From the Division of Molecular and Cellular
Immunology, Medical Institute of Bioregulation, Kyushu University,
3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan,
§ Departments of Internal Medicine (III), Kurume University,
Asahi-machi, Kurume 830-0011, Japan, and ¶ Lady Davis
Institute-McGill University, Jewish General Hospital, Montreal,
Quebec H3T 1E2, Canada
Received for publication, November 1, 2002, and in revised form, November 27, 2002
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ABSTRACT |
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The suppressor of cytokine signaling-3
(SOCS3/CIS-33/SSI-3) is an important negative regulator of cytokine
signaling. Here, we show that an N-terminal truncated isoform
( Cytokines control a wide spectrum of biological responses, but the
duration and intensity of their effects must be tightly regulated (1,
2). Cytokines induce oligomerization of specific cell-surface receptors
and activate the Janus kinase/signal transducer and activator of
transcription (JAK/STAT)1
pathway (3, 4). The strength of cytokine signals is regulated, in part,
by a family of endogenous JAK kinase inhibitor proteins referred to as
suppressors of cytokine signaling (SOCS), cytokine-inducible SH2
proteins (CIS), or STAT-induced STAT inhibitors (SSI) (5-7). Both
SOCS1 and SOCS3 have an N-terminal kinase inhibitory region and inhibit
JAK kinase activity; SOCS1 directly binds to JAKs (8, 9), whereas SOCS3
inhibits JAKs through binding to cytokine receptor tyrosine residues
(10-13). SOCS3 mRNA is induced by various cytokines, and SOCS3
protein negatively regulates immune and inflammatory responses in
vivo (14, 15). However, cytokines exert their actions
concomitantly in stressful situations including inflammation, ER
stress, and viral infections (16, 17). Therefore, the response of SOCS
proteins to cellular stress remains to be investigated.
When cells are under physiological and environmental stress, such as
nutrient deprivation, disturbance of intracellular stores of
Ca2+, or virus infection, the accumulation of incorrectly
folded proteins occurs, which induces a stress response, a process
called the unfolded protein response (UPR). The UPR induces a rapid
repression of protein synthesis to reduce unfolded protein levels or to
interfere with viral replication (16, 17). This repression of protein synthesis occurs mainly through the phosphorylation of eIF2 In the present study, we demonstrate that an isoform of SOCS3 is
generated by alternative translation initiation. This N-terminal truncated product of SOCS3, herein referred as Materials--
SOCS3 mutants were generated by the standard PCR
method and subcloned into pCDNA3 or pMX-IRES-EGFP as described (11,
12). The active PKR construct has been described (25). Human EGF, murine IFN Immunochemical Analysis--
Immunoprecipitation and
immunoblotting analysis were performed as described (11, 12, 26). To
detect WT-SOCS3 and Cells and Mice--
293T cells and RAW264.7 cells were cultured
in Dulbecco's modified Eagle's medium containing 10% fetal bovine
serum. The establishment of Ba/F3 cells expressing a chimeric receptor
consisted of the extracellular domain of the EGF receptor, and the
cytoplasmic domain of the EPO receptor (BF-EGFR/EPOR) was described
previously (27). These cells were maintained in RPMI medium containing 10% fetal bovine serum and 100 ng/ml EGF. For thapsigargin or MG132
treatment, BF-EGFR/EPOR cells were cultured without EGF for 8 h
and pretreated with 5 µM thapsigargin or 50 µM MG132 for 15 min. Then, cells were stimulated with 100 ng/ml EGF for the indicated times, and cell extracts were immunoblotted
with anti-SOCS3(N) or anti-SOCS3(C) antibody. To assess the half-life
of wild-type or mutant SOCS3 protein, infected cells were incubated
with 100 µg/ml cycloheximide for various times. Cell extracts were
then analyzed by immunoblotting with an anti-SOCS3(C) antibody specific to the C terminus of the protein. Transgenic mice expressing the hepatitis C virus (HCV) core protein were described previously (28).
Retrovirus Infection--
The wild-type and mutant SOCS3
cDNAs subcloned in pMX-IRES-EGFP were transfected into a PLAT-E
packaging cell line using the FuGENE 6 (Roche Molecular Biochemicals)
to obtain the viruses (12). Ba/F3 cells expressing the EGFR-EPOR
chimeras (2 × 105 cells) were infected with viruses
on a RetroNectin (TaKaRa)-coated plate for 24 h in the presence of
100 ng/ml EGF. Cells were washed three times with phosphate-buffered
saline, resuspended in RPMI-10% fetal calf serum containing 100 ng/ml
EGF and incubated for the indicated times. Then, cells (1 × 104) were analyzed for EGFP fluorescence on a COULTER
EPICS-XL flow cytometer.
An N-terminal Truncated SOCS3 Form Is Induced by ER Stress--
To
examine the expression of SOCS3 protein by ER stress, we used
BF-EGFR/EPOR cells in which treatment of EGF activates the EPO receptor
cytoplasmic domain and the JAK2 pathway (12, 27). As shown in Fig.
1b (upper panel),
upon EGF stimulation, the endogenous SOCS3 protein was detected as a
single band by a rabbit polyclonal antibody against a peptide from the
N terminus of SOCS3 (anti-SOCS3(N)) (Fig. 1a). SOCS3
was significantly down-regulated after a 2-h stimulation with EGF.
Contrary to this, cells stimulated with EGF in the presence of the 26 S
proteasome inhibitor MG132 showed a sustained increase in SOCS3 protein
levels from 30 min to 2 h. Therefore, according to previous
reports (29), our data verify that the proteasome plays a major role in
the rapid degradation of SOCS3.
Thapsigargin is an inhibitor of the Ca2+ATPase transporter
known to induce the ER stress. We found that the thapsigargin rapidly induced the phosphorylation of eIF2
We also found that the two forms of SOCS3 were not only induced by IL-3
stimulation of Ba/F3 cells, but they were also induced in the mouse
macrophage-like cell line RAW264.7 by either interferon (IFN)
The SOCS3 gene has no intron in the protein-coding region (30).
Therefore, we speculated that N-terminal truncated SOCS3 was derived
from an alternative translation initiation site starting from
methionine at position 12 (Met-12) (Fig. 1a). To examine this possibility, the full-length (WT)-SOCS3 or an N-terminal 11-amino
acid deletion mutant SOCS3 ( Alternative Translation of SOCS3 mRNA Is Induced by Active
PKR--
To verify that N-terminal truncated SOCS3 was produced by
alternative translation through eIF2
Core protein was expressed highly in the thymus of HCV core protein Tg
mice (28). In the core Tg mouse thymus, the levels of The
Because WT-SOCS3 protein levels were low in infected Ba/F3 cells, we
compared the stability of WT-SOCS3 and Lys-6 Is a Major Potential Ubiquitination Site of SOCS3--
To
understand the molecular mechanisms of high stability of
To further examine this hypothesis, we tested ubiquitination of
WT-SOCS3 and K6Q-SOCS3 using Ba/F3 cells infected with SOCS3 viruses.
As shown in Fig. 4c, the presence of MG132 increased the
expression of WT-SOCS3 (bottom panel), which coincided with the appearance of high molecular weight bands containing WT-SOCS3 (middle panel). Ubiquitination of WT-SOCS3 protein was
demonstrated by immunoblotting with anti-ubiquitin antibody (Fig.
4c, upper panel). In contrast, the expression
level of K6Q-SOCS3 was not affected by MG132 (Fig. 4c,
bottom panel), and the ubiquitination of K6Q-SOCS3 was not
detected by anti-ubiquitin antibody. We cannot rule out the possibility
that Lys-6 is not the only ubiquitination site because it may be
involved in ubiquitination of other lysine residues in WT-SOCS3
protein. Nevertheless, our data definitely demonstrate that Lys-6 plays
a major role in the ubiquitination of SOCS3.
It is also noteworthy that the protein stability of SOCS3 is varied in
different cell lines. For example, we found that WT-SOCS3 is relatively
stable in 293 cells and mouse epithelial cell lines, whereas it is
highly unstable in hematopoietic cells including Ba/F3, Raw, and UT-7
cells (Fig. 2 and data not shown). Therefore, ubiquitination of Lys-6
and degradation of SOCS3 may be regulated by a cell type-specific
factor(s) in addition to the SOCS box. Future studies on the mechanisms
of SOCS3 ubiquitination as well as alternative translation initiation
are likely to yield important new information about the regulatory
pathways of cytokine signaling controlled by SOCS3.
In this study, we have demonstrated that ER stress induces the
expression of N-SOCS3) translated from the internal AUG codon 12 was profoundly
induced by endoplasmic reticulum (ER) stress- or active double-stranded
RNA-activated protein kinase PKR, as a result of induction of
eukaryotic initiation factor 2
phosphorylation.
N-SOCS3 exhibited
a stronger cytokine-inhibitory activity and a higher stability than
WT-SOCS3 in Ba/F3 hematopoietic cells. A potential ubiquitination
residue, Lys-6, at the N terminus is evolutionary conserved among SOCS3
species. The K6Q-SOCS3 mutant showed a much longer half-life than
WT-SOCS3 in Ba/F3 cells. Furthermore, inhibition of the 26 S proteasome
pathway increased both ubiquitination and protein levels of WT-SOCS3
but had no effect on K6Q-SOCS3. SOCS3 mutant lacking the
carboxyl-terminal SOCS-box exhibited the same stability as K6Q-SOCS3.
These observations suggest that the short form of SOCS3 is a naturally
occurring stabilized inhibitory protein, whereas WT-SOCS3 is a
short-lived protein modulated by Lys-6 ubiquitination and
proteasome-dependent degradation. Our findings provide
strong evidence for the first time that translational control plays an
important role in stabilization and function of SOCS3.
INTRODUCTION
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ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS AND DISCUSSION
REFERENCES
on serine 51, which interferes with the formation of an active 40 S
translation-initiation complex. Reduction of 40 S initiation complexes
results in suppression of translation initiation of most mRNAs and
reduces protein synthesis of many proteins (18, 19). However, not all
of the proteins are translationally repressed by eIF2
phosphorylation. For example, in mammalian cells expression of
transcription factor ATF4 is induced by eIF2
phosphorylation (20).
Also, in yeast cells induction of eIF2
phosphorylation by the GCN2
kinase plays an important role in the translational induction of
transcription factor GCN4 (21). The UPR also activates transcription
factors ATF6 and XBP1, which induce the ER stress-response genes,
chaperones, and folding catalysts as well as the proapoptotic gene, CHOP/GADD153 (22-24).
N-SOCS3, lacks the
major ubiquitination site lysine 6 of the full-length protein, and
therefore, it is resistant to 26 S proteasome-dependent
degradation. We also provide evidence that
N-SOCS3 is induced by
conditions that enhance eIF2
phosphorylation such as ER stress or
activation of the protein kinase PKR. Furthermore,
N-SOCS3 appears
to be more biologically active than the full-length protein in
suppressing cytokine signaling. Our data provide strong evidence for a
regulation of SOCS3 expression and function at the translational level.
EXPERIMENTAL PROCEDURES
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ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS AND DISCUSSION
REFERENCES
, and IL-10 were purchased from PeproTech EC, Ltd. MG132
and thapsigargin were purchased from Sigma and dissolved in dimethyl
sulfoxide (Me2SO). Affinity-purified polyclonal
rabbit anti-SOCS3(N) or anti-SOCS3(C) antibodies were generated against synthetic peptide SKFPAAGMSRPLDTSLRL or YEKVTQLPGPIREFLDQYDAPL, respectively (Immuno-Biological Laboratories Co., Ltd., Japan). Anti-GFP (FL) and anti-Myc (A-14 and 9E10) antibodies were purchased from Santa Cruz. Rabbit anti-phospho-S51-eIF2
antibody and mouse monoclonal anti-ubiquitin antibody were purchased from Cell Signaling and Zymed Laboratories Inc., respectively.
N-SOCS3, SDS-14% PAGE was used. For the
detection of the ubiquitinated SOCS3, the proteins were transferred
onto polyvinylidene difluoride membranes. The blots were incubated with
Tris-buffered saline containing 0.5% glutaraldehyde for 30 min before
blocking with 10% skim milk in Tris-buffered saline.
RESULTS AND DISCUSSION
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ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS AND DISCUSSION
REFERENCES
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Fig. 1.
Detection of N-terminal truncated SOCS3 in
Ba/F3 cells. a, schematic model of the functional
domains of SOCS3. The first Met and Met-12 are indicated as
solid and open triangles, respectively. Conserved
Lys-6 is shown with an asterisk. b,
EGF-dependent BF-EGFR/EPOR cells were cultured without EGF
for 8 h and then pretreated with Me2SO ( ), 5 µM thapsigargin (Thap.), or 50 µM MG132 for 15 min. Then cells were stimulated with 100 ng/ml EGF for the indicated times, and cell extracts were immunoblotted
(IB) with indicated antibodies. The asterisk
denotes nonspecific bands. Similar results were obtained in three
independent experiments. c, RAW cells were pretreated with
or without 50 µM MG132 for 15 min, then stimulated with
100 ng/ml IFN
or IL-10 for the indicated times. The cell extracts
were immunoblotted with anti-SOCS3(C) antibody. d,
BF-EGFR/EPOR cells pretreated as in b were stimulated with
100 ng/ml EGF for 30 min and analyzed as in b. 293 cell
lysate containing WT-SOCS3 or
N-SOCS3 (12-225 amino acids) were run
on the same gel for comparison.
on serine 51 and slightly increased SOCS3 protein levels 1 h after EGF stimulation as
detected by anti-N-terminal antibody (Fig. 1b). However,
SOCS3 protein was detected as two bands by a rabbit anti-SOCS3(C)
antibody, which recognizes the C terminus of SOCS3 (Fig. 1b,
middle panel). The regions recognized by anti-SOCS3(N) and
anti-SOCS3(C) antibodies are shown in Fig. 1a. The molecular
weight and expression pattern of the two proteins suggested that the
upper band recognized by the anti-SOCS3(C) antibody is identical to the
protein detected by the anti-SOCS3(N) antibody. The levels of the lower
band detected by anti-SOCS3(C) were low after a 1-h EGF stimulation in
the absence of the ER stress inducer. In contrast, this form of SOCS3
was profoundly elevated by the treatment with either thapsigargin or
MG132 for 1 h after EGF stimulation (Fig. 1b). Similar
results were obtained with a mouse monoclonal antibody that recognizes the C terminus of SOCS3 (data not shown). These data implied an involvement of ER stress-induced eIF2
phosphorylation in the induction of the N-terminal truncated SOCS3.
or
IL-10 (Fig. 1c). The same two isoforms of SOCS3 were observed in v-src transformed 3T3 cells or
granulocyte macrophage-colony-stimulating factor-stimulated UT-7 cells
(data not shown). Thus, the N-terminal truncated SOCS3 form was
generated not only by EGF/EPO receptor chimera but also by different cytokines.
N-SOCS3) was expressed in 293 cells
(Fig. 1d). We noticed that the upper and lower bands of
SOCS3 in the Ba/F3 cell lysate migrated at the same positions corresponding to WT-SOCS3 and
N-SOCS3 expressed in 293 cells, respectively (middle panel). We also confirmed that the
anti-SOCS3(C) antibody could recognize both WT- and
N-SOCS3
proteins, whereas the anti-SOCS3(N) antibody reacted only with WT-SOCS3
protein (Fig. 1d). These results indicated that an isoform
of SOCS3 is expressed as a result of alternative translation
initiation. Thus, phosphorylation of eIF2
may play a role in
initiation of translation from the second AUG (Met-12), resulting in
the generation of N-terminal truncated SOCS3.
phosphorylation, we next
examined the effect of the eIF2
kinase PKR (25) on SOCS3 expression. First, we examined whether a similar alternative translation initiation product is expressed in 293 cells transiently transfected with SOCS3
cDNA. As shown in Fig. 2a,
left and middle panels, WT-SOCS3, as well as
N-SOCS3, was detected by an anti-SOCS3(C) antibody as a single band
under normal conditions. On the other hand, WT-SOCS3 but not
N-SOCS3
was expressed as two bands in the presence of thapsigargin. Thus,
alternative initiation in the presence of thapsigargin occurred even
when SOCS3 cDNA was transiently expressed in 293 cells. When SOCS3
cDNA was co-transfected with human PKR cDNA, we found that
expression of the low molecular weight form of SOCS3 was induced. The
lower band detected in WT-SOCS3-transfected cells migrated at the same
level as
N-SOCS3, indicating that N-terminal truncated SOCS3 is
translated from full-length SOCS3 cDNA as a result of PKR
activation and eIF2
phosphorylation. To further confirm that
alternative translation from Met-12 occurred in the presence of
active-PKR, we generated a SOCS3 mutant with substitution of Met-12 to
Ala (M12A-SOCS3). Transient transfection in 293 cells showed that
M12A-SOCS3 was detected as a single band even in the presence of
active-PKR, whereas WT-SOCS3 was detected as two bands (Fig.
2b). Similar results were obtained by thapsigargin treatment
of 293 cells transfected with M12A or WT-SOCS3 cDNA (data not
shown).
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Fig. 2.
N-SOCS3 is derived from an
internal translational site, Met-12. a and
b, the WT-SOCS3,
N-SOCS3, or M12A-SOCS3 cDNA carrying
no upstream untranslated region was transfected into 293 cells with or
without active PKR plasmid. Cells were pretreated with 5 µM thapsigargin for 4 h (a, center
panel). Cell extracts were then analyzed with an anti-SOCS3(C)
antibody. The asterisk denotes a nonspecific band. Similar
results were obtained in two independent experiments. c,
tissue extracts (1 mg of protein/sample) from core-Tg or control
littermates were immunoprecipitated (IP) and analyzed with
an anti-SOCS3(C) antibody. Two separate mice were analyzed.
N-SOCS3 Is Produced in Vivo--
To examine whether
N-SOCS3 is produced in vivo, we measured SOCS3
protein levels by immunoblotting in various tissues from wild-type
(non-Tg) and transgenic (Tg) mice expressing the HCV core protein (28).
We have recently demonstrated that HCV core protein associates with and
activates STAT3, resulting in a higher production of SOCS3 protein
(28). Furthermore, the core protein has been shown to activate PKR
(31). As shown in Fig. 2c, in wild-type mice, WT-SOCS3 was
predominately expressed in the liver and thymus, whereas
N-SOCS3 was
highly expressed in the spleen. Because UPR has been shown to be
activated during differentiation of B lymphocyte to the plasma cells
(32), translation of
N-SOCS3 is likely to be induced in
antigen-stimulated T- and B-lymphocytes in the spleen.
N-SOCS3 were
much higher than those of WT-SOCS3 (Fig. 2c). These data
suggested that HCV core protein not only induces SOCS3 expression
through STAT3 activation but also stimulates alternative translation
initiation of SOCS3 most probably through PKR. These results clearly
indicated that expression of
N-SOCS3 takes place in physiological or
pathological conditions in vivo.
N-SOCS3 Protein Is More Active and Stable Than WT-SOCS3 in
Ba/F3 Cells--
To assess possible functional differences between
WT-SOCS3 and
N-SOCS3, SOCS3 genes were stably introduced into Ba/F3
cells with retrovirus vectors bearing EGFP under the control of an
internal ribosome entry site (IRES). Because SOCS3 inhibits EPO
receptor-JAK2 signaling, the cytokine-suppressing activity of SOCS3 can
be assayed by measuring the reduction of EGFP-positive cells (12, 33). As shown in Fig. 3a, EGFP
levels were similar after 2 days of infection, indicating a similar
SOCS3-EGFP mRNA production in WT-SOCS3- and
N-SOCS3-infected
cells. However, WT-SOCS3 protein levels were less than those of
N-SOCS3. Then we examined possible changes in the population of
EGFP-positive cells (Fig. 3b). The population of
EGFP-positive cells infected with control virus did not change, whereas
EGF-positive cells expressing WT-SOCS3-infected cells decreased
gradually. This indicated that the cytokine signal-suppressing activity
of WT-SOCS3 is weak in Ba/F3 cells, probably due to its low expression.
Contrary to this, the number of
N-SOCS3-infected cells decreased
rapidly, indicating that the suppressing activity of
N-SOCS3 is
higher than that of WT-SOCS3.
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Fig. 3.
N-SOCS3 is a long-lived protein
and possesses a higher inhibitory effect than WT-SOCS3.
a, BF-EGFR/EPOR cells were infected with a control virus
encoding EGFP alone (
) or with retrovirus encoding Myc-tagged
WT-SOCS3 or
N-SOCS3. After 2 days, cells (1 × 105)
were collected for Western blot analysis with anti-Myc or anti-GFP
antibodies. The molecular weight of exogenous SOCS3 was higher than
endogenous SOCS3 because of the 6×Myc tag. b, cells were
maintained in culture in the presence of 100 ng/ml EGF, and the
populations of EGFP-positive cells were counted by FACS analysis on
indicated days after infection. c, BF-EGFR/EPOR cells were
cultured without EGF for 8 h and stimulated with 100 ng/ml EGF for
1 h. The cells were then treated with 100 µg/ml cycloheximide
(CHX.) for the indicated times, and cell extracts were
analyzed with an anti-SOCS3(C) antibody.
N-SOCS3 proteins. We measured
half-life of the endogenous SOCS3 forms in Ba/F3 cells in the presence
of the protein synthesis inhibitor cycloheximide. As shown in Fig.
3c, endogenous WT-SOCS3 is extremely unstable contrary to
N-SOCS3, which is more stable than WT-SOCS3. Similar results were
obtained with transfected WT-SOCS3 or
N-SOCS3 in Ba/F3 cells (data
not shown). As WT-SOCS3 half-life was significantly affected by MG132
(see Fig. 1b), these data suggested that
N-SOCS3 may not
be subjected to proteasome-dependent degradation.
N-SOCS3, we
examined carefully the amino acid sequence within the N-terminal region
of WT-SOCS3. One lysine residue, a potential ubiquitination site, was
found at position 6 (Lys-6) in SOCS3 (Fig. 1a). Thus, we
created a mutant SOCS3 in which Lys-6 was substituted for glutamine
(K6Q-SOCS3). Retroviruses carrying WT- or K6Q-SOCS3-IRES-EGFP were used
to infect BF-EGFR/EPOR cells, and both protein expression and stability
levels were compared and measured. The expression level of EGFP was
similar after 2 days of infection; however, as in
N-SOCS3, the
levels of K6Q-SOCS3 protein were much higher than those of WT-SOCS3
(Fig. 4a). Furthermore, K6Q-SOCS3 protein turnover was much slower than that of WT-SOCS3 (Fig.
4b). We also examined the half-life of C-terminal SOCS-box truncated SOCS3 (
C-SOCS3) using the same retrovirus infection system, based on previous findings that the SOCS-box interacts with
Elongin BC complex to generate the E3 ubiquitin-ligase complex (29,
33). As expected,
C-SOCS3 was as stable as K6Q-SOCS3 (Fig.
4b). These results suggested that Lys-6 of SOCS3 is a key residue for regulating SOCS3 stability through the SOCS-box.
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Fig. 4.
Mutation of Lys-6 diminished the
ubiquitination of SOCS3 and conferred high stability.
a, BF-EGFR/EPOR cells were infected with a retrovirus
encoding either Myc-tagged WT-SOCS3 or K6Q-SOCS3. After 2 days, 1 × 105 cells were collected for Western blot analysis with
anti-Myc and anti-GFP antibodies. b, Ba/F3 cells (1 × 105) infected with the indicated viruses (infection
efficiency was about 80%) were treated with 100 µg/ml cycloheximide
(CHX.) for the indicated times in the presence of 100 ng/ml
EGF, and cell extracts were then analyzed with anti-Myc antibody. To
detect WT-SOCS3 protein, longer exposure of Western blot membrane was
done. c, the infected cells (1 × 106) were
treated with 50 µg/ml MG132 for the indicated times in the presence
of 100 ng/ml EGF, and then cell extracts were immunoprecipitated with a
rabbit anti-Myc antibody. They were then immunoblotted with mouse
anti-Myc (9E10) and anti-ubiquitin (Ub.) antibodies.
N-SOCS3 as a result of alternative translation initiation. It does so through the induction of eIF2
phosphorylation caused by the activation of the protein kinase PERK (17). This is also
strongly supported by our data showing that activation of the eIF2
kinase PKR induces
N-SOCS3 expression. Inasmuch as PKR activity is
induced in virus-infected cells, induction of
N-SOCS3 protein
synthesis by PKR may provide evidence for a differential regulation of
SOCS3 translation by viruses. Our study further substantiates previous
findings showing that translational control plays a role in the
regulation of SOCS proteins. That is, previous data demonstrated that
translation of SOCS1 is suppressed by the presence of the
5'-untranslated region and is regulated in a cap-dependent
manner by the activity of eIF4E-binding proteins (34). Taken together,
these findings suggest that tight control of SOCS protein expression is
required for efficient signaling in response to cytokines or
environmental stress. Further studies with the use of
N-SOCS3
transgenic mice or M12A-SOCS3 knock-in mice are necessary to fully
address the physiological relevance of translational control of SOCS3
in vivo.
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ACKNOWLEDGEMENTS |
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We are grateful to Drs. G. Matsuzaki, H. Nakamura, and T. Nakamura for critical comments and H. Ohgusu for technical assistance.
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FOOTNOTES |
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* This work was supported in part by grants from the Ministry of Science, Education, Culture and Sports of Japan, the Japan Research Foundation for Clinical Pharmacology (to A. Y.), the Fukuoka Cancer Society, the TAKEDA Science Foundation (to A. S.), and by a joint grant from Human Frontier International Program Organization (to A. Y. and A. E. K.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Recipient of a scientist award from the Canadian Institutes
for Health Research (CIHR).
** To whom correspondence should be addressed. Tel.: 81-92-642-6823; Fax: 81-92-642-6825; E-mail: yakihiko@bioreg.kyushu-u.ac.jp.
Published, JBC Papers in Press, November 28, 2002, DOI 10.1074/jbc.C200608200
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ABBREVIATIONS |
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The abbreviations used are: JAK, Janus kinase; STAT, signal transducer and activator of transcription; SOCS, suppressors of cytokine signaling; CIS, cytokine-inducible SH2 proteins; SSI, STAT-induced STAT inhibitors; ER, endoplasmic reticulum; UPR, unfolded protein response; eIF, eukaryotic initiation factor; PKR, protein kinase dsRNA-dependent; GFP, green fluorescence protein; EGFP, enhanced GFP; EGF, epidermal growth factor; IFN, interferon; IL, interleukin; EPO, erythropoietin; EPOR, erythropoietin receptor; HCV, hepatitis C virus; IRES, internal ribosome entry site.
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