The establishment of permissive conditions for IRES-mediated translation during differentiation has been confirmed by our demonstration of the enhanced activity of vascular endothelial gro
Trang 1Phosphorylation of initiation factor-2a is required for activation
of internal translation initiation during cell differentiation
Gabi Gerlitz1, Rosemary Jagus2and Orna Elroy-Stein1
1
Cell Research and Immunology, George S Wise Faculty of Life Sciences, Tel Aviv University, Israel;
2
Center of Marine Biotechnology, University of Maryland Biotechnology Institute, Baltimore, USA
The long uORF-burdened 5¢UTRs of many genes encoding
regulatory proteins involved in cell growth and
differenti-ation contain internal ribosomal entry site (IRES) elements
In a previous study we showed that utilization of the weak
IRES of platelet-derived growth factor (PDGF2) is activated
during megakaryocytic differentiation The establishment of
permissive conditions for IRES-mediated translation during
differentiation has been confirmed by our demonstration of
the enhanced activity of vascular endothelial growth factor,
c-Myc and encephalomyocarditis virus IRES elements
under these conditions, although their mRNAs are not
naturally expressed in differentiated K562 cells In contrast
with the enhancement of IRES-mediated protein synthesis
during differentiation, global protein synthesis is reduced, as
judged by polysomal profiles and radiolabelled amino acid
incorporation rate The reduction in protein synthesis rate
correlates with increased phosphorylation of the translation initiation factor eIF2a Furthermore, IRES use is decreased
by over-expression of the dominant-negative form of the eIF2a kinase, PKR, the vaccinia virus K3L gene, or the eIF2a-S51A variant which result in decreased eIF2a phos-phorylation These data demonstrate a connection between eIF2a phosphorylation and activation of cellular IRES ele-ments It suggests that phosphorylation of eIF2a, known to
be important for cap-dependent transaltional control, serves
to fine-tune the translation efficiency of different mRNA subsets during the course of differentiation and has the potential to regulate expression of IRES-containing mRNAs under a range of physiological circumstances Keywords: differentiation; gene expression; initiation factor 2; IRES; translation initiation
Translation of eukaryotic gene expression is controlled both
by global mechanisms that affect the overall rate of protein
synthesis and by selective control mechanisms that affect the
translation of subsets of mRNA molecules equipped with
appropriate cis-regulatory elements The global mechanisms
are mostly based on controlling the availability of two
rate-limiting components of the initiation step: eIF4E, the 5¢-cap
binding protein, and eIF2, a GTP-binding protein that
mediates the association of Met-tRNAi to the 40 S
ribosomal subunit Control of eIF4E activity is mediated
by influencing its phosphorylation status and/or its
interac-tion with the eIF4E binding proteins, as well as by affecting
the integrity of eIF4G which serves as an adapter protein
that bridges eIF4E, the RNA helicase eIF4A, poly(A)
binding protein and eIF3 (reviewed in [1–3]) Control of
eIF2 activity is mediated through reversible
phosphoryla-tion of its a-subunit When eIF2a is phosphorylated, the
GDP-eIF2 generated at the end of each initiation step
becomes a competitive inhibitor of eIF2B, a rate-limiting
guanine nucleotide exchange factor, resulting in a reduction
of the exchange of eIF2-bound GDP for GTP As GTP binding to eIF2 is a prerequisite to Met-tRNAi binding, phosphorylation of eIF2a effectively inhibits eIF2 recycling and consequently inhibits additional translation initiation steps (reviewed in [4]) The control mechanisms that govern the rate of global protein synthesis are responsive to a variety of conditions including nutrient deprivation, heat shock, apoptosis and viral infection Under conditions that inhibit the initiation of global protein synthesis, subsets of mRNAs remain competent to be recruited by ribosomes Depending on their specific cis-regulatory elements they may gain a translational advantage over other mRNA molecules For instance, mRNAs encoding heat-shock proteins are translated efficiently under conditions of reduced eIF4E/4F activity due to their unstructured 5¢UTR (reviewed in [5]) Another example is the efficient translation of the yeast GCN4 mRNA under conditions of amino acid starvation due to leaky scanning of the upstream ORFs within its 5¢UTR (reviewed in [6])
While much data has been accumulated regarding the control of protein synthesis in response to various stress conditions, less is known about translational control mechanisms that are operative during cellular differenti-ation Cells undergoing terminal differentiation exhibit extensive changes in the pattern of gene expression to acquire a specific biological function that is usually accom-panied by cessation of proliferation In addition to the massive changes at the transcriptional level, mechanisms regulating overall inhibition of protein synthesis release most mRNAs from the polysomes and facilitate the translation of specific mRNAs that are important for the
Correspondence to O Elroy-Stein, Department of Cell Research &
Immunology, George S Wise Faculty of Life Sciences,
Tel Aviv University, Tel Aviv 69978, Israel.
Fax: +972 3 642 2046, Tel.: +972 3 640 9153,
E-mail: ornaes@post.tau.ac.il
Abbreviations: PDGF, platelet-derived growth factor; IRES,
internal ribosomal entry site; VEGF, vascular endothelial growth
factor; CMV, cytomegalovirus; TPA,
12-O-tetradecanoylphorbol-13-acetate; EMCV, encephalomyocarditis virus.
(Received 18 January 2002, revised 3 April 2002,
accepted 2 May 2002)
Trang 2differentiation process [7] However, there is little data
regarding the molecular mechanisms that govern such
global inhibition accompanied by activation of specific
mRNA subpopulations The proliferation-dependent
association of polysomes with 5¢-terminal oligopyrymidine
tract-containing mRNAs [8] and the 3¢UTR-mediated
translational activation of 15-lipoxygenase mRNA during
erythrocytic differentiation [9] are the best characterized
examples
Whereas most mRNAs are translated efficiently, a subset
of mRNAs is poorly translated due to the extraordinarily
long, structured, upstream AUG-burdened 5¢UTRs that
serve as barriers for ribosomal scanning Interestingly, such
translational inhibitors often belong to mRNAs encoding
proteins involved in cell growth and differentiation such as
growth factors, receptors, transcription factors,
proto-oncogenes, and cytokines [10] Using the 1022 nucleotide
long 5¢UTR of platelet-derived growth factor (PDGF2) as a
model, we were able to show previously that the
cumber-some 5¢UTR is not a translational inhibitor, but rather a
translational modulator that is sensitive to changes in the
cellular milieu [11] More specifically, the PDGF2 mRNA
leader was shown to mediate efficient translation under
conditions of megakaryocytic differentiation which provide
a permissive milieu for activation of the PDGF2 internal
ribosomal entry site (IRES) [12,13] During the last decade,
several cellular IRES elements have been discovered
(reviewed in [14,15]), although the mechanisms of cellular
IRES-mediated ribosome recruitment remain unclear To
further understand the mechanism of IRES activation, we
wished to assess whether the mechanisms involved in
regulation of global translation during differentiation have
a role in the translational activation of the IRES-containing
mRNA Since internal translation is independent of the
5¢-cap structure of the mRNA, we did not focus our
attention on the changes in the activity of the 5¢-cap binding
protein, eIF4E, during differentiation However, it
remained of interest to determine eIF2a phosphorylation
status during differentiation and to ascertain whether
phosphorylation of eIF2a is involved in IRES activation
In this study we show that: (a) not only are differentiation
conditions permissive for the recruitment of the PDGF2
IRES, but also for the vascular endothelial growth factor
(VEGF), c-Myc and encephalomyocarditis virus (EMCV)
IRES elements, although their mRNAs are not naturally
expressed in differentiated K562 cells; (b) global protein
synthesis rate is reduced during differentiation, correlating
with increased eIF2a phosphorylation that is known to be
important for cap-dependent translational control; and (c)
inhibition of eIF2a phosphorylation during differentiation
reduces the differentiation-induced IRES activation
M A T E R I A L S A N D M E T H O D S
Plasmids
The pLL vector is composed of a fragment containing
Renilla luciferase from pRL-null (Promega) as the first
cistron, fused to a fragment encoding the cytosolic form of
firefly luciferase from pGL3-basic (Promega) as the second
cistron A 22-base pair fragment containing StuI and NcoI
sites separates the stop codon of the Renilla luciferase and
the ATG initiator codon of the firefly luciferase An NheI
(filled)–SacI fragment containing both cistrons was ligated with a HindIII (filled)–SacI fragment of pCL [12] to generate pLL, which contains the dual luciferase transcrip-tion unit downstream of the cytomegalovirus (CMV) promoter and upstream of the SV40 intron and polyade-nylation sites The 5¢UTRs of VEGF and of PDGF2 were obtained as SpeI (filled)–NcoI fragments from pSKVLUC [16] and pCPL [12], respectively, and were ligated to the StuI–NcoI 7.5-kb fragment of pLL to generate pLVL and pLPL, respectively The EMCV IRES fragment was obtained as Alw26I (filled)–NcoI fragment from pTM1 [17], and was ligated to the StuI–NcoI 7.5-kb fragment of pLL to generate pLEL P2 c-Myc 5¢UTR (GeneBank acces-sion # J00120) was generated by RT-PCR using total RNA from K562 cells and the oligonucleotides 5¢-CCCCACTA GTAATTCCAGCGAGAGGCAGA-3¢ and 5¢-AATACC ATGGTCGCGGGAGGCTGCTG-3¢, and was ligated to the StuI–NcoI 7.5-kb fragment of pLL to generate pLML pcK3L was generated by insertion of the NcoI (filled)– BamHI 0.3-kb fragment of pTM1-K3L [18] into the HindIII (filled)–BamHI sites of pcDNA3 (Invitrogen) under the control of the CMV promoter pcPKRD6¼ p68D6-pcD-NAI/NEO [19] was used for expression of PKRD6, the dominant-negative variant of PKR under the control of the CMV promoter pc2a-S51A (expresses eIF2a with Ser51fi Ala mutation) was generated by PCR of the eIF2a cDNA from p51A and p51D [20], respectively, using the oligonucleotides 5¢-CTGGATATCATGCCGGGTCT AAGTTG-3¢ and 5¢-CTGCTCGAGTTAATCTTCAGCT TTGGC-3¢, followed by ligation into the EcoRV–XhoI sites of pcDNA3 (Invitrogen) under the control of the
expressing GFP under the control of the CMV promoter was used as control plasmid for the cotransfection experiments
Cells and megakaryocytic differentiation The human chronic myelogenous leukemia cell line K562 was grown in RPMI 1640 medium (Biological Industries) supplemented with 50 U penicillinÆmL)1, 50 lgÆmL)1 strep-tomycin, 0.1 mgÆmL)1 kanamycin and 10% fetal bovine serum Cells at a density of 5–7· 105 cellsÆmL)1 or 1.2–1.5· 106cellsÆmL)1were considered as logarithmically growing (log) or growth arrested (dense), respectively Megakaryocytic differentiation was induced by dilution of cells at a density of 1.2· 106cellsÆmL)1, to a final concen-tration of 5· 105cellsÆmL)1, with medium containing 5 nM 12-O-tetradecanoylphorbol-13-acetate (TPA; Calbiochem) for 48 h
Plasmid transfections and luciferase assays Twenty or forty micrograms supercoiled DNA of each of the bi-cistronic vectors or the cotransfected plasmid, respectively, were used per 7.5· 106 K562 cells for each electroporation sample Electroporation was performed in 0.8 mL RPMI 1640 without serum by an electric pulse of
240 V and 1500 mF (Easy Ject1 Electroporator; Equibio) Immediately following the electric pulse, the cells were transferred to 10 mL RPMI 1640 medium supplemented with 20% fetal bovine serum for 24 h The cells were diluted
to a final concentration of 5· 105 viable cellsÆmL)1
Trang 3(as determined by Trypan blue staining) in RPMI 1640
supplemented with 10% fetal bovine serum, with or without
5 nMTPA, for 48 h Transfection efficiency was 50–60%, as
judged by the percentage of fluorescent GFP-expressing
cells The control and differentiated transfectants were
harvested simultaneously, and assayed for Renilla and firefly
luciferase activities using the Dual-luciferase reporter assay
system (Promega) and TD-20e-Luminometer (Turner)
RNA was isolated from cells transfected with the bicistronic
constructs and analysed by Northern blotting using a
LUC-specific probe to ensure that transcripts of the correct size
were produced Bi-cistronic transcript level from all
con-structs was approximately five times higher in differentiated
cells because of the increased activity of the CMV promoter
[12]
32Pilabelling, immunoprecipitation and Western analysis
of eIF2a
A total of 106log, dense or megakaryocytic differentiated
K562 cells were washed twice with Hepes/saline buffer
(50 mM KOH/Hepes pH 7.0, 150 mM NaCl) and
resus-pended in 0.5 mL Dulbecco’s modified Eagle medium
lacking sodium phosphate (Sigma) supplemented with 10%
dialysed fetal bovine serum The cells were labelled for 2 h
with 0.2 mCi32PiÆmL)1( Amersham, #PBS13), followed by
two washes with cold NaCl/Pi containing 10 mM
b-glycerophosphate and 50 mMNaF Proteins were
extrac-ted from the cell pellets by using 470 lL lysis buffer
containing 25 mM KOH/Hepes pH 7.2, 0.5% ElugentTM
(Calbiochem), 100 mMKCl, 0.05% SDS, 1 mM
dithiothre-itol, 2 lMokadaic acid, 10 mMb-glycerophosphate, 50 mM
NaF and protease inhibitor cocktail (CompleteTM, Roche)
For immunoprecipitation the sample was supplemented
with 28 lL 5MNaCl and 0.5 lL anti-eIF2a mAb [21] and
incubated for 1 h at 4C Next, rabbit anti-(mouse IgG) Ig
(Jackson Immuno Research) were added for further
incubation of 1 h, followed by addition of 10 lL packed
protein A-Sepharose (Pharmacia Biotech) for an additional
1-h incubation Following separation of the
immunopre-cipitate by SDS/10% PAGE, the proteins were blotted onto
a nitrocellulose membrane and quantified by
phosphoi-mager The membrane was then used for Western analysis
using antibodies specific for Ser51-phosphorylated eIF2a
(Research Genetics, Inc.), and following stripping mAb
specific for total eIF2a were used
Polysome fractionation
A total of 3.5· 107log, dense, or megakaryocytic
differ-entiated K562 cells were treated with 90 lgÆmL)1
cyclohex-imide for 10 min prior to harvest and used for fractionation
of polysomes by sedimentation through 5–47% sucrose
gradients [22]
Protein synthesis rate
One million log, dense, or megakaryocytic differentiated
K562 cells were re-suspended in 2 mL RPMI medium
containing 10% fetal bovine serum The cells were labelled
for 20 min with 20 lCiÆmL)1 [35S]L-methionine, [35S]L
-cysteine mix (NEN, #NEG072), followed by two washes
with cold NaCl/P Proteins were extracted from the cell
pellets using 50 lL lysis buffer containing 25 mM KOH/ Hepes pH 7.5, 1% Triton X-100, 100 mM KCl, 1 mM dithiothreitol, 2 lM okadaic acid, 10 mM b-glycerophos-phate, 50 mMNaF and protease inhibitor cocktail (Com-pleteTM, Roche) Twenty micrograms total protein were applied onto 3 mM filter papers (Whatman) and washed three times for 1 min in boiling 5% (W/V) trichloroacetic acid containing traces of coldL-methionine andL-cysteine The filters were then rinsed once in ethanol, dried and counted in a scintillation counter (Beckman)
Cell cycle analysis and differentiation markers For cell cycle analysis 5· 105cells were harvested, washed with NaCl/Piand re-suspended in 0.5 mL NaCl/Pi contain-ing 0.1% sodium azide Followcontain-ing addition of 50 lL NaCl/
Pi containing 1% Triton X-100 and 50 lL 1 mgÆmL)1 propidium iodide, the cell-cycle of the cells was analysed by Becton Dickinson FACSort, using the Cell Quest software The Vav protein was used as a marker for differentiation To detect Vav protein level the cells were lysed using a buffer containing 25 mMKOH/Hepes pH 7.5, 1% Triton X-100,
100 mM KCl, 1 mM dithiothreitol, 2 lM okadaic acid,
10 mM b-glycerophosphate, 50 mM NaF and protease inhibitors cocktail (CompleteTM, Roche) 140 lg of total cell proteins were separated by SDS/10% PAGE, and blotted onto a nitrocellulose membrane which was then used for Western analysis using polyclonal antibodies specific for Vav (Santa Cruz) and polyclonal antibodies specific for CKIIa (a gift from D Canaani, Tel Aviv University, Israel)
R E S U L T S
Favorable conditions for IRES-mediated translation are established during differentiation
In previous studies we have demonstrated that in addition to transcriptional activation of PDGF2 during megakaryocytic differentiation, its IRES element undergoes functional activation during the differentiation process [12,13] Interes-ted in elucidating the mechanism of IRES function in general, we have used the differentiation phase to learn more about the possible involvement of trans-acting factors Viewing the differentiated state as a permissive environment for PDGF2-IRES mediated translation, we also wished to check the effect of differentiation conditions on the beha-viour of additional cellular and viral IRES elements Although normally the mRNAs of VEGF and c-Myc are not present in differentiated megakaryocytes, it was still of interest to check the activity of their IRES elements in the differentiated K562 cells that are permissive for PDGF2 IRES use The IRES elements of human VEGF, human c-Myc and EMCV were cloned into a CMV promoter-driven bicistronic vector, between the coding regions of Renilla and firefly luciferases, as illustrated in Fig 1 K562 cells were transfected with each of the recombinant plasmids followed
by incubation under normal or differentiation conditions for
48 h prior to measurements of Renilla and firefly luciferase enzymatic activities As shown in Table 1, in differentiated cells we observed elevation in the activity of both luciferases, because of increased CMV promoter activity in this system which results in a fivefold increase in transcript levels (demonstrated in Fig 3 [12]) However, upon differentiation,
Trang 4the utilization of the first cistron increased only 2.2–2.7-fold
whereas a 6–7.9-fold increase in utilization of the
IRES-mediated second cistron was observed This was in contrast
with utilization of the second cistron from the IRES-less pLL
vector A 2.2- to 3.1-fold increase in the firefly per Renilla
ratio was detected upon differentiation from the
IRES-containing vectors, in contrast with the 0.8-fold increase
observed for the IRES-less transcript from pLL
Reduction of global protein synthesis
during differentiation is accompanied
by eIF2a phosphorylation
The terminal differentiation process is usually accompanied
by arrest of cellular proliferation and by decreased global
protein synthesis [7,8] As megakaryocytic differentiated
cells cease to proliferate [23], we wished to check the status
of their global mRNA translation The rate of
radio-labelled amino acids incorporation in logarithmically
growing cells was compared to that in density-arrested or
differentiated cells The incorporation rate was almost two
times lower in both stationary (dense) and differentiated
cells compared with logarithmically growing cells (Fig 2A)
The decrease in global protein synthesis was also evident
from the differences in the polysomal profiles of the above
cells Fig 2B demonstrates the reduced heavy polysomes levels upon growth arrest due to high density or differen-tiation
As the rate of protein synthesis in higher eukaryotes is commonly regulated at the level of eIF2a phosphorylation [4,24], we wished to check the status of eIF2a phosphory-lation in cells undergoing differentiation For this purpose, the cells were metabolically labelled with 32Pifollowed by immunoprecipitation using antibodies specific for eIF2a Fig 3A shows the radio-labelled phosphorylated eIF2a, the total amount of eIF2a as determined by Western analysis, and the ratio of phosphorylated eIF2a per total eIF2a In dense cells, in which growth arrest was probably induced by depletion of essential nutrients/growth factors in the medium, a two-fold increase in eIF2a phosphorylation was observed compared to logarithmically growing cells However, a more significant, 6.4-fold increase in eIF2a phosphorylation was detected upon growth arrest induced
by the differentiation process As the regulated phosphory-lation of mammalian eIF2a has been shown to occur only
on Ser51 [25], we re-confirmed the32Pilabelling results by using specific antibodies against phosphorylated Ser51 Fig 3B shows the phosphorylated Ser51 and the total eIF2a levels as determined by Western analysis In agreement with the labelling studies, the ratio of Ser51-phosphorylated eIF2a to total eIF2a revealed an increase of 3.3-fold and 7.9-fold in eIF2a phosphorylation level in dense and differentiated cells, respectively, compared with logarith-mically growing K562 cells In summary, growth arrest is accompanied by elevation of eIF2a phosphorylation However, megakaryocytic differentiation involves two- to threefold higher eIF2a phosphorylation than that resulting from growth arrest that is induced by increased cell density
Increased IRES-mediated translation during differentiation requires eIF2a phosphorylation The phenomenon of increased IRES-mediated translation under conditions of increased eIF2a phosphorylation, raised the notion that eIF2a phosphorylation confers a translational advantage on IRES-containing mRNAs To test this hypothesis, we looked at the effect of expression of eIF2a phosphorylation inhibitors on IRES use We used either the vaccinia virus K3L gene that encodes an eIF2a homologue and pseudo-substrate inhibitor of eIF2a protein
Fig 1 The bicistronic transcription units used The bicistronic
tran-scription unit expressing Renilla and Firefly luciferase reporter genes as
the first and second cistrons, respectively, under the control of CMV
promoter The 5¢UTRs of the human PDGF2, VEGF, c-Myc or
EMCV were placed in the intercistronic space as indicated to create
plasmids pLPL, pLVL, pLML and pLEL, respectively The bicistronic
IRES-less vector pLL served as a control plasmid.
Table 1 Effect of differentiation on IRES activity Each of the bicistronic plasmids harbouring the IRES elements indicated in Fig 1 was transfected into K562 cells followed by further incubation under nondifferentiation or differentiation conditions and subsequent analysis of Renilla (R) and Firefly (F) luciferase activity Each value represents the mean ± SE of three independent experiments The fold induction values represent the F/R ratio in differentiated cells relative to the F/R ratio in nondifferentiated cells.
Non-differentiated cells Differentiated cells
Fold F/R induction Renilla
(U per 10 6 cells)
Firefly (U per 10 6 cells) F/R
Renilla (U per 10 6 cells)
Firefly (U per 10 6 cells) F/R
pLL 52 ± 7 0.6 ± 0.08 0.01 ± 0.002 125 ± 15 1.0 ± 0.16 0.008 ± 0.001 0.8 ± 0.2
Trang 5kinases [18,26], or PKRD6, a dominant-negative variant of
the dsRNA activated eIF2a-kinase, PKR [19] To confirm
the connection between eIF2a phosphorylation during
differentiation and IRES activation, we also used a plasmid
encoding a variant form of eIF2a in which Ser51 is replaced
by an alanine residue (eIF2a-S51A) As this variant protein
cannot undergo phosphorylation, it serves as a competitor that reduces the translational inhibitory effect of phosphor-ylated endogenous wild-type eIF2a [20] Plasmids expres-sing K3L, PKRD6, eIF2a-S51A variant or GFP as control, were cotransfected along with the different IRES-containing bi-cistronic vectors into K562 cells followed by their incubation under normal or differentiation conditions for
48 h prior to measurements of Renilla and firefly luciferase enzymatic activities Tables 2 and 3 show the effects of the transfected gene products on the absolute levels of the translation products of both cistrons Overexpression of K3L, PKRD6 or eIF2a-S51A led to enhanced translation of both cistrons in nondifferentiated cells, whereas in differen-tiated cells it led to decreased IRES-mediated translation of the second cistron Fig 4A summarizes the sensitivity of the differentiation-induced IRES activation to the various eIF2a phosphorylation inhibitors Expression of eIF2a-S51A, K3L, or PKRD6 in differentiated cells reduced the level of eIF2a-P to 80%, 70% or 40% compared with GFP-transfected control, respectively (Fig 4B) The effect of the various transfections on eIF2a phosphorylation is underes-timated as not all the cells were successfully transfected The reduction in IRES use in differentiated cells by expression of K3L, eIF2a-S51A and PKRD6 was shown to be correlated with a reduction in the level of eIF2a-P These data suggest that eIF2a phosphorylation is required for more efficient IRES use during the differentiation process
Fig 3 Phosphorylation of eIF2a in logarithmically growing, dense and differentiated K562 cells (A) A total of 106logarithmically growing (Log), density-induced growth arrested (Dense) or differentiated (Dif f ) K562 cells were metabolically labelled with 32 Pi, followed by immunoprecipitation using an antibody specific for eIF2a The immunoprecipitated phospholabeled proteins were separated by SDS/ 10% PAGE and blotted onto a nitrocellulose membrane Phosphor-ylated eIF2a was observed following exposure of the membrane to an X-ray film and the intensities of the bands were determined using a phosphoimager The same membrane was analysed for total eIF2a level by Western analysis and the intensities of the bands were deter-mined by densitometry The [32P]eIF2a/eIF2a ratio in logarithmically growing cells was set as 1 (B) Fifty lg total protein extract from Log, Dense or Diff K562 cells were separated by 10% SDS/PAGE and blotted onto a nitrocellulose membrane Phosphorylated eIF2a was detected using antibodies specific for phosphorylated Ser51 The same membrane was stripped and used for Western analysis using antibodies specific for total eIF2a The eIF2a-P/eIF2a ratio in logarithmically growing cells was set as 1.
Fig 2 The effect of differentiation on the overall protein synthesis level.
(A) Logarithmically growing (Log), density-induced growth arrested
(Dense) or differentiated (Diff) K562 cells were metabolically labelled
with [ 35 S] L -methionine and [ 35 S] L -cysteine followed by determination
of their incorporation level by trichloro-acetic acid precipitation, as
described in Materials and methods The incorporation level (cpmÆlg)1
protein) in log cells was termed 100% The values are means ± SE of
three independent experiments (B) A total of 3.5 · 10 7 log, dense or
differentiated K562 cells were harvested and their cytoplasmic
com-partments were subjected to fractionation on linear 5–47% sucrose
gradients Relative absorbance at 260 nm was monitored continuously
as the gradient was collected The vertical bars on the abscissa indicate
the boundaries of the polysomal (P) and subpolysomal (SP) fractions.
Peaks at the top of the gradient containing the 40 S, 60 S and 80 S
ribosomal subunits are indicated.
Trang 6Reduced phosphorylation of eIF2a has no significant
effect on megakaryocytic differentiation
The differentiation process is a cascade of events leading to
major changes in gene expression Reduced global protein
synthesis is a consequence of upstream events, as inhibition
of protein synthesis per se does not lead to differentiation
However, it seemed important to ascertain the effect of
reduced eIF2a phosphorylation level on the differentiation
process As megakaryocytic differentiation involves growth
arrest and polyploidy [27], DNA content evaluation by flow
cytometry was chosen as a tool to detect the reduced
number of cells in S-phase and enhanced number of cells
harbouring two- to fourfold higher DNA content [23,28]
K562 cells were transfected with plasmids expressing K3L,
PKRD6, eIF2a-S51A, or GFP under similar conditions to
those used to assess the effect of inhibition of eIF2a
phosphorylation on IRES activity The transfection
effi-ciency in these experiments was 50–60%, as judged by the
percentage of fluorescent GFP-expressing cells As shown in
Fig 5A, the differentiation process was not significantly
affected by any of the transfected plasmids, as judged by the
decreased number of cells in S-phase and increased number
of polyploid cells For additional confirmation we checked
the level of the Vav proto-oncogene, which is known to
increase early during megakaryocytic differentiation [27,29]
CKIIa protein level was used as a control As shown in
Fig 5B, the level of Vav protein was increased due to
differentiation, regardless of the transfected plasmid These
results suggest that interference with eIF2a phosphorylation does not prevent the early differentiation steps, e.g the global changes in gene expression upstream of mRNA translation Instead, it interferes with the ability to fine-tune the translation efficiency of specific mRNA groups
D I S C U S S I O N
Cells undergoing terminal differentiation exhibit extensive changes in the pattern of gene expression Much data has been accumulated regarding transcriptional regulation, but less is known about the mechanisms that inhibit the translation of most transcripts while activating the transla-tion of specific mRNAs during the course of differentiatransla-tion During the early developmental stages of Xenopus, Caenor-habditis elegansand Drosophila, the translation of subclasses
of mRNAs is regulated However, during differentiation of mammalian cells, only a few individual mRNAs are known
to be subjected to translational regulation due to their cis-regulatory elements (reviewed in [30]) Initial attempts to identify groups of translationally regulated genes during HL60 cell differentiation towards monocytes/macrophages has revealed that while most mRNAs are released from polysomes early in the differentiation process, a subset of transcripts is retained or even mobilized onto polysomes [7] The data presented in this study suggest that mRNAs harbouring an IRES within long, structured, uORF-burdened 5¢UTRs, comprise a subgroup which is specific-ally translationspecific-ally activated during differentiation (Fig 1
Table 3 Effect of eIF2a-S51A expression on IRES activity Absolute values of Renilla and Firefly activities from experiments described in Fig 4A.
Non-differentiated cells Differentiated cells Renilla
(U per 106cells)
Firefly (U per 106cells) F/R
Renilla (U per 106cells)
Firefly (U per 106cells) F/R
Fold F/R induction pLPL + GFP 87 ± 6 7.8 ± 1.4 0.09 ± 0.02 184 ± 20 59 ± 5 0.32 ± 0.08 3.5 ± 0.3 + eIF2a-S51A 145 ± 11 14 ± 2 0.1 ± 0.02 200 ± 18 42 ± 3 0.2 ± 0.02 2.1 ± 0.2 pLVL + GFP 105 ± 11 42 ± 5 0.4 ± 0.06 273 ± 33 382 ± 37 1.4 ± 0.2 3.5 ± 0.2 + eIF2a-S51A 180 ± 20 72 ± 8 0.4 ± 0.1 304 ± 31 273 ± 28 0.9 ± 0.1 2.2 ± 0.2
Table 2 Effect of K3L and PKRD6 expression on IRES activity Absolute values of Renilla and Firefly activities from experiments described in Fig 4A.
Non-differentiated cells Differentiated cells
Fold F/R induction Renilla
(U per 10 6 cells)
Firefly (U per 10 6 cells) F/R
Renilla (U per 10 6 cells)
Firefly (U per 10 6 cells) F/R pLPL + GFP 53 ± 3 6.4 ± 0.6 0.12 ± 0.02 125 ± 17 50 ± 6 0.4 ± 0.05 3.2 ± 0.2
+ PKRD6 132 ± 12 17 ± 2 0.13 ± 0.02 213 ± 16 37 ± 4 0.17 ± 0.03 1.2 ± 0.15 pLML + GFP 101 ± 12 7.3 ± 1.1 0.07 ± 0 235 ± 23 38 ± 14 0.16 ± 0.02 2.3 ± 0.3 + K3L 303 ± 27 20 ± 3.5 0.06 ± 0.01 318 ± 27 34 ± 3.6 0.1 ± 0.02 1.6 ± 0.2 + PKRD6 222 ± 25 22 ± 3 0.1 ± 0.03 286 ± 28 22 ± 3 0.08 ± 0.01 0.8 ± 0.1 pLVL + GFP 100 ± 11 52 ± 7 0.5 ± 0.1 253 ± 32 368 ± 40 1.43 ± 0.2 2.9 ± 0.2 + K3L 260 ± 21 130 ± 10 0.5 ± 0 390 ± 38 286 ± 20 0.73 ± 0.08 1.5 ± 0.15 + PKRD6 186 ± 20 120 ± 15 0.6 ± 0.1 300 ± 30 191 ± 5 0.6 ± 0.1 1.0 ± 0.1 pLEL + GFP 51 ± 6 2.0 ± 0.9 0.04 ± 0.01 226 ± 25 31 ± 5 0.13 ± 0.01 3.2 ± 0.4 + K3L 228 ± 24 8.5 ± 0.3 0.04 ± 0.01 330 ± 5 33 ± 4 0.11 ± 0.03 2.7 ± 0.25 + PKRD6 98 ± 10 5.7 ± 0.5 0.06 ± 0.01 192 ± 20 17 ± 3.3 0.09 ± 0.01 1.5 ± 0.2
Trang 7and Table 1) under conditions of eIF2a phosphorylation
and substantial inhibition of protein synthesis (Figs 2 and
3) Other recent studies demonstrate a correlation between
differentiation with reduction of global protein synthesis
and enhanced eIF2a phosphorylation [31–34]
The list of recently identified IRES elements within
cumbersome 5¢UTRs of growth factors, cytokines,
tran-scription factors and oncogenes is constantly growing
(reviewed in [14,15]) Cellular IRES elements have been
implied to confer a translational advantage under reduced
levels of active 5¢-cap binding complex However, the
current study shows that under certain physiological
conditions, for instance during differentiation, translation
mediated by cellular IRES elements benefits from
phos-phorylation of eIF2a Supplementary mechanisms for
inhibition of global protein synthesis, such as reduced
availability of the 5¢-cap binding complex, may also take
place during differentiation and contribute to the observed enhancement of 5¢-cap independent translation However, such mechanisms were beyond the focus of this study Phosphorylation of eIF2a which leads to decreased binding
of initiator tRNA to the small ribosomal subunit, has been mostly studied in relation to growth inhibition induced in response to starvation for growth factors/nutrients, heat shock, and virus infection (reviewed in [35]) PKR, the interferon-induced (double-stranded) RNA-activated eIF2a kinase, has been implicated in cellular growth control, as well as in differentiation and apoptosis (reviewed in [36–39])
It seems likely that the regulatory function of eIF2 depends
on the delicate balance of phosphorylated eIF2a with other cellular components, and on the physiological status of the cell The data presented in this study support this idea Inhibition of PKR activity by over-expression of its dominant-negative variant PKRD6, or reduction of eIF2a phosphorylation level by over-expression of its variant form, eIF2a-S51A, or the pseudosubstrate K3L, resulted in reduced IRES activity in differentiated cells However, it did not have any inhibitory effect on IRES activity in nondif-ferentiated cells (Fig 4) The fact that expression of PKRD6 had a greater impact on the levels of eIF2a phosphorylation and IRES-mediated translation compared with the efficient general eIF2a kinase inhibitor K3L suggests that PKR is the primary activated kinase during differentiation Interest-ingly, the over-expression of the eIF2a phosphorylation
Fig 4 Effect of eIF2a phosphorylation inhibitors on IRES activity (A)
Each of the bicistronic vectors pLPL, pLVL, pLML, pLEL (described
in Fig 1A) harbouring the IRES elements of PDGF2, VEGF, c-Myc
or EMCV, respectively, was cotransfected into K562 cells together
with a plasmid expressing the PKRD6, K3L, eIF2a Ser51 fi Ala
mutant (pc2a-S51A), or GFP coding region from the CMV promoter.
The cells were further incubated under normal or differentiation
con-ditions for 48 h and subsequently analysed for Renilla (R) and firefly
(F) luciferase activity The absolute values are presented in Tables 2
and 3 Each value represents the mean ± SE of three independent
experiments The fold induction values represent the F/R ratio in
differentiated cells relative to the F/R ratio in nondifferentiated cells.
The graph demonstrates the effect of K3L (stippled bars), PKRD6
(dark bars), or eIF2a-S51A (hatched bars) on the
differentiation-induced IRES activation relative to the fold induction value with GFP
that was set as 100% (light bars) (B) Fifty lg of total protein extract
from differentiated cells transfected with plasmids expressing GFP,
K3L, PKRD6 or eIF2a-S51A were separated by 10% SDS/PAGE and
blotted onto a nitrocellulose membrane Phosphorylated eIF2a was
detected using antibodies specific for phosphorylated Ser51 The same
membrane was stripped and used for Western analysis using antibodies
specific for total eIF2a The eIF2a-P/eIF2a ratio in GFP-transfected
cells was set as 1.
Fig 5 The effect of eIF2a phosphorylation inhibitors on the differenti-ation process (A) A total of 5 · 10 5 K562 cells were transfected by electroporation with a plasmid expressing GFP, eIF2a-S51A, K3L or PKRD6, as indicated The transfected cells were incubated under normal (–TPA) or differentiation (+ TPA) conditions for 48 h, and subjected to DNA content analysis by flow cytometry (B) One-hun-dred and forty lg of total protein extracted from the transfected cells as detailed in ( A) were separated by SDS/10% PAGE, blotted onto a nitrocellulose membrane, and subjected to Western analysis using antibodies specific for Vav and for CKIIa.
Trang 8inhibitors did not interfere with the global process of
megakaryocytic differentiation as judged by their
morphol-ogy (not shown), cell-cycle, and enhanced Vav protein
expression (Fig 5) The latter result suggests that both
eIF2a phosphorylation and IRES activation are late events
during the differentiation process The peak of PDGF2
IRES activation at 48 h after induction of differentiation
[11,12] is in agreement with this notion It is therefore
conceivable that eIF2a phosphorylation serves to fine-tune
the translation efficiency of specific mRNA groups
Increased translation of certain IRES-containing
mRNAs has also been implicated in apoptosis [40–43], a
cellular process that includes activation of eIF2a
phos-phorylation [44–47] Furthermore, the recently discovered
cell cycle-dependent IRES elements are activated specifically
at the G2/M boundary [48–50], when increased
phosphory-lation of eIF2a is found in correphosphory-lation with decreased
overall rate of protein synthesis [51] Moreover, the IRES
elements of the amino acid transporter protein cat-1 and
c-Myc mRNAs have recently been shown to function
efficiently where there is an increase in eIF2a
phosphory-lation, under conditions of amino acid starvation and
genotoxic stress, respectively [52,53]
What is the mechanism underlying IRES-mediated
translation under conditions of enhanced eIF2a
phosphory-lation? In nondifferentiated cells, in which global translation
is active, the IRES-containing mRNAs compete with the
cap-dependent mRNAs for the translation machinery The
decrease in global protein synthesis and reduced
competi-tion might be beneficial for IRES-mediated translacompeti-tion
during differentiation An interesting possibility may be the
ability of IRES elements to direct efficient translation
initiation in the absence of eIF2 and Met-tRNAi Recently,
internal initiation without Met-tRNAihas been
demonstra-ted in two picorna-like insect viruses, Plautia stali intestinal
virus and Cricket paralysis virus [54–56] It is an open
question whether cellular IRES elements known to contain
conserved secondary and tertiary structural motifs can also
direct internal translation from noncognate initiation
codons in the absence of Met-tRNAi Direct binding to
the 40 S ribosomal subunit followed by joining of the 60 S
subunit may provide a significant advantage to IRES
elements that confer efficient translation under conditions of
global translation inhibition mediated by eIF2a
phosphory-lation Another possibility could be that a mechanism exists
that is similar to the translational regulation of GCN4 in
yeast In this case, the induction of GCN4 translation in
response to eIF2a phosphorylation is modulated by four
short uORFs in the 5¢UTR Reduced rates of ternary
complex formation leads to bypass of the uORFs and
initiation at the downstream GCN4 major ORF [6] A
comparable mechanism of translational regulation was
recently demonstrated for the stress-induced transcription
factor ATF4, in mammalian cells under conditions of
enhanced eIF2a phosphorylation due to stress [57]
Simi-larly, the short ORFs that furnish many of the cellular IRES
elements may have a role in translational regulation, as
seems to be the case for the activation of the cat-1 mRNA
[52] Internal ribosomal binding upstream of the translation
initiator codon may be followed by subsequent scanning to
the initiation codon For instance, in the case of the PDGF2
5¢UTR, which contains three uORFs, the IRES has been
mapped to the central part of the 5¢UTR at the vicinity of
the first uORF [13] The need to scan through the second and third uORFs towards the major coding region cannot
be ruled out at this point Another possibility may be that eIF2a phosphorylation induces the synthesis of a protein that interacts with the IRES This can be achieved by direct translational regulation of its mRNA akin to GCN4/ATF4 mRNAs, or by regulating the translation of a GCN4/ ATF4-like transcription factor that activates the transcrip-tion of the potential IRES activator Current experiments are designed to elucidate the mechanism(s) by which eIF2a phosphorylation serves to enhance IRES-mediated transla-tion
A C K N O W L E D G E M E N T S
This work was supported by the Israel Science Foundation adminis-tered by the Academy of Sciences and Humanities – the Charles H Revson Foundation to O E S., by a grant from the Israeli Chief Scientist’s Office of the Ministry of Health to O E S and NSF 9808401
to R J We thank B White and G Krause for the antibody against phosphorylated eIF2a, D Canaani for antibody against CKIIa,
N Sonenberg for PKRD6 construct, R J Kaufman for eIF2a-S51A construct, and B.-Z Levi for the pSKVLUC construct We are grateful
to T Dever for comments on the manuscript.
R E F E R E N C E S
1 Dever, T.E (1999) Translation initiation: adept at adapting Trends Biochem Sci 24, 398–403.
2 Mathews, M.B., Sonenberg, N & Hershey, J.W.B (2000) Origins and principles of translational control In Translational Control of Gene Expression (Sonenberg, N., Hershey, J.W.B & Mathews M.B., eds), pp 1–32 Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York.
3 Pain, V.M (1996) Initiation of protein synthesis in eukaryotic cells Eur J Biochem 236, 747–771.
4 Hinnebusch, A.G (2000) Mechanism and regulation of initiator methionyl-tRNA binding to ribosomes In Translational Control
of Gene Expression (Sonenberg, N., Hershey, J.W.B & Mathews M.B., eds), 185–244 Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York.
5 Schneider, R.J (2000) Translational control during heat shock In: Translational Control of Gene Expression (Sonenberg, N., Hershey, J.W.B & Mathews M.B., eds), pp 581–594 Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York.
6 Hinnebusch, A.G (1996) Translational control of GCN4: gene-specific regulation by phosphorylation of eIF2 In Translational Control (Hershey, J.W.B., Mathews, M.B & Sonenberg, N., eds),
pp 199–244 Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York.
7 Krichevsky, A.M., Metzer, E & Rosen, H (1999) Translational control of specific genes during differentiation of HL-60 cells.
J Biol Chem 274, 14295–14305.
8 Meyuhas, O (2000) Synthesis of the translational apparatus is regulated at the translational level Eur J Biochem 267, 6321– 6330.
9 Thiele, B.J., Berger, M., Huth, A., Reimann, I., Schwarz, K & Thiele, H (1999) Tissue-specific translational regulation of alter-native rabbit 15-lipoxygenase mRNAs differing in their 3¢-untranslated regions Nucleic Acids Res 27, 1828–1836.
10 Gray, N.K & Wickens, M (1998) Control of translation initiation
in animals Annu Rev Cell Dev Biol 14, 399–458.
11 Bernstein, J., Shefler, I & Elroy-Stein, O (1995) The translational repression mediated by the platelet-derived growth factor 2/c-sis mRNA leader is relieved during megakaryocytic differentiation.
J Biol Chem 270, 10559–10565.
Trang 912 Bernstein, J., Sella, O., Le, S.Y & Elroy-Stein, O (1997) PDGF2/
c-sis mRNA leader contains a differentiation-linked internal
ribosomal entry site (D-IRES) J Biol Chem 272, 9356–9362.
13 Sella, O., Gerlitz, G., Le, S.Y & Elroy-Stein, O (1999)
Differ-entiation-induced internal translation of c-sis mRNA: analysis of
the cis elements and their differentiation-linked binding to the
hnRNP C protein Mol Cell Biol 19, 5429–5440.
14 Carter, M.S., Kuhn, K.M & Sarnow, P (2000) Cellular internal
ribosomal entry site (IRES) elements and the use of cDNA
microarrays in their investigation In Translational Control of Gene
Expression (Sonenberg, N., Hershey, J.W.B & Mathews, M.B.,
eds), pp 615–636 Cold Spring Harbor Laboratory Press, Cold
Spring Harbor, New York.
15 Hellen, C.U.T & Sarnow, P (2001) Internal ribosomal entry sites
in eukaryotic mRNA molecules Genes Dev 15, 1593–1612.
16 Akiri, G., Nahari, D., Finkelstein, Y., Le, S.Y., Elroy-Stein, O.
& Levi, B.Z (1998) Regulation of vascular endothelial growth
factor (VEGF) expression is mediated by internal initiation of
translation and alternative initiation of transcription Oncogene
17, 227–237.
17 Moss, B., Elroy-Stein, O., Mizukami, T., Alexander, W.A &
Fuerst, T.R (1990) New mammalian expression vectors Nature
348, 91–92.
18 Carroll, K., Elroy-Stein, O., Moss, B & Jagus, R (1993)
Recombinant vaccinia virus K3L gene product prevents activation
of dsRNA-dependent, eIF-2a-specific protein kinase J Biol.
Chem 268, 12837–12842.
19 Koromilas, A.E., Roy, S., Barber, G.N., Katze, M.G &
Sonen-berg, N (1992) Malignant transformation by a mutant of the
IFN-inducible dsRNA-dependent protein kinase Science 257,
1685–1689.
20 Kaufman, R.J., Davies, M.V., Pathak, V.K & Hershey, J.W.B.
(1989) The phosphorylation state of eukaryotic initiation factor 2
alters translational efficiency of specific mRNAs Mol Cell Biol 9,
946–958.
21 Scorsone, K.A., Panniers, R., Rowlands, A.G & Henshaw, E.C.
(1987) Phosphorylation of eukaryotic initiation factor 2 during
physiological stresses which affect protein synthesis J Biol Chem.
262, 14538–14543.
22 Meyuhas, O., Thompson, E.A & Perry, R.P (1987)
Glucocorti-coids selectively inhibit translation of ribosomal protein mRNA in
P1798 lymphosarcoma cells Mol Cell Biol 7, 2691–2699.
23 Hoffman, R (1989) Regulation of megakaryocytopoiesis Blood
74, 1196–1212.
24 Hershey, J.W.B & Merrick, W.C (2000) Initiation of Protein
Synthesis In Translational Control of Gene Expression
(Sonen-berg, N., Hershey, J.W.B & Mathews, M.B., eds), pp 33–88 Cold
Spring Harbor Laboratory Press, Cold Spring Harbor,
New York.
25 Colthurst, D.R., Campbell, D.G & Proud, C.G (1987) Structure
and regulation of eukaryotic initiation factor eIF-2 Sequence of
the site in the alpha subunit phosphorylated by the
haem-con-trolled repressor and by the double-stranded RNA-activated
inhibitor Eur J Biochem 166, 357–363.
26 Davies, M.V., Elroy-Stein, O., Jagus, R., Moss, B & Kaufman,
R.J (1992) The vaccinia virus K3L gene product potentiates
translation by inhibiting double-stranded RNA-activated protein
kinase and phosphorylation of the alpha subunit of eukaryotic
initiation factor 2 J Virol 66, 1943–1950.
27 Nagata, Y., Nagahisa, H., Nagasawa, T & Todokoro, K (1997)
Regulation of megakaryocytopoiesis by thrombopoietin and
stromal cells Leukemia 11, 435–438.
28 Cavalloni, G., Dane, A., Piacibello, W., Bruno, S., Lamas, E.,
Brechot, C & Aglietta, M (2000) The involvement of human-nuc
gene in polyploidization of K562 cell line Exp Hematol 28, 1432–
1440.
29 Bustelo, X.R., Rubin, S.D., Suen, K.L., Carrasco, D & Barbacid,
M (1993) Developmental expression of the vav protooncogene Cell Growth Diff 4, 297–308.
30 Wickens, M., Goodwin, E., Kimble, J., Stickland, S & Hentze, M.W (2000) Translational control of developmental decisions In Translational Control of Gene Expression (Hershey, J.W.B., Mathews, M.B & Sonenberg, N., eds), pp 295–370 Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York.
31 Aroor, A.R., Singh, L.P & Wahba, A.J (1995) Hexamethylene bisacetamide-induced differentiation of Friend virus-transformed murine erythroleukemia cells is associated with parallel changes is casein kinase II and guanine nucleotide exchange factor activities Exp Hematol 23, 1204–1211.
32 Hensold, J.O., Barth-Baus, D & Stratton, C.A (1996) Inducers of erythroleukemic differentiation cause mRNAs that lack poly (A)-binding protein to accumulate in translationally inactive, salt-labile 80S ribosomal complexes J Biol Chem 271, 23246–23254.
33 Salzberg, S., Vilchik, S., Cohen, S., Heller, A & Kronfeld-Kinar,
Y (2000) Expression of a PKR dominant-negative mutant in myogenic cells interferes with the myogenic process Exp Cell Res 254, 45–54.
34 Woldehawariat, G., Nekhai, S & Petryshyn, R (1999) Differential phosphorylation of PKR associates with deregulation of eIF-2alpha phosphorylation and altered growth characteristics in 3T3-F442A fibroblasts Mol Cell Biochem 198, 7–17.
35 Rhoads, R.E (1999) Signal transduction pathways that regulate eukaryotic protein synthesis J Biol Chem 274, 30337–30340.
36 Barber, G.N (2000) The interferons and cell death: guardians of the cell or accomplices of apoptosis Semin Cancer Biol 10, 103– 111.
37 Jagus, R., Joshi, B & Barber, G.N (1999) PKR, apoptosis and cancer Int J Biochem Cell Biol 31, 123–138.
38 Kaufman, R.J (2000) Double-stranded RNA-activated protein kinase, PKR In Translational Control of Gene Expression ( Sonenberg, N., Hershey, J.W.B & Mathews, M.B., eds), pp 503–
527 Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York.
39 Kronfeld-Kinar, Y., Vilchik, S., Hyman, T., Leibkowicz, F & Salzberg, S (1999) Involvement of PKR in the regulation of myogenesis Cell Growth Diff 10, 201–212.
40 Coldwell, M., Mitchell, S., Stoneley, M., MacFarlane, M & Willis, A.E (2000) Initiation of Apaf-1 translation by internal ribosome entry Oncogene 19, 899–905.
41 Henis-Korenblit, S., Levi-Strumpf, N., Goldstaub, D & Kimchi,
A (2000) A novel form of DAP5 protein accumulates in apoptotic cells as a result of caspase cleavage and internal ribosome entry-site-mediated translation Mol Cell Biol 20, 496–506.
42 Holcik, M., Lefebvre, C., Yeh, C., Chow, T & Korneluk, R.G (1999) A new internal-ribosomal-entry-site motif potentiates XIAP-mediated cytoprotection Nature Cell Biol 1, 190–192.
43 Stoneley, M., Chappell, S.A., Jopling, C.L., Dickens, M., Mac-Farlane, M & Willis, A.E (2000) c-Myc protein synthesis is initiated from the internal ribosome entry segment during apop-tosis Mol Cell Biol 20, 1162–1169.
44 Balachandran, S., Kim, C.N., Yeh, W.C., Mak, T.W., Bhalla, K.
& Barber, G.N (1998) Activation of the dsRNA-dependent pro-tein kinase, PKR, induces apoptosis through FADD-mediated death signaling EMBO J 17, 6888–6902.
45 Clemens, M.J., Bushell, M., Jeffrey, I.W., Pain, V.M & Morley, S.J (2000) Translation initiation factor modifications and the regulation of protein synthesis in apoptotic cells Cell Death Diff.
7, 603–615.
46 Gil, J., Alcami, J & Esteban, M (1999) Induction of apoptosis by double-stranded-RNA-dependent protein kinase (PKR) involves the alpha subunit of eukaryotic translation initiation factor 2 and NF-kappaB Mol Cell Biol 19, 4653–4663.
Trang 1047 Srivastava, S.P., Kumar, K.U & Kaufman, R.J (1998)
Phos-phorylation of eukaryotic translation initiation factor 2 mediates
apoptosis in response to activation of the double-stranded
RNA-dependent protein kinase J Biol Chem 273, 2416–2423.
48 Cornelis, S., Bruynooghe, Y., Denecker, G., van Huffel, S.,
Tinton, S & Beyaert, R (2000) Identification and characterization
of a novel cell cycle-regulated internal ribosome entry site Mol.
Cell 5, 597–605.
49 Honda, M., Kaneko, S., Matsushita, E., Kobayashi, K., Abell,
G.A & Lemon, S.M (2000) Cell cycle regulation of hepatitis C
virus internal ribosomal entry site-directed translation
Gastro-enterology 118, 152–162.
50 Pyronnet, S., Pradayrol, L & Sonenberg, N (2000) A
cell-cycle-dependent internal ribosome entry site Mol Cell 5, 607–616.
51 Datta, B., Datta, R., Mukherjee, S & Zhang, Z (1999) Increased
phosphorylation of eukaryotic initiation factor 2 alpha at the
G2/M boundary in human osteosarcoma cells correlates with
deglycosylation of p67 and a decreased rate of protein synthesis.
Exp Cell Res 250, 223–230.
52 Fernandez, J., Yaman, I., Merrick, W.C., Koromilas, A., Wek,
R.C., Sood, R., Hensold, J & M.Hatzoglou (2001) Regulation of
internal ribosomal entry site-mediated translation by eIF2a phosphorylation and translation of small uORF J Biol Chem.
277, 2050–2058.
53 Subkhankulova, T., Mitchell, S.A & Willis, A.E (2001) Internal ribosomal entry segment–mediated initiation of c-Myc pro-tein synthesis following genotoxic stress Biochem J 359, 183– 192.
54 Sasaki, J & Nakashima, N (2000) Methionine-independent initiation of translation in the capsid protein of an insect RNA virus Proc Natl Acad Sci USA 97, 1512–1515.
55 Wilson, J.E., Powell, M.J., Hoover, S.E & Sarnow, P (2000) Naturally occurring dicistronic cricket paralysis virus RNA is regulated by two internal ribosome entry sites Mol Cell Biol 20, 4990–4999.
56 Wilson, J.E., Pestova, T.V., Hellen, C.U.T & Sarnow, P (2000) Initiation of protein synthesis from the A site of the ribosome Cell
102, 511–520.
57 Harding, H.P., Novoa, I., Zhang, Y., Zeng, H., Wek, R., Schapira, M & Ron, D (2000) Regulated translation initiation controls stress-induced gene expression in mammalian cells Mol Cell 6, 1099–1108.