Received for publication, September 25, 2000, and in revised form, December 14, 2000
Granulocyte colony-stimulating factor
(G-CSF) plays a major role in the regulation of granulopoiesis.
Treatment of cells with G-CSF has been shown to activate
multiple signal transduction pathways. We show here that Erk5, a novel
member of the MAPK family, and its specific upstream activator MEK5
were activated in response to incubation of cells with G-CSF. Different
from other members of the MAPK family including Erk1/2, JNK, and p38,
maximal activation of Erk5 by G-CSF required the C-terminal region of
the G-CSF receptor. Genistein, a specific inhibitor of protein-tyrosine
kinases, blocked G-CSF-induced Erk5 activation. In contrast, inhibition
of protein kinase C activity increased G-CSF-mediated activation of
Erk5 and MEK5, whereas stimulation of protein kinase C activity
inhibited activation of the two kinases by G-CSF. The proliferation of
BAF3 cells in response to G-CSF was inhibited by expression of a
dominant-negative MEK5 but potentiated by expression of a
constitutively active MEK5. Expression of the constitutively active
MEK5 also increased the survival of BAF3 cells cultured in the absence
of or in low concentrations of G-CSF. Together, these data
implicate Erk5 as an important signaling component in the biological
actions of G-CSF.
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INTRODUCTION |
Granulocyte colony-stimulating factor
(G-CSF)1 is the major
cytokine that regulates the production of neutrophilic granulocytes (1,
2). G-CSF mediates its biological activities by binding to and
activating its cognate receptor, a single transmembrane protein that
belongs to the cytokine receptor superfamily. The G-CSF receptor has no
intrinsic kinase activity in the cytoplasmic domain and thus has to
interact with cytoplasmic kinases for signal transduction. One of the
major signaling events regulated by G-CSF is the activation of the
Janus protein-tyrosine kinases including Jak1, Jak2, and Tyk2 (3-5),
which leads to the tyrosine phosphorylation and subsequent activation
of Stat3, Stat5, and to a lesser extent Stat1 (6-10). Activation of
Stat3 and Stat5 has been shown to be implicated in the regulation of
cell proliferation, differentiation, and survival induced by G-CSF
(11-14).
In addition to the Jak/Stat cascade, G-CSF has also been shown to
activate the mitogen-activated protein kinases (MAPKs), which relay
signals from the cell membrane to the nucleus (15, 16). The MAPK module
is composed of three kinases that establish a sequential activation
cascade. A MAPK kinase kinase (MAPKKK or MEKK) activates a MAPK kinase
(MAPKK or MEK), which in turn activates a MAPK via dual phosphorylation
of a Thr-X-Tyr motif. The MAPK pathways are activated by a
variety of extracellular stimuli via interaction with small GTPases
including Ras and Rho family members and mediate distinct cellular
responses. Members of MAPKs that are activated by G-CSF include Erk1/2,
JNK, and p382 and activation of these kinases appears to be
involved in G-CSF-stimulated cell proliferation (8, 17-21).
Erk5, also known as BMK1, is a novel member of the MAPK family (22,
23). Erk5 shares the Thr-X-Tyr sequence in the dual phosphorylation site with other MAPKs, but has a unique long C-terminal tail, suggesting a distinct regulatory mechanism of its activation. Similar to other members of the MAPK family, Erk5 has been shown to
phosphorylate transcription factors such as c-Myc and members of MEF2
family of transcription factors (24-27). Interestingly, the C-terminal
region of Erk5 has been shown to contain a potent transcription
activation domain that is required for coactivation of MEF2D by Erk5
(28). The direct upstream activator of Erk5 has been identified as MEK5
(23-25), which appears to be activated by MEKK3 and Cot (29, 30).
Although Erk5 was originally shown to be activated by stresses such as
oxidants, hyperosmolarity, and fluid shear stress (24, 31, 32), recent
studies have indicated that Erk5 is also activated by receptors with
intrinsic tyrosine kinase activity such as the receptors for epidermal
growth factor (EGF) and nerve growth factor (NGF) as well as by
G-protein coupled receptors. (27, 33, 34). However, the role of Erk5 in
cytokine receptor signal transduction has not been explored. In this
study, we show that Erk5 is strongly activated by G-CSF stimulation of
cells in a MEK5-dependent manner. Interestingly, unlike
Erk1/2, JNK, and p38, maximal Erk5 activation requires the C-terminal
region of the G-CSF receptor. We demonstrate that protein-tyrosine
kinases and protein kinase C (PKC) exert differential effects on
G-CSF-induced activation of Erk5 and MEK5. Furthermore, we provide
evidence indicating that the MEK5/Erk5 pathway plays a positive role in
the regulation of G-CSF-induced cell proliferation and survival.
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MATERIALS AND METHODS |
Cells--
Murine BAF3 and 32D cells, stably transfected with
cDNAs encoding either the wild-type or the truncated forms of the
human G-CSF receptor, have been described (35, 36). Murine L-G cell line (37) was kindly provided by Dr. T. Honjo (Kyoto University Faculty
of Medicine). Cells were grown in RPMI 1640 medium supplemented with
10% fetal calf serum, 50 µM 2-mercaptoethanol,
gentamicin (50 µg/ml), and 10% WEHI-3B cell-conditioned media. BAF3
cells expressing the wild-type G-CSF receptor (BAF/WT) were
electroporated with cDNAs encoding MEK5 or dominant-active mutant
MEK5DD that were cloned in pCEFL mammalian expression vector (30).
Cells were selected in G418-containing medium, and individual clones were expanded and examined for expression of transfected proteins by
Western blotting.
Reagents--
Wortmannin was from Sigma. PD98059, SB 202190, PP1, genistein, and GF109203X were obtained from CalBiochem. U0126 was
purchased from Promega. Rabbit anti-JNK and p38 antisera used for
immunoprecipitation and kinase assays were raised against GST fusion
proteins of murine JNK and p38. Anti-Erk2 antibody (TR10) was kindly
provided by M. Weber (University of Virginia). Anti-Erk1/2 (Pan-Erk)
antibody was from Transduction Laboratories. Anti-HA and -JNK1
monoclonal antibodies were obtained from PharMingen. Antibodies to Erk5
and MEK5 were purchased from StressGen. Anti-Erk5 antibody used for immunoprecipitation and anti-p38 antibody were from Santa Cruz Biotechnologies. Phosphospecific Erk1/2 antibody was obtained from New
England BioLabs. [
-32P]ATP was purchased from Amersham
Pharmacia Biotech.
Immunoprecipitation and in Vitro Kinase Assays--
Cells were
starved in the absence of serum and conditioned media for 4 h
prior to stimulation with G-CSF for the times indicated. The
preparation of whole cell extracts, immunoprecipitation, and in
vitro kinase assays were essentially as described (38).
GST·MEF2C fusion protein (amino acids 87-467) was used for assay of
Erk5 activity, GST·ATF2(96) fusion protein for JNK and p38
activities, and myelin basic protein (MBP) for Erk2 activity.
GST·Erk5 fusion protein used for assay of MEK5 activity has been
described (34). The GST fusion proteins were prepared as described
(27).
Expression Vectors and Transient Transfection--
The
expression vectors containing the HA-tagged Erk5, MEK5, and
dominant-negative MEK5AA (Ser311 and Thr315 to
Ala) have been previously described (27). BAF3 and 32D cells expressing
the different G-CSF receptor forms were transfected by electroporation
(Electro Square Porator; BTX Genetronics Inc.) (12). Sixteen hours
after transfection, cells were deprived of serum for 4 h prior to
treatment with G-CSF for the indicated times.
Thymidine Incorporation Assay--
105 cells were
incubated in triplicate in 100 µl of serum-free medium in 96-well
plates in the presence or absence of G-CSF (10 ng/ml) for 20 h.
Cells were then pulsed with 1 µCi [3H]thymidine for
4 h and [3H]thymidine incorporation was measured by
liquid scintillation counting.
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RESULTS |
Erk5 Is Activated by G-CSF Treatment of Cells--
To investigate
the involvement of Erk5 in G-CSF-dependent signaling
pathway, we examined Erk5 activation in response to G-CSF in murine
pro-B BAF3 cells expressing the wild-type G-CSF receptor (BAF/WT).
Cells were transiently transfected with HA-tagged Erk5 and were starved
for 4 h prior to G-CSF stimulation for the indicated times. Whole
cell extracts were prepared and subjected to immunoprecipitation with
an anti-HA monoclonal antibody. In vitro kinase assays using MEF2C as a substrate revealed that G-CSF treatment of cells resulted in
transient and strong (more than 8-fold) activation of Erk5 at 15 min of
treatment (Fig. 1A). G-CSF
also induced activation of endogenous Erk5 in BAF/WT cells, myeloid 32D
cells stably transfected with the wild-type G-CSF receptor (32D/WT) and
myeloid L-G cells that expressed the endogenous G-CSF receptor (Fig.
1B). Both 32D/WT and L-G cells were capable of
differentiating into mature granulocytes in response to G-CSF (36, 37).
Activation of endogenous Erk5 appeared weaker than that seen when
assaying the transfected protein because of the poor efficiency of the
anti-Erk5 antibody to immunoprecipitate the protein (data not
shown).

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Fig. 1.
Activation of Erk5 by G-CSF.
A, kinetics of G-CSF-induced Erk5 activation in BAF3 cells
expressing the wild-type G-CSF receptor (BAF/WT). Cells were
transiently transfected with the HA-tagged Erk5 and incubated with
G-CSF (10 ng/ml) for the indicated times. Erk5 was immunoprecipitated
with an anti-HA body, and Erk5 activity was measured by in
vitro kinase assay using the GST·MEF2C fusion protein as a
substrate (upper panel). Equal loading was confirmed by
immunoblotting with an anti-Erk5 antibody (lower panel).
B, activation of endogenous Erk5 by G-CSF in BAF3 and 32D
cells transfected with the wild-type G-CSF receptor as well as in L-G
cells expressing the endogenous G-CSF receptor. Cells were stimulated
with G-CSF for 15 min, and Erk5 activity was determined by in
vitro kinase assay following immunoprecipitation with the
anti-Erk5 antibody.
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Maximal Activation of Erk5 by G-CSF Requires the C-terminal Region
of the G-CSF Receptor--
To determine which cytoplasmic regions of
the G-CSF receptor play a role in the regulation of Erk5 activation, we
assayed Erk5 activity in BAF3 cells expressing the truncated forms of the G-CSF receptor (Fig. 2A).
As shown in Fig. 2B, G-CSF stimulation of BAF3 cells
expressing the D715 receptor induced a weak increase in Erk5 activity.
The kinetics of Erk5 activation was similar to that induced by the
wild-type receptor (data not shown). In contrast, activation of
endogenous or transfected Erk1/2, JNK, and p38 was strongly stimulated
by the D715 receptor (Fig. 2B and data not shown). No Erk5
activation was observed upon G-CSF stimulation of BAF3 cells expressing
the D685 receptor over a period from 5 to 60 min (Fig. 2B
and data not shown). The D685 receptor was still able to mediate a weak
activation of Erk1/2, JNK, and p38 and has previously been shown to
activate Jak2, Stat5, and Akt (12, 35, 38). Together, these results
indicated that the C-terminal 98 amino acids of the G-CSF receptor play an important role in regulating Erk5 activation and that the region between amino acids 686 and 715 of the G-CSF receptor is indispensable for mediating G-CSF-induced Erk5 activation.

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Fig. 2.
Comparison of the requirements of the
different G-CSF receptor cytoplasmic regions for activation of Erk5,
Erk2, JNK, and p38. A, diagram of the wild-type
(WT) and the truncated forms of the G-CSF receptor. Numbers
1, 2, and 3 denote regions conserved in members of the cytokine
receptor superfamily. The four tyrosine residues present in the
cytoplasmic domain of the G-CSF receptor are also indicated.
TM, transmembrane domain. B, activation of Erk5,
Erk2, JNK, and p38 by the WT, D715, and D685 forms of the G-CSF
receptor stably expressed in BAF3 cells. Cells were left unstimulated
or stimulated with G-CSF (10 ng/ml) for 15 min. The kinase activities
of transfected Erk5, or endogenous Erk2, JNK, and p38 were examined by
in vitro kinases using the indicated substrates. The
membranes were probed with appropriate antibodies for equal
loading.
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MEK5 Is Activated by G-CSF and Is Required for G-CSF-stimulated
Activation of Erk5--
MEK5 has been shown to be required for
EGF-induced Erk5 activation (23-25, 27). We examined whether MEK5 was
activated by stimulation of cells with G-CSF. Whole cell extracts were
prepared from BAF/WT cells stimulated with G-CSF for different times.
Endogenous MEK5 was immunoprecipitated and its activity measured with
in vitro kinase assay using GST·Erk5 fusion protein as a
substrate. Incubation of cells with G-CSF stimulated MEK5 activity
(Fig. 3A). Activation of MEK5
by G-CSF occurred earlier than that of Erk5, usually 5 min after
addition of G-CSF, and the signal started to diminish after 20 min and
disappeared 60 min after G-CSF stimulation (data not shown). It
appeared that G-CSF-stimulated activation of MEK5 was slightly more
sustained than Erk5 activation, suggesting that the two kinases are
down-regulated by different mechanisms. Treatment of 32D/WT and L-G
cells with G-CSF also induced activation of endogenous MEK5 (data not
shown). Activation of MEK5 was further examined in BAF3 cells
expressing the truncated forms of the G-CSF receptor. As shown in Fig.
3B, the D715 mutant mediated a weak activation of MEK5
whereas the D685 receptor failed to induce MEK5 activation. Thus, MEK5
activation by G-CSF correlated well with that of Erk5 activation in
cells expressing the different forms of the G-CSF receptor (see Fig.
2). To determine the involvement of MEK5 in G-CSF-induced activation of
Erk5, we cotransfected BAF/WT cells with HA-tagged Erk5 and MEK5AA, a
dominant-negative form of MEK5 (27). Expression of MEK5AA strongly
inhibited Erk5 activation by G-CSF (Fig. 3C), but had no
significant effect on G-CSF-induced Erk1/2 activation (data not shown).
As shown in Fig. 3B, H2O2 treatment
of BAF3 cells stimulated Erk5 activation, which was also blocked by
MEK5AA.

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Fig. 3.
MEK5 is required for G-CSF-induced Erk5
activation. A, BAF/WT cells were unstimulated or
stimulated with G-CSF for the indicated times. Whole cell extracts were
prepared and subjected to immunoprecipitation with an anti-MEK5
antibody. MEK5 activity was measured by in vitro kinase
assay using GST·Erk5 fusion protein as a substrate. B,
G-CSF-induced activation of MEK5 in BAF3 cells expressing the different
G-CSF receptor forms. Cells were stimulated with G-CSF for 15 min.
Activation of endogenous MEK5 was measured by in vitro
kinase assay. C, BAF/WT cells were transiently transfected
with the HA-tagged Erk5 together with either the empty pCEFL vector
(Ctr) or pCEFL vector containing MEK5AA. Cells were
stimulated with G-CSF or H2O2 for 15 min, and
Erk5 activity was determined (upper panel). The membrane was
probed for Erk5 (middle panel). Aliquots of whole cell
extracts were examined for expression of MEK5AA by Western blotting
(lower panel).
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Protein-tyrosine Kinase Activity Is Required for G-CSF-induced Erk5
Activation Whereas PKC Down-regulates the Activity of the Erk5
Pathway--
To investigate whether other signaling pathways have a
role in regulating Erk5 activity, we tested the effects of different protein kinase inhibitors on G-CSF-induced Erk5 activation in BAF/WT
cells (Fig. 4). Pretreatment of cells
with tyrosine kinase inhibitor genistein (GN; lane
8) (39) or MEK inhibitor U0126 (U; lane 5)
(40) almost completely blocked G-CSF-mediated Erk5 activation. In
contrast, another MEK inhibitor PD98059 (PD; lane 4) (41) had no effect on Erk5 activation although it blocked Erk1/2 activation by G-CSF (data not shown). The differential effects
of PD98059 and U0126 on Erk5 activation by G-CSF contrasted to a recent
report that both inhibitors blocked EGF-induced activation of Erk5 in
COS-7 cells (34), and the reason for this discrepancy is unknown. The
phosphatidylinositol 3-kinase (PI3-K) inhibitor wortmannin
(WM; lane 3) (42), the p38 inhibitor SB 202190 (SB; lane 7) (43) or the Src family kinase
inhibitor PP1 (PP; lane 11) (44) had no
significant or only minimal effects. Interestingly, G-CSF-induced Erk5
activation was strongly enhanced by pretreating cells with GF109203X
(GF; lane 6), a specific inhibitor of PKC (45).

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Fig. 4.
Effects of different kinase inhibitors on
G-CSF-induced Erk5 activation. BAF/WT cells transiently
transfected with the HA-tagged Erk5 were unstimulated (lanes
1 and 9) or stimulated with G-CSF for 15 min without
(lanes 2 and 10), or with pre-incubation with
wortmannin (WM, 100 nM; lane 3),
PD98059 (PD, 50 µM; lane 4), U0126
(U, 10 µM; lane 5), GF109203X
(GF, 5 µM; lane 6), SB202190
(SB, 10 µM; lane 7), genistein
(GN, 200 µM; lane 8) or PP1
(PP, 10 µM; lane 11) for 30 min.
Erk5 activation was determined by in vitro kinase
assay.
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We further investigated whether stimulation of PKC activity would
attenuate Erk5 activation by G-CSF. BAF/WT cells were treated with
phorbol 12-myristate 13-acetate (PMA), a well known PKC activator, prior to stimulation with G-CSF. As shown in Fig.
5A, PMA alone did not affect
Erk5 activity but inhibited G-CSF-induced Erk5 activation by more than
60% as determined by phosphorimager analysis. In contrast, PMA alone
induced Erk1/2 activation and further potentiated the activation of
Erk1/2 by G-CSF, presumably by stimulating Raf-1 activity (46, 47).
GF109203X alone induced Erk5 activation in BAF/WT cells and combination
of GF109203X and G-CSF exerted a synergistic effect on Erk5 activation,
leading to an ~4.5-fold greater increase in Erk5 activity as compared
with that induced by G-CSF alone. It appeared that GF109203X slightly
potentiated G-CSF-induced activation of Erk1/2 although its effect on
Erk1/2 activation by G-CSF was significantly weaker than on Erk5
activation. Depletion of PKC through overnight treatment of cells with
PMA also potentiated Erk5 activation by G-CSF. Comparable results were
obtained in 32D/WT cells although GF109203X alone did not stimulate
Erk5 activity in 32D cells (Fig. 5B).

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Fig. 5.
PKC pathway negatively regulates the
activation of Erk5 and MEK5 by G-CSF. A, BAF/WT cells
transiently transfected with the HA-tagged Erk5 were pre-incubated
either with nothing (Ctr), or with PMA for 5 min or
overnight (o/n), or with GF109203X for 30 min. Cells were
then stimulated with G-CSF for 15 min, and Erk5 activation was
determined by in vitro kinase assay (upper
panel). The membrane was probed for Erk5 (middle
panel). Aliquots of whole cell extracts were examined by Western
blotting for Erk1/2 phosphorylation using a phosphospecific antibody
(lower panel). B, 32D cells expressing the
wild-type G-CSF receptor were transiently transfected with the
HA-tagged Erk5 and were then examined for the effects on PMA and
GF109203X on G-CSF-induced Erk5 activation. C, BAF/WT cells
were pretreated with PMA or GF109203X followed by stimulation with
G-CSF as described above. MEK5 was immunoprecipitated and its activity
was determined by in vitro kinase assay using GST·Erk5
fusion protein as a substrate.
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We examined whether G-CSF-induced activation of MEK5 is affected by
alteration of PKC activity. As shown in Fig. 5C, endogenous MEK5 activity was strongly induced by treatment of cells with GF109203X. The combination of G-CSF and GF109203X further dramatically enhanced the activity of MEK5. On the contrary, preincubation of cells
with PMA inhibited G-CSF-induced activation of MEK5. These results
suggest that PKC exerts the negative effect on G-CSF-induced Erk5
activation via inhibiting MEK5 activity either directly or indirectly.
Erk5 Signaling Pathway Plays a Positive Role in G-CSF-induced Cell
Proliferation--
To investigate whether the Erk5 pathway is involved
in the regulation of cell proliferation, we transiently transfected
BAF/WT cells with the dominant-negative mutant MEK5AA because we were unable to stably express MEK5AA in the cells. The transfection efficiency of BAF3 in several experiments was ~50% as determined by
expression of a transfected green fluorescence protein (GFP) construct
(data not shown). As shown in Fig.
6A, expression of MEK5AA
inhibited G-CSF-stimulated cell proliferation by ~30% as measured by
[3H]thymidine uptake assay. In contrast, expression of
MEK5 did not inhibit cell proliferation induced by G-CSF.

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Fig. 6.
Involvement of MEK5/Erk5 pathway in
G-CSF-dependent cell proliferation. A,
inhibition of cell proliferation by dominant-negative MEK5AA. BAF3/WT
cells were transfected with the empty vector (Ctr) or with
the cDNA encoding MEK5 or MEK5AA. 14 h after transfection,
cells were washed and incubated in serum-free medium for 20 h in
the absence or presence of G-CSF (10 ng/ml). Cells were then pulsed
with [3H]thymidine for 4 h, and
[3H]thymidine incorporation was measured. Shown are
representative data of four independent experiments, which gave similar
results. Data are presented as mean ± S.D. of triplicate
determinations. B, MEK5DD stimulated Erk5 activity. BAF/WT
cells were stably transfected with empty vector (Ctr) or
with MEK5 or MEK5DD. Whole cell extracts were prepared from transfected
cells and sequentially immunoblotted for MEK5 (upper panel)
and Stat3 for equal loading (middle panel). Cells were
transiently transfected with HA-tagged Erk5 and were left unstimulated
or stimulated with G-CSF for 15 min. The activity of Erk5 was examined
by in vitro kinase assay (lower panel).
C, MEK5DD potentiated cell proliferation in response to
G-CSF. Cells were cultured in the absence or presence of different
concentrations of G-CSF. Cell proliferation was determined by the
[3H]thymidine incorporation assay. Data are presented as
percentage of maximal proliferation induced by G-CSF at 10 ng/ml.
Analogous results were obtained with two independent clones expressing
the empty vector (Ctr), MEK5, or MEK5DD.
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We stably expressed the constitutively active MEK5DD in BAF/WT cells to
examine its effect on G-CSF biological responses. Expression of MEK5DD
but not MEK5 stimulated Erk5 activity in BAF/WT cells in the absence of
G-CSF (Fig. 6B), but did not affect the activities of
Erk1/2, JNK, p38, or Akt (data not shown). Interestingly, G-CSF
stimulation of cells expressing MEK5DD decreased Erk5 activity, possibly because of activation of a negative regulatory pathway. In the
absence of G-CSF, MEK5DD weakly stimulated the uptake of [3H]thymidine by the BAF/WT cells (about 3-fold increase
as compared with cells not transfected or transfected with MEK5).
Expression of MEK5DD potentiated cell proliferation stimulated by
suboptimal concentrations of G-CSF (0.1-0.3 ng/ml, Fig.
6C). However, when stimulated with optimal concentration of
G-CSF (10 ng/ml), the proliferation of cells expressing MEK5 or MEK5DD
showed no significant differences. Together, these results indicated
that Erk5 pathway is positively involved in regulating G-CSF
stimulated-cell proliferation.
Erk5 Pathway Positively Regulates Cell Survival--
We also
examined the effect of expression of constitutively active MEK5DD on
cell survival. Cells were washed and cultured in serum-free medium
supplemented with no G-CSF or G-CSF at concentrations as indicated in
Fig. 7. Cell viability was determined by
exclusion of trypan blue staining after 30 and 54 h of incubation
(Fig. 7, A and B). Expression of MEK5DD markedly
enhanced the viability of cells cultured in the absence of G-CSF or in
the suboptimal concentrations of G-CSF. The effect of MEK5DD was most
significant at 54 h of culture at which time point MEK5DD reduced
the concentration of G-CSF (as low as 0.03 ng/ml) required for maximal
cell survival (Fig. 7B). MEK5DD did not further
significantly increase the viability of cells that were cultured in
G-CSF at optimal concentrations (10 ng/ml).

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Fig. 7.
MEK5/Erk5 pathway enhances cell
survival. BAF/WT cells transfected with empty vector
(Ctr), MEK5, or MEK5DD were washed and cultured in the
absence or presence of different concentrations of G-CSF as indicated.
Cell viability was determined by trypan blue exclusion at 30 (A) and 54 (B) h of incubation. Two independent
clones expressing the empty vector, MEK5, or MEK5DD were examined, and
similar results were obtained.
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DISCUSSION |
Treatment of cells with G-CSF has been shown to activate the
Jak/Stat, MAPK (Erk1/2, JNK and p38), PI3-K, and Src family kinase cascades (2). Activation of these signaling cascades has been associated with G-CSF-stimulated cell proliferation although each of
these signaling events may also be uniquely involved in eliciting a
specific cellular response to G-CSF (11, 12, 48, 49). In this paper, we
show that a novel MAPK family member Erk5 and its upstream activator
MEK5 are activated upon G-CSF treatment of cells. Furthermore, we
demonstrate that the MEK5/Erk5 cascade positively regulates
G-CSF-stimulated cell proliferation and survival. Although Erk5 has
been shown to be activated by receptor-tyrosine kinases (33, 34), our
data provide the first example that Erk5 pathway is critically
implicated in cytokine receptor signal transduction and plays a central
role in the regulation of cell proliferation and survival that are
stimulated by G-CSF
Erk5 activation by G-CSF is inhibited by pretreatment of cells with
genistein, implying that protein-tyrosine kinases are involved in this
process. It has been shown that significant cross talk occurs between
distinct signal transduction pathways that are activated by a given
cytokine. For instance, activation of Raf/Erk1/2 kinase signaling by
both growth factors and interferons requires Jak2 or Jak1 (50, 51). We
recently showed that G-CSF-induced activation of Akt requires the
activity of Src family kinases whereas activation of Jaks appears to be
not essential (38). The inhibitor of Src family kinases PP1 (44, 52)
has no significant or only a minimal effect on Erk5 activation induced
by G-CSF, suggesting that Src family kinases may not be critically
involved in this activation process. It remains to be determined
whether Jaks are required for G-CSF-stimulated activation of Erk5.
In contrast to protein-tyrosine kinases, Erk5 activation in response to
G-CSF is down-regulated by a PKC-dependent pathway. A
recent report indicated that PKC activator PMA did not stimulate Erk5
activity, however, the role of PKC as a negative regulator of Erk5
activity was not examined (33). PKC stimulates the activities of
Erk1/2, JNK, and p38 (46, 53-55). Consistent with these studies, PMA
activates Erk1/2 and strongly potentiates G-CSF-induced Erk1/2 activation. These data indicate that the PKC pathway, which has been
shown to be activated by G-CSF (56), has differential effects on
G-CSF-induced activation of Erk5 and Erk1/2. It is still unknown which
PKC isoform(s) and by what mechanism the involved PKC isoform down-regulates G-CSF-stimulated Erk5 activation. The fact that G-CSF-mediated activation of MEK5 is also down-regulated by the PKC
pathway indicates that either MEK5 itself or a more upstream molecule
of the Erk5 signaling cascade is the target of regulation by PKC.
The role of Erk5 in mediating cellular responses to extracellular
stimuli is still unclear. Erk5 has been shown to phosphorylate and
activate MEF2 family transcription factors including MEF2A, MEF2C, and
MEF2D (24, 27, 28). We have also observed that G-CSF stimulated MEF2A-
and MEF2C-dependent luciferase reporters (data not shown).
It was shown recently that Erk5 was required for EGF-stimulated cell
proliferation and for Cot-induced transformation of NIH3T3 cells (30,
33). In transient transfection assays, we consistently observed that
MEK5AA inhibited the proliferation of BAF/WT cells in response to
G-CSF. Although the antiproliferative effects of MEK5AA appear modest,
they are likely to be more dramatic than appreciated because only about
50% of cells are transfected under the conditions used for the assay
(data not shown). In addition, expression of constitutively active
MEK5DD significantly potentiated cell proliferation that was stimulated
by suboptimal concentrations of G-CSF. Together, these data suggest
that the MEK5/Erk5 pathway is positively involved in G-CSF-stimulated
cell proliferation. More interestingly, our data also indicate that the
MEK5/Erk5 pathway is an important mediator of the survival signals
transduced by the G-CSF receptor. Expression of MEK5DD in BAF/WT cell
protected cells from apoptosis induced by cytokine withdrawal and
significantly reduced the concentrations of G-CSF required for inducing
maximal cell survival. Notably, maximal survival of cells expressing
MEK5DD required a G-CSF concentration of only 0.03 ng/ml whereas G-CSF at 0.1 ng/ml induced less than 40% of maximal proliferation of the
same cells. It thus appears that the Erk5 pathway has a more significant effect on cell survival than on cell proliferation.
Whether or not Erk5 is also implicated in the regulation of other
biological activities of G-CSF remains to be investigated. Interestingly, the C-terminal region of the G-CSF receptor, a region
required for induction of myeloid differentiation, plays an important
role in the regulation of Erk5 activation in that truncation of this
region markedly compromises the ability of the receptor to mediate Erk5
activation. In contrast, this carboxyl terminus does not play a major
role in G-CSF-induced activation of Erk1/2, JNK, and p38. These
results, together with the differential effects of PMA on Erk2 and Erk5
activation by G-CSF, suggest that distinct mechanisms are involved in
regulating G-CSF-induced activation of Erk5 and other members of the
MAPK family. The results reported here provide a framework to explore
the roles of Ekr5 and its downstream substrates in the biological
actions of G-CSF.
Published, JBC Papers in Press, January 17, 2001, DOI 10.1074/jbc.M008748200
The abbreviations used are:
G-CSF, granulocyte
colony-stimulating factor;
MAPK, mitogen-activated protein kinase;
PI3-K, phosphatidylinositol 3-kinase;
PKC, protein kinase C;
PMA, phorbol 12-myristate 13-acetate;
GFP, green fluorescent protein;
GST, glutathione S-transferase;
HA, hemagglutinin;
Stat, signal
transducers and activators of transcription;
Jak, Janus kinase;
JNK, c-Jun N-terminal kinase;
MEK, MAPK kinase;
Erk, extracellular-regulated
kinase.
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