Báo cáo Y học: Cellular stresses profoundly inhibit protein synthesis and modulate the states of phosphorylation of multiple translation factors potx

Cellular stresses profoundly inhibit protein synthesis and modulate
the states of phosphorylation of multiple translation factors
Jashmin Patel
1
, Laura E. McLeod
2
, Robert G. J. Vries
1
, Andrea Flynn
1
, Xuemin Wang
1,2
and Christopher G. Proud
1,2
1
Department of Biosciences, University of Kent at Canterbury, Canterbury, UK;
2
Division of Molecular Physiology,
School of Life Sciences, University of Dundee, UK
We have examined the effects of widely used stress-inducing
agents on protein synthesis and on regulatory components of
the translational machinery. The three stresses chosen,
arsenite, hydrogen peroxide and sorbitol, exert their effects in
quite different ways. Nonetheless, all three rapidly
( 30 min) caused a profound inhibition of protein syn-
thesis. In each case this was accompanied by dephosphory-
lation of the eukaryotic initiation factor (eIF) 4E-binding
protein 1 (4E-BP1) and increased binding of this repressor
protein to eIF4E. Binding of 4E-BP1 to eIF4E correlated
with loss of eIF4F complexes. Sorbitol and hydrogen per-
oxide each caused inhibition of the 70-kDa ribosomal pro-

tein S6 kinase, while arsenite activated it. The effects of
stresses on the phosphorylation of eukaryotic elongation
factor 2 also differed: oxidative stress elicited a marked
increase in eEF2 phosphorylation, which is expected to
contribute to inhibition of translation, while the other
stresses did not have this effect. Although all three proteins
(4E-BP1, p70 S6 kinase and eEF2) can be regulated through
the mammalian target of rapamycin (mTOR), our data
imply that stresses do not interfere with mTOR function but
act in different ways on these three proteins. All three stresses
activate the p38 MAP kinase pathway but we were able to
exclude a role for this in their effects on 4E-BP1. Our data
reveal that these stress-inducing agents, which are widely
used to study stress-signalling in mammalian cells, exert
multiple and complex inhibitory effects on the translational
machinery.
Keywords: stress; initiation; elongation factor; mRNA
translation; S6 kinase.
The control of mRNA translation in mammalian cells
involves the regulation of a range of components of the
translational machinery, principally by changes in their
phosphorylation, leading to modulation of their activities or
their abilities to interact with one another [1,2].
Initiation factor 4E (eIF4E) plays a key role in mRNA
translation and its control in eukaryotic cells. eIF4E binds
to the 5¢ cap structure (containing 7-methylguanosine
triphosphate; m
7
GTP) which is present at the 5¢ end of all
cellular cytoplasmic mRNAs [3,4]. eIF4E can be regulated

by its own phosphorylation (which occurs at a single major
site (Ser209) [5,6]; and by binding proteins (4E-BPs) that
modulate its availability for initiation complex formation
(reviewed in [7]). eIF4E forms a complex termed eIF4F,
which also contains the translation factors eIF4G (formerly
called p220) and eIF4A. eIF4A has ATP-dependent RNA
helicase activity thought to be required to unwind regions of
self-complementary secondary structure in the 5¢ UTRs of
certain mRNAs [4,8]. Such secondary structure inhibits
translation and therefore mRNAs with 5¢ UTRs that
contain significant secondary structure are often poorly
translated. In contrast to many other cellular mRNAs,
translation of heat shock protein mRNAs appears to be
relatively cap-independent (reviewed in [9–11]), and trans-
lation of the mRNA for the stress-protein BiP/grp78 occurs
by a cap-independent mechanism [12].
The eIF4E binding proteins (4E-BPs) 1 and 2 interact
with eIF4E and inhibit cap-dependent mRNA translation
[13–15]. 4E-BP1 (also termed PHAS-I) competes with
eIF4G for binding to eIF4E, preventing formation of the
eIF4F complex and thus potentially inhibiting the recruit-
ment of eIF4A to the initiation complex on the 5¢ end of the
mRNA [16,17]. 4E-BP1 does not block the translation of
mRNAs that contain features allowing cap-independent
initiation to occur, e.g. internal ribosome-entry elements
derived from picornaviral mRNAs [13,15]. 4E-BP1 is a
phosphoprotein whose state of phosphorylation increases in
response to insulin and other agents that activate translation
(reviewed in [7]). This causes its dissociation from eIF4E.
Studies on 4E-BP2 (PHAS-II) show that its phosphoryla-

tion is also enhanced by insulin and that this causes it to
dissociate from eIF4E [18]. The main signalling pathway
that regulates 4E-BP1 phosphorylation is inhibited by the
immunosuppressant rapamycin (reviewed in [7]), a specific
inhibitor of the FRAP/TOR signalling pathway, which also
leads to activation of p70 ribosomal protein S6 kinase
Correspondence to C. Proud, Division of Molecular Physiology,
School of Life Sciences, MSI/WTB Complex, University of Dundee,
Dow Street, Dundee, DD1 5EH, UK.
Fax: + 44 1382322424; Tel.: + 44 1382344919;
E-mail:
Abbreviations: 4E-BP1, eukaryotic initiation factor 4E-binding protein
1; EF2, elongation factor 2; mTOR, mammalian target of rapamycin;
eIF4E, initiation factor 4E; m
7
GTP, 7-methylguanosine triphosphate;
CaM, calmodulin; eIF, eukaryotic initiation factor; rpS6, ribosomal
protein S6.
(Received 11 March 2002, revised 24 April 2002, accepted 2 May 2002)
Eur. J. Biochem. 269, 3076–3085 (2002) Ó FEBS 2002 doi:10.1046/j.1432-1033.2002.02992.x
(p70 S6k), although 4E-BP1 and p70 S6k appear to lie on
separate branches of this pathway [19]. p70 S6k is activated
by insulin and growth factors (reviewed in [20]) and appears
to play a role in up-regulating the translation of mRNAs
that are characterized by possessing 5¢ terminal tracts of
pyrimidines (and are thus termed 5¢-TOP mRNAs [21,22]).
Agents that activate p70 S6k up-regulate the otherwise low
basal translation of these mRNAs, and evidence has been
presented suggesting a causal link between these events,
although this remains to be confirmed [21]. A third

component of the translational machinery that can be
regulated through mTOR is elongation factor eEF2, the
protein that mediates the translocation step of elongation
[23,24]. Phosphorylation of eEF2 inhibits its activity,
apparently by inhibiting its ability to interact with the
ribosome [25] (reviewed in [24]). Insulin induces the
dephosphorylation of eEF2 and this is blocked by rapamy-
cin, demonstrating a requirement for mTOR dependent
signalling. The effect of insulin appears to involve a decrease
in the activity of the kinase that acts on eEF2, an unusual
calcium/calmodulin (Ca/CaM)-dependent enzyme termed
eEF2 kinase [26–28]. We recently showed that eEF2 kinase
is phosphorylated and inactivated by p70 S6k, thus estab-
lishing a molecular mechanism for the regulation of eEF2
kinase by insulin via mTOR [28].
The initiation factor eIF2 is required to recruit the
initiator methionyl-tRNA (Met-tRNA
i
) to the 40S ribo-
somal subunit [29]. eIF2 is only active when bound to GTP
and an additional protein factor, eIF2B, is required under
physiological conditions to promote recycling of eIF2 to this
form [29,30]. The activity of eIF2B can be modulated in a
variety of ways [29,31] including by its own phosphoryla-
tion, and through phosphorylation of the a subunit of its
substrate, eIF2, at a conserved site (Ser51 in mammals [32]).
Control of eIF2B activity is thought to play a key role in
regulating overall mRNA translation [30].
Here we have investigated the effects of a range of
stressful conditions that are widely employed to study the

roles of stress-activated signalling pathways on cell func-
tion. We find that these stresses rapidly and profoundly
inhibit protein synthesis and markedly alter the phos-
phorylation and/or activity of proteins involved in regula-
ting mRNA translation. These stressful agents exert effects
on several components of the translational machinery, the
common feature being that they cause dephosphorylation
of 4E-BP1 and thus inhibition of eIF4E. In addition to
providing further information on the regulation of a
number of components involved in mRNA translation, our
data show that it is very important, when evaluating the
effects of these stressful agents on cell function, to take into
account their marked effects on translation and on the
translational machinery.
EXPERIMENTAL PROCEDURES
Chemicals, biochemicals and other reagents
m
7
GTP–Sepharose was from Pharmacia Biotech Inc.
[c-
32
P]ATP and
35
S-labelled methionine/cysteine were pur-
chased from Amersham Corp. Chinese hamster ovary
(CHO.K1) cells were kindly provided by L. Ellis (Hannover
School of Medicine, Houston, TX, USA). Materials for
tissue culture were obtained from Gibco. Microcystin-LR
and rapamycin were from Calbiochem. Recombinant
mouse hsp25 was kindly provided by M. Gaestel (Texas

A & M University, Berlin, Germany). The antiserum to
rodent eIF4E has been described previously [33] and that to
4E-BP1 was raised against a synthetic peptide correspond-
ing to residues 101–113 of the human protein and has also
been described earlier [34]. The antisera against eIF4G were
generously provided by S. J. Morley (University of Sussex,
Brighton, UK) or was raised against a synthetic peptide
based on part of the C-terminus of eIF4G
1
[35]. The
antibody for phosphorylated eEF2 was raised against a
synthetic peptide corresponding to the region around Thr56
of mammalian eEF2 and has been described previously [36].
The loading of eEF2 was assessed using an antibody that
reacts with the protein irrespective of its state of phos-
phorylation [37].
Cell culture and stress treatment
Chinese hamster ovary (CHO.K1) cells were cultured as
described previously [38]. Cells were grown to near-conflu-
ence prior to exposure to arsenite, hydrogen peroxide or
sorbitol at the concentrations and for the times indicated.
Where applicable, cells were preincubated with signalling
inhibitors (as described in the text) prior to exposing cells to
stress conditions. In all cases, cell extracts were prepared as
described previously [38] and clarified by centrifugation at
4 °C (13 000 g, 10 min). To assess cell viability, CHO.K1
cells were left untreated or exposed to stress conditions for
specific times. After this, cells were washed with NaCl/P
i
,

removed from the plate by trypsin treatment in a volume of
0.5mL,andtrypanbluewasaddedtoaconcentrationof
0.4% (w/v) to the cell suspension. Cells were transferred to a
haemocytometer and inspected visually for their ability to
exclude the stain. Viability (%) was scored as number of
clear cells/total number of cells · 100.
Analysis of eIF4E and associated proteins eIF4E and
associated proteins were isolated from cell extracts by
affinity chromatography on m
7
GTP–Sepharose and bound
proteins were subjected to SDS/PAGE and Western
blotting as described previously [34,39] (any minor modi-
fications are noted in the text).
Gel electrophoresis, isoelectric focusing
and immunoblotting
For most purposes, samples were subjected to electrophor-
esis on SDS/polyacrylamide gels containing 15% acryla-
mide/0.4% methylene bis-acrylamide. For analysis of eEF2,
gels contained 10% (w/v) acrylamide plus 0.1% methylene
bis-acrylamide. For analysis of the electrophoretically
distinct forms of 4E-BP1, gels contained 13.5% (w/v)
acrylamide and 0.36% (w/v) methylene bis-acrylamide. In
all cases, proteins were transferred to poly(vinylidene
difluoride) membrane (Millipore) and Western blotting
was performed as described earlier [40] using the Enhanced
Chemiluminescence (ECL) system (Amersham plc).
Other assay procedures
Rates of protein synthesis were assayed in CHO.K1 cells by
measuring the incorporation of [

35
S]methionine/cysteine
into acid-insoluble material as described earlier [41].
Ó FEBS 2002 Modulation of translation factors by cellular stress (Eur. J. Biochem. 269) 3077
Approximately 20 lCi of radioisotope (> 1000 CiÆ
mmol
)1
) was used per 60-mm dish of cells.
p70 S6k activity was assayed, following immunoprecip-
itation from cell extracts, using a synthetic peptide substrate
based on the C-terminus of S6 [42]. This peptide binds to
phosphocellulose paper and incorporated radioactivity was
determined by the C
ˇ
erenkov method. Control assays were
performed in each case from which the peptide substrate
was omitted to correct for Ôself-incorporationÕ into the
immunoprecipitated protein; the values thus obtained were
subtracted from those obtained in duplicate assays contain-
ing the peptide substrate.
eEF2 kinase activity was assayed in CHO.K1 cell
extracts using purified eEF2 as a substrate, measuring the
incorporation of
32
P into the protein. The extracts were
incubated with eEF2 (1 lg) for 20 min at 30 °Cinthe
presence and absence of Ca
2+
/CaM. The Ca
2+

/CaM
buffer contained 66 m
M
MgCl
2
,1.2m
M
ATP, 4 m
M
CaCl
2
and 3 lgÆlL
)1
CaM while the Ca
2+
/CaM-free
buffer contained 66 m
M
MgCl
2
,1.2m
M
ATP and 1 m
M
EGTA. These concentrations are for the stock solutions
which were diluted sixfold in the assays. To terminate the
reactions, SDS sample buffer was added and samples were
incubated at 95 °C for at least 5 min. The denatured
samples were analysed 10% SDS/PAGE and the results
visualized by autoradiography.

RESULTS AND DISCUSSION
Stresses markedly inhibit protein synthesis in CHO cells
Treatment of CHO.K1 cells with agents that induce
chemical (arsenite), oxidative (hydrogen peroxide) or
osmotic (sorbitol) stress led to a rapid and marked
inhibition of protein synthesis (Fig. 1A). Each of the
stresses employed inhibited protein synthesis by about
80% under the conditions used here. We have previously
shown that a different stress-condition, heat shock, also
inhibits protein synthesis in these cells [43]. These conditions
arewidelyusedtostimulateÔstress-activatedÕ responses such
as the stress activated protein kinases (p38 MAP kinases
and c-Jun N-terminal kinases, JNKs). There is substantial
interest in the roles of these kinases and signalling pathways
in the transcriptional control of gene expression, although
most of this work ignores possible effects or interference due
to modulation of later stages in gene expression, such as
mRNA translation.
We also analysed the ability of these agents to inhibit
protein synthesis over a range of concentrations. For
arsenite, half-maximal inhibition occurred at 60 l
M
(Fig. 1B), while for hydrogen peroxide and sorbitol this
degree of inhibition was observed at about 0.5 m
M
and
0.2
M
, respectively (Fig. 1B). For arsenite or hydrogen
peroxide, higher concentrations resulted in inhibition by

>90%, while the effect of sorbitol was incomplete even at
the highest concentration tested here, 0.4
M
.Wewere
concerned that these chemical stresses might cause a loss of
cells, but in all cases we saw no evidence of this over the time
periods examined. For example, there was no loss of cellular
material as assessed by the protein content of the resulting
lysates (data not shown). To assess cell viability more
quantitatively, we assessed their ability to exclude trypan
blue. As judged by this criterion, cell viability was > 99.5%
after 25 min and > 99% after 2 h of treatment with the
stress stimuli even up to the highest concentrations of these
agents used in this study. Viability was 99% or higher after
4 h, except for the highest concentration of hydrogen
peroxide tested (3 m
M
) where it was about 94%. It therefore
appears that the effects of the stress conditions on protein
synthesis (and on translation factor phosphorylation, etc.,
see below) are not the consequences of toxic effects leading
to a loss of cell viability.
In view of this substantial inhibition of protein
synthesis, it will be important to consider their effects
on protein synthesis when studying the effects of these
stress conditions on cell physiology. Other agents that
inhibit protein synthesis (such as cycloheximide and
anisomycin [44–46]) cause activation of stress-activated
kinases. Although it may be that the ability of these
conditions to inhibit protein synthesis underlies, or

contributes to, their stimulation of the stress-activated
kinases, the effects of anisomycin on stress-regulated
kinases generally occur at concentrations where this agent
has little effect on overall protein synthesis.
Fig. 1. Cellular stresses inhibit protein synthesis. (A) CHO.K1 cells were incubated with sorbitol (0.4
M
), hydrogen peroxide (3 m
M
), or arsenite
(100 l
M
) for 25 min prior to the addition of [
35
S]methionine for a further 15 min. Cells were then extracted and samples processed to measure
incorporation of label into trichloroacetic acid-precipitable material. Data are expressed as percentage of untreated control cells ± SEM (n ¼ 5,
hydrogen peroxide; n ¼ 6, other conditions). (B) Triplicate plates of CHO.K1 cells were incubated with hydrogen peroxide (0.1, 0.2, 1, 3 m
M
)or
sorbitol (0.2, 0.3, 0.4
M
) for 10 min prior to the addition of 20 lCi [
35
S]methionine for 15 min. The cells were extracted and triplicate samples (60 lg
of protein) were processed to measure the incorporation into trichloroacetic acid-precipitable material. Data are expressed as percentages of
untreated control cells ± SD (for hydrogen peroxide and sorbitol), where n ¼ 9 for all conditions. For arsenite, data are the mean of triplicate
determinations. Incorporation into control samples was typically about 10 000 d.p.m.
3078 J. Patel et al. (Eur. J. Biochem. 269) Ó FEBS 2002
Cellular stresses affect the association of eIF4E
with 4E-BP1 and eIF4G
We have previously shown that the inhibition of protein

synthesis by heat shock was associated with increased
binding of 4E-BP1 with eIF4E [43]. To examine whether
this was a common cellular response to these varied stress
conditions, CHO cells were treated for various times with
the above agents, extracts were prepared and then subjected
to affinity chromatography on m
7
GTP–Sepharose, which
retains eIF4E and associated proteins. The bound proteins
were analysed by SDS/PAGE and Western blotting. As
shown in Fig. 2, treatment with any of the three stresses
used above (arsenite, hydrogen peroxide or sorbitol) caused
a time-dependent rapid increase in the binding of 4E-BP1 to
eIF4E. For sorbitol or hydrogen peroxide, increased
binding was seen as early as 5 min after application of the
stress, and the effect was maximal at 15–20 min (Fig. 2A,B).
For arsenite, the effect was slightly slower, a discernible
increase first being visible at 15 min (Fig. 2C). Dose–
response studies showed that the effect of sorbitol on the
association of 4E-BP1 with eIF4E required 300–400 m
M
,
while that of hydrogen peroxide was already maximal at
0.5 m
M
(Fig. 2D,E). For CHO cells, 4E-BP1 can be
resolved into three electrophoretically distinct species
termed a–c,ofwhicha is the least, and c the most, highly
phosphorylated. 4E-BP1 undergoes phosphorylation at
least six sites that have differing effects on its mobility

and/or binding to eIF4E [7,47–50]. Each ÔbandÕ is therefore
likely to contain a mixture of different species. In particular,
the b form contains some species that bind to eIF4E and
some that do not. This is evident from our earlier work
[52,53] and from Fig. 2D,F, where in control cells both the b
and c species are present but no 4E-BP1 is bound to eIF4E,
while in cells treated with 300 m
M
sorbitol, the protein is
mostly present as the b form, but this form is now bound to
eIF4E. The main effect of the higher concentrations of
sorbitol is to cause the loss of the most phosphorylated
c-form, which is not found in association with eIF4E. Loss
of this species coincides with the marked increase in binding
of 4E-BP1 to eIF4E observed at 300 and 400 m
M
sorbitol
(Fig. 2F). The further dephosphorylation of 4E-BP1 seen at
the highest sorbitol concentration results in the appearance
of the a species, which can be seen (Fig. 2D) to associate
with eIF4E. Similarly, hydrogen peroxide and arsenite each
caused a shift in the behaviour of 4E-BP1 to more mobile,
less phosphorylated species (data not shown), consistent
with the increased binding to eIF4E (Fig. 2A,B).
We have previously shown that, as expected, increased
binding of eIF4E to 4E-BP1 in CHO cells results in loss of
eIF4F complexes in response, e.g. to amino-acid withdrawal
[51,52] or heat shock [43]. As anticipated from these earlier
studies, treatment of CHO cells with sorbitol, arsenite or
higher concentrations of hydrogen peroxide resulted in the

loss of eIF4F complexes, as shown by the loss of eIF4G
bound to eIF4E that occurred concomitantly with the
increased binding of eIF4E to 4E-BP1 (Fig. 3A). In a few
experiments, arsenite was observed to cause an increase in
the binding of eIF4G to eIF4E when used at concentrations
up to 150 l
M
, even though binding of 4E-BP1 to eIF4E was
also enhanced (data not shown). Such findings are hard to
reconcile with the simple model where 4E-BP1 and eIF4G
Fig. 2. Cellular stresses increase the binding of 4E-BP1 to eIF4E. (A–E) CHO.K1 cells were treated with the indicated reagents for the times and/or
using the concentrations shown. After treatment, cell extracts were prepared and samples were subjected to affinity chromatography on m
7
GTP–
Sepharose, and the bound material was then analysed by SDS/PAGE followed by immunoblotting using antibodies for eIF4E and 4E-BP1. The
positions of migration of eIF4E and 4E-BP1 are indicated. 4E-BP1 generally appears as two bands corresponding to the a and b species of 4E-BP1.
The signal for eIF4E serves as a ÔloadingÕ control and should be compared with the signal for 4E-BP1 in each lane. (F) Extracts of cells treated with
the indicated concentrations of sorbitol for 25 min were analysed directly by SDS/PAGE and Western blotting using gels containing 13.5%
acrylamide/0.36% methylene bis-acrylamide. Arrows labelled a, b and c indicated the positions of these electrophoretically distinct species of
4E-BP1.
Ó FEBS 2002 Modulation of translation factors by cellular stress (Eur. J. Biochem. 269) 3079
compete for binding to the same site in eIF4E. Scheper et al.
[53] have also reported that arsenite (at similar concentra-
tions to those used here) increased the binding of eIF4E to
eIF4G. In their case, however, binding of eIF4E to 4E-BP1
was decreased, which is more in line with the expected
reciprocal effects on the binding of these two proteins to
eIF4E. The total cellular content of eIF4G was not affected
by any of these treatments, ruling out the possibility that
degradation of eIF4G was the cause of the loss of signal

(Fig. 3B).
Cellular stresses also affect p70 S6 kinase activity
4E-BP1 phosphorylation is mediated through the rapamy-
cin-sensitive mTOR pathway. To assess whether these
cellular stresses caused a generalized inhibition of mTOR
signalling, we therefore studied their effect on the activity of
p70 S6 kinase. Treatment of cells with hydrogen peroxide or
sorbitol did indeed cause the inactivation of p70 S6 kinase in
a dose-dependent manner (Fig. 4A). For sorbitol, concen-
trations that induced dephosphorylation of 4E-BP1 also
caused inactivation of p70 S6 kinase. For hydrogen perox-
ide, changes in p70 S6 kinase activity were only observed at
relative high concentrations of the compound. In contrast to
the effects of these agents, arsenite had little effect on p70 S6
kinase activity and even caused modest activation at higher
concentrations. This is rather reminiscent of the ability of
arsenite to activate p70 S6 kinase in cardiomyocytes [54].
Activation of S6 kinase by all stimuli so far tested is inhibited
by rapamycin [20]. The finding that arsenite does not inhibit
p70 S6 kinase indicates that arsenite does not cause inhibi-
tion of mTOR signalling, because if it did, p70 S6 kinase
activity would have been decreased by arsenite. We have
previously shown that the activation of p70 S6 kinase by
arsenite in cardiomyocytes is blocked by rapamycin [54]
indicating that arsenite activates p70 S6 kinase in a manner
that still requires the input provided by mTOR.
As an indication of intracellular p70 S6 kinase activity,
we examined the phosphorylation state of ribosomal protein
S6 (rpS6), using an antibody that detects this protein only
when it is phosphorylated [28]. Decreases in rpS6 phos-

phorylation were observed for cells treated with the higher
concentrations of hydrogen peroxide or with sorbitol, where
decreased p70 S6 kinase activity was also observed
(Fig. 4B). Arsenite had little effect on the phosphorylation
of rpS6.
Oxidative stress also modulates the phosphorylation
of elongation factor 2
A third target of mTOR signalling in CHO cells is eEF2
[26]. The phosphorylation of this protein plays an important
role in regulating mRNA translation, by inhibiting the
activity of eEF2 [23,25,55]. Given that cellular stresses
inhibit translation, it was important to study whether they
elicited an increase in the phosphorylation of eEF2. To
assess whether cellular stresses affected the phosphorylation
state of eEF2, we made use of an antibody that detects eEF2
only when it is phosphorylated at its main site of
phosphorylation, Thr56 [36].
The basal level of phosphorylation of eEF2 depends
upon the density of the cells: the more nearly confluent the
cells, the higher the level of phosphorylation (Fig. 5A, top
section, cf. middle section for loading control). The level of
phosphorylation of ribosomal protein S6 was observed to
fall with increasing cell density (Fig. 5C). Both effects are
likely to contribute to a slow-down in protein synthesis
[20,23], which is a logical response for cells approaching
Fig. 4. Effects of cellular stresses on the activity of p70 S6 kinase.
CHO.K1 cells were treated for 25 min with the indicated concentra-
tions of the agents under study, and extracts were then prepared.
p70 S6 kinase activity was measured, using a synthetic peptide sub-
strate, following immunoprecipitation of p70 S6 kinase from the cells

extract using an anti-(p70 S6) kinase serum. All assays were performed
in duplicate. For arsenite, the data are mean ± SEM for four separate
experiments. For hydrogen peroxide and sorbitol, the values shown are
from one set of data that is representative of four to five separate
experiments performed.
Fig. 3. Effects of cellular stresses on the association of eIF4E with
eIF4G and 4E-BP1. CHO.K1cellsweretreatedfor25minwiththe
indicated concentrations of sorbitol, hydrogen peroxide or arsenite,
and extracts were prepared. (A) Samples were subjected to affinity
chromatography on m
7
GTP–Sepharose, and the bound material was
then analysed by SDS/PAGE followed by immunoblotting using
antibodies for eIF4E, eIF4G and 4E-BP1 (positions indicated). (B)
Samples of cell lysate were subjected to SDS/PAGE followed by
Western blotting with an antibody for eIF4G (position shown).
3080 J. Patel et al. (Eur. J. Biochem. 269) Ó FEBS 2002
confluence. The phosphorylation states of S6 and eEF2 are
regulated in opposing directions by mTOR signalling. It
may therefore be that mTOR signalling is repressed at
higher cell densities, although other explanations are
possible. The basis of these effects is not known and further
study of this falls outside the scope of this report. However,
it is important to be aware of this effect when designing
experiments to study the regulation of eEF2 phosphoryla-
tion. For example, hydrogen peroxide elicited a marked
increase in eEF2 phosphorylation in less dense cells where
the initial level of eEF2 phosphorylation is lower, but had
no discernible effect in denser cells where basal eEF2
phosphorylation is high (Fig. 5B). This effect was not

blocked by an inhibitor of the p38a/b MAP kinase pathway,
SB203580 [56], even though this compound effectively
inhibits this pathway at the concentration used (see below).
In fact, in some experiments, SB203580 actually caused a
small increase in the phosphorylation of eEF2 (as seen in
Fig. 5B, top section). eEF2 phosphorylation was sensitive to
low doses of hydrogen peroxide, increases being seen at
concentrations as low as 30 l
M
(Fig. 5C), with the maximal
effect already being seen at about 100 l
M
. It is thus more
sensitive to this agent than either 4E-BP1 or p70 S6 kinase.
Treatment of low density cells with hydrogen peroxide led
to a reproducible increase in the maximal activity of eEF2
kinase (i.e. when measured in the presence of saturating
amounts of calcium ions and calmodulin, Fig. 5D).
Neither sorbitol nor arsenite increased the level of eEF2
phosphorylation in low density cells (Fig. 5B). Sorbitol, but
not arsenite, reproducibly caused a modest decrease in eEF2
phosphorylation in cells where this level is basally high
(Fig. 5B). This appeared to be associated with a decrease in
the activity of eEF2 kinase (Fig. 5E). Because only hydro-
gen peroxide increases the phosphorylation of eEF2, while
all three stresses inhibit translation, it seems that inhibition
of protein synthesis by arsenite or sorbitol is not due
changes in the phosphorylation state of this factor, but
rather to other effects.
Because the phosphorylation of eEF2 is regulated in an

mTOR-dependent manner in CHO cells, the above data
suggest that the cellular stress conditions used here are not
acting to inhibit mTOR function. If this were the case, all
Fig. 5. Effects of stresses on the phosphorylation of elongation factor 2. (A) One plate of confluent (80–90%) CHOK1 cells was trypsinized and then
seeded into new dishes at the indicated approximate dilutions (1 : 2, i.e. 1 part trypsinized cell suspension and 1 part fresh medium, etc.). Each plate
of cells was grown in medium containing serum for 24 h and the cells were then extracted and samples were subjected to 10% SDS/PAGE and
Western blotted with the indicated antisera (probing with anti-eEF2 served as a loading control). (B) Upper and middle sections: CHO.K1 cells
grown to subconfluence (approx. 60–70% confluence) were treated with sodium arsenite (100 l
M
), hydrogen peroxide (3 m
M
) or sorbitol (0.4
M
)
for 25 min, prior to extraction. In some cases (+ SB203580), cells were pretreated with SB203580 (25 l
M
) for 25 min prior to addition of the stress
agent. Samples (30 lg protein) were analysed by SDS/PAGE and Western blotting using antisera specific for eEF2 phosphorylated at Thr56 (top)
or an antibody that recognizes eEF2 irrespective of its state of phosphorylation, as a loading control (middle). The bottom section shows a similar
analysis for cells at 80–90% confluence. Loading controls using anti-eEF2 again confirmed equal loading of cell protein (not shown). (C) CHO.K1
cells were treated for 25 min with a range of concentrations of hydrogen peroxide as indicated. Samples were analysed by SDS/PAGE and Western
blotting using antisera specific for eEF2 phosphorylated at Thr56 (upper section) or an antibody that recognizes eEF2 irrespective of its state of
phosphorylation, as a loading control (loading control, lower section). (D,E) Assays for eEF2 kinase activity. Samples of extracts (20 lgprotein)of
low density (60–70% confluence, D) or higher density cells (80–90%, E) that had been treated with stressful agents as indicated (for 25 min) were
assayed for eEF2 kinase activity using purified eEF2 as substrate. Samples were analysed by SDS/PAGE and autoradiography. The position of the
radiolabelled eEF2 on the autoradiograph is indicated. Similar data were obtained in four (D) or three (E) experiments.
Ó FEBS 2002 Modulation of translation factors by cellular stress (Eur. J. Biochem. 269) 3081
three stresses would be expected to have the same effect on
eEF2 phosphorylation. It is thus unlikely that the
dephosphorylation of 4E-BP1 and the inactivation of

p70 S6 kinase caused by hydrogen peroxide and sorbitol
are due to impairment of the function of mTOR itself, and
perhaps more likely that these stresses interfere with
signalling events downstream of mTOR that impinge on
4E-BP1 and p70 S6 kinase. Knebel et al. [57] have reported
that eEF2 kinase can be phosphorylated and inactivated by
the SB203580-insensitive d-form of p38 MAP kinase. Thus
it is possible that, this enzyme may play a role in the
dephosphorylation of eEF2 and the inactivation of eEF2
kinase caused by sorbitol. However, sorbitol has not been
showntoactivatep38MAPkinase-d, and in the absence of
an inhibitor for this enzyme, it is not possible to test its
involvement. Other mechanisms may also be involved: for
example, an earlier study concluded that osmotic stress
activated a protein phosphatase acting on p70 S6 kinase,
resulting in its inactivation [58]. These authors also reported
that osmotic stress led to dephosphorylation of 4E-BP1.
Stress regulation of 4E-BP1 is not mediated
by the p38 MAP kinase pathway
Dephosphorylation of 4E-BP1 is a common response to the
cell stresses tested here. Sorbitol is known to activate the p38
MAP kinase a/b pathway in other cell types [59]. As
assessed by measuring the activity of the downstream
kinase, MAPKAP-K2 (using hsp27 as substrate), it also did
so in CHO.K1 cells (Fig. 6A). The compound SB203580
inhibits the a and b isoforms of p38 MAP kinase (that
activate MAPKAP-K2 [56]) and did indeed prevent the
activation of MAPKAP-K2 in response to sorbitol, hydro-
gen peroxide or arsenite in CHO.K1 cells (Fig. 6A).
SB203580 did not however, prevent the increase in binding

of 4E-BP1 to eIF4E caused by sorbitol or low concentra-
tions of arsenite, indicating that this effect is not mediated
through p38 MAP kinase a/b (Fig. 6B). SB203580 also
failed to prevent the increase in the binding of 4E-BP1 to
eIF4E induced by hydrogen peroxide (Fig. 6C). It therefore
appears that the effects of stresses on 4E-BP1 phosphory-
lation are not mediated by p38 MAP kinase a/b.
Effects of stress conditions on other translation factors
Other important regulatory proteins for mRNA translation
are eIF2 and its guanine-nucleotide exchange factor, eIF2B.
The activity of eIF2B is important in controlling translation
initiation under a variety of conditions [20,29]. However, in
multiple experiments using a range of concentrations of the
agents studied here, we observed no change in eIF2B
activity under any of the stress conditions tested here (data
not shown), seemingly ruling out a role for this protein in
the inhibitory effects of all three stresses on protein synthesis
in these cells. Heat shock has been reported to inhibit eIF2B
activity in vitro [60].
Concluding comments
All three cell stresses used here cause profound inhibition of
protein synthesis, as also seen for heat shock in these cells.
The three stress conditions studied here have differing
effects on the translation factors studied: these factors are all
those thought to be important in the acute regulation of
mRNA translation in mammalian cells, eIF4F, eIF2B,
eEF2 and p70 S6 kinase. We have previously reported that
osmotic, oxidative or heat stress cause the dephosphoryla-
tion of eIF4E in CHO.K1 cells, while arsenite actually
enhanced eIF4E phosphorylation [61]. None of the stresses

studied here affected eIF2B activity, and they had differing
effects upon p70 S6 kinase and the phosphorylation of
eEF2. However, all these stresses, including, as described
earlier, heat shock [43], caused increased binding of eIF4E
to 4E-BP1 and the consequent loss of eIF4F complexes.
These data suggest this is a common and rapid response of
CHO cells to these stress conditions. Similar, but less
complete, data were published previously for human
embryonic kidney 293 cells [61]. Because no other transla-
tion factor responds in the same way to all the stresses used,
it seems likely that inhibition of eIF4E by increased binding
to 4E-BP1 represents a major mechanism, possibly the
primary mechanism, by which these stresses inhibit mRNA
translation. Anderson and coworkers [62,63] have reported
that certain cell stresses, such as hyperthermia, cause the
formation of stress granules and that this may play an
important role in the inhibition of translation under this
condition. Their data indicate that the formation of such
granules is driven by the phosphorylation of eIF2a [63].
These authors have argued that stress granule formation
may be driven by loss of active eIF2, availability of which is
Fig. 6. The stress-activated p38 MAP kinase pathway is not involved in
the regulation of 4E-BP1 by stresses. CHO.K1 cells were left untreated
(Con) or treated for 25 min with arsenite (50 l
M
), hydrogen peroxide
(1.5 m
M
) or sorbitol (0.4
M

). In some cases, where indicated (+), cells
were preincubated for 60 min with SB203580 (25 l
M
) prior to addition
of the stress stimulus. (A) samples were assayed for MAPKAPK-2
using recombinant hsp27 as substrate; position of radiolabelled hsp27
is shown (figure is an autoradiograph). (B) Samples were analysed
directly by SDS/PAGE and Western blotting using gels containing
13.5% acrylamide/0.36% methylene bis-acrylamide. Positions of the
three electrophoretically separable forms of 4E-BP1 are indicated. (C)
Samples were subjected to affinity chromatography on m
7
GTP–
Sepharose, and the bound material was then analysed by SDS/PAGE
followed by immunoblotting using antibodies for eIF4E and 4E-BP1.
The positions of migration of eIF4E and 4E-BP1 are indicated.
3082 J. Patel et al. (Eur. J. Biochem. 269) Ó FEBS 2002
determined by the activity of eIF2B. In our studies,
however, we saw no effect of the stresses tested upon eIF2B
activity and only sorbitol caused significant phosphoryla-
tion of eIF2, making it unlikely that this pathway is involved
in the inhibition of translation under the other stress
conditions studied here. The absence of an effect of arsenite
on eIF2a phosphorylation, a consistent observation in these
studies, differs from the finding of Anderson and colleagues
that this agent elicited increased eIF2a phosphorylation in
other cell-types.
The loss of eIF4F complexes is expected to strongly
impair de novo initiation of translation of the cap-dependent
mRNAs [15], which are thought to represent the bulk of

cellular mRNAs. Novoa & Carrasco [64] have presented
evidence that reinitiation onto mRNAs that are already
being translated is less dependent on the eIF4F complex
than de novo initiation, consistent with the relatively small
effect of rapamycin on protein synthesis in the short term
[15]. Loss of such complexes should not impair translation
of those cellular mRNAs that possess internal ribosome
entry sequences, because translation of such mRNAs is
independent of the cap and of eIF4E/4F [64] (reviewed in
[65]). A number of cellular proteins are thought to be
encoded by such mRNAs, including stress proteins such as
grp78/BiP, a molecular chaperone whose expression is
increased under stress conditions [66]. Other stress proteins
whose expression rises in response to stressful conditions
include the heat shock proteins. The translation of these
mRNAs shows a low requirement for eIF4F [9] and,
consistent with this, they continue to be translated in cells in
which the level of eIF4E has been reduced by antisense
techniques [67]. Taken together our data suggest that
inactivation of eIF4E, by sequestration by 4E-BP1, is a
common cellular response to stress. It may serve simulta-
neously to impair general cellular translation under stressful
conditions while allowing continued synthesis of stress
proteins whose mRNAs possess internal ribosome entry
sequences or have low requirements for eIF4F for other
reasons. It was recently shown that the 5¢ UTR of the
human hsp70 mRNA contains a potent enhancer of mRNA
translation [68]. This may allow high levels of hsp70
synthesis in the absence of normal eIF4F function, although
this idea remains to be tested.

However, because inhibiting eIF4F formation by treating
cells with rapamycin only has a small effect on the overall
rate of protein synthesis in the short term [15], it is unlikely
that the stress-induced dephosphorylation of 4E-BP1 and
loss of eIF4F complexes is a major cause of the inhibition of
protein synthesis caused by these agents. Indeed, it seems
likely that this involves additional regulatory events, which
remain to be identified, are also important in the stress-
induced inhibition of protein synthesis. Further work will be
required to characterize these events.
Because the stress conditions we have studied have
disparate effects upon the three targets of mTOR that we
have studied (4E-BP1, p70 S6 kinase, eEF2), our data imply
that these stresses do not exert a general inhibitory effect on
mTOR signalling. For example, although hydrogen perox-
ide and sorbitol cause inhibition of p70 S6 kinase and
dephosphorylation of 4E-BP1, arsenite has opposite effects
on these two proteins. In the case of eEF2, arsenite has little
effect, while sorbitol and hydrogen peroxide have opposite
effects. It is more likely therefore that these stress conditions
intervene in different ways to regulate these target proteins,
and that they probably do so by modulating the activities of
the poorly understood signalling components that lie
downstream of ÔdownstreamÕ of mTOR. This could, for
example, involve inactivation of the kinases acting on
4E-BP1, or activation of the corresponding phosphatases.
Lastly, our data reveal a multiplicity of effects of cell
stresses on translation regulators, and their profound
inhibitory effect on protein synthesis. These ÔartificialÕ
stresses are widely used to activate the stress-activated

protein kinases in order to study their roles, e.g. in the
regulation of transcription. It is clearly essential to bear in
mind their effects on mRNA translation and translation
factors when using these agents, and when interpreting data
obtained using them, especially where longer-term effects on
gene expression are being evaluated.
ACKNOWLEDGEMENTS
These studies were supported by Grants (to CGP) from the Medical
Research Council, the Wellcome Trust and the British Heart
Foundation.
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