Download Phosphorylation of eIF2α in response to 26S proteasome inhibition

Biochem. J. (2008) 412, 579–588 (Printed in Great Britain)
579
doi:10.1042/BJ20080324
Phosphorylation of eIF2α in response to 26S proteasome inhibition is
mediated by the haem-regulated inhibitor (HRI) kinase
Azmi YERLIKAYA*, Scot R. KIMBALL† and Bruce A. STANLEY‡1
*Department of Biology, Faculty of Science and Arts, University of Dumlupinar, Kutahya, Turkey, †Department of Cellular and Molecular Physiology, The Pennsylvania State University
College of Medicine, Hershey, PA 17033, U.S.A., and ‡The Pennsylvania State University College of Medicine, Section of Research Resources, Hershey, PA 17033, U.S.A.
The present study demonstrates that even brief inhibition of
degradation by the 26S proteasome inhibits global protein
synthesis, mediated through increased phosphorylation of eIF2α
(eukaryotic translational initiation factor 2α) by the HRI (haemregulated inhibitor) kinase. Exposure of COS-7 cells to the
proteasome inhibitor MG-132 (the proteasome inhibitor carbobenzoxy-L-leucyl-L-leucyl-leucinal) for 4 h resulted in a 55–
60 % decrease in protein synthesis rate compared with control
cells. This repression of protein synthesis after treatment with
MG-132 is not due to induction of apoptosis, which is known
to occur after longer periods of 26S inhibition. Instead, we
observed a significantly increased phosphorylation of eIF2α,
which is known to repress global protein synthesis. In three
MEF (mouse embryonic fibroblast) knockout cell lines lacking
one of the four kinases known to phosphorylate eIF2α, increased
phosphorylation of eIF2α still occurred after inhibition of the
26S proteasome. These three cell lines included a deletion of
the PKR (double-stranded-RNA-dependent protein kinase); a
deletion of the PERK (PKR-like endoplasmic reticulum resident
kinase); or a deletion of the GCN2 (positive general control of
transcription-2) kinase, indicating that none of these kinases was
primarily responsible for the observed phosphorylation of eIF2α.
In contrast, in a fourth MEF knockout cell line, HRI−/− cells
lacking the HRI kinase failed to increase eIF2α phosphorylation
upon proteasome inhibitor treatment (MG-132 or various doses
of Bortezomib), indicating that the HRI kinase is the primary
kinase activated by brief treatment of MEFs with 26S proteasome
inhibitors.
INTRODUCTION
[5,6]. Since intracellular levels of eIF2B are approx. 10–20 %
that of eIF2 in the cytoplasm, phosphorylation of as little as
10 % of eIF2 can be sufficient to sequester virtually all the
available eIF2B, thereby blocking the eIF2B exchange activity
and therefore inhibiting protein synthesis completely [4,6]. eIF2α
is known to be specifically phosphorylated at Ser51 by at least
four different kinases including the interferon-inducible doublestranded RNA-activated PKR (double-stranded-RNA-dependent
protein kinase; in response to viral infection and stress conditions),
the HRI (haem-regulated inhibitor) kinase, the nutrient-regulated
protein kinase GCN2 (positive general control of transcription-2;
in response to uncharged tRNA in nutrient-deprived cells) and
PERK [PKR-like ER (endoplasmic reticulum)-resident kinase; in
response to accumulation of unfolded protein in the ER] [4,5,7].
The 26S proteasome is an ATP-dependent proteolytic system
that is engaged in the selective degradation of short-lived proteins
under normal metabolic conditions, bulk degradation of long-lived
proteins, partial digestion/processing of some proteins [e.g. NFκB (nuclear factor κB)] and antigen presentation. CDK (cyclindependent kinase) inhibitors, M-, S- and G1 -phase-specific
cyclins, p53, ODC (ornithine decarboxylase) and the transcription
factors c-Jun and c-Fos are a few examples of the multitude
of proteins degraded by the 26S proteasome [8]. Previously,
several contradictory studies on the effect of the 26S proteasome
inhibition on protein synthesis rate have been published. For
instance, Schubert et al. [9] indicated that treating HeLa cells
The dominant mechanism of control of global protein synthesis is
phosphorylation/dephosphorylation of translational components,
although other mechanisms, such as cleavage of initiation
factors, can also affect protein synthesis rates, for example
during apoptosis or after viral infection. Phosphorylation of eIF2
(eukaryotic translational initiation factor 2) appears to be a general
mechanism for inhibiting the initiation of protein synthesis [1].
eIF2 is composed of three subunits termed α, β and γ in the
order of increasing molecular mass. The primary role of eIF2 in
translational initiation is to transfer methionyl-tRNA [Met-tRNAi
(initiator methionine tRNA)] to the 40S ribosomal subunits [2].
First, eIF2 forms a ternary complex with Met-tRNAi and GTP;
this ternary complex then binds to the 40S ribosomal subunit
to generate the 43S pre-initiation complex [3]. Upon joining of
the 60S ribosomal subunit to the 43S pre-initiation complex, the
GTP moiety is hydrolysed and an eIF2–GDP complex is released
from the ribosome [4]. In order to promote another round of initiation, the GDP bound to eIF2 must be exchanged for GTP, a
reaction carried out by GEF (guanine-nucleotide-exchange factor)
eIF2B [2].
The global rate of protein synthesis is mainly regulated by the
specific phosphorylation of Ser51 of the eIF2α subunit [5].
eIF2α(P) (phosphorylated eIF2α) cannot undergo GDP/GTP
exchange and forms a non-dissociable eIF2α(P)–eIF2B complex
Key words: apoptosis, eukaryotic translational initiation factor 2α
(eIF2α), protein degradation, protein kinase, protein synthesis,
26S proteasome.
Abbreviations used: AdoMet, S -adenosylmethionine; BCS, biodegradable counting solution; CDK, cyclin-dependent kinase; CHX, cycloheximide;
DMEM, Dulbecco’s modified Eagle’s medium; ECF, enhanced chemifluorescence; eIF2, eukaryotic translational initiation factor 2; eIF2α(P), phosphorylated
eIF2α; ER, endoplasmic reticulum; FCS, fetal calf serum; GCN2, positive general control of transcription-2; HRI, haem-regulated inhibitor; Hsp, heatshock protein; MEF, mouse embryonic fibroblast; PKR, double-stranded-RNA-dependent protein kinase; PERK, PKR-like ER-resident kinase; PITC,
phenylisothiocyanate; TBS-T, Tris-buffered saline with Tween 20; TCA, trichloroacetic acid; UPR, unfolded protein response; Z-Val-Ala-DL-Asp-CH2 F,
benzyloxycarbonyl-valylalanyl-DL-aspartylfluoromethane.
1
To whom correspondence should be addressed (email [email protected]).
c The Authors Journal compilation c 2008 Biochemical Society
580
A. Yerlikaya, S. R. Kimball and B. A. Stanley
with a cocktail of proteasome inhibitors (zLLL/lactacystin) for
2 or 4 h profoundly decreased protein synthesis [9]. Similarly,
Mimnaugh et al. [10] have also shown that the proteasome
inhibitor, lactacystin, decreased the synthesis of most cellular
proteins, while it specifically induced the synthesis of stress
proteins Hsp72 (heat-shock protein 72) and Hsp90 in human
SKBr3 breast tumour cells [10]. Jiang and Wek [11] later indicated
that the reduced levels of translation in response to proteasome
inhibition were caused by increased phosphorylation of eIF2α,
which was mediated through the activation of GCN2 [11]. In
contrast, Bush et al. [12] reported that MG-132 (the proteasome
inhibitor carbobenzoxy-L-leucyl-L-leucyl-leucinal) did not affect
total protein synthesis even after an 18 h treatment of canine
kidney cells.
During our recent studies on the mechanism of degradation of
S-adenosylmethionine decarboxylase (a short-lived, polyamine
biosynthetic enzyme), we found that inhibition of the 26S
proteasome causes a significant increase (>2-fold) in the cellular
level of the enzyme’s substrate, AdoMet (S-adenosylmethionine)
[13]. The present studies trace this increase in AdoMet levels to
an increase in the level of its precursor, methionine. Methionine
levels in turn were increased due to a general increase in
amino acid levels resulting from decreased protein synthesis
and therefore decreased utilization of amino acids. We therefore
decided to investigate the molecular mechanisms responsible for
the decreased protein synthesis rate after inhibition of the 26S
proteasome. Although decreased protein synthesis rates occur
during apoptosis, and inhibition of 26S proteasome has been
reported to induce apoptosis in several different experimental
systems [14–17], our results indicate that loss of protein synthesis
activity after short periods of inhibition of the 26S proteasome is
not accompanied by any signs of induction of apoptosis. Instead,
we show that inhibition of the 26S proteasome significantly
increases eIF2α phosphorylation, which is thus the primary cause
of loss of protein synthesis activity, in agreement with other
published work. By testing four knockout cell lines with individual deletions for each of the four kinases known to
phosphorylate eIF2α, we also demonstrate for the first time that
HRI is the primary kinase responsible for the increased eIF2α
phosphorylation caused by the proteasome inhibitor in MEFs
(mouse embryonic fibroblasts).
EXPERIMENTAL
Materials
ECF (enhanced chemifluorescence) detection reagent for Western
blotting was purchased from Amersham (now GE, Arlington
Heights, IL, U.S.A.). COS-7 cells were obtained from the
A.T.C.C. (Manassas, VA, U.S.A.). Wild-type (PERK+/+ ) and
PERK-knockout cells were kindly provided by Dr David
Ron (New York University School of Medicine). Wild-type
(PKR+/+ ) and PKR−/− MEF cells were a gift from Dr Charles
Weissmann (University of Zurich). Wild-type (GCN2+/+ ) and
GCN2−/− MEF cells were a gift from Dr Douglas Cavener
(The Pennsylvania State University). Bortezomib (also known
as PS-341 or Velcade) was a gift from Dr Vincent Chau
(The Pennsylvania State University College of Medicine).
MG-132 and caspase inhibitor I [Z-Val-Ala-DL-Asp-CH2 F
(benzyloxycarbonyl-valylalanyl-DL-aspartylfluoromethane); ‘ZVAD-FMK’] were purchased from Calbiochem (San Diego,
CA, U.S.A.). DMEM (Dulbecco’s modified Eagle’s medium)
was from Gibco BRL (Gaithersburg, MD, U.S.A.). FCS (fetal
calf serum) was from HyClone (Logan, UT, U.S.A.). PVDF
membranes were obtained from Osmonics (Westborough, MA,
c The Authors Journal compilation c 2008 Biochemical Society
U.S.A.). Anti-caspase 3 and anti-caspase 8 antibodies were
kindly provided by Dr Shao-Cong Sun (Penn State University
College of Medicine, currently at the M.D. Anderson Cancer
Center, Houston, TX, U.S.A.). Alkaline phosphatase-conjugated
goat anti-mouse IgG was from Bio-Rad (Hercules, CA, U.S.A.),
and [methyl-3 H]methionine (84 Ci/mmol), [5-3 H]proline (15–
40 Ci/mmol) and BCS (biodegradable counting solution)
were from Amersham (now GE). PITC (phenylisothiocyanate)
was from Pierce (Rockford, IL, U.S.A.). CHX (cycloheximide),
TCA (trichloroacetic acid) and the protein standard (BSA) were
from Sigma (St. Louis, MO, U.S.A.). All other chemicals were
obtained from Fisher (Pittsburgh, PA, U.S.A.).
Cell culture maintenance
COS-7, wild-type PERK+/+ and PERK−/− knockout cells were
maintained in DMEM (plus 4.5 g/l glucose) supplemented with
10 % (v/v) FCS, 2 mM glutamine, 100 μg/ml streptomycin and
100 units/ml penicillin in a humidified atmosphere at 37 ◦C with
5 % CO2 . Wild-type (PKR+/+ ) and PKR−/− cells were maintained
in DMEM with 10 % FBS, 0.1 mM 2-mercaptoethanol, 1 mM
sodium pyruvate, 100 μg/ml streptomycin and 100 units/ml
penicillin in a humidified atmosphere at 37 ◦C with 5 % CO2 .
Stock cultures were maintained in 75 cm2 Corning flasks, and
experimental cultures were grown in 10 or 6 cm2 Falcon plates.
The cells were subcultured when they were 70–80 % confluent.
Western blotting
The ECF detection system was utilized for Western blotting
according to the manufacturer’s instructions. Proteins were
electrophoretically separated on 12.5 % (w/v) polyacrylamide
gels under denaturing conditions in 0.1 % SDS [18]. The proteins
were then transferred on to a PVDF membrane at 45 V for
50 min at room temperature (20 ◦C) or overnight at 30 V and 4 ◦C
in the transfer buffer [25 mM Tris, 192 mM glycine and 20 %
(v/v) methanol]. When proteins were transferred overnight, the
next morning the voltage was raised to 90 V for an additional
hour to complete the transfer. Membranes were incubated with
a rabbit polyclonal antibody (1:4000 dilution) that specifically
recognizes eIF2α which is phosphorylated at Ser51 , followed by
alkaline phosphatase-conjugated anti-rabbit secondary antibody
(1:4000 dilution) in TBS-T (Tris-buffered saline with Tween
20). The relative amount of total eIF2α in each sample was
determined by probing the membranes with a monoclonal
antibody (1:500 dilution) that recognizes both the phosphorylated
and unphosphorylated forms of eIF2α, followed by alkaline
phosphatase-conjugated anti-mouse secondary antibody (1:4000
dilution). eIF4G was detected with a rabbit polyclonal antibody
(1:3000 dilution). Proenzymes and the cleaved products indicating
activation of caspase 3 or caspase 8 were detected with
rabbit polyclonal antibodies used at 1:2000 or 1:800 dilution
respectively. In each case, the membranes were then extensively
washed with TBS-T and developed with ECF substrate for 3–
5 min. The fluorescence developed was scanned with a Molecular
Dynamics FluorImager using a 570 nm filter, and bands were
quantified using ImageQuant software (Molecular Dynamics).
Determination of protein synthesis rate and amino acid uptake
Cells were seeded in 10 cm2 dishes and treated with MG-132 for
4 h (at approx. 80 % confluence). After 4 h of treatment, cells were
incubated with 0.4 μCi/ml [5-3 H]proline (15–40 Ci/mmol) or with
0.4 μCi/ml [methyl-3 H]methionine (84 Ci/mmol) for 20 min. The
medium was then removed and cells were washed twice with PBS.
Cells were lysed and unincorporated radioactive amino acids were
Phosphorylation of eIF2α by HRI kinase after 26S proteasome inhibition
removed by washing cells once for 10 min and twice for 5 min
each with 10 % (w/v) TCA on ice. After washing once with
methanol, cells lysates were neutralized with 1 ml of 0.3 M NaOH
and 1 % SDS for 30 min at room temperature. The lysate (800 μl)
was added to 10 ml of BCS and counted. To measure amino acids
uptake, cells were similarly labelled with 0.4 μCi/ml [3 H]proline
or 1 μCi/ml [3 H]methionine. After labelling, cells were washed
twice with PBS to remove extracellular (unabsorbed) amino acids,
and then lysed in 1 ml of 0.3 M NaOH and 1 % SDS for 30 min
at room temperature. The intracellular radioactivity released into
the lysate (800 μl) was counted in 10 ml of BCS [19].
Analysis of apoptotic cells by flow cytometry
Apoptosis was analysed based on the translocation of
phosphatidylserine from the inner side of the plasma membrane
to the outer layer during the early stages of apoptosis [20]. Equal
numbers of cells were seeded in 10 cm2 dishes, and grown until
they were 70 % confluent. Then cells were treated with MG132 (50 μM) or DMSO (0.1 %) for 4 h. After treatment, cells
were washed gently with 10 ml of PBS twice, and incubated
with trypsin for 3 min. After removal of trypsin, cells were
incubated at 37 ◦C just until they appeared to be detached.
Cells were then washed with PBS and centrifuged at 200 g for
5 min. Cells were resuspended in 100 μl of annexin V binding
buffer (10 mM Hepes/NaOH, pH 7.4, 5 mM CaCl2 and140 mM
NaCl) containing annexin V–FITC (FITC-labelled annexin V)
and propidium iodide (50 μg/ml). Cells were incubated in this
solution by shaking at room temperature for 15 min. Samples
were then diluted in Hepes buffer and subjected immediately to
flow-cytometry analysis using a FACS Scan II flow cytometer
in the Pennsylvania State University College of Medicine Flow
Cytometry Facility. Early-stage apoptotic cells stain with annexin
V only, while late-stage apoptotic cells as well as necrotic cells
stain with both annexin V and propidium iodide.
Amino acid analysis by HPLC
Cells were lysed in 0.1 M HCl by three quick freeze/thaw cycles.
After centrifugation at 16 000g for 10 min at 4 ◦C, the pellet was
resuspended in 0.3 M NaOH for the protein assay. The supernatant
containing free amino acids was then dried down by centrifugal
evaporation (SpeedVac), resuspended in redry solution [20 %
triethylamine and 40 % (v/v) ethanol], and dried down again.
Amino acids were then resuspended in derivatization solution
(70 % ethanol, 10 % triethylamine and 10 % PITC) for 10 min.
After drying again by centrifugal evaporation, the samples were
resuspended in sample diluent buffer [95 % 2.6 mM Na2 HPO4 ,
pH 7.4 (pH was adjusted with 10 % phosphoric acid), and 5 %
acetonitrile], and were directly injected for HPLC analysis. The
HPLC system consisted of a Waters 600E multisolvent delivery
system equipped with a Waters 484 tunable absorbance detector
and a 30 cm × 3.9 mm C18 column. Amino acids were detected
at 254 nm. The mobile phase A (solution A) consisted of 94 %
140 mM sodium acetate trihydrate, 0.05 % triethylamine (pH
was adjusted to pH 6.4 with acetic acid) and 6 % acetonitrile.
The mobile phase B (solution B) was 60 % acetonitrile, which
was degassed prior to use by sonication (using a Bronson 2200
ultrasonic bath) under vacuum for 20 s. The gradient conditions
were initially 100 % solution A for 13.5 min, followed by an
immediate change to 3 % solution B and then a linear change to
23 % solution B over 16.5 min; then to 71 % solution A and
29 % solution B over 13 min; then to 34 % solution B over
7 min; maintained isocratically at 34 % solution B for 10 min;
then to 100 % solution B over 0.5 min, maintained isocratically
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Table 1 Effect of 26S proteasome inhibition on amino acid levels in
COS-7 cells
Cells were treated with MG-132 (50 μM) or DMSO (0.1 %) for 4 h. The results are presented
as fold increase (MG-132/control). The values are means +
− S.D. (n = 2).
Amino acid
Fold increase
Threonine
Tyrosine
Valine
Methionine
Leucine
Phenylalanine
Lysine
2.07 +
− 0.31
2.12 +
− 0.20
2.09 +
− 0.18
1.70 +
− 0.12
2.14 +
− 0.14
2.18 +
− 0.11
1.82 +
− 0.22
at 100 % solution B for 2.5 min; then to 100 % solution A
over 2 min; and maintained at 100 % solution A for 3 min. The
flow rate was 1 ml/min, but was raised to 1.5 ml/min during
the final 3 min washing of the column with 100 % solution A. To
calculate methionine concentration, a standard curve was obtained
as a function of known concentrations of methionine and the
corresponding peak areas by using linear regression with Prism
3.0 software (GraphPad Software, San Diego, CA, U.S.A.). The
methionine concentration in each sample was then calculated from
the sample attenuance and the standard curve.
Protein determination
The protein concentration was determined using the Bio-Rad dyebinding assay. BSA was used as a standard [21].
Statistical analysis
Statistical analyses were carried out with Prism 3.0 software
(GraphPad Software). Student’s two-tailed t test was used to
determine whether the observed differences were statistically
significant (P < 0.05).
RESULTS
During our studies of the mechanism of S-adenosylmethionine
decarboxylase degradation, we found that a significant increase
in AdoMet level (>2-fold) occurred shortly after inhibition of
the 26S proteasome [13]. One potential mechanism to account
for this observed increase in [AdoMet] could be that inhibition
of the 26S proteasome stabilized AdoMet synthase, the enzyme
that catalyses the production of AdoMet from methionine and
ATP, thereby increasing the amount of AdoMet synthase activity.
Using Western-blot analysis, we determined that the amount of
one of the AdoMet synthase subunits increased only 30 % after
4 h inhibition of the 26S proteasome, whereas other subunits
were not increased at all (results not shown), suggesting that
there must be additional mechanisms involved in producing
the increase in AdoMet level observed after inhibition of the
26S proteasome. A second potential mechanism for the increase
[AdoMet] would be an increase in the availability of the
methionine substrate for the AdoMet synthase-catalysed reaction
after 26S proteasome inhibition. Analysis of amino acid profiles after 4 h MG-132 (50 μM) treatment of COS-7 cells indicated
that inhibition of the 26S proteasome resulted in a 1.7–2.1-fold
increase in the level of all detectable amino acids including
methionine (Table 1), with methionine concentrations in MG132-treated cells rising to 12.8 pmol/mg of protein as compared
with 7.2 pmol/mg of protein in control cells. This strongly
c The Authors Journal compilation c 2008 Biochemical Society
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A. Yerlikaya, S. R. Kimball and B. A. Stanley
Figure 1 Effect of the 26S proteasome inhibitor on protein synthesis in
COS-7 cells
To determine the protein synthesis rate after MG-132 (50 μM) or 0.1 % DMSO treatment for 4 h,
cells were labelled by addition of either 0.5 μCi/ml [3 H]proline or 0.4 μCi/ml [3 H]methionine to
the complete media for 20 min. Cells were washed extensively with PBS and with 10 % TCA for
10 min to remove unincorporated radioactive precursor. Cells were washed with 10 % TCA twice
more for 5 min each. After a final wash with 100 % methanol, cells were lysed in 0.3 M NaOH
and 1 % SDS for 30 min at room temperature. Aliquots were counted by liquid-scintillation
3
counting in 10 ml of BCS. Values are means +
− S.E.M. (n = 4 in the [ H]proline experiment;
n = 3 in the [3 H]methionine experiment). The P values significantly different from that of the
control are indicated (Student’s t test). The S.E.M. is small and not seen in the MG-132-treated
group in the [3 H]methionine incorporation experiment (plotted on the right Y -axis).
suggested that the increase in AdoMet levels observed was
probably due to an increase in the availability of methionine as
a substrate. This increase in intracellular methionine (or other
amino acids) concentration could occur either (i) through an
increase in uptake of methionine from the medium; (ii) through
down-regulation of protein synthesis (less usage of intracellular
amino acids for protein synthesis); or (iii) through an increase
in protein degradation activity, thus supplying more methionine
through release from degraded proteins. This third possibility
was highly unlikely under these experimental conditions, since
the major intracellular protein degradation apparatus (the 26S
proteasome) was inhibited. Using [3 H]methionine or [3 H]proline
in separate experiments, it was determined that inhibition of
the 26S proteasome did not affect amino acid uptake (results
not shown). Therefore we next decided to investigate possible
changes in the rate of protein synthesis after cell exposure to MG132. As seen in Figure 1, the determination of protein synthesis
rate by [3 H]proline incorporation into TCA-precipitable proteins
indicated that treatment of COS-7 cells with MG-132 (50 μM) for
4 h reduced the rate of protein synthesis by ∼ 55 % as compared
with the control cells (treated with 0.1 % DMSO). Likewise, a
∼ 60 % decrease in protein synthesis rate was observed in MG132-treated cells when [3 H]methionine was used as a radiolabel
(Figure 1, plotted on the right side of the graph).
As noted in the Introduction section, it has previously been
shown that apoptosis can be induced by different proteasome
inhibitors, at least at longer exposure times than used in the current
experiments, probably as a consequence of the accumulation
of some crucial proteins that would normally be degraded by
the ubiquitin–proteasome pathway [14–16,22,23]. Therefore we
decided to investigate whether apoptosis was induced in COS-7
cells after short-term inhibition of the 26S proteasome, thereby
possibly accounting for the observed decrease in protein synthesis.
First, we investigated whether the apoptosis executioner caspase
3, which is an enzyme activated in almost every apoptotic process
[14,16], or caspase 8, another early caspase, were cleaved to
their active forms after inhibition of the 26S proteasome. As can
been seen in the Western-blot analyses in Figures 2(A) and 2(B),
no disappearance of caspase 3 or caspase 8 precursor protein
c The Authors Journal compilation c 2008 Biochemical Society
Figure 2 Loss of protein synthesis activity after inhibition of the 26S
proteasome is not due to activation of caspases
(A–B) Analyses of caspase 3 (A) and caspase 8 (B) activation. After treatment of COS-7 cells
with MG-132 (50 μM) or 0.1 % DMSO, equal amounts of protein (50 μg) from cell lysates
were resolved by electrophoresis on a 12.5 % polyacrylamide gel, followed by transfer to a
PVDF membrane at 50 V at room temperature for 45 min. The membranes were probed with
a caspase 3 polyclonal antibody (diluted 1:2000) overnight at 4 ◦C. The membrane was then
washed extensively with TBS-T and incubated with alkaline phosphatase-conjugated anti-rabbit
IgG (diluted 1:4000). The membrane was developed with ECF detection reagent (Amersham) and
visualized with a FluorImager. The same membrane was stripped and reprobed with a caspase
8 polyclonal antibody (diluted 1:800), and developed as described above.
was detected after addition of the proteasome inhibitor MG-132
to COS-7 cells for up to 4 h. Secondly, the percentage of cells
undergoing apoptosis after 4 h treatment of cells with MG-132
was the same as in untreated control cells, as measured by flowcytometry analysis of annexin V and propidium iodide staining
(results not shown), again indicating that no apoptosis was induced
at early time points after MG-132 addition. Pre-incubation of
COS-7 cells with the broad-spectrum caspase inhibitor I (Z-ValAla-DL-Asp-CH2 F; 35 μM) for 1 h before addition of a fresh
medium containing MG-132 and caspase inhibitor I also failed
Phosphorylation of eIF2α by HRI kinase after 26S proteasome inhibition
Figure 3 Effect of the proteasome inhibitor on eIF4G cleavage and
phosphorylation of eIF2α in COS-7 cells
(A) COS-7 cells were treated with MG-132 (50 μM) for time periods indicated. Equal amounts of
protein (50 μg) were separated on an SDS/7.5 % PAGE gel, transferred on to a PVDF membrane
and detected with an eIF4G polyclonal antibody as described in the Experimental section.
(B) To detect eIF2α(P) and total eIF2α, equal amounts of protein (50 μg) were separated on
an SDS/12.5 % PAGE gel, followed by Western-blot analysis as described in the Experimental
section. (C) The results were quantified by ImageQuant software, and the results are expressed
as the level of eIF2α(P) divided by the total level of eIF2α, with zero time set as 1. Values are
means +
− S.E.M. (n = 2). The error bars are small and not visible for some data points.
to prevent the loss of protein synthesis activity 4 h later (results
not shown). This time-dependent experiment also indicated that
the loss of protein synthesis activity occurs as early as 2 h after
addition of MG-132. Altogether, these results indicate that loss
of protein synthesis activity after short-term (2–4 h) inhibition of
the 26S proteasome is independent of and precedes any induction
of apoptosis.
Post-translational processing of some eukaryotic initiation
factors, in particular eIF4G and eIF2, is known to regulate the
global rate of protein synthesis in response to physiological
insults. For example, picarnovirus infections or antitumour agents
cisplatin or etoposide induce the cleavage of eIF4G, which also
occurs during apoptosis and which completely shuts off protein
synthesis [24,25]. Therefore we next examined the stability of
eIF4G after treatment of cells with MG-132. The Western blot
583
in Figure 3(A) shows that the intact eIF4G, which runs as a
set of three closely spaced bands of approx. 220 kDa, was not
cleaved into any visible smaller fragments after 2 h of MG-132
treatment, at which point protein synthesis is already inhibited.
Although a faint immunoreactive band of the anticipated size
(∼ 70 kDa) was observed in treated cells after 4 h of MG-132
treatment, the amount of intact and presumably functional eIF4G
was not significantly affected by MG-132 treatment, eliminating
eIF4G cleavage as a viable explanation of the observed 55–60 %
decrease in protein synthesis rate.
To determine whether 26S proteasome inhibition can cause
an increase in eIF2α phosphorylation, COS-7 cells were treated
with MG-132 (50 μM) in 0.1 % DMSO, and the relative
phosphorylation state of eIF2α was determined by Western-blot
analysis. The blots were probed with a polyclonal antibody that
only recognizes eIF2α(P) (phosphorylated at Ser51 ). The blots
were then stripped and reprobed with a monoclonal antibody that
equally recognizes both phosphorylated and unphosphorylated
forms of the protein, thus measuring total eIF2α levels. As shown
in Figure 3(B) and quantified in Figure 3(C), the relative amount
of eIF2α(P) increased by 20 and 67 % after 2 and 4 h respectively of treatment of COS-7 cells with MG-132. In contrast, the
relative level of eIF2α phosphorylation was slightly decreased in
vehicle-control cells treated with 0.1 % DMSO for the same time
period. The decrease in eIF2α phosphorylation coincides with
the increased rate of protein synthesis observed in control cells
(results not shown), a well-known effect of the addition of fresh
serum-containing medium to cells.
There are several possible mechanisms that could explain
how inhibition of the 26S proteasome may be inducing eIF2α
phosphorylation: (i) eIF2α(P) might normally be rapidly degraded
by the 26S proteasome, and thus accumulate rapidly to higher
levels when the 26S proteasome is inhibited; (ii) inhibition of
the 26S proteasome could result in an increase in the activity
of an eIF2α kinase; or (iii) there could be a decrease in the
activity of an eIF2α phosphatase after inhibition of the 26S
proteasome. Initially, we examined whether increasing eIF2α
phosphorylation might affect the stability of the protein or not.
Our own observations and other reported studies indicated that
treatment of cells with okadaic acid (an inhibitor of phosphatases
1 and 2A) results in an increase in eIF2α phosphorylation [26,27].
Therefore COS-7 cells were pretreated for 1 h with 1 μM okadaic
acid to increase the amount of eIF2α(P) and then eIF2α stability
was determined by CHX chase using Western-blot analysis.
Although eIF2α phosphorylation was significantly increased by
okadaic acid treatment, the half-life of eIF2α in treated cells
was the same as that in the control cells (results not shown). In
addition, stoichiometric phosphorylation of purified eIF2α by an
in vitro kinase treatment did not change its stability in an in vitro
degradation assay (results not shown). These results suggested
that eIF2α(P) is not normally subject to rapid degradation by
the 26S proteasome, and precludes direct changes in eIF2α(P)
stability contributing to its relative increase after 26S proteasome
inhibition.
Next, we investigated which of the known eIF2α kinases might
be mediating the observed phosphorylation of eIF2α after 26S
proteasome inhibition, by testing whether knockout cell lines of
each of the known eIF2α kinases no longer showed increased
phosphorylation of eIF2α after 26S proteasome inhibition. The
first kinase tested was PKR, which is a ubiquitously expressed
serine/threonine protein kinase that has been implicated in
mediating phosphorylation of eIF2α in response to doublestranded RNA, cytokine, growth factor and stress signals [28].
Fibroblasts containing a chromosomal deletion in the PKR
gene (PKR-knockout cells) were utilized to determine whether
c The Authors Journal compilation c 2008 Biochemical Society
584
Figure 4
A. Yerlikaya, S. R. Kimball and B. A. Stanley
Effect of the proteasome inhibitor on eIF2α phosphorylation in wild-type, PKR-knockout and PERK-knockout cells
(A) Wild-type and PKR-knockout cells were treated with MG-132 (50 μM) or 0.1 % DMSO for 4 h. Equal amounts of protein (30 μg) were separated on an SDS/12.5 % PAGE gel, followed by
Western-blot analysis as described in the Experimental section. (B) The results were quantified by ImageQuant software, and expressed as the amount of eIF2α(P) divided by the total eIF2α level,
with the control (DMSO-treated) set as 1. Cells were also treated with MG-132+CHX to determine whether complete inhibition of protein synthesis lowers the increase in eIF2α phosphorylation (right
panel). (C) Wild-type and PERK-knockout cells were similarly treated with MG-132 (50 μM) or 0.1 % DMSO for 4 h, and the total eIF2α and eIF2α(P) were determined by Western-blot analysis.
Wild-type and PERK-knockout cells were also treated with thapsigargin (1 μM) for 1 h, an agent known to activate PERK. (D) Quantification of the membrane shown in (C, left panel). Each bar in
(B, D) is the mean +
− S.E.M. for an experiment run in duplicate. The error bars are small and not visible in certain bars.
this kinase is responsible for phosphorylation of eIF2α after
inhibition of the 26S proteasome. The results presented in
Figures 4(A) and 4(B) indicated that treatment with the MG132 proteasome inhibitor for 4 h resulted in a 1.54-fold increase
in eIF2α phosphorylation in parental wild-type cells (PKR+/+ )
as compared with a 1.65-fold increase in PKR-knockout cells
(PKR−/− ). This indicates that PKR is not the primary kinase
involved in the elevation of eIF2α phosphorylation after inhibition
of 26S proteasome function, since a significant rise in the
amount of eIF2α phosphorylation after MG-132 treatment
still occurred in the cells lacking PKR activity. MG-132 also
stimulated eIF2α phosphorylation even if the protein synthesis
was completely blocked by the CHX treatment in these MEF
cells, suggesting that de novo protein synthesis is not required for
the effect of the proteasome inhibitor on eIF2α phosphorylation
(Figure 4A, right panel).
As stated in the Introduction section, in addition to its role in
the degradation of essentially all short-lived proteins and the great
bulk of long-lived proteins, the 26S proteasome also degrades
proteins that are denatured or improperly folded [29]. In addition
to leading to rapid degradation, the accumulation of unfolded
proteins (e.g. in response to ER stresses) results in increased
transcription of the genes for ER-resident chaperones (which
c The Authors Journal compilation c 2008 Biochemical Society
would increase the protein-folding activity) and repression of
protein synthesis through the increased phosphorylation of eIF2α.
This response to ER stresses is known as the UPR (unfolded
protein response) [30]. Previously, PERK has been shown to be
activated in response to accumulation of misfolded proteins in
the ER, and to phosphorylate eIF2α, implicating PERK in UPRmediated repression of protein synthesis [31,32]. Since inhibition
of 26S proteasome activity could lead to the accumulation of unfolded proteins, we utilized a cell line containing a deletion of
the transmembrane domain in the PERK gene (PERK-knockout
cells) to examine whether PERK plays a role in mediating
phosphorylation of eIF2α under the conditions of blocking the
26S proteasome function. As a positive control, treatment of
cells with thapsigargin, an agent that causes ER stress and
subsequently activates PERK, was shown to significantly induce
phosphorylation of eIF2α in wild-type (PERK+/+ ) cells, but not
in the PERK-knockout (PERK−/− ) cells, a finding consistent with
the deletion of PERK in these cells (Figure 4C). As shown in
Figures 4(C) and 4(D), treatment with MG-132 (50 μM) for
4 h increased the relative eIF2α phosphorylation 1.76-fold in
the parental wild-type (PERK+/+ ) cells and 2.1-fold in PERKknockout (PERK−/− ) cells. Again, since continued induction of
eIF2α phosphorylation by MG-132 treatment occurred even in
Phosphorylation of eIF2α by HRI kinase after 26S proteasome inhibition
Figure 5
585
Analysis of eIF2α phosphorylation in GCN2-knockout and HRI-knockout cells
Cells were treated with MG-132 as in Figure 4, and eIF2α phosphorylation was determined by Western blotting. (A) Induction of eIF2α phosphorylation in GCN2-knockout cells after MG-132
treatment. Total eIF2α and eIF2α(P) were determined by Western-blot analysis after treatment of wild-type (GCN2+/+ ) and GCN2-knockout (GCN2−/− ) cells with 50 μM MG-132 for 4 h.
(B) Quantification of the level of eIF2α phosphorylation seen in (A). The results are plotted as means +
− S.E.M. (n = 3), normalized to the amount of total eIF2α. (C) Phosphorylation of eIF2α in
HRI-knockout cells. The total eIF2α and eIF2α(P) were determined by Western-blot analysis after treatment of wild-type (HRI+/+ ) and HRI-knockout cells (HRI−/− ) with 50 μM MG-132 for 4 h.
(D) Quantification of the level of eIF2α phosphorylation seen in (C). The results are plotted as means +
− S.E.M. (n = 2).
the absence of the PERK kinase, this indicates that PERK is not
involved in mediating eIF2α phosphorylation after inhibition of
26S proteasome function.
A third eIF2α kinase (GCN2) was similarly tested using
GCN2-knockout cell lines, and again the MG-132-induced
increase in eIF2α phosphorylation was not prevented by the
absence of GCN2 upon exposure to 50 μM concentrations
of the proteasome inhibitor MG-132 for 4 h, indicating that
GCN2, usually activated by amino acid starvation, is not the
primary kinase mediating proteasome inhibitor-induced eIF2α
phosphorylation. As seen in Figure 5(A), treatment with MG132 increased eIF2α phosphorylation in both the wild-type and
GCN2-knockout cell lines, and the quantification of normalized
band densities in Figure 5(B) shows a 2.2-fold increase in
eIF2α(P) in the parental wild-type cells and a 5-fold increase
in GCN2-knockout cells after MG-132 exposure. The greater
fold increase in eIF2α phosphorylation in the GCN2-knockout
cells is presumably due to the significantly lower baseline level of
phosphorylation of eIF2α in these GCN2-knockout cells, possibly
indicating a GCN2 role in producing the basal levels of eIF2α
phosphorylation seen in wild-type cells.
Finally, we examined whether the HRI kinase plays a role
in eIF2α phosphorylation using HRI-knockout cells harbouring
a deletion of the HRI genes (HRI−/− cells) after proteasome
inhibition. HRI is the most important eIF2α kinase in erythroid
cells where it is expressed abundantly, becomes activated in
the absence of haem and inhibits protein synthesis through
phosphorylation of eIF2α to prevent accumulation of inactive
(misfolded) globin with no haem cofactor [33]. It was therefore
initially thought that HRI was unlikely to play a role in the
eIF2α phosphorylation observed in non-erythroid fibroblast cells
investigated in the present study. Contrary to our expectations,
MEF cells that lacked HRI showed almost none of the
eIF2α phosphorylation seen in parental MEF cells after 50 μM
MG-132 treatment for 4 h (Figures 5C and 5D). There was a
2.8-fold increase in wild-type cells, but only a non-significant
1.2-fold increase in HRI−/− cells, indicating that HRI is the major
eIF2α kinase involved in the increased phosphorylation of eIF2α
in response to inhibition of the 26S proteasome. While the 20 %
increase in phosphorylation still observed in HRI−/− cells might
be within the range of experimental error, it could also imply that
there is another kinase, probably playing a secondary role, which
phosphorylates eIF2α during 26S proteasome inhibition.
Although MG-132 has been a widely used proteasome inhibitor
and is considered to be quite specific, there has been some
evidence that it may have inhibitory effects on other proteases (e.g.
calpains, cathepsins and cysteine proteases) [34,35]. Therefore
HRI+/+ and HRI−/− cells were also treated with various
concentrations of Bortezomib, a highly selective and novel
dipeptide boronate proteasome inhibitor [36]. Since the effective
concentration of Bortezomib varies in different cell lines, we
tested a wide range of inhibitor concentrations. As can be seen
in Figure 6, Western-blot analysis demonstrated that eIF2α is
significantly phosphorylated in HRI+/+ cells exposed to various
c The Authors Journal compilation c 2008 Biochemical Society
586
A. Yerlikaya, S. R. Kimball and B. A. Stanley
Figure 6 Effect of Bortezomib on eIF2α phosphorylation in HRI wild-type
and HRI-knockout cells
(A) Cells were treated for 4 h with the various concentrations of Bortezomib indicated above
the lanes, and eIF2α phosphorylation level was determined by Western-blot analysis. (B)
Quantification of the level of eIF2α phosphorylation in HRI wild-type and HRI-knockout cells.
The experiment shown in (A) was carried out in triplicate and the results shown in (B) are the
mean +
− S.D. (n = 3).
concentrations of Bortezomib (0.1–10 μM) for 4 h; however, no
change in the level of eIF2α phosphorylation could be seen in the
knockout HRI−/− cells at any Bortezomib concentration. Taken
together, the MG-132 and Bortezomib results indicate that eIF2α
phosphorylation was induced specifically by 26S proteasome
inhibition and that HRI is the primary kinase mediating eIF2α
phosphorylation in response to 26S proteasome inhibition.
DISCUSSION
The 26S proteasome has been shown to be involved in the various
biologically important processes, such as the cell cycle, cellular
metabolism, apoptosis, signal transduction, the immune response
and protein quality control [37], and decreases in protein synthesis
rate caused by inhibition of the 26S proteasome have been
previously reported [9,10]. Decreases in protein synthesis after
longer periods of 26S proteasome inhibition have been noted to
accompany induction of apoptosis [14,16,22,23,39]; however, no
evidence of apoptosis was observed in the current experiments at
the very early time points where inhibition of the 26S proteasome
clearly caused decreased protein synthesis rates, nor were protein
synthesis rates restored upon inhibition of apoptosis-related caspases. These results strongly suggest that the loss of protein synthesis activity after inhibition of the 26S proteasome is independent of the induction of apoptosis. Previously, Brophy et al. [40]
have also indicated that MG-132 treatment does not stimulate apoptosis in COS-7 cells; in fact, they showed that when COS-7 cells
were pretreated with the proteasome inhibitors before addition of
staurosporine (a known inducer of apoptosis in many cell lines),
both caspase 3 activity and percentage of apoptotic cells were reduced as compared with the cells treated with staurosporine alone.
As others have observed, we showed that the apoptosisindependent inhibition of protein synthesis that occurs after brief
inhibition of 26S proteasome function is likely due to elevated
eIF2α phosphorylation [38]. Of the four protein kinases known to
phosphorylate eIF2α, the results presented here indicate for the
c The Authors Journal compilation c 2008 Biochemical Society
first time that HRI is the primary kinase activated by inhibition
of the 26S proteasome. While it is well known that HRI mediates
phosphorylation of eIF2α and is expressed at a significant level in
erythroid cells, several previous studies have also provided evidence that HRI is a ubiquitous eIF2α kinase of mammalian cells
and is expressed in a wide range of non-erythroid cells [41,42].
The present study clearly demonstrates that the HRI kinase can
mediate translational attenuation in non-erythroid cells, although
the mechanism of activation of the kinase is not yet clear.
A previous study reported that eIF2α was significantly phosphorylated in GCN2 wild-type cells treated with 1 μM MG-132
but that MG-132-induced phosphorylation of this translational
initiation factor was greatly decreased in knockout GCN2−/−
cells; however, a closer examination of the results in that
paper shows increasing phosphorylation of eIF2α even in the
knockout GCN2−/− cells at later time points of treatment with
MG-132, e.g. after 6 h [11]. This suggests that another kinase
might also be involved, albeit more slowly, at those lower
MG-132 doses. We did not observe any reduction in eIF2α
phosphorylation in GCN2−/− cells in response to 26S proteasome
inhibition under our experimental conditions; the different results
obtained in the present study may be partly due to the different
concentrations used (1 μM versus 50 μM MG-132 in our study).
The concentration used in our study {a standard dose that has
been used in many studies of 26S proteasome inhibition (see e.g.
[43,44])} may cause more complete or more rapid activation of
HRI than the lower dose used in the study of Jiang and Wek
[11], where possible phosphorylation of eIF2α in the absence of
GCN2 only occurred at later time points (an HRI kinase knockout
was not tested in those experiments); under the conditions tested
in the current experiments, GCN2 kinase activity may still have
been activated, but contributing a relatively minor portion of the
observed eIF2α phosphorylation compared with that arising from
HRI kinase activity. Thus, in the GCN2−/− cells tested herein,
no appreciable decrease in MG-132-stimulated phosphorylation
would be observable because of the still-present and dominant
eIF2α phosphorylation occurring through HRI.
The activation of HRI might be attained by several different
mechanisms, including possible interactions with Hsp proteins.
Previous studies indicated that proteasome inhibitors enhance the
expression of Hsp70 and Hsp27 without heat shock in MEF [45] as
well as Hsp90 expression in lens cells [46]. In addition, another
study showed that Hsp70 or Hsp90 is required for folding and
transformation of HRI into an active kinase in rabbit reticulocyte
lysates and, in living cells, Hsp70 is essential for the activation of
HRI under stress conditions [47]. Further experiments are required
to investigate links between the HRI activation and Hsp proteins
during proteasome inhibition in living cells.
In summary, the results presented here reveal that inhibition
of the 26S proteasome results in repression of global protein
synthesis through elevated phosphorylation of eIF2α, mediated
primarily through activation of HRI kinase activity. Given that
proteasome inhibition induces apoptosis in several experimental
models through the accumulation of a number of pro-apoptotic
proteins such as Bax, Bid, CDK inhibitors (p21 and p27) and p53,
several 26S proteasome inhibitors are currently being tested in
clinical trials as anticancer agents that would induce apoptosis
in rapidly proliferating cells. Specifically, Bortezomib has been
approved for the treatment of multiple myeloma [48]. In spite of
the obvious efficacy of Bortezomib in many cases, a Phase II
trial of Bortezomib treatment in 202 patients with refractory
relapsed multiple myeloma demonstrated that as many as 65 % of
patients did not respond to treatment [49]. Other studies indicate
that resistance to proteasome inhibition, which may account for
some of the non-responsive patients in the Bortezomib study
Phosphorylation of eIF2α by HRI kinase after 26S proteasome inhibition
cited above, may be partly due to induction of Hsp70, which
is known to be an inhibitor of apoptosis acting by preventing
the processing of caspases into active forms [50]. Induction of
Hsp70 would also be expected to enable HRI kinase activation as
mentioned above, and increased phosphorylation of eIF2α and the
resulting decrease in protein synthesis rate in some patients might
prevent the accumulation of normally rapidly degraded proteins,
proteins whose increased levels would play an important and
necessary role in the apoptotic effects of proteasome inhibition.
While inhibitors like Bortezomib appear to be highly specific
for 26S proteasome inhibition, a better understanding of the
downstream effects of 26S proteasome inhibition and the extent
to which they differ between different patients may be important
for understanding the differences between inhibitor-responsive
and inhibitor non-responsive patients in multiple myeloma and
possibly other cancer patients. Further experiments will greatly
increase our understanding of molecular links between the
26S proteasome inhibition and eIF2α phosphorylation, and
information obtained from such experiments may help to define
novel molecular targets and therapeutic strategies to overcome
what appears to be clinical resistance to proteasome inhibition in
some patients.
We thank Dr Isis Rivera-Walsh (Department of Microbiology and Immunology, The
Pennsylvania State University College of Medicine, Hershey, PA, U.S.A.) for excellent
assistance with the flow cytometry-based apoptosis assays, and Dr An Do and
Rick Horetsky (both from the Department of Cellular and Molecular Physiology, The
Pennsylvania State University College of Medicine, Hershey, PA, U.S.A.) for help with
eIF2α phosphorylation analysis in GCN2- and HRI-knockout cells. This work was
supported by the NSF (National Science Foundation) grant MCB-9603740 (to B.A.S.)
and the NIH (National Institutes of Health) grant DK-13499 (to S.R.K.).
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