(Received for publication, February 18, 1997, and in revised form, April 8, 1997)
From the Department of Medicine, Division of Medical Oncology, University of Colorado Health Sciences Center, Denver, Colorado 80262
Phorbol ester tumor promoters, such as phorbol 12-myristate 13-acetate (PMA), are potent activators of extracellular signal-regulated kinase 2 (ERK2), stress-activated protein kinase (SAPK), and p38 mitogen-activated protein kinase (MAPK) in U937 human leukemic cells. These kinases are regulated by the reversible dual phosphorylation of conserved threonine and tyrosine residues. The dual specificity protein phosphatase MAPK phosphatase-1 (MKP-1) has been shown to dephosphorylate and inactivate ERK2, SAPK, and p38 MAPK in transient transfection studies. Here we demonstrate that PMA treatment induces MKP-1 protein expression in U937 cells, which is detectable within 30 min with maximal levels attained after 4 h. This time course coincides with the rapid inactivation of PMA-induced SAPK activity, but not ERK2 phosphorylation, which remains elevated for up to 6 h. To examine directly the role of MKP-1 in the regulation of these protein kinases in vivo, we established a U937 cell line that conditionally expresses MKP-1 from the human metallothionein IIa promoter. Conditional expression of MKP-1 inhibited PMA-induced ERK2, SAPK, and p38 MAPK activity. By titrating the levels of MKP-1 expression from the human metallothionein IIa promoter, however, it was found that p38 MAPK and SAPK were much more sensitive to inhibition by MKP-1 than ERK2. This differential substrate specificity of MKP-1 can be functionally extended to nuclear transcriptional events in that PMA-induced c-Jun transcriptional activity was more sensitive to inhibition by MKP-1 than either Elk-1 or c-Myc. Conditional expression of MKP-1 also abolished the induction of endogenous MKP-1 protein expression in response to PMA treatment. This negative feedback regulatory mechanism is likely due to MKP-1-mediated inhibition of ERK2, as studies utilizing the MEK1/2 inhibitor PD98059 suggest that ERK2 activation is required for PMA-induced MKP-1 expression. These findings suggest that ERK2-mediated induction of MKP-1 may play an important role in preferentially attenuating signaling through the p38 MAPK and SAPK signal transduction pathways.
Mitogen-activated protein (MAP)1 kinases play a key role in transducing various extracellular signals to the nucleus (1). The MAP kinases (MAPKs) consist of three major subgroups that include the ERK, SAPK/JNK, and p38 MAPK families (2). The ERKs, SAPKs, and p38 MAPK are activated by the reversible dual threonine and tyrosine phosphorylation of a conserved TEY, TPY, or TGY motif, respectively (1-5). Although distinct and selective activators of the MAPKs have been cloned and characterized (6-9), less in known about the negative regulation of these kinases. The reversible nature of MAPK phosphorylation suggests that protein phosphatases play an important role in regulating MAPK activity. An expanding subfamily of dual specificity protein tyrosine phosphatases has been identified which is capable of dephosphorylating and inactivating various members of the MAPK family. This class of phosphatases is characterized by MKP-1 (also known as CL100, 3CH134, and Erp) (10-13). MKP-1 expression is highly inducible in response to mitogenic and stress stimuli (10-13). In this regard, the kinetics of serum-induced MKP-1 expression correlate with the inactivation of ERK2, suggesting a physiological role for MKP-1 in the attenuation of ERK2 activity (14). However, the rapid first phase inactivation of ERK2 after growth factor stimulation occurs prior to the induction of MKP-1 protein expression (15). ERK2 inactivation can also occur under conditions where MKP-1 protein synthesis is inhibited (15). Furthermore, the magnitude and duration of serum-induced ERK2 activity are unaltered in MKP-1-deficient mouse embryo fibroblasts (16). Thus, it is currently unclear whether MKP-1 is a physiological regulator of ERK2 in vivo.
Although initial reports indicated that MKP-1 selectively dephosphorylates ERK2 both in vivo and in vitro (14, 17), recent studies suggest that MKP-1 exhibits a broad substrate specificity, being capable of inactivating both SAPK and p38 MAPK in transient transfection studies (5, 18-22). PAC1 and MKP-2 have also been shown to inactivate multiple members of the MAPK family (21, 23, 24). Thus, certain dual specificity protein tyrosine phosphatases appear to exhibit a high degree of cross-reactivity with regard to substrate specificity. In this study we have examined the correlation between MKP-1 expression and the inhibition of ERK2, SAPK, and p38 MAPK in U937 human leukemic cells. To examine the in vivo substrate specificity of MKP-1 in detail we have established a U937 cell line that conditionally expresses MKP-1. We demonstrate that at sufficient levels of expression, MKP-1 is capable of inactivating all members of the MAPK family. However, titration of the levels of MKP-1 expression revealed that MKP-1 preferentially inactivates p38 MAPK and SAPK. This specificity can be extended to nuclear transcriptional events as MKP-1 preferentially inhibits PMA-induced c-Jun transcriptional activity when compared with both Elk-1 and c-Myc activity. Furthermore, we provide evidence that ERK2, and not p38 MAPK or SAPK, mediates MKP-1 induction in U937 cells. These findings suggest the existence of a cross-talk mechanism between the mitogen- and stress-induced signal transduction pathways, whereby ERK2-mediated induction of MKP-1 may play an important role in attenuating signaling through the p38 MAPK and SAPK pathways.
hMTIIa-MKP1(myc) was constructed by
subcloning a 1.1-kilobase HindIII/BamHI fragment
from pCEP4-MKP1(myc) (the gift of N. Tonks and H. Sun) (14, 17), which
encodes the first 314 amino acids of 3CH134 and contains a
COOH-terminal Myc epitope tag, into the pMEP4 expression vector
(Invitrogen). The resulting expression vector contained a hygromycin
B-selectable marker and was driven by the human IIa metallothionein
(hMTIIa) promoter. GST-cJun(5-89) and GAL4-cJun(5-89) have been
described previously (19, 25, 26). pCMV-FLAG-p38 MAPK (7) was provided
by R. Davis. GST-ATF2(1-109) was provided by M. Green, and
GST-cMyc(1-262) was provided by E. Blackwood. Polyclonal SAPK
antiserum was raised in rabbits using a GST-p54 SAPK fusion protein
as antigen (the gift of J. Woodgett). The anti-Myc epitope monoclonal
antibody 9E10 was provided by N. Tonks and H. Sun. The anti-FLAG (M2),
anti-HA (12CA5), and anti-ERK2 (107) antibodies were obtained from
IBI-Kodak, Babco, and Zymed, respectively. The anti-MKP-1 (C19) and
anti-p38 MAPK (C20) antibodies were from Santa Cruz. The 3X-TRE-Luc and
4X-NF
B-Luc reporter plasmids were provided by E. Clark (27). The
Jun-Luc and Fos-Luc reporter plasmids were provided by D. Brenner (28, 29). GAL4-Myc(7-101) and GAL4-Elk-1(83-428) were provided by G. Johnson (30). The MEK1/2 inhibitor PD98059 was purchased from
Calbiochem. The p38 MAPK inhibitor SB203580 was the gift of L. Shapiro,
and C. Dinorello).
U937 cells were cultured in Dulbecco's modified Eagle's medium supplemented with 10% bovine calf serum (31). Transient transfections were performed using the DEAE-dextran procedure as described previously (32). Stable transfections were performed by electroporation (26). Two days post-transfection, cells were cultured in selective media containing 200 µg/ml hygromycin B (Calbiochem). After 3 weeks of selection, resistant cell populations were pooled and cultured continuously in the presence of hygromycin B.
Protein Kinase AssaysWhole cell extracts were prepared by
lysing cells for 30 min on ice in cell lysis buffer (20 mM
Hepes, pH 7.4, 1% Triton X-100, 2 mM EGTA, 10% glycerol,
50 mM -glycerophosphate, 1 mM sodium vanadate, 0.5 mM benzamidine-HCl, 0.5 mM
phenylmethylsulfonyl fluoride, 1 mM dithiothreitol, and 10 µg/ml aprotinin, leupeptin, and pepstatin). SAPK activity was
measured by immunocomplex kinase assay using anti-SAPK
immunoprecipitates and GST-cJun(5-89) as substrate (19). For some
experiments SAPK activity was determined using the GST-cJun(5-89)
solid phase kinase assay (33), which produced identical results.
Epitope-tagged FLAG-p38 MAPK activity was determined by immunocomplex
kinase assay using anti-FLAG immunoprecipitates and GST-ATF2(1-109) as
substrate (5). Alternatively, endogenous p38 MAPK activity was
determined utilizing anti-p38 MAPK immunoprecipitates. Endogenous ERK2
activity was determined by an in-gel kinase assay utilizing myelin
basic protein as substrate (34). Cells transiently transfected with
HA-ERK2 were also used to analyze ERK2 activity. HA-ERK2 was
immunoprecipitated with anti-HA monoclonal antibody (12CA5) and
immunocomplex kinase assays performed using GST-cMyc(1-262) as
substrate.
Whole cell extracts were resolved by SDS-polyacrylamide gel electrophoresis and electroblotted onto nitrocellulose. Endogenous ERK2 and MKP-1 expression were analyzed by immunoblotting with either an ERK2 monoclonal or MKP-1 polyclonal antibody as described previously (19). For analysis of inducible MKP-1(myc) protein expression from the hMTIIa promoter, U937 cells (5 × 106) were metabolically labeled in the absence or presence of 0.5 µM CdSO4 for 4 h in methionine-free Dulbecco's modified Eagle's medium containing 10% dialyzed bovine calf serum and 0.1 mCi/ml Tran35S-label (NEN Life Science Products). Whole cell extracts were prepared as described previously (25, 26, 31) and MKP-1(myc) protein immunoprecipitated for 4 h using 9E10 antibody precoupled to protein A/G-Sepharose (Oncogene Science). Immunoprecipitates were washed extensively in cell lysis buffer, resolved by SDS-polyacrylamide gel electrophoresis, and visualized by fluorography.
Transcriptional StudiesU937 cells (12 × 106) stably expressing either hMTIIa vector or hMTIIa-MKP-1(myc) were transiently transfected in batch with 10 µg of the indicated Luc reporter plasmid. In the GAL4 chimeric activator transcriptional studies, cells were cotransfected with 10 µg of the GAL4 chimeric activator and 10 µg of a 5X-GAL4-Luc reporter, which consists of five GAL4 binding sites upstream of a luciferase reporter gene. Cells were split into 12-well plates and cultured overnight (16-24 h). Cells were treated as indicated and cell extracts analyzed for luciferase activity.
PMA is a potent activator of both ERK2 and SAPK in U937 cells (19,
31). In an attempt to determine whether MKP-1 may be a physiological
regulator of these protein kinases, we compared the kinetics of
PMA-induced MKP-1 expression with the inactivation of PMA-induced ERK2
and SAPK activity in U937 cells. PMA treatment caused a rapid induction
of MKP-1 protein expression (Fig. 1, top
panel). MKP-1 protein could be detected within 30 min, and maximal
levels of expression were observed at 4 h of PMA treatment. PMA
treatment was also found to induce a rapid and transient activation of
SAPK (Fig. 1, middle panel). Furthermore, the attenuation of SAPK activity coincided with the appearance of MKP-1 protein
expression. In contrast to the rapid attenuation of PMA-induced SAPK
activity, PMA treatment resulted in a prolonged phosphorylation of ERK2 as judged by its characteristic shift in electrophoretic mobility (Fig.
1, bottom panel). Although some reduction in ERK2
phosphorylation could be detected after 2 h of PMA treatment, ERK2
phosphorylation remained elevated for up to 6 h. Therefore, the
kinetics of PMA-induced MKP-1 expression correlate more with the
attenuation of SAPK activity than with the dephosphorylation of ERK2 in
U937 cells.
To examine directly the role of MKP-1 in the regulation of SAPK and
ERK2 in vivo, we established a U937 cell line conditionally expressing MKP-1 from the heavy metal-inducible hMTIIa promoter. U937
cells were transfected with either hMTIIa vector alone or hMTIIa-MKP-1
(containing a COOH-terminal Myc epitope tag) and stable cell lines
derived by selection in the presence of hygromycin B. No MKP-1 protein
could be detected in untreated or CdSO4-treated hMTIIa
vector-transfected control cells (Fig. 2). Although a
low basal level of MKP-1 was observed in uninduced hMTIIa-MKP1 cells, MKP-1 expression was enhanced 40-fold after induction with
CdSO4 for 4 h (Fig. 2). The establishment of a U937
cell line conditionally expressing MKP-1 allowed us to examine the
in vivo substrate specificity of MKP-1. As previous and
preliminary studies had demonstrated that PMA was a potent activator of
ERK2, SAPK, and p38 MAPK in U937 cells, we were able to examine whether
conditional expression of MKP-1 functionally inhibited these kinases
under identical cellular conditions. Cells were pretreated with 0.5 µM CdSO4 for 4 h prior to treatment with
PMA for 30 min. Pretreatment of the hMTIIa vector-transfected U937 cell
line with CdSO4 had no effect on PMA-induced ERK2
phosphorylation or ERK2, SAPK, or p38 MAPK activity (Fig.
3, A, B, and C,
respectively). Conditional expression of MKP-1 in the hMTIIa-MKP1 cell
line, however, abolished PMA-induced ERK2 phosphorylation and activity
(Fig. 3A). Conditional expression of MKP-1 was also found to
inhibit PMA-induced SAPK and p38 MAPK activity (Fig. 3, B
and C, respectively).
To examine more closely the in vivo substrate specificity of
MKP-1, we attempted to vary the level of MKP-1 expression from the
hMTIIa promoter. The activity of the hMTIIa promoter is highly sensitive to changes in heavy metal concentrations. To determine whether the magnitude of MKP-1 protein expression from the hMTIIa promoter could be regulated by varying the CdSO4
concentration, MKP-1 was immunoprecipitated from the hMTIIa-MKP1 U937
cell line metabolically labeled for 4 h with
[35S]methionine in the absence or presence of increasing
concentrations of CdSO4. Inducible MKP-1 protein expression
was found to be proportional to the concentration of CdSO4
utilized to induce the hMTIIa promoter (Fig.
4A). Thus, the sensitivities of ERK2, SAPK,
and p38 MAPK to inhibition by various levels of MKP-1 expression were
analyzed. Consistent with the results presented in Fig. 3, maximal
induction of MKP-1 with 1 µM CdSO4 resulted
in the inhibition of PMA-induced ERK2, SAPK, and p38 MAPK activity
(Fig. 4B). However, titration of the level of MKP-1
expression revealed that p38 MAPK and SAPK were significantly more
sensitive to inhibition by MKP-1 than ERK2 (Fig. 4B). In
this regard, PMA-induced p38 MAPK activity was inhibited completely
when little MKP-1 protein expression could be detected. These results
suggest that MKP-1 may play a more selective role in attenuating
signaling through the p38 MAPK and SAPK pathways than the ERK2 pathway
in U937 cells.
SAPK mediates the phosphorylation of the c-Jun NH2-terminal
transcriptional activation domain, which enhances c-Jun transcriptional activity (3, 4, 26, 35). Because the conditional expression of MKP-1
abolished PMA-induced SAPK activity, we sought to determine whether
this effect led to the inhibition of PMA-induced c-Jun transcriptional
activity. To analyze c-Jun transcriptional activity, cells were
transfected with a luciferase (Luc) reporter gene containing five
upstream GAL4 DNA binding sites together with a GAL4-cJun(5-89) chimeric activator plasmid containing the c-Jun
NH2-terminal transcriptional activation domain fused to the
GAL4 DNA binding domain (19, 26). PMA stimulated a 10-15
fold-induction of GAL4-cJun transcriptional activity in the hMTIIa-MKP1
cell line (Fig. 5A). Preinduction of MKP-1
expression with CdSO4, however, abolished PMA-induced GAL4-cJun activity. c-Jun is a major component of the AP-1
transcription factor which mediates the induction of various genes
whose promoters contain TRE sites, including c-jun and
c-fos. Thus, we examined the effect of MKP-1 on a
Luc-reporter containing three TRE sites (3X-TRE-Luc), as well as
Jun-Luc and Fos-Luc reporters, which contain the jun and
fos promoters, respectively. PMA-induced TRE-Luc, Jun-Luc,
and Fos-Luc activity was also inhibited by the conditional expression
of MKP-1 (Fig. 5, B, C, and D,
respectively). MKP-1 does not function as a general inhibitor of
activated transcription as conditional expression of MKP-1 was found to
have no effect on PMA-induced activation of a 4X-NFB-Luc reporter
(Fig. 5E). Furthermore, CdSO4 pretreatment of
U937 cells stably expressing the hMTIIa vector alone had no effect on
any of these PMA-induced transcriptional responses (data not
shown).
The studies described above demonstrate that conditional expression of
MKP-1 at levels that inhibit PMA-induced ERK2, SAPK, and p38 MAPK
activity blocks a wide variety of PMA-induced transcriptional responses. To examine more closely whether the preferential inhibition of SAPK by MKP-1 results in the selective inhibition of specific PMA-induced transcriptional responses, we examined the effect of
conditionally expressing various levels of MKP-1 on PMA-induced activation of several individual transcription factors. SAPK and ERK2
appear to be the sole mediators of c-Jun and c-Myc
NH2-terminal phosphorylation and activation, respectively
(3, 4, 7, 8, 36). Thus, we employed the GAL4-cJun(5-89) activator as an indicator of SAPK-mediated transcription and a GAL4-cMyc(7-101) activator as an indicator of ERK2-mediated transcription. The effect of
MKP-1 on a GAL4-Elk-1(183-428) activator was also examined because of
its coordinate regulation by both SAPK and ERK2 (37). Cells stably
expressing hMTIIa-MKP1 were cotransfected with these GAL4 chimeric
activator plasmids and a 5X-GAL4-Luc reporter. After induction of
increasing levels of MKP-1 protein expression with various
concentrations of CdSO4, cells were treated with PMA for 12 h and luciferase activity determined. PMA-induced GAL4-cJun transcriptional activity was found to be 10-fold more sensitive to
inhibition by conditional expression of MKP-1 than GAL4-cMyc (IC50 = 12 nM and 127 nM
CdSO4, respectively) (Fig. 6). In contrast, GAL4-Elk-1 exhibited an intermediate sensitivity to inhibition by MKP-1
(IC50 = 38 nM CdSO4). These
findings suggest that the differential substrate specificity of MKP-1
with regard to PMA-induced SAPK and ERK2 activity can be functionally
extended to SAPK- and ERK2-mediated nuclear transcriptional events.
MKP-1 is an immediate-early gene product that is induced rapidly in
response to both mitogenic and stress stimuli (10, 11, 13-15, 20,
38-42). Although PMA selectively activates the ERK2 signal
transduction pathway in most cell types (3-5), U937 cells provide a
unique model in which PMA potently stimulates not only ERK2 activity
but also SAPK and p38 MAPK activity (19, 26, 31, 36 and Figs. 3 and 4).
Furthermore, both the SAPK and ERK2 signal transduction pathways have
been implicated in the regulation of MKP-1 expression in response to
various extracellular stimuli (20, 38, 39). We therefore examined the
role of these signal transduction pathways in mediating PMA-induced
MKP-1 expression in U937 cells. To determine the role of SAPK in
mediating the induction of MKP-1 protein, we utilized UV radiation to
activate selectively SAPK and not ERK2 activity in U937 cells. UV
radiation (100 J/m2) elicited a rapid transient activation
of SAPK in U937 cells (Fig. 7A). The time
course and fold activation of SAPK in response to UV radiation closely
paralleled those observed in response to PMA treatment (compare with
Fig. 1). UV radiation also stimulated p38 MAPK activity with a time
course and potency similar to those observed for SAPK (data not shown).
In contrast to PMA treatment (Fig. 7B, lanes 9 and 10), however, UV radiation had no effect on ERK2
phosphorylation (Fig. 7B, upper panel).
Furthermore, although PMA induced a dramatic induction of MKP-1
protein, none could be detected in response to UV radiation (Fig.
7B, lower panel). Prolonged exposure of these
Western blots did not reveal activation of ERK2 or induction of MKP-1
in response to UV radiation (data not shown). These findings suggest
that activation of the SAPK and p38 MAPK pathways is not sufficient to
induce MKP-1 protein expression in U937 cells. These findings also
indicate that inactivation of UV-induced SAPK and p38 MAPK does not
involve induction of MKP-1 protein.
Although activation of p38 MAPK was not sufficient to induce MKP-1
expression in response to UV radiation, we examined directly the
involvement of p38 MAPK activity in mediating PMA-induced MKP-1
expression by utilizing the specific p38 MAPK inhibitor SB203580 (43).
Pretreatment of U937 cells with SB203580 was found to inhibit
PMA-induced p38 MAPK activity in a dose-dependent manner,
with 0.3 µM SB203580 inhibiting p38 MAPK by 80% (Fig. 8, upper panel). Consistent with previous
studies (43), SB203580 had no effect on PMA-induced ERK2 or SAPK
activity in U937 cells (data not shown). Paradoxically, higher
concentrations of SB203580 resulted in less inhibition of PMA-induced
p38 MAPK activity (data not shown). Importantly, inhibiting PMA-induced
p38 MAPK activity by 80% had no effect on PMA-induced MKP-1 expression
(Fig. 8, lower panel). In aggregate, these studies suggest
that activation of the SAPK and p38 MAPK pathway are neither required
nor sufficient for induction of MKP-1 expression in U937 cells.
Many stimuli that induce MKP-1 expression, such as serum and mitogenic
growth factors, potently activate the ERK2 pathway yet have little
effect on SAPK and p38 MAPK activity. We therefore examined whether the
ERK2 pathway was required for PMA-induced MKP-1 expression. PMA-induced
activation of ERK2 is mediated by the sequential activation of Raf-1
and MEK1/2 (1, 2). Thus the MEK1/2-specific inhibitor PD98059 was
utilized to block PMA-induced ERK2 activity (44). Pretreatment of U937
cells with 100 µM PD98059 resulted in a dramatic
inhibition in PMA-induced ERK2 phosphorylation (Fig. 9,
upper panel). Furthermore, this attenuation in PMA-induced ERK2 activation resulted in a near complete inhibition of PMA-induced MKP-1 expression (Fig. 9, lower panel). The residual
induction of MKP-1 in the presence of PD98059 is likely because of the
incomplete inhibition of PMA-induced ERK2 activity. These findings
suggest that the ERK2 pathway predominantly mediates PMA-induced MKP-1 expression in U937 cells.
We have shown that conditional expression of MKP-1 is capable of
inhibiting PMA-induced ERK2 activity and ERK2-mediated gene transcription and that ERK2 mediates PMA-induced MKP-1 expression. It
was thus of interest to determine whether conditional expression of
MKP-1 blocked PMA-induced endogenous MKP-1 expression. Cells stably
expressing hMTIIa-MKP1 were pretreated with increasing concentrations
of CdSO4 to inhibit preferentially the various PMA-induced
protein kinase and transcriptional activities (see Figs. 4 and 6).
Cells were then treated with PMA for 4 h, which resulted in
maximal endogenous MKP-1 protein expression (see Fig. 1). Preinduction
of hMTIIa-MKP-1 resulted in a dose-dependent inhibition of
PMA-induced endogenous MKP-1 protein expression (Fig.
10). The efficacy of this inhibition was similar to the
inhibition of PMA-induced ERK2 activity, although no inhibition of
PMA-induced MKP-1 expression was observed at CdSO4
concentrations that completely inhibited PMA-induced p38 MAPK activity
(compare with Fig. 4). These findings suggest that MKP-1 regulates its
own expression by a classical negative feedback regulatory
mechanism.
MKP-1 was originally identified as a PMA- and mitogen-inducible immediate-early gene product (11, 13). MKP-1 was subsequently found to be a dual specificity phosphatase, exhibiting some selectivity for dephosphorylating and inactivating ERK2 (12, 14). More recent studies utilizing transient transfection techniques, however, demonstrate that MKP-1 exhibits a broad substrate specificity, being capable of inactivating not only ERK2 but also SAPK and p38 MAPK (19-22). In this report we have adopted the approach of stably expressing MKP-1 under the control of the inducible hMTIIa promoter as a means of examining the substrate specificity and functional effects of MKP-1 in vivo. This approach has several advantages over transient transfection protocols. First, induction of the hMTIIa promoter with a given concentration of CdSO4 consistently led to equivalent levels of MKP-1 expression. Second, the level of MKP-1 protein expression could be strictly controlled by varying the concentration of CdSO4 utilized for induction. Finally, in contrast to transient transfection studies, which result in the constitutive expression of MKP-1 for up to 48 h, conditionally expressing MKP-1 in this manner more closely mimics the induction of MKP-1 observed under physiological conditions. This is an important consideration when analyzing the functional effects of MKP-1 as several groups have shown that constitutive expression of MKP-1 blocks cell cycle progression in fibroblasts (13, 17, 38, 45). An additional advantage of the U937 cell system involves the unusual ability of PMA to activate potently not only ERK2 but also the SAPK and p38 MAPK signal transduction pathways. Although the molecular mechanisms mediating PMA-induced SAPK and p38 MAPK activity in U937 cells have not been elucidated, this unique characteristic allowed us to examine the functional effects of MKP-1 on ERK2, SAPK, and p38 MAPK in response to a single extracellular stimulus.
In accordance with previous reports (11, 13, 40), PMA was found to induce MKP-1 protein expression in U937 cells rapidly. The time course of MKP-1 expression coincided with the rapid attenuation of PMA-induced SAPK activity, whereas ERK2 phosphorylation remained elevated for up to 6 h. This correlation led us to hypothesize that MKP-1 may inactivate SAPK preferentially with greater specificity than ERK2 in U937 cells. Although conditional expression of high levels of MKP-1 in U937 cells abolished PMA-induced ERK2, SAPK, and p38 MAPK activity, by titrating the levels of MKP-1 expressed from the hMTIIa-MKP1 expression vector we were able to demonstrate that MKP-1 inactivated p38 MAPK and SAPK preferentially compared with its effect on ERK2 activity. These findings not only substantiate previous studies demonstrating a broad substrate specificity for MKP-1 (5, 18, 19-22) but also indicate that the selectivity of MKP-1 for p38 MAPK, SAPK, and ERK2 is highly dependent on the relative level of MKP-1 expression. These results appear to contradict a recent report that suggests that transiently transfected MKP-1 inactivates these kinases with identical substrate specificity (21). However, only two concentrations of MKP-1 cDNA were utilized to demonstrate substrate specificity, with the lowest amount analyzed inhibiting 50-100% of the measured kinase activity in response to various extracellular stimuli (21). Therefore, this apparent discrepancy may merely be due to our more extensive analysis of this dose-dependent effect. Our findings are consistent with a model in which PMA-induced SAPK and p38 MAPK are inactivated rapidly when PMA-induced MKP-1 expression is submaximal (1-2 h), whereas ERK2 inactivation requires much higher levels of MKP-1 expression such as those observed after 4-6 h of PMA treatment. However, the physiological relevance of this concentration effect is presently unclear. The studies described in this report were performed utilizing a MKP-1 expression vector encoding a COOH-terminal truncation (14, 17). Unfortunately, all commercially available antibodies to MKP-1 are directed against the COOH-terminal region of MKP-1, thus precluding a direct comparison of the levels of MKP-1 expressed from the hMTIIa promoter relative to that observed in response to PMA treatment.
Both mitogenic and stress stimuli have been reported to induce MKP-1 expression in various cell types (10, 11, 13-15, 20, 38-42). Furthermore, both the SAPK and ERK2 signal transduction pathways have been implicated in the regulation of MKP-1 expression (20, 39). Here we report that PMA, which activates both of these pathways potently, induces MKP-1 expression rapidly. The MEK1/2 inhibitor PD98059 dramatically reduced PMA-induced ERK2 activation and MKP-1 expression, suggesting that activation of the ERK2 signal transduction pathway is necessary for PMA-induced MKP-1 expression in U937 cells. Furthermore, the inability of UV radiation, which activates SAPK and p38 MAPK potently and selectively, to induce MKP-1 expression, suggests that the SAPK and p38 MAPK pathways do not play a significant role in mediating MKP-1 expression in U937 cells. These findings are in agreement with those of Brondello et al. (38), who have demonstrated that activation of SAPK by various stress stimuli does not induce MKP-1 protein expression, whereas selective activation of the Raf/MEK/ERK pathway was necessary and sufficient for MKP-1 expression in hamster fibroblasts. However, our results contradict previous reports that suggest that UV-induced SAPK activity is capable of mediating MKP-1 expression in HeLa and NIH3T3 cells (20, 39). This apparent discrepancy may be because of the ability of UV radiation to activate ERK2 in a cell type-specific manner. In this regard, UV radiation has been shown to activate ERK2 in HeLa cells (47). However, we were unable to detect ERK2 activation in U937 cells in response to UV radiation (Fig. 7). Although higher doses of UV radiation may be capable of inducing ERK2 activation in U937 cells, our results indicate that doses that activate both SAPK and p38 MAPK potently have no effect on MKP-1 expression.
Although our data are consistent with a model in which PMA-induced MKP-1 inactivates SAPK and p38 MAPK preferentially, it is presently unclear whether MKP-1 is a physiological phosphatase for either of these kinases. In this regard, UV-induced SAPK is inactivated rapidly in the absence of MKP-1 induction in U937 cells, suggesting that additional dual specificity phosphatases are capable of mediating the attenuation of stress-induced SAPK activity in the absence of ERK2-mediated MKP-1 expression. Recently, several additional MKP-1 family members have been identified which are capable of inactivating SAPK and/or p38 MAPK, including MKP-2 and PAC1 (21, 23, 38). Furthermore, the cytosolically localized dual specificity phosphatase M3/6 has been shown to exhibit an even greater selectivity for inactivating SAPK and p38 MAPK than that demonstrated here for MKP-1 (46). Whether these or other members of the MKP-1 family play a physiological role in attenuating signaling through the SAPK and p38 MAPK pathways remains to be determined. In aggregate, our findings are consistent with a model in which activation of the mitogen-activated signal transduction pathway and subsequent ERK2-mediated MKP-1 expression may play an important role in preferentially attenuating signaling through the stress-activated protein kinases, p38 and SAPK. The existence of such a negative cross-talk mechanism between the mitogen- and stress-activated signal transduction pathways has significant physiological relevance based on the biological effects mediated by the ERK2 and p38/SAPK pathways. There is growing evidence to indicate that these pathways appear to play antagonistic roles in regulating either cellular growth, differentiation and/or survival (ERK2), or cell death (p38/SAPK) (48). ERK2-mediated induction of MKP-1 may play an important role in selectively attenuating stress-induced signaling and directing cell differentiation, growth, or survival rather than cell death. Furthermore, the ability of MKP-1 to inhibit its own expression through a negative feedback mechanism reveals an additional level of regulation whereby MKP-1 not only attenuates signaling through the ERK, SAPK, and p38 MAPK pathways but may also play a role in resensitizing cells to subsequent extracellular stimuli by reducing its own expression and perhaps that of other dual specificity phosphatases.
For graciously providing reagents used in this study we thank N. Tonks, H. Sun, J. Woodgett, R. Davis, M. Green, E. Blackwood, D. Brenner, G. Johnson, L. Shapiro, C. Dinorello, and E. Clark. We also thank F. Wagner for expert technical assistance.