Download Regulation of translation initiation following stress

Oncogene (1999) 18, 6121 ± 6128
ã 1999 Stockton Press All rights reserved 0950 ± 9232/99 $15.00
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Regulation of translation initiation following stress
M Saeed Sheikh*,1 and Albert J Fornace Jr1
1
Division of Basic Sciences, National Cancer Institute, National Institutes of Health, Building 37, Room 5CO9, Bethesda,
Maryland, MD 20892, USA
Recent studies suggest that genotoxic and non-genotoxic
stresses appear to invoke translational checkpoints in
order to inhibit protein synthesis. Depending on the
stress and/or cell type, this downregulation of protein
synthesis may either (i) protect against the deleterious
e€ects of noxious agents and ensure the conservation of
resources that are needed to survive under adverse
conditions or (ii) activate apoptosis. In this article, we
have reviewed several lines of evidence which support the
notion that regulation of translation initiation is an
important component of the cellular stress response.
While the stress-induced post-translational regulation of
translation initiation factors (eIFs) has been well
documented, stress-induced regulation of eIFs at the
mRNA levels, as reviewed here, is only beginning to be
elucidated. Thus, the stress-mediated regulation of eIFs
occurs at multiple di€erent levels involving, transcriptional, post-transcriptional and post-translational controls.
Keywords: translation initiation; eIF2; eIF3; eIF5;
eIF1A121/SUI1; translation checkpoints
Eukaryotic cells have developed multiple mechanisms
to mount a response against endogenous and
exogenous stresses. Cellular responses to genotoxic or
non-genotoxic stress involves transcriptional and posttranscriptional controls a€ecting various cellular
processes (reviewed by Fornace, 1992; Kaufman,
1999). It is now widely held that DNA damage
(genotoxic stress) activates cell cycle checkpoints at
G1/S and G2/M which provide necessary time for cells
to repair damaged DNA prior to progression into next
phase of the cell cycle (reviewed by Fornace, 1992;
Pellegata et al., 1996; Morgan and Kastan, 1997;
Schwartz and Rotter, 1998). Translation regulation in
response to stress is an area that has not been
extensively studied. Several lines of evidence support
the notion that the regulation of protein translation
appears to be an important component of the cellular
stress response. In general, it is conceivable that
genotoxic and non-genotoxic stresses could invoke
translational checkpoints to block protein synthesis,
particularly that of toxic proteins containing errors.
The downregulation of protein synthesis could be an
adaptive response which may protect against the
deleterious e€ects of toxic agents and ensure the
conservation of resources that are needed to survive
*Correspondence: MS Sheikh
under such adverse conditions (reviewed by Hinnebusch, 1994). However, depending on the stress and
cell type, the inhibition of protein synthesis may also
activate apoptosis. In this article, we will review the
current state of published knowledge and some of our
recent data in this emerging area of cellular stress
response.
Protein translation
Eukaryotic mRNAs undergo modi®cations in the
nucleus that involve addition of a unique `cap'
structure at the 5'-end and poly(A)+ tail at the 3'-end
(reviewed by Sachs et al., 1997). The `cap' is an
inverted 7-methylguanosine that is attached via a 5'-5'
triphosphate bridge to the very ®rst nucleotide of
mRNA. The `cap' appears to function in splicing,
polyadenylation, nuclear export, RNA splicing, mRNA
stability and is essential for ecient translation
(reviewed by Lewis and Izaurralde, 1997). Recent
evidence also supports an important role of 3'poly(A)+ tail in protein translation (reviewed by Sachs
et al., 1997). Translation initiation requires the
recruitment of the smaller (40S) ribosomal subunit to
mRNA and involves protein-protein and protein-RNA
interactions (reviewed by Hershey et al., 1996). All
mRNAs are translated via free ribosomes or via
membrane bound ribosomes. The former are unattached cytoplasmic ribosomes that are involved in the
synthesis of a majority of the cellular proteins, while
the latter are bound to the cytosolic side of the
endoplasmic reticulum (ER) membrane and synthesize
membrane proteins or those that are destined for
secretion.
The scanning model of translation initiation
proposes that a 43S subunit complex containing the
initiator methionyl-tRNAi (Met-tRNAi) coupled 40S
ribosomal subunit, eIF2, eIF3 and eIF1A binds to the
`cap' structure (reviewed by Clemens and Bommer,
1999). With the help of eIF4B and eIF4F whose
helicase activities unwind the secondary structure, the
40S ribosome scans mRNA in the 3'-direction in search
of the ®rst initiation codon (AUG) (reviewed by
Clemens and Bommer, 1999). Once the ®rst AUG is
identi®ed, the interactions between AUG and the
anticodon of Met-tRNAi sets the stage for chain
elongation which is the next step in protein synthesis.
The polypeptide chain elongation occurs in three steps
(reviewed by Clemens and Bommer, 1999; Clark, 1980;
Caskey, 1980) (Figure 1). In step 1, the eIFs dissociate
from the smaller ribosomal subunit, the larger
ribosomal subunit binds to the smaller subunit and a
second aminoacyl-tRNA molecule is recruited to the
A-site of the ribosome (reviewed by Clemens and
Translational controls following stress
MS Sheikh and AJ Fornace
6122
Figure 1 An overview showing eukaryotic protein translation in progress
Bommer, 1999; Clark, 1980). Step 2 involves a reaction
catalyzed by peptidyl transferase in which the amino
acid on the second tRNA is joined by a peptide bond
to an amino acid linked to tRNA in the P-site. In step
3, the ribosome moves three nucleotides along mRNA
in the 3'-direction, the tRNA in the A-site is
translocated to the P-site, the A-site becomes vacant
and ready to accept another amino acid-chargedtRNA. The process continues until one of the three
stop codons are reached at which point the translation
is terminated (reviewed by Clemens and Bommer,
1999; Clark, 1980; Caskey, 1980) (Figure 1). On
average, the completion of protein synthesis can take
from 20 ± 60 s.
Post-translational regulation of eIF2 in response to
stress
Mammalian eIF2 is composed of three subunits (a, b,
g) in 1 : 1 :1 stoichiometry (Pathak et al., 1988). The 43S
preinitiation complex contains a small ribosomal
subunit (40S), eIF1A, eIF1A121/SUI1(see below), eIF3
and a ternary complex composed of eIF2, GTP and
Met-tRNAi (reviewed by Clemens and Bommer, 1999;
Sachs et al., 1997). Within the 43S complex, eIF3
interacts with eIF4G, which in turn interacts with the
`cap'-binding subunit eIF4E (reviewed by Clemens and
Bommer, 1999; Sachs et al., 1997). Thus, 43S complex
via various subunits of eIF4 (see below) binds to the
`cap' at the 5'-end of mRNA and slides along mRNA
towards 3'-end until the ®rst AUG is reached (reviewed
by Clemens and Bommer, 1999; Sachs et al., 1997). As
soon as the ®rst AUG is identi®ed, the GTP in the
ternary complex is hydrolyzed to GDP, which results
in the release of eIF2 and GDP as a binary complex
(reviewed by Clemens and Bommer, 1999; Sachs et al.,
1997). In a nucleotide-exchange reaction catalyzed by
eIF2B, the GDP bound to eIF2 is replaced with GTP
and it interacts with the Met-tRNAi, to once again,
form a ternary complex (reviewed by Clemens and
Bommer, 1999; Sachs et al., 1997).
Phosphorylation of eIF2a
Three di€erent types of kinases that phosphorylated
eIF2a have been identi®ed (reviewed by Haro et al.,
1996). All these kinases are activated by various types
of stresses (reviewed by Haro et al., 1996). HRI (heme
regulated inhibitor of translation) was the ®rst eIF2a
kinase that was identi®ed from rabbit reticulocytes and
is activated in response to heme deprivation, heavy
metals and heat shock proteins (reviewed by Chen and
London, 1995). PKR (double-stranded-RNA-activated
protein kinase) is activated by double-stranded-RNA
species, particularly those produced following viral
infection (Schmedt et al., 1995; reviewed by Haro et al.,
1996). The expression of PKR is also induced by
interferon in mammalian cells (Meurs et al., 1990).
GCN2 is a third type of eIF2a kinase, which has been
characterized from yeast Saccharomyces cerevisiae
(reviewed by Hinnebusch, 1994). GCN2 is activated
in response to nutrient deprivation, a condition that
causes the accumulation of uncharged-tRNAs (reviewed by Hinnebusch, 1994). It has been reported
that phosphorylation of eIF2 (Ser51 of a subunit)
inhibits eIF2B such that eIF2B can no longer catalyze
nucleotide exchange and thus, a majority of the eIF2a
exists in binary complex with GDP (reviewed by
Clemens and Bommer, 1999; Haro et al., 1996). It
follows that the phosphorylated eIF2a has a high
anity for eIF2B, which further prevents its ability to
catalyze GDP ± GTP exchange on the unphosphorylated eIF2a (reviewed by Clemens and Bommer, 1999;
Haro et al., 1996). Thus, the eIF2a phosphorylation on
Ser51 is a highly conserved adaptive response that can
cause downregulation of translation initiation under
certain types of stresses (Figure 2).
gadd34/MyD116 and virus connection
HSV-1 (herpes simplex virus) infection is known to
stimulate a host defense response that involves the
activation of host PKR, phosphorylation of eIF2a, and
inhibition of protein synthesis. However, HSV-1 has
evolved a mechanism to overcome the host defense
response (reviewed by Liebermann and Ho€man,
1998). For example, the HSV-1 g34.5 gene (also
known as ICP34.5) product prevents the accumulation
of phosphorylated eIF2a and thereby allows continuous protein synthesis throughout viral infection
(Chou and Roizman, 1992; Chou et al., 1995) (Figure
2). The carboxyl-terminal domain of the g34.5 gene
product, responsible for inducing dephosphorylation of
eIF2a, shows a high degree of homology to the
carboxyl-terminal domain of the Gadd34 protein and
the virulence-associated gene from African swine fever
virus (Figure 3) (Zhan et al., 1994; Chou and Roizman,
1994; reviewed by Liebermann and Ho€man, 1998).
Interestingly, the gadd34 gene was originally cloned as
Translational controls following stress
MS Sheikh and AJ Fornace
Figure 2 Stress-induced phosphorylation of eIF2a can cause
inhibition of translation initiation. Stress activates HRI, PKR and
GCN2 kinases. These kinases phosphorylate the a subunit of
eIF2. Phosphorylated eIF2a (i) inhibits eIF2B and thereby
prevents its ability to catalyze the exchange of GDP to GTP on
eIF2a, (ii) interacts with eIF2B with a higher anity which
further inhibits eIF2B's ability to catalyze GDP-GTP exchange
on eIF2a. Starvation and DNA damage induce gadd34 which
shows homology to HSV-1 viral g34.5 gene. g34.5 gene product
(and perhaps Gadd34) appears to interact with PP1a to
dephosphorylate eIF2a in order to derepress protein synthesis
inhibition. It is also possible that Gadd34 and g34.5 gene product
may play opposing roles since g34.5 gene product appears to
block the growth inhibitory e€ects of Gadd34
a growth arrest and DNA damage-inducible gene from
Chinese hamster ovary cells (Fornace et al., 1988 and
see below). The murine homolog of human and
hamster genes has also been cloned as the myeloid
di€erentiation primary response gene 116 (MyD116)
(reviewed by Liebermann and Ho€man, 1998). The
human, hamster and mouse Gadd34/MyD116 proteins
are highly homologous (Figure 3) and recently the rat
homolog named PEG3 has also been cloned (Su and
Fisher, 1997).
It has been shown that the HSV-1 carrying a
deletion in the Gadd34/MyD116 homologous carboxyl-terminal domain of the g34.5 gene product is unable
to propagate, most likely due to premature termination
of protein synthesis (Chou and Roizman, 1994). Recent
evidence demonstrates that the substitution of the
Gadd34/MyD116 carboxyl-terminal domain with the
corresponding mutant domain of the g34.5 gene
product (Figure 3) is able to preclude the premature
termination of protein synthesis thus suggesting that
the carboxyl-terminal domain of mammalian Gadd34/
MyD116 appears to display a role comparable to that
of viral g34.5 gene product (He et al., 1996; reviewed
by Liebermann and Ho€man, 1998) (Figure 2). The
molecular mechanism via which the conserved carboxyl-terminal domains of g34.5 gene product and
Gadd34/MyD116 induce dephosphorylation of eIF2a
remains unclear. A recent study has demonstrated that
the g34.5 gene product can form a complex with
protein phosphatase 1a (PP1a) and PP1a may be
responsible for the dephosphorylation of eIF2a (He et
al., 1997) (Figure 2). It still remains unclear, however,
whether Gadd34/MyD116 plays a role similar to that
of the g34.5 gene product. Viruses with the mutant
g34.5 gene are unable to grow in the absence of
exogenous Gadd34/MyD116 C-terminal region (reviewed by Liebermann and Ho€man, 1998). This
could be because the endogenous Gadd34/MyD116
protein levels are very low and not sucient to
complement the mutant g34.5 phenotype. However,
this scenario appears paradoxical since overexpression
of exogenous Gadd34 is incompatible with cell survival
(Zhan et al., 1994). We have recently noted that the
upregulation of endogenous Gadd34 is tightly coupled
with the induction of apoptosis; overexpression of
exogenous GADD34 cDNA also induces apoptosis in
human neoplastic cells (Adler et al., in press and our
unpublished results). Furthermore, enforced overexpression of g34.5 gene appears to protect from the
growth inhibitory e€ects of Gadd34 (our unpublished
results). It, therefore, appears as if Gadd34 and g34.5
gene product may play opposing roles i.e. Gadd34
tends to induce apoptosis while the g34.5 gene product
inhibits apoptosis in the virally-infected cells. Whether
indeed the Gadd34-induced apoptotic e€ects are
mediated via alterations in protein translation,
remains to be elucidated.
Regulation of eIF4 in response to stress
The binding of cap structure at the 5'-end is facilitated
by various subunits of eIF4 including eIF4E, -4A, -4B,
-4F and -4G (reviewed by Clemens and Bommer, 1999;
Sachs et al., 1997). Together eIF4E, -4A and -4G form
a large complex known as eIF4F. eIF4E is a capbinding protein, eIF4A has a RNA helicase activity
and eIF4G is a sca€olding subunit that promotes
interactions with eIF3 (reviewed by Clemens and
Bommer, 1999; Sachs et al., 1997). eIF4E is believed
to be a critical subunit of eIF4 since not only does it
promote the direct interaction of 43S complex with
mRNA, but it also exists in limited molar amounts
(reviewed by Clemens and Bommer, 1999; Raught and
Gingras, 1999; Sachs et al., 1997).
Regulation of eIF4E: a MAP kinase connection
eIF4E is an important initiation factor that plays a
critical role in controlling the rate of protein
translation (reviewed by Raught and Gingras, 1999).
Transcriptional upregulation of eIF4E or its phosphorylation via MAPK pathway promotes eIF4E binding
to the `cap' and consequently enhances protein
translation (Wang et al., 1998; Waskiewicz et al.,
1999; reviewed by Clemens and Bommer, 1999; Raught
and Gingras, 1999; Sachs et al., 1997). In general,
eIF4E promotes cell growth and proliferation, inhibits
apoptosis and cause cellular transformation (reviewed
by Raught and Gingras, 1999). Protein kinase Mnk1 is
a substrate for both ERK1 and p38 MAPK (Fukunaga
and Hunter, 1997; Waskiewicz et al., 1997). ERK1 is
6123
Translational controls following stress
MS Sheikh and AJ Fornace
6124
Figure 3 Homology of Gadd34 with viral proteins. The human, hamster and mouse Gadd34 proteins are homologous to each
other and contain a carboxyl-terminal conserved region (dark blue) that is highly homologous to the corresponding regions of HSV1 g34.5 gene product (also known as ICP34.5) and African swine fever virus-NL protein. Lower panel shows the amino acid
sequence alignment of the carboxyl-terminal domain of Gadd34 with the corresponding amino acid sequence from the HSV-1 g34.5
gene product and the African swine fever virus-NL protein. The human, hamster and mouse Gadd34 proteins also contain
moderately conserved 30 ± 40 amino acids long repeats that are not present in viral proteins (Zhan et al., 1994)
activated in response to growth promoting signals
while, p38 is a stress-activated MAPK (Dhanasekaran
and Reddy, 1998). Both ERK1 and p38 can promote
eIF4E phosphorylation via Mnk1 (Wang et al., 1998;
reviewed by Clemens and Bommer, 1999; Raught and
Gingras, 1999). Evidence suggests that MAPKmediated phosphorylation of eIF4E appears to
enhance its ability to bind to the `cap' and thereby
increases the rate of translation initiation (reviewed by
Raught and Gingras, 1999). However, p38 also
promotes phosphorylation of eIF4E inhibitory proteins E4-BPs, which although do not a€ect eIF4E
binding to the `cap', do nevertheless prevent its
interaction with eIF4G and consequently with the
43S complex (Waskiewicz et al., 1999; reviewed by
Clemens and Bommer, 1999; Raught and Gingras,
1999). Thus, the activity of eIF4E can be a€ected
indirectly during stress and can ultimately cause
inhibition of protein synthesis (Figure 4).
Caspase-mediated cleavage of eIF4G
In general, apoptosis can be induced due to overall
shutdown of protein synthesis, which would suggest
that apoptosis can occur in the absence of de novo
protein synthesis. For example, the protein synthesis
inhibitor cycloheximide has been shown to induce
apoptosis (Ishii et al., 1995, 1997). PKR has also been
shown to induce apoptosis by inhibiting protein
synthesis via eIF2a phosphorylation (Lee and Este-
ban, 1994; Srivastava et al, 1998; Gil et al., 1999).
Overexpression of phosphorylation-de®cient eIF2a,
which acts in a dominant-negative manner, has been
shown to override the apoptotic as well as the
translational inhibitory e€ects of PKR (Gil et al.,
1999). A number of studies have reported that, in
certain situations, de novo protein synthesis appears
necessary for apoptosis to occur. For example, p53induced apoptosis has been shown to require new
protein synthesis in order for the full program to be
completed (Polyak et al., 1997). Similarly, nutrient
deprivation (Bulera et al., 1996; Schulz et al., 1996),
ionizing radiation (our unpublished results), and
nonsteroidal anti-in¯ammatory drugs (NSAIDs)
(Chan et al., 1998)-induced apoptosis can be inhibited
by cycloheximide suggesting that ongoing protein
synthesis is needed.
Whether protein synthesis is absolutely required for
apoptosis to occur, remains debatable and may vary
based on the stress and cell type. However, it is
becoming increasingly clear that a majority of the
apoptotic signals engage the preexisting intracellular
apoptotic machinery in the absence of de novo protein
synthesis. Caspases are an integral component of the
preexisting apoptotic machinery and exist in an
inactive state as procaspases that are processed into
catalytically active state in response to apoptotic
stimuli (reviewed by Nunez et al., 1998; Li and
Yuan, 1999). The caspase activation involves (i)
oligomerization-mediated autoactivation of upstream
Translational controls following stress
MS Sheikh and AJ Fornace
eIF4G in response to genotoxic and non-genotoxic
stress appears to occur via caspase 3 (Marissen and
Lloyd, 1998), also known as `the executioner' caspase
which cleaves a number of other death substrates
(reviewed by Nunez et al., 1998; Li and Yuan, 1999).
Thus, eIF4G has been identi®ed as one of many death
substrates and its cleavage during stress-induced
apoptosis appears yet another mechanism which can
modulate translation initiation (Figure 5).
Regulation of eIF1A121/SUI1, eIF3 and eIF5 genes following
stress
Figure 4 Stress-induced phosphorylation of 4E-BPs can indirectly a€ect eIF4E activity to inhibit translation initiation. Stressinduced activation of p38 MAPK promotes eIF4E phosphorylation via Mnk1. Phosphorylation of eIF4E potentiates its
interaction with `cap' and promotes translation initiation. p38
MAPK can also phosphorylate 4E-BPs, phosphorylated 4E-BPs
prevent interaction between eIF4E and eIF4G and thus inhibit
translation initiation
caspases and (ii) cleavage of downstream procaspases
by active upstream caspases (reviewed by Nunez et al.,
1998; Li and Yuan, 1999). Therefore, the activation of
the caspase cascade may occur in the complete absence
of de novo protein synthesis. It is nevertheless possible
that some apoptotic stimuli, albeit in a cell type-speci®c
manner, may still require the synthesis of certain
proapoptotic molecules that appear critical for the
activation of the caspase cascade (Figure 5). This
appears to be the case for p53, which may mediate its
apoptotic e€ects, at least in part, by transcriptionally
upregulating the expression of its downstream target
genes including BAX, DR5, FAS and PIGs (p53inducible genes) (Sheikh et al., 1998 and references
therein).
Recently, it was shown that genotoxic and nongenotoxic stress-induced apoptosis involves the proteolytic cleavage of eIF4G (Marissen and Lloyd, 1998;
Clemens et al., 1998). eIF4G plays a critical role in
translation initiation by bridging the interactions
between eIF4E and eIF3 (reviewed by Clemens and
Bommer, 1999). Thus, the proteolytic cleavage of
eIF4G during stress-induced apoptosis results in
protein synthesis inhibition (Marissen and Lloyd,
1998; Clemens et al., 1998). Therefore, an overall
decrease in the levels of various macromolecules
including the eIF4G following complete shutdown of
protein synthesis is conceivable. However, eIF4G is not
cleaved following complete inhibition of protein
synthesis (Marissen and Lloyd, 1998). Cleavage of
We have recently reported that mammalian eIF5 is a
DNA damage-inducible (DDI) gene (Sheikh et al.,
1997). Expression of the human eIF5 gene is induced
by UV-irradiation albeit in a cell type speci®c manner
(Sheikh et al., 1997). We have also cloned the full
length cDNA of another DDI transcript referred to as
A121 (Sheikh et al., 1997, 1999). The expression of
A121 was induced by UV-irradiation, methylmethane
sulfonate (MMS) and agents that cause endoplasmic
reticulum (ER)-stress (Sheikh et al., 1999). (The ER is
a site for calcium storage, translation, post-translational modi®cations, protein folding, and lipid and
sterol synthesis. ER-stress is induced by agents that
alter ER homeostasis (reviewed by Kaufman, 1999)).
The human A121 cDNA contains a 113 amino acid
long open reading frame which bears a high degree of
homology to yeast translation initiation factor Sui1
(Sheikh et al., 1999; Yoon and Donahue, 1992). To
con®rm that A121 cDNA encodes an eIF that is
equivalent to yeast Sui1p, the human A121 cDNA was
expressed in yeast Saccharomyces cerevisiae harboring
the mutant endogenous sui1 allele (Sheikh et al., 1999).
Expression of exogenous human A121 cDNA corrected
the mutant sui1 phenotype con®rming that human
A121 encodes a bona ®de eIF (Sheikh et al., 1999). The
predicted amino acid sequence of A121 exhibits perfect
identity to the partial length sequence of the recently
puri®ed human eIF1 (Pestova et al., 1998; Sheikh et
al., 1999), further demonstrating that A121 encodes the
human eIF1 (hereafter refer to as eIF1A121/SUI1).
The identi®cation of eIF5 and eIF1A121/SUI1 as
genotoxic stress-inducible genes (Sheikh et al., 1997,
1999) demonstrates, for the ®rst time, that the stressmediated regulation of eIFs can also occur at the
mRNA level. Furthermore, the expression of human
eIF1A121/SUI1 gene is regulated not only by genotoxic
stress but also by ER-stress inducing agents (Sheikh et
al., 1999). Further characterization of the human
eIF1A121/SUI1 gene revealed that it exhibits two transcripts of approximately 0.65 kb and 1.3 kb (Sheikh et
al., 1999). These transcripts contain a common coding
region but di€er in their 3'-untranslated region due to
alternative usage of di€erential poly(A)+ signals
(Sheikh et al., 1999). Interestingly, the long and short
eIF1A121/SUI1 transcripts were di€erentially regulated by
genotoxic and ER stress. For example, UV-irradiation
and the alkylating agent MMS upregulated the
expression of only the larger transcript whereas ERstress regulation of eIF1A121/SUI1 involved upregulation
of both transcripts (Sheikh et al., 1999).
The signi®cance of eIF1A121/SUI1 upregulation in
response to genotoxic and ER-stress is an issue that
6125
Translational controls following stress
MS Sheikh and AJ Fornace
6126
Figure 5 Stress-induced apoptosis may involve activation of caspase cascade; caspases can cleave cellular proteins that control
basic cellular processes including replication, transcription and building of cellular architecture. Stress-induced caspase-dependent
cleavage of eIF4G has recently been reported and can cause inhibition of protein synthesis. Generalized inhibition of any of the
basic cellular processes can activate apoptosis. In general apoptosis can occur in the absence of de novo proteins synthesis. However,
depending on the stimulus and cell type, the ongoing protein synthesis may also be required for apoptosis to occur
is currently being investigated. Yeast sui1 and two
other genes sui2 and SUI3 were originally identi®ed as
translation initiation suppressor loci (Castilho-Valavicius et al., 1990; Cigan et al., 1989). The products of
these genes have been implicated in controlling the
®delity of translation initiation, since mutations in any
one of these loci could restore the expression of the
HIS4 allele lacking the initiation codon AUG (Yoon
and Donahue, 1992; Castilho-Valavicius et al., 1990;
Cigan et al., 1989). SUI2 and SUI3 code for the a and
b subunits of eIF2, respectively (Cigan et al., 1989)
whereas SUI1 encodes the yeast equivalent of human
eIF1A121/SUI1 (Sheikh et al., 1999). A recent report has
described the puri®cation of human eIF1 and eIF1A
(previously known as eIF4C) (Pestova et al., 1998). It
has been reported that eIF1 and eIF1A in association
with the 43S complex form a 48S complex at the
initiation codon (Pestova et al., 1998). eIF1 and eIF1A
act in concert, since the 43S complex lacking eIF1 and
eIF1A does not reach the initiation codon (Pestova et
al., 1998). Human eIF1 alone can recognize and
destabilize the aberrantly formed complexes at the
initiation codon (Pestova et al., 1998), a ®nding which
is consistent with its role in controlling the ®delity of
translation initiation.
It is possible that the upregulation of eIF1A121/SUI1
expression following stress is a defensive response
whereby cells try to control the translation of certain
unwanted mRNAs whose products might otherwise be
deleterious (Figure 6). Support for this hypothesis
comes from recent ®ndings that eIF1A121/SUI1 not only
controls the ®delity of translation initiation (Pestova et
al., 1998; Yoon and Donahue, 1992; Cui et al., 1998)
Figure 6 Genotoxic and endoplasmic reticulum stress may
control translation initiation ®delity by upregulating the levels
of eIF1A121/SUI1
but its overexpression negatively a€ects cell proliferation. For example, when overexpressed, eIF1A121/SUI1
inhibits the colony forming eciency of human cancer
cells (Lian et al., 1999 and our unpublished results).
Recently, it has been shown that the hepatitis B virus
X antigen can downregulate the expression of
eIF1A121/SUI1 (Lian et al., 1999). Analyses of tumors
and matching normal tissues from HBV-infected
Translational controls following stress
MS Sheikh and AJ Fornace
patients su€ering from hepatocellular carcinoma
revealed the absence of eIF1A121/SUI1 gene expression
only in the hepatic tumors but not in the matching
normal hepatic tissues (Lian et al., 1999). It is
interesting that a variety of RNA viruses can
synthesize multiple proteins from a single transcript
by ribosomal frameshifting (Cui et al., 1998 and
references therein). It is not clear whether the hepatitis
B virus can also direct ribosomal frameshifting,
however since yeast sui1 has been implicated in
blocking negative frameshifting (Cui et al., 1998), it is
conceivable that certain viruses may downregulate the
expression of eIF1A121/SUI1 (which might otherwise
prevent negative frameshifting) and thereby gain a
survival advantage.
The molecular mechanisms underlying the DNA
damage repair and replication in the higher eukaryotes
are complex. It is expected that the DNA damage
induced by genotoxic stress in mammalian cells may
also be repaired by the error-prone DNA repair/
replication mechanisms (Hays et al., 1990; Viswanathan et al., 1999 and refs. therein) which can ®x
mutations that may also alter the translation initiation
codon. Recent evidence suggests that certain types of
DNA base modi®cations can cause RNA polymerase
to miscode at the site of lesions which can result in
mutated transcripts (Viswanathan et al., 1999 and
references therein). It is interesting that RNA
polymerase-induced miscoding results in G to A base
substitution (Viswanathan et al., 1999 and references
therein) and may also mutate the start codon (AUG to
AUU). Since eIFA121/SUI1 will not allow translation to
initiate at a non-AUG codon, it is, therefore tempting
to speculate that upregulation of eIF1A121/SUI1 following
genotoxic stress may provide yet another form of
checkpoint to control translation of mRNAs corresponding to genes that have a mutated start codon.
Similarly, it is possible that the upregulation of
eIF1A121/SUI1 following ER-stress may be critical in
order to exert control over ER-coupled translation as
well.
In this article, we have reviewed several lines of
evidence which support the notion that regulation of
translation initiation is an important component of
cellular stress response. While the stress-induced posttranslational regulation of eIFs has been well
documented, stress-induced regulation of eIFs at the
mRNA levels is beginning to be elucidated. Clearly, the
stress-mediated regulation of eIFs occurs at multiple
di€erent levels involving, transcriptional, post-transcriptional and post-translational controls. In the
future, it will be pertinent to investigate whether the
activity and expression of other eIFs, not covered in
this article, are also regulated (negatively or positively)
in response to stress. Our initial evidence demonstrates
that the expression of eIF1A, the interacting partner of
eIF1, is not regulated by stress in a manner similar to
that of eIF1A121/SUI1 (Sheikh et al., 1999 and our
unpublished results). The expression of one of the
subunits of eIF3, by contrast, is downregulated in
response to ionizing radiation in certain cells (our
unpublished results). Given that eIF1A121/SUI1 exhibits
several putative phosphorylation sites (Sheikh et al.,
1999), it will also be important to investigate (i)
whether it is phosphorylated in response to stress
and, if indeed it is, (ii) whether or not its activity is
altered following stress-induced phosphorylation. Finally, the issue of how stress-induced overexpression of
eIF1A121/SUI1 exactly a€ects translation initiation, will
require more in depth studies. We have reviewed only a
few examples of stress-induced regulation of eIFs,
clearly, much more remains to be done. Continued
investigations in this emerging area of cellular stress
response promise to provide more exciting clues, which
will certainly enhance our understanding of the
molecular mechanisms underlying the regulation of
protein translation in context to cellular stress
response.
Acknowledgments
We thank Drs L Augeri, R. Kaufman and Y Huang for
helpful discussions and comments.
References
Adler HT, Chinery R, Wu DY, Kussick SJ, Forance Jr AJ
and Tkachuck DC. Mol. Cell. Biol., (in press).
Bulera SJ, Sattler CA and Pitot HC. (1996). Hepatology, 23,
1591 ± 1601.
Caskey CT. (1980). Trends Biochem. Sci., 5, 234 ± 237.
Castilho-Valavicius B, Yoon HJ and Donahue T. (1990).
Genetics, 24, 483 ± 495.
Chan TA, Morin PT, Vogelstein B and Kinzler KW. (1998).
Proc. Natl. Acd. Sci. USA., 95, 681 ± 686.
Chen JJ and London IM. (1995). Trends Biochem. Sci., 20,
105 ± 108.
Chou J and Roizman B. (1992). Proc. Natl. Acad. Sci. USA,
89, 3266 ± 3270.
Chou J and Roizman B. (1994). Proc. Natl. Acad. Sci. USA,
91, 5247 ± 5251.
Chou J, Chen JJ, Gross M and Roizman B. (1995). Proc.
Natl. Acad. Sci. USA, 92, 10516 ± 10520.
Cigan AM, Pabich EK, Feng L and Donahue TF. (1989).
Proc. Natl. Acad. Sci. USA, 86, 2784 ± 2788.
Clark B. (1980). Trends Biochem. Sci., 5, 207 ± 210.
Clemens MJ, Bushell M and Morley SJ. (1998). Oncogene,
17, 2921 ± 2931.
Clemens MJ and Bommer U-A. (1999). Int. J. Biochem. Cell
Biol., 31, 1 ± 23.
Cui Y, Dinman JD, Kinzy TG and Peltz S. (1998). Mol. Cell.
Biol., 18, 1506 ± 1516.
Dhanasekaran N and Reddy EP. (1998). Oncogene, 17,
1447 ± 1455.
Fornace Jr AJ. (1992). Annu. Rev. Genet., 26, 507 ± 526.
Fornace Jr AJ, Alamo IJ and Hollander MC. (1988). Proc.
Natl. Acad. Sci. USA, 85, 8800 ± 8804.
Fukunaga R and Hunter T. (1997). EMBO J., 16, 1921 ±
1933.
Gil J, Alcami J and Esteban M. (1999). Mol. Cell. Biol., 19,
4653 ± 4663.
Haro CD, Mendez R and Santoyo J. (1996). FASEB J., 10,
1378 ± 1387.
Hays JB, Ackerman EJ and Pang QS. (1990). Mol. Cell. Biol.,
10, 3505 ± 3511.
6127
Translational controls following stress
MS Sheikh and AJ Fornace
6128
He B, Chou J, Liebermann DA, Hofman B and Roizman B.
(1996). J. Virol., 70, 84 ± 90.
He B, Gross M and Roizman B. (1997). Proc. Natl. Acad.
Sci. USA, 94, 843 ± 848.
Hershey JWB, Mathews MB and Sonenberg N. (1996).
Translational control. Cold Spring Habor Laboratory
Press: Cold Spring Harbor, New York.
Hinnebusch AG. (1994). Trends Biochem. Sci, 19, 409 ± 414.
Ishii H, Tamauchi H and Gob GC. (1997). Int. J. Exp.
Pathol., 78, 123 ± 131.
Ishii H, Etheridge MR and Gob GC. (1995). Immunol. Cell
Biol., 73, 463 ± 468.
Kaufman RJ. (1999). Genes Dev., 13, 1211 ± 1233.
Lee SB and Esteban M. (1994). Virology, 199, 491 ± 496.
Lewis JD and Izaurralde E. (1997). Eur. J. Biochem., 247,
461 ± 469.
Liebermann DA and Ho€man B. (1998). Oncogene, 17,
3319 ± 3329.
Lian Z, Pan J, Liu J, Zhang S, Zhu M, Arbuthnot P, Kew M
and Feitelson MA. (1999). Oncogene, 18, 1677 ± 1687.
Li H and Yuan J. (1999). Curr. Opin. Cell Biol., 11, 261 ± 266.
Marissen WE and Lloyd RE. (1998). Mol. Cell. Biol., 18,
7565 ± 7574.
Meurs E, Chong K, Galabru J, Thomas NSB, Kerr IM,
Williams BRG and Hovanessian AG. (1990). Cell, 62,
379 ± 390.
Morgan SE and Kastan MB. (1997). Adv. Cancer Res., 71,
1 ± 25.
Nunez G, Benedict MA, Hu Y and Inohara, N. (1998).
Oncogene, 17, 3237 ± 3245.
Pathak VK, Nielsen PJ, Trachsel H and Hershey JWB.
(1988). Cell, 54, 633 ± 639.
Pellegata NS, Antoniono RJ, Redpath JL and Stanbridge EJ.
(1996). Proc. Natl. Acad. Sci. USA, 93, 15209 ± 15214.
Pestova TV, Borukhov S and Hellen CUT. (1998). Nature,
394, 854 ± 859.
Polyak K, Xia Y, Zweier JL, Kinzler KW and Vogelstein B.
(1997). Nature, 389, 300 ± 305.
Raught B and Gingras A-C. (1999). Int. J. Biochem. Cell
Biol., 31, 43 ± 57.
Sachs AB, Sarnow P and Hentze MW. (1997). Cell, 89, 831 ±
838.
Schmedt C, Green SR, Manche L, Talor DR, Ma Y and
Mathews MB. (1995). J. Mol. Biol., 249, 29 ± 44.
Schulz JB, Weller M and Klockgether T. (1996). J. Neurosci.,
16, 4696 ± 4706.
Schwartz D and Rotter V. (1998). Semin. Cancer Biol., 8,
325 ± 336.
Sheikh MS, Fernandez-Salas E, Yu M, Hussain A, Dinman
JD, Peltz SW, Huang Y and Fornace Jr AJ. (1999). J. Biol.
Chem., 274, 16487 ± 16493.
Sheikh MS, Burns TF, Huang Y, Wu GS, Amundson S,
Brooks KS, Fornace Jr AJ and El-Deiry WS. (1998).
Cancer Res., 58, 1593 ± 1598.
Sheikh MS, Carrier F, Papathanasiou MA, Hollander MC,
Zhan Q, Yu K and Fornace Jr AJ. (1997). J. Biol. Chem.,
272, 26720 ± 26726.
Srivastava SP, Kumar KU and Kaufman RJ. (1998). J. Biol.
Chem., 273, 2416 ± 2423.
Su ZZ and Fisher PB. (1997). Proc. Natl. Acad. Sci. USA, 94,
9125 ± 9130.
Viswanathan A, You HJ and Doetsch PW. (1999). Science,
284, 159 ± 162.
Wang X, Flynn A, Waskiewicz AJ, Webb BLJ, Vries RG,
Baines IA, Cooper JA and Proud CG. (1998). J. Biol.
Chem., 273, 9373 ± 9377.
Waskiewicz AJ, Johnson JC, Penn B, Mahalingam M,
Kimball SR and Cooper JA. (1999). Mol. Cell. Biol., 19,
1871 ± 1880.
Waskiewicz AJ, Flynn A, Proud CG and Cooper JA. (1997).
EMBO J., 16, 1909 ± 1920.
Yoon HJ and Donahue TF. (1992). Mol. Cell. Biol., 12, 248 ±
260.
Zhan Q, Lord KA, Alamo Jr I, Hollander MC, Carrier F,
Ron D, Kohn KW, Ho€man B, Liebermann DA, Fornace
Jr AJ. (1994). Mol. Cell. Biol., 14, 2361 ± 2371.