Mechanism of inhibition of polypeptide chain initiation in heat-shocked Ehrlich ascites tumour cells

Ubiquitination is essential for recovery of cellular activities after heat shock

Eukaryotic cells respond to stress through adaptive programs that include reversible shutdown of key cellular processes, the formation of stress granules, and a global increase in ubiquitination. The primary function of this ubiquitination is thought to be for tagging damaged or misfolded proteins for degradation. Here, working in mammalian cultured cells, we found that different stresses elicited distinct ubiquitination patterns. For heat stress, ubiquitination targeted specific proteins associated with cellular activities that are down-regulated during stress, including nucleocytoplasmic transport and translation, as well as stress granule constituents. Ubiquitination was not required for the shutdown of these processes or for stress granule formation but was essential for the resumption of cellular activities and for stress granule disassembly. Thus, stress-induced ubiquitination primes the cell for recovery after heat stress.

Thermal killing of human colon cancer cells is associated with the loss of eukaryotic initiation factor 5A

Heat-induced cell death appears to be a cell-specific event. Chronic heat stress was lethal to human colon cancer cells (Caco-2, HT29, and HCT116), but not to normal diploid fibroblasts and other cancer cells (BJ-T, WI38, HeLa, ovarian 2008, WI38VA). Acute heat stress (45-51 degrees C, 30 min) caused cell death of colon cancer cells during recovery at physiological temperature. Thermal killing of Caco-2 cells was not mediated via oxidative stress since Caco-2 cells were much more resistant than HeLa and other cancer cells to H(2)O(2)-induced cell death. Acute heat stress caused a striking loss of eukaryotic initiation factor 5A (eIF5A) in colon cancer cells, but not in HeLa and other normal or transformed human fibroblasts. The heat-induced loss of eIF5A is likely to be due to changes in the protein stability. The half-life of eIF5A was changed from >20 h to less than 30 min during the acute heat stress. Sequence analysis of the eIF5A gene from Caco-2 and HeLa cells did not reveal any difference, suggesting that the change in stability in Caco-2 cells was not due to any eIF5A mutation. Pretreatment of cells with protease inhibitors such as phenylmethyl sulfonyl fluoride (PMSF) partially blocked the heat-induced loss of eIF5A and prevented heat-induced cell death. In light of the essential role of eIF5A in cell survival and proliferation, our results suggest that the stability of eIF5A may have an important role in determining the fate of the particular cell type after severe heat stress.

PKR in innate immunity, cancer, and viral oncolysis

The mammalian innate immune system provides a first line of defense against microbial pathogens and also serves to activate an antigen specific acquired immune program. Key components of innate immunity are the interferons (IFNs), a family of related cytokines with potent antimicrobial and immuno-modulatory activities. The IFNs exert their effects through the induction of numerous genes, one of which is the double-stranded RNA-dependent protein kinase (PKR), a pivotal antiviral protein found in most human cells. Following activation by double stranded (ds) RNAs produced during viral replication, PKR phosphorylates the alpha-subunit of eukaryotic translation initiation factor (eIF) 2, causing a severe inhibititon of cellular and viral protein synthesis. Phosphorylation of eIF2alpha and consequent inhibition of protein synthesis is a major cell growth checkpoint utilized by at least three other kinases, in addition to PKR, following exposure to such cellular stresses as amino acid deprivation and the presence of misfolded proteins in the endoplasmic reticulum. Indeed, it has been demonstrated that disruption of the eIF2alpha checkpoint can lead to the transformation of immortalized rodent and human cells, plausibly by increasing the protein synthesis rates of proto-oncogenes. Further, it has been shown that disregulation of the eIF2alpha checkpoint and consequent permissiveness to virus infection may be a common occurrence in tumorigenic mammalian cell lines. These findings have been exploited to develop potent oncolytic RNA viruses that can selectively replicate in and destroy a variety of neoplasias in vitro and in vivo. In this chapter, we describe some of the techniques commonly used in our laboratory to examine PKR activity and eIF2 regulation. Protocols for the generation and use of recombinant vesicular stomatitis virus variants are also described.

Distinctive properties of the 5'-untranslated region of human hsp70 mRNA

A relaxed cap-dependence of translation of the mRNA-encoding mammalian heat shock protein Hsp70 may suggest that its 5'-untranslated region (UTR) possesses an internal ribosome entry site (IRES). In this study, this possibility has been tested in transfected cells using plasmids that express dicistronic mRNAs. Using a reporter gene construct, Renilla luciferase/Photinus pyralis luciferase, we show that the 216-nt long 5'-UTR of Hsp70 mRNA acts as an IRES that directs ribosomes to the downstream start codon by a cap-independent mechanism. The relative activity of this IRES (100-fold over the empty vector) is similar to that of the classical picornaviral IRESs. Additional controls indicate that this high expression of the downstream reporter is not due to readthrough from the upstream cistron, nor is it due to translation of cryptic monocistronic transcripts. The effect of small deletions within the 5'-UTR of Hsp70 mRNA on the IRES activity varies in dependence on their position within the 5'-UTR sequence. With the exception of deletion of nt 33-50, it is small for the 5'-terminal half of the 5'-UTR and rather strong for the 3'-terminal section. However, neither of these small deletions abolishes the IRES activity completely. Excision of larger sections (>50 nt) by truncation of the 5'-UTR from the 5'-end or by internal deleting results in a dramatic impairment of the IRES function. Taken together, these data suggest that the IRES activity of the 5'-UTR of Hsp70 mRNA requires integrity of almost the entire sequence of the 5'-UTR. The data are discussed in terms of a model that allows a three-dimensional rather than linear mode of selection of the initiation region surrounding the start codon of Hsp70 mRNA.

RNA-binding proteins TIA-1 and TIAR link the phosphorylation of eIF-2 alpha to the assembly of mammalian stress granules

In response to environmental stress, the related RNA-binding proteins TIA-1 and TIAR colocalize with poly(A)(+) RNA at cytoplasmic foci that resemble the stress granules (SGs) that harbor untranslated mRNAs in heat shocked plant cells (Nover et al. 1989; Nover et al. 1983; Scharf et al. 1998). The accumulation of untranslated mRNA at SGs is reversible in cells that recover from a sublethal stress, but irreversible in cells subjected to a lethal stress. We have found that the assembly of TIA-1/R(+) SGs is initiated by the phosphorylation of eIF-2alpha. A phosphomimetic eIF-2alpha mutant (S51D) induces the assembly of SGs, whereas a nonphosphorylatable eIF-2alpha mutant (S51A) prevents the assembly of SGs. The ability of a TIA-1 mutant lacking its RNA-binding domains to function as a transdominant inhibitor of SG formation suggests that this RNA-binding protein acts downstream of the phosphorylation of eIF-2alpha to promote the sequestration of untranslated mRNAs at SGs. The assembly and disassembly of SGs could regulate the duration of stress- induced translational arrest in cells recovering from environmental stress.

Effect of hyperthermia on cyclin B expression in a human glioblastoma cell line

Effects of hyperthermia on the cell kinetics of glioblastoma cells were investigated using flow cytometry. Pulse-labeling with 5-bromodeoxyuridine (BUdR) and chasing of the labeled cells revealed temporary accumulation of the labeled cells in G2M phase and a reduction of DNA synthesis. The level of cyclin B rises rapidly in G2 phase and falls at the end of mitosis in normal cycling cells. Cyclin B binds to p34cdc2, resulting in histone kinase activity which is necessary for the initiation of mitosis. The amount of p34cdc2 remains constant throughout the cell cycle. The level of cyclin B was measured using an anti-cyclin B antibody and flow cytometry in order to investigate the cause of the G2 accumulation induced by hyperthermia. A low level of cyclin B, in comparison with that of normal cycling cells, persisted for more than 3 h after hyperthermia. These results indicate that the temporary accumulation of cells in G2M phase after hyperthermia may be caused, at least in part, by an insufficient level of cyclin B.

Translational regulation of the heat shock response

All organisms from bacteria to man respond to an exposure to higher than physiological temperatures by reprogramming their gene expression, leading to the increased synthesis of a unique set of proteins termed heat shock proteins (hsps). The hsps function as molecular chaperones in both normal and stressed cells. The rapid and efficient synthesis of hsps is achieved as a result of changes occurring at gene transcription, RNA processing and degradation, and mRNA translation. With regard to the translational regulation, the emerging picture is that the two key steps of polypeptide chain initiation, namely mRNA binding and Met-tRNA(i) binding to ribosomes, are regulated in heat-shocked mammalian cells. In Drosophila, mRNA binding is regulated by a structural feature of the leader of heat shock mRNAs and by the inactivation of eukaryotic initiation factor- (eIF-) 4F. No clear evidence for changes in Met-tRNA(i) binding has been obtained yet.

Translational control during heat shock

Regulation of translation during heat shock of Drosophila and mammalian cells is reviewed. Protein synthesis is severely inhibited by elevated temperatures but synthesis of heat shock proteins (HSPs) is resistant to this inhibition. The primary site of regulation is polypeptide chain initiation. The activities of two initiation factors, eIF-2 and eIF-4F, are modulated during heat shock. A protein kinase which modulates eIF-2 activity appears to be associated with heat shock proteins (HSPs). Evidence is emerging that HSP70 acts as a heat sensor by detecting the presence of accumulating denatured proteins. In the rabbit reticulocyte lysate denatured proteins bind HSP70 releasing an eIF-2 kinase to shut down protein synthesis. It appears highly likely that a similar mechanism is acting in heat shocked cells. Cell-free protein synthesizing systems prepared from heat shocked cells are deficient in eIF-4F. Modulation of eIF-4F can explain in part the apparent preferential translation of HSP mRNAs.

Regulation of eukaryotic translation initiation factor expression during T-cell activation

Primary T-cells are metabolically quiescent, with little DNA, RNA or protein synthesis. Upon mitogenic stimulation the rate of protein synthesis increases 10-fold. We have studied the role of eIF-2 and eIF-4 alpha (eIF-4E) expression in the mechanism of translational activation. During this period, the levels of eIF-2 alpha and eIF-4 alpha mRNA increase some 50-fold. Similar to the increase in ribosomes and mRNA, the number of eIF-2 alpha, eIF-2 beta, and eIF-4 alpha molecules per cell also increase 2-3-fold. This suggests that in addition to an increase in the pool size of translational components, an additional mechanism exists which results in an increased efficiency of factor utilization. We have looked at initiation factor phosphorylation. We find that eIF-2 alpha does not undergo significant changes in its phosphorylation state nor is there a change in the efficiency of eIF-2 utilization. However, there is a rapid increase in the phosphorylation state of eIF-4 alpha which correlates with the rapid increase in translational activity. It thus appears there are 2 distinct components responsible for the translational activation of quiescent T-cells during mitogenic stimulation. The first is the phosphorylation of eIF-4 alpha, with a concomitant increase in the efficiency of eIF-4 alpha utilization. The second is an increase in the pool sizes of eIF-2 and eIF-4 alpha.

Phosphorylation of translation initiation factor eIF-4E is induced in a ras-dependent manner during nerve growth factor-mediated PC12 cell differentiation

Translation initiation factor eIF-4E, which binds to the 5' cap structure of eukaryotic mRNAs, is believed to play an important role in the control of cell growth. Consistent with this, overexpression of eIF-4E in fibroblasts results in their malignant transformation. The activity of eIF-4E is thought to be regulated by phosphorylation on a single serine residue (Ser-53). Treatment of rat pheochromocytoma (PC12) cells with nerve growth factor (NGF) strongly curtails their growth and causes their differentiation into cells that resemble sympathetic neurons. The present study shows that eIF-4E is rapidly phosphorylated in PC12 cells upon NGF treatment, resulting in a significant increase in the steady-state levels of the phosphorylated protein. In contrast, epidermal growth factor, a factor which elicits a weak mitogenic response in PC12 cells, did not significantly enhance eIF-4E phosphorylation. We also show that although the mitogen and tumor promoter, phorbol 12-myristate-13-acetate, is able to induce phosphorylation of eIF-4E in PC12 cells, the NGF-mediated increase is primarily a protein kinase C-independent response. The NGF-induced enhancement of eIF-4E phosphorylation is abrogated in PC12 cells expressing a dominant inhibitory ras mutant (Ser-17 replaced by Asn), indicating that eIF-4E phosphorylation is dependent on a ras signalling pathway. As phosphorylation of eIF-4E effects translation initiation, these results suggest that NGF-mediated and ras-dependent eIF-4E phosphorylation may play a role in switching the pattern of gene expression during the differentiation of PC12 cells.

Phosphorylation of translation initiation factor eIF-4E is induced in a ras-dependent manner during nerve growth factor-mediated PC12 cell differentiation

Translation initiation factor eIF-4E, which binds to the 5' cap structure of eukaryotic mRNAs, is believed to play an important role in the control of cell growth. Consistent with this, overexpression of eIF-4E in fibroblasts results in their malignant transformation. The activity of eIF-4E is thought to be regulated by phosphorylation on a single serine residue (Ser-53). Treatment of rat pheochromocytoma (PC12) cells with nerve growth factor (NGF) strongly curtails their growth and causes their differentiation into cells that resemble sympathetic neurons. The present study shows that eIF-4E is rapidly phosphorylated in PC12 cells upon NGF treatment, resulting in a significant increase in the steady-state levels of the phosphorylated protein. In contrast, epidermal growth factor, a factor which elicits a weak mitogenic response in PC12 cells, did not significantly enhance eIF-4E phosphorylation. We also show that although the mitogen and tumor promoter, phorbol 12-myristate-13-acetate, is able to induce phosphorylation of eIF-4E in PC12 cells, the NGF-mediated increase is primarily a protein kinase C-independent response. The NGF-induced enhancement of eIF-4E phosphorylation is abrogated in PC12 cells expressing a dominant inhibitory ras mutant (Ser-17 replaced by Asn), indicating that eIF-4E phosphorylation is dependent on a ras signalling pathway. As phosphorylation of eIF-4E effects translation initiation, these results suggest that NGF-mediated and ras-dependent eIF-4E phosphorylation may play a role in switching the pattern of gene expression during the differentiation of PC12 cells.

Graded postischemic reoxygenation ameliorates inhibition of cerebral cortical protein synthesis in dogs

The purpose of this study was to determine the effect of normoxic reperfusion and graded postischemic reoxygenation on cerebral protein synthesis in a cell-free system. Ischemia alone produced a relatively small decrease (15-17%) in activity in all the subcellular systems studied. After a 15-min interval of normoxic reperfusion (75-90 mmHg O2 in arterial blood), a 40% decrease (p less than 0.01) in [14C]leucine incorporation was observed. Reoxygenation with hypoxemic blood containing 37.5 mm Hg O2 at 0-5 min and 56 mm Hg O2 at 6-10 min of recirculation followed by 5 min of normoxic reperfusion resulted in a significant increase (p less than 0.05) of polypeptide chain synthesis in vitro when compared with normoxic reperfusion. The results obtained by this experimental approach tend to show that graded postischemic reoxygenation could be used as a simple and effective neuroprotective tool that substantially diminishes the secondary postischemic damage in nervous tissue, including the newly synthesized proteins.

Enhanced synthesis of stress proteins caused by hypoxia and relation to altered cell growth and metabolism

Cultured cells maintained in very low oxygen levels alter their structure, metabolism and genetic expression. Culture conditions for cells were modified to minimise variation of nutrients and to allow normal survival levels after 24 h of hypoxic exposure. Under these hypoxic conditions, glucose consumption and lactate production rates were similar to aerobic rates until about 12 h after which the hypoxic rates increased. DNA and protein synthesis rates are continuously inhibited to about 48% or 55% of the respective aerobic rates. During this period of decreased protein synthesis, a set of proteins termed oxygen regulated proteins (ORPs), exhibits enhanced relative synthesis. The molecular weights of the five major ORPs are approximately 260, 150, 100, 80 and 33 kDa. While increased relative synthesis of oxygen regulated proteins is partly due to increased levels of mRNA which encode these proteins, the mechanism of enhanced synthesis of ORPs may be more complex.

Frog virus 3-mediated translational shut-off: frog virus 3 messages are translationally more efficient than host and heterologous viral messages under conditions of increased translational stress

Frog virus 3 rapidly and selectively blocks host cell translation while synthesizing more than 60 virus-specific polypeptides. Previous work indicated that virus infection led to activation of a kinase that phosphorylated and, as a consequence, inactivated eIF-2. Although phosphorylation of eIF-2 could explain the rapid decline in host cell translation, it could not explain how viral protein synthesis persisted in the face of host shut-off. To explain this phenomenon, we speculated that viral messages, either as a consequence of their higher translational efficiency or their greater abundance, were able to outcompete host messages for the remaining translational initiation complexes. To test this hypothesis, the relative translational efficiency of three characteristic FV3 messages was measured against that of several model messages. Translational efficiency was determined by monitoring the resistance (and hence the competitiveness) of a given transcript to increasing concentrations of salt in vitro and in vivo. In both rabbit reticulocyte lysates and wheat germ extracts, FV3 messages were more resistant to supra-optimal concentrations of potassium acetate than globin message and three BMV transcripts. In vivo, FV3 polypeptides were synthesized in the presence of salt concentrations that blocked host cell protein synthesis. These results suggest that the selective translation of FV3 messages in virus-infected cells may partly be due to the higher translational efficiency of viral messages. Structural features that contribute to translational efficiency are discussed.

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.

Protein synthesis and protein phosphorylation during heat stress, recovery, and adaptation

Incubating cells at elevated temperatures causes an inhibition of protein synthesis. Mild heat stress at 41-42 degrees C inhibits the fraction of active, polysomal ribosomes from greater than 60% (preheating) to less than 30%. A return to 37 degrees C leads to an increase in protein synthesis, termed "recovery." Continuous incubation at 41-42 degrees C also leads to a gradual restoration of protein synthesis (greater than 70% of ribosomes reactivated by 2-4 h), termed "adaptation". Protein synthesis inhibition and reactivation is prestressed, recovered cells that contain elevated levels of the heat stress proteins occur to the same extent and at the same rate as in "naive" cells. The adaptation response requires transcription of new RNA whereas recovery does not. A large number of phosphorylation changes are induced by severe heat stress and occur with kinetics similar to the inhibition of protein synthesis. These include phosphorylation of eukaryotic protein synthesis initiation factor (eIF)-2 alpha and dephosphorylation of eIF-4B and eIF-4Fp25 (eIF-4E). However, the extent to which the modification occurs is proportional to the severity of the stress, and, under mild (41-42 degrees C) heat stress conditions, these initiation factor phosphorylation changes do not occur. Similarly, under conditions of severe heat stress eIF-2 alpha and eIF-4B frequently recover to their prestress phosphorylation state before the recovery of protein synthesis. eIF-4E dephosphorylation likewise does not occur under mild heat stress conditions. Therefore, these changes in phosphorylation states, which are thought to be sufficient cause, are not necessary for the inhibition of protein synthesis observed.

Isolation and characterization of the promoter and flanking regions of the gene encoding the human protein-synthesis-initiation factor 2 alpha

The promoter region of the gene (eIF-2 alpha) for eukaryotic initiation factor 2 alpha (eIF-2 alpha) was isolated from a human genomic library and its structure was determined by restriction mapping and nucleotide (nt) sequence analysis. The promoter region and twelve in vivo transcriptional start points (tsp) have been identified by endonuclease S1 mapping and their location confirmed by primer-extension analysis, using RNA isolated from human cells. The untranslated leader is 102 to 140 nt long depending upon the tsp, and the 5' region of the mRNA has the potential for forming stable stem-loop structures. The nt sequence of the regions upstream and downstream from the tsp contains neither a 'TATA box' nor a 'CAAT box', but does contain several direct and inverted repeats, as well as palindromic sequences near the tsp. In addition, multiple consensus binding sites for a wide variety of regulatory proteins are present throughout upstream and downstream tsp-flanking regions.

Mechanism of inhibition of polypeptide chain initiation in calcium-depleted Ehrlich ascites tumor cells

Protein synthesis in Ehrlich ascites tumor cells is inhibited when cellular calcium is depleted by the addition of EGTA to the growth medium. This inhibition is at the level of polypeptide chain initiation as evidenced by a disaggregation of polyribosomes accompanied by a significant elevation in 80-S monomers. To identify direct effects of calcium on the protein synthesis apparatus we have developed a calcium-dependent, cell-free protein-synthesizing system from the Ehrlich cells by using 1,2-bis(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA), a recently developed chelator with a high (greater than 10(5)) selectivity for calcium (pKa = 6.97) over magnesium (pKa = 1.77). BAPTA inhibits protein synthesis by 70% at 1 mM and 90% at 2 mM. This effect was reversed by calcium but not by other cations tested. The levels of 43-S complexes (i.e., 40-S subunits containing bound methionyl-tRNAf.eIF-2.GTP) were significantly lower in the calcium-deprived incubations, indicating either inhibition of the rate of formation or decreased stability of 43-S complexes. Analysis of 43-S complexes on CsCl gradients showed that in BAPTA-treated lysates, 40-S subunits containing eIF-3, completely disappeared and the residual methionyl-tRNA-containing complexes were bound to 40-S subunits lacking eIF-3. Our results demonstrate a direct involvement of Ca2+ in protein synthesis and we have localized the effect of calcium deprivation to decreased binding of eIF-2 and eIF-3 to 40-S subunits.

Physiological stresses inhibit guanine-nucleotide-exchange factor in Ehrlich cells

Previously, we have shown that phosphorylation of the eukaryotic initiation factor eIF-2 alpha increases under several physiological stresses in which protein synthesis is inhibited in Ehrlich ascites tumor cells. As phosphorylated eIF-2 [eIF-2(alpha P)] is a potent inhibitor of guanine nucleotide exchange factor (GEF), it seemed likely that it was responsible for the inhibition. We have assayed GEF activity levels in extracts prepared from Ehrlich cells exposed to three such stresses, namely heat shock, serum deprivation and glutamine deprivation. Activity was estimated by the ability of GEF to enhance the release of [alpha-32P]GDP from purified eIF-2 [a modification of the reticulocyte lysate assay of Matts, R. L. & London, I. M. (1984) J. Biol. Chem. 259, 6708]. GEF activity was reduced from control values in extracts of heat-shocked cells and serum-deprived cells, concomitant with increased eIF-2 alpha phosphorylation. Inhibition of GEF activity in heat-shocked and serum-deprived cells was reversed to control levels by increasing the concentration of purified eIF-2.GDP added as substrate in the GEF assay. Since we have shown elsewhere that eIF-2(alpha P).GDP inhibits GEF by competition with eIF-2.GDP, the complete reversal of inhibition of GEF activity in heat-shocked and serum-deprived cells indicates that inhibition is due solely to phosphorylation of eIF-2 alpha. In glutamine-deprived cells phosphorylation of eIF-2 alpha was increased modestly and GEF activity was reduced but GEF activity could not be fully reversed by addition of eIF-2.GDP, suggesting that GEF may also be regulated in other ways. There are greater amounts of GEF relative to eIF-2 in Ehrlich cells (approximately 50%) compared with rabbit reticulocytes (approximately 20%). This explains the efficient rates of protein synthesis in control Ehrlich cells even though they have 30% of their eIF-2 phosphorylated which is enough to inhibit GEF and initiation in reticulocytes completely but only enough to trap approximately 60% of the GEF in Ehrlich cells.

A Neurospora crassa heat-shocked cell lysate translates homologous and heterologous messenger RNA efficiently, without preference for heat shock messages

Cell-free protein synthesis systems were prepared from normally-grown (N-lysate) and heat-shocked (HS-lysate) Neurospora crassa mycelium. Although both lysates translated homologous mRNA, the HS-lysate was more active, yielding a higher incorporation of [35S]-methionine into hot TCA-insoluble material and a vastly superior protein synthesis profile. The optimal temperature for translation by both lysates was 21 degrees C; the HS-lysate did not translate heat-shock mRNA preferentially at any temperature tested. Fortuitously, heterologous messenger RNAs from diverse eukaryotic and viral sources - Drosophila, dog pancreas, rabbit globin mRNA, brome mosaic virus, tobacco mosaic virus - were translated by the HS-lysate with an efficiency comparable to that of the commercial rabbit reticulocyte system and superior to the wheat germ system. The cap analogues, m7G(5')ppp(5')G and m7G(5')Gm, inhibited translation significantly.

Translational repression by chemical inducers of the stress response occurs by different pathways

The mechanism by which chemical inducers of the stress response inhibit protein synthesis was examined. All the chemicals tested principally inhibit the initiation phase of translation. Covalent modification of the initiation factor proteins does not constitute a common mechanism. Eukaryotic initiation factor (eIF)-2 alpha phosphorylation is moderately to strongly induced by Na arsenite and diamide, but only slightly to imperceptibly affected by iodoacetamide, azetidine carboxylic acid, and canavanine. eIF-4B dephosphorylation does not occur in any case. The only consistent change detected is the hyperphosphorylation of the 28,000 Da heat stress protein. These results indicate that these diverse chemicals, all of which enhance the transcription of the stress mRNAs, do not inhibit translation by a common, recognized mechanism; it is likely that several distinct pathways leading to inhibition exist.

The purification and characterization of subunits alpha, beta, and gamma from the rabbit reticulocyte eukaryotic initiation factor 2

Eukaryotic initiation factor 2 (eIF-2) contains three nonidentical subunits, alpha, beta, and gamma. The simultaneous purification of all three subunits was achieved by reverse-phase HPLC using a 0.1% trifluoroacetic acid-acetonitrile binary solvent system. The order of the eluted subunits, beta, alpha, and gamma, was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. After hydrolysis in 6 N HCl, picomole level amino acid composition analysis was achieved by the ninhydrin reaction on a Beckman 6300 system. Using second-derivative spectroscopic analysis, Trp was detected in all three subunits. All three subunits were subjected to amino-terminal sequence analysis. The amino-terminal of eIF-2 alpha from amino acid positions 1 to 23 inclusive was determined. The order of eight amino acids from the amino-terminal of eIF-2 gamma was also determined. This characterization and partial determination of the primary sequence of these subunits permit the utilization of molecular biology techniques in order to elucidate the complete primary structure. Additionally, the partial amino acid sequence data permitted the designation of synthetic gene probes as well as the identification of eIF-2 alpha and gamma cDNA and/or genomic clones.

Regulation of ribosomal protein S6 phosphorylation in heat-shocked HeLa cells

Decreases in energy charge, ribosomal protein phosphorylation and rate of protein synthesis are well-documented facets of the cellular response to hyperthermia in non-vertebrates. We have tried to reproduce this response pattern in 32P-labelled HeLa cells in order to investigate the hypothetical causal relationship between these effects. In HeLa cells shifted from 36 degrees C to 42 degrees C, dephosphorylation of S6 and inhibition of protein synthesis, owing to a decreased initiation rate, were observed, but could not have been mediated by changes in the cells' general energy charge since the ATP and GTP levels were not reduced. In addition, we found that the hyperthermic translation block developed faster than the overall dephosphorylation of S6, showing that S6 dephosphorylation cannot be responsible for the translation block unless site-specific effects play a critical role.

Accumulation of heat shock protein 70 RNA and its relationship to protein synthesis after heat shock in mammalian cells

Heat stress induces a set of heat shock proteins (hsps) in a wide variety of species. In response to either a mild (5 min X 45 degrees C) or severe (30 min X 45 degrees C) heat shock, the timing of expression of the hsps and the recovery of general protein synthesis in rat embryonic fibroblasts was dependent on the duration of the hyperthermic exposure. Synthesis of mRNA coding for an hsp of Mr approximately equal to 70,000 (hsp 70) followed immediately after the mild heat shock but was delayed after the severe heat shock. Appearance of the hsps paralleled the synthesis and decay of RNA and was indicative that new RNA synthesis was required for hsp 70 expression. Inhibition of protein synthesis by cycloheximide after the mild heat shock increased the maximal accumulation of hsp 70 encoding mRNA but did not prevent the subsequent decrease in this mRNA species. These results suggest that mammalian cells control the expression of hsp 70 primarily at the level of transcription, and that the normal pattern of hsp 70 mRNA turnover after an inducing heat stress is not dependent on new protein synthesis.

Induction characteristics of oxygen regulated proteins

Extreme hypoxia induces many changes in the biology of cells, including the enhanced synthesis of oxygen regulated proteins (ORPs). We investigated the conditions required for the induction of ORPs and by modifying culture conditions, eliminated variables other than oxygen concentration. Several exponentially growing rodent and human cell lines were examined before, during, and after various periods of extreme hypoxia. The following responses were analyzed: cell growth, clonogenic survival, glucose consumption, lactate production, media pH, total protein synthesis, and specific protein synthesis. EMT6/Ro cells did not increase in cell number or progress through the cell cycle after initiation of extreme hypoxia. Cell morphology and cell survival were nearly normal for up to 12 hr of hypoxia. During this period, media pH remained constant, with the concentrations of glucose and lactate being virtually indistinguishable from aerobic cultures or initial values. Associated with these conditions, a marked inhibition of total protein synthesis was observed for EMT6/Ro cells, such that the hypoxic protein synthesis rate was about 60% of the aerobic rate. However, enhanced synthesis of a set of proteins, designated as ORPs, was preferentially induced in less than 6 hr. The molecular weights of the five major ORPs are 260, 150, 100, 80 and 33 kD. Under these conditions, the primary inducing agent was a low concentration of oxygen. This set of ORPs was distinctly different from the set of heat induced (heat-shock) proteins, but included the major 100 kD and 80 kD glucose regulated proteins. Although the functions of ORPs are unknown, their induction under conditions that are known to modify the sensitivity of cancer cells to therapeutic agents suggests that the presence of ORPs should be further investigated to determine their possible value in diagnosis and predicting treatment response.

Inhibition of poly(A)-binding protein synthesis in Friend erythroleukemia cells subsequent to heat shock

When Friend erythroleukemia cells (FEC) are incubated at 43 degrees C there is a rapid and nearly complete inhibition of protein synthesis which can be reversed when cells are returned to their normal growing temperature of 37 degrees C. Examination of the recovery of FEC from heat shock indicates that most cellular mRNAs behave as a cohort and return to translation at approximately the same rate. We found a notable exception to this rule in the case of a 78 kDa basic protein (named protein A) whose rate of return to a normal synthetic rate is markedly inhibited subsequent to heat shock. We show that protein A corresponds to the 78 kDa polypeptide commonly found to be associated with the poly(A) tails of mammalian mRNA.