summary, a protocol involving a high dilution of primary antibody and image analysis revealed that the expression of phospho-eIF2alpha within oligodendrocytes was increased in EAE animals compared to control animals.

Immunohistochemical localization of phosphorylated protein kinase R and phosphorylated eukaryotic initiation factor-2 alpha in the central nervous system of SJL mice with experimental allergic encephalomyelitis

Inflammatory cells enter the CNS and target myelin in multiple sclerosis (MS) and experimental allergic encephalomyelitis (EAE), a model of MS, and inflammation is thought to induce stress responses in the CNS. Protein kinase R (PKR) and eukaryotic initiation factor-2 alpha (eIF2 alpha) undergo phosphorylation in response to stress, and the phosphorylated forms of these proteins play a key role in regulating protein synthesis. The objective of this study was to investigate the expression profile of phospho-PKR and phospho-eIF2 alpha during the course of EAE in order to advance the understanding of the stress response in this disease. In control animals (no encephalitogen with no emulsion; no encephalitogen with emulsion) and in preclinical EAE animals, phospho-PKR immunoreactivity was present in oligodendrocytes and some neurons, whereas, in EAE animals with active disease there was widespread labeling of inflammatory cells, and these cells were present during the recovery period of EAE, albeit to a lesser extent. Double-labeling studies revealed that T cells and a few macrophages were phospho-PKR(+). Phospho-eIF2 alpha immunoreactivity was detected in some oligodendrocytes in hindbrain sections of control animals. In EAE animals with active disease, the number of labeled oligodendrocytes increased, and inflammatory T cells also were labeled. Insofar as phospho-PKR activates nuclear factor-kappa B, it may facilitate cytokines expression by T cells. Alternatively, phospho-PKR and phospho-eIF2 alpha may promote apoptosis as a way to regulate T-cell number in the CNS. The expression of phospho-eIF2 alpha in oligodendrocytes during EAE likely is involved with inhibition of protein translation, which is a protective mechanism used to promote cell survival in response to inflammation.

Differential inhibition of Hsc70 activities by two Hsc70-binding peptides

The ability of two high-affinity Hsc70-binding peptides [FYQLALT (peptide-Phi) and NIVRKKK (peptide-K)] to differentially inhibit Hsc70-dependent processes in rabbit reticulocyte lysate (RRL) was examined. Both peptide-Phi and peptide-K inhibited chaperone-dependent renaturation of luciferase in RRL. Peptide-Phi, but not peptide-K, blocked Hsp90/Hsc70-dependent transformation of the heme-regulated eIF2 alpha kinase (HRI) into an active, heme-regulatable kinase. In contrast, peptide-K, but not peptide-Phi, inhibited Hsc70-mediated suppression of the activation of mature-transformed HRI. Furthermore, HDJ2 (Human DnaJ homologue 2), but not HDJ1, potentiated the ability of Hsc70 to suppress the activation of HRI in RRL. Mechanistically, peptide-K inhibited, while peptide-Phi enhanced, HDJ2-induced stimulation of Hsc70 ATPase activity in vitro. The data presented support the hypotheses that peptide-Phi acts to inhibit Hsc70 function by binding to the hydrophobic peptide-binding cleft of Hsc70, while peptide-K acts through binding to a site that modulates the interaction of Hsc70 with DnaJ homologues. Overall, the data indicate that peptide-Phi and peptide-K have differential effects on Hsc70 functions under quasi-physiological conditions in RRL, and suggest that therapeutically valuable peptide mimetics can be designed to inhibit specific functions of Hsc70.

Molecular pathways of protein synthesis inhibition during brain reperfusion: implications for neuronal survival or death

Protein synthesis inhibition occurs in neurons immediately on reperfusion after ischemia and involves at least alterations in eukaryotic initiation factors 2 (eIF2) and 4 (eIF4). Phosphorylation of the alpha subunit of eIF2 [eIF2(alphaP)] by the endoplasmic reticulum transmembrane eIF2alpha kinase PERK occurs immediately on reperfusion and inhibits translation initiation. PERK activation, along with depletion of endoplasmic reticulum Ca2+ and inhibition of the endoplasmic reticulum Ca2+ -ATPase, SERCA2b, indicate that an endoplasmic reticulum unfolded protein response occurs as a consequence of brain ischemia and reperfusion. In mammals, the upstream unfolded protein response components PERK, IRE1, and ATF6 activate prosurvivial mechanisms (e.g., transcription of GRP78, PDI, SERCA2b ) and proapoptotic mechanisms (i.e., activation of Jun N-terminal kinases, caspase-12, and CHOP transcription). Sustained eIF2(alphaP) is proapoptotic by inducing the synthesis of ATF4, the CHOP transcription factor, through "bypass scanning" of 5' upstream open-reading frames in ATF4 messenger RNA; these upstream open-reading frames normally inhibit access to the ATF4 coding sequence. Brain ischemia and reperfusion also induce mu-calpain-mediated or caspase-3-mediated proteolysis of eIF4G, which shifts message selection to m 7 G-cap-independent translation initiation of messenger RNAs containing internal ribosome entry sites. This internal ribosome entry site-mediated translation initiation (i.e., for apoptosis-activating factor-1 and death-associated protein-5) can also promote apoptosis. Thus, alterations in eIF2 and eIF4 have major implications for which messenger RNAs are translated by residual protein synthesis in neurons during brain reperfusion, in turn constraining protein expression of changes in gene transcription induced by ischemia and reperfusion. Therefore, our current understanding shifts the focus from protein synthesis inhibition to the molecular pathways that underlie this inhibition, and the role that these pathways play in prosurvival and proapoptotic processes that may be differentially expressed in vulnerable and resistant regions of the reperfused brain.

Brain ischemia and reperfusion activates the eukaryotic initiation factor 2alpha kinase, PERK

Reperfusion after global brain ischemia results initially in a widespread suppression of protein synthesis in neurons, which persists in vulnerable neurons, that is caused by the inhibition of translation initiation as a result of the phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 (eIF2alpha). To identify kinases responsible for eIF2alpha phosphorylation [eIF2alpha(P)] during brain reperfusion, we induced ischemia by bilateral carotid artery occlusion followed by post-ischemic assessment of brain eIF2alpha(P) in mice with homozygous functional knockouts in the genes encoding the heme-regulated eIF2alpha kinase (HRI), or the amino acid-regulated eIF2alpha kinase (GCN2). A 10-fold increase in eIF2alpha(P) was observed in reperfused wild-type mice and in the HRI-/- or GCN2-/- mice. However, in all reperfused groups, the RNA-dependent protein kinase (PKR)-like endoplasmic reticulum eIF2alpha kinase (PERK) exhibited an isoform mobility shift on SDS-PAGE, consistent with the activation of the kinase. These data indicate that neither HRI nor GCN2 are required for the large increase in post-ischemic brain eIF2alpha(P), and in conjunction with our previous report that eIF2alpha(P) is produced in the brain of reperfused PKR-/- mice, provides evidence that PERK is the kinase responsible for eIF2alpha phosphorylation in the early post-ischemic brain.

Phosphorylation of serine 51 in initiation factor 2 alpha (eIF2 alpha) promotes complex formation between eIF2 alpha(P) and eIF2B and causes inhibition in the guanine nucleotide exchange activity of eIF2B

Phosphorylation of serine 51 residue on the alpha-subunit of eukaryotic initiation factor 2 (eIF2alpha) inhibits the guanine nucleotide exchange (GNE) activity of eIF2B, presumably, by forming a tight complex with eIF2B. Inhibition of the GNE activity of eIF2B leads to impairment in eIF2 recycling and protein synthesis. We have partially purified the wild-type (wt) and mutants of eIF2alpha in which the serine 51 residue was replaced with alanine (51A mutant) or aspartic acid (51D mutant) in the baculovirus system. Analysis of these mutants has provided novel insight into the role of 51 serine in the interaction between eIF2 and eIF2B. Neither mutant was phosphorylated in vitro. Both mutants decreased eIF2alpha phosphorylation occurring in hemin and poly(IC)-treated reticulocyte lysates due to the activation of double-stranded RNA-dependent protein kinase (PKR). However, addition of 51D, but not 51A mutant eIF2alpha protein promoted inhibition of the GNE activity of eIF2B in hemin-supplemented rabbit reticulocyte lysates in which relatively little or no endogenous eIF2alpha phosphorylation occurred. The 51D mutant enhanced the inhibition in GNE activity of eIF2B that occurred in hemin and poly(IC)-treated reticulocyte lysates where PKR is active. Our results show that the increased interaction between eIF2 and eIF2B protein, occurring in reticulocyte lysates due to increased eIF2alpha phosphorylation, is decreased significantly by the addition of mutant 51A protein but not 51D. Consistent with the idea that mutant 51D protein behaves like a phosphorylated eIF2alpha, addition of this partially purified recombinant subunit, but not 51A or wt eIF2alpha, increases the interaction between eIF2 and 2B proteins in actively translating hemin-supplemented lysates. These findings support the idea that phosphorylation of the serine 51 residue in eIF2alpha promotes complex formation between eIF2alpha(P) and eIF2B and thereby inhibits the GNE activity of eIF2B.

Brain ischemia and reperfusion: molecular mechanisms of neuronal injury

Brain ischemia and reperfusion engage multiple independently-fatal terminal pathways involving loss of membrane integrity in partitioning ions, progressive proteolysis, and inability to check these processes because of loss of general translation competence and reduced survival signal-transduction. Ischemia results in rapid loss of high-energy phosphate compounds and generalized depolarization, which induces release of glutamate and, in selectively vulnerable neurons (SVNs), opening of both voltage-dependent and glutamate-regulated calcium channels. This allows a large increase in cytosolic Ca(2+) associated with activation of mu-calpain, calcineurin, and phospholipases with consequent proteolysis of calpain substrates (including spectrin and eIF4G), activation of NOS and potentially of Bad, and accumulation of free arachidonic acid, which can induce depletion of Ca(2+) from the ER lumen. A kinase that shuts off translation initiation by phosphorylating the alpha-subunit of eukaryotic initiation factor-2 (eIF2alpha) is activated either by adenosine degradation products or depletion of ER lumenal Ca(2+). Early during reperfusion, oxidative metabolism of arachidonate causes a burst of excess oxygen radicals, iron is released from storage proteins by superoxide-mediated reduction, and NO is generated. These events result in peroxynitrite generation, inappropriate protein nitrosylation, and lipid peroxidation, which ultrastructurally appears to principally damage the plasmalemma of SVNs. The initial recovery of ATP supports very rapid eIF2alpha phosphorylation that in SVNs is prolonged and associated with a major reduction in protein synthesis. High catecholamine levels induced by the ischemic episode itself and/or drug administration down-regulate insulin secretion and induce inhibition of growth-factor receptor tyrosine kinase activity, effects associated with down-regulation of survival signal-transduction through the Ras pathway. Caspase activation occurs during the early hours of reperfusion following mitochondrial release of caspase 9 and cytochrome c. The SVNs find themselves with substantial membrane damage, calpain-mediated proteolytic degradation of eIF4G and cytoskeletal proteins, altered translation initiation mechanisms that substantially reduce total protein synthesis and impose major alterations in message selection, down-regulated survival signal-transduction, and caspase activation. This picture argues powerfully that, for therapy of brain ischemia and reperfusion, the concept of single drug intervention (which has characterized the approaches of basic research, the pharmaceutical industry, and clinical trials) cannot be effective. Although rigorous study of multi-drug protocols is very demanding, effective therapy is likely to require (1) peptide growth factors for early activation of survival-signaling pathways and recovery of translation competence, (2) inhibition of lipid peroxidation, (3) inhibition of calpain, and (4) caspase inhibition. Examination of such protocols will require not only characterization of functional and histopathologic outcome, but also study of biochemical markers of the injury processes to establish the role of each drug.

The N-terminal region of the heme-regulated eIF2alpha kinase is an autonomous heme binding domain

The N-terminal domain (NTD) of the heme-regulated eukaryotic initiation factor (eIF)2alpha kinase (HRI) was aligned to sequences in the NCBI data base using ENTREZ and a PAM250 matrix. Significant similarity was found between amino acids 11-118 in the NTD of rabbit HRI and amino acids 16-120 in mammalian alpha-globins. Several conserved amino acid residues present in globins are conserved in the NTD of HRI. His83 of HRI was predicted to be equivalent to the proximal heme ligand (HisF8) that is conserved in all globins. Molecular modeling of the NTD indicated that its amino acid sequence was compatible with the globin fold. Recombinant NTD (residues 1-159) was expressed in Escherichia coli. Spectral analysis of affinity purified recombinant NTD indicated that the NTD contained stably bound hemin. Mutational analysis indicated that His83 played a critical structural role in the stable binding of heme to the NTD, and was required to stabilize full length HRI synthesized de novo in the rabbit reticulocyte lysate. These results indicate that the NTD of HRI is an autonomous heme-binding domain, with His83 possibly serving as the proximal heme binding ligand.

Zinc inhibits protein synthesis in neurons. Potential role of phosphorylation of translation initiation factor-2alpha

In the central nervous system, Zn(2+) is concentrated in the cerebral cortex and hippocampus and has been found to be toxic to neurons. In this study, we show that exposure of cultured cortical neurons from mouse to increasing concentrations of Zn(2+) (10-300 microM) induces a progressive decrease in global protein synthesis. The potency of Zn(2+) was increased by about 2 orders of magnitude in the presence of Na(+)-pyrithione, a Zn(2+) ionophore. The basal rate of protein synthesis was restored 3 h after Zn(2+) removal. Zn(2+) induced a sustained increase in phosphorylation of the alpha subunit of the translation eukaryotic initiation factor-2 (eIF-2alpha), whereas it triggered a transient increase in phosphorylation of eukaryotic elongation factor-2 (eEF-2). Protein synthesis was still depressed 60 min after the onset of Zn(2+) exposure while the state of eEF-2 phosphorylation had already returned to its basal level. Moreover, Zn(2+) was less effective than glutamate to increase eEF-2 phosphorylation, whereas it induced a more profound inhibition of protein synthesis. These results suggest that Zn(2+)-induced inhibition of protein synthesis mainly correlates with the increase in eIF-2alpha phosphorylation. Supporting further that Zn(2+) acts at the initiation step of protein synthesis, it strongly decreased the amount of polyribosomes.

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.

Cisplatin inhibits protein synthesis in rabbit reticulocyte lysate by causing an arrest in elongation

The mechanism through which cisplatin (cis-diamminedichloroplatinum) inhibits protein synthesis in rabbit reticulocyte lysate was characterized. Cisplatin and transplatin caused a progressive slowing in the rate of protein synthesis culminating in the complete arrest of translation. Inhibition was dependent upon the aquation of the compounds. Addition of eukaryotic initiation factor eIF-2, eIF-2B, cAMP, MgGTP, or dithiothreitol neither prevented nor reversed the inhibition induced by cisplatin, indicating that the mechanism of cisplatin-induced translational inhibition is distinct from the inhibition induced by other toxic heavy metal ions (Hurst, R., Schatz, J. R., and Matts, R. L. (1987) J. Biol. Chem. 262, 15939-15945; Matts, R. L., Schatz, J. R., Hurst, R., and Kagen, R. (1991) J. Biol. Chem. 266, 12695-12702). Analysis of the polyribosome profile of cisplatin-inhibited reticulocyte lysate indicated that cisplatin arrests the elongation stage of protein synthesis. Agarose gel electrophoresis and Northern blot analysis indicated that mRNA and rRNA become crosslinked to form very high-molecular-weight adducts upon extraction of the RNA from polyribosomes of cisplatin-treated lysates. Diethyldithiocarbamate, which reduces the cytotoxicity of cisplatin in vivo, protects protein synthesis in reticulocyte lysate from inhibition by cisplatin. The data suggest that extensive derivatization of reticulocyte lysate RNA by cis- and transplatin results in the arrest of translating ribosomes. Since arrest of translational elongation is a well-defined mechanism of action of several families of toxins, we suggest that it may contribute to the cytotoxic action of cisplatin observed in certain populations of cells.

The role of the 90-kDa heat-shock protein and its associated cohorts in stabilizing the heme-regulated eIF-2alpha kinase in reticulocyte lysates during heat stress

The heme-regulated eIF-2alpha kinase (HRI) is activated not only in heme-deficient rabbit reticulocyte lysates (RRL), but also in hemin-supplemented RRL treated with heat-shock, N-ethylmaleimide (MalNEt) or heavy metal ions. We have demonstrated previously that heat-shock proteins, Hsp90, Hsp70 and FKBP52, are associated with HRI in RRL; the association of HRI with Hsp90 and FKBP52, but not Hsp70, is enhanced by hemin. To study the role of Hsp90 and its associated cohorts in the regulation of HRI, we examined the interaction of these proteins with HRI in hemin-supplemented RRLs during heat or oxidative stress. The association of HRI with Hsp90, FKBP52 and p23 was maintained in heat-, MalNEt- or Hg2(+)-treated hemin-supplemented RRL. Glycerol gradient centrifugation and gel filtration on Sephacryl S-300 indicated that neither heat shock nor MalNEt-treatment affected the apparent molecular mass of HRI in hemin supplemented RRL. In addition, active HRI was coimmunoprecipitated with 8D3 anti-Hsp90 from both heme-deficient and MalNEt-treated hemin-supplemented RRL. These results demonstrate that activation of HRI in response to heat stress and oxidative stress does not require dissociation of Hsp90 from HRI. Furthermore, HRI activity was inhibited upon addition of hemin to Hsp90-depleted heme-deficient RRL, indicating that inhibition of HRI activity by hemin is not mediated by the reassociation of Hsp90 with HRI. We also examined the dynamics of the interaction of Hsp90 with HRI. Reconstitution of the interaction of Hsp90 with HRI was stimulated by elevated temperature and required both Mg2+ and ATP. Addition of purified Hsp90 to hemin-supplemented RRL which had been treated with MalNEt to inactivate its capacity to chaperone protein renaturation, protected HRI from irreversible denaturation and aggregation upon incubation at 41 degrees C. Our results suggest that Hsp90 interacts with HRI primarily in its capacity as a molecular chaperone, stabilizing HRI from denaturation under conditions of heat stress and oxidative stress.

Differences in the inhibition of translation by cisplatin, transplatin, and certain related compounds

The non-therapeutic cisplatin congeners transplatin and chloroethylenetriamine platinum (dien) inhibited translation to a similar extent as cisplatin did. The IC50 values were: cisplatin 23 microM, transplatin 54 microM, and dien 117 microM. Unlike certain heavy metal inhibitors of translation, the effect of neither cisplatin nor the congeners was reversed by 3':5'-cyclic adenosine monophosphate (cAMP). This suggests that the effect of these platinum compounds does not occur by the heavy metal mechanism. Polyribosomes and ribosomal subunits formed in transplatin-inhibited reactions differed from those in reactions inhibited by cisplatin. Specifically, large polyribosomes and complete 80S ribosomal subunits accumulated in the presence of transplatin. This indicates that while cisplatin slowed initiation of peptide synthesis, the trans-isomer slowed elongation. Substantive differences were not found between cisplatin and the monofunctional compound dien. This congener increased the non-peptidyl disintegrations per minute in the acid precipitates of assays containing [35S]methionine. The high background indicated that an interaction between the label and a precipitable component of the system was induced by dien. However, consumption of methionine by this interaction did not appear to be the cause of the inhibition. Although there may be differences in the mechanisms of the effects, the finding that the non-therapeutic congeners inhibit translation at similar concentrations as cisplatin suggests that this inhibition is not responsible for the anticancer effect. On the other hand, the possibility that decreased translation could play an important role in the toxicity of these compounds in certain quiescent cells cannot be ruled out.

Zinc inhibits turnover of labile mRNAs in intact cells

For immediate early genes such as the c-fos proto-oncogene, mRNA breakdown is very rapid and is largely responsible for the transient nature of mRNA accumulation after transcription is stimulated. We found that in several types of cultured cells and in mice, Zn++ caused marked accumulation of c-fos mRNA and that of another labile mRNA, that encoding the tristetraprolin (TTP) protein. Exposure of TK-L cells to 100 microM ZnSO4 caused an increase of c-fos and TTP mRNA levels within 1 h that reached peak levels in 4-8 h and remained constant to 12 h. Increases in fos protein accumulation were also noted. When the cells were exposed to Zn++ for 4 h and then exposed to actinomycin D, both c-fos and TTP mRNA levels remained constant for up to 10 h, indicating that Zn++ was preventing the breakdown of both c-fos and TTP mRNA. Also, 100 microM ZnSO4 inhibited protein synthesis in TK-L cells, suggesting that the effect on mRNA accumulation could have been an indirect effect resulting from inhibited protein synthesis. Zn++ was unable to inhibit the breakdown of TTP and c-fos mRNA in vitro; however, extracts from cells exposed to Zn++ were less able to cause the breakdown of TTP and c-fos mRNAs than were extracts from control cells, again suggesting that Zn++ indirectly affects mRNA stability through inhibition of protein synthesis. These studies suggest that in addition to their effects on gene transcription, Zn++ and other divalent cations may regulate gene expression by affecting mRNA stability.

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.

Denatured proteins inhibit translation in hemin-supplemented rabbit reticulocyte lysate by inducing the activation of the heme-regulated eIF-2 alpha kinase

The heme-regulated inhibitor (HRI) of protein synthesis becomes activated in rabbit reticulocyte lysates in response to a variety of conditions including heme-deficiency, addition of oxidants, and heat shock. Activated HRI inhibits translation by catalyzing the phosphorylation of the alpha-subunit of eukaryotic initiation factor eIF-2. The molecular nature of the "signal" that leads to the activation of HRI in response to heat shock has not been characterized. We have recently reported that HRI interacts with the 90- and 70-kDa heat shock proteins (hsp) and a 56-kDa protein in hemin-supplemented lysates [Matts, R.L., Xu, Z., Pal, J.K., & Chen, J.-J. (1992) J. Biol. Chem. 267m 18160-18167]. In this report, we demonstrate that addition of denatured proteins, bovine serum albumin (BSA), beta-lactoglobulin, or alpha-lactalbumin, but not the addition of the native proteins, inhibits protein synthesis in hemin-supplemented reticulocyte lysates. The inhibition was reversed upon the addition of 10 mM cAMP or purified eIF-2B, classical criteria for HRI-mediated translational inhibition. Denatured BSA, but not native BSA, stimulated the phosphorylation of the alpha-subunit of eIF-2. This stimulation of eIF-2 alpha phosphorylation was inhibited by a monoclonal antibody to HRI, confirming that denatured BSA was causing the activation of HRI. The concentration of denatured BSA required to inhibit protein synthesis by 50% correlated with the levels of hsp70 present in each lysate preparation. Lysate hsp70 co-immunoadsorbed with denatured BSA, but not with not with native BSA. Hsp70 was co-adsorbed with HRI from lysate in the presence of native BSA, but not in the presence of denatured BSA.(ABSTRACT TRUNCATED AT 250 WORDS)

Suppression of protein synthesis in the reperfused brain

BACKGROUND

Brain ischemia and reperfusion produce profound protein synthesis alterations, the extent and persistence of which are dependent on the nature of the ischemia, the brain region, the cell layer within a region, and the particular proteins studied. After transient ischemia, most brain regions recover their protein synthesis capability; however, recovery in the selectively vulnerable areas is poor. It is unknown whether this phenomenon itself provokes or is a consequence of the process of neuronal death.

SUMMARY OF REVIEW

Protein synthesis suppression during ischemia is due to energy depletion, but this is quickly reversed upon recirculation. Reperfusion does not appear to damage DNA or transcription mechanisms, although there are changes in the profile of transcripts being made. Similarly, purified ribosomes isolated from reperfused brains can make the normal repertoire of proteins and heat-shock proteins. However, during early reperfusion, newly synthesized messenger RNAs appear to accumulate in the nucleus; this alteration in RNA handling could reflect disruption at any of several steps, including posttranscriptional processing, nuclear pore transport, cytoskeletal binding, or formation of the translation initiation complex. Another mechanism that may be responsible for protein synthesis suppression during late reperfusion is progressive membrane destruction, with consequent shifts in the concentration of ions crucial for ribosomal function.

CONCLUSIONS

Protein synthesis suppression after ischemia likely involves a progression of multiple mechanisms during reperfusion. Although the recent work reviewed here offers new insight into the potential mechanisms disrupting protein synthesis, detailed understanding will require further investigation.

Tissue distribution and immunoreactivity of heme-regulated eIF-2 alpha kinase determined by monoclonal antibodies

A highly purified preparation of heme-regulated inhibitor (HRI), an eIF-2 alpha kinase, from rabbit reticulocyte lysates has been used for generating monoclonal antibodies (mAB). Two hybridoma clones secreting HRI-specific antibodies (mAB A and mAB F) were obtained. Both antibodies immunoprecipitated biosynthetically labeled as well as phosphorylated HRI in reticulocyte lysates and also recognized denatured HRI in a Western blot. In in vitro protein kinase assays, preincubation of HRI with the antibodies significantly diminished both autokinase and eIF-2 alpha kinase activities. HRI from reticulocyte lysates could be quantitatively removed by immunoprecipitation with mAB F, and such HRI-depleted lysates were able to maintain protein synthesis under conditions of heme deficiency. With these monoclonal antibodies, HRI was detected only in the reticulocytes and bone marrow of anemic rabbits, among several rabbit tissues tested. The antibodies did not detect cross-reacting HRI in rat or human reticulocytes or in mouse erythroleukemic cells or human K562 cells even after induction of differentiation, although eIF-2 alpha kinase activity was detected in them. Polyclonal anti-rabbit HRI antibody detected HRI in rat reticulocytes. However, no cross-reacting HRI was detected by polyclonal antibody in human reticulocytes or other cell types tested. These findings suggest that HRI is not ubiquitous, and may be erythroid-specific, and that it is antigenically different in different species.

Short-term exposure to cadmium modifies phosphorylation of gill proteins in the sea mussel

1. Sea mussels were exposed to cadmium for short periods of time. The excised gills were incubated with radioactive orthophosphate. The gill proteins were separated by 1- and 2-dimensional gel electrophoresis and the phosphorylation state of the proteins was determined by image analysis of autoradiographs. 2. 1-Dimensional gel electrophoresis revealed that exposure of the animals to cadmium stimulated phosphorylation of the gill proteins in a cadmium concentration-dependent manner. 3. 2-Dimensional gel electrophoresis showed that cadmium differentially affected the phosphorylation of various proteins. Major alterations were observed in the basic, high mol. wt proteins and in the acidic, low mol. wt polypeptides.

An inhibitor(s) of globin mRNA translation in rabbit serum

1. A factor found in rabbit serum inhibits globin mRNA translation in vitro. 2. Inhibition of globin mRNA translation has been demonstrated in a cell-free rabbit reticulocyte lysate. 3. The inactivation of globin mRNA translation is not attributed to either serum albumin or ribonuclease activities. 4. Dialyzing the inhibitor for 24 hr at 4 degrees C does not result in the diminution of the inhibiting activity. However, the activity of the inhibitor is destroyed by heating to 70-80 degrees C for 5 min or by treatment with trypsin for 2 hr. 5. Ion exchange chromatography points to the inhibitor being a neutral protein, whereas, polyacrylamide gel electrophoresis reveals one major band with mol. wt 43 kDa. 6. The activity of the inhibiting material 3-fold greater in anemic serum than in normal serum. 7. These studies suggest that rabbit serum contains a protein inhibitor that may play a physiological role in regulating protein synthesis in red cells.

Protein synthesis initiation factor modifications during viral infections: implications for translational control

Infection of tissue culture cells with certain viruses results in the shutoff of host cell protein synthesis. We have examined virally infected cell lysates using two-dimensional gel electrophoresis and immunoblotting to ascertain whether initiation factor protein modifications are correlated with translational repression. Moderate increases in eukaryotic initiation factor (eIF)-2 alpha phosphorylation are detected in reovirus- and adenovirus-infected cells, as reported previously (Samuel et al., 1984; O'Malley et al., 1989). Neither vesicular stomatitis virus, vaccinia virus, frog virus III, rhinovirus, nor encephalomyocarditis virus caused significantly increased 2 alpha phosphorylation. There were no reproducible, significant changes in eIF-4A, eIF-4B, or eIF-2 beta in cells infected by any of these viruses. The cleavage of eIF-4F subunit p220, such as has been previously demonstrated to occur in poliovirus (Etchison et al., 1982) and rhinovirus (Etchison and Fout, 1985), was not detected in any of the other virus infections analyzed.

Frog virus 3-induced translational shut-off: activation of an eIF-2 kinase in virus-infected cells

Infection of susceptible fathead minnow or Friend erythroleukemia cells with either infectious or heat-inactivated frog virus 3 led to the rapid inhibition of cellular protein synthesis. As seen in other cells, translational shut-off was accompanied by the dissociation of polysomes, but not the degradation of irreversible inactivation of cellular mRNAs. In addition, lysates from cells infected with heat-inactivated FV3 showed a reduced capacity to synthesize protein and to form 43S pre-initiation complexes in vitro. These results indicate that the in vitro systems accurately reflected in vivo events, and suggest that translational shut-off occurred prior to the union of the 40S ribosomal subunit and the [eIF-2.GTP.Met tRNAi] ternary complex. To determine the basis for the translational block, lysates from mock- and FV3-infected cells were assayed in vitro for their ability to phosphorylate the alpha subunit of eIF-2. In contrast to lysates from mock-infected cells, lysates from cells infected with heat-inactivated or infectious FV3 readily phosphorylated the alpha subunit of eIF-2. Since phosphorylation of the alpha subunit of eIF-2 inhibits its catalytic utilization during polypeptide chain initiation, these findings suggest that translational shut-off mediated by FV3 may be due to activation of a kinase that selectively phosphorylates this key initiation factor.

Differential effect of hemin-controlled eIF-2 alpha kinases from mouse erythroleukemia cells on protein synthesis

Cultured mouse erythroleukemia (MEL) cells can be induced to erythroid differentiation by a variety of chemical agents. This differentiation process is marked by the onset of globin mRNA and hemoglobin synthesis. In rabbit reticulocytes, globin synthesis is regulated by a hemin-controlled translational inhibitor (HCI) which acts via phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF-2). From both uninduced and induced MEL cells, hemin-controlled eIF-2 alpha kinases have been partially purified. They resemble HCI with respect to their chromatographic behaviour and their sensitivity towards physiological concentrations of hemin (5-10 microM). Further purification on phosphocellulose, however, reveals that the eIF-2 alpha kinase from uninduced MEL cells is chromatographically distinct from HCI, whilst the eIF-2 alpha kinase activity from induced MEL cells represents a mixture of the former and the HCI-type eIF-2 alpha kinase. The latter inhibits protein synthesis in a fractionated system from rabbit reticulocytes which is free of, but sensitive to, HCI, whereas the eIF-2 alpha kinase from uninduced MEL cells does not show any inhibitory activity. This observation is supported by the finding that induced MEL cells respond in vivo to iron depletion with a shut-off of protein synthesis (as do rabbit reticulocytes), whilst uninduced MEL cells do not.

The phosphorylation of eukaryotic initiation factor 2: a principle of translational control in mammalian cells

In eukaryotic cells, protein biosynthesis is controlled at the level of polypeptide chain initiation. During the initiation process, eukaryotic initiation factor 2 (eIF-2) catalyzes the binding of Met-tRNAf and GTP to the 40S ribosomal subunit. In a later step, eIF-2 is released from the ribosomal initiation complex, most likely as an eIF-2.GDP complex, and another initiation factor termed eIF-2B is necessary to recycle eIF-2 by displacing GDP by GTP. In rabbit reticulocytes, inhibition of protein synthesis is accompanied by the phosphorylation of the alpha-subunit of eIF-2, a process that does not render eIF-2 inactive, but prevents it from being recycled by eIF-2B. First described in rabbit reticulocytes as inhibitors of translation, two distinct eIF-2 alpha kinases are known: the haemin-controlled kinase (termed HCI) and the double-stranded RNA-activated kinase (termed DAI). eIF-2 alpha phosphorylation appears to be a reversible control mechanism since corresponding phosphatases have been described. Recent reports indicate a correlation between eIF-2 alpha phosphorylation and the inhibition of protein synthesis in several mammalian cell types under a range of physiological conditions. In this review, the physical and functional features of the known eIF-2 alpha kinases are described with respect to their role in mammalian cells and the mode of activation by cellular signals. Furthermore, the possible impact of the eIF-2/eIF-2B ratio and of the subcellular compartmentation of these factors (and the eIF-2 alpha kinases) on mammalian protein synthesis is discussed.